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I. Chemical, Crystallographical and Physical Properties of Liquid Paraffins and Paraffin Waxes
I. CHEMICAL, CRYSTALLOGRAPHICAL AND PHYSICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
(A) Liquid paraffins and paraffin waxes from petroleum 1. Composition of petroleum distillation products /
Crude oils and their products contain a large number of individual paraffins. The main physical characteristics of the most frequently occurring alkanes are listed in Table 1-1, indicating that n-pentane is already liquid and n-hexadecane solid at ambient temperature. Table I-I. Physical characteristics of some alkanes occurring in petroleum
Methane Ethane Propane
CH, CZH, CaH,
16 30 44
Butane n-butane iso-butane
C.810
58
Pentane n-pentane 2-methylbutane (iso-pentane) 2,2-dimethylpropane (neopentane)
C,Hn
72
Hexane n-hexane 2-methylpentane
GHi,
86
- 161.5 - 88.5 -42.0
0.424' 0.546' 0.582'
-82 32 96
4.2
- 138.5 - 159.5
- 0.5 - 12.0
0.602' 0.596'
153 134
3.7 3.8
- 129.5
36.0
0.625
197
3.3
- 159.5
28.0
0.620
188
3.4
- 16.5
9.5
0.6139
184
3.5
-94.0 - 153.5
69.0 60.0 63.0
0.659 0.656 0.664
235 228 227
-
-98.0
49.5
0.649
212
-
- 129.0
58.0
0.662
221
3.1
-99.5 between
98.5 19-93
0.684
267
2.7 2.12.9
-118.0
3-methy lpentane
2,2-dimethylbutane 2,3-dimethylbutane Heptane n-heptane iso-heptanes
-182.5 -183.0 -187.0
C7H16
100
and
-25
:.%)
%)
4.6
5.0
2.9 3.2
14
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
I I
Table I-1 (cont.)
I
Compound
I
Formula
~
Octane n-octane 2,2,3-trimethylpentane 2,2,4-trimethylpentane (isooctane) other iso-octanes
$: 114
I I
Melting point, "C (101 kPa)
Boiling point, "C (101 kPa)
-57.0
125.5
0.703
296
2.5
110.5
0.716
285
-
99.5
0.692
268
-
- 107.5 between
Density
at 2ooc
crit. t e m ~ , . Crit. oc press,, MPa
and
Nonane n-nonane iso-nonanes
128
Decane n-decane iso-decanes
142
n-Hendecane n-Dodecane n-Tetradecane n-Hexadecane n-Octadecane n-Eicosane n-Pentacosane n-Triacontane n-Pentatriacontane n-Pentacontane
+ 1046 -53.3
-
150.5
0.718
323
-29.5
173.5 147-1 168
)-\:.0.730
347
-
156 170 198 226 254 282 352 422 492 702
-25.5 -9.5 5.5 18.0 28.0 36.5 53.5 66.0 74.5 92.0
196 216 254 287 308 2054 25g4 3044 3314 4214
0.740 0.749 0.763 0.174 0.782 0.789 0.801 0.810 0.7813 0.7943
-
369 391 429 462 49 1 513
2.3
-
2.1
-
593
2.0 1.9 1.7 1.5 1.4 1.3 1.o
708
0.7
-
-
-
At the boiling point *At0°C
Liquid density at the melting point ' A t 2.1 kPa 104 'C: Hexamethylethane a
+
Even the lower boiling-point fractions of petroleum contain, depending on the source of the crude, in addition to alkanes, varying amounts of other hydrocarbons, namely cycloalkanes and aromatic compounds. Table 1-2 presents the alkane and cycloalkane content of gasoline products over the 40-120 "C distillation range, obtained from different crudes. With increasing average molecular weight, the composition of petroleum fractions is more and more complex. The alkane, cycloalkane and aromatics content of different gasoline and naphtha fractions obtained from three different crudes
15
(A) LIQUID PARAFFINS AND PARAFTIN WAXES FROM PETROLEUM
Table 1-2. Alkane and cycloalkane content in gasolines from different sources Sources
n-Alkanes
Ponca field, Oklahoma Greendale-Kawkalinfield, Michigan Conroe field, Texas Loviszi field, Hungary Budafa field, Hungary
1
Vol- % iso-Alkanes
1
Cyclopentanes
I
Cyclohexanea
35.7
20.5
23.4
20.4
63.1 18.2 26.0 30.8
13.2 20.3 17.2 18.9
8.O 17.3 27.0 29.5
15.7 44.2 29.8 20.8
is shown in Table 1-3. It can be noticed that in the case of the fractions from the Yates field the alkane content decreases, while the cycloalkane content substantially increases with the boiling range. In the case of the other two crudes no such unequivocal change could be observed with regard to alkanes and cycloalkanes, while their aromatics content was the highest in the 95-1 15 "Cfraction. Table 1-4 lists the hydrocarbon composition of the kerosine and light gas oil fractions of Table Z-3. Alkanes, cycloalkanes and aromatics content of petroleum fractions from different sources Boiling point range, O C (101 kPa)
_ _ ~ ___
I
Slaughter field
AI-
kanes
~
Cycloalkanes
Aromatics
1
I
VOl- %
1
Wasson field
Alkanes
Cycloalkanes
Aromatics
I
1
Yates field * Al-
kanes
Cycloalkanes
Table 1-4. Hydrocarbon composition of kerosine and gas oil fractions of petroleum from the Ponca field Hydrocarbon
Boiling point range_ (101 kPa) _ _ 180-23OoC
n-Alkanes Iso-afkanes Monocycloalkanes Bicycloalkanes Tricycloalkanes Monocyclic aromatics* Bicyclic aromatics
23 16 32 11 0 15 3
I
230-300°C
22 8 29 17 4 12 8
Monocyclic aromatics include alkylbenzencs and aromatic cycloalkanc-type hydrocarbons
1
Aromatics
16
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
crude from the Ponca field. It can be observed that the content in bicycloalkanes and bicyclic aromatics increases with the boiling point. The fractions and the distillation residues suitable for the manufacture of lubricating oils have a still more complex composition. In these distillation fractions the carbon atom number of the components varies between 25 and 40. In the residual oil compounds with 50 to 60, and in some cases up to 80 carbon atoms are found. The chemical composition of the lubricating oils obtained by refining these materials differs from the composition of the starting distillates and residual oils actually as a result of the refining operations. This theme, however, is outside the scope of this book. The different compositions, depending on their average molecular weight (viscosity) and extent of refining (viscosity index) are shown in Table 1-5, presenting the carbon atom distribution determined by the so-called Table Z-5. Carbon atom distribution among aromatic, cycloalkane and alkane compounds in refined lubricating oils Viscosity at 98.9 "C
Oil type
_____
High viscosity index aircraft oil High viscosity index motor oil Medium viscosity index motor oil Low viscosity index oil Medical-grade oil
Viscosity index
Carbon atom distribution, % aromatic
1 cycloalkane 1
alkane
101 49
63 62
48
* S.S.U.: flow time in seconds, measured with a Saybolt Universal viscorneter
n-d-M method, that is, the distribution of the total number of carbon atoms contained in the compounds constituting the lubricating oil between the individual groups of hydrocarbons. The highly complex composition of high boiling-point fractions is represented by the data in Table 1-6 referring to a lubricating oil fraction composed of C25-C35 Table 1-6. Composition of a CzsC,, lubricating oil fraction
-
_.____
Compounds
n-Alkanes Iso-alkanes Monocycloalkanes Bicycloalkanes Tri- and polycycloalkanes Monocyclic aromatics with cycloalkane rings Bicyclic aromatics with cycloalkane rings Tricyclic aromatics with cycloalkane rings Polycyclic aromatics with low hydrogen content and non-hydrocarbon compounds
Vol- %
13.7 8.3 18.4 9.9
16.5 10.5 8.1
6.6 8.0
(A) LIQUID PARAFFINS AND PARAFFIN WAXES FROM PFTROLEUM
17
compounds, obtained by fractional distillation from the Ponca field crude. A comparison with the data of Table 1-4 unequivocally confirms that the higher-boiling fractions contain much more cycloalkanes and aromatics than the lower-boiling fractions. From this short summary of the composition of crude petroleums it may be seen that paraffin waxes produced mainly from higher-boiling distillates and residual oils contain normal hydrocarbons as well as large amounts of iso-alkanes. Also, significant amounts of one, or more ring hydrocarbons with straight side chains can be found. 2. Nomenclature of liquid parafis and paraffin waxes All classifications regarding a range of products are more or less arbitrary, or valid only with certain restrictions. It is, however, a basic postulate, when establishing some nomenclature system, that in addition to an endeavour at simplicity, both the technological and application aspects of the products in question should assert themselves. The manufacture of liquid paraffins and paraffin waxes will be discussed in Chapter 11, their application in Chapter 111. In conformity with these chapters we established a nomenclature system, which, in our opinion, satisfies the above basic requirements. This nomenclature will be applied in the course of this book. Widely varying terms are used in the literature, in the technological practice of the petroleum industry and in commerce for different grades of liquid paraffin and paraffin waxes. The terms slab paraffin wax, slack wax, scale wax, and pipeline or tank wax were established in earlier petroleum industrial practice. The term slab wax was used exclusively for paraffin waxes obtained by cooling, pressing and sweating from low-viscosity distillates. Only pressing and sweating were feasible for the separation of the oily part and the solidified paraffin wax, since centrifuging could not be applied. The term slack wax, or slacks, was used for the intermediate product of cooling and pressing without sweating or refining, and the product produced by sweating was called scale wax. On the other hand, petrolatums from residual oils and pipeline or tank waxes cannot be pressed, but only centrifuged in solvent media. This was an important aspect at the time when dewaxing by means of solvents was not yet known. The fraction distilling over between those that could be dewaxed by pressing and sweating and those that could be dewaxed by centrifuging was called the intermediate fraction. This intermediate, that is, paraffinic medium and heavy distillate, could be dewaxed only under great dif€iculties and with very poor yields either by pressing or by centrifuging. The paraffin waxes obtained from the intermediate fraction were called slop wax. The intermediate fraction was often used as fuel without recovering its paraffin wax content. At present, when solvent dewaxing processes have completely conquered the field, these aspects, and the terms connected with them, will obviously lose their importance. 2
18
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Classification of paraffin waxes : Paraffin waxes with macrocrystalline structure can be classified e.g. with respect to their melting point or to the extent of refining. On the basis of the melting p o b t one can distinguish between soft paraffin waxes with melting points below 45 "C, and hard paraffin waxes with melting points between 45 and 60°C and needle penetration values below 20 mm/lO at 25 "C. Depending on the degree of refining, one can classify paraffin waxes as technical, semi-refined and refined grade waxes. Technical grade paraffin waxes usually contain less than 6 wt-% oil; these are products obtained by dewaxing from slacks. Semi-refined paraffin waxes may contain a maximum of 3 wt- % oil, and their colour is light yellow to white. Finally, refined paraffin waxes contain 0.4 to 0.8 wt- % oil, they are completely colourless, odourless and do not contain substances detrimental to health. Our nomenclature system is based on the classification of paraffin waxes into macrocrystalline and microcrystalline groups. The crystal structure of macrocrystalline (slab) paraffin waxes can be observed visually, while that of microcrystalline paraffin waxes only with a microscope. The term amorphous is thus sometimes found in the literature for paraffin waxes obtained from residual oil. As it is known all paraffin waxes obtained from petroleum are crystalline below their setting point. The size of the crystals, however, decreases with the increasing boiling point of the paraffin wax. Microcrystalline para& waxes have higher molecular weights, densities and refractive indices than macrocrystalline paraffin waxes. From the view of both processing and application, it is an important property of microcrystalline para& waxes that they are capable of retaining more oil than macrocrystalline waxes. The structural difference is also confirmed by the observation that blending macrocrystalline slab wax with only a few tenths of a per cent of microcrystalline paraffin wax changes the ease of pressing and sweating the former. After these preliminary remarks, our classification system is shown in Fig. 1-1. The raw materials for liquid paraffins are the distillates obtained by the distillation of petroleum crudes. The raw materials for paraffin waxes are the light, intermediate and heavy hydrocarbon oil distillates obtained by the vacuum distillation of the latter, the residual oils of vacuum distillation, and pipeline and tank waxes. The semiproducts obtained in the first stage from light, intermediate and heavy distillates, from residual oils and from pipeline and tank waxes cannot yet be regarded as paraffin waxes. They are termed slacks and petrolatums, respectively. The difference between paraffin waxes and slacks and petrolatums is in their oil content, and hence in their chemical composition. The differences in chemical composition are obviously affected by the conditions of de-oiling. Macrocrystalline paraffin wax is produced from the slacks obtained from paraffin light oil distillates. Microcrystalline paraffin waxes, both of the brittle and the ductile type, are obtained from petrolatum. Ductile microcrystalline paraffin waxes include two sub-groups, namely plastic and elastic paraffin waxes. Another term used for the low oil-content macrocrystalline paraffin waxes is slab paraffin waxes. The term ceresin is reserved exclusively for brittle microcrystalline paraffin waxes.
Paraffin light distillates (atmospheric
distillation)
Paraffin medium distillates (vacuum distillation)
n
&
c 2 U
0
0 I Slack wax
3 5
* 3
Macrocrystalline paraffins
2 Brittle microcrystalline paraffin waxes
Ductile microcrystalline paraffin waxes
micro crystalline paraffin waxes
Fig. I-I. Sources and classification of liquid paraffins and paraffin waxes from petroleum
Plastic microcrystalline paraffin waxes
4
&L
20
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
The products obtained from slacks and petrolatums, named according to our nomenclature, are products not subjected to further purification. Whatever method is used for further purification, it will not change, or only change insignificantly, apart from colour, odour and content in bi- and tricyclic aromatics, the characteristics of these products. The products that have undergone further purification are distinguished by the attribute "purified". The differences between the characteristics of macrocrystalline, intermediate and microcrystallineparaffin waxes not subjected to purification, and their classification based on these differences is shown in Table 1-7. The basis of our classification is the melting point, kinematic viscosity at 100 "C, penetration at 25 "C, breaking point (Fraass) and oil content (ASTM). In our view, the totality of these characteristics is necessary and sufficient for an unequivocal characterization of the paraffin wax in question, its structure, oil content and mechanical characteristics. Table I-7. Classification of macrocrystalline, intermediate and microcrystalline paraffin waxes by their characteristics
1
I Intermediate
Characteristics
Melting point, "C Viscosity at 100 OC,mm*/s Penetration at 25 O , 0.1 mm (ASTM needle) Breaking point (Fraass), O C Oil content (ASTM), wt-%
brittle (ceresin)
40-60 c5.5
58-70 5.5-10
74-85 >10
12-20 >+25 <0.8
>15 >+15 <5.0
<10 >+25
<2.0
1-
Microcrvstallino ductile elastic
50-60
> 10
20-35 -20-0 0.5-3.0
I I
plastic
50-70
> 10
20-50 -30-+ 10 3.0-7.0
Literature Asinger, F., Paraffins, Chemistry and Technology. Pergamon Press, Oxford (1968). Finck, E., Fette, Seifen, Anstr-Mittel, 62, 502 (1960). Forziati-Willingham-Mair-Rossini: J . Res. natn. Bur. Stund., 82, 11 (1944). Gruse-Stevens: Chemical Technology of Petroleum. MacGraw Hill, New York (1960). Hoffmann, H. J., Erdol, Kohle, 17, 717 (1964). Ivanovszky, L., Chem. Tech. Berf., 11, 315 (1959). Kreuder, W., Seifkn-ale-Fette- Wachse, 84, 665, 699, 735, 773, 849 (1958). - : Seifen-ale-Fette-Wachse, 85, 19, 41, 67, 93 (1959). Mair-Rossini: Ind. Engng. Chem., 47, 1062 (1955). Marx-Presting: Chem. Tech. Berl., 7, 662 (1955). Mazee, W. M., Modern Petroleum Technology, 3rd ed. (Ed. E. B. Evans), Institute of Petroleum, London (1962). Perry, J. H., Chemical Engineers' Handbook. McGraw-Hill, New York (1950). Phillips, J., Petrol. Refiner, 38, 193 (1959). Rossini-Mair: Adv. Chem. Ser., No. 5, 334 (1951).
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
21
Rumberger, J., Symposium on Composition of Petroleum Oils, Determination and Evaluation. ASTM, p. 283 (1958). Teubel-Schneider-Schmiedel :Erddlparajine. VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig (1965). Tuttle, J. B., Petroleum Products Handbook. (Ed. V. B. Guthrie), McGraw-Hill, New York (1960).
Wolff, G., Coating, 9, No. 1. 13 (1976).
(B) Chemical properties of liquid paraffins and paraffin waxes The chemical properties of liquid paraffins and paraffin waxes obtained from petroleum are in relation with the following steps : - preparative and analytical methods for studying the chemical composition of liquid paraffins and paraffin waxes, - determining the chemical composition of the paraffins, - determining the chemical properties of individual paraffin hydrocarbons.
1. Preparative and analytical methods for studying the chemical composition of liquid paraffins and parailin waxes The determination of the chemical composition of liquid paraffins and paraffin waxes can only be carried out after cumbersome separation procedures and subsequent analyses including spectral analysis, gas chromatography, etc. In the case of paraffin waxes with higher average molecular weight it is almost impossible, even using the most laborious operations, to achieve complete separation of individual compounds. In general, the objective is to produce narrow fractions whose components are closely similar or identical with regard to chemical structure. An approach to the chemical nature of a given paraffin wax is also yielded by physical characteristics, whose values are closely related, for a given molecular weight, to the structure of the molecule. For determining and characterizing the chemical composition of paraffin waxes, essentially three groups of preparative and analytical procedures are available: - separation methods, - classification methods based on physical characteristics, - analytical methods for the determination of individual components.
(a) Separation methods For a partial separation of the components differing in molecular weight and chemical structure, the following methods can be considered : - fractional distillation,
22
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
- fractional crystallization,
adduct formation with urea and column chromatography on urea, separation using molecular sieves, - elution chromatography, - thin layer chromatography. Separation by distillation is based on the differing boiling points of the components. This separation method is much limited, since the boiling points of the successive members of the n-alkane series, particularly in the case of compounds containing more than 25 C-atoms, are very close to one another. Therefore; preparative separation by distillation is effective mainly in the case of < C , n-alkanes. This separation method is difficult to apply to iso-alkanes and cycloalkanes, since the boiling points of the members of these two homologous series overlap. If a mixture of pure n-alkanes has been ffrst separated, by some method, from the material to be analyzed, the distribution of the compounds in the mixture can be determined by molecular distillation. It is obvious from what has been said that separation by distillation is much less effective in the case of microcrystalline paraffin waxes than in the case of liquid paraffins and macrocrystalline paraffin waxes. A successful method for the separation of microcrystalline paraffin waxes is fractional crystallization based on differential solubility. Ketones, mixtures of ketones and aromatics, halogenated hydrocarbons and different gasoline grades have been used as solvents in research up to the present. Fractional crystallization yields fractions of both macrocrystalline and microcrystalline paraffin waxes differing in molecular structure and molecular dimension. At higher temperatures of crystallization, fractions containing higher molecular weight and less branched alkanes, as well as cycloalkanes with long side chains will crystallize. With successive lowering of the temperature, the fractions will contain more and more iso-alkanes and cycloalkanes with shorter side chains; simultaneously the average molecular weight of the fractions will decrease. n-Alkanes can also be separated from iso- and cycloalkanes by urea adduct formation. X-ray studies have shown that the long chains of n-alkanes as well as long chains, if present, of iso- and cycloalkanes are enclosed in the tubular channels of the adduct, and this results in a hexagonal urea lattice. Urea crystallizes in the hexagonal system only when an adduct is formed, its normal crystal system being tetragonal. Straight-chain derivatives of n-alkanes, e.g. carboxylic acids, alcohols, esters, amines etc. are also capable of adduct formation. Adduct formation between n-alkanes and urea takes place in solutions of the former in gasoline, benzene or halogenated hydrocarbons when solid urea or an aqueous or alcoholic urea solution is added. When solid urea is applied, a small amount of a wetting agent, i.e. water, alcohol or some other substance with a s h i lar effect is necessary. Adduct formation is inhibited by resins, bituminous substances, sulfur compounds, etc. It is, therefore, important to remove such substances from the material before adduct formation, by elution chromatography or some other method. -
-
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
23
Adduct formation is an equilibrium reaction, the equilibrium being dependent on temperature, concentration of urea and adduct-forming components, and nature of the solvent. Adduct formation is exothermic, the heat of reaction is the higher, the longer the alkane chain. Hence the stability of the adduct is the greater the longer the adduct-forming molecule chain. Short-chain n-alkanes form adducts only at low temperatures, and these products will readily decompose. The following method was used by Hessler and Meinhart. Dilute solutions of macro- and microcrystalline paraffin waxes in carbon tetrachloride were prepared, methyl alcohol saturated with urea was added and the mixture vigorously agitated. The crystalline precipitate formed was filtered, washed with alcohol and dried. The decomposition of the adduct was carried out with distilled water at 70 "C. A diagram of the urea adduct method developed in the Hungarian Oil and Gas Research Institute is shown in Fig. 1-2. As well as urea, thiourea can also successfully be used for studying the chemical composition of complex mixtures of hydrocarbons and their derivatives. Thiourea forms adducts most readily with branched compounds. The essence of columnchromatography, using urea, is as follows. The substance to be studied is introduced, in the form of a solution, into a column filled with urea. Those components of the substance which, under the given conditions, namely thermostatted temperature, presence of activator in the column and percolation time, form an adduct with urea will be bound, 'while the unreacting components will remain in solution and will be eluted from the column by washing with solvent, and determined quantitatively. Subsequently, those components having formed adducts will be eluted by successive stepwise increases of the temperature. The temperatures corresponding to these steps will determine the structure and average molecular weight of the eluate fractions. Molecular sieves are zeolites consisting of aluminium, calcium, alkali and hydrogen orthosilicates. Their characteristic feature is the ready compensation of the negative charges of their tetrahedral and A10i5 crygtal lattices by cation exchange. The interconnected voids in their lattices contain combined water that can reversibly be removed by heating. Dehydrated zeolite is capable of binding molecules having suitable dimensions to fit into the voids. For the separation of n-alkanes from hydrocarbon mixtures, synthetic molecular sieves of the so-called 5 '8, type are suitable. The average diameter of their pores is 5 A, their chemical composition is Me,,/n[(A1OJl, * (Si02)lz] 27 H,O. For chemical group analysis of liquid paraffins and macro- and microcrystalline paraffin waxes, column chromatographic separation methods based on the work of Mair, Rossini, Spengler, Snyder and Heinze are well suited. Silica gel or activated alumina is preferably used as adsorbent. The ratio of adsorbent to sample is between 20 : 1 and 30 : 1. The sample is introduced in the form of a dilute solution in gasoline or hexane. The succession of the eluents is that of the increasing polarity, e.g. hexane, mixture of hexane and benzene, benzene, methanol and chloroform. This method allows separation of saturated hydrocarbons, and mono-, bi- and tricyclic aromatics with satisfactory sharpness.
-
24
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Benzene (solvent for paraffin wax)
Dissolution of paraffin wax (25-30"C).
-.
water (90°C)
Wash and separation
Adduct formation (25"C, 96 h)
Wash liquor ~
Solution of ;so-alkanes Aqueous in benzene solution of urea
with 10 wt-% methanol
Wash
-
Benzene
t
Washed adduct Distilled
Removal of solvent
Is0 - alkanes
Aqueous solution of urea n-Alkanes +solvent
Removal of solvent
n -Alkanes
Fig. 1-2. Method for determining the n-alkane and iso-alkane content in macrocrystalline
and microcrystallineparaffin waxes
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
I
Slack wax
25
]
Separation' of olefins with mercury acetate
i
Olefin - free slack wax
Adduct formation
c
I
n-Alkanes and
1 Iso-alkanes, aromatics, resins
1
I
Column chromatogr.aphy
Column chromatography on silica gel
on silica gel
Iso-alkanes A l k y l - substituted aromat tcs A
Column chromatography on activated carbon
I
Column chrdmatographtj
n-alkanes
a1k y I - substituted aromat ics
Fig. 1-3. Combined separation method of Spengler and Jantzen
A more detailed chemical study of macro- or microcrystalline paraffin wax requires a combination of separation methods. A diagram of a combined separation method developed by Spengler and Jantzen is shown in Fig. 1-3. Separation of macro-and microcrystallineparaffin waxes by thin-layer chromatography was developed, among others, by Dietsche and Sucker. They used a 250
26
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Gm silica gel support layer impregnated with 40 % urea. To avoid recrystallization of the urea, a small amount of sorbite was applied. The paraffin wax to be studied was applied in a 1 % solution in benzene, at 50 to 60 "C. The solvent used for runs was a mixture of carbon tetrachloride and ethanol saturated with urea. By using a suitable solvent composition and temperature (around 50 "C), they succeeded in obtaining satisfactory separation of the paraffin wax with respect to chain length and degree of branching. By using appropriate conditions and simultaneous runs with reference standard materials, they could determine the ratio of n- and iso-alkanes in macrocrystalline paraffin waxes.
(b) Chemical classijcation on the basis of physical characteristics According to Etessam and Sawyer, the relationship between the melting poirlt and the molecular weight for n-alkanes is
M m.p. = 415 ____ - 273 M + 95 where m.p. is the melting point and M the molecular weight. According to Ivanovsky, an analogous relationship can be established between the melting point and the density : lo3 * d y = 716 f 0.75 m.p. (1-2) where d: is the density at 90 "C relative to that of water at 4 "C. From these equations the so-called ring value is derived, since
-
lo3 d: = 511
+ 311 M M+ 95
(1-3)
and the ring value, indicating the density increases due to ring closure as compared to the equimolecular n-alkane, is (1-4)
The so-called asymmetry value is obtained from the Etessam and Sawyer relationship by introducing a factor of 0.75: a.v. = 311
M M
+ 95
- 205 - 0.75 m.p.
(1-5)
The asymmetry value indicates the melting point decrease due to iso-alkanes as compared to the equimolecular n-alkane. For n-alkanes, both the ring value and the asymmetry value are zero. For isoalkanes the ring value differs only slightly from zero, its maximum value can reach 5, while for cycloalkanes the ring value can be as high as 100. In the simultaneous presence of iso-alkanes and cycloalkanes, the ring value will have inter-
27
(B) CHEMICAL PROPERTIES OF LiQUID PARAFFINS AND PARAFFIN WAXES
mediate values, depending on the number and nature of branchings. To decide for mixed paraffin waxes whether they are composed mainly of iso-alkanes or cycloalkanes, it is necessary to know both the ring value and the asymmetry value. In such cases the so-called sum value (s.v.) yields the answer, its value for n-alkanes being zero: S.V. =
r.v.
+ a.v. = lo3
*
d: - 716 - 0.75 m.p.
(1-6)
According to Spengler and Jantzen the relationship between refractive index and melting point permits calculations on the iso-alkane, cycloalkane and alkylsubstituted aromatics content in paraffin waxes. For n-alkanes, this relationship has the form: 70 nD = 4.2 * m.p. 1.4076 (1-7)
+
where n: is the refractive index at 70 "C. Similar relationships are valid for iso- and cycloalkanes and alkyl-substituted aromatics. However, the straight lines representing the latter rekdtionships intersect the substantially steeper straight line for n-alkanes. The point of intersection is not known exactly, but is around the melting point values of 125 to 130°C 70 and nD = 1.4580. Such a refractive index versus melting point diagram is shown in Fig. 1-4. If the measured values of some paraffin wax or of one of its fractions are placed into this diagram, certain conclusions can be made regarding its composition.
Melting point, 'C Fig. 1-4. Relationship between the refraction index and melting point of hydrocarbons
28
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Hersch and Fenske found that the naphthenic ring content of aromatics-free paraffin waxes or their fractions can be determined using the Watterman n-d-M ring analysis modified by them. Their methods are as follows:
- the average number of naphthenic rings per molecule is
RN = 0.284 [(n: - 1.4750)M + 8.79]0*8';
(1-8)
- the number of carbon atoms combined in naphthenic rings is
CN = 2.08 [(n: - 1.4750)M + 8.79]0*73;
(1-9)
- the percentage of naphthenic rings is N = - 2890 [(n: M
-
1.4750)M + 8.79]0-73,
(1-10)
where n z is the refractive index at 20 "C. ( c ) Analytical methods for the determination of individual hydrocarbons or of compositions of their mixtures The methods discussed in the previous paragraphs are suitable to give an overall approach to the chemical composition of macro- and microcrystalline paraffin waxes. This is satisfactory in many cases for manufacturing and application purposes. If, however, individual hydrocarbons must be determined, gas chromatography, mass spectrometry and infrared spectrometry method have to be used. High-temperature gas chromatography and mass spectrometry methods suitable for the analysis of paraffin waxes have been frequently discussed in the recent literature. For the gas chromatography of macrocrystalline paraffin waxes, temperatures between 250 and 350 "Care used. The paraffin wax is retained by stationary liquid phase and individual components are stripped from the column, according to their volatility, using hydrogen or helium as carrier gas. The fractions eluted are recorded with thermal conductivity or flame ionization detectors. Different selective liquid stationary phases are in use, e.g. silicone oils, distillation methyl silicone fluid, carborane (methyl silicone fluid) etc. Gas chromatographic analysis of paraffin waxes will not be discussed in detail here. It will only be mentioned that apparatus and techniques exist that allow the determination of individual hydrocarbons up to CS5. The results of Levy and co-workers are particularly worth mentioning. They combined high-temperature gas chromatography and mass spectrometry methods, and achieved qualitative and quantitative determination of 67 individual components in a refined macrocrystalline paraffin wax. It is, in general, effective to use a suitable form of gas chromatography for separation, and mass spectrometry for subsequent identification.
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
29
There are many reports on infrared spectrometric studies of paraffin waxes. However, no generally accepted analytical method has yet been established for the determination of other than normal hydrocarbons. In the IR spectrometry of paraffin waxes, absorption bands are, in general, within the 600 to 3530 cm-' range. Such analyses were successful in differentiating and detecting primary, secondary and tertiary carbon atoms. The presence of iso-alkanes and cycloalkanes in paraffin waxes forming urea adducts could qualitatively be confirmed. Using IR spectroscopy, some authors succeeded in determining the extent of branching, whilst others determined the numbers of methyl and methylene groups. In addition to these instrumental analytical methods, various chemical analytical procedures were developed for identifying or determining a given group of compounds. The antimony pentachloride method shall be mentioned as an example. This is based on the finding that n-alkanes, in carbon tetrachloride solution, do not react with antimony pentachloride, whereas iso-alkanes form an insoluble, pitch-like substance. Thus, the n-alkane content in macro- and microcrystalline paraffin waxes can be determined. 2. Chemical composition of liquid paraffins and parnffin waxes Liquid paraffins have a relatively simple chemical composition, as they consist almost entirely of n-alkanes. The products manufactured by different companies for different purposes show only slight variations in the molecular weight range. On the other hand, the chemical composition of macrocrystalline and microcrystalline paraffin waxes varies over an almost infinite range of combinations, varying according to the source of the crude petroleum and to processing technology. To characterize the chemical composition of paraffin waxes, let us first summarize the general ideas, and subsequently present the composition of some paraffin waxes from different sources. As demonstrated by spectroscopic studies, paraffin waxes consist mainly of saturated hydrocarbons. The number of aromatic ring compounds, particularly in the case of macrocrystalline paraffin waxes consisting of compounds of lower molecular weight, is so small that they have practically no effect on the properties of the waxes. In fact, the majority of these rings are present as alkylbenzene derivatives and in condensed forms, and hence detrimental to health. In studying the composition of liquid paraffins let us consider the work of Mikhaylov and co-workers who studied the composition of a liquid paraffin obtained by urea dewaxing of a Diesel fuel from a high sulfur-content crude, and subsequent refining by adsorption. This liquid paraffin contained 0.2 wt- % aromatics, the amount of hydrocarbons forming no adducts with urea was relatively small. Table 1-8 lists the most important properties of the liquid paraffin, and the products obtained by twice repeated treatment with urea. As may be seen, the fractions forming no adducts with urea become enriched in iso-, cyclo-
30
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 1-8. Chemical composition and physical properties of liquid paraffin, of its urea-adduct forming parts and parts forming no adduct with urea
1
I
Product
First urea treatment 1 Starting material 2 Adduct-forming compounds 3 Compounds forming no adduct
Total : Second urea treatment 4 Adduct-forming compounds 5 Compounds forming no adduct Total:
1
1
Hydrocarbon content, wt- % relative to starting material Yield, %
n-alkanes
phenylalkanes
Properties
-__
iso-alkanes and cYclOalkanes
~~~~i~~ dl0
1
i
___
Refr. index Melting point
"C
100.0
96.4
0.2
3.4
0.8043
1.4370
24.5
95.0
92.5
traces
2.5
0.8054
1.4370
27.0
5.0
3.7
0.21
1.07
-
-
-
100.0
96.2
0.21
3.57
2.45
2.18
0.027
0.243
0.7743
1.4350
2.55
1.55 3.73
0.165 0.192
0.837 1.080
0.7922
1.4385
5.0
-
-
Table 1-9. Composition of the liquid paraffin in Table 1-8 Hydrocarbons
Identified n-Alkanes Iso-alkanes 2- and 4-methylalkanes 3-methylalkanes 5-methylalkanes 6-methylalkanes Cycloalkanes 1-cyclopentylalkanes 1-cyclohexylalkanes Phenylalkanes 2-phenylalkanes Total identified :
I
Carbon atom range
1
Share, wt-%
I
Number of compounds
C11-Cz4
96.400
14
C,,-Czo
1.168 0.484 0.126 0.061
12 6 6 4
C,,-C1*
0.132 0.299
6 6
c,,-czo
0.047 98.717
6 60
1.283
-
cI7-czo
C11-C24
Non-identified Iso-alkanes
Cycloalkanes Phenylalkanes Total non-identified:
-
-
9.0 7.2
-
31
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
and phenylalkanes. The total hydrocarbon composition of the liquid paraffin is listed in Table 1-9, showing that 60 compounds could be identified, among these 14 different n-alkanes, 28 iso-alkanes, 12 cycloalkanes and 6 phenylalkanes. The amount of non-identified hydrocarbons did not exceed 1.3 %. The authors also stated that the iso-alkanes contained in the fraction forming no adduct with urea are monosubstituted methylalkanes, with the methyl group attached to one of the C, to C, carbon atoms. The cycloalkanes contained in the fraction are rings of five or six carbon atoms, with straight-chain alkyl groups attached. In the phenylalkanes present, the benzene ring is attached to the second carbon atom of the alkane. Paraffin waxes consisting of C,&30 hydrocarbons are mainly composed of n-alkanes. Compounds containing rings, or branched at the end of the chain, are also present, but in small amounts and especially in the higher fractions. In microcrystalline paraffin waxes consisting of >C3,C3, hydrocarbons, obtained from fractions distilling over at higher temperatures or from vacuum distillation residues, the other than normal character dominates. Hydrocarbons other than normal cover the total carbon atom number range from C30 to Cs0. n-Alkanes in microcrystalline paraffin waxes are also mainly within this range. In addition to n-alkanes and iso-alkanes, macrocrystalline and microcrystalline paraffin waxes contain naphthenes, especially alkyl-substituted derivatives of cyclopentane and cyclohexane. Depending on the source of the crude and on the extent of refining, larger or lesser amounts of cyclic sulfur and nitrogen compounds are also present. The decisive factors determining the properties of low oil-content paraffin waxes are hence the distribution, by carbon atom number, of n-, iso- and cycloalkanes and their relative quantities. This appears quite evident, knowing that substantial differences exist between the properties of isomeric n- and iso-alkanes. As an example, Table 1-10 (based on data by Mazee) records the physical properties of two n-alkanes and their iso-alkane isomers, both within the carbon atom number range of macrocrystalline paraffin waxes. To characterize the chemical composition of paraffin waxes some characteristic values for three microcrystalline paraffin waxes from different sources will first be presented, based on data from Ridenour, Spilners and Templin. These values Table I-10. Physical properties of two n-alkanes and their branched isomers in the range of macrocrystalline paraffin waxes Alkane
n-Tetracosane 2-Methyltricosane 2,2-Dimethyl-n-docosane n-Octacosane 10-Nonylnonadecane
1 1 Formula
CZaH,o CZ4H,, CllHSO CzsH,, CzsH,,
Boiling point at 0.5.F
208.6 205.0 201.5 242.0 228.3
Melting point; "C
1 1 Density
di0
50.7 37.6 34.8 61.3 -5.5
0.7562 0.7539 0.7536 0.7639 0.7650
1.4205 1.4201 1.4191 1.4248 1.4247
2.42 2.48 2.71 3.40 2.68
32
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table I-11. Main characteristics of microcrystalline paraffin waxes from different sources Wax identification mark
Characteristics
Melting point, OC Refractive index, &' Density at 70 "C Average molecular weight Ring analysis (n-d-M) RA
RN
Oil content, wt- %
76.0 1.4514 0.8122 629
74.1 1A 8 3 0.8065 574
62.5 1.4547 0.8184 682
0.20 0.60 8.2
0.12 0.48 4.5
0.20 0.90 0.8
are summarized in Table 1-11. Sample A is a typical microcrystalline paraffin wax, sample B a hard, brittle product, sample C a highly flexible, ductile microcrystalline wax with high adhesive power. Portions isolated from these samples by adduct formation with urea, and fractions obtained from these portions by molecular distillation are characterized in Tables 1-12-1-14. The symbols used are: RT = = total ring content per molecule, R, = aromatic ring content per molecule, R, = naphthenic ring content per molecule. In the first eight distillation fractions of the adduct-forming portion of sample A , the total ring content determined by the n-d-M method is less than 0.2 rings/molecule. The value of R N in distillation fraction 9 is 0.4, in the distillation residue 0.8. According to infrared absorption
No. of fraction
Adduct-forming part 1
2
3 4 5 6 7 8 9
Distillation residue
wt-%
M.p.
"C
Refr. ind.
,,g
Dens. at 70°C
Aver.
b:
100 10.5 9.0 11.8 11.8 10.2 9.5 10.0 8.0 10.2
79.0 68.9 71.6 72.8 75.8 77.0 78.6 80.9 83.4 87.4
1.4422 0.7919 1.4374 1.4388 1.4390 1.4408 1.4408 1.4413 1.4426 1.4438 1.4458 0.8028
548 447 486 489 510 537 550 572 590 662
9.0
93.6
1.4531 0.8165
910
Ring analysis (n-d-M)
__ RT
0.0
-
-
I 1 RA
RN
0.0 -
0.0 -
-
-
-
-
-
0.4
0.0
0.4
0.9
0.1
0.8
Corresponding n-alkane
1 vz
~;a:
c,
C.1 C,, C,,
79.4 69.3 72.7 74.0 75.6 78.1 79.5 82.9 82.9 87.5
C,,
98.1
C,, C,, C,, C,, C,,
c,
33
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 1-13. Characteristics of the fractions obtained by molecular distillation from the adduct-forming part of the microcrystalline paraffin wax marked B in Table 1-11 n-alkane
No. of fraction
Adduct-forming part 1 2 3 4 5 6 7 8 9
Distillation residue
No. of fraction
Adduct-forming part 1 2 3 4 5 6 7 8 9
Distillation residue
100 10.1 10.4 10.6 10.5 10.0 10.0 9.1 8.6 11.4
76.0 67.0 69.4 71.2 73.8 76.0 77.6 79.4 80.5 82.2
1.4410 1.4356 1.4368 1.4377 1.4390 1.4399 1A 0 8 1.441 8 1.4425 1.4439
9.3
85.0
1.4488
Yield wt-%
M.P.
100 9.6 11.0 10.4 9.9 11.5 11.7 8.5 9.0 8.9
67.0 62.0 66.2 67.2 68.3 69.6 71.4 72.4 74.2 76.0
1.4400 1.4340 1.4363 1.4371 1.4377 1.4387 1.4402 1.4419 1.4435 1A450
9.5
82.0
1.4529
"C
Refr. ind.
% '"
CS7 C,, C33 Cs C,,
526 439 464 475 509 518 536 557 565 594 0.8066
684
76.8 67.2 70.8 72.7 75.6 (2.37 76.8 Ca8 78.1 Cd0 80.7 Cl0 80.7 Cd2 82.9 0.4
0.05
Ring analysis (n-d-M)
I 1
Dens. at
Aver.
7O"C
Wt.
0.7882 0.7797
0.7959 0.7994
478 407 436 455 458 471 492 520 543 575
0.1 0.3
0.0 0.0
0.3
0.8167
743
1.0
0.0
1.0
-
-
-
mol.
__ RT
0.0 0.0
-
Rk
0.0 0.0
-
-
-
C,
0.35
C
89.0
o
1
RN
~
; !? 72.7 62.3 67.2 69.0 70.8 72.7 74.0 76.8 79.5 81.9
0.0 0.0 -
-
0.1
Cs,
91.7
measurements this consists of monocyclic alkanes and their substituted derivatives. The adduct-formingportion of sample B, and its fractions have a similar composition, the differences showing only in the range of molecular weights and in the total ring content of distillation fraction 9 and of the residue. Fractions from sample C have lower melting points and higher refractive indices than fractions from samples A and B with identical molecular weights. This indicates that 3
-
~
34
1. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
sample C contains a higher percentage of iso-alkanes. However, the slight differences in the melting points demonstrate that these iso-alkanes are branched to a small extent only. The characteristics of the portions of the three samples forming no adduct with urea, as well as of their distillation fractions, are listed in Tables 1-15-1-17. It may be observed that the portion of sample C forming no adduct has a substantially higher average molecular weight, a broader molecular weight range, a lower average melting point and a narrower melting point range than those of the other two samples. This difference is in conformity with its composition: sample C and its portion forming no adduct contains more branched alkanes and the share of aromatic and naphthenic rings is higher than in sample A and B. The ring Table 1-15 Characteristics of the fractions obtained by molecular distillation from the part forming no adduct of the microcrystalline paraffin wax marked A in Table 1-1 1
Part forming no adduct 1 2 3 4 5
6 7 8
100 8.75 9.75 9.75 9.55
10.45 9.85 10.7 10.4
Distillation residue 20.8
63.0-65.0 51.0 57.0 61.0 66.0 66.0 69.0 71.0 72.0
1.4592 1.4580 1.4566 1.4556 1.4559 1.4567 1.4578 1.4595 1.4644
0.8305 0.8291 0.8244 0.8243 0.8239 0.8246 0.8264 0.8310 0.8338
706 490 535 579 623 661 758 783 910
1.9 1.8 1.5 1.6 1.6 1.5 1.6 1.9 1.7
0.15 0.2 0.2
0.6
1.75 1.6 1.3 1.55 1.5 1.35 1.45 1.75 1.1
68.0
1.4677
0.8440
1340
3.0
0.6
2.4
0.05
0.1 0.15 0.15
0.15
Table 1-16. Characteristics of the fractions obtained by molecular distillation from the part forming no adduct of the microcrystalline paraffin wax marked B in Table 1-11
Part forming no adduct 1
6 7
10.1 10.4 10.0 10.1 10.5 9.2 11.9
63.0-63.3 55.6 58.6 60.6 62.7 64.3 65.3 66.0
1.4617 1.4596 1.4559 1.4559 1.4562 1.4573 1.4583 1.4606
0.8325 0.8304 0.8217 0.8217 0.8220 0.8245 0.8255 0.8306
686 500 540 561 602 643 653 712
1.8 1.8 1.3 1.3 1.3 1.4 1.4 1.7
0.25 0.30 0.30 0.35 0.35
1.4 1.55 1.05 1.05 1.00 1.10 1.05 1.35
Distillation residue
27.8
65.6
1.4695
0.8434
930
2.2
0.8
1.4
.
L
3 4 5
100
0.4
0.25 0.25
35
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table Z-17.Characteristics of the fractions obtained by molecular distillation from the part forming no adduct of the microcrystallineparaffin wax marked C in Table 1-11 No. of fraction
Part forming no adduct 1 2 3 4 5 6 7
Distillation residue
Refr. ind. n&O
Dens. at 70 "C
Aver.
Ring analysis RT
I
RA
I N'
100 13.7 9.3 9.8 9.7 9.5 9.6 14.2
55.0
56.0 56.6 56.9 56.6 55.6 55.0 51.0
1.4598 1.4466 1.4481 1.4509 1.4536 1A563 1.4598 1.4644
0.8256 0.8030 0.8087 0.8130 0.8188 0.8232 0.8297 0.8390
773 503 544 603 656 750 839 967
1.2 0.6 0.9 1.0 1.2 1.4 1.7 2.4
0.4 0.1 0.0 0.0 0.0 0.1 0.2 0.4
0.8 0.5 0.9 1.0 1.2 1.3 1.5 2.0
24.2
45.0
1.4727
0.8529
1700
4.0
0.9
3.1
compounds in sample C are concentrated to a higher extent in the higher molecular weight fractions than is the case with samples A and B. The cited authors achieved further separation by thermodiffusion of the distillation fractions obtained from the part of sample A that forms no adduct. As a result of subsequent analyses, they succeeded in determining the chemical composition of this part of the sample. They found that it consists of about 17 wt-% monocyclopentylalkanes, 24 wt- % monocyclohexylalkanes, 6 wt- % dicyclopentylalkanes, 20 wt- % dicyclohexylalkanes, 6 wt- % monocyclic aromatics, 5 wt- % polycyclic aromatics and 22 wt- % polycyclic alkanes. The distribution among these compound types is shown in Fig. 1-5, where, on the one hand, dicyclopentyl-
Distillation yield, wt-%
Fig. 1-5. Distribution of compounds in the part forming no adduct of the microcrystalline
paraffin wax marked A
3+
36
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
and dicyclohexylalkanes, and on the other, aromatics and polycycloalkanes are combined into one group each. Bornemann and Heinze used the combined analytical procedure shown in Fig. 1-6 for characterizing the composition of microcrystalline paraffin waxes. Their starting material was slack wax. In the first stage they prepared - by fractional crystallization - vaseline, plastic and hard microcrystalline paraffin wax (these terms correspond to the classification of Kreuder). The hard paraffin wax was then separated into several fractions by chromatography on columns filled with silica gel. The iso-octane eluates were separated into portions forming adducts and forming no adducts, and these were subsequently subjected to further separation by fractional crystallization from dichloroethane, by chromatography on activated carbon and silica gel, and by molecular distillation. The results that the cited authors obtained with petrolatum from the heavy distillate of Romashkino crude are presented in the following tables. Table 1-18 Table Z-18. Main characteristics of products obtained by fractional crystallization of a petrolatum from the heavy distillate of a Romashkino crude Characteristics
Yield, wt-% Refractive index, ng Density at 90 OC Average molecular weight Melting point, *C Viscosity at 90 O C , mmZ/s Penetration at 25 OC, 0.1 mm Oil content, wt- % Part forming adduct with urea, wt-
%
Part forming no adduct, wt- %
Starting
1.4497 0.8102 450 63.6 16.7 40 (cone) 25.5 27.4 72.6
69.7 1.4546 0.8193 422 40.9 18.1 120 (cone)
-
-
20.7 1A430 0.7977 535 63.4 14.0 27 6.5 48.5 51.5
9.6 1.4393 0.7866 550 77.9 14.4 9 1.7 84.5 15.5
shows the main characteristics of the products obtained by fractional crystallization from the petrolatum. The hard paraffin wax fraction was separated by chromatography on 0.1-0.4 mm silica gel activated at 180 "C.The fractions obtained in this operation are shown in Table 1-19. The iso-octane eluates separated into adduct-forming and non-adduct-forming portions were further separated by fractional crystallization, by molecular distillation at 200-265 "C in a vacuum of 1 cPa, and by chromatography. The narrow fractions obtained by these procedures were analysed by determining the usual physical characteristics (refractive index, density, melting point, etc.), the Hersch-Fenske data and the n, versus b.p. diagram shown in Fig. 1-7. Their experimental results can be summarized as follows. The hard paraffin wax studied consists of C,,-C,, compounds. The n-alkane content represents 25 to 35 %, the majority of these being C,&, compounds. The share of iso-alkanes
Slack wax from heavy distillate I
EIzI Fractional !istillation
crl
a
Plastic paraffin wax
Vase1ine
Hard paraffin wax
Resins
Iso-octane eluate
Iso-octane eluate
Iso-octane eluate IV
Iso-octane eluate
III
I1
I
f
Benzene eiuate
8
, I
distillation
carbon
I I I Chromatographic separation I
carbon
I
carbon
I
I
I
I
carbon
Chromatographic separation on active carbon w
Fig. 1-6.Group analysis of microcrystalline paraffin waxes according to Bornemann and H e i m
4
38
I. PROPERTIES OF LIQUID PARAFFlNS AND PARAFFIN WAXES
Table 1-19. Characteristics of fractions obtained by chromatography on silica gel of the hard microcrystalline paraffin wax (source: Romashkino crude) figuring in Table 1-18 characteristics
Refractive index, :n Density at 90 OC Average molecular weight Melting point, OC Viscosity at 90 OC, mm2/s Penetration at 25 OC, 0.1 mm Oil content, wt- % Part forming a n adduct with urea, wt-% Part forming no adduct, wt- %
1-
___Eluates _ _with. iso-octane~-_ _ _ _Eluate with
__
I
1
111
I I1 _________-___
1.4372 0.7854 620 78.4 12.9 9 0.4
1.4381 0.7868 550 77.4 13.3 9 0.7
87.8 12.2
85.3 14.7
~
1.4402 0.7901 565 77.0 14.4 9 1.2 81.5 18.5
IV
benzene
1.4478 0.8032 610 75.8 16.9 10 2.7
1.4750 0.8553 610 71.2
74.7 25.3
-
15 12.4 -
-
and naphthenes is 55 to 65 wt-%, around 10 wt-% of which are C45-C55 alkyldicycloalkanes and alkyltricycloalkanes. The C35-C50 compounds are iso-alkanes, alkylcyclopentanes and alkylcyclohexanes. Aromatics represent about 1 wt- %, resinous substances around 1.5 wt- %.
0
1.4600
lC200
I
0
20
/ ' "
40
Boiling point,
60
7
80
"C (1 cPa)
Fig. 1-7. Part of the diagram of refractive index versus boiling point. A hard paraffin wax, B I-IV iso-octane eluates, C adduct-forming part of iso-octane eluate I, D part of iso-octane eluate I forming no adduct
The authors of this book studied how and to what extent the chemical composition of macrocrystalline and microcrystalline paraffin waxes from Romashkino crude is changed by the effect of de-oiling and subsequent refining by the hot
39
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 1-20. Characteristics of slack wax and petrolatums from Romashkino crude Characteristics
Macrocrystalline slack wax from light distillate
Microcrystalline petrolatum from heavy distillate
Microcrystalline. petrolatum from residual oil
0.7683 2.75 48.5 27 8+ 1 7.8 1.4269 319 0.19
0.8357 9.91 57.9 146 8 21 42.8 1.4604 480 0.89
0.8264 12.43 65.5 112 4.5+ 6 23.7 1.4551 583 0.65
Density at 80 OC Viscosity at 100 'c,mmZ/s Melting point, OC Penetration at 25 OC, 0.1 mm Colour index ASTM (1/4") Extinction (Pulfrich filter 8, 100 mm) Oil content, wt- % Refractive index &' Molecular weight Sulfur content, wt- % Aromatics, wt- % monocyclic bicyclic tricyclic
1.3 1 .o 0.7
20.0 6.0 3.9
13.1 15.1 1.8
contact method. The main characteristics of the materials investigated are shown in Table 1-20. De-oiling was carried out at + 10 and +30 "C with methyl ethyl ketone, hot contact treatment with 196 m2/g specific surface area activated clay of the bentonite type. The characteristics of the de-oiled products are listed in Table 1-21. Those in Table 1-22 are of products refined by hot contact under Table 1-21. Characteristics of the slack wax and petrolatums figuring in Table 1-20 after deoiling Macrop
Characteristics
Temperature of de-oiling, O C Yield relative to starting material, wt- % Density at 80 "C Melting point, "C Penetration at 25 OC, 0.1 mm Colour index ASTM (1/4") Extinction (Pulfrich filter 8, 10 mm) Oil content, wt- % Refractive index, n? Molecular weight Sulfur content, wt- % Aromatics, wt- % monocyclic bicyclic tricyclic
from paraffinic distillate
10
Microcrystalline paraffin ~ wax from~ heavy distillate
~ ~
A
10
I
B
30
13.0 17.9 37.5 0.7635 0.7884 0.7930 51.9 73.1 66.8 19 15 28 1 8+ 8+ 0.85 1.03 14.2 0.7 5.1 5.7 1.4236 1.4360 1.4360 540 341 517 0.03 0.21 0.10 0.3 0.1 0.1
9.8 4.9 0.5
4.8 1.3 0.2
Microcrystalline paraffin wax from ~ ~ residual oil
_~_______ A
10
J
B
30
21.5 52.9 0.7955 0.8065 78.0 70.5 9 33 5.5+ 5+ 5.0 6.2 1 .o 6.7 1.4391 1.4442 636 694 0.36 0.17 13.7 2.0 1.3
7.0 0.7 0.6
~
40
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 1-22. Characteristics of the macro- and microcrystalline products figuring in Table 1-21 after hot contact refining
Characteristics
Macrocrystalline paraffin wax from light uaraf-
Microcrystalline paraffin wax from heavy distillate
Microcrystalline paraffin wax from residual oil
- - _ _ _ ~ _ _ _ _ _ . _ _
Density at 80 OC Melting point, OC Penetration at 25 OC, 0.1 mm Colour index ASTM (1/4") Extinction (Pulfrich filter 8, 10 mm) Oil content, wt- % Refractive index, ng Molecular weight Sulfur content, wt- % Aromatics, wt- % monocyclic bicyclic tricyclic
0.7900 0.7846 0.8038 0.7892 0.7566 66.8 51.9 73.1 70.5 78 28 19 15 33 9 1 1.5+ 0 1 0.00 0.36 0.20 0.16 0.17 0.7 5.7 5 .O 7.0 1 .o 1.4219 1.4418 1.4403 1.4488 1.4450 517 540 636 694 341 0.03 0.19 0.10 0.36 0.13 0.2 0.1 0.1
8 .O 4.6 0.3
4.0 0.6 0.2
13.3 0.2 0.2
4.4 0.4 0.1
optimum conditions. It may be seen that the aromatics content of all three products is reduced by one order of magnitude as a result of de-oiling, demonstrating that the aromatics and sulfur compounds are contained chiefly in the oily portion, as determined according to ASTM. Hot contact treatment mainly reduces the amount of bicyclic and tricyclic aromatic compounds. The starting materials, the de-oiled products and the products refined by hot contact were separated by chromatography on 0.10-0.18 nim silica gel, the weight ratio of adsorbent to sample being 20 : 1 and temperature 60 "C. Eluents applied successively were aromatics-free gasoline, a mixture of benzene and gasoline, benzene, and a mixture of methanol and chloroform. The mono-, di- and tricyclic aromatics content of the fractions was determined with a H-700 Hilger spectrometer. The distribution (in percentage) of carbon atoms in paraffinic and naphthenic bonds, and the number of methyl and methylene groups per molecule was determined by IR spectrometry. Figure 1-8 shows the refractive index at 80 "C plotted against the chromatographic yield for the petrolatum obtained from the residual oil, for the de-oiled and for the hot-contact purified products. It may be observed that, as was to be expected, both de-oiling and refining by hot-contact result in an increased n-alkane content. The vertical dash lines in the figure indicate the yields corresponding to the refractive index value of 1.4500. Before de-oiling, this yield is only 46 wt-%. After de-oiling at + 10 "C, the yield is 77 wt- %, after de-oiling at +30 "C, it increases to 94 %. By further refining, the 77 wt- % value is increased to 86 wt- %, the 94 wt- % yield to 96 wt- %. The distribution of n-alkanes, sulfur content and aromatics content versus chromatographic yield in the slack wax from light distillate and in the products
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
41
1.51 I 1.50 1.49 1.48 1.47 1.46 1.45
I ,
I
,
I
1.441 I I I I 0 10 20 30 40 50 60 70 80 90 1 0 1.49 I
1
1.47
-
1.46 1.45
X
1.49 I
C
a,
cc
1
De-oiled at t10 O C and refined by hot contact
1
-
440
t
10 20 30 40 50 60 70 80 90 100
1.45 1.46
1'440 1 /.O
10 20 30 40 50 60 70 80 90 100
refined by hot contact
1.46 1.45 1'440
10 20 30 40 50 60 70 80 90 100 Yield, w t - %
Fig. 1-8. Refractive indices of chromatographicalfractions of paraffin waxes from residual oil
42
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
-
'0
20
40 60 60 Yield, w t - %
100
100
"1
De-orled at +10 "C 3 60~a-
I
-
40-
c
-
0
Y
p
9
-3
s
,+ r I
' - 2
4
s
e-oiled at +10T and
I c
3
3
3
20-
-1
2 U I
I
C
20
0
Yield, w t - %
40 60 80 Yield, wt- %
100
Fig. I-9. Distribution of n-alkanes and sulfur in macrocrystalline paraffin waxes
---.---
monocyclic aromatics bicyclic aromatics tricyclic aromatics
a Yield, w t - %
60
1 De-oiled at +10 "C and refined by hot contact
LO
2ol 1 10
0
70
Yield, w t - %
,
,
80 90 Yield, w t - %
100
Fig. I-IO. Distribution of aromatic compounds in macrocrystalline para& waxes
43
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
obtained from this material is shown in Figs 1-9 and 1-10. Similar plots for the microcrystalline paraffin wax from heavy distillates and residual oil are presented in Figs 1-1 1-1-14. The data indicate that the fractions from 0 to 80 wt- % of the macrocrystalline slack wax contain 98 to 75 wt- % n-alkanes, while the corresponding data for petrolatum from heavy distillate are 40 to 55 wt-%, and the n-alkane content of the petrolatum obtained from residual oil in the fractions up to 50 wt- % is as low as 62 to 30 wt-%. 90 I
I
4 3
100
_i 0
-t
1001
I
90 De-oiled at t10 "C 80
P
4 -14 3
80
I I I
:2010
I
I )
[ -22 2 A -11
I
I
-
20
'
!
!
60 80 Vield, wt-"lo
40
'
100
0
I
De-olied ot +30 "C
. 70
< 3 60-I
L-
0 ' 0
10
Yield, w t - %
-
<
..
50
0
LO-
C
20 10 - OO
30-
I I
-1
I I
I
' 804& 0 Yield, wt-
100I
VO I
-De-oiled at +30 "C and 80 - refined by hot contact
70 60 '50A0 30 20 10 -
'0
20
40
60
80
-
*------
-4
z?
0
- s 3 .!3
I
-2 c 3
I I
-1
1I I
' L
3
Ln
100 Yleld, wt- 'fa
Yield, wt-'lo
Fig. I - I I . Distribution of n-alkanes and sulfur in microcrystalline paraffin waxes from heavy
distillates
44
1. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
De-oiling, and, to a lesser extent also refining, lead to an increase in the percentage of n-alkane content in the case of all three starting materials. As to the distribution of aromatic compounds, these are present, in the case of macrocrystalline products, only in the last chromatographic fractions, in the final 7-10 wt-% of the substance, whereas in microcrystalline products, aromatics appear even at a yield of 40 wt- %. De-oiling and refining by hot contact significantly increase this limit. Similar relationships are found in the distribution of sulfur compounds. The sulfur content is maximum in all cases in the fractions where the content in diand tricyclic aromatics is maximum. The sulfur atoms are consequently sited mainly in aromatic compounds. In the following, the chemical composition of some paraffin waxes determined by mass spectrometry is presented, based on data by Edwards. These substances represent the total range of macrocrystalline waxes, and include paraffin waxes 80 70 $ c 60 3 50 6 40 U 30 E 20 10
-monocyclic aromatics ---
-.-.-
bicyclic aromatics tricyclic aromatics
$
$0
50
60 70 80 Yield, wt-%
90
100
7
60-De-orled 50-
at + 3 0 T
w- 40-
2
30209 10Q.
g
Yield, wt-%
Yield, w t- %
?
70
70
6OCDe-oiled at +10 O C and
60-De-oiled at +30 "C and 7 50refined bg hot contact w-
4030-
:
20-
2 10Q. Yield, w t - %
Fig. 1-12. Distribution 9f aromatic compounds in microcrystalline paraffin waxes from heavy
distillates
45
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
whose compositions may be regarded as typical cases. The main characteristics and analytical data of the six paraffin waxes studied are contained in Table 1-23. Figure 1-15 shows the distribution of n- and iso-alkanes relative to the number of carbon atoms. These data also confirm the significant differences in the chemical composition of paraffin waxes depending on their source and manufacturing conditions. The samples investigated consist mainly of n-alkanes ;however, in samples A and F the iso-alkane content is rather high. The distributions demonstrate that hydrocarbons other than normal are concentrated in the higher molecular weight 4
80 70 - Starting material
-3s
* r -2
=
-1 10-
5-
c J
v,
0 Yield, wt-"lo
90 80
De-oiled at +lo "C
4
s
70 -
t
90 I 80 De-oiled at +30 "C 70
c
I
3 50
3
aC
-0 Y
30 20 10 -
--
7C Yield, w t
sn BoI-De-oiled
-
'0 10 "10
at +I0 "C and
70- refined by hot contact
6050 -
-
*----
44
s
30
50
70
Yield, wt-"lo
90
90 t 80 - De-oiled at +30 "C and 7 0 - refined bg hot contact
I
r
s 4030 -a-
40 -
30 -
Y
7C 20 -
20 10 -
- OO
20 10
10 -
Yield, wt-"lo
Yield, wt-"10
Fig. I-13. Distribution of n-alkanes and sulfur in microcrystalline paraffin waxes from residual
oils
46
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
1001
n
1
-monocyclic aromatics --- bicyclic aromatics ----- tricyclic aromatics
s
.3 I
m-
u .c
0
E
2
Q
Yield, w t - %
80 4?
70 - De-oiled at +10 "C
80 I De-oiled at +30
I
I
I
"c
I
3
m-
.-V
a ..
0
30
E 20
0,
a
'40
50
60
70
80
I
$0
Yield, wt-"lo
70 I 6 0 - De-oiled
1
at +10 "C and I c 50- refined b y hot contact 3 40 mu .- 30 -
P
50
60 70 80 Yield, w t - %
70 S 60-De-oiled at +30 "C and 50- refined by hot contact
90
100 I
'3
c
0
E 20 2 .I0 -
4
0
I
Yield, wt-%
Yield, wt-"lo
Fig. 1-14. Distribution of aromatic compounds in microcrystalline paraffin waxes from
residual oils
fractions. Disregarding sample A , the share of hydrocarbons other than normal increases with the melting point of the paraffin wax. Sample A was obtained by blending high and low melting-point paraffin waxes, and hence this sample cannot be directly compared with the others. The above samples were further separated by fractional distillation, and the composition of the fractions were also determined by mass spectrometry. Table 1-24 lists the results of these analyses. Fractions from wax B were not examined since the non-normal hydrocarbon content of this wax was low and the distribution appeared to be of less interest. Figures I-16,1-17 and 1-18 present the dis-
47
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
+
Table I-23. Main characteristics and chemical composition determined by mass spectrometry of various paraffin waxes Characteristics and composition
Melting point, "C Oil content (ASTM), wt- %
n-Alkanes, wt- % Branched alkanes, wt- % Monocycloalkanes, wt- % Polycycloalkanes, wt- % Monocycloaromatics, wt- % Aromatic cycloalkanes
I
--
i
A
50.7 <0.2 75.5 13.5 10.2 0.6
I
i 52.9
57.3
57.5
58.7
62.8
0.3
<0.2
<0.2 81.9 10.4
0.5 66.5 17.9
0.3
<0.2 82.2 8.2 9.0 0.5
0.0 0.0
0.1 0.0
3.4 0.0
86.4 6.3 7.1 0.1
0.0 0.0
traces 0.0
94.0 2.6
0.2 0.0
paraffin wax A
0.3
traces
10
5
Carbon atom number
Carbon atom number
15
15
10
s-
Paraffin wax &
10
I
I
z0
1.9
l5
g
-
13.4
7Paraffin wax D
I
s-
0'
1.4
I Iso-alkanes
I] n-Alkanes I
i
5 1' 5
15
r " 5
20 25 30 35 Carbon atom number Paraffin wax
'15
40
20
25
30
35
40
45
Carbon atom number
c1
15
s
10
8
5
-
Paraffin wax F
I
1' 5
20 25 30 35 Carbon atom number
20 25 30 35 40 Carbon atom number Fig. I-IS. Distribution of n-alkanes and iso-alkanes in various paraffin waxes
40
1' 5
45
48
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table I-24. Composition of distillation fractions of the paraffin waxes
listed in Table 1-23, determined by mass spectrometry
Parafin wax A Components
I-
n-Alkanes Branched alkanes Monocycloalkanes Polycycloalkanes Monocycloaromatics Aromatic cycloalkanes
Parafin wax C Components
n-Alkanes Branched alkanes Monocycloalkanes Polycycloalkanes Monocycloaroma tics Aromatic cycloalkanes
40-45 wt-%
'
82.4 13.9 3.6 0.0 0.1 0.0
1
20-25 wt-%
60-65 wt-%
20-25 wt-%
n-Alkanes Branched alkanes Monocycloalkanes Polycycloal kanes Monocycloaromatics Aromatic cycloalkanes
52.1 28.6 17.7 13 0.3 0.0
1
60-65 wt-%
91.4 7.5 11 0.0 0.0 0.0
l-
20-25 wt-%
5.0 wt-% residua
1
85-90 wt-%
5.0 wt- % residue
60.6
24.0
14.0 1.4 traces 0.0
5.0
0.3 0.0 0.0
60-65 wt-%
I
85-90 wt-%
79.2 13.6 6.5 0.4 0.3 0.0
37.3 34.0 23.8 4.9
traces traces
5.0 wt-%
residue
part of heads
94.5 5.5 0.0 0.0 0.0 0.0
37.6 25.9 30.5 4.4 1.4 0.2
85-90 wt-%
78.7 16.0
1
6.5 wt-% residue
1
part of heads
n-Alkanes Branched alkanes Monocycloalkanes Polycycloalkanes Monocycloaromatics Aromatic cycloalkanes
Components
85-90 wt-%
part of heads
Parafin wax D Components
~
part of heads
95.7 4.2 0.1 0.0 0.0 0.0
I-
60-65 wt-%
64.4 19.4 14.4 1.5 0.2 0.1
48.1 23.0 24.3 3.7 0.8 0.1
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
49
Table I-24 (cont.)
Parafin wax F
20-25 wt-%
Components
1
6C65
1
wt-%
85-90 wt-%
part of heads
n-Alkanes Branched alkanes Monocycloalkanes Polycycloalkanes Monocycloaromatics Aromatic cycloalkanes
78.6 14.9 6.5 traces traces 0.0
54.6 26.7 16.5 1.9 0.2 0.1
42.8 32.4 20.5 3.6 0.6 0.1
5.0 wt-% residue
44.6 23.9 24.1 6.1 1.o
0.3
tribution of normal and branched alkanes in different fractions of samples A, D and F. Turner and co-workers also used mass spectrometry to investigate paraffin waxes. The characteristics of their samples are listed in Table 1-25. Sample A
1n-Alkanes
IIso-alkanes
35 30 25 20 15 10
5
1
35
$ 30-
.t; 25 2 20+ c 15.-
60 %- 65 ' l o head product
r
10 -
85 %- 90 '10
head product
10
10 -
50 15
I
20
I
25
30
35
40
45
Carbon atom number
Fig. 1-16. Distribution of n-alkanes and iso-alkanes in the fractions of the paraffin wax
marked A
4
50
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 2-25. Characteristicsand chemical composition of various paraffin waxes Characteristics, composition
I
Melting point, OC Oil content, wt- % n-Alkanes, wt- % Iso-alkanes, wt- % Cycloalkanes, wt- % Alkylbenzenes, wt-
57.8 -
98.1 1.5
0.4
53.4 0.7 86.1 6.3 7.1 0.5
53.2 0.8 76.4 13.4 10.2 -
51.8 0.1 82.9 7.7 9.4 -
53.7 0.4 91.4 5.6 3.0
54.0 0.3 81.4 10.1 8.5
-
54.4 0.3 92.3 3.5 4.0 0.2
-
60.9 0.2 89.2 5.8 5.0
-
51.0 -
25.0 21.0 54.0 -
was a product obtained by repeated crystallization from a commercial paraffin wax de-oiled by sweating. Samples B, D, N and P were usual commercial grades. Samples E, F and H were obtained from slack waxes by laboratory-scale sweating. Sample J was prepared by a combination of various separations and enriching operations. First, a narrow boiling-point range fraction (b.p. 232°C a t 80 Pa) was obtained by vacuum distillation of a blend of commercial paraffin waxes. The cycloalkane content of this fraction was further enriched by thermodiffusion. The sample J was the bottom product of the thermodiffusion column, obtained with a yield of 7 wt-%. The data in the table and mass spectrometry analyses allowed the following conclusions :
0
n-Alkanes
I
Iso-alkanes ._
-
20% - 2 5 "lo
head product
head product
10 -
Carbon atom number
Fig. 2-17. Distribution of n-alkanes and iso-alkanes in the fractions of the paraffin wax
marked D
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
.-5
25 20 15 10 5
d, 10 -C 5
$
-
t.
0 5 15 --
0 -
51
IIso-alkanes
n-Alkanes
20 % - 25 "lo head product
-. n
I r ~
I
I
60 ' l o - 65 '10 head product
-
0 15
5 101 " 5 0
10 5Carbon atom number
Fig. 1-18. Distribution of n-alkanes and iso-alkanes in the fractions of the paraffin wax
marked F
(i) In all macrocrystalline paraffin waxes with melting points between 53 and 61 "C,the majority of n-alkane molecules are in the C,, to Gorange. It is interesting to note that in spite of the only 1 "Cdifference between the melting points of samples B and D, on the one hand, and sample N , on the other, the share of this fraction is 68 wt- % for the former two samples, in contrast to 86 wt- % for sample N , while the C,,-C,, n-alkane content in sample N is substantially lower than in samples B and D. (ii) The melting point of sample P is substantially higher than that of samples B, D and N . This is due, as shown by mass spectrometric data, to a share of 26 wt- % of C,l-C,, n-alkanes in sample P,as compared to 3-4 % in the other three samples. Sample P also contains 3 wt-% C,,-C,, n-alkanes, and its iso-alkane components, making up 5.8 wt- %, are >C,, compounds. (iii) Sample A is characterized by the finding that its n-alkane share of 98 wt-% is made up almost entirely of C,,-C,, compounds. This is presumably due to the repeated crystallization. The melting point of 58 "C corresponds to this composition. (iv) The melting points of samples E, F and H obtained by laboratory-scale sweating are also in conformity with the distribution by carbon atom number. (v) The melting point and composition of sample J clearly demonstrates that a relatively high melting point can also be reached, even with a low content of n-alkanes, if the iso-alkane and cycloalkane components present in larger amounts are suitable carbon atom number compounds. 4'
52
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Different paraffin wax grades are manufactured by de-oiling of slack waxes and petrolates. It is, therefore, of interest to give a short characterization of the composition of slack waxes. This subject was studied in detail by Kajdas et al., among others. Table 1-26 summarizes their results concerning the composition Table 1-26. Composition of three slack waxes Content, wt-%
Hydrocarbon group
Sample B
[
Sample c
Methods of analysis
I Sample D
+ cycloalkanes
70.7 20.0
33.4 43.5
12.0 48.5
Alkyl-substituted monocyclic aromatics
Adduct formation with urea Deduction of n-alkane content from total alkanes
6.0
16.0
33.5
Alkyl-substituted polycyclic aromatics
Column adsorption on silica gel. Eluent: hexane
2.5
4.0
2.0
Olefins Resins
0.3 0.5
0.6 2.5
1.5 2.5
Column adsorption on silica gel. Eluent : benzene Iodine number Calculated from difference
n-Alkanes Iso-alkanes
~
of three different slack waxes. Sample B was obtained from spindle oil, sample C from lubricating oil, and sample D from residual oil. The slack waxes were separated by a combination of different methods (adduct formation with urea, chromatography, various chemical methods). It may be seen from the data that with increasing average boiling point of the distillate, i.e. average molecular weight of the slack wax, the n-alkane content significantly decreases, while the share of isoand cycloalkanes and alkylated aromatics substantially increases. The composition of a slack wax obtained from a so-called ,,neutral oil 11” fraction of a Polish crude was determined by means of the procedures shown in Fig. 1-19. The results are summarized in Table 1-27. The fractions were analysed by IR spectrometry, NMR, mass spectrometry and UV spectrometry. Table 1-27. Composition of slack wax from neutral oil I1 Hydrocarbons
n-Alkanes Naphthenes with few side chains Iso-alkanes (slightly branched) Naphthenes with many side chains Iso-alkanes (very branched) Naphthenics-aromatics Polycyclic aromatics and resins
I
Carbon atom range
Mark
16 13 9 33 6 17 6
2 1-3 3-4 7-8
-
4-5
-
53
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
r
Extractive crystallization with urea
e Adduct - forming part
Part forming no adduct
Adduct-forming part
b
r
= I
Part forming no adduct
Vacuum distillation
1 10 fractions
I
Chromatographic separation on silica gel
n - Alkanes P
Naphthenes and
Naphthenes and
iso-alkanes
(10 fractions)
(10 fractions)
B
C
D
.
Fig. I-19. Diagram of the separation of slack wax
The hydrocarbon composition and distribution by carbon atom number of group B (Fig. 1-19, Table 1-27) is presented in Fig. 1-20. By further distillation of group C , three different fractions were prepared with boiling ranges of 426-446, 484-492 and 519-557 "C. The IR and NMR spectra of these fractions are very similar, and hence the differences in their structures are presumably explainable from the length of the side chains of the naphthenic rings. An average molecule of the above three fractions consists presumably of one isolated or condensed ring, seven CH, groups, and the longest side chain does not contain more than 20 carbon atoms.
54
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
n Fig. 1-20. Hydrocarbon composition of fraction B obtained from the slack wax from neutral oil 11
3. Chemical properties of individual alkanes and their mixtures Alkanes are neutral compounds fairly resistant to the action of chemicals. This behaviour is reflected in their earlier name paraffins (parum affinis = of little affinity). A more detailed study of the chemical properties of alkanes, however, led to the recognition that they are far from being totally inert to chemicals. Many examples of the radical-type homolytic activation of the C-H bond in alkanes are known. Their most important reactions are summarized in Table 1-28. (a) The reactions of paraffins with halogens Direct halogenation of alkanes takes place according to the following substitution reaction : C"H2, + 2
+&
-+
CnH,,
4.1
- x + HX
This reaction can be considered above all for preparing fluoro-, chloro- and bromoderivatives. Iodine cannot be introduced by a direct substitution reaction, however, this process takes place only a t higher temperatures. The HJ formed in the reaction will dehalogenate, in a reduction reaction, the alkyl iodide formed. Hence, the reaction is reversible and the equilibrium is shifted towards the left-hand side of the above general equation. Therefore, direct substitution with iodine is feasible only in the presence of certain additives (e.g., nitric acid, mercury(I1) oxide, silver
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
55
Table 1-28. Chemical reactions of alkanes No.
~
I
Agent
Process
Product
Halogens Sulfuryl chloride (+dibenzoyl peroxide)
Light-catalysed substitution
Saturated halogensubstituted derivative
Phosgene or oxalyl chloride
Light-catalysed substitution (chlorocarbonylation)
Chloride of carboxylic acid
Sulfur trioxide Sulfur dioxide 4-chlorine Sulfur dioxide oxygen
Substitution
Alkanesulfonic acid, Alkanesulfonic chloride
Nitric acid or nitrogen dioxide
Substitution
Nitroalkane
Oxidizing agents
Oxidation
Alcohol, aldehyde, ketone, carboxylic acid
Chain splitting, Catalytic isomerization, Dehydrogenation
Short-chain hydrocarbons, Branched alkane isomer, Unsaturated hydrocarbons
+
High temperature
perchlorate) that bind or decompose, by oxidation, the hydrogen iodide formed and thereby render the process irreversible. In contrast to iodine, fluorine reacts with explosive violence, resulting in chain splitting of the alkane which will then yield carbon tetrafluoride and hydrogen fluoride. The heat of reaction of the fluorination process is as high as 419 kJ/mol, as compared to the 101-109 kJ/iiiol for chlorination. Therefore, direct fluorination can only be carried out in the presence of retarding catalysts, e.g. silver-plated copper chips, or by using fluorine gas diluted with nitrogen. In such retarded reactions no chain splitting will occur. However, no control of the reaction to obtain monovalent alkyl fluorides is possible, and a mixture of multiple-fluorinated hydrocarbons will be formed. Fluorine derivatives are manufactured by indirect methods, by exchanging chlorine against fluorine in chlorine derivatives. Chlorine and bromine are capable of direct substitution reactions with saturated hydrocarbons. Chlorine reacts with alkanes directly at ambient temperature. With increasing temperature, the rate of the reaction increases. The process can also be accelerated by catalysts or light. Photocatalytic chlorination is a radical chain reaction proceeding according to the following stages : C1, + hv -+ 2 C1* R-H + C1* + R* + HCl R*
+ C1,
-+
R-Cl
+ C1*
56
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
The first reaction step is the absorption of light energy by the chlorine molecule, resulting in the formation of excited chlorine atoms. These immediately react with alkanes, yielding alkyl free radicals. In the next step, the free radicals react with chlorine molecules, and alkyl chlorides or polychlorinated alkanes substituted at different sites will be formed, as well as excited chlorine atoms which maintain the chain reaction. The chain reaction can be broken off as a result of several reactions. The most important reactions are: recombination of excited chlorine atoms on the wall of the equipment (wall effect); reaction of alkyl-free radicals with excited chlorine atoms (instead of chlorine molecules); if oxygen is present, reaction of oxygen with alkyl-free radicals, yielding peroxides (subsequently reacting with one another or with further alkyl-free radicals and thereby forming stable products); finally the reaction of the excited chlorine atoms with oxygen, yielding chlorine dioxide. Irradiation with light in the wavelength range of 2500 to 4000 A, corresponding to the absorption range of chlorine, is used in photochemical chlorination. The energy of such radiation amply provides for the activation energy of the dissociation of chlorine molecules (239 kJ/mol). Photochemical chlorination is mainly of importance in the chlorination of liquid paraffins. In this process, chlorination can be carried out at low temperatures (30-50 "C), since the rate of the reaction depends only sIightIy on temperature. For example, normal dodecane is readily chlorinated already at 30 "C with a chlorine consumption of about 1.7 litres/kg s. Paraffin waxes can also be chlorinated by the photochemical process. Parallel to the higher melting point, chlorination is carried out at somewhat higher temperatures, around 7G90 "C. Paraffin waxes melting below 75 "C are usually chlorinated in the melt, those with higher melting points in solution, using chlorinated hydrocarbons, e.g. carbon tetrachloride, as solvent. Depending on the nature of the paraffin wax and conditions, several products, mono-, di-, tri- and polysubstituted alkanes, are formed simultaneously :
-
+
+ +
C,H,, + ,C1 C1, + C,H,,Cl, HCI C1, .+ C,H,, - C1, HCI etc. C,H,, C1, In the chlorination of alkanes one must reckon with the formation of all theoretically possible chloroalkane isomers. The ratios of the isomers are defined by the number of different-order hydrogen atoms and by the relative rate of reaction of the hydrogen atoms. In liquid-phase chlorination at about 30 "C, the relative rate of reaction of primary, secondary and tertiary hydrogen atoms is approximately 1 :3.25 : 4.43. These proportions are only slightly changed by catalysts or by light irradiation. The values depend above all on the temperature and pressure of the chlorination process. With increasing temperature the difference between the rates decreases. The number of mono- and polysubstituted products of saturated hydrocarbons, also taking into account isomerization, rapidly increases with increasing numbers of carbon atoms in the molecule.
+
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
57
In addition to photocatalytic chlorination, homogeneous catalytic chlorination is also known. This process can be applied both to liquid paraffins and to paraffin waxes. In liquid-phase chlorination, usually catalysts readily soluble in liquid hydrocarbons or in their mixtures with chlorinated hydrocarbons, e.g. carbon tetrachloride, are used. In these processes, chlorine will readily dissociate into active chlorine ions. The most important catalysts are chlorides of iodine, phosphorus, sulfur, antimony, iron and tin. Liquid paraffins are usually chlorinated in the presence of dissolved iodine or phosphorus pentachloride. Bolley was the first, in 1858, to chlorinate paraffin waxes with the cited methods. He obtained a viscous oil-like liquid at ambient temperature. Upon continuing chlorination up to a chlorine content of about 62 wt-%, an amorphous product was formed, which he termed chloroparaffin. In addition to homogeneous catalytic chlorination, heterogeneous catalytic chlorination processes carried out in the gas phase are also known. Among heterogeneous catalysts are copper chloride and iron, usually applied on alumina and silica gel supports. These processes have no great importance for alkane chlorination, since the activity of the catalysts decreases, especially due to coke formation. In thermal chlorination neither catalysts nor light irradiation are used. In this process excited chlorine atoms are formed by the thermal dissociation of chlorine molecules. In the first step, the chlorine radicals attack the C- H bonds and start a chain reaction. Correspondingly, the activation energy is substantially higher, 83.7 kJ/mol, as compared to 50.3 kJ/mol in catalytic chlorination. Thermal chlorination is a chain reaction strongly inhibited by the presence of oxygen. In thermal chlorination, depending on reaction conditions, unsaturated hydrocarbons are also formed besides chloroalkanes. On the other hand, the formation of hydrogen chloride by dehydrochlorination of alkyl chlorides does not take place, or only to a slight extent. Thermal chlorination is used chiefly for lower molecular weight hydrocarbons, but higher molecular weight paraffin hydrocarbons are also chlorinated by this process, at temperatures between 70 and 100 "C, to the desired chlorine content. In fact, even polyethylene can be chlorinated in this manner. Alkanes react slowly with phosgene or oxalyl chloride, yielding the chlorides of the corresponding carboxylic acids according to the following equations: R-H
+ COCl,
R-H+
COCl
1
COCl
hv
R-COCl
+ HCl
R-COCl
+ HCI
or peroxide hv
-co
58
1. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
(6) Sulfochlorination of alkanes
The first mention of the simultaneous reaction of sulfur dioxide and chlorine with liquid alkanes was made in the patent "Procedure for the Halogenation of Hydrocarbons" issued by Cortes F. Reed and Charles Horn in 1936. In this reaction, alkyl sulfochlorides are formed : R-H
+ SO2 + C12
R-SOZ-CI
+ HCl
In addition to the simultaneous introduction of sulfur dioxide and chlorine into the molecule, chlorination without the participation of sulfur can also take place, and subsequently these chlorinated molecules can undergo sulfochlorination. Generally in substitution reactions, di- or polysulfochlorination also takes place, so that finally a mixture of mono-, di- and polysulfochlorides, alkyl chlorides, chloromono- and disulfochIorides and unreacted alkanes is obtained. Similarly to chlorination, sulfochlorination is also a chain reaction :
+ hv Cl* + C1* R-H + C1* -+ R* + HCl R* + SO, -+ R- SOB R-SO,* + C1, -+ R-SOz-Cl + C1* C1,
-+
At identical intensities of irradiation, shorter wave-length light will result in a lower chlorine to sulfur ratio in the products. The chlorination of the hydrocarbon can be avoided by using UV light. Recently some organic catalysts were discovered which are capable of starting and maintaining the sulfochlorination reaction without requiring light. For instance, diazomethane, lead tetraethyl, triphenyl-methylmethane, acetone peroxide, dialkyl peroxides and benzoyl peroxide are such radical-forming substances; their catalytic effect being due to their reaction with chlorine molecules yielding alkylfree radicals and excited chlorine atoms. To suppress chlorination of the alkane chain it is preferable to operate with excess sulfur dioxide pf about 100 mol- % (higher excesses are unfavourable). The formation of polysulfochlorides can be reduced by only partial sulfochlorination. Under optimum conditions, sulfochlorination to 50 wt- % yields 85 wt- % mono- and 15 wt- % disulfochloride, as compared to full sulfochlorination when 60 wt- % mono- and 40 wt- % disulfochlorides are formed. When alkanes are reacted with a 3 : 1 mixture of sulfur dioxide and chlorine, the main product is alkyl monosulfochloride, with minor amounts of polysulfochlorides, alkyl chlorides and chloroalkyl sulfochlorides as side products. Isomeric monosulfochlorides can also be formed in sulfochlorination. Sulfochloride groups are only bound to primary and secondary carbon atoms, the hydrogen atoms bound to tertiary carbon atoms do not enter into reaction. However, the latter are sensitive to chlorination. Therefore, the sulfochlorination
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
59
of compounds containing tertiary carbon atoms results in substantial chlorination of the carbon chain. In higher molecular weight hydrocarbons, the sulfochloride groups are distributed evenly along the methylene groups. Aliphatic sulfochlorides are not heat-resistant : when heated, sulfur dioxide is split off and alkyl chlorides are formed. Such desulfonation reactions take place readily with higher molecular weight aliphatic sulfochlorides. However, in contrast to sulfochlorides, aliphatic sulfofluorides obtained from sulfochlorides with potassium fluoride solution are thermally stable.
(c) Reactiondliquid parafins and parafin waxes with sulfur dioxide, sul&&.rnQkide, sulfuric acid and .fuming suljiuric acid (oleurn) The term sulfonntion designates the chemical reaction in which a bond is 0
I/
formed between the-S=O
I
group and a carbon or nitrogen atom of an organic
OH cornpound.6ulfonic acid groups and sulfonic acids must not be mixed up with the esters of sulfuric acid, that is, with alkyl sulfates having the general formula R-0-SO,-OH, like e.g. ethyl sulfate C,H,-0-SO,-OH, in which the -SO,H group is bound to an oxygen atom and not directly to a carbon or nitrogen atom as in sulfonic acids. In aliphatic hydrocarbons three sulfonic groups can be bound to one and the same carbon atom. Direct sulfonation is usually carried out with concentrated sulfuric acid, oleum, mixtures of sulfur dioxide and chlorine and mixtures of sulfur dioxide and oxygen. At ambient temperature concentrated sulfuric acid, and even fuming sulfuric acid containing less than 15 wt-% sulfur trioxide, does not react with alkanes. Studies by Engler and Hofer showed that in particular medium and high carbon atom number branched alkanes yield alkylsulfonic acids with hot concentrated sulfuric acid according to the following reaction : C,H,,
+2
+ HOSO2OH
+ C,H,,
+ 1-
S 0 2 0 H + H,O
Pure sulfur trioxide, in the liquid or gaseous state, also yields sulfonic acids with alkanes under appropriate conditions. In this case the reaction is essentially an addition: R-H SO, + R-SOaH
+
The dehydrating effect of sulfur trioxide is so vigorous that it carbonizes organic compounds when added directly. Therefore, sulfonation with sulfur trioxide, free of such side reactions, is feasible only at lower temperatures and in the presence of solvents or sulfuric acid.
60
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
With mixtures of sulfur dioxide and oxygen, alkanes yield aliphatic sulfonic acids according to the following reaction : R-H
+ SO, + 1/20,-
light
R-SO,-OH
This sulfoxidation takes place in the presence of UV light or various catalysts, e.g. chlorides or anhydrides of acids, peroxides, peracids. Sulfoxidation of higher alkanes proceeds only under permanent irradiation or the addition of anorganic catalyst. The sulfo group is statistically distributed along the carbon chain. Sulfonic acids with chain-end sulfo groups will be the scarcer the longer the alkane molecule is. In the sulfoxidation of higher alkanes in the presence of acetic anhydride, it is assumed that a mixed anhydride of sulfonperacid and acetic acid is formed, which subsequently reacts with the water present and is transformed into sulfonic acid : R-H
+ (CH3CO),O 0 - 0 - COCH, + HZO
+ SO, +
R- SO,-
0 2
+
R-SO2-O-O-COCH3
R- SOZOH
i
+ CH,COOH
+ CHSCOOH, + 1/2 0 2
(d) Reaction of liquid parafins and parafin waxes with nitric acid Concentrated nitric acid reacts only to a slight extent with alkanes, even at
100 "C. Konovalov found that alkanes can readily be nitrated with dilute nitric acid (13 wt-%) at 130 to 150 "C under pressure, that is, heated in a closed vessel, and yield nitroalkanes :
C,H2,
+2
+ HONO,
-+
C,H,,
+ 1-
NO,
+ HZO
He stated that the reaction is a radical chain reaction accelerated by radicalforming substances, e.g. lead tetraethyl, and retarded by substances inhibiting the chain reaction, e.g. nitrogen monoxide. The nitration of alkanes is accompanied by oxidation, chain splitting and degradation. Alkanes, above all n-alkanes react only slowly with the so-called nitrating mixture, a mixture of concentrated nitric acid and concentrated sulfuric acid. This mixture, eminently suitable for the nitration of aromatic hydrocarbons, when applied to alkanes, results in rapid hydrolysis, by the hot sulfuric acid, of primary nitroalkanes, and conversion of secondary and tertiary nitroalkanes into brown, tar-like products. Nitro compounds are classified by the order of the carbon atom to which they are bound. Correspondingly, primary, secondary and tertiary nitro compounds exist. Iso-alkanes containing tertiary carbon atoms are readily oxidized by hot fuming nitric acid. In this process, called oxidative degradation, carbon dioxide and carboxylic acids with one less carbon atom are formed. Markovnikov observed that hydrogen atoms bound to primary carbon atoms are the most resistant to substitution by nitro groups, hydrogen atoms at secondary
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
61
carbon atoms will enter more readily into reaction, and hydrogen atoms at tertiary carbon atoms react at the highest rate. The most important experimental results of the liquid-phase nitration of alkanes with nitric acid are as follows: (i) The rate of reaction is low, conversion increases With temperature. (ii) Nitration is accompanied by oxidation, and polynitro compounds are also formed, presumably because nitroalkanes are readily soluble, nitratable and hydrolysable. Higher molecular weight alkanes can readily be nitrated in the melt with nitric acid vapour, at relatively low temperatures. At high ratios of nitric acid to alkane, polynitro compounds and fatty acids are primarily formed. When the ratio is reduced to 1 :2, only about 40 wt-% of the alkanes react and the main product consists of mononitro derivatives. The mixtures of alkanes can be nitrated with nitric acid vapour in liquid phase, when their initial boiling point is at least 160-170°C. Alkanes of 7-12 carbon atom numbers cannot be subjected to nitration with nitric acid vapour in liquid phase because of their low boiling points. On the other hand, in the case of gasphase nitration of these alkanes the risk of pyrolysis is present. These alkanes can be nitrated under pressure, at 160-170 O C , with dinitrogen tetroxide.
Table 1-29. Some reactions taking place in the alkane oxidation process
R2/ H \
R1\c/o-o~
">,,-ORz
Rl
R,
R 'z
'CH-0-+
OH
)CH2+R1>
'><:-'3
R
0
R,-C&H
0
0 R-C
+ OH- +
R-C(
-P
+
-
dkyl group
R4
9-
+ Rz-
0 R-C
0
-+ R-CfOOH + R-<
Now: R
R,
+
0 HO) C H - R + 2 R - C 4
0-0
0
\OH
62
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
(e) Oxidation of liquid parafins and parafin waxes
Liquid paraffins and paraffin waxes do not react with oxygen at ambient temperature. The oxidation process begins to a significant extent only at temperatures of 80 to 100 "C. The oxidation of alkanes is a radical chain reaction. R* radicals formed by the effect of light, initiators, etc. react with oxygen to form active peroxide radicals R-00*. The reaction of the latter with alkane molecules R - H results in the formation of alkane hydroperoxide R-OOH and alkyl free radicals R* maintaining the chain reaction. Alkyl-free radicals, peroxide radicals and hydroperoxide are of the greatest significance in the progress of the oxidation process. Some examples of the manifold reactions taking place in the course of the oxidation of alkanes are presented in Table 1-29. Obviously, the number of products is very high, and includes esters, ketones, aldehydes, alcohols, acids, water and hydrogen peroxide. Since all radicals are capable of reacting with all compounds in the reaction product, lactones, hydroxyacids, dicarboxylic acids, etc. can also be formed. It should, however, be stressed that the mechanism of alkane oxidation is not yet fully clarified. Different authors assume various intermediate reactions within the oxidation process. Only some examples shall be mentioned. For example, Rieche maintains that the starting reaction of oxidation is the formation of alkyl hydroperoxides, involving molecular oxygen, according to the following:
R1CH2CH2CH2R2+ 0, -+ RlCH2- CH -CH, - R2
I
0
I
0
I
H The subsequent step is the rearrangement of the alkyl hydroperoxide into the semi-acetal, which is then converted into the aldehyde and alcohol :
RICH, - CH - CH, - R,
I
+
RICH, - CH - 0 - CH2R2
0
OH
I
0
I
H 0 +
1
/I
R,CH,-C-H
+ R2CH2OH
+
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
63
The aldehyde formed in the above reaction yields the peracid with oxygen:
0
0
II
I1
+0 2
R-C-H
-+
R-C-0-OH
The peracid forms a very labile adduct with the aldehyde:
0
0
II
0
+ R-C-H
R-C-0-OH
H
I1
I/
-+ R - C - 0 - 0 - C - R
I
I
OH This adduct is transformed, in the presence of water, into two molecules of fatty acid : 0 H
II
R-C-0-0-C-R
I
I
-+
2R-COOH
OH If no water is present, two molecules of the adduct yield one molecule of the fatty acid anhydride and two molecules of the fatty acid:
0
H
II
I
2R-C-0-0-C-R
-P
I
II
0
OH
+ 2R-COOH + HSO
R-C-0-C-R
I1
0
The primary alcohol formed parallel with the aldehyde reacts with molecular oxygen. The primary product is aldehyde peroxide hydrate, which is rearranged into the ortho-acid, immediately dehydrated to the carboxylic acid :
H
I
R-C-H
I
OH
H
+ 0,
-+
I
R-C-0-OH
I
OH
OH
I
+ R-C-OH
I
-+
RCOOH f H2O
OH
According to Langenbeck and Pritzkow, alcohols, in the course of alkane oxidation, are not oxidized to the corresponding fatty acid homologue, but because oxygen first attacks the hydrocarbon chain, lower fatty acids and hydroxycarboxylic acids are formed. These authors, along with many others, assume that the first stage of the oxidation is the formation of hydroperoxides. In the second stage, however, the secondary hydroperoxides decompose, yielding ketones and alcohols:
64
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
main reaction R1- CH, - CH - CH2 - R2 I \ I side reaction OOH
R1-
CH, - CO - CH2 - Rz
L
Rl-CHz-CH-CH2-R2
I
+ H,O
+ 1/20?
OH The ketone formed as main product is further oxidized, yielding alpha-ketohydroperoxide, which, in the following stage, is rearranged into aldehyde and fatty acid :
Rl-CHZ-CO-CH2-R2
+
0 2
+ R,-CH-CO-CHZ-RZ
I
-+
0
I
+
R1-CHO
+ R2 - CHZ-COOH
OH
Since substantial amounts of esters are also found in the oxidation products, Langenbeck assumed that the esters are formed according to the Bayer-Villiger reaction, from peracids and ketones : 0
I1
R-C-0-OH
+ R1-CO-CH7-RZ
+ RCOOH
+ R,-COO-CH,-R,
Alpha-ketohydroperoxides can, in part, be transformed into diketones, the latter reacting, according to Karrer and Schneider, with peracids, and yielding fatty acid anhydrides and fatty acids: RI-CH-CO-R,
I
--+
R1-CO-CO-R2
+ HZO
0
I
OH R1-CO-CO-Rz
+ R3-COO-OH
+ R1-CO-0-CO-R2
+ R3COOH
These anhydrides form esters with compounds containing hydroxyl groups. In the case of n-alkanes, the rate of oxidation increases with chain length, since the C-H bond energy in -CH,, =CH2 and -CH groups decreases in the ratio of 1 :4 :76, and consequently the effect of -CH2- groups becomes more and more important as the chain length increases. The relative oxidation rates of some n-alkanes are presented in Table 1-30. Another important feature is the higher rate of oxidation of n-alkanes than that of their branched isomers. The relative oxidation rates of n-hexane and of its isomers are listed in Table 1-31. Primary and secondary hydroperoxides are formed more reluctantly and decompose more readily than tertiary hydroperoxides. Tertiary hydrogen reacts relatively readily and rapidly, and the tertiary hydro-
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 1-30. Relative oxidation rates of some n-alkanes 0,(mol) Oxidation rate = alkanes (mol) timi)
,
Hydrocarbons
n-Pentane n-Hexane n-Octane n-Decane
I
oxiZE;ate
1 .o 7.5 200 1380
(
65
Table I-31. Relative oxidation rate of hexane isomers 0 1 (moo Oxidation rate = alkane (mol) time Hydrocarbons
2,3-Dimethylbutane 2,ZDimethylbutane 3-Methylpentane 2-Methylpentane n-Hexane
I
-
Relative oxidation rate
1 12 60 560 1580
peroxides are stable reaction products. Thus, the rate of reformation of the chain transfer radicals is greater in the case of primary and secondary hydroperoxides, and hence the rate of reaction is higher. Salts of multivalent metals, e.g. manganese, cobalt, iron, copper, and radicalforming substances, such as hydrogen bromide, manganese(J1) bromide etc., accelerate the oxidation reactions. Oxidation processes of alkanes can be changed by using so-called modifiers. These are of particular interest in the oxidation of higher alkane mixtures. By using boric acids as modifiers, the ketone to alcohol ratio can be shifted towards the latter in the reaction product. Apparently the effect of boric acids consists not only in their protecting the alcohols against esterification, but also in the reaction of boric acids and their anhydrides, with R-00* radical;, yielding, via intermediate products, alcohols and esters, respectively.
(f)Thermal decomposition and isomerization of alkanes At temperatures above 350 "C,alkanes suffer thermal decomposition. The two major reactions in this process are dehydrogenation and chain splitting :
Chain splitting is thermodynamicallymore feasible than dehydrogenation. According to Haber's rule, the lower molecular weight hydrocarbon formed in chain splitting is the alkane, the higher molecular weight hydrocarbon the alkene (olefin). In the course of thermal decomposition, isomerization reactions also take place as side reactions. Alkylation of alkanes and aromatics formed by the thermal decomposition of olefins, cyclization of alkanes and olefins, further decomposition of olefins formed in primary reactions and polymerization of olefins are also involved. The products of the thermal decomposition that are liquid at ambient temperature, contain in addition to diolefins, naphthenes, and aromatic compounds al5
66
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
kanes and olefins with lower carbon atom numbers than those of the starting alkanes, too. Depending on the reaction conditions, a solid by-product, called petroleum coke, is also formed in greater or lesser amounts. The gaseous products, in quantities depending on the conditions, are hydrogen, methane, ethane, propane, butane, ethylene, acetylene, propylene and but ylene. The major factors affecting the thermal decomposition of alkanes are temperature, time of reaction, and, to a lesser degree, pressure. The temperature of thermal decomposition not only affects the rate of reaction, but also determines the thermodynamically possible processes. With increasing temperature the yield of gaseous reaction products, but also coke formation, increases. Polymerization side-reactionstake place at normal pressure up to about 400 "C. The formation of diolefins and aromatics starts above 600 "C. Increased reaction-time promotes secondary processes, so that the formation of low molecular weight olefins, of coke and of hydrogen then increases. Increased pressure promotes reactions involving volume reduction :it suppresses dehydrogenation, but does not affect chain splitting. Higher pressures are favourable to secondary polymerization and condensation reactions, the share of liquid reaction products increases and the yield of gaseous olefins is reduced. With increasing pressure, the rate of reaction also increases up to rates exceeding the rate corresponding to atmospheric pressure by a factor of 20 to 40. -In conformity to the reactivity of alkanes, the rate of reaction of thermal decomposition increases up to Cl0. However, for >C, alkanes, no difference in activation energy or rate constant is observed. Branched alkanes are cracked more readily than n-alkanes. Compounds containing a single methyl group on the secondary C atom behave similarly to n-alkanes, this similarity increases with molecular weight. The further the methyl group from the end of the molecule, the greater the difference between the behaviour of the iso-alkane and the n-alkane. Two methyl groups on the chain already cause significant differences. Among the various theories postulated for the thermal decompositionmechanism of alkanes, the free radical chain mechanism has been widely accepted. According to this theory, free radicals are formed, when C - C or C - H bonds are thermally split : R-CH2-CH,
+ R-CH,
4-CH,
*,
or
CH3
I
f
CH
R-CH-CHZ-CHZ
I
R - CH- CH,-CH,
\
CH3
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
67
The free radicals react with alkanes forming new radicals:
CH,
*
+ R-CH,-CH,-CH,
+ CH,
+ R-eH-CH,-CH,
The new radical is always formed on internal carbon atoms, since the energy of secondary C - H bonds is lower (372 kJ/mol) than that of primary bonds (398 kJ/mol). Secondary free radicals subsequently form double bonds in the /3-position, and these yield a-olefins, primary free radicals are also formed : R- CH, -CH,- CH2- CH, - 6 H - CH,- CH, + R-CH,-CH,-CH,
R-CH,-CH,-CH,
+
+ CH,=CH-CH,-CH:, R-CH, + CH,=CH,
*
*
3
*
Internai rearrangement of long-chain free radicals leads to the formation of internal free radicals :
R- CH, - CHZ-CHZ- CH, - CH, - CH, - CH, * + --f R - CH,- 6 H - CH, - CH, - CH, - CH, - CH3 . Recombination of free radicals also occurs:
R-CH,
*
+ R-CH,
+ R-CHZ-CHZ-R
However, free radicals are incapable of producing branching and cyclization of the alkane chain. The quantities and the chemical compositions of gaseous, liquid and solid products formed by the thermal decomposition of alkanes are changed by the presence of catalysts. Amorphous catalysts consisting of Si02 and A1,0, can be set to achieve the required effect by varying the alumina content between 13 and 75 wt- %. Crystalline zeolites (molecular sieves) have recently gained importance as catalysts for thermal decomposition. In their presence, as compared to thermal cracking without using catalysts, the C, hydrocarbon content of the gaseous product, and the koalkane, cycloalkane and aromatics content of the liquid product significantly increases. The process of catalytic thermal decomposition is based on the formation of so-called carbonium ions. Carbonium ions are hydrocarbon ions with a free positive charge at one carbon atom, e.g. tertiary butyl cation:
CH,
I
CH3-C+
I
CH, By the use of catalysts, carbonium ions are formed in the course of catalytic thermal decomposition, and their reactions yield the further products.
68
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
By treating n-alkanes containing four or more carbon atoms under relatively mild conditions with metal halides and hydrochloric acid, isomerization into branched alkanes is achieved. The reaction is reversible and accompanied by insignificant amounts of cracking, dehydrogenation and polymerization of the olefins formed. If pure alkanes are used as starting material, small amounts of hydrochloric acid, water, and various promotors act as carbonium ion sources. Table 1-32. Isomerization of n-alkanes Conditions and products c.0
Temperature, OC Pressure, kPa Molar ratio hydrogen/alkane in feed Product distribution in wt- % relative to alkane Gas Liquid cracked Isomers : solid liquid Aromatics Unchanged alkane
I
n-Alkanes
c,.
1
c.0
430 3500 64
430 3500 67
420 3500 32
6 7
5 14
4 32
0 34 1 52
9 22 1 49
13 8 1 42
By their action, the isomerization continues as a chain reaction. High molecular weight n-alkanes can also be isomerized in the presence of Friedel-Craft catalysts. Isomerization conditions of various n-alkanes and the most important characteristics of the products are listed in Table 1-32.
Literature Andreas, F., Chem. Tech. Berl., 16, 449 (1964). Asinger, F., Parufins, Chemistry and Technology. Pergamon Press, Oxford (1 968). Asinger-Fell: Erdd, Kohle, 17, 74 (1964). Azizova-Chernozhukov-Kartinin-Grishin: Nef’i Guz, No. 10, 59 (1970). Bailey-Bannerot-Fetterly: Ind. Engng. Chem., 43, 2125 (1951). Bell-Raley-Rust: Discuss. Furuduy SOC.,10, 242 (1951). Bengen, F., Angew. Chem., 63, 207 (1953). Berty, J., Az asurinyolaj kEmiai feldolgozrisa. (Chemical processing of petroleum). Nehtzipari Konyvkiad6, Budapest (1952). Bornemann-Heinze: Chem. Tech. Berl., 20, 99 (1968). Borzsonyi-E. Kantor-Keszthelyi: MAFKIkiadvdny (Report of the Hungarian Oil and Gas Research Institute), No. 311 (1964). Brandes, G . , Brennst. Chem., 37,263 (1956). Buchler-Graves: Znd. Engng. Chem., 19, 718 (1927). Burwell, A. W., Ind. Engng. Chem., 26, 204 (1934). Burger-Combarnous: Revue Znst. r. Pgtrole, 30, 551 (1975). Clark-Smith: Ind. Engng. Chem., 23,697 (1931).
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
69
Csik6s-E. KAntor-M6zes-Keszthelyi: MT A KCm. Tud. Oszt. K6zl. (Report of the Department of Chemical Sciences of the Hungarian Academy of Sciences) 25, 134 (1966). Dietsche, W., Fette, Seifen, Anstr-Mittel, 72, 778 (1970). Edwards, R. T., Mod. Packag., 26, No. 6. 123 (1953). - : Ind. Engng. Chem., 47, 2555 (1955). - : Ind. Engng. Chem., 49, 750 (1957). - : Petrol. Refiner, 36, 180 (1957). Eggertsen-Groennings: Anulyf. Chem., 33, 1147 (1961). Emanuel, N. M., The Oxidation of Hydrocarbons in the Liquid Phase. Perganion Press, Oxford (1965). Ernanuel-Maizus-Skibida: Angew. Chem., 81, 91 (1969). Etessam-Sawyer: J. Znst. Petrol., 25, 253 (1939). Ferris-Cowles: Ind. Engng. Chem., 37, 1057 (1945). Finkel-Heinze: Chem. Tech. Berl., 14, 299 (1962). Francis-Young: J . Chem. Soc., 73, 928 (1898). Franke, R., Energie Tech., 25, 388 (1975). Freund-BCithory: Erdd, Kohle, 9 , 237 (1956). Freund-Keszthelyi-Mbzes: Chem. Tech. Berl., 17, 582 (1965). Fuchs-Nettesheim: Erdtil, Kohle, 10, 362 (1957). George-Robertson: J. Znst. Petrol., 32, 382, 400 (1946). - : Proc. R. SOC.,185, 288 (1946). George-Walsh: Trans. Faraduy SOC.,42,94, 210, 217 (1946). Gip-Heinze: Acta Chim. Hung., 31, 85 (1962). Grodde, K. H., Erdd, Kohle, 3, 61 (1950). Gross-Grodde: 61, Kohle, 38, 419 (1942). Griinberg, M., Seifen-ble-Fette-Wachse, 90, 478 (1964). Gruse, W. A., Chemical Technology of Petroleum. McGraw-Hill, New York (1960). Guseva-Ashkinadze-Leifman: Izv. Akud. Nuuk. SSSR, Ser. Phys., 27, 104 (1963). Hass-McBee: Znd. Engng. Chem., 23, 352 (1931); 27, 1190 (1935); ,28, 333 (1936); 33, 185 (1941). Heberling, R., Freiberger ForschHff., A 201, 21 (1961). Hessler-Meinhardt : Fette, Seifen, 55, 441 (1953). Hildebrand-Teubel-Peper-Dahlke: Chem. Tech. Berl., 15,482 (1963). Hunter-Segester: US.Pat. 2 670 323 (1954). Isrnaylov-Terteryan: Khimiya Tekhnol. Topl. Musel, 20, No. 12, 6 (1975). Kaiser, R., Fette, Seifen, Anstr-Mittel, 60, 915 (1958). Kajdas, C., Seifen-Ole-Fette- Wachse, 96, 251 (1970). Kajdas-Tiimmler-Berthold: Erdd, Kohle, 23, 663 (1970). Kdntor, E., MdFKZ k6zlemdnyek (Report of the Hungarian Oil and Gas Research Institute), No. 274 (1963). Kharasch-Berkman: J. 01-gChem., 6, 810 (1941). Kisielow-Kajdas: Seifen-Ole-Fette- Wachse, 93, 719 (1967). Krasavchenko-Zemskova-Mikhnovskaya:Neftekhimiya, 11,803 (1971). Krasnova-Malnev-Putshkovskaya: Zzv. Akad. Nauk. SSSR,Ser. Phys., 27, 98 (1963). Kreuder, W., Fette, Seifen, 84, 665, 699, 735, 773, 849 (1958). - : Fette, Seifen, 85, 19, 41, 93 (1959). Larson-Becker: Anulyt. Chem., 32, 1215 (1960). Leibnitz-Hager-Heinze: J. Prukt. Chem., 4. Reihe, 3 , Heft 1-2 (1956). Leibnitz-Hager-Herrnann: J. Prukt. Chem., 4. Reihe, 5, Heft 1-2 (1957). Levy-Doyle-Brown-Melpolder: Analyt. Chem., 33, 698 (1961). Lomrnerzheim, W., Erdd, Kohle, 7,212 (1954). Lund, H. A., Petrol. Process., 7, 326 (1952). McCleary-Degering: Znd. Engng. Chem., 30, 64 (1938). McLaren, F. H., TAPPZ. Bull., 34, 462 (1961).
70
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Mikhaylov-Lulova: Khimiya Tekhnol. Topl. Masel, 20, No. 2, 15 (1975) Mikhaylov-Polyakova-Khmelnitsky : Khimiya Tekhnol. Topl. Masel, 10, No. 8 (1965). Minchin, S. T., J . Inst. Petrol., 34, 541 (1948). Moos-Haas: Erdd, Kohle, 1, 29 (1948). Nogare-Bennett: Analyt. Chem., 30, 1157 (1958) O’Connor-Norris: Analyt. Chem., 32, 701 (1960). Ogilvie-Simmons-Hinds: Analyt. Chem., 30, 25 (1958). O’Neal-Weir: Analyt. Chem., 23, 830 (1951). Padgett, F. W., Oil Gas J., 36, No. 38, 30, 45 (1938). Pavlova-Driyatskaya-Mokhtsiyan: Khimiya Tekhnol. Topl. Masel, 7, No. 3, 58 (1962). Philips, J., Petrol. Refiner, 38, 193 (1959). Postnov-Gafarova-Serikov: Khimiya Tekhnof. Topl. Masel, 17, No. 4, 15 (1972). Postnov-Lulova-Leonteva-Fedoszova: Neftepererab. Neftekhim., No. 2. 11 (1972). Reutner, F., Fette, Seifen, Anstr-Mittel, 70, 162 (1968). Rosner-Teubel: Chem. Tech. Berl., 15, 662 (1963). Schlenk, W., Justus Liebigs Annln. Chem., 565, 204 (1949). - : Fortschr. Chem. Forsch., 2 , 92 (1951). Stewart, R., Oxidation Mechanisms. Benjamin Inc., New York, Amsterdam (1964). Stull, D. R., Ind. Engng. Chem., 39, 518 (1947). Sucker, H., Fette, Seifen, Anstr-Mittel, 70, 849 (1968). Terres-Fischer-Sasse: Brennst. Chem., 31, 193 (1950). Terres-Brinkmann-Fischer : Brennst. Chem., 40, 279 (1959). Teubel-Schneider-Schmiedel : Erd6lparafine. VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig (1965). Thorpe, T. C. G., J. Inst. Petrol., 37, No. 330, 275 (1951). Titschack, G., Fefte, Seijen, 67, 23 (1959). Triems-Heinze: Erdcil, Kohle, 18, 695 (1965). Turner-Brown-Harrison: Ind. Engng. Chem., 47, 1219 (1955). Vimos-M6zes-Keszthelyi-E. Kantor : MAhYI kiadvciny (Report of the Hungarian Oil and Gas Research Institute), No. 249 (1962). Vamos-E. Kintor: Erdd, Kohle, 17, 90 922 (1964). Willis, D. E., J . Chromat., 30, 86 (1967). Zhukhovitsky-Selenkina-Turkeltaub: Khimiya Tekhnof. Topl. Masel, 5 No. 11, 57 (1960). Zimmershied-Dinerstein-Weitkamp: Ind. Engng. Chem., 42, 1300 (1950).
(C) Crystal structure of paraffin waxes Paraffin hydrocarbons - whether individual compounds or their mixtures are always crystalline at temperatures below their melting point or melting range. 1. Crystal structure and crystallization
A crystal structure is one in which the constituent atoms or molecules oscillate about the points of a defined crystallographic arrangement, with frequencies and amplitudes depending on temperature. This arrangement, as regards shape, size and steric position, repeats itself continuously within a crystal. Such a structure is called a lattice structure. The shapes and sizes of crystal faces formed in the crystallization process of a given substance - although the crystallographic system remains unchanged -
(C) CRYSTAL. STRUCTURE OF PARAFFIN WAXES
71
differ according to whether crystallization takes place from the melt or from a solution. Well-developed crystals will only be obtained in the latter case. When crystallizing from the melt, the independently developing crystals will come into contact with one another as they grow, and will interfere with further growth in the places of contact. In the free spaces between the crystals, however, crystallization will continue until the whole of the melt becomes crystalline. As a result, the crystal boundaries - independently of the given crystallographic system - will depend on the relative position and mode of growth of the neighbouring crystals. These crystalline units, to distinguish them from crystals having regular faces, are termed crystallites. The crystals of crystalline organic compounds usually display a low order of symmetry. Only a few compounds crystallize in the cubic system, and tetragonal and hexagonal systems are also rare. The majority of organic crystals are rhombic or monoclinic. In organic molecular lattices with single bonds, the valence directions correqxmd to the tetrahedral arrangement. Hence aliphatic compounds containing no double or triple bonds are composed of carbon chains such as those occurring in the three dimensional lattice of diamond. Thus, aliphatic carbon chains may be regarded as derived from the diamond lattice. The distance between two carbon atoms in a single bond, in aliphatic chains containing no multiple bonds, is always 1.54 A, i.e. the same value as in the diamond lattice. In practice, the questions of crystallization and crystal structure are significant above all in the case of paraffin waxes solid at ambient temperatures. With liquid paraffins, crystallization is of no practical importance either in production or in application. In the course of the manufacture and application of paraffin waxes, crystallization almost always takes place from the melt. However, in the first stage of their manufacture - solvent dewaxing and de-oiling - and in fractional crystallization, parafiin max crystals are formed from solution. During the cooling of the melt, crystallization always starts with nucleus formation. The ability for crystallization of a melt is usually characterized numerically by the number of nuclei formed in the unit volume of the melt during unit time. Nucleus formation always starts below the melting point, that is, at a certain degree of supercooling. A further temperature decrease, that is, further increase in supercooling first leads td an increased capability for crystallization up to a maximum value, after which it decreases. This is due to the fact that by increasing supercooling up to a defined imit, the probability of establishing - by diffusion - the atomic or molecular arrangement required for crystallization will also grow. After, however,r eaching this limit, the viscosity of the melt will increase to such an extent that the viscous medium will hinder diffusion and thereby the establishment of such an arrangement. The slope of the ascending portion of the curve - relating to crystallization ability - changes from substance to substance. but decreases for all substances with lower cooling rates, i.e. slower cooling (Fig. 1-21).
12
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
m. P Extent
of supercooling, "C
Fig. 1-21. Effect of the cooling rate on crystallization ability. I
cooling
- rapid cooling;
2
- slow
Crystal nuclei are already tiny crystals. The atoms or molecules fixed in them by lattice forces exert forces of attraction towards the melt and are capable in this way of binding further atoms or molecules from the melt. This enables the tiny crystals to grow. The rate of growth is called rate of crystallization, defined as linear growth of the crystals, in cm/s units. The rate of crystallization is a function of crystallographic directions, temperature of crystallization and extent of supercooling. Nucleation and growth of crystals are parallel processes. The relative rate of these two processes controls the final structure of the crystalline substance. When studying the relative rates of nucleation and crystal growth as a function of supercooling, several systems differing in properties are encountered in practice (Fig. 1-22). In case a, slight supercooling (1) leads to a great number of slowly growing crystals, resulting in a product consisting of small crystals. The contrary is the case with greater supercooling (2), when few, but large crystals are obtained. In case b, slight supercooling (1) and greater supercooling (3) both yield large crystals, while medium supercooling (2) results in small crystals. In case c, slight supercooling (1) results in large crystals, greater supercooling (2) in small crystals. It is a well-known phenomenon - also occurring with paraffin waxes - that if crystal nuclei of the same species as the melt are initially present, crystallization starts in more points, and the crystal texture of the product will be finer. Consequently, by heating such substances above their melting point and subsequently cooling, crystal sizes will increase, and the texture will become coarser. However, not only nuclei of their own species, but foreign nuclei also affect the texture of substances solidifying in crystals. The nature and extent of the effect of such impurities largely varies: in some cases they act as nuclei, while in others they interfere with the growth of the crystals. The crystallites in substances crystallized from the melt are unoriented. Therefore, the anisotropy of crystals does not appear in such so-called quasi-isotropic systems.
73
(C) CRYSTAL STRUCTURE OF PARAFFIN WAXES
In large paraffin wax castings, fineness of texture within a cross-section changes along the radius. On the surface the texture is finer, owing to rapid cooling, than in the middle. Equations of cooling curves for crystallization from the melt can be derived and considered. If, in a given moment 9, the temperature of a system is t "C and ambient temperature is to "C,the cooling rate of the system is y = - -
dt k - - -(t dB mc
- to)
(1-11)
where m is the mass of the system (g), c the mean specific heat of the substance of the system in the temperature range in question (J/kg"C), and k a constant depending on the surface of the system and on the conditions of cooling.
1
(6) Crystal growth
Crystal growth
I
2 Extent of supercooling, "C 1
3 1 Extent of supercooling;
Extent of supercooling, "C
F&. 1-22. Rates of crystal formation and growth
b
O C
74
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
The functional relationship between the temperature of the system being cooled and time is represented by the so-called cooling curves. Their general equation is
t = (ti, - to)re'+
(1-12)
to
where tin is the initial temperature of the system. If the system is crystalline in the solid state (Fig. I-23b), the cooling curve displays a break at the melting point and proceeds horizontally for some time, indicating that the system stays at the temperature of the melting point as long as crystallization continues. When crystallization is complete, temperature again decreases corresponding to the above logarithmic relationship. If supercooling occurs, a local minimum appears on the cooling curve (Fig. I-23c). If the system consists of several constituents having different melting points (Fig. 1-234, the la) Iu L
3
ea,
c
a
E
I-
Time
-
-
Time
(Cl
L3
:
c
a
E
t-
Melting point
1\ Time
c
Time-
Fig. 1-23. Cooling curves of amorphous and crystalline substances. (a) amorphous substance; (b) crystallizing substance; (c) substance crystallizing from the supercooled melt;
( d ) multicomponent crystallizing substance
(C) CRYSlAL STRUCTURE OF PARAFFIN WAXES
75
crystallization process is indicated on the cooling curve by a straight portion forming an angle with the abscissa. In this case, solidification starts at the temperature tl and is completed at the temperature t,. This is the case with macro- and microcrystalline paraffin waxes. Different types of cooling curves are presented in Fig. 1-23. With many crystalline substances, a further change of the crystal structure takes place in a temperature range below the melting point. These changes - termed allotropic transitions - occur at given, so-called equilibrium transition temperatures, provided that cooling is infinitely slow. Allotropic transitions are always displayed on the cooling curve by further breaks, since they too are accompanied by changes in the heat content. It is an important characteristic of allotropic transitions that they represent reversible processes, i.e. they will be repeated during heating when the corresponding equilibrium temperature is reached, provided that heating is infinitely slow. If the cooling or heating rate is finite, the transitions take place at temperatures lower or higher, respectively, than the equilibrium temperature. Therefore, the transition temperature will differ when reached by cooling or by heating, the difference being a function of the rate of temperature change. The condition for crystallization starting from a solution is supersaturation of the solution by cooling or by removal of part of the solvent. In this case also, the process consists of two parts: nucleation and crystal growth. The number of nuclei will be high when cooling is rapid, the solution is intensely stirred during cooling and its purity is high. Crystal growth is controlled by the diffusion of the solute particles, i.e. finally, for example, by the viscosity and density of the solution at the temperature of crystallization. Industrially, crystallization from solution is implemented in two ways : by solvent removal and by cooling. The latter technique is particularly favoured if the solubility of the substance to be crystallized sharply decreases with decreasing temperature. This technology is, therefore, applied in dewaxing and de-oiling, and in fractional crystallization. Fractionation of paraffin waxes is based on the differing solubility of constituents, having either different molecular weights or being isomers. 2. Crystal structure and habit of individual alkanes and their mixtures According to Mazee, Schaerer, BayK, Smith and others, C,, to C3, normal paraffin hydrocarbons display a well-defined transition point at some centigrades below their melting point. The so-called a-phase, stable below the melting point, is converted into the b-phase, and the transition is accompanied by the release of a relatively high amount of heat. At ambient temperature, normal alkanes between C,, and C,, containing an odd number of carbon atoms have an orthorhombic structure, even-numbered normal alkanes between C,, and c,, have a triclinic structure and those between C,, and c36 a monoclinic structure. At higher tempera-
76
1. PROPERTIES OF
LIQUID PARAFFINS AND PARAFFIN WAXES
tures, the stable structures are crystal systems of higher symmetry, in particular orthorhombic and hexagonal structures. Several authors report that normal alkanes from C,, onwards do not exhibit transition points: the phase stable below the melting point is the Q-phase. In hexagonal structures the long molecules are capable of free rotation around their longitudinal axis. Such paraffin waxes are relatively soft. The hexagonal structure is shown in Fig. 1-24. In the orthorhombic crystals - shown in Fig. 1-25 - free rotation of the molecules is not possible, therefore, such paraffin waxes are more rigid.
Fig. 1-24 Hexagonal structure of paraffin wax crystals
az7.45 A b=L.97W
0
d200=3.725 A dllo=4.14 A
Fig. I-25. Orthorhombic structure of p a r a f i wax crystals
(C) CRYSTAL STRUCTURE OF PARAFFIN WAXES
7 5 1 , ,
, , , ,
Melting point,
I
,
77
;
"C
Fig. 1-26. Transition temperature versus melting point for n-alkanes and their mixtures
Figure 1-26 represents the transition temperatures of individual n-alkanes and of n-alkane mixtures, plotted against their melting points. With increasing carbon atom numbers, i.e. with increasing melting points the temperature difference between the transition point and the melting point decreases and finally disappears at C,, compound. The figure also indicates that for equal melting points, the transition point of the mixture is always lower than that of the individual hydrocarbons. In the case of certain normal alkanes, transition between the different modifications is more complex. For the n-alkane C24H50,between 50.7 and 46.5 "C the stable structure is the hexagonal form, between 46.5 and 42°C the so-called monoclinic I (monoclinic primitive) form, and below 42 "C the triclinic form. In the temperature range below 42 "C, the metastable monoclinic I1 modification can also be formed. As mentioned earlier, substantial amounts of heat are evolved at the transition of the a-phase into the P-phase. Transition points and heats of transition for the transition of some n-alkanes from their P-modification, stable at lower temperatures, into their a-modification, stable at higher temperatures, are listed in Table 1-33. For hydrocarbons containing even numbers of carbon atoms, the transition point is closer to the melting point than for hydrocarbonswith odd carbon numbers. The difference between even-numbered and odd-numbered hydrocarbons also appears in the values of heat of transition.
78
I. PROPERTIES OF LIQUID PARAWINS AND PARAFFIN WAXES
Table 1-33. Transition temperatures and heats of transition of some n-alkanes Nurfber atoms
21 22 23 24 25 26 27 28 29 30 35 36
I
Melting point,
40.4 44.4 47.4 51.1 53.3 57.0 60.0 61.6 64.0 66.0 74.6 75.9
1
Transition temperature, "C
32.5 43.0 40.5 48.1 47.0 53.8 53.0 58.0 58.2 62.0 71.8 73.8
1
Heat of transition, J/mol
15 503 28 240 21 788 31 341 26 103 34 274 28 994 35 489 31 592 37 542
-
The heat of fusion values reported in the literature for the different n-alkanes usually include the heat of transition from the a-phase into the P-phase, since determinations start from the melt, and the total amount of heat evolved during cooling to ambient temperature is measured. In Table 1-34, this heat of fusion, Table 1-34. Melting point, heat of fusion and heat of transition of some n-alkanes Number of carbon atoms
22 24 26 30 34 35
Molecular weight
310 338 366 422 478 492
Melting point, "C
44.4 51.1 57.0 66.0 72.8 74.6
Heat of fusion for the crystal modification stable a t lower temperature, Jjkg
251 065 254 752 256 260 249 598 267 867 259 193
Heat of fusion for the crystal modification stable a t higher temperature, J/kg
157 921 166 762 160 561 163 117 167 349 175 644
Heat of transition, J/kg
93 144 87 990 95 700 86 482 100 728 80 616
the heat of fusion of the crystal modification stable at higher temperature, i.e. the true value of the heat of fusion, and the heat of transition for some n-alkanes are presented in J/kg units. The latter values, when recalculated to J/mol units, differ from those listed in Table 1-33. The reason for this variance is that the data reported in the two tables originate from different authors who used different techniques of measurement.
79
(C) CRYSTAL STRUCTURE OF PARAFFIN WAXES
The specific heat of the 8-modification and of n-alkanes in the liquid state is around 2095 J/kg"C, while the corresponding value for the u-modification is about 4190 J/kg "C. Up till now, branched paraffin hydrocarbons have not been investigated in detail. Much more intricate conditions for transition are to be expected with such can theocompounds, since for example 366 319 isomers of eicosane (C&,,) retically exist. Investigations, however, demonstrated that iso-alkanes display no modification changes in the solid state. A comparison of the edge lengths of the unit cells in normal alkanes demonstrates that the longest edge regularly increases with the number of carbon atoms in the chain. Since even-numbered and odd-numbered alkanes form two series differing in physical and crystallographic properties, even-numbered compounds may only be compared with even-numbered compounds, and odd-numbered compounds with odd-numbered compounds. Alkanes form typical molecular lattices held together by weak van der Waals forces. For this reason it is difficult to prepare single crystals. The usual technique is to apply a thin layer of the compound onto a glass plate by melting. Then the net plane with the longest identity distance will be located parallel to the surface of the glass plate. For some odd-numbered alkanes, this spacing is as follows: C15H32 C17H36 C19H40 &,H,,
21.0 A 23.6 A 26.2 A 38.6 A
The figures indicate that two CH, groups increase the net plane spacing by 2.5 to 2.6 A. The other net plane spacings are much shorter and do not change with chain length. Many researchers have investigated the crystal structure of synthesized n-alkanes. Miiller and Hengstenbergfound the following values for the unit cell dimensions of the rhombic modification of normal C2,H6,: a = 7.45
A;
b = 4.97 A;
c = 77.2
A
They also stated that the unit cell is tetramolecular. Two chain molecules each are located following one another in the direction of the c axis. The C-C bond distances and bond angles in the zig-zag chain are known: 1.54 A and 129"28', respectively. The distance between the CH3 end groups of the chains is 3.1 A. For pentatriacontane (C35H-J and hexacontane (C,pH12,), a and b values were found to be essentiallythe same, while c values were higher, corresponding to what has been said above. According to a later report of Miiller and Lonsdale, the dimensions of the unit cell of n-Cl,H3, at 21 "C are a = 4.28
A;
b = 4.82 A;
c = 23.07
A
80
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
In one of his papers, Mazee reports that x-ray diffraction measurements on n-C,,H, demonstrated the existence of this hydrocarbon in three crystal modifications. He found the following values for the dimensions of the unit cell of the hexagonal modification at 46.5 "C: a = 4.77 A; c = 33.30 A. For the monoclinic modification at 42 "C the corresponding values were found to be: a = 7.50 A; b = 4.99 A; c = 32.70 A; and for the triclinic modification at ambient temperature: a = 7.42 8 ;b = 5.35 A; c = 32.50 A. Based on the results of crystallographic studies, Warth, in the 194Os, came to the conclusion that the unit cells of n-alkanes having higher melting points are bimolecular. For example, n-CS5H,, crystallizes in the orthorhombic system with the unit cell dimensions: a = 7.43 A; b = 4.97 A; c = 46.20 A. Every further - CH, group leads to an increase of c by 1.27 A. It may be seen from the cited data that different values are found in the literature for the edge length c of the unit cell, partly owing to the phenomenon of polymorphism, and partly to the simultaneous presence of several crystal modifications, so that the values found depend largely on the actual modification or mixture of modifications investigated. In addition, experimental data are significantly affected by the purity of the tested material. This is confirmed by a paper by Stanley and Ohlberg which summarizes data found for the long spacings of different crystal modifications in some even-numbered normal paraffin hydrocarbons measured immediately after solidification and after storage for 3 years at 37.5 "C (Table 1-35). Data found by Patton and Simons for the lattice constants of some odd-numbered normal hydrocarbons crystallizing in the orthorhombic system, at 28 "C, are presented in Table 1-36. A comparison with data from other authors indicates that these authors have apparently reversed the positions of the axes a and b.
Compound
Long ing (*0.0'
__~_
*)
Phases initially (relative amounts)
Angle of tilt Equilibrium phase which paraffin chain makes after 3 with basal plane years ~~
__-
81
(C) CRYSTAL. STRUCI'URE OF PARAFFIN WAXES
Table 1-36. Lattice constants of odd-numbered n-alkanes at 28 O Compounds n-C21H44
n-C,,H,* n-CzsH52 n-CP7HE.6 n-C2QH60
1
ao, A
bo,A
4.96f 0.01 4.96f0.01 4.96f 0.01 4.95k0.01 4.95k0.01
7.47f 0.01 7.4650.01 7.4550.01 7.45f0.01 7.44f0.01
1
C
co, A
57.30f 0.08 62.31f0.10 67.41f 0.08 72.59f0.08 77.70f0.18
Studying the crystal structure of cycloalkanes present in microcrystalline paraffin waxes having higher molecular weights, Miiller, Howells, Philip, Rogers and Newman found that their configuration is similar to compressed rings, that is, the molecule, in its crystalline state, consists of two linear chains bound together at the chain ends by a few carbon atoms. The unit cell of the cycloalkane (CH& is unimolecular, the compound crystallizes in the triclinic system and the lattice constants of the unit cell are a = 8.17 A; b = 5.47 A; c = 18.91 A. The corresponding lattice angles are a = 87"18'; /?= 95'10'; y = 106'04'. In cases when the linear chains are sufficiently long, it may be assumed that at the ends of the chain, the carbon atoms are closely packed, and in this way form a subcell. In the case of the above-mentioned cycloalkane, the triclinic subcell has the following lattice constants: a = 4.25 A; b = 5.47 A; c = 2.56A; the correspondingangles are cc = 60'30'; fl = 74'08'; y = 96'47'. In this case the subcell consists of two methylene groups. Since paraffin waxes are mixtures containing mainly n-alkanes, studies related to the crystallization and phase changes of binary, ternary or even more complex n-alkane mixtures are of interest. n-C,,H,Mazee studied the following binary mixtures : n-C,,H,,-n-C,,H,,; -n-C,,H,,; n-C,,H,,-n-C,,H,,; n-C,,H,,-n-C,,H,s; n-C1,H4,-n-C,,H,,. He stated that the constituents of these mixtures are miscible in all proportions, both in the liquid and in the solid state. In the phase diagram of the system n-C,,H,,-n-C&H,, (Fig. 1-27) the two-phase areas (L S,) and (S, + S,) almost shrink
+
SP n-C35 H72
Md-%
nUC36H7L
Hg. 1-27. Phase diagram of the binary system n-CmH7z-n-C96H74
6
82
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Sa
0 n -c19 HLO
Mol-%
n-CZ1Hf.L
Fig. 1-28. Phase diagram of the binary system n-ClsH40-n-C21H44
to single curves. On the other hand, the other systems with lower molecular weight constituents display phase diagrams in which the two-phase areas are more significant and the area (S, + S,) exhibits a minimum value, as may be seen, e.g. in Fig. 1-28. From the phase diagrams of these systems it appeared that a given difference in the carbon atom numbers of the constituents gives rise to a deviation from the ideal behaviour. The difference is greater the lower the molecular weight of the constituents is. With these systems, M a z e observed no melting point depression. For a quantitative evaluation, he calculated the ideal melting point curves of the systems C21H44-C23H48 and C2,H5,-C2,H5,. The calculation was based on Raoult’s law, according to which - assuming that no heat of admixture and no volume change arise - the following relationship is valid between the molar fractions of the and the temperature T where the constituents in the solid phase Xl,s and X2,s mixture totally melts: (1-13)
where XI, I and X2,I are the molar fractions, respectively, of the first and second
83
(C) CRYSTAL STRUCTURE OF PARAFFIN WAXES
constituents in the liquid phase, Tland T2the melting points, and Ql and the heats of fusion. Combining the two equations,
QB
(1-14)
This equation enables us to calculate the value of T. Analogously, the temperature T3 corresponding to the first appearance of melting can be calculated by means of the following equation : (1-15)
The calculated and experimental values are presented in Table 1-37. The data are in good agreement in all cases, the calculated values being slightly higher. L) to Presumably the area (S, + Sp) having a minimum affects the area (S, a certain extent.
+
Table 1-37.Beginning and end of the melting of n-alkane mixtures, determined experimentally and by calculation
I
1
Observed
Calculated
Composition, mol- % _ _ _ _ _ _ _ ~_ _ _ . ~~
CZIHU
__________.
c23H48
50.0 80.0
50.0 20.0
C24H50
CZt3H64
95.0 75.1 50.3 25.2
5.0 24.9 49.1 74.8
43.1 41.6
44.4 42.2
43.6 41.6
44.7 42.4
50.8 51.5 52.9 54.4
50.9 51.8 53.4 54.7
50.9 51.8 53.3 54.1
51.0 52.3 53.8 55.1
Experimental work has shown that volume changes are very small even when normal alkanes whose molecular weights differ greatly are being mixed. For example, the heat of admixture for the system C,H,,-C1,H,, is only about 125.7 J/mol. It has also been observed that the difference between the temperature corresponding to the minimum of the (S, + S,) area and the melting point pertaining to this minimum is the greater the smaller the molecular weights of the constituents (Table 1-38). This finding also confirms that deviation from the ideal behaviour is greater in systems with lower molecular weights than in higher molecular weight systems. The deviation increases with increasing differences between the carbon atom numbers of the constituents. 6*
84
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 1-38. Difference between the temperature minimum of S,) area and the (S, the corresponding melting point for binary mixtures of n-alkanes
+
Temperature differoncs, 'C
Constituents
The depression in the (S, + S,) area indicates that crystallization first appears in the a-form. It has also been confirmed that in binary systems, the structure of the 8-form largely depends on the difference between the chain lengths of the constituents. Deviation from ideal behaviour increases with increasing differences between the chain lengths. This is shown in Fig. 1-29, representing two possible variants of the phase diagram of the C,,H,,-C,,H,, system. In both variants, a m h h u m appears in the liquid area, although less pronounced than in the transition area. The figure indicates that solid solutions in the S, area deviate to a geater extent from the ideal state than in systems where the chain lengths of the constituents differ only by one or two carbon atoms. The lower boundary of the ( S , + S,)
701 L '
68
Y
L
70
-
-
66
?- 64
2 62
c
al a
$ F-
60 58
56
t-
:;I 50
0
I
,
sp
1
100
Fig. 1-29.Two possible variants of the phase diagram of the binary system n-CsoH6s-n-C8,H,P
85
(C) CRYSTAL STRUCTURE OF PARAFTIN WAXES
area is remarkably flat, and the lowest temperature within this broad range is relatively high. Presumably this finding is associated either with a series of mixed crystals, or with phase separation in the solid phase. X-ray studies did not confirm the latter assumption. However, in systems with greater chain length differences, e.g. n-C,,Hu-C,lH,,, phase separation takes place, i.e. a normal eutectic will arise. Figure 1-30 represents yet another phase diagram type for binary n-alkane systems. Here compositions exist that are monotropic, i.e. undergo only a single phase change. Commercial macrocrystalline paraffin waxes consist, at ambient temperatures, mainly of hard orthorhombic crystals, but the hexagonal structure is also always present, even if only in small amounts. In addition, liquid branched hydrocarbons also participate in building up the final structure. The amount of the hexagonal form depends on the molecular mass range of the constituents. The melting rangL and the transition range become broader with more complex paraffin wax composition. For narrower molecular mass range fractions, the transition range is narrower. When this transition is abrupt, contraction of the paraffin wax during cooling can cause cracks due to deformation. The difference between the melting point and the transition range is the greater the broader the molecular mass range of the paraffin wax. Figure 1-31 demonstrates that the transition temperatures of commercial macrocrystalline paraffin waxes with broad molecular weight ranges and complex
‘i
18 38
I
50 Mol-Olo
11 0 n-C20h2
Fig. 1-30. Phase diagram of the binary system n-Ci8Hcn-C&H,s
86
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
c
.-C 0
a
C 0
.+ ._ m C
0,
t-
Melting point, "C Fig. 1-31. Transition point versus melting point of paraffin waxes representing various molecular weight ranges. I approach curve for n-alkanes, 2 mixtures of n-alkanes, 3 narrow fractions obtained by sweating from 57-60 OC melting range paraffin wax, 4 commercial paraffin waxes having different melting points
compositions, are lower - at identical melting points - than those of individual n-alkanes, their mixtures and narrower molecular weight range products obtained by fractional crystallization. The true heat of fusion, related to the modification stable at the higher temperature, of commercial macrocrystalline paraffin waxes is 167 kJ/kg. The total heat of fusion, including heat of transition from the modification stable at the higher temperature into the modification stable at the lower temperature, is around 251 kJ/kg. An increased ratio of branched alkanes in the paraffin wax can act in two different ways : if these compounds are liquid, they soften the wax and reduce cohesion stresses. If, however, they are solid, they will plasticize the wax. With suitable types of branched alkanes, these will fill the voids between crystals and correct the cracks arising due to contraction in the transition range. According to recent studies, crystal structure transformation in the n-alkane content of commercial paraffin waxes is not complete and often takes place only to a limited extent. The various properties of commercial paraffin waxes change with the relative amounts of the polymorphous modifications. Macrocrystalline slab paraffin waxes contain C,,-C,, n-alkanes, microcrystaln-alkanes. In the solld phase no transition occurs line paraffin waxes C&,, with n-alkanes above C3,, as at this carbon atom number the transition point becomes identical with the melting point. Hence in microcrystalline paraffins the importance of polymorphism is much less than in macrocrystalline slab parafin waxes.
(C) CRYSTAL STRUCTURE OF PARAFFIN WAXES
87
Fig. 1-32. Habits of paraffin wax crystals. ( a ) plate crystals; ( b ) malcrystals; (c) needle crystals
In crystalline substances, the boundaries of individual crystals are often independent of the crystal systems in which the substance crystallizes. This is also the case with individual alkanes and their mixtures. Many hundreds of photographs confirm that paraffin wax crystals appear in three different habits, namely plates, needles and so-called mal-shapes. The latter are small-size, underdeveloped crystals, which often agglomerate. The cross-section of needle crystals is similar to the annual rings of trees, while plate crystals resemble paved roads. The different crystal shapes are shown in Fig. 1-32, The conditions for the formation of plate and needle crystals have been studied by many researchers. Based on different studies and theories, Ferris and other researchers have come to the following conclusions: ( i ) The three crystal habits of paraffin waxes are the resul4 of different factors. For a given wax, both the conditions of crystallization and the chemical composition of the wax are of decisive importance. (ii) As a first approach, it has generally been confirmed that within given molecular weight limits, the constituents having higher melting points crystallize in plates. The low-melting constituents crystallize in needles, the medium-melting constituents in mal-shapes. Also, as a first approach, one may say that this relationship is independent of the presence of solvent in the crystallization process and of solvent concentration, if solvent was a t all present. (iii) The above statement is closely related to the observation that normal alkanes crystallize in plates and the crystal lattice is simplest in these crystals. Needle crystals contain both aliphatic and cyclic hydrocarbons, while mal-shaped crystals are characterized by their content of branched hydrocarbons. With the latter, the crystal lattice is obviously more complex than with needle crystals. (iv) From the view of transformability into one another, the three crystal habits exhibit a preferential order of succession. Plate crystals will always readily be transformed into needle- and mal-shaped crystals. Within the two latter habits, needle crystals will be transformed - under appropriate conditions - into malshaped crystals. Consider a molten mixture of high-melting constituents that - in the pure state - crystallize in plates, with low melting constituents that - in the pure state - crystallize in needles or mul-shapes. Cooling of this melt
88
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
will lead first to plate crystal formation, while the still liquid low-melting constituents that yield needle- and mal-shaped crystals will be present as diluting medium. Further cooling will then result in the slow formation of needle- or mal-shaped crystals on top of the existing plate-shaped crystals, occasionally transforming the latter. (v) Cooling rate during crystallization is also an important factor in the resulting crystal habit. For both plate and needle crystals, low cooling rates will result in large crystals. The size of mal-shaped crystals, however, only slightly depends on the cooling rate. Presumably their crystal lattice is so complex that the probability of the incorporation of further molecules into the lattice, i.e. crystal growth is very slight. (vi) When crystallization takes place from a solvent, the relative solubility of individual hydrocarbons must be taken into account. The solubility of alkanes - independently of the form in which they crystallize - is inversely proportional to their melting point. In the presence of solvent, alkane mixtures begin to crystallize at relatively low temperatures. However, the constituents crystallizing in needles are more soluble than those crystallizing in plates. Therefore, needle crystals will appear only at lower temperatures and higher concentrations. Crystallization starts with the appearance of plate crystals. They are followed by malshape crystals which in this respect occupy a medium position. For any given alkane mixture and solvent, one can always find a concentration limit and a corresponding crystallization temperature limit, below and above which, respectively, crystallization in needles can be suppressed. If, however, conditions are such that concentration and temperature are above and below, respectively, these limits, needle and mal-shaped crystals are capable of transforming plate crystals. In needle crystals, a vein system or ribbing proceeding along the total length of the crystal can always be detected. The location and origin of this vein system has been investigated by several researchers. According to Ferris and co-workers it consists of a central hole system within the crystals. As regards mal-shaped crystals, Ferris and co-workers found that as a result of very rapid cooling, aggregates of tiny crystals are formed from paraffin wax melts, and these appear as mal-shaped crystals. According to another assumption, mal-shaped crystals are actually plate crystals piled on one another. For instance, n-hexacosane crystallizes in this manner from both melt and solution. Earlier its crystals were believed to be mal-shaped. Certain relationships can be detected between the crystal habits of paraffin waxes and the manufacturing process of the lubricating oil fractions from which they are won. Plate crystals are obtained from paraffinic distillates having lower average boiling points. Compounds present in such fractions and crystallizing in needle and mal-shapes have melting points so low that they behave as oils at the usual dewaxing and de-oiling temperatures, and are, therefore, almost completely absent from the de-oiled paraffin waxes. In distillates having higher boiling points and in distillation residues, needle and mal-shaped crystals appear, and their
(C) CRYSTAL STRUCTURE OF PARAFFIN WAXES
89
proportion increases with increasing boiling points. This also indicates that macrocrystalline paraffin waxes mainly crystallize in plates consisting of n-alkanes, while microcrystalline paraffin waxes crystallize in needle- and mal-shaped crystals containing branched alkanes. Table 1-39 lists the distribution of crystal habits in microcrystalline paraffins as a function of the boiling point. Table 1-39. Per cent distribution of crystal habits in microcrystalline paraffin waxes of different boiling points Average boiling point at 1.4 kPa, "C
80 95 105 115 130 140 150
Crystal habits ~
plates
~
mu/-shapes
100 99 98 91 78 54 37
0 1 2 7 12 17 27
1
needles
0 0 0 2 10 29 36
The crystal habits of alkanes are important factors in paraffin wax manufacture. Cakes from light paraffinic distillates are easy to press. This can be attributed to the fact that they contain plate crystals at the temperature of dewaxing. Such cakes are also easy to sweat, since at sweating temperatures they crystallize in large interlacing needle-shaped crystals. The paraffin wax is identical at both dewaxing and sweating, but the conditions of crystallization largely differ. At dewaxing, the amount of oil present is still relatively high, so that it keeps in solution the compounds that tend to crystallize in needles to such an extent, that a plate crystal structure will be established. In the sweating operation, however, when the wax cake is being cooled and crystallizes, the amount of oil present is much less, so that both crystal habits will crystallize simultaneously, and the needle crystals will alter the shape of the plate crystals. With increasing average boiling points of the distillates, the proportion of mal-shaped crystals substantially increases in the paraffin wax. Mal-shaped crystals crystallize simultaneously with plate crystals and alter their shape. Therefore, such distillates cannot be pressed.
Literature Asinger, F., Paraffins. Chemistry and Technology. Pergamon Press, Oxford (1968). Brooks-Dunstan: The Science of Petroleum. Vol. V, Part 111, Oxford University Press, London (1955). Brown, R. G., J . Appl. Phys., 34, 2382 (1963). Buchler-Graves: Znd.Engng. Chem., 19, 718 (1927). Clarke, E. W., Znd. Engng. Chem., 43, 2526 (1951). Coffey, S., Rodd's Chemistry of Carbon Compounds. Vol. I, Part A, Elsevier Publishing Company, Amsterdam (1964).
90
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFlN WAXES
Edwards, R . T., Petrol. Refiner, 36, 180 (1957). - : Znd. Engng. Chem., 49, 750 (1957). - : TAPPZ. Bull., 41, 267 (1958). Evans, U. R., Discuss. Faraday SOC.,N o . 5, 77 (1949). Ferris-Cowles-Henderson: Znd. Engng. Chem., 23,681 (1931). Ferris-Cowles: Znd. Engng. Chem., 37, 1054 (1945). Fontana, B. J., J . Phys. Chem., 57, 222 (1953). Gray, C. G., J. Inst. Petrol., 29, 226 (1943). - : Petroleum Lond., 7, 94, 98 (1944). Hoffman-Smyth: J. Am. Chem. SOC.,72, 171 (1950). Hoffman-Decker: J. Phys. Chem , 57, 22 (1953). Howells-Phillips-Rogers: Acta Crystallogr., 3, 210 (1950). Johnson, J. E., Znd. Engng. Chem., 46, 1046 (1954). Katz, E., J. Inst. Petrol., 17, 37 (1931). Kinsel-Phillips: Znd. Engng. Chem., 17, 152 (1945). Kirk-Othmer : Encyclopedia of Chemical Technology.Vol. 15, Wiley Interscience, New York (1968).
Magill-Pollack-Wyman: J . Polym. Sci. A3, 3781 (1965). Mazee, W. M., R e d . Trav. chim. Pays-Bas, Belg., 67, 197 (1948). Miiller, A., Proc. R. SOC. A 120,437 (1928). - : Proc. R . Soc., A 38, 514 (1932). Newman, B. A., J . Appl. Phys., 38, 4105 (1967). Niegisch-Swan: J. Appl. Phys., 31, 1906 (1960). Ohlberg, S. M., J. Phys. Chem., 63, 248 (1959). Padgett-Hefley-Hendrickson: Znd. Engng. Chem., 18,832 (1926). Padgett-Killingsworth: Pap. Trade J., 122, No. 5, 9, 37 (1946). Piper-Brown-Dyment: J. Chem. SOC.,127, 2194 (1925). Rhodes-Mason-Sutton: Znd. Engng. Chem., 19, 935 (1927). Sachanen, A. N., The Chemical Constituents of Petroleum. Reinhold Publ. Co., New York (1945).
Schaerer-Bayle-Mazee: Recl. Trav. chim. Pays-Bas, Belg., 75, 513 (1956). Smith, A. E., J. Chem. Phys., 21, 2229 (1953). Sullivan-McGill-French: Znd. Engng. Chem., 19, 1042 (1927). Templin, P. R., Znd. Engng. Chem., 48, 154 (1956). Teubel-Schneider-Schmiedel : Erd6iparafine. VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig (1965). Turner, A. J., J . Polym. Sci., 62, 53 (1962). Turner-Brown-Harrison: Znd. Engng. Chem., 47, 1219 (1955). Vand-Aitken-Campbell: Acta Crystullogr., 2, 398 (1949). Vand-Boer: Proc. K. Ned. Akad. Wet., 50.991 (1947). Warth, A. H., The Chemistry and Technology of -Waxes. Reinhold Publishing Co., New York (1956).
Wilman-, H., Proc. Phys. SOC.,64, 329 (1951). Wilson-Lipson: Proc. Phys. SOC.,53, 245 (1941).
(D) Physical properties of paraffin waxes It is a very complex task to establish relationships between the chemical composition of paraffin waxes and their applicability in various fields. In many cases these relationships cannot unambiguously be determined. It is, therefore, of great importance to know the physical properties of paraffin waxes, and to develop suitable methods for their measurement.
91
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
The literature has dealt extensively with such functional properties as tensile strength, blocking point, sealing strength, modulus of rupture etc. A knowledge of these physical properties allows us to determine with greater certainty whether the paraffin wax in question is suited for a given application. 1. Melting point, boiling point and melt viscosity Some important physical properties of normal paraffins, liquid or solid at ambient temperature, are summarized in Table 1-40. The melting point of n-alTable 1-40. Physical constants of normal alkanes
Pentane Hexane Heptane Octane Nonane Decane Undecane Dodecane Tridecane Tetradecane Pentadecane Hexadecane
- 129.7 -94.0 - 90.5 - 56.8 - 53.7 29.7 -25.6 -9.7 6.0 5.5 10.0 18.1
36.1 68.7 98.4 125.7 150.8 174.1 195.9 216.3 235.5 253.6 270.7 287.1
Heptadecane Octadecane Nonadecane Eicosane
22.0 28.0 32.0 36.4
302.6 317.4 331.6 345.1
-
-
0.6263 0.6594 0.6838 0.7026 0.7177 0.7301 0.7402 0.7487 0.7563 0.7627 0.7684 0.7733
1.3577 1.3749 1.3876 1.3974 1.4054 1.4119 1.4172 1.4216 1.4256 1.4290 1.4319 1.4345
d at m.p.
0.7767 0.7767 0.7776 0.7777
1.4360 (at 1.4367 (at 1.4336 (at 1.4346 (at
25 "C)
28 "C) 38 "C) 40 "C)
0.7782 0.7782 0.7797 0.7786 0.7785 0.7587 (at 90 "C) 0.7788 (at 60 "C) 0.7792 0.7797 0.7795 (at 70 "C)
1.4240 (at 1.4358 (at 1.4270 (at 1.4283 (at 1.4380 (at
70 "C) 45 "C) 70 "C) 70 "C) 60 "C)
b.p. at 2.1 kPa
Heneicosane Docosane Tricosane Tetracosane Pentacosane Hexacosane Heptacosane Octacosane Nonacosane Triacontane Tetracontane Pentacontane Hexacontane HeDtacontane
40.4 44.4 47.4 51.1 53.3 57.0 60.0 61.6 64.0 66.0 81.4 91.9-92.3 98.5-99.3 105-105.5
215 224-225 234 243 259 262 270 279-281 286 304
-
420-422 -
-
-
92
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
kanes increases with molecular weight. The melting point of hectane, the highest molecular weight normal alkane isolated in a pure state, is 115 "C.Over the carbon atom number range from C , to CZ5,the relationship between the melting point and the carbon atom number cannot be described with one single function for even-numbered and odd-numbered n-alkanes, i.e. the melting points are higher and lower, respectively, than the average values calculated from even- and oddnumbered n-alkane melting points. It should be stressed that branching of the carbon chain, at identical molecular weights, results in an important decrease of the melting point, since high melting points are inseparable from high symmetry of the crystals, and this condition will be satisfied above all in the case of straight-chain alkanes. The melting points of various normal and branched paraffins are presented in Table 1-41. For example it may be seen, that among C,,HS, alkanes, the melting point of n-hexacosane is 56.4 "C as compared to 0.0 "C for 11-n-butyldocosane. Figure 1-33 represents average melting point versus molecular weight for n-alkanes and various types of waxes, including petroleum waxes obtained by rectification and fractional crystallization. Several of these narrow boiling-pointrange fractions have very similar molecular weights, and yet, as demonstrated by the figure, their melting points differ substantially. The boiling points of identical molecular weight straight-chain and branched alkanes also differ, but to a much lesser degree than the melting points. In the
82 77 71
66
y
60 , 54
'g 49 C
.-
5
43
38 32 27
21 16
b-4 I
ns of soft wax from rnperature carbonof brown coal fractions of total wax from I t c of brown coal
' '
10 I I I 1 1 ~ 1 ~ ' l L l , 1 ~ 1 ~ ' ~ 1 ' 1 ~ ~ ' I 1 11 1 1, ~ 1 1 I 200 300 400 500 600 700 800900 Molecular weight
Fig. 1-33. Melting point versus molecular weight for normal alkanes and waxes from different sources
(D) PHYSICAL. PROPERTIES OF PARAFFIN WAXES
93
Table 1-41. Melting points of various normal and branched alkanes found in waxes ~~
~
Hydrocarbon
~UHXl
n-Tetracosane 2-Methyltricosane 2,2-Dimethyldocosane 5-n-But yleicosane
c26H64
n-Hexacosane 5-n-Butyldocosane 7-n-Butyldocosane 9-n-Butyldocosane 11-n-Butyldocosane
CZIlH58
Melting point by capillary tubs method O C
51.5 42.0 34.6 8.0
56.4 20.8 3.2 1.3 0.0
10-Nonylnonadecane
- 5 to - 6
GPHIO 22-Methyltritetracontane
66.6-66.7
C,,-C,, range, the boiling point of the branched alkanes is lower by only 4 to 15 "C (depending on the number, length and position of the side chains) than that of the corresponding n-alkane. No definite relationship between the viscosities of normal and branched paraffinic hydrocarbons can be established. While alkanes with one side chain often have lower viscosities than that of the normal alkane, the viscosity of alkanes with two or three side chains ishigher than that of the corresponding normal alkane. The temperature coefficient of the viscosity of normal alkanes decreases with increasing molecular weight. Among isomeric branched alkanes the temperature coefficient of viscosity is highest for the compound with the shortest side chain, while this coefficient is substantially lower for alkanes with long side chains and for normal alkanes. Paraffin liquids at ambient temperature and melts of paraffins solid at ambient temperature behave as Newtonian systems. For certain applications, the viscosity of macrocrystalline and microcrystalline paraffin waxes, above all in the 100150 "C temperature range, is of importance. Fig. 1-34 shows melt viscosities, measured at 100, 120 and 140"C, for blends prepared from a macrocrystalline paraffin wax (drop melting point 53 "C) and a microcrystalline paraffin wax (drop melting point 70 "C).As to rheological properties of macrocrystalline and microcrystalline paraffin wax melts, it should be noted that they behave as nonNewtonian systems within their melting range : they exhibit structural viscosity and rheodestruction. In such systems viscosity is a function of the shear stress applied and of load time.
94
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
20 40 60 80 I( 0
Macrocry st wax, wt-% L . A L - J
100 80 60 40 20 0
Microcryst wax, wt-Oh
Fig. 1-34. Melt viscosities of blends of a macrocrystalline and a microcrystalline paraffin wax
2. Density and thermal expansion
The density of paraffin waxes increases with their melting point. Table 1-42 lists the densities of several macrocrystalline paraffin waxes with different melting points, over a temperature range starting much below and ending much above the melting point. Figure 1-35 represents density versus melting point curves for narrow boilingrange fractions obtained by repeated vacuum distillation from a commercial Table 1-42. Densities of macrocrystalline paraffin waxes having different melting points V
0
2
-
; ~1 7 14 27 38 66 74 85
50
52
1
Melting point, "C 55
1
57
I
61
Density, g/cm'
0.906 0.903 0.897 0.872 (28 "C) 0.849 0.776 (63 "C) 0.769 0.762
0.915 (2 "C) 0.911 (10°C) 0.909 (15 "C) 0.897 0.873 0.775 0.768 (77 "C)
-
0.917 0.914 (10 "C) 0.910 (15 "C) 0.902 0.877 0.780 (60 "C) 0.774 (71 "C) 0.766 (82 "C)
0.922 0.919 0.914 0.911 0.896 (35 "C) 0.779 (63 "C) 0.772 0.765
0.922 0.919 0.915 0.905 (28 "C) 0.903 (35 "C) 0.783 (63 "C) 0.775 0.769
95
(D) PHYSICAL PROPERTIES OF PARAPFIN WAXES
Melting point, "C
Fig. 1-35 Density versus melting point for narrow boiling-range fractions obtained by vacuum distillation from a commercial paraffin wax (melting point 65 "c)
paraffin wax (melting point 65°C). At identical boiling points and molecular weights, the density of these fractions is generally lower than that of the pure n-alkanes. For comparison, Table 1-43 lists the densities of some pure normal and branched alkanes measured at 70 and 90 "C. The coefficient of cubical expansion does not change in the C21-C31range. Expansion coefficients for branched 2,2-dimethyldocosane and 10-nonylnonadecaneare lower than for n-alkanes having the same molecular weight. Table I-43. Densities of some individual alkanes Density, glcm' Compound
~
at 70°C
CzlH,, n-Heneicosane Cz3H,, n-Tricosane C,,H,, n-Tetracosane CZ8Hs8 n-Octacosane C3,HB2n-Triacontane C,lH,, n-Hentriacontane Cs4H,, n-Tetratriacontane C3,H,, n-Pentatriacontane C38H,, n-Hexatriacontane Cd8Ha n-Tritetracontane C,,H,, 2-Methyltricosane C,,H,, 2,2-Dimethyldocosane C,,H,, 13-Methylpentacosane C,,H,, 10-Nonylnonadecane C,4H,, 22-Methyltritetracontane C,,H,, 1-Cyclohexyloctadecane
0.7587 0.7654 0.7682 0.7759 0.7795 0.7827
-
-
-
0.7662 0.7642 0.7720 0.7170 0.7938 0.7997
1
at 90°C
0.7468 0.7531 0.7562 0.7639 0.7676 0.7709 0.7728 0.7734 0.7783 0.7812 0.7539 0.7536 0.7595 0.7650 0.7816 0.7874
Temp. coeff. of density in the liquid phase dd/At
0.00060 0.00060 0.00060 0.00060 0.00060 0.00059
-
0.00061 0.00053 0.00063 0.00055 0.00061 0.00062
96
I. PROPERTm OF LIQUID PARAFFINS AND PARAF'FW WAXES
As with the macrocrystalline paraffin waxes, the densities of microcrystalline waxes in the liquid and solid state differ greatly. Density data of a microcrystalline paraffin wax with a melting point of 73 "C are presented in Table 1-44. Plots of paraffin wax density versus temperature always exhibit several sharp breaks. If starting from the melt, the first break in the cooling curve appears at the liquid-solid phase transformation. As generally known, this transformation Table 1-44. Densities of a microcrystalline paraffin wax (melting point 73 "C)in the solid and liquid phase Temperature, "C
6.7 15.6 26.9 52.8 71.1 76.7 82.2 93.3 100.0 115.6 137.8 160.0 182.2 204.4 225.6
1
Density, g/cm*
0.9343 0.9294 0.9236 0.8921 0.8300 0.8025 0.7999 0.7935 0.7890 0.7805 0.761 1 0.7535 0.7410 0.7380 0.7140
is isothermal in the case of individual hydrocarbons. In paraffin waxes, however, these being mixtures of individual hydrocarbons, the liquid-solid transformation takes place over a temperature range indicated by the break. After solidification of the wax, further cooling results in solid-phase transitions between different crystal modifications. These transitions appear on the curve as subsequent breaks, their number and temperature depending on molecular weight. The breaks indicate that the transformations in question are accompanied by greater or lesser volume changes. These changes can also be measured directly. Table 1-45 summarizes volume expansion data for various macro- and microcrystalline paraffin waxes. Expansion data include volume changes occurring at liquid-solid and solid-solid transformations as well as thermal expansion of the liquid and solid phases. Expansion values were measured using a dilatometer. Figure 1-36 shows, based on the above data, the specific volumes of these paraffin waxes against temperature. The curves referring to macrocrystalline waxes resemble each other. On each curve, a sharp break at about 10°C below the melting point can be observed indicating a change in crystal structure. The crystallization points listed in the table are defined as the temperatures corresponding to
97
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
Table 1-45. Volume changes of macro- and microcrystalline paraffin waxes Characteristics
i
I
Macrocrystalline paraffin waxes
Density a t 100 OC, dAo0 0.7531 Viscosity a t 98.9 'C, mmZ/s 3.19 Needle penetration a t 25 O C , 100 g/5s, 0.1 mm; ASTM D-5 33 Melting point (cooling curve), OC; ASTM D-87 51 0.34 Oil content, wt- %; ASTM D-721 Average molecular weight 376 Crystallization point, O C 51.6 a-p phase transition point, O C 34.0 Temperature change required for total phase transformation, OC 8.6 at liquid-solid transform. at a-p phase transition 10.5 Expansion coefficient, cm3/g * OC in the liquid phase 0.0011 in the a solid phase 0.0016 in the p solid phase 0.0010 Expansion at melting,* 0.1228 cm3/g vol- x 10.6 ,_ Expansion at a-,!I phase transition,* cm*/g 0.0356 vol- % 3.2 Volume change occurring between the transition point of the a phase and the crystallization point of the liquid, cma/g 0.1716 vol- % 15.5
0.7547 3.22 28
0.7543 3.51
0.7555 3.59
19
19
55
Microcryst. wax
0.7945 12.59 36
53 0.48 358 53.2 35.5
0.22 377 55.9 38.4
56 0.36 387 56.8 40.5
62 8.52 587 74.0 -
8.5 10.3
8.8 11.4
7.7 12.0
35.0 -
0.0011 0.0014 0.0009
0.0010 0.0013 0.0008
0.0010 0.0014 0.0008
0.0010 o.oO09
0.1237 10.7
0.1264 11.0
0.1262 11.0
0.1372 12.5
0.0369 3.3
0.0387 3.5
0.0390 3.5
0.1735 15.7
0.1763 16.0
0.1770 16.1
-
0.1372 12.5
* Since paraffin waxes are mixtures of several components, these processes are non-isothermal.
the initial deviations from the curves representing the liquid phase. The actual initial temperature of crystallization, i.e. at which the first crystals appear in the melt, is some degrees centigrade higher. Its accurate measurement, however, is difficult. The volume-temperature curve for the microcrystalline wax differs from the former curves above all in that the solidification process is not indicated by a wellobservable break : the process takes place within a much broader temperature range than in the case of macrocrystalline waxes. Also, no transformation in the solid phase can be observed, although such transformations may actually take place, but will be masked by the broad solidification range. When crystal forma7
98
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
.
0 1 3000
0
E
u 1 2500
d E 1 1 2000 0
r" 1 1500 u
aJ
0
m l 1000
20
25
30
35
40
L5
50
55
60
65
70
75
00
Temperature. "C Fig. 1-36. Specific volume versus temperature for various paraffin waxes
1.3500
. 0
1.3000
rn
1.2500
= .U U
1.1 500
Qi CL
CT)
1.1000 1.0500 Temperature,
"C
Fig. 1-37. Specific volume versus temperature for some n-alkanes
tion starts, the volume of macrocrystalline waxes sharply decreases. In the case of the microcrystalline wax, however, the corresponding break occurs at some centigrades below the crystallization point, and the break, as noted before, is only slight. These differences between the two paraffin wax types are caused by the higher oil content and molecular weight range as well as by the iso- and cycloalkane content of microcrystalline paraffin waxes. It may also be seen from the data in Table 1-45 that the values of the expansion coefficients for macro- and microcrystalline waxes in the liquid and solid state are approximately identical. Total expansion of microcrystalline wax, however, is much less than that of macrocrystalline waxes. The difference found in total expansion is in good agreement with the volume change observed at the solid-phase transition in macrocrystalline paraffin wax. Hence the smaller expan-
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
99
sion of microcrystalline wax is due above all to the absence of solid-phase transition. It is obvious that for a given commercial wax, the character of the relationship between specific volume and temperature will depend on the carbon atom number and structure of the constituting individual alkanes, as well as on the nature and amount of non-paraffinic compounds present in the wax. To demonstrate this relationship in the case of individual alkanes, Fig. 1-37 is presented, in which, by way of example, specific volume versus temperature is plotted for normal C,, CZ4,CZB,C,,, C,, and C,, alkanes. These expansion curves resemble those in the foregoing figure, but certain differences should be noted. In the case of C24,C2,, C,, and C,, alkanes, thesolid-phase transition takes place at some "C below the melting point. The curve for C,, displays no solid-phase transition, while the curve for C,, exhibits two such transitions. It is clear from the foregoing that if a sufficiently wide temperature range is being considered, a difference will be observed between the total change in specific volume of macro- and microcrystalline paraffin waxes. Kinsel and Phillips proposed to utilize this phenomenon for classifying petroleum waxes into macroand microcrystalline waxes, by introducing a so-called crystallinity index. The volume contraction occurring during cooling a liquid paraffin wax from a given temperature to another temperature consists of three parts : (i) The contraction Kl occurring while the liquid cools from the given temperature to the melting point. Its value depends on molecular weight, the macroor microcrystalline nature of the wax being of no importance. (ii) The contraction K, taking place during solidification and, if any, solidphase transition. Its value does not only depend on molecular weight, but on the crystallization tendency of the wax. In the extreme case, when the substance is amorphous, the value of K, will be zero. Its value is the higher, the greater the crystallization tendency of the wax. (iii) The purely thermal contraction Ks of the solidified wax when cooled from the melting point to the given final temperature. Its value depends primarily on the molecular weight range. Total contraction Kt is equal to the algebraic sum of the above partial contractions : Kt ( ~ 0 1 - x = ) KI K, K, (1-16)
+
+
Hence the value of K,, i.e. contraction taking place during solidification and solid-phase transition can be obtained by measuring K, and K, and subtracting their sum from the measured value of K,. Kinsel and Phillips cooled 100 cm3 paraffin wax samples of various origin and structure from 93.4 "C, at a cooling rate of 5.56 "C (10 O F ) per hour, to temperatures exceeding their melting points by 2.7 "C. The measured contractions for the various waxes yielded an average Kl value of 0.072 vol-%/"C. This value could be used for all paraffin waxes with a maximum error of k0.008 vol- %/T. In contrast, the values of K, for the solid state changed with temperature. Large con'7
100
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
tractions were observed for all paraffin waxes immediately below their melting points, the values decreasing with further cooling. KJ'C values over different temperature ranges, for a 73 "C melting point macrocrystalline wax and a 71 "C melting point microcrystalline wax are presented in Table 1-46. Table 1-46. K , values of a macrocrystallineand a microcrystallineparaffin wax over different temperature ranges
I
Macrocrystallineparafin wax, melting point 73 "C Temperature range, "C
K , for 0.56 "C
Microcrystalline paraffin
wax. melting Doint 71 "C
Temperature range, "C
temperature decrease
71-66 66-60 60-54 54-49 49-27
0.530 0.333 0.144 0.078 0.040 Average value 0.155
K* for 0'56
temperature decrease
69-64 64-58 58-53 53-47 47-27
0.230 0.188 0.170 0.130 0.077 Average value 0.130
The large contraction observed immediately below the melting point is the combined effect of both thermal contraction and structural change occurring in the solid phase. Hence, it would be incorrect to subtract the average value of K, from Kt in order to determine Kc. Kc includes, of course, contraction due to structural changes, but this cannot be experimentally determined. Kinsel and Phillips assumed that for amorphous waxes the purely thermal contraction per "C is identical in the solid and liquid state. Therefore, contraction of different waxes should be compared over temperature ranges identical with respect to their melting points and not over an absolute pre-set temperature range. The chosen ranges should be wide enough to include all contraction due to changes in crystal structure. Kinsel and Phillips chose a temperature range A t , = 5.56 "C (10 OF) above the melting point and A t , = 27.80 "C (50 OF) below the melting point for their experiments. The value of Kc and the crystallinity index serving to characterize the crystalline character of the structure were then derived as follows: For the above conditions (1-17) Since, as assumed above,
-K.- -- 4 At2
(1-18)
At,
Equation (1-17) will assume the following simplified form:
Kc = Kt - K1 A t, -
0
33.36
(1-19)
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
101
Using the value K J d t , = 0.072 accepted to be valid for all paraffin waxes, it follows that K, = Kt - 0.072 * 33.36 = Kt - 2.4 (1-20) Kinsel and Phillips defined the crystallinity index as ten times the value of K,:
C.Z. = lo(& - 2.4)
(1-21)
where Kt is total contraction of the wax measured over the range from 5.56 "C above its melting point to 27.80 "C below its melting point; K, is that portion of the contraction of the solid wax which is due to thermal contraction only and independent from changes in crystal structure, and Kl is the contraction of the wax in the liquid state. Table 1-47 lists total contraction and crystallinity index values calculated from the former for microcrystalline, intermediate and macrocrystalline paraffin waxes. Table I-47.Total contraction and crystallinity index for macrocrystalline, intermediate and microcrystalline paraftin waxes Melting point, "C
Total contraction, vol- %
Crystallinity index
Macrocrystalhe parafin waxes 55 52 60 73 51 56 64
13.3 13.7 13.7 13.9 14.0 14.1 14.1
109 113 113 115 116 117 117
Intermediate parafin waxes 70 74 72 74 66
11.4 11.7 12.5 S2.5 12.7
90 93 101
Microcrystalline paraf i n waxes 71 82 71 64 83 71 83 60 86 86
8.9 9.2 9.3 9.3 9.4 9.4 9.5 9.8 10.0 10.4
65 68 69 69 70 70 71 74 76 80
101
103
102
1. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
The crystallinity index of microcrystalline waxes does not exceed 80, while macrocrystalline waxes have values above 110. However, even the use of the crystallinity index does not allow the drawing of a sharp dividing line between macro- and microcrystalline structures. Any classification based on this index must always take into account the oil content of the Table f-48.Total contraction of mixtures of paraffin waxes with various percentages of white mineral oil Oil content, wt-%
Melting point,
Total contraction, vol- %
Macrocrystalline parafin white oil waxes 0 5 10 20 30 40
53 52 51 50 47 45
13.30 12.50 11.50 10.50 9.05 1.25
Microcrystalline para f i n waxes white oil 0 5 10 20 30 40 50 0 5 10 20 30 40 50
74 73 73 12 70 68 65 86 85 85 84 82 81 79
10.05 10.34 10.28 9.16 8.18 1.49 6.79 11.40 11.40 11.40 10.52 10.24 8.76 8.08
+
+
wax. As demonstrated by the data in Table 1-48, total contraction for microcrystalline waxes with 5 to 10 wt- % oil content, these being still acceptable grades, is practically identical with total contraction for waxes containing no oil, whereas 5 wt- % oil in macrocrystalline waxes will reduce total contraction by 0.8 vol- %. This appears a significant decrease, but since oil content in commercial refined macrocrystalline paraffin waxes is normally below 1 wt- %, its effect on the crystallinity index of such waxes will be negligible.
3. Optical properties The refractive index is a physical property frequently used for identifying substances and for determining their compositions. The refractive index of paraffin waxes is usually measured at 80 to 85 "C.
103
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
The relationship between density, refractive index (both measured at the same temperature) and molecular weight is given by the Lorentz-Lorenz formula :
(1-22) where M , is molar refraction, n refractive index, M molecular weight and d density. Table 1-49 lists refractive indices in the solid state of some macrocrystalline paraffin waxes having different melting points. Table Z-49. Refractive indices of some macrocrystalline paraffin waxes in the solid state Melting point, "C
At 1 0 ° C At 20°C At 25 OC At 40 "C At 5 0 ° C
50
I
'
52
55
1
57
_
I
_ 61~
_
Refractive index
1.5306 1.5268 1.5169 1.5040
-
1.5321 1.5278 1.5256 1.5071 -
1.5348 1.5305 1.5277 1.5103
-
1.5366 1.5332 1.5311 1.5163 1.5049
1.5350 1.5328 1.5241 1.5087
The refractive index and the molar refraction calculated from the former are functions of chemical composition and molecular weight. Relationships between the molar refraction and molecular weight of alkanes and alkenes, cycloalkanes and aliphatic esters are plotted in Fig. 1-38. Within each homologous series, molar refraction is the function of molecular weight. The refractive indices of macrocrystalline paraffin waxes at 84 "C vary between 1.4210and 1.4315, their molar refraction between 100 and 154. The corresponding values for microcrystalline waxes are 1.4320 to 1.4480 and 160 to 195. In Table 1-50, refractive index data measured a t 70 and 90 "C and molar refraction are listed for a number of hydrocarbons which are usual constituents of macrocrystalline and microcrystalline paraffin waxes. The data indicate that the temperature coefficient of the refractive index slightly decreases with rising molecular weight in the C,,-C,, normal alkane range. The value of this coefficient is identical for normal and branched alkanes having the same molecular weight. According to Mazee, the molar refraction of the compounds in Table 1-50 can be calculated, with a knowledge of the molecular weight, by using the following relationship : M , = 0.33063M -t 1.6165 (1-23) Plots of the refractive index against temperature always indicate liquid-solid and solid-solid phase transformations. Therefore, the refractive index can be utilized to a certain extent for studying the crystal structure of paraffin waxes. Figure 1-39 represents such a plot for a macrocrystalline wax having a melting point of 48 "C. In the liquid phase, one refractive index value is measured for
104
1. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
9
160
0
c
150
0
5
140 ._ c V
2 130 .&? -2 120
P
'"3ZO
310
,d,
4AO I 4!?@*4b 4!iO G!O
Molecular weight Fig. 1-38. Molar refraction versus molecular weight for some hydrocarbons and aliphatic
esters. I alkanes and alkenes, 2 cycloalkanes, 3 esters
Table I-50.Refractive indices at 70 and 90 OC and molar refraction values for various hydrocarbons
I
I
Compound
CZ1H,, n-Heneicosane C,,H,, n-Tricosane CZ4H,,n-Tetracosane C,,H,, n-Octacosane C,,H,, n-Triacontane C,,H,, n-Hentriacontane C34H70 n-Tetratriacontane C,,H,, n-Pentatriacontane C,,H,, n-Hexatriacontane C,,H,, n-Tritetracontane Cs4H,, 2-Methyltricosane CZ4H,,2,2-Dimethyldocosane C,,H,, 13-Methylpentacosane CZsH,, 10-Nonylnonadecane C14HB022-Methyltritetracontane CtaH,, 1-Cyclohexyloctadecane
*n +*
-
refractive index,
f =
Refractive index at 7 0 ° C
1.4240 1.4210 1.4283 1.4324 1.4342 1.4354
-
-
1.4216 1.4269 1.4303 1.4322 1.4417 1.4416
1
at 9 0 ° C
1.4160 1.4190 1.4205 1.4248 1.4266 1.4278 1.4296 1.4301 1.4308 1.4340 1.4201 1.4192 1.4229 1.4247 1.4346 1.4339
I 1
An * At
~~-
0.00040 0.00040 0.00039 0.00038 0.00038 0.00038
-
0.00038 0.00039 0.00037 0.00038 0.00036 0.00039
99.46 108.64 113.23 131.83 141.03 146.16 159.65 164.38 168.25 201.37 113.48 113.31 122.68 131.60 206.17 111.09
temperature
M = molecular weight, d = density
every temperature. In the a- and /?-phaseof the solid state, double refraction appears owing to anisotropy. From the two curves referring to the solid state, it can clearly be determined that the transition from the a-phase into the /?-phase takes place in this case between 30.6 and 24.4 "C.The less the constituents that are contained in the wax in question, the narrower the transition temperature range. Individual alkanes have sharp transition temperatures, as has been shown in Fig. 1-37.
105
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
1.58 156
1.54 1.52
- 1.50 a
2
1.48 146
5 (L
144 1.42 140
15
25
35
45
55
65
Temperature, "C
75
a5
95
Fig. 1-39. Refractive index kersus temperature of a macrocrystalline paraffin wax (melting point 48 "C)
When measuring refractive index as a function of temperature, it is more accurate to use polarized light, since when using non-polarized light there is a possibility of double refraction. Measurements of the refractive index can be used for technological control in paraffin wax manufacture. Fig. 1-40 shows Tiedje's experimental results. His starting material was a microcrystalline wax originating from a residual oil. From this he prepared fractions I , II and III by distillation. Subsequently he separated the starting material, as well as the three distillates, by fractional crystallization into fractions having different melting points. Simultaneously he prepared, as a
Boundary of naphthenes Boundary of is0 -alkanes
Melting point, "C
Fig. 1-40. Refractive indices of fractions prepared from a microcrystalline paraffin wax by distillation and crystallization
106
1. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
first stage, a narrow melting-range fraction by crystallization of the initial microcrystalline wax. Its melting point was in the 58 to 67 "C range. From this material he prepared several further fractions by distillation (marked by IV in the figure). He then measured the refractive index of all fractions at 80 "C and plotted them against their melting points. The objective of this investigation was to find out whether the sequence of operations, when both crystallization and distillation are applied, affects the sharpness of separation. As seen in the figure, the relationships between melting points and refractive indices give a clear answer. The refractive indices of final fractions having identical melting points, that is, their chemical compositions vary largely according to which separating operation, crystallization or distillation had first been applied. Among further optical properties, the colour and colour stability of paraffin waxes are important characteristics in many applications. Among instruments used for measuring colour, are the Lovibond Tintometer, the Saybolt Chromometer and the Tag Robinson instrument. The colour of macrocrystalline waxes in the liquid phase is best determined with the Lovibond Tintometer or the Saybolt Chromometer. To measure the darker colour of microcrystalline waxes in the liquid state, the Lovibond Tintometer in which thin layers of the material are used, and the Tag Robinson instrument are suitable. The problem of measuring the colour of paraffin waxes in the solid state reappears from time to time. However, up to the present no generally accepted methods, like those for liquid-state measurement, have been developed. Most specifications require that paraffin waxes do not change their colour during storage or application. Colour stability is connected primarily with oxidative stability, since oxidation causes discoloration of the wax. No generally accepted method for evaluating colour stability has as yet been developed, owing to the fact that oxidation causing discoloration depends on many variables like temperature, oxygen concentration, intensity and spectrum of light, effect of metal catalysts, etc. The methods developed so far can be classified into two groups. In one group measurements are made in the liquid state, in the presence of metal catalysts and air. In the other group, colour stability is investigated in the solid phase, in daylight, at a specified temperature. For applications in the paper industry a glossy surface of the paraffin wax film is required. Glossy films are obtained by rapid cooling which results in the formation of small-size crystallites. These reflect a large proportion of the incident light, whereas relatively large-size crystallites obtained by slow cooling yield dull surfaces. Initially glossy surfaces of paraffin waxes become duller with time, due to recrystallization and oxidation. The extent of gloss is measured by photometry of the reflected light. However, no method yielding satisfactorily reproducible results has as yet been developed.
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
107
4. Rheological properties
The rheological behaviour of macrocrystalline and microcrystalline paraffin waxes in the temperature range below their melting points depends on the lattice structure, on the forces between the molecules forming the lattice and on the chemical composition defining the former. Thus, even in the case of identical origin, a given macrocrystalline or microcrystalline paraffin wax, at temperatures below its melting point, can behave as an elastic, viscoelastic or viscoplastic system, depending on crystallization conditions, especially on the cooling rate, moreover on the actual temperature, on the magnitude of the acting forces and on the mode of load. Since paraffin waxes are polycrystalline substances, no sharp boundaries between these deformation behaviour types depending on the above conditions can be drawn. Except for chemical uses, in almost all applications of macrocrystalline and microcrystalline paraffin waxes, definite rheological properties are required. It is noteworthy that none the less these properties had up to the present not yet been properly characterized and purposefully studied. Many wide-spread qualifying parameters, e.g. hardness, flexibility and tensile strength yield important pertinent information. In our research work in these directions we developed methods effectuating static and dynamic loads, corresponding to the crystal structure of paraffin waxes. In discussing the elastic and plastic deformation of crystalline substances, the following relationships are, to a greater or lesser extent, depending on conditions, also valid for paraffin waxes. (i) It is a general rule that the finer the crystal grains in a polycrystalline system, the higher its strength. The size of the crystal particles and the crystal structure can be modified by thermal and mechanical treatment resulting in recrystallization. The crystal structure can also be changed by adding a suitably selected additive. (ii) When a melt of a polycrystalline substance is being cooled, the system will in most cases assume an oriented structure below the melting point. In the direction of the highest temperature gradient during cooling, coarser particles will be formed than in the other directions. (iii) In the case of elastic deformation, the atoms, ions or molecules forming the crystal lattice will be shifted to a small extent against one another. The field of intermolecular forces or the field of atomic and ionic forces will be thereby disturbed, and the system will accumulate such an amount of potential energy as to keep equilibrium with external forces. When the external load ceases, internal stresses will return molecules, atoms or ions to their original places, and the initial lattice structure will instantaneously be re-established. (iv) If structural elements acquire higher energies than characteristic for their potential trough, they will pass the potential barrier and arrive into a neighbouring potential trough. If this process is successively repeated, it will manifest itself macroscopically in reaching the yield point, i.e. in plastic deformation of the
108
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
system. In test specimens made of polycrystalline substances plastic deformation in various parts of the specimen will appear at different loads, indicating that although macroscopically the deformation appears elastic, plastic deformation may already have taken place in some parts. (v) Except in the case of ideal elastic deformation, the internal stresses caused by deformation relax with time. In crystalline systems, this relaxation is accompanied by the shift of numerous atoms or molecules along definite crystallographic planes. (vi) In polycrystalline systems, in addition to simultaneous elastic and plastic deformation and to relaxation of internal stresses, the so-called elastic post-effect usually also appears. This manifests itself in hysteresis of the stress-strain curves, i.e. these curves differ when stress is repeatedly applied. The fatigue phenomenon of polycrystalline systems is related to elastic hysteresis. Fatigue means that periodically changing loads, after a certain number of cycles, result in creep or break of the test specimen at loads lower than the load corresponding to the yield point. For evaluation of the rheological properties of crystalline substances, including paraffin waxes, testing methods belonging essentially to three groups can be considered :
(i) strength tests, (ii) hardness tests, (iii) fatigue tests. Strength tests include tensile, compression, bending, torsion and impact-bending tests. Hardness is the resistance against the penetration of a body (needle, or cone, or plunger rod) under a defined load, this body being made of a harder material than the substance being tested. To measure the hardness of paraffin waxes, penetration tests are widely accepted. It is a common feature of strength and hardness tests that the test specimens are subjected to short-time stresses. In addition to the characteristics that are obtained in this manner, long-period fatigue tests yield important information. For all solids a fatigue limit exists: this is the stress below which the substance is able to resist an infinite number of fatigue cycles without breaking. For paraffin waxes the fatigue phenomenon is of greatest interest in wax-plastics compounds containing relatively high amounts of plastics. The most wide-spread strength test for paraffin waxes is tensile testing. We investigated the tensile strength of macro- and microcrystalline paraffin waxes in great detail. Our results indicated that tensile strength of both macroand microcrystalline waxes in the temperature range from -20 "C to +25 "C increases with the speed of stretching. This finding demonstrates that waxes behave in this temperature range as viscoelastic systems. A great number of tests has confirmed that tensile strength can vary greatly even within relatively narrow melting point and oil content ranges, and between macrocrystalline paraffin waxes from the same origin, but chosen from different production batches.
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
109
The limiting values measured with macrocrystalline paraffin waxes obtained from Romashkino crude, having a melting range of 52-54 "C and an oil content of 0.5-1.03 wt-%' are presented as an example in Table 1-51. Table 1-51. Maximum and minimum tensile strength values of macrocrystallineparaffin waxes from Romashkino crude (melting range 52-54 "C, oil content 0.5-1.03 wt- %) -~
Temperature,
~
-20
+ +
Tensile strength, N/cm* at 50 mrnlrnin
O C
-49.0 9.8-58.8 45.1-78.4 -39.2
0 25 30
1
-
at 500 mmlmin
196.0 14.7-1 86.2 65.7-107.8 -39.2
According to our investigations, tensile strength values as listed in Table 1-52 should be expected from macrocrystalline paraffin waxes with good tensile properties. (The values are valid for stretching rates of 500 mmlmin and for test specimens as used by us.) Table I-52. Tensile strength values for macrocrystalline paraffin waxes specified as "good tensile" (Testing speed : 500 mmlmin) "C
- 20 0 25 30
+ +
Tensile strength, N/cm'
196.0-215.6 176.4-186.2 78.4-107.8 39.2-49.0
It may be seen from Tables 1-51 and 1-52 that tensile strength increases with lower temperatures (at least within the temperature range from - 20 "Cto +30 "C). This is valid for both macrocrystalline and microcrystalfine waxes. Tensile strength also reacts sensitively to oil content: it decreases with higher oil content, particularly at low temperatures. A comparison of tensile strengths of macro- and microcrystalline waxes does not allow the establishment of an unambiguous and generally valid relationship. Apparently the effect of other factors on tensile strength, e.g. oil content and temperature, is greater than the effect of macro- or microcrystalline crystal structure. Blends of macro- and microcrystalline paraffin waxes have tensile strengths which, when plotted against composition of the blend, yield curves exhibiting
110
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
maxima. Figure 1-41 represents tensile strength versus composition of blends prepared from a macrocrystalline wax (melting point 52 "C, oil content 1.0 wt-%) and a microcrystalline wax (melting point 69 "C, oil content 13.0 wt-x), measured at different stretching rates at 0 "C. Compressive stresses and deformations are similar, apart from the sign, to those of tensile tests. In compression, the length of the circular or quadratic cross-sectional test specimen will decrease, and its specific value yields the deforniation corresponding to the given stress conditions. We developed a method for measuring compressive deformation in which we utilized test specimens with 1 cm2 circular cross-sections and 8.21 mm length. Temperatures are variable between -20 "C and +40 "C, compressive stresses between 2.45 N/cm2 and 981 N/cm2. After stress removal, also as a function of time, elastic recovery can be studied. The measured results can be evaluated based on the specific characteristics and functional relationships summarized in Table 1-53. Deformation and elastic recovery as a function of time, after removing the compressive stress, for a macrocrystalline paraffin wax (drop melting point 53 "C, oil content 0.18 wt-%) at I
I
Microcryst. wax, w t - % L
100
l
~
i
l
l
l
80 60 40 20 Macrocryst. wax, wt-%
~
l
0
l
l
Fig. 1-41. Tensile strength versus composition for blends of a macrocrystalline and a microcrystalline paraffin wax. at 0 O C (v = stretching rate, mm/min)
111
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
Table I-53. Specific characteristics and functional relationships for characterizing isothermal compressive deformation Symbols
height of the initial test specimen, mm height of the deformed test specimen, mm height of the test specimen after recovery, mm acorn,, compressive stress, N/cm* tR time of recovery, min tD time of deformation, min T temperature, OC
h0 h h,
Definitions ho- h D=deformation ratio related to the initial height of the test specimen (specific deho formation) h,- h R =recovery ratio related to the initial height of the test specimen, h0 D,= (D- R) permanent deformation
h,- h R’ = -ratio of recovery and deformation ho-h Functional relationships t D = const. aCOlllDI = f ( O ) R = f (tR) acomDr = const. a,,,,, = const. D~ = f ( t R ) %om,, = f(R’) tD = const.
t D = const. t R = const.
Table 1-54 Compression characteristics of a macrocrystalline paraffin wax at 25 “C (drop melting point 53 OC, oil content 0.18 wt-%)
I
Characteristics
Initial height of test specimen ho, mm Height of the deformed specimen h, mm Height of the test specimen after recovery h,, mm ho-h D = -h0 h,- h R =h0 D,= D - R
h,- h R’ = _ _ ho- h
-l
Compressive stress, Nlcm’
I
29.4
I
49.0
58.8
Time of deformation, min
8.21
8.02
8.04
7.98
7.93
8.33
8.11
7.85
7.89
1.73
7.16
8.03
8.17
7.91
7.97
7.82
7.85
8.13
0.012
0.021
0.020
0.031
0.021
0.036
0.007
0.007
0.010
0.011
0.011
0.012
0.005
0.014
0.010
0.020
0.010
0.024
0.60
0.35
0.53
0.36
0.53
0.33
112
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
25 "C is shown in Table 1-54. The values for elastic recovery always relate to a recovery time of 10 minutes. Numerous measurements have confirmed that the deformation of paraffin waxes resulting from a given compressive stress is, over the temperature range in question, i.e. -20 to f40 "C, a function of the loading time. After removal of the load, deformation will partly or totally disappear as a function of time. This finding also indicates that paraffin waxes that are crystalline at ambient temperature behave as vkcoelastic systems in this temperature range. Compressive deformation properties depend not only on external conditions, but also on crystal structure. Figure 1-42 represents compressive stresses leading to D = 0.02 specific deformation in a 3-min loading time against composition of blends prepared from a macrocrystalline wax (drop melting point 51 "C, oil content 1.0 wt-%) and a microcrystalline wax (drop melting point 69 "C, oil content 13 wt-%). By increasing compressive stress in crystalline substances, the yield point will be reached at a given value, and subsequent deformation will be plastic flow. However, under certain external conditions the test specimens will break before reaching the yield point. In such cases the rheological behaviour of the substance
l
I
I
f
l
l
l
l
l
l
l
100 90 80 70 60 50 LO 30 20 10 0 Macrocryst. wax, wt-"10 Fig. 1-42. Compressive stress resulting in 0.02 specific deformation versus composition for blends of a macrocrystalline and a microcrystalline wax (loading time 3 min)
(D) PWSICAL PROPERTIES OF PARAFFIN WAXES
113
will be characterized by its compressive strength, that is, by the ratio of the force resulting in break and the initial cross-sectional area of the test specimen. Since the deformation of paraffin waxes is viscoelastic or plastoelastic, their compressive strength can unequivocally be determined only when conditions for the loading time are also defined. Compressive strength was defined by us as the compression force acting on a surface of 1 cm2 that leads to the break of the test specimen within 5 seconds. According to these tests, the compressive strength of macrocrystalline paraffin waxes at ambient temperature is higher than that of microcrystalline waxes. Oil content always reduces the value of the compressive strength. In Fig. 1-43 the compressive strength of two macrocrystalline and two microcrystalline waxes differing in oil content is plotted against temperature. An unambiguous relationship also exists between crystal structure, chemical composition and compressive strength. Compressive strength values for microcrystalline paraffin waxes obtained by fractional crystallization are summarized in Table 1-55. The starting material was a refined residual oil petrolatum from Romashkino crude. Its compressive strength could not be measured at 25 and 0 "C, since it underwent plastic deformation and did not break. Information on the behaviour of paraffin waxes to dynamic stresses is obtained from impact-bending tests. We developed a method for the evaluation of the toughness of paraffin waxes based on the use oethe Charpy pendulum. Without going into details of the method, it shall be noted that impact-bending strength Lfis defined as the ratio of the energy L required to break the test specimen and
350 300
250
200 150 100
,Macrocrystalline
waxes
50
,Microcrystalline
waxes
0 -
1 -20 -10
0
+I0 +20 + 10
Temperature, "C
Fig. 1-43. Compressive strength versus temperature for macrocrystallineand microcrystalline paraffin waxes 8
114
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table Z-55.Compressive strength values at different temperatures, for microcrystalline paraffin waxes obtained by fractional crystallization of a petrolatum from a refined residual oil
I
Tested material
Compressive strength, N/cm*
Petrolatum Two-stage fractionation: Stage I: Fraction of components having melting points above 20 "C
+
323.4
588.0
910.2
Stage 11: Fraction of components having melting points between 20 and 0O C
166.6
539.0
744.8
Fraction of components having melting points above 5 O C
214.4
431.2
901.6
+
+
the cross-sectional area F of the specimen :
L,=-L [J/cm21
(1-24)
The impact-bending strength of paraffin waxes depends on the dimensions of the test specimen and on test conditions. In our method we use test specimens having 16 mm diam. circular cross-sections and 110 mm lengths. The results obtained indicate that impact-bending strength of paraffin waxes increases with temperature and reaches a maximum value at a given temperature. From this temperature on, plastic flow appears and the test specimen does not break in the impact-bending test. Figure 1-44 shows impact-bending strength against temperature for macrocrystalline paraffin waxes, each having drop melting points around 50 "C, but differing in oil content. The test was carried out using a striker with an energy maximum of 3.92 J. This must be mentioned, since, particularly in dynamic tests, the viscoelastic or plastoelastic nature of paraffin waxes must be taken into account. It follows from this fact that the value of impact-bending strength will, under otherwise equal conditions, depend on the kinetic energy of the striker in the moment when it strikes the specimen, i.e. on the speed of deformation at that moment. Increasing speeds result in increasingly elastic behaviour : the wax becomes tougher, and finally, with increase from a given speed of deformation, it will behave as a brittle system. Figure 1-45 represents impact-bending strength against composition of blends prepared from macro- and microcrystalline paraffin waxes and measured with 0.98 and 3.92 J strikers at 0 and 25 "C. The figure demonstrates how sensitively impact-bending strength reacts to changes of the crystal structure. It should be noted that in the test at 25 "C using the 0.98 J striker, the blends containing 50 to 80 wt- % macrocrystalline paraffin wax did not break, but were only bent. However,
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
115
0.40
0.35 Oil content, wt-%
6 .
0.30
T - 1.0 - 4.2 - 6.2 - 9.1 H - 11.4
a b c
7
L-
5 0.25 c
:
c
Ln
0
C ._
0.2c
U
C a,
J?
c U
0.1E
0
Q
E
-
0.1c
0.0:
c
-20
-10
0 10 20 Temperature,
"c
30
40
50
Fig. 1-44. Impact-bending strength versus temperature for macrocrystalline paraffin waxes differing in oil content
the blends containing less than 50 wt-% macrocrystalline wax, and the pure macrocrystalline wax itself exhibited well-defined breaks. On theother hand, the blends containing 50 to 80 wt-% macrocrystalline wax, when using the 3.92 J striker, also had defined impact-blending strength values. Hence, it may be assumed that the crystallite systems formed over the 50 to 80 wt- % macrocrystalline wax range show plastic flow at deformation speeds attainable with the 0.98 J striker, while a t the higher speeds realized with the 3.92 J striker, they behave as tough systems, and show some elasticity. Among methods and characteristics to be considered for the evaluation of strength properties of paraffin waxes, there is the Fraass breaking point used for the specification control of bituminous materials. We found that this method can also be applied for paraffin waxes : the breaking point satisfactorily characterizes low-temperature flexibility of waxes, i.e. the temperature limit where brittleness follows. Table 1-56 contains the Fraass breaking point of two-stage fractional crystallization products obtained from a refined residual oil petrolatum of Romashkino origin. In the first stage of fractionation, hard paraffin wax was separated at 30 and 10°C. In the second crystallization, soft microcrystalline 8*
116
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES 1.01
0.1 I 100
I
I
I
I
I
I
I
I
80 60 40 20 Macrocryst. wax, wt-%
I
20 40 60 80 Microcryst. wax, w t - %
0
I
100
Fig. 1-45. Impact-bending strength versus composition for blends of a macrocrystalline and
a microcrystalline paraffin wax
Table 1-56. Fraass breaking point for paraffin waxes obtained by two-stage fractional crystallization of petrolatum from refined residual oil Fractiolu
Stage Z
Fraction obtained at Fraction obtained at
+ 30 + 10 OC
O C
Stage I1 Fractions from the filtrate of stage I at 30 OC Fraction obtained at -20 OC Fraction obtained at - 10 OC Fraction obtained at 0 OC Fractions from the fdtrate of stage I at 10 O C Fraction obtained at -20 O C Fraction obtained at 0 "C
I
Bredti% point,
++2525 -30 -30 -26 -30
-30
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
117
wax fractions were separated at 0, - 10 and -20 "C. It may be seen from the data in the table that the second crystallization of the filtrate obtained at + 30 "C yielded, at 0 and - 10 "C,products plastic and flexible at low temperatures, while the products obtained in the first fractionation stage were already brittle at ambient temperature. Penetration measurement is the most wide-spread method for determining the hardness of paraffin waxes. Penetration measurements carried out at different temperatures are suitable to characterize the thermal sensitivity of paraffin waxes. Our investigations showed that in the temperature range below the solidification point the penetration of macrocrystalline waxes changes to a greater extent with temperature than that of microcrystalline waxes. An increase in oil content results in increased penetration values for both macro- and microcrystalline waxes. Besides these factors the penetration also depends to a great extent on the modscation of the crystalline structure. Figure 1-46 shows the penetration values against composition of blends prepared from macro- and microcrystalline waxes, measured at 0 "C and 25 "C. The tests were carried out with the needle specified in the ASTM standard. The macrocrystalline paraffin wax used had a drop melting point of 53 "C and an oil content of 0.6 wt-%. The corresponding values of the microcrystalline wax were 70 "C and 10.0 wt-%. 100 1
I
' 2b
I
!
I
I
40 60 '
8'0 'Id0
Macrocryst. wax, wt-"lo I I I I I I I I I ' A 100 80 60 40 20 0 Microcryst. wax, wt-"b
Fig. 1-46. Needle penetration versus composition for blends of a macrocrystalline and a
microcrystalline paraffin wax
118
I. PROPERTIES OF LIQUID PARAFFINS A N D PARAFFIN WAXES
5. Thermal properties A knowledge of specific heat is almost always necessary in engineering calculations. A value of 2.1 kJ/kg "C is being generally accepted for the specific heat of C2?-C40alkanes, both in the solid and liquid state. This value is in fact acceptable in many cases for industrial practice. It must, however, be remembered that it is only an approximate value. The specific heat of alkanes depends, in addition to temperature, on molecular weight, and in the solid state on crystal structure. According to Cragoe the specific heat of oils with high paraffin content can be calculated by using the following formula :
4.2 c = -(0.388-O.OOO45t)
d
(1-25)
where c is specific heat, kJ/kg * "C, d density at 15.6115.6 "C, and t the temperature in OF. This empirical formula is also suitable for calculating the specific heat of alkanes liquid at ambient temperature. The specific heat against temperature curves for some n-alkanes in the C,o-C,5 range are shown in Fig. 1-47. The figure demonstrates that the 2.1 kJ/kg * "C value is acceptable as an approach only in the C25
Fig. 1-47. Specific heat versus temperature for solid-state n-alkanes in the CI,-C,, range
119
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
to C,, range, and even in this range only at temperatures between 30°C and 50°C. This, as well as the effect of crystal structure, are shown in Table 1-57. The temperature ranges indicated in the table include the temperature of the structural change taking place in the solid phase. It can also be stated from these data that specific heat changes over a very wide range with temperature. The specific heat of the crystalline modification stable above the a + /? transition temperature is substantially higher in all cases than that of the modification stable below the transition point, and also than that of the liquid phase. For alkanes which have no solid-phase transition, e.g. n-C,H,, and n-CUHs8, specific heat monotonouslyincreases up to themelting point. The specific heat of the liquid phase Tuble 1-57. Specific heat values of some n-alkanes over different temperature ranges n-Alkanes
C,,H,, n-Heneicosane
40.30-40.40
32.8
15-21 21-32 34-39 45-55
1.89 1.97 5.70 2.39
Cg4H,, n-Tetracosane
50.70-50.80
47.0
25-35 35-45 45-50 52-61
1.80 2.01 4.06 2.43
C30D61n-Triacontane
65.80-65.95
59.2
25-35 35-45 45-57 60-65 67-77
1.97 2.18 2.51 4.11 2.56
C36H74n-Hexatriacontane
75.80-75.85
73.5
25-35 35-45 45-55 55-65
65-72 74-75 77-86
1.80 2.01 2.35 3.10 3.77 5.12 2.56
C,,H,, n-Tetracontane
81.35-81.45
-
30-40 40-50 50-60 60-70 70-80 82-90
1.72 1.89 2.23 3.35 4.11 2.51
C,,H,, n-Tritetracontane
85.25-85.35
-
43-50 50-60 60-70 70-76 76-83 86-91
2.09 2.35 2.60 3.10 3.75 2.64
120
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
is smaller in these cases too than the specific heat of the solid phase close to the melting point. The average heat of fusion of n-alkanes occurring in paraffin waxes is 167.6 kJ/kg. For hydrocarbons that have two crystal modifications in the solid phase, the thermal effect of a + p phase transition is around 83.8 kJ/kg. In macrocrystalline paraffin waxes both phase transition and melting take place over a broad temperature range, and these ranges may overlap, due to the numerous hydrocarbon constituents. Hence a separate determination of heat of fusion and heat of phase transition would meet with difficulties. The total thermal effect of the transformation from the liquid phase to the solid phase stable at the lower temperature is, according to the above-mentioned data, 230.5 to 251.4 kJ/kg. Heat of fusion and heat of transition values for some n-alkanes occurring in macrocrystalline paraffin waxes have been discussed in the foregoing chapter and are listed in Tables 1-33 and 1-34. The true heat of fusion varies between 159.2 and 167.6 kJ/kg. Heat of transformation, for even-numbered alkanes, is around 92.2 kJ/kg, i.e. a substantial fraction of total heat of fusion. In Table 1-58, heat of fusion values for the crystal modification stable at ambient temperature of macrocrystalline paraffin waxes having different melting points are listed. It should be noted from the data that heat of fusion values of macrocrystalline waxes are substantially lower than those of individual n-alkanes. The figures in the table Table 1-58. Heat of fusion values for macrocrystalline paraffin waxes Melting Point,
"C
51.7 52.2 52.4 65.3
Heat of fusion for the crystal modification stable at ambient temperature, kJ/kg
168.86 163.00 147.07 183.52
do not exceed 184 kJ/kg, whereas the values for the total heat of fusion, relative to the crystal structure stable at ambient temperature for n-alkanes having closely similar melting points, are around 247 kJ/kg. Some authors explain this difference by assuming that in parafin waxes that are composed of hydrocarbon mixtures, the n-alkanes retain the crystal structure stable at the higher temperature, and no solid-phase transition occurs. Another explanation would involve the presence of signscant amounts of soft paraffin wax components in the tested macrocrystalline paraffin waxes, these components reduce the heat of fusion. Similar results were obtained by Further, Parks and Todd with three highly refined macrocrystalline waxes that are shown in Table 1-59.
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
121
Table 1-59. Heat of fusion values for various average molecular mass macrocrystalline paraffin waxes Average number of Melting carbon point, "C atoms
20 25 33
36.4 53.3 71.1
Heat o f fusion for the crystal modification stable at the lower temperature, Mfkg
217.88 224.58 226.26
For microcrystalline paraffin waxes, owing to the total or partial absence of solid-phase transition, the values of heat of fusion vary between 146 and 168 kJ/kg. According to Trouton's rule, the molar entropy of evaporation at lo5Pa pressure approaches to a constant value for all substances: Mrs s, = = const = 21
Ts
(1-26)
where M is the molecular weight, r, the heat of evaporation in kcal/kg, and T, the boiling point in K. For C,+, alkanes, the value of the constant varies between 20 and 22. Kistiakowsky replaced the Trouton constant by the following function:
Mrs - - 8.82 + 4.575 lg T, Ts
(1-27)
for hydrocarbon mixtures. Heat of evaporation depends on external pressure according to the ClausiusClapeyron equation : dP r, = AT(V" - v') (1-28)
dr
where rs is the heat of evaporation in kcal/kg, T the boiling point in K, v" the specific volume of the vapour in m3/kg, uf the specific volume of the liquid in m3/kg,-dP the slope of the vapour pressure curve in kgf/m2 * K, and A is 11427
dT
kcal/m * kgf. To calculate the heat of evaporation for hydrocarbons at pressures differing from 1 bar (lo5 Pa), Schumacher introduced the following equation: MrS - -
T
8.82 + 4.575 lg- T V,
(1-29)
122
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
where 0, is the so-called apparent molar volume of the liquid. Its value is 1.2 for n-alkanes, 1.1 for iso-alkanes and 0.9 for naphthenes and their derivatives. These us values are, however, valid only at 1 bar (lo5 Pa) pressure. For other pressures Mr, T -= 8.82 + 4.575 lg(1-30) T V,(P>” where the value of s depends on evaporation temperature. For n-alkanes the following function is valid : s =
0.685 lg T - 0.8
(1-31)
105
84
0
. E
x
7
63 C“
.c
0
2
0 Q
9 a,
-
Lc
0
0
42
aJ
I
21
0 Temperature, “C Fig. 1-48. Molar heat of evaporation versus temperature for n-alkanes liquid and solid at ambient temperature
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
123
The evaporation process of alkane mixtures proceeds, of course, at successively changing temperatures at constant pressure, and at successively changing pressures at constant temperature. For this reason engineering calculations apply average boiling points. After determining the average boiling point, calculations are carried out with the individual hydrocarbon having the same boiling point as the average boiling point of the mixture. Figure 1-48 represents molar heat of evaporation of a number of n-alkanes liquid or solid at ambient temperature against temperature at different pressures. A further thermal property of paraffin waxes to be mentioned is thermal conductivity. This can be calculated for macrocrystalline paraffin waxes having melting points in the 50 to 54 "C range, over the temperature range from - 180 "C to +30 "C, using the following empirical relationship:
1 = 0.005 . (1
- 0.0016t)
(1-32)
-
where I is thermal conductivity in Jim "C * s, and f is temperature in "C. The above relationship is acceptable as an approach for microcrystalline waxes and for paraffLns liquid at ambient temperature.
6. Solubility Petroleum waxes are obtained, following the usual manufacturing processes, by crystallization, in the presence of various solvents, from pafaffinic distillates and refined paraffinic lubricant oil fractions. In some applications paraffin waxes are used in solution, and in many cases they come into contact with solvents during their service life. Thus solubility is a very important physical characteristic of paraffin waxes. The solubility values of macrocrystalline paraffin waxes, having 50-55 "C melting ranges, in solvents significant from the view of both manufacture and application are listed in Table 1-60. The data should be considered as average values, since, at identical melting points, the chemical composition of macrocrystalline waxes from different origins can vary, and hence their solubilities in a given solvent under given conditions will also vary. Figure 1-49 represents solubility against temperature data for a refined p a r a f i wax (melting point 51 "C) in various solvents. Figure 1-50 shows solubility versus melting point data for different paraffin waxes, measured at 20 "C, in various solvents. As a general rule it can be stated that solubility increases in all solvents with decreasing melting point. Solubility values of paraffin waxes in different boiling-range petroleum distillates can be determined by using the nomogram presented in Fig. 1-5 l . The lefthand scale of the nomogram represents the average boiling point of the solvent, the right-hand scale the difference between the melting point of the paraffin wax and the temperature of dissolution.
L
Table 1-60. Solubility of macrocrystallineparaffin waxes in various solvents (g wax/100 cms solvent)
I
Solvent
Propane n-Pentane n-Hexane n-Heptane n-Octane n-Decane Naphtha (boiling range 96-146 "C) Kerosene (boiling range
1 1 1 1 -15
0.18
0.27
178-271 "C)
Methanol Ethanol n-Propanol Isopropanol n-Butanol Iso-butanol Acetone Methyl ethyl ketone Methyl propyl ketone Methyl butyl ketone Methyl isobutyl ketone Dichloroethane Benzene Toluene 30 % acetone- 35 % benzene- 35 % toluene
-10
0.011 0.024 0.045 0.034
-5
0.30
0
0.95
2.77 1.37 0.99
3.69 2.18 1.69 0.94
0.93
1.70
0.28
0.50
0.89
0.13
0.045 0.095 0.180 0.14 0.29
5
0.55
0.52
0.023 0.048 0.089 0.067
I
1
1
Temperature, "C 10
5.11 4.81 3.55 2.90 1.44
15
~
6.94 6.07 5.06 4.24 2.74
20
1
9.53 8.31 7.18 5.93 4.98
25
1
30
40
1
45 ~
6C
0.047 0.21 0.18 0.54 0.543 0.135 0.650
0.008 0.08 0.44 0.33 0.94 0.97 0.258 1.31
0.024 0.17 0.71 0.61 1.86 2.14 0.52 2.53
0.032 0.145 0.11 0.31 0.315 0.095 0.306
0.62
0.8 1.4
0.29 1.90 3.05
0.40 4.4 6.6
0.91 10.5 14.5
2.24 24.5 32.0
5.10 56.0 69.0
0.07
-0.15
0.34
0.8
1.8
4.0
9.0
0.366 0.666 0.488
1
8
1.60
0.037
P
17.16 16.23 14.36 11.66 9.17
0.024 0.096 0.063 0.18 0.185 0.063 0.202 0.775
0.018 0.087 0.188 0.333 0.253 0.078
35
h,
0.063 0.318 1.35 1.23 3.60 4.03 1.07 7.48
0.103 0.55 2.65 2.36 7.05 9.60 2.30
0.16 0.97 4.95 4.63 16.7 30.0
E
52 135
8
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
0.11 I I ' I ! I 1 -18 -16 -14 -12 -10 -8-6 -4 -2
125
' ' '
' " I I I I 0 +2 +4 +6 t 8 +I0 +12 +14+16 +I8
Temperature, "C
Fig. I-49. Solubility versus temperature for a refined macrocrystalline paraffin wax (melting point 51 "C). I cyclohexane, 2 methylcyclohexane, 3 95-150 "C boiling-range petroleum distillate, 4 160-200 "C boiling-range petroleum distillate, 5 180-270 "C boiling-range petroleum distillate, 6 toluene, 7 propane, 8 mixture of 65 wt- % benzene and 35 wt- % acetone
Melting point,
O C
Fig. I-50. Solubility, of paraffin waxes versus melting point in various solvents at 20°C. I nitrobenzene, 2 ethylene dichloride, 3 paraffinic distillate, 4 naphtha
126
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
24 Fig. I-51. Solubility of paraffin waxes in petroleum distillates having different average boiling points. Instruction f o r the use of the nomogram: identical sides of the solubility scale and the right-hand scale must be used together. Example: average boiling point 200 OC, temperature difference 19 'C. Then solubility is 47 g/100 cm3
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
127
Several authors developed equations for calculating the solubility of paraffin waxes. Berne and Allen found the following relationship for hydrocarbon solvents, particularly for non-aromatic distillation fractions : C = (1120 - 2.97Y) 1.357"-")
(1-33)
where C is the solubility of the paraffin wax in g/100 cm'; Y , in "C, is the average of the temperatures at which yields of distillation of the solvent carried out according to ASTM standard are 10, 30, 50, 70 and 90%, respectively; m is the melting point of the wax in "C, and t is the temperature of the solution in "C. The equation is strictly valid only for solvents boiling between 60 and 300 "C, for waxes with melting points between 45 and 70 "C, and for solution temperatures between 0 and (m - 10) "C. Pool and his co-workers found that the solubility of paraffin waxes can be described by the following empirical equation : lg W = A (lg T - K)
(1-34)
where W is the solubility of the wax in g/100 g; T the temperature of the solution in K; A is a constant depending on the properties of the wax, and K is a constant depending on the properties of the solvent. The dependence of the solubility of solids on temperature can also be described by utilizing the Clausius-Clapeyron equation. Regarding the molar fraction of the solute, the following relationship can be derived : lgxi = - -.H F 4.575
HF T 4- 4.575Tf 1
-
(1-35)
where xi is the molar fraction of the solute; HF is the molar heat of fusion of the solute, in cal/mol; T is the saturation temperature of the solution in K, and Tf is the melting point of the solid, in K. The solution power of solvents is particularly important in solvent dewaxing processes. The requirement to the solvent is that it dissolve the wax only to a very slight extent or not at all at the operating temperature, while dissolving the oil readily. Solution power is usually varied by utilizing mixed solvents. We studied the solubilities of a macrocrystalline wax having a melting point of 53 "C and of the fractions obtained from the former by fractional crystallization. The solvents used were mixtures of acetone, benzene and toluene. The acetone content varied between 15 and 45 vol-%. Figure 1-52 shows the dependence of solubility on temperature for the original macrocrystalline wax in the pure individual solvents. The wax concentration is plotted on a logarithmic scale against the reciprocal of the absolute temperature. This mode of presentai ion corresponds to the Clausius-Clapeyron equation, with the difference that the unit of concentration is g/100 cn1' instead of molar fraction. Figure 1-53 represents the solubility of the same wax in mixed solvents containing 15, 30 and 45 vol-% acetone, respectively, the rest of the solvent being a 1 : 1 mixture of benzene and
128
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES c
$ 30
m 0,
10 8 6 4
0 0
z 2 a"
5
g
.-
5 c
1
0.8 0.6
0.4
C
a, V
c
0
0.2
V
;j 0.1 3 C .+ ..+.
2P
0.03
3.3 3.4 3.5 3.6 3.7 3.8
Reciprocal of cloud point, (l/K).103
3
Fig. 2-52. Solubility of a commercial macrocrystalline paraffin wax in pure solvents. 2 acetone, 2 benzene, 3 toluene
I
I
I
I
I
I
3.3 3 4 3.5 3.6 37 3.8 : 3
Reciprocal of cloud point, (l/K),103
Fig. 2-53.Solubility of the same wax as in Fig. 1-52 in mixed solvents containing 1 5 , 30 and 45 vol-x, resp., of acetone and made up to 100% with 1 : 1 benzene/toluene
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
' i aJ
-> $
129
10 8
6
4
0,
.2
0
0
X
0
3 1 m 0.8 c- 0.6
--0,
0.4
$ C
02
P
c
U 0
g 3
01
&
+ 0
b 003 I I I I I I a 3.3 3.4 3.5 3.6 3.7 3.8 : 3 ReciDrocal of cloud point, (l/K).103
Fig. 1-54 Solubility of the same wax as in Fig. 1-52 and its fractions in a mixed solvent (30 vol- % acetone, 35 vol- % benzene, 35 vol- % toluene). I hard wax, 2 initial wax, 3 intermedier wax, 4 soft wax
oluene. Figure 1-54 represents the solubilities of the initial wax and of the fractions prepared from it, in a mixed solvent consisting of 30 vol-% acetone, 35 vol-% benzene and 35 vol- % toluene. The melting points of the fractions were 56 "C for the hard wax, 50 "C for the intermedier wax and 46 "C for the soft wax. Many of the commercial paraffin waxes contain natural or synthetic waxes or other types of additives. It is, therefore, an important characteristic of their miscibility. In the molten state, petroleum waxes are normally well miscible with vegetable waxes, e.g. carnauba and candelilla wax. At melt temperatures only slightly above the melting point of the paraffin wax, however, solubility of vegetable waxes having higher melting points is rather poor. When such mixtures are stored in the temperature range between the melting points of the pure components, crystallization of the higher-melting component can occur. In the solid state of these systems two phases are present, the higher-melting vegetable wax crystals being uniformly distributed in the paraffin wax. This is presumably the reason for the significant rise in hardness of paraffin waxes even at small concentrations of vegetable wax. It has been observed that high-melting vegetable waxes readily soluble in lowermelting paraffin waxes cause smaller changes in properties than poorly soluble vegetable waxes. This is in agreement with the well-known fact that mixing of a low-melting and a high-melting paraffin wax, readily soluble in one another, 9
130
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
causes only slight changes in properties, except for large concentrations of the higher-melting paraffin wax. The majority of natural resins are well miscible with paraffin waxes. However, the stability of such mixtures is not always satisfactory, because the resins sometimes separate already in the melt or become oxidized. Substantial property changes will take place again only if the solubility of the resin in the paraffin wax is poor. Miscibility of paraffin waxes with synthetic resins is a more complex question. Products containing substantial amounts of oxygen in the macromolecule are poorly soluble or insoluble, whereas polymers consisting essentially of hydrocarbon chains are, owing to their similarity of structure, well miscible with paraffin waxes.
7. Adhesive properties Two adhesive properties of paraffin waxes are sealing strength and blocking point. Laminated packaging materials consisting of two or more layers of usually different materials, e.g. paper, plastic films, metal foil, are frequently manufactured with paraffin wax as sealing agent. Both macro- and microcrystalline paraffin waxes, and products based on such waxes and containing various additives, are used for such purposes. Sealing strength is an important characteristic of laminates. It is numerically expressed by the force per unit length required to separate two bonded layers (paper-paper, paper-metal foil, etc.). The sealing strength of laminated packaging materials depends on the properties of the paraffin wax as well as on the amount of wax and on the laminating process. When laminating paper to paper, sealing strength will depend almost exclusively on the cohesion of the paraffin wax, since the latter readily penetrates into the pores of the paper and will be firmly anchored there. In paper to metal foil laminates, the strength of the bond will be chiefly determined by the adhesion of the wax to the metal, since the wax cannot penetrate into the metal layer. In metal to metal laminates, the bond strength will depend solely on the adhesion of the wax. In laboratory tests for measuring sealing strength, in addition to the above variables, testing conditions will affect results. The most significant testing conditions are angle of separation, speed of pulling the layers apart and the elapsed time between lamination and testing. In addition to suitable cohesive properties, flexibility is an important requirement with which laminating waxes must comply. As well as bonding, the wax film must also act as moisture barrier. To satisfy these various and partly contradictory requirements, laminating paraffin waxes contain various additives, e.g. polyisobutylene, ethylene-vinyl acetate copolymers, vegetable waxes, etc.
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
Testing temperature, "C
Pulling speed, cmlmin
Paraffin wax load on paper, mN/mz
268.8 206.0 206.0 206.0 188.4 157.0 160.9
131
Sealing strength, mN/cm
14.7 13.7 12.8 11.8 10.6 10.1 10.1
Sealing strength of microcrystalline paraffin waxes usually greatly surpasses that of macrocrystalline refined waxes. Macrocrystalline waxes in the 360-420 molecular weight range and 5 M O "C melting range, and containing less than 1 wt-% of oil, are considered by many as not suitable for laminating, due to the low sealing strength. Among microcrystalline paraffin waxes, the harder types having higher melting points exhibit lower sealing strength values than the softer, plastic types with lower melting points. Table 1-61 lists the sealing strength of a macrocrystalline paraffin wax (melting point 62 "C)at different thicknesses of the paraffin wax film, i.e. mass of applied wax per unit surface area. The same results are also presented in Fig. 1-55. In the example referred to, it may be observed that above values of 157 mN/m2 sealing strength increases with the thickness of the wax film. However, this increase is presumably due to the fact that, upwards from a defined film thickness, the readings of the testing instrument will include the stiffness of the wax film-paper system in the sealing strength value.
E 2 Z E
15
5 cn
6
L
m
10
4 -
cn
-._0 C
0
rn
5 Wax load, mN/m2
Fig. I-55. Sealing strength versus paraffin wax load on paper
9'
132
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 2-62. Effect of waxing temperature on the sealing strength of macrocrystalline paraffin wax Testing temperature, "C
Waxing temperature,, "C
25 24 26
71 88 99
Paraffin wax load on paper, mN/ma
Sealing strength, mN/cm
Sealing strength of this paraffin wax, at film thicknesses around 157 mN/m2, changes little with temperature of waxing (Table 1-62). The effect of waxing temperature depends on the properties of the wax. Additives in the paraffin wax may result in increased dependence of the sealing strength on waxing temperature. Lamination temperature significantly affects sealing strength even when using relatively homogeneous waxes. Listed in Table 1-63 are sealing strength values, at different laminating temperatures, for the macrocrystalline wax having a melting point of 62 "C. Table 1-63. Effect of laminating temperature on the sealing strength of macrocrystalline paraffin wax
Laminating temperature, O C
Sealing strength, mN/cm
93 120 150
10.1 10.5 13.7
In continuous film formation, sealing strength values increase with rising laminating temperatures, and decrease, at identical laminating temperatures, with rising final cooling temperatures. This effect is more significant with waxes containing additives. Table 1-64 lists sealing strengths at different laminating and Table I-64. Sealing strength of paraffin wax containing polyethylene wax at different laminating and final cooling temperatures Polyethylene wax, wt- %
1.1 1.1 1.1 1.1
Laminating temperature, "C
97 107 97 107
Final cooling temperature, OC
Sealing strength, mN/cm
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
133
cooling temperatures for a macrocrystalline paraffin wax (melting point 60 "C) containing 1.1 wt- % polyethylene wax. Adhesion of microcrystalline paraffn waxes is substantially higher than that of macrocrystalline paraffin waxes. For this reason laminating waxes always contain microcrystalline paraffin waxes. Table 1-65 contains adhesion values for Table 1-65. Adhesion of micro- and macrocrystalline para& waxes Sample
A
Drop melting point, "C (ASTM D 127)
Needle penetration at 25 "C, 0.1 mm (ASTM D 5)
Adhesion at
25 Oc9 mN'cma
J K
73 69 75 12 76 14 67 73 65 63 89
24 24 27 21 30 22 24 25 38 41
I
486.6 449.3 440.5 410.1 410.1 410.1 358.1 358.1 281.5 182.5 68.7
L
57
15
0.0
B
C D
E F G H
Z
various microcrystalline paraffin waxes (samples A-K), and one macrocrystalline wax (sample L), having various drop melting points and needle penetration values. Adhesion is here defined as the force required to separate unit surface area bonded layers. The values refer to the separation of a cellophane film coated on both sides with wax and laminated on both sides with cellophane, i.e. to the separation of two laminations. The data in the table exhibit great differences between the adhesion of microcrystalline waxes having closely similar drop melting points or needle penetration values (e.g. samples A and H, samples A and F). Using this test method, the macrocrystalline refined wax L - owing to its largely differing crystal structure - yields an adhesion value of zero. Adhesion of the high drop melting point, hard microcrystalline wax K is also small. Another important adhesive property of paraffin waxes is the so-called blocking point, characterizing the sticking of waxed paper and self-sticking of waxes. The blocking point is defined, according to ASTM standard, as the lowest temperature at which waxed papers will stick together sufficiently to injure the surface films and performance properties. Since blocking may occur over a temperature range, a 50% blocking point is measured. For paraffin waxes having identical melting points, increased oil content substantially reduces the blocking point. Additives having lower melting points than the paraffin wax, if present in larger amounts, will also lead to low blocking points.
134
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
With macrocrystalline paraffin waxes, a lower transition temperature also lowers the blocking point, since the crystal modification stable at the higher temperature is always softer than the modification stable at the lower temperature. In Table 1-66, blocking points for a number of low oil content macro- and microcrystalline paraffin waxes are shown, together with their melting points. The data indicate that the blocking temperature is usually lower by 10 to 20 "C than the melting point. Table 1-66. Melting and blocking points of paraffin waxes Difference be-
and blocking point, "C ~
A B C D E F G H I J K L M N 0
P R S T U V
51 50 51 52 54 55 55 56 55 56 58 59 59 58 59 61 62 64 67 71 84
35.6 32.2 32.2 33.9 33.9 37.2 36.7 36.1 39.1 36.7 39.7 43.0 41.9 42.4 43.0 44.1 45.2 52.9 54.0 53.4 63.9
15.4 17.8 18.8 18.1 20.1 17.8 18 3 19.9 15.9 19.3 18.3 16.0 17.1 15.6 16.0 16.9 16.8 11.1 13.0 17.6 20.1
Paraffin waxes are crystallized from a variety of solvents. The separation temperature of crystalline waxes is termed de-oiling temperature in the petroleum refining industry. From one and the same starting material, waxes differing in properties are obtained with different yields if the de-oiling operation is carried out a t different temperatures. Table 1-67 contains the blocking points of waxes obtained from a semirefined wax by crystallization from a mixture of methyl ethyl ketone and toluene, at de-oiling temperatures changing from -7 "C to 27 "C.Raising the de-oiling temperature results in paraffin waxes having higher blocking points. However, the temperature difference between the melting point and the blocking point decreases. At identical melting points, the blocking point of paraffin waxes largely depends on molecular weight distribution. A broader distribution results in lower blocking
+
135
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
Table 1-67. Relationship between blocking point and temperature of de-oiling
1
~ f ~ ~ - ~ Paraffin ~ & wax f "C
yield, wt-%
-7 -1 4 16 21 27
1
Melting point,
"c
1
Blocking Point, "c
58
84 77 71 60 46 12
Difference between m.p. and blocking I Doint. "c ~
38.0 40.2 42.4 45.8 49.6 51.2
60
61 62 64 66
20.0 19.8 18.6 16.2 14.4 14.8
points. Table 1-68 presents blocking points for n-alkane mixtures. The data reveal that at identical melting points of the mixtures, the blocking point increases with narrower molecular weight distribution. At the same time, the difference between the melting point and the blocking point is lowered. Both changes improve the usefulness of the paraffin waxes. Table 2-68. Blocking points of n-alkane mixtures differing in molecular weight distribution
Components
1
I
I C22H46-Ca4H70
C24HS0-cS2H66 C25H52-C20H6CI
C2,H,,--C2sH,s C27H56
Carbon atom number range
13 9 5
3
1
58.0 58.5 58.0 58.0 59.0
I
I
Difference
I
1
point, "C
41.9 45.2 47.4 51.2 54.0
~-
16.1 13.3 10.6 6.8 5 .O
Lower-melting components substantially reduce the blocking point. In order to decide whether the blocking-point-reducing effect of iso-alkanes and naphthenes is due to their lower melting point or to their chemical structure, the blocking points of a commercial refined wax, its blend with an iso-alkane-naphthene mixture (melting point 45 "C, 28 wt- % iso-alkane, 66 wt- % monocycloalkane, 6 wt- % dicycloalkane), and its blend with an n-alkane mixture (melting point also 45 "C) were measured. The data are presented in Table 1-69, demonstrating that the effects of the added components were essentially the same in both cases, both with regard to blocking point and to the difference between blocking point and melting point. This points to the conclusion that the blocking point-reducing effect of hydrocarbons, other than n-alkanes, is mainly the result of their lower melting point. However, presumably this conclusion is not valid for paraffin wax products containing hydrocarbons other than n-alkanes in large amounts.
136
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 1-69. Effect of iso-alkanes and naphthenes on the blocking point of paraffin waxes
! '
Blocking point, "C
60.0
44.0
16.0
90 % base paraffin wax 10 % isoalkane-naphthene mixture melting at 45 O c
58.5
42.4
15.0
90% base paraffin wax 10% nalkane mixture melting at 45 "C
58.0
41.9
16.1
Melting point, "C
Materials
Difference between m.p. and blocking point, "C
~ _ _ _ _ _ _ _ _ _
-
Base paraffin wax (70 % n-alkanes, 24 % iso-alkanes, 6 % monocycloalkanes)
+
+
8. Water vapour permeability
In the manufacture of packaging materials, paraffin waxes are chiefly used to reduce water vapour permeability of paper. This is valid for both coating and laminating waxes, the latter being often applied for the sake of the low vapour permeability of the paraffin wax film. Water vapour permeability tests are usually carried out not with the wax itself, but with coated paper, carton etc., that is, with the prepared packaging material, since paraffin wax is used directly for coating only in exceptionalcases (e.g. fruits cheese). The water vapour permeability of waxed packaging materials is affected by a number of variables, including the properties of the paraffin wax, conditions of wax application, properties of the paper and mechanical stresses. No realistic picture can be obtained concerning permeability, if paper specimens treated under similar coating conditions, but using different macro- and microcrystalline paraffin waxes are compared. Instead, coating should be carried out under specific conditions for each paraffin wax, these being optimum conditions for the wax in question, and only such coated papers should be submitted to water vapour permeability testing. Permeability to water vapour of coated or laminated paper specimens, prepared under optimum conditions and measured prior to being subjected to mechanical stress, is essentially the same whether macro- or microcrystalline paraffin wax has been used, since, for unbroken films, these two wax types are equally resistant to water vapour. However, water vapour permeability measured prior to mechanical stress (folding) is insufficient to evaluate the quality of packaging materials. Papers coated or laminated with paraffin wax will, almost without exception, be subjected to various mechanical loads, especially to flexing, during their service life.
137
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
As regards flexing, macrocrystalline and microcrystalline paraffin waxes behave quite differently. Refined paraffin waxes having macrocrystalline structures are brittle, easily cracking products. Under the effect of folding, the film coat will crack, the continuity of the film will be disrupted to a greater or smaller extent. Water vapour will readily permeate through the porous paper lying below the cracks. Hence, water vapour permeability will substantially increase under the effect of folding in a coating made with macrocrystalline paraffin wax. Among the great variety of microcrystalline paraffin waxes, coatings made of flexible products exhibit cracks to only a slight extent or not at all under the effect of folding, the continuity of the film remains largely undisrupted, and hence no significant change in vapour permeability will result. Plastic and flexible microcrystalline paraffin waxes would appear to be the most favourable for the manufacture of coatings and laminates with a low water vapour permeability. However, the tack of such soft microcrystalline waxes is too high, Table 1-70. Water vapour permeability values of paper coated with paraffin wax (sulphite paper, wax coating 25 wt- %) Coating material
Melting OC
j
-
Water vapour permeability* g/m* 24 h
(
folding
After folding
-
No coating
-
Refined macrocrystalline wax
54
0.20 0.45
12.4 16.2
Refined macrocrystalline wax
60
0.26 0.51
18.6 12.3
Plastic microcrystalline paraffin wax
67
0.27 0.39
3.9 4.8
* At
406.0
23 "C and 50% rei. humidity
their blocking point is low, their surface is dull and attracts much dust. For these reasons, they cannot be used in the pure state for coating, but are very suitable for blending to obtain low water vapour permeability coatings. Water vapour permeability values of paper coated with macro- and niicrocrystalline paraffin waxes, before and after folding, are presented in Table 1-70, 9. Water resistance
Water and aqueous solutions come into direct contact with waxed paperboard used for containers of deep-frozen food, milk cartons, paper cups, etc. Therefore, resistance to water is an important requirement in those applications where no water must be allowed to penetrate through the paraffin wax film.
I38
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Similarly to water vapour permeability tests, resistance to water is usually measured not with the pure wax, but with wax coated paperboard. Resistance to water of the composite material depends to a much greater extent on the properties of the paper substrate than in the case of water vapour permeability. When coating paper with a mottled surface (e.g. blotting paper), some of the wax will be absorbed between the fibres, but some fibres will protrude through the wax film to the surface. These fibres will then act as wicks in which moisture can easily penetrate through the wax film. Two types of water barrier tests are in use. In absorption tests, weighted waxcoated paper cups are filled with a 1 wt-% lactic acid solution coloured with methylene blue. The filled cups are then stored for 72 hours a t -4 "C or +20 "C, according to standard specification. Subsequently the cups are emptied and weighed. The amount of solution absorbed is determined from the weight increase. Simultaneously, the internal surfaces of the cups are visually checked for staining with methylene blue (location and magnitude of stained areas is observed). Resistance to water is considered satisfactory only if slight staining occurs, and that only at junctions and folds. The other test consists of a fatigue test. Waxed cartons, filled with water and sealed, are dropped at a rate of 144 per minute from a height of 9.6 mm, at 3 "C, by means of a suitable apparatus. The endurance of the waxed cartons is measured by the period before which 50% of the tested cartons (usually 24 specimens) exhibit cracks or break. In general, tests show that resistance to water of lower-melting microcrystalline paraffin waxes is superior to that of macrocrystalline refined waxes. This difference is presumably due to the denser crystal structure and higher flexibility of microcrystalline waxes. Blends of refined macrocrystalline and microcrystalline waxes are frequently used for waterproof coatings. 10. Electrical properties Owing to their relatively high dielectric strength, low water vapour permeability and good resistance to water, paraffin waxes, especially the higher-melting microcrystalline waxes, are used for insulating purposes in electrical engineering. Insulating coatings of paraffin wax provide efficient protection against both water vapour and water. The use of paper and textiles impregnated with paraffin wax for wire insulation has been known for a long time past. Paper impregnated with paraffin wax is used in capacitors as a dielectric; paper tubes coated with paraffin wax are placed inside the metal casing of dry cells to reduce desiccation of the cathode mixture. Pure paraffin wax or its blend with bitumen is frequently used in capacitors to keep components in place. Ceramic insulators are also frequently impregnated with paraffin wax to reduce water vapour permeability. The volume resistivity of paraffin waxes is in the order of 1016-5 . I d * ohm/cm3. The values normally decrease with rising temperature. However, in the solid-phase
139
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
transition range of macrocrystalline paraffin waxes, this change is frequently inverse. Data found in the literature on the dielectric strength of paraffin waxes vary within very wide limits. The inordinately large differences are due to variations in chemical composition and purity, as well as to the different methods of measurement used by the various authors. Table 1-71 lists relative permittivity data for paraffin waxes and, for comparison, for some natural waxes. Table 1-72 contains relative permittivities and, by way of example, some dielectric strength values measured with two microcrystalline paraffin waxes differing in properties. Table 1-71.Relative permittivities of paraffin waxes and some natural waxes Substance
Paraffin waxes Ceresins Candelilla wax Carnauba wax Beeswax
Initial value
After 6 months storage in 3.5 Wt-% NaCl solution
2.19-2.24 2.1 6-2.24 2.38-2.49 2.66-2.8 3 2.87-2.88
2.31-2.55 2.29-2.32 2.50-2.62 3.84-4.19 3.11-3.26
'
' 1
after repeated
2.24-2.30 2.28-2.29 2.45-2.56 2.82-2.83 2.84-2.90
Table 1-72. Dielectric properties of two microcrystalline paraffin waxes Properties
1
i
WaxA
i
88
Melting point, OC Needle penetration at 25'C, 0.1 mm Average molecular weight Oil content, wt- %
WaxB
2 890 0.6
84 7 804 10.4
I ~
Temperature, "C 80 60 40 20
Temperature, O C 20 40 60
2.46 2.37 2.31 2.16
2.30 2.39 2.25 2.09
2.38 2.42 2.56 2.62
2.47 2.44 2.44 2.45
2.49 2.53 2.74 2.79
Dielectric strength, kV/cm 82 1500 71 780 22 625
!
I
2.48 2.50 2.74 2.79
140
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Literature ASTM D 1465-57 T. ASTM D 1500-58 T. ASTM D 156-53 T. ASTM D 1321-61 T . ASTM D 937-58 T. ASTM D 1320-60 T. Berne-Allen: Ind. Engng. Chem., 30,806 (1938). Brooks, K. W., Oil Gas J., 58,89 (1960). Buchler-Graves: Ind. Engng. Chem., 19, 718 (1927). Davis, D. S., Ind. Engng. Chem., 32, 1293 (1940). Edwards, R. T., Petrol. Refiner, 36, 180 (1957). Eucken, A., Elektrochemie, 45, 126 (1939). Ferris-Cowles: Znd. Engng. Chem., 37, 1054 (1945). Gray, C. B., J. Inst. Petrol., 29, 236 (1943). Gruse-Stevens: Chemical Technology of Petroleum. McGraw-Hill Co., New York (1960). Hickel, A, E., Petrol. Refiner, 24, 207 (1945). Johnson, J. F., Ind. Engng. Chem., 46, 1046 (1954). Kinsel-Phillips: Znd. Engng. Chem., 17, 152 (1945). Kolvoort, E. C. H., J . Znst. Petrol., 24, 338 (1938). Lord, H. D., J. Inst. Petrol., 25, 263 (1939). Mazee, W. M., Red. Trav. chim. Pays-Bas, Belg., 67, 197 (1948). - : J. Znst. Petrol., 35, 97 (1949). M6zes-V8mos: Reoldgia 6s reometria. (Rheology and rheometry), Miiszaki Konyvkiad6, Budapest (1968). M6zes-Zsida-FBnyinB: MA-FKZ K6zI. (Report of the Hungarian Oil and Gas Research Institute), 9, 241 (1968). - : Chem. Tech. Berl., 20, 481 (1968). Miiller, A,, Proc. R . Soc., A 120, 437 (1928); A 127,417 (1930); A 138, 514 (1932). Padgett, F. W., Oil Gas J., 36, N o . 38, 30, 45 (1938). Padgett-Hefley-Hendrikson: Ind. Engng. Chem., 18, 832 (1926). Phillips, J., TAPPZ. Bull., 41, 291 (1958). - : Petrol. Refiner, 38, 193 (1959). Scott-Harley: J . Znst. Petrol., 25, 238 (1939). Seyer-Fordyce: J. Am. chem. SOC.,58,2029 (1936). Seyer-Morris: J. Am. chem. SOC.,61, 1114 (1939). Seyer-Patterson: J . Am. chem. Soc., 66, 179 (1944). Smith, A. E., J. chem. Phys., 21 2229 (1953). Templin, P. R., Znd. Engng. Chem., 48 154 (1956). Thorpe, T. C. G., J . Inst. Petrol, 37 275 (1951). Tiedje, J. L., Proceed. Fourth World Petroleum Congr., Section W/B, Preprint 1 (1955). Turner-Brown-Harrison: Ind. Engng. Chem., 47, 1219 (1955). Ubbelohde, A. R., Trans. Faraday Soc., 34, 282, 292 (1938). Van Vinkle, M., Petrol. Refiner, 27, 291 (1948). Warth, A. H., The Chemistry and Technology of Waxes. Reinhold Publishing Co., New York (1956). Watson, K. M.. Znd. Engng. Chem., 23, 360 (1931). West, C. D., J. Am. chem. Soc., 59 742 (1937). Wibaut-Langedijk: Red. Trav. chim. Pays-Bas, Belg., 59, 1220 (1940).
(A) Liquid paraffins and paraffin waxes from petroleum 1. Composition of petroleum distillation products /
Crude oils and their products contain a large number of individual paraffins. The main physical characteristics of the most frequently occurring alkanes are listed in Table 1-1, indicating that n-pentane is already liquid and n-hexadecane solid at ambient temperature. Table I-I. Physical characteristics of some alkanes occurring in petroleum
Methane Ethane Propane
CH, CZH, CaH,
16 30 44
Butane n-butane iso-butane
C.810
58
Pentane n-pentane 2-methylbutane (iso-pentane) 2,2-dimethylpropane (neopentane)
C,Hn
72
Hexane n-hexane 2-methylpentane
GHi,
86
- 161.5 - 88.5 -42.0
0.424' 0.546' 0.582'
-82 32 96
4.2
- 138.5 - 159.5
- 0.5 - 12.0
0.602' 0.596'
153 134
3.7 3.8
- 129.5
36.0
0.625
197
3.3
- 159.5
28.0
0.620
188
3.4
- 16.5
9.5
0.6139
184
3.5
-94.0 - 153.5
69.0 60.0 63.0
0.659 0.656 0.664
235 228 227
-
-98.0
49.5
0.649
212
-
- 129.0
58.0
0.662
221
3.1
-99.5 between
98.5 19-93
0.684
267
2.7 2.12.9
-118.0
3-methy lpentane
2,2-dimethylbutane 2,3-dimethylbutane Heptane n-heptane iso-heptanes
-182.5 -183.0 -187.0
C7H16
100
and
-25
:.%)
%)
4.6
5.0
2.9 3.2
14
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
I I
Table I-1 (cont.)
I
Compound
I
Formula
~
Octane n-octane 2,2,3-trimethylpentane 2,2,4-trimethylpentane (isooctane) other iso-octanes
$: 114
I I
Melting point, "C (101 kPa)
Boiling point, "C (101 kPa)
-57.0
125.5
0.703
296
2.5
110.5
0.716
285
-
99.5
0.692
268
-
- 107.5 between
Density
at 2ooc
crit. t e m ~ , . Crit. oc press,, MPa
and
Nonane n-nonane iso-nonanes
128
Decane n-decane iso-decanes
142
n-Hendecane n-Dodecane n-Tetradecane n-Hexadecane n-Octadecane n-Eicosane n-Pentacosane n-Triacontane n-Pentatriacontane n-Pentacontane
+ 1046 -53.3
-
150.5
0.718
323
-29.5
173.5 147-1 168
)-\:.0.730
347
-
156 170 198 226 254 282 352 422 492 702
-25.5 -9.5 5.5 18.0 28.0 36.5 53.5 66.0 74.5 92.0
196 216 254 287 308 2054 25g4 3044 3314 4214
0.740 0.749 0.763 0.174 0.782 0.789 0.801 0.810 0.7813 0.7943
-
369 391 429 462 49 1 513
2.3
-
2.1
-
593
2.0 1.9 1.7 1.5 1.4 1.3 1.o
708
0.7
-
-
-
At the boiling point *At0°C
Liquid density at the melting point ' A t 2.1 kPa 104 'C: Hexamethylethane a
+
Even the lower boiling-point fractions of petroleum contain, depending on the source of the crude, in addition to alkanes, varying amounts of other hydrocarbons, namely cycloalkanes and aromatic compounds. Table 1-2 presents the alkane and cycloalkane content of gasoline products over the 40-120 "C distillation range, obtained from different crudes. With increasing average molecular weight, the composition of petroleum fractions is more and more complex. The alkane, cycloalkane and aromatics content of different gasoline and naphtha fractions obtained from three different crudes
15
(A) LIQUID PARAFFINS AND PARAFTIN WAXES FROM PETROLEUM
Table 1-2. Alkane and cycloalkane content in gasolines from different sources Sources
n-Alkanes
Ponca field, Oklahoma Greendale-Kawkalinfield, Michigan Conroe field, Texas Loviszi field, Hungary Budafa field, Hungary
1
Vol- % iso-Alkanes
1
Cyclopentanes
I
Cyclohexanea
35.7
20.5
23.4
20.4
63.1 18.2 26.0 30.8
13.2 20.3 17.2 18.9
8.O 17.3 27.0 29.5
15.7 44.2 29.8 20.8
is shown in Table 1-3. It can be noticed that in the case of the fractions from the Yates field the alkane content decreases, while the cycloalkane content substantially increases with the boiling range. In the case of the other two crudes no such unequivocal change could be observed with regard to alkanes and cycloalkanes, while their aromatics content was the highest in the 95-1 15 "Cfraction. Table 1-4 lists the hydrocarbon composition of the kerosine and light gas oil fractions of Table Z-3. Alkanes, cycloalkanes and aromatics content of petroleum fractions from different sources Boiling point range, O C (101 kPa)
_ _ ~ ___
I
Slaughter field
AI-
kanes
~
Cycloalkanes
Aromatics
1
I
VOl- %
1
Wasson field
Alkanes
Cycloalkanes
Aromatics
I
1
Yates field * Al-
kanes
Cycloalkanes
Table 1-4. Hydrocarbon composition of kerosine and gas oil fractions of petroleum from the Ponca field Hydrocarbon
Boiling point range_ (101 kPa) _ _ 180-23OoC
n-Alkanes Iso-afkanes Monocycloalkanes Bicycloalkanes Tricycloalkanes Monocyclic aromatics* Bicyclic aromatics
23 16 32 11 0 15 3
I
230-300°C
22 8 29 17 4 12 8
Monocyclic aromatics include alkylbenzencs and aromatic cycloalkanc-type hydrocarbons
1
Aromatics
16
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
crude from the Ponca field. It can be observed that the content in bicycloalkanes and bicyclic aromatics increases with the boiling point. The fractions and the distillation residues suitable for the manufacture of lubricating oils have a still more complex composition. In these distillation fractions the carbon atom number of the components varies between 25 and 40. In the residual oil compounds with 50 to 60, and in some cases up to 80 carbon atoms are found. The chemical composition of the lubricating oils obtained by refining these materials differs from the composition of the starting distillates and residual oils actually as a result of the refining operations. This theme, however, is outside the scope of this book. The different compositions, depending on their average molecular weight (viscosity) and extent of refining (viscosity index) are shown in Table 1-5, presenting the carbon atom distribution determined by the so-called Table Z-5. Carbon atom distribution among aromatic, cycloalkane and alkane compounds in refined lubricating oils Viscosity at 98.9 "C
Oil type
_____
High viscosity index aircraft oil High viscosity index motor oil Medium viscosity index motor oil Low viscosity index oil Medical-grade oil
Viscosity index
Carbon atom distribution, % aromatic
1 cycloalkane 1
alkane
101 49
63 62
48
* S.S.U.: flow time in seconds, measured with a Saybolt Universal viscorneter
n-d-M method, that is, the distribution of the total number of carbon atoms contained in the compounds constituting the lubricating oil between the individual groups of hydrocarbons. The highly complex composition of high boiling-point fractions is represented by the data in Table 1-6 referring to a lubricating oil fraction composed of C25-C35 Table 1-6. Composition of a CzsC,, lubricating oil fraction
-
_.____
Compounds
n-Alkanes Iso-alkanes Monocycloalkanes Bicycloalkanes Tri- and polycycloalkanes Monocyclic aromatics with cycloalkane rings Bicyclic aromatics with cycloalkane rings Tricyclic aromatics with cycloalkane rings Polycyclic aromatics with low hydrogen content and non-hydrocarbon compounds
Vol- %
13.7 8.3 18.4 9.9
16.5 10.5 8.1
6.6 8.0
(A) LIQUID PARAFFINS AND PARAFFIN WAXES FROM PFTROLEUM
17
compounds, obtained by fractional distillation from the Ponca field crude. A comparison with the data of Table 1-4 unequivocally confirms that the higher-boiling fractions contain much more cycloalkanes and aromatics than the lower-boiling fractions. From this short summary of the composition of crude petroleums it may be seen that paraffin waxes produced mainly from higher-boiling distillates and residual oils contain normal hydrocarbons as well as large amounts of iso-alkanes. Also, significant amounts of one, or more ring hydrocarbons with straight side chains can be found. 2. Nomenclature of liquid parafis and paraffin waxes All classifications regarding a range of products are more or less arbitrary, or valid only with certain restrictions. It is, however, a basic postulate, when establishing some nomenclature system, that in addition to an endeavour at simplicity, both the technological and application aspects of the products in question should assert themselves. The manufacture of liquid paraffins and paraffin waxes will be discussed in Chapter 11, their application in Chapter 111. In conformity with these chapters we established a nomenclature system, which, in our opinion, satisfies the above basic requirements. This nomenclature will be applied in the course of this book. Widely varying terms are used in the literature, in the technological practice of the petroleum industry and in commerce for different grades of liquid paraffin and paraffin waxes. The terms slab paraffin wax, slack wax, scale wax, and pipeline or tank wax were established in earlier petroleum industrial practice. The term slab wax was used exclusively for paraffin waxes obtained by cooling, pressing and sweating from low-viscosity distillates. Only pressing and sweating were feasible for the separation of the oily part and the solidified paraffin wax, since centrifuging could not be applied. The term slack wax, or slacks, was used for the intermediate product of cooling and pressing without sweating or refining, and the product produced by sweating was called scale wax. On the other hand, petrolatums from residual oils and pipeline or tank waxes cannot be pressed, but only centrifuged in solvent media. This was an important aspect at the time when dewaxing by means of solvents was not yet known. The fraction distilling over between those that could be dewaxed by pressing and sweating and those that could be dewaxed by centrifuging was called the intermediate fraction. This intermediate, that is, paraffinic medium and heavy distillate, could be dewaxed only under great dif€iculties and with very poor yields either by pressing or by centrifuging. The paraffin waxes obtained from the intermediate fraction were called slop wax. The intermediate fraction was often used as fuel without recovering its paraffin wax content. At present, when solvent dewaxing processes have completely conquered the field, these aspects, and the terms connected with them, will obviously lose their importance. 2
18
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Classification of paraffin waxes : Paraffin waxes with macrocrystalline structure can be classified e.g. with respect to their melting point or to the extent of refining. On the basis of the melting p o b t one can distinguish between soft paraffin waxes with melting points below 45 "C, and hard paraffin waxes with melting points between 45 and 60°C and needle penetration values below 20 mm/lO at 25 "C. Depending on the degree of refining, one can classify paraffin waxes as technical, semi-refined and refined grade waxes. Technical grade paraffin waxes usually contain less than 6 wt-% oil; these are products obtained by dewaxing from slacks. Semi-refined paraffin waxes may contain a maximum of 3 wt- % oil, and their colour is light yellow to white. Finally, refined paraffin waxes contain 0.4 to 0.8 wt- % oil, they are completely colourless, odourless and do not contain substances detrimental to health. Our nomenclature system is based on the classification of paraffin waxes into macrocrystalline and microcrystalline groups. The crystal structure of macrocrystalline (slab) paraffin waxes can be observed visually, while that of microcrystalline paraffin waxes only with a microscope. The term amorphous is thus sometimes found in the literature for paraffin waxes obtained from residual oil. As it is known all paraffin waxes obtained from petroleum are crystalline below their setting point. The size of the crystals, however, decreases with the increasing boiling point of the paraffin wax. Microcrystalline para& waxes have higher molecular weights, densities and refractive indices than macrocrystalline paraffin waxes. From the view of both processing and application, it is an important property of microcrystalline para& waxes that they are capable of retaining more oil than macrocrystalline waxes. The structural difference is also confirmed by the observation that blending macrocrystalline slab wax with only a few tenths of a per cent of microcrystalline paraffin wax changes the ease of pressing and sweating the former. After these preliminary remarks, our classification system is shown in Fig. 1-1. The raw materials for liquid paraffins are the distillates obtained by the distillation of petroleum crudes. The raw materials for paraffin waxes are the light, intermediate and heavy hydrocarbon oil distillates obtained by the vacuum distillation of the latter, the residual oils of vacuum distillation, and pipeline and tank waxes. The semiproducts obtained in the first stage from light, intermediate and heavy distillates, from residual oils and from pipeline and tank waxes cannot yet be regarded as paraffin waxes. They are termed slacks and petrolatums, respectively. The difference between paraffin waxes and slacks and petrolatums is in their oil content, and hence in their chemical composition. The differences in chemical composition are obviously affected by the conditions of de-oiling. Macrocrystalline paraffin wax is produced from the slacks obtained from paraffin light oil distillates. Microcrystalline paraffin waxes, both of the brittle and the ductile type, are obtained from petrolatum. Ductile microcrystalline paraffin waxes include two sub-groups, namely plastic and elastic paraffin waxes. Another term used for the low oil-content macrocrystalline paraffin waxes is slab paraffin waxes. The term ceresin is reserved exclusively for brittle microcrystalline paraffin waxes.
Paraffin light distillates (atmospheric
distillation)
Paraffin medium distillates (vacuum distillation)
n
&
c 2 U
0
0 I Slack wax
3 5
* 3
Macrocrystalline paraffins
2 Brittle microcrystalline paraffin waxes
Ductile microcrystalline paraffin waxes
micro crystalline paraffin waxes
Fig. I-I. Sources and classification of liquid paraffins and paraffin waxes from petroleum
Plastic microcrystalline paraffin waxes
4
&L
20
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
The products obtained from slacks and petrolatums, named according to our nomenclature, are products not subjected to further purification. Whatever method is used for further purification, it will not change, or only change insignificantly, apart from colour, odour and content in bi- and tricyclic aromatics, the characteristics of these products. The products that have undergone further purification are distinguished by the attribute "purified". The differences between the characteristics of macrocrystalline, intermediate and microcrystallineparaffin waxes not subjected to purification, and their classification based on these differences is shown in Table 1-7. The basis of our classification is the melting point, kinematic viscosity at 100 "C, penetration at 25 "C, breaking point (Fraass) and oil content (ASTM). In our view, the totality of these characteristics is necessary and sufficient for an unequivocal characterization of the paraffin wax in question, its structure, oil content and mechanical characteristics. Table I-7. Classification of macrocrystalline, intermediate and microcrystalline paraffin waxes by their characteristics
1
I Intermediate
Characteristics
Melting point, "C Viscosity at 100 OC,mm*/s Penetration at 25 O , 0.1 mm (ASTM needle) Breaking point (Fraass), O C Oil content (ASTM), wt-%
brittle (ceresin)
40-60 c5.5
58-70 5.5-10
74-85 >10
12-20 >+25 <0.8
>15 >+15 <5.0
<10 >+25
<2.0
1-
Microcrvstallino ductile elastic
50-60
> 10
20-35 -20-0 0.5-3.0
I I
plastic
50-70
> 10
20-50 -30-+ 10 3.0-7.0
Literature Asinger, F., Paraffins, Chemistry and Technology. Pergamon Press, Oxford (1968). Finck, E., Fette, Seifen, Anstr-Mittel, 62, 502 (1960). Forziati-Willingham-Mair-Rossini: J . Res. natn. Bur. Stund., 82, 11 (1944). Gruse-Stevens: Chemical Technology of Petroleum. MacGraw Hill, New York (1960). Hoffmann, H. J., Erdol, Kohle, 17, 717 (1964). Ivanovszky, L., Chem. Tech. Berf., 11, 315 (1959). Kreuder, W., Seifkn-ale-Fette- Wachse, 84, 665, 699, 735, 773, 849 (1958). - : Seifen-ale-Fette-Wachse, 85, 19, 41, 67, 93 (1959). Mair-Rossini: Ind. Engng. Chem., 47, 1062 (1955). Marx-Presting: Chem. Tech. Berl., 7, 662 (1955). Mazee, W. M., Modern Petroleum Technology, 3rd ed. (Ed. E. B. Evans), Institute of Petroleum, London (1962). Perry, J. H., Chemical Engineers' Handbook. McGraw-Hill, New York (1950). Phillips, J., Petrol. Refiner, 38, 193 (1959). Rossini-Mair: Adv. Chem. Ser., No. 5, 334 (1951).
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
21
Rumberger, J., Symposium on Composition of Petroleum Oils, Determination and Evaluation. ASTM, p. 283 (1958). Teubel-Schneider-Schmiedel :Erddlparajine. VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig (1965). Tuttle, J. B., Petroleum Products Handbook. (Ed. V. B. Guthrie), McGraw-Hill, New York (1960).
Wolff, G., Coating, 9, No. 1. 13 (1976).
(B) Chemical properties of liquid paraffins and paraffin waxes The chemical properties of liquid paraffins and paraffin waxes obtained from petroleum are in relation with the following steps : - preparative and analytical methods for studying the chemical composition of liquid paraffins and paraffin waxes, - determining the chemical composition of the paraffins, - determining the chemical properties of individual paraffin hydrocarbons.
1. Preparative and analytical methods for studying the chemical composition of liquid paraffins and parailin waxes The determination of the chemical composition of liquid paraffins and paraffin waxes can only be carried out after cumbersome separation procedures and subsequent analyses including spectral analysis, gas chromatography, etc. In the case of paraffin waxes with higher average molecular weight it is almost impossible, even using the most laborious operations, to achieve complete separation of individual compounds. In general, the objective is to produce narrow fractions whose components are closely similar or identical with regard to chemical structure. An approach to the chemical nature of a given paraffin wax is also yielded by physical characteristics, whose values are closely related, for a given molecular weight, to the structure of the molecule. For determining and characterizing the chemical composition of paraffin waxes, essentially three groups of preparative and analytical procedures are available: - separation methods, - classification methods based on physical characteristics, - analytical methods for the determination of individual components.
(a) Separation methods For a partial separation of the components differing in molecular weight and chemical structure, the following methods can be considered : - fractional distillation,
22
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
- fractional crystallization,
adduct formation with urea and column chromatography on urea, separation using molecular sieves, - elution chromatography, - thin layer chromatography. Separation by distillation is based on the differing boiling points of the components. This separation method is much limited, since the boiling points of the successive members of the n-alkane series, particularly in the case of compounds containing more than 25 C-atoms, are very close to one another. Therefore; preparative separation by distillation is effective mainly in the case of < C , n-alkanes. This separation method is difficult to apply to iso-alkanes and cycloalkanes, since the boiling points of the members of these two homologous series overlap. If a mixture of pure n-alkanes has been ffrst separated, by some method, from the material to be analyzed, the distribution of the compounds in the mixture can be determined by molecular distillation. It is obvious from what has been said that separation by distillation is much less effective in the case of microcrystalline paraffin waxes than in the case of liquid paraffins and macrocrystalline paraffin waxes. A successful method for the separation of microcrystalline paraffin waxes is fractional crystallization based on differential solubility. Ketones, mixtures of ketones and aromatics, halogenated hydrocarbons and different gasoline grades have been used as solvents in research up to the present. Fractional crystallization yields fractions of both macrocrystalline and microcrystalline paraffin waxes differing in molecular structure and molecular dimension. At higher temperatures of crystallization, fractions containing higher molecular weight and less branched alkanes, as well as cycloalkanes with long side chains will crystallize. With successive lowering of the temperature, the fractions will contain more and more iso-alkanes and cycloalkanes with shorter side chains; simultaneously the average molecular weight of the fractions will decrease. n-Alkanes can also be separated from iso- and cycloalkanes by urea adduct formation. X-ray studies have shown that the long chains of n-alkanes as well as long chains, if present, of iso- and cycloalkanes are enclosed in the tubular channels of the adduct, and this results in a hexagonal urea lattice. Urea crystallizes in the hexagonal system only when an adduct is formed, its normal crystal system being tetragonal. Straight-chain derivatives of n-alkanes, e.g. carboxylic acids, alcohols, esters, amines etc. are also capable of adduct formation. Adduct formation between n-alkanes and urea takes place in solutions of the former in gasoline, benzene or halogenated hydrocarbons when solid urea or an aqueous or alcoholic urea solution is added. When solid urea is applied, a small amount of a wetting agent, i.e. water, alcohol or some other substance with a s h i lar effect is necessary. Adduct formation is inhibited by resins, bituminous substances, sulfur compounds, etc. It is, therefore, important to remove such substances from the material before adduct formation, by elution chromatography or some other method. -
-
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
23
Adduct formation is an equilibrium reaction, the equilibrium being dependent on temperature, concentration of urea and adduct-forming components, and nature of the solvent. Adduct formation is exothermic, the heat of reaction is the higher, the longer the alkane chain. Hence the stability of the adduct is the greater the longer the adduct-forming molecule chain. Short-chain n-alkanes form adducts only at low temperatures, and these products will readily decompose. The following method was used by Hessler and Meinhart. Dilute solutions of macro- and microcrystalline paraffin waxes in carbon tetrachloride were prepared, methyl alcohol saturated with urea was added and the mixture vigorously agitated. The crystalline precipitate formed was filtered, washed with alcohol and dried. The decomposition of the adduct was carried out with distilled water at 70 "C. A diagram of the urea adduct method developed in the Hungarian Oil and Gas Research Institute is shown in Fig. 1-2. As well as urea, thiourea can also successfully be used for studying the chemical composition of complex mixtures of hydrocarbons and their derivatives. Thiourea forms adducts most readily with branched compounds. The essence of columnchromatography, using urea, is as follows. The substance to be studied is introduced, in the form of a solution, into a column filled with urea. Those components of the substance which, under the given conditions, namely thermostatted temperature, presence of activator in the column and percolation time, form an adduct with urea will be bound, 'while the unreacting components will remain in solution and will be eluted from the column by washing with solvent, and determined quantitatively. Subsequently, those components having formed adducts will be eluted by successive stepwise increases of the temperature. The temperatures corresponding to these steps will determine the structure and average molecular weight of the eluate fractions. Molecular sieves are zeolites consisting of aluminium, calcium, alkali and hydrogen orthosilicates. Their characteristic feature is the ready compensation of the negative charges of their tetrahedral and A10i5 crygtal lattices by cation exchange. The interconnected voids in their lattices contain combined water that can reversibly be removed by heating. Dehydrated zeolite is capable of binding molecules having suitable dimensions to fit into the voids. For the separation of n-alkanes from hydrocarbon mixtures, synthetic molecular sieves of the so-called 5 '8, type are suitable. The average diameter of their pores is 5 A, their chemical composition is Me,,/n[(A1OJl, * (Si02)lz] 27 H,O. For chemical group analysis of liquid paraffins and macro- and microcrystalline paraffin waxes, column chromatographic separation methods based on the work of Mair, Rossini, Spengler, Snyder and Heinze are well suited. Silica gel or activated alumina is preferably used as adsorbent. The ratio of adsorbent to sample is between 20 : 1 and 30 : 1. The sample is introduced in the form of a dilute solution in gasoline or hexane. The succession of the eluents is that of the increasing polarity, e.g. hexane, mixture of hexane and benzene, benzene, methanol and chloroform. This method allows separation of saturated hydrocarbons, and mono-, bi- and tricyclic aromatics with satisfactory sharpness.
-
24
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Benzene (solvent for paraffin wax)
Dissolution of paraffin wax (25-30"C).
-.
water (90°C)
Wash and separation
Adduct formation (25"C, 96 h)
Wash liquor ~
Solution of ;so-alkanes Aqueous in benzene solution of urea
with 10 wt-% methanol
Wash
-
Benzene
t
Washed adduct Distilled
Removal of solvent
Is0 - alkanes
Aqueous solution of urea n-Alkanes +solvent
Removal of solvent
n -Alkanes
Fig. 1-2. Method for determining the n-alkane and iso-alkane content in macrocrystalline
and microcrystallineparaffin waxes
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
I
Slack wax
25
]
Separation' of olefins with mercury acetate
i
Olefin - free slack wax
Adduct formation
c
I
n-Alkanes and
1 Iso-alkanes, aromatics, resins
1
I
Column chromatogr.aphy
Column chromatography on silica gel
on silica gel
Iso-alkanes A l k y l - substituted aromat tcs A
Column chromatography on activated carbon
I
Column chrdmatographtj
n-alkanes
a1k y I - substituted aromat ics
Fig. 1-3. Combined separation method of Spengler and Jantzen
A more detailed chemical study of macro- or microcrystalline paraffin wax requires a combination of separation methods. A diagram of a combined separation method developed by Spengler and Jantzen is shown in Fig. 1-3. Separation of macro-and microcrystallineparaffin waxes by thin-layer chromatography was developed, among others, by Dietsche and Sucker. They used a 250
26
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Gm silica gel support layer impregnated with 40 % urea. To avoid recrystallization of the urea, a small amount of sorbite was applied. The paraffin wax to be studied was applied in a 1 % solution in benzene, at 50 to 60 "C. The solvent used for runs was a mixture of carbon tetrachloride and ethanol saturated with urea. By using a suitable solvent composition and temperature (around 50 "C), they succeeded in obtaining satisfactory separation of the paraffin wax with respect to chain length and degree of branching. By using appropriate conditions and simultaneous runs with reference standard materials, they could determine the ratio of n- and iso-alkanes in macrocrystalline paraffin waxes.
(b) Chemical classijcation on the basis of physical characteristics According to Etessam and Sawyer, the relationship between the melting poirlt and the molecular weight for n-alkanes is
M m.p. = 415 ____ - 273 M + 95 where m.p. is the melting point and M the molecular weight. According to Ivanovsky, an analogous relationship can be established between the melting point and the density : lo3 * d y = 716 f 0.75 m.p. (1-2) where d: is the density at 90 "C relative to that of water at 4 "C. From these equations the so-called ring value is derived, since
-
lo3 d: = 511
+ 311 M M+ 95
(1-3)
and the ring value, indicating the density increases due to ring closure as compared to the equimolecular n-alkane, is (1-4)
The so-called asymmetry value is obtained from the Etessam and Sawyer relationship by introducing a factor of 0.75: a.v. = 311
M M
+ 95
- 205 - 0.75 m.p.
(1-5)
The asymmetry value indicates the melting point decrease due to iso-alkanes as compared to the equimolecular n-alkane. For n-alkanes, both the ring value and the asymmetry value are zero. For isoalkanes the ring value differs only slightly from zero, its maximum value can reach 5, while for cycloalkanes the ring value can be as high as 100. In the simultaneous presence of iso-alkanes and cycloalkanes, the ring value will have inter-
27
(B) CHEMICAL PROPERTIES OF LiQUID PARAFFINS AND PARAFFIN WAXES
mediate values, depending on the number and nature of branchings. To decide for mixed paraffin waxes whether they are composed mainly of iso-alkanes or cycloalkanes, it is necessary to know both the ring value and the asymmetry value. In such cases the so-called sum value (s.v.) yields the answer, its value for n-alkanes being zero: S.V. =
r.v.
+ a.v. = lo3
*
d: - 716 - 0.75 m.p.
(1-6)
According to Spengler and Jantzen the relationship between refractive index and melting point permits calculations on the iso-alkane, cycloalkane and alkylsubstituted aromatics content in paraffin waxes. For n-alkanes, this relationship has the form: 70 nD = 4.2 * m.p. 1.4076 (1-7)
+
where n: is the refractive index at 70 "C. Similar relationships are valid for iso- and cycloalkanes and alkyl-substituted aromatics. However, the straight lines representing the latter rekdtionships intersect the substantially steeper straight line for n-alkanes. The point of intersection is not known exactly, but is around the melting point values of 125 to 130°C 70 and nD = 1.4580. Such a refractive index versus melting point diagram is shown in Fig. 1-4. If the measured values of some paraffin wax or of one of its fractions are placed into this diagram, certain conclusions can be made regarding its composition.
Melting point, 'C Fig. 1-4. Relationship between the refraction index and melting point of hydrocarbons
28
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Hersch and Fenske found that the naphthenic ring content of aromatics-free paraffin waxes or their fractions can be determined using the Watterman n-d-M ring analysis modified by them. Their methods are as follows:
- the average number of naphthenic rings per molecule is
RN = 0.284 [(n: - 1.4750)M + 8.79]0*8';
(1-8)
- the number of carbon atoms combined in naphthenic rings is
CN = 2.08 [(n: - 1.4750)M + 8.79]0*73;
(1-9)
- the percentage of naphthenic rings is N = - 2890 [(n: M
-
1.4750)M + 8.79]0-73,
(1-10)
where n z is the refractive index at 20 "C. ( c ) Analytical methods for the determination of individual hydrocarbons or of compositions of their mixtures The methods discussed in the previous paragraphs are suitable to give an overall approach to the chemical composition of macro- and microcrystalline paraffin waxes. This is satisfactory in many cases for manufacturing and application purposes. If, however, individual hydrocarbons must be determined, gas chromatography, mass spectrometry and infrared spectrometry method have to be used. High-temperature gas chromatography and mass spectrometry methods suitable for the analysis of paraffin waxes have been frequently discussed in the recent literature. For the gas chromatography of macrocrystalline paraffin waxes, temperatures between 250 and 350 "Care used. The paraffin wax is retained by stationary liquid phase and individual components are stripped from the column, according to their volatility, using hydrogen or helium as carrier gas. The fractions eluted are recorded with thermal conductivity or flame ionization detectors. Different selective liquid stationary phases are in use, e.g. silicone oils, distillation methyl silicone fluid, carborane (methyl silicone fluid) etc. Gas chromatographic analysis of paraffin waxes will not be discussed in detail here. It will only be mentioned that apparatus and techniques exist that allow the determination of individual hydrocarbons up to CS5. The results of Levy and co-workers are particularly worth mentioning. They combined high-temperature gas chromatography and mass spectrometry methods, and achieved qualitative and quantitative determination of 67 individual components in a refined macrocrystalline paraffin wax. It is, in general, effective to use a suitable form of gas chromatography for separation, and mass spectrometry for subsequent identification.
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
29
There are many reports on infrared spectrometric studies of paraffin waxes. However, no generally accepted analytical method has yet been established for the determination of other than normal hydrocarbons. In the IR spectrometry of paraffin waxes, absorption bands are, in general, within the 600 to 3530 cm-' range. Such analyses were successful in differentiating and detecting primary, secondary and tertiary carbon atoms. The presence of iso-alkanes and cycloalkanes in paraffin waxes forming urea adducts could qualitatively be confirmed. Using IR spectroscopy, some authors succeeded in determining the extent of branching, whilst others determined the numbers of methyl and methylene groups. In addition to these instrumental analytical methods, various chemical analytical procedures were developed for identifying or determining a given group of compounds. The antimony pentachloride method shall be mentioned as an example. This is based on the finding that n-alkanes, in carbon tetrachloride solution, do not react with antimony pentachloride, whereas iso-alkanes form an insoluble, pitch-like substance. Thus, the n-alkane content in macro- and microcrystalline paraffin waxes can be determined. 2. Chemical composition of liquid paraffins and parnffin waxes Liquid paraffins have a relatively simple chemical composition, as they consist almost entirely of n-alkanes. The products manufactured by different companies for different purposes show only slight variations in the molecular weight range. On the other hand, the chemical composition of macrocrystalline and microcrystalline paraffin waxes varies over an almost infinite range of combinations, varying according to the source of the crude petroleum and to processing technology. To characterize the chemical composition of paraffin waxes, let us first summarize the general ideas, and subsequently present the composition of some paraffin waxes from different sources. As demonstrated by spectroscopic studies, paraffin waxes consist mainly of saturated hydrocarbons. The number of aromatic ring compounds, particularly in the case of macrocrystalline paraffin waxes consisting of compounds of lower molecular weight, is so small that they have practically no effect on the properties of the waxes. In fact, the majority of these rings are present as alkylbenzene derivatives and in condensed forms, and hence detrimental to health. In studying the composition of liquid paraffins let us consider the work of Mikhaylov and co-workers who studied the composition of a liquid paraffin obtained by urea dewaxing of a Diesel fuel from a high sulfur-content crude, and subsequent refining by adsorption. This liquid paraffin contained 0.2 wt- % aromatics, the amount of hydrocarbons forming no adducts with urea was relatively small. Table 1-8 lists the most important properties of the liquid paraffin, and the products obtained by twice repeated treatment with urea. As may be seen, the fractions forming no adducts with urea become enriched in iso-, cyclo-
30
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 1-8. Chemical composition and physical properties of liquid paraffin, of its urea-adduct forming parts and parts forming no adduct with urea
1
I
Product
First urea treatment 1 Starting material 2 Adduct-forming compounds 3 Compounds forming no adduct
Total : Second urea treatment 4 Adduct-forming compounds 5 Compounds forming no adduct Total:
1
1
Hydrocarbon content, wt- % relative to starting material Yield, %
n-alkanes
phenylalkanes
Properties
-__
iso-alkanes and cYclOalkanes
~~~~i~~ dl0
1
i
___
Refr. index Melting point
"C
100.0
96.4
0.2
3.4
0.8043
1.4370
24.5
95.0
92.5
traces
2.5
0.8054
1.4370
27.0
5.0
3.7
0.21
1.07
-
-
-
100.0
96.2
0.21
3.57
2.45
2.18
0.027
0.243
0.7743
1.4350
2.55
1.55 3.73
0.165 0.192
0.837 1.080
0.7922
1.4385
5.0
-
-
Table 1-9. Composition of the liquid paraffin in Table 1-8 Hydrocarbons
Identified n-Alkanes Iso-alkanes 2- and 4-methylalkanes 3-methylalkanes 5-methylalkanes 6-methylalkanes Cycloalkanes 1-cyclopentylalkanes 1-cyclohexylalkanes Phenylalkanes 2-phenylalkanes Total identified :
I
Carbon atom range
1
Share, wt-%
I
Number of compounds
C11-Cz4
96.400
14
C,,-Czo
1.168 0.484 0.126 0.061
12 6 6 4
C,,-C1*
0.132 0.299
6 6
c,,-czo
0.047 98.717
6 60
1.283
-
cI7-czo
C11-C24
Non-identified Iso-alkanes
Cycloalkanes Phenylalkanes Total non-identified:
-
-
9.0 7.2
-
31
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
and phenylalkanes. The total hydrocarbon composition of the liquid paraffin is listed in Table 1-9, showing that 60 compounds could be identified, among these 14 different n-alkanes, 28 iso-alkanes, 12 cycloalkanes and 6 phenylalkanes. The amount of non-identified hydrocarbons did not exceed 1.3 %. The authors also stated that the iso-alkanes contained in the fraction forming no adduct with urea are monosubstituted methylalkanes, with the methyl group attached to one of the C, to C, carbon atoms. The cycloalkanes contained in the fraction are rings of five or six carbon atoms, with straight-chain alkyl groups attached. In the phenylalkanes present, the benzene ring is attached to the second carbon atom of the alkane. Paraffin waxes consisting of C,&30 hydrocarbons are mainly composed of n-alkanes. Compounds containing rings, or branched at the end of the chain, are also present, but in small amounts and especially in the higher fractions. In microcrystalline paraffin waxes consisting of >C3,C3, hydrocarbons, obtained from fractions distilling over at higher temperatures or from vacuum distillation residues, the other than normal character dominates. Hydrocarbons other than normal cover the total carbon atom number range from C30 to Cs0. n-Alkanes in microcrystalline paraffin waxes are also mainly within this range. In addition to n-alkanes and iso-alkanes, macrocrystalline and microcrystalline paraffin waxes contain naphthenes, especially alkyl-substituted derivatives of cyclopentane and cyclohexane. Depending on the source of the crude and on the extent of refining, larger or lesser amounts of cyclic sulfur and nitrogen compounds are also present. The decisive factors determining the properties of low oil-content paraffin waxes are hence the distribution, by carbon atom number, of n-, iso- and cycloalkanes and their relative quantities. This appears quite evident, knowing that substantial differences exist between the properties of isomeric n- and iso-alkanes. As an example, Table 1-10 (based on data by Mazee) records the physical properties of two n-alkanes and their iso-alkane isomers, both within the carbon atom number range of macrocrystalline paraffin waxes. To characterize the chemical composition of paraffin waxes some characteristic values for three microcrystalline paraffin waxes from different sources will first be presented, based on data from Ridenour, Spilners and Templin. These values Table I-10. Physical properties of two n-alkanes and their branched isomers in the range of macrocrystalline paraffin waxes Alkane
n-Tetracosane 2-Methyltricosane 2,2-Dimethyl-n-docosane n-Octacosane 10-Nonylnonadecane
1 1 Formula
CZaH,o CZ4H,, CllHSO CzsH,, CzsH,,
Boiling point at 0.5.F
208.6 205.0 201.5 242.0 228.3
Melting point; "C
1 1 Density
di0
50.7 37.6 34.8 61.3 -5.5
0.7562 0.7539 0.7536 0.7639 0.7650
1.4205 1.4201 1.4191 1.4248 1.4247
2.42 2.48 2.71 3.40 2.68
32
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table I-11. Main characteristics of microcrystalline paraffin waxes from different sources Wax identification mark
Characteristics
Melting point, OC Refractive index, &' Density at 70 "C Average molecular weight Ring analysis (n-d-M) RA
RN
Oil content, wt- %
76.0 1.4514 0.8122 629
74.1 1A 8 3 0.8065 574
62.5 1.4547 0.8184 682
0.20 0.60 8.2
0.12 0.48 4.5
0.20 0.90 0.8
are summarized in Table 1-11. Sample A is a typical microcrystalline paraffin wax, sample B a hard, brittle product, sample C a highly flexible, ductile microcrystalline wax with high adhesive power. Portions isolated from these samples by adduct formation with urea, and fractions obtained from these portions by molecular distillation are characterized in Tables 1-12-1-14. The symbols used are: RT = = total ring content per molecule, R, = aromatic ring content per molecule, R, = naphthenic ring content per molecule. In the first eight distillation fractions of the adduct-forming portion of sample A , the total ring content determined by the n-d-M method is less than 0.2 rings/molecule. The value of R N in distillation fraction 9 is 0.4, in the distillation residue 0.8. According to infrared absorption
No. of fraction
Adduct-forming part 1
2
3 4 5 6 7 8 9
Distillation residue
wt-%
M.p.
"C
Refr. ind.
,,g
Dens. at 70°C
Aver.
b:
100 10.5 9.0 11.8 11.8 10.2 9.5 10.0 8.0 10.2
79.0 68.9 71.6 72.8 75.8 77.0 78.6 80.9 83.4 87.4
1.4422 0.7919 1.4374 1.4388 1.4390 1.4408 1.4408 1.4413 1.4426 1.4438 1.4458 0.8028
548 447 486 489 510 537 550 572 590 662
9.0
93.6
1.4531 0.8165
910
Ring analysis (n-d-M)
__ RT
0.0
-
-
I 1 RA
RN
0.0 -
0.0 -
-
-
-
-
-
0.4
0.0
0.4
0.9
0.1
0.8
Corresponding n-alkane
1 vz
~;a:
c,
C.1 C,, C,,
79.4 69.3 72.7 74.0 75.6 78.1 79.5 82.9 82.9 87.5
C,,
98.1
C,, C,, C,, C,, C,,
c,
33
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 1-13. Characteristics of the fractions obtained by molecular distillation from the adduct-forming part of the microcrystalline paraffin wax marked B in Table 1-11 n-alkane
No. of fraction
Adduct-forming part 1 2 3 4 5 6 7 8 9
Distillation residue
No. of fraction
Adduct-forming part 1 2 3 4 5 6 7 8 9
Distillation residue
100 10.1 10.4 10.6 10.5 10.0 10.0 9.1 8.6 11.4
76.0 67.0 69.4 71.2 73.8 76.0 77.6 79.4 80.5 82.2
1.4410 1.4356 1.4368 1.4377 1.4390 1.4399 1A 0 8 1.441 8 1.4425 1.4439
9.3
85.0
1.4488
Yield wt-%
M.P.
100 9.6 11.0 10.4 9.9 11.5 11.7 8.5 9.0 8.9
67.0 62.0 66.2 67.2 68.3 69.6 71.4 72.4 74.2 76.0
1.4400 1.4340 1.4363 1.4371 1.4377 1.4387 1.4402 1.4419 1.4435 1A450
9.5
82.0
1.4529
"C
Refr. ind.
% '"
CS7 C,, C33 Cs C,,
526 439 464 475 509 518 536 557 565 594 0.8066
684
76.8 67.2 70.8 72.7 75.6 (2.37 76.8 Ca8 78.1 Cd0 80.7 Cl0 80.7 Cd2 82.9 0.4
0.05
Ring analysis (n-d-M)
I 1
Dens. at
Aver.
7O"C
Wt.
0.7882 0.7797
0.7959 0.7994
478 407 436 455 458 471 492 520 543 575
0.1 0.3
0.0 0.0
0.3
0.8167
743
1.0
0.0
1.0
-
-
-
mol.
__ RT
0.0 0.0
-
Rk
0.0 0.0
-
-
-
C,
0.35
C
89.0
o
1
RN
~
; !? 72.7 62.3 67.2 69.0 70.8 72.7 74.0 76.8 79.5 81.9
0.0 0.0 -
-
0.1
Cs,
91.7
measurements this consists of monocyclic alkanes and their substituted derivatives. The adduct-formingportion of sample B, and its fractions have a similar composition, the differences showing only in the range of molecular weights and in the total ring content of distillation fraction 9 and of the residue. Fractions from sample C have lower melting points and higher refractive indices than fractions from samples A and B with identical molecular weights. This indicates that 3
-
~
34
1. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
sample C contains a higher percentage of iso-alkanes. However, the slight differences in the melting points demonstrate that these iso-alkanes are branched to a small extent only. The characteristics of the portions of the three samples forming no adduct with urea, as well as of their distillation fractions, are listed in Tables 1-15-1-17. It may be observed that the portion of sample C forming no adduct has a substantially higher average molecular weight, a broader molecular weight range, a lower average melting point and a narrower melting point range than those of the other two samples. This difference is in conformity with its composition: sample C and its portion forming no adduct contains more branched alkanes and the share of aromatic and naphthenic rings is higher than in sample A and B. The ring Table 1-15 Characteristics of the fractions obtained by molecular distillation from the part forming no adduct of the microcrystalline paraffin wax marked A in Table 1-1 1
Part forming no adduct 1 2 3 4 5
6 7 8
100 8.75 9.75 9.75 9.55
10.45 9.85 10.7 10.4
Distillation residue 20.8
63.0-65.0 51.0 57.0 61.0 66.0 66.0 69.0 71.0 72.0
1.4592 1.4580 1.4566 1.4556 1.4559 1.4567 1.4578 1.4595 1.4644
0.8305 0.8291 0.8244 0.8243 0.8239 0.8246 0.8264 0.8310 0.8338
706 490 535 579 623 661 758 783 910
1.9 1.8 1.5 1.6 1.6 1.5 1.6 1.9 1.7
0.15 0.2 0.2
0.6
1.75 1.6 1.3 1.55 1.5 1.35 1.45 1.75 1.1
68.0
1.4677
0.8440
1340
3.0
0.6
2.4
0.05
0.1 0.15 0.15
0.15
Table 1-16. Characteristics of the fractions obtained by molecular distillation from the part forming no adduct of the microcrystalline paraffin wax marked B in Table 1-11
Part forming no adduct 1
6 7
10.1 10.4 10.0 10.1 10.5 9.2 11.9
63.0-63.3 55.6 58.6 60.6 62.7 64.3 65.3 66.0
1.4617 1.4596 1.4559 1.4559 1.4562 1.4573 1.4583 1.4606
0.8325 0.8304 0.8217 0.8217 0.8220 0.8245 0.8255 0.8306
686 500 540 561 602 643 653 712
1.8 1.8 1.3 1.3 1.3 1.4 1.4 1.7
0.25 0.30 0.30 0.35 0.35
1.4 1.55 1.05 1.05 1.00 1.10 1.05 1.35
Distillation residue
27.8
65.6
1.4695
0.8434
930
2.2
0.8
1.4
.
L
3 4 5
100
0.4
0.25 0.25
35
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table Z-17.Characteristics of the fractions obtained by molecular distillation from the part forming no adduct of the microcrystallineparaffin wax marked C in Table 1-11 No. of fraction
Part forming no adduct 1 2 3 4 5 6 7
Distillation residue
Refr. ind. n&O
Dens. at 70 "C
Aver.
Ring analysis RT
I
RA
I N'
100 13.7 9.3 9.8 9.7 9.5 9.6 14.2
55.0
56.0 56.6 56.9 56.6 55.6 55.0 51.0
1.4598 1.4466 1.4481 1.4509 1.4536 1A563 1.4598 1.4644
0.8256 0.8030 0.8087 0.8130 0.8188 0.8232 0.8297 0.8390
773 503 544 603 656 750 839 967
1.2 0.6 0.9 1.0 1.2 1.4 1.7 2.4
0.4 0.1 0.0 0.0 0.0 0.1 0.2 0.4
0.8 0.5 0.9 1.0 1.2 1.3 1.5 2.0
24.2
45.0
1.4727
0.8529
1700
4.0
0.9
3.1
compounds in sample C are concentrated to a higher extent in the higher molecular weight fractions than is the case with samples A and B. The cited authors achieved further separation by thermodiffusion of the distillation fractions obtained from the part of sample A that forms no adduct. As a result of subsequent analyses, they succeeded in determining the chemical composition of this part of the sample. They found that it consists of about 17 wt-% monocyclopentylalkanes, 24 wt- % monocyclohexylalkanes, 6 wt- % dicyclopentylalkanes, 20 wt- % dicyclohexylalkanes, 6 wt- % monocyclic aromatics, 5 wt- % polycyclic aromatics and 22 wt- % polycyclic alkanes. The distribution among these compound types is shown in Fig. 1-5, where, on the one hand, dicyclopentyl-
Distillation yield, wt-%
Fig. 1-5. Distribution of compounds in the part forming no adduct of the microcrystalline
paraffin wax marked A
3+
36
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
and dicyclohexylalkanes, and on the other, aromatics and polycycloalkanes are combined into one group each. Bornemann and Heinze used the combined analytical procedure shown in Fig. 1-6 for characterizing the composition of microcrystalline paraffin waxes. Their starting material was slack wax. In the first stage they prepared - by fractional crystallization - vaseline, plastic and hard microcrystalline paraffin wax (these terms correspond to the classification of Kreuder). The hard paraffin wax was then separated into several fractions by chromatography on columns filled with silica gel. The iso-octane eluates were separated into portions forming adducts and forming no adducts, and these were subsequently subjected to further separation by fractional crystallization from dichloroethane, by chromatography on activated carbon and silica gel, and by molecular distillation. The results that the cited authors obtained with petrolatum from the heavy distillate of Romashkino crude are presented in the following tables. Table 1-18 Table Z-18. Main characteristics of products obtained by fractional crystallization of a petrolatum from the heavy distillate of a Romashkino crude Characteristics
Yield, wt-% Refractive index, ng Density at 90 OC Average molecular weight Melting point, *C Viscosity at 90 O C , mmZ/s Penetration at 25 OC, 0.1 mm Oil content, wt- % Part forming adduct with urea, wt-
%
Part forming no adduct, wt- %
Starting
1.4497 0.8102 450 63.6 16.7 40 (cone) 25.5 27.4 72.6
69.7 1.4546 0.8193 422 40.9 18.1 120 (cone)
-
-
20.7 1A430 0.7977 535 63.4 14.0 27 6.5 48.5 51.5
9.6 1.4393 0.7866 550 77.9 14.4 9 1.7 84.5 15.5
shows the main characteristics of the products obtained by fractional crystallization from the petrolatum. The hard paraffin wax fraction was separated by chromatography on 0.1-0.4 mm silica gel activated at 180 "C.The fractions obtained in this operation are shown in Table 1-19. The iso-octane eluates separated into adduct-forming and non-adduct-forming portions were further separated by fractional crystallization, by molecular distillation at 200-265 "C in a vacuum of 1 cPa, and by chromatography. The narrow fractions obtained by these procedures were analysed by determining the usual physical characteristics (refractive index, density, melting point, etc.), the Hersch-Fenske data and the n, versus b.p. diagram shown in Fig. 1-7. Their experimental results can be summarized as follows. The hard paraffin wax studied consists of C,,-C,, compounds. The n-alkane content represents 25 to 35 %, the majority of these being C,&, compounds. The share of iso-alkanes
Slack wax from heavy distillate I
EIzI Fractional !istillation
crl
a
Plastic paraffin wax
Vase1ine
Hard paraffin wax
Resins
Iso-octane eluate
Iso-octane eluate
Iso-octane eluate IV
Iso-octane eluate
III
I1
I
f
Benzene eiuate
8
, I
distillation
carbon
I I I Chromatographic separation I
carbon
I
carbon
I
I
I
I
carbon
Chromatographic separation on active carbon w
Fig. 1-6.Group analysis of microcrystalline paraffin waxes according to Bornemann and H e i m
4
38
I. PROPERTIES OF LIQUID PARAFFlNS AND PARAFFIN WAXES
Table 1-19. Characteristics of fractions obtained by chromatography on silica gel of the hard microcrystalline paraffin wax (source: Romashkino crude) figuring in Table 1-18 characteristics
Refractive index, :n Density at 90 OC Average molecular weight Melting point, OC Viscosity at 90 OC, mm2/s Penetration at 25 OC, 0.1 mm Oil content, wt- % Part forming a n adduct with urea, wt-% Part forming no adduct, wt- %
1-
___Eluates _ _with. iso-octane~-_ _ _ _Eluate with
__
I
1
111
I I1 _________-___
1.4372 0.7854 620 78.4 12.9 9 0.4
1.4381 0.7868 550 77.4 13.3 9 0.7
87.8 12.2
85.3 14.7
~
1.4402 0.7901 565 77.0 14.4 9 1.2 81.5 18.5
IV
benzene
1.4478 0.8032 610 75.8 16.9 10 2.7
1.4750 0.8553 610 71.2
74.7 25.3
-
15 12.4 -
-
and naphthenes is 55 to 65 wt-%, around 10 wt-% of which are C45-C55 alkyldicycloalkanes and alkyltricycloalkanes. The C35-C50 compounds are iso-alkanes, alkylcyclopentanes and alkylcyclohexanes. Aromatics represent about 1 wt- %, resinous substances around 1.5 wt- %.
0
1.4600
lC200
I
0
20
/ ' "
40
Boiling point,
60
7
80
"C (1 cPa)
Fig. 1-7. Part of the diagram of refractive index versus boiling point. A hard paraffin wax, B I-IV iso-octane eluates, C adduct-forming part of iso-octane eluate I, D part of iso-octane eluate I forming no adduct
The authors of this book studied how and to what extent the chemical composition of macrocrystalline and microcrystalline paraffin waxes from Romashkino crude is changed by the effect of de-oiling and subsequent refining by the hot
39
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 1-20. Characteristics of slack wax and petrolatums from Romashkino crude Characteristics
Macrocrystalline slack wax from light distillate
Microcrystalline petrolatum from heavy distillate
Microcrystalline. petrolatum from residual oil
0.7683 2.75 48.5 27 8+ 1 7.8 1.4269 319 0.19
0.8357 9.91 57.9 146 8 21 42.8 1.4604 480 0.89
0.8264 12.43 65.5 112 4.5+ 6 23.7 1.4551 583 0.65
Density at 80 OC Viscosity at 100 'c,mmZ/s Melting point, OC Penetration at 25 OC, 0.1 mm Colour index ASTM (1/4") Extinction (Pulfrich filter 8, 100 mm) Oil content, wt- % Refractive index &' Molecular weight Sulfur content, wt- % Aromatics, wt- % monocyclic bicyclic tricyclic
1.3 1 .o 0.7
20.0 6.0 3.9
13.1 15.1 1.8
contact method. The main characteristics of the materials investigated are shown in Table 1-20. De-oiling was carried out at + 10 and +30 "C with methyl ethyl ketone, hot contact treatment with 196 m2/g specific surface area activated clay of the bentonite type. The characteristics of the de-oiled products are listed in Table 1-21. Those in Table 1-22 are of products refined by hot contact under Table 1-21. Characteristics of the slack wax and petrolatums figuring in Table 1-20 after deoiling Macrop
Characteristics
Temperature of de-oiling, O C Yield relative to starting material, wt- % Density at 80 "C Melting point, "C Penetration at 25 OC, 0.1 mm Colour index ASTM (1/4") Extinction (Pulfrich filter 8, 10 mm) Oil content, wt- % Refractive index, n? Molecular weight Sulfur content, wt- % Aromatics, wt- % monocyclic bicyclic tricyclic
from paraffinic distillate
10
Microcrystalline paraffin ~ wax from~ heavy distillate
~ ~
A
10
I
B
30
13.0 17.9 37.5 0.7635 0.7884 0.7930 51.9 73.1 66.8 19 15 28 1 8+ 8+ 0.85 1.03 14.2 0.7 5.1 5.7 1.4236 1.4360 1.4360 540 341 517 0.03 0.21 0.10 0.3 0.1 0.1
9.8 4.9 0.5
4.8 1.3 0.2
Microcrystalline paraffin wax from ~ ~ residual oil
_~_______ A
10
J
B
30
21.5 52.9 0.7955 0.8065 78.0 70.5 9 33 5.5+ 5+ 5.0 6.2 1 .o 6.7 1.4391 1.4442 636 694 0.36 0.17 13.7 2.0 1.3
7.0 0.7 0.6
~
40
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 1-22. Characteristics of the macro- and microcrystalline products figuring in Table 1-21 after hot contact refining
Characteristics
Macrocrystalline paraffin wax from light uaraf-
Microcrystalline paraffin wax from heavy distillate
Microcrystalline paraffin wax from residual oil
- - _ _ _ ~ _ _ _ _ _ . _ _
Density at 80 OC Melting point, OC Penetration at 25 OC, 0.1 mm Colour index ASTM (1/4") Extinction (Pulfrich filter 8, 10 mm) Oil content, wt- % Refractive index, ng Molecular weight Sulfur content, wt- % Aromatics, wt- % monocyclic bicyclic tricyclic
0.7900 0.7846 0.8038 0.7892 0.7566 66.8 51.9 73.1 70.5 78 28 19 15 33 9 1 1.5+ 0 1 0.00 0.36 0.20 0.16 0.17 0.7 5.7 5 .O 7.0 1 .o 1.4219 1.4418 1.4403 1.4488 1.4450 517 540 636 694 341 0.03 0.19 0.10 0.36 0.13 0.2 0.1 0.1
8 .O 4.6 0.3
4.0 0.6 0.2
13.3 0.2 0.2
4.4 0.4 0.1
optimum conditions. It may be seen that the aromatics content of all three products is reduced by one order of magnitude as a result of de-oiling, demonstrating that the aromatics and sulfur compounds are contained chiefly in the oily portion, as determined according to ASTM. Hot contact treatment mainly reduces the amount of bicyclic and tricyclic aromatic compounds. The starting materials, the de-oiled products and the products refined by hot contact were separated by chromatography on 0.10-0.18 nim silica gel, the weight ratio of adsorbent to sample being 20 : 1 and temperature 60 "C. Eluents applied successively were aromatics-free gasoline, a mixture of benzene and gasoline, benzene, and a mixture of methanol and chloroform. The mono-, di- and tricyclic aromatics content of the fractions was determined with a H-700 Hilger spectrometer. The distribution (in percentage) of carbon atoms in paraffinic and naphthenic bonds, and the number of methyl and methylene groups per molecule was determined by IR spectrometry. Figure 1-8 shows the refractive index at 80 "C plotted against the chromatographic yield for the petrolatum obtained from the residual oil, for the de-oiled and for the hot-contact purified products. It may be observed that, as was to be expected, both de-oiling and refining by hot-contact result in an increased n-alkane content. The vertical dash lines in the figure indicate the yields corresponding to the refractive index value of 1.4500. Before de-oiling, this yield is only 46 wt-%. After de-oiling at + 10 "C, the yield is 77 wt- %, after de-oiling at +30 "C, it increases to 94 %. By further refining, the 77 wt- % value is increased to 86 wt- %, the 94 wt- % yield to 96 wt- %. The distribution of n-alkanes, sulfur content and aromatics content versus chromatographic yield in the slack wax from light distillate and in the products
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
41
1.51 I 1.50 1.49 1.48 1.47 1.46 1.45
I ,
I
,
I
1.441 I I I I 0 10 20 30 40 50 60 70 80 90 1 0 1.49 I
1
1.47
-
1.46 1.45
X
1.49 I
C
a,
cc
1
De-oiled at t10 O C and refined by hot contact
1
-
440
t
10 20 30 40 50 60 70 80 90 100
1.45 1.46
1'440 1 /.O
10 20 30 40 50 60 70 80 90 100
refined by hot contact
1.46 1.45 1'440
10 20 30 40 50 60 70 80 90 100 Yield, w t - %
Fig. 1-8. Refractive indices of chromatographicalfractions of paraffin waxes from residual oil
42
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
-
'0
20
40 60 60 Yield, w t - %
100
100
"1
De-orled at +10 "C 3 60~a-
I
-
40-
c
-
0
Y
p
9
-3
s
,+ r I
' - 2
4
s
e-oiled at +10T and
I c
3
3
3
20-
-1
2 U I
I
C
20
0
Yield, w t - %
40 60 80 Yield, wt- %
100
Fig. I-9. Distribution of n-alkanes and sulfur in macrocrystalline paraffin waxes
---.---
monocyclic aromatics bicyclic aromatics tricyclic aromatics
a Yield, w t - %
60
1 De-oiled at +10 "C and refined by hot contact
LO
2ol 1 10
0
70
Yield, w t - %
,
,
80 90 Yield, w t - %
100
Fig. I-IO. Distribution of aromatic compounds in macrocrystalline para& waxes
43
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
obtained from this material is shown in Figs 1-9 and 1-10. Similar plots for the microcrystalline paraffin wax from heavy distillates and residual oil are presented in Figs 1-1 1-1-14. The data indicate that the fractions from 0 to 80 wt- % of the macrocrystalline slack wax contain 98 to 75 wt- % n-alkanes, while the corresponding data for petrolatum from heavy distillate are 40 to 55 wt-%, and the n-alkane content of the petrolatum obtained from residual oil in the fractions up to 50 wt- % is as low as 62 to 30 wt-%. 90 I
I
4 3
100
_i 0
-t
1001
I
90 De-oiled at t10 "C 80
P
4 -14 3
80
I I I
:2010
I
I )
[ -22 2 A -11
I
I
-
20
'
!
!
60 80 Vield, wt-"lo
40
'
100
0
I
De-olied ot +30 "C
. 70
< 3 60-I
L-
0 ' 0
10
Yield, w t - %
-
<
..
50
0
LO-
C
20 10 - OO
30-
I I
-1
I I
I
' 804& 0 Yield, wt-
100I
VO I
-De-oiled at +30 "C and 80 - refined by hot contact
70 60 '50A0 30 20 10 -
'0
20
40
60
80
-
*------
-4
z?
0
- s 3 .!3
I
-2 c 3
I I
-1
1I I
' L
3
Ln
100 Yleld, wt- 'fa
Yield, wt-'lo
Fig. I - I I . Distribution of n-alkanes and sulfur in microcrystalline paraffin waxes from heavy
distillates
44
1. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
De-oiling, and, to a lesser extent also refining, lead to an increase in the percentage of n-alkane content in the case of all three starting materials. As to the distribution of aromatic compounds, these are present, in the case of macrocrystalline products, only in the last chromatographic fractions, in the final 7-10 wt-% of the substance, whereas in microcrystalline products, aromatics appear even at a yield of 40 wt- %. De-oiling and refining by hot contact significantly increase this limit. Similar relationships are found in the distribution of sulfur compounds. The sulfur content is maximum in all cases in the fractions where the content in diand tricyclic aromatics is maximum. The sulfur atoms are consequently sited mainly in aromatic compounds. In the following, the chemical composition of some paraffin waxes determined by mass spectrometry is presented, based on data by Edwards. These substances represent the total range of macrocrystalline waxes, and include paraffin waxes 80 70 $ c 60 3 50 6 40 U 30 E 20 10
-monocyclic aromatics ---
-.-.-
bicyclic aromatics tricyclic aromatics
$
$0
50
60 70 80 Yield, wt-%
90
100
7
60-De-orled 50-
at + 3 0 T
w- 40-
2
30209 10Q.
g
Yield, wt-%
Yield, w t- %
?
70
70
6OCDe-oiled at +10 O C and
60-De-oiled at +30 "C and 7 50refined bg hot contact w-
4030-
:
20-
2 10Q. Yield, w t - %
Fig. 1-12. Distribution 9f aromatic compounds in microcrystalline paraffin waxes from heavy
distillates
45
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
whose compositions may be regarded as typical cases. The main characteristics and analytical data of the six paraffin waxes studied are contained in Table 1-23. Figure 1-15 shows the distribution of n- and iso-alkanes relative to the number of carbon atoms. These data also confirm the significant differences in the chemical composition of paraffin waxes depending on their source and manufacturing conditions. The samples investigated consist mainly of n-alkanes ;however, in samples A and F the iso-alkane content is rather high. The distributions demonstrate that hydrocarbons other than normal are concentrated in the higher molecular weight 4
80 70 - Starting material
-3s
* r -2
=
-1 10-
5-
c J
v,
0 Yield, wt-"lo
90 80
De-oiled at +lo "C
4
s
70 -
t
90 I 80 De-oiled at +30 "C 70
c
I
3 50
3
aC
-0 Y
30 20 10 -
--
7C Yield, w t
sn BoI-De-oiled
-
'0 10 "10
at +I0 "C and
70- refined by hot contact
6050 -
-
*----
44
s
30
50
70
Yield, wt-"lo
90
90 t 80 - De-oiled at +30 "C and 7 0 - refined bg hot contact
I
r
s 4030 -a-
40 -
30 -
Y
7C 20 -
20 10 -
- OO
20 10
10 -
Yield, wt-"lo
Yield, wt-"10
Fig. I-13. Distribution of n-alkanes and sulfur in microcrystalline paraffin waxes from residual
oils
46
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
1001
n
1
-monocyclic aromatics --- bicyclic aromatics ----- tricyclic aromatics
s
.3 I
m-
u .c
0
E
2
Q
Yield, w t - %
80 4?
70 - De-oiled at +10 "C
80 I De-oiled at +30
I
I
I
"c
I
3
m-
.-V
a ..
0
30
E 20
0,
a
'40
50
60
70
80
I
$0
Yield, wt-"lo
70 I 6 0 - De-oiled
1
at +10 "C and I c 50- refined b y hot contact 3 40 mu .- 30 -
P
50
60 70 80 Yield, w t - %
70 S 60-De-oiled at +30 "C and 50- refined by hot contact
90
100 I
'3
c
0
E 20 2 .I0 -
4
0
I
Yield, wt-%
Yield, wt-"lo
Fig. 1-14. Distribution of aromatic compounds in microcrystalline paraffin waxes from
residual oils
fractions. Disregarding sample A , the share of hydrocarbons other than normal increases with the melting point of the paraffin wax. Sample A was obtained by blending high and low melting-point paraffin waxes, and hence this sample cannot be directly compared with the others. The above samples were further separated by fractional distillation, and the composition of the fractions were also determined by mass spectrometry. Table 1-24 lists the results of these analyses. Fractions from wax B were not examined since the non-normal hydrocarbon content of this wax was low and the distribution appeared to be of less interest. Figures I-16,1-17 and 1-18 present the dis-
47
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
+
Table I-23. Main characteristics and chemical composition determined by mass spectrometry of various paraffin waxes Characteristics and composition
Melting point, "C Oil content (ASTM), wt- %
n-Alkanes, wt- % Branched alkanes, wt- % Monocycloalkanes, wt- % Polycycloalkanes, wt- % Monocycloaromatics, wt- % Aromatic cycloalkanes
I
--
i
A
50.7 <0.2 75.5 13.5 10.2 0.6
I
i 52.9
57.3
57.5
58.7
62.8
0.3
<0.2
<0.2 81.9 10.4
0.5 66.5 17.9
0.3
<0.2 82.2 8.2 9.0 0.5
0.0 0.0
0.1 0.0
3.4 0.0
86.4 6.3 7.1 0.1
0.0 0.0
traces 0.0
94.0 2.6
0.2 0.0
paraffin wax A
0.3
traces
10
5
Carbon atom number
Carbon atom number
15
15
10
s-
Paraffin wax &
10
I
I
z0
1.9
l5
g
-
13.4
7Paraffin wax D
I
s-
0'
1.4
I Iso-alkanes
I] n-Alkanes I
i
5 1' 5
15
r " 5
20 25 30 35 Carbon atom number Paraffin wax
'15
40
20
25
30
35
40
45
Carbon atom number
c1
15
s
10
8
5
-
Paraffin wax F
I
1' 5
20 25 30 35 Carbon atom number
20 25 30 35 40 Carbon atom number Fig. I-IS. Distribution of n-alkanes and iso-alkanes in various paraffin waxes
40
1' 5
45
48
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table I-24. Composition of distillation fractions of the paraffin waxes
listed in Table 1-23, determined by mass spectrometry
Parafin wax A Components
I-
n-Alkanes Branched alkanes Monocycloalkanes Polycycloalkanes Monocycloaromatics Aromatic cycloalkanes
Parafin wax C Components
n-Alkanes Branched alkanes Monocycloalkanes Polycycloalkanes Monocycloaroma tics Aromatic cycloalkanes
40-45 wt-%
'
82.4 13.9 3.6 0.0 0.1 0.0
1
20-25 wt-%
60-65 wt-%
20-25 wt-%
n-Alkanes Branched alkanes Monocycloalkanes Polycycloal kanes Monocycloaromatics Aromatic cycloalkanes
52.1 28.6 17.7 13 0.3 0.0
1
60-65 wt-%
91.4 7.5 11 0.0 0.0 0.0
l-
20-25 wt-%
5.0 wt-% residua
1
85-90 wt-%
5.0 wt- % residue
60.6
24.0
14.0 1.4 traces 0.0
5.0
0.3 0.0 0.0
60-65 wt-%
I
85-90 wt-%
79.2 13.6 6.5 0.4 0.3 0.0
37.3 34.0 23.8 4.9
traces traces
5.0 wt-%
residue
part of heads
94.5 5.5 0.0 0.0 0.0 0.0
37.6 25.9 30.5 4.4 1.4 0.2
85-90 wt-%
78.7 16.0
1
6.5 wt-% residue
1
part of heads
n-Alkanes Branched alkanes Monocycloalkanes Polycycloalkanes Monocycloaromatics Aromatic cycloalkanes
Components
85-90 wt-%
part of heads
Parafin wax D Components
~
part of heads
95.7 4.2 0.1 0.0 0.0 0.0
I-
60-65 wt-%
64.4 19.4 14.4 1.5 0.2 0.1
48.1 23.0 24.3 3.7 0.8 0.1
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
49
Table I-24 (cont.)
Parafin wax F
20-25 wt-%
Components
1
6C65
1
wt-%
85-90 wt-%
part of heads
n-Alkanes Branched alkanes Monocycloalkanes Polycycloalkanes Monocycloaromatics Aromatic cycloalkanes
78.6 14.9 6.5 traces traces 0.0
54.6 26.7 16.5 1.9 0.2 0.1
42.8 32.4 20.5 3.6 0.6 0.1
5.0 wt-% residue
44.6 23.9 24.1 6.1 1.o
0.3
tribution of normal and branched alkanes in different fractions of samples A, D and F. Turner and co-workers also used mass spectrometry to investigate paraffin waxes. The characteristics of their samples are listed in Table 1-25. Sample A
1n-Alkanes
IIso-alkanes
35 30 25 20 15 10
5
1
35
$ 30-
.t; 25 2 20+ c 15.-
60 %- 65 ' l o head product
r
10 -
85 %- 90 '10
head product
10
10 -
50 15
I
20
I
25
30
35
40
45
Carbon atom number
Fig. 1-16. Distribution of n-alkanes and iso-alkanes in the fractions of the paraffin wax
marked A
4
50
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 2-25. Characteristicsand chemical composition of various paraffin waxes Characteristics, composition
I
Melting point, OC Oil content, wt- % n-Alkanes, wt- % Iso-alkanes, wt- % Cycloalkanes, wt- % Alkylbenzenes, wt-
57.8 -
98.1 1.5
0.4
53.4 0.7 86.1 6.3 7.1 0.5
53.2 0.8 76.4 13.4 10.2 -
51.8 0.1 82.9 7.7 9.4 -
53.7 0.4 91.4 5.6 3.0
54.0 0.3 81.4 10.1 8.5
-
54.4 0.3 92.3 3.5 4.0 0.2
-
60.9 0.2 89.2 5.8 5.0
-
51.0 -
25.0 21.0 54.0 -
was a product obtained by repeated crystallization from a commercial paraffin wax de-oiled by sweating. Samples B, D, N and P were usual commercial grades. Samples E, F and H were obtained from slack waxes by laboratory-scale sweating. Sample J was prepared by a combination of various separations and enriching operations. First, a narrow boiling-point range fraction (b.p. 232°C a t 80 Pa) was obtained by vacuum distillation of a blend of commercial paraffin waxes. The cycloalkane content of this fraction was further enriched by thermodiffusion. The sample J was the bottom product of the thermodiffusion column, obtained with a yield of 7 wt-%. The data in the table and mass spectrometry analyses allowed the following conclusions :
0
n-Alkanes
I
Iso-alkanes ._
-
20% - 2 5 "lo
head product
head product
10 -
Carbon atom number
Fig. 2-17. Distribution of n-alkanes and iso-alkanes in the fractions of the paraffin wax
marked D
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
.-5
25 20 15 10 5
d, 10 -C 5
$
-
t.
0 5 15 --
0 -
51
IIso-alkanes
n-Alkanes
20 % - 25 "lo head product
-. n
I r ~
I
I
60 ' l o - 65 '10 head product
-
0 15
5 101 " 5 0
10 5Carbon atom number
Fig. 1-18. Distribution of n-alkanes and iso-alkanes in the fractions of the paraffin wax
marked F
(i) In all macrocrystalline paraffin waxes with melting points between 53 and 61 "C,the majority of n-alkane molecules are in the C,, to Gorange. It is interesting to note that in spite of the only 1 "Cdifference between the melting points of samples B and D, on the one hand, and sample N , on the other, the share of this fraction is 68 wt- % for the former two samples, in contrast to 86 wt- % for sample N , while the C,,-C,, n-alkane content in sample N is substantially lower than in samples B and D. (ii) The melting point of sample P is substantially higher than that of samples B, D and N . This is due, as shown by mass spectrometric data, to a share of 26 wt- % of C,l-C,, n-alkanes in sample P,as compared to 3-4 % in the other three samples. Sample P also contains 3 wt-% C,,-C,, n-alkanes, and its iso-alkane components, making up 5.8 wt- %, are >C,, compounds. (iii) Sample A is characterized by the finding that its n-alkane share of 98 wt-% is made up almost entirely of C,,-C,, compounds. This is presumably due to the repeated crystallization. The melting point of 58 "C corresponds to this composition. (iv) The melting points of samples E, F and H obtained by laboratory-scale sweating are also in conformity with the distribution by carbon atom number. (v) The melting point and composition of sample J clearly demonstrates that a relatively high melting point can also be reached, even with a low content of n-alkanes, if the iso-alkane and cycloalkane components present in larger amounts are suitable carbon atom number compounds. 4'
52
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Different paraffin wax grades are manufactured by de-oiling of slack waxes and petrolates. It is, therefore, of interest to give a short characterization of the composition of slack waxes. This subject was studied in detail by Kajdas et al., among others. Table 1-26 summarizes their results concerning the composition Table 1-26. Composition of three slack waxes Content, wt-%
Hydrocarbon group
Sample B
[
Sample c
Methods of analysis
I Sample D
+ cycloalkanes
70.7 20.0
33.4 43.5
12.0 48.5
Alkyl-substituted monocyclic aromatics
Adduct formation with urea Deduction of n-alkane content from total alkanes
6.0
16.0
33.5
Alkyl-substituted polycyclic aromatics
Column adsorption on silica gel. Eluent: hexane
2.5
4.0
2.0
Olefins Resins
0.3 0.5
0.6 2.5
1.5 2.5
Column adsorption on silica gel. Eluent : benzene Iodine number Calculated from difference
n-Alkanes Iso-alkanes
~
of three different slack waxes. Sample B was obtained from spindle oil, sample C from lubricating oil, and sample D from residual oil. The slack waxes were separated by a combination of different methods (adduct formation with urea, chromatography, various chemical methods). It may be seen from the data that with increasing average boiling point of the distillate, i.e. average molecular weight of the slack wax, the n-alkane content significantly decreases, while the share of isoand cycloalkanes and alkylated aromatics substantially increases. The composition of a slack wax obtained from a so-called ,,neutral oil 11” fraction of a Polish crude was determined by means of the procedures shown in Fig. 1-19. The results are summarized in Table 1-27. The fractions were analysed by IR spectrometry, NMR, mass spectrometry and UV spectrometry. Table 1-27. Composition of slack wax from neutral oil I1 Hydrocarbons
n-Alkanes Naphthenes with few side chains Iso-alkanes (slightly branched) Naphthenes with many side chains Iso-alkanes (very branched) Naphthenics-aromatics Polycyclic aromatics and resins
I
Carbon atom range
Mark
16 13 9 33 6 17 6
2 1-3 3-4 7-8
-
4-5
-
53
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
r
Extractive crystallization with urea
e Adduct - forming part
Part forming no adduct
Adduct-forming part
b
r
= I
Part forming no adduct
Vacuum distillation
1 10 fractions
I
Chromatographic separation on silica gel
n - Alkanes P
Naphthenes and
Naphthenes and
iso-alkanes
(10 fractions)
(10 fractions)
B
C
D
.
Fig. I-19. Diagram of the separation of slack wax
The hydrocarbon composition and distribution by carbon atom number of group B (Fig. 1-19, Table 1-27) is presented in Fig. 1-20. By further distillation of group C , three different fractions were prepared with boiling ranges of 426-446, 484-492 and 519-557 "C. The IR and NMR spectra of these fractions are very similar, and hence the differences in their structures are presumably explainable from the length of the side chains of the naphthenic rings. An average molecule of the above three fractions consists presumably of one isolated or condensed ring, seven CH, groups, and the longest side chain does not contain more than 20 carbon atoms.
54
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
n Fig. 1-20. Hydrocarbon composition of fraction B obtained from the slack wax from neutral oil 11
3. Chemical properties of individual alkanes and their mixtures Alkanes are neutral compounds fairly resistant to the action of chemicals. This behaviour is reflected in their earlier name paraffins (parum affinis = of little affinity). A more detailed study of the chemical properties of alkanes, however, led to the recognition that they are far from being totally inert to chemicals. Many examples of the radical-type homolytic activation of the C-H bond in alkanes are known. Their most important reactions are summarized in Table 1-28. (a) The reactions of paraffins with halogens Direct halogenation of alkanes takes place according to the following substitution reaction : C"H2, + 2
+&
-+
CnH,,
4.1
- x + HX
This reaction can be considered above all for preparing fluoro-, chloro- and bromoderivatives. Iodine cannot be introduced by a direct substitution reaction, however, this process takes place only a t higher temperatures. The HJ formed in the reaction will dehalogenate, in a reduction reaction, the alkyl iodide formed. Hence, the reaction is reversible and the equilibrium is shifted towards the left-hand side of the above general equation. Therefore, direct substitution with iodine is feasible only in the presence of certain additives (e.g., nitric acid, mercury(I1) oxide, silver
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
55
Table 1-28. Chemical reactions of alkanes No.
~
I
Agent
Process
Product
Halogens Sulfuryl chloride (+dibenzoyl peroxide)
Light-catalysed substitution
Saturated halogensubstituted derivative
Phosgene or oxalyl chloride
Light-catalysed substitution (chlorocarbonylation)
Chloride of carboxylic acid
Sulfur trioxide Sulfur dioxide 4-chlorine Sulfur dioxide oxygen
Substitution
Alkanesulfonic acid, Alkanesulfonic chloride
Nitric acid or nitrogen dioxide
Substitution
Nitroalkane
Oxidizing agents
Oxidation
Alcohol, aldehyde, ketone, carboxylic acid
Chain splitting, Catalytic isomerization, Dehydrogenation
Short-chain hydrocarbons, Branched alkane isomer, Unsaturated hydrocarbons
+
High temperature
perchlorate) that bind or decompose, by oxidation, the hydrogen iodide formed and thereby render the process irreversible. In contrast to iodine, fluorine reacts with explosive violence, resulting in chain splitting of the alkane which will then yield carbon tetrafluoride and hydrogen fluoride. The heat of reaction of the fluorination process is as high as 419 kJ/mol, as compared to the 101-109 kJ/iiiol for chlorination. Therefore, direct fluorination can only be carried out in the presence of retarding catalysts, e.g. silver-plated copper chips, or by using fluorine gas diluted with nitrogen. In such retarded reactions no chain splitting will occur. However, no control of the reaction to obtain monovalent alkyl fluorides is possible, and a mixture of multiple-fluorinated hydrocarbons will be formed. Fluorine derivatives are manufactured by indirect methods, by exchanging chlorine against fluorine in chlorine derivatives. Chlorine and bromine are capable of direct substitution reactions with saturated hydrocarbons. Chlorine reacts with alkanes directly at ambient temperature. With increasing temperature, the rate of the reaction increases. The process can also be accelerated by catalysts or light. Photocatalytic chlorination is a radical chain reaction proceeding according to the following stages : C1, + hv -+ 2 C1* R-H + C1* + R* + HCl R*
+ C1,
-+
R-Cl
+ C1*
56
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
The first reaction step is the absorption of light energy by the chlorine molecule, resulting in the formation of excited chlorine atoms. These immediately react with alkanes, yielding alkyl free radicals. In the next step, the free radicals react with chlorine molecules, and alkyl chlorides or polychlorinated alkanes substituted at different sites will be formed, as well as excited chlorine atoms which maintain the chain reaction. The chain reaction can be broken off as a result of several reactions. The most important reactions are: recombination of excited chlorine atoms on the wall of the equipment (wall effect); reaction of alkyl-free radicals with excited chlorine atoms (instead of chlorine molecules); if oxygen is present, reaction of oxygen with alkyl-free radicals, yielding peroxides (subsequently reacting with one another or with further alkyl-free radicals and thereby forming stable products); finally the reaction of the excited chlorine atoms with oxygen, yielding chlorine dioxide. Irradiation with light in the wavelength range of 2500 to 4000 A, corresponding to the absorption range of chlorine, is used in photochemical chlorination. The energy of such radiation amply provides for the activation energy of the dissociation of chlorine molecules (239 kJ/mol). Photochemical chlorination is mainly of importance in the chlorination of liquid paraffins. In this process, chlorination can be carried out at low temperatures (30-50 "C), since the rate of the reaction depends only sIightIy on temperature. For example, normal dodecane is readily chlorinated already at 30 "C with a chlorine consumption of about 1.7 litres/kg s. Paraffin waxes can also be chlorinated by the photochemical process. Parallel to the higher melting point, chlorination is carried out at somewhat higher temperatures, around 7G90 "C. Paraffin waxes melting below 75 "C are usually chlorinated in the melt, those with higher melting points in solution, using chlorinated hydrocarbons, e.g. carbon tetrachloride, as solvent. Depending on the nature of the paraffin wax and conditions, several products, mono-, di-, tri- and polysubstituted alkanes, are formed simultaneously :
-
+
+ +
C,H,, + ,C1 C1, + C,H,,Cl, HCI C1, .+ C,H,, - C1, HCI etc. C,H,, C1, In the chlorination of alkanes one must reckon with the formation of all theoretically possible chloroalkane isomers. The ratios of the isomers are defined by the number of different-order hydrogen atoms and by the relative rate of reaction of the hydrogen atoms. In liquid-phase chlorination at about 30 "C, the relative rate of reaction of primary, secondary and tertiary hydrogen atoms is approximately 1 :3.25 : 4.43. These proportions are only slightly changed by catalysts or by light irradiation. The values depend above all on the temperature and pressure of the chlorination process. With increasing temperature the difference between the rates decreases. The number of mono- and polysubstituted products of saturated hydrocarbons, also taking into account isomerization, rapidly increases with increasing numbers of carbon atoms in the molecule.
+
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
57
In addition to photocatalytic chlorination, homogeneous catalytic chlorination is also known. This process can be applied both to liquid paraffins and to paraffin waxes. In liquid-phase chlorination, usually catalysts readily soluble in liquid hydrocarbons or in their mixtures with chlorinated hydrocarbons, e.g. carbon tetrachloride, are used. In these processes, chlorine will readily dissociate into active chlorine ions. The most important catalysts are chlorides of iodine, phosphorus, sulfur, antimony, iron and tin. Liquid paraffins are usually chlorinated in the presence of dissolved iodine or phosphorus pentachloride. Bolley was the first, in 1858, to chlorinate paraffin waxes with the cited methods. He obtained a viscous oil-like liquid at ambient temperature. Upon continuing chlorination up to a chlorine content of about 62 wt-%, an amorphous product was formed, which he termed chloroparaffin. In addition to homogeneous catalytic chlorination, heterogeneous catalytic chlorination processes carried out in the gas phase are also known. Among heterogeneous catalysts are copper chloride and iron, usually applied on alumina and silica gel supports. These processes have no great importance for alkane chlorination, since the activity of the catalysts decreases, especially due to coke formation. In thermal chlorination neither catalysts nor light irradiation are used. In this process excited chlorine atoms are formed by the thermal dissociation of chlorine molecules. In the first step, the chlorine radicals attack the C- H bonds and start a chain reaction. Correspondingly, the activation energy is substantially higher, 83.7 kJ/mol, as compared to 50.3 kJ/mol in catalytic chlorination. Thermal chlorination is a chain reaction strongly inhibited by the presence of oxygen. In thermal chlorination, depending on reaction conditions, unsaturated hydrocarbons are also formed besides chloroalkanes. On the other hand, the formation of hydrogen chloride by dehydrochlorination of alkyl chlorides does not take place, or only to a slight extent. Thermal chlorination is used chiefly for lower molecular weight hydrocarbons, but higher molecular weight paraffin hydrocarbons are also chlorinated by this process, at temperatures between 70 and 100 "C, to the desired chlorine content. In fact, even polyethylene can be chlorinated in this manner. Alkanes react slowly with phosgene or oxalyl chloride, yielding the chlorides of the corresponding carboxylic acids according to the following equations: R-H
+ COCl,
R-H+
COCl
1
COCl
hv
R-COCl
+ HCl
R-COCl
+ HCI
or peroxide hv
-co
58
1. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
(6) Sulfochlorination of alkanes
The first mention of the simultaneous reaction of sulfur dioxide and chlorine with liquid alkanes was made in the patent "Procedure for the Halogenation of Hydrocarbons" issued by Cortes F. Reed and Charles Horn in 1936. In this reaction, alkyl sulfochlorides are formed : R-H
+ SO2 + C12
R-SOZ-CI
+ HCl
In addition to the simultaneous introduction of sulfur dioxide and chlorine into the molecule, chlorination without the participation of sulfur can also take place, and subsequently these chlorinated molecules can undergo sulfochlorination. Generally in substitution reactions, di- or polysulfochlorination also takes place, so that finally a mixture of mono-, di- and polysulfochlorides, alkyl chlorides, chloromono- and disulfochIorides and unreacted alkanes is obtained. Similarly to chlorination, sulfochlorination is also a chain reaction :
+ hv Cl* + C1* R-H + C1* -+ R* + HCl R* + SO, -+ R- SOB R-SO,* + C1, -+ R-SOz-Cl + C1* C1,
-+
At identical intensities of irradiation, shorter wave-length light will result in a lower chlorine to sulfur ratio in the products. The chlorination of the hydrocarbon can be avoided by using UV light. Recently some organic catalysts were discovered which are capable of starting and maintaining the sulfochlorination reaction without requiring light. For instance, diazomethane, lead tetraethyl, triphenyl-methylmethane, acetone peroxide, dialkyl peroxides and benzoyl peroxide are such radical-forming substances; their catalytic effect being due to their reaction with chlorine molecules yielding alkylfree radicals and excited chlorine atoms. To suppress chlorination of the alkane chain it is preferable to operate with excess sulfur dioxide pf about 100 mol- % (higher excesses are unfavourable). The formation of polysulfochlorides can be reduced by only partial sulfochlorination. Under optimum conditions, sulfochlorination to 50 wt- % yields 85 wt- % mono- and 15 wt- % disulfochloride, as compared to full sulfochlorination when 60 wt- % mono- and 40 wt- % disulfochlorides are formed. When alkanes are reacted with a 3 : 1 mixture of sulfur dioxide and chlorine, the main product is alkyl monosulfochloride, with minor amounts of polysulfochlorides, alkyl chlorides and chloroalkyl sulfochlorides as side products. Isomeric monosulfochlorides can also be formed in sulfochlorination. Sulfochloride groups are only bound to primary and secondary carbon atoms, the hydrogen atoms bound to tertiary carbon atoms do not enter into reaction. However, the latter are sensitive to chlorination. Therefore, the sulfochlorination
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
59
of compounds containing tertiary carbon atoms results in substantial chlorination of the carbon chain. In higher molecular weight hydrocarbons, the sulfochloride groups are distributed evenly along the methylene groups. Aliphatic sulfochlorides are not heat-resistant : when heated, sulfur dioxide is split off and alkyl chlorides are formed. Such desulfonation reactions take place readily with higher molecular weight aliphatic sulfochlorides. However, in contrast to sulfochlorides, aliphatic sulfofluorides obtained from sulfochlorides with potassium fluoride solution are thermally stable.
(c) Reactiondliquid parafins and parafin waxes with sulfur dioxide, sul&&.rnQkide, sulfuric acid and .fuming suljiuric acid (oleurn) The term sulfonntion designates the chemical reaction in which a bond is 0
I/
formed between the-S=O
I
group and a carbon or nitrogen atom of an organic
OH cornpound.6ulfonic acid groups and sulfonic acids must not be mixed up with the esters of sulfuric acid, that is, with alkyl sulfates having the general formula R-0-SO,-OH, like e.g. ethyl sulfate C,H,-0-SO,-OH, in which the -SO,H group is bound to an oxygen atom and not directly to a carbon or nitrogen atom as in sulfonic acids. In aliphatic hydrocarbons three sulfonic groups can be bound to one and the same carbon atom. Direct sulfonation is usually carried out with concentrated sulfuric acid, oleum, mixtures of sulfur dioxide and chlorine and mixtures of sulfur dioxide and oxygen. At ambient temperature concentrated sulfuric acid, and even fuming sulfuric acid containing less than 15 wt-% sulfur trioxide, does not react with alkanes. Studies by Engler and Hofer showed that in particular medium and high carbon atom number branched alkanes yield alkylsulfonic acids with hot concentrated sulfuric acid according to the following reaction : C,H,,
+2
+ HOSO2OH
+ C,H,,
+ 1-
S 0 2 0 H + H,O
Pure sulfur trioxide, in the liquid or gaseous state, also yields sulfonic acids with alkanes under appropriate conditions. In this case the reaction is essentially an addition: R-H SO, + R-SOaH
+
The dehydrating effect of sulfur trioxide is so vigorous that it carbonizes organic compounds when added directly. Therefore, sulfonation with sulfur trioxide, free of such side reactions, is feasible only at lower temperatures and in the presence of solvents or sulfuric acid.
60
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
With mixtures of sulfur dioxide and oxygen, alkanes yield aliphatic sulfonic acids according to the following reaction : R-H
+ SO, + 1/20,-
light
R-SO,-OH
This sulfoxidation takes place in the presence of UV light or various catalysts, e.g. chlorides or anhydrides of acids, peroxides, peracids. Sulfoxidation of higher alkanes proceeds only under permanent irradiation or the addition of anorganic catalyst. The sulfo group is statistically distributed along the carbon chain. Sulfonic acids with chain-end sulfo groups will be the scarcer the longer the alkane molecule is. In the sulfoxidation of higher alkanes in the presence of acetic anhydride, it is assumed that a mixed anhydride of sulfonperacid and acetic acid is formed, which subsequently reacts with the water present and is transformed into sulfonic acid : R-H
+ (CH3CO),O 0 - 0 - COCH, + HZO
+ SO, +
R- SO,-
0 2
+
R-SO2-O-O-COCH3
R- SOZOH
i
+ CH,COOH
+ CHSCOOH, + 1/2 0 2
(d) Reaction of liquid parafins and parafin waxes with nitric acid Concentrated nitric acid reacts only to a slight extent with alkanes, even at
100 "C. Konovalov found that alkanes can readily be nitrated with dilute nitric acid (13 wt-%) at 130 to 150 "C under pressure, that is, heated in a closed vessel, and yield nitroalkanes :
C,H2,
+2
+ HONO,
-+
C,H,,
+ 1-
NO,
+ HZO
He stated that the reaction is a radical chain reaction accelerated by radicalforming substances, e.g. lead tetraethyl, and retarded by substances inhibiting the chain reaction, e.g. nitrogen monoxide. The nitration of alkanes is accompanied by oxidation, chain splitting and degradation. Alkanes, above all n-alkanes react only slowly with the so-called nitrating mixture, a mixture of concentrated nitric acid and concentrated sulfuric acid. This mixture, eminently suitable for the nitration of aromatic hydrocarbons, when applied to alkanes, results in rapid hydrolysis, by the hot sulfuric acid, of primary nitroalkanes, and conversion of secondary and tertiary nitroalkanes into brown, tar-like products. Nitro compounds are classified by the order of the carbon atom to which they are bound. Correspondingly, primary, secondary and tertiary nitro compounds exist. Iso-alkanes containing tertiary carbon atoms are readily oxidized by hot fuming nitric acid. In this process, called oxidative degradation, carbon dioxide and carboxylic acids with one less carbon atom are formed. Markovnikov observed that hydrogen atoms bound to primary carbon atoms are the most resistant to substitution by nitro groups, hydrogen atoms at secondary
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
61
carbon atoms will enter more readily into reaction, and hydrogen atoms at tertiary carbon atoms react at the highest rate. The most important experimental results of the liquid-phase nitration of alkanes with nitric acid are as follows: (i) The rate of reaction is low, conversion increases With temperature. (ii) Nitration is accompanied by oxidation, and polynitro compounds are also formed, presumably because nitroalkanes are readily soluble, nitratable and hydrolysable. Higher molecular weight alkanes can readily be nitrated in the melt with nitric acid vapour, at relatively low temperatures. At high ratios of nitric acid to alkane, polynitro compounds and fatty acids are primarily formed. When the ratio is reduced to 1 :2, only about 40 wt-% of the alkanes react and the main product consists of mononitro derivatives. The mixtures of alkanes can be nitrated with nitric acid vapour in liquid phase, when their initial boiling point is at least 160-170°C. Alkanes of 7-12 carbon atom numbers cannot be subjected to nitration with nitric acid vapour in liquid phase because of their low boiling points. On the other hand, in the case of gasphase nitration of these alkanes the risk of pyrolysis is present. These alkanes can be nitrated under pressure, at 160-170 O C , with dinitrogen tetroxide.
Table 1-29. Some reactions taking place in the alkane oxidation process
R2/ H \
R1\c/o-o~
">,,-ORz
Rl
R,
R 'z
'CH-0-+
OH
)CH2+R1>
'><:-'3
R
0
R,-C&H
0
0 R-C
+ OH- +
R-C(
-P
+
-
dkyl group
R4
9-
+ Rz-
0 R-C
0
-+ R-CfOOH + R-<
Now: R
R,
+
0 HO) C H - R + 2 R - C 4
0-0
0
\OH
62
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
(e) Oxidation of liquid parafins and parafin waxes
Liquid paraffins and paraffin waxes do not react with oxygen at ambient temperature. The oxidation process begins to a significant extent only at temperatures of 80 to 100 "C. The oxidation of alkanes is a radical chain reaction. R* radicals formed by the effect of light, initiators, etc. react with oxygen to form active peroxide radicals R-00*. The reaction of the latter with alkane molecules R - H results in the formation of alkane hydroperoxide R-OOH and alkyl free radicals R* maintaining the chain reaction. Alkyl-free radicals, peroxide radicals and hydroperoxide are of the greatest significance in the progress of the oxidation process. Some examples of the manifold reactions taking place in the course of the oxidation of alkanes are presented in Table 1-29. Obviously, the number of products is very high, and includes esters, ketones, aldehydes, alcohols, acids, water and hydrogen peroxide. Since all radicals are capable of reacting with all compounds in the reaction product, lactones, hydroxyacids, dicarboxylic acids, etc. can also be formed. It should, however, be stressed that the mechanism of alkane oxidation is not yet fully clarified. Different authors assume various intermediate reactions within the oxidation process. Only some examples shall be mentioned. For example, Rieche maintains that the starting reaction of oxidation is the formation of alkyl hydroperoxides, involving molecular oxygen, according to the following:
R1CH2CH2CH2R2+ 0, -+ RlCH2- CH -CH, - R2
I
0
I
0
I
H The subsequent step is the rearrangement of the alkyl hydroperoxide into the semi-acetal, which is then converted into the aldehyde and alcohol :
RICH, - CH - CH, - R,
I
+
RICH, - CH - 0 - CH2R2
0
OH
I
0
I
H 0 +
1
/I
R,CH,-C-H
+ R2CH2OH
+
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
63
The aldehyde formed in the above reaction yields the peracid with oxygen:
0
0
II
I1
+0 2
R-C-H
-+
R-C-0-OH
The peracid forms a very labile adduct with the aldehyde:
0
0
II
0
+ R-C-H
R-C-0-OH
H
I1
I/
-+ R - C - 0 - 0 - C - R
I
I
OH This adduct is transformed, in the presence of water, into two molecules of fatty acid : 0 H
II
R-C-0-0-C-R
I
I
-+
2R-COOH
OH If no water is present, two molecules of the adduct yield one molecule of the fatty acid anhydride and two molecules of the fatty acid:
0
H
II
I
2R-C-0-0-C-R
-P
I
II
0
OH
+ 2R-COOH + HSO
R-C-0-C-R
I1
0
The primary alcohol formed parallel with the aldehyde reacts with molecular oxygen. The primary product is aldehyde peroxide hydrate, which is rearranged into the ortho-acid, immediately dehydrated to the carboxylic acid :
H
I
R-C-H
I
OH
H
+ 0,
-+
I
R-C-0-OH
I
OH
OH
I
+ R-C-OH
I
-+
RCOOH f H2O
OH
According to Langenbeck and Pritzkow, alcohols, in the course of alkane oxidation, are not oxidized to the corresponding fatty acid homologue, but because oxygen first attacks the hydrocarbon chain, lower fatty acids and hydroxycarboxylic acids are formed. These authors, along with many others, assume that the first stage of the oxidation is the formation of hydroperoxides. In the second stage, however, the secondary hydroperoxides decompose, yielding ketones and alcohols:
64
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
main reaction R1- CH, - CH - CH2 - R2 I \ I side reaction OOH
R1-
CH, - CO - CH2 - Rz
L
Rl-CHz-CH-CH2-R2
I
+ H,O
+ 1/20?
OH The ketone formed as main product is further oxidized, yielding alpha-ketohydroperoxide, which, in the following stage, is rearranged into aldehyde and fatty acid :
Rl-CHZ-CO-CH2-R2
+
0 2
+ R,-CH-CO-CHZ-RZ
I
-+
0
I
+
R1-CHO
+ R2 - CHZ-COOH
OH
Since substantial amounts of esters are also found in the oxidation products, Langenbeck assumed that the esters are formed according to the Bayer-Villiger reaction, from peracids and ketones : 0
I1
R-C-0-OH
+ R1-CO-CH7-RZ
+ RCOOH
+ R,-COO-CH,-R,
Alpha-ketohydroperoxides can, in part, be transformed into diketones, the latter reacting, according to Karrer and Schneider, with peracids, and yielding fatty acid anhydrides and fatty acids: RI-CH-CO-R,
I
--+
R1-CO-CO-R2
+ HZO
0
I
OH R1-CO-CO-Rz
+ R3-COO-OH
+ R1-CO-0-CO-R2
+ R3COOH
These anhydrides form esters with compounds containing hydroxyl groups. In the case of n-alkanes, the rate of oxidation increases with chain length, since the C-H bond energy in -CH,, =CH2 and -CH groups decreases in the ratio of 1 :4 :76, and consequently the effect of -CH2- groups becomes more and more important as the chain length increases. The relative oxidation rates of some n-alkanes are presented in Table 1-30. Another important feature is the higher rate of oxidation of n-alkanes than that of their branched isomers. The relative oxidation rates of n-hexane and of its isomers are listed in Table 1-31. Primary and secondary hydroperoxides are formed more reluctantly and decompose more readily than tertiary hydroperoxides. Tertiary hydrogen reacts relatively readily and rapidly, and the tertiary hydro-
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 1-30. Relative oxidation rates of some n-alkanes 0,(mol) Oxidation rate = alkanes (mol) timi)
,
Hydrocarbons
n-Pentane n-Hexane n-Octane n-Decane
I
oxiZE;ate
1 .o 7.5 200 1380
(
65
Table I-31. Relative oxidation rate of hexane isomers 0 1 (moo Oxidation rate = alkane (mol) time Hydrocarbons
2,3-Dimethylbutane 2,ZDimethylbutane 3-Methylpentane 2-Methylpentane n-Hexane
I
-
Relative oxidation rate
1 12 60 560 1580
peroxides are stable reaction products. Thus, the rate of reformation of the chain transfer radicals is greater in the case of primary and secondary hydroperoxides, and hence the rate of reaction is higher. Salts of multivalent metals, e.g. manganese, cobalt, iron, copper, and radicalforming substances, such as hydrogen bromide, manganese(J1) bromide etc., accelerate the oxidation reactions. Oxidation processes of alkanes can be changed by using so-called modifiers. These are of particular interest in the oxidation of higher alkane mixtures. By using boric acids as modifiers, the ketone to alcohol ratio can be shifted towards the latter in the reaction product. Apparently the effect of boric acids consists not only in their protecting the alcohols against esterification, but also in the reaction of boric acids and their anhydrides, with R-00* radical;, yielding, via intermediate products, alcohols and esters, respectively.
(f)Thermal decomposition and isomerization of alkanes At temperatures above 350 "C,alkanes suffer thermal decomposition. The two major reactions in this process are dehydrogenation and chain splitting :
Chain splitting is thermodynamicallymore feasible than dehydrogenation. According to Haber's rule, the lower molecular weight hydrocarbon formed in chain splitting is the alkane, the higher molecular weight hydrocarbon the alkene (olefin). In the course of thermal decomposition, isomerization reactions also take place as side reactions. Alkylation of alkanes and aromatics formed by the thermal decomposition of olefins, cyclization of alkanes and olefins, further decomposition of olefins formed in primary reactions and polymerization of olefins are also involved. The products of the thermal decomposition that are liquid at ambient temperature, contain in addition to diolefins, naphthenes, and aromatic compounds al5
66
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
kanes and olefins with lower carbon atom numbers than those of the starting alkanes, too. Depending on the reaction conditions, a solid by-product, called petroleum coke, is also formed in greater or lesser amounts. The gaseous products, in quantities depending on the conditions, are hydrogen, methane, ethane, propane, butane, ethylene, acetylene, propylene and but ylene. The major factors affecting the thermal decomposition of alkanes are temperature, time of reaction, and, to a lesser degree, pressure. The temperature of thermal decomposition not only affects the rate of reaction, but also determines the thermodynamically possible processes. With increasing temperature the yield of gaseous reaction products, but also coke formation, increases. Polymerization side-reactionstake place at normal pressure up to about 400 "C. The formation of diolefins and aromatics starts above 600 "C. Increased reaction-time promotes secondary processes, so that the formation of low molecular weight olefins, of coke and of hydrogen then increases. Increased pressure promotes reactions involving volume reduction :it suppresses dehydrogenation, but does not affect chain splitting. Higher pressures are favourable to secondary polymerization and condensation reactions, the share of liquid reaction products increases and the yield of gaseous olefins is reduced. With increasing pressure, the rate of reaction also increases up to rates exceeding the rate corresponding to atmospheric pressure by a factor of 20 to 40. -In conformity to the reactivity of alkanes, the rate of reaction of thermal decomposition increases up to Cl0. However, for >C, alkanes, no difference in activation energy or rate constant is observed. Branched alkanes are cracked more readily than n-alkanes. Compounds containing a single methyl group on the secondary C atom behave similarly to n-alkanes, this similarity increases with molecular weight. The further the methyl group from the end of the molecule, the greater the difference between the behaviour of the iso-alkane and the n-alkane. Two methyl groups on the chain already cause significant differences. Among the various theories postulated for the thermal decompositionmechanism of alkanes, the free radical chain mechanism has been widely accepted. According to this theory, free radicals are formed, when C - C or C - H bonds are thermally split : R-CH2-CH,
+ R-CH,
4-CH,
*,
or
CH3
I
f
CH
R-CH-CHZ-CHZ
I
R - CH- CH,-CH,
\
CH3
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
67
The free radicals react with alkanes forming new radicals:
CH,
*
+ R-CH,-CH,-CH,
+ CH,
+ R-eH-CH,-CH,
The new radical is always formed on internal carbon atoms, since the energy of secondary C - H bonds is lower (372 kJ/mol) than that of primary bonds (398 kJ/mol). Secondary free radicals subsequently form double bonds in the /3-position, and these yield a-olefins, primary free radicals are also formed : R- CH, -CH,- CH2- CH, - 6 H - CH,- CH, + R-CH,-CH,-CH,
R-CH,-CH,-CH,
+
+ CH,=CH-CH,-CH:, R-CH, + CH,=CH,
*
*
3
*
Internai rearrangement of long-chain free radicals leads to the formation of internal free radicals :
R- CH, - CHZ-CHZ- CH, - CH, - CH, - CH, * + --f R - CH,- 6 H - CH, - CH, - CH, - CH, - CH3 . Recombination of free radicals also occurs:
R-CH,
*
+ R-CH,
+ R-CHZ-CHZ-R
However, free radicals are incapable of producing branching and cyclization of the alkane chain. The quantities and the chemical compositions of gaseous, liquid and solid products formed by the thermal decomposition of alkanes are changed by the presence of catalysts. Amorphous catalysts consisting of Si02 and A1,0, can be set to achieve the required effect by varying the alumina content between 13 and 75 wt- %. Crystalline zeolites (molecular sieves) have recently gained importance as catalysts for thermal decomposition. In their presence, as compared to thermal cracking without using catalysts, the C, hydrocarbon content of the gaseous product, and the koalkane, cycloalkane and aromatics content of the liquid product significantly increases. The process of catalytic thermal decomposition is based on the formation of so-called carbonium ions. Carbonium ions are hydrocarbon ions with a free positive charge at one carbon atom, e.g. tertiary butyl cation:
CH,
I
CH3-C+
I
CH, By the use of catalysts, carbonium ions are formed in the course of catalytic thermal decomposition, and their reactions yield the further products.
68
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
By treating n-alkanes containing four or more carbon atoms under relatively mild conditions with metal halides and hydrochloric acid, isomerization into branched alkanes is achieved. The reaction is reversible and accompanied by insignificant amounts of cracking, dehydrogenation and polymerization of the olefins formed. If pure alkanes are used as starting material, small amounts of hydrochloric acid, water, and various promotors act as carbonium ion sources. Table 1-32. Isomerization of n-alkanes Conditions and products c.0
Temperature, OC Pressure, kPa Molar ratio hydrogen/alkane in feed Product distribution in wt- % relative to alkane Gas Liquid cracked Isomers : solid liquid Aromatics Unchanged alkane
I
n-Alkanes
c,.
1
c.0
430 3500 64
430 3500 67
420 3500 32
6 7
5 14
4 32
0 34 1 52
9 22 1 49
13 8 1 42
By their action, the isomerization continues as a chain reaction. High molecular weight n-alkanes can also be isomerized in the presence of Friedel-Craft catalysts. Isomerization conditions of various n-alkanes and the most important characteristics of the products are listed in Table 1-32.
Literature Andreas, F., Chem. Tech. Berl., 16, 449 (1964). Asinger, F., Parufins, Chemistry and Technology. Pergamon Press, Oxford (1 968). Asinger-Fell: Erdd, Kohle, 17, 74 (1964). Azizova-Chernozhukov-Kartinin-Grishin: Nef’i Guz, No. 10, 59 (1970). Bailey-Bannerot-Fetterly: Ind. Engng. Chem., 43, 2125 (1951). Bell-Raley-Rust: Discuss. Furuduy SOC.,10, 242 (1951). Bengen, F., Angew. Chem., 63, 207 (1953). Berty, J., Az asurinyolaj kEmiai feldolgozrisa. (Chemical processing of petroleum). Nehtzipari Konyvkiad6, Budapest (1952). Bornemann-Heinze: Chem. Tech. Berl., 20, 99 (1968). Borzsonyi-E. Kantor-Keszthelyi: MAFKIkiadvdny (Report of the Hungarian Oil and Gas Research Institute), No. 311 (1964). Brandes, G . , Brennst. Chem., 37,263 (1956). Buchler-Graves: Znd. Engng. Chem., 19, 718 (1927). Burwell, A. W., Ind. Engng. Chem., 26, 204 (1934). Burger-Combarnous: Revue Znst. r. Pgtrole, 30, 551 (1975). Clark-Smith: Ind. Engng. Chem., 23,697 (1931).
(B) CHEMICAL PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
69
Csik6s-E. KAntor-M6zes-Keszthelyi: MT A KCm. Tud. Oszt. K6zl. (Report of the Department of Chemical Sciences of the Hungarian Academy of Sciences) 25, 134 (1966). Dietsche, W., Fette, Seifen, Anstr-Mittel, 72, 778 (1970). Edwards, R. T., Mod. Packag., 26, No. 6. 123 (1953). - : Ind. Engng. Chem., 47, 2555 (1955). - : Ind. Engng. Chem., 49, 750 (1957). - : Petrol. Refiner, 36, 180 (1957). Eggertsen-Groennings: Anulyf. Chem., 33, 1147 (1961). Emanuel, N. M., The Oxidation of Hydrocarbons in the Liquid Phase. Perganion Press, Oxford (1965). Ernanuel-Maizus-Skibida: Angew. Chem., 81, 91 (1969). Etessam-Sawyer: J. Znst. Petrol., 25, 253 (1939). Ferris-Cowles: Ind. Engng. Chem., 37, 1057 (1945). Finkel-Heinze: Chem. Tech. Berl., 14, 299 (1962). Francis-Young: J . Chem. Soc., 73, 928 (1898). Franke, R., Energie Tech., 25, 388 (1975). Freund-BCithory: Erdd, Kohle, 9 , 237 (1956). Freund-Keszthelyi-Mbzes: Chem. Tech. Berl., 17, 582 (1965). Fuchs-Nettesheim: Erdtil, Kohle, 10, 362 (1957). George-Robertson: J. Znst. Petrol., 32, 382, 400 (1946). - : Proc. R. SOC.,185, 288 (1946). George-Walsh: Trans. Faraduy SOC.,42,94, 210, 217 (1946). Gip-Heinze: Acta Chim. Hung., 31, 85 (1962). Grodde, K. H., Erdd, Kohle, 3, 61 (1950). Gross-Grodde: 61, Kohle, 38, 419 (1942). Griinberg, M., Seifen-ble-Fette-Wachse, 90, 478 (1964). Gruse, W. A., Chemical Technology of Petroleum. McGraw-Hill, New York (1960). Guseva-Ashkinadze-Leifman: Izv. Akud. Nuuk. SSSR, Ser. Phys., 27, 104 (1963). Hass-McBee: Znd. Engng. Chem., 23, 352 (1931); 27, 1190 (1935); ,28, 333 (1936); 33, 185 (1941). Heberling, R., Freiberger ForschHff., A 201, 21 (1961). Hessler-Meinhardt : Fette, Seifen, 55, 441 (1953). Hildebrand-Teubel-Peper-Dahlke: Chem. Tech. Berl., 15,482 (1963). Hunter-Segester: US.Pat. 2 670 323 (1954). Isrnaylov-Terteryan: Khimiya Tekhnol. Topl. Musel, 20, No. 12, 6 (1975). Kaiser, R., Fette, Seifen, Anstr-Mittel, 60, 915 (1958). Kajdas, C., Seifen-Ole-Fette- Wachse, 96, 251 (1970). Kajdas-Tiimmler-Berthold: Erdd, Kohle, 23, 663 (1970). Kdntor, E., MdFKZ k6zlemdnyek (Report of the Hungarian Oil and Gas Research Institute), No. 274 (1963). Kharasch-Berkman: J. 01-gChem., 6, 810 (1941). Kisielow-Kajdas: Seifen-Ole-Fette- Wachse, 93, 719 (1967). Krasavchenko-Zemskova-Mikhnovskaya:Neftekhimiya, 11,803 (1971). Krasnova-Malnev-Putshkovskaya: Zzv. Akad. Nauk. SSSR,Ser. Phys., 27, 98 (1963). Kreuder, W., Fette, Seifen, 84, 665, 699, 735, 773, 849 (1958). - : Fette, Seifen, 85, 19, 41, 93 (1959). Larson-Becker: Anulyt. Chem., 32, 1215 (1960). Leibnitz-Hager-Heinze: J. Prukt. Chem., 4. Reihe, 3 , Heft 1-2 (1956). Leibnitz-Hager-Herrnann: J. Prukt. Chem., 4. Reihe, 5, Heft 1-2 (1957). Levy-Doyle-Brown-Melpolder: Analyt. Chem., 33, 698 (1961). Lomrnerzheim, W., Erdd, Kohle, 7,212 (1954). Lund, H. A., Petrol. Process., 7, 326 (1952). McCleary-Degering: Znd. Engng. Chem., 30, 64 (1938). McLaren, F. H., TAPPZ. Bull., 34, 462 (1961).
70
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Mikhaylov-Lulova: Khimiya Tekhnol. Topl. Masel, 20, No. 2, 15 (1975) Mikhaylov-Polyakova-Khmelnitsky : Khimiya Tekhnol. Topl. Masel, 10, No. 8 (1965). Minchin, S. T., J . Inst. Petrol., 34, 541 (1948). Moos-Haas: Erdd, Kohle, 1, 29 (1948). Nogare-Bennett: Analyt. Chem., 30, 1157 (1958) O’Connor-Norris: Analyt. Chem., 32, 701 (1960). Ogilvie-Simmons-Hinds: Analyt. Chem., 30, 25 (1958). O’Neal-Weir: Analyt. Chem., 23, 830 (1951). Padgett, F. W., Oil Gas J., 36, No. 38, 30, 45 (1938). Pavlova-Driyatskaya-Mokhtsiyan: Khimiya Tekhnol. Topl. Masel, 7, No. 3, 58 (1962). Philips, J., Petrol. Refiner, 38, 193 (1959). Postnov-Gafarova-Serikov: Khimiya Tekhnof. Topl. Masel, 17, No. 4, 15 (1972). Postnov-Lulova-Leonteva-Fedoszova: Neftepererab. Neftekhim., No. 2. 11 (1972). Reutner, F., Fette, Seifen, Anstr-Mittel, 70, 162 (1968). Rosner-Teubel: Chem. Tech. Berl., 15, 662 (1963). Schlenk, W., Justus Liebigs Annln. Chem., 565, 204 (1949). - : Fortschr. Chem. Forsch., 2 , 92 (1951). Stewart, R., Oxidation Mechanisms. Benjamin Inc., New York, Amsterdam (1964). Stull, D. R., Ind. Engng. Chem., 39, 518 (1947). Sucker, H., Fette, Seifen, Anstr-Mittel, 70, 849 (1968). Terres-Fischer-Sasse: Brennst. Chem., 31, 193 (1950). Terres-Brinkmann-Fischer : Brennst. Chem., 40, 279 (1959). Teubel-Schneider-Schmiedel : Erd6lparafine. VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig (1965). Thorpe, T. C. G., J. Inst. Petrol., 37, No. 330, 275 (1951). Titschack, G., Fefte, Seijen, 67, 23 (1959). Triems-Heinze: Erdcil, Kohle, 18, 695 (1965). Turner-Brown-Harrison: Ind. Engng. Chem., 47, 1219 (1955). Vimos-M6zes-Keszthelyi-E. Kantor : MAhYI kiadvciny (Report of the Hungarian Oil and Gas Research Institute), No. 249 (1962). Vamos-E. Kintor: Erdd, Kohle, 17, 90 922 (1964). Willis, D. E., J . Chromat., 30, 86 (1967). Zhukhovitsky-Selenkina-Turkeltaub: Khimiya Tekhnof. Topl. Masel, 5 No. 11, 57 (1960). Zimmershied-Dinerstein-Weitkamp: Ind. Engng. Chem., 42, 1300 (1950).
(C) Crystal structure of paraffin waxes Paraffin hydrocarbons - whether individual compounds or their mixtures are always crystalline at temperatures below their melting point or melting range. 1. Crystal structure and crystallization
A crystal structure is one in which the constituent atoms or molecules oscillate about the points of a defined crystallographic arrangement, with frequencies and amplitudes depending on temperature. This arrangement, as regards shape, size and steric position, repeats itself continuously within a crystal. Such a structure is called a lattice structure. The shapes and sizes of crystal faces formed in the crystallization process of a given substance - although the crystallographic system remains unchanged -
(C) CRYSTAL. STRUCTURE OF PARAFFIN WAXES
71
differ according to whether crystallization takes place from the melt or from a solution. Well-developed crystals will only be obtained in the latter case. When crystallizing from the melt, the independently developing crystals will come into contact with one another as they grow, and will interfere with further growth in the places of contact. In the free spaces between the crystals, however, crystallization will continue until the whole of the melt becomes crystalline. As a result, the crystal boundaries - independently of the given crystallographic system - will depend on the relative position and mode of growth of the neighbouring crystals. These crystalline units, to distinguish them from crystals having regular faces, are termed crystallites. The crystals of crystalline organic compounds usually display a low order of symmetry. Only a few compounds crystallize in the cubic system, and tetragonal and hexagonal systems are also rare. The majority of organic crystals are rhombic or monoclinic. In organic molecular lattices with single bonds, the valence directions correqxmd to the tetrahedral arrangement. Hence aliphatic compounds containing no double or triple bonds are composed of carbon chains such as those occurring in the three dimensional lattice of diamond. Thus, aliphatic carbon chains may be regarded as derived from the diamond lattice. The distance between two carbon atoms in a single bond, in aliphatic chains containing no multiple bonds, is always 1.54 A, i.e. the same value as in the diamond lattice. In practice, the questions of crystallization and crystal structure are significant above all in the case of paraffin waxes solid at ambient temperatures. With liquid paraffins, crystallization is of no practical importance either in production or in application. In the course of the manufacture and application of paraffin waxes, crystallization almost always takes place from the melt. However, in the first stage of their manufacture - solvent dewaxing and de-oiling - and in fractional crystallization, parafiin max crystals are formed from solution. During the cooling of the melt, crystallization always starts with nucleus formation. The ability for crystallization of a melt is usually characterized numerically by the number of nuclei formed in the unit volume of the melt during unit time. Nucleus formation always starts below the melting point, that is, at a certain degree of supercooling. A further temperature decrease, that is, further increase in supercooling first leads td an increased capability for crystallization up to a maximum value, after which it decreases. This is due to the fact that by increasing supercooling up to a defined imit, the probability of establishing - by diffusion - the atomic or molecular arrangement required for crystallization will also grow. After, however,r eaching this limit, the viscosity of the melt will increase to such an extent that the viscous medium will hinder diffusion and thereby the establishment of such an arrangement. The slope of the ascending portion of the curve - relating to crystallization ability - changes from substance to substance. but decreases for all substances with lower cooling rates, i.e. slower cooling (Fig. 1-21).
12
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
m. P Extent
of supercooling, "C
Fig. 1-21. Effect of the cooling rate on crystallization ability. I
cooling
- rapid cooling;
2
- slow
Crystal nuclei are already tiny crystals. The atoms or molecules fixed in them by lattice forces exert forces of attraction towards the melt and are capable in this way of binding further atoms or molecules from the melt. This enables the tiny crystals to grow. The rate of growth is called rate of crystallization, defined as linear growth of the crystals, in cm/s units. The rate of crystallization is a function of crystallographic directions, temperature of crystallization and extent of supercooling. Nucleation and growth of crystals are parallel processes. The relative rate of these two processes controls the final structure of the crystalline substance. When studying the relative rates of nucleation and crystal growth as a function of supercooling, several systems differing in properties are encountered in practice (Fig. 1-22). In case a, slight supercooling (1) leads to a great number of slowly growing crystals, resulting in a product consisting of small crystals. The contrary is the case with greater supercooling (2), when few, but large crystals are obtained. In case b, slight supercooling (1) and greater supercooling (3) both yield large crystals, while medium supercooling (2) results in small crystals. In case c, slight supercooling (1) results in large crystals, greater supercooling (2) in small crystals. It is a well-known phenomenon - also occurring with paraffin waxes - that if crystal nuclei of the same species as the melt are initially present, crystallization starts in more points, and the crystal texture of the product will be finer. Consequently, by heating such substances above their melting point and subsequently cooling, crystal sizes will increase, and the texture will become coarser. However, not only nuclei of their own species, but foreign nuclei also affect the texture of substances solidifying in crystals. The nature and extent of the effect of such impurities largely varies: in some cases they act as nuclei, while in others they interfere with the growth of the crystals. The crystallites in substances crystallized from the melt are unoriented. Therefore, the anisotropy of crystals does not appear in such so-called quasi-isotropic systems.
73
(C) CRYSTAL STRUCTURE OF PARAFFIN WAXES
In large paraffin wax castings, fineness of texture within a cross-section changes along the radius. On the surface the texture is finer, owing to rapid cooling, than in the middle. Equations of cooling curves for crystallization from the melt can be derived and considered. If, in a given moment 9, the temperature of a system is t "C and ambient temperature is to "C,the cooling rate of the system is y = - -
dt k - - -(t dB mc
- to)
(1-11)
where m is the mass of the system (g), c the mean specific heat of the substance of the system in the temperature range in question (J/kg"C), and k a constant depending on the surface of the system and on the conditions of cooling.
1
(6) Crystal growth
Crystal growth
I
2 Extent of supercooling, "C 1
3 1 Extent of supercooling;
Extent of supercooling, "C
F&. 1-22. Rates of crystal formation and growth
b
O C
74
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
The functional relationship between the temperature of the system being cooled and time is represented by the so-called cooling curves. Their general equation is
t = (ti, - to)re'+
(1-12)
to
where tin is the initial temperature of the system. If the system is crystalline in the solid state (Fig. I-23b), the cooling curve displays a break at the melting point and proceeds horizontally for some time, indicating that the system stays at the temperature of the melting point as long as crystallization continues. When crystallization is complete, temperature again decreases corresponding to the above logarithmic relationship. If supercooling occurs, a local minimum appears on the cooling curve (Fig. I-23c). If the system consists of several constituents having different melting points (Fig. 1-234, the la) Iu L
3
ea,
c
a
E
I-
Time
-
-
Time
(Cl
L3
:
c
a
E
t-
Melting point
1\ Time
c
Time-
Fig. 1-23. Cooling curves of amorphous and crystalline substances. (a) amorphous substance; (b) crystallizing substance; (c) substance crystallizing from the supercooled melt;
( d ) multicomponent crystallizing substance
(C) CRYSlAL STRUCTURE OF PARAFFIN WAXES
75
crystallization process is indicated on the cooling curve by a straight portion forming an angle with the abscissa. In this case, solidification starts at the temperature tl and is completed at the temperature t,. This is the case with macro- and microcrystalline paraffin waxes. Different types of cooling curves are presented in Fig. 1-23. With many crystalline substances, a further change of the crystal structure takes place in a temperature range below the melting point. These changes - termed allotropic transitions - occur at given, so-called equilibrium transition temperatures, provided that cooling is infinitely slow. Allotropic transitions are always displayed on the cooling curve by further breaks, since they too are accompanied by changes in the heat content. It is an important characteristic of allotropic transitions that they represent reversible processes, i.e. they will be repeated during heating when the corresponding equilibrium temperature is reached, provided that heating is infinitely slow. If the cooling or heating rate is finite, the transitions take place at temperatures lower or higher, respectively, than the equilibrium temperature. Therefore, the transition temperature will differ when reached by cooling or by heating, the difference being a function of the rate of temperature change. The condition for crystallization starting from a solution is supersaturation of the solution by cooling or by removal of part of the solvent. In this case also, the process consists of two parts: nucleation and crystal growth. The number of nuclei will be high when cooling is rapid, the solution is intensely stirred during cooling and its purity is high. Crystal growth is controlled by the diffusion of the solute particles, i.e. finally, for example, by the viscosity and density of the solution at the temperature of crystallization. Industrially, crystallization from solution is implemented in two ways : by solvent removal and by cooling. The latter technique is particularly favoured if the solubility of the substance to be crystallized sharply decreases with decreasing temperature. This technology is, therefore, applied in dewaxing and de-oiling, and in fractional crystallization. Fractionation of paraffin waxes is based on the differing solubility of constituents, having either different molecular weights or being isomers. 2. Crystal structure and habit of individual alkanes and their mixtures According to Mazee, Schaerer, BayK, Smith and others, C,, to C3, normal paraffin hydrocarbons display a well-defined transition point at some centigrades below their melting point. The so-called a-phase, stable below the melting point, is converted into the b-phase, and the transition is accompanied by the release of a relatively high amount of heat. At ambient temperature, normal alkanes between C,, and C,, containing an odd number of carbon atoms have an orthorhombic structure, even-numbered normal alkanes between C,, and c,, have a triclinic structure and those between C,, and c36 a monoclinic structure. At higher tempera-
76
1. PROPERTIES OF
LIQUID PARAFFINS AND PARAFFIN WAXES
tures, the stable structures are crystal systems of higher symmetry, in particular orthorhombic and hexagonal structures. Several authors report that normal alkanes from C,, onwards do not exhibit transition points: the phase stable below the melting point is the Q-phase. In hexagonal structures the long molecules are capable of free rotation around their longitudinal axis. Such paraffin waxes are relatively soft. The hexagonal structure is shown in Fig. 1-24. In the orthorhombic crystals - shown in Fig. 1-25 - free rotation of the molecules is not possible, therefore, such paraffin waxes are more rigid.
Fig. 1-24 Hexagonal structure of paraffin wax crystals
az7.45 A b=L.97W
0
d200=3.725 A dllo=4.14 A
Fig. I-25. Orthorhombic structure of p a r a f i wax crystals
(C) CRYSTAL STRUCTURE OF PARAFFIN WAXES
7 5 1 , ,
, , , ,
Melting point,
I
,
77
;
"C
Fig. 1-26. Transition temperature versus melting point for n-alkanes and their mixtures
Figure 1-26 represents the transition temperatures of individual n-alkanes and of n-alkane mixtures, plotted against their melting points. With increasing carbon atom numbers, i.e. with increasing melting points the temperature difference between the transition point and the melting point decreases and finally disappears at C,, compound. The figure also indicates that for equal melting points, the transition point of the mixture is always lower than that of the individual hydrocarbons. In the case of certain normal alkanes, transition between the different modifications is more complex. For the n-alkane C24H50,between 50.7 and 46.5 "C the stable structure is the hexagonal form, between 46.5 and 42°C the so-called monoclinic I (monoclinic primitive) form, and below 42 "C the triclinic form. In the temperature range below 42 "C, the metastable monoclinic I1 modification can also be formed. As mentioned earlier, substantial amounts of heat are evolved at the transition of the a-phase into the P-phase. Transition points and heats of transition for the transition of some n-alkanes from their P-modification, stable at lower temperatures, into their a-modification, stable at higher temperatures, are listed in Table 1-33. For hydrocarbons containing even numbers of carbon atoms, the transition point is closer to the melting point than for hydrocarbonswith odd carbon numbers. The difference between even-numbered and odd-numbered hydrocarbons also appears in the values of heat of transition.
78
I. PROPERTIES OF LIQUID PARAWINS AND PARAFFIN WAXES
Table 1-33. Transition temperatures and heats of transition of some n-alkanes Nurfber atoms
21 22 23 24 25 26 27 28 29 30 35 36
I
Melting point,
40.4 44.4 47.4 51.1 53.3 57.0 60.0 61.6 64.0 66.0 74.6 75.9
1
Transition temperature, "C
32.5 43.0 40.5 48.1 47.0 53.8 53.0 58.0 58.2 62.0 71.8 73.8
1
Heat of transition, J/mol
15 503 28 240 21 788 31 341 26 103 34 274 28 994 35 489 31 592 37 542
-
The heat of fusion values reported in the literature for the different n-alkanes usually include the heat of transition from the a-phase into the P-phase, since determinations start from the melt, and the total amount of heat evolved during cooling to ambient temperature is measured. In Table 1-34, this heat of fusion, Table 1-34. Melting point, heat of fusion and heat of transition of some n-alkanes Number of carbon atoms
22 24 26 30 34 35
Molecular weight
310 338 366 422 478 492
Melting point, "C
44.4 51.1 57.0 66.0 72.8 74.6
Heat of fusion for the crystal modification stable a t lower temperature, Jjkg
251 065 254 752 256 260 249 598 267 867 259 193
Heat of fusion for the crystal modification stable a t higher temperature, J/kg
157 921 166 762 160 561 163 117 167 349 175 644
Heat of transition, J/kg
93 144 87 990 95 700 86 482 100 728 80 616
the heat of fusion of the crystal modification stable at higher temperature, i.e. the true value of the heat of fusion, and the heat of transition for some n-alkanes are presented in J/kg units. The latter values, when recalculated to J/mol units, differ from those listed in Table 1-33. The reason for this variance is that the data reported in the two tables originate from different authors who used different techniques of measurement.
79
(C) CRYSTAL STRUCTURE OF PARAFFIN WAXES
The specific heat of the 8-modification and of n-alkanes in the liquid state is around 2095 J/kg"C, while the corresponding value for the u-modification is about 4190 J/kg "C. Up till now, branched paraffin hydrocarbons have not been investigated in detail. Much more intricate conditions for transition are to be expected with such can theocompounds, since for example 366 319 isomers of eicosane (C&,,) retically exist. Investigations, however, demonstrated that iso-alkanes display no modification changes in the solid state. A comparison of the edge lengths of the unit cells in normal alkanes demonstrates that the longest edge regularly increases with the number of carbon atoms in the chain. Since even-numbered and odd-numbered alkanes form two series differing in physical and crystallographic properties, even-numbered compounds may only be compared with even-numbered compounds, and odd-numbered compounds with odd-numbered compounds. Alkanes form typical molecular lattices held together by weak van der Waals forces. For this reason it is difficult to prepare single crystals. The usual technique is to apply a thin layer of the compound onto a glass plate by melting. Then the net plane with the longest identity distance will be located parallel to the surface of the glass plate. For some odd-numbered alkanes, this spacing is as follows: C15H32 C17H36 C19H40 &,H,,
21.0 A 23.6 A 26.2 A 38.6 A
The figures indicate that two CH, groups increase the net plane spacing by 2.5 to 2.6 A. The other net plane spacings are much shorter and do not change with chain length. Many researchers have investigated the crystal structure of synthesized n-alkanes. Miiller and Hengstenbergfound the following values for the unit cell dimensions of the rhombic modification of normal C2,H6,: a = 7.45
A;
b = 4.97 A;
c = 77.2
A
They also stated that the unit cell is tetramolecular. Two chain molecules each are located following one another in the direction of the c axis. The C-C bond distances and bond angles in the zig-zag chain are known: 1.54 A and 129"28', respectively. The distance between the CH3 end groups of the chains is 3.1 A. For pentatriacontane (C35H-J and hexacontane (C,pH12,), a and b values were found to be essentiallythe same, while c values were higher, corresponding to what has been said above. According to a later report of Miiller and Lonsdale, the dimensions of the unit cell of n-Cl,H3, at 21 "C are a = 4.28
A;
b = 4.82 A;
c = 23.07
A
80
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
In one of his papers, Mazee reports that x-ray diffraction measurements on n-C,,H, demonstrated the existence of this hydrocarbon in three crystal modifications. He found the following values for the dimensions of the unit cell of the hexagonal modification at 46.5 "C: a = 4.77 A; c = 33.30 A. For the monoclinic modification at 42 "C the corresponding values were found to be: a = 7.50 A; b = 4.99 A; c = 32.70 A; and for the triclinic modification at ambient temperature: a = 7.42 8 ;b = 5.35 A; c = 32.50 A. Based on the results of crystallographic studies, Warth, in the 194Os, came to the conclusion that the unit cells of n-alkanes having higher melting points are bimolecular. For example, n-CS5H,, crystallizes in the orthorhombic system with the unit cell dimensions: a = 7.43 A; b = 4.97 A; c = 46.20 A. Every further - CH, group leads to an increase of c by 1.27 A. It may be seen from the cited data that different values are found in the literature for the edge length c of the unit cell, partly owing to the phenomenon of polymorphism, and partly to the simultaneous presence of several crystal modifications, so that the values found depend largely on the actual modification or mixture of modifications investigated. In addition, experimental data are significantly affected by the purity of the tested material. This is confirmed by a paper by Stanley and Ohlberg which summarizes data found for the long spacings of different crystal modifications in some even-numbered normal paraffin hydrocarbons measured immediately after solidification and after storage for 3 years at 37.5 "C (Table 1-35). Data found by Patton and Simons for the lattice constants of some odd-numbered normal hydrocarbons crystallizing in the orthorhombic system, at 28 "C, are presented in Table 1-36. A comparison with data from other authors indicates that these authors have apparently reversed the positions of the axes a and b.
Compound
Long ing (*0.0'
__~_
*)
Phases initially (relative amounts)
Angle of tilt Equilibrium phase which paraffin chain makes after 3 with basal plane years ~~
__-
81
(C) CRYSTAL. STRUCI'URE OF PARAFFIN WAXES
Table 1-36. Lattice constants of odd-numbered n-alkanes at 28 O Compounds n-C21H44
n-C,,H,* n-CzsH52 n-CP7HE.6 n-C2QH60
1
ao, A
bo,A
4.96f 0.01 4.96f0.01 4.96f 0.01 4.95k0.01 4.95k0.01
7.47f 0.01 7.4650.01 7.4550.01 7.45f0.01 7.44f0.01
1
C
co, A
57.30f 0.08 62.31f0.10 67.41f 0.08 72.59f0.08 77.70f0.18
Studying the crystal structure of cycloalkanes present in microcrystalline paraffin waxes having higher molecular weights, Miiller, Howells, Philip, Rogers and Newman found that their configuration is similar to compressed rings, that is, the molecule, in its crystalline state, consists of two linear chains bound together at the chain ends by a few carbon atoms. The unit cell of the cycloalkane (CH& is unimolecular, the compound crystallizes in the triclinic system and the lattice constants of the unit cell are a = 8.17 A; b = 5.47 A; c = 18.91 A. The corresponding lattice angles are a = 87"18'; /?= 95'10'; y = 106'04'. In cases when the linear chains are sufficiently long, it may be assumed that at the ends of the chain, the carbon atoms are closely packed, and in this way form a subcell. In the case of the above-mentioned cycloalkane, the triclinic subcell has the following lattice constants: a = 4.25 A; b = 5.47 A; c = 2.56A; the correspondingangles are cc = 60'30'; fl = 74'08'; y = 96'47'. In this case the subcell consists of two methylene groups. Since paraffin waxes are mixtures containing mainly n-alkanes, studies related to the crystallization and phase changes of binary, ternary or even more complex n-alkane mixtures are of interest. n-C,,H,Mazee studied the following binary mixtures : n-C,,H,,-n-C,,H,,; -n-C,,H,,; n-C,,H,,-n-C,,H,,; n-C,,H,,-n-C,,H,s; n-C1,H4,-n-C,,H,,. He stated that the constituents of these mixtures are miscible in all proportions, both in the liquid and in the solid state. In the phase diagram of the system n-C,,H,,-n-C&H,, (Fig. 1-27) the two-phase areas (L S,) and (S, + S,) almost shrink
+
SP n-C35 H72
Md-%
nUC36H7L
Hg. 1-27. Phase diagram of the binary system n-CmH7z-n-C96H74
6
82
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Sa
0 n -c19 HLO
Mol-%
n-CZ1Hf.L
Fig. 1-28. Phase diagram of the binary system n-ClsH40-n-C21H44
to single curves. On the other hand, the other systems with lower molecular weight constituents display phase diagrams in which the two-phase areas are more significant and the area (S, + S,) exhibits a minimum value, as may be seen, e.g. in Fig. 1-28. From the phase diagrams of these systems it appeared that a given difference in the carbon atom numbers of the constituents gives rise to a deviation from the ideal behaviour. The difference is greater the lower the molecular weight of the constituents is. With these systems, M a z e observed no melting point depression. For a quantitative evaluation, he calculated the ideal melting point curves of the systems C21H44-C23H48 and C2,H5,-C2,H5,. The calculation was based on Raoult’s law, according to which - assuming that no heat of admixture and no volume change arise - the following relationship is valid between the molar fractions of the and the temperature T where the constituents in the solid phase Xl,s and X2,s mixture totally melts: (1-13)
where XI, I and X2,I are the molar fractions, respectively, of the first and second
83
(C) CRYSTAL STRUCTURE OF PARAFFIN WAXES
constituents in the liquid phase, Tland T2the melting points, and Ql and the heats of fusion. Combining the two equations,
QB
(1-14)
This equation enables us to calculate the value of T. Analogously, the temperature T3 corresponding to the first appearance of melting can be calculated by means of the following equation : (1-15)
The calculated and experimental values are presented in Table 1-37. The data are in good agreement in all cases, the calculated values being slightly higher. L) to Presumably the area (S, + Sp) having a minimum affects the area (S, a certain extent.
+
Table 1-37.Beginning and end of the melting of n-alkane mixtures, determined experimentally and by calculation
I
1
Observed
Calculated
Composition, mol- % _ _ _ _ _ _ _ ~_ _ _ . ~~
CZIHU
__________.
c23H48
50.0 80.0
50.0 20.0
C24H50
CZt3H64
95.0 75.1 50.3 25.2
5.0 24.9 49.1 74.8
43.1 41.6
44.4 42.2
43.6 41.6
44.7 42.4
50.8 51.5 52.9 54.4
50.9 51.8 53.4 54.7
50.9 51.8 53.3 54.1
51.0 52.3 53.8 55.1
Experimental work has shown that volume changes are very small even when normal alkanes whose molecular weights differ greatly are being mixed. For example, the heat of admixture for the system C,H,,-C1,H,, is only about 125.7 J/mol. It has also been observed that the difference between the temperature corresponding to the minimum of the (S, + S,) area and the melting point pertaining to this minimum is the greater the smaller the molecular weights of the constituents (Table 1-38). This finding also confirms that deviation from the ideal behaviour is greater in systems with lower molecular weights than in higher molecular weight systems. The deviation increases with increasing differences between the carbon atom numbers of the constituents. 6*
84
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 1-38. Difference between the temperature minimum of S,) area and the (S, the corresponding melting point for binary mixtures of n-alkanes
+
Temperature differoncs, 'C
Constituents
The depression in the (S, + S,) area indicates that crystallization first appears in the a-form. It has also been confirmed that in binary systems, the structure of the 8-form largely depends on the difference between the chain lengths of the constituents. Deviation from ideal behaviour increases with increasing differences between the chain lengths. This is shown in Fig. 1-29, representing two possible variants of the phase diagram of the C,,H,,-C,,H,, system. In both variants, a m h h u m appears in the liquid area, although less pronounced than in the transition area. The figure indicates that solid solutions in the S, area deviate to a geater extent from the ideal state than in systems where the chain lengths of the constituents differ only by one or two carbon atoms. The lower boundary of the ( S , + S,)
701 L '
68
Y
L
70
-
-
66
?- 64
2 62
c
al a
$ F-
60 58
56
t-
:;I 50
0
I
,
sp
1
100
Fig. 1-29.Two possible variants of the phase diagram of the binary system n-CsoH6s-n-C8,H,P
85
(C) CRYSTAL STRUCTURE OF PARAFTIN WAXES
area is remarkably flat, and the lowest temperature within this broad range is relatively high. Presumably this finding is associated either with a series of mixed crystals, or with phase separation in the solid phase. X-ray studies did not confirm the latter assumption. However, in systems with greater chain length differences, e.g. n-C,,Hu-C,lH,,, phase separation takes place, i.e. a normal eutectic will arise. Figure 1-30 represents yet another phase diagram type for binary n-alkane systems. Here compositions exist that are monotropic, i.e. undergo only a single phase change. Commercial macrocrystalline paraffin waxes consist, at ambient temperatures, mainly of hard orthorhombic crystals, but the hexagonal structure is also always present, even if only in small amounts. In addition, liquid branched hydrocarbons also participate in building up the final structure. The amount of the hexagonal form depends on the molecular mass range of the constituents. The melting rangL and the transition range become broader with more complex paraffin wax composition. For narrower molecular mass range fractions, the transition range is narrower. When this transition is abrupt, contraction of the paraffin wax during cooling can cause cracks due to deformation. The difference between the melting point and the transition range is the greater the broader the molecular mass range of the paraffin wax. Figure 1-31 demonstrates that the transition temperatures of commercial macrocrystalline paraffin waxes with broad molecular weight ranges and complex
‘i
18 38
I
50 Mol-Olo
11 0 n-C20h2
Fig. 1-30. Phase diagram of the binary system n-Ci8Hcn-C&H,s
86
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
c
.-C 0
a
C 0
.+ ._ m C
0,
t-
Melting point, "C Fig. 1-31. Transition point versus melting point of paraffin waxes representing various molecular weight ranges. I approach curve for n-alkanes, 2 mixtures of n-alkanes, 3 narrow fractions obtained by sweating from 57-60 OC melting range paraffin wax, 4 commercial paraffin waxes having different melting points
compositions, are lower - at identical melting points - than those of individual n-alkanes, their mixtures and narrower molecular weight range products obtained by fractional crystallization. The true heat of fusion, related to the modification stable at the higher temperature, of commercial macrocrystalline paraffin waxes is 167 kJ/kg. The total heat of fusion, including heat of transition from the modification stable at the higher temperature into the modification stable at the lower temperature, is around 251 kJ/kg. An increased ratio of branched alkanes in the paraffin wax can act in two different ways : if these compounds are liquid, they soften the wax and reduce cohesion stresses. If, however, they are solid, they will plasticize the wax. With suitable types of branched alkanes, these will fill the voids between crystals and correct the cracks arising due to contraction in the transition range. According to recent studies, crystal structure transformation in the n-alkane content of commercial paraffin waxes is not complete and often takes place only to a limited extent. The various properties of commercial paraffin waxes change with the relative amounts of the polymorphous modifications. Macrocrystalline slab paraffin waxes contain C,,-C,, n-alkanes, microcrystaln-alkanes. In the solld phase no transition occurs line paraffin waxes C&,, with n-alkanes above C3,, as at this carbon atom number the transition point becomes identical with the melting point. Hence in microcrystalline paraffins the importance of polymorphism is much less than in macrocrystalline slab parafin waxes.
(C) CRYSTAL STRUCTURE OF PARAFFIN WAXES
87
Fig. 1-32. Habits of paraffin wax crystals. ( a ) plate crystals; ( b ) malcrystals; (c) needle crystals
In crystalline substances, the boundaries of individual crystals are often independent of the crystal systems in which the substance crystallizes. This is also the case with individual alkanes and their mixtures. Many hundreds of photographs confirm that paraffin wax crystals appear in three different habits, namely plates, needles and so-called mal-shapes. The latter are small-size, underdeveloped crystals, which often agglomerate. The cross-section of needle crystals is similar to the annual rings of trees, while plate crystals resemble paved roads. The different crystal shapes are shown in Fig. 1-32, The conditions for the formation of plate and needle crystals have been studied by many researchers. Based on different studies and theories, Ferris and other researchers have come to the following conclusions: ( i ) The three crystal habits of paraffin waxes are the resul4 of different factors. For a given wax, both the conditions of crystallization and the chemical composition of the wax are of decisive importance. (ii) As a first approach, it has generally been confirmed that within given molecular weight limits, the constituents having higher melting points crystallize in plates. The low-melting constituents crystallize in needles, the medium-melting constituents in mal-shapes. Also, as a first approach, one may say that this relationship is independent of the presence of solvent in the crystallization process and of solvent concentration, if solvent was a t all present. (iii) The above statement is closely related to the observation that normal alkanes crystallize in plates and the crystal lattice is simplest in these crystals. Needle crystals contain both aliphatic and cyclic hydrocarbons, while mal-shaped crystals are characterized by their content of branched hydrocarbons. With the latter, the crystal lattice is obviously more complex than with needle crystals. (iv) From the view of transformability into one another, the three crystal habits exhibit a preferential order of succession. Plate crystals will always readily be transformed into needle- and mal-shaped crystals. Within the two latter habits, needle crystals will be transformed - under appropriate conditions - into malshaped crystals. Consider a molten mixture of high-melting constituents that - in the pure state - crystallize in plates, with low melting constituents that - in the pure state - crystallize in needles or mul-shapes. Cooling of this melt
88
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
will lead first to plate crystal formation, while the still liquid low-melting constituents that yield needle- and mal-shaped crystals will be present as diluting medium. Further cooling will then result in the slow formation of needle- or mal-shaped crystals on top of the existing plate-shaped crystals, occasionally transforming the latter. (v) Cooling rate during crystallization is also an important factor in the resulting crystal habit. For both plate and needle crystals, low cooling rates will result in large crystals. The size of mal-shaped crystals, however, only slightly depends on the cooling rate. Presumably their crystal lattice is so complex that the probability of the incorporation of further molecules into the lattice, i.e. crystal growth is very slight. (vi) When crystallization takes place from a solvent, the relative solubility of individual hydrocarbons must be taken into account. The solubility of alkanes - independently of the form in which they crystallize - is inversely proportional to their melting point. In the presence of solvent, alkane mixtures begin to crystallize at relatively low temperatures. However, the constituents crystallizing in needles are more soluble than those crystallizing in plates. Therefore, needle crystals will appear only at lower temperatures and higher concentrations. Crystallization starts with the appearance of plate crystals. They are followed by malshape crystals which in this respect occupy a medium position. For any given alkane mixture and solvent, one can always find a concentration limit and a corresponding crystallization temperature limit, below and above which, respectively, crystallization in needles can be suppressed. If, however, conditions are such that concentration and temperature are above and below, respectively, these limits, needle and mal-shaped crystals are capable of transforming plate crystals. In needle crystals, a vein system or ribbing proceeding along the total length of the crystal can always be detected. The location and origin of this vein system has been investigated by several researchers. According to Ferris and co-workers it consists of a central hole system within the crystals. As regards mal-shaped crystals, Ferris and co-workers found that as a result of very rapid cooling, aggregates of tiny crystals are formed from paraffin wax melts, and these appear as mal-shaped crystals. According to another assumption, mal-shaped crystals are actually plate crystals piled on one another. For instance, n-hexacosane crystallizes in this manner from both melt and solution. Earlier its crystals were believed to be mal-shaped. Certain relationships can be detected between the crystal habits of paraffin waxes and the manufacturing process of the lubricating oil fractions from which they are won. Plate crystals are obtained from paraffinic distillates having lower average boiling points. Compounds present in such fractions and crystallizing in needle and mal-shapes have melting points so low that they behave as oils at the usual dewaxing and de-oiling temperatures, and are, therefore, almost completely absent from the de-oiled paraffin waxes. In distillates having higher boiling points and in distillation residues, needle and mal-shaped crystals appear, and their
(C) CRYSTAL STRUCTURE OF PARAFFIN WAXES
89
proportion increases with increasing boiling points. This also indicates that macrocrystalline paraffin waxes mainly crystallize in plates consisting of n-alkanes, while microcrystalline paraffin waxes crystallize in needle- and mal-shaped crystals containing branched alkanes. Table 1-39 lists the distribution of crystal habits in microcrystalline paraffins as a function of the boiling point. Table 1-39. Per cent distribution of crystal habits in microcrystalline paraffin waxes of different boiling points Average boiling point at 1.4 kPa, "C
80 95 105 115 130 140 150
Crystal habits ~
plates
~
mu/-shapes
100 99 98 91 78 54 37
0 1 2 7 12 17 27
1
needles
0 0 0 2 10 29 36
The crystal habits of alkanes are important factors in paraffin wax manufacture. Cakes from light paraffinic distillates are easy to press. This can be attributed to the fact that they contain plate crystals at the temperature of dewaxing. Such cakes are also easy to sweat, since at sweating temperatures they crystallize in large interlacing needle-shaped crystals. The paraffin wax is identical at both dewaxing and sweating, but the conditions of crystallization largely differ. At dewaxing, the amount of oil present is still relatively high, so that it keeps in solution the compounds that tend to crystallize in needles to such an extent, that a plate crystal structure will be established. In the sweating operation, however, when the wax cake is being cooled and crystallizes, the amount of oil present is much less, so that both crystal habits will crystallize simultaneously, and the needle crystals will alter the shape of the plate crystals. With increasing average boiling points of the distillates, the proportion of mal-shaped crystals substantially increases in the paraffin wax. Mal-shaped crystals crystallize simultaneously with plate crystals and alter their shape. Therefore, such distillates cannot be pressed.
Literature Asinger, F., Paraffins. Chemistry and Technology. Pergamon Press, Oxford (1968). Brooks-Dunstan: The Science of Petroleum. Vol. V, Part 111, Oxford University Press, London (1955). Brown, R. G., J . Appl. Phys., 34, 2382 (1963). Buchler-Graves: Znd.Engng. Chem., 19, 718 (1927). Clarke, E. W., Znd. Engng. Chem., 43, 2526 (1951). Coffey, S., Rodd's Chemistry of Carbon Compounds. Vol. I, Part A, Elsevier Publishing Company, Amsterdam (1964).
90
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFlN WAXES
Edwards, R . T., Petrol. Refiner, 36, 180 (1957). - : Znd. Engng. Chem., 49, 750 (1957). - : TAPPZ. Bull., 41, 267 (1958). Evans, U. R., Discuss. Faraday SOC.,N o . 5, 77 (1949). Ferris-Cowles-Henderson: Znd. Engng. Chem., 23,681 (1931). Ferris-Cowles: Znd. Engng. Chem., 37, 1054 (1945). Fontana, B. J., J . Phys. Chem., 57, 222 (1953). Gray, C. G., J. Inst. Petrol., 29, 226 (1943). - : Petroleum Lond., 7, 94, 98 (1944). Hoffman-Smyth: J. Am. Chem. SOC.,72, 171 (1950). Hoffman-Decker: J. Phys. Chem , 57, 22 (1953). Howells-Phillips-Rogers: Acta Crystallogr., 3, 210 (1950). Johnson, J. E., Znd. Engng. Chem., 46, 1046 (1954). Katz, E., J. Inst. Petrol., 17, 37 (1931). Kinsel-Phillips: Znd. Engng. Chem., 17, 152 (1945). Kirk-Othmer : Encyclopedia of Chemical Technology.Vol. 15, Wiley Interscience, New York (1968).
Magill-Pollack-Wyman: J . Polym. Sci. A3, 3781 (1965). Mazee, W. M., R e d . Trav. chim. Pays-Bas, Belg., 67, 197 (1948). Miiller, A., Proc. R. SOC. A 120,437 (1928). - : Proc. R . Soc., A 38, 514 (1932). Newman, B. A., J . Appl. Phys., 38, 4105 (1967). Niegisch-Swan: J. Appl. Phys., 31, 1906 (1960). Ohlberg, S. M., J. Phys. Chem., 63, 248 (1959). Padgett-Hefley-Hendrickson: Znd. Engng. Chem., 18,832 (1926). Padgett-Killingsworth: Pap. Trade J., 122, No. 5, 9, 37 (1946). Piper-Brown-Dyment: J. Chem. SOC.,127, 2194 (1925). Rhodes-Mason-Sutton: Znd. Engng. Chem., 19, 935 (1927). Sachanen, A. N., The Chemical Constituents of Petroleum. Reinhold Publ. Co., New York (1945).
Schaerer-Bayle-Mazee: Recl. Trav. chim. Pays-Bas, Belg., 75, 513 (1956). Smith, A. E., J. Chem. Phys., 21, 2229 (1953). Sullivan-McGill-French: Znd. Engng. Chem., 19, 1042 (1927). Templin, P. R., Znd. Engng. Chem., 48, 154 (1956). Teubel-Schneider-Schmiedel : Erd6iparafine. VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig (1965). Turner, A. J., J . Polym. Sci., 62, 53 (1962). Turner-Brown-Harrison: Znd. Engng. Chem., 47, 1219 (1955). Vand-Aitken-Campbell: Acta Crystullogr., 2, 398 (1949). Vand-Boer: Proc. K. Ned. Akad. Wet., 50.991 (1947). Warth, A. H., The Chemistry and Technology of -Waxes. Reinhold Publishing Co., New York (1956).
Wilman-, H., Proc. Phys. SOC.,64, 329 (1951). Wilson-Lipson: Proc. Phys. SOC.,53, 245 (1941).
(D) Physical properties of paraffin waxes It is a very complex task to establish relationships between the chemical composition of paraffin waxes and their applicability in various fields. In many cases these relationships cannot unambiguously be determined. It is, therefore, of great importance to know the physical properties of paraffin waxes, and to develop suitable methods for their measurement.
91
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
The literature has dealt extensively with such functional properties as tensile strength, blocking point, sealing strength, modulus of rupture etc. A knowledge of these physical properties allows us to determine with greater certainty whether the paraffin wax in question is suited for a given application. 1. Melting point, boiling point and melt viscosity Some important physical properties of normal paraffins, liquid or solid at ambient temperature, are summarized in Table 1-40. The melting point of n-alTable 1-40. Physical constants of normal alkanes
Pentane Hexane Heptane Octane Nonane Decane Undecane Dodecane Tridecane Tetradecane Pentadecane Hexadecane
- 129.7 -94.0 - 90.5 - 56.8 - 53.7 29.7 -25.6 -9.7 6.0 5.5 10.0 18.1
36.1 68.7 98.4 125.7 150.8 174.1 195.9 216.3 235.5 253.6 270.7 287.1
Heptadecane Octadecane Nonadecane Eicosane
22.0 28.0 32.0 36.4
302.6 317.4 331.6 345.1
-
-
0.6263 0.6594 0.6838 0.7026 0.7177 0.7301 0.7402 0.7487 0.7563 0.7627 0.7684 0.7733
1.3577 1.3749 1.3876 1.3974 1.4054 1.4119 1.4172 1.4216 1.4256 1.4290 1.4319 1.4345
d at m.p.
0.7767 0.7767 0.7776 0.7777
1.4360 (at 1.4367 (at 1.4336 (at 1.4346 (at
25 "C)
28 "C) 38 "C) 40 "C)
0.7782 0.7782 0.7797 0.7786 0.7785 0.7587 (at 90 "C) 0.7788 (at 60 "C) 0.7792 0.7797 0.7795 (at 70 "C)
1.4240 (at 1.4358 (at 1.4270 (at 1.4283 (at 1.4380 (at
70 "C) 45 "C) 70 "C) 70 "C) 60 "C)
b.p. at 2.1 kPa
Heneicosane Docosane Tricosane Tetracosane Pentacosane Hexacosane Heptacosane Octacosane Nonacosane Triacontane Tetracontane Pentacontane Hexacontane HeDtacontane
40.4 44.4 47.4 51.1 53.3 57.0 60.0 61.6 64.0 66.0 81.4 91.9-92.3 98.5-99.3 105-105.5
215 224-225 234 243 259 262 270 279-281 286 304
-
420-422 -
-
-
92
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
kanes increases with molecular weight. The melting point of hectane, the highest molecular weight normal alkane isolated in a pure state, is 115 "C.Over the carbon atom number range from C , to CZ5,the relationship between the melting point and the carbon atom number cannot be described with one single function for even-numbered and odd-numbered n-alkanes, i.e. the melting points are higher and lower, respectively, than the average values calculated from even- and oddnumbered n-alkane melting points. It should be stressed that branching of the carbon chain, at identical molecular weights, results in an important decrease of the melting point, since high melting points are inseparable from high symmetry of the crystals, and this condition will be satisfied above all in the case of straight-chain alkanes. The melting points of various normal and branched paraffins are presented in Table 1-41. For example it may be seen, that among C,,HS, alkanes, the melting point of n-hexacosane is 56.4 "C as compared to 0.0 "C for 11-n-butyldocosane. Figure 1-33 represents average melting point versus molecular weight for n-alkanes and various types of waxes, including petroleum waxes obtained by rectification and fractional crystallization. Several of these narrow boiling-pointrange fractions have very similar molecular weights, and yet, as demonstrated by the figure, their melting points differ substantially. The boiling points of identical molecular weight straight-chain and branched alkanes also differ, but to a much lesser degree than the melting points. In the
82 77 71
66
y
60 , 54
'g 49 C
.-
5
43
38 32 27
21 16
b-4 I
ns of soft wax from rnperature carbonof brown coal fractions of total wax from I t c of brown coal
' '
10 I I I 1 1 ~ 1 ~ ' l L l , 1 ~ 1 ~ ' ~ 1 ' 1 ~ ~ ' I 1 11 1 1, ~ 1 1 I 200 300 400 500 600 700 800900 Molecular weight
Fig. 1-33. Melting point versus molecular weight for normal alkanes and waxes from different sources
(D) PHYSICAL. PROPERTIES OF PARAFFIN WAXES
93
Table 1-41. Melting points of various normal and branched alkanes found in waxes ~~
~
Hydrocarbon
~UHXl
n-Tetracosane 2-Methyltricosane 2,2-Dimethyldocosane 5-n-But yleicosane
c26H64
n-Hexacosane 5-n-Butyldocosane 7-n-Butyldocosane 9-n-Butyldocosane 11-n-Butyldocosane
CZIlH58
Melting point by capillary tubs method O C
51.5 42.0 34.6 8.0
56.4 20.8 3.2 1.3 0.0
10-Nonylnonadecane
- 5 to - 6
GPHIO 22-Methyltritetracontane
66.6-66.7
C,,-C,, range, the boiling point of the branched alkanes is lower by only 4 to 15 "C (depending on the number, length and position of the side chains) than that of the corresponding n-alkane. No definite relationship between the viscosities of normal and branched paraffinic hydrocarbons can be established. While alkanes with one side chain often have lower viscosities than that of the normal alkane, the viscosity of alkanes with two or three side chains ishigher than that of the corresponding normal alkane. The temperature coefficient of the viscosity of normal alkanes decreases with increasing molecular weight. Among isomeric branched alkanes the temperature coefficient of viscosity is highest for the compound with the shortest side chain, while this coefficient is substantially lower for alkanes with long side chains and for normal alkanes. Paraffin liquids at ambient temperature and melts of paraffins solid at ambient temperature behave as Newtonian systems. For certain applications, the viscosity of macrocrystalline and microcrystalline paraffin waxes, above all in the 100150 "C temperature range, is of importance. Fig. 1-34 shows melt viscosities, measured at 100, 120 and 140"C, for blends prepared from a macrocrystalline paraffin wax (drop melting point 53 "C) and a microcrystalline paraffin wax (drop melting point 70 "C).As to rheological properties of macrocrystalline and microcrystalline paraffin wax melts, it should be noted that they behave as nonNewtonian systems within their melting range : they exhibit structural viscosity and rheodestruction. In such systems viscosity is a function of the shear stress applied and of load time.
94
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
20 40 60 80 I( 0
Macrocry st wax, wt-% L . A L - J
100 80 60 40 20 0
Microcryst wax, wt-Oh
Fig. 1-34. Melt viscosities of blends of a macrocrystalline and a microcrystalline paraffin wax
2. Density and thermal expansion
The density of paraffin waxes increases with their melting point. Table 1-42 lists the densities of several macrocrystalline paraffin waxes with different melting points, over a temperature range starting much below and ending much above the melting point. Figure 1-35 represents density versus melting point curves for narrow boilingrange fractions obtained by repeated vacuum distillation from a commercial Table 1-42. Densities of macrocrystalline paraffin waxes having different melting points V
0
2
-
; ~1 7 14 27 38 66 74 85
50
52
1
Melting point, "C 55
1
57
I
61
Density, g/cm'
0.906 0.903 0.897 0.872 (28 "C) 0.849 0.776 (63 "C) 0.769 0.762
0.915 (2 "C) 0.911 (10°C) 0.909 (15 "C) 0.897 0.873 0.775 0.768 (77 "C)
-
0.917 0.914 (10 "C) 0.910 (15 "C) 0.902 0.877 0.780 (60 "C) 0.774 (71 "C) 0.766 (82 "C)
0.922 0.919 0.914 0.911 0.896 (35 "C) 0.779 (63 "C) 0.772 0.765
0.922 0.919 0.915 0.905 (28 "C) 0.903 (35 "C) 0.783 (63 "C) 0.775 0.769
95
(D) PHYSICAL PROPERTIES OF PARAPFIN WAXES
Melting point, "C
Fig. 1-35 Density versus melting point for narrow boiling-range fractions obtained by vacuum distillation from a commercial paraffin wax (melting point 65 "c)
paraffin wax (melting point 65°C). At identical boiling points and molecular weights, the density of these fractions is generally lower than that of the pure n-alkanes. For comparison, Table 1-43 lists the densities of some pure normal and branched alkanes measured at 70 and 90 "C. The coefficient of cubical expansion does not change in the C21-C31range. Expansion coefficients for branched 2,2-dimethyldocosane and 10-nonylnonadecaneare lower than for n-alkanes having the same molecular weight. Table I-43. Densities of some individual alkanes Density, glcm' Compound
~
at 70°C
CzlH,, n-Heneicosane Cz3H,, n-Tricosane C,,H,, n-Tetracosane CZ8Hs8 n-Octacosane C3,HB2n-Triacontane C,lH,, n-Hentriacontane Cs4H,, n-Tetratriacontane C3,H,, n-Pentatriacontane C38H,, n-Hexatriacontane Cd8Ha n-Tritetracontane C,,H,, 2-Methyltricosane C,,H,, 2,2-Dimethyldocosane C,,H,, 13-Methylpentacosane C,,H,, 10-Nonylnonadecane C,4H,, 22-Methyltritetracontane C,,H,, 1-Cyclohexyloctadecane
0.7587 0.7654 0.7682 0.7759 0.7795 0.7827
-
-
-
0.7662 0.7642 0.7720 0.7170 0.7938 0.7997
1
at 90°C
0.7468 0.7531 0.7562 0.7639 0.7676 0.7709 0.7728 0.7734 0.7783 0.7812 0.7539 0.7536 0.7595 0.7650 0.7816 0.7874
Temp. coeff. of density in the liquid phase dd/At
0.00060 0.00060 0.00060 0.00060 0.00060 0.00059
-
0.00061 0.00053 0.00063 0.00055 0.00061 0.00062
96
I. PROPERTm OF LIQUID PARAFFINS AND PARAF'FW WAXES
As with the macrocrystalline paraffin waxes, the densities of microcrystalline waxes in the liquid and solid state differ greatly. Density data of a microcrystalline paraffin wax with a melting point of 73 "C are presented in Table 1-44. Plots of paraffin wax density versus temperature always exhibit several sharp breaks. If starting from the melt, the first break in the cooling curve appears at the liquid-solid phase transformation. As generally known, this transformation Table 1-44. Densities of a microcrystalline paraffin wax (melting point 73 "C)in the solid and liquid phase Temperature, "C
6.7 15.6 26.9 52.8 71.1 76.7 82.2 93.3 100.0 115.6 137.8 160.0 182.2 204.4 225.6
1
Density, g/cm*
0.9343 0.9294 0.9236 0.8921 0.8300 0.8025 0.7999 0.7935 0.7890 0.7805 0.761 1 0.7535 0.7410 0.7380 0.7140
is isothermal in the case of individual hydrocarbons. In paraffin waxes, however, these being mixtures of individual hydrocarbons, the liquid-solid transformation takes place over a temperature range indicated by the break. After solidification of the wax, further cooling results in solid-phase transitions between different crystal modifications. These transitions appear on the curve as subsequent breaks, their number and temperature depending on molecular weight. The breaks indicate that the transformations in question are accompanied by greater or lesser volume changes. These changes can also be measured directly. Table 1-45 summarizes volume expansion data for various macro- and microcrystalline paraffin waxes. Expansion data include volume changes occurring at liquid-solid and solid-solid transformations as well as thermal expansion of the liquid and solid phases. Expansion values were measured using a dilatometer. Figure 1-36 shows, based on the above data, the specific volumes of these paraffin waxes against temperature. The curves referring to macrocrystalline waxes resemble each other. On each curve, a sharp break at about 10°C below the melting point can be observed indicating a change in crystal structure. The crystallization points listed in the table are defined as the temperatures corresponding to
97
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
Table 1-45. Volume changes of macro- and microcrystalline paraffin waxes Characteristics
i
I
Macrocrystalline paraffin waxes
Density a t 100 OC, dAo0 0.7531 Viscosity a t 98.9 'C, mmZ/s 3.19 Needle penetration a t 25 O C , 100 g/5s, 0.1 mm; ASTM D-5 33 Melting point (cooling curve), OC; ASTM D-87 51 0.34 Oil content, wt- %; ASTM D-721 Average molecular weight 376 Crystallization point, O C 51.6 a-p phase transition point, O C 34.0 Temperature change required for total phase transformation, OC 8.6 at liquid-solid transform. at a-p phase transition 10.5 Expansion coefficient, cm3/g * OC in the liquid phase 0.0011 in the a solid phase 0.0016 in the p solid phase 0.0010 Expansion at melting,* 0.1228 cm3/g vol- x 10.6 ,_ Expansion at a-,!I phase transition,* cm*/g 0.0356 vol- % 3.2 Volume change occurring between the transition point of the a phase and the crystallization point of the liquid, cma/g 0.1716 vol- % 15.5
0.7547 3.22 28
0.7543 3.51
0.7555 3.59
19
19
55
Microcryst. wax
0.7945 12.59 36
53 0.48 358 53.2 35.5
0.22 377 55.9 38.4
56 0.36 387 56.8 40.5
62 8.52 587 74.0 -
8.5 10.3
8.8 11.4
7.7 12.0
35.0 -
0.0011 0.0014 0.0009
0.0010 0.0013 0.0008
0.0010 0.0014 0.0008
0.0010 o.oO09
0.1237 10.7
0.1264 11.0
0.1262 11.0
0.1372 12.5
0.0369 3.3
0.0387 3.5
0.0390 3.5
0.1735 15.7
0.1763 16.0
0.1770 16.1
-
0.1372 12.5
* Since paraffin waxes are mixtures of several components, these processes are non-isothermal.
the initial deviations from the curves representing the liquid phase. The actual initial temperature of crystallization, i.e. at which the first crystals appear in the melt, is some degrees centigrade higher. Its accurate measurement, however, is difficult. The volume-temperature curve for the microcrystalline wax differs from the former curves above all in that the solidification process is not indicated by a wellobservable break : the process takes place within a much broader temperature range than in the case of macrocrystalline waxes. Also, no transformation in the solid phase can be observed, although such transformations may actually take place, but will be masked by the broad solidification range. When crystal forma7
98
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
.
0 1 3000
0
E
u 1 2500
d E 1 1 2000 0
r" 1 1500 u
aJ
0
m l 1000
20
25
30
35
40
L5
50
55
60
65
70
75
00
Temperature. "C Fig. 1-36. Specific volume versus temperature for various paraffin waxes
1.3500
. 0
1.3000
rn
1.2500
= .U U
1.1 500
Qi CL
CT)
1.1000 1.0500 Temperature,
"C
Fig. 1-37. Specific volume versus temperature for some n-alkanes
tion starts, the volume of macrocrystalline waxes sharply decreases. In the case of the microcrystalline wax, however, the corresponding break occurs at some centigrades below the crystallization point, and the break, as noted before, is only slight. These differences between the two paraffin wax types are caused by the higher oil content and molecular weight range as well as by the iso- and cycloalkane content of microcrystalline paraffin waxes. It may also be seen from the data in Table 1-45 that the values of the expansion coefficients for macro- and microcrystalline waxes in the liquid and solid state are approximately identical. Total expansion of microcrystalline wax, however, is much less than that of macrocrystalline waxes. The difference found in total expansion is in good agreement with the volume change observed at the solid-phase transition in macrocrystalline paraffin wax. Hence the smaller expan-
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
99
sion of microcrystalline wax is due above all to the absence of solid-phase transition. It is obvious that for a given commercial wax, the character of the relationship between specific volume and temperature will depend on the carbon atom number and structure of the constituting individual alkanes, as well as on the nature and amount of non-paraffinic compounds present in the wax. To demonstrate this relationship in the case of individual alkanes, Fig. 1-37 is presented, in which, by way of example, specific volume versus temperature is plotted for normal C,, CZ4,CZB,C,,, C,, and C,, alkanes. These expansion curves resemble those in the foregoing figure, but certain differences should be noted. In the case of C24,C2,, C,, and C,, alkanes, thesolid-phase transition takes place at some "C below the melting point. The curve for C,, displays no solid-phase transition, while the curve for C,, exhibits two such transitions. It is clear from the foregoing that if a sufficiently wide temperature range is being considered, a difference will be observed between the total change in specific volume of macro- and microcrystalline paraffin waxes. Kinsel and Phillips proposed to utilize this phenomenon for classifying petroleum waxes into macroand microcrystalline waxes, by introducing a so-called crystallinity index. The volume contraction occurring during cooling a liquid paraffin wax from a given temperature to another temperature consists of three parts : (i) The contraction Kl occurring while the liquid cools from the given temperature to the melting point. Its value depends on molecular weight, the macroor microcrystalline nature of the wax being of no importance. (ii) The contraction K, taking place during solidification and, if any, solidphase transition. Its value does not only depend on molecular weight, but on the crystallization tendency of the wax. In the extreme case, when the substance is amorphous, the value of K, will be zero. Its value is the higher, the greater the crystallization tendency of the wax. (iii) The purely thermal contraction Ks of the solidified wax when cooled from the melting point to the given final temperature. Its value depends primarily on the molecular weight range. Total contraction Kt is equal to the algebraic sum of the above partial contractions : Kt ( ~ 0 1 - x = ) KI K, K, (1-16)
+
+
Hence the value of K,, i.e. contraction taking place during solidification and solid-phase transition can be obtained by measuring K, and K, and subtracting their sum from the measured value of K,. Kinsel and Phillips cooled 100 cm3 paraffin wax samples of various origin and structure from 93.4 "C, at a cooling rate of 5.56 "C (10 O F ) per hour, to temperatures exceeding their melting points by 2.7 "C. The measured contractions for the various waxes yielded an average Kl value of 0.072 vol-%/"C. This value could be used for all paraffin waxes with a maximum error of k0.008 vol- %/T. In contrast, the values of K, for the solid state changed with temperature. Large con'7
100
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
tractions were observed for all paraffin waxes immediately below their melting points, the values decreasing with further cooling. KJ'C values over different temperature ranges, for a 73 "C melting point macrocrystalline wax and a 71 "C melting point microcrystalline wax are presented in Table 1-46. Table 1-46. K , values of a macrocrystallineand a microcrystallineparaffin wax over different temperature ranges
I
Macrocrystallineparafin wax, melting point 73 "C Temperature range, "C
K , for 0.56 "C
Microcrystalline paraffin
wax. melting Doint 71 "C
Temperature range, "C
temperature decrease
71-66 66-60 60-54 54-49 49-27
0.530 0.333 0.144 0.078 0.040 Average value 0.155
K* for 0'56
temperature decrease
69-64 64-58 58-53 53-47 47-27
0.230 0.188 0.170 0.130 0.077 Average value 0.130
The large contraction observed immediately below the melting point is the combined effect of both thermal contraction and structural change occurring in the solid phase. Hence, it would be incorrect to subtract the average value of K, from Kt in order to determine Kc. Kc includes, of course, contraction due to structural changes, but this cannot be experimentally determined. Kinsel and Phillips assumed that for amorphous waxes the purely thermal contraction per "C is identical in the solid and liquid state. Therefore, contraction of different waxes should be compared over temperature ranges identical with respect to their melting points and not over an absolute pre-set temperature range. The chosen ranges should be wide enough to include all contraction due to changes in crystal structure. Kinsel and Phillips chose a temperature range A t , = 5.56 "C (10 OF) above the melting point and A t , = 27.80 "C (50 OF) below the melting point for their experiments. The value of Kc and the crystallinity index serving to characterize the crystalline character of the structure were then derived as follows: For the above conditions (1-17) Since, as assumed above,
-K.- -- 4 At2
(1-18)
At,
Equation (1-17) will assume the following simplified form:
Kc = Kt - K1 A t, -
0
33.36
(1-19)
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
101
Using the value K J d t , = 0.072 accepted to be valid for all paraffin waxes, it follows that K, = Kt - 0.072 * 33.36 = Kt - 2.4 (1-20) Kinsel and Phillips defined the crystallinity index as ten times the value of K,:
C.Z. = lo(& - 2.4)
(1-21)
where Kt is total contraction of the wax measured over the range from 5.56 "C above its melting point to 27.80 "C below its melting point; K, is that portion of the contraction of the solid wax which is due to thermal contraction only and independent from changes in crystal structure, and Kl is the contraction of the wax in the liquid state. Table 1-47 lists total contraction and crystallinity index values calculated from the former for microcrystalline, intermediate and macrocrystalline paraffin waxes. Table I-47.Total contraction and crystallinity index for macrocrystalline, intermediate and microcrystalline paraftin waxes Melting point, "C
Total contraction, vol- %
Crystallinity index
Macrocrystalhe parafin waxes 55 52 60 73 51 56 64
13.3 13.7 13.7 13.9 14.0 14.1 14.1
109 113 113 115 116 117 117
Intermediate parafin waxes 70 74 72 74 66
11.4 11.7 12.5 S2.5 12.7
90 93 101
Microcrystalline paraf i n waxes 71 82 71 64 83 71 83 60 86 86
8.9 9.2 9.3 9.3 9.4 9.4 9.5 9.8 10.0 10.4
65 68 69 69 70 70 71 74 76 80
101
103
102
1. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
The crystallinity index of microcrystalline waxes does not exceed 80, while macrocrystalline waxes have values above 110. However, even the use of the crystallinity index does not allow the drawing of a sharp dividing line between macro- and microcrystalline structures. Any classification based on this index must always take into account the oil content of the Table f-48.Total contraction of mixtures of paraffin waxes with various percentages of white mineral oil Oil content, wt-%
Melting point,
Total contraction, vol- %
Macrocrystalline parafin white oil waxes 0 5 10 20 30 40
53 52 51 50 47 45
13.30 12.50 11.50 10.50 9.05 1.25
Microcrystalline para f i n waxes white oil 0 5 10 20 30 40 50 0 5 10 20 30 40 50
74 73 73 12 70 68 65 86 85 85 84 82 81 79
10.05 10.34 10.28 9.16 8.18 1.49 6.79 11.40 11.40 11.40 10.52 10.24 8.76 8.08
+
+
wax. As demonstrated by the data in Table 1-48, total contraction for microcrystalline waxes with 5 to 10 wt- % oil content, these being still acceptable grades, is practically identical with total contraction for waxes containing no oil, whereas 5 wt- % oil in macrocrystalline waxes will reduce total contraction by 0.8 vol- %. This appears a significant decrease, but since oil content in commercial refined macrocrystalline paraffin waxes is normally below 1 wt- %, its effect on the crystallinity index of such waxes will be negligible.
3. Optical properties The refractive index is a physical property frequently used for identifying substances and for determining their compositions. The refractive index of paraffin waxes is usually measured at 80 to 85 "C.
103
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
The relationship between density, refractive index (both measured at the same temperature) and molecular weight is given by the Lorentz-Lorenz formula :
(1-22) where M , is molar refraction, n refractive index, M molecular weight and d density. Table 1-49 lists refractive indices in the solid state of some macrocrystalline paraffin waxes having different melting points. Table Z-49. Refractive indices of some macrocrystalline paraffin waxes in the solid state Melting point, "C
At 1 0 ° C At 20°C At 25 OC At 40 "C At 5 0 ° C
50
I
'
52
55
1
57
_
I
_ 61~
_
Refractive index
1.5306 1.5268 1.5169 1.5040
-
1.5321 1.5278 1.5256 1.5071 -
1.5348 1.5305 1.5277 1.5103
-
1.5366 1.5332 1.5311 1.5163 1.5049
1.5350 1.5328 1.5241 1.5087
The refractive index and the molar refraction calculated from the former are functions of chemical composition and molecular weight. Relationships between the molar refraction and molecular weight of alkanes and alkenes, cycloalkanes and aliphatic esters are plotted in Fig. 1-38. Within each homologous series, molar refraction is the function of molecular weight. The refractive indices of macrocrystalline paraffin waxes at 84 "C vary between 1.4210and 1.4315, their molar refraction between 100 and 154. The corresponding values for microcrystalline waxes are 1.4320 to 1.4480 and 160 to 195. In Table 1-50, refractive index data measured a t 70 and 90 "C and molar refraction are listed for a number of hydrocarbons which are usual constituents of macrocrystalline and microcrystalline paraffin waxes. The data indicate that the temperature coefficient of the refractive index slightly decreases with rising molecular weight in the C,,-C,, normal alkane range. The value of this coefficient is identical for normal and branched alkanes having the same molecular weight. According to Mazee, the molar refraction of the compounds in Table 1-50 can be calculated, with a knowledge of the molecular weight, by using the following relationship : M , = 0.33063M -t 1.6165 (1-23) Plots of the refractive index against temperature always indicate liquid-solid and solid-solid phase transformations. Therefore, the refractive index can be utilized to a certain extent for studying the crystal structure of paraffin waxes. Figure 1-39 represents such a plot for a macrocrystalline wax having a melting point of 48 "C. In the liquid phase, one refractive index value is measured for
104
1. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
9
160
0
c
150
0
5
140 ._ c V
2 130 .&? -2 120
P
'"3ZO
310
,d,
4AO I 4!?@*4b 4!iO G!O
Molecular weight Fig. 1-38. Molar refraction versus molecular weight for some hydrocarbons and aliphatic
esters. I alkanes and alkenes, 2 cycloalkanes, 3 esters
Table I-50.Refractive indices at 70 and 90 OC and molar refraction values for various hydrocarbons
I
I
Compound
CZ1H,, n-Heneicosane C,,H,, n-Tricosane CZ4H,,n-Tetracosane C,,H,, n-Octacosane C,,H,, n-Triacontane C,,H,, n-Hentriacontane C34H70 n-Tetratriacontane C,,H,, n-Pentatriacontane C,,H,, n-Hexatriacontane C,,H,, n-Tritetracontane Cs4H,, 2-Methyltricosane CZ4H,,2,2-Dimethyldocosane C,,H,, 13-Methylpentacosane CZsH,, 10-Nonylnonadecane C14HB022-Methyltritetracontane CtaH,, 1-Cyclohexyloctadecane
*n +*
-
refractive index,
f =
Refractive index at 7 0 ° C
1.4240 1.4210 1.4283 1.4324 1.4342 1.4354
-
-
1.4216 1.4269 1.4303 1.4322 1.4417 1.4416
1
at 9 0 ° C
1.4160 1.4190 1.4205 1.4248 1.4266 1.4278 1.4296 1.4301 1.4308 1.4340 1.4201 1.4192 1.4229 1.4247 1.4346 1.4339
I 1
An * At
~~-
0.00040 0.00040 0.00039 0.00038 0.00038 0.00038
-
0.00038 0.00039 0.00037 0.00038 0.00036 0.00039
99.46 108.64 113.23 131.83 141.03 146.16 159.65 164.38 168.25 201.37 113.48 113.31 122.68 131.60 206.17 111.09
temperature
M = molecular weight, d = density
every temperature. In the a- and /?-phaseof the solid state, double refraction appears owing to anisotropy. From the two curves referring to the solid state, it can clearly be determined that the transition from the a-phase into the /?-phase takes place in this case between 30.6 and 24.4 "C.The less the constituents that are contained in the wax in question, the narrower the transition temperature range. Individual alkanes have sharp transition temperatures, as has been shown in Fig. 1-37.
105
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
1.58 156
1.54 1.52
- 1.50 a
2
1.48 146
5 (L
144 1.42 140
15
25
35
45
55
65
Temperature, "C
75
a5
95
Fig. 1-39. Refractive index kersus temperature of a macrocrystalline paraffin wax (melting point 48 "C)
When measuring refractive index as a function of temperature, it is more accurate to use polarized light, since when using non-polarized light there is a possibility of double refraction. Measurements of the refractive index can be used for technological control in paraffin wax manufacture. Fig. 1-40 shows Tiedje's experimental results. His starting material was a microcrystalline wax originating from a residual oil. From this he prepared fractions I , II and III by distillation. Subsequently he separated the starting material, as well as the three distillates, by fractional crystallization into fractions having different melting points. Simultaneously he prepared, as a
Boundary of naphthenes Boundary of is0 -alkanes
Melting point, "C
Fig. 1-40. Refractive indices of fractions prepared from a microcrystalline paraffin wax by distillation and crystallization
106
1. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
first stage, a narrow melting-range fraction by crystallization of the initial microcrystalline wax. Its melting point was in the 58 to 67 "C range. From this material he prepared several further fractions by distillation (marked by IV in the figure). He then measured the refractive index of all fractions at 80 "C and plotted them against their melting points. The objective of this investigation was to find out whether the sequence of operations, when both crystallization and distillation are applied, affects the sharpness of separation. As seen in the figure, the relationships between melting points and refractive indices give a clear answer. The refractive indices of final fractions having identical melting points, that is, their chemical compositions vary largely according to which separating operation, crystallization or distillation had first been applied. Among further optical properties, the colour and colour stability of paraffin waxes are important characteristics in many applications. Among instruments used for measuring colour, are the Lovibond Tintometer, the Saybolt Chromometer and the Tag Robinson instrument. The colour of macrocrystalline waxes in the liquid phase is best determined with the Lovibond Tintometer or the Saybolt Chromometer. To measure the darker colour of microcrystalline waxes in the liquid state, the Lovibond Tintometer in which thin layers of the material are used, and the Tag Robinson instrument are suitable. The problem of measuring the colour of paraffin waxes in the solid state reappears from time to time. However, up to the present no generally accepted methods, like those for liquid-state measurement, have been developed. Most specifications require that paraffin waxes do not change their colour during storage or application. Colour stability is connected primarily with oxidative stability, since oxidation causes discoloration of the wax. No generally accepted method for evaluating colour stability has as yet been developed, owing to the fact that oxidation causing discoloration depends on many variables like temperature, oxygen concentration, intensity and spectrum of light, effect of metal catalysts, etc. The methods developed so far can be classified into two groups. In one group measurements are made in the liquid state, in the presence of metal catalysts and air. In the other group, colour stability is investigated in the solid phase, in daylight, at a specified temperature. For applications in the paper industry a glossy surface of the paraffin wax film is required. Glossy films are obtained by rapid cooling which results in the formation of small-size crystallites. These reflect a large proportion of the incident light, whereas relatively large-size crystallites obtained by slow cooling yield dull surfaces. Initially glossy surfaces of paraffin waxes become duller with time, due to recrystallization and oxidation. The extent of gloss is measured by photometry of the reflected light. However, no method yielding satisfactorily reproducible results has as yet been developed.
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
107
4. Rheological properties
The rheological behaviour of macrocrystalline and microcrystalline paraffin waxes in the temperature range below their melting points depends on the lattice structure, on the forces between the molecules forming the lattice and on the chemical composition defining the former. Thus, even in the case of identical origin, a given macrocrystalline or microcrystalline paraffin wax, at temperatures below its melting point, can behave as an elastic, viscoelastic or viscoplastic system, depending on crystallization conditions, especially on the cooling rate, moreover on the actual temperature, on the magnitude of the acting forces and on the mode of load. Since paraffin waxes are polycrystalline substances, no sharp boundaries between these deformation behaviour types depending on the above conditions can be drawn. Except for chemical uses, in almost all applications of macrocrystalline and microcrystalline paraffin waxes, definite rheological properties are required. It is noteworthy that none the less these properties had up to the present not yet been properly characterized and purposefully studied. Many wide-spread qualifying parameters, e.g. hardness, flexibility and tensile strength yield important pertinent information. In our research work in these directions we developed methods effectuating static and dynamic loads, corresponding to the crystal structure of paraffin waxes. In discussing the elastic and plastic deformation of crystalline substances, the following relationships are, to a greater or lesser extent, depending on conditions, also valid for paraffin waxes. (i) It is a general rule that the finer the crystal grains in a polycrystalline system, the higher its strength. The size of the crystal particles and the crystal structure can be modified by thermal and mechanical treatment resulting in recrystallization. The crystal structure can also be changed by adding a suitably selected additive. (ii) When a melt of a polycrystalline substance is being cooled, the system will in most cases assume an oriented structure below the melting point. In the direction of the highest temperature gradient during cooling, coarser particles will be formed than in the other directions. (iii) In the case of elastic deformation, the atoms, ions or molecules forming the crystal lattice will be shifted to a small extent against one another. The field of intermolecular forces or the field of atomic and ionic forces will be thereby disturbed, and the system will accumulate such an amount of potential energy as to keep equilibrium with external forces. When the external load ceases, internal stresses will return molecules, atoms or ions to their original places, and the initial lattice structure will instantaneously be re-established. (iv) If structural elements acquire higher energies than characteristic for their potential trough, they will pass the potential barrier and arrive into a neighbouring potential trough. If this process is successively repeated, it will manifest itself macroscopically in reaching the yield point, i.e. in plastic deformation of the
108
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
system. In test specimens made of polycrystalline substances plastic deformation in various parts of the specimen will appear at different loads, indicating that although macroscopically the deformation appears elastic, plastic deformation may already have taken place in some parts. (v) Except in the case of ideal elastic deformation, the internal stresses caused by deformation relax with time. In crystalline systems, this relaxation is accompanied by the shift of numerous atoms or molecules along definite crystallographic planes. (vi) In polycrystalline systems, in addition to simultaneous elastic and plastic deformation and to relaxation of internal stresses, the so-called elastic post-effect usually also appears. This manifests itself in hysteresis of the stress-strain curves, i.e. these curves differ when stress is repeatedly applied. The fatigue phenomenon of polycrystalline systems is related to elastic hysteresis. Fatigue means that periodically changing loads, after a certain number of cycles, result in creep or break of the test specimen at loads lower than the load corresponding to the yield point. For evaluation of the rheological properties of crystalline substances, including paraffin waxes, testing methods belonging essentially to three groups can be considered :
(i) strength tests, (ii) hardness tests, (iii) fatigue tests. Strength tests include tensile, compression, bending, torsion and impact-bending tests. Hardness is the resistance against the penetration of a body (needle, or cone, or plunger rod) under a defined load, this body being made of a harder material than the substance being tested. To measure the hardness of paraffin waxes, penetration tests are widely accepted. It is a common feature of strength and hardness tests that the test specimens are subjected to short-time stresses. In addition to the characteristics that are obtained in this manner, long-period fatigue tests yield important information. For all solids a fatigue limit exists: this is the stress below which the substance is able to resist an infinite number of fatigue cycles without breaking. For paraffin waxes the fatigue phenomenon is of greatest interest in wax-plastics compounds containing relatively high amounts of plastics. The most wide-spread strength test for paraffin waxes is tensile testing. We investigated the tensile strength of macro- and microcrystalline paraffin waxes in great detail. Our results indicated that tensile strength of both macroand microcrystalline waxes in the temperature range from -20 "C to +25 "C increases with the speed of stretching. This finding demonstrates that waxes behave in this temperature range as viscoelastic systems. A great number of tests has confirmed that tensile strength can vary greatly even within relatively narrow melting point and oil content ranges, and between macrocrystalline paraffin waxes from the same origin, but chosen from different production batches.
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
109
The limiting values measured with macrocrystalline paraffin waxes obtained from Romashkino crude, having a melting range of 52-54 "C and an oil content of 0.5-1.03 wt-%' are presented as an example in Table 1-51. Table 1-51. Maximum and minimum tensile strength values of macrocrystallineparaffin waxes from Romashkino crude (melting range 52-54 "C, oil content 0.5-1.03 wt- %) -~
Temperature,
~
-20
+ +
Tensile strength, N/cm* at 50 mrnlrnin
O C
-49.0 9.8-58.8 45.1-78.4 -39.2
0 25 30
1
-
at 500 mmlmin
196.0 14.7-1 86.2 65.7-107.8 -39.2
According to our investigations, tensile strength values as listed in Table 1-52 should be expected from macrocrystalline paraffin waxes with good tensile properties. (The values are valid for stretching rates of 500 mmlmin and for test specimens as used by us.) Table I-52. Tensile strength values for macrocrystalline paraffin waxes specified as "good tensile" (Testing speed : 500 mmlmin) "C
- 20 0 25 30
+ +
Tensile strength, N/cm'
196.0-215.6 176.4-186.2 78.4-107.8 39.2-49.0
It may be seen from Tables 1-51 and 1-52 that tensile strength increases with lower temperatures (at least within the temperature range from - 20 "Cto +30 "C). This is valid for both macrocrystalline and microcrystalfine waxes. Tensile strength also reacts sensitively to oil content: it decreases with higher oil content, particularly at low temperatures. A comparison of tensile strengths of macro- and microcrystalline waxes does not allow the establishment of an unambiguous and generally valid relationship. Apparently the effect of other factors on tensile strength, e.g. oil content and temperature, is greater than the effect of macro- or microcrystalline crystal structure. Blends of macro- and microcrystalline paraffin waxes have tensile strengths which, when plotted against composition of the blend, yield curves exhibiting
110
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
maxima. Figure 1-41 represents tensile strength versus composition of blends prepared from a macrocrystalline wax (melting point 52 "C, oil content 1.0 wt-%) and a microcrystalline wax (melting point 69 "C, oil content 13.0 wt-x), measured at different stretching rates at 0 "C. Compressive stresses and deformations are similar, apart from the sign, to those of tensile tests. In compression, the length of the circular or quadratic cross-sectional test specimen will decrease, and its specific value yields the deforniation corresponding to the given stress conditions. We developed a method for measuring compressive deformation in which we utilized test specimens with 1 cm2 circular cross-sections and 8.21 mm length. Temperatures are variable between -20 "C and +40 "C, compressive stresses between 2.45 N/cm2 and 981 N/cm2. After stress removal, also as a function of time, elastic recovery can be studied. The measured results can be evaluated based on the specific characteristics and functional relationships summarized in Table 1-53. Deformation and elastic recovery as a function of time, after removing the compressive stress, for a macrocrystalline paraffin wax (drop melting point 53 "C, oil content 0.18 wt-%) at I
I
Microcryst. wax, w t - % L
100
l
~
i
l
l
l
80 60 40 20 Macrocryst. wax, wt-%
~
l
0
l
l
Fig. 1-41. Tensile strength versus composition for blends of a macrocrystalline and a microcrystalline paraffin wax. at 0 O C (v = stretching rate, mm/min)
111
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
Table I-53. Specific characteristics and functional relationships for characterizing isothermal compressive deformation Symbols
height of the initial test specimen, mm height of the deformed test specimen, mm height of the test specimen after recovery, mm acorn,, compressive stress, N/cm* tR time of recovery, min tD time of deformation, min T temperature, OC
h0 h h,
Definitions ho- h D=deformation ratio related to the initial height of the test specimen (specific deho formation) h,- h R =recovery ratio related to the initial height of the test specimen, h0 D,= (D- R) permanent deformation
h,- h R’ = -ratio of recovery and deformation ho-h Functional relationships t D = const. aCOlllDI = f ( O ) R = f (tR) acomDr = const. a,,,,, = const. D~ = f ( t R ) %om,, = f(R’) tD = const.
t D = const. t R = const.
Table 1-54 Compression characteristics of a macrocrystalline paraffin wax at 25 “C (drop melting point 53 OC, oil content 0.18 wt-%)
I
Characteristics
Initial height of test specimen ho, mm Height of the deformed specimen h, mm Height of the test specimen after recovery h,, mm ho-h D = -h0 h,- h R =h0 D,= D - R
h,- h R’ = _ _ ho- h
-l
Compressive stress, Nlcm’
I
29.4
I
49.0
58.8
Time of deformation, min
8.21
8.02
8.04
7.98
7.93
8.33
8.11
7.85
7.89
1.73
7.16
8.03
8.17
7.91
7.97
7.82
7.85
8.13
0.012
0.021
0.020
0.031
0.021
0.036
0.007
0.007
0.010
0.011
0.011
0.012
0.005
0.014
0.010
0.020
0.010
0.024
0.60
0.35
0.53
0.36
0.53
0.33
112
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
25 "C is shown in Table 1-54. The values for elastic recovery always relate to a recovery time of 10 minutes. Numerous measurements have confirmed that the deformation of paraffin waxes resulting from a given compressive stress is, over the temperature range in question, i.e. -20 to f40 "C, a function of the loading time. After removal of the load, deformation will partly or totally disappear as a function of time. This finding also indicates that paraffin waxes that are crystalline at ambient temperature behave as vkcoelastic systems in this temperature range. Compressive deformation properties depend not only on external conditions, but also on crystal structure. Figure 1-42 represents compressive stresses leading to D = 0.02 specific deformation in a 3-min loading time against composition of blends prepared from a macrocrystalline wax (drop melting point 51 "C, oil content 1.0 wt-%) and a microcrystalline wax (drop melting point 69 "C, oil content 13 wt-%). By increasing compressive stress in crystalline substances, the yield point will be reached at a given value, and subsequent deformation will be plastic flow. However, under certain external conditions the test specimens will break before reaching the yield point. In such cases the rheological behaviour of the substance
l
I
I
f
l
l
l
l
l
l
l
100 90 80 70 60 50 LO 30 20 10 0 Macrocryst. wax, wt-"10 Fig. 1-42. Compressive stress resulting in 0.02 specific deformation versus composition for blends of a macrocrystalline and a microcrystalline wax (loading time 3 min)
(D) PWSICAL PROPERTIES OF PARAFFIN WAXES
113
will be characterized by its compressive strength, that is, by the ratio of the force resulting in break and the initial cross-sectional area of the test specimen. Since the deformation of paraffin waxes is viscoelastic or plastoelastic, their compressive strength can unequivocally be determined only when conditions for the loading time are also defined. Compressive strength was defined by us as the compression force acting on a surface of 1 cm2 that leads to the break of the test specimen within 5 seconds. According to these tests, the compressive strength of macrocrystalline paraffin waxes at ambient temperature is higher than that of microcrystalline waxes. Oil content always reduces the value of the compressive strength. In Fig. 1-43 the compressive strength of two macrocrystalline and two microcrystalline waxes differing in oil content is plotted against temperature. An unambiguous relationship also exists between crystal structure, chemical composition and compressive strength. Compressive strength values for microcrystalline paraffin waxes obtained by fractional crystallization are summarized in Table 1-55. The starting material was a refined residual oil petrolatum from Romashkino crude. Its compressive strength could not be measured at 25 and 0 "C, since it underwent plastic deformation and did not break. Information on the behaviour of paraffin waxes to dynamic stresses is obtained from impact-bending tests. We developed a method for the evaluation of the toughness of paraffin waxes based on the use oethe Charpy pendulum. Without going into details of the method, it shall be noted that impact-bending strength Lfis defined as the ratio of the energy L required to break the test specimen and
350 300
250
200 150 100
,Macrocrystalline
waxes
50
,Microcrystalline
waxes
0 -
1 -20 -10
0
+I0 +20 + 10
Temperature, "C
Fig. 1-43. Compressive strength versus temperature for macrocrystallineand microcrystalline paraffin waxes 8
114
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table Z-55.Compressive strength values at different temperatures, for microcrystalline paraffin waxes obtained by fractional crystallization of a petrolatum from a refined residual oil
I
Tested material
Compressive strength, N/cm*
Petrolatum Two-stage fractionation: Stage I: Fraction of components having melting points above 20 "C
+
323.4
588.0
910.2
Stage 11: Fraction of components having melting points between 20 and 0O C
166.6
539.0
744.8
Fraction of components having melting points above 5 O C
214.4
431.2
901.6
+
+
the cross-sectional area F of the specimen :
L,=-L [J/cm21
(1-24)
The impact-bending strength of paraffin waxes depends on the dimensions of the test specimen and on test conditions. In our method we use test specimens having 16 mm diam. circular cross-sections and 110 mm lengths. The results obtained indicate that impact-bending strength of paraffin waxes increases with temperature and reaches a maximum value at a given temperature. From this temperature on, plastic flow appears and the test specimen does not break in the impact-bending test. Figure 1-44 shows impact-bending strength against temperature for macrocrystalline paraffin waxes, each having drop melting points around 50 "C, but differing in oil content. The test was carried out using a striker with an energy maximum of 3.92 J. This must be mentioned, since, particularly in dynamic tests, the viscoelastic or plastoelastic nature of paraffin waxes must be taken into account. It follows from this fact that the value of impact-bending strength will, under otherwise equal conditions, depend on the kinetic energy of the striker in the moment when it strikes the specimen, i.e. on the speed of deformation at that moment. Increasing speeds result in increasingly elastic behaviour : the wax becomes tougher, and finally, with increase from a given speed of deformation, it will behave as a brittle system. Figure 1-45 represents impact-bending strength against composition of blends prepared from macro- and microcrystalline paraffin waxes and measured with 0.98 and 3.92 J strikers at 0 and 25 "C. The figure demonstrates how sensitively impact-bending strength reacts to changes of the crystal structure. It should be noted that in the test at 25 "C using the 0.98 J striker, the blends containing 50 to 80 wt- % macrocrystalline paraffin wax did not break, but were only bent. However,
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
115
0.40
0.35 Oil content, wt-%
6 .
0.30
T - 1.0 - 4.2 - 6.2 - 9.1 H - 11.4
a b c
7
L-
5 0.25 c
:
c
Ln
0
C ._
0.2c
U
C a,
J?
c U
0.1E
0
Q
E
-
0.1c
0.0:
c
-20
-10
0 10 20 Temperature,
"c
30
40
50
Fig. 1-44. Impact-bending strength versus temperature for macrocrystalline paraffin waxes differing in oil content
the blends containing less than 50 wt-% macrocrystalline wax, and the pure macrocrystalline wax itself exhibited well-defined breaks. On theother hand, the blends containing 50 to 80 wt-% macrocrystalline wax, when using the 3.92 J striker, also had defined impact-blending strength values. Hence, it may be assumed that the crystallite systems formed over the 50 to 80 wt- % macrocrystalline wax range show plastic flow at deformation speeds attainable with the 0.98 J striker, while a t the higher speeds realized with the 3.92 J striker, they behave as tough systems, and show some elasticity. Among methods and characteristics to be considered for the evaluation of strength properties of paraffin waxes, there is the Fraass breaking point used for the specification control of bituminous materials. We found that this method can also be applied for paraffin waxes : the breaking point satisfactorily characterizes low-temperature flexibility of waxes, i.e. the temperature limit where brittleness follows. Table 1-56 contains the Fraass breaking point of two-stage fractional crystallization products obtained from a refined residual oil petrolatum of Romashkino origin. In the first stage of fractionation, hard paraffin wax was separated at 30 and 10°C. In the second crystallization, soft microcrystalline 8*
116
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES 1.01
0.1 I 100
I
I
I
I
I
I
I
I
80 60 40 20 Macrocryst. wax, wt-%
I
20 40 60 80 Microcryst. wax, w t - %
0
I
100
Fig. 1-45. Impact-bending strength versus composition for blends of a macrocrystalline and
a microcrystalline paraffin wax
Table 1-56. Fraass breaking point for paraffin waxes obtained by two-stage fractional crystallization of petrolatum from refined residual oil Fractiolu
Stage Z
Fraction obtained at Fraction obtained at
+ 30 + 10 OC
O C
Stage I1 Fractions from the filtrate of stage I at 30 OC Fraction obtained at -20 OC Fraction obtained at - 10 OC Fraction obtained at 0 OC Fractions from the fdtrate of stage I at 10 O C Fraction obtained at -20 O C Fraction obtained at 0 "C
I
Bredti% point,
++2525 -30 -30 -26 -30
-30
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
117
wax fractions were separated at 0, - 10 and -20 "C. It may be seen from the data in the table that the second crystallization of the filtrate obtained at + 30 "C yielded, at 0 and - 10 "C,products plastic and flexible at low temperatures, while the products obtained in the first fractionation stage were already brittle at ambient temperature. Penetration measurement is the most wide-spread method for determining the hardness of paraffin waxes. Penetration measurements carried out at different temperatures are suitable to characterize the thermal sensitivity of paraffin waxes. Our investigations showed that in the temperature range below the solidification point the penetration of macrocrystalline waxes changes to a greater extent with temperature than that of microcrystalline waxes. An increase in oil content results in increased penetration values for both macro- and microcrystalline waxes. Besides these factors the penetration also depends to a great extent on the modscation of the crystalline structure. Figure 1-46 shows the penetration values against composition of blends prepared from macro- and microcrystalline waxes, measured at 0 "C and 25 "C. The tests were carried out with the needle specified in the ASTM standard. The macrocrystalline paraffin wax used had a drop melting point of 53 "C and an oil content of 0.6 wt-%. The corresponding values of the microcrystalline wax were 70 "C and 10.0 wt-%. 100 1
I
' 2b
I
!
I
I
40 60 '
8'0 'Id0
Macrocryst. wax, wt-"lo I I I I I I I I I ' A 100 80 60 40 20 0 Microcryst. wax, wt-"b
Fig. 1-46. Needle penetration versus composition for blends of a macrocrystalline and a
microcrystalline paraffin wax
118
I. PROPERTIES OF LIQUID PARAFFINS A N D PARAFFIN WAXES
5. Thermal properties A knowledge of specific heat is almost always necessary in engineering calculations. A value of 2.1 kJ/kg "C is being generally accepted for the specific heat of C2?-C40alkanes, both in the solid and liquid state. This value is in fact acceptable in many cases for industrial practice. It must, however, be remembered that it is only an approximate value. The specific heat of alkanes depends, in addition to temperature, on molecular weight, and in the solid state on crystal structure. According to Cragoe the specific heat of oils with high paraffin content can be calculated by using the following formula :
4.2 c = -(0.388-O.OOO45t)
d
(1-25)
where c is specific heat, kJ/kg * "C, d density at 15.6115.6 "C, and t the temperature in OF. This empirical formula is also suitable for calculating the specific heat of alkanes liquid at ambient temperature. The specific heat against temperature curves for some n-alkanes in the C,o-C,5 range are shown in Fig. 1-47. The figure demonstrates that the 2.1 kJ/kg * "C value is acceptable as an approach only in the C25
Fig. 1-47. Specific heat versus temperature for solid-state n-alkanes in the CI,-C,, range
119
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
to C,, range, and even in this range only at temperatures between 30°C and 50°C. This, as well as the effect of crystal structure, are shown in Table 1-57. The temperature ranges indicated in the table include the temperature of the structural change taking place in the solid phase. It can also be stated from these data that specific heat changes over a very wide range with temperature. The specific heat of the crystalline modification stable above the a + /? transition temperature is substantially higher in all cases than that of the modification stable below the transition point, and also than that of the liquid phase. For alkanes which have no solid-phase transition, e.g. n-C,H,, and n-CUHs8, specific heat monotonouslyincreases up to themelting point. The specific heat of the liquid phase Tuble 1-57. Specific heat values of some n-alkanes over different temperature ranges n-Alkanes
C,,H,, n-Heneicosane
40.30-40.40
32.8
15-21 21-32 34-39 45-55
1.89 1.97 5.70 2.39
Cg4H,, n-Tetracosane
50.70-50.80
47.0
25-35 35-45 45-50 52-61
1.80 2.01 4.06 2.43
C30D61n-Triacontane
65.80-65.95
59.2
25-35 35-45 45-57 60-65 67-77
1.97 2.18 2.51 4.11 2.56
C36H74n-Hexatriacontane
75.80-75.85
73.5
25-35 35-45 45-55 55-65
65-72 74-75 77-86
1.80 2.01 2.35 3.10 3.77 5.12 2.56
C,,H,, n-Tetracontane
81.35-81.45
-
30-40 40-50 50-60 60-70 70-80 82-90
1.72 1.89 2.23 3.35 4.11 2.51
C,,H,, n-Tritetracontane
85.25-85.35
-
43-50 50-60 60-70 70-76 76-83 86-91
2.09 2.35 2.60 3.10 3.75 2.64
120
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
is smaller in these cases too than the specific heat of the solid phase close to the melting point. The average heat of fusion of n-alkanes occurring in paraffin waxes is 167.6 kJ/kg. For hydrocarbons that have two crystal modifications in the solid phase, the thermal effect of a + p phase transition is around 83.8 kJ/kg. In macrocrystalline paraffin waxes both phase transition and melting take place over a broad temperature range, and these ranges may overlap, due to the numerous hydrocarbon constituents. Hence a separate determination of heat of fusion and heat of phase transition would meet with difficulties. The total thermal effect of the transformation from the liquid phase to the solid phase stable at the lower temperature is, according to the above-mentioned data, 230.5 to 251.4 kJ/kg. Heat of fusion and heat of transition values for some n-alkanes occurring in macrocrystalline paraffin waxes have been discussed in the foregoing chapter and are listed in Tables 1-33 and 1-34. The true heat of fusion varies between 159.2 and 167.6 kJ/kg. Heat of transformation, for even-numbered alkanes, is around 92.2 kJ/kg, i.e. a substantial fraction of total heat of fusion. In Table 1-58, heat of fusion values for the crystal modification stable at ambient temperature of macrocrystalline paraffin waxes having different melting points are listed. It should be noted from the data that heat of fusion values of macrocrystalline waxes are substantially lower than those of individual n-alkanes. The figures in the table Table 1-58. Heat of fusion values for macrocrystalline paraffin waxes Melting Point,
"C
51.7 52.2 52.4 65.3
Heat of fusion for the crystal modification stable at ambient temperature, kJ/kg
168.86 163.00 147.07 183.52
do not exceed 184 kJ/kg, whereas the values for the total heat of fusion, relative to the crystal structure stable at ambient temperature for n-alkanes having closely similar melting points, are around 247 kJ/kg. Some authors explain this difference by assuming that in parafin waxes that are composed of hydrocarbon mixtures, the n-alkanes retain the crystal structure stable at the higher temperature, and no solid-phase transition occurs. Another explanation would involve the presence of signscant amounts of soft paraffin wax components in the tested macrocrystalline paraffin waxes, these components reduce the heat of fusion. Similar results were obtained by Further, Parks and Todd with three highly refined macrocrystalline waxes that are shown in Table 1-59.
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
121
Table 1-59. Heat of fusion values for various average molecular mass macrocrystalline paraffin waxes Average number of Melting carbon point, "C atoms
20 25 33
36.4 53.3 71.1
Heat o f fusion for the crystal modification stable at the lower temperature, Mfkg
217.88 224.58 226.26
For microcrystalline paraffin waxes, owing to the total or partial absence of solid-phase transition, the values of heat of fusion vary between 146 and 168 kJ/kg. According to Trouton's rule, the molar entropy of evaporation at lo5Pa pressure approaches to a constant value for all substances: Mrs s, = = const = 21
Ts
(1-26)
where M is the molecular weight, r, the heat of evaporation in kcal/kg, and T, the boiling point in K. For C,+, alkanes, the value of the constant varies between 20 and 22. Kistiakowsky replaced the Trouton constant by the following function:
Mrs - - 8.82 + 4.575 lg T, Ts
(1-27)
for hydrocarbon mixtures. Heat of evaporation depends on external pressure according to the ClausiusClapeyron equation : dP r, = AT(V" - v') (1-28)
dr
where rs is the heat of evaporation in kcal/kg, T the boiling point in K, v" the specific volume of the vapour in m3/kg, uf the specific volume of the liquid in m3/kg,-dP the slope of the vapour pressure curve in kgf/m2 * K, and A is 11427
dT
kcal/m * kgf. To calculate the heat of evaporation for hydrocarbons at pressures differing from 1 bar (lo5 Pa), Schumacher introduced the following equation: MrS - -
T
8.82 + 4.575 lg- T V,
(1-29)
122
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
where 0, is the so-called apparent molar volume of the liquid. Its value is 1.2 for n-alkanes, 1.1 for iso-alkanes and 0.9 for naphthenes and their derivatives. These us values are, however, valid only at 1 bar (lo5 Pa) pressure. For other pressures Mr, T -= 8.82 + 4.575 lg(1-30) T V,(P>” where the value of s depends on evaporation temperature. For n-alkanes the following function is valid : s =
0.685 lg T - 0.8
(1-31)
105
84
0
. E
x
7
63 C“
.c
0
2
0 Q
9 a,
-
Lc
0
0
42
aJ
I
21
0 Temperature, “C Fig. 1-48. Molar heat of evaporation versus temperature for n-alkanes liquid and solid at ambient temperature
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
123
The evaporation process of alkane mixtures proceeds, of course, at successively changing temperatures at constant pressure, and at successively changing pressures at constant temperature. For this reason engineering calculations apply average boiling points. After determining the average boiling point, calculations are carried out with the individual hydrocarbon having the same boiling point as the average boiling point of the mixture. Figure 1-48 represents molar heat of evaporation of a number of n-alkanes liquid or solid at ambient temperature against temperature at different pressures. A further thermal property of paraffin waxes to be mentioned is thermal conductivity. This can be calculated for macrocrystalline paraffin waxes having melting points in the 50 to 54 "C range, over the temperature range from - 180 "C to +30 "C, using the following empirical relationship:
1 = 0.005 . (1
- 0.0016t)
(1-32)
-
where I is thermal conductivity in Jim "C * s, and f is temperature in "C. The above relationship is acceptable as an approach for microcrystalline waxes and for paraffLns liquid at ambient temperature.
6. Solubility Petroleum waxes are obtained, following the usual manufacturing processes, by crystallization, in the presence of various solvents, from pafaffinic distillates and refined paraffinic lubricant oil fractions. In some applications paraffin waxes are used in solution, and in many cases they come into contact with solvents during their service life. Thus solubility is a very important physical characteristic of paraffin waxes. The solubility values of macrocrystalline paraffin waxes, having 50-55 "C melting ranges, in solvents significant from the view of both manufacture and application are listed in Table 1-60. The data should be considered as average values, since, at identical melting points, the chemical composition of macrocrystalline waxes from different origins can vary, and hence their solubilities in a given solvent under given conditions will also vary. Figure 1-49 represents solubility against temperature data for a refined p a r a f i wax (melting point 51 "C) in various solvents. Figure 1-50 shows solubility versus melting point data for different paraffin waxes, measured at 20 "C, in various solvents. As a general rule it can be stated that solubility increases in all solvents with decreasing melting point. Solubility values of paraffin waxes in different boiling-range petroleum distillates can be determined by using the nomogram presented in Fig. 1-5 l . The lefthand scale of the nomogram represents the average boiling point of the solvent, the right-hand scale the difference between the melting point of the paraffin wax and the temperature of dissolution.
L
Table 1-60. Solubility of macrocrystallineparaffin waxes in various solvents (g wax/100 cms solvent)
I
Solvent
Propane n-Pentane n-Hexane n-Heptane n-Octane n-Decane Naphtha (boiling range 96-146 "C) Kerosene (boiling range
1 1 1 1 -15
0.18
0.27
178-271 "C)
Methanol Ethanol n-Propanol Isopropanol n-Butanol Iso-butanol Acetone Methyl ethyl ketone Methyl propyl ketone Methyl butyl ketone Methyl isobutyl ketone Dichloroethane Benzene Toluene 30 % acetone- 35 % benzene- 35 % toluene
-10
0.011 0.024 0.045 0.034
-5
0.30
0
0.95
2.77 1.37 0.99
3.69 2.18 1.69 0.94
0.93
1.70
0.28
0.50
0.89
0.13
0.045 0.095 0.180 0.14 0.29
5
0.55
0.52
0.023 0.048 0.089 0.067
I
1
1
Temperature, "C 10
5.11 4.81 3.55 2.90 1.44
15
~
6.94 6.07 5.06 4.24 2.74
20
1
9.53 8.31 7.18 5.93 4.98
25
1
30
40
1
45 ~
6C
0.047 0.21 0.18 0.54 0.543 0.135 0.650
0.008 0.08 0.44 0.33 0.94 0.97 0.258 1.31
0.024 0.17 0.71 0.61 1.86 2.14 0.52 2.53
0.032 0.145 0.11 0.31 0.315 0.095 0.306
0.62
0.8 1.4
0.29 1.90 3.05
0.40 4.4 6.6
0.91 10.5 14.5
2.24 24.5 32.0
5.10 56.0 69.0
0.07
-0.15
0.34
0.8
1.8
4.0
9.0
0.366 0.666 0.488
1
8
1.60
0.037
P
17.16 16.23 14.36 11.66 9.17
0.024 0.096 0.063 0.18 0.185 0.063 0.202 0.775
0.018 0.087 0.188 0.333 0.253 0.078
35
h,
0.063 0.318 1.35 1.23 3.60 4.03 1.07 7.48
0.103 0.55 2.65 2.36 7.05 9.60 2.30
0.16 0.97 4.95 4.63 16.7 30.0
E
52 135
8
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
0.11 I I ' I ! I 1 -18 -16 -14 -12 -10 -8-6 -4 -2
125
' ' '
' " I I I I 0 +2 +4 +6 t 8 +I0 +12 +14+16 +I8
Temperature, "C
Fig. I-49. Solubility versus temperature for a refined macrocrystalline paraffin wax (melting point 51 "C). I cyclohexane, 2 methylcyclohexane, 3 95-150 "C boiling-range petroleum distillate, 4 160-200 "C boiling-range petroleum distillate, 5 180-270 "C boiling-range petroleum distillate, 6 toluene, 7 propane, 8 mixture of 65 wt- % benzene and 35 wt- % acetone
Melting point,
O C
Fig. I-50. Solubility, of paraffin waxes versus melting point in various solvents at 20°C. I nitrobenzene, 2 ethylene dichloride, 3 paraffinic distillate, 4 naphtha
126
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
24 Fig. I-51. Solubility of paraffin waxes in petroleum distillates having different average boiling points. Instruction f o r the use of the nomogram: identical sides of the solubility scale and the right-hand scale must be used together. Example: average boiling point 200 OC, temperature difference 19 'C. Then solubility is 47 g/100 cm3
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
127
Several authors developed equations for calculating the solubility of paraffin waxes. Berne and Allen found the following relationship for hydrocarbon solvents, particularly for non-aromatic distillation fractions : C = (1120 - 2.97Y) 1.357"-")
(1-33)
where C is the solubility of the paraffin wax in g/100 cm'; Y , in "C, is the average of the temperatures at which yields of distillation of the solvent carried out according to ASTM standard are 10, 30, 50, 70 and 90%, respectively; m is the melting point of the wax in "C, and t is the temperature of the solution in "C. The equation is strictly valid only for solvents boiling between 60 and 300 "C, for waxes with melting points between 45 and 70 "C, and for solution temperatures between 0 and (m - 10) "C. Pool and his co-workers found that the solubility of paraffin waxes can be described by the following empirical equation : lg W = A (lg T - K)
(1-34)
where W is the solubility of the wax in g/100 g; T the temperature of the solution in K; A is a constant depending on the properties of the wax, and K is a constant depending on the properties of the solvent. The dependence of the solubility of solids on temperature can also be described by utilizing the Clausius-Clapeyron equation. Regarding the molar fraction of the solute, the following relationship can be derived : lgxi = - -.H F 4.575
HF T 4- 4.575Tf 1
-
(1-35)
where xi is the molar fraction of the solute; HF is the molar heat of fusion of the solute, in cal/mol; T is the saturation temperature of the solution in K, and Tf is the melting point of the solid, in K. The solution power of solvents is particularly important in solvent dewaxing processes. The requirement to the solvent is that it dissolve the wax only to a very slight extent or not at all at the operating temperature, while dissolving the oil readily. Solution power is usually varied by utilizing mixed solvents. We studied the solubilities of a macrocrystalline wax having a melting point of 53 "C and of the fractions obtained from the former by fractional crystallization. The solvents used were mixtures of acetone, benzene and toluene. The acetone content varied between 15 and 45 vol-%. Figure 1-52 shows the dependence of solubility on temperature for the original macrocrystalline wax in the pure individual solvents. The wax concentration is plotted on a logarithmic scale against the reciprocal of the absolute temperature. This mode of presentai ion corresponds to the Clausius-Clapeyron equation, with the difference that the unit of concentration is g/100 cn1' instead of molar fraction. Figure 1-53 represents the solubility of the same wax in mixed solvents containing 15, 30 and 45 vol-% acetone, respectively, the rest of the solvent being a 1 : 1 mixture of benzene and
128
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES c
$ 30
m 0,
10 8 6 4
0 0
z 2 a"
5
g
.-
5 c
1
0.8 0.6
0.4
C
a, V
c
0
0.2
V
;j 0.1 3 C .+ ..+.
2P
0.03
3.3 3.4 3.5 3.6 3.7 3.8
Reciprocal of cloud point, (l/K).103
3
Fig. 2-52. Solubility of a commercial macrocrystalline paraffin wax in pure solvents. 2 acetone, 2 benzene, 3 toluene
I
I
I
I
I
I
3.3 3 4 3.5 3.6 37 3.8 : 3
Reciprocal of cloud point, (l/K),103
Fig. 2-53.Solubility of the same wax as in Fig. 1-52 in mixed solvents containing 1 5 , 30 and 45 vol-x, resp., of acetone and made up to 100% with 1 : 1 benzene/toluene
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
' i aJ
-> $
129
10 8
6
4
0,
.2
0
0
X
0
3 1 m 0.8 c- 0.6
--0,
0.4
$ C
02
P
c
U 0
g 3
01
&
+ 0
b 003 I I I I I I a 3.3 3.4 3.5 3.6 3.7 3.8 : 3 ReciDrocal of cloud point, (l/K).103
Fig. 1-54 Solubility of the same wax as in Fig. 1-52 and its fractions in a mixed solvent (30 vol- % acetone, 35 vol- % benzene, 35 vol- % toluene). I hard wax, 2 initial wax, 3 intermedier wax, 4 soft wax
oluene. Figure 1-54 represents the solubilities of the initial wax and of the fractions prepared from it, in a mixed solvent consisting of 30 vol-% acetone, 35 vol-% benzene and 35 vol- % toluene. The melting points of the fractions were 56 "C for the hard wax, 50 "C for the intermedier wax and 46 "C for the soft wax. Many of the commercial paraffin waxes contain natural or synthetic waxes or other types of additives. It is, therefore, an important characteristic of their miscibility. In the molten state, petroleum waxes are normally well miscible with vegetable waxes, e.g. carnauba and candelilla wax. At melt temperatures only slightly above the melting point of the paraffin wax, however, solubility of vegetable waxes having higher melting points is rather poor. When such mixtures are stored in the temperature range between the melting points of the pure components, crystallization of the higher-melting component can occur. In the solid state of these systems two phases are present, the higher-melting vegetable wax crystals being uniformly distributed in the paraffin wax. This is presumably the reason for the significant rise in hardness of paraffin waxes even at small concentrations of vegetable wax. It has been observed that high-melting vegetable waxes readily soluble in lowermelting paraffin waxes cause smaller changes in properties than poorly soluble vegetable waxes. This is in agreement with the well-known fact that mixing of a low-melting and a high-melting paraffin wax, readily soluble in one another, 9
130
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
causes only slight changes in properties, except for large concentrations of the higher-melting paraffin wax. The majority of natural resins are well miscible with paraffin waxes. However, the stability of such mixtures is not always satisfactory, because the resins sometimes separate already in the melt or become oxidized. Substantial property changes will take place again only if the solubility of the resin in the paraffin wax is poor. Miscibility of paraffin waxes with synthetic resins is a more complex question. Products containing substantial amounts of oxygen in the macromolecule are poorly soluble or insoluble, whereas polymers consisting essentially of hydrocarbon chains are, owing to their similarity of structure, well miscible with paraffin waxes.
7. Adhesive properties Two adhesive properties of paraffin waxes are sealing strength and blocking point. Laminated packaging materials consisting of two or more layers of usually different materials, e.g. paper, plastic films, metal foil, are frequently manufactured with paraffin wax as sealing agent. Both macro- and microcrystalline paraffin waxes, and products based on such waxes and containing various additives, are used for such purposes. Sealing strength is an important characteristic of laminates. It is numerically expressed by the force per unit length required to separate two bonded layers (paper-paper, paper-metal foil, etc.). The sealing strength of laminated packaging materials depends on the properties of the paraffin wax as well as on the amount of wax and on the laminating process. When laminating paper to paper, sealing strength will depend almost exclusively on the cohesion of the paraffin wax, since the latter readily penetrates into the pores of the paper and will be firmly anchored there. In paper to metal foil laminates, the strength of the bond will be chiefly determined by the adhesion of the wax to the metal, since the wax cannot penetrate into the metal layer. In metal to metal laminates, the bond strength will depend solely on the adhesion of the wax. In laboratory tests for measuring sealing strength, in addition to the above variables, testing conditions will affect results. The most significant testing conditions are angle of separation, speed of pulling the layers apart and the elapsed time between lamination and testing. In addition to suitable cohesive properties, flexibility is an important requirement with which laminating waxes must comply. As well as bonding, the wax film must also act as moisture barrier. To satisfy these various and partly contradictory requirements, laminating paraffin waxes contain various additives, e.g. polyisobutylene, ethylene-vinyl acetate copolymers, vegetable waxes, etc.
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
Testing temperature, "C
Pulling speed, cmlmin
Paraffin wax load on paper, mN/mz
268.8 206.0 206.0 206.0 188.4 157.0 160.9
131
Sealing strength, mN/cm
14.7 13.7 12.8 11.8 10.6 10.1 10.1
Sealing strength of microcrystalline paraffin waxes usually greatly surpasses that of macrocrystalline refined waxes. Macrocrystalline waxes in the 360-420 molecular weight range and 5 M O "C melting range, and containing less than 1 wt-% of oil, are considered by many as not suitable for laminating, due to the low sealing strength. Among microcrystalline paraffin waxes, the harder types having higher melting points exhibit lower sealing strength values than the softer, plastic types with lower melting points. Table 1-61 lists the sealing strength of a macrocrystalline paraffin wax (melting point 62 "C)at different thicknesses of the paraffin wax film, i.e. mass of applied wax per unit surface area. The same results are also presented in Fig. 1-55. In the example referred to, it may be observed that above values of 157 mN/m2 sealing strength increases with the thickness of the wax film. However, this increase is presumably due to the fact that, upwards from a defined film thickness, the readings of the testing instrument will include the stiffness of the wax film-paper system in the sealing strength value.
E 2 Z E
15
5 cn
6
L
m
10
4 -
cn
-._0 C
0
rn
5 Wax load, mN/m2
Fig. I-55. Sealing strength versus paraffin wax load on paper
9'
132
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 2-62. Effect of waxing temperature on the sealing strength of macrocrystalline paraffin wax Testing temperature, "C
Waxing temperature,, "C
25 24 26
71 88 99
Paraffin wax load on paper, mN/ma
Sealing strength, mN/cm
Sealing strength of this paraffin wax, at film thicknesses around 157 mN/m2, changes little with temperature of waxing (Table 1-62). The effect of waxing temperature depends on the properties of the wax. Additives in the paraffin wax may result in increased dependence of the sealing strength on waxing temperature. Lamination temperature significantly affects sealing strength even when using relatively homogeneous waxes. Listed in Table 1-63 are sealing strength values, at different laminating temperatures, for the macrocrystalline wax having a melting point of 62 "C. Table 1-63. Effect of laminating temperature on the sealing strength of macrocrystalline paraffin wax
Laminating temperature, O C
Sealing strength, mN/cm
93 120 150
10.1 10.5 13.7
In continuous film formation, sealing strength values increase with rising laminating temperatures, and decrease, at identical laminating temperatures, with rising final cooling temperatures. This effect is more significant with waxes containing additives. Table 1-64 lists sealing strengths at different laminating and Table I-64. Sealing strength of paraffin wax containing polyethylene wax at different laminating and final cooling temperatures Polyethylene wax, wt- %
1.1 1.1 1.1 1.1
Laminating temperature, "C
97 107 97 107
Final cooling temperature, OC
Sealing strength, mN/cm
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
133
cooling temperatures for a macrocrystalline paraffin wax (melting point 60 "C) containing 1.1 wt- % polyethylene wax. Adhesion of microcrystalline paraffn waxes is substantially higher than that of macrocrystalline paraffin waxes. For this reason laminating waxes always contain microcrystalline paraffin waxes. Table 1-65 contains adhesion values for Table 1-65. Adhesion of micro- and macrocrystalline para& waxes Sample
A
Drop melting point, "C (ASTM D 127)
Needle penetration at 25 "C, 0.1 mm (ASTM D 5)
Adhesion at
25 Oc9 mN'cma
J K
73 69 75 12 76 14 67 73 65 63 89
24 24 27 21 30 22 24 25 38 41
I
486.6 449.3 440.5 410.1 410.1 410.1 358.1 358.1 281.5 182.5 68.7
L
57
15
0.0
B
C D
E F G H
Z
various microcrystalline paraffin waxes (samples A-K), and one macrocrystalline wax (sample L), having various drop melting points and needle penetration values. Adhesion is here defined as the force required to separate unit surface area bonded layers. The values refer to the separation of a cellophane film coated on both sides with wax and laminated on both sides with cellophane, i.e. to the separation of two laminations. The data in the table exhibit great differences between the adhesion of microcrystalline waxes having closely similar drop melting points or needle penetration values (e.g. samples A and H, samples A and F). Using this test method, the macrocrystalline refined wax L - owing to its largely differing crystal structure - yields an adhesion value of zero. Adhesion of the high drop melting point, hard microcrystalline wax K is also small. Another important adhesive property of paraffin waxes is the so-called blocking point, characterizing the sticking of waxed paper and self-sticking of waxes. The blocking point is defined, according to ASTM standard, as the lowest temperature at which waxed papers will stick together sufficiently to injure the surface films and performance properties. Since blocking may occur over a temperature range, a 50% blocking point is measured. For paraffin waxes having identical melting points, increased oil content substantially reduces the blocking point. Additives having lower melting points than the paraffin wax, if present in larger amounts, will also lead to low blocking points.
134
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
With macrocrystalline paraffin waxes, a lower transition temperature also lowers the blocking point, since the crystal modification stable at the higher temperature is always softer than the modification stable at the lower temperature. In Table 1-66, blocking points for a number of low oil content macro- and microcrystalline paraffin waxes are shown, together with their melting points. The data indicate that the blocking temperature is usually lower by 10 to 20 "C than the melting point. Table 1-66. Melting and blocking points of paraffin waxes Difference be-
and blocking point, "C ~
A B C D E F G H I J K L M N 0
P R S T U V
51 50 51 52 54 55 55 56 55 56 58 59 59 58 59 61 62 64 67 71 84
35.6 32.2 32.2 33.9 33.9 37.2 36.7 36.1 39.1 36.7 39.7 43.0 41.9 42.4 43.0 44.1 45.2 52.9 54.0 53.4 63.9
15.4 17.8 18.8 18.1 20.1 17.8 18 3 19.9 15.9 19.3 18.3 16.0 17.1 15.6 16.0 16.9 16.8 11.1 13.0 17.6 20.1
Paraffin waxes are crystallized from a variety of solvents. The separation temperature of crystalline waxes is termed de-oiling temperature in the petroleum refining industry. From one and the same starting material, waxes differing in properties are obtained with different yields if the de-oiling operation is carried out a t different temperatures. Table 1-67 contains the blocking points of waxes obtained from a semirefined wax by crystallization from a mixture of methyl ethyl ketone and toluene, at de-oiling temperatures changing from -7 "C to 27 "C.Raising the de-oiling temperature results in paraffin waxes having higher blocking points. However, the temperature difference between the melting point and the blocking point decreases. At identical melting points, the blocking point of paraffin waxes largely depends on molecular weight distribution. A broader distribution results in lower blocking
+
135
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
Table 1-67. Relationship between blocking point and temperature of de-oiling
1
~ f ~ ~ - ~ Paraffin ~ & wax f "C
yield, wt-%
-7 -1 4 16 21 27
1
Melting point,
"c
1
Blocking Point, "c
58
84 77 71 60 46 12
Difference between m.p. and blocking I Doint. "c ~
38.0 40.2 42.4 45.8 49.6 51.2
60
61 62 64 66
20.0 19.8 18.6 16.2 14.4 14.8
points. Table 1-68 presents blocking points for n-alkane mixtures. The data reveal that at identical melting points of the mixtures, the blocking point increases with narrower molecular weight distribution. At the same time, the difference between the melting point and the blocking point is lowered. Both changes improve the usefulness of the paraffin waxes. Table 2-68. Blocking points of n-alkane mixtures differing in molecular weight distribution
Components
1
I
I C22H46-Ca4H70
C24HS0-cS2H66 C25H52-C20H6CI
C2,H,,--C2sH,s C27H56
Carbon atom number range
13 9 5
3
1
58.0 58.5 58.0 58.0 59.0
I
I
Difference
I
1
point, "C
41.9 45.2 47.4 51.2 54.0
~-
16.1 13.3 10.6 6.8 5 .O
Lower-melting components substantially reduce the blocking point. In order to decide whether the blocking-point-reducing effect of iso-alkanes and naphthenes is due to their lower melting point or to their chemical structure, the blocking points of a commercial refined wax, its blend with an iso-alkane-naphthene mixture (melting point 45 "C, 28 wt- % iso-alkane, 66 wt- % monocycloalkane, 6 wt- % dicycloalkane), and its blend with an n-alkane mixture (melting point also 45 "C) were measured. The data are presented in Table 1-69, demonstrating that the effects of the added components were essentially the same in both cases, both with regard to blocking point and to the difference between blocking point and melting point. This points to the conclusion that the blocking point-reducing effect of hydrocarbons, other than n-alkanes, is mainly the result of their lower melting point. However, presumably this conclusion is not valid for paraffin wax products containing hydrocarbons other than n-alkanes in large amounts.
136
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Table 1-69. Effect of iso-alkanes and naphthenes on the blocking point of paraffin waxes
! '
Blocking point, "C
60.0
44.0
16.0
90 % base paraffin wax 10 % isoalkane-naphthene mixture melting at 45 O c
58.5
42.4
15.0
90% base paraffin wax 10% nalkane mixture melting at 45 "C
58.0
41.9
16.1
Melting point, "C
Materials
Difference between m.p. and blocking point, "C
~ _ _ _ _ _ _ _ _ _
-
Base paraffin wax (70 % n-alkanes, 24 % iso-alkanes, 6 % monocycloalkanes)
+
+
8. Water vapour permeability
In the manufacture of packaging materials, paraffin waxes are chiefly used to reduce water vapour permeability of paper. This is valid for both coating and laminating waxes, the latter being often applied for the sake of the low vapour permeability of the paraffin wax film. Water vapour permeability tests are usually carried out not with the wax itself, but with coated paper, carton etc., that is, with the prepared packaging material, since paraffin wax is used directly for coating only in exceptionalcases (e.g. fruits cheese). The water vapour permeability of waxed packaging materials is affected by a number of variables, including the properties of the paraffin wax, conditions of wax application, properties of the paper and mechanical stresses. No realistic picture can be obtained concerning permeability, if paper specimens treated under similar coating conditions, but using different macro- and microcrystalline paraffin waxes are compared. Instead, coating should be carried out under specific conditions for each paraffin wax, these being optimum conditions for the wax in question, and only such coated papers should be submitted to water vapour permeability testing. Permeability to water vapour of coated or laminated paper specimens, prepared under optimum conditions and measured prior to being subjected to mechanical stress, is essentially the same whether macro- or microcrystalline paraffin wax has been used, since, for unbroken films, these two wax types are equally resistant to water vapour. However, water vapour permeability measured prior to mechanical stress (folding) is insufficient to evaluate the quality of packaging materials. Papers coated or laminated with paraffin wax will, almost without exception, be subjected to various mechanical loads, especially to flexing, during their service life.
137
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
As regards flexing, macrocrystalline and microcrystalline paraffin waxes behave quite differently. Refined paraffin waxes having macrocrystalline structures are brittle, easily cracking products. Under the effect of folding, the film coat will crack, the continuity of the film will be disrupted to a greater or smaller extent. Water vapour will readily permeate through the porous paper lying below the cracks. Hence, water vapour permeability will substantially increase under the effect of folding in a coating made with macrocrystalline paraffin wax. Among the great variety of microcrystalline paraffin waxes, coatings made of flexible products exhibit cracks to only a slight extent or not at all under the effect of folding, the continuity of the film remains largely undisrupted, and hence no significant change in vapour permeability will result. Plastic and flexible microcrystalline paraffin waxes would appear to be the most favourable for the manufacture of coatings and laminates with a low water vapour permeability. However, the tack of such soft microcrystalline waxes is too high, Table 1-70. Water vapour permeability values of paper coated with paraffin wax (sulphite paper, wax coating 25 wt- %) Coating material
Melting OC
j
-
Water vapour permeability* g/m* 24 h
(
folding
After folding
-
No coating
-
Refined macrocrystalline wax
54
0.20 0.45
12.4 16.2
Refined macrocrystalline wax
60
0.26 0.51
18.6 12.3
Plastic microcrystalline paraffin wax
67
0.27 0.39
3.9 4.8
* At
406.0
23 "C and 50% rei. humidity
their blocking point is low, their surface is dull and attracts much dust. For these reasons, they cannot be used in the pure state for coating, but are very suitable for blending to obtain low water vapour permeability coatings. Water vapour permeability values of paper coated with macro- and niicrocrystalline paraffin waxes, before and after folding, are presented in Table 1-70, 9. Water resistance
Water and aqueous solutions come into direct contact with waxed paperboard used for containers of deep-frozen food, milk cartons, paper cups, etc. Therefore, resistance to water is an important requirement in those applications where no water must be allowed to penetrate through the paraffin wax film.
I38
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Similarly to water vapour permeability tests, resistance to water is usually measured not with the pure wax, but with wax coated paperboard. Resistance to water of the composite material depends to a much greater extent on the properties of the paper substrate than in the case of water vapour permeability. When coating paper with a mottled surface (e.g. blotting paper), some of the wax will be absorbed between the fibres, but some fibres will protrude through the wax film to the surface. These fibres will then act as wicks in which moisture can easily penetrate through the wax film. Two types of water barrier tests are in use. In absorption tests, weighted waxcoated paper cups are filled with a 1 wt-% lactic acid solution coloured with methylene blue. The filled cups are then stored for 72 hours a t -4 "C or +20 "C, according to standard specification. Subsequently the cups are emptied and weighed. The amount of solution absorbed is determined from the weight increase. Simultaneously, the internal surfaces of the cups are visually checked for staining with methylene blue (location and magnitude of stained areas is observed). Resistance to water is considered satisfactory only if slight staining occurs, and that only at junctions and folds. The other test consists of a fatigue test. Waxed cartons, filled with water and sealed, are dropped at a rate of 144 per minute from a height of 9.6 mm, at 3 "C, by means of a suitable apparatus. The endurance of the waxed cartons is measured by the period before which 50% of the tested cartons (usually 24 specimens) exhibit cracks or break. In general, tests show that resistance to water of lower-melting microcrystalline paraffin waxes is superior to that of macrocrystalline refined waxes. This difference is presumably due to the denser crystal structure and higher flexibility of microcrystalline waxes. Blends of refined macrocrystalline and microcrystalline waxes are frequently used for waterproof coatings. 10. Electrical properties Owing to their relatively high dielectric strength, low water vapour permeability and good resistance to water, paraffin waxes, especially the higher-melting microcrystalline waxes, are used for insulating purposes in electrical engineering. Insulating coatings of paraffin wax provide efficient protection against both water vapour and water. The use of paper and textiles impregnated with paraffin wax for wire insulation has been known for a long time past. Paper impregnated with paraffin wax is used in capacitors as a dielectric; paper tubes coated with paraffin wax are placed inside the metal casing of dry cells to reduce desiccation of the cathode mixture. Pure paraffin wax or its blend with bitumen is frequently used in capacitors to keep components in place. Ceramic insulators are also frequently impregnated with paraffin wax to reduce water vapour permeability. The volume resistivity of paraffin waxes is in the order of 1016-5 . I d * ohm/cm3. The values normally decrease with rising temperature. However, in the solid-phase
139
(D) PHYSICAL PROPERTIES OF PARAFFIN WAXES
transition range of macrocrystalline paraffin waxes, this change is frequently inverse. Data found in the literature on the dielectric strength of paraffin waxes vary within very wide limits. The inordinately large differences are due to variations in chemical composition and purity, as well as to the different methods of measurement used by the various authors. Table 1-71 lists relative permittivity data for paraffin waxes and, for comparison, for some natural waxes. Table 1-72 contains relative permittivities and, by way of example, some dielectric strength values measured with two microcrystalline paraffin waxes differing in properties. Table 1-71.Relative permittivities of paraffin waxes and some natural waxes Substance
Paraffin waxes Ceresins Candelilla wax Carnauba wax Beeswax
Initial value
After 6 months storage in 3.5 Wt-% NaCl solution
2.19-2.24 2.1 6-2.24 2.38-2.49 2.66-2.8 3 2.87-2.88
2.31-2.55 2.29-2.32 2.50-2.62 3.84-4.19 3.11-3.26
'
' 1
after repeated
2.24-2.30 2.28-2.29 2.45-2.56 2.82-2.83 2.84-2.90
Table 1-72. Dielectric properties of two microcrystalline paraffin waxes Properties
1
i
WaxA
i
88
Melting point, OC Needle penetration at 25'C, 0.1 mm Average molecular weight Oil content, wt- %
WaxB
2 890 0.6
84 7 804 10.4
I ~
Temperature, "C 80 60 40 20
Temperature, O C 20 40 60
2.46 2.37 2.31 2.16
2.30 2.39 2.25 2.09
2.38 2.42 2.56 2.62
2.47 2.44 2.44 2.45
2.49 2.53 2.74 2.79
Dielectric strength, kV/cm 82 1500 71 780 22 625
!
I
2.48 2.50 2.74 2.79
140
I. PROPERTIES OF LIQUID PARAFFINS AND PARAFFIN WAXES
Literature ASTM D 1465-57 T. ASTM D 1500-58 T. ASTM D 156-53 T. ASTM D 1321-61 T . ASTM D 937-58 T. ASTM D 1320-60 T. Berne-Allen: Ind. Engng. Chem., 30,806 (1938). Brooks, K. W., Oil Gas J., 58,89 (1960). Buchler-Graves: Ind. Engng. Chem., 19, 718 (1927). Davis, D. S., Ind. Engng. Chem., 32, 1293 (1940). Edwards, R. T., Petrol. Refiner, 36, 180 (1957). Eucken, A., Elektrochemie, 45, 126 (1939). Ferris-Cowles: Znd. Engng. Chem., 37, 1054 (1945). Gray, C. B., J. Inst. Petrol., 29, 236 (1943). Gruse-Stevens: Chemical Technology of Petroleum. McGraw-Hill Co., New York (1960). Hickel, A, E., Petrol. Refiner, 24, 207 (1945). Johnson, J. F., Ind. Engng. Chem., 46, 1046 (1954). Kinsel-Phillips: Znd. Engng. Chem., 17, 152 (1945). Kolvoort, E. C. H., J . Znst. Petrol., 24, 338 (1938). Lord, H. D., J. Inst. Petrol., 25, 263 (1939). Mazee, W. M., Red. Trav. chim. Pays-Bas, Belg., 67, 197 (1948). - : J. Znst. Petrol., 35, 97 (1949). M6zes-V8mos: Reoldgia 6s reometria. (Rheology and rheometry), Miiszaki Konyvkiad6, Budapest (1968). M6zes-Zsida-FBnyinB: MA-FKZ K6zI. (Report of the Hungarian Oil and Gas Research Institute), 9, 241 (1968). - : Chem. Tech. Berl., 20, 481 (1968). Miiller, A,, Proc. R . Soc., A 120, 437 (1928); A 127,417 (1930); A 138, 514 (1932). Padgett, F. W., Oil Gas J., 36, N o . 38, 30, 45 (1938). Padgett-Hefley-Hendrikson: Ind. Engng. Chem., 18, 832 (1926). Phillips, J., TAPPZ. Bull., 41, 291 (1958). - : Petrol. Refiner, 38, 193 (1959). Scott-Harley: J . Znst. Petrol., 25, 238 (1939). Seyer-Fordyce: J. Am. chem. SOC.,58,2029 (1936). Seyer-Morris: J. Am. chem. SOC.,61, 1114 (1939). Seyer-Patterson: J . Am. chem. Soc., 66, 179 (1944). Smith, A. E., J. chem. Phys., 21 2229 (1953). Templin, P. R., Znd. Engng. Chem., 48 154 (1956). Thorpe, T. C. G., J . Inst. Petrol, 37 275 (1951). Tiedje, J. L., Proceed. Fourth World Petroleum Congr., Section W/B, Preprint 1 (1955). Turner-Brown-Harrison: Ind. Engng. Chem., 47, 1219 (1955). Ubbelohde, A. R., Trans. Faraday Soc., 34, 282, 292 (1938). Van Vinkle, M., Petrol. Refiner, 27, 291 (1948). Warth, A. H., The Chemistry and Technology of Waxes. Reinhold Publishing Co., New York (1956). Watson, K. M.. Znd. Engng. Chem., 23, 360 (1931). West, C. D., J. Am. chem. Soc., 59 742 (1937). Wibaut-Langedijk: Red. Trav. chim. Pays-Bas, Belg., 59, 1220 (1940).