Chapter Four Additives

CHAPTER FOUR ADDITIVES

It should be apparent from the first three paragraphs of this book that many of the properties which all but the simplest lubricants need in order to fulfil their functions in the design and operation of machines can only be obtained - in absolute terms or economically - by using chemical additives. In this chapter, we examine the nature, composition and properties of many different types and species of addititive in all the classes of lubricants and special fluids already surveyed. Much of this examination relates not only to the effect of the indidual additives themselves, but also the product of the interaction between additives and lubricant bases, between additives and between all of these and the surfaces to be lubricated.

Additives can be defined as substances which improve the performance of lubricants, either by imparting new properties to a base oil or grease, or by enhancing properties already present. The performance of lubricating oils is classified according the effects of base oil and additives respectively is summarised in Table 4.1 below. Table 4.1. Influence of Base Oil Stock and Additives on the Properties of

Lubricating Oils

Effects dominated by base oil choice

Effects common to base oil and additives

Effects dominated by additives

Density Evaporation rate Flash- and fire-points Compressibility

Viscosity Viscosity-temperature characteristics Pour-point Low temperature rheology Resistance to foaming Thermal stability Oxidation stability Carbonisation residue Miscibility and solvent power Colour Physiological effects Odour

Detergent-dispersanteffects Emulsibility and demulsibility Friction properties (lubricity) Anti-wear properties Anti-corrosion and anti-rust properties Adhesion to metal surfaces Alkalinity Ash content

Specific heat Thermal conductivity Electrical conductivity Resistance to radiation

The need for improvement in the performance of lubricants by the use of additives has been brought about largely by the increased technical specifications 255

of machines - more severe conditions of extended operation, attempts to prolong the service life of machines and decrease the consumption of lubricants and the adoption of new and more sophisticated processes. Additives offer very important improvement possibilities and are indispensable in the production of modem lubricants. The use of additives, in the modem sense, began in the 1930’s and enormous growth has been seen since in both their production rates and the scope of their applications. Annual world-wide consumption of additives for lubricants and special oils is estimated at more than 2.5 million tonnes (including, in 1990, 0.2 million te zinc dialkyldithiophosphates, 0.8 million te metallic detergents, 0.5 million te ashless dispersants and 0.5 million te VI improvers). The major portion is used in the production of engine oils. For example, in 1990, the US additive consumption could be broken down into 64% used for blending engine oils (37% for multi-grade automotive engine oils, 20% for monograde engine oils and 7% for other engine oils) and the remaining 36% for blending industrial oils and greases (13% for hydraulic and automatic transmission fluids, 8% for cutting and metal-forming fluids, 2% for gear oils, 13% for other types of oils and greases (396). The increased dosage of additives in different lubricants (especially in engine oils) has led to a much higher increase in the consumption of additives than that of finished lubricants. The production of additives has become an important and innovative branch of the speciality chemical industry.

Usage of most lubricating oil and grease additives may be divided into two groups according to their function: (1) additives which improve the physical properties of the lubricant, (2) chemically-acting additives, whose effects can usually be measured by some efficiency characteristic of the lubricant. However, the boundary between these two groups is not clearly defined. The type of additive determines the importance and extent of its application. Hence, additives may be classified as primary and special. Primary additives are commonly employed in the most frequently used groups of lubricating oils, e.g., automotive oils. Special additives impart specific properties to certain types of lubricants. The largest quantity and number of additives are used for the improvement of petroleum-based oils in the manufacture of finished engine oils. Many lubricant additives exhibit multifunctional properties and, for instance, some typical antioxidants simultaneously act as corrosion and rust inhibitors, whilst also acting as metal deactivators and anti-wear agents. Many viscosity index improvers also act as pour-point depressants and some increase the adhesivity of the oil to metal surfaces. Some film-strength improvers also improve lubricity. Both synergistic and antagonist effects can occur when a number of different additives is used together. This must be taken into account when selecting both the appropriate compounds for a particular application and the correct dosage to achieve the overall desired effect. Manufacture of additive components themselves is an important area of chemical technology, but selection of the best balance of component species and relative dosages for each application is an equally demanding and expensive field of study. The identification of additives present in oils and, especially. quantitative determination of individual components can be a very difficult task, in both multi-component concentrates and in finished oils.

256

Dialysis, together with suitable spectroscopic and chromatographic techniques, can be recommended as a convenient analytical tool (182,192,319). In dialysis, similar in principle to ultra-filtration, smaller molecules diffuse relatively rapidly through the pores of a membrane, such as a rubber diaphragm, from a solution of the multi-component additive dissolved in petroleum ether or n-heptane, whilst larger molecules dialyse very slowly, if at all. Base oil, alkyl- and arylphosphates, esters of fatty acids, alkylphenols, alkyl borates, dialkyldithiophosphates and metal dithiocarbamates have small and easily dialysed molecules. Molecules of metal phenolates and sulphonates dialyse slowly. Molecules of basic metal phenol sulphides, sulphonates and salicylates, barium phosphonates, polyisobutenes and polymethacrylates do not dialyse at all. Sometimes, only the lower molecular weight portions of additive mixtures can dialyse, e.g., in polymethacrylates and succinimides. The individual components of the dialysate and the residue can be determined by IRspectroscopy, NMR or other methods, advantageously in combination (320). Some types of oil additives can be identified by the amount of the characteristic chemical elements. For example, zinc indicates the likely presence of antioxidants of the zinc dialkyldithiophosphate (ZDDP) type; higher amounts of nitrogen the presence of a succinimide-type ashless dispersant or other compounds with amino or amido groups. Calcium, barium and magnesium indicate the presence of ashforming detergents; both chlorine and lead are typical for high-pressure anti-wear agents, and silicon for anti-foams. Elemental analysis can thus reveal some of the characteristic elements typical in individual additive types and may prove useful in the identification of additives in the treated oil. Both emission and atomic absorption spectrometry are convenient methods for the determination of the metal content of lubricants, and have to a large extent supplanted in routine use the more traditional gravimetric, volumetric, chelatometric, colorimetric and photometric methods which are specified in many standard specifications.

In practical applications, additives must be fully soluble in the oil, with a high resistance to cloud and sediment (deposit) formation, even after long-term storage conditions and after temperature fluctuations. Some additives are mutually incompatible and, when they are used together, cloud and sediment can be formed. The same can happen when oils are mixed which contain incompatible additives. Compounded oils must therefore be tested not only for storage stability but also for blending with other finished oils and storage. The solvent properties of the base oil into which additives are compounded can have important effects. High viscosity index, paraffinic and hydrocracked stocks present particular blending problems. It is normal practice in the industry to test additives by blending into selected, “difficult” base stocks, as well as for miscibility with other blends which may be encountered in the application when lubricant reservoirs are replenished from other sources of supply. Storage stability can be tested either in accelerated tests under artificial conditions (the oil is heated to 120-150 “C, then cooled to the ambient temperature of storage and the amount of sediment measured by centrifugation), or under long-term storage conditions (the oil is, typically, stored at ambient temperature for 60 days, without mixing and without exposure to light. The homogeneity of the oil and the amount of sediment are then determined.). Additive compatibility in the case of oil-mixing is typically tested by mixing test oils in various ratios, from 1:l to 9:1,and testing the resultant mixtures by similar methods to those used for testing storage stability.

It is particularly important during additive manufacture, handling and storage to prevent quality deterioration - by over-heating, contact with moisture, mixing with incompatible additives - and to protect operators from possible harmful physiological effects ( I 72). 257

4.1 ANTIOXIDANTS (OXIDATION INHIBITORS) Two types of antioxidants are used, either separately or in combination, to improve the oxidation stability (or resistance to oxidation) of finished lubricants: (1) radical acceptors or scavengers, (2) peroxide decomposers. Antioxidants are sometimes combined with metal deactivators or passivators (1-3, 136). Current antioxidants are less effective or even ineffective in unrefined or under-refined oils. Such oils are less responsive to the normal types of antioxidants due to their high aromatic content. Some oil constituents, especially resinous materials derived from the crude oil, may make antioxidants ineffective.

Oils for turbine, hydraulic, electro-insulating and similar, relatively low temperature applications, are mostly blended with radical acceptors and metal deactivators, peroxide decomposers being used to a lesser extent. Low temperature oxidation, i.e., at temperatures up to about 150 "C,is involved under these operating conditions. In engine and other oils which work at high temperatures, peroxide decomposers are predominantly used, sometimes in combination with metal passivators and, to a lesser extent, radical acceptors. Radical acceptors also improve the storage stability of such oils. Radical acceptors, used in combination with peroxide decomposers,may markedly improve the overall oxidation stability of oils. Oils exposed to strong radiation may be blended with anti-rads.

4.1.1 Radical Acceptor-Qpe Low Temperature Antioxidants Common low temperature antioxidants which are based on radical acceptors or scavengers, InH, interrupt the oxidation chain reaction both by entering into the propagation phase and by hydrogen transfer to R* and RO,* radicals (I):

- ROOH + In*

ROO* + InH

- RH + In*

R* + InH

(4.1) (4.2)

Depending on the strength of the InH bond and the character of the In* radical, other reactions can occur. For instance: initiation: InH + 0,

- In* + HO,

(4.3)

RH + In*

- R* + InH

(4.4)

- In - In

(4.5)

propagation: terminations: 2 In* 25 8

2 In* + ROO*

In* + R*

- InOOR

(4.6)

- InR

(4.7)

During termination reactions, inactivation of the radicals produced takes place; the higher the reaction rate and the less reactive is the In* radical, the more effective is the inhibitor and the slower the oxidation of the oil. By contrast, a higher reactivity of the In* radical enhances chain transfer reactions and the oxidation proceeds according to reaction (4.4)despite the presence of the inhibitor, albeit at a lower rate. Initiation by reaction (4.3) is also possible at high inhibitor concentrations (2,3,136).Because of the dependence of all the individual reaction rate constants on temperature and oil composition, the effects of inhibitors varies with oil type. In extreme cases, the inhibitor may even exhibit oxidising properties. Reactions in oils are even more complex because of the presence of “natural” inhibitors, already present in the virgin oil or formed during its oxidation. Moreover, the interaction of various oil constituents or their oxidation products with inhibitors and their oxidation products (I)may make the situation even less clear. The most frequently used low temperature antioxidants of the free radicalacceptor type are electron-donor compounds with particular reference to their reactions with peroxy radicals ROO.. As examples, phenols, aromatic amines and compounds which form phenol and amines by their decomposition may be cited. Phenols may either contain one ring with one or more functional groups, or a multiplicity of rings condensed or linked by bridges. Electron-acceptor compounds are less common as inhibitors. The alkylperoxy radicals do not readily lose their electrons and add to double bonds, except where these are activated. For example, in some polyaromatic hydrocarbons such as alkylnaphthalenes, anthracene or phenanthrene (4, 6), the following reactions are believed to occur:

+ROO*-

HOO

H

Disproportionation products

>

&@ ‘ H

‘OOR

102

@ ROO

H

In reality, the action of these aromatics is to slow down oxidation, rather than inhibit it.

259

Phenols Phenols alone do not act as antioxidant. Even mono-alkyl phenols show little activity (5, 136). The most important compounds, in this context, are sterically-hindered phenolic rings of the type: OH

R3

in which R,, R,, R, represent alkyls. These compounds react with ROOand ROO. radicals and eventually form quinones, via relatively stable semiquinone radicals (6-9): OH 0. 0

+ ROO.

(4.9)

(-ROOH)

I

I

R3

R3

InH + ROO* OH

R*+Ro;*

ROO

- InH-OOR -In-OOR + ROOH +ROO*

(4.10)

OH

- R+ooR I

R3

R3

R3

InH + ROO.

- n-complex - In*+ ROOH + ROO*

R3

0

In* + ROO.

260

R,

ROO

- InOOR

R3

(4.12)

(4.13) R3 (4.14)

(4.15)

2 In*

- In -In

(4.16)

The following alkylphenols are examples of antioxidants of this type:

tBU

tBu

Me

tBu

Me

where Me represents methyl and tBu tert-Butyl. In hydrocarbon medium, the effectiveness of these phenols increases from (I) to (V) (10) and is enhanced by the presence of at least one tert-butyl group in the ortho position with respect to the hydroxyl (10, 11). Both di-tert-butyl-4-methylphenol (111) and 2,4-dimethyl-6-tert-butylphenol(V) are commonly used. Methyl groups in positions 2 and 4 and tert-butyl groups in positions 2 and 6 stabilise the transition radicals by inductive, hyperconjugative and steric effects, increase the solubility of phenols in hydrocarbons, decrease their solubility in water and protect them from the direct influence of oxygen. The presence of the bulky tertiary alkyl group in position 4 is undesirable, as it hinders the reactivity of the ROO* radical in this position (7, 10). There is an optimum steric hindrance of the hydroxyl group; too intensive stenc hindrance depresses the course of the reaction, whilst weak steric hindrance enhances chain-transfer reactions. Substituents, such as alkyl, aryl and alkaryl, which increase the electron-donor character of the inhibitor, enhance its activity when present in the para or ortho positions. Electron-acceptor substituents (-Cl, -NO,, -COOH, etc.) have a negative effect on inhibition (12). 2,6-ditertbutyl-4-methylphenolis the most frequently used monohydroxy phenol used in mineral oils. It acts efficiently in both refined and highly-refined oils. Its efficiency is strongly depressed in poorly-refined oils. However, in contrast to some other antioxidants, it does not enhance the formation of sludges in the course of ageing ( I ) . Its disadvantage is its high volatility, which limits its usage at higher 26 1

temperatures. This phenol can also be used in synthetic oils and lubricating greases, provided they are not basic. The same holds true for other phenols of a similar type. 2- and 3-tert-butyl-4-methoxyphenolis used as a special inhibitor for oils or waxes which come into is also believed to be non-toxic. contact with food products. 2,6-di-tert-butyl-4-methylphenol

Poly-hydroxy single ring phenols, such as pyrocatechol, hydroquinone and pyrogallol and their derivatives, have not found application in mineral oils, but they can be used as inhibitors in some synthetic oils such as polyglycols. The same is true for hydroxylated, condensed aromatics, e.g., polyalkylated naphthols and dihydroxylated anthraquinones, which are used in ester oils. Bis-phenols with inter-ring bridges ortho or para to the hydroxyls are very thio-bis-phenols important. 2,2’-methylene-bis-phenol,4,4’-methylene-bis-phenol, and dithio-bis-phenols can be cited as examples. In these compounds, one alkyl in the ortho or para positions with respect to the hydroxyl should be tert-butyl. The activity of bis-phenols, which are obviously analogues of hindered monohydroxy phenols, is higher than would result from the doubled hydroxyls. The molar concentrations of bis-phenols required to achieve the same effect is therefore less than half that for mono-hydroxylic phenols. Another advantage is their much lower volatility. However, they have a greater tendency to form oxidation sludges, which increases in the order methylene-bis-phenolscthiobisphenolscdithiobisphenols(I). Examples of this type of inhibitor are 2,2’-methylene-bis-(4-methyl-6-tertbutylphenol) and 4,4’-methylene-bis-(2-methyl-6-tert-butylphenol), the latter being normally a little less effective. Both phenols are typical radical scavengers, unable to decompose hydroperoxides. They actually inhibit peroxide decomposition, as evidenced by studies with cumene hydroperoxide as model compound. On the other hand, analogous 2,2’-or 4,4’-thio- and dithio-bis-phenols impart both radical acceptor and peroxide decomposer properties and are, therefore, even more effective than methylene-bis-phenols (I).Their tendency to form sludges can be suppressed by other additives. It has been found that, in combination with dialkyldithiophosphates, these ashless bis-phenols effectively suppress thickening of engine oils, acid formation, bearing corrosion and the formation of insoluble materials in heavily-loaded passenger car gasoline engines (13). It is well-known that the replacement of sulphur by selenium can be advantageous in some antioxidants (38).In the case of bis-phenols, however, this substitution did not prove to be useful ( I ) .

Amines Primary, secondary and tertiary aromatic amines and diamines are effective antioxidants. The effectiveness of aliphatic amines is doubtful; they are used in lubricants only as colour stabilisers. Several views exist on the mechanism of amine action. In the case of primary and secondary amines, hydrogen transfer is assumed to be the first step, followed by the formation of comparatively stable iminoxyl radicals (13,15):

262

- R’N*R” + ROOH R’N*R” + ROO* - ROO+ R’R”N.0

ROO* + R’NHR”

(4.17) (4.18)

The R’N*R” radicals react further with peroxy radicals to form an end-product of the hydroxylamine type. Although this mechanism cannot be applied to tertiary amines, the possibility of hydrogen splitting from the ortho position with respect to nitrogen cannot be excluded. Theories which assume the existence of ionic transition states or ionic complexes formed by the electron shift have been postulated (14): ROO*+fi(-RO6+*6(,

or [ R O 6 * 6 C ]

(4.19)

or, alternatively, the following complex:

This complex further reacts with another peroxy radical to form molecular compounds (I6,17). Generally, the antioxidant properties of amines are enhanced by three factors: (i) effective delocalisation of the unpaired electron formally placedat the nitrogen atom, (ii) high electron density at the nitrogen atom to facilitate electron transfer to the radical, (iii) sufficient steric hindrance of the nitrogen to suppress chain transfer by the arylamine radical. In both mineral and synthetic lubricants, secondary amines are the most often used. Diphenylamine and its C-alkylated or N-alkylated, arylated or alkarylated derivatives, such as 4.4’-dioctyldiphenylamine,phenyl- 1-naphthylamine, phenyl-2naphthylamine, are the most important amines from this group. Both phenylnaphthylamines are very effective low temperature antioxidants, especially in mineral, non-aromatic oils and oils with optimum aromaticity. They act as radical acceptors and moderate peroxide decomposers. Their primary decomposition products act as inhibitors in the later period of ageing by oxidation. The inferior colour-stability and tendency to enhance the formation of insoluble products ( I ) is one disadvantage of these antioxidants. The established carcinogenicity of 2-naphthylamine makes the application of phenyl-2-naphthylamine dubious, in spite of its higher antioxidant efficiency as compared with pheny 1- 1-napthy lamine. Phenyl- 1-naphthy lamine and alky lated diphenylamines can even be used for the stabilisation of slightly basic synthetic oils and lubricating greases. For example, a mixture of 50 parts of phenyl-1-napthylamine, 25 parts of diphenyl-pphenylenediamine and 25 parts of p-diisopropyldiphenylamineis recommended as a universal inhibitor for lubricating greases.

263

Tertiary amines, such as N,N’-tetramethyldiaminodiphenylmethane(4,4’methylene-bis-[N,N-dimethylaniline])can be used in lubricating oils: Me

.Me

This amine is recommended for use in industrial oils. It is noteworthy that it belongs to a small group of antioxidants which, when used at a high enough concentration, is able to increase the stability of oils with a higher than optimum aromatics content. However, this inhibitor adds colour to oils, and like all antioxidants which do this, it enhances the formation of oil-insoluble oxidation products. Furthermore, its primary oxidation products seem to have only sIight antioxidant effects in mineral oil lubricants. Among the cyclic amines, 2,2,4-trimethyl- 1,2-dihydroquinoline, substituted in the 6- position, and its polymers are recommended for mineral, ester and polyglycol oils and greases: R

This inhibitor also possesses anti-ozone properties, important in plastics and elastomers. At the time of writing, its use is allowed in products which come into contact with food. In addition to phenols and amines, aminophenolic inhibitors are used. One example is 2,6-di-tert-butyl-4-N-dimethylaminomethylphenol: OH

This compound is used in industrial oils, e.g., turbine oil, in diester lubricants and also in lubricating greases. In some respects, it is similar to N,N’-tetramethyldiaminodiphenylmethane, already mentioned. At a suitable concentration, it can improve the properties of oils with higher aromatic contents, but its efficiency is lower. As in the case of 2,6-di-tert-butyl-4-methylphenol, it impedes the formation of insoluble oxidation products. However, it is very volatile, and its effectiveness, especially in non-aromatic oils and oils with optimum aromaticity, is lower. Contrary to expectations, a lower tendency, in comparison with other antioxidants, to form acid oxidation products has not been observed (I). Another aminophenol, commonly used in the USSR, is 4-hydroxydiphenylamine: 264

This has proved especially useful in oils of lower viscosity, e.g., in transformer oils. Many other compounds have been proposed as low temperature antioxidants of the radical acceptor type. Some of them have been used for special purposes, but they have not achieved broad acceptance (270). The following general conclusions can be postulated for the use of synthetic low temperature antioxidants in mineral oil lubricants: Synthetic antioxidants increase the oxidation stability, especially, of non-aromatic oils and oils of optimum aromaticity. The optimum composition of oils with respect to aromaticity shifts towards oils of lower aromaticity if both the inhibitor concentration and the severity of oxidation increase. Consequently, non-aromatic oils are more responsive to inhibitors during the early stages of oxidation, i.e., in the early stages of the life of the oil, whereas oils of optimum aromaticity, adequately blended with additives, are more responsive to inhibitors when the oil is more fully oxidised, i.e., in the later stages of its working life. Oils of high aromaticity are exceptional, in that they can only be blended with a few special inhibitors; inhibitors which can be widely and generally applied are not yet available for highly aromatic oils. When the more common inhibitors are used in these oils, they do not improve oxidation stability and can even cause it to deteriorate. Generally, it is necessary to avoid over-treating with inhibitors, whether they are “natural” or “synthetic” (1). General recommendations on similar lines regarding the choice of inhibitor for synthetic oils cannot yet be formulated. The choice of inhibitor for both polyester oils, based on polyols, and diester oils, based on mono-alcohols or polyglycols, depends on the temperature conditions under which the oil is to be used. Stericallyhindered phenols are recommended for temperatures below 125 “C, amines, selenides and phenothiazines for temperatures up to 150 and 175 OC (28).Recently, amines such as phenyl-1-naphthtylamine at a concentration of 1-2%, in combination with C- or N-substituted diphenylamines have been applied at temperatures up to 200 “C. For higher temperatures (up to 260 “C), polyalkylated naphthols, polyhydroxylated biphenyls and hydroxylated benzophenones (19) have been suggested. C- and N-alkylated phenothiazines, phenothiazine carboxylic acids (20) and some silicon compounds, such as 5-ethyl-10,lO’-diphenylazasilazene(I) ( 2 4 , and boron derivatives, such as lO-hydroxy-9,1O-boroxazophenthrene, also exhibit the required properties (11) (22).

265

These are frequently used in combination with both phenyl- 1-naphthylamine and a suitable metal deactivator (23).Alternatively, some metallic substances showing a synergistic effect, such as organo-tin compounds, e.g., (III) and (IV) (24) and alkali metal (Li,Na, K, Rb,Cs) salts of carboxylic acids (25), can be used together with compounds (I) and (11) above. Rl\

/R2

R3/ sn\R4

(111)

iF [ Sn

x-

[1:

2

- Sn / R1

(IV)

Phosphate ester oils are usually very resistant to oxidation, so antioxidants are not necessary. This is even more valid for polymers of trifluorochloroethylene. On the other hand, the diesters of fluorinated alcohols and dicarboxylic acids are somewhat less stable, and, moreover, almost insensitive, even antagonistic to common antioxidants. Both phenyl- 1-naphthylamine and phenothiazines have proved useful in polyglycol ester oils, as have some phenols, e.g., dihydroxyphenols (hydroquinone) and dibutyl-4,4’-diphenylolpropane,and some amines, such as substituted pphenylenediamines (N,N’-di-(2-naphthyl)-p-phenylenediamine,and in polyglycols, trimethyl- 1,2-dihydroxyquinolineand its polymers. Antioxidants developed for polyphenylether oils are very interesting in that they also inhibit the oxidation of aromatic compounds. Useful compounds include some organometallics, especially transition metal (Ti, Mn, Fe, Co, Ni) acetylacetonates. Similar compounds can be recommended for aromatic ester oils (26). The inhibition of silicone oils against oxidation is very difficult. The solubility of compounds of a different nature from the silicone is limited and special techniques sometimes need to be used. Silicone oils show good resistance to oxidation, but tend to become gelatinous at higher temperatures. Suitable inhibitors such as polyaromatics (e.g.,1.1 ’-dinaphthyl-1,Zbenzanthrene,fluoroanthrene)are effective up to a maximum temperature of 285 “C (27).Some organometallic compounds are especially recommended - ferrocene (cyclopentadienylmanganesetricarbonyl)(28). tin dibutyl laurate (29) and cerium (30) and selenium (dodecylselenide)derivatives. It is interesting to note that polymethylsilicones themselves retard the oxidation of mineral oils under static conditions. This is probably a surface phenomenon; the thin superficial layer of silicone hinders the transport of oxygen from the surface into the bulk oil (31). In contrast to silicone oils, the oxidation stability of silicon acid esters is relatively low. Nevertheless, they are very responsive to antioxidant treatment. The most effective antioxidants in this case are aromatic amines (especially phenyl-lnaphthylamine). Because peroxide formation is suppressed, hydrolytic stability increases at the same time as oxidation stability: the formation of peroxides and their 266

decomposition products, particularly water, is effectively controlled.

The Susceptibility to Oxidation Inhibition of Oil Lubricants During the oxidation of an oil, the sequence of reactions 2.82a and 2.82b which lead to non-branching oxidation chains must be stopped. An inhibitor interferes with the oxidation by reaction 4.1 (page 258) when a hydroperoxide is formed which is stable under the conditions of the oxidation. In reaction 4.2 which is atypical - the radical R* reacts very rapidly with oxygen - reactions 2.82a and 4.6 - with the formation of unreactive products. These reactions can be summarised in the general scheme: InH + nROO* = stable, unreactive products

(4.20a)

The stoichiometric coefficient n represents the number of ROO. radicals reacting with one inhibitor molecule. Inhibitors of the radical-acceptor type can be natural, synthetic or formed during oil oxidation. Very often, a mixture cif inhibitors is present, together with other additives, fresh or aged, in laboratory, static or road tests. The total amount of inhibitor present can be determined from the oxygen uptake of the oil, measured over time under suitable conditions (tig. 4.14. When effective antioxidants are present in the oil, its properties (viscosity, acidity, deposit formation) should not change during the tests (fig. 4.1b). (abs 1 I021

h

nllnl

b

a

Fig. 4.1. a) Determination of the induction period T~and T~ b) Relations between the induction capacity and the changes characterising the oil ageing (e.g., of viscosity, acidity, deposits) in dependence on time, on length of path (idealized model) - the broken line after the addition of antioxidant The “titration” of antioxidants by peroxy radicals is based on these principles (384). The rates of oxygen absorption by the oil under investigation and cyclohexene (Cy-H) are determined in an inert solvent (hexadecane) and the reaction is initiated by the decomposition of azobisisobutyronitrile (ANNA). The titration is carried out at a temperature such that the decomposition of primary hydroperoxides is negligible. 1 cm3 of Cy-H plus 9 cm3 of n-hexadecane is placed in a suitable reactor (39Z).0.5 cm3 of antioxidant InH solution in n-hexadecane (2-20.104 mol) is added at 60 “C. In a separate test, an equivalent amount of the oil under investigation in hexadecane plus 0.5 crn3 of AN-NA solution in chlorobenzene (0.03 0.1 mol) is used. The rate of oxygen absorption is measured until all the antioxidant is consumed. The following reactions occur: AN-NA = 2Aa + N2

(4.20b)

267

A. + 02=AOO* A 0 0 0 + CyH = AOOH + Cy*

(4.20~) (4.2Od)

The cyclohexenyl radical is further converted by the following reaction sequence:

cy. + 0, = cyoo. CyOO.

+ CyH = CyOOH + C y

(4.20e) (4.200

in the presence of the inhibitor, reaction 4.20f is suppressed: CyOO* + InH = CyOOH + In* CyOO. otherwise a chain reaction would ensure. Generally,

+ In*= CyOO-In

InH + nCyOO* = non-reactive products

(4.2Oi)

The oxygen absorption rate is plotted and the induction periods determined graphically for strong (z,) and total (3 inhibitor content. The length of the induction period is proportional to the inhibitor concentration and inversely proportional to the initiator concentration.The antioxidantconcentration can be calculated as follows: z R. [In HJ = (m~l.dm-~) (4.20j)

n

where Ri is the the rate of radical formation, for azobisisobutyronitrile Ri = 4.6.10-8 dm3.s-' at 60 OC. For a given antioxidant at a known concentration, equation (4.20j) enables comparison to be made of the effectiveness of different antioxidants in the same medium or the same antioxidant in different media. The stoichiometric coefficient n can be determined by means of equation (4.20j) only if the antioxidant is not substantially destroyed during the induction period. For hindered phenols n = 2, for bis-phenols n = 4, for zinc dialkyldithiophosphates which also act as radical acceptors n = 1 to 4, typically 3. From the value of the stoichiometriccoefficient, the mechanism of antioxidant action can be deduced. When there is insufficient information on the nature of the antioxidant, the total inhibition capacity of the oil can be determined by equation (4.20k): 7 R. n[ln HJ = 2 (mol.dm3) (4.20k) [OI where [O]is the concentration of oil in the test solution. The expression n[ln HJ applies to all antioxidants of the radical acceptor type. Metal deactivators and peroxides are not included in this definition. Over a period of time, determination of the value of this expression can be used to follow the gradual consumption of antioxidants, to monitor synergistic and antagonistic interactions between antioxidants and other additives and to study the influence of the conditions under which the oil has operated (e.g., temperature). In addition, the conditions can be determined under which it is necessary to recondition or rejuvenate the oil by addition of fresh antioxidant, or estimation of the oil drain interval (385,389).

4.1.2 Metal Deactivators The presence of metals and their compounds, even trace amounts present as impurities, is a very important factor in the ageing of lubricants. The catalytic action of metals and especially transition metals and their compounds depends on several factors which are still not understood in detail - metal type, nature of the oxidised substrate, oxidation products which are able to coordinate their bonding to metal,

268

etc. All these factors influence the redox potential of the metal compound - probably the most important variable - and the solubility of the metal compound and can be involved in steric hindrance (32). Metals and their compounds can influence the oxidation in the initiation, propagation and termination phases. The following processes can occur in the initiation phase: - reaction of metal compounds with the substance, e.g., RH + M"+

- R* + H+ + M("-') - R* + M"+

(4.21)

- reaction of metal compounds with hydroperoxides, e.g., ROOH + M("-')+

- RO* + HO- + Mn+

(4.22)

HOa + M("-1)+

(4.23)

HO- + M"+

- in summary: ROOH

- RO* + HO.

(4.24)

- ROO* + H+ + M("-')+

(4.25)

- or at higher peroxide concentrations: ROOH + Mn+

- ROO+ HO- + M"+ 2ROOH - RO* + ROO* + H20

ROOH + M("-')+

- in summary,

(4.26) (4.27)

- activation of the oxygen molecule:

02+

M"+/

M("+')+ 00,

+ RH

/

R* + HOO* + M"+

(4.28)

Mn++ 00 In the propagation phase, the activation of oxygen by metals can occur, leading to:

- ROO* ROO* + RH - ROOH + R* R * + 0,

(4.29) (4.30)

In the termination phase, radicals can be converted into anions by the action of metal compounds: ROO* + M@-')+

- ROO- + M"+

(4.31)

and metal compounds can accelerate the conversion of peroxy radicals into nonradical products: 2R00* (+ M"+)

- molecules (+M("-')+)

(4.32)

Note that the metals act as antioxidants in reactions (4.3 1) and (4.32). 269

Oxidation can be influenced by metals without any changes in their valency; zinc compounds, for example, interact in this way. The following reactions are believed to occur:

- ROOMX + HX

ROOH + MX,

- RO* + MXO* MXO* + ROOH - MXOH + ROO* ROOMX

MXOH + HX

or, in summary: 2ROOH

- MX, + H,O

- ROO*+ ROa + H,O

(4.33) (4.34) (4.35) (4.36) (4.37)

Copper, iron and lead are the metals which most often come into contact with lubricants and their power to influence oxidation in lubricants normally descends in this order (34). However, this order itself may change, depending on the reaction conditions. It is different for homogeneous and heterogeneous catalytic action and depends on temperature. For instance, below 150 "C, copper is more active than iron, whereas above 180 OC, iron becomes more active than copper (36). The metal-catalysed oxidation of lubricants is greatly influenced by electrondonor substances. They can act as oxidation promoters, if their complexes with metals are unstable, or as metal deactivators if these complexes are stable. Moreover, they act as antioxidants if they irreversibly transfer an electron to the metal, or decompose the peroxides. Finally, if the electron-donor group is sterically-hindered, they are ineffective as additives (35). The prevention of oxidative ageing enhanced by metal ions can be achieved with the so-called metal deactivators. The most effective are nitrogen compounds, which convert the ions into catalytically inactive chelates. Metal passivators are substances which reduce the solubility of metals by interaction with its surface. Some examples of Schiff's bases very important as metal deactivators, include N,N' -bis-salicylideny1- 1,2-diaminopropane: H I CH=N-CH-CH,N=C

q

OH

I

CH3

D

HO

and the analogous, but less soluble, diaminoethane (BSED) derivative, anthranilic acid, some acid amides (oxamides), thiadiazoles, imidazoles, pyrazoles (mercaptobenzimidazole), mercaptobenzthiazole, mercaptobenz-imidazoline,etc. Metal deactivators are commonly combined with radical acceptors in lower temperature applications, at lower concentrations (5 - 30 p.p.m.). Synergisticeffects can frequently be observed in these systems. Some metal deactivators are multi-

270

functional; for instance, anthranilic acid acts as radical scavenger, metal deactivator and anti-corrosive agent (37). The use of metal deactivators can, however, bring some problems. Their effect depends on the type of metal, their concentration, the oil environment in which they operate and other, often unclear factors. An example of this variety of behaviour is provided by the low temperature oxidation of tetralin (used as a model cycloalkane aromatic), initiated by azobisisobutyronitrile.Oxygen absorption was found to be hindered by BSED in the presence of Cu,Zn, Ni and Co acetonylacetonates, but BSED was less effective in the presence of Fe acetonylacetonateor caprylate and Cu stearate. Under similar conditions, anthranilic acid was effective in the presence of both elemental copper and its salts, as well as the acetonylacetonates of Ni and Zn. However, it was almost ineffective in the presence of Fe and Co acetonylacetonates and Fe caprylates. N-phenylanthranilicacid, acting simultaneously as a diphenylamine antioxidant, was effective in the presence of Co, Ni and Zn acetonylacetonatesand Cu stearate, but was practically ineffective in the presence of iron salts, Cu acetonylacetonate and elementary copper. The effect of the additive is expressed either by the reduction of oxygen absorption without change in the induction period, or by a marked prolongation of the induction period after which rapid oxygen absorption may follow (35).

The deactivator must therefore be selected with much care, because it is unusual to know the form in which metal is likely to be present in the lubricant. Certain chelates, such as the Co-chelate of BSED, activate the oxygen and can act as oxidants. They are recommended, for example, as catalysts for the oxidative “sweetening” of gasolines (39).

4.1.3 Peroxide Decomposer Antioxidants ROO. radicals are produced and disappear very quickly in oils at high temperatures, so that phenols or amines have insufficient time to intervene. Thus, another type of additive must be used if the oxidation proceeds beyond this stage. Oxidation retarders or high-temperature antioxidants, also called peroxide decomposers, are effective inhibitors. They act by decomposing hydroperoxides and peroxides immediately they have been formed during the propagation phase of the oxidation chain reaction, in such a way that the products of decomposition are molecular compounds rather than radicals. The propagation stage is thus eliminated and the reaction terminates in stable products after initiation. A typical termination reaction is the conversion of aromatic hydroperoxides to molecular compounds, by either (a) radical or (b) cationic mechanisms. Phenols formed in this way have antioxidant effects: -

R-oH

+ ’*

(4.38.1)

+ 9”

(4.38.2)

R-Co-R‘ Common types of peroxide decomposer inhibitor include sulphur and phosporus compounds, with both elements often simultaneously present in the same molecule. 27 1

As a rule, peroxide decomposers are themselves or produce substances of acidic character, acting by a cationic mechanism. Anionic mechanisms, resulting from basic decomposers, are less commonly encountered.

R\ R\ CHOOCHOOH + H O ' R/ R/ or, in the case of tertiary amines, ROOH + R',N

+ H20

mR'+ R/

CO

- ROH + R;NO

HO-

(4.39)

(4.40)

The majority of these compounds simultaneously exhibit both anti-corrosion and anti-wear properties. However, some sulphur compounds, such as sulphides with mixed alkyls, cycloalkyls and cycloalkql substituents, certain mercaptans and tiobis-propionic acid esters, may increase the corrosiveness of the oil. This corrosivity is caused by the gradual oxidation of the sulphur compound to sulphoxide (noncorrosive), sulphone (non-corrosive) and sulphonic acid (corrosive). Whereas the conversion to sulphoxide and sulphone is a desirable reaction because simultaneous reduction of the peroxide occurs (e.g., of ROOH to ROH), severe oxidation is dangerous. Sulphoxides are also peroxide decomposers, but are sometimes less effective than sulphides.

4.1.3.1 Sulphur-containing Decomposers Certain simple aromatic sulphides and disulphides, such as benzyl sulphide and dibenzyldisulphide, are peroxide decomposers, although they are more often employed as anti-wear additives. Sulphur-containing alkylphenols and aromatic amines, which also act as radical acceptors, are more efficient sulphide antioxidants. Examples of alkylphenol sulphides include: OH OH

where R and R' are C1-CI2alkyls or cycloalkyls, R" - alkyl, cycloalkyl, oxyalkyl, arylalkyl (alkyl C1-CI2), R"' - alkyl C,-C,,, n=1-2. 2,2'-dithio-bis-(3-methyl-4,6-di tertbutylphenol: (CH3)2C OH

(CH&C 212

CH3

CH3

C(CH3)2

and 4,4’-thio-bis-(2-methy1-6-tertbutylphenol) (1 ). Aromatic thiols and their derivatives are known to be corrosion and oxidation inhibitors and also anti-seizure additives:

Q R/&)y+ SH

SH

SH

SH

RO

SH

R

where (a) are .I-alkylphenols, R = alkyl C, - C,,, (b) are 2,5-dialkylphenols, R = alkyl C, - C,, (c) are 2,5-dialkyloxythiophenols,R = alkyl C, - C,, (d) is dithiohydroquinone. The sulphur compounds contained in base oils also have antioxidant action and are often referred to as “natural” antioxidants.

Selenium compounds, such as dialkyl and diarylselenides,e.g., dilaurylselenide, are even more effective. Some of them are effective up to 270 “C,others react with peroxides in the same way as sulphur decomposers by forming selenoxides, from which selenides are regenerated at higher temperatures (4 1). The disadvantages of selenides are their corrosivity and toxicity. 4.1.3.2 Sulphur- and Nitrogen-containing Decomposers

Phenothiazines and dithiocarbamates are the most important compounds in this category which contain both sulphur and nitrogen. Phenothiazines tend to form oxidation sludges in oils and hence find their applications mainly in the form of C- or N-alkylated derivatives of longer alkyls, such as the following:

where R groups are C,,

- C,, alkyls, and

R

273

where R,R’ are alkyls, aryls and cycloalkyls, etc.. R:’ R”’ the same or hydrogen. Metal dialkyldithiocarbamates(I), the products of the reactions of organic mines with CS, and metal hydroxide (most frequently zinc), are efficient oxidation and corrosion inhibitors, e.g., for silver bearings (404) and good metal deactivators. Some have anti-wear properties. According to Sanin (84), they are more efficient than the zinc dialkyldithiophosphates(ZDDP) and their thermal stability is 50-60 “C higher. They can be used in both oils and greases, but have not, so far, found as broad application as the ZDDP. They are, however, important as synergists for

ZDDP:

r

1

(1) where R are alkyls (identical or different) or heterocyclic radicals and M is Zn, Cd, Pb, Ni, Ba, Mg or other metals, like Co, As, Bi, Sb, Se and Mo. Compounds of this type which have been commercialised as additives include cadmium diamyldithiocarbamate (withdrawn because of cadmium toxicity), lead and zinc diamyldithiocarbamates.The zinc salt has been used as a complete or partial replacement of ZDDP in engine oils, for improvement of “factory fill” engine oils (engine, gear-box and final-drivehousings) and as oxidation inhibitors in lubricating greases for operation at elevated temperatures. Lead diamyldithiocarbamate is a good multifunctional antioxidant, with anti-wear, anti-seizure and anti-corrosion properties; it is also valuable as a friction modifier. Antimony dialkyldithiocarbamate has similar properties. Having a low ash content, it is suitable for two-stroke oils. The molybdenum analogue has been claimed to improve the anti-wear properties of lubricating greases twice to four times as much as MoS2 (399). Other sulphur- and nitrogen-containing oxidation inhibitors with anti-wear and anti-corrosion properties include derivatives of 2J-dimercapto- 1,3,4-thiodiazoles:

N-N

I I

R - (S)x - C C - (S),, - R ‘S’ These are predominantly used in gear oils, but also in turbine and hydraulic oils and fluids for automatic transmissions. Incorporated in emulsion fluids, they prevent welding of metal surfaces during metal-working processes at high pressures and act as rust inhibitors and biocides (42). 4.1.3.3 Phosphorus-containing Decomposers Some compounds of phosphorus (111), for instance trialkylphopshites, 274

triarylphosphites and trialkarylphosphites, can decompose peroxides. The peroxides are reduced, whilst the phosphites are converted into phosphates:

/ \ OR

O = P - O ROR

(R = butyl or longer alkyl or aryl group). Phosphites show very marked anti-corrosion properties, but they are insufficiently stable in the presence of moisture. They are used in plastics more often than in oils.

4.1.3.4 Decomposers Containing both Sulphur and Phosphorus Dithiophosphates The most widely-used peroxide decomposer inhibitors contain both sulphur and phosphorus. Sulphur provides the antioxidant properties whilst the anti-corrosion effect is supplied by the phosphorus. Certain simple sulphur-containing alkylphosphonates belong to this group of decomposers: OR

,

RS - CH,- P’

I \ OR

0

(R is alkyl C,, - C,,, R is hydrogen or C,

- C,,

alkyl)

Complex esters of phosphoric acid and bis-(alkylpheno1)-disuphides or polysulphides are other examples (43):

However, the most widely used types are the 0.0’-dialkyl-, diaryl- or alkylaryldithiophosphates (DDP) of metals, of which zinc is the most frequently encountered (perhaps more properly referred to as phosphorodithioates, but the term di-(alkylor aryl) dithiophosphates - abbreviated to DDP - is now almost universally used except by purists and will be used throughout here). They are produced by the reaction of alcohols (ROH), phenols (ArOH), alkylphenols (RArOH) or mixtures of these with phosphorus pentasulphide: 4 ROH + P2S5

- 2(RO)2P(S)SH + H2S

(4.4 1)

275

The acid produced by this reaction is then neutralised by a metal or its oxide or hydroxide. Another route is via the double-decomposition of alkali dithiophosphates by heavy-metal salts. If a metal oxide is used for neutralisation, a mixture of normal and basic salts is formed. Both salts are good anti-oxidants; of the two, only the basic salt is insoluble in methanol. Small amounts of other esters are formed in the reaction of PzS, with alcohols: OH I S=P-OR I SH

SH I S=P-SR I SR

OR OR I I S=P-s-P=S I I OR OR

OH SR OR OR I I I I S=P-OR S=P-OR S=P-s-s-P=S I I I I OR OR OR OR The extent of the side reactions by which these products are formed depends on the purity of the P,S, and alcohols and on the temperature of the neutralisation step. The presence of these by-products reduces the efficiency of the MDDP, they are less thermally stable, some are corrosive and have an unpleasant smell. The melting-point of the P,S, should be close to 138 "C, P-contents 28.2-28.5% by weight, Scontent to 100%. with the lowest attainable organics content. Alcohols used should be water-free. To prevent the acid decomposing during the neutralisation,the temperature should not exceed 82 "C during this stage in the process.

The following structures have been attributed to metal DDP (62):

M is a divalent metal and R can be alkyl, alkaryl or aryl. In solution, MDDP molecules combine together in associated form (57, 268). Complexes can be formed from MDDP and some compounds, such as phenol (46). amines (47). pyridine and picolines (48). 1- 10-phenanthroline (45) and other nitrogen-containing bases (49). Complexes are also formed with peroxy compounds.

Zinc is the most frequently used metal in DDP, but other metals can be used. Dithiophosphates of the alkali metals are water-soluble, promote the formation of emulsions and are corrosive. This last property excludes them from practical use.

276

Alkaline earth metals (Ca, Ba) impart detergency and dispersancy. Barium and nickel DDP also act as anti-corrosion agents. Nickel DDP improves the stability of hydrocracked oils against light. The iron derivatives do not have anti-oxidant properties. Lead DDP increases the strength of the lubricating film. Antimony and bismuth DDP's are effective high-pressure additives in industrial gear oils and the oil-components of those lubricating greases where the lubricant film must have a very high load-carrying capacity. They also have good anti-wear properties, but their thermal stabilities are lower than those of ZDDP's. Molybdenum dithiophosphates are used as friction modifiers. Salts of some organic amines, such as guanidine, are effective antioxidants (84). Thermal Stability of DDP There is a close connection between the antioxidant and anti-wear efficiency and the thermal stability of MDDP. Conclusions from investigation of the relationship between the thermal stability and composition of DDP can be summarised as follows: impurities in DDP decrease thermal stability (64), thermal stability decreases in the order arybn-alkybisoalkybbenzyl (40, 60), generally, thermal stability increases with increasing length of constituent alkyl groups, a similar effect is brought about by the presence of a quaternary carbon atom in the /%position in the alkyl group, the type of cation also affects thermal stability (66). in alkyl derivatives, thermal stability decreases in the order Zn>Ni>Ba (280), in aryl derivatives, thermal stability decreases in the order Zn>Ni>Cd>NH4 (281), decomposition temperatures established by differential thermal analysis (DTA) varied in their dependence on experimental conditions in the range 135 - 190 "C for alkyl (85) and around 250 "C for aryl compounds (fig. 4.2), thermal stability is also influenced by other factors, such as the chemical nature of the solvent, or the oil environment; thus, the di-n-butylamine derivative remains almost unchanged after 5 hours at 175 "C in decalin, but decomposes partially in a white oil and totally in engine oil. The following decomposition products have been identified in different media: O,O', S - tri-n-butyldithiophosphatein vaseline oil, O,O', S - tri-n-butyltetrathiophosphate in engine oil, S, S', S" - tri-n-butyltetrathiophosphate in the absence of solvent (54, 55, 282). The final zinc- and phosphorus-containing product of thermal decomposition is a metathiophosphate (84)or a polymeric, glassy, water-soluble residue to which has been attributed the ability to form load-bearing films on metal surfaces (64). However, the zinc compounds may remain oil-soluble without precipitation and formation of solid deposits after oxidation of use in an engine (72),despite then higher molecular weight of the decomposition products. In addition, a variety of 277

gaseous products after thermal decomposition have been identified by various authors: at lower temperatures di-n-butylsulphide (55), sulphane (hydrogen sulphide) and alkenes (84, 283), di-n-butylsulphide, butyl mercaptans; mercaptan and sulphane (60), alkene and mercaptans, and at higher temperatures, sulphane, sulphides and disulphides (64).

I

I

,

OTA 1

3

L

mg

TG

0

100

200 300 400 500 600

700 800

900 o0l-

Fig. 4.2. Derivatogram of ZnDDP with varying alkyls 1 - isobutyl-isoamyl, 2 - dihexyl, 3 - diisooctyl, 4 - di-n-cetyl

There has been much less investigation of the decomposition products of zinc diaryldithiophosphates.Decomposition occurs at a much higher temperature and the mechanism differs from that of the dialkyl compounds. The residue is oil-soluble and has a much lower load-carrying capacity than its analogue for alkylated derivatives (65). Fig. 4.2.I illustrates the changes in ZDDP concentration in an experimental lubricant formulation with time during an engine test. This also shows the appearance of the decomposition products and the disulphide form of the dialkyldithiophosphates.

-

TIME INTO TEST

Fig. 4.2.1. Changes in phosphorous chemistry of an experimental engine oil in a test engine as monitored by P-31 N M R (418)

278

The accurate determination of decomposition temperature is difficult and rather imprecise. Some deviations between results described by various authors for substances of the same chemical composition can be accounted for by various factors, all of which have important consequences for the storage and use of these additives. The results obtained depend on the rate of heating, heat input, amount of the sample, the concentration of MDDP in solution in the sample tested, the detailed arrangement of the apparatus, etc. The decomposition of MDDP starts with evolution of gas at about 70 "C. At this temperature, the degree of decomposition can be observed as weight loss after 24 hours of thermal exposure. Storage and handling temperature of concentrated solutions of MDDP should not exceed 50-55 "C. When heating MDDP, the skin temperature of the container and heating surfaces should not exceed 90 O C and only warm water or a similar heat exchange fluid should be used (not steam). Dilute hydrocarbon solutions of MDDP, such as finished oil blends containing around 3% of additive concentrate, are less heat sensitive (especially in properly balanced formulations with other additives) and can therefore be exposed to higher sustained temperatures (e.g., in engines), but the decomposition-temperaturdcomposition relationship is an important factor in the selection of MDDP additives for particular applications.

Antioxidant Effectiveness The antioxidant properties of dithiophosphates depend on the cation, the nature of their alkyl or aryl groups, the oil environment and the temperature conditions to which they are exposed. Zinc is the preferred cation. Nickel, barium and calcium are effective up to 170 "C (280).However, their effectiveness decreases at higher temperatures, and eventually a pro-oxidation effect can be observed (50). The following conclusions appear to hold for the alkyl and aryl derivatives (69):a short alkyl chain is more effective than a longer one (unfortunately, the opposite conclusion also appears in the literature (84);a branched alkyl chain is more effective than a straight chain with the same carbon number (in this respect, propyl and isopropyl alkyls show anomalous behaviour - n-Pr is more effective than iso-Pr); alkylaryl and diary1 dithiophosphates are less effective than dialkyl dithiophosphates synthesised from both primary and secondary alcohols. It is generally accepted that the thermal stability of dithiophosphates is the main factor which governs their effectiveness as antioxidants - the lower their stability, the higher their efectiveness and consequently the lower the concentration of DDP required to achieve the same effect in oil (68). The operating temperature in an engine for an oil blended with DDP should not be far above its decomposition temperature, otherwise excessive amounts of deposits are liable to form in those parts of the mechanism which are at a higher temperature. In an operating engine, the lubricant meets its maximum temperature at the piston crown and in the piston-ring zone, with a temperature gradient down the piston defined by the mode of operation and the thermal design of the piston. This must be particularly taken into account when dialkyldithiophosphates are used (71). Heavy inorganic residues and carbonaceous deposits are formed at temperatures which greatly exceed the decomposition temperatures of dialkyldithiophosphates. Both types of residue are insoluble in oil and enhance the formation of deposits on the upper parts of the piston. The decomposition temperatures of alkyl-aryl- and

279

diaryldithiophosphates are, in contrast, higher and therefore closer to the operating temperature in this part of the engine (about 250 "C). Furthermore, the final decomposition products are semi-solid, soluble in oil and do not form deposits. This difference can be especially significant in the formulation of oils for high-output diesel engines (see below). Another advantage of diaryldithiophosphates is that they form oil-soluble hydrolysis products, whereas dialky ldithiophosphates hydrolyse in the presence of water or steam to a white, oil-insoluble product, which participates in deposit formation. These factors must be balanced against the lower anti-oxidant and anti-wear properties of the diaryldithiophosphates. Resistance to hydrolysis among dithiophosphates decreases in the order shorter alkyls (secondary> primary) >longer alkyls>alkyl-aryl. The differing thermal stability of ZDDP thus very strongly affects piston cleanliness of engines operating at high temperatures. This has been demonstrated i n Petter AVB engine test (Table 4.2) and in Caterpillar 1-G engine tests. Although this engine is now obsolete as the basis of a test procedure (the level of severity being much less than those of engines in many vehicles), these results clearly demonstrate the effects of this aspect of additive chemistry. Table 4.2. Results of Petter AVB Tests of API CD-level Oils Containing Different ZDDP Additive content

Oil A

Oil B

Oil C

ZDDP (0.1% weight Zn in oil)

Zn dialkyldithiophosphate C,+Ca iso-a1kyls;low thermal stability 8% weight

Zn dialkyldithiophosphate Ca+C,, n-alkyls; higher thermal stability 8% weight

Thermally stable Zn alkylaryldithiophosphate

6.518.314.8 8.316.2

6.519.4110 9.719.6

7.0

7.8

7.2l9.619.4 9.919.1 9.0

70

80

90

Detergen t-dispersant additive (0.5% weight 300 TBN Ca-sulphonate, 2.5% 250 TBN Ca phenolate, 5.0% succinimide

8% weight

Merit rating of piston (10 max.) Piston grooves Piston lands Piston rings (average) Total rating

The higher thermal stability of zinc alkylaryl- and diaryldithiophosphatesis the reason why they are preferred for use in oils for heavy-duty engines, despite their inferior antioxidant and anti-wear properties. Additionally, they have the advantage of some detergent-dispersantcontribution,the magnitude of which depends on the size of the molecule (these properties can also be observed in some DDP with long alkyls). The first engine oils which contained zinc dialkyldithiophosphates and met the MIL-C specification have been satisfactorily used in both diesel and gasoline engines for many years. However, oils containing zinc diaryldithiophosphates have been used more often since the introduction of the Caterpillar engine

280

clutch test. They were shown to be effective in diesel engines, but exhibited somewhat worse anti-wear properties in gasoline engines. A compromise can be found with zinc alkylaryldithiophosphates.

The antioxidant properties of DDP also depend on the composition of the oil in which they are used. The presence of aromatic and cycloalkanoaromatic hydrocarbons in oils enhances these properties (285). However, even here the general rule is valid, that an optimum concentration exists for both natural and synthetic antioxidants; above the optimum, the antioxidant effect starts to decrease (1, 36). Many opinions have been published on the mechanism of the antioxidant action of ZDDP (including 44,50, 51, Z37,287). This property has been investigated by following the decomposition of cumene hydroperoxide, tetralin hydro-peroxide and di-tert-butyl hydroxide in the presence of ZDDP. Some authors presume that the active form of the antioxidant is the product of reaction of ZDDP with a peroxy compound. According to Bums (53),decomposition of a peroxide in the presence of ZDDP proceeds in three stages at different rates: the first step is rapid, the second slow and the third and final step is again rapid. According to this mechanism, bis(dialkyloxythiophosphory1)-disulphide is formed as first intermediate: RO\

4s

Sq, / O R +Roo. RO, 4 SOOR Sq, / O R

P RO1p\ S - Z n - S "OR 0

P

J

RO/'S-Zn-S

SOOR Sq, / OR +Roo. RO\/ P - P / p\ RO S - Z n - S "OR RO/ 'S-Zn-S RO\

P "OR

\

/

(4.42)

OR

(4.43)

/'\ OR

+ 2R00- + Zn2+ Rib1 et al. (54, SS), in similar terms, described the three-stage decomposition of cumene hydroperoxide in excess zinc di-n-butyldithiophosphate.In the first step, a complex of the hydroperoxide with the ZDDP is rapidly formed. Slow decomposition of the complex, accompanied by the precipitation of zinc compounds, occurs in the second step. In the third, DDP decomposition products cause rapid decomposition of further quantities of hydroperoxide which have been added. Bums et al. (50, 51,52, 103) assumed that there was hydroperoxide interaction with the sulphur atom in the dithiophosphate molecule, whilst Rhbl concluded that a peroxide-zinc bond had been formed in the complex. This conclusion is based on the observation that complexes are not formed from dithiophosphate decomposition products, which are effective antioxidants. Rhbl found the following phosphorus containing decomposition products, among others: 0,O:S-tri-n-butyldithiophosphate, 0 , O :S-tri-n-butyl-thiophosphateand 0,O:O ', S-tetra-nbutyldithiophosphate (286). The same products were formed from bis(dibutoxythiophosphory1)-disulphide. This implies that this disulphide is itself a 28 1

transient product in the decomposition chain. Decomposition products containing thionic sulphur can decompose peroxides very rapidly, whilst sulphur-free decomposition products decompose peroxides only very slowly. The activity of thiocontaining products is intermediate between these extremes. The following phosphorus-free decomposition products were formed: di-n-butyl-sulphide, disulphide, sulphoxide and sulphones, the first three of which are well-known peroxide decomposers. Sanin and co-workers (84) reached similar conclusions. According to them, both thionic and thiolic sulphur act in depressing the rate of oxidation, but in different stages in the reaction sequences; thiolic sulphur acts in the early phase whilst thionic sulphur influences more advanced stages. The reaction mechanism of MDDP interaction with hydroperoxides is radical in the opening phase, changing to ionic in the advanced phase. The simultaneouspresence of cumyl alcohol, a-methyl styrene, phenol and acetone in the products of ZDDP interaction with cumene hydroperoxide suggests that radical and ionic mechanisms participate in the decomposition of peroxides. However, at lower temperatures, the radical scavenging function of ZDDP cannot be excluded from consideration (36,54,288,320).

Neutral zinc salts seem to act as radical acceptors. The same property can be attributed to DDP acids which are formed in the thermal decomposition of ZDDP. However, basic salts are inactive in this respect, but become active after interaction with decomposition products. Another product of thermal oxidation, bis(dialkyldithiophosphory1)-disulphide,is not active as a radical scavenger (385). From this, it is possible to conclude that neutral ZDDP’s are active antioxidantseven during the early stages of operation of an oil, whereas this property is lacking in the basic salts. There is less information available about the mechanism of action of diaryldithiophosphates. It is known that no complex is formed from zinc diphenyldithiophosphate and cumene hydroperoxide (54).

Anti-wear Properties The presence of sulphur or phosphorus in MDDP imparts to these additives pronounced anti-wear and sometimes even extreme pressure characteristics. However, these attributes are also influenced by the metal, alkyl and aryl groups, and by thermal stability. The terms “anti-wear’’ and “extreme pressure (EP)” properties are conventionally used to describe additive action in several levels of severity of conditions, lubrication regimes and potential wear in those cases where the lubricated surfaces are not uniformly separated by a lubricant film. In this text, “extreme pressure” action is used to describe that sector of this range of situations in which the original lubricant film has virtually ceased to exist in the contact zone; catastrophic failure of the mechanism is prevented by the local establishmentof low-shear layers of high thermal stability between the surfaces. Surface-to surface contact has occurred and with it significant surface damage, but the process has been arrested. The term “anti-wear agent” covers behaviour over a wide range of circumstances, including those in

282

which a partial film of the original lubricant is retained. Anti-wear agents can thus act by enhancing adhesion of the lubricant to the lubricated surfaces and by wholly or partly replacing it. The differences between the various regimes may be manifest principally in terms of a difference in the local temperature in the contact zone (the EP region denoting a much higher temperature), but the magnitude of these temperature boundaries varies considerably among lubricated systems. The practical result of moving from one regime to another is often apparent as a change of friction coefficient - the EP region being characterised by p much higher than the lower end of the anti-wear additives’ sphere of influence. “Friction modifier” effects can result from influence on both these phenomena.

Ability to reduce wear by MDDP decreases in the following order of cations: Cd>Zn>NilFe>Ag>Pb>Sb; Sn>Bi. Extreme pressure properties follow the following sequence: BiZAg, Pb>Se, Sn> Cd, Fe, Ni>Zn (289) and for the organic substituents: sec-alkybp-alkyl>alkaryl>aryl (69,290). Investigation on the four-ball apparatus (289) of the load-carrying capacity of films formed by di-4methyl-2-pentyl-dithiophosphatesshowed that whereas zinc was the most effective under mild load, it was the least effective under high load. However, when used in conjunction with cadmium, its anti-seizure properties in a valve-train system gave better results than when nickel was used (289). This emphasises that choice of dialkyldithiophosphates must be guided by consideration of the actual regime which is dominant. This is further supported by the observation that some zinc dialkyldithiophosphatesin engine oil reduce wear - but increase friction coefficient (370,375).

The best anti-wear properties among zinc dithiophosphates have proved to be obtained with mixed secondary and primary C, - c6 alcohols, or even better, C, - C , - the former may cause pitting corrosion (to an extent dependent on the type of material lubricated). Individual short-chain alcohols can be ruled out; they produce crystalline ZDDP of limited solubility, separating from oil on storage. MDDP may not be soluble in oil if the alkyl chain has fewer than five carbon atoms. If a mixture of alcohols is employed, one of them may have fewer than five carbon atoms and the resultant additive remain oil-soluble.

ZDDP prepared from mixed C, - c6 secondary alcohols have given good results in the FZG test (11th to 12th load stages can be achieved in a normal run) and in four-ball machine tests. Such additives are incorporated mostly into gear, hydraulic and cutting oils to provide anti-wear properties.They were also used in engine oils for higher anti-wear properties if these were especially desirable, whereas ZDDP prepared from primary alcohols were used primarily in engine oils because of their better thermal stability. ZDDP with primary and secondary alkyls with a carbon number average of 8 are regarded as universal for oils in gasoline and lightly-loaded diesels. Zinc dialkyldithiophosphates of long-chain primary alcohols (Cs - C,o) and, particularly, mixtures of zinc dialkyl- and diaryldithiophosphatesare used in engine oils to meet higher specifications, relating to supercharged engines. The high anti-wear efficiency of the ZDDP’s and their ability to increase the strength of the lubricating film are also used in other applications in the field. For instance, reduction of scuffing in engine valve-trains can be achieved by increased 283

dosage of ZDDP of lower thermal stability, although this is chiefly a problem of design and metallurgy. Some of the more recent studies of the influence of additives on the anti-wear efficiency of oils have concentrated mainly on the mechanism and kinetic effect of the metal (especially zinc) ZDDP and their decomposition products. These efforts have been based on the theory accepted hitherto that the thermal decomposition of the dithiophosphatesoccurs at temperatutes prevailing on the friction pair surface and hence that their decomposition products from a protectivd coat either by direct reaction with the materials of the friction surface or by “in situ” polymerisation on the friction surface. Research and field experience have shown that the decomposition kinetics and chemical compositiotl of the decomposition products depend to a great extent on the types and compositions of other additives simultaneouslypresent, as well as on the composition of the base oils, the presence of oxygen in the environment and other factors and interactions among the decomposition products of the MDDP and other additives present, and that there is a considerable amount of mutual influence of all these effects (321‘-325,382).

I

0.7-,

-..-..-..- ..-..-..-.. -..-..

0.5a3 0.l. 1

80

120

1$0

l77

& OC

$3

w

9i 120 MINUTES

Fig. 4.3. Decomposition kinetics of ZnDDP in the presence of differing DD-additives 1 - ZnDDP-1.2 (1.08)% wt in base oil. 2 - ZnDDP-1.2 (1.08)% wt + Ca-suplhonate (TBN 295)-6.0 (1.6)% wt in base oil, 3 - ZnDDP-1.2 (1.08)% wt + Ca-natural sulphonate (TBN 5)-6.0 (2.0)%wt in base oil, 4 - ZnDDP-1.2 (1.08)% wt + Ca-phenolsulphide(TBN 250)-6.0 (3.66)Wwt in base oil, 5 - ZnDDP-1.2 (1.08)% wt + bissuccinimide (TBN 25)-6.0 (3.0)%wt in base oil

The decomposition of three ZDDP‘s was investigated (326,383)by examining the extinction of the P-0-C region in their infra-red spectra as a function of temperature and time in base oils from different sources. Kinetics of the decomposition of ZDDP was affected by all the above factors, and different detergent-dispersant (DD) additives - which are significant synergists of the ZDDP anti-wear effect affected anti-wear properties in different ways (fig. 4.3).Essentially,those DD-additives which accelerate and intensify the thermal decomposition of ZDDP have a significant influence. The greatest effect is that of multi-component DD-additives. However, the concentration of particular constituents plays a part, as does the type of polymer modifier, if present. It is worth noting that oils containing ZDDP lose their antiwear properties after their antioxidant effect has been exhausted (386).

The anti-weareffect of ZDDP’s also depends on the character of the base oil. For example, they have only a very smalE effect in polypropylene oils; in solvent raffinates and hydrocracked oils their effect increases with increasing aromatic content. 284

It may be speculated that this interaction is influenced by the solvent properties of the base oil. The EP activity of thiophosphates has been shown by several workers to involve the deposition of various solid products of thermal decomposition on the surface of the metal. The polarity of the oil, acting as solvent, may influence the course of the decomposition reactions and the rate at which deposition occurs.

Effects of DDP on the Friction Properties of Oils ZDDP’s reduce the friction coefficient of oils. At the same concentration of ZDDP in oil, the magnitude of this effect depends on: - the chemical structure of the ZDDP, and hence the value of its decomposition temperature; ZDDP with shorter and mainly secondary alkyls are therefore more effective than ZDDP with longer alkyls or alkylaryls (419), - the electrochemical reactivity of the ZDDP with the friction surface; the effect of the ZDDP on the friction coefficient of the oil increases with increase in reactivity (420), - the temperature of the oil and the contact pressure; friction coefficients of oils decrease as a result of ZDDP action more as temperature and contact pressure increase.

Anti-corrosion Properties The pronounced, favourable effect of ZDDP of lower thermal stability is demonstrated, for example, in the observed reduction of wear of Pb-Cu or Pb-Cd bearing bushes. Bearing metals containing lead can be attacked, especially, by peroxides and organic acids. The action of peroxide on lead produces lead oxides, which with organic acids give lead salts. These catalyse the further oxidation of lead. ZDDP in behaving as an antioxidant suppresses the formation of peroxides and organic acids; its decomposition products react with lead to form complex inorganic products, which contain sulphur and phosphorus and form a protective film on the bearing surface. This film reduces contact of the acidic, corrosive compounds with the surface of the metal. Other compounds containing sulphur and phosphorus, such as phospho-sulphurised unsaturated hydrocarbons (terpenes, alkenes) may react with lead in a similar fashion to form a protective coating.

However, mixtures of oils containing ZDDP with oils containing other anti-wear additives containing sulphur and phosphorus sometimes cause increased corrosivity, particularly towards copper, It is, therefore, advisable to use an adequate corrosion inhibitor at the same time as the anti-wear additive. The mechanism of formation of this protective coating has not been satisfactorily explained, as yet. According to prevalent opinion, it is produced by an adsorption process and by the exchange of atoms between the bearing material and the decomposition products of ZDDP.Measurement of the phosphorus concentration in the surface film with radioactive 32Phas shown that the phosphorus concentration is almost 100 times as high under mechanical load than without the load (29Z). The rate of growth of the film is proportional to the friction produced at the point of contact (292). The surface film contains, in addition to phosphorus, sulphur and zinc (70).

285

ZDDP is not suitable for engines with bearing metals containing silver because it causes corrosion; it can also promote the corrosion of bearings made from older types of phosphor-bronze.

Zinc Dithiophosphates in Engine Oils ZDDP's have so far been unsurpassed as additives for engine oils. Their merit is their multi-functionality - they act as antioxidant, anti-wear, passivator and anticorrosion additives. They are synergistic with other types of antioxidants, such as alkylphenols, metal deactivatorsand zinc dialkyldithiocarbamates and they improve the efficiency of some detergent-dispersant additives. Zinc, phosphorus and sulphur are now being introduced into all types of lubricating oils for 4-stroke engines, except oils for engines with silver bearing bushes (73).They cannot be used above a low maximum concentration (0.4 - 0.5%) in 2-stroke engine oils in which the oil is mixed with gasoline. In this case, the ZDDP, if used at higher concentrations,acts as a source of deposits on the spark plugs, causing failure and reducing service life. Oils containing low ZDDP concentrations may thicken when exposed to severe conditions, when the oil in the crankcase heats up to about 150 O C . The reason for this is the accumulation of high-temperature sludges, as well as the evaporation of light components. An increase in the concentration of ZDDP may only suppress the formation of sludge, and it is advantageousto use, simultaneously,other antioxidants such as akylphenols (ZO6). The evaporation of light components can only be avoided by a judicious choice of base oil. The concentration of ZDDP in engine oils depends on the required zinc content. This varies, as a rule, between 0.04 and 0.1% by weight, but it can be as high as 0.15% when, for example, improved anti-wear properties or lower thickening rate are desirable (Table 4.3). The modem type of gasoline engine oil, produced since 1972, usually contain 0.10 to 0.15% zinc; in diesel engine oils, it is recommended not to exceed 0.12% zinc. Table 4.3. Effects of Increased Zinc Content in Oil on Results in the 4-hour Sequence IIID Test in a Gasoline Engine Zn concentration in oil (% weight) 0.08 0.12 0.15

SE specification limit

1180

124

32

375 max.

Piston varnish rating

8.6

9.5

9.7

9.1 min.

Oil sludge rating

6.8

8.1

9.8

9.2 min.

Viscosity increase (%)

The properties of ZDDP are affected by the metal: phosphorus ratio. The metal should be in a slight stoichiometric excess and the ZDDP reaction at most only slightly acidic. Currently-available products have Zn:P ratio of 105-1lo%,pH 4.5286

5.0 - this value should not fall below 4.0.The thermal stability of ZDDP improves with increasing Zn:P ratio. The presence of excess alcohols and their oxidation products is undesirable. It has been alleged that the concentration of zinc on the roads exceeds, in some areas, the threshold value of toxicity (74).The maximum admissible concentration of zinc in air is, at present, 5 mg.m3. This value is far from being reached even at a consumption of 1 litre of oil per 1,600km.However, zinc on the road can originate from other sources, such as tyre wear. Zinc dithiophosphates are slightly toxic. They are effective fungicides (265).The LD50 of commercial products is about 2.5g per kg of the living organism. A number of ZDDP’s has been found to be toxicologically active as skin and eye irritants.

The Determination of Zinc, Phosphorus and Sulphur in DDP’s and in Oils Dialkyldithiophosphate additive concentrates usually contain up to 9% by weight Zn, 8.5% P and 17% S; the alkylaryl- and dialkyl- compounds contain up to 3.6% Zn, 3.3% P and 7% S. The zinc content of both ZDDP and oil can be determined by IP 117/47; the sample is incinerated and Zn determined gravimetrically as zinc hydroxyquinolate. ASTM D-1549 and GOST 14330-69 specify a polarographic method of determination of zinc in oil and additives (containing up to 2% Zn and in the absence of Cd). Measurement by atomic absorption spectroscopy (e.g., by ASTM D-4628-86) is currently routinely used and the use of isotopic dilution analysis is envisaged (310). Phosphorus in additives and oils can be determined by ASTM D-1091 and IP 148/72, IP 149 and IP245 (photometric methods for determination of P at 0.002-2% weight). The sample is mineralised with H,SO, and the phosphorus transformed into orthophosphoric anions with HNO, and HCIO, or H,O,. According to the photometric method, ammonium vanadate and molybdenate solutions are added to form a yellow complex, which is measured photometrically at a wave-length of 420-470 nm. According to the titration method, phosphate anions are precipitated as quinoline phosphomolybdate from boiling HCI by sodium hydroxide, the excess of which is titrated with HCI. The photometric method is also described in CSN 65 6253. The standard emission spectroscopic method for P, Zn, Ba and Ca is specified in 1P 187. The total sulphur content in oils and additives can be determined by CSN 65 6228, GOST 1437-47. ASTM D-1551, IP 63 and DIN 51786, in which the sample is burned in an air-jet in a quartz tube. The sulphur oxides produced are absorbed in 3% H,O, and converted into H,SO,, which is determined by a chelatometrical method, titration with NaOH or gravimetrically as barium sulphate. ASTM D-1552 is more intricate. The sample is burned in an oxygen flame by the Wickbold method (DIN 51409 and IP 243). and absorption of SO, in H,O, gives H,SO,, which is then determined as sulphate by titration with barium perchlorate, or by nephalometric, gravimetric or photometric methods. According to KiriEenko, sulphur can be determined by hydrogenation, followed by coulometric determination of H,S (123).

4.1.3.5 Ashless Peroxide Decomposers Containing Sulphur, Phosphorus and Nitrogen Where metal-containing antioxidants cannot be used, ashless antioxidants containing phosphorus, nitrogen and thionic sulphur can be employed (79,for example:

287

/OR S=P-OR \ SNR'R"

/OR S=P-OR \ SCH,NHR'

/ NHR S=P-NHR \NHR

/(C18H37)

[

S = P - OC6H5 NHRR' or

S = P -FS H3 ' S I

NHRR

where R is octyl, - CO - (CH2)8- CONH,, R',R" - hydrogen or methyl). These additives are stable up to comparatively high temperatures (about 300 "C) and suitable for high specification engine oils. Some effectively suppress oil thickening whilst avoiding contamination of exhaust gas catalysts. Inexpensive antioxidants with good anti-wear properties can be derived from Sderivatives of 0,O'-dialkyldithiophosphoricacids. As stated earlier, some of them are products of decomposition of ZDDP. The S-alkyl, S-alkoxymethyl-, Salkylthiomethyl- and S-hydroxybenzyl- derivatives can be cited as examples (7679). The S-carbamoyldithiophosphates,reaction products of mono- and diisocyanates with dialkyldithiophosphoricacid, are also good antioxidant additives (as effective as ZDDP) and have good anti-wear properties (better than ZDDP) (80): R' - NCO + HS - P(S)(OR)L-

R'

- NH - CO - S - P(S)(OR)z

(4.42)

where R' is an alkyl with 12 or more C atoms. These derivatives of the comparatively unstable S-carbamoyl group have lower decomposition temperatures than ZDDP; their deficiency is their tendency to revert, during storage, into gels. They fail to protect metals from corrosion and need to be combined with a corrosion inhibitor.

4.1.3.6 Future Developments Prospects for future development of oils containing ZDDP and S- and P-containing ashless antioxidantscan to some extent be jeopardised by the introduction of exhaust gas post-combustion catalysts. The deleterious elements are phosphorus, which acts as a poison for catalysts based on noble metals (Pt, Pd) and sulphur, which poisons the NO,-reduction catalysts. Unless sufficiently resistant catalysts are developed, or some other way found of separation of the harmful elements from the exhaust gases, 288

massive departures from present additive systems and the adoption of phosphorusand sulphur-free additives may be the consequence. This holds true not only for antioxidants, but also for other types of engine-oil additives (59, 68, 73). The question of the biodegradabilityof ZDDP and other phophorus-containingadditives, and their effects on the environment in waste oil-disposal, may bring about future changes in the range of additives in large-scale use.

4.2 DETERGENTS AND DISPERSANTS Detergents and dispersants (DD) are essential additives, particularly for engine oils. Their chief function is to avoid or suppress the formation of deposits on the hot surfaces of internal combustion engines - particularly on pistons - and disperse the corrosive cold sludges into the oil in those parts of the engine operating at relatively low temperatures.They are,in addition, required to neutralise the acidic components contained in the oil and acid substances entering the oil, and thus reduce corrosive wear of the metal surfaces and protect ferrous surfaces from rusting. They can increase the thermal stability of the oil and affect the process of its oxidation. They have also made considerable progress in gasoline formulations, where they keep the inlet manifold from the fuel reservoir to the inlet valves clean, and diesel oils, where they improve clean combustion.

4.2.1 Significance of Detergent-Dispersant Additives Mechanisms of their Action In addition to efficient oil filtration, the detergent-dispersant ability of the oil is a major determinant of the extent and rate of the deposition on the working parts of the machine of substances formed in or entrained into the oil. In the case of an internal combustion engine, these parts may especially include the piston-ring grooves or on the piston surface; insoluble substances produced during incomplete fuel combustion can form deposits and varnishes, which may obstruct the operation of vital parts of the engine, impede heat flow, etc. In essence, two types of deposits can be formed: deposits caused by heat (the thermooxidation products of fuel and oil which, because of flocculation promoted by heat, deposit on the hot parts of the engine) and - cold sludges caused by phenomena taking place at lower temperatures (carbon particles depositing by co-action of water on colder parts of the engine). Organic acids condensing on the hot parts of the engine are responsible for the formation of varnishes and lacquers. The rate of deposit formation can also be affected to a considerable extent by mechanical contaminants (dust) entering the oil through poor filtration and to abrasion of the metallic friction surfaces. Deposits and varnishes are, together with wear of the engine by solid particles, the decisive factors in ensuring good operation, output and reliability of the engine.

-

289

Since there are differences between diesel and gasoline engines in terms of fuel composition, the thermal regimes in the engine and operating conditions, the conditions giving rise to the formation and the nature of insoluble substances in the oil, and for the formation of deposits and varnishes differ also. Anderson (81) illustrates the problem in the charts depicted below:

Diesel Engines fuel

incomplcte combustion and diffusion products of ignition

$-

soot

+

combustion H,O + SO, + NOx

pre-ignition products

H2S0, + oxidation products

low-molecular weight oxidised,polymers

1. t

varnishes

Gasoline Engines PISTON VARNISH

t

fuel

oil

blow -by

+ heat + NO + 0,

oxidised gaseous

oxidation products

products (droplets)

DEPOSITS

piston ring-zone

I

t

carbon deposits

d

varnish

inorganic b sats

water SLUDGES

These charts illustrate that one of the significant promoters of deposit formation in diesel engines is the sulphuric acid produced via sulphur oxides from the sulphur-compounds present in diesel fuel and oil. Nitrogen oxides, originating form nitrogenous substances present in fuel and lubricant and. predominantly, from the direct combination of nitrogen and oxygen at high engine temperatures,promote the production of sulphuric acid and also contribute themselves to oxidative transformations in the

290

lubricant. These transformations,together with other reactions between sulphuric acid and oil, cause the formation of insoluble products which adhere strongly to the hot metal surfaces. The amount of deposit grows with increasing fuel sulphur content, with increased engine temperature and the rate of formation of nitrogen oxides. The acidic products from burnt fuel may condense on the relatively cool cylinder walls and on the piston surfaces, and they can catalyse the polymerisation of substances produced by thermooxidation reactions to sludge and varnish deposits. If deposition of solids (which besides carbon can contain sulphate crystals, additive residues, wear debris and oxidised fuel and lubricant) becomes excessive, the piston grooves can become full, which, together with varnishes, immobilises them; in consequence, combustion products may penetrate past the piston at a greater rate into the sump, or the oil may penetrate into the combustion chamber. This again causes formation of deposits, increases oil consumption and decreases output and, eventually, inevitable stoppage of the engine. Varnish deposits on the piston also impede heat transfer, which can cause an increase in piston temperature and decreases the service life of the oil. Varnish in spark-ignition engines causes an increase in the octane requirement of the engine. Burgess and colleagues (83).in studying the oxidation products of oil in gasoline engines, found that the action of oxygen on oil first produces low-molecular weight liquid products (oxidised monomers) of low oil-solubility in oil (so-called red oil). When these monomers are exposed to constant heating, highmolecular weight, oil-insoluble products result. When these products come into contact with the piston surface, they harden and form varnishes. According to prevalent opinion, 90% of the deposits on the piston come from the oil and only 10% from the fuel. The formation of deposits in the gasoline engine depends on the type and operating conditions. Essentially, there is a difference between the oxidation of the oil in the low temperature and high temperature in the engine. At low temperatures,e.g., during stop-go running of an insufficiently warmedup engine, the main sources of low-temperature deposits (cold sludges) are the products of incomplete fuel combustion - partially cracked and oxidised hydrocarbons, olefins, dienes, aldehydes, acids, phenolics etc., produced in the relatively cool flame region near the relatively cold walls of the combustion chamber. The chemical composition of low temperature deposits has not been sufficiently investigated. A part of the product of incomplete fuel combustion penetrates as blow-by into the crankcase. Here, the products condense and form oil-insoluble droplets. These droplets either return to the piston surface with the oil, directly or through positive crankcase ventilation. Because they are more polar than oil, they are more readily bonded by adhesive forces to the hot metal surface and reactions between their active components occur. The liquid is first transformed into an adhesive, half-solid substance, which further hardens and forms varnish deposits. In addition, these droplets react with water and lead compounds derived from anti-knocks, if present, to form semi-solid resin deposits - cold sludges. Nitrogen oxides take part in the formation of deposits and cold sludges (81).This can be demonstrated by analysis of the carbonaceous products in which the nitrogen is substantially higher than in the fuel or the lubricant. Although the conditions affecting the formation of NO, and their concentration is known in, for instance, diesel engine exhaust gases, their influence on the formation of deposits is considerably less understood. It is supposed that they are a positive factor, although considerably less significant, than air or sulphur oxides.

Suppression of the lay-down of deposits and varnishes from fuel oxidation products, and the neutralisation of substances which enhance the formation of these oxidation products, can be achieved by: - neutralisation of acidic substances and suppression of polymerisation of oxidised polymers, - solubilisation, i.e., transformation of liquid but poorly-soluble or insoluble substances such as asphaltenes into soluble form, - peptisation of solid particles, e.g., carboides and soot, by converting them into

29 1

stable suspensions in oil,

- preventing the particles from depositing on the metal surface in relatively nonturbulent sites. DD additives provide chemical neutralisation of acids, mainly strong acids, and prevent polymerisation of oxidised monomers, as long as the additive retains an alkaline reserve, until its alkalinity is exhausted. During this neutralisation process, excess metal hydroxide or carbonate - which constitutes the alkaline reserve of the detergent, is converted into metal sulphate. Thus, the higher the sulphur content of the fuel, and the more the tendency for acid products from the oxidation of fuel and oil to accutnulate in the oil, the higher the alkalinity of the additive to be used needs to be. From the commentary earlier, it is apparent that oils for lubricating diesel engines generally need to be compounded with DD additives of higher alkaline reserve than those for gasoline engines; as the sulphur content of the diesel fuel and the operating temperature to which the oil is exposed increase, the alkaline reserve of the DD additive should also increase. High performance, high compression ratio gasoline engines used for high-load highway operation for prolonged periods on leaded gasoline also require oils with basic additives. In these cases, chemical neutralisation of inorganic acids, HCl and HBr, originating from anti-knock additives, and organic acids is necessary. Over recent years, developments have been aimed at reduce diesel engine piston temperatures for a given power output, e.g., by improved cooling through piston design, and at the use of low-sulphur diesel fuel. On the other hand, the specific output and long-term loading of gasoline engines has been increased, without designing to reduce oil temperature and other potential sources and promoters of oil oxidation (e.g., by introducing positive crankcase ventilation, increasing the aromatic and/or anti-knock agent content of the gasoline). These factors, in respect of which the requirements of diesel and gasoline engines have tended to move closer, determine the alkaline reserve necessary in DD additives, and have led, amongst other needs, to the development of “mixed-fleet’’ oils for lubricating both gasoline and diesel engines.

The total base number (TBN) and total acid number (TAN) of “over-based” additives in oils is associated with the concentration and composition of MDDPtype antioxidants, detergents and dispersants (TAN measures the extent to which the additive reacts with strong base, whilst TBN is the amount of strong acid; additives may and do have, simultaneously, both high TAN and TBN values). MDDP’s have very high acidity. Metal dialkyldithiophosphates with mainly shorter alkyls have higher acidities than alkylaryl- and diaryldithiophosphates. The following TAN’S have been measured, by ASTM D-664, in commercial ZDDP concentrates: 146 mg KOHlg for ZDDP with C,-C, primary alkyls, 124 mg KOWg in those with C , alkyls and 70 mg KOWg for alkylaryl compounds.

Among detergents and dispersants, metal alkarylsulphonates have relatively high acidities, even when their total alkalinity is also high. The TAN depends on the method of manufacture (extent of neutralisation of sulpho-acids or residual SO,) and may exceed 30 mg KOH/g, particularly in low-base sulphonates, in which the

292

TAN can even be higher than the TBN. By contrast, basic metal alkylphenates and, even more, succinimide dispersants, have very low acidities (the alkalinity of succinimides or Mannich products results from amine groups, chiefly secondary amines, which have higher alkalinities than primary amines; for this reason, bissuccinimides have higher alkalinities than mono-succinimides). Acidity i n succinimides results from the presence of unreacted alkenylmaleic anhydrides. Metal thiophosphonates have relatively low acidities (e.g., commercial Ba thiophosphonate with TBN 65 mg KOWg has TAN 5.5 mg KOWg). The TAN of metal salicylates is a little higher (e.g., commercial Ca akylsalicylate with TBN 142 mg KOWg has TAN 12 mg KOWg). High oil acidity, caused by the acidity of some additives as well as by organic acids, when it cannot be neutralised by strong bases, is obviously undesirable in that it causes corrosion, promotes fatigue wear of friction surfaces and accelerates oil ageing (some oxidation reactions of hydrocarbons and their derivatives proceed more rapidly in an acid environment). For these reasons, very low-base petroleum sulphonates (TBN <30) with a high TAN can be an undesirable component of DD packages for engine oils. To prolong the interval between oil changes, the difference between TBN and TAN of the fresh oil should be as high as possible. The level of alkaline reserve in the oils for gasoline engines is limited by the ash content of the oil. This increases with the alkaline reserve of the additive. Gasoline engines are more sensitive to oil ash content than diesel engines. Similarly, conventionally-lubricated two-stroke gasoline engines, in which the oil is mixed with the gasoline, are sensitive to ash content, particularly water-cooled engines. Low-ash oils are mandatory for rotary gasoline engines. Fouling of the combustion chamber causes the conditions for combustion and the quality of the exhaust gases to deteriorate and leads to deposit formation on the exhaust valves; this impairs their operation and leads to a drop in output. Ash, acting as thermal insulator, causes the thermal regime in the engine to advance, leading to pre-ignition. When the combustion chamber is fouled with deposits derived from ash, better antiknock properties (higher octane rating) of the gasoline are required. The base oil itself induces a higher octane requirement as its heavy component (brightstock) concentration increases. However, metal DD additives, the main ash precursors in oil, have a far more significant influence. The type of metal is also a contributor to this problem. For example. tests of mono-grade SAE 30 oils with and without additives in a Fiat 125B engine fueled with lead-free gasoline showed an octane requirement increases after 216 hours as follows (87): Base oil (free of ZDDP and metal detergents) Oil containing ashless dispersant Oil containing ZDDP Oil containing ZDDP + Mg-containing detergent (0.63% ash) Oil containing ZDDP + Ca-containing detergent (0.68%) ash Oil containing ZDDP + Ba-containing detergent (0.82% ash)

3-4 5-6 6 6-7 7-8 8

Other authors (88)confirmed this effect with SAE low140 SE oils of differing ash content on octane requirement measurements in a Ford Cortina 1600 engine fueled with lead-free and leaded fuels (see Table 4.4).

293

Table 4.4. Influence of Ash Content in Oil on Octane Requirement Increase Ash content of oil (96 weight)

leaded

1.29 1.15 0.27 0.02

Gasoline lead-free

8.1 8 .O 5.0 2.8

5.6 5.3 3.9

As a result of the polymeric VI improvers they contain, multi-grade oils have less influence on octane requirement than mono-grade oils. Polyolefins have proved to have the best performance in this respect (89). Other authors (90.92) have also reported on the effects of ash content of lubricating oils on engine octane requirement. The tendency to knock can, in these cases, be explained by low thermal conductivity of the deposits in the combustion chamber and on the piston crown, which causes a general increase in engine temperature and also local over-heating. These hot-spots cause irregular ignition (pre-ignition). The occurrence and extent of this phenomenon depends on the composition of the ash deposits and/or the type of metal they contain. For example, lead-containing deposits originating from the gasoline were shown to have a lesser effect than barium-containing deposits produced from the oil. This is associated with the decomposition temperature of the deposits. Table 4.5 illustrates the decomposition temperature of various compounds which occur in combustion chamber deposits.

Table 4.5. Melting-points and Decomposition Temperatures of Compounds Found in Combustion Chamber Deposits in Gasoline Engines ( "C) Cation

Anion Sulphate Carbonate Chloride Bromide Nitrate Oxides

Lead

Magnesium

Calcium

Barium

1170 (d)315-400 50 1 513 (d)470 888

(d)l124 (d)350 708 700

1193-1450 900 772 730

2800

2580

1580 (d)1360 963 847 592 1923

(d) indicates decomposition

Combustion chamber temperatures vary between 1,000 and 1,300 "C, so all lead compounds melt or decompose, whereas barium sulphate and carbonate are unchanged (lead is first deposited as carbonate, from which oxides are gradually formed; these are then converted into chlorides,bromides and sulphates). The melting-point and decomposition temperature clearly suggest that magnesium and calcium salts should give rise to less trouble than barium salts. A higher content of calcium and magnesium in detergents is more acceptable, in this respect, than barium. The apparent importance of lubricant-derived deposits on octane requirement increase in 4-stroke engines has diminished somewhat in response to a downwards trend in oil consumption, itself driven by emission control regulations. For much of its life, the engine of a modem passenger car consumes virtually no oil between drains. The opportunity for ash-forming additive residues from the lubricant to penetrate the combustion zone has therefore been much reduced.

Fewer problems with ash occur in diesel oils when high alkaline reserve oils are used. Some problems have been noted and linked to ash deposition on exhaust valves 294

and turbo-compressor blades when API CD oils have been used in highlysupercharged, medium-speed diesel engines (92).Ash also constitutes a considerable proportion (around 20% by weight) of deposits in the first piston groove; up to 58% ash has been identified in crown land deposits in Cummins NTC 400 tests (421). Diesel oil development efforts have included attempts to reduce ash levels. For example, Daimler-Benz allows 1.8-2.0% maximum by weight of ash (depending on lubricant performance level) in oils approved for lubricating their supercharged diesel engines; Detroit Diesel permits a maximum of 1%. The reason for this is the very close tolerance between the piston grooves and rings, which are liable to be fouled by deposits derived from ash in the oil.

Modern mixed DD additives offer two ways of resolving these problems: combination of ash-containing metal detergents and ash-free dispersants (chiefly succinimides) and the combination of different metals in ash-containing metallic detergents. For instance, in formerly used combinations of calcium and barium, calcium generally produced less sulphate-ash than barium, but the bulk of the calcium ash is higher than that of the barium ash. Calcium salts adhere more strongly to metal surfaces than barium salts, so that barium salts can be expelled with the exhaust gases and more readily leave the engine. Magnesium salts produce the least amount of ash. Basic metallic detergents specifically neutralise inorganic and organic acids in oil and inhibit the polymerisation of oxidised monomers. On the other hand, ashfree dispersants, being of low basicity, cannot neutralise strong acids. Their beneficial effect is due to another mechanism. The mechanism of the solubilising and peptising effects of DD additives is associated with the structure of the DD additive and with the properties of its components. Oils containing DD additives are, in fact, colloidal solutions in which the molecules of the additives form micelles of up to 10 to 500 molecules. The size of the micelles is dependent on the concentration of the additive in oil and on the temperature. Polar compounds, such as water, alcohols, etc., increase the size of the micelles. Excess basicity in over-based detergents is solubilised in the detergent micelles. The ratio of number of molecules to number of ions is around 30; this ratio varies with temperature, concentration of the additive and the concentration of polar components in the oil. The presence of ions or electrically-charged particles is a significant factor in the stabilisation of contaminants in used oils. Different DD additives present the same essential chemical structural features, comprising polar components connected to long, oleophilic hydrocarbon chains. The polar component enables the molecule to become bound to polar targets (e.g., contaminant particles) whilst the non-polar oleophile imparts solubility in oil and prevents the coagulation and sedimentation of the particles. Solubilisation occurs when the polar components of the molecule become bound to polar intermediate products of oil and fuel oxidation, which otherwise tend to agglomeration and 295

flocculation form the oil as the temperature increases. Peptisation by the DD additive takes place during the process of binding to solid particles, products of thermooxidation reactions of fuel and oil, dust particles, metallic wear debris, etc.. The oil-DD additive system involves a number of forces besides the generalised dispersive forces (82): induction forces generated by the interaction of the permanent dipoles of the additives and the induced dipoles in their surroundings; orientation forces generated by hydrogen bonds and the transfer of charge from electron-donor molecules (transformation of electron-donor-acceptor complexes) and, sometimes, chemical bonds. Dispersive forces operate particularly between additive and oil. Interactions occur among additive molecules, in which all these forces are involved except chemical bonding, and give rise to the formation of micelles. Similar forces act between the micelles and their surroundings: polar, oil-insoluble liquids (oxidation products of hydroxy acids, sulphuric acid salts) are solubilised. The solubilised products may enter into the interior of the micelle or its shell, or be adsorbed on to its surface. Thus, solubilisation may be inter-micellar, micellar or super-micellar, depending on the strength of the interaction and the nature of the substances involved. Interaction may occur between micelles, insoluble oxidation products and contaminants (“carbon”). The resultant effects are the adsorption of the micelles on to the surface of the contaminant particles, establishment of electrical double-layers (the so-called electrostatic barrier), mutual repulsion of particles of like charge and peptisation of the particles. A similar barrier system may be formed by adsorption of micelles on to the metal surface. Such barriers prevent settlement of the contaminant and the formation of varnish on the metal surface; interactions with the metal can also have anti-corrosion and anti-friction and anti-wear effects. Dispersive forces do not change with temperature; the chemical forces become stronger, whilst the remaining interactions become weaker. The weakening effect dominates overall. Temperature increase has, in general, a deleterious effect which requires the use of higher quality additives. Some highly polar contaminants in the additives or the environment in which they operate, such as hydrophilic substances (e.g., low molecular weight sulphonates) and, especially, water, have adverse effects. They dissolve if they are present in small amounts. At higher concentrations, they are preferentially adsorbed on to surfaces and repel the additives from them. The processes taking place in these systems are varied and intricate. Many deficiencies can be overcome by using additives in combination, such as metal detergents with ashless dispersants. Such combinations also minimise the risk of a sudden collapse of the system, followed by separation of sludges from oil and settlement of these sludges on the metal surface (sludge dumping).

Solubilising and peptising effects of DD additives are associated with the magnitude of the charge on and the size of the micelles. Additives forming large micelles of small charge (e.g., succinimides) usually have solubilising effects, whilst additives which form small micelles of high charge (such as polymeric DD additives, metal sulphonates and alkylsalicylates) mostly have peptising effects. The latter types, unlike the former, are adsorbed on the surface of the metal and on the sooty particles, forming electrostatic barriers and preventing the adsorption of other contaminants on the metals and suppressing the agglomeration of insoluble particles. The effectiveness of detergent-dispersant additives depends on their type, the size of their non-polar moieties, and on their constitution and basicity. That of ashless additives depends principally on the nature and amount of their active organic groups. The concentration of the additive is also important. An optimum concentration exists at which maximum efficiency can be achieved. Electrical phenomena are important, but they are not the sole criteria. The nature and presence of polar compounds in the oil are very important; they generally reduce the effect 296

of DD additives by becoming adsorbed on particles in suspension and reducing the detergent surface, solubilising additive micelles and changing the colloidal properties of the system. During service, both oil and additive undergo chemical changes, the amount of additive in oil diminishes and the oil loses its detergentdispersant capacity. Test of solubilising effects on oil-insoluble hydroxy- and keto-acids have shown that ashless dispersants have solubilisation effects 10 times greater than those of metal detergents (84,94).On the other hand, metal detergents are excellent peptisers of solid particles of diameter 2-1 5 nm dispersed in oil, forming a layer 3-5 nm thick which is physically adsorbed on the surface of the particles. The peptising power of sulphonates and phosphonates is drastically suppressed when water is present. The reason for this may be the partial removal of the film from the particle surface by hydration. Thus, the effective dispersive capacity of sulphonates and phosphonates is lower in a moist environment, e.g., in an engine crankcase, than in water-free oil. This explains why the efficiency of these additives is substantially lower under cold-running conditions or stop-go operation when the amount of condensed watcr in the oil increases. Polymeric dispersant VI improvers peptise 2-100 nm particles with a 2-3 nm thick film by means of hydrogen bonds between the dispersant group in the polymer and the polar sites on the particle surface. This film forms a barrier between the particles and prevents them coagulating. Unlike in the case of the metallic detergents, water does not reduce the peptising effects of polymeric dispersants. These dispersants are also preferable to metallic detergents, i n that they peptise solid particles i n the critical size region of 1-5 nm, which is typical for soot (3-5 nm) produced during the partial combustion of fuel (95).

Succinimides not only possess solubilising but also peptising effects. They peptise particles between 2 and 1.5 nm by forming a 10 nm thick surface film. This is formed with 2-50 nm particles by means of hydrogen bonds and with larger particles (up to 1.5 nm) by salt formation or physical adsorption (96).

It is evident from the different solubilising and peptising abilities of different DD additive species that it is desirable to fortify the oil with a combination of detergent and dispersants of different types. These must also be chosen according to the engine type and field conditions for which the oil is intended, and the additive dosage must be adjusted so as to match their concentration to the oxidation products and contaminants which are expected.

Anticorrosion Ability (Control of Corrosive Wear) The acid products discussed in the previous section are corrosive; they can become a primary source of corrosive wear of engine components, especially the metal surfaces of pistons and piston rings, cylinder liners, valves, valve- lifters and bearings, because of direct chemical reaction between the acids and the metals. The effect of oxidation products on copper-lead bearing bushes is not, however, direct and wear is not always of the corrosive type. According to Dennison ( 9 9 ,the first phase of attack is the conversion of the lead into oxides by the action of peroxides produced by oil oxidation; this oxide is then atacked by the acids. A lead-free composition rich in copper is of low strength and is prone to pitting corrosion. So although bearing corrosion can also be caused by their metallurgy, the extent of their corrosive wear is usually affected by the amount and nature of the acidic substances produced during the oxidation of

297

the fuel and the oil. It is therefore necessary to identify the sources of and conditions for the occurrence of corrosion phenomena in both diesel and gasoline engines.

The main sources of corrosion and corrosive wear in diesel engines, particularly of the piston and its rings, are acids of sulphur, if the fuel is sulphurous and the temperature of the metal surfaces lies below the dew-point of the combustion products. These conditions are especially characteristic of the design and operation of slow-speed diesels in marine service. On the other hand, the main agents of corrosion in gasoline engines run on leaded fuels are the halogen hydride acids from organic chlorides and bromides used as lead-scavengers in lead-based anti-knock compositions. Chlorine, in particular, is highly corrosive. Liquid acid oxidation products also participate in corrosion in both types of engine. The degree of corrosion is considerably influenced by engine temperature - as temperature decreases, corrosive wear increases. During engine operation at low temperatures, acid-containing blow-by of fuel combustion products causes cylinder and piston-ring corrosion, if the temperature lies below its dew-point. Gasoline engines are more sensitive than the smaller diesels in this respect.

The most effective way of reducing corrosive wear of engine components is the neutralisation of the acids (i.e., organic acids and sulphuric acid) by the alkaline reserve of the DD additives in the oil. It has been established (99) that these basic detergents reduce corrosive wear substantially up to oil pH 6 in gasoline engines and 4.5 in diesel engines. It has been clearly demonstrated that the extent of corrosive wear decreases with increasing TBN of the oil. This is evident fromfig 4.4, which illustrates the correlation between wear of the first piston-ring of a stationary diesel engine running on diesel fuels of varying sulphur content and alkaline reserve of the lubricants used. Tests by Cook (ZOO) and Bolnes (ZOZ) showed that there was no difference between sulphonates, calcium phosphonate and barium phosphonate in this respect. However, the contrary opinion appears to prevail, that higher efficiency is obtainable by the use of package detergents, mainly basic sulphonates. Ashless detergents are also beneficial. This synergistic effect of ashless detergents can be explained by

$5. a-

3

4-

2a 3 z 2-

e

P ’-0-

r

0.1

. .

I

I

.

I

0.2 0.3 0.4 0.5 0.6 0.7

s

08

-

09 % S

Fig. 4.4. Correlation between the piston rings wear (measured by radioactive method) of a stationary diesel engine, lubricating oils of varying alkalinous reserve (TBN) and diesel fuels with increasing sulphur content

298

so-called physical dispersion, in which the ashless dispersants enclose the sulphuric and organic acids in comparatively strongly-bound micelles, so that the acids cannot act freely. Tests made with Caterpillar L-1 engines showed that ashless dispersants are effective up to oil pH 1-1.5. Ashless dispersants are believed to be effective up to oil pH 4-4.5.

Neutralisation of the acidic products from the products of combustion and thermooxidation products gradually exhausts the alkaline reserve of the oil. The process of exhaustion depends on the quantity of acid produced, the nature of operation of the engine, the oil composition and other factors. After some time, the level of oil alkalinity becomes stabilised. However, the alkalinity of the fresh oil must be high enough so as not to drop into the acid region throughout the entire operating life of the oil.

Anti-rust Protection Favourable conditions for the rusting of steel arise whenever the engine temperature drops below the dew-point of water when the engine is stationary. Basic sulphonates and phenolates are effectiveanti-rust agents because they neutralise the acids which, in the presence of water, promote rusting. However, neutral sulphonate has better ability to become adsorbed on the metal surfaces and thus prevent them coming into contact with water directly (102). The best results have been obtained with detergents containing a combination of metals. Zinc and lead sulphonateshave also given good results. Anti-rust boosters comprising alkylphenolethoxyethanol(I) or copolymers of ethylene-propylene oxide (11) types are sometimes incorporated into mixed packages containing alkaline metallic detergents:

0 - CH2CH2(OCH2CH2)nOH

R HO fCH2CH2 .+CH

(R is usually iso-octyl and n = 4 -9)

I CH3

- CH2 - 0 -j+CH,CH,O

+OH

(11)

(x is about 4 and y about 55)

If these products are used alone, they do not act as rust inhibitors. However, combination with basic detergents causes interactions and this and the products of the interactions neutralise acid contaminants in oil very effectively. Dosages used should be such that the concentration in oil is 0.1 to 0.5% by weight.

Thermal Stability Improvement The correlation between the amount of deposits and oil temperature can be studied using a modified Panel Coker Test (PCT). In this test, the temperature is gradually 299

raised and the quantity of deposit determined at each temperature level after 4 hours of exposure at that temperature level until a temperature is reached at which the oil breaks down. This break-down is manifested by an abnormal increase in deposit formation. Tests have confirmed that the thermal stability of oil increase with increasing DD additive concentration and increasing oil TBN (103). Tests run in parallel between modified PCT and diesel engines have established that the thermal stability of oil containing DD additives gradually decreases with time in service (104).This also shows that thermal stability is related to DD additive concentration; as these additives and their alkaline reserves become exhausted, the thermal stability of the oil decreases.

Effects on the Rate of Oil Oxidation Tests made in model engines such as the Petter W1 have established that the oxidation stability of the oil increases if suitable DD additives are present in addition to antioxidants. Comparison of laboratory oxidation tests together with tests made in model engines has shown that the oxidation stability of oils dosed with ZDDP additives at conventional levels is increased with increasing dosages of calcium petroleum sulphonate and with its alkaline reserve (99).This leads to the conclusion that conventional DD additives with a high alkaline reserve actually improve the antioxidant properties of oils in general, because unlike phenolates, salicylates and phosphonates, petrosulphonates do not themselves behave as antioxidants (106) on the contrary, low-base petroleum sulphonates, in the absence of other additives, act as pro-oxidants and only exhibit antioxidant behaviour at higher concentrations and higher alkalinity (320). There are various interpretations of the mechanism of this effect of DD additives. Sechter et al. (105) attribute it to the solvent power of the detergent-dispersants for the oxidation products and their incorporation into DD micelles, thus rendering them inactive. Phenolates, phosphonates and salicylates (unlike sulphonates), are also believed to produce - after reaction with acidic components in the oil substances which themselves act as antioxidants.

Behaviour towards Water All detergent-dispersant additives are amphipatic, oil-soluble surface-active compounds containing both hydrophobic and hydrophilic groups, able to reduce the inter-facial tension in the oil-water interface. Contact of oil concentrates of DD additives leads to hydrotropy and the formation of emulsions, the stability of which depends on the interfacial surface tension. Differences in polarity result in differences in the effects of particular compounds on the decrease in interfacial surface tension and on emulsion stability. The most effective, in both respects, are those compounds which contain hydrophilic groups of high polarity, such as S0,X (sulphonates), -NH-(e.g., succinimides)and -COOX(carboxylates).

300

This is the reason why DD additives and oils containing them must be protected from contact with water. The DD additive itself does not, in fact, lose its efficiency when it is transferred into the emulsion phase, however, contact of the oil containing the additives causes partial dissolution of the additive in water, which again causes a decrease in the concentration of the DD additive in the oil. The solubility in water of different compounds used as DD additives varies. Hence, resistance of DD additives to water is one of the tests in use. This test consists in determining the decrease in sulphated ash content of the oil after it has been washed with water.

4.2.2 Detergents Detergents mostly used at present comprise calcium, magnesium and, to a much lesser extent, barium salts of petroleum-based and synthetic alkarylsulphonic acids, alkylphenols, and alkenylphosphonic,thiophosphonic and alkylsalicylic acids (oncepopular barium salts have now almost disappeared from additive treatments, on toxicity and other grounds). It has been suggested by Crawford that mixtures of different metals may exhibit synergistic effects and so provide a performance improvement. Additives intended for other purposes may exhibit detergent effects if they have long alkyl chains. For example, zinc dialklydithiophosphates with alkyls around C, have marked detergent properties which can even surpass the performance of ZDDP’s with shorter alkyls in combination with detergents. Thus, at the same molal concentration, the following numerical ratings have been observed in Petter W1 tests (141): base oil + 2.2% weight ZDDP (Cz0) base oil + 0.8% weight ZDDP (C,) base oil + 0.5% weight ZDDP (C,) + 0.6% weight 10 TBN petroleum sulphonate base oil + 0.5% weight ZDDP (C,) + 0.6% weight 10 TBN synthetic Ca sulphonate

26.1 18.4 23.2 20.0

Detergents can either be neutral or basic (super-basic, hyperbasic), i.e., containing super-stoichiometric amounts of metals. The latter are more frequently termed overbased detergents, or detergents with an alkaline reserve. The alkaline reserve or degree of overbasing is expressed by the TBN; this number represents the total alkalinity expressed as amount of KOH per gram. ASTM D-2896 and DIN 51 596-Blatt 1 specify the potentiometric method for determination of total alkalinity. The oil sample is dissolved in a water-free mixture of chlorobenzene and glacial acetic acid and tiuated potentiometrically with a solution of perchloric acid in acetic acid. The number of mg of KOH equivalent to the perchloric acid consumed per gram of sample is the TBN. A coulometric method is also available (123). ASTM D-4739-87 (ASTM D-664 was formerly used) specifiesTBN determinationby potentiometric titration of the oil sample dissolved in a mixture of toluene and isopropanol with an alcoholic solution of hydrochloric acid. Compared with ASTM D-2896, it gives lower values for TBN, since it involves the neutralisation of strong acids with weak bases (bases are stronger in D-2896, since the medium is

30 1

glacial acetic acid); also, the reproducibility of the results is poorer as the transition-point is less distinct, which can be especially important in the case of used oils.

Overbased detergents containing as much as 30 times the stoichiometric amount of metals have been developed over recent years. They can be prepared by heating a mixture of an acid or neutral substrate with a large excess of metal base in solution in a solvent in the presence of a promoter, followed by filtration. Carbonation of the reaction mixture with CO, before filtration is often used to increase the incorporation of metal in the form of carbonate, which is colloidally dispersed andor bound in complexes in the filtered basic detergent (110-114,260).The promoters can be C, - C5 alcohols (mostly methanol) because of their high efficiency and low price, other alcoholates, polyglycols, phenols, mercaptans, ketones, nitro-compounds, carboxylic acids, phosphorus thioacids, oximes and, generally, organic compounds at least 0.0005% soluble in water at 50 "C, with an ionisation constant above about 1.10-lo in the presence of water at 25 "C and a pH at least 7 in saturated water solution, also at 25 "C. The basic detergent shows good solubility when the carbonate is produced in amorphous form; crystalline carbonates (except magnesium carbonate) diminish the solubility of the detergent in oil. The detergent over-basingprocess occurs in two stages: hydration of MO to M(OH), and carbonation with C 0 2 in a water-free environment. Ammonia is used in the hydration process to neutralise acidic components (e.g., sulphonic acid in the manufacture of overbased sulphonates). The solvents employed for the neutralisation and carbonation include volatile hydrocarbons of boiling-point below 150 "C, e.g., hexane, heptane, light gasoline, benzene and xylenes, which are vaporised by the heat developed during the exothermic neutralisation and carbonation reactions. The colloidal dispersion of over-based salts may contain both hydrated and non-hydrated carbonates. These differ in respect of their stability and their ability to form colloidal dispersions. For instance, an overbased magnesium detergent was found to contain the following salts: MgCO, . Mg(OH), . 3H20, MgCO,. MgO. 5H2Q, MgCO3.3H20, MgCO3.5H2O.

A detergent can be characterised by a number of parameters: relative molecular mass, constitution of its organic portion, length of the alkyl chains, total base number (TBN), ratio of metal in the alkaline reserve to metal in the neutral molecule ("salt to soap ratio"), type of metal and, in some cases, sulphur and phosphorus content.

Table 4.6. Limiting Values of Some Currently-used Detergents Sulphonates Relative molecular mass

Phenolates Whenolsubhides)

Salicylates

375-700

160-600

(sulphonic acid)

(alkylphenol)

250- 1,OOO (alkylsalicylic acid)

TBN (mg KOWg) 0-500 Ratio of metal in alkaline reserve to metal in neutral compound 1-30 Type of metal Ca,Mg,Ba Neutral compound content (56 weight) 10-45

50-400

64-345

0.8-10 Ca,Mg,Ba

1-10 Ca,Mg

30-50

10-45

302

Increased detergent alkalinity reduces deposits and wear, giving improved oil consumption figures and longer engine life. Selection of metallic detergent components is critical in securing anti-wear protection and lowest oil consumption. Limiting values of these variables in some currently-used types of detergents are shown in Table 4.6. These variables are not, however, sufficient for assessing the effectiveness of a detergent in oil, which can only be determined by field tests.

4.2.2.1 Alkarylsulphonates

Petroleum sulphonates or, often, synthetic sulphonates are used. Either can be neutral or overbased, calcium or magnesium or sometimes barium alkaryl-sulphonates soluble in oil. They can provide the essential, thermally stable components of detergent additives in all types of oil. The composition of neutral (also referred to as “normal”) sulphonates can be represented by the general formula: (RAr.SO,),M where M is Ca, Mg or Ba in stoichiometric ratio to the acidic group, and RAr is an alkylaromatic group of petroleum or synthetic origin. The alkalinity of basic and overbased sulphonates is predominantly produced by a dispersion of carbonates of the metal concerned in a neutral sulphonate. This has been confirmed by electron microscopy studies (107). The general formula of overbased sulphonates is thus: (RAr.SO,),M.nM’CO, In basic sulphonates, 3 to 15 metal atoms in the alkaline components are associated with one in the neutral component. M and M’ need not be identical; for example M can be Ca and M’ Ba or Mg. The excess metal in basic sulphonates can also be bound to hydroxyl; some authors (108,209)quote the following formula: (RArS0,)MOH Kuliev (2) suggests the following formula for the two forms: (RArSO,),M.MO.M(OH),,

(RArS03),M.M0.MC0,

The most frequently used formula for basic sulphonates is: RR Ar - SO, -0

0 II M- 0 - C - 0 --+M-.OH

+

Overbased sulphonates can be prepared by a great variety of processes. The most common comprises neutralisinga sulphonic acid with excess calcium oxide or calcium oxide in the presence of a promoter, typically methanol, then carbonating the mixture to produce a colloidal dispersion of what is term a calcium carbonated overbased sulphonate.

303

The dispersed calcium carbonate is amorphous, with a particle size from 20 to 150 Angstroms. It is solubilised in a mineral oil camier by the calcium sulphonate, giving bright, clear dispersions. These products are characterised by being relatively non-viscous and having no thixotropic properties. When an overbased calcium sulphonate is treated with an active hydrogen compound such as water, alcohol or a lower carboxylic acid, the amorphous calcium carbonate can be converted into the crystalline form. The predominantly calcium carbonate may exist in three modifications: calcite, vaterite and aragonite. Crystalline calcium carbonate has much larger particles, from 150 up to as much as 500 Angstroms. X-ray diffraction data have confirmed that these calcite particles consist of thin, wafer-like platelets, with a pronounced tendency to orient parallel to the calcium sulphonate in mineral oil. A pronounced change in rheological properties on modification of the calcium carbonate into calcite is attributed to a high degree of association between these parallel platelets (403).Because of this, thixotropic overbased calcium sulphonates with crystalline calcium carbonate have higher viscosities, are more resistant to mechanical break-down, possess good extreme pressure properties and provide better corrosion protection.

Mineral-oil based sulphonates, so-called petroleum sulphonates, were the first detergents used for engine oils during World War 11. They are valuable by-products of white-oil production or can be deliberately produced by sulphonating suitable oil distillates. The following structure can be used as a model for the hydrocarbon portion (215):

-

CH19H39

Ca

According to Danvik et al. (280). the chemical composition of mineral oil sulphonates is strongly affected by the choice of sulphonating conditions.

Sulphonate-type detergents used today in lubricating and other oils (e.g, in diesel fuel, heating and quenching oils) are predominantly calcium salts of relatively low alkalinity (TBN up to about 25), exceptionally Ba salts of TBN up to about 70, together with over-based calcium and magnesium petroleum sulphonates of TBN as high as 400, with TBN 500 rare but available. Supply problems with petroleum sulphonates, their comparatively high price and the manufacture of synthetic alkarylsulphonates for laundry detergents have encouraged petroleum sulphonates to be complemented by synthetic sulphonates. These are predominantly alkylbenzene sulphonates with at least 20 carbon atoms in the alkyl chain (e.g., polydodecylbenzene sulphonate), manufactured either intentionally by alkylating benzene, naphthalene or their alkyl derivatives with propene or isobutene 1-alkeno-oligomers, or extracted as by-products from the manufacture of laundry detergents (261).Their chemical structure can be illustrated by the formula: 304

where M is Ca, Mg, Ba, R are C,, to C3, alkyls, x > 1. Synthetic sulphonate detergents are manufactured and used chiefly as calcium salts at TBN up to around 400, and to a much lesser extent, barium salts up to TBN 80 and magnesium salts at TBN up to about 500. The magnesium salts are more expensive, sensitive to water, with which they can form gels when stored. They have. however, very good anti-rust properties. They are therefore particularly suitable for engines with hydraulic valve-lifters, which are prone to rust failure, which causes valve-knocking. Another advantage is their relatively low ash content, which is important in avoiding pre-ignition.

Examples of commercial products are shown in Table 4.7. Barium content in oils and additives is usually determined by 1P 1 lot743 the sulphated ash is dissolved i n perchloric acid and the Ba is determined gravimetrically as sulphate. Metals which form insoluble sulphates such as lead must not be present. After Ba has separated, Ca and Al may be determined. IP 1 1 1/74 specifies a particular method for determining Ca in the presence of P, Fe, Al, Ba and Mg. Oil is incinerated, the ash dissolved in HCI and the Ca precipitated as oxalate, which can then be determined by titration with KMnO,. ASTM D-811 specifies a process for separating Ba,Ca, Mg, Zn, Sn. Si and Al in both fresh and used oils in the presence of S, P and Cl; the sulphate ash is dissolved in HCl and the individual metals determined by the wet method. Methods for analysing the composition of sulphonates by liquid chromatography are specified in ASTM D-2548 (for Na sulphonates) and ASTM D-2894 (for Ca and Ba sulphonates). For the determination of Na sulphonates, the sample is dissolved in chloroform and adsorbed on silica gel. Oil is eluted with chloroform and sulphonate with alcohol; the amounts present are determined gravimetrically. For the determination of Ca and Ba sulphonates, the sample is dissolved in diethyl ether and the sulphonates converted with HCI into sulphonic acid. The latter is converted into sodium sulphonate. The further analysis proceeds according to ASTM D-2548. ASTM D-855 specifies the detailed analysis of sulphates. Factors for the conversion of metal concentration in oil into sulphated ash are: Metal

Conversion factor

Ca Mg Ba Zn

3.4 4.95 1.7 1.24

The ash value is obviously dependent on the metal (Ba sulphonate has the highest, Ca sulphonate lower and Mg sulphonate the lowest ash content) and on the alkaline reserve. The relative molecular weight for good solubility of metal sulphonates used as detergents and anti-corrosion additives for lubricating oils should be between about 900 and 1100. Detergent capability and solubility both improve as it increases. It is

305

Table 4.7. Composition of Commercial Petroleum and Synthetic Sulphonates Used as Components in Engine Oil Detergent Additives* Properties and Compositions

Density at 15°C (kg.m-)) (approx.) Content (%weight) of: Ba

Petroleum sulphonates calcium calcium neutral slightly basict

calcium neutral medium basic

Synthetic sulphonates barium highly neutral medium basic basic

magnesium highly basic

1100

1 loo

930

940

950

lo00

1 100

-

-

-

-

11-13 (16.2) 1.8-2.0 28-32

Mg Ca S Active sulphonate (%)

1.4-2.1 1.7-2.4

I .5-2.4 1.6-2.4 35-47

1.7-1.9 2.7-3.0

TBN (mg KOWg)

max.10

10-30

max. 10

Sulphated ash (% weight)

4.8-7.2

5.1-8.5

5.8-6.5

-

2.84.0 2.7-3.0 42-47

20-60 280-310 (400) 9.5-13.6

13.U.2(55)

900 5.5-6.1

-

2.6-2.9

max.10 9.4-10.4

-

12-15 2.6-2.9 4247

1.8-2.0 min. 29

50-80

to 500

20.4-25.5

7-10

-

45-50

35-509b solutions of metal sulphonates.

Commercial, medium-base (TBN 10-12) products are also available derived from petroleum, also over-based (TBN 300-400). Their preparation is substantially more difficult.

also desirable that the dispersion of molecular weights in the product is as low as possible. Detergent power, naturally, depends on the effective concentration of active sulphonate. Sulphonates which have low inorganic salt and sediment content (below 0.1% by weight) as a result of purification with low molecular weight alcohols such as isopropanol are more effective as detergents. The anti-corrosion and anti-rust properties of barium and magnesium salts are better than those of calcium salts. This property, inherent in neutral sulphonates, can be impaired by high metal chloride content, e.g., CaC1, present as residue from the conversion of sodium into calcium salts. The chlorine content of petroleum sulphonates should not exceed 0.03% by weight. According to Anand and co-workers (272,272), the most effective detergent-dispersantsare Ca-Ba petroleum sulphonates prepared from sodium petroleum sulphonates with an average molecular weight of 460-482 (this may be achieved either by fractionation, e.g., according to Fische et al. (273). or by mixing petroleum sulphonates of different ranges of molecular weight obtained from different oil fractions). The chemical nature of the oil fraction used as starting material for sulphonation has a significant effect on the dispersive power of the finished sulphonate. The best detergent-dispersant efficiency in engine oils is that of Ca-Ba petroleum sulphonate produced from oil fractions with a CN:CAratio of 4 - 5 . The petroleum sulphonate molecule should contain only one aromatic ring and the alkanic side-chain should have at least 13 carbon atoms. Tests by Bowden’s microscopic method have shown that the detergent-dispersanteffect of oils do not increase linearly with the concentration of Ca-Ba petroleum sulphonate, but that there are optimum concentrations at which maximum efficiency is achieved (efficiency varies with gradually increasing sulphonate concentration in oil, up an optimum). The efficiency variation of petroleum sulphonates with good detergent-dispersant ability is minimal and such petroleum sulphonates possess good efficiency, even when the optimum concentration is exceeded, whereas the efficiency of medium- or low-quality sulphonates deteriorates rapidly with further increases of concentration in oil.

The desirable cleansing, dispersant and neutralising power of metal synthetic alkarylsulphonates depends mainly on: - the length and configuration of the alkyl chains, - the ratio of polar to non-polar parts of the molecule, - basicity. Solubility in oil and detergent-dispersant action of the sulphonate increases with length of the alkyls. This is illustrated in Table 4.8: The detergent efficiency of these sulphonates decreases with the number of substituents on the alkyl chain. Also, di- and polyphenyl derivatives cause deterioration in the beneficial effects of monophenyl derivatives. These phenomena are the consequence of adverse changes in the polar and non-polar portions of the molecule (85).Basicity is obviously the determinant of the neutralising power of the alkaryl sulphonate. Neutralising capacity improves with alkaline reserve, but the detergent-dispersant efficiency decreases, because of the decrease in active sulphonate concentration. These factors must be taken into account in formulating oils with sulphonates. 307

Table 4.8. Petter AV1 Engine Test Results with Oils Treated with Barium Monoalkarylsulphonatesof Various Alkyl Chain Lengths Alkyls

Relative molecular weight'

Average number of C atoms

c10-c14

167

11.8

c15-c22 c22-c33 c24-c35

280 360 390

19.8 26.0 27.7

c,,-c,* (n-dkyl) CZ6-C2,(polyisobutenyl)

518 510

-

Total test merit rating points

Not measured - sulphonate insoluble in oil 86 91.7 93 91 80

* Relative molecular weight of sulpho-acids.

Metal sulphonates usually have good thermal stabilities. Therefore, relatively high temperatures can be used in handling them, namely, up to 80 OC bulk temperature and up to 120 "C skin temperature. Contamination with water must be avoided in order to prevent emulsion formation. The biological activity and human skin effects are similar to those of mineral oils, and the same regulations are applicable. However, recent work has raised questions of possible skin-sensitisation potential with certain sulphonates. \

4.2.2.2 Alkylphenolates (Alkylphenolsulphides) The first detergents of this type (also referred to as phenates, sulphurised phenates and sulphurised alkylphenates) were first used during World War I1 in the form of calcium and barium tert-octylphenolsulphide and tert-amylphenolsulphide (116),the condensation products of calcium tert-amylphenolate with formaldehyde (117)and calcium and barium phenolates with long alkyl substituents (118).The presence of a labile sulphur atom in the alkylphenolsuphide enhances its antioxidant properties and imparts anti-wear properties. The detergent action depends on the size and configuration of the alkyl group. The patent literature quotes a number of alkylphenolate and alkylphenol-sulphide type detergent additives. Some alkylphenolsulphides have, in addition, anti-foam effects. The superiority of the phenolate detergents resides in their ability to alleviate oxidation and even corrosion. Therefore, they enable the amount of additives in oil to be reduced. In lubricants in which MDDP are not permitted, they perform the full function of antioxidant. The high neutralising power of magnesium overbased phenolates can makes them preferred additives in lubricants for marine diesel engines requiring high alkalinity, although their use in this application has been suspected of causing cylinder wear. All detergents decrease bearing weight loss, but, on an equal metal and base molal content, phenates are slightly better than salicylates and sulphonates. Some advantages of phenates stem from the fact that they are derived from relatively weak acids (phenols and C02). On reaction with strong acids, 308

sulphurised alkylphenols are liberated, which can be expected to provide antioxidant and anti-wear characteristics. Phenolates have good high-temperature properties and are therefore suitable as detergent components in oils for diesel engines and highly thermally-loaded gasoline engines. Phenolates with longer alkyl groups improve the solubility of polymeric viscosity modifiers in oil. This is particularly important for high VZ hydrocracked and polyolefin oils, in which the solubility of polymeric modifiers is poorer. The synergistic effect of phenolates and sulphonates is also of considerable importance. Calcium, barium and magnesium alkylphenolates are produced and used either as hydroxylated salts: OMOH

OMOH

OMOH

or, with alkaline reserve, in the form of metal carbonates: M -0 - CO- 0 - M 0I

0I

I

!-o-Co-o-M

I 0

where M is Ca, Ba or Mg, x is 1 - 5 , R is a straight-chain or branched C,-C,, alkyl, typically nonyl or decyl. The alkaline reserve in the calcium phenolates available now is as high as 400 TBN, but in barium phenolates below 120, because of the high ash content in high TBN Ba products. The preparation and production of basic alkylphenolates is simpler than that of sulphonates and phosphonate, in that the phenol itself (without promoter) enhances the production of the excess metal oxide in the colloidal form (119). Promoters are used in commercial processes; these include methanol, methoxyethanol and ethylene glycol. The presence of petroleum or synthetic sulphonate has a beneficial effect. This interaction between phenolates and sulphonates can be utilised in the preparation of package overbased DD additives with alkylphenolates and sulphonates in suitable ratios. This is advantageous particularly in the case of those petroleum sulphonates where the alkaline reserve is difficult to produce or to introduce into finished oils without problems of oil-solubility.

Examples of commercial products are illustrated in Table 4.9. Alkylphenolates can be handled safely at temperatures up to 120 "C (skin temperature up to 180 "C). Calcium-containing products have a similar effect on the human organism as mineral oils. Bariumcontaining products are toxic and must be handled more carefully; this toxicity has led to the virtual removal of barium-containing additives from commercial use.

309

Recently (1988-9), high TBN sulphurised calcium alkylphenolates have appeared on the market, with TBN around 400 mg KOWg (ADX410from Adibis). These represent a departure from the phenate materials hitherto available. During manufacture, long-chain naturally-occurring acids are introduced into the phenate molecule. The viscometric properties of these high TBN products are unusual in that in the concentrate form they have viscosity index over 150, compared about 70 for conventional phenates at 250 TBN. The incorporation of long-chain acids also imparts significant friction modification properties, again unusual in phenates. Normal treatment levels in automotive lubricant formulations reduces the friction coefficient from levels of 0.13 to 0.085.

Table 4.9. Properties and Composition of Commercial Alkylphenolates and Phenol Sulphides Used as Components of Detergent Additives in Engine Oils* ROpertY

Calcium type medium high basicity

neutral

1100

1000

Barium type medium overbased basicity 1000

1100

9.3-14.0

4.5 8.5-10 3.5-5.5 2.6-2.8 2.8-3.5 85-120 250-400 10max. 70-80 9.9-16 31.6-47.6 7.7-1 1.9 14.5-17 * All are solutions in suitable diluent oil raftinate; actives content around 50%weight.

-

-

11-13 3-4 95-120 19.5-25.5

Density at 15 "C (kgm3) Composition (% weight) Ca Ba S TBN (mg KOWg) Sulphated ash (% weight)

1100 2.9-4.7

-

4.2.2.3 Alkenyl Phosphonates and Thiophosphonates These DD additives have almost entirely lost their market position because of their inferior thermal stability, their high ash content as barium salts, their high cost of manufacture and the general effort to reduce the phosphorus content of oils for gasoline engines fitted with catalytic converters. Since they were largely useful as barium salts, the toxicity of barium has also tended to discourage their use. They were used for the first time during World War II, when it was found that metal salts of the alkenylphosphonic acids produced by the reaction of phosphorus pentasulphide with liquid polyalkenes of 500-2000 relative molecular weight, mainly polybutenes (99,I22),have highly detergent-dispersant effects on high- and low-temperature sludges in lubricating oils. One of the first commercial products was the potassium salt prepared by the reaction product of PzSs with liquid polyisobutene of viscosity about 120 mm2.s-' at 38 "C (220). Later commercial products were salts of alkaline earth metals, particularly barium salts (122).

The action of steam or a prolonged hydrolysis prior to or during the neutralisation stage can cause substitution of part of the sulphur in the thiophosphonate group, PS(SM),, by oxygen, so that a mixture of thiophosphonate and phosphonate is

3 10

produced. For simplicity, the former commercial products are sometimes called phosphonates (although they contain sulphur); they were given the general formula:

x x II/

R P

\

M.M(OH),

\ /

or

MCO,

X

where R is a long alkenyl radical with minimum relative molecular weight 500, X is oxygen or sulphur, M is a bivalent metal. Calcium salts are difficult to prepare and only soluble in oil to a limited extent. In packaged detergent engine oil formulations, now obsolete, medium-base barium (thio)phosphonates of TBN up to about 120 were used, of the following type:

X X It II R-P-S-P-R I

I

where R is any alkenyl C,,-C,, and X is S or 0. The commercial products usually had 12.0% by weight barium, 0.8-1.2% phosphorus, 1-3% sulphur and up to 20% sulphated ash. TBN varied around 70 and the additive concentrate usually contained about 50% ac'tive component. Combination products of the following type were more frequently used: /O\ R e O - PI

P-O-@R I

"\ /"

M.M(OH),

or

MCO,

where R is an alkyl C,-C,,, M is Ca. Phosphonates of higher alkaline reserve may also be prepared, but such a reserve in the barium compounds would be associated with an extremely high ash content. Similar promoters or solvents as in the production of sulphonates are employed for other over-based products, including phenols, phenolsulphides (124,alcohols (125) and acids, such as formic acid (226).

Although now largely obsolete and replaced by sulphonate/phenate combinations on grounds of toxicity and ash content, the phosphonates and thiophosphonates were 31 1

highly efficient detergent-dipersants suitable for improving piston cleanliness. They were particularly suitable for oils intended for lubricating gasoline engines, less suitable for diesel oils, because they have lower stability at higher temperatures. Phosphonates could sometimes be used in such cases as DD additive components, mainly phosphonates of higher Ba/P ratio (above about 5/1), because they contributed efficiently to the reduction of carbon deposits in the piston grooves. They were, to some extent, able to prevent the formation of "cold sludge", particularly thiophosphonates with low Ba/P ratios. The relative molecular weight, the BaP ratio, the method of preparation and the number of unsaturated linkages in the alkenyl chain determine the efficiency of thiophosphonates. According to Karl1 et al. (262),the efficiency of Ba thiophosphonates grows at the same molal concentration up to a relative molecular weight of 1,250;according to MAFKI staff (263),Ba thiophosphonates alkylated with polyisobutene of relative molecular weight 850 to 2,300 have similar efficiencies. Because of the neutralisation effect, the efficiency of thiophosphonates increases with increasing metal content up to a Ba/P atomic ratio of 1.5/1. Phosphonic acids can be prepared from polyalkene and P,S, at 200 to 250 "C. The barium salt is then obtained by neutralisation with barium hydroxide, after steam treatment at 150-200 "C and filtration. According to the MAFKI work (263),a better product was obtained if the thiophosphonic acids were purified by sedimentation only, without steam treatment, provided sediment content does not exceed 1.5 to 2% weight (the barium content of the sediment is substantially higher, and exists largely in the phosphonate form, which reduces the efficiency of the additive). High efficiency barium thiophosphonates cannot be made from alkenes with more than one double bond in the chain - dienes reduce the effectiveness of the additive.

Alkenylthiophosphonates are the most thermally sensitive of all the ashcontaining detergent-dispersants. They should not be heated above 60 "C and only liquid heat-transfer media should be used for warming them. Thiophosphonates are .toxic and ingestion or long-term inhalation should be avoided. 4.2.2.4 Carboxylates

The first detergents used were the soaps of high molecular weight carboxylic acids, such as calcium dichlorostearate: [CH, - (CH2)7- CH- CH I I c1 Cl or calcium chorophenylstearate:

- (CH,),

0 II - C - 01Ca

0 II [CH, - (CH2)7 - CH - CH - (CH2)7 - CI

I

01 Ca

Cl Although these substances had good detergent capacity, they were abandoned because of they promoted oxidation and caused corrosion of copper-lead bearings.

312

Calcium naphthenate with a high alkaline reserve is still used occasionally, e.g., in diesel cylinder lubricants in cross-head marine engines operating on a total oilloss system:

r IR

0

1

II (CH2)n- C - 0 Ca.CaCO3

L

J2

Alkylsalicylates form a separate group of general formula:

where M is a bivalent metal, usually Ca, but also Mg, and R is a c,-C,o alkyl. The salicylate esters originally used, such as zinc diisopropyl salicylate, were later substituted by calcium salicylates with longer chains, such as octyl, attached to the aromatic nucleus. Basic calcium and magnesium salicylates are now used. Typical products, made by Shell, are characterised as follows: Shell Code SAP001 SAP002 SAP005 SAP007 SAP008*

Metal type Ca Ca Ca Mg Mg

%w/w 6.0 2.3 10.0 7.4 6.0

TBN (mgKOWg)

165

64 280 345 280

* Also contains 2.9 weight boron. Magnesium alkylsalicylates are mainly used in marine engine oils, because of their good anti-corrosion properties. They help achieve high alkalinity values at low ash-levels, but they tend to have lower thermal stability then the calcium salts. Alkylsalicylates are prepared by carboxylation of water-free metal alkylphenolates with carbon dioxide at elevated pressure (227). Preferred starting materials are rnonoalkylphenolates substituted, preferably, in the para-position;2,6-dialkylphenolsand 2,4,6-trialkylphenolsare not suitable, in that they fail to carboxylate, or only carboxylate at low yield at high tempratures and pressures (228).The alkaline reserve is usually in the form of carbonate. Alkylsalicylates of exceptional purity can be made by carboxylation in wiped-film reactors (387).

313

Treatment with carbon dioxide (carbonation) of a mixture of salicylate and added metal hydroxide in the presence of a promoter, e.g., an alcohol, is a general procedure for the preparation of overbased detergents. Alternatively. a colloidal complex of a metal carbonate can be prepared in siru and then combined with the acid or its salt to give the required excess basicity. This chemical route is followed in the case of magnesium phenates, where the former reaction path is less satisfactory. In both cases, the degree of basicity can be expressed as metal ratio or "basicity index", which is the ratio of the metal present to the metal contained in the neutral salt.

An advantage of the alkylsalicylates is their high thermal stability, which makes them particularly suitable for oils used in lubricating heavy-duty diesel engines. Overbased alkylsalicylates have good anti-corrosion properties, but they have the disadvantage of incompatibility with some mineral and synthetic sulphonates unless other DD additives are present. Magnesium sulphonates are less sensitive in this respect, but have the disadvantage of lower thermal stabilities. Salicylates also have a significant ability to reduce friction. The basic metal salts of sulphurised alkylsalicylic acids are also good detergents (294). Alkyl esters, e.g., ethylhexyl esters, of alkylsalicylates,act as antioxidants. Because of their antioxidant and detergent actions, alkylsalicylatesmay be used alone, however, they are usually combined with ZDDP to improve anti-wear properties and with succinimides to prevent the formation of low-temperature deposits, to improve piston cleanliness and to reduce the ash content of the oil. Like phenolates, they show synergistic effects with alkaryl sulphonates. Combinations with phenolates can be extremely effective.

With regard to handling temperatures and effects on the human organism, the same guide-lines apply as for alkylphenolates. According to Grjaznov et al. (295),neither basic Ca alkylsalicylate nor the low-base Ba alkylsalicylate decomposed when heated at 180 "C for 2 hours in a nitrogen atmosphere. After heating at 300 "C, Ca salicylate retained some of its properties by forming a CaCO, dispersion, in spite of decomposition; on the other hand, Ba alkylsalicylatelost its detergent and anti-corrosion properties. Sodium alkylsalicylate decomposed at 180 "C.

The composition of the commonly-used ash-containing detergents, sulphonates, phenolates and/or salicylates, has tended to stabilise somewhat in recent years, except for the continued pressure towards ever-higher overbasing, in some cases involving novel process chemistry. A comparison of constitution and properties, and effects in gasoline and diesel engines, is given in Table 4.10. There has also been comparatively little development evident from the patent literature in this area. Among newer compounds may be noted: - basic calcium polyisobutyl sulphonates by the reaction of PIB (r.m.w. around 950) with chlorosulphonic acid (327), - sulphurised basic magnesium alkylphenolate with TBN around 250 mg/KOWg (320), - basic metal polyarylaminosulphate and polyarylaminophenolsulphides (329), - very high TBN (>400) overbased sulphurised calcium alkylphenolate.

314

Table 4.10. Comparison of Ash-containing Detergents in Engine Oils Effect Diesel engine oils -deposit reduction above piston rings -deposit reduction below piston rings -increase in TBN per 8 ash -TBN retention Gasoline engine oils -prevention of oil-thickening (MS IIID) -rust prevention (MS IID) -reduction in varnish (MS VD) Both diesel and gasoline engines -increase in TBN per Q ash -prevention of CuPb type bearing corrosion -effect on Friction coefficient

Sulphonates Phenolates Salicylates

3 3

2 2 1

1 2 1

2 3 3

1 1 2

1 2 1

1

1 =best 2 = medium

3 =worst Note: Calcium alkylsalicylates have a beneficial effect on piston cleanliness in diesel engines, particularly in engines with high temperatures in the first ring groove (e.g., Caterpillar 1G-2) and on the reduction of carbon in that groove. Similar effects can also be achieved with mixtures of overbased calcium alkylphenolate and low-base calcium sulphonate in a suitable ratio. By contrast. basic calcium phenolate by itself is the least effective in this respect.

4.2.3 Dispersants The term dispersant is used in this context to denote ash-free additives capable, among many other useful functions, of dispersing cold sludge-forming substances in the crankcases of gasoline engines. The tendency to form low temperature sludges and, consequently, to increase viscosity is also displayed by oils in smaller diesel engines. Special methods for testing the resistance of these oils to the formation of cold sludges have been developed (e.g.,formerly, the Perkins Diesel Oil Thickening Test and Daimler Benz OM 616 test, a replacement test now under consideration by CCMC, with the OM 602A as the likely candidate). The sludge formed in this type of engine is different from that formed in gasoline engines, in that it arises as a result of temperature distribution in the flame-front in the diesel combustion chamber and the volatility of diesel fuel. In small diesels, the surface area of metal exposed per unit volume of combustion space is relatively high, leading to soot being formed in the cooler parts of the flame. Soot build-up in the crankcase oil of engines of this type can be quite rapid and can cause it to become thixotropic, leading to engine failure as a result of inadequate oil-flow. Whilst cold sludges in gasoline engine also contain sooty material, the starting materials from which the soot is made are different and the overall chemistry of the deposits is also different. However, the phenomena in both types of engine are amenable to dispersant treatment.

Dispersants are, moreover, capable of stabilising the products of thermooxidation, peptising solid contaminants and preventing them from settling; they are synergistic with detergents and are satisfactorily stable at elevated temperatures. For these reasons, they are conventionally used as components in DD additive packages. Modem high-quality engine oils usually contain 4 to 8% by weight of ashless dispersant. The first compound with proven ability to suppress significantly the

315

formation of cold sludges in gasoline engines was the copolymer of lauryl methacrylate with diethylaminoethyl methacrylate (130) (see also under VI improvers). A number of dispersants suitable as components of DD additives has been developed since. The mechanism of one aspect of DD action - maintaining contaminants in a peptised state - can be deduced from consideration of the probable nature of the peptised particles. In the case of ashless dispersants such as succinimides, relatively small (0-500 A) particles may be produced, by forming a thick, non-polar film around the contaminant (which is polar), thus preventing coagulation. Polymeric dispersants achieve a similar effect, producing larger particles (0-1.000 A). The structure and shape of the polymer chain may exert an important influence. This action of ashless dispersant can also be supposed to operate with metallic dispersants in the case of the formation of small particles (0-200 A). However, metallic dispersants can also form large particles (5,000-15,000 A) on which a surface charge prevents coagulation by electro-static repulsion.

4.2.3.1 Succinimides and Bis-succinimides Succinimides and bis-succinimines are the reaction products of alkenylsuccinic anhydrides and polyethylene polyamines (131): R - C H = C H - C H -CO I CH, - CO’

‘

N - (CH,), - (NHCH, - CH,), - (CH& - NH,

(mono) succinimide R - C H = C H - C H -CO, I CH, - CO

,\

N (CH,),

- (NHCH,

- CH,), - (CH,), - N

/CO - CH - CH = CH - R I - CH,

’ ‘CO

bis - succinimide where R is usually a polybutenyl fragment of relative molecular weight 800 to 2,500 (mostly 800 to 1,200) and x = 2 to 5. The alkenylsuccinic anhydrides are made by the reaction of a polyalkene with maleic anhydride. The distribution of molecular weights in the alkenyl should be as namow as possible, in order to reduce its resistance to cleavage, which causes the formation of products of only limited solubility in oil or partial solubility in water. The alkenyl anhydride must therefore be tested prior to use in reactions with polyethylenepolyamines for the concentration of water-soluble constituents (with alkyls up to about Cl0) and oil-soluble constituents (with alkyls above C,d. Copolymers of polyisobutene (PIB) with styrene or PIB monomers with functional groups, or other polyalkenes, e.g., poly-n-butenes, copolymers of ethylene-propylene, propylene-isobutene, etc., have been used in place of polyisobutene. Since the reaction of polyalkene with maleic anhydride is never fully complete, some residual polyalkene remains which is virtually impossible to extract in a practical industrial process; this polyalkene therefore becomes part of the resulting product and affects its viscosity. This is one reason why the viscosities of commercial products varies over such a wide range.

316

The addition reaction of maleic anhydride with PIB is normally carried out with excess maleic anhydride in the initial charge by simply heating the starting materials together. At the end of the reaction, the excess maleic anhydride is removed by distillation from the crude polyisobutenyl succinic anhydride. The addition proceeds vigorously at first, but slows after a substantial proportion of the PIB has been converted. The conversion of PIB in the later stages of the reaction can be considerably increased by the addition of haloalkanes or halogens. Chlorinated PIB reacts much more readily than the original polymer. This method allows polyisobutenyl succinic anhydride to be produced with significantly less unconverted residual PIB, thus enabling a more concentrated succinimide product to be made, which has lower viscosity (especially at low temperatures) than products containing a higher concentration of unreacted PIB. PIB succinimides are used as the principal dispersant component in many lubricating oil formulations for a variety of reasons. Prominent among the advantageous factors are high thermal stability, superior dispersant activity, flexibility in modification of the molecule to meet particular performance requirements, ready availability and moderate cost of raw materials and ease of handling. Perhaps the biggest disadvantages in the longer-term are oil-thickening at low temperatures and the presence of chlorine (normally 50 - 150 p.p.m.). The degree of low temperature thickening is largely a function of the structure of the molecule and its restricted rotation due to the methyl groups. If large amounts of dispersant are needed in the final oil to meet engine test standards, this can emphasise the problem. Chlorine content in PIB succinimides is a function of the method of manufacture. Some can result from catalyst residues present in PIB, but it can also be present as a result of the method of manufacture of the polyisobutenyl succinic anhydride intermediate. Depending on the routes chosen, it can vary between 50 and 1,500 p.p.m. Chlorine in lubricating oils is an emotive issue because there it can lead to the possibility of emission of dioxins in vehicle exhausts. At least one PIB manufacturer has offered chlorineand bromine-free PIB, which may well be the preferred raw material in the future. The manufacture of mono- and bis-succinimides depends on the accurate determination of the acidity of the polyalkylenyl anhydride which forms the non-polar part of the molecule. This determines the conditions for the reaction of the anhydride with the polyalkylene polyamine, especially the molar ratio of the two starting materials. Smaller inaccuracies in this determination can lead to differences in the ratio of mono- and bis-succinimides in the final product. Technical monosuccinimides always contain some bis-succinimide and vice versa. The manufacture of packaged succinimides containing different amounts of mono- and bis-succinimides is based on this principle. In addition, mono-succinimides can form amide complexes, similar to bis-succinimides, of the following type: R - C H = C H - C H -CO I

CH2 - CO'

'

N - (CH2)2- (NHCH, - CHz),- (CH,),- NH

\

CO- CH - CH = CH - R

CO - CH2

by proton-transfer to the tertiary nitrogen in one succinimide ring from a primary amine nitrogen in the mono-succinimide, causing ring-opening and condensation. The condensation is reversible and the monosuccinimide is probably re-formed at higher temperatures (150-180 "C). It is possible that this sort of mechanism is responsible for the observed variation in the viscosity of mono-succinimide additive concentrates after storage at intermediate temperatures (50-90 "C). It seems unlikely that this condensation affects the properties of the dispersant in finished oil, although this possibility cannot be entirely ruled out.

Mono-succinimideshave excellent dispersant action on low temperature sludges in oil and the products of incomplete gasoline combustion, as well as improving the cleanliness of the entire piston surface in gasoline engines. Their effect on the high 317

molecular weight products of diesel fuel combustion is less marked, in that they do not sufficiently prevent the formation of deposits in the first ring groove, although they do materially contribute to the reduction of varnish deposition on the lower (skirt) portion of diesel pistons. These products of diesel operation are better dealt with by bis-succinimides, which have higher molecular weight. The bis- compounds also efficiently disperse gasoline combustion by-products, provided a certain amount of the basic nitrogen is present as mono-succinimide. Since the nitrogen content in bis-succinimides is significantly lower than in mono-succinimides, nitrogen is usually present in higher concentration in gasoline engine oils than in diesel engine oils. Compared with bis-succinimides. mono-succinimides are more aggressive towards sealing materials of the Viton (@ Dupont) and copper-lead type bearing materials. Bis-succinimides have higher thermal and oxidation stabilities than monosuccinimides.Moreover, the cleavage of mono-succinimidesgives rise to potentially insoluble products, which must be dispersed by the remaining succinimide. On the other hand, the cleavage of bis-succinimides produces two oil-soluble, potentially beneficial succinimides. For this reason, it is good practice to formulate with a combination of both succinimide types in a suitable ratio. The use of such combinations is essential for oils formulated to lubricate both gasoline and diesel engines, such as oils to meet MIL-L-46152 E, class API SG/CD, which must pass both MS Sequence VE (gasoline test) and Caterpillar 1-G 2 (high-output diesel test). Generally, the thermal stability of succinimides is high. The -NH, groups cleave at temperatures up to 250 to 350 “C; polyisobutene is stable up to about 310 “C. the decomposition of succinimides accelerates above 250 OC if the oil also contains ZDDP above a critical ratio because of reactions with -NH, and -NH- groups.

The nitrogen content of commercial succinimide concentrates (about 60-70% actives in oil) varies between 1.4 and 2.1% by weight, depending on the lengths of the polyalkenyl and polyethyleneamine chains. The nitrogen content of bissuccinimides is in the range 1.1 to 1.4%. Nitrogen can be determined by ter-Meulen hydrogenation, Kjeldhahl or Dumas volumetric methods (252) (ASTM D-3228).

Succinimides are only slightly basic, and their chemical neutralising power is therefore lower than that of overbased detergents. They are, however, able to neutralise acidic products by so-called “physical neutralisation”. The dispersant power and oil-solubility of succinimides is affected mainly by the length of the polyalkenyl and polyethyleneamine chains. The presence of other compounds, like ZDDP, or higher polyamines of the type: is also important. 318

The reaction products of pol yalkylenepolyamineswith a mixture of alkenylsuccinic anhydrides and aliphatic mono-carboxylic acids also have good dispersant properties and increase the thermal stability of ZDDP’s.

These types of succinimides have no anti-wear properties. They must therefore be used together with wear- and corrosion-inhibitors. The most frequently used inhibitors are ZDDP, n-alkenylsuccinates, calcium bis-phenolates and metal dialkyldithiocarbamates. Mutli-functional succinimides are also available with anticorrosion and anti-wear properties; their molecular structure contains functional groups of phosphorus, sulphur, boron and similar atoms. Combinations of succinimides (and primary amines in general) with ZDDP are very sensitive to concentration in relation to their anti-wear properties. For a given ZDDP concentration, a critical amine concentration exists above which the anti-wear efficiency of the ZDDP is drastically reduced. On the other hand, it increases synergistically up to this point. ZDDP’s with long alkyl chains are more sensitive to this phenomenon; the aryl dithiophosphates are insensitive (381). Succinimidesby themselves have a corrosive effect on copper and its alloys because of the formation of chelate bonds between the copper and the amide and amine nitrogens in the succinimide molecule. This reaction can be inhibited by adding a small amount of acid, e.g., succinic acid or terephthalic acid, or by incorporating boron in the succinimide molecule.

The presence of boron or sulphur improves both the dispersant efficiency and anti-wear properties of succinimides. The boron content, expressed by the mass ratio to nitrogen, can vary between 0.1 and 5.5:l. Succinimides containing boron can be prepared by reaction with various boron compounds, e.g., boron trioxide or boric acid and its esters and halogenides (277, 421). The patent literature describes of other types of succinimide dispersants (270). Interesting examples include those prepared by the reaction of alkenylsuccinic anhydrides with condensation products of urea with polyalkylenepolyamine(278) of the type: H2N-(ANH),-CO-(NH-A),-NH,

t”

and the reaction products of piperidine derivatives (296) of the general formula - CHR

I

c -z II 0

where n is 2 to 200 and Z is a linkage of the type:

-0-N

,CH2CHz\ ‘CH,CH2 / N - M - o -

319

0-A-N

,

/ CHZCH2\

CHZCHZ\

\ CH2CH2/ C H - D - C H

\ CHzCH2/ N - M - o -

where A,M,D are bivalent saturated aliphatic hydrocarbon radicals with 2 - 10 carbon atoms, and R is a univalent alkenyl radical of 30 to 200 carbon atoms. Similar examples include the reaction products of alkenylsuccinic anhydrides with derivatives of 1(aminoethyl) piperazine, 1,4-bis-(aminopropyl)piperazineand dipropylenetriamine. The halogen-assisted reaction, mentioned earlier, of polyalkenes with maleic anhydride, can produce multiple substitution on the polyalkene chain. This presents the possibility of increasing the number of polar groups in the molecule and an increase in dispersant effect.

4.2.3.2 Miscellaneous Polyalkenepolyamine Derivatives Dispersant additives with pronounced antioxidant properties can be made by Mannich reactions from long-chain alkylphenols (most often from polyisobutenes or polypropylenes of molecular weight about 1,OOO), polyethylenepolyamines and formaldehyde (279):

CH2 - NH - (CH,CH,NH-),

R OH

R

- CH2CH2 - NH2

CH, - NH - (CH2- CH2- NH), - CH2 - CH2

OH

Their effectiveness depends on the average molecular weight of the akyls and on the mutual ratio of the string materials. The thermal stability of these Mannich base products is lower than those of the succinimides. Other dispersants of the general formula: OH

CHZ- NH - (A -NH),-

X II A - NH - C -NH

- A - (A - NH),- NH,

R-& OH

X II CH, -NH - (A - NH),- A - NH - C

- NH - A - (A - NH),- NH- CH,

OH

can be prepared from Mannich bases by condensation with urea and thiourea (A is -CH2-CH2-and x is oxygen or sulphur).

320

These additives, like succinimides, can be improved by introduction of boron into the molecule, which also gives them antioxidant properties. Polyethylenepolyamines and carboxylic, sulphonic or organophosphoric acids can be used for preparation of amidic dispersants, e.g., of the following type: R-CO-NH-(CH2-CH2-NH-).&H2-CH2-NH2, where R is an alkyl up to C35. These products are thermally stable up to about 260 "C and, because of the high polarity of the mnolecule, have very good dispersive power. The ester of isostearic acid and tetraethylenepentamine is a well-proven dispersant for use in oils for watercooled two-stroke gasoline engines. Although these products are thermally stable, the upper limit is below that required for air-cooled two-stroke engine oils, such as for chain-saw and similar duties. In these cases, the even higher stability and detergent qualities of ash-containing additives are required. As mentioned earlier, additives which contain amine groups, especially mono-succinimides, can damage fluoro-elastomers, making them brittle. Fluorocarbons are used for sealings in automobile engines because of their very good thermal stability (up to 200 "C) and resistance to oxidation. One such product, Viton A (8Dupont), is a vinylidene fluoridehexafluoropropylene copolymer: - ( - CF - CF2),- ( - CH2 - CF2- )YI CF3

and analogous to ter- and quadripolymers of vinylfluoride, tetrafluorethylene,hexafluor-propylene and the oxygencontaining, hydrogen-free copolymer: - ( - CF - CF,), - [- CF2 - CF(OCF3) -IyI CF,

Some engine manufacturers have developed their own bench tests to evaluate the compatibility of fluoro-elastomer sealing materials with lubricating oils. The conditions of these monitoring tests and admissable borderline values are shown in Table 4.11. It has become evident that the results of test conducted on fresh oil may lead to wrong conclusions, since oils which have been in use in the engine for longer periods of time have exhibited substantially better results (398). The possible effects of additives on cross-linking must not be overlooked (405).

Table 4.11. Test Conditions for Compatibility of Lubricating Oils with Fluoro-elastomerSeal Materials - Permitted Limits Engine manufacturer

Daimler Benz & MAN

ope1 Volkswagen Peugeot Citroen Caterpillar*

Test conditions Seal performance parameters Temperature Duration ElongationTensile Volume Hardness strength (GM60256) ( "C) (h) ..................(% change) .................

150

I68 150 72-1000

150 150 177

96 168 96

-45 +10 -40

-

-40 -30

-40 4 0 -10

f5 E3

f5

E3

f25

-

-

-15

-5

f5

-

-

-

*Test of effect of lubricating oil on clutch plates faced with glass fibre and fluoro-elastomer.

32 1

It has been observed that base-oils of different Vl's have different effects on fluoro- elastomer packings. Base-oil VI changes of 70 to 96 can cause up to 40% differences in the elongation of fluoroelastomers. Compounds containing active sulphur have adverse effects on nitrile-rubber packings. On the other hand, nitrogen-free succinate esters, which in some respects are excellent dispersants, show good compatibility with fluoro-elastomer packings.

4.2.3.3 Esters of Alkenylsuccinic Acids Succinate esters, containing neither nitrogen nor basicity as TBN, are used commercially as ashless dispersants, and have the following general fomulse:

R - CH = CH - CH - CO - C2H4OH I CH2COOH R - CH = CH - CH - CO - C2H4OH I CH2 - CO - 0 - C2H4OH R - CH = CH- CH -CO - C2H40 - C2H4 - 0 -CO - CH -CH = CH - R I I CH2COOH HO - CO - CH2 To form these compounds, polyisobutenylsuccinic anhydride is condensed with a polyol to give the succinate ester. An excess of hydroxyl over acid is generally used, so that the succinate ester contains free, unreacted hydroxyl groups.

The alcohols can contain 1-40 carbon atoms and many different subtituents, such as chlorine, bromine, phenyl, alkoxy groups and ester groups (e.g., ethylene glycol monooleate). The most usual are polyvalent alcohols containing at least three hydroxyls, of which a portion is esterified with c8 - C,, carboxylic acids (e.g., sorbitol monooleate, sorbitol distearate, glycerol monooleate or monostearate and erythritol didocanoate) as well as unsaturated alcohols, aminoalcohols, phenols, mono- to tri-alkylphenols with alkyl groups (of relative molecular weight as high as 1,000). 2-chlorophenol, resorcinol, pyrocatechol, bis-alkylphenols with methylene, sulphide and polysulphide bridges and a- and pnaphthols. R is a polyalkene or alkene copolymer containing at least 50 carbon atoms. In order to have sufficient stability and for the ester to be soluble in oil, the R group must contain at least 80% of aliphatic a-mono-alkenes and the proportion of double bonds must not exceed 5% of all the -C-C- bonds. In addition to the polybutene currently employed, copolymers containg, e.g., 95% isobutene and 5% styrene, or 98% isobutene, I% piperylene and I % chloroprene, or 80% ethylene and 20% propylene may be used. The relative molecular weight may be 700 to 5,000. Polymers of relative molecular weight 10,000 to 1 million may be used if VI improver properties are required. The polyalkyl chain may also contain hetero-atoms or other groups, such as chlorine, bromine, sulphur, keto- or nitrogroups, but the amount should not exceed a limit of about 10%.at which the unsaturated nature of the alkene would be lost.

322

The most recent compounds of this type to find wide application are the esters of polyalkylene succinic acids and pentaerythritol:

R - CH = CH - CH - CO - 0 - CH,C(CH,OH), - CH2O I CH2 - CO - 0 - CH,C(CH,OH),

- CH2O

They have good thermal and oxidation stabilities, poorer abilities to disperse products of the incomplete combustion of engine fuel but a fair ability to disperse cold sludges. They are therefore suitable as ashless dispersants for gasoline and diesel engine oils, as well as for “mixed fleet” oils, in combination with bissuccinimide an a ratio of about 60:40. They can also be used for partially and fully synthetic oils. Other esters which may be used include those which result from the reaction of alkylsuccinic acid anhydrides with epoxides or mixtures of epoxide with water (e.g., with ethylene, propylene, butylene, styrene, cyclohexene, 1,2-0ctene oxides and butadiene monoxide). The effects in gasoline and diesel oils of the main types of ashless dispersants are compared in Table 4.12. Table 4.12. Comparative Data on the Performance of the Main Types of Ashless Dispersant in Gasoline and Diesel Oils Function monosuccinimide Gasoline engines varnish sludge cold sludge Diesel engines deposits above piston rings deposits below piston rings soot dispersion Gasoline/diesel CuPb bearings fluoro-elastomer compatibility 1 = best

2 = medium

1 1 2 2 3 1

3 3

Dispersant Type bispentaerythritol/ succinimide alkenylsuccinic acid ester

2 2 3

3 3 1

1

2 2

3 1 3

2

1

2

1

3 =worst

4.2.3.4 Nitrogen-containing Copolymers In most of these products, one comonomer molecule provides oil-solubility, whilst the other contains the functional, basic nitrogen group. These products combine VI improver and dispersant properties and are further discussed in a later section (4.5). They may be represented by a general formula (133):

323

- CH- CH2- CH- CH2- CH - CH2- CH - CH2 - CH - CH2 I L I

I

L I

R

I L I B

R

I

I

L I

L I

R

R

where R is an oleophilic group, e.g.,- CnH2n+,,

B is a basic nitrogen group, e.g., -(CH2),-NR2 with n > 2

L is a group providing a bond between R or B and the polym-r ch in: -0-

0 II

0 II

c - , - c- NH -, - 0 -

The most commonly encountered commercial products are copolymers of: dodecylmethacrylate (90%)and n-diethylaminoethylmethacrylate(1 0%)(134): CH3 I

CH-CI

c=o I

OC 12H25

CH3 I CH2-CI

c=o

I CH2 I

CH2 I

N

/\

cHf n

dodecylmethacrylate (90%)and vinylpyridine or N-vinylpyrrolidone(10%):

-.CH

324

dodecylfumarate (95%) and N-dimethylaminoethylmethacry late (5%):

I

c=o I

CH - CH

c=o I

-

12’25

Jm

A separate category of polymeric products which have powerful dispersant and VI improver properties is now widely used, comprising “functionalised” olefin

copolymers (OCP) and ethylene-propylene-dienecopolymer mixtures (EPDM). Materials derived from polyolefins represent, by a large margin, the cheapest VI improvers. Since these substances contain at least one double bond per molecule, dispersant VI improvers can be produced from such polymers by thermal reaction with maleic anhydride followed by reaction with polyalkylene polyamines. The reaction with maleic anhydride can also be carried out under high-shear conditions to generate reactive molecular fragments. Several commercial dispersant VI improvers, prepared by this and other routes, are now avaliable. In terms of formulating technology, the dispersant polymers enable solutions to be found to particular problems associated with blending multi-grade oils to meet high performance specifications, particularly for gasoline engine oils. Monomeric dispersants are viscous and have a pronounced thickening effect on lubricating oils. At the high dosages required to meet modem engine-cleanliness requirements, monomeric dispersants thicken the oil so much that, when combined with the dose of polymeric W improver needed to achieve the required high temperature (100 “C) viscosity of the lubricant grade, low temperature viscosity limits may be exceeded. Polymeric dispersants, having both VI improver and dispersant properties, can be used to achieve the required viscosity limits by effecting a reduction in the dosage of monomeric ashless dispersant needed to meet the engine performance stipulations.

4.2.3.5 Miscellaneous Dispersants

Included under this category are, for example, polyethyleneglycol dibenzoate, polyvinyl polystearate and dialkylnonyl borate. Over-based sulphonates and medium-TBN Ba phosphonates can, at high concentrations, reduce the formation of sludges produced during “cold” operation of engines (Z29). Their use in this way is, however, expensive and raises problems associated with high ash content.

Unlike ash-containing metallic detergents, the development of ashless dispersants is still relatively active (330). In addition to esters of polyalkenylsuccinic acid with polyamines or polyvalent alcohols, Mannich compounds and carboxylated amides, new types of compounds have appeared, including biscarbamides (331), polyaminocycloalkanes (332),substituted pyrimidine and triazine compounds (333), 325

polyesters containing carboxypyrolidone (334, reaction products of chlorinated paraffins (336, esters of alkylsalicylic acids (e.g., with pentaerythritol) (337) and polyalkylene amides of alkylsalicylic acids (338).

4.2.4 Combining Antioxidants with Detergent-Dispersants Modem heavy-duty (HD) engine oils are generally formulated with mixed detergentdispersant additives together with antioxidants such as ZDDP and alkylphenols or aromatic amines, and frequently with added rust inhibitors, demulsifiers and antifoams. The DD components are mostly combinations of two or three types of detergents, for example, high- and low-base calcium (exceptionally magnesium) sulphonates plus overbased or medium-based calcium phenolate or calcium (less frequently magnesium) salicylate and, almost always, ashless dispersant, usually succinimide. Different packaged additives are produced for different specifications, comprising oils mainly for lubricating gasoline or diesel engines, particularly for high piston temperature operation, and also for water- or air-cooled two-stroke gasoline engines (i.e., operating over different temperature ranges). The metal used in detergents is usually calcium, less frequently magnesium, and, very rarely, barium. The value of the alkaline reserve (TBN) in particular detergent components is chosen in relation to the desired effect and also in respect of the required ash content and finished oil TBN. All components contribute alkalinity proportional to their TBN and concentration in the package. The different ash content and TBN of engine oils differ is illustrated in Table 4.13. These data show the correlation between severity of duty, alkalinity and ash content; alkalinity and ash increase with increasing severity. Overbased oils, required for lubricatinglarge stationarydiesel engines fed with heavy sulphur fuels (such as crosshead marine engines), can contain magnesium, in order to achieve lower ash contents at very high TBN, as compared with calcium.

Table 4.13. Ash Limits and TBN in Engine Oils Oil specification

Ash content (% weight max.)

All season oils for gasoline engines MIL-L-46152E Diesel Oils: MIL-L-21WE Medium-basicity oils High-basicity oils Low-speed engine oils CCMC D-5

TBN (mgKOWg)

1.o

9-10

1.5

10-12 up to 30 up to 70 about 40 about 15

3.5 8.5 4.7 up to 2.0

4.2.5 Packaged Additives The need to combine different kinds of additives in engine oils, as mentioned above, led to the development of so-called packaged additives. The components in these packages of additives differ in type and dosage, according to the nature and quality 326

of the base oil, the desired quality and performance of the finished engine oils and the operating conditions of particular engines. Polymer viscosity modifiers and pourpoint depressants may, exceptionally, be incorporated into the packages. The use of package additives simplifies and improves handling, storage and quality control of additives, makes dosage more precise and reduces losses. An interesting approach to package additives is the use of “universal”, “cascade” or “booster” packages. This is a means of rationalising production of a series of oils for different performance levels by the use of combinations of additives designed to enhance particular performance characteristics of oils.

The following section discusses the dosages and characteristics of the mostfrequently used antioxidants and detergent-dispersants in package additives.

Zinc Dialkyldithiophosphates (ZDDP) Zinc dialkyldithiophosphates are a regular component of package additives intended for oils for the lubrication of gasoline and naturally-aspirated and lightlysupercharged diesel engines. The ZDDP content of the package usually varies between 10 and 15%, so that the zinc or phosphorus contents in oil are 0.04 to 0.15% weight according to the antioxidant and anti-wear effects required for the oil. The ZDDP content also depends on the presence of alkylphenolate or alkylsalicylate in the package, because both have antioxidant properties. Because of their relatively high decomposition temperatures, ZDDP’s of longchain alkyls or, better still, alkyl-aryl or diaryldithiophosphates are preferred in package additives intended for use in heavily-loaded diesel engines. However, ZDDP with good anti-wear properties (dialkyldithiophosphates of C,-C, alkyls) must also be present in amounts equivalent to 0.3 - 0.5 weight % in the finished oil. In oils containing dispersants with NH- groups, the ZDDP dosage must be very carefully adjusted. The zinc content must not exceed the equivalent of amino- or amido- groups, since this would cause excessive formation of varnishes on the pistons if the first piston ring temperature exceeds 220 OC (297). On the other hand, free -NH2 and =NH groups, in the absence of or with low concentrations of ZDDP, form chelate bonds with copper and thus cause corrosion of copperflead bearing materials. The ratio of ZDDP to succinimide or, in general, to any dispersant which contains amino- and/or amido- groups must be so adjusted so that all the -NH, and =NH groups can form complexes with ZDDP’s; theoretically, one ZDDP molecule should correspond to one -NH2 or =NH group. Phosphorus-based ZDDP is the principal source of phosphorus in an engine oil. This is very important in connection with catalysts used for the reduction of harmful exhaust emissions. One of the main sources of catalyst poison are oils with a high ZDDP content. Over the years, a number of workers have demonstrated the adverse effect on catalyst metals of lubricant-derived phosphorus, using combustor rigs and engine dynamometers. One poisoning mechanism is the formation of an impervious glaze of zinc pyrophosphate on the catalyst surface.

3 27

Catalyst deterioration is highest on exposure to exhaust gases containing unburnt oils which contain ZDDP's while operating at catalyst temperature below about 480 "C. When both phosphorus and lead are present, plugging of catalyst pores by lead phosphate has also been mentioned as a means of further action by phosphorus. Much of the earlier work on phosphorus contamination of catalysts was done using base stocks and ZDDP alone. More recent studies on fully-formulated lubricants have indicated that the behaviour of ZDDP is influenced by the metallic additives used as detergents and that the adverse effect of the ZDDP on catalyst life is much reduced in practice. However, metallic additives themselves have a deleterious effect on catalysts as a result of the front face becoming blocked by ash. Lubricant-derived phosphorus has also been found to have adverse effects on the oxygen sensor in the system. The poisoning mechanism here appears to cause an increase in sensor response time, so that the engine is more likely to operate outside its aidfuel control window. High ash content oils or high oil consumption can also cause sensor-blocking.

Catalyst poisoning can be reduced in four main ways: - engine oil consumption should be minimised. In particular, oil access to the exhaust system should be reduced by improving the effectiveness of exhaust valve stems and guides, - to avoid zinc phosphate formation, the catalyst should operate at higher temperatures, above about 540 OC, in a warmed-up engine, perhaps by moving the catalytic converter nearer to the exhaust gas manifold, - use of catalysts which are more resistant to phosphorus, - cutting down the ZDDP in the lubricant so as to minimise phosphorus content; supplementary anti-weadantioxidant additives may be necessary. Workers in this field have found that adequate oxidation and wear control, as measured in the Sequence IIlE test can be achieved by the use of additional antioxidant supplements to compensate for the reduced phosphorus level. Caterpillar 1H-2 and 1G-2 diesel tests as well as the CRC L-38 bearing corrosion test do not appear to be affected by lower phosphorus levels. In Japan, SG/CD quality engine oils have been available for some years with phosphorus levels as low as 0.08% weight. No in-service problems have been reported with these oils. Some engine manufacturers (e.g., Volkswagen) require a minimum phosphorus content in engine oils of 0.08% weight in the interests of engine durability.

Low-temperature Antioxidants The antioxidant effect of a package can be enhanced by the presence of a lowtemperature antioxidant of the radical-scavenger type together with the ZDDP. Package additives in engine oils contain antioxidants of the hindered phenol type or aromatic amines. Their concentration in the package should correspond to about 0.30.5% weight in the finished oil.

Sulphonates Calcium sulphonates of 5 to 300 TBN (mg KOWg) are mostly used. Magnesium sulphonates of TBN up to 400 have proved useful mainly in gasoline engine oils, where they are efficient rust-inhibitors and agents for the reduction of wear in valve328

train components. However, they aggravate bore-polishing when used in diesel oils. There is virtually no difference between the detergent effects of calcium and magnesium sulphonates. Mixed fleet oils can therefore contain both calcium and magnesium salts. Low-base sulphonate of TBN up to about 25 can be incorporated particularly when anti-rust characteristics need to be enhanced; they have also proved useful in oils for high-performance diesel engines, especially in combination with high-TBN phenolates. Overbased sulphonates are more often used in gasoline engine oils, but their concentration in oil should not exceed 1.5% weight, because they can promote more severe deposit formation in the piston grooves. Phenolates (Phenol Sulphides) Calcium salts (70 to 400 TBN) are much more often used than magnesium salts. The latter are mostly found in oils for marine engines. Barium salts of 70 to 120 TBN formerly found limited application in two-stroke gasoline engines. The concentration of phenolates in package additives is about 20 to 60 % weight. The higher concentration is recommended in oils for highly thermally-loaded engines, where they sometimes perform the function of antioxidants. Higher dosage of package additives with a high proportion of phenolates can to some extent improve the performance of base oils of poor oxidation stability. Salicylates Much the same comments apply as for the phenolates. They are competitive with the more widely-used phenolates largely because of their strongly positive effects on engine cleanliness in the 1 G-2 diesel test and on friction reduction. Again, combinations of salicylates and phenolates together with sulphonates and succinimides can be used to produce very effective DD packaged additives for oils to meet all performance specifications. Phosphonates (Thiophosphonates) These must now be regarded as obsolete in packaged DD additives. They were regarded as especially suitable for gasoline engine oils, in which they contributed significantly to piston cleanliness. Packages typically contained up to 40% by weight of phosphonates, usually with basic sulphonates and succinimides. Ashless Dispersants Succinimides predominate in this category, with bis-succinimides in diesel oils and mono-succinimides in gasoline engine oils. Except for dispersant VI improvers, other ashless dispersant types are found much less often. Package additives usually contain 30-60% of succinimide concentrate, depending on the duty required (gasoline engine oils, especially “all-season oils”, containing the higher proportion). 329

Gasoline engine oils to meet API SF performance standards usually contain 4-5% by weight succinimide in the finished oil, whilst SG performance demands 7-8% succinimide or equivalent. Typical total dosages of detergent-dispersant additives in engine oils for different performance specifications are illustrated in Table 4.14. Multigrade oils usually contain 20-30% more DD additive than mono-grades in order to meet the same performance standards. Table 4.14. Concentration of Detergent-DispersantAdditives in Engine Oils for Different Specifications API classification

Other classifications

DD-additive content (% weight in finished oil)

SA SB SC SD SE SF,SG CA CB

cc

CD, CD+, CE,CF-2

Ford M,C lOlA Ford M,C IOIB, GM 6041M Ford M,C IOIC, GM 6136M, MIL-L-46152A MIL-L-46152B MIL-L-2 l04A, DEF-2 1OlC (without ashless dispersant) Supplement 1, DEF-2IOID MIL-L-2104B Caterpillar Series 3, MIL-L-45199B. MIL-L- 2104C, D &E

0.5-1.0 2.5-4.0 4.0-6.0 6.0-9.5 up to 12.0 1.5-2.2 2.2-4.5 4.5-8.0 UP to 20.0

The total amount of DD additive needed to meet the required performance level depends on the synergistic effects, if any, of the components selected, the quality of the base oil and its response to additives and the type and concentration of other additives present, among other, less important factors.

As mentioned earlier, the composition of DD additives used for two-stroke gasoline engine oils with conventional lubrication systems differs from those of DD additives in oils for four-stroke engines, and depends on the type of engine and its thermal load. Oils dosed with ash-containing dispersant-detergents (mainly highly thermally-stable low- and high-base calcium sulphonates and (formerly) mediumto high-base barium phenolates (barium ash being more readily carried out of the engine) or (currently) calcium phenolates (calcium producing less ash and, consequently, causing less spark plug fouling) are more suitable for lubricating low cubic capacity, air-cooled engines (e.g., motor-cycle, chain-saw and mower engines) which operate at a first ring temperature around 260 "C. The alkaline-reserve of these ash-containing detergents is present as metal hydroxide in preference to metal carbonate. For water-cooled engines with a lower temperature on the first piston ring, such as sports boats, ashless dispersants are preferred, which can be succinimides, high molecular weight carboxylate condensation products with polyethylenepolyamines. These oils are also suitable for lubricating two-stroke 330

automobile gasoline engines. The DD additive content of two-stoke engine oils is usually in the range 3 to 8% by weight. Package additives normally also contain antifoams (e.g., polysiloxanes) of a suitable composition and molecular weight to suppress foaming in the finished oil.

Diluent Oils Package additives are supplied and used as solutions in oil to facilitate handling. The diluent oil content of packages usually varies between 10 and 50%, depending on the composition of the package and the additive components. Preferably, the diluent oil should be moderately highly-refined and should not affect perceptibly the composition and physical properties of the base oil in which the package is mixed. Since the additive components are frequently viscous and may be at the limit of their solubility, the diluent oil must have good solvent characteristics, low viscosity, reasonably high VI and low volatility. A 100 Solvent Neutral is a common choice for this purpose, preferably of narrow boiling range and high naphthenics content. Detergent packages which contain both ZDDP and overbased detergents are very sensitive to water. Even a low water content (0.4% weight), at temperatures above about 60 O C , can cause hydrolysis of the ZDDP and reaction of the hydrolysis products with the alkaline reserve of the detergents, giving calciumcontaining compounds of limited oil-solubility. These compounds contribute to cloud-formation in the oil and can lead to deposit-formation which is difficult to remove by filtration: it causes deterioration in the antioxidant and anti-wear properties of the additive. The diluent oil specification therefore normally includes restrictions on water content; care must be exercised in handling and storing the diluent oil to avoid contamination with water. Depending on the crude oil source of the diluent oil, it may have an appreciable sulphur content (0.05 - 1.0% by weight). This sulphur may be present in a form which has significant antioxidant effect and this may be synergistic with antioxidants deliberately added in the package. Package additives made from different diluent oils may therefore exhibit different performance properties in finished oil.

4.2.6 Effects of Antioxidant and Detergent-Dispersant Additives on Oil Quality Parameters The presence of oxidation inhibitors and DD additive packages in finished oil not only affects its thermooxidation stability and detergent-dispersant behaviour, but also other properties. Colour: package additives darken oils; the effect of metallic detergents, particularly phenolates and salicylates, can be very marked. Kscosiry: package additives increase the viscosity of the finished oil. This varies with the nature of the package, as the viscosities of individual additive concentrate components also vary considerably, as do their concentrations. The viscosities of zinc dialkyldithiophosphates are about 10 mm*.s-l at 100 "C, whereas the viscosities of zinc diaryldithiophosphates can be twice these values. The viscosity of ashcontaining detergents depends on the nature of the organic moiety, on the type and concentration of the metal and on the TBN. Whereas the viscosity of calcium petroleum sulphonate of TBN up to 10 is usually about

33 1

20 mm2.s-' at 100 O C , it increases to about three times this value at TBN 20-25. Synthetic calcium sulphonates have relatively low viscosities - 300 TBN calcium sulphonates 30-50 mm*.s-' at 100 "C. The commonly-used succinimide ashless dispersants can have very high viscosities (150 - 800 mm2.s-* at 100 "C)- see also note earlier (page xxxx) on dispersantlVI improver formulation effects. Viscosities of commercial package additives are normally between 40 and 150 mm2.s" at 100 "C. The contribution by the additive package to the viscosity of the finished oil can be a significant constraint on formulation of the additive package, especially in low-viscosity multi-grade oils.

Pour-point: the pour-point of the compounded finished oil is principally affected by the pour-point of the diluent oil, rather than any effects of the additive materials themselves, unless the package contains polymeric pour-point depressant or viscosity modifier. In some packages, a significant amount of polymer may be present not only as pour-point depressant (usually at a concentration equivalent to about 0.5% by weight of the finished oil, but also in the form of dispersantlV1 improver (see page 325). Blending practice varies; in many instances, the VI improver is blended separately from the rest of the package.

Flush-point: the flash-point of the finished oil is also mainly affected (in relation to the contribution of the additive package) by that of the diluent oil; currently, it usually varies around 200 "C and hence is close to that of the base oil. Both flashand pour-points are important factors involved in the choice of diluent oil for additive manufacture (it should be noted that the additive concentrates themselves contain up to about 50% diluent oil in order to be handlable during manufacture. The diluent oil content of the finished oil is, therefore, the sum of that in the components and any further quantities used to adjust the viscosity and composition of the package additive). Ash-content: the principal source is obviously ash-containing additives. This depends on TBN and the atomic weight of the metals used, which vary in the order Zn
4.3 SYNERGISM BETWEEN ANTIOXIDANT AND DETERGENT-DISPERSANT ADDITIVES Every type of additive used in lubricating oils is represented by a series of chemical compounds of similar effects. If these compounds are present in the oil alone, mainly as functional additives, then their individual effects can be measured unequivocally, 332

and will be found to depend on the composition of the additive species, its concentration and purity and the nature of the oil, which determines the susceptibility of the oil to additive treatment. However, when packaged additives are employed, synergistic and antagonistic additive effects may be involved (135). In the case of synergism, the effects of two or more substances present simultaneously exceeds the expected sum of the effects of the individuals applied separately, or exceeds the effect of the most active substances applied at the same concentration. In the case of antagonism, the opposite effects occur; the individuals appear to “interfere” with each other’s actions. A practical consequence of this can be that a substance which is difficult or expensive to obtain can be replaced by a cheaper synergistic combination which has a similar or better effect. Synergism cannot be exactly identified and proved unless it can be expressed quantitatively. The physical or physico-chemical effects may not be sufficient. Only some types of additives, especially antioxidants, show a clear chemical effect; others, such as detergent-dispersants,show physical effects - their quantitative evaluation is difticult to prove and is inaccurate. Therefore, only a few additive types can be termed “true” synergists (or antagonists), whereas other are quasi-synergistic, presenting effects similar to synergism.

The terms “homosynergism” and “heterosynergism” are used to distinguish different types of action (136).In homosynergism each component acts according to the same mechanism; for example, in the case of radical scavenger antioxidants, one phenol may deliver a missing hydrogen to another, or in the case of peroxide decomposers, more effective decomposers may arise by reaction with the substrate. In heterosynergism, each of the substances present may act by a different mechanism, for example, one accelerates termination of radical chains by acting as radical acceptor, whilst the other retards propagation by acting as peroxide decomposer. The result, in each case, is reduction of the production and accumulation of radicals.

4.3.1 Antioxidant Synergism The efficiency of 2,6-di-tert-butyl-4-methylphenolcan be enhanced by a homosynergistic effect by admixture with simpler phenols which are unsubstituted in the ortho- position. Maximum synergistic effect of the phenol mixture (such that the phenols being complementary in the ortho- position) is achieved if one phenol has at least one ortho- tert-alkyl group while this position is vacant in the other position. A synergistic effect also exists between alkylphenols and dialkyl phosphites (269) or alkylphenols and aromatic amines, and between aromatic secondary amines (e.g., phenyl-1-naphthylamine) and aromatic diamines (e.g., diaminonaphthalenes), basic carboxylates (and fluorinated analogues), enolates and partially saponified tetraminoacetic acid esters, polyhydrobenzophenones and diarylthioureas (388). The interaction between hindered and un-hindered phenols in the presence of ROO. has been examined by e.s.r. spectroscopy (390).In a mixture of both types of phenol, the concentration of radicals

333

arising from unhindered phenols increases. Evidently, the unsaturated phenol delivers hydrogen to phenoxy radical from the hindered phenol, so that its other reaction is suppressed and the initial, efficient antioxidantis regenerated. Earlier experience of synegism in hinderdunhindered phenol mixtures (389) is c o n f i e d by this finding.

The efficiency of phenolic antioxidants can also be enhanced by the heterosynergism of ZDDP. For example, a stronger antioxidant effect (identified by laboratory oxidation tests) in highly-refined oils can be achieved with a combined dose of 2,6-ditert-butyl-4-methylphenoland ZDDP than with the alkylphenol alone. The efficacy of ZDDP can also be increased by the heterosynergistic effect of some alkylphenols, such as di-tert-butyl-bis-phenol (137). The heterosynergism between ZDDP and mine-type antioxidants is much weaker. This combination has, however, proved useful in improving the performance of oils of low aromatic content made by hydrocracking (103). Mixtures of ZDDP with alkylphenols substituted in the ortho- position, particularly bis-phenols in socalled "ashless antioxidants", can not only reduce the ash content of engine oils, but also provide an economic solution to the problem of oil-thickening caused by thermooxidation stress, for instance in gasoline engines in high performance passenger cars operating over prolonged periods of time (106). This type of performance can be simulated in laboratory engines (see Table 4.15). Table 4.15. Engine Oil Viscosity Increase in the Petter W1-ET 325 Engine Test as a Result of Different Antioxidant Treatments (140) S A E 20W-50 oil, to former Ford M,C lOlB specification Viscosity increase at 37.8 "C Antioxidant comprising:

2.55 % (weight) Zn dinonylphenyl-dithiophosphate 1.O % Zn dinonylphenyldithiophosphate+ 1.O % alkylphenol 1.O% alkylphenol Inhibitor-free oil (test terminated after 41 hours because of stuck piston ring) Crankcase sump oil temperature Coolant temperature Test duration 25g oil replenished after Total oil charge

(a)

925 208 407 2700

163 "C 177 "C 48 hours 16 & 36 hours 90Og)

The combination of ZDDP and N,N'-disalicylidenethylene diamine with conventional DD additives is also synergistic. Quite low doses (about 0.1%) of this typical metal deactivator can distinctly improve the result in engine tests on treated oils (139). Engine oils often exhibit interesting synergism of ZDDP and detergentdispersants; the results are frequently surprising and difficult to explain (Z38). 334

Forbes and Wood (142) reported the favourable effect of ZDDP on enhancing the dispersing and solubilising power of succinimides; this effect has also been observed by Russian and other researchers (84, 264, 265); it is, however, accompanied by a slight diminution in the ability of succinimide to stabilise the dispersion of solid matter in oil. The rules which govern the synergism of succinimides and antioxidants have not been satisfactorily explained. The influence of MDDP on the micellar structure of the colloidal solution of succinimides in oil, or the interaction between succinimides may be involved. The reason for the reduction in the stabilising power of succinimides in the presence of MDDP may be the growth of the charge on its micelles (electrical conductivity grows if succinimides are mixed with MDDP), which could result in the decrease in stabilising power (266). The decrease in adsorptive capacity may be caused otherwise.

Synergistic effects between added antioxidants and “natural” antioxidants (usually sulphur compounds) in the base oil may also be encountered, depending on the crude source of the lubricant. Synergy may amplify the effect of base oil sulphur content variation, requiring the adjustment of addtive treatment to allow for variations caused by the use of very low-sulphur crudes, e.g., some North Sea crudes.

4.3.2 Detergent-Dispersant Synergism The effects of antioxidants can be identified by laboratory methods (e.g., length of induction period, growth of acidity in the oil, fall in oxygen pressure) but it is considerably more difficult to establish detergent-dispersant effects and the results are always less objective. Laboratory methods available typically measure the ability of the DD additive to disperse a solid, such as soot or TiO,, or the tendency to form deposits on a hot-plate. It is doubtful if such methods have a sufficiently valid chemical basis on which to base conclusions regarding synergism. It is therefore necessary and reasonable to rely on the evaluation of the detergent-dispersant ability of additives from engine test results. In the case of engine oils, their effects can be quantified by numerical rating of piston deposits. For many years, detergents were used individually. Later, with the development of more heavily thermally-loaded engines, package DD additives or detergents with metal combinations, proved more useful. Both types of additives act to produce an increase in efficiency, and in both cases phenomena similar to synergy appear. It is, in fact, necessary to take advantage of this effect in formulating oils for heavily thermally-loaded engines (138,143). The advantages of the package additive approach to DD formulation can be demonstrated in many ways. A single basic calcium sulphonate mixed with ZDDP cannot satisfy requirements more severe than MIL-L-2 104A/Supplement 1 specification, whereas with a number of detergent types combined into a package with ZDDP and other additives, almost any desired performance level may be achieved by gradually increasing the dosage. Other examples (see Table 4.16) can be used to demonstrate how the total additive content of the oil to reach a given performance level may be reduced if different detergents and dispersants are

335

suitably combined. Metal alkylphenolates can produce a remarkable synergistic effect in combination with both sulphonates and succinimides. For example, all the present specification limits can be satisfied with overbased phenolate or salicylate plus overbased calcium sulphonate, ashless dispersant, ZDDP and other additives in minor amounts by gradually increasing the dosage and optimising the base oil composition. It is also worth mentioning the distinctly favourable effect of overbased metallic detergents on the low- and medium- temperature dispersancy properties of succinimide-containing oils, whether or not they also contain ZDDP. This effect grows with increasing TBN of the detergents. By contrast, neutral and low-base sulphonates have negative effects (144). To meet modem gasoline engine oil specifications in wide-range multi-grade oils in which ash and zinc contents is controlled to low limits, a disproportionate amount of dispersant may be required. The polymeric viscosity modifier itself may introduce problems similar to base oil effects, particularly the ethylene-propylene copolymers (OCP) which provide cost-effective control of viscosity-temperature characteristics. This has led to attempts to introduce limited VI improver characteristics into the dispersant molecule (as well as dispersancy into the Vl improver) to reduce the amount of hydrocarbon VI improver required.

Efforts continue to develop tests which provide information on the physical and physico-chemical basis of DD additive action in oil solution; laboratory test so far available are unreliable and engine tests are expensive. The objective is to provide, above all, simple, inexpensive, but reliable methods. Although basic DD additives have neutralising capabilities, their effect is mostly of a physical or physico-chemical nature. DD additives are surface-active substances which carry positive or negative charges and which have electron donor and electron-acceptor properties. These affect the interaction between additive and oil, and among additive molecules, among micelles formed from additives and solvents, among micelles themselves and between micelles and the surfaces of the structural material or insoluble contaminants (82). Interesting studies of DD additive solutions in oils have been carried out using methods which have proved useful elsewhere in studies of similar substances in a water environment. These studies involve electrical phenomena, such as determination of electrokinetic potential (potential difference between interface of solid phase with adsorbed ions and the interior of the solution); the type, size and density of particle charges; structure and composition of micelles and the thickness of diffusion double layers; mobility of micelles or solvated ions and variation in electrical

Table 4.16. Composition of Detergent-DispersantPackage Additives to Satisfy Engine Oil Specifications at 0.05% Zn Component ratios (% weight)

Alternative 1

2

ZDDP Ba thiophosphonate Ca phenolate (100 TBN) Ca sulphonate (IOOTBN) Succinimide

13 87 -

8 weight of additive package required to meet DEF 2101D

5.4

4.8

336

3

4

14

18

60 26

37

-

18 40 12

3.7

3.7

15

30

30

conductivity of oil solutions with time, voltage, concentration and combination of additive species (145157,249).The results are then compared with those from other tests: detergent capability (e.g., by COST 10734-64, where the oil solution is oxidised at 250 "C in the presence of a standard substance and the rate of oil filtration and degree of filter-fouling are evaluated), polarisation and polarisability of the additives, solubilisation of substances insoluble in oil (e.g., pigments, soot) and the results of engine tests. Although the results gained in water or polar environments are not comparable with those in non-polar oils of varied composition and dissociation of additives into ions several orders of magnitude lower, some remarkable results have been obtained (146,148-150,249). It has been established that potential changes with temperature, the temperature gradient up to 100 O C can be both positive (e.g., that of petroleum sulphonates) and negative (e.g., that of alkylphenolates and phosphonates). Differences in potential and the magnitude of micellar charge increase with temperature. The temperature thus significantly affects the surface properties of the additive (145). This effect is positive at the beginning and negative at the end, when desorption processes prevail on the surface, The conductivity of oil solutions is low but measurable; measurements are reproducible - conductance grows with increasing additive concentration. The relationship between conductance and time indicates that some particles possess surface charges and can discharge themselves rapidly on electrodes (e.g., petroleum sulphonates and succinimide micelles), whereas other particles possess internal charges (e.g., solvated ions) and discharge themselves slowly, generating non-homogeneous electric fields (e.g., alkylphenolates, phosphonates (145,249)).This demonstrates that the charged particles are attributable to the additives, which generate an electrostatic barrier layer on the surfaces of the metal or the solid particles (e.g., soot). When substances possess their own detergent effect they are characterised by small micelles with large charges; when the additives have a solubilising effect, the micelles are large with small charges (150,152,249). The conductivity of oil solutions of additives usually decreases with increasing applied voltage. However, with some additives, the decrease is small (e.g., petroleum sulphonates) or zero (succinimides), and with some discontinuous (alkylphenolates) or eventually stabilising at a boundary value (phopshonates) (249).These changes may be linked to the rate of dissociation of the additive to ions in the oil environment. The dissociation is most rapid in succinimides and petroleum sulphonates, slower in phenolates and phosphonates. The former type have a predominantly solubilising effect, whilst the latter have a barrier effect. It is possible to provide some logical basis for the use of additives in combination in terms of these different mechanisms of solubilisation and barrier-like effects, which may be synergistic or antagonistic. The phenomena have been followed by measuring the conductance of mixed additive solutions. In some cases, the conductance is additive, in others there may be evidence of slight antagonism, e.g., in the case of combination of phenolates and phosphonates. The combination of sulphonates with phenolates or phosphonates can be shown to be distinctly synergistic (145,146,249).The presence of ZDDP, in terms of conductance, is generally synergistic. The degree of interaction is, moreover, influenced by the ratio of concentration of the additives. It is, however, remarkable that dithiophosphates, phenolates and phosphonates actually reduce the otherwise excellent solubilising power of petroleum sulphonates and succinimides (249). The relationships between electrokinetic properties and the results of engine tests have also been studied. Results to date have not provided any clear conclusions. Some authors have found a correlation (146,155).some a certain amount and some none at all (154).The reasons for these discrepancies appear to be that the test conditions vary, that lubricant and colloidal systems change during the test and that the research has not been sufficiently intensive or systematic. Nevertheless, the study of electrokinetic processes has contributed to our knowledge of these systems and has the potential to enable the best use to be made of the surface properties of DD additives and, in particular, their interactions.

331

4.4 RUST, CORROSION AND FATIGUE INHIBITORS

4.4.1 Rust Inhibitors Additive-free mineral and synthetic oils and greases do not usually provide sufficient protection of the surfaces of ferrous metals from the effects of water or moisture, atmospheric oxygen and acidic contaminants which may - particularly if the machine is at rest, cause corrosion, i.e., rust-formation. Rust-inhibitors improve the protective capacity of lubricants in this specific respect and are incorporated into special rustpreventive oils. The main task of these oils is to prevent rusting of machine components and materials. However, these inhibitors are also incorporated into many types of lubricating oils, such as turbine, hydraulic, gear and instrument oils, as well as into most high-quality lubricating greases, especially those with lithium and synthetic bases, and into the so-called dual-purpose or double function oils. These dual-purpose oils, for example for automotive engines and gears (in which case they are frequently known as “factory-fill” oils) act as preventatives during stationary storage of the equipment and also as conventional lubricants when the automobile is set in motion. By this means, costly stripping of the preventive oil is avoided. Sheet metal rolling-oils also have a double function, acting as lubricants during the rolling process and as preventatives during transport and storage of the sheets. These oils need not be stripped off the sheets before further processing.

Suitable compounds for use as rust inhibitors include organic compounds, readily soluble in mineral and synthetic oils, which are amphipatic, resistant to hydrolysis and have good adhesion to metal surfaces. The mechanism of their protective action consists in adsorption of the polar group on to the steel surface, so that the hydrocarbon portion of the molecule forms a hydrophobic, water-repellent film. This protective film can be strong enough to withstand the effects of organic and inorganic acids and protect the surface from galvanic corrosion. Rust inhibitors currently used include low molecular weight alkylsuccinimides (258)and alkenylsuccinic acid esters (162)and alkylenesuccinic anhydrides (mainly for turbine and hydraulic oils). A typical product is made by the reaction of alkenylsuccinic acid with propylene oxide (400): R - CH = CH - CH - COOH I CH2COO - CH - CH2 - OH I

(R = CI,H21)

CH,

- derivatives of alkylthioacetic acids (259): R-S-CH,COOH

-

(where R is a long alkyl)

N-acyl derivatives of sarcosine: R-CO-N(CH,)-CH,-COOH

and of similar fatty mines;

- substituted imidazohnes (260) (chiefly for preventive oils): 338

/N R-C (X can be (CH, residue);

- CH, - NY),Y,

CH I 'NX - CH2 -

n = 0 to 7, Y is a mono- or di-carboxylic acid

-

amine or alkanolamine salts of dialkylphosphates (161),which also have antiwear properties;

-

non-ionic esters of fatty acids and polyhydroxy alcohols (e.g., penta-erythritol, sorbitol), used chiefly in preventive oils;

- 4-alkyl-phenoxycarboxylicacids, e.g., 4-(nony1phenoxy)-acetic acid: R

0 (CH,), - COOH

- amides or sulphamides of the type: R-CO-NH-(CH,),-COOH Ar-S02-NH-(CH,),-C00H

(Na) (R is a longer alkyl) (Na)

(Ar is an aryl group, n = 1-5)

- petroleum and synthetic alkaryl sulphonates soluble in oil, including, for example, sodium and calcium salts (both neutral and basic), lead, and zinc sulphonates and ethylenediamine sulphonates. The addition of 0.2% by weight of sodium petroleum sulphonate reduces the surface tension of water from 76 to 42 mN.m-', and 0.1% weight reduces the interfacial tension between mineral oil and water from 51 to 2.1 mN.m-'. These petroleum sulphonates are prepared from petroleum sulphonic acids of relative molecular weight 400 to 550. The oil-soluble sodium petroleum sulphonates are also used as emulsifiers, water-repelling agents, wetting agents and stabilisers for the structure of lubricating greases, and contain around 60% by weight of water, up to 0.5% of inorganic salts, 14-164 ash (as Na2S04) and 16-19%of SO,. The synthetic sulphonates can be prepared from dinonylnaphthalene sulphonic acid, e.g., its ethylenediamine salt: r

1

or, more frequently, by neutralisation of sulphonated alkylbenzenes of which the alkyl is a polypropylene of 40-520 relative molecular weight. Synthetic sulphonates are regarded as being more efficient than petroleum sulphonates. Each of the above examples has its own specific properties and applications (163). Ammonium sulphonate is a good rust inhibitor for automatic transmission fluids and flushing oils and for greases, as well as a corrosion inhibitor against HBr and HCI for gasoline engine oils. Sodium sulphonate is suitable for cutting oils, lithium greases, aluminium complex greases and greases based on

339

diester synthetic oils. In water-free media, it is also an effective inhibitor of galvanic corrosion, able to solubilise water produced. In aqueous media, it is displaced from the surface. It is inefficient in light hydrocarbons (403). Neutral calcium sulphonates are used as rust inhibitors in preventive, cutting and metal-working oils which contain chlorinated paraffins, rolling-mill and rinse oils and in lithium greases. They are characterised by high thermal stability, and can also act as inhibitors of galvanic corrosion. Mediumbase calcium sulphonates (40 - 50 TBN) are virtually all-purpose rust inhibitors suitable for all types of mineral and synthetic oils and greases, high chlorinated paraffin cutting-oils, oils and greases with a high load-carrying capacity, MoS2-containing lithium greases and other, similar preparations. They can neutralise organic and inorganic acids. Lead sulphonate can partly or completely replace lead soaps in oils with a high load-carrying film, and is thus a suitable rust inhibitor for gear oils. It also acts as a de-watering agent and is therefore a convenient additive for lubricants for Morgoil type bearings and for hydraulic oils. Zinc sulphonate is useful in hydraulic oils because of its demulsifier properties, thermal stability above 190 "C and its capacity to reduce sludge formation in oils containing ZDDP. It is also suitable for greases, particularly those based on bentonite, which are softened by other sulphonates. Ethylenediamine sulphonate is particularly useful as a rust inhibitor in lithium and calcium-based greases prepared from 12-hydroxystearic acid, aluminium complex greases and emulsion oils for metalworking processes, because of its good emulsifying properties in hard water. The concentration of these rust inhibitors in both oils and greases varies between 0.5 and 2% by weight; greases usually have the higher concentration. Sarcosine and imidazoline derivatives are of considerable importance. They are used in preventive, turbine, hydraulic (both mineral and synthetic) fluids and cutting oil emulsions, non-flammable water/glycol fluids, greases (particularly bentonite and MoS2-containing types) and even in light fuels, both alone and in mixtures with other types of rust inhibitors, in concentrations from 0.01 to 2% by weight. In engine oils, they improve the capacity to neutralise acids (e.g., HBr and HCI produced by the decomposition of alkyl halides present in lead anti-knocks). Substituted imidazolines dissolved in light hydrocarbons are used for de-watering metal surfaces to speed up drying. Lauryl- and oleyl-sarcosines and their sodium and/or ammonium salts and esters and their reaction products with alkylamines are also used. Imidazolines and sarcosines display marked synergism. The advantage of the imidazolines is their high thermal stability. They are compatible with many other types of additives. However, they are very sensitive to oxidation. They form quaternary ammonium compounds with alkyl halides, and react with acids to form salts. Their salts with low molecular weight acids, e.g., acetic, hydrochloric and phosphoric acids, are oil-soluble. Their salts with higher fatty and naphthenic acids and alkyl- and alkaryl-sulphonic acids are insoluble in water but soluble in oil.

Rust protection is also essential for some water solutions and oil emulsions in water. Water-soluble inorganic salts, such as alkali metal or ammonium borates, metaphosphates, vanadates, molybdates and tungstates have good anti-rust properties. They have even proved to be satisfactory under elasto-hydrodynamic and mixed lubrication conditions. Nitrates and fluorosilicates also provide corrosion protection. Among organic compounds, water-soluble benzoates and acetylenic compounds such as propargyl alcohol and butynediol possess outstanding anti-rust and anti-corrosion properties.

4.4.2 Corrosion Inhibitors and Metal Passivators Corrosion inhibitors are substances which provide protection to both ferrous and non-ferrous metal components - mostly bearings - against attack by acidic contaminants contained in or produced by oils and greases. They also act as metal

340

passivators, inhibiting the catalytic effect of metals in oil oxidation by forming surface layers on the metal and impeding the emission of metal ions into the oil. The mechanism of this corrosion inhibitor effect consists in chemical reaction of the corrosion inhibitor with the metal surfaces. This reaction produces a protective film which is resistant to corrosion agents (264,165).The film must firmly adhere to the metal surface - it must be adhere sufficiently firmly to prevent other surfaceactive oil components, such as detergents and dispersants and acidic substances, from destroying it. The formation of a protective film is an intricate process whose nature, speed and thickness depend on the composition of the additive and the metal type and their interaction. For example, sulphurcontaining additives are believed to promote three typical reactions with metals: formation of mercaptide or thio-acid salts, formation of metal sulphide and metal-additive complexes (166,167).The reactions of metals with polysulphides, particularly those with a high sulphur content, causes the formation of M,S, metal sulphides. The protective film is destroyed with an increasing concentration of acidic substances. However, if sufficient inhibitor is present, it may re-form. Destruction and re-establishment of the film proceed simultaneously; the destructive process does not become prevalent until the inhibitor is exhausted. This process is accelerated by temperature (168).

The majority of compounds used as corrosion inhibitors are peroxide decomposer-type antioxidants, compounds containing sulphur, phosphorus or both. The earliest corrosion inhibitors for engine oils were organic phosphites. They were mostly mixtures of mono-, di- and tri-organophosphites, made by the reaction of alcohols or hydroxy esters, e.g., methyl lactate or trimethyl citrate, with PCI, (269,170),with the general formula (RO),P(OH)3-n, R being an organic radical and n =I, 2 or 3. These phosphites were widely used in premium-types of engine oils in the late 1930’s and the early 1940’s often in combination with film-strength improvers such as methyl chlorostearate or hexachloro-diphenyl ether (271).By the mid-1 940’s, the majority of these inhibitors had been replaced by new substances which contained sulphur or phosphorus or both, which became prototypes for the products now in use. These may be classified as follows: dialkyl or diaryldithiophosphates of metals, chiefly zinc and nickel; metal diorganodithiocarbamates, chiefly zinc; sulphurised terpenes or higher (C,2) alkenes produced by heating elemental sulphur with the terpene and washing the crude product with caustic soda or alkaline sodium sulphide to remove dissolved, corrosive sulphur (273); phosphosulphurised terpenes - produced by heating terpenes with P2S, (2 74); thiazoles (I) and triazines (11):

which act as chelating agents, forming, with copper and silver, a complex insoluble in water and many organic substances. This complex produces a transparent film on the metal surface. These compounds are, therefore, used as special corrosion

34 1

inhibitors for copper and silver alloys and are suitable for turbine (including aircraft turbine) oils, hydraulic fluids, electro-insulating oils, cutting and rolling-mill oils and special lubricating greases. The concentration in oil used is very low, from 0.01 to 0.05 by weight. They act during oil oxidation as passivators of copper and its alloys in oils which come into contact with copper. They are also used as anti-stain inhibitors in gear oils, preventing changes in the colour of copper resulting from the presence of corrosive sulphur. Substituted triazines (339),the reaction products of benzotriazine and alkylsulphides (34.9, also act as oxidation and corrosion inhibitors.

4.4.3 Anti-fatigue Additives The primary cause of metal fatigue appears to be subsurface and surface cracks in the solid friction surface. These cracks are first generated by repeated cyclic stressing of the surface which leads in its first stage to strengthening of the material by work-hardening but then to the loss of elasticity and cracking. The formation of cracks is aggravated by defects in the material, irregularities on the surface, the presence of a second, foreign phase (oxides, carbides, sulphides, brittle impurities) on and under the surface, defective treatments leading to residual stress and unequal distribution of surface energy, discontinuities in contact geometry and operational factors such as elastohydrodynamic friction, tangential forces without shearing, rolling with shearing, elastic deflection of the bearing, oscillations, stick-slip effects etc. (407).As soon as they form, the cracks tend to form a branching network, and the surface disintegrates with flaking and pitting (372,408).The formation of cracks can be reduced by plating both friction surfaces with very thin protective layers (e.g., O.lpm Cd on steel). Lubricants have an undoubted effect on wear by fatigue. Oil penetrates into the cracks and enlarges them by hydrostatic pressure (408). It appears that higher viscosity oils apparently impede fatigue by forming thicker films on the surface and penetrating more slowly into the cracks. However, the chemical constitution of the oil can be more important in the fatigue-failure mechanism than its viscosity (408). Reactive oils such as esters are more problematical than the less reactive mineral oils. Oils which tend to release hydrogen by dehydrogenation are worse than oils with a lesser tendency to dehydrogenate. Acid products of oil deterioration encourage fatigue failure. The same may hold for some additives. For instance, some EP additives reduce the life expectancy of gears (420). Atmospheric oxygen does not seem to influence fatigue, but dissolved oxygen seems to be active. One of the most deleterious substances is dissolved or dispersed water, present as an impurity or produced as a product of oil deterioration (372).Another hypothesis suggests the decomposition of water to atomic oxygen and hydrogen on fresh surfaces of the cracks. The oxygen then acts corrosively and the hydrogen causes hydrogen embrittlement and corrosion (242). The relative stability of additive species, particularly ZDDP, can influence fatigue failure of metal surfaces in severe contact conditions. Pitting of cam-followers and cam-noses has been observed as an

342

oil-related problem when low-viscosity multi-grade (SAE 1 OW-30) oils were used in over-stressed valvetrains in prototype gasoline engines. The problem was more severe with relatively thermally unstable, lower alkyl-substituted ZDDP than with the more stable higher branched-chain alkyl, or diaryl, additives. However, the less stable additives gave significantly better protection against scuffing wear of the flanks of the cams. This problem was largely overcome metallurgically but the case illustrates the possibility of additive interactions with metal surfaces leading to fatigue failure in severe EHD conditions.

In summary, all corrosive substances which may penetrate into the cracks and all factors which promote corrosion aggravate fatigue phenomena and all factors which suppress corrosion can be beneficial, e.g., non-corrosive antioxidants and inhibitors, non-corrosive thickening agents (e.g., polyolefins), and non-corrosive solid lubricants which smooth the friction surface and products with similar properties. Special attention must be paid to water. The life-expectancy, L, of a material depends on the concentration of water ( c ) in p.p.m. according to the empirical relationship: (4.43) If, for example, the durability is 1 at 100 p.p.m., then at 25 p.p.m. it is 2.6 and at 400 p.p.m. it is 0.52 (422). The principal function of anti-fatigue additives is to suppress the effect of water. They include: compounds which neutralise protons, e.g., isopropylaminoethanol (423,373); compounds which produce a hydrophobic film on the surface, e.g., higher fatty alcohols (such as n-octadecanol or the salts of octadecanoic acid and organic amines); compounds which bind free water which is present; for example, organic amine salts of carboxylic acids such as succinates prevent water adsorption on the metal surface and its diffusion into the extremities of the cracks (374); compounds which produce compact surface films, e.g., alkyl-phosphonates with organic cations (375). Particular combinations of the above types can be devised so as to produce synergism.

4.5 MODIFIERS OF VISCOSITY AND VISCOSITYTEMPERATURE CHARACTERISTICS Additives of this type affect the rheological properties of the oil, increasing its viscosity at higher temperatures without degrading its other properties, mainly lowtemperature flow and pumpability and thermal and chemical stability, and without interfering with the effects of other additives. They are mainly used in engine, hydraulic and gear oils, but are also applied in other lubricants. They are mostly linear, non-crystalline or atactic polymers and copolymers, with a variety of chemical compositions, in the mean relative molecular weight range from 5,000 to 2 million. 343

Unlike pure substances of lower molecular weight, polymers contain macromolecules of varying relative molecular weight. They are, therefore, characterised by mean molecular weights, of which the most significgt is number average molecular weight, M,, and mass average molecular weight, M,. Relevant equations are:

where Niis the number of molecules in the system and M iis their relative molecular weight. For example, if a system contains 5 molecules each of 20,000,40,000,60,000,80,000 and 1OO.OOO molecular weight (g.mol-I) respectively, then:

The k, value can be determined osmometrically, cryoscopically or ebullioscopicdly and the Gw value from light diffusion or the rate of sedimentation in polymer solutions (302). The precursors of the synthetic polymers were animal oils, blown vegetable oils, condensed oils and voltolised oils. The viscosity of these oils can be substantially increased by blowing with air at 70-120 "C. The viscosity of mineral oils can be increased by compounding them with blown animal and vegetable oils, such as rape-seed, cotton, castor and sperm oils, which also improve their lubricity, but at the cost of oxidation stability. A considerable increase in viscosity can be achieved by voltolisation.

The oldest viscosity and viscosity index improvers are the polybutenes, above all the polyisobutenes (I), which are commercial products (known as,e.g., Paratone, Exanol, Oppanol and Hyvis) (I75) and polyalkylstyrenes (11) (e.g., Santodex), used as early as before World War 11.

where R is an alkyl c&, radical. These additives are still in use. Atactic polypropylene with iw around 30,000, fully separated from isotactic constituents and stereoblocks can also be used. It has similar properties to polyisobutene, but lower thickening power at identical Mw.

Additives of the above type have been improved on considerably in respect of some of their properties by other polymer types, including: 344

-73ry ;# 1

- copolymers of methacrylate esters (polymethacrylates or PMA) (111) (Z76), marketed as Acryloid, Garbacryl, Plexol and Viscoplex:

{CH2

COORI

CH2 - COOR2

(111)

with i w o f about 6.0.104to 1.0.106; - similar polyacrylates (PA) (IV) (177,Z78)(e.g.,Glissoviscal)

{CH2

-

CH2 - COOR2

(IV)

COORl

m n where n = 100 - 3,000, R, and R, are C, - C,, aliphatic radicals (identical or different), average C, to CI2; - ethylene-propylene copolymers (EPC or OCP) (V)

-

styrene-ethylene-propylenecopolymers (Va)

of different mole ratios of ethylene to propylene, ranging from 1.5 to 0.43:1 (311) and with Gwaround 8.10. The standard ethylene content in these copolymers is 50 to 70%; if it reaches 808, the copolymer becomes partially crystalline and the stability of its solutions in oil is affected.

345

- copolymers of ethylene, propylene and a diene (EPDM) (VI)

where x,y and z can be 1 - 100.

-

The content of non-conjugated dienes is 5 10%. The dienes most frequently used are trans-1,C hexadiene, 5-ethylidene-2-norborneneand dicyclopentadiene.

-

hydrogenated block co-polymers of styrene and butadiene or isoprene (VII) with M,around 1. lo5, containing about 50-70% of styrene

-ri

-

The high polymer block of the hydrogenated polydiene olcfin chain is terminated by two lower polymer polystyrene blocks. This structure allows a super-molecular arrangement, which decisively affects the properties of block polymers.

- copolymers of alkyl fumarates and vinyl acetates (VIII) (279)

and mixtures between these and other polymers. The order of monomer units in the polymers can be of the block type, as shown, or random. Each of these products is a mixture of polymers of different molecular size. The distribution of the relative molecular weights differs in different types of polymeric viscosity modifiers, as shown in fig.4.5.

Sn

The difference between and k, characterises the extent of the distribution or the heterogeneity of the relative molecular weights, which can be defined by the equation: (4.44)

346

0

3

5 7 9 BISSUCCINIMIDE,%wt

Fig. 4.5. Correlation between bissuccinimite content (% wt) in oil and merit ratings at Petter AVB engine test The patent literature indicates a series of other compounds suitable as viscosity and viscosity index modifiers for lubricating oils, for example (298): - ethylene copolymers with C, to C,, a-olefins containing 60 to 80 mole % ethylene (313), - ethylene, propylene and 1,4-Fexadiene terpolymers containing 25 to 5% ethylene (314, - cis-l,4-polybutadiene with M, 7.5.104to 2.106 (3Z5), - among adamantyl polymers, poly-3,5-dimethyl-l-adamantylacrylate with G, 80,000to 285,000; this should be a more effective VI improver than conventional polymethacrylates (180).

Recent developments in VI improvers have taken place in the following directions: - developments of polyolefin-types. These include, for example, hydrogenated polybutadienes of G, about 20,000: r

cH2-i;21 1

(CH,

- CH, - CH2 - CH,), - (CH, - CH = CH - CH,),

-

Their properties are similar to those of polybutene (high thermal and mechanical stabilities and good thickening power, especially at lower temperatures); - copolymers of olefins with alkyl methacrylates. The aim here is to combine the advantages and neutralise the deficiencies of the respective homopolymers. Such products include alkyl methacrylate/ethylene-propylene copolymers (341). styrene/alkyl methacrylate block copolymers (342)containing 550% of styrene -as 95% - of lauryl methacrylate, with a M , of 10,OOO to 50,000 and and as much a ratio of M , / M , of less than 2, and similar products; - new polymeric components acting as both dispersant and VI improvers, including, for example, tetramer mixtures of three alkyl methacrylates and an N,N'-dialkylaminoalkylmethacrylamide(343), epoxidised terpolymers of ethylene, C3-C8 a-olefins and unconjugated dienes, oxidised copolymers of acrylonitrile, reaction products of polyolefins (e.g., atactic polypropylene) and polyethylene polyamines, terpolymers of ethylene, propylene and Nvinylimidazolines or cyclic imides, etc.;

347

-

polymers with multi-functional groups, such as sulphonic-copolymers of allyl derivatives, e.g., allyl alcohol, allyl acetate, allyl esters of c12+8 carboxylic acids and I-alkenes, e.g., 1-hexadecene. These compounds can act as viscosity and VI modifiers, pour-point depressants, dispersants, EP additives and rust and oxidation inhibitors. Other compounds have also become available, which combine the properties of viscosity and VI modifiers and synthetic oils, such as: - adducts of amines (e.g., methylbenzylamine) with phosphoric acid esters of the

OH I

R-O-P-OR II 0 (R is hydrogen or an alkyl up to C,,, R is hydrogen or an alkyl up to C7),operating as viscosity and VI modifiers, anti-wear and anti-corrosion inhibitors; - polyoxyalkyleneglycol diethers (345) of the following general formula:

CH3 [RO-(CH2dH

72%

CH,CH -O++

CH3 CH, (!XI -&H-o&CH,

(R is C, to Ctl alkyl and a+b+c = 5 to 100). Mixed ViscositylVImodifiers now in use include, for example, polymethacrylates with polyolefins, styrene-diene copolymers with poly -isobutenes (or with ethylenepropylene copolymers)and others. The aim is to achieve more complex end-results, including easier handling and economy. The advantages of ester polymers, particularly polymethacrylates, include a strongly beneficial effect on the viscosity-temperature curve, adaptability to use in a variety of oils and compatibility with DD additives; their main demerit (besides a somewhat high cost) is their tendency to form deposits in thermally loaded diesel engines. The advantage of the ethylene-propylene and hydrogenated styrenediene polymers is their high thickening effect at low doses and hence their cost-effectiveness as VI improvers. They have good mechanical shear stability and form less carbon deposit at elevated temperatures; they are, therefore, especially recommended for use in multigrade oils for supercharged diesel engines. Their disadvantages include a lesser effect than PMA on V1, a higher viscosity increase, mainly at low temperatures and low shear rates and, in particular, their inability to reduce pour-point.

All these polymers are normally handled as concentrates in raffinate oil solutions of 2 - 30 mm2.s-l average viscosity at 50 "C. The polymer concentration in commercial products depends on the solubility of the polymer in oil. Commercial polymethacrylates contain 30-808 active polymer. alkene copolymersup to about 18% and styrene-butadiene copolymers only 5 to 7%. The viscosity of the diluent oil is also important. Styrene copolymers are hence mostly supplied in the solid state for direct dilution in the oil they are intended to modify at elevated temperatures (about 130 - 150 "C).

348

These polymers increase both oil viscosity and viscosity index; some also act as pour-point depressants, antioxidants and, in some cases, anti-wear agents (326).

Thickening Effects of Polymers When a polymer is added to an oil solvent, its macromolecules assume the shape of random clusters in varying degrees of development depending on the nature of the oil and the temperature of the solution and on the structure and rigidity of the polymer chain. The less rigid polymers (according to Kuhn, those having shorter chain segments), being more flexible, tend to form thicker clusters. Polymer rigidity increases with increasing structural branching from polyolefinsthrough polystyrenes to polyalkylmethacrylates,as illustrated in Table 4.17. Fig.4.6 shows the cluster structures of the most important examples of polymeric viscosity modifiers. PMA and ethylene-propylene copolymer (OCP) clusters have roughly the same size, however OCP clusters are formed by longer and more flexible chains, whereas PMA clusters are formed by thicker and more rigid chains. The central polyolefin block in hydrogenated polystyrene-diene copolymers (SDC) is more or less the same as that of OCP. However, unlike OCP, SCD does not exist in oil solution as an isolated individual form but in an associated form around the insoluble terminal blocks, so that a much more developed structure is formed.

The viscosity increase at a given temperature and shear stress of an oil due to the presence of a polymer can be expresses by the specific viscosity, qSFF= e, or by the reduced viscosity, Vred = e.c-l,or, most accurately, by the intrinsic viscosity [ q ] . This is a fundamental parameter characteristic of the polymer/solvent pair at a given temperature and shear rate ( D ) in macromolecular chemistry. The thickening power of polymers is connected with the nature of the oiVpolymer interaction and depends on the polymer type and the composition of the base stock. Table 4.17. Lengths of Chain Segments of Selected Polymers, According to Kuhn Polymer Structure Length (A) Polyethylene

-CH,-CH,-

14.5

Polypropylene

-CHZ-CHI CH3

15.2

Poly-(n-pentene-1)

-CHZ-CH-

20.4

I

C3H7 Polystyrene

Polyalkyl methacrylate

- CH,

- CH -

21.6

7H3

-30

349

t

a

b

W(M'

C

Fig. 4.6. The distribution of the relative molecular weights of different types of polymeric viscosity modifiers a - PMA, b - OCP, c - HSBCP

In the oiVpolymer system, two main interactions operate (181);polymer with polymer, Ipp,leading to the formation of the structure in the oil, and polymer with oil, IP. leading to the solution of the polymer in the oil. If no polymer/oil interaction occurs, i.e., if the oil is a poor polymer solvent, the polymer chain tends to form intermolecular aggregates, coiled clusters, as the most likely conformation of the macromolecules, and avoid contact with the oil. In polymer/oil interaction when the oil is a good polymer solvent, the chains interact with the oil and straighten due to solvation, so that the clusters expand, their density decreases and the viscosity of the system increases, according to the following equation: (4.45) this is the so-called which can be derived from the Einstein expression for relative viscosity q/qo.pes,rq!; equivalent density of the cluster under limiting conditions, i.e., the density of the equivalent spheres which would - with respect to solution viscosity increase - be present, as well as the shape of the dissolved macromolecule, of which the shape cannot be determined. The oiVpolymer interaction depends on the nature of the oil and the structure of the polymer and can be expressed as the hydrodynamicJolume [q]. M,where [q] is the intrinsic viscosity and M the average of the relative molecular weight M,. For example, the different dissolving powers of oils of various compositions for PMA, expressed as hydrodynamic volume, are illustrated in Table 4.18 (347).

Table 4.18. Solution Properties of PMA in Different Oils Oil type Cycloalkanic oil Highly aromatic oil Synthetic (ester) oil Cycloalkanic + 6% (weight) synthetic oil

[q].M.

(cm3.mol-') 4.3 5.8 7.0

6.1

Fundamental differences exist between polymeric viscosity modifiers, in respect of the relationships between polymer thickening power (related to its cluster bulk) and temperature. Fig.4.7 illustrates the

350

differences in the cluster bulks of different oils (expressed as the hydrodynamic bulk in relation to temperature within the range 10-160 OC (347). Whereas the bulk of PMA increases within this range, those of olefin (OCP) and styrene-isoprene copolymers (SIC) decreases. The shapes of the curves of reduced viscosities of different viscosity modifiers, as shown infig. 4.8, is in agreement with this (348). The thickening power of PMA mostly increases within this temperature range, whereas that of OCP, SIC and PIB sytematically decrease.

C

Fig. 4.7. The cluster structures of different types of polymeric viscosity modifiers u - PMA, b - OCP, c - HSBCP

12 10 8 6

4

Fig. 4.8. The differences in the dimensions of the hydrodynamic bulks of different types of polymers in relation with different temperatures a OCP, b - HSBCP, c - PMA

-

These differences have significant practical consequences on the viscosity at low and high temperatures, for example, of multigrade engine oils made from different viscosity modifiers, as shown infig. 4.9.

The operating temperature range involved can be identified from the ratio of by the index Q specific, reduced or intrinsic viscosities at two temperatures defined by the relationship: (4.46)

351

qred

3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6

1.4 1.2 1.0 0.8 0.6 0 .4 0.4

0.3 0.2 0.1

0

- 0.4

I :

I

I

1

-30

-10

0

20

40

60

I

80 100 TEMPERATURE ( O C )

Fig. 4.9. Characters of curves of reduced viscosities of oils of different origin with different viscosity modifiers 1 - PMA, 2 - HSICP, 3 - HSBCP, 4 - OCP,5 - PIB

_____

in cycloalkanicraffinate (3.8 m * . ~ - ~ / l O "C) o in paraffinicraffinate (6.5 m 2 . s ~ 1 / 1 0"C) 0

At e>l, the solution process is endothermic and the solvent power within the temperature range considered is poor, but it improves with increasing temperature. The polymer has low solvent power, but increases the VI; ester polymers such as PMA provide an example. At Q
352

Fig. 4.10. Comparison of the thickening power of OCP and PMA at a) 98.9 "C and b) -17.8 "C (CCS)

Fig. 4.1 I . Comparison of the thickening power of HSBCP and PMA at a) 98.9 "C and b) -17.8 "C (CCS)

Fig. 4.12. Comparison of the thickening power of PIB and PMA at a) 98.9 "C and b) -17.8 "C (CCS) The compatibility of polymer with oil can be tested by centrifuging the solution for 1 hour at 0 "C at 25,000 G. The solution is then stored for one month at -20 "C. The following observations can result: - the solution remains homogeneous, - the solution converts to a gel at -20 "C, - the polymer separates from the solution at -20 "C, - the polymer separates from the solution at 0 OC or even at normal temperature. Incompatibility can be observed when the oil and the polymer we. very dissimilar (250).

353

Table 4.19. Effects of Different Polymers and Brightstock in the Same Base Oil (133) Viscosity

Viscosity increase Specific viscosity Q VI at ("C) 37.8 98.9 37.8 98.9 37.8 98.9 37.8l98.9 35.15 5.41 96 45.9 7.96 10.75 2.55 0.305 0.379 1.239 130 50.05 7.47 14.9 2.06 0.424 0.381 0.900 119 56.99 7.47 21.84 2.06 0.622 0.381 0.613 101

Oil type

(mm2.s-') at ( "C)

Base oil +1.13% PMA 4.84% PIB +lo% Brightstock

Table 4.20. Thickening Power of Various Polymers over the Temperature Range -17.8 to 98.9 "C Polymer type*

Thickening effect of 10%weight of polymer in SAE 10 Engine Oil qmd at 98.9 "C

PMA-1 PMA-2 OCP PIB SBC

18.3 15.5

8.9 9.0 8.6

q,d at -17.8 "C 2.41 5.2 4.8 5.2 9.65

Q98.91-17.8 7.6 2.38 1.85 1.73 0.89

.

PMA = polymethacrylate OCP = ethylene-propylene copolymer PIB = polyisobutene SBC = styrene-butadiene copolymer

The Influence of Base Oil Base oil affects solvent power and compatibility in the oiVpolymer system. The Q values in the same oil differ according to the polymer type and vice versa. Polymer type being the same, Q can be affected by the chemical composition (alkanicity, aromaticity) or viscosity of the oil; a more distinct effect is normally achieved in oils of higher alkanicity or lower viscosity. Highly polar oils from cycloalkanic or aromatic types are more susceptible to polymethacrylate modifiers, whereas alkanic oils of lower polarity (particularly hydrocracked oils or PAO) are receptive to polyolefin types. This suggests that the thickening power of PMA is higher in the cyclic oils and that of polyolefins in alkanic oils (401). Currently available commercial oils are fairly compatible with most commercially available polymers. Exceptions are hydrocarbon oils of very high VI (above 130) and low aromatic content, in which the highly polar polymers may separate, e.g., PMA with average side-chain carbon number below 10 and C4 alkyls and shorter below 30% (276).Compatibility may also be expected to be low in the systems comprising hydrocarbon-free (e.g., ester) oils and non-polar polymers (e.g., some polyalkenes).

354

Influence of Polymer Composition The thickening power of polymethacrylates, polyacrylates and polyfumarates is influenced by the length and configuration of the R substituent; it increases with the length of the alkyl chains and is greater with branched than with straight-chain alkyls of the same carbon number. Ester polymers with short alkyls have a greater influence on VZ increase, since their Q is higher and their solubility in oil at lower temperatures is lower (Table 4.21). To ensure solubility in and compatibility with oils, the average size of alkyls in ester polymers should not be shorter than C4. This is particularly applicable to hydrogenates. Thus, those ester polymers containing isoalkyls which are more soluble in oil are better VI improvers for hydrogenates. The differing effect of alkyls of different chain-lengths and configurations is used in the selection of alcohols for ester polymers of the required thickening power and/or effect on oil VZ. Hydrocarbon polymers exist as crystalline and amorphous types. Only the amorphous linear-chain polymers which are soluble in both mineral and synthetic oils under conventional operational conditions are used as viscosity and VZ modifiers. They all have Q
PMA PMA PMA PIB OCP

Mw.10-3Dispersion Alkyls Index C, C,,, M,JMn (%) 4.5 52 40 8.7 500 22 4.5 34 8.8 500 100 4.8 0 13.8 530 3 125 2.7 94

c,

Q

qred(110cm3.g-')at ( "C) 0

37.8

0.238 0.219 0.300 0.622 1.12

0.380 0.310 0.360 0.548 1.01

98.9

0.470 0.410 0.390 0.500 0.85

150

0.430 1.97 0.450 1.87 0.410 1.30 0.455 0.80 0.74 0.76

*PMA = polymethacrylate EPC = ethylene-propylene copolymer PIB = polyisobutene SBC = styrene-butadiene copolymer

Influence of Relative Molecular Weight Relative molecular weight has a decisive effect on the thickening power of polymers. This can be generally expressed by the Mark-Houwink equation: 355

[ql =K[,].Ma

(4.47)

where Kiql and a are constants depending on the solvent, polymer and temperature. As a rule, a is greater than 0.5 (it is generally 0.65 to 0.75). Attempts have been made to establish experimentallythe relationship between K and a and properties such as polymer type, rigidity of the chain, size and disposition of blocks in block polymers, affinity between solvent and monomer, etc. (377,378).

At the same concentration of polymer in oil, the effect on VZ increases with increasing relative molecular weight. For example, 1% of polymer of higher relative molecular weight (30,000 - 50,000) or 5-10% of the same polymer of lower relative molecular weight (5,000 - l0,OOO) produce the same increase in VZ. Reduced viscosity and the index Q increase with increasing relative molecular weight at any temperature between 0 and 150 “C. The difference in relative molecular weight can, to some extent, explain the lower effect of brightstock (of 600 to 6,000 relative molecular weight) on oil VI increase compare with the synthetic polymers, the relative molecular weight of which are usually several orders of molecular weight higher.

Effects of Polymer Concentration in the Oil Viscosity increases with increasing polymer concentration. The effect of concentration c on reduced viscosity can be expressed by the empirical Huggins equation: (4.48)

where kH is a constant. Viscosity increase is proportional to concentration at lower polymer concentrations and to as much as the fifth power of the polymer concentration at higher concentrations. This change in the relationship can be explained by “entangling” of the polymer chains at higher concentrations. Viscosity index also increases with concentration in systems with Q>l, but the progression of oil thickening increment with increasing polymer concentration differs from that of viscosity index increment. Whilst thickening increases constantly with increasing amount of polymer, the maximum VZ increase is achieved at an optimum concentration. Below this point, VZ increases with the amount of polymer incorporated but the VZ increment per unit polymer decreases; above it, VZ changes little or may even decrease. This phenomenon must be taken into account when multigrade engine oils of different viscosity classifications are being formulated. The decrease of the effect of VZ increase at increasing polymer concentration has, moreover, economic consequences which must be considered, since polymers are relatively costly and the achievement of high VI can be expensive. It is important to select an oil with the maximum response to polymer treatment and never to exceed the optimum concentration for the particular polymer used.

356

Effects of Temperature The thickening power of a polymer is strongly affected by the Flory temperature 0 when 0 = 1 (299). Reduced viscosity decreases above and increases below this temperature. Changes are greatest close to the temperature 0. Every polymer type has a different 0; for alkene polymers, it is below 20 "C, for styrene-diene copolymers slightly higher and for polymethacrylates the highest, above 129 "C. Changes in intrinsic viscosity [ q ]with temperature are highest in oils of lower solvent power for polymers, and with polymers with short side-chains (e.g., polymethacrylates). In oils of good solvent power for polymers e.g., polyalkenes, the dependence of [ q] on temperature is slightly lower. Within the temperature range 20 - 120 OC, the thickening power of polyolefins is higher than that of polymethacrylates, although it decreases with increasing temperature (reduced viscosity decreases). The thickening power of polyolefins within this temperature range decreases in the order OCP > SIC > SBC > PIB. Thickening power is distinctly higher in the lower temperature ranges (-10 to 20 "C) and decreases in the order OCP > PIB > SBC > SIC (401). An increase in intrinsic viscosity with temperature can be observed in some systems where Q > 1; after the maximum has been exceeded, intrinsic viscosity decreases. This phenomenon has not yet been satisfactorily explained. Some authors suggest the existence of a second temperature in the higher temperature region (the Prigozhin temperature), or the existence of interactions between the main and side-chains of the polymer which are affected by temperature. Decreased thickening power at higher temperature can have adverse effects on, for example, the piston-ring region in internal combustion engines.

Effects of Pressure The pressure-viscosity relationship in oils thickened with polymers was discussed in Chapter 2.1.2.2. Thickening power increases with increasing pressure. However, the relationship between the viscosity coefficient a and the thickening effect of polymers has not yet been sufficiently investigated (379, 380).

Viscosity Changes with Shear Stress Oils containing polymeric additives lose their initial Newtonian nature and become pseudoplastic with, in the ideal case, viscosity reversible with changing shear stress and shear rate, D.Polymers of different types show different behaviour, in this respect, in oil, which is illustrated infig.4.13, which shows the changes in apparent viscosities of oils thickened with various polymers (274). The viscosities of these oils at different temperatures are listed in Table 4.22.

357

Table 4.22. Viscosities at Different Temperatures of Multigrade Engine Oils (Oil Viscosity around 17 mrn2.s-') Containing Different Polvmeric VZ Modifiers Polymer *

PMA I PMA I1 OCP SBC PIB I PIB I1

Kinematic viscosity (mm2.s-') at ("c)

180

150

98.9

5.86 5.23 4.46 4.35 4.38 4.7

8.1 7.79 6.96 6.55 6.6 6.99

17.06 16.97 16.88 16.84 17.26 17.18

Dynamic viscosity (mPa.s) at ("C) 37.8

VI

-17.8 (CCS)

84.7 101.4 130.0 157.7 163.8 135.1

1.650 2.870 1.875 2.250 12.700 6.180

231 193 151 124 123 149

Pour-point

("(3 -38 -34 -14 -14 -14 -14

* PMA = polymethacrylate

PIB = polyisobutene SBC = styrene-butadiene copolymer

OCP = olefin copolymer

BASE STOCK

10

lo2

lo3

lo4

1 0 ~ 2

Fig. 4.13. Dependence of apparent viscosities of oils containing different polymers on the shear rate Knowledge of the dependence (decrease) of viscosity on shear rate is particularly important for polymeric VI modifiers for use in oils exposed to high shear stress, such as engine, hydraulic and gear oils, since the viscosity at high shear may fall below the critical limit for increased wear (about 5 mm2.s-') or below the break-down limit (about 2 mm2.s-').

Polymers of different chemical composition, configuration and molecular weight yield oils of different sensitivity to shear stress, and they therefore exhibit different temporary viscosity loss and different degrees of thixotropy on being returned to the rest state. Temporary viscosity loss may be expressed as a temporary shear stability index (TSSI): TSSI =

d' .1 "d.1

358

- 'd,h - 'd.0

. loo

(4.49)

is the dynamic viscosity of the fully-formulated oil measured at low shear-rate (mPa.s), vd,h the same measured at a high shear-rate, vd,o the dynamic viscosity of the oil formulated without the VI improver, all in the same units.

vd,l

The temporary decrease in the viscosity of an oil which contains polymeric additives under increasing shear stress is the result of two factors acting simultaneously: the orientation of the polymer molecules and heating up of the oil by internal friction. Molecular orientation, which is influenced by the chemical nature of the polymers, is the dominant factor at low shear rates, while at high shear rates it is accompanied by temperature rise in the oil which becomes the dominant influence. Tests with straight mineral oil have shown that the oil temperature increase is minimal at low shear rates - about 2 "C at 1 .2.104s-', but increases with increasing shear rate, reaching about 17 O C at 6.6. 105.s-*.This relationship is probably dependent on the nature of the oil. The result of these effects is that SAE 50 engine oil, for example, behaves as an SAE 30 oil around the piston rings and SAE 20 in the main engine bearings (187).

However, the magnitude of the temporary shear loss in viscosity in oils thickened with different polymers depends not only on the shear rate but also on the temperature at which the oil viscosity is measured at a given shear rate. These differences are shown in Table 4.23. The decrease in the apparent viscosity of an oil thickened with polyethylene-propylene and, especially, polystyrene-diene, at elevated temperature and at a given shear rate is substantially lower than that of an oil thickened with polymethacrylate. This is an important factor in field service, when it is necessary to sustain a sufficiently high engine oil viscosity in sites of high shear rate and high temperature, for example in the piston ring zone.

Mechanical Stability (Shear Stability) Temporary viscosity loss in oils containing polymeric VZ improvers must be distinguished from the permanent viscosity loss caused either by partial mechanical depolymerisation attributable to the effects of shear stress (the subject of this section) or by the thermooxidation of the polymer at elevated temperatures in the presence of oxygen (see fig 4.14). The former is the result of mechanical resistance to shear and the latter of resistance to oxidation and the effect of heat, i.e., the thermooxidative stability of the polymer in the oil. 'II ORIGIN OIL

' I 1 ORIGIN OIL

D 0

b

D

Fig. 4.14. Difference between a) temporary and b) permanent loss of oil viscosity containing a polymeric VI modifier

359

Table 4.23. Temporary Viscosity Loss of Oils Containing Different Polymeric VZ Improvers and Dependence on Shear Rate and Oil Temperature Polymer Type Oil viscosity (mm2.s-') at 37.8 "C 98.9 "C 130 "C @Pas) at -17.8 "C Oil temperature ( "C)

PMA

OCP

SBC

74.97 14.11 8.67

106.77 15.44 8.86

1 10.06 16.14 8.80

1.650

Shear rate

1.950

1.,900

Temporary viscosity loss (%)

(~-').105

70

2.99 1.01 0.4

38.1 29.8 24.7

34.1 26.9 18.4

37.8 25.9 19.3

100

3.44 I .44 0.5 1

36.6 20.3 9.4

32.0 22.7 16.0

22.5 11.4 1.9

I30

3.61 1.24 0.22

28.5 26.2 20.4

27.8 27.8 19.9

14.8 11.4 11.4

I50

3.75 1.08 0.67

30.8 18.2 21.4

26.8 15.4 18.7

7.1 3.8 3.8

Mechanical stability is affected by lubricant stress, which is a function of shear stress and shear rate, by the time over which the stress is acting, by the frequency of changes in tension, by the resistance to flow, which is dependent on temperature and viscosity of the base oil and concentration and average molecular weight of the polymer (257) and, especially, by the type of polymer. All effects which tend to increase the viscosity of the additive-containing oil also tend to diminish mechanical stability. Under comparable conditions, polymers which contain an aromatic nucleus exhibit higher stabilities than copolymers of alkenes or aliphatic esters, such as methacrylates. Other effects are involved which may reverse this order. Molecular size is of primary importance; the larger the molecule, the lower is its mechanical stability. For example, if polymethacrylate with an average molecular 100,OOO to weight M, up to 100,000 degrades& about 5%, then that of 200,000 degrades by 10%and that of M , over 400,000 by more than 20%. At equal M,, polyalkenes (e.g.,ethylene-propylenecopolymers) are less stable than PMA (250,253). This may be explained in terms of the better solubility of polyalkenes in oil, which results in larger particles more susceptible to stress. At equal length of the main chain and equal thickening power, all polyalkenes are, however, of lower and more stable than PMA. Mechanical stability of a polymer is also _ influenced _ by the dispersion of polymer sizes, which can be expressed by the ratio M , / M,,the dispersion index. A small

kw

Mw

3 60

ratio indicates a narrow distribution of molecular sizes and a high mechanical stability. Styrene-diene copolymers have a low dispersion index, between 1 and 2. Ethylene-propylene copolymers and polymethacrylates mostly have high dispersion indices, up to 10. The larger molecules degrade first, so that the dispersion becomes narrower and the G,+,/ M , ratio smaller (318).Mechanical stability decreases with increasing length of alkyl chains in the polymers. The better solubility of the lower ester polymers is evident here also. Cross-linking reduces mechanical stability. Base oil effects also play a part in these phenomena; as a rule, mechanical stability diminishes in good solvents (Q < 1). It should be noted that those factors which lessen mechanical stability usually improve thickening power of the polymer. The choice of a suitable polymer is hence a compromise between these two properties. The mechanical stability of polymers or oil thickened with polymers can be expressed by the shear stability index (SSZ): SSI =

v1

- v2

.loo

(4.50)

"1 - v o

in terms of the viscosities - usually expressed in mm2.s-I - where: v1 - oils containing polymer at the start of the test, v2 - the same at the end of the test, vo - base oil viscosity. The lower the SSI, the more stable is the product towards mechanical shear. The mechanical stability of polymer-containing oils can be established by standard laboratory tests, of which the most commonly used is the Bosch injector test. In this, the oil is repeatedly driven through a narrow orifice under standard conditions (CEC-L-14A-78, DIN 51-382). In the magnetostriction test, the oil is exposed to high-frequency vibrations for 5 to 30 minutes (ASTM D-2603-70 (1988)). There are also other tests available, such as the Orbahn injector., the Nieman gear test rig (Z89), the power steering pump test and others. Results from the different tests are not interchangeable (e.g., polyalkenes are more stable in a sonic test than in an injector) and do not correlate with engine tests. However, they offer useful information on the mechanical stability of oils and demonstrate that polymer molecules become smaller with continuing mechanical stress until they stabilise at a certain point, e.g., within 30 cycles in the Bosch injector, or 5 minutes in the oscillator. The results of tests on engine oils containing different polymers (349) show that there is good agreement between results obtained from the Orbahn injector and the Nieman gear-box (in the FZG test). There is no correlation between the results of the Peugeot 204, and those from the Orbahn injector and Nieman gear-box tests. The results differ with every polymer. The main reason for this may be thermooxidation, which is involved in the Peugeot engine test but not in the rig tests. (The Peugeot 204 test is now obsolete and does not appear in the European specifications, because no cars are now manufactured with the integral gear-box which formed the basis of this test. However, these observations have been retained in the text to illustrate the effect of an environment in which the oil is simultaneously exposed to mechanical shear and oxidative deterioration.)

Thermal and Thermooxidation Stability Unlike mechanical stress, thermal or thermooxidation stress affects all polymer molecules and the viscosity loss has no lower limit.

36 1

It would be expected that polymethacrylates would have higher oxidation stabilities than ethylenepropylene copolymers; the molecule contains no tertiary carbon, no carbon adjacent to an aromatic nucleus and no carbon in the a-position to a double bond, as in the case of the other polymers mentioned. It is well known that these carbons bring about the lowest resistance to the effects of oxygen. However, the effects of structural factors on the oxidation stabilities of polymers in, for example, engine operation has not been satisfactorily explained entirely. Engine and other oils also contain antioxidants with different synergistic and antagonistic effects, which strongly affect the resistance to degradation of these polymers (303). Thermal stability alone can be followed thermogravimetrically (Table 4.24) at a given rate of temperature increase (e.g., 2 "Clminute) and in an inert or oxygen atmosphere. Studies of the behaviour of polymethacrylates in nitrogen have shown that the thermal resistance of polymethacrylates in nitrogen increases with molecular size, so long as their alkyls are identical. The effects of side chains are small, and polymethacrylates prepared by anionic synthesis have lower dispersion indices, higher purities and better stabilities. The presence of nitrogen in the molecule increases the thermal stability of polymethacrylates. Some polyalkenes have higher stabilities (250,254,257)(Tables 4.26, 4.27) than the polymethacrylates. The reason for these differences have not been explained: there may be differences in manufacture, in the effect of base oil, additives and by failure, in comparing the properties of polymers, to select the optimum type of polymer for the conditions of the application. It can, therefore, be desirable to verify and place in context much of the data contained in both manufacturers' and scientific papers.

The thermooxidation and mechanical stabilities of oils can be tested under laboratory or field conditions in full-scale engines (188). The results may differ considerably from those obtained in laboratory rigs, although similar relationships between polymer compositions and thermooxidation stability may be observed; the thermooxidation stability of a given type of polymer decreases with increasing molecular size, and the degree of branching, cross-linking, dispersion and length of the polymer side-chains. However, as shown in Table 4.28, the presence and type of antioxidants is also important. Suitable standard test engines for this work include CRC L-38 (10 hour test), Caterpillar 1432 and 1-H2 (see page 447) and the now obsolete Peugeot 204. The engine which was used in the Peugeot 204 (CEC L-25-A-78) test was a 1.1 litre 4-cylinder OHC gasoline engine in which the transmission, final drive gears and the engine were all lubricated with the same oil. The test was operated for SO hours at an engine speed of 4,100 r.p.m. and an oil temperature of 1 15 "C. Mechanical and thermooxidation stability were evaluated from differences in viscosity after removal of fuel contaminants before and after the test.

Polymer-containing oils are road-tested by driving a minimum distance of 800 km; this distance is regarded as sufficient to demonstrate the extent of polymer degradation. The greatest viscosity and VI losses are, however, observed after a minimum distance of 1,800 km. Road tests can, unfortunately, provide results which rank polymers in different orders, depending on the polymer type and the effects of the type and design of engine and transmission, and, of course, the operating conditions which produce the stress on the lubricant. Gel permeation chromatography tests on oil samples from "hot tests" in four-cylinder engines operating with sump temperatures in the region have enabled distribution curves to be obtained which show the break-down of the largest molecules in polymethacrylates (fig. 4.154 and a broadening of the distribution in ethylene-propylene (fig. 4.1%) and ) (186). styrene-butadiene (fig. 4 . 1 5 ~copolymers

362

Table 4.24. Thermal Stability of Polymers by ThermogravimetricAnalysis Side-chain composition (mol %) c, M, ,104 M, I M,

Polymer Type

cl

Polymethacrylate (commercial products)

Polymethacrylate (anionic preparation) Polymethacrylate Polyisobutene Ethylene-propylene copolymer

clZ

26

33

-

100

‘14

19

Decompositiontemperature (“C) for 2% 10% weight loss

‘18

‘16

16

6

10.5

55 155 286 1130

1.8 2.4 3.2 9.0

245 244 260 276

270 272 288 308

-

10

588

1.56

90 94

2.5 2.7

305 300-320 332 400

332 340-350 370 432

Table 4.25. Comparison of Typical Additive Polymers Polymer Type

v

spec. at (“C )

SSI

Q 98.91 -17.8”C at polymer concentration (%)

at O 149 PMA-1

PMR-2

Styrene-diene

w m w

Styrene-diene+PIB PIB Polyalkene

1.05 2.6

-

-17.8 0.3 0.7

-

5 6.6 2.0 3.5 1.5 1 2.5

10 5.0 2.1 4.7 1.7 0.95 2.7

15

20

4.3 4.1 2.3 2.75 6.2 >7 1.8 2.05 0.9 0.89 3.1 3.6

Thennooxidation stability (160 “C, 5 1 air), viscosity at 98.9% change (%) after

(%I 150 42

26 0 2 0 14

C

98.9

37.8 -17.8

35 29 24 12 25 8 1 0 0 0 16 18 40

0

24h

72h

18

35

-

0 0

-12

0

-3

-23 -2.5

0

-3

-6

0

-

-

TGA

Viscosity at 98.9% loss by DIN 51-382

% loss

at

O C

300

-

22 22 15 11 18

400 -

89 91 75 66 82

(%)

42.2 17.1 3.3 7.7 -

15.7

1 ,,,,_,A, ,.._..."

lo3

10'

b

' .' .. -... lo5 lo6

*

MW

C

Fig. 4.15. Envelope curves of: a - PMA, b - OCP, c - HSBCP contained in engine oil subjected to the "hot test"

Table 4.26. Viscosity Changes in Oxygen and Nitrogen Atmospheres of Oils Containing Different VZ Improvers (Z85) Polymer Type Polymer concentration Oil viscosity Viscosity loss (a) (% by weight)

polymethacrylate dispersant p l y methacrylate ethylene-propylenecopolymer styrene-butadienecopolymer un-thickened oil

2.0 2.0 2.0 2.0

-

(mm2.s-') at in nitrogen* in oxygen* 98.9 "C (24h) (48h) (72h) (72h) 10.73 3.5 3.3 3.7 2.9 11.16 8.0 9.3 9.7 7.6 25.37 20.5 36.5 44.0 67.5 16.1 21.6 25.7 34.8 12.54 6.19

* 5 1 /hournitmgen or oxygen. In standard Caterpillar diesel engine tests, styrene copolymers have proved superior to polyrnethacrylates(255 and others). In general. polyrnethacrylateshave poorer thermlioxidationstabilities than polyalkene and polystyrene copolymers in thermally highly-stressed, particularly in supercharged diesel engines, They lose their solubility in oil, forming varnishes which adversely affect engine cleanlinessalthough, dispersant polymethacrylates can be exceptions. However, good engine cleanliness results (particularly in piston ratings) cannot be achieved unless - even with hydrocarbon polymers - the composition of the dispersant-detergent additives and their concentration in oil matches the VI improver type. Tests in automotive engines with integral gear-boxes, when these were current, were very severe. The oil was subjected to much more severe conditions than in engines with separate gear-boxes and final drive units. Viscosity losses in oils containing polymethacrylates and polyalkenes can be reliably forecast from FZG tests in the Niemann gear-box (189,190).Oils are also subjected to higher stress in automobiles with rear-mounted, aircooled engines.

364

Table 4.27. Tests on SAE 10W/50 VI Improver-containing Oils in the NSU Prinz Engine (Hot Test) Comparison with DIN 51-383 (185)

-

Copolymer Type Engine Test Duration (hours) 0 5 15 25

-

styrene-butadiene average viscosity molecular loss weight* (%) 98,000 94,000 76,000 55,000

30.0

ethylene-propylene

p l y methacrylate

average viscosity molecular loss weight* (%) 90,000 85,000 60,000 50,000

average viscosity molecular loss weight* (%)

-

12.0

240,000 240,000 225,000 200,000

30.5

DIN 51-383 (30 cycles) 5.7 - 18.8 * Measured by gel permeation chromatography on the oil after the prescribed test period.

~~

-

9.5

Table 4.28. Oxidation Stability of Polymeric VI Improvers in the Presence of Antioxidants (Results from Tests at 150 "C with FdCdPb Catalysts)(276) Polymer Type

M.103

Induction period (minutes)

Oxygen Oil viscosity absorption change (mm2.s-') ( m o l 0 9 ) after (96) at 37.8 OC after 420 minutes 420 minutes

(a) in the presence of phenolic antioxidant: polymer-free oil N-free PMA

4.0% 4.4% 6.0% N-containing PMA 4% 5% Anionic PMA 4% PIB 2.4% OCP 1.5%

270 155 392

-

90 94

294 265 215 >420 >420 340 340 285

0.9

+35 (+ insoluble substances) 0.71 +37 1 +34 1.1 +52 0.20 -7 0.12 -1 0.6 +25 0.6 -6 0.8 -24

.o

(b) in the presence of ZDDP: polymer-free oil N-free PMA

4% 4.4% 6% N-containing PMA 4% 5% Anionic PMA 4% PIB 2.4% OCP 1.5%

-

155 392

-

-

-

90 94

344 463 405 355 306 32 1 392 360 360

0.24 0.13 0.13 0.25 0.62 0.46 0.2 0.26 0.2

+5 4 +1 +9 -1 +15 +8 +5.5 -15

365

Thermooxidation effects not only cause the breakdown of polymer molecules but also their oxidation, so that both oxidation and condensation processes occur. These usually cause viscosity increase as the lubricant ages. In this respect, a certain amount of degradation of the polymer may have a beneficial effect by acting to counteract the oil-thickening process. These are, however, complex circumstances, which depend on the nature of the base oil, the type and concentration of additives, the type and operating regime of the engine etc. For this reason, different results are obtained in laboratory engine and road tests, which may obscure the basic mechanisms of the processes involved. Thermooxidative instability of VZ improvers at higher concentrations can become a major cause of deterioration in engine oil performance, frequently manifest in the form of piston fouling. Such problems with multi-grade oils with high polymer concentrations must be countered by increased dosage of detergent-dispersant additives; polymers of high stability towards shear and thermooxidation are obviously preferred on both technical and economic grounds. Considerable practical effort has been devoted to the development and manufacture of multi-grade oils for both gasoline and diesel engines which show small viscosity and VI decrease throughout their working lives (so-called “stay-in-grade” oils). Mixed fleet oils for fleets involving both gasoline and diesel engines must incorporate stable VI improvers. These highly stable VI improvers are also important in the formulation of shear-stable oils for all-season hydraulic fluids, gear and other types of oils.

Low-temperature Properties An important factor in the evaluation of viscosity modifiers and VZimprovers is their effects on the changes in oil viscosity at low temperature and variable shear-rates. All polymeric modifiers increase oil viscosity at low temperatures. At low shear stress, the differences in the thickening power of particular polymers at low temperatures may be very large. However, these values tend to level off with increasing shear stress because of the different susceptibilities of different polymers to shear stress, manifest as differences in the temporary viscosity decrease. These differences are illustrated in Table 4.29. The thickening power of every polymer at low temperature depends on its molecular weight and the molecular weight distribution curve. It also depends , in copolymers, on the concentration ratios of the component monomers, and in those polymers which themselves have no pour-point depressant effect, on the efficiency of any added pour-point depressant. Table 4.30. provides a comparison of the effects on base oil viscosity of commercial polymethacrylates and ethylene-propylene copolymers with different shear stabilities, within a defined low temperature range, as measured at the shear rate of the cold cranking simulator (CCS). It shows that changes in the specific viscosities of mixtures in the same polymer type in the same temperature range are relatively small. It is interesting to note that changes among ethylene-propylene copolymer (OCP) types are negative, so that the viscosities of oils thickened with polymer at these temperatures and shear rates are lower than the base oil viscosities. 366

Table 4.29. Viscosities of Oils Thickened with Commercial Polymeric Additives of Different Shear Stabilities (246) Product Type PMA PMA PMA OCP OCP PMNOCP PMNOCP PMAPIB

Concentration

in oil (% weight)

Polymer SSI

8.0 9.9 10.1 14.0 13.5 11.0 10.8 11.2

Viscosities (mPa.s) at -28.9 "C Brookfield (low shear)* CCS (high shear)t

45 24 15 22 12 23 23 30

15,480 23,000 17,100 876,000 93,000 818,000 43,000 24,400

13,500 13,250 13,250

not measurable 12,750

not measurable 13,250 14,250

* The Brookfield viscometer measures the viscosity at shear rates up to 1.5 s-l by ASTM 2983-87. It comprises a ~~

~~~

~

~~~

~

~

~~

cylinder or disc suspended in a ruby thrust bearing; the cylinder or disc rotates, powered by a synchronous electric motor through a beryllium-copper spring, in the liquid under test. The deviation of the spring is read off on a dial. In order to measure the viscosity, the reading must be multiplied by a simple calibration constant.

t The CCS (cold cranking simulator) measures viscosity at a shear rate of about 104 to 16 s-' by ASTM D-2602-86 and DIN 5 1-377; a universal constant-voltage electric motor powers the rotor, which is close-mounted in the stator of the measuring unit. A small amount of oil is charged into the annulus between the rotor and the stator and adjusted to the specified temperature. At constant motor input, the rotor velocity is a function of the oil viscosity, which can be deduced from a calibration curve plotted from measured viscosities of calibrating oils.

Table 4.30. Effect of Polymethacrylatesand Ethylene-PropyleneCopolymers of Different SSI on Oil Viscosity at Low Temperatures by CCS (246) Oil A B

Polymer SSI % weight Viscosity (CCS) (mPa.s) at ( "C) Viscosity change at ( "C)

OCP OCP c OCP D PMA E PMA F PMA Base oils for: A-C D-F -

29 25 19 23 17 1

12.6 13.0 14.0 9.0 9.0 16.6

98.9

-17.8 -12.2

-6.7

-1

15.07 14.91 15.05 14.94 15.12 15.46

2675 2350 2075 1950 2075 2225

1560 1220 1000 1030 1090 1220

985 800 660 700 750 820

645 535 440 470 500 555

- 1060 - 860

690 570

445 385

-17.8 -12.2 -6.7 0.18 0.03 0.09 0.26 0.35 0.15

0.47 0.15 -0.06 0.20 0.27 0.42

0.34 0.16 -0.04 0.23 0.32 0.44

-1 0.45 0.20 -0.01 0.22 0.30 0.44

This phenomenon is due either to exceptional properties of the polymer, or to the effect of the oil contained in the commercial polymer concentrate. The specific viscosities of oils thickened with OCP's are usually lower than those of oils thickened with PMA's, because of the greater influence of shear rate on the apparent viscosity decrease of the OCP or the oil thickened with it. The thickening power of PMA in the low temperature region increases with increasing shear strength of the PMA whereas the reverse applies in the case of OCP, so that there also appear to be differences in this aspect of behaviour among polymer types. Differences in the thickening power of polymeric VI improvers of different types at low temperatures and at high and low shear rates are illustrated in Table 4.31. 367

Table 4.3 1. Low Temperature Viscosities at High (CCS) and Low (Brookfield) Shear Rates of Engine Oils Containing Different Polymers (183) Additive type:

Styrene-isoprene copolymer

PMA-1

PMA-2

OCP

SAE viscosity grade 1OWl50 1ow140 1OWl40 1OWl40 1OWl40 Viscosity (mm*.s-'/98.9 "c) 21.7 15.3 14.1 14.5 17.3 (mPa.s/-17.8"C)(CCS) 2550 2250 1410 1450 1730 (mPa.s)(Brookfield)at "C -1 1 3360 2340 1680 1460 2550 -18 7890 5430 3890 3840 5400 -22 171000 124000 7300 6400 46500 -26 333000 251000 127000 394000 1235000 -31 121oooO 893000 192oooO ..not measurable..

In selecting polymers for use as viscosity and VZ modifiers, other properties or potential effects of the polymers on the behaviour and properties of the oil, must also be considered. This includes effects on the demulsibility of the oil, pour-point effects (many alkene and akene copolymers fail in this respect) and compatibility with other additives.

4.5.1 Dispersant VZ Improvers Tri-functional polymethacrylates prepared by the copolymerisation of certain polar monomers - particularly N-containing compounds - form a special group, together with bi-functional olefin copolymers and olefin-diene copolymers, which also contain polar groups (introduced either by copolymerisation with a functional comonomer or by post-copolymerisationreaction of the copolymer). PMA-type VI improvers with both pour-point depressant and dispersant effects include the following types:

1

CH,

CH2

-

I

c=o

I 0 - (-CH2)2

0 - (-CH,),CH,

(Older type, X is - OH, -N

C2H5 2' H5

368

-r3

or-N

/

-X

C H 2 - C H 2\

\CH2 - CH2

0)

I

CH,

-

Other nitrogenous comonomers such as vinylpyridine are N-vinylmorpholine are also suitable. Polyolefin types of dispersant VZ improvers tend to lack pour-point depressant characteristics to the same degree as PMA's and may require the addition of additional lower molecular weight PMA's to correct this deficiency. These compounds include styrene and ester copolymers: CH -CH I I

c = oc = o n

OR

OR

c=

c

\ /

m

N I

The motive for the development of these additives, apart from a general effort to achieve multi-functionality, was to reduce the adverse effects of high concentrations of polymethacrylate VI improvers on engine cleanliness. This had led to the need for a higher dosage of DD additives in multi-grade engine oils, with economic penalties and, as mentioned earlier, formulation problems resulting from the high viscosity contribution from both DD additive and VZ improver; this made the achievement of low viscosity multi-grades difficult without recourse to expensive, close-cut base oils or volatility problems with normal base oils. The earlier versions of dispersant VZ improvers, mostly nitrogenous PMA types, had the additional capability - which was a prime reason for their introduction - of dispersing cold sludge in gasoline engine oils. They also provided some dispersancy in oils used for the lubrication of normally-aspirated diesel engines. Later developments aimed at improving the dispersant power of these additives at higher operating temperatures. The shear strength of dispersant copolymers varies. Their thickening power is generally lower than that of non-dispersant types. The ability of these copolymers to disperse cold sludges in gasoline engine oils is beneficial in the formulation of DD packages, in that they reduce the ashless

369

dispersant concentration required in order to achieve better engine cleanliness; this is shown by the values in Table 4.32, in which results of Sequence VC tests are compared. Table 4.32. Dispersant Polymethacrylates - Effects on the Reduction of Ashless Dispersant Concentration in SAE 1OW/40Engine Oils (191) Composition of oil (% weight): Base oil (mixture of selected raffinates) Detergent and antioxidant Ashless dispersant Polymethacrylate: - non-dispersant type I - dispersant type I - non-dispersant type I1 - dispersant type I1 Oil properties: viscosity (mm2.s-') at 98.9 "C viscosity (CCS)(mPa.s.) at -17.8 "C active elements concentration Ca P Zn Sequence VC engine test results (192 hours) sludge rating* varnish rating* piston-crown varnish rating* carbon groove fill (%) oil screen clogging (%)

* Merit rating on a scale of 0 - 10.

A

B

83.5 4.8 2.5

84.75 4.8 0.75

C

D

84.27 4.8 3.13

95.92 4.8 1.08

9.2 9.7 7.8 8.2 14.75 2170

14.81 2150

15.08 2240

14.4 2150

0.23 0.11 0.12

0.23 0.1 1 0.12

0.23 0.11 0.12

0.23 0.11 0.12

7.1 6.4 7.0 2.7

9.6 8.0 8.1 0

0

0

8.6 6.2 6.9 2.0 0

9.6 7.3 7.2 1.5 0

In effect, about 4% of the dispersant polymeric VI improver was able to replace about 1% of ashless dispersant. The contrast between the effects of dispersant and non-dispersant VZ improvers on piston cleanliness in supercharged diesel engines is evident from the Caterpillar 1 G test results shown in Table 4.33. These clearly indicate that the dispersant types are more effective in these engines than the non-dispersant types. Table 4.33. Results of Caterpillar 1 G Tests of SAE 20W/40Oils Containing Dispersant and Non-dispersant VZ Improvers (292) VI improver type* Concentration Sulphated in oil (%)

Ash (Wwt.)

Test Duration (hours)

Groove Fill Land Deposits (8 fill) (% area) 1 2 3 4 1 2 3

1.8 1.5 480 61 0 1.8 1.5 120 48 23 Polymers with the same shear strength, 9% viscosity loss by DIN 51382.

Dispersant Nondispersant

370

0 17

0 3

0 66

0 98

0 94

In the selection of polymethacrylate VZ improvers, particularly dispersant types, base oil properties and the presence and concentration of other viscosity modifiers must be taken into account. Dispersant polymethacrylates can produce cloudiness in hydrocracked oils at lower temperatures, which fails to disappear after the oil has been heated. These products are not always miscible with polybutenes.

4.5.2 Polymers as Anti-wear Additives Special anti-wear additives are normally incorporated into oils to reduce wear. Conventional anti-wear additives include sulphur- and phosphorus-containing compounds, but polymers are also becoming available (316). As early as 1961, it was found that frictional losses in internal combustion engines were lower with thickened oils than with non-thickened oils of the same viscosity (184,185).These lower frictional losses are accounted for not only by the temporary loss of viscosity at higher shear stress and shear rate, but also by the visco-elastic properties of oils containing polymers (258).This effect finds application in bearings and other components exposed to shock loads. A significant proportion of the stress is absorbed by elastic deformation of the lubricant film and so, as was established by Harnoy (304). The load-carrying capacity of the film is independent of viscosity. On the other hand, in the static stress condition, the viscous properties of the lubricant dominate. This is applicable, for example, to the crankshaft bearings in engines, of which the wear-rate is dependent on the oil viscosity at a given temperature and shear rate, and hence also on the mechanical stability of the polymers (305,317). The anti-wear properties of polymers are affected by the molecular size, concentration and chemical composition of the polymers and the base oil (295).At the same concentration, polymers of lower relative molecular weight are more effective; in the case of polymethacrylates, a polymer of M , 42,000 is more effective than that with 2, 75,000 - 350,000. Wear decreases with increasing polymer concentration. Less wear occurs with SAE 10W/30 multi-grade oils than with unthickened SAE 30 oils; a 10W/30 oil with 6% polymer is better than the same oil with 1.7% polymer and a lOW/50 oil is better than a 10W/30, because it contains more polymer. Polymers which form a tougher structure in oil - the measure of which is the unit chain-length segment - are more effective, according to Kuhn (251). The toughness of the chain is the result of interactions in the oil/polymer system. In mineral oils of the same conventional viscosity classification, such as 10W/50, polymethacrylates are more effective than styrene-diene or ethylene-propylene copolymers. Dispersant polymethacrylates, when used at higher concentrations,.are very effective. Diester oils further increase the effectiveness of polymethacrylates, but further study is required to produce more accurate correlations.

37 1

4.6 POUR-POINT DEPRESSANTS Pour-point depressants are added in order to overcome the effects of residual solid hydrocarbons (waxes, paraffins and ceresines) which have not been separated from the oil in the de-waxing process and so reduce the fluidity limit (true pour-point) of the oil. Pour-point depressants do not prevent the cry stallisation of residual paraffins and ceresines, but do prevent them forming interlocking networks and separating from the oil in the form of felt-like lattice. The mechanism of the pour-point depressant effect can be explained either by the adsorption of a thin film of the pour-point depressant on to the surface of the nascent crystals of paraffin and ceresine (polyalkyl naphthalene acts in this way) or by co-crystallisation with them (as in the case of polymethacrylates, polyacrylates, polyacrylamides, alkene copolymers). This prevents the formation of undesirable, extended structures of needles and platelets (293, 294). Pour-point depressants prevent paraffin crystals from forming networks and ceresine crystals from swelling. This mechanism also explains their lack of effect in cycloalkanic oils (in which the solid phase does not form), their limited effect in high pour-point oils and their marked effect in partly de-waxed oils (containing ceresine microcrystals). Pour-point depressants do not affect cloud-point to a notable degree, so that a difference of more than about 10 "C between cloud-point and pour-point indicates the presence of a pour-point depressant.

The oldest known pour-point depressants are the condensation products of chlorinated paraffins with naphthalene (Paraflow) or phenol (195, 296):

(where R is a long-chain alkyl and R' is hydrogen or a short alkyl). These pour-point depressants were used in the early 1930's. They are now used to a limited extent for decreasing the pour-point of oils, but more often to improve the filtrability of paraffins in solvent de-waxing processes. They enhance the free formation of compact crystals moving in the oil.

Tetra-alkylphenyl and bis-(tetra-alkylphenyl) phthalates are similar, older types of pour-point depressants:

/R

aco0 coo

/R

(R is a long alkyl, examples include Santopour and Xylopour). The most frequently used oil pour-point depressants are the polymethacrylates (Acryloid, Plexol, Viscoplex, Garbacryl, etc.) and polyalkylacrylates (Glissoviscal) of higher alcohols (197, 298):

(where n = 2,000 to 8,000, R is a C, to C,, alkyl, and R’ is H or -CH,). Their advantage is their multi-functional effect, in that they simultaneously act as both pour-point depressant and viscosity index improvers. A number of polymers and copolymers have similar properties, for example polyacrylamides (Z99), vinyl carboxylate and dialkyl fumarate copolymers (200 - 202), polyalkylstyrenes (203), some alkene polymers and copolymers (204) and maleic acid polyesters (3Z6). Pour-point depressants have relatively low mean molecular weights, ranging between 500 and 10,000. More-recently developed pour-point depressants for oil include: - alpha-olefin copolymers (60 - 95 mol % 1-hexene and 5 - 4 mol 8 1-octadecene) (350). - p-alkylbenzylchloride polymers (e.g.,p-dodecylbenzylchloride)or its copolymers with styrene, propylene or 1 -hexene (35Z), - ethylene and vinyl ester copolymers of saturated C, to C, carboxylic acids (353).

The reduction of the pour-point of an oil depends on the composition and properties of the oil and on the pour-point depressant type, its constitution, relative molecular weight and concentration in oil. Pour-point depressants have little or no effect on non-refined oils which contain polyaromatic hydrocarbons and resins; also these compounds have themselves some pour-point depressant effect, they act as antagonists towards synthetic pour-point depressants. In strongly aromatic oils with higher pour-points, depressants have only a slight effect, however the effect is more pronounced in lower viscosity oils. The use of pour-point depressants in very high viscosity oils is totally without effect, since when these oils are cooled, they are immobilised because of the considerable increase in viscosity which occurs and on which the pour-point depressant has no influence; the result is a “viscosity” or “false” pour-point. Generally, pour-point depressants have no effect on engine oils in the viscosity classifications above SAE 30. Different types of pour-point depressants influence pour-point in different ways. However, in all cases, there is an optimum concentration in oil. If this optimum is exceeded, the pour-point increases again. In the region of the optimum, the pourpoint reduction with increasing dosage decreases. The optimum concentration can only be established empirically. Within a polymer group, the pour-point depressant capability depends on the composition of the monomers and their concentration in oil (fig. 4.16). The distribution of molecules by size has more influence than molecular weight; to be

373

efficient, the substances should be as homogeneous as possible and no molecules should be present which are too large or too small.

--- WLYMETHACRYLATE WLYACRYLATE

- 40 I

0.1

0.2

0.3

DEPRESSANT To w t

Fig. 4.16. Effect of the C , , alkyl polyacrylate and polymetacrylate on the pour points of oil

The nature of the side-substituent is also of importance in the cases of polymethacrylates and polyacrylates; n-akyls are currently preferred. These alkyls should be long enough to ensure good solubility of the polymers in oil and their length also affects the efficiency as pour-point depressants (133).If good efficiency is to achieved, the choice of optimum length can be very important. However, the efficiency is still higher if alkyls of different lengths but whose mean carbon number corresponds to the optimum are employed. This factor is important in commercial manufacturing practice. The use of some types of pour-point depressants, particularly the older types, in insufficiently dewaxed or bright-stock-containing oils can cause a considerable increase in pour-point when such oils are stored for an extended period of time at temperatures close to their cloud-points.The same effect may occur if alternate solution and precipitation of paraffins takes place. This phenomenon is termed pourpoint reversal. The pour-point may increase in this way by 20-30 "C. This behaviour has not been completely explained. It has been suggested that heating the oil causes the desorption of the pour-point depressant and that the solid hydrocarbons, becoming exposed, redissolve in oil; when the oil cools down again, the pour-point depressant fails to re-adsorb on to the hydrocarbon crystals. The magnitude of this phenomenon varies, and it depends on the composition and concentration of the solid phase in oil, the concentration and nature of the pour-point depressant and on the pattern of the temperature changes. It may also be provoked by the presence of some detergents, which can form a gel in the oil. It is interesting to note that it has never been observed in multi-grade oils which do not contain brightstock and in which polymetacrylates are used as VI improvers and pour-point depressants. It may be concluded from this that pour-point reversal is connected with the concentration of ceresine, which are mainly present in brightstock, and with the type of pour-point depressant which is used.

The interaction of oil, paraffin, polymer and pour-point depressant leads, at low temperatures, to a gel of differing degrees of robustness. Such gels contribute to an 374

increase in viscosity and plasticity of the lubricant, and so-called gel-viscosityoccurs (306). The gel structure breaks down and the gel viscosity decreases with increasing shear rate D; the structure partially regenerates as D decreases, as a result of pseudoplasticity and thixotropy. Gel viscosity can be calculated from the area delineated by the hysteresis curve representing the change of viscosity with change and extent of the shear stress variation within which the viscosity has been followed. Its share in low temperature viscosity with a given base oil and paraffin composition is strongly affected by the composition of the polymer and the pour-point depressant. Antioxidant and detergent-dispersant additives have a lesser influence (307). Some modem hydrocarbon copolymer V l improvers, particularly ethylene-propylene copolymers, can exhibit problems of gel formation at low temperatures which have in some situations led to engine failures. These have been observed when automobiles have been allowed to stand in severe winter weather conditions and, especially, when the oil in the crankcase has been subjected to repeated temperature cycle below and above about -15 "C. Gelling of the lubricant has occurred to a sufficient extent to seriously impede circulation of the oil by the oil pump through the filter screen and the relatively narrow oil-ways. The use of supplementary electrical power from an auxilliary system (when the automobile's own battery is unable, because of the low temperature) to turn the engine causes the engine to turn virtually unlubricated and damage to the moving components has resulted. In response to this problem, the engine builders have insisted on the inclusion of pumpability tests in the June, 1989 up-date of SAE 5300.

4.7 ANTI-FOAMS Anti-foam additives became important with the extensive use of detergent-dispersant additives, which enhance the formation and stability of foam. The presence of antifoams is therefore highly desirable in engine oils, especially those containing DD additives in high dosages. Foams are particularly liable to be generated in engines with well-cooled crankcase and rocker-box covers; it is well known that lower or higher susceptibility to foaming is associated with the design of the oil circulation systems in engines.

Anti-foam additives are also essential for other oil types used in circulation systems wherever extensive contact with air is involved, such as turbine, hydraulic and automatic transmission oils and fluids. Considerable foaming may also occur in gear trains; anti-foam additives are also indicated for gear-oils. Many factors are involved in oil foaming and, while empirical rules exist, they do not provide a sufficient explanation for this undesirable effect. Foaming may be caused by changes in viscosity and surface tension in oils and hence by changes in oil temperature, and by contamination of the oil by surfaceactive substances (see Chapter 2.4.1 for details).

It is necessary to distinguish between surface foaming and the formation of emulsions of air in oil. Surface foaming can be limited or eliminated by the addition of anti-foam agents. The earliest known were calcium and lithium soaps, lanolin, alkylsulphates,

375

potassium oleate and other compounds, which are no longer in use. Polysiloxanes, mainly polymethylsiloxanes and polyvinylsiloxanes and other compounds of these types can now be regarded as universal anti-foams. They are capable not only of preventing the formation of foam but also of breaking existing foam (205 -210). To be efficient, an anti-foam must be insoluble in oil, have a lower surface tension than the oil and be very finely dispersed in it. In order to be oil-insoluble, the viscosity of silicone oils (polydimethylsiloxane) must exceed 80 mm2.s-*at 40 "C. Their viscosity increases with relative molecular weight. Silicone oils of viscosity 10,000to 15,000 mm2.s-I are generally even more effective. The surface tension of silicone oils in these viscosity ranges is as low as 20.9 to 21.5 mN.m-', as compared with mineral oil at 28 to 35 mN.m-I. In order to achieve maximum effect and stability during prolonged storage of the oil, the size of the dispersed silicone oil particles should not exceed 1 pm. This requirement can be met by first dispersing the silicone oil in a low-boiling aromatic liquid and then adding it to the oil. The dosage level of silicones in oil is important. The practical dosage used is 35 p.p.m. in engine oils, 1-10 p.p.m. in hydraulic oils and 15-20 p.p.m. in automatic transmission fluids. If the polysiloxane dose is too low, the effect achieved may be opposite to that desired, because true solutions may result in which the same foaming tendency increase may occur as is observed with other soluble contaminants. On the other hand, excessive dosage with polysiloxane may result in a slight clouding effect due to the limited solubility in oil of the polysiloxane. The mechanism of the action of these additives is unclear. They may reduce the strength of the surface films separating the gaseous phase from the liquid phase, which causes the gas bubbles to burst. These additives must, therefore, have only limited solubility in oil and a lower surface tension.

Polysiloxanes are effective in acidic, neutral and slightly basic environments, however their effectiveness diminishes sharply in the presenece of strong base. They are highly stable thermally and have a beneficial effect on other oil properties, reducing the saturation vapour pressure and thus the volatility. When oil containing polysiloxane is oxidised, formation of resins and acids decreases (36),the oxidation induction period is extended, varnish formation is reduced and, to a lesser extent, the thermal stability and detergent capability of the oil is enhanced (209). In addition to polysiloxanes, other organo-silicon compounds have an anti-foam effect, for example CH2=CH-SiR,(OC,H,),, where (x+y) = 3. Also available on the market are some alkyl acrylate homo- and copolymers, such as 2-ethylhexyl acrylate copolymers with ethyl acrylate, which are recommended for use in gear oils at 100300 p.p.m.. Other products specified in the patent literature include calcium and potassium oleates, sodium alkylsulphonates, various fluorinated compounds, salts of alkylalkylenedithiophosphates, trialkylmonothiophosphates,esters of sulphated ricinoleic acid and mixtures of polyethylene-glycol ethers with polyethylene sulphides. It seems possible that fluorinated compounds may become of major importance.

376

Anti-foam additives do not suppress the formation of emulsions of air in oil. On the contrary, decreasing foaming at the oil surface may retard the desorption of air from oil, and this factor must be considered in the choice of an anti-foam for a particular system. This is especially true for oils such as hydraulic fluids in which absorbed air may be a problem. For instance, some fluorinated compounds and polyether siloxanes - in contrast to polymethylsiloxanes - when used so as to exert significant anti-foam effect exhibit a substantially lower tendency to prevent the desorption of air from oil (402).

4.8 EMULSIFIERS AND DEMULSIFIERS Emulsifiers (ionic and non-ionic surfactants or tensides) reduce the interfacial surface tension at the water-oil phase boundaries to about 10 mN.m-l and form at these sites thin, continuous, elastic films which prevent the agglomeration of the droplets. A water-emulsifier-oil interface replaces that of water-oil. In order to form a stable emulsion, it is necessary for the continuous (external) phase containing the dissolved emulsifier to envelop completely the dispersed (internal) phase droplets with emulsifier to prevent agglomeration (220, 221). In determining the nature of the emulsion, the decisive factors are the ratio of water to oil and the hydrophiliclipophilic nature of the emulsifier. An excess of water and hydrophilic emulsifiers produce oil-in-water emulsions, whilst excess oil and oleophilic or lipophilic emulsifiers result in water-in-oil emulsions (fig.4.27).

-- - ----_ TYPE WIO

Fig. 4.17. Schematic representation of the “oil in water” and “water in oil” types of emulsions 1 - emulsifier molecule Some hydrophilic emulsifiers must be soluble in oil, so that so-called emulsion oils can be made. These emulsion oils form the concentrate used for the preparation of oil-in-water emulsions. This solubilisation of hydrophilic emulsifiers which are normally insoluble in oil is brought about by the use of auxilliary solvents, e.g., alcohols and glycols.

377

Hydrophilic emulsifiers are used as components of metal-working and rollingmill emulsions and, mostly, of the so-called non-flammable oils. Lipophilic emulsifiers are used as components of oils which are capable of emulsifying water so as to overcome the harmful effects of moisture. Rust inhibitors and some detergent additives have lipophilic emulsifier properties. Therefore, oils which contain additives such as these must be prevented from coming into contact with water, which would otherwise cause emulsification. The size of the particles dispersed in the emulsion depends on the emulsifier ratio. The proportionality of the soap content to the total surface can be determined in emulsions with a very low concentration of soap-type emulsifiers. Further reduction of particle size can be achieved by increasing the emulsifier content from a threshold value; excess emulsifier molecules form colloidal micelles (353).Emulsions can be classified by droplet diameter (in nm) into very coarse (25-lo), coarse (10-1),coarse-colloidal(10.1 I), medium-colloidal (0.1-0.01), fine-colloidal (0.01-0.001) and transparent micro-emulsions (less than 0.001). The coarser the droplets, the more rapidly they settle. By Stokes’ Law, the settling rate of 0.1pm diameter drops at a viscosity of 1 mPa.s is 4.7.10-3c d d a y , and at a viscosity of 1 P a s it is 4.7.10-6c d d a y . In a coarse emulsion with 25 nm diameter droplets, the rates are 294 and 0.294 respectively, in the same units.

In order to evaluate the nature of an emulsifier, the HLB system (hydrophiliclipophilic balance) has been established. Surface-active substances can be classified in this way by the HLB from 0 to 40 (210)(see fig. 4.18). This system is, however, only applicable to non-ionic emulsifiers. The HLB values of surface-active agents can be estimated roughly, e.g., from their composition, the properties of the tenside-water mixture, comparison of the properties of the emulsion and by paper, gas and liquid chromatography. For example, with non-ionic surface active substances, increased content of polyethylene glycol groups and free hydroxyls, higher neutralisation number of the fatty acid and a low ester saponification number increase the HLB value (267). HLB values can also be determined indirectly by titrating with water a mixture of 4% benzene and 96% dioxane with 1% emulsifier until the mixture becomes cloudly. The amount of water added (the water value of the emulsion) is proportional to the HLB value. Each homologous series has an inherent proportionality constant.

Substances of low HLB value are more lipophilic and those of high HLB value more hydrophilic. The nature of any given emulsion of a specified type depends on the hydrophilic-lipophilicbalance of the emulsifier. The most commonly used watersoluble emulsifiers are sodium, potassium and ammonium soaps of higher fatty, naphthenic, alkylarylsulphonic and sulphonaphthenic acids. The most widely used oil-soluble emulsifiers are calcium, barium and magnesium salts of the same acids and phenols. The emulsion type can also be changed by substitution of the cation in the external phase - that in which the emulsifier is dissolved. The two types act as antagonist in their emulsifying effect and admixture can cause the emulsion system to break down. Some solid substances may form oil-in-water emulsions. For instance, hydrophilic SiO, can form oil-in-water emulsions, whilst oleophilic soot forms water-in-oil emulsions. However, no emulsion can form at a certain Si02-to-soot ratio, since the effects of these substances neutralise each other.

378

Emulsifiers may be classified in terms of ionogenicity into ionic and non-ionic types. In ionic emulsifiers, the emulsifier molecule dissociates into a capillarily active anion or cation. In the first case, this is referred to as an anion-active emulsifier and in the second to a cation-active emulsifier. Non-ionic emulsifiers do not dissociate. In anionic or anion-active emulsifiers, the hydrophobic anion tends to pass or effectively passes, during the ionisation process, into water solution in the oil phase, whereas the cation remains in the water phase. Examples of these emulsifiers are the sodium, potassium and ammonium salts of fatty acids (I): CH,-(CH2),-COO- Na'

(K', NH,')

(1)

(n > 16, most frequently stearic or unsaturated oleic acid),

or of naphthenic acids: CH3 I CH

/ \

H2C

CH - (CH2), - COO- Na'

I H2C -CH, I

(K', NH;)

or amine soaps, e.g., di- and triethanolamine salts of fatty acids (111): CH,(CH,),-COO-

[NH(CH2CH,OH),]'

(111)

Sodium and amine salts of sulphated fatty oils or fatty alcohols and alkyl- and alkylarylsulphonic acids are also very widely used. The common deficiency of sodium and ammonium soaps of fatty acids is their tendency to form soaps which are insoluble in water with calcium and magnesium ions. The solid soaps can precipitate to form scums and deposits which coat machine surfaces, eventually causing abrasive wear, and block filter screens and oil-ways. This deficiency can be limited or even eliminated by adding complexing agents, such as ethylenediaminotetracetic (EDTA) salts (247). On the other hand, naphthenic acid soaps are almost insensitive to electrolytes, but promote oil foaming. The difference between the sodium and ammonium soaps of fatty acids used as emulsifiers consists in the stability of the emulsions formed towards pH effects; the sodium soaps form more stable emulsions at higher pH than the ammonium soaps. The effectiveness of sulphonates is influenced by ions which make water hard to a much lesser extent than is the case with fatty acid soaps. These drawbacks can be practically eliminated by the use of a suitable combination with non-ionic emulsifiers, of which a further advantage is the protection of ferrous surfaces against corrosion.

Cation-active or cationic emulsifiers contain hydrophobic cations which tend to pass into the oil phase during the ionisation process. Examples of such substances include quaternary ammonium, pyridinium and similar salts containi'ng a long alkyl in the cation residue [(CH,)3N-(CH,),-CH,]+ Hal-@ > 11) (N) , the so-called

379

inverse soaps. Examples include cetylpyridinium bromide and chloride (V), cetyltrimethylammonium bromide or chloride and some nitrogen derivatives of fatty acids (e.g., imidazolines):

Quaternary salts form stable emulsions in the neutral or acid regions. In non-ionic emulsifiers, the hydrophobic and hydrophilic moieties are mutually bound in a polar, undissociated molecule. Examples include ethoxylated substances with active hydrogens: RXf CH,CH2-0 % H (VI), where R is a long alkyl or alkaryl, X is -0-, -COO-, -CONH-, etc., and n > 6, (for example ethoxylated higher fatty alcohols, alkylphenols, sugars, amines, acids and their amides). Whereas the long polyethylene glycol residue is hydrophilic, the similar polypropylene glycol chain is hydrophobic. Use of non-ionic emulsifiers has substantially increased. Their advantage is their ability to form stable and non-corrosive emulsions at pH close to or higher than 7 even at a low concentration in oil (up to 3%); they are less sensitive to water hardness, only mildly irritant to the skin and not as readily susceptible to bacterial decomposition as other types of emulsifiers. Because of these qualities, their increasing use is unsurprising. A summary of commercially available emulsifiers is given in Table 4.34. These emulsifiers differ in their solubility in organic substances - propylene glycol, isopropanol, perchlorethylene, xylene and the others. The presence of such substances may therefore affect their emulsifying capablities.

Table 4.34. Some Qpes of Hydrophilic and Oleophilic Non-ionic Emulsifiers Hydrophilic emulsifiers

HLB value

Oleophilic emulsifiers

polyoxyethylene esters of stearic acid

16.9 - 17.9

mono- and diacyl glycerides of comestible fats*

2.8 - 3.5

polyoxyethylene ethers

14.5 - 15.4

sorbitol and fatty acid esters (lauric, palmitic, stearic, oleic)

1.8 - 8.6

polyoxyethylene esters of acylglycerols

1 1 - 18.1

polyoxyethylene -alkaryl esterst

13 - 13.3

polyoxyethylene esters of sorbitol and oleic acid 9.2 - 10.2

Insoluble in water at higher temperatures (75 "C). t Insoluble in mineral oil at normal temperatures (25 "C).

380

HLB value

Demulsifiers Unwanted emulsions may be produced when oil comes into contact with water, for instance in manufacturing (e.g., refining with acid or alkali) or in applications (e.g., turbine oils). These emulsions have to be broken and demulsifiers may be used for this purpose. Both emulsifiers and demulsifiers are surface-active substances. The difference between them resides in their constitution, in the length and configuration of their alkyl substituents. In emulsifiers these tend to be long and straight-chain, in demulsifiers shorter and often branched. In this respect, they are similar to wetting agents. Emulsifiers form a relatively tough and strong film on the phase boundary of the droplets, whilst demulsifiers destroy this film and replace with a thin film which does not impede the coalescence of the droplets. Typical examples are anionic alkali metal and ammonium salts of alkarylsulphonic acids, e.g., didocylbenzene- or dinonylnaphthalenesulphonatesfor oil-in-water emulsions and the calcium analogues for water-in-oil emulsions. Antimony salts have also been reported. Lead naphthenates act as anti-emulsion agents. Ampholytic reaction products of polyalkyleneamines and dicarboxlic acids (415) or of polyisocyanates with polyamines and sulphuric acid (416) have been proposed. A11 these products must be used at very low concentrations. A particular problem arises in engine oils which contain detergent-dispersant additives, especially those in which high dosages of ashless dispersants, e.g., succinimide, are used. The dispersants can act as effective non-ionic emulsifiers and in those parts of the engine in which water is present in liquid form can give rise to water-in-oil emulsions, which resemble butter in appearance, frequently referred to as emulsion sludge or cold sludges. The problem can be aggravated by certain types of engine design, in which blow-by gases from the combustion chamber (gas, containing water resulting from fuel oxidation, which passes the sealing piston-rings in the cylinder bores) are vented via cooler parts of the engine, such as the rocker-box, in order to minimise oil-losses to atmosphere. This problem can be largely overcome by engine designs in which the blow-by gases are recirculated into the fuel-air intake system, but it can be alleviated by incorporation of demulsifiers into the engine oil.

4.9 EXTREME PRESSURE, ANTI-SEIZURE, LUBRICITY AND ANTI-WEAR ADDITIVES Extreme pressure additives include compounds which react chemically with the material of the friction pair, so that new compounds are formed which create a lubricating film firmly bound to the surface and resistant to rupture. This film increases the effectiveness of the lubricant and prevents direct metal-to-metal contact, welding and seizure of the friction surfaces. The objective is not necessarily the reduction of the friction coefficient. Lubricity or “oiliness” additives are compounds which, by physical adsorption and chemisorption of their polar molecules on to the surfaces of the friction pair form boundary films of higher load-carrying capacity and lower friction coefficients than the unfortified oil.

381

Extreme pressure additives often act as anti-seizure agents, at high surface temperatures. A sudden increase in load causing rupture of the lubricant film produces momentary temperature flashes and the surface becomes very hot locally. Under such conditions, the additive decomposes and forms a reactive, predominantly inorganic layer (especially chloride, sulphide, phosphide, phosphite or low-melting eutectics). This layer is resistant to micro-welding and wear is arrested. Simultaneously,the temperature flashes are suppressed, the hot-spot temperature falls below the decomposition point of the additives and reactive layer-formation ceases. The process is repeated if the layer is broken. This self-controlling mechanism is only set in operation when the local temperature rises above the decomposition temperature of the additive. Lubricity additives, which are mostly organic compounds, are affective only up to the temperatures at which they desorb from the surfaces. These temperatures are lower in the case of physisorption and higher for chemisorption; as a general rule, they do not exceed 150 "C. The transition between the two types of additive may be continuous if chemisorption is translated into chemical reaction as temperature rises, so that anti-wear action becomes anti-seizure protection.

Both groups of additive contribute to the reduction of galling and abrasion and, consequently, to the reduction of wear of the friction surfaces in shearing and rolling motion, particularly under mixed friction, extreme pressure and shear.

Fig. 4.18. Illustration of dependence of friction coefficients on the temperature

Fig.4.18 (356)illustrates the variation of friction coefficient p with temperature. The behaviour of the base oil is represented by curve I. The inevitably weak bonds between the non-polar oil and the surface weaken with increasing temperature and the friction coefficient increases. The friction coefficient remains low until the softening point (T,) of, e.g., the soap produced from the additive, e.g., a fatty acid, is reached (curve II). The extreme pressure additive (curve 111) strongly affects the friction coefficient after a reaction temperature (T,) is exceeded; at this point, the extreme pressure additive starts to react with the surface. A hypothetical combination of lubricity and extreme pressure additives should behave according to curve IV. 382

Fig.4.29 (359) illustrates the behaviour of additives in terms of wear in relation to normal load FN expressed as the Archard coefficient K (m3.N-’.m“). Dry friction After this point has been passed, causes slight abrasion up to a critical point FNcrit. abrasion increases drastically and also affects sub-surface layers (curve a). With additive-free oil (curve b), abrasion reduces to lower values down to a certain loadpoint. If an anti-wear additive is added, the coefficient of abrasion reduces by AK‘ (curve c). The extreme pressure additive allows the load to be increased by a value AFN (curve d).

I

I

I

FN

Fig. 4.19. Illustration of the correlation between the behaviour of friction modifiers, extreme-pressure additives and the normal load There is no unified nomenclature in the technical literature for the description and classification of these additives. Soviet literature uses the terms “prisadki antifrikcionnyje” (anti-friction additives), “protivoiznosnyje” (extreme pressure, anti-seizure). The terms oiliness agents, friction modifiers, film strength agents, anti-wear agents, extreme pressure (EP) agents, anti-seizure agents and anti-welding agents can all be encountered. One reason for this is that the substances employed for any particular purpose often act in a number of ways; however, some are more effective as lubricant film strength improvers, some as friction modifiers, and so on.

4.9.1 Extreme Pressure (EP) Additives EP additives are compounds with one or more reactive sites in the molecule which contain atoms or group which, when certain temperatures are reached, react with the metal surfaces of the friction pair. Since these active groups principally contain -C1, -S-, Pc and/or =Pc, reaction with the metal surface produces chloride, sulphide, phosphide or phosphate layers, which prevent the metal surfaces from welding or seizing. The local temperature, which accelerates the reaction between the metal surface and the active atoms of the EP additive, increases with increasing load on the friction surfaces by increasing the surface pressure. The temperature of the friction surfaces drops as soon as a sufficiently thick layer of reaction products is formed. At this point, further decomposition is suspended as long as the protective layer is able to prevent the temperature increase or local over-heating, unless rupture 383

of the protective layer due to further pressure rises, etc. occurs. The surface decomposition temperatures of EP additives differ; for example, those of chlorinecontaining additives are about 300 OC whilst those of sulphur-containing additives are about 600 to 800 “C. The combination of different EP additives with different active elements together with lubricity additives enables reduced wear of the friction surfaces over a wide pressure and temperature range to be achieved (213). Substances capable of increasing the strength of lubricant films or establishing a firm lubricating layer between the surfaces of the friction pair can be used as additives for lubricants exposed to boundary or mixed friction. In this situation, lubricants without such additives can expose the surfaces to severe wear, extensive development of frictional heat and surface damage. This can result in short operating life, impaired functioning and possible breakdown of the machine parts. Typical locations of such problems include the surfaces of gear teeth, so these types of additives are above all found in gear oils. However, they are also found in other oils, for example hydraulic fluids, oils used for lubricating pneumatic tools and in oils used for metal-working, such as cutting, drawing and rolling-mill oils. Many types of compounds have been used or recommended for use as EP additives for liquid lubricants and greases, including compounds of sulphur, phosphorus, chlorine, bromine, fluorine, iodine, boron, lead and many others. Iodine-based aromatic complexes are suitable for lubricating “un-lubricatable” metals such as titanium and some stainless steels. Titanium-to-titanium pairs seize and stainless steel pairs are hard to lubricate. A constantly regenerating layer of metal di-iodides (Ti12, FeI,) with a lamellar structure is supposed to be formed. The iodine-organic complex can be used alone or in oil solution (214-216).

Not all compounds with these active elements are suitable for lubricant additive use. Factors to be considered include, not only anti-wear activity, but other properties, such as solubility, volatility, thermal and oxidation stability, miscibility with the base oil involved, compatibility with other additives (synergism and antagonism occur), reactivity with water, effects on different metals and packing and sealing materials, activity over a prescribed temperature range, physiological activity - and, of course, price. The thermal stability of the additive is particularly important for gear oils. The decomposition temperature must be high enough to prevent the additive decomposing at the bulk oil temperature in the gear system and the decomposition products must not be corrosive to the metal surfaces at these temperatures. On the other hand, the rate of decomposition and reaction with the metal must be high enough to form a chemical layer immediately with increasing load on the bare surface under conditions of boundary friction between the surfaces of the teeth. Commercial products in use contain one or more active elements or comprise combinations of compounds containing various active elements. These compounds are usually more effective and their action more complex because particular active elements become active at different temperatures and thus complement one another over the whole operating temperature range. 3 84

Since the phosphorus-, chlorine- and sulphur-based compounds used do not react until higher temperatures are reached, they may be combined advantageously with additives which react at lower temperatures (up to about 180 "C), such as fatty acids or lead soaps. The temperature range of the package additive can be extended in this way and friction coefficients reduced; synergistic phenomena identifiable by radiotracer techniques - can be utilised to the maximum extent. The compounds used in such combinations also complement each other in respect of shear strength and reduction of friction coefficient. Shear strength increases in the order PCI>S. This sequence is, however, also dependent on temperature.

In some cases, the effective compound cannot be formed until the active elements in the mixture of compounds reaches certain temperature and pressure conditions. For instance, chlorine- and sulphur-based additives are chemically more active. However, metal chloride films with a melting-point of about 680 O C , suitable for lubricating steel surfaces at temperatures over 800 "C, are formed more rapidly in combination with sulphur compounds as well as chlorine (227). Combinations of lead soaps and sulphur compounds form active lead sulphide under high pressure, which is sufficiently plastic under pressure at 700 'C and which has, under these conditions, a low shear strength. In the absence of lead, sulphur compounds on their own produce iron sulphide, which melts at around 1180 "C.

It is very important to match the reactivity of the additive to the application. The requirements of gear oils - when the contact time is longer and the compound must be less aggressive - are very different in this respect from those of metalworking fluids, where the contact time can be very short and reactivity must be higher. Among gear and metal-working operations themselves, there are marked differences in the operating environment. In general, EP additives should only be reactive enough to ensure that chemical wear (corrosion) does not exceed mechanical wear. With respect to chemical composition, EP additives currently available may be classified as follows: - compounds containing one active element, such as chlorine or other halogens, phosphorus, sulphur, selenium, tellurium, lead or boron; - compounds containing two or more active elements, such as chlorine and phosphorus, chlorine, phosphorus and sulphur, or sulphur, chlorine, lead or molybdenum. The effect of chlorine-based compounds depends on the lability of chlorine in the compound and on the degree of chlorination (201). A certain lability of the -C-C1 bond is essential for the effectiveness of the additives. Thus, chlorinated paraffins containing about 40% chlorine in aliphatic bonds have found wider application than, for example, chlorinated naphthalenes with relatively strong aromatic bonds (disregarding the higher animal toxicity of the chlorinated naphthalenes). The -CCl, group is highly effective in this type of EP additive. According to Daney (220) compounds containing phosphorus and -CCl, groups, like tris(trichlorethy1)-phosphate or tris-(trichloro-ten-butyl)phosphate exhibit excellent ability to reduce friction under high loads. In addition to these compounds, many chlorine-containing substances have been proposed and patented, such as trichloroni tropropanol, chlorodibenzyldisulphide,dichlorodiphenyltrichloro-ethaneand

385

chlorinated kerosene, as being suitable for use as EP additives, but not all have found any wide application. The human toxicity of many of them is an important factor. Chlorinated substances become effective on ferrous surfaces at temperatures around 200 "C and above, giving rise to active surface layers of iron chlorides.These substances are virtually without effect under low loads (365).Their effectiveness increases with increasing load, probably due to tribological activation of the surface. The additive itself undergoes chemical change during its action, for example, chloroform is converted into hexachlorethane (220).

Chlorine-based compounds are still used in exceptional cases as EP additives, mainly in cutting and metal-working oils, for examples for machining titanium and its alloys. They are less-used in gear-oils, because of their relatively high susceptibility to hydrolysis and low thermal stability, together with their tendency to from inactive decomposition products and corrosive hydrogen chlorides in the presence of water. Classic chlorinated paraffins have been recommended, e.g., in cutting oils, with 300-400TBN Ca (Mg) sulphonate in combination with ZDDP or other S-P compounds and natural or synthetic esters. However, the use of chlorinebased additives has declined on ecological and health grounds, because burning of chlorparaffins in the presence of aromatic oils can give rise to potentially carcinogenic and mutagenic compounds, including the formation of toxic furans and dioxins. Methods for the determination of chlorine in oils and additives depend on burning the sample and determining the hydrogen chloride produced gravimetrically or by titration. According to CSN 65 6234, combustion products are absorbed in a 3% H202 solution and HCI is determined either by titration with mercury (11) nitrate or gravimetrically as AgCI. ASTM D-808-87 and DIN 51-577 Method A specify methods for the determining chlorine in oils, additives and greases involving oxidation of the sample in a compressed oxygen bomb. The hydrogen chloride evolved is absorbed in Na2C0, solution and measured gravimetrically as AgCI. DIN 51-577 involves burning the sample according to the GroteKreckeler method in a stream of air and absorbing the combustion products in 10% and 5% ammonia solution. Chloride ion is determined gravimetrically or volumetrically (the method cannot be used in the presence of other halogens, with the exception of fluorine). Chlorine in oils, greases and additives can be determined in the absence of other halogens by ASTM D-1317-89 and IP 118;the sample is dissolved in low-boiling hydrocarbons and refluxed with sodium metal and n-butanol. The sodium chloride produced is determined by titration with AgNO,.

Other compounds containing halogens have been tested alone or in mixtures with sulphur compounds (222). Iodine-, bromine- and fluorine-based compounds are very effective but rather expensive. They are, therefore, only normally considered for special oils; for example, fluorine compounds have been used in special silicone gear oils. Phosphorus-based compounds used are mostly alkyl and aryl phosphites and phosphates. Phosphites are more effective than phosphates and the aliphatic esters are better than aromatic esters (220). According to other authors (222,223), tertiary phosphites are weak EP additives and the effects of alkyl phosphites decreases with the size of the alkyls. Phosphorus compounds are preferred as EP additives to chlorine- and sulphur-based compounds at low velocities. In order to increases adhesion of phosphorus compounds to metal surfaces, other active groups such as hydroxyl or chlorine and at least one aryl or alkyl group must be incorporated.

386

Compounds containing phosphorus alone are not used much except in cases of low torqueAow speed applications. The most commonly encountered additives contain phosphorus in combination with other active elements. Some triaryl phosphates, such as triphenyl, tricresyl and trixylyl phosphates and mixtures are marketed. It is not entirely clear how triaryl phosphates function. At one time, it was supposed that they formed metal phosphides, analogous to sulphides and chlorides, of lower melting-points and shear strengths to those of the metals comprising the friction surfaces (220,224). Later, it was suggested that they form metal phosphates on the friction pair surfaces and that the EP effect of triaryl phosphates was connected with their tendency to hydrolysis (225). Thus, the presence of minor amounts of water and oxygen is necessary to bring about hydrolysis, eventually to phosphoric acid. The function of the neutral phosphates is to provide solubility in oiI(336).

Although triaryl phosphates are not as effective as many sulphur and chlorine compounds, they are still used, because they do not corrode metals and they substantially reduce wear in machines operated for along time under low load. They are also effective anti-corrosion agents and suitable as anti-wear agents in all types of circulating oils in greases, gas-turbine synthetic oils, compressor oils and metalworking fluids. In the last example, the polishing effect of triaryl phosphates is used; they also provide a smooth surface no machined materials which does not tend to rust. The concentration of triaryl phosphates used varies between 0.5 and 2% depending on the type of lubricant. Tricresyl and trixylyl phosphates are also recommeded as rubber swelling agents. They are incorporated into alkanic oils which come into contact with packings and seals and would otherwise cause shrinkage of the rubber and consequent leakage. The phosphate concentration in this application is about 2% by weight. Liquid trialkyl phosphates are also used as non-flammable components in aircraft hydraulic fluids, ashless hydraulic oils and as liquid components for special lubricating greases.

Sulphur-basedcompounds are much more widely used, as EP additives in metalworking fluids and gear and other oils. The most widely-used substances are the disulphides, e.g., dibenzyl disulphide, butylphenyl disulphide, polysulphides, sulphurised sperm oil (now almost totally replaced for conservation reasons, e.g., by jojoba oil), sulphurised terpenes and alkene polymers, alkylxanthates, dialkyldithiocarbamates and others. The compounds usually contain a reactive sulphur, able to form iron sulphides on iron and steel. These sulphides form a film of their own which withstands high loads, although the friction coefficient, which is higher than that of a chloride or a phosphate film (2Z8), remains comparatively high (at around 0.5) and should be complemented by another film, such as a chloride film. Tests of abrasion particles collected from iron surfaces have shown that they contain more iron oxide than iron sulphide, although the sulphides are, in fact, the protective agents (367). The more effective the additive, the more sulphide is formed relative to oxide. For example, 1 1 to 14% S and 15% 0 is formed from diphenyl disulphide and 8 to 18% S and 8% 0 from di-tert-butyl disulphide. This is probably connected with the strength of the -C-S bond, which decreases in disulphides in the order diphenybdisec-butybdi-tert-butybdibenzyl, whereas the effectiveness decreases in the same order (368). Mercaptans and sulphides can provide an effective protective layer for lighter loads.

387

Sulphur-based compounds are regarded as more effective EP additives for highspeed applications than chlorine- or phosphorus-containing compounds (226). This can be explained in terms of the autocatalytic reaction between sulphur and iron and by the dependence of this reaction on temperature. Some authors have asserted that sulphides and disulphides are effective additives and mercaptans ineffective (227). Other claim that disulphides and polysulphides are effective, whilst monosulphides fail to act as EP additives. These controversies could reflect differences in the conditions of test used for different compounds, as well as the type of sulphur compound. Selenium- and tellurium-based compounds act in the same way as sulphur compounds. They are, however, expensive and toxic and therefore not easily used. Like sulphur compounds, some of them have anti-oxidant properties and also act as anti-rads.

Combined chlorine-phosphorus based compounds are very little used. Di-(2-chlorethyl)vinyl phosphonate (228), the bis-(p-chlorphenyl) ester of phosphorous acid, trichloromethanechlorophosphorousacid and their salts have been proposed as additives for gear oils.

Chlorine-sulphur bused compounds and mixtures of them can be effective EP additives, chiefly in gear oils intended for heavily-loaded hypoid gears and cutting fluids. In this combination, one active element complements the other over the range of operating conditions. The metal chlorides form more rapidly in the presence of sulphur and the “anti-seizure’, film comprises either a mixed layer of iron chlorides and sulphides or a complex of both. Substances containing these two active elements with useful EP properties include chlorobenzyl disulphide, sulphochlorinated sperm oil and its substitutes, pinene and mineral oil, and chlorobenzylalkyl xanthogenates. Phosphorus-sulphur compounds are at present the most widely used EP additives, particularly for automotive gear oils, as well as for industrial gear oil, hydraulic oils and oils used for moderate severity metal-working applications. They are, however, also suitable for other applications. Automotive gear oils now incorporate S / P compounds as their only EP additives. They meet the requirements imposed by increasing speed and load of the gear trains of modem automobiles, as well as at the high speeds of passenger cars and the high torques of trucks (230,231).They also possess other virtues; they are not corrosive in the presence of water, indeed they perform as corrosion inhibitors, they improve the thermooxidation stability of oils, suppress - in the presence of suitable friction modifiers - vibration in limited-slip differentials and they do not adversely affect packing and sealing materials, in contrast to chlorinated additives (232). Sulphurphosphorus based EP additives, having good oxidation stability and quite good thermal stability and demulsifier properties, are employed in industrial oils, where their lubricant film load-carrying capacity matches that of conventional EP additives of the sulphurised sperm oiVlead naphthenate type. They are virtually the only antiwear and antioxidant additive in hydraulic oils. Sulphur-phosphorus based EP additives used now are mainly of two types: metal dialkyldithiophosphates (mainly zinc and/or antimony and tin - especially for compounds containing shorter alkyls) and similar compounds of molybdenum or

388

tungsten, such as:

(Molyvan L manufactured by R. T. Vanderbilt Co.) and polyalkenes with a bridge containing sulphur and phosphorus, e.g., products prepared from polybutene and P2S5:

H3C - C?H3 I

CH3

_...qn CH3

s

s

CH3

II

I

YH3 II ‘s s‘P-CH=C-CH, CHZ-C=CH-P’ ’

I

C-CH,

C-CH,

CH3

CH3

I

The ratio of sulphur to phosphorus differs among these additives, but the sulphur content is usually many times larger. Chlorine-phosphorus-sulphur based compounds and their mixtures are highly efficient EP additives, suitable for lubricating heavily-loaded gears and oils for the more demanding metal-working operations. Compounds made from all three active elements include, for example, condensation products of chlorinated kerosene, chlorinated paraffins and the salt of dialkyldithio-phosphoric acid containg 33.7% C1,0.99% P and 2.2% S (233). Lead soaps have been effectively used as EP additives for many years and include the first additives to oils used for lubricating hypoid gears. They are characterised by the ability to suppress wear in gears where both sliding and rolling action occurs (e.g., hypoid and worm gears) and behave as steel corrosion inhibitors in the presence of water (acting as de-watering agents). In the presence of a compound containing active sulphur, they form sulphurised lead soaps; these form anti-seizure films effective at high temperatures and extend the range of phosphorus-chlorinesulphur additives from 90 to 180 “C. The most commonly-used soaps are lead naphthenates, since they are cheap and readily soluble in oils, particularly cycloalkanic and dark oils, less so in high VZ oils. Commercial concentrates of lead naphthenates in oil contain 20-30% lead. Lead oleates, 12-hydroxystearatesand similar compounds are also used, but some authors claim that lead naphthenates are better EP additives than other lead soaps, especially in the presence of water, because they suppress the formation of oil-inwater emulsions (234). However, they impair the oxidation stability of oils. Lead soaps are not usually used as sole EP additives in gear oils, but are complemented by other additives containing active sulphur or phosphorus. EP additives comprising Pb soaps and sulphur and chlorine-based compounds are recommended as special additives for oils intended for lubricating hypoid axles in trucks operating under high torque/low speed conditions. The relative merits and 389

demerits of lead soaps mixed with sulphurised fatty oils and sulphur- and phosphorus compounds are illustrated in Table 4.35. Table 4.35. Comparison of Gear Oils to MIL-L-2105B Specification with Various EP Additives EP additive type:

Lead naphthenatd sulphurised fatty oil

EP properties: 4-ball tests: wear index 59-74 weld load (N) 3188-5395 Timken: OK load (N) 333 Falex: No. of teeth (fewer teeth, less wear) 21

SdphW-phoSphomsbased additive

50-80 2452-3924 333 10

Thermal stability (50 h at 162.8 "C): Viscosity increase (%) Insolubles content (%) Benzene-insolubles (%)

220 9.5 3.5

9.7 0.08 0.05

Corrosion stability (ASTM 130-56): 6 h at 98.9 "C 3 h a t 121.1 "C

Ib 2c

lb 2c

Foaming test (ASTM 982-46) Sequence I - tendency - stability Sequence I1 - tendency - stability Sequence I11 - tendency - stability

10 0 50 0 0 0

10 0 10 0 0 0

Demulsification (4001111 water, 200g oil, stirred for 10 min. at 51.7 "C) - quantity of water separated (cm3) after: 10 min. 200 315 Ih 220 360 24 h 245 385 Lead content in oils and additives can be determined by CSN 65 237 and IP 120; copper and iron can be measured at the same time. The sample is fully oxidised with H,SO, + HNO, + H,O,, and the filtered PbSO, is boiled with Na,C03 solution in order to separate other sulphates. Pb is precipitated from the solution with H,S, PbS is oxidised to sulphate and measured gravimetrically. Iron is separated from the filtrate as hydroxide and determined colorimetrically with thioglycollic acid or o-phenanthroline; copper is also determined colorimetrically as yellow diethyldithiocarbamate. In lubricating greases, lead is determined by ASTM D-1262-81: the sample is oxidised with the same mixture of sulphuric and nitric acids and hydrogen peroxide. The solution is mixed with dilute sulphuric acid and alkali acetate, the precipitate filtered and boiled with aqueous ammonia. After acidification with nitric acid, the lead is determined by electrolysis as lead peroxide. These elements are now more conveniently measured by atomic emission spectroscopy (ASTM D4951-89).

In addition to the compounds containing the main active elements typical of EP additives, other compounds are also used, such as organic borates (chlorophenyl390

boric acid dibutyl ester) which, like ZDDP, are also effective antioxidants, and various complexes containing halogenides of two different metals, such as silver, copper, tin, manganese and cobalt, and arsenates or antimonates (235). EP additives may also incorporate other compounds such as tartaric acid, boric acid, lecitin, alkylsuccinic acids, nitro-compounds, epoxidised aliphatics, phenylisocyanates and many more which are, by themselves, able to increase the strength of the lubricating film and also bring about other beneficial effects. Polymers, such as fluorinated polymers, polyamides, polyvinyl chloride, high density polyethylene and polyisobutenes, can be used as EP additives in some special cases, for example in lubricating greases. They not only increase the strength of the lubricating film but they also increase the adhesion and resistance to washout with water of the lubricants and increase their drop-points (236). The loadcarrying and antiwear capacities of both liquid and plastic lubricants can be increased by combining oil-soluble EP additives with dispersions of solid lubricants, particularly graphite, MoS2 and poly-p-phenylene (see below) at a concentration of r 1 5-2596 (4Z4).

However, the effect of particular solid lubricants varies in this respect and differs with concentration and composition of the oil-soluble additive, i.e., with the different type and concentration of the active elements. It is not manifest in combinations with EP additives containing active sulphur and chlorine (237), and the operating conditions must be taken into account in applying these additives. They cannot be employed in oils which must be clear and bright, as the addition of even a small amount of graphite or MoS2 makes the oil dark; the possibilities must always be taken into account of the partial separation of the solid lubricant from the oil when it is undisturbed or partially caught on filters. Possible synergism and antagonism must be allowed for when mixtures of additives of different effects are used. For instance, it is well known that some detergent and dispersant additives neutralise the effect of extreme pressure additives, when the adhesion of the detergent or dispersant to the metal surface is stronger than that of the EP additive. Such interactions must be monitored experimentally.

4.9.2 Lubricity Additives (Friction Modifiers) These mostly comprise compounds with long, straight hydrocarbon chains (about 10 carbons or more) - which provide oil-solubility - and an end polar group which has sufficient adsorption and/or chemisorption capacity towards the friction surface, and which reduces the friction coefficient of the lubricant. The mildest additives of this type are C,, - C,, alcohols; long-chain carboxylic and hydroxycarboxylic acids and their esters, higher amides and imides, phosphites, phosphates, phosphonates, the important dithiophosphates (Mo), thiocarbamates and dithiocarbamates (Sb) (2Z9), and other derivatives, including those of boron. Sperm oil used to be a

39 1

frequent component in lubricating oil additives, but its scarcity and conservation pressures stimulated the development of substitutes, e.g., esters prepared by the selective hydrogenation of soya bean and linseed oils, the hydrogenolysis of the resulting fatty acids to the fatty alcohols and esterification of the acids and alcohols obtained to long-chain esters with unsaturated bonds in the acid group (296). The oil derived from the seed of the jojoba is very similar in composition to sperm oil. Some authors mention a beneficial effect of cholesterol type liquid crystals on the lubricity of oils (394). Each of the compounds mentioned exerts a specific influence on the change in static or dynamic friction coefficient. In a package additive, the so-called friction modifier is able to change the friction profile of the lubricant - the curve in which is plotted the change in friction coefficient from nil shear rate (static friction) to a certain value of shear rate (kinetic or dynamic friction) - and to suppress the “stickslip” phenomenon at low shear rates, which can be achieved by careful blending. Modern transmission systems, such as automatic transmissions, limited slip axles, wet brakes and many slide-ways (e.g., in machine tools) require a lubricant with specific properties to secure a specific relationship between friction coefficient and shear rate. This relationship - the ratio between static and dynamic friction coefficients -can vary even in mechanisms which are apparently identical but with small design differences; friction coefficients are affected by a number of factors such as pressure, shear rate, type of materials and surface finish in the friction couple, temperature and products of reaction of the components of the lubricant (238,239,246).The wrong ratio may adversely affect the performance and operation of the mechanism. A high static coefficient combined with a low dynamic coefficient may cause difficulty in gear movement and a distinct squawking noise in automatic transmissions; on the other hand, too low a static coefficient may cause sliding and delayed gear engagement, and inadequate transmission of power. Limited-slip axles require oils which combine EP additives and additives which provide a sufficiently high static friction coefficient to prevent the gears from vibrating. It is not easy to meet these conflicting requirements, especially since antagonistic effects between sulphur- and phosphorus-based EP additives and some friction modifiers have been observed (258,259).The wrong relationship between friction coefficient and shear rate can cause stick-slip motion in slide-ways and adversely affect machining accuracy in machine tools.

SLIDING VELOCITY

Fig. 4.20. Effect of different types of lubricating additives on the change in static and dynamic friction coefficients of the base stock A - additive-free, B - with 1 % glycolate, C - with 1 % oleic acid, D - with 1 % dioleylphosphite

392

Neither mineral nor synthetic base oils provide the right friction profile for many such applications and friction modifier additives must be added in the right combination to achieve the correct ratio between static and dynamic friction coefficients. Fig. 4.20 illustrates how particular additives affect the ratio of these coefficients in different ways. Some compounds which are suitable for use as lubricity additives and components of package friction modifiers are used for special purposes. For example, aminodithiophosphates are used as vibration suppressors in oils designed for lubricating limited-slip axles (24Z). and N-acylsarcosine derivatives (24Z), sulphonated fatty acids and their esters (242, combinations of organo-phosphoric and fatty acids (243), esters of dimerised fatty acids (244) etc. in oils to suppress squawking in automatic transmissions. Lubricity additives are also important in automotive gear and engine oils. Decreasing friction coefficient decreases the resistance to motion between the friction surfaces and reduces fuel consumption. Ash-containing friction modifiers are also used including dispersions of MoS2 or organo-metallic compounds forming active sulphides in situ, as are ashless compounds containing active elements or groups, which do not affect the colour of the oil and do not deposit dark coatings on machine parts, such as primary akylamines of the laurylamine long-chain type (381). other oil-soluble amines, esters of long-chain carboxylic and hydroxycarboxylic acids, oxazolines, imidazolines and, especially, phosphorus and boron derivatives (393).

4.10 MISCELLANEOUS ADDITIVES Special additives are encountered which are designed for very well-defined duties, or which are only used for some specially-designed lubricants. Disinfectants are used to prevent the growth of bacteria and moulds in oil emulsions used for machining processes and in non-flammable hydraulic emulsions. The oil-in-water emulsions used for metal-working repeatedly and rapidly undergo undesirable changes, including decomposition, pH decrease, increased corrosivity, oil separation, colour change, generation of offensive odours and increased biological activity towards the human skin. These phenomena may be caused by microorganisms, such as pseudomonas oleovorans, p . aeruginosa, escherichia coli, micrococcus aureus, candida albicans and aspergillus niger, which often occur in the water used to make the emulsion but may also enter the emulsion by contamination during operation.

Appropriate disinfectants (used at dosages up to about 0.2% by weight) include a variety of phenols, chlorinated compounds, organic nitrogen-bases (e.g., alkylated hexahydrotriazines), biocidally-active zinc dialkyldithiophosphates and atkylphenyltributylstannates (279).The most effective treatments for metal-working emulsions and coolants are claimed to be condensed aldehydes and heterocyclics

393

containing sulphur, nitrogen or both (354). Preferred phenols, on grounds of economy, toxicity and environmental impact are phenol, cresols and xylenols, preferably containing C, - C, alkyls. Polychlorinated phenols are banned on ecological grounds. The mechanism of the antimicrobial effect of phenols consists in two reactions of phenols within the cell of the organism: halting the oxidation-phosphorylationprocess and coagulation of the cell protein at higher concentrations. The advantage of phenols is the rapid destruction of bacteria; their disadvantage is their ready solubility in water (in systems operating at a very high water content they must be used as their alkali metal salts), their loss of activity in the presence of non-ionic emulsifiers and the reaction of some phenols (those unsubstituted in the ortho- and para-positions) with formaldehyde, resulting in inactive products. Objections from hygienists are expected.

The most effective aldehyde is formaldehyde as formalin, a 30-40% solution in water. However, because of the problems of using free formaldehyde (volatility, corrosivity of its oxidation products and more recently identified carcinogenicity), condensed forms have been developed, O-formals and N-formals. The oxygen formals can be produced by the reaction of alcohols and glycols (e.g., 1,2-propylene glycol (I)) or benzyl alcohol (11) with formaldehyde to produce semi-formals - full formals are more stable chemically and consequently have little or no effect. H I H,C - C - CH2OH I 0 - CHZOH

@

CH2 - OCH20H

(1)

Nitrogen-formals are produced by the reaction of formaldehyde with amines and amides. The substances relevant here are various substituted hexahydrotriazines, condensation products of formaldehyde with primary alkyl amines, hydroxyalkyl amines and alkoxyalkyl amines, e.g.,

/cHfN - CH2 - CH2 - OR

RO - CH2CH2 - N I H2C

\

/

I CH,

N I CH2 - CH2 - OR

(hexahydro- 1,3,5-trialkoxyethy1-1,3,5-triazine). In contrast to formaldehyde, they are less volatile, longer-acting, anti-corrosive and harmless to the skin. They are effective at concentrations of 400 -1,500 p.p.m. (163).

Compounds which are more efficient and quicker acting have been developed, including methylene-bis-oxazoline (I), methylene-bis-oxazine (11) and aminal (III), condensation products of 2- and 3-hydroxyalkylamines with excess formaldehyde:

394

CH? L

-

0 - CH2

CH-0

/CH,-CH2\

\

CH, - CH2

/O CH2 - CH,

(111)

In aqueous solution, these compounds are less alkaline than hexahydrotriazines, and hence less harmful to the skin. The third group of aldehyde disinfectants are the reaction products of formaldehyde with urea and/or chlorinated urea, such as dimethylol urea (IV) and N-methylolchloroacetamide(V): HOCH2-NH-CO-NH-CH,OH

(IV)

ClCH,-CO-NH-CH*OH

(V)

The latter is the more effective; however, it has the disadvantage that it hydrolyses in water to produce corrosive hydrochloric acid. It must therefore be used in conjunction with a suitable corrosion inhibitor. The effects of aldehyde-type disinfectants derives from their reaction with carboxylic. amino, hydroxyl and sulph-hydryl groups in cell wall proteins. This reaction destroys the osmotic balance which kills the microorganism. Formaldehyde-based biocides have little effect on moulds or yeasts, but this effect can be increased by combining them with a small amount of phenolic biocides. However, the pH of the mixture must not exceed 9 - 9.5 (397).

A further group of effective disinfectants is compounds containing sulphur and nitrogen, nitrogen, phosphorus and metal dithiocarbamate and dithiophosphate (the metal may be zinc, manganese or sodium). Effective substances containing sulphur and nitrogen, which can also contain halogen, are the substituted derivatives of

395

and 1,Zbenzthiazole

0

where R is H, alkyl or aryl. Some newer, very effective biocides include organic boron compounds. Biocides must meet the following requirements (309):disinfecting effect on a wide range of microorganisms, stability under field conditions, absence of odour, zero human toxicity in the concentration employed, no irritating effect on the skin and respiratory system and no adverse effects on machine components, machined material and paints used on the machinery. The use of mercury-based compounds (phenyl mercuric chloride, mercury phenyl borates etc.) formerly applied in conjunction with phenols - has ceased because of health hazards. In any case, mercury-based disinfectants are unsuitable for emulsions used for machining aluminium and its alloys because these react with mercury and the work-pieces can be damaged.

Biocides have to be changed from time to time, because the microorganisms become resistant to one type continuously applied. Inevitably, biocides are harmful to the human organism, and contaminate waste water after the emulsions in use have decomposed. For this reason, efforts have been made to find alternative methods of bacterial control, including ionisation and pasteurisation of emulsions, the use of bactericidal ion exchangers (358,359)and the oligodynamic effect of metallic silver. The principal advantage of ion exchangers is that the ion exchanger particles anchor the reactive, ionic bactericidal compound and the concentration of this compound in solution is negligible. As the bacterial suspension flows through the active layer of the fixed-bed exchanger, the cells come into contact with the particle surface and the cells absorb a small amount of the bactericidal compound and become inactive. Bactericidal ion exchangers can be prepared by impregnating a cation-active exchanger of 0.3 - 0.6 nm diameter particles with an alkaline aqueous solution of bis-tributyl zinc oxide (360):

Some chlorinated hydrocarbons are recommended for suppressing the formation of deposits in two-stroke engine spark plugs (245). Tackiness impmvers can be added to lubricating oils and greases to improve adhesion to the lubricated surface and reduce the separability of oils from greases. Important groups of compounds used for this purpose comprise aluminium soaps of high molecular weight fatty acids and high molecular weight polyalkenes, mainly polybutenes. They are particularly used in lubricants which can stain the materials being processed by the lubricated machinery, e.g., lubricants for textile machinery. 396

However, they can also reduce splash-out of greases from bearings and leakage of oils from gear-boxes. Quench oils,particularly those used for warm quench baths, incorporate special additives to increase the quenching-rate, increase the oxidation stability of the oil and prevent the formation of deposits on the hardened work-pieces. These preparations include mixtures of ashless and ash-containing antioxidant and detergent-dispersant additives containing calcium, phosphorus, sulphur and nitrogen.

Additives Enhancing Swelling of Packings and Sealants Elastomers, for example butadiene-acrylonitrile copolymers, are often used for sealing rings and packings for automatic transmissions and gear-boxes. The sealing materials must be compatible with the oil used and they must not shrink or soften. Slight swelling is desirable in order to achieve a tight seal. If the oil itself fails to produce such mild swelling, a suitable aromatic additive - usually an aryl phosphate - will be incorporated in the oil.

Additives Producing Low-melting Eutectics Organo-metallicadditives have been recommended recently for liquid lubricants and greases which are supposed to form eutectics with the metal asperities on the sliding surfaces. This reduces the time for running-in, decreases friction, wear and temperature and reduces the dependence of lubrication on viscosity. The so-called third-generation products aim to replace conventional lubricants with EP additives and solid lubricants in certain situations (194).

Tribopolymers These products are now attracting increased attention. They are additives formed in situ, either directly in oils by polymerisation, or on the tribochemically activated surfaces from monomers present in the oil (361).Tribopolymers may, for instance, be products of zinc dialkyldithiophosphate transformations (362).According to Sor et al. (363), ZDDP forms a boundary film of a colloidal nature on the friction surfaces and, simultaneously, the metal undergoes a chemical transformation. These films may be associated with the aromatic components of the mineral oil and their oxidation products, as well as micelles of other functional additives present in the oil. Close attention has been paid to tripolymers in the USSR (364). Some tribopolymers are claimed to surpass the anti-wear properties of MoS2 and also protect surfaces containing lead and copper from corrosion.

397

DY= Lubricants are sometimes dyed to enable the different types of liquid lubricants and greases to be distinguished. Oil-soluble and water-insoluble azo-dyes are used, including:

-

a typical red azo-dye ( 1-(2,4-dimethyl-benzoazo)-2-hydroxy-naphthy11-azo)azobenzene HO ,C*3

N%

- a typical brown-red monoazo-dye (1-(2,4-dimethyl-benzazo)-2-hydroxynaphthalene)

Dosage of these materials in oil seldom exceeds 0.05% by weight. The appearance of the oil may also be improved by the addition of fluorescent organic substances. Some solid lubricants also used as additives are coloured, e.g., the indanthrenes (blue) and the phthalocyanines (blue and green). Several other special additive types are worth mentoning: - anti-ruds - which suppress the destructive effects of radiation on lubricants, - de-watering agents - e.g., alkylarylsulphonates of Pb and Zn, - unti-creep agents - soaps of fatty acids, fluorinated polyacylates (477), - anti-static agents for textile oils - cationic surfactants, - water-repelling agents - e.g., (for greases) metallic soaps (Na,Ca) of C, - C, polymethacry lic acids.

398

Chapter 4 - References 1 . VESELY’, V. : Ropa a Uhlie, 11, 1969, 297. 2. KUUJEV, A. M. : Chimija i technologija prisadok k maslam i toplivam. Moscow, Izd. Chimija 1972. J. : Antioxidanty. Raha, Academia 1968. 3. POSPISIL, 4. DUNN,J. R. et al. : J. Chem. Soc., 1954, 580. 5. BEAN,X. : Rev. GCn. Caoutch., 31, 1954,49. R. B. et al. : Ind. Eng. Chem., 44, 1962, 102. 6. SPACHT, INGO GOLD, K. N. : J. Inst. Petrol., 45, 1959,244; J. Phys. Chem., 64,1960, 1636. %COOK,J. et al. : J. Amer. Chem. SOC.,78, 1956, 4159. ~ . D E M S OE.VT. , - EMANUEL, N. M. : Usp. Chim., 27, 1958,365. lO.Sco?r, G. : Chem. and Ind., 1963,271. 1 ~.ROSENWALD, R. - HOADSON, I. : Ind. Eng. Chem., 42, 1950, 162. A. A. : Neftechimija, 3, 1963, 594. 12. BOGDANOV, G. N. - BOLDIN, J. P. : J. Amer. Chem. SOC.,82, 1960, 595. 13. THOMAS, 14. MYERS,H. : Reprints ACS, Div. Petrol. Chem., 13, 2, 1968, B61. 15.MAHONEY, L. R. : Angew. Chem., 8, 1969,547. LOO BOOZER, C. E. - HAMMOND, G. S. : J. Amer. Chem. SOC.,76, 1954,3861. V. V. : Usp. Chim., 39, 1970, 1687. 17.LEVIN, P.J . - MICHAJLOV, 18.ZISMAN, W. A. : Ind. Eng. Chem., 45, 1963, 1406. 19. U.S. Patents 3,326,802, 3,493,570. 20. STRANGE, H. D. et al. : Ind. Eng. Chem., Prod. Res. Dev., 6, 1967, 33. 21. U.S. Patent 3,287,269. R. F. et al. : Ind. Eng. Chem., Prod. Res. Dev., 5, 1966, 226. 22. BRIDCER, J. S. - EDVARDS, E. D. : Synthetic Lubricants for Gas Turbines. IP Reprint 1960. 23. ELLIOT, 24. HRONEC, M. et al. : J. Oxidn. Commun., 8, 1985,51; Ropa a Uhlie, 28, 1986, 303. 25. CHAO,T. S . et al. : Preprint ACS, Petrol Div., 27 (2), 1982, 362; Ind. Eng. Chem., Prod. Res. Dev., 23, 1984,21 26. U.S. Patents 3,399,139, 3,413,223, 3,422,014. 27. ACTON,E. M. et al. : J. Chem. Eng. Data, 6, 1961, 64. 28. SCHIEFER, H. M. et al. : 137 ACS Nat. Meeting 1960. 29. KOEIZOVA,R. 1. et al. : Chim. Technol. Topl. Masel, 11 (1 I), 1966,52. 30. Hydrocarbon Processing - Petrol. Refiner, 40 ( I I), 1961,41. 31. BLAGOVIDOV,I. F. - BOROVAJA, M. S. : Chim. Technol. Topl. Masel, 8 (6), 1963,52. 32. SHELDON, R. A. - KOCHI,J. K. : Oxid. Comb. Revs., 5, 1973, 135. 33. OCHIDI, E. : Tetrahedron, 20, 1964, 1819. 34. CHAKRAVARTY, N. K. : J. Inst. Petrol., 49. 1963, 353. 35. HRONEC, M. - VESELY,V. : Ropa a Uhlie, 15, 1973, 169; 16, 1974,353,437. 36. VESELY,V. et al. : Niektort probltmy vq’skumu a spracovania ropy (Some Problems of Crude Oil). Bratislava 1978, 11. 37. CS Patent 95903, (1959) (Vesely, V. et a].). 38. ZORN,H. : ErdoI u. Kohle, 3, 1950, 171. R. J. : Petrol Refiner, 41 (2). 1962, 139. 39. LOOMIS, 40. LARSON, R. : Sci. Lubr., 10 (8),1954, 12. 41. DENISON, G. H. - CONDIT, P. C. : Ind. Eng. Chem., 41, 1949,945. 42. U.S. Patents 2,719,125, 2,983,716, 2,719,126. 43. KUUIEV,A. M. et al. : Azerb. Neft. Choz., 7, 1962, 23. R. : SAE Meeting. Tulsa (Oklahoma), Nov. 1958. 44, SCANLEY, C. S. - LARSON, M. : J. Chem. SOC., 1971,350. 45. FERNANDO, Q. - SHIRO, 46. IVANOV, S. K. - KORSALYKOV, Ch. : Erdol u. Kohle, 24,1917,516.

399

47. DHINGRA, M. M. et al. : J. Magn. Reson., 6, 1972, 577. 48. ANGUS, J. R. et al. : J. Inorg. Nucl. Chem., 33, 1971, 3967. 49. MARSHALL, T. - FERNANDO, Q. : Anal. Chem., 44,1972, 1346. 50. SOPOV, D. et al. : Acta chim. Acad. Sci. Hung., 36, 1963, 371; Erdol u. Kohle, 19, 1966, 732; Revue de l’lnst. Franpis de Pktrole, 22, 1967, 1520. S . K. - KATEVA, J. : Neftechimija, 11, 1971,290. 51. IVANOV, 52. IVANOV, S. K. et al. : Neftechimija, 12, 1972,606. 53. BURN,A. J. : Advances in Chem. Ser., No. 75, 1968,325; J. Inst. Petrol, 57, 1971,319. 54. RABL,V. - KOZAK,P. : Ropa a Uhlie, 17, 1975, 256, 545. 55. RABL,V. : Thesis. Institute of Chemical Technology, Prague 1977. 56. SOFQV,D. - IVANOV, S . : Bulg. Acad. Sci., 11, 1969,619. 57. BURN,A. J. : Tetrahedron, 22, 1966,2153. 58. HANNEMAN, W. W. - PORTER, R. S . : J. Org. Chem., 29, 1964,2996. 59. ROW, C. N. - Dickert, J. J. : Preprints ACS, Div. Petrol. Chem., 10, 2, 1965, D 71. 60. LUTHER, H. - SINHA,S . K. : Erdol u. Kohle, 17, 1964, 91. 61. ASHFORD, J. S . et al. : J. Appl. Chem., 15, 1965, 170. 62. GALLOPOULOS, N. E. : ASLE Trans., 7, 1964,55; Preprints ACS, Div. Petrol. Chem., 11,3, 1966.21. 63. FENG,I. M. et al. : ASLE Trans., 6, 1963, 60. 64. BRAZIER, A. D. - ELLIOT, J. S . : J. Inst. Petrol., 53, 1970, 63. 65. JAYNE, G. J. J. - ELLIOT, J. S . : Preprints ACS, Div. Petrol. Chem., 14, 2, 1969, A59. 66. MOSTECKY, J. - RABL,V. : Bulletin of the Institute of Chem. Technology. Prague 1972, D26. 67. RAVIKOVIC, A. M. et al. : Chim. Technol. Topl. Masel, 9 (7), 1974,44. 68. WOOD,G. P. : Symposium on Engine Oils. London 1969. 69. JAYNE, G. J. J. - ELLIOT,J. S. : J. Inst. Petrol., 56, 1970, 547. 70. BAUMGARTEN, E. : Erdol u. Kohle, 23, 1972, 577. 7 1. Discussion on the Brazier and Elliot Lecture at the IP Symposium. London 1967. 72. Unpublished Works. Prague Inst. of Chemical Tech. and Res. Inst. of Benzina. Prague 1973. 73. DARLING, S. M. et al. : Chem. Tech., 4, 1974,356. 74. CRAIG,W. S. : Air Water News, 25, 1972, 38. ~ ~ . T I S K OV. VA N., et al. : Prisadki k maslam i toplivam. Moscow, GTTI 1961. G. R. et al. : J. Amer. Chem. SOC.,74, 1952, 161. 76. NORMAN, 77. U. S. Patent 2,614,988. 78. U. S. Patent 2,589,675. 79. U. S . Patent 2,34333 1. 80. U. S. Patents 3,317,426, 3,537,998. 81. ANDERSON, D. A. : Japanese Petr. Inst., Tokyo, 10, 1%8, 23. 82. SECHTER, Ju. H. - KRWN.S . E. : Poverchnostno-aktivnyje veSEestva iz neftjanogo syja. Moscow, Chimija 1971. 83. BURGES, J. E. et al. : Abstr. Amer. Chem. Soc.Meeting. San Francisco, April 1968. 84. SANIN.I. P. et al. : Prisadki k maslam. Moscow, Chimija 1966. 85. CSIKOS, R. et al. : COMECON Symposium on Additives. HallelSaale 1976. 86. LANDEN, E. W. : SAE Paper, 1963, No. 7 14B. ~ ~ . M A R C I AA. N T- E , CHAMPO, P. : SAE Paper, 1972,720 945. ~ ~ . P L O N S KL.Eet R ,al. : Amer. Chem. SOC.Meeting. Chicago 1974. 89. KALINOWSKI, M. I. et al. : SAE Joint Automot. Lubric. Meeting, 1958. 90. WILSON, J. V. D. - JAYNE, G.J. J. : SAE Paper, 1972,720 152. ~ ~ . G U I B J. ET C., - DUVALL, J. J. : SAE Paper, 1972,720 114. 92. Cox, W. A. - LOCK,J. K. : SAE Meeting. Pittsburg 1957. 93. PEN, J. B. : J. Amer. Oil. Chem. Soc.,35, 1958, 110. 94. WIDROTER,M. : 1 lth Symposium on Lubricants. Magdeburg 1969.

~ ~ . M A ~ H J.E B. w :sConference , on Lubrication and Wear. London 1957. 96. BARWELL, F. J. et al. : Proc. Inst. Mech., 1954 - 1955, E(AD). 97. DENISON, G. H. : Ind. Eng. Chem., 36, 1964,477. 98. DEVINE, M. J. - SANDER, L. F. : Amer. Chem. Soc.Meeting. San Francisco, April 1968. 99. STEWART, W. T. - STUART, F. A. : Advances in Petroleum Chemistry and Refining. Vol. 7, New York, Interscience Publ. 1963. 1oo.cOOK, B. B. : Conference on Engine Oils. London 1969. I01.Internal Papers of Orobis Ltd. (1969). 102.DENISON, G. H. - CLAYTON, J. 0. : SAE Journal, 53, 1945,264. 103.KATEVA. J. - IVANOV, S. K. : Erdol u. Kohle, 26,1973,77. ~ ~ ~ . B A z IV. K :ARopa , a Uhlie, 15, 1973,315. IOS.SECHTER, J. H. eta]. : Maslorastvorimyje sulfonaty. Moscow, G'ITI 1963. IO~.RETZLOFF, J. B. - SNEED, R. B. : Conference on Fuels and Lubricants. New York, September 1971. 107.SMALHEER,C. V. - SMITH, R. K. : Lubricant Additives. Cleveland, The Lesins-ales Co. 1967. 108.U. S. Patent 2,426,340. 109.U. S. Patent 2,451,346. llO.U. S. Patent 2,616,924. 1 1 1.U. S. Patent 2,616,925. I12.U. S. Patent 2,585,520. 113.U. S. Patent 3,027,325. 114.U. S. Patent 3,006,925. I IS.WOOD,G. P. : J. Inst. Petrol., 55, 1969, 194. 116.U. S. Patent 2,362,289. 117.U. S. Patent 2,250,188. 118.U. S. Patent 2,252,663. 119.U. S. Patent 3,036,971. 120.U. S. Patent 2,316,079. 121.U. S. Patent 2,843,579. 122.KOSOLAFQFF,G. M. : Phosphates, Halophosphates and Thioanalogs, Organophosphorus Compounds. New York 1950. 123.KIRltENKOVA,s. eta]. : Ropa a Uhlie, 11, 1969, 649; 12, 1970,646; 13,1971, 181,289; Chem. Listy, 67, 1973, 31 1; In: Some Problems of Crude Oil Research and Processing. Slovak Technical University, Bratislava 1978, 81. 124.U. S. Patent 3,108,960;UK Patent 975,360. 125.U. S. Patent 3,139,729. 126.U. S. Patent 2,965,569. 127.U. S. Patent 2,252,662. 128.hUEV. T. N. et al. : Chim. Techno]. Topl. Masel, 12 (2), 1967, 16. 129.Unpublished Works of Lubrizol Co. (1962). I~O.WILLIS, R. W. - BALLARD, E. C. : SAE Trans., 63,1955,777. 131.U. S.Patent 3,018,250,3,172,892, 3,272,746 (Shell). HELM, J. : Testing of Crude Oil and its Products. Prague, SNTL 1957. 133.SCHILLINC3, A. : Les huiles pour moteurs et le graissage des moteurs. I. Park, Editions Technip 1975. 134.COLEMAN,L. E. : Petroleum Additives. Encyclopaedia of Polymer Science and Technology. New York 1968. ~ ~ ~ . S T ~V.P -IMAmfCEK, NA, M. : COMECON Symposium on Mechanism of Additive Effects. Moscow 1973. 136.ScoTr,G. : Atmospheric Oxidation and Antioxidants. Amsterdam, Elsevier 1965. I ~ ~ . K E N N E R L YG. , W. - PATERSON,W. L. Jr. : Ind. Eng. Chem., 48, 1956, 1917. 138.Unpublished Works, Res. lnst. of Benzina. Prague 1972.

401

I39.Unpublished Works of the Slovak Technical University and Slovnaft Works. Bratislava 1972. 140.DEBUAN. F. : Mineraloltechnik, 18, 1973, 11. I~I.STEPINA, V. et aI. : Ropa a Uhlie, 14, 1972,565. 142.FORBES. E. S. - WOOD,J. M. : Congress on Surface-Active Substances. Barcelona, September 1968. 143.UnpublishedWorks of E. Cooper Co. (1969). 144.FORBES, E. S. - WOOD,J. M. : Symposium on Engine Oils. London, April 1969. Slovak Technical University, Bratislava 1974. ~~~.MIN J. c: Thesis. A, 146.zASLAVSKIJ. J. S. et al. : Chim. Technol. Topl. Masel, 7 (8). 1962, 58. J. C: Proceedings of Electrotechnical Faculty. Slovak Technical University, Bratislava, ~~~.MIN A, September 1974. I48.BERNELIN, B. : Rev. h t . Fr. Petrole, 10, 1959, 1295. 149.BRIANT, J. : Note aux Comptes Rendus, 246, 1958, 3634. 1. F. et al. : Chim. Technol. Topl. Masel, 16 ( I ) , 1971, 37. ~50.BL~GovrDov, J. S. - SOR,G. 1. : Chim. Technol. Topl. Masel, 6, 1, 1961, 52. I~I.ZASLAVSKIJ. 152.30~,G. I. et al. : Chim. Technol. Topl. Masel, 11 (1 I), 1966, 38. 1 5 3 . h ~Jr.~ H.~ et~ al. . : Neftechimija, 5, 1971, 787. I~~.KNISPEL, B. - JAGER,G. : Schmierungstechnik,2, 1971,3,75, 83; KNISPEL, B. : Schmierungstechnik,8, 1977.4, 124. 155.30~,G. I. et al. : Chim. Technol. Topl. Masel, 7 (8), 1962, 58; 16 (6), 1971, 26. ~ ~ ~ . K RS. E E. J Net, al. : Chim. Technol. Topl. Masel, 16 (6), 1971, 31. 1. : Ropa a Uhlie, 17, 1975, 12. 157.MINCA, J. - NEPRAS, 158.U. S. Patent 2,962,443. 159.U. S. Patent 2,884,379. 160.U. S . Patent 3,088,910. 161.U. S. Patent 2,371,851. BAKER, H. R. et al. : Ind. Eng. Chem., 40, 1948,2338; 41, 1949, 137. 163.Pamphletsof R. T. Vanderbilt Co. C. E. : Lubric. Eng.. 4, 1948, 125. 164.GUITENPLAN,J. D.- PRUITON, T. w.: Petrol. Pr0C., 7, 1952, 1780. 165.SMALHEER. c . v. - MASTIN, 166.Losi~ov,B. V. : Neft. Choz.. 1954 (5). 61. 167.Losr~ov,B. V. et al. : Osnovy primenenija nefteproduktov. Moscow, G'lTI 1959. 168.Losi~ov,B. V. et al. : Neft. Choz., 1953 (4), 53. 169.U. S . Patent 2,084,270. 170.U. S. Patent 2,252,675. I~~.MUSGRAVE, F. F. : Iron Steel Eng., 20, 5 , 1943, 4. 172.3TEPINA. v. : Ropa a Uhlie, 19, 1977, 507. 173.U. S. Patent 2,537,297. 174.U. S . Patent 2,379,453. I75.THOMAS, R. M. et al. : lnd. Eng. Chem., 32, 1940,299. 176.U. S. Patent 2,125,885. 177.U. S. Patent 2,486,493. 178.U. S . Patent 2,604,453. 179.U. S . Patent 2,936,300. 180.U. S. Patent 3,533,947. 181.ZLAMAL, Z. - KAZDA,A. : Ropa a Uhlie, 1, 1959, 37. 182.Auos. R. : Z. Anal. Chem., 236, 1968,350. 183.Publicationof Shell Chemicals Co., 1977. 184.OKRENT. E. A. : ASLETrans., 4, 1961,97; 251.7, 1964, 147. 185.SCHODEL, U. et al: ErdoI u. Kohle, 25, 1972, 261. I86.NEUDORFL. P. : Internal Publication of Rohm Co., 1974. 187.MARCIANTE. A. - BUSEITO,G . : European Automotive Symposium. Roma 1974. 188.ShtrrH. M. F. Jr. et al. : Conference on Fuels and Lubricants. Tulsa, Oct. - Nov. 1972.

402

189.BRAMHALL, A. D. - WRIGHT,B. : Performance Testing of Lubricants for Automotive Engines and Transmissions. Ed. McCue, C. F. et al., Barking, Applied Science Publ. 1974,487. 190.LONSTRUP, T. F. - SEQUEIRA, J. S . : Performance Testing of Lubricants for Automotive Engines and Transmissions. Barking, Applied Science Publ. 1974,475. 191.Publicationsof Rohm and Rohm & Haas Co., 1973. 1 9 2 . H ~M. ~ . W. - MUNSELL, M. W. : Preprints Amer. Chem. SOC., Petrol. Div., 5, 1960, 115. 193.LORENSEN, L. E. : Preprints Amer. Chem. SOC.,Petrol. Div., 7, 1962, B61 B71. 194.lnternal Publ. of Optimol Oelwerke Co. (West Germany). 195.U. S. Patent 1,815,022. 196.U. S. Patent 2,191,498. 197.U. S. Patent 2,091,627. 198.U. S. Patent 2,655,479. 199.U. S. Patent 2,387,501. 200.U. S. Patent 2,666,746. 201.U. S. Patent 2,721,877. 202.U. S. Patent 2,721,878. 203.U. S. Patent 2,651,628. 204.U. S. Patent 2,895,915. 205.U. S . Patent 4,059,581. 206.UK Patent 856,434. G. et al. : Schmieningstechnik,4, 1973 ( I ) , 19. ~O~.JAGER, 208.MAMEDALIJEV, J. G. et al. : Z . prikl. Chim., 26, 1953, 854. 209.CERNOTUKOV, N. J. - KRWN,S . E. : Okisljaemost mineralnych masel. Moscow, G'ITI 1955. 210.BERCHER, P. : Emulsion Theory and Practice. New York 1965. al.J: ,Tenzidy (Tensides). Bratislava, Alfa 1977. ~ ~ ~ . B L A. A et ~E 21 2.Catalogue of Surface-Active Substances of Honeywill-Atlas Ltd. ~ ~ ~ . M O L ND.AetRal. , : Ropa a Uhlie, 17, 1975,270. 214.Oil Gas Journal, 1%5,44, 125. 215.lnd. Eng. Chem., 1966, 10. 216.Chem. Week, 97 (22), 1965, 52, 54. 217.BONER, C. J. : Gear and Transmission Lubricants. New York, Reinhold Publ. 1964. BOWDEN, DEN, F. P. : Ann. N. Y. Acad. Sci., 53,1951,805. 21 9.RUMPF. K. K. : Viscositat und Fliessvermogen v. Mehrbereichs-Motorenolen. Esslingen, Techn. Akademie 1978, Bericht 2. ~~~.DAw W.E :YInd. , Eng. Chem., 42, 1950, 1841. ~~~.DAw W.E :YJ. ,Inst. Petrol., 31. 1945, 73; 33, 1947,673. 222.UK Patent 132,839. 223.TINKEL, E. D. : J. Inst. Petrol., 34, 1948,743. 2 2 4 . B ~ 0. c ~et al. : Proc. Royal Soc., 177A, 1940, 103. BA BAR CROFT, F. T. - DANIEL, S. G. : ASMWASLE Conference on Lubrication, October 1964. 226.BORSOFF, V. N. : Bureau of Aeronautics Report, 1954 - 1955. HAM HA MILT ON, L. A. - WOODS,W. W. : Ind. Eng. Chem., 42, 1950,503. 228.U. S . Patent 2,824,839. 229.French Patent 851 237. ~ ~ O . K E LC. L YI. , : J. Inst. Petrol., 1928, 14. M. L. - SMITH,D. B. : SAE Journal Preprint, 1970.700 87 1. 23 1.LEWTHER, ~ ~ ~ . H A V I L M. A NL.Det, al. : SAE Journal Preprint, 1967,660 778. 233.U.S. Patent 2,511,731. 234.U.S. Patent 2,388,083. 235.U.S. Patent 2,645,613. 236.Bis~,J. M. : NLGI Spokesman, 1970, 3. GLUSING, H. : Mineraloltechnik,20, 1975, 3. ~ ~ ~ . H c K MJ.A- N ,

403

238.sMITH, G. R. et al. : SAE Paper, 1961,363A. 239.HAVILAND. M. L. : S A E Meeting. Detroit, Jan. 1963. 240.U.S. Patent 3,022,014. 241.U.S. Patent 3,156,652. 242.U.S. Patent 2,855,366. 243.U.S. Patent 2,913,415. 244.U.S. Patent 3,039,967. 245.U.S. Patent 3,083,162. PAP PA PAY, A. S. - STIMMING, P. N. : Performance Testing of Lubricants for Automotive Engines and Transmissions. Barking, Appl. Science Publ. 1974,769 247.BECHER, P. : Principles of Emulsion Technology. New York,Reinhold Publ. 1955. 248.MOLYNEUX, P. H. : Sci. Lubr., 3, 1963, 144. 249.BAtovs~Y~ M. : Thesis. Res. Inst. of Organic Technology, Bratislava 1975. 250,ARLIE. J. P. et al. : Inst. of Petroleum, 1975, IP-005 - IP-006. 25 I.VOLLMERT, B. : Fundamentals of Macromolecular Chemistry. Prague, Academia 1970. 252.NEUDORFL, P. : Viscoplex-Seminar ‘75. Publication of Rohm GmbH Co. 253.SCHt)DEL,U. : Viscoplex-Seminar ‘75. Publication of Rohm GmbH Co. 254.Publications of Orobis Co., London 1974 - 1976. 255.LILLYWHITE. J. et al. : Mineraloltechnik, 20, 1975,9. 256.PEARCE, A. F: SAE Paper, 1968,680 069. 257.BAR?z, W. J. - WISSUSSEK, D. : Performance Testing of Lubricants for Automotive Engines and Transmissions. Barking, Appl. Science Publ. 1974, 505. 258.BRANNEN, W. T. : Performance Testing of Lubricants for Automotive Engines and Transmissions. Barking, Appl. Science Publ. 1974, 783. 259.Various authors: In : Performance Testing of Lubricants for Automotive Engines and Transmissions. Barking, Appl. Science Publ. 1974. 260.DAwsoN, P. H. et al. : Preprints ACS, Div. Petrol. Chem., 4 (3), 1959, 45. 261.U.S. Patent 4,137,184 (1979). 262.KARLL. R. E. - PETERSON, J. V. : 6th World Petroleum Congress. Sect. VI, Frankfurt a. M. 1963, Paper 19. ~~~.BARBOCZKY, E. et al. : COMECON Symposium on Additives. HalldSaale 1976. 264.VIPPER, A. B. et al. : Neftechimija, 6, 1968,922. 265.BLAGovIDov, J. F. et al. : Chim. Technol. Topl. Masel, 13 (7), 1968,41. 266.ALHERS. W. - OVERBECK, J. : Coll. sci., 15, 1960,489. 267.GRIFFIN. W. C. : J. SOC.Cosmet. Chem., 1, 1949, 311. ~ ~ ~ . H E I L W EI.I LJ., : Preprint ACS, Div. Petrol. Chem., 4, 1965, 19. 269.Gu~c,R. A. et al. : Neftepererab. i Neftechim., 8, 1977,40. 270.RANNEY, M. W. : Lubricant Additives. London, Noyes Data Corp. 1973. 271.ANAND, 0. N. et a]. : Indian Inst. of Petroleum Publication,Dehra-Dun, IP 74-006. 272.ANAND. K. s. et al. : J. Inst. Petrol., 58, (564), 1972, 11. 273.FIsH, K. R. et al. : Lubric. Eng., 14 (2), 1958, 64. 274.BARTZ, W. J. : Tribology International,2, 1976, 13. PIK PIKE, W. C. : CEC-IGL 9th Meeting, 1976. 276.ARLIE, J. P. - DENIS,J. : Colloques Internationaux du CNRS. Brest 1974, No. 233. 277.U.S. Patents 3,254,025, 3,284,409. 278.U.S. Patents 3,449,362, 3,491,025. 279.U.S. Patents 3,697,574,3,704,308, 3,756,953,3,751.365. 280.GRISINA,0. N. et al. : Neftechimija, 10, 1970,297; 11, 1971,298. 281.STANTON, G. M. - BARRALL, E. M. : Preprint ACS, Div. Petrol. Chem., 14, (4), 1969, A59. 282.KuzMI~A,G. M. et al. :Neftechimija, 12, 1972. 112. 283.M1~cFENG,1. et al. : ASLE Trans., 6, 1963.60. 284.Row~,C. N. - DICKERT, J. J. : ASLE Trans., 10, 1964,85. 285.BOURDONCLE, B.- PARC, G . : Rev. Inst. Fr. P&role, 24,1969,1073.

404

286.KozA~.P. et al. : Proceedings of the Prague Inst. of Chemical Technology, D33, 1976, 367. H. : Schmierstoffe u. Maschinen-schmierung, 31, 1968, 25. 287.HOERDING D. - FISCHER, 288.COLCLOUGH, J. P. - CUNEON, J. 1. : J. Chem. SOC.,1964,4790. 289.AL~uM.K. G. et al. : Proc. Inst. Mech. Eng., 183, 1968-69, PI3, P7. 290.ZAMBERLIN, 1. et al. : Nafta (Zagreb), 20, 1969, 351. 291.LOESER, E. H. et al. : ASLE Trans., 1, 1958, 329. 292.FUREY, N. J. : ASLE Trans., 2, 1959,91. 293.DANVIK, E. 1. et al. : Soc. Petr. Eng. J., 17, (3), 1977, 184. 294.U.S. Patent 3,595,791. 295.GRJAZNOV. B. V. et al. : Trudy VNIINP, 14, 1976.99, 296.U.S. Patent 3,630,903. 297.ST!&NA, V. : Symposium on Mechanism of Additive Effects. Bratislava 1978. 298.U.S. Patents 3,547,821, 3,551,336, 3,598,738, 3,642,636. 299.FLORY, P. J. : Principles of Polymer Chemistry. Ithaca (N.Y.), Cornell Univ. Press 1953. 3Oo.sELBY, T. W. : ASLE Trans., 1, 1958.68. RAWLOWS LOW SKI, W. et al. : Przemysl Chem., 51, 1972,781. 302.LAZAR, M. - MIKULASOVA, D. : Synthesis and Properties of Macromolecular Substances. Bratislava, Alfa 1976. 303.Mti~LER,H. G. : Viscosity and Flow Properties of Multigrade Engine Oils. Esslingen, Techn. Akademie 1978, Paper 4. ~O~.HARNOY, A. : Rheolog. Acta, 16, 1977, 51. R. J. : Viscosity and Flow Properties of Multigrade Engine ~~~.STAMBO R.UL.G-HKOPKO, , Oils. Esslingen, Techn. Akademie 1978, Paper 16. G. D. : SAE Trans., 74, 1966, Paper 650 442. 3 0 6 . s ~ T. ~ W. ~ ~- .STAFFIN, 307.pLS~0,S. - BAXA,J. : In: Some Problems of Research and Processing of Crude Oils. Bratislava, Ed. Centre of the Slovak Technical Univ. 1978,29. BELL, E. W. et al. : J. Amer. Oil. Chem. SOC.,54, 1977, 259. ~O~.VLASAK, R. et al. : Occupational Medicine, 19.4, 1967, 167. 3 I0.Unpublished Works of the Petroleum Chemistry and Technology Dept., Slovak Technical Univ. Bratislava. 3 1 1. U S . Patent 3,634,249. 312.U.S. Patent 3,554,91I . 31 3.U.S. Patent 3,55 1,336. 314.U.S. Patent 3,598,738. 315.U.S. Patent 3,547,821. ~ ~ ~ . L O N G S T. T RF.UetPal. , : 10th World Petroleum Congress. Rev. Paper 10, Bucharest 1979. G.I L. et ,al. : 10th World Petroleum Congress. Rev. Paper 10, Bucharest 1979. ~~~.HART NG 318.WUNDERLICH, W. - JOSl, H. SAE Paper 780 732, 1978. 319.JENKINS, G . I. - HUMPHRI-YS, C. M. A. J. Inst. Of Petrol., 51, 1965, 493. 320.VERGOS. Th.- STEPINA, V. : Ropa a Uhlie, 23,1981,393. 321.MINAMITAN1, H.et al. : Msiuzen Oil Tech. Journal, 23, 1978, 54. 322.sLASTER. B. B. : SAE Paper, 750 866, 1975. Ch. K. : ASLE Trans., 14, 1970, I . ~ ~ ~ . G A L L O P ON. U LE.O- SMURPHY, , GAL ROUND^, F. G. : ASLE Trans., 18, 1973.79. 325.Fox, W. J. : Unpublished Works of Orobis Co., 1968. ~ ~ ~ . & % PV.I eta]. N A ,: Ropa a Uhlie, 23, 1981, 174. 327.U.S. Patents 4,137,184, 4,159,958. 328.U.S. Patents 3,932,289,4,104,180. 329.U.S. Patent 4,083,792. 330.RANNEY, M. W. : Synthetic Oils and Additives for Lubricants. Noyes Date Corp., Park Ridge 1980.

405

331.US. Patent 4,029,702. 332.U.S. Patent 4,153,507. 333.U.S. Patent 4,113,725. 334.U.S. Patent 4,070,370. 335.U.S. Patent 4,100,086. 336.U.S. Patent 4,083,791. 337.U.S. Patent 4,098,708. 338.U.S. Patent 4,090,971. 339.U.S. Patent 4,081,386. 340.U.S. Patent 4,153,563. 341.U.S. Patent 4,021,357. 342.U.S. Patent 4,136,047. 343.U.S. Patent 4,021,357. 344.U.S. Patent 4,070,295. 345.U.S. Patent 3,966,625. 346.U.S. Patent 3,979,308. 347.WUNDERLICH. W. : Spekbrum 23, Publication of Rohm & Co., 1981. 348.TkEBlCK*, v1. - STkPINA, V. : 23rd Annual Conf. on Petroleum. Bratislava 1981. 349.DmMANN. L. P. - MARSDEN,K. : International Symposium CEC. Rome 1981. 350.U.S. Patent 4,132,663. 351.U.S. Patent 4,142,865. 352.U.S. Patent 3,947,368. 353.MANG, T. : Wassermischhare Kuhlschmierstoffe fur Zerspannung, Kontakt und Studium, Bd. 61, Expert Verlag Grafenau, 1980 354.DIEHL, K. H. : ibid. 355.KELL6, V. - TKAC,A. : Fyziksllna chCmia (Physical Chemistry). Bratislava, Alfa 1977. D. : Friction - an Introduction to Tribology. Heinemann, London 356.BOWDEN, F. P. - TABOR, 1973. 357.CZICHOS, H. : Tribology. Amsterdam, Elsevier 1978. 358.TAYLOR, S. L. et al. : Appl. Microbiol., 20, 5, 1970,720. 359.JERMOLENK0, I. N.et al. : Z.M.E.J. 2, 1970, 107. 36O.So~oL,D. - STUMR, A. : Seminary on Safety and Hygienical Problems of Metalworking Emulsions. CS Society for Science and Technology, D e b 6 1981. 361.HuGEL, G. : Lubr. Eng., 14, 1958,523. ~~~.MING-FENG, 1. : Wear, 3, 1%0, 309. 363.so~.G. I. et al. : Eurotrib ‘81. Warszawa 1981. 364.ZASLAvSKIJ, Jr. S. : TribopolimerobrazujuE.smazof. materialy. Moscow, Nauka 1979. MIN BOW DEN, F. P. - TABOR, D. : The Friction and Lubrication of Solids. Oxford, Calendon Press, Vol. 1, 1950, Vol. 2, 1964. ~~~.BIEB R et , al. : ASLE Trans., 11, 1968, 155. H.EE. ~~~.GODFREY. D. : ASLE Trans., 5, 1%2,57. 368.A~LuM.K. G. et al. : J. Inst. Petrol., 51, 1965, 145; 53, 1967, 173; ASLE Trans., 11, 1968, 162. 369.BENNET, P. A. - KABEL,R. A. : Lubr. Eng., 19,1963,365. 370.WADDEY, E. w.et al. : SAE Paper 780599, 1978. ~ ~ ~ . K E P PR. L EK., - JOHNSON, M. F. : SAE Paper 790329, 1979. P. : PttrOk et Technique, 277, 1981,74. %!.ARMSTRONG, E. L. - BAUWIN, ~ ~ ~ . S C H A ? Z P. B E: Lubr. R G , Technol., Trans. ASME, 93,1971, Series F, 231. 374.BAUGHAM, R. A. : ASME Paper A235. 1958 (58). 375.FOWLER, P. A. et al. : ASLE Paper 79 22 4A2, ASLEIASME Lubr. Cfr., Dayton, Oct. 1979. 376.MuRP~y.W. R. : ASLE Paper A235, No. 58. 377.DONDOS, A. et al. :J. Chim. Phys., 64, 1967, 1012; Macromolecules, 4, 1971,479. 378.B~~Ez4, S.V. et al. : Vysokomol. sojed., A10,1968,2356.

406

379.SIRINOV. Ch. D. et al. : Izv. VUZ Neft. i Gaz, 8/10, 1965.75, 12/2, 1%9, 53. W. D. : J. Lubr. Technol., Trans. ASME, Series F, 90, 1968,580. 380.NOvAK. J. D. - WINER. F. G. : ASLE Trans., 24,198 1,431. 38 1 .ROUNDS, 382.KAPSA. Ph. et al. : Trans. ASME, 103, 10, 1981,486. 383.STEPINA. v . et a]. : Ropa a Uhlie, 24, 10, 1982, 596. 384.MAHONEY, L. R. et al. : Ind. Eng. Chem., Prod. Res. Dev., 17, 1978,250. 385.KORCEK. S. et al. : SAE Paper 7380955 (1978); 810014 (1981). 386.WILLERMET, P. A. et a]. : 1978 Annual ASLE Meeting, Paper No. 78-AM-18-1. 387.T~At,A. et al. : CS Patents 193 886,193 890, 193 892; Ropa a Uhlie, 22,1980,297. J. W. : Preprint ACS, Petrol. Div., 10 (2), 1965, 127; Ind. Eng. 388.DAv1s.T. G. - THOMPSON, Chem., Prod. Res. Dev., 5, 1966,76. 389.MAHONEY, L. R. - DAROOGE, M. A. : J. Amer. Chem. SOC.,89,1967,5619. 390.TKAt, A. et al. : 10th Annual Meeting of French-Czechoslovak Cooperation on Oxidative Ageing and Burning of Polymers. CS Society for Science and Technology, HlubokP, Oct. 19-23, 1976. 391.MAHONEY. L. R. : Angew. Chem., Intern., Ed., 8, 1969,547. 392.MAHONEY, L. R. et al. : Prepflnts ACS, Petrol. Div., 24 (3), 1979,802. 393.HORODYSKY, A. G. et al. : Preprints ACS, Petrol. Div., 27 (2), 1982,589. 394.GRIBAJLQ A. P. eta]. : cH?TM, 7, 1983, 18. 395.SIEBER. I. : Schmierungstechnik 10, 1983,299-306. 396.US International Trade Commission “Synthetic Organic Chemicals, US Production and Sales”. 397.PAULUS, W. : ErdoI u. Kohle, 37, 1984, 303. 398.KILLER, C. : Mineraloltechnik, 3, 1984.3. 399.ZAJMOVSKNA, T. A. et d.: Chim. Technol. Top]. Masel, (4), 1984, 38. 4W.FALBE, J. - HASSERODT, U. : Katalysatoren, Tenside und Mineraloladditive. Stuttgart, G. Thieme Verl. 1978. ~ O ~ . T R E B I C K YV. , et al. : Ropa a Uhlie, 29, 1987, 160. ~O~.WEBER, k.- RICHMAN,U. : Schmierungstechnik, 16, 1985, 51 403.Mur~,R. J. : NLGI Spokesman, 7,1988, 140. ~W.BOWMAN, L. D. - SAVAGE, M. W. : SAE Reprints, 1 IOB, 1960. STREI RE IT, G. - DUNSE, S. : Kautschuk and Gummi, Kunststoffe, 38, 1985,471. 406.KENNEDY, P. J. : ASLE Trans., 23, 1980.370. 407.LITmAN. W. E. : In : Approach to Lubrication of Concentrated Contacts. Ku, P. M.,(Ed.) Washington, NASA 1970,309. 408.S~.S. P. : Wear, 25, 1973, 111; et al. : Wear, 32, 1975. 33. 409.APPELWORN,J. R. : In : Approach to Lubrication of Concentrated Contacts. Ku, P. M., (Ed.) Washington, NASA 1970, 309. M. F. : SAE Paper 790 329, 1976. 410.KF.PPLER, R. K. - JOHNSON, 41 ~.VESELY, V. : Ropa a Uhlie, 24, 1982,99. 412.Chem. Week, 27 (6). 1979. 413.KLUCKS. P. - KEKENABY, J. : Ropa a Uhlie, 20,1978,520. 414.GAYLE, J. : J. Appl. Polymer Sci., 22, 1978, 1971. 415.U.S. Patent 4,151,173. 416.U.S. Patent 4,016,131. 417.AHLBORN. G. H. : In : Intern. Jahrbuch d. Tribologie 1982. Bartz, W. J., (Ed.), GrafenauHanover, Expert-Vincent2 Verl. 1982. 418.Lubrizol Newsline, Vol. 3, No. 1, (1985). ~ ~ ~ . V A L DJ.AetUal. F , : Ropa a Uhlie, 31,584 (1989). 4 2 0 . w ~S~. S~. .et a]. : Tribology Trans., 31, 381, (1988). 421.Lubrizol Newsline, Vol. 6, 3 (1988); U.S. Patent 1,565,627.

407