Aliphatic Polyamides

Chapter 18

Aliphatic Polyamides Professor Marianne Gilbert

18.1 INTRODUCTION There are a very wide range of materialsdfibers, crystalline plastics, amorphous plastics, adhesives, and rubbersdwhich are classified as polyamides. They have the common feature that the amide (eCONHe) group occurs repeatedly in the polymer. This chapter considers aliphatic polyamides. Various aliphatic/aromatic compositions for polyamides are shown in Table 21.4. Those known as polyphthalamides that contain aromatic groups are discussed in Section 21.2. Fully aromatic polyamides (e.g., Kevlar, Nomex, and Vectran) have liquid crystal structures, and are discussed in Section 21.9.2. Polymers with some structural similarities, the polyimides, and polyamide-imides are also covered in Chapter 21. In the past, the bulk of polyamide materials have been used in the form of fibers, but their use in engineering and other applications has increased to about 50%; it is anticipated that less than 50% of the polyamides produced will be used in fibers and textiles by 2020 (see Table 18.1). Polyamides are now very important engineering plastics. The fiber-forming polyamides and their immediate chemical derivatives and copolymers were often referred to as nylons. This was a generic name introduced when DuPont first produced “nylon 66.” (It has been suggested that this name was derived from a combination of New York and LONdon.) However, the term polyamide is now preferred. The early development of the polyamides was largely due to the work of W. H. Carothers and his colleagues, who first synthesized polyamide 66 (PA66) in 1935 after extensive and classical researches into condensation polymerization. Commercial production of this polymer for subsequent conversion into fibers was commenced by the DuPont Company in December 1939. The first polyamide moldings were produced in 1941, but the polymer did not become well known in this form until about 1950. In an attempt to circumvent the DuPont patents, German chemists investigated a wide range of synthetic fiber-forming polymers in the late 1930s. This work resulted in the successful introduction of polyamide 6 (PA6) (and incidentally in the

TABLE 18.1 Polyamide Market Size by Application, 2013e2020 (kt) (Momin, 2015) Application

2013

2014

2015

2020 (Predicted)

Fibers and textiles

3888.2 (52%)

3967.2 (51.5%)

4058.3 (50.5%)

4527.6 (46.5%)

Automotive

1283.2 (17%)

1346.5 (17.5%)

1431.0 (18%)

1951.3 (20%)

Electricals and electronics

842.5 (11%)

880.8 (11.5%)

921.3 (11.5%)

1181.5 (12%)

Film and coating

496.9 (7%)

521.9 (7%)

549.6 (7%)

716.1 (7%)

Industrial/machinery

411.7 (5.5%)

427.3 (5.5%)

444.6 (5.5%)

550.5 (6%)

Consumer goods and appliances

364.1 (5%)

383.7 (5%)

405.2 (5%)

541.4 (5.5%)

Others

176.2 (2.5%)

187.0 (2%)

199.3 (2.5%)

281.0 (3%)

Total

7462.8

7714.2

8009.2

9749.4

Brydson’s Plastics Materials. http://dx.doi.org/10.1016/B978-0-323-35824-8.00018-9 Copyright © 2017 Elsevier Ltd. All rights reserved.

487

488 Brydson’s Plastics Materials

evolution of the polyurethanes), and today PA66 and PA6 account for nearly all of the polyamides produced for fiber applications, and the majority of the plastics applications. Very many other aliphatic polyamides have been prepared in the laboratory, and a few have become of specialized interest as plastics materials including PAs 11, 12, 46, 610, 612, and copolymers such as 66/610 and 66/610/6. DuPont remains a major supplier of polyamides, marketed under the trade name Zytel. Of the many possible methods for preparing linear polyamides (i)e(iii) below are of commercial importance for the production of aliphatic polyamides: i. The reaction of diamines with dicarboxylic acids, via a “polyamide salt” (Figure 18.1). R and R1 may both be aliphatic, producing an aliphatic polyamide; if R is aromatic (e.g., in terephthalic or isophthalic acid), a polyphthalamide is produced (see Section 21.2). COO

nHOOC • R • COOH + nH2NR1NH2

H3N

n R

R1

HOOC OCR CONH R 1 NH

n

NH2

+ 2n H2O

FIGURE 18.1 Reaction of diamines with dicarboxylic acid.

ii. Self-condensation of an u-amino acid (Figure 18.2). nNH2R COOH

NHRCO n

+ nH2O

FIGURE 18.2 Self-condensation of u-amino acid.

iii. Opening of a lactam ring (Figure 18.3). NH nR

NHRCO n

CO FIGURE 18.3

Opening of a lactam ring.

iv. The reaction of diamines with diacid chlorides. This reaction is used to produce poly(p-phenylene terephthalamide) (better known as Kevlar, see Section 21.9.2). An example of route (i) is given in the preparation of PA66, which is made by reaction of hexamethylenediamine with adipic acid. The first “6” indicates the number of carbon atoms in the diamine and the second, the number of carbon atoms in the acid. Thus, as a further example, PA610 is made by reacting hexamethylenediamine with sebacic acid (HOOC(CH2)8COOH). (In this context, the numbers 10,11, and 12 are considered as single numbers.) Where the material is denoted by a single number, for example, PA6 and PA11, preparation from either an u-amino acid or a lactam is indicated. The polymer PA66/610 (60:40) indicates a copolymer using 60 parts of PA66 salt with 40 parts of PA610 salt. An interesting example of a polymerization where routes (i) and (iii) are combined is in the preparation of the glassy polyphthalamide Grilamid TR55 (Section 21.2). Isophthalic acid (a) and the diamine bis-(4-amino-3-methylcyclohexyl)methane (b) combine to form a “polyamide-type salt,” which reacts with lauryl lactam (c) after ring opening (Figure 18.4).

COOH

CO (CH2)10 NH (III)

COOH

(IV)

CH3 H2N

CH3 CH2

(V)

FIGURE 18.4 Monomers used to produce Grilamid TR55.

NH2

Aliphatic Polyamides Chapter | 18

489

18.2 INTERMEDIATES FOR ALIPHATIC POLYAMIDES 18.2.1 Adipic Acid It is possible to produce adipic acid by a variety of methods from such diverse starting points as benzene, acetylene, and waste agricultural products. In practice, however, benzene is the favored starting point, and some of the more important routes for this material are illustrated in Figure 18.5. A typical route is via cyclohexane and cyclohexanol. To produce cyclohexane, benzene is subjected to continuous liquid phase hydrogenation at a pressure of 2.34 MPa and temperature of 210
C using a Raney nickel catalyst. After cooling and separation of the catalyst, the produce is fed to the cyclohexane store. In the next stage of the operation, cyclohexane is preheated and continuously oxidized in the liquid phase by air using a trace of cobalt naphthenate as catalyst. This gives about 70% yield of a mixture of cyclohexanol and cyclohexanone with a small quantity of adipic acid. The cyclohexanolecyclohexanone mixture is converted into adipic acid by continuous oxidation with 50% HNO3 at about 75
C using a copper-ammonium vanadate catalyst. The adipic acid is carefully purified by subjection to such processes as steam distillation and crystallization. The pure material has a melting point of 151
C.

18.2.2 Hexamethylenediamine Hexamethylenediamine may be conveniently prepared from adipic acid via adiponitrile:

HOOC(CH 2 )4 COOH → NC(CH 2 )4 CN → H2 N(CH 2 )6 NH2 In a typical process, adiponitrile is formed by the interaction of adipic acid and gaseous ammonia in the presence of a boron phosphate catalyst at 305e350
C. The adiponitrile is purified and then subjected to continuous hydrogenation at 130
C and 28 MPa pressure in the presence of excess ammonia and a cobalt catalyst. By-products such as hexamethyleneimine are formed but the quantity produced is minimized by the use of excess ammonia. Pure hexamethylenediamine (boiling point at 1.9 kPa pressure, 90e92
C, melting point 39
C) is obtained by distillation.

18.2.3 Sebacic Acid and Azelaic Acid Sebacic acid is normally made from castor oil, which is essentially glyceryl ricinoleate. The castor oil is treated with caustic soda at high temperature, for example 250
C, so that saponification, leading to the formation of ricinoleic acid, is followed by a reaction giving sebacic acid and octan-2-ol:

Castor oil

NaOH

glycerol + CH3(CH2)5CH(OH)CH2CH=CH(CH2)7COOH

NaOH + H2O

CH3(CH2)5CH(OH)CH3 + HOOC(CH2)8COOH + H2 Because of the by-products formed, the yield of sebacic acid is necessarily low, and in practice, yields of 50e55% (based on the castor oil) are considered to be good. Sebacic acid may also be produced by an electrooxidation process developed by Asahi Chemical Industry in Japan (Yamataka et al., 1979), and also piloted by BASF in Germany. It produces high purity sebacic acid from readily available adipic acid. The process consists of three steps. Adipic acid is partially esterified to the monomethyl adipate. Electrolysis of the potassium salt of monomethyl adipate in a mixture of methanol and water gives dimethyl sebacate. The last step is the hydrolysis of dimethyl sebacate to sebacic acid. Overall yields are reported to be about 85% (Castor Oil, 2015). Sebacic acid is used for PA610.

O

OH

OH HOOC(CH2)4COOH FIGURE 18.5 Routes to the formation of adipic acid.

490 Brydson’s Plastics Materials

NOSO3H

NOH

+ H2SO4 + SO3 NOSO3H CO + 2NH2OH

(CH2)5

+ (NH4)2SO4 + H2O NH

FIGURE 18.6 Production of caprolactam from cyclohexanone oxime.

18.2.4 Caprolactam Caprolactam (CL) is preferred to u-aminocaproic acid for the manufacture of PA6 because it is easier to make and to purify. Over the years, many routes for the manufacture of CL itself have been developed. Of these routes, the bulk of manufacture is via cyclohexanone and cyclohexanone oxime. Cyclohexanone is normally prepared either from phenol or from cyclohexane as shown in Figure 18.5. The alternative route involves the air oxidation of cyclohexane and proceeds via the production of a mixture of cyclohexanol and cyclohexanone often known as KA oil. It was in the cyclohexane oxidation section of the CL plant of Nypro Ltd that the huge explosion occurred at Flixborough, England in 1974. The conversion of cyclohexanone to cyclohexanone oxime is brought about by the use of hydroxylamine sulfate, (NH2OH),½H2SO4. The sulfuric acid is neutralized with ammonia to ammonium sulfate and this is separated from the oxime. In the presence of oleum, the oxime undergoes the process known as the Beckmann rearrangement to yield the crude CL. After further neutralization with ammonia, the CL and further ammonium sulfate are separated by solvent extraction (Figure 18.6). In one process, the resulting solution is continuously withdrawn and cooled rapidly to below 75
C to prevent hydrolysis and then further cooled before being neutralized with ammonia. After phase separation, the oil phase is then treated with trichlorethylene to extract the CL, which is then steam distilled. Pure CL has a boiling point of 120
C at 0.0013 MPa pressure. In the above process 5.1 tons of ammonium sulfate are produced as a by-product per ton of CL. Developments have been carried out to reduce or eliminate this by-product. One route which eliminates the production of ammonium sulfate, at the expense of complicated purification processes, is the photonitrozation process involving nitrosyl chloride and is used by Toray in Japan. In the mid-1990s efforts were made to manufacture CL from butadiene or adiponitrile. DSM, working initially with DuPont and later with Shell, developed a process called Altam using butadiene and carbon monoxide feedstocks to make CL without ammonium sulfate production. DSM claims cost reductions of 25e30%, simplified plant operations, and lower energy consumption. In the late 1990s, BASF and DuPont investigated the feasibility of investing in a process in China converting butadiene to adiponitrile/hexamethylene diamine (HMDA)/caprolactone (ICIS, 2007). Rhodia, meanwhile, developed its own alternative approach to CL manufacture called Capucine. Both the BASF and Rhodia processes involve the hydrogenation of adiponitrile to make 6-aminocapronitrile with an HMDA coproduct, using different operating conditions and catalysts. Adiponitrile can be manufactured from butadiene and hydrogen cyanide, and by electrolysis from acrylonitrile. A more recent approach, originally developed by EniChem (now Syndial) and commercialized by Sumitomo in Japan (2003), completely eliminates the production of ammonium sulfate. The chemical reaction in this case is a so-called ammoximation reaction, whereby cyclohexane is reacted with ammonia and hydrogen peroxide at around 90
C in the presence of a titanosilicate catalyst.

18.2.5 u-Aminoundecanoic Acid The starting point for this amino acid, from which PA11 is obtained, is the vegetable product castor oil, composed largely of the triglyceride of ricinoleic acid, or ricinoleic acid itself. Castor oil is first subjected to treatment with methanol or ethanol to form the appropriate ricinoleic acid ester. Cracking of the ester at about 500
C leads to the formation of the undecylenic acid ester together with such products as heptyl alcohol, heptanoic acid, and heptaldehyde. Undecylenic acid may then be obtained by hydrolysis of the ester.

Aliphatic Polyamides Chapter | 18

491

Treatment of the acid by HBr in the presence of a peroxide leads to u-bromoundecanoic acid together with the 10-isomer, which is removed. Treatment of the u-bromo derivative with ammonia leads to u-aminoundecanoic acid, which has a melting point of 50
C.

Castor oil

CH3(CH2)5CH(OH)CH=CH(CH2)7COOR

CH2CH(CH2)8COOR + C6H13CHO

hydrolysis

pyrolysis ~ 500ºC

CH2=CH(CH2)8COOH NH3

CH2BrCH2(CH2)8COOH + CH3CHBr(CH2)8COOH

bromination

NH2(CH2)10COOH

18.2.6 Lauryl Lactam (Laurolactam, Dodecanelactam) PA12 first became available on a semicommercial scale in 1963. The monomer, dodecanelactam, is prepared from butadiene by a multistage reaction. In one process, butadiene is treated with a Ziegler-type catalyst system to yield the cyclic trimer, cyclododeca-1, 5, 9-triene. This may then be hydrogenated to give cyclododecane, which is then subjected to direct air oxidation to give a mixture of cyclododecanol and cyclododecanone. Treatment of the mixture with hydroxylamine yields the corresponding oxime, which on treatment with sulfuric acid rearranges to form the lactam (Figure 18.7). CH CH2 CH2 CH CH CH2

CH2

CH2 CH

CH2

CH

CH2

CH

CH2 H2

CH2

CH2

CH2

CH2

Air

CH CH2

CH2

CH2

CH2

CH

CH2 CH2

CH2

CH2

CH2

CO and

(CH2)10

(CH2)10

CH

C O

OH

CO NH2OH

CH2

(CH2)10

H2SO4

(CH2)11

NH

C NOH FIGURE 18.7 Preparation of lauryl lactam.

Other methods of producing cycloalkanone oximes are still being investigated, e.g. the use of a photochemical reaction of a cycloalkane with a photonitrosating agent in a liquid by light irradiation (Takahashi et al., 2014).

18.3 POLYMERIZATION OF ALIPHATIC POLYAMIDES As already indicated, polyamides are produced commercially by reacting diamines with dibasic acids, by self-condensation of an amino acid or by opening of a lactam ring. Whatever method is chosen, it is important that there should be an equivalence in the number of amine and acid groups for polymers of the highest molar mass to be obtained. In the case of the amino acids and lactams, this is ensured by the use of pure monomer, but when diamines and dibasic acids are used, it is necessary to form a salt to ensure such an equivalence. Small quantities of monofunctional compounds are often used to regulate molar mass.

492 Brydson’s Plastics Materials

18.3.1 Polyamides 46, 66, 610, and 612 The PA66 salt is prepared by reacting hexamethylenediamine and adipic acid in boiling methanol, the comparatively insoluble salt (melting point 190e191
C) precipitating out. A 60% aqueous solution of the salt is then run into a stainless steel autoclave together with a trace of acetic acid to limit the molar mass (9e15 kgmol1). The vessel is sealed and purged with oxygen-free nitrogen and the temperature is raised to about 220
C. A pressure of 1.7 MPa is developed. After 1e2 h, the temperature is raised to 270e280
C and steam bled off to maintain the pressure at 1.7 MPa. The pressure is then reduced to atmospheric for 1 h, after which the polymer is extruded, then granulated for melt processing, or melt spun to produce fibers. PA610 is prepared from the appropriate salt (melting point 170
C) produced from hexamethylenediamine and sebacic acid by a similar technique. As the latter is produced from castor oil, PA610 is a renewable sourced PA610 containing a minimum of 60% renewably sourced ingredient by weight. PA612 uses decane-1,10-dicarboxylic acid. PA46, introduced in the late 1970s as Stanyl by DSM, is prepared by reacting 1,4-diaminobutane with adipic acid.

18.3.2 Polyamide 6 Both batch and continuous processes have been used for the manufacture of PA6. In a typical batch process, the CL, water (which acts as a catalyst), and a molar mass regulator (e.g., acetic acid) are charged into the vessel and reacted under a nitrogen blanket at 250
C for about 12 h. Ring opening occurs as shown in Figure 18.3, and the product consists of about 90% high polymer and 10% low molar mass material such as the monomer. In order to achieve the best physical properties, the low molar mass materials may be removed by leaching and/or vacuum distillation. In the continuous process, the reactants are maintained in reservoirs, which continuously feed reaction columns kept at a temperature of about 250
C. The polymerization casting of PA6 in situ in the mold has been developed in recent years. Anionic polymerization is normally employed. The polymerization reaction consists of three steps, and reactions are illustrated in Figure 18.8: 1. Catalyst formationdThe catalyst sodium caprolactam (NaCL) is produced by the reaction between sodium hydride and CL. 2. InitiationdNaCL forms a complex with acetylcaprolactam (AcCL), which then reacts with CL monomer. 3. PropagationdPolymer chains start growing. A typical system (Wichterle et al., 1961; Neuhäusl, 1968) uses as a catalyst 0.1e1 mol% of AcCL and 0.15e0.50 mol % of the sodium salt of CL. The reaction temperature initially is normally between 140 and 180
C but during polymerization, this rises by about 50
C. Moldings up to one ton in weight are claimed to have been produced by these casting techniques. The possibility of using the anionic polymerization of CL in additive manufacturing was investigated by Khodabakhshi (2011). Reaction injection-molding techniques, developed primarily for polyurethanes (see Chapter 28), have also been adapted for PA6 in what must be considered as a variation of the polymerization casting technique.

18.3.3 Polyamide 11 This polymer may be prepared by stirring the molten u-aminoundecanoic acid at about 220
C. During condensation 0.4e0.6% of a 12-membered ring lactam may be formed by intramolecular condensation, but this is not normally removed since its presence has little effect on the properties of the polymer. Since u-aminoundecanoic acid is produced from castor oil, PA11 has excellent environmental credentials and is marketed as a polymer of 100% renewable origin (e.g., RilsanÒ PA11). In this context it is discussed further in Chapter 23.

18.3.4 Polyamide 12 The opening of the CL ring for PA6 involves an equilibrium reaction, which is easily catalyzed by water. In the case of PA12 from capryl lactam, higher temperatures (i.e., above 260
C), are necessary for opening the ring structures, but since in this case the condensation is not an equilibrium reaction, the process will yield almost 100% of high polymer (Aélion, 1962).

Aliphatic Polyamides Chapter | 18

493

I Catalyst formation CO (CH2)5

NaH

(CH2)5 – CO + H2

NH

N–Na+

II Activated intiation with initiator CO (CH2)5

CH3CON–(CH2)5CON

(CH2)5 – CO

+ NCOCH3

(CH2)5

N–

OC CO

CH3CON–(CH2)5CON (CH2)5

(CH2)5

+

NH

OC

CO

CH3CONH(CH2)5CON (CH2)5

+

(CH2)5 N–

OC III Propagation (CH2)5

CO +

CH3CO[NH(CH2)5CO]1N

(CH2)5 N–

CO (CH2)5 CH3CO[NH(CH2)5CO]1N–(CH2)5CON CO (CH2)5 CH3CO[NH(CH2)5CO]1

N–(CH

CO +

2)5CON

(CH2)5

CO (CH2)5

CO +

CH3CO[NH(CH2)5CO]2N

NH

(CH2)5

CO

etc. N–

FIGURE 18.8 Anionic polymerization of caprolactam.

18.3.5 Copolymers It should be noted that using the types of intermediate described above, the preparation of copolymers is relatively straightforward. Commercial materials available include aliphatic/aliphatic (e.g., 66/6), aliphatic/aromatic (e.g., 66/6T, where T represents terephthalic acid), and aromatic/aromatic (e.g., 6I/6T, where I represents isophthalic acid.).

18.4 STRUCTURE OF ALIPHATIC POLYAMIDES Aliphatic polyamides such as PA46, PA66, PA6, PA610, and PA11 are linear polymers and thus thermoplastic. Because they are generally made using step reaction polymerization, molar masses of polyamides are relatively low, with number average molar mass in the range 12e20 kgmol1. (PA6 produced by anionic polymerization is an exception, when molar masses of 40e100 kgmol1 can be produced, depending on polymerization temperature (Van Rijswijk et al., 2006)). Different molar mass grades are characterized by viscosity number VN (ISO 307), defined as   h 1 18.1 1 VN ¼ hr c

494 Brydson’s Plastics Materials

(a)

(b) N-H

O C

O C C

C

C

C

C

C

C

C

C

C

H-N C C

C

C C

C

C N-H

N-H N-H

C

C C

C

C

C C

C

C

C O

C

C

C

C C O

O C C

C

H-N

H-N C

C

C

C C O

C C

C C

N-H

O C

C

C C

O C

Polyamide 66

C Polyamide 6

FIGURE 18.9 Hydrogen bonded planes in (a) PA66 and (b) PA6.

where hhr is the relative viscosity of a solution of the polymer in a specified solvent and c is the solution concentration in gcm3. VN is in cm3g1, and typical values for PAs produced by step reaction polymerization range from 120 to 290. Backbone chains of aliphatic polyamides consist of small atoms, so planar zig-zag structures are formed when crystallization occurs. Molecules align themselves so that they are fully hydrogen bonded as shown in Figure 18.9 (Meares, 1965). The hydrogen-bonded planes pack together to form unit cells. It is seen from Figure 18.9 that the PA66 molecule is very symmetrical, while the PA6 molecule is less so, so the molecules have to be arranged in alternating directions in order to become fully aligned, and fully H-bonded. This explains the lower crystallization rate, and also the lower melting temperature of PA6 (see Section 4.3). It has been observed that polymers from intermediates with an even number of methylene groups have higher melting temperatures than similar polymers with an odd number of methylene groups. This is demonstrated clearly in Figure 18.10 (Coffman et al., 1947), where it is seen that PA66 has a higher melting temperature than either PA56 or PA76.

POLYAMIDE MEETING POINT IN °C

280 260 240 A 220 200 180 160

B

4

10 6 8 12 NUMBER-OF CARBON ATOMS IN DIAMINE CHAIN

FIGURE 18.10 Melting temperatures of polyamides from aliphatic diamines: A with adipic acid and B with sebacic acid.

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495

With polymers from amino acids or lactams, the same rule applies, PA7 having a higher melting temperature (w227
C) than either PA6 (w215
C) or PA8 (w180
C) (see also Figure 4.9). These differences are due to the differences in the crystal structure of polymers with odd and even methylene groups that develop in order that oxygen atoms in one molecule are adjacent to amino groups of a second molecule, enabling hydrogen bonding. Hydrogen bonds with an NHeO distance of 0.28 nm are produced and are the reason for the high melting temperatures of polyamides such as PA6, 66, and 7. The crystal structures of the polyamides differ according to the type of polymer and in some cases, such as with PA66, two crystal forms can coexist. These structures have been discussed in detail elsewhere (Hill, 1953; Holmes et al., 1955; Geil, 1963; Mandelkern, 1964).

18.5 PROPERTIES OF ALIPHATIC POLYAMIDES 18.5.1 Introduction There are a number of structural variables that can considerably affect the properties of the aliphatic polyamides: 1. The distance between the eCONHe groups: It is the presence of the eCONHe groups that causes the aliphatic polyamides to differ from polyethylene; and the higher their concentration, the greater the difference. As a rule, the higher the amide group concentration (i.e., the shorter the distance between eCONHe groups), the higher the density and the forces required to mechanically separate the polymer molecules. This results in a higher tensile strength, rigidity, hardness, and resistance to creep, Tm, and heat deflection temperature, and resistance to hydrocarbons. PA11 has twice the distance between amide groups than PA6, and consequently is intermediate in properties between PA6 and polyethylene. 2. Molar mass: Increasing molar mass increases melt viscosity, the more viscous grades being more suitable for processing by extrusion techniques. 3. N-Substitution: Replacement of the hydrogen atom in the eCONHe group by such groups as eCH3 and eCH2OCH3 will cause a reduction in the interchain attraction and a consequent decrease in softening temperature. This is considered in more detail in Section 18.10. 4. Copolymerization: Except in those rare cases where monomer segments are isomorphous (e.g., 66 and 6T polyamides, which cocrystallize into one crystal structure), copolymerization, as usual, leads to less crystalline and frequently amorphous materials. As might be expected, these materials are tough, leather-like, flexible and, when unfilled, reasonably transparent.

18.5.2 Transition Temperatures and Crystallinity The high intermolecular attraction caused by H-bonding leads to polymers of high melting temperature. However, above the melting temperature, the melt viscosity is low because of the relatively low molar mass of these polymers. As is commonly the case with crystalline polymers, the glass transition temperature is of only secondary significance for aliphatic polyamide homopolymers. There is even considerable uncertainty as to the numerical values. Rigorously dried polymers appear to have Tgs of about 50
C, these figures dropping toward 0
C as water is absorbed. It has also been shown (Boyer, 1963) that polyamides possess a b-transition at about 130
C due to rotation in (eCH2e)n sequences. At room temperature, PA66 containing the usual amount of absorbed water appears to be slightly above the Tg, and crystallization may occur only very slowly. This can lead to after-shrinkage effects, which may occur for periods up to 2 years. With PA6 that has a lower crystallization rate, the effect is less marked. The after-shrinkage process may be accelerated by annealing the samples at an elevated temperature, typically that which corresponds to the maximum crystallization rate for that polymer (see also Section 4.3). The properties of the polyamides are considerably affected by the amount of crystallinity. While in some polymers (e.g., the polyacetals and PCTFE) processing conditions have only a minor influence on crystallinity, in the case of some of the polyamides, the crystallinity of a given polymer may vary by as much as 40%. Thus, a molding of PA6, slowly cooled and subsequently annealed, may be 50e60% crystalline, while rapidly cooled thin-wall moldings may be only 10% crystalline. As with other crystalline polymers, properties are dependent not only on total percentage crystallinity but also on the size of morphological structures such as spherulites. According to the method of processing, different morphological structures will be produced (Müller and Pfluger, 1959). Slowly cooled melts may form spherulites, whereas rapidly cooled polymers may form only fine aggregates. It follows that in an injection molding, the morphological form of rapidly cooled

496 Brydson’s Plastics Materials

surface layers may be quite different from that of the slower cooled centers. Smaller spherulites can be obtained by the use of nucleating agents as described in Section 7.9.6 and this can give a more uniform structure in an injection molding. Several years ago it was found that seeding the polymer with about 0.1% of a fine silica resulted in polyamides with greater tensile strength, hardness, and abrasion resistance but with some reduction in impact strength and elongation at break. Subsequent developments using phosphorus compounds as nucleating agents were found to give profoundly shortened molding cycles, in a typical instance down from 30 to 4 seconds. It was also found that overnucleation tended to give a crystalline surface layer with some undesirable properties, and the current aim is to produce a polymer, which on molding is of a two-phase structure, the bulk consisting of a uniform crystal structure with a very thin near-amorphous surface layer. The suppliers of PA46 have laid particular emphasis on the fact that this polymer, with its highly symmetrical chain structure and short repeat distance, leads to both a high level of crystallinity and a high rate of nucleation. In turn the high nucleation rate leads to a fine crystalline structure, which in this case is claimed to lead to a higher impact strength (dry, as molded) than for PA6 and PA66. The greater the degree of crystallinity, the less the water absorption and hence the less will be the effect of humidity on the properties of the polymer. The degree of crystallinity also has an effect on electrical and mechanical properties. In particular, high crystallinity leads to high abrasion resistance.

18.5.3 Chemical Resistance PA46, PA6, PA66, PA610, PA11, and PA12 are materials with exceptionally good resistance to swelling and dissolution in hydrocarbons, because of their high cohesive energy density, H-bonding, and crystallinity. Esters, alkyl halides, and glycols have little effect. Alcohols generally have some swelling action and may in fact dissolve some copolymers (e.g., PA66/610/6). There are few solvents for the polyamides, of which the most common are formic acid, glacial acetic acid, phenols, and cresols, which are capable of hydrogen bonding with the polymers and have a similar high solubility parameter. Mineral acids attack the polyamides, but the rate of attack depends on the type of polyamide and the nature and concentration of the acid. Nitric acid is generally active at all concentrations. The polyamides have very good resistance to alkalis at room temperature. Resistance to all chemicals is more limited at elevated temperatures. Due to their capacity for H-bonding, the water absorption of polyamides is relatively high, and increases as the concentration of hydrogen bonds increases, so is highest for PA46, and much lower for PA11 and PA12 (Table 18.2). Absorbed water effectively acts as a plasticizer for polyamides, reducing their Tg, thus increasing their impact resistance but reducing their modulus at room temperature as seen in Table 18.2. Factors affecting water absorption are shown in Figures 18.11 and 18.12. The polyamides are hygroscopic. Figure 18.11 shows how the equilibrium water absorption of different polyamides varies with humidity at room temperature. Figure 18.12 shows how the rate of moisture absorption is affected by the environmental conditions.

18.5.4 General Properties Typical properties of some commercial grades of polyamide are given in Table 18.2. The figures given in the table are obtained on moldings relatively free from orientation and tested under closely controlled conditions of temperature, testing rate, and humidity. Changes in these conditions or the use of additives may profoundly affect these properties, particularly impact resistance. The effects of temperature and humidity on tensile properties of PA66 are illustrated in Figure 18.13. Dried samples are brittle at temperatures below their glass transition temperatures of about 50
C; at higher temperatures, yield and failure strengths are decreased and elongation at break increases, behavior typical of most semicrystalline plastics (Figure 18.13(a)). However, when samples are conditioned in a moist atmosphere, modulus is reduced, and elongation is higher at lower temperatures (Figure 18.13(b)). These figures illustrate the plasticization effect of water, and show that PA66 has a measure of flexibility in spite of its high crystallinity under general conditions of service. Mechanical property data in Table 18.2 clearly show the effect of moisture on properties. Tensile strength and modulus are decreased, while impact resistance and elongation at both yield and break are increased. Modulus can be increased by the use of fillers, and toughened grades with higher impact resistance are available. Laboratory tests and experience during use have demonstrated that the polyamides have extremely good abrasion resistance. This may be further improved by addition of external lubricants and by processing under conditions which

TABLE 18.2 Properties of Unfilled Untoughened Polyamides (Materials Data Center)

Unit

Test Method

46 (Pentamid AHT H1)

Tensile stress at yield

MPa

ISO 527

100/55

82/55

80/e

64/e

62/52

40

41

Tensile stress at break

MPa

ISO 527

e

e

e

e

e

e

54

Elongation at break/yield

%

ISO 527

y8/25

b25/>50

y4.5/e b25/e

y5/e b >50/ e

y4.3/19 b35/>50

e

>10

Tension modulus

GPa

ISO 527

3.2/1.0

3.1/1.4

2.9/1.5

2.0/1.2

2.4/1.5

1.38

1.38

0.096

0.053

Property

66 (ZytelÒ 101)

6 (ZytelÒ 7301)

610 (ZytelÒ RSLC3060)

612 (ZytelÒ 158)

11 (RTP 200C)

12 (RTP 200F)

Mechanical Properties

1

ASTM D256

0.053/e

Impact strength Notched Izod 23
C

kJm

Notched Charpy 23
C

kJm2

ISO 179

10/35

5.5/15

6/e

6.8/e

4.2/8

kgm3

ASTM D792

1180

1140

1130

1090

1060

1040

1020

cm g

ISO 307

160

145

150

150

120

e

e

Water absorption(23 C/ 50%RH)

%

ISO 62

3.6

2.6

3.0

1.4

1.3

0.2

0.1

UL94

Rating

ISO 1210 (1.5/ 1.6 mm)

HB

V-2

HB

HB

HB

e

HB

Melting temperature




C

ISO 11357

285

262

221

223

218

221

224

Heat deflection temperature under load




C

ISO 75 1/e2 160

70

55

55

62

54.4

e

275

190

160

155

135

e

e

e

100

70

e

120

e

e

Physical Properties Density

3 1

Viscosity number


1.8 MPa 0.45 MPa Coefficient of linear expansion

6


10 1

C

ISO 11359 1/e2

Where data are shown as x/y, x refers to dry samples and y refers to conditioned samples (ambient temperature and RH).

Aliphatic Polyamides Chapter | 18

Thermal Properties

497

498 Brydson’s Plastics Materials

12 MOISTURE CONTENT AT EQUILIBRIUM IN %

Polyamide Polyamide 6/610/66

10

8

6

4

Polyamide 66 Polyamide 610

2 Polyamide II 0

50

100

RELATIVE HUMIDITY IN % FIGURE 18.11 Effect of relative humidity on the water absorption of the polyamides.

develop a highly crystalline hard surface: for example, by use of hot injection molds and by annealing in a nonoxidizing fluid at an elevated temperature (150e200
C for PA66). The polymers have fairly sharply defined melting temperatures and above Tm the homopolymers have low melt viscosities. Some thermal properties of the polyamides are given in Table 18.2. All figures quoted are for unfilled polyamides. Incorporation of glass fiber as a filler will considerably narrow the gap between the two values for deflection temperature obtained at different loadings (see Table 18.6). The high figure for the deflection temperature of PA46 is in part due to the high Tm consequent upon the small and even number of methylene groups between the eCONHe groups but also to the high levels of crystallinity, which have a high stiffening effect. The heat deflection temperature of PA46 depends on molding conditions, hence crystallinity. A lower figure of 150
C can be obtained when a mold temperature of 80
C is used. If the mold temperature is increased to 120
C, facilitating faster nucleation and crystallization, the deflection temperature can be as high as 170
C.

MOISTURE ADSORPTION IN %

10 8

IMMERSION IN BOILING WATER

IMMERSION IN WATER (ROOM TEMPERATURE)

6 STANDING IN AIR (100 % R.H.)

4

2 STANDING IN AIR (65 % R.H.)

0

1

10 TIME IN days

100

1,000

FIGURE 18.12 Effect of environmental conditions on rate of moisture absorption of PA66 (3.2-mm-thick specimens).

Aliphatic Polyamides Chapter | 18

499

FIGURE 18.13 Tensile curves for PA66 (ZytelÒ 101 NC010) (a) after drying and (b) after conditioning at atmospheric temperature and relative humidity. Reproduced with permission of Du Pont de Nemours.

18.5.5 Frictional Properties The coefficients of friction of the polyamides are somewhat higher than the acetal resins (Chapter 19). Results obtainable will depend on the method of measurement, but typical properties are given in Table 18.3. The effect of lubricants on the kinetic coefficient of friction of PA66 with similar surfaces is shown in Table 18.4. For bearing applications the upper working limits are determined by frictional heat built-up, this being related to the coefficient of friction under working conditions. A measure of the upper working limits of a material for this application is the maximum PV value (the product of load in MPa on the projected bearing area and the peripheral speed in ms1) which TABLE 18.3 Kinetic Coefficient of Friction of Polyamide 66 (Riley, 1958) Moving Surface

Stationary Surface Polyamide (Molded)

Polyamide (Machined Surface)

Mild Steel

Polyamide (molded)

0.63

0.52

0.31

Polyamide (machined surface)

0.45

0.46

0.33

Mild steel

0.41

0.41

0.6e1.0

500 Brydson’s Plastics Materials

TABLE 18.4 Effect of Lubricants on the Kinetic Coefficient of Friction of Polyamide 66 (Riley, 1958) Lubricant

Coefficient of Friction

None

0.46

Water

0.24

Liquid paraffin

0.13

Graphite

0.28

can be tolerated. Maximum PV values of 0.017e0.035 are suggested for continuous operation of unlubricated PA66 bearings while initially oiled polyamide bearings can be used intermittently at PV values of 0.28. Higher maximum PV values can be employed with continuously lubricated bearings (see also Chapter 19 for data on polyacetals).

18.5.6 Electrical Properties The polyamides are reasonably good electrical insulators at low temperatures and under conditions of low humidity but the insulation properties deteriorate as humidity and temperature increase. Because of the polar structure they are not good insulators for high-frequency work and since they absorb water they are also generally unsuitable under humid conditions. The effect of the amount of absorbed water on the volume resistivity of PA66 is shown in Figure 18.14. This effect is even greater with PA6 but markedly less with PA11. Some typical electrical properties of the polyamides are given in Table 18.5.

18.5.7 Effect of Weathering When in service indoors or otherwise protected from sunlight polyamides show no appreciable change of properties on aging at room temperature. Care should be taken in the use of the polymers when exposed to direct sunlight, particularly in film and filament applications, where embrittlement is liable to occur. Some improvement may be achieved if stabilized compounds are used. Continuous exposure to air at temperatures above 60
C will also cause surface discoloration and a lower impact strength for moldings. The useful life of a molding in service at 100
C will be of the order of only 4e 6 weeks. If the molding is immersed in oil, or otherwise shielded from oxygen, a considerably longer life-time may be expected. Heat-stabilized grades have markedly improved resistance.

VOLUME RESISTIVITY IN ohm cm

1015 1014 1013 1012 1011 1010 100

0

1

2

3

4

5

6

7

8

WATER CONTENT IN % W/W FIGURE 18.14 Effect of moisture content on the volume resistivity of PA66.

Aliphatic Polyamides Chapter | 18

501

TABLE 18.5 Electrical Properties of the Polyamidesa Property

Units

PA66

PA6

PA11

PA610

PA66/610/6 (40:30:30)

PA66/610 (35:65)

Volume resistivity

U m (dry)

>1017

>1017

e

>1017

e

e

Dielectric constant

U m 50% RH

10

e

e

10

10

1015

U m 65% RH

1014

e

1013e1014

e

e

e

10 Hz dry

3.6e6.0

3.6e6.0

e

3.7

e

e

103 Hz 65% RH

e

e

3.7

e

e

e

6

3.4

e

e

e

e

e

3

0.04

0.02e0.06

e

0.02

e

e

e

e

0.06

e

e

e

>100

>100

e

>100

e

e

3

10 Hz 50% RH Power factor

10 Hz dry 6

10 Hz 65% RH Dielectric strength

15

16

15

kVcm1 50% RH 25
C

a

The data on PA11 are from trade literature on Rilsan, those on the other polymers from information supplied by ICI.

18.6 ADDITIVES An extremely wide range of different polyamides is available on the market. By varying type of polyamide, using copolymers and blends, changing molar mass and incorporating additives, grades have been designed for many different applications and processing methods. For example, heat resisting grades, flame retardant grades, lubricated grades and toughened grades are available. Additives used in polyamides can be grouped as follows: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

heat stabilizers, light stabilizers, plasticizers, lubricants, reinforcing fillers, pigments, fungicides, nucleating agents, flame retarders, impact modifiers.

With the possible exception of pigments, which may be dry-blended by the processor, additives are incorporated by the manufacturers and only a limited amount of information about them is normally made available. It is important that all additives are stable at the high processing temperatures used for polyamides. Various heat stabilizers have been used; the hindered phenol N,N0 -hexamethylene bis[3-(3,5-di-t-butyl-4 -hydroxyphenyl)propionamide] is an example of one used currently (see Section 7.2.3). Light stabilizers include carbon black and various phenolic materials. Plasticizers are comparatively uncommon but plasticized grades are supplied by some manufacturers. Plasticizers lower the melting temperature and improve toughness and flexibility, particularly at low temperatures. For polyamides, very polar compounds are necessary. The two compounds which have been used most frequently (Kohan, 1995) are (1) N-butylbenzenesulfonamide and (2) 2-ethylhexyl-4-hydroxybenzoate, as shown in Figure 18.15. The effectiveness of a broad range of possible plasticizers was discussed in 2012 (Belous et al., 2012). Plasticized polyamide compositions are typically prepared by compounding the polyamide and other additives with the plasticizer in an extruder. A new method of introducing plasticizers, involving polymerizing monomers in the presence of plasticizer to produce particles of plasticized polyamide, melt-blending polyamide with one or more additives to produce particles then cube-blending particles of plasticized polyamide and particles of polyamide melt-blended with one or more additives (Fish and Mestemacher, 2005).

502 Brydson’s Plastics Materials

(b)

(a)

O S N H O

CH3

OH

O

O CH3

CH3 FIGURE 18.15 Typical plasticizers used in polyamides.

Self-lubricating grades are of particular value in some gear and bearing applications. Molybdenum disulfide and/or graphite added in quantities of 1e2% and silicone oil (2e5%) are used for this purpose (Section 7.5). Lubricants may also be used to enhance flow and mold release. Materials used are usually of low molar mass, contain a hydrocarbon component and an amide component, and are typified by ethylene bis(stearamide). Polyamide 6 crystallizes relatively slowly so the use of nucleating agents (Section 7.9.6) is beneficial. In addition to the fine silica discussed in Section 18.5.2, many other materials will have nucleating effects. Seeding with powdered PA66 which has a higher melting temperature than PA6 is particularly effective. As with many other plastics materials there have been substantial efforts to improve the resistance of polyamides to burning. Fire retardant additives for polyamides usually function by increasing charring. Bromine/antimony combinations have been used regularly. Halogen compounds synergized by zinc oxide or zinc borate have also been used. Halogen-free fire retardants include compounds containing red phosphorus which exhibit very good electrical insulation properties as well as improved flame resistance. They are, however, dark in color. Furthermore, grades of polyamides for electrical and electronic applications need to be phosphorus and halogen-free to conform to UL94-V0 requirements, and melamine cyanurate is used as a flame retardant in this instance with unreinforced grades. With glass-filled grades (see below) magnesium hydroxide may be used as a fire retardant but it is required in substantial quantities and is less effective than typical halogen and phosphorus-containing additives. Recently, phosphinates have been introduced (Horold, 2014), and the introduction of nanoclays is being investigated (Section 7.7.4). Toughened grades of polyamides are readily available. Impact modifiers used include butadiene-based block copolymers (SBS, SIS) and functionalized EPM or EPDM (Table 7.4). Their use is discussed in Section 7.6.2. A variety of particulate mineral fillers, typically used at levels of 20e40% have been used to enhance PA properties. Compounds available include those containing calcined clay, wollastonite, talc and chalks (see Chapter 8). As PAs are crystalline, these can modify polymer microstructure. In some cases, mechanical properties can be improved by the use of silane coupling agents. Fiber-reinforced polyamide compounds have become available in recent years and are dealt with in the next section.

18.7 FIBER-FILLED POLYAMIDES 18.7.1 Glass-Filled Polyamides There are a number of properties in which the thermoplastics show weaknesses when compared with metals. These include: 1. 2. 3. 4. 5. 6.

low rigidity and tensile strength; dimensional instability due to a high temperature coefficient of expansion and a high water absorption; low impact strength to fracture; low maximum service temperature; low creep resistance; low hardness and scratch resistance.

In an attempt to minimize these disadvantages glass-filled varieties of a number of thermoplastics have been successfully introduced. Of these the glass-filled polyamides are a very important group, and these in turn can be subgrouped

Aliphatic Polyamides Chapter | 18

503

into glass-fiber-filled grades and glass-bead-filled varieties. The glass reinforced polyamides constitute a significant proportion of the polyamide plastics market. The glass-fiber-filled types can be obtained in two ways. One route (the “long-glass” process) involves passing continuous lengths of glass fiber (as rovings) through a polymer melt or solution to produce a glass-reinforced strand that is chopped into pellets about 0.3 cm diameter and 6e12 mm in length. In this case the fibers will be parallel to the ‘long’ axis of the pellet. The alternative route involves blending a mixture of resin and glass fibers (w6 mm length) in an extruder. From 20e40% glass is commonly used, usually of the electrical grade (E-grade) and with diameter of about 10 mm. Before blending with the polyamide the glass fibers are often treated with a lubricant to improve mechanical handling of the roving, a coupling agent such as a silane to improve the resin-glass bond and some poly(vinyl acetate) resin to hold the filaments together as a strand. Such reinforcement leads to a substantial increase in tensile strength, modulus, hardness, creep resistance, ASTM deflection temperature under load and a sharply reduced coefficient of expansion. Typical figures are shown in Table 18.6. The glass-fiber polyamides have a resistance to creep at least three times as great as unfilled polymers. In the case of impact strength the situation is complex since unfilled polyamides tend to break showing tough fracture whereas the filled polymers break with a brittle fracture. On the other hand, the glass-filled polymers are less notch sensitive and in some tests and service conditions the glass-filled polyamides may prove the more satisfactory. As with other crystalline polymers, the incorporation of glass fibers narrows the gap between the heat deflection temperatures and the crystalline melting temperature. While most glass-fiber-reinforced polyamides were initially of the short-glass type, both short and long types are now available. The Verton materials, based on both PA6 and PA66 as well as a number of other thermoplastics, were originally introduced by ICI and are now marketed by Sabic are an example of long glass-fiber compounds. They have been claimed to have better impact resistance and higher stiffness than short-glass compounds, together with better impact resistance retention at low temperatures and stiffness retention at higher temperatures. A comparison of some properties of long-and short-fiber materials is given in Table 18.7. However, in view of the many other additives which are now incorporated in glass-fiber reinforced polyamides (see below), the effects of fiber length represent just one of the factors controlling product properties, and manufacturer’s data should be consulted. The presence of glass-fiber fillers can to some extent mask the differences between the two most widely used PA6 and PA66. For example, an advantage of unfilled PA66 in injection molding is that the high Tm leads to a high solidification temperature and shorter cycle times. However, in glass-filled grades the more rapid cooling and crystallization can lead to a poorer surface finish than obtained with corresponding PA6 compounds. It is also considered that abrasive wear on screws is greater with PA6. Water absorption decreases with increasing glass-fiber content at about the same rate with both PA66 and PA6. Additives used for unfilled polyamides are also used for glass-fiber-filled polyamides. Thus, again, a wide variety of grades are available including toughened grades and flame retardant grades. Mineral fillers are sometimes used in addition

TABLE 18.6 Comparison of Glass-Fiber-filled and Unfilled Polyamide 66 Property

ASTM Test

Glass Filled

Unfilled

kgm

D792

1380

1140

Tensile strength

MPa

D638

159

79

Elongation at break

%

D638

3e5

80e100

Flexural modulus

GPa

D790

8.0

3.0

24 h water absorption at saturation

%

D570

5.6

8.9

D696

28

99

Density

Coefficient of linear expansion

Units 3

6


10

1

C

Deflection temperature under load 0.45 MPa




C

D.648

254

200

1.8 MPa




C

D.648

245

75

504 Brydson’s Plastics Materials

TABLE 18.7 Comparison of Short- and Long-Fiber Glass-Reinforced Polyamide 66 (50% Fiber Loading) Property

ISO Test

Condition

Units

PA66 D 50% Short Fiber

PA66 D 50% Long Fiber

Tensile strength

527

D

MPa

200

230

C

MPa

155

165

Elongation

527

D

%

3

4

C

%

6

6

D

GPa

12

15.8

C

GPa

9.6

11.2

2

11

27

2

Flexural modulus

Izod impact

Deflection temp.

178

180

kJm

C

kJm

13

37

D




C

250

261

D




C

263

263

75

(1.8 MPa) Melting temperature

D

1218

D, dry as molded; C, conditioned according to ISO 1110, accelerated method: 70
C/62% RH.

to glass fibers, so a very wide range of compositions are produced. To produce self-lubricating grades, PTFE is sometimes blended into glass-filled compounds. Polyamides filled with glass beads were also introduced in the late 1960s. Grades filled with 40% of glass spheres have compressive strength some eightfold higher than those of unfilled polyamides as well as showing good improvement in heat distortion temperature, tensile strength and modulus. Compared with glass-fiber filled grades they are easier to process, have lower melt viscosity, uniform and predictable shrinkage and minimum warpage. They are also more isotropic in their mechanical properties. Some grades of PA66 currently available are, for example, TechnylÒ A 218 S30 is an injection molding grade supplied by Solvay which contains 30% glass beads, and is recommended for mechanical components which require a very good surface finish with low warpage, and good compression strength (bearing housings). Thermocomp RX06027 is supplied by SABIC IP is a UV stabilized easy molding grade; the glass bead content is not disclosed. Glass beads are also used in combination with glass fibers to optimize properties. Both fibers and sphere fillers tend to improve self-extinguishing characteristics.

18.7.2 Aramid-Fiber-Filled Polyamides Some polyamide suppliers provide grades containing aramid fibers (e.g., Kevlar and Nomex, see Section 21.9.2) as alternative reinforcements. ZytelÒ 70K20HSL BK284 is a heat stabilized injection molding grade of PA66 containing 20% Kevlar fibers, and claimed to have excellent wear resistance. RTP company produces a range of PA66 grades containing 10e20% aramid fiber, including some lubricated by PTFE. Luvocom 1-7139 is also a lubricated molding grade containing aramid fibers and PTFE marketed by Lehmann Voss.

18.7.3 Carbon-Fiber-Filled Polyamides and Related Products Many polyamide suppliers include carbon-fiber-filled grades in their portfolio. Both short (3e6 mm) and long (6e12 mm) fibers are used. Carbon fibers result in substantially increased strength and modulus, and can also increase thermal and electrical conductivity. The effect of glass and carbon fibers on modulus are illustrated in Figure 18.16. Carbon-fiber/polyamide powders are also available for producing parts by additive manufacturing using 3D printing, for example, WindformÒ XT 2.0 for selective laser sintering (SLS), produced by CRP Technology in Modena, Italy, and CarbonMide, produced by EOS GmbH - Electro Optical Systems, Munich, Germany. Stainless steel fibers (5e20%) have been used to provide electromagnetic/radiofrequency (EMI/RFI) shielding (RTP Co.EMI 260 series).

Aliphatic Polyamides Chapter | 18

505

Tensile modulus (MPa)

50 40 30 20 10 0 0

10

20

30

40

50

60

70

% Fiber Glass fiber (flame retardant)

Carbon fiber

(Series RTP 201 and RTP 281) FIGURE 18.16 Effect of fiber concentration on tensile modulus (RTP Co.): I catalyst formation, II activated initiation with initiator, and III propagation.

TABLE 18.8 Specialty Grades of Polyamides (RTP Company) Grade

Features

Composition Details

RTP 200 TC-C-21 FR

Thermally conductive, electrically conductive, flame retardant, UL94 V-0

e

RTP 200 TC-C-55

Thermally conductive, electrically conductive

e

RTP 200 TC- I -15 FR

Thermally conductive, electrically insulative, flame retardant, UL94 V-0

e

EMI 263

EMI attenuation/shielding

20% mineral filler

ESD A 280

Antistatic

Carbon fiber

PermaStatÒ 205

Permanently antistatic

Glass fiber

RTP 283 HEC

Electrically conductive EMI/RFI/ESD protection

Ni coated carbon fiber

18.7.4 Available Grades The wide variety of additives discussed above enables an enormous range of polyamide grades with different properties to be produced. RTP company alone offer more than 300 grades based on PA66. These include a wide range of specialty compounds, providing fire retardancy, ESD (electrostatic dissipation), EMI/RFI shielding and thermal conductivity, illustrated by examples shown in Table 18.8.

18.8 PROCESSING OF POLYAMIDES In the processing of polyamides consideration should be given to the following points: 1. 2. 3. 4. 5.

The The The The The

tendency of the material to absorb water. high melting temperature of the homopolymers. low melt viscosity of the homopolymers. tendency of the material to oxidize at high temperatures when oxygen is present. crystallinity of the solid polymer and hence the extensive shrinkage during cooling.

The above features are particularly marked with PA46, PA6, PA66, and PA610 and less marked with PA11 and PA12. Providing they are dry the copolymers may be processed in much the same way as conventional thermoplastics.

506 Brydson’s Plastics Materials

In the injection molding of PA66, for example, it is necessary that the granules be dry. The polymer is normally supplied in sealed containers but should be used within an hour of opening. If reworked polymer is being used, or granules have become otherwise damp, the polymer should be dried in an oven at about 70e90
C. Too high a temperature will oxidize the surface of the granules and result in inferior moldings. Injection molding cylinders should be free from dead spots and a temperature gradient along the cylinder is desirable. Feeding units may also be fitted with a recirculating hot air drying loop. Because many commercial polyamides have a relatively low melt viscosity at typical process melt temperatures and consequently, slight loss of material when the injection carriage is retracted (so-called “drooling”) may occur through normal injection nozzles, even when the screw is retracted axially (in order to reduce pressure ahead of the screw tip) at the end of the screw rotation phase. Several types of nozzle have been specially designed for use with polyamides and all function by sealing the end of the nozzle, at this stage in the injection cycle. Examples which have been used include spring-loaded needle valves, sliding side-closure nozzles or hydraulic nozzle valves activated at the appropriate stages of the molding cycle. Additional instrumentation, for example the incorporation of a nozzle pressure transducer, is a mechanism of obtaining real-time data to study these effects more thoroughly. As discussed previously crystalline polymers exhibit a higher molding shrinkage than that generally observed with amorphous polymers. With average molding conditions this is about 1.8% for PA66 but by increasing the injection pressure and the injection time the shrinkage may be halved. This is because a high initial mold cavity pressure is developed and a large part of the crystallization process will be complete before the cavity pressure has dropped to zero. The shrinkage will also be affected by the melt temperature, the mold temperature, the injection speed and the design of the mold as well as by the type of polyamide used. The polyamides, PA66 in particular, may also exhibit a certain amount of after-shrinkage. Further dimensional changes may occur as a result of molding stresses being relieved by the plasticizing effect of absorbed water. It is consequently often useful to anneal moldings in a non-oxidizing oil for about 20 min at a temperature 20
C higher than the maximum service temperature. Where this is not known a temperature of 170
C is suitable for PA66, with somewhat lower temperatures for the other polyamides. When dimensional accuracy is required in a specific application the effect of water absorption should also be considered. Manufacturers commonly supply data on their products showing how the dimensions change with ambient humidity. The particular features of the polyamides should also be taken into account in extrusion. Dry granules must be used unless a devolatilizing extruder is employed. Because of the sharp melting temperature resulting from the relatively low molar mass of polyamides, it is found appropriate to use a screw with a very short compression zone. Polymers of the lowest melt viscosity are to be avoided for extrusion since they are difficult to handle. Provision should be made to initiate cooling immediately the extrudate leaves the die. The polymerization casting process mentioned in Section 18.3.2 has been adapted to reaction injection molding (RIM), a process originally developed for polyurethanes. In this process the reacting ingredients are mixed together by impingement of jets of the materials in a small mixing chamber adjacent to the mold cavity into which the reacting material is then injected. Because of the low injection pressures much lower locking forces are possible than in conventional injection molding, making the process attractive for large area moldings. The first polyamides specifically developed for RIM were introduced by Monsanto in 1981 as Nyrim. They are block copolymers of a polyether (such as a poly(ethylene glycol), poly(propylene glycol) or polybutadiene containing hydroxyl groups) with CL. The reaction components comprise the polyether, CL, adipyl-bis-caprolactam as chain propagator and a CLemagnesium bromide complex as catalyst. The latter has to be protected against moisture, carbon dioxide and oxygen, and thus requires special care in handling. Other polyamide-RIM systems have been developed by Upjohn and Allied Fibers and Plastics. Special grades of polyamides are available for RIM processes. TECARIM is the Ensinger trade name for the product group of tough, highly resistant PA6-block copolymers manufactured using the reaction injection molding process from Nyrim. Unlike polyurethane-RIM processes, polyamide-RIM reactions are endothermic and require temperatures of 130e140
C. In contrast to the polyurethane-RIM systems, this enables thick wall parts to be made. Cycle times of 2e3 min are comparable to those for polyurethane-RIM.

18.9 APPLICATIONS OF POLYAMIDES Current global consumption of polyamides is about 8 Mt per annum as shown in Table 18.1. Recent figures show that the annual world production of PA6 is 4.3 Mt, and that of PA66 is 3.4 Mt (CIEC, 2016), so the production of the remaining

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aliphatic polyamides (mainly PA46, PA610, PA11, and PA12) is much lower. The latter group are primarily used as plastics and are usually restricted to applications where PA6 and PA66 are unsuitable. (In the 1990s it was estimated that in Western Europe, plastics usage was split as follows: PA6, 48%; PA66, 40%; PA11 and PA12, 10%; and the rest, 2%.) For historical reasons, usage of the various types varies from country to country. For example, in the United States and the United Kingdom, PA66 was the first to be developed and remains well entrenched, while for similar reasons PA6 is more dominant in Germany. The substantial market penetration of PA11 and PA12 in France also reflects long-standing French commercial activity with these types. The polyamides are one of the major engineering thermoplastics in volume terms. Table 18.1 shows that the major plastics application for polyamides is the automotive sector, followed by the electrical and electronic industries. Most products supplied to these industries will be injection molded; the same is true for the industrial/machinery and consumer goods, illustrating the predominance of injection molding for the processing of polyamides. There is extensive usage of the PA6, PA66, PA610, PA11, and PA12 in mechanical engineering. Well-known applications include gears, cams, bearings, bushes and valve seats. The polyamides have found steadily increasing application as plastics materials for specialty purposes where their toughness, rigidity, abrasion resistance, good hydrocarbon resistance and reasonable thermal stability are important. Because of their high cost they have not become general purpose materials such as polyethylene and polystyrene, which are about a third of the price of the polyamides.

18.9.1 Automotive Applications Many of the automotive applications have involved metal replacement, thus achieving weight reduction and energy saving in use and offering the opportunity to mold a product in one piece when previously a metal part required assembling of several parts, or alternatively, extensive machining with consequent waste of material. For example, intake manifolds in polyamide are tough, corrosion resistant, lighter and cheaper than aluminum (once tooling costs are covered) and offer better air flow due to a smooth internal bore instead of a rough cast one. Under-the-hood applications have shown particularly high growth. Typical examples include air intake manifolds, rocker covers, engine covers, reservoirs, ducts and air induction systems, radiator end tanks, fuel rails, electrical connectors and others. Glass-filled polyamides are used for some of the more demanding applications when long-term exposure to high temperatures (180
C and above) is required. The self-lubricating properties of PAs make them useful for gears and bearings. Polyamide moving parts may be frequently operated without lubrication, and are silent running. Other automotive applications include door handles and radiator grills. As an integral part of the vehicle’s body the door handles have many competing requirements. They must excellent surface appearance, paintability and UV resistance, but also good mechanical properties like stiffness and toughness. Polyamide film is used in the production of safety airbags. Extruded tubing is used for fuel transportation. PA46 is the polyamide exhibiting the highest temperature resistance (Hürschnitz et al., 1990). Its HDT at 1.8 MPa is 160
C, and 285
C when filled with 30% of glass fibers. Its applications in the automotive industry mainly consist of parts like chain tensioners, oil filter parts, signaling lamp bases, gear-shift forks, clutch components, speedometer gear wheels and fuel distributors.

18.9.2 Electrical and Electronic Applications Electrical insulation, corrosion resistance and toughness make polyamides a good choice for high load parts in general electrical applications as insulators, switch housings and the ubiquitous cable ties. Another major application is for power tool housing. Flame retardant grades of PA6 and PA66 are useful in applications such as electrical and electronic housings, enclosures, sockets, switches and wiring components. The high creep resistance at elevated temperatures, low coefficient of friction, and high heat distortion temperature of PA46 has made it useful for manufacturing connectors, end laminates in electric motors, surface-mount devices, and brush holders in electric motors. Glass-filled polyamides are extensively used in the telecommunications field for relay coil formers and tag blocks. Glass reinforced and toughened blends of PA66 with aromatic polyesters have been developed by Du Pont and are marketed under the trade name ZytelÒ HTN. These make it possible to produce thinner and lighter components for the latest generation of electrical and electronic devices including electronic connectors, relays, light-emitting diode components and various other electrical parts.

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18.9.3 Consumer Goods and Appliances This is a very broad area. Some examples are given below: l l l l l l l l l l l

Covers for laptop and mobile phone housings. Engine covers for mowers and snowmobiles. Sports shoes: golf, soccer, cycling, basketball, and so on. Ski boots. Tennis racket bumpers. Decks of racing boats. Components for coffee machines, push chairs, chain saws. Furniture: offices, stadium seats. Curtain rails. Hair combs. Sterilizable moldings have found application in medicine and pharmacy.

Polyamides and acetal resins (Chapter 19) are competitors in many applications, the latter being superior in fatigue endurance, creep resistance and water resistance. Under normal conditions of humidity the polyamides are superior in impact toughness and abrasion resistance. When a polyamide is considered appropriate it is necessary to consider the relative importance of mechanical properties, water resistance and ease of processing. While PA66 has the best mechanical properties, this material is probably the more difficult to process and has a high water absorption value. PA6 is easier to process but has slightly inferior mechanical properties. PA11 and PA12 have the lowest water absorption, and are easy to process, but have significantly poorer mechanical properties.

18.9.4 Films and Coating Polyamide film has been used increasingly for packaging applications for foodstuffs and for pharmaceutical and medical products. Cast and biaxially oriented films are available. Food packaging polyamide films offer toughness, high tensile strength and elastic modulus, flex-crack resistance, low gas permeability and low odor transmission. Film of high brilliance and clarity, particularly from PA11, is available for point-of-sale displays. Due to their polar characteristics they can be printed without surface treatments, so are often used as outer layers in laminated films. Food products packed in polyamide include fresh pasta, cheese, fresh meat and fish and pet food. Because of their temperature resistance, PAs can also be used for boil-in-the-bag food packaging. Medical applications include packages for surgical instruments and blister packs for tablets. Examples of other miscellaneous applications are bags for aggressive chemical products, novelty balloons filled with helium gas and book covers. PA11 is also used in powder form in spraying and fluidized bed dipping to produce chemical-resistant coatings. Although more expensive than the polyolefin and PVC powders, it is of interest because of its hardness, abrasion resistance and petrol resistance.

18.9.5 Fibers and Filaments As well as for conventional textile applications and carpets, polyamide fibers and filaments are used in a number of specialist applications. Polyamide monofilaments have found application in brush tufting, wigs, surgical sutures, sports equipment, braiding and outdoor upholstery. PA610 and PA11 have found extensive application in these fields because of their flexibility but PA66 is also used for brush tufting <90 mm in diameter. PA 66/610 copolymer is used in the manufacture of a monofilament for angling purposes.

18.9.6 Extruded Products Extruded applications of polyamide, other than film and monofilament, are less commonly encountered because of the low melt viscosity of the polymers. Uses include cable sheathing which requires resistance to abrasion and/or chemical attack, flexible tubing for conveying petrol and other liquids, piping for chemical plant, rods for subsequent machining, as the tension member of composite belts for high-duty mechanical drive and for bottles requiring resistance to hydrocarbons.

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PA11 and PA12 are frequently preferred because of their ease of processing but the high molar mass PA6, PA66, and PA610 polymers find occasional use.

18.9.7 Cast Polyamides As previously mentioned, moldings can be produced by the polymerization casting of CL. This enables very large objects to be produced. Such polymerization-cast polymers also possess certain advantageous properties. The polymers tend to have a much higher molar mass, and also 45e50% crystallinity, again higher than for melt processed materials. This leads to a higher tensile strength, hardness, modulus and resistance to creep. The comparatively stress-free moldings also have a reasonably consistent morphological structure. A disadvantage is the shrinkage of 4e4.5% which occurs during polymerization. Nylacast, a major producer of cast products provides a range of grades of cast PA6. Nylacast Natural PA6, available in a variety of colors is the base for other grades. Oilon was first introduced into the market in 1974. It is a lubricated nylon having a blended liquid lubricant system built in during the process stages, resulting in a substantial increase in bearing life more than 5 times that of natural cast nylon and more than 25 times that of phosphor bronze. Nylube is a development of Oilon, having excellent wear resistance. Nylacast Moly contains MoS2 providing good wear and abrasion resistances, combined with lower water absorption. Aquanyl is a copolymer of PA6 and PA12 and has reduced water absorption and improved stability and impact strength. Heat stabilized and toughened grades, glass-filled grades and antistatic grades are also available. Industries in which cast PA6 products are used for engineering applications include off-shore oil and gas, construction, agriculture, quarrying and mining, and shipping and ports, railways and transport. Nylacast polymers are routinely supplied as strips, billets, discs and rings up to 2.5 m in diameter. Custom built items can also be produced readily using CAD/CAM techniques. Among the products made by polymerization casting are propellers for small marine craft, conveyor buckets used in the mining industry, liners for coal washing equipment and main drive gears for use in the textile and papermaking industries. A very wide range of engineering components are available, such as seal rings, washers, bearings, clamps, wear pads, brake blocks, rollers and nozzles. Sheaves and pulleys for carrying ropes and cables have been manufactured for over 40 years at Nylacast. They are used in many different industries and offer lower friction, lower weight and less corrosion than their steel counterparts and also reduce cable and rope wear. They are available in diameters from 25 mm to 0.2 m.

18.9.8 Polyamide Reaction Injection Molding There is a continuing interest in polyamide-RIM materials as alternatives to polyurethane-RIM. This method provides a particularly suitable option if the piece numbers are too large to permit manufacture by machining, and where an excessive wall thickness or size precludes the use of injection moulding. Molded component weights are typically from 0.5 to 18 kg. Complex moldings can be produced; wall thickness variations can be accommodated, and inserts and reinforcements readily integrated. There are no flow lines in the component so moldings are free of stress.

18.9.9 Recycling As with all thermoplastics, methods of recycling polyamides are being developed. Because of the importance of fiber applications for PA6 and PA66, considerable attention has been given to fiber recycling (Page, 2000; Silva, 2016).

18.10 POLYAMIDES OF ENHANCED SOLUBILITY Polyamides such as PA6, PA66, PA610, PA11, andPA12 exhibit properties which are largely due to their high molecular order and the high degree of interchain attraction which is a result of their ability to undergo hydrogen bonding. It is, however, possible to produce polymers of radically different properties by the following modifications of the molecular structure: 1. Replacement of some or all of the eCONHe hydrogens by alkyl or alkoxy-alkyl groups to reduce hydrogen bonding which results in softer, lower melting temperature and even rubber polymers (N-substitution). 2. Use of acids or amines containing large bulky side groups which prevent close packing of the molecules. 3. Use of trifunctional acids or amines to give branched structures.

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4. Copolymerization to give irregular structures (e.g., 66/610/6 copolymer, which is soluble in alcohols and many other common polar solvents.) 5. Reduction in molar mass. Treatment of a polyamide with formaldehyde leads to the formation of N-methylol groups but the polymers are unstable. If, however, the polyamide is dissolved in the solvent such as 90% formic acid and then treated with formaldehyde and an alcohol in the presence of an acidic catalyst such as phosphoric acid a process of alkoxymethylation occurs to produce a polymer containing the structure: CON CH2OCH3

Methylmethoxy polyamides have been produced in which about 33% of the NH groups have been substituted. Such materials are soluble in the lower aliphatic alcohols, e.g. ethanol, and in phenols. They also absorb up to 21% of moisture when immersed in water. They have been used as components in both solution based and hot melt adhesives. If methylmethoxy polyamide is heated with 2% citric acid at elevated temperatures, typically for 20 min at 120
C, cross-linking will take place. This material finds a limited application in films and coatings which require good abrasion and flexing resistance. In the early 1950s, a new class of polyamides became available differing from the polyamides in that they contained bulky side groups, had a somewhat irregular structure and were of low molar mass (2.0e5.0 kgmole1). These products were marketed by BASF under the trade name VersamidÒ, but were sold to Gabriel Polymers in 2015. A typical example of this class of polymer is VersamidÒ 140, which is a semisolid, reactive polyamide resin based on dimerized fatty acid and polyamines. This material is used with solid or liquid epoxy resins (see Chapter 27) to form tough chemically resistant thermoset coatings which can be cured at room temperature. It is also used in primers and enamel paint formulations and adhesives.

18.11 OTHER ALIPHATIC POLYAMIDES Although less than a dozen aliphatic polyamide types together with a few miscellaneous copolymers have become available commercially, a very large number have been prepared and investigated. Of the many diamineedibasic acid combinations those based on intermediates with less than four carbon atoms are unsuitable either because of the tendency to form ring structures or because the melting temperatures are too high. Obviously development of a new polymer is costly, and only justified if the new product offers significant advantages over existing ones. Polyamides have also been produced from intermediates with lateral side groups. One particular type of polyamide produced from intermediates containing lateral side groups are the poly-(a-amino acids). The proteins may be considered as a special class of such polymers in that they are long chain molecules containing the residues of some 25e30 amino acids arranged in a highly specific way in the molecular chain, but are outside the scope of this book.

18.12 POLYAMIDE BLENDS In order to increase further the available properties of polyamides, several commercial blends are available. These include NORYL GTX blends of polyamide and modified polyphenylene ether supplied by SABIC, and TerezÒ PA/PP blends from Ter Hell Plastic Gmbh which offer lower water absorption.

REFERENCES Aélion, R., 1962. Nylon 6 and related polymers. Industrial & Engineering Chemistry 53 (10), 826e828. Belous, A., Tchoudakov, R., Tzur, A., Narkis, M., Alperstein, D., 2012. Development and characterization of plasticized polyamides by fluid and solid plasticizers. Polymers Advanced Technologies 23 (6), 938e945. Boyer, R.F., 1963. The relation of transition temperatures to chemical structure in high polymers. Rubber Chemistry and Technology 36, 1303e1421. Castor Oil, 2015. Sebacic Acid. Available: http://www.castoroil.in/castor/castor_seed/castor_oil/sebacic_acid/sebacic_acid.html (Last accessed 11.03.16.). Centre for Industry Education Collaboration (CIEC), 2016. Polyamides. University of York. Available: http://www.essentialchemicalindustry.org/ polymers/polyamides.html (Last accessed 11.03.16.). Coffman, D.D., Nerchet, G.J., Peterson, W.R., Spanagel, E.W., 1947. Polymeric amides from diamines and dibasic acids. Journal of Polymer Science 2 (3), 306e313. Fish, R., Mestemacher, S., 2005. Process for Efficiently Producing Highly Plasticized Polyamide Blends. US 20050038146 A1.

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Geil, P.H., 1963. Polymer Single Crystals. Interscience, New York. Hill, R. (Ed.), 1953. Fibers from Synthetic Polymers. Elsevier, London. Holmes, D.R., Bunn, C.W., Smith, D.J., 1955. The crystal structure of polycaproamide - nylon-6. Journal of Polymer Science 17 (83), 159e177. Horold, S., 2014. Chapter 6 Phosphinate fire retardants. In: Papaspyrides, D., Kiliaris, P. (Eds.), Polymer Green Flame Retardants. Elsevier, Amsterdam, pp. 237e248. Hürschnitz, R., Heather, P.H., Derks, W., Van Leeuwendal, R., 1990. Nylon 46-a design material for demanding engineering applications. Kunstoffe 80 (11), 1272e1276. ICIS, 2007. Caprolactam Production and Manufacturing Process. Available: http://www.icis.com/resources/news/2007/11/01/9075186/caprolactamproduction-and-manufacturing-process/ (Last accessed 12.02.16.). Kohan, M.I. (Ed.), 1995. Nylon Plastics Handbook. Hanser, Munich, p. 365. Khodabakhshi, K., 2011. Anionic Polymerization of Caprolactam: an Approach to Optimising the Polymerisation Conditions to Be Used in a Jetting Process (Ph.D. thesis). Loughborough University. Mandelkern, L., 1964. Crystallization of Polymers. McGraw-Hill, Inc, New York. Meares, P., 1965. Polymers: Structure and Bulk Properties. Van Nostrand, London. Momin, P., 2015. Polyamide market size, by application, 2013e2020. In: Polyamide Market by Type (PA 6, PA 66, Bio-based and Specialty), by Application (Automotive, Films & Coatings, Industrial/machineries, Consumer Goods & Appliances, Fibers & Textiles, and Others), by Process and by Region. Markets and Markets, Maharashtra, India. Müller, A., Pfluger, R., 1959. Morphologie krystalliner polyamide 6. Plastics 24, 350e356. Neuhäusl, E.R., 1968. Plastics and Polymers 36, 93. Page, I.B., 2000. Polyamides as Engineering Thermoplastic Materials. Smithers Rapra, pp. 24e26. Riley, J.L., 1958. Engineering Materials Design 1, 132. RTP Company Corporate Headquarters, 2015. Standard Nylon 6/6 (PA) Compounds e Data Sheets. Available: http://web.rtpcompany.com/info/data/ 0200/index.htm (Last accessed 15.03.16.). Silva, E., 2016. Recycled Polyamides, a Literature Review and Research Opportunities, pp. 1e15. Academia. http://www.academia.edu/. Takahashi, T., Nishikawa, Y., Morita, S., 2014. Method of Producing Cycloalkanone Oxime. US20140158522 A1. Toray Industries, Inc. Van Rijswijk, K., Bersee, H.E.N., Beukers, A., Picken, S.J., Van Geenen, A., 2006. Optimisation of anionic polyamide -6 for vacuum infusion of thermoplastic composites: influence of polymerisation temperature on matrix properties. Polymer Testing 25 (3), 392e404. Wichterle, O., Sebenda, Kralice, J., 1961. Anionic polymerization of caprolactam. In: Ferry, J.D., Overberger, C., Schulz, G.V., Staverman, A.J., Stuart, H.A. (Eds.), Fortschr. Hochpolymer.-Forsch. Springer-Verlag, pp. 578e595. Yamataka, K., Matsuoka, Y., Isoya, T., 1979. Electrolytic Methods for Production of Sebacic Acid from Adipic Acid. Asahi Chemical Industry Co., Ltd. DE2830144.

BIBLIOGRAPHY BPF, 2016. Polymer Thermoplastics. Available: http://www.bpf.co.uk/plastipedia/polymers/polymer-thermoplastics.aspx#nylonspolymides. Kohan, M.I., 1995. Polyamide Plastics Handbook. Carl Hanser Verlag Munich, Vienna, New York. Palmer, R.J., 2014. Polyamides, Plastics. In: Mark, H.F. (Ed.), Encyclopaedia of Polymer Science and Technology, fourth ed. John Wiley and Sons, Inc.