I
RENE AELION Research and Development Laboratory, Foster Grant Co., Inc., Leominster, Mass.
Nylon 6 and Related Polymers M a n y higher nylon polymers have interesting and useful properties, but r a w material costs and ease of production will be the key factors in determining volume production existing differences are due mainly to the physical and chemical properties of the monomers and the effects they have on the polymerization process. The principal factor is the production cost of the monomer. In this respect, the anticipated expansion of the production of caprolactam arising from a downward trend in processing costs should give a definite advantage to nylon 6 over the others for large scale development.
I N THE LINEAR ALIPHATIC POLYAMIDE
field, four types of nylon have been produced on a large industrial scale for more than 10 years. Two of them, nylon 6,6 and nylon 6,10, are prepared from a diacid, diamine salt formulation: HJi”CH2)6NH*, HOOC(CH2)aCOOH + nylon 6.6 H,N-(CH~)G-NH~, HOOC-(CHt),-COOH -+ nylon 6,lO and the others, nylon 6 and nylon 11, from a lactam and an amino acid : HN(CH2)s-CO L
J
-+
R a w Materials
nylon 6
A general method for the synthesis of lactams has been developed. The starting material is a cyclic hydrocarbon with one less atom in the ring than exists in the corresponding lactam. A ketone group is fixed on this ring (Q), then the nitrogen atom is introduced by oximation (7), and the lactam group is formed by the Beckmann rearrangement
H1N-(CH,)lo-COOH -+ nylon 11 In each class one member has a short aliphatic chain and the other member a much longer one. These four basic types of nylon already cover a relatively wide range of properties. I n recent years, extensive research in several countries has been carried out to determine if these four types of nylon were the only ones presenting interesting commercial applications, either in the plastic or in the textile fields. Other types of nylon, mainly prepared from amino acid and lactams, were investigated from the standpoints of costs and properties in comparison with those already produced. A few have reached a pilot plant evaluation, and it has been found that their properties, although interesting, vary within the limits set up by the four basic nylons. The
(4 ’
For caprolactam the commonly employed raw materials are benzene, phenol, and cyclohexane. I n the case of the higher lactams listed (below), an additional step is required to obtain first the 8- and 12-membered ring materials which are not readily available. I n the case of caprylactam, this ring is obtained either by polymerization of 4 molecules of acetylene (6) or by the dimerization of butadiene (70). I n a similar way, the 12-member ring for the preparation of dodecalactam can
Monomers Used in the Preparation of Nylons Formulas
Monomer
Valerolactam
Nylon Types
HN-(CH2)4CO
Nylon 5
L--I
Caprolactam
HN-(CH2)5CO I
7-Aminoheptanoic acEd Caprylactam
_
H2N-(CH2)6COOH HN-(CHz,)vCO L
9-Aminopelargonic acid 1 1-Aminoundecanoic acid Dodeca lacta m
826
Nylon 6
:
i
H2N-(CH2)sCOOH H2N-(CHz)laCOOH HN-(CHz)11CO I J
INDUSTRIAL AND ENGINEERING CHEMISTRY
Nylon 7 Nylon 8 Nylon 9 Nylon 1 1 Nylon 12
be obtained by trimerization of butadiene (70). The same route can then be used to synthesize these higher lactams, but production costs are higher than for caprolactam. New methods have been developed for caprolactam and are now being used on a large scale. increasing the price advantage of caprolactam. Du Pont’s process is based on the nitration of the saturated cyclohexane ring, followed by a reduction-dehydration in the presence of a borophosphoric acid catalyst to give caprolactam directly (2). Snia Viscosa’s process is based on the reactivity of a tertiary carbon atom on a 6-member ring. Cyclohexane carboxylic acid, which can be prepared from toluene ( 8 ) , is an example of this type of compound. Reaction with a nitrosation agent such as nitrosyl sulfuric acid in the presence of H 2 S 0 4 gives caprolactam directly with evolution of COa. These two syntheses comprise fewer steps than the conventional method and should accentuate the downward trend of the caprolactam selling price. In the case of the amino acids, specific methods are employed, for no general synthesis has been found. For example, the 7-aminoheptanoic acid and the 9-aminopelargonic acid can be prepared by telomerization of ethylene in the presence of a halogenated compound such as CC14 or bromoacetic acid. I n a first step, a mixture of saturated aliphatic chains is obtained, the 7- and 9-carbon members being prominent (5). I n a second step, the end groups are modified into amine and carboxyl groups to form the w-amino acids. Amino acids can also be obtained by using raw agricultural materials, such as fatty acids. The long aliphatic chain is split into two parts by oxidation or pyrolysis, and the amine and acid terminal groups are fixed on one of the shorter chains, the other remaining as a by-product. For example, 9-aminopelargonic acid can be synthesized by oxidation of oleic acid, and ll-aminoundecanoic acid may be formed by pyrolysis of ricinoleic acid, leaving as by-product the saturated aldehydes, nonanal and heptanal, respectively ( I ) .
Of dl1 the monomers, amino acids, or lactams considered to be of commercial interest in large scale manufacture of nylon, none has reached so far a production cost as low as caprolactam. In addition to monomer cost, other factors have to be taken into account in the final evaluation of the polymer. Physical Properties
T h e differences in physical properties between amino acids and lactams are important in the polymerization process from a theoretical and technological point of view. Amino acids are high-melting point solids which cannot be purified readily by distillation. I n some cases, purification is accomplished through the distillation of their methyl or ethyl esters, which are distillable at high vacuum. T h e most commonly used method is crystallization from water, in which amino acids have limited solubility. This solubility decreases as the number of methylene groups in the amino acid becomes larger, but the solubility increases fast enough with temperature to permit easy purification by crystallization in water. Temperatures above 100' C. are used to dissolve the amino acid or the sodium salt of the amino acid, which is then crystallized by cooling. I n the final step the monomer is obtained as a water suspension. Therefore, the most convenient way of feeding a nylon monomer into the polymerization equipment is to use the water slurry directly at room temperature or its homogeneous solution at elevated temperature and pressure. I t would be difficult to feed a monomer melt, since the polymerization proceeds very rapidly as soon as the monomer melts (with the evolution of water). Lactams, however, are obtained in a dry state after purification, either by vacuum distillation for the low members or by crystallization in an organic solvent for the higher molecular weight compounds in the series. For example, caprolactam is distilled and dodecalactam can be purified by crystallization in cyclohexane. Since lactams are stable a t their melting points, they are very easy to handle and can be fed directly into the polymerization equipment. T h e polymerization does not start without the introduction of a catalyst and, therefore, continuous or discontinuous polymerization processes are very easy to operate as far as feeding is concerned. The picture is somewhat reversed, however, from the standpoint of the polymerization itself. Polymerization Kinetics
The polymerization of an amino acid is a straight-forward dehydration reaction which offers limited possibility of process modifications. O n the other
hand, the polymerization of a lactam is a much more complicated process with greater versatility. Characteristics of the polymerization reaction for the amino acids are: Yields vary from 86 to 89%, depending on the molecular weight of t h e amino acids. T h e r e is no large amount of byproducts, except water. After polymerization, the polymer does not need a n y additional treatment. Characteristics of the polymerization reaction for lactams are: T h e yields vary from 95 to 100'%, depending on the molecular weight of the lactam. T h e r e is an equilibrium reaction for nylon 5 and nylon 6, with substantial amounts of by-products involved. Polycondensation of Amino Acids. T h e polycondensation of amino acids is a second order exothermic reaction. No catalyst is necessary to start the reaction, but a sufficiently high temperature is required. Water is usually removed at a temperature of about 260" C., and the reaction is accelerated by applying vacuum. I n a discontinuous process, the monomer is fed as a water suspension or as a solution under pressure. By releasing the pressure in the polymerization kettle, this extra water is removed first, followed by the water coming from the condensation reaction. The polymer is extruded in the molten form and processed into pellets. In a continuous process, the monomer is fed usually as a water slurry a t room temperature, but in this case removal of the large amount of extra water has to be carried out quickly. In the case of the polycondensation of the 11-aminoundecanoic acid, a convenient process which has been in industrial operation involves spraying this suspension in the form of very fine droplets on the walls of a jacketed kettle maintained a t high temperature. Upon contact with the heated walls, the water in the droplets is instantaneously vaporized, and the molten amino acid flows down as polymerization starts. The problems of rapid heat transfer are solved in this case because the monomer can drop only when molten or, in other words, when it has reached a sufficiently high temperature. The molten low molecular weight polymer can then be driven continuously into a vertical polymerization tube. At the bottom of the tube, polymerization is completed, and the liquid product may be processed directly. Theoretical yields would be obtained except for slight traces of monomer carried over during the removal of water. By continuous polymerization of nylon 11, for example, yields of 89% are ob-
.
tained, computed as weight of the dry polymer to 100% of monomer, compared with a theoretical yield of about 91%, which takes into account the loss of one molecule of water for one molecule of amino acid. REACTION BY-PRODUCTS.The water formed during the polymerization reaction can come either from an inter- or an intramolecular reaction-that is, by loss of water across 2 molecules of amino acid or from the same molecule. I n the case of nylon 7 and nylon 11, the probability of intramolecular dehydration by forming an 8- or a 12-membered ring lactam is very remote, but, nevertheiess, this possibility does exist. At the end of the polymerization reaction, a very small amount of lactam is formed, about 1% in the case of nylon 7 and 0.4 to 0.6% for nylon 11, These lactam by-products are not extracted from the polymer because they do not alter the properties of the nylon, except in some special applications where even very small amounts of impurities might be objectionable. Once the lactam is extracted from the polymer, there is no danger of its reformation during processing of the polymer, T h e fact that the polymer has reached its final stage directly at the bottom of the polymerization tube and requires no further treatment permits extensipn of the continuous process all the way to the end product. I n the case of textile operations, for example, the molten polymer can be driven directly into spin heads and extruded as yarn or staple fiber continuously. Polymerization of Lactams. The mechanism of the polymerization of lactams has been extensively investigated in the past few years. Several studies have been published, mainly by Hermans and his coworkers ( 3 ) . A catalyst is always needed to start the reaction. I n this discussion, we are concerned only with water-catalyzed reactions. The mechanism of these reactions has been well elucidated now. I t is considered to involve an addition reaction of an open lactam ring into the growth chain initiated by the combined catalytic effect of water and acid groups. Contrary to the amino acid polycondensation, this reaction does not involve any significant removal of water since only very small amounts are used as catalyst. T h e ease of opening the lactam ring is obviously a function of the number of methylene groups in it. Only in the case of a 6- or 7-membered ring is the opening easy. This is an equilibrium reaction. I n the case of higher lactams, the water-catalyzed reaction is much more difficult to accomplish with high degrees of conversion, because of the greater stability of the large rings. With the higher lactams, opening the ring is not VOL. 53, NO. 10
OCTOBER 1961
827
an equilibrium reaction, and once it is achieved there is very little tendency to reformation of the lactam. Therefore, the yield of the reaction approaches theoretical. T h e polymerization of dodecalactam is a good example of this type of reaction. At the usual polymerization temperature of 260’ C. and polymerization of 30 hours, more than 50% of the lactam is unreacted. Much higher temperatures have to be used to carry the polymerization to completion, but a degree of conversion of almost 100% can be obtained by processing the lactam at these high temperatures. I n the case of the 7-membered dng, caprolactam, polymerization is easy to carry out but leads to a n equilibrium reaction. T h e amount of residual lactam is a function of temperature. At usual polymerization temperatures, values of about 9 to 10% of residual monomer are obtained. T h e important work of Zahn and his coworkers (77) has led to the identification of small amounts of the linear and cyclic dimers and trimers in addition to the caprolactam. These low molecular weight substances modify substantially the properties of nylon 6 and usually are objectionable for further processing of the polymer. TherTfore, the nylon 6 obtained after polymerization has to go through additional treatment to remove these oligomers. T h e usual procedure is a water extraction, followed by a thorough drying. If the lactam coming from the extraction process is fully recovered, a yield of 95% can be obtained in the polymerization. This yield reflects the loss of the unrecovered oligomers and takes into account the slight resinification occurring during the distillation. Extensive research has been carried out on methods for the removal of these low molecular weight substances. High vacuum and superheated steam have proved to be convenient ways of devolatilizing the polymer. T o obtain good results, the devolatilization has to be carried out at high temperature, and it is, therefore, very difficult to obtain complete extraction because of the monomer reversion reaction. Only very fast processes can bring down the amount
of extractables to less than 2%, which is the maximum limit for further good processing of the nylon. These two processes provide a much easier and less expensive recovery of the lactam, compared with its isolation in the water extraction process. Concerning the polymerization reaction itself, continuous or discontinuous processes are easy to carry out so long as a n unextracted polymer is the final material desired. This is the case, for example, in the production of staple fiber where the unextracted polymer is spun directly and the extraction process is applied to the finished product, because of the ease of handling such material. Nevertheless, complete continuous processes have been set u p with the vacuum extraction technique, and the spinning of yarns has been accomplished without any further extraction. Polymer Characteristics
In the polymerization of amino acids, the degree of polymerization is controlled by the use of a small amount of a compound having one kind of functional group, either basic or acidic. Higher degrees of polymerization are obtained by carrying the reaction without any stabilizer and pushing the reaction as far as possible by the use of high vacuum. Nevertheless, the degree of polymerization is never very high. In the polymerization of lactams, acid groups are necessary to start the reaction, and the same compound can be used simultaneously as a catalyst and a stabilizer. When high degrees of polymerization are desired, the stabilizing effect of the acid catalyst should be avoided. T h e usual procedure is to catalyze with an amino acid or diacid diamine salt which has no limiting effect on the growing of the polymer chain. With nylon 6 , a very high degree of polymerization can be obtained conveniently by post polymerization. Following the normal polymerization, this after-treatment can be applied on the nylon 6 a t a temperature just below the melting point. If this nylon has not been stabilized, a marked increase in the molecular weight will be obtained after a few hours.
I Comparative Properties of Nylon Nylon Type
Nylon Nylon Nylon Nylon Nyion Nylon
6
7 8 9 11 12
Me!ting Poinf, c.
Specific Gravity, Gi.ams/Cc.
215 225 195 200 185 180
1 .13 1.10 1 .08 1.06 1 .04 1 .03
’
Moisture Regain,
70
4.7 2.4 1 .40 1.2 1.18 1-10 I
828
INDUSTRIAL AND ENGINEERING CHEMISTRY
Comparative Properties
Physical properiies of these various nylons follow a definite pattern and are closely related to the number of methylene groups of the polymer chain, as shown (below). Moisture regain and specific gravity decrease with a n increasing number ofmethylene groups. The more distant the functional amide groups are in the polymer chain, the closer is the nylon to a polyolefin. The mechanical properties in the plastics and textile applications are not very different. T h e real differences in behavior appear mainly when the properties are measured a t extreme conditions, such as thoroughly wet us. dry atmosphere and high compared with very low temperatures. The most attractive polyamide is nylon 7, taking into account the characteristics of the polymerization reaction and the general properties of the polymer. So far, its cost compared to that of nylon 6 does not overcome the advantages found in the processing and its properties. For this reason, so far no new nylon is produced on a large industrial scale, other than the four nylons already mentioned. Wylon 6,lO and nylon 11, which are more expensive to make than nylon 6,6 or nylon 6, are produced on a commercial scale because they are sufficiently different from nylon 6 and nylon 6,6 and have found specific applications. Bui competition between nylon 6 and nylon 6,6 might become very acute when production of the former reaches a large enough scale. I n this case, too, there seems to be a definite advantage in the production cost of nylon 6 comparcd with nylon 6,6. literature Cited (1) Aelion, R., Ann. chim., Puris3, 5 (1948). ( 2 ) England, D. C. (to E. I. du Pont de Nemours and Co.), U. S. Patent 2,634,269 (Sept. 21, 1951). (3) Hermans, P. H., Heikens, D., and van Velden, P. F., J . Poiymer Sci. 30, 81 (19 58).
(4) Klar, R., Hilgetag, G. (to I. G. Farbenind. A.G.), Ger. Patent 735,727 (April 15, 1943) (addition to Ger. Patent (5)686,902). Ncsmejanow, A., Chem. Tech. (Berlin) I
9. 139 (1957).
.
(6) ’Reppe, .W:, “Neue Entwicklungen auf dem Gebiete der Chemie der Acetylens und Kohlenoxyds,” Springer-Verlag, Berlin, 1949. (7) Schlack, P. (to E. I. d u Pont de Nemours and Co.), U. S. Patent 2,283,150 (May 12, 1942). (8) Snia Viscosa: Belg. Patent 582,793 (Sept. 18, 1 9 5 9 ) . (9) Wallach, Ann 312, 187 (1900). (10) Wilke, G., Angem. Chem. 69, 397 (1957). (11) Zahn, H., Rathgeber, Peter, Rexroth, E., Krzikalla, R., Lauer, W., Mir6, P., Spoor, H., Schmidt, Franz. Seidel, B., Hildebrand, D., Zbid., 68, 229 (1956). RECEIVED for review November 14, 1960 ACCEPTEDMarch 13, 1961 Division of 138th Meeting, ACS. New York, September 1960
