Nylon MORTON SHOR The College of the City of New York, New York City*
HISTORY
I .
T WAS not until the 17th century that the alchemsts stopped trying to make gold from the baser metals and turned their efforts to the more scientific domain which we now call chemistry. They did not, however, cease in their efforts to bring forth great riches from their laboratories. They merely refocused their interests and turned their energies to another symbol of wealth. For centuries silk had been a mark of wealth and prestige, and now scientists were trying to devise methods whereby they could synthesize this substance. Robert Hooke, in 1664, and R. A. F. Rhumur, in 1734, suggested methods and materials for the spinning of artificial silk (1). It was not until the 1840's, however, when a Manchester silk manufacturer named Schwabe designed the forerunner of our modem rayon spinneret, that any artificial fibers were actually produced. Schwabe spun fibers from the natural gums and resins, but these fibers were weak and not nearly so attractive as the threads produced by Mercer. Mercer had introduced his process for the "mercerization" of cotton in 1844, and i t took hold almost immediately, being used down to this very day (2). What was probably the first rayon ever produced was a thin strand of nitrocellulose exhibited a t the Paris Exposition of 1889. The Count Hilaire de Chardonnet had been an assistant of Louis Pasteur during the latter's experiments on diseases of the silkworm, and he had become interested in the spinning of silk. Despite their fragility and generally poor physical properties, these new threads, which de Chardonnet made by extrnding nitrocellulose through a fine orifice, created a furor in the textile world (I). The rayons eventually became the "artificial s i l k of commerce. They offered no serious threat to the silk market, however, since the only point of semblance between the two fibers was that of appearance, and even here there were serious objections. The early rayons were weak and flammable, and lacked water resistance. They had a cheap, gaudy luster and feel, and the acetate rayons were soluble in many drycleaning fluids. Recent improvements such as the use of staple-length rayon to decrease the sheen, control of the degree of acetylation to improve the strength and flexibility and control the sheen of acetate rayons, chemical treatment to decrease the inflammability, have minimized some difficulties and obviated others. *present address: Central ~ ~ Corporation. Hohoken, New Jersey.
b
Rayon, despite all of these improvements, however, is still far from being the ideal fiber. In 1927, Dr. Wallace Hume Carothers, working in the du Pont research laboratories, became interested in the synthetic fiber problem. During the course of his investigations he synthesized various ethers and esters of cellulose but found that these offered no advantage over the existing rayons. He then discarded cellulose as a starting material and began work on a program which finally brought forth the fiber we now know as nylon. This program can be divided into three stages : 1. Fundamental research period. Period of concentration on polyamides, leading to the synthesis of the polymer with the required properties. 3. Period of development of the practical manufacturing process. 2.
In this first period of research, Carothers conducted investigations in the field of polycoudensation. By 1928 he had produced polyesters with average molecular weights in the neighborhood of 5000 by combining dihydric alcohols with dibasic acids. By placing a sample of the polyester in the molecular still (a device for the distillation of substances with extremely low vapor pressures) i t was found that the removal of water was accompanied by an increase in molecular weight of the substance to values between 10,000 and 25,000 (2). In addition to this remarkable change in the molecular weight of the substance there were equally remarkable changes in its physical properties. It changed from a semiopaque, waxy, brittle solid to one which was tough, elastic, and rather transparent. Fibers drawn from this "super polyester" were far superior to those spun from the polyester. They were more flexible and had a higher tensile strength and a higher degree of elastic recovery. But the main advantage of the fibers drawn from the super polyester lay in the fact that they could be "cold drawn." When a fiber is cold drawn i t is stretched longitudinally. This stretching orients the molecules into approximate parallelism, with the axis of the fiber. The orientation of the molecules in this manner results in an improvement of the physical properties of the fiber (15). The fibers spun from these super polyesters had excellent physical properties but unfortunately had too low a softening point to be of practical use. In an attempt to obviate this difficulty, Carothers made the super polyamide of e-amino caproic acid. This had the required ~ h ~ s i c ~ a rl o ~ e r t i e s ~ ~~~~~l ~ ~ poods t ~ substance ~ i ~ also , but lay a t the other extreme with regard to its soften88
ing point and solubility. It had an extremely high softening point and was insoluble in practically all organic solvents, phenols and formamide being notable exceptions. All of his attempts to find a middle ground between these two sets of compounds, to produce a readily spinnable, practical fiber, met with failure, and in 1930, he actually gave up the synthetic fiber project. Evidently Carothers did not believe that his search was completely hopeless, for several months later he resumed work. Working with 9-amino nonanoic acid, he obtained super polymers with properties very close to those he was seeking. With this degree of success he decided to limit his research to super polyamides, since only these compounds seemed to have softening points higb enough for practical use. He subsequently prepared super polyamides from various amino acids and diamines and dibasic acids, always aiming for improvement over the polymeric product obtained from 9-amino nonanoic acid. On February 28, 1935, a now historic date, he produced the super polyamide of hexamethylene diamine and adipic acid. This compound, referred to as "66" (the first numeral refers to the number of carbon atoms in the diamine and the second to those in the acid) could be spun into fibers that, upon cold drawing, were superior to any fiber then known, in tensile stren,gth, elasticity, flexibility, and in many other ways. This &as nylon. The first two ohases of the work were com~leted. of' the third phase, that of t h i comThe mercial production of nylon, now presented themselves. Hexamethylene diamine and adipic acid were mere laboratory curiosities, and there was no factory anywhere in the world equipped to turn out these compounds in the tremendous quantities needed for the commercial production of nylon. By 1936, however, Roger Williams, of the Ammonia Department of the du Pont organization, had developed a process for the production of adipic acid from phenol. The diamine was then produced using the acid as a starting point. In taking the process out of the laboratory and bringing it through the semiworks stage into the factory, enormous difficulties were met and overcome. By the middle of 1938 a pilot plant was in full operation, making brush bristles and textile yarns of this magic new material. Unfortunately Carothers was not present to see the commercial success of his invention. He died on April 29, 1937, a few short weeks after the last of his patents was applied for (9). PRODUCTION
The following definitions are quoted directly from the patent papers of Dr. Carothers. These papers are written in so clear and lucid a manner that the author could never hope to improve upon the language used therein. 1.
"A condensation is any reaction that occurs with the for-
Molecular Orientation in A- Undrawn Nylon B-Cold Drawn Nylon
Undrawn Nylon
Nylon Being Cold Drawn
Drawn Nylon mation of new bonds between atoms not already joined, that proceeds with the elimination of elements (H2, NI, etc.) or simple molecules (HsO, CH8CHsOH, HCI, etc.). For example, esterification, amide formation, ether formation, and anhydride formation" (15). 2. "Condensation polymers are compounds formed by the mutual condensation of a number of (functionally> .. similar molecules to form asinglemolecule" (15). 3. "Linear polymers are compounds whose molecules are long chains built up from repeating units. This type of stmcture may be symbolized by the following general formula: -A-A-A-A-A-A-A-A-A-A+The unit, or radical, PA-, I called the stntcturd unit of the polymer" (15).
The chemistry of synthetic fibers is the chemistry of high-polymer materials; and with a knowledge of high-polymer chemistry Carothers was able to "design" the exact type of molecule for which he was searching. In order to be spinnable into a tough, flexible, elastic, lustrous fiber, the molecules would have to meet the following specifications: 1. The molecules must be enomously large. Molecular weights of the order of tens of thousands, and lengths of about 1000 A.U. are required. Long molecules are necessary for the spinning of a strong fiber just as long sections of staple-lengh cotton are necessary for the spinning of strong cotton thread. 2. These long molecules must have a higb degree of linear symmetry. This is necessary in order to permit the orientation of the molecules into approximate parallelism. 3. The structural unit must have a high degree of polarity. This makes for strong secondary intermolecular forces which strengthen the fiber and maintain the induced orientation. 4. The molecules should be highly oriented. This orientation
90
JOURNAL OF
gives a stronger, more flexible, more transparent, more lustrous fiber than is obtainable from the same material unoriented.
the requirement Carothers resorted to the condensation reaction. By the reaction of ethylene glycol and succinic acid he produced polyesters with molecular weights in the neighborhood of 3000' By use of the to eliminate the water of esterification the molecular weight could be brought up to 10,000, and often as high as 25,000 (15). If the reacting mass could have its molecular weight run up to 25,000, then why not 50,000 or 100,000? did the entire mass become One gigantic Or this possiCarothers bility and made a study of the factors involved in ~ o ~ ~ He found ~ ~that ~there ~are three ~ factors t i which may stop the process: 1. The reaction may become intramolecular at some stage, with consequent ring formation. Polymerization usually cesses when five-membered rings form; sometimes cease3 when sixmembered rings form; and rarely ceases with the formation of three- and seven-membered rings (15). 2. The terminal functional groups responsible for the condensation may hecome "lost or mutilated" through side reactions. 3. Mechanical effects such as viscosity and solubility and other kinematic effects may come into play.
By use of the molecular still Carothers had made molecules of the required size. By the use of alphaomega straight chain dihydric alcohols, and dicarboxylic acids, the requirement of linear symmetry was satisfied. The use of the hydroxyl group and the amino group in conjunction with the carboxylic group degree of into the introduced the chain. There yet remained, however, the requirement of molecular orientation to be satisfied. The need for orientation can be seen from an analogy to a system of short wirelike magnets. If these magnets are randomly placed in a mass there will be little strength to the system. If, however, they are all lined up parallel the system will have much greater strength. The method of producing this required orientation is given by Carothers in one of his patents. In his own words: "An especially valuable and remarkable property of the synthetic compounds of the present invention resides in their capacity to be drawn into strong, flexible fibers, which are in some respects, especially in their elastic properties and high ratio of wet strength to dry strength, superior t o any artificial fibers that have been prepared hitherto. This capacity appears t o depend upon the extraordinary facility with which the polymers of this invention accept a very high degree of permanent orientation under the action of stress. . . . E-x c e ~~ tin those ~ ~ - - mechanical - ~ ~ ~ ~ ~ ~ . instances where the melting point of the polymer is so high that melting is accompanied by decomposition, threads of the polymer are readily obtained b y touching the molten specimen with a rod and drawing the rod away, men this drawing is done very slowly, the threads closely resemble the mass from which they were drawn; that is.. thev as . are oDaaue . . and show the same melting point before. Very fine threads prepared in this way are frequently lacking in pliability and are somewhat fragile. However, if such threads are subjected t o stretching a t ordinary or slightly elevated temperatures, they are profoundly changed in their physical properties. The stretching results in a permanent ~
CHEMICAL EDUCATION
elongation; the original thread first separates into two sections joined by a thinner, transparent section, and as the stretching continues, the transparent section grows until the opaque sections are completely exhausted. he fiber produc& in this way is very much stronger than the thread from which i t W- drawn. It is also more pliable and elastic. I t s melting point is changed and its transparency and luster increased. It exhibits a high degree of birefringence and Parallel extinction between crossed Nicol's prisms, and furnishes a typical oriented fiber diagram when examined by x-ray methods in the usual way. This method of imparting new properties t o the fibers is referred t o as 'cold drawing' " (15).
~ finvestigating t ~ ~ scores of super polymers tarothem found that he could draw the following generalizations: The higher the molecular, weight and length, the degree of linear symmetty, the polarity of the unit, ~ the ~ degree . of molecular orientation, the higher will and be the melting point, the insolubility, the tensile stren@h, the flexibility, transparency, elasticity, and luster of the resultant fiber. The variation of melting point, tensile strength, and spinnability with weight and length are shown in ~ ~ b TABLE I* Avcragr Mol. W I .
A.rro8-r A.V.
M. g . , T.
Sginnobili~y
780 3,190 5~870
60 188 440
66 t o 6 7 74 t o 7 5 73 to 75
7,330
570
74 to 75
Absnt ~brent short fiber., not drawable L O ~ Zfibers, not .
g,330
130
75 to 76
16,900
1320
77 to 78
- Long drawable fibers,
Tenrile Slrcngth
... ... .. . weak Weak
drawable Spinnable,dranable
18,600 p. s,i.
*Table i furnished through the courtesy of Mr. A. X. Schmidt, Departmvlt of chemical ~ogineering. he college of T ~ Ccity of N ~ Wvork.
By application of these fundamental generalizations Carothers found that he could make fibers with the required properties in either of two ways: by the condensation of amino acids of the formula R-NHR'-COOH, where R' is a straight chain of at least five carbon atoms (17); or by the reaction of dibasic acids and diamines (18). Nylon is the name given to the class of s ~ f polyamides f produced in these two ways, and is a generic term, much as the terms wood and glass. The production of is carried out as follows: Hexamethylene diamine, 144 parts, and adipic acid, 174 parts, are combined in a solvent made up of 1300 parts of 95 per cent alcohol and 210 parts of water. The mixture is warmed to Promote the formation of the salt, and then cooled to orecidtate i t (19). The salt is then delivered to the top of a tower dbwh which it flows, emerging as the finished nylon at the bottom. The salt is first autoclaved to complete the polymerization, and a constant stream of nitrogen is bubbled throuzh the mass. This removes the water of condensation,, accomplishing what the molecular still did in the laboratory. This, and many other operations, including the actual spinning, are carried out under a blanket of nitrogen to prevent atmospheric A
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l
~
oxidation and the consequent discoloration of the product. The molten polymerized mass from the autoclave is extruded in the form of a ribbon onto a rotating drum, where it is cut into chips. These chips are then charged into the hopper of the spinning unit, onto a heating grid, where the polymer melts. The melt is pumped and metered by specially designed pumps, and filtered through filter packs, before entering the spinneret. Here, it is extruded under a nitrogen pressure of 8 to 10 p. s. i. gage, a t a temperature of 210°C. Upon leavina the sdnneret, the uolymer strikes the cdld air and solidifies. The solG nylon filaments are taken up onto a rotating drum and are then ready for the final step in the process. The fiber is cold drawn by feeding it off the takeup drum onto a drum rotating a t a higher s u e d . This stretches the threads to 400 cent iftheir original length (2). For use in hosiery, or other textile manufacture, the nylon fibers must be treated with a sizing to prevent snagging during subsequent handling. Silk presents no problem in this respect, since it contains the natural gum sericin, which acts as a size 612). The technical difficulties involved in carrying out this apparently simple series of operations were enormans. Pumps had to be designed to work in the neighborhood f,, 3 0 0 0 ~ .with no lubricant save the t ~ air had ~ to~ be molten polymer itself. ~ completely excluded from the process. Apparatus had to be designed that would take up the nylon filaments, spun at the rate of 3000 feetper minute, and cold draw these filaments to four times their original length without snapping them.
basis of strength per unit weight the natural fibers are superior to the metals. Nylon, as can be seen, is high up in the scale of fibers, being surpassed only by flax. Since the stress-strain curve for nylon is practically linear up to 4 per cent strain (9),the recovery test was made by stretching the fibers 4 per cent, holding for 100 seconds, and permitting 60 seconds for recovery. Du Pant gives the following figures for nylon (9): Tensile strength, dry: 5 to 7 g. per denier Tensile strength, wet: 4.4 to 6.0 g. per denier loo^ streneth: about 90 oer cent of tensile streneth " ~ o d u l u ofhlasticity: s 300 kg./cm.' (4.6.10' Ib./in.2) Density: 1.14 g./m.a Average heat capacity: 0.555 cal./g."C. (20' to 265'C.) Heat of fusion: 22 d / g . Moisture content: 4.5per cent at 24Y!., 72 percent re^. hum. Refractive index: 1.53 to 1.57 Volume resistivity: 4.10Mohm-cm.at 18percent re]. hum. 6.10Yohm-cm. when wet Dielfftric 4 at 1000 cycles. 22'C.. 18 per cent rel. h,.m ---.... Power factor: 5 per cent under the conditions above
Nylon is practically unaffected by most organic acids, alcohols, halogenated hydrocarbons, cold dilute alkalis, soaps, aldehydes, ketones, cold bleaches, and organic solvent mixtures. It is, however, readily soluble in the phenols and formic acid. It is virtually and ~noninflammable, h ~ ~ willi melt and ~ bum only reluctantly when held in a flame. Nylon is completely inert ph~siologicall~.The only known case of toxic effects resulting from the wearing of nylon is that of a woman whose feet blistered after wearing nylon hose. Investigation showed, however, that the blistering was due to phenols inPROPERTIES completely removed from the hose by the manuThe following physical properties refer to Nylon facturer (14). 66, the super polyamide of hexamethylene and adipic acid RECENT DEVELOPMENTS In his patents, Carothers has indicated the various -(CH,),NH-OC-(CH,),-CO-HN-(CHP)~NH-OCacids and amines that he used to produce nylons (cH,),-CWHN(18). He lists about 15 diamines and about an equal TABLE 2' number of acids, giving a total number of simple comTenriia Slrenglk, s''e"~'h: olio, T / D binations over 200. This does not include mixed l~nt~~iol in L L / I X . ~ SO. G.. D nylons, i. e., nylons produced by using more than one 23,000 '1.8 steel 180,000 DU~I 55,000 2.8 19,700 kind of amine in conjunction with one or more acids. Spruce 10,000 0.5 20,000 FISX 140.000 1.5 93,000 Despite the apparent perfection of the existing Cotton 36,000 1.5 24,000 nylons, work is continually being done to improve them. Silk 50,000 1.4 36,000 NYIO~ 71,000 1.1 65,000 A recent British patent (,525,516) tells of nylons pro* Table 2 furni~hedthrough the eourtuy of Mr. A. X.Schmidt. Depart- duced from aromatic diamines. The compounds disment of Chemical Engineering. he college of The City of New ~ o r k . cussed in this paper are all meta-diamines, and a typical structure of such a nylon is : T A B L E 3 (7)
Fiber Nylon silk
A&te Cordura Viseose
R ~ Par Cenl 100 50 50 40 30
~ ~~
~~ ~ ~ .
Dry
Wd
5.0 4.6 3.4 1.8 1.5
4.5 3.8 2.2 0.9 1.0
c~/ iwas, ~ ~~ A~~~ S ~O.~ . ~ , ~~ ~~ ~-~,O* C, -t ( C H ~ ~ C O N H - ~ H N O C - ( C H ~ ) ~ - C O N H + per cmr HNOC-. 3.5 11.0 6.5
....
12.0
* A t 60 per c a t humidity. t A one-denier fiber is one ofsuch size that 9000 meters of the fiber weigh one gram. A onedenier nylon fiber has a diameter of 4.10-4 in. (9).
Table 2 points out the surprising fact that on the
..
The patent states that these reactions differ from aliphatic nylon reactions in the following respects: 1. Aliphatic nylon reactions are of the second order, while those of the aromatic nylons are third order reactions. 2. Aromatic diamines do not form nylon salts, since they are too weak as bases for salt formation. 3. The reactions of aromatic diamines are much slower than thoseaf thealiphatics.
P o n t . Rep,orts on Nylon. Rayon Textile Monthly, 22, The patent goes - on to list catalysts that will in- ( 5 ) D u650-2 (1941). crease the aromatic diamine reaction rate five- to (6) GERsEn AND LATKRoP, .lEStimBticn of nylons in mixtures,M tenfold, and gives, finally, a method of stopping the ~ e r t i l eColorist, 63, 253 (1941). (7) Horn. "Nylon as a textile fibre," Ind. Eng. Ckem., 32, 1560 polymerization reaction at any desired point. T& is r,n*n\ , . " 1 " , . adding a of ( 8 ) LEGGIS,"Preboardins.nylon hosiery." Tatile World. 91. . either reactant. 115-6 (1941). (9) "Nylon, Versatile Product of Du Pont Chemistry;' issued There have been many other types of nylons inDecember. 1941, by Nylon Division, E. I. du Pont dc vestigated, including compounds bearing the -4% Nemours and Company, Inc., Wilmington, Delaware. group, the --CN group, etc. But these (10) "Progress in nylon research;' Silk I.and Rayon World, 17, are mere laboratow curiosities. so far. 15-6 (1941). "Polyamides, a note on the origin of nylon," Further and more detailed information on the (11) ~cnr~nowm; Chem. Age (London), 44, 293-4 (1941). new experimental nylons can be found in British (12) SCHIP~ER, "How nylon yam is sized in preparation for patents 525,516, 523,506, and 524,795. knitting," Rayon T a t & Monthly, 22, 2 3 5 4 (1941). LITERATURE CITED
(1) BOLTON, "The development of nylon," Chemistry and Industry,61,31-5 (1942). (2) BOLTON, "The development of nylon," Ind. Eng. Ckem., 34,53-8 (1942). (3) BOXSER, "Higher polymer chemistry of fibres made by Carothers, Am. Dyestuff Reptr., 31, 373-5 (1942). (4) DOLE,"The structure of synthetic fibres." ibid., 30, 85 ff, (1941).
(13) "Treatment for improving nylon," Textile Colorist, 63, 666 (1941). (14) WARRINGTON. "Nylon, properties in relation to silk and rayon in knitted fabrics," Can. Ckem. Process Ind., 25, 113 (1941). (15) U. S. Patent 2,071,250. (16) U. S. Patent 2,071,251. (17) U. S. Pstent 2,071,253. (18) U.S.Patent2,130.523. (19) U. S. Patent2,130,948.
