Nylon- 72-Prep aration, lor the past three to four years, a novel plastic maon a commercial scale in Europe, specifically in Germany and Switzerland. I n Japan a small trial plant producing approximately five tons a month of the same item is on stream, and here in the United States the license for its production has already been obtained by a well-known firm, where full-scale production is expected to start in the near future. This plastic material, a polyamide, is known as nylon12, so named because it has a straight-chain structure with acid amide groups and 12 carbon atoms in its monomer unit (Figure 1). I n principle, nylon-12 wcan be prepared from w-aminolauric acid-i.e., aminododecanoic acid as well as from lauryl lactam. The possibility of its preparation was already indicated by Carothers’ patent application, especially by U.S. patent 2,071,253, in 1935 and also in 1938 by Schlack’s patent application DRP 748,253. After the development of a method of Ziegler and Wilke in 1957 ( 3 )for the preparation of cyclododecatriene by trimerizing butadiene, its industrial production was considered. From cyclododecatriene, as in the case of caprolactam, there are several possible routes leading to lauryl lactam. The three more important routes are shown in Figure 2. Thus, oxosynthesis with cyclodecatriene yields oxymethylcyclododecane in the first step. Oxymethylcyclododecane can then be oxidized to cyclododecanoic acid, which, in turn, in presence of nitrososulfuric acid, forms lauryl lactam through the intermediate cyclododecanone oxime which cannot be isolated. Both of the other two syntheses lead first to cyclododecane as a result of hydrogenation of cyclododecatriene. I n one method cyclododecanone oxime is produced by light-catalyzed introduction of the nitrosogroup into cyclododecane; then, by means of Beckmann rearrangement, this oxime yields lauryl lactam. We are of the opinion that the most economic way, and therefore the one utilized by Chernische Werke Huls and by Emser Werke, is to convert cyclododecanol first to cyclododecanone. Then, by reaction with hydroxylamine it forms the oxime which, as before, through Beckmann rearrangement and with the aid of concentrated sulfuric acid and oleum, yields lauryl lactam.
F terial has been in production
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WOLFGANG G R I E H L DJAVID RU ESTEM
Because this polyamide can be prepared from w-aminolauric acid and lauryl lactam, routes to the latter are discussed
Properties, and Applications Polymerization As mentioned, nylon-12 can be prepared not only from lauryl lactam but may also be obtained from the corresponding w-amino acid. Nylon-11, Rilsan, which resembles nylon-12 in its properties, is prepared from the corresponding w-amino acid namely w-aminoundecanoic acid. However, contrary to aminoundecanoic acid, which is obtained from castor oil, no economic way has been found until now for the preparation of w-aminododecanoic acid. Therefore, in considering the preparation of polymers, we can limit ourselves to lactam as the starting material. Lauryl lactam is a colorless, easily crystallized substance which melts a t 153OC; it is sparingly soluble in water but is readily soluble in organic solvents. The s!ight solubility in water even at high temperatures requires a relatively high temperature and pressure to bring about rupture of the lactam ring in the so-called hydrolytic polymerization, as a first step to the reaction. The generated w-aminolauric acid, in the presence of water a t first produces a low molecular polyamide. After the removal of the water from the reaction vessel, the polymer is obtained easily. I n practice the reaction temperature is kept a t approximately 30OoC. T h e chain length of the polymer can be determined by the addition of mono- or dicarboxylic acids, or mono- or diamines, such as sebacic acid or benzylamine, respectively. Theoretically, addition of 1/lo0 mol stabilizer, for example, is expected to result in a number-average = 100. Howdegree of polymerization of 100--i.e., ever, traces of water, which cannot be removed completely, act as a stabilizer and therefore the degree of polymerization is found to be lower than the expected value. T o find what relationship exists between the actual degree of polymerization and important solution viscosities, we determined, together with Dr. Zarate (I), the intrinsic viscosities and through end group titrations the number-average degree of polymerization of various nylon-12 polymers. Taking m-cresol as a solvent and utilizing the Mark-Houwink relationship, we obtained = 52.2 = 52.2 [ 9 ] ’ ~ ~ ~ . the following expression: Naturally this equation holds only when the “most probable” distribution is assured. For hydrolytically polymerized nylon-12 samples, in spite of high poly-
Figure 7 .
Two production methods leading to nylon-12
Figure 2. Flow sheet i n production of lauryl lacturn VOL. 6 2
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Figure 3.
Weight fraction distribution of nylon- 72 in hydrolytic polycondensation and postcondensation
merization temperature, the “most probable” distribution can be assumed. By fractionating various polymer samples we could show that we were correct in our assumption. Conversely, for the samples which are further polymerized in solid state and for branched products as well as for anionically polymerized nylon-12, this assumption is notjustified (2). Fractionation results in solid-state postpolymerized nylon-12 are given in Figure 3. The initial product of polymerization, with a number-average degree of polymerization of 88 ( E = 88)) was fractionated into 50 fractions. I t gave an experimental distribution curve that was in perfect agreement with the theoretical Flory distribution curve. The distribution curve of the afterpolymerized product was similar in its form to that of the initial condensation product, but in this case a shift to higher molecular weights was obvious. The curve = 145 is not, however, as flat as the Flory curve for that corresponds to a degree of polymerization of afterpolymerized product calculated from 62 fractions.
AUTHORS Wovgang Griehl is the Director of Forschungsab-
Figure 4. DSC diagrams of various crystalline aliphatic polyamides
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teilung fuer Polymer Chemie, with Emser Werke A G , 7073 D o m a t l E M S , Switzerland. Djavid Ruestem is diplomatechemist at Emser Werke. This paper was presented at the 158th meeting of the American Chemical Society, September 7-?2, 7969, IVew York, N . Y., as part of the Symposium on Novel Processes and Technology of the European and Japanese Chemical Industries.
TABLE I.
PHYSICAL PROPERTIES O F NYLON-I2 COMPARED W I T H THOSE OF NYLON-6 AND POLYETHYLENE I N CONDITIONED STATE
Properties Melting point range, O C Specific gravity, g/cc Water absorption, 65% RH at 2OoC, yo Tensile strength a t break, psi Notched impact strength, cm psi Indentation hardness (Ball test), psi Modulus of elasticity, psi Linear swelling in water, yo Specific resistivity, ohm. cm Dielectric loss factor at 106 c/s Water vapor permeability (0.05-mm thick films), g/m2 24 h Oxygen permeability (0.05 mm thick films), cc/m2 24 h 1 atm Aroma impermeability Grease impermeability Solvent absorption in 1000 h at 20°C Petroleum ether, yo Methanol, % Water,
Nylon-12 can also be obtained through anionic polymerization of lauryl lactam. This method must be carried out strictly in the absence of water, whereas in the case of nylon-12, this did not present exceptional difficulties. To effect polymerization it is necessary to use alkali or alkaline earth metals together with cocatalysts. Certain difficulties arise when-as is normally the case-a working temperature below the melting point of polymer is desired. These difficulties are the result of the relatively high melting point of lactam (153"C), as well as the melting point of 180'C for the polymer.
Properties
Nylon-12 combines the excellent mechanical properties, such as hardness, tensile strength, and resistance to abrasion, of the well-known polyamides-Le., of nylon4 and -66, with other outstanding properties, namely low water-sensitivity and density of polyolefins. Some of its physical characteristics can be seen in the thermograms given in Figure 4. For purposes of comparison, the DSC diagrams of nylon-6, -11, and -66 are also included. Also nylon-12, with 18OoC, has the lowest melting
Polyethylene 125-130 0.96 0.01 3560 213 5690 142,000 0 1 x 10'6 0,0006 1
Nylon-6 215-220 1.14 4.4 8530 421 7110 212,000 2.5 0 . 5 x 1012 0.11 35
Nylon-72 178-180 1.01 0.85 9240 427-711 9240 170,000 0.2 i x 1014 0.05 9
130 Good Good
300 Good Good
1500 Not good Not good
0.6 15.4 10.5
1.4 8.5 1.5
8.6 0.1 0.08
point among all the important polyamides-a melting point, however, which is high enough for most practical purposes. T h e glass transition temperatures (Tg)of these polyamides are more or less the same, though a tendency toward lower glass transition temperature with the increasing hydrocarbon chain length between the amide groups is apparent. Here it might be of interest to point out that, in comparison with other polyamides, nylon-6 has the lowest solidification rate. Because of its relatively long hydrocarbon chain, nylon-12 has a density of only 1.01, in contrast to 1.15 for nylon-6 and -66. Similarly, the relatively low rate of water absorption of nylon-12 is a property due to its structure (almost a paraffin structure). I n Figure 5, the rate of water absorption of various polyamides as a function of the number of "C" atoms in the monomer unit are shown. These figures were obtained from the experiments which were carried out a t room temperature and a relative humidity of 65%. T h e fact that all important properties of nylon-12 lie between those of nylon-6 and' polyethylene can be attributed to the comparatively long hydrocarbon chain which constitute the repeat units of this polyamide. I n Table I we tried to compare the properties of nylon-1 2 with those of low-pressure polyethylene and VOL. 6 2
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Figure 5.
Water absorption of aliphatic homopolpmides.
R h 65%,
T 2OoC
Figure 6. Reequilibration of nylon- 72 at various temperatures
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nylon-6. Here it is worth mentioning that the properties of nylon-66 are very similar to those of nylon-6. All these figures were obtained from those products which were conditioned before the measurements. The physical properties of nylon-12, even its melting point, lie almost always between those of nylon-6 and polyethylene. Referring to the tensile strength a t break and notched impact strength, we note that these two properties are strongly dependent on the molecular weight of the sample. Products of a higher degree of polymerization-i.e., products with higher melt and solution viscosities-possess better qualities, as far as hardness and impact strength are concerned, than the products of lower degree of polymerization. I n the case of nylon-6 and -66, however, these important properties are affected significantly by the water content of the material and the experimental temperature. For the sake of clarity, Table I gives as few figures as possible, and therefore, some of the details are omitted; but it is worth mentioning that notched impact strengths of unconditioned samples of nylon-6 or -66 (samples which were dried), for example, fall to a value which is only 10% of the value given by conditioned material. Cooling a sample from 20°C to -20°C reduces its notched impact strength by half. We remember the time when the injection molding technique was first used in the plastic industry; then we had a pocket comb made of nylon-6. Normally it could be bent so that its two ends touched each other. One day it was very cold and the air was dry; on that day a slight bending strain was enough to break the comb in two. Contrary to what was said for nylons 6 and 66, notched impact strength of nylon-12 is affected only very slightly by dryness or extensive cooling. Please note the slight linear swelling of nylon-12 in water. Here again nylon-12 lies between nylon-6 and polyethylene. This low linear swelling confers extraordinary dimensional stability on nylon-12, and a glass-fiber reinforced product possesses still better properties. The electrical properties shown in Table I are significant. Although they are inferior to those of polyethylenes, their applications in the electrical industry are still justified. The water vapor permeabilities of nylon12 films are measured a t a pressure of 1 atm and a relative humidity of 65y0. I n contrast to polyethylene, the aroma impermeability of this polyamide is rather good, and therefore nylon-12 films are gaining increasing importance in the food industry as packing material. Frequently nylon-12 is added to polyethylene films to improve their water vapor permeability and aroma impermeability properties. Among its chemical properties, nylon-12 maintains
good stability in petrolether and methanol. The same stability is maintained in other organic solvents, such as butylacetate, benzene, and plant and mineral oils, but phenolic solvents and some chlorinated hydrocarbons have a deterrent effect on it. A gradual split of amide groups of nylon-12 takes place in mineral acids and concentrated alkalies. T o differentiate between nylon-12 and nylon-6 (and -66), concentrated formic acid normally can be used as a test substance. In contrast to the ready solubility of nylon-6 (and -66), nylon-12 remains insoluble. Unfortunately, all aliphatic polyamides, including nylon-12, are only moderately stable to heat and light. However, addition of a small amount of a stabilizer considerably improves this property. Because of its high processing temperature and little water affinity, a special technique is necessary in working with nylon-12. Despite its low water uptake, nylon-12 is sensitive to hydrolysis. Therefore, in working with this polyamide, especially during processing in extruders, this point has to be borne in mind. For example, a water content of O.lYc lowers the melt viscosity of nylon-12 within 20 min a t 270°C from 5700 to 3300 P. Water content higher than this value brings about a significant degradation, and a lower water content may result in further condensation if the products are free from stabilizers. Like all polycondensates, nylon-12 contains a certain percentage of short-chain components depending on the degree of polymerization which corresponds to a Flory distribution function. These oligomers, in contrast to nylon-6 oligomers, are practically insoluble in water, and their value, like polyester and nylon-66, lies in the region of l.Oyc. (Dacron and nylon-66 contain 1.5% oligomers.) As they are almost insoluble in water and because of their insignificant amount, it is not necessary to remove them from nylon-12 even when the product is to be used as packing material in food industry. However, they can be removed by repeated refluxing with an organic solvent such as methanol. I n nylon-6, the classical polyamide prepared from a lactam, the amount of oligomers not only depends on the degree of polymerization but is also influenced, even to a greater extent, by the polymerization (or processing) temperature. Whereas nylon-6 polymerized a t 180°C contains only 3% oligomers, at 28O"C, polymerized nylon-6 contains more than 12% oligomers. Here a n equilibrium between nylon-6 and its oligomers is involved. After the elapse of some time, a t the above-mentioned temperatures, oligomer-free nylon-6 depolymerizes to form the same amount of oligomers again. We wanted to establish the existence of such a tempera ture-dependent oligomer-polymer equilibrium in the
Figure 7. Reequilibration of nylon- 72 with diferent water contents at 280°C
case of nylon-12, too. For this purpose, after extracting the oligomers from a nylon-12 sample of a degree of = 80, and allowing it to dry in vacpolymerization of uum a t 9O"C, we heated small amounts of this sample a t various temperatures in an atmosphere of nitrogen and in sealed glass tubes. Figure 6 shows that the methanol soluble compounds, after the establishment of the equilibrium, always lie in the region of l.Oyc,and this value is independent of temperature. I t follows, therefore, that temperature has no influence on the ultimate establishment of the equilibrium. I t is clear that the establishment of an equilibrium takes a long time and that the rate of re-formation of oligomers depends on the experimental temperature. Further, this rate of reequilibration is influenced strongly by the amount of water present in the sample (Figure 7). All polymer samples under investigation had the same degree of polymerization-i.e. P, = 80. As a stabilizer they contained sebacic acid. After extracting the methanol-soluble compounds, the samples were so treated that they contained various amounts of water. (The actual amount of water in each sample can be seen in Figure 7 . ) The experiment was carried out as before in sealed tubes, and the temperature chosen for this investigation was 280°C. Analogous to nylon-6, the greater the amount of water in the polymer sample, the higher was the rate of reequilibration. Application
The use of nylon-12 is essentially connected with its exceptional characteristics : Exceptional toughness and impact strength which result from its polyamide structure VOL. 6 2
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Low water absorption due to its rather long aliphatic chain between the -CONH- groups Good resistance to chemicals Relatively low melting point Because of these characteristics the main field of application of nylon-12 is in the plastic industry including the preparation of films, as well as sheets and sinter powder for coating metals. The preparation of nylon-12 can easily be controlled to yield a product which satisfies all requirements, particularly in connection with degree of polymerization and melting behavior. Unlike many other polymeric materials such as vinyl polymers or polyolefins, processing nylon-12 involves no difficulty in maintaining the same molecular weight distribution and the same reproducible viscosities. As in its preparation, here also the low water content of the polymer plays a major role. This novel polymer owes its applicability in the electrical field to this low water content. 'The same can also be said for its application in other fields where dimensional stability--a strongly moisturedependent property-is a t premium. As a result of this characteristic, nylon-12 has secured its place in the European electrical industry. A great quantity of it is being prepared for covering cables. Other applications of nylon-12 are made in the automobile industry and in hydraulic systems. I n the former, it is used to prepare oil- and gasoline-resistant tubes. Components produced in nylon-1 2, especially odorfree films, are finding increasing applications in the food industry. For instance, in one section of the food industry, blown nylon-12 films are being used as sausage skins. Sterilized films and bags are also used in the pharmaceutical and medical fields. Finally, the use of nylon-12 films in electrical engineering as insulating material must not be underestimated. Further applications of nylon-12 in plastic engineering by means of injection molding are in machining precision parts such as cogwheels, screws, injection syringes, and insulation material, as well as in the manufacture of ship propellers. In this connection, the authors mention that the usual polyamide additives, such as plasticizers, crystallization promoters, pigments, and heat and light stabilizers can also be added to nylon-12, either during the polymerization or afterward, to modify the product and make it suitable for its ultimate usage. Glass-fiber reinforced polymer can also be prepared and finds application where exceptional rigidity and hardness are desired. Another application field for nylon-12 involves its use in coating metals. For this purpose the polymer used must be in powder form. T o obtain such a powder, the material is either pulverized in a grinder or precipitated from its solution in a solvent such as cyclohexanol or caprolactam. The metals are then coated either by a whirl-sintering process or by electrostatic methods. I n the former the particle size should be between 10022
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200p, whereas in the latter a particle size of less than 100p is quite adequate. I n addition to coating metals, powdered nylon-12 is also used in the clothing industry for use as an adhesive powder to fix the stiffening materials and in tailoring stitch-free hems. But, here, to meet the meltingpoint requirement between 120-140°C, one of the copolyamides of nylon-12 is used. The conditions in preparing this copolyamide have to be so arranged so that the resulting adhesive powder will not be dissolved by common cleaning agents. The survey of applications of nylon-12 will not be complete if we do not add a few words about the uses of this plastic material in the textile industry. Being a polyamide with a straight-chain structure, nylon-12 can, naturally, be spun without difficulty. However, because of its low melting point and flat stress-strain curve, its use in technical fields, for instance in tire cords, is out of the question. O n the contrary, despite its low water absorption, it is suitable for the preparation of clothing, even of socks and underwear, where its soft touch leaves the wearer with a comfortable feeling. For a long time it was thought that synthetic fibers were suitable only for clothing fabrics such as natural wool, silk, and cotton fibers when they possessed a high rate of water absorption, namely 20 to 30%. Even today research continues for polyamides with high water absorption properties. But experience has shown that for a comfortable wear, the water transport property of clothing materials is more important than a high water absorption ability. This water transport can be influenced extensively by the shape of the fiber surface (such as profile fibers) and by crimping (as in texturing) in conjunction with the ingeniously constructed fabric structure. Here it is opportune to draw attention to the fact that the most celebrated polyester fibers absorb only O,GYo water at normal temperature and 65y0 relative humidity. Because of its good dimensional stability, nylon-12 should be suitable for well-fitting swimwear. To verify this property, swimwear was prepared from nylon-12 fabrics and tested. Results showed that the swimwear preserved its well-fitting properties and forms even in a wet condition. It was observed that the drying of the swimwear, prepared from voluminous yarn, was extremely slow. This shows once again that the surface forces, the volume, and the fabric structure of the material and not the water absorption property of the fiber, are responsible for the exhibition of the abovementioned characteristics of a textile product. We hope that with this limited paper, everyone will become acquainted with nylon-12. We feel certain it will help fill the many gaps in plastics engineering. REFERENCES (1) Grichl, W., and Zarate, J., Plarluerarbeiter, 18, 527-531 (1967). ( 2 ) Griehl, W., and Zarate, J., ibid., p 891-896. (3) Wilke, G., Angew. Chemie, 69, 397-%8 (1957).
