INDUSTRIAL AND ENGINEERING CHEMISTRY
MARCH, 1940
about this rearrangement of 1,4-dihydronaphthalene at low temperatures just as sodium ethylate does a t higher temperatures. The fact that mixtures of 1,4 and 1,Bdihydronaphthalene containing up to 70 per cent of the latter are polymerized to clear soluble resins containing none of the insoluble 1.2-uolvmer lends credence to this theow. Although this
thalene in a satisfactory manner.
315
Literature Cited (1) Bamberger and Lodter, Ber., 20, 3705 (1887). (2) Braun, van, and Kirsohbaum, Ibid., 54,604-10 (1921).
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(5) Scott, u. s. Patent 2,146,447(1939). ( 6 ) Scott and Walker, Ibid., 2,055,708(1936).
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ill) Walker,'U. S: Patent 2,168,011(1939).
VINYL RESINS Influence of Chemical Composition upon Properties and Uses S. D. DOUGLAS Carbide and Carbon Chemicals Corporation, South Charleston, W. Va.
In common with other synthetic resins, the characteristics and usefulness of the vinyl resins may be varied by suitable changes in manufacturing conditions. As an example of resins of this type, the vinyl chloride-vinyl acetate copolymers may be modified by controlling degree of poly0 ONE familiar with the complexities of synthetic resins i t is apparent that wide variations in physical and mechanical properties may be attained by appropriate modifications of the manufacturing process. I n studying the particular class of resins formed by the conjoint or simultaneous polymerization of two typical vinyl compounds, vinyl chloride and vinyl acetate, an excellent opportunity has been offered to consider the effect of variation in chemical composition upon the characteristics and practical usefulness of the resin. In order to discuss these vinyl chloride-vinyl acetate resins adequately, it will be desirable first to describe briefly the synthesis, structure, and characteristics of the vinyl resins in general and of polyvinyl chloride and polyvinyl acetate in particular.
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Synthesis and Structure of Vinyl Resins Vinyl resins in general are prepared by polymerization rather than by a condensation reaction typified by the phenolic, urea, and alkyd resins. The resin molecule consists of a linear chain in which the monomers have reacted with one another a t the double bond to form high molecular weight polymers. The reaction may be brought about by irradiation with ultraviolet light or by addition of a small amount of a peroxide, ozone, or tetraethyllead (3). Vinyl compound polymerization is a chain reaction in which a large number of molecules react in rapid sequence to form one macromolecule. Polymerization is influenced by several factors. For example, traces of certain impurities act as inhibitors and either retard the rate of polymerization or lower the molecular weight of the resin formed, or both.
merization and vinyl chloride-vinyl acetate composition. The properties of the resulting resinshandtheir industrial applications are described in detail. A brief discussion of polyvinyl chloride and polyvinyl acetate, and their relation to the vinyl chloridevinyl acetate copolymers is included. Rate of polymerization varies directly as the square root of the catalyst concentration and doubles with every 8" C. rise in temperature. It is also directly proportional to the concentration of the vinyl compounds present. I n other words, solvent lowers the rate of polymerization, and the amount of reduction is specific for each solvent. Average molecular weight, or degree of polymerization of the resin produced, is directly proportional to the solvent concentration in the charge. Compounds that are solvents for the monomer but not for the polymer affect the molecular weight in the same way as do solvents for the polymer. Each solvent has a specific effect upon the average molecular weight. The latter decreases with increase in temperature and in catalyst concentration. The properties of the resulting resin are closely associated with its molecular weight and with the relative quantities of the various polymer bands of which the resin is composed. Certain characteristics vary with the average molecular weight, others are independent of it. Thus tensile and impact strengths, abrasion resistance, and viscosity in solution increase, while water absorption, refractive index, hardness, and electrical properties remain practically constant. Solubility in organic solvents rises with decrease in molecular weight.
Polyvinyl Chloride and Acetate Turning now to a consideration of specific vinyl resins, some of the properties and applications of polyvinyl chloride will be mentioned. This material is unusually strong and water- and chemical-resistant, but i t softens so slowly with rise in tem-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
perature that decomposition begins before it can be properly molded. For this reason the resin is always plasticized by mixing on a hot roll mill or in a jacketed dough mixer with a plasticizer, usually a compatible high-boiling liquid such as tricresyl phosphate. Upon cooling, a resilient rubberlike mass is formed which has been used successfully in numerous a p
plications requiring greater resistance to sunlight, oxidation, oils, and greases than is exhibited by robber. Such uses include wire insulation, tubing, impregnated cloth for raincoats and shower curtains, and linings for acid-resistant containers. The poor solubility of polyvinyl chloride in most organic solvents limits its use in the surface coatings field. Polyvinyl acetate, on the other hand, has very dinerent properties. It softens a t a temperature but little above atmospheric, is brittle, and has a relatively high water absorption. It does, however, have excellent heat stability and clarity. Because of the limitations mentioned, polyvinyl acetate is not particularly suitable for molding or for surface coatings, but it is used widely as a water-insoluble adhesive.
Preparation of V i n y l Chloride-Vinyl Acetate Copolymers
It has been pointed out that polyvinyl chloride is strong but must be internally plasticized, and that polyvinyl acetate is brittle but softens a t a low temperature. Upon comparing the characteristics of these two resins, it would appear that mixtures of them should make an interesting and useful product. But when such mixtures were prepared, it was quickly discovered that the two polymers were incompatible and that the mixtures were weak and useless. It was then found (6) that when the monomeric vinyl compounds were polymerized simultaneously, a resin was fonned that retained the strerigtli and water resistance of polyvinyl chloride but that had been sufficiently plasticized intcrnally by the combined polyvinyl acetate to make i t moldable at temperatures within the range of its heat stability and soluble in many organic solvents. Thus a resin molecule was formed that consisted of a linear chain in which monomeric vinyl chloride and vinyl acetate
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had reacted with themselves and with one another a t the double bond to form a conjoint polymer. The relative proportions of vinyl chloride and vinyl acetate in the chain depend upon the composition of the reaction mixture; and the length of the molecule, as in the case of other vinyl polymerizations, is governed by reaction conditions a t the instant of formation (9). Average molecular weight may also be controlled cy fractionation Grid extraction (8). Extraction is accomplished by partial precipitationorby treating thefinely divided dry resin with an appropriate solvent. nonsolvent mixture that dissolves out the lower polymers and leaves the higher hands intact as a granular powder. The oopolymerization of these compounds makes possible the synthesis of a wide range of resins suitable for a variety of industrial applications. Not only is it possible to vary the average molecular weight as in the case of other vinyl resins to suit the application involved but to alter vinyl chloride-vinyl acetate ratio as well. Thus, in common with other vinyl resins, increase in the average molecirlar weight or degree of polymerization of these copolymers results in greater strength and toughness but makes the resin more difficult to mold or to put into solution. Electrical properties, 8pcifie gravity, refractive index, and water resistance remain constant. On the other hand, increase in vinyl chloride content improves water resistance and raises the softening temperature, hut yields a less soluble and less easily molded product. In deciding upon a resin for any specific application, a type is naturally oliosen that combines as many desirable properties as possible with a minimum of undesirable ones. Resins representing many of the numerous possible comhinations of average molecular weight and composition have been made, and uses discovered for them; others have not yet found a,pplication,and still others have yet to be prepared and examibed. At this time the resins included in the first of these three groups will be discussed. In order to properly classify these materials, it will be convenient to identify them by their combined vinyl chloride content and by their relative average molecular weight. The term “relative” is used because it is not claimed that the values given are necessarily the actual average molecular weights of the resins under discussion, but that the figures are a t least comparative and serve the purpose adequately. The method employed for the determination of average molecular weight is a combination of the familiar freezing point depression measurement and viscosity in dilute solution. Resins of low molecular weight are evaluated cryoscopically, and those more highly polymerized, by measuring their viscosities in dilute solution in an Ostwald viscometer. Values are calculated from the familiar Staudinger equation. Properties and Applications of V i n y l Chloride Vinyl Acetate Resins The copolymer resins have excellent strength and resistance to water, acids, alkalies, alcohol, and oil, and good electrical insulating properties. Heat stability is good, although continued exposure to elevated temperatures causes an appreciable darkening in color. For this reason various types of stabilizers are used, including calcium stearate, lead stearate, or white lead (6).
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INDUSTRIAL AND ENGINEERING CHEMISTRY
SURFACE COATINGS.I n outlining the results that have reached practical application, it may be appropriate to begin with those at the lower end of the average molecular weight range and vinyl chloride content. Accordingly, the resin used in the surface-coatings field will be discussed (7). It has a molecular weight of 8500 to 9500 and contains 85 to 87 per cent vinyl chloride. This polymeric molecule is large enough to give a film of excellent toughness but small enough to permit adequate solubility. As solvents it is preferable to use ketones, which yield solutions of minimum viscosity, s u p plemented by aromatic hydrocarbons (I). The latter are swelling agents rather than solvents but do not appreciably decrease the solvent action of the ketones when used in moderate amounts. All surface coatings made from these copolymer resins must be baked a t relatively high temperatures in order to secure proper adherence. These lacquers have fouud extensive use in lining cans and in coating aluminum foil, concrete, asbestos hoard, and labels, among others. They dry by evaporation rather than by oxidation or further polymerization. COATEDPAPER. The next copolymer in the series is a resin similar in composition to the type just described, but having an average molecular weight of 9500 to 10,500. This material was developed for use in coating paper. 111 this application the resin containing suitable modifiers, including pigment, plasticizer, and stabilizer, is calendered onto the paper and forms a film about 0.002 inch thick. The paper thus treated is widely used in lining various types of closures. If a thinner film is desired, as in the coating of paper labels
317
or wrappers, the resin is applied by roller coating from solution, the lacquer resins described above being used for this purpose. PHONOGRAPH RECORDS, FLOORTILE. Almost identical with this paper-coating resin is a variety used in the manufacture of phonograph records (4). It has been found over a period of years that these records give better sound reproduction than do those made from shellac, and that they have greater mechanical strength and toughness. For these reasons radio transcription records are molded from this material. I n addition to the resin, fillers, stabilizers, and pigment are present in the finished article. This resin has also been used successfuUy in the manufacture of floor tile. For this purpose it is mixed with slate flour and pigmented to any desired color. INJECT~ON MOLDINQ.Among the more interesting and important applications for a thermoplastic material is the rapidly growing injection-molding industry. Here a resin is required that flows readily a t the molding temperature, shrinks slightly in the mold, and yields a strong sound casting. Most suitable for this purpose is a copolymer having a molecular weight of 9500 to 10,500 and a vinyl chloride content of 85 to 87 per cent. This resin provides maximum strength and toughness in the molded article and adequate fluidity a t the molding temperature. Low water absorption characteristics and absence of high concentrations of fugitive plaaticizer result in freedom from warpage and in low shrinkage. They also provide greater accuracy, better siae control, higher finish, and a more perfect reproduction of the mold surface. On the
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ISDUSTRIAL AND ENGINEERING CHEMISTRY
other hand, softening temperature is not so high as desired for some purposes, a l t h o u g h this situation is being steadily improved. Limited heat stability has been a handicap, but recently developed stabilizers have largely overcome this difficulty. Because of this heat stability problem the conventional heating cylinders of the automatic injection-molding machines have been completely redesigned. As a result several of the machines now on the m a r k e t a r e provided w i t h these new streamlined cylinders XThich are usable with all types of injection - molding resins, gen er all y with improved efficiencies. These injectionmolding compounds flow best at a generally lower temperature than do other similar materials. I n fact, this desirable plastic behavior is not improved by the use of higher inj e c t ion tempera(Above) THE“VIKYLITE”RESINSHEETS tures. This USEDIN THISNICKEL-PLATING BARREL ACTIONOF THE RESISTTHE CORROSIVE feature, combined PLATING BATH with inherent low gravity and speci(Below) TENSIONTESTINGA JOINT MADE WITH “VINYLITE” RESINADfic h e a t , means HESIVE “XL-5041” that appreciably The steel panel is tearing while the less h e a t is rebond remains intact. quired during the injection cycle, a measurable factor in operating economy. The mix used in injection molding includes lubricant, plasticizer, stabilizer and pigment, or dye. Small articles, such as combs and toothbrushes, made by the injection-molding process are now in commercial production, and larger items have been made experimentally. COMPRESS~ON MOLDING.For conventional compressionmolding application, a material having an average molecular weight of 12,000 to 13,000 and containing 85 to 88 per cent vinyl chloride is generally used. It is somewhat stiffer a t the molding temperature than any of those previously mentioned. The development of injection molding has resulted in
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a corresponding decrease in the use of compression-molding technique, and for this reason resin of this type is not used in quantity at present. SHEET-STOCK’ RES~N. In the manufacture of plastic sheets, outstanding strength and toughness are required. At the same time it is not necessary that the resin have as high plasticity at molding temperatures as is required in injection molding. Accordingly, a grade having a higher average molecular weight (15,000 to 16,000) and containing 88 to 90 per cent vinyl chloride is used. The strength and water resistance of this resin have made possible the production of a tough, nonclouding, nonwarping sheet that is finding use in bookbindings, radio dials, advertising novelties, and numerous other applications. As in the case of other types of thermoplastic sheet, the finished article is fabricated by warming the resin until it can be readily cut and shaped. In addition to resin, these sheets usually contain suitable plasticizer, pigment, and heat stabilizer. Fillers may be used but are ordinarily unnecessary. Unlike thermosetting resins, fillers such as wood flour or alpha-pulp weaken the resin mechanically rather than strengthen it, and since they also increase water absorption and cloudiness, they are rarely used. SYNTHETIC TEXTILES. One of the most interesting and important uses for a resin of the general type under discussion lies in the broad field of synthetic textiles. It is apparent that only thermoplastic linear polymers, as exemplified by the new diamine-dibasic acid resins, may be used. It is likewise evident that the greater the length or molecular weight of the macromolecules comprising this polymer, the greater will be the strength and resiliency of the resulting fiber. Accordingly, copolymers are chosen that are still more highly polymerized than any previously described (9). As will be mentioned later, two types have been developed. One has a molecular weight of 16,000 to 19,000, and the other, 20,000 to 23,000; both resins contain 88 t o 90 per cent vinyl chloride. The fiber is made by extrusion from solution through spinnerets, solvent being removed by warm air. Multifilament yarn of widely varying denier, including filament deniers considerably under that of natural s a , may be fabricated. Because of its low water absorption, wet and dry tensile strengths are practically the same. As would be expected, this value can be greatly increased by stretching without embrittling or otherwise lowering the quality of the yarn. The latter is “set” by heating under tension to about 65” C.; then the yarn is stable with respect t o shrinkage, up to the temperature of the r‘set”. Above this temperature, shrinkage occurs, accompanied by a slight reduction in tensile strength and a corresponding increase in elongation. Shrinkage or crimpage may also be controlled by use of a solvent-nonsolvent bath maintained a t a suitable temperature. By this device it is possible to reduce runs in knitted goods or thread slippage in woven goods. Controlled porosity of filtering fabrics is an important application of this principle. Copolymer resin woven into other materials, such as cotton, wool, or rayon staple, and heated appropriately, gives crease permanency, crease resistance, and improved strength and body, particularly when the article is wet. The thermoplastic nature of this material makes it of special interest when woven into cotton or other fabrics as an interlayer for starchless shirt collars, similar to the familiar Trubenizing process. Copolymer fabrics are not attacked by mold or bacteria, a characteristic particularly valuable in damp climates or in special uses such as awnings, fish lines, and fish nets. It was established during a recent experiment near the coast of Florida that fish nets made from copolymer resin caught twice as many fish as did the conventional impregnated cotton nets. Furthermore, in a &month test, nautical twines made of this
MARCH, 1940
INDUSTRIAL AND ENGIYEERING CHEMISTRY
material showed no deterioration, while all of the others tested disintegrated either partly or completely during this period. Because of the chemical inertness of the copolymer resin, industrial uses have been largely investigated but other outlets are being developed. The lower molecular weight type is designed for use in the manufacture of staple fiber which has shown considerable promise in the fabrication of wool, cotton, and glass felts. Upon heating, the thermoplastic resin acts as an efficient binder, gives added strength, and permits the manufacturing time to be greatly shortened. The other grade mentioned is somewhat stronger and is used for continuous filament. This material offers possibilities in the manufacture of long-lasting knitted and woven goods and braided articles, especially such items as chemically resistant hose, sport goods, and nautical appliances used under such conditions that the thermoplastic properties of the yarn offer no disadvantages. Other valuable characteristics of these fibers include true elasticity, obtainable only in a linear polymer, noncombustibility, and low electrical conductivity. Although it is possible to attain in commonly used fabrics most of the properties desired, such a result can be accomplished only by a series of impractical treatments. These treatments lack permanency and are prohibitively expensive. The use of these copolymer fibers, which appear to have numerous attractive characteristics not common to other textile materials, will probably reach considerable Volume in this field. COATEDWIRE. I n the discussion of polyvinyl chloride, the use of this resin in plasticized form as a substitute for rubber in wire insulation, tubing, and cloth coating Tvas described. Although this resin has a number of interesting and valuable characteristics, it is believed that another member of the copolymer series looks still more promising. This product, having a molecular weight of 20,000 to 22,000 and containing 95 per cent vinyl chloride and 5 per cent vinyl acetate, may be plasticized on the roll mill with any one of a large number of plasticizers to form a rubberlike mass particularly suitable
319
for the purposes mentioned. Coated wire, for example, is usually fabricated by extruding the plasticized resin around the wire in any predetermined dimension. Copolymer-insulated mire offers a number of advantages over the conventional rubber-covered variety. By appropriate pigmentation, any desired color or combination of colors may be used, either in opaque or transparent grades. Small bores, thin wall thicknesses to close dimensions, and uniformity of section may be obtained. Varying but controlled hardness may be secured by adjusting the concentration of plasticizer. Electrical properties are excellent, and chemical inertness is much superior to that of rubber. There is no corrosion of the wire and there is no aging or oxidation upon long exposure t o sunlight and air. This resin is also used in the manufacture of electrical insulating tape. The choice of plasticizer depends upon the nature of the application. Plasticizers are available that are almost completely insoluble in gasoline and oils, and others that are especially suitable for use at extremely low temperatures. Since these copolymer resins may be regarded as polyvinyl chloride internally plasticized with polyvinyl acetate, it is evident that comparatively low concentrations of external plasticizer are sufficient to produce the rubberlike characteristics needed.
Literature Cited (All citations are to United States patents.) Doolittle, A. K., 2,136,378 (1938). Douglas, S. D., 2,055,468 (1936); Reid, E. W., 2,064,565 (1936). Douglas, S. D., 2,075,575 (1937); Young, C. O., and Douglas, 8 . D., 1,775,882 (1930); Shriver, L. C., 1,938,870 (1933). Groff, F.. 1.932,889 (1933). Groff, F., 1,966,856 (1934); Young, C. O., and Douglas, S. D., 2,013,941 (1935); Duggan, F. W,,2,126,179 (1938); Carruthers, T. F., and Blair, C. M., 2,157,068 (1939). Reid, E. W.,1,935,577 (1933); Young, C. O., and Douglas, S. D., 2,011,132 (1935); Douglas, S. D., 2,075,429 (1937). Reid, E. IT., 2,052,658 (1936); Doolittle, A. K., 2,140,518 (1938), 2,141,126 (1938), and 2,161,024 (1939). Robertson, H. F., 1,921, 326 (1933); Young, C. O., and Douglas, S.D., 1,990,685(1935). Feild, T. A , , Jr., and Codon, J. F., 2,161,766 Rugeley, E. W., (1939).
“VIKYLITE”
RESIN
ADHESIVES BOTH BONDA N D INSULATE
LAMINATIOKS FOR THE CORES OF SMALL ELECTRIC MOTORS Left, before; center, during; right, after ~ t & sembly by heat and pressure. C o u r t e s y , A i a z Electrothermic Corporation
