VINYLIDENE CHLORIDE POLYMERS W. C. GOGGIN AND R. D. LOWRY The Dow Chemical Company, Midland, Mich.
Vinylidene chloride plastic as a new and unusual material is discussed from the standpoint of history, chemical and physical structure, outstanding characteristics, methods of fabrication, and applications. These resins differ from familiar plastic materials in that they exhibit crystallinity. This crystallinity is demonstrated by x-ray diffraction patterns. While presenting some mechanical problems, the control of this crystallinity offers a wide range of prop-
S
OME hundred years ago the French chemist Regnault encountered a strange new fluid (6) which was later determined to be unsym-dichloroethylene, now more commonly known as vinylidene chloride. From that time until the beginning of the last decade, this material has been mentioned in the literature only rarely. I n 1922 Brooks (2) indicated that halogenated ethylenes other than vinyl chloride and vinyl bromide show a tendency toward polymerization. Staudinger and Feisst (7), reporting in 1930 on the polymerization of an apparently impure unsymdichloroethylene, indicated that the liquid polymerizes quickly in light or slowly when kept in the dark. This polymeric material is completely saturated, and its structure is represented by the long chain -CHrCCl*-CH2-CClr. Fiesst reported the polymer to be crystalline, as later confirmed by Natta and Rigamonti (6).
erties and unique fabrication techniques. The extrusion and continuous orientation of vinylidene chloride plastic is now a commercial accomplishment. Injection molding of these resins, along with control of molding properties, is presented for the f i s t time. Applications are cited illustrating some fabrication methods as well as the unbsual characteristics of chemical inertness, water resistance, high strength, toughness, and abrasion resistance. As a result, it was possible to introduce the first vinylidene chloride polymers commercially early in 1940.
Chemical and Physical Structure Petroleum and brine are the basic raw msterials. Ethylene, made by cracking petroleum, and chlorine, from the electrolysis of brine, combine to form trichloroethane, which is converted t o vinylidene chloride as shown in Figure 1. It is a clear, colorless liquid having a boiling point of 31.7' C. and the structural formula CH2=CC12 (Figure 2). Vinylidene
PETROLEUM
TRICHLORETHANE
EQUIPMENT
VINYLIDENE
-) SPECIALLY OESlONED MECHANICAL DEVICES
I
CHLORIDE MONOMER
POLYMERIZER
I MOLDING
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GRANULES
1
FOR THE PRODUCTION FIGURE 1. FLOWSHEET OF VINYLIDENE CHLORIDE
The development of vinylidene chloride by The Dow Chemical Company began actively over a decade ago. Research men investigated the material while working on chlorinated aliphatic compounds. Based on that original study, a thorough program on vinylidene chloride was instituted ( 1 ) .
have sottening FIGURE 2. STRUCTURAL FORMULA FOR points ranging VINYLIDENE CHLORIDE (CH,=CCl,) from about 70"to Large spheres represent chlorine and small a t least 180' C. ones, hydrogen Soft flexible materials to hard rigid materials can be obtained. Present commercialpolymers have softening points from about 120' to 140' C., with a molecular weight of approximately 20,000. These polyvinylidene chloride plastics are known by the trade name, "Saran". To a greater or lesser degree Saran exhibits regions of crystal structure. This property can be demonstrated readily by its x-ray diffraction pattern. Most organic thermoplastics exist in an amorphous state and do not exhibit crystallinity. Under special conditions Saran can be made amorphous. Figure 4 (top) is a diffraction pattern of Saran substantially in this state. When allowed to return to room temperatures, Saran gradually changes to its normal crystalline state and shows the ringlike diffraction pattern of Figure 4 (center). 327
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FIGURE3. STRUCTURAL FORMULA FOR POLYVIKYLIDENE CHLORIDE (-CH~-CC1~-CH~-CC1~-) Amorphous Saran can be converted by mechanical working to an oriented crystalline state, showing the lattice diffraction pattern of Figure 4 (bottom). The three physical conditions of Saran (amorphous, normal crystalline, and oriented) can be seen by viewing specimens through crossed polaroids. Figure 5 is a picture of a sample in the amorphous state; one part has been recrystallized by heat treatment, and the other section has been recrystallized and oriented by mechanical working.
Genera1 Properties Saran can be obtained in a wide range of properties (4). For simplicity, a typical Saran formulation has been selected, and its general properties are shown in Table I. Specific properties of various fabricated forms will be discussed under the section on fabrication development. TABLEI. GENERAL PROPERTIES OF Effect of Effeot of Effect of Effect of
weak acids strong acids weak alkalies strong alkalies
Effect of organic solvents Water absorption Water permeability Burning r a t e Thermal conductivity Specific heat Index of refraction S ecific gravity &me resistivity (d. 0.1 Dielectric strength (BO cvcles) Dielectric oonstint (60 iycles) Power factor (60 cycles) Effect of age Effect of sunlight Machinability Color possibilities
A
SARAN FORMVLATION
None Darkens in HISOI; otherwise none None Affected b y ",OH, darkens in caustic: otherwise none Highly resistant 0.00 % Very low None 2.2 X 10-4 cal./sec./sq. cm./O C./ om. 0.316 tal./' C./gram 1.61 1.70 1014 - l O l e ohm-om. 500-2500 volts/mil. 4 0.03-0.08 None Slight Good Unlimited; transparent to opaque
One of the outstanding characteristics of Saran is its resistance to chemicals and solvents. At room temperature it is extremely resistant to all acids and to all common alkalies, except for concentrated ammonium hydroxide. Slight discoloration with little change in mechanical properties will occur when exposed t o concentrated sulfuric acid or caustic over long periods. It is substantially unaffected by both aliphatic and aromatic hydrocarbons, alcohols, esters, ketones, and nitroparaffins. It is swelled or softened only by oxygen-bearing organic solvents such as cyclohexanone and dioxane. The resistance to chemicals or solvents decreases with rise in temperature. The resistance of Saran to any chemical is in part a function of the crystallinity of the polymer. It is chemically more resistant in the crystallized form than in the amorphous state. A second important characteristic of Saran is its extremely low water absorption and vapor transmission. According to A. S. T. M. test D570-40T over a period of 24 hours, it
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tions. The low water vapor transmission or permeability of Saran is exemplified in the following test: A 0.001-inch-thick Saran film tested in a Thwing-Albert vapometer a t 45' C. (113" F.) transmitted only 1 to 3 grams per square meter per 24 hours. These tests were run with the apparatus in dried recirculated air, using water in the vapometer. This resistance to moisture transmission is a t least five times that of the best commercially available organic film. Saran is thermoplastic and has a defihite softening point that limits the temperature a t which it can be used. Since softening points vary with composition, the upper limit of operating temperature can be varied from 150" F. (66" C . ) or lower, to 250" F. (121" C.), From the standpoint of fire hazard, exposures t o much higher temperatures are not dangerous since Saran is not flammable. The basic polymers are odorless, tasteless, and nontoxic. Their high refractive index enhances their many color possibilites, while their toughness and abrasion resistance are of a high order; and the retention of these properties upon aging ensures excellent wearing qualities.
powder, as originally manufactured, is conveniently and inexpensively plasticized, .pigmented, and colored by ball milling. Thorough melting IS of prime importance in the extrusion of Saran. To accomplish this, it is necessary to heat the material to above its crystallite melting point. I n this condition the product becomes completely amorphous. Saran has a narrower softening range than most other thermoplastic materials. Curves in Figure 6 portray this relationshipgraphicallv. When the material is heated above this narrow s o f t e n i n g range, it can be extruded in a very fluid state. Under these conditions no plastic memory is encountered. and the retention of b I the desired shape and surface -is Fabrication Development and Applications thus made possible. The fabrication of Saran presents many interesting probWhen first exlems. It can be fabricated by conventional thermoplastic truded and practice; because of its normal crystalline state, it is suscooled, the prodINCREISINO TEMPERATURE ceptible to working by methods described later. I n order to uct is soft, weak, conserve the time and investment necessary for the fabricator FIQURE 0. PLASTICS FLOWWRSUS and pliable. If to produce a finished product, as well as to supply a conTEMPERATURE OF SEVERAL TEIERMO- it is allowed t o tinually expanding background of technical service, the PLASTICS remain at room materials, processes, and equipment are being subjected to temperature intensive study and development. without further treatment, it will gradually harden while Saran is adapted to the conventional method for extrusion partially recrystallizing at a slow rate with a random crystal of plastics, Modified screw-type extrusion equipment is used. arrangement. By heat treatment, recrystallization can be While these modifications are relatively minor, they are abproduced a t controlled rates. Recrystallization time can be solutely necessary for the successful extrusion of Saran. regulated from a few seconds to several weeks. For any one Designs permitting streamline plastic flow are desirable. formulation the rate of recrystallization is a function of the Since iron- and copper-base metals catalyze the thermal detemperature (Figure 7). It is interesting to note that an incomposition of Saran in hot zone8 above 130' C., it becomes crease in specific gravity occurs during recrystallization, as necessary to select other suitable metals for thwe heated shown in Figure 8. sections. The metals which can be used in contact with The control of extrusion and heat treatment of Saran perSaran above about 130' C. are magnesium alloys, nickel, mits a range of properties to be obtained. Tensile strengths Z nickel, Hastelloy B, Stellite 10, and impervious nickel plate. can thus be controlled from 4000 to 12,000 pounds per square The extrusion of Saran includes supplying a uniform rate inch, hardness from 60 to 95 (Rockwellsuperficial 15Y), elastic feed to the screw machine hopper, mixing and heating the elongations from 10 to 40 per cent. Such products have good plastic as it is forced along by the screw, and then forming it fatigue life as illustrated by the following example: Specias it passes through the die. The extruded shape may be mens of S/le-inch tubing having a wall thickness of '/le inch cooled and subsequently heat-treated. are unruptured when flexed through a 15" arc 1750 times per Saran may be fed to the extrusion unit as a powder or as minute for 2,500,000 cycles. I n comparison, standard I/,granules. The powder presents no bulk problem with the inch copper tubing failed after about 500 cycles in the same screw machine and eliminates a costly milling operation. This test. A few of the auulications of extruded Saran which appear of -interest include: rods for making gaskets, valve seats, and ball checks; medicinal probes; chemical-resistant, flexible tubing and pipe; tape for wrapping joints; chemical conveyor belts; tape and strips for die cutting; and various items of wearing apparel. Injection molding offers a second fabrication method for Saran. It makes possible the production of intricate shapes having properties similar to those obtained by extrusion as mentioned above. Here again the equipment involved consists of the standard, convenUnoriented Amorphoue Orientqd tional-type injection molding machines modicrystalline crystalline fied only as to contact metals and designs FIGURE 5. CROSS-POLAROID PICTURE OF PHYBICAL STATES OF SARAN
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through the heating tunnel or cylinder. The fundamentals of design include the same contact metals listed for extrusion, strict streamlining, and the reduction of the thickness of plastic sections in the heating zone. Conventional injection molding die designs and die metals can be used.
1
5 2
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l
I
I
l
l
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l
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0
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20
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-10
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40
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I
l 60
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-
I 80
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FIGURE 7. RECRYSTALLIZATION TIME OF SARAN VERSUS TEMPERATURE
The injection molding of Saran is unique. With other commercial thermoplastics the use of cold dies hastens the cooling of the plastic part and shortens the cycle. With Saran the reverse is true. Cold dies produce soft, flexible, amorphous pieces. Rapid hardening is accomplished by heat treatment (heated dies) t o produce recrystallization. For normal sections this permits short cycles. When heavy sections are involved, heated dies allow the section to retain its heat, and thus it recrystallizes rapidly. Undw these conditions Saran can be ejected from the die a t temperatures as high as 80" to 100" C. in a strain-free, warp-free, dimensionally stable form. This, of course, promotes very rapid cycles with heavy sections. In fact, moldings have been produced with quarter-inch sections in a 17-second cycle. Injection-molded Saran is playing a part in national defense. Many of its %pplications have been replacements of metals and not other thermoplastics. Comparative tests showed that in many instances Saran was the only material that would satisfy the industrial requirements for the replacement of such strategic materials as nickel, aluminum, stainless steel, and rubber in applications requiring chemical resistance. A few examples of industrial applications are: 1. Spray-gun handles, chosen because of solvent and abrasion resistance t o replace aluminum. Here the practice of cleaning the equipment by washing in potent lacquer solvents does not affect Saran. 2. Valve seats, chosen because of freedom from channeling based on abrasion resistance, of seating qualities based on its resilience, and, in some applications, of resistance to corrosive gases and liquors; it replaces precision-machined metal seats and imported horn seats. In one type of valve Saran, far outperforming any previous metal seat, showed no air leakage after 15,000,000 cycles of operation. 3. Acid dipper, chosen because of acid resistance and toughness t o replacing glass in hard usage where much breakage was encountered. 4. Moldings for the rayon industry, such as spinneret couplings, gasket holders, filter parts nozzle tips, rollers, guides, etc., chosen for its inertness and stability t o chemicals and solvents used in processing rayon; it replaces special formulations of hard rubber. In addition, Saran is now being called on for the more familiar general type of plastic application. Its good mechanical characteristics, range of attractive colors, and favor-
able economic outlook point t o a promising future in the injection molding field. Like other thermoplastic materials, Saran can be compression-molded. It differs from the other thermoplastic materials only in that it requires the same selection of metals for the dies as are above listed for hot zones in extrusion equipment. However, like other thermoplastic materials, the economic considerations usually favor injection over compression molding. A third method of fabrication, that of crystal orientation, brings out new properties of Saran. The oriented form is produced by extrusion, subsequent plastic deformation (as by stretching), and heat treatment. It is thus possible to obtain long, continuous extrusions of monofilaments, tapes and other shapes which have exceptional properties. While the method is not complex, it does require special control and careful attention to the techniques involved. A brief description of equipment and techniques used will clarify this point. In the production of strong, continuous sections, the plastic must be uniformly heated to a temperature above its crystallite melting point and thereafter cooled below this point. Preferably the material is mechanically formed to a shape similar to that of the section desired before being cooled. One method of fusing and shaping involves the use of a screwtype extrusion machine as described previously. The degree of cooling may be sufficient to reduce the temperature of the fused, shaped mass to about room temperature. The material will remain in this amorphous or supercooled condition for a sufficient time t o permit cold-working operations t o be carried out. The Saran is now ready for the orientation step.
1.69
167
2
0
FIGURE 8.
6
4
8
CHANGE IN SPECIFIC GRAVITY WITH RECRYSTALLIZATIOK
The orientation process (Figure 9) provides a method of effecting plastic deformation and partial recrystallization of the shaped, supercooled material. It involves an accurately controlled, predetermined rate of withdrawal of the plastic from the extrusion unit. The ultimate size and uniformity of the continuous section is governed by controlling the withdrawal rate. The shaped, supercooled material is then elongated under controlled conditions, preferably a t room temperature. During this mechanical stretching, there is a partial recrystallization of the material and orientation of the crystallites
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EXTRUSION
OUENCH OR HEAT TREAT
1
I
STRETCH
REEL OR PACKAGE B HEAT TREAT.
FOR SARAN FIGURE 9. ORIENTATIOSPROCESS
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SEAT COVERSOF SARANARE DURABLE AND EASILY CLEANED
along the major axis of the strand. The reduction in crosssectional area is approximately proportional to the elongation which takes place. This reduction, along with a comparison of unoriented and oriented Saran, is shown in Figure 10. The material may be heat-treated after or during stretching to affect the degree of crystallization and thus control the properties of the oriented article. After winding and packaging, the plastic strand is ready for use. The above method will produce orientation in a single direction with resulting unidirectional properties. The resulting tensile strength is a function of the elongation (Figure 11). These unidirectional properties of high tensile strength, great flexibility, long fatigue life, and good elasticity are particularly desirable for small monofilament sections where the load is along the longitudinal axis. In larger, noncircular sections, such as tapes, ovals, and semicircles, the desired degree of transverse orientation can be realized and accompanying transverse properties controlled through the introduction of other factors in the continuous process. For example, a rolling operation incorporated after the quenching step produces some flattening and transverse elongation of the strand with a resultant increase in transverse strength. Other processes involving mechanical or recrystallization control are also available for desired modifications of directional properties. With a range of plastic formulations and the flexibility of the above process, a variety of continuous sections with required properties can be produced.
I
Cnstretched unoriented, ,800010,000 Ib./&. in. tensile, impact low, flexibility low.
FIQURE10.
Stretched, oriented, ~0.000-60,000 lb./aQ. in. teqe!le, !mpact high, flexibility high.
COMPARISONOF UNORIENTED AND ORIENTED SARAN
Extruded and oriented sections are now being produced for textile uses, ranging in size from 0.007 to 0.100 inch in circular monofilaments; in other shapes their maximum dimensions are 0.200 inch. These materials have shown adapt-
ability to standard textile operations and have been fabricated by braiding, weaving, knitting, and twisting. While many uses now exist for monofilaments in the sizes commercially available, still greater fields await single and multiple fine fibers having the properties of oriented Saran.
FIGURE11. TENSILESTRENQTH OF SARAN MONOFILAMENTS VERSUS ELONGATION
Many uses of Saran monofilaments fall in fields formerly supplied by imported natural products, such ~ t 9 hemp, long-fiber paper, reed, rattan, horsehair, Spanish silkworm gut, and linen. From the results obtained, it appears that these substitutions will be permanent. A range of typical examples might include: FILTERFABRICS.Saran’s extreme chemical resistance suits it for this use. Since it is thermoplastic, the question of upper o erating temperature limits immediately arises. Figure 12 sfows that even at a temperature above that of boiling water, Saran still retains half its original tensile Ptrength. Normal strength is regained on cooling t o room temperature. SPECIAL ROPES.Saran’s high wet strength and its chemical and fungal resistance have directed its use to special ropes and cores for wire ropes.
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In addition to the methods of fabrication discussed, other operations are based on Saran’s combination of thermoplasticity and crystallinity, which have been used in forming the plastic. These operations include drawing, forging, blowing, rolling, stamping, and welding.
Literature Cited (1) I
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[TEMPERATURE-
DEGREES
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10,000
\ c.
0
I5
30
45
60
70
90
105
I20
FIGURE 12. TENSILESTRENGTH OF SARAN MONOFILAMENTS VERSUS
TEMPERATURE
(2)
ARTICLESOF APPAEEL. The general attractiveness and range of color possibilities fit Saran for such apparel accessories as belts, suspenders, handbags, and shoes. UmoLsTERY FABRICS.Long life, ease of cleaning, abrasion resistance, and flexibility, as well as color possibilities, have attracted upholstery fabric applicationst o Saran. Saran in subway transDortation seatine shows no material wear and amears like new after a hard se&ce period of over a year (3). ’saran upholstery fabric is now being used in train and bus seats, as well as household furniture and automobile seat covers.
(3) (4) (5) (6) (7)
Britton, E. C., et al., U. 8.Patents 2,121,009-12 (1938), 2,160,9403, 2,160,946 (1939), 2,206,022 (1940), 2,235,796 (1941); Coleman, G. H., et al., Ibid., 2,136,122-3, 2,136,333-4 (1938), 2,160,944 (1939); Wiley, R. M., et al., Ibid., 2,136,347-9 (1938). 2,160,931-5, 2,160.945, 2,160,948, 2,183,602 (1939), 2,205,449 (1940), 2,232,933,2,233,442,2,235,782(1941) ; Reilly, J. H.. et al.. Ibid., 2,140,548 (1938). 2,160,903-4, 2,160,936-8 (1939), 2,237,315 (1941); Reinhardt, R. C., et al., Ibid., 2,160,939, 2,160,947 (1939), 2,196,579, 2,220,545 (1940), 2,249,915-17 (1941); MoClurg, R. S., and Gibb, D. L., Zbid., 2,176,091 (1939); Sebrell, L. B., IbicE., 2,215,379 (1940); Hanson, A. W., and Goggin, W. C., Ibid., 2,238,020 (1941). Brooks, B. T., “Chemistry of the Non-benzenoid Hydrocarbons”, New York, Chemical Catalog Co., 1922. Goggin, W. C., Modern Plastics, 18, 36-9 (1940). Goggin, W. C., News Ed. (Am. Chem. Soc.), 18, 923-4 (1940). Natta, G., and Rigamonti, R., Atti. accad. Lincei, Classe aci. j i s . mat. nat., 24, 381-8 (1936). Regnault, V., Ann. chim. phys., [2] 69,151 (1838). Staudinger, H., and Feisst, W., Helv. Chim. A d a , 13,832 (1930).
PRESENTED as part of the Symposium on Recent Progress in High Polymer Plastics before the Division of Paint, Varnish, and Plastics Chemistry at the 102nd Meeting of the AMERICAN CHEXICAL SOCIETYAtlantic City, N. J.
Effects of Low Temperatures on Neoprene Vulcanizates FELIX L. YERZLEY’ AND DONALD F. FRASER E.I. du Pont de Nemours & Company, Inc., Wilmington, Del.
HE development of rubberlike materials to resist the effect of extreme cold is becoming increasingly important to the progress of engineering design, particularly in the automotive and aviation industries. While this development has progressed rapidly, more rapid progress will be possible when the properties to be improved are more clearly defined and when standard methods of evaluation have been adopted. Comparisons of results between laboratories have been made extremely difficult by the variety of tests and methods of interpretation. It is recognized that the physical factors are too complicated to permit a ready and practical solution of the problem by a single test procedure. The purpose of this paper is to describe and discuss the advantages and limitations of various test procedures used by the du Pont rubber laboratories. Certain features of the tests have been unique and are described in the hope that they may suggest further improvements in test methods. Yet in spite of the limitations of the tests, they have suggested compounding improvements of familiar types of neoprene and the development of an entirely new type of neoprene with superior resistance to the effects of low-temperature exposure.
T
1. Present addreas, Pioneer Instrument Division of Bendix Aviation Corporation, Bendix. N. J.
The elasticity of materials may be used as a basis for measurements a t low temperatures to determine the effects of continued exposure. The two aspects which are most obvious are changes in modulus and changes in resilience. The indications of change in modulus can be obtained by tests in tension, torsion, bending, or compression. Tests for changes in resilience can be made, with the proper temperature control, by one of the familiar types of resiliometer. In addition to the foregoing methods, which represent a direct approach t o the engineering problem by the measurement of mechanical properties, there are the indirect method of the T-50 test and methods which may be important in fundamental studies, including the measurements of heat capacity (2) and dimensional changes (6) a t various temperatures. The indirect methods generally imply a change in physical structure of materials as the result of transition from an amorphous form to crystalline form on continued exposure to cold. Two distinct demands must be met by rubberlike materials in order to perform satisfactorily at low temperatures. The first is that the material remain sufficiently flexible to resist fracture by impact or by probable mechanical strains. The second is the maintenance of functional characteristics-
