NEEDLES Courlasy. R. C. A . Menulac1lrrinp Compnn”, 1nc.
Some recent developments in the chemibtry are reviewed briefly. Consideration is then given to the physical and chemical properties of the polymer in relation to use as a material for injection molding. Flow characteristics and thermal stability are of primary importance in this connection. Study of these properties shows that polystyrene is particularly well suited to injection molding. Recent work on the control of the polymerization has made practical the production of styrene resins which are readily solublc in drying oils. Possible applications of these products in the field of coatings are discussed. Attention is also directed to the various possible sources for monomeric styrene, either as a by-product from various cracking processes or its direct produrtion by synthetic means. of styrene polymerization
HE polymerization of styrene has been the subject of extensive chemical investigation during the past fifteen years, and the long-chain polymer hypothesis originally proposed by Staudinger has met with general acceptance. The work prior to the past four or five years was effectively summarized by Ellis (5) and by Burk, Thompson, Weith, and Williams (8). I n brief, styrene polymerizes to give solid products of high molecular weight which are soluble in aromatic hydrocarbons and some solvents of other types, and which can be shown to be polymer homologous mixtures COYering a wide range of molecular weight. Polymerization at moderate temperatures and without catalysts yields products which are of high average molecular weight and which impart high viscosity to their solutions. Increase in temperature and addition of catalyst cause a reduction in the average moleciilar weight and intrinsic viscosity of the resulting polyrnrv. The viscosit,p which it imparts to a sohition thus
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1302
becomes a n indication as to the average molecular weight of any particular sample of polystyrene. The reaction mechanism of styrene polymerization and the polymer distribution in the products were investigated by Schulz (9) and by Schulz and Husemann (IO),but there is still some uncertainty as to the exact mechanism involved in the reaction. It appears highly probable, however, that the process can be considered as involving a t least three stepsnamely, chain initiation, chain propagation, and chain termination-and that the second step is very rapid in comparison to the first. The configuration of the polymer has also been a matter for debate a t times, but the recent work of Marvel and Moon (8) supports the original view of Staudinger that the phenyl residues are attached to every second carbon atom, thus: -CH-CH~-CH-CHZ-CH-CHZ-
I
CBH~
I
CsH6
LsH.5
The formula is in good agreement with the products of the destructive distillation of polystyrene, among which Staudinger and Steinhofer (11) found, 1,3-diphenylpropane, 1,3,5triphenylpentane, and 1,3,5-triphenylbenzene. Destructive distillation becomes evident when polystyrene is heated to 330-350" C. and is quite rapid at 360-375" C. Recent work in this laboratory has shown that toluene and isoprophenylbenzene are also products of the decomposition, and the formation of the latter compound is most simply explained on the basis of the formula given. I n addition to the viscosity relation already mentioned, a number of other physical characteristics show wide variation with change in average molecular weight. This is well illustrated by the low mechanical strength and brittleness of the lower polymers (20,000-30,000 molecular weight or below), as contrasted with the toughness of the higher members of the series. Solubility in ether is lost as the molecular weight increases, and the thermal softening point tends to become less and less sharply defined, although the A. S. T. M. heat distortion point shows little variation. Specific gravity and refractive index show very slight change with molecular weight.
Injection Molding of Thermoplastic Resins The commercial application of polystyrene had until the past two or three years failed, in the United States, to keep pace with the scientific studies of the polymerization reaction and of the structure of the polymers. However, this picture is now undergoing rapid change. The lower polymers are of some interest in certain applications, particularly as an ingredient in surface coatings, because of their good color, water resistance, and solubility properties, but greatest interest a t present centers on the products of higher molecular weight because their superior mechanical properties make them of great value in the molding field. The excellent electrical properties, very low water absorption, and high degree of chemical resistance of these higher polymers have long been known but could not be used to good advantage with the older type of molding technique, known as compression molding. The newer method, known as injection molding, which is especially suited to the handling of thermoplastics, has been making rapid strides in this country within the past few years, and the use of polystyrene is expanding with it. I n simple terms, injection molding consists essentially in heating a thermoplastic material in a specially constructed chamber until i t becomes plastic, then forcing it through a small orifice into a cooled mold. The mold is kept closed until the plastic has cooled and hardened. It is then opened, the molded plastic piece removed, and the process repeated. The
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temperatures involved are usually much higher than those used in compression molding. There is considerable variation with different machines and different types of thermoplastics but the temperature in the plastic a t the point of injection usually falls somewhere in the range 160-230" C. The flow characteristics and the stability of the material at these comparatively high temperatures are therefore of prime interest in a practical way. A critical examination of polystyrene with regard to these two factors shows that i t possesses flow properties which make it particularly well suited to injection molding, and that its thermal stability makes it compare favorably with other thermoplastic resins.
Thermal Stability of Polystyrene The breakdown which occurs on prolonged heating above 100" C., but below the temperature of destructive distillation, was investigated and the effects obtained at 135' and 160" C. were shown to be due to surface oxidation. The effect is negligible in the interior portion of the mass or when the heating is carried out under nitrogen. Even in air a t 160" C. i t develops very slowly and takes several hours to become readily detectable. It has also been found that a rapid decrease in molecular weight throughout a molten mass of resin does not begin to occur until a temperature of about 250' C. is reached. I n one experiment polystyrene was molded in a familiar type of commercial injection-molding machine under conditions such that the temperature of the material at the injection orifice was about 240" C. The material used gave a viscosity of 53.1 centipoises in 10 per cent toluene solution. With molding cycles up to 2 minutes, the molded pieces, when broken u p and tested, showed viscosities practically the same as that of the original material. When a 4-minute cycle was used, the viscosity dropped from 53.1 to 42.8 centipoises. I n practice the molding cycle is seldom more than 20 or 30 seconds, and the temperature reached by the material is rarely higher than 200-220' C. Under such conditions the effect of the molding operation on the molecular weight of the material could probably not be detected at all unless a very high-molecular-weight sample was used. It is of some interest to compare the result just described with that of a somewhat similar experiment in which the polystyrene was forced by means of a variable-pitch screw through a heated orifice, "8 inch in diameter. The temperature of the resin as i t issued from the orifice was 207' C . The viscosity dropped from 59 centipoises to 50. From this it would appear that the work done in the plastic by the action of the screw tends to increase the molecular breakdown. Actually, the effect of the screw may be due largely to the fact that it serves to bring oxygen from the air into more intimate contact with the molten resin mass. There is still a great deal of uncertainty concerning the relation between the three factors of heat, oxygen, and mechanical work as to the part each plays in molecular breakdown. A study of the effects of mechanical work under controlled conditions with rigorous exclusion of oxygen would be an important contribution to an understanding of the subject.
Flow Characteristics of Polystyrene The viscosity or flow characteristics of a plastic may be expressed in practice in a variety of ways, none of which is well adapted to cover a wide range such a s is passed through when polystyrene is heated from room temperature up to the temperature of injection molding. The resin changes during heating from a hard solid state, first, to a rubbery condition in which it is highly elastic, then gradually to what appears to be a true liquid of high viscosity. I n the interests of clarity it is desirable to select certain points throughout this range and
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describe the physical state at tliese points in terms best adapted to each particular state, with special coirsideration to the requirement.s of injection molding. Polystyrene a t room temperature is becoming familiar as a hard solid in tlre form of colorless transparent cast pieces or as molded ohjccts in a variety of shapes and colors. Some Of the forms in which it is now being used have recently been described by Humphrey ( 6 ) . The cold flow, or gradual distortion under heavy load, is very low a t room temperatiire and does not lmome appreciable until about 60" C. is reaclied. It is also int,ercsting to note that samples cooled in solid carbon dioxide have about the same impact strength as at room teinpcrature. (In the actual test, notched Izod compression-molded test bars were placed on a block of solid carbou dioxide for 2 hours and then broken in the normal way; an average valuc of about 0.275 foot-pound of energy t.o break was obtained. Injection-molded test bars under similar conditions give an average impact value of about 0.4 foot-pound energy to break.) The tliermal softening point, as measured liy tlie A. S. T. M. k a t distortion test,, is a t about 80" C. This is tire point at which distortion under moderate load first reacbes a small specified value. In the region of the tlrermal softcuing p i n t , polystyrene shows behavior strikingly resembling that of raw rubber. This phenomenon has been commented on by a number of investigators and was studied extensively by Wliitby (19). According to him, the minimum temperature a t which polystyrene (made by polymerization at room temperature) shows rubberlike elasticity is 65" C. In one experiment which is of particular interest, a strip of polystyrene was heated to 95' C., stretched to an elongation of 450 per cent, and immediately released. The strip contracted at once to an extension of only 50 per cent, and after 200 seconds it had returned to its origiiial length. When held in the extended form for 10 minutes, the extension immediately after relcase was 175 per cent, and after 200 seconds was 75 per cent. This illustrates clearly the limited flow propcrties of the resin a t 95" C. As the temperature was increased above this point, the properties of a true fluid gradually became more pronuunced, and tlie elastic property slowly disappeared. To demonstrate in simple fashion what is taking place between 95" C. and the temperature of injection molding, i t is of mme interest to describe the appearance of samples heated at various intervening temperatures. Granular samples of commercial polystyrene were heated in evacuated flasks a t various selected temperatures. I n each case the flask was heated in aii oil bath for an hour, and temperature was determined by a tliermometer wit11 the bulb a t the interior of the resin mass. When the heating pcriod was over, the polystyrcne was allowed to cool and the resin removed by breaking t,lie flask. The prodiict obtained by carrying out the above procedure at 135" C. looked somewhat like a loosely packed snowball with a thin coating of ice over its surface. The individual granules were clearly visible and appeared to be simply fused together at the points of contact as they existed in the original loosely packed granular material. Obviously there was little real liquid flow. Increasing t.he temperature to 165" C. produced definite changes in appearance, although the opaque white character of the snowball was still retained. Some of the spaces between the particles assumed the shape of bubbles, indicating that the particles fused enough to form continuous walls around what were fomierly the spaces between discrete particles, and tlie glassy or icy appearance at the surface appeared to go somewhat deeper into tire mass. At 185" C. the fusion was practically complete, but the viscosity was so high that bubbles were trapped in t,Iie mass and gave it a ahite t,ranslncent appearance.
Couiteay, .Shew Inrulolor Company
CnwHon
~ I O X I D S IxorcaTOR FOR CHECXIXCI I>IrEE C4s
COSCl:NTRATION
The products obtained a t 210" and 225' C. present a different appearance from those just described. Complete fusion has finally bcen obtained, and most of the bubbles or voids are eliminated, leaving the material with a clear, transparent,, glasslike form. Even a t tlrese temperatures, however, the viscosity was found to be high, and the molten resin could not be poured out of the flask. The rate of flow was so slow that the mass coolcd before i t moved more than a fraction of the way down the side of the container. The behavior of the resin between 185" and 225" C. is of particular interest because, as already pointed out, this region is in the operating range of most commercial injection-molding equipment. For best results in molding it is necessary that the material be fairly fluid at the molding temperature so that the pressure applied will be transmitted throughout and will force the resin into all parts of the mold. On the other hand, if the viscosity is too low, the plastic will flow out through the cracks between different sections of tire mold and will cause what is known as flash. This flash makes necessary an expensive finishing operation on the molded piece. A third and most important consideration in the economics of injection molding is the time cycle required for the complete molding operation. Obviously this is a function of the time required to fill the mold and the time required to cool the resin to a state suc11 that it can he discharged from the mold without deforming. The description just given of the flow characteristics at various temperatures demonstrates that polystyrene is peculiarly adapted to the process 01 injection molding, combining as i t does the factors which mako for the production of clean, sliarply defined molded pieces and those which make for maximum speed of molding. Tlris view is now being confirmed in practice in the production of a wide variety of molded objects. P o l y s t y r e n e in I'rotective C o a t i n g s The excellent color and chemical stability of polystyrene resins early led to attempts to use tliem in protective and decorative coatings. A numher of such applications have been made successfully from solutions of high-molecular-
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weight polystyrene, usually plasticized with one of the chemical plasticizers such as tricresyl phosphate, dibutyl phthalate, or the like. High-molecular-weight polystyrene is, however, incompatible with most of the ingredients commonly used in paints and varnishes, particularly the drying oils-a fact which until recently has prevented them from being utilized widely for coating purposes. Recent work upon the control of the styrene reaction has now made practical the production of styrene polymers of relatively low molecular weight which are readily soluble in the drying oils to produce oleoresinous varnishes, Perhaps the most outstanding feature these new styrene resins impart to oleoresinous varnish films is the combination of excellent flexibility and film strength with high resistance to moisture absorption and moisture permeability. These two properties are seldom found together in resin-oil varnishes, but are necessary for many important applications, including the fabric and paper coating fields, waterproof sandpaper, varnished cambric and tapes for electrical insulation, and coatings for metal food containers and for improving the moisture resistance of transparent cellulose derivatives used in packaging foods, tobacco, and other products. These new oil-soluble styrene products also retain the desirable properties of good color, heat stability, and high resistance to alkalies and acids. The extent to which these properties are preserved in the varnish film depends largely upon the ratio of resin and drying oil used in preparing the varnish. A varnish containing as much as 50 per cent resin and 50 per cent tung oil, for instance, yields a tough, flexible film of outstanding moisture and alkali resistance and of unusually good gloss and leveling properties. Varnishes containing two to four parts oil to one part styrene resin are still more flexible, though of somewhat lower moisture and chemical resistance, and are well suited for electrical insulating purposes since they have a high dielectric breakdown of around 2200 volts per mil.
Sources for Monomeric Styrene The increasing commercial importance of polystyrene as a plastic and in coating materials is directing attention more and more towards the various possible sources for monomeric styrene. The synthetic production through the stage of ethylbenzene has long been known, but styrene has become commercially available in large quantity in this country only within the last few years. The issuance of several recent
patents (4, 7) on the pyrolysis of ethylbenzene indicates that this process is still of interest. I n addition to the synthetic sources, a large amount of styrene is also formed as a by-product in various hydrocarbon cracking processes. Cambron and Bayley ( 3 ) studied the pyrolysis of propane and found that a n appreciable yield of styrene was obtained a t 800’ C. Birch and Hague (1) carried out somewhat similar studies on gas mixtures rich in propane, and estimated, on the basis of their results, that a plant capable of dealing with a throughput of 5,000,000 cubic feet of propane-rich gas per day could produce 59,000 liters of motor benzene containing 3400 liters of styrene. The presence of styrene in considerable amounts in the drip oil from carbureted water gas was demonstrated by Ward, Jordan, and Fulweiler ( I d ) . It is not unreasonable to suppose that in the near future there will be a large supply of material available from such sources as these a t a cost relatively low in comparison with other thermoplastic materials. Under such conditions we may expect to see a considerable expansion in the use of polystyrene.
Literature Cited Birch, S. F., and Hague, E. N., IND. ENO. CHEM.,26, 1008 (1934). Burk, R. E., Thompson, H. E., Weith, A. J., and Williams, I., “Polymerization”, New York, Reinhold Publishing Corp., 1935. Cambron, A., and Bayley, C. H., Can. J. Research, 10, 145 (1934). Dreisbach, R.R. (to Dow Chemical Co.), U.S. Patent 2,110,829 (March 8, 1938). Ellis, C., “Chemistry of Synthetic Resins”, New York, Reinhold Publishing Corp., 1935. Humphrey, L. E.,Modern Plastics, 17,No.2. 100 (1939). Mark, H.,and Wulff, C. (to I. G . Farbenindustrie), U. S. Patent 2,110,833(March 8, 1938). Marvel, C. S.,and Moon, N . S., J. Am. Chem. Soc., 62, 45 (1940). Schulz, G . V., 2.physik. Chem., B32, 27-45 (1936). Schulz, G. V., and Husemann, E., Ibid., B34, 187-213 (1936); B36, 184-94 (1937). Staudinger, H., and Steinhofer, A., Ann.,517, 35 (1935). Ward, A. L.,Jordan, C. W., and Fulweiler, W. H., IND. ENG. CHEM., 24,969,1238 (1932). Whitby, G. S.,J. Phys. Chem., 36, 198 (1932). PRESENTED as part of the joint symposium on Plastics and Resins from Hydrocarbons before the Divisions of Petroleum Chemistry, of Paint and Varnish Chemistry, and of Rubber Chemistry, at the 98th Meeting of the American Chemical Society, Boston, MRSS. Other papers in this symposium appeared in March, 1940. pages 293-323.
Vat Dyes as Pigments E. I. du Pont
CRAYTON K. BLACK de Nemours & Company, Inc., Wilmington,
AT dyes are well known for the excellent properties they impart to textiles. These include not only resistance to light and washing, but also excellent fastness to chlorine bleach, dry cleaning, rubbing, perspiration, hot pressing, and other color-destroying agencies encountered in textile use. The term “vat” dye is derived from the method of application of these colors rather than from any chemical family. Structurally, several chemical groups are included. The indigoids are characterized by the presence of the structure,
Del.
0
0
C
C
I!
II
s r y l e n e / b = d‘arylene
d\
where
X
b/
= 0, S, NH, CHI, etc.
On reduction with such agents as alkaline sodium hydrosulfite, the leuco or soluble form is produced, the carbonyl group becoming
