9. POLYSTYRENE RESINS
The polystyrene resins are thermoplastic products discovered about 100 years ago and are therefore the oldest synthetic resins known. Their practical application has been greatly retarded by the lack of inexpensive raw materials of high purity and by the difficulties experienced in their manufacture.
Ethylene, from petroleum or natural gas, is combined with benzene, from byproduct coke-oven operations, to form ethyl benzene, which is cracked to vinyl benzene or styrene. This monomer is polymerized by heat at 100°-150° C. The resin may be extremely tough or very brittle, depending on the conditions of polymerization. Products having different properties are obtained by (a) low temperature polymerization, (b) high temperature polymerization, and (c) catalytic polymerization.
The low-temperature polymers, sometimes designated as alpha-metastyrol, are produced by polymerizations of vinyl benzene at temperatures under 175° C. A transparent resin, colorless to light yellow, is produced which is remarkably tough, has excellent tensile strength, unusually good dielectric properties, and is resistant to most chemicals.
Polymerization at high temperatures (above 175° C.) yields a brittle resin designated as beta-metastyrol. This type is transparent but usually dark in color, has low tensile strength and shock resistance.
When vinyl benzene is polymerized in the presence of catalysts, the resulting resin is similar to resins obtained at high temperatures, except that it is lighter in color. It is sometimes designated as gamma-metastyrol. Oxidizing agents are usually the catalysts. Clear, colorless, vitreous resins are obtained by excluding air during polymerization.
Polystyrene resin is a clear, colorless, highly thermoplastic molding material with high insulating property, moisture resistance, inertness, dimensional stability, and impact strength. It can be molded directly by heat and pressure, and the molded articles are remarkably resistant to discoloration by light. Polystyrene has a dielectric constant of 2.6, a power factor of 0.02 percent, and is equivalent to fused quartz as an electrical insulator of low dielectric loss. Films of 0.002 inch thickness have a dielectric strength of more than 2,000 volts per mil thickness, which is better than that of any other available synthetic resin and even better than that of shellac. The tensile strength of the resin is 5,500 to 7,000 pounds per square inch, and its impactresistance remains unchanged at temperatures as low as minus 70° C. It transmits all wave lengths of light down to 3,000 Angstrom units.
Polystyrene is adapted to large scale production of transparent, translucent, and opaque moldings in a wide variety of colors. It is easily molded by injection processes, softening at about 150° F. and is molded at 300° to 375° F., under 3,000 to 30,000 pounds pressure per square inch. As much as 40 percent filler may be used without seriously affecting the tensile strength, although the filler does affect the dielectric properties. Since the resin is thermoplastic there is no waste in the molding operation; scrap material may be reground and used again.
The unusual properties of these polystyrene resins should give them widespread applications when the cost is low enough to make them competitive with other materials. Potentially large volume outlets are in radio frequency insulation; in dentures because of the strength, low specific gravity, ease of coloring, and absence of odor and taste of the material; in electrical parts for submarine and aircraft storage battery cases and separators; and for the manufacture of glass eyes.
Other possible applications of polystyrene resins are in metal lacquers and in light colored enamels. Their toughness and light color, together with their solubility in cheap solvents, suggest their use for these purposes. Such lacquers are said to be quick-drying, resistant to water, and moderately so to acids and alkalies.
For a number of years, the Naugatuck Chemical Division of the United States Rubber Co. produced small quantities of polystyrene resins, which were marketed under the trade name Victron when for general purposes and under the trade name Marvelyn when for use in dentures. Little progress was made because of high costs and failure to produce a water-white product. The sales price was between $1.50 and $2 per pound. Early in 1937 the Naugatuck Chemical Division transferred its patents on polystyrene resins to the Carbide and Carbon Chemicals Corporation.
The Dow Chemical Co., Midland, Mich., late in 1937 announced commercial production of clear, colorless polystyrene in several forms. Styron is the trade name for the resin from this source. In January 1938, the Bakelite Corporation announced Bakelite Polystyrene. The plants manufacturing polystyrene have a capacity in excess of 2,000,000 pounds a year, and the resin is currently offered at 72 cents per pound.
At least one other domestic firm is doing research on the polystyrenes and expects to produce commercially in the near future.
At least two commercial types of polystyrene resins are produced abroad. Both are made in Germany and marketed under the trade names Resoglas and Trolitul. Resoglas is a water-white, transparent thermoplastic resin softening at about 150° C. Its water absorption is low, it is nonoxidizing, and does not discolor on weathering and baking. Appreciable quantities are produced in Germany and the sales price there was reported to have been 40 cents per pound during 1936.
Molded Polystyrene Resins.Source: Bakelite Corporation, 247 Park Avenue, New York, N. Y.
Molded Polystyrene Resins.
Source: Bakelite Corporation, 247 Park Avenue, New York, N. Y.
Small quantities of Resoglas and Trolitul have been imported from Germany in recent years. Table9shows the quantities imported in recent years.
Table 9.—Resoglas and Trolitul: United States imports for consumption, 1933-37
1Not available.2Preliminary.Source: Analyses of invoices of paragraph 28, act of 1930—U. S. Tariff Commission.
1Not available.
2Preliminary.
Source: Analyses of invoices of paragraph 28, act of 1930—U. S. Tariff Commission.
With the more advanced development of polystyrol resins in Germany prior to 1938, evidenced by larger commercial production, by wider application, by the marketing of a water-white product at a considerably lower price, it might be expected that imports into the United States would have been in considerably larger amount than shown in table9. That they were small was probably due to the high rate of duty which made them expensive as compared with other synthetic resins in the United States and thus limited their market to uses in which the others were less satisfactory. Resoglas was reported to have been selling for 40 cents per pound in Germany. The imported resin is assessed for duty under the provisions of paragraph 28 of the Tariff Act of 1930 at 45 percent ad valorem based on American selling price (as a competitive product) and 7 cents per pound. The American selling price of the resin made in the United States until late in 1937, as determined by the Bureau of Customs, Treasury Department, was $1.85 per pound. The duty was therefore 90 cents per pound. Imports of Trolitul were valued at 75 cents per pound, giving a cost of $1.75 per pound laid down, duty paid, in domestic markets. With the present American selling price of 72 cents per pound, the duty would be approximately 36 cents per pound.