Appearance A clear hard resin. Melting point 110° to 120° C. A. S. T. M. (Ring & Ball). Acid value 0 to 1. Iodine value 125 to 135. Specific gravity (at 20° C.) 1.02 or 8.5 lbs. per gallon. Color (50 percent solution by weight in toluol) 13 to 15 Gardner Holt standard. Odor slight when cold, sweet and aromatic when melted. Soluble in aromatic hydrocarbons, petroleum thinners, turpentine, and varnish oils. Insoluble in alcohols, esters, ketones, and not completely compatible with nitrocellulose. Production. In the United States two makers of petroleum resins are producing in commercial quantities and several others are carrying on extensive research. Production was small in 1935, but increased in 1936 and in 1937. The development and expansion of these resins over the past 2 years indicate that they will become important. Imports into and exports from the United States. There has been no importation of petroleum resins into the United States. Exports have been confined to samples and experimental quantities. 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. Properties and uses.