SYNTHETIC RUBBER No product identical to natural rubber in its physical and chemical properties has ever been artificially synthesized. The term "synthetic rubber" has, however, been used indiscriminately to designate materials possessing to some degree the physical properties of natural rubber. Some of these closely resemble the natural product, others fracture under sudden loading and exhibit little or no retractive. ability. To merit the name "synthetic rubber" a material should be susceptible at room temperature of considerable elongation and retract forcefully upon release of stretching forces. The first notable synthetic rubber was pro duced in Germany during the World War when supplies of natu ral rubber were cut off. This "methyl rubber" could not, how ever, compete with natural rubber in either cost or quality and was not used after the close of the war. More recently a syn thetic rubber, used commercially in Russia, has been made from butadiene produced from alcohol or separated from petroleum. Reports indicate that its quality is that of a good grade of re claimed rubber and that it is not used alone but is blended with natural rubber. The latest commercial development in Europe is the production in Germany of several synthetics, known as Buna, of good quality and used rather extensively. Production has re cently been increased to 2,000 tons per month and facilities for further increases are planned. The Bunas are made by polymer ization of mixtures of butadiene with other polymerizable mate rials. Buna S, polymer of butadiene and styrene is used for auto tires and for replacing rubber in a variety of products. Perbunan, butadiene-acrylonitrile polymer, is available on the American market and has been adopted in the United States for the manu facture of certain oil resistant compositions. These Bunas, like rubber, are vulcanized with sulphur.
In the United States several synthetic rubbers have been de veloped. Neoprene (formerly called Duprene) is made by react
ing vinyl acetylene with hydrogen chloride and polymerizing the resulting chloroprene. Unlike rubber, Neoprene can be vulcan ized by heat alone without addition of sulphur. Usually, how ever, sulphur or metallic oxides—litharge, magnesia or zinc oxide —are mixed with it to promote vulcanization. Properly designed Neoprene compositions are highly resistant to oils and petroleum hydrocarbons and to deterioration by sunlight or ozone. Various rubber-like Thiokols are produced by reaction with alkali poly sulphides of organic di-halides such as ethylene dichloride or di-chloro ethers. They are very resistant to oils and lacquer solvents but are inferior to rubber in strength and toughness. Neoprene, Thiokol, and Perbunan are all utilized by American manufacturers. Vistanex, polymer of iso butylene, is rubbery in character and, although it cannot be vulcanized, its chemical in ertness makes it of value for certain purposes. Koroseal, plasti cized gamma polymer of vinyl chloride, cannot replace rubber but supplements the use of rubber in sheathing electrical con ductors, in coating fabrics for clothing or curtain material, or paper for waterproof and oil-resistant packaging, and in lining metal equipment for handling corrosive chemicals such as nitric, hydrofluoric or chromic acids. As replacements for hard rubber, molding plastics such as bakelite, glyptal resins, urea-formalde hyde products, poly methacrylates and asphalt have all been used to some extent. The intelligent selection and use of vari ous synthetics by rubber manufacturers have enabled them to improve the quality of many articles and to meet the require ments of a greater variety of service than was possible with natu ral rubber alone. (J. W. Sc.)