ELASTOMER

any member of a class of polymeric substances that possess the quality of elasticity, i.e., the ability to regain shape after deformation. Elastomers are the base material for all rubber products, both natural and synthetic, and for many adhesives. Polymers are chemical compounds whose molecules consist of several thousand smaller molecules, called monomers, that are linked together to form long chains. In elastic polymers these chains are highly flexible, disordered, and intertwined. In chemical terms, elastomers are extremely viscous fluids, lacking the rigidity of a glass or the ordered arrangement of a crystal. When stretched, the molecules are pulled into alignment and often take on aspects of a crystalline arrangement, but upon release they return spontaneously to their naturally disordered, entangled state. This return to natural disorder distinguishes elastomers from plastic polymers, which are normally glassy or crystalline and therefore retain much of the shape to which they are deformed. Almost all elastomers are hydrocarbons; i.e., they are composed principally of carbon and hydrogen and their compounds. Some occur naturallye.g., polyisoprene, which is formed in the latex of the rubber tree and is processed into natural rubber. Most elastomers, however, are produced synthetically from derivatives of petroleum and natural gas. Monomers such as isoprene, butadiene, and butylene are subjected to various polymerization reactions in which they are built up into large molecules. In many cases other chemical elements or compounds are incorporated into the polymer in order to modify basic propertiese.g., chlorine in polychloroprene (neoprene) and sulfur in polyalkylene polysulfide (Thiokol), which contribute to the oil-resistance of these rubbers. Properties can also be modified by producing elastomers as copolymers, i.e., polymers made up of more than one type of monomer. Examples include nitrile rubber (an acrylonitrile-butadiene copolymer) and butyl rubber (a copolymer of isobutylene and isoprene). In another method, some elastomers are blended with various plastic polymers such as polypropylene or polystyrene; the resultant materials, known as thermoplastic elastomers, retain the resilience of rubber but can be remolded and reprocessed upon the application of heat (a property important in recycling). In order to be made into useful rubber products, elastomeric materials must be subjected to various modifications. These include: strengthening of the material by cross-linking the polymer chains (for instance, by sulfur atoms in the process known as vulcanization); further strengthening by fillers such as carbon black; and treatment with chemicals that provide resistance to weathering and chemical attack. For fabrication into adhesives, elastomers are often dissolved in organic solvents and treated with various other additives to improve their application, adhesion, and durability. Additional reading Jacqueline I. Kroschwitz (ed.), Encyclopedia of Polymer Science and Engineering, 2nd ed., 17 vol. (198590), is the most comprehensive source of information on polymer science and includes articles on the major topics treated in this article; it is also available in a condensed, 1-vol. edition, Concise Encyclopedia of Polymer Science and Engineering (1990). Two additional reference works are Geoffrey Allen and John C. Bevington (eds.), Comprehensive Polymer Science: The Synthesis, Characterization, Reactions & Applications of Polymers, 7 vol. (1989); and Joseph C. Salamone (ed.), Polymeric Materials Encyclopedia, 12 vol. (1996). Books on polymer science for the nonscientific reader are Hans-Georg Elias, Mega Molecules (1987; originally published in German, 1985); and Raymond B. Seymour and Charles E. Carraher, Giant Molecules: Essential Materials for Everyday Living and Problem Solving (1990).Maurice Morton (ed.), Rubber Technology, 3rd ed. (1987), is a suitable introduction for persons entering the industry. Anil K. Bhowmick and Howard L. Stephens (eds.), Handbook of Elastomers: New Developments and Technology (1988), is a somewhat more advanced survey of rubber technology. Colin Barlow, Sisira Jayasuriya, and C. Suan Tan, The World Rubber Industry (1994), reviews the history and production of synthetic and natural rubber. James L. White, Rubber Processing: Technology, Materials, Principles (1995), describes and analyzes the processes used to manufacture rubber articles.J.A. Brydson, Rubbery Materials and Their Compounds (1988), reviews the science of rubber compounding, giving current recipes and the sometimes complex reasons behind them. Attilio Bisio, Synthetic Rubber: The Story of an Industry (1990), briefly recounts the chemical processes used to produce synthetic elastomers. The Vanderbilt Rubber Handbook, 13th ed., edited by Robert F. Ohm (1990), the standard reference for rubber technologists, provides information on materials, recipes, test methods, properties, and uses of rubber compounds. Alan N. Gent (ed.), Engineering with Rubber: How to Design Rubber Components (1992), surveys design principles for various products, showing how to select materials, to determine stiffness, strength, and durability in selected cases, and to specify manufacturing quality. Annual Book of ASTM Standards, section 9, Rubber, published by the American Society for Testing and Materials, has full details of standard test methods for rubber and rubber products, including hardness, strength, and resistance to heat. Worldwide Rubber Statistics (annual), published by the International Institute of Synthetic Rubber Producers, contains detailed information on the production and consumption of natural rubber and synthetic elastomers. Alan N. Gent