PLASTIC

any of numerous synthetic materials that consist of giant molecules called polymers, with extremely long chains of repeating units derived from short molecules. Plastics can be formed into products by molding or otherwise shaping. A significant property of most plastics is that they soften when heated, so that they can be formed into shapes, then become rigid on cooling. This property derives from their physical structure, which consists of a network of polymers, the constituent units of which separate under heat sufficiently to slide apart, but on cooling become firmly entangled again. The first plastic developed was celluloid, consisting of nitrocellulose softened by vegetable oils and camphor, patented in 1870 by an American printer, John W. Hyatt. The first completely synthetic plastic was Bakelite, produced from phenol and formaldehyde by Leo Baekeland in 1909. Advances in basic understanding of molecular physics facilitated the subsequent development of nylon, polyethylene, and other plastics. All plastics are manufactured by some method of polymerization, the process of forming the long chains and networks of molecules. The two major divisions of plastics are the thermosetting resins and thermoplastic resins. Thermosetting resins become insoluble and infusible on heating. Among thermosetting resins are the phenolic resins, furan resins, aminoplastics, alkyds and polyesters of unsaturated acids, epoxy resins, polyurethanes, and silicones. Thermoplastic resins, which can be melted and solidified repeatedly, unlike thermosetting resins, include cellulose derivatives, addition polymers (polyethylene, polypropylene, vinyls, acrylics, fluorocarbon resins, and polystyrenes), and condensation polymers (nylons, polyethylene terephthalate, polycarbonates, and polyamides). Other types of resins include oil-soluble or modified resins, plastics such as casein and lignin extracted from natural products, and special-application synthetics such as resins used as adhesives and as additives to paper and textiles. Raw materials for plastics include coal and cellulose, but by far the chief source is petroleum. Plastics are shaped by a variety of means, including extrusion, blow-molding, calendering between rollers, thermosetting in hydraulic presses, injection molding, rotational molding using centrifugal action, thermoforming, vacuum molding, laminating by press, and casting. Foamed plastics are produced by forming gas bubbles in the molten material. Plastic products are further shaped and finished by means ranging from mechanical through laser machining, ultrasonic welding, and radiation processing. Because of their easy manipulation, economical manufacture, low specific gravity, and resistance to corrosion, plastics have replaced metal, wood, glass, and other materials in many applications. An immense array of plastic industrial and consumer goods is available. See also resin. 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).Modern Plastics Encyclopedia (annual) contains individual articles on plastics and fabrication processes written primarily for nonspecialists. Molding processes are described in Michael L. Berins (ed.), SPI Plastics Engineering Handbook of the Society of the Plastics Industry, Inc., 5th ed. (1991); and Charles A. Harper (ed.), Handbook of Plastics, Elastomers, and Composites, 3rd ed. (1996). Ferdinand Rodriguez, Principles of Polymer Systems, 4th ed. (1996), includes coverage of the fabrication of plastics and has detailed bibliographies. Ferdinand Rodriguez