INDUSTRIAL POLYMERS, MAJOR

chemical compounds used in the manufacture of synthetic industrial materials. In the commercial production of plastics, elastomers, man-made fibres, adhesives, and surface coatings, a tremendous variety of polymers are used. There are many ways to classify these compounds. In the article industrial polymers, chemistry of, polymers are categorized according to whether they are formed through chain-growth or step-growth reactions. In plastic (thermoplastic and thermosetting resins), polymers are divided between those that are soluble in selective solvents and can be reversibly softened by heat (thermoplastics) and those that form three-dimensional networks which are not soluble and cannot be softened by heat without decomposition (thermosets). In the article man-made fibre, fibres are classified as either made from modified natural polymers or made from entirely synthetic polymers. In this article, the major commercially employed polymers are divided by the composition of their backbones, the chains of linked repeating units that make up the macromolecules. Classified according to composition, industrial polymers are either carbon-chain polymers (also called vinyls) or heterochain polymers (also called noncarbon-chain, or nonvinyls). In carbon-chain polymers, as the name implies, the backbones are composed of linkages between carbon atoms; in heterochain polymers a number of other elements are linked together in the backbones, including oxygen, nitrogen, sulfur, and silicon. Carbon-chain polymers Polyolefins and related polymers By far the most important industrial polymers (for example, virtually all the commodity plastics) are polymerized olefins. Olefins are hydrocarbons (compounds containing hydrogen and carbon ) whose molecules contain a pair of carbon atoms linked together by a double bond. Most often derived from natural gas or from low-molecular-weight constituents of petroleum, they include ethylene, propylene, and butene (butylene). Olefin molecules are commonly represented by the chemical formula CH2=CHR, with R representing an atom or pendant molecular group of varying composition. As the repeating unit of a polymeric molecule, their chemical structure can be represented as: The composition and structure of R determines which of the huge array of possible properties will be demonstrated by the polymer. Polyethylene (PE) Ethylene, commonly produced by the cracking of ethane gas, forms the basis for the largest single class of plastics, the polyethylenes. Ethylene monomer has the chemical composition CH2=CH2; as the repeating unit of polyethylene it has the following chemical structure: Figure 1: The linear form of polyethylene, known as high-density polyethylene (HDPE). Figure 2: The branched form of polyethylene, known as low-density polyethylene (LDPE). This simple structure can be produced in linear or branched forms such as those illustrated in Figures 1 and 2. Branched versions are known as low-density polyethylene (LDPE) or linear low-density polyethylene (LLDPE); the linear versions are known as high-density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE). In 1899 a German chemist, Hans von Pechmann, observed the formation of a white precipitate during the autodecomposition of diazomethane in ether. In 1900 this compound was identified by the German chemists Eugen Bamberger and Friedrich Tschirner as polymethylene (n), a polymer that is virtually identical to polyethylene. In 1935 the British chemists Eric Fawcett and Reginald Gibson obtained waxy, solid PE while trying to react ethylene with benzaldehyde at high pressure. Because the product had little potential use, development was slow. As a result, the first industrial PEactually an irregularly branched LDPEwas not produced until 1939 by Imperial Chemical Industries (ICI). It was first used during World War II as an insulator for radar cables. In 1930 Carl Shipp Marvel, an American chemist working as a consultant at E.I. du Pont de Nemours & Company, Inc., discovered a high-density product, but DuPont failed to recognize the potential of the material. It was left to Karl Ziegler of the Kaiser Wilhelm (now Max Planck) Institute for Coal Research at Mlheim an der Ruhr, Ger., to win the Nobel Prize for Chemistry in 1963 for inventing linear HDPEwhich Ziegler actually produced with Erhard Holzkamp in 1953, catalyzing the reaction at low pressure with an organometallic compound henceforth known as a Ziegler catalyst. By using different catalysts and polymerization methods, scientists subsequently produced PEs with various properties and structures. LLDPE, for example, was introduced by the Phillips Petroleum Company in 1968. LDPE is prepared from gaseous ethylene under very high pressures (up to 350 megapascals, or 50,000 pounds per square inch) and high temperatures (up to 350 C, or 660 F) in the presence of peroxide initiators. These processes yield a polymer structure with both long and short branches. As a result, LDPE is only partly crystalline, yielding a material of high flexibility. Its principal uses are in packaging film, trash and grocery bags, agricultural mulch, wire and cable insulation, squeeze bottles, toys, and housewares. Some LDPE is reacted with chlorine (Cl) or with chlorine and sulfur dioxide (SO2) in order to introduce chlorine or chlorosulfonyl groups along the polymer chains. Such modifications result in chlorinated polyethylene (CM) or chlorosulfonated polyethylene (CSM), a virtually noncrystalline and elastic material. In a process similar to vulcanization, cross-linking of the molecules can be effected through the chlorine or chlorosulfonyl groups, making the material into a rubbery solid. Because their main polymer chains are saturated, CM and CSM elastomers are highly resistant to oxidation and ozone attack, and their chlorine content gives some flame resistance and resistance to swelling by hydrocarbon oils. They are mainly used for hoses, belts, heat-resistant seals, and coated fabrics. LLDPE is structurally similar to LDPE. It is made by copolymerizing ethylene with 1-butene and smaller amounts of 1-hexene and 1-octene, using Ziegler-Natta or metallocene catalysts. The resulting structure has a linear backbone, but it has short, uniform branches that, like the longer branches of LDPE, prevent the polymer chains from packing closely together. The main advantages of LLDPE are that the polymerization conditions are less energy-intensive and that the polymer's properties may be altered by varying the type and amount of comonomer (monomer copolymerized with ethylene). Overall, LLDPE has similar properties to LDPE and competes for the same markets. HDPE is manufactured at low temperatures and pressures using Ziegler-Natta and metallocene catalysts or activated chromium oxide (known as a Phillips catalyst). The lack of branches allows the polymer chains to pack closely together, resulting in a dense, highly crystalline material of high strength and moderate stiffness. Uses include blow-molded bottles for milk and household cleaners and injection-molded pails, bottle caps, appliance housings, and toys. UHMWPE is made with molecular weights of 3 million to 6 million atomic units, as opposed to 500,000 atomic units for HDPE. These polymers can be spun into fibres and drawn, or stretched, into a highly crystalline state, resulting in high stiffness and a tensile strength many times that of steel. Yarns made from these fibres are woven into bulletproof vests. Carbon-chain polymers Acrylic polymers Acrylic is a generic term denoting derivatives of acrylic and methacrylic acid, including acrylic esters and compounds containing nitrile and amide groups. Polymers based on acrylics were discovered before many other polymers that are now widely employed. In 1880 the Swiss chemist Georg W.A. Kahlbaum prepared polymethyl acrylate, and in 1901 the German chemist Otto Rhm investigated polymers of acrylic esters in his doctoral research. A flexible acrylic ester, polymethyl acrylate, was produced commercially by Rohm & Haas AG in Germany beginning in 1927 and by the Rohm and Haas Company in the United States beginning in 1931; used in sheets for laminated safety glass, it was sold under the trademarked name Plexigum. In the early 1930s a more rigid plastic, polymethyl methacrylate, was discovered in England by Rowland Hill and John Crawford at Imperial Chemical Industries, which gave the material the trademarked name Perspex. At the same time, Rhm attempted to produce safety glass by polymerizing methyl methacrylate between glass layers; the polymer separated from the glass as a clear plastic sheet, which Rhm gave the trademarked name Plexiglas. Both Perspex and Plexiglas were commercialized in the late 1930s. (DuPont subsequently introduced its own product under the trademark Lucite.) During the 1940s an oil-resistant acrylate elastomera copolymer of ethyl acrylate and 2-chloroethyl vinyl etherwas produced by Charles H. Fisher at U.S. Department of Agriculture laboratories. In 1950, after R.C. Houtz had discovered spinning solvents that could dissolve polyacrylonitrile, DuPont introduced its trademarked Orlon, the first acrylic fibre to be produced in commercial quantities. Polyacrylonitrile (PAN) Acrylonitrile (CH2=CHCN), a compound obtained by reacting propylene with ammonia (NH3) and oxygen in the presence of catalysts, is polymerized to polyacrylonitrile through suspension methods using free-radical initiators. The structure of the polymer repeating unit is: Most of the polymer produced is employed in acrylic fibres, which are defined as fibres that contain 85 percent or more PAN. Because PAN is difficult to dissolve in organic solvents and is highly resistant to dyeing, very little fibre is produced containing PAN alone. On the other hand, a copolymer containing PAN and 2 to 7 percent of a vinyl comonomer such as vinyl acetate can be readily spun to fibres that are soft enough to allow penetration by dyestuffs. Acrylic fibres are soft and flexible, producing lightweight, lofty yarns. Such properties closely resemble those of wool, and hence the most common use of acrylics in apparel and carpets is as a wool replacementfor example, in knitwear such as sweaters and socks. Acrylics can be sold at a fraction of the cost of the natural fibre, and they offer better light resistance, mildew resistance, and resistance to attack by moths. Acrylic fibres are also used as precursors for the production of carbon and graphite fibres, as replacements for asbestos in cement, and in industrial filters and battery separators. Acrylics modified by halogen-containing comonomers such as vinyl chloride or vinylidene chloride are classified as modacrylics. (By definition, modacrylics contain more than 35 and less than 85 percent PAN.) Chlorine imparts a notable flame resistance to the fibrean advantage that makes modacrylics desirable for such products as children's sleepwear, blankets, awnings, and tents. However, they are not as widely used as the simple acrylics because of their higher cost and because they are somewhat sensitive to heat (for instance, from ironing). Carbon-chain polymers Fluorinated polymers Polytetrafluoroethylene (PTFE) PTFE was discovered serendipitously in 1938 by a DuPont chemist, Roy Plunkett, who found that a tank of gaseous tetrafluoroethylene (CF2=CF2) had polymerized to a white powder. During World War II it was applied as a corrosion-resistant coating to protect metal equipment used in the production of radioactive material. DuPont released its trademarked Teflon-coated nonstick cookware in 1960. PTFE is made from the gaseous monomer tetrafluoroethylene, using high-pressure suspension or solution methods in the presence of free-radical initiators. The polymer is similar in structure to polyethylene, consisting of a carbon chain with two fluorine atoms bonded to each carbon: The fluorine atoms surround the carbon chain like a sheath, giving a chemically inert and relatively dense product with very strong carbon-fluorine bonds. The polymer is inert to most chemicals, does not melt below 300 C (575 F), and has a very low coefficient of friction. These properties allow it to be used for bushings and bearings that require no lubricant, as liners for equipment used in the storage and transportation of strong acids and organic solvents, as electrical insulation under high-temperature conditions, and in its familiar application as a cooking surface that does not require the use of fats or oils. Fabrication of PTFE products is difficult because the material does not flow readily even at elevated temperatures. Compression molding of fine powders in the presence of volatile lubricants is one successful technique. In the coating of metal cooking surfaces, aqueous dispersions of fine particles are used. Fluoroelastomers A number of fluorinated polymers or copolymers having elastomeric properties are produced that incorporate the monomers vinylidene fluoride (CH2=CF2), hexafluoropropylene (CF2=CFCF3), and chlorotrifluoroethylene (CF2=CFCl) in addition to tetrafluoroethylene. These elastomers have outstanding resistance to oxygen, ozone, heat, and swelling by oils, chlorinated solvents, and fuels. With service temperatures up to 250 C (480 F), they are the elastomers of choice for use in industrial and aerospace equipment subjected to severe conditions. However, they have a relatively high density, are swollen by ketones and ethers, are attacked by steam, and become glassy at temperatures not far below room temperature. Also, their low reactivity makes interlinking the polymer chains a long and complex process. Principal applications are as temperature-resistant O-rings, seals, and gaskets. Carbon-chain polymers Diene polymers Dienes are compounds whose molecules contain two carbon-carbon double bonds separated by a single bond. The most important diene polymerspolybutadiene, polychloroprene, and polyisopreneare elastomers that are made into vulcanized rubber products. Polybutadiene (butadiene rubber, BR) Butadiene (CH2=CH-CH=CH2) is produced by the dehydrogenation of butene or butane or by the cracking of petroleum distillates. It is polymerized to polybutadiene by solution methods, using anionic or Ziegler-Natta initiators. Like the other diene polymers, polybutadiene is isomericit can be produced with more than one molecular structure. A common elastomeric structure is cis-1,4 polybutadiene, whose repeating unit has the following structure: Two other structures are the trans-1,4 and the 1,2 side vinyl isomers. Polybutadienes are made either with high cis content (95 to 97 percent) or with only 35 percent cis content along with 55 percent trans and 10 percent side vinyl. The properties of the two polymers are quite different. Although both display much higher resilience than other elastomers, the resilience of the mixed-isomer polymer is somewhat lower. In addition, the mixed polymer never crystallizes, so that, without reinforcing fillers such as carbon black, its products are weak and brittle. Both materials show good abrasion resistance. Much of the polybutadiene produced is blended with natural rubber (polyisoprene) or with styrene-butadiene rubber to give improved resilience and lower rolling resistance. More than half of all usage is in tires; other applications are footwear, wire and cable insulation, and conveyor belts. Carbon-chain polymers Vinyl copolymers In addition to the copolymers mentioned in previous sections (e.g., fluoroelastomers, modacrylics), a number of important vinyl (carbon-chain) copolymers are manufactured. These include most of the important synthetic elastomers not described in Diene polymers, along with several specialty plastics and thermoplastic elastomers. These copolymers are described in this section. Acrylonitrile-butadiene-styrene (ABS) ABS is a graft copolymer made by dissolving styrene-butadiene copolymer in a mixture of acrylonitrile and styrene monomers, then polymerizing the monomers with free-radical initiators in an emulsion process. Grafting of acrylonitrile and styrene onto the copolymer chains occurs by chain-transfer reactions. ABS was patented in 1948 and introduced to commercial markets by the Borg-Warner Corporation in 1954. ABS is a tough, heat-resistant thermoplastic. The three structural units provide a balance of properties, the butadiene groups (predominantly trans-1,4) imparting good impact strength, the acrylonitrile affording heat resistance, and the styrene units giving rigidity. ABS is widely used for appliance and telephone housings, luggage, sporting helmets, pipe fittings, and automotive parts. Heterochain polymers Cellulosics Cellulose (C6H7O23) is a naturally occurring polymer made up of repeating glucose units. In its natural state (known as native cellulose), it has long been harvested as a commercial fibreas in cotton, flax, hemp, kapok, sisal, jute, and ramie. Wood, which consists of cellulose in combination with a complex network polymer called lignin, is a common building material. Paper is also manufactured from native cellulose. Although it is a linear polymer, cellulose is thermosetting; that is, it forms permanent, bonded structures that cannot be loosened by heat or solvents without causing chemical decomposition. Its thermosetting behaviour arises from strong dipolar attractions that exist between cellulose molecules, imparting properties similar to those of interlinked network polymers. In the 19th century, methods were developed to separate wood cellulose from lignin chemically and then to regenerate the cellulose back to its original composition for use as both a fibre (rayon) and a plastic (cellophane). Ester and ether derivatives of cellulose were also developed and used as fibres and plastics. The most important compounds were cellulose nitrate (nitrocellulose, made into celluloid) and cellulose acetate (formerly known as acetate rayon but now known simply as acetate). Both of these chemical derivatives were based on the cellulose structure with X being NO2 in the case of the nitrate and COCH3 in the case of the acetate. Rayon Rayon is a generic term, coined in 1924, for artificial textile material composed of reconstituted, regenerated, and purified cellulose derived from plant sources. Developed in the late 19th century as a substitute for silk, this first semi-synthetic fibre is sometimes misnamed artificial silk. The first practical steps toward producing a synthetic fibre were represented by attempts to work with the highly flammable nitrocellulose, produced by treating cotton cellulose with nitric acid (see below Cellulose nitrate). In 1884 and 1885 in London, Joseph Wilson Swan exhibited fibres made of nitrocellulose that had been treated with chemicals in order to change the material back to nonflammable cellulose. Swan did not follow up the demonstrations of his invention, so that the development of rayon as a practical fibre really began in France, with the work of Louis-Marie-Hilaire Bernigaud, comte de Chardonnet, who is frequently called the father of the rayon industry. In 1889 Chardonnet exhibited fibres made by squeezing a nitrocellulose solution through spinnerettes, hardening the emerging jets in warm air, and then reconverting them to cellulose by chemical treatment. Manufacture of Chardonnet silk, later known as rayon, the first commercially produced man-made fibre, began in 1891 at a factory in Besanon. Although Chardonnet's process was simple and involved a minimum of waste, it was slow, expensive, and potentially dangerous. In 1890 another French chemist, Louis-Henri Despeissis, patented a process for making fibres from cuprammonium rayon. This material was based on the Swiss chemist Matthias Eduard Schweizer's discovery in 1857 that cellulose could be dissolved in a solution of copper salts and ammonia and, after extrusion, be regenerated in a coagulating bath. In 1908 the German textile firm J.-P. Bemberg began to produce cuprammonium rayon as Bemberg (trademark) silk. A third type of celluloseand the most popular type in use todaywas produced in 1891 from a syrupy yellow liquid that three British chemists, Charles Cross, Edward Bevan, and Clayton Beadle, discovered by the dissolution of cellulose xanthate in dilute sodium hyroxide. By 1905 Courtaulds Ltd., the British silk firm, was producing this fibre, which became known as viscose rayon (or simply viscose). In 1911 the American Viscose Corporation began production in the United States. Modern manufacture of viscose rayon has not changed in its essentials. Purified cellulose is first treated with caustic soda (sodium hydroxide). After the alkali cellulose has aged, carbon disulfide is added to form cellulose xanthate, which is dissolved in sodium hydroxide. This viscous solution (viscose) is forced through spinnerettes. Emerging from the holes, the jets enter a coagulating bath of acids and salts, in which they are reconverted to cellulose and coagulated to form a solid filament. The filament may be manipulated and modified during the manufacturing process to control lustre, strength, elongation, filament size, and cross section as demanded. Rayon fibre remains an important fibre, although production has declined in industrial countries because of environmental concerns connected with the release of carbon disulfide into the air and salt by-products into streams. It has many properties similar to cotton and can also be made to resemble silk. In apparel, it is used alone or in blends with other fibres in applications where cotton is normally used. High-strength rayon, produced by drawing (stretching) the filaments during manufacture to induce crystallization of the cellulose polymers, is made into tire cord for use in automobile tires. Rayon is also blended with wood pulp in paper making. Cellulose nitrate Heterochain polymers Polyamides A polyamide is a polymer that contains recurring amide groups (R-CO-NH-R) as integral parts of the main polymer chain. Synthetic polyamides are produced by a condensaton reaction between monomers, in which the linkage of the molecules occurs through the formation of the amide groups. They may be produced by the interaction of a diamine (a compound containing two amino groupse.g., hexamethylenediamine) and a dicarboxylic acid (containing two carboxyl groupse.g., adipic acid), or they may be formed by the self-condensation of an amino acid or an amino-acid derivative. The most important amide polymers are the nylons, an extremely versatile class of material that is an indispensable fibre and plastic. In this section the aramids, aromatic polyamides that contain benzene rings in their carboxylic-acid portions, are also described. Nylon In October 1938, DuPont announced the invention of the first wholly synthetic fibre ever produced. Given the trade name Nylon (which has now become a generic term), the material was actually polyhexamethylene adipamide, also known as nylon 6,6 for the presence of six carbon atoms in each of its two monomers. Commercial production of the new fibre began in 1939 at DuPont's plant in Seaford, Del., U.S., which in 1995 was designated a historic landmark by the American Chemical Society. Soon after the DuPont fibre was marketed, nylon 6 (polycaprolactam) was produced in Europe based on the polymerization of caprolactam. Nylon 6 and nylon 6,6 have almost the same structure and similar properties and are still the most important polyamide fibres worldwide. Their repeating units have the following structure: Nylon 6,6 was first synthesized at DuPont in 1935 by Wallace Hume Carothers by the condensation reaction of adipic acid and 1,6-hexamethylenediamine: As developed by Carothers, Julian Hill, and coworkers, the production process involved the use of a molecular still, which allowed polymerization to proceed more nearly to completion by eliminating water produced in the condensation reaction. Nylon arrived on the scene just in time to replace silk (a natural polyamide), whose East Asian supply sources had been cut off by imperial Japan. Women's stockings made of the new fibre were exhibited at the Golden Gate International Exposition in San Francisco and at the New York World's Fair in 1939. The next year they went on sale throughout the United States, touching off a nylon mania that survived diversion of the fibre to military use during World War II and continued after the war with such intensity that nylon virtually established the synthetic-fibre industry. The high strength, elasticity, abrasion resistance, mildew resistance, lustre, dyeability, and shape-holding properties of the material made it ideal for innumerable applications in apparel, home furnishings, automobiles, and machinery. In addition, extruded and molded plastic parts made of nylon exhibited high melting points, stiffness, toughness, strength, and chemical inertness; they found immediate use as gear wheels, oil seals, bearings, and temperature-resistant packaging film. Nylon is still a very important fibre, and its market has grown greatly since its introduction. However, it has yielded some market share to fibres of polyethylene terephthalate (see the section on Polyesters), which are cheaper to produce and display many superior properties. In apparel and home furnishings, nylon is an important fibre, especially in hosiery, lingerie, stretch fabrics and sports garments, soft-sided luggage, furniture upholstery, and carpets. (For carpeting the nylon fibre is made in large-diameter filaments.) Industrial uses of nylon fibre include automobile and truck tires, ropes, seat belts, parachutes, substrates for coated fabrics such as artificial leather, fire and garden hoses, nonwoven fabrics for carpet underlayments, and disposable garments for the health-care industry. As plastics the nylons still find employment as an engineering plasticfor example, in bearings, pulleys, gears, zippers, and automobile fan blades. Unlike rayon and acetate, nylon fibres are melt-spuna process described in the article man-made fibre. Other polyamides of commercial importance include nylons 4,6; 6,10; 6,12; and 12,12each prepared from diamines and dicarboxylic acids; nylon 11, prepared by step-growth polymerization from the amino acid H2N(CH2)10COOH; and nylon 12, made by ring-opening polymerization of a cyclic amide. Heterochain polymers Polysulfides Polysulfides are polymers that contain one or more groups of sulfur atoms in their backbones. They fall into two types: compounds containing a single sulfur atom per repeating unit and compounds containing two or more. Of the former type, polyphenylene sulfide is the most important. The latter type is known generically as polysulfide rubber or by its trade name, thiokol. Polyphenylene sulfide (PPS) PPS is a high-strength, highly crystalline engineering plastic that exhibits good thermal stability and chemical resistance. It is polymerized by reacting dichlorobenzene monomers with sodium sulfide at about 250 C (480 F) in a high-boiling, polar solvent. Polymerization is accompanied by loss of sodium chloride. When electron-donor or electron-acceptor dopants are added to PPS, the polymer becomes a conductor of electricity. PPS is used principally in automotive and machine parts, appliances, electronic and electrical processing equipment, and coatings. Heterochain polymers Polyurethanes Polyurethanes are a class of extremely versatile polymers that are made into flexible and rigid foams, fibres, elastomers, and surface coatings. They are formed by reacting an isocyanate (a compound having the functional group NCO) with an alcohol (having the functional group OH). Polyurethane molecules can adopt a linear or a network architecture. Linear polyurethanes are formed by reacting a dialcohol with a diisocyanate, whereas network polyurethanes are formed from polyfunctional alcohols or isocyanates. Dialcohol monomers include ethylene glycol (HOCH2CH2OH); diethylene glycol (HOCH2CH2OCH2CH2OH); 1,4-butanediol (HOCH2CH2CH2CH2OH); 1,6-hexanediol (HO6OH); alcohol-terminated polyethers such as polyethylene oxide and polypropylene oxide (see Aliphatic polyethers); and flexible, alcohol-terminated polyesters such as poly-1,4-butylene adipate: The alcohol-terminated polyethers and polyesters are known as polyols. Isocyanates commonly used to prepare polyurethanes are toluene diisocyanate (TDI), methylene-4,4-diphenyl diisocyanate (MDI), and a polymeric isocyanate (PMDI). These isocyanates have the following structures: During the late 1930s Otto Bayer, manager of the IG Farben laboratories in Leverkusen, Ger., prepared many polyurethanes by condensation reaction of dihydric alcohols such as 1,4-butanediol with difunctional diisocyanates. A major breakthrough in the commercial application of polyurethane did not occur until 1941, when a trace of moisture reacted with isocyanate to produce carbon dioxide. The production of this gas resulted in many small empty areas, or cells, in the product (which was subsequently called imitation Swiss cheese). In 1953 Bayer and the Monsanto Chemical Company (now Monsanto Company) formed the Mobay Chemical Corporation to produce polyurethane in the United States. Polyurethane foams The largest segment of the market for polyurethanes is in rigid and flexible foams. Flexible foams are usually made with polyols and an excess of TDI. Foam is manufactured by adding water, which reacts with the terminal isocyanate groups to increase the molecular weight through urea linkages while simultaneously releasing carbon dioxide. The carbon dioxide gas, referred to as the blowing agent, is trapped as bubbles in the increasingly viscous polymer. The principal uses of flexible foam are in upholstery, bedding, automobile seats, crash panels, carpet underlays, textile laminates, and sponges. Rigid foams are made with PMDI and polyether glycols, along with low-molecular-weight dialcohols to increase the rigidity. Use of PMDI, which contains a larger number of reactive functional groups, results in a network polyurethane. A blowing agent such as pentane is normally added to augment the foaming. (Chlorofluorocarbons such as Freon [trademark] used to be employed as blowing agents before they were declared unacceptable for depleting ozone in the stratosphere.) Rigid polyurethane foam is used in insulation, packaging, marine flotation equipment, and lightweight furnishings. Heterochain polymers A wide variety of heterochain polymersthat is, polymers in which the backbone contains elements such as oxygen, nitrogen, sulfur, or silicon in addition to carbonare in commercial use. Many of these compounds are complex in structure. In this section the major heterochain polymer families are presented in alphabetic order, with important representatives of each family described in turn. Aldehyde condensation polymers Aldehyde condensation polymers are compounds produced by the reaction of formaldehyde with phenol, urea, or melamine. The reaction is usually accompanied by the release of water and other by-products. The monomers have the following structures: The polymerization reactions of these monomers produce complex, thermosetting network polymers with the following general structures (in which CH2 groups connected to the units are provided by the formaldehyde): Figure 4: The network architecture and molecular structure of phenol-formaldehyde resin. The network structure of phenol-formaldehyde resin is also illustrated in Figure 4. Heterochain polymers Polysiloxanes (silicones) Polysiloxanes are polymers whose backbones consist of alternating atoms of silicon and oxygen. Although organic substituents are attached to the silicon atoms, lack of carbon in the backbones of the chains makes polysiloxanes into unusual inorganic polymers. They can exist as elastomers, greases, resins, liquids, and adhesives. Their great inertness, resistance to water and oxidation, and stability at high and low temperatures have led to a wide range of commercial applications. Siloxanes were first characterized as macromolecules by the English chemist Frederic Stanley Kipping in 1927. Because Kipping thought that the structure of the repeating unit was essentially that of a ketone (that is, the polymer chains formed by silicon atoms, with oxygen atoms attached by double bonds), he incorrectly called them silicones, a name that has persisted. In 1943 Eugene George Rochow at the General Electric Company Laboratories in Schenectady, N.Y., U.S., prepared silicones by the hydrolysis of dialkyldimethoxysilanea ring-opening process that he patented in 1945 and that remains the basis of modern polymerization methods. The most common siloxane polymer, polydimethylsiloxane, is formed when the chlorine atoms of the monomer, dichlorodimethylsilane (Cl2Si2), are replaced by hyroxyl (OH) groups by hydrolysis. The resultant unstable compound, silanol (Cl2Si2), condenses in step-growth fashion to form the polymer, with concomitant loss of water. Some cyclic products are also formed, and these are purified by distillation and converted to polysiloxane by ring-opening polymerization. The repeating unit of polydimethylsiloxane has the following structure: Siloxane molecules rotate freely around the Si-O bond, so that, even with vinyl, methyl, or phenyl groups attached to the silicon atoms, the molecule is highly flexible. In addition, the Si-O bond is highly heat-resistant and is not readily attacked by oxygen or ozone. As a result, silicone rubbers are remarkably stable, and they have the lowest glass transition temperature and the highest permeability to gases of any elastomer. On the other hand, the Si-O bond is susceptible to hydrolysis and attack by acids and bases, and the rubber vulcanizates are relatively weak and readily swollen by hydrocarbon oils. Nonvulcanized, low-molecular-weight polysiloxanes make excellent lubricants and hydraulic fluids and are known as silicone oils. Vulcanized silicone rubber is prepared in two principal forms: (1) as low-molecular-weight liquid room-temperature-vulcanizing (RTV) polymers that are interlinked at room temperature after being cast or molded into a desired shape or (2) as heat-curable, high-temperature-vulcanizing (HTV) elastomers of higher viscosity that are mixed and processed like other elastomers. RTV elastomers are usually interlinked using reactive vinyl end-groups, whereas HTV materials are usually interlinked by means of peroxides. Silicone rubber is used mainly in O-rings, heat-resistant seals, caulks and gaskets, electrical insulators, flexible molds, and (owing to its chemical inertness) surgical implants. Heterochain polymers Polyimides Polyimides are polymers that usually consist of aromatic rings coupled by imide linkagesthat is, linkages in which two carbonyl (CO) groups are attached to the same nitrogen (N) atom. There are two categories of these polymers, condensation and addition. The former are made by step-growth polymerization and are linear in structure; the latter are synthesized by heat-activated addition polymerization of diimides and have a network structure. Typical of the condensation type is the polyimide sold under the trademarked name of Kapton by DuPont, which is made from a dianhydride and a diamine. When the two monomers react, the first product formed is a polyamide. The polyamide can be dissolved in solvents for casting into films, or it can be melted and molded. Conversion to polyimide occurs when the intermediate polyamide is heated above 150 C (300 F). Unlike the polyamide, the polyimide is insoluble and infusible. Kapton is stable in inert atmospheres at temperatures up to 500 C (930 F). Related commercial products are polyamideimide (PAI; trademarked as Torlon by Amoco Corporation) and polyetherimide (PEI; trademark Ultem); these two compounds combine the imide function with amide and ether groups, respectively. Network polyimides are formed from bismaleimide and bisnadimide precursors. At temperatures above 200 C (390 F), bismaleimides undergo free-radical addition polymerization through the double bonds to form a thermosetting network polymer. Bisnadimides react somewhat differently at elevated temperatures. The nadimide group first decomposes to yield cyclopentadiene and maleimide, which then copolymerize to form the network polyimide structure. Polyimides are amorphous plastics that characteristically exhibit great temperature stability and high strength, especially in the form of composites. They are used in aircraft components, sporting goods, electronics components, plastic films, and adhesives. Heterochain polymers Polyethers Polyethers are polymers that are formed by the joining of monomers through ether linkagesi.e., two carbon atoms connected to an oxygen atom. A variety of polyethers are manufactured, ranging from engineering plastics to elastomers. The compounds also differ markedly in structure, though they all retain the C-O-C linkage. Polyacetal Also called polyoxymethylene (POM) or simply acetal, polyacetal has the simplest structure of all the polyethers. It is manufactured in a solution process by anionic or cationic chain-growth polymerization of formaldehyde (H2C=O), a reaction analogous to vinyl polymerization. By itself, the polymer is unstable and reverts to monomer on heating to 120 C (250 F); for this reason the commercial product is reacted further with acetic anhydride to cap the ends of the chains (where depolymerization is initiated on heating) with acetate groups. The end-capped polymer is marketed by DuPont under the trademarked name of Delrin. It is a high-strength, highly crystalline engineering plastic that exhibits a low coefficient of friction and excellent resistance to oils, greases, and solvents. Also marketed is a copolymer (trademarked as Celcon by Hoechst Celanese Corp.) prepared from trioxane (a trimer of formaldehyde) and small amounts of ethylene oxide to prevent the polymer from decomposing to formaldehyde on heating. Both polyacetal and the copolymer have been used as a replacement for metal in plumbing and automotive parts. Principal uses include appliance parts, electronics components, gears, bushings, bearings, plumbing fixtures, appliances, toys, toiletry and cosmetic articles, food-processing equipment, zippers, and belt buckles. Heterochain polymers Polyesters Polyesters are polymers made by a condensation reaction taking place between monomers in which the linkage between the molecules occurs through the formation of ester groups. The esters, which in almost all cases link an organic alcohol to a carboxylic acid, have the general structure where R and R are any organic combining groups. The major industrial polyesters include polyethylene terephthalate, polycarbonate, degradable polyesters, alkyds, and unsaturated polyesters. Polyethylene terephthalate (PET) PET is produced by the step-growth polymerization of ethylene glycol and terephthalic acid. The presence of the large benzene rings in the repeating units gives the polymer notable stiffness and strength, especially when the polymer chains are aligned with one another in an orderly arrangement by drawing (stretching). In this semicrystalline form, PET is made into a high-strength textile fibre marketed under such trademarked names as Dacron (DuPont) and Terylene (Imperial Chemical Industries Ltd.). The stiffness of PET fibres makes them highly resistant to deformation, so that they impart excellent resistance to wrinkling in fabrics. They are often used in durable-press blends with other fibres such as rayon, wool, and cotton, reinforcing the inherent properties of those fibres while contributing to the ability of the fabric to recover from wrinkling. PET is also made into fibre filling for insulated clothing and for furniture and pillows. When made in very fine filaments, it is used in artificial silk, and in large-diameter filaments it is used in carpets. Among the industrial applications of PET are automobile tire yarns, conveyor belts and drive belts, reinforcement for fire and garden hoses, seat belts (an application in which it has largely replaced nylon), nonwoven fabrics for stabilizing drainage ditches, culverts, and railroad beds, and nonwovens for use as diaper top sheets and disposable medical garments. PET is the most important of the man-made fibres in weight produced and in value. At a slightly higher molecular weight, PET is made into a high-strength plastic that can be shaped by all the common methods employed with other thermoplastics. Recording tape and magnetic film is produced by extrusion of PET film (often sold under the trademarks Mylar and Melinex). Molten PET can be blow-molded into a transpare