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reactions result in branching and crosslinking of the polymer. Equation 2-183 shows how a diisocyanate crosslinks the urethane and urea groups from two different polymer chains. The
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NH COO + NH CO NH N COO Allophanate OC NH R Biuret NH CO N CO NH
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relative amounts of allophanate and biuret crosslinks in the polymer depend on the relative amounts of urea and urethane groups (which, in turn, depends on the relative amounts of diamine and diol) and reaction conditions. There is a greater tendency to biuret linkages since the urea N is more reactive than the urethane N H H. Trimerization of isocyanate groups to form isocyanurates also occurs and serves as an additional source of branching and crosslinking:
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These reactions present a complexity in carrying out polymerization. Simultaneously, we have the ability to vary polymer properties over a wide range by control of the relative amounts of reactants and the polymerization conditions. Further control of the nal product is achieved by choice of monomers. Most polyurethanes involve a macroglycol (a low-molecular-weight polymer with hydroxyl end groups), diol or diamine, and diisocyanate. The macroglycol (also referred to as a polyol) is a polyether or polyester synthesized under conditions that result in two or more hydroxyl end groups. (Details of polyurethane synthesis involving macroglycols are described in Sec. 2-13c.) The diol monomers include ethylene glycol, 1,4-butanediol, 1,6-hexanediol, and p-di(2-hydroxyethoxy)benzene. The diamine monomers include diethyltoluenediamine, methylenebis(p-aminobenzene), and 3,30 dichloro-4,40 -diaminodiphenylmethane. The diisocyanate monomers include hexamethylene diisocyanate, toluene 2,4- and 2,6-diisocyanates, and naphthalene 1,5-diisocyanate. Alcohol and isocyanate reactants with functionlity greater than 2 are also employed. The extent of crosslinking in polyurethanes depends on a combination of the amount of polyfunctional monomers present and the extent of biuret, allophanate, and trimerization reactions [Dusek, 1987]. The latter reactions are controlled by the overall stoichiometry and the speci c catalyst present. Stannous and other metal carboxylates as well as tertiary amines are catalysts for the various reactions. Proper choice of the speci c catalyst result in differences in the relative amounts of each reaction. Temperature also affects the extents
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of the different reactions. Polymerization temperatures are moderate, often near ambient and usually no higher than 100 120 C. Signi cantly higher temperatures are avoided because polyurethanes undergo several different types of degradation reactions, such as
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2-185 2-186
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as well as decomposition back to the alcohol and isocyanate monomers. Overall control in the synthesis of polyurethane foamed products also requires a balance between the polymerization crosslinking and blowing processes. An imbalance between the chemical and physical processes can result in a collapse of the foamed structures (before solidi cation by crosslinking and/or cooling) or imperfections in the foam structures, which yields poor mechanical strength and performance. The wide variations possible in synthesis give rise to a wide range of polyurethane products including exible and rigid foams and solid elastomers, extrusions, coatings, and adhesives. Polyurethanes possess good abrasion, tear, and impact resistance coupled with oil and grease resistance. The global production of polyurethanes was more than 15 billion pounds in 1997. Flexible foamed products include upholstered furniture and auto parts (cushions, backs, and arms), mattresses, and carpet underlay. Rigid foamed products with a closedcell morphology possess excellent insulating properties and nd extensive use in commercial roo ng, residential sheathing, and insulation for water heaters, tanks, pipes, refrigerators, and freezers. Solid elastomeric products include forklift tires, skateboard wheels, automobile parts (bumpers, fascia, fenders, door panels, gaskets for trunk, windows, windshield, steering wheel, instrument panel), and sporting goods (golf balls, ski boots, football cleats). Many of these foam and solid products are made by reaction injection molding (RIM), a process in which a mixture of the monomers is injected into a mold cavity where polymerization and crosslinking take place to form the product. Reaction injection molding of polyurethanes, involving low-viscosity reaction mixtures and moderate reaction temperatures, is well suited for the economical molding of large objects such as automobile fenders. (Many of the elastomeric polyurethane products are thermoplastic elastomers; see Sec. 2-13c.)
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2-12f Polysiloxanes Polysiloxanes, also referred to as silicones, possess an unusual combination of properties that are retained over a wide temperature range ( 100 to 250 C). They have very good lowtemperature exibility because of the low Tg value ( 127 C). Silicones are very stable to high temperature, oxidation, and chemical and biological environments, and possess very good low-temperature exibility because of the low Tg values ( 127 C for dimethylsilox ane), a consequence of the long Si bond (1.64 A compared to 1.54 A for C and O C) the wide Si Si bond angle (143 compared to 109.5 for C C). The Si bond is O C O stronger than the C bond ($450 vs. 350 kJ/mol) and siloxanes are very stable to high C temperature, oxidation, chemical and biological environments, and weathering, and also possess good dielectric strength, and water repellency. The United States production of polysiloxane uids, resins, and elastomers was about 800 million pounds in 2001. Fluid applications include uids for hydraulics, antifoaming, water-repellent nishes for textiles,
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