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Polyurethane (; often abbreviated PUR and PU) is a class of composed of organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane term does not refer to the single type of polymer but a group of polymers. Unlike polyethylene and polystyrene, polyurethanes can be produced from a wide range of starting materials resulting in various polymers within the same group. This chemical variety produces polyurethanes with different chemical structures leading to many different applications. These include rigid and flexible , and coatings, adhesives, electrical potting compounds, and fibers such as spandex and polyurethane laminate (PUL). Foams are the largest application accounting for 67% of all polyurethane produced in 2016.
A polyurethane is typically produced by reacting a polymeric isocyanate with a polyol. Since a polyurethane contains two types of monomers, which polymerize one after the other, they are classed as alternating copolymers. Both the isocyanates and polyols used to make a polyurethane contain two or more per molecule.
Global production in 2019 was 25 million metric tonnes, accounting for about 6% of all polymers produced in that year.
DuPont introduced polyethers, specifically poly(tetramethylene ether) glycol, in 1956. BASF and Dow Chemical introduced polyalkylene glycols in 1957. Polyether polyols were cheaper, easier to handle and more water-resistant than polyester polyols. Union Carbide and Mobay, a U.S. Monsanto/Bayer joint venture, also began making polyurethane chemicals. In 1960 more than 45,000 metric tons of flexible polyurethane foams were produced. The availability of chlorofluoroalkane blowing agents, inexpensive polyether polyols, and methylene diphenyl diisocyanate (MDI) allowed polyurethane rigid foams to be used as high-performance insulation materials. In 1967, urethane-modified polyisocyanurate rigid foams were introduced, offering even better thermal stability and flammability resistance. During the 1960s, automotive interior safety components, such as instrument and door panels, were produced by back-filling thermoplastic skins with semi-rigid foam.
In 1969, Bayer exhibited an all-plastic car in Düsseldorf, Germany. Parts of this car, such as the fascia and body panels, were manufactured using a new process called reaction injection molding (RIM), in which the reactants were mixed and then injected into a mold. The addition of fillers, such as milled glass, mica, and processed mineral fibers, gave rise to reinforced RIM (RRIM), which provided improvements in flexural modulus (stiffness), reduction in coefficient of thermal expansion and better thermal stability. This technology was used to make the first plastic-body automobile in the United States, the Pontiac Fiero, in 1983. Further increases in stiffness were obtained by incorporating pre-placed glass mats into the RIM mold cavity, also known broadly as resin injection molding, or structural RIM.
Starting in the early 1980s, water-blown microcellular flexible foams were used to mold gaskets for automotive panels and air-filter seals, replacing PVC polymers. Polyurethane foams are used in many automotive applications including seating, head and arm rests, and headliners.
Polyurethane foam (including foam rubber) is sometimes made using small amounts of to give less dense foam, better cushioning/energy absorption or thermal insulation. In the early 1990s, because of their impact on ozone depletion, the Montreal Protocol restricted the use of many chlorine-containing blowing agents, such as trichlorofluoromethane (CFC-11). By the late 1990s, blowing agents such as carbon dioxide, pentane, 1,1,1,2-tetrafluoroethane (HFC-134a) and 1,1,1,3,3-pentafluoropropane (HFC-245fa) were widely used in North America and the EU, although chlorinated blowing agents remained in use in many developing countries. Later, HFC-134a was also banned due to high ODP and GWP readings, and HFC-141B was introduced in early 2000s as an alternate blowing agent in developing nations.
Chemistry
Polyurethanes are produced by reacting di with , n ≥ 2 (1992). 9780195209334, Oxford University Press. ISBN 9780195209334 often in the presence of a catalyst, or upon exposure to ultraviolet radiation.
Common catalysts include tertiary , such as DABCO, DMDEE, or , such as dibutyltin dilaurate. The stoichiometry of the starting materials must be carefully controlled as excess isocyanate can trimerise, leading to the formation of rigid . The polymer usually has a highly molecular structure, resulting in a thermosetting material which does not melt on heating; although some thermoplastic polyurethanes are also produced.
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