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A polyurethane, commonly abbreviated '''PU''', is any Polymer consisting of a chain of Organic units joined by Urethane links. Polyurethane polymers are formed by reacting a Monomer containing at least two Isocyanate Functional Group s with another monomer containing at least two Alcohol groups in the presence of a Catalyst . Polyurethane formulations cover an extremely wide range of stiffness, hardness, and densities. These materials include low density flexible Foam used in Upholstery and bedding, low density rigid foam used for Thermal Insulation , soft solid Elastomers used for gel pads and print rollers, and hard solid plastics used as electronic instrument bezels and structural parts. Polyurethanes are widely used in high resiliency flexible foam seating, rigid foam insulation panels, microcellular foam Seal s and Gasket s, durable elastomeric wheels and tires, electrical potting compounds, high performance Adhesive s and sealants, Spandex Fibers , seals, gaskets, Carpet underlay, and hard plastic parts. Polyurethane products are often called "urethanes". They should not be confused with the specific substance Urethane , also known as Ethyl Carbamate . Polyurethanes are not produced from ethyl carbamate, nor do they contain it. HISTORY The pioneering work on polyurethane polymers was conducted by Otto Bayer and his coworkers in 1937 at the laboratories of I.G. Farben in Leverkusen, Germany see German Patent 728.981 (1937) I.G. Farben . They recognized that using the polyaddition principle to produce polyurethanes from liquid diisocyanates and liquid Polyether or Polyester diols seemed to point to special opportunities, especially when compared to already existing plastics that were made by polymerizing olefins, or by Polycondensation . The new monomer combination also circumvented existing patents obtained by Wallace Carothers on Polyester s ''Polyurethanes: A Class of Modern Versatile Materials'' Raymond B. Seymour George B. Kauffman J. Chem. Ed. 69, 909 1992 Initially, work focused on the production of fibers and flexible foams. With development constrained by World War II (when PU's were applied on a limited scale as aircraft coating ), it was not until 1952 that polyisocyanates became commercially available. Commercial production of flexible polyurethane foam began in 1954, based on Toluene Diisocyanate (TDI) and polyester polyols. The invention of these foams (initially called ''imitation Swiss Cheese '' by the inventors ) was thanks to water accidentally introduced in the reaction mix. These materials were also used to produce rigid foams, gum rubber, and Elastomer s. Linear fibers were produced from Hexamethylene Diisocyanate (HDI) and 1,4-butanediol (BDO). The first commercially available polyether polyol, Poly(tetramethylene Ether) Glycol ), was introduced by DuPont in 1956 by polymerizing Tetrahydrofuran . Less expensive polyalkylene glycols were introduced by BASF and Dow Chemical the following year, 1957. These polyether polyols offered technical and commercial advantages such as low cost, ease of handling, and better hydrolytic stability; and quickly supplanted polyester polyols in the manufacture of polyurethane goods. Another early pioneer in PU's was the Mobay Corporation . In 1960 more than 45,000 tons of flexible polyurethane foams were produced. As the decade progressed, the availability of Chlorofluoroalkane blowing agents, inexpensive polyether polyols, and Methylene Diphenyl Diisocyanate (MDI) heralded the development and use of polyurethane rigid foams as high performance insulation materials. Rigid foams based on polymeric MDI (PMDI) offered better thermal stability and combustion characteristics than those based on TDI. In 1967, urethane modified Polyisocyanurate rigid foams were introduced, offering even better thermal stability and Flammability resistance to low density insulation products. Also 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 AG exhibited an all plastic car in Dusseldorf, Germany. Parts of this car were manufactured using a new process called RIM, Reaction Injection Molding . RIM technology uses high-pressure impingement of liquid components followed by the rapid flow of the reaction mixture into a mold cavity. Large parts, such as automotive Fascia and body panels, can be molded in this manner. Polyurethane RIM evolved into a number of different products and processes. Using Diamine Chain Extender s and Trimerization technology gave poly(urethane urea), poly(urethane isocyanurate), and polyurea RIM. The addition of fillers, such as milled glass, Mica , and processed mineral fibers gave arise to RRIM, reinforced RIM, which provided improvements in Flexural Modulus (stiffness) and thermal stability. This technology allowed production of the first plastic-body automobile in the United Sates, the Pontiac Fiero , in 1983. Further improvements in flexural modulus were obtained by incorporating preplaced glass mats into the RIM mold cavity, also known as SRIM, or structural RIM. Starting in the early 1980s, water-blown microcellular flexible foam was used to mold gaskets for panel and radial seal air filters in the automotive industry. Since then, increasing energy prices and the desire to eliminate PVC plastisol from automotive applications have greatly increased market share. Costlier raw materials are offset by a significant decrease in part weight and in some cases, the elimination of metal end caps and filter housings. Highly filled polyurethane elastomers, and more recently unfilled polyurethane foams are now used in high-temperature oil filter applications. |
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