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Polyurethane - Wikipedia. Polyurethane synthesis, wherein the urethane groups . While most polyurethanes are thermosetting polymers that do not melt when heated, thermoplastic polyurethanes are also available. Polyurethane polymers are traditionally and most commonly formed by reacting a di- or polyisocyanate with a polyol. Both the isocyanates and polyols used to make polyurethanes contain, on average, two or more functional groups per molecule.
Some noteworthy recent efforts have been dedicated to minimizing the use of isocyanates to synthesize polyurethanes, because the isocyanates raise severe toxicity issues. Non- isocyanate based polyurethanes (NIPUs) have recently been developed as a new class of polyurethane polymers to mitigate health and environmental concerns. Polyurethanes neither contain nor are produced from ethyl carbamate. Polyurethanes are used in the manufacture of high- resilience foam seating, rigid foam insulation panels, microcellular foam seals and gaskets, durable elastomeric wheels and tires (such as roller coaster, escalator, shopping cart, elevator, and skateboard wheels), automotive suspension bushings, electrical potting compounds, high performance adhesives, surface coatings and surface sealants, synthetic fibers (e. Spandex), carpet underlay, hard- plastic parts (e. These materials were also used to produce rigid foams, gum rubber, and elastomers. Linear fibers were produced from hexamethylene diisocyanate (HDI) and 1,4- butanediol (BDO).
In 1. 95. 6 Du. Pont introduced polyether polyols, specifically poly(tetramethylene ether) glycol, and BASF and Dow Chemical started selling polyalkylene glycols in 1. Polyether polyols were cheaper, easier to handle and more water- resistant than polyester polyols, and became more popular. Union Carbide and Mobay, a U. S. Monsanto/Bayer joint venture, also began making polyurethane chemicals.
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 1. 96. 7, urethane- modified polyisocyanurate rigid foams were introduced, offering even better thermal stability and flammability resistance. During the 1. 96. In 1. 96. 9, Bayer exhibited an all- plastic car in D. 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 fibres, 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 1.
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 1. PVC plastisol from automotive applications have greatly increased market share.
In the early 1. 99. Montreal Protocol restricted the use of many chlorine- containing blowing agents, such as trichlorofluoromethane (CFC- 1. By the late 1. 99. HFC- 1. 34a) and 1,1,1,3,3- pentafluoropropane (HFC- 2. North America and the EU, although chlorinated blowing agents remained in use in many developing countries.
Long, flexible segments, contributed by the polyol, give soft, elastic polymer. High amounts of crosslinking give tough or rigid polymers. Long chains and low crosslinking give a polymer that is very stretchy, short chains with lots of crosslinks produce a hard polymer while long chains and intermediate crosslinking give a polymer useful for making foam. The crosslinking present in polyurethanes means that the polymer consists of a three- dimensional network and molecular weight is very high. In some respects a piece of polyurethane can be regarded as one giant molecule. One consequence of this is that typical polyurethanes do not soften or melt when they are heated; they are thermosetting polymers.
The choices available for the isocyanates and polyols, in addition to other additives and processing conditions allow polyurethanes to have the very wide range of properties that make them such widely used polymers. Isocyanates are very reactive materials.
This makes them useful in making polymers but also requires special care in handling and use. The aromatic isocyanates, diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI) are more reactive than aliphatic isocyanates, such as hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI). Most of the isocyanates are difunctional, that is they have exactly two isocyanate groups per molecule. An important exception to this is polymeric diphenylmethane diisocyanate, which is a mixture of molecules with two, three, and four or more isocyanate groups. In cases like this the material has an average functionality greater than two, commonly 2. Polyols are polymers in their own right and have on average two or more hydroxyl groups per molecule. Polyether polyols are mostly made by co- polymerizing ethylene oxide and propylene oxide with a suitable polyol precursor.
The polyols used to make polyurethanes are not . Despite them being complex mixtures, industrial grade polyols have their composition sufficiently well controlled to produce polyurethanes having consistent properties. As mentioned earlier, it is the length of the polyol chain and the functionality that contribute much to the properties of the final polymer. Polyols used to make rigid polyurethanes have molecular weights in the hundreds, while those used to make flexible polyurethanes have molecular weights up to ten thousand or more. PU reaction mechanism catalyzed by a tertiary amine. Generalized urethane reaction.
The polymerization reaction makes a polymer containing the urethane linkage, . Alternatively, it can be promoted by ultraviolet light. This reaction is referred to as the blowing reaction and is catalyzed by tertiary amines like bis- (2- dimethylaminoethyl)ether.
A third reaction, particularly important in making insulating rigid foams is the isocyanate trimerization reaction, which is catalyzed by potassium octoate, for example. One of the most desirable attributes of polyurethanes is their ability to be turned into foam. Making a foam requires the formation of a gas at the same time as the urethane polymerization (gellation) is occurring.
The gas can be carbon dioxide, either generated by reacting isocyanate with water or added as a gas; it also be produced by boiling volatile liquids. In the latter case heat generated by the polymerization causes the liquids to vaporize. The liquids can be HFC- 2. HFC- 1. 34a (1,1,1,2- tetrafluoroethane), and hydrocarbons such as n- pentane. Carbon dioxide gas formed by reacting water and isocyanate.
The balance between gellation and blowing is sensitive to operating parameters including the concentrations of water and catalyst. The reaction to generate carbon dioxide involves water reacting with an isocyanate first forming an unstable carbamic acid, which then decomposes into carbon dioxide and an amine. The amine reacts with more isocyanate to give a substituted urea. Water has a very low molecular weight, so even though the weight percent of water may be small, the molar proportion of water may be high and considerable amounts of urea produced. The urea is not very soluble in the reaction mixture and tends to form separate . The concentration and organization of these polyurea phases can have a significant impact on the properties of the polyurethane foam. Rigid foam surfactants are designed to produce very fine cells and a very high closed cell content.
Flexible foam surfactants are designed to stabilize the reaction mass while at the same time maximizing open cell content to prevent the foam from shrinking. An even more rigid foam can be made with the use of specialty trimerization catalysts which create cyclic structures within the foam matrix, giving a harder, more thermally stable structure, designated as polyisocyanurate foams. Such properties are desired in rigid foam products used in the construction sector.
Careful control of viscoelastic properties . Open- cell foams feel soft and allow air to flow through, so they are comfortable when used in seat cushions or mattresses. Closed- cell rigid foams are used as thermal insulation, for example in refrigerators.
Microcellular foams are tough elastomeric materials used in coverings of car steering wheels or shoe soles. Raw materials. Other materials are added to aid processing the polymer or to modify the properties of the polymer. Isocyanates. The most commonly used isocyanates are the aromatic diisocyanates, toluene diisocyanate (TDI) and methylene diphenyl diisocyanate, MDI. TDI and MDI are generally less expensive and more reactive than other isocyanates. Industrial grade TDI and MDI are mixtures of isomers and MDI often contains polymeric materials. They are used to make flexible foam (for example slabstock foam for mattresses or molded foams for car seats). The isocyanates may be modified by partially reacting them with polyols or introducing some other materials to reduce volatility (and hence toxicity) of the isocyanates, decrease their freezing points to make handling easier or to improve the properties of the final polymers.
Aliphatic and cycloaliphatic isocyanates are used in smaller quantities, most often in coatings and other applications where color and transparency are important since polyurethanes made with aromatic isocyanates tend to darken on exposure to light. They can be further classified according to their end use. Higher molecular weight polyols (molecular weights from 2,0.