Polyurethane resins

(organic chemistry) Any resin resulting from the reaction of diisocyanates (such as toluene diisocyanate) with a phenol, amine, or hydroxylic or carboxylic compound to produce a polymer with free isocyanate groups; used as protective coatings, potting or casting resins, adhesives, rubbers, and foams, and in paints, varnishes, and adhesives.

Polyurethane (or polyisocyanate) resins are produced by the reaction of a diisocyanate with a compound containing at least two active hydrogen atoms, such as a diol or diamine. Linear, fiber-forming polymers are formed by the addition of diisocyanates to diols, while cross-linking is made possible by the use of polyols or isocyanates having more than two functional groups. Unique elastomeric or stretch fibers can be made from polyester prepolymers, in which rubbery polyester blocks alternate with rigid urethane units and terminal isocyanate groups provide sites for further chain extension and cross-linking.

There are three major types of polyurethane elastomers. One type is based on ether- or ester-type prepolymers that are chain-extended and cross-linked using polyhydroxyl compounds or amines; alternately, unsaturated groups may be introduced to permit vulcanization with common curing agents such as peroxides. A second type is obtained by first casting a mixture of prepolymer with chain-extending and cross-linking agents, and then cross-linking further by heating. The third type is prepared by reacting a dihydroxy ester- or ether-type prepolymer, or a diacid, with a diisocyanate such as diphenylmethane dissocyanate and a diol.

Polyurethane resins can be produced in forms varying from hard, glossy, solvent-resistant coatings, to abrasion- and solvent-resistant rubbers, fibers, and flexible-to-rigid foams. The foams have found the widest use. The more flexible foams are employed as upholstery material for furniture, as rug backing, insulation, and crash pads. The more rigid foams are employed as the core in structural and insulating laminates and as insulation in refrigerated appliances and vehicles. Polyurethanes are also used as adhesives, for example, in the bonding of rubber and of nylon. The flexible polyurethanes may be used for coating rubber articles to give them additional resistance to abrasion and solvents. Wire insulated with polyurethane resin can be soldered directly without previously removing the coating because the polymer decomposes at the soldering temperature to yield a clean wire surface. See also Plastics processing; Polyester resins; Polyether resins; Polymerization.