Polyurethane Coatings

THE CHEMISTRY OF POLYISOCYANATES WAS FIRST described by Professor Otto Bayer in the laboratories of the I. G. Farbenindustry, today's Farbenfabriken Bayer, in Leverkusen, Germany. Polyurethanes are mainly characterized by the linkage -NH-C(CO)-O-, though they may also contain other functional groups such as ester, ether, urea, and amide. The most important commercial route for the synthesis of such polymers is the addition polymerization that occurs when di- or higher functionality isocyanates and di- or higher functionality hydroxyl compounds, such as hydroxyl-terminated acrylics, polyesters, or polyethers, are combined and undergo rearrangement reactions. When di-functional reactants are used, linear or thermoplastic polyurethanes are produced. Cross-linked or thermoset polyurethanes are formed if the functionality of at least one of the reactants is greater than 2. The historical and commercial developments as well as the chemistry and applications of polyurethanes have been reviewed by a number of authors [1–23]. Polyurethanes have found extensive applications in the coating industry due to the fact that they exhibit excellent abrasion resistance, toughness, chemical and corrosion resistance, as well as a wide range of useful mechanical properties. Polyurethanes are widely used in coatings, adhesives, sealants, foams, elastomers, and RIM (reaction injection molding, composites, fibers, etc.). Excluding coatings, the 1988 United States consumption of polyurethanes was about 2,750 million pounds (1.25 million metric tons). In 1991, the national market for polyurethane coatings was about 209 million pounds (95,000 metric tons) [24, 25]. Although the market for polyurethane coatings is large and growing, it is readily apparent that it represents only about 5 %–10 % of the total domestic polyurethane market. The growth in this industry is exemplified by the fact that in 2002 the amount used in the United States was about 5,500 million pounds (2.51 million metric tons), and about 6,390 million pounds (2.90 metric tons) in North America. Reasons for the use of polyurethane coatings include high performance characteristics such as flexibility, toughness, strength, abrasion, chemical, and stain resistance, good light stability when aliphatic isocyanates are used, and good low temperature properties. The latter factor is an important reason for use of polyurethane coatings on plastic substrates.