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Polymer Science and Engineering: The Shifting Research Frontiers
generally used for alcohol-water separations, which become more important as biotechnology processes come into practice. Europe and Japan seems to be the leaders of research in this field. Major breakthroughs in membrane materials and fabrication are needed and appear to be possible.
The U.S. coatings (paint) industry uses about 2.6 × 109 pounds of polymers per year and converts these to coatings having about $11B in sales. These coatings are applied onto goods valued at about $1T to $2T. For example, the typical cost of the paint on a house is about $500; on a car, $200; and on a refrigerator, $50.
Until the early 1970s most coatings contained only 15 to 30 percent paint solids, the remaining 70 to 85 percent being organic solvents, which were released as air pollutants when the films dried. Since then, reduction of solvent emissions has been the most important single driving force for technology change. Two kinds of 100 percent solids coatings are now being sold for specialized applications. Powder coatings are electrostatically applied and subsequently heat fused. Radiation-cured coatings are based on solventless liquid oligomers responding to ultraviolet or electronbeam cure. Solvent-borne high-solids coatings play an ever-increasing role. These are often based on oligomers containing hydroxyl groups as cross-linkable sites. Cross-linking is accomplished by formaldehyde-based methylolated or alkoxy-methylolated nitrogenous (amino) compounds. Acid-catalyzed heat cure causes the formation of multiple ether-based cross-links. Alternately, hydroxyl functional oligomers are cross-linked with isocyanates to form urethanes. Other common high-solids coatings have drying oil functionality and are cross-linked by air oxidation. Such high-solids coatings now contain about 20 to 50 percent volatile organic compounds (VOCs). The VOCs include solvents, by-products of the cross-linking reaction, and amines used to block catalysts. The role of aqueous coatings in reducing emissions of VOCs has increased greatly in the past few years. These coatings are based mostly on high-molecular-weight latex polymers and still contain some organic solvents to help film formation and wetting. Alternately, aqueous coatings are based on lower-molecular-weight hydrophilic polymers, which, unlike latexes, are synthesized not in water but in organic solvents and are subsequently dispersed into water. The ratio of VOCs to paint solids in aqueous coatings is now about 1:1 to 2:1. The environmental concerns leading to a reduction in VOCs were met by newly developed coatings based on new polymer chemistry that often provide better performance than the traditional coatings. In particular, resistance to weathering and to corrosive environments (including acid rain) was improved, not only for heat-cured but also for ambient-cured coatings. Lowering the cure temperature has allowed the application of