Metal Finishing Guide Book


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Page 183 of 903

Table II. Comparison Between Methyl and Phenyl Groups Methyl Groups Phenyl Groups Hardness while hot Flexibilty Water repellency Low temperature properties Chemical resistance Rate of cure Thermal shock Heat and oxidation resistance Heat and oxidation resistance Better shelf life Less thermoplastic In preparing moisture-cured polyurethanes it is critical that all of the coating pigments and solvents are totally free of water, because traces of moisture in the package can cause the coating to cure before application. In addition, the fluid hose leading to the spray gun and the head space above the coating in the pressure pot or reservoir must be free of moisture. A nitrogen blanket or a desiccant can be used to keep the head space dry. The advantages of high-solids polyurethane, moisture-cured industrial coatings with less than 420 g/L (3.5 lb/gal) VOCs are the following: (1) This is a single-component system, hence no pot-life limitations. (2) It has all of the advantages of two-component polyurethane coatings. (3) It achieves chemically resistant properties more quickly than some two-component polyurethanes. (4) It is readily available in military camouflage coatings complying with MIL-C-53039. Disadvantages are the following: (1) This is a technology with less field history than two-component polyurethanes. (2) Currently only a few companies supply military-specification-approved camouflage coatings. (3) They are not yet available in wide range of colors or in small quantities and may have a limited shelf life (less than 1 year). (4) They are very sensitive to moisture contamination, therefore requiring a special effort to keep moist air from the packaged or stored coating. (5) Drying time is affected by moisture in air. In very dry climates, the drying time may be longer than usual. (6) Currently they may be more expensive than two-component polyurethanes, with an average cost greater than $25/gal. (7) They have other disadvantages of two-component polyurethanes. Typical polyurethane end uses are the following: Aircraft skins Missiles and other aerospace products Over-the-road trucks Buses Rail cars Automotive refinishing Automotive (original equipment manufacturer): newly introduced high-gloss clear coat over metallic base coat; chip-resistant primer surfacers; flexible coatings for plastic facias bumpers Military ground support equipment, such as tanks personnel carriers,vehicles, with resistance to live chemical agents (chemical agents-resistant coatings) Architectural and Maintenance Structures and vessels in chemical plants 178

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