Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 92 of 331

Fig. 4. Powder coating blocked polyisocyanate. few major exceptions are plastic products used in transportation applications, such as automotive exterior or interior parts. In such applications, where a high degree of surface aesthetic quality such as gloss, distinctness of image (DOI), and color matching/harmony are required, this can be achieved only by painted finishes. In some aspects, coatings formulated for plastic substrates may require different considerations compared with the coatings designed for metal substrates. For ex- ample, plastics have lower surface tension, which makes the wetting and leveling of coatings on plastic substrates more difficult than on metal or wood sub- strates. Unlike metals, plastics will deform or melt at high temperature; therefore, the curing of painted plastic parts is carried out either at room temperature or at lower temperatures than metal parts. The typical industrial practice is 180°F for low-bake plastics, and 250°F for high-bake plastics. Some plastic products, such as those made from the SMC (sheet molding compounds) and glass- or fiber-reinforced composites, may need special preparation to obtain good surface evenness before painting. Furthermore, some plastics may be sensitive to cer- tain types of coating solvents. For these reasons, coating formulators have to be aware of these differences when designing coatings for plastics. Today, the designers in the transportation industry not only need to select appropriate plastics to produce components, they also need to select appropriate coatings to paint these plastic components. In addition to obtaining high gloss and DOI, other factors such as weatherability, acid-rain etching resis- tance, scratch/mar-resistance,and chemical resistance are also important in determining the appropriate coatings. Polyurethane coatings, with a desirable 91

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