Metal Finishing Guide Book


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5. The coatings are relatively expensive as packaged, but competitive with solvent-based epoxies. 6. Pot life can vary from about 6 hours to more than 8 hours. 7. Sometimes the coatings can be difficult to clean up when stuck to equipment and skin. POLYURETHANE DISPERSIONS Advantages 1. Coatings made from polyurethane dispersions dry to tough films of dependable hardness and flexibility. 2. Coatings for metals, textiles, leather, wood, glass, paper, and rigid plastics are made from these versatile products. 3. Application can be done with most commonly used equipment with water thinning and water cleanup. 4. Very little, if any, solvent and only very small quantities of coalescing aids are used. 5. Films dry to predetermined gloss and color and, because they do not chalk, both gloss and color retention are excellent. 6. Like all lacquers, no chemical change occurs during drying and exposure; the dry films retain their original properties for long periods. Disadvantages 1. These coatings have the typical problems of waterborne finishes: drying time is dependent on temperature and relative humidity. 2. Like most lacquers, total nonvolatiles are quite low (35���40% by weight), so multiple coats are necessary for good film buildup. 3. Unlike solvent-borne lacquers, care must be taken to ensure good intercoat adhesion because the topcoat does not tend to dissolve any previous coats. 4. Surface cleanliness and freedom from any oil or grease, both on the surface and in application equipment, is essential for good film quality and adhesion. BAKED ALKYDS, MODIFIED ALKYDS AND ACRYLICS Alkyds, modified alkyds and acrylics should be cured using a water-reducible bake at temperatures less than 90��C (194��F). Advantages 1. Coatings are available at VOCs less than 360 g/L (3.0 lb/gal), less water; and some even below 275 g/L (2.3 lb/gal), less water. 2. The coatings have excellent film performance, often equivalent to polyurethanes. 3. The coatings are made for use on metal substrates only due to high temperature of bake, except perhaps for high-temperature plastics. 4. There are no pot life limitations; the coatings are a one-component materi188

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