Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 82 of 331

8.Topcoats can be made in a range of colors and gloss levels. Disadvantages 1. The coatings are two- or three-component systems comprising a base, a cat- alyst (curing agent) and, water. 2.The coatings must be applied to properly prepared surfaces. 3.Depending on formulation, it can be difficult to mix the base and catalyst com- ponents due to their high viscosity. After adding water, the viscosity drops to manageable levels. 4.The coatings must be mixed using prescribed procedures. 5. The coatings are relatively expensive as packaged, but competitive with sol- vent-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 equip- ment and skin. POLYURETHANE DISPERSIONS Advantages 1.Coatings made from polyurethane dispersions dry to tough films of de- pendable 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 mul- tiple 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. 81

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