Metal Finishing Guide Book

2012-2013

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may not be compatible with airless or air-assisted airless spray guns. Disadvantages 1. If coatings are dried in low-temperature and/or high-humidity environments, poor films may form leading to degraded physical and chemical properties. 2. The coatings must be applied over exceptionally clean, oil- and grease-free surfaces. 3. Typical problems of improper surface preparation or mixing can include edge pull, substrate wetting, and cratering. 4. Unprimed or poorly primed ferrous metals will produce severe flash rusting with most latex paints. 5. The film remains thermoplastic and soluble in any solvent that would dissolve the basic polymer. 6. Latex paints contain appreciable quantities of water-soluble, non volatile materials, such as thickeners, surfactants, and other addi tives that remain in the dried film. On exposure, these leach out and leave the film less continuous and more permeable. 7. Splashes and spills must quickly be cleaned up with water before the coating dries. Once the coating is dry, solvents are required for cleanup. 8. Equipment cleanup often requires three steps: water, solvent, water. 9. Storage areas should be protected. 10.Storage areas should be protected from excessively low temperatures to prevent freezing and from excessively high temperatures to prevent degradation of the coating. 11.General housekeeping must be rigorous to prevent bacterial or fungal growth. AIR AND FORCE-DRIED ACRYLIC-EPOXY HYBRIDS Acrylic-epoxy hybrid coatings should be air- or forced-air dried at temperatures less than 90��C (194��F). Advantages 1. Coatings cross-link to form insoluble, thermoset films. 2. The acrylic portion provides ultraviolet (UV) resistance, which appreciably improves chalk resistance, gloss, and color retention on exterior exposure. 3. The epoxy portion improves adhesion and alkali resistance. 4. The very long pot life (36 hours at normal ambient temperature) of the mixed and catalyzed coatings greatly adds to their utility. 5. The low precure molecular weight helps gloss development so that high gloss finishes are possible. 6. Water is used for thinning and cleanup reducing costs of cleanup. 7. Increased safety due to low flammability may result in lower insurance costs and better working conditions. 8. The coatings may be applied by most conventional spray equipment, but may not be compatible with airless and air-assisted airless spray guns. 186

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