Metal Finishing Guide Book

2012-2013

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ventional two-component polyurethanes, the primary focus at the present time is in the wood-finishing industry. All coating systems discussed so far can be dried and cured either at room temperature, or force dried in low-temperature ovens, less than 90��C (194��F). As was pointed out earlier, the alkyds, modified alkyds, acrylic latexes, and acrylic epoxy hybrids are excellent choices for applications in which superior physical and chemical properties may not be required. However, there is a wide range of waterborne resin technologies that perform extremely well when exposed to chemical environments. Among them are the alkyd, alkyd-modified, polyester, and acrylic water-reducible coatings, which cure at temperatures of 121��C (250��F) and above. The upper temperature limit is usually in the 162 to 204��C (350���400��F) range. This group of coating formulations are water reducible or water thinnable and are similar to their solvent-based counterparts in chemical structure. Like their solvent-based counterparts, these resins are modified with aminoplast resins to allow for cross-linking at elevated temperatures. Typically these coatings are used where any of the following properties may be required; hardness, mar and abrasion resistance, excellent color and gloss retention, particularly when the coating is exposed to sunlight, chemical, detergents, and solvents. End uses include metal office furniture, large appliances, supermarket shelving, computer mainframes and metal hardware for the computer and business machines industry, metal laboratory equipment, bicycle frames, lighting fixtures, automotive, and transportation applications where the components can withstand the relatively high baking temperatures. As regards compliance with VOC regulations, formulations are readily available with 360 g/L (3.0 lb/gal) VOCs, less water, and some are even below 275 g/L (2.3 lb/gal) levels. AIR AND FORCE-DRIED ALKYDS Both water-reducible alkyds and modified alkyds are either air-cured, or forcedair dried at temperatures less than 90��C (194��F). Advantages 1. VOCs are generally below 2.0 lb/gal (240 g/L), less water. 2. The coatings are made for use on steel, aluminum, and plastics. 3. The coatings are composed if a one-component material with no pot life limitations and can be touched up and repaired. 4. Cleanup is relatively easy, being comprised of flushing hoses with water, then solvent, then water. 5. The coatings have low curing temperatures and air-dry at ambient temperatures in less than an hour, or force dry at 65��C (150��F) for 10���20 minutes. 6. The coatings can be spray-applied with standard equipment, but may not be compatible with airless and air-assisted airless spray guns. New formulation and spray application technologies are overcoming these limitations. 7. The coatings are ideal as dipping primers and enamels and excel lent for general purpose primers and topcoats. 8. Available in a wide range of color and gloss levels, the coatings can also be tex184

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