Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 76 of 331

are in order. The EPA has defined air- or force-dried coatings as those that dry or cure below these temperatures, and many rules establish special VOC limits for this category. In contrast, coatings that cure above 194°F are often regulated as "baked" coatings for which other VOC limits are established. Mostly the lim- its for air- or force-dry category are higher than for the baked. Probably the most common waterborne coatings used for application to met- als are air- or force-dried at temperatures below 90°C (194°F).A wide range of coat- ing formulations fall into this broad category. The most commonly available technologies are water-reducible alkyds and modified alkyds, acrylic latexes, and acrylic epoxy hybrids. Often consumers are unaware of which of these tech- nologies they are purchasing, as the coatings are frequently sold as generic "wa- terborne" products. A brief descriptive overview of the basic differences follows, and the most notable advantages and disadvantages of each resin system are specifically highlighted. As a generic group, water reducible formulations, dispersions, and latexes are ide- al for companies that need to get into compliance with VOC regulations, yet do not require the coatings to have sophisticated properties. As a group, the waterbornes tend to have VOC contents well below 2.0 lb/gal, less water and some are even below 1.0 lb/gal (120 g/L). Moreover they are readily available in a wide range of colors and gloss levels. Generally they exhibit good performance properties, but are probably not as durable or chemical- and solvent-resistant as two-component polyurethanes, epoxies, or baked fin- ishes. They should be considered for applications such as dipping primers and topcoats,general-purpose shop primers, and spray applied enamels. Typical end uses include steel roof trusses, steel building support structures,farm im- plements (not combines or tractors), electrical cabinets, boxes,frames, fence posts, and similar products. When applying the coatings in humid and/or cold environments they should be force dried at a low oven temperature of approximately 120 to 150°F. If an oven is not available, consider blowing air over the parts to promote the evaporation of water from the coating film. Failure to do so can lead to a poor quality film that is easily damaged by handling and susceptible to the early onset of corrosion and other premature failures. Water-reducible, or water-thinnable alkyds and modified alkyds are similar to the solvent-based alkyds. Like the solvent-based coatings, they are modified polyesters, but have high acid values and employ special chemical blocking agents, such as carboxylic acid functionalities. When the alkyds are neutralized with ammonia or volatile amines, it is possible to use water as the reducing liquid. Although they may take longer to dry, the resulting coatings have similar gloss, flow, and leveling properties compared with their solvent-borne counter- parts. The acrylic latexes include other polymers such as vinyl acrylic and styrene acrylic. The resins are characterized as high-molecular-weight polymers dis- persed as discreet particles in water. Those formed by polymerizing a single monomer are called homopolymers, whereas those polymerized from a blend of two or more different monomers are called co-polymers. Because desired film characteristics can be built into the resin by the choice of the monomers used, most of the latexes used for coating miscellaneous met- al parts are co-polymers. Latex coatings, like lacquers, do not go through a chemical change as they 75

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