Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 63 of 331

coating materials HIGH-SOLIDS, LOW-VOC, SOLVENT-BASED COATINGS BY RON JOSEPH (1944 - 2011) RON JOSEPH & ASSOCIATES INC., SAN JOSE, CALIF. This article focuses on the most common high-solids, solvent-based coating technologies. Each of the resin systems discussed is available to meet the Rea- sonable Available Control Technology (RACT) limits of most state volatile organic compounds (VOCs) regulations. Most systems are also available to satisfy the more stringent regulations of California and the Best Available Control Technology (BACT) requirements of other states. The discussions and tables presented in this article provide sufficient detail to enable a company that is out of compliance with state regulations to make an initial selection of potential coatings that may satisfy the company's per- formance and production requirements; however, it will still be necessary to ask vendors for samples and to conduct comprehensive production-type tests be- fore implementing the technology of choice. AIR AND FORCE-DRIED ALKYDS These alkyds are air- or force-dried at temperatures less than 90°C (194°F). Alkyd resins are essentially oil-modified polyesters and are made by reacting an alcohol with an acid. [Eq. (1)} Acid + Alcohol = Oil-free polyester (1) Typical acids used are phthalic anhydride and maleic anhydride. Typical al- cohols used are pentaerythratol, glycerine, ethylene glycol, trimethylol ethane and trimethylol propane. These acids and alcohols can be combined in various com- binations under very precise and controlled conditions. They form a wide range of alkyd resins, each of which, either alone or in combination, has its own dis- tinctive chemical and physical properties. The coating formulator chooses the ap- propriate resin, or combination of resins according to customer requirements (see Table I). Alkyds can also be modified with other resins to change or improve their fi- nal properties, such as hardness, gloss retention, color retention and sunlight re- sistance. [Eq. (2)] 62

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