Metal Finishing Guide Book

2012-2013

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equipment, it is customary to use epoxy coatings as the primer and undercoat and then apply a more UV-resistant top coat, such as an acrylic or polyurethane. Epoxies should not be applied at low ambient temperatures, usually less than 50���60��F (10���15��C), because they will not cure properly. The common air- or force-dried, two-component epoxies that are used in the general metals, plastics ,and industrial maintenance industries comprise two separate packages, of which component A consists of the epoxy resin and component B can be a polyamine (for example, diethylene triamine, triethylene tetramine, tetraethylene pentamine), polyamide, polysulfide, or some other resin. In the case of baked epoxy coatings that cure during a high-temperature bake, usually above 60���204��C (140���400��F), the two resins are preblended by the coating manufacturer and are supplied as a single-component package. Examples include blends of epoxy resin with amine, urea formaldehyde, or melamine formaldehyde resins. Only when the applied coating attains an elevated temperature do the two resin systems react to form the cured finish. When clear coatings are required, neither component A nor component B is colored; however, for colored finishes, component A will usually contain the pigments and other additives, and component B will be clear. Industrial Maintenance Coatings If maximum chemical resistance is required, such as in industrial maintenance coatings that are used in chemical plants and refineries, component B is usually a polyfunctional amine. Unfortunately, these coatings tend to be very hard and sacrifice flexibility. If the painting operators do not wear proper protective clothing and appropriate respirators during the mixing and application of the coating, and if the unreacted amine comes into contact with their skin or is inhaled, the operators can experience severe dermatitis; therefore, stringent safety procedures must be followed. Epoxy-polyamine coatings have a relatively short pot life and must be used within a short time after the two components have been mixed. The manufacturers' technical data sheets will provide further details. General-Purpose Industrial Epoxy Coatings Improved toughness and flexibility are obtained when epoxy resins are reacted with polyamide resins. Unlike the polyamines, they do not cause severe dermatitis, and their pot life tends to be longer. Availability as VOC-Compliant Coatings For the general metals and plastics industries, several coating vendors supply VOCcompliant primers and top coats. Depending on the application, VOC contents are available as 168���420 g/L (1.4���3.5 lb/gal). Compliant epoxies are available that meet military specifications, such as MIL-P-23377 (primer), MIL-P-53022 (primer), MILC-22750 (top coat), and MIL- P-24441 (primer and top coat systems). High-solids epoxies, with very low VOC contents, often well below the regulated limits, are also available for the industrial maintenance industry. The advantages of high solids, solvent-based catalyzed epoxies, with VOCs less than 420 g/L (3.5 lb/gal), are the following: (1) They are used primarily as a primer because of excellent adhesion properties to metals, plastics, composites, wood, masonry, ceramics, glass, paper and other substrates. (2) They are available 174

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