Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 68 of 331

tics ,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 coat- ing 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 tem- perature 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 pig- ments 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 usual- ly 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 appropri- ate respirators during the mixing and application of the coating, and if the unre- acted amine comes into contact with their skin or is inhaled, the operators can experience severe dermatitis; therefore, stringent safety procedures must be fol- lowed. Epoxy-polyamine coatings have a relatively short pot life and must be used with- in 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 der- matitis, and their pot life tends to be longer. Availability as VOC-Compliant Coatings For the general metals and plastics industries, several coating vendors supply VOC- compliant 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), MIL- C-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 to meet many military primer and top coat specifications (MIL-P-23377, MIL-P- 53022, MIL-C-22750, MIL-P-24441). (3) They can be formulated into a wide range of colors and gloss levels. (4) Depending on the choice of curing agent (component B) they can achieve excellent hardness and chemical resistance, par- ticularly alkali resistance. (5) They exhibit excellent resistance to many solvents, 67

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