Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 71 of 331

meet performance standards for top-of-the-line products, such as computers, busi- ness machines, aircraft and truck cabs, and meet various military top-coat and camouflage specifications. (10) For polyester urethanes, formulations are avail- able with VOCs lower than 2.8 lb/gal. Coatings often have better chemical re- sistance; therefore, they are more commonly used on laboratory instruments, ma- chine tools, computers, business machines, aircraft (where resistance to hy- draulic fluids is important), industrial maintenance coatings for chemical plants, refineries, etc. (11) For acrylic urethanes, formulations are available with VOCs under 3.5 lb/gal and in some cases, under 2.8 lb/gal. Coatings are often report- ed to exhibit better exterior UV (sunlight) resistance; therefore, they are usually used on transportation equipment, such as automobiles, trucks, buses, and some private and commercial aircraft. Disadvantages are the following: (1) The two-component system requires mixing in prescribed proportions. (2) They have a limited pot life, sometimes less than four hours, particularly in high-solids formulations. (3) Like many high-solids coatings, it can be difficult to achieve a uniform film thickness on complex- shaped parts. (4) Equipment must be cleaned before coating begins to set. (5) They are relatively expensive (usually $30/gal). Aliphatic polyurethanes for exterior exposure are more expensive than aromatic polyurethanes for interior exposure. (6) They must be handled with care, and paint operators must use appropriate respirators. (Consult coating vendor for appropriate recommendation). (7) Polyurethanes can have allergic sensitization in some people, particularly if they do not wear appropriate respirators. (8) They may need to be applied over epoxy primer, and like most other coatings, they must be applied over clean, pretreat- ed surfaces. (9) At the present time, low-VOC, high-gloss acrylic polyurethanes are not readily available in small quantities of "automotive" colors, but are avail- able in "fleet" colors. New formulation polyurethane technologies are emerging. They will have very low VOC contents, while retaining manageable viscosities. MOISTURE-CURED POLYURETHANES In the previous section on two-component polyurethanes, we discussed the re- action of the polyhydroxy resin, such as the polyester, acrylic or polyether, with the polyisocyanate. It was explained that the chemical reaction commences as soon as the two components, A and B, are mixed; however, if the polyhydroxy resin is prereacted with a polyisocyanate but the reaction is not taken to completion, leav- ing some unreacted isocyanate groups, the coating then cures in the presence of moisture from the air. Such materials are called moisture cured polyurethanes. The coating is supplied in one package (the second component being atmos- pheric moisture) [Eq. (7)]: Prereacted polyhydroxylisocyanate (clear or pigmented) + Atmospheric moisture = Polyurethane (7) In preparing moisture-cured polyurethanes it is critical that all of the coating pigments and solvents are totally free of water, because traces of moisture in the package can cause the coating to cure before application. In addition, the flu- id hose leading to the spray gun and the head space above the coating in the pressure pot or reservoir must be free of moisture. A nitrogen blanket or a des- iccant can be used to keep the head space dry. 70

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