Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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coating materials POLYURETHANE COATINGS FOR METAL AND PLASTIC SUBSTRATES BY AUGUSTIN T. CHEN AND RONALD T. WOJCIK OLIN CORP., CHESIRE, CONN. Polyurethane coatings have excellent gloss, hardness, flexibility, abrasion resis- tance, chemical resistance, and ultraviolet (UV) durability. In addition, higher solids polyurethane coatings meet current regulations for volatile organic compound (VOC) emissions without compromising a coating's performance. Because polyurethane coatings offer so many advantages, they are the market leader for coating various substrates in the following end-use markets: automotive refin- ish; aircraft; and chemical agent resistant coatings (CARC). Moreover, polyurethane coatings have especially rapid growth in automotive original equipment manufacturing (OEM); industrial maintenance (such as bridges and bulk storage tanks); and appliances. This article describes polyurethane chemistry, applications, and perfor- mance requirements for coating metallic and plastic substrates. The authors will limit discussion to two-component solventborne, one-component solvent borne, and powder coatings based on polyurethanes as applied to the automotive refinish, automotive OEM, aircraft, and industrial maintenance end-use mar- kets for metal substrates. Due to the strong growth of plastic components for automotive OEM applications, polyurethane coatings for automotive OEM plastic components are also discussed. POLYURETHANE COATINGS FORMULATION The performance of any coatings can be divided into five elements: processing applications, coating appearance, mechanical performance, heat and UV resistance, and chemical resistance. Figure 1 outlines the elements, including test methods, used to measure per- formance. Coating scientists use Fig. 1 as a laboratory blueprint defining the important steps in the formulation process. A formulator will modify this blue- print, as required, to meet specific performance objectives. For example, in au- tomotive refinish, formulators focus first on optimizing the application and appearance, before maximizing the mechanical performance and resistance test- ing (UV and chemical). In industrial maintenance, formulators are most concerned with optimizing application and chemical resistance, whereas mechanical per- formance and appearance are less important. If a performance property fails the specification, the formulator adjusts the concentrations of the chemical reactants and restarts the design process. SOLVENTBORNE COATINGS Coating scientists begin the formulation process by selecting chemical reactants that form a good film. They then blend, adjust, and readjust concentrations fol- lowing the formulation outline, until the finished product has met all per- formance requirements. Polyurethane solventborne coatings contain some or all of the following chemical reactants: aliphatic polyisocyanates, polyols (hy- droxylated acrylic or polyester), solvents, catalyst, pigments, and additives (e.g., 84

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