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Page 179 of 903

Fig. 2. Reaction for typical 2-component polyester-polyurethane, R��� = aliphatic hydrocarbon chain; R��� = aliphatic polyester chain. Component A = polyester polyol (clear or pigmented); component B = aliphatic polyisocyanate (clear). Typically, polyfunctional polyisocyanates used in two-component polyurethanes are homopolymers or copolymers of toluene diisocyanate (TDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), dicyclomethane diisocyanate (HMDI). Components A and B can be batch mixed by manually mixing immediately before the coating is applied. Alternatively, special proportioning equipment can be set to automatically measure out each component according to its prescribed ratio. Each component is then pumped separately to a manifold, where they both come together in the fluid hose leading to the spray gun. Downstream of the manifold is a short static mixer that comprises a short plastic or stainless steel tube located in the fluid hose only a few inches or feet from the spray gun. Small baffles in the tube cause the components to be thoroughly mixed immediately before entering the spray gun. In-line mixing allows components A and B to be mixed on a continuous basis. The primary advantage is that the viscosity of the coating remains constant throughout the day and is used before it can outlive its pot life. The most important advantages of two-component, high-solids polyurethanes [polyester urethanes with VOCs less than 340 g/L (2.8 lb/gal); acrylic urethanes with VOCs less than 420 g/L (3.5 lb/gal)] are the following: (1) They offer excellent physical film performance ��� abrasion resistance, toughness, hardness up to pencil hardness of 6H. (2) They have excellent resistance to most solvents and chemicals and excellent outdoor durability (primarily the aliphatic polyurethanes). (3) They can cure at ambient (room) temperatures, elevated temperatures or even under subzero conditions. (4) They can be used for application to steel, 176

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