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Forming Stable Polyallophanate Powder Coatings With Excellent Weathering Stability

SpecialChem / PCI Magazine – Oct 15, 2008

Polyurethane powder coatings are known for excellent properties such as weathering resistance, superior flow characteristics, high flexibility and chemical resistance. The curing of polyurethane powder coatings proceeds by reaction between an OH-functionalized polyester and an isocyanate crosslinker. Due to the high reactivity of the isocyanate groups it is necessary to use blocked isocyanate crosslinkers to prevent the premature reaction of the NCO-groups and the OH-groups of the polyester.In the case of externally blocked isocyanate crosslinkers, the blocking agent, e. g., ε-caprolactam, is set free at temperatures above 170 °C. On the one hand, the free blocking agent improves the flow properties of the coating but the main disadvantage is its emission during the curing process. In contrast, internally blocked uretdione crosslinkers are completely emission-free. The initial disadvantage of the high curing temperatures (>190 °C) due to the dissociation of the uretdione ring was eliminated using various catalysts that enable curing temperatures of 130 °C.1

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