Appearance of Automotive Powder Clearcoats over Waterborne Basecoats:Effects of Amines Contained in Waterborne Basecoats on Unevenness of Powder Clearcoat Films

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A waterborne basecoat was sprayed onto aluminum strips and stainless steel sheets with a mirror-like surface, and heated at 80°C for 0, 1, 2, 3, 4, 10, and 20 min, respectively. Contents of N, N-dimethylaminoethanol (DMAE) and N-methylmor-pholine (NMM) in the heated waterborne basecoats on the strips were measured by pyrolysis-gaschromatography. An automotive powder clearcoat formulated with a glycidyl functional acrylic resin and dodecanedioic acid was applied to the basecoats on the sheets, baked at 150°C for 30min, and orange peel, that is the film unevenness with about 1-5mm wavelength was ranked by two human inspectors. The powder clearcoat was exposed to DMAE vapor and NMM vapor at room temperature for a week, and flow behavior during baking was determined with an oscillating rheometer.<BR>Both the contents of DMAE and NMM in the basecoat and the film unevenness were reduced with increase in heated flash time. The magnitude of complex viscosity of powder clearcoats treated with amines increased with increase in amounts of amines. This result is explained in terms of the base catalysis of amines for the crosslinking reaction of glycidyl group of acrylic resin and dodecanedioic acid. It is concluded that amines in heated waterborne basecoats migrate into the powder clearcoat layer during baking and promote the crosslinking reaction which raises the viscosity, to make the film surface uneven.
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