Modeling the depthwise gradient in curing and skin formation in wrinkling coatings

A theoretical model of solidification of a thermoset coating was constructed to examine the effects of a depth wise gradient in curing and skin formation on the coating's wrinkling behavior. The effects of coating thickness and catalyst concentration on the wrinkle wavelength of an acrylic-melamine coating were investigated with the model and mechanical profilometry. The model predicted that thin coatings with low and high catalyst concentrations gel quickly from top downward. Such coatings therefore have little time for the liquid below to diffuse into the already gelled skin to produce enough compressive stress to wrinkle the skin. The predicted time for complete gelation, as well as measured wrinkle wavelength in thicker coatings, rose as coating thickness was increased and showed an optimum as catalyst concentration was raised. Although time for complete gelation does not directly correlate with the wrinkle wavelength, the match of the trends of experimentally observed and theoretically predicted behavior is evidence that the essentials have been identified and represented by the model.