Degradation of scratch resistance of clear coatings by outdoor weathering

Abstract

Scratch resistance is important for maintaining car appearance. It is thus important to understand how scratch characteristics change over time. Although accelerated weathering tests are commonly used, there is no specific test optimized for estimating scratch resistance degradation. The present study investigates the relationship between chemical changes and scratch resistance in the outdoor weathering test. To estimate the similarity in degradation process between outdoor and artificial weathering, three kinds of automotive clear coating (two isocyanate and one epoxy acid curing types) were subjected to outdoor weathering and xenon lamp accelerated weathering tests. The scratch properties and chemical changes were examined using Fourier transform infrared spectroscopy (FT-IR). Scratch resistance was measured in terms of the critical fracture force, which is defined based on the microscope observation of cracks in a scratch trail after a scratch test with increasing normal load applied to the scratch tip. For all coatings, the critical fracture force decreased after one month of outdoor exposure. The FT-IR spectra showed that crosslink scission occurred in the coating surface layer due to oxidation and hydrolysis during exposure. Scratch resistance degradation was found to be caused by chemical changes over a short time in the surface layer. Although the xenon test also lowered scratch resistance and led to the same kinds of chemical changes observed for outdoor weathering for all coatings, the extent of each reaction was not proportional to outdoor reactions.