Dependence of dirt resistance of steel topcoats on their surface characteristics

Contamination of exterior coatings with dirt is a significant problem in seriously polluted environments. To correlate the dirt resistance of exterior coatings with surface characteristics (wettability, roughness, and composition), steel topcoats were evaluated using both dirt pick-up tests according to a national standard testing method of China (GB/T 9757-2001) and outdoor exposure experiments in Shanghai, China. These topcoats included polyvinylidene fluoride (PVDF) paints with various water contact angles (WCAs), polyurethane (PU) paint, polysiloxane (PSiO) paint, and 2K fluorocarbon (FPU) paint. Opposite dirt resistance results were obtained from these two dirt pick-up methods. In the outdoor exposure experiments, superhydrophilic PVDF paints showed excellent self-cleaning performance and thus superior dirt resistance. Superhydrophobic PVDF paint exhibited self-cleaning performance initially, but became dirty finally due to a piling up of PM2.5 pollutants. PVDF paints with WCAs of 85A degrees-125A degrees, PU, PSiO, and FPU paints all exhibited poor dirt resistance in the outdoor environment. These results indicate that dirt resistance depends on surface wettability and the characteristics of pollutants rather than on the surface roughness and composition.