Ageing behavior of acrylic polyurethane varnish coating in artificial weathering environments

Acrylic polyurethane varnish coatings were exposed in two artificial weathering environments produced by a fluorescent UV/condensation weathering device and a xenon lamp exposure and weathering equipment for different time periods. The ageing behavior of the coating was evaluated by using film thickness gauge, glossmeter, spectrophotometer, scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR) and electrochemical impedance spectroscopy (EIS). The results showed that the Xenotest protocol had a significant effect on the thickness loss and gloss. However, the yellow index measurements and low frequency impedance showed that greater degradation was caused by the UV exposure protocol. These findings were supported by the results obtained from FTIR analysis. In the two artificial weathering environments, the low frequency impedance modulus of intact coatings decreased exponentially with increase of ageing time, and correlated with the changes of other properties of ageing coatings, which can be used to predict coatings lifetime.