Studying Long-term Performance of a Nano-ZnO filled Waterborne Polyurethane Coating using Spectroscopies and Microscopies
Xiaohong Gu, Dongmei Zhe, Stephanie S. Watson, Guodong Chen, Minhua Zhao, Paul E. Stutzman, Deborah S. Jacobs, Tinh Nguyen, Joannie W. Chin, Jonathan W. Martin
Although inorganic UV absorbers such as zinc oxide (ZnO) and cerium oxide (CeO2) exhibit numerous advantages compared to organic UV absorbers, the mechanisms of how these nanoparticles affect the long-term performance of polymeric coatings are less than clear. Researchers have different views on this issue. In this study, the effect of ZnO nanoparticles on the performance of a waterborne polyurethane coating exposed to UV radiation was assessed and the photodegradation mechanism involved with ZnO nanoparticles was investigated. Specimens of nano-filled polyurethane thin films were prepared by spin casting a mixture of waterborne polyurethane with different loadings of waterborne ZnO dispersion onto calcium fluoride substrates. Exposure was carried out on the NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) at both dry (0% RH) and humid (75% RH) conditions at 45 oC. The chemical degradation was examined by FTIR and UV-visible spectroscopies. Changes in surface morphology were characterized by confocal microscopy, atomic force microscopy (AFM), electric force microscopy (EFM) and scanning electron microscopy combined with energy dispersive X-ray spectroscopy microanalysis (SEM/EDX). The results clearly showed that the photo-catalytic effect of ZnO nanoparticles was dominant in the photodegradation of the studied nano-filled polyurethane system. This effect was greatly influenced by the loadings of the nanoparticles and the relative humidity of the exposure environment. The origin of the photo-catalytic effect of ZnO nanoparticles was further investigated by electron paramagnetic resonance (EPR) spectroscopy.