Long-Term Performance of Nano-Filled Polymeric Materials: Effect of ZnO Nanoparticles on Photodegradation of a Waterborne Polyurethane Coating
Xiaohong Gu, Dongmei Zhe, Minhua Zhao, Guodong Chen, Nhieu Ly, Paul E. Stutzman, Li Piin Sung, Tinh Nguyen, Joannie W. Chin
Polymers are widely used as exterior coatings in buildings, bridges, aircrafts and automobiles for both protection and aesthetic purposes. However, the service life of the applied coatings is limited due to the photodegradation of polymers induced by ultraviolet (UV) irradiation. Recent development in nano-particle technology has initiated the use of inorganic UV absorbers such as zinc oxide (ZnO) for UV protection. However, the mechanisms of how these nanoparticles affect the long-term performance of polymeric coatings are not well-understood. The objective of this study is to investigate the effect of ZnO nanoparticles on the performance of a waterborne polyurethane (PU) coating during UV exposure. A one-component waterborne PU and a waterborne ZnO dispersion were used to prepare the ZnO/PU nanocomposite films. UV exposure was conducted on the NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) under both dry (0 % RH) and wet (75 % RH) conditions at 45 C. To study the photolytic effect of nanoparticles, the degradation of ZnO/PU films exposed to the same conditions in the absence of UV was investigated as a comparison. Specimens were taken from the exposure chamber at different time intervals and their chemical and physical properties were measured. The chemical changes of the films were examined by Fourier Transform Infrared Spectroscopy (FTIR) and UV-visible spectroscopies. Changes in surface morphology were characterized by 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 show that the ZnO nanoparticles act as a catalyst to accelerate the photodegradation of the PU. This photo-catalytic effect is dependent on the ZnO concentration as well as the exposure condition. The photo-catalytic effect of ZnO nanoparticles on the physical degradation mode of the ZnO/PU films is discussed.