Polymers are widely used as exterior coatings in buildings, bridges, aircrafts, and automobiles for both protection and aesthetic purposes. However, the sustainability and service life of the applied coatings is limited due to the photodegradation of polymers induced by ultraviolet (UV) irradiation. Recent developments in nano-particle technology have initiated the use of inorganic UV absorbers such as zinc oxide (ZnO) for UV protection. Nevertheless, the mechanisms of how these nanoparticles affect the long-term performance of polymeric nanocomposites are not well understood. The objective of this study is to investigate the effect of ZnO nanoparticles on the long-term performance of a polyurethane (PU) nanocomposite during UV exposure. 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. Specimens were taken from the exposure chamber at different time intervals; their chemical and physical properties were measured for the assessment of the sustainability of the ZnO/PU nanocomposites. The chemical changes of the films were examined by Fourier Transform Infrared Spectroscopy (FTIR) and UV-visible spectroscopy. 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 nanocomposites. This photo-catalytic effect is dependent on the ZnO concentration as well as the exposure condition (e.g. RH). The photo-catalytic effect of ZnO nanoparticles is further confirmed by electron paramagnetic resonance (EPR) spectroscopy. Further, the physical degradation mode including the aggregation and the release of nanoparticles in ZnO/PU nanocomposites during UV exposure was also discussed.
Citation: Polymer Degradation and Stability
Pub Type: Journals