Effect of TiO2 pigment type on the UV degradation of filled coatings

The objective of this study was to investigate the effect of the photoreactivity of titanium dioxide (TiO2) pigments on the photodegradation of polymeric coatings used in exterior applications. Two polymer matrices, an amine-cured epoxy (EP) and an acrylic urethane (AU), containing three types of TiO2 pigments, classified by different levels of photoreactivity, were studied. Specimens were exposed on an ultraviolet (UV) weathering chamber, the Simulated Photodegradation by High Energy Radiant Exposure device at the National Institute of Standards and Technology. Two exposure conditions were used: ambient, dry condition [25°C and 0% relative humidity (RH)] and high temperature, wet condition (55°C and 75% RH), which is similar to more severe outdoor exposures. The physical and chemical degradations of the filled coatings were monitored at periodic intervals using a combination of laser scanning confocal microscopy (LSCM) and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). Progression of degradation on the coating surfaces was characterized by LSCM in terms of changes in surface roughness and morphology, pigment agglomerate size, and the occurrence of pits or holes in the coatings. The observed physical changes were correlated to the chemical changes measured by ATR-FTIR as a function of UV exposure time. Both EP and AU systems showed less degradation in terms of surface roughness and morphological changes under the dry conditions compared to the wet exposure conditions. It was observed that both the pigment type (and hence photoreactivity) and particle dispersion strongly affected the degradation of both EP and AU systems.