Effect of UV Radiation on Surface Mechanical Properties of NanoTiO₂–Acrylic Urethane Coatings

Nanoindentation was used to monitor the changes in surface mechanical property of nanotitanium dioxide (nanoTiO2)/acrylic urethane (AU) coatings exposed to high-intensity UV radiation at 55°C and dry conditions. Five TiO2 nanoparticles of similar size but different crystal phases and surface treatments were selected in this study. Preliminary results indicate that two competing behaviors (hardening vs. softening) were found in surface modulus and hardness changes, as UV exposure time was increased. The relationship between photodegradation-induced surface mechanical behavior and the characteristics of nanoTiO2 (crystal phases and surface treatments) was examined. It appears that AU coatings containing rutile nanoTiO2, regardless of surface treatment, exhibit the typical hardening behavior. However, the AU coatings containing anatase or anatase/rutile mix (high anatase ratio) nanoTiO2 show a softening behavior. It implies that photoreactivity of nanoTiO2 plays a major role in the photodegradation-induced surface mechanical behavior. Other key physical parameters, such as surface morphology and filler dispersion measured using laser scanning confocal microscope, and bulk mechanical properties measured by tensile testing were obtained. The correlation between these physical quantities and the surface mechanical properties may provide a fundamental understanding of the photodegradation-induced surface mechanical behavior for different nanoTiO2/AU coatings.