UV stability of polyurethane binding agent on multilayer photocatalytic TiO2 coating

Degradation of a heat cured polyurethane (PU) dispersion was studied on BK7 glass substrates coated with titanium dioxide. Two titanium dioxide layers on the substrate were bound with waterborne polyurethane dispersion that was diluted with water and then cured. Four dilutions (PU dispersion/H2O mass ratios of 1:12, 1:8, 1:4, and 1:2) were tested. The study showed that TiO2 photocatalysis causes polyurethane to degrade as a function of UV-illumination time. Degradation of polyurethane was analyzed by FTIR-ATR, surface electron microscopy imaging and gravimetric analysis. Both the FTIR method and gravimetric analysis showed the degradation of polyurethane to follow pseudo-first-order reaction kinetics. The reaction rate constants determined for degradation of polyurethane at dilution 1:4 were kobs = 0.021 h−1 by FTIR-ATR and kobs = 0.0023 h−1 by gravimetric analysis. Gravimetric analysis indicated that the rate of degradation is slower when the amount of polyurethane is increased (dilution 1:2 kobs = 0.0013 h−1). This is because the titanium dioxide becomes covered by polyurethane, as verified by SEM imaging. When both the photocatalytic efficiency of TiO2 in self-cleaning coatings and the binding of titanium dioxide to the substrate are important, all three analytical methods suggest that the optimal dilution of PU binder is 1:4.