Insight into the dependence of dirt adsorption/desorption on the surface wetting behavior of TiO2-based nanocomposite coatings

TiO2-based nanocomposite coatings were fabricated using two silicon-oxo hybrid binders and further treated with UV-irradiation to acquire coatings with a wide range of static water contact angles (SWCA) (from 5 degrees to 118.6 degrees) and different dynamic surface wetting behaviors. The dirt adsorption and desorption behavior of these coatings were measured after applying modified rice-straw ash and washing with a water-spray. The results were correlated with static surface wettability and dynamic surface wettability values. It was found that the coatings with an SWCA of about 75 degrees displayed the highest dirt adsorption regardless of the type of binder used. Lower SWCA and receding water contact angles (RWCA) allowed for better dirt desorption and hence better dirt pickup resistance. The coatings with RWCA values close to 0 degrees exhibited self-cleaning performance even when the surface was not superhydrophilic. In addition, the amount of dirt removed from the surface of the coatings by dirt desorption tests was evaluated and used as an indicator of the ability of the coating to remove dirt from polluted air. Specific SWCA and low RWCA values seem to be critical to the coating's ability to remove dirt from the air. In addition, the dependence of the dirt removal ability on the hysteresis of water contact angle (HWCA) was different for the two silicon-oxo hybrid binders.