Preparation of photocatalytic TiO^sub 2^-based self-cleaning coatings for painted surface without interlayer

Photocatalytic TiO2 provides a promising route to produce self-cleaning coating surfaces, but it often causes decomposition of organic substrates and other coating components. An interlayer is often employed to insulate photocatalytic activity from sensitive substrate layers, complicating the application of these self-cleaning coatings. Herein, photocatalytic TiO2-based coatings were prepared based on an aqueous TiO2 dispersion in a hybrid binder synthesized from tetraethyl orthosilicate and methyl trimethoxy silane via a sol-gel process. Coatings with various levels of nano TiO2 were prepared and found to be transparent to visible light; i.e., the clear coating could be directly cast on architectural latex coatings without changing its original appearance. The photocatalytic coating system was composed of a TiO2-based clear top coat and an opaque latex film underneath. The coatings were evaluated through outdoor exposure studies and accelerated weathering tests. Results of these tests showed that the photocatalytic coatings with a TiO2 content range of 33-45% exhibited excellent self-cleaning performance, while displaying none of the expected degradation. In fact, this nano TiO2-based clear coating actually protected the latex film from UV-induced damage. The increased stability of the organic film may have resulted from reduced UV transmission through the photo-catalyst containing clear coat as well as reduced water permeability. It was demonstrated that the combination of appropriate TiO2 content and a suitable binder are crucial for the fabrication of robust photocatalytic self-cleaning coatings for painted surfaces without the need for an interlayer.