It is well known that photo-catalytic coatings presenting an optical quality are easily obtained by thermal crystallization of amorphous sol-gel TiO2 films. Such crystallization usually requires a post-deposition thermal treatment at relatively high temperatures. This restrains photo-catalytic applications to supports of reasonably good thermal stability, such as fused silica or mineral glasses. Photo-catalytic properties of TiO2 films deposited on polymers have rarely been studied in the sol-gel literature. To exploit the photo-catalytic properties of sol-gel TiO2 films on thermally sensitive substrates, such as polymer supports, low temperature crystallization approaches are necessary.
Patent application EP 1 167 296 proposes such an approach. In this document, three inventions having different aims and effects are presented. The two first inventions propose to obtain Anatase titanium oxide by a heat treatment in a closed vessel. These two first inventions are mainly intended to obtain a crystallized powder. A coating is nevertheless realized with the obtained slurry to prove the photo-catalytic activity, but these coatings are thick and don't allow the optical properties of the coated substrate to be preserved. Such inventions are more intended for ceramic applications, the crystalline powder being recovered by centrifugation after thermal treatment of the solution. No data in these two first inventions mentioned the sol-gel preparation of stable suspensions compatible with the deposition of optical quality photo-catalytic TiO2 films.
The third invention proposes to perform an ozone treatment to obtain a coatable solution. This coatable solution allows obtaining optical quality coatings. Nevertheless the photo-catalytic activity appears to be quite weak. The addition of some slurry obtained according to one of the first two inventions is proposed to enhance it. Moreover it has to be underlined that ozone has to be fabricated on-site which is constraining, as it is an irritant oxidant that causes problems in the security of the industrial process.