A photocatalytic self-cleaning titanium dioxide (TiO 2 ) coating was prepared as a multilayer coating structure by the spin-coating method. Three substrate materials (two thermoplastics and one ceramic) were used: (1) high-density polyethylene (HDPE), (2) poly(vinyl chloride), and (3) borosilicate glass (BK7). The multilayer structure consisted of a polyurethane protective layer on the substrate, two layers of photocatalytic TiO 2 on the protective layer, and finally immobilized TiO 2 particles bound in a diluted polyurethane dispersion. Photocatalytically active surfaces were achieved by reactive oxygen-plasma surface etching of the fabricated coatings. The structure and properties of the coating surfaces were characterized with scanning electron microscopy and contact-angle measurements. The coatings on HDPE and BK7 were rendered superhydrophilic by an oxygen-plasma treatment. The photocatalytic activity and self-cleaning properties of the prepared surfaces were studied with palmitic acid (model soil), the degradation of which was confirmed by contact-angle measurements and gas chromatography analysis.
Multilayered photocatalytic TiO 2 -based coating was prepared by spin coating on a high-density polyethylene (HDPE) substrate. The multilayered coating consisted of a polyurethane (PU) barrier layer and two layers of TiO 2 nanoparticles bound with PU. The adhesion between the HDPE substrate and protective PU coating was enhanced by oxygen plasma treatment of the substrate. The improved adhesion contributed to the photocatalytic degradation of palmitic acid. Long-term activity of the photocatalytic coating in degradation of palmitic acid under UV illumination was followed by FTIR-ATR. The catalytic activity of the coating was maintained in three identical cycles where palmitic acid was added and UV-irradiated for 6 h. According to FTIR measurements, the palmitic acid was almost completely decomposed after 6 h, but gas chromatography (GC) analysis showed total decomposition to require 12 h UV illumination ($ 97% of palmitic acid decomposed in 12 h). Study of the degradation of palmitic acid by GC as a function of time indicated that the degradation kinetics was pseudofirst order, and the rate constant obtained was 0.31 h À1 .
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