Behaviour of nanoparticle (ultrafine) titanium dioxide pigments and stabilisers on the photooxidative stability of water based acrylic and isocyanate based acrylic coatings

Abstract

A series of nano-particle grade anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their photochemical activites have been determined and compared in water based acrylic and isocyanate acrylic coatings with typical benzophenone and hindered piperidine light stabilisers. Their performance on wood and aluminium substrates is assessed by FTIR, colour and gloss change and mass loss following artifical weathering. UV absorption analysis of benzophenone and benzotriazole chromophore based UV absorbers shows that they absorb more strongly in the near UV below 350 nm. However, nanoparticle rutile absorbs more strongly above this range and therefore, operates as a strong opacifier. Anatase also possesses opacifying behaviour, but to a lesser degree than rutile above 380 nm. In water based acrylics the absorbers undergo decomposition during irradiation whereas the nanoparticles, by virtue of their inorganic nature are inherently stable. Mass loss experiments indicated that anatase is a photosensitiser, though the intensity of the effect was found to be dependent upon the nature of the coating. Rutile was found to be an effective stabiliser with performance greater than or equal to the organic absorbers and HALS. Combinations of anatase HALS were found to be antagonistic. Little or no synergy was observed between rutile and HALS. The outstanding performance of both anatase and rutile nanoparticles is visibly and colourimetrically evident on clear acrylic wood coatings with rutile being the more effective. The 70 nm particles are more effective than the 90 nm particles. However, this data for anatase is in marked contrast to the chemical changes from FTIR analysis. Here the difference may be due to the colour bleaching effect on the lignin products through the photosensitising activity of the anatase. From a commercial point-of-view coated nanoparticles offer a significant opportunity for cost-effective benefits over conventional organic absorbers and HALS for the photoprotection of acrylic coating systems.