John Stratton

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

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.

Photocatalytic Coatings for Environmental Applications

Abstract A series of nano- and micronparticle-grade anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their photocatalytic activites have been determined in a series of paint films by FTIR, chalking, color, gloss change and weight loss after artifical weathering. The pigments have also been examined by rapid assessment methodologies using photodielectric microwave spectroscopy, 2-propanol oxidation and hydroxyl analysis. The microwave response under light and dark cycles provides an extended timescale probe of charge-carrier dynamics in the pigments. Pigment particle size, surface area and properties clearly play an important role in dispersion and any polymer–pigment interactions. Photooxidation studies on several types of paint films show a clear demarcation between nanoparticle- and pigmentary-grade titanium dioxide, with the former being more active because of their greater degree of catalytic surface activity. The photosensitivity of titanium dioxide is considered to arise from localized sites on the crystal surface (i.e. acidic OH), and occupation of these sites by surface treatments inhibits photoreduction of the pigment by ultraviolet radiation; hence, the destructive oxidation of the binder is inhibited. Coatings containing 2–5% by weight alumina or alumina and silica are satisfactory for general-purpose paints. If greater resistance to weathering is desired, the pigments are coated more heavily to about 7–10% weight. The coating can consist of a combination of several materials, e.g. alumina, silica, zirconia, aluminum phosphates of other metals. For example, the presence of hydrous alumina particles lowers van der Waals forces between pigments particles by several orders of magnitude, decreasing particle–particle attractions. Hydrous aluminum oxide phases appear to improve dispersibility more effectively than most of the other hydroxides and oxides. Coated nanoparticles are shown to exhibit effective light stabilization in various water- and oil-based paint media in comparison with conventional organic stabilizers. Hindered piperidine stabilizers are shown to provide no additional benefits in this regard, often exhibiting strong antagonism. The use of photocatalytic titania nanoparticles in the development of self-cleaning paints and microbiological surfaces is also demonstrated in this study. In the former case, surface erosion is shown to be controlled by varying the ratio of admixture of durable pigmentary-grade rutile (heavily coated) and a catalytic-grade anatase nanoparticle. For environmental applications in the development of coatings for destroying atmospheric pollutants such as nitrogen oxide gases (NOX), stable substrates are developed with photocatalytic nanoparticle-grade anatase. In this study, porosity of the coatings through calcium carbonate doping is shown to be crucial in the control of the effective destruction of atmospheric NOX gases. For the development of microbiological substrates for the destruction of harmful bacteria, effective nanoparticle anatase titania is shown to be important, with hydrated high surface area particles giving the greatest activity.

Interrelationship of spectroscopic properties with the thermal and photochemical behaviour of titanium dioxide pigments in metallocene polyethylene and alkyd based paint films: micron versus nanoparticles

Abstract A series of nano and micron particle grade anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their thermal and photochemical activites have been determined in monomodal metallocene polyethylene (with a monomodal mass distribution) and alkyd based paint films. Their performance has been assessed by FTIR and hydroperoxide analysis, chalking, gloss change and weight loss following artifical weathering. The pigments have also been examined by rapid assessment methodologies using photodielectric microwave spectroscopy, luminescence, 2-propanol oxidation and hydroxyl analysis. The microwave response under light and dark cycles provides an extended timescale probe of charge carrier dynamics in the pigments. Photo-oxidation studies on polyethylene containing nano-particle and pigmentary grade titanium dioxide pigments show that in general the former are more photo-active with anatase and rutile forms exhibiting high activity. Pigment particle size and surface area clearly play an important role in dispersion and any polymer–pigment interactions. This relationship is also extrapolated to the thermal sensitisation activities of the pigments on processing and oven ageing. Nano-particle pigments induce oxidation of the polymer during processing and long-term thermal ageing accelerating the formation of hydroperoxide and carbonyl groups. This behaviour can play a major role in controlling the photo-activity of the pigments in a polymer matrix. Photo-oxidation studies on paint films show a clear demarcation between nano-particle and pigmentary grade titanium dioxide with the former being more active. SEM analysis shows the formation of pits and holes emanating out from the particles. Model system studies based on 2-propanol oxidation and hydroxyl analysis go some way to predicting pigment activities but clear correlations do not exist. On the other hand the real-time microwave cavity perturbation method can be used as a gauge of photo-activity in titanium dioxide pigments. This, we believe, is a useful application, which could form the basis of an effective characterisation technique for commercial systems.