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

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.