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Degradation and stabilisation of polymers and coatings: nano versus pigmentary titania particles

Allen, Norman S. and Edge, Michele and Ortega, Amaya and Sandoval, Gonzalo and Liauw, Chris and Verran, Joanna and Stratton, John and McIntyre, Robert B. (2004) Degradation and stabilisation of polymers and coatings: nano versus pigmentary titania particles. Polymer degradation and stability, 85 (3). pp. 927-946. ISSN 0141-3910

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Abstract

A series of nano and micron particle size 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 a metallocene polyethylene with a monomodal molar mass distribution and alkyd/acrylic based paint films. Their performance has been assessed by FTIR and hydroperoxide analysis, chalking, colour, gloss change and weight loss following artificial 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. Photooxidation studies on polyethylene containing nano-particle and pigmentary grade titanium dioxide pigments show that in general the former are more photoactive 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 photoactivity of the pigments in a polymer matrix. Photooxidation studies on paint films show a clear demarcation between nano-particle and pigmentary grade titanium dioxide with the former being more active. Here nano and micron particle size anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. The photosensitivity of titanium dioxide is considered to arise from localised sites on the crystal surface, and occupation of these sites by surface treatments inhibits photo-reduction of the pigment by ultraviolet radiation and hence the destructive oxidation of the binder is inhibited. Coatings containing 2–5% w/w 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% w/w. The coating can consist of a combination of several materials, e.g. alumina, silica, zirconia, aluminium phosphates of other metals. For example, the presence of hydrous aluminium oxide particles lowers Van der Waals forces between pigments particles by several orders of magnitude, decreasing particle-particle attractions. Hydrous aluminium oxide phases appear to improve dispersibility more effectively than most of the other hydroxides and oxides. Coated nanoparticles are shown to exhibit effective light stabilisation in various water and oil based paint media in comparison with conventional organic stabilisers. Hindered piperidine stabilisers 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.

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