Comparison of various thermal and photoageing conditions on the oxidation of titanium dioxide pigmented linear low density polyethylene films

The thermoxidative (oven aging) and photooxidative degradation of linear low density polyethylene (LLDPE) film materials (100 mu m thick) containing five different types of titanium dioxide pigments are studied by Fourier Transform infra-red (FTIR) spectroscopy. Rates of thermal and photooxidative degradation are determined by measuring the formation of non-volatile carbonyl oxidation products which absorb in the infra-red region of the spectrum with a maximum absorbance at 1710 cm(-1). On thermal oxidative degradation at a temperature of 90 degrees C the rates of carbonyl formation increase autocatalytically with most of the pigments acting as thermal (catalytic) sensitisers. Comparison of the effects of photo- and photothermal oxidation at 90 degrees C showed that the pigments are powerful photoactivators at high temperatures. Rates of polymer oxidation were found to be closely related to the nature of the surface treatments on the pigment particles. Using selective narrow band irradiation at 365 nm all the pigments except the heavily coated rutile grade behaved as sensitisers, while using a narrow band 254 nm source the pigments behaved as UV screeners and stabilised the polymer. Under high intensity polychromatic irradiation all the pigments behaved as photosensitisers. The effects are discussed in relation to the role of the thermal and photogeneration of carriers (traps) on the surface of the pigment particles in terms of controlling pigment activity in a polymer substrate. (C) 1996 Elsevier Science Limited