INFLUENCE OF TITANIUM-DIOXIDE PIGMENTS ON THE THERMAL AND PHOTOCHEMICAL OXIDATION OF LOW-DENSITY POLYETHYLENE FILM

The thermal (oven ageing) and photoxidative degradation of low density polyethylene films (100-mu-m thick) containing nine types of titanium dioxide pigments have been studied by i.r. spectroscopy and hydroperoxide analysis, as a function of temperature; apparent activation energies have been calculated. Rates of thermal and photooxidative degradation have been determined by measuring the formation of non-volatile carbonyl oxidation products which absorb in the i.r. with a maximum absorbance at 1710 cm-1. On thermal oxidative degradation over the temperature range 50-90-degrees, the rates of carbonyl formation increase autocatalytically with most of the pigments acting as thermal (catalytic) sensitizers. The nature of the pigment appears to control the rate of oxidation at lower temperatures. On photooxidation, the rates of carbonyl formation are autocatalytic, the effect increasing with increasing temperature of the irradiation unit with many of the pigments behaving as stabilizers. The stabilizing effect of many of the pigments is however gradually converted into a sensitizing effect as the temperature is increased during irradiation. The role of the sensitizing or stabilizing effects of the pigment on the rate of polymer oxidation is more important during photooxidation. From the rates of carbonyl formation, activation energies were determined and found to be significantly higher for thermal ageing (191.9-128.4 kJ mol-1) than photooxidation (43.1-30.8 kJ mol-1). In the former case, all the pigments lowered the activation energy for carbonyl formation while in the latter case many of the pigments increased the energy. The behaviour of the pigments as catalysts for the thermal oxidative breakdown of the polymer is confirmed by hydroperoxide analysis. All the pigmented polymer films are found to exhibit higher concentrations of hydroperoxide species before and after thermal oxidative ageing compared with the control unpigmented polymer film. Thus, titanium dioxide pigments are concluded to play an important role in the photothermal oxidation of polyethylene through their ability to catalyse the thermal oxidation of the polymer.