Titanium dioxide photocatalysed oxidation of plasticisers in thin poly(vinyl chloride) films

Abstract

A closed system incorporating FTIR continuous monitoring and GC-MS sampling has been developed to study the generation under UVA illumination of volatile photodegradation products from model plasticised and TiO₂ pigmented films. The presence of the plasticiser dibutyl adipate (DBA), at levels of 0-88 phr, has been shown to slow the onset of yellowing of PVC caused by the formation of polyene sequences through photodegradation of the polymer matrix. However, increasing the plasticisation increases the amount of volatile byproducts emitted under intense UVA illumination as a result of TiO₂ catalysed photodegradation. The rate of CO₂ production from irradiated films increases linearly by up to a factor of 21 as the concentration of DBA is raised from 0 to 88 phr. This increase in the rate of CO₂ production suggests increasing photoactivity within the film and this, accompanied by a decreased yellowing rate of the polymer backbone, implies that the plasticiser is photo-oxidised in preference to the polymer matrix. In addition, small quantities of organic molecular fragments (up to a maximum of ∼10 mumol m -² ) are released into the gas phase above the films and have been trapped using adsorption tubes. The molecular structures of the emitted species form an homologous series, the parent of which is the plasticiser, each with one fewer carbon atoms. All the fragments are produced at similar levels suggesting the mode of fragmentation is via random scission. The production of volatile organic carbon compounds (VOCs) peaks after 1 h irradiation, which corresponds to the initial rapid photo-oxidation of plasticiser moieties bound to the TiO₂ surface. The VOC emissions account for only ∼3% of the total carbon emission with ∼97% accounted for by CO₂, suggesting that the major breakdown pathway is via complete oxidation with the majority of retained molecular fragments within the irradiated polymer film being subsequently oxidised. The efficiency of such oxidation is similar for each fragment as determined by separate complete oxidation studies over irradiated pure TiO₂ films. In each case the major product is CO₂ with only small quantities of other VOCs being produced.