Mechanism of Action of Titanium Dioxide Pigment in the Photodegredation of Poly(Vinyl Chloride) and Other Polymers

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Abstract
The extensive use of titanium dioxide as a pigment and optical agent in polymers, particularly poly(vinyl chloride), polyethylene and alkyd resins, has prompted many investigations of the various deleterious effects (loss of gloss and mechanical properties, chalking, pinking) of long-term exposure to sunlight of the pigmented polymers. This short review summarises the fundamental semiconductor processes undergone by TiO2 on UV irradiation and the consequent chemical action of the reactive intermediates so generated. It also describes the strategies developed to counteract photodegradation induced by TiO2, in particular its encapsulation in a coating by near-UV transparent metal oxides, such as Al2O3 and SiO2, with wide band gaps. The effects of doping TiO2 with transition metal ions are also noted.