Rapid measurement of polymer photo-degradation by FTIR spectrometry of evolved carbon dioxide

Abstract

A novel method, allowing simultaneous UV exposure of a polymer sample and IR interrogation of the vapour in a specially constructed cell, has been applied to polyethylene (PE) samples containing TiO 2 pigments with different photoactivities. Measurements of the CO 2 generated by films exposed to ultraviolet irradiation (UV) were completed in 5 h – very much less than conventional accelerated tests. The TiO 2 pigments used included anatase and rutiles with different surface treatments. Anatase pigmented material gave significantly higher CO 2 emission than unpigmented PE whilst rutile-pigmented PEs either gave reduced CO 2 emission or enhanced emission, according to the surface treatment. The ranking of the pigments as protectants or pro-degradants correlated well with the carbonyl index measured after more than 300 h exposure to UVA fluorescent tubes in a QUV machine. The method was then used to probe mechanistic aspects of the photo-oxidation of pigmented polyethylene (PE) film. For unpigmented polymer the photo-degradation was sensitive to changes in the small fraction of incident UV below 300 nm, but for pigmented films this was much less important. This is because unpigmented film degrades by direct photochemical attack whereas, for pigmented film, photocatalysis by TiO 2, which absorbs in the 300–400 nm region, is important. For films whose photo-oxidation was dominated by photocatalysis by the TiO 2 the rate of oxidation was shown to vary as the square root of the UV intensity. By contrast, for unpigmented films the rate of direct photochemical oxidation was linearly proportional to UV intensity. The difference is a consequence of the controlling role of electron–hole recombination in photocatalytic processes. For both unpigmented and pigmented films the rate of oxidation was shown to increase with increasing humidity and oxygen content of the atmosphere.