FTIR studies of TiO₂ : pigmented polymer photodegradation
A novel method (in-situ quantitative infrared spectroscopy of evolved C02) for studying photo-degradation has been developed and applied to the study of several different polymers including a series of polyethylene and poly (vinyl chloride) samples containing Ti02 pigments with different photo-activitics. Infrared (IR) analysis was used to monitor carbon dioxide emitted from samples exposed to ultraviolet irradiation (UV) in atmospheres of differing composition. The experiments were conducted in a specially constructed cell that permits simultaneous UV exposure of the sample and IR interrogation of the vapour in the cell. It has been demonstrated that the in-situ gas-phase method permits a fast and convenient way of assessing the durability of pigmented and unpigmented polymer. A single test on one material occupied about 7 hours - very much less than conventional artificial weathering exposures. Excellent correlation between the C02 method and the conventional method for measuring carbonyl groups in the polyethylene films has been demonstrated, The Ti02 pigments used included anatase and rutiles with different surface treatments. Anatase-pigmented material gave significantly higher C02 emission than unpigmented polyethylene. The rutile-pigmented polyethylenes either gave reduced C02 emission or enhanced emission, according to the surface treatment. The ranking of the pigments as protectants or pro-degradants coincided with that obtained from much more time consuming laboratory testing and field experience. Similar results were obtained for the poly (vinyl chloride). The CO2 method is a convenient way to study the factors (humidity, oxygen concentration and UV intensity or wavelength), which influence the rate of carbon dioxide evolution i. e. the rate of photo-oxidation of the polymers. Blown polyethylene(PE) film was exposed to UV irradiation while held under tensile stress in a specially designed metal frame. The chemical degradation was followed using the FTIR carbonyl index. Tests were conducted on unpigmented PE and on a series of five PEs containing TiO2 pigments with different photo-activities. The yield strengths measured in tensile tests on unexposed films were typically 10 % greater in the longitudinal( machine)direction than in the transverse direction. UV exposures were conducted with stress applied both parallel and transverse to the machine direction.For all six materials, tensile stress accelerated carbonyl group development, Some samples cracked during UV exposure in tension and did so in a shorter time if the stress was applied in the machine direction than if it was transverse to it. The carbonyl index at the onset of cracking was lower for longitudinal samples than for transverse samples. Unpigmented PE was slightly more sensitive to transverse strain and the anatasepigmented PE slightly more sensitive to longitudinal strain.