Abstract
Most organic coatings are pigmented with Titanium Dioxide (TiO2) to increase their covering power. TiO2 can be photoactivated under UV irradiation causing the formation of free radicals leading to accelerated degradation of the polymeric coating and release of Carbon Dioxide (CO2). An important consideration is the role of relative humidity (RH) of the atmosphere contacting the irradiated panels. This has been studied for pigmented UPVC films using a novel closed-loop flat panel irradiation reactor and measuring the CO2 evolution kinetics by FTIR spectroscopy. The atmospheric humidity has been controlled (RH 0% to 98%) in the flow system using saturated salt solutions and the work has shown that the peak in degradation rate occurs at RH of 32% which coincides with a peak in photo-generated hydrochloric acid concentration. PVC films can be stabilised by incorporating hydrotalcite mineral pigments (2-10%) which replace hydrochloric acid with carbonic acid and deactivate the TiO2.
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