Novel flat panel reactor for monitoring photodegradation

Abstract

A closed system incorporating FTIR continuous monitoring of the gas phase and a novel gas cell for the UV irradiation of flat panels of organic coatings is described. This has been used to probe various aspects of the photocatalysed degradation of TiO2 pigmented polymer films under UVA illumination. The flat panel cell is machined from a single block of aluminium and is specifically designed to minimise dead volumes, enable rapid gas mixing and be suitable for the investigation of flat coupons of material up to 220 × 100 mm in size. The cell incorporates a gas input and output channel in its base feeding and fed by small bore holes drilled in such a way as to maximise the exhaustive effects on the contents of the cell. To demonstrate the effectiveness of the design the effect of TiO2 pigment photoactivity on the rates of degradation of a commercial acrylic emulsion paint using commercial photostable TiO2 grades is reported. In testing using this new reactor configuration it has been possible to correlate photoactivities obtained over periods of several thousand hours Xenon Arc exposure, which is the most reliable test currently available. The test cell has also been used to evaluate other coating components. Using unplasticised PVC films we have found that the rate of photogenerated CO2 falls by up to 40% when a simple molecular UV absorber (2 hydroxybenzophenone) is added at levels of up to levels of 1.5% per hundred resin (PHR). Beyond this there is little further reduction in photoactivity. Finally the test cell has been used to investigate the effects of TiO2 loading on the rates of CO2 evolution from irradiated unplasticised PVC films pigmented with 0–70 PHR TiO2. The initial rate of photodegradation (over 100 min) follows a linear trend to increased CO2 evolution rates up to 50 PHR TiO2. There is a transition to a secondary rate after the production of 1.8 mmol m-2 of CO2 suggesting that after a set amount of degradation there is a change in the reaction rate. This rate is still proportional to the concentration of TiO2. It is likely that this is due to a combination of acid catalysis and the generation of porosity in the irradiated surface, the latter enabling greater oxygen access to the irradiated TiO2.