The effects of humidity on photodegradation of poly(vinyl chloride) and polyethylene as measured by the CO2 evolution rate

The effects of relative humidity on polymer photodegradation have been studied using a closed loop photoreactor which allows measurements of evolved CO2. Two different polymers which were expected to have different responses to relative humidity were studied; PVC and polyethylene, both containing photoactive TiO2 pigment. Relative humidity was controlled in the photoreactor by means of salt solutions and bypass control. With both polymers, there was an initial CO2 evolution rate which was less dependent on humidity. After a short incubation time, the CO2 evolution changed to a higher secondary rate which was more sensitive to the relative humidity. For the polyethylene, the secondary rate increased with relative humidity, though above a humidity of 58%, the CO2 evolution rate was constant. This is consistent with an increase in the formation of hydroxyl radicals. The PVC photodegradation was more complex; with very low humidity, the CO2 evolution rate was low, but increased significantly to a peak at around 35% RH. At higher humidity levels, the CO2 evolution rate decreased. This effect is attributed to the catalytic effect of hydrochloric acid. This explanation was confirmed by studying the effects of hydrotalcite additions on PVC degradation. The hydrotalcite replaces the hydrochloric acid with carbonic acid and reduces the CO2 evolution rate and the effects of humidity. The results clearly demonstrate that the CO2 evolution method is a very effective way of studying the effects of humidity in a fairly rapid manner.