Physicochemical aspects of the environmental degradation of poly(ethylene terephthalate)

The degradation of amorphous poly(ethylene terephthalate) bottle and amorphous sheet materials are investigated under different environmental conditions (wet soil, 100 and 45% relative humidity and UV irradiation) by measuring the rate of chain scission, using viscometric analysis, end-group analysis by FTIR and crystallinity via density measurements at different temperatures. Using the Arrhenius expression the lifetime of the biaxially oriented polyester bottle material and activation energy for degradation are found to be very dependent upon the environmental conditions, with hydrolysis being a dominant process. Negligible degradation is observed at temperatures below the glass transition (c. 80°C) in dry conditions. From density measurements at 45 and 100% relative humidity the crystallinity exhibits an initial facile increase only at temperatures about 70°C, due to plasticisation by the moisture and annealing, followed by an inflection which increases from 32 to 35% with the severity of the degradation conditions. This inflection is consistent under all degradation conditions at 0·5 of a chain scission and is then followed by a much slower rate due to combined hydrolytic degradation/oxidation of the polymer chains. The initial rapid increase is found to be faster at lower rates of chain scission due to an annealing/plasticisation effect by the moisture. This is confirmed by the fact that, under both dry conditions and UV irradiation, where significant rates of chain scission are observed, especially at high temperatures and prolonged times (>500 days), the crystallinity shows no significant increase. Hydrolytic degradation on thermal ageing is confirmed using end-group analysis, which shows that hydroxyl and carboxyl rates are synonymous and increase significantly only under high humidities and temperatures above the Tg. Upon UV exposure, however, chain scission is accelerated in the presence of wet soil with the rate of carboxyl formation exceeding that of the hydroxyl group. The latter is associated with the importance of a Norrish Type II intramolecular hydrogen atom abstraction and unzipping mechanism. Metal ion contents in the polyester film material are found to vary significantly with the ageing condition. Thus, whilst antimony appears to be extracted the copper content is enhanced significantly, which may contribute to the accelerated degradation of the polyester. The implications of these results in terms of the ageing of bottle material are discussed.