PHYSICOCHEMICAL ASPECTS OF THE ENVIRONMENTAL DEGRADATION OF POLY(ETHYLENE-TEREPHTHALATE)

The degradation of amorphous poly(ethylene terephthalate) bottle and a morphous sheet materials are investigated under different environmenta l conditions (wet soil, 100 and 45% relative humidity and UV irradiati on) by measuring the rate of chain scission, using viscometric analysi s, end-group analysis by FTIR and crystallinity via density measuremen ts at different temperatures. Using the Arrhenius expression the lifet ime of the biaxially oriented polyester bottle material and activation energy for degradation are found to be very dependent upon the enviro nmental conditions, with hydrolysis being a dominant process. Negligib le degradation is observed at temperatures below the glass transition (c. 80-degrees-C) in dry conditions. From density measurements at 45 a nd 100% relative humidity the crystallinity exhibits an initial facile increase only at temperatures about 70-degrees-C, due to plasticisati on by the moisture and annealing, followed by an inflection which incr eases 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 i nitial rapid increase is found to be faster at lower rates of chain sc ission due to an annealing/plasticisation effect by the moisture. This is confirmed by the fact that, under both dry conditions and UV irrad iation, where significant rates of chain scission are observed, especi ally at high temperatures and prolonged times (>500 days), the crystal linity shows no significant increase. HydrolytiC degradation on therma l ageing is confirmed using end-group analysis, which shows that hydro xyl and carboxyl rates are synonymous and increase significantly only under high humidities and temperatures above the T(g). Upon UV exposur e, however, chain scission is accelerated in the presence of wet soil with the rate of carboxyl formation exceeding that of the hydroxyl gro up. The latter is associated with the importance of a Norrish Type II intramolecular hydrogen atom abstraction and unzipping mechanism. Meta l ion contents in the polyester film material are found to vary signif icantly with the ageing condition. Thus, whilst antimony appears to be extracted the copper content is enhanced significantly, which may con tribute to the accelerated degradation of the polyester. The implicati ons of these results in terms of the ageing of bottle material are dis cussed.