A novel approach for studying the thermal degradation, and for estimating the rate of acetaldehyde generation by the chain scission mechanism in ethylene glycol based polyesters and copolyesters

A novel approach to studying the thermal degradation of ethylene glycol based polyesters and copolyesters is described. The method involves the use of a gas chromatograph to measure the amount of acetaldehyde (a degradation by-product) generated over time at an elevated constant temperature for an extended period. We have found that at high temperatures, polyester resin show gradual decrease with time in the amount of acetaldehyde that is generated, and that this decline eventually approaches a near asymptotic value. A generalized polyester degradation mechanism for the generation and consumption of acetaldehyde is proposed which is consistent with the known literature. In this mechanistic picture, it is proposed that acetaldehyde is generated by three different routes involving first the hydroxyl-end groups, second the vinyl-end groups and finally the mid-polymer chain-scission. It is further suggested that of these, the hydroxyl and the vinyl end-group based acetaldehyde generation routes deplete with time leaving behind the last, but inexhaustible, chain-scission route. From the data in this study, it is possible to estimate the rate of acetaldehyde generation by this mid-polymer chain-scission route to be approximately 1.10-1.46 ppm per min at 280 degrees C. For the various polyesters examined in this study, the above information was applied together with the (a) amount of residual acetaldehyde in the resin, (b) KO end-group concentration and (c) vinyl end-group concentration, to calculate the amount of acetaldehyde generation possible over a given period of time. This calculated [acetaldehyde] compared very well with the actual observed [acetaldehyde] over the same period of time as obtained by integrating the area under the curve in the [time]-[acetaldehyde] profiles, thereby validating the proposed mechanisms. In agreement with the thermodynamic principles, a near doubling of the chain-scission acetaldehyde generation rate was observed when the temperature was increased by 10 degrees centigrade. A further support for the mechanisms came from the observation that significant amounts of anhydride linkages are generated in polyesters upon exposure to heat for extended periods of time. (C) 1999 Elsevier Science Ltd. All rights reserved.