Kishan Chand Khemani

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

Abstract 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°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) HO 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.

CO2-Blown PETG Foams

Publisher Summary This chapter reports investigations on the foamability of PETG using CO2 as the blowing agent. The use of CO2 as a blowing agent offers many advantages—it is easily available, has a relatively low critical temperature, its equation of state is accurately known, and is soluble to considerable extents in polymers. The latter allows CO2 to easily plasticize the polymer matrix, thus making it the blowing agent of choice in making microcellular foams. The solubility of CO2 in polymer melts remains appreciably high to allow production of foams with larger cells. Thus, a whole range of foams can be made using the same blowing agent. The use of CO2 in producing microcellular morphology in several polymers with relatively low Tgs has been well established. The procedure normally used is to contact the polymer with high-pressure CO2 to obtain a saturated solution. PETG is an amorphous, glycol modified poly(ethylene terephthalate) made from terephthalic acid, ethylene glycol, and cyclohexanedirnethanol by a polycondensation process. However, unlike PET, it does not undergo crystallization on heating or on plasticization by the dissolved species; the comonomer, cyclohexanedimethanol, is responsible for the completely amorphous nature of this polymer. PETG has a low Tg and high-melt strength, and has been successfully evaluated for producing medium density foamed sheets by extrusion using CO2-evolving chemical blowing agents. PETG has high affinity for CO2, and is easily processed into foams with varying cell sizes.