Thermal degradation of poly(butylene terephthalate) at the processing temperature

In an accompanying paper [Samperi et al., Polym Degrad Stab (in press)] we have reported an investigation on the isothermal degradation of poly(ethylene terephthalate) (PET) and present here a parallel study on poly(butylene terephthalate) (PBT). Although the two aromatic polyesters are structurally quite similar, our results show that the presence of the butylene unit in PBT is apparently able to induce significant changes in the isothermal degradation of PBT compared to PET. A rationalisation of the differences observed is offered. Several isothermal degradation experiments on PBT were conducted in the temperature range of 270-350 degreesC, in order to simulate the reactions that take place in the actual processing of PBT. The structural characterisation of the reaction products was performed by MALDI Mass Spectrometry and by NMR analysis. The results indicate that the cyclic oligomers of PBT formed at temperatures below 290 degreesC by ring-chain equilibration mechanism undergo thermal decomposition at higher temperature by a beta-hydrogen transfer mechanism, as well as the open PBT chains. The formation of unsaturated oligomers was detected by MALDI and also by H-1 and C-13 NMR techniques, whereas in contrast to the thermally degraded PET sample terephthalic anhydride-containing oligomers were not observed. Formation of butadiene was suggested by H-1 NMR data on PBT samples processed at various temperatures. (C) 2003 Published by Elsevier Ltd.