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

We previously reported an investigation on the isothermal degrdn. of poly(ethylene terephthalate) (PET) and present here a parallel study on poly(butylene terephthalate) (PBT). Although the two 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 degrdn. of PBT compared to PET. A rationalization of the differences obsd. is offered. Several isothermal degrdn. expts. on PBT were conducted at 270-350°C in order to simulate the reactions that take place in its actual thermal processing of. Structural characterization of the reaction products was performed by MALDI mass spectrometry and by NMR anal. The results indicate that cyclic oligomers of PBT formed at temps. below 290°C by ring-chain equilibration undergo thermal decompn. at higher temps. by a ß-hydrogen transfer mechanism, as well as the open PBT chains. The formation of unsatd. oligomers was detected by MALDI and also by 1H and 13C NMR techniques, whereas in contrast to the thermally degraded PET sample, terephthalic anhydride-contg. oligomers were not obsd. Formation of butadiene was suggested by 1H NMR data on PBT samples processed at various temps.