Primary thermal degradation mechanisms of PET and PBT

Abstract

The primary thermal degradation mechanisms of polyethyleneterephthalate (PET) and polybutyleneterephthalate (PBT) have been studied by direct pyrolysis mass spectrometry using negative chemical ionization (NCI). The NCI mass spectra of PET and PBT are dominated by series of ions which might be assigned either to cyclic or open-chain oligomers with olefin and carboxylic end-groups. The structures of the pyrolysis products have been identified by comparison of their daughter ion spectra with those of authentic samples of cyclic and open-chain oligomers with olefin and carboxyl end-groups. The authors' results provide strong evidence that cyclic oligomers are the primary pyrolysis products of PET and PBT, formed by intramolecular exchange (ionic) reactions. These further decompose by a β-CH hydrogen transfer reaction, which involves a six-membered cyclic transition state, and generates open chain oligomers with olefin and carboxylic end-groups. The pyrolysis of PET and PBT was also carried out in the presence of ammonium polyphosphate (APP) (an acid precursor). APP affects the pyrolysis by increasing the rate of decomposition and lowering the Polymer Decomposition Temperatures (PDT)s of both polymers. However, it does not change the nature of the compounds obtained in the pyrolysis of the pure polymers. This fact provides a firm argument in favour of the belief that the β-CH hydrogen transfer occurs by an ionic process.