Characterisation of the species responsible for yellowing in melt degraded aromatic polyesters .1. Yellowing of poly(ethylene terephthalate)

Changes in the luminescence, intra-red and visible spectra of poly(ethylene terephthalate) (PET) when degraded at melt temperatures in air and nitrogen have been examined. Main chain-scission and subsequent aggregation of terephthalate fragments is illustrated by an increase in dimer fluorescence, while the presence of a longer wavelength fluorescence is attributed to aromatic hydroxylation. A longer residence time in the melt leads to a reduction in the intensity of hydroxylated bands consistent with their further oxidation. Phosphorescence studies on oxidised PET and its extracts indicate stilbenequinone-type impurities. Infra-red spectra are consistent with the development of hydroxylated terephthlate units, vinyl esters dicarboxylic acids and quinones. Lack of symmetry in the charge-transfer bands of the visible spectra of degraded PET reinforce the findings from luminescence and infra-red spectra, indicating that for short residence times in the melt polyconjugated species are not responsible for yellowing. (C) 1996 Published by Elsevier Science Limited