Spectroscopic, physical and product studies of the thermal degradation of poly(ethylene glycol) containing a 1,3-disubstituted phenolic group

Abstract

The high-temperature heat-exchanging oil Lutron KS1, a poly(ethylene glycol) containing a single benzene ring in the chain with 1,3-substitution, has been characterised by 1H and 13C NMR spectroscopy, IR, matrix-assisted laser desorption ionisation time-of-flight (MALDI-TOF) mass spectrometry and size exclusion chromatography (SEC); the latter shows it to have a low PDI (1.06) while 1H NMR and MALDI-TOF indicate a peak molecular mass of 816 and 682 respectively. The 20-fold rise in viscosity of Lutron during industrial use at 170–210°C over prolonged periods is shown to be due to the effect of oxygen in causing degradation to give carbonisation and the release of a range of volatile degradation products including CH 3CHO, H 2O, and di- and tri(ethylene glycols). Thermal degradation under N 2 gives a much more complex array of volatiles, including 18-crown-6; the formation of most of these involves simple homolysis followed by disproportionation. The SEC chromatograms of residual Lutron were bimodal, indicating extensive formation of high mass products. The 1% addition of various well-known antioxidants had only, at best, a marginal effect on reducing the oxygen-induced degradation; the most effective method of extending the service life of Lutron is to exclude oxygen during the heating process.