Relationship between hydroperoxide concentration and average molar mass in thermo-oxidized polyethylene

The aim of this research project is to investigate the oxidation mechanism of high density polyethylene (HDPE) used in outdoor applications, in order to establish in a near future, a non-empirical kinetic model for lifetime prediction. The present paper focuses on the changes in the hydroperoxide (POOH) concentration induced by thermo-oxidative ageing, and on their relationship with the evolution of the weight average molar mass (Mw) due both to chain scission and crosslinking processes.Thin HDPE films were aged at 110 and 140°C in air under atmospheric pressure. In a first part, changes in the POOH concentration versus ageing time were assessed by three different analytical methods previously reported in the literature: modulated differential scattering calorimetry (MDSC), Fourier transform Infra-Red spectrometry after chemical derivatization treatment with gaseous sulfur dioxide (SO2–FTIR), and iodometry. A comparison of experimental results revealed that these three methods provide very similar quantitative data on POOH accumulation, whereas iodometry tends to strongly underestimate the subsequent stage of POOH decomposition. It was thus suspected that iodometry does not only titrate POOH, but also other chemical species (presumably double bonds) formed when POOH decompose. Therefore, only MDSC and SO2–FTIR were considered as relevant methods for POOH titration.In a second part, changes in Mw versus ageing time were monitored by size exclusion chromatography (SEC). A sharp drop of Mw was first observed at the beginning of exposure, which was assigned to an intensive chain scission process. Then, in a second stage, a stabilization or even a substantial re-increase in Mw was observed, suggesting a competition between chain scission and crosslinking processes. As this second stage starts at the same time as POOH decomposition, it was concluded that there is a strong correlation between both phenomena, occurring respectively at the macromolecular and molecular scales.