The effect of thermal and thermo-oxidative degradation conditions on rheological, chemical and thermal properties of HDPE

This research evaluates the effect that thermal and thermo-oxidative degradation conditions exert on the rheological, chemical and thermal properties of high-density polyethylene (HDPE). To that end, dynamic oscillatory rheology, solubility tests in xylene, TGA, DSC and FTIR were conducted on HDPE samples subjected to different degradation conditions: atmosphere (air or nitrogen), temperature (150, 175, 200, 225 or 250 degrees C) and time (10, 30 or 60 min). Thus, under nitrogen atmosphere, chain scission mechanism prevails over the formation of long chain branching/crosslinking phenomena, which is reflected in a decrease in complex viscosity (vertical bar eta*vertical bar) and an increase in crystallinity (chi(c)). Interestingly, under air atmosphere, two rheological responses were observed: a) a well-developed rubbery region, at degradation temperatures of 225 and 250 degrees C and, b) a so-called second plateau in G' and G, for lower degradation temperatures. Solubility tests in xylene point out that the rubbery region is consequence of both branching phenomena and the presence of crosslinldng; however, the second plateau should be solely ascribed to the increase in the branching mechanism. In this case, the decrease in chi(c), is consequence of both increase in branching and formation of degradation products (accordingly supported by FTIR results). Finally, TGA results reveal that thermal and thermo-oxidative degradation shift the characteristic temperatures (T-5% and T-max) to lower values, compared to virgin HDPE. (C) 2017 Elsevier Ltd. All rights reserved.