Methodology to Evaluate Oxidation Degradation of High-Density Polyethylene Corrugated Pipe Resin

A methodology is presented to evaluate the oxidative degradation of compression-molded samples made from high-density polyethylene corrugated pipe resin. The test samples contained 2.5 wt% of carbon black and an antioxidant (AO) package that consisted of 0.05% Irganox 1010 and 0.1% Irgafox 168. Samples were incubated in forced-air ovens at temperatures of 65°C, 75°C, and 85°C (149°F, 167°F, and 185°F) and in a water bath at 85°C (185°F). The amount of AOs in the incubated samples was measured by using both the oxidative induction time (OIT) test and the induction temperature (IT) test. It was found that the OIT test was much more sensitive than the IT test in assessing the AO depletion of incubated samples. For the evaluation of oxidation degradation, both melt index (MI) and tensile break elongation were used to detect changes in molecular weight and mechanical property, respectively. For air incubation, AO depletion rates obeyed a first-order reaction at all three temperatures, and the AOs underwent two simultaneous reactions according to the second-order reaction at the 85°C water incubation. At 85°C (185°F) air incubation, oxidation degradation was detected as reflected by a decrease in MI and tensile break elongation. The decrease of MI indicated an increase in the molecular weight due to the cross-linking of the polymer. According to the Arrhenius equation, the predicted lifetime of this particular AO package under continuous flow of air is 117 years at 23°C (73.4°F).