Evaluation of the oxidative degradation mechanism of corrugated high density polyethylene pipe and the pipe resin

High density polyethylene (HDPE) pipe has become one of the fastest growing engineering products. However, HDPE is susceptible to oxidative degradation which leads to strength loss and brittle cracking. This research is to evaluate some of the factors such as exposure environments, antioxidants (AO) package(s), and carbon black (CB), which affect the rate of degradation. Two types of materials were selected: (1) custom made HDPE plaque with known AO(s) concentrations and/or CB (2) commercially available corrugated HDPE pipes. Samples were subjected to air, water, aerobic, and anaerobic environments. Oxidative induction time (OIT) test is used to monitor the AO depletion; Fourier Transform Infrared (FTIR), melt index (MI), and tensile test are implemented to evaluate the change of polymer structural and physical strength. For the custom made samples, effects of primary AO (I-1010), secondary AO (I-168) and CB on oxidation behavior were evaluated. The combination of 2:1 ratio of I-168 to I-1010 is proven to be less effective against thermo-oxidation compared with other AO combinations. Depending on the type of AO, OIT depletion trend in water can be expressed in two term first order reaction which represents the hydrolysis and diffusion mechanism. Strong interaction between AO and CB were found, causing a rapid decrease of OIT, ultimately leading to faster degradation. CB loading and particle sizes also found to have an impact on OIT retained.