Oxidative degradation of corrugated HDPE pipe in water and air [PE]

This paper presents a study on the oxidative resistance (OR) of a corrugated HDPE pipe. The oxidation process was carried out in forced air ovens and water baths, to simulate various environments. Elevated temperatures were employed to accelerate the oxidation reactions. Three temperatures (65, 75, and 85oC) were used so that the Arrhenius equation could be applied to predict properties at ambient temperatures lower than the incubation temperatures. The depletion of antioxidants (AOs) was measured by the oxidative induction time (OIT) test and changes of polymer properties due to oxidation were evaluated by melt index (MI) and tensile tests. The AOs decreased shortly after the incubation started as indicated by the OIT data. The AO depletion rate was faster in water than in air. The AO depleted exponentially with time in both water and air incubation conditions. The oxidation degradation of the HDPE pipe was detected by changes of MI and tensile break elongation. However, MI decreased steadily as incubation time increased while tensile break elongation decreased irregularly with time. By correlating OIT, MI and tensile break elongation test data, the boundaries of three oxidation stages were established based on various MI values. The Stage A is 113 year, Stage B is 310 years, and Stage C is 97 years. The total lifetime of the pipe is predicted to be 520 years.