Long term durability of crosslinked polyethylene tubing used in chlorinated hot water systems

The objectives of the present research are to understand better the degradation of crosslinked polyethylene tubing subjected to pressurised, hot chlorinaled water in flow through systems (e.g. home plumbing) and to estimate its service lifetime. Crosslinked polyethylene tubing was subjected to pressure testing with flowing hot, chlorinated water at elevated temperatures from 90 to 120"C, with chlorine levels between 0 and 5 ppnri. The combined effects of free chlorine, pH, and metal ion content on the time to failure were found to correlate better with the oxidation-reduction potential than with the free chlorine content directly. Consumption of the antioxidant was analysed using the oxidation induction time as a function of fraction of the time to failure in the tests, with the antioxidant being found to be completely consumed in less than half the time to failure at 115"C. Fractographic observations made using a scanning electron microscope indicated no degradation until the antioxidant was completely consumed. The nature of the damage at failure was investigated using highly polished cross-sections of pipe, which failed by a pinholc leak.