Reaction mechanism for oxidation and degradation of high density polyethylene in chlorinated water

Polyethylene is increasingly used for potable water pipe even though polyethylene is susceptible to oxidative degradation. Accelerated aging conditions with chlorinated water solutions which minimized variations in solution chemistry were used in a 160-day (3840 h) immersion study of high density polyethylene (HDPE) pipe and HDPE resin. Samples were periodically characterized for changes in visual appearance and surface chemistry using infrared spectroscopy. Surface carbonyl bonds were detected for both HDPE pipe and HDPE resin samples. Experiments with isotopic 18O2 gas demonstrated that molecular oxygen is partly responsible for formation of the carbonyl oxygen. Both HDPE pipe and HDPE resin samples were demonstrated to form 4-chloro-2-methylbutan-2-ol and 2,3-dichloro-2-methylbutane as novel HDPE breakdown products which leached into chlorinated water. From these data, reactive species involved in the breakdown of HDPE pipe have been identified. A proposed mechanism for the breakdown of HDPE is Cl
- or HO
- initiated H-abstraction to produce a carbon-center radical that reacts with triplet oxygen to form a peroxyl, which upon decomposition yields a carbonyl and further propagates the formation of radicals.