Kinetics of chlorine-induced polyethylene degradation in water pipes

The presence of chlorine in drinking water supplies in many countries creates the undesirable side effect of causing a relatively under investigated degree of polymer degradation in the polyethylene pipes used for transport. In order to predict pipe lifetimes and ensure safe water supplies, a kinetic model using data for the degradation rates of polyethylenes immersed in chlorine solutions, was developed. In order to replicate phenomena that normally occur very slowly at low concentrations of chlorine, accelerated ageing studies were necessary. These were carried out at high chlorine concentrations under well-defined experimental conditions (70, 400 and 4000 ppm). Results showed that, for the chlorine concentrations studied, a chain scission process associated with carbonyl formation is occurring. It was also shown that the rate of this degradation does not depend on the presence of stabilizer. A kinetic model, taking into account the chlorine concentration, is proposed in order to simulate the molar mass changes occurring. This will facilitate the prediction of the degree of polyethylene embrittlement and ultimately the lifetime.