Degradation of polyethylene pipes by water disinfectants

Abstract

Polyethylene (PE) pipes are commonly used for the transport of drinking water under pressures of few bars. There is an abundant literature on the results of isobaric and isothermal ageing tests made in pure water. These results show that, at ambient temperature, in the pressure domain of practical interest, the pipes perish by brittle fracture with lifetimes exceeding 50 years. There were considerable research efforts, in the past half century, to optimize the polymer structure and processing conditions in order to improve the pipe durability. In the last decade, however, it was discovered that disinfectants, of which the effect was ignored in previous studies, attack the antioxidants and, presumably, shorten the pipe lifetime because PE can undergo an oxidative degradation. This deleterious effect is especially pronounced when chlorine dioxide is used as a disinfectant, but chlorine and bleach lead also to measurable effects. The present chapter is aimed to answer the following question: Is it possible to build a kinetic model leading to a prediction of the time to failure for a given PE pipe, at given couple of temperature and pressure, for a given disinfectant in a given concentration in water? Such an investigation must cover three very distinct domains which will constitute the three main sections of this chapter: 1. Chemical aspects: Mechanisms and kinetics of disinfectant-antioxidant and disinfectant-polymer reactions, role of oxygen, macromolecular processes (chain scission) having consequences on the mechanical behaviour; 2. Transport phenomena: Penetration of disinfectants or their by-products in PE, loss by extraction and evaporation of antioxidants, couplings between these processes, composition and structure gradients resulting from diffusion controlled reactions; 3. Mechanical aspects: Kinetic modelling of pipe fracture under pressure. Coupling with chemical degradation