The Effects of Chlorine Depletion of Antioxidants in Polyethylene

Abstract

For many polyethylene products, their working life-span depends on retention by the material of its antioxidant. In dry air, when the working and environmental stresses remain within defined limits, the life-span of the material can be many decades. Immersed in water, for example, the diffusion and loss of antioxidant from the material's surface can increase. Also, some types of aggressive agents, if in the water, can enter the material's surface and migrate into the material to increase the depletion of the antioxidant population. Researched in this study, is the depletion of antioxidants in polyethylene when exposed to water containing different chlorine concentrations. This research relates to the world-wide use of polyethylene pipes in water treatment and distribution networks.

A kinetic model for predicting the oxidative degradation of additive free polyethylene in bleach desinfected water

“…1, it can be concluded that oxidation is initiated by radical species coming mainly from ClOH, the eventual contribution of Cl 2 being clearly insufficient to be detectable. In addition, based on the mechanisms proposed in the literature [18,20,27,32,33], it can be concluded that these radicals are formed according to scenario S3.…”

“…On the contrary, for other authors [18,20,32,33], Cl 2 and ClOH could directly dissociate into Cl% and HO% radicals (presumably within the PE matrix) because Cl−Cl and Cl−O bonds are characterized by a very low dissociation energy (of respectively 242 and 247 kJ mol …”

“…In the last two decades however, it was discovered that the water disinfectants do not only destroy the organic substances in water by oxidizing processes (among which radical processes play a key role), but also consume the stabilizing function of phenolic antioxidants and initiate a radical chain oxidation of the polymer matrix in the inner wall of pipes, thus leading to a significant reduction the pipe lifetime. These deleterious effects are especially pronounced when chlorine dioxide is used as a disinfectant [12][13][14], but measurable effects were also evidenced in the case of chlorine and bleach [15][16][17][18][19][20][21].…”

Critical considerations for the accelerated ageing of high-density polyethylene potable water materials

“…Another problem with reported ageing studies is the magnitude of free available chlorine concentrations examined and the omission of initial and/or final free available chlorine concentration. For example, Dear and Mason [7,11] found drastic differences between polyethylene pipe failure and antioxidant profiles when aging was conducted at 80 C with 0-5000 ppm versus 15,000-125,000 ppm chlorine solutions, whereby uniform antioxidant reduction and slow crack growth through the pipe wall were observed at lower chlorine levels only. Unfortunately, data were not provided to describe solution chemistry during or at the completion of the ageing process.…”

“…Chemical interaction can be examined by immersing polymer samples in neat contaminant solution [67][68][69] or by exposing them to dilute aqueous solutions [70][71][72][73]. Most accelerated ageing techniques [7,9,11,14,16,17,22,25,43] Source: Dietrich and WheltonÓ2008 Awwa Research Foundation. Reprinted with Permission.…”