Accelerated aging of polyethylene pipe grades in aqueous chlorine dioxide at constant concentration

Abstract

The impact of disinfected water on the degradation of polyethylene (PE) was investigated with immersion tests of two PE pipe materials in 10 and 5 ppm chlorine dioxide (ClO2) medium at 60 °C. Aging experiments in 1 ppm ClO2 at 60, 50 and 40 °C were also carried out to study the effect of different aging temperatures. During conditioning, the pH was kept at 6.8. A specific exposure device with continuous concentration control and adjustment has been applied in order to generate reliable and reproducible aging conditions. Sample characterization applying scanning electron microscopy (SEM), tensile test, dynamic oxidation tests as well as FTIR-spectroscopy indicated accelerated antioxidant consumption and polymer degradation. Material aging at 50 °C and above was found to be much faster than at 40 °C applying 1 ppm ClO2 concentration. An optimized testing condition for fast material characterization in case of 1 mm thick specimens was found to be a concentration of 1 ppm ClO2 at 50 °C. The simultaneously increasing material embrittlement and the consumption of active antioxidant molecules imply an apparent unselective reaction of ClO2 with the polymer and the stabilizers molecules. Moreover, the radical nature and the high reactivity of ClO2 led to the formation of carbon-chlorine species, which are assumed to originate from degraded antioxidant molecules.