Degradation of high-density polyethylene (HDPE) exposed to chlorine dioxide-containing water: The effect of co-monomer and crystalline structure

Throughout the world, polyethylene (PE) pipes are widely used for the distribution of drinking water. As a result of exposure to drinking water containing ClO₂, practical experience has demonstrated that the lifetime of PE pipes can be significantly reduced, and PE pipes in water distribution systems may prematurely fail due to chemical degradation. In this research, the effect of co-monomer type and distribution upon the stability of PE pipes against water-containing ClO₂ has been explored. For this purpose, three pipe grades of polyethylene resins were analyzed, two of which were copolymerized by 1-butene (J-100 and AK-80 samples) and one of which was copolymerized by 1-hexene (AS-80 sample). At first, Samples were aged at 60 °C for 5 weeks in aging solutions containing 10 ppm of ClO₂ disinfectant in DI water. A correlation was found between the changes in physiochemical properties of the samples and their resistance against ClO₂. The crystalline phases of a polymer are impervious to harsh chemicals and plays a role as barriers against ClO₂ molecules. Therefore, the higher crystalline a polymer is, the slower the kinetic of degradation against ClO₂ will be. Moreover, the strong chain entanglement in amorphous phase, suppress the crack growth and enhance the resistance of the pipe against chemical degradation. Consequently, AS-80 resin, which has 1-hexene co-monomers and the uniform distribution of comonomers, exhibits higher stability against ClO_2, as a result of its higher crystallinity and strong chain interaction in amorphous region, and it is preferable in drinking water-containing ClO₂ pipe systems.