Márton Bredács, Anita Redhead, Andreas Frank, Amaia Bastero, Gerald Pinter
Papers # 2016 Berlin
To maintain the high quality of potable water chlorine based disinfectants are frequently dosed into the drinking water network. Considering the oxidative nature of these species their long-term impact on the aging of polyolefin pipes is a matter of interest. In this study four polyolefin pipe grades were objected to an accelerated aging in 10 ppm of chlorine dioxide (ClO2) solution at 60 °C in order to gain information about the penetration behavior of ClO2. After a few weeks of conditioning the surface analysis with scanning electron microscopy (SEM) revealed several micro cracks, confirming a highly degraded superficial layer. The cross section SEM images displayed the degraded surface layer with a thickness of about a 100 µm. Furthermore thermal analysis such as the Oxidation Onset Temperature (OOT) and IR-spectroscopy (FTIR) indicated a significant material degradation in various depths for each sample.
In the past decades polyolefin (PO) pipes have been successfully applied in the water distribution network due to their numerous advantages. To ensure the nowadays expected 100 years of service lifetime the possible effect of operation conditions on the long-term performance of the PO pipe must be thoroughly analyzed. Several drinking water disinfection techniques such as ozonation, ultraviolet (UV) radiation, chlorination, the addition of chloramines, and chlorine dioxide (ClO2) are used to eliminate diseasecausing microorganisms in potable water [1–3]. The addition of disinfectant to potable water creates a highly oxidative environment, which may lead to an accelerated consumption of stabilizers and even to the degradation of the materials.
Chlorine based disinfectant are the most widely applied species as a result of their high effectivity against various pathogens and remarkably lower implementation and operation cost compared to ozonation and UV radiation. Considering the strong oxidizing potential of ClO2, it is the most effective chemical substance against microorganisms among the chlorine based disinfectants [4,5] When ClO2 enters water it does not hydrolyze but remains as a dissolved gas. This gas is a stable radical which reacts fast due to single-electron transfer with an organic compound, forming mostly chlorite ions, but no chlorinated by-products. Beside the effectivity, another advantageous property of ClO2 is that it does not produce carcinogenic trihalomethanes [6,7]. The applied amount of ClO2 for drinking water disinfection is generally in the range of 1-1,4 mg/l in the USA  and in the EU it is usually kept between 0,05-1 mg/l . At higher concentrations chlorite and chlorate ions are proved to have an adverse health effect  the maximum allowed amount of it is 0,8 mg/l . Premature pipe failure was observed in several regions in France [11,12] and in the USA , which could be directly linked to the application of ClO2 in drinking water network. Some recently published studies [14–23] confirmed a fast consumption of different antioxidants (AOs) in ClO2 solution. Furthermore in the work of Colin et al. [11,12,23] the presented data indicate that oxidizing agents derived from chlorine dioxide could react with polymer molecules too. In contrast based on the investigations of Steven and Seeger  the reactivity of ClO2 to saturated hydrocarbons should be zero.
Despite the obvious impact of disinfectants on PO lifetime the accurate degradation mechanism has not been understood yet. The aim of the current study is to gain a deeper insight into the relevant degradation process by accelerated laboratory aging of PO pipe grades. In order to investigate the effect of this disinfectant on polyethylene (PE) and polypropylene (PP) materials, an accelerated immersion test was performed. In the present work the main focus was put on the analysis of material property changes in various depths.