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Aging mechanism of polyethylene pipe material in chlorine dioxide and hypochlorite solution

 

Márton Bredács, Andreas Frank, Amaia Bastero, Alex Stolarz, Gerald Pinter

Papers # 2018 Las-Vegas

Accelerated aging experiments by the immersion of a commercial polyethylene (PE) pipe grade in 10 ppm of chlorine dioxide (ClO2) and in 100 ppm of hypochlorite (HOCl) solution at 60 °C were performed. An additional immersion test was conducted in 1 ppm of ClO2 at 40 °C. The exposure parameters were continuously controlled especially the disinfectant concentration during the aging of 1 mm thick tensile specimens. Analysis techniques such as SEM, tensile test, FTIR and DSC-OOT, revealed significant differences in the aging mechanism caused by the two disinfectants. On the one hand, the presence of ClO2 induced an apparent parallel material embrittlement with the consumption of active stabilizers, displaying surface cracks and a few hundred microns thick degraded surface layer. On the other hand, the results of HOCl exposure indicate an accelerated auto-oxidative degradation, displaying an etched-like, rough surface characteristic with numerous cavities, without the appearance of distinct surface cracks and degraded layer. 

Chlorine dioxide (ClO2) and hypochlorite (HOCl) are the most used oxidizing agents for secondary drinking water disinfection due to their high effectivity against pathogenic microorganisms. Considering the strong oxidative nature of these disinfectants, accelerated antioxidant consumption and material degradation can be expected in the affected pipe surface layer. The presence of chlorine-based oxidizing agents will shorten the crack initiation time and lead to premature pipe failure. 

To accurately study the effect of ClO2 and HOCl on polyethylene (PE) pipes materials, during laboratory aging stable exposure parameters are essential to generate reliable and reproducible data. Unfortunately, accelerated experiments with constant concentration are quite challenging in laboratory scale due to the fast reaction with organic and inorganic compounds as well as the considerable thermal decomposition of both disinfectants. To overcome this obstacle, an exposure device was developed at PCCL, which allows stable and well-controlled aging experiments in ClO2 and HOCl solutions.

The objective of this work is to provide further insight into the relevant degradation process of PE in chlorinated media. Tensile specimens with a thickness of 1 mm were periodically removed after immersion in 10 ppm of ClO2, 100 ppm of HOCl solution at 60°C and in 1 ppm of ClO2 at 40°C. Scanning electron microscopy (SEM) images showed surface cracks and a few hundred micron thick embrittled material layer due to immersion in ClO2, while exposure to HOCl resulted in an etched like surface without micro-cracks and degraded surface layer. Based on Oxidation Onset Temperature (OOT), tensile tests and IR-spectroscopy (FTIR) analyses, distinct aging processes were observed for each disinfectant. In case of ClO2, the results indicate that the oxidizing agent attacks simultaneously the polymer and the antioxidant molecules. Exposure to HOCl led to material embrittlement only after the complete loss of active antioxidants. Profound knowledge of the dominating aging mechanisms is essential to develop PE pipe materials and antioxidant packages with increased performance in contact with chlorinated water. 

Related keywords : Polyethylene pipes, Aging mechanism, Chlorine dioxide, Hypochlorite solution.
Please note that the whole article content is available on PPCA website only


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