Aging Mechanism and Material Ranking of PE Pipe Grades in Aqueous Chlorine Dioxide and Hypochlorite Solutions

Marton Bredacs

Research output: ThesisDoctoral Thesis

Abstract

With the ever-growing application of polyethylene (PE) as a piping material and the wide spread usage of secondary disinfection in the drinking water system, the effect of chlorine dioxide (ClO2) and hypochlorite (HOCl) on the degradation of PE pipes has become a matter of interest. Considering the fast concentration decline due to decomposition and the oxidation reaction of ClO2 and HOCl, precisely controlled aging experiments were found to be essential when investigating the effect of disinfectants. Standardized pipe pressure tests with disinfected aqueous solutions are used to assess the effect of disinfectants. Due to high costs and long testing times, there is a clear need for accelerated, well-controlled laboratory aging experiments, which provide reliable and reproducible data within a short amount of time. Therefore, a newly developed exposure device was used. When studying the consumption of AO, the Oxidation Induction Times (OIT) and Oxidation Onset Temperatures (OOT) of aged samples were compared. The latter was found to be more sensitive and faster than the OIT. Tensile tests and Scanning Electron Microscopy (SEM) analyses indicate that aging of 1 mm thick specimens in 1 ppm ClO2 at 50°C or a less aggressive medium provides suitable results to evaluate the progress of PE degradation. Immersion in HOCl solution led to a highly surface-limited aging process; hence the application of thinner, 0.3 mm thick specimens was found to be advantageous. SEM analyses indicate that ClO2 generates surface cracks and a degraded material layer, while immersion in HOCl resulted in surface erosion. The different aging mechanisms were further confirmed by comparing the elongation at break, the OOT and the Carbonyl Index (CI) evaluated over the aging time. The ClO2 aged samples suggest that polymer chains are attacked before the complete loss of active AO. In the case of HOCl immersion, an accelerated auto-oxidative aging mechanism was detected. Considering the distinct PE aging products identified in the FTIR-ATR spectrum for each medium and chain scission reactions shown by GPC analyses, polymer degradation was presumably dominated by the following reactions. The decomposition of α keto hydroperoxides in ClO2 and the decomposition of allylic-hydroperoxides in HOCl seem to govern the chain scission of PE in the affected surface layer. The evolution of molecular weight over the aging time implies that reactive chlorine species of HOCl attack selectively oxidized tertiary carbon atoms. The presence of ClO2 resulted in a quasi-competitive reaction between chain defects, oxidized chain parts and tertiary carbon atoms of PE molecules. The changes in OOT, degree of crystallinity and CI depth profiles recorded during the immersion test in ClO2 of three PE grades suggest that the rate determining step is the ClO2 diffusion. The increased crystallinity of a 200 µm surface layer points out the gradual decrease in the ClO2 diffusion rate, so with advanced chemical degradation, AO consumption became slower. Consequently, the material property profiles represent a high potential to rank various material formulations regarding their relative ClO2 resistance. The degree of crystallinity and CI profiles of HOCl-exposed samples remained unchanged affirming the highly surface limited polymer degradation. The results also indicate that carbonyl products are predominately derived from polymer degradation. Since a clear material ranking based on these property profiles is rather complicated, immersion tests were performed with six PE grades in 1 ppm and 0.5 ppm ClO2 at 50°C and 60°C. Recording the decline of elongation at break and OOT in 1 ppm ClO2 at 50°C or lower concentration and temperature was found to be an optimal method for performance screening of various PE pipe grades. Based on mechanical and thermal characterization, three distinct groups with different ClO2 resistance we
Translated title of the contributionAlterungsmechanismen und Material Ranking von PE Rohrwerkstoffen im Chlor Dioxide und Hypochlorit Lösungen
Original languageEnglish
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Aust, Nicolai, Assessor B (internal)
  • Pinter, Gerald Gerhard, Assessor A (internal)
Publication statusPublished - 1800

Bibliographical note

embargoed until null

Keywords

  • Polyethylene
  • Aging Mechanism
  • Disinfectants
  • Chlorine Dioxide
  • Material Ranking

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