Analyzing the Dehydrochlorination of Poly-(vinyl chloride) Piping Under Oxidative Stresses
- Steve Kuznetsov
- Avi Prakash
- Beverly George
- Mark Rowzee
- Mark Rivers (University of Chicago, GSECARS)
- Nestor Zaluzec (Argonne National Laboratory, Electron Microscopy Center, Materials Science Division)
Polyvinyl chloride (PVC) is a long-chain polymer, often used as piping for potable water transport. PVC is easily degradable by thermal, tensile and chemical stresses; and electromagnetic radiation. In potable water transport, the PVC is exposed to various disinfectants, such as sodium hypochlorite or gaseous chlorine. In wastewater services, the piping is exposed to a much higher concentration of disinfectants. In both cases, the chlorine in the disinfectant acts as an oxidizing agent and degrades the plastic by a process known as dehydrochlorination. In this study, we proposed to (1) assess the dehydrochlorination of PVC that was exposed to hypochlorite ion disinfectants in use, and (2) attempt to create a model of this degradation of PVC in correlation with our own timed exposures to calcium hypochlorite. This would allow us to quantify the degradation of PVC pipe over time.
We obtained samples of PVC that had been in exposure to sodium hypochlorite for 10 years, and we prepared our own timed exposures to a calcium hypochlorite solution. With an optical binocular microscope, we took a general survey of the exposed surfaces of the pipe at 10x, 20x, 40x, and 70x. This level of magnification, however, proved to be of no quantifiable use. Scanning electron micrographs (SEM images) of the same surfaces, with varying magnifications of 9000 – 10000x showed various structural blemishes could be observed and catalogued, including micro-fissures and “pustule”-like structures. X-ray tomography imaging (XRT) of those same samples was used to obtain a high-resolution image of the PVC at the micron level. At this resolution, the various chemical inconsistencies were described with the aid of digital image processing. Our goal was to correlate these aberrations with those that were visible to us in the SEM images, to understand the mechanism by which dehydrochlorination takes place.