Analyzing the Dehydrochlorination of Poly-(vinyl chloride) Piping Under Oxidative Stresses

Abstract

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) at-tempt 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 mecha-nism by which dehydrochlorination takes place.

Flue pipingVent piping

Hypochlorite tank

Delivery piping (installed 2001)Delivery piping (installed 2010)

Analysis of Scanning Electron Micrographs

Distinct changes on the surfaces of poly-(vinyl chloride) that was exposed to calcium hy-pochlorite were noticed in the electron micrograms of our piping sample. Samples were imaged at numerous points; the most striking differences began to appear after 20 days of exposure. As a base-line, we also imaged “new” piping (from the flue of the plant). In the untreated piping (Figure 1), a relatively smooth inner surface is shown to be heavily littered with what appears to be crystal or particulate residues.

Figure 1. Unused, untreated pipe taken from the most sheltered section of the gas flue; as such, exposure to hy-pochlorite fumes would have been minimal and this pipe section may be referred to as “unused.” Residues or salts are visible on the SEM as well as a smooth inner surface.

Figure 2. Exposed piping, sample 51. Exposed for 34 days. Structures that appear to have held some residue or salt (pus-tule-like structures) are visible on the surface, with diameters ranging from 2-8µm.

Figure 3. Exposed piping, sample 21. Exposed for 34 days. The density of these structures, even at this level of exposure, is very apparent in this micrograph. Larger structures of di-ameters up to 15µm are visible.

Figure 4. Scanning electron micrograph of piping exposed to oxidative stress in the form of sodium hypochlorite 12% solution for ten years (2001-2011). Pustule-like structures are readily apparent and seem to be more densely distributed than on the exposed piping.

After exposure, structures which resemble craters or fissured pustules (where some residue or particulate could have been lodged earlier) were noticed on all samples, re-gardless of exposure length (Figure 2,3).

These same pustule-like structures were observed on the surfaces of piping exposed to ten years of oxidative stress with a much higher frequency than in the piping samples exposed in our lab(Figure 4). It is notable that the back-ground surface of the poly-(vinyl chloride) was relatively smooth (Figure 2), the background surface has begun to deteriorate.

Pustule-like structures were not visible on untreated piping, only samples exposed to our calcium hypochlorite treatment and those exposed to sodium hypochlorite while in use displayed any of these structures.

We were not able to compare our scanning electron micrographs to those found in literature, as our magnifi-cations (~10000x) were nowhere near to those in the lit-erature (~30x). Binocular micrograms at magnifications of 20x and 70x were examined to ascertain if similar struc-tures had appeared on the “used” samples as had to those exposed to oxidative stresses in literature (Figure 5).

The two samples are dissimilar. As the poly-(vinyl chloride) in the literature was exposed to a constant ten-sile stressor and relatively high temperatures (200°C), de-hydrochlorination and microfissuring were pronounced. Regardless of the conditions, microfissuring in our samples may well have developed after ten years of oxidative stress. We did not find analogous structures at the 20x or 70x level of magnification of the piping in use for ten years.

Figure 5. A comparison between the 70x binocular view of a used piping sample and an analogous 38x and 35x scanning electron micrographs of dehydrochlorinated poly-(vinyl chlo-ride) from Chung, et. al.

Acknowledgements This research was made possible through the Exemplary Student Research Program, supported by Argonne National Laboratory’s Edu-cational Programs (CEPA), the Center for Advanced Radiation Sources (CARS) – University of Chicago, and the Electron Microscopy Center at Argonne. GSECARS is supported by NSF EAR-0622171 and DOE DE-FG02-94ER14466. Argonne National Laboratory is a U.S. Depart-ment of Energy laboratory managed by UChicago Argonne, LLC. We would also like to give our thanks specifically to Mrs. Beverly George and Mr. Mark Rowzee (corresponding author, mrowzee@na-perville203.org) for their help at NNHS; Mr. Lou Harnisch for his help at Argonne throughout this endeavour; and to Anne Hunter for work-ing on our team until January.

Steve Kuznetsov, Avi Prakash, Naperville North High SchoolDr. Nestor Zaluzec, Materials Science Division, Electron Microscopy Center, Argonne National Laboratory;

Dr. Mark Rivers, GSECARS, University of Chicago

For our studies of the dehydrochlorination of PVC pipe, we worked with piping samples from the Springbrook Water Reclamation facility in Naperville, Illinois. The po-ly-(vinyl chloride) pipes that we selected for our experiment were from three groups: 1. Piping used as part of a sodium hypochlorite delivery system for ten years, most of which had suffered significant degradation. 2. Piping purposed for flue ventilation – these pipes had relatively minimal contact with oxidative compounds and were chosen for possible use as a baseline. 3. Piping that had been installed as a replacement in 2010 and then barely used there-after, as the tank to which this piping was connected failed, for possible use as a baseline. We will be referring to pipe from the first group as “used,” the second, “flue,” the third, “new.” We quickly recognized that the flue piping was more degraded than the unused, recently-installed piping. For this reason, we chose to use the recently-installed piping as our control.

X-Ray Tomography Analysis

X-Ray tomography (XRT) was used to analyze three samples of piping: one sample exposed for 77 days to 21g/100mL calcium hypo-chlorite solution, one sample that had been in use at the Springbrook Water Reclamation Facility for ten years, and one sample from our “new” sample batch. Each image has a resolution of five micrometers. Each pixel represents a small section of our pipe; the intensity of each pixel is directly proportional to the intensity of the x-rays absorbed and re-emitted at that site. Higher levels of intensity signify pixels containing heavier ele ments. Pixel intensity values were extracted and a histogram was created for each sample. Intensity values for the pipe and defects within the pipe were almost normally distributed in each of the samples, so a significance level of six σ (α= 9.867 x 10-10) was selected to select only pixels which were visually obvious defects. Counts of pixels with intensities above this threshold are plotted below, as a function of depth within the sample, moving from the inside of the pipe to the outside. This is analogous to moving from the surface with greatest contact with oxidation to the surface untouched by the same process. As our team did not have data from a nuclear magnetic resonance (NMR) or X-ray fluorescence (XRF) analysis to correlate with this XRT data, none of the pixels which we deem to be significantly bright can be definitively matched with their chemical makeup.

Figure 1. A sample distribution of pixel intensities, for the “new” sample that was analyzed. A normal trend is clearly evident. This distribution is approximately normal with µ=1629.6277 and σ=232.1360.

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For the new pipe, there is no trend in pixel intensity values. This is indicative of the random dispersion of additives to the PVC, and of the relatively un-degraded state of the pipe. The distribution is approximately normal with µ=1629.6277 and σ=232.1360, the threshold value is i*=3022. Slices deeper within the sample than slice 800 are discarded as they contain large amounts of modeling clay that was used to hold the sample in the path of the APS beam.

Figure 2. A plot of the count of pixels above the six σ thresh-old as a function of depth within the sample for the new pipe sample.

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For the treated pipe, there is a clear trend in pixel intensity values: there are a large number of defects on the surface exposed to calcium hypochlorite for the lon-gest time, and this number quickly decreases and remains low for the rest of the sam-ple. This is clearly indicative of our exposure, as the inner surface of the PVC would have accumulated the most debris from our treatment. The residues would remain even after prolonged washes in deionized water on the nanoscale. The distribution is approximately normal with µ= 1528.8238 and σ= 326.4696, the threshold value is i*=3489. Slices deeper within the sample than slice 800 are discarded as they contain large amounts of modeling clay that was used to hold the sample in the path of the APS beam. Figure 3. A plot of the count of pixels above the six σ thresh-

old as a function of depth within the sample for the treated pipe sample.

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For the used pipe, there is no clear trend in pixel intensity values: counts of intense pixels spike 2.5mm into the sample, and these levels gradually return to pre-vious values afterwords. This variation could be caused by artifacts created from the reconstruction of our data. The pattern could also represent the degradation pattern of chemical additions to the PVC. Five general categories exist, (1) inorganic metal salts, particularly lead metal; (2) metal soap, or other salts of organic acids, particu-larly calcium, zinc, barium; (3) organotin compounds; (4) auxiliary thermal stabiliz-ers, particularly phosphites, β-diketone, and (5) antioxidants (Xiang et. al.). As the pipe undergoes degradation from the inside out, these compounds’ depletion patterns follow suit: we would expect the lowest levels in the most degraded PVC. This might create a pattern similar to the one seen in this analysis. The distribution is approximately normal with µ= 1700.6950 and σ= 256.1009, the threshold value is i*=3237. Slices deeper within the sample than slice 1200 are dis-carded as they contain large amounts of paint on the outside of the pipe, which does not relate to this analysis. The lowest 200 rows of pixels in each image are ignored as they contain modeling clay that was used to hold the sample in the path of the APS beam.

Figure 4. A plot of the count of pixels above the six σ thresh-old as a function of depth within the sample for the used pipe sample.

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Future Directions

Nuclear Magnetic Resonance imaging is the best analysis candidate. PVC samples should be dissolved in deuterated tetrahydrofuran in order to use liquid-state NMR, as gradients of dehydrochlorination may affect results if solid-state NMR were used. NMR spectra should be analyzed as per Si Kun (specifically from 2D HMQC and HMBC spectra) to identify structural groups repre-sentative of dehydrochlorinated PVC. The concentration of the following structural groups will be correlated to the overall dehydrochlorina-tion: 1-chloro-2-propenyl end groups, head-to-head addition fragments, 4-chloro-2-butenyl end groups, 1,2-dichloroethyl end groups, chlo-romethyl branches, 2,4-dichlorobutyl branches, 1,2-dichloroethyl branches. The dehydrochlorination of PVC may be quantified by using 1D 1H NMR to look at the concentrations of internal and external allylic chlorines (-CH=CH-CHCl- and -CHCl-CH=CH2) and tertiary chlorines (>CClCH2CHCl-CH2-CH2Cl). This data will allow a much more conclusive test for dehydrochlorination. A much more efficient exposure may be achieved with an ultraviolet source of radiation, as this would not leave the residues we noticed with XRT analysis of the treated sample and would allow for a much more even dehydrochlorination pattern. Our exposure relied on the fact that water flow was even and constant throughout the 77 days.

Sources Cited

Chung, Sarah, Ken Oliphant, Patrick Vibien, and Jingguo Zhang. "An Examination of the Relative Impact of Common Potable Water Disinfectants (Chlorine,chloramines,and Chlorine Dioxide) on Plastic Piping System Components." Jana Laboratories Inc., Aurora, Ontario, CanadaSi, Kun. Kinetics and Mechanism of Vinyl Chloride Polymerization Effects of Additives on Polymerization Rate, Molecular Weight and Defect Concentration in the Polymer. Diss. Case Western Reserve University, 2007.

Figure 6. A sample image of a middle slice of the new sam-ple. Brighter dots signify a defect, a grey color outside of the areas bounded by white dots is air.

Note 1: Axes are not given units as values are arbitrary and only relevant in comparison between these three XRT volumes of images. Note 2: Analysis of twenty smaller subsections of the signifi-cantly bright pixels showed no difference in trends between the smaller sections and the overall trend of all twenty.

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