The PVC Geomembrane Institute (PGI) is working toward the advancement of the use of PVC geomembranes in appropriate applications. One of the things the Institute is involved in is providing information on topics of interest to our readers. This issue of the Geomembrane Technical Bulletin contains two articles that address some commonly asked questions about PVC geomembranes - durability and plasticizer loss. The first article written by James A. McKelvey, III, an Associate Geotechnical Engineer with Roy F. Weston, Incorporated, takes an in-depth look at the loss of plasticizer and its effects on PVC geomembranes. The second article, 'PVC Geomembrane Case Study 25 Years Later", found on page 5 discusses the physical properties and serviceability of the material after 25 years in the ground.
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Jack Haynes recently reported that the Bureau of Reclamation Canal Lining Demonstration project has been constructed and the ten-year monitoring period has begun. The Bureau has written a report about the specifics of this project. To get a copy contact: Mr. Jack Haynes U.S. Bureau of Reclamation, 1150 North Curtis Road, Boise, ID 83706-1234. Telephone: 208-378-5093, Fax: 208-378-5066.
Demonstrating PVC Geomembrane Durability Landfill Closures
By: James A. McKelvey, III
Roy F. Weston, Inc.
The New York State Department of Environmental Conservation (NYSDEC) had asked WESTON to demonstrate that the PVC geomembrane proposed within a municipal solid waste landfill cover system would have sufficient long term durability so as to be considered accept able in serving as the barrier layer within this cover system. In response to this request, WESTON performed a literature study regarding the long term durability of PVC geomembranes when used in landfill closure systems. This article was
initially an inter-office memorandum, of which NYSDEC was copied, and serves to summarize the findings of the aforementioned literature study. The article should allay any concerns NYSDEC has with PVC geomembrane in this application.
It is widely believed by some within the geosynthetic community that PVC geomembranes degrade in the presence of landfill leachate, and as such should be considered inferior to polyethylene geomembranes. While it is true that PVC may in fact degrade when in contact with some chemicals, the same holds true for polyethylene. There is no perfect geomembrane in terms of chemical compatibility. For example, neither PVC nor polyethylene is compatible with benzene, however urethane geomembranes are available for retaining this chemical. Another example is black liquor, an effluent derived from certain processes within the paper industry. It has been shown that this chemical is particularly aggressive to high-density polyethylene. However, PVC has excellent resistance to this chemical. Although examples could be cited demonstrating polyethylene's compatibility to many chemicals, it should be left to the engineer to determine through chemical compatibility studies what geomembranes may be used for a particular application. In order to ensure that the geomembrane meets the design intent of the application, it is essential that the mechanisms of degradation are fully understood.
Degradation of polymeric materials is the decomposition of the chemical composition of the polymer, resulting in an alteration to the physical properties of the material. Polymer scientists consider the onset of degradation of PVC to be the release of hydrogen chloride (HCL) from the material. The release of this gas indicates that the polymer backbone structure of the PVC is being changed by either chain scission, depolymerization, cross-linking, bond changes or dehydrohalogenation (which is the primary effect of heating this polymer). As PVC is inert to many chemicals as demonstrated by the confidence evident in engineers specifying unplasticized (in comparison to thin films) PVC pipe in harsh chemical environments, the degradation associated with PVC geomembranes in landfill environments is normally not the degradation discussed above, but rather the migration and volatilization of plasticizers from the geomembrane.
PVC resins are hard, brittle compounds (which is not an indication of low strength, only that rupture occurs without any noticeable prior change in the rate of elongation) due to the strong attraction bonds between hydrogen and chlorine atoms of adjacent polymer chains, resulting in secondary bonding between the polymer chains. In order to allow processing of thin films or geomembranes, plasticizers must be added to the resin so as to increase low temperature flexibility and softness. Plasticizers are clear, organic liquids which alter processing and physical properties. These compounds fall into two categories based on their compatibility with the resin; primary plasticizers used for end use plasticization and secondary plasticizers used for processing. There are many different types of primary plasticizers used in PVC, of which phthalates are the most common in PVC geomembrane production. Phthalates consist of a polar ester group and a linear group. The ester group bonds to the hydrogen-chlorine atoms of the PVC through Van der Waals bonding, while the linear group acts as a buffer between the polymer chains. As the lineal group separates the polymer chains, the attraction between the hydrogen and chlorine bonds between adjacent chains is reduced, resulting in a reduction of chain secondary bonding, and ultimately providing greater flexibility of the overall material. Secondary plasticizers are typically aromatics which are used to aid in the manufacturing of some PVC products. However, secondary plasticizers are not used in the manufacture of geomembranes, and therefore no further discussion is required of these compounds.
Typical PVC geomembranes contain 30 to 35 percent plasticizers per weight. Plasticizers on the surface of the geomembrane are subject to migration out of the product. The plasticizers within the sheet, which are secondarily bonded to the PVC chains, will require encouragement to migrate. The plasticizer loss is a function of plasticizer type, temperature, sheet thickness, environmental conditions and exposure time. The worst case for plasticizer loss is when there is a large gradient of organic compounds between the geomembrane and the surrounding environment. Here the gradient must be of sufficient energy to overcome the Van der Waals bonding of the ester group of the compound, allowing the linear group to separate the chains and provide migration paths between the chains. As the percentage of plasticizer is reduced, secondary bonding between the polymer chains increases, 'locking in’, the remaining plasticizers. Studies by the U.S. Bureau of Reclamation on 1O-mil PVC geomembranes used in canal linings how that 54 percent of the initial plasticizer content remain after 19 years of service. In this application, the organic gradient was very high due to running water within the canals, minimizing organic concentrations on the geomembrane surface. Even with a 46 percent reduction in plasticizer, the geomembrane still met most of the original design specifications.
The effects of plasticizer loss on the physical properties of PVC geomembranes is a loss in total weight, a slight reduction in sheet thickness, an increase of elastic modulus, an increase in tensile strength and a reduction in ultimate strain. Of these physical property changes, only the strain to failure is a cause of concern for landfill cover systems. However, laboratory testing indicates that if as much as 75 percent of the plasticizer is lost, the ultimate strain still exceeds
100 percent. This amount of strain to failure would easily accommodate typical differential settlements associated with municipal solid waste landfill cover systems. The laboratory study discussed above was extremely aggressive in terms of plasticizer loss in light of the Bureau of Reclamation study and several case histories of PVC geomembranes used in landfill applications.
Two recent case histories look at the use of PVC geomembranes in actual usage. The first article, "PVC Geomembranes and MSW Leachate," by Francis X. Taylor, looks at PVC geomembranes in a MSW landfill in Pennsylvania. Samples of 20-mil geomembrane were retrieved from the sump area of the landfill after being constantly exposed to leachate for 13 years. The break strain of these samples was in excess of 300 percent. Similar results were obtained from samples retrieved from a MSW landfill closure system in Florida after five years of service. This paper entitled "Examination of PVC in a 'Top Cap' Application" by Samuel B. Levin and Mark D. Hammond. Based on these observations as well as the excellent physical properties ( e.g. puncture resistance and multi-axial burst) and CQA inherent to PVC geomembranes, it is easily concluded that this material is an excellent choice for barrier layers within MSW landfill closure systems.
James A. McKelvey, III is an Associate Geotechnical Engineer with Roy F Weston, Inc., West Chester, PA.
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