It is widely believed by some within the geosynthetic community that PVC geomembranes degrade after installation. While it is true that PVC may degrade when in contact with some chemicals, the same holds true for other geomembranes such as 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.
PVC resins by themselves are hard, brittle compounds 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 facilitate the processing of geomembranes, plasticizers are added to the resin to increase low temperature properties, elongation, and flexibility. Plasticizers are clear, organic liquids that improve processability and provide the physical properties associated with PVC geomembranes. These compounds fall into two categories based on their compatibility with the resin; primary plasticizers, those that have a high degree of compatibility with the PVC resin matrix, and secondary plasticizers, which are used in some markets other than geomembranes to lower the overall cost. There are many different types of primary plasticizers used in PVC, of which phthalates are the most common in PVC geomembrane production, because they provide the geomembrane with the best balance of properties.
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 that are secondarily bonded to the PVC chains, 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 have sufficient energy to overcome the Vander 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 10-mil PVC geomembranes used in canal linings show that 54 percent of the initial plasticizer content remained after 19 years of service. In this application, the organic gradient was very high due to running water within the canals minimizing organic concentrations from the geomembrane surface. Even with a 46 percent reduction in plasticizer, the geomembrane still met the original design specifications.
Case studies of geomembranes under actual conditions contain some of the most important information that can be gathered to determine long term performance. Opportunities to evaluate the performance of the geomembrane under these conditions are limited because of the expense of excavating the geomembrane and the fear of disturbing the geomembrane in an attempt to obtain a sample. This leaves only limited opportunities to investigate the durability, such as when sites are being expanded or require modification. To increase the number of case studies the PGI has initiated a research project with the Minnesota Department of Natural Resources. The main objective of the project is to investigate the long-term (30 year) durability of PVC geomembranes and seams. Samples of different PVC geomembranes and seams are being obtained annually from a double lined settling basin that contains mine drainage. Since this project is in the second of the 30 year duration, other case histories were sought to provide an insight to the long-term durability of PVC geomembranes. The following paragraphs describe such a case history.
In 1993 a golf course pond was being enlarged and the existing PVC geomembrane was excavated in the process. The site was at the Lake of the North Golf Course located in the northern part of the lower peninsula of Michigan. According to Jerry Matthews, the golf course architect who originally designed the project, the PVC geomembrane was installed in the summer of l968. The material, a 10 mil PVC geomembrane, was originally covered by twelve inches of sand. Approximately six to eight inches of silt had accumulated over the sand during the 25 year period from 1968 to 1993.
The climate is harsh with winter temperatures falling well below 0° F and summer temperatures rising to above 90° F. Also based upon some other previous studies of plasticizer extraction, rainwater may be more severe than a typical municipal landfill leachate. This phenomenon is due to the fact that there is a larger gradient for plasticizer migration in water than leachate, due to the lack of organic compounds in the water. Lastly, this geomembrane was only 10 mil thick. Changes to a PVC geomembrane will occur more quickly with this gauge than the thicker gauges, 20 to 40 mils, that are typically used on projects today. For all of these reasons, this site provided some meaningful information relating to the long-term performance of PVC geomembranes.
Samples were taken to evaluate the physical properties of the parent material and the factory and field seams. In 1968, all seams were made using a chemical fusion weld. Physical testing according to NSF Standard 54 for PVC geomembranes was conducted, along with chemical analysis of the film. The specific tests that were conducted included thickness, specific gravity, tensile, elongation, 100% modulus, and tear resistance. Peel and shear tests were conducted on both the factory and field seams. The test results are summarized in Figures 1 and 2. There are several things that are immediately apparent from this data.
- The physical properties still exceed the requirement of NSF Standard 54 - 83 even after 25 years. In fact they exceed the NSF 54 Standard by a large margin.
- There was no deterioration of the seams by the peel and shear values in either the field or factory seams. All of the peel tests on the factory and field seams resulted in a film-tearing bond.
The analytical tests confirm that the formulation of this PVC geomembrane has changed very little over the 25 year period. Existing PVC geomembrane formulas have about 30% plasticizer, versus the 27.8% found in this geomembrane. (An unexposed sample of the geomembrane was not available for comparative purposes so the remaining plasticizer content was compared to the formulation of a current geomembrane, i.e., 30% plasticizer.) It may be concluded that the geomembrane had reached a steady state with the surrounding harsh environment and it was not losing any additional plasticizer.
The samples themselves were still very flexible with no sign of deterioration or cracking of the surface. There appeared to be no physical signs that would indicate that the geomembrane had not functioned as designed for the 25 years it was in service.
This study only adds to the growing amount of information that suggests PVC geomembranes are a viable choice for a wide range of applications. The fact that they do contain plasticizer is not the problem that some people would believe, but gives the geomembrane the flexibility that is important in geosynthetic design, installation, and long-term service.
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