POSITION PAPER ON THE USE OF PVC LINER IN LANDFILLS AND WASTE WATER TREATMENT PLANTS FOR THE MICHIGAN DEPARTMENT OF NATURAL RESOURCES
In the choice of a flexible membrane liner or cap, it is generally accepted that there is not a universal material that provides the best performance in every requirement for this application. It is the position of OxyChem, in offering this flexible PVC liner material, that OxyFlex™ PVC offers the best overall performance in applications involving landfill liners, landfill caps and waste water treatment liners, based upon actual performance of PVC liners and technical data relating to these particular end uses.
In this paper, I will address the concerns that you have voiced in the choice of PVC over other FML materials. I will try to keep this as brief as possible and still state the facts in a clear fashion. There is much more material concerning this subject, some of which I have attached for more in depth reading. Of course, I would welcome an opportunity to further discuss these issues at your convenience.
I will address the following topics for which you have a concern.
1. Light degradation
2. Loss of plasticizer
3. Insufficient strength in material under 50 mils
4. Pinholes in material under 50 mils
5. Insufficient seam strength
6. Chemical compatibility
It is a fact that long term exposure to UV radiation can lead to the reduction of physical properties of PVC materials. It should be noted that PVC films are routinely used in outdoor applications such as graphic/decorative decals, examples being automotive pinstripes, truck fleet marking decals, tents, industrial tarpaulins and outdoor advertising. PVC is the material of choice for all theses applications, one reason being its excellent outdoor performance characteristics.
To examine this particular application, the FML is exposed only as long as it takes to place it at the site and cover it with a protective soil cover. This usually is a short period of time, say 3 or 4 months. However, to determine the actual endurance of OxyFlex PVC as compared to HDPE, a 40 mil sample of the OxyFlex PVC and a 60 mil sample of HDPE were exposed to 1,000,000 Langleys (a unit of light measurement) in real time EMMAQUA testing in Arizona by the outside test firm, DSET. This testing correlates to 4 to 6 years of actual outdoor exposure depending upon the particular area of the country. Even after this exposure, the PVC exceeded the requirements for NSF Standard 54 for the properties tested; 100% modulus, tensile and elongation. The HDPE, on the otherhand, could not be tested for physical properties because the material was destroyed in the testing by the sunlight. The US Bureau of Reclamation was commissioned by the EPA to conduct the same real time EMMAQUA test using 5" wide seam samples, which included PVC and HDPE materials. Again the HDPE samples were destroyed by the sunlight, but the PVC samples were still sound. Light exposure of the PVC during installation would have no measurable effect on its physical properties.
The next concern, loss of plasticizer, again has lead to some misconceptions. To better understand what plasticized PVC is, one needs to understand the relationship between PVC resin and plasticizer. Plasticizer is a liquid which is very compatible with the PVC resin molecule. The plasticizer imparts the flexibility into the PVC liner. Both the plasticizer and the PVC resin are very long chained organic molecules and as such are difficult to separate once mixed, due both to their chemical affinities and their size. The plastizer acts as a lubricant allowing the PVC molecules to slide over each other. This allows them to slide back and forth when put under stress. This gives the PVC the ability to rebound when stresses are removed. The HDPE and the VLDPE on the other hand, are long crystalline chains, which once stress is introduced, do not rebound but stay in their elongated form causing extreme thinning and stress cracking associated with HDPE.
It is true that plasticizer can be extracted from PVC using certain concentrated organic solvents. In situations where the liner would be exposed to this environment, we would not recommend OxyFlex PVC. However, in the typical landfill the levels of organic solvents are very low, usually less then 1%. These very small levels would not have the same effect on liners because the other components would interact in a complex manner.
PVC liners have been in use since the 1950's and samples of the liners have been examined and remain totally functional after many years.
Dr. Henry Haxo recovered 20 mil PVC liner from the Lycoming County Pennsylvania Landfill. The sample was taken (July, 1984) from near a leachate collection sump during the excavation of a leachate pipe and in all probability had been fully immersed in leachate for the entire 6 years the facility had been in service (since June, 1978).
A full set of physical property tests along with analytical tests were performed on the field sample and compared with similar test results from a new unexposed 20 mil PVC sample obtained from the same manufacturer. The exposed sample test results were also compared with the NSF Standard 54 test values for 20 mil PVC liner.
Quoting from the Haxo report:
"The PVC FML remains flexible and useful with almost 90% retention of all original physical properties. It can also be seen that the physical properties after the exposure exceeded the minimum values specified for a 20 mil PVC in the NSF Standard 54, Flexible Membrane Liners."
In another report, a 20 mil PVC FML that was used to line a 5 acre pond, was tested after 17 years under water with no earth cover. Again, the material still met all the requirements for the NSF Standard 54 for Flexible Membrane Liners.
In testing of 30 mil OxyFlex PVC, using the EPA Method 9090, the recognized test method for liner compatibility, the PVC again passes the requirements for the NSF Standard 54 after testing. The PVC was immersed in a leachate from the Grand Central Landfill for a period of 120 days at 23°C and 50°C. Additional testing is in progress for 40 and 50 mil OxyFlex PVC. The results of which will be available in December, 1989.
According to Bob Landreth of the EPA,
"It has long been recognized in the industry that PVC materials have the longest history of containing municipal solid waste leachate."
The next concern is the insufficient strength of material under 50 mils. This is a little puzzling to me since a reference to a specific physical property is not made. PVC liners at 20 mil have been successfully used in the containment industry for over 35 years with a great deal of success. I realize that thicker liners are now specified to provide an extra margin of safety. The puncture resistance of PVC liners compared to HDPE has been shown to be superior in real world three dimensional stress testing. In a paper by Ronald K. Frobel of the US Department of the Interior, he reports in testing conducted on various FML for puncture resistance placed under pressure over a subgrade configuration that:
"The HDPE reaches its yield point rapidly and ruptures over a sharp protrusion, whereas the PVC continues to elongate and form over a protrusion. "
In a similar study commissioned by the EPA, 30 mil PVC was compared to 100 mil HDPE. The results were the same. Even with the gauge differential, the 30 mil PVC material did not puncture, whereas the 100 mil HDPE failed at cone heights of 1 inch.
The manufacturers of HDPE do recommend that 60 mil HDPE liners be used to help in bonding the seams due to difficulty in bonding this crystalline material and the stress cracks that occur at the seams. This may have carried over into all FML.
The physical properties of PVC along with test results such as those above are the basis for our recommendation for OxyFlex PVC FMLs under 50 mils. The gauge recommendation will vary depending upon the specific application.
Next, concerning pinholes in PVC film. I would like to know what study was done to provide this information. The chance of pinholes in OxyFlex PVC FML of 20 to 60 mil is nonexistent. Our technology allows us to produce millions of pounds of PVC films yearly in the thickness down to 3 mils for other applications, where the need for a pinhole free film is critical. Many of our PVC films are used in inflatable applications, swimming pools and other medical applications which require film integrity to perform, at much lower gauges than used in FMLs. To suggest that PVC FMLs have a pinhole problem is total unfounded and not true.
Seam strength is certainly a major consideration in choosing a FML liner. PVC liners, because of their flexibility, can be factory seamed into much larger panels than more rigid polymers. This reduces the number of field seams by 80% compared to a HDPE liner. Because the factory seams can be made under a controlled environment, the factory seams have higher bond strength. Additionally the field seams, using a solvent or adhesive, are very simple to accomplish and conduct QA testing. OxyFlex PVC is formulated specifically to allow the material to obtain a seam bond stronger than the virgin material. HDPE on the other hand, requires sophisticated equipment to seam. Even then the grinding process used and the stresses induced into the liner at the seam points has lead to numerous failures due to stress cracks of the HDPE. Also checking the seam integrity is difficult and time consuming.
A memorandum issued by the EPA states the following:
" As a result of documented failures of liner seams to contain waste at some surface impoundments, EPA is evaluating the potential for stress cracking of high/medium density polyethylene (HDPE) liner materials."
The report continues:
"Two HDPE properties may also weaken the long-term stress-crack resistance of the polymer significantly. First, as an HDPE liner ages, the amount of crystallinity increases, and the number of tie molecules decreases. And second, elevated temperatures promote oxidation of stabilizers added to the HDPE to retard the liner's breakdown. HDPE liners are seamed using heat to fuse together adjacent sheet along the edges where they overlap. Material next to the seams may be made more brittle and weaker due to the very high temperature required for fusion. "
The report goes on to offer practices to reduce the chance of this occurrence and how to look for and report stress crack failures. PVC, since it is not a crystalline polymer is not subject to stress cracking failures.
Lastly, the concept of chemical compatibility. I have already touched upon this in my discussion on loss of plasticizer. I would like to elaborate on this slightly. The OxyFlex PVC passes the typical landfill leachate with flying colors in the EPA 9090 testing. In this test the FML in immersed in the leachate for ultimate contact. In reality the FML is covered by a protective soil cover of 1 foot or more. This forms a barrier and protects to liner from intimate exposure to the leachate, further improving its performance.
In laboratory testing, OxyFlex PVC has been exposed to pH's ranging from 2 to 12. The material was tested for three months at temperatures of 73°F, 100°F and 158°F. Physical testing of the material showed no significant change in any physical property. The OxyFlex PVC met the requirements for NSF Standard 54 for PVC Flexible Liner after the exposure period.
I would agree that for an application where the PVC FML is exposed to concentrated organic solvents which would attack the polymer, we would not recommend it. However, for landfill liners and caps, waste water treatment plants and related applications, this is not the case and OxyChem guarantees it!
In summary, I would like to quote from a article by Buranek and Pacey presented to the Geosynthetic '87 Conference on geomembranes:
"The choice of liner system materials continues to be very limited. HDPE and LLDPE have excellent durability, chemical/biological resistance, and weldability; however, their mechanical properties leave much to be desired. Specifically; the are too stiff and therefore very difficult to handle in the field; their sensitivity to notches and scratches limits the design strains to 5-8% in spite of their ability to elongate 500-700% in the un-notched state; and they have relatively high coefficients of thermal expansion/contraction and consequently are very difficult to place in varying temperature conditions (it is common for exposed HDPE/LLDPE geomembrane to be taut and perfectly wrinkle-free after a cool night and completely "cross-hatched" with large wrinkles by that afternoon). Some manufacturers are working to develop new products which will combine the durability, chemical/biological resistance and weldability of HDPE/LLDPE with the excellent mechanical properties of other materials, say PVC. "
HDPE suffers from being a crystalline material susceptible to stress cracking. It can be a time bomb waiting to go off in a landfill. When you consider that the thermal expansion of HDPE is three times that of PVC, it is not a logical choice in a environment such as Michigan which can have large temperature swings. And when it does stretch, it cannot rebound, but becomes very thin at points where it does yield, unlike PVC.
Our stand is that there is no FML presently that meet all the requirements. We continue development to find one. But lets examine the benefits of OxyFlex PVC FML:
1. PVC has superior puncture resistance in actual usage. It conforms to ground settling and irregular subgrades without yielding.
2. PVC has a reliable proven history of performance in gauges down to 20 mil since the 1950s.
3. PVC has the ability to produce a strong reliable seam consistently.
4. PVC provides excellent chemical resistance in landfill and waste water treatment applications.
5. PVC offers UV protection which assures that the polymer does not lose its original physical properties while it is being installed.
6. PVC has the flexibility for efficient installation in the field.
7. PVC allows for large factory seamed panels, which reduces the number of field seams by 80%
I certainly appreciate this opportunity to explain OxyChem's position on PVC flexible membrane liners. I look forward to further discussions on this subject in person on some future date at your convenience.
David C. Lauwers
Manager, Product Development
Occidental Chemical Corporation
For more information call 800-OK-LINER today!