Comparison of 30 mil PVC and 60 mil HDPE Geomembranes

 

 

 MARCH 1999

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Comparison of 30 mil PVC and 60 mil HDPE Geomembranes

According to ASTM D 4439 Standard Test Method, a geomembrane is a geosynthetic membrane or barrier with low permeability used with any geotechnical engineering related material so as to control fluid migration from a man-made project, structure, or system. Two common materials used to manufacture geomembranes are polyvinyl chloride (PVC) and high density polyethylene (HDPE). At present HDPE and very flexible polyethylene (VFPE) account for 60 to 65% of the geomembrane market while PVC geomembranes account for 20 to 25%. Other types of geomembranes, e.g. chlorosulphonated polyethylene and flexible polypropylene, account for the remainder of the market. The main objective of this technical bulletin is to illuminate the advantages of PVC and HDPE geomembranes. This will be accomplished by comparing various properties for each of these materials.
 

Thickness:

Of course, the first difference between PVC and HDPE geomembranes is that the US EPA regulations require a thickness of 30 mil for PVC and 60 mil for HDPE geomembranes. A thicker HDPE geomembrane is required for a number of reasons including the ability to weld without damage to the liner, increased strain to tensile yield, greater stress crack resistance (discussed subsequently), and less susceptibility to folding which can lead to stress cracking.
 

Formulation:

The commonly used formulations for HDPE and PVC geomembranes are shown in the following table.

It can be seen that the HDPE formulation contains no plasticizer and that the majority (96-97%) of the formulation is the HDPE resin. It should be noted that the composition of the HDPE resin differs from manufacturer to manufacturer. Manufacturers have tried to develop HDPE resins that are more resistant to stress cracking, chemical attack, oxidation, and more cost effective. This allows HDPE geomembrane manufacturers to differentiate their products. It should also be noted that HDPE is a misnomer in the industry because the density of the HDPE resin ranges from 0.934 to 0.938 mg/l (Koerner 1998), which is actually in the medium-density range (MDPE). Of course, geomembrane durability increases with increasing resin density; however increased resin density also results in increased stress cracking and cost. In fact, two major reasons for using MDPE instead of HDPE for geomembranes is increased stress crack resistance and lower cost.

In summary, HDPE is a geomembrane "trade" name and different resins and formulations are used to create MDPE, i.e. HDPE, geomembranes. Conversely, a plasticizer is added to impart flexibility and processability to PVC geomembranes. If a significant amount of plasticizer is lost due to improper use, one of the most important advantages of PVC geomembranes, flexibility, will be reduced. (The yield point for HDPE geomembranes occurs at a strain of 5-15% whereas PVC geomembranes undergo 200-300% strain before yield.) As a result, the new generation manufacturers utilize primary plasticizers that enhance plasticizer retention. Based on this discussion it is recommended that site specific testing of geomembranes be conducted to ensure that the desired properties will be achieved because each geomembrane is different.
 

Chemical Compatibility:

The main advantage of HDPE geomembranes is their better chemical resistance to hydrocarbons and solvents (Vandervoort 1992). However, this data was generated using EPA Method 9090, which does not consider chemical resistance under the stresses that will be imposed on the liner system. The semi-crystalline nature of HDPE may make it more susceptible to stress cracking when tested under stress in the presence of leachate. In addition, this testing was conducted using HDPE and not MDPE geomembranes. MDPE geomembranes are less chemically resistant than HDPE geomembranes because chemical resistance increases with resin density. While the chemical resistance of HDPE to hydrocarbons is desirable, it results in a number of less than desirable characteristics including stress cracking, poor subgrade conformance, low interface friction, and poor axi-symmetric tensile elongation (Koerner 1998). Vandervoort (1992) showed the difference in chemical resistance between HDPE and PVC may be significant for allphatic and aromatic hydrocarbons and chlorinate, oxygenated, and crude petroleum solvents. Since municipal solid waste (MSW) leachate has been found to be fairly neutral, pH around 7, and the major inorganic constituents are lead and cadmium (Oweis and Khera 1998), PVC geomembranes are very suitable for MSW landfills (see photo below). Successful 9090 testing of numerous PVC geomembranes supports this conclusion and PVC geomembranes have been used for MSW landfill liners since approximately 1980. It should be noted that PVC geomembranes can be formulated to provide chemical resistance to specific environments, e.g., oil-resistant PVC, which will be the subject of a future PGI Technical Bulletin. In summary, HDPE and PVC geomembranes should be formulated to resist the site-specific environment but it appears that both geomembranes will provide adequate chemical resistance for MSW landfills.
 

Stress Cracking:

Polyethylene is formed by the polymerization of compounds containing an unsaturated bond between two carbon atoms. This results in a high crystallinity that makes it resistant to a wide range of chemicals but also increases its tendency to rupture under stress. Stress cracking, which has been frequently observed in the field, refers to failure of the geomembrane under stress in a brittle manner exhibiting little or no elongation adjacent to the failure surface (Hsuan 1998). The fundamental governing factor for stress cracking is the polymer characteristics, among which crystallinity and molecular weight are most important (Hsuan 1998). Of course, PVC is an amorphous thermoplastic, and not a crystalline thermoplastic, and thus is not susceptible to stress cracking. However, it can be susceptible to plasticizer migration, which has been addressed by the manufacturers who have developed new primary plasticizers and stabilizers to enhance plasticizer retention.
 

Installation and Wrinkles:

It is universally recognized that field prepared seams are potentially the most problematic. PVC geomembrane rolls are factory seamed to produce large panels that greatly reduce the number of field seams. In a given liner area, the length of field seam required in a PVC liner may be 80% less than that required for a similar polyethylene geomembrane. This reduces the potential problems associated with field seaming and the cost and duration of field installation. PVC and polyethylene geomembranes can be seamed using either single or dual-track wedge welding equipment. This allows long continuous seams to be constructed in a wider range of environmental conditions. In addition, the use of a dual-track wedge welder allows air-channel testing to be conducted to nondestructively test seams of both PVC and polyethylene geomembranes. In summary, the same techniques for conducting CQA and CQC programs can be used for both PVC and HDPE geomembranes because both liners can be seamed using a dual-track wedge welder.

HDPE geomembranes provide less conformance to subgrade materials due to their stiffness and relatively high coefficient of thermal expansion. This can lead to large wrinkles or waves in the liner system (Koerner et al. 1997) that preclude conformance with regulations which require intimate contact between the geomembrane and underlying CCL or GCL. In addition, these waves can disrupt the leachate collection and removal system by creating obstructions to flow and result in less than 1 foot of drainage material being placed above the wave. The presence of these waves can also delay construction because it may be undesirable to place the soil or geosynthetic drainage material over the geomembrane during the hottest part of the day. At the hottest part of the day, the waves are the greatest and they can be increased by the placement or spreading of the drainage material. It is more desirable to wait for a cooler time of the day so the HDPE waves will be smaller and the equipment is less likely to encounter and damage a wave. Giroud (1995) showed that the amount of wrinkling decreases with an increase in interface strength between the geomembrane and the subgrade. As a result, Giroud (1995) concludes that flexible geomembranes, such as PVC, exhibit smaller wrinkles than smooth HDPE geomembranes because of the higher interface strength. PVC also has a lower coefficient of thermal expansion and its high elongation allows for better field performance, especially where differential settlement is a concern.
 

Interface Strength:

A number of slope failures, e.g. Boschuk (1991) and Seed et al. (1990), have shown the importance of soil/geomembrane and geosynthetic/geomembrane interface strengths on the stability of geomembrane lined slopes. A large amount of literature is available on geomembrane interface strength and it is clear that the smoother, harder geomembranes, e.g., HDPE, exhibit lower interface friction values than softer and rougher geomembranes, e.g., PVC. In recent years, textured HDPE geomembranes have been used to meet or in some cases exceed the interface strength developed with a smooth PVC geomembrane.
 

Summary:

PVC and HDPE geomembranes have performed well in the past. If they are designed and installed properly they will continue to perform well in MSW landfills in the future. However, when considering construction and performance factors, such as placing cover soil over wrinkles, stress cracking at folds, wrinkles, and welds, interface friction, and installation CQA/CQC, PVC may be the liner of choice.
 

References:

Boschuk, J. (1991). "Landfill Covers – An Engineering Perspective," Geotechnical Fabrics Report, Vol. 9, No. 2, March, pp. 23-24.

Giroud, J.P. (1995). "Wrinkle Management for Polyethylene Geomembranes," Geotechnical Fabrics Report, Vol. 13, No. 3, April, pp. 14-17.

Hsuan, Y.G. (1998) "Data Base of Field Incidents Used to Establish HDPE Geomembrane Stress Crack Resistance Specification", Proceedings of 12th GRI Conf. on Field Installation of Geosynthetics, Drexel Univ., Philadelphia, PA, pp. 153-176.

Koerner, R.M. (1998). Designing with Geosynthetics, Fourth Edition, Prentice-Hall, Inc., NJ, 761 p.

Koerner, G.R., Eith, A.W., and Tanese, M. (1997) "Properties of Exhumed HDPE Field Waves and Selected Aspects of Wave Management", Proceedings of 11th GRI Conf. on Field Installation of Geosynthetics, Drexel Univ., Philadelphia, PA, pp. 152-162.

Oweis, I.S. and Khera, R.P. (1998). Geotechnology of Waste Management, Second Edition, PWS Publishing Company, MA, 472 p.

Seed, R.B., Mitchell, J.K., and Seed, H.B. (1990). "Kettleman Hills Waste Landfill Slope Failure. II: Stability Analysis." Journal of Geotechnical Engineering, ASCE, Vol. 116, No. 4, pp. 669-689.

Vandervoort, J. (1992). The Use of Extruded Polymers in the Containment of Hazardous Wastes, Schlegel Lining Technology, Inc., The Woodlands, TX.

 

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