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INTRODUCTION

IDENTIFICATION AND BEHAVIOR OF PVC

The identification of PVC Geomembranes can be accomplished by looking at the Geomembrane as a whole, or examining the individual components of the formula. The extent of identification should be governed by the degree of criticality of the application since each additional level of identification adds cost to the project. In most applications, the certifications supplied against the National Sanitation Foundation Standard (NSF) 54 would be acceptable. However, there may be some critical applications where additional testing such as EPA 9090 is required to compare geomembrane samples used for testing, versus geomembrane that is used at the project site. This paper discusses the various identification tests that can be conducted. It also discusses the effect of different raw materials in the geomembrane formula on its performance and physical properties.

RAW MATERIALS IN PVC GEOMEMBRANES

PVC geomembrane formulas are composed of several raw materials. The PVC resin and plasticizer make up the major raw materials. It is also comprised of several other raw materials including pigment, stabilizers, lubricants, possibly fillers, and biocides. To better understand the effect of these components, this paper will describe the individual raw materials used in PVC Geomembranes and what effect they have on the overall performance and physical properties of the PVC geomembrane.

PVC Resin

PVC resin is by far the largest component of the PVC geomembrane formula. PVC resin unlike some other polymers does not have variability in structure and composition. PVC resin used in the manufacture of geomembranes is referred to as a homopolymer. This means that the resin is polymerized using only vinyl chloride monomer. Suspension technology is used in the polymerization. Suspension technology in vinyl chloride monomer is dispersed in water prior to the reaction. The monomer forms small droplets within the water upon agitation. An initiator is added and the resin polymerizes within these droplets. Because only suspension resin is used by all manufacturers of geomembrane, the process and the physical properties in the finished product are very similar. The only property that may differ is the molecular weight of the resin. PVC resin requires simple testing to determine the property that effects the performance of the geomembrane. In the United States, this is generally referred to as the relative viscosity of the resin. In other countries, it is usually called inherent viscosity. It is determined by making a dilute solution of the PVC resin in cyclohexanone. The flow rate of the solvent and the polymer solution is then compared using a viscometer. The test method is ASTM 1243 - Standard Test Method for Dilute Solution Viscosity of Vinyl Chloride Polymers.

The relative viscosity of the resin affects both processing and physical properties. The higher the relative viscosity, the higher the temperature needed to process and fuse the compound. This can be modified by process aids and plasticizer, but primarily remains dependent upon the relative viscosity. High relative viscosity gives better physical properties.

Tensile strength, elongation, cold crack resistance, chemical compatibility, puncture resistance, and long term durability are all improved. For this reason, higher molecular weight PVC is used in the manufacture of the geomembrane. The relative viscosity is usually in the range of 2.3 to 2.6.

There is a limit to the molecular weight of the PVC resin that can be used. If relative viscosity is too high, the temperatures at which the compound will have to be processed will be so high that it will be impossible to stabilize the compound. Also, high relative viscosity resins are expensive, thus adding substantial cost to the formula.

Plasticizer

Plasticizer is the next major component of the formula. In PVC geomembrane, only primary plasticizers are used. Secondary plasticizers are not suitable for PVC geomembranes because of high volatility and poor compatibility. The primary plasticizers have the highest degree of compatibility with the PVC resin.

Primary plasticizers are grouped into two classes, monomeric or polymeric. They vary in their molecular weight. Included on the monomeric plasticizers are phthalates, adipates, and trimellitates. The polymeric plasticizers include various molecular weight plasticizers polymerized from adipic and glutaric acids in combination with various glycols. The end groups can also vary, including alcohols, esters and acids. Both the molecular weight and the end groups will affect the performance of the polymeric plasticizer. The molecular weight can be so high that the plasticizers are solid at room temperature, and are sometimes called alloys.

Primary monomeric plasticizers are used in the majority of the PVC geomembranes. It offers the best overall combination of performance, processing, efficiency and cost. Efficiency is a measurement of the amount of plasticizer that is required to reach a given flexibility. They provide good physical properties, and excellent chemical compatibility in most environments that are associated with geosynthetic applications. Hydrogen bonding between the PVC resin and the plasticizers gives plasticizers their high degree of permanence. If the geomembrane loses some plasticizer, the remaining plasticizer is even more tightly bonded due to additional hydrogen bonding, since the plasticizer that is remaining comes in closer contact with the PVC polymer chains. Manufacturers can supply specific information on the chemical compatibility of their geomembrane in different environments.

Polymeric plasticizers are typically used in oil resistant PVC geomembrane. Because of the large polymer chains associated with them, they are very tightly entangled with the PVC resin chains. This gives the oil resistant PVC geomembrane its excellent chemical compatibility in environments that contain higher levels of solvents and oils. The higher the molecular weight of the polymeric plasticizer is, the better its chemical resistance will be. There are, however, some properties that are sacrificed. The higher the molecular weight of the plasticizer, the lower its efficiency. Also, the low temperature resistance is not as good with polymeric plasticizers in comparison with monomeric plasticizer. A higher level of the polymeric plasticizer is required to reach the same flexibility and low temperature properties. With certain polymeric plasticizers, it is not possible to reach certain low temperature properties. Some types of polymeric plasticizer freeze at higher than the cold crack temperature specified.

Fillers

Fillers are sometimes used in PVC geomembranes and typically in low levels. They may be added to provide some functional purposes such as assisting in providing improved seaming properties, acting as biocides, improving processing, or acting to improve stability of the formula.

Stabilizers

Stabilizers serve several important purposes in a PVC geomembrane. The primary function is to stabilize the formula so that it can be calendered at elevated temperatures, 350 F or higher, into the geomembrane without degrading to the point that the physical properties are affected. It also provides UV stability that is needed for a specifically designed function. The stabilizer system also provides protection for the geomembrane during its expected lifetime. The stabilizers are metallic/organic complexes, usually metallic esters. They also contain antioxidants, phosphites, and acid scavengers. Those used in geomembranes are formulated to produce compounds with low volatility and good resistance to water extraction.

Lubricants

Lubricants are used to provide metal release of the compound during the calendering process. They are used at a level that while providing the needed lubrication does not interfere with the seaming process of the geomembrane during fabrication. They are generally complex esters.

Pigments

Pigments are used in PVC geomembranes to provide the obvious, color. They also act as UV stabilizers. Most PVC geomembranes are either black or shades of gray. Carbon black and titanium dioxide are the two pigments used to make these colors. Carbon black is an excellent UV protector, absorbing most of the UV radiation that strikes the geomembrane and converting it to heat. Titanium dioxide reflects almost all UV radiation. Hence, together they offer excellent UV protection. PVC geomembranes can be exposed for years with minimal UV degradation, using only these pigments and the normal stabilizer systems used in a PVC geomembrane. Tests conducted on PVC geomembranes exposed for over six months during installation showed absolutely no degradation of physical properties. (See Table 1 next page)

                        Table.1 – PVC Geomembrane Exposed Six Months

Samples, which contained the proper pigment levels, taken from geosynthetic projects exposed over 10 years can still pass NSF 54 Standard Specifications. (See Table 2.)

Miscellaneous

There are several miscellaneous raw materials that are used in PVC geomembranes to meet specific needs. They are not necessarily in every PVC geomembrane formula. These include biocides, UV additives, process aids, and impact modifiers. Biocides are added to resist any biological attack that the geomembrane may experience in the field and to meet the soil burial requirements for NSF standard 54. UV additives are added to PVC geomembrane specifically designed for outdoor exposure. Impact modifiers may be added to improve low temperature resistance, for those PVC geomembranes designed for low temperature applications.

IDENTIFICATION TESTS FOR THE PVC GEOMEMBRANE

The easiest and least expensive way to identify the PVC geomembrane is to examine the geomembrane intact, without extracting its individual components. This can be accomplished using the following methods:

Use the certification data according to NSF 54 standard.

Conduct Infrared (IR) analysis of the geomembrane.

Require Certificate of Analysis for the PVC resin used in the geomembrane.

Examine the physical appearance of the PVC geomembrane.

These methods are discussed below.

NSF 54 Standard Certification Data

There is a significant amount of information that can be used from the certification data generated on the PVC geomembrane according to the NSF 54 standard. The following physical and mechanical properties provide information about identifying the geomembranes: Specific Gravity, Tensile Strength, Modulus, Tear Resistance, Low Temperature Resistance, Dimensional Stability, Water Extraction, Volatile Loss.  These properties, if taken together, should provide the information necessary to assure that a sample of geomembrane used for testing, and one used in the actual project are the same material. These properties are discussed below.

Specific Gravity

The specific gravity of the final product is dependent on the specific gravities of all raw materials used in a formula and their percentages. The PVC resin, with a specific gravity of 1.4, is the largest contributor. The plasticizer, with the specific gravity about 1.0 is the next largest factor. A decrease of the specific gravity below the intended value is an indication of an increase in the plasticizer content. On the other hand, an increase of the specific gravity may be due to a decrease in the plasticizer or an increase in the pigment. A variation in the level of plasticizer would be reflected on the modulus value. In general, changes in the specific gravity of more than 0.05 can be interpreted as the result of a change in the formula.

Tensile or Break Strength

The tensile strength depends not only on the amount of plasticizer in the formula, but also the molecular weight of the PVC resin. For the same amount of plasticizer an increase of the molecular weight of the resin causes an increase in strength. On the other hand, a decrease in tensile strength suggests a decrease in the molecular weight of the resin. Certainly, there will be some variation in the tensile properties for individual samples. However, a statistical shift will be noticeable in the case of a change in the resin. In general, a variation in tensile strength is considered significant if it differs from the average strength by 300 psi.

Modulus

The amount and type of plasticizer control this property. Unlike the tensile strength, the modulus is not significantly affected by a change in the molecular weight of the PVC resin. A statistical change in the modulus of 200 to 300 psi may indicate a change in the type or amount of the plasticizer. The modulus at 100 percent strain is usually used being also indicative of the flexibility of the geomembrane.

Tear Resistance

The tear resistance is also a measure of the amount of plasticizer and the molecular weight of the resin. Their effects on the tear resistance follow the same trend as their effects on tensile strength.

Low Temperature Resistance

The low temperature resistance is governed by the resin molecular weight and the plasticizer type and amount. Higher molecular weight PVC resins give improved low temperature resistance. Monomeric plasticizers give better low temperature resistance than polymer plasticizers. Some monomeric plasticizers perform better than others. The higher the level of plasticizer in the geomembrane, the better the low temperature properties.

Dimensional Stability

Usually there are slight differences in the dimensional stability of geomembranes produced by different manufacturers. There is some variability in this property, so this should only be looked at if the change in dimensional stability is above 1 percent statistically and in combination with changes in other physical properties.

Water Extraction

Water extraction is determined by the types of plasticizer, stabilizer, lubricants, and other soluble material that could be part of the formulation. Meeting the requirements of the NSF-54 standard assures that proper raw materials have been used.

Volatile Loss

The volatile loss is determined by the plasticizer and stabilizer systems. The low values required by NSF-54 standards assure that secondary plasticizers and stabilizers with high volatility are not used.  A minor change in any of the properties does not mean that there has been a change to the formula. However, if several properties are affected, there is a possibility that the formulas are different. A further discussion is given later in the paper.

Infrared Analysis of the Geomembrane

Infrared (IR) analysis of the geomembrane provides information about the composition of the major raw materials in the formula. The IR of one geomembrane sample can be compared against another to determine if they have the same composition. It will not only pick up those peaks associated with the PVC resin, but also the plasticizer type. It may also show some indications of pigment type. Although by itself, it cannot prove the formulation is the same; differences in the IR spectra will show differences in formula.

As part of any good quality control program, manufacturers of PVC geomembranes will request the Certificates of Analysis be provided on the PVC resin that they use. Note that the PVC resin is the largest component of the PVC geomembrane formula. These certificates will show the relative viscosity (molecular weight), plasticizer absorption, manufacturer, contamination, color, percent volatiles as well as the manufacturer name. This is a good check to assure that the type of PVC resin used in various samples of the geomembrane are the same.

Physical Appearance

The surface appearance and color of the PVC geomembrane provided by the different PVC geomembrane manufacturers may be slightly different. Just by looking at the top and bottom of the geomembrane and comparing the color, you can usually determine if the geomembranes are supplied by the same manufacturer.

LABORATORY STUDY USING PVC GEOMEMBRANE

To determine the effect that changes in the formula have on the physical properties of the film, a laboratory study was conducted by Occidental Chemical. In the study, four variations of a flexible PVC formula were compounded and calendered using laboratory equipment. The formulas contain PVC resin, plasticizer, filler, pigment, stabilizer A, stabilizer B and lubricant. The levels of these raw materials were varied as outlined in Table 3.

 The films were produced at 20 mils to simulate a typical geomembrane. The film was then tested for specific gravity, tensile strength, 100 percent modulus, graves tear, volatility and low temperature impact. The test results are summarized in Table 4.

Comparison of data in Table 4 in view of the PVC formulations in Table 1 proves how the NSF 54 testing can be used to identify and compare PVC geomembranes.

For example, in formula one through four, all levels of raw materials are the same. The only difference is the type of stabilizer. Stabilizer A is manufactured to have low volatility, whereas stabilizer B is just a general purpose stabilizer. Examination of the physical properties indicates that only two parameters are significantly affected, namely, specific gravity and volatility, with the largest effect being on volatility.

The fact that stabilizer B is not a low volatility type, means that there are components in the stabilizer that are more volatile and show up in the volatility results. The increase in specific gravity indicates that there is a higher level of metals in the geomembrane containing stabilizer A versus stabilizer B.

Comparison between formulas one, two and three, indicates that the major difference in these formulas is the level of plasticizer. Although it affects several properties, the modulus is shown to provide the best measure of plasticizer level. If we examine formula one versus two, it can be seen that the 100 percent modulus increases from 1500 psi to 1900 psi as the level of plasticizer decreases from 31.3 percent to 27.8 percent. Similarly, comparing formula one to three suggests that the 100 percent modulus decreases from 1500 psi to 1200 psi as the level of plasticizer increases from 31.3 percent to 34.3 percent.

The change in plasticizer level also affects other properties of the film including specific gravity, tensile strength, graves tear, and low temperature resistance. Specific gravity, it can be seen increases as the plasticizer level is lowered and decreases as the plasticizer is raised. This is because the specific gravity of the plasticizer is about 1.0 compared to 1.4 for the PVC resin.

The tensile strengths, and graves tear, both follow the same trend. As the plasticizer increases, both decrease. As the plasticizer decreases, both increase. They follow the flexibility of the film.

Concerning the low temperature resistance, the higher the plasticizer level is,
the lower the temperature at which the film has a good crack resistance. It should be noted that the volatility did not change with the change in the levels of plasticizer. This is due to the fact that a primary plasticizer that had a low volatility was used.

IDENTIFICATION TESTS - INDIVIDUAL COMPONENTS

These include mainly the PVC resin, plasticizer, and pigment. The other minor components such as lubricants, stabilizer, biocide, and other miscellaneous components are difficult to analyze without expensive testing. Also, minor differences are difficult to determine.

Another method to identify and compare PVC geomembranes is by dissolving the geomembranes in an appropriate solvent, and separating the individual components. However, the method requires additional time and expense to conduct. It should be used only when the critical nature of the project justifies the added expense or in research work.

There are several levels of complexity for this analysis. Each level requires additional time and expense. They are:

Identification of Resin and Plasticizer

This method involves dissolving the geomembrane and separating the resin from the plasticizer using the appropriate wet chemistry. The process can be conducted according to ASTM D2124 -Standard Recommended Practice for Extraction and Analysis of Plasticizer Mixtures from Vinyl Chloride Plastics. The resin and plasticizer can then be analyzed using the appropriate test methods. The chemical composition of the PVC resin can be determined using infra red analysis and comparing it to a known chemical standards or curve. The molecular weight can be determined using ASTM 1243 - Standard Test Method for Dilute Solution Viscosity of Vinyl Chloride. The plasticizer can be analyzed using infrared and gas chromatography. Again the plasticizer must be compared against known chemical standards or curves. Polymeric plasticizers can be difficult to identify and may require gel permeation chromatography.

Identification of Major Components

This method uses the same process, ASTM D2124, to dissolve the geomembrane into the individual components of the formula. The major components, including the resin, plasticizer, filler pigments, and miscellaneous raw materials can then be separated and analyzed. The resin and plasticizer can be identified as in the above methods. The fillers, pigments and miscellaneous raw materials require infrared and gas chromatography as well as various wet chemistry methods to identify the individual components.

Identification of All Components

This last method takes the procedure used in "Identification of Major Components" but uses other more sophisticated methods to identify minor components of the formula, such as metal analysis. This uses analytical techniques such as atomic absorption, and other state of the art analysis techniques, such as microwave digestion and inductively coupled plasma analysis. This is by far the most expensive type of analysis.

LABORATORY STUDY TO DETERMINE THE ACCURACY OF ANALYSIS FOR INDIVIDUAL RAW MATERIALS

To determine the accuracy of the analysis method, we analyzed the laboratory films using the methods outlined in "Identification of Major Components." The results of this analysis are summarized in Table 5.

Comparison between tables one and four shows that there is agreement between the actual percentages of the raw materials in the formulas and the percentages based upon analysis. The data agrees within one to two percent with all raw materials and all formulas, except for the minor components, which includes the stabilizer and lubricants. Note that the accuracy of this data will be dependent on the quality of the analysis.

CONCLUSION

The identification or comparison of PVC geomembranes can be performed using several methods. For most applications, this can be accomplished using information supplied with the PVC geomembrane, i.e., the certifications according to NSF 54 standards. This data provides a wealth of information that allows comparison between different samples of PVC geomembrane.

If additional identification work is considered for a specific project, it should be discussed with the PVC geomembrane manufacturer during the time that the specification is being planned. The manufacturer can help to determine what would be the best and least expensive approach in providing the information that is needed.

This article is reprinted with permission from the Geosynthetics Research Institute and IFAI. For a copy of the proceedings from the 7th GRI Seminar, ''Geosynthetic Liner Systems: Innovations, Concerns and Designs'", contact Mary Snavely, IFAI, 345 Cedar Street, Suite 800, St. Paul, MN. Telephone: 612-222-2508 or Fax: 612-222-8215.

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