Chemical resistance of geosynthetic materials

Geotextiles and Geomembranes I I (1992) 61-98

Chemical Resistance of Geosynthetic Materials

P.E. C a s s i d y , M. M o r e s , D J . K e r w i c k , D.J. K o e c k Department of Chemistry,SouthwestTexas StateUniversity,San Marcos,Texas 78666,USA K.L. V e r s c h o o r & D.F. W h i t e TRl/lnternational, Inc., 9063 Bee Caves Road, Austin, Texas 78733, USA (Received I1 November 1990: accepted 2 January 1991)

ABSTRACT In our technical society, environmental safety through controlled technological advances has become increasingly more important. As a result, the need to study the long-term durability and resistance of geosynthetics in contact with hazardous waste has been evident for some time. For this purpose, a study to develop a technological base for geotextiles, geonets and pipes used in containment facilities was undertaken. Information gathered from literature. government agencies, academic institutions and industry provided the necessary background for assessment. Geosynthetic materials, exposure media, mechanical and chemical test methods are reviewed.

1 INTRODUCTION Geosynthetic materials provide m a n y of the design functions necessary for construction o f h a z a r d o u s waste storage facilities at a fraction of the cost o f previously used natural construction materials. Although extensively studied for other engineering applications, these materials must n o w sustain new stresses not previously assessed in their earlier applications. Geosynthetics must demonstrate resistance to environmental a n d chemical degradation while in contact with aggressive chemicals a n d still maintain physical a n d mechanical properties 61 Geotextiles and Geomembranes 0266-1144/91/$03.50© 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain.

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PE. Cassidy et al.

inherent to their design specifications. Recognizing this, the US Environmental Protection Agency (EPA) has provided standards to ensure that minimum technological requirements for safe landfill operation are met both now and in the future (Schwope et al., 1985; Bellen et al.. 1987; Matrecon Inc., 1988; Landreth, 1989). EPA Method 9090 for geomembranes assesses the chemical resistance of polymeric liner materials with waste liquids by laboratory simulations of physical and mechanical stresses that liner materials would experience in situ. Although the specification for geotextiles, geonets and pipes used in hazardous waste containment facilities has increased in recent years, no comprehensive guidelines for standardization testing are currently available. The scope of this article will be to review both those test methods currently being used and the test methods proposed for future use. Although adaptations of EPA Method 9090 have been used extensively in analyzing geotextiles, geonets, geogrids and pipes used in geotechnical applications, no industry-wide standards are currently available to compare the design capabilities of geosynthetics (Hoare, 1986; Montalvo, 1989: Tisinger, 1989a,b: White & Verschoor, 1989: Landreth, 1990). Microstructural techniques such as thermal analysis, spectroscopy, chromatography and microscopy are currently being tested to evaluate the changes in the base polymer when subjected to aggressive chemical media for extended periods. The relationship between the structural changes experienced by the polymer and the changes in the mechanical properties of the manufactured geosynthetic material can then be compared to better predict possible failure modes of construction materials when designing containment facilities. Accelerated aging laboratory tests need to be developed to aid in the prediction of geosynthetic performance after long-term exposure to hazardous chemicals and environmental stress. These test methods must correlate changes in chemical and physical properties during the aging period to real-time serviceability in actual engineering applications. Geotextiles, geonets, geogrids and pipes were studied to better understand their composition, manufacturing techniques and design functions. Chemical resistance studies ofgeosynthetics were reviewed to clarify the current criteria in use in characterization of degradation processes in these materials. Exposure procedures presently used were studied to facilitate recommendation of a standard procedure for industry-wide application. Specialized analytical chemical techniques which examine microstructural changes in the base polymer were scrutinized to determine which procedures could be adapted for base polymers as well as manufactured geosynthetic materials. A test method that could be utilized by the geosynthetic industry must accommodate

Chemical resistance of geosynthetic materials

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the above restraints while simulating actual field conditions and giving meaningful results within a reasonable time.

2 GEOSYNTHETIC MATERIALS Koerner (1990) has provided an excellent and in-depth review of the function and structure of geosynthetic materials. The following sections are summarized primarily from this source. 2.1 Geotextiles

The use of geotextiles has grown phenomenally, both in number and in application, in the past l0 years. Although made of synthetic rather than natural fibers, these materials are textiles in the traditional sense, but are not as subject to biodegradation as are natural analogs. Geotextiles are porous to water flow both normal to the manufactured plane and within the plane of the fabric itself. The degree of porosity, which may vary widely, is used to determine the applications of the specific fabric. Geotextiles vary in the type of polymer used, the type of fiber and the fabric style. Fiber types and fabric styles have been developed for use in a number of general and specific applications. The vast majority of polymers used in geotextiles are derived from hydrocarbons. The chemical and environmental endurance required of many of the fabric applications can be traced to the type of polymer used in the geotextile's construction. Fibers used in geotextiles are predominantly made from polypropylene (PP), polyethylene (PE), poly (ethylene terephthalate) (PET) and polyamide. The polymers are formed into fibers by first being melted and then forced or extruded through a small opening (spinneret), known as melt spinning. The resulting filaments are then hardened or solidified by cooling. Subsequent to or during the cooling process, the fibers are stretched or ~drawn'. Drawing increases the fiber's strength as the polymer molecules in the fibers align themselves in a more orderly fashion and subsequently crystallize when possible. The fiber bundles are formed into monofilament yarn and then processed. Unlike monoor multifilaments, staple fibers are formed by chopping a continuous filament into lengths of one to four inches which are then formed into a rope-like bundle called a 'tow'. These staple fibers are then twisted or spun into yarns for subsequent fabric manufacture. Of completely different origin, slit-film fibers are made from a continuous polymer sheet and then cut with knives or lanced by air jet. The resulting ribbon-

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PE. Cassidy et al.

like slit-film fibers are then converted into geotextile fabrics by conventional processes (Koerner, 1990). The finished fibers are made into fabrics that may be woven, nonwoven or knit. Fabrics are woven on conventional weaving machines to produce a wide diversity of fabric weaves. Kaswell (1963) gives an excellent review of weaving technology with clear illustrations of various fabric weaves. The numerous variations have a major influence on the physical, mechanical and hydraulic properties of the resulting geotextile. Developed to serve in at least 80 application areas, geotextiles always perform at least one of five distinct functions: separation of layers, reinforcement, filtration, drainage, and moisture barrier (in coated form). 2.2 Geonets

Geonets, unlike geotextiles, are relatively stiff, netlike materials with large open spaces (0.9-5.0 cm) between the structural ribs and serve primarily in a drainage role. Although structural stability is afforded in many applications, geonets may cause considerable instability on side slopes. Geonets are generally extruded and have three-dimensional structures (USEPA, 1984, 1985a, 1986a; Netlon, 1987; USEPA, 1988). Nondeformed nets are used primarily as a core material to provide planar flow in drainage systems. Geonets are commonly used as components in geocomposite systems applications utilized for drainage or reinforcement (USEPA, 1985b, 1988; GRI, 1989a.b.c: Yazdani & Norbert, 1989). 2.3 Geogrids Geogrids are stiffnetlike materials that exhibit obvious differences from geonets. Geogrids are generally open structures with high strength junctions that have uniaxial or orthogonal biaxial strength and are used primarily to reinforce or support (USEPA, 1984, 1985a, 1986a: Netlon, 1987; USEPA, 1988). There are two different methods from which geogrids can be produced. First, deformed grids are hardened after extrusion to enhance their physical properties. This type is found in separation and reinforcement applications. Secondly, geogrids are also formed from joined polymeric strips laid in a grid-like pattern and bonded or joined at the intersections. This type is also used in reinforcement applications. As with geonets, geogrids are commonly found in geocomposite systems. (USEPA, 1985b, 1988: GRI, 1989a.b.c', Yazdani & Norbert, 1989).

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2.4 Pipes Plastic pipes used in containment facilities for hazardous waste are often slotted or perforated. Although technically not a ~geosynthetic' material, pipes are composed of polymeric materials which are grouped into two basic classes: thermoplastics and thermosets. Thermoplastic pipes soften when heated and reharden upon cooling. Commonly used thermoplastics are acrylonitrile-butadiene-styrene (ABS), chlorinated polyvinyl chloride (CPVC), polybutylene (PB), polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC) and polyvinylidene fluoride (PVDF or PVF2). Thermosets which are permanently shaped when cured by heat (chemical reaction) are classified as reinforced thermosetting resin pipe (RTRP), with the reinforcement most commonly being fiberglass (FRP), and reinforced plastic mortar pipe (RPMP), which consists of composite layers of thermosetting resin-sand aggregates. Thermoplastics are the most commonly used in waste systems due to their low cost and resistance to chemical and environmental degradation (Smith & Bayless, 1983; Brockschmit, 1985: Anon. 1986, 1987, 1988a,b; Baily, 1988: Leonhardt, 1988; Lord & Halse, 1988). Like geomembranes, pipes must exhibit flexibility and durability in hazardous waste applications and should be made from polymers that provide these characteristics. In addition, pipes must also maintain rigidity. In pipe applications, PVC in its rigid form has a tendency to become brittle (due to lack ofplasticizers) and therefore reduce its ability to maintain strength in any capacity. Overall, in applications involving geomembranes and pipe, PE is used because of its ability to accommodate the requirements of these products. PB is used in the manufacture of pipe due to its superior creep properties and maintains properties similar to PE regarding chemical resistance.

2.5 Geoeomposites Geocomposites consist of various combinations of geotextiles, geogrids, geonets and geomembranes. The geocomposites in the present context will exclude naturally occurring materials. Geocomposites give a higher system performance than can be attained by the specific geosynthetic material functioning alone (Koerner, 1990). Geotextile--geonet a n d geotextile--geogrid composites - - Separation and filtration are improved when a geotextile 'sandwich' is utilized. Reinforcement is enhanced to at least the sum of the strengths of the individual components. These 'sandwich' geocomposites are used to provide barriers and drainage interceptions. Placed horizontally they

66

PE. Cassidy et al.

make excellent barriers to upward moving water in a capillary zone. Horizontal flow can also be encouraged or depressed based upon the anisotropy of the specific geotextile utilized. Geocomposites have been used in trapping and conveying leachate in landfills and in conducting gases from beneath geomembrane liners of various types (GRI, 1989a.b.c). Geotextile-geomembrane composites - - Geotextiles provide increased

resistance to puncture, tear propagation and friction-related sliding when laminated to one or both sides ofa geomembrane. Tensile strength can also be increased with proper choice ofgeotextile (GRI, 1989a.b.c). Geonet-geomembrane composites - - Geonet-geomembrane systems can be fabricated from like materials and therefore can be effectively bonded together to form a barrier to liquid and/or vapor. This composite is enhanced in both strength and frictional resistance over its component parts. A geomembrane-geonet-geomembrane"sandwich' system can be made where the interior net acts as a drain to the leak detection system (GRI. 1989a.b,c). Geomembrane-low permeability soil composites - - The reasons for this type of composite are that geomembranes decrease the leakage rate while low-permeability soil increases the breakthrough time. In addition, the low permeability soil can reduce the leakage rate from any holes that might develop in the geomembrane, while the geomembrane will prevent cracks in the soil due to changes in moisture content (Giroud & Bonaparte, 1989).

3 STRUCTURE OF BASE POLYMER RESINS The durability of geosynthetic materials is dependent to a great extent upon the composition of the polymers from which they are made. To quantify the properties of these materials, a knowledge of their structures on the chemical, molecular and supermolecular level is necessary. 3.1 Chemical structure

The chemical composition and structure of the polymer from which the geosynthetic is made is directly related to the properties that the finished materials exhibit. These structure-property relationships allow manufacturers to custom-design polymeric materials for specific applications (Chart'on, 1989). PE (Fig. la), the simplest organic polymer, has the least reactive chemical structure of all commercial thermoplastics. PE produces

67

Chemical resistance of geosynthetic materials

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is similar to PE, but possesses better flexural properties (Schneider, 1988). Polyester, in its most common form, PET (Fig. lc) is a condensation polymer of a dibasic acid and a dialcohol. The ester group, --CO2--, the important polymeric link. can be hydrolyzed under certain alkaline conditions. The planar aromatic group stiffens the polymer backbone and increases the inherent flexibility of PET, as compared to aliphatic polyesters (Cooke & Rebenfeld, 1988; Rebenfeld, 1988: Rollin & Lombard, 1988; Rebenfeld & Cooke, 1989). PVC (Fig. I d) is a linear addition polymer that is used extensively in its syndiotactic form. Rigid PVC, as used in pipes, is a filled, unplasticized material while flexible applications require the polymer to be compounded with plasticizers (oils or waxes). PVC is known for its ability to absorb organic liquids and to consequently experience a softening (lower Tg). It is also susceptible to UV light- or heat-induced degradation to become brittle and darken (Cooke & Rebenfeld, 1988; Rebenfeld, 1988: Rollin & Lombard, 1988; Schneider, 1988; Charron, 1989; Daniel, 1989; Rebenfeld & Cooke, 1989). CPVC (Fig. le) is produced by free radical chlorination of swollen PVC, resulting in a higher softening temperature (Alger, 1989a). PB or butyl rubber (BR) (Fig. l f) is produced commercially from 1-butene. Compared to PE and PP, PB exhibits superior creep properties, which makes it quite useful as pipe material. The mechanical properties for PB, however, lie between low and high density PE. PB has the capability to form thin-walled pipe (Alger, 1989b). ABS (Fig. lg), a terpolymer comprised of these three monomers, is often compared to styrene in terms of physical properties but possesses much greater impact strength. Typically, ABS contains approximately 20% rubber, 25% acrylonitrile and 55% styrene. Variation of these ratios allows for preferred changes in tensile modulus and impact strength. Substituting butadiene with saturated rubber such as chlorinated polyethylene or acrylate to produce acrylonitrile-chlorinated polyethylenestyrene copolymer (ACS) or acrylonitrile-styrene-acrylate copolymer (ASA), respectively, improves oxidation resistance (Alger, 1989c). PVDF (Fig. lh) is prepared by high pressure free radical polymerization in aqueous systems. It demonstrates moderate tensile and impact strength as well as good chemical and solvent resistance (Alger, 1989d). Chlorosulfonated polyethylene (CSPE or CSM) (Fig. l i), a crosslinkable PE derivative, is obtained by reacting PE with a mixture of chlorine and sulfur dioxide under UV irradiation, As expected, the crystallinity of this substituted PE is destroyed. Commercially, CSPE contains 1-1.5% sulfonyl chloride and 25-43% chlorine (Alger, 1989e). Chlorinated polyethylene (CPE or CM) (Fig. l k) is reacted with

Chemical resistance of geosynthetic materials

69

chlorine in solution or in suspension containing approximately 25% chlorine. Again, the structural irregularity of the backbone results in decreased crystallinity and therefore a rubbery product. CPE exhibits good weathering~ ozone~ oil and heat resistance (Alger, 1989a). Ethylene interpolymer alloy (EIA) is a blend of ethylene vinyl acetate (EVA) copolymer and PVC resulting in a thermoplastic polymer (Matrecon Inc., 1988). EVA is prepared by a high pressure free radical ethylene polymerization process. Depending on monomer content~ structural regularity is altered and therefore crystallinity is increased or decreased accordingly. Incorporating vinyl acetate increases flexibility and resembles other polymers gradually becoming rubbery (Alger~ 1989f). 3.2 Molecular structure

The degree of polymerization or average molecular weight of polymers refers to the average number of monomer units in the chain or its length. Molecular weights of the polyolefins. PE and PP, which are produced by addition polymerizations can range from 200 000 to 1 000 000. Polyesters~ produced from condensation reactions, usually produce molecular weights in the l0 000 to 30 000 range. The polyolefins require higher molecular weights due to the absence of functional groups which would provide inherent chain stiffening. The physical and chemical properties of the polymers used in geosynthetic materials require sufficiently high molecular weight: however~ too high a molecular weight will lead to processing problems (Rollin & Lombard, 1988: Schneider~ 1988: Charron, 1989: Rebenfeld & Cooke~ 1989). The sequencing of asymmetric or chiral carbons which contain a substituent group at each monomer residue refers to the stereoregularity or tacticity of the polymer. This capacity to form stereoregular polymers results in three primary possibilities which are characterized by nuclear magnetic resonance (NMR) spectroscopy. Isotactic (Fig. 2a) sequences have the same configuration along the polymer backbone: syndiotactic (Fig. 2b) sequences exhibit pendant groups about the chiral center arranged in an alternating manner: and atactic (Fig. 2c) sequences demonstrate a random or irregular arrangement along the backbone (Billmeyer, 1971a). Tacticity is determined by the bond-making process during polymerization and is solely dependent on polymerization conditions and catalyst type (Alger, 1989g). This occurrence of stereochemical configurational isomerism markedly affects the bulk physical properties. Isotactic and syndiotactic isomers result in high-melting compounds that crystallize readily, whereas atactic sequences produce materials that do not crystallize readily and are low-melting or even

P.E. Cassidy et al.

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rubbery (Allcock & Lampe, 1981a; Seymour & Carraher, 1981a). Due to the random nature of polymerization involving synthetic polymers, a mixture of polymer chains with different molecular weights (broad molecular weight distribution, MWD) is produced and referred to as polydispersity. Physical properties of polymer products such as melt viscosity, tensile strength, modulus, impact strength or toughness, and resistance to heat and corrosives are dependent on the molecular weight and the M W D (Seymour & Carraher, 1989b). The size distribution is described quantitatively by the M W D function. The distribution may be established either by determination of both n u m b e r (Mn) and weight average (Mw) molecular weights or by fractionation by gel permeation chromatography (GPC) (Alger, 1989h). 3.3 S u p e r m o l e c u l a r

structure

The three-dimensional structure or fine structure of macromolecules is important for all geosynthetics, but particularly so for fiber-forming

Chemical resistance of geosynthetic materials

71

polymers used to make geotextiles. The manner in which the polymer organizes to form well oriented, closely-packed crystalline regions and randomly coiled amorphous regions must be properly balanced to produce the correct physical properties necessary for fibers used in geotextiles (Cooke & Rebenfeld, 1988: Rebenfeld, 1988: Rollin & Lombard, 1988: Rebenfeld & Cooke, 1989). Polymers crystallize, to a limited extent, only if molecular structure is ordered, whereas, nonpolymeric solids are essentially 100% crystalline. In polymeric compounds, the remaining noncrystalline portion constitutes coiled, disordered (amorphous) chains. Polymers with structurally regular chains, such as high density polyethylene (HDPE), possess the ability to disentangle and form crystals. Polymer crystallization depends on the presence or absence of tacticity and the minimum-energy conformation of the chain (Allcock & Lampe, 1981b). As the degree of crystallinity increases, structural irregularity decreases. Thus, compounds that suggest highly ordered, i.e. isotactic or syndiotactic, sequences with minimal branching are highly crystalline. Crystallinity has a profound effect on polymer properties, especially mechanical properties, due to the smaller polymeric crystals in which the evenly and tightly packed molecules result in high intermolecular forces and dense regions. Therefore, an increase in the degree of crystallinity directly correlates to an increase in modulus, stiffness, yield and tensile strength, hardness and softening point and a decrease in permeability (Alger, 1989i). Microcrystalline polymers maintain a regular packing of chains in small domains in a matrix of amorphous polymer. This type of orientation results in a higher density than polymers that are totally amorphous. Microcrystalline regions held together by dipolar, hydrogen bonding, or van der Waals forces provide what could be called "physical crosslinks" for the amorphous regions. (The chains do not have primary chemical interchain bonds.) When introduced to an amorphous polymer, microcrystalline "crosslinks" stiffen and toughen the polymer and a rubbery, elastomeric polymer changes into a tough flexible material. Microcrystalline polymers bend better without breaking, resist impact better and are less affected by temperature changes or solvent penetration than completely amorphous polymers (Allcock & Lampe, 1981c). However, except in those cases where branching is purposely introduced to inhibit crystal formation and lower density, increased branching on the backbone is undesirable since branching disrupts crystal formation. Orientation (such as cold drawing) of crystalline polymers results in improved physical properties when fibers are stretched and/or processed. As fibers stretch, the molecules become more oriented, tend to crystallize and in turn, are stronger, tougher and

72

PE. Cassidy et al.

somewhat more elastic than unoriented fibers. High molecular symmetry and high cohesive energies between chains, which both require a fairly high degree of polymer crystallinity, result in high tensile strength and high modulus in fibers. Order also increases when polymer films are biaxially oriented (Seymour & Carraher, 1981a). At the molecular level, the crystal structure is characterized by wide angle X-ray diffraction. The properties of geosynthetic materials are directly related to the structure of the polymer from which the materials are made. The chemical properties of the base polymer, the average molecular weight, the molecular weight distribution, and the crystallinity combine to contribute to the chemical and environmental durability of the geosynthetic (Cooke & Rebenfeld, 1988: Rebenfeld, 1988; Rollin & Lombard, 1988; Daniel, 1989: Rebenfeld & Cooke, 1989).

4 EXPOSURE CONDITIONS The performance of geosynthetic components depends upon the longterm stability of the polymeric materials" physical, mechanical and hydraulic properties. Degradation caused by interactions with the waste stream can reduce a component's ability to perform its original design function. Although the USEPA has provided Method 9090 and other guidance documents (USEPA, 1986b), performance criteria for the chemical resistance of geomembranes are not evaluated. EPA Method 9090, discussed in Section 4. l, addresses test methods for geomembranes and the EPA'S Flexible Liners Expert System (FLEX) suggests performance criteria. FLEX, a computerized expert system, supplements EPA Method 9090 in the evaluation of chemical resistance of geomembranes (Schwope et al.. 1985; USEPA, 1986b; Landreth, 1989). Other geosynthetics are now being tested in nonstandardized adaptations of EPA Method 9090. 4.1 EPA Method 9090 for geomembranes

Developed in the early 1980s, EPA Method 9090 provides a basis for determining the resistance of geomembranes with waste liquids by artificially simulating field conditions. Rectangular geomembrane samples are immersed in a chemical environment representative of the waste liquids or leachates to be contained. Minimum periods of 120 days at room temperature (22°C) and elevated temperatures (50°C) are used. Samples are immersed in exposure tanks that are designed to support the samples such that they touch neither other samples nor the sides or

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bottom of the tanks. All surfaces must be available. The leachate and the tank must be sealed to prevent the loss of volatile leachate components. Physical properties of the geosynthetic samples are monitored before immersion and every 30 days thereafter to evaluate the resistance of the selected geomembranes with the representative exposure media. Some membranes, depending on their structure may be tested differently than others, as shown in Table I. EPA Method 9090 was developed to assess resistance ofgeomembranes only and provides no guidance for testing other geosynthetics (Tisinger, 1989a: White & Verschoor, 1989). 4.2 Leachate composition When a testing facility is contracted to perform chemical resistance testing, it is usually the responsibility of the contractor to supply the leachate solution(s). The leachate should be representative of the waste that will be stored in their facility in which the materials will finally be installed. As part of the landfill permitting process, the resistance studies are performed using leachate from actual landfills (Coia et al.. 1987). Chemical resistance testing of geosynthetics should employ worstcase exposure conditions. This is necessary to ensure that when in actual use, the materials will not be subjected to conditions worse than those experienced in the testing laboratory. Standardized leachate solution Table 1 Applicable Physical Property Tests for Various Geomembrane Types" Physical

Geomembranes

property. test

Semi-c~stalline

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thermoplastic Weight Dimensions Hardness Modulus of elasticity Tear resistance Tensile properties Puncture resistance Specific gravity Volatiles content Extractables content Hydrostatic resistance aUSEPA

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can be 'spiked' using chlorinated and aromatic organic chemicals to produce an aggressive chemical leachate for adapted EPA Method 9090 testing for materials other than geomembranes. In a study by Haxo et al. (1988) trichloroethylene (TCE) and 1,l,l-trichloroethane (TCA) as well as benzene, toluene and xylene (BTX) were used as spiking agents in 120 day experiments testing geotextiles, geogrids and leachate collection pipes. The leachate was changed monthly and the fresh leachate was spiked at the beginning of each monthly cycle. The test cells and stirrers were made of stainless steel and sealed with Teflon gaskets to prevent loss ofvolatiles (Cadwallader, 1986a.b: Texas Water Commission, 1986: USEPA, 1986c: Haxo et al., 1988).

5 GEOSYNTHETIC CHARACTERIZATION METHODS As previously mentioned, no standardized methods exist for analyzing the durability of geosynthetic materials (other than geomembranes) upon exposure to the environment or aggressive chemicals. However, methods are currently being formulated by ASTM for the respective geosynthetic materials. In the past, the durabilities of the geosynthetics have traditionally been assessed on the basis of mechanical property test results, not on the microstructural changes that cause the changes in the mechanical properties. This evaluation aids in the determination of changes in the molecular structures of the base polymers used to manufacture the geosynthetic. Specialized analytical techniques have been used to examine the molecular structure ofgeosynthetic materials (Tisinger, 1989c). 5.1 Mechanical test methods

Before discussing the testing methods for each type of geosynthetic, it is worthwhile mentioning index and performance tests. Index tests are used in quality control and quality assurance. They are also used to monitor changes that may occur after a material has had some sort of exposure. Most important, index tests do not reflect design features or applications. Performance tests, on the other hand, are used to test a material for a desired design feature. The difference between index and performance tests is illustrated by the results from two, seemingly very similar, techniques: grab strength and wide-width tensile strength. Both tests measure the breaking load and elongation to break. However, due to the small width of the sample and the nonuniform stress state in a grab test, the plane-strain strength of the material cannot be determined. The

Chemical resistance of geosynthetic materials

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wide-width tensile test, with its larger sample and uniform stress conditions, does reflect the properties of the entire material and could be considered as a plane-strain performance test. While the grab test is not a performance test, it does make an excellent index test due to the small sample size required. Of the two types of tests, index tests are used in chemical resistance studies. Afortiori. a change in index results dictates a change in macromolecular structure which, in turn, changes the performance of the polymer. In addition to the following sections on test methods used in geosynthetic applications, Table 2 offers an overview of these methods, advantages and disadvantages. 5.1.1 Geotextiles

Geotextiles vary randomly in thickness and weight in any given sample roll due to normal manufacturing techniques. In order to obtain representative samples, testing laboratories have developed various shuffling techniques to ensure that all areas of the sample roll and a full variation of the product are represented within each sample group at each given exposure interval. Each of the adapted tests requires m a n y replicates due to the duration of the test (typically 120 days) and the frequency of testing (every 30 days). There is some controversy over whether the samples should be tested wet or dry after exposure. The samples may be rinsed with water and tested wet, tested while still coated with leachate, or dewatered with a roller wrapped in a thin PE film and then tested. Most methods exhibit potential problems with contamination of equipment and the work place. The following physical tests (ASTM, 1984) are c o m m o n l y specified for geotextiles for a chemical resistance program: Mass per unit area - - ASTM D-3776, ISO 9864-1990 (ISO, 1990) Thickness - - ASTM D-1777, ISO 9863-1990 Grab strength - - ASTM D-4632 Puncture m ASTM D-3787 Trapezoidal tear m ASTM D-4533 Hydrostatic burst strength - - ASTM D-3786 Water permeability - - ASTM D-4491 Transmissivity - - ASTM D-4716 Apparent opening size - - ASTM D-4751, CW-02215 (Anon., 1977) Wide-width tensile strength m ASTM D-4595 5.1.1.1 Mass per unit area - - ASTM-D3776, I S O 9864-1990. Mass per

unit area is the proper term for weight ofa geotextile. Sometimes referred to as 'basis weight', it is a measure of fabric mass per unit area usually

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Fiber/yarn tensile

Geotextiles Grab strength

Test method

Small sample size. Universal test.

Small sample size.

Small sample size. Universal test.

Small sample size. High precision. Universal test among geotextile manufacturers. Small sample size. Universal test.

Small sample size. Universal test.

Advantages

Table 2 Mechanical Test Methods

Gripping procedure causes variability in test results caused by edge effects. Many replicates required. Untried in chemical compatibility. Staple fibers too small to test. High variability in test results caused by dimensional fluctuations inherent in product. High variability in test results caused by dimensional fluctuations along roll width and sample prep inconsistency. Not very reliable as endorsed by Task Force #25. Dropping this test. Some inter-lab variability reported due to apparatus design. Samples must be tested dry. Variability in test results caused by static electricity and retention of leachate.

Disadvantage~

Single opening 2-3" wide strip Square (dimensions not standardized) 2" wide strip

ASTM D751

ASTM I)4595 8" wide strip

ASTM D4716 6 X 6" square

Strip tensile

Creep compliance

Strip tensile

Wide width tensile

Hydraulic transmissivity

ASTM D4716 6 × 6" square

Hydraulic transmissivity

Geonets Aperture size

ASTM D4595 8" wide strip

Wide width tensile

2" wide strip

ASTM D751

Strip tensile

Large sample size decreases effects of dimensional variability.

Small sample size. Common specification in geonet industry. Small sample size. Good reproducibility. Generates results for application to design. Small sample size. Accurate product thickness data provided. Small sample size. Gripping along full width of sample decreases variability caused by side wall effects. Large size decreases variability caused by dimensional fluctuations along roll width.

Large sample size decreases effects of dimensional variability.

Small sample size. Gripping along full width of sample decreases variability caused by side wall effects. Large size decreases variability caused by dimensional fluctuations along roll width.

Large sample size decreases number of test replicates. Exposure requires large volumes of leachate. Test itself requires large volume of leachate for flow rate determination.

No formal standard.

No formal standard.

No formal standard.

Large sample size decreases number of test replicates• Exposure requires large volumes of leachate. Test itself requires large volume of leachate for flow rate determination.

Geotextiles rope up, giving artificial high values•

ASTM D638

Ring tensile

I-6" wide arc

!-6" wide tube section

ASTM D1238 3-6g

Melt flow index

~-i" wide strip

ASTM D638 (modified)

Three point bend or torsion

Sample

ASTM D2412 3-12" long specimens

ASTM DI238 3-6 g

Standard

Strip tensile

Pipe Pipe stiffness

Melt flow index

Test method

Advantages

Small sample size. Simulates loading modes experienced in installation.

Small sample size. Universal test.

Small sample size. Universal test.

Universal test. Small smaple size with 3" specimens. High reproducibility. Small sample size. Universal test.

Small sample size. Universal test.

TABLE 2--contd.

Pipe geometries require large volumes of leachate for exposures. Samples must be machine cut for thick wall pipes. Modifications are necessary for corrugated and slotted pipe. Necessary melt temperature may cause decomposition of some polymers. Test results vary for short specimens due to side wall effects. Pipe geometry requires large volume of exposure liquid. No formal standard.

Necessary melt temperature may cause decomposition of some polymers.

Disadvantages

g~

".,.d OO

Small sample size.

EPA 9090

Wide width tensile

Small sample size. Small sample size.

!-3 g

EPA 9090

i

Single node

Small sample size enabling several test replicates. Small sample size enabling several test replicates.

Small sample size.

Volatiles and extractables content

All Geosynthetics Solution viscosity

Junction node strength

Single rib

ASTM D3015 Slide preparation

Carbon dispersion

Geogrids Single rib tensile

ASTM D I 180

Hydraulic burst strength

-3" arc

ASTM D!693

Environmental stress cracking

No formal standard associated with geosynthetics community. High variability in test results due to large inherent error in test method• High variability in test results due to large inherent error in test method•

No formal standard.

No formal standard.

Test specimens must be machine cut from thick wall pipe. Test does not monitor stresses simulating field installation and usage. Test does not monitor relative changes experienced in field applications.

-.....I ~D

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given in units of grams per square meter (g/m 2) or ounces per square yard (oz/yd2). The mass of the fabric should be measured to the nearest 0-01% of the total sample mass and the length and width should be measured under zero fabric tension. Typical values for geotextiles vary from 135 to 680 g/m 2 (4-20 oz/yd2). 5.1.1.2 Thickness - - A S T M D-1777, I S O 9863-1990. The thickness of a geotextile is measured as the distance between the upper and lower surfaces of the material measured at a specified pressure. Thickness of commonly used geotextiles ranges from 10 to 300 mils (1 mil = 0.001 inch). 5.1.1.3 Grab strength - - A S T M D-4632. The breaking load and elongation to break is determined by the grab strength test. Although procedures exist for testing samples wet or dry, the dry conditions are usually used. The specimen is pulled apart at a constant rate of extension. 5.1.1.4 Puncture - - A S T M D - 3 7 8 7 . The puncture test assesses geotextile

resistance to objects such as sticks and rocks under quasi-static conditions. A blunt-ended metal rod (0.79cm (~6 in) in diameter) is pushed through the fabric which is firmly clamped in an empty cylinder (4.4cm (1] in) in diameter) by a compressed testing apparatus. The resistance to puncture is then measured in pounds of force. 5.1.1.5 Trapezoidal tear - - A S T M D-4533. The trapezoidal tearing load is the force required to successfully break individual fibers in a fabric. The fabric is inserted into a tensile testing machine on the bias so that the fibers tear progressively. An initial 1.6 cm (~ in) cut is made to start the process. The individual fibers are actually stressed rather than the entire fabric system. 5.1.1.6 Hydrostatic bur~t strength m A S T M

D-751 or A S T M

D-3786.

Several methods exist to load fabrics out of plane and stress them to failure. The fabric is deformed in various ways by use of rubber diaphragm until the central portion of the geotextiles, lying along the minor axis of the fabric, yields to strain. Although a difficult test to perform, the pressure (of the diaphragm) versus strain response yields an accurate modulus. 5.1.1.7 Water permeability (permittivity) - - A S T M D-4491. The permittivity

or cross-plane permeability of a geotextile is a critical assessment of the filtration ability of the fabric. Geotextiles deform under load. Therefore, a new term, permittivity (Koerner, 1990) is defined

Chemical resistance of geosynthetic materials _

81

k. t

where $ is the permittivity, k. is the permeability coefficient normal to fabric, and t is the thickness of the fabric. Used in Darcy's formula q = kiA q = k AhA t k. _ ~ t

q AhA

_

where q is the flow rate, Ah is the pressure head difference, and A is the area of fabric under test. The formula is used in conjunction with constant pressure heads, q is measured for one value of Ah and then q is calculated for other values of Ah. When plotted ( A h A versus q) the slope of the resultant line yields the permittivity (~t'). When ~' is multiplied by the thickness of the geotextile, the traditional permeability coefficient is obtained. Preconditioning of the fabric, temperature, and used or deaired or deionized water significantly affect test results. 5.1.1.8 Transmissivity - - ASTM-D4716. The transmissivity or in-plane permeability of the geotextile quantifies the drainage function of the fabric. As with permittivity, the variation of the fabric thickness under compressive load must be considered. Transmissivity ( 0 ) is introduced and developed using Darcy's formula as follows: 0

= kpt

q = kpiA q = kp~(W)(t) kpt = 0

-

qL Ah(gO

where 0 is the transmissivity, kp is the permeability coefficient (in-plane), L is the length of the fabric, and W is the width of fabric. Thicker nonwoven fabrics are best suited to convey drainage liquids, but are subject to variation of fabric thickness (compression under load).

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Although transmissivity decreases exponentially with applied stress, most geotextiles reach constant values after approximately 24 kPa (500 lb/ft2). At pressures beyond these, the material has the necessary mechanical strength yet retains some transmissivity (Koerner, 1990). 5.1.1.9 Apparent opening size - - A S T M D-4751, CW-02215. In this test, a series of sets of beads are used to determine the opening size of the fabric. Apparent opening size (AOS) is given as the size of the set of beads which passes 5% or less through the fabric. The beads are assigned US standard sieve numbers and these correspond to the AOS. 5.1.1.10 Wide-width tensile strength - - D-4595. This method is used to determine the plane-strain strength of a material. Although similar to other strip tensile tests (i.e. grab strength), the wide-width strip method minimizes the tendency of the sample to neck down or contract. It accomplishes this by having a sample which is wider than it is long. This produces results which bear closer relationship to the field performance of the material in plane-strain conditions than does the grab strength. This test approximates potential field stress more closely than the grab test because the sample is constrained along the entire width of the sample. 5.1.2 Geonets

Geonets are primarily constructed of HDPE. They must maintain their drainage capacity over extended periods of time while subjected to degradation by the waste stream. Tests for geonets include: Mass per unit area - - ASTM D-3776, ISO 9864-1990 Volatiles and extractables - - SW-870, MTM-I&2 (Matrecon Inc., 1988) Specific gravity - - ASTM D-792 Transmissivity - - ASTM D-4716 Wide-width tensile strength - - ASTM D-4595 Compressive strength Creep compliance 5.1.2.1 Mass per unit area - - A S T M D-3776. Geonets are evaluated the

same as geotextiles (see Section 5.1.1.1). 5.1.2.2 Volatiles and extractables - - SW-870, M T M - I &2. These methods are used to measure the amount of volatiles and extractables in a

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material before and after exposure to leachate. Samples are weighed, heated (150 + 2°C) and weighed again to determine percent weight loss. Any change in weight during the heating cycle is due to loss of volatile substance(s) which were absorbed from the leachate. If a weight loss is observed by virtue of exposure to leachate an extraction of a soluble component is indicated. 5.1.2.3 Specific gravity - - ASTM-D792. This test is not used to assess chemical resistance, but is instead used for material identification. Since PE is less dense than water, water displacement cannot be used. Specific gravity, therefore, is the relation of the material's weight in air versus its weight submerged in water. 5.1.2.4 Transmissivity - - A S T M D-4716. Geonets are evaluated using basically the same transmissivity method as geotextiles, but the effects of higher overburden pressures can be very different (see Section 5.1.1.8). 5.1.2.5 Wide-width tensile strength - - D-4595. Wide-width testing, as performed for geotextiles or geogrids, may be applied to geonets although this is not widely done. As a performance test, the measurement of wide-width tensile strength may not be appropriate for net since the application is usually to provide planar drainage, rather than to provide any extra physical strength to an engineered structure. 5.1.2.6 Miscellaneous methods. The following test methods are commonly used by testing laboratories to evaluate geonets; however, there are no published standard methods. They are as follows: Compressive strength - - Compressive strength tests have been proposed to measure the resistance of geonet products to deformation under compressive stress. 'Layover' of strands is a known failure mode for geonet and may be assessed by this technique through evaluation of compressive stress/strain properties. Compressive strength methods have been developed as a quality control tool and are under study by ASTM as a new standard. Creep compliance -- A related compressive property is creep compliance, or the tendency ofa geonet product to take on a permanent deformation under long-term loading. This property is related to the geonet's hydraulic transmissivity, since the net's effective thickness is reduced by permanent deformation. Proposed methods for measurement of creep compliance combine transmissivity measurements with the compressive stress/strain profile of the product.

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5.1.3 Geogrids Geogrids are constructed primarily of H D P E and may contain PP fibers. Tests for geogrids include Mass per unit area - - ASTM D-3776, ISO 9864-1990 Volatiles and extractables - - SW-870, MTM-I&2 Specific gravity - - ASTM D-792 Wide-width tensile strength - - ASTM D-4595 Node junction strength Rib strength 5.1.3.1 Mass per unit area (ASTM D-3776). Geogrids are evaluated the same as geotextiles (see Section 5.1.1.1). 5.1.3.2 Volatiles and extractables (SW-870. MTM-I&2). Geogrids are evaluated the same as geonets (see Section 5.1.2.2). 5.1.3.3 Specific gravity (ASTMD-792). Geogrids are evaluated the same as geonets (see Section 5.1.2.3). 5.1.3.4 Wide-width tensile strength (ASTM D-4595). Geogridsareevaluated the same as geotextiles with appropriate adjustments (see Section 5.1.1.10). 5.1.3.5 Miscellaneous methods. The following test method is commonly used by testing laboratories to evaluate geogrids, however, there are no published standard methods. Node junction and rib strength -- At present, the usefulness of this test is in question, since it does not relate to any specific mode of loading expected for geogrids. 5.1.4 Pipes A wide diversity of methods for testing pipes have been evolved and published by ASTM, the Plastic Pipe Institute, the Gas Research Institute and the National Sanitation Foundation. Pipes composed of PVC and H D P E have found considerable use in the handling of chemicals due to their good resistance to chemical degradation. (Pipes are used in leak detection systems (LDS) and leachate recovery and collection systems (LCRS) in Resource Conservation Act (RCRA) approved hazardous waste cells). To assess long-term durability, sections (cross and longitudinal) of the pipe sample are exposed to leachate under conditions similar to EPA Method 9090 and are tested after exposure in accordance with a method selected from ASTM D-2412. Physical testing

Chemical resistance of geosynthetic materials

85

should simulate the primary stress that pipes experience, which is static compressive loading. Wall thickness, volatiles and extractable content, hardness and specific gravity may also be appropriate when assessing long-term durability of pipes composed of geosynthetic materials.

5.1.5 Summary Of the list of tests for geotextiles, some are not applicable for various reasons. First, weight and thickness are merely basic tests that are not an accurate gauge of property changes in these materials. These data have widespread inconsistency due to machine variation used for such measurements. Grab strength is generally favored but variability may result from sample grip technique. Puncture testing is also rather vague since the probe does not interact and tear the fibers of the designated region, but may only separate them. Since the number of those being broken and those being pushed out of the way cannot be monitored, puncture testing should be used with caution or use a cone-shaped or flatended probe. Trapezoidal tear data are varied due to sample preparation. When a trap tear test specimen is prepared, it is difficult to accurately reproduce slits. Further, the mounting of the sample in the test machine is difficult to keep consistent. Hydrostatic burst strength is a sensitive and repeatable index test, but its reliability is questionable. Permittivity presents some variability, most notably, in apparatus design in different testing laboratories. Transmissivity for geotextiles requires a large sample size and, as a result, a large amount of leachate, which affects flow rate determination. AOS is highly variable due to sample preparation. For example, samples must be tested completely dry and such practice is contrary to EPA Method 9090 testing. Also, a large sample size is required and the flow of testing beads is hindered by any particulate matter retained within the fibrous network. Wide-width tensile strength is limited by its required large sample size but is regarded as an accurate test. In addition to the tests already mentioned, volatiles, extractables and specific gravity are widely accepted for geonets as they are for geomembranes. The importance of evaluating deformation of compressive stress of geonets is evident as compressive stress/strain properties are examined. However, compressive strength is under study, awaiting approval for standardization. Combining design factors and thickness evaluation, creep compliance is another proposed method where a formal standard needs to be finalized. For geogrids, as mentioned in Section 5.1.3.5, node junction and rib strength is under question and needs further evaluation. ASTM D-2412 for pipes is widely accepted among the geosynthetic

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community. However, certain variations, such as large pipes used in hazardous waste containment must be addressed.

5.2 Specialized analytical techniques Specialized techniques examine the molecular structure ofgeosynthetic materials and elucidate the relationship between microstructural changes in the base polymer and physical and mechanical property changes ofgeosynthetics. They comprise analytical methods commonly used to characterize components on the molecular level. Table 3 includes a compilation of analytical methods applicable to geosynthetics with advantages and disadvantages.

5.2.1 Thermal analysis Thermal analysis includes a range of techniques for determining the temperature dependence of the polymer property changes. Mass, heat of reaction and volume are the most commonly observed physical property changes examined by thermal analytical methods. A good indicator of structure-behavior relationships in polymer applications, this measurement of property changes versus temperature has prompted the development of these techniques especially for polymer work (Alger, 1989j). 5.2.1.1 Differential scanning calorimetry. Differential scanning calorimetry (DSC) is a thermal analytical technique in which the difference in the amount of heat absorbed or emitted by a polymer sample is measured by the power consumed as the temperature is increased (Seymour & Carraher, 1981c). As a transition occurs in the sample, thermal energy is added to the sample or reference material (an inert standard) to maintain the same temperature for both. This energy, which is recorded, compensates for that lost or gained as a consequence of endothermic (absorbed energy) or exothermic (emitted energy) reactions taking place in the sample. This provides a direct calorimetric measurement of the transition occurring in the polymer (glass transition or crystalline melting point) (Willard et al., 1981a; Skoog, 1985a). The resulting thermal curve displays energy (MJ/s or Mcal/s) as a function of temperature (°C). Endotherms provide information used to calculate melting point ranges and the degree of crystallinity. Exotherms allow the assessment of the oxidative stability of the material based on the oxidative induction time or the oxidative induction temperature (Thomas & Verschoor, 1988a,b). DSC also measures decomposition onset temperatures.

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5.2.1.2 Thermogravimetry. Thermogravimetry (TG) furnishes information about the composition of a geosynthetic as elucidated by its thermal degradation. TG involves gradual heating of the sample in an inert atmosphere or air to temperatures of as much as 1000°C while monitoring the weight loss as a function of temperature. The weight loss corresponds to the volatilization of various components of the test sample, there being additives or products of decomposition of the actual polymer. Data commonly derived from TG are concentration of additives, polymer, carbon black, ash, and onset of decompositon temperatures (Thomas & Verschoor, 1988a, b). 5.2.1.3 Thermomechanical analysis. Thermomechanical analysis (TMA) is an analytical technique in which the penetration, expansion, contraction or extension of a substance is measured as a function of temperature. This technique measures the deformation of the material under nonoscillatory compressive load as it is subjected to a controlled temperature program (Wendlandt & Gallagher, 1981a; Willard et al.). Interchangeable sample probes are used to determine these changes in the materials being tested. Polymer swelling and dissolution in liquids can be detected by TMA operated in its isothermal mode. Information may be readily obtained for a variety of solvent swelling agent systems that usually require more sophisticated experimental techniques (Machin & Rogers, 1970). Softening, heat distortion and glass transition temperatures may be detected with a small-tip diameter probe and a loaded weight tray. Coefficients of thermal expansion and dimensional changes due to stress relief may be examined by probes of large tip diameter and zero loading in the expansion mode. Samples may be tested in the form of plugs, films, pelletized powders or fibers. The temperature range falls between that of liquid nitrogen and 850°C. 5.2.1.4 Dynamic mechanical analysis. Dynamic mechanical analysis (DMA) determines the mechanical properties of materials as they are deformed under periodic stresses. This most sensitive of thermal analytical techniques measures the resonant frequency and mechanical damping as a function of temperature. The sample (a thin film) is subjected to a controlled temperature program as it is forced to flex at a selected amplitude. The temperature range at which samples are tested is - 1 5 0 to 500°C. DMA detects transitions associated with the movement of polymer chains. This method generates information about the viscoelastic properties of a material (usually semicrystalline) such as storage modulus, loss modulus, stress relaxation and creep compliance.

Spectroscopy Fourier transform infrared spectroscopy (FTIR)

Dynamic mechanical analysis (DMA)

Thermomechanical analysis (TMA)

Thermogravimetry (TG)

Thermal analysis Differential scanning calorimetry (DSC)

Test method

Physical and chemical changes in polymer structure.

Melting point, degree of crystallinity, oxidative induction time and temperature. Concentration of polymer, additives and carbon black, onset decomposition temperature. Polymer swelling and dissolution. softening, heat distortion and glass transition temperatures. Storage modulus, loss modulus, stress relaxation and creep compliance. effectiveness of reinforcing agents and fillers.

Properties examined

Unless test conditions duplicate performance conditions, data can only used for index or QC. Data not always consistent with DSC.

Small sample size, simple procedure.

Observe changes on a molecular level.

Changes may not be noticed unless changed molecules retained within bulk polymer.

Large database of TG curves required.

Small sample size, QC/ QA fingerprinting.

Smaller sample size and easier testing than tensile tests.

Results not always reproducable on different instruments.

Disadvantages

Small sample size.

Advantages ~

Table 3 Analytical Test Methods

O0 O0

Only qualitative analysis possible.

Difficult sample preparation only qualitative analysis possible.

Resolution of 4-5 nm, magnification up to 60000.

Solvent/column interaction may produce bad data, qualitative identification limited•

Can only observe surface changes.

Short and simple procedure.

Excellent quantitative determination.

Quantitative and qualitative determination of volatile components.

Flow patterns, microcracks and air channels can be found, dispersion of carbon black can be observed. Microstructural changes such as microcracks can be observed.

Good comparative test.

Observe changes in samples too thick or solid for FTIR

Molecular weight distribution.

Degradation of the surface of a polymer.

aAll of these methods have the common advantage of very small sample size (in comparison to tests listed in Table l) and the possibility of performing many replicate tests.

Scanning electron microscopy (SEM)

Microscopic analysis optical microscopic analysis

(GC)

Chromatography Gel permeation chromatography (GPC) Gas chromatography

Attenuated total reflectance-FTIR

OO ~D

90

P.E. Cassidy et al.

DMA is helpful in determining the effectiveness of reinforcing agents and fillers used in thermoset resins (Willard et al.. 1981c; Wendlandt & Gallagher, 1981b). 5.2.2 Spectroscopy Spectroscopic techniques provide information on the compositional and structural characteristics of a polymeric material. IR spectroscopy is particularly useful in the identification of characteristic functional groups and molecular configurations by simple inspection and reference comparison. The sample is subjected to IR radiation of successively decreasing frequencies. A series of spectral bands is generated, each of which correlates to a particular frequency or range of frequencies where the organic (plastic) material absorbs radiation. These characteristic absorption frequencies indicate certain functional groups in the polymer (ASTM, 1984). Comparative IR techniques can be a sensitive method to detect changes in the polymer. 5.2.2.1 Fourier transform IR spectroscopy. Fourier transform has been applied to various spectroscopic methods to enhance the sensitivity of these methods using a high speed computer that isolates very weak signals from environmental noise (Skoog, 1985b). Fourier transform IR (FTIR) spectroscopy involves the splitting of the incident beam into waves followed by their recombination. Spectral information is obtained from the phase difference between these two waves by a Michelson interferometer. The information is digitized, transformed mathematically from the time domain and converted to a conventional IR spectrum (Alger, 1989k). Special advantages in FTIR spectroscopy include energy limited, time limited or signal-to-noise limited situations. FTIR has the ability to look at intractable, thick and intensely absorbing materials which has led to its particularly wide application in polymer characterization. In addition, this technique observes physical and chemical changes in the polymer structure as they occur (Grasselli et ai.. 1987). This method enhances the capability to evaluate ongoing chemical and physical property changes of polymeric materials on a small scale upon exposure to hazardous materials. 5.2.2.2 Attenuated total reflectance Fourier transform IR spectroscopy. Attenuated total reflectance (ATR) FTIR spectroscopy provides information on the composition of complex samples and the surface chemistry of polymeric materials. The sample is placed in contact with a transmitting crystal which is then fixed in the sample compartment. The distance to which the infrared radiation appears to penetrate in internal reflection depends on the wavelength, but is of the order of 4-10 ~ m. In

Chemical resistance of geosynthetic materials

91

the case of ATR, the energy is absorbed and not transmitted. It is possible, by scanning over a range of IR frequencies, to correlate specific vibrational absorption maxima with the atomic groupings responsible for the absorption. These correlations provide a powerful tool for the identification of particular covalent bonds. If the surface of the polymeric material has been degraded upon exposure to hostile chemicals, this technique gives information regarding changes in surface chemistry resulting from the exposure (Harrick, 1967: Grasselli et al., 1987).

5.2.3 Chromatography Chromatographic methods allow the separation, isolation and identification of closely related components in complex mixtures. Due to the diversity of these separation techniques, other analytical methods do not possess the ability to identify components from mixtures with such accuracy. Chromatography identifies components in the gaseous, liquid or solid state, which may include substances that have been absorbed into the geosynthetic material itself. This valuable information can be used to identify various components of leachates or products that result from polymer degradation (Allcock & Lampe, 1981d). 5.2.3.1 Gel permeation chromatography. GPC is an extremely valuable analytical technique for determination of molecular weight distribution. Used for a wide variety of solvents (tetrahydrofuran, benzene, xylene, chloroform, dimethylformamide or fluorinated alcohols) and polymer systems, GPC requires only small samples (typically 0.5-3 ml of a 0.050.1% solution of polymer). The technique utilizes a rigid porous column (1-3 m, or 3-10 feet, packed with highly crosslinked, porous, solventswelled polystyrene). The degree of swelling of the stationary phase determines the resolution of the separation. The column works as a sieve with the larger, higher molecular weight polymers passing through the gel column more quickly than the smaller, lower molecular weight molecules due to the retention of the latter in the pores of the stationary phase (Billmeyer, 1971b; Skoog & West, 1982). A detector is then used to determine the difference in refractive index (differential refractometer) or absorbance (UV spectrophotometer), which relate to the amount of polymer in the eluted solvent, as a function of time to produce the molecular weight distribution. 5.2.3.2 Gas chromatography. GC fractionates the components of a vaporized sample as a consequence of partition between a mobile gaseous phase and a stationary phase held in a column (Skoog & West, 1982). Headspace GC is an important analytical technique for qualitative

92

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and quantitative determination of volatile components of complex samples, even in minute concentrations (Lattimer & Pausch, 1980; USEPA, 1980; Ettre et al.. 1983). The method requires vaporization of the volatiles in a confined headspace and analysis of the resulting gas by GC. Separation of the sample is achieved by elution using an inert carder gas (helium) and a stationary solid phase that has different affinities for the sample components. Results are received in graphical form displaying peaks with varying retention times (identifying the component) and peak area (from which amount is calculated) (Skoog, 1985c). 5.2.4 Microscopic analysis The use of magnification to evaluate geosynthetic materials is provided by various types of microscopic techniques. Microscopy supplies rapid, direct observational information about the surface as well as the internal microstructure and defect distribution within the materials. Microscopic analysis is also a valuable tool for the examination of geosynthetic materials before and after exposure to aggressive chemical media. 5.2.4.1 Optical microscopic analysis. Cross sections of polymeric materials are examined using microtome specimen samples or in an innovative technique utilized by Rollin et al. (1989) where optical fibers are used to carry the light directly into the geosynthetic. Cross-sections can be observed and photographed using minimum resolution (100200 nm) and magnifications of up to 2000X. Flow patterns of the polymer, microcracks and air channels can be found in substantially less time and with greater precision than before realized. According to Peggs and Charron (1989) the microtome technique of microstructural examination has been found to contribute to the development of new and modified materials, and subsequent processing methods. The distribution and relative magnitude of residual stresses can be identified by crossed or partially crossed polarizing filters. Characteristics of fracture phenomena can also be determined during failure analysis. Microstructural evaluation of the basic geomembrane facilitates the improvement of the standard method of determining the dispersion of carbon black in PE geomembranes. 5.2.4.2 Scanning electron microscopy. A powerful and versatile analytical tool for determining microstructural changes in geosynthetic materials, the scanning electron microscope (SEM) has resolution limits of 4-5 nm and magnifications of up to 60 000X. Microcracks that undermine the strength and durability of geosynthetics are easily visible within the analytical parameters of SEM (Rollin et al., 1989).

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5.2.5 Summary Thermal analytical techniques thus far have become the most versatile, reliable and established methods to follow the effects of exposure to aggressive media of geosynthetics on the molecular level. The small sample size, common to all analytical methods that have been discussed, offers a distinct advantage over traditional mechanical methods. Conversely, instrumental errors encountered using analytical techniques must also be taken into consideration. Although the application of FTIR spectroscopy seems to present itself as an accurate method of compound identification and changes that may occur upon exposure, it is subject to great variance according to the instrument. Geosynthetics are not comprised of one substance: they contain additives for increased chemical, environmental, and biological resistance. The data obtained from FTIR spectroscopy are therefore littered due to undesirable results from additives. However, if an appropriate apparatus were employed, such noise could be eliminated using the computer which is interfaced with the spectrometer. Chromatography provides a good comparative test to determine substance content and examines molecular weight distribution. However, it relies upon specific solvent requirements for the column, which may not allow for appropriate preparation of the geosynthetic sample. Microscopy allows excellent sample inspection via magnification, but sample preparation can be tedious. The results are qualitative, as no information on the molecular level is generated.

6 CONCLUSIONS Geosynthetic materials provide a myriad of applications that can be made by proper combination with other geosynthetics and/or natural materials. The resultant designs have found widespread use in hazardous waste containment facilities and their functions are well-documented. However, the chemical resistance of geosynthetic materials other than geomembranes is not well-established. Although EPA Method 9090 (USEPA, 1986b) details evaluation of the physical properties of exposed geomembranes, it fails to provide vital information as to the chemical changes that lead to the physical changes. Supplementing physical testing of geosynthetics with chemical and thermal analysis can provide information not available in the basic chemical resistance test (e.g. EPA Method 9090). It is important to recognize that the physical properties of a given material are governed by its chemical structure; a change in the chemical structure may constitute a change in physical properties. Through spectral, thermal and

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quantitative analysis of geosynthetics subjected to EPA Method 9090 tests and similar exposure conditions, one can better assess the changing structural property relationships of the polymer comprising the geosynthetic. In addition to utilizing chemical tests to supplement acquired physical test data, a correlation must be established between them in order to validate field performance.

ACKNOWLEDGEMENTS The authors appreciate the support for this project by the USEPA (Grant No. G R 815495). The authors would also like to thank Robert E. Landreth, Project Officer, USEPA; Robert M. Koerner, Drexel University, Philadelphia, Pennsylvania; Sam Allen, Rock Rushing and Rick Thomas, Texas Research Institute, Austin, Texas for their comments and helpful suggestions. We are also greatfui to Keith Brewer, Matt Adams, Kevin Kane and Kurt French, SWTSU, for technical assistance.

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