GEOSYNTHETIC CLAY LINER WITH ELECTRICALLY CONDUCTIVE PROPERTIES

Abstract

An electrically conductive geosynthetic clay liner incorporating an electrically conductive textile graphene.

Claims

1. A geosynthetic clay liner (GCL) incorporating an electrically conductive textile.

2. The GCL of claim 1, wherein said electrically conductive textile incorporates fibres coated with graphene.

3. The GCL of claim 1, wherein said electrically conductive textile is coated with graphene.

4. The GCL of claim 1, wherein said electrically conductive textile is made from fibres containing graphene.

5. The GCL of claim 1, wherein the electrical conductivity of a circuit formed therefrom may be measured over a distance of at least 1 metre.

6. The GCL of claim 5, wherein the distance is at least 10 metres.

7. The GCL of claim 5, where in the distance is at least 100 metres.

8. The GCL of claim 1, wherein the graphene content of the textile is less than or equal to 20% by mass.

9. The GCL of claim 8, wherein the graphene content of the textile is less than or equal to 10% by mass.

10. The GCL of claim 8, wherein the graphene content of the textile is less than or equal to 5% by mass.

11. The GCL of claim 8, wherein the graphene content of the textile is less than or equal to 2% by mass.

12. The GCL of claim 1, wherein the fibres of the textile are polymer fibres.

13. The GCL of claim 12, wherein said textile polymer is PET, PP or PE.

14. A multi-layer construction incorporating the GCL of claim 1.

15. The multi-layer construction of claim 14, further incorporating a water barrier layer.

16. The multi-layer construction of claim 15, wherein said water barrier layer is an electrical insulator.

17. A multi-layer construction, according to claim 14, for use as part of an inspection process to determine whether the water barrier is intact.

18. A method of inspecting the integrity of a water barrier, wherein said water barrier incorporates a multi-layer sheet according to claim 14, said method including the steps of: applying a voltage to one side of the sheet proximal to said electrically conductive textile component of the GCL; detecting whether an electrical circuit is thereby formed in the GCL.

19. The method of claim 18, wherein the resistance of said textile is less than 2500 Ohms per square.

20. The method of claim 18, wherein the resistance of said textile is less than 1000 Ohms per square.

21. The method of claim 18, wherein the resistance of said textile is less than 500 Ohms per square.

22. The method of claim 18, wherein the resistance of said textile is less than 50 Ohms per square.

23. The method of claim 18, wherein the measurement method employs a discontinuous electrical circuit via a capacitance and the resistance of the textile is less than 500,000 Ohms per square.

24. The method of claim 18, wherein the measurement method employs a discontinuous electrical circuit via a capacitance and the resistance of the textile is less than 200,000 Ohms per square.

25. The method of claim 18, wherein the measurement method employs a discontinuous electrical circuit via a capacitance and the resistance of the textile is less than 100,000 Ohms per square.

26. The method of claim 18, wherein the measurement method employs a discontinuous electrical circuit via a capacitance and the resistance of the textile is less than 50,000 Ohms per square.

27. An electrically conductive geosynthetic clay liner (GCL) incorporating a geotextile that contains graphene.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a schematic of an inspection circuit used to detect defects in a waterproof geomembrane that acts as a barrier layer, according to the state of the art.

[0029] FIG. 2 is a schematic of an alternative inspection circuit used to detect defects in a waterproof geomembrane that acts as a barrier layer, according to the state of the art.

[0030] FIG. 3 is a schematic of the use of an electrically conductive GCL in an inspection circuit used to detect defects in a waterproof geomembrane, according to the invention.

[0031] FIG. 4 is a schematic of an electrically conductive GCL suitable for use in an inspection circuit used to detect defects in a waterproof geomembrane, according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0032] The invention resides in the use of graphene as an electrically conducting component of a polymer fibre for a geotextile that is incorporated in a multi-layer geosynthetic clay liner, for use as one part of a water barrier for man-made earthworks where another part of the water barrier is an electrically insulating plastic geomembrane. The invention enables a way to test the geomembrane for defects, such as holes, via the electrical properties imbued by the graphene.

[0033] Turning to the figures, we note that FIG. 1 is a schematic illustration of a traditional inspection circuit used to detect for defects in a barrier layer (11) using a voltage/current source (14). When the inspection probe (13) is close to a defect (16), such as a hole, current will flow through the defect (16) into the earthwork base (12) via the earth contact (15) to form a continuous circuit. This circuit can only be formed when the earthwork base (12) is electrically conductive, which is often not the case, and is thus unreliable.

[0034] FIG. 2 is a schematic illustration of an alternative configuration of the inspection system of FIG. 1. Instead of direct contact by the earth (25) to the earthwork base (22), a relatively large area earth pad (27) is used to provide indirect electrical contact via a capacitance, where the barrier layer (21) provides a dielectric between the earth pad (27) and the earthwork base (22).

[0035] FIG. 1 illustrates an example of the circuit formed when electrical leak detection is performed on a simple water barrier assembly with a conductive under-layer such as a water-containing clay base. Clay is used in many cases to prepare the ground for water retention (e.g. dams and ponds) and water direction (e.g. canals and drainage). Clay also provides a good medium for electrical conduction due to its water and ionic content. If the clay base is partially or completely dry this process is not reliable and may not work at all. Also, if there is poor physical contact between the barrier layer and the clay base, caused by for example, air or water pockets, the inspection process can be unreliable. In the absence of a clay base, or equivalent, the inspection process is unreliable.

[0036] Traditional earthworks utilising clay bases as water barriers require substantial thicknesses of clay, sometimes measuring in the hundreds of centimetres of thickness. These traditional earthworks can be replaced by geosynthetic clay liners with as little as one centimetre of clay thickness.

[0037] Electrical inspection techniques are typically either low voltage or high voltage. Low voltage techniques typically require an electrically conductive layer on both sides of the membrane. This is provided by water being present in the area being inspected (often referred to as water lance or water puddle techniques). High voltage techniques (often referred to as arc or spark techniques) do not require a conductor on the side of the barrier layer being inspected (typically the top layer) and can use many thousands of volts to ensure that small holes, even pinholes, can be detected.

[0038] Two principal mechanisms of forming an earth connection are illustrated in FIG. 1 and FIG. 2. In FIG. 1 an earth (25) is formed where the electrical conductor is connected to the conducting under-layer (not shown in FIG. 1), e.g. by inserting a metal rod into the clay base, or by attaching to the conductive textile under-layer. In FIG. 2 an area of conductor, the earth pad (27) rests on top of the nominally insulating barrier layer (21). In some instances, the barrier layer (21) is not a perfect insulator so over a large contact area such as that formed by the earth pad (27) enough current can flow to through the circuit between the probe (23) and the earth (25). In other instances, the barrier layer (21) acts as a dielectric and the earth pad (27) acts as one electrode of a capacitor.

[0039] FIG. 3 is a schematic illustration embodying the application of the invention. An inspection circuit is used to detect for defects in a barrier layer (31) using a voltage/current source (34). When the inspection probe (33) is close to a defect (36), current flows through the defect (36) into and through the electrically conductive GCL (38) via the earth contact (35, 37) to form a circuit.

[0040] FIG. 4 is a schematic illustration of the three layers used to construct a geosynthetic clay liner. The Electrically Conductive Geotextile (41) and the backing layer (43) of textile or net, sandwich the clay barrier layer (42).

[0041] If, as illustrated according to the invention in FIG. 3, a conducting layer is added underneath the barrier Layer (31) as part of the GCL (38), the earthwork base (32) may be any material and no other conductivity beneath or in the barrier layer (31) is required. The incorporation of graphene into or onto the geotextile used in the GCL will tend to make the GCL sufficiently electrically conductive to allow both low and high voltage inspection techniques to be performed depending on the thickness of the barrier layer (31) and the size of the defect (36) that needs to be detected. The larger the defect (36) and the thinner the barrier layer (31) the lower the voltage required for inspection. FIG. 3 illustrates this configuration with the electrically conductive GCL (38) and the inspection configuration.

[0042] Electrical inspection for defects in the barrier layer can be performed by many methods. Industrial standards have been set to normalise the inspection conditions. These are embodied in the following international standards documents: ASTM D6747, ASTM D7002, ASTM D7007, ASTM D7240, ASTM D7703 and ASTM D7852.

[0043] Electrical inspection methods rely on electrical conductivity to form a circuit. Sufficient conductivity depends on the size and length of the conductive path and the conductivity of the media (water, earth, conductive textile, barrier layer). This combination of variables gives a wide range over which the inspection methods can be effective. Tuning the inspection method to the desired outcome and conditions is required. This allows the electrical conductivity of the conductive GCL to also be tailored to the desired application and inspection methods. In some cases, the electrical conductivity of the conductive GCL can be quite low, such as where the inspection voltage is high, the defect size is large and the circuit path is short.

[0044] Geotextiles are permeable fabrics which, when used in association with soil, have the ability to separate, filter, reinforce, protect, or drain. Typically made from synthetic fibres, such as polypropylene or polyester but potentially including other synthetic fibres, such as: polyamide; acrylonitrile; polylactide; polyester; cellulose; polyurethane; polyethylene and/or semi-synthetic fibres, such as: regenerated cellulose and/or natural fibres, which are primarily cellulosic, such as: abaca; coir; cotton; flax; jute; kapok; kenaf; raffia; bamboo; hemp; modal; piha; ramie; sisal, or; soy protein. Natural fibres are often biodegradable while synthetic fibres are not. Thus, fibre selection depends on the application.

[0045] Geotextile fabrics, like other fabrics, can be formed from fibres by many methods, including: weaving, knitting, knotting, braiding and non-woven overlay techniques where further steps, such as inter-tangling (e.g. needle punch, felting, hydro-entanglement, spun-lacing, water needling) and can include various steps to improve the desired properties, such as carding and heat bonding.

[0046] Geotextiles, in the context of the present invention, are advantageously made from fibres and are typically either woven or non-woven. Non-woven geotextiles are usually either continuous fibre, also known as filaments, or staple fibre. Staple fibres are shorter lengths that can be formed into a textile. In some cases, the staple fibres are unique fibres and in other clusters of fibres.

[0047] Geosynthetics are so named for their use in civil engineering applications including: airfields; bank protection; canals; coastal engineering; dams; debris control; embankments; erosion; railroads; retaining structures, reservoirs; roads; sand dune protection; slope stabilisation; storm surge; stream channels; swales and; wave action.

[0048] Various forms of graphene exist. Ideal graphene is pure carbon and is the best electrical conductor in the graphene family. It tends to be free of defects and other chemical substituents, such as oxygen. Graphene oxide (GO) is a highly-oxidised form of graphene that is an electrical insulator. Intermediary species can be referred to by various descriptions, such as partially reduced graphene oxide (prGO) or functionalised graphene, where various chemical groups are attached to the edges and/or basal planes of the graphene.

[0049] This functionality allows tailoring of the electrical and physical properties of the graphene, for example to make it easier to incorporate into or onto materials, such as plastics, in order to form composites. Incorporation of heteroatoms, wherein carbon atoms are replaced by other atoms, such as nitrogen, and other covalently bonded atoms can also be used to tailor the properties of graphene.

[0050] Graphene can also come in various dimensions, whether it be single layers of graphene or multiple layers. Various terminologies have been used to describe the structural permutations and some attempts have been made at standardising terminology. Regardless of terminology, these single-layer and multi-layer structures of graphene have useful conductivity that give rise to the properties of composite polymers, fibres and textiles as described herein. These various permutations of graphene are generalised herein as graphene unless otherwise detailed and their properties described.

[0051] The forms of graphene that can facilitate a scale from electrically conductive to electrically insulating means many forms of graphene can be used as an electrical conductor. Even relatively poorly conducting graphene can serve the purpose, especially where it's other properties make it desirable for use.

[0052] Graphene can be produced by many methods, including: anodic bonding; carbon nanotube cleavage; chemical exfoliation; chemical synthesis; chemical vapour deposition; electrochemical exfoliation; electrochemical intercalation; growth on silicon carbide; liquid phase exfoliation; micromechanical cleavage; microwave exfoliation; molecular beam epitaxy; photo-exfoliation; precipitation from metal, and; thermal exfoliation.

[0053] Some of these routes give rise to materials referred to as: chemically converted graphene; few-layer graphene; GO; graphene; graphene oxide; graphene nanoflakes; graphene nanoplatelets; graphene nanoribbons; graphene nanosheets; graphite nanoflakes; graphite nanoplatelets; graphite nanosheets; graphite oxide; LCGO; liquid crystal graphene oxide; multi-layer graphene; partially reduced graphene oxide; partially reduced graphite oxide; prGO; rGO; reduced graphene oxide; reduced graphite oxide.

[0054] Incorporation of graphene into a textile can be achieved by many methods, but in each case the properties of the fibre and textile will depend on the fibre chemistry, graphene chemistry, graphene shape and processes used to incorporate the graphene into or onto the fibres and the process of forming a textile.

[0055] Preferred methods include mixing the graphene into the polymer prior to forming the fibre. However, it is also possible to coat fibres or a textile with graphene to make the conductive textile. The graphene can be present as a powder or as a dispersion in a fluid to facilitate dispersion of the graphene in the polymer. Coating the graphene is preferably from a dispersion of graphene in a fluid.

[0056] Methods of incorporation of graphene into the polymer can include: Melt-compounding of graphene into the polymer; in-situ polymerisation of the polymer with the graphene, and; solution blending. Whichever technique is used, it is desirable that the graphene is sufficiently dispersed to achieve electrical conductivity with a minimum of graphene. In some cases, additives are required to reduce phase separation of the graphene and the polymer.

[0057] Other conductive additives can be added to the graphene coating or to the graphene-containing polymer. These conductive additives can improve the effectiveness of the graphene in providing electrical conductivity. For example, carbon blacks, carbon fibres and carbon nanotubes are all conductive carbons that can assist with the dispersion of the graphene in the coating liquid or in the polymer mixture and provide further interconnectivity.

[0058] A preferred embodiment exists where the conductive geotextile is formed from a fibre that includes graphene, wherein the fibre is formed by melt extrusion from pellets or powders of the polymer. The graphene is added to the melt extrusion in a concentrated form dispersed in a carrier polymer, which may be the same as the bulk polymer, or may be different. The concentrated form of the graphene polymer dispersion is mixed and diluted in the melt extrusion process to obtain the desired concentration of graphene in the fibres.

[0059] In an alternative embodiment, the concentrated form of the graphene is dispersed in a fluid, such as: oil, solvent or water.

[0060] In another embodiment, the fibres are made by wet-spinning solutions of polymer containing graphene or wet-spinning polymer fibres into coagulation baths containing graphene to produce a surface coating of graphene on the fibre.

[0061] In another embodiment, the GCL can be made electrically conductive by the addition of graphene to the clay before incorporation into the GCL.

[0062] In another embodiment, the GCL can be made electrically conductive by making the strengthening textile or net electrically conductive by addition of graphene either in the polymer or coated onto the formed textile or net.

EXAMPLE 1

[0063] Approximately 100 cm.sup.2 rectangles of GCL were made by needle punching conductive geotextile through powdered bentonite clay into a backing of woven non-conductive geonet. The punched geotextile fibres were sealed to the backing geonet by flame melting the protruding fibres. The conductive geotextile was made by coating a non-woven, low weight (150 grams per square meter) PET geotextile with a solution containing a dispersion of graphene to attain 2 weight percent loading of graphene on the geotextile. The electrical resistance of the conductive geotextile was measured to be 2000 Ohms/sq and this was maintained in the assembled GCL.

EXAMPLE 2

[0064] The sample from Example 1 was placed beneath a waterproof geomembrane with a deliberately created hole punched through it. The hole was approximately one millimetre in diameter. When inspected with a spark detector at circa 15,000 volts, the sample of GCL from Example 1 proved a suitable electrical conductor to allow spark testing of the waterproof geomembrane and the hole was reliably detected.

EXAMPLE 3

[0065] 100 cm.sup.2 squares of commercial GCL were used as received and conductive geotextile was adhered to the surface of the existing non-woven, non-conducting geotextile surface by needle punching through the existing GCL. The electrically conductive geotextile was the same material as used in Example 1. Testing of the samples as per Example 2, gave the same result as found in Example 2.

EXAMPLE 4

[0066] 100 cm.sup.2 squares of commercial GCL were used as received and conductive geotextile was adhered to the surface of the existing non-woven, non-conducting geotextile surface by gluing. The electrically conductive geotextile was the same material as used in Example 1. Testing of the samples as per Example 2, gave the same result as found in Example 2.

EXAMPLE 5

[0067] Similarly to Example 1, a GCL was assembled from conductive geotextile where the geotextile was made from staple fibre and had been coated with graphene to be made conductive prior to assembly into the GCL.

EXAMPLE 6

[0068] 100 cm.sup.2 squares of commercial GCL were used as received and the non-conductive geotextile on the surface of the GCL was made conductive by coating with a solution of graphene. Testing of the samples as per Example 2, gave the same result as found in Example 2.

[0069] It will be appreciated by those skilled in the art that the above described embodiments are merely several examples of how the inventive concept can be implemented. It will be understood that other embodiments may be conceived that, while differing in their detail, nevertheless fall within the same inventive concept and represent the same invention.