MXPA03009116A

MXPA03009116A - Material and process useful for preparing embossed flexible graphite article.

Info

Publication number: MXPA03009116A
Application number: MXPA03009116A
Authority: MX
Inventors: Klug Jeremy
Original assignee: Advanced Energy Tech
Priority date: 2001-04-04
Filing date: 2002-04-02
Publication date: 2004-11-22
Also published as: BR0208668B1; BR0208668A; TW592968B; WO2002084760A3; EP1385783A2; WO2002084760A2; AU2002303216A1; EP1385783A4

Abstract

A material useful in a process for embossing a flexible graphite sheet is presented. The inventive material is a flexible graphite sheet which has a preselected void condition which provides the capability of controlling the morphology, and thus the functional characteristics, of the resulting embossed sheet.

Description

WO 01/084760 A2 llliM »« fflillliiiPMBiiMI¾i (BF, BJ, CF, CO., GA. GN. OQ, GW. ML, MR. For tvo-ltlter cades and Olher abbrtylaUs. Rftr to the "Guid- NE, SN T, TO > "ie &nt; Note on adta and Abbreviattons'apptaring at iht (xgtn- * i» góféixh regular issue efíhe PCTGacette. Publf »h« d: - wítouí ttnwtíimal eeKh rtport and bá repubfhhed., Upan rtctipl of tat report Ol / p e6e¿ | '\ ZZZ ??? S \ Q iuosjeaiBd s? T ???? : As lúes MATERIAL AND USEFUL PROCESSES FOR THE PREPARATION OF FLEXIBLE ARTICLES OF GRAPHITE RECORDED Field of the Invention The present invention relates to a process for materials that can be used in the preparation of a flexible graphite article that is etched with a particular pattern thereon. Through the practice of the present invention, a material is provided that acts as a substrate to form a flexible article (such as a sheet) of graphite having a pattern etched therein. Uses of the material of the present invention include the formation of a recorded article that can be used as a component in an electrochemical fuel cell. Background of the Invention An ion exchange membrane fuel cell, more specifically a proton exchange membrane (PEM) fuel cell, produces electricity through the chemical reaction of hydrogen and oxygen in the air. Within the fuel cell, electrodes denoted in the form of anodes and cathodes surround a polymer electrolyte to form what we refer to generally as a membrane electrode assembly (or MEA). Frequently, the electrode also functions in the form of gas diffusion layers, or GDL, of the fuel cell. A catalytic material stimulates the hydrogen molecules to split into hydrogen atoms, and subsequently into the membrane, where each of the atoms is divided into a proton and an electron. Electrons are used as electrical energy. The protons migrate through the electrolyte and combine with oxygen and electrons to form water. A fuel cell is conveniently formed PE of an assembly of membrane electrodes sandwiched between two graphite flux field plates. Conventionally, the membrane electrode assembly consists of randomly oriented carbon fiber paper electrodes (anodes and cathodes) within a layer of a catalyst material, particularly platinum or a metal of the platinum group coated on particles of isotropic carbon, such as carbon black, bonded to either side of a proton exchange membrane placed between the electrodes. In the operation, the hydrogen flows to the anode through the channels in one of the plates of the flow field, where the catalyst promotes its separation into hydrogen atoms and subsequently into protons that pass through the membrane and electrons that they flow through an external load. The air flows through the channels in the other plate of the flow field to the cathode, where the oxygen found in the air is separated into oxygen atoms, which bind with the protons through the membrane. exchange of protons and electrons through the circuit, and combine to form water. Since the membrane is an insulator, the electrons travel through an external circuit where electricity is used, and they bond with protons at the cathode. In a current of air in the part of the cathode there is a mechanism by means of which the water formed by the combination of hydrogen and oxygen can be eliminated. The combinations of such fuel cells are used in a row of fuel cells to provide the desired voltage. A limiting factor for the use of flexible graphite materials in the form of a PEM fuel cell component is the definition of a pattern etched into the material, which, if not sufficient, can interfere with the operation of the fuel cell, allowing either fluid filtering or not allowing enough fluid to flow through the fuel cell. In addition, the thermal and electrical properties of flexible graphite materials can still be optimized, although they are superior to prior art materials. Graphites are made from planar layers or networks of hexagonal carbon atom formations. These planes of layers of carbon atoms placed in hexagonal form are substantially planar and are oriented, or arranged so as to be substantially parallel and equidistant from one another. The sheets, or layers of equidistant, parallel, substantially planar carbon atoms normally referred to as graphene layers or basal planes, are bonded or bonded together and their groups are placed in crystallites. The graphites arranged in an upper form consist of crystallites of considerable size: the crystallites being aligned or oriented with respect to one another and having well ordered carbon layers. In other words, graphites ordered in an upper form have a high degree of preferred crystallite orientation. It should be noted that, by definition, graphites have anisotropic structures and therefore exhibit or possess many properties that are highly directional, for example, thermal and electrical conductivity and fluid diffusion. In short, graffiti can be characterized as laminated carbon structures, that is, structures consisting of superimposed layers or layers of carbon atoms joined together by weak Van Der Waals forces. In consideration of the graphite structure, normally two axes or directions are observed, to wit, the axis or direction "c" and the axes or directions "a". For simplicity, the axis or direction "c" can be considered as the direction perpendicular to the carbon layers. The axes or directions "a" can be considered as the directions parallel to the carbon layers or the directions perpendicular to the "c" direction. Graphites suitable for making flexible graphite sheets have a very high degree of orientation. As noted above, the bonding forces that hold the layers of parallel carbon atoms together are only van Der Waals weak forces. The natural graphites can be chemically treated so that the space between the superimposed layers or layers of carbon can be clearly open to provide an expansion mark in the direction perpendicular to the layers, that is, in the "c" direction, and therefore form an expanded or numbed graphite structure, in which the laminar character of the carbon layers is retained substantially. The graphite flake that has been expanded chemically or thermally, and more particularly, expanded to have a thickness dimension or the final "c" direction which is up to about 80 or more times the original dimension of the "c" direction , can be formed without the use of a linker in cohesion sheets or integrated expanded graphite, for example, coils, papers, strips, tapes or the like (usually referred to as "flexible graphite"). The formation of graphite particles that have been expanded to have a final "c" thickness or direction that is as much as about 80 times or more the original "c" direction dimension within the integrated flexible sheets by compression is considered possible. , without the use of any bonding material, due to the mechanical interlock, or cohesion that is achieved between the graphite particles expanded in voluminous form.

As noted above it has been found that in addition to flexibility, the sheet material possesses a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to the orientation of the graphite particles expanded substantially parallel to the opposite faces of the sheet resulting from a very high compression, for example, roller pressing. Therefore, the material of the produced sheet has excellent flexibility, good strength and a very high degree of orientation. In summary, the process for producing an anisotropic, non-binder, anisotropic graphite sheet material, eg, coil, paper, strip, tape, sheet, sphere or the like, comprises compressing or compacting under a predetermined load, and in the absence of a linker, expanded graphite particles having an "c" direction dimension which is as large as about 80 or more times that of the original particles, to thereby form a flexible, substantially planar integrated graphite sheet. Once compressed the expanded graphite particles that generally have the appearance of a worm or bugs, will maintain compression and alignment with the opposite major surfaces of the leaf. The density and thickness of the sheet material can be varied, controlling the degree of compression. The density of the sheet material may be in the range of from about 0.04 g / cc to about 1.4 g / cc. The material of the flexible graphite sheet exhibits a degree of appreciable anisotropy due to the alignment of the graphite particles parallel to the greater parallel and opposite surfaces of the sheet, increasing, at the moment of roller pressing of the sheet material, the degree of anisotropy to achieve an increased density. In an anisotropic roller-pressed sheet material, the thickness, for example, the direction perpendicular to the surfaces of the parallel and opposite sheets, comprises the "c" direction and the directions fluctuate along the length and width, for example , along parallel to the opposite major surfaces that comprise the "a" directions and usually by orders of magnitude, the thermal, electrical and fluid diffusion properties of the sheet are very different, for the "c" and "to". This considerable difference in properties, for example, anisotropy, which is directionally dependent, may be inconvenient in some applications. For example, in joint applications where the flexible graphite sheet is used as the joint material and in use is held tight between the metal surfaces, the diffusion of fluids, eg bases or liquids, occurs more easily in the form parallel and between the larger surfaces of the flexible graphite sheet. In most cases, higher joint performance could be provided if the fluid flow resistance is increased parallel to the larger surfaces of the graphite sheet ("a" direction), even at the cost of reduced strength to the transverse fluid diffusion flow of the larger faces of the graphite sheet (direction "c"). With respect to electrical properties, the strength of the anisotropic flexible graphite sheet in the transverse direction to the larger surfaces ("c" direction) of the flexible graphite sheet is high, and substantially less in the direction parallel to the larger faces of the flexible graphite blade (direction "a"). In applications such as electrodes for fuel cells, it may be desirable to decrease the transverse electrical resistance to the larger surfaces of the flexible graphite sheet (direction "c"), including use in the case of increased capacity. of electrical resistance in the direction parallel to the larger faces of the flexible graphite blade (direction "a"). With regard to thermal properties, the thermal conductivity of a flexible graphite sheet in a direction parallel to the larger surfaces of the flexible graphite sheet is relatively high, although it is relatively low in the direction "c" transverse to the larger surfaces. The flexible fibrous sheet may be supplied with channels, which have preferably smooth sides, which pass between the opposite and parallel surfaces of the flexible graphite sheet and are separated by walls of compressed expanded copper. When the flexible graphite sheet functions as an electrode in an electrochemical fuel cell, it is placed so that it rests on the ion exchange membrane, so that "the top parts" of the graphite sheet walls Flexible lean against the ion exchange membrane. SUMMARY OF THE INVENTION The present invention is directed to material to be used in a process that allows the engraving of a high definition pattern on a flexible graphite sheet, to provide a material with the unique ability to be used as a component in a cell. PEM fuel already a process that allows the engraving of a high definition pattern on a flexible graphite sheet. The present invention provides a material suitable for being used to form a recorded article and a process useful for forming a recorded article. The material and process are useful for manufacturing PEM fuel cells. The material is formed from a sheet of a compressed mass of expanded graphite particles. The particles may have a controlled condition (and, concomitantly in many cases density) controlled to allow the formation of a recorded article having a controlled morphology. This can be achieved, for example, by satining or pressing the flexible graphite before engraving. By controlling the hollow condition of the sheet, and thus the morphology of the engraved article, certain characteristics of the engraved article can be controlled. For example, the thermal anisotropy ratio (that is, the ratio of the thermal conductivity in plane to the thermal conductivity across the plane) can be controlled to provide a determined thermal anisotropy to provide a desired heat dissipation capacity. Similarly, electrical anisotropy can be controlled in the same way. The etched pattern is conveniently formed in the material of the present invention, mechanically impacting an opposite surface of the graphite sheet to displace the graphite within the sheet at predetermined locations to provide a channel pattern. The process of the present invention comprises providing an engraving apparatus generally comprising two opposing elements, one of the two opposing elements comprising an engraving element having a pattern of engraving therein, the engraving pattern formed through them having valleys. the formation of a series of walls (for example, upper part of the walls) which have a predetermined height from the surface of the engraving element and the floors of the channel, around the engraving element; and the other of the two opposing elements comprising a capture element having an impact surface, wherein the engraving element and the capture element are formed in the engraving apparatus, so that the impact surface of the capture element is separate from the channel floors of the engraving element by a distance "d" which is at least equal to (and preferably greater than) the height of the valleys; wherein the material of the flexible graphite sheet of the present invention is recorded by passing it between the engraving element and the capture element of the engraving apparatus, so that the valleys of the engraving element exert pressure on the flexible graphite sheet, wherein the flexible graphite sheet has a thickness in the region of the engraving pattern before engraving, which is less than the distance "d", although greater than the distance between the impact surface of the capture element and the valleys of the etching element, thereby forming a gap between the flexible graphite sheet and the channel floors of the engraving element, wherein in addition, the engraving of the flexible graphite sheet in the engraving apparatus causes the material to flow from the area of the flexible graphite sheet encountering pressure from the valleys of the engraving element to the gap between the flexible graphite sheet and the channel floors of the engraving element. The sheet of the present invention is preferably impregnated with resin, such as a thermoplastic or thermosetting resin. For example purposes, the thermosetting resin can be selected from polycarbodiimide resins, phenolic resins, acrylic resins, furfuryl alcohol resins, epoxy resins, cellulose, urea resins, melamine resins and diallyl festalate resins. The thermosetting resin may also include resins of elastic rubber polymers, such as styrene-butarene rubber resins, acrylonitrile-butarene rubber resins and chloroprene rubber resins; polysiloxane resins, such as silicone elastomer resins and silicone rubber resins of the curing type at room temperature; and polyurethane resins. The thermoplastic resin can be selected from olefin resins, styrene resins, vinyl resins, ethylene-vinyl acetate copolymer resins, amide resins, ether resins, carbonate resins, acetate resins and acrylic resins. More specifically, the thermoplastic resins may include polyethylene resins, polystyrene resins, polypropylene resins, polymethyl metalacrylate resins, polyethylene terephthalate resins, polybutylene terephthalate resins, polyethersulfone resins, polycarbonate resins, polyoxamethylene resins, polyamide resins, polyimide resins, polyamideimide resins, polyvinyl alcohol resins, polyvinyl chloride resins, fluororesin resins, polyphenylsulfone resins, polyetherketone resins, polysulfone resins, polyetherketone resins, polyarylate resins, polyetherimide resins and polymethylpentene resins, ethylene-propylene resin copolymer resins, ethylene-acrylate ester copolymer resins, polystyrene resins, acrylonitrile-styrene copolymer resins, acrylonitrile-butadine-styrene copolymer resins, chloride resins vinyl, polymethyl methacrylate resins , polyester resins, nylon-6 resins, nylon-66 resins, polystyrene terephthalate resins and polybutylene terephthalate resins. The thermoplastic resin may be a mixture, a copolymer, or a modified polymer thereof. Also specifically included are acetate resins and acrylic resin. Preferably, the resin is a thermosetting resin, more preferably a resin system based on acrylic, epoxy or phenol with which the sheet is impregnated before etching and the resin is conveniently cured once the flexible graphite sheet is etched. . The resin content of the flexible graphite sheet material impregnated with resin is preferably at least about 5%, and more preferably at least about eM O% by weight. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood and its advantages will be better appreciated by virtue of the detailed description that follows, especially when read with reference to the accompanying drawings, wherein: Figure 1 is a view in plan of partial cross-section of one side of an engraved graphite article produced in accordance with the present invention. Figure 1 (A) is a top plan view of the sheet of Figure 1; Figure 2 is a partial cross-sectional view of an embodiment of an engraving apparatus useful in the process of the present invention. Fig. 2 (A) is a partial cross-sectional view of an embodiment of an engraving apparatus useful in the process of the present invention, observed immediately after the beginning of the engraving process; Figure 2 (B) is the engraving apparatus of Figure 2, observed as the engraving occurs; Figure 2 (C) shows a perspective view of the engraving apparatus of Figure 2; Figures 3, 4 are photomicrographs at a 50x magnification of a cross section of one of the walls of a flexible graphite sheet prepared in accordance with the present invention, showing morphologies that can be achieved using a free flexible graphite sheet of holes (figure 3) and with holes (figure 4); and Figure 5 is a side plan view of a sheet of embossed flexible graphite having an "edge" or "flange", as described above. Detailed Description of the Invention Graphite is a form of crystalline carbon that comprises atoms covalently linked in planes with flat layers with weaker bonds between them. blueprints. By treating the graphite particles, such as graphite flakes, with an interleaver of, for example, a solution of sulfuric and nitric acid, the crystal structure of the graphite reacts to form a graphite compound and intercalator. The treated graphite particles will be referred to hereinafter as "interleaved graphite particles". When exposed to high temperature, the intercalator within the graphite decomposes and volatilizes causing the particles of intercalated graphite to expand in dimensions as large as approximately 80 or more times their original volume in a manner similar to an accordion in the direction "c ", for example, in the direction perpendicular to the crystalline planes of the graphite. The exfoliated graphite particles look like bugs, and are therefore commonly referred to as worms. The worms can be compressed together into flexible sheets, so that, unlike the original graphite flakes, they can be formed and cut into various shapes and supplied with small transverse openings by mechanical impact deformation. Graphite starting materials suitable for use in the present invention include carbonaceous materials with high graphite content with the ability to intercalate organic and inorganic acids as well as halogens and subsequently expand when exposed to heat. These carbonaceous materials with a high content of graphite more preferably have a graphite content of about 1.0. As used in the present description, the "grade of graphite content" refers to the value g according to the formula: g - 3.45 - d (002 0.095 where d (002) is the space between the graphite layers of the carbons found in the crystal structure measured in Angstrom units The space between the graphite layers is measured by standard X-ray diffraction techniques, and the positions of the diffraction peaks corresponding to the iller indexes are measured ( 002), (004) and (006), and standard minimum squares techniques are used to derive the space that minimizes the total error for all these peaks.Examples of carbonaceous materials with high graphite content include natural graffiti from several sources, as well as other carbonaceous materials such as carbons prepared by vapor deposition and the like.Natural graphite is the most preferred.The graphite starting materials used in the present The invention may contain components without carbon, provided that the crystal structure of the starting materials maintains the required degree of graphite content and has the ability to exfoliate. Usually, any material containing carbon whose crystal structure possesses the required degree of graphite content and which can be interleaved and exfoliated, will be suitable for use with the present invention. The graphite preferably has an ash content of less than 6% by weight. More preferably, the graphite used for the present invention will have a purity of at least about 98%. In the most preferred embodiment the graphite employed will have a purity of at least about 99%. A common method for manufacturing a graphite sheet is described in the Shane and Associates publication in US Patent No. 3,404,061, the disclosure of which is incorporated herein by reference. In the normal practice of the Shane and associates method, the natural graphite flakes are interspersed by dispersing the flakes in a solution containing, for example, a mixture of nitric and sulfuric acid, conveniently at a level of from about 20 to about 300 parts by weight. weight of intercalator solution per 100 parts by weight of graphite flakes (pph). The interleaving solution contains oxidation agents and other interleaving agents known in the art. Examples include those oxidizing agents and oxidation mixtures, such as solutions containing nitric acid, potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, perchloric acid and the like or mixtures, such as for example , concentrated nitric acid and chlorate, chromic acid and phosphoric acid, sulfuric acid and nitric acid or mixtures of a strong organic acid, for example, trifluoroacetic acid and a strong oxidizing agent soluble in the organic acid. Alternatively, an electrical potential can be used to provide the oxidation of the graphite. The chemical species that can be introduced into the graphite crystal using electrolytic oxidation, include sulfuric acid, as well as other acids. In a preferred embodiment, the intercalating agent is a solution of a mixture of sulfuric acid, based on sulfuric acid and phosphoric acid, and an oxidizing agent, for example, nitric acid, perchloric acid, chromic acid, potassium permanganate, hydrogen peroxide, yodic and periodate acids or the like. The interleaving solution may also contain metal alures, such as ferric chloride, and ferric chloride mixed with sulfuric acid, or an aluride, such as bromine in the form of a solution of bromine and sulfuric acid or bromine in an organic solvent. The amount of interleaving solution can range from about 20 to about 150 pph and more usually from about 50 to about 120 pph. Once the flakes are interspersed, any excess solution of the flakes is removed and the flakes are washed with water. Alternatively, the amount of interleaving solution may be limited to between about 10 and about 50 pph, which allows the washing step to be eliminated as considered and described in US Patent No. 4,895,713, the description of which is also incorporated herein by reference. the present invention as a reference. The particles of the graphite flake treated with interleaving solution can be contacted optionally, for example, combined with an organic reducing agent selected from alcohols, sugars, aldehydes and asters, which are reactive with the surface film of the solution of intercalation of oxidation at temperatures that fluctuate from 25 ° C to 125 ° C. Suitable specific organic agents include hexadecanol, octadecanol, 1-octanol, 2-octanol, decyl alcohol, 1, 10 decanediol, decyl aldehyde, 1-propanol, 1,3-propanediol, ethylene glycol, polypropylene glycol, dextrose, fructose, lactose, sucrose, potato starch, ethylene glycol monostearate, diethylene glycol dibenzoate, propylene glycol monostearate, glycerol monostearate, dimethyl oxylate, diethyl oxylate, methyl formate, ethyl formate, ascorbic acid and lignin-derived compounds, such as sodium lignosulfate. The amount of organic reducing agent is from about 0.5 to 4% by weight of the graphite flake particles. The use of an expansion aid applied before, during or immediately after interleaving can also provide improvements. Among these improvements can be found a reduced exfoliation temperature and an increased expanded volume (also referred to as "volume of worms"). An expansion aid within this context will conveniently be an organic material sufficiently soluble in the interleaving solution to achieve an improvement in expansion. To a lesser extent, organic materials of this type containing carbon, hydrogen and oxygen can preferably be used exclusively. The carboxylic acids have been found to be especially effective. A suitable carboxylic acid useful as the expansion aid can be selected from aromatic, aliphatic or cycloaliphatic acids, straight or branched chain monocarboxylic acids, saturated or unsaturated, dicarboxylic acids and polycarboxylic acids which have at least one carbon atom, and preferably up to about 15 carbon atoms, which are soluble in the interleaving solution in effective amounts to provide an improvement that can be measured from one or more of the exfoliation aspects. Suitable organic solvents can be used to improve the solubility of an organic expansion aid in the interleaving solution. Representative examples of saturated aliphatic carboxylic acids are acids, such as those of the formula H (CH2) nCOOH, wherein n is a number from 0 to about 5, including formic, acetic, propionic, butyric, pentanoic, haxanoic, and Similar. Instead of the carboxylic acids, reactive anhydrides or carboxylic acid derivatives, such as alkyl esters, can also be used. Representatives of alkyl esters are methyl formate and ethyl formate. Sulfuric acid, nitric acid and other known aqueous intercalators have the ability to decompose formic acid, finally to water and carbon dioxide. Because of this, formic acid and other sensitive expansion aids are conveniently contacted with the graphite glue prior to the immersion of the flake in the aqueous intercalator. Representatives of dicarboxylic acids are aliphatic dicarboxylic acids having from 2 to 12 carbon atoms, in particular oxalic acid, fumaric acid, malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid, 1,5-pentanedicarboxylic acid, acid 1,6-hexanedicarboxylic acid, 1,1-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid and aromatic dicarboxylic acids, such as phthalic acid or etherphthalic acid. Representatives of alkyl esters are dimethyl oxylate and diethyl oxylate. The representatives of cycloaliphatic acids are cyclohexane carboxylic acid and aromatic carboxylic acids are benzoic acid, naphthoic acid, anthranilic acid, p-aminobenzoic acid, salicylic acid, or acids, myp tolyl, methoxy and ethoxybenzoic acids, acetoacetamibenzoic acids and acetamidobenzoic acids, phenylacetic acid, and naphthoic acids. The representatives of hydroxy aromatic acids are hydroxybenzoic acid, 3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid, 4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid, 5-hydroxy acid. -2-naphthoic, 6-hydroxy-2-naphthoic acid and 7-hydroxy-2-naphthoic acid. The most prominent among the polycarboxylic acids is citric acid. The interleaving solution will be aqueous and will preferably contain an amount of expansion aid of from about 1 to 10%, the amount being effective to increase the exfoliation. In the embodiment where the expansion aid is contacted with the graphite flake before or after immersion in the aqueous intercalating solution, the expansion aid can be mixed with the graphite through suitable means, such as a mixer. V, usually from an amount of from about 0.2% to about 10% by weight of the graphite flake. After intercalating the graphite flake, and after mixing the interleaved graphite flake interleaved with the organic reduction agent, the combination can be exposed to temperatures within the range of 25 ° C to 125 ° C to promote the reaction of the reducing agent and interlayer coating. The heating period is up to about 20 hours, with shorter heating periods, for example, at least about 10 minutes, for higher temperatures within the aforementioned range. You can use times of half an hour or less, for example, in the order of 10 to 25 minutes, at higher temperatures. The graphite particles treated in this manner are sometimes referred to as "interleaved graphite particles". By exposing the interleaved graphite particles at high temperatures, for example, at temperatures of at least about 160 ° C, especially from about 700 ° C to 1000 ° C and above, they expand as much as about 80 to 1000 times or plus its original volume in a manner similar to an accordion in the c-direction, for example, in the direction perpendicular to the crystalline planes of the constituent graphite particles. The expanded graphite particles, for example, exfoliated, have a bug appearance and are now commonly referred to as worms. The worms can be compressed together into flexible sheets, so that unlike the original graphite flakes, they can be formed and cut into various shapes and supplied with small transverse openings by mechanical impact deformation, as will be described later. The sheet and the flexible graphite sheet are coherent, with good resistance to handling and are suitably compressed, for example, by roller pressing, until obtaining a thickness from about 0.075 mm to 3.75 mm and a typical density of about 0.1. to 1.4 grams per cubic centimeter (g / cc). From about 1.5% to 30% by weight of the ceramic additives can be combined with interleaved graphite flakes, such as described in US Patent No. 5,902,762 (which is incorporated herein by reference). ) to provide an improved resin impregnation in the final flexible graphite product. The additives include ceramic fiber particles having a length from about 0.15 to about 1.5 millimeters. The width of the particles is conveniently from about 0.04 to 0.004 mm. The ceramic fiber particles are non-reactive and do not adhere to the graphite and are stable at temperatures of up to about 1 100 ° C, preferably about 1400 ° C or higher. Suitable ceramic fiber particles are formed from macerated quartz glass fibers, carbon and graphite fibers, zirconia, boron nitride, silicon carbide and magnesia fibers, and naturally occurring mineral fibers such as fibers of calcium metasilicate, calcium aluminum silicate fibers, aluminum oxide fibers and the like. As it is observed, sometimes the flexible graphite sheet can also be conveniently treated with resin and the absorbed resin, after curing, increases the resistance to moisture and resistance to handling, for example, rigidity, the flexible graphite sheet, as well as the "fixation" of the morphology of the sheet. The suitable resin content is preferably at least about at least 5% by weight, more preferably about 1 0 to 35% by weight, and suitably up to about 60% by weight. Resins found to be especially useful in the practice of the present invention include resin systems based on acrylic, epoxy and phenolic or mixtures thereof. Suitable epoxy resin systems include those based on bisphenol A glycidyl ether (DGEBA) and other multifunctional resin systems; Phenolic resins that can be employed include resole and phenolic novolak. Normally, although not necessarily, the resin system is soldered to facilitate application on the flexible graphite sheet. In a typical resin impregnation step, the flexible graphite sheet is passed through a container and impregnated with the resin system from, for example, sprinkler nozzles, with the resin system being "extracted through the mat. "conveniently by means of a vacuum chamber. The resin is subsequently preferably dried, reducing the stickiness of the resin and the sheet impregnated with the resin, which has a starting density of about 0.1 to about 1.1 g / cc, is subsequently processed to change the condition of holes in the resin. the sheet. The term "hole condition" means the percentage of the sheet represented by voids, which are generally in the form of trapped air. Generally, this is achieved by applying pressure to the sheet (which has the effect of densifying the sheet) to reduce the level of gaps in the sheet, for example in a satin mill or in a platen press. Conveniently, the flexible graphite sheet is densified to a density of at least about 1.3 g / cc (although the presence of resin in the system can be used to reduce gaps without requiring densification to the level). The hole condition can be conveniently used to control and adjust the morphology and functional characteristics of the final engraved article. For example, conductivity, thermal and electrical, permeability range and chemical washout characteristics can be produced and controlled in a potential way, controlling the condition of the holes (and, usually the density) of the previous sheet recorded . Therefore, if a group of desired characteristics of the final engraved article is recognized prior to the manipulation of the hole condition, the hole condition may be custom designed to achieve these characteristics to the greatest extent possible. As noted above, this can be achieved, for example, by satining or pressing the flexible graphite before engraving. More conveniently, especially when the final etched article is intended to be used as a component in an electrochemical fuel cell, the flexible graphite sheet impregnated with resin is manipulated to be relatively free of voids, to use thermal and electrical conductivities for applications of fuel cell. Generally, this is accomplished by achieving a density of at least about 1.4 g / cc, more preferably at least about 1.7 g / cc, which indicates a relatively void-free condition, which leads to production of a recorded article having a relatively high anisotropy ratio (potentially of the order of about 150 and greater). When a lower proportion of anisotropy is desired, such as in certain heat dispersion applications, a high density of the void condition is preferred, which generally corresponds to a density in the range of about 1.1 to about 1.3. g / cc (depending again on the presence / level of resin in the system). Referring now to Figures 3,4, photomicrographs of a cross section of a wall of each of the two sheets, prepared using the material of the present invention, are presented. The sheet of figure 3 was manipulated before engraving until a relatively void-free condition was obtained. The sheet of figure 4 was not manipulated at all before engraving. The differences in morphology are apparent. It can be readily appreciated in Figure 3 that the graphene layers are more aligned, (eg, in parallel) wall surfaces. In fact, a region of "inverted triangle" in the upper part of the wall is evident and an intersection line appears where the frontal parts of the graphite flow meet, essentially dividing the internal structure of the wall into relatively symmetrical parts. When this is contrasted with the wall of Figure 4, the structure created by the control of the hole condition is apparent. As will be familiar to those skilled in the art and as described above, the relative amount of structure in a recorded flexible graphite wall can and will lead to different properties. Subsequently the satin flexible graphite sheet passes through an etching apparatus, as will be described later, and is subsequently heated in a furnace to cure the resin. Depending on the nature of the resin system employed, and especially the type and level of solvent employed (which is suitably designed to suit the specific resin system, as will be familiar to those skilled in the art), it may be included a vaporisation drying step before the etching step. In this drying step, the flexible graphite sheet impregnated with resin is exposed to the heat to vaporize and thereby remove part of all or the solvent, without carrying out the curing of the resin system. In this way, the creation of bubbles during the curing step is avoided, which can be caused by the vaporization of the solvent trapped inside the sheet by the densification of the sheet during the forming of the surface. The degree and time of heating will vary with the nature and amount of the solvent, and preferably at a temperature of at least about 65 ° C and more preferably from about 80 ° C to about 95 ° C for from about 3 to about 20 minutes for this purpose. In International Publication No. WO 00/64808, the description of which is incorporated herein by reference, there is shown an embodiment of an apparatus for continuously forming a sheet of flexible graphite impregnated with resin and satin. As illustrated in Figure 2-2 (C), an engraving apparatus 10 generally comprises two opposing elements 20 and 30, at least one of which is in an engraving element 20 and has a pattern of engraving therein. . The engraving pattern is elaborated, forming a series of walls 22 having upper parts or valleys 22a that have a predetermined height from the surface of the engraving element 20, is separated by floors of the channel 24, around the surface of the element etching 20. Normally, the floors of the channel 24 are in fact the surface of the engraving element 20. The capture element 30 preferably comprises an element with generally flat surface against which the engraving element 20 operates to force the engraving pattern On the flexible graphite sheet impregnate with resin. The impact surface 32 of the capture element 30 may also have texture or other artifacts (not shown) to facilitate the etching process or apply a desired texture or pattern to the non-engraved surface of the flexible graphite sheet. In addition, since it is anticipated that the pattern engraved on the flexible graphite sheet may vary, the situation may potentially occur when certain portions of the flexible graphite sheet have different cross-sectional areas to each other. In such cases, it may be desirable to apply a structure to the impact surface 32 of the capture elements 30 (such as a depression) (not shown), to allow the flow of graphite / resin in the depression in lower cross-sectional regions of the flexible graphite sheet 1 00, to keep the upper (or operating) surface of the flexible graphite sheet 10000 relatively uniform. The resulting flexible graphite sheet 1 00 will have a "rim" or "rim" 102a in FIG. its bottom, as shown in Figure 5, which can be used or eliminated (such as by machinery). The engraving element 20 and the capture element 30 may comprise rollers, plates or a combination thereof, or other structures, provided they have the ability to cooperate to engrave a pattern onto a flexible graphite sheet, and preferably comprise rolls, as shown in Figure 2 (C). The engraving element 20 and the capture element 30 are formed in the engraving apparatus 10, so that the surface 32 of the capture element 30 is separated from the floors of the channel 24 of the engraving element 20 by a distance "d" , which is at least equal to the height of the walls 22. In fact, in the most preferred embodiment, the surface 32 of the capture element 30 is separated from the floors of the channel 24 of the engraving element 20 by a distance "d "which is equal to the height of the walls 22 plus the desired thickness of the flexible graphite sheet 1 00 in the place of the floors of the sheet 102 of the flexible graphite sheet 100. The sheet of flexible graphite satin impregnated with resin 100a, it is formed to have a thickness in the region of the engraving pattern before engraving, which is less than the distance "d", but greater than the distance between the surface 32 of the capture element 30 and the valleys 22a of the walls 22 of the engraving element 20 , as illustrated in FIG. 2. During engraving, the material (eg, graphite and resin) found in the sheet 100a flows from the area of the sheet 100a which encounters pressure from the valleys 22a of the element 10a. engraving 20, pressing against the sheet 100a towards the opening between the sheet 1 00a and the floors of the channel 24 of the engraving element 20, as illustrated in Figures 2-2 (B). This "readjustment" of graphite / resin of the flexible satin graphite sheet impregnated with 1 00a resin is surprising and leads to a sheet of flexible graphite 100 that has sheet flats 102 and valleys of sheet 1 04 that form a channel pattern corresponding to the engraving pattern of the engraving element 20 (as shown in Figures 1 and 1 (A)), the channel pattern of sheet 1 00 having an improved channel definition compared to the processes of the prior art. The resulting engraved graphite sheet can be used in a variety of applications, including in the form of a component in an electrochemical fuel cell. The above description is intended to enable those skilled in the art to carry out the present invention. It is not intended to detail all the possible variations and modifications that may be appreciated for those skilled in the art to read the present description. However, it is intended that all such modifications and variations be included within the scope of the present invention, which is defined by the claims that follow. The claims are intended to cover the elements and steps indicated in any manner and sequence that is effective to meet the intended objectives of the present invention, unless the context specifically indicates otherwise.

Claims

32 R E I V I N D I C A C I O N E S 1. A material useful as a substrate for a recorded flexible graphite sheet, wherein the material comprises a flexible graphite sheet having a void condition selected to produce a desired morphology at the time of engraving. 2. The material according to claim 1, wherein the flexible graphite sheet is relatively free of voids before etching. 3. The material according to claim 1, wherein the flexible graphite sheet is subjected to the application of pressure to provide the selected void condition. 4. The material according to claim 3, wherein the flexible graphite sheet is densified to obtain a density of at least about 1.1 g / cc before etching. 5. The material according to claim 1, wherein the flexible graphite sheet is impregnated with resin. 6. The material according to claim 5, wherein the resin is present at a level of at least about 5% in the flexible graphite sheet. 7. The material according to claim 5, wherein the resin comprises a thermoplastic or thermo-setting resin. The material according to claim 6, wherein the resin comprises an acrylic-based resin system, an epoxy-based resin system or a phenol-based resin system. 33 9. The material according to claim 5, wherein the flexible graphite sheet will be etched in an etching apparatus comprising an engraving element comprising walls having valleys and a capture element comprising a surface, and the graphite sheet flexible has a thickness less than the height of the walls of the engraving element used. The material according to claim 9, wherein the flexible graphite sheet has a thickness less than the height of the walls of the engraving element, but greater than the distance between the surface of the capture element and the valleys of the engraving element. the walls of the engraving element. eleven . A process for producing a recorded flexible graphite sheet having leaf and leaf valleys, which comprises: a) providing an engraving apparatus that generally comprises two opposing elements, i. one of the two opposing elements comprising an engraving element having a pattern of engraving therein, the engraving pattern being formed by the formation of a series of walls having a predetermined height from the surface of the engraving element and floors of the channel, around the surface of the engraving element, the walls having valleys; and ii. the other of the two opposing elements comprising a capture element having an impact surface; wherein the engraving element and the capture element are formed in the engraving apparatus so that the impact surface of the capture element is separated from the floors of the channel of the engraving element by a distance "d" which is at least equal to the height of the walls; b) engraving a flexible graphite sheet by passing it between the engraving element and the capturing element of the engraving apparatus, so that the valleys of the engraving element exert pressure on the flexible graphite sheet; wherein the flexible graphite sheet has a thickness in the region of the engraving pattern before the engraving process, which is less than the distance "d", but greater than the distance between the impact surface of the capture element and the valleys of the walls of the engraving element, thereby forming a gap between the flexible graphite sheet and the channel floors of the engraving element. 12. The process according to claim 1, wherein the flexible graphite sheet is impregnated with resin before etching. 13. The process according to claim 12, wherein the resin is a thermoplastic or thermo-setting resin. The process according to claim 12, wherein the resin used to impregnate the flexible graphite sheet comprises a system based on acrylic, epoxy or phenol, or mixtures thereof. 15. The process according to claim 14, wherein the resin is cured after the graphite sheet is etched. flexible. 16. The process according to claim 14, wherein the resin content of the flexible graphite sheet impregnated with resin is at least about 5% by weight. 17. The process according to claim 12, wherein the flexible graphite sheet is glazed before the etching process. The process according to claim 17, wherein the flexible graphite sheet is relatively free of voids. 19. The process according to claim 18, wherein the flexible graphite sheet becomes relatively void free by saturating it until a density of at least about 1.6 g / ce is obtained before etching. 20. The process according to claim 1, wherein the impact surface of the capture element is separated from the floors of the channel of the engraving element by a distance "d", which is equal to the height of the walls. plus the desired thickness of the flexible graphite sheet engraved in the place of the floors of the flexible graphite sheet. twenty-one . The process according to claim 1, wherein the impact surface of the capture element comprises a pattern designed and formed to compensate for the difference of the cross-sectional areas of the recorded flexible graphite sheet. 22. A process for producing a flexible graphite sheet 36 engraving having a controlled morphology wherein the process comprises: a) providing an engraving apparatus that generally comprises two opposing elements, i. one of the two opposing elements comprising an engraving element having a pattern of engraving therein, the engraving pattern being formed by the formation of a series of walls having a predetermined height from the surface of the engraving element and floors of the channel, around the surface of the engraving element, the walls having valleys; and ii. the other of the two opposing elements comprising a capture element having an impact surface; wherein the engraving element and the capture element are formed in the engraving apparatus so that the impact surface of the capture element is separated from the floors of the channel of the engraving element by a distance "d" which is equal at the height of the walls; b) manipulate the hole condition of the flexible graphite sheet by saturating it until a predetermined density is obtained; c) engraving the manipulated flexible graphite sheet by passing it between the engraving element and the capturing element of the engraving apparatus, so that the valleys of the engraving element exert pressure on the flexible graphite sheet; where the graphite sheet 37 flexible handled has a thickness within the region of the engraving pattern before the engraving process, which is less than the distance "d", but greater than the distance between the impact surface and the capture element and the walls of the engraving element. etching, thereby forming a gap between the flexible graphite sheet and the channel floors of the engraving element. And where also the hole condition of the flexible graphite sheet is selected to achieve a desired morphology of the recorded flexible graphite sheet. 23. The process according to claim 22, wherein the flexible graphite sheet is impregnated with resin before etching. 24. The process according to claim 23, wherein the resin is a thermo-setting resin or thermoplastic. 25. The process according to claim 23, wherein the resin used to impregnate the flexible graphite sheet comprises a system based on acrylic, epoxy or phenol, or mixtures thereof. 26. The process according to claim 25, wherein the resin is cured after the flexible graphite sheet is etched. 27. The process according to claim 22, wherein the flexible graphite sheet becomes relatively void free by saturating it to a density of at least about 1.6 g / ce before etching. 38 28. The process according to claim 22, wherein the impact surface of the capture element is separated from the floors of the channel of the engraving element by a distance "d", which is equal to the height of the walls plus the thickness desired of the flexible graphite sheet engraved in the place of the floors of the flexible graphite sheet.