MXPA99012037A - Flexible graphite sheet with decreased anisotropy - Google Patents

Abstract

Flexible graphite sheet is made by compressing a mixture of relatively large particles of intercalated, exfoliated, expanded natural graphite with smaller particles of intercalated, exfoliated expanded, expanded particles of natural graphite. The resulting sheet of flexible graphite exhibits increased electrical conductivity through the thickness ("c"direction) of the sheet and improved sealability perpendicular to the"c"direction.

Description

LAMINA D, E FLEXIBLE GRAPHITE WITH REDUCED ANISOTROPIA FIELD OF THE INVENTION This invention relates to a method for making flexible graphite sheets having reduced anisotropy, with respect to electrical resistivity, to provide increased electrical conductivity across the thickness of the sheet, with respect to the sealing capacity, to provide improved sealing capacity (fewer spills) perpendicular to the thickness of the flexible graphite sheet.

BACKGROUND OF THE INVENTION Graphites are made from layer planes of configurations or hexagonal networks of carbon atoms. These layer planes of the carbon atoms that are arranged in a hexagonal manner are substantially planar and are oriented or ordered so as to be substantially parallel and equidistant from one another. The sheets or layers substantially flat, parallel equidistant carbon atoms, which are usually referred to as basal planes, are linked or joined together and the groups thereof are accommodated in crystallites. The highly ordered graffiti consist of *** crystallites of considerable size: the crystallites highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graffiti has a high degree of preferred crystallite orientation. It should be noted that graphites have anisotropic structures and therefore exhibit or possess many properties that are highly directional. Briefly, graffiti can be characterized as laminated carbon structures, that is, structures consisting of superimposed layers or sheets of carbon atoms joined by weak van der Waals forces. When considering the structure of the graphite, two axes or directions are usually noticed, namely 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. Natural graphite has a high degree of orientation. As noted above, the bonding forces that hold together the parallel layers of carbon atoms are only van der Waals weak forces. The natural graphites can be treated so that the spacing between the superimposed layers or layers of carbon can be opened appreciably in order to provide a marked expansion in the direction perpendicular to the layers, that is, in the "c" direction and thus forming an expanded or swollen graphite structure in which substantially the laminar character is maintained. The natural graphite foil which has expanded greatly and which has expanded in a particular manner so as to have a final thickness or direction dimension "c", which is at least 80 or more times than the direction dimension " c "original, can be formed without the use of a binder within the cohesive or integrated sheets, for example, fabrics, papers, strips, tapes, or the like. It is believed that the formation of graphite particles that have expanded to have a final thickness or dimension "c" that is at least 80 times the dimension of the original "c" direction, within the integrated sheets without the use of no binder material is possible due to the excellent mechanical entanglement, or cohesion, which is achieved between the graphite particles expanded in a bulky manner. In addition to flexibility, it has also been found that the material of the sheet, as noted above, possesses a high degree of anisotropy, for example, with respect to electrical and thermal properties. Sheet material having excellent flexibility, good strength and a high degree of orientation can be produced. Briefly, the process for producing flexible graphite sheet material, without binder, for example, fabrics, paper, strip, tape, sheet metal, mats, or the like, comprises compression or compaction under a predetermined load and in the absence of a binder, of expanded graphite particles having a "c" direction dimension which is at least 80 times the original particles, so as to form an integrated, substantially flat, flexible graphite sheet. Expanded graphite particles, which are generally wormlike or vermiform in appearance, once compressed, will maintain compression fit. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material may be in the range of from about 5 pounds per cubic foot to about 125 pounds per cubic foot. The material of the flexible graphite sheet exhibits an appreciable degree of anisotropy, for example, with respect to electrical resistivity, increasing the degree of anisotropy after roller pressing of the sheet material for increased density. In the roll-pressed anisotropic sheet material, the thickness, ie, the direction perpendicular to the surface of the sheet comprises the "c" direction and the direction extending along the length and width, is say, along or parallel to the surfaces, it comprises the "a" directions.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a scanning electron microscope view (SEM) at an original 100X magnification showing the natural graphite lamellae dimensioned in the 20 x 50 mesh range; and Figure 2 is a scanning electron microscope (SEM) view at an original 100X magnification showing the natural graphite lamellae sized at 50 x 100 mesh.

DETAILED DESCRIPTION OF THE INVENTION Graphite is a crystalline form of carbon comprising atoms joined in planes in flat layers with weaker bonds between the planes. By treating the graphite particles, such as the natural graphite foil, with the intercalation of, for example, a solution of sulfuric and nitric acid, the crystal structure of the graphite reacts to form a graphite compound and what is interspersed Hereinafter, the treated particles of graphite are referred to as "intercalated graphite particles". After exposure to high temperature, interleaved graphite particles expand in dimension as much as 80 times or more of their original volume in a manner similar to accordion in the "c" direction, that is, in the direction perpendicular to the crystalline planes of graphite.

The exfoliated graphite particles are vermiform in appearance, and are therefore referred to as worms. The worms can be compressed together into flexible sheets which, unlike the original graphite lamellae, can be formed and cut into different shapes. In US Pat. No. 3,404,061 to Shane et al., The disclosure of which is incorporated herein by reference, a common method for manufacturing the graphite metal sheet from flexible graphite is described. In the typical practice of the Shane et al. Method, the natural graphite lamellae are interspersed by dispersing the lamellae in a solution containing an oxidizing agent of, for example, a mixture of nitric and sulfuric acid. The intercalation solution contains oxidizing agents and other intercalation agents known in the art. Examples include those containing oxidizing agents and oxidizing 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.

In a preferred embodiment, the intercalating agent is a solution of a mixture of sulfuric acid or sulfuric acid and phosphoric acid, and an oxidizing agent, ie, nitric acid, perchloric acid, chromic acid, potassium permanganate, peroxide hydrogen, iodic or periodic acids, or the like. Although less preferred, the intercalation solutions may contain metal halides such as ferric chloride, and ferric chloride mixed with sulfuric acid, or a halide, such as bromine as a solution of bromine and sulfuric acid or bromine in an organic solvent. After the lamellae are interspersed, any solution in excess of the lamellae is drained. The amount of intercalation solution that is retained in the lamellae after runoff can range from 20 to 150 parts of solution by weight per 100 parts by weight of graphite lamella (pph) and more typically from about 50 to 120 pph. Alternatively, the amount of intercalation solution may be limited to between 10 to 50 parts of solution per one hundred parts of graphite by weight (pph) which allows the washing step to be eliminated as taught and described in the patent. of the United States of America Number 4, 895, 713, the description of which is also incorporated herein by reference. The graphite lamellae interspersed with flexible graphite are exfoliated by exposing them to a flame for only a few seconds at a temperature greater than 700 ° C, more typically 1000 ° C or higher. Then, the exfoliated graphite particles, or worms, are compressed and subsequently roller-pressed into a sheet of densely compressed flexible graphite metal sheet of the desired density and thickness and of substantially increased anisotropy with respect to electrical resistivity and other physical properties In US Pat. No. 3,404,061 to Shane et al mentioned above, these methods of exfoliation and methods for compressing the exfoliated graphite particles into thin metal sheets are described. It is conventional to compress the worms exfoliated in stages, the technique being referred to as the first or early stage product as "flexible graphite mat" having a density of about 3 to 10 pounds / ft3 and a thickness of from 0.1 to 1 inch. . Then, the flexible graphite mat is further compressed by roller pressing into a sheet or sheet of standard density metal previously selected. A flexible graphite mat can thus be compressed into a sheet or sheet of thin metal between 2-180 mils in thickness with a density approaching the theoretical density, although a density of about 70 pounds / ft3 is acceptable for most applications and suitably 10 to 100 pounds / ft3. In a particular embodiment of the present invention, a first batch of natural graphite lamella particles, i.e., naturally occurring graphite lamellae, as shown in Figure 1, (original 100X magnification), at least 80 percent in weight sized in 20 x 50 mesh (through 20 mesh over 50 mesh), it is treated by dispersing the naturally occurring lamellae in an intercalation solution such as the one described above. After the lamellae of the first batch are interspersed, the excess solution is drained from the lamellae, which are then washed with water and dried. A second batch of smaller natural graphite lamellae sized as shown in Figure 2 (original magnification 100X), sized to at least 80 percent by weight in 50 mesh (through 50 mesh over 100 mesh) ), is treated with an intercalation solution in the same manner as the first batch and washed with water and dried in a similar manner. These non-exfoliated natural graphite lamellae, when less than 80 weight percent in 50 x 100 mesh, are mixed and combined with the non-exfoliated particles of the first batch to provide from about 25 percent to 75 percent by weight. weight of the smaller non-exfoliated natural graphite lamellae sized in the mixture that was combined. Particles of the non-exfoliated natural interspersed graphite lamellae are easily mixed to provide a substantially uniform mixture of non-exfoliated, unexpanded lamella particles. This can be achieved, for example, by spreading the finer, non-exfoliated natural graphite particles onto a bed of the larger non-exfoliated natural graphite particles, which are placed on a vibrating table. The mixture of dried lamellae is exposed to a flame for only a few seconds and the particles of the lamella interspersed, ie exfoliated, into vermicular-like particles, which are approximately 80 to 1000 times the volume of the initial dry interspersed lamellae. It has been found that the use of more than 80 weight percent of the smaller sized particles results in a brittle sheet product that does not have good tensile strength.; the use of particle amounts of smaller size of less than 25 weight percent does not significantly affect the anisotropy of the resulting flexible graphite sheet, in terms of electrical resistivity. The mixture of large and small exfoliated graphite particles is roller-pressed into a sheet or sheet metal typically 0.002 to 0.180 inches thick and having a density of at least 10 pounds / ft3. The sheet, or resulting sheet metal, is characterized in that it has a reduced electrical resistivity, ie, increased electrical conductivity, through the thickness ("c" direction) of the sheet or metal sheet. As the proportional amount of the smaller sized particles increases (50 x 100 mesh), the electrical conductivity in the "c" direction of the metal sheet or sheet is also increased, which is important when the sheet or sheet metal is used as a component of a fuel cell electrode, as described in U.S. Patent Number 5,300,370 with reference to "GRAFOIL", which is the trade designation for flexible graphite products of the UCAR Coal Company Inc.

EXAMPLE I (PREVIOUS TECHNIQUE) A natural graphite sheet, sized at 80 percent by weight in 20 x 50 mesh was treated (Figure 1) in a mixture of sulfuric acid (90 weight percent) and nitric acid (10 weight percent). The naturally occurring interspersed lamella which was treated in this manner at about 1 weight percent water was washed and dried. A portion of the heat-treated, interleaved, heat-expandable natural graphite foil was introduced into an oven at 2500 ° F to obtain rapid expansion of the foil in one pound of worm-like, vermicular particles having a volume of about 325 times that of the lamella interspersed without expanding. The interleaved graphite sheet, heat expanded, worm-shaped, was rolled into a sheet approximately 0.030 inches thick and 24 inches wide and a density of 45 pounds / ft3. Samples of the 0.030 inch thick sheet had an electrical resistivity of 10,500 μm O m (micro ohm), in the direction of the thickness of the sheet (direction "c").

EXAMPLE II (This Invention) A first batch of natural graphite lamella, sized at 80 percent by weight in 20 x 50 mesh (Figure 1) in a mixture of sulfuric acid (90 percent by weight) and nitric ( 10 percent by weight). The interleaved natural graphite lamella which was treated in this manner at about 1 weight percent water was washed and dried. A second batch of natural graphite lamella, of smaller size, sized to 80 weight percent in 50 x 100 mesh (Figure 2) in a mixture of sulfuric and nitric acid, was treated and washed in the same manner as the first batch of natural graphite of larger size to obtain the natural graphite foil expandable to heat, unexpanded, interspersed. Different amounts of the expandable natural graphite foil were mixed into the heat, unexpanded, interspersed with the material from the second smallest sized particle batch with one (1) pound of natural heat expandable, unexpanded graphite particles, interspersed with the first batch, to provide blended blends containing from about 25 to 75 by weight of the interleaved, unexpanded natural graphite foil of smaller size. The mixture of the dried lamellae is exposed to a flame for only a few seconds and the particles of the interspersed lamella expand, that is, they are exfoliated, into vermicular-like particles, which are approximately 80 to 1000 times the volume of the initial dry interspersed lamellae. The mixtures of natural graphite particles, heat-expanded, worm-shaped, were pressed into a sheet in a sheet approximately 0.030 inches thick and 24 inches wide and 45 pounds / foot3. Samples (2.5 inch diameter) of the sheet of this Example II were tested for electrical resistivity, as compared to Example I, as shown in the following Table. Also, samples were tested in the form of packing density of 80 pounds / foot3, in the form of rings (50 mm ID x 90 mm OD, as specified in DIN 28090-1) of thickness of 0.030 inches by the Sealing capacity, with the results shown in the Table: Sealing Capacity% in Weight of Starting Particles (DIN 28090-) mI / min Smallest (50 x 100 mesh) Electrical Resistivity 0 10,500 m O m 0.77 25 5,200 m 0 m 75 2,800 m 0 m 0.48 As mentioned earlier, the electrical resistivity decreases with the addition of 25 percent, approximately half of the resistivity, 75 percent addition had about a quarter of the resistivity, while the manageability (strength and flexibility) required was maintained substantially to use the material in a commercial way. The electrical resistivity was obtained using a 2001 Keithley Multimeter and gold-plated, four-probe 225 psi pressure plates on the samples. * The sizes of the mesh in the present are the series of screens of the United States of North America.

Claims (4)

1. CLAIMS 1. The method for making flexible graphite sheet having reduced electrical resistivity through improved thickness and sealing capacity perpendicular to the thickness of the sheet ("c" direction), comprising: (i) providing a first batch of lamella of natural graphite of relatively large size sized at least 80 weight percent in 20 x 50 mesh; (ii) providing a second batch of natural graphite lamella of smaller size sized at least 80 percent by weight in 50 x 80 mesh; (iii) combining the first batch and the second batch to provide a blended mixture containing from about 25 to 75 weight percent of the smaller size natural graphite sheet of the second batch; (iv) treating the combined mixture with an intercalation solution to obtain the intercalated, heat expandable graphite lamella mixture; (v) exposing the interleaved natural graphite lamella mixture of step (iii) at an elevated temperature, to exfoliate the natural graphite lamella sandwiched in a mixture of relatively large, expanded, vermicular worm-like graphite particles. of smaller size; and (vi) passing the combined mixture of step (v) through the pressure rollers to form a compressed, roller pressed, coherent sheet that is formed of the combined mixture of predetermined thickness, decreasing the electrical resistivity through the thickness of the compressed sheet with the increasing amounts in the combined mixture of step (iii) of the smallest natural graphite sheet of step (ii). The method according to claim 1, wherein the mixture of step (vi) is roller pressed to form a sheet having a thickness of from 2 to 180 mils and a density of at least 10 to 100 pounds / ft3 . 3. The flexible graphite sheet that is formed by compressing a combined mixture of relatively large particles of expanded natural graphite, exfoliated, interspersed with smaller particles of expanded, exfoliated particles, interspersed with natural graphite, characterized by having increased electrical conductivity through the thickness of the sheet, compared to the flexible graphite sheet which is formed only from relatively large particles of expanded natural graphite, exfoliated, interleaved. The flexible graphite sheet according to claim 3, wherein the smallest particles are sized to at least 80 percent in 20 x 50 mesh and are present in the amount of 25 to 75 weight percent, and the larger particles are sized to at least 80 weight percent in 20 x 50 mesh.