MXPA99011545A - Flexible graphite composite article for protection against thermal damage - Google Patents

Abstract

Composite article comprising a sheet of flexible graphite in conductive heat relationship with a sheet of heat shielding thermally conductive non-graphitic material (20) in which the heat damage effect of a localized high temperature light source (30) is avoided.

Description

COMPU ARTICLE OF FLEXIBLE GRAPHITE FOR PROTECTION AGAINST THERMAL DAMAGE FIELD OF THE INVENTION This invention relates to a composite article comprising adjacent layers of non-graphite material, i.e., metal, or in some cases, plastic, and flexible graphite sheet. The composite article minimizes the damaging effect to the article composed of a localized source of high temperature, v. g. , a flame, or a stream of hot gas, which is adjacent to the non-graphite layer of the composite article, and also to any substrate adjacent to the flexible graphite sheet of the composite article.

BACKGROUND OF THE INVENTION Graphites are made from planar layers of hexagonal arrays or networks of carbon atoms. These layers of hexagonally arranged carbon atoms are substantially planar and oriented or ordered to be substantially parallel and equidistant from one another. The leaves or layers of carbon atoms parallel, equidistant, substantially planar, usually referred to as elementary planes, are linked or joined together and groups thereof are arranged in crystals. Highly ordered graffiti consist of crystals of considerable size: crystals that are highly aligned or oriented relative to each other and that have well oriented carbon layers. In other words, highly ordered graffiti have a high degree of preferred crystal orientation. It should be noted that graphites have anisotropic structures and 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 layers of carbon atoms joined by weak van der Waals forces. When considering the structure of 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 graphites have a high degree of orientation. As noted above, the bonding forces that hold the parallel layers of carbon atoms together are only van der Waals weak forces. The natural graffiti can be treated such that the space between the overlapping carbon layers or sheets can be appreciably opened to provide a marked expansion in the direction perpendicular to the layers, ie, in the "c" direction and thus form a structure of expanded or intumescent graphite in which the laminar character is retained substantially. The natural graphite foil which has been greatly expanded and more particularly expanded to have a final thickness or dimension in the "c" direction which is at least 80 or more times the dimension in the original "c" direction can be formed into sheets cohesive or integrated without the use of a binder, v. g. , networks, papers, strips, tapes, or the like. The formation of graphite particles that have been expanded to have a final thickness or dimension "c" which is at least 80 times the original dimension in the "c" direction in integrated sheets without the use of any binder material is believed to be possible due to the excellent mechanical interlock, or cohesion that is achieved between the voluminously expanded graphite particles. In addition to flexibility, the sheet material, as noted above, has been found to also possess a high degree of anisotropy. Sheet material can be produced which has excellent flexibility, good strength and a high degree of orientation. Briefly, the process to produce material in flexible graphite sheets, without binder, v. g., network, paper, strip, tape, sheet, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles having a dimension in the "c" direction which is at least 80 times that of the original particles to form an integrated, flexible, substantially flat graphite sheet. Expanded graphite particles that are generally wormlike or vermiform in appearance, once compressed, will maintain the compression set. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be in the range of approximately 80 kilograms per cubic meter to approximately 2000 kilograms per cubic meter.

The flexible graphite sheet material exhibits an appreciable degree of anisotropy, with the degree of anisotropy being increased by the roll pressing of the sheet material for increased density. In roll-anisotropic sheet material, the thickness, ie, the direction perpendicular to the surface of the sheet comprises the "c" direction and the directions that fluctuate along the length and width, i.e., throughout or parallel to the surfaces that comprise the "a" directions.

BRIEF DESCRIPTION OF THE DIAMETERS Figure 1 shows a continuous sheet of flexible graphite contiguous to and in conductive heat relation with a continuous metal sheet; Figure 2 shows an elevation view of the arrangement of Figure 1 in combination with a localized high temperature heat source; Figure 3 shows a continuous sheet of flexible graphite intermediates contiguous to metal sheets, with the arrangement being held in place by clamps; Figure 4 shows the arrangement of Figure 3 mechanically formed in a tray-like configuration; Figure 5 shows a composite article formed of internal and external concentric metal tubes adjoining a flexible graphite intermediate sheet; and Figure 6 shows a composite article formed of a plastic sheet adjoining a flexible graphite sheet.

DETAILED DESCRIPTION OF THE INVENTION Graphite is a crystalline form of carbon comprising atoms joined in flat laminated planes with weaker bonds between the planes. Treating graphite particles, such as natural graphite lamella, with an intercalator of, v. g. , a solution of sulfuric and nitric acid, the crystalline structure of the graphite reacts to form a graphite compound and the intercalant. The treated particles of graphite are referred to hereinafter as "interleaved graphite particles". By exposure to high temperature, interleaved graphite particles expand in dimension as much as 80 or more times their original volume in an accordion manner 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 commonly 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 several shapes. A common method for making graphite sheet from flexible graphite is described by Shane et al. In U.S. Patent No. 3,404,061, the disclosure of which is incorporated herein by reference. In the typical practice of the Shane et al. Method, lamellae of natural graphite are interspersed by dispersing the lamellae in a solution containing an oxidizing agent of, v. g. , a mixture of nitric and sulfuric acid. The intercalation solution contains oxidation and other intercalation agents known in the art. Examples include those containing oxidation 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, v. g. , 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, hydrogen peroxide, Yodic or periodic acids, or the like. Although less preferred, 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 excess solution is drained from the lamellae. The amount of intercalation solution retained in the lamellae after draining can range from 20 to 150 parts of solution by weight per 100 parts by weight of graphite lamellae (pph) and more typically approximately 50 to 120 pph. Alternatively, the amount of the intercalation solution can 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 US Pat. . OR . , No. 4,895,713 whose description is also incorporated herein by reference. The interleaved graphite lamellae are exfoliated in flexible graphite by exposing them to a flame for only a few seconds at a temperature greater than 700 ° C, more typically 1000 ° C or higher. The exfoliated graphite particles, or worms, are then compressed and subsequently roller-pressed into a sheet of densely compressed flexible graphite sheet of desired density and thickness and anisotropy increased substantially with respect to thermal conductivity and other physical properties. Suitable exfoliation methods and methods for compressing the exfoliated graphite particles into thin sheets are described in the above-mentioned U.S. Patent No. 3,404,061 to Shane et al. It is conventional to compress the worms exfoliated in stages with the product of the first stage or initial compression step referred to in the art as "flexible graphite mat". The flexible graphite mat is then further compressed by roll compression into a sheet or sheet of standard density of preselected thickness. A flexible graphite mat can thus be compressed by roller compression into a sheet or sheet between 0.0508 to 1.778 mm thick with a density approaching the theoretical density, although a density of approximately 104.2 kg / m is acceptable for most applications. Flexible roller-compressed graphite is known to be a relatively good thermal barrier with thermal conductivity along and parallel to its surface, which is approximately twenty (20) or more times greater than its thickness. Certain thermal properties of interleaved, non-exfoliated graphite have been used in the manufacture of wall or floor coverings (U.S. Patent No. 5, 176,863) and in packaging applications (U.S. Patent No. 5,494,506). In the present invention the highly anisotropic thermal conductivity characteristics of flexible compressed graphite roller are employed in the treatment of high temperature protection applications. Figure 1 shows a flexible graphite sheet compressed by roller at 10, ie without holes or openings, contiguous to a sheet of metal 20 in a similar manner without holes or openings. The sheet 10 of flexible graphite compressed by roller is formed of particles 12 of exfoliated graphite, compressed, interleaved aligned such that the direction of the axis "c" of the particles 12, and the sheet 10, is transverse to the parallel surfaces 14, 16 plane of the sheet 10 continuous, that is, transverse to the thickness of the sheet 10. The directions "a" of the particles 12 and sheet 10 of exfoliated, compressed, interleaved graphite are along and between the flat surfaces 14, 16 of the sheet 12 and extend in all directions along and within the sheet 12 parallel to the flat surfaces 14, 16 as shown in 18 of Figure 1. The flow of heat, or thermal transfer by conduction occurs when heat flows through a body by transferring the kinetic energy of individual atoms or molecules without mixing the atoms or molecules. When a material is heated, the atoms or molecules are given greater vibratory movement, that is, greater kinetic energy; in some way, probably because of the shocks, these atoms or molecules share this increased energy with their neighbors, who in turn pass the energy to the backs, and so on.

For the measurement of heat flow by conduction, the basic law of heat transfer can be written in the form of a regime equation: Regime = Force Impulse Resistance in which the driving force is the temperature difference through a solid body, since it is apparent that heat can only flow when there is a temperature unevenness. This law, known as Fourier's Law, states that the regime of heat flow through a body is proportional to the fall in temperature, to the area, and inversely proportional to the thickness of the body. The mathematical expression of Fourier's Law is: where "Q" is the amount of heat energy transmitted at time "t", "A" is the area of the body perpendicular to the direction of heat flow, and "(T2-T?)" is the temperature difference between opposite sides or ends of the body, "L" is the thickness of the body in the direction of heat flow, and "k" is a constant that is defined by this equation and is called the thermal conductivity of the particular substance that constitutes the body. If "Q" is measured in kcal, "t" in hours, "A" in square meters, "T2" and "T" in degrees centigrade, and "L" in meters, then "k" is expressed as kcal per hour per cubic meter per degree centigrade per meter; "k" can also be expressed in scientific units such as watts per meter degree Kelvin (W / m ° K). When each of the terms in the equation defining "k" is equal to one, "k" is named the coefficient of thermal conductivity. The numerical value of the coefficient of thermal conductivity depends on the substance of which the body is made and its average temperature. When the thickness of the body is very small, in the case of small air pockets, or with thin adhesive coatings or layers, the effect on heat transfer by conduction through surrounding bodies is only slightly affected by such pockets or layers and coatings, that is, there is substantially no thermal barrier to conduction heat transfer. With reference to Figure 2, which is an elevation view of the compound of Figure 1, a localized high temperature heat source 30 is positioned adjacent closely to the continuous metal sheet 20, of substantially isotropic conductivity, i.e. uniform heat which may be for example simple or alloy carbon steel which is intended to protect a substance that can be damaged by the heat indicated generally at 40. Other metals, such as aluminum, copper, noble metals, and their metals may be used. alloys Referring to Figure 2, the localized heat source 30, which may be a flame, an automobile exhaust through which hot gases and the like are flowing, establishes a temperature Ti on the adjacent surface 19 of the sheet 20. of continuous steel. Since T2 is smaller than Ti, on the opposite surface 21, the heat will travel from Ti to T2 at a rate determined by the substantially uniform, ie non-anisotropic, thermal conductivity of the steel, v. g. , 10 to 20 watts per meter per degree Kelvin in all directions, and, without the presence of contiguous graphite sheet 10, continuous flexible, compressed by roll, the low temperature T2 will increase, due to the conduction of heat energy, to approximately the high value of Ti. With the flexible graphite sheet 10 contiguous in place as shown in Figure 2, in conductive heat transfer relationship with the steel sheet 20, i.e., there is no substantial barrier to conduction heat transfer, and the temperature T2 rising will establish a corresponding temperature T3 in the portion of the flexible graphite sheet 10 bordering the steel sheet 20. The temperature T3 is applied to two very different thermal conductivity paths (due to the anisotropy of the sheet 10 of flexible graphite compressed to roll, v. g. , at least 20: 1, with respect to thermal conductivity), the direction "c" through the thickness of the flexible graphite sheet 10 from the flat surface 16 to the flat surface 14 and the "a" directions. In the path of the "c" direction, the thermal conductivity is relatively low, about 1/3 of that corresponding to the steel sheet 20. The other path of thermal conductivity, the "a" directions, parallel to the flat surfaces 16, 14 of the flexible graphite sheet 10 has a relatively high thermal conductivity.

In these "a" trajectories, the thermal conductivity is typically at least 10 times that of steel. Consequently the heat, i.e. thermal energy, passes slowly by conduction through the thickness of the flexible graphite sheet 10 (direction "c"), but very quickly through the sheet 10 in directions parallel to the flat surfaces 16, 14 (address "a"). As a result the heat moves rapidly by conduction from T3 to T4, keeping T2 relatively low, while the temperature T increases and causes the heat, thermal energy, to flow from T to T5 by conduction in the steel sheet 20, placed that the heat passing from T2 to T5 through the steel sheet 10 moves at a rate of about 1/10 of the heat regime passing from T3 to T4 in the "a" direction of the flexible graphite sheet 10 . The result of the configuration described above is to diffuse the conduction heat energy from the source 30 onto the continuous sheet of steel, substantially thermally non-anisotropic (ie, isotropic) and also the sheet 10 of thermally compressed continuous anisotropic flexible graphite. by roller. As a consequence of the thermal phenomenon described above, a high temperature heat point located at T2 is avoided and the temperature at the surface 16 of the steel sheet 20 is very uniform and remains much lower than Ti, as does the temperature of the flexible graphite sheet 10 which is in conductive thermal transfer relationship with the steel sheet 20. In order to achieve the thermal performance described above, it is necessary that the respective sheets 10 and 12 be continuous, that is, without holes, cuts or other openings that would interfere with the heat conduction inside and through the respective sheets. Figure 3 shows a sheet 23 of additional steel overlapping and contiguous to and in conductive thermal transfer relationship with the flexible graphite sheet 10 which is intermediate between the steel sheets 20 ', 23 and the composite is anchored in place by the clamps 26. In Figure 4, the compound of Figure 3 has been mechanically deformed, v. g. , by stamping, to a tray-like configuration, with the addition of a carpet or cover mat that can be damaged by heat. Figure 5 shows a composite formed of an inner tube 120 of steel sheet wound around an automotive exhaust pipe 150 through which the exhaust gases of the engine, indicated at 130, pass at very high temperatures, v. g. , up to 538 ° C and constitutes a source of high temperature heat. Adjacent to the internal steel sheet 120 is the flexible graphite sheet 10, compressed to an intermediate roll which is enclosed within the adjacent tube 123 of external steel sheet. In all the foregoing embodiments of Figures 2-5, the thermal phenomenon corresponds to that described in relation to Figures 1 and 2. That is, localized high temperature hot spots are avoided and the carpets or mats 40 'are not subject to heat damage. The composite article of Figure 6 comprises a continuous sheet of plastic 60 contiguous with the sheet 70 of continuous flexible graphite which rests on the adjacent substrate 80 which can be damaged by heat, v. g. , felt, plastic, rubber. The sheet 60 of plastic has a substantially uniform thermal conductivity, ie, isotropic which is greater than the thermal conductivity of the thermally anisotropic flexible graphite sheet in its "c" direction, but substantially less than the thermal conductivity of the sheet. graphite in the "a" direction. Heat from the localized high temperature source 300 could ordinarily melt the plastic sheet 60 to T1 and damage the substrate 80. However, the heat that passes through the plastic sheet 60 is "diffused" due to the presence of the sheet 70 of continuous, anisotropic, flexible graphite, compressed by roll in conductive heat transfer relationship with the same, which results in the same thermal phenomenon described in relation to Figures 1 and 2, and heat damage to substrate 80 is avoided. The graphite and non-graphite components of the composite article of the present invention are placed and held together in conductive heat transfer relationship by staples, clamps, as described above, or by the use of very thin adhesive coatings or layers, v. g. , from 5 to 10 microns thick, which does not significantly affect the heat transfer by conduction. In addition to the configurations in the form of a tray and concentric tubular, the composite heat protection article can be formed as parabolic or spherical reflectors, oven covers and enclosures, handles, seat covers, curtains and screens.

Claims (6)

1. REVIVAL DICTIONS 1. In combination, an article composed in the form of a thermally compressed roll anisotropic flexible graphite continuous sheet arranged in conductive heat transfer relationship with a continuous substrate comprising a non-graphitic material for protection against conductive heat thermally substantially uniform thermal conductivity, and a high temperature heat source adjacent to thermally conductive non-graphitic heat-shielding substrate, said heat-conductive non-graphically conductive continuous substrate that is intermediate said graphite web flexible and said high temperature heat source.
2. 2. A composite device for heat protection in the form of an anisotropic continuous sheet thermically compressed to contiguous flexible graphite roll and fixed to a continuous metal sheet of substantially uniform thermal conductivity, said sheet of continuous flexible graphite compressed by roll and said metal sheet continues to be arranged in a conductive heat transfer ratio.
3. 3. A composite heat shield device in the form of a thermally compressed anixotropic flexible graphite continuous sheet to intermediate roller first and second continuous metal sheets affixed thereto, said flexible graphite sheet being in a transfer relationship of conductive heat with said first and second continuous metal sheets.
4. 4. A heat protection composite device according to claim 3 wherein the first and second continuous metal sheets are deformed and mechanically congruent. A composite heat protection device according to claim 3 wherein said first and second metal sheets are in the form of an outer tube and an inner tube which are arranged concentrically. 6. A composite heat shield device in the form of an anisotropic flexible graphite continuous sheet thermally compressed to an intermediate roll to a continuous sheet of plastic and a substrate that can be damaged by heat, said sheet of flexible graphite fixed to the sheet of plastic in conductive heat transfer ratio.