METHOD FOR SEALING CARBON AND GRAPHITE SURFACES

Abstract: A method for sealing the surface of a porous carbon or graphite article which comprises the steps of: (a) applying a thermosetting treating solution upon the surface of a porous carbon or graphite article, wherein the temperature of the surface of the article is below the cure temperature of the treating solution, and the viscosity of the treating solution upon the surface of the article is sufficiently low to permit infiltration of the treating solution into the pores of the article at ambient pressure; (b) allowing the treating solution to infiltrate into the pores of the article, wherein the treating solution is applied for a sufficient number of times for the rate of infiltration of the treating solution into the pores of the article to decrease to a predetermined minimum; (c) removing any excess treating solution from the surface of the article; and (d) heating the surface of the article to slowly raise the temperature of the impregnated resin to the curing temperature of the resin to polymerize the resin in situ within the pores of the article to form a substantially non-porous surface on the article; steps (a) to (d) inclusive being acrried out at ambient pressure.

Description: The present invention relates to topical treatments for sealing carbon and graphite surfaces. More particularly, the present invention relates to an improved method of impregnating and coating the surfaces of porous carbon and graphite articles in order to eliminate porosity. In the production of parts of composite materials such as, for example, graphite-epoxy composite parts for use in the aircraft and space industry, it is necessary to employ moulds which are devoid of open porosity on the working surfaces. The reason for this requirement is substantially, two-fold, namely, (1) to prevent intrusion of liquid resin into the mold media and (2) to prevent gas infiltration through the mould when a high pressure differential is applied across the mould cross-section during the vacuum forming of the composite part. Because of its superior dimensional stability over wide ranges of temperature variation and its high temperature tolerance, graphite is an ideal candidate for use as the mould forming material in the production of these composites. However, fine-grain graphite, as manufactured and machined, has undesirable surface porosity as well as dusting and poor wear characteristics which severely limit its use in this particular application. A common method of eliminating porosity in graphite materials is to impregnate the graphite shape with a liquid resinous material which, when cured, fills or blocks virtually all of the open pores. In these impregnation processes, the graphite article is placed in an autoclave The autoclave is evacuated to remove gases from the porous structure of the shape, and ensure impregnation by the liquid resinous material. Generally, the electrode is then contacted with the resinous material under pressure to assist infiltration of the resin into the pores of the electrode. Following impregnation, the resin is cured by heating the impregnated electrodes. Multiple impregnation/curing cycles, usually three or more, are required to achieve complete sealing of porosity. US-A- 2 909 450 discloses a method of treating a porous article, such as a carbon electrode, to eliminate its porosity. The article is impregnated with a solution containing 65-99% by weight of furfuryl alcohol, 0-25% by weight of furfural, 0.5-10% by weight of a zinc chloride catalyst and the balance water. The impregnation is carried out by immersing a carbon electrode in the solution within a treating cylinder and carbon electrode is subjected to a vacuum. Thereafter, the electrode, while immersed in the solution, is subjected to a pressure of 1034 kPa (150 psi) for one hour. The impregnated electrode is then removed from the treating cylinder and placed in an oven at 95°C to 100°C to cure for 18 to 24 hours. This procedure may be repeated one or more times until the porosity of the carbon electrode is substantially eliminated. These prior art methods, however, suffer from the disadvantage in that they require treatment of the graphite articles under vacuum and then pressure in autoclaves or similar equipment. Certain precision machined graphite shapes and large parts are not easily processed in autoclave type equipment because of their configuration or size. Thus, these parts are difficult or impossible to treat by these prior-art methods. Another problem is that application of the resinous material to the part surface is uneven with some excess resinous material typically remaining upon the surface, forming an uneven coating with surface accumulations of resin. After curing of the resin, these surface accumulations and the uneven coating on the surface result in loss of the fine dimensional tolerances to which the graphite parts are machined. It has now been found possible to provide a method for impregnating graphite articles to eliminate the surface porosity of the articles. It has also been found possible to provide a method for impregnating graphite articles which provides thin, uniform, level applications of the resinous material on the surface of the porous carbon or graphite article, such that the machined tolerances of the article are not lost when the resin is cured. It has further been found possible to provide a method for impregnating carbon and graphite articles which does not require vacuum and/or pressure equipment. According to the present invention there is provided a method for sealing a porous carbon or graphite article to a predetermined depth below its exposed surface without disturbing the surface finish of the article which comprises the steps of: (a) applying a thermosetting treating solution, comprising a heat polymerizable resin selected from polyimides, polybenzimidazoles, bismaleimides, polyarylketones and polyphenylene sulfides dissolved in a solvent, upon the surface of a porous carbon or graphite article, wherein the temperature of the surface of the article is below the cure temperature of the treating solution, and the viscosity of the treating solution upon the surface of the article is sufficiently low to permit infiltration of the treating solution into the pores of the article at ambient pressure; (b) allowing the treating solution to infiltrate into the pores of the article, wherein the treating solution is applied for a sufficient number of times for the rate of infiltration of the treating solution into the pores of the article to decrease to a predetermined minimum; (c) removing any excess treating solution from the surface of the article; and (d) heating the surface of the article to slowly raise the temperature of the impregnated resin to the curing temperature of the resin to polymerize the resin in situ within the pores of the article to form a substantially non-porous surface on the article; wherein steps (a) to (d) inclusive are carried out at ambient pressure. The treating solution of the invention comprises a suitable polymerizable resin which has a sufficiently low viscosity upon the surface of the article to allow infiltration of the pores of the article at ambient pressure, i.e. without assistance of an applied pneumatic pressure to assist in the infiltration of the solution. The treating solution is drawn by capillary action into the open pores of the carbon or graphite article without assistance from pressure, or without previously drawing gasses out the pores with vacuum. For fine grained graphite and carbon articles, a viscosity below about 100 milli Pascals seconds (100 centipoise), measured at the temperature of the surface of the article, is generally suitable. The treating solution is thermosetting, i.e. or curable by heating to a cure temperature, at which temperature there is a rapid polymerization of the treating solution. The cure temperature is higher that the temperature of the surface when the treating solution is applied, that is, the treating solution is sufficiently stable or nonreactive at the temperature of the surface so as to permit infiltration of the solution into the pores of the article and removal of the excess solution. The polymerizable resin used to form the treating solution, should in its cured state be stable at temperatures to which the impregnated article is exposed during use. In addition the resin must be capable of being formed into a treating solution of sufficiently low viscosity at the surface temperature of the article. As resins and polymerizable solutions are typically solid or have a high viscosity, the viscosity of such must be reduced, preferably by heating the surface of the article before application of the treating solution. In addition, the viscosity of the treating solution may be reduced by dilution with a suitable solvent. The treating solution is applied to the surface and allowed to soak or infiltrate into the pores of the surface. The cycle of applying the treating solution and allowing the solution to infiltrate is preferably repeated to maximize the amount of treating solution infiltrated into the pores. The number or repetitions can be determined by routine experimentation by observing the rate at which the treating solution is absorbed into the article surface, and repeating application of the treating solution until the rate of infiltration slows to a predetermined value. Any excess solution upon the surface which has not infiltrated into the pores is then removed to provide a surface substantially free of accumulations of the treating solution. After removal of the excess solution, the infiltrated treating solution is then cured by heating the surface of the article up to the curing temperature to rapidly polymerize or cure the resin, forming a substantially non-porous surface on the article. Preferably, the article is burnished to provide a smooth finish coating upon the article by applying treating solution to the non-porous surface of the article to form a layer or coating of treating solution upon the surface, burnishing the layer of treating solution to achieve level uniform filling of pores upon the surface, repeating the application and burnishing until the porosity at the surface is substantially eliminated, and thereafter heating the coated layer to slowly raise the temperature thereof to the polymerization temperature of the resin, thereby curing the resin to produce a level, impervious surface film which is chemically cross-linked with the resin impregnant filling the pores of the article. It has been unexpectedly found in accordance with the invention that the surfaces of a porous carbon or graphite article can be effectively sealed by application of a treating solution containing a high temperature polymerizable liquid resin at ambient or atmospheric pressure. In addition, the sealing can be accomplished without uneven coatings and residues upon the surface which alter the outer dimensions of the article. The unexpected results of the invention are achieved by applying polymerizable treating solution upon the surface of the article which has a low viscosity upon the surface, and allowing it to infiltrate the pores of the surface. The process of the invention is carried out at ambient pressure. This eliminates the need for high-pressure autoclaves required in prior-art impregnation processes. With no autoclave to limit the size or the configuration of the article, it is possible to treat articles which are much larger and of a more complex configuration. In addition, the treating solution can be applied by methods not generally available, or which are impractical in an autoclave environment, such as, for example, complex spraying techniques, brushing, wiping, and hand application of the solution. This allows a more uniform application of the treating solution to the surface, and permits treatment of graphite and carbon parts with complex geometries which cannot be evenly treated in an autoclave. In addition, without the autoclave, it is possible to remove excess treating solution from the surface before the treating solution is polymerized. This further permits a more even coating upon the surface of the article than is possible in methods using an autoclave. With an even uniform coating, with no accumulations of the treating solution on the surface, it is, therefore, possible to maintain fine machined tolerances to a degree not possible with prior art methods. It is unexpected that, notwithstanding the fact that a pressure and vacuum cycle was not used to assist impregnation, that the surface of the carbon or graphite shape is effectively sealed. In the typical practice of the invention, using graphite usually used for tools, the surface is sealed to a depth estimated to be approximately 30-50 millimetres of the cured or thermoset resin. In contrast, prior methods of pressure impregnation and curing such as in the case of US-A- 2 909 450 (referred to hereinabove), utilizing autoclave equipment and the like, ordinarily result in filling of substantially all of the pores throughout the body of the article. By sealing only the surface portions, the present invention permits a significant saving in equipment and material costs. The method of the present invention constitutes a significant improvement in that precision machined shapes and large parts which are not easily processed in autoclave equipment may be treated to fill or block all surface porosity, providing apparent total imperviousness. Furthermore, precision machined carbon or graphite surfaces may be treated to achieve thin, uniform, level coatings which do not significantly alter the dimensions of the article, which is normally not possible with autoclave processing. A number of high temperature polymerizable liquid resins of low viscosity can be used in the method of the invention. For temperature stability up to about 400°C, the treating solution comprises resins selected from polyimides, polybenzimidazoles, bismaleimides, polyarylketones, and polyphenylene sulfides. A preferred class of high-temperature polymeric resins are fluorinated polyimides. High temperature polymerizable resins are often in the form of a solid, and must be dissolved in a compatible solvent to reduce the viscosity. Suitable solvents for reducing the viscosity of the treating solution include any solvent compatible with the particular resin being used. Suitable solvents for solutions containing fluorinated polyimides, include, for example, methylethylketone, and 1-methyl-2-pyrolidinone. A suitable treating solution based upon fluorinated-polyimide resins is a solution as used in Example I, below. Before application of the treating solution, the surfaces of the porous carbon or graphite article are thoroughly cleaned using known procedures in order to remove any surface contaminants as well as any loose graphite particles and dust that may remain after the machining operation. If the surface of the article is to be heated to reduce the viscosity, the cleaned carbon or graphite article is heated after cleaning by placing it in an oven, e.g., a forced draft oven. The article is heated at ambient pressure to an elevated temperature which is high enough to reduce the viscosity of the treating solution upon contact with the heated surface of the article. The preheat temperature to be used with any given treating solution must be chosen carefully to achieve maximum viscosity reduction, or "thinning" of the treating solution while avoiding rapid cross-linking of the resin. The treating solution is applied uniformly to the surfaces of the porous carbon or graphite article at ambient pressure using any one of several known techniques. The solution may be applied by spraying or wiping with a brush or cloth, for example, or by immersing the article within a bath of the treating solution. In any case, the low viscosity treating solution will be drawn by capillary action into the open pores of the carbon or graphite article. The application of treating solution is applied to the surface until the surface appears to be substantially saturated with treating solution. This typically requires repeated applications as the treating solution infiltrates into the surface. Generally, the treating solution is applied to the surfaces of the article 10 to 14 times, or until absorption slows to a predetermined minimum, for example, where about five minutes pass before dry spots appear. After the treating solution is applied to the article, any excess solution remaining upon the surface, i.e. solution which has not soaked or infiltrated into the surface, is removed to ensure an even surface upon the final article and to prevent undesirable accumulations which change the outer dimension of the article. The solution may be removed by any suitable means, such as, for example, wiping by cloth rags. The carbon or graphite article with its saturated but dry surface is then placed in an atmospheric pressure oven and heated slowly to an elevated temperature which is sufficient to cure the impregnated resin within the pores of the article. Preferably, the article is heated to curing temperatures using different heating rates in a step-wise fashion with various hold periods at intermediate temperatures. For polyimide-based treating solutions the curing temperature is about 240°C. Successful curing has been achieved with heating rates ranging from about 5°C/hr. to about 25°C/hr. Table I shows a suitable schedule for heating the impregnated carbon or graphite article with fluorinated polyimide-based treating solutions. If the article with the non-porous surface is to used in temperatures above 240°C, the solution treated surface must be further post-cured to a temperature between about 350°C and about 450°C, preferably near about 410°C, at a heating rate between about 25°C/hr and about 60°C/hr. It may be necessary, where a very low degree of porosity is required, to repeat one or more times the cycle of applying coat of treating solution, removing excess solution, and curing the solution. A "finish" coating may be applied to the impregnated carbon or graphite article after curing, which serves to fill any residual porosity and to level the article surfaces. The heated resin impregnated surfaces of the article are then coated uniformly with a layer of the treating solution, after which the surface is hand burnished with a damp cloth. During the burnishing the polymerization may advance due to the heat in the article, which may be further heated to assist further polymerization of the resin. By hand burnishing, it is possible to produce a thin, level, glassy surface film on the article surfaces. The solution is reapplied and burnished several or more times until the porosity at the surfaces is substantially eliminated. 'the applied solution is then heated to slowly raise its temperature to complete the polymerization of the resin, thereby curing the resin to produce a thin, level, uniform, liquid and gas impervious film or coating on the surface of the article. The resulting film on the surface is not a superficial coating, but is chemically cross-linked with the resin impregnant filling the pores of the article. Preferably the same treating solution is used throughout treatment of the article. However, different treating solutions may be used in repeated applications of the treating solutions, or a different treating solution may be used to form a burnished finish coat. The present invention is further illustrated by reference to the following Example, but is in no manner limited thereto. Example I A treating solution was prepared by admixing approximately 14 weight percent of a fluorinated polyimide resin, and 86 weight percent of a solvent. The fluorinated polyimide resin was available under the trademark "Thermid FA-700", from National Starch and Chemical Corporation. The solvent was a mixture of 80 weight percent methylethylketone and 20 weight percent 1-methyl-2-pyrolidinone. This treating solution was used to surface impregnate a number of graphite articles which had been previously machined to the desired shape and thoroughly cleaned to remove all surface contaminants and loose graphite particles and dust. The articles were preheated to a temperature of 65°C. A layer of the treating solution was then applied to the preheated surfaces of graphite articles at room temperature by brushing the solution uniformly onto the surfaces thereof. The solution was allowed to soak into the article surfaces and another layer of the solution was applied and allowed to absorb in the same manner. This procedure was repeated an average of 10 to 14 times until absorption became quite slow, i.e., five minutes to show dry spots, for example. After the final application of the treating solution, the articles were allowed to set for about 20 minutes to one-half hour. Any remaining liquid was then wiped off the surfaces. In those cases where liquid resin built up on the surfaces, it was softened and removed using a treating solution dampened cloth. The graphite articles were then heated in an atmospheric oven to cure the impregnated resin using the same heating schedule or cycle set forth in Table I. The cured articles were then removed from the oven and allowed to cool to room temperature. Although the experimental work described in the foregoing Example was carried out using fine-grained graphite articles, which typically have a maximum grain size of about 0.1524 mm (0.006 inches), the method of the invention is not so limited but is broadly applicable as well to the impregnation and sealing of coarser grade carbon and graphite materials.

Claims: A method for sealing a porous carbon or graphite article to a predetermined depth below its exposed surface without disturbing the surface finish of the article which comprises the steps of: (a) applying a thermosetting treating solution, comprising a heat polymerizable resin selected from polyimides, polybenzimidazoles, bismaleimides, polyarylketones, and polyphenylene sulfides dissolved in a solvent, upon the surface of a porous carbon or graphite article, wherein the temperature of the surface of the article is below the cure temperature of the treating solution, and the viscosity of the treating solution upon the surface of the article is sufficiently low to permit infiltration of the treating solution into the pores of the article at ambient pressure; (b) allowing the treating solution to infiltrate into the pores of the article, wherein the treating solution is applied for a sufficient number of times for the rate of infiltration of the treating solution into the pores of the article to decrease to a predetermined minimum; (c) removing any excess treating solution from the surface of the article; and (d) heating the surface of the article to slowly raise the temperature of the impregnated resin to the curing temperature of the resin to polymerize the resin in situ within the pores of the article to form a substantially non-porous surface on the article; steps (a) to (d) inclusive being carried out at ambient pressure. A method according to claim 1, wherein the viscosity of the treating solution is below about 100 mPa s (100 centipoise) at the temperature of the surface of the article. A method according to claim 1 or 2, wherein steps (a) and (b) are executed a plurality of times. A method according to claim 3, wherein steps (a) and (b) are executed from 10 to 14 times. A method according to any of claims 1 to 4, wherein the article is a fine grain graphite having a maximum grain size of about 0.152 mm (0.006 inches). A method according to any of claims 1 to 5, wherein the treating solution is applied in step (a) by spraying the treating solution on the surface, or by wiping the treating solution on the surface, or by immersing the article within the treating solution. A method according to any of claims 1 to 6, wherein the impregnated surface of the article is heated at a rate of from about 5°C to about 25°C per hour. A method according to any of claims 1 to 7, wherein the polymerizable resin is solid resin before being dissolved in the solvent. A method according to any of claims 1 to 8, wherein the solvent comprises methylethylketone and/or 1-methyl-2-pyrolidinone. A method according to claim 9, wherein the solvent comprises 80 weight percent methylethylketone and 20 weight percent 1-methyl-2-pyrolidinone, based upon the weight of the solvent. A method according to any of claims 1 to 10, wherein the polymerizable resin is a fluorinated polyimide. A method according to claim 10 or 11, wherein the treating solution comprises 14 weight percent of a fluorinated polyimide, and 86 weight percent of the solvent, based upon the total weight of the treating solution. A method according to any of claims 1 to 12, wherein the impregnated surface of the article is heated at a rate of about 25°C per hour to a final temperature of about 240°C. A method according to any of claims 1 to 13, wherein the impregnated surface of the article is heated according to the schedule set forth in Table I, namely A method according to any of claims 1 to 14, wherein the non-porous surface produced in step (d) is further heated to a temperature above the cure temperature. A method according to claim 15, wherein the non-porous surface produced in step (d) is further heated to a temperature between about 350°C and 450°C at a heating rate between about 25°C per hour and 60°C per hour. A method according to claim 16, wherein the non-porous surface is further heated to about 410°C, at a heating rate between about 25°C per hour and 60°C per hour. A method according to any of claims 1 to 17, wherein the surface of the article is heated before coating with the treating solution. A method according to any of claims 1 to 18, wherein before the application of the treating solution in (a) the surface of the article is heated to a temperature sufficiently high to reduce the viscosity of the treating solution upon contact with the heated surface sufficiently to permit the infiltration of the treating solution into the pores of the article. A method according to claim 18 or 19, wherein the treating solution is at ambient temperature when applied to the heated surface. A method according to any of claims 18 to 20, wherein the viscosity of the treating solution is about 5 mPa s (5 centipoise) at 25°C. A method according to any of claims 18 to 21, wherein the surface of the article is heated to a temperature of about 60°C before application of the treating solution. A method according to any of claims 1 to 22, further comprising the steps of: (e) applying the treating solution to the surface of the article to form a layer of the treating solution upon the surface of the article; (f) burnishing the layer of treating solution to achieve level uniform filling of any pores present at the surface; wherein steps (e) and (f) are executed for a sufficient number of times such that the porosity at the surface is substantially eliminated; and (g) heating the coated layer to slowly raise the temperature thereof to the polymerization temperature of the resin, thereby curing the resin to produce a level, impervious surface film which is chemically cross-linked with the resin impregnant filling the pores of the article. A method according to claim 23, wherein the impregnated surface of the article is burnished in step (f) with a cloth dampened with the treating solution. A method according to claim 23 or 24, wherein the non-porous surface produced in step (d) is heated to a temperature below the cure temperature of the treating solution before applying a layer of polymerizable treating solution in step (e).

Inventors: CHIU, CHARLES CHI-CHIEH, NOE, JAMES BEAM
Applicants: UNION CARBIDE CORPORATION
Application year: 1989
Main classification: C04B41
IPC codes: C09C1, C04B41, C08K9, C09C3, C01B31