Abstract:
An apparatus and method for forming a densified carbon-carbon composite. The apparatus includes: a green part molding station for forming a green part; a carbonization station for carbonizing the green part; and an impregnation station for impregnating the carbonized part with a substantially curing by-product free, high carbon yield resin. The impregnation station includes a mold forming a sealed enclosure configured in the shape of the carbonized part for receiving the carbonized part and a vacuum source for evacuating the mold. At least one resin injection port is in the mold and a supply of substantially curing by-product free, high carbon yield resin is connected to the resin injection port for injection into the mold. In the preferred embodiment, the substantially curing by-product free, high carbon yield resin is a cyanate ester having a viscosity of less than about 100 cps at 250° F. and a carbon yield value of greater than about 60 wt. %. The combination of a high carbon yield resin and a pressurized impregnation station configured to the shape of the composite part for use with a curing by-product free resin provides for a substantial reduction in the number of carbonization cycles required to reach final density while, at the same time avoids the environmental and safety problems of a solvent-based system.

Description:
This invention was made with government support under various contracts awarded through the Department of Defense. The government has certain rights in this invention. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the manufacturing of composite materials, and, more particularly to the manufacture of carbon-carbon composites by densifying carbon-carbon composites with a polymeric matrix. 
     2. Description of the Prior Art 
     Carbon-carbon composites are comprised of a carbonaceous matrix reinforced by carbon fibers. Such composites are used to combine the advantages of fiber-reinforced composites with the refractory properties of structural ceramics. These composites maintain their properties at high temperatures making them suitable for uses such as space vehicle heat shields, rocket nozzles and aircraft brakes. Additional applications have been found for these composites in the medical and industrial areas given the composite&#39;s biocompatibility, chemical inertness and thermal conductivity. 
     A common method of manufacturing carbon/carbon composites is by pyrolyzing a carbon fiber/polymeric composite. One example would be a polymeric composite based on autoclave cured carbon fiber/phenolic prepreg. The prepreg is cut and stacked on a molding tool. The assembly of the prepreg and mold are bagged and placed in a autoclave. A vacuum is pulled on the bag and the temperature is raised in the autoclave at a specified rate. At the time the resin softens, the autoclave pressure is increased to about 100 psi. The autoclave is then held at about 350° F. for about two hours. Next, the autoclave is cooled to room temperature and the cured composite is removed from the tool. 
     The cured composite is placed in a carbonization furnace and heated slowly at a specified rate in the absence of oxygen to about 1000° C. This process often requires a number of days. The heating step volatilizes all of the organic portion of the phenolic molecule except for the carbon. The phenolic resin looses about 35% of its weight during the carbonization process creating microvoids or porosity in the composite. Significantly, these voids extend to the center of the item being manufactured and must be filled in order to maximize the strength of the final product. 
     The voids are then filled with a solvent diluted resin by series of vacuum and pressure cycles. The carbon/carbon composite is submerged in the solvent solution of phenolic resin (or other high carbon content material such as furan thermoset resin) and evacuated. While still submerged in the solution, pressure is added to further push the solution into the composite porosity. The impregnated composite is then dried to remove the solvent. Care must be taken to remove all the solvent without losing the resin. After all the solvent is removed, the resin must then be cured or otherwise stabilized in the composite pores. 
     The resin impregnated carbon/carbon composite is then recarbonized by essentially the same carbonization process. If the process fully performs its purpose, the micro void content is reduced by another increment. The amount of this reduction depends on the solvent/resin ratio and the carbon content of the resin used. The solvent/resin solution typically is about 50% resin by volume and a resin with a high carbon yield looses about 35% of its volume during carbonization. This results in about 18% of void volume that is filled by repeated re-impregnation and recarbonization. This process is called densification. 
     The purpose of densification is primarily to obtain good mechanical properties. As the density increases, so do the mechanical properties of the resulting composite. Three or more re-impregnations (and carbonizations) are sometimes required to achieve a useful product depending on the end use for the composite. 
     Although the use of carbon-carbon composites has become widespread, the market for these materials has been limited due to their high cost of production. These costs arise in part from the need for the multiple re-impregnations and carbonizations steps. As discussed above, these steps can take days and are susceptible to high production loss rates from minor imperfections in the process. 
     Thus, there remains a need for an apparatus for forming a densified carbon-carbon composite which provides for a substantial reduction in the number of carbonization cycles required to reach final density while, at the same time avoids the environmental and safety problems of a solvent-based system. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus and method for forming a densified carbon-carbon composite. The invention improves on prior art approaches by impregnating either a green part or a carbon-carbon composite using a low viscosity resin that is thinned with heat as opposed to prior art solvent processes. The heated resin is forced into the interstices of the item in an evacuated mold under pressure developed in a resin transfer molding apparatus. This injection pressure is maintained throughout the duration of the curing process. This continuous pressure is made possible by the fact that the resin contains no solvents and thus produces no gaseous by-products during curing. 
     The apparatus of the present invention includes: a green part molding station for forming a green part; a carbonization station for carbonizing the green part; and an impregnation station for impregnating the carbonized part with a substantially curing by-product free, high carbon yield resin; and a vacuum system for evacuating the mold. Desirably, the green part molding station and the impregnation station are essentially the same station. 
     In the preferred embodiment, the impregnation station includes a mold forming a sealed enclosure for receiving the carbonized part and configured in the shape of the carbonized part. At least one resin injection port is in the mold and a supply of substantially curing by-product free, high carbon yield resin is connected to the resin injection port for injection into the mold. The impregnation station includes a heater for curing the resin impregnated carbonized body. In addition, the impregnation station includes a press for maintaining the carbonized part under pressure during impregnation and curing. 
     A pump is connected between the resin injection port and the supply of resin. The pump is operated at a pressure just sufficient to fill the sealed enclosure with the resin to prevent damage to the carbonized part. Then, a second, packing pressure further impregnates the carbonized part with the resin. 
     In the preferred embodiment, the substantially curing by-product free, high carbon yield resin is a cyanate ester having a viscosity of less than about 100 cps at 175° F. and, most preferably a viscosity of less than about 50 cps at 175° F. Also, preferably, the substantially curing by-product free, high carbon yield resin has a carbon yield value of greater than about 25 wt. % or, in the most preferred embodiment a carbon yield value of greater than about 60 wt. %. 
     The carbonization station includes an enclosed chamber, a heater and a non-oxidizing atmosphere. The non-oxidizing atmosphere desirably is maintained at a positive pressure in the enclosed chamber. In the preferred embodiment, the non-oxidizing atmosphere is a non-reactive gas, such as nitrogen. A filtering system downstream from the carbonization station cleans the exiting gas before it is discharged. 
     The combination of a high carbon yield resin and a pressurized impregnation station configured to the shape of the composite part for use with a curing by-product free resin provides for a substantial reduction in the number of carbonization cycles required to reach final density without the environmental and safety problems of a solvent-based system. 
     Accordingly, one aspect of the present invention is to provide an apparatus for forming a densified carbon-carbon composite. The apparatus includes: a green part molding station for forming a green part; and an impregnation station for impregnating the green part with a substantially curing by-product free, resin. 
     Another aspect of the present invention is to provide an impregnation station for impregnating a carbonized part with a substantially curing by-product free, high carbon yield resin to form a densified carbon-carbon composite. The apparatus includes: a mold forming a sealed enclosure for receiving the carbonized part and configured in the shape of the carbonized part; at least one resin injection port in the mold; and a supply of substantially curing by-product free, high carbon yield resin connected to the resin injection port. 
     Still another aspect of the present invention is to provide an apparatus for forming a densified carbon-carbon composite. The apparatus includes: a green part molding station for forming a green part; a carbonization station for carbonizing the green part; and an impregnation station for impregnating the carbonized part with a substantially curing by-product free, resin, the impregnation station including: a mold forming a sealed enclosure for receiving the carbonized part and configured in the shape of the carbonized part; at least one resin injection port in the mold; and a supply of substantially curing by-product free, high carbon yield resin connected to the resin injection port. 
     These and other aspects of the present invention will become apparent to those skilled in the art after a reading of the following description of the preferred embodiment when considered with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of the green part molding station of the present invention; 
     FIG. 2 is a schematic illustration of the impregnation station of the present invention; 
     FIGS. 3A-3C is an illustration of the resin transfer molding device of the present invention; 
     FIG. 4 is an illustration of the carbonization station of the present invention; 
     FIG. 5 is a flow chart illustrating the method of the present invention; and 
     FIG. 6 is a graphical illustration of the improved results of the present invention as compared to the prior art processes. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, like reference characters designate like or corresponding parts throughout the several views. Also in the following description, it is to be understood that such terms as “forward,” “rearward,” “left,” “right,” “upwardly,” “downwardly,” and the like are words of convenience and are not to be construed as limiting terms. 
     Referring now to Table 1 below, there is shown a comparison of the steps of commonly known processes for making a carbon-carbon composite to the process of the present invention. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 SOLVENT 
                   
               
               
                 CVD 
                 IMPREGNATION 
                 PRESENT INVENTION 
               
               
                   
               
             
             
               
                 Carbon Fabric 
                 Carbon Fabric 
                 Carbon Fabric 
               
               
                 Polymeric Matrix 
                 Polymeric Matrix 
                 Polymeric Matrix 
               
               
                   
                 Green Part 
                 Green Part 
               
               
                 Carbonization Step 
                 Carbonization Step 
                 Carbonization Step 
               
               
                   
                 Go to Vat 
                 Go to Impregnation Station 
               
               
                 CVD Impregnation 
                 Vacuum/Pressure of 
                 Vacuum/Pressure 
               
               
                   
                 solvated resin 
                 No Gaseous By-products 
               
               
                   
                   
                 during Resin Curing 
               
               
                   
                   
                 High Carbon Yield Resin 
               
               
                   
                 Dry to Remove 
                 Cure 
               
               
                   
                 Solvent 
               
               
                   
                 Cure 
                 Further Carbonization Step 
               
               
                   
                 Further Carbonization 
                 Impregnation/Cure 
               
               
                   
                 Step 
               
               
                   
               
             
          
         
       
     
     The known processes include, among others, chemical vapor deposition (CVD) and a solvent-based system that uses a solvent to liquefy the polymeric resin for impregnation into the carbon fiber structure. Both these processes and the present invention use a carbon fabric and a polymer resin as basic components. These components are combined to from a “green part” which will have a carbon and a non-carbon component. The green part then goes through a first carbonization step after which other steps are conducted depending on the method employed. The impregnation and carbonization steps are repeated until a desired level of density is achieved. 
     With the CVD process, it is difficult to density the central regions of thick items. This is because the initial densification steps tend to block the outer regions of the part. As these outer regions become blocked, it becomes every more difficult to force the gas to the center of the part. 
     In a preferred embodiment, the green part is constructed from a preform comprised of multiple layers of a woven carbon fabric that are tacked together. In one approach, the layers are tacked together with a low melt, powdered epoxy material such as Shell Chemical&#39;s Epon 1031. The layers with the powdered epoxy therebetween are compacted in a vacuum bag and then heated until the melting temperature of the epoxy is reached. The relatively stiff, bonded layers are referred to as a preform. It will be appreciated by one of ordinary skill that for composites having a flat shape, the epoxy bonding step may be omitted. However, for more complicated shapes, creating a preform is desirable. The preform is then impregnated with a resin to create a carbon-carbon green part that will be transformed into a carbon-carbon composite. 
     Referring now to the drawings in general and FIG. 1 in particular, it will be understood that the illustrations are for the purpose of describing a preferred embodiment of the invention and are not intended to limit the invention thereto. As best seen in FIG. 1, an apparatus for forming a densified carbon-carbon composite, generally designated  10 , is shown constructed according to the present invention. The apparatus  10  includes three major subassemblies: a green part molding station  12  (FIG.  1 ); a impregnation station  14  (FIG.  2 ); and a carbonization station  16  (FIG.  4 ). 
     The green part molding station  12  is a resin transfer molding (RTM) station and includes a mold  18  having at least one surface configured to the shape of the part to be made. As shown in FIGS. 3A-3C, preferably the mold  18  is enclosed and includes an upper part  18   a  and a lower part  18   b . A seal  19  (See FIG. 3A) is provided in the lower part  18   b  to provide an airtight seal between the mold components during use. The mold  18  further includes a resin injection port  26  and a second evacuation port  30  for providing entrance and exit points, respectively, for the matrix. A resin reservoir  23  holds a supply of resin and communicates with the mold  18  via entrance conduit  17  and pump  25 . A resin reservoir heating means is also provided to lower the viscosity of the resin sufficiently so that it may be pumped. A vacuum source  24  is connected to the mold  18  at the evacuation port  30 . A preferred vacuum source is a vacuum pump. Alternatively, the vacuum source  24  may be connected to the mold at the injection port  26  via a “T” fitting to reduce the number of openings provided in the mold. In yet another embodiment, the vacuum source  24  may be connected to an external chamber for evacuating the mold  18 . The green part molding station  12  further includes a platen press  22 , which is provided with a heating means  20  for curing the resin. 
     The present invention further includes an impregnation station  14  (FIG. 2) that, in a preferred embodiment, is substantially identical to the green part molding station. That is, essentially the same station may be used to form the molded green part and to impregnate the carbon-carbon composite. The impregnation station includes a mold  218  having at least one surface configured to the shape of the part to be made. This mold  218  is substantially identical to the mold  18  described above. The impregnation station  14  includes a resin reservoir  223 , inlet conduit  217 , platen  222 , platen heater  220 , exit conduit  215 , pump  223 , resin heater  225 , and vacuum source  224  as described above with respect to the green part molding station  12 . 
     Both the green part molding station  12  and the impregnation station  14  further include appropriate valving, gauges and temperature sensors as needed to control the process of the present invention. These items have been omitted from the illustrations for the purpose of clarity but are properly within the scope of the claimed invention. 
     In a preferred embodiment, the matrix is a cyanate ester resin, which has a high carbon yield. The matrix carbon yield is an important determinant of process efficiency. This characteristic is determined by heating a known weight of matrix precursor in an inert atmosphere at a given temperature. The remaining carbon residue is then weighed. Carbon yield is the ratio of the weight of the carbon residue to the original weight. Desirably, the resin will have a carbon yield of at least about 25 percent after carbonization. More desirably, the resin will have a carbon yield of at least about 60 percent after carbonization. 
     In addition to having the carbon yield described above, the resin should be able to be heated to a high enough temperature for use in a RTM-like process. At the same time, the resin must not give off any gaseous by-product during cure and should have a suitable pot life. Desirably, the resin has a viscosity of less than about 100 centipose (cps) at 175° F. More desirably, the resin has a viscosity of less than about 50 cps at 175° F. A suitable resin is Primaset PT-30 available from Lonza Corporation. 
     Referring now to FIG. 4, the present invention further includes a carbonization station  30  comprised of a furnace  31  which includes an enclosed chamber  34 , a heating means  36  for heating the chamber and a non-oxidizing atmosphere within the chamber. The non-oxidizing atmosphere can be provided by drawing a vacuum in the chamber  34  or, alternatively, by pressurizing the chamber using a non-reactive gas supply  40 . Preferably, the non-reactive gas is nitrogen, however, any suitable such gas such as argon may be used. Preferably, the chamber  34  is pressurized to a pressure gas pressure during carbonization. Preferably, the heating means is adapted to heat the enclosed chamber  34  to a temperature of greater than about 650° C. More preferably, the heating means is adapted to heat the enclosed chamber  34  to a temperature of greater than about 950° C. 
     The carbonization station further includes a filtering system for cleaning the pyrolysis gases emitted by charring of the matrix in the enclosed chamber  34 . The filtering system includes a water bath  44  and an activated carbon filter  42  connected to the enclosed chamber  34 . The water bath  44  is interposed between the enclosed chamber  34  and the activated carbon filter  42 . 
     A method of forming a densified carbon-carbon composite according to the present invention is illustrated in the flow chart of FIG.  5 . The method begins with the creation of a preform (Block  50 ) as described above. The preform is then impregnated to create a green part (Block  52 ). Desirably, the step may be carried out using a resin transfer molding apparatus. Next, the green part is carbonized to form a carbon-carbon composite (Block  54 ). The microvoids of the carbon-carbon composite are impregnated using a molten resin and without the use of any type of solvent to make the resin flowable (Block  56 ). The carbon-carbon composite is then carbonized to transform the resin to carbon. (Block  58 ) The impregnation and carbonization steps are repeated until a desired density for the carbon-carbon composite is reached (Block  59 ). As was discussed above, the step of creating the preform  50  is optional and the use of this step may vary depending on the shape of the item to be created. 
     The impregnation/densification steps of the present invention include placing the preform or carbon-carbon composite item  213  in the impregnation mold  218  and heating the mold to a temperature of between about 300-350° F. while being held closed by the platen  22 . In order to maintain the proper viscosity and flowability of the matrix, the inlet conduit  217  and the outlet conduit  215  which connects the vacuum source  224  are heated. In a preferred embodiment, the inlet conduit  217  is maintained at a temperature of about 175° F. and the outlet conduit  215  is maintained at a temperature of about 200° F. Other temperatures may be used depending on the makeup of the resin being used. Next, a vacuum is pulled in the mold. The amount of this vacuum should be sufficient to ensure that proper impregnation is achieved. A vacuum of less than about 25 torr is desirable with a vacuum of less than about 10 torr being preferred. More preferably, a vacuum of less than about 2 torr is achieved. After a proper vacuum is achieved, the matrix is pumped into the mold. Desirably, the matrix is pumped at a pressure sufficient to fill the mold, about 150 psi. The pressure may then be raised to a packing pressure that is substantially higher than the pressure used to fill the mold. A packing pressure of about 300 psi is desirable in the practice of the present invention. It will be appreciated that the impregnation of both the green part and the carbon-carbon composite is carried out solely by the hydraulic pressure created when the resin is introduced into the mold. The resin temperature, initial filling pressure and the packing pressure should be selected to achieve the desired densification. The parameters discussed herein are exemplary only and may be varied depending on many factors apparent to one of ordinary skill. 
     After the mold is full, it is burped to remove any trapped air with any matrix exiting the mold being caught in the matrix trap  27  connected to the exit conduit. The mold is then isolated using appropriate valving. The platen  222  serves as a means to hold the mold  218  closed while the resin is injected and cured and as a source of heat to the mold  218 . In a preferred embodiment using the resin described above, a curing time of about 5 hours is used. However, it will be readily apparent that other curing times may be used depending on a number of factors to include the size of the article being created, the resin selected and the equipment available for this step. 
     The method of the present invention provides a number of advantages over solvent-based impregnation/densification processes. For example, the environmental and worker health concerns involved with handling and disposing of large volumes of hazardous solvents are eliminated. Also, solvents can present a significant fire risk. The process of the present invention provides for placing pressure on the impregnating fluid during the entire impregnation process. Further, there is no impregnate volume loss due to solvent loss. The absence of a solvent drying step reduces processing time significantly. 
     As will be appreciated by one of ordinary skill, the solvent drying step required by prior art processes is highly variable and difficult to manage. The solvent must escape from the innermost interstices of the part. If solvents are trapped in the composite structure at this stage, solvent gas generated by the resin curing process temperature can force the resin out of the porosity or even worse, blister or delaminate the part. 
     The resin transfer molding (RTM) approach to making a carbon-carbon composite relies on an established, controllable and reproducible technology. RTM is a closed process that is capable of minimizing worker exposure to potentially hazardous chemicals. As can be seen in FIG. 6, for a given number of densification cycles, a higher density may be achieved by the method of the present invention. Curve  60  illustrates the density achieved using solvent-based approaches. Curve  62  shows the improved results achieved through the practice of the present invention. Experience with the present invention has shown that it will create a carbon-carbon composite having 95 percent of theoretical density after about three carbonization cycles. 
     Accordingly, significant cost reductions are achievable because of the reduced number of impregnations required. The use of RTM permits lower vacuums to be used during impregnation along with higher resin injection and holding pressures. All of these advantages combine to provide a more efficient manufacturing process and associated higher output. 
     Certain modifications and improvements will occur to those skilled in the art upon a reading of the foregoing description. By way of example, the mold may be provided with bolt fasteners to hold it closed while it is heated and evacuated. The bolted mold may then be placed in a oven for the step of curing the resin therein. Also, the resin may be transferred to the mold using a pressure pot rather than a pump. Alternatively, the resin may be pulled into the evacuated mold relying on vacuum alone for this step. It should be understood that all such modifications and improvements have been deleted herein for the sake of conciseness and readability but are properly within the scope of the following claims.