Plastic article or earthenware shaping or treating: apparatus – Distinct means to feed – support or manipulate preform stock... – Female mold type means
Reexamination Certificate
1999-05-17
2001-12-04
Davis, Robert (Department: 1722)
Plastic article or earthenware shaping or treating: apparatus
Distinct means to feed, support or manipulate preform stock...
Female mold type means
C264S029500, C264S571000, C425S405100
Reexamination Certificate
active
06325608
ABSTRACT:
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'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
Abali Felix
Avva Vishnu Sarma
Cunningham Anthony
Sadler Robert L.
Shivakumar Kunigal N.
,MacCord Mason PLLC
Davis Robert
North Carolina A&T State University
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