1. Field of the Invention
The present invention relates to composite materials technology, and more specifically to a relatively light-weight, inexpensive, durable, high performance structural laminate composite material for use to 1000xc2x0 F., and above, which can advantageously be used in high temperature environments. More particularly, the preferred embodiment of the present invention relates to a graphite-fiber/phenolic-resin composite material which retains relatively high strength and modulus of elasticity at temperatures as high as 1,000xc2x0 F. (538xc2x0 C.). The material costs only 5 to 20 percent as much as refractory materials do. The fabrication of the composite includes a curing process in which the application of full autoclave pressure is delayed until after the phenolic resin gels. This modified curing process allows moisture to escape, so that when the composite is subsequently heated in service, there will be much less expansion of absorbed moisture and thus much less of a tendency toward delamination. In contrast, internal pressure caused by the expansion of moisture absorbed in other prior art composite materials like prior art graphite/epoxies and prior art graphite/polyimides causes delamination at temperatures in the range of 500 to 700xc2x0 F. (260 to 370xc2x0 C.).
2. General Background
At the request of NASA/MSFC, Martin Marietta Manned Space Systems has performed an extensive development/verification activity for a composite nose cone for the external tank (ET). At the time of the initiation of this effort, there was no materials technology available to provide a nose cone which could withstand the high heating and structural loading of the ET nose cone without (a) requiring the use of secondary heat shield materials, (b) increasing the weight of the existing nose cone, and (c) significantly increasing the cost over the existing nose cone cost. There were high temperature polymeric composite materials available; however, none met all requirements. Carbon/phenolic laminates have been proven in rocket nozzle applications to be able to withstand extreme heating conditions; however, these materials did not possess the specific strength and stiffness required for a weight-effective structure. Also, recent data shows that the materials on the market today have the potential to xe2x80x9cply lift,xe2x80x9d or delaminate due to internal pressure caused by absorbed moisture, at about 500xc2x0 F. Graphite/polyimide laminates showed promising mechanical properties, but suffered from the moisture-induced delamination problem (also known as xe2x80x9cthermal shockxe2x80x9d) at temperatures below 700xc2x0 F. in laminates of the thickness required for a composite nose cone. Other technologies such as ceramic matrix composites and carbon/carbon were considered too expensive for this application. Therefore, a program was initiated to develop laminate material which could meet all requirements.
U.S. Pat. No. 3,724,386 for xe2x80x9cAblative Nose Tips and Method for their Manufacturexe2x80x9d discloses in Example II heating graphite yarn impregnated with phenolic resin slowly to 160xc2x0 F. to slowly evaporate solvent from the resin (see column 8, lines 16-18).
U.S. Pat. Nos. 4,100,322 and 4,215,161 for xe2x80x9cFiber-Resin-Carbon Composites and Method of Fabricationxe2x80x9d disclose impregnating graphite yarn with phenolic resin under vacuum and a temperature of about 150xc2x0 F. until the solvent has gone and the resin gels, then further heating the composite to cure it. However, the solvent stripping process was interrupted twice and each time pressure of 200 psig was applied to the composite material. It is then subjected to pyrolysis, and then pores of the composite are impregnated with phenolic resin. After this, the phenolic resin is cured at about 350xc2x0 F. The resulting structure is said to be graphite/carbon/phenolic composite, and its porosity is disclosed to be 4%. A carbon/carbon/phenolic composite described therein is said to have a porosity of 5.8%.
U.S. Pat. No. 4,659,624 for xe2x80x9cHybrid and Unidirectional Carbon-Carbon Fiber Reinforced Laminate Compositesxe2x80x9d discloses a method similar to the method disclosed in U.S. Pat. Nos. 4,100,322 and 4,215,161 (and with similar materials), but one in which more resin is added and pyrolized up to 5 times. This patent points out at column 2, line 50 through column 3, line 2 that it is important to properly initially cure laminate materials to provide interconnecting pores which allow the escape of gases formed during post-cure pyrolysis.
U.S. Pat. No. 4,957,801 for xe2x80x9cAdvance Composites with Thermoplastic Particles at the Interface Between Layersxe2x80x9d discloses a resin-impregnated fiber layer with outer layers of resin thereon. The fiber can comprise, for example, graphite.
U.S. Pat. No. 5,288,547 for xe2x80x9cToughened Resins and Compositesxe2x80x9d discloses a composite in which a porous membrane film of thermoplastic material is sandwiched between two layers of resin-impregnated fibers, and then the composite is cured in an autoclave, for example. The resin can be, for example, phenolic resin.
U.S. Pat. No. 5,359,850 for xe2x80x9cSelf Venting Carbon or Graphite Phenolic Ablativesxe2x80x9d discloses a resin-impregnated reinforcing cloth made of, for example, graphite fibers with degradable fibers interwoven therewith. The degradable fibers are chosen such that they degrade at a temperature of about 400xc2x0 F. to 500xc2x0 F. so that they will provide passageways for the gaseous decomposition products produced as the resin matrix approaches the char temperature. In this patent, foreign material is introduced to create porosity. The fabric weave is altered by introducing a low-temperature degradable thread which may not assure fabric strength properties. The porosity which is created by this process is uniform. There is a definite pattern when the foreign material is replaced by voids. It is believed that the addition of these special degradable fibers will add to the cost of the material. Further, it is believed that in some cases the degradable fibers might not burn away before the plies blow apart.
U.S. Pat. No. 5,360,500 for xe2x80x9cMethod of Producing Light-Weight High-Strength Stiff Panesxe2x80x9d discloses a panel made by a pair of surface members separated and supported by an internal core in which spaces or interconnected pores provide vents to an edge of the panel so that gas can flow through the vents during a pyrolysis process. The vents are on the order of 10 mm in diameter.
None of these patents discloses a composite material with a weight, thickness, structural performance, and pore structure as advantageous for use in a nose cone of the external tank of the space shuttle, or other high temperature structural applications, as the material of the present invention.
A novel materials technology has been developed and demonstrated for providing a high modulus composite material for use to 1000xc2x0 F. The material of the present invention can be produced at 5-20% of the cost of refractory materials, and has higher structural properties. This technology successfully resolves the problem of xe2x80x9cthermal shockxe2x80x9d or xe2x80x9cply lift,xe2x80x9d which limits traditional high temperature laminates (such as graphite/polyimide and graphite/phenolic) to temperatures of 550-650xc2x0 F. in thicker (0.25xe2x80x3 and above) laminates. The technology disclosed herein is an enabling technology for the nose for the External Tank (ET) of the Space Shuttle, and has been shown to be capable of withstanding the severe environments encountered by the nose cone through wind tunnel testing, high temperature subcomponent testing, and full scale structural, dynamic, acoustic, and damage tolerance testing.
In the present invention, cure conditions (temperature, pressure, vacuum) and cure apparatus (specific vacuum bag methodology) are manipulated to produce a graphite/phenolic composite laminate with a permeable microstructure comprising an interconnected network of pores which allows moisture to escape from the composite material when the composite material is heated; this helps prevent delamination (xe2x80x9cply liftxe2x80x9d or xe2x80x9cthermal shockxe2x80x9d) when the material is heated to temperatures above 500xc2x0 F. The graphite/phenolic composite of the present invention can be used for components for applications requiring high strength and stiffness upon exposure to very high heating (e.g. rocket nozzles for missiles or launch boosters, fire walls, heat shields, circuit boards, secondary structure on missiles or launch vehicles which see high aerodynamic heating, and parts to be used on the leading edge of aerodynamic products (airplanes, jets, rockets, fuel tanks for aerospace structures, etc.)).
The present invention comprises a method of producing a composite material, comprising the steps of:
impregnating a fiber material with a resin to create a resin-impregnated fiber material;
without applying pressure, heating the resin-impregnated fiber material under vacuum at a sufficient temperature for a sufficient amount of time until the resin gels; and
applying temperature (and, optionally, pressure) for a sufficient period to cure the resin-impregnated fiber material. The starting percentage by weight of fiber material (before being cured) is preferably 30-80%, with the balance resin. The resulting porosity of the composite material is preferably at least 3% by volume, more preferably about 3-25% by volume, and most preferably about 7-14% by volume.
The preferred embodiment of the method of the present invention of producing a composite material comprises the steps of:
(i) impregnating a graphite fiber material with a phenolic resin to create a resin-impregnated fiber material, in a ratio of 30-80% by weight graphite fiber and 20-70% by weight phenolic resin;
(ii) placing the resin-impregnated fiber in an autoclave or oven;
(iii) applying full vacuum and/or pressure;
(iv) raising the temperature to cause the resin to flow and initiate cure,
(v) holding the material at a temperature to allow gellation of resin while volatiles are being released;
(vi) raising the temperature for final cure if required;
(vii) cooling the material;
(viii) removing the material from the autoclave or oven;
(ix) post-curing the composite laminate material removed from the autoclave, if required.
The present invention includes the composite material made by the method of the present invention disclosed herein, as well as a composite material, produced by any method, having a composition and structure which is the same as the composite material produced by the method of the present invention disclosed herein.
The material of the present invention comprises a high performance structural laminate composite material for use in high temperature applications, consisting essentially of resin-impregnated fiber, the resin-impregnated fiber consisting essentially of:
(a) preferably 50-80% by weight fiber, and
(b) preferably 20-50% by weight cured resin, the composite material having:
(c) a permeability sufficient to allow moisture to escape from the composite material, without causing plylift, when the composite material is heated to temperatures up to 1000xc2x0 F. More preferably, the permeability is sufficient to allow moisture to escape from the composite material, without causing plylift, even when the composite material is heated to temperatures above 1000xc2x0 F. The material of the present invention has a microscopic construction which provides permeability that is sufficient to allow moisture to escape therefrom as it is heated to temperatures up to 1000xc2x0 F. and above without exhibiting ply-lift.
The composite material preferably has an across-ply permeability having a Darcys constant of at least 10xe2x88x9215 cm2. More preferably, the across-ply permeability of the composite material has a Darcy""s constant of at least 10xe2x88x9214 cm2. Most preferably, the across-ply permeability of the composite material has a Darcy""s constant of at least 10xe2x88x9213 cm2.
The material of the present invention comprises a high performance structural laminate composite material for use in high temperature applications, consisting essentially of phenolic resin-impregnated graphite fiber, the phenolic resin-impregnated graphite fiber consisting essentially of:
(a) preferably 50-80% by weight graphite fiber; and
(b) preferably 20-50% by weight cured phenolic resin, the composite material having:
(c) a permeability sufficient to provide a network of pores which allows moisture to escape from the composite material, without causing plylift, when the composite material is heated.
The percentage by weight of graphite fiber is more preferably 60-80%, and the percentage by weight of cured phenolic resin is more preferably 20-40%. Most preferably, the percentage by weight of graphite fiber cloth is 65-75%, and the percentage by weight of cured phenolic resin is 25-35%.
Preferably, the porosity is 3-25% by volume. Most preferably, the porosity is 7-14% by volume.
Preferably, the compressive strength of the material after exposure to temperatures above 700xc2x0 F. for several minutes is at least 50% of the compressive strength of the material immediately after being cured, the shear strength of the material after exposure to temperatures above 700xc2x0 F. for several minutes is at least 50% of the shear strength of the material immediately after being cured, and the compressive strength of the material at 900xc2x0 F. is at least 25% of the compressive strength of the material at room temperature.
The graphite fiber cloth was selected to have a combination of high strength, high modulus, good thermo-oxidative stability, and moderate cost. The optimum fiber type to provide this balance is a fiber made from a polyacrylynitrile (PAN) precursor, such as the Toho G30-5001 fiber used in the development documented herein. Similar fibers are Hercules AS4 and IM-7, and Amoco T300, T650-35, and T650-45. Fiber types which were not selected were fibers based on pitch precursors (e.g., Amoco P-75 and P-100), or fibers based on rayon precursors. Pitch based fibers are much more expensive and do not have adequate strength. Rayon based fibers do not have the desired strength or modulus. The selected fiber was woven into an 8-harness satin fabric to facilitate part fabrication. The selected fiber can be, for example, an eight-harness fabric woven from Toho G-30/500-3K graphite fiber. The resin is advantageously selected from a group consisting of phenolics, bismaleimides (BMIs), polyimides, cyanate esters, epoxies, or any blend of these resins. The resin can comprise Cytec 506 phenolic resin.
Preferably, the graphite fiber material has a minimum tensile strength of at least 300 KSI, more preferably at least 400 KSI, and most preferably at least 500 KSI, a minimum modulus of at least 20 MSI, more preferably at least 25 MSI, and most preferably at least 30 MSI, and relatively low cost. Most preferably, the resin is phenolic resin.
The material can consist of phenolic resin-impregnated graphite fiber cloth, and the phenolic resin-impregnated graphite fiber cloth can consist of graphite fiber cloth and cured phenolic resin.
The present invention also includes apparatus comprising a component which requires high strength and stiffness upon short term exposure to very high heating, made of the material of the present invention. The component can be a rocket nozzle, a part for an aerodynamic vehicle, or some other component exposed to high heating. The component can be part of a fire wall or heat shield.
Further, the present invention comprises vacuum bag apparatus for producing a composite laminate material having a network of pores. This vacuum bag apparatus can comprise:
(a) a base for receiving the laminate material thereon;
(b) a non-stick layer to be received on the laminate material for helping to prevent the laminate material from sticking to layers above the non-stick layer;
(c) a first volatiles flow and resin retaining layer above the non-stick layer for allowing volatiles, but not the majority of the resin, to escape from the laminate material through the first volatiles flow and resin retaining layer as heat is applied and the vacuum is drawn in the bag apparatus;
(d) a bleeder layer on the first volatiles flow and resin retaining layer for absorbing most of the resin which flows through the first volatiles flow and resin retaining layer;
(e) a second volatiles flow and resin retaining layer on the bleeder layer for allowing volatiles, but very little resin, to flow through the bleeder layer as heat is applied and the vacuum is drawn in the vacuum bag apparatus;
(f) a first gas-flow layer on the second volatiles flow and resin retaining layer for allowing gas to flow evenly through the vacuum bag apparatus when a vacuum is drawn in the apparatus;
(g) a lateral gas-flow layer surrounding the laminate material to ensure that volatiles can flow out of the laminate in virtually any direction;
(h) a vacuum bag layer attached to the base in an air-tight manner, the base and the vacuum bag layer enclosing the laminate material and the non-stick layer, the first volatiles flow and resin retaining layer, the bleeder layer, the second volatiles flow and resin retaining layer, the gas-flow layer, and the lateral gas-flow layer. In certain circumstances, one or more of the layers can be omitted, as described further below. The vacuum bag apparatus preferably also comprises a port in the vacuum bag layer communicating with a vacuum source for allowing a vacuum to be pulled in the bag. In a room at standard temperature and pressure, the vacuum causes a pressure of about 15 psi to be applied to the laminate in the vacuum bag. The application of additional pressure may not be a necessary step to make the present invention work.
It is an object of the present invention to provide a high-strength, low weight, high temperature material which has sufficient permeability to allow moisture to exit therefrom, even when the material has a thickness of more than 0.40xe2x80x3, when heated to temperatures of above 500xc2x0 F., without damaging the internal structure of the material.
It is an object of the present invention to provide a high-strength, low weight, high temperature material which has sufficient permeability to allow moisture to exit therefrom, even when the material has a thickness of more than 0.40xe2x80x3, when heated to temperatures of above 1000xc2x0 F., without damaging the internal structure of the material.
It is another object of the present invention to provide a method of making such material.
A further object of the present invention is to provide components made of such material.
It is also an object of the present invention to provide a material which can withstand the high heating and structural loading of the ET nose cone without (a) requiring the use of secondary heat shield materials, (b) increasing the weight of the existing nose cone, and (c) significantly increasing the cost over the existing nose cone cost.
Another object of the present invention is to provide an ET nose cone made of this material.
Unlike many prior art methods of producing composite material, in the method of the present invention, there is no pyrolizing step (the composite material of the present invention is not pyrolized). In the method of the present invention, unlike the method of U.S. Pat. No. 5,359,850: no foreign material is introduced to create porosity; the fabric weave is not altered and areal weight of fabric is constant, which assures strength properties; the porosity which is created by the process of the present invention is random and spread over the composite laminate; no material is decomposed by the method of the present invention; and the cost of creating the porosity is relatively low.
Because of the high permeability of the material of the present invention, it is believed by the inventors that there will be no ply lift at any thickness, whether the laminate is at least 0.1 inch thick, at least 0.2 inch thick, at least 0.4 inch thick, or even more than 4 inches thick.
Although the specific examples described herein relate to graphite fiber and phenolic resin, other appropriate fibers and resins could be used in conjunction with the present invention.