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[0001] This is a continuation of U.S. patent application Ser. No. 08/724,177 filed Sep. 30, 1996, which is a continuation of U.S. patent application Ser. No. 08/591,363 filed Jan. 25, 1996.
[0002] 1. Field of the Invention
[0003] The invention relates to graphite foam material, of the type used for high temperature insulation and the like, and to a method of making the same. The invention also relates to a graphite material that may be used and added to provide protection from electrostatic discharge (ESD) or shielding from electromagnetic and radio interference (EMI/RFI).
[0004] 2. Art Background
[0005] In the prior art, various forms of graphite material have been used as insulating materials in high temperature applications including industrial ovens and furnaces, vacuum furnaces and controlled atmosphere heating apparatus and the like.
[0006] One of the first such graphite insulating materials was powdered carbon black which had an appropriate amount of insulating capacity, but was very difficult to handle, relatively heavy, and extremely time consuming to replace. As a result with the advent of foam and resin chemistry, a number of newer materials were developed which were lighter and easier to handle, but which had the requisite insulating capacity. One such material is a carbon fiber insulating material made of a carbon fiber held in a matrix by a phenolic resin material, and formed into a board or block.
[0007] Static electricity and electrostatic discharge (ESD) are naturally occurring phenomena. Simply stated, static electricity is electrical energy at rest on a surface. It is generally created by the rubbing together and separating of two materials, one of which is usually non-conductive. Typically, one material gives up electrons and becomes positively charged; the other takes on the electrons and becomes negatively charged. ESD may be defined as the sudden discharge of an electrostatic potential from one body to another. A good example may be the shock one receives when touching a metal door knob after walking across a carpeted floor.
[0008] In many environments, ESD may damage or destroy sensitive electronic components, erase or alter magnetic media, or set off explosions or fires in flammable environments. These discharges may be caused by a variety of sources, most commonly there is a direct discharge from a person or equipment into a sensitive object.
[0009] One way of preventing ESD is to reduce the generation of charges in the first place. A second way of preventing ESD is to provide a ground path for the safe dissipation of accumulated charges to ground. A third method is to provide shielding or protection of devices and equipment from discharge through packaging. ESD may also be controlled with materials, such as conductive plastics, that do not generate high levels of charge, that dissipate charges before they accumulate to dangerous levels, or that provide electrostatic shielding to prevent charges from reaching the sensitive product.
[0010] Electromagnetic Interference (EMI) is electrical energy, either electromagnetic or in the radio frequency (RF) range in the case of radio frequency interference (RFI) that is radiated by specific sources. Some of these sources include computer circuits, radio transmitters, fluorescent lamps, color TV oscillators, electric motors, automotive ignition coils, overhead power lines, lightning, TV games, and many other resources. EMI/RFI may interfere with the operation of simple household appliances such as causing the unwanted operation of garage door openers. On another level, EMI/RFI may corrupt data in large scale computer systems, cause inaccurate readings and output in aircraft guidance systems, and interrupt the functioning of medical devices, such as pacemakers.
[0011] Proper shielding may prevent products from emitting electromagnetic or radio frequency energy to other susceptible equipment. Shielding may also protect susceptible equipment from the effects of externally radiated EMI/RFI as the shielding absorbs the energy emitted, converting it to thermal energy.
[0012] EMI thermoplastic composites are used primarily for shielding against emission or reception of EMI and RFI. Traditionally, shielding has been accomplished by encasing sensitive electronic parts in metal housings or by using metallic coatings on the inside of plastic housings. Thermoplastic compounds with appropriate shielding additives are cost effective alternatives in many applications due to their ability to take on complex shapes and maintain tight tolerances.
[0013] It is desirable to provide, at relatively low cost, a compound/s that may dissipate charges before they accumulate to dangerous levels, that provide electrostatic shielding to prevent charges from reaching the sensitive product. Moreover, it is desirable to provide, at relatively low cost, a material for shielding against emission or re-emission of electrostatic.
[0014] The present invention is a composition of matter, and specifically, a material comprising cryogenically treated graphite or carbon particles which are then expanded by thermal shock/gas expansion. The expanded particles are then combined with a phenolic resin, or the like, and then thermoset under pressure at an elevated temperature to form a hardened sheet or plate. The carbon or graphite particles can be obtained from previously expanded graphite which has been made into flexible graphite foil, and therefore, the present invention permits the recycling of graphite foil which is not otherwise commercially distributed. The method of making said material is also described and claimed.
[0015] The material has generally the same insulating and other physical characteristic as the prior art carbon fiber insulation materials, and it is less expensive than prior art materials.
[0016] Another advantage of the present invention is that it can utilize, without any drawbacks, recycled flexible graphite material, as a starting material. Such recycled flexible graphite material is currently typically being landfilled. Thus, the present invention is particularly advantageous as a benefit to the environment. Additionally, the advantageous method of making the material and the quality of the material made in accordance with the present invention provide additional benefits.
[0017] Another advantage is the reduced weight loss due to oxidation, resulting in longer furnace life between successive rebuilding of the furnace.
[0018] According to the present invention, one may use finished low density blocks, boards, billets, etc. to make higher density parts by cutting (i.e., using, for example, a cork bore or saw) to shape or mold the material into a desired shape and pressing using different pressures to the obtain desired density. For example, die formed rings can be made using the present invention. The compressive strength of the die formed rings was greater than or equal to the strength of some monolithic graphites of the prior art.
[0019] The present invention also includes a graphite material and a method for making the graphite material by using a thermoplastic material mixed with re-expanded graphite. The compound of thermo plastic material and re-expanded graphite is fed into an injection molding system at a relatively high temperature and injected into a mold where a plastic material is formed. The plastic material is then removed from the mold when the material is still very hot but hard set.
[0020] The present invention further includes in one embodiment thereof a method of making a graphite material. Flexible graphite is ground into a powder having a particle size in the approximate range of 25 to 80 mesh. The graphite powder is mixed in an amount ranging between approximately 10% - 90% graphite powder by weight, with a resin, in an amount ranging between approximately 10% - 90% by weight. The graphite powder, mixed with the resin is hot pressed.
[0021] The present invention also provides in another embodiment thereof a method of making a graphite material. A flexible graphite is ground into a powder having a particle size in an approximate range of 25 - 80 mesh. The graphite powder is soaked in a cryogenic liquid. The soaked graphite powder is expanded. The resulting graphite powder is mixed in an amount ranging between approximately 10% - 90% by weight with a resin, in an amount ranging between approximately 10% - 90% by weight. The expanded soaked graphite powder that has been mixed with the resin is then hot pressed.
[0022] The present invention further provides in another embodiment thereof a method of making a graphite material. A flexible graphite is ground into a powder having a particle size in an approximate range of 25 - 80 mesh. The graphite powder is then soaked in a cryogenic liquid. The soaked graphite powder is then expanded. The expanded soaked graphite powder is ground into a powder having a particle size in an approximate range of 48 to 100 mesh. The resulting graphite powder is mixed, in an amount ranging between approximately 10% - 90% by weight with a resin in an amount ranging between approximately 10% - 90% by weight. The graphite powder mixed with the resin is then hot pressed.
[0023] The present invention provides in another embodiment thereof a method of making a graphite material. Initially, graphite flakes are soaked in an acid. The soaked graphite flakes are then expanded. The soaked expanded graphite flakes are precompacted. The precompacted expanded soaked graphite is then ground into a powder that has a particle size in a range of approximately 25 to 80 mesh. The graphite powder is mixed an amount ranging between approximately 10% - 90% by weight with a resin in an amount ranging between approximately 10% - 90% by weight. The graphite powder that has been mixed with the resin is then hot pressed.
[0024] The objects, features and advantages of the present invention will become apparent to one skilled in the art from reading the following detailed description in which:
[0025]
[0026]
[0027]
[0028]
[0029]
[0030]
[0031]
[0032]
[0033]
[0034]
[0035] In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, one having ordinary skill in the art should recognize that the invention may be practiced without these specific details. In some instances, well-known circuits, structures, and techniques have not been shown in detail to avoid obscuring the present invention.
[0036] The present invention relates to a material and methods of making materials, with different densities, the materials having superior heat insulation capacity for use in furnaces and other apparatus. The method of making the present invention is shown in the flow chart in
[0037] The starting material is preferably recycled flexible graphite, such as the type which may be obtained as a byproduct from the manufacture of flexible graphite rolls.
[0038] The flexible graphite material is sometimes referred to as vermiculated graphite. It is a graphite material which has already been subjected to an expansion process, typically an acid treatment of graphite followed by a heat shock treatment. The starting material can be in the form of chunks, bricks, strips, or any other form which may be obtained.
[0039] The flexible graphite is first ground to a very fine powder having a particle size in the range of 35 to 80 mesh and a tap density of approximately 0.177 - 0.230 g/cc. Somewhat smaller or larger mesh can be used as well, but the particle size is preferably within the range of 35 to 80 mesh as specified. The flexible graphite can be ground in a cone mill grinder or hammer mill grinder or other grinder known in the art.
[0040] In the next step, the powdered carbon particles are poured into a container of liquid nitrogen, and permitted to absorb sufficient liquid nitrogen so that they sink in the container below the surface of the liquid nitrogen. It is believed that other cryogenic liquids may also work, and are deemed to be within the scope of the present invention. The sinking of the particles, while not deemed critical to the subject process, appears to be an adequate indicator of sufficient absorption of the liquid nitrogen into the particles, which is important in the following expansion step.
[0041] Alternatively, one may treat ground flexible graphite with acid such as fuming nitric acid, sulfuric acid, etc., and then heat the compound, of the respective acid and ground flexible graphite, thereby causing graphite to expand. For example, an experiment was conducted using 4 grams of ground flexible graphite and 6 centimeters cube (cc) of fuming nitric acid. The compound was then heated to 1100° Celsius (C.). The expansion ratio obtained was 8 to 10 times.
[0042] The liquid nitrogen soaked carbon particles or the ground flexible graphite particles treated with one of the above-mentioned acids are next injected into a hot air burner in an oven with an air stream flowing therethrough, the oven temperature being approximately 650° Fahrenheit (F.). One such acceptable hot air burner is a propane burner such as a Universal® 40,000 BTU per hour propane heater. One possible arrangement is shown in
[0043] If the recycled graphite particles are used, this heat treatment or thermal shock/gas expansion expands the particles to about 4 to 8 times their original size, and the density of the particles is in the range of 0.080 to 0.030 grams per cubic centimeter. The expanded particles may be compressed and molded to the desired size, shape and density without using the next steps.
[0044] The expanded, thermally-shocked, carbon material is then mixed with a resin, and preferably a phenolic resin, and most preferably a phenolic resin such as Borden Durite RD-2414 in a preferred ratio of about 60% carbon to 40% resin by weight. Other ratios may be used, and the selection of a ratio is within the skill of persons of ordinary skill in the art.
[0045] The mixture is then thermoset at a temperature of 350° F. and a pressure dependent upon the density required for about 1 hour. The shape of the product can be any shape and size as required for the intended purpose.
[0046] The thermoset product is then heat treated in a furnace. The temperature of the heat treatment is preferably about 2000° F., but may vary from 1000 - 5000° F. depending upon final usage.
[0047] The density may be in the range from less than 0.1 g/cc to approximately theoretical density.
[0048]
[0049]
[0050] In another embodiment of the present invention, a method for making a substantially high density graphite material with plastic characteristics (graphite-plastic) is provided. The main steps of this method are shown in
[0051] In the embodiment, of the method according to the present invention, described herein, the compound material, i.e., pellets, include a thermoplastic material mixed with 45-60% re-expanded graphite, but the present invention is not limited in scope in this respect. The compound material is then injected into a mold at a temperature of approximately 650° F. The mold may have an approximate dimension of 6″×9″×0.125″, but the present invention is not limited in scope in this respect. The injection of the compound may be performed for approximately 1 minute. Graphite material with plastic characteristics is then formed into plates, or other type of shapes in the mold at a temperature of 250° Fahrenheit and removed therefrom when the material is still hot, but hard set.
[0052] The resulting graphite material with plastic characteristics produced may be formed into different geometry's due to its high density of the produced which allows the graphite-plastic materials produced to be shaped better. The graphite-plastic plates obtained have a density of approximately 1.5 grams per cc. It is believed that any thermoplastic material may be used in this process instead of the PPS.
[0053] A graphite-plastic material according to the present invention may also be obtained by using a thermosetting plastic such as phenolic resin, epoxy resin, and mixing it with graphite powder. The compound of thermoset material and the graphite powder may then be heated to a temperature that is below the thermosetting temperature (350° F.) for approximately an hour and then introduced in a mold by using a process of hot pressing which is well-known by one skilled in the art. It will be noted that instead of graphite powder, re-expanded graphite may be used in this process.
[0054] The present invention also provides a method for making an electrically conductive plastic that has a relatively low resistivity. According to this process, a PPS material or a liquid crystal polymer (LCP) resin is mixed with re-expanded graphite to make pellets as explained above. The pellets are then mixed in a tumbler with 20% by weight nickel coated carbon fibers. The nickel coated carbon fibers may be a mixture of 50% carbon particles and 50% nickel in various concentrations. The addition of nickel coated carbon fibers to the pellets causes a decrease the bulk resistance of the pellets from approximately 0.100 ohm inches to 0.00085 ohm inches. Then this compound is subject to an injection molding process as explained in the foregoing. The material produced has both a low electrical resistivity and a high corrosion resistance. Moreover, the material produced may be used for bipolar plates in photon exchange membrane (PEM) type fuel cells, gaskets such as intake manifold, flange gaskets, etc. for automotive devices.
[0055]
[0056] The process then flows to block TABLE 1 Graphite Resin Resistivity 90% 10% 17.8 Microhms/meter square 80% 20% 26.5 Microhms/meter square 70% 30% 33.0 Microhms/meter square 60% 40% 61.1 Microhms/meter square 50% 50% 90.2 Microhms/meter square 40% 60% 166.0 Microhms/meter square 30% 70% 328.0 Microhms/meter square 20% 80% 2294.0 Microhms/meter square 10% 90% N/A
[0057] Next, at block
[0058] The resulting graphite material obtained by way of the process described above has a density of approximately 1.5 g/cm
[0059]
[0060] Next, at block
[0061] Next at step TABLE 2 Graphite Resin Resistivity 90% 10% 10.0 Microhms/meter square 80% 20% 17.0 Microhms/meter square 70% 30% 33.0 Microhms/meter square 60% 40% 38.7 Microhms/meter square 50% 50% 60.9 Microhms/meter square 40% 60% 84.5 Microhms/meter square 30% 70% 134.0 Microhms/meter square 20% 80% 248.0 Microhms/meter square 10% 90% 736.0 Microhms/meter square
[0062] Next the graphite mixture is hot pressed after initially introducing the graphite mixture into a first mold. Hot pressing is performed in a manner as explained above in the description in connection with the embodiment of the process illustrated in
[0063]
[0064] Table 3 illustrates possible mixture ratios between graphite and resin and the corresponding resistivities of the resulting graphite materials.
TABLE 3 Graphite Resin Resistivity 90% 10% 15.0 Microhms/meter square 80% 20% 23.6 Microhms/meter square 70% 30% 25.8 Microhms/meter square 60% 40% 46.1 Microhms/meter square 50% 50% 116.9 Microhms/meter square 40% 60% 161.8 Microhms/meter square 30% 70% 186.8 Microhms/meter square 20% 80% 1219.9 Microhms/meter square 10% 90% N/A
[0065]
[0066] Table 4 illustrates possible combination ratios between graphite and resin, and the corresponding resistivity of the resulting graphite materials.
TABLE 4 Graphite Resin Resistivity 90% 10% 18.05 Microhms/meter square 80% 20% 21.3 Microhms/meter square 70% 30% 32.2 Microhms/meter square 60% 40% 73.0 Microhms/meter square 50% 50% 83.0 Microhms/meter square 40% 60% 119.0 Microhms/meter square 30% 70% 4136.0 Microhms/meter square 20% 80% N/A 10% 90% N/A
[0067]
[0068] The graph designated by the symbol “▪” is in connection with the embodiment of the process explained in connection with
[0069] In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will however be evident that various modifications and changes can be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.