WO2022103517A1

WO2022103517A1 - Improved method to manufacture carbon foam at atmospheric pressure

Info

Publication number: WO2022103517A1
Application number: PCT/US2021/053352
Authority: WO
Inventors: Rudolph Andrew Olson, Iii
Original assignee: Cfoam, Llc
Priority date: 2020-11-12
Filing date: 2021-10-04
Publication date: 2022-05-19

Abstract

An improved method to manufacture carbon foam at atmospheric pressure which eliminates the need to utilize an autoclave or similar device to pressurize the product during manufacture. Low viscosity liquid and offgas emerging during the heating steps escapes from the heating container through perforations. The method can further perform the step of adding at least one of a particulate pore stabilizer and an active polymer particulate to the starting material to form an admixture in the bed. A carbon foam, pore stabilized carbon foam, and polymer particulate modified carbon foam prepared at atmospheric pressure by these methods is also taught.

Description

IMPROVED METHOD TO MANUFACTURE CARBON FOAM

AT ATMOSPHERIC PRESSURE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present invention claims priority to U.S. Provisional Patent Application No. 63/112,693, filed November 12, 2020, incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

[0002] The present invention is directed to a method for producing a carbon foam and a pore stabilized carbon foam from coal, with all steps performed at atmospheric pressure.

BACKGROUND OF THE DISCLOSURE

[0003] CFOAM® carbon foam is created using coal as a precursor. In the manufacturing process, coal is heated in an inert atmosphere (e.g. nitrogen gas) so that it does not burn. The precursor first melts as it is heated, then evolves gases that cause the material to foam. The foaming step is typically done under high pressure (about 400 psi) to help regulate the rate at which the precursor transforms into liquid phase, forms bubbles, and crosslinks to form a solid foam, which enables the generation of high quality foam. This foaming step is performed over a temperature range of about 25-470°C, during which oxygen, nitrogen, and some hydrogen are eliminated from the coal. The foamed product is then fired in a kiln at atmospheric pressure under nitrogen to eliminate most of the remaining hydrogen and further crosslink the carbon until only vitreous carbon remains at 1000°C.

[0004] Forming the foam in an autoclave at high pressure is expensive, time consuming, capital intensive, dangerous, and limited to a batch-type operation. In addition, the size of the product is limited to the size of the autoclave that is allowed by the process conditions, or of course the autoclave that a business can afford. The higher the pressure, the more expensive the autoclave and the more the size is limited by engineering constraints.

[0005] A primary reason carbon foam cannot be manufactured at lower pressures is because, when pressure is reduced, the bubbles within the foam tend to coarsen and the foam tends to break down, forming larger cavities within the product. The size of the cavities tends to get larger as the pressure at which foaming is performed is decreased. This was demonstrated in example experiments performed on a series of foams produced at 400 psi (typical of production), 300, 200, 100 and 50 psi. A large cavity formed near the container boundary at about 200 psi and the foam partially collapsed into the cavity at 100 psi as the cavity coarsened.

BRIEF SUMMARY OF THE INVENTION

[0006] An improved method for producing carbon foam at atmospheric pressure is taught using the steps of: partially filling a heating container with starting material to form a bed, the heating container having perforations; heating the bed to a first elevated temperature sufficient to form a substantially homogeneous, plastic, open cell carbon material; heating the substantially homogeneous, plastic, open cell carbon material to a second elevated temperature sufficient to form a carbon foam, wherein the carbon foam is substantially free to expand in at least one direction during formation; wherein low viscosity liquid and offgas emerging during the heating steps escapes from the heating container through the perforations; and wherein the method is conducted at about 0 psig. The method can further comprise the step of adding at least one of a particulate pore stabilizer and an active polymer particulate to the starting material to form an admixture in the bed.

[0007] A carbon foam, pore stabilized carbon foam, and polymer particulate modified carbon foam prepared at atmospheric pressure by the methods above is also taught.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Fig. 1 shows photograph cross-sections of carbon foam samples made using foaming pressures of 400 psi (typical of manufacturing process), 300, 200, 100, and 50 psi; [0009] Fig. 2 illustrates an embodiment of the container used in making carbon foam at atmospheric pressure;

[0010] Fig. 3 is photographic structure comparison of a foam made at atmospheric pressure as compared to foam made using the conventional pressurized process at high temperature;

[0011] Fig. 4 is a photograph of a cross-section of a carbon foam sample with a large cavity formed on the bottom;

[0012] Fig. 5 is a photograph cross-section comparison of a carbon foam sample run under similar conditions with and without perforations on the bottom liner;

[0013] Fig. 6 is a photograph showing the result of 2 wt% phenolic resin addition to the carbon foam starting material;

[0014] Fig. 7 is a photograph showing the result of 2 wt% polyethylene oxide addition to the carbon foam starting material; [0015] Fig. 8 is a photograph showing a cross-section of a carbon foam sample containing 2 wt% phenolic resin and 4 wt% alumina powder made at atmospheric pressure; [0016] Fig. 9 is a photograph of a large carbon foam panel produced containing 2 wt% phenolic resin and 4 wt% alumina powder made at atmospheric pressure.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Some embodiments of the present invention are described in this section in detail sufficient for one skilled in the art to practice the present invention without undue experimentation. It is to be understood, however, that the fact that a limited number of preferred embodiments are described does not in any way limit the scope of the present invention as set forth in the claims.

[0018] It is to be understood that whenever a range of values is described herein, i.e. whether in this section or any other part of this patent document, the range includes the end points and every point there between as if each and every such point had been expressly described. Unless otherwise stated, the words “about” and “substantially” as used herein are to be construed as meaning the normal measuring and/or fabrication limitations related to the value or condition which the word “about” or “substantially” modifies. Unless expressly stated otherwise, the term “embodiment” is used herein to mean an embodiment of the present invention.

[0019] An improved method to manufacture carbon foam at atmospheric pressure is disclosed herein, which eliminates the need to utilize an autoclave or similar device to pressurize the product during manufacture. This also enables the process to be run in a continuous fashion instead of batch. At the least, this improved method allows one to use autoclaves designed for much lower pressures, which are much lower cost and capable of being built in much larger sizes, enabling the production of larger carbon foam products. [0020] CFOAM® carbon foam is created using coal as a precursor. In the manufacturing process, coal is heated in an inert atmosphere (e.g. nitrogen gas) so that it does not burn. The precursor first melts as it is heated, then evolves gases that cause the material to foam. The foaming step is typically done under high pressure (about 400 psi) to help regulate the rate at which the precursor transforms into liquid phase, forms bubbles, and crosslinks to form a solid foam, which enables the generation of high quality foam. This foaming step is performed over a temperature range of about 25-470°C, during which oxygen, nitrogen, and some hydrogen are eliminated. The foamed product is then fired in a kiln at atmospheric pressure under nitrogen to eliminate most of the remaining hydrogen and further crosslink the carbon until only vitreous carbon remains at 1000°C.

[0021] The starting coal materials may include bitumen, anthracite, or even lignite, or blends of these coals that exhibit a “free swell index” as determined by ASTM D720 of between about 3.5 and about 9.0, but are preferably bituminous or subbituminous, agglomerating coals that have been comminuted to an appropriate particle size, preferably to a fine powder below about -60 to -80 mesh. Additionally, according to further highly preferred embodiments of the present invention, the coal starting materials of the present invention possess all or at least some of the following characteristics: 1) a volatile matter content (dry, ash-free basis) of between about 23% and about 45% as defined by ASTM D3175, “Test Method for Volatile Matter in the Analysis of Coal and Coke”; 2) a fixed carbon (dry basis) between about 50% and about 60% as defined by ASTM D3172, “Practice for Proximate Analysis of Coal and Coke”; 3) a Gieseler initial softening temperature of between about 380°C and about 415°C as determined by ASTM D2639, Test Method for Plastic Properties of Coal by the Constant-Torque Gieseler Plastometer”; 4) a plastic temperature range above about 50°C as determined by ASTM D2639; 5) a maximum fluidity of at least 300 ddpm (dial divisions per minute) and preferably greater than about 2000 ddpm as determined by ASTM D2639; 6) expansion greater than about 20% and preferably greater than about 100% as determined by Amu Dilatation; 7) vitrinite reflectance in the range of from about 0.8 to about 1.3 as determined by ASTM D2798, “Test Method for Microscopical Determination of the Reflectance of Vitrinite in Polished Specimens of Coal”; 8) less than about 34% inert maceral material such as semifusinite, micrinite, fusinite, and mineral matter as determined by ASTM D2798; and 9) no significant oxidation of the coal (0.0 vol % moderate or severe oxidation) as determined by ASTM D 2798 and non-maceral analysis. The low softening point (380-415°C) is important so that the material softens and is plastic before volatilization and coking occur. The large plastic working range or “plastic range” is important in that it allows the coal to flow plastically while losing mass due to volatilization and coking. Vitrinite reflectance, fixed carbon content and volatile matter content are important in classifying these coal starting materials as “high-volatile” bituminous coals that provide optimum results in the process of the present invention.

[0022] Thus, according to various embodiments of the present invention, a coal particulate starting material characterized as a high-volatile bituminous or subbituminous coal containing from about 23% to about 45% by weight (dry, ash-free basis) volatile matter, as defined by ASTM D3175, is a basic requirement for obtaining optimum results in the form of optimum carbon foaming in accordance with the process of the present invention. The various parameters derived from the Gieseler plasticity evaluations form the second highly important set of characteristics of the starting material coal if optimum results are to be obtained. Thus, a softening point in the range of from about 380°C and about 415°C, a plastic range of at least about 50°C and preferably between about 75 and 100°C, and a maximum fluidity of at least several hundred and preferably greater than 2000 ddpm (dial divisions per minute) are highly important to the successful optimized practice of the present invention. Accordingly, in order to obtain the carbon foams exhibiting the superior properties described herein, it is important that the coal starting material be a relatively high volatility bituminous or subbituminous coal having a softening point as just described and a plastic range on the order of above about 50°C all with the indicated Gieseler fluidity values described.

Exhibition of Arnu dilatation values greater than about 20% and preferably above about 100% when combined with the foregoing characteristics provide indications of a highly preferred high volatility bituminous or subbituminous coal starting material.

[0023] The cellular coal -based products described herein are semi-crystalline or more accurately turbostratically-ordered and largely isotropic, i.e., demonstrating physical properties that are approximately equal in all directions. The cellular coal-based products typically exhibit average pore sizes of about 400-600 pm, although average pore sizes of 700 pm are possible within the operating parameters of the process described. The thermal conductivities of the cellular coal-based products are generally less than about 1.0 W/m-K. Typically, the cellular coal-based products of the present invention demonstrate compressive strengths on the order of from about 600 to about 3000 psi at densities of from about 0.27 to about 0.53 g/cm³. The coal starting material can exhibit the previously specified free swell index of between about 3.5 and about 9.0 and preferably between about 7 and about 8.5. Selection of starting materials within these parameters was determined by evaluating a large number of coals characterized as ranging from high to low volatiles. In general, it has been found that bituminous and subbituminous coals exhibiting free swell indexes within the previously specified ranges provided the best foam products in the form of the lowest calcined foam densities and the highest calcined foam specific strengths (compressive strength/density). Such bituminous or subbituminous coals that also possess the foregoing set of properties, high volatile content (23% to 45% by weight), large plastic range (at least about 50°C), etc., and are thus characterized as high volatile bituminous or subbituminous coals, form the preferred starting materials of the process of the present invention. Coals having free swell indices below the specified preferred ranges may not agglomerate properly and will leave a powder mass or sinter, but not swell or foam, while coals exhibiting free swell indices above these preferred ranges may heave upon foaming and collapse upon themselves, leaving a dense compact.

[0024] The production method of the present invention comprises: 1) heating a high volatile bituminous or subbituminous coal admixture of preferably small (i.e., less than about 100-pm particle size) in a “mold” and under a non-oxidizing atmosphere at a heat up rate of from about 0.1 to about 20°C per minute to a temperature of between about 300 and about 700°C; 2) soaking at a temperature of between about 300 and 700°C for about 10 minutes up to about 12 hours to form a preform or finished product; and 3) controllably cooling the preform or finished product to a temperature below about 100°C. The non-oxidizing atmosphere may be provided by the introduction of inert or non-oxidizing gas into and around the “mold” at a pressure of from about 0 psi above atmospheric pressure, i.e., free flowing gas, up to about 500 psi. The inert gas used may be any of the commonly used inert or non-oxidizing gases such as nitrogen, helium, argon, CO2, etc.

[0025] As the mold pressure is increased from atmospheric pressure to 500 psi, as imposed by the non-oxidizing gas, the way in which the chemical reactions within the coal will evolve as it is heated will change, which in turn will impact the pore structure and density of the carbon foam. Further, the heat-up rate will also affect density; a faster heat-up rate results in a less dense expanded coal product with a slightly larger pore size on average versus that for a slower heat-up rate. These phenomena are, of course, due to the kinetics of the volatile release reactions which are affected, as just described, by pressure, temperature and the rate at which that temperature is achieved. These process variables can be used to custom produce the expanded coals of the present invention in a wide variety of controlled densities, strengths, etc.

[0026] A known method of improving the quality of carbon foam made at atmospheric pressure is as described in US Patent 7,767,183, portions of which are taught below. The process, as shown in Figure 2, teaches placing comminuted coal 18, also referred to as a bed 18, into a container 12. In some embodiments the container has a flat bottom 14. The container may utilize a lid 24. A thin metal foil or plate liner 16, which may be referred to as a bottom liner 16, may be perforated to provide for offgas 32 and liquid 34 evolution from the bottom of the coal mass. In some embodiments, such perforations may be provided by a series of pin holes. Optionally, a top surface covering 20 can be placed on the top surface of the coal bed. For example, aluminum foil may be placed on the top surface of the coal bed, to provide a top surface covering 20. A loose-fitting metal cover 22 rides on the surface of the coal bed during foaming and a loose-fitting lid 24 is disposed on the container 12.

[0027] The loose-fitting cover 22 likely helps regulate the foaming reactions, similar to the function of high pressure in an autoclave. However, the cover 22 does not function quite as well as a pressurized autoclave. Figure 3 demonstrates the structure of a foam made at atmospheric pressure according to US Patent 7,767,183, as compared to foam made using the conventional process at high temperature, using the container elements shown in Figure 2 and White Forest coal as the starting material. The container 12 was a steel beaker. Aluminum foil was placed in the container as a bottom liner 16 and a top liner 20. An aluminum disk cover 22 of about 1-mm thickness was placed on the coal bed. Note the atmospheric foam sample shown in Figure 3 has much better structure in comparison to those produced at low pressure in Figure 1, but still does not have the homogeneity of material produced at -400 psi; some larger voids are apparent where bubbles tended to ripen.

[0028] Another problem encountered when producing carbon foam at atmospheric pressure is as represented in Figure 4. During the reaction, low viscosity liquids tend to emerge from the coal at a different time and to a different extent than they do at high pressure. These liquids can become trapped on the bottom of the coal bed. If the liquids then vaporize at higher temperature when the coal has become semi-molten, they can expand by roughly a factor of 1,000 and form large cavities on the bottom of the material, as shown in Figure 4.

[0029] A structural modification of the container 12 was used to eliminate large cavities on the bottom of the foam. Perforations were added to the bottom liner 16 of the pan that were large enough and prevalent enough to allow the low viscosity liquid 34 and offgas 32 emerging from the coal to escape. A range of perforation specifications are suitable for applying to the container that vary with the specific carbon foam being formed. Hole (perforation) sizes, hole centers, holes/sq. in., and open area (%) can be in accordance with the Industrial Perforations Association (IP A) numbers that range between 100 and 115, which corresponds to perforation hole sizes in the range of 0.02 inches to 0.125 inches. Perforations can be all the same size or a mixture of sizes to fit the forming requirements. The equivalent strength of the perforated material is used in place of the strength of the solid material of the container 12. The effect of the perforations on the yield strength of the material can be obtained as a function of the yield strength of the solid or unperforated material. Thus, safety margins for the perforated material for any geometry of application and any loading conditions can be determined using the equivalent solid material concept taught in the IPA Handbook. Using a perforated container 12, the liquid 34 is not allowed to become trapped and later vaporize. Figure 5 is a cross-section of a foam sample run under similar conditions as those in Figure 4, and described in Figure 2, yet perforations were included on the bottom liner 16 of the container 12.

[0030] To tackle the problems associated with the coarsening of porosity with reduced pressure, the change in the reaction rate of coal as a function of temperature was researched. If low viscosity liquids 34 tend to preferentially escape from the coal at atmospheric pressure, then they would decrease the viscosity of the liquid phase fraction that is forming during the foaming step, thereby enhancing bubble growth. Possibly a preferential generation of low viscosity liquid 34 is simply adding to the total fraction of liquid phase, which may also enhance bubble growth. The addition of a polymeric additive such as phenolic resin (with activator) could potentially speed the solidification of the liquid phase through reaction, thereby reducing the amount of time bubbles would have to grow, or potentially increase the viscosity of the liquid phase, also limiting growth. As an added benefit, the phenolic resin would tend to be converted to vitreous carbon in the process, adding to the yield of the foam. In another similar concept, the addition of polyethylene oxide polymer was also explored in an effort to achieve the same outcome, though the only intention of adding this material was to increase the viscosity of the liquid phase through thickening, not necessarily reduce the amount of liquid phase or speed its cross-linking rate.

[0031] Figure 6 shows the result of 2 wt% phenolic resin (Georgia-Pacific, 501C01) addition to White Forest coal. Note the reduction of coarse pores relative to samples shown in Figure 5. Figure 7 shows the result of 2 wt% polyethylene oxide (Dow Chemical, PolyOx WSR 303), again showing a reduction in the size of coarse pores. An essentially identical result was achieved when using a PolyOx WSR 308 with slightly higher molecular weight. Thus, both phenolic resin and polyethylene oxide had a similar impact on the structure of the foam. There is no doubt that other polymers with similar properties and performance could have as good or better impact on controlling the structure of the foam as these two materials. Addition of an inorganic crosslinking agent, such as boric acid, could also produce similar result. Optimization of the foam properties could be performed through designed experiments utilizing coal, thermal profile, inorganic crosslinking additives, and/or polymer additives as variables.

[0032] When used in conjunction with the technology described in US Patent Application 16/587,816, published as US 2020/0115232A1, portions of which are herein taught below, incorporation of fine ceramic powder, an even more uniform pore structure was observed. Figure 8 shows a cross-section of a foam sample containing 2 wt% phenolic resin and 4 wt% alumina powder made at atmospheric pressure. Figure 9 shows a large panel produced using the same conditions. Further experiments using a mixture of polymer additives, ceramic or metal powders, and thermal profile will enable the optimization of a carbon foam product for a given coal.

[0033] Typically, the cells in carbon foams are of a size that is readily visible to the unaided human eye. Also, the void volume of carbon foams is such that it typically occupies much greater than one-half of the carbon foam volume. The density of carbon foams typically is less than about 1.0 g/cm³ and generally less than about 0.8 g/cm³. In some embodiments, the density for carbon foam may range from about 0.05 g/cm³ to about 0.8 g/cm³. In some embodiments, carbon foams may exhibit compressive strengths ranging up to about 10,000 psi. In other embodiments, the compressive strength for carbon foam may range from about 100 psi to about 10,000 psi. In certain other embodiments, compressive strengths for carbon foam may range from about 400 psi to about 7,000 psi. The carbon foam incorporated in a tool body may be carbonized carbon foam. Alternatively, if desired, the carbon foam incorporated in a tool body may be carbon foam containing graphite.

[0034] Carbon foams have been produced by a variety of methods. Some of these methods include producing carbon foams directly from particulate coal. For example, US Patent Nos. 6,749,652; 6,814,765; and 7,588,608; describe methods for producing carbon foam directly from particulate coal. To produce carbon foam from particulate coal, typically, a suitable swelling coal, such as bituminous coal, is heated in an essentially closed vessel. The particulate coal is placed in a mold and is heated in an inert atmosphere under high pressure, typically greater than ambient, where the maximum pressure can reach about 500 psi or greater. The particulate coal is heated to temperatures sufficient to cause the coal to become plastic and swell, forming a carbon foam. In many instances, heating the particulate coal to a temperature between about 300°C and about 500°C is sufficient to form a carbon foam material. The temperatures and pressures are varied, depending upon the characteristics of the particulate coal. The resultant carbon foam may subsequently be heated under an essentially inert, or otherwise non-reactive, atmosphere to temperatures as great as about 3000°C. Heating of the carbon foam to such elevated temperatures has been found to improve certain properties of the foam. Such properties have included, but are not limited to, electrical resistance, thermal conductivity, thermal stability, and strength. [0035] During heating, the particles begin to melt and evolve gases that cause the material to foam. The foaming step is done under high pressure to help regulate bubble formation. As oxygen, nitrogen, and hydrogen are eliminated from the precursor during heat up, the carbon continues to cross-link until only a glassy like carbon material remains at 1000°C.

[0036] The typical foaming process must be tightly controlled in order to provide relatively uniform pore sizes and distribution throughout the foam. Even with tight controls on the process, non-uniformity of the pore sizes and distribution can occur. During the process, smaller pores tend to grow into bigger ones, and the bigger pores tend to rise throughout the foam more quickly than smaller ones. This tendency during the foaming process can induce the formation of a gradient in pore size from the bottom to the top of the foam.

[0037] When using mesophase pitch as the precursor, the material completely melts. As a result, nucleation of bubbles may be promoted at the sides and bottom of the metal pan used to contain the melt. This excessive localized bubble formation can result in regions of the foam that contain very coarse porosity. Additionally, large pore sizes are often observed in regions along the bottom and sides of the pan used for the foaming process. Addition of particulate pore stabilizer can help promote nucleation of bubbles across the volume of the foam and limit excessive bubble formation coming from the sides and bottom of the pan.

[0038] The present invention is directed to adding a particulate pore stabilizer to the coal particulate starting material such that bubble formation and resulting pore formation is stabilized during the foaming process to provide a more uniform distribution of pores through the carbon foam body.

[0039] Without intending to be bound by theory, the heights or overall volume of the carbon foam is believed to be a function of bubble stability during the foaming process. If the bubbles have a propensity to grow, their tendency to rise increases, which reduces the stability of the foam. In general, if the growth of the bubbles can be inhibited, the foam tends to be more stable, which enables better foam heights and greater manufacturing yields.

[0040] There are several advantages in stabilizing the pore formation and controlling pore size distribution throughout the carbon foam. Uniform pore size and distribution provides more consistent properties throughout the carbon foam. Additionally, stabilizing the pore formation during the foaming process allows for a greater volume of carbon foam to be produced. [0041] In aqueous systems, improving the stability of bubbles is often done through the use of surfactants, which are molecules that have a hydrophilic head and a hydrophobic tail. Surfactants will tend to align at the bubble interface with the hydrophobic head oriented into the vapor side and the hydrophilic end oriented to the aqueous phase. Stabilizing bubbles in liquefied coal is not straightforward, as it must be accomplished in a complex mix of organic polymers at relatively high temperatures, greater than 340°C. Surfactants are generally not a suitable option for stabilizing bubbles and controlling pore formation during the carbon foam making process.

[0042] The present invention includes introducing a particulate pore stabilizer to the coal particulate starting material. The particulate pore stabilizer is a material that increases the stability of the bubbles formed during the foaming process and preferably helps promote nucleation of bubbles throughout the volume of the foam instead of just at the sides and bottom of the carbon foam panel. In some embodiments, the particulate pore stabilizer preferably provides a wetting angle, or contact angle, between the particulate and the liquid phase of between about 50 degrees and about 105 degrees. The wetting (contact) angle is measured at the foam-bubble interface to better quantify wettability.

[0043] In some embodiments, the particulate pore stabilizer may include, but is not limited to, carbon black; oxides such as alumina, silica, kyanite, boric acid and titania; ceramics such as silicon carbide, aluminosilicate clay, and fine metal powders. Preferably, the particulate pore stabilizer does not have a significant detrimental impact on the resultant properties of the carbon foam. Alternatively, the particulate pore stabilizer may be selected to add or tailor a property for the carbon foam. In some embodiments, the particulate pore stabilizer is able to withstand the thermal treatment temperatures during the carbon foam making process without melting or otherwise decomposing. Further combinations or mixtures of different particulate pore stabilizers may be used.

[0044] The particulate pore stabilizer may be added in amounts ranging from about 0.1% to about 10% by weight. In certain embodiments, the particulate pore stabilizer may be added in amounts less than about 1% by weight. In some embodiments, the majority of the particle sizes in the particulate pore stabilizer are preferably less than 10 pm. Preferably, the particle sizes are less than about 5 pm. In some embodiments, the particle sizes in the particulate pore stabilizer may range from about 3 to about 4 pm.

[0045] In practice, the desired amount of particulate pore stabilizer is added to or blended with a carbon foam starting material such as a particular bituminous coal, subbituminous coal, or combinations thereof to form an admixture. The admixture is added to a mold or pan and heated under controlled temperature and pressure conditions as described above to form carbon foam. In some embodiments, the use of the particulate pore stabilizer during the foaming process may increase the volume of carbon foam from about 5% to about 50% above non-pore stabilized carbon foam.

[0046] Example 1 : The production of carbon foam from mesophase pitch using about 1% carbon black as the particulate pore stabilizer resulted in a carbon foam having an overall finer pore structure with a uniform distribution of pores, as well as an increase in foam volume of about 15%. Close examination of the non-pore stabilized structure reveals some large pores within 1-inch of the top, whereas the material with 1% carbon black contains none. Also noted was the height and volume of the pore stabilized carbon foam containing 1% carbon black was considerably higher, signifying a volume increase.

[0047] Example 2: The production of carbon foam from mesophase pitch using about 1% silica fume as the particulate pore stabilizer resulted in a carbon foam having an overall finer pore structure and an increase in uniformity of the pores with very limited large pores and exhibited an increase in carbon foam volume of about 40%. The silica fume appeared to generate some type of microstructural defect near the bottom (possibly agglomerates of silica), but was still very effective in limiting large pores near the top and also generating much greater height and volume.

[0048] Example 3 : The production of carbon foam from particulate coal using about 4% calcined alumina (about 3 to 4 pm, d50) as the particulate pore stabilizer resulted in a carbon foam having an overall finer pore structure with a uniform distribution of pores. Pore stabilized carbon foam with 4% fine calcined alumina from Aluchem, versus non-pore stabilized carbon foam, was produced. The average pore size of the pore stabilized sample is about half the size of the non-pore stabilized pore size. The use of silica fume and carbon black as the particulate pore stabilizer did not appear to have a major impact on pore structure and uniformity when used with a particulate coal.

[0049] It is generally not desirable that the reaction chamber be vented or leak during the heating and soaking operation when processing variables are set to run at high pressure. The pressure of the chamber and the increasing volatile content therein tends to retard further volatilization while the cellular product sinters at the indicated elevated temperatures. If the furnace is vented or leaks during soaking, an insufficient amount of volatile matter may be present to permit inter-particle sintering of the coal particles, thus resulting in the formation of a sintered powder as opposed to the desired cellular product. Thus, according to a preferred embodiment of the present process, venting or leakage of non-oxidizing gas and generated volatiles is inhibited, consistent with the production of an acceptable cellular product. Additionally, more conventional blowing agents may be added to the particulate prior to expansion to enhance or otherwise modify the pore-forming operation.

[0050] Cooling of the preform or product after soaking is not particularly critical, except as it may result in cracking of the preform or product as the result of the development of undesirable thermal stresses. Cooling rates less than 10°C/min to a temperature of about 100°C are typically used to prevent cracking due to thermal shock. Somewhat higher, but carefully controlled cooling rates may however, be used to obtain a “sealed skin” on the open cell structure of the product as described below. The rate of cooling below 100°C does not influence the final product.

[0051] After expanding the high volatile bituminous admixture as just described, the porous or foamed coal product is an open celled material. Several techniques have been developed for “sealing” the surface of the open celled structure to improve its adhesive capabilities for further fabrication and assembly of a number of parts. For example, a layer of a commercially available graphitic adhesive can be coated onto the surface and cured at elevated temperature or allowed to cure at room temperature to provide an adherent skin. Alternatively, the expansion operation can be modified by cooling the expanded coal product or preform rapidly, e g. at a rate of 10°C/min or faster after expansion. It has been discovered that this process modification results in the formation of a more dense skin on the preform or product which presents a closed pore surface to the outside of the preform or product. At these cooling rates, care must be exercised to avoid cracking of the preform or product.

[0052] After expanding, the porous coal -based preform or product (i.e. carbon foam in accordance with the present invention), is readily machineable, sawable and otherwise readily fabricated using conventional fabrication techniques.

[0053] Subsequent to production of the preform or product as just described, the preform or product may be subjected to carbonization and/or graphitization according to conventional processes to obtain particular properties desirable for specific applications of the type described hereinafter. Ozonation may also be performed, if activation of the coal-based expanded product would be useful in a final product application such as in filtering of air or water, or a high surface area catalyst. Additionally, a variety of additives and structural reinforcers may be added to the coal based preforms or products either before or after expansion to enhance specific mechanical properties such as fracture strain, fracture toughness, and impact resistance. For example, particles, whiskers, fibers, plates, etc. of appropriate carbonaceous or ceramic composition can be incorporated into the porous coalbased preform or product to enhance its mechanical properties.

[0054] The open celled, coal -based preforms or products (i.e. carbon foams of the present invention) can additionally be impregnated with, for example, petroleum pitch, epoxy resins or other polymers using a vacuum assisted resin transfer type of process. The incorporation of such additives provides load transfer advantages similar to those demonstrated in carbon composite materials. In effect, a 3-D composite is produced that demonstrates enhanced impact resistance and load transfer properties.

[0055] The cooling step in the expansion process results in some relatively minimal shrinkage on the order of less than about 5% and generally in the range of from about 2% to about 3%. This shrinkage must be accounted for in the production of near net shape preforms or final products of specific dimensions and is readily determinable through trial and error with the particular coal starting material being used. The shrinkage may be further minimized by the addition of some inert solid material such as coke particles, ceramic particles, ground waste from the coal expansion process, etc., as is common practice in ceramic fabrication.

[0056] Carbonization, sometimes referred to as calcining, is conventionally performed by heating the preform or product under an appropriate inert gas at a heat-up rate of less than about 5°C per minute to a temperature between about 800°C and about 1700°C and soaking for from about 1 hour to about three or more hours. Appropriate inert gases are those described above that are tolerant of these high temperatures. The inert atmosphere is supplied at a pressure from about 0 psi up to a few atmospheres. The carbonization/calcination process serves to remove all of the non-carbon elements present in the preform or product such as sulfur, oxygen, hydrogen, etc.

[0057] Graphitization, commonly involves heating the preform or product either before or after carbonization at heat-up rate of less than about 10°C per minute, preferably from about 1°C to about 5°C per minute, to a temperature of between about 1700°C and about 3000°C in an atmosphere of helium or argon and soaking for a period of less than about one hour. Again, the inert gas may be supplied at a pressure ranging from about 0 psi up to a few atmospheres.

[0058] The porous coal -based preforms or products resulting from processing in accordance with the foregoing procedures can be used in a broad variety of product applications, some, but not all, of which will now be broadly described.

[0059] Products that could be fabricated using the coal-based porous preforms or products of the present invention are various lightweight sheet products useful in the construction industry. Such products may involve the lamination of various facing materials to the surface of a planar sheet of the preform material using an appropriate adhesive. For example, a very light and relatively inexpensive wall board would simply have paper laminated to its opposing planar surfaces, while a more sophisticated curtain wall product might have aluminum sheet, polymer or fiber-reinforced polymer sheets or even stainless steel sheet laminated thereto. A wide variety of such products that have lightweight, low cost and adequate strength can easily be envisioned for wallboard, structural wallboard, bulkheads, etc. The materials of the present invention exhibit sound insulation and vibration resistance due to excellent sound and vibration damping properties, and good thermal insulating properties (less than about 1 watt per meter K thermal conductivity).

[0060] Laminates of these materials may even be used to produce heating element incorporating members, since a carbonized core could serve as an electrical heating element when connected to an appropriate source of electrical energy.

[0061] Similar surface laminated, porous preform, core based products could also find use in the transportation industry where lighter, and especially fire retardant, walls, bulkheads, containers, etc. are in constant demand. Such products would of course require that the expanded coal-based porous core be carbonized as described hereinabove prior to application of the exterior skins, if fire resistance or retardancy is desired.

[0062] Yet another product application for the porous coal products or carbon foams of the present invention is as a replacement for the ceramic foam filters currently applied in the filtering of molten metal, such as aluminum, for the removal of contaminating particulates also called inclusions. The current ceramic foam materials are relatively expensive and extremely friable. In addition, making ceramic foam filters economically at a pore size finer than about 60 pores per inch (ppi) is extremely difficult. Some aluminum filtration applications desire a pore size finer than 60 ppi to produce a purer metal. It is possible to produce a porous coal-based preform of the type described herein having a pore size finer than 60 ppi and of the same size and shape as the ceramic foam filter using the above described fabrication process, to serve as a molten metal filter of this type. The cost of such a more robust, i.e., less friable, filter would also be less than that of a comparable ceramic foam filter.

[0063] Yet other product applications for the carbon foam materials of the present invention reside in the field of heat exchangers. In this application, the heat transfer properties of a graphitized porous coal-based material can be exploited to produce a heat exchanger capable of extracting heat from or adding heat to a fluid (gas or liquid) flowing through the pore structure.

[0064] The foregoing explanations, descriptions, illustrations, examples, and discussions have been set forth to assist the reader with understanding this invention and further to demonstrate the utility and novelty of it and are by no means restrictive of the scope of the invention. It is the following claims, including all equivalents, which are intended to define the scope of this invention.

Claims

1. A method for producing carbon foam at atmospheric pressure, comprising the steps of: partially filling a heating container with starting material to form a bed, said heating container comprising perforations; heating said bed to a first elevated temperature sufficient to form a substantially homogeneous, plastic, open cell carbon material; heating said substantially homogeneous, plastic, open cell carbon material to a second elevated temperature sufficient to form a carbon foam, wherein the carbon foam is substantially free to expand in at least one direction during formation; wherein low viscosity liquid and offgas emerging during said heating steps escapes from said heating container through said perforations; and wherein said method is conducted at about 0 psig.

2. The method of claim 1, wherein said perforations are positioned on the bottom surface of the heating container and arranged in accordance with Industrial Perforations Association (IP A) numbers between 100 and 115.

3. The method of claim 1, wherein said starting material comprises comminuted high volatile bituminous coal, subbituminous coal, coal particulate, isotropic pitch, and mixtures thereof.

4. The method of claim 1, wherein said heating is conducted in a substantially non-oxidizing atmosphere.

5. The method of claim 1, wherein said second elevated temperature is a temperature at which the carbon foam has expanded to an essentially maximum volume and formed an open cell, non-plastic, carbon foam.

6. The method of claim 1, further comprising the step of heating said carbon foam to a third elevated temperature of greater than about 450°C, and less than about 700°C.

7. The method of claim 6, further comprising the step of heating of said carbon foam to a temperature greater than said third elevated temperature.

8. The method of claim 1, wherein said first elevated temperature is above about 340°C.

9. The method of claim 1, wherein said second elevated temperature is at least about 20°C. greater than said first elevated temperature.

10. The method of claim 1, further comprising the steps of: placing a perforated bottom liner inside the container adjacent the container bottom surface; positioning a top surface covering on the bed; placing a loose-fitting cover on top of the top surface covering; and placing a lid on the container.

11. The method of claim 1, further comprising the step of: adding at least one of a particulate pore stabilizer and an active polymer particulate to the starting material to form an admixture in the bed.

12. The method of claim 11, wherein the particulate pore stabilizer comprises; carbon black, fine oxides of alumina, silica, boric acid and titania, aluminosilicate clay or kyanite, fine nonoxide powders of silicon carbide or metal powders, and mixtures thereof.

13. The method of claim 11, wherein the active polymer particulate comprises; phenolic resin, polyethylene oxide, reactive modifying polymers, viscosity modifying polymers, inorganic crosslinking agents, and mixtures thereof.

14. The method of claim 11, wherein said first elevated temperature is between about 300°C and about 500°C.

15. The method of claim 11, further comprising; heating the bed in a non-reactive atmosphere to a temperature of about 3000°C.

16. The method of claim 11, further comprising; promoting nucleation of bubbles throughout the volume of the carbon foam thereby providing a wetting angle, or contact angle, between the particulate and the liquid phase ranging from about 50 degrees and about 105 degrees.

17. The method of claim 11, wherein the particulate pore stabilizer is admixed in amounts ranging from about 0.1% to about 10% by weight, preferably less than 5% by weight.

18. The method of claim 11, wherein the active polymer particulate is admixed in amounts ranging from about 0.1% to about 10% by weight, preferably less than 5% by weight.

19. The method of claim 11, wherein the particulate pore stabilizer particle size is less than 10 pm, preferably less than about 5 pm, and more preferably in the range of about 3 to about 4 pm.

20. The method of claim 11, further comprising; increasing the volume of the carbon foam from about 5% to about 50% above a non-pore stabilized carbon foam.

21. The method of claim 12, wherein the admixture comprises a particulate coal starting material and about 4% by weight calcined alumina particulate pore stabilizer, resulting in a finer and more uniform distribution of pores.

22. The method of claim 12, wherein the pore stabilized carbon foam density is in the range of about 0.1 to about 0.8 g/cm³, preferably from about 0.2 to about 0.6 g/cm³ and most preferably from about 0.3 to about 0.5 g/cm³.

23. The method of claim 11, wherein the starting material comprises a high-volatile coal of bitumen, anthracite, lignite, and mixtures thereof, comprising about 23% to about 45% by weight volatile matter.

24. The method of claim 11, wherein the heating step is performed in a non-oxidizing atmosphere at a heat up rate of about 0.1 °C to about 20°C per hour.

25. The method of claim 11, further comprising: soaking the admixture at a temperature of between 300°C and 700°C for about 10 minutes to about 12 hours.

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26. The method of claim 11, further comprising: controllably cooling the admixture to a temperature below about 100°C.

27. The method of claim 11, further comprising: sealing the surface of the pore stabilized carbon foam with a graphitic adhesive.

28. The method of claim 11, further comprising: carbonizing and graphitizing the admixture.

29. A carbon foam prepared at atmospheric pressure by the process comprising the steps of Claim 1.

30. A pore stabilized carbon foam prepared at atmospheric pressure by the process comprising the steps of Claim 12.

31. A polymer particulate modified carbon foam prepared at atmospheric pressure by the process comprising the steps of Claim 13.

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