Patent Publication Number: US-2002009585-A1

Title: Organic, low density microcellular materials, their carbonized derivatives, and methods for producing same

Description:
BACKGROUND OF THE INVENTION  
       [0001] Low density microcellular materials (“LDMMs”) are known and have been used in a variety of applications including, but not limited to, thermal barriers and insulation, acoustical barriers and insulation, electrical and electronic components, shock and impact isolators, and chemical applications. See, e.g., Materials Research Society, vol. 15, no. 12 (December 1990); Lawrence Livermore National Labs Materials, Science Bulletin UCRL-TB-117598-37; U.S. Pat. No. 4,832,881.  
       [0002] In general, an LDMM is a type of foam, which may be thought of as a dispersion of gas bubbles (having diameters usually smaller than 1000 nm) within a liquid, solid or gel. See IUPAC Compendium of Chemical Terminology (2d ed. 1997). Specifically, and as used herein, an LDMM is a foam having a density of less than about 1000 kg/m 3  and a microcellular structure in which the average pore size is less than about 1000 nm.  
       [0003] The usefulness of any particular foam depends on certain properties, including, but not limited to, bulk density, bulk size, cell or pore structure, and/or strength. See, e.g., “Mechanical Structure-Property Relationship of Aerogels,” Journal of Non-Crystalline Solids, vol. 277, pp. 127-41 (2000); “Thermal and Electrical Conductivity of Monolithic Carbon Aerogels,” Journal of Applied Physics, vol. 73 (2), Jan. 15, 1993; “Organic Aerogels: Microstructural Dependence of Mechanical Properties in Compression,” Journal of Non-Crystalline Solids, vol. 125, pp. 67-75 (1990). For example, density affects, among other things, a foam&#39;s solid thermal conductivity, mechanical strength (elastic modulus), and sound velocity. In general, lowering the density of a foam will also lower its solid thermal conductivity, elastic modulus, and longitudinal sound velocity. However, a foam&#39;s density cannot be too low otherwise it will not satisfy the mechanical stability of its intended application.  
       [0004] In addition, a foam will generally be more useful and better suited to more applications if it can be produced in a variety of shapes and sizes. Further, pore structure affects, among other things, the gas thermal conductivity within a foam, as well as mechanical strength and surface area. In general, smaller pore sizes improve a foam&#39;s physical properties in these areas if the density of the material does not increase. It is therefore desirable in most cases to lower density and pore size until a minimum is reached for both cases. This can be difficult to achieve since, in most materials, these properties counteract each other so that decreasing density leads to larger pore sizes.  
       [0005] Other important properties, at least for purposes of commercialization, include ease and flexibility of manufacture, for example, the ability to withstand the stresses that typically exist during manufacture that cause degradation (e.g., shrinkage and/or cracking), and the ability to make foams having a broad range of properties, sizes and shapes that can also be made in situ.  
       [0006] Generally, foams can be classified by their pore size distribution. Average pore size may fall within three ranges: (1) micropore, in which the average pore size is less than about 2 nm; (2) mesopore, in which the average pore size is between about 2 nm and about 50 nm; and (3) macropore, in which the average pore size is greater than about 50 nm. See IUPAC Compendium of Chemical Terminology (2d ed. 1997). An example of a foam having a micropore structure is a xerogel. An example of a foam having a mesopore structure, and a particularly useful foam, is an aerogel. Generally, an aerogel is a type of foam in which gas is dispersed in an amorphous solid composed of interconnected particles that form small, interconnected pores. The size of the particles and the pores typically range from about 1 to about 100 nm. Specifically, and as used herein, an aerogel has an average pore size of between about 2 nm and about 50 nm.  
       [0007] Another way to classify foams is by the number of closed or open pores they have. For example, closed pore foams have a high number of sealed or encapsulated pores that trap the dispersed gas such that the gas cannot easily escape. See, e.g., U.S. Pat. Nos. 6,121,337; 4,243,717; and 4,997,706.  
       [0008] Open pore foams have a lower number of sealed or encapsulated pores and, as such, the interior spaces and surfaces are accessible and the gas within them may be evacuated. Thus, foams with more open pores are more desirable for evacuated thermal insulation, chemical and catalytic reactions, and electrical applications. For example, only open pore materials can be evacuated for increased thermal insulation commonly known as vacuum insulation, many chemical and catalytic reactions operate by accessing activated surfaces on the interior of foams thus more open spaces and surfaces increase reaction efficiencies, and many electrical applications also operate by accessing conducting surfaces thus more open surfaces increase electrical efficiencies. In general, the known aerogel foams are open pore foams in which nearly all the pores are open. Non-aerogel foams typically have fewer open pores, in which generally less than about 80% of the pores are open.  
       [0009] Aerogel foams may be further classified, for example, by the type of components from which they are made. Inorganic aerogel foams may be made using silica, metal oxides or metal alkoxide materials and typically exhibit high surface area, low density, optical transparency and adequate thermal insulation properties. See, e.g., U.S. Pat. Nos. 5,795,557; 5,538,931; 5,851,947; 5,958,363. However, inorganic aerogels have several problems. For example, the precursor materials are relatively expensive, sensitive to moisture, and exhibit limited shelf-life. See, e.g., U.S. Pat. No. 5,525,643. Also, the processes used to make inorganic aerogels are typically expensive and time-consuming requiring multiple solvent-exchange steps, undesirable supercritical drying (discussed in more detail below) and/or expensive reagents for the modification of the gel surfaces. See, e.g., “Silica Aerogel Films Prepared at Ambient Pressure by Using Surface Derivatization to Induce Reversible Drying Shrinkage,” Nature, vol. 374, no. 30, pp. 439-43 (March 1995); “Mechanical Strengthening of TMOS-Based Alcogels by Aging in Silane Solutions,” Journal of Sol-Gel Science and Technology, vol. 3, pp. 199-204 (1994); “Synthesis of Monolithic Silica Gels by Hypercritical Solvent Evacuation,” Journal of Materials Science, vol. 19, pp. 1656-65 (1984); “Stress Development During Supercritical Drying,” Journal of Non-Crystalline Solids, vol. 145, pp. 3-40 (1992); and U.S. Pat. No. 2,680,696.  
       [0010] In contrast, organic aerogel foams typically exhibit lower solid thermal conductivity and can be readily converted into low density, high surface area carbonized-foams that exhibit high electrical conductivity. Moreover, the precursor materials used to make organic aerogels tend to be inexpensive and exhibit long shelf-lives. See, e.g., “Aerogel Commericalization: Technology, Markets, and Costs,” Journal of Non-Crystalline Solids, vol. 186, pp. 372-79 (1995). Further, organic aerogels can be opaque (useful to reduce radiative thermal transfer) as well as transparent. As a result, generally, organic aerogels are more desirable, especially for electronic applications and thermal applications in which optical transparency is not desired.  
       [0011] Foams, including aerogel foams, can also be classified by their bulk properties. Monolithic foams, or monoliths, can be defined as being bulk materials having volumes greater than 0.125 mL, which corresponds to a block of material having a volume greater than 125 mm 3  (i.e., 5 mm×5 mm×5 mm). Thin film and sheet foams can be defined as a coating, less than 5 mm thick, formed on a substrate. Granular or powder foams can be defined as comprising particle sizes of having volumes less than 0.125 mL. In general, foams that can be made in monolithic form have advantages over thin film or granular foams. For example, monolithic foams can be made for a wide variety of applications in which thin films, sheets or granulars would not be practical. For example, most thermal insulation, acoustical attenuation and kinetic (shock absorption) applications require thicker insulating material that cannot be provided by thin films or sheets. And, granular materials tend to settle and are not mechanically stable. Many chemical and catalytic applications also require more material than can be provided by thin films or sheets. Even some electrical applications require monolithic materials such as fuel cell and large capacitor electrodes.  
       [0012] In general, organic LDMMs made using non-critical drying methods have been limited to thin film or granular shapes. Organic, monolithic LDMMs generally have not been made using non-critical drying methods with one exception which took four days to prepare. See U.S. Pat. No. 5,945,084)  
       [0013] Further, although large monolithic inorganic aerogels have been made, such shapes and sizes have been limited and these inorganic aerogels have been made using undesirable supercritical drying methods (as explained below). For example, silica aerogels have been made in the following shapes and sizes: (1) a sheet 1 cm thick and having a length and width of 76 cm (corresponding to a volume of 5.776 liters); and (2) a rod 12 inches long having a diameter of 8 inches (corresponding to a volume of 9.884 liters).  
       [0014] Organic aerogels made using supercritical drying methods, however, have much more limited shapes and sizes, e.g.: (1) a sheet 1 inch thick and having a length and width of 12 inches (corresponding to a volume of 0.155 liters); and (2) a rod 3 inches long having a diameter of 8 inches (corresponding to a volume of 1.471 liters). No organic monolithic aerogel is known whose smallest dimension is greater than 3 inches. Further, no organic monolithic aerogel is known made using non-critical drying techniques where the smallest diameter is greater than 5 mm. In addition, many of the known organic monolithic foams lack sufficient structural strength to withstand the stresses arising during manufacture. As a result, these foams tend to shrink and some also crack during manufacture.  
       [0015] In general, foams can be made using a wide variety of processes. See, e.g., U.S. Pat. Nos. 6,147,134; 5,889,071; 6,187,831; and 5,229,429. However, aerogels have been typically made using well known “sol-gel” processes. The term “sol” is used to indicate a dispersion of a solid in a liquid. The term “gel” is used to indicate a chemical system in which one component provides a sufficient structural network for rigidity, and other components fill the spaces between the structural units. The term “sol-gel” is used to indicate a capillary network formed by interlinked, dispersed solid particles of a sol, filled by a liquid component.  
       [0016] The preparation of foams by such known sol-gel processes generally involves two steps. In the first step, the precursor chemicals are mixed together and allowed to form a sol-gel under ambient conditions, or, more typically, under conditions of temperature higher than ambient. In the second step, commonly referred to as the “drying step,” the liquid component of the sol-gel is removed. See, e.g., U.S. Pat. Nos. 4,610,863; 4,873,218; and 5,476,878. The ability to dry the sol-gel is in part dependent on the size of the foam. A larger foam will require more intensive drying because of the longer distance the solvent must pass from the interior of the foam to the exterior. A sol-gel that is dried in a mold or container will require that the liquid travel through the sol-gel to the open surface of the mold or container in order for the liquid component to be removed.  
       [0017] Conventional supercritical drying methods usually require the undesirable and potentially dangerous step of supercritical extraction of the solvent. In the case of direct supercritical extraction (a process wherein the solvent in which the sol-gel is formed is removed directly without exchanging it for another solvent), the solvent that is being extracted is most typically an alcohol (e.g., methanol), which requires high temperatures and pressures for extraction. Such conditions require the use of highly pressurized vessels. Subjecting alcohols to the high temperatures and pressures increases the risk of fire and/or explosion. Methanol poses the additional risk of toxicity.  
       [0018] Known sol-gel processes have several additional problems. In many instances, the precursor materials used are expensive and can be dangerous under the conditions used in conventional supercritical drying. Also, the resulting foams have been made in limited sizes and shapes due to constraints inherent in the known manufacturing processes and also tend to exhibit cracking and/or shrinkage.  
       [0019] Another problem with conventional drying methods is that the drying step is time consuming and frequently quite tedious, typically requiring one or more solvent exchanges. See, e.g., U.S. Pat. Nos. 5,190,987; 5,420,168; 5,476,878; 5,556,892; 5,744,510; and 5,565,142. Another problem is that conventional drying methods sometimes require the additional step of chemically modifying the sol-gel. See, e.g., U.S. Pat. No. 5,565,142; “Silica Aerogel Films Prepared at Ambient Pressure by Using Surface Derivatization to Induce Reversible Drying Shrinkage,” Nature, vol. 374, no. 30, pp.439-43 (March 1995).  
       [0020] For example, the most common process for aerogel production involves exchanging the organic solvent in which the aerogel is formed (typically alcohol or water) with liquid carbon dioxide, which is then removed by supercritical extraction. Although the supercritical extraction of carbon dioxide requires relatively low temperatures (under 40° C.), it requires very high pressures (generally above 1070 psi). And, although carbon dioxide is non-flammable, the solvent-exchange step is very time consuming.  
       [0021] Moreover, even the known processes using ambient (non-critical) drying methods have deficiencies in that they do not produce low density monolithic foams, but rather thin films or granules.  
       [0022] As explained above, the known processes tend to produce organic aerogels having limited shapes and sizes. One reason for this is that the mold or container in which the foam is made is limited in size and/or shape. As a result, such processes do not allow for the extraction of foams where the distance the solvent must pass is very large.  
       [0023] An example of a known process for making foams is U.S. Pat. No. 5,565,142, which describes certain inorganic foams produced using evaporative drying methods. The described process requires solvent exchange and a further step wherein the sol-gel is chemically modified. Similarly, U.S. Pat. No. 5,945,084 describes the production of resorcinol foams by evaporative drying processes in which the lowest reported density of these foams is greater than 400 kg/m 3 . However, these foams exhibit relatively high thermal conductivity and require an excessive amount of time to gel, cure and dry. One example in this patent took more than four days to complete.  
       [0024] Although known foams may exhibit some of the above-described useful properties, no known foam exhibits all of these properties. Thus, an organic, low density, open cell foam that can have a wide variety of monolithic forms with sufficient structural strength and that optionally can be formed in situ is still needed.  
       SUMMARY OF THE INVENTION  
       [0025] One objective of this invention is to provide an organic LDMM comprising a large, monolithic foam having a size that is not limited by the method in which it is made. The only limit as to the size and shape of these foams is the application in which they will be used. By way of example only, the LDMMs of this invention can be made in situ in the walls or in insulated barriers used in refrigerated trucks, buildings, and aircraft.  
       [0026] It is another objective of this invention to provide large, monolithic aerogels with large bulk shapes and sizes whose smallest dimension (e.g., width, height, length, thickness, diameter) is greater than about 3 inches; and/or sufficient structural strength to withstand the stresses arising during manufacture such that they are substantially free of cracks.  
       [0027] It is another objective of this invention to provide organic LDMMs comprising a monolithic aerogel prepared using non-critical drying processes. Such aerogels have sufficient structural strength to withstand the stresses arising during manufacture such that they are substantially free of cracks.  
       [0028] It is a further objective of this invention to provide organic LDMMs having an average pore size between about 50 nm and about 1000 nm. Such LDMMs have densities less than about 300 kg/m 3 , pore structures in which greater than about 80% of the pores are open, and/or low thermal conductivities under vacuum.  
       [0029] Additional objectives include providing carbonized-forms of the above-described LDMMs useful in electronic and chemical applications, among others; providing methods for making these LDMMs, including methods that do not require supercritical drying and yet still yield large, monolithic foams.  
       [0030] These objectives are merely exemplary and are not intended to limit the scope of the inventions described in more detail below and defined in the claims.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0031] In order that this invention may be more fully understood, the following detailed description is set forth. However, the detailed description is not intended to limit the inventions that are defined by the claims. It will be appreciated by one of skill in the art that the properties of the LDMMs, as well as the steps and materials used in the manufacture of LDMMs may be combined and/or varied without departing from the scope of the basic invention as disclosed herein.  
       [0032] Properties of the LDMMs  
       [0033] The LDMMs of this invention comprise organic foams having unique and/or improved properties. Such properties include, but are not limited to, low and/or variable densities; pore structures having small pore sizes and/or a large portion of open pores; large monolithic shapes and sizes; sufficient structural strength to withstand the stresses that arise during manufacture; low thermal conductivities; and/or the ability to be formed in situ.  
       [0034] As defined above, an LDMM is a foam having a density less than about 1000 kg/m 3  and pore sizes less than about 1000 nm. The LDMMs of this invention preferably have a density less than about 500 kg/m 3 , more preferably less than about 300 kg/m 3 , even more preferably less than about 275 kg/m 3 , and yet even more preferably less than about 250 kg/m 3 , and yet further even more preferably less than about 150 kg/m 3 . LDMMs with even lower densities (e.g., less than 100 kg/m 3  are especially preferred because, as discussed in more detail below, they may exhibit additional preferred properties such as lower thermal conductivity.  
       [0035] The LDMMs of this invention preferably have small average pore sizes, between about 2 nm and about 1000 nm. More preferably, the LDMMs of this invention have average pore sizes between about 2 nm and 50 nm. LDMMs with small pore sizes (e.g., between about 2 nm and about 20 nm) are especially preferred because, as discussed in more detail below, they may exhibit additional preferred properties such as lower thermal conductivity.  
       [0036] The LDMMs of this invention also comprise an open cell structure in which greater than about 80% of the cells or pores are open. The amount of open pores that LDMMs have can be calculated by measuring the absoption of liquid nitrogen or by using standard nitrogen gas adsorption measurements (BET analysis). In general, the greater the open cell structure of the LDMM, the greater the evacuated thermal insulation, chemical, catalytic, and electrical properties the LDMM exhibits. Thus, preferably, the LDMMs of this invention comprise an open cell structure in which at least about 90% of the cells or pores are open, and more preferably substantially all of the pores are open.  
       [0037] The LDMMs of this invention may further comprise monolithic shapes and sizes. Such LDMMs have volumes greater than about 0.125 mL in which no single dimension is less than about 5 mm. Thus, for example, in the case of an LDMM having a generally rectangular shape, the length, width and height of the material must each be no less than about 5 mm. Similarly, for generally round, spherical, or elliptical shapes, the smallest diameter must be no less than about 5 mm. An LDMM of this invention may be a large monolithic foam whose smallest dimension is greater than about 3 inches. The maximum size of the LDMMs of this invention, however, are not limited and can take any size, shape or form. For example, the LDMMs of this invention can be made in situ in the walls or insulated barriers used in refrigerated trucks, buildings and aircraft.  
       [0038] Such bulk properties differentiate the LDMMs of this invention from known thin film, sheet, granular or powder foams. The limitations of thin film, sheet, granular and powder foams are known. For example, most thermal insulation, acoustical attenuation and kinetic (shock absorption) applications require thicker insulating material that thin films or sheets cannot provide. And, granular materials tend to settle and are not mechanically stable. Also, many chemical and catalytic applications require larger shapes (monolithic materials) than thin films or sheets can provide. Even some electrical applications such as fuel cell and large capacitor electrodes require monolithic materials  
       [0039] An LDMM of this invention may also have sufficient structural strength to minimize degradation during manufacture. Thus, for example, they exhibit substantially no cracking. The LDMMs may also exhibit minimal shrinkage (i.e., the final product is nearly the same physical size as the precursor solution from which it is derived). For example, in the case of aerogels formed using a sol-gel process, the aerogels of this invention exhibit minimal shrinkage compared to the sol-gel. Preferably, the LDMMs exhibit less than about 25% shrinkage, and more preferably do not substantially shrink at all.  
       [0040] The enhanced structural strength of these LDMMs may be achieved by the inclusion of a suitable solvent that strengthens the solid network by, for example, providing strong hydrogen bonding and/or covalent modifications within the LDMM network. An example of this interaction would be, in the case of an aerogel, a complex between one or more hydroxylated aromatics and one or more hydrogen-bonding agents. A preferred solvent is a material that provides strong hydrogen bonding such as an aliphatic carboxylic acid, including acetic acid, formic acid, propionic acid, butyric acid, and pentanoic acid, with acetic acid being most preferred. Thus, an LDMM of this invention comprises a hydrogen bonding agent (e.g., acetic acid) to provide sufficient structural strength to minimize degradation.  
       [0041] Another unique and/or improved property that may be exhibited by an LDMM of this invention includes low thermal conductivity or thermal transfer. The lower the thermal conductivity the better thermal insulation properties (i.e., lower thermal transfer) the LDMM exhibits. Thus, a preferred LDMM may exhibit a thermal conductivity of less than about 0.0135 W/(m° K) up to pressures of 10 Torr, and even more preferred, less than 0.008 W/(m° K) up to pressures of 10 Torr. Another preferred LDMM may exhibit a thermal conductivity of less than about 0.009 W/(m° K) up to about 1 Torr, and even more preferred, less than about 0.007 W/(m° K) up to about 1.0 Torr. And, a further preferred LDMM may exhibit a thermal conductivity of less than about 0.005 W/(m° K) up to about 0.1 Torr, and even more preferred, less than about 0.0035 W/(m° K) up to about 0.1 Torr. A more preferred LDMM of this invention exhibiting these thermal conductivities is a monolithic LDMM formed using a non-critical drying method.  
       [0042] Additional, and optional, properties of the LDMMs of this invention include high surface areas (greater than about 10 m 2 /g, preferably greater than about 50 m 2 /g, more preferably greater than about 100 m 2 /g, and even more preferably greater than about 200 m 2 /g); low resistivities (less than about 0.02 ohm meter, preferably less than about 0.002 ohm meter); high acoustical impedance; high compressive strength; high shock absorption; and/or high chemical resistance to minimize solvent swelling.  
       [0043] Having described the properties that the LDMMs of this invention may exhibit, exemplary embodiments of unique combinations of these properties are provided. In one embodiment, the organic LDMM of this invention comprises a foam having an average pore size of between about 50 nm and about 1000 nm; a density of less than about 300 kg/m 3 ; and greater than about 80% of the pores are open pores. Preferably, all of the pores are open pores and the density is less than about 275 kg/m 3 .  
       [0044] In another embodiment, the organic LDMM of this invention is a monolithic structure that has been non-critically dried and has a thermal conductivity of less than about 0.0135 W/(m° K) up to pressures of 10 Torr, and more preferrably, less than 0.008 W/(m° K) up to pressures of 10 Torr. Another such LDMM has a thermal conductivity of less than about 0.009 W/(m° K) up to about 1 Torr, and more preferably, less than about 0.007 W/(m° K) up to about 1.0 Torr. And, a further such LDMM has a thermal conductivity of less than about 0.005 W/(m° K) up to about 0.1 Torr, and more preferably, less than about 0.0035 W/(m° K) up to about 0.1 Torr.  
       [0045] In a preferred embodiment, the organic LDMM of this invention comprises an aerogel foam—defined above as having an average pore size of between about 2 nm and 50 nm—that is prepared using non-critical drying processes. This aerogel has a monolithic form while maintaining sufficient structural strength such that it is substantially free of cracks.  
       [0046] In another preferred embodiment, the organic LDMM of this invention comprises a monolithic aerogel whose smallest dimension is greater than about 3 inches while maintaining sufficient structural strength such that it is substantially free of cracks.  
       [0047] Process of Making LDMMs  
       [0048] In general, organic LDMMs, including those of the present invention, may be prepared using an improved two-step sol-gel polymerization process. The first step comprises reacting an hydroxylated aromatic or a polymer resin comprising an hydroxylated aromatic with at least one electrophilic linking agent in a solvent. The solvent comprises at least one compound, which is a liquid that dissolves the organic precursor, precipitates the cross-linked product, and serves to strengthen the solid network during the second step (i.e., drying). Mechanisms for this strengthening interaction may include strong hydrogen bonding and/or covalent modifications that stiffen the polymer backbone so as to minimize (and preferably prevent) cracking and shrinking during drying. The reaction may take place in the presence of a catalyst that promotes polymerization and/or cross-linking and produces sol-gel formation at a rate consistent with or more rapid than other LDMMs known in the art.  
       [0049] The second step, comprises drying the sol-gel to remove the liquid components. Unlike other sol-gel processes, the drying step does not require supercritical extraction and/or does not cause substantial degradation. Although supercritical extraction methods optionally may be used alone or in combination with ether drying methods, they are not preferred.  
       [0050] More particularly, in the first step of the inventive process, the hydroxylated aromatic or polymer resin comprising the hydroxylated aromatic may be added in an amount from about 0.5% to about 40% (by weight based on the resulting solution), preferably from about 1% to about 20%, and more preferably from about 1% to about 8%. The electrophilic agent may be added in an amount from about 1% to about 40% (by weight based on the resulting solution), preferably from about 3% to about 20%, and more preferably from about 4% to about 8%. The solvent may be added in an amount from about 30% to about 97% (by weight based on the resulting solution), preferably from about 50% to about 94%, and more preferably from about 60% to about 85%.  
       [0051] The precursor chemicals are mixed together and allowed to form a sol-gel in an environment maintained at an ambient pressure and a temperature between about 20° C. and about 100° C., and preferably between about 40° C. and about 80° C. It is believed that such temperatures provide rapid thorough cross-linking of the chemical matrix, which results in stronger, higher quality, finished LDMMs. The processing temperatures tend to be limited by the boiling point of the precursor chemical solution and by the vessel or mold in which the gel is formed. However, if the process is conducted at pressures greater than ambient, then the processing temperature may be increased (if a more temperature-tolerant vessel or mold is used).  
       [0052] Further, it is also believed that increasing temperature to the higher end of the range increases the rate of cross-linking, however, it also increases pore size. Whereas, lowering the temperature increases the time it takes to prepare the sol-gel. Therefore, to form small pores, it may be desirable to allow gelation to occur at, for example, 40° C., after which the temperature may be increased, possibly in stages to, for example, 80° C., to provide the most thoroughly cross-linked, strong and rigid finished product in the least amount of time. As discussed below, other variables may be adjusted or changed to allow for smaller pores without the need for incremental temperature increases.  
       [0053] Optionally, the chemical precursors may be preheated prior to gelation to prevent, or reduce, expansion of the pore fluid during gelation and curing. Furthermore, in order to prevent premature drying of the gel, it is important to ensure that the container within which the gel is formed is capped, or kept pressurized, substantially at all times prior to the drying step(s) so that the sol-gel does not begin to dry prematurely.  
       [0054] According to one drying process methodology, the liquid component of the finished sol-gel may be removed by evaporative methods. For example, it has been determined that an evaporation cycle at a reduced (vacuum) pressure and at a temperature of between about 50° C. and 100° C. for about 2 to about 20 hours, depending upon sample size and formulation, is effective to remove the liquid component of the sol-gel.  
       [0055] According to another drying process methodology, most of the liquid component of the finished sol-gel may be removed by centrifugation, and the remaining liquid may be removed by evaporative methods. The solid matrix of the foams of the present invention have been observed to be sufficiently strong to withstand processing by centrifugation at approximately 2000 rpm, more preferably up to 1000 rpm and even more preferably up to 500 rpm.  
       [0056] According to yet another drying process methodology, most of the liquid component of the finished sol-gel may be removed by applying a pressure differential across the sol-gel; thereby, forcing the liquid component out of the sol-gel by displacing the liquid component with the gas. This can be accomplished by applying gas pressure to one side of the sol-gel with the other side exposed to atmospheric pressure. Alternatively, a reduced pressure (vacuum) can be applied to one side (with the other side exposed to atmospheric pressure). The remaining liquid may be removed by evaporative methods, as above. The gas, such as air, also may be heated in order to speed evaporation.  
       [0057] According to still another drying process methodology, the liquid component of the finished sol-gel may be removed by freeze drying (i.e., sublimation drying). First, the wet gel is frozen. Next, the gel is subjected to reduced pressure, and the frozen solvent sublimes, or changes directly from solid to gas without passing through a liquid phase.  
       [0058] A further, and preferred, drying process involves vacuum purging/washing the sol-gel using a low surface tension solvent. First, the solvent is supplied to one side of the sol-gel. A pressure differential is then applied across the sol-gel to remove the pore fluid and force the low surface tension solvent through the sol-gel. The low surface tension solvent aids in the extraction of the pore fluid by “washing” it out of, and replacing it in, the pores. Because the solvent has low surface tension, it is readily extracted from the sol-gel. Suitable low surface tension solvents include, but are not limited to, hexane, ethyl ether, pentane, and isopentane (2 methylbutane), with hexane being preferred. Also, it is contemplated that in the case where the solvent is acetic acid, because hexane and acetic acid are miscible, hexane easily can be added to the sol-gel. And, because hexane is also very volatile, it easily can be extracted by evaporation. It is also contemplated that because surface tension decreases as temperature increases, it is desirable to preheat the solvent and/or the sol-gel.  
       [0059] The inventive processes yield LDMMs having a unique and/or improved combination of properties including, but not limited to, foams with a wide range of densities (e.g., from about 50 mg/cm 3  to about 500 mg/cm 3 ), having open cell structures, in monolithic forms, and/or exhibiting minimal degradation (i.e., shrinkage or cracking) and without apparent size or shape limitations.  
       [0060] Although sol-gel polymerization processes of an hydroxylated aromatic and an electrophilic linking agent are known, such processes have been conducted in the absence of a solvent capable of strengthening the gel network. See, e.g., U.S. Pat. Nos. 5,945,084; 5,476,878; 5,556,892; and 4,873,218. Such known processes require time-consuming drying protocols and/or do not yield foams in monolithic forms. This limits their use to the production of thin films or supporting substrates, or to the production of granules or thin wafers. And, although some known sol-gel processes have produced unshrunken monolithic gels capable of withstanding the pressures induced by non-critical drying, these processes require lengthy drying protocols and yield foams that do not exhibit the unique properties of this invention. See, e.g., U.S. Pat. Nos. 5,945,084; and 5,565,142. Specifically, these materials have higher bulk densities, larger particle and pore sizes, and/or a significant fraction of closed pores within the solid structure. Further, some of these known materials cannot be carbonized, and thus, cannot be used in electrical applications.  
       [0061] Preferably, the hydroxylated aromatics useful in the inventive process may be selected from the group comprising phenol, resorcinol, catechol, hydroquinone, and phloroglucinol. More preferably, the hydroxylated aromatic comprises a phenol compound. Even more preferably, the hydroxylated aromatic comprises part of a soluble polymer resin in which the hydroxylated aromatic has been co-polymerized with a linking agent such as formaldehyde.  
       [0062] Preferably, the electrophilic linking agent may be selected from the group comprising aldehydes and alcohols. More preferably, the aldehyde may be furfural or formaldehyde, and even more preferably, furfural. A suitable alcohol may be furfuryl alcohol. However, furfural is a more preferred electrophilic linking agent.  
       [0063] Commonly available, partially pre-polymerized forms of the hydroxylated aromatic may also be used. For example, liquid phenolic resins may be used, such as FurCarb LP520 (QO Chemicals, Inc., West Lafayette, Ind.) as well as phenolic-novolak resins GP-2018C, GP-5833 and GP-2074, with GP-2018c being more preferred (Georgia-Pacific Resins, Inc., Decatur, Ga.). Those with higher average molecular weights (e.g., GP-2018c) appear to produce the strongest, most rigid finished product. Such products are solid flakes which must be dissolved in a liquid solvent prior to use in the processes of this invention. Alternatively, a liquid resin may be used such as FurCarb LP520 (QO Chemicals, Inc., West Lafayette, Ind.) which comprises a phenolic-novolak that has been dissolved in an approximately equal weight amount of furfural. In that case, the liquid resin comprises not only the hydroxylated aromatic but also the electrophilic linking agent. Preferably, however, the solid-form of the phenolic resin material is used because it allows more flexibility for adjustment of the phenol/furfural ratio, a variable that affects the properties of the finished product. Where pre-polymerized forms of the hydroxylated aromatic and electrophilic linking agents are used (e.g., phenolic-novolak flakes), the ratio of novolak/furfural should be adjusted to maximize the amount of cross-linking between phenolic-novolak and furfural and to minimize the cross-linking of furfural to itself. It is contemplated that each cross-link uses a furfural molecule and a phenolic novolak site. For a given novolak, there is a certain amount of sites available to cross-link, and as such, it would be desirable to provide sufficient furfural to achieve as complete cross-linking as possible without providing too much excess. Thus, under certain conditions, the excess furfural may cross-link to itself forming a furfural foam having inferior properties.  
       [0064] Preferably, the solvent comprises a reactive compound acting as both a hydrogen-bond donor and acceptor capable of interacting with multiple sites on the polymer backbone. Suitable solvents include aliphatic carboxylic acids. More preferably, the solvent is selected from the group consisting of acetic acid, formic acid, propionic acid, butyric acid, and pentanoic acid, with acetic acid being even more preferred.  
       [0065] Without wishing to be bound to any particular theory, it is believed that, in the case of a solvent comprising a hydrogen-bonding solvent, the solvent dissolves the precursor, precipitates the cross-linked product, and forms hydrogen-bonded adducts with the hydroxylated aromatics in the backbone of the cross-linked product. This hydrogen-bonding interaction involves two or more hydroxylated aromatics and constitutes an additional cross-linking mechanism, resulting in a more robust sol-gel which is relatively more tolerant of stresses from evaporative, centrifugal, gas pressure, or vacuum drying methods than are prior art sol-gels.  
       [0066] A catalyst may also be used in the preparation of the sol-gel. The catalyst promotes polymerization and produces sol-gel formation at a rate consistent with or more rapid than other LDMMs known in the art. See, e.g., U.S. Pat. Nos. 5,556,892 and 4,402,927. Examples of preferred catalysts that may be used include mineral acids, such as, but not limited to, hydrochloric acid, hydrobromic acid, sulfuric acid, and Lewis acids, such as, but not limited to, aluminum trichloride and boron trifluoride. More preferred catalysts include hydrochloric acid, hydrobromic acid and sulfuric acid.  
       [0067] In general, increasing the amount of catalyst substantially reduces the time required for gelation and/or curing and tends to yield stronger foams (but too much catalyst degrades the quality of the product). However, increasing the amount of catalyst may also increase pore size.  
       [0068] Although the mineral acids are preferred, other commercially available catalysts having similar chemical properties, for example QUACORR 2001 catalyst (QO Chemicals, Inc., West Lafayette, Ind.), may also be used. It will be recognized by one ordinarily skilled in the art that a compatible catalyst in accordance with the present formulation will increase the rate of the electrophilic aromatic substitution reaction constituting the cross-linking process above the rate exhibited in the absence of the catalyst. It has been found in relation to the present formulations that increased amounts of catalyst, for example, up to approximately seven percent (7%) by weight fur some formulations, increases hardness of the resulting solid matrix; but also increases average pore size within the resulting organic foam.  
       [0069] The reaction mixture may also include other suitable agents to enhance certain useful properties of the LDMM or to assist in the reaction. For example, optional alcohol may be added to reduce the average pore size within, and to increase the strength of, the resulting organic LDMM. The amount of the optional alcohol to be added to the reaction mixture is preferably between about 3% and about 13% (by weight of the total mixture).  
       [0070] The effect of adding alcohol or increasing the alcohol content is a very useful and pronounced means of reducing pore size. However, adding or increasing alcohol content also tends to increase gelation time. But, the effect of alcohol may be used in combination with adjustments or changes to other variables to offset the undesirable effects. For example, it may be desirable to increase the gelation and/or curing temperature (or increase the amount of acid catalyst) while at the same time increasing the alcohol content. In this way, the increased alcohol content will more than offset the larger pore size caused by the increased temperature (or amount of acid catalyst). And, the increased temperature (or amount of acid catalyst) will offset the longer gelation time caused by the increased alcohol content.  
       [0071] There may be, however, a maximum allowable amount of alcohol that can be added to a particular formulation that is processed at a particular gelation temperature. If more than this maximum allowable amount of alcohol is added, the pore size becomes too small and the sol-gel may shrink during the drying step.  
       [0072] Examples of useful alcohols include aliphatic alcohols and polyalcohols. Preferred aliphatic alcohols include ethyl, 1- or 2-propyl, some butyls (not t-butyl), and most pentyl alcohols, with isopropanol being more preferred due to its low toxicity and being relatively inexpensive. Preferred polyalcohols include ethylene glycol, propylene glycol and glycerine. Polyalcohols tend to form LDMMs with very small pore size. However, polyalcohols tend to be more difficult to extract by evaporation (but may be more readily extracted by solvent purging techniques described below), and they tend to produce gels that shrink when dried. Accordingly, aliphatic alcohols are more preferred.  
       [0073] The reaction mixture may also include surfactants to further reduce, or prevent, shrinkage upon drying, presumably by reducing the surface tension of the pore fluid and thereby making extraction of the pore fluid (i.e., the drying step) easier, specially when dried by evaporative processes. The surfactant allows for the production of unshrunken monoliths with smaller pore sizes than is possible without the use of this component while maintaining the same unshrunken characteristic. However, depending on the processing conditions, some amount of the surfactant may remain after removal of the pore fluid. Thus, for some applications (e.g., applications or insulation), it may not be desirable to use a surfactant in which case, other variables (e.g., material formulation and/or processing parameters) should be adjusted to avoid shrinkage (without resorting to the use of surfactants). For example, where the LDMM is pyrolized to form a carbonized-derivative useful in electrical applications, surfactants may be useful because any residual surfactant will be removed during pyrolization.  
       [0074] Examples of useful surfactants include low molecular weight, non-ionic, primary alcohol ethoxylates. One such family of surfactants is NEODOL (Shell Chemical Company, Houston, Tex.), such as NEODOL 23-3 and NEODOL 23-5. Tergitol XL-80N or Tergitol 15-S-7 (Union Carbide Co.) is another example that may also be used.  
       [0075] If desired, doping agents, as known and defined in the prior art, may be added to chemically activate the foam. Examples of useful dopants include metal powders, metal oxides, metal salts, silica, alumina, aluminosilicates, carbon black, fibers, and the like. See, e.g., U.S. Pat. Nos. 5,476,878 and 5,358,802.  
       [0076] Further, additives comprising novoloid fibers (organic polymers made from phenol and formaldehyde and available from American Kynol, Pleasantville, N.Y.) may be used to further strengthen the LDMM. Such novoloid fiber additives may provide structural strength to the gel, and allow for the preparation of lighter, less dense materials than can be made without the fibers. Because novoloid fibers are compatible with the base resins of the present invention, the gels may better cross-link to the novoloid fibers, forming a more coherent matrix. Additionally, the novoloid fibers can be completely pyrolized into a carbonized form compatible with the pyrolized foams of the present invention.  
       [0077] It is contemplated that the fibers can be added in such a way that they settle and produce a very hard base at the bottom of the finished foam that can be used for mechanical attachment to other devices. Also the gels can be slowly rotated so that the fibers are evenly distributed throughout the sol-gel or the fibers can be added when the viscosity of the sol-gel is high enough to prevent the fibers from settling.  
       [0078] Fire resistant additives may also be added. Typically, flame-retarding chemicals are based on combinations of bromine, chlorine, antimony, boron, and phosphorus. Many of these retardants emit a fire-extinguishing gas (halogen) when heated. Others react by swelling or foaming, forming an insulation barrier against heat and flame. Accordingly, one such exemplary fire retardant useful in the present invention is 2,3-dibromopropanol.  
       [0079] Although the formulations described herein produce LDMMs with no observable shrinkage (i.e., the final product is substantially the same physical size as the sol-gel from which it is derived), if the formulations are not balanced correctly, the LDMMs will shrink during the drying process. The factors that affect the tendency to shrink are the overall strength of the sol-gel and the sizes of the pores therein. The strength of a foam is related to density (i.e., all other variables being equal, a higher density foam will be stronger than a lower density foam). The tendency of the sol-gel to shrink upon drying is related to pore size (i.e., all other variables being equal, a foam with smaller pores will be more prone to shrinkage than one with larger pores). Thus, a sol-gel with a relatively strong and well-formed solid capillary network has less tendency to shrink upon drying, and a sol-gel with micropores has more tendency to shrink upon drying.  
       [0080] The formulation may be tailored to obtain the desired mix of properties. For many applications, the ideal material is a relatively strong, rigid foam which is also of a relatively low density, and also has relatively small pore sizes. Oftentimes, therefore, when producing the organic LDMMs of the present invention, the goal is to maximize strength and rigidity of the LDMM material while, at the same time, producing a relatively low-density product, and further minimizing pore size such that the pores are of the smallest diameter that will still permit production of an unshrunken product.  
       [0081] In the case where the LDMM is to be used in a thermal insulation application, lowering density and/or reducing pore size may decrease thermal conductivity or thermal transfer. In general, there are three types of thermal transfer: solid conduction, gas conduction and radiative conduction. See, e.g., “Thermal Properties of Organic and Inorganic Aerogels,” Journal of Materials Research, vol. 9, no. 3 (March 1994), incorporated by reference herein. Low density porous materials, such as LDMMs, typically have low solid conduction. LDMMs with higher density generally have higher solid conduction. Opaque LDMMs also typically have low radiative conduction. As the LDMM becomes more transparent, radiative conduction increases. A preferred LDMM of this invention is black, which does not use an opacifier, in order to reduce radiative conduction. Thus, to achieve an LDMM with useful thermal insulation properties, it is desirable to minimize gas conduction.  
       [0082] Gas conduction is produced by gas molecules bouncing into one another and transferring heat from the “hot side” to the “cold side” of a thermal insulator. One way to eliminate gas conduction is to completely remove all of the gas (e.g., keeping the LDMM under high vacuum). However, because this is not practical, it is desirable that the LDMM have low conduction without resorting to high vacuum. This can be achieved by making the average pore size smaller and preferably less than the mean free path or MFP (i.e., the average distance a gas molecule must travel before bounces into another gas molecule) at a given pressure.  
       [0083] At ambient pressures, the MFP is quite short and it becomes more difficult to produce an LDMM that has low gas conductivity with an average pore size smaller than the MFP. However, as pressure is lowered, the MFP becomes longer and LDMMs can be made more easily with pore sizes smaller than the MFP. The LDMMs of the present invention exhibit very low gas thermal conductivity starting below about 10 Torr.  
       [0084] However, although smaller pore size is generally desirable to achieve lower thermal conductivity, the amount of time and effort required for fluid extraction (drying) increases. Further, with all things equal, smaller pore size may increase the risk of shrinkage.  
       [0085] The processes according to the present invention allow for the production of LDMMs having small pore size with minimal shrinkage. For example, the above-described vacuum-purge process yields unshrunken monoliths with substantially smaller pores than is possible with evaporative drying. And, if evaporative drying is to be used, the inclusion of a surfactant yields unshrunken monoliths with substantially smaller pores than is possible without a surfactant. Thus, the formulation and/or processing conditions are tailored to obtain the desired mix of properties.  
       [0086] Density can be altered, and thus thermal conductivity can be altered, by using precursor formulations that have a lower or higher solid content. Although LDMMs with lower density have lower solid conduction, at ambient conditions, LDMMs have fairly low solid conduction and gas conduction dominates. Thus, LDMMs with higher density typically have lower overall thermal conductivity at ambient conditions. However, when gas conduction is mostly eliminated by lower gas pressure, lower density LDMMs exhibit lower overall thermal conductivity.  
       [0087] Density may also be altered to alter pore size. With all other variables being equal, higher density generally results in smaller pores. Higher density LDMMs tend to gel and cure faster, thereby reducing production times. However, higher density LDMMs require more precursor chemicals, weigh more for the same volume and tend to be more expensive to make. Thus, the formulation and/or processing condition must be tailored to achieve a good balance between density, pore size and thermal conductivity.  
       [0088] A preferred formulation used to prepare an LDMM of this invention comprises (all in weight %) from about 70% to about 80% acetic acid (as the solvent); from about 5% to about 11% isopropyl alcohol (as an additive); from about 2% to about 7% hydrobromic acid (as the catalyst), from about 4% to about 8% novolak (as the hydroxylated aromatic); and from about 2% to about 7% furfural (as the electrophilic linking agent). An even more preferred formulation comprises 77% acetic acid, 7% isopropyl alcohol, 5% hydrobromic acid, 6% novolak and 5% furfural.  
       [0089] The isopropanol component of the above formulation may be replaced, with no obvious change in the finished material, by an equal amount of 1-propanol or an approximate molar equivalent (1.1 g) of ethanol. Other alcohols may also be used with success.  
       [0090] Increasing the acid component of the above-described formulation produces, up to a point, stronger materials. As an example, if hydrobromic acid is used, it can be increased up to about seven percent (7%) by weight without any obvious deleterious effect (e.g., reaction occurs too quickly and yields large particles and pores and may produce a gel that is cosmetically inferior), although above a certain amount, the tendency to produce stronger gels diminishes. Hydrochloric acid, which is less expensive, may be used in place of the hydrobromic acid, but the resultant LDMM materials are not quite as strong and have larger pores than those produced using hydrobromic acid. Sulfuric acid may also be used and produces gels that are relatively strong and rigid. However, in the case of some glass or plastic molds, the use of sulfuric acid may interfere with the ability to form a sol-gel.  
       [0091] It may now be seen by one ordinarily skilled in the art that variations within the above-described process parameters, including but not limited to those of formulation, temperature, and drying methods, may result in LDMMs having controlled average pore size and improved solid network strength that can be tailored to meet the needs of the application. Such LDMMs may be formed into large, uncracked, net shaped monoliths.  
       [0092] The LDMMs of this invention, including those formed by the above-described improved procedures, can be further processed. For example, the LDMMs may be pyrolized to yield carbon forms. Such carbonized-foams have particularly useful electrical properties. For example, carbonized-foams exhibit low electrical resistance and are electrically conductive. By virtue of their high surface areas, such LDMMs have exceptional charge-storing capacities. Any of the well known pyrolysis processes can be used. See, e.g., U.S. Pat. No. 5,744,510.  
       [0093] Additionally, in the case where the LDMMs are formed in a standardized shape, the LDMMs may be readily cut, machined, or otherwise formed to adjust the shape of the monolith to fit the application. Preferably, the LDMMs of this invention are formed in situ within a cast or mold in a variety of shapes and/or sizes to fit the final product exactly. Under these circumstances, the LDMM should exhibit substantially no shrinkage such that upon in situ formation, the LDMM maintains the dimensions of the application. Thus, for example, where the LDMM is being formed in situ in walls or insulated barriers (e.g., used in refrigerated trucks, buildings, or aircraft), the formed LDMM should substantially occupy the space within the walls or insulated barriers.  
       [0094] In order that this invention may be better understood, the following examples are set forth. 
     
    
    
     EXAMPLES  
     [0095] Several samples of the LDMMs of this invention were prepared using a sol-gel polymerization process. The specific process by which they were made, and the precursor materials used, are described below. Unless otherwise indicated, each of the LDMMs that was prepared had the following dimensions: a cylinder 25 mm long with a 36 mm diameter (25.5 mL). Also, each of the LDMMs that was prepared was black except for those Examples using resorsinol.  
     [0096] After the samples were prepared, they were subjected to a series of tests and/or visually examined and compared to samples that had been analytically tested. The tests that were conducted are standard analyses that are described below in more detail. The visual examination of the samples provided information as to pore size, strength and rigidity. For example, it has been observed that an LDMM that is free of visual defects and has a glassy appearance indicates a microporous structure. in LDMM that is free of visual defects and has a smooth but not highly reflective surface indicates a more preferred mesoporous structure. However, a grainy surface indicates a macroporous structure. Other physical examinations include, for example, whether the LDMM exhibited any shrinkage; whether the top of the LDMM is flat or concave (concave indicates a weaker solid network); whether and to what extent the top of the LDMM can be pushed inward (as a measure of the strength and rigidity); and whether and to what extent the LDMM, upon breaking, leaves a clean or cleaved break at the fracture point (a cleaner break indicating a comparatively stronger and mesoporous LDMM).  
     [0097] In general, each of the samples was prepared using one of the drying methods shown in Table 1 below (unless otherwise indicated). The total amount time required to prepare the samples (gelation, curing and drying) was less that about 24 hours, with the exception of some of the samples prepared using Method I. As one of skill in the art will appreciate, in the examples dried using Method I, the time required to dry the samples can be reduced using other drying methods herein described.  
               TABLE 1                          Drying Methods                     Method           No.   Drying Method               [   Enhanced Evaporation: the sample is placed in a vacuum oven at           between 40° C. to 80° C., and under a vacuum of about 50 Torr,           until the sample is dried to completion.       II   Centrifugation: most of the pore fluid is removed by centri-           fugation at 500 rpm for 10 minutes, after which the sample is           dried to completion by evaporation as described in Method I.       III   Vacuum-Induced Pressure Differential: the sample is formed in a           bottle or tube, and a reduced pressure of 500 Torr is applied to           one side of sample. Most of the pore fluid is removed in about           15 minutes, after which the sample is dried to completion by           evaporation as described in Method I.       IV   Pressure-Induced Pressure Differential: the sample is formed in a           bottle or tube, and gas pressure of less than about 10 psi is           applied to one side of sample. Most of the pore fluid is removed           in about 20 minutes, after which the sample is dried to           completion by evaporation as described in Method I.                  
 
     [0098] Examples 1-5, as shown in Table 2 below, were prepared using a liquid phenolic-novolak resin for the hydroxylated aromatic and electrophilic linking agent components. These formulations were mixed in plastic bottles. The alcohol (where present) was mixed with the acetic acid, the FurCarb was then dissolved in the acetic acid solution, and the acid was then slowly added with mixing. The bottle was then capped and hand shaken for about one minute. The sample was then placed in a 60° C. oven for 6 to 8 hours, after which the pore fluid was removed by the specified drying method.  
               TABLE 2                          Formulations with liquid resin                         Example Number                                     Component (wt %)   1   2   3   4   5                                             Acetic Acid   81.1   81.1   81.1   81.1   76.1       FurCarb UP-520*   13.5   13.5   13.5   13.5   14.1       Isopropyl Alcohol   0   0   0   0   4.2       Hydrochloric Acid   5.4   5.4   5.4   5.4   5.6       Method of Pore Fluid Removal   I   II   III   IV   I                          
 
     [0099] Examples 1-5 are LDMMs. Based on the examination of the resulting foams, it was observed that the addition of alcohol produced higher quality foams of greater rigidity and smaller pore size) as compared to formulations that did not comprise alcohol.  
     [0100] Examples 6-27, as described in Tables 3-7 below, were prepared using a solid phenolic-novolak flake-resins. These formulations were mixed in plastic bottles. The alcohol component was added to the acetic acid, then the acid catalyst was added, followed by gentle mixing. The surfactant component (if present) was then added, followed by the resin, followed by the cross-linking agent (furfural or formaldehyde). The bottle was then capped and hand shaken for about one minute. The sample was then placed in a 40° C. gelation oven for 8 hours, then transferred to an 80° C. curing oven for 8 hours, after which the pore fluid was removed by Method I as described above.  
               TABLE 3                          Formulations with solid phenolic-novolak flake resin                         Example Number                                     Component (wt %)   6   7   8   9   10                                             Acetic Acid   77.3   74.8   78.7   75.6   80.6       GP-2056   7.4       GP-2074       7.8       GP-5833           7.4       GP-2018C               6.1   6.1       Isopropyl Alcohol   6.7   6.7   3.3   6.7   5       Hydrochloric Acid               6.7       Hydrobromic Acid   3.3   6.7   3.3       3.3       Furfural   5.3   2.3   7.3   5   5       Formaldehyde (37% aqueous)       1.7                  
 
     [0101] Examples 6-10 were LDMMs prepared using several different phenolic-novolak flake resins from Georgia Pacific, listed above from the lowest to highest average molecular weight. It was observed that as the molecular weight of the resin increased, the average pore size decreased and the resulting LDMM was a more rigid finished product. It was also observed that the use of hydrobromic acid as the catalyst produced more rigid LDMMs with smaller pore sizes as compared to those LDMMs prepared using hydrochloric acid as the catalyst.  
               TABLE 4                          Formulations with solid phenolic-novolak flake resin                         Example Number                                         Component (wt %)   11   12   13   14                                                     Acetic Acid   80.2   78.9   77.6   77.6           GP-5833 novolak flake resin   6.1   6.1   6.1   6.1           Ethyl alcohol   3.7           n-Propyl Alcohol       5           1-Butyl Alcohol           6.3            Isobutyl Alcohol               6.3           NEODOL 23-5   1.7   1.7   1.7   1.7           Hydrobromic acid   3.3   3.3   3.3   3.3           Furfural   5   5   5   5                      
 
     [0102]               TABLE 5                          Formulations with solid phenolic-novolak flake resin                         Example Number                                     Component (wt %)   15   16   17                                                 Acetic Acid   78.9   78.9   78.9           GP-5833 novolak flake resin   6.1   6.1   6.1           1-Pentanol   5               Iso-amyl alcohol       5           Cyclohexanol           5           NEODOL 23-5   1.7   1.7   1.7           Hydrobromic acid   3.3   3.3   3.3           Furfural   5   5   5                        
     [0103]               TABLE 6                          Formulations with solid phenolic-novolak flake resin                         Example Number                                         Component (wt %)   18   19   20   21                                                     Acetic Acid   78.9   78.9   78.9   78.9           GP-5833   6.1   6.1   6.1   6.1           2-Ethoxy-ethanol (cellosolve)   5           Ethylene Glycol       5           Propylene Glycol           5           Glycerol               5           NEODOL 23-5   1.7   1.7   1.7   1.7           Hydrobromic acid   3.3   3.3   3.3   3.3           Furfural   5   5   5   5                        
     [0104] Examples 11-21 are LDMMs using several different alcohol additives. In general, all of these formulations produced good, monolithic foams that were unshrunken with the exception of the samples prepared using polyalcohol (Examples 19-21), which exhibited shrinkage.  
               TABLE 7                          Formulations with solid phenolic-novolak flake resin                         Example Number                                         Component (wt %)   22   23   24   25   26   27                                                 Acetic Acid   74   70   77.5   79.3   80.7   78.9       GP-2018C novolak flake resin   0   0   0   5   4.3   6.1       GP-2074 novolak flake resin   8.9   13.3   0   0   0   0       GP-5833 novolak flake resin   0   0   6.1   0   0   0       Isopropyl alcohol   6.7   0   0   5   5   5       Glycerol   0   0   6.7   0   0   0       Tergitol XL-80N   0   0   0   1.7   1.7   1.7       Hydrobromic acid   6.7   0   0   5   5   0       Hydrochloric acid   0   10   6.7   0   0   0       Sulfuric acid   0   0   0   0   0   3.3       Furfural   0   0   3   4   3.3   5       Formaldehyde (aqueous, 37%)   3.7   0   0   0   0   0       Furfuryl Alcohol   0   6.7   0   0   0   0                  
 
     [0105] Examples 22-27 are formulations that resulted in unshrunken monolithic LDMMs having a good appearance and rigidity.  
     [0106] Examples 28-33, as described in Table 8 below, were prepared in the same manner as for Examples 6-27, except that the phenolic resin component was replaced by either a non-phenolic resin (Example 28) or a monomeric hydroxylated aromatic Examples 29-33).  
               TABLE 8                          Formulations with a non-phenolic resin       or a monomeric hydroxylated aromatic                         Example Number                                         Component (wt %)   28   29   30   31   32   33                                                 Acetic Acid   91.2   81.3   70.3   69.9   77.3   77       B-19-S resorcinol flake resin*   3.1   0   0   0   0   0       Resorcinol   0   4   0   0   0   7.3       Hydroquinone   0   0   7.3   0   0   0       Phenol (crystalline)   0   0   0   6.7   3.7   0       Isopropyl Alcohol   0   5   5   5   5   3.3       NEODOL 23-5    0   1.7   1.7   1.7   1.7   1.7       Hydrobromic Acid   0   1   5   5   5   0       Sulfuric Acid   1   0   0   0   0   0       Furfural   4.7   7   0   0   0   0       Furfuryl Alcohol   0   0   0   0   7.3   0       Formaldehyde (37% aqueous)   0   0   10.7   11.7   0   10.7                          
 
     [0107] Examples 28-33 are formulations that used a variety of hydroxylated aromatics other than phenolic resins. It was observed that although these formulations produced suitable LDMMs, formulations using phenolic resins resulted in higher quality materials. The monomeric resorsinol formulations (Examples 29 and 33) produced well-formed sol-gels which shrank and cracked upon drying. The other formulations exhibited little or no shrinkage or cracking.  
     [0108] Examples 34-39, as described in Table 9 below, were prepared in the same manner as Examples 6-27 except that they were gelled and cured at a single temperature for 8 hours total, after which the pore fluid was removed by solvent-flushing with hexane and a vacuum-induced pressure differential.  
               TABLE 9                          Formulations processed using solvent-flushing drying technique                         Example Number                                         Component (wt %)   34   35   36   37   38   39                                                 Acetic Acid   75.6   74.3   74.9   73.6   75.2   74       GP-2018C novolak flake resin   6.1   5   6.1   5   6.1   5       Isopropyl Alcohol   8.3   11.7   7.3   10.7   7   10.3       Sulfuric Acid   5   5   6.7   6.7       Hydrobromic Acid                   6.7   6.7       Furfural   5   4   5   4   5   4       Temperature of Gelation/   70   70   60   60   60   60       Curing                  
 
     [0109] Examples 34-39 are formulations that produced unshrunken LDMMs. These LDMMs did not have any visual defects and the rate of fluid flow through the samples indicated that they had very small pore sizes that exhibited by Example 51 described below. Also, this drying technique produced dried samples faster than any of the other drying methods used.  
     [0110] Examples 40-41, as shown in Table 10 below, were prepared by gelling the formulation at 40° C. for 8 hours and then curing at 60° C. for 8 hours, followed by drying using Method I. These Examples demonstrate that the processes of this invention can be used to prepare LDMMs have a wide range of properties, including densities.  
               TABLE 10                          Formulations Resulting In Relatively High Density Foams                             Ex. No.                                     Component (wt %)   40*   41                                             Acetic Acid   71.7   47.8           GP-2018C novolak flake resin   12   28           Isopropyl Alcohol   5   0           Hydrobromic Acid   3.3   1.5           Furfural   8   22.7           Density (mg/cc)   238   510                                  
 
     [0111] Examples 42-44, as shown in Table 11 below, were prepared in the same manner as for Examples 6-27. Each of these samples had a solids content of 11% and a density of about 110 kg/m 3 . These samples were then subjected to solid state  13 C NMR spectrometry. This test is designed to detect the presence of organic molecules containing the  13 C isotope, which is naturally occurring in an abundance of approximately 1.1%. This technique provides information on the organic compounds in the dried gel and the structural features comprising the gel network; specifically, NMR can also provide information on the bonding patterns responsible for the presence of a particular molecule.  
               TABLE 11                          NMR Analyses                         Example Number                             Component (wt %)   42   43   44                                     Acetic Acid   78.9   81.6   85.6       GP-2018C   6.1   6.1   0       GP-5833   0   0   6.1       Isopropyl Alcohol   5   5   0       NEODOL 23-5   1.7   0   0       Hydrobromic Acid   3.3   3.3   3.3       Furfural   5   5   5       NMR Analysis (wt %) in Dried LDMM)       Acetic Acid   4-6   6-8   6-8       NEODOL 23-5   1-2       Furfural (unreacted)   1-3       Furfural (cross-linked)       12-18   10-15                  
 
     [0112] Examples 42-44 show that acetic acid is retained in the dried gel, even after extended drying. This suggests that it is strongly anchored to the network by hydrogen-bonding, or it would have evaporated during drying. This is consistent with the hypothesis that acetic acid strengthens the gel by way of the hydrogen-bonding mechanism.  
     [0113] Examples 42-43 show no evidence of the incorporation of isopropanol. Isopropanol is known to be a weaker hydrogen-bonding species than is acetic acid, and it is more easily removed by evacuation.  
     [0114] Example 42 used the surfactant NEODOL; the presence of this material is indicated in the NMR spectrum, confirming that NEODOL remains in the dried sol-gel. Surfactants are desirable for the production of the large monolithic gels described in Examples 25-27 (used Tergitol XL-80N) and 29-33 (used NEODOL 23-5), and the NMR data for Example 42 confirm the presence of the surfactant in the dried gel. Since resonances for the NEODOL overlap with those of cross-linked furfural, it proved impossible to quantify the amount of the latter. However, the spectra clearly show the presence of NEODOL in Example 42.  
     [0115] Examples 45-49, as shown in Table 12 below, were prepared in the same manner as for Examples 6-27. The foams that were produced were then pyrolized to produce carbonized-derivatives, particularly useful in electrical applications. Specifically, the foams were placed into a ceramic tube, which was then sealed and purged for several hours with argon gas. The tube was then placed in a high temperature tube oven which was programmed as follows: heat from 22° C. to 250° C. in 2 hours; dwell at 250° C. for 4 hours; heat from 250° C. to 1050° C. in 9.5 hours; and dwell at 1050° C. for 9.5 hours.  
     [0116] As can be seen in table 12, the carbonized-derivatives exhibited volume losses of between about 48-56%, and mass losses of about 51-67%. Shrinkage is expected from pyrolysis. However, the LDMMs of this invention exhibited a considerable improvement over the prior art, which typically exhibit more than about 70% shrinkage.  
               TABLE 12                          Carbonized-Derivatives                         Example Number                                     Composition (wt %)   45   46   47   48   49                                             Acetic Acid   83.5   78.9   80.2   78.9   78.9       GP-2018C               6.1   6.1       GP-5833       6.1   6.1       FurCarb UP-520   13       Isopropyl Alcohol   0.9   5       5   5       Ethyl Alcohol           3.7       NEODOL 23-5       1.7   1.7   1.7   1.7       Hydrochloric Acid   2.6       Hydrobromic Acid       3.3   3.3   3.3       Sulfuric Acid                   3.3       Furfural       5   5   5   5       Density before carbonization (mg/cc)   110   148   100   119   177       Density after carbonization (mg/cc)   112   108   90   118   127       Volume Shrinkage (%)   52   55.3   51.0   55.9   48       Main Loss (%)   51.5   67.5   56.0   56   63.2       Resistivity (ohm meter)   0.013   0.015           0.017                  
 
     [0117] Examples 50 and 51, as shown in Table 13 below, were also prepared. Example 50 was prepared in the same manner as Examples 1-5, and it was dried using Method IV. Example 51 was prepared in the same manner as Examples 6-27. Average pore size, surface area and density were then determined for each of these samples. Average pore size for these samples were calculated using standard multipoint BJH (Barrett, Joyner and Halenda) analysis of nitrogen desorption curves. Surface area calculations were made using standard multipoint BET (Brunauer Emmett Teller) analysis of nitrogen adsorption curves. Bulk densities were calculated from the measured weight and volumes of the porous solids.  
                           TABLE 13                                      Example Number                                     Composition (wt %)   50   51                                             Acetic Acid   67.6   78           GP-2018C       6.1           FurCarb UP-520   14.1           Isopropyl Alcohol   8.4   5           NEODOL 23-5       1.7           Hydrochloric Acid   9.9           Hydrobromic Acid       4.2           Furfural       5           Density (mg/cc)   140   110           Average Pore Size (nm)   12   41           Surface Area (m 2 /g)   66   40                      
 
     [0118] Examples 52-60, as shown in Tables 14-15 below, were prepared and their thermal conductivities were determined. Examples 52-54 and 60 were prepared in the same manner as for Examples 1-5, and then dried using Method I. Examples 55-59 were prepared in the same manner as Examples 6-27. Example 55 was cut using a bandsaw from the sample prepared in Example 61 (described in Table 16). Prior to determining its thermal conductivity, Example 55 was heated in an oven at 100° C. for 5 hours to remove residual surfactant.  
     [0119] Thermal conductivity was measured using two techniques: hot wire and steady-state thin heater. In the hot wire technique, cylindrical samples of LDMM were made with a 0.001 inch diameter tungsten wire running the length of the cylinder. The samples were typically 2.0 cm in diameter and 5.0 to 7.0 cm in length. The samples were then placed within a vacuum chamber and measurements of the current through and voltage for the wire were made as a function of applied power. The resistance of the wire, and hence the temperature of the wire, were then calculated and graphed as a function of time and fit to theoretical models. Thermal conductivity was then calculated from fit functions. e.g. “The hot-wire method applied to porous materials of low thermal conductivity,” High Temperature High Pressures, 1993, vol. 25, pp. 391-402, 13 th  ECTP Proceedings pp 219-230. In this fashion, thermal conductivities were calculated as a function of pressure.  
     [0120] In the steady-state thin heater technique, a 0.04 cm thick 4.5 cm square heater is sandwiched between two 1 cm thick×6 cm diameter LDMM samples. Thermocouples are placed on the interior and exterior surfaces of the samples. Aluminum heat sinks are then used to hold the samples and heater together and eliminate any gap between the samples. Thermal conductivity is then calculated by fitting both the temperature increase and decrease versus time curve as the heater is powered to thermal equilibrium and then turned off See. e.g. ASTM C1114-00. As in the hot wire technique, the samples are put into a vacuum chamber during these measurements so that the thermal conductivity can be calculated as a function of pressure.  
               TABLE 14                          Thermal Conductivity Analyses                         Example Number                                     Composition (wt %)   52   53   54   55   56                                             Acetic Acid   77.4   76.0   67.6   78   0       GP-2018C   0   0   0   6.1   5       FurCarb UP-520   14.1   14.1   14.1   0   0       Isopropyl Alcohol   0   4.2   8.4   5   5       Hydrochloric Acid   8.5   6.7   9.9   0   0       Hydrobromic Acid   0   0   0   4.2   3.3       Furfural   0   0   0   5   4.1       Density (mg/cc)   140   140   140   84   91       W/m ° K. @ Torr*   0.0053   0.0028   0.0016   0.0050   0.0016           @   @   @   @   @           0.017   0.004   0.006   0.080   0.054       W/m ° K. @ Torr*   0.0070   0.0035   0.0036   0.0060   0.040           @   @   @   @   @           0.100   0.100   0.100   0.425   760       W/m ° K. @ Torr*   0.0088   0.0065   0.007   0.0070           @   @   @   @           0.800   1.00   1.00   1.00       W/m ° K. @ Torr*       0.0132   0.0135   0.0161               @   @   @               10.0   10.0   10.0       W/m ° K. @ Torr*       0.041   0.0445   0.062               @   @   @               760   760   760                          
 
     [0121]               TABLE 15                          Thermal Conducitivity Analyses                         Example Number                                 Composition (wt %)   57   58   59   60                                         Acetic Acid   67.6   77.4   80.6   80.6       GP-2018C   0   7.9   6.1   6.1       FurCarb UP-520   14.1   0   0   0       Isopropyl Alcohol   8.4   5   5   5       Hydrochloric Acid   9.9   0   0   0       Hydrobromic Acid   0   3.3   3.3   3.3       Furfural   0   6.4   5   5       Density (mg/cc)   144   179   123   112       W/m ° K. @ Torr*   0.004   0.0043   0.0025   0.005           @   @   @   @           0.676   0.070   0.080   0.028       W/m ° K. @ Torr*   0.004   0.030   0.037   0.005           @   @   @   @           0.980   760   760   0.040       W/m ° K. @ Torr*   0.008           0.05           @           @           10.0           760       W/m ° K. @ Torr*   0.039           @           760                            
     [0122] Example 61, as shown in Table 16 below, was prepared in the same manner as for Examples 6-27, except that the chemicals were mixed in 1000 ml bottles, then combined in a 8.3 liter TUPPERWARE container, which was filled to slightly more than about half full. The resulting foam was an unshrunken, monolithic aerogel having the following dimensions: 6.2 cm×23 cm×34 cm.  
     [0123] Also, from the same chemical mixture, a smaller sample was prepared (Example 51). As shown in Table 13, that sample (and thus Example 61) had a density of 110 mg/cc; an average pore size of 41 nm; and a surface area of 40 m 2 /g.  
               TABLE 16                          Large, Monolithic Aerogel                                 Ex. No.           Composition (wt %)   61                                         Acetic Acid   78           GP-2018C   6.1           Isopropyl Alcohol   5           Hydrobromic Acid   4.2           NEODOL 23-5   1.7           Furfural   5           Density (mg/cc)   112                      
 
     [0124] Examples 62 and 63, as shown in Table 17 below, were prepared in the same manner as Example 6-27. These examples show that by adding a surfactant to an LDMM, shrinkage can be considerably reduced or eliminated.  
                       TABLE 17                                      Example Number                                 Composition (wt %)   62   63                                             Acetic Acid   80.6   78.9           GP-2018C   6.1   6.1           Isopropyl Alcohol   5   5           NEODOL 23-5   0   1.7           Hydrobromic Acid   3.3   3.3           Furfural   5   5           Shrinkage of dried material (vol. %)   20   0                      
 
     [0125] As described above, materials exhibiting both low density and microcellular open porosity have many favorable physical properties. The tests and measurements reported in this application indicate that the materials disclosed herein exhibit both of these characteristics. In addition, the materials disclosed herein can be produced in a wide variety of shapes and sizes, and the process may be completed in time frames shorter than those reported for prior art materials. Additionally, the current application discloses new compositions of matter and formulation processes that use less expensive starting materials and easier processing conditions than those described previously.  
     [0126] While particular materials, formulations, operational sequences, process parameters, and end products have been set forth to describe and exemplify this invention, such are not intended to be limiting. Rather, it should be noted by those ordinarily skilled in the art that the written disclosures are exemplary only and that various other alternatives, adaptations, and modifications may be made within the scope of the present invention. Accordingly, the present invention is not limited to the specific embodiments illustrated herein, but is limited only by the following claims.  
     [0127] All references cited within the body of the instant specification are hereby incorporated by reference in their entirety.