MXPA99009251A - Process for producing low density gel compositions - Google Patents

Abstract

Se revelan procesos para producir una composición de gel que puede ser utilizada para producir composiciones de gel de baja densidad sin necesidad de secado supercrítico, tratamiento térmico o tratamiento superficial. El proceso comprende secar un gel húmedo que comprende sólidos de gel y un agente de secado para eliminar al agente de secado al tiempo que se reduce al mínimo la contracción del gel durante el secado.

Description

PROCESS TO PRODUCE LOW-DENSITY GEL COMPOSITIONS FIELD OF THE INVENTION The present invention relates to a process for producing low density gel compositions, including aerogels, xerogels and the like, without the need for a supercritical drying step or heat treatment, heat treatment or chemical surface treatment.

BACKGROUND OF THE INVENTION The term "gel" encompasses wet gels, including hydrogels and alcogels and gels dried from wet gels including aerogels and xerogels. The term "airgel" which was coined by S.S. Kistler in U.S. Patent No. 2,188,007 and is generally used to refer to a gel that has been dried under supercritical temperature / pressure conditions. The term "xerogel" is generally used to refer to a gel that has been dried by the evaporation of the solvent. The gel composition refers to a composition comprising a gel which may additionally include other components, for example, an opacifying agent or a coloring agent. Gel compositions are used in a wide variety of applications, including thermal and acoustic insulation; supports and vehicles of catalysts; filters and molecular meshes; agents for rheology control; reinforcing agents; thickeners and electronic compounds; adsorbents; opacifying agents; particulate additives; membranes; filters; radiation detectors; coatings and dielectrics and other applications disclosed herein and / or known to those of ordinary skill in the art. Gel compositions having lower bar densities and / or higher surface areas and / or a higher structure are more advantageous for use in many applications. The bar density of the gel composition is related to the porosity of the gel composition, wherein the gel composition having lower bar density will generally have higher porosity. Gel compositions are generally produced by combining a gel precursor and a suitable solvent to form a sol and then initiate gelation of the sol to form a "wet" gel comprising the solid gel structure and the liquid solvent. The liquid solvent is then removed to form a dry gel composition. Aerogels that are produced using a supercritical drying step will generally have lower bar densities than the gel compositions known hitherto and which are produced without using supercritical drying and which then become the gel of choice for many applications . However, the supercritical drying step necessary for the production of an airgel may require the use of relatively expensive and / or complex processing equipment and conditions and, therefore, may be disadvantageous. In addition to the approaches that use supercritical drying, at least several other approaches have been proposed for producing gels. Alexander et al., In U.S. Patent No. 2,765,242, discloses a process for producing gels that utilize development or standing in water at high temperature, followed by thermal treatment in alcohols at temperatures significantly above the boiling point with the in order to esterify the surface. The gel granules can then be ground until a fine powder is obtained. The disadvantages of the approach disclosed in the patent of Alexander et al. include the cost of the high pressure esterification step. WO 94/25149 discloses a process for the preparation of xerogels by surface chemical modification. The agents for chemical surface modification disclosed have the formula RxMXy / wherein R is an organic group, such as CH3, C2H5 etc .; X is a halogen and M is Si or Al. The potential disadvantages of the approach disclosed in WO 94/25149 include the high cost of the reagents and potential problems related to the disposal of the by-products of the reaction. United States Patent No. 5, 270,027, discloses a process for preparing silica xerogels using alkanolamines. The developed process produces xerogels that have a total variable pore volume of 2 to 3 cc / g. The equivalent density of the individual granules is 0.29 to 0.37 g / cc. The potential disadvantages of the approach disclosed in U.S. Patent No. 5,270,027 include the complicated steps disclosed as part of the process, in particular, the heat treatment step and that is not disclosed to the process as a producer of aerogels with sufficiently low densities to certain applications.

SUMMARY OF THE INVENTION The present invention provides processes for producing gel compositions having advantageously low bar densities without the need for supercritical drying, heat treatment or chemical modification of the surface. The present invention provides processes for producing gel compositions comprising: drying a wet gel comprising gel solids and a drying agent to remove the drying agent under drying conditions sufficient to minimize shrinkage of the gel during drying. In a first aspect, the present invention provides a process for producing gel compositions, comprising: drying a wet gel comprising gel solids and a drying agent to remove the drying agent at a pressure of less than 300 pounds per square inch absolute (psia) sufficient to minimize shrinkage of the gel during drying. Preferably, the bar density and / or the compacted density of the dry gel compositions (Bar Density) is less than or equal to 115% of the theoretical density of the solid gels in the reaction solution (Theoretical Density) as follows : (Bar Density / Theoretical Density) < 115%. In another aspect, the present invention provides a process for producing gel compositions comprising: drying a wet gel comprising gel solids and a drying agent to remove the drying agent at a pressure of less than 300 psia to produce a composition of dry gel that has a bar density less than or equal to 0.27 grams / cubic centimeter (g / cc). In a further aspect, the present invention provides a process for producing gel compositions comprising: drying a wet gel comprising gel solids and a drying agent to remove the drying agent at a pressure less than or equal to 300 psia to produce a dry gel composition having a compacted density less than or equal to 0.2 grams / cubic centimeter (g / cc). In a further aspect, the present invention provides a process for producing gel compositions comprising: drying a wet gel comprising gel solids and a drying agent to remove the drying agent, wherein the chemical properties of the drying agent reduce to a minimum the contraction of the gel during drying. An advantage of the aspects of the present invention is that the processes can be carried out at ambient temperatures and / or ambient pressures. The particularities and advantages of the processes of the present invention are described in more detail in the following sections.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of one embodiment of a process of the present invention for producing gel compositions.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides processes for producing gel compositions comprising drying a wet gel comprising gel solids and a drying agent to remove the drying agent.

In accordance with one aspect of the present invention, a process for producing gel compositions comprising: drying a wet gel comprising gel solids and a drying agent to remove the drying agent under drying conditions sufficient to minimize the shrinkage of the gel during drying at a pressure less than or equal to 300 psia, preferably at a pressure less than or equal to 100 psia, more preferably less than or equal to 30 psia, most preferably less than or equal to 16. Preferably, the bar density and / or the compacted density of the dry gel compositions (Bar Density) is less than or equal to 115%, more preferably, less than or equal to 110%, and most preferably less than or equal to at 105% of the theoretical density of the gel solids in the reaction solution (Theoretical Density) as shown below: (Bar Density or Compacted / Theoretical Density) < 115%, preferably < 110%, more preferably < 105%. The density in bar, the compacted density and the theoretical density can be determined in the ways that are discussed below. Preferred products of the present invention have a bar density less than or equal to 0.27 g / cc, preferably less than or equal to 0.22 g / cc, more preferably, less than or equal to 0.15 g / cc and / or a compacted density less than or equal to 0.2 g / cc, preferably, less than or equal to 0.15 g / cc, and more preferably, less than or equal to 0.10 g / cc. In another aspect, the present invention provides a process for producing gel compositions comprising: drying a wet gel comprising gel solids and a drying agent to remove the drying agent under drying conditions sufficient to produce a dry gel composition. having a bar density less than or equal to 0.27 g / cc, preferably less than or equal to 0.22 g / cc, more preferably less than or equal to 0.15 g / cc, at a pressure less than or equal to 300 psia, preferably, at a pressure less than or equal to 100 psia, more preferably, less than or equal to 30 psia and most preferably, less than or equal to 16. In a further aspect, the present invention provides a process for producing gel compositions, comprising: drying a wet gel comprising gel solids and a drying agent to remove the drying agent under drying conditions sufficient to produce a dry gel composition having a compacted density less than or equal to at 0.2 g / cc, preferably, less than or equal to 0.15 g / cc, more preferably, less than or equal to 0.10 g / cc, at a pressure less than or equal to 300 psia, preferably at a pressure less than or equal to 100 psia, more preferably, less than or equal to 30 psia and most preferably, less than or equal to 16. The processes of the present invention can be performed using mixing vessels and equipment for handling gels and conventional laboratory and industrial gel compositions. . The choice of the particular equipment used to practice the processes of the present invention is considered to be within the skill of anyone having ordinary skill in the art and, therefore, will not be described in detail below. As will be recognized by those of ordinary skill in the art from the description and examples set forth herein, the processes of the present invention may be carried out as continuous or batch processes. The chemical properties of the drying agent of relevance to the process of the present invention include: ratio of the density of the liquid phase to the density of the solid phase at the freezing point (ratio of HCl / Psyido); vapor pressure at the freezing / melting point; heat of vaporization per unit volume; melting point; molecular weight and solubility in water. Suitable drying agents for use in the processes of the present invention have: a Piiguido / Psyido ratio at the freezing point of 0.95-1.05, preferably 0.97-1.03; and a vapor pressure at the freezing / melting point greater than or equal to 1 Torr, preferably greater than or equal to 10 Torr, more preferably, greater than or equal to 25 Torr. Preferably, the drying agent for use in the process of the present invention additionally has one or more of the following properties: a heat of vaporization per unit volume less than 200 calories per cubic centimeter (cal / cc), preferably, less than or equal to 125 cal / cc, more preferably, less than or equal to 100 cal / cc (? H (cal / cc) < _ 200, preferably 125, more preferably _ < 100); a melting point within 15 ° C, preferably within 5 ° C of the temperature at which the drying is conducted; a molecular weight less than or equal to 300, preferably less than or equal to 100; and / or water solubility (ie, the water is soluble / miscible in the drying agent). A further embodiment of a process of the present invention comprises drying a wet gel composition comprising gel solids and a drying agent to remove the drying agent, wherein the drying agent has a Piiquid / Psyido ratio at the point of freezing from 0.95-1.05, preferably 0.97-1.03; and / or a vapor pressure at the freezing / melting point greater than or equal to 1 Torr, preferably greater than or equal to 10 Torr, more preferably, greater than or equal to 25 Torr. In preferred embodiments, the drying agent additionally has one or more of the following properties: a heat of vaporization per unit volume less than 200 calories per cubic centimeter (cal / cc), preferably, less than or equal to 125 cal / cc, more preferably, less than or equal to 100 cal / cc (? H (cal / cc) < 200, preferably < 125, more preferably < 100); a melting point within 15 ° C, preferably within 5 ° C of the temperature at which the drying is conducted; a molecular weight less than or equal to 300, preferably less than or equal to 100; and / or water solubility (ie, the water is soluble / miscible in the drying agent). The vapor pressure at the freezing / melting point of a drying agent is related to the rate, that is, to the speed, at which the gel solids are dried. The rate of sublimation (drying) is directly proportional to the vapor pressure at the solid-vapor interface. If the vapor pressure is low, the rate or speed of drying is insufficient to maintain the temperature at the interface at or below the speed or freezing rate. Preferred drying agents for use in the processes of the present invention have vapor pressures at their freezing / melting point greater than or equal to 1 Torr, preferably, greater than or equal to 10 Torr, more preferably, greater than or equal to 25 Torr. Torr. The total amount of energy that must be introduced to a "wet" gel to remove the liquid is directly proportional to the vaporization heat property per unit volume of the drying agent. Although, in the processes of the present invention, the vapor can be sublimated, the net energy is from the liquid phase change to the vapor phase even though the process path can be from liquid to solid to vapor. Preferred drying agents for use in a process of the present invention have the vaporization heat property per unit volume less than or equal to 200 cal / cc, preferably less than or equal to 125 cal / cc, more preferably, less than or equal to at 100 cal / cc. The melting point of the drying agent will affect the temperature conditions at which the drying step of a process of the present invention is conducted. Preferably, the temperature conditions during drying are within 15 ° C, more preferably, within 5 ° C of the freezing / melting point of the drying agent. To facilitate processing, it is desirable that a process of the present invention be carried out almost at room temperature, therefore, it is preferred that the freezing / melting point of the drying agent be within 15 ° C, more preferably, within of 5 ° C of the room temperature. The molecular weight of a drying agent will generally affect the freezing / melting point of the drying agent. When the drying agent is in the pores of a metal oxide and an organometallic oxide, the freezing point decreases as the molecular weight of the solvent increases exponentially. Thus, preferred drying agents for use in a process of the present invention have molecular weights less than or equal to 300, preferably less than or equal to 100. Examples of drying agents suitable for use in a process of the present invention are set forth in the next box. A preferred drying agent for use in a process of the present invention is t-butanol (Piiquid / Psyido ratio at the freezing point is 1.00) due to its high vapor pressure at the melting / freezing point, compared to other drying agents.

V.P. = vapor pressure; ? H = Vaporization Heat; ? H cal / cm3 = heat of vaporization per unit volume; MW = molecular weight A conventional method for drying wet gels is to remove the liquid solvent by evaporation, which results in the presence of a liquid-vapor interface. The driving force for evaporation is a lower vapor pressure of the solvent in the gas phase above the sample than the vapor pressure of the solvent at the liquid-vapor interface. This driving force can be obtained by increasing the temperature or by lowering the vapor pressure of the gas phase using a vacuum or a carrier gas or gaseous vehicle. As a result of the surface tension of the liquid, the interface is curved and exerts pressure by capillarity on the pore wall which can cause the contraction of the material to be dried. The magnitude of this capillary pressure that is exerted is inversely proportional to the pore size. For the pore size of gels and fine powders (1-100 nm), this capillarity pressure can exceed one thousand atmospheres. During evaporation of the solvent, the sample will continue to contract until the strength of the material is sufficient to withstand the capillarity pressure. Because of this contraction, the dry gel composition will have a bulk density greater than that of the dry gel composition in the absence of shrinkage, making the gel composition less advantageous for certain applications.

The aerogels produced using a supercritical drying step in general will have lower bar densities and larger surface areas than the gel compositions known up to now, produced without using supercritical drying and, therefore, have become the gel of choice for many applications. In supercritical drying, the temperature of the pore fluid increases under pressure until the critical temperature and pressure are exceeded. In that case, the liquid-vapor interface is not presented and hence, shrinkage is avoided. However, the supercritical drying step, necessary for the production of an airgel, may require the use of relatively expensive and / or complex processing equipment and conditions and, therefore, is disadvantageous. Another possible approach to the production of gel compositions is to freeze dry the liquid gel wet solvent. In freeze drying, the solid containing the liquid freezes first. The pressure around the sample is then reduced to eliminate by sublimation the frozen liquid. For freeze drying, only solid-vapor interfaces exist and the shrinkage is usually negligible. However, when dried either with compliant materials, with materials that have small pore sizes or fine powders, the use of drying by P1606 / 99 X freezing generally causes contraction and deformation of the material. The freezing point of a fluid in a pore is less than the freezing point of the bulk liquid. Hence, as the sample cools, first freezing occurs on the outside of the sample. The liquid is expelled from the pores to feed the continuous freezing of liquid and exerts a compressive force on the sample, similar to the evaporation ^ LO of the solvent. Only when the sample is strong enough to withstand this compressive force is that the contraction stops. The processes of the present invention differ from the drying processes and methods analyzed previously. In accordance with the present invention, the liquid in the gel-sol prior to drying comprises a drying agent having the properties discussed herein. The "wet" gel is then dried under conditions, which in The combination with the properties of the drying agent, which minimize shrinkage of the solids of the gel composition to produce gel compositions. In the case where the wet gel composition comprises another solvent different from the agent of After drying, the liquid in the wet gel is replaced by a drying agent having the properties described above. The drying of the gel can be achieved using P1606 / 99MX a variety of process trajectories. In general, it is preferred in the processes of the present invention that drying be initiated at a temperature above the freezing point of the drying agent. Preferably, the wet gel composition comprising gel solids and a drying agent is placed in a dryer at a temperature approximately equal to or higher than the freezing point of the drying agent. Quick drying can then be initiated by establishing either a vacuum or the flow of a gaseous vehicle or stripping gas into the sample. Preferably, the drying conditions are maintained in such a way that the temperature of the vapor-liquid interface is rapidly cooled below the freezing point of the drying agent. This causes the formation of a "crust" frozen in the gel solids, which means that there are liquid-solid and solid-vapor interfaces but minimal or no liquid-vapor interfaces. Drying continues as the steam removal continues. The temperature of the dryer can even be increased as long as the temperature at the interface remains below the freezing point of the liquid. The temperature of the interface (assuming that the solid is fully saturated) is related to the rate of energy transport to the sample and the mass transfer of the vapor away from the sample. In P1606 / 99MX balance, the temperature of the interface is the so-called wet bulb temperature and is calculated from: n (.tdrier "t et bulb> = DH k3 (pdrier-woodface) = DH k9 (pdrier" F < twet bulb)) where: h = heat transfer coefficient tdrier = dryer temperature twet bulb = wet bulb temperature DH = latent heat of vaporization kg = mass transfer coefficient pdrier = partial pressure of the solvent in the dryer. pinterface = vapor pressure of the solvent at the wet bulb temperature (^ e ^ a interface) In a process of the present invention, the wet bulb temperature at the interface will preferably be less than the dryer temperature (the temperature of the environment in which the wet gel is drying). As the partial pressure driving force increases, the temperature difference between the dryer and the wet bulb temperature increases.

The combination of the drying rate and the heat of vaporization should be sufficient to reduce the interfacial temperature to the freezing point. In the last stages of drying, the temperature of the dryer can be increased, since an increase in the heat and in the resistance to the transfer of mass inside the solid allows a greater P1606 / 99MX dryer temperature with an interface temperature that is still at freezing point. The processes of the present invention can be used to produce gel compositions comprising: silica; titanium; aluminum; zirconium; other metal oxides and / or organometallic oxides or mixtures thereof. The gel compositions may additionally comprise fillers which include, but are not limited to: carbonaceous materials; oxides of iron; Al203; FeTi03; Ti02; Zr02 and / or other fillers known in the art. Carbon materials include: carbon black, activated carbon; graphite; compounds comprising carbon black and metal oxide (e.g., silica) and blends including such carbonaceous fillers. A preferred carbon black has a surface area of nitrogen (N2SA) of at least 10 m2 / g, preferably from 15 to 500 m2 / g. A schematic diagram of one embodiment of a process of the present invention for producing a gel composition comprising silica is set forth in Figure 1. As shown in Figure 1, the process steps of the embodiments of the present invention can be performed after the initial gelation of a solution comprising a gel precursor. Gel precursors include in form P1606 / 99I1X enunciative, oxide, polymeric and particulate gel components, known in the art, such as for example: Component of Oxide Gel Form (s) as SiO_ Alkoxide Gel Metallic Precursor, Silicate, Colloidal, Pyrogenic Compositions, Silicon halides TiO_ Alkoxide, Colloidal, Pyrrogenic, Titanate Compositions, Titanium Halides A1203 Alkoxides, Colloidal, Aluminate Compositions, Salts, Pyrrogenic, Aluminum Halides ZrO Alkoxide, Colloidal, Zirconate Compositions, Salts, Pyrrogenic, Zirconium Halides Compounds Oxide Combinations of the Metallic Previous Precursors Organometallic Oxide Organometallic forms of the previous precursor The metal oxide compounds refer to composite materials comprising combinations of metal oxides and / or organometallic oxides. The term organometallic oxide refers to a composition comprising a metal oxide and an organic material (i.e., a material comprising the CHX function) which may additionally comprise other chemical groups. The choice of a particular gel precursor is made based on the type of P1606 / 99 X desired composition. A preferred gel component for certain applications is SiO2 wherein the sodium silicate is the preferred precursor or the preferred form. The initial gel can be produced from a stock solution. The stock solution may comprise the gel precursor and a solvent. The amounts of each component will vary depending on the density and structure desired in the final gel composition. Suitable solvents will depend on the particular gel precursor. For a sodium silicate precursor a preferred solvent is water. The stock solution can be prepared by mixing the gel precursor and the solvent. To obtain advantageously low densities in the final gel composition, the initial gelling step can be carried out from a solution with a sufficiently low percentage of solids by weight to obtain the desired bar density in the final gel composition after making a process of the present invention and when processing the solution, using sol-gel processing techniques, in a form wherein a low concentration of solids is maintained in the final gel composition. In particular, the initial gelling step can be carried out at the start of gelation in a solution comprising the gel precursor or the gel precursor and solids P1606 / 99MX, for example, an opacifying agent, in an initial concentration of solids that obtains the desired solids concentration in the final gel composition. As will be understood by those of ordinary skill in the art, the solids concentration of the solution, which may comprise, for example, gel precursor solids and the opacifying agent, is sufficient to obtain the desired densities in the final composition. In a preferred method for producing the compositions of the present invention, the solids concentration of the solution is less than or equal to 10%, preferably less than or equal to 8% to obtain in the final composition the desirably low bar densities. The solution comprising the gel component can be produced and the initial gelation can be carried out by conventional processes to produce gel compositions using, for example, conventional sol-gel processing techniques. In particular, the solution comprising the gel precursor can be produced and the initial gelation can be carried out by the processes disclosed herein in the examples. Various solutions, including metal alkoxides, colloidal suspensions and combinations thereof, can be used with a variety of gelling mechanisms to reach the stage of P1606 / 99I1X initial gelation. By varying the processing conditions such as time, temperature, pH, pore fluid, the microstructure of the composition can be altered. The initiation of gelation can be effected in any manner known in the art, including: manipulation of the pH of the stock solution by means of the addition of an acid or a base; manipulation of the temperature and pressure of the stock solution by means of environmental controls and the use of a gelation catalyst, for example, an acid or a base. As shown in Figure 1, after gelation by means of pH manipulation and the use of a gelation catalyst, for example, sulfuric acid (H2SO), the gel can be washed to remove residual salts. For example, in the case of the sodium silicate gel precursor and the H2S04 catalyst, after gelation the gel can be washed with water to remove the sodium sulfate (Na2SO4). The washing steps can be repeated until the desired amount of salts has been eliminated. For example, to a point where the concentration of sodium in the liquid phase is less than 100 parts per million. After washing the resulting gel, it can be left to stand in water to achieve the desired mechanical characteristics in the composition PI fififi / «« re of final gel. As shown schematically in Figure 1, after washing and / or restoring, the rest of the solution in the gel can be exchanged with a solution comprising at least one drying agent. The exchange step can be repeated several times if desired. Preferably, after the step or exchange steps have been completed, the initial gel solution (eg, water) has been virtually completely replaced by the solution comprising the drying agent. In particular, it is preferred that the wet gel comprising the drying agent and the wet solids comprises less than 5%, by weight, preferably less than 2%, by weight, more preferably less than 1%, by weight, of water. The solution of the drying agent comprises the drying agent and may additionally comprise a solvent. Intermediate compounds and suitable solvents include, but are not limited to, methanol, ethanol, n-propanol, iso-propanol, n-pentane, n-hexane, n-heptane. The solution of the drying agent may comprise equal to or more than 90%, preferably equal to or more than 98%, by weight, of the drying agent, wherein the remainder is a solvent or a combination of solvents. In a preferred embodiment of the process of the present invention, the drying agent solution comprises 100% by weight of the drying agent.

P1606 / 99MX dried. After exchange of the drying agent, the resulting gel composition is dried. The drying step is carried out in a manner sufficient to minimize shrinkage of the solids portion of the wet gel. A suitable method for effecting the drying step is to dry the gel composition under vacuum at a pressure of from about 0 psi to the vapor pressure of the drying agent at the freezing / melting point of the drying agent. Another suitable method, which may be advantageous in a large-scale production process, is to dry the gel using a fluidized bed. In In general, fluidized bed drying can be achieved by placing the wet gel composition in a fluidized bed reactor and by passing a dry, inert gas (with respect to the gel composition) through the gel composition. The speed of In the case of fluidization, the velocity of the gas stream necessary to maintain fluidization will depend on the physical characteristics and the volume of the wet gel, but it should be sufficient to maintain fluidization. The temperature of the gas can be Approximately room temperature, for example, 16-25 ° C. After drying, the gel composition can be further processed into the forms P1606 / 99MX known in the art. For example, the gel composition can be milled or ground to produce a powder comprising the gel composition or the gel composition can be heated above the boiling point of the drying agent to remove the residual drying agent. In addition to the steps discussed above and / or shown schematically in Figure 1, in a process of the present invention, additional steps of washing, drying and / or developing or resting, where desirable, may be included to produce a particular gel composition. . In particular, a process of the present invention may include one or more of the following steps: washing the wet gel prior to exchange of the drying agent; aging to the wet gel after exchange of the drying agent and before drying; exchange (replace) the wet gel fluid with other solvents before exchanging the fluid for the drying agent solution. In addition, particular aging steps can be carried out at elevated temperature and / or pressure. In general, the washing or exchange steps will comprise the exchange of the solution within the gel by another solution. In general, the steps of aging will include keeping the gel, with or without a solution present inside P1606 / 99MX of the gel, at particular temperature and pressure conditions. Depending on the desired characteristics in the final gel composition, in the process of Present invention may include optional steps such as for example thermal (or hydrothermal) aging before drying. The processes of the present invention can also be used advantageously to produce gel compositions with a wide range of surface areas, for example, 40-1000 m ^ / g, wherein the choice of the particular surface area will depend on the application that is applied. intended for the gel composition. In particular, where desired, the process of the present invention can advantageously produce gel compositions having BET surface areas equal to or greater than 200 m2 / g, preferably equal to or greater than 400 m ^ / g, more preferably , equal or greater than 500 m ^ / g. The BET surface area can be determined using the test procedure D1993 of the ASTM. The processes of the present invention may additionally be used advantageously to produce gel compositions having a greater porosity or equal 0.86, preferably greater than or equal to 0.91, more preferably, greater than or equal to 0.93. Porosity can be determined in the form P1606 / 99MX discussed later. In addition, the processes of the present invention can be used to produce gel compositions having a volume of greater than or equal to 3 g / cc, preferably greater than or equal to 4.5 g / cc, more preferably, greater than or equal to 8. g / cc. The pore volume is the inverse of the bar density and can be determined in the manner set out below. In addition, the processes of the present invention can be used to produce hydrophilic gel compositions. The gel compositions produced by the processes of the present invention can be used for applications that include, but are not limited to, thermal and acoustic insulation; catalyst supports and vehicles; filters and molecular meshes; rheology control agents; reinforcing agents; thickeners and electronic compounds; adsorbents; opacifying agents, particulate additives, membranes; filters; radiation detectors; coatings and dielectrics and other applications disclosed herein and / or known to those of ordinary skill in the art. The particularities and advantages of the processes of the present invention and of the gel compositions produced by the process of the present invention are described further in the P1606 / 99MX following Examples. The following analytical procedures can be used to determine the properties of a gel composition and were in fact used in the Examples described below. Porosity and Bar Density The porosity of a gel composition can be determined by determining the bar density of the composition and calculating the porosity by the following method. To determine the bar density, the gels were cast and formed into cylindrical molds. The total gel volume was determined by physically measuring the dimensions of a dry gel. Bar density was determined by weighing the dry gel and dividing it by geometric volume. In cases where a bar-like geometry is not maintained or, as a verification of the above method, the mercury displacement was used. The bar density of the gel compositions measured by mercury displacement was effected as follows. A clean, empty glass cell was filled with mercury to a specific height and the cell was weighed. The mercury was then removed and the cell was cleaned again. Next, a dry gel sample of known weight was placed in the glass cell and mercury was added to the cell at the same specific height as before. The weight of the cell containing the mercury and the sample was measured. The weight of the mercury in both cases then became a volume based on the density of the mercury. The difference between the volume of mercury that fills an empty cell and the volume of mercury that fills the cell containing the sample is known as the volume displaced after subtracting the weight of the sample. Since mercury does not wet the sample, this volume is equal to the total volume of the sample. The density is then determined by dividing the weight of the sample between the displaced volume. The porosity is defined as the fraction of the sample volume, ie the pores, both in the particulate material and around it and can be determined by the following formula: Porosity = 1- (bar density measured in porous form) (density of material in solid form) (in the case of a silica gel = > e = l-pen bar / Psi.02) The density of a solid mass of material is determined with reference to the composition of the material. In the case of a silica gel composition, without agents or opacifiers, the density of the solid mass of the material was assumed to be the density of a solid mass of silica which is 2.2 g / cc (220 kg / m3). In the case of a gel composition that P1606 / 99MX includes opacifying agents, the density of the solid mass of the material was assumed to be a weighted average of the densities of each component. For example, in the case of a gel composition comprising a silica gel precursor and a carbon black opacifying agent, the density of the solid mass of the material was assumed to be a weighted average density of a solid mass of silica (2.2 g / cc) and the density of a solid mass of carbon black (1.8 g / cc).

Pore Volume The pore volume of a gel sample can be calculated from the bar density, as determined by the preceding procedure, using the following ratio for a silica gel: bar density = 1 / (pore volume) + 1 / PSÍO2) Compacted Density The compacted density of the gel samples was determined by the following procedure. 1.0 g of the material to be analyzed was placed in an oven at 140 SC for 4-6 hours to physically remove the combined water. The dried material was milled slightly to produce a fine powder. Then approximately 0.1-1.0 g of the Bl K fi fi O OMir powder and were emptied into a graduated cylinder of 10 ce. The cylinder was struck lightly 200 times in total in order to efficiently compact the material. The volume occupied by the material was noted. The density compacted was obtained by dividing the weight of the material by the volume occupied.

Determination of Theoretical Density The theoretical density refers to the density of a dry sample that would be obtained if there were no contraction of the sample during drying. The theoretical density is calculated from the solids content (percentage by weight of the sample) in the solution, the density of the solid phase of the sample and the density of the liquid phase of the liquid in the solution. In the case of a wet gel, the theoretical density would be as shown below: Theoretical Density = wt% solids / [wt% s61j_dos / psolid + (100-wt% solids / pdrying agent)] where: t% solids = Percent by weight of the solids of the gel in the solution. Psolid + = the density of the solid phase of the gel Pdrying agent = the density of the liquid phase of the drying agent.

Smoke Black Analytical The nitrogen surface area (N2SA) of the carbon blacks used in the examples, expressed as square meters per gram (m2 / g) was determined in accordance with the test procedure D3037 Method A of the ASTM. The dibutyl phthalate (DBP) adsorption value of the carbon blacks used in the examples, expressed as millimeters per 100 grams of carbon black (ml / 100g), was determined in accordance with the procedure set forth in ASTM D2414.

Properties of the Black Smoke The carbon black CB-A used in the following examples is a carbon black produced by Cabot Corporation, Boston, Massachusetts, which has an N2SA of 24 m2 / g and a DBP of 132 ml / 100g. A modified CB-A carbon black was produced using the following procedure. Two hundred grams of CB-A was added to a solution of 10.1 g of sulphanilic acid and 6.23 g of concentrated nitric acid in 21 g of water. A solution of 4.87 g of NaN02 in 10 g of water was added to the mixture in rapid stirring. The internal salt 4-sulfobenzenediazonium hydroxide was formed in situ, which reacts with the carbon black. After 15 minutes, the dispersion was dried in an oven at 125 C.

The resulting carbon black product was designated as "Modified CB-A" and is a carbon black having 4-C6H4S03- groups attached.

BET Surface Area The BET surface area of a gel composition can be determined using the test procedure D1993 of the ASTM.

Sodium Analysis The sodium content analysis of the wet gels described in the following examples was performed using a specific sodium ion electrode Model 710A, manufactured by Orion Research of Boston, Massachusetts.

Residual Water Content The residual water content of the wet gels was determined by gas chromatography using Hewlett Packard Model 5890 Gas Chromatography, manufactured by Hewlett Packard, Inc., Palo Alto, California. The features and advantages of the process of the present invention are further illustrated by the following examples.

Examples 1-26 Examples 1-26 illustrate processes for P1606 / 99MX produce gel compositions that include processes of the present invention and comparative processes. The examples illustrating a process of the present invention are identified as "Example #", the comparative examples, which illustrate other processes, are identified as "Comparative Example #". Except where indicated, the processes were performed at ambient temperatures of approximately 20 ° C and ambient pressures of approximately 12.2 psia in Albuquerque, New Mexico. The drying agents used in each process were selected from the following group and had the properties set out below: DI ene / QQM-V V.P. = vapor pressure; ? H = Vaporization Heat; ? ? H cal / cm3 = heat of vaporization per unit volume; MW = molecular weight Three different techniques were used to dry the wet gels. The first drying technique used was vacuum drying, where the wet gels were placed in a vacuum chamber at 25 ° C and dried while sucking a vacuum approximately up to 10 Torr. The second drying technique used was oven drying, where the wet gels were placed in an oven that was maintained at a temperature of 140 ° C. The third drying technique used was the fluidized bed drying. The wet gels were milled to an average size of approximately 250 microns and placed in a 10 cm diameter fluidized bed dryer. Gaseous, dry nitrogen was blown through the wet gel granules, which had an initial temperature of 16-20 ° C at a drying rate of 400 cubic feet / hour. As the gels dried, the gel particles were collected and analyzed by the techniques described herein.

EXAMPLE 1 This example illustrates a process of the present invention for producing a gel composition comprising silica and carbon black. The carbon black used was the modified CB-A that has the properties described above.

Si cc / OQ Y A silica stock solution was prepared by mixing a commercially available sodium silicate (with a molar ratio of SiO 2 / Na 2 de of 3.3: 1 from the PQ Corporation), with deionized water in a volume ratio of 5%. water to sodium silicate in such a way that the weight percent of silica when neutralized with mineral acid was 5%. A separate solution consisting of 2M H2SO4 was prepared by diluting concentrated sulfuric acid (J.T. Baker ^ LO 98%) with water. An aliquot of the sodium silicate stock solution was then added slowly to an appropriate amount of stirred 2M acid, such that the resulting silica sol had a pH of about 1.3 to 2.0. At this point, added carbon black CB-A to the sun, so that the total solids content (silica + carbon) remained at 5% and the carbon content as a percent of the total solids was 15%. The addition rate of silica remained constant at 1 milliliter / minute and the acid solution was maintained at 15 ° C in a jacketed beaker. The gelation was achieved by the continuous addition of sodium silicate solution until the pH of the sun rose to 5.0. In this point, The sun was stirred vigorously for 2-5 minutes and then slipped or emptied into cylindrical tubes. The gelation occurred from 5 to 15 minutes and the tubes were sealed to prevent premature drying. HE P1606 / 99HX allowed the gels to rest for 1-2 hours at 50 ° C in the molds, after which they were removed and placed in sealed tubes containing deionized water and kept at room temperature atmosphere. New water was added every 5 hours for a total of 20 hours at which time it was determined, by using a sodium electrode (previously described), that the sodium sulfate salt had been removed from the gel sufficiently. The ^ pLO gels then rested at 80 ° C in deionized water for 1 hour. After the removal of the stove, the gels were rinsed several times with deionized water and placed in tubes sealed with tert-butanol and exchange of the pore fluid was allowed for 6 hours at 50 ° C. This is repeated until the residual water content of the gel reaches approximately 0.5% by volume. The gels were then placed in a chamber and dried under vacuum. The resulting materials had a bulk density of 0.10 g / cm3. These results illustrate that the process of the present invention can be used to produce gel compositions with apparent densities below 0.27 g / cm3.

Comparative Employment 2 PlßOß / QQMY The steps of Example 1 were repeated essentially with minor exceptions. The water was removed from the gel by washes with tert-butanol, until the residual water content of the gel reached about 5% by volume. The gels were then placed in a vacuum chamber and dried. The resulting materials had a bulk density of 0.35 g / cm3. These results illustrate that the increase in flLO the water content of the wet gel will reduce the effectiveness of the process of the present invention to produce gel compositions with apparent densities below 0.27 g / cm3.

Example Co parati o 3_ The steps in example 1 were repeated essentially with minor exceptions. Ethanol was used instead of tert-butanol to remove water from the gel by washing. This was repeated until the The residual water content of the gel reached approximately 0.5% by volume. The gels were then placed in a chamber and dried under vacuum. The resulting materials had a bulk density of 0.34 g / cm 3. These results illustrate that gel compositions with higher bulk density were obtained when a drying agent with properties outside the preferred ranges is used.

Comparative Example 4 The steps of Example 1 were repeated essentially with minor exceptions. Acetone was used instead of tert-butanol to remove water from the gel by washing. This was repeated until the residual water content of the gel reached approximately 0.5% by volume. The gels were then placed in a chamber and dried under vacuum. The resulting materials had a bulk density of 0.35 g / cm3. These results illustrate that gel compositions with higher bulk density were obtained when a drying agent with properties outside the preferred ranges is used.

Comparative Example 5_ The steps of Example 1 were repeated essentially with minor exceptions. Isopropyl alcohol was used instead of tert-butanol to remove water from the gel by washing. This was repeated until the residual water content of the gel reached approximately 0.5% by volume. The gels were then placed in a chamber and dried under vacuum. The resulting materials had a bulk density of 0.32 g / cm3. These results illustrate that gel compositions with higher bulk density were obtained when a drying agent with properties outside the preferred ranges is used.

Comparative Example 5 The steps of Example 1 were repeated essentially with minor exceptions. After exchanges in tert-butanol, the gels were placed in a conventional oven at 140 ° C to dry. The resulting materials had a bulk density These results indicate that drying in an oven with ter-butanol drying agent produced gel compositions with higher apparent densities than with vacuum drying. Comparative Example 7 The steps of Example 6 were essentially repeated with minor exceptions. Ethanol was used instead of tert-butanol to remove the water gel per wash. This was repeated until the residual water content of the gel reached approximately 0.5% by volume, after which the gels were placed in a conventional oven at 140 ° C to dry. The resulting materials had a bulk density of 0.31 g / cm 3. These results indicate that oven drying with ethanol drying agent produced gel compositions with lower apparent densities than with vacuum drying.

Comparative Example 8_ 5 The steps of Example 6 were repeated essentially with minor exceptions. Acetone was used instead of tert-butanol to remove water from the gel by washing. This was repeated until the residual water content of the gel reached approximately 0.5% by volume, after which the gels were placed in a conventional oven at 140 ° C to dry. The resulting materials had a bulk density of 0.29 g / cm3. These results indicate that drying in an oven with acetone drying agent produced gel compositions with lower apparent densities than with vacuum drying.

Comparative Example 9_ The steps of Example 6 were repeated essentially with minor exceptions. Isopropyl alcohol was used instead of tert-butanol to remove water from the gel by washing. This was repeated until the residual water content of the gel reached approximately 0.5% by volume, after which the gels were placed in a conventional oven at 140 ° C to dry. The resulting materials had a bulk density of 0.30 g / cm3. These results indicate that oven drying with isopropyl alcohol drying agent produced gel compositions with lower apparent densities than with vacuum drying.

Example 10 The steps in Example 1 were essentially doubled with a few minor exceptions. In this example, the appropriate amounts of sodium silicate and sulfuric acid were combined where the solids content was kept at 5%, however, no carbon black was added to this sample. After replacing water with tert-butanol, the gels were then placed in a chamber and dried under vacuum. The resulting materials had a bulk density of 0.09 g / cm3. These results indicate that a process of the present invention can be used to produce a gel composition with low bulk density.

Example 11 The steps in Example 1 were essentially doubled with a few minor exceptions. In this example, the appropriate amounts of sodium silicate and sulfuric acid were combined in P1606 / 99MX where the solids content was adjusted to 8%. No carbon black was added to this sample. After replacing water with tert-butanol, the gels were then placed in a chamber and dried under vacuum. The resulting materials had a bulk density of 0.11 g / cm 3. These results indicate that a process of the present invention can be used to produce a gel composition with low bulk density.

Example 12 The steps of Example 1 were essentially doubled with a few minor exceptions. In this example, the appropriate amounts of sodium silicate and sulfuric acid were combined where the solids content was adjusted to 10%. No carbon black was added to this sample. After replacing water with tert-butanol, the gels were then placed in a chamber and dried under vacuum. The resulting materials had a bulk density of 0.13 g / cm3. These results indicate that a process of the present invention can be used to produce a gel composition with low bulk density.

E emplo 13 P1606 / 99MX The steps in Example 1 were essentially doubled with a few minor exceptions. In this example, the appropriate amounts of sodium silicate and sulfuric acid were combined where the solids content was adjusted to 12%. No carbon black was added to this sample. After replacing water with tert-butanol, the gels were then placed in a chamber and dried under vacuum. The resulting materials had a bulk density of 0.15 g / cm3. These results indicate that a process of the present invention can be used to produce a gel composition with low bulk density.

Example 14 The steps of Example 1 were essentially doubled with a few minor exceptions. After replacing the water with tert-butanol, the gels were placed in a fluidized bed chamber and dried by passing dry nitrogen through the wet material. The temperature of the incoming nitrogen was maintained between 16 and 25 ° C. The nitrogen flow used was 100 SCFH for 50g of initial wet material. After removing the sample from the chamber, it was placed in a convection oven at 140 ° C for 1-2 hours. The resulting materials had a compacted density of 0.065 g / cm3. These results indicate that a process of the present invention can be advantageously used in conjunction with fluidized bed drying to produce gel compositions with low densities.

Example 15 The steps of Example 14 were duplicated essentially with a few minor exceptions. The temperature of the incoming nitrogen is set at 30 ° C. The nitrogen flow used was 100 SCFH for 50g of initial wet material. After removing the sample from the chamber, it was placed in a convection oven at 140 ° C for 1-2 hours. The resulting materials had a compacted density of 0.05 g / cm3. These results indicate that a process of the present invention can be used in a Advantageous together with fluidized bed drying to produce gel compositions with low densities.

Example 16 The steps of Example 14 were essentially doubled with a few minor exceptions. The temperature of the incoming nitrogen is set at 40 ° C. The nitrogen flow used was 100 SCFH for 50g of initial wet material. After removing the sample from the chamber, it was placed in a convection oven at 140 ° C for 1-2 hours. The resulting materials had a compacted density of 0.085 g / cm 3. These results indicate that a process of the present invention can be advantageously used in conjunction with fluidized bed drying to produce gel compositions with densities ^ pA low.

Example 17 The steps of Example 14 were essentially doubled with a few minor exceptions.

The temperature of the incoming nitrogen is set at 50 ° C. The nitrogen flow used was 100 SCFH for 50g of initial wet material. After removing the sample from the chamber, it was placed in a convection oven at 140 ° C for 1-2 hours.

The resulting materials had a compacted density of 0.09 g / cm3. These results indicate that a process of the present invention can be advantageously used in conjunction with fluidized bed drying to produce gel compositions with low densities.

Example 18 The steps of Example 14 were essentially doubled with a few minor exceptions. The temperature of the incoming nitrogen is set at 60 ° C. The nitrogen flow used was 100 SCFH for 50g of initial wet material. After removing the sample from the chamber, it was placed in a convection oven at 140 ° C for 1-2 hours. The resulting materials had a compacted density of 0.10 g / cm3. These results indicate that a process of the present invention can be advantageously used in conjunction with fluidized bed drying to produce gel compositions with low densities. 15 Example 19 The steps in Example 14 were essentially doubled with a few minor exceptions. The temperature of the incoming nitrogen is set at 20 ° C for 3-5 minutes and subsequently rises to 2-3 ° C / min. The nitrogen flow used was 100 SCFH for 50g of initial wet material. After removing the sample from the camera, this was placed in a convection oven at 140 ° C for 1-2 hours. 25 The resulting materials had a compacted density of 0.05 g / cm3. These results indicate that a process of the present invention can be advantageously used in conjunction with fluidized bed drying to produce gel compositions with low densities.

Co-operative Example 20 The steps in Example 10 were essentially doubled with minor exceptions. Ethanol was used instead of tert-butanol to remove water from the gel by washing. This was repeated until the residual water content of the gel reached approximately 0.5% by volume, after which they were placed in a fluidized bed chamber and dried by passing dry nitrogen through the wet material. The resulting materials had a compacted density of 0.252 g / cc. These results illustrate that gel compositions of higher compacted density are obtained when a drying agent with properties outside the preferred ranges is used.

Comparative Example 21 The steps of Example 10 were essentially doubled with minor exceptions. Acetone was used instead of tert-butanol to remove water from the gel by washing. This was repeated until the residual water content of the gel reached approximately 0.5% by volume, after which they were placed in a fluidized bed chamber and dried by passing dry nitrogen through the wet material. The resulting materials had a compacted density of 0.324 g / cc. These results illustrate that gel compositions of higher compacted density are obtained when a drying agent with properties outside the preferred ranges is used.

Comparative Example 22 The steps of Example 10 were essentially doubled to form a wet gel comprising gel and water solids. The wet gel was then dried directly by placing the wet gel in a fluidized bed chamber and drying it by passing dry nitrogen through the wet material. The resulting materials had a compacted density of 0.663 g / cc. These results illustrate that gel compositions of higher bulk density are obtained when a drying agent with properties outside the preferred ranges is used.

Comparative Example 23 The steps of Example 10 were essentially doubled with a few minor exceptions.

In this example, the appropriate amounts of sodium silicate and sulfuric acid were combined and the solids content was kept at 5%. In addition, the synthesis of this material was adjusted in such a way that 22 liters of sol were formed which eventually became a gel. This material was then reduced in size by a reasonable number of times 5 of washing and solvent exchange. After the water was replaced by tert-butanol, a portion of the gels was placed in a fluidized bed chamber and dried by passing dry nitrogen through the wet material. The resulting ÉV-0 materials had a compacted density of 0.07 g / cm3 and a surface area of 826 m2 / g. These results indicate that a process of the present invention can be used advantageously together with bed drying fluidized to produce gel compositions with low densities and high surface areas.

Comparative Example 24 The steps of Example 10 were doubled with essentially a few minor exceptions. In this example, the appropriate amounts of sodium silicate and sulfuric acid were combined, such that the resulting solids content was 8% based on the silica. In this composition no carbon black was included and the gels were left for 18 hours at 60 ° C. The rest of the steps were kept equal to the steps of Example 14. The resulting materials had a compacted density of 0.09 g / cm 3 and a surface area of 655 m2 / g. These results indicate that a process of the present invention can be advantageously used in conjunction with fluidized bed drying to produce gel compositions with low densities and high surface areas.

Comparative Example 25 The steps of Example 10 were essentially doubled with a few minor exceptions. In this example, the appropriate amounts of sodium silicate and sulfuric acid were combined, in such a way that the resulting solids content was 10% based on the silica. In this composition no carbon black was included and the gels were left for 18 hours at 60 ° C. The rest of the steps were kept equal to the steps of Example 14. The resulting materials had a compacted density of 0.12 g / cm3 and a surface area of 655 m2 / g. These results indicate that a process of the present invention can be advantageously used in conjunction with fluidized bed drying to produce gel compositions with low densities and high surface areas.

Comparative Example 26 The steps of Example 10 were essentially doubled with a few minor exceptions.

In this example, the appropriate amount of sodium was 12% based on the silica. In this composition no carbon black was included and the gels were left for 18 hours at 60 ° C. The rest of the other steps remained the same as in Example 14. The resulting materials had a compacted density of 0.15 g / cm 3 and a surface area of 765 m2 / g. These results indicate that a process of the present invention can be advantageously used in conjunction with fluidized bed drying to produce gel compositions with low densities and high surface areas.

Comparative Example 27 The steps of Example 10 were essentially doubled with a few minor exceptions. In this example, the appropriate amounts of sodium silicate and sulfuric acid were combined and the solids content was kept at 5%. In addition, the synthesis of this material was adjusted in this way that 22 liters of the sol were formed, which eventually became a gel. This material was then reduced in size for a number of reasonable times of washing and solvent exchange. After the water was replaced by tert-butanol, the gel was placed in a freezer at -14 ° C for 1 or 2 days to freeze the gel. A portion of the frozen gel pellets was placed in a fluidized bed chamber and dried by passing dry nitrogen through the wet material. The resulting materials had a compacted density of 0.147 g / cm3. These results indicate that a process of the present invention can be advantageously used in conjunction with freeze drying. The results of each of the preceding examples are summarized in the following Table 1.

TABLE 1 »4 co * = water content of 5% by weight ** = frozen Eg. = Example Ex. Comp. = Comparative example NM = CB = carbon black, 15% by weight was not measured These results were analyzed in the description of each example. It should be clearly understood that the forms of the present invention described herein are illustrative only and are not intended to limit the scope of the invention.

Claims (18)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following CLAIMS t 1 is claimed as property. A process for producing gel compositions comprising: drying a wet gel comprising gel solids and a drying agent to remove the drying agent under drying conditions sufficient to minimize shrinkage of the gel during drying at a pressure less than or equal to 300 psia.
2. 2. The process according to claim 1, wherein the bar density of the wet gel compositions is less than or equal to 115% of the theoretical density of the gel solids in the wet gel.
3. 3. The process according to claim 1, wherein the drying conditions are sufficient to produce a dry gel composition having a bar density less than or equal to 0.27 g / cc.
4. 4. The process according to claim 1, wherein the drying conditions are sufficient to produce a dry gel composition having a compacted density less than or equal to 0.2 g / cc.
5. 5. The process for producing gel compositions comprising: drying a wet gel comprising gel solids and a drying agent to remove the drying agent, wherein the Piiquid / Psyidium ratio at the freezing point of the drying agent is 0.95 to 1.05.
6. The process according to claim 5, wherein the vapor pressure at the freezing / melting point of the drying agent is equal to or greater than 1 Torr.
7. 7. The process according to claim 5, wherein the drying is conducted at a pressure less than or equal to 3000 psia.
8. The process according to claim 5, wherein the drying conditions are sufficient to produce a dry gel composition having a bar density less than or equal to 0.27 g / cc.
9. The process according to claim 5, wherein the drying conditions are sufficient to produce a dry gel composition having a compacted density less than or equal to 0.2 g / cc.
10. 10. The process according to claim 1, wherein the dry gel composition is hydrophobic.
11. 11. The process according to claim 5, wherein the dry gel composition is hydrophobic.
12. 12. The process according to claim 1, wherein the drying agent is t-butanol.
13. The process according to claim 5, wherein the drying agent is t-butanol.
14. The process according to claim 1, wherein the gel composition comprises silica.
15. 15. The process according to claim 14, P160S / 99MX wherein the gel composition further comprises carbon black.
16. 16. The process according to claim 5, wherein the gel composition comprises silica.
17. 17. The process according to claim 16, wherein the gel composition further comprises carbon black.
18. 18. A product produced by the process of claim 1.