Method for manufacturing silica aerogel

Abstract

This invention provides a method for producing powdered silica aerogel using a sol-gel reaction, which allows for the production of silica aerogel powder with relatively uniform particle size without the need for grinding. [Solution] A method for producing silica aerogel comprises: a sol production step of mixing a silica raw material containing an alkyl group-containing alkoxysilane with water, an acid catalyst, and a lower alcohol to produce a sol; a gel precursor solution preparation step of adding a basic catalyst solution, obtained by dissolving a basic catalyst that is poorly soluble in a hydrophobic solvent in a lower alcohol, to the sol to prepare a gel precursor solution; an emulsion production step of adding a hydrophobic solvent to the gel precursor solution and stirring, causing the polycondensation reaction of the sol to proceed and the gel precursor solution to thicken, thereby producing an emulsion in which gel particles are dispersed in the hydrophobic solvent; and a drying step of filtering the emulsion and drying the filtered solid to obtain silica aerogel powder.

Description

Technical Field

[0001] The present disclosure relates to a method for producing silica aerogel.

Background Art

[0002] Silica aerogel is useful as a material such as a heat insulating material because of its low thermal conductivity. Silica aerogel is used in a powdered form, compression-molded into a predetermined shape, or added to a paint or the like. As a method for producing powdered silica aerogel, for example, Patent Documents 1 and 2 describe a method in which alkoxysilane or the like is hydrolyzed to produce a sol, which is then gelled, aged, solvent-exchanged, washed, dried, and finally pulverized. Patent Document 3 describes a method in which alkoxysilane is hydrolyzed to produce a sol, which is then gelled, pulverized, subjected to a hydrothermal reaction and solvent exchange, and then dried.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] As described in the above patent documents, in order to produce silica aerogel powder, a pulverization treatment is required after gelation or after drying. However, according to the pulverization treatment, there is a problem that the particle size distribution of the obtained powder becomes broad and it is difficult to align it to have a desired particle diameter. There is also a problem that the shape is likely to become uneven and there are many angular particles and the like.

[0005] Furthermore, in conventional manufacturing methods, solvent replacement is performed beforehand using an organic solvent with low surface tension to suppress gel shrinkage when removing the solvent from the gel during the drying process. Also, if impurities remain in the gel during drying, they may act as flocculants, potentially preventing the shrunken pores from returning to their original state. Therefore, the gel may be washed with an organic solvent to remove impurities (specifically, basic catalysts used for gelation, unreacted raw materials, etc.). However, solvent replacement and washing use large amounts of organic solvents, resulting in a significant environmental burden.

[0006] This disclosure has been made in view of the above circumstances, and its first objective is to provide a method for producing powdered silica aerogel using a sol-gel reaction that can produce silica aerogel powder with relatively uniform particle size without the need for grinding. Its second objective is to provide a method that does not require solvent replacement after gelation. [Means for solving the problem]

[0007] (1) The present disclosure is a method for producing silica aerogel, comprising: a sol production step of mixing a silica raw material containing an alkyl group-containing alkoxysilane with water, an acid catalyst, and a lower alcohol to produce a sol; a gel precursor solution preparation step of adding a basic catalyst solution, obtained by dissolving a basic catalyst that is poorly soluble in a hydrophobic solvent in a lower alcohol, to the sol to prepare a gel precursor solution; an emulsion production step of adding a hydrophobic solvent to the gel precursor solution and stirring, thereby causing a polycondensation reaction of the sol to proceed and thicken the gel precursor solution, thereby producing an emulsion in which gel particles are dispersed in the hydrophobic solvent; and a drying step of filtering the emulsion and drying the filtered solid to obtain silica aerogel powder.

[0008] In the gel precursor liquid preparation step, a basic catalyst solution, obtained by dissolving a basic catalyst in a lower alcohol, is added to the sol. The basic catalyst is a catalyst for gelling the sol. In the manufacturing method of this disclosure, the basic catalyst used is one that is soluble in solvents that are easily miscible with water (hydrophilic solvents) such as lower alcohols, but is sparingly soluble in solvents that are not easily miscible with water (hydrophobic solvents). As a result, even if the gel precursor liquid containing the basic catalyst solution is mixed with the hydrophobic solvent in the subsequent emulsion formation step, the basic catalyst does not migrate to the hydrophobic solvent side and is used in the polycondensation reaction (gelling reaction) of the sol.

[0009] In the emulsion formation process, when a hydrophobic solvent is added to the gel precursor liquid and stirring is started, it initially separates into two layers: the upper layer is the hydrophobic solvent and the lower layer is the gel precursor liquid. After a predetermined time, the polycondensation reaction of the sol proceeds, and the viscosity of the gel precursor liquid increases.

[0010] Generally, when two polymer components 1 and 2 with different viscosities η are blended in volume fractions φ1 and φ2, it is known that if α, defined by equation (i), is approximately 1, both phases become continuous phases; if α is greater than 1, component 1 becomes a dispersed phase and component 2 becomes a continuous phase; and if α is less than 1, component 1 becomes a continuous phase and component 2 becomes a dispersed phase (GM Jordhamo, JA Manson, and LH Sperling, Polym. Eng. Sci., 26, 517 (1986)).

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[0011] When this finding is applied to the emulsion process, with the gel precursor liquid as component 1 and the hydrophobic solvent as component 2, if the volume fraction φ1 of the gel precursor liquid is equal to or less than the volume fraction φ2 of the hydrophobic solvent, the viscosity η1 of the gel precursor liquid increases to become greater than the viscosity η2 of the hydrophobic solvent, at which point the gel precursor liquid becomes a dispersed phase and the hydrophobic solvent becomes a continuous phase. That is, a sea-island structure is formed between a dispersed phase consisting of gel particles from the gel precursor liquid and a continuous phase consisting of the hydrophobic solvent. In this way, an emulsion in which gel particles are dispersed in the hydrophobic solvent is produced. Then, in the drying process, silica aerogel powder can be obtained by drying the solid material filtered from the emulsion. In other words, according to the manufacturing method of this disclosure, silica aerogel powder with relatively uniform particle size can be produced without grinding.

[0012] (2) In the above configuration, at least a portion of the gel particles produced in the emulsion production step may be spherical. In the emulsion produced, gel particles dispersed in a hydrophobic solvent tend to become spherical in an attempt to reduce their surface energy. According to this configuration, silica aerogel powder having spherical silica aerogel particles can be obtained.

[0013] (3) In any of the above configurations, the gel particles produced in the emulsion production step may be hydrophobic, and at least a portion of the lower alcohol contained in the gel particles may be replaced with the hydrophobic solvent.

[0014] The silica raw material used in the sol formation process contains an alkoxysilane having an alkyl group. By using an alkoxysilane having an alkyl group, hydrophobicity is imparted to the gel particles produced in the emulsion formation process. As a result, in the emulsion formation process, the solvent (lower alcohol) in the gel particles mixes with the hydrophobic solvent, and it is thought that at least a portion of it is replaced by the hydrophobic solvent. Therefore, with this configuration, shrinkage of the gel particles in the drying process can be suppressed and the pore structure can be maintained without performing a separate solvent replacement treatment. In addition, when the solvent in the gel particles is replaced with the hydrophobic solvent, it is thought that the basic catalyst contained in the gel particles is also released to the outside along with the solvent. In this case, it is not necessary to perform a washing treatment to remove the basic catalyst.

[0015] (4) In any of the above configurations, the silica raw material may include an alkoxysilane without an alkyl group, and the content ratio of the alkoxysilane with an alkyl group to the alkoxysilane without an alkyl group in the silica raw material may be 5:5 to 9:1 in molar ratio.

[0016] By using a predetermined amount of alkoxysilane without alkyl groups, the gel particle skeleton (a three-dimensional network structure formed by the linkage of primary particles) can be strengthened. Therefore, this configuration is suitable for suppressing shrinkage of gel particles during drying and maintaining the pore structure.

[0017] (5) In any of the above configurations, the lower alcohol in the sol generation step and the gel precursor liquid preparation step may be one or more selected from methanol, ethanol, and isopropanol.

[0018] (6) In any of the above configurations, the hydrophobic solvent in the emulsion formation step may be one or more selected from hydrocarbon solvents.

[0019] (7) In the configuration of (6) above, the hydrocarbon solvent may be configured to include hexane, heptane, and toluene.

[0020] (8) In any of the above configurations, the basic catalyst may be one or more selected from quaternary ammonium salts, aqueous ammonia, and alkali metal hydroxides.

[0021] (9) In any of the above configurations, in the emulsion formation step, the amount of the hydrophobic solvent added to the gel precursor liquid may be 1.5 times or more and 3 times or less in volume ratio with respect to the amount of the lower alcohol contained in the gel precursor liquid.

[0022] When the gel precursor liquid and the hydrophobic solvent are mixed, in order to form a dispersion phase composed of gel particles obtained by gelation of the gel precursor liquid and a continuous phase composed of the hydrophobic solvent, it is desirable that the amount of the hydrophobic solvent added to the gel precursor liquid is more than the amount of the lower alcohol contained in the gel precursor liquid. On the other hand, even if the amount of the hydrophobic solvent is increased excessively, the influence on the formation of the two phases is small, and only an increase in the amount of waste liquid is caused. This configuration is suitable for generating an emulsion in which gel particles are dispersed in the hydrophobic solvent.

[0023] (10) In any of the above configurations, the stirring speed of the liquid in the emulsion formation step may be 400 rpm or more and 2000 rpm or less. This configuration is suitable for generating gel particles having a desired particle size.

[0024] (11) In any of the above configurations, the emulsion formation step may be performed at a temperature of 25°C or more and 60°C or less. According to this configuration, the polycondensation reaction of the sol can easily proceed at an appropriate rate.

[0025] (12) In any of the above configurations, after the emulsion formation step, it may have an aging step of allowing the emulsion to stand or be stirred to age the gel particles. According to this configuration, the polycondensation reaction can further proceed in the gel particles to strengthen the skeleton. As a result, the effect of suppressing the shrinkage of the gel particles during drying is enhanced.

[0026] (13) In any of the above configurations, the average particle size of the silica aerogel powder may be 30 μm or more and 300 μm or less. This configuration makes it easy to use the silica aerogel powder when it is compressed into a predetermined shape or added to paints, etc., to manufacture heat insulating materials. It is also preferable from the viewpoint of suppressing the shedding of silica aerogel particles. [Effects of the Invention]

[0027] According to the method for producing silica aerogel described herein, silica aerogel powder with relatively uniform particle size can be produced without grinding. Furthermore, silica aerogel powder can be produced without solvent replacement treatment after gelation. [Brief explanation of the drawing]

[0028] [Figure 1] This is an SEM image (magnification 300x) of silica aerogel particles obtained by the manufacturing method of Example 1. [Modes for carrying out the invention]

[0029] The method for producing silica aerogel described herein will be described in detail below. The method for producing silica aerogel described herein is not limited to the following forms and can be implemented in various forms with modifications and improvements that can be made by those skilled in the art, without departing from the gist of this disclosure. Numerical ranges using "~" in this specification indicate a range that includes the numbers written before and after "~" as the minimum and maximum values, respectively. In numerical ranges described stepwise in this specification, the lower or upper limit of one numerical range may be replaced with the lower or upper limit of another numerical range described stepwise.

[0030] The method for producing silica aerogel according to this disclosure comprises a sol generation step, a gel precursor liquid preparation step, an emulsion generation step, and a drying step. Each step will be described below.

[0031] <Sol formation process> This process involves mixing a silica raw material containing an alkyl group-containing alkoxysilane with water, an acid catalyst, and a lower alcohol to produce a sol.

[0032] The silica raw material may be one or more types, as long as it contains an alkoxysilane having an alkyl group. An alkoxysilane having an alkyl group is a silane compound with 1 to 3 siloxane bonds (the number of oxygen atoms (O) bonded to one silicon atom (Si)). These silane compounds are also called monofunctional silane compounds, difunctional silane compounds, and trifunctional silane compounds. By using an alkoxysilane having an alkyl group, hydrophobicity is imparted to the gel particles produced in the emulsion formation process.

[0033] From the perspective of easily designing the skeleton and pore structure of the resulting silica aerogel to a desired state, it is desirable to use two or more types of alkoxysilanes with different numbers of siloxane bonds as silica raw materials. For example, using alkoxysilanes without alkyl groups can strengthen the skeleton of the gel particles. In addition, it is possible to increase the proportion of pores with relatively small diameters, which is effective in improving heat insulation. Alkoxysilanes without alkyl groups are silane compounds with 4 siloxane bonds. On the other hand, if the content of alkoxysilanes with alkyl groups is high, the ratio of "-O-Si-O-" bonds decreases, and the proportion of pores with relatively large diameters increases. For example, it is desirable that the content ratio of alkoxysilanes with alkyl groups to alkoxysilanes without alkyl groups be 5:5 to 9:1 in molar ratio. In other words, it is desirable that the content of alkoxysilanes without alkyl groups (tetrafunctional silane compounds) be between 100 mol% and 50 mol% when the total silane compound is 100 mol%.

[0034] Among alkoxysilanes having alkyl groups, monofunctional silane compounds include methoxytrimethylsilane, isopropoxytrimethylsilane, ethoxytrimethylsilane, tert-butoxytrimethylsilane, ethoxytriethylsilane, methoxydimethyl(phenyl)silane, trimethyl(vinyloxy)silane, and isopropenyloxytrimethylsilane. Difunctional silane compounds include dialkoxysilane and diacetoxysilane. The number of carbon atoms in the alkoxy group of dialkoxysilane is preferably 1 to 9. Examples include dimethyldimethoxysilane, diethyldimethoxysilane, and diisobutyldimethoxysilane. Trifunctional silane compounds include trialkoxysilane and triacetoxysilane. The number of carbon atoms in the alkoxy group of trialkoxysilane is preferably 1 to 9. Examples include methyltrimethoxysilane, ethyltrimethoxysilane, propyltrimethoxysilane, hexyltrimethoxysilane, octyltrimethoxysilane, methyltriethoxysilane, ethyltriethoxysilane, propyltriethoxysilane, pentyltriethoxysilane, hexyltriethoxysilane, and octyltriethoxysilane.

[0035] Examples of tetrafunctional silane compounds that are alkoxysilanes without alkyl groups include tetraalkoxysilanes and tetraacetoxysilanes. The alkoxy group of tetraalkoxysilanes preferably has 1 to 9 carbon atoms. Examples include tetramethoxysilanes, tetraethoxysilanes, tetrapropoxysilanes, and tetraisopropoxysilanes.

[0036] The acid catalyst is a catalyst used to promote the hydrolysis of the silica raw material, and can be selected from inorganic acids, organic acids, and organic acid salts. Examples of inorganic acids include hydrochloric acid, sulfuric acid, sulfurous acid, nitric acid, hydrofluoric acid, phosphoric acid, phosphorous acid, hypophosphorous acid, bromate, chloric acid, chlorous acid, and hypochlorous acid. Examples of organic acids include carboxylic acids such as acetic acid, formic acid, propionic acid, oxalic acid, malonic acid, succinic acid, citric acid, malic acid, adipic acid, and azelaic acid. Examples of organic acid salts include acidic aluminum phosphate, acidic magnesium phosphate, and acidic zinc phosphate. One or more types of acid catalysts may be used. The amount of acid catalyst should be between 0.01% and 0.1% by mass, based on 100% by mass of the total silica raw material.

[0037] As the lower alcohol, any alcohol with 5 or fewer carbon atoms may be used. From the viewpoint of excellent miscibility with water, methanol, ethanol, and isopropyl alcohol are examples. The method of mixing the silica raw material, water, acid catalyst, and lower alcohol is not particularly limited. For example, an aqueous solution of the acid catalyst may be prepared in advance, the silica raw material may be added and stirred, and then the lower alcohol may be added. Alternatively, the silica raw material may be added to an aqueous solution containing water, acid catalyst, and lower alcohol. The temperature and time suitable for sol formation should be appropriately determined depending on the type and amount of silica raw material and acid catalyst used, for example, it may be carried out at a temperature of 15 to 50°C for several minutes to about one hour.

[0038] <Gel precursor solution preparation process> This process involves preparing a gel precursor solution by adding a basic catalyst solution, obtained by dissolving a basic catalyst that is poorly soluble in a hydrophobic solvent in a lower alcohol, to the resulting sol.

[0039] The basic catalyst is one that is poorly soluble in the hydrophobic solvent used in the subsequent emulsion formation step. For example, it can be selected from quaternary ammonium salts such as tetramethylammonium hydroxide, tetraethylammonium hydroxide, and tetrapropylammonium hydroxide, alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide, and aqueous ammonia. Among these, quaternary ammonium salts are preferred for their ease of handling. One or more basic catalysts may be used. The amount of basic catalyst should be between 0.07% and 0.7% by mass, based on 100% by mass of the total silica raw material. The lower alcohol may be the same as or different from the one used in the previous sol formation step.

[0040] Basic catalysts are catalysts that promote the polycondensation reaction of sols, and the time it takes for the polycondensation reaction to begin varies depending on the concentration of the basic catalyst solution (amount of basic catalyst used). For example, it is preferable to prepare the gel precursor solution so that the time until the polycondensation reaction begins (gelation start time) at room temperature is about 20 to 30 minutes. In this specification, unless otherwise specified, room temperature is assumed to be 20°C ± 5°C.

[0041] <Emulsion Formation Process> This process involves adding a hydrophobic solvent to the prepared gel precursor liquid and stirring it, which causes a polycondensation reaction of the sol to proceed, increasing the viscosity of the gel precursor liquid and generating an emulsion in which gel particles are dispersed in the hydrophobic solvent.

[0042] This step should preferably be started before the gel precursor liquid begins the polycondensation reaction at room temperature. As a hydrophobic solvent, an organic solvent with a relatively low surface energy is desirable, considering the ease of drying in the subsequent drying step. For example, solvents with a surface tension of 30 mN / m or less at 20°C are suitable. Examples of low surface tension solvents include aliphatic hydrocarbons such as pentane, hexane, and heptane; aromatic hydrocarbons such as benzene, toluene, and xylene; ethers such as ethyl ether, propyl ether, and isopropyl ether; and ketones such as acetone and methyl ethyl ketone. Among these, hydrocarbon solvents are preferable from the viewpoint of excellent workability. For example, hexane, heptane, and toluene are suitable.

[0043] When a gel precursor solution is mixed with a hydrophobic solvent, the gel precursor solution forms two phases: a dispersed phase consisting of gelled gel particles and a continuous phase consisting of the hydrophobic solvent. Therefore, it is desirable that the amount of hydrophobic solvent added to the gel precursor solution is greater than the amount of lower alcohol contained in the gel precursor solution. For example, the amount of hydrophobic solvent added should be 1.5 to 10 times the volume of lower alcohol contained in the gel precursor solution. Using 3 times or less is preferable as it reduces the amount of waste liquid.

[0044] The particle size of the resulting silica aerogel particles can be adjusted by controlling the stirring speed in this process. For example, to produce gel particles (ultimately silica aerogel particles) with a particle size of approximately 30-300 μm, the stirring speed in this process should be set to between 400 rpm and 2000 rpm. From the viewpoint of allowing the polycondensation reaction of the sol to proceed sufficiently and producing spherical gel particles, and ultimately spherical silica aerogel particles, the stirring time should be at least three times the gelation start time of the gel precursor liquid. For example, if the gelation start time is 20 minutes, stirring should be performed for 60 minutes or more. It is even more preferable to set the stirring time to four times or more, or even six times or more, the gelation start time. Furthermore, from the viewpoint of allowing the polycondensation reaction of the sol to proceed at an appropriate rate and producing gel particles of the desired size, this process should be carried out at a temperature between 25°C and 60°C.

[0045] When stirring is initiated in this process, the mixture initially separates into two layers: a gel precursor liquid at the bottom and a hydrophobic solvent at the top. Continuing the stirring process, as the gel precursor liquid reaches the gelation start time, a polycondensation reaction of the sol proceeds, increasing the viscosity of the gel precursor liquid. This forms a two-phase system consisting of a continuous phase of the hydrophobic solvent and a dispersed phase of gel particles, generating an emulsion in which gel particles are dispersed in the hydrophobic solvent. Because the gel particles are dispersed in the hydrophobic solvent, they tend to become spherical in an attempt to reduce their surface energy. In this specification, "spherical" is not limited to perfectly spherical shapes, but includes shapes close to spherical (almost spherical). In this process, the lower alcohols contained in the gel particles mix with the hydrophobic solvent, and it is thought that some or all of them are replaced by the hydrophobic solvent. Additionally, the basic catalyst contained in the gel particles may also be released to the outside along with the lower alcohols.

[0046] After this process, a maturation step may be performed in which the emulsion is allowed to stand or stirred to mature the gel particles. Performing a maturation step strengthens the skeleton of the gel particles by further polycondensation reactions, thereby increasing the effect of suppressing shrinkage of the gel particles during drying. The maturation step is preferably performed while applying energy such as heat to the emulsion for a predetermined time. The maturation step is preferably performed at a temperature below the boiling point of the hydrophobic solvent. For example, it is preferable to perform it at a temperature of 50°C or higher. On the other hand, to avoid deterioration of the working environment due to excessively high temperatures and decomposition of the basic catalyst, it is preferable to perform it at a temperature of 120°C or lower. The standing time should be between 12 hours and 60 hours, or even between 24 hours and 48 hours. The maturation step is preferably performed in a sealed container to suppress evaporation of the solvent.

[0047] <Drying process> This process involves filtering the generated emulsion and drying the filtered solid to obtain silica aerogel powder.

[0048] Suitable filter media for filtration include filter paper and mesh made from cellulose, glass fiber, nylon, etc. The type of filter paper should preferably be type 1 or type 2 as specified in JIS P3801:1995. Using type 3 or higher will increase the filtration time. For nylon mesh, a mesh opening of 15-28 μm is preferable.

[0049] Drying of the filtered solid can be carried out under atmospheric pressure, but to shorten the drying time, it may be carried out under reduced pressure or under pressure below the critical point of the hydrophobic solvent. When drying under atmospheric pressure, it is preferable to dry at a temperature of room temperature to 150°C or below, preferably 100°C or below. Drying should preferably be carried out at a temperature above the boiling point of the liquid used (such as the hydrophobic solvent). Drying may be carried out in multiple stages at different temperatures. The drying time will vary depending on the amount of solid, drying temperature, and other conditions, but it should be about 2 to 6 hours under heating.

[0050] The resulting silica aerogel powder can be used by compressing it into a molded body or by preparing a liquid composition. The average particle size of the silica aerogel powder can be adjusted as appropriate depending on the application, but from the viewpoint of thermal insulation and ease of handling, it is preferable to have an average particle size of 30 μm to 300 μm. In particular, considering the stability of the liquid composition, ease of coating, and suppression of particle shedding, an average particle size of 200 μm or less, and even 120 μm or less, is desirable. The average particle size of the silica aerogel powder can be determined from the median diameter (D) obtained from the volume-based particle size distribution measured by laser diffraction-scattering. 50 ) can be adopted. As mentioned above, in the emulsion formation process, gel particles tend to become spherical. Therefore, the resulting silica aerogel powder will have spherical silica aerogel particles. [Examples]

[0051] Next, the present disclosure will be described in more detail with reference to examples.

[0052] <Example 1> First, a silica raw material consisting of 134.1 g of methyltrimethoxysilane (MTMS) and 36.5 g of tetramethoxysilane (TMOS) (with a molar ratio of MTMS:TMOS of 8:2) was mixed with 6.0 g of a 0.07 mol / L aqueous acetic acid solution as an acid catalyst. The mixture was then stirred at room temperature for 3 minutes to carry out hydrolysis. Next, 400 g of methanol was added to this solution to produce a sol (sol production step).

[0053] Next, a commercially available tetramethylammonium hydroxide (TMAH) aqueous solution was diluted with pure water to prepare a 25% by mass TMAH aqueous solution as a basic catalyst. 4 g of this TMAH aqueous solution and 20 g of methanol were mixed to prepare a basic catalyst solution. This basic catalyst solution was added to the sol and stirred at 400 rpm for 1 minute to prepare a gel precursor solution (gel precursor solution preparation step). The gel precursor solution was prepared so that the time until the polycondensation reaction begins at room temperature (gelation onset time) was approximately 20 minutes.

[0054] Next, 42.5 g of the gel precursor solution was taken out, and 50 g of heptane, a hydrophobic solvent, was added to it. Stirring was then started at 400 rpm at 25°C. The amount of heptane added was twice the volume of methanol contained in the gel precursor solution. Upon starting the stirring, the mixture initially separated into two layers: the lower layer was the gel precursor solution and the upper layer was heptane. After the stirring time had elapsed, the gel precursor solution's gelation onset time, fine spherical gel particles became visible. Stirring was continued for 2 hours to produce an emulsion in which gel particles were dispersed in heptane (emulsion production step).

[0055] Next, the emulsion was transferred to a pressure vessel, and the vessel was placed in a 90°C oil bath and left to stand for another 48 hours to allow the gel particles to mature (maturation process). After that, the pressure vessel was cooled to room temperature, and the emulsion was filtered through filter paper to obtain a solid. The obtained solid was left at room temperature for 24 hours, and then dried at 100°C for 2 hours to remove the solvent contained in the solid. In this way, silica aerogel powder was produced (drying process). When the dried solid (silica aerogel powder) was observed with an optical microscope, spherical particles with a particle diameter of 30 to 150 μm were confirmed. Figure 1 shows a photograph of one of the obtained silica aerogel particles observed with a scanning electron microscope (SEM). Before SEM observation, the sample was treated with platinum deposition, and the observation conditions were an acceleration voltage of 15 kV and a magnification of 300x. As shown in Figure 1, the shape of the silica aerogel particles was almost spherical, and the particle diameter was about 150 μm.

[0056] <Example 2> The only difference between the manufacturing method of Example 2 and the manufacturing method of Example 1 is the change in the maturation process after the emulsion generation process. Specifically, in the manufacturing method of Example 2, the generated emulsion was transferred to a reaction vessel with a stirring mechanism, and the reaction vessel was placed in a 70°C water bath and stirred for a further 200 hours to mature the gel particles. When the obtained silica aerogel powder was observed with an optical microscope, spherical particles with a particle size of 30 to 150 μm were confirmed.

[0057] <Example 3> The main differences between the manufacturing method of Example 3 and the manufacturing method of Example 1 are that the silica raw materials used in the sol formation process were mixed so that the molar ratio of MTMS to TMOS was 6:4, and the maturation method was changed. Specifically, in the manufacturing method of Example 3, the silica raw materials were a mixture of 134.1g of MTMS and 97.4g of TMOS. In the manufacturing method of Example 3 as well, the gel precursor liquid was prepared so that the gelation start time was approximately 20 minutes.

[0058] Then, in the emulsion generation process, 47g of the gel precursor liquid was taken out, and 55g of heptane, a hydrophobic solvent, was added to it, and stirring was started at 400rpm at 25°C. The amount of heptane added was 1.5 times the volume of methanol contained in the gel precursor liquid. In the manufacturing method of Example 3, as in the manufacturing method of Example 1, when stirring was started, it initially separated into two layers: the lower layer was the gel precursor liquid and the upper layer was heptane. After the stirring time had elapsed to the time when the gelation of the gel precursor liquid began, fine spherical gel particles became visible to the naked eye. Stirring was continued for 2 hours to generate an emulsion in which gel particles were dispersed in heptane. Then, the generated emulsion was transferred to a reaction vessel with a stirring mechanism, and the reaction vessel was placed in a 70°C water bath and stirred for a further 200 hours to mature the gel particles. When the obtained silica aerogel powder was observed with an optical microscope, spherical particles with a particle size of 30 to 150 μm were confirmed.

[0059] <Reference example> The only difference between the manufacturing method of the Reference Example and the manufacturing method of Example 1 is that the stirring time in the emulsion formation process was shortened to 40 minutes. That is, in the manufacturing method of the Reference Example as well, stirring was started at 400 rpm at 25°C as part of the emulsion formation process, and after a two-layer separation state, fine spherical gel particles became visible to the naked eye when the stirring time exceeded the time required for the gelation of the gel precursor liquid. However, stirring was limited to 40 minutes, the emulsion was transferred to a pressure vessel, and the pressure vessel was placed in a 90°C oil bath and left to stand for another 48 hours to allow the gel particles to mature. After that, the pressure vessel was cooled to room temperature, and an attempt was made to filter the emulsion with filter paper, but the emulsion had solidified into a mass at the bottom of the pressure vessel, so it was scraped out with a metal spatula. In this manufacturing method, it is thought that the gelation reaction could not proceed sufficiently because the stirring time was short, and the gel particles coalesced into a mass during maturation. The extracted solid was left at room temperature for 24 hours, then dried at 100°C for 2 hours to remove the solvent contained in the solid. This manufacturing method did not yield silica aerogel in powder form. Observation of the dried solid under an optical microscope revealed no spherical particles.

Claims

1. A sol production step involves mixing a silica raw material containing an alkyl group-containing alkoxysilane with water, an acid catalyst, and a lower alcohol to produce a sol. A gel precursor solution preparation step involves adding a basic catalyst solution, obtained by dissolving a basic catalyst that is poorly soluble in a hydrophobic solvent in a lower alcohol, to the sol to prepare a gel precursor solution. An emulsion generation step is to add a hydrophobic solvent to the gel precursor liquid and stir, causing the polycondensation reaction of the sol to proceed and the gel precursor liquid to thicken, thereby generating an emulsion in which gel particles are dispersed in the hydrophobic solvent. A drying step is performed to filter the emulsion and dry the filtered solid to obtain silica aerogel powder. A method for producing silica aerogel, characterized by having the following features.

2. A method for producing silica aerogel according to claim 1, wherein at least a portion of the gel particles produced in the emulsion generation step are spherical.

3. In the emulsion generation step, the gel particles generated are hydrophobic. A method for producing silica aerogel according to claim 1, wherein at least a portion of the lower alcohol contained in the gel particles is replaced with the hydrophobic solvent.

4. The silica raw material comprises an alkoxysilane that does not have an alkyl group. A method for producing silica aerogel according to claim 1, wherein the content ratio of the alkoxysilane having an alkyl group to the alkoxysilane not having an alkyl group in the silica raw material is 5:5 to 9:1 in molar ratio.

5. The method for producing silica aerogel according to claim 1, wherein in the sol generation step and the gel precursor liquid preparation step, the lower alcohol is one or more selected from methanol, ethanol, and isopropanol.

6. The method for producing silica aerogel according to claim 1, wherein in the emulsion formation step, the hydrophobic solvent is one or more selected from hydrocarbon solvents.

7. The method for producing silica aerogel according to claim 6, wherein the hydrocarbon solvent is hexane, heptane, or toluene.

8. The method for producing silica aerogel according to claim 1, wherein the basic catalyst is one or more selected from quaternary ammonium salts, aqueous ammonia, and alkali metal hydroxides.

9. The method for producing silica aerogel according to claim 1, wherein in the emulsion generation step, the amount of the hydrophobic solvent added to the gel precursor liquid is 1.5 times or more and 3 times or less by volume relative to the amount of the lower alcohol contained in the gel precursor liquid.

10. The method for producing silica aerogel according to claim 1, wherein the stirring speed of the liquid in the emulsion generation step is 400 rpm or more and 2000 rpm or less.

11. The method for producing silica aerogel according to claim 1, wherein the emulsion formation step is performed at a temperature of 25°C or higher and 60°C or lower.

12. A method for producing silica aerogel according to claim 1, further comprising a maturation step of allowing the emulsion to stand or stirring after the emulsion generation step to mature the gel particles.

13. The method for producing silica aerogel according to claim 1, wherein the average particle size of the silica aerogel powder is 30 μm or more and 300 μm or less.

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