Aerogel powder and its manufacturing method

Abstract

To provide aerogel powder which can be manufactured at a lower cost per unit occupied volume and has excellent thermal insulation properties.SOLUTION: The aerogel powder of the present invention has a particle size distribution peak at 0.1 to 1.0 μm as measured by laser diffraction type particle size distribution measurement. The aerogel powder of the present invention preferably has two particle size distribution peaks, either having a smaller particle size peak at 0.1 to 1.0 μm or 50% or more of the total number of particles having a particle size of 0.1 to 1.0 μm. The aerogel powder of the present invention is further preferably modified with an organic group, the organic group being, for example, a trimethylsilane group or a triethylsilane group.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a heat insulating material using aerogel and a method for producing the same, and in particular to a method for producing weakly bonded ultrafine aerogel powder, a method for producing weakly bonded ultrafine aerogel powder hybridized with hollow particles, and a heat insulating material using them. [Background technology]

[0002] Aerogel is a material that was first described in 1931 (Non-Patent Document 1) and is generally defined as a gel consisting of a microporous solid with a gas as the dispersed phase.

[0003] Although aerogels are very low-density materials, they are delicate, brittle, and highly fragile, which has made them difficult to apply in many practical situations. Therefore, new types of mechanically robust aerogels are needed for engineering applications such as novel filters, highly insulated windows, ultra-thin walls for refrigerators, and high-performance insulation for buildings. To achieve this goal, hybrid aerogels reinforced with fibers or other organic molecules have been studied. For example, it has been proposed to modify the surface of silica aerogel with organic groups and then naturally dry it to produce a low-density porous aerogel comparable to that produced by supercritical drying. Furthermore, Patent Documents 1 and 2 propose aerogel composites that not only have heat insulating properties but also flexibility.

[0004] However, aerogels suitable for many practical applications have not yet been realized. In particular, aerogels produced by supercritical drying are expensive, and the price factor is also an obstacle to practical applications. On the other hand, the storage and transportation of cryogenic liquids, such as liquid hydrogen and liquid helium, which have lower boiling points (-252.8°C for liquid hydrogen) than the boiling point of liquid nitrogen (-196°C), is becoming increasingly important. Therefore, the emergence of aerogels suitable for the storage and transportation of cryogenic liquids such as liquid hydrogen and liquid helium is anticipated. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] WO2017 / 170498 publication [Patent Document 2] Patent No. 6288382 [Non-patent literature]

[0006] [Non-Patent Document 1] SS Kistler, Nature 1931,127,741. Summary of the Invention [Problem to be solved by the invention]

[0007] As mentioned above, silica-based aerogels have been developed over many years and exhibit a porous structure, resulting in ultra-low thermal conductivity compared to other insulating materials in the world. However, the biggest drawback to applying aerogel is its high manufacturing cost, which limits the uses of aerogel. An object of the present invention is to provide a method for producing weakly bonded ultrafine aerogel powder that can reduce the production cost per unit occupied volume and has excellent heat insulating properties, a method for producing weakly bonded ultrafine aerogel powder hybridized with hollow particles, and a heat insulating material using such aerogel powder. [Means for solving the problem]

[0008] The present inventors came up with the idea that if an aerogel is made to have a very low density, the volume occupied by the same weight can be increased, thereby reducing the production cost, and thus arrived at the present invention. [1] As shown in FIG. 1, the method for producing a weakly bonded ultrafine particle aerogel powder of the present invention includes the steps of: a mixing step of mixing a metal alkoxide with a solvent and hydrolyzing the mixture to produce a sol; a gelling step of gelling the sol obtained in the mixing step; an aging step of aging the gel obtained in the gelling step to produce a wet gel; a solvent substitution step of replacing the solvent in the wet gel with a predetermined substitution solvent; a modification step of modifying the surface of the network structure with a predetermined organic group; a washing step of the modified wet gel obtained in the wet gel production step; a drying step of the washed modified wet gel; and a pulverization step of the dried wet gel. [2] In the method for producing a weakly bonded ultrafine particle aerogel powder of the present invention, the metal of the metal alkoxide preferably contains at least one of silicon (Si), aluminum (Al), titanium (Ti), zirconium (Zr), hafnium (Hf), yttrium (Y), vanadium (V), cerium (Ce), lanthanum (La), neodymium (Nd), samarium (Sm), praseodymium (Pr), holmium (Ho), and molybdenum (Mo). [3] In the method for producing a weakly bonded ultrafine particle aerogel powder of the present invention, the metal alkoxide is preferably a silicon alkoxide. [4] In the method [3] for producing weakly bonded ultrafine particle aerogel powder of the present invention, it is preferable to use at least one of tetraethoxysilane, trimethoxysilane, tetramethoxysilane, triethoxysilane, tripropoxysilane, tetrapropoxysilane, and tributoxysilane as the silicon alkoxide.

[0009] [5] In the method for producing a weakly bonded ultrafine particle aerogel powder according to the present invention, preferably, in the mixing step, the ratio of the silicon alkoxide to the solvent is within a predetermined range. In the mixing step, an acid catalyst may be added to the solvent to promote the hydrolysis reaction. [6] In the method [3] for producing a weakly bonded ultrafine particle aerogel powder of the present invention, it is preferable that an acid catalyst and a base catalyst are added to the mixture of the silicon alkoxide and the solvent in the gelation step. [7] In the method [3] for producing a weakly bonded ultrafine particle aerogel powder of the present invention, preferably, in the aging step, the aging temperature is 15 to 70°C, and the aging time is more than 0 hours and less than 24 hours. [8] In the method [3] for producing a weakly bonded ultrafine particle aerogel powder of the present invention, preferably, in the modification step, the reactive group modifying the surface of the network structure with an organic group has only one or more of halogen, amino group, imino group, carboxyl group, alkoxyl group, hydroxyl group, alkyl group, phenyl group, alkyl fluoride, and phenyl fluoride. [9] In the method [8] for producing weakly bonded ultrafine particle aerogel powder of the present invention, preferably, the compound added as a reagent having a reactive group in the modification step is hexamethyldisilazane, hexamethyldisiloxane, trimethylchlorosilane, trimethylmethoxysilane, trimethylethoxysilane, triethylethoxysilane, triethylmethoxysilane, dimethyldichlorosilane, dimethyldiethoxysilane, methyltrichlorosilane, ethyltrichlorosilane, acetic acid, formic acid, succinic acid, or methyl chloride.

[10] In the method for producing weakly bonded ultrafine particle aerogel powder according to the present invention [9], preferably, the wet gel washing step includes washing the wet gel that has undergone the modification step with a washing liquid so as to remove the reagent having the reactive group from the gel.

[11] In the method for producing a weakly bonded ultrafine particle aerogel powder according to the present invention [3], the drying step is preferably carried out under atmospheric pressure.

[12] In the method [3] for producing a weakly bonded ultrafine aerogel powder of the present invention, preferably, in the pulverization step, the aerogel dried by the method described in

[11] is pulverized so that the weakly bonded ultrafine aerogel contains 50% or more aerogel particles having primary particles as skeleton units, and the remainder is aerogel particles having secondary particles as skeleton units.

[0010]

[13] In the method for producing weakly bonded ultrafine particle aerogel powder according to the present invention [3], the solvent used in the mixing step is preferably at least one of methanol, ethanol, n-propanol, 2-propanol, n-butanol, 2-butanol, and t-butanol.

[14] In the method for producing weakly bonded ultrafine particle aerogel powder according to the present invention [3], the substitution solvent is preferably at least one organic solvent selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, acetone, methyl ethyl ketone, 1,2-dimethoxyethane, acetonitrile, hexane, toluene, diethyl ether, chloroform, ethyl acetate, tetrahydrofuran, methylene chloride, N,N-dimethylformamide, dimethyl sulfoxide, acetic acid, and formic acid, used alone or in combination of two or more.

[0011]

[15] The method for producing a weakly bonded ultrafine particle aerogel powder hybridized with hollow particles of the present invention is the method for producing a weakly bonded ultrafine particle aerogel powder described in [1] or [2], further comprising, between the mixing step and the gelling step, a step of adding hollow particles to a mixture of the metal alkoxide and the solvent in the mixing step.

[16] The method for producing a weakly bonded ultrafine particle aerogel powder hybridized with hollow particles according to the present invention is the method for producing a weakly bonded ultrafine particle aerogel powder according to any one of [3] to

[14] , further comprising a step of adding hollow particles to the mixture of the silicon alkoxide and the solvent prepared as a silica precursor in the mixing step.

[17] In the method for producing a weakly bonded ultrafine particle aerogel powder hybridized with hollow particles according to the present invention

[15] or

[16] , the hollow particles preferably include at least one of nanoparticles having an outer diameter of 30 nm to 360 nm and a shell thickness of 7.5 nm to 65 nm, and microparticles having an outer diameter of 1 μm to 23 μm and a shell thickness of 0.35 μm to 3 μm.

[18] In the method for producing a weakly bonded ultrafine particle aerogel powder hybridized with hollow particles according to the present invention

[17] , the amount of hollow nanoparticles added is preferably 0.01 wt % to 30 wt % of the total weight of the hybridized aerogel.

[19] In the method for producing a weakly bonded ultrafine particle aerogel powder hybridized with hollow particles according to the present invention

[17] , the amount of the hollow microparticles added is preferably 0.01 wt % to 30 wt % of the total weight of the hybridized aerogel.

[0012]

[20] The thermal insulating material of the present invention is characterized in that it uses an aerogel having a three-dimensional network structure whose skeleton is formed by clusters, which are aggregates of primary particles, as a raw material, and contains fine particles having a three-dimensional network structure whose skeleton is formed by the primary particles. Here, primary particles will be explained. In the three-dimensional network structure of conventional aerogel powder particles, the units that make up the skeleton are called secondary particles (see, for example, paragraph 0033 of Patent Document 1). Primary particles are smaller units of particles that gather together to form secondary particles. According to the same document, the diameter of secondary particles is generally 2 nm to 50 μm, while the diameter of primary particles is 0.1 nm to 5 μm. However, as common technical knowledge, there is no universally defined range for the absolute values ​​of the particle diameters of primary particles and secondary particles.

[21] In the heat insulating material

[20] of the present invention, preferably, the aerogel has a three-dimensional network structure having a skeleton formed of primary particles of a metal oxide, and the metal of the metal oxide is at least one oxide of silicon (Si), aluminum (Al), titanium (Ti), zirconium (Zr), hafnium (Hf), yttrium (Y), vanadium (V), cerium (Ce), lanthanum (La), neodymium (Nd), samarium (Sm), praseodymium (Pr), holmium (Ho), or molybdenum (Mo).

[22] In the insulating material

[20] or

[21] of the present invention, preferably, 50% or more of the microparticles are dispersed with a mode particle diameter of 0.1 μm or more and 1.0 μm or less. The particle diameter referred to here is a value measured using a laser diffraction particle size distribution analyzer. Laser diffraction particle size distribution measurement is abbreviated as PSD (particle size distribution) measurement in this specification. In this specification, particle diameters will be explained assuming PSD measurement. However, in PSD measurement, not only the diameter of the particles themselves but also particle agglomerations are observed as particle diameters, so the true particle diameter is likely to be smaller than the measured value. If there are differences in particle diameter depending on the measurement method, the particle diameter may be converted appropriately.

[23] In the heat insulating materials

[20] to

[22] of the present invention, preferably, the heat insulating material further contains hollow particles.

[24] In the heat insulating material

[23] of the present invention, the hollow particles preferably comprise at least one of nano-hollow particles having an outer diameter of 30 nm or more and 360 nm or less, and micro-hollow particles having an outer diameter of 1 μm or more and 23 μm or less.

[25] In the heat insulating material

[23] or

[24] of the present invention, preferably, the hollow particles have a shell, and a gas having a thermal conductivity lower than that of air is sealed in the hollow part inside the shell. [Effects of the Invention]

[0013] The method for producing weakly bonded ultrafine aerogel powder of the present invention can provide ultrafine aerogel powder with an expanded volume occupied by the aerogel powder. The expanded volume ultrafine aerogel powder can fill a larger volume with the same weight, thereby reducing the material cost of the heat insulating material. The method for producing a weakly bonded ultrafine aerogel powder hybridized with hollow particles according to the present invention can provide an ultrafine aerogel powder containing hollow particles, which can be filled into a larger volume with the same weight, thereby reducing the material costs of the heat insulating material. The heat insulating material of the present invention uses ultrafine aerogel powder or weakly bonded ultrafine aerogel powder hybridized with hollow particles, so the powder size is smaller than that of conventional aerogels, and the unit volume occupied by the aerogel material can be increased. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a manufacturing process diagram illustrating an example of an aerogel synthesis process, showing the synthesis process of weakly bonded ultrafine particle aerogel powder. [Figure 2] FIG. 10 is a sequence diagram illustrating a high-speed crushing process. [Figure 3] 1 is an explanatory diagram schematically illustrating the structure of a general aerogel and the aerogel powder produced by pulverizing the general aerogel. [Figure 4] 1 is an explanatory diagram schematically illustrating the aerogel of the present invention and the ultrafine particle aerogel powder produced by pulverizing the aerogel. FIG. [Figure 5] FIG. 1 shows SEM images of commercially available aerogel and weakly bonded ultrafine particle aerogel powder. [Figure 6] FIG. 1 shows the particle size distribution of commercially available aerogel and weakly bonded ultrafine particle aerogel powder. [Figure 7] 1 is an external view of a weakly bonded ultrafine particle aerogel powder according to one embodiment of the present invention. [Figure 8] FIG. 1 is a diagram showing the thermal conductivity of commercially available aerogel and weakly bonded ultrafine particle aerogel powder. [Figure 9] FIG. 1 illustrates the difference in density of aerogel powders under different grinding mechanisms. [Figure 10] 1 shows numerical data of particle size distribution data of weakly bonded ultrafine particle aerogel powder according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] <Definitions> In this specification, a numerical range indicated with "to" indicates a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In numerical ranges described in stages in this specification, the upper or lower limit of a numerical range of a certain stage may be replaced with the upper or lower limit of a numerical range of another stage. In numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with a value shown in the examples. "A or B" may include either A or B, or may include both. Unless otherwise specified, the materials exemplified in this specification can be used alone or in combination of two or more. In this specification, the content of each component in a composition means the total amount of multiple substances present in the composition if multiple substances corresponding to each component are present in the composition, unless otherwise specified.

[0016] <Aerogel> In a narrow sense, a dry gel obtained by applying supercritical drying to a wet gel is called an aerogel, a dry gel obtained by drying under atmospheric pressure is called a xerogel, and a dry gel obtained by freeze-drying is called a cryogel. However, in this embodiment, a low-density dry gel obtained from a wet gel regardless of these drying methods is called an aerogel. That is, in this embodiment, aerogel means a "gel comprised of a microporous solid in which the dispersed phase is a gas," which is an aerogel in a broad sense. <Aerogel Powder> Aerogel powder is a powder obtained by pulverizing aerogel.

[0017] Hereinafter, each step of the method for producing the weakly bonded ultrafine particle aerogel powder of this embodiment will be described. Figure 1 is a manufacturing process diagram illustrating an example of the aerogel synthesis process, showing the synthesis of weakly bonded ultrafine particle aerogel powder. The synthesis process for weakly bonded ultrafine particle aerogel powder includes the mixing, gelation, aging, solvent exchange, modification, washing, and grinding steps. Silica aerogel is produced through two main steps: the formation of a wet gel using sol-gel chemistry and the drying of the wet gel. The wet gel consists of a liquid solvent and a three-dimensional network framework of silica nanostructures formed by the hydrolysis and condensation of silica precursor molecules. By controlling the parameters of the aging and grinding processes, the volume of the aerogel powder can be expanded.

[0018] (Mixing process) The mixing step is a step of mixing a metal alkoxide (e.g., silicon alkoxide) with a solvent and hydrolyzing the mixture to produce a sol. In the mixing step, an acid catalyst may be added to the solvent to promote the hydrolysis reaction. In the case of a method for producing a weakly bonded ultrafine particle aerogel powder hybridized with hollow particles, the mixing step refers to a step of mixing a metal alkoxide (e.g., silicon alkoxide) with a solvent containing hollow particles and hydrolyzing the mixture to produce a sol.

[0019] The metal alkoxide preferably contains at least one metal element selected from the group consisting of silicon (Si), aluminum (Al), titanium (Ti), zirconium (Zr), hafnium (Hf), yttrium (Y), vanadium (V), cerium (Ce), lanthanum (La), neodymium (Nd), samarium (Sm), praseodymium (Pr), holmium (Ho), and molybdenum (Mo). The silicon alkoxide may have at least one of a hydrolyzable functional group and a condensable functional group, or may have both a hydrolyzable functional group and a condensable functional group. As the silicon alkoxide, for example, at least one of tetraethoxysilane, trimethoxysilane, tetramethoxysilane, triethoxysilane, tripropoxysilane, tetrapropoxysilane, and tributoxysilane may be used.

[0020] Examples of hydrolyzable functional groups include alkoxy groups. Examples of condensable functional groups (excluding functional groups that fall under the category of hydrolyzable functional groups) include hydroxyl groups, silanol groups, carboxyl groups, and phenolic hydroxyl groups. The hydroxyl group may be contained in a hydroxyl group-containing group such as a hydroxyalkyl group. Each of the hydrolyzable functional groups and condensable functional groups may be used alone or in combination of two or more types.

[0021] As the solvent, for example, water or a mixture of water and an alcohol can be used. Examples of alcohols include methanol, ethanol, n-propanol, 2-propanol, n-butanol, 2-butanol, and t-butanol. Among these, alcohols with low surface tension and low boiling points, such as methanol, ethanol, and 2-propanol, are preferred in terms of reducing the interfacial tension with the gel wall. These may be used alone or in combination of two or more. The weight ratio of the solvent to the silicon alkoxide is preferably, for example, 1 to 1.5 times.

[0022] Examples of base catalysts include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide; ammonium hydroxide, etc. Among these, ammonium hydroxide (aqueous ammonia) is highly volatile and unlikely to remain in the aerogel after drying, which makes it less likely to impair water resistance, and is also advantageous from the standpoint of economy. The above base catalysts may be used alone or in combination of two or more.

[0023] Examples of acid catalysts include inorganic acids such as hydrofluoric acid, hydrochloric acid, nitric acid, sulfuric acid, sulfurous acid, phosphoric acid, phosphorous acid, hypophosphorous acid, bromic acid, chloric acid, chlorous acid, and hypochlorous acid; acid phosphates such as aluminum acid phosphate, magnesium acid phosphate, and zinc acid phosphate; and organic 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. Among these, organic carboxylic acids are preferred as acid catalysts that further improve the water resistance of the resulting aerogel composite. Examples of such organic carboxylic acids include acetic acid, but they may also include formic acid, propionic acid, oxalic acid, and malonic acid. These may be used alone or in combination of two or more. The method for producing a weakly bonded ultrafine particle aerogel powder hybridized with hollow particles will be described in detail later.

[0024] (Gelling process) The gelation step is a step in which the sol obtained in the mixing step is gelled and then aged to obtain a wet gel. In this step, a base catalyst can be used to promote gelation. Examples of base catalysts include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide; and ammonium hydroxide. Among these, ammonium hydroxide (aqueous ammonia) is highly volatile and unlikely to remain in the aerogel after drying, and is also advantageous in terms of economy. The above base catalysts may be used alone or in combination of two or more.

[0025] The use of a base catalyst can accelerate the dehydration condensation reaction or dealcoholization condensation reaction of the metal alkoxide in the sol, thereby enabling the gelation of the sol to be completed in a shorter time. In particular, the use of ammonia as a base catalyst can easily produce a weakly bonded ultrafine particle aerogel.

[0026] (ripening process) The aging step may be carried out in a sealed container to prevent the solvent and the base catalyst from volatilizing. The aging strengthens the bonds between the components that make up the wet gel, resulting in a wet gel with sufficient strength to suppress shrinkage during drying. Table 1 shows the conditions for the manufacturing process of the weakly bonded ultrafine particle aerogel according to one embodiment of the present invention, and defines the range of variables in the aging process. As shown in Table 1, the aging temperature can be, for example, 15 to 70° C., preferably 20 to 70° C., and more preferably 25 to 60° C. By setting the aging temperature to 15° C. or higher, a wet gel with higher strength can be obtained, and by setting the aging temperature to 70° C. or lower, evaporation of the solvent (especially alcohols) can be easily suppressed, allowing aging to be performed while suppressing volumetric shrinkage. [Table 1]

[0027] The aging time varies depending on the aging temperature. In the case of the method for producing a weakly bonded ultrafine particle aerogel powder hybridized with hollow particles, the aging time can be particularly shortened compared to conventional aerogel manufacturing methods because hollow particles are contained in the sol. The aging time can be, for example, 1 second to 24 hours, preferably 1 second to 12 hours, and more preferably 1 to 3 hours. If the aging time is 1 second to 3 hours, a weakly bonded wet gel is more likely to be obtained, and if it is 3 to 24 hours, a stronger bonded wet gel is more likely to be obtained.

[0028] In the aging step, the aging temperature may be increased within the above ranges or the aging time may be increased within the above ranges to increase the density or bond strength of the resulting aerogel. Alternatively, the aging temperature may be decreased within the above ranges or the aging time may be decreased within the above ranges to decrease the density or bond strength of the resulting aerogel composite.

[0029] (Solvent substitution process) The solvent substitution process is a process in which the solvent in the wet gel is replaced with a predetermined substitution solvent. Heating the process can improve substitution efficiency. Specifically, the substitution solvent can be a low-surface-tension solvent when the drying process is performed at a temperature below the critical point of the solvent used for drying under atmospheric pressure.

[0030] In the solvent substitution step, a solvent with low surface tension can be used. Low surface tension solvents generally have extremely low mutual solubility with water. Therefore, when a low surface tension solvent is used in the solvent substitution step, various organic solvents can be used, such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, acetone, methyl ethyl ketone, 1,2-dimethoxyethane, acetonitrile, hexane, toluene, diethyl ether, chloroform, ethyl acetate, tetrahydrofuran, methylene chloride, N,N-dimethylformamide, dimethyl sulfoxide, acetic acid, and formic acid. The above organic solvents may be used alone or in combination of two or more. Alternatively, a hydrophilic organic solvent having high mutual solubility in both water and a solvent having low surface tension may be used. Examples of hydrophilic organic solvents include methanol, ethanol, 2-propanol, acetone, and methyl ethyl ketone. From the viewpoint of economy, methanol, ethanol, or methyl ethyl ketone may be used.

[0031] (modification process) The modification process involves modifying the surface of the network structure with organic groups, and using the repulsive forces between the organic groups after drying to create a low-density porous structure. TMCS (Trimethylchlorosilane) is an example of an organic compound used in the modification process. Details of the modification process are described, for example, in Koji Tajiri's "Research on the Preparation of Silica Aerogel and Its Modified Forms, and Evaluation of Their Structure and Thermal and Mechanical Properties" (Doctoral Dissertation, Nagoya Institute of Technology (2002), pp. 56-73).

[0032] The reactive group modifying the surface of the network structure with an organic group is preferably a hydrolyzable functional group or a condensable functional group, but is not limited thereto. It may also be a reactive group in a silicon alkoxide having a hydrolyzable functional group or a condensable functional group, which further has a reactive group different from the hydrolyzable functional group and the condensable functional group (a functional group that does not fall under the category of the hydrolyzable functional group or the condensable functional group). Examples of the reactive group include an epoxy group, a mercapto group, a glycidoxy group, a vinyl group, an acryloyl group, a methacryloyl group, and an amino group. The epoxy group may be contained in an epoxy group-containing group such as a glycidoxy group.

[0033] Examples of reactive groups that modify the surface of the network structure with organic groups include halogens, amino groups, imino groups, carboxyl groups, alkoxyl groups, hydroxyl groups, alkyl groups, phenyl groups, fluorides of alkyl groups, and fluorides of phenyl groups. Only one type of these reactive groups may be present, or two or more types may be present. Specific examples include organic silane compounds such as hexamethyldisilazane, hexamethyldisiloxane, trimethylchlorosilane, trimethylmethoxysilane, trimethylethoxysilane, triethylethoxysilane, triethylmethoxysilane, dimethyldichlorosilane, dimethyldiethoxysilane, methyltrichlorosilane, and ethyltrichlorosilane. In addition to these, organic compounds such as carboxylic acids such as acetic acid, formic acid, and succinic acid, and alkyl halides such as methyl chloride can also be used.

[0034] (Cleaning process) In the washing step, the wet gel obtained in the wet gel forming step is washed. This washing can be performed repeatedly using, for example, water or an organic solvent. In this case, heating can improve the washing efficiency.

[0035] Examples of the organic solvent that can be used include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, acetone, methyl ethyl ketone, 1,2-dimethoxyethane, acetonitrile, hexane, toluene, diethyl ether, chloroform, ethyl acetate, tetrahydrofuran, methylene chloride, N,N-dimethylformamide, dimethyl sulfoxide, acetic acid, formic acid, etc. The organic solvents listed above may be used alone or in combination of two or more.

[0036] The amount of water or organic solvent used in the washing step can be an amount that can sufficiently replace the solvent in the wet gel and wash it. The temperature environment in the washing step can be a temperature below the boiling point of the solvent used for washing, and for example, when hexane is used, the temperature can be raised to about 30 to 60°C.

[0037] (drying process) In the drying step, the wet gel that has been washed and (if necessary) solvent-substituted as described above is dried, thereby obtaining a weakly bonded aerogel. The drying method is not particularly limited, and known methods such as atmospheric drying, supercritical drying, or freeze drying can be used. Among these, atmospheric drying or supercritical drying can be used from the viewpoint of ease of producing a low-density weakly bonded aerogel. Furthermore, atmospheric drying can be used from the viewpoint of low-cost production. In this embodiment, atmospheric pressure means 0.1 MPa (atmospheric pressure).

[0038] The aerogel of this embodiment can be obtained by drying the solvent-substituted and washed wet gel under atmospheric pressure at a temperature below the critical point of the solvent used for drying. The drying temperature varies depending on the type of substituted solvent. In particular, considering that drying at high temperatures can accelerate the evaporation rate of the solvent and cause large cracks in the gel, the drying temperature can be increased stepwise up to 40 to 150°C, or even 40 to 120°C. The drying time varies depending on the volume of the wet gel and the drying temperature, but can be 4 to 36 hours. In this embodiment, applying a pressure below the critical point to accelerate drying within a range that does not impair productivity is also considered to be atmospheric pressure drying.

[0039] (Crushing process) The pulverization step involves pulverizing the dried aerogel to destroy the secondary particle structure and mechanically reduce it to the primary particle structure, and it is preferable to use a pulverizer. A pulverizer is a unit operation used to break down solid materials into small pieces, and there are many different types of pulverizers, including grinding mills and pulverizers. Crushers use rotating blades to crush particles, and are available in high-speed and low-speed rotations. High-speed rotations can crush particles into smaller pieces. On the other hand, low-speed rotations are suitable for crushing particles that are tightly bound together. Dried aerogel has low thermal conductivity, so there is a high probability that it will become very hot due to the heat generated during crushing. Therefore, it is advisable to equip the crusher with a water-cooling mechanism.

[0040] Grinding mills include bead mills, ball mills, and rod mills. A bead mill is a media agitation grinder that uses beads to nano-disperse and finely grind powders. Slurry and beads (media) are placed in a grinding chamber (vessel), which rotates at high speed with an agitation mechanism, imparting energy to the beads through centrifugal force, and grinding the material particles through shear stress, frictional force, and impact force. It can grind powders of about 100 to 150 μm in size to 1 to several μm, and by using microbeads, it is possible to achieve fine particle size down to several nm. A ball mill is a typical grinding machine for obtaining fine particles. In a ball mill, balls, usually made of sand or metal, are packed into a slightly tilted or horizontally rotating cylinder, and grinding occurs through collisions and friction with the balls. The material to be crushed is placed into one side of the cylinder, and the crushed material is discharged from the other side. Rod mills use rods (metal cylinders) instead of balls as the grinding medium. The material is crushed by the impact of the rods on the material in a rotating drum (body), which makes it less likely to be over-pulverized compared to ball mills and produces products with a relatively uniform particle size.

[0041] Table 2 shows an example of the conditions for the manufacturing process of weakly bonded ultrafine particle aerogel, which is one embodiment of the present invention. Here, the range of variables for the grinding process is defined when a Wonder Crusher WC-3 manufactured by Osaka Chemical Co., Ltd. is used as the fine grinder. As shown in Table 2, the range of variables in the pulverization step is preferably 10,000 to 28,000 rpm, more preferably 10,000 to 25,000 rpm, and most preferably 11,000 to 22,000 rpm. The pulverization time is preferably 1 to 120 minutes, more preferably 3 to 60 minutes, and most preferably 5 to 45 minutes. [Table 2] [Example]

[0042] [Embodiment 1] <Weakly bonded ultrafine particle airgel powder> This embodiment will be described again with reference to the manufacturing process diagram shown in FIG. Silica aerogel is produced mainly through the following two steps: forming a wet gel using the sol-gel method, and then drying the wet gel. The wet gel consists of a nanostructured solid silica network and a liquid solvent, and is produced by hydrolyzing and condensing silica precursor molecules. This silica precursor is produced by mixing TEOS (Tetraethoxysilane) and methanol (mixing process). A total of 6.3 g of oxalic acid (0.01 M) is then added to the mixture, followed by 1.5 g of ammonium hydroxide (NH4OH 0.5 M) to form an alcosol, which gels when left at room temperature (gelation step). Following gelation, the alcogels were aged in methanol at 60°C for 3, 6, and 12 hours (aging step). To prevent complete evaporation of the methanol during the aging step, an excess amount of methanol was added to the gel. The amount of excess methanol added should be determined taking into account the amount of methanol evaporated during the aging step, as well as the aging temperature and time.

[0043] In the solvent substitution process, to avoid reverse reactions during surface modification, the alcogel was immersed in hexane at 60°C for 10 hours. To modify the surface, the hexane-only solvent was replaced with a mixture of hexane and TMCS (Trimethylchlorosilane). The volume ratio of hexane to TMCS was kept constant at 4. In the modification process, the alcogel was immersed in the mixture of hexane and TMCS at 60°C for 24 hours (modification process). Before drying the alcogel, the sample was immersed in pure hexane at 60°C for 6 hours to remove excess TMCS (washing process).

[0044] The final step in creating aerogel is drying (drying process). The drying process consists of three steps, one at 40°C for 4 hours, one at 80°C for 2 hours, and one at 120°C for 1 hour, as shown in Table 3 below. The first step was held at 40°C for 4 hours, the second step at 80°C for 2 hours, and the third step at 120°C for 1 hour, after which the entire furnace was cooled. [Table 3]

[0045] After the drying process, the aerogel sample was subjected to high-speed pulverization (pulverization process). Using a Wonder Crusher WC-3 manufactured by Osaka Chemical Co., Ltd., which corresponds to a fine pulverizer, a program of approximately 5 minutes at a high speed of 11,200 rpm to 21,000 rpm was carried out three times, as shown in Figure 2.

[0046] It should be noted that it is practically difficult to completely distinguish between gelation and aging, and that the aggregation of fine particles proceeds simultaneously with gelation, leading to a reaction similar to aging. Here, the temperature of the alcosol is raised from room temperature to 60°C to promote aging, but in applications where fine particles are preferred, the alcosol temperature may be maintained at room temperature. Furthermore, the aging process may be a short process, such as from one second to one minute, in which case it cannot be clearly distinguished from the gelation process.

[0047] Next, the weakly bonded ultrafine aerogel powder after high speed crushing of the present invention will be described. Each ultrafine aerogel powder of the present invention has a three-dimensional network structure. Conventional aerogel powder particles also have a three-dimensional network structure, but the ultrafine aerogel powder of the present invention has a different skeleton unit. That is, as stated in paragraph 0033 of Patent Document 1, "Aerogel particle 1 is considered to be in the form of a secondary particle composed of a plurality of primary particles," the three-dimensional network structure of conventional aerogel powder particles is composed of secondary particle units, whereas the ultrafine aerogel powder of the present invention is characterized in that the skeleton unit is a primary particle.

[0048] FIG. 3 is an explanatory diagram showing the structure of a general aerogel and the aerogel powder produced by pulverizing it. (A) shows the structure of a general aerogel 30 and a cross section 40 produced when pulverizing it, (B) shows secondary particles 20 that form the skeleton of the three-dimensional network structure, and (C) shows aerogel powder 50 produced by pulverizing the aerogel 30 of (A). As shown in Figure 3(A), in a typical aerogel 30, the colloids contained in the gel before drying become secondary particles 20, forming a three-dimensional network structure 30 with the secondary particles 20 as skeletal units. In the aerogel created in this way, the skeleton forms approximately 10% of the volume of the three-dimensional network structure, and the remaining approximately 90% is made up of pores. When the size of the pores is smaller than the mean free path of the gas, such as air, that fills the pores, almost no heat conduction occurs due to collisions between gas molecules. For this reason, aerogel is used as a thermal insulating material.

[0049] Figure 3(C) shows a schematic diagram of the structure of aerogel powder 50 obtained when a typical aerogel is crushed. When the typical aerogel shown in Figure 3(A) is crushed, the cut surface 40 produced by the crusher is not the secondary particles 20 themselves, but the areas where the secondary particles are connected. This is thought to be because, as shown in Figure 3(B), the secondary particles are tightly agglomerated from primary particles, resulting in strong bonds, while the bonds between secondary particles are much weaker. As a result, when a typical aerogel is crushed, the resulting aerogel powder 50 has a three-dimensional network structure in which the skeleton is formed by the secondary particles 20 (Figure 3(C)).

[0050] FIG. 4 is an explanatory diagram showing the structure of the aerogel 31 of the present invention and the ultrafine aerogel powder 51 produced by pulverizing the aerogel 31, in which (A) shows the three-dimensional network structure of the aerogel 31 produced after the aging process in the process of producing the ultrafine aerogel powder 51 of the present invention, (B) shows the secondary particles 21 that form the skeleton of the three-dimensional network structure of the aerogel shown in (A), (C) shows a cross section 40 when the aerogel 31 shown in (A) is pulverized, and (D) shows the ultrafine aerogel powder 51 of the present invention.

[0051] In the present invention, aerogel is produced with less aging than in conventional processes. As a result, the three-dimensional network structure of the resulting aerogel 31 is formed around a skeleton of secondary particles 21 (FIG. 4(B)), in which primary particles 11 are more sparsely packed than in conventional processes (FIG. 4(A)). When an aerogel having such a three-dimensional network structure 31 (FIG. 4(A)) is subjected to ultra-high-speed pulverization, not only the connections between the secondary particles 21 that make up the skeleton but also the secondary particles 21 themselves, as shown in FIG. 4(C), exhibit cut surfaces 40 caused by the pulverizer, and the secondary particles 21 themselves are pulverized. As a result, the ultrafine particle aerogel powder 51 of the present invention has a three-dimensional network structure in which the skeleton is formed by primary particles 11, as shown in FIG. 4(D). Because the secondary particles 21 are formed by loosely agglomerating primary particles 11 as shown in FIG. 4(B), the outer edges of the actual secondary particles 21 are unclear. However, for ease of understanding, the outer edges are indicated by dashed circles in FIG. 4(C).

[0052] FIG. 5(A) shows an SEM image of a commercially available aerogel, and FIG. 5(B) shows an SEM image of a weakly bonded ultrafine particle aerogel powder according to one embodiment of the present invention.

[0053] Figure 6 is a distribution diagram showing the particle size distribution after the high-speed grinding process. Figure 10 shows the numerical particle size distribution data for the weakly bonded ultrafine particle aerogel powder, an embodiment of the present invention, shown in Figure 6. For each sample with an aging time of 3, 6, and 12 hours, the particle size after the high-speed grinding process is plotted on a logarithmic scale on the horizontal axis, and the frequency (left vertical axis) and cumulative value (right vertical axis) of the relative particle amount are shown. For comparison, data for conventional (commercially available) aerogel powder are also shown. The particle size was measured using PSD measurement. More specifically, Figure 6 shows the results measured using a laser diffraction particle size distribution analyzer, SALD-2300, manufactured by Shimadzu Corporation. Note that in PSD measurement, not only the particle size itself but also particle agglomerations are observed as particle size, so the measured values ​​are biased toward the positive direction (there are many errors in measuring values ​​larger than the true value). However, as described below, sufficient information has been obtained to explain the characteristics of the weakly bonded ultrafine particle aerogel powder of the present invention.

[0054] Conventional aerogel powders have a single peak in the relative particle size distribution, with a mean particle size of approximately 300 μm. In contrast, for the weakly bonded ultrafine particle aerogel powder of this embodiment, the frequency of the relative particle size distribution after the high-speed milling process exhibits a bimodal peak for samples aged for 3, 6, and 12 hours. For the sample aged for 3 hours, the first peak averaged 0.32 μm with a standard deviation of 0.10, and the second peak averaged 21.14 μm with a standard deviation of 0.14. For the sample aged for 6 hours, the first peak averaged 0.66 μm with a standard deviation of 0.15, and the second peak averaged 31.89 μm with a standard deviation of 0.40. For the sample aged for 12 hours, the first peak averaged 0.96 μm with a standard deviation of 0.13, and the second peak averaged 38.52 μm with a standard deviation of 0.21.

[0055] The split into two peaks strongly suggests that there are essential differences in the particles that make up each peak. If there were no essential changes and only the particle size of the resulting particles changed depending on the aging conditions, it would be unlikely that two peaks would appear, even if the peak positions changed accordingly. Therefore, it is natural to assume that the particles that make up the second peak, which have larger particle sizes, have a three-dimensional network structure with secondary particles as the skeletal unit, as in the past, while the particles that make up the first peak, which have smaller particle sizes, have a three-dimensional network structure with primary particles as the skeletal unit. This result supports the explanation given above with reference to Figures 3 and 4.

[0056] Furthermore, by changing the aging conditions, we were able to significantly change, or control, the properties of the particles generated after high-speed milling, i.e., whether secondary particles or primary particles form the structural units of the skeleton. The mode of the dispersion for the larger particle size was 10 μm or greater, while the mode of the dispersion for the smaller particle size was 1 μm or less. For samples aged for 6 and 12 hours and then high-speed milled, the cumulative relative particle mass exceeded 50% on the larger particle size peak side. The cumulative relative particle mass exceeded 50% at approximately 20 μm for the sample aged for 6 hours and then high-speed milled, and at approximately 40 μm for the sample aged for 12 hours and then high-speed milled, both of which were near the second peak side. On the other hand, for the sample aged for 3 hours and then high-speed milled, the cumulative relative particle mass exceeded 50% at approximately 0.3 μm on the smaller particle size peak side (first peak side). From another perspective, in the samples aged for 6 and 12 hours and then crushed at high speed, 60% to 70% of the particles had a diameter of 10 μm or more, and based on their size, they were predominantly composed of secondary particles as their skeleton units. In contrast, in the sample of the weakly bonded ultrafine particle aerogel powder of this embodiment aged for 3 hours, approximately 80% of the particles had a diameter in the range of 0.1 μm to 1.0 μm, meaning that based on their size, they were predominantly composed of primary particles as their skeleton units.

[0057] FIG. 7 is an image illustrating the bulk of the weakly bonded ultrafine aerogel powder of this embodiment, showing the case where 5 g of aerogel is placed in a 200 cc or 500 cc beaker. From the left, commercially available aerogel and ultrafine aerogel powder samples aged for 12 hours, 6 hours, and 3 hours are shown. The ultrafine aerogel powder samples are the same as those described in FIG. 6. When the aging time is short, at 3 hours, the bulk of the ultrafine aerogel powder increases to more than 10 times that of the commercially available aerogel.

[0058] Figure 8 compares the thermal conductivity of commercially available aerogel and weakly bonded ultrafine aerogel powder. The thermal conductivity of commercially available aerogel granules is 20 mW / mK. In contrast, the thermal conductivity of the weakly bonded ultrafine aerogel powder, which is one embodiment of the present invention, is 23 mW / mK.

[0059] FIG. 9 shows the difference in density of aerogel powders using different crushing mechanisms, showing high-speed crushing and ball milling, which are embodiments of the present invention, and low-speed crushing, which is a comparative example. As shown in Figure 9, the density in the high-speed grinder is 0.0179 g / cm 3 The density measured by ball milling was 0.0548 g / cm 3 The density measured using a low-speed grinder was 0.0790 g / cm 3 In other words, the bulk density of the ultrafine particle aerogel powder can be reduced by pulverizing it using a high-speed pulverizer. [Embodiment 2]

[0060] <Weakly bonded ultrafine aerogel powder with hollow particles added> Hollow particles may be further added to the ultrafine particle aerogel powder of the first embodiment, thereby reducing the thermal conductivity of the heat insulating material. By using aerogel as an insulating material, the size of the pores in the aerogel is smaller than the mean free path of air, suppressing heat conduction through gas due to collisions or convection between gas molecules. This is expected to result in thermal insulation performance similar to that of a vacuum. However, in reality, the performance does not match that of a vacuum. The inventors investigated the cause and found that insulation filled with aerogel powder retains fine interconnected pores, and the aforementioned heat conduction occurs through these interconnected pores. Therefore, the inventors developed a technology to reduce thermal conductivity by adding hollow particles to aerogel to create a hybrid (patent application filed as JP 2020-120921). The added hollow particles block these fine interconnected pores, suppressing the slight heat conduction through the gas, thereby reducing thermal conductivity.

[0061] Furthermore, since the spherical shells that make up the hollow particles are highly airtight, the thermal conductivity of the heat insulating material can be further reduced by sealing a gas with a lower thermal conductivity than air inside the spherical shells. The hollow particles to be added are not particularly limited, but may be nanoparticles, microparticles, or both. The nanoparticles are preferably prepared to have an outer diameter of 30 nm to 360 nm and a shell thickness of 7.5 nm to 65 nm. The outer diameter corresponds to a range of approximately 1 / 2 to approximately 5 times the mean free path of air at room temperature and normal pressure. Because the hollow size of the nanoparticles is prepared to be on the same order as the mean free path of air, when added to an aerogel, they contribute significantly to the heat insulating effect. The microparticles are preferably prepared to have an outer diameter of 1 μm to 23 μm and a shell thickness of 0.35 μm to 3 μm. The outer diameter is greater than 15 times the mean free path of air at room temperature and normal pressure, which not only contributes to the heat insulating effect but also enhances the structural strength of the network. Aerogels still have the fine interconnected pores described above, but the added hollow particles block these pores, suppressing heat conduction through gas, such as convection, which would otherwise occur through the pores, thereby improving the insulating effect.

[0062] Hollow nanoparticles can be produced, for example, by the soft template method. Specifically, the surface of a polymer electrolyte is modified with ammonia in ethanol and then coated with silica (SiO2), producing particles consisting of a core and a spherical shell. The medium trapped in the core is removed by washing or calcining the resulting particles, producing hollow particles. The double emulsion method is suitable for producing hollow microparticles. A dispersed multiphase system consisting of an immiscible liquid, such as an oil phase containing a surfactant and an aqueous phase containing a precursor and surfactant, is emulsified to produce an emulsion with the oil phase as the continuous phase and aqueous phase-centered droplets. Adding aqueous phase to this emulsion transforms it into an emulsion with gel-centered droplets in the aqueous phase as the continuous phase. The resulting emulsion is then washed / filtered or calcined to produce hollow microparticles.

[0063] In the method for producing ultrafine particle aerogel powder of the present invention, hollow particles are preferably added to a mixture of TEOS and methanol prepared as a silica precursor (mixing step) before the gelation step in the manufacturing process diagram shown in Figure 1. After the addition, the mixture is preferably thoroughly stirred by ultrasonic vibration to uniformly disperse the added hollow particles. The amount of hollow particles added is adjusted to achieve the following composition relative to the total hybridized aerogel:

[0064] The content of the hollow nanoparticles is preferably 0.01 to 30% by weight, more preferably 0.10 to 15% by weight, and most preferably 1.00 to 10% by weight. Similarly, the content of the hollow microparticles is preferably 0.01 to 30% by weight, more preferably 0.10 to 15% by weight, and most preferably 1.00 to 10% by weight.

[0065] As shown in the second embodiment, by adding hollow particles to the ultrafine particle aerogel powder, the thermal conductivity of the heat insulating material can be reduced.

[0066] In the embodiments of the method for producing weakly bonded ultrafine aerogel powder or the method for producing weakly bonded ultrafine aerogel powder hybridized with hollow particles of the present invention, the case of producing silica aerogel powder has been shown, but the present invention is not limited to the case of using silicon aerogel as the metal aerogel. Various metal elements, namely, metal oxide aerogel containing at least one of silicon (Si), aluminum (Al), titanium (Ti), zirconium (Zr), hafnium (Hf), yttrium (Y), vanadium (V), cerium (Ce), lanthanum (La), neodymium (Nd), samarium (Sm), praseodymium (Pr), holmium (Ho), or molybdenum (Mo) may also be used. Furthermore, although the embodiment of the heat insulating material of the present invention has been described using silica aerogel powder, the present invention is not limited to the use of silicon aerogel as the metal oxide aerogel, and various metal elements, namely, metal oxides containing at least one of aluminum (Al), titanium (Ti), zirconium (Zr), hafnium (Hf), yttrium (Y), vanadium (V), cerium (Ce), lanthanum (La), neodymium (Nd), samarium (Sm), praseodymium (Pr), holmium (Ho), and molybdenum (Mo) may also be used. [Industrial Applicability]

[0067] The method for producing weakly bonded ultrafine particle aerogel powder of the present invention is suitable for use in producing weakly bonded ultrafine particle aerogel powder. Also, the method for producing weakly bonded ultrafine particle aerogel powder hybridized with hollow particles of the present invention is suitable for use in producing weakly bonded ultrafine particle aerogel powder hybridized with hollow particles. The method for producing a weakly bonded ultrafine aerogel powder of the present invention is suitable for use in heat insulating materials. The weakly bonded ultrafine aerogel powder to which hollow particles have been added of the present invention is also suitable for use in heat insulating materials. [Explanation of symbols]

[0068] 11 Primary particles 20 Secondary particles 21 Secondary particles formed by sparsely packed primary particles 30 Three-dimensional network structure formed by a skeleton of secondary particles 31 A three-dimensional network structure formed by a skeleton of secondary particles, each of which is made up of sparsely packed primary particles. 40 Cut surface by crusher 50 General aerogel powder 51 Ultrafine particle aerogel powder of the present invention

Claims

1. forming a gel by hydrolyzing a metal alkoxide; Aging the resulting gel; drying the aged gel; crushing the dried gel; and In the aging step, the aging temperature is 15 to 70°C and the aging time is 12 hours or less, In the pulverizing step, the dried gel is pulverized at a rotation speed of the pulverizing blade of the pulverizing device of 10,000 to 28,000 rpm. A method for producing aerogel powder.

2. 2. The method for producing an aerogel powder according to claim 1, wherein the pulverization step is carried out so that, when a laser diffraction particle size distribution measurement is performed on the obtained aerogel powder, the particle size distribution has a peak in the range of 0.1 to 1.0 μm in terms of the number of particles.

3. 3. The method for producing an aerogel powder according to claim 1, wherein the aging time is 3 hours or less.

4. Further, a step of modifying the gel with an organic group The method for producing an aerogel powder according to claim 1 or 2, comprising:

5. forming a gel by hydrolyzing a metal alkoxide; Aging the resulting gel; a step of drying the aged gel; and The aging temperature in the aging step is 15 to 70°C, and the aging time is 12 hours or less. The aerogel powder thus obtained is pulverized so that, when a laser diffraction particle size distribution measurement is carried out, the particle size distribution has a peak in the range of 0.1 to 1.0 μm in terms of the number of particles.

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