JP7660414B2 - Method for producing spherical silica aerogel - Google Patents

Description

The present invention relates to a method for producing spherical silica aerogel.

Aerogel is a material with high porosity and excellent oil absorption. Aerogel here means a solid material with a porous structure and gas as a dispersion medium, and particularly means a solid material with a porosity of 60% or more. The porosity is the amount of gas contained in the apparent volume expressed as a volume percentage. Aerogel has excellent oil absorption due to the high porosity.
Among them, silica aerogel has various applications, but is useful as a cosmetic material, and in the case of foundation, for example, it is used as an additive to improve the appearance durability when applied to the skin. More specifically, the porous structure of silica aerogel absorbs sebum well, and prevents the skin from getting wet with sebum, which increases the regular reflectance of light and causes shine. Moreover, if silica aerogel is produced by being hydrophobic, it has good affinity with organic components of cosmetic materials such as foundation and disperses uniformly, further enhancing the appearance durability effect of preventing shine.
In addition, it is desirable that the particle size of these silica aerogels be 1 to several tens of μm in order to obtain a smooth feel when blended into cosmetics, and that their shape be spherical in order to improve rolling properties on the skin.

As a method for producing such spherical silica aerogel having an appropriate particle size, for example, the following method has been proposed.
Patent Document 1 discloses a method for producing spherical silica aerogel, which includes the steps of preparing an aqueous silica sol, dispersing the aqueous silica sol in a hydrophobic solvent to form a W/O emulsion, gelling the silica sol to convert the W/O emulsion into a dispersion of a gelled body, replacing the water in the gelled body with a solvent having a surface tension of 30 mN/m or less at 20° C., subjecting the gelled body to a hydrophobizing treatment (silylation treatment) with a hydrophobizing agent (silylating agent), and removing the substituted solvent, in the above order. Note that in Patent Document 1, the silylation treatment of the gelled body is performed in an organic solvent, and there is a problem in that the surface treatment of the gelled body with the silylation agent is insufficient.
Therefore, Patent Document 2 discloses a method for producing spherical silica aerogel, which includes the steps of separating the gelled dispersion obtained in the step of converting the W/O emulsion into a gelled dispersion into two layers, an O phase and a W phase, adding a basic substance to the W phase to age the gelled dispersion in the W phase, silylation of the gelled dispersion in the W phase, extracting the gelled dispersion with a hydrophobic organic solvent, and recovering the gelled dispersion to obtain a powder of hydrophobic spherical silica aerogel. In Patent Document 2, the silylation of the gelled dispersion is carried out in an aqueous solvent, and the surface treatment of the gelled dispersion with a silylation agent can be sufficiently carried out.

International Publication No. 2012/057086 JP 2018-177620 A

However, in the method of Patent Document 2, a large amount of acid is added during the hydrophobization treatment (silylation treatment), so the dispersion becomes strongly acidic, and the inside of a stainless steel reaction tank that is normally used must be lined with glass, resulting in a problem of extremely high equipment costs. In addition, the reaction time is long, so large equipment must be used, which is uneconomical.
However, there was a problem that the reaction did not proceed smoothly when the silylation treatment was performed under neutral conditions, and there was also a problem that the silica particles were dissolved and the physical properties of the obtained silica aerogel were deteriorated when the silylation treatment was performed without adding an acid because the solution became strongly basic (pH 10 to 13).
Therefore, an object of the present invention is to provide a method for producing spherical silica aerogel that can be efficiently silylated using a conventional reaction apparatus.

As a result of extensive research to solve the above problems, the inventors discovered that by adjusting the pH of the dispersion liquid in the silylation treatment step to a range of 8 or more and 10 or less, the reaction can be carried out in a conventional reaction apparatus and the silylation treatment can be carried out efficiently, thereby completing the present invention.
That is, the present invention provides:
(1) a step of preparing an aqueous silica sol; (2) a step of dispersing the aqueous silica sol in a hydrophobic solvent to form a W/O type emulsion; (3) a step of converting the W/O type emulsion into a dispersion of a gel; (4) a step of separating the dispersion into two layers, an O phase and a W phase; (5) a step of adding a basic substance to the W phase to age the gelled body dispersed in the W phase; (6) a step of removing the O phase; (7) a step of silylation-treating the gelled body dispersed in the W phase with a silazane; (8) a step of adjusting the pH of the W phase to 1.0 to 7.0; (9) a step of extracting the silylated gelled body with a hydrophobic organic solvent; and (10) a step of recovering the silylated gelled body to obtain a powder of hydrophobic spherical silica aerogel.

In the method for producing spherical silica aerogel according to the present invention, the reaction in the silylation process, which was previously carried out under acidic conditions, is carried out under basic conditions (pH 8 to 10). This makes it possible to use reaction equipment made of materials that are sensitive to acid, and also allows the use of highly reactive silylation agents, improving the reaction efficiency.

The method for producing spherical silica aerogel of the present invention includes the following steps:
(1) a step of preparing an aqueous silica sol; (2) a step of dispersing the aqueous silica sol in a hydrophobic solvent to form a W/O type emulsion; (3) a step of converting the W/O type emulsion into a dispersion of a gel; (4) a step of separating the dispersion into two layers, an O phase and a W phase; (5) a step of adding a basic substance to the W phase to mature the gelled body dispersed in the W phase; (6) a step of removing the O phase; and (7) a step of subjecting the gelled body dispersed in the W phase to a silylation treatment with silazanes (adjusting the pH of the dispersion to 8 or more and 10 or less).
(8) a step of adjusting the pH of the W phase to 1.0 to 7.0; (9) a step of extracting the silylated gelled body with a hydrophobic organic solvent; and (10) a step of recovering the silylated gelled body and obtaining a powder consisting of hydrophobic spherical silica aerogel.
The above manufacturing method will be described in detail below in an orderly manner.

(1) Step of preparing aqueous silica sol In the manufacturing method of the present invention, an aqueous silica sol is first prepared. Here, silica refers to silicon dioxide, which is a general term for materials composed of silicon dioxide and is represented as _SiO2 .
The aqueous silica sol can be prepared by any known method. One example is a method using an alkali metal silicate, as described below.
When an alkali metal silicate is used, it is preferable to prepare a silica sol by neutralizing it with a mineral acid such as hydrochloric acid or sulfuric acid. Specifically, a method of adding an aqueous solution of an alkali metal silicate to an aqueous solution of an acid while stirring the aqueous solution, or a method of impinging and mixing an aqueous solution of an acid and an aqueous solution of an alkali metal silicate in a pipe (see, for example, JP-B-4-54619). More specifically, the amount of acid used in preparing an aqueous silica sol is preferably 1.05 to 1.2 in terms of the molar ratio of hydrogen ions to the alkali metal content of the alkali metal silicate. When the amount of acid is within this range, the pH of the prepared silica sol is about 1 to 5. More preferably, the amount of acid is adjusted so that the pH of the prepared silica sol is 2.5 to 3.5.

The concentration of the silica sol prepared by the above method is preferably 50 g/L or more in terms of silica content ( _SiO2 equivalent concentration) in order to complete gelation in a relatively short time, to sufficiently form the skeletal structure of the silica particles, to suppress shrinkage during drying, and to easily obtain a large pore volume. On the other hand, in order to relatively reduce the density of the silica particles, to obtain a good pore volume, and to easily increase the oil absorption, it is preferably 160 g/L or less, and more preferably 100 g/L or less. It is even more preferably 90 to 100 g/L.
By setting the concentration of the aqueous silica sol to the above lower limit or more, it becomes easy to set the pore volume of the aerogel obtained by the BJH method to 8 mL/g or less, and to set the peak pore radius of the aerogel by the BJH method to 50 nm or less. In addition, by setting the concentration of the aqueous silica gel to the above upper limit or less, it becomes easy to set the pore volume of the aerogel by the BJH method to 2 mL/g or more, and to set the peak pore radius of the aerogel by the BJH method to 10 nm or more.

(2) Step of dispersing the aqueous silica sol in a hydrophobic solvent to form a W/O type emulsion To produce the spherical silica aerogel of the present invention, the aqueous silica sol prepared as described above is dispersed in a hydrophobic solvent to form a W/O emulsion. By forming such a W/O emulsion, the silica sol becomes spherical due to surface tension, etc., so that the silica sol dispersed in the hydrophobic solvent in the spherical shape can be gelled to obtain a spherical gelled body. In this way, by going through the emulsion formation step of forming a W/O emulsion, it is possible to produce an aerogel having a high circularity of 0.8 or more.

The hydrophobic solvent may be a solvent having hydrophobicity to the extent that it can form a W/O emulsion with the aqueous silica sol. For example, organic solvents such as hydrocarbons and halogenated hydrocarbons can be used as such solvents. More specifically, hexane, heptane, octane, nonane, decane, dichloromethane, chloroform, carbon tetrachloride, dichloropropane, etc. can be mentioned. Among these, heptane having a suitable viscosity can be particularly preferably used. If necessary, a plurality of solvents may be mixed and used. In addition, it is also possible to use a hydrophilic solvent such as lower alcohols in combination (as a mixed solvent) as long as it can form a W/O emulsion with the aqueous silica sol.
The amount of the hydrophobic solvent used is not particularly limited as long as it is an amount that makes the emulsion a W/O type, but generally, the amount of the hydrophobic solvent used is about 1 to 10 parts by volume per part by volume of the aqueous silica sol.

When forming the above W/O emulsion, it is preferable to add a surfactant. The surfactant to be used may be any of anionic surfactants, cationic surfactants, and nonionic surfactants. Among these, nonionic surfactants are preferred because they are easy to form a W/O emulsion. In the present invention, since the silica sol is aqueous, a surfactant having an HLB value of 3 or more and 6 or less, which is a value indicating the degree of hydrophilicity and hydrophobicity of the surfactant, can be suitably used. In the present invention, the "HLB value" means the HLB value according to the Griffin method.
As described above, in the present invention, the shape of the aerogel particles is determined almost entirely by the shape of the droplets of the W/O emulsion. Specific examples of surfactants that can be suitably used include sorbitan monooleate, sorbitan monostearate, and sorbitan monosesquioleate.
The amount of surfactant used is the same as the general amount used to form a W/O emulsion. Specifically, a range of 0.05 g to 10 g per 100 ml of aqueous silica sol can be suitably adopted. If a large amount of surfactant is used, the droplets of the W/O emulsion tend to become finer, and conversely, if a small amount of surfactant is used, the droplets of the W/O emulsion tend to become larger. Therefore, it is possible to adjust the average particle size of the aerogel by increasing or decreasing the amount of surfactant used.

When forming a W/O emulsion, a known method for forming a W/O emulsion can be used as a method for dispersing an aqueous silica sol in a hydrophobic solvent. From the viewpoint of ease of industrial production, it is preferable to form the emulsion by mechanical emulsification, and specifically, a method using a mixer, homogenizer, etc. can be exemplified. A homogenizer can be preferably used. The average particle size of the silica sol droplets in the W/O emulsion and the average particle size of the aerogel are roughly in correspondence with each other. At the same time, by making the particle size of the silica sol droplets in the emulsion sufficiently small in this way, the shape of the silica sol droplets is less likely to be disturbed, making it easier to obtain spherical silica aerogel with a higher circularity.

(3) Step of converting the W/O emulsion into a dispersion of a gel To produce the spherical silica aerogel of the present invention, the silica sol in the W/O emulsion is gelled. Silica sol in the acidic range is easily gelled by heating. Therefore, the gelling can be achieved by heating the W/O emulsion.
The gelation temperature is preferably 50° C. to 80° C., and more preferably 60° C. to 70° C. If the gelation temperature is higher than the above range, the specific surface area will be low, and if it is lower, gelation will not proceed sufficiently.
The gelation time is preferably 30 minutes to 24 hours, and more preferably 5 to 12 hours.
By gelling, the W/O type emulsion is converted into a dispersion of a gel.

(4) Step of separating the dispersion into two layers, O phase and W phase To produce the spherical silica aerogel of the present invention, the gelled body thus obtained is separated and recovered. The operation of separating the O phase and the W phase is generally called demulsification. After demulsification, the gelled body obtained by the above step is dispersed in the W phase.
The demulsification method may be any known method, and specifically may be one or a combination of methods selected from the following: addition of a water-soluble organic solvent, addition of a salt, application of centrifugal force, addition of an acid, filtration, change in volume ratio (addition of water or a hydrophobic solvent), etc. Preferably, a certain amount of a water-soluble organic solvent can be added to the emulsion together with water as necessary to separate it into an O phase and a W phase. After the demulsification process, the upper layer is generally an O phase (organic layer), and the lower layer is a W phase (aqueous layer).

Examples of the water-soluble organic solvent include acetone, methanol, ethanol, isopropyl alcohol, etc. Among these, isopropyl alcohol is preferably used since it is effective in increasing the efficiency of the silylation treatment described below.
The amount of the water-soluble organic solvent added is preferably adjusted depending on the type and amount of the surfactant used in forming the emulsion. For example, when sorbitan monooleate is used as the surfactant for the W/O emulsion, the water-soluble organic solvent is added in an amount of about 0.1 to 0.4 times by mass relative to the amount of the O phase, and the mixture is stirred as necessary and then allowed to stand, whereby the mixture can be suitably separated into the O phase and the W phase. In addition to the water-soluble organic solvent, water is preferably added in an amount of about 0.6 to 0.9 times by mass relative to the amount of the O phase. The temperature at which the separation operation is carried out is not particularly limited, but it can usually be carried out at about 20 to 70°C.

(5) Step of adding a basic substance to the W phase and maturing the gelled material dispersed in the W phase In the production method of the present invention, after obtaining a gelled material dispersed in the W phase as described above, the gelled material is maturated by adding a basic substance to the W phase (in which the gelled material is dispersed) separated from the O phase to adjust the pH of the W phase to a weak acidic or basic state, and maturing the gel.
By adding a basic substance to the W phase, the pH of the W phase, which is in the acidic range, increases to a state of weak acidity or basicity, and specifically, the pH of the W phase is preferably 4.5 to 10, more preferably 5.0 to 8.0, and particularly preferably 6.0 to 7.5. As the basic substance, ammonia, caustic soda, alkali metal silicate, etc. can be used. Among them, caustic soda is preferably used because it can easily adjust the pH.

The gel can be aged by maintaining the aging temperature at room temperature to about 80° C. The aging time may be appropriately set depending on the pH of the W phase and the aging temperature, and is about 0.5 to 12 hours.
As described above, a liquid in which the hydrophilic gel is dispersed in the W phase can be obtained, but if the gel is simply collected by filtration or the like and then dried, the pores will be crushed due to shrinkage and it will not be possible to obtain an aerogel. Methods for suppressing this shrinkage include supercritical drying and a method of hydrophobizing the gel and then drying it, as described below.

(6) Step of Removing O Phase When the gelled body is subjected to silylation treatment, the W phase is separated from the O phase in order to improve the efficiency of the treatment. The method of separation is not particularly limited, but the O phase can be removed by decantation or the like to recover the W phase, which is separated into two phases, the O phase and the W phase. This separation may be performed before the aging.
Here, it is not necessary to completely separate and remove the O phase, but in order to carry out the silylation treatment efficiently in the process of silylation treatment of the gelled body contained in the W phase, it is better for the proportion of the O phase to be as small as possible, and it is preferable for it to be 20 wt % or less of the amount of the W phase, and even more preferably 10 wt % or less.

(7) Step of subjecting the gelled body dispersed in the W phase to silylation treatment with silazanes The gelled body dispersed in the W phase is hydrophobized by silylation treatment with a silylating agent. The spherical silica aerogel obtained by the silylation treatment exhibits hydrophobicity, and shrinkage is suppressed when the gelled body is dried, making it possible to obtain a powder that retains the porous structure of the aerogel.
In addition, when performing the silylation treatment, an acid is added to adjust the pH of the solution to 8 or more and 10 or less, thereby producing a silica aerogel with good physical properties. Examples of the acid that can be used include hydrochloric acid, sulfuric acid, and acetic acid. Of these, hydrochloric acid is preferably used. The order in which the silylation agent and the acid are added does not affect the physical properties.

Here, the silylating agent that can be used in the present invention is a silanol group:
M-OH (2)
[In the formula, M represents a Si atom forming a gelled body. In formula (2), the remaining valences of M are omitted. The same applies to all the following formulas.]
This reacts with (M-O-) _(4-n) SiR _n (3)
[In formula (3), n is an integer of 1 to 3, R is a hydrocarbon group, and when n is 2 or more, the multiple Rs may be the same or different from each other.] can be given as an example. By performing silylation treatment using such a silylation agent, the hydroxyl groups on the surface of the aerogel powder are end-capped with hydrophobic silyl groups and inactivated, so that dehydration condensation reaction between the surface hydroxyl groups can be suppressed. Therefore, even if drying is performed under conditions below the critical point, drying shrinkage can be suppressed, so that it is possible to obtain a silica aerogel having a BJH pore volume of 2 mL/g or more.

Known examples of the above silylating agent include compounds represented by the following general formulas (4) and (5).
R _n SiX _(4-n) (4)
[In formula (4), n represents an integer of 1 to 3; R represents a hydrophobic group such as a hydrocarbon group; and X represents a group (leaving group) that can be cleaved from the molecule by cleaving the bond with the Si atom in a reaction with a compound having a hydroxy group. When n is 2 or more, multiple Rs may be the same or different. Also, when n is 2 or less, multiple Xs may be the same or different.]

［式（５）中、Ｒ^１はアルキレン基を表し；Ｒ^２及びＲ^３は各々独立に炭化水素基を表し；Ｒ^４及びＲ^５は各々独立に水素原子又は炭化水素基を表す。］

[In formula (5), R ¹ represents an alkylene group; R ² and R ³ each independently represent a hydrocarbon group; and R ⁴ and R ⁵ each independently represent a hydrogen atom or a hydrocarbon group.]

In the above formula (4), R is a hydrocarbon group, preferably a hydrocarbon group having 1 to 10 carbon atoms, more preferably a hydrocarbon group having 1 to 4 carbon atoms, and particularly preferably a methyl group.
Examples of the leaving group represented by X include a group represented by -NH- _SiR3 (wherein R has the same meaning as R in formula (4)).
Specific examples of the silylating agent represented by the above formula (4) include hexamethyldisilazane, 1,3-bis(3,3,3-trifluoropropyl)-1,1,3,3,-tetramethyldisilazane, 1,3-bis(chloromethyl)tetramethyldisilazane, 1,1,3,3-tetramethyldisilazane, 1,3-diphenyltetramethyldisilazane, 1,3-divinyl-1,1,3,3-tetramethyldisilazane, etc. Hexamethyldisilazane is particularly preferred in terms of good reactivity.
Depending on the number of leaving groups X (4-n), the number of bonds with hydroxy groups on the aerogel powder skeleton changes. For example, if n is 2:
(MO-) ₂ SiR ₂ (7)
If n is 3, then:
M-O-SiR ₃ (8)
The following bond is formed. By silylating the hydroxy group in this manner, a silylation treatment is carried out.

In the above formula (5), R ¹ is an alkylene group, a group represented by --SiR ₂ R ₃ or a group represented by --(SiR ₂ --NH--SiR ₂ ) _m -- (wherein R has the same meaning as R in formula (4), and m is 1 to 5).
In the above formula (5), ^R2 and ^R3 are each independently a hydrocarbon group, and preferred groups include the same groups as R in formula (4). ^R4 represents a hydrogen atom or a hydrocarbon group, and in the case of a hydrocarbon group, preferred groups include the same groups as R in formula (4). When a gelled body is treated with the cyclic silazanes represented by formula (5), the Si-N bond is cleaved by reaction with the hydroxyl group, and the following structure is formed on the surface of the aerogel powder skeleton in the gelled body ^: (M-O-) _2SiR2R3 ( ⁹ )
In this way, the cyclic silazanes of the above formula (5) also silylate the hydroxyl groups, thereby carrying out the silylation treatment.
Specific examples of the cyclic silazanes represented by the above formula (5) include 2,2,4,4,6,6-hexamethylcyclotrisilazane, octamethylcyclotetrasilazane, 2,4,6-trimethyl-2,4,6-trivinylcyclotrisilazane, and the like.

The amount of the silylating agent used in the above silylation treatment varies depending on the type of the treating agent, but when hexamethyldisilazane is used as the silylation agent, it is preferably 10 to 150 parts by mass per 100 parts by mass of silica (the amount of _SiO2 calculated from the amount of silica sol used), more preferably 20 to 130 parts by mass, and even more preferably 30 to 120 parts by mass.
The above silylation treatment can be carried out by adding a silylation agent to the W phase and reacting for a certain period of time. For example, when hexamethyldisilazane is used as the silylation agent and the treatment temperature is set to 60° C., the treatment can be carried out by maintaining the mixture for about 2 to 3 hours.
In the silylation process, it is preferable to add a water-soluble organic solvent for the purpose of increasing the solubility of the silylation agent in the W phase and increasing the efficiency of the reaction. Examples of the water-soluble organic solvent include acetone, methanol, ethanol, and isopropyl alcohol. Of these, isopropyl alcohol is preferably used. The water-soluble organic solvent is preferably added so that its concentration in the W phase containing the gel is about 20 to 80 wt %.

(8) Step of adjusting the pH of the W phase to 7 or less In the production method of the present invention, an acidic substance is added to the silylated dispersion (W phase) to adjust the pH of the W phase to 7 or less (neutral to acidic). By adding an acidic substance to the W phase, the pH of the W phase, which is in the basic range, drops to a neutral or acidic state. Specifically, the pH of the W phase is preferably 1.0 to 7.0, more preferably 1.5 to 5.0, and particularly preferably 1.5 to 3.0. As the acidic substance, hydrochloric acid, sulfuric acid, etc. can be used. Among them, the use of hydrochloric acid is preferable because the pH adjustment can be easily performed.
The step of adjusting the pH of the W phase to 7 or less can be carried out by holding the W phase at room temperature to about 80° C. The time can be appropriately set depending on the temperature of the W phase, but is about 0.25 to 1 hour.

(9) Step of extracting the silylated gelled body with a hydrophobic organic solvent After adjusting the pH of the W phase to 7 or less, the gelled body is extracted into the hydrophobic organic solvent. This makes it easier to suppress shrinkage during drying. That is, since the hydrophobized gelled body has a high affinity for the hydrophobic organic solvent, it moves to the hydrophobic organic solvent side by adding a hydrophobic organic solvent and stirring or the like. The selection criterion for the hydrophobic organic solvent used for extracting the gelled body is that it has a small surface tension so as not to cause drying shrinkage during subsequent drying. Specifically, hexane, heptane, nonane, decane, dichloromethane, methyl ethyl ketone, toluene, etc. can be used, and hexane, heptane, decane, and toluene can be preferably used. The amount of the hydrophobic organic solvent used may be appropriately set to be 0.5 to 10 times the volume of the W phase.
After extraction into the hydrophobic organic solvent, it is preferable to wash the organic solvent with water or an aqueous solution of alcohol in order to remove salts contained in the gelled body and sulfates contained in the hydrophobic organic solvent. This washing operation can be performed by a known method, and a well-known method can be adopted as a so-called liquid/liquid extraction operation. In order to increase the washing efficiency, it is preferable to use an aqueous solution of isopropyl alcohol of about several tens of wt %. In addition, in order to increase the washing efficiency, it is preferable to raise the temperature to a high level within a range not exceeding the boiling point of the hydrophobic organic solvent. Usually, the washing can be performed in the range of 45 to 70°C.

(10) Step of recovering the silylated gelled body and obtaining a powder of hydrophobic spherical silica aerogel The gelled body from which impurities such as salts have been removed as described above is recovered from the liquid. In the recovery, the gelled body dispersed in the hydrophobic organic solvent may be filtered off. Next, the hydrophobic organic solvent is removed (i.e., dried) from the gelled body obtained by filtration. The drying temperature is preferably equal to or higher than the boiling point of the solvent and equal to or lower than the decomposition temperature of the surface treatment agent such as the silylation agent, and the pressure is preferably normal pressure or reduced pressure.
In this manner, spherical silica aerogel can be produced.

(Spherical silica aerogel)
The powder produced as described above is composed of spherical silica aerogel. The term "silica aerogel powder" means that the powder is composed of 50% or more of silica by mass. The content is preferably 60% or more, and more preferably 75% or more. Components other than silica include components derived from the hydrophobizing agent.

(Average circularity)
Furthermore, according to the above-mentioned production method, the silica aerogel particles are usually spherical with an average circularity of 0.8 or more, and preferably 0.85 or more.
The "average circularity" is a value obtained by obtaining an SEM image observed with a scanning electron microscope (SEM), determining a value C (circularity) defined by the following formula (1) for each particle by image analysis, and calculating the arithmetic mean value of this circularity C for 2000 or more particles. In this case, a particle group forming one aggregate particle is counted as one particle.
C=4πS/L ² (1)
[In the above formula, S represents the area (projected area) that the particle occupies in the image, and L represents the length (perimeter) of the outer periphery of the particle in the image.]
The closer the average circularity is to 1, the closer the particle shape is to a perfect sphere.

The spherical silica aerogel powder produced by the above-mentioned manufacturing method of the present invention usually has the following characteristics in that it is easy to express the inherent properties of aerogel.

(D50)
Regarding the particle size, the volume cumulative 50% diameter (D50) measured by the Coulter counter method is usually in the range of 1 to 30 μm, and can be in the range of 1 to 20 μm, or even in the range of 1 to 10 μm. The particle size depends on the particle size at the time of forming the W/O emulsion.

(Specific surface area)
The specific surface area measured by the BET method using a nitrogen adsorption method is usually 400 to 1000 ^{m 2} /g. The larger the specific surface area of the spherical silica aerogel, the smaller the particle size of the primary particles that make up the porous structure (network structure) of the independent particles, and the greater the thickening effect when used as an additive in cosmetics. A high thickening effect makes it possible to prevent dripping when applied to the skin or hair. Therefore, the specific surface area is preferably 500 m ² /g or more, and more preferably 550 m ² /g or more.
On the other hand, if the specific surface area of the spherical silica aerogel powder is too large, the pore volume becomes small and the oil absorption becomes small, so it is preferably 850 m ² /g or less, and more preferably 750 ^{m 2} /g or less. Usually, it is difficult to obtain an aerogel with a specific surface area exceeding 1000 m ² /g.
In the present invention, the specific surface area measured by the BET method is a value obtained by drying a sample to be measured at a temperature of 150° C. for 2 hours or more under a vacuum of 1 kPa or less, and then obtaining an adsorption isotherm only on the nitrogen adsorption side at liquid nitrogen temperature, and analyzing it by the BET method, and the range of partial pressure (P/P ₀ ) during analysis is 0.1 to 0.25.

(Pore volume)
The pore volume according to the BJH method is usually 2 to 8 ml/g. The larger the pore volume, the better the oil absorption performance, which is preferable. The lower limit is more preferably 2.5 ml/g or more, and particularly preferably 4 ml/g or more. The upper limit is more preferably 6 ml/g or less. If the pore volume is 2 ml/g or less, it is not possible to obtain good oil absorption performance. Also, it is usually difficult to obtain a pore volume exceeding 8 ml/g.
In the present invention, the pore volume of the spherical silica aerogel powder by the BJH method is obtained by obtaining an adsorption isotherm in the same manner as in the BET specific surface area measurement, and analyzing it by the BJH method (Barrett, EP; Joyner, LG; Halenda, PP, J. Am. Chem. Soc. 73, 373 (1951) (hereinafter, sometimes referred to as "BJH pore volume"). The pores measured by this method are pores with a radius of 1 to 100 nm, and the integrated value of the volume of the pores in this range is the pore volume in the present invention.

(Pore radius peak)
The peak pore radius of the spherical silica aerogel powder of the present invention measured by the BJH method is usually in the range of 10 to 50 nm. The peak pore radius is also obtained by obtaining an adsorption isotherm in the same manner as in the BET specific surface area measurement and analyzing it by the BJH method. The peak pore radius is the value of the pore radius at which the cumulative pore volume (volume distribution curve) based on the logarithm of the pore radius has the maximum peak value.

(Oil Absorption)
Silica aerogel is usually characterized by having a high oil absorption, and the spherical silica aerogel of the present invention also has an oil absorption of preferably 400 mL/100 g or more, more preferably 550 mL/100 g or more, and particularly preferably 650 mL/100 g or more. The higher the oil absorption, the more effective it is in preventing shine when used in cosmetics, so it is preferable. The upper limit of the oil absorption is not particularly limited, but is about 750 mL/100 g at most.
In the present invention, the oil absorption is measured by the method described in JIS K6217-4 "Determination of oil absorption."

(Confirmation of hydrophobicity)
Being hydrophobic reduces the adsorption of moisture, which causes deterioration over time, and improves compatibility with hydrophobic resins, making it extremely useful when dispersed in hydrophobic resins. The hydrophobicity of aerogels is also significant from the viewpoint that they can be produced without supercritical drying or solvent replacement. Specifically, the hydrophobization of spherical silica aerogels can be achieved by treating the spherical silica aerogels with a silylating agent to introduce organic silyl groups onto the surface.
Here, whether or not silica aerogel is hydrophobic can be confirmed very easily by putting the powder in a container together with pure water and stirring, etc. If the powder is hydrophobic, it will not disperse in water, and when left to stand, it will regain its two-layer structure with water as the lower layer and powder as the upper layer.

(M value)
Hydrophobicity and its degree can also be evaluated by the M value. The M value is a value measured according to the measurement method described in the Examples. The M value of the powder made of the spherical silica aerogel of the present invention is preferably 30 to 55 vol%, more preferably 35 to 55 vol%, and particularly preferably 40 to 55 vol%.

(Carbon Content)
The carbon content can be cited as one of the indicators showing that the spherical silica aerogel of the present invention is hydrophobic. The carbon content in the hydrophobic spherical silica aerogel is derived from the surface treatment agent and can be measured by quantifying the amount of carbon dioxide generated when the spherical silica aerogel is subjected to oxidation treatment in air or oxygen at a temperature of about 1000 to 1500°C.
The spherical silica aerogel of the present invention preferably has a carbon content of 5 to 12% by mass, more preferably 6 to 10% by mass. A higher carbon content is preferable because it can prevent makeup from being ruined by sweat when the hydrophobic spherical silica aerogel of the present invention is used as an additive for foundation, but it is difficult to obtain a powder made of the hydrophobic spherical silica aerogel of the present invention with a carbon content of more than 12% by mass by a general hydrophobization treatment.

(Application)
The spherical silica aerogel produced by the method of the present invention has an appropriate particle size distribution and specific surface area for use as a cosmetic additive, and when used as a foundation additive, it has excellent appearance retention and a smooth feel. In addition, the silica aerogel has a high oil absorption capacity, efficiently absorbs oil from the skin and scalp surface, and is hydrophobic and has the effect of repelling sweat, so it can be suitably used in cosmetics other than the foundation, such as paste and cream type makeup and skin care cosmetics, and even deodorant products and hair styling products.
Of course, since the toner has the appropriate particle properties, it can be suitably used as a material for various applications such as a heat insulating agent and a matting agent.

In the following, examples are shown to specifically explain the present invention, but the present invention is not limited to these examples. The examples and comparative examples were evaluated by the following methods.
<Evaluation method>
The hydrophobic spherical silica aerogels produced in Examples 1 to 3 and Comparative Examples 1 to 3 were subjected to tests for the following items.

(D50)
The silica aerogel powder was added to ethanol and ultrasonically dispersed for 30 minutes. The obtained ethanol dispersion was measured for D50 using a precision particle size distribution measuring device Multisizer 3 manufactured by Beckman Coulter, Inc., with a 50 μm aperture tube.

(Specific surface area, pore volume and oil absorption)
The BET specific surface area and the BJH pore volume were measured according to the above definitions using a BELSORP-max manufactured by Microtrac-Bell Co., Ltd. The oil absorption was measured according to JIS K6217-4 "Determination of oil absorption."

(M value)
Hydrophobic silica aerogel floats in water but completely dissolves in methanol. Taking advantage of this, the degree of modified hydrophobicity measured by the following method was taken as the M value and used as an index of the hydrophobic treatment by the hydrophobic groups on the silica aerogel surface.
0.2 g of silica aerogel was added to 50 ml of water in a 200 mL beaker and stirred with a magnetic stirrer. Methanol was added to the mixture using a burette, and the end point was when the entire amount of silica aerogel was wetted and suspended in the solvent in the beaker. At this time, the methanol was introduced into the solution using a tube so that it would not come into direct contact with the sample. The volume percent of methanol in the methanol-water mixed solvent at the end point was taken as the hydrophobicity (M value).
M value = amount of methanol dropped (ml) / (amount of methanol dropped (ml) + 50 ml)

(Average circularity)
The silica aerogel powder was observed using a Hitachi High-Technologies SEM (S-5500) at an acceleration voltage of 3.0 kV, secondary electron detection, and a magnification of 1000 times. The obtained SEM image was analyzed to calculate the circularity of the silica aerogel particles using the following formula. The average circularity was calculated by averaging the circularities of 2000 or more silica aerogel particles.
C = 4πS/ ^L2
[In the above formula, S represents the area (projected area) that the particle occupies in the image, and L represents the length (perimeter) of the outer periphery of the particle in the image.]

(Carbon Content)
The carbon content was measured using an elemental analyzer (vario MICRO cube) manufactured by Elementor Japan Co., Ltd.

Example 1
(1) 100 g of sodium silicate was gradually added to 100 g of sulfuric acid while stirring with a stirring blade to prepare an aqueous silica sol. At this time, the pH was 2.9.
(2) 129 g of heptane was added to 139 g of the aqueous silica sol prepared above, and 1.5 g of sorbitan monooleate was added. The solution was stirred for 2.5 minutes at 8600 rpm using a homogenizer to form a W/O emulsion.
(3) The obtained W/O emulsion was gelled at 70° C. for 1 hour while being stirred with a stirring blade.
(4) Subsequently, 77 g of isopropyl alcohol and 52 g of ion-exchanged water were added, and the mixture was stirred with a stirring blade to separate the O phase and the W phase.
(5) Subsequently, 4.83 g of a 0.5 mol/L aqueous sodium hydroxide solution was added. At this time, the pH of the W phase was 6.7. The gel was aged at 60° C. for 1 hour.
(6) The O phase was removed by decantation, and the W phase was recovered.
(7) 1.5 g of 35% hydrochloric acid and 7.7 g of hexamethyldisilazane were added to the W phase obtained, and the mixture was stirred and held in a water bath at 60° C. for 2 hours to carry out a silylation treatment. The pH at this time was 9.3.
(8) After the silylation treatment, 6.6 g of 35% hydrochloric acid was added while stirring with a stirring blade to carry out neutralization treatment. The pH at this time was 2.1.
(9) Next, 90 g of heptane was added to extract the gel, and 77 g of isopropyl alcohol and 52 g of ion-exchanged water were added to wash the extract twice.
(10) The gelled body obtained after silylation was filtered using a suction filter, and dried by heating at 150° C. for 12 hours or more under vacuum pressure, to obtain a powder of hydrophobic spherical silica aerogel of the present invention.
Table 1 shows the pH during the surface treatment, the D50 of the obtained powder, the specific surface area, the pore volume, the peak pore radius, the oil absorption amount, the M value, the carbon content, the average circularity, the surface treatment temperature, and the surface treatment time (the same applies to the following examples).

Example 2
The same procedure as in Example 1 was carried out, except that in the emulsion formation step, stirring was carried out at 4,600 rpm, and in the silylation treatment, the pH was adjusted to 8.7.
Example 3
The same procedure as in Example 1 was carried out except that in the silylation treatment, the pH was adjusted to 8.2.

<Comparative Example 1>
During the silylation treatment, 10 g of 35% hydrochloric acid and 12 g of hexamethyldisiloxane as a silylation treatment agent were added, and the mixture was kept in a water bath at 70° C. for 12 hours with stirring.
The same procedure as in Example 1 was repeated, except that after the silylation treatment, 14.2 g of a 24% aqueous sodium hydroxide solution was added with stirring to carry out neutralization.
<Comparative Example 2>
The same operations as in Example 1 were carried out except that 35% hydrochloric acid was not added during the silylation treatment and that the neutralization treatment was not carried out.
<Comparative Example 3>
The same procedure as in Example 1 was carried out, except that 35% hydrochloric acid was not added during the silylation treatment.
<Comparative Example 4>
The same procedure as in Example 1 was carried out except that in the silylation treatment, the pH was adjusted to 7.1.
<Comparative Example 5>
The same procedure as in Example 1 was carried out except that in the silylation treatment, the pH was adjusted to 0.4.

In Examples 1 to 3, silica aerogel powder with good physical properties could be obtained by performing the surface treatment under basic conditions with a pH of 8 to 10. In addition, by using a highly reactive silylating agent, the surface treatment time, which was previously required 12 hours or more, could be shortened to 2 hours, improving the reaction efficiency.
In Comparative Example 1, the physical properties were good because of the conventional method for producing silica aerogel powder, but the surface treatment time required 12 hours. In addition, the pH during the surface treatment was strongly acidic, so it was necessary to consider the equipment.
In Comparative Example 2, the surface treatment was performed at a pH of 10 or more, and no neutralization treatment was performed after the surface treatment, so the physical properties were deteriorated. This is because the silica aerogel was partially dissolved due to the pH being too high.
In Comparative Example 3, the surface treatment was performed at a pH of 10 or more, and therefore the physical properties were deteriorated. This is because the silica aerogel was partially dissolved due to the pH being too high.
In Comparative Example 4, the surface treatment was performed at a pH of 8 or less, and therefore the physical properties were deteriorated. This is because more hydrochloric acid than necessary was added, which resulted in consumption of hexamethyldisilazane, resulting in insufficient surface treatment.
In Comparative Example 5, the surface treatment was performed at a strongly acidic pH, which resulted in poor physical properties. This is thought to be due to the same reasons as in Comparative Example 4. Furthermore, since the treatment was strongly acidic, it was necessary to consider the equipment.

Claims (2)

(1) a step of preparing an aqueous silica sol; (2) a step of dispersing the aqueous silica sol in a hydrophobic solvent to form a W/O type emulsion; (3) a step of converting the W/O type emulsion into a dispersion of a gelled body; (4) a step of separating the dispersion into two layers, an O phase and a W phase; (5) a step of adding a basic substance to the W phase to age the gelled body dispersed in the W phase; (6) a step of removing the O phase; (7) a step of subjecting the gelled body dispersed in the W phase to a silylation treatment with a silazane; (8) a step of adjusting the pH of the W phase to 1.0 to 7.0 ; (9) a step of extracting the silylated gelled body with a hydrophobic organic solvent; and (10) a step of recovering the silylated gelled body to obtain a powder of hydrophobic spherical silica aerogel,
The method for producing a spherical silica aerogel, wherein in the silylation treatment, the pH of the dispersion is adjusted to 8 or more and 10 or less. The method for producing spherical silica aerogel according to claim 1, wherein the silazane is hexamethyldisilazane in the silylation process.