JP3520601B2 - Aerogel manufacturing method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an airgel having a porous skeleton of silica, and more specifically, for example, sunlight which is required to have daylighting property, light transmitting property, transparency and high heat insulating property at the same time. The present invention relates to a method for producing an airgel that can be used in various applications such as a light-transmitting heat insulating material useful for collecting heat, a heat insulating material for an opening in a house, and a peep window for a high temperature device typified by a furnace.

[0002]

2. Description of the Related Art Conventionally, an airgel made of silica has been known as a material having a low thermal conductivity and a light transmitting property. This airgel is available from USP 4402927.
Alkoxysilanes (also referred to as silicon alkoxides, alkyl silicates, etc.), as disclosed in Japanese Patent Nos. 4,432,956 and 4,610,863.
The gelled compound in the wet state consisting of a silica skeleton obtained by hydrolyzing and polymerizing the above, in the presence of a solvent (dispersion medium) such as alcohol or liquefied carbon dioxide, is supercritical above the critical point of this solvent. It can be produced by drying in the state, and then, if necessary, further subjecting it to heat treatment at about 500 to 750 ° C. or higher. Also, USP5
As described in JP-A-137927 and JP-A-5124364, a gel-like compound is similarly obtained using sodium silicate as a raw material, and the gel-like compound can also be produced by supercritical drying. The airgel obtained by such a manufacturing method is a material useful as, for example, a heat insulating material having light transmittance.

However, since the airgel as described above is very hydrophilic, its performance deteriorates with time due to moisture in the air under practical conditions. That is, the transparency was lowered, and the shrinkage and deformation occurred. For this reason, in order to put the airgel into practical use, it was essential to devise a method such that the airgel itself would not come into contact with the outside air by methods such as perfect packing with a film or sandwiching it with glass plates and then performing a complete sealing. . In addition, it is known that the airgel obtained by supercritical drying has a medium during supercritical drying on the surface of the silica, and an alkoxyl group reacted with alcohol is present, which further improves the transparency of the obtained airgel. For this purpose, a heat treatment is performed at 500 to 750 ° C. to remove the alkoxyl group. However, according to this method, even the trivial hydrophobicity of the alkoxyl group is eliminated, and the hydrophilicity of the airgel becomes more remarkable.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above facts, and an object thereof is to provide a method for producing an airgel which is excellent in transparency, heat insulation and hydrophobicity. .

[0005]

The method for producing an airgel according to claim 1 of the present invention comprises a step of forming a gel-like compound by gelling a sol obtained by hydrolyzing a polycondensable alkoxysilane by a polycondensation reaction. In a method of producing an airgel in which a gel compound is subjected to a hydrophobizing treatment and a supercritical drying by a reaction with a hydrophobizing agent, the hydrophobizing treatment and the supercritical drying are performed, and then, at a temperature equal to or higher than the decomposition temperature of the alkoxyl group, and hydrophobizing. It is characterized in that the heat treatment is carried out below the decomposition temperature of the hydrophobic group imparted by the treatment.

The method for producing an airgel according to claim 2 of the present invention is characterized in that the hydrophobizing agent is an organic silane compound.

In the method for producing an airgel according to claim 3 of the present invention, the temperature of the heat treatment after the supercritical drying is 270 to 40.
It is characterized in that it is 0 ° C.

The present invention will be described in detail below. The alkoxysilane used in the present invention is an alkoxysilane represented by the following general formula, and more specifically, a bifunctional alkoxysilane represented by the following general formula, represented by the following general formula. A trifunctional alkoxysilane, a tetrafunctional alkoxysilane represented by the following general formula, and an alkoxysilane oligomer represented by the following general formula, and the like.

[0009]

[Chemical 1]

[0010]

[Chemical 2]

[0011]

[Chemical 3]

[0012]

[Chemical 4]

[0013]

[Chemical 5]

The gel-like compound (wet alcogel) according to the present invention is represented by at least one selected from the group consisting of the alkoxysilanes represented by the above general formulas to the general formulas, or the above general formulas to the general formulas. It is obtained by hydrolyzing and polycondensing a mixture containing at least one selected from the group consisting of alkoxysilanes represented by the formula and an alkoxysilane represented by the above general formula.

Specific examples of the alkoxysilanes represented by the above general formulas to general formulas used in the present invention are as follows:
Examples of the bifunctional alkoxysilane include dimethyldimethoxysilane, dimethyldiethoxysilane, diphenyldiethoxysilane, diphenyldimethoxysilane, methylphenyldiethoxysilane, methylphenyldimethoxysilane, diethyldiethoxysilane, and diethyldimethoxysilane. Examples of trifunctional alkoxysilanes include methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, phenyltrimethoxysilane, and phenyltriethoxysilane. Examples of tetrafunctional alkoxysilanes include: , Tetramethoxysilane, tetraethoxysilane, etc., but are not particularly limited.

The alkoxysilane oligomer represented by the above general formula preferably has a degree of polymerization of 10 (hereinafter, a monomer having a degree of polymerization of n is referred to as an n-mer) or less, but is a colorless transparent liquid. If so, it is not limited to this. The above-mentioned alkoxysilane oligomer does not need to be a compound having a uniform degree of polymerization, and may have a distribution of the degree of polymerization or have a mixed molecular structure in the form of chain, branch or ring. Considering the stability as a substance and the reaction time for producing a gel-like compound, those having 2 to 6 mers are preferable. R in the alkoxysilane oligomer represents an alkyl group or a phenyl group, and among them, a methyl group (—CH
_3), ethyl group (-C ₂ H ₅₎ are preferred. Specifically, in the case of an oligomer of methoxysilane, the average molecular weight is 2
In the case of an oligomer of ethoxysilane of 50 to 700, an oligomer having an average molecular weight of 300 to 900 is preferable.

In order to efficiently hydrolyze the alkoxysilane and perform polycondensation in the present invention, it is preferable to add a catalyst to the reaction solution containing the alkoxysilane in advance. Examples of such catalysts include acidic catalysts and basic catalysts. Specifically, as the acidic catalyst, hydrochloric acid, citric acid, nitric acid, sulfuric acid, ammonium fluoride or the like is used, and as the basic catalyst, ammonia, piperidine or the like is used, but is not limited to these. Absent.

The solvent used for the hydrolysis and polycondensation of the alkoxysilane is usually an alcohol, in order to uniformly dissolve and mix the starting alkoxysilane and water.
Acetone or the like is used, but the solvent is not limited to these and may be any solvent in which both alkoxysilane and water are easily dissolved. However, considering that alcohol is produced by the hydrolysis reaction in the production process of the gel-like compound and that supercritical drying is performed, alcohol is preferable as the solvent.

The mixing ratio of the reaction solution is not limited,
Brightness, heat insulation, specific surface area, density, optical refractive index, etc. are required.
It can be changed in various ways depending on the performance. This
The density of the airgel is not particularly limited, but is 0.01 to
0.3 g / cm³Is preferable, and further,
0.04-0.2g / cm³Is more preferred to be
Yes. That is, the density of this airgel is 0.01 g / c
m³If less than, it takes a lot of time to gel.
Or shrinkage during supercritical drying is unavoidable and
It is so lightweight that it may come into contact with workers' hands, containers, members, etc.
Those that are difficult to handle in practice, such as sticking to and breaking
There is a problem that becomes 0.3g / cm ³Cross over
In some cases, the thermal conductivity of the airgel will increase slightly and the
It is no longer a very good material in terms of heat,
Performance also deteriorates, and it is difficult to prepare airgel.
It Therefore, the density after supercritical drying is in this range.
Thus, the compounding ratio of the reaction solution is determined.

The resulting gel compound is subjected to a hydrophobizing treatment. As the hydrophobizing agent, one having a functional group that reacts with the silanol group of the gel-like compound and a hydrophobic group is used. Examples of the functional group that reacts with the silanol group include halogen, amino group, imino group, carboxyl group, alkoxyl group and hydroxyl group. Examples of the hydrophobic group include an alkyl group, a phenyl group and their fluorides. The hydrophobizing agent may have only one kind of each of the functional group and the hydrophobic group, or 2
It may have more than one species. Specifically, hexamethyldisilazane, hexamethyldisiloxane, trimethylchlorosilane, trimethylmethoxysilane, trimethylethoxysilane, triethylethoxysilane, triethylmethoxysilane, dimethyldimethoxysilane, dimethyldichlorosilane, dimethyldiethoxysilane, methyltrimethoxy. Examples thereof include organic silane compounds such as silane, methyltrichlorosilane, and ethyltrichlorosilane.

Here, the hydrophobizing treatment is performed by using a liquid as a medium in advance before performing supercritical drying, or by using a supercritical fluid as a medium during supercritical drying which will be described later. The medium is not particularly limited as long as it has low reactivity with the hydrophobizing agent and dissolves the hydrophobizing agent. When the liquid is subjected to the hydrophobizing treatment, a solution obtained by dissolving the hydrophobizing agent in a solvent such as alcohol is prepared as a hydrophobizing solution, and the gel compound is immersed in the hydrophobizing solution. Further, the reaction is heated if necessary.

The solvent used for the supercritical drying is not particularly limited, but for example, ethanol, methanol, isopropanol, dichlorodifluoromethane, carbon dioxide, water or the like alone or in combination of two or more. Mention may be made of mixed systems. When supercritical drying is performed using a single solvent instead of a mixed system, generally, the solvent and the gel compound obtained by solvent substitution in the same solvent are put together in an autoclave, and the solvent is used. After the temperature and pressure are raised to the critical point or higher, the solvent is gradually removed, and finally the temperature and temperature are returned to the normal temperature and normal pressure to complete the drying. When supercritical drying is carried out in a mixed system of two or more kinds, a method of raising the temperature and pressure set in the drying container to a supercritical state in the mixed system, There is a method in which the first solvent is replaced with a second solvent desired to be in a supercritical state, the solvent replacement is almost completed, and then the solvent is removed in a supercritical state of the second solvent.

When the hydrophobic treatment is carried out during supercritical drying,
This is performed by injecting a hydrophobizing agent in an amount corresponding to the solubility into the medium in the drying container forming a supercritical state and maintaining the temperature and pressure.

The airgel obtained by subjecting it to hydrophobic treatment and supercritical drying as described above is composed of a homogeneous porous silica skeleton, and has the properties of a porous body such as high heat insulation and high surface area, and is transparent. It is a material with properties. In the method of producing an airgel according to the present invention, the airgel thus obtained is further heat-treated in the present invention. In general, in order to make an airgel obtained by supercritical drying more excellent in transparency, 1200 to 1300 ° C. or 500 as described in USP 432956 or USP 4327065 is used.
Heat treatment is performed at a high temperature such as ˜750 ° C. The heat treatment decomposes and removes the alkoxyl group (≡Si—OR, R is an alkyl group) contained in the airgel,
It is intended to improve the transparency by using high-purity silica airgel. However, the airgel produced by this method has another problem. That is, since silica constituting the airgel has a silanol group (≡Si—OH) on the surface of the silica, it has hydrophilicity and shows remarkable deterioration with time in performance in a humidity atmosphere. However, especially those obtained by supercritical drying in the supercritical state of alcohol and not subjected to the above-mentioned heat treatment show a hydrophobic property because they also have many alkoxyl groups on the silica surface, and this deterioration with time It has the effect of softening the degree. Nevertheless, if the above-mentioned heat treatment is carried out to remove the alkoxyl group, some of the airgel's hydrophobicity will also disappear, and it will deteriorate severely over time under humidity conditions, no matter how excellent the transparency is. It was a material that was difficult to use.

In the present invention, by selecting the treatment temperature in the heat treatment, the transparency can be enhanced without impairing the excellent hydrophobicity, and a hydrophobic aerogel having excellent transparency can be produced. The treatment temperature is selected such that the alkoxyl group is thermally decomposed and removed, and the alkylsilyl group (≡Si—R) provided by the hydrophobizing agent remains without being decomposed. Specifically, 270-400 ℃
It is preferably about the same. That is, as is clear from the differential thermal analysis curve of the typical airgel sample after supercritical drying shown in FIG. 1, the exothermic peak 1 at which the alkoxyl group is decomposed is 271 ° C. and the alkylsilyl group which is a hydrophobic group. Exothermic peak 2 due to decomposition of is 478 ° C. Therefore, it can be seen that at about 270 to 400 ° C., the alkoxyl group is thermally decomposed and removed, and the alkylsilyl group (≡Si—R) provided by the hydrophobizing agent remains without being decomposed.

The airgel of the present invention is a structure composed of very fine silica particles, and its particle size is much smaller than the wavelength of light, and the void structure is also very homogeneous, so it is also a porous body. Regardless, it has transparency. Here, the light transmissivity is, for example, visual transparency in the visible light wavelength region or the like, or transmissivity in the infrared region, but is not limited thereto. Moreover, since it is hydrophobic, it has excellent moisture resistance, and its performance and dimensions are stable over time.

[0027]

The manufacturing method of the airgel according to claim 1 of the present invention is as follows.
A sol obtained by hydrolyzing a polycondensable alkoxysilane is gelated by a polycondensation reaction to form a gel-like compound, and the gel-like compound is hydrophobized by reaction with a hydrophobizing agent and subjected to supercritical drying. In the manufacturing method, supercritical drying is performed after the hydrophobic treatment, and then,
Since the heat treatment is performed at the decomposition temperature of the alkoxyl group or higher and lower than the decomposition temperature of the hydrophobic group imparted by the hydrophobizing treatment, the transparency, heat insulation and hydrophobicity are excellent.

The method for producing an airgel according to claim 2 of the present invention is excellent in hydrophobicity because the hydrophobizing agent is an organic silane compound. It is characterized by

In the method for producing an airgel according to claim 3 of the present invention, the temperature of the heat treatment after the supercritical drying is 270 to 40.
Since it is 0 ° C, it is excellent in transparency, heat insulation and hydrophobicity.

[0030]

EXAMPLES The present invention will be specifically described below with reference to examples.

Specific examples and comparative examples of the present invention will be shown below, but the present invention is not limited to the following examples.

Example 1 An oligomer of tetramethoxysilane having an average molecular weight of 470 (manufactured by Colcoat Co., Ltd .; trade name: methyl silicate 51), ethanol (manufactured by Nacalai Tesque, Inc .; special grade reagent), water and 28% by weight of ammonia. By gradually adding a mixture with an aqueous solution at room temperature, the molar ratio of the tetramethoxysilane oligomer: ethanol: water: ammonia = 1: 70: 20:
A reaction solution (hereinafter referred to as a sol) having a mixing ratio of 0.6 was obtained. After the sol was poured into the container, the sol was allowed to stand at room temperature for about 10 minutes to wait for the sol to gel. After that, the gel-like compound was taken out of the container and the size was 5 mm or less.
A gel compound having a thickness of 1 mm and a thickness of 10 mm was obtained.

Next, the gel-like compound was placed in a high-pressure container and further filled with ethanol. 18 in this container
After adding carbon dioxide at 55 ° C. and 55 kg / cm ² to replace ethanol in the gel and in the container with carbon dioxide, the inside of the container is in a supercritical condition of carbon dioxide, 8
Supercritical drying (solvent removal) was performed at 0 ° C and 160 kg / cm ² . Next, dimethyldimethoxysilane [Toray Dow Corning Silicone Co., Ltd .; AY43-004] was added as a hydrophobizing agent to the atmosphere in the supercritical state so that the concentration in the high-pressure container was 0.6 mol / liter. Then, the hydrophobizing agent was diffused into the supercritical fluid over 1 hour, and the gel-like compound was left in the supercritical fluid to perform the hydrophobizing treatment. After that, carbon dioxide in a supercritical state was circulated and then depressurized to remove ethanol and the hydrophobizing agent contained in the gel compound. Then, the sample was taken out from the high-pressure container to obtain an airgel sample. Finally, this airgel sample was placed in an electric furnace, heated to 300 ° C over 6 hours, and kept at 300 ° C for 6 hours. After cooling, the airgel sample was taken out. The size of the sample was 51 mm in diameter and 10 mm in thickness.

(Example 2) A gel-like compound obtained in the same manner as in Example 1 was mixed with hexamethyldisilazane (manufactured by Toray Dow Corning Silicone Co., Ltd .; S).
Z6079] was immersed in a 1.2 mol / liter ethanol solution. After leaving it as such at room temperature (20 ° C.) for one day, the gel compound was taken out and further washed with ethanol.

Next, this gel-like compound was placed in a high-pressure container and further filled with ethanol. 80 in this container
° C., after the 160 kg / cm ^2, the vessel 80 ° C., 1
It was replaced with carbon dioxide of 60 kg / cm ² , further depressurized, and supercritical drying (solvent removal) was performed. Then, the sample was taken out from the high-pressure container to obtain an airgel sample. Finally, the airgel sample was placed in an electric furnace for 3 hours for 3 hours.
The temperature was raised to 00 ° C and kept at 300 ° C for 6 hours. After cooling, the airgel sample was taken out. The size of the sample was 51 mm in diameter and 10 mm in thickness.

Example 3 Example 3 was repeated except that the hydrophobizing agent was replaced with hexamethyldisilazane and dimethyldimethoxysilane dimethyldimethoxysilane [Toray Dow Corning Silicone Co., Ltd .; AY43-004] was used. An airgel sample was obtained in the same manner as in Example 2.
Similarly, the size of the sample was 51 mm in diameter and 10 mm in thickness.

Example 4 An airgel sample was obtained in the same manner as in Example 2 except that the heat treatment after supercritical drying was carried out at 350 ° C. Similarly, the size of the sample was 51 mm in diameter and 10 mm in thickness.

(Comparative Example 1) After obtaining a gel compound in the same manner as in Example 2, the same hydrophobization and supercritical drying as in Example 1 were carried out to obtain an airgel sample. The subsequent heat treatment was not performed.

(Comparative Example 2) A gel-like compound was obtained in the same manner as in Example 2, and then supercritical drying was carried out in the same manner as in Example 2 without hydrophobizing to obtain an airgel sample. The size of the sample was 49.5 mm in diameter and 9.5 mm in thickness.

(Comparative Example 3) The airgel sample obtained in Comparative Example 2 was placed in an electric furnace and heated to 500 ° C for 8 hours,
The temperature was kept at 500 ° C. for 6 hours and then allowed to cool to obtain an airgel sample. The sample size is 48 mm in diameter and 9 m in thickness
It was m.

The density, visible light transmittance and thermal conductivity of the airgel obtained in Examples 1 to 4 and Comparative Examples 1 to 3 were measured. Further, in order to confirm the deterioration of the performance of the airgel under the humidity condition, the density and the visible light transmittance after the humidity resistance test were measured.

Here, the thermal conductivity was measured by ASTM- using a thermal conductivity measuring device manufactured by Eihiro Seiki Co., Ltd. according to a stationary method.
It measured by the method based on C518. The thermal conductivity at 30 ° C. was measured under the conditions of set temperatures of 20 ° C. and 40 ° C. The visible light transmittance was obtained by measuring the light transmittance distribution in the visible light region and determining the visible light transmittance based on JIS-R3106. All the thicknesses are converted back to 10.0 mm. The humidity resistance test conditions were a temperature of 60 ° C. and a relative humidity of 90%, and the test time was 48 hours. The main contents of the sample and the results of the initial physical properties are shown in Table 1. Table 2 shows changes in physical properties before and after the moisture resistance test. The presence or absence of the alkoxyl group and the alkylsilyl group, which is a hydrophobic group, was confirmed by using NMR. That is, in the airgel of Examples 1 to 4, there was no alkoxyl group but a hydrophobic group. For Comparative Example 3, neither the alkoxyl group nor the hydrophobic group was present.

[0043]

[Table 1]

[0044]

[Table 2]

The results shown in Tables 1 and 2 show that the airgel of the example is superior in transparency and hydrophobicity as compared with the comparative example, so that it is stable with respect to size and light transmittance over time. It was a good material.

The airgel obtained according to the present invention is excellent in the functions peculiar to the porous material such as the heat insulating property and the light transmittance. Moreover, since the airgel obtained by the production method of the present invention is also excellent in hydrophobicity, its performance is stable over time even under high humidity conditions. The airgel produced by the present invention can be used in various applications such as a light-transmissive heat insulating material useful for solar heat collection, an acoustic material, a catalyst carrier, and a Cherenkov counter medium.

[0047]

According to the method for producing an airgel according to claim 1 of the present invention, a hydrophobic group is formed by subjecting to supercritical drying after the hydrophobizing treatment and then at or above the decomposition temperature of the alkoxyl group and by the hydrophobizing treatment. Since it is heated below the decomposition temperature of, it is excellent in transparency, heat insulation and hydrophobicity.
Performance is stable over time even under high humidity conditions.

According to the method for producing an airgel according to claim 2 of the present invention, since the hydrophobizing agent is an organic silane compound, the hydrophobicity is excellent, and therefore the performance is stable over time even under high humidity conditions.

According to the method for producing an airgel according to claim 3 of the present invention, the temperature of the heat treatment after the supercritical drying is 270.
Since the temperature is up to 400 ° C., it is excellent in transparency, heat insulation and hydrophobicity, so that the performance is stable with time even under high humidity conditions.

[Brief description of drawings]

FIG. 1 is a differential thermal analysis curve for a sample before heat treatment with hydrogel and supercritically dried airgel.

[Explanation of symbols]

1 Exothermic peak due to decomposition of alkoxyl group 2 Exothermic peak due to decomposition of alkylsilyl group which is a hydrophobic group

─────────────────────────────────────────────────── ─── Continuation of front page (56) References Tokuhyo HEI 11-505499 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C08L 83/00-83/16 C08G 77 / 00-77/62

Claims (3)

(57) [Claims] 1. A gel compound is formed by gelling a sol obtained by hydrolyzing a polycondensable alkoxysilane by a polycondensation reaction, and the gel compound is hydrophobized by a reaction with a hydrophobizing agent and supercritical. In the method for producing an airgel to be dried, the hydrophobic treatment and supercritical drying are performed, and then a heat treatment is performed at a decomposition temperature of the alkoxyl group or higher and below the decomposition temperature of the hydrophobic group imparted by the hydrophobic treatment. An airgel manufacturing method characterized by the above. 2. The method for producing an airgel according to claim 1, wherein the hydrophobizing agent is an organic silane compound. 3. The temperature of the heat treatment after the supercritical drying is 2
70-400 degreeC, The manufacturing method of the hydrophobic airgel of Claim 1 or Claim 2 characterized by the above-mentioned.