JP3048276B2 - Airgel production 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 excellent heat insulation and light transmission.

[0002]

2. Description of the Related Art As a method for producing a light-transmitting inorganic porous material, a gel-like compound obtained by hydrolyzing and polycondensing an alkoxysilane (also referred to as silicon alkoxide, alkyl silicate, or the like) is treated with a supercritical solvent. There is a method of drying in a state (US Pat. Nos. 4,402,927 and 443).
Nos. 2956 and 4610863).

[0003]

In order to obtain such a porous body, it is necessary to remove the solvent therein without destroying the structure of the gel-like compound, and to remove the solvent in a supercritical state of the solvent. That is, a so-called supercritical drying method is used. However, this method has various steps such as a step of increasing the pressure and temperature to a supercritical state of a solvent or a mixed solvent of a solvent and an extraction solvent (such as carbon dioxide), and a step of lowering the pressure to normal temperature and normal pressure. Therefore, it took a very long time in the drying process.

[0004] In view of such circumstances, an object of the present invention is to provide a method for producing a porous body (aerogel) having light transmittance by short-time supercritical drying.

[0005]

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention provides a compound represented by the following general formula (I): SiR ¹ _n (OR ² ) _4-n (I) wherein R ¹ and R ² Are independently of each other
5 represents an alkyl group or a phenyl group. R ¹ and R
^When there are two or more ² each, two or more R ¹ and two or more R ² may be the same as each other,
It may be different. n = 0 to 2] in a method for producing an airgel by hydrolyzing an alkoxysilane represented by the following formula and subjecting a gelled product (gelled compound) obtained by condensation polymerization to supercritical drying. Provided is a method for producing an airgel, wherein the temperature in the container is maintained at a temperature equal to or higher than the boiling point of the solvent before removing the airgel from the high-pressure container used for supercritical drying after the removal.

[0006] The aerogel obtained by the production method of the present invention is a light-permeable porous body. Aerogel (aerogel) generally refers to a porous material obtained by removing a solvent or the like from a gel-like compound containing a solvent before drying, such as a wet alcogel, and removing the solvent by supercritical extraction. And the resulting dried porous gel.

The alkoxysilane used in the present invention is:
Represented by the above general formula (I), and more specifically
Is represented by the following general formula (II)[Wherein, R^Three, R^FourAnd R^FiveAre, independently of each other, carbon
Represents an alkyl group of the formulas 1 to 5 or a phenyl group. Two
R^FiveMay be the same or different from each other
No. A bifunctional alkoxysilane represented by the following general formula:
(III) R⁶−Si (OR⁷)_Three … (III) [wherein, R⁶And R⁷Are independently of each other
5 represents an alkyl group or a phenyl group. Three R⁷Is
They may be the same or different. 〕so
A trifunctional alkoxysilane represented by the following general formula (IV): Si (OR⁸)_Four … (IV) [wherein, R⁸Is an alkyl group having 1 to 5 carbon atoms or
Represents a nyl group. Four R ⁸May be the same as each other
And may be different. ] 4-functional alkoxy
Silan, which is represented by the general formulas (III) and (IV)
At least one of the alkoxysilanes represented by
Or represented by the general formulas (III) and (IV), respectively
At least one of the alkoxysilanes represented by the general formula
Hydrolysis of a mixture of alkoxysilanes represented by (II)
Then, a gel body (for example, wet
Lucogel) is obtained.

The formulas (II) and (II) used in the present invention
The bifunctional, trifunctional and tetrafunctional alkoxysilanes represented by I) and (IV), respectively, are not particularly limited. Specific examples thereof include bifunctional alkoxysilanes such as dimethyldimethoxysilane,
Dimethyldiethoxysilane, diphenyldiethoxysilane, diphenyldimethoxysilane, methylphenyldiethoxysilane, methylphenyldimethoxysilane, diethyldiethoxysilane, diethyldimethoxysilane and the like are used. As the trifunctional alkoxysilane, for example, methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane and the like are used. As the tetrafunctional alkoxysilane, for example, tetramethoxysilane, tetraethoxysilane, or the like is used.

In order to efficiently hydrolyze the above-mentioned alkoxysilane and carry out polycondensation in the present invention, it is preferable to add a catalyst in advance to a reaction system containing the alkoxysilane. Examples of such a catalyst include an acidic catalyst and a basic catalyst. Specifically, as the acidic catalyst, hydrochloric acid, citric acid, nitric acid, sulfuric acid, ammonium fluoride and the like are used, and as the basic catalyst, ammonia, piperidine and the like are used, but not limited thereto. Absent.

The solvent used for the hydrolysis and condensation polymerization of alkoxysilane is usually alcohol, in order to uniformly dissolve and mix the raw material alkoxysilane and water.
Acetone or the like is used, but is not limited thereto, and any substance can be used as long as both alkoxysilane and water are easily dissolved. However, alcohol is most preferable in consideration of the fact that alcohol is generated by a hydrolysis reaction in the process of forming a gel-like compound, and in consideration of supercritical drying described later.

The solvent used for supercritical drying is not particularly limited, and examples thereof include a single system of ethanol, methanol, dichlorodifluoromethane, carbon dioxide, water and the like or a mixture of two or more thereof. it can. When supercritical drying is performed using a single solvent instead of a mixed system, the solvent is generally replaced in a high-pressure vessel (or pressure vessel, also called a drying vessel) such as an autoclave, and the solvent is replaced with the same solvent. The gelled compound subjected to the above is put together, and the temperature and pressure are raised to a temperature not lower than the critical point of the solution. Then, the solvent is gradually removed, and finally the state is returned to the normal temperature and normal pressure state, thereby completing 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 to a supercritical state in the mixed system in a drying vessel, a method of forming a gel-like compound in the drying vessel Is replaced with the solvent (second solvent) that is to be brought into a supercritical state from the solvent contained therein, and after substantially completing the solvent replacement, the solvent is removed in the supercritical state of the second solvent There is a method to do so.

In the supercritical drying, the following is common to both single and mixed systems. By making high temperature and high pressure, the solvent in the gel compound to be dried is brought into a supercritical state, and while maintaining the temperature and pressure in the supercritical state,
The solvent is removed by using an inert gas as necessary. When the solvent is almost completely removed, the pressure and temperature are gradually lowered to normal temperature and normal pressure, and then the airgel (which may be a sample) is taken out. At this time, as the rate of increase or decrease in pressure or temperature increases, the stress applied to the solvent itself or the gel-like compound increases, and problems such as cracks in the gel tend to occur. If the pressure and temperature are reduced while the solvent is not sufficiently removed in the supercritical state, liquefaction and dew condensation of the solvent occur in the process, which also causes cracking and shrinkage of the gel.

From the above, supercritical drying requires an extremely long time (for example, several tens of hours) due to the need for a somewhat moderate temperature increase, pressure increase, and sufficient solvent extraction time. Was the actual situation. In order to solve such a problem, the method for producing the airgel of the present invention is not particularly limited, but is carried out, for example, as follows.

First, an alcohol, water, and the above-mentioned catalyst are added to and mixed with the above-mentioned alkoxysilane, and the alkoxysilane is hydrolyzed and polycondensed. The alcohol used at this time is, for example, methanol, ethanol, isopropanol, butanol, etc., and is not particularly limited. When the polycondensation reaction proceeds sufficiently, the reaction mixture (sol) gels, and a gel compound is obtained.

Next, an alcohol is added to the gel-like compound and heated, that is, so-called ripening is performed. In this case, if necessary, the aging step may be omitted. This gel-like compound is preferably one in which water and unreacted substances have been removed, and the solvent portion has been completely replaced with alcohol (also referred to as an alcogel). Next, the gel compound in this state is supercritically dried to remove the solvent (mainly alcohol).

As a method of performing supercritical drying, for example, the following method can be mentioned. That is, a method in which the gel compound of alkoxysilane obtained as described above is heated and pressurized to a temperature and pressure higher than the critical point of alcohol, and the alcohol is dried in that state. (Approximately 60 atm) and then drying the carbon dioxide in a supercritical state.

In any case, in the present invention, after drying the solvent in a supercritical state, the pressure is returned to normal pressure, but the temperature is kept at a temperature higher than the boiling point of the solvent until the airgel is finally removed. It is characteristic. As described above, it takes a very long time to completely remove the solvent. If the temperature and pressure are lowered to room temperature and normal pressure while the solvent is not sufficiently removed, the gel may be broken due to liquefaction or dew condensation of the solvent. However, according to the method of the present invention, the airgel may be taken out by lowering the pressure where solvent removal is appropriate. The appropriate location is determined by the type of the solvent and the temperature conditions during drying in the supercritical state, and is determined by the residual ratio of the solvent in the high-pressure vessel.

By performing such supercritical drying and removing the contained solvent from the gelled compound, a porous body having light transmittance can be obtained. When the supercritical drying is performed by the above method, the extraction step in the supercritical drying step can be achieved in only 1 to 2 hours, and the total drying step time is several hours (actually, about 3 to 5 hours). Will be able to do it.

[0020]

The gel-like compound obtained by hydrolysis and condensation polymerization of alkoxysilane is supercritically dried. The supercritical drying is to remove the solvent contained in the gel compound in a critical point of the solvent contained in the gel-like compound or in an atmosphere at a higher temperature and a higher pressure than the critical point. In such an atmosphere, phase transition (evaporation, condensation) of the solvent does not occur, so that the destruction and aggregation of the structure of the gel compound at the time of solvent removal can be suppressed.
For this reason, airgel obtained by supercritical drying,
It becomes porous. On the other hand, when normal heating and drying are performed, the solvent changes from a liquid to a gas, and when the gel is removed from the structure of the gel compound, the structure is destroyed by the surface energy of the solvent. Or agglomerate.

From the solvent extraction step by supercritical drying, the temperature is maintained at or above the boiling point of the solvent until the airgel is removed from the container after the pressure is reduced after the completion of the step (the temperature does not fall below the boiling point of the solvent). Even if a certain amount of solvent remains in the container, only a phase transition from a supercritical fluid to a gas occurs and the solvent does not become a liquid but exists as a complete gas. Does not destroy the structure of the airgel. Accordingly, there is no need to completely remove the solvent, and supercritical drying can be completed in a very short extraction time, and an aerogel free from structural destruction such as shrinkage and cracks can be obtained.

The aerogel obtained by the production method of the present invention is a structure composed of very fine silica particles, and is porous because its particle diameter and interparticle space are much smaller than the wavelength of light. Nevertheless, it has transparency. Here, the transparency is, for example, visual transparency in a visible light wavelength region or the like, but is not limited thereto.

[0023]

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-Tetramethoxysilane (a reagent manufactured by Toray Dow Corning Silicone Co., Ltd.) was added to ethanol (Nacalai Tesque, Inc.).
(Special grade reagent) and a 0.01 mol / l aqueous ammonia solution were gradually added. At this time, the reaction was performed at room temperature, and the mixing ratio was tetramethoxysilane: ethanol:
Aqueous ammonia = 1: 5: 4 (molar ratio). After stirring for about 2 hours, the mixture was allowed to stand to gel. After gelation, ethanol was added, the mixture was heated at 50 ° C., and the addition of ethanol was repeated to accelerate (age) the polycondensation reaction so that the gel was not dried.

Next, the gel was placed in carbon dioxide at 18 ° C. and 55 kg / cm ² , and the operation of replacing the ethanol in the gel with carbon dioxide was performed for 2 to 3 hours. Thereafter, the inside of the container is set at 80 ° C., 160 kg / cm, which is a supercritical condition of carbon dioxide.
^Then , supercritical drying (solvent removal) was performed for 2 hours. After that, only the pressure is reduced to normal pressure while keeping the inside of the container at 80 ° C,
An airgel sample was removed from the container. The sample had a thickness of 5 mm and a diameter of 50 mm.

Example 2 Instead of supercritical drying using carbon dioxide as an extraction solvent in Example 1, ethanol was replaced with ethanol under supercritical conditions (250 ° C., 80 kg / cm ² ) for 2 hours. Drying is performed, and then the pressure is increased to normal pressure and the temperature is increased to 80 ° C.
After that, the airgel sample was obtained in the same manner as in Example 1 except that the airgel sample was taken out.

Example 3 In Example 1, tetraethoxysilane (a reagent manufactured by Toray Dow Corning Silicone Co., Ltd.) was used instead of tetramethoxysilane, and 0.04 mol / mol of 0.01N ammonia water was used. l An airgel sample was obtained in the same manner as in Example 1 except that an aqueous solution of ammonium fluoride was used, and the mixing ratio was set to tetraethoxysilane: ethanol: aqueous solution of ammonium fluoride = 1: 5.5: 5.

Example 4 In Example 1, supercritical drying was carried out at 80 ° C. and 160 kg / cm.
² hours at 100 ° C. and 160 kg / cm ² instead of condition ^2. Then, the pressure is reduced to normal pressure,
An airgel sample was obtained in the same manner as in Example 1, except that the airgel sample was taken out at 00 ° C.

Example 5 In Example 2, supercritical drying was performed under supercritical conditions (275 ° C., 90 kg / cm ² ) using methanol instead of ethanol. 70
An airgel sample was obtained in the same manner as in Example 2, except that the airgel sample was taken out after the temperature was lowered to ° C.

Example 6 In Example 1, 2 was replaced by tetramethoxysilane.
An airgel sample was obtained in the same manner as in Example 1, except that a mixture of dimethyldimethoxysilane and tetramethoxysilane, which were functional alkoxysilanes, in a ratio of 1: 9 (molar ratio) was used. -Example 7-In Example 1, 3 was used instead of tetramethoxysilane.
An airgel sample was obtained in the same manner as in Example 1, except that a mixture of 5: 5 (molar ratio) of methyltrimethoxysilane and tetramethoxysilane, which were functional alkoxysilanes, was used.

Comparative Example 1 In the drying step of Example 1, 80 ° C., 160 kg / cm ²
Instead of performing supercritical drying for about 2 hours under the supercritical condition and taking out the airgel sample at 80 ° C.,
A sample was obtained in the same manner as in Example 1 except that drying was performed under supercritical conditions of kg / cm ² for 2 hours to lower the temperature and pressure to room temperature and normal pressure, and then the airgel sample was taken out.

Comparative Example 2 In the drying step of Example 1, after performing supercritical drying, 8
Instead of taking out the airgel sample at 0 ° C., a sample was obtained in the same manner as in Example 1 except that after supercritical drying, the temperature and pressure were lowered to room temperature and normal pressure, and the airgel sample was taken out. .

Comparative Example 3 A sample was obtained in the same manner as in Comparative Example 1 except that in the drying step of Comparative Example 1, drying was performed for 24 hours instead of 2 hours. Comparative Example 4 In Example 1, the gelled product was aged, and then heated to 80 to 100 ° C. at normal pressure and dried to obtain a dried product sample.

Table 1 shows the main contents of the samples obtained in Examples 1 to 7 and Comparative Examples 1 to 4 and the items concerning the operating conditions for obtaining them.

[0034]

[Table 1]

With respect to the samples obtained in Examples 1 to 7 and Comparative Examples 1 to 4, the bulk density, the specific surface area, the thermal conductivity, the light transmittance, and the state of the dried sample were examined. The specific surface area was determined using a BET method based on a nitrogen adsorption method. The thermal conductivity was measured by a method based on ASTM-C518 using a thermal conductivity measuring device based on a steady method manufactured by Eiko Seiki Co., Ltd.
The measurement was performed under the conditions of 0 ° C and 40 ° C.

The light transmittance is measured by measuring the spectral distribution of the transmitted light of the sample in the visible light range, and measuring the visible light transmittance according to JIS-R3.
106. The state of the dried sample was examined by visual observation to see if the sample had no cracks (indicated as crack-free) or no shrinkage. The results are shown in Table 2.

[0037]

[Table 2]

As can be seen from the table, the airgel of the embodiment is
Since it has excellent porosity, it has excellent heat insulation and light transmittance. In contrast, the comparative example had the following disadvantages. In Comparative Example 4, remarkable shrinkage occurred during drying. In Comparative Examples 1 and 2, supercritical drying was performed. However, since the removal of the solvent was insufficient, a small amount of the solvent liquefied at the time of temperature decrease, the sample shrinked, and the performance as a porous body was reduced. Structural destruction such as crack generation occurred. In Comparative Example 3, the same aerogel as that of Example was obtained, but it took a long time for supercritical drying.

[0039]

According to the aerogel production method of the present invention, an aerogel can be obtained by short-time supercritical drying, and no destruction due to shrinkage, cracks or the like occurs during the drying. The aerogel obtained by the present invention is excellent in functions such as heat insulation and the like, which are unique to porous materials, and in light transmittance.

The aerogel obtained by this manufacturing method is, for example, a heat insulating material, an acoustic material, a Cherenkov element,
It can be used for various uses such as a catalyst carrier.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C08G 77/32 C08G 77/06 C08G 77/02

Claims (1)

(57) [Claims] 1. A compound represented by the following general formula (I): SiR ¹ _n (OR ² ) _4-n (I) wherein R ¹ and R ² independently of one another have 1 to 1 carbon atoms.
5 represents an alkyl group or a phenyl group. R ¹ and R
^When there are two or more ² each, two or more R ¹ and two or more R ² may be the same as each other,
It may be different. n = 0 to 2] in a method of hydrolyzing an alkoxysilane represented by the formula, and supercritically drying a gelled product obtained by condensation polymerization to produce an airgel,
Manufacturing the airgel, wherein the temperature in the container is maintained at a temperature equal to or higher than the boiling point of the solvent, until the airgel is removed from the high-pressure container used for supercritical drying after removing the solvent in the supercritical state. Method.