JPH08198615A - Production of silica aerogel - Google Patents

Abstract

PURPOSE: To produce silica aerogel having high transparency. CONSTITUTION: Isopropanol is substd. for the liq. phase of wet silica gel and supercritical drying is carried out using the isopropanol as a medium to produce the objective hydrophobic silica aerogel having high transparency. This silica aerogel is used as it is or heat treatment is further carried out at 300-600 deg.C for several hr in an oxidizing atmosphere, e.g. in the air to obtain hydrophilic silica aerogel having higher transparency than the hydrophobic silica aerogel.

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 silica aerogel having high transparency.

[0002]

2. Description of the Related Art Silica aerogel is a substance having an extremely high porosity and a large surface area, which is obtained by preparing a wet gel of silica mainly by hydrolysis of silicon alkoxide and drying it by supercritical drying. From the characteristics of the structure, it is known to exhibit unique physical properties such as transparency, heat insulation, low density, and low refractive index. Therefore, conventionally, with respect to silica airgel having such characteristics, its production method, characteristic evaluation, applied technology, and the like have been actively studied.

[0003] Conventionally, a general method for producing silica airgel is as follows. As a raw material of silica, methyl silicate or ethyl silicate, or an oligomer thereof is exclusively used because of easy handling of gelation reaction and availability such as price. In order to obtain a silica wet gel having a desired silica concentration and therefore a silica airgel having a desired density, methyl silicate or its oligomer is diluted with methanol, and ethyl silicate or its oligomer is diluted with ethanol at an appropriate ratio, and then water and a catalyst are diluted. Is added to cause hydrolysis and gelation reaction to produce a silica wet gel. The liquid phase of the obtained silica wet gel is methanol when diluted with methanol and ethanol when diluted with ethanol. After the aging treatment, the silica wet gel is placed in an autoclave, and when the liquid phase of the silica wet gel is methanol, the interior of the autoclave is filled with methanol and with ethanol. After slowly heating the autoclave to a temperature and pressure above the critical point of each alcohol (for example, 300 ° C., 80 atm), the alcohol is slowly released from the autoclave to zero while maintaining the temperature above the critical temperature. Allow to drain until completely removed. Then, the temperature is lowered and the produced silica airgel is taken out. (This is called supercritical drying with alcohol.) Alternatively, a silica wet gel whose liquid phase is alcohol or acetone is placed in an autoclave, and the inside of the autoclave is filled with the same alcohol or acetone as the liquid phase of the wet gel and then liquefied After replacing the liquid in the autoclave with carbon, and then heating to a temperature and pressure above the critical point of carbon dioxide (for example, 40 ° C, 90 atm), slowly add carbon dioxide while maintaining the temperature above the critical temperature. Then, the temperature is lowered and the silica airgel produced is taken out. (This is called supercritical drying with carbon dioxide.)

[0004] In supercritical drying with an alcohol, a silica airgel is obtained in which an alkyl group remains on the silica surface and becomes hydrophobic, and this is dried in an oxidizing atmosphere such as air.
By performing a heat treatment at a temperature of about 0 ° C. for several hours, the residual alkyl group is removed to make the film hydrophilic and improve the light transmittance (transparency). Supercritical drying with carbon dioxide gives hydrophilic silica aerogels from the beginning.

Conventionally, supercritical drying has been considered as a means for removing a liquid phase without causing the solid phase network structure of a wet gel to shrink as it is. Therefore, the type of medium for supercritical drying (alcohol for supercritical drying with alcohol, carbon dioxide for supercritical drying with carbon dioxide, etc.)
In the case of supercritical drying with alcohol, it does not affect the structure and properties of the obtained airgel except that the surface is covered with an alkyl group corresponding to alcohol. Therefore, if the surface alkyl group is removed by heat treatment, the same properties are obtained. It was thought that an aerogel could be obtained. Therefore, in order to eliminate the need to replace the liquid phase of the wet gel, methanol is used when methyl silicate or its oligomer is used as the silica raw material, and methanol is used when ethyl silicate or its oligomer is used as the silica raw material. Ethanol has been used as a medium during supercritical drying. Carbon dioxide, which can be dried at a lower temperature when there is a problem in the device such as temperature limitation, has been used as a medium for supercritical drying.

By the way, silica aerogels which have been put to practical use or are under research and development are used for transparent low refractive index materials for Cherenkov detectors, transparent heat insulating materials, transparent low density materials for collecting cosmic dust, and the like. There are many things that make use of their features. The silica airgel obtained by the conventional production method has considerable transparency, but has a considerably low transmittance particularly in a short wavelength region, and looks somewhat cloudy in appearance.

[0007]

Under the circumstances described above, the present invention has been made in order to produce a silica airgel having higher transparency.

[0008]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to achieve the above-mentioned object, and as a result of detailed examination of the influence of the type of medium during supercritical drying, which has not been considered in the past, By using isopropyl alcohol as a medium for supercritical drying, even if silica wet gel prepared in the same manner is used, the resulting silica aerogel is superhydrophobic with methanol or ethanol as a medium in the hydrophobic state as it is taken out from the autoclave. Transparency is higher than that after critical drying, and in the state where heat treatment is performed to remove organic groups on the surface to make it hydrophilic, it is significantly higher than that after supercritical drying with methanol, ethanol or carbon dioxide as a medium. I found that. All of these are mainly due to the improvement of the light transmittance on the short wavelength side, and therefore, especially in the heat-treated silica airgel, almost no cloudiness was observed.

In the present invention, a general method is used such as, for example, diluting methyl silicate to an appropriate concentration with methanol and adding an appropriate amount of water and ammonia as a catalyst to carry out hydrolysis and gelation. The prepared silica wet gel is first immersed in isopropyl alcohol. The liquid phase of the silica wet gel is completely replaced with isopropyl alcohol, for example, by repeating the operation of replacing the isopropyl alcohol with a new one after a few hours or more after the immersion or the like. The substituted silica wet gel is placed in an autoclave, and the inside of the autoclave is filled with isopropyl alcohol. Then, supercritical drying is performed using isopropyl alcohol as a medium. When the obtained silica airgel is used as a hydrophobic airgel, it is used as it is, and when it is desired to further increase the transparency because it may be hydrophilic, a temperature range of 300 to 600 ° C. in an oxidizing atmosphere such as air,
For example, heat treatment is performed at 500 ° C. for about 2 hours before use.

[0010]

According to the present invention, starting from the same silica wet gel, a silica airgel having higher transparency than that obtained by the existing method can be obtained.

[0011]

EXAMPLE In order to compare the influence of the medium for supercritical drying, silica aerogels obtained by performing supercritical drying on methanol, ethanol, isopropyl alcohol or carbon dioxide as a medium were used for silica wet gels prepared by the same method. Light transmittance (transparency) was compared for both (before and after heat treatment).

A silica wet gel having two types of silica concentrations was prepared. After diluting with 10 mol or 20 mol of methanol per 1 mol of methyl silicate, add 6 mol of water (actually, use 0.1 N ammonia water since ammonia needs to be added as a catalyst) and stir well. 85
The mixed solution was poured into a disk-shaped mold having a thickness of 10 mm.
The liquid gelled into a transparent jelly in about 30 minutes to 2 hours after standing at room temperature. After standing still for 1 to 2 hours, the obtained silica wet gel was taken out of the mold and immersed in methanol, ethanol, or isopropyl alcohol. The operation of leaving the mixture at room temperature for about 12 hours and then replacing each alcohol with a new one was repeated twice to completely replace the liquid phase of the silica wet gel with the immersed alcohol. The sample for supercritical drying using carbon dioxide as a medium was completely replaced with ethanol or acetone at this stage.

The critical temperature and critical pressure of the supercritical drying medium are:
Methanol is 239 ° C 80 atm, ethanol is 243 ° C
63 atm, 235 ° C. and 47 atm for isopropyl alcohol, and 31 ° C. and 73 atm for carbon dioxide. As an autoclave for supercritical drying, an external heating type using an electric heater was used. In the case of supercritical drying using alcohol as a medium, the silica wet gel in which the liquid phase has been completely replaced with the target alcohol is placed in an autoclave as described above, and the inside of the autoclave is filled with the target alcohol. The temperature was raised to 200 ° C. in 4 hours, and to 300 ° C. in 4 hours, and then heated. The pressure increases as the temperature inside the autoclave rises.
00 atm, 80 to 1 when using other alcohol
A certain temperature (usually 150 ° C.) to maintain 00 atm.
When it reached (before and after), the alcohol was discharged little by little. When the temperature of the electric heater reaches 300 ° C, the temperature is maintained at 300 ° C. In order to ensure that the alcohol is in a supercritical state, a thermometer installed at the center of the autoclave lid indicating the lowest temperature inside the autoclave should be read. When the temperature exceeds 250 ° C (usually 30 minutes to 1 hour after the temperature of the electric heater reaches 300 ° C), the amount of alcohol discharged is increased and the autoclave pressure is started to decrease. Removed completely. Then about 10
After cooling to around room temperature over a period of time, the produced silica airgel was taken out.

When carbon dioxide is used as a supercritical drying medium, a silica wet gel whose liquid phase is ethanol or acetone is placed in an autoclave and filled with ethanol or acetone, while maintaining a temperature of 20 ° C. or lower and a pressure of 70 atm or higher. Inject liquid carbon dioxide with a high-pressure liquid delivery pump,
The operation of discharging ethanol was repeated intermittently over about one and a half days to completely replace the liquid phase in the autoclave with liquid carbon dioxide. Thereafter, while maintaining the pressure at 90 to 100 atm, the temperature of the electric heater was raised to 100 ° C. in one hour to maintain the temperature. When the reading of the thermometer at the center of the autoclave lid exceeded 40 ° C., the pressure dropped. Start and about 2
Atmospheric pressure over time. Thereafter, the resulting silica airgel was cooled to around room temperature and taken out.

A part of the obtained silica airgel sample was used as it was for measuring the light transmittance, and a part was subjected to a heat treatment and then the light transmittance was measured. Heat treatment in air atmosphere 5
The temperature was raised to 00 ° C. in 3 hours, maintained at 500 ° C. for 2 hours, and cooled to around room temperature in 5 hours.

FIG. 1 compares the light transmission spectra in the visible light region of a sample of a silica airgel prepared from a silica wet gel having a silica concentration obtained by diluting methyl silicate 10 times with methanol according to the type of supercritical drying medium. Although the carbon dioxide medium has a higher transmittance, the one using isopropyl alcohol as a medium has the highest transmittance among the alcohol-based solvents that can give hydrophobic silica airgel. It is shown that. The density of silica airgel is 0.10 to 0.14 g / cm3.
(Different depending on the size of the residual alkyl group, the one using isopropyl alcohol as the supercritical drying medium has the highest density), and the thickness is 9 mm.

FIG. 2 shows a similar comparison of the samples after heat treatment of the above-mentioned silica aerogel. In particular, the sample using isopropyl alcohol as the supercritical drying medium is particularly superior to the sample using another medium. It can be seen that the light transmittance is remarkably high in the short wavelength region. The density is around 0.14 g / cm3 and the thickness is about 8.5 mm. FIG. 3 shows a comparison of light transmittance of a sample after heat treatment of a silica airgel prepared from a silica wet gel having a silica concentration obtained by diluting methyl silicate 20 times with methanol. Density is 0.10
It is around g / cm3 and the thickness is about 7.7 mm. Even if the densities are different, the one in which isopropyl alcohol is used as a supercritical drying medium has a remarkably high light transmittance in the short wavelength region as compared with the ones in which other mediums are used as shown in FIG. I understand.

[Brief description of drawings]

FIG. 1 is a view showing the difference in the light transmission spectrum in the visible light region of a silica aerogel before heat treatment prepared from the same silica wet gel according to the type of supercritical drying medium.

FIG. 2 is a view showing a difference in a light transmission spectrum in a visible light region of a silica aerogel prepared from the same silica wet gel and subjected to a heat treatment depending on a type of a supercritical drying medium.

FIG. 3 shows the difference in the light transmission spectrum in the visible light region of the silica aerogel prepared from a silica wet gel having a lower silica concentration than that of the sample shown in FIG. FIG.

Claims (2)

[Claims] 1. A method for producing a silica airgel, wherein a liquid phase of a silica wet gel is replaced with isopropyl alcohol, and isopropyl alcohol is used as a medium for supercritical drying. 2. A silica airgel produced by the method according to claim 1 at 300 to 600 ° C. in an oxidizing atmosphere such as air.
A method for producing silica airgel by heat treatment in.