Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
1997-11-21
2001-03-06
Hendrickson, Stuart (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C502S233000, C065S017200
Reexamination Certificate
active
06197270
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing an aerogel. More specifically, the present invention relates to a process for producing a silica aerogel, which has properties such as heat insulating properties peculiar to porous materials. Additional properties of the aerogel include and transparency, as well as excellent hydrophobic nature.
2. Description of the Prior Art
A process for producing an aerogel which is useful as a translucent heat insulator has been disclosed in U.S. Pat. No. 4,402,927, U.S. Pat. No. 4,432,956, and U.S. Pat. No. 4,610,863. These patents disclosed a following process for obtaining an aerogel. That is, hydrolysis and condensation polymerization of alkoxysilane (also named silicon alkoxide or alkyl silicate) are carried out, and then the resulting gel is dried in the presence of a dispersion medium under a supercritical condition over a critical point of the dispersion medium (such a drying method is called supercritical drying), to obtain the aerogel.
In this process, the dispersion medium used in supercritical drying is carbon dioxide or organic solvents such as ethanol and the like. However, in the case of using carbon dioxide as the dispersion medium, since the resulting aerogel retains silanol groups on the surface therefore, it manifests hygroscopicity and therefore adsorbs moisture in the atmosphere. Further, silanol groups having high hydrophilicity are formed on the surface of the aerogel by adsorbed water, which adsorbs more moisture in the atmosphere over time. On the other hand, in the case of using organic solvents such as ethanol and the like as the dispersion medium of supercritical drying, since the resulting aerogel retains alkoxyl groups on the surface, the aerogel retains hydrophobicity immediately after the supercritical drying. But, the alkoxyl groups are hydrolyzed by moisture in the atmosphere over time, so that silanol groups are formed on the surface of the aerogel and, consequently adsorbs moisture in the atmosphere in the same way as the above case.
Therefore, there is a problem that optical properties such as transparency or thermal properties such as heat insulating properties of the aerogel are lowered, and dimensional change or cracks arises from shrinkage caused by moisture adsorption, so that deterioration of quality or performance arises, resulting in lack of practical use.
The present inventors have proposed a process for producing an aerogel, which can prevent properties therefore from deteriorating over time by imparting hydrophobicity thereto, in JPA-HEI 5-279,011 and JPA-HEI 7-138,375. In these processes, alkoxysilane compounds are used as a starting material and an aerogel is produced through the steps of hydropbobilizing treatment and supercritical drying.
However, in a process for producing an aerogel using alkoxysilane, there is the following problem. In the process, where alkoxysilane is hydrolyzed in an organic solvent such as an alcohol with water and a catalyst, the resulting product is condensation-polymerized, and a gel is prepared through a sol state; but, in this case, since organic solvents such as alcohol used as a solvent have relatively high volatility, when the gel obtained is exposed to the air, the organic solvents contained in the gel is volatilized in a short time, so that the resulting aerogel is liable to manifest cracks or shrinkage before supercritical drying. This particularly occurs in the steps of preparation of the sol, gelation and curing of the gel. Therefore, there is a problem that the resulting aerogels obtained after supercritical drying include some off-qualities such as cracks and the like, and the yield of non-detectives thereof becomes lower.
In order to prevent an organic solvent of an alcohol and the like used in the sol or the gel from volatilizing, it is necessary to attain the airtight condition or to fill the atmosphere with the vapor of the same organic solvent from the time point of preparing the sol. Curing the gel is effective for strengthening the gel structure to heat at a range from room temperature to several tens of ° C. However, it is not preferable from a safety viewpoint to heat under the condition of filling the atmosphere with the vapor of an organic solvent.
In view of these circumstances, the present inventors investigated starting materials as alternatives alkoxysilane and paid attention to water glass as a promising material. Another process for producing an aerogel made from water glass is disclosed in JPA-SHO47-15,398, U.S. Pat. No. 5,137,927 and JPA-HEI2-304,299. In the process, since supercritical drying is performed using an alcohol as a dispersion medium, the resulting aerogel has transient hydrophobicity, but deteriorates over time, as described above.
SUMMARY OF THE INVENTION
The present invention to provide a process for efficiently producing an aerogel which has excellent heat insulating properties and transparency, and which can prevent deterioration of these properties with time due to water or moisture.
A process of the present invention comprises the steps of, preparing a sol as a hydrosol from a water glass solution by lowering a pH value thereof; gelling the sol to obtain a gel as a hydrogel; reacting the gel with a hydrophobilizing agent having hydrophobic groups as well as functional groups reactive with silanol groups; and supercritically drying the gel to obtain an aerogel. According to the present invention, by performing supercritical drying, an aerogel is produced an aerogel that is formed into a porous structure without shrinking and is excellent in such properties as heat insulating properties peculiar to porous materials and transparency. Moreover, by hydrophobilizing the aerogel, the above properties are deteriorate difficulty with time due to water or moisture. Further, according to the present invention, non-defective aerogels are more efficiently obtained in comparison with a process using alkoxysilanes as a starting material. Since a water glass solution is used as a starting material in the present invention, the sol and the gel respectively become a hydrosol and a hydrogel containing water as a dispersion medium, the water is not easily volatilized in a short time even when the sol or the gel is exposed to the air. Therefore, the resulting aerogel does not manifest cracks or shrinkage prior to supercritical drying.
The present invention includes two aspects, each having a different way of reacting the gel with the hydrophobilizing agent.
In the first aspect of the invention, after replacing water in the gel with an organic solvent, this gel is reacted with a hydrophobilizing agent in liquid phase. The solvent contained in the resulting gel is then removed by supercritically drying, to obtain a hydrophobic aerogel.
On the other hand, in the second aspect of the invention, the gel is reacted with a hydrophobilizing agent in a dispersion medium in the state of a supercritical fluid while supercritically drying the gel. In this case, since a hydrophobilizing treatment is carried out while supercritically drying the gel, there is the following merit in comparison with the first aspect. That is, in the first aspect in which supercritical drying is performed after hydrophobilizing treatment in liquid phase, a diffusion rate of the hydrophobilizing agent into the gel is slow and the reaction of the hydrophobilizing treatment requires a long time. Also, the extent of the hydrophobilizing treatment is not always uniform between outside portion and inner portion of the gels. Furthermore, it is necessary to use a large quantity of an organic solvent for washing the gels before and after the hydrophobilizing treatment. In the second aspect, a diffusion rate of the hydrophobilizing agent into the gel is high, and the hydrophobilizing treatment can be and rapidly uniformly done in the inner portion of the gels. Also, it is not necessary to use a large quantity of an organic solvent, as in the first aspect.
The present invention includes the third aspect in which the st
Sonoda Kenji
Tsubaki Kenji
Yokogawa Hiroshi
Yokoyama Masaru
Birch & Stewart Kolasch & Birch, LLP
Hendrickson Stuart
Matsushita Electric & Works Ltd.
LandOfFree
Process for producing aerogel does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for producing aerogel, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for producing aerogel will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2493185