Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Patent
1997-05-01
1998-06-02
Marcantoni, Paul
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
203 57, 502233, B01D 334, C01B 3312
Patent
active
057595062
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
The invention relates to a process for preparing modified SiO.sub.2 gels, referred to below as "aerogels".
Aerogels in the wider sense, i.e. in the sense of "gel with air as dispersion medium", are prepared by drying a suitable gel. The term "aerogel" in this sense includes aerogels in the narrower sense, xerogels and cryogels. A dried gel is referred to as an aerogel in the narrower sense if the gel liquid is removed at temperatures above the critical temperature, and starting at pressures above the critical pressure. If the gel liquid is, on the other hand, removed subcritically, for example with formation of a liquid-vapor boundary phase, the resultant gel is referred to as a xerogel. It should be noted that the gels prepared according to the invention are aerogels in the sense of gel with air as dispersion medium. Since these gels are prepared by subcritical drying, they may, however, also be referred to as xerogels.
SiO.sub.2 aerogels are known for their excellent heat insulation efficiency. They can be prepared, for example, by acid hydrolysis of tetraethyl orthosilicate in ethanol. The hydrolysis gives a gel whose structure can be influenced by the temperature, the pH and the duration of the gelling process. However, the gel structure generally collapses when the wet gel is dried, since the capillary forces arising during the drying are extremely large. Collapse of the gel can be prevented by carrying out the drying at a temperature above the critical temperature and at a pressure above the critical pressure of the solvent. Since the liquid/gaseous phase boundary disappears in this region, capillary forces are also absent, and the gel does not change its nature during the drying process, i.e. there is also no shrinkage of the gel during drying. Preparation processes based on this drying technique are known, for example, from EP-B-0 396 076 and WO 92/03378. This technique requires, however, for example if ethanol is used, a temperature greater than 240.degree. C. and pressures of up to 40 bar.
Although the drying temperature is reduced to about 40.degree. C. if ethanol is replaced by CO.sub.2 before drying, the necessary pressure, however, is then 80 bar.
WO 92/20623 discloses a process in which SiO.sub.2 aerogels are prepared by hydrolysis and polycondensation of tetraalkoxysilanes, wherein the aqueous/alcoholic gel is aged in a solution of tetraalkoxysilanes in order to increase the strength of the gel structure. However, the prior art has the disadvantages that extremely expensive raw materials, such as tetraalkoxysilanes, are employed both for preparing and for aging of the gels, and that the dwell times necessary for aging of the gel and strengthening of the structure are very long.
It has now been found that deionized SiO.sub.2 gels, prepared from inexpensive water glass, can be dried under sub-critical conditions if, before drying, they are aged in an aqueous/organic solution of alkyl and/or aryl ortho-silicate and/or free silicic acid. The products obtained are referred to below as "aerogels", and have outstanding heat insulation efficiency.
The invention therefore relates to a process for preparing aerogels, which comprises acid ion exchanger or a mineral acid, give an SiO.sub.2 gel, and, if a mineral acid was used in step a), washing the gel with water until it is free of electrolytes, or organic solvent until the water content of the gel is <20% by weight, condensable alkyl and/or aryl ortho-silicate of the formula R.sup.1.sub.4-n Si(OR.sup.2).sub.n, where n is from 1 to 4 and R.sup.1 and R.sup.2, independently of one another, are C.sub.1 -C.sub.6 -alkyl, cyclohexyl or phenyl, or using an aqueous silicic acid solution, in order to strengthen the gel structure,
In step a), an acid ion-exchanger resin is preferably employed, those containing sulfonic acid groups being particularly suitable. If mineral acids are employed, hydrochloric and sulfuric acid are especially suitable. The water glass used is in general sodium water glass and/or potassium water glass.
The base used in
REFERENCES:
patent: 3794713 (1974-02-01), Aboutboul et al.
patent: 5647962 (1997-07-01), Jansen et al.
Jansen Rolf-Michael
Zimmermann Andreas
Hoechst Aktiengesellschaft
Marcantoni Paul
LandOfFree
Process for preparing aerogels 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 preparing aerogels, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for preparing aerogels will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-1456400