Compositions – Electrolytes for electrical devices
Patent
1996-02-29
1998-04-14
Lovering, Richard D.
Compositions
Electrolytes for electrical devices
2523156, 423338, 502233, 524493, 5245755, F04B 174, B01J 2108, C01B 33158, C08K 336
Patent
active
057388016
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of PCT/EP 94/02822 filed Aug. 25, 1994 which is based on Fed. Rep. Germany P 4329294.1 filed Aug. 21, 1993.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to novel hydrophobic silica aerogels obtainable by reacting a waterglass solution with an acid at a pH of from 7.5 to 11, substantially removing ionic constituents from the resulting silica hydrogel by washing with water or dilute aqueous solutions of inorganic bases, while the pH of the hydrogel is kept in the range from 7.5 to 11, displacing the aqueous phase contained in the hydrogel by an alcohol, and subsequent supercritical drying of the resulting alcogel.
The invention furthermore relates to the preparation of these aerogels and to the use thereof as thermal insulating material, as support material for catalysts and as additive in rubber.
2. Description of the Related Art
Silica aerogels are outstandingly suitable, because they have only low thermal conductivity, as thermal insulating material. As is known, they are obtainable, for example, by precipitating silica from waterglass using an acid, washing the ionic constituents out of the hydrogel formed, displacing the water by a lower-boiling, water-soluble organic liquid, especially an alcohol such as methanol, heating the resulting "alcogel" containing the alcohol under supercritical pressure to a temperature above the critical temperature of the organic liquid and removing the liquid by flash evaporation at the supercritical temperature. If required, the water-soluble liquid can be replaced, before this supercritical drying, by a water-insoluble organic liquid or else liquid carbon dioxide (US-A-2 093 454, DE-A 34 29 671).
The large-volume and large-pore silica aerogel produced in this drying process can be converted into powder form by milling. A special method for producing small aerogel particles comprises, according to DE-A 21 03 342, spraying waterglass and acid from a mixing nozzle, which results in drop-shaped particles.
The aerogels prepared by the known processes must also be subjected to a subsequent treatment to render them hydrophobic because otherwise they show a drastic deterioration, owing to a rapid onset of uptake of water, in their insulation properties.
Normally used for this purpose are organosilicon compounds, silanes and chlorosilanes, with which the dry gel is treated in the gas phase or which may also be present even during the precipitation or supercritical drying. However, these water repellents are costly, and when chlorosilanes are used it is necessary to remove and neutralize the hydrogen chloride produced.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide hydrophobic silica aerogels which can be prepared in a simple and economic manner.
We have found that this object is achieved by hydrophobic silica aerogels which are obtainable by reacting a waterglass solution with an acid at a pH of from 7.5 to 11, substantially removing ionic constituents from the resulting silica hydrogel by washing with water or dilute aqueous solutions of inorganic bases, while the pH of the hydrogel is kept in the range from 7.5 to 11, displacing the aqueous phase contained in the hydrogel by an alcohol, and subsequent supercritical drying of the resulting alcogel.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
We have also found the process defined hereby for preparing these aerogels.
Last but not least, we have found the use of these aerogels as thermal insulating material, as support material for catalysts and as additive to rubber.
The hydrogels which are used as starting materials for the silica aerogels can advantageously be prepared from alkali metal waterglass, especially sodium waterglass. This entails a waterglass solution, which is normally from 10 to 30, preferably 12 to 20, % by weight, being mixed homogeneously with a dilute acid, in particular a mineral acid, preferably sulfuric acid, so that the pH of the mixed product is adjusted to, as a rule, from 7.5 to 11, preferably 8 to 11, pa
REFERENCES:
patent: 2093454 (1937-12-01), Kistler
patent: 3346507 (1967-10-01), Taulli
patent: 4053435 (1977-10-01), Lynch
patent: 4256682 (1981-03-01), Denton
patent: 4610863 (1986-09-01), Tewari et al.
patent: 4954327 (1990-09-01), Blount
patent: 5294480 (1994-03-01), Mielke et al.
Herrmann Gunter
Mronga Norbert
Teich Friedhelm
Ziegler Bernd
BASF - Aktiengesellschaft
Lovering Richard D.
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