Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Patent
1995-10-27
1998-01-06
Foelak, Morton
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
2523156, 2523157, 423338, 521180, 521181, 521187, 521188, 521915, C08J 928
Patent
active
057055351
DESCRIPTION:
BRIEF SUMMARY
A "sol" is taken to mean a colloidal, liquid system in which the dispersed particles (size 1 to 1000 nm) are either solids or macromolecules.
A "gel" is a colloidal system having solid character in which the colloidal constituents form a continuous (interpenetrating) network in a dispersing medium whose kinetics are slower than those of the colloidal constituents. The dispersed constituents are held together by covalent bonds, hydrogen bonds, dipole forces, van der Waals forces and/or by mechanical intertwining.
The gels are subdivided with respect to their dispersing medium (water, alcohol, organic solvents, air) into hydrogels for water, alkogels for alcohol, lyogels for organic solvents and aerogels for air.
If air is contained as a dispersant, the gels are additionally differentiated according to the type of drying/preparation:
If the gel liquid was removed by simple drying, forming a liquid/vapor interface, the dried gel is termed a "xerogel".
If the gel liquid was removed above its critical point and pressure (supercritical conditions), the dried gel is termed an "aerogel". Because of the supercritical conditions, no interface is formed.
If the liquid was removed by freeze drying, the dried product is termed a "cryogel". Here, the solid/gas interface is overcome by sublimation.
Hereafter, aerogels are also taken to mean xerogels and cryogels in accordance with the above definition.
Generally, aerogels are highly porous materials made of silica or metal oxide which are distinguished by particularly low densities of 20 to 300 kg/m.sup.3 with extremely high internal surface areas of over 1000 m.sup.2 /g. Because of these properties, aerogels are outstandingly suitable as heat insulators, sound insulators, catalyst supports and as adsorbents.
Aerogels can be prepared according to the prior art by two different routes via a sol-gel process with subsequent supercritical drying.
In the Kistler method (S. S. Kistler, J. Phys. Chem. 36 (1932), pp. 52 to 64), water glass is used as starting material. Acidifying water glass with HCl or H.sub.2 SO.sub.4 produces a silica hydrogel which is then freed of alkali metal ions by washing them out with water. Water contained in the hydrogel is then completely exchanged in one step for 95% pure alcohol (ethanol, methanol). The resulting alkogel is then dried supercritically in an autoclave.
Since the drying of alkogels requires high temperatures and high pressures, solvent being exchanged for CO.sub.2 prior to the supercritical drying. The supercritical drying from CO.sub.2 proceeds at substantially lower temperatures (T.sub.k =31.1.degree. C., p.sub.k =73.9 bar).
The second method for preparing aerogels according to the prior art starts SiO.sub.2 aerogels, a precisely measured amount of water and catalyst are added to tetramethoxysilane in methanol or in ethanol. In the hydrolysis, with elimination of alcohol, silicic acid is formed which in turn forms an SiO.sub.2 gel with elimination of H.sub.2 O (sol/gel process). The alkogel formed in this manner is supercritically dried in an autoclave. This method can also be used to prepare organic aerogels from No. 5,086, 085, U.S. Pat. No. 5,081, 163, U.S. Pat. No. 4,997, 804, U.S. Pat. No. 4,873, 218!.
The disadvantages of supercritical drying methods are the temperature and pressure conditions and the discontinuous mode of operation. When water-containing gels are dried, temperatures of at least 370.degree. C. and pressures of 220 bar are necessary. When methanol-containing gels are dried, temperatures of at least 240.degree. C. and pressures of at least 81 bar are necessary. Even when the organic solvent is exchanged with CO.sub.2, drying thereof proceeds at pressures of at least 74 bar and temperatures of at least 31.degree. C. The disadvantages of supercritical drying at atmospheric pressure and with supply of heat by contactor by convection are that the resulting capillary forces lead to gel collapse. This hazard exists particularly in the case of hydrogels or lyogels having a low solids content.
In the supercritical dryi
REFERENCES:
patent: 4190457 (1980-02-01), McDaniel
patent: 4970397 (1990-11-01), Green et al.
patent: 5081163 (1992-01-01), Pekala
Chemical Abstracts, vol. 80, No. 10, 11 Mar. 1974, Columbus, Ohio; Abstract No. 52623b, Butsko `Effect of an electric field on silica gel structure`.
Jansen Rolf-Michael
Kessler Birgit
Wonner Johann
Zimmermann Andreas
Foelak Morton
Hoechst Aktiengesellschaft
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