Coating processes – Particles – flakes – or granules coated or encapsulated – Inorganic base
Patent
1998-01-08
2000-12-12
Lovering, Richard D.
Coating processes
Particles, flakes, or granules coated or encapsulated
Inorganic base
516101, 428405, 423338, B01D 700, B32B 2102, B01J 1200, C01B 3312
Patent
active
06159539&
DESCRIPTION:
BRIEF SUMMARY
RELATED APPLICATION
This application is filed under 35 U.S.C. .sctn.371 and claims priority to PCT Application No. PCT/EP96/02894 filed Jul. 7, 1996, which claims priority to German Application No. 19525021.4 filed Jul. 5, 1995.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for preparing organically modified SiO.sub.2 aerogels and their use.
2.Description of the Related Art
Aerogels, in particular those having porosities above 60% and densities below 0.6 g/cm.sup.3, have an extremely low thermal conductivity and are therefore used as heat insulation material, such as described in EP-A-0 171 722.
SUMMARY OF THE INVENTION
Aerogels in the broad sense, i.e. in the sense of "gel having air as dispersion medium", are prepared by drying a suitable gel. The term "gel" in this sense includes aerogels in the narrower sense, xerogels and cryogels. A dried gel is termed an aerogel in the narrower sense here if the liquid of the gel is removed at temperatures above the critical temperature and starting from pressures above the critical pressure. If, in contrast, the liquid of the gel is removed subcritically, for example with formation of a liquid-vapor interface, the resulting gel is frequently also termed xerogel. It must be noted that the gels according to the invention are aerogels, in the sense of gel having air as dispersion medium. Since these gels are prepared by subcritical drying, they could also be described as xerogels, however.
SiO.sub.2 aerogels can be prepared, for example, by acid hydrolysis of tetraethyl orthosilicate in ethanol. In the course of the hydrolysis, a gel is formed whose structure is determined, inter alia, by the temperature, the pH and the duration of the gelation process. However, the gel structure generally collapses during drying of the wet gels, since the capillary forces occurring during drying are extremely strong. The collapse of the gel can be prevented by carrying out the drying above the critical temperature and critical pressure of the solvent. Since the liquid/gaseous phase boundary disappears in this range, the capillary forces are also absent and the gel does not change during drying, i.e. the gel also does not shrink during drying. Preparation processes based on this drying technique are disclosed, for example, by EP-A-0 396 076 or WO 92/03378. However, this technique requires, for example when ethanol is used, a temperature of about 240.degree. C. and pressures of over 60 bar. Although exchanging ethanol by CO.sub.2 prior to drying lowers the drying temperature to approximately 30.degree. C., the pressure required is then over 70 bar.
An alternative to the above drying is offered by a process for subcritical drying of SiO.sub.2 gels if these are reacted with a chlorine-containing silylation agent prior to drying. The SiO.sub.2 gel can be obtained in this case by means of water, for example by acid hydrolysis of tetraalkoxysilanes, preferably tetraethoxysilane (TEOS), in a suitable organic solvent, preferably ethanol. After exchanging the solvent for a suitable organic solvent, in a further step, the resulting gel is reacted with a chlorine-containing silylation agent. As silylation agent, use is preferably made of methylchlorosilanes (Me.sub.4-n SiCl.sub.n where n=1 to 3), on account of their reactivity. The resulting SiO.sub.2 gel which is modified on the surface by methylsilyl groups can then be air-dried from an organic solvent. By this means aerogels having densities below 0.4 g/cm and porosities above 60% can be attained.
The preparation process based on this drying technique is described in detail in WO 94/25149.
Furthermore, the above-described gels can be admixed with tetraalkoxysilanes in the alcoholic aqueous solution prior to drying and aged in order to increase the gel network strength, as disclosed, for example, in WO 92/20623.
However, the tetraalkoxysilanes used as starting materials in the above-described processes represent an extraordinarily high cost factor.
A considerable reduction in costs can be achie
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Journal of Liquid Chromatography, vol. 11, No. 16, pp 3375-3384, 1988, month unknown.
Schwertfeger Fritz
Zimmermann Andreas
Hoeschst Aktiengesellschaft
Lovering Richard D.
Metzmaier Daniel S.
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