Process for preparing organically modified aerogels using alcoho

Details Process for preparing organically modified aerogels using alcoho Process for preparing organically modified aerogels using alcoho

1998-10-26

2000-10-10

Lovering, Richard D.

Coating processes

Particles, flakes, or granules coated or encapsulated

Inorganic base

516 87, 516100, 423338, C01B 3316, C01B 3318, B01J 1300

Patent

active

061299499

DESCRIPTION:

BRIEF SUMMARY
Process for the preparation of organically modified aerogels by using alcohols, in which process the salts formed are precipitated.
The invention relates to a process for the preparation of organically modified SiO.sub.2 aerogels with the use of alcohols, in which process the salts formed are precipitated.


BACKGROUND OF THE INVENTION

1. Field of the Invention
Aerogels, particularly those having porosities over 60% and densities below 0.6 g/cm.sup.3 have an extremely low thermal conductivity and for this reason are used as thermal insulating materials, as described e.g. in EP-A-0 171 722.
2. Discription of the Related Art
Aerogels in the broader sense of the term, i.e. in the sense of a "gel with air as dispersing agent," are prepared by drying a suitable gel. Understood by the term "aerogel" in this sense are aerogels considered in the narrow sense, xerogels and cryogels. A dried gel is considered an aerogel in the narrow sense of the term when the liquid of the gel is removed at temperatures above the critical temperature and starting from pressures above the critical pressure. However, if the liquid of the gel is removed under subcritical conditions, e.g. with the formation of a liquid-vapor boundary phase, then the resulting gel is designated as a xerogel. It should be noted that the gels according to the invention are aerogels in the sense of gels with air as dispersing agent.
SO.sub.2 aerogels can be prepared e.g. by acid hydrolysis of tetraethyl orthosilicate in ethanol. During the hydrolysis a gel is formed whose structure is determined, among other things, by the temperature, the pH and the duration of the gelation process. However, during the drying of the wet gel the gel structure generally collapses because the capillary forces resulting during drying are extremely great. Collapse of the gel can be prevented by carrying out the drying above the critical temperature and critical pressure of the solvent. Since in this range the liquid/gas phase boundary disappears, the capillary forces also vanish and the gel does not change during the drying process, i.e. no shrinking of the gel during drying will occur, either. Methods of preparation based on this drying technology are disclosed e.g. in EP-A-0 396 076 or WO 92/03378. However, e.g. when ethanol is used, this technique requires a temperature of about 240.degree. C. and pressures over 60 bar. Although the exchange of ethanol against CO.sub.2 before drying does reduce the drying temperature to about 30.degree. C., the pressure required is then over 70 bar.
An alternative for the above drying method is offered by a process of subcritical drying of SiO.sub.2 gels, if, before drying, the latter are reacted with a chlorine-containing silylating agent. In that case the SiO.sub.2 gel can be obtained e.g. by acid hydrolysis of tetraalkoxysilanes, preferably tetraethoxysilane (TEOS) in a suitable solvent, preferably ethanol, by means of water. In a further step, after exchange of the solvent against a suitable organic solvent, the resulting gel is reacted with a chlorine-containing silylating agent. Used as silylating agents, because of their reactivity, are preferably methylchlorosilanes (Me.sub.4-n SiCl.sub.n, with n=1 to 3). Thereupon the resulting SiO.sub.2 gel whose surface has been modified by methylsilyl groups, can be dried in air from an organic solvent. In this way aerogels having densities of less than 0.4 g/cm.sup.3 and porosities over 60% can be obtained. WO 94/25149 gives a detailed description of the method of preparation based on this drying technique.
Furthermore, before drying, the above-described gels can be treated in the aqueous alcoholic solution with tetraalkoxysilanes, and then aged, in order to increase the strength of the gel network, as disclosed e.g. in WO 92/20623.
However, the tetraalkoxysilanes used as starting materials in the above-described process are extremely expensive. Furthermore, during silylation with chlorine-containing silylating agents hydrogen chloride (HCL) and a plurality of side products associated therewit

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