Coating processes – With post-treatment of coating or coating material – Liquid extraction of coating constituent or cleaning coating
Patent
1999-08-30
2000-12-05
Cameron, Erma
Coating processes
With post-treatment of coating or coating material
Liquid extraction of coating constituent or cleaning coating
427379, 427387, B05D 302
Patent
active
061563862
DESCRIPTION:
BRIEF SUMMARY
The invention is concerned with a method for the preparation of organically modified SiO.sub.2 aerogels.
Aerogels, especially those with porosities greater than 60% and densities below 0.6 g/cm.sup.3, have an extremely low thermal conductivity and therefore are used as thermal insulation material, for example, as described in EP-A-0 171 722.
In the broadest sense, that is, in the sense of "gel with air as dispersing agent", aerogels are produced by drying of a suitable gel. In this sense, the term "aerogel" includes aerogels in the narrow sense, xerogels and cryogels. A dry gel is called aerogel in the narrowest sense when the liquid of the gel is removed above the critical temperature and starting at pressures above the critical pressure. On the other hand, if the liquid of the gel is below the critical state and is removed, for example, with the formation of a liquid-vapor boundary phase, then the gel is frequently called a xerogel. It should be pointed out that the gels according to the invention are aerogels in the sense of a gel with air as dispersing agent.
SiO.sub.2 aerogels can be prepared, for example, by acid hydrolysis of tetraethylorthosilicate in ethanol. During the hydrolysis, a gel is formed, the structure of which is determined among others by the temperature, the pH value and the duration of the gelling process. However, generally, the gel structure collapses during the drying of wet gels, since the capillary forces that occur during drying are extremely large. Gel collapse can be prevented by carrying out the drying above the critical temperature and critical pressure of the solvent. Since, in this region, the liquid-gas phase boundary disappears, capillary forces are also absent and the gel does not change during drying, that is, no shrinking of the gel occurs during drying. Manufacturing methods based on this drying technique are known, for example, from EP-A-0 396 076 or WO 92/03378. However, this technique requires, for example when using ethanol, a temperature of about 240.degree. C. and pressure above 60 bar. By replacing the ethanol by CO.sub.2 before drying, the drying temperature can be reduced to about 30.degree. C. but then the pressure required is above 70 bar.
An alternative to the above drying is a method for the below-critical drying of SiO.sub.2 gels, in which these are reacted with a chlorine-containing silylating agent before drying. The SiO.sub.2 gel can be obtained, for example, in this method by acid hydrolysis of tetraalkoxysilanes, preferably tetraethoxysilane (TEOS) in a suitable organic solvent, preferably ethanol, using water. After the exchange of the solvent for a suitable organic solvent, in a next step, the obtained gel is reacted with a chlorine-containing silylating agent. Due to their reactivity, methylchlorosilanes (Me.sub.4-n SiCl.sub.n with n=1 to 3) are used as silylating agent. The SiO.sub.2 gel thus obtained, the surface of which is modified with methylsilyl groups, is then dried from an organic solvent in air. Thus, aerogels with densities of 0.4 g/cm.sup.3 and porosities above 60% can be obtained.
The preparation method based on this drying technology is described in detail in WO 94/25149.
The gels described above can also be reacted with tetraalkoxysilanes in the alcoholic-aqueous solution before drying and aged in order to increase the strength of the gel network, as disclosed, for example, in WO 92/20623.
In the method described above, the tetraalkoxysilanes, which are used as starting materials, represent an extraordinarily high cost factor.
A significant reduction in cost can be achieved by using water glass as starting material for the production of SiO.sub.2 gels. For this purpose, for example, starting with an aqueous water glass solution, with the aid of an ion-exchange resin, a silicic acid can be prepared which polycondenses upon the addition of a base to an SiO.sub.2 gel. After exchange of the aqueous medium for a suitable organic solvent, in the next step, the obtained gel is reacted with a chlorine-containing silylating agent. Due to
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Cabot Corporation
Cameron Erma
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