Compositions – Electrolytes for electrical devices
Patent
1997-02-21
1999-02-02
Bell, James J.
Compositions
Electrolytes for electrical devices
2523152, 428920, 501 12, F04B 174
Patent
active
058660275
DESCRIPTION:
BRIEF SUMMARY
This application is a .sctn.371 of PCT/EP95/03275, filed Aug. 17, 1995, based on Fed. Rep. Germany Application No. P 44 30 669.5, filed Aug. 29, 1994.
The invention relates to a process for producing modified, fiber-reinforced xerogels having porosities greater than 60% and densities below 0.6 g/cm.sup.3, hereinafter termed "fiber-reinforced xerogels", and to their use.
Xerogels having porosities above 60% and densities below 0.6 g/cm.sup.3 , like supercritically dried aerogels, have very low thermal conductivity because of their very low density and high porosity. However, the high porosity also leads to low mechanical stability not only of the gel from which the xerogel is obtained by drying but also of the xerogel itself.
Aerogels in the wider sense, i.e. in the sense of "gel with air as dispersion medium", are formed by drying a suitable gel. The term "aerogel" in this sense encompasses aerogels in the narrower sense, xerogels and cryogels. A dried gel is an aerogel in the narrower sense when the liquid of the gel is substantially removed at temperatures above the critical temperature and starting from pressures above the critical pressure. If, by contrast, the liquid of the gel is removed suberitically, for example through formation of a liquid-vapor boundary phase, the resulting gel is termed a xerogel.
On drying conventional gels, capillary forces give rise to very high stresses which, in conjunction with the low stability of the highly porous gel structure, lead to collapse in the course of drying. The capillary pressure P.sub.c which arises during the drying process is determined by the surface tension Y.sub.LV of the surface between pore liquid 2 and its vapor 4, the contact angle .theta. between liquid meniscus and pore wall 3 and the radius r of pore 1 (FIG. 1): ##EQU1##
The collapse can be avoided if the gel is dried super-critically, as described for example in EP-A-0382310, EP-A-0018955 and U.S. Pat. No. 4,610,863, and hence the surface tension Y.sub.LV is reduced to zero. However, the aerogels thus produced are mechanically not very stable.
However, the mechanical stability can be improved by fiber-reinforcing the aerogel as described in WO93/06044. However, the production process described in WO93/06044 is technically very difficult because of the need for supercritical drying. It involves drying a gel from an alcohol, for example methanol, at temperatures of 250.degree. to 260.degree. C. and pressures between 9.7 and 15.9 MPa.
DE-A-43 42 548 and WO 94/25149 describe processes for producing xerogels having densities below 0.3 g/cm.sup.3 and porosities above 60% where there is no need for supercritical drying of gels. The gels are modified by treatment of the internal surface area, for example by silylation in the case of SiO.sub.2 xerogels, in such a way that they can be air dried without collapsing. The thus-produced xerogels are likewise mechanically not very stable and break easily.
It is an object of the present invention to provide a process for producing mechanically stable xerogels having porosities above 60% and xerogel matrix densities below 0.6 g/cm.sup.3 whereby the above-described disadvantages are avoided.
It has now been found that mechanically stable xerogels can be produced if, before or during the formation of the gel, fibers are incorporated into the sol in suitable form and amount, the pore surface of the the gel produced from the sol is modified in suitable form, and the gel is then dried under subcritical conditions. The products obtained are hereinafter termed "fiber-reinforced xerogels".
The invention accordingly provides a process for producing fiber-reinforced xerogels having porosities greater than 60% by volume and xerogel matrix densities less than0.6 g/cm.sup.3, which comprises way that a sufficiently large proportion of the surface groups of the gel is replaced by groups of the surface-modifying substance as to substantially suppress any further condensation between the surface groups on different pore surfaces and/or reduce the capillary forces by changing th
REFERENCES:
patent: 5306555 (1994-04-01), Ramamurthi et al.
6001 Chemical Abstracts, 119 (1993) Jul. 26, No. 4, Columbus, Ohio, U.S.
Frank Dierk
Kessler Birgit
Zimmermann Andreas
Bell James J.
Hoechst Aktiengesellschaft
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