Aerogel composites, process for producing the same and their use

Details Aerogel composites, process for producing the same and their use Aerogel composites, process for producing the same and their use

1997-02-20

1998-08-04

Bell, James J.

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Composite having voids in a component

252 62, 25231501, 264 42, 264 48, 264344, 423325, 4282934, 4283179, 428902, 428920, 442334, 442381, 442387, 442391, 501 12, 501 35, 501154, B32B 326, C01B 33113, E04B 174

Patent

active

057890757

DESCRIPTION:

BRIEF SUMMARY
The invention relates to aerogel composites, a process for producing same and their use.
Aerogels having porosities above 60% and densities below 0.6 g/cm.sup.3 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 aerogel is obtained by drying but also of the dried aerogel itself. The aerogels are not very flexible and are very brittle.
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 narrow 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 subcritically, for example through formation of a liquid-vapor boundary phase, the resulting gel is termed a xerogel. The term "aerogel" as used herein means, unless otherwise stated, aerogels in the wider sense, i.e. in the sense of "gel with air as dispersion medium."
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 supercritically, 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.
WO93/06044 discloses how fiber reinforcement of the aerogel makes it possible to obtain a fiber-reinforced, monolithic aerogel whose mechanical stability is appreciably superior to that of an unreinforced aerogel. However, the fiber reinforcement further reduces the flexibility of the fiber-reinforced, monolithic material compared with that of the pure aerogel; the resulting sheets have a high modulus of elasticity. In addition, the production process described in WO93/06044 is technically very difficult because of the need for supercritical drying. It involves drying the 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.
There are a number of applications, for example use as a thermal insulating material for curved surfaces, where it would be desirable to be able to use aerogels in a flexible form, i.e. in the form of very flexible sheets or mats. However, the great brittleness of prior art aerogels prevents this.
It is an object of the present invention to provide very flexible aerogels having porosities above 60% and densities below 0.6/cm.sup.3.
The present invention accordingly provides a mat-shaped composite comprising an aerogel having porosities above 60% and densities below 0.6 g/cm.sup.3 and fibers dispersed therein, the aerogel having cracks and the aerogel fragments enclosed by the cracks, whose average volume is 0.001 mm.sup.3 to 1 cm.sup.3, being held together by the fibers.
The composites of this invention preferably comprise aerogel fragments having an average volume of 0.1 to 30 mm.sup.3. The smaller the ave

REFERENCES:
patent: 4447345 (1984-05-01), Kummermehr et al.
patent: 5306555 (1994-04-01), Rammurthi et al.
Chemical Abstracts, vol. 119, No. 4, Jul. 26, 1993.

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