Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Patent
1997-08-13
2000-06-27
Speer, Timothy M.
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
428 365, 4282947, 428323, 264640, 264642, B32B 516, B32B 1310
Patent
active
060804750
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a composite material comprising from 10 to 95% by volume of aerogel particles and at least one inorganic binder, a process for its preparation, and its use.
Most non-porous, inorganic solids have relatively high thermal conductivity, since heat is conducted efficiently through the solid material. In order to achieve lower thermal conductivities, use is therefore often made of porous materials, e.g. based on vermiculites. In a porous body, there remains only a solid framework which can transmit heat efficiently, whereas the air in the pores, is in comparison with the solid body, transmits less heat.
However, pores in a solid generally lead to a deterioration in its mechanical stability, because stresses can be transmitted only through the framework. Therefore, porous, but still mechanically stable materials likewise have relatively high thermal conductivity.
For many applications, however, a very low thermal conductivity in conjunction with good mechanical strength, i.e. high compressive and flexural strengths, is desirable. Firstly, shaped articles need to be machined, and secondly, depending on the application, they must be able to withstand mechanical loads without fracture or cracking, even at elevated temperatures.
Because of their very low density, high porosity and small pore diameter, aerogels, especially those having porosities greater than 60% and densities less than 0.6 g/cm.sup.3, have extremely low thermal conductivities and therefore find application as heat-insulating materials, as described, for example, in EP-A-0 171 722. The small pore diameters, less than the mean free path of air molecules, are particularly important for low thermal conductivity, since they give rise to a lower thermal conductivity for the air in the pores than for air in macropores. Therefore, the thermal conductivity of aerogels is even smaller than that of other materials having similar porosity values but larger pore diameters, such as e.g. foams or materials based on vermiculites.
However, the high porosity also gives rise to low mechanical stability, both of the gel from which the aerogel is dried and also of the dried aerogel itself.
Aerogels in the widest sense, i.e. in the sense of "gels containing air as dispersion medium" are produced by drying a suitable gel. The term "aerogel" in this sense includes aerogels in the narrower sense, xerogels and cryogels. A dried gel is referred to as an aerogel in the narrower sense if the gel liquid is removed at temperatures above the critical temperature and starting from pressures above the critical pressure. If, by contrast, the gel liquid is removed subcritically, for example with formation of a liquid-vapor boundary phase, the resulting gel is often also referred to as a xerogel. It should be noted that the gels according to the invention are aerogels in the sense of gels containing air as dispersion medium.
For many applications, however, it is necessary to use aerogels in shaped articles having adequate mechanical stability.
EP-A-0 340 707 discloses an insulating material having a density of from 0.1 to 0.4 g/cm.sup.3 comprising at least 50% by volume of silica aerogel particles having a diameter between 0.5 and 5 mm, bound together by at least one organic and/or inorganic binder. The comparatively coarse particle size has the result that shaped articles produced from the insulating material have an inhomogeneous distribution of the aerogel material. This applies especially if the smallest typical dimensions of the shaped articles, which in films or sheets is the thickness, is not very much greater than the typical diameter of aerogel particles. Especially at the perimeters, an increased binder proportion would be necessary, which would have an adverse effect on the thermal conductivity of the shaped article, particularly at the surface.
Furthermore, in a shaped article made from this insulating material, regions of low mechanical stability with diameters between 0.5 and 5 mm and comprising aerogel material would appear at the surface and, un
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Frank Dierk
Stuhler Helmut
Zimmermann Andreas
Hoechst Aktiengesellschaft
Speer Timothy M.
Stein Stephen
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