Fabric (woven – knitted – or nonwoven textile or cloth – etc.) – Nonwoven fabric – Including particulate material other than strand or fiber...
Reexamination Certificate
2000-01-20
2002-11-12
Juska, Cheryl A. (Department: 1771)
Fabric (woven, knitted, or nonwoven textile or cloth, etc.)
Nonwoven fabric
Including particulate material other than strand or fiber...
C442S381000, C442S389000, C442S393000, C442S394000, C442S415000
Reexamination Certificate
active
06479416
ABSTRACT:
The present invention relates to a composite material that contains at least one formation of fibrous material and aerogel particles, a process for the production of this, and the use thereof.
Because of their very low density, high porosity, and small pore diameters, aerogels, in particular those with porosities of greater than 60% and densities of less than 0.4 g/cm
3
, exhibit very low thermal conductivity and for this reason are used as thermal insulating materials as is described, for example, in EP-A-O 171 722.
However, their great porosity results in very poor mechanical stability, both of the gel from which the aerogel is dried, as well as of the dried aerogel itself.
In the broadest sense, i.e., when regarded as “gels with air an the dispersed material,” aerogels are manufactured by curing a suitable gel. When used in this sense, the term “aerogel” includes aerogels in the narrower sense, such as xerogels and cryogels. A gel is designated as an aerogel in the narrower sense if the liquid is removed from the gel at temperatures above the critical temperature and starting from pressures that are above the critical pressure. In contrast to this, if the liquid is removed from the gel sub-critically, for example with the formation of a liquid-vapour boundary phase, then the resulting gel is, in many instances, referred to as xerogel. It should be noted that the gels according to the present invention are aerogels in the sense that they are gels with air as the dispersed material.
The process that shapes the aerogel in concluded during the sol-gel transition. Once the solid gel structure has been formed, the external shape can only be changed by size reduction, for example, by pulverizing. The material is too brittle for any other form of processing.
For many applications, however, it is necessary to use the aerogel in certain shapes. In principle, the production of shaped parts is possible even at the gel is being formed. However, the replacement of solvents, typically required during production, and which is governed by diffusion (with respect to aerogels, see, for example, U.S. Pat. No. 4 610,863 and EP-A 0 396 076; with respect to aerogel composite materials, sea, for example, WO 93/06044), and the drying—which is similarly governed by diffusion—lead to production times that are economically unacceptable. For this reason, it is appropriate to carry out a shaping stage after the production of the aerogel, which is to say, after it has been dried, and to do this without any essential change of the internal structure of the aerogel taking place with respect to the particular application.
For many applications, e.g., in order to insulate curved or irregularly shaped surfaces, it is necessary to use flexible panels or mats of insulating material.
DE-A 33 46 180 describes rigid panels from shaped bodies based on silicic acid aerogel obtained by flame pyrolysis combined with reinforcement by long mineral fibres. However, this silicic acid aerogel that is extracted from flame pyrolysis is not an aerogel in the above sense since it is not manufactured by curing a gel, and for this reason it has a completely different pore structure. Mechanically, it is much more stable and for this reason can be pressed without destruction of the microstructure, although it has a greater thermal conductivity than typical aerogels in the above sense. The surface of a moulded body such as this is extremely delicate and for this reason must be hardened, as by the use of a binder, or by being covered with a film. Furthermore, the resulting shaped body cannot be compressed.
DE-A-44 18 843 describes a mat of a fibre-reinforced aerogel. It is true that, because of the very high proportion of aerogel, these mats display a very low level of thermal conductivity, but they require relatively protracted production times because of the above-described diffusion problems. In particular, it is only possible to manufacture thicker mats by combining a number of thinner mats, and this involves additional outlays.
It is the task of the present invention to produce a composite material that is based on aerogel granulate, that has a lower level of thermal conductivity, and is both mechanically stable and easy to manufacture in the form of mats or panels.
This problem has been solved by a composite material that contains at least one formation of fibrous materials and aerogel particles, which is characterised in that the fibrous formation contains at least one thermoplastic fibrous material with which the aerogel particles are connected and by which the fibres are connected to each other in the formation in such a way that the thermoplastic fibres at the surface are fused and on cooling result in a joining of the fibres to each other and to the aerogel particles. This thermal consolidation ensures a stable fibrous formation and ensures that the aerogel particles are bonded to the fibres.
Here, a fibrous formation in understood to be any formation that can be produced using a surface-forming technique. Examples of such surface formations are textile fabric, random-fibre matting, knitted fabrics, and fleeces, with fleeces being preferred.
Fleeces understood to include the so-called stable fibre mats, i.e., random-fibre mats of fibres that are of finite length, as well as spun-fibre mats, i.e., those that are of continuous fibres.
In the case of thermoplastic fibres, hereinafter referred to as the first fibre material, these can be fibres of a thermoplastic, organic material such as, for example, polyolefin fibres, polyamide fibres, or preferably polyester fibres. The fibres can be round, trilobal, pentalobal, octalobal, in the form of strips, or be shaped like fir trees, dumb bells, or otherwise. Hollow fibres can also be used. The first fibre materials can be a smooth or crimped.
In addition, the fibrous formation can contain at least one extra fibre material that is bonded to the first fibres of thermoplastic material during the thermal consolidation process. To this end, the melting point of the material from which these fibres are made may not be lower than the temperature at which the fleece becomes thermally consolidated. In the case of the fibres, these can be both inorganic fibres, such as mineral or glass fibres, or organic fibres, such as polyolefin, polyamide, or polyester fibres or mixtures thereof. It is preferred that the additional fibres be of the identical material as the first fibres, although of another profile, another diameter, or display another type of crimping and/or another degree of elongation.
The fibres can be modified by conventional additives, for example anti-static agents such as carbon black. The fibres that are contained in the formation can contain IR opacifiers, such an carbon black, titanium dioxide, iron oxide, or zirconium dioxide, as well as mixtures of these, in order to reduce the radiation contribution to thermal conductivity. The fibres may also be dyed in order that they are coloured.
The diameter of the fibres that are used in the composite material should preferably be smaller than the mean diameter of the aerogel particles, so that a high proportion of aerogel can be bound into the composite material. The selection of very fine fibres makes it possible to produce mats that are very flexible, whereas the use of thicker fibres results in bulkier mats that are stiffer on account of their greater resistance to bending.
The denier of the fibres is preferably between 0.8 and 40 dtex.
Mixtures of fibres that are of different materials, with different profiles and/or different deniers can also be used. Mixing in thicker fibres results in greater resistance to bending. In order to achieve good consolidation of the fleece, on the one hand, and to ensure good adhesion of the aerogel granulate on the other hand, the proportion by weight of the first, thermoplastic fibrous material should be between 10 and 100%-wt, preferably between 40 and 100%-wt, relative to the total quantity of fibre.
Of the spun fleeces, those that are of fibres of synthetic polymers, the so called spun bonds,
Frank Dierk
Thönnessen Franz
Zimmermann Andreas
Cabot Corporation
Juska Cheryl A.
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