Aerogel-coated sheet

Details Aerogel-coated sheet Aerogel-coated sheet

1998-12-07

2001-11-13

Copenheaver, Blaine (Department: 1773)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Composite having voids in a component

C428S312200

Reexamination Certificate

active

06316092

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to thermal insulation materials that are based on aerogels, a process for manufacturing these, and the use thereof.
2. Description of the Related Art
Aerogels are highly porous, low-density materials that are manufactured by forming a gel and then removing the liquid therefrom, whilst retaining the gel structure to a large extent when this is done.
According to a narrow definition (for example, see Gesser and Goswanni,
Chemistry,
rev. 1989, 89, 767), aerogels are materials in which the liquid is removed from the gel under super-critical conditions, whereas if the gel is dried under sub-critical conditions one refers to xerogels, and in the event that the liquid is removed from the frozen state by sublimation, one speaks of cryogels.
Within the context of the present intention, aerogels include all these materials and can contain any other gases apart from air. The gases can also be removed from the aerogel in a vacuum.
In particular, aerogels with porosities of greater than 60% and densities of less than 0.6 g/cm
3
exhibit very low thermal conductivities and for this reason are well suited for thermal insulation materials. Because of their low density, such materials exhibit little mechanical stability, however.
Formulations containing aerogels that also incorporate a binder in order to overcome this disadvantage are described, for example, in EP-A 0 340 707 and DE-A 44 37 424. In order to obtain highly effective aerogels that can additionally be transparent, the aerogel can be reinforced by incorporating fibres (see, for example, DE-A 44 30 642). However, all of these processes entail the disadvantage that the incorporation of alien substances increases the thermal conductivity of the aerogel material.
Brinker et al. (
Nature,
1995, 374, 439) describe a process for manufacturing thin aerogel films on rigid substrates, with thermally insulating cladding for buildings being seen as one possible application.
Surprisingly, it was found that aerogels can be applied to flexible films as a thin layer, which means that aerogel materials of good mechanical stability can be manufactured.
DE-A 38 44 003 describes films that are coated with silica gel, said films additionally containing photochromic compounds. In order to manufactured these, a washing liquid that contains up to 5% polyvinylalcohol in addition to the photochromic substances is placed in a liquor tank. A transparent film, for example of polyacrylate film, is passed through the liquor, and waterglass and sulphuric acid solution is sprayed onto the film of liquid that is formed by doing this, when the silica gel is precipitated out. The film that is so coated is dried at approximately 80° C. at a pressure of 0.1 to 0.5 bar. However, this manufacturing process does not produce a product with a gel structure and for this reason this does not involve an aerogel-coated film.
BRIEF SUMMARY OF THE INVENTION
For this reason, the object of the present invention is a coated film that contains an aerogel layer.
DETAILED DESCRIPTION OF THE INVENTION
In a special embodiment of the film according to the present invention, the aerogel, preferably in the form of powder or granulate, is incorporated into the film, preferably in the form of powder or granulate. The aerogel can be evacuated, although this does not necessarily have to be done.
In addition to low thermal conductivity, the film coated according to the present invention exhibits good mechanical stability.
In a simple manner, it permits a high level of light transmission, and is thus suitable for translucent and opaque thermal insulation.
Because of a low dialectric constant, in conjunction with a low loss factor, it is also suitable for electronic applications.
All common film materials such as polyesters, for example polyethyleneterephthalate (PET), cellulose and its derivatives, for example Cellophane®, polyolefins, for example polyethylene, polypropylene, polystyrene, polymethyl(meth)acrylates, polyvinylchloride, polyamides, and polycarbonates are suitable as films. Generally speaking, molar weight and orientation are not critical. In addition, the films can contain fibres (e.g., glass fibres). In the same way, metal films, for example of aluminum, are similarly suitable, although in this case opaque applications will be pursued.
Preferred film materials are polypropylene and polyethyleneterephthalate, with biaxially oriented films being especially preferred. Films that exhibit the lowest possible flammability are also advantageous.
The films can be imprinted or coloured.
In order to provide for better adhesion of the aerogel layer, it is preferred that the films be pre-treated by using processes available to the practitioner skilled in the art.
Chemical pre-treatment, for example with aminosiloxanes or imines, or a pre-treatment by a corona discharge procedure are preferred.
In principle, all available coating techniques such as dip coating, spin coating, and roller coating, are suitable for this purpose.
The films that have been prepared in this way are generally from 5 to 1000 &mgr;m thick.
In the case of transparent films, at least one of the films can be textured or structured in order to achieve particular visual effects. In one preferred embodiment, the film is of such a structure that the transmission of directed light will vary as a function of the angle of incidence. This effect can be achieved, for example, by using a film that has been shaped to a suitable profile (e.g., a saw-tooth profile), which means that laminations are formed by this profile. It is preferred that the laminations be so arranged that their edges are, as far as possible, perpendicular to the direction of incidence of the light. Depending on the angle subtended between the source of the light and the film, there will be a more or less pronounced reflection on the inclined surfaces and thus more or less transmission of light. Because of this, for example, in the case of an application in the domain of transparent thermal insulation, transmission can be high when the Sun is low and—conversely—transmission may be low when the Sun is high. The structuring of the film can be covered by an additional flat film on the side that is remote from the aerogel in order to avoid soiling by the accumulation of dust.
The coated film according to the present invention preferably contains aerogels with porosities of greater than 60%, especially above 70%, and in particular above 80%. Generally speaking, the density of the aerogel is less than 0.6 g/cm
3
, and preferably less than 0.4 g/cm
3
. The porosity is defined as a percentage proportion of the pore volume to the total volume of the aerogel.
The thermal conductivity of the aerogel should preferably be less than 40 mW/mK and particular less than 25 mW/mK.
The aerogels that are used according to the present invention are generally obtained in that
a) a sol is produced;
b) if required, fibres are added to the sol;
c) the sol obtained as in b) is converted to a gel;
d) the liquid contained in the gel is exchanged; and
e) the gel obtained as in d) is dried such that an aerogel is formed.
Metal oxide compounds, in particular Si and Al compounds, in particular Si or Al alkoxides, waterglass, organic polycondensates based on melamine formaldehyde resins (U.S. Pat. No. 5,086,085) or resorcin formaldehyde resins (U.S. Pat. No. 4,873,218) or mixtures thereof, which are suitable for the sol-gel technique, serve as the starting material for step a) (C. J. Brinker, G. W. Scherer,
Sol
-
Gel Science,
1990, Chapters 2 and 3). In addition, the sol can consist of particulate or colloidal Si or Al oxide. It is preferred that Si compounds, in particular waterglass, be used.
On the surface, the sol particles have functional groups that are capable of condensation. From this point on, these groups will be referred to as surface groups. Typically, sol particles carry hydrophilic hydroxyl groups (OH) from Si or Al compounds. Depending on the manufacturing process, radicals of alkoxy groups (OR)

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