METHOD FOR PRODUCING AN AEROGEL MATERIAL

Abstract

The invention relates to a method for producing an aerogel material with a porosity of at least 0.55 and an average pore size of 10 nm to 500 nm, having the following steps: a) preparing and optionally activating a sol; b) filling the sol into a casting mold (10); c) gelling the sol, whereby a gel is produced, and subsequently aging the gel; at least one of the following steps d) and e), d) substituting the pore liquid with a solvent; e) chemically modifying the aged and optionally solvent-substituted gel (6) using a reaction agent; followed by f) drying the gel, whereby the aerogel material is formed. The casting mold used in step b) is provided with a plurality of channel-forming elements (2) which are designed such that the sol filled into the casting mold lies overall at a maximum distance X from a channel-forming element over a specified minimum length L defined in the channel direction of the elements, with the proviso that X<15 mm and L/X>3.

Claims

1. A process for the production of an aerogel material with a porosity of at least 0.55 and an average pore size of 10 nm to 500 nm, comprising the following steps: a) preparing and optionally activating a sol; b) filling the sol into a casting mold; c) gelling the sol, whereby a gel is produced, and subsequently aging the gel; at least one of the following steps d) and e) d) exchanging the pore liquid with a solvent e) chemically modifying the aged and optionally solvent-exchanged gel using a reacting agent; followed by f) drying the gel, whereby the aerogel material is formed; characterized in that the casting mold used in step b) is provided with a plurality of channel-forming elements, which are configured such that, along a specified minimum length L defined in the channel direction of the elements, every location of the sol filled into the casting mold has a maximum distance X from a channel-forming element fulfilling the provision that X≦15 mm and L/X>3.

2. The process according to claim 1, wherein the channel-forming elements are configured as bundles of pipes arranged parallel to each other, wherein the casting mold for the sol is formed by the interior spaces of the pipes, and wherein the solvent exchange d) and/or the chemical modification of the gel e) is carried out directly in the casting mold across an interspace between the gel and the channel-forming element formed as a result of a shrinkage during the aging of the gel c.

3. The process according to claim 2, wherein all of the pipes have an identical cross-section.

4. The process according to claim 2, wherein the optionally solvent-exchanged and optionally chemically modified gel is removed as gel rods from the casting mold and wherein subsequently the drying f) is carried out by means of subcritical drying.

5. The process according to claim 1, wherein the channel-forming elements are configured as bundles of rod elements arranged parallel to each other, wherein the casting mold for the sol is formed by a space located between the rod elements, and wherein the rod elements are withdrawable from the casting mold in channel direction after gelation and aging in such manner that a plate-shaped gel body with continuous channels is formed, wherein the solvent exchange d) and/or the chemical modification of the gel e) is carried out by applying solvent or reaction agent.

6. The process according to claim 5, wherein the application of solvent or reaction agent is carried out by forced convection by placing the gel body onto a suction plate that is at least partially permeable and applying on the underside thereof a vacuum so as to draw off the solvent or reaction agent, and wherein new solvent or reaction agent is supplied from above the gel body.

7. The process according to claim 1, wherein the sol is prepared as a silicon oxide sol in an alcoholic solvent mixture containing at least one acid-catalytically activatable hydrophobicization agent, wherein the volume fraction of the hydrophobicization agent in the sol is 5 to 60%, the gelation of the sol is initiated by addition of a base; a chemical modification of the aged gel is carried out, wherein the chemical modification is a hydrophobicization initiated by the release or the addition of at least one hydrophobicization catalyst interacting with the hydrophobicization agent; and the drying of the gel is carried out by means of subcritical drying.

8. The process according to claim 7, wherein the catalytically activatable hydrophobicization agent is hexamethyldisitoxane (HMDSO).

9. The process according to claim 7, wherein the volume fraction of the hydrophobicization agent in the sol is 20 to 50%, particularly 25% to 40% and more particularly 34% to 38%.

10. The process according to claim 7, wherein the hydrophobicization catalyst is trimethytchlorosilane (TMCS) and/or HCl in an alcoholic solution.

11. The process according to claim 1, wherein the gel is a polymer-based gel, preferably a polyisocyanate-based gel.

12. The process according to claim 1, wherein the optionally activated sol is added to a fiber-based matrix before the gelation.

3. A first precursor product for producing an aerogel plate, consisting of an aerogel plate provided with longitudinal holes, which plate can be produced according to claim 5.

14. A first precursor product for producing an aerogel plate, consisting of a plurality of aerogel rods, which rods can be produced according to claim 2.

15. An aerogel plate, consisting of a first precursor product according to claim 13, into the longitudinal holes of which are inserted correspondingly shaped aerogel rods of a second precursor product according to claim 14.

16. The process according to claim 3, wherein all of the pipes have a hexagonalshaped cross-section.

17. The process according to claim 8, wherein the hydrophobicization catalyst is trimethylchlorosilane (TMCS) and/or HCl in an alcoholic solution.

18. The process according to claim 9, wherein the hydrophobicization catalyst is trimethylchlorosilane (TMCS) and/or HCl in an alcoholic solution.

19. The process according to claim 1, wherein the gel is a polyisocyanate-based gel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Examples of the invention will henceforth be described in more detail with reference to the drawings, which show:

[0057] FIG. 1 a schematic view of distance relations in various arrangements: (a) square pipe profile, (b) circular pipe profile, (c) arrangement with several circular pipe profiles, (d) hexagonal pipe profile, (e) arrangement with several hexagonal pipe profiles, (f) orthonormal arrangement of circular rods and (g) hexagonal arrangement of circular rods;

[0058] FIG. 2 (a) to (d) the step sequence of a first embodiment of the process; and

[0059] FIG. 3 (a) to (e) the step sequence of a second embodiment of the process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0060] FIG. 1 illustrates some basic geometric shapes and relations. In the figures, the innermost point which has the distance farthest away from the next channel-forming element is shown with a cross. Also shown is the maximum distance X defined in the above-mentioned sense, which is the shortest distance that the innermost point has from the next channel-forming element.

[0061] FIGS. 1a to 1e show a situation in which the pipe components 2 used as channel-forming elements and also a sol contained therein or a still unaged gel 4 formed therefrom can be seen. For better illustration, these figures also show a solvent or a reaction agent 5 for the steps d) or e) described above, which should penetrate into the previously aged gel after removal of the pipe components. FIGS. 1f and 1g show another situation in which the channel formation in an aged gel material 6 by means of rod elements has already been completed: the rod elements were removed and circular channels 7 were formed into which the reaction agent 5 was filled.

[0062] In the square pipe profiles with inner edge length a shown in FIG. 1a, the maximum distance X is =a/2. As already mentioned, this is the shortest distance from the innermost point within the profile. In the circular profile with internal diameter d shown in FIG. 1b the maximum distance X is d/2. In the regular hexagonal pipe profile with inner edge length b shown in FIG. 1d the maximum distance X is =b/2 √3.

[0063] FIGS. 1c and 1e show arrangements of tightly packed circular or hexagonal pipe profiles.

[0064] In the case of the orthonormal lattice grid indicated in FIG. 1f, in the lattice points of which the channel-forming circular rods are arranged and which consists of a plurality of square elementary cells with side length A, the maximum distance is given by X=½ (A√2−ds).

[0065] In the case of the hexagonal lattice grid indicated in FIG. 1g, in the lattice points of which the channel-forming circular rods are arranged and which consists of a plurality of square elementary cells with side length B, the maximum distance is given by X=B−½ ds.

[0066] The process sequence shown in FIGS. 2a to 2d first shows in FIG. 2a a bundle of circular cylindrical pipes 2, which is still empty initially and which, in particular, rests on the bottom surface of a confinement tray not shown. In FIG. 2b the pipe bundle is filled with a sol or with a gel 4 formed therefrom which is still unaged. In FIG. 2c an aging of the gel with accompanying shrinkage has occurred, whereby a gap-like interspace 8 filled with syneresis fluid has formed between the cylindrical rods 6 made of aged gel and the pipes 2. In FIG. 2d the gel rods 6 are shown with pipes 2 partially pulled upwards. These are now ready for further processing.

[0067] The process sequence shown in FIGS. 3a to 3e first shows in FIG. 3a a cuboid confinement tray 10 with a base plate 12 provided with an arrangement of cylindrical rods 14 in a nail board manner. In the example shown, all rods are approximately of the same length. In FIG. 3b the confinement tray contains a filled sol or a gel formed therefrom which is still unaged, the filling level of which lies just below the rod tips. In FIG. 3c an aging of the gel with accompanying shrinkage has occurred, whereby an interspace 8 is formed between the cylindrical rods 14 and the plate-shaped body 16 made of aged gel. In FIG. 3d a lid part 18 of the confinement tray has been lifted upwards, whereby a base part 20 of the confinement tray with the aged gel body 16 contained therein is uncovered. In FIG. 3e the aged gel body 16 provided with through holes 22 has been lifted out of the base part 20 provided with rods 14 and is ready for further processing.

[0068] Production of an Inorganic Organic Hybride Aerogel Granulate

[0069] A silicon oxide sol in alcohol is activated by the addition of dilute ethanolic ammonia solution at room temperature. The sol contains 2% aminopropyltriethoxysilane (APTES) as a side component which is added together with the ammonia. This sol is now filled into an open vessel which, as shown in FIG. 2, is provided with a pipe bundle package insert with a pipe inner diameter d=13 mm, a wall thickness h.sub.w=1 mm and a length L=90 cm. This insert fills the entire vessel volume. After gelation, the gel pack is aged for 12 h. Thereafter, the pipe bundle insert is removed and excess liquid is decanted off. Thereafter, a diluted solution containing a polymer cross-linking agent reacting with amine groups and a hydrophobicization agent is added. The mixture is allowed to diffuse into the gel for a further 12 hours and to react within the vessel, whereupon excess liquid is removed again. The resulting gel rods are then placed in an autoclave, exchanged for CO.sub.2 and subsequently supercritically dried. As a product, X-aerogel rods with a density of 0.14 g/cm.sup.3 and a compressive strength of >10 MPa remain.

[0070] Highly Efficient Production of a Silicate-Based Aerogel Granulate

[0071] A silica sol is produced in a continuous process and diluted with HMDSO from an SiO.sub.2 content of 10% to a content of 6.6%. This sol is activated at a temperature of 35° C. by admixing diluted ammonia solution at a filling station. At the filling station, there are present 200 I containers which are provided with a honeycomb-like insert filling the cavity completely. The honeycomb mold has a wall thickness of 0.5 mm and a cell diameter of 8 mm. The containers are now individually filled and hermetically closed by means of covers, and then they are stored for 18 h at 70° C. During this time, the mixture undergoes gelling and the gel bodies formed in the honeycomb channels undergo aging, whereby the latter shrink slightly. As a result of the shrinkage, interspaces are formed in which the liquid can circulate (analogously to FIG. 2c). After aging, the containers are opened and the syneresis liquid is drained off. Thereafter, 20 I of diluted mineral acid are added as a catalyst into each vessel, whereby the catalyst is evenly distributed in the interspaces between the gel and the honeycomb wall. The containers are again closed and stored for 8 h at 90° C., whereby the gels undergo hydrophobicization. Thereafter the containers are emptied and the hydrophobicized gel rods are dried in an oven at 150° C. During drying the gel rods spontaneously break up to form an aerogel granulate with a grain size between 4 and 7 mm. The density of the aerogel granulate thus obtained is 0.096 g/cm.sup.3 and the thermal conductivity of the loose material is 17.8 mW/mK. By virtue of the processing according to the present invention the gel bodies remain unchanged in the mold until the drying step, thus resulting in a yield of granulate of at least 95%. Compared to mechanically crushed gels, this results in significantly less aerogel dust, which must be regarded as an inferior product.

[0072] In an alternative embodiment, the inserts in a large-scale process are not introduced into individual containers, but rather are introduced closely following each other in an elongated process tunnel and thus pass with the gel through the entire production process on a conveyor belt, whereby the syneresis liquid is drawn off in a certain region at the bottom and shortly thereafter the hydrophobicization catalyst is dosed in from the ceiling through an injection system.

[0073] Production of a Structured Polyurethane Aerogel Plate

[0074] Two freshly prepared solutions in an organic solvent mixture consisting of an isocyanate mixture (component 1) and a polyol with a catalyst (component 2) are mixed with each another and placed into a tray mold into which a uniform, covering arrangement of cylindrical rods according to FIG. 3a) has been inserted. The individual bars have a diameter ds=20 mm, a length Ls=331 mm and a shortest center-to-center distance A=35 mm. The filling level of the sol mixture consisting of components 1 and 2 is H=315 mm. On the upper side, the sol is covered with a suitable perforated plate which engages the rods. After gelation and aging of the gel, the perforated plate is removed and the individual rods are withdrawn. The gel body is then removed from the mold and transferred to an autoclave. The pore liquid contained in the gel body is now extracted in this autoclave by means of supercritical CO.sub.2 and the gel is subsequently subjected to subcritical drying. In the end, a polyurethane aerogel perforated plate of 273 mm thickness remains.

[0075] In an alternative embodiment, the mixtures 1 and 2 consist of a solution of resorcinol with a small admixture of acid catalyst and a diluted aqueous formaldehyde solution. In this case, however, it is necessary before supercritical drying to replace the aqueous pore liquid by a suitable solvent medium such as, for example, acetone or ethanol, which is done by solvent exchange.

[0076] Industrial Production of an Aerogel Plate

[0077] A silicon oxide sol produced in a continuous through-flow reactor is adjusted to a silicate content of 5.7% (measured as SiO.sub.2). The sol is provided with ammonia as a gelling catalyst and is placed in a shell mold in which a nailboard-like insert is present. The insert consists of a base plate onto which has been placed a regular hexagonal arrangement of needle-like rods normally extending to the surface analogously to FIG. 1g with a diameter ds=1.5 mm and a length Ls=70 mm and a shortest center-to-center distance B=10 mm, which corresponds to the edge length of the hexagon. The filling level H of the sol mixture is also 70 mm so that the tips of the rods are just covered. The sol is then covered up with a second plate (cover plate, not shown). After gelation and aging of the gel, the cover plate is removed, the gel plate is removed from the mold and the insert is carefully removed. The gel plate provided with through holes is transferred onto a slow running (7.3 m/h) conveyor belt. This gel body is sprayed from above with a fresh mixture of hydrophobicization agents consisting of 85% HMSO and 15% hydrochloric-acid-diluted ethanol, with the excess liquid forming on the plate being continuously suctioned off via the gas- and liquid-permeable membrane material of the conveyor belt by means of a pump providing a slight underpressure. After an exchange and hydrophobicization time of 6 h at 75° C., the plate is dried by means of solvent drying at 150° C.

[0078] Comparative Example

[0079] According to a standard procedure without channel-forming elements, which is customary today, the exchange and hydrophobicization time to be expected under otherwise identical conditions is approximately 25 times longer, i.e. 150 h, which is unacceptable for an industrial process.

[0080] In a further embodiment, the aerogel plate described in the above example and produced according to the process of the present invention is loaded with aerogel cylinders that fit into the holes. The gel cylinders required for this purpose were prepared previously from a suitably selected polyurethane gel formulation and subsequently dried supercritically from CO.sub.2.