MXPA99011049A - Method for producing substantially globular lyogels and aerogels - Google Patents

Abstract

The invention relates to a method for producing substantially globular lyogels, wherein the constituents which make up the gel are mixed, whereupon lyosol is introduced into an agitated medium which does not dissolve in said lyosol in a noticeable manner in order to form said gel. The present invention also relates to a method for producing substantially globular aerogels, wherein the lyogels produced according to said method are converted into an aerogel.

Description

Method to produce substantially globular lyogels and aerogels Aerogels, particularly those with porosity above 60% and a density of less than 0.6 g / cm3, exhibit extremely low thermal conductivity and are therefore used as a thermal insulating material, as described for example in EP-AO 171 722. Further, by virtue of its very low refractive index for solid substances, it is known that they are used for Cerenkov detectors. In addition, because of its particular acoustic impedance, the literature describes a possible use as a means of impedance matching, for example in the alpha sound range. It is also possible that they are used as carriers for effective substances in pharmacy or agriculture. Aerogels in the broadest sense, for example in the sense of "gel with air as the dispersing agent" are produced by dehydrating a suitable gel. The term "airgel" in this sense encompasses aerogels in the most limited sense, xerogels and cryogels. In this regard, a dehydrated gel is termed an airgel in the most limited sense when the gel liquid is removed above the critical temperature and starting from pressures above the critical pressure. On the other hand, if the liquid is removed from the gel under subcritical conditions, for example with the formation of a liquid / flavor interface, then the resulting gel is often also referred to as a xerogel. When the term airgel is used in the present invention, these are aerogels in the broadest sense, for example, in the sense of "gel with air as the dispersion medium". The term does not include aerogels known from the first literature and which are obtained, for example, by precipitation of silicic acid (for example, DE 3025437, DD 296 898) or which occur as a pyrogenic silica, for example Aerosil (Trade Mark) . In these cases, during manufacturing, no three-dimensional gel crystal network is developed which is homogeneous over relatively large distances. As regards aerogels, it is basically possible to differentiate between inorganic and organic aerogels. Inorganic aerogels have been known since 1931 (S. S. Kistler, Nature 1931, 127, 741). Since then, aerogels of various raw materials have been emerging. In this regard, for example, are aerogels of SiO2_? Ti02_? Zr02_, Sn02_? Li20_? Ce02_, V206 and mixtures thereof (H. D. Geaser, P. C. Goswami, Chem. Rev. 1989, 89, 765 et seq.). For some years, organic aerogels have also been known which are derived from most widely diverse raw materials, for example melamine formaldehyde (R. W. Pekala, J. Mater, Sci. 1989, 24, 3221).

Therefore, inorganic aerogels can be produced in different ways. On the one hand, the Si02 aerogels can, for example, be produced by acid hydrolysis and condensation of the tetra-ethyl orthosilicate in ethanol. During this process, a gel is produced which can be dehydrated by supercritical dehydration while maintaining its structure. Production methods based on this dehydration technique are known, for example from EP-A-0 396 076, WO 92/03378 or WO 95/06617. However, the high pressure technique included in the supercritical dehydration of aerogels is an expensive process and involves a high safety risk. In addition, however, supercritical dehydration of aerogels is a very expensive production method. In principle, a method for the supercritical dehydration of Si02 gels gives an alternative to supercritical dehydration. The costs involved in supercritical dehydration are substantially lower because of the simplest technology, the lowest energy costs and the lowest safety risk. For example, Si02 gels can be obtained by acid hydrolysis of tetra-alkoxy silanes in a suitable organic solvent by means of water. Once the solvent has been exchanged for an adequate organic solvent, the gel obtained, in an additional step, reacts with a silylating agent. The Si02 gel resulting from this can then, from an organic solvent, dehydrate in the air. In this way, aerogels with a density of less than 0.4 g / cm3 and porosity above 60% can be achieved. The production method based on this dehydration technique is described in detail in WO 94/25149. In addition, the gels described above can, before dehydration and in the aqueous alcohol solution, be mixed with tetra-alkoxy silanes and allowed to stand, in order to increase the strength of the crystal lattice of the gel, as revd in FIG. WO 92/20623. However, the tetra-alkoxy silanes used as raw materials in the processes described above also represent an extremely high cost factor. A considerable reduction in costs can be achieved by using water glass as a raw material for the production of Si02 gels. For this purpose, it is possible, for example, to produce a silicic acid from an aqueous solution of water glass with the help of an ion exchange resin, the silicic acid then being polycondensed by the addition of a base to produce a Si02 gel. . After exchange of the aqueous medium for a suitable organic solvent, then it is possible, in a further step, to react the resulting gel with a chlorinating silylating agent. The Si02 gel which is surface-modified, for example with methyl silyl groups, can then also, like an organic solvent, dehydrate in the air. The production method based on this technique is known from DE-A-43 42 548. Alternative methods with respect to the production of an Si02 airgel in a water glass base with subsequent supercritical dehydration are described in Patent Application 195 41 715.1 and 195 41 992.8 of Germany. Patent application 196 48 798.6 from Germany discloses a method for producing aerogels in which aerogels are surface modified without prior exchange of solvent, i.e., essentially with water in the pores, after which they are dehydrated. From DE-PS 896 189, it is known that globular hydrogels of silicic acid can be produced from a raw material, for example, water glass, which contains silicic acid and by reaction with an acid, for example sulfuric acid, a hydrosol of silicic acid forming gel which then is in the form of individual drops, passed through a gaseous or liquid medium, for example, a mineral oil, which is not miscible with water and hydrosol. The drops of hydrosol therefore acquire a more or less globular shape and remain in the oil layer long enough for the conversion of the sol to the liquid hydrogel to take place. However, the hydrogel base produced by the indicated method contains contaminations of mineral oil which can not be completely eliminated even by very expensive washing processes. In the case of this method, if the mixture is injected into a gaseous medium, then the adopted method is one by means of which first the drops of hydrosol are produced by a jet of mixture of water glass, sulfuric acid and aluminum sulfate, These drops are then injected into a container full of air. Under the applied conditions, the conversion of the hydrosol to the hydrogel occurs within 1 second so that the tiny droplets of hydrogel can be retained in a layer of water at the bottom of the vessel and further processed. DE-C-21 03 243 describes a method and apparatus for producing substantially globular hydrogels which contain silicic acid, the hydrosol of silicic acid being formed in a special mixture jet of a silicic acid-containing raw material and an acid solution. The hydrosol thus formed is, for the purpose of droplet formation, sprayed to a gaseous medium which does not dissolve markedly in the hydrosol, for example air. However, because of the necessary time of fall as a reaction time for the formation of the gel, dependent on the particle size, the overall height of the apparatus to which the hydrosol is injected is unfortunately considerable. As a result, this prior art method is very expensive since correspondingly substantial material is required, the space requirement is considerable and it takes longer to manufacture the apparatus. Furthermore, in the case of this known method, lyogel particles of a non-homogeneous size distribution arise which possibly then have to be classified according to their size so that everything in this method becomes more expensive and takes more time . It is common for all the aforementioned methods that in order to initiate gel formation, two or more initial components, for example, water glass solution and mineral acid, have to meet. In this regard, it is apparently favorable for the properties of the gel particles, in particular for their subsequent stability, if the shape and size of the particles can still be adjusted before the gel-forming process. It is particularly convenient for the subsequent steps of the process which goes after the formation of gel and molding, such as for washing, for possibly later reactions and for the last dehydration process, if the particles are present in an easy to handle form , in other words, for example, as balls. Globular particles are in terms of stability, superior to all other forms. By virtue of the regular geometry and the lack of edges and corners, it is substantially possible to avoid undesirable abrasion during the following stages of the process. The substantially globular lyogels have the advantage that the particle size distribution of the finished product of the lyogel can be adjusted particularly easily by the molding process. Therefore, the present invention is based on the problem of providing a method for producing substantially globular lyogels in which the disadvantages of prior art methods are avoided. This problem is solved according to the fact that the gel forming components are mixed together to form a liosol and then the lyosol is introduced to a mobile medium to form the lyogel, the medium not being notably in the liosol. In the present Application, it is to be understood that the term liosol or lyogel means a sol or a gel in which the sol or gel interstices are filled with fluid. If the fluid consists essentially of water, then one speaks of a hydrosol or hydrogel, as the case may be. Incorporation into a mobile atmosphere greatly increases the drying time of the lyosol particles in the medium so that the overall height of the apparatus can be markedly reduced. Therefore, the apparatuses require considerably less material and space so that the costs of the method according to the invention are considerably reduced. Ideally, the medium is an air atmosphere, by means of which other substances can be added to the liosol before it is introduced into the air atmosphere. Therefore, the air can also contain other gaseous media. All the apparatuses known to a person skilled in the art for this purpose can be used to mix the gel forming component and to incorporate the liosol. Conveniently, the liosol is poured drop by drop or sprayed into the air, preferably in the direction of gravity. According to a preferred embodiment, the liosol is added to a stream of air which flows substantially against the direction of gravity. The air flow may also contain speed components otherwise directed. Thus, the drying time of the particles in the air can be increased under control, which results in additional savings in the overall height of the apparatus to which the liosol is introduced. The air flow that is opposite to the direction of gravity can also be used for any classification of droplets or particles during gel formation. Particles with a diameter below the grain boundary diameter which corresponds to the flow velocity are delivered upward considering that the larger particles are delivered downwards. Accordingly, no additional step is necessary to classify the gel particles according to their size so that the costs of the method according to the invention are still more reduced. A further development of this embodiment provides the liosol that will be introduced to an air flow, the speed of which decreases in the flow direction. Another preferred embodiment of the method is that the lyosol drops, after becoming lyogel, are retained in a layer of water. An additional effect of the reduced fall speed of the balls is due to the air flowing against the direction of the fall; The effect is to soften the introduction of the lyogel balls in a layer of water for example. The starting substances suitable for the method according to the invention are basically some substances which are useful for the prior art forms of synthesizing lyogels, for example as a preliminary stage for an airgel (see for example J. Brinker, GW Schere, Sol-Gel Science, The Physics and Chemistry of Sol / Gel Processing Academic Press Ltd., London 1990, DE-A-43 42 548, US-A-5 081 163, US-A-4 873 218). Therefore, the previous steps of the Si02 hydrosols, for example silicic acid and mineral acid, are preferred. Soluble water glass solutions and hydrochloric acid are particularly preferred. Another problem on which the present invention is based lies in providing a method for producing substantially globular aerogels. This problem is solved by a method in which a substantially globular lyogel, as can be produced in accordance with the present invention, becomes an airgel. The method to convert the lyogel into an airgel is by no means limited. All alternative methods known to one skilled in the art can be applied. In a preferred embodiment, the substantially globular lyogel reacts with a silylating agent. All silylating agents can be used. such as, for example, trimethyl chlorosilane, which are known to a person skilled in the art. Prior to silylation, the lyogels can be washed and / or the lyogel solvent can be exchanged for another organic solvent. The washing of the lyogel or hydrogel and the exchange of solvent can also be carried out by methods described in the state of the art. The dehydration can also be carried out by methods known to a person skilled in the art. In this regard, supercritical, as well as subcritical dehydration processes known for aerogels are preferred, with supercritical dehydration being particularly preferred. The method according to the invention is described in detail hereinafter with reference to an example of the embodiment. Example 1 A sodium-water glass solution is produced by diluting S3.5 kg of commercially available sodium-water glass solution with 25.5% Si02 and 7.6% Na20 with 31.7 kg of deionized water. A dilute hydrochloric acid is produced by diluting 19.3 kg of 25% hydrochloric acid commercially available with 65.8 kg of deionized water. In each case, 30 kg / hr of the diluted hydrochloric acid and the sodium water glass solution are supplied to a mixing and spraying apparatus in precisely measured quantities. The outlet of the mixing nozzle is located at the upper end of a tube through which the heated air flows vertically upwards. The third bottom of the tube is filled with water. Above the surface of the water, the tube has air inlet openings. The air flow is adjusted to an empty tube speed of 4 m / second. The temperature inside the tube is 100 ° C. The hydrogel spheres are captured in the water layer, settle through the water layer and are delivered from the spraying tower in a stream of water. The small hydrogel balls are continuously washed with 0.1 mol of hydrochloric acid and then with deionized water. Subsequently, the lyogel balls are washed with acetone in several stages until the water content in the gel is less than 1%. The gel moistened with acetone is exposed to a mixture of acetone and 5% trimethyl chlorosilane for 10 hours. Then, again in several stages, the gel is washed with acetone. The gel balls moistened with acetone are dehydrated in a fluidized bed with nitrogen at 180 ° C for 5 minutes. The obtained airgel balls have a density of 130 kg / m3 and their heat conductivity is 0.01 W / mK.

Claims (12)

1. Claims 1. A method for producing substantially globular lyogels in which the gel forming components are mixed to form a lyosol after which, in order to form the lyogel, the lyosol is introduced to the mobile medium which does not dissolve notably in the liosol.
2. 2. A method according to claim 1, characterized in that the medium is air.
3. 3. A method according to claim 2, characterized in that the air contains at least one other gaseous medium.
4. 4. A method according to claim 2 or 3, characterized in that the liosol is supplied by dripping into the air moving.
5. A method according to claim 2 or 3, characterized in that the liosol is sprayed into the air moving.
6. 6. A method according to at least one of claims 2 to 5, characterized in that the liosol is introduced into a stream of air directed substantially in opposition to the force of gravity.
7. 7. A method according to claim 6, characterized in that the liosol particles are selected according to size by the flow of air which is directed in opposition to the force of gravity.
8. 8. A method according to claim 6 or 7, velocity of the air stream decreases in the direction of flow. 8. A method according to at least one of the preceding claims, characterized in that the liosol particles are retained in a layer of water.
9. 9. A method according to at least one of the preceding claims, characterized in that the liosol is formed of silicic acid and mineral acid.
10. 10. A method according to at least one of claims 1 to 8, characterized in that the liosol is formed of a solution of sodium water glass and hydrochloric acid.
11. 11. The use of substantially globular lyogels, produced according to at least one of the preceding claims, for the production of aerogels.
12. 12. A method for producing substantially globular aerogels in which a substantially globular lyogel, produced according to at least one of claims 1 to 10, is converted into an airgel.