METHOD FOR PRODUCING AEROGELS AND AEROGELS OBTAINED USING SAID METHOD

Abstract

The invention relates to a method for producing an aerogel under increased pressure, to the aerogel obtained using said method and to their use.

Claims

1. A method for producing a silica aerogel by means of a sol-gel process, comprising producing a lyogel from a sol; and converting the lyogel into an aerogel, wherein the production of the lyogel is carried out at least partially at a pressure of more than 30 bar.

2. The method according to claim 1, wherein the production of the lyogel is carried out in a compressed gas, a supercritical substance, or a mixture of both.

3. The method according to claim 1, wherein: the pressure is selected from more than 40 bar, more than 50 bar, more than 60 bar, more than 70 bar, and more than 74 bar; and/or the production of the lyogel is carried out at a temperature selected from above 50° C., 60° C., 70° C., and 80° C.

4. The method according to claim 1, wherein converting the lyogel into an aerogel is carried out at a pressure of more than 50 bar.

5. The method according to claim 1, wherein the sol is a solution or a dispersion of a precursor.

6. The method according to claim 5, wherein the precursor is selected from silicic acids, in particular colloidal silicic acid, colloidal silica, silanes, silica sols, tetraalkoxysilanes, siloxanes and mixtures thereof.

7. The method according to claim 1, wherein the sol comprises a hydrophobing silanizing agent.

8. The method according to claim 1, wherein the production of the lyogel is carried out by introducing the sol into a pressurized apparatus in the form of droplets.

9. The method according to claim 1, wherein after the production of the lyogel a solvent exchange is performed.

10. The method according to claim 9, wherein the solvent exchange occurs by contacting the lyogel with an organic solvent at elevated pressure.

11. The method according to claim 10, wherein the organic solvent is brought into contact with the lyogel together with a hydrophobing agent.

12. The method according to claim 11, wherein subsequent to contact with the organic solvent and hydrophobing agent the lyogel is converted into an aerogel.

13. An aerogel produced by the method of claim 1, wherein the aerogel is in the form of particles having a substantially circular cross-section.

14. The aerogel according to claim 13, wherein the particles are spherical or cylindrical.

15. The aerogel according to claim 13, wherein particle size is selected from 0.1 to 10 mm, 0.2 to 8 mm, 0.3 to 7 mm, and 0.5 to 5 mm.

16-17. (canceled)

18. An apparatus for producing an aerogel at pressure, comprising: (a) at least one reactor which can be pressurized, (b) at least one inlet opening arranged at the reactor for introducing fluids into the reactor, and (c) at least one outlet opening arranged at the reactor for removing liquids or solids from the reactor.

19. (canceled)

Description

[0221] The figures show according to

[0222] FIG. 1 a cross-section of an apparatus according to the invention for carrying out the method according to the invention,

[0223] FIG. 2 sorption isotherms of the commercially available aerogel P300 and of the hydrophobized aerogel H-8 according to the invention.

[0224] A further subject-matter of the present invention—according to a second aspect of the present invention—is an aerogel, in particular obtainable according to the method previously described, wherein the aerogel is provided in the form of particles having an in particular substantially circular cross-section.

[0225] As previously stated, the aerogels according to the present invention are characterized by an in particular circular cross-section, whereby on the one hand the mechanical load-bearing capacity and on the other hand the ability to produce dense sphere packings is significantly increased.

[0226] In the context of the present invention, it is usually provided that the aerogel particles are spherical or cylindrical.

[0227] Because of their shape, the aerogels according to the invention offer advantages in processing. For example, the spherical aerogels are much easier to mix into powder mixtures. Due to their improved flowability, higher strengths under uniaxial compressive loading and higher packing density compared to conventional aerogel powders, which are based on shapeless or cubic particles, the preferably spherical aerogels according to the invention can be used preferentially in powder blends or powder mixtures, such as thermal insulation plasters.

[0228] As far as the particle size of the aerogel particles is concerned, this can naturally vary over a wide range. However, it has been well proven if the aerogel comprises particle sizes in the range of from 0.1 to 10 mm, in particular from 0.2 to 8 mm, preferably from 0.3 to 7 mm, more preferably from 0.5 to 5 mm.

[0229] Similarly, it may be envisioned in the context of the present invention that the aerogel particles comprise a monodisperse particle size distribution.

[0230] However, it is also possible within the scope of the present invention for the aerogel particles to comprise a polydisperse particle size distribution. In particular, the particle size distribution can be selectively controlled by varying the conditions of spraying or dripping into the reactor.

[0231] The aerogel particles according to the invention are highly porous solids. Typically, the aerogel comprises a porosity above 94%, in particular above 95%, preferably above 96%.

[0232] Similarly, it may be envisaged that the aerogel comprises a porosity of from 94 to 99.5%, in particular from 95 to 99%, preferably from 96 to 98%.

[0233] Furthermore, the aerogels according to the invention comprise high internal surface areas. Thus, it may be provided that the aerogel comprises a BET surface area of at least 500 m.sup.2/g, in particular 600 m.sup.2/g, preferably 650 m.sup.2/g, more preferably 700 m.sup.2/g, more preferably 800 m.sup.2/g.

[0234] Similarly, it may be provided that the aerogel comprises a BET surface area in the range of from 500 to 1,000 m.sup.2/g, in particular 600 to 1,050 m.sup.2/g, preferably 650 to 1,000 m.sup.2/g, more preferably 700 to 950 m.sup.2/g, particularly preferably 800 to 900 m.sup.2/g.

[0235] Now, as far as the thermal conductivity of the aerogel is concerned, this can vary over a wide range. Usually, however, the aerogel comprises very low thermal conductivities in the context of the present invention. Particularly good results are obtained when the aerogel comprises a thermal conductivity of at most 0.025 W/mK, in particular at most 0.022 W/mK, preferably 0.020 W/mK, more preferably 0.019 W/mK.

[0236] Typically, the aerogel comprises a thermal conductivity in the range of from 0.012 to 0.025 W/mK, in particular from 0.013 to 0.022 W/mK, preferably from 0.014 to 0.020 W/mK, more preferably from 0.015 to 0.019 W/mK.

[0237] Furthermore, it may be provided in the context of the present invention that the aerogel comprises a density in the range of from 0.01 to 0.60 g/cm.sup.3, in particular from 0.11 to 0.55 g/cm.sup.3, more preferably from 0.12 to 0.50 g/cm.sup.3, particularly preferably from 0.13 to 0.50 g/cm.sup.3.

[0238] For further details on the aerogel according to the invention, reference can be made to the above explanations on the method according to the invention, which apply accordingly with respect to the aerogel according to the invention.

[0239] Again, a further subject-matter of the present invention—according to a third aspect of the present invention—is the use of the aerogel described above for insulation purposes, in particular for sound insulation, electrical insulation or thermal insulation, in particular for heat insulation.

[0240] For further details on the use according to the present invention, reference can be made to the explanations on the further aspects of the invention, which apply accordingly with respect to the use according to the present invention.

[0241] Again, a further subject-matter of the present invention—according to a fourth aspect of the present invention—is the use of an aerogel as previously described for insulating purposes, in particular as or in thermal insulations.

[0242] In this context, it may be envisaged that the aerogel is used in loose filling, in a powder mixture or in an insulating composition, for example an insulating plaster.

[0243] For further details on the use according to the invention, reference can be made to the above explanations on the further aspects of the invention, which apply accordingly with respect to the use according to the invention.

[0244] Again, a further subject-matter of the present invention—according to a fifth aspect of the present invention—is an apparatus for producing aerogel at pressure, wherein the apparatus comprises [0245] (a) at least one reactor which can be pressurized, [0246] (b) at least one inlet opening arranged at the reactor, in particular a nozzle, for introducing fluids, in particular liquids, into the reactor, and [0247] (c) at least one outlet opening arranged at the reactor, in particular a sluice, for removing fluids or solids from the reactor.

[0248] In the context of the present invention, it may in particular be provided that via at least one inlet opening a sol for producing a lyogel is dripped or sprayed into the reactor. Preferably, however, the reactor comprises a plurality of inlet openings for the introduction of fluids, in particular liquids, namely at least one nozzle for introducing the sol into the reactor and at least one nozzle for introducing further solvents.

[0249] The outlet opening of the reactor is preferably in the form of a sluice, in order to be able to quickly remove the lyogel or aerogel from the reactor or also to ensure multiple solvent exchange by covering and then draining the contaminated solvent from the reactor.

[0250] According to a preferred embodiment of the present invention, it is provided that the apparatus comprises at least one inlet and/or outlet opening arranged at the reactor for introducing and/or removing gases into and/or from the reactor.

[0251] Preferably, the pressure in the reactor is regulated by the amounts of substances, in particular in the gas phase and/or a supercritical phase and/or the temperature. Pressure regulation may be performed, for example, such that gas is introduced into or removed from the reactor.

[0252] Furthermore, in the context of the present invention, it is usually provided that the apparatus comprises a device for temperature regulation. Temperature regulation can also be used to specifically influence and control the processes in the reactor and thus in the apparatus as a whole. In particular, it is possible for the reactor to be heated or cooled.

[0253] Usually, the apparatus also has a control device, in particular for controlling the pressure and/or the temperature in the reactor.

[0254] The apparatus according to the invention can either comprise one reactor or, however, also comprise a plurality of reactors, in particular successive and/or interconnected reactors, so that the individual method steps of the method according to the invention are each carried out in separate reactors. In this way, continuous aerogel production can be carried out.

[0255] For further details on the apparatus according to the invention, reference can be made to the above explanations on the further aspects of the invention, which apply accordingly with respect to the apparatus according to the invention.

[0256] Finally, again further subject-matter of the present invention—according to a sixth aspect of the present invention—is a method for producing a lyogel by means of a sol-gel process, wherein the production of the lyogel is carried out at least partially at a pressure of more than 30 bar.

[0257] With regard to the production of the lyogel, all advantages, particularities and embodiments previously mentioned in the context of the method for the production of an aerogel with respect to the lyogel apply accordingly.

[0258] For further details on the method for producing a lyogel according to the present invention, reference can be made to the above explanations on the further aspects of the invention, which apply accordingly with respect to the method for producing a lyogel according to the present invention.

[0259] The subject-matter of the present invention will be illustrated below in a non-limiting manner and by way of example with reference to the figure representations as well as the working examples in an exemplary and non-limiting manner.

[0260] FIG. 1 schematically shows an apparatus 1 according to the invention with a reactor 2. The reactor 2 comprises several inlet openings, in particular nozzles 3, 4, 5 for the inlet of liquids and/or gases and has an outlet opening 6 for the removal of substances from the reactor 2, such as, for example, aerogels or lyogels or liquid solvents.

[0261] To carry out the method according to the invention, a precursor solution 7 is provided, which is placed in a container 8 and is introduced or sprayed into the reactor 2 by means of the inlet opening 7, in particular a nozzle. The precursor solution 7 is in particular an aqueous solution of a silicic acid, a silica sol or a silane hydrolysate, which comprises a pH value in the basic range, preferably between 8.5 and 10.

[0262] The reactor 2 preferably comprises an atmosphere 9 of supercritical CO.sub.2, in particular with a pressure of 80 to 120 bar and a temperature of 120° C. As a result, an almost spherical and dimensionally stable lyogel 10 forms immediately from the sol. The lyogel particles 10 collect on the bottom of the reactor 2 and can either be removed from the reactor 2 or further processed in the reactor. Preferably, after production of the lyogel 10, a solvent exchange is carried out with simultaneous hydrophobing of the lyogel 10 by means of a suitable organic solvent as well as a hydrophobing agent, in particular a silanizing agent Solvent and hydrophobing agent are introduced into the reactor 2 via the inlet opening 5. Here, it is more preferably the case that the organic solvent is soluble in CO.sub.2, in order to enable an enclosing supercritical drying with CO.sub.2. Gases, such as CO.sub.2, can be introduced into the reactor via the inlet opening 5 and, if necessary, removed again. After solvent exchange has taken place, the lyogel 10 is dried, or in particular by first draining the solvent through the outlet opening 6 and then carrying out supercritical drying of the lyogel using CO.sub.2, so that an aerogel is obtained.

[0263] The subject-matter of the present invention is explained below with reference to examples of embodiments in a non-limiting manner:

Working Examples

[0264] Silica aerogels are produced from silicic acids and examined for their properties:

1. Production of the Aerogels

[0265] Preparation of the Starting Material:

[0266] The silicic acid is prepared from sodium silicate by means of ion exchangers. The solids content is adjusted to 5 to 10 wt. %, preferably 7 to 8 wt %. For storage of the silicic acid, it can be stabilized at a pH of 1 to 2 using HCl. The pH of the silicic acid is then adjusted to a pH of 8.5 to 10.5 a few minutes before use by adding NH.sub.3.

[0267] Method Description:

[0268] The silicic acid produced is dripped into a container pressurized with CO.sub.2 by means of a high-pressure pump. Depending on the capillary selected, droplets with a diameter of 2 to 6 mm are produced. The pressure inside the container can be varied between 30 bar and 300 bar for gelation, with a minimum temperature of 60° C. The droplets are formed as soon as they enter the container. Gelation occurs immediately when the silicic acid enters the pressure vessel due to the change in pH caused by CO.sub.2 diffusing into the water.

[0269] The hydrogel particles in spherical form collect at the bottom of the vessel. The water present in the hydrogels interferes with the drying process and must therefore be exchanged for a suitable CO.sub.2-soluble solvent. For this purpose, the pressure is preferably set in the supercritical range, for example to 140 bar, and ethanol containing 5% hexamethyldizisilazane (HDMZ) is metered into the vessel. This initially leads to the formation of a liquid ethanol phase at the bottom of the vessel and a CO.sub.2 phase saturated with ethanol. It has been shown that both covering of the gels with the liquid ethanol-HDMZ mixture and exclusive contact of the gels with the ethanol-saturated gas phase results in sufficient solvent exchange. The simultaneous addition of HDMZ leads to hydrophobing of the gels. After a residence time of 30 minutes, the liquid ethanol is drained from the vessel. This is followed by two more cycles of adding ethanol to the vessel with the aim of saturating the CO.sub.2 phase with ethanol. After 20 minutes each, the saturated gas phase and the liquid ethanol phase are exchanged. It has been shown to be particularly advantageous that the first solvent exchange is carried out in such a way that the gels are covered with the liquid ethanol phase.

[0270] At the end of the solvent exchange under pressure, the gels contain less than 5% water and can be dried supercritically. For this purpose, the pressure in the column is preferably varied for 45 minutes between 100 to 160 bar, preferably 120 bar and 160 bar, at a vessel temperature of 80° C. to 120° C. The supercritical drying can be either continuous or discontinuous. In the discontinuous one, the gel is brought into contact with a defined quantity of the drying medium, in particular carbon dioxide, in the column and, after an adjustable residence time, the drying medium enriched with solvent is partially or completely removed from the column and replaced by fresh drying medium, wherein the process is repeated as often as necessary until the desired degree of drying is achieved. Alternatively, in continuous drying, the column can be continuously flushed with the drying medium, in particular carbon dioxide. In continuous drying, the pressure can either be kept constant or varied, in particular varied periodically. After completion of the drying step, dry spherical aerogels can be removed.

2. Properties of the Aerogels

[0271] By investigating the solvent exchange in hydrogels using compressed carbon dioxide, the influence of silanizing agents and their time of addition during the manufacturing process is examined.

[0272] It is found that the addition of silanizing agents prior to gel formation comprises positive effects on the forming gel matrix. The silanizing agent is incorporated into the forming Si—O network. This leads to partial elastification of the network, which is reflected in smaller pore radii and accelerated solvent exchange and lower shrinkage.

[0273] To evaluate the degree of hydophobicity, the prepared aerogel samples are stored in liquid water and 98% relative humidity.

[0274] As a result, it is found that pre-silanization is beneficial for structural build-up but often insufficient for complete silanization. By post-silanization during the drying process, only small amounts of moisture are absorbed in the pores of the aerogels over a storage period of 4 weeks in water

TABLE-US-00001 TABLE 1 Overview of the results from the nitrogen adsorption BET. BET Average Porosity surface area pore radius Samples [%] [m.sup.2/g] [nm] Remark ENOVA 93.78 696.3 13 Reference 3110 ENOVA P- 94.80 754.6 19 Reference 300 Aerogel 96.45 707.8 29 Unsilanized A-1 Aerogel 95.79 802.2 24 Pre-silanized A-6 Aerogel 94.95 866.7 24 Pre-silanized A-7 Aerogel 96.04 728.7 23 Pre-silanized A-8 Aerogel 97.07 705.5 18 Pre-silanized + H-6 Post-silanized Aerogel 96.63 673.1 15 Pre-silanized + H-7 Post-silanized Aerogel 97.23 656.3 21 Pre-silanized + H-8 Post-silanized

[0275] Samples A5 to A8 are pre-silanized using hexadimethyldisilazane at pH values of 7.0, in contrast samples H-5 to H-8 are pre- and post-silanized. As a result, the average pore radius can be varied between 30 and 15 nm. The shrinkage due to the drying performed could be reduced for the samples with pre- and post-silanization. In addition, H-8 comprises the lowest shrinkage and the highest porosity.

[0276] Thermal Conductivity

[0277] For the determination of the thermal conductivity a device from C3 Prozess und Analysetechnik GmbH of the type Hot Disk with a sensitivity up to 0.005 W/m*K is used. The Hot Disk sensor here consists of a nickel double spiral, which serves both as a heat source and for measuring the temperature rise during the measurement

TABLE-US-00002 TABLE 2 Overview of results from thermal conductivity measurements Measured thermal Calculated thermal Sample conductivity [W/m*K] conductivity [W/m*K] Aerogel UMSICHT 0.019 to 0.032 0.0169

[0278] Pore Volume and Density

[0279] To determine the density and pore volume, investigations were carried out using mercury porosimetry. Here, the sample is pressurized up to 400 MPa, which destroys the sample, but thereby also allows a complete detection of the inner pore volume.

TABLE-US-00003 TABLE 3 Overview of results from mercury porosimetry Average Density Porosity pore radius Sample [g/cm.sup.3] [%] [nm] Remark ENOVA 3110 0.147 94.3 76 Reference ENOVA P-300 0.142 95.1 75 Reference Aerogel 0.133-0.149 91.2-97.3 68-111 160222

[0280] The commercially available, subcritically dried and hydrophobized aerogel Enova P300 (Cabot Corporation), which comprises an average density of 150 kg/m3 according to the data sheet, and the aerogel Enova 3110 (Cabot Corporation) are used as reference.

[0281] The measured values of the analogously performed sorption measurement are shown together with the hydrophobic aerogel H-8 in FIG. 2F. Both aerogels comprise a similar course of the isotherms. Likewise, no constant value of the adsorbed volume is achieved. This behavior shows that the P300 also contains pores that are not detected by the sorption measurement and the evaluation according to BET or BJH. In addition, the P300 comprises an extended hysteresis at lower pressures, which is caused by an increased inhibition of the desorption of nitrogen.

TABLE-US-00004 TABLE 4 Properties derived from the sorption isotherms of the commercial aerogel P300 and sample H-8 Sam- S.sub.BET V.sub.P, Sorption d.sub.P, Geo d.sub.P, BET d.sub.P, BJH, A d.sub.P, BJH, D ple [m.sup.2/g] [cm.sup.3/g] [nm] [nm] [nm] [nm] P300 754.6 3.61 37 19 25 8 H-8 656.3 3.38 97 20.6 40 10

REFERENCE SIGNS

[0282] 1 apparatus [0283] 2 reactor [0284] 3 inlet opening [0285] 4. inlet opening [0286] 5. inlet opening [0287] 6. outlet opening [0288] 7. precursor solution [0289] 8. container [0290] 9. carbon dioxide atmosphere [0291] 10. lyogel particle