METHOD FOR PRODUCING AEROGELS AND AEROGELS OBTAINED USING SAID METHOD

Abstract

The invention relates to a method for producing an aerogel using a sol-gel process, in which first a lyogel is formed from at least two precursor sols and the lyogel is then converted to an aerogel.

Claims

1. A method for producing a silica aerogel using a sol-gel process, in which first a lyogel is formed and the lyogel is then converted into an aerogel, wherein for producing the lyogel at least two precursor sols, preferably two precursor sols, are mixed with each other, wherein a first precursor sol comprises an acidic pH value or a basic pH value and a second precursor sol comprises a pH value different from the first precursor sol.

2. The method according to claim 1, wherein the precursor sols are mixed with one another and are supplied, in particular immediately after mixing, into a reaction apparatus, preferably in the form of droplets, more preferably dropwise.

3. The method according to claim 1, wherein the precursor sols for producing the lyogel are continuously mixed with each other, in particular by means of a feed system, preferably by means of a two-substance feed.

4. The method according to one claim 1, wherein the precursor sols are provided separately from each other.

5. The method according to claim 1, wherein the first precursor sol comprises an acidic pH and the second precursor sol comprises a basic pH.

6. The method according to claim 1, wherein the acidic pH is in a range of pH 0 to 6, in particular pH 1 to 4, preferably pH 1.5 to 2.5.

7. The method according to claim 1, wherein the basic pH is in a range of pH 7 to 13, in particular pH 8 to 12, preferentially pH 9 to 11.

8. The method according to claim 1, wherein the precursor sols mixed with each other comprise a pH value in a range from pH 4.5 to 9.5, in particular pH 5 to pH 9, preferably pH 5.3 to 8.5.

9. The method according to claim 8, wherein the precursor sols mixed with each other comprise a weakly acidic pH, in particular in a range from pH 4.5 to 6.8, preferably pH 5 to 6.5, or a weakly basic pH, in particular in a range from pH 7.5 to 9.5, preferably pH 7.8 to 9.

10. The method according to claim 1, wherein the production of the lyogel is carried out at a pressure of less than 40 bar, in particular less than 30 bar, preferably less than 20 bar, more preferably less than 10 bar, particularly preferred at atmospheric pressure.

11. The method according to claim 1, wherein the production of the lyogel from the mixed precursor sols takes place within less than 60 seconds, in particular less than 30 seconds, preferably less than 20 seconds, more preferably less than 10 seconds, further preferably less than 5 seconds.

12. The method according to claim 1, wherein the precursor sols comprise silicon-based precursors.

13. The method according to claim 12, wherein the precursor sols contain silicon in amounts in a range from 3 to 20 wt. %, in particular 4 to 15 wt. %, preferably 5 to 10 wt. %, based on precursor sols.

14. The method according to claim 12, wherein the precursors are selected from the group of silicas, in particular colloidal silicic acid, silica sols, silicic acid sols, silanes, in particular tetraalkoxysilanes, siloxanes, silicates and mixtures thereof.

15. A silica aerogel, in particular obtainable according to claim 1, wherein the silica aerogel is in the form of particles with an in particular at least substantially circular cross-section and in that the silica aerogel comprises particle sizes in the range of 0.1 to 10 mm.

16. The silica aerogel according to claim 15, wherein the silica aerogel comprises particle sizes in the range of 0.2 to 8 mm, preferably 0.3 to 7 mm, more preferably 0.5 to 5 mm.

17. The use of a silica aerogel according to claim 15 for insulation purposes, in particular for sound insulation, electrical insulation or thermal insulation, in particular for thermally insulating purposes, or as a carrier material, as an absorbent or as an adsorbent.

18. The use of a silica aerogel according to claim 15 for insulation purposes, in particular as or in thermally insulating materials.

19. An apparatus for producing aerogel and/or obtainable by a method according to claim 1, wherein the apparatus comprises (a) at least one reactor, (b) at least one inlet opening arranged on the reactor, in particular a nozzle, for supplying fluids, in particular liquids, to the reactor, (c) at least two feeds connected to the inlet opening via a mixing device, and (d) at least one outlet opening arranged on the reactor, in particular a sluice, for the removal of liquids or solids from the reactor.

20. A method for producing a silica-lyogel using a sol-gel process, wherein for producing the lyogel at least two precursor sols, preferably two precursor sols, are mixed with each other, wherein a first precursor sol comprises an acidic pH or a basic pH and a second precursor sol comprises a pH value different from the first precursor sol.

Description

WORKING EXAMPLES

[0263] Silica aerogels are produced from silicic acids as precursors using the method according to the invention and the properties of the obtained aerogel particles are investigated:

[0264] 1. Producing the Aerogels

[0265] Producing the Acidic Precursor Sols:

[0266] For producing the acid adjusted precursor sols, silicic acid is prepared from sodium silicate using ion exchange against protons. The solids content is adjusted to 5 to 10 wt. %, preferably 7 to 8 wt. %.

[0267] The silicic acid thus produced comprises a pH of 1.8 to 2.4 to preferably 2.0. For storage of the silicic acid, it can be stabilized to a pH of 1 to 1.5 using HCl.

[0268] Producing the Basic Precursor Sols:

[0269] For producing the basic precursor sols, silicic acid is again first produced from sodium silicate using ion exchange. The solids content is adjusted to 5 to 10 wt. %, preferably 7 to 8 wt. %.

[0270] The pH of the silicic acid is adjusted to a pH of 9 to 11, preferably 10.5, a few minutes before use by using NH3 (approx. 25%, approx. 4.5 g/100 g solution). Very rapid addition and very rapid mixing of the aqueous NH3 solution are essential to prevent immediate gelation.

[0271] The ammonia addition is 7.0 ml ammonia 25 wt. % per 100 g silicic acid solution. The storage time of such a solution is usually only a few hours, preferentially a maximum of 2 hours.

[0272] Alternatively, commercial silica sols can be used as a basic solution. For example, Ludox SM with a solids content of 30 wt. % and a pH of about 10, or LUDOX AS-40, which comprises a lower sodium content, or mixtures of these are suitable.

[0273] Mixing Ratios of Precursor Sols for Lyogel Formation:

[0274] The following table lists the mixing ratios in which the above precursor sols are used in the method of the invention for producing the lyogel.

TABLE-US-00001 TABLE 1 Mass balances of the silicic acid solutions to be mixed and the corresponding pH values Proportion Proportion basic solution acidic solution B/S NH.sub.3 obtained in [g] in [g] ratio wt.-% pH-value Silicic Silicic acid solution acid solution 7 wt. %, pH 11 7 wt. %, ca. pH 2 3.50 50.0 0.070 0.39 5.75 3.75 50.0 0.075 0.41 6.11 4.00 50.0 0.080 0.45 6.55 Silica sol Silicic LUDOX SM acid solution 7 wt. %, pH 10.5 7 wt. %, ca. pH 2 10.0  50.0 0.20 6.15 Silica sol Silicic LUDOX SM acis solution 10 wt. %, pH 9.1 7 wt. %, pH 2 25.0  50.0 0.50 6.45

[0275] Procedure for the Formation of Lyogels and Aerogels

[0276] The silicic acid solutions prepared as described above are mixed in an apparatus according to the invention, preferentially via a two-substance feed using a T-piece and subsequently a static mixer in the form of a tube insert and using two pumps. In particular, models for low-viscosity systems are suitable as static mixers, e.g. with a diameter-length ratio of 1:5, for example according to the “Kenics” design. Preferentially, gelation occurs immediately upon mixing of the two solutions, so that the mixed precursor solutions can be dripped with short gelation times at resulting pH values close to the neutral range. The formed lyogel particles, in particular hydrogel particles, collect at the bottom of the apparatus, preferentially in the form of spherical particles.

[0277] Since the water present in the lyogels or in particular hydrogels would interfere with the drying process for conversion into an aerogel, it is preferentially exchanged for a suitable, in particular CO.sub.2-soluble, solvent, e.g. ethanol. For this purpose, the solvent exchange preferably takes place at a pressure of about 1 to 30 bar and temperatures between 30° C. and 150° C. The gel stored in the reaction apparatus according to the invention, in particular the autoclave, is covered with the solvent, for example ethanol, in liquid form. For this purpose, the autoclave is pressurized in particular to prevent boiling of the solvent, wherein for ethanol the vapor pressure of 5.6 bar is at 130° C., so that a pressurization of 10 bar is suitable here.

[0278] Furthermore, hydrophobing of the lyogel can also be performed as part of the solvent exchange. For this purpose, the gels are overlaid with a liquid mixture of ethanol and hexamethyldisilazane (HDMZ), wherein the simultaneous addition of HDMZ leads to hydrophobing of the gels. Alternatively, exclusive contact of the gels with a gas phase saturated with ethanol can also lead to sufficient solvent exchange.

[0279] After a residence time of 30 min, the liquid ethanol is drained from the container. Further washing cycles can then follow by adding ethanol to the container, wherein the objective is to bring the water content of the gels below 10% by volume. In this case, the ethanol phase is replaced after every 20 min. It has proved particularly advantageous if the first solvent exchange is carried out in such a way that the gels are covered with the liquid ethanol phase.

[0280] After the end of the solvent exchange, in particular if the gels contain less than 5 wt. % water, the gel particles can be supercritically dried. For this purpose, compressed carbon dioxide is fed in as a drying fluid, wherein the gas flow can be used for targeted continuous drying of the organogels and single-stage aerogel particle generation can be ensured. Based on the spherical particle shape and a typical particle diameter between 1 and 6 mm, supercritical drying can be carried out in a time window of 10 to 60 min at a pressure in a range of 120 to 160 bar and a temperature in a range of 60 to 120° C.

[0281] 2. Characterization of the Aerogels

[0282] The characterization of the aerogels obtained according to the invention is carried out as described below:

[0283] Particle Sizes:

[0284] The sieve analysis method is used to determine the size of the obtained aerogel particles according to the invention. Smaller particles in the size range below 1 mm are additionally measured by light microscopy.

[0285] The particle size analysis has shown that aerogel particles according to the invention comprise particle sizes in the range of 0.1 to 10 mm, in particular 0.2 to 8 mm, preferably 0.3 to 7 mm.

[0286] Thermal Conductivity:

[0287] To determine the thermal conductivity, an instrument from C3 Prozess und Analyse-technik GmbH of the Hot Disk type with a sensitivity of 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.

[0288] For aerogels according to the invention, thermal conductivities in the range of 0.012 to 0.025 W/mK, in particular 0.013 to 0.022 W/mK, preferably 0.014 to 0.020 W/mK, are measured.

[0289] Pore Volume and Density:

[0290] To determine the density and pore volume, studies were carried out using mercury porosimetry. Here, the sample is subjected to up to 400 MPa pressure, which destroys the sample, but thereby also allows complete detection of the internal pore volume.

[0291] Commercially obtained, subcritically dried and hydrophobized aerogel Enova P300 (Cabot Corporation, average density according to data sheet approx. 150 kg/m.sup.3) and aerogel Enova 3110 (Cabot Corporation) serve as reference.

[0292] The density of aerogel particles according to the invention is in a range of 0.01 to 0.60 g/cm.sup.3, in particular 0.11 to 0.55 g/cm.sup.3, preferably 0.12 to 0.50 g/cm.sup.3, according to the results of mercury porosimetry,

[0293] Porosity, BET Surface Area and Mean Pore Radius:

[0294] To determine the porosity, BET surface area and mean pore radius of the aerogels according to the invention, the nitrogen adsorption of the aerogels was measured and/or determined using the BET method. For this purpose, the measurement is generally carried out according to DIN ISO 9277:2003-05 (“Determination of the specific surface area of solids by gas adsorption using the BET method”).

[0295] According to the aforementioned method, values for porosity of 94 to 99.5%, in particular 95 to 99%, preferably 96 to 98%, are obtained for aerogels according to the invention. Furthermore, the aerogels comprise a BET surface area in the range of 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.

REFERENCE SIGNS

[0296]

TABLE-US-00002 1 apparatus 2 reactor 3 precursor sol 4 precursor sol 5 feed 6 valve 7 metering device 8 mixing device 9 static mixing elements 10 inlet opening 11 lyogel particles 12 inlet opening 13 inlet opening 14 outlet opening 15 measuring device