METHOD FOR PRODUCING AN AEROGEL MATERIAL

Abstract

The present invention relates to a process for producing an aerogel material based on amorphous silica, comprising the following steps:

a) preparing a mixture comprising silica sol, alcohol, and a hydrophobizing agent activatable by acid catalysis;

b1) adding a base to the mixture formed in step a) and mixing the resulting mixture;

b2) gelation of the mixture comprising silica sol obtained in step b2), resulting in the formation of a silica gel, and optional aging of the gel;

c) adding a hydrophobization catalyst to the silica gel formed in step b2) and optionally aged, in-situ formation or controlled release of a hydrophobization catalyst, and initiation of the catalyzed hydrophobization of the silica;

d) removing the volatile constituents of the mixture formed in step c) by subcritical drying, resulting in the formation of the aerogel material, wherein at least steps b2) to d) are carried out in the same reactor.

Claims

1-18. (canceled)

19. A process for producing an aerogel material based on amorphous silica, comprising the following steps: a) preparing a mixture comprising silica sol, alcohol, and a hydrophobizing agent activatable by acid catalysis; b1) adding a base to the mixture formed in step a) and mixing the resulting mixture; b2) gelation of the mixture comprising silica sol obtained in step b2), resulting in the formation of a silica gel, and optional aging of the gel; c) adding a hydrophobization catalyst to the silica gel formed in step b2) and optionally aged, in-situ formation or controlled release of a hydrophobization catalyst, and initiation of the acid-catalyzed hydrophobization of the silica gel; d) removing the volatile constituents of the mixture formed in step c) by subcritical drying, resulting in the formation of the aerogel material, characterized in that at least steps b2) to d) are carried out in the same reactor.

20. The process of claim 19, wherein step a) is carried out by hydrolysis of an alcoholic solution of an organosilicate selected from the group consisting of tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate (TMOS), tetraisopropyl orthosilicate (TPOS), and mixtures thereof and subsequent dilution using an organic solvent mixture consisting of alcohol, a hydrophobizing agent activatable by acid catalysis, and water.

21. The process of claim 19, wherein the alcohol is selected from the group consisting of methanol, ethanol, n-propanol, isopropanol, butanol, and mixtures thereof

22. The process of claim 19, wherein the hydrophobizing agent activatable by acid catalysis is selected from the group consisting of hexamethyldisiloxane, trimethylethoxysilane, trimethylmethoxysilane, and mixtures thereof

23. The process of claim 19, wherein the base used in step b1) is selected from the group consisting of ammonia, ammonium fluoride, and aminosilanes.

24. The process of claim 19, wherein step b2) is carried out at a temperature of 80 to 120 C. within 20 to 180 minutes.

25. The process of claim 19, wherein the hydrophobization catalyst is selected from the group consisting of hydrogen chloride, nitric acid, sulfuric acid, trimethylchlorosilane, and mixtures thereof

26. The process of claim 19, wherein hydrophobization of the silica according to step c) is carried out at a temperature of 80 to 130 C., at a pressure of 1.2 to 4 bar, within 10 to 180 minutes.

27. The process of claim 19, wherein step d) is carried out at a temperature of 100 to 200 C. and at a pressure of 0.1 to 4 bar.

28. The process of claim 19, wherein, during the performance of step d), an optionally preheated carrier gas is passed into the reactor continuously at a gas hourly space velocity of 150 to 1500 h.sup.1 and, after mixing with the gaseous constituents of the reactor, in turn exits the reactor.

29. The process of claim 19, wherein the reactor in which steps b) to d) are carried out is a tube having a diameter of 5 to 50 mm or a bundle of a plurality of such tubes arranged parallel to one another.

30. The process of claim 19, wherein an axis of symmetry of the axially symmetric reactor used in steps b) to c) forms a horizontal angle of 10 to 45 degrees during the performance of this step.

31. The process of claim 19, wherein the aerogel material has a density of less than 0.3 g/cm.sup.3 and a thermal conductivity of 12 to 30 mW/(mK).

32. A process for producing a composition that provides thermal or acoustic insulation, wherein the composition is formed from aerogel material produced by the process of claim 19.

33. The process of claim 32, wherein the composition is a thermal and/or acoustic insulation panel.

34. The process of claim 32, wherein the composition is plaster, mortar, or concrete formulations for thermal insulation.

35. The process of claim 32, wherein the composition is bulk material for thermal insulation.

36. The process of claim 32, wherein the composition is a thermal insulation container.

37. The process of claim 32, wherein the composition is a coating for thermal and/or acoustic insulation.

38. The process of claim 32, wherein the composition is a thermally insulating textile or a film membrane for lightweight architectural construction.

Description

EXAMPLES

Example 1

Preparation According to the Invention of an Aerogel in a Laboratory Reactor Block Provided with Appropriate Holes

[0051] A tetraethyl orthosilicate-based (TEOS-based) sol concentrate having a degree of hydrolysis of 75% and an SiO.sub.2 equivalent content of 20% by weight was prepared from TEOS according to J. Non-Cryst. Solids, 1995, 186, 1-8. The sol concentrate was diluted with ethanol and hexamethyldisiloxane (HMDSO) as hydrophobizing agent to about 6% by weight of SiO.sub.2 equivalent content, the volume fraction of HMDSO in the sol being about 30% by volume. 1370 mL of this sol was preheated to 35 C. and activated by addition of 2 molar ethanolic ammonia solution and immediately used to fill the laboratory reactor block (diameter: 20 mm, length 250 mm) provided with appropriate holes. The reactor had been preheated beforehand to a plate temperature of 70 C. Gelation occurred after approx. 2-3 minutes, whereupon the reactor was closed pressure-tight and heated to a plate temperature of 110 C. This was accompanied by a rise of about 2.5-3 bar in the overpressure in the reactor. After aging for 2.5 hours, the heating was switched off and the reactor cooled for 45 minutes until the overpressure was <0.5 bar. The residual overpressure was carefully released and the reactor lid was removed. The bottom plate was loosened and approx. 250 mL of syneresis liquid (pore liquid) was drained off and recovered, which corresponded to shrinkage of the aged gel of approx. 18%. The bottom plate was then screwed back on tightly and 350 mL of dilute ethanolic H.sub.2SO.sub.4 solution (hydrophobization catalyst) was added, completely covering the gel rods in the reactor block with the hydrophobization catalyst liquid. The lid was then screwed back on medium-tight and the reactor heated to a nominal temperature of 110 C. by means of hotplates. The gel rods were then hydrophobized for 2.5 hours, with the measured overpressure being approx. 1.7 bar. The heating was then switched off again. After a cooling period of approx. 30 minutes, during which the residual overpressure fell below 0.5 bar, the reactor lid was carefully opened again and removed. The excess hydrophobization catalyst solution was drained off by loosening the bottom plate.

[0052] During the gelation, aging, and hydrophobization processes mentioned above, the reactor was positioned vertically (orientation of the holes/gel rods formed). For the drying process, the reactor lid was then screwed back on medium-tight and the reactor was laid on its side, resulting in a horizontal orientation of the gel rods. The locking screws for the bottom plate were loosened for the drying process so that there was a gap of approx. 1-2 mm between the reactor block and bottom plate through which the drying gases could escape.

[0053] Hot nitrogen (T=200 C.) was then introduced into the top part at a flow rate of 20 L/min and the reactor was heated to 180 C. (nominal hotplate temperature). Drying in the reactor was complete after approx. 1 hour, after which the nitrogen supply and reactor heating were switched off. The reactor was cooled for 45 minutes and approximately 1250 ml of a bluish-white, particulate, hydrophobic aerogel was obtained as bulk material. Analysis of the product revealed a bulk density of 0.11-0.13 g/cm.sup.3 and a thermal conductivity of 17.8 mW/m K for the bulk material. The particle size distribution showed that over 90% of the particles had a particle size between 1.0 mm and 7.0 mm. Because the reactor design was not optimized for drying, slight differences in the quality of the material were detected at the top and bottom (approx. 10% difference in density). The total process time was 7.5 hours.

Comparative Example 1

Preparation of a Gel in a Plastic Tube-Bundle Reactor with Drying in a Drying Cabinet

[0054] A silica sol concentrate in accordance with example 1 was diluted with ethanol and HMDSO to 5.7% by weight of SiO.sub.2 equivalent content SiO.sub.2. The proportion of HMDSO in the sol mixture was 33% by volume. 410 mL of this sol was mixed at room temperature with 10 mL of 2 M aqueous NH.sub.3 solution and, after stirring briefly, transferred to a beaker that was filled completely with a bundle of cut-to-length plastic drinking straw tubes having an internal diameter of approx. 8 mm and made of polypropylene. The latter served as a mold for the (aero)gel rods being prepared.

[0055] The beaker was covered with a watch glass, sealed with Parafilm, and placed in a heating cabinet at 65 C. Gelation occurred after about 10-12 minutesThe gel rods were allowed to age at 65 C. for 14 hours. Approximately 80 mL of syneresis liquid was then decanted off and the drinking straws serving as molds were removed, leaving behind the vertical gel rods. A hydrophobization catalyst solution consisting of 250 mL of HMDSO, 10 mL of ethanol, and 7.5 mL of 37% by weight hydrochloric acid solution was then added, covering the gel rods generously (approx. 1.5 cm) with liquid. The beaker covered with a watch glass and sealed with Parafilm was incubated again in a heating cabinet at 65 C. for 24 hours for the purposes of the hydrophobization reaction. The excess hydrophobization catalyst solution was then decanted off, after which the gel rods were dried in a drying cabinet under a nitrogen atmosphere for 3 hours at 150 C.

[0056] The product was obtained in the form of relatively large rod fragments of hydrophobic aerogel having a length typically of between 4 mm and 15 mm and a diameter of approx. 6-6.5 mm. The bulk density of the material thus obtained was 0.113 g/cm.sup.3. The thermal conductivity of the unchanged sample was 22-23 mW/m K. The relatively high value was due to the large fragments and the resulting proportion of large air holes. The total process time was 42 hours.

[0057] Compared to example 1, comparative example 1 has the disadvantage of needing an additional transfer of hydrophobized gel to the drying unit and associated additional investment costs in industrial equipment.

Example 2

Preparation According to the Invention of an Aerogel in a Single-Tube Reactor

[0058] An electrically-heatable tube made of stainless steel 1.4571 (internal diameter 20 mm, length 150 mm) with flange and media-tight valves at the top and bottom was chosen as the test reactor. In addition, a reservoir vessel was flange-mounted at the top unit and one at the bottom unit. Pressure equalization could likewise be achieved via a gas-displacement line. The reactor was at an inclination of 22 to horizontal throughout the test.

[0059] First of all, a P750 sol concentrate having an SiO.sub.2 equivalent content of 20.0% by weight was prepared from Dynasilan 40 (manufacturer: Evonik Ressource Efficiency GmbH) and diluted to 5.8% by weight with ethanol and HMDSO (30% by volume in the sol). A dilute ammonia solution was then added at room temperature with stirring, 470 ml of the activated sol mixture was transferred to the single-tube reactor preheated to 65 C., and the latter was closed pressure-tight. 15 minutes after filling the reactor, the nominal temperature was raised to 100 C., which was accompanied by a rise in overpressure to approx. 2.0 bar. After aging for 1 hour, the heating was switched off. 90 mL of syneresis liquid was then drained off into a reservoir vessel likewise connected to the gas-displacement line and removed from the system. The top reservoir was then filled with 200 ml of a dilute ethanolic nitric acid solution and this was added slowly via the top reservoir. The tube reactor was closed again pressure-tight and heated to a nominal temperature of 100 C. The gel rod in the tube was now hydrophobized for 90 minutes, with the overpressure being approx. 1.5 bar. With the heating switched on, the residual hydrophobization catalyst liquid was then removed in accordance with the procedure described above and the pressure in the reactor was slowly vented to the atmosphere. Hot nitrogen (T=200 C.) having a flow rate of 9 L/min set on the mass-flow controller was then introduced at the reactor base and the offgas was conducted away via the top part and a condenser. The single-tube reactor was then heated to 165 C. within 10 minutes. Drying in the reactor was complete after approx. 45 minutes. The yield was 500 ml of a particulate, hydrophobic aerogel bulk material, which corresponds to a yield of >95%. Analysis of the product revealed a bulk density of 0.124 g/cm.sup.3 and a thermal conductivity of 17.6 mW/m K for the bulk material. The particle size distribution showed a symmetrical distribution with a diameter in the range between 0.3 mm and 5 mm. The total process time lasted 5 hours.

Example 3

Preparation According to the Invention of an Aerogel in a Single-Tube Reactor: Altering the Inclination of the Single-Tube Reactor During the Process

[0060] A test analogous to example 2 was carried out using the same starting sol and identical process parameters. On filling the reactor, this was in the vertical position. At the end of aging and after draining the syneresis liquid, the hydrophobization catalyst solution was added at the top in the manner described in example 2 while still vertically oriented. The hydrophobization catalyst was recovered via the top part by tilting into the vertical position with the reactor open.

[0061] The reactor was rotated into the horizontal position to dry.

[0062] This example shows that all process steps (gelation, aging, hydrophobization, and drying) should preferably be carried out at a slight inclination to the horizontal.

Example 4

Preparation According to the Invention of an Aerogel in a Tube-Bundle Reactor

[0063] The pilot plant used consisted of a stirred-tank reactor for the preparation of the sol and a tube-bundle reactor with top and lid unit, and also appropriate auxiliary units (heating, heat exchanger, condenser) and tanks/reservoirs for the reagents used. The tube-bundle reactor consisted of a heat exchanger of parallel tubes, each with an internal diameter of 18 mm, and a jacket that can be flushed with heat-transfer fluid. The reactor was screwed firmly to the floor at a fixed angle to the horizontal of 19.

[0064] First of all, 76 L of a sol according to example 2 was prepared by diluting the sol concentrate with ethanol and HMDSO in a stirred-tank reactor and preheated to 45 C.

[0065] Dilute ethanolic ammonia solution was then added and the sol thus activated was transferred to the tube-bundle reactor preheated to 60 C. via a transfer line with pressure equalization. . In an alternative embodiment, a dilute ammonia solution and the sol mixture diluted with ethanol and HMDSO were fed into the reactor in the desired ratio by means of two separate pipelines and, during the filling process, mixed homogeneously in the reactor by means of a mixing device situated at the reactor inlet, such as a set of apertures or a static mixer. The top and bottom valves of the reactor were then closed, whereby the heat-exchanger tubes, together with the gel rods that were forming, formed a pressure-tight closed system.

[0066] The reactor temperature was then raised to 112 C. by heating the heat-exchanger fluid. This was accompanied by a rapid rise in pressure to a value of 2.5 bar. After aging for a period of 60 minutes, the bottom and top valves were carefully opened and the syneresis fluid was collected in the reservoir. 18.5 L of a dilute solution of nitric acid in ethanol was then preheated to 60 C. in the stirred-tank reactor and afterwards pumped into the reservoir just mentioned. The heating of the heat exchanger of the tube-bundle reactor was set to 95 C. The hydrophobization catalyst was then pumped into the tube-bundle reactor slowly, in portions, and the nominal temperature for the heating was raised to 112 C. After a hydrophobization time of 70 minutes, the excess hydrophobization catalyst liquid was removed from the system. The system consisting of reactor and peripheral circuit was vented slowly against a nitrogen atmosphere, with a nitrogen flow of 1.36 m.sup.3/min set at the end of this. The volatile constituents of the offgas were recovered via a condenser. At the same time as the flushing with nitrogen, the nominal heating temperature was set to 160 C. Drying was complete after 50 minutes.

[0067] At the end, the bottom of the reactor was slowly opened and the aerogel granules were discharged by a pulse of nitrogen. This left behind 6.1 kg of aerogel granules of extremely homogeneous quality having a particle size between 0.5 and 6 mm. Analysis of the material revealed a bulk density of 0.112 g/cm.sup.3 and a thermal conductivity of 17.8 mW/(m K) for the bulk material. The total process time was about 4 hours.

Comparative Example 2

Preparation of an Aerogel in a Honeycomb Reactor and Subsequent Drying in a Drying Cabinet

[0068] A section of hexagonal polypropylene plastic honeycomb (Tubus Waben, cell size 8 mm, length 450 mm) was inserted in a vertical orientation into a stainless steel drum body having an approximate capacity of 45 L, coated on the inside with ethylene tetrafluoroethylene (ETFE), and having a hermetically closable lid, so as to fill the entire vessel volume with the honeycomb block except for an approximately 4 cm air gap at the lid. The lid and base of the drum were each equipped with a ball valve; the drum was mounted on an aluminum frame that allowed it to rotate freely over a horizontal axis. A sol concentrate prepared from Dynasylan-40 (manufacturer: Evonik Ressource Efficiency GmbH) was then diluted to a silicate content of 6.0% by weight in analogous manner to example 2, the volume fraction of HMDSO in the sol being 29.2% by volume, activated by addition of dilute ethanolic ammonia solution, and homogenized by stirring. The drum body loaded with honeycomb was then filled with 41.1 L of this sol at room temperature. The drum was tightly sealed and transferred to a heating cabinet of sufficient capacity preheated to 65 C. Gelation commenced after approx. 10 minutes, as measured by a residual amount of the activated sol at room temperature. After an aging time of 72 hours at 65 C., the drum was removed from the heating cabinet, opened carefully, and the lid removed. 11.8 L of syneresis liquid was then carefully drained off via the bottom valve, which was closed again at the end. 14 L of a dilute nitric acid hydrophobization catalyst solution (solvent composition ethanol:HMDSO in a volume ratio of 2:1) was then poured into the honeycomb body from above, so that the liquid covered it by approx. 5 mm. The drum reactor was closed, transferred back to the heating cabinet, and the nominal temperature increased to 80 C. The gel rods were hydrophobized in this manner for 24 hours. The drum reactor was then removed from the heating cabinet and opened carefully (slight overpressure). The excess hydrophobization catalyst liquid was drained off via the bottom valve. The honeycomb block was then removed from the drum using a dedicated auxiliary structure and the hydrophobized gel rods were drained by tapping.

[0069] The gel pieces were then dried in a drying cabinet under a nitrogen atmosphere at 150 C. with circulating air (air exchange rate: approx. 10per hour) for 3 hours. This afforded 2.3 kg of an aerogel material, which consisted largely of relatively large cylindrical rod fragments. The composition of the fragments was very similar to that from example 1. The bulk density of the material thus obtained was 0.115 g/cm.sup.3. A sample of the aerogel material was mechanically comminuted, resulting in a broad distribution of particles between 0.2 mm and 6 mm, and the thermal conductivity of the bulk material was determined as 16.7 mW/(m K) with the aid of a two-plate device.

[0070] Compared to example 4, comparative example 2 has the disadvantage of needing an additional transfer, for example via an airlock system, of hydrophobized gel to an additional drying unit that would need to be purchased, as well as associated additional investment costs.