SILICA AEROGEL WITH INCREASED ALKALINE STABILITY

Abstract

The present invention relates to a process for producing a hydrophobized silica aerogel, comprising the following steps: a) preparing a hydrophobized silica gel comprising alkoxy groups; b) drying of the hydrophobized silica gel obtained in step a); c) treatment of the product obtained in step b) with a gas mixture comprising water and a base or an acid. A hydrophobized silica aerogel with a reduced alkoxy group content, suitable for thermal insulation applications, is also provided.

Claims

1-16. (canceled)

17. A process for producing a hydrophobized silica aerogel, comprising the following steps: a) preparing a hydrophobized silica gel comprising alkoxy groups; b) drying the hydrophobized silica gel obtained in step a); c) treating the hydrophobized silica gel dried in step b) with a gas mixture comprising water and a base or an acid.

18. The process of claim 17, wherein the alkoxy groups are alkylalkoxy groups selected from the group consisting of: methoxy (OCH.sub.3); ethoxy (OC.sub.2H.sub.5); propoxy (OC.sub.3H.sub.7); butoxy (OC.sub.4H.sub.9); and mixtures thereof.

19. The process of claim 17, wherein the silica gel is obtained by hydrolysis of an organosilicate selected from the group consisting of: tetraethyl orthosilicate (TEOS); tetramethyl orthosilicate (TMOS); tetraisopropyl orthosilicate (TPOS); and mixtures or oligomers thereof.

20. The process of claim 17, wherein the silica gel is obtained by gelification of an inorganic precursor selected from the group consisting of: sodium silicate; waterglass; ion exchanged waterglass; silicic acid; colloidal silica; and mixtures thereof.

21. The process of claim 17, wherein step a) is carried out using a hydrophobizing agent activatable by an acid catalyst selected from the group consisting of: hydrogen chloride; nitric acid; sulfuric acid; trimethylchlorosilane; and mixtures thereof.

22. The process of claim 17, wherein step a) is carried out using a hydrophobizing agent activatable by an acid catalyst selected from the group consisting of: hexamethyldisiloxane; trimethylethoxysilane; trimethylmethoxysilane; and mixtures thereof.

23. The process of claim 17, further comprising a gelation of the silica sol and optionally aging of the resulting silica gel in the presence of a base catalyst selected from the group consisting of ammonia; ammonium fluoride; aminosilanes; and mixtures thereof.

24. The process of claim 17, wherein step b) is carried out under subcritical conditions.

25. The process of claim 17, wherein the base used in step c) of the process is selected from the group consisting of ammonia; lower aliphatic alkylamines; and mixtures thereof.

26. The process of claim 17, wherein the acid used in step c) of the process is selected from the group consisting of: a volatile mineral acid; a carboxylic acid; a halosilane; and mixtures thereof.

27. The process of claim 26, wherein the volatile mineral acid is selected from the group consisting of: hydrochloric acid (HCl); hydrofluoric acid (HF); hydrobromic acid (HBr); hydroiodic acid (HI); and nitric acid (HNO.sub.3); the carboxylic acid is formic acid or acetic acid; and the halosilane is trimethylchlorosilane.

28. The process of claim 17, wherein step c) is conducted at the temperature of 50° C.-250° C.

29. The process of claim 17, wherein the duration of step c) is 1 minute-1000 minutes.

30. The process of claim 18, wherein the silica gel is obtained by hydrolysis of an organosilicate selected from the group consisting of: tetraethyl orthosilicate (TEOS); tetramethyl orthosilicate (TMOS); tetraisopropyl orthosilicate (TPOS); and mixtures or oligomers thereof.

31. The process of claim 18, wherein the silica gel is obtained by gelification of an inorganic precursor selected from the group consisting of: sodium silicate; waterglass; ion exchanged waterglass; silicic acid; colloidal silica; and mixtures thereof.

32. The process of claim 18, wherein step a) is carried out using a hydrophobizing agent activatable by an acid catalyst selected from the group consisting of: hydrogen chloride; nitric acid; sulfuric acid; trimethylchlorosilane; and mixtures thereof.

33. The process of claim 18, wherein step a) is carried out using a hydrophobizing agent activatable by an acid catalyst selected from the group consisting of: hexamethyldisiloxane; trimethylethoxysilane; trimethylmethoxysilane; and mixtures thereof.

34. A silica aerogel, comprising trimethylsiloxyl (≡SiOSiMe.sub.3), alkoxysilyl (≡SiOR) and silanol (≡SiOH) groups, wherein the silica aerogel comprises: an envelope density of at most 0.17 g/cm.sup.3; a ratio of the amount of the trimethylsiloxyl groups (≡SiOSiMe.sub.3) to the sum of the amount of the trimethylsiloxyl, the alkoxysilyl and the silanol groups: N.sub.SiOSiMe3/(N.sub.SiOSiMe3+N.sub.SiOR+N.sub.SiOH) of more than 0.5; and a ratio of the amount of the alkoxysilane groups to the sum of the amount of the alkoxysilane and the silanol groups N.sub.SiOR/(N.sub.SiOR+N.sub.SiOH) of 0.05 to 0.35; wherein R is an alkyl group; the amount of the trimethylsiloxyl (≡SiOSiMe.sub.3) and alkoxysilyl (≡SiOR) groups is determined by .sup.1H-NMR and the amount of the silanol (≡SiOH) groups is determined by .sup.29Si-NMR.

35. The silica aerogel of claim 34, wherein R is methyl or ethyl.

36. A composition for thermal and/or acoustic insulation comprising the silica aerogel of claim 34.

Description

EXAMPLES

Comparative Example 1

[0084] A silicon dioxide sol concentrate was prepared by alcoholic hydrolysis of TEOS using a molar ratio of TEOS/water/sulfuric acid of 1:3.0:0.001 in ethanol at room temperature and an equivalent silica content, expressed as SiO.sub.2 of 20 wt. % at 35° C. The sol concentrate was then allowed to rest overnight before use. The sol concentrate was then diluted with HMDSO and ethanol to the final sol concentration of 5.7 wt. % SiO.sub.2 equivalents such that the HMDSO content in the sol was 30 wt. %. Thereafter, 2% by volume of a diluted ethanolic ammonia solution was added to this sol, which resulted in gelation within 8-10 minutes. The fresh organogel was placed inside a sealed steel tube pressure vessel, covered with a small amount of ethanol and aged at 95° C. for 2 h.

[0085] Following aging, the sealed tube was cooled to room temperature and carefully opened. The aged gel was then mechanically crushed, and the gel granulate particles again placed inside the steel tube. Subsequently, a mixture of dilute nitric acid, ethanol and HMDSO was added to barely cover the gel particles. The steel tube was sealed again and placed inside an oil bath at 100° C. where it was kept for hydrophobization for 2 h. Upon completion of the hydrophobization and cooling to 45° C., the gel particles were recuperated and subsequently dried in a convective drying oven under nitrogen at 150° C.

[0086] The final aerogel material had an envelope density of 0.115 g/cm.sup.3 and a typical packed bed thermal conductivity of the granulate specimen of 0.0184 W/(m K). Other chemical data of the aerogel are summarized in Table 1.

Example 1: Production of an Aerogel Granulate Material at Laboratory Scale

[0087] The aerogel material with residual alkoxy groups obtained in comparative example 1 was then kept inside the same oven and the temperature of the oven was raised to 165° C. Once the new set temperature was reached, the oven was briefly opened, and a thin capillary tube connected to a syringe pump positioned under the aerogel specimen and the door closed again. The hydrolysis of ethoxy groups was now initiated by dosing a 6M aqueous ammonium hydroxide solution at a rate of 300 mL/h. The dosing was stopped after 40 minutes, and residual water/ammonia mixture inside the oven purged with nitrogen flow over the course of another 10 minutes. The final aerogel material was then recuperated and characterized, having an envelope density of 0.118 g/cm.sup.3 and a typical packed bed thermal conductivity of the granulate specimen of 0.0184 W/(m K). Other chemical data of the aerogel are summarized in Table 1.

Example 2

[0088] A procedure was identical to the one described in example 1 with the exception that the drying temperature was 160° C. and that the ethoxy group hydrolysis step was carried out directly at the end of the drying step by beginning dosing of the ammonium hydroxide solution with the capillary having been placed at the beginning of the drying step. Physico-chemical data of the aerogel are summarized in Table 1.

Example 3

[0089] A procedure was identical to the one described in example 1 with the exception that the ethoxy group hydrolysis step was performed by injection of a 3M aqueous trifluouroacetic acid solution at a rate of 200 mL/h and the dosing was maintained over a period of 25 minutes. Physico-chemical data of the aerogel are summarized in Table 1.

Example 4: Industrial Production of an Aerogel Granulate Material

[0090] The pilot plant used consisted of a stirred batch reactor for the production of the sol and a tube bundle reactor with top and bottom hermetically sealed lids, as well as a downstream and additionally heated hydrophobization reactor vessel, a phase separation unit and a hybrid convection/contact dryer unit. The device periphery was made up of the appropriate auxiliary units (heating, heat exchanger, condenser) and solvent reservoirs as well as storage for the various reagents. The tube bundle reactor consisted of a heat exchanger of parallel tubes, each with an inner diameter of 23 mm, and a jacket that could be purged with a heat transfer fluid medium. The reactor was mounted to the floor of the factory site at a fixed angle of 19° to the horizontal.

[0091] At the beginning, 76 L of a sol consisting of 30 kg of ethanol, 30 kg of sol concentrate, 4.3 kg of water and 25.8 kg of HMDSO were prepared by diluting a PEDS sol concentrate with ethanol and HMDSO in a stirred reactor and preheated to 45° C. Then dilute ethanolic ammonia solution was added and the sol thus activated was transferred via a transfer line with pressure equalization to the tube bundle reactor preheated to 60° C. bith both lids closed. Following transfer of the sol, top and bottom valves to the reactor were also closed, as a result of which the heat exchanger tubes, together with the gel rods that formed, were isolated inside a tightly sealed reactor.

[0092] The temperature of the reactor was then raised to 112° C. by heating the heat exchanger medium. The pressure rose rapidly to a value of 2.5 bar. After an aging time of 60 minutes, the bottom and top valves were carefully opened and the syneresis fluid was released into the release vessel. The heating of the tube bundle heat exchanger was now reduced to 102° C. Now 18.51 of a dilute sulfuric acid solution in ethanol which had been mixed and preheated to 60° C. inside the stirred batch reactor was pumped into the reactor and circulated at a constant flow of 4 l/min. The hydrophobization catalyst was then pumped for an additional 75 minutes at the same temperature.

[0093] Once the hydrophobization was complete, the reactor was quickly cooled to a temperature of 73° C. and vented to ambient pressure. The reactor bottom lid was opened and the resulting hydrophobized gel bars were separated from the liquid phase by the phase separation unit and transferred to the dryer unit, where the gels were dried under a stream of nitrogen at 160° C. to constant weight. After the drying unit, the aerogel granulate was sent through a heated tunnel with a residence time of 17 minutes containing an atmosphere of 1.1 vol % of ammonia and 350 mbar of water vapour. At the end of this tunnel, the final aerogel material was recuperated through a load lock system.

[0094] The analysis of the material showed an envelope density of 0.108 g/cm.sup.3 and a thermal conductivity of the bed of 17.9 mW/(m K). Other chemical data of the aerogel are summarized in Table 1.

TABLE-US-00001 TABLE 1 Physico-chemical properties of the prepared aerogels. Envelope density N.sub.SiOSiMe3 N.sub.SiOR N.sub.SiOH N.sub.SiOSiMe3/(N.sub.SiOSiMe3 + N.sub.SiOR/(N.sub.SiOR + Example [g/cm.sup.3] [mmol/g] [mmol/g] [mmol/g] N.sub.SiOR + N.sub.SiOH) N.sub.SiOH) Comparative 0.115 2.8 0.55 0.95 0.65 0.37 Example 1 Example 1 0.118 2.8 0.42 1.20 0.65 0.26 Example 2 0.122 2.8 0.48 1.25 0.62 0.28 Example 3 0.128 2.6 0.35 1.42 0.59 0.20 Example 4 0.108 2.7 0.31 1.52 0.60 0.17

[0095] The envelope density was measured by powder pycnometry using a GeoPyc 1360 device (Micromeritics).

[0096] The amount of the trimethylsiloxyl (≡SiOSiMe.sub.3) and alkoxysilyl (≡SiOR) groups is determined by .sup.1H-NMR and the amount of the silanol (≡SiOH) groups is determined by .sup.29Si-NMR, as described in W. J. Malfait et al. describe in Chem. Mater. 2015, 27, 6737-6745.