PROCESS FOR SYNTHESISING A "ONE POT" HYDROPHOBIC SILICA AEROGEL FROM A SILICA PRECURSOR

Abstract

Disclosed is a simple, inexpensive method for preparing a hydrophobic silica aerogel by using a sodium silicate solution as a silica precursor.

Claims

1. A method for preparing a hydrophobic silica aerogel by ‘one-pot’ synthesis using an aqueous silica precursor, comprising the following steps: i) mixing the silica precursor, and one or more alcohols and adjusting the mixture to a pH between 0 and 5; ii) adding an organosilane; iii) wet hydrophobic gelling in the presence of a basic solution as a catalyst; and iv) drying, in order to obtain the hydrophobic silica aerogel.

2. The method according to claim 1, such that the aqueous silica precursor is selected from sodium silicate solutions, colloidal silica solutions, silica solutions extracted from a silica-rich source, waste from silica-based insulation material, glass, and mixtures thereof.

3. The method according to claim 1, such that the aqueous precursor solution contains between 4 and 8 wt % SiO.sub.2.

4. The method according to claim 1, such that the alcohol is selected from the group consisting of ethanol, methanol, isopropyl alcohol, and mixtures thereof.

5. The method according to claim 1, such that the pH adjustment is carried out by adding an inorganic acid selected from hydrochloric acid, nitric acid, sulphurous acid, and oxalic acid, and mixtures thereof.

6. The method according to claim 5, such that the concentration of the inorganic acid is between 0.1 and 0.2 mo1.1.sup.-1.

7. The method according to claim 1, such that the organosilane is selected from compounds of formula (I): ##STR00002## wherein each of the R.sub.1 — R.sub.4 groups is identical or different and independently selected from linear or branched C1-C12 alkyl groups and linear or branched C2-C12 alkenyl groups.

8. The method according to claim 1, such that the organosilane is selected from methyltrimethoxysilane, methyltriethoxysilane, vinyltrimethoxysilane, isobutyltriethoxysilane, and isobutyltrimethoxysilane.

9. The method according to claim 1, such that the organosilane is included in the amount of 1 - 50 wt % of the wet gel.

10. The method according to claim 1, such that the synthesis and gelling are carried out at a controlled temperature and pressure between 15 and 30° C. and 1 and 200 bar, respectively.

11. The method according to claim 1, such that the basic solution is an ammonia solution having a concentration between 0.1 and 2 mol.l.sup.-1.

12. The method according to claim 1, such that ripening and/or washing is carried out prior to contacting the wet gel with a supercritical fluid.

13. The method according to claim 12, such that the washing is carried out with ethanol at a temperature and pressure between 20 and 50° C. and 1 and 200 bar, respectively.

14. The method according to claim 1, such that the wet gel is prepared in the form of a monolith, granules, or composite with organic or inorganic fibres.

15. The method according to claim 1, such that the drying is carried out at ambient pressure or in one or more fluids in supercritical conditions.

16. The method of claim 1, wherein prior to mixing the silica precursor, the silica precursor is pretreated by passing an aqueous solution of the silica precursor containing between 4 and 31 wt % SiO.sub.2 through an ion exchanger resin.

17. The method according to claim 2, such that the alcohol is selected from the group consisting of ethanol, methanol, isopropyl alcohol, and mixtures thereof.

18. The method according to claim 3, such that the alcohol is selected from the group consisting of ethanol, methanol, isopropyl alcohol, and mixtures thereof.

19. The method according to claim 2, such that the pH adjustment is carried out by adding an inorganic acid selected from hydrochloric acid, nitric acid, sulphurous acid, and oxalic acid, and mixtures thereof.

20. The method according to claim 3, such that the pH adjustment is carried out by adding an inorganic acid selected from hydrochloric acid, nitric acid, sulphurous acid, and oxalic acid, and mixtures thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] FIG. 1 is a schematic representation of the method for preparing a hydrophobic silica aerogel according to one embodiment of the invention.

[0048] FIG. 2 is an SEM micrograph of a hydrophobic silica aerogel prepared according to one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] The following examples are provided to illustrate this invention, without limiting its scope in any way.

Example 1: Synthesis of a Silica Aerogel

[0050] 30 ml of a sodium silicate solution (containing app. 27 wt % SiO.sub.2) is diluted in 143 ml deionised water in order to obtain a sodium silicate solution containing app. 6 wt % SiO.sub.2. Then, the sodium silicate solution is passed through an ion exchanger resin (Amberlite IR-120 H+) in order to eliminate the Na+ ions and obtain silicic acid. 130 ml silicic acid is mixed with 52 ml ethanol, and 1 ml hydrochloric acid (1 N) is then added. 18 ml of silylating agent (isobutyltriethoxysilane) is added and mixed. Following 1 h of stirring, 5 ml of an ammonia solution (1 N) is added, and gelling is carried out over 10 min. Following ripening and washing, the silica hydrogel is dried by LTSCD.

Example 2: Properties

[0051] The hydrophobic silica aerogel obtained in Example 1 has the following characteristics: [0052] Bulk density: 100 kg/m.sup.3 [0053] Contact angle: 130° [0054] Thermal conductivity: 0.0157 W/m.K

[0055] Bulk density is defined by the ratio between mass and the volume of the geometrical envelope.

[0056] Measuring contact angles consists of measuring the angle formed by a drop of water at its point of contact with the surface of a solid (the sample) and the gaseous phase (here, the atmosphere). The device used to measure contact angles is a Digidrop goniometer.

[0057] Flow measurement is used to measure thermal conductivity. Two plates on either side of the sample may be heated or cooled, allowing for precise determination of the temperature difference between the hot plate and the cold plate. A data acquisition system allows for the development of flows and temperatures to be monitored and the thermal conductivity to be determined.

[0058] Microscopic observation was carried out by means of a scanning electronic microscope (Philips, XL30).

[0059] The nanostructure of the hydrophobic silica aerogels obtained is shown in FIG. 2.