Method for producing silica aerogel and silica aerogel produced thereby

Abstract

The present invention relates to a method for producing a silica aerogel and a silica aerogel produced thereby. The present invention provides a two-step process of a nucleation step of forming silica colloid particles by using a low-concentration silica precursor and a growth step of further adding a relatively high-concentration silica precursor to form a silica wet gel by using the silica colloid particles as a seed. Thus, the present invention provides a method for producing a silica aerogel of which mechanical stability is improved to enhance pore characteristics, and physical properties are readily controllable, and also provides a silica aerogel produced thereby.

Claims

1. A method for producing a silica aerogel, comprising: 1) adding an acid catalyst and a first water glass solution to a reactor to prepare a water glass dispersion solution for forming silica colloid particles; 2) adding a surface modifier solution to the water glass dispersion solution for forming silica colloid particles, to form the silica colloid particles; 3) mixing an acid catalyst with a second water glass solution to prepare a water glass dispersion solution for forming a silica wet gel; 4) adding the water glass dispersion solution for forming the silica wet gel to the reactor including the silica colloid particles to form the silica wet gel; and 5) drying the silica wet gel; wherein the first water glass solution and the second water glass solution contain a silicon dioxide.

2. The method of claim 1, wherein a concentration of the silicon dioxide in the second water glass solution is equal to or greater than a concentration of the silicon dioxide in the first water glass solution.

3. The method of claim 1, wherein a concentration of the silicon dioxide in the first water glass solution is 0.01 to 2 wt %.

4. The method of claim 1, wherein a concentration of the silicon dioxide in the second water glass solution is 1 to 11 wt %.

5. The method of claim 1, wherein a concentration ratio of the silicon dioxide in the first water glass solution to the silicon dioxide in the second water glass solution is 1:1 to 1:1100.

6. The method of claim 1, wherein the acid catalyst in step 1) and the acid catalyst in step 3) are selected from the group consisting of nitric acid, hydrochloric acid, sulfuric acid and hydrofluoric acid.

7. The method of claim 1, wherein the surface modifier solution is a solution in which a surface modifier is added to a nonpolar organic solvent.

8. The method of claim 7, wherein the surface modifier is at least one selected from the group consisting of trimethylchlorosilane (TMCS), hexamethyldisilazane (HMDS), methyltrimethoxysilane, trimethylethoxysilane, ethyltriethoxysilane, and phenyltriethoxysilane.

9. The method of claim 7, wherein the nonpolar organic solvent is at least one selected from the group consisting of hexane, heptane, toluene, and xylene.

10. The method of claim 1, wherein the surface modifier solution is added in an amount that a molar ratio of a surface modifier to a silicon dioxide in the first water glass solution becomes 0.05 to 20.

11. The method of claim 1, wherein gelation, solvent exchange, and surface modification are simultaneously performed in step 4).

12. The method of claim 1, wherein the silica wet gel is grown by using the silica colloid particles as a seed.

13. The method of claim 1 further comprising, during step 4), a step of adding an ammonium hydroxide (NH.sub.4OH).

14. The method of claim 13, wherein the ammonium hydroxide is added in an amount that a molar ratio of the ammonium hydroxide to the silicon dioxide in the second water glass solution becomes 0.5 to 25.

15. A silica aerogel produced by the method of claim 1.

Description

EXAMPLE 1

(1) 1.5 g of a nitric acid was previously prepared in a beaker, and a 0.5 wt % of a first water glass solution (1.7 g of water glass) is added in a volume of 100 ml to prepare a water glass dispersion solution for forming silica colloid particles. The dispersion solution was then added to a reactor at 55° C. and stirred to maintain the temperature.

(2) Thereafter, a surface modifier solution, which was obtained by adding 10 g of hexamethyldisilazane (HMDS) to 200 ml of n-hexane and stirring, was added into the reactor and reacted to produce silica colloid particles.

(3) 6.5 g of a nitric acid was previously prepared in another beaker, and a 4 wt % of a second water glass solution (12.4 g of water glass) is added in a volume of 100 ml to prepare a water glass dispersion solution for forming a silica wet gel.

(4) The water glass dispersion solution for forming a wet gel is added to the reactor including the silica colloid particles and causing a reaction to grow the silica wet gel.

(5) After initiation of the reaction, the silica wet gel in an aqueous solution layer was surface-modified and floated onto the top of the organic solvent layer of n-hexane. Then, in order to adjust the degree of surface modification, 3 ml of ammonium hydroxide was added at 30 minutes after the addition of the water glass solution for forming a wet gel surface modifier solution. When the surface modification was completed and a hydrophobic silica wet gel was completely floated onto the organic solvent layer of the n-hexane, 400 ml of n-hexane was further added, and then the aqueous solution layer remaining in the lower portion of the reactor was discharged through an outlet of the reactor. After 2 hours, the silica wet gel dispersed in an n-hexane layer was completely dried in a forced convection oven at 150° C. for 6 hours to produce a hydrophobic silica aerogel.

EXAMPLES 2 to 5

(6) Silica aerogels were produced in the same manner as in Example 1, except that first and second water glass solutions, and a nitric acid were used in respective amounts described in Table 1 below.

COMPARATIVE EXAMPLE 1

(7) Silica aerogels were produced in the same manner as in Example 1, except that 8 g of a nitric acid, 4.5 wt % of a water glass solution (14.1 g of water glass), and a surface modifier solution were added at a time.

COMPARATIVE EXAMPLES 2 to 5

(8) Silica aerogels were produced in the same manner as in Example 1, except that first and second water glass solutions and a nitric acid were used in respective amounts described in Table 1 below.

EXPERIMENTAL EXAMPLE 1

Tap Density Measurement

(9) For comparative analysis of physical properties of hydrophobic silica aerogels produced in Examples 1 to 5 and

(10) Comparative Examples 1 to 5, the tap density (g/ml) and the specific surface area (BET, m.sup.2/g) of each aerogel were measured, and the results were shown in Table 1 below.

(11) 1) Tap Density (g/ml)

(12) Tap density was measured by using a tap density measuring instrument (STAV II, Engelsman AG). Specifically, each of the aerogels was placed into a standardized cylinder (25 ml) and weighted, then the cylinder was fixed to the tap density measuring instrument, a noise damping hood was closed, and 2500-times tapping was set. After the tapping measurement, the volume of each aerogel in the cylinder was measured, and a ratio of the previously measured weight to the above volume was calculate to measure the density.

(13) 2) Specific surface area (BET, m.sup.2/g)

(14) The specific surface area was analyzed by the adsorption/desorption amount of nitrogen according to partial pressure (0<p/p.sub.0<1) by using a 3FLEX apparatus (Micrometrics Company).

(15) Specifically, 100 mg of each aerogel was placed in a cylinder and pretreated at 200° C. for 8 hours, and then measured by using a specific surface area measuring apparatus.

(16) TABLE-US-00001 TABLE 1 Nucleation step Growth step Silicon Nitric Silicon Nitric Surface Tap dioxide acid dioxide acid modifier density BET (wt %) (g) (wt %) (g) (g) (g/ml) (m.sup.2/g) Comparative 4.5 8 — — 10 0.112 589 Example 1 Comparative 0.005 1.5 4.445 6.5 10 0.118 557 Example 2 Comparative 2.1 1.5 2.4 6.5 10 0.231 327 Example 3 Comparative 0.5 1.5 0.5 6.5 10 0.283 225 Example 4 Comparative 0.5 1.5 12 6.5 10 0.320 232 Example 5 Example 1 0.5 1.5 4 6.5 10 0.113 684 Example 2 0.1 1.5 4.4 6.5 10 0.107 653 Example 3 0.2 1.5 4.3 6.5 10 0.105 672 Example 4 1 1.5 3.5 6.5 10 0.115 621 Example 5 1.5 1.5 3 6.5 10 0.114 625

(17) As shown in Table 1, it may be ascertained that as compared with the hydrophobic silica aerogel of Comparative Example 1 in which the same amount of silica precursor as in Example was added, the hydrophobic silica aerogel of Examples to 5 produced by the production method according to an embodiment of the present invention exhibited a high specific surface area as a whole while maintaining the tap density in the same or similar level.

(18) In addition, it can be seen that even if the silica precursor was added in two steps as in the present invention, when the concentrations of the first and second water glass solutions fall outside the numerical range of the present invention as in Comparative Examples 2 to 4, the effect of improving the tap density and specific surface area is not good.

(19) This is because an excessively low concentration of the first water glass causes the silica colloid particles not to be formed properly or the number of formed particles to be too small to properly serve as a seed of the silica wet gel, and an excessively high concentration of the first water glass causes the silica wet gel to be immediately formed instead of forming the silica colloid particles serving as a seed, which is not different from a one-step adding process.

(20) In addition, this is because an excessively low concentration of the second water glass solution causes the silica precursor to be insufficient and the colloid particles to be used as a seed to hardly grow the silica wet gel, and an excessively high concentration of the second water glass solution causes the pore structure of the produced silica wet gel to reduce and the specific surface area to excessively lower.

(21) The above results show that the silica aerogel was produced by a two-step process of a nucleation step of forming silica colloid particles by using a low-concentration silica precursor in the specific range of the present invention and a growth step of forming a silica wet gel by using the silica colloid particles as a seed by adding a relatively high-concentration silica precursor, so that the mechanical stability was improved and the pore characteristics was improved.

(22) The foregoing description of the present invention has been presented for purposes of illustration. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.