Preparation method of high-strength and large-size silica aerogel

Abstract

The application provides a preparation method of high-strength and large-size silica aerogel, relates to the field of aerogel material preparation, and comprises the following steps: S1, wet gel preparation; S2, aging; S3, surface modification; and S4, drying; wherein, the step S2 is twice aging of liquid aging and anhydrous ethanol aging which are sequentially performed; and the step S3 is modification by using a modification liquid of an ethanol solution of a modifier with a volume fraction of 8-12%. The method can effectively solve the problems of low strength, difficulty in forming, non-uniform microstructure, easy moisture absorption and the like of the silica aerogel, so that the aerogel structure with stable microstructure, good uniformity, low thermal conductivity, excellent heat insulation and heat preservation performance is obtained.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of aerogel material preparation, in particular to a preparation method of high-strength and large-size silicon dioxide aerogel. BACKGROUND

[0002] The silicon dioxide aerogel is a light nanometer porous material, which has the characteristics of small density, high porosity (the porosity is 80%-99.8%), large specific surface area, low thermal conductivity (the thermal conductivity is 0.013 W / (m*K)), and the like. In recent years, due to the excellent heat insulation performance of the silicon dioxide aerogel, and the non-toxicity, harmlessness and excellent environmental protection performance, the silicon dioxide aerogel is widely applied to the fields of heat insulation, high-temperature resistance and fire prevention, and has a good application prospect in the field of heat insulation materials.

[0003] However, the silicon dioxide aerogel has low strength and poor toughness, so that the prepared silicon dioxide aerogel cannot be directly applied to a heat insulation layer, and the application field of the silicon dioxide aerogel is seriously restricted. Meanwhile, the gel skeleton structure with low strength will have problems such as shrinkage and collapse in the drying process, and further causes the heat insulation performance of the aerogel structure to be reduced. In addition, due to the problems such as shrinkage and collapse, the microstructure of the silicon dioxide aerogel is non-uniform, and further causes the heat insulation effect to be non-uniform when the aerogel structure is used for fire prevention and heat insulation, and a local temperature collection point is generated, which further affects the overall heat insulation performance of the aerogel structure. In addition, the silicon dioxide aerogel also has the problem that the skeleton structure is expanded and broken when absorbing water, which also causes the heat insulation performance of the aerogel structure to be reduced. SUMMARY

[0004] In view of the problems in the prior art, the purpose of the present application is to provide a preparation method of high-strength and large-size silicon dioxide aerogel, which can effectively solve the problems of low strength, poor forming, non-uniform microstructure and easy moisture absorption of the silicon dioxide aerogel, so as to obtain an aerogel structure with stable microstructure (i.e. not easy to have problems such as shrinkage, collapse and skeleton structure rupture), good uniformity, low thermal conductivity and excellent heat insulation performance.

[0005] The purpose of the present application is achieved by the following technical scheme:

[0006] A preparation method of high-strength and large-size silicon dioxide aerogel, comprising:

[0007] Step S1, wet gel preparation: uniformly mixing tetraethyl orthosilicate, anhydrous ethanol, deionized water and an acid solution, then performing hydrolysis, adding ammonia water after hydrolysis and stirring, and then standing for a period of time to obtain a wet gel;

[0008] Step S2, aging: the wet gel is added into the aging solution for primary aging at room temperature, and then is added into the ethanol solution for secondary aging in a water bath environment;

[0009] Step S3, surface modification: the aged gel is added into the modification solution for more than twice modification, and then is replaced by the ethanol solution after the modification is completed;

[0010] Step S4, drying: the modified gel solution is dried by the supercritical drying to obtain the silica aerogel block.

[0011] Based on the further optimization of the above scheme, the volume ratio of the tetraethyl orthosilicate, the anhydrous ethanol, the deionized water and the acid solution is 4.5-6.5:13-17:0.8-1.2:0.3-1; the acid solution is any one of nitric acid, hydrochloric acid or phosphoric acid.

[0012] Based on the further optimization of the above scheme, the hydrolysis step in the step S1 is to be placed in a water bath with a temperature of 45-65℃ for 8-16h.

[0013] Based on the further optimization of the above scheme, the volume ratio of the ammonia water and the acid solution is 1:0.3-1, and the stirring time after the ammonia water is added is 3-5min.

[0014] Based on the further optimization of the above scheme, the standing time in the step S1 is 5-30min.

[0015] Based on the further optimization of the above scheme, the aging solution for the primary aging includes the tetraethyl orthosilicate, the anhydrous ethanol and the ammonia water, and their volume percentages are 12%-16%:82%-88%:0.8%-1.2%; the time for the primary aging is 5-10h.

[0016] Based on the further optimization of the above scheme, the temperature of the water bath environment for the secondary aging is 40-60℃, the aging time is 48-72h, and the anhydrous ethanol is replaced every 12-16h.

[0017] Through the aging step of first aging in the aging solution and then aging in the anhydrous ethanol, firstly, the polymers in the aging solution enter the network structure of the gel and continuously react with the polymers in the gel, so that the gel structure is uniform and the gel strength is improved; secondly, the water in the gel is fully replaced by the sealing effect of the anhydrous ethanol on the water, so that the capillary force in the network structure is reduced, the gel network skeleton is not thick, and problems such as shrinkage hole and collapse of the gel in the drying process are avoided.

[0018] Based on further optimization of the above scheme, the modification liquid is an ethanol solution with a volume fraction of 8% to 12% of the modifier; wherein the modifier is composed of trimethylethoxysilane, dimethyldiethoxysilane and methyltriethoxysilane, and the volume ratio among them is 0.7 to 1.3: 1.8 to 2.2: 6.5 to 7.5; the modification time of the modification liquid is 22 to 26 hours each time.

[0019] The application modifies the aged wet gel by using a modifier composed of trimethylethoxysilane, dimethyldiethoxysilane and methyltriethoxysilane. Firstly, the hydrophobic group Si-CH3 reacts with the silicon hydroxyl Si-OH in the gel, thereby being grafted to the surface of the gel to present hydrophobicity, avoiding the skeleton cracking caused by the moisture absorption of the gel; secondly, the modifier does not produce excess by-products in the reaction process, which is green and environmentally friendly; thirdly, the grafted oxysilane enhances the stability of the gel surface; fourthly, the modifier with multiple groups uses multiple hydrophobic groups to enhance the skeleton strength of the gel and improve the overall strength of the gel, and uses single hydrophobic groups to increase the chain length of the gel and restrict the shrinkage hole problem caused by the continuous reaction of multiple hydrophobic groups, thereby ensuring the uniformity of the porosity and microstructure of the finally prepared silica aerogel.

[0020] Based on further optimization of the above scheme, the supercritical drying step is specifically:

[0021] Firstly, the modified gel solution is slowly put into the drying kettle, and the drying kettle is sealed; then, the drying kettle is heated to keep the temperature at 40 to 60 DEG C, and then the drying kettle is pressurized by pumping CO2 into the drying kettle to make the pressure of the drying kettle rise to 10 to 15 MPa, realizing the extraction of anhydrous ethanol, and the extracted anhydrous ethanol is discharged from the separation kettle; the drying time is 4 to 12 hours, and after drying, the pressure relief valve is opened to release the pressure, and the pressure relief rate is 0.2 to 2.0 MPa / h.

[0022] The following are the effects of the technical scheme of the application:

[0023] The present application prepares silica aerogel with high specific surface area, porosity, large size, high strength, green and environmental protection through hydrolysis and polycondensation reaction of tetraethyl orthosilicate, twice aging (i.e. one aging of aging liquid and twice aging of anhydrous ethanol) and modification of modification liquid and supercritical drying. Through twice aging, not only the tetraethyl orthosilicate is fully hydrolyzed and polycondensed, but also the uniformity of the gel is improved, and the water in the gel is effectively replaced, and the capillary force in the skeleton structure is reduced. Then, the specific proportion of the modifier is added, which not only enhances the skeleton structure of the gel, but also solves the water absorption of the gel surface, and further improves the surface stability and internal porosity of the gel, thereby effectively improving the fireproof and heat insulation performance of the aerogel structure. The overall process of the present application is simple and feasible, the materials used are common and easy to obtain, and the preparation cost is low, which is suitable for industrialized and large-scale production. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 It is a sample physical map of the silica aerogel prepared in the embodiment of the present application.

[0025] Figure 2 It is a microstructure diagram of the silica aerogel prepared in the embodiment of the present application.

[0026] Figure 3 It is a schematic diagram of the device structure of supercritical drying in the embodiment of the present application.

[0027] Figure 4 It is the N2 adsorption and desorption curve of the silica aerogel prepared in the embodiment of the present application.

[0028] Figure 5 It is the pore size distribution diagram of the silica aerogel prepared in the embodiment of the present application.

[0029] Figure 6 It is the microstructure and physical map of the silica aerogel prepared in the comparative example 3 of the present application; wherein, Figure 6 (a) is the microstructure, Figure 6 (b) is the physical map.

[0030] Wherein, 10 is a drying kettle; 20 is a separation kettle; 30 is a CO2 gas cylinder. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical scheme and advantages of the embodiments of the present application more clear, the technical scheme of the embodiments of the present application will be clearly described below. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments.

[0032] Example 1:

[0033] A preparation method of high-strength and large-size silica aerogel, comprising:

[0034] Step S1, wet gel preparation: first, uniformly mix tetraethyl orthosilicate, anhydrous ethanol, deionized water and acid solution, wherein the volume ratio of tetraethyl orthosilicate, anhydrous ethanol, deionized water and acid solution is 4.5:13:0.8:0.3, and the acid solution uses nitric acid; then, put it into a water bath kettle with a temperature of 45℃ for hydrolysis for 16h; after hydrolysis, add ammonia water and stir for 3min (the stirring rate is set according to the actual situation, as long as the ammonia water and the mixed solvent are fully contacted), the volume ratio of ammonia water and acid solution is 1:0.3; then stand for 10min to obtain a wet gel.

[0035] Step S2, aging: the wet gel is added into the aging liquid for primary aging at room temperature (the room temperature in this embodiment is 24-26℃), and the aging liquid comprises tetraethyl orthosilicate, anhydrous ethanol and ammonia water, and their volume percentages are 12%:87.2%:0.8%; the time of primary aging is 10h.

[0036] Then, secondary aging of anhydrous ethanol is carried out in a water bath environment, the water bath environment temperature is 40℃, the aging time is 64h, and the anhydrous ethanol is replaced every 16h (i.e. the anhydrous ethanol is replaced three times).

[0037] Step S3, surface modification: the aged gel is added into the modification liquid for two times of modification, and the modification liquid is an ethanol solution with a volume fraction of 8% modifier; wherein the modifier is composed of trimethylethoxysilane, dimethyldiethoxysilane and methyltriethoxysilane, and their volume ratio is 0.7:1.8:6.5; the modification liquid is replaced every 26h (i.e. the modification liquid is replaced every 26h).

[0038] After modification, anhydrous ethanol solution is used for replacement.

[0039] Step S4, drying: the modified gel solution is dried by supercritical drying (the supercritical drying device is shown in Figure 3 , and specifically:

[0040] Firstly, the modified gel solution is slowly put into the drying kettle, and the drying kettle is sealed; then, the drying kettle is heated to keep the temperature at 40℃, and then CO2 is pumped into the drying kettle to realize pressurization by (CO2 cylinder and opening V4 valve), so that the pressure of the drying kettle rises to 10MPa, realizing the extraction of anhydrous ethanol, and the extracted anhydrous ethanol is discharged from the separation kettle; the drying time is 12h, after drying, the pressure relief valve (i.e. V5 valve shown in Figure 3 ) is opened for pressure relief, and the pressure relief rate is 1.0MPa / h.

[0041] Finally, a silica aerogel block is obtained.

[0042] Example 2

[0043] A method for preparing a high-strength and large-size silica aerogel, comprising:

[0044] Step S1, wet gel preparation: first, uniformly mix tetraethyl orthosilicate, anhydrous ethanol, deionized water, and an acid solution, wherein the volume ratio of tetraethyl orthosilicate, anhydrous ethanol, deionized water, and the acid solution is 5.5:15:1:0.6, and the acid solution is hydrochloric acid; then, place it in a water bath with a temperature of 55°C for hydrolysis for 12 hours; after hydrolysis, add ammonia water and stir for 4 minutes (the stirring rate is set according to the actual situation, as long as the ammonia water and the mixed solvent are fully contacted), and the volume ratio of ammonia water to acid solution is 1:0.6; then, stand for 20 minutes to obtain a wet gel.

[0045] Step S2, aging: add the wet gel to an aging liquid for primary aging at room temperature (the room temperature in this embodiment is 24-26°C), and the aging liquid comprises tetraethyl orthosilicate, anhydrous ethanol, and ammonia water, and their volume percentages are 14%, 85%, and 1% respectively; the primary aging time is 7 hours.

[0046] Then, perform secondary aging of anhydrous ethanol in a water bath environment, the water bath environment temperature is 50°C, the aging time is 56 hours, and the anhydrous ethanol is replaced every 14 hours (i.e., the anhydrous ethanol is replaced three times).

[0047] Step S3, surface modification: add the aged gel to a modification liquid for three times of modification, and the modification liquid is an ethanol solution with a volume fraction of 10% of a modifier; wherein the modifier is composed of trimethylethoxysilane, dimethyldiethoxysilane, and methyltriethoxysilane, and their volume ratio is 1:2:7; the modification liquid is replaced every 24 hours (i.e., the modification liquid is replaced every 24 hours).

[0048] After the modification is completed, replace it with an anhydrous ethanol solution.

[0049] Step S4, drying: dry the modified gel solution by using supercritical drying (a supercritical drying device is shown in FIG. 1), specifically as follows: Figure 3

[0050] ​Firstly, the modified gel solution is slowly put into the drying kettle, and the drying kettle is sealed; then, the drying kettle is heated and kept at 50℃, and CO2 is pumped into the drying kettle to pressurize the drying kettle to 12MPa by opening the V4 valve, so as to realize the extraction of anhydrous ethanol, and the extracted anhydrous ethanol is discharged from the separation kettle; the drying time is 8h, and after drying, the pressure relief valve (i.e. Figure 3 the V5 valve shown in the figure) is opened to release pressure at a rate of 1.5MPa / h.

[0051] Finally, a silica aerogel block is obtained.

[0052] Example 3:

[0053] A preparation method of a high-strength and large-size silica aerogel, comprising:

[0054] Step S1, wet gel preparation: first, tetraethyl orthosilicate, anhydrous ethanol, deionized water and acid solution are uniformly mixed, wherein the volume ratio of tetraethyl orthosilicate, anhydrous ethanol, deionized water and acid solution is 6.5:17:1.2:1, and the acid solution is phosphoric acid; then, put into a water bath kettle with a temperature of 65℃ for hydrolysis for 8h; after hydrolysis, add ammonia water and stir for 5min (the stirring rate is set according to the actual situation, only need to meet the full contact of ammonia water and mixed solvent), the volume ratio of ammonia water and acid solution is 1:1; then, stand for 30min to obtain a wet gel.

[0055] Step S2, aging: the wet gel is added into the aging liquid at room temperature (the room temperature in this embodiment is 24-26℃) for primary aging, the aging liquid includes tetraethyl orthosilicate, anhydrous ethanol and ammonia water, and their volume percentages are: 16%:82.8%:1.2%; the time of primary aging is 5h.

[0056] Then, secondary aging of anhydrous ethanol is carried out in a water bath environment, the water bath environment temperature is 60℃, the aging time is 48h, and the anhydrous ethanol is replaced every 12h (i.e. the anhydrous ethanol is replaced three times).

[0057] Step S3, surface modification: the aged gel is added into the modification liquid for twice modification, and the modification liquid is an ethanol solution with a volume fraction of 12% modification agent; wherein the modification agent is composed of trimethylethoxysilane, dimethyldiethoxysilane and methyltriethoxysilane, and their volume ratio is 1.3:2.2:7.5; the modification liquid is replaced every 22h (i.e. the modification liquid is replaced every 22h).

[0058] After modification, anhydrous ethanol solution is used for replacement.

[0059] Step S4, drying: the modified gel solution is dried by supercritical drying (a supercritical drying device is shown in Figure 3 , specifically as follows.

[0060] First, the modified gel solution is slowly placed into the drying kettle, and the drying kettle is sealed. Then, the drying kettle is heated to keep the temperature at 60℃, and CO2 is pumped into the drying kettle to pressurize the drying kettle to 15 MPa by a CO2 cylinder and opening the V4 valve, so as to realize the extraction of anhydrous ethanol, and the extracted anhydrous ethanol is discharged from the separation kettle. The drying time is 4 h, and after drying is completed, the pressure relief valve (i.e. the V5 valve shown in Figure 3 ) is opened to release pressure at a rate of 2.0 MPa / h.

[0061] Finally, a silica aerogel block is obtained.

[0062] Comparative Example 1

[0063] A preparation method of a silica aerogel, comprising:

[0064] Step S1, preparation of a wet gel: consistent with step S1 in Example 2.

[0065] Step S2, aging: the wet gel is aged in an anhydrous ethanol water bath environment at a temperature of 50℃ for 56 h, and the anhydrous ethanol is replaced every 14 h (i.e. three times).

[0066] Step S3, surface modification: consistent with step S3 in Example 2.

[0067] Step S4, drying: consistent with step S4 in Example 2.

[0068] Comparative Example 2

[0069] A preparation method of a silica aerogel, comprising:

[0070] Step S1, preparation of a wet gel: consistent with step S1 in Example 2.

[0071] Step S2, aging: consistent with step S2 in Example 2.

[0072] Step S3, surface modification: the aged gel is added to a modification liquid for three times of modification, and the modification liquid is an ethanol solution with a volume fraction of 10% of a modifier; wherein the modifier is trimethylethoxysilane; and the modification liquid is replaced every 24 h for each modification (i.e. every 24 h).

[0073] After modification, the anhydrous ethanol solution is replaced.

[0074] Step S4, drying: consistent with step S4 in Example 2.

[0075] Comparative Example 3:

[0076] A preparation method of a silica aerogel, comprising:

[0077] Step S1, wet gel preparation: consistent with step S1 in Example 2.

[0078] Step S2, aging: consistent with step S2 in Example 2.

[0079] Step S3, surface modification: the aged gel is added to a modification liquid, and modified three times, the modification liquid is an ethanol solution with a volume fraction of 10% modifier; wherein the modifier is composed of methyl triethoxysilane; the modification time of the modification liquid is 24h each time (i.e. the modification liquid is replaced every 24h).

[0080] After the modification is completed, anhydrous ethanol solution is used for replacement.

[0081] Step S4, drying: consistent with step S4 in Example 2.

[0082] Comparative Example 4:

[0083] A preparation method of a silica aerogel, comprising:

[0084] Step S1, wet gel preparation: consistent with step S1 in Example 2.

[0085] Step S2, aging: consistent with step S2 in Example 2.

[0086] Step S3, surface modification: the aged gel is added to a modification liquid, and modified three times, the modification liquid is an ethanol solution with a volume fraction of 10% modifier; wherein the modifier is composed of trimethylethoxysilane and dimethyldiethoxysilane, and the volume ratio between them is 1:2; the modification time of the modification liquid is 24h each time (i.e. the modification liquid is replaced every 24h).

[0087] After the modification is completed, anhydrous ethanol solution is used for replacement.

[0088] Step S4, drying: consistent with step S4 in Example 2.

[0089] The aerogel samples prepared in Examples 1-3 and Comparative Examples 1-4 are tested by N2 adsorption-desorption under the same conditions (i.e. the same environmental temperature, humidity, etc.) by using a full-automatic specific surface area analyzer, and the corresponding pore parameters are calculated according to the BET model, and the pore rate calculation formula is:

[0090] ;

[0091] wherein: ρ b ρ s apparent density, skeleton density, respectively, ρ s 2.2 g / cm 3 ;

[0092] Meanwhile, the thermal conductivity of the aerogel samples prepared in Examples 1-3 and Comparative Examples 1-4 at 25°C was tested according to the standard GB / T 10294-2008 "Determination of Steady-State Thermal Resistance and Related Properties of Thermal Insulation - Guarded Hot Plate Method", and the test results are as follows:

[0093]

[0094] In addition, the N2adsorption-desorption curve and pore size distribution of the silica aerogel prepared in Example 2 of the present application are shown in Figure 4 , Figure 5 The N2adsorption-desorption curve and pore size distribution of the silica aerogels prepared in Examples 1 and 3 are consistent with Example 2, and thus the average pore size of the silica aerogel in the present application is 13.23 nm.

[0095] In addition, silica is difficult to form due to its high porosity, and is usually in the form of a broken block (see Figure 6 (b)), and the actual photographs of the silica aerogels prepared in Examples 1-3 are shown in Figure 1 , which are a whole piece of complete structure (i.e., without collapse, shrinkage and other problems during supercritical drying); the actual photographs of the silica aerogels prepared in Comparative Examples 1, 2 and 4 are substantially the same as those in the present application; and the actual photograph of the silica aerogel prepared in Comparative Example 3 is shown in Figure 6 (b), which is in the form of a broken block, i.e., the overall dispersion is caused by collapse and shrinkage. Therefore, it is proved that the silica aerogel prepared by the method of the present application not only has a larger specific surface area and higher porosity, but also has higher strength to avoid problems such as collapse and shrinkage during drying.

Claims

1. A method for preparing high-strength, large-size silica aerogel, characterized in that: include: Step S1, wet gel preparation: Tetraethyl orthosilicate, anhydrous ethanol, deionized water and acid solution are mixed evenly and then hydrolyzed. After hydrolysis, ammonia water is added and stirred. After standing for a period of time, wet gel is obtained. Step S2, Aging: The wet gel is added to the aging solution at room temperature for the first aging, and then subjected to a second aging in anhydrous ethanol in a water bath. The aging solution for the first aging includes tetraethyl orthosilicate, anhydrous ethanol and ammonia, with a volume percentage of 12%–16%: 82%–88%: 0.8%–1.2%. The aging time for the first aging is 5–10 hours. Step S3, Surface Modification: The aged gel is added to the modification solution and modified more than twice. After modification, it is replaced with anhydrous ethanol solution. The modification solution is an ethanol solution with a volume fraction of 8% to 12% modifier. The modifier is composed of trimethylethoxysilane, dimethyldiethoxysilane and methyltriethoxysilane, and their volume ratio is 0.7 to 1.3: 1.8 to 2.2: 6.5 to 7.

5. The modification time for each modification is 22 to 26 hours. Step S4, Drying: The modified gel solution is dried using supercritical drying to obtain silica aerogel blocks.

2. The method for preparing a high-strength, large-size silica aerogel according to claim 1, characterized in that: The volume ratio of tetraethyl orthosilicate, anhydrous ethanol, deionized water and acid solution is 4.5–6.5:13–17:0.8–1.2:0.3–1; the acid solution is any one of nitric acid, hydrochloric acid or phosphoric acid.

3. The method for preparing a high-strength, large-size silica aerogel according to claim 1 or 2, characterized in that: The hydrolysis step in step S1 is as follows: hydrolyze in a water bath at a temperature of 45-65°C for 8-16 hours.

4. The method for preparing a high-strength, large-size silica aerogel according to claim 3, characterized in that: The volume ratio of ammonia to acid solution is 1:0.3-1, and the stirring time after adding ammonia is 3-5 minutes.

5. The method for preparing a high-strength, large-size silica aerogel according to claim 3, characterized in that: The settling time in step S1 is 5 to 30 minutes.

6. The method for preparing a high-strength, large-size silica aerogel according to claim 1, characterized in that: The secondary aging process involves a water bath temperature of 40–60°C and an aging time of 48–72 hours, with anhydrous ethanol being replaced every 12–16 hours.

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