Method for manufacturing silica aerogel

Abstract

The present invention relates to a method for manufacturing silica aerogel, which comprises: (1) manufacturing a hydrogel composite; (2) washing the manufactured hydrogel composite with a washing solvent; and (3) drying the washed hydrogel composite, wherein impurities are removed from the hydrogel composite in step (2), and the washing solvent is heated to a temperature above the boiling point (b.p.) of the washing solvent.

Description

Technical Field

[0001] Cross - reference to related applications

[0002] This application claims the priority and benefit of Korean Patent Application No. 10-2020-0092415, filed on Jul. 24, 2020, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0004] The present invention relates to a method for manufacturing a silica aerogel, and more particularly, to a method for manufacturing a silica aerogel capable of effectively washing an aerogel during the manufacturing process of the silica aerogel. Background Art

[0005] Aerogels are highly porous materials composed of nanoparticles, and thus have attracted attention as efficient thermal insulation materials, sound insulation materials, etc. because they have high porosity, high specific surface area, and low thermal conductivity. Since such aerogels have very low mechanical strength due to their porous structure, aerogel composites have been developed in which an aerogel is impregnated into a fiber mat formed of existing thermal insulation fibers (e.g., organic or inorganic fibers) so that the aerogel is bonded to the fiber mat.

[0006] As an example, a mat containing a silica aerogel using a silica aerogel is manufactured through a silica sol preparation step, a gelation step, an aging step, a surface modification step, and a drying step. In particular, in the prior art, a small amount of NH4OH is used in the aging step, and hexamethyldisilazane (HMDS) is used as a surface modifier. In this case, NH3 is generated as HMDS decomposes into trimethylsilanol (TMS) or trimethylethoxysilanol. NH4OH or NH3 reacts with carbon dioxide used as an extraction solvent in the supercritical drying process to form an ammonium carbonate salt. Then, as the temperature decreases, the ammonium carbonate salt precipitates to form a solid-phase powder, which causes problems such as fouling, clogging of pipes or valves, etc. in subsequent processes. By pre-removing the residual NH4OH and NH3 in the solvent during the washing process before the supercritical drying process, an additional removal process can be omitted, and the problem regarding the generation of ammonia odor can be solved.

[0007] The process of removing ammonia by washing is carried out by diffusing ammonia from the aerogel into the washing tank. In this case, methods such as increasing the temperature of the washing tank to increase the diffusion rate or decreasing the ammonia concentration in the washing tank to increase the ammonia concentration difference can be used. Methods such as increasing the amount of washing water used to decrease the ammonia concentration in the washing tank or discharging the washing water in the gas phase to increase the ammonia content in the discharge stream are used to remove ammonia in the washing tank with a small amount of washing water. However, because of using conventional methods, the temperature may not be raised above the boiling point of the washing solvent under normal pressure, so the increase in the diffusion rate caused by the temperature increase may be limited, and when the amount of washing water used is increased to decrease the ammonia concentration in the washing tank, it may lead to an increase in the energy for purifying the washing solvent.

[0008] When the washing process is carried out in a supercritical drying apparatus, in order to prevent an increase in the size of the apparatus, the washing time cannot be increased above a certain level. Therefore, a more effective washing method is needed to achieve the required level of ammonia removal rate.

[0009] Prior art documents

[0010] Patent documents

[0011] JP 2018-111803 A Summary of the Invention

[0012] Technical Problem

[0013] Therefore, an object of the present invention is to provide a method for manufacturing a silica aerogel, which can effectively remove impurities from the aerogel by increasing the diffusion rate of the washing solvent from the aerogel to the washing tank during the washing process.

[0014] Technical Solution

[0015] To achieve the above object, according to one aspect of the present invention, there is provided a method for manufacturing a silica aerogel, which includes: (1) manufacturing a hydrogel composite material; (2) washing the manufactured hydrogel composite material with a washing solvent; and (3) drying the washed hydrogel composite material, wherein, in step (2), impurities are removed from the hydrogel composite material, and the washing solvent is heated to a temperature above the boiling point (b.p.) of the washing solvent.

[0016] Advantageous Effects

[0017] In the method for manufacturing a silica aerogel of the present invention, the hydrogel composite material can be washed with a washing solvent heated to a temperature above the boiling point of the washing solvent under normal pressure to increase the diffusion rate of the washing solvent from the aerogel to the washing tank during washing, so that impurities can be effectively removed from the hydrogel composite material. Brief Description of the Drawings

[0018] Figure 1 These are photos of the filter and the housing in the supercritical dryer used in Example 1.

[0019] Figure 2 These are photos of the filter and the housing in the supercritical dryer used in Comparative Example 1.

[0020] Figure 3 These are photos of the filter and the housing in the supercritical dryer used in Comparative Example 3. Detailed Description of the Invention

[0021] Hereinafter, the present invention will be described in more detail to facilitate understanding of the present invention. In this case, the terms and words used in this specification and the appended claims should not be construed as having common and dictionary meanings, but may be interpreted as having meanings and concepts corresponding to the technical spirit of the present invention based on the principle that the inventor can appropriately define the terms and words in order to describe his invention in the best way.

[0022] A disadvantage of silica aerogel, which is widely used as a thermal insulation material in construction or industrial sites, is that when the surface of the silica aerogel is not hydrophobized, due to the hydrophilicity of the silanol group (Si-OH), the silica aerogel absorbs water in the air and gradually increases its thermal conductivity. In addition, a problem with silica aerogel is that it is difficult to predict the rebound phenomenon due to the accelerated collapse of pores during the drying process, which makes it difficult to manufacture a super-insulating product with micropores.

[0023] Therefore, the surface of the silica aerogel must be hydrophobically modified to inhibit the absorption of moisture in the air and thus maintain a low thermal conductivity. Generally, silica aerogel is manufactured through a silica sol preparation step, a gelation step, an aging step, a surface modification step, and a drying step.

[0024] In the silica solation step and / or the aging step, an alkali catalyst can be used, and the cation of the alkali catalyst used herein can react with carbon dioxide in a subsequent drying step to form a carbonate. In addition, the surface modifier used in the surface modification step forms ammonium ions (NH4 + ) during the process of hydrophobizing the surface of the silica aerogel, and the ammonium ions formed can also react with carbon dioxide in a subsequent drying step to form ammonium carbonate salts. The ammonium salts thus formed may clog the pipes in the drying system, and some ammonium salts can form carbonates in the final silica aerogel.

[0025] For example, when ammonium hydroxide is used as an alkali catalyst, the remaining ammonium ions, together with the ammonium ions formed by the decomposition of the surface modifier, react with carbon dioxide to form ammonium carbonate salts. When the produced silica aerogel is dried by supercritical drying, the ammonium ions can react with carbon dioxide in the supercritical drying step to form ammonium carbonate salts, which may block the pipelines in the supercritical drying system, and some ammonium carbonate salts can produce hydrophilic ammonium carbonate salts in the final silica aerogel, thereby enhancing the thermal conductivity of the final silica aerogel and causing the adsorption of moisture, which results in an increase in thermal conductivity and heat insulation performance. Moreover, the ammonia remaining in the supercritical waste liquid may produce an odor during the reuse of the solvent or the wastewater treatment process and may cause problems due to its high pH.

[0026] Therefore, for example, residual impurities such as ammonia (NH3) and ammonium ions (NH4 + ) etc. must be removed to reduce the manufacturing cost of the silica aerogel and prevent the reduction of the heat insulation performance of the final product. When the residual impurities are preferentially removed during the washing process before the supercritical drying process, the regeneration step of reheating and decomposing the ammonium carbonate salts that will be generated in the supercritical drying system can be omitted, and the problem of ammonia odor occurring during the supercritical drying process can be solved.

[0027] The method for manufacturing a silica aerogel according to the present invention includes: (1) manufacturing a hydrogel composite material; (2) washing the manufactured hydrogel composite material with a washing solvent; and (3) drying the washed hydrogel composite material, wherein, in step (2), impurities are removed from the hydrogel composite material, and the washing solvent is heated to a temperature above the boiling point (b.p.) of the washing solvent.

[0028] In the method for manufacturing a silica aerogel according to the present invention, the washing solvent used for washing the manufactured hydrogel composite material can be heated to a temperature above the boiling point of the washing solvent and can be added to the hydrogel composite material so that the hydrogel composite material can be washed with the washing solvent. As a result, by increasing the diffusion rate of the washing solvent from the aerogel to the washing tank, ammonium ions (NH4 + ) can be more effectively removed from the hydrogel composite material.

[0029] According to the present invention, the manufacturing of the hydrogel composite material is not particularly limited and may include a silica sol preparation step, an aging step, and a surface modification step. According to an embodiment of the present invention, the hydrogel composite material may be an aged silica gel - fiber composite material, but the present invention is not specifically limited thereto.

[0030] In step (2), the process of washing the produced hydrogel composite material can be carried out by placing the produced hydrogel composite material in a washing space (such as a washing tank) and introducing a washing solvent heated to a temperature above the boiling point of the washing solvent into the washing tank. According to an embodiment of the present invention, the washing step may further include: discharging the washing solvent from the washing tank after introducing the washing solvent into the washing tank. In this way, impurities can be removed from the hydrogel composite material.

[0031] According to an embodiment of the present invention, heating the washing solvent to a temperature above the boiling point (b.p.) of the washing solvent means heating the washing solvent to a temperature above the boiling point (b.p.) of the washing solvent under the pressure during the process of heating the washing solvent and the washing process.

[0032] According to an embodiment of the present invention, the washing solvent heated to a temperature above the boiling point of the washing solvent may have a temperature of 80°C to 200°C, specifically a temperature of 80°C to 150°C, and more specifically a temperature of 80°C to 120°C. At the same time, the energy required to heat the washing solvent can be calculated by theoretically determining the energy required to heat the liquid-phase washing solvent and the energy required to convert the washing solvent into a gas phase, and then adding the energies for the two purposes. The energy required to heat the washing solvent calculated in this way can be 1 MJ / m 2 / hr to 50 MJ / m 2 / hr, specifically 2 MJ / m 2 / hr to 30 MJ / m 2 / hr, and more specifically 10 MJ / m 2 / hr to 30 MJ / m 2 / hr.

[0033] When the washing solvent is heated to the above temperature range, some or all of the washing solvent may evaporate. Therefore, the washing solvent added to the hydrogel composite material may include the heated and evaporated washing solvent. When the temperature of the washing solvent is too low, the washing solvent does not evaporate to an appropriate extent. On the other hand, when the temperature of the washing solvent is too high, too much energy may be consumed to heat the washing solvent. Therefore, when the temperature of the washing solvent meets the above temperature range, the silica hydrogel can be washed more effectively, and the energy consumption can be maintained at an appropriate level.

[0034] According to one embodiment of the present invention, after washing the hydrogel composite material, the discharged waste washing solvent may have a temperature of 30°C to 120°C, particularly 50°C to 115°C, and more particularly 70°C to 110°C. When the temperature of the discharged waste washing solvent is too low, the diffusion of the washing solvent does not reach an appropriate level during the washing process, resulting in poor removal efficiency of ammonium ions. On the other hand, when the temperature of the discharged waste washing solvent is too high, the hydrogel composite material may be exposed to high temperatures during the washing process, causing changes in the physical properties of the aerogel and potentially consuming excessive energy to wash the hydrogel composite material.

[0035] According to one embodiment of the present invention, the washing solvent added to the hydrogel composite material may be added to the hydrogel composite material in a heated and evaporated gas phase. However, the washing solvent added to the hydrogel composite material may be a mixed phase of gas phase and liquid phase, depending on the heating temperature conditions. When the washing solvent is in a mixed phase of gas phase and liquid phase, based on the total weight of the washing solvent, the content of the washing solvent contained in the gas phase may be 10% by weight to 90% by weight, specifically 10% by weight to 70% by weight, and more specifically 20% by weight to 50% by weight.

[0036] The washing solvent may include ethanol. Specifically, the washing solvent may be ethanol or a mixture containing ethanol and water. When the washing solvent is a mixture containing ethanol and water, the washing solvent may be a water-containing ethanol containing 85% to 99% by volume of ethanol. The volume percentage is measured when the washing solvent is in the liquid phase and is based on the content of the liquid phase.

[0037] Ethanol is preferably used as the washing solvent because ethanol has excellent solubility in CO2, which is used as a supercritical solvent in the subsequent supercritical drying process. Compared with other alcohol solvents having a higher number of carbon atoms, ethanol may have excellent solubility in CO2 and may exhibit excellent drying efficiency during supercritical drying. In addition, ammonium ions are present in a relatively large amount in the hydrogel composite material because ammonium ions ionize in water. Therefore, when ethanol, which exhibits high solubility in the water contained in the hydrogel composite material, is used as the washing solvent, ammonium ions can be more effectively removed from the hydrogel composite material. In addition, since ethanol has a lower surface tension than other alcohol solvents having a higher number of carbon atoms, adverse effects during drying (such as shrinkage applied to the hydrogel composite material during supercritical drying after the washing step, etc.) can be minimized.

[0038] According to one embodiment of the present invention, when using a mixture containing ethanol and water as a washing solvent, depending on the heating temperature range of the washing solvent, the washing solvent may be a mixture containing gaseous ethanol and water and liquid ethanol and water, a mixture containing gaseous ethanol and liquid water, or a mixture containing gaseous ethanol and water.

[0039] According to one embodiment of the present invention, the washing in step (2) can be carried out at atmospheric pressure. When washing, the washing solvent can be heated to a temperature above the boiling point of the washing solvent at atmospheric pressure.

[0040] According to one embodiment of the present invention, the impurities may include one or more selected from the group consisting of residues derived from an alkali catalyst used in the manufacturing process of the hydrogel composite material, residues derived from a surface modifier, and silica microparticles.

[0041] The alkali catalyst may be a catalyst used in the gelation process of silica gel, a catalyst used in the aging process of silica gel, or an alkali catalyst used in both the gelation and aging processes of silica gel.

[0042] The alkali catalyst may include one or more selected from the group consisting of sodium hydroxide, potassium hydroxide, ammonium hydroxide (NH4OH), tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH), tetrabutylammonium hydroxide (TBAH), methylamine, ethylamine, isopropylamine, monoisopropylamine, diethylamine, diisopropylamine, dibutylamine, trimethylamine, triethylamine, triisopropylamine, tributylamine, choline, monoethanolamine, diethanolamine, 2-aminoethanol, 2-(ethylamino)ethanol, 2-(methylamino)ethanol, N-methyldiethanolamine, dimethylaminoethanol, diethylaminoethanol, triethanolamine, 2-(2-aminoethoxy)ethanol, 1-amino-2-propanol, triethanolamine, monopropanolamine, and dibutanolamine. Specifically, the alkali catalyst may be ammonium hydroxide (NH4OH).

[0043] In addition, the surface modifier may include silazane compounds. Specifically, the silazane compound may be hexamethyldisilazane. The silazane compound can react with an alcohol to form two molecules, namely an alkoxysilane compound and ammonia, and the formed ammonia can be retained as an impurity.

[0044] According to one embodiment of the present invention, the residues derived from the alkali catalyst and the residues derived from the surface modifier may include ammonia (NH3), ammonium ions (NH4 + ) and their mixtures.

[0045] Meanwhile, the silica microparticles may be impurities generated during the manufacturing process of the hydrogel composite material, that is, silica aerogel particles detached from the hydrogel composite material through the removal in step (2). In the method for manufacturing silica aerogel of the present invention, the hydrogel composite material is washed with a washing solvent heated to a temperature above the boiling point of the washing solvent, and the washing solvent is in a gas phase evaporated by heating or in a mixed phase including a gas phase and a liquid phase. Therefore, the washing solvent contained in the gas phase can effectively remove the silica microparticles weakly attached to the hydrogel composite material and included as impurities from the hydrogel composite material.

[0046] According to an embodiment of the present invention, the flow rate of the washing solvent may be 10 mL / m 2 / min to 800 mL / m 2 / min, specifically 30 mL / m 2 / min to 750 mL / m 2 / min, and more specifically 40 mL / m 2 / min to 700 mL / m 2 / min. When the flow rate of the washing solvent satisfies the above range, impurities can be effectively removed from the hydrogel composite material. In particular, the silica microparticles included as impurities in the hydrogel composite material can be effectively removed from the hydrogel composite material. When the flow rate of the washing solvent is too small, the washing efficiency using the washing solvent may be poor. On the other hand, when the flow rate of the washing solvent is too large, excessive energy may be consumed, and unnecessary solvent waste may be aggravated.

[0047] In addition, the usage amount of the washing solvent can be adjusted according to the amount of the hydrogel composite material to be washed. Specifically, the volume flow rate ratio of the hydrogel composite material and the washing solvent in step (2) may be 1:1 to 1:9, specifically 1:1 to 1:8.5, and more specifically 1:1 to 1:8.3. The volume flow rate ratio represents the ratio of the volume (L / h) of the washing solvent added to the washing tank per hour to the volume (L) of the hydrogel composite material provided in the space (such as the washing tank) for washing the hydrogel composite material.

[0048] In step (2), the process of washing the hydrogel composite with a washing solvent can be carried out for 30 minutes to 200 minutes, specifically 60 minutes to 200 minutes. In the method for manufacturing silica aerogel of the present invention, a washing solvent heated to a temperature above the boiling point (b.p.) of the washing solvent under atmospheric pressure is added to the hydrogel composite, so that the hydrogel is washed with the washing solvent by increasing the diffusion rate of the washing solvent. Therefore, compared with the conventional method in which a washing solvent heated to a temperature below the boiling point of the washing solvent is used for washing, the method for manufacturing silica aerogel of the present invention can have excellent ammonium ion removal efficiency even when the washing is carried out for a relatively short washing time.

[0049] In step (3), the drying of the hydrogel composite can be carried out by supercritical drying, atmospheric drying or a combination of both. When drying is carried out by both supercritical drying and atmospheric drying, supercritical drying can be carried out first, and then atmospheric drying can be further carried out.

[0050] Supercritical drying can be carried out using supercritical carbon dioxide. Supercritical carbon dioxide refers to carbon dioxide in a critical state, in which, since no evaporation process occurs when the temperature and pressure exceed a predetermined temperature and pressure limit called the supercritical point, the gas and liquid cannot be distinguished from each other.

[0051] Supercritical carbon dioxide has a molecular density close to that of a liquid, but has a low viscosity, its properties are close to those of a gas, and it exhibits high drying efficiency due to the rapid diffusion of supercritical carbon dioxide and its high thermal conductivity, so that the drying time can be shortened. Specifically, supercritical drying can be carried out as follows: the hydrogel composite is placed in a supercritical drying reactor, the supercritical drying reactor is filled with liquid-phase CO2 to replace the solvent in the hydrogel composite with CO2, the resulting mixture is heated to 40°C to 50°C at a certain heating rate, specifically 0.1°C / min to 1°C / min, and the pressure is maintained above the pressure at which carbon dioxide becomes supercritical, specifically 100 bar to 150 bar, so that the pressure maintains the supercritical state of carbon dioxide for a certain time, specifically 20 minutes to 1 hour. Generally, carbon dioxide becomes supercritical at a temperature of 31°C and a pressure of 73.8 bar. In this case, supercritical drying can be carried out as follows: by maintaining carbon dioxide at a certain temperature and pressure at which carbon dioxide becomes supercritical for 2 hours to 12 hours, specifically 2 hours to 6 hours, and gradually reducing the pressure.

[0052] Atmospheric drying can be carried out using a conventional method such as natural drying under the conditions of a temperature of 70°C to 200°C and atmospheric pressure (1±0.3 atm).

[0053] According to one embodiment of the present invention, after washing the hydrogel composite material in step (2), the method for manufacturing the silica aerogel may further include: recovering the waste washing solvent to purify the recovered waste washing solvent.

[0054] Purifying the recovered waste washing solvent in step (2) may include: introducing the recovered waste washing solvent into a purification tower, and reusing the purified washing solvent as the washing solvent in step (2).

[0055] The purification process carried out in the purification tower can be carried out under normal pressure. The recovered waste washing solvent can be introduced into the purification tower in a state where the recovered waste washing solvent is cooled by passing through a heat exchanger. Therefore, before the recovered waste washing solvent is transferred to the purification tower, a process of further cooling the recovered waste washing solvent by the heat exchanger to reduce the temperature of the recovered waste washing solvent can be carried out. Specifically, a process of cooling the recovered waste washing solvent to a temperature below the boiling point of the washing solvent can be carried out, and more specifically, a process of cooling the recovered waste washing solvent to the boiling point of the washing solvent.

[0056] Examples

[0057] Hereinafter, exemplary embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement the present invention. However, it should be understood that the present invention can be implemented in various forms and is not intended to be limited to the exemplary embodiments described herein.

[0058] Example 1

[0059] Tetraethyl orthosilicate (TEOS), ethanol, and distilled water as silica precursors are mixed at a weight ratio of 1:0.9:0.22 to prepare a silica sol. Additionally, ethanol, NH4OH (30% aqueous solution), and tetramethylethoxysilane (TMES) are mixed at a weight ratio of 1:0.054:0.154 to prepare a gelation catalyst solution. The silica sol and the gelation catalyst solution thus prepared are mixed at a volume ratio of 1:1 to prepare a catalytic sol.

[0060] A glass fiber mat is immersed in the catalytic sol and then gelated for 10 minutes. After the gelation is completed, the gelated hydrogel felt is aged at 70 °C in a chamber for 24 hours.

[0061] By continuously injecting aqueous ethanol (91.8 vol%) heated to 90 °C into the aged hydrogel felt at a flow rate of 50 mL / m 2 / min and discharging the aqueous ethanol from the aged hydrogel felt, the aged hydrogel felt is washed for 200 minutes.

[0062] When the washing is completed, the hydrogel felt is placed in a supercritical extractor, and then supercritical drying is carried out by injecting CO2 at 75 °C and 150 bar into the supercritical extractor. Then, the silica aerogel dried by supercritical drying is collected.

[0063] Examples 2 to 6

[0064] The silica aerogel is manufactured in the same manner as in Example 1 by performing washing and supercritical drying, except that the temperature, flow rate, and washing time of the washing solvent are changed as listed in Table 1 below.

[0065] Comparative Example 1

[0066] The silica aerogel is prepared in the same manner as in Example 1, except that the washing process is not performed.

[0067] Comparative Example 2

[0068] The silica aerogel is manufactured in the same manner as in Example 1, except that the aged hydrogel felt is washed for 360 minutes by continuously injecting water-containing ethanol (91.8 vol%) heated to 70 °C into the aged hydrogel felt at a flow rate of 50 mL / m 2 / min and discharging the water-containing ethanol from the aged hydrogel felt.

[0069] Comparative Example 3

[0070] The silica aerogel is manufactured in the same manner as in Example 1, except that the aged hydrogel felt is washed for 240 minutes by continuously injecting water-containing ethanol (91.8 vol%) heated to 70 °C into the aged hydrogel felt at a flow rate of 670 mL / m 2 / min and discharging the water-containing ethanol from the aged hydrogel felt.

[0071] Experimental Example

[0072] 1) Ammonia removal rate

[0073] After measuring the initial ammonia amount contained in the silica gel-fiber composite material and the remaining ammonia amount after washing, the ammonia removal rate in each of the silica aerogels of Examples 1 to 6 and Comparative Examples 1 to 3 is calculated according to the following Mathematical Expression 1.

[0074] [Mathematical Expression 1]

[0075] (Initial ammonia amount in silica gel-fiber composite material - Remaining ammonia amount after washing) / Initial ammonia amount in silica gel-fiber composite material × 100

[0076] 2) Heating load

[0077] The load required to heat the washing solvent in the washing processes shown in Examples 1 to 6 and Comparative Examples 1 to 3 was calculated by adding the energy required to heat the liquid-phase washing solvent and the energy required to convert the washing solvent into the gas phase.

[0078] 3) Moisture impregnation rate

[0079] Three samples (125 mm × 125 mm, thickness less than 10 mm) were prepared using each of the silica aerogels produced in Examples 1 to 6 and Comparative Examples 1 to 3, and the weight (W1) of the samples was measured.

[0080] The samples were floated on distilled water at 21 ± 2 °C and immersed 127 mm underwater by placing a 6.4 mm sieve on each sample.

[0081] After 15 minutes, the sieve was removed. When the samples rose to the water surface, the samples were picked up with a clamp and vertically suspended for 60 ± 5 seconds. Then, the weight (W2) of the samples was measured.

[0082] The moisture impregnation rate was calculated using the following Mathematical Expression 2.

[0083] [Mathematical Expression 2]

[0084] Moisture impregnation rate = (W2 - W1) / W1 × 100

[0085] where W1 represents the weight of the sample before immersion in water, and W2 represents the weight of the sample after immersion in water.

[0086] 4) Dust emission rate

[0087] After applying a constant vibration to the samples under the following conditions, the dust emission rate of each of the manufactured samples as described in the moisture impregnation rate was calculated using Mathematical Expression 3.

[0088] Sample: 125 mm × 125 mm, thickness less than 10 mm

[0089] Vibration: 24 Hz

[0090] Amplitude: 3 mm

[0091] Time: 12 hours

[0092] [Mathematical Expression 3]

[0093] Dust emission rate (P v ) = (W c - W v / Wc )×100

[0094] Wherein, W c represents the weight of the sample before vibration, and W v represents the weight of the sample after vibration.

[0095] 5) Measurement of thermal conductivity

[0096] The thermal conductivities of the individual silica aerogel felt rolls produced in Examples 1 to 6 and Comparative Examples 1 to 3 were measured at room temperature (25 °C) using an HFM 436 device commercially available from NETZSCH.

[0097] Table 1

[0098]

[0099] Compared with Comparative Example 1 without separate washing and Comparative Examples 2 and 3 using a washing solvent at 70 °C, the silica aerogel felts of Examples 1 to 6 have a significantly excellent ammonia removal rate even when the silica aerogel felts are washed for a short washing time. The silica microparticles contained in the silica aerogel felt in a state of being weakly bound to the silica aerogel felt can be scattered during the operation of using the silica aerogel felt, and are scattered due to the impact and vibration on the product to which the silica aerogel felt is applied. The degree of dust emission of the silica aerogel felt can be evaluated by a method of measuring the weight reduction of the silica aerogel felt after applying vibration to the silica aerogel felt. As shown in Table 1, it can be seen that a small amount of silica microparticles are detached from the silica aerogel felts of Examples 1 to 6 after vibration, and the degree of dust emission caused by vibration is low because the weight reduction of the silica aerogel felts of Examples 1 to 6 is significantly smaller than the weight reduction of the silica aerogels of Comparative Examples 1 to 3. The silica aerogel of Comparative Example 1 without separate washing has the highest dust emission rate, while the silica aerogels of Comparative Examples 2 and 3 have a dust emission rate lower than that of the silica aerogel of Comparative Example 1, but significantly higher than that of the silica aerogel felts of Examples 1 to 6.

[0100] It can be seen from the evaluation of the dust emission rate that the silica microparticles weakly bound to the aerogel felt are removed during the washing process, and the silica microparticles are more effectively removed during the washing process of the present invention. Therefore, it can be seen that silica aerogels having a smaller dust emission rate, such as the silica aerogel felts of Examples 1 to 6, are produced by using the method for producing silica aerogel of the present invention.

[0101] Figures 1 to 3 Pictures taken of the filter and the housing in the supercritical dryer used in Example 1 and Comparative Examples 1 and 3 are shown respectively. SeeFigures 1 to 3 As can be seen, in Comparative Example 1 where no separate washing was performed, a large amount of ammonium salts generated during the supercritical drying process were deposited in the filter and the outer shell. On the contrary, it can be seen that in the case of Example 1 and Comparative Example 3 where ammonium ions were removed, a very small amount of ammonium salts were deposited in the filter and the outer shell. In particular, it can be seen that the silica aerogel of Example 1 has excellent results compared to Comparative Example 3 because no ammonium salts deposited in the outer shell were observed in the case of Example 1.

[0102] In addition, it can be seen that the silica aerogels of Examples 1 to 6 have a lower moisture impregnation rate compared to the silica aerogels of Comparative Examples 1 and 2. The method of manufacturing a silica aerogel necessarily requires a step of hydrophobically modifying the surface of the silica aerogel to suppress the water absorption of the silica aerogel and thus maintain a low thermal conductivity. However, even when the surface of the silica aerogel is hydrophobically modified to impart hydrophobicity to the silica aerogel, when hydrophilic substances such as impurities (such as salts, etc.) are contained in the silica aerogel, the moisture impregnation rate increases in proportion to the amount of the hydrophilic substances. The moisture impregnation rate is one of the physical properties that can reflect the durability when the silica aerogel is used as a thermal insulation material in scenarios such as rain or underwater. When the moisture impregnation rate is high, the silica aerogel is easily impregnated with water, which causes the thermal conductivity to deteriorate over time. Therefore, when the moisture impregnation rate is high, although the silica aerogel has a low thermal conductivity immediately after being manufactured, the thermal conductivity increases over time. Therefore, based on the fact that the silica aerogels of Examples 1 to 6 have a low moisture impregnation rate, it can be seen that the silica aerogels manufactured by the method of manufacturing a silica aerogel of the present invention have high durability.

Claims

1. A method for manufacturing silica aerogel, which comprises: (1) manufacturing a hydrogel composite material; (2) washing the manufactured hydrogel composite material with a washing solvent; and (3) drying the washed hydrogel composite material, wherein, in step (2), impurities are removed from the hydrogel composite material, heating the washing solvent to a temperature above the boiling point of the washing solvent, the washing solvent being a mixture of ethanol and water in a gas phase and a liquid phase, a mixture of ethanol and water in a gas phase and a liquid phase, or a mixture of ethanol and water in a gas phase, and the washing solvent is a mixture of ethanol and water.

2. The method according to claim 1, wherein The temperature of the heated washing solvent is from 80°C to 200°C.

3. The method according to claim 1, wherein, The washing solvent is a mixed phase of a gas phase and a liquid phase.

4. The method according to claim 1, wherein The washing solvent contains 85 vol% to 99 vol% of ethanol.

5. The method according to claim 1, wherein, The temperature of the heated washing solvent is from 80°C to 120°C.

6. The method according to claim 1, wherein, The impurities include one or more selected from the group consisting of residues derived from an alkali catalyst used in the manufacturing process of the hydrogel composite material, residues derived from a surface modifier, and silica microparticles.

7. The method according to claim 6, wherein, The alkali catalyst includes one or more selected from the group consisting of sodium hydroxide, potassium hydroxide, ammonium hydroxide (NH4OH), tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH), tetrabutylammonium hydroxide (TBAH), methylamine, ethylamine, isopropylamine, monoisopropylamine, diethylamine, diisopropylamine, dibutylamine, trimethylamine, triethylamine, triisopropylamine, tributylamine, choline, monoethanolamine, diethanolamine, 2-aminoethanol, 2-(ethylamino)ethanol, 2-(methylamino)ethanol, N-methyldiethanolamine, dimethylaminoethanol, diethylaminoethanol, nitrilotriethanol, 2-(2-aminoethoxy)ethanol, 1-amino-2-propanol, triethanolamine, monopropanolamine, and dibutanolamine.

8. The method according to claim 6, wherein, The surface modifier includes silazane compounds.

9. The method according to claim 6, wherein, The silica microparticles are silica aerogel particles that are separated from the hydrogel composite material by removal in step (2) as impurities generated during the manufacturing process of the hydrogel composite material.

10. The method according to claim 6, wherein, Residues derived from the base catalyst and residues derived from the surface modifier include ammonia (NH3), ammonium ions (NH4 + ) and mixtures thereof.

11. The method according to claim 1, wherein, The flow rate of the washing solvent is 10 mL / m 2 / min to 800 mL / m 2 / min.

12. The method according to claim 1, wherein The volume flow rate ratio of the hydrogel composite material to the washing solvent in step (2) is from 1:1 to 1:

9.

13. The method according to claim 1, wherein, Step (2) is carried out for 30 minutes to 200 minutes.

14. The method according to claim 1, wherein, Step (3) is carried out by supercritical drying, atmospheric drying, or a combination of both.

15. The method according to claim 1, which further comprises: purifying the waste washing solvent recovered in step (2).

16. The method according to claim 15, wherein, Purifying the waste washing solvent recovered in step (2) includes: introducing the recovered waste washing solvent into a purification tower to purify the waste washing solvent; and reusing the purified washing solvent as the washing solvent in step (2).

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