Supercritical drying method for silica wet gel mat

Abstract

The present invention relates to a supercritical drying method for a silica wet gel mat, which can improve drying efficiency, and a production method of a silica aerogel mat including the same. According to the supercritical drying method of the present invention, since the use amount of carbon dioxide is reduced without additional equipment investment and energy input, the efficiency of the supercritical drying step is improved, and thus the silica wet gel mat can be dried in a more economical manner.

Description

[0001] This case is a divisional application of the patent application filed on December 13, 2019, with application number 201980037972.9 and invention title "Supercritical Drying Method for Silica Wet Gel Mat". Technical Field

[0002] Cross-reference to related applications

[0003] This application claims the benefit of Korean Patent Application No. 2018-0160982, filed on December 13, 2018, with the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0005] This invention relates to a supercritical drying method for silica wet aerogel mats and a method for producing silica aerogel mats including the method, wherein the supercritical drying method can improve drying efficiency. Background Technology

[0006] Aerogels are highly porous materials composed of nanoparticles, possessing high porosity, specific surface area, and low thermal conductivity, thus attracting attention as efficient thermal and sound insulation materials. However, due to their porous structure, aerogels exhibit very low mechanical strength. Therefore, an aerogel composite material has been developed that binds aerogels to fiber felts by impregnating the fiber felt, such as inorganic or organic fibers, both of which are conventional thermal insulation fibers.

[0007] Silica aerogel is a highly porous material with high porosity, specific surface area, and low thermal conductivity, making its application in various fields, such as thermal insulation materials, catalysts, sound-absorbing materials, and interlayer insulation materials for semiconductor circuits, foreseeable. Although the commercialization of silica has been very slow due to its complex manufacturing process and low mechanical strength, stabilization studies of silica aerogel have led to the release of early application products, and its market expansion, including in insulation materials, is gradually increasing. Because of its very low mechanical strength due to its porous structure, silica aerogel is typically bonded to a substrate and commercialized in the form of aerogel mats or sheets.

[0008] For example, silica aerogel mats using silica aerogel are prepared through a silica sol preparation process, a silica sol gelation process, a silica sol aging process, a silica sol surface modification process, and a silica sol drying process.

[0009] The drying process is a step in the manufacture of silica aerogel felts and can be carried out by removing solvents while maintaining the porous structure of the silica aerogel felt. The drying process can be performed using either ambient drying or supercritical drying.

[0010] Environmental drying processes do not require high-pressure reaction conditions or the high-pressure equipment specific to supercritical drying, making them simple and economical. However, compared to supercritical drying, environmental drying suffers from a problem: due to the evaporation of water or organic solvents at high temperatures, the thermal insulation performance is drastically reduced because the porous structure inside the gel is significantly disrupted. Furthermore, these problems are exacerbated when drying is performed directly without the use of organic solvents with low surface tension.

[0011] To maintain the porosity of aerogels and prevent shrinkage during drying, supercritical drying using supercritical carbon dioxide can be employed. Supercritical drying involves introducing supercritical carbon dioxide into a high-pressure reactor to extract the solvent from the gelled felt product. Because supercritical drying preserves the three-dimensional structure of the aerogel, products with excellent thermal insulation properties can be produced.

[0012] Meanwhile, to further improve the drying efficiency of supercritical drying processes, attempts have been made to increase the diffusion rate of carbon dioxide to promote the displacement of solvent and carbon dioxide. In one instance, a technique for increasing the carbon dioxide diffusion rate by applying pulses in a supercritical drying process has been investigated. However, this technique requires the development of separate equipment or apparatus, and therefore, a method to improve drying efficiency in a simpler way is still needed.

[0013] [Existing technical documents]

[0014] [Patent Literature]

[0015] [Patent Document 1] JP 2013-067521 A Summary of the Invention

[0016] Technical issues

[0017] One aspect of the present invention is to reduce the amount of carbon dioxide used and improve drying efficiency in the supercritical drying step of aerogel mats. Specifically, in the supercritical drying step, a supercritical drying method for silica wet gel mats is provided by repeatedly supplying and stopping the supply of carbon dioxide to two supercritical extractors in an alternating manner, using a small amount of carbon dioxide to maximize drying efficiency.

[0018] Another aspect of the present invention is to provide a method for producing silica aerogel mats using a supercritical drying method for silica wet gel mats.

[0019] Another aspect of the present invention is a silica aerogel felt produced by the aforementioned production method, the silica aerogel felt having excellent material properties and having low thermal conductivity and water repellency at its inner ends.

[0020] Technical solution

[0021] According to one aspect of the present invention, a supercritical drying method for a silica wet gel mat using a first supercritical extractor and a second supercritical extractor is provided, the method comprising: placing the silica wet gel mat in each of the first and second supercritical extractors, (a) supplying carbon dioxide to the first supercritical extractor and not supplying carbon dioxide to the second supercritical extractor, and (b) not supplying carbon dioxide to the first supercritical extractor and supplying carbon dioxide to the second supercritical extractor, wherein steps (a) and (b) are performed alternately, each step being performed at least twice.

[0022] According to another aspect of the present invention, a method for producing silica aerogel felt is provided, the method comprising preparing silica sol, impregnating silica sol in a felt substrate, then gelling the impregnated silica sol, surface modifying the silica wet gel felt formed by the gelation reaction, and supercritically drying the silica wet gel felt, wherein the supercritical drying step is performed by the supercritical drying method described above.

[0023] According to another aspect of the present invention, a silica aerogel felt is provided, wherein the difference in thermal conductivity between the inner and outer ends of the silica aerogel felt in the wound state at room temperature (25°C) is less than 1.0 mW / mK, wherein the inner and outer ends of the felt refer to the two ends of the felt when the felt is divided into three parts in the longitudinal direction in the unwound state.

[0024] Beneficial effects

[0025] According to the supercritical drying method of the present invention, the efficiency of the supercritical drying step is improved by reducing the use of carbon dioxide without additional equipment investment and energy input, thus allowing for the drying of silica wet gel mats in a more economical manner.

[0026] Furthermore, when using the supercritical drying method to produce silica aerogel felt, it has the advantage of being able to produce aerogel felt with uniform and excellent physical properties. Attached Figure Description

[0027] Figure 1 The silica aerogel felt (1 batch roll) according to the present invention is shown;

[0028] Figure 2A method for supplying and stopping the supply of carbon dioxide in a supercritical drying step is shown according to an embodiment of the present invention;

[0029] Figure 3 A method for supplying and stopping the supply of carbon dioxide in a supercritical drying step is shown according to a comparative example of the present invention;

[0030] Figure 4 The ethanol recovery rate (%) is shown in Example 1 and Comparative Example 1 of the present invention. Detailed Implementation

[0031] The invention will now be described in more detail to facilitate understanding of the invention.

[0032] It should be understood that the words or terms used in the specification and claims of this invention should not be construed as limited to their meanings as defined in common dictionaries. It will be further understood that, based on the principle that the inventors can appropriately define the meanings of words or terms to best interpret the invention, the words or terms are interpreted as having meanings consistent with their meanings in the context of the relevant art and the technical concept of the invention.

[0033] When drying silica aerogel mats using supercritical drying, although carbon dioxide has excellent diffusivity, its diffusion takes a long time due to the mesoporous structure of the silica aerogel mat. Furthermore, when supercritical drying is performed by continuously supplying carbon dioxide into the supercritical extractor, a channeling phenomenon occurs, causing carbon dioxide to flow unevenly to certain areas. This degrades the drying efficiency of the supercritical drying process and makes it difficult to dry the silica aerogel mat uniformly. Consequently, the physical properties of the final silica aerogel mat deteriorate.

[0034] Therefore, in this invention, by reducing the absolute amount of carbon dioxide used while increasing the amount of alcohol solvent to replace carbon dioxide, the drying efficiency of the supercritical drying process is greatly improved. Furthermore, by reducing the absolute amount of carbon dioxide used, the energy required for cooling and heating to recover carbon dioxide, as well as the associated costs, are also reduced.

[0035] Furthermore, the present invention not only improves the drying efficiency of the supercritical drying step, but also produces a silica aerogel felt with a small difference in thermal conductivity between its inner and outer ends, thus exhibiting excellent uniform thermal insulation, and low water repellency, thus exhibiting high hydrophobicity, and can be effectively used in certain insulation applications without additional treatment.

[0036] The present invention will be described in detail below.

[0037] Supercritical drying method for silica wet gel mats

[0038] The present invention relates to a supercritical drying method for a silica wet gel mat using a first supercritical extractor and a second supercritical extractor, the method comprising: placing the silica wet gel mat in each of the first and second supercritical extractors; (a) supplying carbon dioxide to the first supercritical extractor and not supplying carbon dioxide to the second supercritical extractor; and (b) not supplying carbon dioxide to the first supercritical extractor and supplying carbon dioxide to the second supercritical extractor, wherein steps (a) and (b) are performed alternately, each step being performed at least twice.

[0039] The supercritical drying method of the present invention is a method using a first supercritical extractor and a second supercritical extractor, wherein the supercritical extractor refers to a high-pressure device that uses supercritical fluid to perform supercritical drying on silica wet gel felt.

[0040] When two or more supercritical extractors are used as described in this invention, two or more silica wet gel mats can be dried simultaneously, and since two supercritical extractors can be operated in a single system, such as a carbon dioxide injection pump, separator, carbon dioxide separator and heat exchanger, investment costs and operating costs can be reduced.

[0041] The supercritical drying method of the present invention includes (a) supplying carbon dioxide to a first supercritical extractor and not supplying carbon dioxide to a second supercritical extractor, and (b) not supplying carbon dioxide to the first supercritical extractor and supplying carbon dioxide to the second supercritical extractor.

[0042] That is, step (a) is performed by supplying carbon dioxide to the first supercritical extractor while not supplying carbon dioxide to the second supercritical extractor, followed by step (b) by supplying carbon dioxide to the second supercritical extractor while not supplying carbon dioxide to the first supercritical extractor. Steps (a) and (b) may be performed sequentially or in reverse order, and may be performed alternately and repeatedly based on the first step.

[0043] Specifically, in the supercritical drying method of the present invention, steps (a) and (b) are performed alternately, and each step (a) and step (b) is repeated at least twice.

[0044] By alternating between steps (a) and (b), supplying and stopping the supply of carbon dioxide to the first and second supercritical extractors, the amount of carbon dioxide used can be reduced by half compared to the case where carbon dioxide is supplied to each extractor simultaneously and continuously.

[0045] Furthermore, each of steps (a) and (b) may be repeated at least twice. For example, this means performing steps (a), (b), (a), and (b).

[0046] When carbon dioxide is continuously supplied to the supercritical extractor, the temperature inside the extractor decreases due to the continuous flow of carbon dioxide, thus degrading the efficiency of the supercritical drying step. Furthermore, a channeling phenomenon, where carbon dioxide flow is deflected, occurs inside the supercritical extractor. Channeling refers to the phenomenon where the fluid does not flow uniformly within the packing layer, but instead uses a portion as a channel for uneven flow. Carbon dioxide in the supercritical extractor also tends to flow to certain parts of the extractor. Therefore, when carbon dioxide is continuously supplied throughout the entire supercritical drying process, this channeling phenomenon accelerates and becomes uncontrollable, ultimately leading to a sharp decline in the drying efficiency of the supercritical drying process.

[0047] In this invention, by alternately performing steps (a) and (b) at least twice each, carbon dioxide is prevented from flowing continuously in one extractor, and cross-flow is prevented by repeatedly causing the flow and cessation of carbon dioxide. Furthermore, by providing sufficient time for the carbon dioxide to displace the solvent contained in the mesopores within the silica wet gel felt, the supercritical drying efficiency can be further improved compared to continuous operation.

[0048] According to one embodiment of the supercritical drying method of the present invention, steps (a) and (b) may be performed alternately at least four times each. Furthermore, steps (a) and (b) may be performed alternately at least four times each, at least five times each, at least six times each, etc.

[0049] According to one embodiment of the present invention, while maintaining the total time spent on the supercritical drying step, when the supercritical drying process is carried out by adjusting the time spent on each of steps (a) and (b) to change their repetition number, the drying efficiency and physical properties of silica aerogel felt can be improved at different repetition numbers.

[0050] In the supercritical drying method of the present invention, before steps (a) and (b), a silica wet gel mat is placed in each of the first and second supercritical extractors.

[0051] Specifically, a silica wet gel mat is placed in a first supercritical extractor, and then step (a) is performed; a silica wet gel mat is placed in a second supercritical extractor, and then step (b) is performed; then steps (a) and (b) are repeated alternately. Alternatively, a silica wet gel mat is placed in a second supercritical extractor, and then step (b) is performed; a silica wet gel mat is placed in a first supercritical extractor, and then step (a) is performed; then steps (b) and (a) are repeated alternately.

[0052] That is, the silica wet gel mat can be placed inside the supercritical extractor just before supplying carbon dioxide to the supercritical extractor via step (a) or step (b). When the silica wet gel mat is placed inside the supercritical extractor and no carbon dioxide is supplied for an extended period, as described later, the silica wet gel mat is stored in a heated supercritical extractor without a carbon dioxide supply, causing the silica wet gel mat to dry at high temperatures without carbon dioxide, resulting in structural deformation or deterioration of its physical properties.

[0053] According to one embodiment of the invention, the felt is placed in the first supercritical extractor before the step of supplying carbon dioxide to the first supercritical extractor (step (a)) and before the step of supplying carbon dioxide to the second supercritical extractor (step (b)), thereby shortening the interval between the time point of introducing the silica wet gel felt and the time point of supplying carbon dioxide, so as to prevent the silica wet gel felt from drying out due to the high temperature in the supercritical extractor in the absence of carbon dioxide.

[0054] In the supercritical drying method of the present invention, before placing the silica wet gel felt into the interior of each of the first and second supercritical extractors, a step of raising the temperature inside each of the first and second supercritical extractors may be performed.

[0055] In the step of increasing the temperature, the temperature inside the first supercritical extractor and the second supercritical extractor can be increased to 40°C to 100°C, specifically 50°C to 90°C, more preferably 60°C to 80°C.

[0056] In repeating steps (a) and (b), the first step (a) and step (b) may involve increasing the internal pressure of the first or second supercritical extractor to 75 bar to 250 bar, specifically 100 bar to 200 bar, more specifically 150 bar to 170 bar, while simultaneously supplying carbon dioxide to the first or second supercritical extractor.

[0057] That is, the supercritical drying method of the present invention includes: raising the temperature inside the first and second supercritical extractors to the aforementioned temperature; placing a silica wet gel mat in one supercritical extractor; then, while supplying carbon dioxide, raising the pressure to the aforementioned pressure; and placing the silica wet gel mat in the other supercritical extractor; then, while supplying carbon dioxide, raising the pressure to the aforementioned pressure. After the above steps, the silica wet gel mat is subjected to supercritical drying while alternately and continuously supplying carbon dioxide to the two supercritical extractors.

[0058] Carbon dioxide is gaseous at room temperature and atmospheric pressure. However, when the temperature and pressure exceed predetermined limits known as the supercritical point, evaporation does not occur, and carbon dioxide enters a critical state in which it is indistinguishable from a liquid. Carbon dioxide in this critical state is called supercritical carbon dioxide. Typically, in supercritical drying processes using supercritical carbon dioxide, silica gel is placed in a supercritical drying reactor, and then liquid carbon dioxide is supplied to it. Subsequently, by increasing the temperature and pressure inside the supercritical extractor, the carbon dioxide is converted to a supercritical state, and then the supercritical drying process is carried out. In this case, it takes a long time to bring the carbon dioxide to a supercritical state, and there is a temperature difference between the central part and the outside of the supercritical extractor, which reduces drying efficiency and increases operating costs.

[0059] According to one embodiment of the present invention, by first increasing the temperature and pressure of the supercritical extractor without supplying liquid carbon dioxide to the supercritical extractor, the supercritical state is reached immediately after the carbon dioxide is supplied to the supercritical extractor. Therefore, the time required for the supercritical drying step can be shortened, and the temperature deviation between the central part of the supercritical extractor and the outside environment can be reduced.

[0060] The temperature of the carbon dioxide supplied to the first and second supercritical extractors can be from 50°C to 100°C, preferably from 60°C to 80°C, and more preferably from 70°C to 80°C.

[0061] In the supercritical drying method of the present invention, steps (a) and (b) can each be performed for 5 to 30 minutes. According to one embodiment of the present invention, even when the supercritical drying process is performed by performing steps (a) and (b) for 5 minutes, 10 minutes, 15 minutes, 20 minutes, etc., respectively, the drying efficiency of the supercritical drying step and the physical properties of the produced silica aerogel felt have been proven to be excellent. That is, the supercritical drying method of the present invention can be used in various ways by changing the timing of alternating supply and cessation of carbon dioxide supply.

[0062] In the supercritical drying method of the present invention, the amount of carbon dioxide supplied to the first supercritical extractor or the second supercritical extractor can be from 300 kg / batch to 700 kg / batch, specifically from 300 kg / batch to 600 kg / batch, more preferably from 300 kg / batch to 400 kg / batch. Specifically, the capacity of the first supercritical extractor and the second supercritical extractor can be 70 L.

[0063] As described above, in this invention, when two supercritical extractors are used, carbon dioxide is not added to both extractors simultaneously and continuously. Instead, the supply and cessation of carbon dioxide are repeated alternately, thus requiring only about half the amount of carbon dioxide compared to the case where carbon dioxide is added continuously to both supercritical extractors.

[0064] In the supercritical drying method of the present invention, the solvent recovery rate calculated by Equation 1 below can be 90% or more.

[0065] [Equation 1]

[0066] Solvent recovery rate (%) = [(Amount of solvent discharged and recovered from the supercritical extractor) / (Amount of solvent introduced into the silica wet gel mat in the supercritical extractor)] × 100

[0067] The supercritical drying method of the present invention exhibits a solvent recovery rate equal to or higher than that achieved when carbon dioxide is continuously supplied to both the first and second supercritical extractors. Despite using approximately half the amount of carbon dioxide, the exchange reaction between the solvent and carbon dioxide occurs more actively, resulting in a drying efficiency more than twice that of the previous method, based on the same amount of carbon dioxide.

[0068] The aforementioned effects are achieved by alternating and repeating the supply and cessation of carbon dioxide to the first and second supercritical extractors. Because carbon dioxide is blocked and diffuses into the mesopores, solvent displacement is facilitated, resulting in high drying efficiency even with only small amounts of carbon dioxide.

[0069] Method for manufacturing silica aerogel mat

[0070] The present invention provides a method comprising: 1) preparing silica sol; 2) impregnating silica sol in a felt substrate, and then gelling the impregnated silica sol to produce silica wet gel felt; 3) surface-modifying the silica wet gel felt; and 4) supercritically drying the surface-modified silica wet gel felt, wherein the supercritical drying step is performed by the supercritical drying method of the present invention.

[0071] 1) Preparation of silica sol

[0072] The silica sol of the present invention can be prepared by mixing a silica precursor, an alcohol, and an acidic aqueous solution.

[0073] The silica precursor can be a silicon-containing alkoxy compound, and specifically a tetraalkyl silicate, such as tetramethyl orthosilicate (TMOS), tetraethyl orthosilicate (TEOS), methyl triethyl orthosilicate, dimethyl diethyl orthosilicate, tetrapropyl orthosilicate, tetraisopropyl orthosilicate, tetrabutyl orthosilicate, tetrasec-butyl orthosilicate, tetratert-butyl orthosilicate, tetrahexyl orthosilicate, tetracyclohexyl orthosilicate, tetra(dodecyl) orthosilicate, trimethylethoxysilane (TMES), or mixtures thereof, but not limited thereto.

[0074] The amount of silica precursor used can range from 0.1% to 30% by weight, but is not limited to, the silica sol containing silica (SiO2) content. If the silica content is less than 0.1% by weight, the silica aerogel content in the final felt is too low to achieve the required level of insulation. If the silica content is greater than 30% by weight, excessive silica aerogel formation may degrade the mechanical properties of the felt, particularly its flexibility.

[0075] The alcohol can be specifically a monohydric alcohol, such as methanol, ethanol, isopropanol, and butanol; or a polyhydric alcohol, such as glycerol, ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, and sorbitol, and any one or a mixture of two or more of these can be used. When considering miscibility with water and aerogels, the alcohol can be a monohydric alcohol having 1-6 carbon atoms, such as methanol, ethanol, isopropanol, and butanol.

[0076] Considering the degree of hydrophobicity of the silica aerogel ultimately produced while promoting the surface modification reaction, those skilled in the art can use alcohols (polar organic solvents) as described above in appropriate amounts.

[0077] Acidic aqueous solutions can promote the gelation of silica sol, which will be described later. The acidic precursors in the acidic aqueous solution can specifically include one or more inorganic acids, such as nitric acid, hydrochloric acid, acetic acid, sulfuric acid, and hydrofluoric acid, and can be used in an amount that promotes the subsequent hydration of the silica precursors.

[0078] 2) Impregnate the felt substrate with silica sol and then gel it.

[0079] This step is used to produce silica gel composite materials, and can be carried out by adding an alkaline catalyst to the silica sol and impregnating the silica sol with the added alkaline catalyst into a substrate for use as a felt.

[0080] In this invention, the gel can be formed into a network structure from a silica precursor material. The network structure can be a planar mesh structure with one or more types of atomic arrangements where specific polygons are interconnected, or a structure in which specific polyhedra share their vertices, edges, faces, etc. to form a three-dimensional skeleton structure.

[0081] Alkaline catalysts that can be used to induce gelation reactions promote gelation by increasing the pH of silica sol.

[0082] The base catalyst can be an inorganic base, such as sodium hydroxide and potassium hydroxide; or an organic base, such as ammonium hydroxide. However, in the case of an inorganic base, the metal ions contained in the compound can coordinate to the Si-OH compound. Therefore, an organic base is preferred. Specifically, the organic base can be 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, methamintriethanol, 2-(2-aminoethoxy)ethanol, 1-amino-2-propanol, triethanolamine, monopropanolamine, or dibutanol, and mixtures of two or more of them can be used. More specifically, the base can be ammonium hydroxide (NH4OH).

[0083] The amount of alkali catalyst can be such that the pH of the silica sol is between 4 and 8. If the pH of the silica sol exceeds this range, gelation will be difficult to achieve, or the gelation rate will be too slow, resulting in deterioration of processing performance. Furthermore, since alkali may precipitate when added in the solid phase, it is preferable to add alkali in the form of a solution diluted with the aforementioned alcohol (polar organic solvent).

[0084] The gelation of the silica sol of the present invention can occur while the silica sol is impregnated in a felt substrate.

[0085] Impregnation can be carried out in a reaction vessel capable of holding the felt substrate, either by pouring silica sol into the reaction vessel or by placing the felt substrate in a reaction vessel containing the silica sol to be impregnated. To improve the bonding between the felt substrate and the silica sol, the felt substrate can be gently pressed down for thorough impregnation. Subsequently, the substrate for the felt can be pressed to a predetermined thickness under constant pressure to remove any remaining silica sol, thereby shortening the drying time.

[0086] The felt substrate can be a film, sheet, mesh, fiber, porous body, foam, nonwoven fabric, or a laminate of two or more layers thereof. Furthermore, depending on the intended use of the substrate for the felt, surface roughness can be formed or patterned on its surface. Specifically, the felt substrate can be a fiber capable of further improving thermal insulation performance by incorporating spaces or voids, through which silica aerogel can be easily inserted.

[0087] Specifically, the felt substrate can be polyamide, polybenzimidazole, polyaramid, acrylic resin, phenolic resin, polyester, polyetheretherketone (PEEK), polyolefin (such as polyethylene, polypropylene or their copolymers), cellulose, carbon, cotton, wool, hemp, nonwoven fabric, glass fiber or ceramic wool, etc.

[0088] 3) Surface modification of silica wet gel mat formed by gelation reaction

[0089] In silica aerogel felts, silanol groups (Si-OH) exist on the silica surface. Due to their hydrophilicity, these silanol groups absorb water from the air, resulting in a gradual increase in thermal conductivity. Therefore, to maintain low thermal conductivity, the surface of the wet silica aerogel felt needs to be modified to be hydrophobic beforehand.

[0090] Surface modification can be achieved using surface modifiers that include polar solvents and organosilane compounds.

[0091] The polar solvent can be methanol, ethanol or isopropanol, and the organosilane compound can be trimethylchlorosilane (TMCS), hexamethyldisilazane (HMDS), methyltrimethoxysilane, trimethylethoxysilane, ethyltriethoxysilane, phenyltriethoxysilane, etc.

[0092] Surface modification can be omitted when the precursor containing hydrophobic groups is used alone or in mixtures used to prepare silica sol. An aging process can be performed before surface modification. Aging is an optional step that keeps the wet silica gel at an appropriate temperature to allow it to undergo complete chemical changes, thereby forming a more robust network structure and enhancing mechanical stability.

[0093] The aging step of the present invention can be carried out by placing an alkaline catalyst, such as sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonium hydroxide (NH4OH), triethylamine, pyridine, etc., in an organic solvent diluted to a concentration of 1-10% for 1 to 10 hours at a temperature of 50°C to 90°C.

[0094] 4) Supercritical dried silica wet gel

[0095] The supercritical drying process for silica wet gel is carried out by the supercritical drying method for silica wet gel mats of the present invention.

[0096] The supercritical drying method for silica wet gel mats is the same as described above.

[0097] Furthermore, the method for producing silica aerogel felt according to one embodiment of the present invention can also include a washing step before the supercritical drying step. Washing is a step to remove impurities (unreacted substances, byproducts, etc.) generated during the reaction process and residual ammonia, which can be converted into ammonium carbonate salts by reaction with carbon dioxide during the supercritical drying process, in order to obtain high-purity hydrophobic silica aerogel, and can be carried out by a dilution process or an exchange process using a nonpolar organic solvent.

[0098] Silica aerogel mat

[0099] The present invention provides a silica aerogel felt, wherein the difference in thermal conductivity between the inner and outer ends of the silica aerogel felt in the wound state at room temperature (25°C) is less than 1.0 mW / mK. The inner and outer ends of the felt refer to the two ends of the felt when the wound is unwound and the felt is divided into three parts based on the longitudinal direction.

[0100] Silica aerogel felt refers to a material made of silica aerogel, which is combined with fibers and other materials to form a mat or sheet as a substrate for felting. Due to its flexibility, it possesses properties that allow it to be bent, folded, or cut. Silica aerogel imparts thermal insulation properties due to its porosity, and the substrate used for the felt, such as fibers, enhances the flexibility and mechanical strength of the silica aerogel felt.

[0101] Thermal conductivity refers to the degree of heat transfer between two surfaces of a material or composite material through a specific medium on either side of a material or composite material where a temperature difference exists between the two surfaces. Specifically, thermal conductivity is measured by dividing the heat energy transferred per unit hour and per unit area by the temperature difference.

[0102] When the thermal conductivity is low, it means that there is a strong tendency to retain heat without dissipating heat; when the thermal conductivity of silica aerogel felt is low, it means that its insulation performance is excellent.

[0103] Figure 1 A silica aerogel felt according to one embodiment of the present invention is shown. In the case of the silica aerogel felt 10 of the present invention, in the wound state, i.e., in the form of a roll, the difference in thermal conductivity between the inner end 11 and the outer end 12 is low, thus exhibiting generally uniform insulation properties. Specifically, the thermal conductivity of the inner end 11, which typically does not exhibit excellent insulation properties, can be low, which may be an effect achieved by drying according to the supercritical drying method of the present invention described above.

[0104] Specifically, when the silica aerogel felt 10 is wound up, at room temperature (25°C), the difference in thermal conductivity between the inner end 11 and the outer end 12 can be less than 1.0 mW / mK, preferably less than 0.7 mW / mK, less than 0.5 mW / mK, or less than 0.3 mW / mK.

[0105] In this case, the inner end 11 and the outer end 12 refer to the two ends of the felt when it is unwound and divided into three parts based on the longitudinal direction. The inner end 11 refers to the section located on the side of the inner end 11a inside the roll when the felt is wound, and the outer end 12 refers to the section located on the side of the outer end 12a outside the roll when the felt is wound.

[0106] The longitudinal direction refers to the direction orthogonal to the inner end 11a or the outer end 12a for a roll of silica aerogel felt in a batch (a roll of a batch). The length measured in the corresponding direction can be defined as the total length L of the silica aerogel felt.

[0107] In this context, "a roll of one batch" can refer to a silica aerogel felt obtained by supercritical drying of a silica wet gel felt placed in a first or second supercritical extractor. This means that its dimensions can vary depending on the size of the supercritical extractor or the degree of felt winding; therefore, the total length L of the roll of one batch can vary. Those skilled in the art can appropriately modify the size of the roll of one batch, taking into account the internal volume of the supercritical extractor, the intended use of the felt, etc. Because it refers to the difference in thermal conductivity of segments divided into three parts based on the total length of the roll of one batch, rather than the absolute size of the roll of one batch, the effect of the thermal conductivity of the inner and outer ends according to the invention applies to all rolls of various sizes.

[0108] Furthermore, the silica aerogel felt satisfies the difference in thermal conductivity between the inner and outer ends, and the thermal conductivity of the inner end at room temperature (25°C) can be from 13 Mw / mK to 20 Mw / mK, preferably from 15 Mw / mK to 19 Mw / mK, and more preferably from 15 Mw / mK to 18 Mw / mK.

[0109] The main factors affecting heat transfer in porous materials are density and structure. That is, even materials with the same microstructure can have different thermal conductivity depending on their pore structure if their density is high. Furthermore, factors in the microstructure that influence heat transfer include grain morphology, phase mass, grain orientation, and porosity.

[0110] In the supercritical drying process, if carbon dioxide is continuously supplied instead of repeatedly supplying and stopping it, the silica wet gel will not dry its pores sufficiently due to the carbon dioxide cross-flow phenomenon, resulting in the deterioration of the physical properties of the silica aerogel felt and the presence of high thermal conductivity.

[0111] Furthermore, the silica aerogel felt produced in this invention can exhibit low water repellency, which can be less than 5%, preferably less than 4%, and more preferably less than 2%.

[0112] Water repellency can be calculated using Equation 2 below.

[0113] [Equation 2]

[0114] Water repellency (%) = {[Weight of silica aerogel mat after water impregnation (wt%) - Weight of silica aerogel mat before water impregnation (wt%)] / [Weight of silica aerogel mat before water impregnation (wt%)]} × 100

[0115] Hydrophobicity refers to the degree to which silica aerogel felt absorbs water. Low hydrophobicity means high hydrophobicity, and low thermal conductivity can be maintained through low hydrophobicity.

[0116] Therefore, the aerogel felt of the present invention can be effectively used as a heat insulation material, thermal insulation material or non-combustible material for aircraft, ships, automobiles, building structures, etc., as well as for insulation and cooling of factory equipment, such as pipes and industrial boilers in various industrial facilities.

[0117] Example

[0118] The present invention will be described in detail below with reference to embodiments. However, the following embodiments are merely illustrative and are not intended to limit the scope of the invention.

[0119] Example 1

[0120] A silica sol was prepared by mixing 75% hydrated tetraethyl orthosilicate (HTEOS) (19% to 20% by weight of silica), ethanol, and water in a weight ratio of 1:2.25:0.35. Based on HTEOS, 0.44% by weight of an alkaline catalyst solution in which ethanol and ammonia were mixed in a weight ratio of 210:1 was added to the silica sol. The silica sol containing this solution was then impregnated in glass fibers to induce gelation. After gelation, the gelled silica sol was aged by placing it in an 80% to 90% ammonia solution (2 to 3% by volume) at a temperature of 50°C to 70°C for 1 hour. Subsequently, based on the silica sol, an 80% to 90% hexamethyldisilazane (HMDS) solution (2 to 10% by volume) is placed at a temperature of 50°C to 70°C for 4 hours to re-age the aged silica sol in order to produce rolls of silica wet gel mat.

[0121] A roll of surface-modified silica wet gel was introduced into a 70L first supercritical fluid extractor heated to 70°C, and then carbon dioxide at 75°C was added and maintained for 20 minutes until 160 bar was reached. Subsequently, another roll of silica wet gel was introduced into a 70L second supercritical fluid extractor heated to 70°C, and then carbon dioxide at 75°C was added and maintained for 20 minutes until 160 bar was reached. Thereafter, carbon dioxide was supplied to the first supercritical fluid extractor at a rate of 5.5 L / min for 20 minutes, while no carbon dioxide was supplied to the second supercritical fluid extractor. After 20 minutes, the supply of carbon dioxide to the first supercritical fluid extractor was stopped, and carbon dioxide was started to be supplied to the second supercritical fluid extractor at a rate of 5.5 L / min for 20 minutes.

[0122] The aforementioned 40-minute process is considered as one process, and this process is repeated a total of 4 times. That is, for a total of 160 minutes (2 hours and 40 minutes), in each supercritical extractor, the supply and cessation of carbon dioxide are repeated four times at 20-minute intervals.

[0123] Subsequently, the ethanol extracted through the bottom of the extractor is recovered. Then, carbon dioxide is discharged from the bottom of the extractor over a period of 1 hour, followed by further environmental drying in an oven at 150°C to produce silica aerogel mats.

[0124] Example 2

[0125] Except for repeating the supply and stop of carbon dioxide five times at 15-minute intervals (150 minutes), and then once more at 5-minute intervals (10 minutes), the same operation is performed in the same manner as in Example 1.

[0126] Example 3

[0127] Except for repeating the supply and stop of carbon dioxide at 10-minute intervals 8 times (160 minutes), the same operation was performed in the same manner as in Example 1.

[0128] Example 4

[0129] Except for repeating the supply and stop of carbon dioxide 16 times (160 minutes) at 5-minute intervals, the same operation as in Example 1 was performed.

[0130] Example 5

[0131] Except for repeating the supply and stop of carbon dioxide twice at 30-minute intervals (120 minutes), and then once at 20-minute intervals (40 minutes), the same operation is performed in the same manner as in Example 1.

[0132] Comparative Example 1

[0133] Except for the continuous supply of carbon dioxide to both the first and second supercritical extractors for 160 minutes, the same operation was performed in the same manner as in Example 1.

[0134] Comparative Example 2

[0135] Except for the continuous introduction of carbon dioxide into both the first and second supercritical extractors for 3 hours, the same operation was performed in the same manner as in Example 1.

[0136] Comparative Example 3

[0137] Except that carbon dioxide was continuously introduced into both the first and second supercritical extractors for 2 hours and 40 minutes, and the flow rate of carbon dioxide was changed to 6.5 L / min, the same operation was performed in the same manner as in Example 1.

[0138] Comparative Example 4

[0139] Except for supplying liquid carbon dioxide to the supercritical extractor by omitting the steps of raising the temperature and internal pressure of the supercritical extractor, and using a single supercritical extractor, the same operation is performed in the same manner as in Example 1.

[0140] Specifically, a roll of surface-modified silica wet gel was introduced into a 70L first supercritical extractor, and then liquid carbon dioxide was injected into the extractor to reach 25°C and 70 bar. Subsequently, the temperature inside the extractor was raised to 70°C over a period of 1 hour, and when it reached 150 bar, carbon dioxide was continuously supplied for 160 minutes.

[0141] Experimental Example 1

[0142] The amount of carbon dioxide used and the physical properties of the manufactured silica aerogel mats were compared during the supercritical drying steps according to the examples and comparative examples.

[0143] 1) Carbon dioxide usage

[0144] Table 1 below shows the amount of carbon dioxide supplied to the first or second supercritical extractor in each embodiment and comparative example. This represents the total amount of carbon dioxide supplied to the first or second supercritical extractor, and since the amount in the first and second supercritical extractors is the same, it is represented as a single value in Table 1 below.

[0145] 2) Measurement of thermal conductivity

[0146] The thermal conductivity of the rolls of silica aerogel felt produced in each example and comparative example at room temperature (25°C) was measured using an HFM 436 from NETZSCH Co, Ltd.

[0147] - Measurement of inner end thermal conductivity: With the coil unwound, the thermal conductivity was measured at a point one-sixth of the total length of the silica aerogel felt in the longitudinal direction from the inner end. Figure 1 ).

[0148] - Measurement of thermal conductivity at the outer end: With the coil unwound, the thermal conductivity was measured at a point one-sixth of the total length of the silica aerogel felt in the longitudinal direction from the outer end. Figure 1 ).

[0149] 3) Measurement of water repellency

[0150] Calculate the water repellency of silica aerogel felt using the following equation.

[0151] - Water repellency (%) = {[Weight of silica aerogel mat after water impregnation (wt%) - Weight of silica aerogel mat before water impregnation (wt%)] / [Weight of silica aerogel mat before water impregnation (wt%)]} × 100

[0152] Immersion time in water is 15 minutes. Sample instructions and test methods are in accordance with ASTM C 1511.

[0153] [Table 1]

[0154]

[0155] As shown in Table 1 above, in Examples 1 to 5, where carbon dioxide was alternately supplied and stopped in the first and second supercritical extractors, the total amount of carbon dioxide used was reduced by about half compared to Comparative Examples 1 to 3, where carbon dioxide was continuously supplied to both the first and second supercritical extractors, and the thermal conductivity and water repellency of the inner end of the silica aerogel felt were lower. Furthermore, in Comparative Example 4, where liquid carbon dioxide was supplied by omitting the steps of raising the temperature and internal pressure of the supercritical extractor, the thermal conductivity and water repellency of the inner end were higher, thus confirming a deterioration in the physical properties of the silica aerogel felt. In other words, when using the supercritical drying method of alternately supplying and injecting carbon dioxide into the two supercritical extractors, it was demonstrated that not only was the amount of carbon dioxide used reduced, thereby improving the efficiency of the supercritical drying process, but the thermal conductivity of the inner end was also lower. Therefore, silica aerogel felt with excellent insulation properties, excellent physical properties, and low water repellency can be produced.

[0156] Meanwhile, Example 1 involved repeating the supply and stop of carbon dioxide four times at constant intervals of 20 minutes, allowing the supercritical drying step to proceed for a total of 160 minutes. Examples 2 to 5 involved the supercritical drying step proceeding for a total of 160 minutes, but with the intervals between each step varying to 15 minutes, 10 minutes, and 5 minutes, respectively. As shown in Table 1, despite the variation in the intervals between each step in the alternating supply and stop of carbon dioxide, silica aerogel mats with excellent physical properties, low internal thermal conductivity, and low water repellency can still be produced.

[0157] That is, when the supercritical drying step is performed by alternating and stopping the supply of carbon dioxide in two supercritical extractors, even if the timing of alternating and stopping the supply of carbon dioxide varies, the effects of reducing the amount of carbon dioxide used and improving the physical properties of silica aerogel mat are still demonstrated.

[0158] Experimental Example 2

[0159] Using the silica aerogel felt produced in Example 1 and Comparative Example 1, the ethanol recovery rate (%) in each supercritical drying process was calculated as follows.

[0160] - Ethanol recovery rate (%) = [(Amount of ethanol discharged and recovered from the supercritical extractor) / (Amount of ethanol introduced into the silica wet gel mat in the supercritical extractor)] × 100

[0161] The ethanol recovery rate (%) in the supercritical drying process of Example 1 and Comparative Example 1 as a function of time was calculated and shown in the figure. Figure 4The ethanol recovery was calculated at 40, 80, 120 and 160 minutes after extraction, and is shown in Table 2 below.

[0162] [Table 2]

[0163]

[0164] like Figure 4 As shown in Table 2, in the case of Example 1, although only half the amount of carbon dioxide was used compared to Comparative Example 1, the ethanol recovery rate was higher than that of Comparative Example 1. That is, in Example 1, although only half the amount of carbon dioxide was used, the amount of residual ethanol was also reduced, and the efficiency of the supercritical drying process was further improved.

[0165] [Explanation of reference numerals in the attached figures]

[0166] 10: Silica aerogel mat

[0167] L: Total length

[0168] 11: Inner End

[0169] 12: External end

[0170] 11a: Inner end

[0171] 12a: outer end

[0172] P1: Measurement point for internal thermal conductivity

[0173] P2: Measurement point for thermal conductivity at the outer end

Claims

1. A silica aerogel felt, wherein the difference in thermal conductivity between the inner and outer ends of the silica aerogel felt in the wound state at 25°C is less than 1.0 mW / mK. in, The inner and outer ends of the felt refer to the two ends of the felt when it is unwound and divided into three parts along the longitudinal direction. The silica aerogel felt is prepared by a method for producing silica aerogel felt, the method comprising: Preparation of silica sol; The silica sol is impregnated in a felt substrate, and then the impregnated silica sol is gelled. Surface modification of silica wet gel mat formed by gelation reaction; and The silica wet gel felt is subjected to supercritical drying. The supercritical drying is carried out by the following method, including: The silica wet gel felt is placed in each of the first and second supercritical extractors; (a) Supplying carbon dioxide to the first supercritical extractor but not to the second supercritical extractor, and (b) Not supplying carbon dioxide to the first supercritical extractor but supplying carbon dioxide to the second supercritical extractor. Steps (a) and (b) are performed alternately, with each step performed at least twice.

2. The silica aerogel felt according to claim 1, wherein, The thermal conductivity of the inner end of the silica aerogel felt at 25°C is 13 mW / mK to 20 mW / mK.

3. The silica aerogel felt according to claim 1, wherein, The water repellency of the silica aerogel felt, calculated using Equation 2 below, is less than 5%. [Equation 2] Water repellency = {[Weight of silica aerogel felt after water immersion - Weight of silica aerogel felt before water immersion] / [Weight of silica aerogel felt before water immersion]} × 100%.

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