WO2009038393A2 - Method for preparing surface-modified transparent bead type aerogel and aerogel prepared therefrom - Google Patents

Method for preparing surface-modified transparent bead type aerogel and aerogel prepared therefrom Download PDF

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Publication number
WO2009038393A2
WO2009038393A2 PCT/KR2008/005570 KR2008005570W WO2009038393A2 WO 2009038393 A2 WO2009038393 A2 WO 2009038393A2 KR 2008005570 W KR2008005570 W KR 2008005570W WO 2009038393 A2 WO2009038393 A2 WO 2009038393A2
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WIPO (PCT)
Prior art keywords
wet gel
water glass
bead type
silica wet
glass solution
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PCT/KR2008/005570
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French (fr)
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WO2009038393A3 (en
Inventor
Mi Sun Yoo
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Neb, Co., Ltd.
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Publication of WO2009038393A2 publication Critical patent/WO2009038393A2/en
Publication of WO2009038393A3 publication Critical patent/WO2009038393A3/en

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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/14—Colloidal silica, e.g. dispersions, gels, sols
    • C01B33/157—After-treatment of gels
    • C01B33/159—Coating or hydrophobisation
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/14—Colloidal silica, e.g. dispersions, gels, sols
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/14—Colloidal silica, e.g. dispersions, gels, sols
    • C01B33/152—Preparation of hydrogels
    • C01B33/154—Preparation of hydrogels by acidic treatment of aqueous silicate solutions
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/14—Colloidal silica, e.g. dispersions, gels, sols
    • C01B33/157—After-treatment of gels
    • C01B33/158—Purification; Drying; Dehydrating

Definitions

  • the present invention relates to a method for preparing a surface- modified transparent bead type aerogel having permanent hydrophobicity, and a bead type aerogel prepared therefrom, and more particularly, to a method for preparing a surface-modified transparent bead type aerogel without any of additional impurity removal and washing processes by removing a sodium component from a water glass in advance, and a surface-modified bead type aerogel prepared therefrom.
  • the bead type aerogel is an ultralow-density advanced material having very excellent transparency, a porosity, of 90 % (percent) or more as in powder, and a specific surface area of several hundreds up to approximately 1500 m Ig, and also has a nanoporous structure.
  • the bead type aerogel may be used in the fields, such as super-insulating dual glass, which has limited the use of powdery aerogel since the bead type aerogel should satisfy the requirement of transparency, extending to the field of heat-insulating materials of a LNG carrier, building insulation materials, soundproof materials, etc., in which the use of the powder has been restricted since dusts and the like are raised in the use of the powder.
  • the bead type aerogel has the same low heat conductivity as the powder, it is a very useful heat-insulating material that may be used in the field of ultra-low dielectrics, catalyst, electrode material, soundproof material, etc. or in the filed of a refrigerator, an automobile, an aircraft, etc.
  • the bead type aerogel may be manufactured using various methods.
  • W095/06617 describes that hydrophobic silicic acid aerogels are obtained by reacting a solution of water glass with sulfuric acid at a pH value from 7.5 to 11; removing NaCl components from the obtained silicic acid hydrogel by washing it with water or diluted aqueous solutions of inorganic bases (sodium hydroxide or ammonia) at a pH value from 7.5 to 11; removing water with an exchange of alcohol; and drying the obtained alcohol gel at 240 to 280°C (degrees centigrade) under a supercritical condition of 55 to 90 bar, and also discloses a method for preparing hydrophobic silicic acid aerogels through a supercritical drying process without a siIyIation process.
  • An aspect of the present invention provides a novel method for preparing a surface-modified bead type aerogel having permanent hydrophobicity and transparency.
  • Another aspect of the present invention provides a method for preparing a surface-modified bead type aerogel having excellent properties such as insulating property, soundproofing property and high specific surface area that the conventional powder aerogels have.
  • Still another aspect of the present invention provides a surface- modified bead type aerogel prepared from the method according to one exemplary embodiment of the present invention, the surface-modified bead type aerogel having a high specific surface area, as well as excellent properties such as insulating property, soundproofing property, permanent hydrophobicity and transparency.
  • a method for preparing a surface-modified bead type aerogel having permanent hydrophobicity comprising: a) removing a sodium component from a water glass solution, in which a water glass is diluted to a concentration of 5 to 15 % (percent) by weight, by contacting a cation exchange resin with the water glass solution at a room temperature; b) forming a silica wet gel by adjusting pH of the sodium-free water glass solution to a pH range of 4-6 and aging the sodium-free water glass solution!
  • silica wet gel c) cutting the formed silica wet gel into the form of beads; d) adding the cut silica wet gel to a silylation solution comprising a hydrophobic alcohol solvent and a silylating agent, and simultaneously performing silylation and solvent substitution processes on the cut silica wet gel ; and e) drying the silica wet gel.
  • a method for preparing a surface-modified bead type aerogel having permanent hydrophobicity comprising: a) removing a sodium component from a water glass solution, in which a water glass is diluted to a concentration of 5 to 15 % (percent) by weight, by contacting a cation exchange resin with the water glass solution at a room temperature; b) forming a silica wet gel by adjusting pH of the sodium-free water glass solution to a pH range of 4 to 6 and aging the sodium-free water glass solution by adding the sodium-free water glass solution in a state of right before the formation of silica wet gel dropwise to a 70 to 90°C (degrees centigrade) oil ; c) adding the formed silica wet gel to a silylation solution comprising a hydrophobic alcohol solvent and a silylating agent, and simultaneously performing silylation and solvent substitution processes on the formed silica wet gel ; and d) drying
  • a surface-modified bead type aerogel prepared according to the method according to one exemplary embodiment of the present invention, the bead type aerogel having a particle size of 0.1 to 0.5 cm (centimeters).
  • the formation of sodium chloride (NaCl) that is formed as a by-product may be prevented in the formation of a silica wet gel by a conventional reaction of a water glass with hydrochloric acid by using a cation exchange resin to remove a sodium component from a solution of water glass in advance. Therefore, the method of the present invention does not require an additional NaCl removal process. As a result, it is possible to obtain a bead type aerogel having excellent transparency since the damage in a shape of the bead type silica gel caused by the conventional NaCl removal process may be prevented.
  • the surface-modified bead type aerogel prepared from the method according to one exemplary embodiment of the present invention has excellent properties such as insulating property, soundproofing property and transparency.
  • the bead type aerogel prepared from the method according on exemplary embodiment of the present invention has a solid structure with a particle size of approximately 0.1 to 0.5 cm (centimeters), and also has a high porosity and a high specific surface area of 600 to 1300 mVg (square meters per gram) .
  • the bead type aerogel according to one exemplary embodiment of the present invention is easy to handle and has excellent transparency, as well as an insulating property corresponding to the conventional powder aerogels, and therefore it may be easily used to manufacture dual glasses requiring transparency, ultra-low dielectrics, catalysts, electrode materials, soundproof materials, building heat-insulating materials, insulating panels of an LNG carrier, etc.
  • FIG. 1 is a schematic diagram illustrating the removal of sodium using an ion exchange resin.
  • FIG. 2 is a photograph illustrating a bead type aerogel prepared in Example 1.
  • FIG. 3 is a photograph illustrating a bead type aerogel prepared in Example 2.
  • the present invention is characterized in that a lifespan of aerogel is extended by surface-modifying (permanently hydrophobising) the aerogel in order to protect the aerogel from moisture in the air that obstructs a porous structure of the aerogel. Also, it is characterized in that the aerogel is prepared in the form of bead so that it can be easy to apply to the field of various applications. In addition, the present invention is characterized in that the aerogel may be economically and easily prepared by performing silylation and solvent substitution(exchange) processes at the same time.
  • the surface-modified bead type aerogel prepared from the method according to one exemplary embodiment of the present invention has excellent insulating property, high porosity, high specific surface area, soundproofing property and transparency.
  • An embodiment of the present invention is related to a method for preparing a hydrophobically surface-modified bead type aerogel using a water glass.
  • a major component of water glass is sodium silicate (SiC>2 # Na2 ⁇ ), but a sodium (Na) component should be removed from the sodium silicate so as to obtain a pure silica component.
  • a sodium component is removed from the water glass using a cation exchange resin. That is to say, a water glass is diluted to a constant concentration, and passed through a cation exchange resin to exchange a sodium (Na) component in the water glass with cation such as hydrogen, etc., which leads to the removal of the sodium component in the water glass.
  • a cation exchange resin that is to say, a water glass is diluted to a constant concentration, and passed through a cation exchange resin to exchange a sodium (Na) component in the water glass with cation such as hydrogen, etc., which leads to the removal of the sodium component in the water glass.
  • pH of a water glass solution is reduced since the sodium component is removed from the water glass and exchanged with a hydrogen component.
  • the pH of a water glass solution is in a range of approximately pH 3 to 4, and silica is solated within this pH range.
  • the pH of the sodium-free water glass solution is adjusted to a suitable pH range to form a wet gel, and aged to obtain a transparent silica wet gel.
  • a sodium silicate sodium silicate
  • a sodium component in the water glass is substituted with hydrogen as represented by the following Formula 1, and removed from the water glass solution to obtain pure silica.
  • the water glass may be generally diluted with water or distilled water, etc.
  • concentration of the water glass is less than 5 % (percent) by weight, a reaction apparatus is too large compared to an amount of the obtained silica, which leads to the inefficient manufacturing process.
  • pH of the water glass solution that is subjected to the cation exchange process is in a range of approximately pH 3 to 4.
  • a silica wet gel is formed by adjusting the pH of the water glass solution to a pH range of approximately 4 to 6. This is why a wet gel is effectively formed within the pH range of approximately 4 to 6, and the pH of the water glass may be adjusted, for example, by adding an ammonia solution, but the present invention is not particularly limited thereto.
  • silica in the water glass solution is polymerized to form a wet gel.
  • the water glass solution may be aged at approximately 20 to 40°C (degrees centigrade) for approximately 2 to 6 hours to obtain a wet gel .
  • the aging temperature of the water glass solution When the aging temperature of the water glass solution is below 20°C (degrees centigrade), the aging time of the water glass solution is too long. On the contrary, the aging temperature of the water glass solution may be shortened in the temperature (i.e. 50 to 60°C (degrees centigrade)) exceeding 40°C (degrees centigrade), but silica particles are conglomerated since silica is suddenly formed in a too high temperature, which makes it impossible to obtain a bead type aerogel silica having a high specific surface area. Therefore, it is desirable to age the water glass solution at 20 to 40°C (degrees centigrade) for approximately 2 to 6 hours in consideration of the wet gel formation efficiency.
  • the bead type silica aerogel obtained from the above- mentioned silica wet gel is not uniform in shape, and difficult to form in the form of perfect bead. Therefore, a silica wet gel-forming solutionCwater glass solution) adjusted to a pH range of 4 to 6 in a state of right before the formation of wet gel is added dropwise to a 70 to 90°C (degrees centigrade) oil and aged in the oil to obtain bead type aerogel with more uniform size and shape.
  • a silica wet gel-forming solutionCwater glass solution adjusted to a pH range of 4 to 6 in a state of right before the formation of wet gel is added dropwise to a 70 to 90°C (degrees centigrade) oil and aged in the oil to obtain bead type aerogel with more uniform size and shape.
  • the droplets of the water glass solution may be undesirably broken.
  • the wet gel-forming solutionCwater glass solution is aged in the oil, the conglomeration and/or growth of silica particles are suppressed even at a high temperature since a water glass solution in a state of right before the formation of wet gel (i.e. in a state of a wet gel is formed to some degree) is added to the oil, thus to obtain a silica wet gel having a relatively high specific surface area.
  • the temperature of the oil exceeds 90°C (degrees centigrade)
  • the wet gel-forming solution is aged very rapidly, which leads to a low specific surface area of some wet gel.
  • the bead type silica wet gel is separated from the oil, and the oil is recovered and re-used.
  • Any of animal or vegetable edible oils and the like that are widely known in the art may be used as the oil, and specific examples of the oil that may be used herein include, but are particularly limited to, vegetable oils extracted from corn, olive, bean, cottonseed, etc., animal oils extracted from cattle, pig, fish, etc. Chemically synthetic oils may be also used as the oil.
  • Silica particles are polymerized into a silica backbone while being grown into nanoparticles during the aging, and the silica backbone is then formed into a silica wet gel having a high specific surface area.
  • the silica wet gel is subject to the above-mentioned operation to obtain a silica wet gel having a high specific surface area of 600 nf/g (square meters per gram) or more, and preferably 600 to 130OmVg (square meters per gram).
  • the silica wet gel prepared thus is composed of silica particles forming a backbone, and a large amount of water. In this case, it is very important to effectively remove water without contraction of the silica particles forming a backbone.
  • the pH of the water glass solution is reduced since a sodium component in the water glass is substituted with hydrogen by means of the cation exchange resin and removed from the water glass according to the method of the present invention. Therefore, the method of the present invention does not require a reaction with hydrochloric acid as described in the method for preparing a silica wet gel using a conventional water glass.
  • by-products such as NaCl which are formed together by the reaction with hydrochloric acid in the conventional method for preparing a silica wet gel, are not formed in the method of an embodiment in the present invention, the method of an embodiment in the present invention does not need a separate washing process, and therefore damages in powdering and transparency of wet gel in the washing process may be prevented.
  • the obtained silica wet gel is translucent, and has transparency.
  • the transparency of the silica wet gel is lost due to the broken backbone structure of silica gel in the wet gel. Therefore, in order to prepare a transparent bead type aerogel, a wet gel should be sliced (cut) into bead type pieces and added into a silylation solution used in the next silylation and solvent substitution processes. Meanwhile, since the obtained wet gel in the oil has its bead type shape, it is unnecessary to slice the wet gel into bead type pieces.
  • the hydrophobicity (silylation) and solvent substitution processes are performed at the same time in the present invention. That is to say, the bead type silica wet gel is added to a silylation solution (a solution comprising a hydrophobic alcohol solvent and a silylating agent), and refluxed to hydrophobically modify a surface of the silica wet gel through the silylation, and to remove water from the silica wet gel through its substitution with a solvent.
  • silylation solution a solution comprising a hydrophobic alcohol solvent and a silylating agent
  • the hydrophobicity and solvent substitution processes may be performed at a temperature around a boiling temperature of the silylation solution for approximately 2 to 24 hours under an atmospheric pressure.
  • the reflux time is less than 2 hours, the silica wet gel is not sufficiently silylated according to the kind of the silylating agents, whereas when the reflux time exceeds 24 hours, undesired side reactions may occur.
  • the hydrophobicity and solvent substitution processes may be performed under a reduced pressure.
  • the hydrophobicity and solvent substitution processes under the reduced pressure may be performed at the same time by adjusting a pressure of a reactor containing the silylation solution and the wet gel to a pressure range of 30 to 200 mmHg (millimeters of mercury) and refluxing the reaction solution(reactants ⁇ containing solution) at 45 to 60°C (degrees centigrade).
  • the hydrophobicity and solvent substitution processes under the reduced pressure may be shortened, and particularly completed within 60 minutes, compared to those under an atmospheric pressure.
  • the pressure in the reactor is less than 200 mmHg (millimeters of mercury)
  • the reaction time may be shortened within 60 minutes, and the reaction rate becomes faster with a decreasing pressure in the reactor.
  • moisture is removed from the silica wet gel and the wet gel is surface-hydrophobisized with a silylating agent, for example, by putting a mixture of the silica wet gel and a silylation solution comprising the hydrophobic alcohol solvent and a silylating agent into a reactor, separating a solvent and water from the resulting mixture while refluxing the mixture under a temperature of the reactor from 45 to 60 ° C (degrees centigrade) and a pressure of the reactor from 200 to 30 mmHg (millimeters of mercury), thus to remove the moisture and re-reflux the solvent, followed by repeating the operations until the moisture is completely removed from the silica wet gel.
  • the discharged solvent is separated using a cooling tube or a centrifuge, and re-used in the si IyIation/solvent substitution co-process.
  • the silylation solution used in the silylation (hydrophobicity) and solvent substitution processes is a mixture comprising 1 to 10 % (percent) by weight of a silylating agent and 90 to 99 % (percent) by weight of a hydrophobic alcohol solvent.
  • a silylating agent in the silylation solution is less than 1 % (percent) by weight, surface-unmodified aerogel may be formed, whereas when the content of the silylating agent in the silylation solution exceeds 10 % (percent) by weight, the non-reacted silylating agent may be present, which is made undesirable in an economical aspect .
  • Ri is hydrogen or Cl-ClO, preferably C1-C5 alkyl , aromatic alkyl, or heteroaromatic alkyl
  • X is halogen selected from the group consisting of F, Cl, Br and I, and preferably Cl, or Cl-ClO, preferably C1-C5 alkoxy group, aromatic alkoxy group, or heteroaromatic alkoxy group), and/or disiloxane represented by Formula of RsSi-O-SiRs (wherein, two R3 groups are identical to, or different from each other, and are Cl-ClO alkyl, preferably C1-C5 alkyl, aromatic alkyl, hetero aromatic alkyl, or hydrogen) may be used as the silylating agent.
  • silylating agent examples include, but are not particularly limited to, hexamethyldisilane, ethyltriethoxysilane, trimethoxysilane, triethylethoxysilane, methyltrimethoxysi lane, ethyltrimethoxysi 1ane, methoxytrimethylsi 1ane, trimethy1chlorosi 1ane, hexamethyldisiloxane (HMDSO) and triethylchlorosilane, and they may be used alone or in combinations thereof.
  • HMDSO hexamethyldisiloxane
  • At least one selected from the group consisting of n-butanol, n- pentanol, n-hexanol and n-octanol may be used as the hydrophobic alcohol solvent.
  • the silica wet gel When the silica wet gel is subject to the above-mentioned silylation process, it has a heat-insulation equal to or greater than that of the conventional aerogel powder even when the silylating agent is used at a low content, and a surface of the silica wet gel is permanently silylated as represented by the following Formula 2 since a silylation reaction condition is modified to a strong acid condition to get rid of powder that does not react with the silylating agent.
  • a continuous process is possible by performing the silylation and solvent substitution at the same time, and the used hydrophobic alcohol solvent may be transferred to a distillation process and used again for simultaneous process of silylation and solvent substitution.
  • the solvent that may be used for the substitution of a wet gel with a solvent in the preparation of aerogel should satisfy requirements of (1) effectively removing water from pores of the wet gel, and (2) evaporating a solvent while applying a possibly low capillary pressure to a gel structure when the wet gel is dried under an atmospheric pressure. That is to say, the solvent has polarity as high as possible so as to satisfy the requirement of (1), and the solvent has a low surface tension, that is, should be nonpolar if possible so as to satisfy the requirement of (2).
  • the solvent should not be high in polarity as in methanol, ethanol, THF, acetone, etc., and also be not nonpolar as in heptane, pentane, etc. That is to say, when the solvent is polar as in the former, a very high capillary pressure generated in a gas- liquid interface during the drying may be applied to the gel structure, and, when the solvent is nonpolar as in the latter, it is difficult to effectively remove water from the pores of the wet gel since it has poor compatibility with water.
  • n-butanol, n-pentanol, n-hexanol, n-octanol and the like are used in the present invention as the solvent satisfying these requirements.
  • the hydrophobisized silica wet gel prepared in the hydrophobicity and solvent substitution is dried to obtain a bead type aerogel.
  • the drying of the silica wet gel may also be performed under an atmospheric pressure or a reduced pressure.
  • the drying may be performed at a temperature of approximately 100 to 250°C (degrees centigrade), and preferably 120 to 150°C (degrees centigrade).
  • the drying rate is very slow, whereas when the silica wet gel is dried at a temperature greater than 250 ° C (degrees centigrade), a surface- modified silylating group may be lost due to its thermal cracking.
  • a suitable time to dry a wet gel may be varied according to the structure and particle size of obtained aerogel, the kind of used solvents, the content of remainders in the gel structure, etc.
  • the optimum drying time may be determined by measuring a time that a remaining solvent is not detected from the dried silica particles using a therraogravimetric analyzer (TGA).
  • TGA therraogravimetric analyzer
  • the drying process may optionally include: completely removing moisture from the wet gel, evaporating a solvent from a reduced-pressure distilling apparatus to such an extent that a weight ratio of a solid material and the solvent is in a range of 1:3 to 1:5, transferring the wet gel to a dehydrator to remove the solvent as large as possible, and drying the wet gel in a shelf-type (atmospheric-pressure or reduced-pressure) oven dryer.
  • the wet gel When the weight ratio of the solid material and the solvent is less than 1:3, the wet gel may be excessively powdered, whereas when the weight ratio of the solid material and the solvent exceeds 1:5, unnecessary energies may be increasingly consumed.
  • the method has a problem that two operations are additionally required compared to the process of closing a reflux line and drying a wet gel, but has an advantage that an energy that is used to completely evaporate a solvent from a large number of fine pores in the silica may be reduced by passing a wet gel through a physical dehydrator.
  • the drying of the wet gel by the closing the reflux line and the drying of the wet gel using a shelf-type oven dryer may be used.
  • the drying time is too long, whereas when the wet gel is dried at a temperature over 200°C (degrees centigrade), a surface-hyhydrophobizied silylating group may be lost due to its thermal cracking.
  • a suitable time to dry a wet gel may be determined as a time point that there is no solvent used in the formed aerogel.
  • the surface-modified bead type aerogel prepared from the method according to one exemplary embodiment of the present invention has a particle size of 0.1 to 1.0 cm (centimeters), preferably 0.1 to 0.8 cm (centimeters), and more preferably 0.1 to 0.5 cm (centimeters), and has a
  • the sodium component in the water glass solution is completely exchanged and removed by the ion exchange process, and the water glass solution is gradually changed from an alkaline state to an acidic state, that is, the pH of the water glass solution is reduced to a pH range of 3 to 4.
  • the water glass solution is then solated.
  • 4 % (percent) by weight of an ammonia solution was added to the solated water glass solution to adjust pH of the sodium-free water glass solution to a pH range of 4 to 6, thus to form a silica wet gel.
  • silica wet gel was aged at a room temperature, silica particles started to be formed, and a slightly bluish transparent silica wet gel was then formed within approximately 2 hours.
  • the formed wet gel was sliced into bead type pieces with a diameter of approximately 0.5 to 8 cm (centimeters), and the bead type wet gel was carefully kept to prevent the bead type wet gel from being broken, and added to 1500 ml (milliliters) of n-butanol, in which 5 % (percent) by weight of trimethoxymethylsilane (TMMS) is dissolved, thus to perform a surface modification (silylation) and a solvent substitution.
  • TMMS trimethoxymethylsilane
  • the used solvent was recovered under the same conditions as described above. Then, the recovered solvent was re-refluxed for 20 minutes to completely separate and remove moisture from the wet gel, thus to surface- modify a silica wet gel. In this case, moisture may be effectively removed since phase separation between water and a hydrophobic solvent (n-butanol) in the wet gel appears very clearly.
  • a reflux line was closed, and the wet gel was continuously distilled under a reduced pressure to evaporate a solvent.
  • a reflux line was closed, and the wet gel continued to be refluxed at 50 to 55°C (degrees centigrade) under a pressure of 120 mmHg (millimeters of mercury) to obtain approximately 53 g (grams) of a bead type aerogel whose surface is hydrophobisized with si lane within 10 to 20 minutes.
  • the hydrophobic solvent including n-butanol and trimethoxymethylsilane (TMMS) was recovered and re-used.
  • the bead type aerogel prepared in this example was shown in FIG. 2. It was revealed that the bead type aerogel prepared in this example has a specific surface area of 920 raVg (square meters per gram) and a particle size of 0.1 to 1 cm (centimeters) .
  • the sodium component in the water glass solution is completely exchanged and removed by the ion exchange process, and the water glass solution is gradually changed from an alkaline state to an acidic state, that is, the pH of the water glass solution is reduced to a pH range of 3 to 4.
  • the water glass solution is then solated.
  • 4 % (percent) by weight of an ammonia water was added to the solated water glass solution to adjust pH of the sodium-free water glass solution to a pH range of 4 to 6, thus to form a silica wet gel.
  • the sodium-free water glass solution i.e.
  • sodium-free water glass solution in a state of right before the formation of silica wet gel was added dropwise to a 90°C (degrees centigrade) edible oil extracted from corn.
  • Droplets in the form of solution were changed into a gel state immediately (within approximately 30 minutes) due to the temperature of the edible oil to form a bead type wet gel having a constant size. Since the formed bead type wet gel was precipitated from the oil, it was easily separated from the oil, and the oil was recovered and reused.
  • the resulting bead type wet gel was carefully kept to prevent the bead type wet gel from being broken, and added to 1500 ml (milliliters) of n- butanol, in which 3.5 % (percent) by weight of methoxytrimethylsilane (MTMS) is dissolved, thus to perform a surface modification (silylation) and a solvent substitution at the same time.
  • MTMS methoxytrimethylsilane
  • the used solvent was recovered under the same conditions as described above. Then, the recovered solvent was re-refluxed for 20 minutes to completely separate and remove moisture from the wet gel, thus to surface- modify a silica wet gel. In this case, moisture may be effectively removed since phase separation between water and a hydrophobic solvent (n-butanol) in the wet gel appears very clearly.
  • Example 3 500 ml (milliliters) of a 10%(percent by weight) water glass solution, which was prepared by diluting a water glass solution (based on 35% (percent) of a sodium silicate solution) with distilled water, was allowed to flow through a column filled with 1 L (liter) of a cation exchange resin Aemberlite IR-120H (commercially available from Aldrich) to perform cation exchange of sodium in a water glass. In this case, the water glass solution was in repeated contact with a cation exchange resin until a sodium component is not detected in the water glass solution passed through the column filled with the cation exchange resin.
  • a cation exchange resin Aemberlite IR-120H (commercially available from Aldrich)
  • the sodium component in the water glass solution is completely exchanged and removed by the ion exchange process, and the water glass solution is gradually changed from an alkaline state to an acidic state, that is, the pH of the water glass solution is reduced to a pH range of 3 to 4.
  • the water glass solution is then solated.
  • 4 % (percent) by weight of an ammonia solution was added to the solated water glass solution to adjust pH of the sodium-free water glass solution to a pH range of 4 to 6, thus to form a silica wet gel.
  • the sodium-free water glass solution i.e.
  • sodium-free water glass solution in a state of right before the formation of silica wet gel was added dropwise to a 90 0 C (degrees centigrade) edible oil extracted from corn.
  • Droplets in the form of solution were changed into a gel state immediately (within approximately 30 minutes) due to the temperature of the edible oil to form a bead type wet gel having a constant size. Since the formed bead type wet gel was precipitated in the oil, it was easily separated from the oil, and the oil was recovered and re ⁇ used.
  • the resulting bead type wet gel was carefully kept to prevent the bead type wet gel from being broken, added to 3000 ml (milliliters) of n-butanol, in which 3.5 % (percent) by weight of methoxytrimethylsilane (MTMS) is dissolved, and then refluxed at 120 to 150°C (degrees centigrade) for 6 to 7 hours to perform its hydrophobicity and solvent substitution processes.
  • MTMS methoxytrimethylsilane
  • a time point that moisture is separated from the solvent is when approximately 1000 ml (milliliters) of n-butanol in addition to the wet gel was present in the reactor, and therefore the solvent in a condenser should be adjusted to a constant solvent content so as to maintain a content of n-butanol in the reactor to a content range of approximately 1000 ml (milliliters).
  • the content of n-butanol was adjusted to approximately 1000 ml (milliliters) from the beginning, it was impossible to mix the n-butanol with the wet gel.
  • the silica wet gel was filtered and dried at 150°C (degrees centigrade) for 2 hours under an atmospheric pressure.
  • a surface-modified bead type aerogel was obtained in a yield of approximately 50 g (grams), and the n-butanol and methoxytrimethylsilane (MTMS) were recovered during the filtration process, and re-used. It was revealed that the bead type aerogel prepared in this example has a specific surface area of 930 mVg (square meters per gram) and a particle size of 0.1 to 0.5 cm (centimeters) .

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Abstract

There is provided a method for preparing a surface-modified transparent bead type aerogel having permanent hydrophobicity without any of additional impurity removal and washing processes by removing a sodium component from a water glass in advance, and a surface-modified bead type aerogel prepared therefrom. The method for preparing a surface-hydrophobisized bead type aerogel includes: removing a sodium component from a water glass solution, in which a water glass is diluted to a concentration of 5 to 15 % (percent) by weight, by contacting a cation exchange resin with the water glass solution at a room temperature; forming a silica wet gel by adjusting pH of the sodium-free water glass solution to a pH range of 4-6 and aging the sodium-free water glass solution; cutting the formed silica wet gel into the form of beads, when necessary; adding the cut silica wet gel to a silylation solution comprising a hydrophobic alcohol solvent and a silylating agent, and simultaneously performing silylation and solvent substitution processes on the cut silica wet gel; and drying the silica wet gel. Also, the bead type aerogel prepared from the method has a permanent hydrophobicity, and a particle size of 0.1 to 0.5 cm (centimenters). The bead type aerogel may be used for an insulating panel of a LNG carrier, transparent dual glass, etc. since it has porosity, high transparency and high specific surface area.

Description

[DESCRIPTION] [Invention Title]
METHOD FOR PREPARING SURFACE-MODIFIED TRANSPARENT BEAD TYPE AEROGEL AND AEROGEL PREPARED THEREFROM [Technical Field]
The present invention relates to a method for preparing a surface- modified transparent bead type aerogel having permanent hydrophobicity, and a bead type aerogel prepared therefrom, and more particularly, to a method for preparing a surface-modified transparent bead type aerogel without any of additional impurity removal and washing processes by removing a sodium component from a water glass in advance, and a surface-modified bead type aerogel prepared therefrom.
[Background Art]
Recently, concerns regarding aerogels have been raised with development of industrial technologies. In general, there have been attempts to prepare a powdery aerogel. This is why the powdery aerogel is easily applicable to various fields since it is easily used as a filler in various functional plastic products such as functional (insulating) paints, and mixed homogeneously. However, this powdery aerogel has disadvantages that it is not transparent enough, and dusts and the like are raised in the use of a large amount of powdery aerogel. For this reason, a bead type aerogel is required in certain fields. The bead type aerogel is an ultralow-density advanced material having very excellent transparency, a porosity, of 90 % (percent) or more as in powder, and a specific surface area of several hundreds up to approximately 1500 m Ig, and also has a nanoporous structure.
As a result, the bead type aerogel may be used in the fields, such as super-insulating dual glass, which has limited the use of powdery aerogel since the bead type aerogel should satisfy the requirement of transparency, extending to the field of heat-insulating materials of a LNG carrier, building insulation materials, soundproof materials, etc., in which the use of the powder has been restricted since dusts and the like are raised in the use of the powder. Since the bead type aerogel has the same low heat conductivity as the powder, it is a very useful heat-insulating material that may be used in the field of ultra-low dielectrics, catalyst, electrode material, soundproof material, etc. or in the filed of a refrigerator, an automobile, an aircraft, etc.
The bead type aerogel may be manufactured using various methods. For example, W095/06617 describes that hydrophobic silicic acid aerogels are obtained by reacting a solution of water glass with sulfuric acid at a pH value from 7.5 to 11; removing NaCl components from the obtained silicic acid hydrogel by washing it with water or diluted aqueous solutions of inorganic bases (sodium hydroxide or ammonia) at a pH value from 7.5 to 11; removing water with an exchange of alcohol; and drying the obtained alcohol gel at 240 to 280°C (degrees centigrade) under a supercritical condition of 55 to 90 bar, and also discloses a method for preparing hydrophobic silicic acid aerogels through a supercritical drying process without a siIyIation process. In order to obtain a pure silica from the silica wet gel obtained by reacting a solution of water glass with an acid, sodium chloride that is formed as a by-product should, however, be removed from the silica wet gel. Since, a bead type of the silica gel may be damaged due to the presence of the NaCl removal process, the bead type aerogel is difficult to be obtained, and its transparency may be spoiled.
[Disclosure]
[Technical Problem]
An aspect of the present invention provides a novel method for preparing a surface-modified bead type aerogel having permanent hydrophobicity and transparency.
Another aspect of the present invention provides a method for preparing a surface-modified bead type aerogel having excellent properties such as insulating property, soundproofing property and high specific surface area that the conventional powder aerogels have.
Still another aspect of the present invention provides a surface- modified bead type aerogel prepared from the method according to one exemplary embodiment of the present invention, the surface-modified bead type aerogel having a high specific surface area, as well as excellent properties such as insulating property, soundproofing property, permanent hydrophobicity and transparency.
[Technical Solution]
According to an aspect of the present invention, there is provided a method for preparing a surface-modified bead type aerogel having permanent hydrophobicity, comprising: a) removing a sodium component from a water glass solution, in which a water glass is diluted to a concentration of 5 to 15 % (percent) by weight, by contacting a cation exchange resin with the water glass solution at a room temperature; b) forming a silica wet gel by adjusting pH of the sodium-free water glass solution to a pH range of 4-6 and aging the sodium-free water glass solution! c) cutting the formed silica wet gel into the form of beads; d) adding the cut silica wet gel to a silylation solution comprising a hydrophobic alcohol solvent and a silylating agent, and simultaneously performing silylation and solvent substitution processes on the cut silica wet gel ; and e) drying the silica wet gel.
According to another aspect of the present invention, there is provided a method for preparing a surface-modified bead type aerogel having permanent hydrophobicity, comprising: a) removing a sodium component from a water glass solution, in which a water glass is diluted to a concentration of 5 to 15 % (percent) by weight, by contacting a cation exchange resin with the water glass solution at a room temperature; b) forming a silica wet gel by adjusting pH of the sodium-free water glass solution to a pH range of 4 to 6 and aging the sodium-free water glass solution by adding the sodium-free water glass solution in a state of right before the formation of silica wet gel dropwise to a 70 to 90°C (degrees centigrade) oil ; c) adding the formed silica wet gel to a silylation solution comprising a hydrophobic alcohol solvent and a silylating agent, and simultaneously performing silylation and solvent substitution processes on the formed silica wet gel ; and d) drying the silica wet gel.
According to still another aspect of the present invention, there is provided a surface-modified bead type aerogel prepared according to the method according to one exemplary embodiment of the present invention, the bead type aerogel having a particle size of 0.1 to 0.5 cm (centimeters).
[Advantageous Effects]
As described above, the formation of sodium chloride (NaCl) that is formed as a by-product may be prevented in the formation of a silica wet gel by a conventional reaction of a water glass with hydrochloric acid by using a cation exchange resin to remove a sodium component from a solution of water glass in advance. Therefore, the method of the present invention does not require an additional NaCl removal process. As a result, it is possible to obtain a bead type aerogel having excellent transparency since the damage in a shape of the bead type silica gel caused by the conventional NaCl removal process may be prevented. Also, the surface-modified bead type aerogel prepared from the method according to one exemplary embodiment of the present invention has excellent properties such as insulating property, soundproofing property and transparency. In particular, the bead type aerogel prepared from the method according on exemplary embodiment of the present invention has a solid structure with a particle size of approximately 0.1 to 0.5 cm (centimeters), and also has a high porosity and a high specific surface area of 600 to 1300 mVg (square meters per gram) .
The bead type aerogel according to one exemplary embodiment of the present invention is easy to handle and has excellent transparency, as well as an insulating property corresponding to the conventional powder aerogels, and therefore it may be easily used to manufacture dual glasses requiring transparency, ultra-low dielectrics, catalysts, electrode materials, soundproof materials, building heat-insulating materials, insulating panels of an LNG carrier, etc.
[Description of Drawings]
FIG. 1 is a schematic diagram illustrating the removal of sodium using an ion exchange resin.
FIG. 2 is a photograph illustrating a bead type aerogel prepared in Example 1.
FIG. 3 is a photograph illustrating a bead type aerogel prepared in Example 2.
[Best Mode]
The present invention is characterized in that a lifespan of aerogel is extended by surface-modifying (permanently hydrophobising) the aerogel in order to protect the aerogel from moisture in the air that obstructs a porous structure of the aerogel. Also, it is characterized in that the aerogel is prepared in the form of bead so that it can be easy to apply to the field of various applications. In addition, the present invention is characterized in that the aerogel may be economically and easily prepared by performing silylation and solvent substitution(exchange) processes at the same time. The surface-modified bead type aerogel prepared from the method according to one exemplary embodiment of the present invention has excellent insulating property, high porosity, high specific surface area, soundproofing property and transparency.
An embodiment of the present invention is related to a method for preparing a hydrophobically surface-modified bead type aerogel using a water glass. A major component of water glass is sodium silicate (SiC>2#Na2θ), but a sodium (Na) component should be removed from the sodium silicate so as to obtain a pure silica component.
In an embodiment of the present invention, a sodium component is removed from the water glass using a cation exchange resin. That is to say, a water glass is diluted to a constant concentration, and passed through a cation exchange resin to exchange a sodium (Na) component in the water glass with cation such as hydrogen, etc., which leads to the removal of the sodium component in the water glass.
pH of a water glass solution is reduced since the sodium component is removed from the water glass and exchanged with a hydrogen component. Particularly, the pH of a water glass solution is in a range of approximately pH 3 to 4, and silica is solated within this pH range. Then, the pH of the sodium-free water glass solution is adjusted to a suitable pH range to form a wet gel, and aged to obtain a transparent silica wet gel.
As shown in FIG. 1, when a water glass (sodium silicate) is diluted to a concentration of 5-15 % (percent) by weight, and the diluted water glass solution is in contact with a cation exchange resin, a sodium component in the water glass is substituted with hydrogen as represented by the following Formula 1, and removed from the water glass solution to obtain pure silica. The water glass may be generally diluted with water or distilled water, etc. When the concentration of the water glass is less than 5 % (percent) by weight, a reaction apparatus is too large compared to an amount of the obtained silica, which leads to the inefficient manufacturing process. On the contrary, when the concentration of the water glass exceeds 15 % (percent) by weight, it is difficult to easily exchange components in the cation exchange resin in a short time. The water glass becomes in contact with the cation exchange resin until the sodium component is completely removed from the water glass.
Formula 1
(ReSiTi)-SO3-H+ + Na2O-SiO2 → H9O + SiO,
Figure imgf000009_0001
Polystyrene-divinylbenzene copolymers having a sulfonate group (-SO3H), more particularly Diaion or Aemberlite-based strong acid resins, and even more particularly at least one of SKlB, SKIlO, SK112, IR-120H, IR-120B, IR- 120L, IR-122, IR-124 may be used as the cation exchange resin.
pH of the water glass solution that is subjected to the cation exchange process is in a range of approximately pH 3 to 4. A silica wet gel is formed by adjusting the pH of the water glass solution to a pH range of approximately 4 to 6. This is why a wet gel is effectively formed within the pH range of approximately 4 to 6, and the pH of the water glass may be adjusted, for example, by adding an ammonia solution, but the present invention is not particularly limited thereto. When the water glass solution is aged within the pH range, silica in the water glass solution is polymerized to form a wet gel. The water glass solution may be aged at approximately 20 to 40°C (degrees centigrade) for approximately 2 to 6 hours to obtain a wet gel .
When the aging temperature of the water glass solution is below 20°C (degrees centigrade), the aging time of the water glass solution is too long. On the contrary, the aging temperature of the water glass solution may be shortened in the temperature (i.e. 50 to 60°C (degrees centigrade)) exceeding 40°C (degrees centigrade), but silica particles are conglomerated since silica is suddenly formed in a too high temperature, which makes it impossible to obtain a bead type aerogel silica having a high specific surface area. Therefore, it is desirable to age the water glass solution at 20 to 40°C (degrees centigrade) for approximately 2 to 6 hours in consideration of the wet gel formation efficiency.
Meanwhile, the bead type silica aerogel obtained from the above- mentioned silica wet gel is not uniform in shape, and difficult to form in the form of perfect bead. Therefore, a silica wet gel-forming solutionCwater glass solution) adjusted to a pH range of 4 to 6 in a state of right before the formation of wet gel is added dropwise to a 70 to 90°C (degrees centigrade) oil and aged in the oil to obtain bead type aerogel with more uniform size and shape. The expression "in a state of right before the formation of wet gel" used in this specification means a state of a water glass solution in which a wet gel starts to be formed at pH 4-6 and is formed to some degree within approximately 30 minutes to 1 hour, but is still in a liquid state.
In general, it takes approximately 2 to 6 hours to form a wet gel in the wet gel-forming solution at approximately 20 to 40°C (degrees centigrade), as described above, but the aging time is shortened in a high- temperature oil. When the oil has a temperature of 70 to 90"C (degrees centigrade), a wet gel is generally formed within approximately 30 minutes to 4 hours. When the temperature of the oil is below 70°C (degrees centigrade), the aging time in which a droplet is converted into a wet gel is too long. In this case, when a small reactor is used, silica conglomerated before the droplet is converted into a wet gel, thus to form a non-uniform bead type wet gel. When the temperature of the oil exceeds 90"C (degrees centigrade), the droplets of the water glass solution may be undesirably broken. Meanwhile, when the wet gel-forming solutionCwater glass solution) is aged in the oil, the conglomeration and/or growth of silica particles are suppressed even at a high temperature since a water glass solution in a state of right before the formation of wet gel (i.e. in a state of a wet gel is formed to some degree) is added to the oil, thus to obtain a silica wet gel having a relatively high specific surface area. Also, when the temperature of the oil exceeds 90°C (degrees centigrade), the wet gel-forming solution is aged very rapidly, which leads to a low specific surface area of some wet gel. When a uniform bead type silica wet gel is formed as described above, the bead type silica wet gel is separated from the oil, and the oil is recovered and re-used. Any of animal or vegetable edible oils and the like that are widely known in the art may be used as the oil, and specific examples of the oil that may be used herein include, but are particularly limited to, vegetable oils extracted from corn, olive, bean, cottonseed, etc., animal oils extracted from cattle, pig, fish, etc. Chemically synthetic oils may be also used as the oil.
Silica particles are polymerized into a silica backbone while being grown into nanoparticles during the aging, and the silica backbone is then formed into a silica wet gel having a high specific surface area. The silica wet gel is subject to the above-mentioned operation to obtain a silica wet gel having a high specific surface area of 600 nf/g (square meters per gram) or more, and preferably 600 to 130OmVg (square meters per gram).
The silica wet gel prepared thus is composed of silica particles forming a backbone, and a large amount of water. In this case, it is very important to effectively remove water without contraction of the silica particles forming a backbone. The pH of the water glass solution is reduced since a sodium component in the water glass is substituted with hydrogen by means of the cation exchange resin and removed from the water glass according to the method of the present invention. Therefore, the method of the present invention does not require a reaction with hydrochloric acid as described in the method for preparing a silica wet gel using a conventional water glass. Also, by-products such as NaCl, which are formed together by the reaction with hydrochloric acid in the conventional method for preparing a silica wet gel, are not formed in the method of an embodiment in the present invention, the method of an embodiment in the present invention does not need a separate washing process, and therefore damages in powdering and transparency of wet gel in the washing process may be prevented.
The obtained silica wet gel is translucent, and has transparency. When the silica wet gel is pulverized, the transparency of the silica wet gel is lost due to the broken backbone structure of silica gel in the wet gel. Therefore, in order to prepare a transparent bead type aerogel, a wet gel should be sliced (cut) into bead type pieces and added into a silylation solution used in the next silylation and solvent substitution processes. Meanwhile, since the obtained wet gel in the oil has its bead type shape, it is unnecessary to slice the wet gel into bead type pieces.
The hydrophobicity (silylation) and solvent substitution processes are performed at the same time in the present invention. That is to say, the bead type silica wet gel is added to a silylation solution (a solution comprising a hydrophobic alcohol solvent and a silylating agent), and refluxed to hydrophobically modify a surface of the silica wet gel through the silylation, and to remove water from the silica wet gel through its substitution with a solvent.
The hydrophobicity and solvent substitution processes may be performed at a temperature around a boiling temperature of the silylation solution for approximately 2 to 24 hours under an atmospheric pressure. When the reflux time is less than 2 hours, the silica wet gel is not sufficiently silylated according to the kind of the silylating agents, whereas when the reflux time exceeds 24 hours, undesired side reactions may occur. Also, the hydrophobicity and solvent substitution processes may be performed under a reduced pressure. The hydrophobicity and solvent substitution processes under the reduced pressure may be performed at the same time by adjusting a pressure of a reactor containing the silylation solution and the wet gel to a pressure range of 30 to 200 mmHg (millimeters of mercury) and refluxing the reaction solution(reactants~containing solution) at 45 to 60°C (degrees centigrade). The hydrophobicity and solvent substitution processes under the reduced pressure may be shortened, and particularly completed within 60 minutes, compared to those under an atmospheric pressure. When the pressure in the reactor is less than 200 mmHg (millimeters of mercury), the reaction time may be shortened within 60 minutes, and the reaction rate becomes faster with a decreasing pressure in the reactor. However, it is substantially difficult to perform the processes under a pressure of 30 mmHg (millimeters of mercury) or less due to the loss in pressures of connection lines or a vacuum pump connected to the reactor. A temperature of the reactor is maintained to a temperature range of 45 to 60°C (degrees centigrade). This is why the temperature of the reactor is spontaneously reduced due to the rapid evaporation of heat from the solvent, and it is desirable to supply heat from the outside in order to facilitate the rapid reaction rate.
In the si IyIation/solvent substitution co-process, moisture is removed from the silica wet gel and the wet gel is surface-hydrophobisized with a silylating agent, for example, by putting a mixture of the silica wet gel and a silylation solution comprising the hydrophobic alcohol solvent and a silylating agent into a reactor, separating a solvent and water from the resulting mixture while refluxing the mixture under a temperature of the reactor from 45 to 60°C (degrees centigrade) and a pressure of the reactor from 200 to 30 mmHg (millimeters of mercury), thus to remove the moisture and re-reflux the solvent, followed by repeating the operations until the moisture is completely removed from the silica wet gel. The discharged solvent is separated using a cooling tube or a centrifuge, and re-used in the si IyIation/solvent substitution co-process.
The silylation solution used in the silylation (hydrophobicity) and solvent substitution processes is a mixture comprising 1 to 10 % (percent) by weight of a silylating agent and 90 to 99 % (percent) by weight of a hydrophobic alcohol solvent. When the content of the silylating agent in the silylation solution is less than 1 % (percent) by weight, surface-unmodified aerogel may be formed, whereas when the content of the silylating agent in the silylation solution exceeds 10 % (percent) by weight, the non-reacted silylating agent may be present, which is made undesirable in an economical aspect .
A si lane compound represented by Formula of Ri 4-n-SiXn (wherein, n is
integer from 1 to 3; Ri is hydrogen or Cl-ClO, preferably C1-C5 alkyl , aromatic alkyl, or heteroaromatic alkyl; and X is halogen selected from the group consisting of F, Cl, Br and I, and preferably Cl, or Cl-ClO, preferably C1-C5 alkoxy group, aromatic alkoxy group, or heteroaromatic alkoxy group), and/or disiloxane represented by Formula of RsSi-O-SiRs (wherein, two R3 groups are identical to, or different from each other, and are Cl-ClO alkyl, preferably C1-C5 alkyl, aromatic alkyl, hetero aromatic alkyl, or hydrogen) may be used as the silylating agent.
Specific examples of the silylating agent include, but are not particularly limited to, hexamethyldisilane, ethyltriethoxysilane, trimethoxysilane, triethylethoxysilane, methyltrimethoxysi lane, ethyltrimethoxysi 1ane, methoxytrimethylsi 1ane, trimethy1chlorosi 1ane, hexamethyldisiloxane (HMDSO) and triethylchlorosilane, and they may be used alone or in combinations thereof.
At least one selected from the group consisting of n-butanol, n- pentanol, n-hexanol and n-octanol may be used as the hydrophobic alcohol solvent.
When the silica wet gel is subject to the above-mentioned silylation process, it has a heat-insulation equal to or greater than that of the conventional aerogel powder even when the silylating agent is used at a low content, and a surface of the silica wet gel is permanently silylated as represented by the following Formula 2 since a silylation reaction condition is modified to a strong acid condition to get rid of powder that does not react with the silylating agent.
Formula 2
Figure imgf000015_0001
A continuous process is possible by performing the silylation and solvent substitution at the same time, and the used hydrophobic alcohol solvent may be transferred to a distillation process and used again for simultaneous process of silylation and solvent substitution.
In general, the solvent that may be used for the substitution of a wet gel with a solvent in the preparation of aerogel should satisfy requirements of (1) effectively removing water from pores of the wet gel, and (2) evaporating a solvent while applying a possibly low capillary pressure to a gel structure when the wet gel is dried under an atmospheric pressure. That is to say, the solvent has polarity as high as possible so as to satisfy the requirement of (1), and the solvent has a low surface tension, that is, should be nonpolar if possible so as to satisfy the requirement of (2). In order to satisfy the two conflicting requirements, the solvent should not be high in polarity as in methanol, ethanol, THF, acetone, etc., and also be not nonpolar as in heptane, pentane, etc. That is to say, when the solvent is polar as in the former, a very high capillary pressure generated in a gas- liquid interface during the drying may be applied to the gel structure, and, when the solvent is nonpolar as in the latter, it is difficult to effectively remove water from the pores of the wet gel since it has poor compatibility with water.
Therefore, n-butanol, n-pentanol, n-hexanol, n-octanol and the like are used in the present invention as the solvent satisfying these requirements.
The hydrophobisized silica wet gel prepared in the hydrophobicity and solvent substitution is dried to obtain a bead type aerogel. The drying of the silica wet gel may also be performed under an atmospheric pressure or a reduced pressure.
Where the silica wet gel is dried under an atmospheric pressure, the drying may be performed at a temperature of approximately 100 to 250°C (degrees centigrade), and preferably 120 to 150°C (degrees centigrade). When the silica wet gel is dried at a temperature less than 100°C (degrees centigrade), the drying rate is very slow, whereas when the silica wet gel is dried at a temperature greater than 250°C (degrees centigrade), a surface- modified silylating group may be lost due to its thermal cracking. A suitable time to dry a wet gel may be varied according to the structure and particle size of obtained aerogel, the kind of used solvents, the content of remainders in the gel structure, etc. Therefore, the optimum drying time may be determined by measuring a time that a remaining solvent is not detected from the dried silica particles using a therraogravimetric analyzer (TGA). Where the silica wet gel is dried under a reduced pressure, a reflux line is closed when moisture is completely removed in the hydrophobicity and solvent substitution processes, and the silica wet gel may then be dried within 20 minutes by maintaining the pressure and temperature of the reactor to the same conditions, such as a pressure of 30 to 200 mmHg (millimeters of mercury) and a temperature of 45 to 60°C (degrees centigrade), as in the hydrophobicity and solvent substitution processes, thus to obtain a bead type aerogel .
When the silica wet gel is dried under a reduced pressure, the drying process may optionally include: completely removing moisture from the wet gel, evaporating a solvent from a reduced-pressure distilling apparatus to such an extent that a weight ratio of a solid material and the solvent is in a range of 1:3 to 1:5, transferring the wet gel to a dehydrator to remove the solvent as large as possible, and drying the wet gel in a shelf-type (atmospheric-pressure or reduced-pressure) oven dryer.
When the weight ratio of the solid material and the solvent is less than 1:3, the wet gel may be excessively powdered, whereas when the weight ratio of the solid material and the solvent exceeds 1:5, unnecessary energies may be increasingly consumed. The method has a problem that two operations are additionally required compared to the process of closing a reflux line and drying a wet gel, but has an advantage that an energy that is used to completely evaporate a solvent from a large number of fine pores in the silica may be reduced by passing a wet gel through a physical dehydrator.
Optionally the drying of the wet gel by the closing the reflux line and the drying of the wet gel using a shelf-type oven dryer may be used. For example, it is desirable in aspect of energy efficiency to dry a wet gel at a temperature of 140 to 200°C (degrees centigrade) in a shelf-type oven dryer under an atmospheric pressure. When the wet gel is dried at a temperature below 140°C (degrees centigrade), the drying time is too long, whereas when the wet gel is dried at a temperature over 200°C (degrees centigrade), a surface-hyhydrophobizied silylating group may be lost due to its thermal cracking. Finally, a suitable time to dry a wet gel may be determined as a time point that there is no solvent used in the formed aerogel.
By performing a drying process after the silylation and solvent substitution process according to an embodiment in the present invention, a structure of the surface-modified aerogel is permanently maintained, and its drying rate is rapid. The surface-modified bead type aerogel prepared from the method according to one exemplary embodiment of the present invention has a particle size of 0.1 to 1.0 cm (centimeters), preferably 0.1 to 0.8 cm (centimeters), and more preferably 0.1 to 0.5 cm (centimeters), and has a
2 high specific surface area of 600 to 1300 m /g (square meters per gram).
[Mode for Invention]
Hereinafter, the present invention will be described in more detail referring to the exemplary embodiments of the present invention. However, it should be understood that the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention.
Example 1
500 ml (milliliters) of a water glass solution containing 10 % (percent) by weight of water glass (i.e. 10 %(percent) by weight water glass solution), which was prepared by diluting a water glass solution (based on 35% (percent) of a sodium silicate solution) with distilled water, was allowed to flow through a column filled with 1 L (liter) of a cation exchange resin Diaion SKl BH (commercially available from Aldrich) to perform cation exchange of sodium in a water glass. In this case, the water glass solution was in repeated contact with a cation exchange resin until a sodium component is not detected in the water glass solution passed through the column filled with the cation exchange resin. The sodium component in the water glass solution is completely exchanged and removed by the ion exchange process, and the water glass solution is gradually changed from an alkaline state to an acidic state, that is, the pH of the water glass solution is reduced to a pH range of 3 to 4. In this case, the water glass solution is then solated. Then, 4 % (percent) by weight of an ammonia solution was added to the solated water glass solution to adjust pH of the sodium-free water glass solution to a pH range of 4 to 6, thus to form a silica wet gel. When the resulting solution of silica wet gel was aged at a room temperature, silica particles started to be formed, and a slightly bluish transparent silica wet gel was then formed within approximately 2 hours. The formed wet gel was sliced into bead type pieces with a diameter of approximately 0.5 to 8 cm (centimeters), and the bead type wet gel was carefully kept to prevent the bead type wet gel from being broken, and added to 1500 ml (milliliters) of n-butanol, in which 5 % (percent) by weight of trimethoxymethylsilane (TMMS) is dissolved, thus to perform a surface modification (silylation) and a solvent substitution.
Moisture was separated and removed in a condenser by refluxing the resulting reaction mixture under a temperature of a reactor from 50 to 55°C (degrees centigrade) and a pressure of the reactor of 120 mmHg (millimeters of mercury) during the modification and a solvent substitution processes. At the same time, the used solvent was recovered under the same conditions as described above. Then, the recovered solvent was re-refluxed for 20 minutes to completely separate and remove moisture from the wet gel, thus to surface- modify a silica wet gel. In this case, moisture may be effectively removed since phase separation between water and a hydrophobic solvent (n-butanol) in the wet gel appears very clearly. Then, when the moisture was completely removed, a reflux line was closed, and the wet gel was continuously distilled under a reduced pressure to evaporate a solvent. A reflux line was closed, and the wet gel continued to be refluxed at 50 to 55°C (degrees centigrade) under a pressure of 120 mmHg (millimeters of mercury) to obtain approximately 53 g (grams) of a bead type aerogel whose surface is hydrophobisized with si lane within 10 to 20 minutes. The hydrophobic solvent including n-butanol and trimethoxymethylsilane (TMMS) was recovered and re-used. The bead type aerogel prepared in this example was shown in FIG. 2. It was revealed that the bead type aerogel prepared in this example has a specific surface area of 920 raVg (square meters per gram) and a particle size of 0.1 to 1 cm (centimeters) .
Example 2
500 ml (milliliters) of a 10%(percent by weight) water glass solution (based on 35% (percent) of a sodium silicate solution) with distilled water, was allowed to flow through a column filled with 1 L (liter) of a cation exchange resin Aemberlite IR-120H (commercially available from Aldrich) to perform cation exchange of sodium in a water glass. In this case, the water glass solution was in repeated contact with a cation exchange resin until a sodium component is not detected in the water glass solution passed through the column filled with the cation exchange resin. The sodium component in the water glass solution is completely exchanged and removed by the ion exchange process, and the water glass solution is gradually changed from an alkaline state to an acidic state, that is, the pH of the water glass solution is reduced to a pH range of 3 to 4. In this case, the water glass solution is then solated. Then, 4 % (percent) by weight of an ammonia water was added to the solated water glass solution to adjust pH of the sodium-free water glass solution to a pH range of 4 to 6, thus to form a silica wet gel. Right before a wet gel is formed after approximately 30 minutes, the sodium-free water glass solution (i.e. sodium-free water glass solution in a state of right before the formation of silica wet gel) was added dropwise to a 90°C (degrees centigrade) edible oil extracted from corn. Droplets in the form of solution were changed into a gel state immediately (within approximately 30 minutes) due to the temperature of the edible oil to form a bead type wet gel having a constant size. Since the formed bead type wet gel was precipitated from the oil, it was easily separated from the oil, and the oil was recovered and reused. The resulting bead type wet gel was carefully kept to prevent the bead type wet gel from being broken, and added to 1500 ml (milliliters) of n- butanol, in which 3.5 % (percent) by weight of methoxytrimethylsilane (MTMS) is dissolved, thus to perform a surface modification (silylation) and a solvent substitution at the same time.
Moisture was separated and removed in a condenser by refluxing the resulting reaction mixture under a temperature of a reactor from 50 to 55°C (degrees centigrade) and a pressure of the reactor of 120 mmHg (millimeters of mercury) during the modification and a solvent substitution processes. At the same time, the used solvent was recovered under the same conditions as described above. Then, the recovered solvent was re-refluxed for 20 minutes to completely separate and remove moisture from the wet gel, thus to surface- modify a silica wet gel. In this case, moisture may be effectively removed since phase separation between water and a hydrophobic solvent (n-butanol) in the wet gel appears very clearly. Then, when the moisture was completely removed, a reflux line was closed, and the wet gel was continuously distilled at 50 to 55°C (degrees centigrade) under a pressure of 120 mmHg (millimeters of mercury) to evaporate a solvent. A reflux line was closed, and the wet gel continued to be distilled under a reduced pressure at the same conditions to obtain approximately 50 g (grams) of a bead type aerogel whose surface is hydrophobisized with si lane within 10 to 20 minutes. The hydrophobic solvent including n-butanol and trimethoxymethylsilane (TMMS) was recovered and re¬ used. The bead type aerogel prepared in this example was shown in FIG. 3. It was revealed that the bead type aerogel prepared in this example has a specific surface area of 940 nf/g (square meters per gram) and a particle size of 0.1 to 0.3 cm (centimeters).
Example 3 500 ml (milliliters) of a 10%(percent by weight) water glass solution, which was prepared by diluting a water glass solution (based on 35% (percent) of a sodium silicate solution) with distilled water, was allowed to flow through a column filled with 1 L (liter) of a cation exchange resin Aemberlite IR-120H (commercially available from Aldrich) to perform cation exchange of sodium in a water glass. In this case, the water glass solution was in repeated contact with a cation exchange resin until a sodium component is not detected in the water glass solution passed through the column filled with the cation exchange resin. The sodium component in the water glass solution is completely exchanged and removed by the ion exchange process, and the water glass solution is gradually changed from an alkaline state to an acidic state, that is, the pH of the water glass solution is reduced to a pH range of 3 to 4. In this case, the water glass solution is then solated. Then, 4 % (percent) by weight of an ammonia solution was added to the solated water glass solution to adjust pH of the sodium-free water glass solution to a pH range of 4 to 6, thus to form a silica wet gel. Right before a wet gel is formed after approximately 30 minutes, the sodium-free water glass solution (i.e. sodium-free water glass solution in a state of right before the formation of silica wet gel) was added dropwise to a 900C (degrees centigrade) edible oil extracted from corn. Droplets in the form of solution were changed into a gel state immediately (within approximately 30 minutes) due to the temperature of the edible oil to form a bead type wet gel having a constant size. Since the formed bead type wet gel was precipitated in the oil, it was easily separated from the oil, and the oil was recovered and re¬ used. The resulting bead type wet gel was carefully kept to prevent the bead type wet gel from being broken, added to 3000 ml (milliliters) of n-butanol, in which 3.5 % (percent) by weight of methoxytrimethylsilane (MTMS) is dissolved, and then refluxed at 120 to 150°C (degrees centigrade) for 6 to 7 hours to perform its hydrophobicity and solvent substitution processes.
Moisture was completely removed from the bead type silica during the hydrophobicity and solvent substitution processes. A time point that moisture is separated from the solvent is when approximately 1000 ml (milliliters) of n-butanol in addition to the wet gel was present in the reactor, and therefore the solvent in a condenser should be adjusted to a constant solvent content so as to maintain a content of n-butanol in the reactor to a content range of approximately 1000 ml (milliliters). When the content of n-butanol was adjusted to approximately 1000 ml (milliliters) from the beginning, it was impossible to mix the n-butanol with the wet gel. When the moisture was completely separated, the silica wet gel was filtered and dried at 150°C (degrees centigrade) for 2 hours under an atmospheric pressure. In this case, a surface-modified bead type aerogel was obtained in a yield of approximately 50 g (grams), and the n-butanol and methoxytrimethylsilane (MTMS) were recovered during the filtration process, and re-used. It was revealed that the bead type aerogel prepared in this example has a specific surface area of 930 mVg (square meters per gram) and a particle size of 0.1 to 0.5 cm (centimeters) .

Claims

[CLAIMS] [Claim 1]
A method for preparing a surface-modified bead type aerogel having permanent hydrophobicity, comprising: a) removing a sodium component from a water glass solution, in which a water glass is diluted to a concentration of 5 to 15 % (percent) by weight, by contacting a cation exchange resin with the water glass solution at a room temperature; b) forming a silica wet gel by adjusting pH of the sodium-free water glass solution to a pH range of 4-6 and aging the sodium-free water glass solution; c) cutting the formed silica wet gel into the form of beads! d) adding the cut silica wet gel to a silylation solution comprising a hydrophobic alcohol solvent and a silylating agent, and simultaneously performing silylation and solvent substitution processes on the cut silica wet gel; and e) drying the silica wet gel.
[Claim 2]
The method of claim 1, wherein the forming of the silica wet gel is carried out at a temperature of 20 to 40°C (degrees centigrade) for 2 to 6 hours.
[Claim 3]
A method for preparing a surface-modified bead type aerogel having permanent hydrophobicity, comprising: a) removing a sodium component from a water glass solution, in which a water glass is diluted to a concentration of 5 to 15 % (percent) by weight, by contacting a cation exchange resin with the water glass solution at a room temperature; b) forming a silica wet gel by adjusting pH of the sodium-free water glass solution to a pH range of 4 to 6 and aging the sodium-free water glass solution by adding the sodium-free water glass solution in a state of right before the formation of silica wet gel dropwise to a 70 to 90"C (degrees centigrade) oil; c) adding the formed wet gel to a silylation solution comprising a hydrophobic alcohol solvent and a siIyIating agent, and simultaneously performing silylation and solvent substitution processes on the formed silica wet gel ; and d) drying the silica wet gel.
[Claim 4]
The method of claim 1 or 3, wherein the cation exchange resin is a polystyrene-divinylbenzene copolymer having a sulfonate group (-SO3H) .
[Claim 5]
The method of claim 4, wherein the cation exchange resin is at least one selected from the group consisting of SKlB, SKIlO, SK112, IR-120H, IR- 120B, IR-120L, IR-122 and IR-124.
[Claim 6]
The method of claim 3, wherein the oil is at least one selected from the group consisting of a vegetable oil, an animal oil and a synthetic oil.
[Claim 7]
The method of claim 1 or 3, wherein the silylating agent is at least one selected from the group consisting of a si lane compound represented by
Formula of Ri 4-n-SiXn (wherein, n is integer from 1 to 3; Ri is hydrogen, Cr
C10, preferably C1-C5 alkyl , aromatic alkyl , or heteroaromatic alkyl ; and X is halogen selected from the group consisting of F, Cl, Br and I, and preferably Cl, Ci-Cio, preferably CrC5 alkoxy group, aromatic alkoxy group, or heteroaromatic alkoxy group), and a disiloxane represented by Formula of R3Si- 0-SiR3 (wherein, two R3 groups are identical to, or different from each other, and are Ci-Ci0 alkyl , aromatic alkyl, or hetero aromatic alkyl , or hydrogen).
[Claim 8]
The method of claim 7, wherein the silylating agent is at least one selected from the group consisting of hexamethyldisilane, trimethoxysilane, ethy11riethoxysi 1ane, triethylethoxysi1ane, methy11rimethoxysi 1ane, ethyltrimethoxysi 1ane, methoxytrimethy1si 1ane, trimethy1chlorosi 1ane, hexamethyldisiloxane (HMDSO) and triethylchlorosilane.
[Claim 9]
The method of claim 1 or 3, wherein the hydrophobic alcohol solvent is selected from the group consisting of n-butanol, n-pentanol, n-hexanol and n- octanol .
[Claim 10]
The method of claim 1 or 3, wherein the silylation solution in operation (c) comprises 1-10 % (percent) by weight of the silylating agent and a 90-99 % (percent) by weight of the hydrophobic alcohol solvent.
[Claim 11]
A bead type aerogel having a particle size of 0.1 to 0.5 cm (centimeters) prepared according to the method defined in claim 1 or 3.
[Claim 12]
The bead type aerogel of claim 11, wherein the bead type aerogel has a
2 specific surface area of 600 to 1300 m /g (square meters per gram).
PCT/KR2008/005570 2007-09-19 2008-09-19 Method for preparing surface-modified transparent bead type aerogel and aerogel prepared therefrom WO2009038393A2 (en)

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