Aerogel and its manufacturing method

Abstract

The present invention provides: an aerogel which is able to be suppressed in the synthesis cost, while having excellent mechanical strength and a tunable composition; and a method for producing this aerogel.　An aerogel which has a structural unit represented by general formula (1). In general formula (1), each of R1 and R2 independently represents a hydrogen atom or an alkyl group having from 1 to 6 carbon atoms; R3 represents a divalent group that is selected from among alkylene groups which have from 1 to 10 carbon atoms, while optionally having a (meth)acryloyl group or a (meth)acryloyloxy group, and optionally having a nitrogen atom, and alkylene oxy groups represented by (CxH2xO)y (wherein x represents an integer from 1 to 4, and y represents an integer from 1 to 15); each A independently represents a single bond, NH or O; and * represents a bond with another structural unit.

Description

[Technical Field]

[0001] The present invention relates to an aerogel and a method for producing the same. [Background technology]

[0002] Currently, polymer foams widely used for purposes such as thermal insulation, shock absorption, and gas adsorption have large pores exceeding a micrometer in size, leaving much room for improvement in thermal insulation, gas adsorption, and mechanical strength.

[0003] In contrast, aerogels with pores of about 100 nanometers generally have excellent heat insulation and gas adsorption properties. Aerogels made of silica gel have long been studied as a typical aerogel (Non-Patent Document 1), but they are brittle materials that crumble with even slight deformation and can withstand extremely small stresses. Research has been reported on the use of methyltrimethoxysilane instead of the conventionally used tetramethoxysilane and tetraethoxysilane as silica gel raw materials, aiming to reduce crosslink density and improve brittleness (Non-Patent Document 2). However, the mechanical strength is unsatisfactory; the material can only be compressed up to 80% without plastic deformation, and the stress at that point is only about 9 MPa. Furthermore, the synthesis of aerogel requires multiple steps, prolonged heating, and strict control of pH conditions. Furthermore, the raw materials tetramethoxysilane and tetraethoxysilane lack structurally variable sites, making it difficult to tune (adjust and design) the network structure to suit specific purposes.

[0004] In addition, research has been reported that aims to improve brittleness by using aerogels that are carbonized after cross-linking polymerization of resorcinol and formaldehyde instead of silica gel (Non-Patent Document 3), but the mechanical strength is not satisfactory. Aerogels with normal density crack when compressed by about 3%, with a stress of about 0.8 MPa. Lowering the density allows for compression up to about 50%, but the stress at that point is extremely small, at 0.08 MPa. Furthermore, the synthesis of aerogels requires multiple steps and prolonged heating. Furthermore, the network structure can be tuned by changing the pH and catalyst amount during the reaction of resorcinol and formaldehyde, but this is not a direct tuning method. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Samuel Stephens Kistler, "Coherent expanded aerogels and jellies," Nature, May 16, 1931, vol. 127, p. 741 [Non-patent document 2] Kazuyoshi Kanamori et al., "New Transparent Methylsilsesquioxane Aerogels and Xerogels with Improved Mechanical Properties," Advanced Materials, 2007, vol. 19, p. 1589-1593 [Non-patent document 3] R.W. Pekala, "Organic aerogels from the polycondensation of resorcinol with formaldehyde", Journal of Materials Science, 1989, vol. 24, p. 3221-3227 Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, an object of the present invention is to provide an aerogel that has excellent mechanical strength, can reduce synthesis costs in terms of raw material prices, number of steps, reaction time, and condition control, and further allows for tuning of the composition, and a method for producing the same. [Means for solving the problem]

[0007] As a result of intensive research to solve the above problems, the present inventors have found that the following aerogel can solve the above problems, and have completed the present invention. That is, the present invention provides the following inventions.

[0008] <1> An aerogel having a structural unit represented by general formula (1). [ka] In general formula (1), R 1 and R 2 each independently represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, and R 3 may have a (meth)acryloyl group or a (meth)acryloyloxy group, an alkylene group having 1 to 10 carbon atoms which may have a nitrogen atom, and (C x H 2x O) y (wherein x is an integer of 1 to 4, and y is an integer of 1 to 15), each A independently represents a single bond, NH, or O, and * represents a bonding site. <2> A polymer having 40 or more structural units represented by general formula (1) in one molecule. <1> The aerogel according to claim 1. <3> Pores of 100 nm or less <1> or <2> The aerogel according to claim 1. <4> A step of reacting a compound represented by general formula (1') in a solvent in the presence of a polymerization initiator to obtain a gel having a structural unit represented by general formula (1); and drying the gel having <1> ~ <3> 10. The method for producing the aerogel according to claim 9 . [ka] In general formula (1'), R 1 and R 2 each independently represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, and R 3 may have a (meth)acryloyl group or a (meth)acryloyloxy group, an alkylene group having 1 to 10 carbon atoms which may have a nitrogen atom, and (C x H 2x O) y (wherein x is an integer of 1 to 4, and y is an integer of 1 to 15), and each A independently represents a single bond, NH, or O. <5> The solvent used in the step of obtaining the gel is at least one selected from dimethylformamide, dimethyl sulfoxide, methanol, isopropyl alcohol, and water. <4> A method for producing the aerogel described above. [Effects of the Invention]

[0009] The aerogel of the present invention can be compressed up to 95% without fracture and has excellent mechanical strength, with a maximum stress of 138.7 MPa. Furthermore, the aerogel of the present invention can be synthesized in a short time by radical polymerization at room temperature using inexpensive raw materials with structurally variable sites, thereby reducing synthesis costs and enabling the aerogel network structure to be tuned according to the purpose. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a scanning electron microscope image of an aerogel of the present invention. [Figure 2]2A is the stress-strain curve of an aerogel made from a difunctional monomer, FIG. 2B is the stress-strain curve of an aerogel made from a trifunctional monomer, and FIG. 2C is the stress-strain curve of an aerogel made from a tetrafunctional monomer. DETAILED DESCRIPTION OF THE INVENTION

[0011] aerogel The aerogel of the present invention has a structural unit represented by general formula (1). [ka]

[0012] In general formula (1), * represents a bonding site.

[0013] In general formula (1), R 1 and R 2 are each independently a hydrogen atom or an alkyl group having 1 to 6 carbon atoms. Examples of the alkyl group having 1 to 6 carbon atoms include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, an s-butyl group, a t-butyl group, a pentyl group, and a hexyl group. R 1 and R 2 is preferably a hydrogen atom or a methyl group.

[0014] In general formula (1), R 3 may have a (meth)acryloyl group or a (meth)acryloyloxy group, an alkylene group having 1 to 10 carbon atoms which may have a nitrogen atom, and (C x H 2x O) y (wherein x is an integer of 1 to 4, and y is an integer of 1 to 15) The alkylene group having 1 to 10 carbon atoms may be linear or branched, and specific examples thereof include a methylene group, an ethylene group, a propylene group, a trimethylene group, a tetramethylene group, a pentamethylene group, a neopentylene group, a hexamethylene group, a heptamethylene group, an octamethylene group, a nonamethylene group, and a decamethylene group. Some of the hydrogen atoms in the alkylene group having 1 to 10 carbon atoms may be substituted with (meth)acryloyl groups or (meth)acryloyloxy groups. Furthermore, some of the carbon atoms in the alkylene group having 1 to 10 carbon atoms may be substituted with nitrogen atoms. When the alkylene group has a nitrogen atom, it is preferable that one or two carbon atoms in the alkylene group are substituted with nitrogen atoms, and further it is preferable that a hydrogen atom bonded to the nitrogen atom is substituted with a (meth)acryloyl group. (C x H 2x O) y The alkyleneoxy group represented by the following formula may be linear or branched. Specific examples of the repeating unit enclosed in parentheses in the alkyleneoxy group include the following units (groups). CH2O CH2CH2O CH2CH2CH2O C(CH3)HCH2O CH2C(CH3)HO CH2CH2CH2CH2O When y is 2 or more, the repeating unit may be composed of a plurality of types. R 3 Preferred examples of the alkyl group include a methylene group, a pentamethylene group, a neopentylene group, a 2-ethyl-2-methyl-trimethylene group, a hexamethylene group, and an octamethylene group, as well as groups in which some of the hydrogen atoms in these groups have been substituted with (meth)acryloyl groups or (meth)acryloyloxy groups and / or groups in which some of the carbon atoms in these groups have been substituted with nitrogen atoms. R 3 Specifically, divalent groups represented by the following structural formulas are preferred. [ka] In the above structural formula, R 4 each independently represents a (meth)acryloyl group or a (meth)acryloyloxy group, and "~" represents the bonding position to A in general formula (1).

[0015] In the general formula (1), A's each independently represent a single bond, NH, or O.

[0016] The aerogel of the present invention has a structural unit represented by the general formula (1) above. It is estimated that the number of repeating units in one molecule must be 40 or more to form an aerogel. However, it is difficult to directly measure the molecular weight of the polymer in the aerogel. Therefore, it is impossible or impractical to directly determine the number of structural units represented by the general formula (1) in the aerogel of the present invention. The aerogel of the present invention may be a polymer in which the structural unit represented by general formula (1) in one molecule is the same unit, or may be a polymer in which multiple types of units are present.

[0017] Manufacturing method The aerogel of the present invention can be produced, for example, by a method having the following two steps. Process 1 A step of reacting a compound represented by general formula (1') in a solvent in the presence of a polymerization initiator to obtain a gel having a structural unit represented by general formula (1). Process 2 drying the gel

[0018] Process 1 As a source of the structural units of the aerogel having the structural units represented by the general formula (1) of the present invention, a compound (monomer) represented by the following general formula (1') is used.

[0019] [ka]

[0020] In general formula (1'), R 1 and R2 are each independently a hydrogen atom or an alkyl group having 1 to 6 carbon atoms. In general formula (1'), R 3 is an alkylene group having 1 to 10 carbon atoms and (C x H 2x O) y (wherein x is an integer of 1 to 4, and y is an integer of 1 to 15) In the general formula (1′), A's each independently represent a single bond, NH or O.

[0021] R in general formula (1') 1 , R 2 and R 3 Specific examples of the group include R 1 , R 2 and R 3 The same groups as those exemplified above are also included.

[0022] Specific examples of the compound represented by general formula (1') include: N,N'-methylenebisacrylamide, N,N'-methylenebismethacrylamide, N,N'-ethylenebisacrylamide, N,N'-ethylenebismethacrylamide, N,N'-propylene bismethacrylamide, N,N'-butylenebismethacrylamide Bisacrylamide, etc.; ethylene glycol diacrylate, ethylene glycol dimethacrylate, 1,3-propanediol diacrylate, 1,3-propanediol dimethacrylate, 1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, Neopentyl glycol diacrylate, Neopentyl glycol dimethacrylate, 3-methyl-1,5-pentanediol diacrylate, 3-methyl-1,5-pentanediol dimethacrylate, 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, 1,9-nonanediol diacrylate, 1,9-nonanediol dimethacrylate, 1,10-decanediol diacrylate, 1,10-decanediol dimethacrylate, diethylene glycol diacrylate, diethylene glycol dimethacrylate, triethylene glycol diacrylate, triethylene glycol dimethacrylate, Polyethylene glycol diacrylate (4 to 15 ethylene glycol repeating units), Polyethylene glycol dimethacrylate (ethylene glycol repeating units: 4 to 15), Polypropylene glycol diacrylate (number of propylene glycol repeating units: 4 to 15), Polypropylene glycol dimethacrylate (number of propylene glycol repeating units: 4 to 15) and polyfunctional acrylamides or (meth)acrylates represented by the following formulas. [ka]

[0023] To promote the polymerization reaction of the compound represented by general formula (1'), a polymerization initiator is used in step 1. The polymerization initiator used in producing the aerogel of the present invention is not particularly limited as long as it is an initiator that can polymerize the above-mentioned monomers, but photopolymerization initiators are preferred, such as 2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone, 1-hydroxycyclohexyl phenyl ketone, and 2-hydroxy-2-methylpropiophenone.

[0024] The polymerization reaction of the compound represented by general formula (1') is carried out in a solvent that can dissolve the compound represented by general formula (1') but does not react with the compound represented by general formula (1'). Examples of such solvents include dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methylpyrrolidone, and isopropyl alcohol. These solvents may be used alone or as a mixed solvent of two or more. Dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidone are particularly preferred as the solvent.

[0025] Furthermore, to improve the mechanical strength of the aerogel or to impart functionality to the aerogel, additives may be added to the reaction system in step 1, provided that the effects of the present invention are not impaired. Examples of additives include nanoparticles, nanorods, or nanosheets of metals, metal oxides, metal hydroxides, alloys, etc.; various nanofibers; and nanocarbon materials such as carbon nanodots, carbon nanotubes, graphene, and graphene oxide.

[0026] If the concentration of the compound represented by general formula (1') in the reaction system in step 1 is 40 mg / mL or less, the polymer will be too soft to form an aerogel, so the concentration is preferably greater than 40 mg / mL.If the concentration is 120 mg / mL or more, the polymer will become hard like a resin, making it difficult to form an aerogel, so the concentration is preferably less than 120 mg / mL. The amount of polymerization initiator in the reaction system is appropriately determined depending on the amount of the compound represented by general formula (1') and the type of polymerization initiator, etc. For example, when the concentration of the compound represented by general formula (1') in the reaction system is 60 mg / mL, the concentration of the polymerization initiator is preferably 1 to 5 mg / mL, particularly 2 mg / mL.

[0027] Furthermore, the reaction conditions for the polymerization reaction of the compound represented by general formula (1') are not particularly limited as long as a gel can be obtained, and the reaction temperature, reaction time, etc. can be in the same range as for conventional gels. The polymerization reaction is preferably carried out in an atmosphere of an inert gas such as nitrogen or argon.

[0028] The above-described reaction produces a gel (hydrogel or organogel) in which the reaction solvent is the dispersion medium. Furthermore, before step 2, solvent substitution of the dispersion medium of the gel produced in step 1 may be carried out, if necessary. Solvent substitution may facilitate drying in the subsequent step 2. When solvent substitution is carried out, examples of the solvent to be used include ethanol, methanol, and 2-propanol. Examples of solvent substitution methods include a method in which the step of immersing the gel in the substitution solvent for a certain period of time is carried out once or repeatedly several times.

[0029] Process 2 The gel obtained in step 1 is dried to obtain an aerogel. Any known method for drying aerogels can be used. Examples include atmospheric drying, supercritical drying, and freeze-drying. However, supercritical drying is preferred because it does not cause destruction or shrinkage of the gel skeleton. When the gel obtained in step 1 is dried by supercritical drying, the dispersion medium is removed at a temperature and pressure equal to or higher than the critical point of the dispersion medium in the gel. In the case of supercritical drying, the dispersion medium in the gel is preferably replaced from the reaction solvent in step 1 to ethanol and then to liquefied carbon dioxide by the solvent substitution described above.

[0030] The aerogel of the present invention has pores with a size of 1 nm or more and 100 nm or less. The pore size is a value measured by measuring the aerogel with a scanning electron microscope.

[0031] The density of the aerogel of the present invention can be changed depending on the application, for example, 0.1 to 0.2 g / cm 3 The range can be:

[0032] Aerogel Applications The aerogel of the present invention is expected to be applied as a heat insulating material, shock absorbing material or gas adsorbing material to construction materials, helmet materials, blankets, space materials including space suits, and the like. [Example]

[0033] Example 1 [Preparation of aerogels from bifunctional monomers] 180 mg of N,N'-methylenebisacrylamide and 2.0 μL of 2,2-diethoxyacetophenone were dissolved in 3.0 mL of N,N-dimethylformamide, and the solution was bubbled with nitrogen and then sealed in a cylindrical transparent polyethylene container with a diameter of 10 mm. The container was irradiated with a 500 W high-pressure mercury lamp at room temperature for 30 minutes to obtain a precursor gel. This gel (3.0 cm 3 The gel was immersed in ethanol (100 mL) at room temperature for 6 hours, and the ethanol was replaced with the same amount of pure water. This process was repeated three times to replace the dispersion medium inside the gel with ethanol. The gel was then placed in a supercritical carbon dioxide dryer, and the ethanol inside the gel was replaced with liquid carbon dioxide (16 °C, 5 atm) for 6 hours. The gel was then left to stand in a supercritical state (40 °C, 10 atm) for 1 hour. The pressure was then reduced for 1 hour to obtain aerogel A1. Aerogel A2 was also obtained in the same manner as in the preparation of aerogel A1, except that the amount of N,N'-methylenebisacrylamide was changed to 240 mg. [ka]

[0034] Example 2 [Preparation of aerogels from trifunctional monomers] 180 mg of N,N',N''-triacryloyldiethylenetriamine and 2.0 μL of 2,2-diethoxyacetophenone were dissolved in 3.0 mL of N,N-dimethylformamide, and after bubbling with nitrogen, the solution was placed in a cylindrical transparent polyethylene container with a diameter of 10 mm. The container was irradiated with a 500 W high-pressure mercury lamp at room temperature for 30 minutes to obtain a precursor gel. This gel (3.0 cm 3The gel was immersed in ethanol (100 mL) at room temperature for 6 hours, and the ethanol was replaced with the same amount of pure water. This process was repeated three times to replace the dispersion medium inside the gel with ethanol. The gel was then placed in a supercritical carbon dioxide dryer, and the ethanol inside the gel was replaced with liquid carbon dioxide (16 °C, 5 atm) for 6 hours. The gel was then left to stand in a supercritical state (40 °C, 10 atm) for 1 hour. The pressure was then reduced for 1 hour to obtain aerogel B1. Aerogel B2 was also obtained in the same manner as in the preparation of aerogel B1, except that the amount of N,N',N''-triacryloyldiethylenetriamine was changed to 240 mg. [ka]

[0035] Example 3 [Preparation of aerogels from tetrafunctional monomers] 180 mg of N,N',N'',N'''-tetraacryloyltriethylenetetramine and 2.0 μL of 2,2-diethoxyacetophenone were dissolved in 3.0 mL of N,N-dimethylformamide, and after bubbling with nitrogen, the solution was placed in a cylindrical transparent polyethylene container with a diameter of 10 mm. The container was irradiated with a 500 W high-pressure mercury lamp at room temperature for 30 minutes to obtain a precursor gel. This gel (3.0 cm 3 The gel was immersed in ethanol (100 mL) at room temperature for 6 hours, and the ethanol was replaced with the same amount of pure water. This process was repeated three times to replace the dispersion medium inside the gel with ethanol. The gel was then placed in a supercritical carbon dioxide dryer, and the ethanol inside the gel was replaced with liquid carbon dioxide (16 °C, 5 atm) for 6 hours. The gel was then left to stand in a supercritical state (40 °C, 10 atm) for 1 hour. The pressure was then reduced for 1 hour to obtain aerogel C1. Furthermore, aerogel C2 was obtained in the same manner as in the preparation of aerogel C1, except that the amount of N,N',N'',N'''-tetraacryloyltriethylenetetramine was changed to 240 mg. [ka]

[0036] [Evaluation of aerogel properties] The obtained aerogel was evaluated for the following physical properties.

[0037] density The density of the aerogel was measured by dimensional and weight measurements, and the results are shown in Table 1. [Table 1]

[0038] Scanning electron microscope (SEM) An arbitrary cross section of the aerogel A1 was observed using a scanning electron microscope. Preparation method of the sample to be measured: The sample was cut into thin slices using a razor, and platinum was vacuum-deposited on it. Measuring device: Hitachi High-Technologies SU8010 Measurement conditions: vacuum, room temperature The results are shown in Figure 1. A three-dimensional network of N,N'-methylenebisacrylamide polymers and pores of several tens to several hundred nanometers were confirmed.

[0039] Mechanical property evaluation The mechanical strength of the aerogel was evaluated by a compression test. Method for preparing the sample to be measured: Cut with a razor and process into a cube with a side length of 10 mm. Measuring device: Shimadzu AGS-10kNS precision universal testing machine Measurement conditions: room temperature, compression speed 10 mm min -1 The results are shown in Figure 2. All aerogels exhibited excellent mechanical strength, and aerogel A1 of Example 1 had a compressive modulus of 2.12 MPa calculated from a strain range of 0 to 10%, and could be compressed up to 95% without fracture, with a stress of 130 MPa or more. [Industrial Applicability]

[0040] The aerogel of the present invention is expected to be applied as a heat insulating material, shock absorbing material or gas adsorbing material to construction materials, helmet materials, blankets, space materials including space suits, and the like.

Claims

1. a step of reacting a compound represented by general formula (1') by irradiating it with light in a solvent at room temperature in an inert gas atmosphere in the presence of at least one photopolymerization initiator selected from the group consisting of 2,2-diethoxyacetophenone, 2,2-dimethoxy-2-phenylacetophenone, 1-hydroxycyclohexyl phenyl ketone, and 2-hydroxy-2-methylpropiophenone, to obtain a gel having a structural unit represented by general formula (1); and drying the gel 1. A method for producing an aerogel, the aerogel comprising: The pores are in the range of 1 to 100 nm, and A material that satisfies at least one of the following conditions: density of 0.1 to 0.31 g / cm 3 and compressibility up to 95% without breaking A method for producing an aerogel. 【Chemical 1】 In general formula (1'), R 1 and R 2 each independently represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms; R 3 may have a (meth)acryloyl group or a (meth)acryloyloxy group, an alkylene group having 1 to 10 carbon atoms which may have a nitrogen atom, and (C x H 2x O) y (wherein x is an integer of 1 to 4, and y is an integer of 1 to 15), and each A independently represents a single bond, NH, or O. 【Chemistry 2】 In general formula (1), R 1 and R 2 each independently represent a hydrogen atom or an alkyl group having 1 to 6 carbon atoms; R 3 represents a divalent group selected from alkylene groups having 1 to 10 carbon atoms which may have a (meth)acryloyl group or a (meth)acryloyloxy group and which may have a nitrogen atom, and alkyleneoxy groups represented by (C x H 2x O) y (x is an integer of 1 to 4, and y is an integer of 1 to 15); A each independently represents a single bond, NH, or O; * represents a bonding site.

2. 2. The method for producing an aerogel according to claim 1, wherein the solvent used in the step of obtaining the gel is at least one selected from the group consisting of dimethylformamide, dimethyl sulfoxide, methanol, isopropyl alcohol, and water.

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