Water dispersion of hydrophobic silica aerogel particle as well as solid composite material, heat insulation material and soundproofing material

Abstract

To provide a silica aerogel dispersion as well as a solid composite material, a heat insulation material and a soundproofing material, wherein a material obtained by heating and drying water dispersion of silica aerogel particles has excellent flexibility and wet strength and can be molded into a thick large area sheet material which does not generate cracking and shrinkage or warping.SOLUTION: The water dispersion of silica aerogel particles of the present invention contains organic nanofibers with anionic functional groups, organic short fibers with an average fiber diameter larger than 0.5 μm and less than 30 μm, surfactants, water, and silica aerogel particles. The solid composite is a strong solid composite with excellent thermal insulation and sound absorption properties, formed by drying the water dispersion of the silica aerogel particles of the present invention.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an aqueous dispersion of hydrophobic silica aerogel particles, as well as a solid composite material, a heat insulating material, and a sound insulating material.

Background Art

[0002] Powdery or particulate silica aerogel is much cheaper than the sheet form, and thus its use in heat insulating materials has attracted much attention. As a prior art regarding the dispersion of hydrophobic silica aerogel particles in an aqueous solution containing an organic nanofiber having an anionic functional group and drying the dispersion to obtain a heat insulating material and a sound insulating material containing silica aerogel, there is Patent Document 1. However, the solid composite material obtained by drying the silica aerogel dispersion of Patent Document 1 has limitations in molding thickness and area, and care is required when handling materials with poor flexural resistance or flexibility. Also, the wet strength is low and there are limitations in load resistance.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] An object of the present invention is to provide an aqueous dispersion of silica aerogel particles, a solid composite material, a heat insulating material, and a sound insulating material, wherein a material obtained by heat-drying the aqueous dispersion of silica aerogel particles has excellent flexibility and wet strength, and can form a thick large-area sheet material without cracks, shrinkage, or warping.

Means for Solving the Problems

[0005] The present invention is characterized by the following configuration and solves the above problems. [1] An aqueous dispersion of silica aerogel particles containing an organic nanofiber having an anionic functional group, an organic short fiber having an average fiber diameter greater than 0.5 μm and less than 30 μm, a surfactant, water, and silica aerogel particles. [2] The aqueous dispersion of silica aerogel particles according to [1], wherein the organic short fiber contains at least one selected from natural fibers or synthetic fibers. [3] A solid composite material containing an organic nanofiber having an anionic functional group, an organic short fiber having an average fiber diameter greater than 0.5 μm and less than 30 μm, and silica aerogel particles. [4] A heat insulating material of the solid composite material according to claim 3 described in [3]. [5] A sound insulating material of the solid composite material according to claim 3 described in [3].

Advantages of the Invention

[0006] By including organic short fibers, the present invention not only maintains the dispersibility and stability of the aqueous dispersion of hydrophobic silica aerogel particles, but also avoids shrinkage, warping or cracking problems after heat drying even for thick large-area molded bodies, and can impart flexibility, wet strength, and impact resistance (cushioning property) to the obtained composite sheet material. Furthermore, by including organic short fibers, the amount of organic nanofibers having anionic groups can be reduced, drying can be promoted, and costs can be reduced.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Figure 3

Figure 4

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Figure 6

Mode for Carrying Out the Invention

[0008] The aqueous dispersion of silica aerogel particles of the present invention is an aqueous dispersion of hydrophobic silica aerogel particles obtained by dispersing hydrophobic silica aerogel particles in a dispersion containing anionic functional group-containing organic nanofibers, organic short fibers with a diameter of 0.51 to 30 μm, nonionic surfactants, and water, and a composite obtained by drying the same

[0009] The anionic functional group-containing organic nanofibers used in the present invention are not particularly limited, and examples thereof include organic nanofibers having anionic functional groups such as carboxyl groups or their carboxylate groups, sulfate groups or their sulfate groups. The counterions of the carboxylate group or sulfate group are not particularly limited, and may be alkali metal cations, alkaline earth metal cations, ammonium cations, amines, or the like

[0010] Examples of the anionic functional group-containing organic nanofibers used in the present invention include nanofibers having anionic functional groups obtained by natural formation or artificial processing, such as modified cellulose nanofibers or chitin nanofibers obtained by sulfation, TEMO oxidation, succinic acid monoesterification, maleic acid monoesterification, phthalic acid monoesterification, phosphoric acid esterification, phosphorous acid esterification, etc. Furthermore, anionic polymers having anionic functional groups or their bases such as saccharan, hyaluronic acid, agar, gelatin, alginic acid, CMC, and polyacrylic acid, and having thickening properties and thixotropic properties similar to nanofibers, obtained by natural formation or artificial synthesis, are also included. Among them, cellulose nanofibers and chitin nanofibers having anionic functional groups on the surface by modification or modification are more preferable because they have high crystallinity, high thixotropic properties, and high viscosity

[0011] The crystallinity of the organic nanofibers is preferably 30% or more, more preferably 40% or more. Since the organic nanofibers with high crystallinity have high thixotropy and viscosity, they can stabilize the micelles into which the aerogel particles are incorporated, and the dried composite is preferable because it is strong and has excellent wet strength. The most preferable organic nanofibers having an anionic functional group used in the present invention are cellulose nanofibers and chitin nanofibers having an anionic functional group on the surface by modification or modification. As the ionic functional group of the organic nanofiber having the anionic functional group, it is particularly preferable that it is at least one or more groups such as a carboxyl group or a salt group thereof with a monovalent cation and a sulfate group or a salt group thereof with a monovalent cation.

[0012] Specific examples of particularly preferable organic nanofibers having an anionic functional group include TEMPO-oxidized cellulose nanofibers, dicarboxylic acid monoesterified modified cellulose nanofibers such as succinic acid monoesterification, maleic acid monoesterification, and phthalic acid monoesterification, carboxymethyl-modified cellulose nanofibers, sulfuric acid esterified modified cellulose nanofibers, or chitin nanofibers.

[0013] In addition, the higher the degree of substitution of the anionic functional group on the surface of the organic nanofiber, the higher the water affinity of the organic nanofiber, so the effect of improving the viscosity and thixotropy of the aqueous dispersion becomes higher, and the intrusion into the liquid silica aerogel particles can be avoided. Therefore, the average degree of substitution of the anionic functional group on the surface of the organic nanofiber is preferably 0.05 to 1.00, more preferably 0.10 to 0.95, and even more preferably 0.15 to 0.90. When it is 0.05 or less, the thickening effect and thixotropy become low. On the other hand, when it is 1.0 or more, the crystallinity of the nanofiber decreases, the thixotropy decreases, or the fibrous shape collapses, which may lead to a decrease in the thickening effect and a decrease in the strength of the composite material after drying.

[0014] The counter ion of the base of the anionic functional group is not particularly limited, but is cellulose nanofibers or chitin nanofibers having an anionic functional group selected from at least one of monovalent metal cations such as sodium cations, potassium cations, and lithium cations, ammonium cations, tetraalkylammonium cations, and organic cations such as primary amines, secondary amines, and tertiary amines.

[0015] The average fiber diameter of the organic nanofibers having an anionic functional group is preferably 1 to 500 nm, more preferably 2 to 400 nm, and further preferably 3 to 300 nm. If it is 1 nm or less, the manufacturing cost of the nanofiber increases and the heat resistance decreases. In addition, the crystallinity may decrease. On the other hand, if it is 500 nm or more, the surface area decreases and the thixotropy may be lost. Furthermore, if the fiber diameter is large, the nanofiber becomes rigid and the surface area decreases, so that a uniform and strong network cannot be formed and silica aerogel particles cannot be incorporated and fixed, and therefore the composite material after drying may become brittle or dust may be generated due to the falling off of silica aerogel particles.

[0016] Furthermore, the average fiber length of the organic nanofibers having an anionic functional group is preferably 0.1 to 50 μm, more preferably 0.2 to 40 μm, even more preferably 0.3 to 30 μm, and most preferably 0.35 to 20 μm. If the fiber length is too short, the network formed by the nanofibers is weak, and the resulting silica aerogel composite is brittle, which is not preferable. On the other hand, if the fiber length exceeds 50 μm, the viscosity of the dispersion is high, making it impossible to produce a high-concentration silica aerogel dispersion.

[0017] The purpose of the organic short fibers used in the present invention is to eliminate problems such as cracks, shrinkage, warping, and deformation that may occur when drying a silica aerogel dispersion to produce a composite material, particularly a thick composite material, and to efficiently produce a flexible or strong composite material. Although the function of the organic short fibers is not yet clearly understood, if the silica aerogel composite material of the present invention is compared to reinforced concrete, the nanofibers with anionic functional groups play the role of cement, the silica aerogel particles play the role of aggregate, and the organic short fibers play the role of steel bars. That is, the role of the organic short fibers is to reinforce the composite material of the silica aerogel and the organic nanofibers with anionic functional groups. In addition to the above effects, the organic short fibers have the effect of improving the impact resistance, load resistance, flexibility, and wet strength of the composite material. Further, by adding the organic short fibers, the required mixing ratio of the organic nanofibers with anionic functional groups, which have high water absorption and high price, can be reduced, so that the drying speed can be increased and the manufacturing price of the silica aerogel composite material can be reduced. Furthermore, when adding organic short fibers with flame retardancy such as aramid fibers and zylon fibers, the flame retardancy of the composite material can also be improved. Furthermore, when adding hydrophobic fibers, the hygroscopicity of the composite material can also be suppressed.

[0018] It is presumed that the organic short fibers are randomly dispersed in the composite sheet or cross the aggregates of the silica aerogel particles rather than forming a network together with the nanofibers having anionic functional groups.

[0019] The organic short fibers used in the present invention are not particularly limited, but the fiber diameter is preferably larger than 0.5 μm and less than 30 μm. When it is 0.5 μm or less, it is not preferable because the effects of suppressing cracks and warping and improving flexibility and wet strength are low. On the other hand, when it is 30 μm or more, it is difficult to maintain the smoothness and uniformity of the obtained composite sheet material. Also, the heat insulation and sound insulation properties of the composite material may be reduced. More preferably, the fiber diameter is 0.55 to 25 μm, still more preferably 0.6 to 20 μm. On the other hand, the aspect ratio of the organic fiber is more preferably an organic short fiber of 20 or more and 1000 or less. Regardless of whether the organic short fiber is natural or synthetic, if the fiber length is too long, aggregation of the organic short fiber may occur during the preparation of the dispersion liquid, or the uniformity of the dried sheet may decrease, so the effects of suppressing cracks and shrinkage may be lost. The fiber length of the organic short fiber is preferably 10 to 10000 μm. More preferably, it is 20 to 9000 μm, still more preferably 30 to 8000 μm, and still more preferably 50 to 7000 μm.

[0020] Specific examples of the organic short fibers include natural cellulose fibers such as wood pulp fibers and cotton linter pulp fibers, regenerated cellulose fibers such as viscose produced by the wet forming method, polyamide fibers such as nylon, aromatic polyamide fibers or aramid fibers such as para-aramid and meta-aramid, zylon fibers or polyparaphenylene benzobisoxazole fibers, and short fibers obtained by cutting long fibers such as polyester fibers and polypropylene fibers. Among them, cellulose-based short fibers are preferable in terms of the environment and availability. On the other hand, when flame retardancy and heat resistance are required, flame-retardant short fibers such as polyamide fibers or zylon fibers are preferable. Also, in order to impart hydrophobicity to the composite material, surface-hydrophobized modified cellulose short fibers, polypropylene and polyethylene short fibers are preferable. On the other hand, polyester short fibers are preferable in terms of versatility and low cost. The organic fiber may be appropriately selected according to the application field of the final composite material. When the fiber diameter of the organic short fiber is larger than the predetermined upper limit, it is refined by using chemical and mechanical or physical methods to loosen it. The refining method is not particularly limited, and examples thereof include methods such as mechanical pulverization, homogenizer treatment, colloid mill treatment, or ultrasonic treatment.

[0021] The nonionic surfactant of the present invention is a component for dispersing silica aerogel particles in an aqueous solution. In the present invention, the nonionic surfactant includes a hydrophilic nonionic surfactant that dissolves in water, even if it has low solubility and does not mix completely with water.

[0022] The nonionic surfactant is not particularly limited, but ether-type surfactants such as ethylene glycol or polyethylene glycol alkyl ether, propylene or polypropylene glycol alkyl ether, glycerin or polyglycerin ether, alkyl glycoside, etc., ester-type surfactants such as sucrose fatty acid ester, polyglycerin fatty acid ester, etc. can be used. Among them, ether-type nonionic surfactants represented by the following formula (1) or (2), which are nonionic ether-type surfactants excellent in water solubility, are preferred. R1-O-(CH2-CH2-O) n -R2 (1) R1-O-(CH 2- CH 2- CH 2- O) n -R2 (2) (In formula (1) and formula (2), R1 is a hydrogen atom, an alkyl group having 10 or less carbon atoms, a cycloalkyl group having 5 to 10 carbon atoms, or an alkenyl group having 10 or less carbon atoms, R2 is an alkyl group having 3 to 10 carbon atoms, a cycloalkyl group having 5 to 10 carbon atoms, or an alkenyl group having 10 or less carbon atoms, and n is 1 to 20)

[0023] Furthermore, the nonionic surfactant is preferably at least one or more of amphiphilic alcohols having 4 to 7 carbon atoms represented by the following formula (3). HO-R3 (3) (In formula (3), R3 is an alkyl group having 4 to 7 carbon atoms) Methanol, ethanol, 2-propanol, or isobutyl alcohol with a high polarity and 3 or fewer carbon atoms has a weak surfactant function and is difficult to conform to hydrophobic silica aerogel. Therefore, even if a small amount (for example, 15% or less) is added, hydrophobic silica aerogel cannot be dispersed, and if a large amount (for example, 15% or more) is added, the surfactant and water will enter the pores of the silica aerogel together, and the silica aerogel will shrink severely after drying, which is not preferable. Alcohols with 8 or more carbon atoms are not preferable because they are hardly soluble or insoluble in water and a uniform silica aerogel dispersion cannot be obtained.

[0024] By using a nonionic ether-type surfactant represented by formula (1) or formula (2) or an amphiphilic alcohol represented by formula (3), hydrophobic silica aerogel particles are dispersed as aggregates, which is particularly preferable. After drying the aqueous dispersion of such silica aerogel particles, the silica aerogel particles remain as aggregates and are incorporated into the network of organic nanofibers having anionic functional groups. Therefore, even if the content of the organic nanofibers is small, the silica aerogel does not shrink or fall off, and a strong composite excellent in heat insulation and sound absorption can be obtained. Furthermore, the nonionic ether-type surfactant and / or the amphiphilic alcohol having 4 or more carbon atoms can also be used in combination with an ester-type nonionic surfactant.

[0025] Considering the production of the composite material by drying the dispersion, a surfactant with a boiling point of 300 °C or lower is preferable. If the boiling point of the surfactant is too high, it is difficult to evaporate, and when drying and forming into a composite material, there is a risk that the surfactant remains in the composite, reducing the heat resistance of the composite, which is not preferable.

[0026] Among the ether-type nonionic surfactants represented by the above formulas (1) and (2), an ether-type nonionic surfactant with a low carbon number of R1 and R2 and a small n and a boiling point of 300 °C or lower is preferable.

[0027] Examples of ether-type nonionic surfactants having a boiling point of 300°C or lower include at least one or more selected from the group consisting of ethylene glycol monoalkyl ethers or dialkyl ethers, diethylene glycol monoalkyl ethers or dialkyl ethers, triethylene glycol monoalkyl ethers or dialkyl ethers, propylene glycol monoalkyl ethers or dialkyl ethers, dipropylene glycol monoalkyl ethers or dialkyl ethers, and tripropylene glycol monoalkyl ethers or dialkyl ethers.

[0028] The amphiphilic alcohol represented by the above formula (3) is preferably a hydrophilic alkyl alcohol in which the number of carbon atoms of R3 in the formula (3) is 4 to 7. Examples include at least one or more selected from the group consisting of 1-butanol (n-butyl alcohol, normal butanol), 2-methyl-1-propanol (isobutyl alcohol), 2-butanol (sec-butyl alcohol), 2-methyl-2-propanol (tert-butyl alcohol), all isomers of pentanol, all isomers of hexanol, 1-heptanol, 2-heptanol, 3-heptanol, and 4-heptanol.

[0029] [Silica aerogel] The silica aerogel used in the present invention is preferably a hydrophobic aerogel having a hydrophobic group on its surface.

[0030] As the aerogel has an increasing porosity, there are many air pores of several tens of nanometers that inhibit the mean free path, so the thermal conductivity becomes small. Since it has a hydrophobic group on the particle surface, even when dispersed in an aqueous medium, the penetration and intrusion of water into the pores are prevented. This means that the original high porosity of the aerogel can be maintained in the state of the composition and even in the state of the heat insulating material after molding, and thus excellent heat insulating properties can be exhibited.

[0031] Thus, silica aerogel having a hydrophobic group on its surface cannot be uniformly dispersed in an aqueous medium alone, but can be dispersed in an aqueous medium in the presence of a surfactant. However, when the content of the surfactant is low, the silica aerogel cannot be dispersed. On the other hand, when the concentration of the surfactant is increased until the silica aerogel can be dispersed, the surfactant and water tend to penetrate into the pores. The present invention combines an organic nanofiber having an anionic functional group and a surfactant so that the liquid does not penetrate into the pores of the silica aerogel, and thus the porosity of the heat insulating material after molding can be highly maintained, and a composite material having high heat insulating properties and sound absorbing properties can be obtained.

[0032] The silica aerogel used in the present invention has pores with a porosity of 50% by volume or more, preferably 70% by volume or more, more preferably 80% by volume or more, and even more preferably 90% by volume or more.

[0033] The particle size of the silica aerogel is not particularly limited. However, if the particle size is too small, the overall porosity of the silica aerogel decreases, and it becomes difficult to exhibit the effects of heat insulation and soundproofing, which is not preferable. On the other hand, if the particle size is too large, the size of the inter-particle gaps becomes large, and air may convect between the large voids, reducing the heat insulation and sound absorption effects, which is not preferable. A particle size range of 0.2 μm to 50 mm is preferable. More preferably, it is 0.3 μm to 40 mm. Even more preferably, it is 0.5 μm to 30 mm. Using a silica aerogel with a particle size of 50 mm or more is not preferable because the gaps between the particles become larger than the mean free path of air, not only reducing the heat insulation and soundproofing effects, but also making the composite material brittle. Furthermore, in order to reduce the gaps between the particles, a silica aerogel in which particles of various particle sizes are mixed rather than a single particle size is preferable.

[0034] For the silica aerogel having such a particle size, commercially available products having the particle size range may be used, or silica aerogel having a particle size larger than the above range may be appropriately pulverized and used.

[0035] The aqueous dispersion of the silica aerogel particles of the present invention is prepared by blending the above components in a predetermined ratio. The aqueous dispersion of the hydrophobic silica aerogel particles of the present invention is paste-like or liquid depending on the solid content concentration, and the silica aerogel particles are dispersed as aggregates. The aggregates of silica aerogel particles correspond to the micelles of an O / W type emulsion. The shape and size of the micelles or aggregates (the number of silica aerogels incorporated per micelle) depend on the mixing ratio of the silica aerogel and the organic nanofiber having an anionic functional group, the concentration of the surfactant, and the dispersion process and conditions. Generally, the higher the mixing ratio of the silica aerogel, the larger the micelles. On the other hand, the higher the amount of the surfactant or the more it is dispersed, the smaller the micelles. When the composition is constant, the smaller the size of the aggregates of silica aerogel particles, so-called micelles, the larger the specific surface area, and as a result, the film thickness of the network formed by the organic nanofiber having an anionic functional group can be reduced. As a result, the resulting composite material becomes weaker.

[0036] When preparing an aqueous dispersion of silica aerogel particles, the mixing order of each component is not particularly limited. However, considering that the surface of silica aerogel is hydrophobic and it is difficult to disperse silica aerogel in water in the absence of a nonionic surfactant, a method is preferred in which an organic nanofiber having an anionic functional group, an organic short fiber, a nonionic surfactant, and water are mixed first, and then silica aerogel particles are added and mixed. The addition order of each component is not particularly limited when preparing an organic nanofiber having an anionic functional group, an organic short fiber, a nonionic surfactant, and water. Also, the mixing equipment is not particularly limited, as long as each component is uniformly dispersed in water. For example, mixing equipment such as a mixer, a magnetic stirrer, a stirrer, or a homogenizer can be used. Next, when adding silica aerogel, either a method of adding a predetermined amount at once and mixing or a method of adding and mixing in small portions is possible. When adding and mixing silica aerogel particles, if the shear force is too high or the mixing time is too long, the silica aerogel particles may collapse. Therefore, a shaker or a mechanical stirrer is preferred over a stirrer having a high shear force such as a homogenizer. Also, when the viscosity of the silica aerogel dispersion is high, after preparing the dispersion, it can be diluted by adding water or a mixture of water and a surfactant.

[0037] The mixing method for preparing an aqueous dispersion of silica aerogel particles is not particularly limited, and known methods such as shaking, magnetic stirrer stirring, and mechanical stirring can be applied. However, if the stirring speed is too fast, the pore structure of silica aerogel may collapse due to the strong shear force, which is not preferred. Generally, a preferred stirring speed is 10,000 rpm or less, more preferably 5,000 rpm or less, and even more preferably 2,500 rpm.

[0038] The stirring time is not particularly limited, but it may be appropriately adjusted according to the stirring method and speed. If the stirring time is too long, the composite material may become brittle after drying the silica aerogel dispersion, which is not preferable. As a guideline, stirring may be continued until the silica aerogel particles are completely dispersed in the mixed solution and no undispersed silica aerogel is visible on the liquid surface. The temperature of the dispersion during stirring is not particularly limited, but it may be appropriately adjusted according to the construction environment and application. If the temperature is too high, there is a risk of decomposition and volatilization of the organic components, which is not preferable. A temperature range of 15 to 90 °C is preferable. More preferably, it is 20 to 80 °C, and even more preferably, it is 23 to 70 °C.

[0039] The concentration of the organic nanofiber having an anionic functional group with respect to water may be appropriately adjusted according to the length of the nanofiber. The shorter the nanofiber, the lower the viscosity of the aqueous dispersion of the organic nanofiber having an anionic group, so the concentration can be increased up to 2% by weight or more. On the other hand, when the nanofiber is long, since the viscosity of its aqueous dispersion is high, it is preferable to keep the concentration of the nanofiber at 0.5% by weight or less. Therefore, the preferable concentration range is 0.05 to 2% by weight. Within this range, the silica aerogel particles can be easily dispersed in the aqueous solution, and the micelles of the dispersed silica aerogel particles can maintain high stability, which is preferable. If the thixotropy or viscosity of the dispersion is less than 0.05% by weight, the silica aerogel dispersion is unstable, which is not preferable. On the other hand, if it is 2% by weight or more, the viscosity of the dispersion is high, so the silica aerogel particles cannot be well dispersed, and it is difficult to obtain a uniform dispersion, which is not preferable. More preferably, it is 0.1 to 1.5% by weight, and even more preferably, it is 0.2 to 1.0% by weight.

[0040] The concentration of the nonionic surfactant with respect to water depends on the type of surfactant and the concentration of the silica aerogel particles. Without particular limitation, 0.2 to 20% by weight is preferred. More preferably, it is 0.5 to 15% by weight. Most preferably, it is 1.0 to 10% by weight. If the concentration of the nonionic surfactant is too low, it becomes difficult to uniformly disperse the silica aerogel. However, if the concentration of the nonionic surfactant is too high, due to the decrease in surface tension, the solution containing the surfactant can not only wet the surface of the silica aerogel particles but also penetrate into the pores of the silica aerogel. Therefore, when drying the aqueous dispersion of the silica aerogel, the evaporation of water and the surfactant causes the pores of the silica aerogel to contract, resulting in a decrease in heat insulation and sound insulation properties.

[0041] The concentration of the organic short fibers with respect to water is not particularly limited and may be appropriately adjusted according to the predetermined mixing ratio with the organic nanofibers having an anionic functional group. For example, it is 0.05 to 2% by weight. More preferably, it is 0.1 to 1.5% by weight, and even more preferably, it is 0.2 to 1.0% by weight.

[0042] The concentration of the silica aerogel particles with respect to water is not particularly limited and may be appropriately adjusted according to the predetermined mixing ratio with the organic nanofibers having an anionic functional group. For example, it is 0.5 to 15% by weight.

[0043] When producing a composite from the aqueous dispersion of the silica aerogel particles of the present invention, the strength and wet strength of the composite can be improved by adding a resin. As the form of the resin, a form that does not contain an organic solvent such as an aqueous emulsion type and an aqueous solution type is preferred. In such a form, the type of resin is not particularly limited. For example, O / W emulsion type polyurethane resin, a mixture of polyol and isocyanate which is a precursor of polyurethane, phenolic resin, epoxy resin, emulsion type acrylic resin, polyethylene glycols, polypropylene glycols, resins such as polyvinylpyrrolidone and polyvinyl alcohol can be mentioned. When adding a curable resin such as epoxy or phenolic resin, it is preferable to include the respective curing agent and accelerator together.

[0044] In addition to the above composition, the aqueous dispersion of the silica aerogel particles of the present invention may contain an infrared absorber. By including these infrared-acting materials, it is possible to absorb thermal energy from a heat source or reduce thermal energy by repeating reflection within a heat insulating material, so that the heat insulation property can be increased.

[0045] The aqueous dispersion of the silica aerogel particles can be dried to evaporate water and the surfactant to form a solid composite, which can be used as a solid composite material.

[0046] In that case, since the aqueous dispersion of the silica aerogel particles has a high viscosity and is likely to contain air bubbles in the dispersion, it is preferable to defoam before molding. The solid composite obtained from the defoamed aqueous dispersion of the silica aerogel particles is preferable because it has fewer voids larger than the mean free path of air, resulting in a composite having high heat insulation, sound absorption, and strength. The defoaming method is not particularly limited, and filtration method, pressurization method, etc. can be considered.

[0047] When producing a composite material by drying the aqueous dispersion, methods such as normal temperature and pressure drying, heating and normal pressure drying, spray drying, drying using dry air or gas, etc. can be used alone or in combination. Since the thermal conductivity of silica aerogel is low, heating drying or dry gas drying is preferable. The drying temperature and time depend on factors such as the boiling point of the surfactant, the dimensions of the mold, normal pressure or reduced pressure. Examples of drying methods that combine heating and dry air or spray drying can be mentioned from the viewpoints of productivity and construction.

[0048] The drying temperature and time are not particularly limited and may be adjusted according to the boiling point of the surfactant and the shape of the mold. For example, 20 to 200 °C is preferable. More preferably, it is 50 to 180 °C, and even more preferably 75 to 150 °C. If the temperature is too low, the drying time for removing water and the surfactant becomes long, and the productivity is low, which is not preferable. On the other hand, if the drying temperature is too high, the volatilization of water and the surfactant is too intense, so there is a possibility of forming bubbles or defects, which is not preferable. Also, if the drying temperature is too high, the organic nanofibers or organic short fibers may decompose or their performance may deteriorate, which is not preferable.

[0049] The solid composite material of the present invention is a solid composite material containing organic nanofibers having an anionic functional group, organic short fibers having an average fiber diameter greater than 0.5 μm and less than 30 μm, and a surfactant.

[0050] The content rates of the respective components of the solid composite material containing organic nanofibers having an anionic functional group, organic short fibers, and silica aerogel particles are not particularly limited. However, if the content rates of organic materials such as organic nanofibers having an anionic functional group and organic short fibers are too high, the density of the composite material becomes high, and there is a risk of deterioration in heat insulation and sound insulation. On the other hand, if the content rate is too low, cracks or fissures may occur or the bendability may become poor. Therefore, the preferable content rate of the organic nanofibers having an anionic functional group is 0.5 to 40% by weight, preferably 1.0 to 30% by weight, and more preferably 1.5 to 20% by weight. The preferable content rate of the organic short fibers is 0.5 to 40% by weight, preferably 1.0 to 30% by weight, and more preferably 1.5 to 20% by weight. On the other hand, the heat insulation and sound insulation improve as the content rate of the silica aerogel particles increases. However, if it is too high, the composite material becomes brittle and the bendability becomes poor. Therefore, the content rate of the silica aerogel is 20 to 99% by weight, preferably 30 to 98% by weight, and more preferably 40 to 97% by weight. The silica aerogel particle composite material of the present invention may also contain an organic polymer according to the application.

[0051] The organic polymer is not particularly limited, and examples thereof include polyurethane, acrylic resin, epoxy resin, and phenol resin. By including these organic polymers, the mechanical properties and water resistance of the silica aerogel composite material can be improved, which is preferable. The content of the organic polymer may be appropriately adjusted according to the application. For example, the total weight of the three components of the organic nanofiber, organic short fiber, and silica aerogel particles having an anionic functional group / the weight ratio of the organic polymer can be 50 / 50 to 99 / 1.

[0052] The solid composite material of the present invention can be formed into a film having a thickness of less than 2 mm, and can be made into a molded body having a thickness of 2 mm or more. The upper limit of the thickness of the molded body is not particularly limited, but it is preferably 100 mm or less in consideration of ease of manufacture and practicality. A more preferable thickness of the molded body is 3 to 50 mm, and still more preferably 3 to 30 mm.

[0053] When forming a thick composite material, a lamination molding method can be promoted. For example, a composite sheet of 1 to 10 mm is dried, and then a new dispersion liquid is applied thereon and dried. The feature of the repeated lamination molding method is that it can reduce the drying time and form a thick composite material. The spray drying method can also be applied according to the application.

[0054] The solid composite obtained by drying the aqueous dispersion of the silica aerogel particles has a low specific gravity and a low thermal conductivity, and is therefore suitable as a heat insulating material.

[0055] The aqueous dispersion of silica aerogel containing organic short fibers of the present invention is suitable for molding a large-area and thick composite. The dried composite has flexibility and flexural resistance, and has high wet strength, so it can be used in various application fields. In addition, by including short fibers, cracks, shrinkage, and warping do not occur during the dry forming process, and the defective rate of the product can also be reduced. Furthermore, by adding organic short fibers, the amount of organic nanofibers having an anionic functional group can be reduced, so that the material cost of the composite can also be reduced. Furthermore, when adding organic short fibers having flame retardancy such as aramid fibers and zylon fibers, the flame retardancy of the composite can be improved. Furthermore, when adding hydrophobic fibers, the hygroscopicity of the composite can also be suppressed.

[0056] Furthermore, the solid composite obtained by drying the aqueous dispersion of the silica aerogel particles is also suitable as a soundproof material because it has a low specific gravity and the nanopores of the silica aerogel are maintained.

[0057] In addition, the aqueous dispersion of the silica aerogel particles can also be directly applied to a heat generating surface or a soundproof surface as a paint and dried.

Examples

[0058] The present invention will be further described using examples. Note that the present invention is not limited to only these examples. The production methods, physical property evaluation methods used, organic nanofibers having anionic functional groups, manufacturers of organic nanofibers, and their preparation methods for each example and each comparative example are shown below.

[0059] (TEMPO-oxidized cellulose nanofibers) Nippon Paper Industries TEMPO-oxidized cellulose nanofibers are cellulose nanofibers in which a part or all of the hydroxymethyl groups of carbon at the 6th position on the surface of the cellulose nanofibers are converted to carboxyl groups by TEMPO oxidation. In the present invention, Celenpia TC-01A manufactured by Nippon Paper Industries Co., Ltd. (aqueous dispersion with a solid content of 1 wt%) was used. Before use, distilled water was added to dilute the solid content concentration to a predetermined range, for example, up to 0.4 wt%, and then used.

[0060] (Cellulose nanofiber modified by succinic acid monoesterification) The cellulose nanofiber modified by succinic acid monoesterification was prepared according to the method described in JP 2022-030446. The fiber diameter of the obtained cellulose nanofiber modified by succinic acid esterification was about 4 nm, and the average degree of substitution of the succinic acid functional group was 0.3. Before use, distilled water was added to dilute the solid content concentration to a predetermined range, for example, up to 0.35 wt%, and then it was used.

[0061] Roughness (Organic short fiber) Cellulose microfibril (CMF) was obtained by subjecting a mixed solution of 50 g of linter pulp and 950 g of distilled water to 5 passes using a supermasscolloider and a coarse grindstone. Observed by FE-SEM, the fiber diameters of 50 randomly selected CMF fibers were measured, and as a result of calculating the average fiber diameter as the average value, it was 0.99 μm. Before using the obtained CMF dispersion, the solid content concentration was diluted to a predetermined value by dilution with distilled water or squeezing using a nylon mesh. Para-aramid microfiber was prepared by subjecting a mixed solution of 50 g of Kevlar (registered trademark) fiber and 950 g of distilled water to 3 passes with a coarse grindstone using the same supermasscolloider and then defibrating it by subjecting it to 3 passes with a fine grindstone. As a result of evaluating the average fiber diameter by the same SEM observation, it was 0.65 μm. The solid content concentration of the obtained aqueous dispersion of para-aramid microfiber was diluted to a predetermined range, for example, up to 0.48 wt%, and used. Alternatively, the solid content concentration was squeezed to a predetermined range using a nylon mesh. Meta-aramid short fiber was prepared using commercially available meta-aramid short fiber. The average fiber diameter was 15 μm and the average length was 9 mm. An aqueous dispersion was prepared by immersing it in distilled water for 10 minutes and then stirring it with a mixer for 1 minute. Before using the obtained dispersion, the solid content concentration was diluted to a predetermined value by dilution with distilled water or squeezing using a nylon mesh, and then used. Filter paper: ADVANTEC Toyo's quantitative filter paper No. 5B was used. The average fiber diameter was 20 μm. After being immersed in distilled water for 10 minutes, it was stirred with a mixer for 1 minute to prepare an aqueous dispersion. Before using the obtained dispersion, it was diluted with distilled water or squeezed using a nylon mesh to dilute the solid content concentration to a predetermined value before use. Laurylated modified CMF: The above-mentioned CMF was replaced with DMF, and after preparing 50 ml of a 1 wt% DMF dispersion, 5 ml of vinyl laurate and 0.5 g of calcium carbonate were added thereto, and it was placed in an oil bath at 60 °C and stirred for 3 hours to be laurylated. After the reaction, it was washed twice each with ethanol and distilled water in that order, and then redispersed in water to prepare an aqueous dispersion of a predetermined concentration. In order to evaluate the degree of substitution of lauryl groups, a part of the sample was put into a 1N aqueous sodium hydroxide solution and hydrolyzed at room temperature for 3 hours, and then lauric acid was quantified by titration. As a result of quantifying the number of moles of lauryl groups per 162 g of cellulose, it was 0.25.

[0062] (Surfactant) The surfactant used in the examples or comparative examples of the present invention was ethylene glycol monobutyl ether (EGMBE). The addition amount of EGMBE was 3.6 parts per 100 parts of water.

[0063] (Silica aerogel particles) As silica aerogel particles, CABOT's ENOVA AEROGEL MT1100 and P200 were used respectively. For MT1100, the particle size was 2 - 24 μm, and the bulk specific gravity was 25 - 50 kg / m 3 For P200, the particle size was 0.01 - 1.2 mm, and the bulk specific gravity was 75 - 95 kg / m 3 respectively.

[0064] (Evaluation of thermal conductivity of heat dissipation material) The thermal conductivity λ (W / (m·K)) of the heat dissipation material is obtained from the following formula. The thermal diffusivity α (m 2 / s), specific heat c (J / kg·K), and specific gravity ρ (kg / m 3 ) were obtained using the respective methods shown below. c = λ / (ρ × α) The measurement of the thermal diffusivity was carried out by the laser flash method (measurement device: LFA467 HyperFlash thermal conductivity measurement device manufactured by Netzsch Japan). The specific heat of the heat dissipation material of the present invention is calculated from below by summing the product of the specific heat c of each single material constituting it and the mass ratio r per unit volume. c = c s r s + c n r n + c a r a Here, the subscript s represents silica aerogel particles, the subscript n represents organic nanofibers and organic short fibers, and the subscript a represents air. Among these, the term of air can be ignored because it is sufficiently small compared to the others. Here, the specific heat of silica aerogel was cited as 675 J / kg·K, and the specific heat of cellulose nanofibers was cited as 1260 J / kg·K.

[0065] (Evaluation of the specific gravity of the composite sheet) A regular hexahedron with a certain area (S) was cut out from the composite sheet, its mass (W) was measured with a microelectronic balance, and its thickness (d) was measured with a micrometer, and it was obtained by the following formula. Specific gravity = W / (d × S)

[0066] (Specific gravity of cellulose nanofibers and organic short fibers) The specific gravity of cellulose nanofibers and organic short fibers was used as 1.6 with reference to the literature value regardless of the surface modification species for the calculation of the thermal conductivity.

[0067] (Measurement of the thickness of the composite sheet) Using a micrometer, the thicknesses of three random parts of the prepared sheet were measured, and the average of the three values was taken as the thickness of the sheet.

[0068] (Maximum thickness that can be formed) The mold was fixed, and the thickness of the formed sheet was increased by increasing the thickness of the cast layer of the aqueous dispersion of silica aerogel particles. After drying and forming, the appearance of the sheet was observed, and the maximum thickness at which cracks and fractures began to occur was evaluated as the maximum thickness.

[0069] (Evaluation of flexural fatigue resistance) The dried composite sheet material was cut into a rectangle of 3 - 4 cm × 7 - 8 cm, and the appearance was observed while bending the longer side of the rectangle. The bending radius until it cracked was evaluated.

[0070] (Evaluation of wet strength) The dried composite sheet material was cut into a rectangle of 3 - 4 cm × 7 - 8 cm, immersed in distilled water for 3 hours, then picked up with tweezers and the appearance was observed. If the appearance of the sheet was straight and without damage as before immersion, the wet strength was rated as ○; if it bent without breaking, it was rated as △; if it was damaged, it was rated as ×.

[0071] [Comparative Example 1] According to the composition shown in Table 1a, Nippon Paper's Celenia TC - 01A, surfactant, EGBE, and distilled water were weighed, placed in a 120 ml polypropylene wide - mouth bottle, stirred with a magnetic stirrer until uniform, then silica aerogel (MT1100) was added, the lid was closed, and it was further stirred with a magnetic stirrer for 10 minutes. Then, a 120 mm × 120 mm frame was set on the PP sheet, cast into the frame, and dried in a forced - air dryer at 105°C for 5 hours. The dried sheet showed no deformation or cracks, and the thickness was 1.8 mm. The composite sheet and the appearance photos after evaluation are shown in Figure 1. As a result of evaluating the flexural fatigue resistance, although it had flexural properties, cracks occurred when bent to 90° or less. The bending radius was 90° or more. Also, when the sheet was put into water, it did not sink and remained floating on the liquid surface, but after 3 hours, the composite sheet became soft and bent when picked up with tweezers, so the wet strength was low. The evaluation results and the evaluation results of the thermal conductivity are shown together in Table 1b. In addition, as a result of evaluating the maximum formable thickness, it was less than 2 mm. When it reached 2 mm or more, cracks and warping occurred.

[0072] [Example 1] According to the composition shown in Table 1a, Nippon Paper's Selenia TC-01A, organic short fibers, surfactant EGBE, and distilled water were weighed and placed in a 120 ml polypropylene wide-mouth bottle. After stirring uniformly with a magnetic stirrer, silica aerogel (MT1100) was added, the lid was closed, and stirring was continued with a magnetic stirrer for another 10 minutes. Then, a 120 mm × 120 mm frame was provided on a PP sheet, and after casting into the frame, it was dried in a forced-air dryer at 105 °C for 5 hours. The appearance photo of the dried sheet is shown in Figure 2. The thickness was 2.5 mm. The bending radius, wet strength, and thermal conductivity were evaluated respectively, and the results are shown in Figure 2 and Table 1b. After soaking in water for 3 hours as in Comparative Example 1, when picked up with tweezers, the sheet was straight and did not bend, showing excellent wet strength. Also, even when the sheet was bent to 0°, cracks or splits did not occur, showing excellent flexural fatigue resistance. Also, as a result of evaluating the formable thickness, it was confirmed that it could be formed up to 100 mm. Depending on the forming method, it can also be formed to a greater thickness.

[0073] [Examples 2 - 4] According to the composition shown in Table 1a, composite sheets were prepared in the same manner as in Example 1. In all cases, no cracks, shrinkage, or curling occurred. The results of evaluating the respective bending radii, wet strengths, and thermal conductivities, etc. are shown in Table 1b. As a result of evaluating the formable thickness, it was confirmed that in all cases, it could be formed up to 100 mm. Depending on the forming method, it can also be formed to a greater thickness.

[0074]

Table 1a

[0075]

Table 1b

[0076] [Comparative Example 2] A silica aerogel composite material was prepared in the same manner as in Comparative Example 1, except that succinic acid monoesterified cellulose nanofiber (CNF) was used instead of Selenpia TC-01A. The thickness of the obtained sheet (Figure 3) was 1.7 mm. When bent up to 90°, cracks occurred and the bending radius was 90° or more. When the composite sheet was picked up with tweezers after immersion in water, it became soft and curved, and the wet strength was low. Photographs of the appearance before and after the test are shown in Figure 3. The evaluation results and the evaluation results of the thermal conductivity are shown together in Table 2b. Also, as a result of evaluating the maximum thickness that can be formed, it was less than 2 mm.

[0077] [Example 5] A composite sheet was prepared according to the composition shown in Table 2a in the same manner as in Example 1. A rectangular (100 mm × 140) molding frame was used. There were no cracks, deformations or warps in the obtained composite sheet. The thickness was 3.2 mm. As shown in Figure 4, when the immersed composite sheet was picked up, it did not bend and was as straight as before immersion. Also, no cracks or fissures occurred even when bent up to 0°. It was excellent in flexural strength or flexibility and wet strength. The evaluation results of the thermal conductivity are shown in Table 2b. Also, as a result of evaluating the thickness that can be formed, it was confirmed that it could be formed up to 100 mm. Depending on the molding method, it can be molded to a greater thickness.

[0078] [Examples 6 - 10] Each composite sheet was prepared according to the composition shown in Table 2a in the same manner as in Example 5. There was no significant difference in thickness, which was 2.0 - 3.3 mm. Also, it was excellent in flexural strength or flexibility and wet strength, and there were no cracks, warps or deformations. The maximum thickness that could be formed was 3 mm or more. The evaluation results of the thermal conductivity are shown in Table 2b together.

[0079] [Examples 11 - 12] Instead of using MT1100, a composite material was prepared in the same manner as in Example 1 except that P200 was used and the composition in Table 2a was followed. By using silica aerogel particles with a large particle size, even when the blending ratio of silica aerogel was increased to 94% by weight or more, cracks and warping did not occur, and a composite sheet with a thickness of 10 mm or more was obtained. The obtained composite sheet was so rigid that it could not be bent and had excellent load-bearing properties. Also, after immersion in water, the sheet could be maintained in a rigid state and had excellent wet strength. Further, as a result of confirming the formable thickness, it was confirmed that it could be formed up to 100 mm. Depending on the forming method, it can also be formed to a greater thickness. The composite sheet prepared in Example 11 and a photograph of its pressure resistance test are shown in Fig. 5. It did not deform even when a 100 g weight was placed on it. By using large-grained silica aerogel particles of P200, the maximum formable thickness was greatly improved.

[0080]

Table 2a

[0081]

Table 2b

[0082] The surfaces of the composite sheets obtained in Comparative Example 2, Example 5, and Example 8 were observed by SEM, and the results are shown in Fig. 6. Example 5 and Example 8 containing organic short fibers had clearly different surface morphologies compared to Comparative Example 2. There were many bumps and irregularities on the surface of the composite sheet obtained in Comparative Example 2, which caused the silica aerogel particles to be exposed due to the rupture of the CNF network having anionic groups. On the other hand, the surfaces of Example 5 and Example 8 to which organic short fibers were added had no bumps and irregularities seen in Comparative Example 2, and a flat surface was formed by the reinforcement of the organic short fibers. Also, it was clearly seen that the organic short fibers were stretched on the surface of the sheet.

[0083] The evaluation results of the silica aerogel dispersion obtained in the examples and comparative examples and the composites dried therefrom are summarized in Table 1b and Table 2b. As is clear from the results shown in the table, the silica aerogel particle / organic nanofiber / organic short fiber composite obtained in the examples was excellent in flexural resistance or flexibility, wet strength, and heat insulation properties, and no cracks or curling occurred when a thick and large-area sheet was created. The composite sheets obtained in Comparative Examples 1 and 2 clearly had lower flexural resistance, wet strength, and maximum formable thickness compared to the examples in which organic short fibers were added. The effect of the organic short fibers was verified from the comparison between the comparative examples and the examples.

Industrial Applicability

[0084] The composite obtained by drying the aqueous dispersion of the silica aerogel particles of the present invention is excellent in flexural resistance or flexibility, water resistance, and heat insulation properties, and can be used as a heat insulating material or a sound insulating material. In addition, since the aqueous dispersion of silica aerogel has excellent stability, it can be expected to be used in a wide range of fields as a paint.

Claims

1. An aqueous dispersion of silica aerogel particles containing an organic nanofiber having an anionic functional group, an organic short fiber having an average fiber diameter greater than 0.5 μm and less than 30 μm, a surfactant, water, and silica aerogel particles.

2. The aqueous dispersion of silica aerogel particles according to claim 1, wherein the organic short fiber contains at least one selected from natural fibers and synthetic fibers.

3. A solid composite material containing an organic nanofiber having an anionic functional group, an organic short fiber having an average fiber diameter greater than 0.5 μm and less than 30 μm, and silica aerogel particles.

4. A heat insulating material of the solid composite material according to claim 3.

5. A sound insulating material of the solid composite material according to claim 3.

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