Fiber-reinforced hollow particle

Abstract

To provide a fiber-reinforced hollow particle that can retain sufficient strength even upon exposure to high temperatures.SOLUTION: A fiber-reinforced hollow particle includes an outer shell portion containing an inorganic compound and inorganic nanofibers, and a hollow portion surrounded by the outer shell portion, wherein the inorganic nanofibers have an aspect ratio of 3 or more and a surface area of 500 nm2 or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to fiber-reinforced hollow particles. [Background technology]

[0002] Hollow particles have a cavity surrounded by a shell composed of organic or inorganic compounds. Due to their excellent insulating properties and light weight, they have been widely used as lightweight fillers and insulating paint fillers in the field of building materials. However, conventional hollow particles have large particle diameters of several tens of micrometers or more, making them difficult to use in small components or thin-film paints. The present inventors have therefore developed inorganic oxide hollow particles with a fine average particle diameter of approximately 0.5 to 10 micrometers, which are applicable to a wide range of applications (Patent Documents 1 to 3). However, the present inventors discovered a new problem with these inorganic oxide hollow particles: even a single crack in the outer shell can become the fracture origin and cause instantaneous fracture. To address this problem, the present inventors focused on fibrous materials used as reinforcing materials in fields such as building materials. By incorporating cellulose nanofibers into the outer shell of inorganic oxide hollow particles, they successfully developed inorganic oxide hollow particles that can maintain sufficient strength for practical use even when cracks are present in the outer shell (Patent Document 4). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6324247 specification [Patent Document 2] Patent No. 6389373 specification [Patent Document 3] Patent No. 6389431 specification [Patent Document 4] Patent No. 7266498 specification Summary of the Invention [Problem to be solved by the invention]

[0004] However, since the fiber material of the inorganic oxide hollow particles is cellulose, which is an organic substance, the particle strength may decrease in high temperature environments. An object of the present invention is to provide fiber-reinforced hollow particles that can maintain sufficient strength even when exposed to high temperatures, and a method for producing the same. [Means for solving the problem]

[0005] In order to solve the above problems, the present inventors have focused on inorganic nanofibers and conducted research, and have found that, by incorporating only inorganic nanofibers having specific properties into the outer shell of hollow particles, sufficient strength can be maintained even when exposed to high temperatures.

[0006] That is, the present invention provides the following [1] to

[10] . [1] An outer shell portion containing an inorganic compound and inorganic nanofibers; a hollow portion surrounded by the outer shell portion; Equipped with The inorganic nanofibers have an aspect ratio of 3 or more and a surface area of ​​500 nm 2 That's all. Fiber reinforced hollow particles. [2] The fiber-reinforced hollow particles according to [1], wherein the content of the inorganic nanofibers in the hollow particles is 1% by mass or more and 20% by mass or less. [3] The inorganic nanofiber has an aspect ratio of 3 to 400 and a surface area of ​​1000 nm 2 More than 20000nm 2 is as follows: The fiber-reinforced hollow particles according to [1] or [2], wherein the content of the inorganic nanofibers in the hollow particles is 1% by mass or more and 13% by mass or less. [4] The fiber-reinforced hollow particle according to any one of [1] to [3] above, wherein the inorganic nanofibers are one or more selected from aluminum oxide nanofibers, titanium oxide nanofibers, silicon carbide nanofibers, silicon nitride nanofibers, and boron nitride nanofibers. [5] The fiber-reinforced hollow particle according to any one of [1] to [4] above, which has a hollowness of 50% or more. [6] A method comprising the steps of: sending a raw material liquid containing raw material compounds and inorganic nanofibers contained in a stirring vessel equipped with stirring blades, the ratio (D2 / D1) of the inner diameter D1 of the stirring vessel to the rotational diameter D2 of the stirring blades being 0.3 or more, to a spraying device while stirring at a rotational speed of 500 rpm or more; and spraying and heating the raw material liquid from the spraying device; The inorganic nanofibers have an aspect ratio of 3 or more and a surface area of ​​500 nm 2 That's all. Method for producing fiber-reinforced hollow particles. [7] The method for producing fiber-reinforced hollow particles according to [6] above, wherein the raw material compounds are compounds containing one or more elements selected from Group 1 elements of the periodic table, Group 2 elements of the periodic table, Group 4 elements of the periodic table, Group 8 elements of the periodic table, Group 9 elements of the periodic table, Group 10 elements of the periodic table, Group 11 elements of the periodic table, Group 12 elements of the periodic table, Group 13 elements of the periodic table, Group 14 elements of the periodic table, and Group 15 elements of the periodic table, and the raw material compounds contain one or more compounds selected from inorganic salts, organic salts, and alkoxides. [8] The method for producing fiber-reinforced hollow particles according to [6] or [7], wherein the raw material compound contains one or more compounds selected from sodium salts, potassium salts, magnesium salts, calcium salts, barium salts, titanium salts, zirconium salts, zinc salts, borates, boric acid, aluminum salts, aluminum alkoxides, and silicate alkoxides. [9] The method for producing fiber-reinforced hollow particles according to any one of [6] to [8], wherein the inorganic nanofibers are one or more selected from aluminum oxide nanofibers, titanium oxide nanofibers, silicon carbide nanofibers, silicon nitride nanofibers, and boron nitride nanofibers.

[10] The method for producing fiber-reinforced hollow particles according to any one of [6] to [9], wherein the amount of inorganic nanofibers used is 1% by mass or more and 20% by mass or less in the hollow particles. [Effects of the Invention]

[0007] The fiber-reinforced hollow particles of the present invention have an outer shell reinforced with inorganic nanofibers, and therefore can maintain sufficient strength even when exposed to high temperatures. Therefore, the fiber-reinforced hollow particles of the present invention are useful as fillers for heat-insulating and heat-shielding coatings, etc. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram showing an X-ray diffraction pattern of the fiber-reinforced hollow particles obtained in Example 1. [Figure 2] FIG. 2 is a diagram showing the X-ray diffraction pattern of the hollow particles obtained in Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0009] <Fiber-reinforced hollow particles> The fiber-reinforced hollow particle of the present invention has an outer shell containing an inorganic compound and inorganic nanofibers, and a hollow portion surrounded by the outer shell, and the inorganic nanofibers have an aspect ratio of 3 or more and a surface area of ​​500 nm 2 As a result, it is possible to exhibit sufficient strength even when exposed to high temperatures, and even if a crack occurs in part of the outer shell, it will not break and will be able to maintain a strength that can withstand practical use.

[0010] As used herein, "hollow particles" refer to particles having an internal cavity (hollow structure) and an outer shell that separates the hollow portion. Hollow particles may also have a plurality of independent spaces, each of which is separated by one or more partition walls, and each of these independent spaces may be formed by non-communicating bubbles (hereinafter also referred to as "closed bubbles") separated by the partition walls. Herein, the term "outer shell" refers to the wall located on the outermost surface of a particle that contacts only one closed bubble inside the particle, and the term "partition wall" refers to the wall that separates adjacent closed bubbles within a particle. The outer shell is composed of an inorganic compound and inorganic nanofibers. When a particle has multiple closed bubbles, the outer shell and the partition walls are also composed of an inorganic compound and inorganic nanofibers. Therefore, hollow particles differ from porous particles, which have multiple pores extending from the particle surface to the interior. Hollow particles can be distinguished from porous particles by transmission electron microscope (TEM) images. In addition, in this specification, the term "fiber-reinforced hollow particles" refers to hollow particles whose strength, particularly compressive strength, is reinforced by fibers.

[0011] [Inorganic compounds] The fiber-reinforced hollow particles of the present invention contain an inorganic compound in the outer shell portion that defines the hollow portion. The inorganic compound is not particularly limited as long as it can constitute the outer shell portion of the hollow particle, and examples thereof include inorganic carbonates, inorganic phosphates, and inorganic oxides. The inorganic oxide may be amorphous, and examples thereof include inorganic glass. The inorganic compound may contain one or more types.

[0012] Examples of inorganic carbonates include carbonates containing one or more elements selected from Group 1 elements and Group 2 elements of the periodic table. Examples of inorganic phosphates include phosphates containing one or more elements selected from Group 1 elements and Group 2 elements of the periodic table. Examples of inorganic oxides include inorganic oxides containing one or more elements selected from Group 1 elements of the periodic table, Group 2 elements of the periodic table, Group 4 elements of the periodic table, Group 8 elements of the periodic table, Group 9 elements of the periodic table, Group 10 elements of the periodic table, Group 11 elements of the periodic table, Group 12 elements of the periodic table, Group 13 elements of the periodic table, Group 14 elements of the periodic table, and Group 15 elements of the periodic table. Examples of inorganic glass include inorganic glasses containing one or more elements selected from Group 1 elements, Group 2 elements, Group 13 elements, Group 14 elements, and Group 15 elements of the periodic table.

[0013] Examples of Group 1 elements in the periodic table include lithium, sodium, potassium, and cesium. Examples of Group 2 elements in the periodic table include magnesium, calcium, strontium, and barium. Examples of Group 4 elements in the periodic table include titanium and zirconium. Examples of Group 8 elements in the periodic table include iron and ruthenium. Examples of Group 9 elements in the periodic table include cobalt, rhodium, and iridium. Examples of Group 10 elements in the periodic table include nickel, palladium, and platinum. Examples of Group 11 elements in the periodic table include copper, silver, and gold. Examples of Group 12 elements in the periodic table include zinc and cadmium. Examples of Group 13 elements in the periodic table include boron, aluminum, gallium, indium, and thallium. Examples of Group 14 elements in the periodic table include silicon, germanium, tin, and lead. Examples of Group 15 elements of the periodic table include phosphorus, arsenic, antimony, and bismuth.

[0014] Specific examples of inorganic carbonates include sodium carbonate, potassium carbonate, calcium carbonate, and magnesium carbonate. One or more inorganic carbonates may be contained. Specific examples of inorganic phosphates include tetrasodium pyrophosphate, disodium dihydrogen pyrophosphate, sodium metaphosphate, sodium polyphosphate, calcium pyrophosphate, calcium metaphosphate, magnesium pyrophosphate, and magnesium metaphosphate. One or more inorganic phosphates may be contained. Specific examples of inorganic oxides include sodium oxide, potassium oxide, magnesium oxide, barium oxide, calcium oxide, zinc oxide, copper oxide, boron oxide, aluminum oxide, iron oxide, and silicon oxide, and may also be composite oxides that combine inorganic oxides. One or more inorganic oxides may be contained. Examples of inorganic glass include aluminosilicate glass, aluminoborosilicate glass, borosilicate glass, phosphate glass, silica glass, and borate glass. One or more types of inorganic glass may be contained.

[0015] Among these, from the viewpoint of exhibiting sufficient strength even when exposed to high temperatures, the inorganic compound preferably contains one or more selected from inorganic oxides and inorganic glasses, more preferably contains inorganic glass, and even more preferably contains one or more inorganic glasses selected from aluminosilicate glass, aluminoborosilicate glass, borosilicate glass, borate glass, and phosphate glass.

[0016] In terms of making it easier to enjoy the effects of the present invention, the inorganic compound preferably contains one or more inorganic oxides containing an element selected from Group 1 elements, Group 2 elements, Group 13 elements, and Group 14 elements of the periodic table; more preferably contains one or more inorganic oxides containing an element selected from Group 1 elements and Group 2 elements of the periodic table, and one or more inorganic oxides containing an element selected from Group 13 elements and Group 14 elements of the periodic table; and even more preferably contains one or more elements selected from sodium oxide, potassium oxide, magnesium oxide, barium oxide, calcium oxide, and zinc oxide, one or more elements selected from boron oxide and aluminum oxide, and silicon oxide.

[0017] Suitable chemical compositions of the inorganic compounds include the following. (i) Contains 20% by mass or less of an oxide of a Group 1 element of the periodic table, 30% by mass or less of an oxide of a Group 2 element of the periodic table, 40% by mass or less of an oxide of a Group 13 element of the periodic table, and 30% by mass or more of silicon oxide. Note that the content of the oxide of a Group 1 element of the periodic table may be 0% by mass. (ii) Contains 15% by mass or less of an oxide of a Group 1 element of the periodic table, 3 to 20% by mass of an oxide of a Group 2 element of the periodic table, 5 to 35% by mass of an oxide of a Group 13 element of the periodic table, and 35 to 60% by mass of silicon oxide. Note that the content of the oxide of a Group 1 element of the periodic table may be 0% by mass. (iii) Contains 15% by mass or less of an oxide of a Group 1 element of the periodic table selected from sodium oxide and potassium oxide, 3 to 20% by mass of an oxide of a Group 2 element of the periodic table selected from magnesium oxide and calcium oxide, 5 to 35% by mass of an oxide of a Group 13 element of the periodic table selected from aluminum oxide and boron oxide, and 35 to 60% by mass of silicon oxide. Note that the content of the oxide of a Group 1 element of the periodic table may be 0% by mass. (iv) Contains 5 to 15 mass% of an oxide of an element of Group 1 of the periodic table selected from sodium oxide and potassium oxide, 3 to 20 mass% of an oxide of an element of Group 2 of the periodic table selected from magnesium oxide and calcium oxide, 5 to 35 mass% of an oxide of an element of Group 13 of the periodic table selected from aluminum oxide and boron oxide, and 35 to 60 mass% of silicon oxide.

[0018] In this specification, the content of each inorganic compound described above is a value obtained by measuring the amount of the inorganic compound in terms of its oxide using X-ray fluorescence analysis and calculating the chemical component. Each chemical component is calculated by correcting the amount of the oxide of the element being analyzed using the following formula so that the total amount of the oxide of the element being analyzed is 100%.

[0019] Chemical composition (corrected) (%) = Chemical composition (uncorrected) × 100 / (100 - impurities (%)) The impurity (%) is calculated by subtracting the total value of the chemical composition of the oxides mentioned above from 100.

[0020] [Inorganic nanofibers] The fiber-reinforced hollow particles of the present invention further contain inorganic nanofibers in the outer shell that defines the hollow portion, and the inorganic nanofibers are characterized by having the following properties, which enable them to exhibit sufficient strength even when exposed to high temperatures: The inorganic nanofibers may be solid or hollow (e.g., tubular) as long as they are made of inorganic material and are pulverized to nanometer size. Among these, solid nanofibers are preferred from the viewpoint of ensuring sufficient strength when exposed to high temperatures.

[0021] (aspect ratio) The inorganic nanofibers used in the present invention must have an aspect ratio of at least 3. If the aspect ratio is less than 3, the strength decreases when exposed to high temperatures, resulting in an insufficient reinforcing effect. The aspect ratio of the inorganic nanofiber is 3 or more, and from the viewpoint of improving strength when exposed to high temperatures, it is preferably 3.5 or more, more preferably 4 or more, and even more preferably 4.5 or more. If the aspect ratio is too high, the inorganic nanofibers become entangled, reducing adhesion to the inorganic compound and tending to reduce the initial strength (strength before exposure to high temperatures). Therefore, the aspect ratio of the inorganic nanofiber is preferably 1000 or less, more preferably 700 or less, even more preferably 550 or less, and even more preferably 400 or less.

[0022] The fiber diameter and fiber length of the inorganic nanofibers can be selected as appropriate as long as the aspect ratio is 3 or more, but for example, the fiber diameter is preferably 1.0 nm or more, more preferably 1.5 nm or more, even more preferably 2.0 nm or more, even more preferably 2.5 nm or more, and particularly preferably 3 nm or more, and preferably 60 nm or less, more preferably 50 nm or less, even more preferably 40 nm or less, even more preferably 30 nm or less, and particularly preferably 20 nm or less. The fiber length is preferably 10 nm or more, more preferably 20 nm or more, even more preferably 30 nm or more, and even more preferably 40 nm or more, and preferably 3500 nm or less, more preferably 1500 nm or less, even more preferably 500 nm or less, even more preferably 250 nm or less, and particularly preferably 100 nm or less. The fiber diameter of the inorganic nanofibers is determined by measuring the fiber diameters of 100 inorganic nanofibers randomly selected within the observation field when observing the inorganic nanofibers using a transmission electron microscope (TEM, for example, JEM-2100F (manufactured by JEOL Ltd.)) and calculating the average value. The fiber length of the inorganic nanofibers is determined by measuring the fiber lengths of 100 inorganic nanofibers randomly selected within the observation field when observing the inorganic nanofibers using a scanning electron microscope (SEM, for example, JSM-7001F (manufactured by JEOL Ltd.)) and calculating the average value. When using commercially available inorganic nanofibers, the fiber diameter and fiber length listed in the catalog can be referenced.

[0023] (Surface area) The inorganic nanofibers used in the present invention have a surface area of ​​500 nm 2 The surface area must be 500nm or more. 2 If the thickness is less than this, the adhesion to the inorganic compound decreases, resulting in an insufficient reinforcing effect. The surface area of ​​the inorganic nanofiber is set to 1000 nm in order to ensure sufficient strength even when exposed to high temperatures. 2 More than 1200nm is preferable. 2 More preferably, 1400 nm or more 2The upper limit of the surface area of ​​the inorganic nanofiber is not particularly limited, but from the same viewpoint as above, it is preferably 40,000 nm 2 Preferably below 25000nm 2 Less than 18000nm is more preferable 2 The following is more preferable: The surface area of ​​the inorganic nanofiber can be calculated by the following formula (1) assuming that the fiber is cylindrical and the fiber diameter is the diameter of the circle at the base.

[0024] Surface area of ​​inorganic nanofibers =π×〔(fiber diameter) / 2〕 2 ×2+π×(fiber diameter)×(fiber length) (1)

[0025] The inorganic nanofibers are not particularly limited as long as they have heat resistance of 600°C or higher and the above-mentioned properties, but examples thereof include aluminum oxide nanofibers, aluminum hydroxide nanofibers, titanium oxide nanofibers, zirconium oxide nanofibers, silicon carbide nanofibers, aluminum silicate nanofibers, silicon nitride nanofibers, boron nitride nanofibers, and carbon nanofibers. One or more types of inorganic nanofibers may be contained.

[0026] Among these, the inorganic nanofibers are preferably one or more selected from aluminum oxide nanofibers, titanium oxide nanofibers, silicon carbide nanofibers, silicon nitride nanofibers, and boron nitride nanofibers, as these nanofibers can more easily achieve the effects of the present invention.

[0027] In addition, from the viewpoint of improving affinity with the inorganic compound and exhibiting sufficient strength even when exposed to high temperatures, it is preferable to use inorganic nanofibers containing the same elements as the constituent elements of the inorganic compound. For example, the following suitable combinations can be mentioned. (i) When the outer shell of the hollow particle contains aluminum oxide, it preferably contains aluminum oxide nanofibers. (ii) When the outer shell of the hollow particle contains silicon oxide, it is preferable that it contains one or more selected from silicon carbide nanofibers and silicon nitride nanofibers. (iii) When the outer shell of the hollow particle contains boron oxide, it is preferable that it contains boron nitride nanofibers. (iv) When the outer shell of the hollow particles contains aluminosilicate glass, it preferably contains one or more selected from aluminum oxide nanofibers, silicon carbide nanofibers, and silicon nitride nanofibers. (v) When the outer shell of the hollow particles contains aluminoborosilicate glass, it is preferable that the outer shell contains one or more nanofibers selected from aluminum oxide nanofibers, silicon carbide nanofibers, silicon nitride nanofibers, and boron nitride nanofibers. (vi) When the outer shell of the hollow particles contains aluminosilicate glass or aluminoborosilicate glass, it preferably contains at least aluminum oxide nanofibers.

[0028] From the viewpoint of exhibiting sufficient strength even when exposed to high temperatures, the content of inorganic nanofibers in the fiber-reinforced hollow particles of the present invention is preferably 1% by mass or more, more preferably 3% by mass or more, even more preferably 6% by mass or more, and even more preferably 8% by mass or more. Furthermore, if the content of inorganic nanofibers is too high, the inorganic nanofibers become entangled, reducing adhesion to the inorganic compound and tending to reduce initial strength (strength before exposure to high temperatures). Therefore, the content of inorganic nanofibers is preferably 20% by mass or less, more preferably 15% by mass or less, even more preferably 13% by mass or less, and even more preferably 11% by mass or less.

[0029] (Hollow rate) The hollowness of the fiber-reinforced hollow particles of the present invention is typically 50% or more, preferably 65% ​​or more, more preferably 70% or more, even more preferably 75% or more, and even more preferably 80% or more. Such a hollowness allows for sufficient performance in various physical properties, such as thermal insulation and dielectric properties. The upper limit of the hollowness is preferably 95% or less, more preferably 90% or less, from the viewpoint of ensuring sufficient strength. Herein, the "hollowness" can be calculated using the following formula (2) after measuring the apparent density of the particles using a dry automatic densimeter and calculating the true density of the outer shell based on the chemical composition measured using an X-ray fluorescence analyzer. Since it is difficult to measure the hollowness of individual particles, the hollowness refers to the hollowness of the particle group. Herein, the "apparent density" is measured using a dry automatic densimeter in accordance with JIS R 1620. For example, an Accupyc (Shimadzu Corporation) can be used as the dry automatic densimeter.

[0030] Hollowness (%) = [1 - (apparent density) / (true density)] x 100 (2)

[0031] The average particle diameter of the fiber-reinforced hollow particles of the present invention is not particularly limited, but is preferably 1 μm or more from the viewpoint of ensuring sufficient voids, and is preferably 10 μm or less, more preferably 5 μm or less, from the viewpoint of application to small components and thin film coatings. Here, in this specification, the "average particle diameter" refers to the particle diameter (D 50 ) For measuring the particle size distribution, for example, a laser diffraction / scattering particle size distribution measuring device can be used.

[0032] The thickness of the outer shell of the fiber-reinforced hollow particles of the present invention can be appropriately set depending on the hollowness. For example, when the hollowness is 50% or more, it is preferably 20% or less of the particle diameter; when the hollowness is 70% or more, it is preferably 10% or less of the particle diameter; and when the hollowness is 80% or more, it is preferably 7% or less of the particle diameter. Here, the "thickness of the outer shell" in this specification refers to the thickness of the wall located on the outermost surface of the particle. Thirty randomly selected particles are observed with a scanning electron microscope (SEM), and the thickness of the outer shell is measured using the following method. That is, an SEM image is taken for each particle, three points on the outer shell are randomly selected from the SEM image, the thickness of the center of each of the three points is measured, and the average value is calculated to determine the shell thickness of that particle. The average value for the 30 particles is then calculated. Note that, for example, a JSM-7001F (manufactured by JEOL Ltd.) can be used as the scanning electron microscope.

[0033] The fiber-reinforced hollow particles of the present invention can have the following properties: (i) The particle strength after heating at 600°C for 1 hour can be preferably 6 MPa or more, more preferably 7 MPa or more, and even more preferably 8 MPa or more. The upper limit of the particle strength is not particularly limited, but is, for example, preferably 20 MPa or less, more preferably 18 MPa or less, and even more preferably 15 MPa or less. Here, in this specification, "particle strength" refers to the particle strength when pressure is applied to hollow particles using a pressure molding machine and the hollow structure retention rate is 50%. Specifically, it can be measured by the method described in the Examples below. (ii) When heated at 600°C for 1 hour, the change in particle strength between before and after heating can be preferably 10% or less, more preferably 5% or less, and even more preferably 2% or less. The change in particle strength (%) can be calculated using the following formula (3).

[0034] Particle strength change rate (%) = |(AB)| / A × 100 (3)

[0035] (In the formula, A represents the particle strength (MPa) before heating, and B represents the particle strength (MPa) after heating.)

[0036] The fiber-reinforced hollow particles of the present invention can exhibit sufficient strength even when exposed to high temperatures, and can maintain a strength sufficient for practical use without breaking even if cracks occur in part of the outer shell. Therefore, the fiber-reinforced hollow particles of the present invention can be used, for example, as heat insulating materials, heat shielding materials, catalyst carriers, building materials, electronic materials, etc.

[0037] <Method for manufacturing fiber-reinforced hollow particles> The method for producing fiber-reinforced hollow particles of the present invention is characterized by comprising the steps of: feeding a raw material liquid containing raw material compounds and inorganic nanofibers contained in an agitation tank equipped with an agitation blade having a ratio (D2 / D1) of the inner diameter D1 of the agitation tank to the rotational diameter D2 of the agitation blade of 0.3 or more, while stirring at a rotational speed of 500 rpm or more, to a spraying device; and spraying and heating the raw material liquid from the spraying device. By feeding the raw material liquid contained in the agitation tank to the spraying device while stirring under predetermined conditions in this way, it is possible to suppress the settling of inorganic nanofibers into the raw material liquid, thereby enabling the efficient production of hollow particles containing inorganic nanofibers in the outer shell. The method for producing the fiber-reinforced hollow particles of the present invention can be any known method as long as it is possible to control the above-mentioned ratio (D2 / D1) and the stirring rotation speed of the raw material liquid, and examples of such methods include spray pyrolysis and spray drying.

[0038] (Preparation of raw material solution) The raw material solution may be prepared in a stirring tank, or a previously prepared raw material solution may be added to the stirring tank. The order in which the raw materials are added when preparing the raw material solution is not particularly limited, and the raw material compounds, inorganic nanofibers, and solvent may be added in any order or simultaneously, but from the viewpoint of handleability, it is preferable to mix the raw material compounds and solvent and then mix the mixed solution with the inorganic nanofibers.

[0039] Examples of the raw material compound include compounds containing one or more elements selected from Group 1 elements of the periodic table, Group 2 elements of the periodic table, Group 4 elements of the periodic table, Group 8 elements of the periodic table, Group 9 elements of the periodic table, Group 10 elements of the periodic table, Group 11 elements of the periodic table, Group 12 elements of the periodic table, Group 13 elements of the periodic table, Group 14 elements of the periodic table, and Group 15 elements of the periodic table, and one or more compounds selected from inorganic salts, organic salts, and alkoxides.

[0040] Suitable raw material compounds include, for example, inorganic salts and alkoxides. Examples of inorganic salts include sodium salts, potassium salts, magnesium salts, calcium salts, barium salts, titanium salts, zirconium salts, zinc salts, borates, boric acid, and aluminum salts. Examples of alkoxides include aluminum alkoxides and silicate alkoxides.

[0041] Examples of sodium salts include sodium nitrate, sodium sulfate, sodium chloride, and sodium hydroxide. Examples of potassium salts include potassium nitrate, potassium sulfate, potassium chloride, and potassium hydroxide. Examples of magnesium salts include magnesium nitrate, magnesium sulfate, magnesium chloride, magnesium phosphate, and magnesium hydroxide. Examples of calcium salts include calcium nitrate, calcium chloride, calcium hydroxide, calcium formate, calcium acetate, and calcium propionate. Examples of borates include metaborates such as sodium borate and potassium borate, tetraborates such as sodium tetraborate and potassium tetraborate, and pentaborates such as sodium pentaborate and potassium pentaborate. Examples of aluminum salts include aluminum nitrate, aluminum sulfate, aluminum chloride, aluminum phosphate, aluminum hydroxide, aluminum acetate, and aluminum oxalate. Examples of silicate alkoxides include tetramethyl orthosilicate (TMOS), tetraethyl orthosilicate (TEOS), tetrapropyl orthosilicate (TPOS), and tetrabutoxysilane. Furthermore, a solution in which aluminum oxide or silicon oxide is dispersed in a solvent, or a sol solution of aluminum oxide or silicon oxide can also be used as the raw material solution.

[0042] Among these, as the raw material compound, from the viewpoint of easily enjoying the effects of the present invention, one or more compounds selected from sodium salts, potassium salts, magnesium salts, calcium salts, barium salts, titanium salts, zirconium salts, zinc salts, borates, boric acid, aluminum salts, aluminum alkoxides, and silicate alkoxides are preferred, one or more compounds selected from sodium salts, potassium salts, magnesium salts, calcium salts, borates, boric acid, aluminum salts, aluminum alkoxides, and silicate alkoxides are more preferred, and compounds containing at least sodium salts, magnesium salts, calcium salts, boric acid, aluminum salts, and silicate alkoxides are even more preferred.

[0043] The solvent is not particularly limited as long as it can dissolve the raw material compounds, and examples thereof include water and organic solvents. Among these, water is preferred from the viewpoints of environmental impact and production costs. The method of mixing the raw material compounds and the solvent is not particularly limited, and they may be mixed by adding both simultaneously or by adding one to the other.

[0044] The amounts of the raw material compounds and the solvent used are such that the total concentration of the raw material compounds in the raw material solution is usually 0.01 to 1.0 mol / L, and preferably 0.1 to 0.9 mol / L. The content of each raw material compound in the raw material solution may be any amount that satisfies the stoichiometric composition based on the preset hollow particles.

[0045] The inorganic nanofibers may be appropriately selected as long as they satisfy the above-mentioned aspect ratio and surface area. Specific embodiments are as described above. The amount of inorganic nanofibers used is an amount that will result in the above-mentioned content of inorganic nanofibers in the hollow particles.

[0046] The liquid temperature of the raw material liquid may be set as appropriate depending on the type of solvent as long as it is higher than the freezing point of the solvent, but is preferably 1 to 50°C, more preferably 5 to 40°C, and even more preferably 10 to 30°C.

[0047] (stirring tank) The shape of the stirring vessel is not particularly limited as long as it can uniformly stir the raw material liquid, and examples thereof include a vertical cylindrical type, a semi-elliptical type, a semi-circular type, and a conical type. The volume of the stirring vessel can be appropriately selected depending on the production scale, but the volume of the raw material liquid usually contained in the stirring vessel is preferably 40 to 100%, more preferably 50 to 98%, of the volume of the stirring vessel.

[0048] (stirring blade) The stirring blades may be appropriately selected from propeller blades, paddle blades, triple-swept blades, anchor blades, screw blades, turbine blades, ribbon blades, etc. Among these, propeller blades, paddle blades, and turbine blades are preferred. The stirring blades may be installed on the stirring shaft in one stage or multiple stages, and when installed in multiple stages, they may be the same or different. The number of blades constituting the stirring blade is preferably two or more, and more preferably two to four.

[0049] When installing a single-stage agitator, it is preferable to install the agitator below the agitator tank. When installing multiple stages, the first stage should be installed below the agitator tank, and the second and subsequent stages should be installed above that. The agitator blades from the third stage onwards may be spaced at equal intervals to or different from the intervals between the first and second agitator blades.

[0050] (Impeller rotation diameter D2 / tank inner diameter D1) In the present invention, it is necessary to use stirring blades in which the ratio (D2 / D1) of the inner diameter D1 of the stirring vessel to the rotational diameter D2 of the stirring blades is 0.3 or more. If this ratio (D2 / D1) is less than 0.3, sufficient centrifugal force is not applied to the raw material liquid, causing the inorganic nanofibers to settle and resulting in a raw material liquid containing no inorganic nanofibers being sent to the spraying device, making it impossible to produce hollow particles containing inorganic nanofibers in the outer shell. Here, in this specification, "inner diameter of a stirring vessel" refers to the maximum inner diameter of the stirring vessel on the same horizontal plane as the stirring blades fixed to the stirring shaft. Also, "rotation diameter of the stirring blade" refers to the diameter of the circumference formed by the stirring blade when the stirring blade is rotated. Note that when stirring blades are installed in multiple stages, it is sufficient that the ratio (D2 / D1) of each stirring blade satisfies the above range. From the viewpoint of suppressing the settling of the inorganic nanofibers in the raw material liquid, the ratio (D2 / D1) is preferably 0.35 or more, and more preferably 0.4 or more, and from the viewpoint of suppressing the entrainment of air and foaming of the raw material liquid, the ratio is preferably 0.9 or less, more preferably 0.8 or less, and even more preferably 0.7 or less.

[0051] (rotation speed of stirring blade) The rotation speed of the stirring blade must be 500 rpm or more. If the rotation speed is less than 500 rpm, sufficient shear force is not applied to the raw material liquid, causing the inorganic nanofibers to settle and resulting in the raw material liquid containing no inorganic nanofibers being sent to the spraying device, making it impossible to produce hollow particles containing inorganic nanofibers in the outer shell. The rotation speed of such stirring blades is preferably 600 rpm or more, more preferably 700 rpm or more, from the viewpoint of suppressing sedimentation of inorganic nanofibers in the raw material liquid, and is preferably 1300 rpm or less, more preferably 1100 rpm or less, and even more preferably 900 rpm or less, from the viewpoint of suppressing air entrainment and foaming of the raw material liquid. When stirring blades are installed in multiple stages, the rotation speed of each stirring blade may be the same or different as long as it is within the above range.

[0052] Next, the raw material liquid in the stirring tank is sent to a spraying device by a liquid sending pump, and a mist of the raw material liquid is sprayed from the spraying device into the heating furnace. Here, in this specification, "mist" refers to fine droplets dispersed and suspended in a gas, in other words, a mist-like liquid. Any material that is used as a furnace material can be used for the heating furnace, and the material may be selected taking into consideration the heating temperature, etc. The shape of the heating furnace is preferably a vertical cylindrical type, and the size of the heating furnace can be appropriately selected depending on the production scale.

[0053] The spraying device is not particularly limited, and for example, a fluid nozzle such as a two-fluid nozzle, a three-fluid nozzle, or a four-fluid nozzle can be used. Fluid nozzle systems include an internal mixing system in which gas and raw material liquid are mixed inside the nozzle, and an external mixing system in which gas and raw material liquid are mixed outside the nozzle, and either can be used. Examples of gases that can be supplied to the nozzle include air and inert gases such as nitrogen and argon. Among these, air is preferred from the viewpoint of economy. One or more spraying devices can be installed, and they can be installed at either the bottom or top of the heating furnace.

[0054] The flow rate of the raw material liquid is preferably 1 L / h or more, more preferably 3 L / h or more, even more preferably 5 L / h or more, and is preferably 100 L / h or less, more preferably 80 L / h or less, even more preferably 60 L / h or less. The ejection speed of the raw material liquid is preferably 1 m / s or more, more preferably 5 m / s or more, even more preferably 10 m / s or more, and is preferably 50 m / s or less, more preferably 35 m / s or less, even more preferably 20 m / s or less.

[0055] The mist of the raw material liquid sprayed from the spraying device is heated by a heating device in the heating furnace to form a film containing the inorganic compound and inorganic nanofibers, and fiber-reinforced hollow particles are formed from this as the starting point. Examples of heating devices include combustion burners, hot air heaters, and electric heaters. One or more heating devices can be installed. Any combustion burners, hot air heaters, and electric heaters that are commonly available on the market can be used. The heating temperature is preferably 200°C or higher, more preferably 400°C or higher, even more preferably 600°C or higher, even more preferably 800°C or higher, and is preferably 1800°C or lower, more preferably 1600°C or lower, even more preferably 1400°C or lower, and even more preferably 1200°C or lower.

[0056] Next, the fiber-reinforced hollow particles are recovered. The fiber-reinforced hollow particles can be recovered, for example, by moving them from the downstream side of the heating furnace to a powder recovery device using an induction fan. Furthermore, dust removal and purification equipment such as a scrubber can be installed downstream of the powder recovery device, as needed. Examples of powder recovery devices include a cyclone powder recovery machine and a bag filter. Furthermore, when recovering the fiber-reinforced hollow particles, the particle size can be adjusted by passing them through a filter. [Example]

[0057] The following examples will explain the present invention in more detail, but the present invention is not limited to the examples below.

[0058] The raw materials used in this example are as follows: (1) Raw material compounds for forming inorganic compounds Tetraethyl orthosilicate: Tama Chemicals Co., Ltd. Aluminum nitrate nonahydrate: Osaki Kogyo Co., Ltd. Calcium nitrate tetrahydrate: Osaki Kogyo Co., Ltd. Magnesium nitrate hexahydrate: Ako Kasei Co., Ltd. Sodium nitrate: Nissan Chemical Co., Ltd. Boric acid: Etimaden (2) Aluminum oxide nanofiber Examples 1 to 3: AS-200, manufactured by Nissan Chemical Industries, Ltd., fiber diameter (minor diameter) 10 nm, fiber length (major diameter) 50 nm (all catalog values) Example 4: Aluminasol F1000, manufactured by Kawaken Fine Chemicals Co., Ltd., fiber diameter (minor diameter) 4 nm, fiber length (major diameter) 1400 nm (all values ​​listed in the catalog) Example 5: Aluminasol F3000, manufactured by Kawaken Fine Chemicals Co., Ltd., fiber diameter (minor diameter) 4 nm, fiber length (major diameter) 3000 nm (all values ​​listed in the catalog) Comparative Example 2: NP-ALO-1-100 (nano alumina particles, spherical), manufactured by EM Japan, fiber diameter (minor axis) 80 nm, fiber length (major axis) 80 nm (all values ​​listed in the catalog) (3) Cellulose nanofiber Comparative Example 3: BMa-100, manufactured by Sugino Machine Co., Ltd., fiber diameter (minor diameter) 30 nm, fiber length (major diameter) 1000 nm (all values ​​listed in the catalog)

[0059] 1. Analyzing the Surface Area of ​​Nanofibers The shape of the nanofiber was considered to be cylindrical, and the fiber diameter (listed in the catalog) was calculated using the diameter of the circle at the base using the following formula (1). Note that the nanoalumina particles of Comparative Example 2 are spherical, so 4πr 2 was calculated from

[0060] Nanofiber surface area =π×〔(fiber diameter) / 2〕 2 ×2+π×(fiber diameter)×(fiber length) (1)

[0061] 2. Chemical composition analysis The hollow particles were pressed into briquettes, which were then measured in oxide equivalents using an X-ray fluorescence analyzer (ZSX primus II, manufactured by Rigaku Corporation). The chemical components of each element were calculated by correcting the total value of the oxides of the analyzed elements (SiO2, Al2O3, BO3, CaO, MgO) using the following formula so that it was 100%.

[0062] Chemical composition (corrected) (%) = Chemical composition (uncorrected) × 100 / (100 - impurities (%)) (In the formula, the impurity (%) is calculated by subtracting the total value of the chemical composition of the oxides described above from 100.)

[0063] 3. Hollowness analysis The apparent density of the hollow particles was measured using a dry automatic density meter (Accupyc, manufactured by Shimadzu Corporation), and the true density of the outer shell was calculated based on the chemical composition measured using an X-ray fluorescence analyzer. The hollow ratio was then calculated using the following formula (2).

[0064] Hollowness (%) = [1 - (apparent density) / (true density)] x 100 (2)

[0065] 4. Particle Intensity Measurement The particle strength was measured by the following procedure for particles before and after heating at 600°C for 1 hour in a box-type electric furnace. (1) The particles and ethanol were mixed in a mass ratio of 5:1 to prepare a sample (ethanol was mixed because it is difficult to form pellets by pressure using only the particles). (2) A certain amount of sample was placed into the jig. (3) The sample was placed in a pressure molding machine and hydraulic pressures (2 MPa, 5 MPa, 10 MPa, 30 MPa) were applied. (4) The specimen was left standing for 1 minute with the pressure applied. (5) The pellets were removed from the pressure molding machine. (6) The pellets were dried at 105°C for 2 hours (using a hot air dryer to remove the ethanol mixed in (1)). (7) The density of the particles was measured using a microcompression tester (MCT-510, manufactured by Shimadzu Corporation).

[0066] Then, the remaining rate of the hollow structure was calculated from the obtained density, mass, volume before destruction, and volume after destruction according to the following formulas (a) and (b). The remaining rate p of the hollow structure was calculated as follows. Let m be the mass, V be the volume before fracture, and v be the volume after fracture. In this case, the density before fracture (apparent density) is x = m / V, and the density after fracture (true density) is y = m / v. If the remaining rate of the hollow structure is p, the apparent density ρ is expressed as follows:

[0067] m / ((V×p+v×(1-p))=ρ (a)

[0068] Solving this for p gives us the following:

[0069] p=(1-ρ / y) / ρ×(1 / x-1 / y) (b)

[0070] Furthermore, the survival rate (linear) of the hollow structure is shown as the survival rate P calculated by the following formula. The survival rate (linear) may also be calculated by formula (c). x×P+y×(1-P)=ρ (c)

[0071] A graph showing the relationship between the residual rate p of the hollow structure and the pressure was created, and the pressure at which 50% remained was determined from the graph, and this value was taken as the 50% remaining strength of the hollow particles.

[0072] Examples 1 to 5 and Comparative Examples 2 to 3 The raw material compounds (tetraethyl orthosilicate, aluminum nitrate nonahydrate, magnesium nitrate hexahydrate, sodium nitrate, and boric acid) were mixed in 250 kg of ion-exchanged water to the molar concentrations shown in Table 1, and the resulting solution was placed in a stirring tank.

[0073] [Table 1]

[0074] Next, a raw material solution was prepared by adding the nanofibers shown in Table 3 to the solution in the stirring tank so that the content of each nanofiber in the particles was the amount shown in Table 3. Next, the raw material solution in the stirring tank was stirred for 1 hour under the conditions that the ratio (D2 / D1) of the inner diameter D1 of the stirring tank to the rotation diameter D2 of the stirring blade was 0.3 and the rotation speed of the stirring blade was 600 rpm. Next, while continuing to stir the raw material liquid, a pump was used to send the raw material liquid to a two-fluid nozzle at a speed of 0.15 m / s or more, and the raw material liquid was sprayed from the two-fluid nozzle into a heating furnace. The spraying conditions for the two-fluid nozzle were a nozzle air flow rate of 500 L / min and a liquid flow rate of 470 mL / min. The internal temperature of the heating furnace was set to 1000°C. The particles were then quenched using a cooling mechanism installed at the furnace outlet, and the hollow particles were then recovered using a bag filter. The analysis results of the obtained hollow particles are shown in Table 3. The chemical composition of the hollow particles obtained in Example 1 is shown in Table 2. Furthermore, when the X-ray diffraction spectrum of the hollow particles was measured using an X-ray diffractometer (D8 ADVANCE, manufactured by Bruker), a broad XRD pattern was observed along with sharp peaks derived from aluminum oxide nanofibers, as shown in Figure 1, confirming that the outer shell of the hollow particles was composed of aluminoborosilicate glass and aluminum oxide nanofibers.

[0075] [Table 2]

[0076] Comparative Example 1 Hollow particles were produced in the same manner as in Example 1, except that aluminum oxide nanofibers were not added to the raw material liquid. The X-ray diffraction spectrum of the obtained hollow particles was measured using an X-ray diffractometer in the same manner as in Example 1. As shown in Figure 2, no sharp peaks derived from aluminum oxide nanofibers as in Figure 1 were observed, confirming that the outer shell of the hollow particles was composed of aluminoborosilicate glass and did not contain aluminum oxide nanofibers.

[0077] Comparative Example 4 An aluminum oxide nanofiber-containing solution was prepared by the same procedure as in Example 1, except that the stirring blade was changed to one in which the ratio (D2 / D1) of the inner diameter D1 of the stirring vessel to the rotation diameter D2 of the stirring blade was 0.1. Next, aluminum oxide nanofibers had settled in the raw material liquid in the stirring vessel, and hollow particles were produced using this solution by the same procedure as in Example 1. The X-ray diffraction spectrum of the obtained hollow particles was measured using an X-ray diffractometer in the same manner as in Example 1. As a result, no sharp peak derived from aluminum oxide nanofibers was observed, as in Comparative Example 1. This revealed that the outer shell of the hollow particles was composed of aluminoborosilicate glass and did not contain aluminum oxide nanofibers, and further analysis was therefore abandoned.

[0078] Comparative Example 5 An aluminum oxide nanofiber-containing solution was prepared in the same manner as in Example 1, except that the rotation speed of the stirring blade was changed to 300 rpm. Next, aluminum oxide nanofibers had settled in the raw material liquid in the stirring tank, and hollow particles were produced using this solution in the same manner as in Example 1. The X-ray diffraction spectrum of the obtained hollow particles was measured using an X-ray diffractometer in the same manner as in Example 1. As a result, no sharp peak derived from aluminum oxide nanofibers was observed, as in Comparative Example 1. This revealed that the outer shell of the hollow particles was composed of aluminoborosilicate glass and did not contain aluminum oxide nanofibers, and further analysis was therefore abandoned.

[0079] [Table 3]

[0080] Table 3 shows that by incorporating inorganic nanofibers with controlled aspect ratios and surface areas into the outer shell of hollow particles, a fiber reinforcement effect (toughness) is achieved, and sufficient strength can be achieved even when exposed to high temperatures of 600°C. Furthermore, it can be seen that hollow particles containing inorganic nanofibers in the outer shell can be efficiently produced by controlling the ratio of the inner diameter D1 of the stirring tank to the rotational diameter D2 of the stirring impeller and the rotational speed of the stirring impeller for the raw material liquid in the stirring tank before supplying it to the spraying device, and then spraying and heating this raw material liquid from the spraying device.

Claims

1. an outer shell portion including an inorganic compound and inorganic nanofibers; a hollow portion surrounded by the outer shell portion; Equipped with The inorganic nanofibers have an aspect ratio of 3 or more and a surface area of ​​500 nm 2 That's all. Fiber reinforced hollow particles.

2. The fiber-reinforced hollow particle according to claim 1, wherein the content of the inorganic nanofibers in the hollow particle is 1% by mass or more and 20% by mass or less.

3. 3. The fiber-reinforced hollow particle according to claim 1, wherein the inorganic nanofibers are one or more selected from the group consisting of aluminum oxide nanofibers, titanium oxide nanofibers, silicon carbide nanofibers, silicon nitride nanofibers, and boron nitride nanofibers.

4. 3. The fiber-reinforced hollow particle according to claim 1, wherein the hollowness is 50% or more.

5. The method includes a step of feeding a raw material liquid containing raw material compounds and inorganic nanofibers, which is contained in a stirring vessel equipped with a stirring blade having a ratio (D2 / D1) of an inner diameter D1 of the stirring vessel to a rotational diameter D2 of the stirring blade, to a spraying device while stirring at a rotational speed of 500 rpm or more, and spraying and heating the raw material liquid from the spraying device, The inorganic nanofibers have an aspect ratio of 3 or more and a surface area of ​​500 nm 2 That's all. Method for producing fiber-reinforced hollow particles.

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