Core-shell particle, method for producing hollow particle, core-shell particle, and hollow particle

Abstract

In order to produce core-shell particles and hollow particles having a sharp particle size distribution by suppressing the generation of solid particles and also suppressing particle aggregation and coalescence, this method for producing core-shell particles according to one embodiment of the present disclosure involves mixing an oil phase containing a hydrophobic organic solvent and a metal alkoxide into an aqueous phase comprising water and / or a hydrophilic organic solvent while in the presence of a surfactant, forming an O / W emulsion, and producing core-shell particles having a solvent as a core and a metal oxide as a shell.

Description

Method for producing core-shell particles and hollow particles, core-shell particles and hollow particles

[0001] The present invention relates to a method for producing core-shell particles having a solvent as the core and a metal oxide as the shell, and a method for producing hollow particles having air as the core and a metal oxide as the shell.

[0002] Hollow particles have a lower specific gravity than solid particles and, due to their hollow shape, possess properties such as a low dielectric constant, a low dielectric dissipation factor, and a low thermal conductivity. Therefore, by incorporating hollow particles into resins, the resin compositions can be made lightweight and have a low dielectric constant, a low dielectric dissipation factor, and a low thermal conductivity, leading to their use in a wide range of applications. From an industrial perspective, a useful method for producing hollow particles is to use an emulsion to produce core-shell particles having a solvent as the core and a metal oxide as the shell, and then calcining the resulting particles to remove the solvent. Here, Patent Document 1, for example, discloses a method for producing core-shell particles that encapsulate the organic material inside, by preparing an aqueous solution containing a quaternary ammonium salt, a silica source (an example of a metal oxide source), a hydrophobic organic material, and water, and stirring the resulting solution at a temperature of 10 to 100°C. Furthermore, Patent Document 2 discloses a method for synthesizing core-shell particles by converging and mixing a solvent containing a hydrophobic organic substance and a hydrophilic organic solvent with an aqueous medium to obtain an emulsion containing emulsified oil droplets of the hydrophobic organic substance, and then adding and mixing a silica source to the obtained emulsion to form a shell made of silica on the surface of the emulsified oil droplets.

[0003] Japanese Patent Publication No. 2008-150229 Japanese Patent Publication No. 2012-246187

[0004] However, the methods for producing core-shell particles described in Patent Documents 1 and 2 both have the problem of generating solid particles other than core-shell particles because the silica source (metal oxide source) reacts outside the oil droplets, and the problem of particles associating or coalescing to form irregularly shaped particles. The solid particles generated have significantly smaller particle sizes than core-shell particles containing organic matter inside the core, and irregularly shaped particles formed by association or coalescence have larger particle sizes. Therefore, metal oxide powders using core-shell particles obtained by this production method tend to have a broad particle size distribution. In light of the above background, one aspect of the present invention is to provide a method for producing core-shell particles and hollow particles with a narrow particle size distribution by suppressing the generation of solid particles and irregularly shaped particles.

[0005] The present inventors have conducted extensive research to solve the above-mentioned problems and have found that the generation of solid particles and irregular-shaped particles can be suppressed by mixing an oil phase containing a hydrophobic organic solvent and a metal alkoxide with an aqueous phase consisting of water and / or a hydrophilic organic solvent in the presence of a surfactant. A method for producing core-shell particles according to one aspect of the present invention involves mixing an oil phase containing a hydrophobic organic solvent and a metal alkoxide with an aqueous phase consisting of water and / or a hydrophilic organic solvent in the presence of a surfactant to form an O / W emulsion, thereby producing core-shell particles having a solvent core and a metal oxide shell. The core-shell particles according to one aspect of the present invention are core-shell particles having a hydrophobic organic solvent core and a metal oxide shell, and have a mode diameter D ｍｏｄ and cumulative 10% diameter D １０ Relative to D ｍｏｄ / D １０ is 20 or less, and the cumulative 50% diameter D ５０ and cumulative 90% diameter D ９０ Relative to D ５０ / D ９０ The hollow particles according to one embodiment of the present invention are hollow particles having a metal oxide shell, and have a mode diameter D ｍｏｄ and cumulative 10% diameter D １０ Relative to D ｍｏｄ / D１０ is 20 or less, and the cumulative 50% diameter D ５０ and cumulative 90% diameter D ９０ Relative to D ５０ / D ９０ is 0.6 or more.

[0006] The present invention makes it possible to provide a method for producing core-shell particles and hollow particles, such as core-shell silica particles and hollow silica particles, having a sharp particle size distribution by suppressing the generation of solid particles and irregularly shaped particles.

[0007] Modes for carrying out the present invention are described in detail below. However, the present invention is not limited to the following embodiments. A method for producing core-shell particles according to one aspect of the present invention is characterized by mixing an oil phase containing a hydrophobic organic solvent and a metal alkoxide with an aqueous phase containing water and / or a hydrophilic organic solvent in the presence of a surfactant to form an O / W emulsion, thereby producing core-shell particles having the solvent as the core and the metal oxide as the shell. A method for producing core-shell particles according to a preferred aspect of the present invention is characterized by mixing an oil phase containing a hydrophobic organic solvent, a metal alkoxide, a basic compound, and a surfactant with an aqueous phase containing water and / or a hydrophilic organic solvent to form an O / W emulsion, thereby producing core-shell particles having the solvent as the core and the metal oxide as the shell. One aspect of the present invention is described below.

[0008] <Method for Producing Core-Shell Particles> A method for producing core-shell particles according to one embodiment of the present invention is characterized in that an aqueous phase consisting of water and / or a hydrophilic organic solvent is mixed with an oil phase containing a hydrophobic organic solvent and a metal alkoxide in the presence of a surfactant to form an O / W emulsion, thereby producing core-shell particles with the solvent as the core and the metal oxide as the shell. The oil phase is micronized in the aqueous phase by mixing, generating oil droplets. The surfactant stabilizes the interface between the oil droplets and the aqueous phase, thereby forming an O / W emulsion in which the oil droplets are dispersed in the aqueous phase. In the O / W emulsion, the metal alkoxide present on the surface of the oil droplets first comes into contact with water in the aqueous phase and hydrolyzes, forming a shell made of metal oxide on the surface of the oil droplets. The metal alkoxide disperses from the center of the oil droplets to the surface, so as to level the metal alkoxide concentration gradient within the oil droplets generated by the consumption of the metal alkoxide through hydrolysis. The shell continues to grow thick until water and the metal alkoxide no longer diffuse within the shell. In this way, a shell made of metal oxide is formed on the surface of the oil droplets through hydrolysis and dispersion. Since the other components remain inside the shell as a solvent, it is possible to produce core-shell particles in which the solvent is the core and the metal oxide is the shell.

[0009] A preferred embodiment of the present invention relates to a method for producing core-shell particles. This method involves mixing an oil phase containing a hydrophobic organic solvent, a metal alkoxide, a basic compound, and a surfactant with an aqueous phase containing water and / or a hydrophilic organic solvent to form an O / W emulsion, thereby producing core-shell particles with the solvent as the core and the metal oxide as the shell. The oil phase is refined in the aqueous phase by mixing, generating oil droplets. The surfactant stabilizes the interface between the oil droplets and the aqueous phase, forming an O / W emulsion in which the oil droplets are dispersed in the aqueous phase. The hydrolysis of the metal alkoxide inside the oil droplets is promoted by the catalytic action of the basic compound, but the metal alkoxide component seeks water necessary for hydrolysis and accumulates in the outer layer inside the oil droplets. This hydrolysis and accumulation results in the formation of a shell made of metal oxide on the outermost layer of the oil droplets. Since other components remain inside the shell as solvent, core-shell particles with the solvent as the core and the metal oxide as the shell can be produced.

[0010] [Mixing method] The mixing method is not limited, and any conventionally known mixing method for forming an emulsion can be adopted. Examples of the mixing method include batch mixing using a homomixer or the like, continuous mixing using a line mixer or the like, and membrane mixing using an SPG membrane or the like. Depending on the mixing method, the particle size of the formed emulsion and the processing capacity for forming the emulsion also differ, but in any method, by mixing an oil phase with an aqueous phase, it is possible to produce core-shell particles with a solvent as the core and a metal oxide as the shell.

[0011] The metal alkoxide is added to the oil phase. Conventionally known methods have been reported in which core-shell particles can be obtained by adding a metal alkoxide to the aqueous phase after forming an O / W emulsion. However, this method results in the aggregation of core-shell particles, resulting in the generation of irregularly shaped particles or coalesced particles. In the present invention, by having the metal alkoxide present inside the oil droplets, the active surface of the reaction is the interface between the shell and the oil droplets, and therefore exists inside the core-shell particles. This prevents the reaction from proceeding on the outer surface of the shell, which would otherwise cause aggregation or coalescence. As a result, the generation of particles resulting from aggregation or coalescence of core-shell particles can be suppressed. Furthermore, since the metal alkoxide is already present in the oil phase when the oil phase and aqueous phase are mixed, the metal oxide derived from the metal alkoxide is formed along the surface of the oil phase. Therefore, according to one aspect of the present invention, the generation of solid particles made of metal oxide can also be suppressed.

[0012] In the present disclosure, particle association and coalescence are distinguished as follows. Specifically, particle association refers to the phenomenon in which, in an O / W emulsion, multiple core-shell particles formed by a certain degree of shell-forming reaction come into contact while maintaining their shell shape, and then the shell-forming reaction proceeds further at the contact points, resulting in the bonding of multiple core-shell particles with the outer shells as the bonding points. Meanwhile, coalescence refers to the phenomenon in which multiple oil droplets in which the shell-forming reaction has not progressed much come into contact with each other at the portions where no shells have been formed, and combine into a single oil droplet. Core-shell particles formed by association have multiple cores and a collection of spherical shells surrounding each core, while core-shell particles formed by coalescence have a single core and a non-spherical shell covering the core. Therefore, while it is generally preferable to reduce both types of core-shell particles as much as possible, the practical advantage of reducing core-shell particles having non-spherical shells formed by coalescence is greater.

[0013] The aqueous phase and the oil phase are mixed in the presence of a surfactant. The surfactant may be added to the aqueous phase or the oil phase before mixing, or may be added as a third phase other than the aqueous phase and the oil phase at the same time as mixing. It is preferable that the surfactant be added to the oil phase before mixing.

[0014] When the surfactant is a hydrophilic compound, the oil phase may be prepared by mixing the surfactant with a hydrophilic organic solvent, and then mixing the resulting mixture with a hydrophobic organic solvent and a metal alkoxide. By using a hydrophilic surfactant as a component of the oil phase, the hydrophobic groups of the surfactant are located inside the oil phase, and the hydrophilic groups are located at the emulsion interface (outside the oil phase), thereby forming a more stable O / W emulsion. Hydrophilic surfactants are suitable for forming O / W emulsions, but are poorly compatible with hydrophobic organic solvents. Therefore, to incorporate a hydrophilic surfactant into the oil phase, it is advisable to first mix the surfactant with a hydrophilic organic solvent that is compatible with both the surfactant and the hydrophobic organic solvent. Furthermore, the hydrophilic surfactant contributes to stabilization by being located at the interface (outside the oil droplets) in the O / W emulsion. Therefore, when a hydrophilic surfactant is contained in the oil phase, the surfactant is present at a higher concentration near the interface where it is ultimately preferably deployed, compared to when it is contained in the aqueous phase or mixed as a third phase, which is expected to stabilize the emulsion shape early and suppress unintended reactions.

[0015] In the present disclosure, the phrase "a surfactant is a hydrophilic compound" refers to an ionic surfactant such as a cationic surfactant, an anionic surfactant, or an amphoteric surfactant, or, if the surfactant is a nonionic surfactant, refers to the HLB (Hydrophile-Lipophile Balance) of the nonionic surfactant being closer to the hydrophilic side than to the lipophilic side. While not limiting the present disclosure, for example, when the HLB value of a surfactant is 8 or more, or 10 or more, the nonionic surfactant can be said to be a hydrophilic compound. In the present disclosure, the HLB value is calculated using the following mathematical formula (A):

[0016] When mixing the aqueous phase and the oil phase, it is preferable to incorporate a basic compound into the oil phase in advance. As described above, the basic compound promotes the hydrolysis reaction of the metal alkoxide through its catalytic action, thereby shortening the time required from mixing to the formation of a shell on the oil droplets. Therefore, the basic compound can reduce the opportunity for association or coalescence between oil droplets in which a shell has not yet formed and the oil phase is exposed to the aqueous phase, resulting in a reduction in coarse core-shell particles.

[0017] The basic compound may be contained in the aqueous phase. On the other hand, from the viewpoint of reducing the total amount of the basic compound required to achieve a predetermined catalyst concentration, it is advantageous to contain the basic compound in the oil phase, which usually has a smaller total volume than the aqueous phase in an O / W emulsion. Also, from the viewpoint of making the resulting core-shell particles have a shape closer to a sphere, it is advantageous to contain the basic compound in the oil phase.

[0018] When mixing the aqueous phase and the oil phase, the final ratio of the aqueous phase to 100 parts by volume of the oil phase is preferably 200 parts by volume or more, more preferably 1000 parts by volume or more. The higher the ratio of the aqueous phase within this range, the more stable the O / W emulsion that is less likely to associate or coalesce with other oil droplets can be formed. The final ratio of the aqueous phase to 100 parts by volume of the oil phase is preferably 5000 parts by volume or less, more preferably 3000 parts by volume or less. The lower the ratio of the aqueous phase within this range, the higher the yield of core-shell particles. It is preferable to set the volumes of the aqueous phase and oil phase to be mixed so as to satisfy the above ratio.

[0019] In the present disclosure, expressions such as "final oil phase" and "final aqueous phase" refer to the oil phase and aqueous phase, respectively, in the formed O / W emulsion, and are distinguished from the oil phase and aqueous phase to be mixed. Here, the components and volume of the oil phase and aqueous phase in the formed O / W emulsion may differ from the components and volume of the oil phase and aqueous phase to be mixed. This is because, when the O / W emulsion is formed, components of each phase may migrate to the other phase due to differences in solubility, etc. In particular, the hydrophilic organic solvent contained in the oil phase to be mixed usually migrates to the aqueous phase in the O / W emulsion. Therefore, the "final oil phase" is a mixture of components contained in the oil phase to be mixed, excluding the hydrophilic organic solvent. On the other hand, the "final aqueous phase" is a mixture of the hydrophilic organic solvent contained in the oil phase to be mixed with the aqueous phase to be mixed.

[0020] [Aging Step] The core-shell particles according to one embodiment of the present invention may be prepared by a aging step after emulsion formation. In the aging step, for example, by continuously stirring the formed O / W emulsion for a certain period of time, the metal alkoxide of the oil phase component undergoes quantitative hydrolysis and dehydration condensation, thereby increasing the yield of the core-shell particles. The O / W emulsion may also be heated in the aging step. Examples of heating methods include commonly known methods such as heater heating and microwave heating. Heating the formed O / W emulsion promotes the hydrolysis and dehydration condensation of the metal alkoxide of the oil phase component, thereby enabling the production of core-shell particles having a metal oxide shell in a short period of time. Two or more aging steps may also be combined.

[0021] [Aqueous Phase] In one embodiment of the present invention, the aqueous phase can be made of distilled water, ion-exchanged water, ultrapure water, or the like. From the viewpoint of achieving a more uniform and stable emulsion formation of the hydrophobic organic substance, a hydrophilic organic solvent compatible with water can be added to the water within a range that does not impair the objectives of the present invention. Examples of hydrophilic organic solvents that can be added to the water include alcohols having 1 to 3 carbon atoms, such as methanol, ethanol, and isopropanol, and acetone. The amount of hydrophilic organic solvent added is within a range that allows separation of the aqueous phase and the oil phase after the addition of the hydrophilic organic solvent, and is typically less than 100% by mass, for example, 0% by mass or more and 20% by mass or less, relative to 100% by mass of the final aqueous phase.

[0022] In order to promote the hydrolysis reaction, the aqueous phase preferably contains at least water, and more preferably contains water and a hydrophilic organic solvent.

[0023] [Oil Phase] In one embodiment of the present invention, the oil phase comprises a hydrophobic organic solvent and a metal alkoxide, and in a preferred embodiment, the oil phase comprises the following components: a hydrophobic organic solvent, a metal alkoxide, a basic compound, and a surfactant. After mixing the components, the oil phase preferably has low solubility in water and is separated from the aqueous phase. Each component in the oil phase will be described below.

[0024] [Hydrophilic Organic Solvent in Oil Phase] From the viewpoint of dissolving each component in the oil phase, a hydrophilic organic solvent compatible with the hydrophobic organic solvent can be added to the oil phase within a range that does not impair the object of the present invention. Examples of hydrophilic organic solvents that can be added include C1-C3 alcohols such as methanol, ethanol, and isopropanol, and acetone.

[0025] The amount of hydrophilic organic solvent added to the oil phase is within a range that allows separation of the oil phase and the aqueous phase after the addition of the hydrophilic organic solvent, and is usually less than 1000% by mass, for example, 500% by mass or less, and preferably 200% by mass or less, relative to 100% by mass of the final oil phase. From the viewpoint of obtaining an oil phase in which each component is highly compatible, the amount of hydrophilic organic solvent added is preferably 20% by mass or more, more preferably 50% by mass or more, relative to 100% by mass of the final oil phase. When the surfactant is a hydrophilic compound, in order to more uniformly dissolve the surfactant in the oil phase, the amount of hydrophilic organic solvent added to the oil phase is preferably 350% by mass or more, more preferably 1000% by mass or more, relative to 100% by mass of the surfactant.

[0026] [Hydrophobic Organic Solvent] In one embodiment of the present invention, the hydrophobic organic solvent is preferably a compound that has low solubility in water and separates from the aqueous phase. Examples of the hydrophobic organic solvent used include oils selected from hydrocarbon compounds, ester compounds, fatty acids having 6 to 22 carbon atoms, and silicone oils. Examples of hydrocarbon compounds include alkanes or cycloalkanes having 5 to 18 carbon atoms, liquid paraffin or liquid petroleum jelly, squalane, squalene, perhydrosqualene, trimethylbenzene, xylene, toluene, and benzene. Of these, alkanes or cycloalkanes having 5 to 18 carbon atoms are preferred. Examples of ester compounds include oils and fats such as glycerin esters of fatty acids having 6 to 22 carbon atoms. Examples include mink oil, turtle oil, soybean oil, sweet almond oil, beauty leaf oil, palm oil, grapeseed oil, sesame seed oil, corn oil, pearl oat oil, arara oil, rapeseed oil, sunflower oil, cottonseed oil, apricot oil, castor oil, avocado oil, jojoba oil, olive oil, and cereal germ oil. Ester compounds include condensates of fatty acids having 4 to 22 carbon atoms with monohydric alcohols having 1 to 22 carbon atoms, and condensates of fatty acids having 4 to 22 carbon atoms with polyhydric alcohols other than glycerin. Specific examples include isopropyl myristate, isopropyl palmitate, butyl stearate, hexyl laurate, isononyl isononanoate, 2-ethylhexyl palmitate, 2-hexyldecyl laurate, 2-octyldecyl palmitate, and 2-octyldodecyl myristate. Other ester compounds include esters of polycarboxylic acid compounds and alcohols. Specific examples include diisopropyl adipate, 2-octyldodecyl lactic acid ester, di-2-ethylhexyl succinate, diisostearyl malate, glycerin triisostearate, and diglycerin tetraisostearate.

[0027] Examples of fatty acids having 6 to 22 carbon atoms include myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, linoleic acid, linolenic acid, and isostearic acid. Examples of silicone oils include polydimethylsiloxane (PDMS), polysiloxanes modified with fatty acids, aliphatic alcohols, or polyoxyalkylenes, fluorosilicones, and perfluorosilicone oils. The polydimethylsiloxane (PDMS) may be phenylated (e.g., phenyl trimethicone). Alternatively, the PDMS may be optionally substituted with aliphatic and / or aromatic groups. Furthermore, PDMS is preferably a hydrocarbon-based oil or silicone oil, and is preferably a linear or cyclic silicone containing 2 to 7 silicon atoms, optionally containing alkyl or alkoxy groups in pendant or terminal silicone chains, with octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, hexadecamethylcyclohexasiloxane, heptamethylhexyltrisiloxane, heptamethyloctyltrisiloxane, etc. Among the above oils, hydrocarbon compounds are preferred because they are easily emulsified and dispersed in the presence of cationic surfactants such as quaternary ammonium salts, enhance the utilization efficiency of hydrophobic organic substances, and form a highly regular mesopore structure in the outer shell. Alkanes or cycloalkanes having 5 to 18 carbon atoms and squalene are preferred, with alkanes or cycloalkanes having 5 to 10 carbon atoms being more preferred. The above hydrophobic organic solvents can be used alone or in combination of two or more in any desired ratio.

[0028] From the viewpoint of making the oil phase incompatible with the aqueous phase, the amount of the hydrophobic organic solvent added is preferably 5% by mass or more, more preferably 10% by mass or more, based on 100% by mass of the final oil phase. From the viewpoint of maintaining a stable O / W emulsion form, the amount of the hydrophobic organic solvent added is preferably 90% by mass or less, more preferably 80% by mass or less, based on 100% by mass of the final oil phase.

[0029] [Metal Alkoxide] In one embodiment of the present invention, the metal alkoxide is preferably a substance that generates a metal compound having a hydroxyl group upon hydrolysis. Specific examples include compounds represented by the following general formulas (1) to (5): XY ４ (1) R ３ XY ３ (2) R ３ ２ XY ２ (3) R ３ ３ XY (4) Y ３ X-R ４ -XY ３ (5) (wherein X represents a metal atom, R ３ each independently represents an organic group in which a carbon atom is directly bonded to a metal atom, and R ４ represents a hydrocarbon group having 1 to 4 carbon atoms or a phenylene group, and Y represents a monovalent hydrolyzable group that becomes a hydroxy group upon hydrolysis.) The metal atom represented by X is selected from metal atoms that form metal alkoxides, such as silicon, titanium, and zirconium.

[0030] In the general formulas (1) to (4), R ３ are each independently preferably a hydrocarbon group having 1 to 22 carbon atoms in which some of the hydrogen atoms may be substituted with fluorine atoms, a phenyl group, or a benzyl group, more preferably an alkyl group having 1 to 22 carbon atoms, even more preferably an alkyl group having 4 to 18 carbon atoms, still more preferably an alkyl group having 6 to 18 carbon atoms, and particularly preferably an alkyl group having 8 to 16 carbon atoms. ４ is preferably an alkanediyl group having 1 to 4 carbon atoms (e.g., methylene, ethylene, trimethylene, propane-1,2-diyl, or tetramethylene) or a phenylene group. Y is preferably an alkoxy group having 1 to 22 carbon atoms, more preferably an alkoxy group having 1 to 8 carbon atoms, and particularly preferably an alkoxy group having 1 to 4 carbon atoms.

[0031] Examples of metal alkoxides include the following compounds: Tetraalkoxymetal compounds in which Y is an alkoxy group having 1 to 3 carbon atoms in the general formula (1), such as tetraalkoxysilane and tetraalkoxytitanium, particularly tetraalkoxysilane; ３ is a phenyl group, a benzyl group, or a hydrocarbon group having 1 to 20 carbon atoms, preferably 1 to 10 carbon atoms, and more preferably 1 to 5 carbon atoms, in which some of the hydrogen atoms have been substituted with fluorine atoms, and is particularly a trialkoxy metal compound or dialkoxy metal compound, particularly a trialkoxy silane or dialkoxy silane. Among these, tetramethoxysilane, tetraethoxysilane, phenyltriethoxysilane, 1,1,1-trifluoropropyltriethoxysilane, and tetra-i-propoxytitanium are particularly preferred. The above metal alkoxides can be used alone or in combination of two or more in any desired ratio.

[0032] From the viewpoint of making the metal oxide shell thicker in the produced core-shell particles, the amount of metal alkoxide added is preferably 50% by mass or more, more preferably 60% by mass or more, based on 100% by mass of the final oil phase. From the viewpoint of better exhibiting the function of the core-shell particles, the amount of metal alkoxide added is preferably 90% by mass or less, more preferably 80% by mass or less, based on 100% by mass of the final oil phase.

[0033] [Basic Compound] In one embodiment of the present invention, the basic compound promotes the hydrolysis of the metal alkoxide and further promotes the dehydration condensation of the hydroxyl groups after hydrolysis. Examples of the basic compound include amines. Examples of the amine include at least one selected from primary amines, secondary amines, tertiary amines, and quaternary ammonium hydroxides, which contain a hydrogen atom or an organic group selected from alkyl groups, alkenyl groups, and hydroxyalkyl groups having 1 to 5 carbon atoms. The number of hydrogen atoms in the amines is determined by the number of organic groups, i.e., the grade of the amine. Among these, quaternary ammonium hydroxides are preferred from the viewpoint of compatibility with the oil phase. Specific examples of primary amines include ethylamine, n-propylamine, n-butylamine, 1,3-diaminopropane, and pentylamine. Specific examples of secondary amines include diethylamine, di-n-propylamine, di-n-butylamine, and dipentylamine, with diethylamine and di-n-propylamine being more preferred. Specific examples of tertiary amines include dimethylethylamine, diethylmethylamine, triethylamine, tripropylamine, tributylamine, and tripentylamine. From the viewpoint of obtaining high recovery efficiency and hollow silica particles with a high air content, triethylamine and tripropylamine are more preferred. Substituted amines such as alkanolamines, such as monoethanolamine, diethanolamine, triethanolamine, diethylethanolamine, and propanolamine, can also be used. Specific examples of quaternary ammonium hydroxides include tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, tributylmethylammonium hydroxide, trimethylhydroxyethylammonium hydroxide (choline), tetraethanolammonium hydroxide, and methyltriethanolammonium hydroxide. From the viewpoint of obtaining high recovery efficiency and hollow silica particles with a high air content, tetramethylammonium hydroxide and tetraethylammonium hydroxide are more preferred.The basic compounds may be used alone or in combination of two or more.

[0034] From the viewpoint of sufficiently promoting hydrolysis, the amount of the basic compound added is preferably 1% by mass or more, more preferably 2% by mass or more, relative to 100% by mass of the final oil phase, and is usually 10% by mass or less, preferably 5% by mass or less, relative to 100% by mass of the final oil phase.

[0035] [Surfactant] In one embodiment of the present invention, the type of surfactant is not limited. Examples of surfactants include cationic surfactants, anionic surfactants, amphoteric surfactants, and nonionic surfactants. From the viewpoint of stably forming an O / W emulsion, the surfactant is preferably a cationic surfactant. The cationic surfactant is preferably a quaternary ammonium salt, more preferably at least one selected from the group consisting of quaternary ammonium salts represented by the following general formula (6) or general formula (7), and even more preferably at least one selected from alkyltrimethylammonium salts and dialkyldimethylammonium salts. [R １ R ３ ３ N] ＋ X － (6) [R １ R ２ R ３ ２ N] ＋ X － (7)

[0036] In the general formula (6) and the general formula (7), R １ and R ２ each independently represents a linear or branched alkyl group having 4 to 22 carbon atoms; R ３ represents an alkyl group having 1 to 3 carbon atoms, and a plurality of R ３ may each be a different group, and X －represents a monovalent anion. Examples of alkyl groups having 4 to 22 carbon atoms include various butyl groups, various pentyl groups, various hexyl groups, various heptyl groups, various octyl groups, various nonyl groups, various decyl groups, various dodecyl groups, various tetradecyl groups, various hexadecyl groups, various octadecyl groups, and various eicosyl groups. Examples of alkyl groups having 1 to 3 carbon atoms include methyl groups, ethyl groups, n-propyl groups, and isopropyl groups. In general formula (6) and general formula (7), R ３ is preferably a methyl group.

[0037] X in general formulas (6) and (7) － is preferably at least one monovalent anion selected from the group consisting of halogen ions, hydroxide ions, and nitrate ions, from the viewpoint of being easily decomposed and volatilized during firing. － is more preferably a halide ion, more preferably a chloride ion or a bromide ion, and even more preferably a bromide ion.

[0038] Examples of the alkyltrimethylammonium salt represented by general formula (6) include butyltrimethylammonium chloride, hexyltrimethylammonium chloride, octyltrimethylammonium chloride, decyltrimethylammonium chloride, lauryltrimethylammonium chloride (dodecyltrimethylammonium chloride), tetradecyltrimethylammonium chloride, hexadecyltrimethylammonium chloride, stearyltrimethylammonium chloride, behenyltrimethylammonium chloride, butyltrimethylammonium bromide, hexyltrimethylammonium bromide, octyltrimethylammonium bromide, decyltrimethylammonium bromide, lauryltrimethylammonium bromide, tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, stearyltrimethylammonium bromide, and behenyltrimethylammonium bromide.

[0039] Examples of the dialkyldimethylammonium salt represented by the general formula (7) include dibutyldimethylammonium chloride, dihexyldimethylammonium chloride, dioctyldimethylammonium chloride, dihexyldimethylammonium bromide, dioctyldimethylammonium bromide, dilauryldimethylammonium bromide, and ditetradecyldimethylammonium bromide.

[0040] The quaternary ammonium salts are preferably lauryltrimethylammonium chloride, stearyltrimethylammonium chloride, and behenyltrimethylammonium chloride, more preferably stearyltrimethylammonium chloride and behenyltrimethylammonium chloride.

[0041] Examples of surfactants that are hydrophilic compounds include ionic surfactants.

[0042] From the viewpoint of further stabilizing the O / W emulsion, the amount of surfactant added is preferably 3% by mass or more, more preferably 5% by mass or more, relative to 100% by mass of the final oil phase. From the viewpoint of preparing a uniform oil phase, the amount of surfactant added is preferably 15% by mass or less, more preferably 10% by mass or less, relative to 100% by mass of the final oil phase. The above-mentioned preferred amount is applied when the surfactant is previously incorporated into the oil phase before mixing the aqueous phase and the oil phase. Note that even when the surfactant is added simultaneously with mixing, the above-mentioned preferred amount may be the mass % calculated assuming that the surfactant is added to the oil phase.

[0043] [Core-Shell Particles] The core-shell particles according to one embodiment of the present invention are characterized by being core-shell particles having a core made of a solvent, particularly a hydrophobic organic solvent, and a shell made of a metal oxide. The core-shell particles may exist in the form of a powder of aggregated core-shell particles, i.e., a core-shell particle powder. The core-shell particles according to one embodiment of the present invention include a solvent core. Examples of the solvent include hydrophobic organic solvents such as hexane, heptane, and decane. The core-shell particles may also include, in addition to the hydrophobic organic solvent, any of a hydrophilic organic solvent such as methanol, ethanol, and 2-propanol, a metal alkoxide, a basic compound, and a surfactant. The core-shell particles according to one embodiment of the present invention include a metal oxide shell. The metal oxide is preferably an oxide of a metal capable of forming a metal alkoxide, and specific examples include oxides of silicon, aluminum, titanium, zirconium, etc.

[0044] The core-shell particles according to one embodiment of the present invention have a very sharp particle size distribution, with a reduced number of solid particles of metal oxide and irregularly shaped particles formed by the association or coalescence of particles.

[0045] The sharpness of the particle size distribution of core-shell particles is determined by the mode diameter D ｍｏｄ and cumulative 10% diameter D １０ Relative to D ｍｏｄ / D １０ The more solid particles there are in the powder of core-shell particles, the more likely the solid particles are to have a modal diameter D ｍｏｄ Since the particle size is significantly smaller than that of ｍｏｄ In this case, the ratio D ｍｏｄ / D １０ The ratio D tends to be high and is useful as an indicator of the amount of solid particles. ｍｏｄ / D １０ is preferably 20 or less, more preferably 10 or less.

[0046] The sharpness of the particle size distribution of core-shell particles is the cumulative 50% diameter D ５０ and cumulative 90% diameter D ９０ Relative to D ５０ / D ９０The more irregular-shaped particles there are in the powder of core-shell particles, the larger the cumulative 50% diameter D ５０ Since the particle size is significantly larger than that of ５０ In this case, the ratio D ５０ / D ９０ The ratio D tends to be low and is useful as an index of the amount of irregularly shaped particles. ５０ / D ９０ is preferably 0.6 or more, and more preferably 0.65 or more.

[0047] The sharpness of the particle size distribution of the core-shell particles is determined by the above-mentioned ratio D ｍｏｄ / D １０ and comparison D ５０ / D ９０ The product of (D ｍｏｄ ×D ５０ ) / (D １０ ×D ９０ ) can be expressed as one of the indices. The more either solid particles or irregularly shaped particles are present in the powder of core-shell particles, the more the particle size distribution is biased towards either the small particle size side or the large particle size side. In such cases, the product (D ｍｏｄ ×D ５０ ) / (D １０ ×D ９０ ) tends to be a high value and is useful as an index representing the total amount of solid particles and irregular-shaped particles. ｍｏｄ ×D ５０ ) / (D １０ ×D ９０ ) is preferably 20 or less, more preferably 10 or less.

[0048] In the present disclosure, the particle size distribution of the core-shell particles is a number-based particle size distribution determined by electron microscope image analysis. Specifically, 1,000 core-shell particles are photographed using a scanning electron microscope, and the photographed images are analyzed using image analysis software to calculate the primary particle diameter (circle-equivalent diameter) of each particle, thereby obtaining a particle size distribution graph with the primary particle diameter on the horizontal axis and the number-based frequency on the vertical axis. As the scanning electron microscope, for example, a field emission scanning electron microscope S-5500 (manufactured by Hitachi High-Technologies Corporation) can be used. As the image analysis software, for example, WINROOF2018 (manufactured by Mitani Corporation) can be used.

[0049] In the particle size distribution of the core-shell particles, the cumulative 10% diameter D １０ The cumulative 50% diameter D is the 100th smallest primary particle diameter (i.e., the smallest 10% of the number of particles photographed) of the calculated primary particle diameters of 1000 particles. ５０ The cumulative 90% diameter D is the 500th smallest primary particle diameter (i.e., the 50% order of the number of particles photographed) among the calculated primary particle diameters of 1000 particles. ９０ The most common diameter D is the 100th largest primary particle diameter (i.e., the order of 10% of the number of particles photographed) among the calculated primary particle diameters of 1000 particles. ｍｏｄ is the cumulative 50% diameter D starting from the particle diameter 0 nm. ５０ When the particle size distribution is divided into class widths of 1 / 100 nm (rounded up for values ​​less than 1 nm), the class with the highest frequency is used as the median value between the start point and end point of the class (rounded up for values ​​less than 1 nm).

[0050] <Method for producing hollow particles> The solvent inside the core-shell particles can be removed from the core-shell particles according to one embodiment of the present invention. Methods for removing the solvent include a separation treatment such as filtration, and a heat treatment such as drying or firing. Hollow particles can be obtained by removing the solvent inside the core-shell particles.

[0051] [Drying Treatment] In one embodiment of the present invention, hollow particles can be obtained by drying the core-shell particles to remove the solvent therefrom. The drying method is not particularly limited, and known methods such as air drying, reduced-pressure drying, and spray drying can be used. However, because drying under reduced pressure tends to more effectively suppress particle coalescence and aggregation than drying under atmospheric pressure, reduced-pressure drying or spray drying is preferred. Furthermore, the drying temperature is preferably 35 to 200°C, more preferably 50 to 200°C, particularly preferably 80 to 200°C, and particularly preferably 120 to 200°C. A drying temperature of 35 to 200°C is advantageous from the viewpoint of obtaining a powder of hollow particles in which particle coalescence and aggregation are suppressed.

[0052] [Separation Treatment] In one aspect of the present invention, the core-shell particles may be recovered from the aqueous dispersion derived from the O / W emulsion by solid-liquid separation, if necessary, before the drying or calcination treatment. Alternatively, a coagulant may be added to form weak aggregates of a plurality of core-shell particles, followed by solid-liquid separation. Adding the coagulant allows for solid-liquid separation, making it possible to easily recover the core-shell particles. The filtration method is not particularly limited, and known methods such as vacuum filtration, pressure filtration, and centrifugal filtration can be applied. Furthermore, the coagulant to be added is not particularly limited. However, from the viewpoint of preventing the coagulant components from being mixed into the resulting hollow particles, coagulants composed of compounds that do not contain metal element components, such as carbon dioxide, ammonium carbonate, ammonium bicarbonate, and ammonium carbamate, are preferred. In this embodiment, a drying treatment and a separation treatment may be combined to remove the solvent from the core-shell particles. Performing multiple treatments can result in a more efficient and productive production method.

[0053] [Caustic Treatment] In one embodiment of the present invention, the hollow particles can be calcined as needed. Calcination is typically performed after drying the core-shell particles. Hollow particles from which the solvent has been removed may contain residual hydroxyl groups and pores. Depending on the intended use, further calcination is preferably performed to thoroughly remove the dispersion medium (i.e., the aqueous phase) from the particles, crush the hydroxyl groups, and remove the pores to obtain hollow particles. That is, the calcined hollow particles are preferred not only because the amount of hydroxyl groups on the particle surface is reduced, but also because the dispersion medium remaining in the particles is completely removed. The calcination temperature during the calcination is preferably 300 to 1300°C, more preferably 600 to 1200°C, because too low a temperature makes it difficult to remove the dispersion medium components, while too high a temperature can cause particle fusion. The calcination time is not particularly limited as long as the remaining dispersion medium is removed, but if it is too long, productivity will decrease, so it is sufficient to heat the mixture to the desired calcination temperature and then maintain the temperature for 0.5 to 48 hours, more preferably 2 to 24 hours. The atmosphere during calcination is also not particularly limited, and the calcination can be carried out under an inert gas such as argon or nitrogen, or in the air.

[0054] In one embodiment of the present invention, the hollow particles can be subjected to a crushing treatment using a known crushing means, if necessary, to further reduce the number of agglomerated particles formed by aggregation of primary particles. The crushing method is not particularly limited, and known methods such as a ball mill or a jet mill can be used. In one embodiment of the present invention, the hollow particles can be subjected to a classification treatment using a known classification means, if necessary, to further reduce the number of agglomerated particles formed by aggregation of primary particles. The classification method is not particularly limited, and known methods such as sieving or air classification can be used.

[0055] [Hollow Particles] Hollow particles according to one embodiment of the present invention are characterized by having a metal oxide shell and air filling the voids in the core. The hollow particles may exist in the form of a powder in which the hollow particles are aggregated, i.e., a hollow particle powder. The metal oxide is preferably an oxide of a metal capable of forming a metal alkoxide, specifically an oxide of silicon, aluminum, titanium, zirconium, or the like. The core of the hollow particle according to one embodiment of the present invention may further contain a solvent in addition to air. Examples of the solvent include hydrophobic organic solvents such as hexane, heptane, and decane. Furthermore, the core of the hollow particle may contain, in addition to the hydrophobic organic solvent, a hydrophilic organic solvent such as methanol, ethanol, or 2-propanol, a metal alkoxide, a basic compound, and a surfactant. The hollow particles according to one embodiment of the present invention have a metal oxide shell. The metal oxide is preferably an oxide of a metal capable of forming a metal alkoxide, specifically an oxide of silicon, aluminum, titanium, zirconium, or the like.

[0056] The hollow particles according to one embodiment of the present invention have a very sharp particle size distribution, with a reduced amount of solid particles of metal oxide and irregularly shaped particles formed by the association or coalescence of particles.

[0057] The hollow particles have a particle size distribution derived from the core-shell particles. The preferred values ​​of each index in the particle size distribution of the hollow particles are the same as those described above for the core-shell particles, and therefore the description thereof will not be repeated. Furthermore, the method for determining the particle size distribution of the hollow particles is also the same as that described above for the core-shell particles, and therefore the description thereof will not be repeated.

[0058] Example 1 An aqueous phase was prepared by adding 1797 g of methanol to 12576 g of ion-exchanged water. Next, a solution of 75 g of hexadecyltrimethylammonium bromide dissolved in 2042 g of methanol was added to 174 g of heptane, and 20 g of triethylamine and 672 g of tetramethoxysilane were further added to prepare an oil phase. While stirring the aqueous phase with a disper mixer (15,000 rpm), the oil phase was added to the aqueous phase, and mixing was continued for 15 minutes. After mixing, the resulting O / W emulsion was stirred with a paddle stirrer (100 rpm) for 60 minutes to obtain an aqueous dispersion containing core-shell particles. The resulting aqueous dispersion was filtered through a filter to obtain a cake containing core-shell particles. The cake was dried at 130°C for 24 hours to obtain a core-shell particle powder. This core-shell particle powder was calcined at 800°C for 3 hours to obtain a hollow particle powder containing hollow particles. The obtained hollow silica particles were subjected to SEM observation and elemental analysis by EDS analysis, and were found to be hollow silica particles having voids inside and silica in the shell portion. Furthermore, the hollow silica particles had a uniform spherical shape and were free of aggregated irregularly shaped particles.

[0059] Example 2: 14,373 g of ion-exchanged water was weighed to prepare an aqueous phase. Next, a solution of 75 g of hexadecyltrimethylammonium bromide dissolved in 265 g of methanol was added to 174 g of heptane, and 20 g of triethylamine and 919 g of tetraethoxysilane were further added to prepare an oil phase. While stirring the aqueous phase with a disper mixer (15,000 rpm), the oil phase was added to the aqueous phase, and mixing was continued for 15 minutes. After mixing, the resulting O / W emulsion was stirred with a paddle stirrer (100 rpm) for 60 minutes to obtain an aqueous dispersion containing core-shell particles. The resulting aqueous dispersion was filtered with a filter to obtain a cake containing core-shell particles. The cake was dried at 130°C for 24 hours to obtain a core-shell particle powder. This core-shell particle powder was calcined at 800°C for 3 hours to obtain a hollow particle powder containing hollow particles. The obtained hollow silica particles were subjected to SEM observation and elemental analysis by EDS analysis, and were found to be hollow silica particles having voids inside and silica in the shell portion. Furthermore, the hollow silica particles had a uniform spherical shape and were free of aggregated irregularly shaped particles.

[0060] Example 3: 1797 g of methanol was added to 12576 g of ion-exchanged water, and then 75 g of hexadecyltrimethylammonium bromide was dissolved therein to prepare an aqueous phase. Next, 174 g of heptane was added to 2042 g of methanol, and then 20 g of triethylamine and 672 g of tetramethoxysilane were added to prepare an oil phase. While stirring the aqueous phase with a disper mixer (15,000 rpm), the oil phase was added to the aqueous phase, and mixing was continued for 15 minutes. After mixing, the resulting O / W emulsion was stirred with a paddle stirrer (100 rpm) for 60 minutes to obtain an aqueous dispersion containing core-shell particles. The resulting aqueous dispersion was filtered with a filter to obtain a cake containing core-shell particles. The cake was dried at 130°C for 24 hours to obtain a core-shell particle powder. This core-shell particle powder was calcined at 800°C for 3 hours to obtain a hollow particle powder containing hollow particles. The obtained hollow silica particles were subjected to SEM observation and elemental analysis by EDS analysis, and were found to be hollow silica particles having voids inside and silica in the shell portion. Furthermore, the hollow silica particles had a uniform spherical shape and were free of aggregated irregularly shaped particles.

[0061] Example 4 An aqueous phase was prepared by adding 1797 g of 2-propanol to 12576 g of ion-exchanged water. Next, a solution of 75 g of hexadecyltrimethylammonium bromide dissolved in 2042 g of 2-propanol was added to 174 g of heptane, and 20 g of triethylamine and 1254 g of tetra-i-propoxytitanium (TTIP) were further added to prepare an oil phase. While stirring the aqueous phase with a disper mixer (15,000 rpm), the oil phase was added to the aqueous phase, and mixing was continued for 15 minutes. After mixing was completed, the resulting O / W emulsion was stirred with a paddle stirrer (100 rpm) for 60 minutes to obtain an aqueous dispersion containing core-shell particles. The resulting aqueous dispersion was filtered through a filter to obtain a cake containing core-shell particles. The cake was dried at 130°C for 24 hours to obtain a core-shell particle powder. The core-shell particle powder was calcined at 800°C for 3 hours to obtain a hollow particle powder containing hollow particles. The obtained hollow titania particles were subjected to SEM observation and elemental analysis using EDS analysis, and were found to be hollow silica particles having voids inside and titania in the shell. The hollow titania particles had a uniform spherical shape, and no associated irregularly shaped particles were present.

[0062] Example 5 The same procedure as in Example 4 was carried out except that triethylamine was not used, to obtain a powder of core-shell particles and hollow titania particles.

[0063] Comparative Example 1: 14,373 g of ion-exchanged water was weighed to prepare an aqueous phase. Next, a solution of 75 g of hexadecyltrimethylammonium bromide dissolved in 265 g of methanol was added to 174 g of heptane, and 20 g of triethylamine was further added to prepare an oil phase. The oil phase was added to the aqueous phase while stirring the aqueous phase with a disper mixer (15,000 rpm), and mixing was continued for 15 minutes. After mixing was completed, 919 g of tetraethoxysilane was added to the resulting O / W emulsion while stirring with a paddle stirrer (100 rpm). Stirring was then continued for 60 minutes, yielding an aqueous dispersion containing core-shell particles. The resulting aqueous dispersion was filtered with a filter to obtain a cake containing core-shell particles. The cake was dried at 130°C for 24 hours to obtain a core-shell particle powder. This core-shell particle powder was calcined at 800°C for 3 hours to obtain a hollow particle powder containing hollow particles. The obtained hollow silica particles were subjected to SEM observation and elemental analysis by EDS analysis, and were found to be hollow silica particles having voids inside and silica in the shell portion.

[0064] Comparative Example 2 An aqueous phase was prepared by adding 1797 g of 2-propanol to 12576 g of ion-exchanged water. Next, a solution of 75 g of hexadecyltrimethylammonium bromide dissolved in 2042 g of 2-propanol was added to 174 g of heptane, and 20 g of triethylamine was further added to prepare an oil phase. The oil phase was added to the aqueous phase while stirring the aqueous phase with a disper mixer (15,000 rpm), and mixing was continued for 15 minutes. After mixing was completed, 1254 g of tetra-i-propoxytitanium (TTIP) was added to the resulting O / W emulsion while stirring with a paddle stirrer (100 rpm). Stirring was then continued for 60 minutes, yielding an aqueous dispersion containing core-shell particles. The resulting aqueous dispersion was filtered through a filter to obtain a cake containing core-shell particles. The cake was dried at 130°C for 24 hours to obtain a core-shell particle powder. The core-shell particles were calcined at 800° C. for 3 hours to obtain a particle powder. When the obtained titania particles were subjected to SEM observation and elemental analysis by EDS analysis, it was found that amorphous solid titania particles accounted for the majority, and no hollow particles were obtained.

[0065] <Analysis of particle size distribution> The hollow particles of the examples and comparative examples were photographed using a field emission scanning electron microscope S-5500 (manufactured by Hitachi High-Technologies Corporation). The photographed images were analyzed using image analysis software WINROOF2018 (manufactured by Mitani Shoji Co., Ltd.) to calculate the primary particle diameters (circle-equivalent diameters) of 1,000 particles, thereby obtaining a particle size distribution graph with the primary particle diameter on the horizontal axis and the number-based frequency on the vertical axis.

[0066] In the particle size distribution of the core-shell particles, the cumulative 10% diameter D １０ The cumulative 50% diameter D was calculated by using the smallest 100th primary particle diameter out of the calculated primary particle diameters of 1000 particles. ５０ The cumulative 50% diameter D was calculated by using the smallest 500th primary particle diameter of the 1000 particles. ５０ The cumulative 90% diameter D was calculated by using the smallest 500th primary particle diameter out of the calculated primary particle diameters of 1000 particles. ９０ The most common diameter D was the 100th largest primary particle diameter among the calculated primary particle diameters of 1000 particles. ｍｏｄ is the cumulative 50% diameter D starting from the particle diameter 0 nm. ５０ When the particle size distribution was divided into class widths of 1 / 100 nm (rounded up for values ​​of 1 nm or less), the median value between the start point and end point of the class with the highest frequency was adopted (rounded up for values ​​of 1 nm or less).

[0067] <Results> The results of particle size distribution analysis are shown in Table 1. In Comparative Example 2, amorphous solid titania particles accounted for the majority of the powder, and hollow particles were not obtained, so particle size distribution analysis was not performed.

[0068]

[0069] As can be seen from Table 1, the hollow particles of Examples 1 to 5 have a ratio D ｍｏｄ / D １０ is 20 or less, ratio D ５０ / D ９０ The hollow particles of Comparative Example 1 had a particle size distribution that was sharper than that of the hollow particles of Comparative Example 1. ５０ / D ９０The value of σ was 0.81, which is a good value. This is thought to be due to the broad tail of the particle size distribution resulting from the presence of small solid particles, which caused the peak of the particle size distribution to appear biased toward larger particle sizes compared to when this tail did not exist.

[0070] <Summary> As can be understood from the above description, the present invention includes the following aspects.

[0071] [1] A method for producing core-shell particles, comprising mixing an aqueous phase comprising water and / or a hydrophilic organic solvent with an oil phase containing a hydrophobic organic solvent and a metal alkoxide in the presence of a surfactant to form an O / W emulsion, thereby producing core-shell type particles having the solvent as the core and the metal oxide as the shell.

[0072] [2] The method for producing core-shell particles according to [1], wherein the oil phase further contains a basic compound.

[0073] [3] The method for producing core-shell particles according to [1] or [2], wherein the oil phase contains the surfactant.

[0074] [4] The method for producing core-shell particles according to any one of [1] to [3], wherein the O / W emulsion is formed and then aged.

[0075] [5] The method for producing core-shell particles according to any one of [1] to [4], wherein when the surfactant is a hydrophilic compound, the surfactant is mixed with a hydrophilic organic solvent, and the resulting mixture is then mixed with the hydrophobic organic solvent and the metal alkoxide to prepare the oil phase.

[0076] [6] A method for producing hollow particles, comprising removing a solvent from within core-shell particles obtained by the method for producing core-shell particles according to any one of [1] to [5].

[0077] [7] The method for producing hollow particles according to [6], wherein the solvent is removed by drying or by baking after drying.

[0078] [8] The method for producing hollow particles according to [6] or [7], wherein solid-liquid separation is carried out before removing the solvent.

[0079] [9] Core-shell particles having a core of a hydrophobic organic solvent and a shell of a metal oxide, wherein the core-shell particles have a mode diameter D ｍｏｄ and cumulative 10% diameter D １０ Relative to D ｍｏｄ / D １０ is 20 or less, and the cumulative 50% diameter D ５０ and cumulative 90% diameter D ９０ Relative to D ５０ / D ９０ is 0.6 or more.

[0080]

[10] Hollow particles having a metal oxide shell, wherein the most frequent diameter D in a number-based particle size distribution determined by electron microscope image analysis is ｍｏｄ and cumulative 10% diameter D １０ Relative to D ｍｏｄ / D １０ is 20 or less, and the cumulative 50% diameter D ５０ and cumulative 90% diameter D ９０ Relative to D ５０ / D ９０ is 0.6 or more.

Claims

1. A method for producing core-shell particles, comprising mixing an aqueous phase consisting of water and / or a hydrophilic organic solvent with an oil phase containing a hydrophobic organic solvent and a metal alkoxide in the presence of a surfactant to form an O / W emulsion, thereby producing core-shell type particles with the solvent as the core and the metal oxide as the shell.

2. The method for producing core-shell particles according to claim 1, wherein the oil phase further contains a basic compound.

3. The method for producing core-shell particles according to claim 2, wherein the oil phase contains the surfactant.

4. The method for producing core-shell particles according to any one of claims 1 to 3, wherein the O / W emulsion is formed and then aged.

5. The method for producing core-shell particles according to any one of claims 1 to 3, wherein when the surfactant is a hydrophilic compound, the surfactant is mixed with a hydrophilic organic solvent, and the resulting mixture is then mixed with the hydrophobic organic solvent and the metal alkoxide to prepare the oil phase.

6. A method for producing hollow particles, comprising removing the solvent from the core-shell particles obtained by the method for producing core-shell particles according to any one of claims 1 to 3.

7. The method for producing hollow particles according to claim 6, wherein the solvent is removed by drying or by baking after drying.

8. The method for producing hollow particles according to claim 6, wherein solid-liquid separation is carried out before removing the solvent.

9. Core-shell particles having a core of a hydrophobic organic solvent and a shell of a metal oxide, and in the number-based particle size distribution determined by electron microscope image analysis, the most frequent diameter D ｍｏｄ and cumulative 10% diameter D １０ Relative to D ｍｏｄ / D １０ is 20 or less, and the cumulative 50% diameter D ５０ and cumulative 90% diameter D ９０ Relative to D ５０ / D ９０ is 0.6 or more.

10. Hollow particles with a metal oxide shell, having a mode diameter D in the number-based particle size distribution determined by electron microscope image analysis. ｍｏｄ and cumulative 10% diameter D １０ Relative to D ｍｏｄ / D １０ is 20 or less, and the cumulative 50% diameter D ５０ and cumulative 90% diameter D ９０ Relative to D ５０ / D ９０ is 0.6 or more.

Images