Method for producing hollow silica particles

Abstract

To provide hollow silica particles each comprising a dense silica shell layer.SOLUTION: A method for producing hollow silica particles involves adding a first silica material to an oil-in-water emulsion with a pH of 3.0 or less, and then maintaining the pH of the oil-in-water emulsion at 5 or more. After this process, a second silica material is added to the oil-in-water emulsion with a pH of 8 or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing hollow silica particles. [Background technology]

[0002] Hollow silica particles are particles with voids inside a shell layer formed from silica. Hollow silica particles are widely used in catalysts, catalyst supports, cosmetic pigments, resin fillers, adsorbents, desiccants, heat insulating materials, paints, drug delivery systems, optical filters, and more, due to their diverse particle sizes, shell pore structures, and surface properties. Furthermore, due to the low refractive index of their hollow shape, they are also useful as anti-reflective coating materials.

[0003] One example of a method for producing hollow silica particles is to prepare an oil-in-water emulsion in which an oil phase is dispersed in an aqueous phase, then attach a silica raw material to the oil droplets to produce oil-core-silica-shell particles, and then remove the oil droplet components from these particles to obtain hollow silica particles.Since they are produced from an emulsion, hollow silica particles with particle diameters of several tens of nanometers to 10 μm can be formed. On the other hand, small-diameter hollow silica particles have a thinner silica shell layer, which reduces particle strength and can lead to particle cracking during use or storage. If the shell layer is weak, the particles may break when mixed with other raw materials, such as ceramic raw materials. Furthermore, if the shell layer is porous, the internal hollow structure may not be maintained when mixed with solvents, etc.

[0004] Patent Document 1 proposes a method for producing hollow silica particles, in which an initial bottom charge containing water, sodium chloride, precipitated calcium carbonate, and sodium silicate is stirred at pH 9, followed by adding an aqueous sodium silicate solution and an aqueous sulfuric acid solution, followed by aging, filtration, and drying to obtain dried particles, which are then treated with concentrated hydrochloric acid to remove calcium carbonate.

[0005] Patent Document 2 proposes hollow silica microcapsules with an average pore diameter of 1.6 to 10 nm. Patent Document 2 proposes a method for producing hollow silica microcapsules by obtaining a W / O type emulsion or an O / W / O type emulsion containing an alkali metal silicate in the aqueous phase, adding a precipitant to this emulsion to form hollow silica microcapsules, washing with water as required, drying, and calcining the microcapsules, and further converting the calcined microcapsules into mesoporous microcapsules.

[0006] Non-Patent Document 1 proposes a method for synthesizing hollow silica particles by combining an emulsion using a block copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) with sodium silicate. In Non-Patent Document 1, ethanol in which trimethylbenzene has been dissolved is added to an aqueous solution to which a block copolymer has been added to obtain an emulsion, to which an aqueous solution of sodium silicate is added, the pH is adjusted to 5.2, and the emulsion is aged. The white powder is isolated, dried, and calcined to synthesize hollow silica particles. This method produces hollow silica particles with an average diameter of approximately 1 μm or less and a BET specific surface area of 426 m. 2 Hollow silica particles with a particle size of about / g have been obtained. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Qianyao Sun et al, "The Formation of Well-Defined Hollow Silica Spheres with Multilamellar Shell Structure", Advanced Materials, 2003, 15, No.13, July 4. [Patent documents]

[0008] [Patent Document 1] Special Publication No. 2000-500113 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-102592 Summary of the Invention [Problem to be solved by the invention]

[0009] In Patent Document 1, the core particles are calcium carbonate, and the emulsion is made alkaline to allow a silica raw material to adhere to the core particles. In this method, the particle size and shape of the resulting hollow particles are limited by the particle size and shape of the solid calcium carbonate core particles. Furthermore, when calcium carbonate is removed with acid, pores are generated in the shell layer, preventing the shell layer from being sufficiently densified. Furthermore, the acid treatment may reduce the strength of the shell layer. Patent Document 1 lists liquid materials such as vegetable oil as other examples of core particles, but there is a problem in that the silica raw material does not adhere sufficiently to core particles made of liquid materials under alkaline conditions.

[0010] As in Patent Document 2, the hollow silica obtained from a W / O type emulsion is formed by droplets of an aqueous phase containing an alkali metal silicate, and therefore the particles are not those in which voids are clearly present in the hollow portion, but rather are particles in which the silica density decreases gradually from the outermost shell to the center. In addition, hollow silica obtained from W / O / W emulsions has a hollow core made up of oil droplets in the center, and the silica raw material contained in the aqueous phase in the middle layer forms a shell layer. Because this shell layer is formed from the aqueous phase in the middle layer, it is difficult to densify.

[0011] In Non-Patent Document 1, sodium silicate is attached to oil droplets in an oil-in-water emulsion under acidic conditions, followed by aging, and then a white powder is isolated. This method has the problem that the shell layer of the resulting hollow silica particles cannot be sufficiently densified, resulting in poor strength of the hollow silica particles.

[0012] An object of the present invention is to provide hollow silica particles having a dense silica shell layer. [Means for solving the problem]

[0013] The present invention provides the following. [1] A method for producing hollow silica particles, comprising: adjusting the pH of an oil-in-water emulsion containing an aqueous phase, an oil phase, and a surfactant to 3.0 or less; adding a first silica raw material to the oil-in-water emulsion; adding a second silica raw material to the emulsion containing the first silica raw material in the presence of alkali metal ions at a pH of 8 or more; obtaining a hollow silica precursor dispersion; obtaining hollow silica precursors from the hollow silica precursor dispersion; and then obtaining hollow silica particles from the hollow silica precursors. [2] The method for producing hollow silica particles according to [1], wherein the first silica raw material and the second silica raw material each independently comprise one or more species selected from the group consisting of alkali metal silicates, active silicic acid, and silicon alkoxides. [3] The method for producing hollow silica particles according to [2], wherein the alkali metal silicate is sodium silicate. [4] The method for producing hollow silica particles according to any one of [1] to [3], wherein an aqueous solution of an alkali metal silicate is used as the first silica raw material.

[0014] [5] The method for producing hollow silica particles according to any one of [1] to [4], wherein at least one of an aqueous solution of an alkali metal silicate and an aqueous solution of active silicic acid is used as the second silica raw material. [6] The method for producing hollow silica particles according to any one of [1] to [5], wherein the second silica raw material is added to the heated emulsion. [7] The method for producing hollow silica particles according to any one of [1] to [6], wherein an acid is added to the oil-in-water emulsion to adjust its pH to 3 or more, and then an aqueous alkali metal silicate solution is added as a first silica raw material. [8] The method for producing hollow silica particles according to any one of [1] to [7], wherein a base is added to the emulsion after the addition of the first silica raw material, and then a second silica raw material is added.

[0015] [9] The method for producing hollow silica particles according to any one of [1] to [8], wherein the hollow silica precursor is calcined to obtain the hollow silica particles.

[10] The method for producing hollow silica particles according to [9], wherein the firing temperature is 300°C to 800°C.

[11] The method for producing hollow silica particles according to any one of [1] to

[10] , wherein the surfactant is a polyoxyethylene-polyoxypropylene copolymer.

[12] The average primary particle diameter of the obtained hollow silica particles is 10 nm to 10 μm, and the BET specific surface area is 300 m 2 / g or less, [1] to

[11] .

[13] The average primary particle diameter is 10 nm to 10 μm, and the BET specific surface area is 150 m 2 / g or less, and the shell layer contains 500 mass ppm or more of an alkali metal component. [Effects of the Invention]

[0016] According to the present invention, hollow silica particles having a dense silica shell layer can be provided. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 shows a transmission electron microscope image (TEM image) of the hollow silica particles obtained in Example 1. [Figure 2] FIG. 2 shows a transmission electron microscope image (TEM image) of the hollow silica particles obtained in Example 1. [Figure 3] FIG. 3 shows a scanning electron microscope (SEM) image of the hollow silica particles obtained in Example 1 after crushing. [Figure 4] FIG. 4 shows a transmission electron microscope image (TEM image) of the hollow silica particles obtained in Example 21. [Figure 5] FIG. 5 shows a scanning electron microscope (SEM) image of the hollow silica particles obtained in Example 21 after crushing. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be described below, but the present invention is not limited to the examples in the following description. The method for producing hollow silica particles of the present invention is characterized by adjusting the pH of an oil-in-water emulsion containing an aqueous phase, an oil phase, and a surfactant to 3.0 or less, adding a first silica raw material to the oil-in-water emulsion, adding a second silica raw material to the emulsion to which the first silica raw material has been added in the presence of alkali metal ions at a pH of 8 or more to obtain a hollow silica precursor dispersion, obtaining hollow silica precursors from the hollow silica precursor dispersion, and then obtaining hollow silica particles from the hollow silica precursors. This method allows the production of hollow silica particles with a dense silica shell layer. The dense shell layer increases particle strength and allows the particles to maintain their shape even when mixed with other materials. Furthermore, the dense shell layer makes it difficult for solvents and foreign substances from the outside to penetrate into the hollow interior. Furthermore, there is no limitation on the silica raw material, and even if an alkali metal silicate is used, hollow silica particles having a dense silica shell layer can be provided.

[0019] This method uses an oil-in-water emulsion containing an aqueous phase, an oil phase, and a surfactant. This oil-in-water emulsion is an emulsion in which an oil phase is dispersed in water, and when a silica raw material is added to this emulsion, the silica raw material adheres to the oil droplets, forming oil-core-silica-shell particles. Hereinafter, oil-in-water emulsions will also be simply referred to as emulsions. The dispersion of oil core-silica shell particles formed by adding the first silica raw material and before the second silica raw material is added, and the dispersion of oil core-silica shell particles after the second silica raw material is added, may also be referred to as emulsion. The latter dispersion of oil core-silica shell particles after the second silica raw material is added may be equivalent to the hollow silica precursor dispersion.

[0020] The aqueous phase of the emulsion mainly contains water as a solvent. The aqueous phase may further contain additives such as a water-soluble organic liquid or a water-soluble resin. The proportion of water in the aqueous phase is preferably 50 to 100% by mass, more preferably 90 to 100% by mass.

[0021] The oil phase of the emulsion preferably comprises a water-insoluble organic liquid that is incompatible with the aqueous phase components, which forms droplets in the emulsion and form the oil-core portion of the hollow silica precursor. Examples of organic liquids include aliphatic hydrocarbons such as n-hexane, isohexane, n-heptane, isoheptane, n-octane, isooctane, n-nonane, isononane, n-pentane, isopentane, n-decane, isodecane, n-dodecane, isododecane, and pentadecane, or mixtures thereof, such as paraffinic base oils; alicyclic hydrocarbons such as cyclopentane, cyclohexane, and cyclohexene, or mixtures thereof, such as naphthenic base oils; benzene, toluene, xylene, ethylbenzene, propylbenzene, cumene, and methacrylic acid; Examples of suitable solvents include aromatic hydrocarbons such as ethylene, tetralin, and styrene; ethers such as propyl ether and isopropyl ether; esters such as ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, n-amyl acetate, isoamyl acetate, butyl lactate, methyl propionate, ethyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, and butyl butyrate; vegetable oils such as palm oil, soybean oil, and rapeseed oil; and fluorine-based solvents such as hydrofluorocarbons, perfluorocarbons, and perfluoropolyethers. Polyoxyalkylene glycols that become hydrophobic liquids at the shell formation reaction temperature can also be used. Examples include polypropylene glycol (molecular weight 1000 or more) and polyoxyethylene-polyoxypropylene block copolymers containing less than 20% by mass of oxyethylene units and having a cloud point (1% by mass aqueous solution) of 40°C or less, preferably 20°C or less. Among these, polyoxypropylene-polyoxyethylene-polyoxypropylene block copolymers are preferred. These may be used alone or in combination of two or more kinds so long as they form a single oil phase.

[0022] The organic liquid is preferably a hydrocarbon having 8 to 16 carbon atoms, particularly 9 to 12 carbon atoms. The organic liquid is selected taking into consideration a comprehensive range of factors, including operability, safety against fire, separability between the hollow silica precursor and the organic liquid, shape characteristics of the hollow silica particles, and solubility of the organic liquid in water. The hydrocarbon having 8 to 16 carbon atoms may be linear, branched, or cyclic, as long as it has good chemical stability, and hydrocarbons with different carbon numbers may be mixed and used. As the hydrocarbon, saturated hydrocarbons are preferred, and linear saturated hydrocarbons are more preferred.

[0023] The flash point of the organic liquid is preferably 20 to 90°C, and more preferably 30 to 80°C. When an organic liquid with a flash point of less than 20°C is used, the flash point is too low, so fire prevention and work environment measures are necessary. On the other hand, an organic liquid with a flash point of more than 90°C has low volatility, so there is a risk that a large amount of the organic liquid will adhere to the obtained hollow silica particles.

[0024] The emulsion contains a surfactant to enhance emulsion stability. The surfactant is preferably water-soluble or water-dispersible and is preferably added to the aqueous phase. A nonionic surfactant is preferred. Examples of the nonionic surfactant include the following surfactants. Polyoxyethylene-polyoxypropylene copolymer surfactants Polyoxyethylene sorbitan fatty acid ester surfactants: polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan tristearate, polyoxyethylene sorbitan monooleate Polyoxyethylene higher alcohol ether surfactants: polyoxyethylene lauryl ether, polyoxyethylene cetyl ether, polyoxyethylene stearyl ether, polyoxyethylene oleyl ether, polyoxyethylene octylphenol ether, polyoxyethylene nonylphenol ether Polyoxyethylene aliphatic ester surfactants: Polyoxyethylene glycol monolaurate, polyoxyethylene glycol monostearate, polyoxyethylene glycol monooleate Glycerin fatty acid ester surfactants: stearic acid monoglyceride, oleic acid monoglyceride Furthermore, polyoxyethylene sorbitol fatty acid ester surfactants, sucrose fatty acid ester surfactants, polyglycerin fatty acid ester surfactants, polyoxyethylene hydrogenated castor oil surfactants, etc. may also be used. These may be used alone or in combination of two or more.

[0025] Among the nonionic surfactants described above, polyoxyethylene-polyoxypropylene copolymer surfactants can be preferably used. Polyoxyethylene-polyoxypropylene copolymers are block copolymers in which polyoxyethylene blocks (EO) and polyoxypropylene blocks (PO) are bonded. Examples of block copolymers include EO-PO-EO block copolymers and EO-PO block copolymers, with EO-PO-EO block copolymers being preferred. The proportion of oxyethylene units in the EO-PO-EO block copolymer is preferably 20% by mass or more, more preferably 30% by mass or more. The weight average molecular weight of the polyoxyethylene-polyoxypropylene copolymer is preferably 3,000 to 27,000, more preferably 6,000 to 19,000. The total amount of polyoxyethylene blocks is preferably 40 to 90% by mass, and the total amount of polyoxypropylene blocks is preferably 10 to 60% by mass, based on the entire polyoxyethylene-polyoxypropylene copolymer.

[0026] The amount of surfactant used varies depending on conditions such as the type of surfactant, its HLB (Hydrophile-Lipophile Balance), which is an index of the surfactant's hydrophilicity or hydrophobicity, and the particle size of the target silica particles. However, a content of 500 to 20,000 ppm by mass in the aqueous phase is preferred, with 1,000 to 10,000 ppm by mass being more preferred. A content of 500 ppm by mass or more can further stabilize the emulsion. Furthermore, a content of 20,000 ppm by mass or less can reduce the amount of surfactant remaining in the hollow silica particles (the final product).

[0027] The water phase and the oil phase may be mixed at a mass ratio of 200:1 to 5:1, preferably 100:1 to 9:1.

[0028] Methods for preparing oil-in-water emulsions are not limited to the following. The aqueous and oil phases can be prepared in advance, and then the oil phase is added to the aqueous phase and thoroughly mixed or stirred. Other methods that apply strong physical shear forces include ultrasonic emulsification, stirring emulsification, and high-pressure emulsification. Other methods include membrane emulsification, in which a finely divided oil phase is dispersed in an aqueous phase through a membrane with micropores; phase inversion emulsification, in which a surfactant is dissolved in an oil phase and then an aqueous phase is added to emulsify; and phase inversion temperature emulsification, in which a surfactant changes from water-soluble to oil-soluble at a temperature near its cloud point. These emulsification methods can be appropriately selected depending on the desired particle size, particle size distribution, and the like. In order to reduce the particle size of the resulting hollow silica particles and narrow the particle size distribution, it is preferable that the oil phase is sufficiently dispersed and emulsified in the aqueous phase. For example, the mixture can be emulsified using a high-pressure homogenizer at a pressure of 100 bar, preferably 400 bar or more.

[0029] In this method, a first source of silica is added to an oil-in-water emulsion. Examples of the first silica raw material include an aqueous solution of water-soluble silica, an aqueous dispersion of solid silica, a mixture of these, and one or more selected from the group consisting of alkali metal silicates, active silicic acid, and silicon alkoxides, or their aqueous solutions or dispersions. Among these, one or more selected from the group consisting of alkali metal silicates, active silicic acid, and silicon alkoxides, or their aqueous solutions or dispersions, are preferred because of their high availability. Examples of solid silica include silica sol obtained by hydrolyzing an organosilicon compound and commercially available silica sol. Examples of alkali metals in alkali metal silicates include lithium, sodium, potassium, and rubidium, with sodium being preferred due to its availability and economical advantages. That is, sodium silicate is preferred as the alkali metal silicate. Sodium silicate has a composition expressed as Na2O·nSiO2·mH2O. The ratio of sodium to silicate, expressed as the molar ratio n of Na2O / SiO2, is preferably 1.0 to 4.0, and more preferably 2.0 to 3.5.

[0030] Activated silicic acid is obtained by cation exchange of alkali metal silicate, replacing the alkali metal with hydrogen. An aqueous solution of this activated silicic acid exhibits weak acidity. A hydrogen cation exchange resin can be used for the cation exchange. The alkali metal silicate and active silicic acid are preferably dissolved or dispersed in water before being added to the emulsion. The concentration of the alkali metal silicate and active silicic acid aqueous solution is preferably 3 to 30 mass %, more preferably 5 to 25 mass %, in terms of SiO concentration.

[0031] As the silicon alkoxide, for example, tetraalkylsilanes such as tetramethoxysilane, tetraethoxysilane, and tetrapropoxysilane can be preferably used. Furthermore, composite particles can also be obtained by mixing other metal oxides with the silica raw material. Examples of other metal oxides include titanium dioxide, zinc oxide, cerium oxide, copper oxide, iron oxide, and tin oxide.

[0032] The first silica raw material may be any of the above silica raw materials, or a mixture of two or more of them. Among them, an aqueous solution of an alkali metal silicate, particularly an aqueous solution of sodium silicate, may be preferably used as the first silica raw material.

[0033] In this method, the addition of the first silica raw material to the oil-in-water emulsion is carried out when the pH of the oil-in-water emulsion is 3.0 or less. Preferably, the pH of the oil-in-water emulsion containing the aqueous phase, the oil phase, and the surfactant is adjusted to 3.0 or less by adding an acid. An example of adding the first silica source is to add an acid to the emulsion and then add an aqueous solution of sodium silicate. The neutral emulsion is first acidified and then the aqueous sodium silicate solution, which is an alkaline component, is added, so that the entire emulsion can be kept acidic when the first silica raw material is added. In the addition of the first silica raw material, it is preferable to adjust the pH of the emulsion after adding the acid to 2 or less, and then to add the aqueous sodium silicate solution and adjust the pH to 3.0 or less. The pH when the first silica raw material is added to the emulsion is preferably 3.0 or less, more preferably 2.4 or less, which makes it possible to make the thickness of the first coating layer formed by the silica raw material on the oil droplets in the emulsion via the surfactant more uniform and to make the silica shell layer of the resulting hollow silica more dense. The pH of the emulsion upon addition of the first silica source may be 1 or greater.

[0034] Examples of the acid include hydrochloric acid, nitric acid, sulfuric acid, acetic acid, perchloric acid, hydrobromic acid, trichloroacetic acid, dichloroacetic acid, methanesulfonic acid, and benzenesulfonic acid.

[0035] When adding the first silica raw material, the amount of SiO2 in the first silica raw material is preferably 1 to 50 parts by mass, more preferably 3 to 30 parts by mass, per 100 parts by mass of the oil phase contained in the emulsion. When adding the first silica raw material, the pH of the emulsion after adding the first silica raw material is maintained at 3.0 or less for preferably 1 minute or more, more preferably 5 minutes or more, and even more preferably 10 minutes or more.

[0036] Next, it is preferable to maintain the pH of the emulsion to which the first silica raw material has been added at 5 or higher, thereby making it possible to immobilize the first silica raw material on the surface of the oil droplets. For example, there is a method of adjusting the pH of the emulsion to 5 or higher by adding a base to the emulsion to which the first silica raw material has been added. Examples of the base include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, alkaline earth metal hydroxides such as magnesium hydroxide and calcium hydroxide, ammonia, and amines. Alternatively, a method of exchanging anions such as halogen ions for hydroxide ions by anion exchange treatment may be used.

[0037] When adding the base, it is preferable to gradually add the base while stirring the emulsion to which the silica raw material has been added, thereby gradually increasing the pH of the emulsion. If the stirring is weak or a large amount of base is added all at once, the pH of the emulsion will become uneven, which may result in an uneven thickness of the first coating layer. The emulsion is preferably maintained, preferably with stirring, at a pH of 5 or higher for a period of 10 minutes or longer, preferably 1 hour or longer, or may be maintained for 4 hours or longer. During the holding period, the pH of the emulsion is preferably 7 or less.

[0038] Next, a second silica raw material is added in the presence of alkali metal ions at an emulsion pH of 8 or higher, thereby obtaining a hollow silica precursor dispersion. Here, the hollow silica precursor is an oil core-silica shell particle.

[0039] The second silica raw material may be the same as the first silica raw material described above, either alone or in combination. Among them, at least one of an aqueous solution of sodium silicate and an aqueous solution of activated silicic acid is preferably used as the second silica raw material. When adding the second silica raw material at an emulsion pH of 8 or higher, a method of adding an alkali metal hydroxide simultaneously with the second silica raw material may be used. Alternatively, a method of using sodium silicate as an alkali metal silicate in the second silica raw material may be used. In this case, the sodium silicate component, which is an alkaline component, is added to the weakly acidic emulsion whose pH has been adjusted to 5 or higher after the addition of the first silica raw material, so that the pH of the emulsion can be maintained at an alkaline level of 8 or higher while the second silica raw material is being added. In addition, alkali metal ions become present in the emulsion.

[0040] The pH of the emulsion when the second silica raw material is added to the emulsion is preferably 8 or higher, and may be 9 or higher. This allows a denser second coating layer to be formed on the first coating layer made of the first silica raw material. When adding the first silica raw material, the emulsion is first acidified and then adjusted to a pH of 5 or higher to ensure more uniform adhesion of the first silica raw material to the oil droplets. The first silica layer obtained using this method is porous and lacks density, resulting in low strength. When adding the second silica raw material, the emulsion is made alkaline, allowing a high-density second silica layer to form on top of the first silica layer obtained earlier. A dense silica shell layer can be formed from the silica layers formed in these two stages. The pH of the emulsion when the second silica raw material is added to the emulsion is not particularly limited, but may be 13 or less, or may be 11 or less. If the pH becomes too high when an aqueous sodium silicate solution is used as the second silica raw material, an acid may be added to adjust the pH. The acid used here may be the same acid as that used when the first silica raw material is added.

[0041] The second silica raw material is added in the presence of alkali metal ions. These alkali metal ions may be derived from the first silica raw material, the second silica raw material, or a base added for pH adjustment. They can also be incorporated by adding additives to the emulsion. For example, an alkali metal silicate may be used as at least one of the first and second silica raw materials. Alternatively, an alkali metal halide, sulfate, nitrate, fatty acid salt, or the like may be used as an additive to the emulsion.

[0042] The second silica raw material may be added, for example, by adding either an aqueous sodium silicate solution or an aqueous active silicic acid solution to the emulsion after the addition of the first silica raw material, or by adding both. When both are added, the aqueous sodium silicate solution and the aqueous active silicic acid solution may be added all at once or in order.

[0043] For example, the addition of the second silica raw material can be performed by repeating the steps of adding an aqueous sodium silicate solution and adding an aqueous activated silicic acid solution once or twice or more times while adjusting the pH to promote adhesion of the silica raw material onto the first silica layer.

[0044] The second silica raw material is preferably added to a heated emulsion to promote adhesion of the silica raw material onto the first silica layer. The heating temperature is preferably 30 to 100°C, more preferably 50 to 80°C. When a heated emulsion is used, after the addition of the second silica raw material, the resulting emulsion is preferably slowly cooled to room temperature (23°C).

[0045] In the addition of the second silica raw material, the amount of the second silica raw material added is preferably adjusted so that the amount of SiO2 in the second silica raw material is 20 to 500 parts by mass, and more preferably 40 to 300 parts by mass, per 100 parts by mass of the oil phase. In the addition of the second silica raw material, it is preferable to maintain the emulsion at a pH of 8 or higher for 10 minutes or longer after the addition of the second silica raw material.

[0046] Through the addition of the first silica raw material and the second silica raw material, the total amount of the first silica raw material and the second silica raw material added is preferably adjusted so that the sum of the SiO2 in the first silica raw material and the SiO2 in the second silica raw material is 30 to 500 parts by mass, and more preferably 50 to 300 parts by mass, per 100 parts by mass of the oil phase.

[0047] Although the silica shell layer of the present invention is mainly composed of silica, other metal components such as Ti and Zr may be contained as necessary for refractive index adjustment, etc. The method for containing other metal components is not particularly limited, and for example, a method such as adding a metal sol liquid or a metal salt aqueous solution simultaneously in the step of adding the silica raw material can be used.

[0048] Hereinafter, the steps of obtaining hollow silica precursors from a hollow silica precursor dispersion and then obtaining hollow silica particles from the hollow silica precursor will be described.

[0049] Methods for obtaining hollow silica precursors from hollow silica precursor dispersions include, for example, filtering the dispersion, heating to remove the aqueous phase, and separating the precursors by sedimentation or centrifugation. One example is a method in which the dispersion is filtered using a filter of about 0.1 μm to 5 μm, and the filtered hollow silica precursor is dried. If necessary, the obtained hollow silica precursor may be washed with water, an acid, an alkali, an organic solvent, or the like.

[0050] Methods for obtaining hollow silica particles by removing the oil core from a hollow silica precursor include, for example, a method of calcining the hollow silica precursor to burn and decompose the oil, a method of volatilizing the oil by drying, a method of decomposing the oil by adding an appropriate additive, and a method of extracting the oil using an organic solvent or the like. For example, the hollow silica precursor is preferably heated at a temperature of 300 to 800°C, particularly 400 to 600°C, for a heating time of 1 to 8 hours, particularly 3 to 6 hours. In this case, the temperature rise rate is preferably 1 to 20°C / min, particularly 2 to 10°C / min. The obtained hollow silica particles may be aggregated during the drying or calcination process, and may be crushed to obtain an aggregate size that is easy to handle. Examples of the crushing method include a method using a mortar, a method using a dry or wet ball mill, a method using a shaking sieve, or a method using a crusher such as a pin mill, a cutter mill, a hammer mill, a knife mill, or a roller mill.

[0051] The fact that the resulting hollow silica particles have voids inside the shell layer can be confirmed by observation with a transmission electron microscope (TEM). Spherical particles with internal voids that can be confirmed by TEM observation are defined as "primary particles." Because the primary particles partially bond together during the firing and drying processes, the resulting hollow silica is often an aggregate of secondary particles that are agglomerations of primary particles. The size of primary particles can be determined by directly observing their particle diameters using TEM observation. Specifically, a region is observed in a magnified manner where there are no particles with extremely large or small diameters and the distribution of primary particle diameters within the observation range is thought to be similar to the distribution of primary particle diameters for the entire hollow silica particles obtained, the size of each primary particle is measured, and the resulting distribution of primary particle sizes is estimated to be the distribution of the entire primary particle size. The average size of the primary particles is preferably 10 nm to 10 μm, more preferably 50 nm to 2 μm, and even more preferably 100 nm to 1 μm.

[0052] The hollow silica particles have a BET specific surface area of 300m 2 / g or less is preferable, and 200m 2 / g or less is more preferable, and 150m 2 / g or less is more preferable. Here, the BET specific surface area was measured using a specific surface area measuring device "Tristar II3020" manufactured by Shimadzu Corporation. After pre-treatment, the sample was dried at 230°C until the pressure reached 50 mTorr, and then the measurement was performed by the multi-point method using liquid nitrogen.

[0053] The shell thickness of the hollow silica particles is preferably 0.01 to 0.3, more preferably 0.02 to 0.2, and even more preferably 0.03 to 0.1, relative to the size of the primary particle. If the shell thickness is less than 0.01 times the primary particle size, the strength of the hollow silica particles may decrease. If this ratio is greater than 0.3, the internal voids become too small, and the properties of the hollow shape are lost. Here, the shell thickness is measured by measuring the shell thickness of each particle by TEM observation, in the same manner as for the primary particle diameter.

[0054] The hollow silica particles are formed using oil droplets of an oil-in-water emulsion as core particles, and therefore the shape of the hollow silica particles is spherical, preferably a perfect sphere.

[0055] In the present invention, the shell layer of the obtained hollow silica particles contains an alkali metal component, which is hardly observed when silicon alkoxide is used as the silica raw material. For example, when a sodium silicate aqueous solution is used as the silica raw material, the mass concentration of the Na component in the shell of the obtained hollow silica particles is 500 ppm by mass or more, and even 1000 ppm by mass or more in some cases.On the other hand, the mass concentration of the Na component in the shell of general hollow silica particles made using tetraethyl orthosilicate as the silica raw material is 100 ppm by mass or less. The Na component can be measured by adding perchloric acid and hydrofluoric acid to the obtained hollow silica, igniting the mixture to remove the main component silicon, and then measuring the Na component by ICP emission spectrometry. Furthermore, when an alkali metal silicate is used as the silica raw material, the shell layer of the resulting hollow silica particles contains less carbon (C) components derived from the raw material than when a silicon alkoxide is used as the silica raw material.

[0056] The hollow silica particles produced according to the method of the present invention are characterized by the denseness of the shell layer, and can have the property of settling in water but floating in an oil component, for example. A portion of the hollow silica particles produced by the method of the present invention can be hollow silica having one pore (hollow silica having one pore). This pore, as observed by scanning electron microscope (SEM), has a pore size of 5 nm to 3 μm, preferably 1 / 3 or less of the particle size of the hollow silica particles, and more preferably 10 nm to 1 μm, preferably 1 / 5 or less of the particle size of the hollow silica particles. Such single-pore hollow silica particles can be used as sustained-release agents for perfumes and drugs. Specifically, a specific drug (preferably a liquid or solution) is incorporated into the particles under reduced pressure. Under normal pressure, the drug is released from the single pore. These single-pore hollow silica particles can be selected from the particle group of the present invention, for example, by the following method. First, the particles are placed in a liquid medium under atmospheric pressure, and the particles that float up are collected. If a fluorine-based solvent is used as the liquid medium, particle groups with an apparent specific gravity lighter than the liquid medium are easily collected. Next, the collected particle group is placed in a liquid medium under reduced pressure. The single-pore hollow silica particles sink into the liquid medium because the liquid medium fills the hollow portion. Therefore, the particles that have settled in the liquid medium are collected and dried, and the single-pore hollow silica particles can be collected.

[0057] The hollow silica particles of the present invention have an average primary particle diameter of 10 nm to 10 μm and a BET specific surface area of 300 m 2 / g or less, and the shell layer contains an alkali metal component. The mass concentration of the alkali metal component in the shell of the hollow silica particles according to the present invention is preferably 500 ppm by mass or more, more preferably 1000 ppm by mass or more. If the alkali metal component is too small, the shell becomes porous, making it impossible to obtain a dense, strong shell. The hollow silica particles of the present invention can be produced by the above-mentioned production method, but are not limited to those produced by the above-mentioned production method. Details of the average primary particle size, BET specific surface area, etc. of the hollow silica particles are as described above. [Example]

[0058] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these. In the following description, the same components are used. Unless otherwise specified, "%" represents "% by mass." Examples 1 to 17 are examples, and Example 21 is a comparative example.

[0059] (Example 1) "Preparation of emulsion" 3.6 g of EO-PO-EO block copolymer (Kolliphor P188 manufactured by BASF) was added to 1778 g of pure water and stirred until dissolved. To this aqueous solution, 18 g of n-dodecane (special grade reagent, manufactured by Wako Pure Chemical Industries, Ltd.) was added, and the solution was stirred using an IKA homogenizer until the entire solution became homogenous, producing a crude emulsion. This coarse emulsion was emulsified three times at a pressure of 400 bar using a high-pressure emulsifier (LAB1000 manufactured by SMT Co., Ltd.) to produce a fine emulsion.

[0060] "First stage shell formation" To 1600 g of the resulting fine emulsion, 3.4 g of 35% hydrochloric acid (special grade reagent, manufactured by Wako Pure Chemical Industries, Ltd.) was added to adjust the pH to 1.7. Next, 34.8 g of a diluted aqueous solution of sodium silicate (SiO2 concentration 6.9 mass %, Na2O concentration 2.3 mass %) was added and stirred thoroughly, and the pH was adjusted to 2.4 and maintained for 15 minutes. While thoroughly stirring this liquid, 46.1 g of a 0.1 M aqueous solution of sodium hydroxide was slowly added dropwise, and the stirring state was maintained for 4 hours, after which an oil core-silica shell particle dispersion liquid with a pH of 5.2 was obtained.

[0061] "Activated silicic acid preparation" 380 g of purified water was added to 270 g of well-washed hydrogen-type cation exchange resin (SK1BH manufactured by Mitsubishi Plastics, Inc.), and the temperature was cooled to 5°C. While the resin dispersion was thoroughly stirred and the temperature was maintained at 5°C, 360 g of a diluted aqueous sodium silicate solution (SiO2 concentration 10.3 mass %, Na2O concentration 3.5 mass %) was added dropwise in small amounts. After the dropwise addition was completed, the resin was removed by filtration to obtain a 5% by mass aqueous solution of active silicic acid.

[0062] "Second stage shell formation" 1,300 g of the oil core-silica shell particle dispersion obtained in the first shell formation was heated to 70°C, and 35.2 g of a diluted sodium silicate aqueous solution (SiO2 concentration 3.5 mass%, Na2O concentration 1.2 mass%) was added with stirring to adjust the pH to 9.6. While stirring this suspension and maintaining it at 70°C, 200 g of the prepared 5% by mass aqueous solution of active silicic acid was slowly added dropwise to the suspension to adjust the pH to 9.2. Next, 1.4 g of a diluted aqueous sodium silicate solution (SiO2 concentration 3.5 mass %, Na2O concentration 1.2 mass %) was added to adjust the pH to 9.5. Again, 200 g of a 5% by mass aqueous solution of active silicic acid was slowly added dropwise to adjust the pH to 9.2. Next, 5.2 g of diluted sodium silicate aqueous solution (SiO2 concentration 3.5 mass %, Na2O concentration 1.2 mass %) was added again to adjust the pH to 9.4. Again, 195 g of a 5 mass % aqueous solution of active silicic acid was slowly added dropwise to adjust the pH to 8.9, and the mixture was slowly cooled to room temperature to obtain a hollow silica precursor dispersion.

[0063] "Filtering, drying, and baking" 1200 g of the hollow silica precursor dispersion was filtered under pressure (pressure 0.28 MPa) using a 0.45 μm hydrophilized PTFE membrane filter, and then dried at 80° C. for 8 hours to obtain a hollow silica precursor. The obtained precursor was calcined at 550°C for 4 hours (heating rate: 10°C / min) to obtain 6 g of hollow silica particles.

[0064] "evaluation" The BET specific surface area of the hollow silica obtained in Example 1, measured by nitrogen adsorption, was 118 m 2 / g. Transmission electron microscope images (TEM images) of the hollow silica particles obtained in Example 1 are shown in FIGS. 116 primary particles with clear outlines were selected from the TEM image shown in Figure 1, and their diameters were measured and tallied to find that the average was 278 nm. Furthermore, the shell thickness was measured from the TEM image shown in Figure 2 and found to be 15 nm. A scanning electron microscope (SEM) image of the hollow silica particles obtained in Example 1 after crushing them in an agate mortar is shown in Figure 3. From Figure 3, it can be seen that the hollow silica particles of Example 1 maintain their hollow particle shape even after crushing in a mortar. The mass concentration of Na in the shell of the hollow silica obtained in Example 1 was 3800 ppm by mass.

[0065] (Example 21) "Preparation of emulsion" 18 g of EO-PO-EO block copolymer (Kolliphor P188 manufactured by BASF) was added to 1765 g of pure water and stirred until dissolved. 18 g of n-dodecane was added to this aqueous solution, and the mixture was stirred using an IKA homogenizer until the entire solution became homogenous, producing a crude emulsion. This coarse emulsion was emulsified three times at a pressure of 400 bar using a high-pressure emulsifier (LAB1000 manufactured by SMT Co., Ltd.) to produce a fine emulsion.

[0066] "Formation of hollow silica precursor" To 1700 g of the resulting fine emulsion, 9.2 g of 35% hydrochloric acid was added to adjust the pH to 1.5. Next, 123.3 g of a diluted aqueous solution of sodium silicate (SiO2 concentration 6.9 mass %, Na2O concentration 2.3 mass %) was added and stirred thoroughly, and the pH was adjusted to 2.2 and maintained for 15 minutes. While thoroughly stirring this liquid, 127.1 g of a 0.1 M aqueous solution of sodium hydroxide was slowly added dropwise, and the stirring state was maintained for 4 hours, after which a hollow silica precursor dispersion liquid with a pH of 5.2 was obtained.

[0067] "Filtering, drying, and baking" 600 g of the hollow silica precursor dispersion was filtered under reduced pressure using a 0.45 μm hydrophilized PTFE membrane filter, and then dried at 80° C. for 8 hours to obtain a hollow silica precursor. The obtained precursor was calcined at 550°C for 4 hours (heating rate: 10°C / min) to obtain 3 g of hollow silica particles.

[0068] "evaluation" The BET specific surface area of the hollow silica obtained in Example 21, measured by nitrogen adsorption, was 554 m 2 / g. A TEM image of the hollow silica particles obtained in Example 21 is shown in FIG. In the TEM image shown in FIG. 4, the shadow of the shell layer is lighter than in the TEM image shown in FIG. 2, suggesting that the shell is less dense than in Example 1. Figure 5 shows an SEM image of the hollow silica particles obtained in Example 21 after crushing them in an agate mortar. Figure 5 shows that the hollow silica particles of Example 21 had insufficient silica shell strength, so most of the particles had collapsed, and many fragments of the silica shell were present.

[0069] (Example 2) "Preparation of emulsion" 2.4 g of EO-PO-EO block copolymer (Kolliphor P188 manufactured by BASF) was added to 480 g of pure water and stirred until dissolved. 16 g of n-dodecane was added to this aqueous solution, and the whole solution was stirred using an IKA homogenizer until it became homogenous, producing a crude emulsion. This coarse emulsion was emulsified three times at a pressure of 400 bar using a high-pressure emulsifier (LAB1000 manufactured by MST Corporation) to produce a fine emulsion.

[0070] "First stage shell formation" To 442 g of the resulting fine emulsion, 9 g of 2M hydrochloric acid was added to adjust the pH to 1.5. Next, 12.3 g of a diluted sodium silicate aqueous solution (SiO2 concentration 10.4 mass %, Na2O concentration 3.6 mass %) was added and stirred thoroughly, and the pH was adjusted to 2.1 and maintained for 15 minutes. While thoroughly stirring this liquid, 4 g of a 1 M aqueous solution of sodium hydroxide was slowly added dropwise, and after maintaining the state under stirring for 1 hour, an oil core-silica shell particle dispersion liquid with a pH of 5.8 was obtained.

[0071] "Second stage shell formation" 400 g of the oil core-silica shell particle dispersion obtained in the first shell formation was heated to 30°C, and 3 g of diluted sodium silicate aqueous solution (SiO2 concentration 10.4 mass%, Na2O concentration 3.6 mass%) was slowly added while stirring to adjust the pH to 9. Next, 127 g of a diluted aqueous solution of sodium silicate (SiO2 concentration 10.4 mass %, Na2O concentration 3.6 mass %) was gradually added together with 0.5 M hydrochloric acid to adjust the pH to 9. This suspension was kept at 30°C for 2 days and then slowly cooled to room temperature to obtain a hollow silica precursor dispersion.

[0072] "Filtering, drying, and baking" 770 g of the hollow silica precursor dispersion was filtered under pressure (pressure 0.28 MPa) using a 0.45 μm hydrophilic PTFE membrane filter. The filtered cake was dried in a nitrogen atmosphere at 60°C for 1 hour and then at 400°C for 4 hours (heating rate 5°C / min) to obtain a hollow silica precursor. The obtained precursor was calcined at 550°C for 4 hours (heating rate: 5°C / min) to obtain 13.8 g of hollow silica particles. The specific surface area of the hollow silica particles obtained in Example 2 is shown in Table 1.

[0073] (Examples 3 to 5) Silica particles were produced in the same manner as in Example 2. However, the reaction temperature and holding temperature during the second shell formation were changed. Table 1 shows the reaction temperature and holding temperature during the second-stage shell formation in Examples 3 to 5, and the specific surface area of the resulting hollow silica particles. The higher the reaction temperature and holding temperature during the second shell formation, the smaller the specific surface area, which suggests that hollow silica particles with a dense shell can be produced. [Table 1]

[0074] (Examples 6 to 13) Silica particles were prepared in the same manner as in Example 5. However, the type of EO-PO-EO block copolymer used was changed in Examples 6 to 13. The EO-PO-EO block copolymer used in each example is shown in Table 2. In all of Examples 6 to 13, hollow silica particles were obtained, but in Examples 6, 7, and 11, many hollow silica particles with one hole were observed. [Table 2]

[0075] (Example 14) Silica particles were prepared in the same manner as in Example 5, except that the amount of EO-PO-EO block copolymer (Kolliphor P188 manufactured by BASF) added was 10 times the amount. The specific surface area of the hollow silica particles obtained in Example 14 was 140 m 2 / g.

[0076] (Example 15) "Preparation of emulsion" 2.4 g of EO-PO-EO block copolymer (ADEKA F-68) was added to 480 g of pure water and stirred until dissolved. 16 g of n-dodecane was added to this solution, and the whole solution was stirred using an IKA homogenizer until it became homogenous, producing a crude emulsion. This coarse emulsion was subjected to ultrasonic irradiation twice for 1 minute at an intensity of V-Level 3.4 using an ultrasonic disperser (Ginsen Co., Ltd., GSCVP-600) to produce a fine emulsion.

[0077] After the formation of the first shell, silica particles were produced in the same manner as in Example 5. However, after the formation of the first shell, the liquid containing the silica raw material was not maintained at pH 5, but instead, a 1 M aqueous sodium hydroxide solution was added to bring the pH to 9. The specific surface area of the hollow silica particles obtained in Example 15 was 170 m 2 / g.

[0078] (Example 16) "Preparation of emulsion" 4.9 g of EO-PO-EO block copolymer (Pluronic PE10400 manufactured by BASF) was added to 462 g of pure water and stirred until dissolved. To this aqueous solution, 33 g of a fluorine-based solvent, Asahiklin AC-6000 (AGC: 1,1,1,2,2,3,3,4,4,5,5,6,6-tridecafluorooctane), was added, and the solution was stirred using an IKA homogenizer until the entire solution became homogenous, producing a crude emulsion. This coarse emulsion was subjected to ultrasonic irradiation twice for 1 minute at an intensity of V-Level 3.4 using an ultrasonic disperser (Ginsen Co., Ltd., GSCVP-600) to produce a fine emulsion.

[0079] "First stage shell formation," "Second stage shell formation," and "Filtration" The first and second shell formations and filtration were carried out in the same manner as in Example 5 to produce silica particles.

[0080] "Dry" The cake after filtration was dried at 60°C for 2 days to remove AC-6000, and 8 g of hollow silica particles was obtained. The specific surface area of the hollow silica particles obtained in Example 16 was 80 m 2 / g.

[0081] (Example 17) "Preparation of emulsion" 13 g of EO-PO-EO block copolymer (PE10500 manufactured by BASF) was added to 174 g of pure water and dissolved. This solution was kept at 5°C, and 13 g of PO-EO-PO block copolymer (25R-1 manufactured by ADEKA) was added and dissolved, yielding a clear aqueous solution. 300 g of pure water was heated to 60°C, and the block copolymer mixed solution was added dropwise with stirring. This yielded a fine emulsion with droplets of approximately 300 nm in diameter.

[0082] After the first shell formation, silica particles were produced in the same manner as in Example 5. However, the first shell formation was carried out at 60°C, and after that, the liquid to which the silica raw material had been added was not maintained at pH 5, but instead a 1 M aqueous sodium hydroxide solution was added to bring the pH to 9. The specific surface area of the hollow silica particles obtained in Example 17 was 160 m 2 / g. The entire contents of the specification, claims, abstract and drawings of Japanese Patent Application No. 2017-248972, filed on December 26, 2017, are incorporated herein by reference as part of the disclosure of the specification of the present invention.

Claims

1. A method for producing hollow silica particles, comprising: adding a first silica raw material to an oil-in-water emulsion having a pH of 3.0 or less, the oil-in-water emulsion containing an aqueous phase, an oil phase, and a surfactant; maintaining the pH of the oil-in-water emulsion at 5 or more; and then adding a second silica raw material to the oil-in-water emulsion having a pH of 8 or more in the presence of alkali metal ions.

2. 2. The method for producing hollow silica particles according to claim 1, wherein the first silica raw material and the second silica raw material each independently comprise at least one selected from the group consisting of alkali metal silicates and active silicic acids.

3. The method for producing hollow silica particles according to claim 2, wherein the alkali metal silicate is sodium silicate.

4. The method for producing hollow silica particles according to claim 1 , wherein an aqueous solution of an alkali metal silicate is used as the first silica raw material.

5. 5. The method for producing hollow silica particles according to claim 1, wherein at least one of an aqueous solution of an alkali metal silicate and an aqueous solution of an active silicic acid is used as the second silica raw material.

6. The method for producing hollow silica particles according to claim 1 , wherein the second silica raw material is added to the heated emulsion.

7. 7. The method for producing hollow silica particles according to claim 1, wherein a base is added to the emulsion after the addition of the first silica raw material, and then a second silica raw material is added.

8. 8. The method for producing hollow silica particles according to claim 1, further comprising obtaining a hollow silica precursor from the hollow silica precursor dispersion obtained after the addition of the second silica raw material, and calcining the hollow silica precursor.

9. The method for producing hollow silica particles according to claim 8, wherein the firing temperature is 300°C to 800°C.

10. A method for producing hollow silica particles according to any one of claims 1 to 9, wherein the surfactant is a polyoxyethylene-polyoxypropylene copolymer.

11. The average primary particle diameter of the obtained hollow silica particles is 10 nm to 10 μm, and the BET specific surface area is 300 m ２ The method for producing hollow silica particles according to claim 1 , wherein the surface roughness is 1 / g or less.

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