Method for manufacturing hollow silica particles

Abstract

This invention provides a simple method for producing spherical, hollow silica particles with excellent monodispersibility, having an average particle diameter of 1 μm or less (nano-size). [Solution] A method for producing hollow silica particles comprising the steps of: dispersing AB block copolymer and organic solvent (i) in water to form emulsion particles; forming a silica layer by a sol-gel reaction to obtain silica particles with the emulsion particles as a core; and firing the silica particles to obtain hollow silica particles, wherein the AB block copolymer satisfies the following conditions (a) to (c). (a): Polymer block A contains 80% by mass or more of constituent units derived from alkyl methacrylates, etc., having an alkyl group having 6 to 18 carbon atoms. (b): Polymer block B contains 50% by mass or more of constituent units derived from methacrylate having a quaternary ammonium base. (c): The mass ratio of polymer block A to polymer block B is 50:50 to 80:20.

Description

TECHNICAL FIELD

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

[0002] Hollow particles are particles having voids inside, and have properties different from those of ordinary particles, such as low density, low refractive index, high specific surface area, and substance encapsulation ability. Taking advantage of these properties, hollow particles are widely applied as functional materials such as lightweight materials, heat insulating materials, and coloring materials. In addition to resins and carbon, metal oxides such as alumina, titania, zirconia, and tin oxide are often used as materials for forming hollow particles. Among these, hollow particles formed of silica (hollow silica particles) are inexpensive, easy to handle, and have many excellent properties such as low refractive index, low dielectric constant, low thermal conductivity, low density, and biocompatibility. Therefore, many studies have been conducted on their applications to antireflective materials, low dielectric materials, heat insulating materials, resin fillers, catalysts, drug delivery systems (DDS), and the like.

[0003] One of the typical production methods for hollow silica particles is the template method. The template method is a method in which a silica shell is formed on the surface of template particles such as inorganic particles or organic particles, and then the template is selectively removed to obtain hollow silica particles. This method is capable of adjusting the particle size of the obtained hollow silica particles according to the particle size of the particles used as the template, and many studies have been made on this method.

[0004] For example, a method has been proposed in which after a silica shell is formed on the surface of calcium carbonate particles serving as a template, an acid is added to dissolve the calcium carbonate to produce hollow silica particles (Patent Document 1). However, since inorganic particles are often crystalline and amorphous, when they are used as a template, there is a problem that the obtained hollow silica particles are less likely to be spherical.

[0005] Furthermore, a method has been proposed for producing hollow silica particles by forming a silica shell on the surface of polystyrene particles used as a template, and then removing the polystyrene by calcination (Patent Document 2). Using organic particles such as polystyrene as a template makes it easier to produce spherical hollow silica particles compared to using inorganic particles. However, the synthesis process of organic particles is often complicated, which has presented challenges in terms of cost and other factors.

[0006] As a method that can solve the above-mentioned problems, for example, a method has been proposed in which a polymer emulsion is used as a template, a silica shell is formed on the surface of the emulsion particles, and then the polymer is removed by calcination to produce hollow silica particles (Patent Document 3). [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2005-263550 [Patent Document 2] Special Publication No. 2009-504632 [Patent Document 3] International Publication No. 2021 / 010243 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] The method proposed in Patent Document 3 allows for the production of hollow silica particles with a wider variety of structures compared to the method using solid particles such as inorganic particles as templates. However, there was a problem in that the particle size distribution of the resulting hollow silica tended to be broad.

[0009] This invention has been made in view of the problems of the prior art, and its objective is to provide a simple method for producing spherical hollow silica particles with excellent monodispersibility, having an average particle diameter of 1 μm or less (nano-size). [Means for solving the problem]

[0010] In other words, the present invention provides a method for producing hollow silica particles as shown below. [1] A method for producing hollow silica particles, comprising: a first step of dispersing an AB block copolymer containing polymer block A and polymer block B and a water-insoluble organic solvent (i) in water to form emulsion particles containing the AB block copolymer and the organic solvent (i); a second step of adding a silica source compound and forming a silica layer on the surface of the emulsion particles by a sol-gel reaction to obtain silica particles with the emulsion particles as a core; and a third step of washing the obtained silica particles with an organic solvent (ii) that dissolves the AB block copolymer and the organic solvent (i), or firing them at 500°C or higher to remove the AB block copolymer and the organic solvent (i) to obtain hollow silica particles, wherein the AB block copolymer satisfies the following (a) to (c), and the mass ratio of the AB block copolymer to the organic solvent (i) used in the first step is 20:80 to 50:50. (a) The polymer block A contains 80% by mass or more of constituent units derived from at least one methacrylate selected from the group consisting of alkyl methacrylate having an alkyl group having 6 to 18 carbon atoms, cycloalkyl methacrylate having a cycloalkyl group having 6 to 18 carbon atoms, and arylalkyl methacrylate having an arylalkyl group having 6 to 18 carbon atoms, and has a number average molecular weight of 5,000 to 40,000 and a molecular weight distribution (weight average molecular weight / number average molecular weight) of less than 1.8. (b): The polymer block B contains 50% by mass or more of constituent units derived from methacrylate having a quaternary ammonium base. (c): The mass ratio of polymer block A to polymer block B is 50:50 to 80:20. [2] The method for producing hollow silica particles according to [1], wherein the organic solvent (i) is a hydrocarbon having 6 to 18 carbon atoms that is liquid at 25°C. [3] A method for producing hollow silica particles according to [1] or [2], wherein the average particle diameter of the hollow silica particles is 10 to 1,000 nm. [Effects of the Invention]

[0011] According to the present invention, a simple method for producing spherical hollow silica particles with excellent monodispersibility, having an average particle diameter of 1 μm or less (nano-size), can be provided. [Brief explanation of the drawing]

[0012] [Figure 1] This is a transmission electron microscope image of the hollow silica particles obtained in Example 1. [Modes for carrying out the invention]

[0013] <Method for producing hollow silica particles> The embodiments of the present invention will be described below, but the present invention is not limited to the embodiments described below. One embodiment of the method for producing hollow silica particles of the present invention comprises a first step, a second step, and a third step. The first step is to disperse an AB block copolymer containing polymer block A and polymer block B and a water-insoluble organic solvent (i) in water to form emulsion particles containing the AB block copolymer and the organic solvent (i). The second step is to add a compound that serves as a silica source and form a silica layer on the surface of the emulsion particles by a sol-gel reaction to obtain silica particles with the emulsion particles as a core. The third step is to wash the obtained silica particles with an organic solvent (ii) that dissolves the AB block copolymer and the organic solvent (i), or to calcine them at 500°C or higher to remove the AB block copolymer and the organic solvent (i) and obtain hollow silica particles. The AB block copolymer satisfies the following conditions (a) to (c), and the mass ratio of the AB block copolymer to the organic solvent (i) used in the first step is 20:80 to 50:50. The following describes the details of the method for producing hollow silica particles according to this embodiment. (a): Polymer block A contains 80% by mass or more of constituent units derived from at least one methacrylate selected from the group consisting of alkyl methacrylate having an alkyl group having 6 to 18 carbon atoms, cycloalkyl methacrylate having a cycloalkyl group having 6 to 18 carbon atoms, and arylalkyl methacrylate having an arylalkyl group having 6 to 18 carbon atoms, has a number average molecular weight of 5,000 to 40,000, and a molecular weight distribution (weight average molecular weight / number average molecular weight) of less than 1.8. (b): Polymer block B contains 50% by mass or more of constituent units derived from methacrylate having a quaternary ammonium base. (c): The mass ratio of polymer block A to polymer block B is 50:50 to 80:20.

[0014] (1st step) In the first step, the AB block copolymer and a water-insoluble organic solvent (i) are dispersed in water. This forms emulsion particles containing the AB block copolymer and the organic solvent (i). That is, emulsion particles formed by dispersing the organic solvent (i) in water in the presence of the AB block copolymer are used as a template.

[0015] [AB Block Copolymer] The AB block copolymer comprises polymer block A (hereinafter also referred to as "chain A") and polymer block B (hereinafter also referred to as "chain B"). Preferably, the AB block copolymer is a diblock type copolymer substantially composed only of polymer block A and polymer block B.

[0016] Polymer block A is a block that forms the core of a core-shell emulsion in water through hydrophobic interaction. Therefore, polymer block A is composed of structural units derived from highly hydrophobic monomers. Specifically, polymer block A contains 80% by mass or more of structural units derived from at least one methacrylate selected from the group consisting of alkyl methacrylates having an alkyl group with 6 to 18 carbon atoms, cycloalkyl methacrylates having a cycloalkyl group with 6 to 18 carbon atoms, and arylalkyl methacrylates having an arylalkyl group with 6 to 18 carbon atoms, preferably 80% by mass or more, more preferably 90% by mass or more, and particularly preferably 95% by mass or more. Note that polymer block A may be substantially composed only of structural units derived from these methacrylates.

[0017] Examples of the above methacrylates for constituting the structural units of polymer block A include monofunctional methacrylates having a substituent such as hexyl, heptyl, octyl, nonyl, decyl, dodecyl, tetradecyl, hexadecyl, octadecyl, 2-ethylhexyl, isooctyl, cyclohexyl, trimethylcyclohexyl, t-butylcyclohexyl, benzyl, isobornyl, dicyclopentanyl, dicyclopentenyl, and dicyclopentenyloxyethyl.

[0018] Polymer block A may further contain structural units derived from methacrylates other than the above methacrylates (other methacrylates). Examples of the other methacrylates include monofunctional methacrylates having a substituent such as methyl, ethyl, propyl, butyl, pentyl, methoxyethyl, butoxyethyl, phenoxyethyl, nonylphenoxyethyl, glycidyl, 2-hydroxyethyl, 2-hydroxypropyl, 4-hydroxybutyl, polyethylene glycol, polypropylene glycol, polyethylene glycol monomethyl ether, polypropylene glycol monomethyl ether, poly-ε-caprolactone, and polydimethylsiloxane.

[0019] The number-average molecular weight (Mn) of polymer block A, measured by gel permeation chromatography (GPC), is 5,000 to 40,000, preferably 10,000 to 30,000. By using an AB block copolymer containing polymer block A with a number-average molecular weight within the above range, emulsion particles suitable for use as a template can be formed. If the number-average molecular weight of polymer block A is less than 5,000, the hydrophobic interaction with organic solvent (i) becomes insufficient, and the emulsion particles may not be formed stably. On the other hand, if the number-average molecular weight of polymer block A exceeds 40,000, polymerization requires an excessive amount of time, and the resulting emulsion particles may become too large in size, making them unsuitable as a template.

[0020] The molecular weight distribution (weight-average molecular weight (Mw) / number-average molecular weight (Mn)) of polymer block A (hereinafter also referred to as "PDI") is less than 1.8, preferably 1.5 or less. If the molecular weight distribution of polymer block A is 1.8 or higher, it becomes difficult to form stable emulsion particles in water.

[0021] Polymer block B is a water-soluble polymer block that forms the shell of a core-shell type emulsion. Therefore, polymer block B is composed of structural units derived from highly hydrophilic monomers. Furthermore, because it selectively precipitates silica on the surface of the emulsion particles to form a silica layer, polymer block B, which forms the shell, has a positive charge. Specifically, polymer block B contains 50% by mass or more, preferably 70% by mass or more, structural units derived from methacrylate having a quaternary ammonium base. If the content of structural units derived from methacrylate having a quaternary ammonium base in polymer block B is less than 50% by mass, hydrophilicity becomes insufficient, making it difficult to form emulsion particles, and AB block copolymer may precipitate. Polymer block B may be substantially composed only of structural units derived from methacrylate having a quaternary ammonium base. However, polymer block B may have amino groups that are not quaternary ammonium chloride.

[0022] Methacrylates containing a quaternary ammonium base can be obtained, for example, by reacting an amino group-containing methacrylate, such as dimethylaminoethyl methacrylate and diethylaminoethyl methacrylate, with a quaternizing agent to produce a quaternary ammonium chloride. Specifically, by reacting dimethylaminoethyl methacrylate with benzyl chloride, the chloride salt of benzyldimethyl-2-methacroyloxyethylammonium can be obtained. Note that it is not necessary to react all amino groups of the amino group-containing methacrylate with the quaternizing agent.

[0023] Furthermore, methacrylates having a quaternary ammonium base can also be obtained by reacting a compound having a primary or secondary amino group and a tertiary amino group with a methacrylate having a glycidyl group or isocyanatoethyl methacrylate to introduce a tertiary amino group, and then reacting it with a quaternizing agent. In addition, a quaternary ammonium salt may be obtained by reacting a methacrylate having a glycidyl group with a tertiary amine, or a quaternary ammonium salt may be obtained by reacting a methacrylate having a halogenated alkyl group such as 3-chloro-2-hydroxypropyl methacrylate with a tertiary amine.

[0024] In the AB block copolymer, the mass ratio (A:B) of polymer block A to polymer block B is 50:50 to 80:20, preferably 60:40 to 70:30. The mass ratio of polymer block A to polymer block B is designed taking into account the overall compositional balance of the AB block copolymer.

[0025] AB block copolymers can be synthesized by conventionally known methods. From the viewpoint of producing polymers with more controlled structures, it is preferable to produce them by living polymerization methods such as living anionic polymerization, living cationic polymerization, and living radical polymerization. Among these, living radical polymerization is particularly preferred from the viewpoint of conditions, materials, and equipment.

[0026] Living radical polymerization methods include atom transfer radical polymerization (ATRP), reversible addition-cleavage chain transfer polymerization (RAFT), nitroxide polymerization (NMP), organotellurium polymerization (TERP), reversible transfer catalytic polymerization (RTCP), and reversible catalyst-mediated polymerization (RCMP). Among these, the RTCP and RCMP methods, which use organic compounds as catalysts and organioidides as polymerization initiators, are preferred. These methods are advantageous in terms of cost and purification because they use relatively safe commercially available compounds and do not use heavy metals or special compounds. Furthermore, by using tertiary iodine at the growth end, a highly accurate block structure can be easily formed using general equipment.

[0027] The polymerization reaction may be either thermal polymerization or photopolymerization, and azo radical generators, peroxide radical generators, and photosensitizers may be added to the polymerization reaction system. The polymerization method may be solvent-free, solution polymerization, or emulsion polymerization, with solution polymerization being preferred. The solvent used in solution polymerization is not particularly limited, but it is preferable to select a type and amount of solvent that does not significantly affect the formation of emulsion particles.

[0028] In producing the AB block copolymer, either polymer block A or polymer block B may be polymerized first. In particular, it is preferable to polymerize polymer block A first, followed by polymer block B. If polymer block B is polymerized first, and the polymerization rate is not 100%, the methacrylate containing the quaternary ammonium base is more easily introduced into polymer block A. The quaternary ammonium base in polymer block B may be formed by polymerizing the amino group-containing methacrylate and then performing quaternary chlorination, or by polymerizing the amino group-containing methacrylate after quaternary chlorination.

[0029] [Emulsion particles] The AB block copolymer has a hydrophobic polymer block A and a hydrophilic polymer block B, and therefore self-emulsifies in the presence of a large amount of water to form core-shell type emulsion particles. If a water-insoluble organic solvent (i) is present during self-emulsification, the organic solvent (i) is incorporated into the core and dispersed as part of the emulsion particles. The average particle size of the formed emulsion particles is preferably 30 to 500 nm, and more preferably 50 to 350 nm. The average particle size of the emulsion particles is measured and calculated by a dynamic light scattering particle size distribution analyzer as the volume-based cumulative 50% particle size (median diameter (D)). 50 )) is.

[0030] The organic solvent (i) is preferably a hydrocarbon having 6 to 18 carbon atoms that is liquid at room temperature (25°C). Specific examples of such organic solvent (i) include hexane, heptane, octane, nonane, decane, undecane, dodecane, tetradecane, hexadecane, benzene, toluene, xylene, ethylbenzene, cumene, cyclohexane, cycloheptane, phenylcyclohexane, 1,5-hexadiene, 1,9-decadien, cyclooctatetraene, and pentamethylcyclopentadiene.

[0031] The mass ratio of AB block copolymer to organic solvent (i) (AB block copolymer:organic solvent (i)) is 20:80 to 50:50, preferably 30:70 to 40:60. By keeping the mass ratio of AB block copolymer to organic solvent (i) within the above range, stable emulsion particles can be formed.

[0032] When forming emulsion particles, the amount of AB block copolymer and organic solvent (i) used, based on the total mass of water, AB block copolymer, and organic solvent (i), is preferably 0.5 to 10% by mass, and more preferably 1 to 5% by mass. If it is less than 0.5% by mass, the amount (number) of emulsion particles formed tends to decrease, and there is a higher possibility that solid silica particles, not hollow, will be formed. On the other hand, if it exceeds 10% by mass, it may become difficult to stably form emulsion particles, and oily components may separate or polymers may precipitate over time.

[0033] A method for forming emulsion particles by emulsifying and dispersing AB block copolymer and a water-insoluble organic solvent (i) in water is conventionally known. For example, emulsion particles can be formed by adding a polymerization solution of AB block copolymer to a large amount of water while stirring to disperse and emulsify, then adding the organic solvent (i) dropwise and stirring thoroughly. Emulsification and dispersion methods using ultrasonic devices or high-pressure homogenizers that physically apply strong shear force may also be applied, and heating may be used as necessary.

[0034] (2nd process) In the second step, a silica source compound is added to the emulsion particles formed in the first step, and a silica layer is formed on the surface of the emulsion particles by a sol-gel reaction. This yields silica particles in which the emulsion particles are the core (core component) and the silica layer is the shell (silica shell). The precursor of the silica layer is negatively charged, while the surface of the emulsion particles is positively charged because polymer block B contains a quaternary ammonium base. Therefore, the sol-gel reaction proceeds selectively on the surface of the emulsion particles due to electrostatic action, and the silica layer is formed.

[0035] Examples of silica sources include silicon alkoxides and sodium silicate. Examples of silicon alkoxides include tetraalkoxysilanes such as tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetrabutoxysilane, as well as their oligomers. Sodium silicate, also known as "water glass," is used in the manufacture of various chemical products, and is preferred because type 1 silicic acid, type 2 silicic acid, and type 3 silicic acid are inexpensive and readily available.

[0036] The content of the silica-source compound during the sol-gel reaction is preferably 0.3 to 5% by mass, and more preferably 0.5 to 3% by mass, when converted to SiO2 concentration. If it is less than 0.3% by mass, the total amount of silica is small, which may make it somewhat difficult to form stable and uniform silica particles. On the other hand, if it is more than 5% by mass, the amount of silica generated in the system becomes excessive, and solid silica particles may be formed outside the surface of the emulsion particles. By appropriately controlling the amount of silica-source compound relative to the emulsion particles and appropriately selecting the reaction conditions, the thickness of the silica layer formed on the particle surface can be controlled, and the thickness of the silica shell of the hollow silica particles obtained can be controlled.

[0037] The temperature during the sol-gel reaction is preferably 0 to 90°C, and more preferably 10 to 50°C. While higher temperatures increase the reaction rate, the sol-gel reaction may proceed rapidly, potentially leading to variations in particle uniformity and stability. It is also preferable to appropriately set the reaction temperature according to the properties and reactivity of the silica source compound. The reaction may be carried out at a low temperature for a while before raising the temperature to increase reactivity, and in some cases, a hydrothermal reaction at 100°C or higher may be applied using a heat-resistant container.

[0038] The hydrolysis rate of alkoxysilanes is usually faster under acidic or basic conditions. Therefore, the pH during the sol-gel reaction may be adjusted as appropriate to improve reactivity and shorten the reaction time, and should be set appropriately depending on the properties of the silica source compound used.

[0039] Among the compounds that serve as silica sources, sodium silicate and tetramethoxysilane are relatively reactive. Therefore, when using a silica source compound with relatively high reactivity, the reaction time for the sol-gel reaction can be set to 5 minutes to 12 hours, and preferably to 30 minutes to 4 hours. On the other hand, when using a silica source compound with relatively low reactivity, the reaction time for the sol-gel reaction may be set to 1 hour or more, preferably to 4 hours or more, and in some cases to about a week.

[0040] After the sol-gel reaction, if necessary, the silica particles with the generated emulsion particles as the core are separated from the reaction system. Methods of separation include, for example, solid-liquid separation by suction filtration, solid-liquid separation by centrifugation or sedimentation, and heating the reaction system (dispersion) to remove the liquid.

[0041] (3rd step) In the third step, the silica particles obtained in the second step are either (1) washed with an organic solvent (ii) that dissolves the AB block copolymer and organic solvent (i), or (2) calcined at 500°C or higher. This removes the core components, the AB block copolymer and organic solvent (i), from the silica particles, thereby obtaining hollow silica particles.

[0042] For example, a large amount of organic solvent (ii) is added to the silica particles obtained in the second step and stirred to extract the core components. Then, hollow silica particles can be obtained by solid-liquid separation using methods such as filtration or centrifugation. To thoroughly remove any remaining AB block copolymer and organic solvent (i), it is preferable to increase the stirring time after adding organic solvent (ii), heat if necessary, or repeat washing multiple times.

[0043] As the organic solvent (ii), a solvent capable of dissolving the AB block copolymer and organic solvent (i) is used. Examples of organic solvents (ii) include hydrocarbon solvents such as hexane, toluene, and xylene; alcohol solvents such as methanol, ethanol, isopropanol, butanol, and dodecanol; ketone solvents such as acetone, methyl ethyl ketone, diethyl ketone, and isobutyl methyl ketone; ester solvents such as ethyl acetate, butyl acetate, amyl acetate, dimethyl succinate, dimethyl adipate, methyl lactate, and dimethyl lactate; ether solvents such as dipropyl ether, tetrahydrofuran, and dioxane; carbonate solvents such as dimethyl carbonate, ethylene carbonate, and propylene carbonate; N,N-dimethylformamide, N,N-dimethylacetamide, pyrrolidone, N-methylpyrrolidone, and 3-methoxy-N,N-dimethyl Examples include amide solvents such as propanamide and 3-butoxy-N,N-dimethylpropanamide; urea solvents such as tetramethylurea and dimethylimidazolidinone; sulfoxide solvents such as dimethyl sulfoxide; glycol monoether solvents such as ethylene glycol, propylene glycol, diethylene glycol, ethylene glycol methyl ether, ethylene glycol butyl ether, diethylene glycol methyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, and dipropylene glycol monomethyl ether; glycol diether solvents such as ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, and diethylene glycol diethyl ether; and so on.

[0044] Furthermore, hollow silica particles can also be obtained by, for example, calcining the silica particles obtained in the second step at 500°C or higher to remove the AB block copolymer and organic solvent (i) by thermal decomposition or volatilization. Calcining the silica particles is preferable because it allows for more effective removal of the AB block copolymer and organic solvent (i).

[0045] The firing temperature is 500°C or higher, preferably 600°C or higher. The firing time is preferably 1 to 12 hours, and more preferably 3 to 6 hours. The heating rate is preferably 1 to 10°C / min, and more preferably 2 to 5°C / min. The atmosphere inside the firing furnace can be, for example, air, nitrogen, and helium. Among these, firing in air is preferred.

[0046] (Hollow silica particles) The hollow silica particles obtained through the above process may agglomerate and form aggregates due to drying or calcination. In such cases, the aggregates may be broken up using any crushing method, such as using a mortar and pestle, bead mill, roll mill, or hammer mill, to obtain hollow silica particles. However, if strong shearing is applied when the silica shell thickness of the hollow silica particles is thin, the hollow structure may be destroyed. Therefore, when using a crushing method that applies strong shearing, it is preferable to appropriately control the crushing conditions.

[0047] The average particle diameter of the hollow silica particles produced by the manufacturing method of this embodiment is preferably 10 to 1,000 nm, more preferably 20 to 500 nm, and particularly preferably 50 to 200 nm. The thickness of the silica shell constituting the hollow silica particles is preferably 2 to 200 nm, and more preferably 5 to 100 nm, depending on the particle diameter.

[0048] The apparent volume and void volume of hollow silica particles can be calculated from the particle size and the thickness of the silica shell. The ratio of the void volume to the apparent volume of the hollow silica particle can then be calculated as the "porosity (%)". The porosity of hollow silica particles is preferably 20-80%, and more preferably 30-60%. If the porosity exceeds 80%, the silica shell is thin, which tends to reduce the strength of the hollow silica particles. On the other hand, if the porosity is less than 20%, the proportion of the hollow portion decreases, which may make it difficult for the hollow particle to function properly.

[0049] The average particle diameter of the hollow silica particles and the thickness of the silica shell can be observed, measured and calculated by observing the particles using a transmission electron microscope (TEM). Specifically, the particle diameter and silica shell thickness of 50 particles are measured from the observed TEM image, and the average values thereof are taken as the "average particle diameter" and "silica shell thickness". EXAMPLES

[0050] Hereinafter, the present invention will be specifically described based on examples, but the present invention is not limited to these examples. In addition, "parts" and "%" in the examples and comparative examples are based on mass unless otherwise specified.

[0051] <Production of AB Block Copolymer> (Synthesis Example 1) 298.6 parts of diethylene glycol monobutyl ether (BDG), 1.0 part of iodine, 3.0 parts of 2,2-azobis(2,4-dimethylvaleronitrile) (trade name "V-65", manufactured by FUJIFILM Wako Pure Chemical Corporation) (V-65), 0.2 parts of diphenylmethane (DPM), 117.0 parts of benzyl methacrylate (BzMA), and 117.0 parts of 2-ethylhexyl methacrylate (EHMA) were placed in a reaction vessel. The mixture was stirred while bubbling nitrogen, and polymerized at 65°C for 4 hours to synthesize a polymer (A chain). The solid content measured by sampling a part of the reaction solution was 40.1%, and the polymerization conversion rate calculated based on the solid content was 90.3%. The polystyrene-equivalent number average molecular weight (Mn) of the A chain measured by GPC using tetrahydrofuran (THF) as a developing solvent was 19,800, and the dispersity (PDI = weight average molecular weight (Mw) / number average molecular weight (Mn)) was 1.28.

[0052] Next, 1.5 parts of V-65 and 58.5 parts of 2-(dimethylamino)ethyl methacrylate (DMAEMA) were added, and polymerization was carried out at 65°C for a further 4 hours. The solid content measured by sampling a portion was 49.6%, and the polymerization conversion rate was approximately 100%. After diluting the sample with toluene and 2-propanol, the amine value of the product was measured using a potentiometric automatic titrator with a 0.1 mol / L 2-propanolic hydrochloric acid solution. As a result, the amine value of the product was 70.6 mgKOH / g.

[0053] The reaction solution was cooled to room temperature, diluted with 172.4 parts of BDG, and then a mixed solution of 46.2 parts of benzyl chloride (BzCl) and 46.2 parts of BDG was added dropwise over 30 minutes. The temperature was raised to 80°C and the reaction was allowed to proceed for 5 hours, during which the amino groups derived from DMAEMA were chlorinated with quaternary ammonium chloride to obtain a solution of AB block copolymer C-1. The solid content of the obtained solution was 39.2%, and the amine value of AB block copolymer C-1 was approximately 0 mgKOH / g. This confirmed that virtually all amino groups had been chlorinated with quaternary ammonium chloride.

[0054] (Synthesis examples 2-5, comparative synthesis examples 1-3) Solutions of AB block copolymers C-2 to C-5 and H-1 to H-3 were obtained in the same manner as in Synthesis Example 1 described above, except for the formulations shown in Tables 1 and 2. The meanings of the abbreviations in Tables 1 and 2 are shown below. • DMQ: A quaternary ammonium salt composed of DMAEMA and BzCl. CHMA: Cyclohexyl methacrylate LMA: Lauryl methacrylate IBXMA: Isobornyl methacrylate MMA: Methyl methacrylate HMA: Hexyl methacrylate

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[0057] <Manufacturing of hollow silica particles> (Example 1) 15.3 parts of a solution of AB block copolymer C-1 were placed in a reaction vessel, and 616.9 parts of deionized water were added and the mixture was stirred for a while. Then, 20 parts of dodecane were slowly added dropwise using a dropper while stirring. After that, the mixture was irradiated with ultrasound for 1 hour to obtain a white aqueous dispersion of emulsion particles. The average particle size of the emulsion particles, measured using a dynamic light scattering particle size distribution analyzer (product name "nanoSAQRA", manufactured by Otsuka Electronics Co., Ltd.), was 120 nm.

[0058] To the obtained white aqueous dispersion, 1.0 part of 28% aqueous ammonia was added to adjust the pH to 10. 26.7 parts of tetraethoxysilane (TEOS) were added while stirring at 25°C, and the mixture was stirred at 25°C for 4 hours. The precipitated white solid was separated by suction filtration. The obtained white solid was thoroughly washed with deionized water and dried at 100°C for 12 hours. Then, it was placed in a calcination furnace and calcined in air at 600°C for 4 hours. The heating rate was 3°C / min. The calcined white solid was crushed in a mortar to obtain 6.7 parts of hollow silica particles S-1. A transmission electron microscope image of the obtained hollow silica particles is shown in Figure 1.

[0059] (Examples 2-12, Comparative Examples 1-6) Hollow silica particles S-2 to S-12 and HS-1 to HS-6 were obtained in the same manner as in Example 1 described above, except for the formulations shown in Tables 3 to 5. The meanings of the abbreviations in Tables 3 to 5 are shown below. TMOS: Tetramethoxysilane • 3-SA: Silicic acid No. 3 (product name "Sodium Silicate Solution (No. 3)", manufactured by Kishida Chemical Co., Ltd.)

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[0063] (Example 13) 15.3 parts of a solution of AB block copolymer C-1 were placed in a reaction vessel, and 616.9 parts of deionized water were added and the mixture was stirred for a while. Then, 20 parts of dodecane were slowly added dropwise using a dropper while stirring. After that, the mixture was irradiated with ultrasound for 1 hour to obtain a white aqueous dispersion of emulsion particles. The average particle size of the obtained emulsion particles was 120 nm. 1.0 part of 28% aqueous ammonia was added to the obtained white aqueous dispersion to adjust the pH to 10. 26.7 parts of TEOS were added while stirring at 25°C, and the mixture was stirred at 25°C for 4 hours. The precipitated white solid was separated by suction filtration. The obtained white solid was thoroughly washed with deionized water to obtain a paste-like white solid. 50 parts of ethanol were added to the obtained paste-like white solid, and after thorough stirring, solid-liquid separation was performed by suction filtration to obtain a white paste. The process from ethanol addition to solid-liquid separation (washing) was repeated a total of 5 times to remove the core components. After drying at 100°C for 12 hours, the material was ground in a mortar to obtain 6.2 parts of hollow silica particles S-13.

[0064] (Examples 14-16) Hollow silica particles S-14 to S-16 were obtained in the same manner as in Example 13 described above, except that the formulation shown in Table 6 was used. The meaning of the abbreviations in Table 6 is shown below. IPA: Isopropanol • MEK: Methyl ethyl ketone

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[0066] <Rating> The obtained hollow silica particles were observed using a transmission electron microscope (TEM, model number "H-7650", manufactured by Hitachi High-Technologies Corporation), and TEM images were obtained. Fifty particles were randomly extracted from the obtained TEM images, and the particle diameter and silica shell thickness of the extracted particles were measured, and the average particle diameter (nm) and average silica shell thickness (nm) were calculated, respectively. Furthermore, the apparent volume of the particles and the volume of the voids were calculated from the calculated average particle diameter and average silica shell thickness, and the ratio of the volume of voids to the apparent volume of the particles (porosity (%)) was calculated. In addition, the hollow structure of the hollow particles was evaluated by observing the obtained TEM images according to the evaluation criteria shown below. The results are shown in Table 7. ◎: Hollow structure can be confirmed in all particles. ○: Hollow structures can be observed in most particles, but not in some particles. △: Hollow structures can be observed in some particles, but not in the majority of particles. ×: The presence of particles with a hollow structure cannot be confirmed.

[0067] TIFF2026144448000007.tif152170

[0068] Some of the particles produced in Comparative Example 3 had a hollow structure, but due to the low firing temperature, the decomposition and removal of the core components were insufficient, resulting in a brownish powder with burnt organic components. [Industrial applicability]

[0069] The hollow silica particles produced by the manufacturing method of the present invention are useful as materials for applications such as weight reduction, low dielectric constant, heat insulation, low refractive index, catalysts, and drug delivery systems (DDS), taking advantage of the unique functions of their hollow structure.

Claims

1. A first step involves dispersing an AB block copolymer containing polymer block A and polymer block B and a water-insoluble organic solvent (i) in water to form emulsion particles containing the AB block copolymer and the organic solvent (i), A second step involves adding a compound that serves as a silica source, forming a silica layer on the surface of the emulsion particles by a sol-gel reaction, and obtaining silica particles with the emulsion particles as the core. The third step involves washing the obtained silica particles with an organic solvent (ii) that dissolves the AB block copolymer and the organic solvent (i), or firing them at 500°C or higher to remove the AB block copolymer and the organic solvent (i) and obtain hollow silica particles. The AB block copolymer satisfies the following conditions (a) to (c): A method for producing hollow silica particles, wherein the mass ratio of the AB block copolymer to the organic solvent (i) used in the first step is 20:80 to 50:

50. (a) The polymer block A contains 80% by mass or more of constituent units derived from at least one methacrylate selected from the group consisting of alkyl methacrylate having an alkyl group having 6 to 18 carbon atoms, cycloalkyl methacrylate having a cycloalkyl group having 6 to 18 carbon atoms, and arylalkyl methacrylate having an arylalkyl group having 6 to 18 carbon atoms, and has a number average molecular weight of 5,000 to 40,000 and a molecular weight distribution (weight average molecular weight / number average molecular weight) of less than 1.

8. (b): The polymer block B contains 50% by mass or more of constituent units derived from methacrylate having a quaternary ammonium base. (c): The mass ratio of polymer block A to polymer block B is 50:50 to 80:

20.

2. The method for producing hollow silica particles according to claim 1, wherein the organic solvent (i) is a hydrocarbon having 6 to 18 carbon atoms that is liquid at 25°C.

3. A method for producing hollow silica particles according to claim 1 or 2, wherein the average particle diameter of the hollow silica particles is 10 to 1,000 nm.

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