Method for producing hollow silica particles

Abstract

The present invention provides a new method for manufacturing hollow silica particles whereby hollow silica particles can be produced by an inexpensive manufacturing process having a low environmental impact, and the particle diameter of the obtained hollow silica particles can be reduced to less than 200 nm. A method for producing hollow silica particles according to the present invention comprises: obtaining an emulsion by using two or more non-ionic surfactants having different cloud points to disperse temperature-responsive core / shell particles at a temperature which is greater than or equal to the cloud point of the non-ionic surfactant having the lowest cloud point among the non-ionic surfactants and is less than the cloud point of the non-ionic surfactant having the highest cloud point; adding a silica starting material while feeding the emulsion at a flow rate that produces a laminar flow of 0.3 mL / min or greater; and coating the silica onto the temperature-responsive core / shell particles, which serve as cores, to form a hollow silica particle precursor.

Description

Method for producing hollow silica particles

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

[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] Various methods for producing hollow silica particles have been proposed, including a method in which a silica coating layer is formed around a solid core particle and then the core particle is removed using an acid to obtain hollow silica particles, and a method in which an oil-in-water emulsion is prepared in which an oil phase is dispersed in an aqueous phase, and then a silica raw material is attached to the oil droplets to produce oil-core-silica shell particles, and then the oil droplet component is removed from these particles to obtain hollow silica particles.

[0004] For example, Patent Document 1 discloses a method for producing silica by simultaneously adding an aqueous solution of silicate and / or an acidic silicic acid solution and an aqueous solution of an alkali-soluble inorganic compound to an alkaline aqueous solution of pH 10 or more, or an alkaline aqueous solution of pH 10 or more in which seed particles are dispersed as needed, to produce silica in the form of SiO ２ and the molar ratio MOX / SiO when inorganic compounds other than silica are expressed as MOX. ２ (b) preparing a core particle dispersion liquid in which a silica content is in the range of 0.3 to 1.0; (b) adding a silica source to the core particle dispersion liquid to form a first silica coating layer on the core particles; and (c) adding an acid to the dispersion liquid to remove some or all of the elements that constitute the core particles.

[0005] Patent Document 2 also describes a method for producing hollow silica particles, which comprises 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 precursor.

[0006] Japanese Patent Application Publication No. 2001-233611 International Publication No. 2019 / 131658

[0007] The method described in Patent Document 1 involves complicated steps and limited production volume, resulting in high production costs. Furthermore, ultrafiltration is used to replace the solvent with ethanol, which results in the discharge of large amounts of ethanol during the production process, potentially placing a burden on the environment. The method described in Patent Document 2 allows hollow silica particles to be produced inexpensively, but discharges oil and organic solvents when calcining a hollow silica precursor to obtain hollow silica particles, potentially placing a burden on the environment. Furthermore, while this method is suitable for producing hollow silica particles of 200 nm or larger, it is difficult to obtain particles smaller than this size.

[0008] Therefore, an object of the present invention is to provide a new method for producing hollow silica particles, which is inexpensive and has a low environmental impact, and which can reduce the particle diameter of the obtained hollow silica particles to less than 200 nm.

[0009] As a result of extensive research, the present inventors discovered that two or more nonionic surfactants with different cloud points can be used to form temperature-responsive core-shell particles with at least one of these nonionic surfactants as a core, and that these can then be used as nuclei to form hollow silica particles, thereby completing the present invention.

[0010] That is, one aspect of the present invention relates to a method for producing hollow silica particles, comprising: using two or more nonionic surfactants having different cloud points, obtaining an emulsion in which temperature-responsive core-shell particles are dispersed at a temperature that is equal to or higher than the cloud point of the nonionic surfactant with the lowest cloud point among the nonionic surfactants and lower than the cloud point of the nonionic surfactant with the highest cloud point; and adding a silica raw material to the emulsion while feeding the emulsion at a flow rate of 0.3 mL / min or higher that results in a laminar flow, thereby coating the temperature-responsive core-shell particles as nuclei with silica to form hollow silica particle precursors.

[0011] According to the manufacturing method of the present invention, hollow silica particles can be produced inexpensively and easily. Furthermore, since temperature-responsive core-shell particles are used as the core, calcination is not required for hollowing, and the environmental impact is low because no alcohol or oil is emitted. Furthermore, by changing the type of nonionic surfactant used as the core, the particle size can be controlled to any desired value, allowing the particle size of the hollow silica particles to be reduced to less than 200 nm.

[0012] FIG. 1 is a transmission electron microscope (TEM) image of the hollow silica particles obtained in Example 1.

[0013] The present invention will be described below, but the present invention is not limited to the examples in the following description. In this specification, the term "to" indicating a range of values ​​means that the values ​​before and after it are included as the lower and upper limits. In this specification, "mass" is synonymous with "weight."

[0014] The method for producing hollow silica particles of the present invention (hereinafter also referred to as the present production method) includes: using two or more nonionic surfactants having different cloud points, obtaining an emulsion in which temperature-responsive core-shell particles are dispersed at a temperature that is equal to or higher than the cloud point of the nonionic surfactant with the lowest cloud point among the nonionic surfactants and lower than the cloud point of the nonionic surfactant with the highest cloud point; adding a silica raw material to the emulsion while feeding the emulsion at a flow rate of 0.3 mL / min or higher that results in a laminar flow; and coating the temperature-responsive core-shell particles with silica as nuclei to form hollow silica particle precursors.

[0015] According to this method, temperature-responsive core-shell particles made of a nonionic surfactant are coated with silica as cores, and then the nonionic surfactant is hydrophilized by adjusting the environmental temperature, thereby removing the nonionic surfactant from the pores of the shell layer formed by silica. Therefore, calcination is not required during hollowing, and hollow silica particles can be produced inexpensively and easily. Furthermore, since no alcohol or oil is emitted during hollowing, the environmental impact is low. Furthermore, since the particle diameter can be controlled to any desired value by changing the type of nonionic surfactant, hollow nanosilica particles with a particle diameter of less than 200 nm can be easily produced.

[0016] (Preparation of Temperature-Responsive Core-Shell Particles) In this production method, an emulsion is first formed in which temperature-responsive core-shell particles composed of two or more nonionic surfactants with different cloud points are dispersed in an aqueous phase. The emulsion is formed at a temperature (hereinafter also referred to as the emulsification temperature) that is equal to or higher than the cloud point of the nonionic surfactant with the lowest cloud point among the two or more nonionic surfactants and lower than the cloud point of the nonionic surfactant with the highest cloud point. The nonionic surfactant with a cloud point lower than the emulsification temperature becomes a hydrophobic molecule and forms droplets in the emulsion, while the nonionic surfactant with a cloud point higher than the emulsification temperature enhances the emulsion stability in the emulsion, thereby forming a so-called oil-in-water emulsion.

[0017] Examples of nonionic surfactants used in the present production method 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: monoglyceride stearate, monoglyceride oleate. 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.

[0018] Among the nonionic surfactants described above, polyoxyethylene-polyoxypropylene copolymer surfactants are preferably used from the viewpoints of particle size and stability during emulsion of two or more nonionic surfactants with different cloud points. Polyoxyethylene-polyoxypropylene copolymers are block copolymers in which a polyoxyethylene block (EO) and a polyoxypropylene block (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.

[0019] In the present production method, two or more types of nonionic surfactants with different cloud points are used. From the viewpoints of emulsion stability and particle size, it is preferable to use 2 to 5 types of nonionic surfactants, and more preferably 2 to 3 types.

[0020] In this production method, the two or more nonionic surfactants with different cloud points preferably include a nonionic surfactant with an HLB of less than 14 and a nonionic surfactant with an HLB of 14 or more. Using both a nonionic surfactant with an HLB of less than 14 and a nonionic surfactant with an HLB of 14 or more improves the stability of the emulsion particles. The HLB values ​​used are those calculated at 25°C. Among nonionic surfactants with an HLB of less than 14, it is more preferable to use a nonionic surfactant with an HLB of less than 12, even more preferable to use a nonionic surfactant with an HLB of less than 11, and particularly preferable to use a nonionic surfactant with an HLB of less than 10. Among nonionic surfactants with an HLB of 14 or more, it is more preferable to use a nonionic surfactant with an HLB of 18 or more, even more preferable to use a nonionic surfactant with an HLB of 19 or more, and particularly preferable to use a nonionic surfactant with an HLB of 20 or more.

[0021] From the viewpoints of ease of preparation during emulsion particle formation and prevention of solubilization, the combination of two or more nonionic surfactants with different cloud points is preferably one in which the difference in HLB value is 5 or more, more preferably 7 or more, and even more preferably 10 or more. Specifically, it is most preferable to use both a nonionic surfactant with an HLB of less than 10 and a nonionic surfactant with an HLB of 20 or more.

[0022] In the present production method, the difference in cloud point between the nonionic surfactant with the highest cloud point (hereinafter also referred to as the low-cloud-point-side high-cloud-point nonionic surfactant) among the group of nonionic surfactants having a cloud point lower than the emulsification temperature (hereinafter also referred to as the low-cloud-point-side nonionic surfactant) and the nonionic surfactant with the lowest cloud point (hereinafter also referred to as the high-cloud-point-side low-cloud-point nonionic surfactant) among the group of nonionic surfactants having a cloud point equal to or higher than the emulsification temperature (hereinafter also referred to as the high-cloud-point-side nonionic surfactant) is preferably 10° C. or more. When the difference in cloud point between the low-cloud-point-side high-cloud-point nonionic surfactant and the high-cloud-point-side low-cloud-point nonionic surfactant is 10° C. or more, emulsion particles can be stably obtained even when many types of nonionic surfactants are used. The difference in cloud point is more preferably 15°C or more, and even more preferably 20°C or more, and from the viewpoint of uniformity of the particle size distribution of the emulsion particles, is preferably 250°C or less, more preferably 230°C or less, and even more preferably 200°C or less.

[0023] When two types of surfactants are used, the amount of nonionic surfactant used is preferably such that the mass ratio of the nonionic surfactant with the lowest cloud point to the nonionic surfactant with the highest cloud point is in the range of 1:1 to 30:1, more preferably 2:1 to 20:1, and even more preferably 3:1 to 15:1. The particle size of the obtained hollow silica particles varies depending on the combination of nonionic surfactants, but if there is too much of the nonionic surfactant with the lowest cloud point, the particle size will become too large, so using them in the above range makes it easy to adjust the particle size to the desired size.

[0024] Similarly, when three or more types of nonionic surfactants are used, the group of nonionic surfactants having a cloud point lower than the emulsification temperature (low cloud point nonionic surfactants) and the group of nonionic surfactants having a cloud point equal to or higher than the emulsification temperature (high cloud point nonionic surfactants) are used so that the mass ratio of the former to the latter is preferably in the range of 1:1 to 30:1, more preferably 2:1 to 20:1, and even more preferably 3:1 to 15:1.

[0025] The aqueous phase of the emulsion mainly contains water as a solvent. To the aqueous phase, additives such as a water-soluble organic liquid and a water-soluble resin may be further added. The proportion of water in the aqueous phase is preferably 50 to 100% by mass, more preferably 90 to 100% by mass.

[0026] In this production method, an emulsion is formed by a temperature-transfer emulsification method. Specifically, two or more nonionic surfactants with different cloud points are added to an aqueous phase, and the mixture is heated to a temperature (emulsification temperature) above the cloud point of the nonionic surfactant with the lowest cloud point and below the cloud point of the nonionic surfactant with the highest cloud point, followed by stirring to emulsify. When heated to the emulsification temperature, the nonionic surfactant with a cloud point lower than the emulsification temperature changes from water-soluble to oil-soluble at a temperature near its cloud point, and the hydrophilic nonionic surfactant attaches to the surfactant, forming temperature-responsive core-shell particles.

[0027] For example, when ADEKA Corporation's "L-62" (EO-PO-EO block copolymer, cloud point: 32°C, HLB: 7) and ADEKA Corporation's "F-68" (EO-PO-EO block copolymer, cloud point: >100°C, HLB: 29) are used as nonionic surfactants, the emulsification temperature is preferably 34 to 100°C, more preferably 40 to 60°C.

[0028] In order to reduce the particle size of the resulting hollow silica particles and narrow the particle size distribution, it is preferable that the temperature-responsive core-shell particles are sufficiently dispersed and emulsified in the aqueous phase. For example, the mixture can be emulsified using a homogenizer at a speed of preferably 150 to 10,000 rpm, more preferably 1,000 to 6,000 rpm.

[0029] In the present production method, the temperature-responsive core-shell particles are preferably formed by emulsification in a batch or continuous manner, and particularly, the continuous emulsification is preferred from the viewpoint of reducing unreacted particles in the system and enabling the reaction to occur under uniform temperature conditions.

[0030] The resulting temperature-responsive core-shell particles preferably have a 50% cumulative average diameter D50 of 10 nm to 1 μm in volume equivalent. When the 50% cumulative average diameter D50 is 10 nm or more, the particle diameter of the resulting hollow silica particles can be reduced, and when it is 1 μm or less, dispersibility can be maintained. The 50% cumulative average diameter D50 is more preferably 12 nm or more, even more preferably 15 nm or more, more preferably 900 nm or less, and even more preferably 800 nm or less.

[0031] The particle size of the temperature-responsive core-shell particles in the emulsion can be measured by a dynamic light scattering meter.

[0032] (Preparation of Thermoresponsive Silica Composite Particles) In this production method, a silica raw material is added to an emulsion in which thermoresponsive core-shell particles are dispersed, and the thermoresponsive core-shell particles are coated with silica as cores to form hollow silica particle precursors. The emulsion is fed at a flow rate of 0.3 mL / min or more that results in a laminar flow.

[0033] Examples of silica raw materials include aqueous solutions of water-soluble silica, aqueous dispersions of solid silica, mixtures thereof, and at least one selected from the group consisting of alkali metal silicates, active silicic acid, and silicon alkoxides, or their aqueous solutions or dispersions.Of these, at least one selected from the group consisting of alkali metal silicates, active silicic acid, and silicon alkoxides, or their aqueous solutions or dispersions, are preferred due to their high availability.Examples of solid silica include silica sol obtained by hydrolyzing organosilicon compounds and commercially available silica sols.Examples of alkali metals in alkali metal silicates include lithium, sodium, potassium, rubidium, etc., and sodium is preferred due to its availability and economical reasons.That is, sodium silicate is preferred as the alkali metal silicate.Sodium silicate is preferably Na ２ O.nSiO ２ ・mH ２ The composition is represented by the formula: Na SiO ... ２ O / SiO ２ The molar ratio n is preferably 1.0 to 4.0, more preferably 2.0 to 3.5.

[0034] Activated silicic acid is obtained by replacing alkali metal with hydrogen by cation exchange treatment of alkali metal silicate, and an aqueous solution of this activated silicic acid exhibits weak acidity. For example, a hydrogen cation exchange resin is used for cation exchange. The alkali metal silicate and activated silicic acid are preferably dissolved or dispersed in water and then added to the emulsion. The concentrations of the alkali metal silicate and activated silicic acid aqueous solution are determined by the SiO ２ The concentration is preferably 3 to 30% by mass, more preferably 5 to 25% by mass.

[0035] As the silicon alkoxide, for example, tetraalkylsilanes such as tetramethoxysilane, tetraethoxysilane, and tetrapropoxysilane are 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.

[0036] As the silica raw material, the above-mentioned silica raw materials may be used alone or in combination of two or more. Among them, an aqueous solution of an alkali metal silicate, particularly an aqueous solution of sodium silicate, is preferably used as the silica raw material.

[0037] In this production method, the emulsion and silica raw material are brought into contact in a laminar flow region. A laminar flow is a flow in which a fluid moves regularly in the direction of the flow. Unlike a turbulent flow, which becomes disturbed and diffuses midway through the flow, a laminar flow flows in a nearly linear fashion. Because the temperature-responsive core-shell particles are soft particles produced by the soft template method, they may stick together or their shape may be deformed if they are in a turbulent flow region. However, by bringing the emulsion into contact with the silica raw material in a laminar flow region, a silica shell layer can be formed while maintaining the particle shape.

[0038] The flow rate of the emulsion in the laminar flow is 0.3 mL / min or more. When the flow rate of the emulsion is 0.3 mL / min or more, the emulsion can be reacted uniformly with the silica raw material. The flow rate of the emulsion is preferably 0.35 mL / min or more, more preferably 0.4 mL / min or more, and from the viewpoint of emulsion stability, is preferably 5.0 mL / min or less, more preferably 2.0 mL / min or less.

[0039] The silica raw material is also preferably added to the emulsion while being sent as a laminar flow. The flow rate when adding the silica raw material is preferably 0.2 to 2.0 mL / min. When the flow rate of the silica raw material is 0.2 mL / min or more, a silica layer can be formed evenly around the temperature-responsive core-shell particles, and when the flow rate is 2.0 mL / min or less, the formation of self-condensed silica particles can be suppressed. The flow rate of the silica raw material is more preferably 0.3 mL / min or more, and more preferably 1.0 mL / min or less.

[0040] The reaction between the emulsion and the silica raw material can be carried out using a flow-type reactor, and examples of a method for sending the liquid in a laminar flow include using a microreactor capable of forming a laminar flow or a continuous liquid sending device. Specifically, a Y-shaped microreactor is preferably used as a microreactor capable of forming a laminar flow.

[0041] In this production method, the silica raw material is preferably added to the emulsion containing the temperature-responsive core-shell particles under acidic conditions. By treating under acidic conditions, the silica raw material is hydrolyzed, and the surface charges of the silica particles in the silica raw material and the temperature-responsive core-shell particles become opposite, resulting in the silica particles being adsorbed around the temperature-responsive core-shell particles and coating the temperature-responsive core-shell particles with silica. If the emulsion is not acidic, it is preferable to make it acidic by adding an acid to the emulsion. One example of adding the silica raw material is to add an acid to the emulsion and then add an aqueous sodium silicate solution. Since the emulsion is first acidified and then the aqueous sodium silicate solution, which is an alkaline component, is added, the entire emulsion can be kept acidic during the addition of the silica raw material. When adding the silica raw material, it is preferable to adjust the pH of the emulsion to 2.0 or less after adding the acid, and then adjust the pH to 3.0 or less after adding the aqueous sodium silicate solution. The pH during the addition of the silica raw material to the emulsion is preferably 3.0 or less, more preferably 2.4 or less. This allows the thickness of the coating to be more uniform when the coating is formed from the silica raw material on the temperature-responsive core-shell particles in the emulsion, and makes the silica shell layer of the resulting hollow silica particles denser.

[0042] 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.

[0043] The amount of silica raw material added is 100 parts by mass of the temperature-responsive core-shell particles contained in the emulsion. ２ is preferably 1 to 50 parts by mass, more preferably 3 to 30 parts by mass.

[0044] When adding the silica raw material, the laminar flow state is preferably maintained for 1 second or more, more preferably 5 seconds or more, and even more preferably 10 seconds or more, under the condition that the pH of the emulsion after adding the silica raw material is 3.0 or less.

[0045] Next, it is preferable to maintain the pH of the emulsion to which the silica raw material has been added at 5 or higher. This allows the silica raw material to be immobilized on the surface of the temperature-responsive core-shell particles. For example, a method can be used in which the pH of the emulsion to which the silica raw material has been added is adjusted to 5 or higher by adding a base to the emulsion to which the 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, amines, etc. Alternatively, a method can be used in which anions such as halogen ions are exchanged for hydroxide ions by anion exchange treatment.

[0046] 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 if a large amount of base is added all at once, the pH of the emulsion may become non-uniform, which may result in non-uniform thickness of the coating. It is preferable to hold the emulsion at a pH of 5 or higher, preferably while stirring. This holding time may be 10 minutes or more, preferably 1 hour or more, or even 4 hours or more. During the holding time, the pH of the emulsion is preferably 7 or less.

[0047] The silica coating may be performed once or multiple times, and by repeating the silica coating, a silica shell layer having a desired thickness can be formed.

[0048] In this production method, it is preferable to install a receiving liquid downstream of the laminar flow region and use the receiving liquid to receive the hollow silica particle precursors formed by coating the temperature-responsive core-shell particles with silica. If the silica raw material is added to the emulsion in which the temperature-responsive core-shell particles are dispersed and the reaction liquid is left as is, aggregation and coalescence due to unreacted silica raw material may occur, so by contacting the receiving liquid, particle growth can be suppressed.

[0049] Examples of the receiving liquid include water, ethanol, propanol, isopropyl alcohol (IPA), N-methyl-2-pyrrolidone (NMP), methyl ethyl ketone (MEK), and mixtures thereof. Among these, from the viewpoint of inhibiting particle growth and controlling aggregation, a mixture of water and ethanol, IPA, NMP, or MEK is preferred, a mixture of ethanol and an aqueous solution having a pH of 9 to 14, a mixture of IPA and an aqueous solution having a pH of 9 to 14, or a mixture of NMP and an aqueous solution having a pH of 9 to 14 is more preferred, and a mixture of ethanol and an aqueous solution having a pH of 9 to 14 in a ratio of 1:1 to 20:1 (mass ratio), or a mixture of IPA and an aqueous solution having a pH of 9 to 14 in a ratio of 1:1 to 20:1 (mass ratio) is even more preferred.

[0050] (Formation of Hollow Thermoresponsive Silica Composite Particles) In this production method, hollow silica particle precursors are preferably formed by maintaining them at a temperature lower than the cloud point of the nonionic surfactant with the lowest cloud point, which hydrophilizes the nonionic surfactant and allows it to pass through the pores in the silica shell layer and be removed from the interior of the particles, thereby obtaining hollow silica particles.

[0051] The hollowing process is preferably carried out in an aqueous solution having a pH of 7 or higher. By adjusting the pH of the aqueous solution to 7 or higher, particle growth can be suppressed. A method for adjusting the pH of the aqueous solution to 7 or higher includes adding a base, and examples of the base include the above-mentioned alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, alkaline earth metal hydroxides such as magnesium hydroxide and calcium hydroxide, ammonia, amines, etc. The pH of the aqueous solution is preferably higher than 7, and more preferably 9 or higher.

[0052] The hollowing is preferably carried out in the presence of a solvent. By carrying out the hollowing in the presence of a solvent, unreacted surfactant can be washed away. Examples of the solvent include lower alcohols such as ethanol, propanol, and IPA; NMP; MEK; and the like, and one or more of these can be used. Among these, ethanol, IPA, and NMP are more preferably used. When an infusion solution containing alcohol is used as the receiving solution, the hollowing can be carried out in the receiving solution whose pH has been adjusted to alkaline.

[0053] The solvent is contained in the aqueous solution in an amount of preferably 0.1 to 80% by mass, more preferably 0.5 to 50% by mass, and even more preferably 1.0 to 40% by mass.

[0054] After hollowing out, the hollow silica particles are separated by filtration or the like.

[0055] The hollow silica particles are preferably dried to remove moisture, and the drying conditions are 70 to 120° C. for 1 to 24 hours until the hollow silica particles are completely dehydrated.

[0056] (Densification of Hollow Silica Particles) The hollow silica particles may be calcined to tighten the shell and densify the shell layer. The calcination conditions are preferably 300 to 1000°C, more preferably 400 to 950°C, and the calcination is preferably carried out for 0.5 to 10 hours, more preferably 1 to 6 hours.

[0057] (Surface treatment of hollow silica particles) The surface of hollow silica particles may be treated with a silane coupling agent. By treating the surface of hollow silica particles with a silane coupling agent, the amount of remaining silanol groups on the surface is reduced, and the surface is made hydrophobic, which, for example, reduces moisture adsorption to improve dielectric loss, and improves affinity with resin when forming a resin composition, thereby improving dispersibility in resin.

[0058] The conditions for the surface treatment are not particularly limited, and general surface treatment conditions may be used, and a wet treatment method or a dry treatment method may be used. From the viewpoint of uniform treatment, the wet treatment method is preferred.

[0059] Examples of the silane coupling agent used in the surface treatment include aminosilane coupling agents, epoxysilane coupling agents, mercaptosilane coupling agents, silane coupling agents, organosilazane compounds, etc. These may be used alone or in combination of two or more.

[0060] Specific examples of the surface treatment agent include aminosilane-based coupling agents such as aminopropyl methoxysilane, aminopropyl triethoxysilane, ureidopropyl triethoxysilane, N-phenylaminopropyl trimethoxysilane, and N-2(aminoethyl)aminopropyl trimethoxysilane; epoxysilane-based coupling agents such as glycidoxypropyl trimethoxysilane, glycidoxypropyl triethoxysilane, glycidoxypropyl methyldiethoxysilane, glycidyl butyl trimethoxysilane, and (3,4-epoxycyclohexyl)ethyl trimethoxysilane; mercaptosilane-based coupling agents such as mercaptopropyl trimethoxysilane and mercaptopropyl triethoxysilane; silane-based coupling agents such as methyltrimethoxysilane, vinyltrimethoxysilane, octadecyltrimethoxysilane, phenyltrimethoxysilane, metachloroxypropyl trimethoxysilane, imidazole silane, and triazine silane; and CF ３ (CF ２ ) ７ CH ２ CH ２ Si(OCH ３ ) ３ , C.F. ３ (CF ２ ) ７ CH ２ CH ２ SiCl ３ , C.F. ３ (CF ２ ) ７ CH ２ CH ２ Si(CH ３ ) (OCH ３ ) ２ , C.F. ３ (CF ２ ) ７ CH ２ CH ２ Si(CH ３ ) C1 ２ , C.F. ３ (CF ２ ) ５ CH ２ CH ２ SiCl ３ , C.F. ３ (CF ２ ) ５CH ２ CH ２ Si(OCH ３ ) ３ , C.F. ３ CH ２ CH ２ SiCl ３ , C.F. ３ CH ２ CH ２ Si(OCH ３ ) ３ , C ８ F １７ SO ２ N (C ３ H ７ ) CH ２ CH ２ CH ２ Si(OCH ３ ) ３ , C ７ F １５ CONHCH ２ CH ２ CH ２ Si(OCH ３ ) ３ , C ８ F １７ CO ２ CH ２ CH ２ CH ２ Si(OCH ３ ) ３ , C ８ F １７ -O-CF(CF ３ )CF ２ -O-C ３ H ６ SiCl ３ , C ３ F ７ -O-(CF(CF ３ )CF ２ -O) ２ -CF (CF ３ )CONH-(CH ２ ) ３ Si(OCH ３ ) ３ fluorine-containing silane coupling agents such as hexamethyldisilazane, hexaphenyldisilazane, trisilazane, cyclotrisilazane, 1,1,3,3,5,5-hexamethylcyclotrisilazane, and other organosilazane compounds.

[0061] Among these, it is preferable to use a silane coupling agent containing a methacrylic group or a vinyl group.

[0062] The amount of the silane coupling agent used for treatment is preferably 1 part by mass or more, more preferably 2 parts by mass or more, and is preferably 500 parts by mass or less, more preferably 400 parts by mass or less, and even more preferably 300 parts by mass or less, per 100 parts by mass of the hollow silica particles.

[0063] Examples of methods for treating with a silane coupling agent include a dry method in which a silane coupling agent is sprayed onto hollow fired silica particles, and a wet method in which hollow fired silica particles are dispersed in a solvent and then a silane coupling agent is added to cause a reaction.

[0064] The hollow silica particles obtained by this production method may be aggregated during the drying and firing processes, and may be crushed to reduce the aggregate size to an easy-to-handle size. Examples of the crushing method include a method using a mortar, a bead mill, a dry or wet ball mill, a shaking sieve, or a crusher such as a pin mill, a cutter mill, a hammer mill, a knife mill, a roller mill, or a jet mill.

[0065] (Physical Properties of Hollow Silica Particles) The fact that the obtained hollow silica particles have a space inside the shell layer can be confirmed by observation with a transmission electron microscope (TEM). Spherical particles with a space inside that can be confirmed by TEM observation are defined as "primary particles." Because the primary particles are partially bonded to each other during the baking and drying processes, the obtained hollow silica may become an aggregate of secondary particles formed by aggregation of primary particles, but it is preferable to crush them to form primary particles as described above. The size of the primary particles is determined by directly observing their particle diameters using TEM observation. Specifically, a portion where there are no particles with extremely large or small particle diameters and where the primary particle diameter distribution within the observation range is thought to be similar to the primary particle diameter distribution of the obtained hollow silica particles as a whole is observed by magnifying it, measuring the size of each primary particle, and tallying the primary particle size distribution obtained, which is then estimated as the overall primary particle size distribution. The average size of the primary particles is preferably 10 nm to 1 μm, more preferably 11 to 900 nm, and even more preferably 12 to 800 nm.

[0066] The hollow silica particles have a BET specific surface area of ​​1000 m ２ / g or less, and ２ / g or less is more preferable, and 800m ２ / g or less is more preferable. ２ / g or less, the BET specific surface area is easily dispersed when composited with a resin. In addition, the lower limit of the BET specific surface area is not particularly limited because it affects the reactivity with a silane coupling agent or a surface treatment agent and the physical properties of the compound.

[0067] Here, the BET specific surface area is measured by a multipoint BET method using nitrogen gas with a specific surface area measuring device (for example, a specific surface area / pore distribution measuring device "3-Flex" manufactured by Shimadzu Corporation) in a state where moisture is completely removed from the hollow silica particles.

[0068] The shell thickness of the hollow silica particles is preferably 1 to 50 nm. When the shell thickness is 1 nm or more, the hollow particles are less likely to break, and when it is 50 nm or less, a low refractive index can be achieved. The shell thickness is more preferably 5 nm or more, even more preferably 10 nm or more, and more preferably 40 nm or less, even more preferably 30 nm or less.

[0069] Furthermore, the shell thickness of the hollow silica particles is preferably 0.01 to 0.3 relative to the primary particle diameter. When the shell thickness is 0.01 or more relative to the primary particle diameter, a decrease in the strength of the hollow silica particles can be prevented. When this ratio is 0.3 or less, the volume of the internal space can be secured, allowing the properties of the hollow shape to be exhibited. The shell thickness is more preferably 0.02 or more relative to the primary particle diameter, even more preferably 0.03 or more, and more preferably 0.2 or less, even more preferably 0.1 or less.

[0070] 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.

[0071] Since the hollow silica particles are formed using the temperature-responsive core-shell particles in the emulsion as nuclei, the hollow silica particles have a spherical shape, preferably a true sphere. The sphericity of the hollow silica particles is preferably 0.75 to 1.0. A low sphericity may make the hollow silica particles more susceptible to breakage. The sphericity can be expressed as the average value obtained by measuring the maximum diameter (DL) and the minimum diameter (DS) perpendicular to the maximum diameter (DL) for 100 particles in a photographic projection obtained by photographing with a scanning electron microscope (SEM), and calculating the ratio (DS / DL) of the minimum diameter (DS) to the maximum diameter (DL). From the viewpoint of dispersibility, the sphericity is more preferably 0.80 or more, even more preferably 0.82 or more, even more preferably 0.83 or more, particularly preferably 0.85 or more, even more preferably 0.87 or more, and most preferably 0.90 or more.

[0072] In the present invention, alkali metal components may be contained in the shell layer of the obtained hollow silica particles. These alkali metal components are 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 Na components in the shell of the obtained hollow silica particles is 500 mass ppm or more, and in some cases 1000 mass ppm or more. On the other hand, the mass concentration of Na components in the shell of general hollow silica particles made using tetraethyl orthosilicate as the silica raw material is 100 mass ppm or less. This 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 it by ICP optical emission spectrometry. Furthermore, when an alkali metal silicate is used as the silica raw material, the shell layer of the obtained hollow silica particles contains less carbon (C) components derived from the raw materials than when silicon alkoxide is used as the silica raw material.

[0073] The hollow silica particles obtained by this production method have a particle density (hereinafter also referred to as He density) of 1.5 g / cm as determined by density measurement using a dry pycnometer using helium gas. ３ Since helium gas permeates minute voids, it is possible to obtain a density corresponding to the true density of the silica portion of silica particles having internal voids. ３ When the density of He is 2.30 g / cm or more, the particle strength can be ensured. ３ To obtain a silica-based material exceeding this, firing at a fairly high temperature is required, and the particles are prone to breakage. Therefore, the He density is 2.30 g / cm ３ Preferably, it is 2.25 g / cm or less. ３ More preferably, 2.20 g / cm or less ３ The following is even more preferred:

[0074] The hollow silica particles obtained by this production method preferably have a refractive index of 1.35 or less. Hollow silica particles with a refractive index of 1.35 or less can be used as a low-refractive index filler for anti-reflection coatings, etc. Such a refractive index is more preferably 1.33 or less, and even more preferably 1.30 or less.

[0075] The hollow silica particles obtained by this production method can be used as various fillers, and are particularly suitable as fillers for resin compositions used in the production of electronic substrates used in electronic devices such as personal computers, notebook computers, and digital cameras, and communication devices such as smartphones and game consoles, as well as automobile panels, etc. In particular, taking advantage of their low refractive index, they are suitable as low-refractive index materials for anti-reflection coatings used in electronic devices, automobiles, camera lenses, etc.

[0076] As described above, the present specification discloses the following: (1) A method for producing hollow silica particles, comprising: using two or more nonionic surfactants with different cloud points to obtain an emulsion in which temperature-responsive core-shell particles are dispersed at a temperature equal to or higher than the cloud point of the nonionic surfactant with the lowest cloud point and lower than the cloud point of the nonionic surfactant with the highest cloud point; and adding a silica raw material to the emulsion while feeding the emulsion at a flow rate of 0.3 mL / min or higher that results in a laminar flow, thereby coating the temperature-responsive core-shell particles with silica as nuclei to form hollow silica particle precursors. (2) The method for producing hollow silica particles according to (1), wherein the two or more nonionic surfactants with different cloud points include a nonionic surfactant with an HLB of less than 14 and a nonionic surfactant with an HLB of 14 or higher. (3) The method for producing hollow silica particles according to (1) or (2), wherein the temperature-responsive core-shell particles have a 50% cumulative average diameter D50, calculated on a volumetric basis, of 10 nm to 1 μm. (4) A method for producing hollow silica particles according to any one of (1) to (3), wherein the temperature-responsive core-shell particles are formed by emulsifying in a batch or continuous manner. (5) A method for producing hollow silica particles according to any one of (1) to (4), wherein the temperature-responsive core-shell particles are coated with silica under acidic conditions. (6) A method for producing hollow silica particles according to any one of (1) to (5), wherein the temperature-responsive core-shell particles are coated with silica using a microreactor or a continuous liquid delivery device. (7) A method for producing hollow silica particles according to any one of (1) to (6), further comprising hollowing the hollow silica particle precursor by maintaining the hollow silica particle precursor at a temperature below the cloud point of the nonionic surfactant with the lowest cloud point. (8) A method for producing hollow silica particles according to (7), wherein hollowing is carried out in an aqueous solution with a pH of 7 or higher. (9) A method for producing hollow silica particles according to (7) or (8), wherein hollowing is carried out in the presence of a solvent. (10) The method for producing hollow silica particles according to any one of (1) to (9), further comprising treating the surfaces of the hollow silica particles with a silane coupling agent containing a methacrylic group or a vinyl group. (11) The method for producing hollow silica particles according to any one of (1) to (9), further comprising treating the surfaces of the hollow silica particles with a silane coupling agent containing a methacrylic group or a vinyl group. (12) The method for producing hollow silica particles according to any one of (1) to (9), further comprising treating the surfaces of the hollow silica particles with a silane coupling agent containing a methacrylic group or a vinyl group. (13) The method for producing hollow silica particles according to any one of (1) to (9), further comprising treating the surfaces of the hollow silica particles with a silane coupling agent containing a methacrylic group or a vinyl group. (14) The method for producing hollow silica particles according to any one of (1) to (9), further comprising treating the surfaces of the hollow silica particles with a silane coupling agent containing a methacrylic group or a vinyl group. (15) The method for producing hollow silica particles according to any one of (1) to (9), further comprising treating the surfaces of the hollow silica particles with a silane coupling agent containing a methacrylic group or a vinyl group. (16) The method for producing hollow silica particles according to any one of (1) to (9), further comprising treating the surfaces of the hollow silica particles with a silane coupling agent containing a methacrylic group or a vinyl group.３ (12) The method for producing hollow silica particles according to any one of (1) to (11) above, wherein the refractive index of the obtained hollow silica particles is 1.35 or less. (13) The method for producing hollow silica particles according to any one of (1) to (12) above, wherein the shell thickness of the obtained hollow silica particles is 1 to 50 nm.

[0077] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the following description, the same components are used. Examples 1 to 5, 12 to 16, 23 to 26, 29 to 31, and 34 to 36 are working examples, and Examples 6 to 11, 17 to 22, 27 to 28, and 32 to 33 are comparative examples.

[0078] <Method for measuring specific surface area> The particles were dried under reduced pressure at 230°C to completely remove moisture and used as a sample. The specific surface area of ​​this sample was determined by the multipoint BET method using nitrogen gas with a specific surface area and pore distribution measuring device "3-Flex" manufactured by Shimadzu Corporation.

[0079] <Method for measuring density> The density was measured using a dry pycnometer (AccuPycII 1340 manufactured by Micromeritics). The measurement conditions were as follows: Sample cell: 10 cm ３ Cell Sample weight: 1.0 g Measurement gas: Helium Number of purges: 10 times Purge treatment filling pressure: 135 kPag Number of cycles: 10 times Cycle filling pressure: 135 kPag Rate to complete pressure equilibration: 0.05 kPag / min

[0080] <Method for measuring refractive index> (1) The particles were dried at 120°C to obtain a sample powder. (2) A few drops of a standard refractive index liquid with a known refractive index were dropped onto a glass plate, and the above sample powder (0.1 g) scraped off with a spatula was mixed with this. (3) The above operation (2) was performed with various standard refractive index liquids, and the refractive index of the standard refractive index liquid when the mixed liquid became transparent was taken as the refractive index of the particles.

[0081] <Evaluation of hollow particles> Particles were observed using a transmission electron microscope (TEM) to obtain TEM images. Observation was performed using a Hitachi High-Technologies Corporation "HT7700" under the following conditions: acceleration voltage: 100 kV, imaging mode: HR (high resolution), and magnification: 30,000 times. When the ratio of hollow particles to all particles in the TEM image was 80% or more, the evaluation was "A" (hollow particles formed), when it was less than 80%, the evaluation was "B" (solid particles formed), and when no particles were formed, the evaluation was "C".

[0082] <Method for Measuring Shell Thickness> Hollow silica particles were observed using a transmission electron microscope (TEM) to obtain a TEM image. Observation was performed using a Hitachi High-Technologies Corporation "HT7700" under the following conditions: acceleration voltage: 100 kV, imaging mode: HR (high resolution), and magnification: 30,000x. The particle diameter and diameter of the particle interior (space portion) of 30 randomly selected particles were measured, and the shell thickness was calculated using the following formula to obtain an average value. Shell thickness (nm) = (particle diameter (nm) - particle interior diameter (nm)) ÷ 2

[0083] Example 1 Preparation of Emulsion 7.8 g of EO-PO-EO block copolymer A (ADEKA Corporation's "ADEKA (registered trademark) Pluronic L-62", HLB: 7, cloud point: 32°C) and 0.9 g of EO-PO-EO block copolymer B (ADEKA Corporation's "ADEKA (registered trademark) Pluronic F-68", HLB: 29, cloud point: >100°C) were added to 171.3 g of pure water, and the mixture was stirred at 5,000 rpm at 40°C using an IKA Corporation homogenizer for 10 minutes or more until the entire liquid became homogenous, thereby preparing an emulsion in which temperature-responsive core-shell particles were dispersed.

[0084] "Shell formation" 2.5 g of 1 M sulfuric acid (special grade reagent, manufactured by Wako Pure Chemical Industries, Ltd.) was added to 90 g of the obtained emulsion to adjust the pH to 2.0, and the mixture was kept warm in a thermostatic chamber at 40°C. Next, in a separate container, No. 3 sodium silicate aqueous solution (SiO ２ Concentration 28.9% by mass, Na ２ O concentration 9.4 mass % aqueous solution (SiO ２ Concentration 0.14% by mass, Na ２A solution containing 0.05% by mass of ethanol was prepared and placed in a 40°C thermostatic chamber for incubation. Subsequently, a PTFE (polytetrafluoroethylene) tube (inner diameter 0.5 mm, outer diameter 1 / 16 inch) was placed in each of the prepared solution containers and connected to a Smoothflow Pump Q manufactured by TACMINA Corporation. This was then connected to a Y-shaped microreactor manufactured by MAC Engineering (stainless steel (SUS), Y-shaped impact angle 44°, flow path width 0.5 mm, hole diameter 1 mm). The outlet side of the PTFE tube was connected to a container containing a solution (receiving solution) consisting of 106.7 g of ethanol and 13.1 g of a pH 12 aqueous solution. The emulsion and a diluted No. 3 sodium silicate aqueous solution were pumped at 0.4 mL / min for 3 hours, and the reaction solution was dripped into the receiving solution to obtain hollow silica particle precursors in which temperature-responsive core-shell particles were coated with silica.

[0085] "Filtration and washing" Since some of the hollow silica particle precursors were aggregated, the hollow silica particle precursors were suction filtered and washed at room temperature with a 1:1 mixed solution of water and ethanol (mass ratio, pH 7) using a hydrophilic PTFE membrane filter (pore size 0.45 μm) to obtain 0.4 g of hollow silica particles.

[0086] "Evaluation" The BET specific surface area of ​​the hollow silica particles obtained in Example 1 was 34.9 m ２ / g. A transmission electron microscope (TEM) image of the hollow silica particles obtained in Example 1 is shown in Figure 1. 80% or more of the total number of particles were hollow particles. Furthermore, 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 they were approximately 20 nm. The results are shown in Table 1.

[0087] (Examples 2 to 11) Silica particles were prepared in the same manner as in Example 1. However, in Examples 2 to 11, the liquid delivery rates of the temperature-responsive emulsion and the diluted No. 3 sodium silicate aqueous solution were changed as shown in Table 1. The results are shown in Table 1.

[0088]

[0089] As shown in Table 1, TEM observation showed that hollow particles were obtained in all of Examples 1 to 5, but solid particles were formed in Examples 6 to 11.

[0090] (Examples 12 to 22) Silica particles were prepared in the same manner as in Example 1. However, in Examples 12 to 22, an aqueous solution of sodium silicate No. 3 (SiO ２ Concentration 28.9% by mass, Na ２ The dilution concentration of the temperature-responsive emulsion was changed to a 0.1% by mass aqueous solution, and the liquid delivery rates of the temperature-responsive emulsion and the diluted No. 3 sodium silicate aqueous solution were changed as shown in Table 2. The results are shown in Table 2.

[0091]

[0092] As shown in Table 2, Examples 12 to 16 all yielded hollow particles, while Examples 17 to 22 formed solid particles.

[0093] (Examples 23 to 30) Silica particles were prepared in the same manner as in Example 1. However, in Examples 23 to 28, the type of low HLB EO-PO-EO block copolymer A used was changed as shown in Table 4. In Examples 29 and 30, the type of high HLB EO-PO-EO block copolymer B used was changed as shown in Table 4. The physical properties of the EO-PO-EO block copolymer are shown in Table 3. The results are shown in Table 4.

[0094]

[0095]

[0096] Hollow silica particles were obtained in all of Examples 23 to 26, 29, and 30. In contrast, no particles were formed in Examples 27 and 28, which were reacted at temperatures lower than the cloud points of the nonionic surfactants used.

[0097] (Examples 31 to 33) Silica particles were produced in the same manner as in Example 1. However, in Examples 31 to 33, the type and material of the continuous reaction reactor used in the shell formation step were changed as shown in Table 5. The reactor used in Example 31 was a Y-shaped microreactor that formed a laminar flow, the reactor used in Example 32 was a Helix-type reactor that formed a turbulent flow, and the reactor used in Example 33 was a static mixer that formed a turbulent flow. The results are shown in Table 5.

[0098]

[0099] In Example 31, where the liquid was delivered in a laminar flow, hollow silica particles were obtained, whereas in Examples 32 and 33, where the liquid was delivered in a turbulent flow, solid particles were formed.

[0100] (Examples 34 to 36) Silica particles were produced in the same manner as in Example 1. However, in Examples 34 to 36, the type and composition of the receiving liquid used in the shell formation step were changed as shown in Table 6. The results are shown in Table 6.

[0101]

[0102] Hollow silica particles were also obtained in Examples 34 to 36 in which the type of receiving liquid was changed.

[0103] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on a Japanese patent application (Patent Application No. 2023-124792) filed on July 31, 2023, the contents of which are incorporated herein by reference.

Claims

1. A method for producing hollow silica particles, comprising: using two or more nonionic surfactants having different cloud points, obtaining an emulsion in which temperature-responsive core-shell particles are dispersed at a temperature that is equal to or higher than the cloud point of the nonionic surfactant with the lowest cloud point among the nonionic surfactants and lower than the cloud point of the nonionic surfactant with the highest cloud point; adding a silica raw material while feeding the emulsion at a flow rate of 0.3 mL / min or more that results in a laminar flow, and coating the temperature-responsive core-shell particles with silica as nuclei to form hollow silica particle precursors.

2. The method for producing hollow silica particles according to claim 1, wherein the two or more nonionic surfactants having different cloud points include a nonionic surfactant having an HLB of less than 14 and a nonionic surfactant having an HLB of 14 or more.

3. The method for producing hollow silica particles according to claim 1 or 2, wherein the temperature-responsive core-shell particles have a 50% cumulative average diameter D50 of 10 nm to 1 μm in terms of volume.

4. The method for producing hollow silica particles according to claim 1 or 2, wherein the temperature-responsive core-shell particles are formed by emulsification in a batch or continuous manner.

5. The method for producing hollow silica particles described in claim 1 or 2, wherein the temperature-responsive core-shell particles are coated with silica under acidic conditions.

6. The method for producing hollow silica particles according to claim 1 or 2, wherein the temperature-responsive core-shell particles are coated with the silica using a microreactor or a continuous liquid delivery device.

7. A method for producing hollow silica particles according to claim 1 or 2, further comprising making the hollow silica particle precursor hollow by maintaining the precursor at a temperature lower than the cloud point of the nonionic surfactant having the lowest cloud point.

8. The method for producing hollow silica particles according to claim 7, wherein hollowing is carried out in an aqueous solution having a pH of 7 or higher.

9. The method for producing hollow silica particles according to claim 7, wherein the hollowing is carried out in the presence of a solvent.

10. The method for producing hollow silica particles according to claim 1 or 2, further comprising treating the surfaces of the hollow silica particles with a silane coupling agent containing a methacryl group or a vinyl group.

11. The density of the obtained hollow silica particles is 1.5 g / cm3 as determined by density measurement using a dry pycnometer with helium gas. ３ The method for producing hollow silica particles according to claim 1 or 2, wherein 12. The method for producing hollow silica particles according to claim 1 or 2, wherein the refractive index of the resulting hollow silica particles is 1.35 or less.

13. The method for producing hollow silica particles according to claim 1 or 2, wherein the shell thickness of the obtained hollow silica particles is 1 to 50 nm.