Method for producing hollow resin particles

Abstract

Provided is a method for producing hollow resin particles which each comprise a shell that contains a resin and a hollow part that is surrounded by the shell, the method including: a mixed liquid preparation step for preparing a mixed liquid that contains a polymerizable monomer, a hydrophobic organic solvent, a polymerization initiator, and an aqueous medium; a suspension step for suspending the mixed liquid so as to prepare a suspension in which droplets of a polymerizable monomer composition that contains the polymerizable monomer, the hydrophobic organic solvent, and the polymerization initiator are dispersed in the aqueous medium; a polymerization step for subjecting the suspension to a polymerization reaction so as to prepare a precursor composition which includes precursor particles that each have a hollow part and contain the hydrophobic organic solvent in the hollow part; a flotation separation step for floating and separating the precursor particles in the precursor composition; and a solvent removal step for removing the hydrophobic organic solvent, which is contained in the precursor particles, in a state in which the precursor particles are floated and separated.

Description

Method for producing hollow resin particles

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

[0002] Hollow particles, such as hollow resin particles produced by polymerizing polymerizable monomers, are particles that have a cavity inside them, and compared to solid particles whose interiors are substantially filled with resin or the like, they scatter light well and have low light transmittance. Therefore, they are widely used as organic pigments with excellent optical properties such as opacity and whiteness, or as hiding agents in water-based paints, paper coating compositions, and the like, and are also used as additives (additives added to resins for molding) for molded articles such as light reflectors, heat insulating materials, and sound insulating materials.

[0003] As a technique relating to such hollow resin particles, for example, Patent Document 1 describes a method for producing hollow resin particles having a shell containing a resin and a hollow portion surrounded by the shell, the method comprising a mixed solution preparation step, a suspension step, a polymerization step, and a flotation, separation and washing step in which precursor particles in a precursor composition obtained in the polymerization step are floated and separated while metal components in the precursor composition are removed, thereby performing flotation, separation and washing, in which the precursor particles in the precursor composition have a floating velocity V at 25°C of 0.1 m / h or more as calculated by a specific formula (1).

[0004] Japanese Patent Application Laid-Open No. 2023-067861

[0005] According to the manufacturing method described in Patent Document 1, hollow resin particles with a small amount of residual metal can be manufactured with high production efficiency. However, as a result of intensive research, the present inventors have found that, depending on the structure of the hollow resin particles, the composition of the shell, and the conditions for removing the hydrophobic organic solvent, the manufacturing method described in Patent Document 1 may cause breakage of the hollow resin particles.

[0006] The present inventors have therefore investigated conditions for removing the hydrophobic organic solvent and found that damage to hollow resin particles can be prevented by removing the hydrophobic organic solvent encapsulated in precursor particles in the presence of an aqueous medium. However, the present inventors have found that methods for removing the hydrophobic organic solvent in the presence of an aqueous medium tend to cause excessive foaming during removal of the hydrophobic organic solvent, which makes it necessary to limit the amount of hollow resin particles supplied to the solvent removal equipment or to limit the conditions for removing the hydrophobic organic solvent to milder conditions, resulting in poor productivity of hollow resin particles.

[0007] The present invention has been made in consideration of the above-described circumstances, and its object is to provide a method for producing hollow resin particles that can suppress foaming during removal of a hydrophobic organic solvent and that can produce hollow resin particles with high productivity.

[0008] The present inventors have conducted research to achieve the above-mentioned object and have found that the above-mentioned object can be achieved by a production method including a flotation separation step of floating and separating precursor particles in a precursor composition, and a solvent removal step of removing the hydrophobic organic solvent contained in the precursor particles while the precursor particles are in a floated and separated state, thereby completing the present invention.

[0009] That is, the present invention provides the following manufacturing method: [1] A method for manufacturing hollow resin particles having a shell containing a resin and a hollow portion surrounded by the shell, comprising: a mixed solution preparation step of preparing a mixed solution containing a polymerizable monomer, a hydrophobic organic solvent, a polymerization initiator, and an aqueous medium, a suspending step of suspending the mixed solution to prepare a suspension in which droplets of a polymerizable monomer composition containing the polymerizable monomer, the hydrophobic organic solvent, and the polymerization initiator are dispersed in the aqueous medium, a polymerization step of subjecting the suspension to a polymerization reaction to prepare a precursor composition containing precursor particles having hollow portions and encapsulating the hydrophobic organic solvent in the hollow portions, a flotation separation step of floating and separating the precursor particles from the precursor composition, and a solvent removal step of removing the hydrophobic organic solvent encapsulated in the precursor particles while the precursor particles are in a floated and separated state. [2] The method for producing hollow resin particles according to [1], further comprising a washing step of washing the hollow resin particles obtained in the solvent removal step from which the hydrophobic organic solvent encapsulated in the precursor particles has been removed. [3] The method for producing hollow resin particles according to [1] or [2], further comprising a drying step of drying the hollow resin particles obtained in the solvent removal step from which the hydrophobic organic solvent encapsulated in the precursor particles has been removed. [4] The method for producing hollow resin particles according to any one of [1] to [3], wherein the floatation and separation step involves pressurizing the precursor composition and then reducing the pressure from the pressurized condition to float the precursor particles. [5] The method for producing hollow resin particles according to any one of [1] to [4], wherein the solvent removal step involves removing the hydrophobic organic solvent encapsulated in the precursor particles under a pressurized environment of 0.1 to 2.0 MPa gauge pressure. [6] The method for producing hollow resin particles according to any one of [1] to [5], wherein the solvent removal step involves removing the hydrophobic organic solvent encapsulated in the precursor particles under a gas stream. [7] The method for producing hollow resin particles according to any one of [1] to [6], wherein the floatation separation step is a step of floating the precursor particles to separate them into an upper layer (A) and a lower layer (B) having a lower content of precursor particles than the upper layer (A).[8] The method for producing hollow resin particles according to [7], wherein the upper layer (A) contains 0.1 to 70 parts by volume of gas per 100 parts by volume of the precursor particles in the upper layer (A). [9] The volume (V) of the aqueous medium in the upper layer (A) is Ａｗ ) and the volume of the precursor particle (V Ａｐ ) to the total volume of the precursor particles (V Ａｐ ) ratio [V Ａｐ / (V Ａｗ +V Ａｐ ) )] is 40% or more, and the volume (V Ｂｗ ) and the volume of the precursor particle (V Ｂｐ ) to the total volume of the precursor particles (V Ｂｐ ) ratio [V Ｂｐ / (V Ｂｗ +V Ｂｐ

[10] The method for producing hollow resin particles according to [7] or [8], wherein the average specific gravity (D Ｂ ) the average specific gravity (D Ａ ) ratio [D Ａ / D Ｂ

[11] The method for producing hollow resin particles according to any one of [7] to [9], wherein the ratio of the crosslinkable monomer to the total weight of the precursor particles is greater than 0.3 and less than 0.97.

[11] The method for producing hollow resin particles according to any one of [7] to

[10] , wherein, in the solvent removal step, when removing the hydrophobic organic solvent encapsulated in the precursor particles, an aqueous medium having an electrical conductivity of 2 μS / cm or less is added to the upper layer (A) while the aqueous medium is discharged from the lower layer (B).

[12] The method for producing hollow resin particles according to [4], wherein the pressurizing condition in the floatation separation step is a gauge pressure of 0.1 to 2.0 MPa.

[13] The method for producing hollow resin particles according to any one of [1] to

[12] , wherein the proportion of the crosslinkable monomer in 100% by mass of the polymerizable monomer is 40% by mass or more and 100% by mass or less.

[14] The method for producing hollow resin particles according to any one of [1] to

[13] , wherein the mixed solution further contains a dispersion stabilizer.

[0010] According to the present invention, it is possible to provide a method for producing hollow resin particles, which can suppress foaming during removal of a hydrophobic organic solvent and can produce hollow resin particles with high productivity.

[0011] The present invention provides a method for producing hollow resin particles having a resin-containing shell and a hollow portion surrounded by the shell. The method includes: (A) a mixed solution preparation step of preparing a mixed solution containing a polymerizable monomer, a hydrophobic organic solvent, a polymerization initiator, and an aqueous medium; (B) a suspending step of suspending the mixed solution to prepare a suspension in which droplets of a polymerizable monomer composition containing the polymerizable monomer, the hydrophobic organic solvent, and the polymerization initiator are dispersed in the aqueous medium; (C) a polymerization step of subjecting the suspension to a polymerization reaction to prepare a precursor composition containing precursor particles having hollow portions and encapsulating the hydrophobic organic solvent in the hollow portions; (D) a flotation separation step of floating and separating the precursor particles from the precursor composition; and (E) a solvent removal step of removing the hydrophobic organic solvent encapsulated in the precursor particles while the precursor particles are in a floated and separated state.

[0012] The manufacturing method of the present invention is a method for manufacturing hollow resin particles having a shell (outer shell) containing a resin and a hollow portion surrounded by the shell. In the present invention, the hollow portion is a hollow space that is clearly distinguishable from the shell of the hollow resin particle formed from a resin material. The shell of the hollow resin particle may have a porous structure, and in that case, the hollow portion has a size that is clearly distinguishable from the numerous minute spaces uniformly dispersed within the porous structure.

[0013] The hollow portion of the hollow resin particles can be confirmed, for example, by SEM observation of the particle cross section or by TEM observation of the particles as they are. The hollow portion of the hollow resin particles is usually filled with a gas such as air. Furthermore, the hollow resin particles obtained by the production method of the present invention usually have a shell that is free from interconnecting pores and shell defects, and the hollow portion is isolated from the outside of the particle by the shell. However, the shell may have one or more interconnecting pores, and the hollow portion may be connected to the outside of the particle via the interconnecting pores.

[0014] (A) Mixed Liquid Preparation Step The mixed liquid preparation step in the production method of the present invention is a step of preparing a mixed liquid containing a polymerizable monomer, a hydrophobic organic solvent, a polymerization initiator, and an aqueous medium.

[0015] [Polymerizable Monomer] The polymerizable monomer is a polymerizable monomer used to form the shell of the hollow resin particle. Examples of the polymerizable monomer include crosslinkable monomers and non-crosslinkable monomers. The non-crosslinkable monomer is a polymerizable monomer having only one polymerizable functional group, while the crosslinkable monomer is a polymerizable monomer having two or more polymerizable functional groups and forming a crosslinked bond in the resin by polymerization reaction. As the polymerizable monomer, a compound having an ethylenically unsaturated bond as the polymerizable functional group is generally used.

[0016] In the present invention, the crosslinkable monomer is preferred because it has a plurality of polymerizable functional groups, which can link the monomers together and increase the crosslink density of the shell. Furthermore, by using the crosslinkable monomer in combination with a non-crosslinkable monomer, the mechanical properties of the shell of the resulting hollow resin particles can be improved.

[0017] Crosslinkable monomers can be classified according to the number of polymerizable functional groups. Crosslinkable monomers can also be classified into aromatic group-containing crosslinkable monomers and non-aromatic group-containing crosslinkable monomers. Examples of crosslinkable monomers include bifunctional aromatic group-containing crosslinkable monomers such as divinylbenzene, divinyldiphenyl, divinylnaphthalene, and diallyl phthalate; allyl (meth)acrylate (meaning allyl acrylate and / or allyl methacrylate; the same applies hereinafter); ], ethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, 1,10-decanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, bisphenol A di(meth)acrylate, polyphenylene ether vinyl-modified at both ends, and polyphenylene ether methacrylate-modified at both ends and tri- or higher functional crosslinkable monomers such as trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol poly(meth)acrylate, etc. Among these, bifunctional aromatic group-containing crosslinkable monomers and bifunctional aromatic group-free crosslinkable monomers are preferred, with divinylbenzene, ethylene glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate being more preferred, divinylbenzene and ethylene glycol di(meth)acrylate being even more preferred, and divinylbenzene and ethylene glycol dimethacrylate being particularly preferred. In addition, from the viewpoint of facilitating the production of a precursor composition having cohesive properties, an aromatic group-containing crosslinkable monomer may be used. These crosslinkable monomers may be used alone or in combination of two or more.

[0018] The non-crosslinkable monomer means a compound having only one polymerizable functional group, and specifically includes a monovinyl monomer, and more specifically includes a hydrophilic monomer and a non-hydrophilic monomer. The hydrophilic monomer preferably has a solubility in water of 1% by mass or more, and the non-hydrophilic monomer preferably has a solubility in water of less than 1% by mass.

[0019] Examples of the hydrophilic monomer include monomers having a hydrophilic group, such as acid group-containing monomers, hydroxyl group-containing monomers, amide group-containing monomers, and polyoxyethylene group-containing monomers.

[0020] The acid group-containing monomer refers to a monomer containing an acid group. The acid group here includes both a proton-donating group (Brønsted acid group) and an electron pair-accepting group (Lewis acid group). By using an acid group-containing monomer as a hydrophilic monomer, the heat resistance of the resulting hollow resin particles can be further improved.

[0021] The acid group-containing monomer is not particularly limited as long as it has an acid group, and examples thereof include carboxyl group-containing monomers and sulfonic acid group-containing monomers. Examples of carboxyl group-containing monomers include ethylenically unsaturated carboxylic acid monomers such as acrylic acid, methacrylic acid, crotonic acid, cinnamic acid, itaconic acid, fumaric acid, maleic acid, and butenetricarboxylic acid; and monoalkyl esters of unsaturated dicarboxylic acids such as monoethyl itaconate, monobutyl fumarate, and monobutyl maleate. Among these, acrylic acid, methacrylic acid, and maleic acid are preferred, and acrylic acid and methacrylic acid are more preferred. Examples of sulfonic acid group-containing monomers include styrenesulfonic acid.

[0022] Examples of hydroxyl group-containing monomers include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, etc. Examples of amide group-containing monomers include acrylamide, dimethylacrylamide, etc. Examples of polyoxyethylene group-containing monomers include methoxypolyethylene glycol (meth)acrylate, etc.

[0023] Examples of non-hydrophilic monomers include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, ethoxypolyethylene glycol (meth)acrylate, propoxypolyethylene glycol (meth)acrylate, butoxypolyethylene glycol (meth)acrylate, hexaoxypolyethylene glycol (meth)acrylate, octoxypolyethylene glycol polypropylene glycol (meth)acrylate, lauroxypolyethylene glycol (meth)acrylate, stearoxypolyethylene glycol (meth)acrylate, phenoxypolyethylene glycol polypropylene glycol (meth)acrylate, and polyethylene glycol mono(meth)acrylate. acrylic monovinyl monomers such as polypropylene glycol mono(meth)acrylate, polyethylene glycol propylene glycol mono(meth)acrylate, polyethylene glycol tetramethylene glycol (meth)acrylate, propylene glycol polybutylene glycol mono(meth)acrylate, monoethylene glycol mono(meth)acrylate, and 2-aminoethyl (meth)acrylate; aromatic vinyl monomers such as styrene, ethylvinylbenzene, vinyltoluene, α-methylstyrene, p-methylstyrene, and halogenated styrenes; monoolefin monomers such as ethylene, propylene, and butylene; diene monomers such as butadiene and isoprene; carboxylic acid vinyl ester monomers such as vinyl acetate; halogenated vinyl monomers such as vinyl chloride; vinyl halide monomers such as vinylidene chloride; vinylpyridine monomers; and the like.

[0024] The non-crosslinkable monomers can be used either alone or in combination of two or more.

[0025] The proportion of the crosslinkable monomer in 100% by mass of the polymerizable monomer is preferably 40% by mass or more and 100% by mass or less, more preferably 50% by mass or more and 100% by mass or less, even more preferably 70% by mass or more and 100% by mass or less, and particularly preferably 80% by mass or more and 100% by mass or less. Furthermore, the proportion of the non-crosslinkable monomer in 100% by mass of the polymerizable monomer is preferably 0% by mass or more and 60% by mass or less, more preferably 0% by mass or more and 50% by mass or less, even more preferably 0% by mass or more and 30% by mass or less, and particularly preferably 0% by mass or more and 20% by mass or less. When the content of the crosslinkable monomer is within the above range, the content of the crosslinkable monomer units in the shell of the hollow resin particle is sufficiently high, and a covalent bond network is densely spread throughout the shell. As a result, the hollow resin particle has excellent strength, is resistant to crushing, and is resistant to deformation even when exposed to heat or the like from the outside. On the other hand, when the content of the non-crosslinkable monomer is within the above range, the occurrence of interconnected pores and shell defects in the shell is more likely to be suppressed.

[0026] The content of the polymerizable monomer (total amount of crosslinkable monomer and non-crosslinkable monomer) in the mixed solution prepared in the mixed solution preparation step in the production method of the present invention is not particularly limited, but from the viewpoint of the balance between the porosity, particle size, and mechanical strength of the hollow resin particles, it is preferably 15 to 55 mass%, and more preferably 25 to 50 mass%, relative to 100 mass% of the total mass of the components in the mixed solution excluding the aqueous medium.

[0027] [Hydrophobic Organic Solvent] In the present invention, a hydrophobic organic solvent is used as a non-polymerizable, poorly water-soluble organic solvent, which acts as a spacer material that forms hollow spaces inside the particles.

[0028] The hydrophobic organic solvent is not particularly limited, but hydrocarbon solvents can be suitably used, and specific examples thereof include saturated hydrocarbon solvents such as pentane, butane, normal hexane, cyclohexane, heptane, and octane, aromatic hydrocarbon solvents such as benzene, toluene, and xylene, and relatively volatile solvents such as carbon disulfide and carbon tetrachloride. These hydrophobic organic solvents can be used either alone or in combination of two or more.

[0029] As the hydrophobic organic solvent, it is preferable that the proportion of saturated hydrocarbon solvents is 50% by mass or more in a total amount (100% by mass) of hydrophobic organic solvents. This allows sufficient phase separation to occur within the droplets of the polymerizable monomer composition prepared in the suspension step described below, making it easier to obtain hollow resin particles having only one hollow portion, and suppressing the generation of porous particles. From the viewpoint of further suppressing the generation of porous particles and from the viewpoint of making the hollow portion of each hollow resin particle more uniform, the proportion of saturated hydrocarbon solvents is preferably 60% by mass or more, more preferably 80% by mass or more, in a total amount (100% by mass) of hydrophobic organic solvents.

[0030] Furthermore, the hydrophobic organic solvent is preferably a hydrocarbon solvent having 5 to 8 carbon atoms. A hydrocarbon solvent having 5 to 8 carbon atoms is easily encapsulated in the precursor particles during the polymerization step described below, and can be easily removed from the precursor particles during the solvent removal step described below. Among these, a hydrocarbon solvent having 6 to 8 carbon atoms is more preferred, with hexane, heptane, and cyclohexane being even more preferred.

[0031] Furthermore, from the viewpoint of ease of removal in the solvent removal step described below, the hydrophobic organic solvent preferably has a boiling point of 99°C or less at atmospheric pressure, more preferably 98°C or less, while from the viewpoint of ease of inclusion in the precursor particles, the hydrophobic organic solvent preferably has a boiling point of 30°C or more at atmospheric pressure.

[0032] In the present invention, when the hydrophobic organic solvent is a mixed solvent containing multiple types of hydrophobic organic solvents and has multiple boiling points, the boiling point of the hydrophobic organic solvent is the boiling point of the solvent with the highest boiling point among the solvents contained in the mixed solvent, i.e., the highest boiling point among the multiple boiling points.

[0033] Furthermore, the hydrophobic organic solvent preferably has a relative dielectric constant of 3 or less at 20°C. The relative dielectric constant is one of the indicators showing the degree of polarity of a compound. When the relative dielectric constant of the hydrophobic organic solvent is sufficiently small, 3 or less, it is considered that phase separation proceeds quickly in the droplets of the polymerizable monomer composition prepared in the suspension step described below, and hollow portions are likely to be formed.

[0034] Examples of hydrophobic organic solvents having a dielectric constant of 3 or less at 20°C include pentane (1.8), hexane (1.9), heptane (1.9), octane (1.9), cyclohexane (2.0), benzene (2.3), and toluene (2.4) (the values ​​in parentheses are the dielectric constant values). For the dielectric constant at 20°C, values ​​described in known literature (e.g., "Chemical Handbook: Basics," edited by the Chemical Society of Japan, Revised 4th Edition, Maruzen Co., Ltd., published September 30, 1993, pages II-498 to II-503) and other technical information can be referenced. Examples of methods for measuring the dielectric constant at 20°C include a dielectric constant test conducted in accordance with JIS C 2101:1999-23 at a measurement temperature of 20°C.

[0035] The porosity of the hollow resin particles can be adjusted by changing the amount of the hydrophobic organic solvent in the mixed solution. In the polymerization step described below, the polymerization reaction proceeds with the droplets of the polymerizable monomer composition encapsulating the hydrophobic organic solvent, so the porosity of the resulting hollow resin particles tends to increase as the content of the hydrophobic organic solvent increases.

[0036] The content of the hydrophobic organic solvent in the mixed solution is preferably 50 to 500 parts by mass, more preferably 60 to 400 parts by mass, even more preferably 80 to 350 parts by mass, and particularly preferably 100 to 300 parts by mass, per 100 parts by mass of the total mass of the polymerizable monomers. By setting the content of the hydrophobic organic solvent within the above range, it is possible to appropriately increase the porosity while maintaining the strength of the hollow resin particles.

[0037] [Polymerization initiator] In the present invention, it is preferable to use an oil-soluble polymerization initiator as the polymerization initiator. By using an oil-soluble polymerization initiator as the polymerization initiator, the polymerization initiator can be suitably incorporated into the interior of droplets of the polymerizable monomer composition in the suspension obtained in the suspension step described below.

[0038] The oil-soluble polymerization initiator is not particularly limited as long as it is lipophilic and has a solubility in water of 0.2% by mass or less, and examples of the oil-soluble polymerization initiator include benzoyl peroxide, lauroyl peroxide, t-butyl peroxide-2-ethylhexanoate, 2,2'-azobis(2,4-dimethylvaleronitrile), and azobisisobutyronitrile.

[0039] The content of the polymerization initiator is preferably 0.5 to 20 parts by mass, more preferably 1 to 15 parts by mass, and even more preferably 2 to 12 parts by mass, relative to 100 parts by mass of the total mass of the polymerizable monomers in the mixed solution. By setting the content of the polymerization initiator within the above range, the polymerization reaction can be sufficiently progressed, and there is little risk of the polymerization initiator remaining after completion of the polymerization reaction, and there is also little risk of an unexpected side reaction proceeding.

[0040] [Aqueous Medium] The aqueous medium may be a medium selected from the group consisting of water, a hydrophilic solvent, and a mixture of water and a hydrophilic solvent.

[0041] The hydrophilic solvent is not particularly limited as long as it is sufficiently miscible with water and does not cause phase separation, and examples thereof include alcohols such as methanol and ethanol, tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), etc. These hydrophilic solvents can be used either alone or in combination of two or more.

[0042] Among aqueous media, water is preferred due to its high polarity. When a mixture of water and a hydrophilic solvent is used, it is preferable that the polarity of the entire mixture is not too low, from the viewpoint of properly forming droplets of a polymerizable monomer composition containing a polymerizable monomer, a hydrophobic organic solvent, and a polymerization initiator. When a mixture of water and a hydrophilic solvent is used, it is preferable that the mixing ratio (mass ratio) of water to hydrophilic solvent is 99:1 to 50:50.

[0043] In addition, in the mixed solution preparation step of the production method of the present invention, it is preferable to use a dispersion stabilizer in addition to the polymerizable monomer, hydrophobic organic solvent, polymerization initiator, and aqueous medium. That is, the mixed solution preparation step is preferably a step of preparing a mixed solution containing the polymerizable monomer, hydrophobic organic solvent, polymerization initiator, aqueous medium, and dispersion stabilizer. By including a dispersion stabilizer, it is possible to further improve the dispersion stability of droplets of the polymerizable monomer composition in the suspension step described below.

[0044] The dispersion stabilizer is a compound that disperses droplets of the polymerizable monomer composition in an aqueous medium in the suspension step described below, and may be either an inorganic dispersion stabilizer or an organic dispersion stabilizer.

[0045] Examples of inorganic dispersion stabilizers include colloidal silica, magnesium hydroxide, calcium phosphate, calcium carbonate, barium sulfate, calcium oxalate, calcium carbonate, magnesium carbonate, barium carbonate, tricalcium phosphate, aluminum hydroxide, magnesium hydroxide, ferric hydroxide, hydroxyapatite, diatomaceous earth, clay, and bentonite.

[0046] Examples of organic dispersion stabilizers include methyl cellulose, hydroxypropyl methyl cellulose, carboxymethyl cellulose, and starch.

[0047] Among these, inorganic dispersion stabilizers are preferred from the viewpoint of having a high dispersion stabilizing effect and making it easier to control the particle size of droplets of a polymerizable monomer composition containing a polymerizable monomer, a hydrophobic organic solvent, and a polymerization initiator. Among inorganic dispersion stabilizers, metal-containing dispersion stabilizers are preferred, and poorly water-soluble inorganic metal salts are more preferred. Furthermore, poorly water-soluble inorganic metal salts are preferably inorganic metal salts having a solubility of 0.5 g or less in 100 g of water, such as magnesium hydroxide, calcium hydroxide, barium hydroxide, calcium phosphate, etc., and among these, magnesium hydroxide is more preferred. Each dispersion stabilizer can be used alone or in combination of two or more.

[0048] Furthermore, from the viewpoint of further enhancing the dispersion stabilizing effect, the dispersion stabilizer is preferably used in the form of a dispersion or solution by dispersing or dissolving it in an aqueous medium. That is, in the mixed solution preparation step of the production method of the present invention, the mixed solution is preferably obtained by mixing the dispersion stabilizer in the form of a dispersion or solution with the polymerizable monomer, the hydrophobic organic solvent, and the polymerization initiator.

[0049] The aqueous medium may be any medium selected from the group consisting of water, a hydrophilic solvent, and a mixture of water and a hydrophilic solvent. The hydrophilic solvent is not particularly limited as long as it is sufficiently miscible with water and does not cause phase separation, and examples thereof include alcohols such as methanol and ethanol; tetrahydrofuran (THF); dimethyl sulfoxide (DMSO); and the like.

[0050] Among aqueous media, water is preferred due to its high polarity. When a mixture of water and a hydrophilic solvent is used, it is preferable that the polarity of the entire mixture is not too low, from the viewpoint of properly forming droplets of a polymerizable monomer composition containing a polymerizable monomer and a hydrophobic organic solvent. When a mixture of water and a hydrophilic solvent is used, it is preferable that the mixing ratio (mass ratio) of water to hydrophilic solvent is 99:1 to 50:50.

[0051] When preparing a dispersion or solution of the dispersion stabilizer, the mixing ratio of the dispersion stabilizer to the aqueous medium, in terms of the weight ratio of "dispersion stabilizer:aqueous medium", is preferably 0.7:100 to 7:100, more preferably 1.0:100 to 4.0:100, and even more preferably 1.4:100 to 3:100. By setting the mixing ratio of the dispersion stabilizer to the aqueous medium within the above range, the dispersion stabilization effect can be more appropriately enhanced.

[0052] In the mixed solution preparation step of the production method of the present invention, the above-mentioned components can be mixed by stirring or the like to obtain a mixed solution. In this case, other materials may be mixed as needed in addition to the above-mentioned components. According to the mixed solution preparation step of the production method of the present invention, a mixed solution is prepared in which an oil phase containing a polymerizable monomer, a hydrophobic organic solvent, and a lipophilic material such as a polymerization initiator is dispersed in an aqueous medium and an aqueous phase containing a dispersion stabilizer used as needed, with particles having a particle size of about several mm. The dispersion state of these components in the mixed solution can be observed with the naked eye, depending on the type of each component.

[0053] In addition, in the mixed solution preparation step, from the viewpoint that the composition of the shell portion is likely to be uniform, it is preferable to prepare the mixed solution in advance by preparing an oil phase containing a polymerizable monomer, a hydrophobic organic solvent, and a polymerization initiator, and mixing this with a dispersion or solution obtained by dispersing or dissolving a dispersion stabilizer in an aqueous medium.

[0054] (B) Suspension Step The suspension step is a step of preparing a suspension in which droplets of a polymerizable monomer composition containing a polymerizable monomer, a hydrophobic organic solvent, and a polymerization initiator are dispersed in an aqueous medium by suspending the mixed liquid obtained in the mixed liquid preparation step described above.

[0055] The suspension method for forming droplets of the polymerizable monomer composition is not particularly limited, but a method in which the mixed solution obtained in the mixed solution preparation step described above is stirred using a stirrer capable of strong stirring is preferred. The stirrer used in the suspension step is not particularly limited, but for example, a stirrer equipped with a stirring blade or a rotor and a supply tank for supplying the mixture to the stirrer can be used. The stirrer is not particularly limited as long as it is equipped with a stirring blade or a rotor. However, from the viewpoint of efficiently forming a suspension, a stirrer having a combination of a rotor and a stator that are comb-tooth concentric rings is preferred, in which the rotor is rotated at high speed to circulate the dispersion from the inside of the rotor to the outside of the stator, and the dispersion is stirred in the gap between the rotor and the stator.

[0056] An example of an agitator having such a configuration is an in-line emulsifying disperser, and examples of the in-line emulsifying disperser include those with the product name "Cavitron" (manufactured by Eurotech), the product name "Milder" (manufactured by Pacific Machinery Works), the product name "Ebara Milder" (manufactured by Ebara Corporation), the product name "TK Pipeline Homomixer" (manufactured by Tokushu Kika Kogyo Co., Ltd.), the product name "Colloid Mill" (manufactured by Kobe Steel Pantech Co., Ltd.), the product name "Slasher" (manufactured by Nippon Coke and Engineering Co., Ltd.), the product name "Trigonal Wet Fine Pulverizer" (manufactured by Mitsui Miike Chemical Engineering Co., Ltd.), and the product name "Fine Flow Mill" (manufactured by Pacific Machinery Works).

[0057] In the suspending step of the production method of the present invention, a suspension can be obtained in which droplets of the polymerizable monomer composition containing the lipophilic material are uniformly dispersed in an aqueous medium. Such droplets of the polymerizable monomer composition are difficult to observe with the naked eye and can be observed using known observation equipment such as an optical microscope. Furthermore, in the suspending step, phase separation occurs in the droplets of the polymerizable monomer composition, which makes it easy for the low-polarity hydrophobic organic solvent to collect inside the droplets. As a result, the resulting droplets contain the hydrophobic organic solvent in their interiors and materials other than the hydrophobic organic solvent distributed around their peripheries.

[0058] (C) Polymerization Step The polymerization step is a step of preparing a precursor composition containing precursor particles having hollow portions and encapsulating a hydrophobic organic solvent in the hollow portions by subjecting the suspension prepared in the suspension step described above to a polymerization reaction.

[0059] In the polymerization process, the polymerizable monomer in the droplets of the polymerizable monomer composition is polymerized while the droplets still contain the hydrophobic organic solvent, thereby forming precursor particles having a shell containing a resin, which is a polymer of the polymerizable monomer, and a hollow portion filled with the hydrophobic organic solvent.

[0060] In the polymerization step, droplets of the polymerizable monomer composition are subjected to the polymerization reaction while encapsulating the hydrophobic organic solvent, which facilitates the polymerization reaction while maintaining the shape, and facilitates the adjustment of the size and porosity of the precursor particles. Furthermore, since the polymerizable monomer and the hydrophobic organic solvent are used in combination, the polarity of the hydrophobic organic solvent is low relative to the shell of the precursor particles, and the hydrophobic organic solvent is not easily compatible with the shell, which leads to sufficient phase separation and the formation of only one hollow portion.

[0061] The polymerization method is not particularly limited, and for example, a batch method, a semi-continuous method, a continuous method, etc. can be used. The polymerization temperature is preferably 40 to 90°C, more preferably 50 to 80°C. The polymerization reaction time is preferably 1 to 48 hours, more preferably 3 to 24 hours.

[0062] The polymerization step yields a precursor composition in which precursor particles encapsulating a hydrophobic solvent are dispersed in an aqueous phase containing an aqueous medium as the main component.

[0063] An antifoaming agent may be added to the obtained precursor composition. The antifoaming agent is not particularly limited, and examples thereof include polyether-based antifoaming agents, mineral oil-based antifoaming agents, and alcohol-based antifoaming agents. Among these, water-soluble antifoaming agents such as polyether-based antifoaming agents and alcohol-based antifoaming agents are preferably used because they are less likely to remain in the hollow resin particles. Residual antifoaming agents in the hollow resin particles may increase the dielectric loss tangent of the hollow resin particles. However, polyether-based antifoaming agents are particularly preferably used because they are less likely to increase the dielectric loss tangent of the hollow resin particles.

[0064] The amount of antifoaming agent added is not particularly limited, but from the viewpoint of more effectively suppressing foaming during removal of the hydrophobic organic solvent, it is preferably 0.01 parts by mass or more, more preferably 0.1 parts by mass or more, per 100 parts by mass of the precursor composition, while from the viewpoint of reducing the amount of residual antifoaming agent and suppressing an increase in the dielectric loss tangent of the hollow resin particles, it is preferably 50 parts by mass or less, more preferably 20 parts by mass or less, and even more preferably 0.5 parts by mass or less. Note that an antifoaming agent may also be added in the floatation separation step or solvent removal step described below, and in that case, it is preferable that the total amount of antifoaming agent added per 100 parts by mass of the precursor composition used be within the above-mentioned range.

[0065] (D) Flotation Separation Step The flotation separation step is a step of floating and separating precursor particles in the precursor composition obtained by the polymerization step.

[0066] The method for floating and separating the precursor particles in the precursor composition is not particularly limited, but examples thereof include a method of attaching air bubbles to the precursor particles, a method of adjusting the composition of the mixed liquid used in the mixed liquid preparation step to make the specific gravity of the precursor particles smaller than the specific gravity of the aqueous medium, etc. Among these, the method of attaching air bubbles to the precursor particles is preferred from the viewpoint of enabling the precursor particles to be floated and separated with high efficiency.

[0067] The method for attaching bubbles to the precursor particles is not particularly limited, but a method of pressurizing the precursor composition and then depressurizing it from the pressurized condition (pressurization / depressurization operation) is preferred. That is, in the floatation separation step, it is preferable to pressurize the precursor composition and then depressurize it from the pressurized condition (pressurization / depressurization operation) to float the precursor particles. By such pressurization / depressurization operation, fine bubbles can be uniformly generated, and the precursor particles can be floated and separated with high efficiency. Note that the reason why bubbles are generated by the pressurization / depressurization operation is not necessarily clear, but it is presumed that this is because part of the gas in the gas layer in the container dissolves in the aqueous medium during pressurization in the pressurization / depressurization operation, and then at least part of the gas dissolved in the aqueous medium vaporizes when the pressure is reduced.

[0068] When pressurizing and depressurizing operations are performed, the pressure (gauge pressure) during pressurization is preferably 0.1 to 2.0 MPa, and more preferably 0.1 to 1.0 MPa. By setting the pressure during pressurization at or above the lower limit, a large amount of fine bubbles can be uniformly generated during depressurization, allowing the precursor particles to be floated and separated with high efficiency, and foaming during the removal of the hydrophobic organic solvent in the solvent removal step can be further suppressed. On the other hand, by setting the pressure during pressurization at or below the upper limit, excessive foaming due to pressurization and depressurization operations can be suppressed, allowing a relatively large amount of precursor composition to be supplied to the floatation separation equipment, and further improving productivity.

[0069] A preferred method of pressurizing is to inject a compressed gas into the system. Examples of the compressed gas include air, oxygen, nitrogen, and argon. Among them, an inert gas is preferred, and nitrogen is more preferred.

[0070] The temperature and pressure conditions during pressurization are preferably such that the saturated solubility of the gas in the gas layer in the container in water falls within a range of 0.0001 g / 100 g of water to 0.01 g / 100 g of water. By setting the above conditions, a large amount of fine bubbles can be uniformly generated during depressurization, allowing the precursor particles to be floated and separated with high efficiency, and foaming during removal of the hydrophobic organic solvent in the solvent removal step can be further suppressed.

[0071] The pressing time is preferably within the range of 1 to 10 minutes.

[0072] When performing pressurization and depressurization operations, the pressure during depressurization is not particularly limited as long as it is lower than the pressurization conditions (pressure during pressurization), and may be higher than atmospheric pressure or lower. The pressure during depressurization is preferably atmospheric pressure or lower, more preferably 110 kPa or lower in absolute pressure, and even more preferably 105 kPa or lower in absolute pressure. By setting the pressure during depressurization to the above upper limit or lower, a large amount of fine bubbles can be uniformly generated during depressurization, allowing the precursor particles to be floated and separated with high efficiency, and foaming during removal of the hydrophobic organic solvent in the solvent removal step can be further suppressed. The lower limit of the pressure during depressurization is not particularly limited, but may be, for example, 1 kPa or higher.

[0073] The decompression time is preferably within the range of 1 minute to 10 minutes.

[0074] When pressurizing and depressurizing the system, the temperature during heating and depressurizing is preferably 20 to 90° C., more preferably 20 to 80° C., even more preferably 20 to 70° C., particularly preferably 20 to 60° C., particularly preferably 20 to 50° C., and most preferably 20 to 40° C. By keeping the temperature within the system at or below the upper limit mentioned above, excessive foaming due to pressurizing and depressurizing operations can be suppressed, and the amount of precursor composition supplied to the flotation separation equipment can be relatively increased, thereby further improving productivity.

[0075] In the flotation separation step, the pressurization and depressurization operation may be carried out only once or may be carried out multiple times.

[0076] The floatation separation step is preferably a step of floating the precursor particles to separate them into an upper layer (A) and a lower layer (B) having a lower content of precursor particles than the upper layer (A).

[0077] The upper layer (A) is a layer formed on the upper part of the lower layer (B), and contains a higher proportion of precursor particles than the lower layer (B).

[0078] The upper layer (A) contains precursor particles and an aqueous medium. The volume (V Ａｗ ) and the volume of the precursor particle (V Ａｐ ) to the total volume of the precursor particles (VＡｐ ) ratio [V Ａｐ / (V Ａｗ +V Ａｐ ) is preferably 40% or more, more preferably 45% or more, even more preferably 50% or more, particularly preferably 55% or more, and most preferably 60% or more. When the ratio is within the above range, foaming during removal of the hydrophobic organic solvent in the solvent removal step can be further suppressed, thereby further increasing productivity. The upper limit of the ratio is not particularly limited, but is preferably 80% or less. The ratio can be adjusted by adjusting the floatation separation conditions, such as the conditions for pressurization and depressurization operations. The ratio can be measured by the method described in the Examples.

[0079] The weight ratio of the precursor particles contained in the upper layer (A) to the total precursor particles used in the flotation separation step is preferably 80 wt% or more, more preferably 90 wt% or more. When the weight ratio of the precursor particles contained in the upper layer (A) is within the above range, foaming during removal of the hydrophobic organic solvent in the solvent removal step can be further suppressed, thereby further increasing productivity. The above ratio can be adjusted by adjusting the flotation separation conditions, such as the conditions for pressurization and depressurization operations.

[0080] The upper layer (A) preferably further contains a gas, and more preferably is a foam layer containing precursor particles, an aqueous medium, and a gas.

[0081] The upper layer (A) preferably contains 0.1 to 70 parts by volume of gas per 100 parts by volume of precursor particles in the upper layer (A), more preferably 10 to 70 parts by volume of gas, and even more preferably 20 to 70 parts by volume of gas. When the amount of gas in the upper layer (A) is within the above range, a relatively large amount of precursor composition can be supplied to the flotation separation equipment, and foaming during removal of the hydrophobic organic solvent in the solvent removal step can be further suppressed, thereby further increasing productivity. The amount of gas in the upper layer (A) can be adjusted by adjusting the flotation separation conditions, such as the conditions for pressurization and depressurization operations. The amount of gas in the upper layer (A) can be measured by the method described in the Examples.

[0082] The lower layer (B) is a layer formed below the upper layer (A) and contains a smaller proportion of precursor particles than the upper layer (A).

[0083] The lower layer (B) contains an aqueous medium. The lower layer (B) may further contain precursor particles. The volume (V Ｂｗ ) and the volume of the precursor particle (V Ｂｐ ) to the total volume of the precursor particles (V Ｂｐ ) ratio [V Ｂｐ / (V Ｂｗ +V Ｂｐ ) is preferably 20% or less, more preferably 15% or less, even more preferably 10% or less, and particularly preferably 5% or less. The above ratio can be adjusted by adjusting the floatation separation conditions, such as the conditions for pressurization and depressurization operations. The above ratio can also be measured by the method described in the Examples.

[0084] The weight ratio of the precursor particles contained in the lower layer (B) to the total precursor particles used in the flotation separation step is preferably 20 wt% or less, more preferably 10 wt% or less. When the weight ratio of the precursor particles contained in the lower layer (B) is within the above range, foaming during removal of the hydrophobic organic solvent in the solvent removal step can be further suppressed, thereby further increasing productivity. The above ratio can be adjusted by adjusting the flotation separation conditions, such as the conditions for pressurization and depressurization operations.

[0085] The lower layer (B) generally does not substantially contain a gas, and the gas content per 100 parts by volume of the lower layer (B) is generally 0.1 part by volume or less.

[0086] After the floatation separation, a boundary layer (C) may be formed between the upper layer (A) and the lower layer (B). The boundary layer (C) is formed by dividing the volume (V Ｃｗ ) and the volume of the precursor particle (V Ｃｐ ) to the total volume of the precursor particles (V Ｃｐ ) ratio [V Ｃｐ / (V Ｃｗ +V Ｃｐ ) is more than 20% and less than 40%.

[0087] By the floatation separation step, the precursor composition is preferably separated into two or three layers consisting of an upper layer (A), a lower layer (B) and an optional boundary layer (C).

[0088] The volume proportion of the upper layer (A) in the precursor composition after flotation separation is preferably 60% by volume or less, more preferably 20 to 60% by volume, and even more preferably 40 to 60% by volume. When the volume proportion of the upper layer (A) is equal to or less than the above upper limit, the amount of precursor composition supplied to the flotation separation equipment can be relatively increased, thereby further improving productivity.

[0089] The volume fraction of the lower layer (B) in the precursor composition after flotation separation is preferably 39% by volume or more, more preferably 39 to 80% by volume, and even more preferably 39 to 60% by volume. When the volume fraction of the lower layer (B) is equal to or greater than the above lower limit, the amount of precursor composition supplied to the flotation separation equipment can be relatively increased, thereby further improving productivity.

[0090] The total volume ratio of the upper layer (A) and the lower layer (B) in the precursor composition after floatation separation is preferably 95% by volume or more, more preferably 97% by volume or more. When the volume ratio is within the above range, foaming during removal of the hydrophobic organic solvent in the solvent removal step can be further suppressed, thereby further improving productivity.

[0091] The volume fraction of the boundary layer (C) in the precursor composition after flotation separation is preferably 5% by volume or less, more preferably 3% by volume or less. When the volume fraction of the boundary layer (C) is within the above range, foaming during removal of the hydrophobic organic solvent in the solvent removal step can be further suppressed, and productivity can be further improved.

[0092] The volume proportions of the upper layer (A), the lower layer (B), and the boundary layer (C) in the precursor composition after flotation separation can be adjusted by adjusting the flotation separation conditions, such as the conditions for pressurization and depressurization operations, etc. The above proportions can be measured by the method described in the Examples.

[0093] The average specific gravity of the lower layer (B) (D Ｂ ) the average specific gravity (D Ａ ) ratio [D Ａ / D Ｂ ] is usually less than 1, preferably more than 0.3 but less than 0.97, more preferably more than 0.3 but less than 0.80, and even more preferably more than 0.3 but less than 0.67. When the ratio is within the above range, foaming during removal of the hydrophobic organic solvent in the solvent removal step can be further suppressed, thereby further increasing productivity. The ratio can be adjusted by adjusting the floatation separation conditions, such as the conditions for pressurization and depressurization operations. The ratio can also be measured by the method described in the Examples.

[0094] A defoaming agent may be added to the precursor composition after flotation separation. The defoaming agent may be added to any of the upper layer (A), the lower layer (B), and the boundary layer (C), but is preferably added to at least the upper layer (A). The addition method is not particularly limited, but it is preferable to spray the defoaming agent, diluted as necessary, from above the upper layer (A).

[0095] The defoaming agent is not particularly limited, and examples thereof include polyether-based defoaming agents, mineral oil-based defoaming agents, and alcohol-based defoaming agents. Among them, water-soluble defoaming agents such as polyether-based defoaming agents and alcohol-based defoaming agents are preferably used because they are less likely to remain in the hollow resin particles. Although the dielectric loss tangent of the hollow resin particles may increase if the defoaming agent remains in the hollow resin particles, polyether-based defoaming agents are particularly preferably used because they are less likely to increase the dielectric loss tangent of the hollow resin particles.

[0096] The amount of antifoaming agent added is not particularly limited, but from the viewpoint of more effectively suppressing foaming during removal of the hydrophobic organic solvent, it is preferably 0.01 parts by mass or more, more preferably 0.1 parts by mass or more, per 100 parts by mass of the precursor composition before flotation separation, while from the viewpoint of reducing the residual amount of antifoaming agent and suppressing an increase in the dielectric loss tangent of the hollow resin particles, it is preferably 50 parts by mass or less, more preferably 20 parts by mass or less, and even more preferably 0.5 parts by mass or less. Note that an antifoaming agent may also be added in the above-mentioned polymerization step or the solvent removal step described below, and in that case, it is preferable that the total amount of antifoaming agent added per 100 parts by mass of the precursor composition before flotation separation be within the above-mentioned range.

[0097] (E) Solvent Removal Step The solvent removal step is a step of removing the hydrophobic organic solvent contained in the precursor particles while the precursor particles are in a floated and separated state. In the production method of the present invention, the precursor particles floated and separated in the floatation separation step are subjected to a solvent removal treatment while the precursor particles are in a floated and separated state.

[0098] The production method of the present invention, which includes such a solvent removal step, can suppress foaming during removal of the hydrophobic organic solvent, allowing a relatively large amount of hollow resin particles to be supplied to the solvent removal equipment and allowing removal conditions that promote removal of the hydrophobic organic solvent to be adopted. As a result, the production method of the present invention can produce hollow resin particles with high productivity. Furthermore, the production method of the present invention can prevent breakage of the hollow resin particles during removal of the hydrophobic organic solvent.

[0099] In the solvent removal step, the method for removing the hydrophobic organic solvent contained in the precursor particles is not particularly limited, but a method in which the precursor particles are heated to replace the hydrophobic organic solvent contained in the precursor particles with a gas is preferred.

[0100] The heating temperature in the solvent removal step is set to a temperature higher than the boiling point of the hydrophobic organic solvent under the pressure environment in the solvent removal step and lower than the boiling point (T ｂ The boiling point of the hydrophobic organic solvent is usually lower than the boiling point of the aqueous medium. The heating temperature in the solvent removal step is preferably lower than the boiling point (T ｂ ), more preferably within a temperature range of 2°C to 35°C lower, and even more preferably within a temperature range of 5°C to 20°C lower. By setting the heating temperature in the solvent removal step within the above range, foaming during removal of the hydrophobic organic solvent can be further suppressed, and productivity can be further improved.

[0101] The pressure in the solvent removal step may be normal pressure, increased pressure, or reduced pressure. In particular, it is preferable to remove the hydrophobic organic solvent contained in the precursor particles under a pressurized environment in the solvent removal step. The pressure condition is preferably a gauge pressure of 0.1 to 2.0 MPa, more preferably 0.3 to 1.5 MPa, and even more preferably 0.7 to 1.0 MPa. By setting the pressure in the solvent removal step within the above range, foaming during removal of the hydrophobic organic solvent can be further suppressed, thereby further increasing productivity.

[0102] The heating time in the solvent removal step can be determined depending on the desired level of hydrophobic organic solvent removal. The desired level of hydrophobic organic solvent removal may be, for example, a level at which the amount of hydrophobic organic solvent in the hollow resin particles is 0.01 g / g or less. The desired level of residual monomer removal may be, for example, a level at which the amount of residual monomer in the hollow resin particles is 0.001 g / g or less. The heating time for achieving such a removal level is typically about 10 to 60 hours. The amounts of hydrophobic organic solvent and residual monomer in the hollow resin particles can be measured by the method described in the Examples.

[0103] In the solvent removal step, it is preferable to remove the hydrophobic organic solvent contained in the precursor particles under a gas flow. Specifically, it is preferable to inject and discharge gas into the gas phase above the precursor composition after floatation separation. By removing the hydrophobic organic solvent under a gas flow, it is possible to promote removal of the hydrophobic organic solvent and further increase productivity.

[0104] When the hydrophobic organic solvent is removed under a gas stream, it is preferable to use an inert gas such as nitrogen or argon as the gas. Use of an inert gas can improve the electrical insulation properties of the resulting hollow resin particles.

[0105] When removing the hydrophobic organic solvent under a gas flow, the flow rate of the gas is determined based on the volume (V ０ The flow rate of the gas is preferably determined according to the volume (V) of the precursor composition before flotation separation per minute. ０ ) or more (0.5V ０ / min or more), and the volume (V ０ ) or more than 1.0 times (V ０ By setting the gas flow rate within the above range, it is possible to promote removal of the hydrophobic organic solvent, and it is possible to further increase productivity.

[0106] In the solvent removal step, when removing the hydrophobic organic solvent contained in the precursor particles, it is preferable to perform a washing operation in which an aqueous medium having an electrical conductivity of 2 μS / cm or less is added to the upper layer (A) while the aqueous medium is discharged from the lower layer (B). By performing such a washing operation, the amount of residual metal in the obtained hollow resin particles can be reduced, and electrical insulation can be improved.

[0107] In the production method of the present invention, the amount of residual metal in the resulting hollow resin particles can be reduced and electrical insulation can be improved by washing the hollow resin particles in a washing step after the solvent removal step without performing the above-mentioned washing operation. On the other hand, by performing the above-mentioned washing operation in the solvent removal step of the production method of the present invention, the amount of residual metal in the resulting hollow resin particles can be reduced and electrical insulation can be improved even if the washing time in the washing step after the solvent removal step is shortened or the washing step after the solvent removal step is omitted. In other words, by performing the above-mentioned washing operation in the solvent removal step of the production method of the present invention, hollow resin particles with a low amount of residual metal and excellent electrical insulation can be produced with very high productivity.

[0108] In the washing operation, an aqueous medium having an electrical conductivity of 2 μS / cm or less (hereinafter sometimes referred to as washing water) is used as washing water. The electrical conductivity of the washing water is not particularly limited as long as it is 2 μS / cm or less, but it is preferably 1 μS / cm or less. When the electrical conductivity of the washing water is within the above range, washing efficiency can be improved. The temperature of the washing water is preferably within the temperature range described above as the heating temperature in the solvent removal step, and is preferably equal to the heating temperature in the solvent removal step.

[0109] The method for adding the washing water to the upper layer (A) is not particularly limited, but a method in which the washing water is added from above the upper layer (A) is preferred. The washing water added from above the upper layer (A) passes through the upper layer (A) and moves to the lower layer (B). At this time, the metal components in the upper layer (A) are dispersed or dissolved in the washing water moving through the upper layer (A), thereby enabling the metal components to be removed from the upper layer (A) with high efficiency. As a result, hollow resin particles with a small amount of residual metal and excellent electrical insulation can be produced with very high productivity.

[0110] The addition of the washing water is preferably carried out by spraying. By adding the washing water by spraying, the precursor particles can be kept in a good floating and separated state. As a result, the frequency of contact between the particles and the washing water can be increased while suppressing foaming during removal of the hydrophobic organic solvent, thereby improving the washing efficiency.

[0111] A preferred method for discharging the aqueous medium from the lower layer (B) is to discharge a clear liquid from the bottom of a container containing the precursor composition.

[0112] The operation of adding washing water to the upper layer (A) and the operation of discharging the aqueous medium from the lower layer (B) may each be carried out only once, multiple times, or continuously.

[0113] In the solvent removal step, it is preferable to carry out the operation of adding washing water to the upper layer (A) and the operation of discharging the aqueous medium from the lower layer (B) continuously or intermittently for at least a part of the time during which the hydrophobic organic solvent encapsulated in the precursor particles is removed, and it is preferable to carry out the operation continuously or intermittently for the entire time during which the hydrophobic organic solvent encapsulated in the precursor particles is removed.

[0114] In this case, it is preferable that the amount of washing water added to the upper layer (A) and the amount of aqueous medium discharged from the lower layer (B) per unit time are approximately equal. Here, the amount of washing water added to the upper layer (A) and the amount of aqueous medium discharged from the lower layer (B) per unit time are each determined based on the volume (V ０) per hour. ０ ) to 0.01 to 2.00 times (0.01V ０ / hour ~ 2.00V ０ / hour), and the volume of the precursor composition before flotation separation (V ０ ) to 0.5 to 1.5 times (0.5V ０ / hour ~ 1.5V ０ / hour) is more preferable.

[0115] In the solvent removal step, a defoaming agent may be added to the system. The defoaming agent may be added to any of the upper layer (A), the lower layer (B), and the boundary layer (C), but is preferably added to at least the upper layer (A). The addition method is not particularly limited, but the defoaming agent may be diluted as needed and sprayed from above the upper layer (A). When the above-mentioned washing operation is performed, the defoaming agent may be added together with the washing water.

[0116] As the antifoaming agent, the same antifoaming agent as that usable in the floatation separation step can be mentioned, and the preferred ones are also the same.The amount of antifoaming agent added is not particularly limited, but from the viewpoint of more effectively suppressing foaming when removing the hydrophobic organic solvent, it is preferably 0.01 parts by mass or more, more preferably 0.1 parts by mass or more per 100 parts by mass of the precursor composition before floatation separation.On the other hand, from the viewpoint of reducing the residual amount of antifoaming agent and suppressing the increase in the dielectric loss tangent of the hollow resin particles, it is preferably 50 parts by mass or less, more preferably 20 parts by mass or less, and even more preferably 0.5 parts by mass or less.In addition, an antifoaming agent may also be added in the above-mentioned polymerization step or solvent removal step, and in this case, it is preferable that the total amount of antifoaming agent added per 100 parts by mass of the precursor composition before floatation separation is within the above-mentioned range.

[0117] (F) Other Steps The production method of the present invention may include other steps in addition to the above-mentioned mixed solution preparation step, suspension step, polymerization step, floatation separation step, and solvent removal step.

[0118] (F-1) Washing Step The production method of the present invention preferably further includes a washing step. The washing step is a step of subjecting the hollow resin particles obtained in the solvent removal step described above, from which the hydrophobic organic solvent encapsulated in the precursor particles has been removed, to a washing treatment. By including the washing step in the production method of the present invention, the amount of residual metal in the obtained hollow resin particles can be reduced, and electrical insulation properties can be improved.

[0119] When a dispersion stabilizer is used, it is preferable to wash the hollow resin particles obtained in the solvent removal step using an acid or alkali in order to remove the remaining dispersion stabilizer. When the dispersion stabilizer used is an acid-soluble dispersion stabilizer, it is preferable to wash the hollow resin particles obtained in the solvent removal step with an acid. On the other hand, when the dispersion stabilizer used is an alkali-soluble dispersion stabilizer, it is preferable to wash the hollow resin particles obtained in the solvent removal step with an alkali.

[0120] For example, when an acid-soluble dispersion stabilizer is used as the dispersion stabilizer, it is preferable to add an acid to a system containing hollow resin particles and an aqueous medium, adjust the pH to preferably 6.5 or less, more preferably 6 or less, and then perform washing. The acid to be added may be an inorganic acid such as sulfuric acid, hydrochloric acid, or nitric acid, or an organic acid such as formic acid or acetic acid, but sulfuric acid is particularly preferred because it has a high efficiency in removing the dispersion stabilizer and places a small burden on the production equipment.

[0121] In the washing step, it is preferable to wash the hollow resin particles obtained in the solvent removal step with washing water. As the washing water, it is preferable to use the same washing water as that used when performing the washing operation in the above-mentioned solvent removal step. Note that when washing with the above-mentioned acid or alkali is performed, it is preferable to subsequently wash the hollow resin particles obtained in the solvent removal step with washing water. Washing the hollow resin particles with washing water can reduce the amount of residual metal in the obtained hollow resin particles and improve electrical insulation properties.

[0122] In the washing step, solid-liquid separation is preferably performed to separate and recover the washed hollow resin particles obtained in the washing step from the aqueous medium. Examples of solid-liquid separation methods include centrifugation, filtration, and static separation, with centrifugation and filtration being preferred.

[0123] The hollow resin particles recovered by solid-liquid separation may be further washed with wash water. Repeated washing with wash water and solid-liquid separation can reduce the amount of residual metal in the resulting hollow resin particles and improve their electrical insulation. Repeated washing with wash water and solid-liquid separation can be performed until the electrical conductivity of the aqueous medium removed during solid-liquid separation (or the filtrate, if solid-liquid separation is performed by filtration) reaches, for example, 10 μS / cm or less.

[0124] When the production method of the present invention does not include a washing step, it is preferable to carry out solid-liquid separation in order to separate and recover the hollow resin particles obtained in the solvent removal step from the aqueous medium.

[0125] (F-2) Drying Step The production method of the present invention preferably further includes a drying step. The drying step is a step of drying the hollow resin particles obtained by removing the hydrophobic organic solvent encapsulated in the precursor particles in the solvent removal step described above. When the production method of the present invention includes the washing step described above, the drying step may be a step of drying the washed hollow resin particles recovered in the washing step.

[0126] The drying method is not particularly limited as long as it can remove the aqueous medium, and examples of the drying method include reduced pressure drying, heat drying, flash drying, and combinations thereof.

[0127] When using a heat drying method, the drying conditions are not particularly limited as long as they allow removal of the aqueous medium. The drying temperature is not particularly limited, but is preferably 50 to 250°C, more preferably 75 to 220°C, and even more preferably 100 to 200°C. The drying time is not particularly limited, but is preferably 1 to 48 hours, more preferably 3 to 24 hours. The drying atmosphere is also not particularly limited, and can be appropriately selected depending on the application of the hollow resin particles. Examples of the drying atmosphere include air, oxygen, nitrogen, and argon.

[0128] (F-3) Hollow Portion Re-Substitution Process The hollow portion re-substitution process is a process of substituting the gas or liquid inside the hollow resin particles with another gas or liquid. This substitution can change the environment inside the hollow resin particles, selectively confine molecules inside the hollow resin particles, or modify the chemical structure inside the hollow resin particles according to the application.

[0129] <Hollow Resin Particles> The shape of the hollow resin particles obtained by the production method of the present invention is not particularly limited as long as a hollow portion is formed inside. The external shape of the hollow resin particles is not particularly limited, but a spherical shape is preferred for ease of production. Furthermore, the hollow resin particles obtained by the production method of the present invention may have one or more hollow portions, but preferably have only one hollow portion in order to maintain a good balance between high porosity and mechanical strength. The shell of the hollow resin particle and, when the hollow resin particles have two or more hollow portions, the partition walls separating adjacent hollow portions may be porous. The hollow resin particles obtained by the production method of the present invention preferably have an average circularity of 0.950 to 0.995.

[0130] The external shape of the hollow resin particles can be confirmed, for example, by observing the particles with an SEM or TEM, and the internal shape of the hollow resin particles can be confirmed, for example, by observing the cross section of the particles with an SEM or TEM.

[0131] The volume average particle diameter (Dv) of the hollow resin particles obtained by the production method of the present invention is preferably 1 to 50 μm, more preferably 2 to 50 μm, and even more preferably 3 to 50 μm. Furthermore, the particle size distribution (volume average particle diameter (Dv) / number average particle diameter (Dp)) is preferably 1.05 to 2.0, more preferably 1.05 to 1.5, and even more preferably 1.05 to 1.3. The volume average particle diameter (Dv) and number average particle diameter (Dp) of the hollow resin particles can be determined, for example, by measuring the particle diameter of the hollow resin particles using a laser diffraction particle size distribution analyzer, calculating the number average and volume average, respectively, and using the resulting values ​​as the number average particle diameter (Dp) and volume average particle diameter (Dv) of the particles. The particle size distribution is calculated by dividing the volume average particle diameter by the number average particle diameter.

[0132] The porosity of the hollow resin particles obtained by the production method of the present invention is preferably 50% or more, more preferably 50 to 95%, and even more preferably 55 to 90%. By setting the porosity within the above range, the hollow resin particles can be made to have excellent lightness, heat resistance, and heat insulation properties. The porosity (%) of the hollow resin particles is calculated based on the apparent density D of the hollow resin particles. １ and true density D ０ The porosity of the hollow resin particles can be expressed as the proportion of the hollow portion in the specific gravity of the hollow resin particles. Porosity (%) = 100 - (apparent density D １ / True density D ０ ) x 100

[0133] The hollow resin particles obtained by the production method of the present invention can be used, for example, as additives in components such as low dielectric materials, heat insulating materials, sound insulating materials, and light reflecting materials used in various fields such as automobiles, electricity, electronics, construction, aviation, and space, food containers, footwear such as sports shoes and sandals, home appliance parts, bicycle parts, stationery, tools, etc.

[0134] In particular, when the manufacturing method of the present invention includes one or both of the cleaning operation and the cleaning step during the solvent removal step, the amount of residual metal is reduced, and hollow resin particles with excellent electrical insulation can be produced with high productivity. The hollow resin particles thus obtained are suitable for use as additives for achieving low transmission loss in the electrical or electronic fields. For example, such hollow resin particles are suitable for use as materials for electronic circuit boards. Specifically, by incorporating the hollow resin particles into the insulating resin layer of an electronic circuit board, the transmission loss of the electronic circuit board can be reduced.

[0135] The hollow resin particles obtained by the production method of the present invention are also suitable for use as additives in semiconductor materials such as interlayer insulating materials, dry film resists, solder resists, bonding wires, bonding sheets, magnet wires, semiconductor encapsulants, epoxy encapsulants, mold underfills, underfills, die bond pastes, buffer coating materials, copper-clad laminates, flexible substrates, high-frequency device modules, antenna modules, and automotive radar. Among these, they are particularly suitable as additives in semiconductor materials such as interlayer insulating materials, solder resists, bonding sheets, magnet wires, epoxy encapsulants, underfills, buffer coating materials, copper-clad laminates, flexible substrates, high-frequency device modules, antenna modules, and automotive radar. The bonding sheet is an insulating adhesive layer-forming material used to bond a conductor layer and an organic insulating layer when producing a multilayer printed wiring board.

[0136] Furthermore, the hollow resin particles obtained by the production method of the present invention are suitable as additives for molded products because, when added to molded products, they have excellent effects as a weight-saving material, heat insulating material, soundproofing material, vibration damping material, light diffusing agent, etc. For example, they can be used as an additive for resin molded products, and can also be contained as a filler in fiber-reinforced molded products formed using resin and reinforcing fibers. Furthermore, when used as a light diffusing agent, the hollow resin particles obtained by the production method of the present invention are suitable as additives for light diffusing materials such as light diffusing films and light diffusing plates.

[0137] Furthermore, the hollow resin particles obtained by the production method of the present invention have high porosity, are difficult to crush, and have excellent heat resistance, so they satisfy the heat insulation and shock-absorbing properties (cushioning properties) required for undercoating materials and also satisfy the heat resistance required for thermal paper applications.Furthermore, the hollow resin particles obtained by the production method of the present invention are also useful as plastic pigments excellent in gloss, hiding power, etc.

[0138] Furthermore, the hollow resin particles obtained by the manufacturing method of the present invention can be used for various purposes depending on the components contained therein, since useful components such as fragrances, medicines, agricultural chemicals, and ink components can be encapsulated inside the hollow resin particles by means of immersion treatment, vacuum or pressure immersion treatment, etc.

[0139] The hollow resin particles obtained by the production method of the present invention are also suitable for use as rust inhibitors. The hollow resin particles obtained by the production method of the present invention are also useful as additives that reduce electrical conductivity. Therefore, for example, paints containing the hollow resin particles obtained by the production method of the present invention can be used as rust-preventive paints (paint primers, lubricating paints, etc.) for improving the corrosion and rust resistance of steel materials and the like. Furthermore, rust-preventive additives can also be encapsulated in the hollow resin particles added to the rust-preventive paints.

[0140] The hollow resin particles obtained by the production method of the present invention can be mixed with a resin or rubber to form a resin composition or a rubber composition.

[0141] The resin composition may be a liquid resin composition or a resin molded product. Examples of liquid resin compositions include those containing a liquid matrix resin before a curing reaction, those obtained by dissolving or dispersing each component in a solvent, and those in which the matrix resin is a thermoplastic resin and the resin is in a liquid state due to the resin being melted. Examples of resin molded products include those obtained by molding the above-mentioned liquid resin composition by a known method.

[0142] The matrix resin contained in the resin composition is not particularly limited and may be, for example, a thermosetting resin or a thermoplastic resin. The resin contained in the resin composition may be an unreacted monomer, a prepolymer, or a macromonomer, a polymer, or a precursor of a cured resin such as polyamic acid. The matrix resin contained in the resin composition may contain a thermoplastic elastomer as a resin. Furthermore, the resin composition may contain rubber.

[0143] The thermosetting resin may be any known one and is not particularly limited, but examples thereof include phenolic resins, melamine resins, urea resins, unsaturated polyester resins, epoxy resins, polyurethane resins, silicon resins, alkyd resins, thermosetting modified polyphenylene ether resins, thermosetting polyimide resins, benzoxazine resins, urea resins, allyl resins, aniline resins, maleimide resins, bismaleimide triazine resins, liquid crystalline polyester resins, vinyl ester resins, unsaturated polyester resins, cyanate ester resins, polyetherimide resins, etc. These thermosetting resins may be used alone or in combination of two or more.

[0144] The thermoplastic resin may be any known one, and is not particularly limited, but examples thereof include polyolefins such as polypropylene and polyethylene; polyamides such as PA6, PA66, and PA12; polyimide, polyamideimide, polyetherimide, polyetherketoneketone, polyvinyl chloride, polystyrene, poly(meth)acrylate, polycarbonate, polyvinylidene fluoride, acrylonitrile-butadiene-styrene copolymer (ABS), acrylonitrile-styrene copolymer (AS), polyphenylene ether, polyphenylene sulfide, polyester, polytetrafluoroethylene, thermoplastic elastomers, etc. These thermoplastic resins may be used either alone or in combination of two or more.

[0145] In applications requiring a low dielectric constant or a low dielectric loss tangent, insulating resins such as epoxy resins, thermosetting modified polyphenylene ether resins, thermosetting polyimide resins, silicon resins, benzoxazine resins, melamine resins, urea resins, allyl resins, phenol resins, unsaturated polyester resins, polyurethane resins, and aniline resins are preferably used as the resin, and among these, epoxy resins, thermosetting polyimide resins, modified polyphenylene ether resins, silicon resins, benzoxazine resins, and melamine resins are preferably used. These insulating resins can be used either alone or in combination of two or more.

[0146] As the thermoplastic elastomer, a thermoplastic elastic polymer that has conventionally been used as a molding resin can be used, such as a urethane-based elastomer, a styrene-based elastomer, an olefin-based elastomer, an amide-based elastomer, and an ester-based elastomer. A thermoplastic elastomer generally exhibits rubber elasticity at room temperature (25°C) and has the property of being plasticized and moldable at high temperatures. The thermoplastic elastomer may be used alone or in combination of two or more types.

[0147] The matrix rubber contained in the rubber composition and the rubber that may be contained in the resin composition are not particularly limited, and examples thereof include natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene copolymer rubber (SBR), acrylonitrile-butadiene copolymer rubber (NBR), ethylene-propylene-diene terpolymer (EPDM), etc. The rubbers may be used alone or in combination of two or more.

[0148] The resin content of the resin composition (100% by mass) is not particularly limited, but is preferably 50 to 95% by mass or less. When the resin content is within this range, the hollow resin particles can fully exert the effects of lowering the dielectric loss tangent, while also providing excellent moldability of the resin composition and mechanical strength of the resulting molded article.

[0149] The rubber content in 100% by mass of the total solid content of the rubber composition is not particularly limited, but is preferably 50 to 95% by mass or less. When the rubber content is within the above range, the hollow resin particles can fully exhibit the effects of lowering the dielectric tangent, etc., while the rubber composition can also have excellent moldability and rubber elasticity.

[0150] The resin composition or rubber composition may further contain additives such as a curing agent, curing catalyst, or initiator for promoting the curing reaction, depending on the type of resin or rubber. Examples of curing agents include amines, acid anhydrides, imidazoles, thiols, phenols, naphthols, benzoxazines, cyanate esters, and carbodiimides. The content of the curing agent is not particularly limited and may be, for example, 5 to 120 parts by mass per 100 parts by mass of the resin or rubber.

[0151] The content of the hollow resin particles obtained by the production method of the present invention in the resin composition or rubber composition is not particularly limited, but is preferably 5 to 50 mass %. When the content of the hollow resin particles is within this range, the effects of the hollow resin particles, such as low dielectric tangent, can be fully exhibited, and the moldability of the composition and the mechanical properties of the resulting molded article can also be excellent.

[0152] The resin composition or rubber composition may further contain additives such as a compatibilizer, an ultraviolet absorber, a colorant, a heat stabilizer, a filler, a solvent, etc., as needed, within a range that does not impair the effects of the present disclosure. In addition, the resin composition may further contain organic or inorganic fibers such as carbon fibers, glass fibers, aramid fibers, and polyethylene fibers.

[0153] The resin composition or rubber composition can be obtained, for example, by mixing the hollow resin particles obtained by the production method of the present invention with a resin or rubber, and further additives, solvents, etc. that are added as needed. For example, when the resin in the resin composition is a thermoplastic resin, the hollow resin particles and further additives that are added as needed can be added to a molten thermoplastic resin and mixed by melt kneading. Furthermore, for example, the resin composition can be a liquid resin composition, or a resin molded product obtained by molding the liquid resin composition by a known method.

[0154] The method for producing a resin composition from a liquid resin composition is not particularly limited. For example, a liquid resin composition obtained by incorporating hollow resin particles or the like into a liquid matrix resin before a curing reaction, or a liquid resin composition obtained by dissolving or dispersing each component in a solvent, can be applied to a support, and if necessary, dried and cured to obtain a resin molded body.

[0155] Examples of the material for the support include resins such as polyethylene terephthalate and polyethylene naphthalate, and metals such as copper, aluminum, nickel, chromium, gold, and silver. The surface of these supports may be coated with a release agent.

[0156] The liquid resin composition can be applied by any known method, such as dip coating, roll coating, curtain coating, die coating, slit coating, and gravure coating.

[0157] Alternatively, a resin molded product can be obtained by impregnating a substrate with the liquid resin composition, and then drying and curing the composition as necessary. Examples of the substrate include inorganic fibers such as carbon fiber, glass fiber, metal fiber, and ceramic fiber, and organic synthetic fibers such as polyamide fiber, polyester fiber, polyolefin fiber, and novoloid fiber. Glass fiber (glass cloth) is particularly preferred. The form of the substrate is not limited, and woven fabrics and nonwoven fabrics can be used.

[0158] When the liquid resin composition contains a solvent, it is preferable to dry the resin composition after the coating or impregnation. The drying temperature is preferably a temperature at which the matrix resin does not harden, and is usually 20° C. to 200° C., preferably 30° C. to 150° C. The drying time is usually 30 seconds to 1 hour, preferably 1 minute to 30 minutes.

[0159] The curing reaction of the resin composition is carried out by a method appropriate for the type of resin, and is not particularly limited. When a resin that cures upon heating is included, the heating temperature for the curing reaction is adjusted appropriately depending on the type of resin, and is not particularly limited, but is usually 30°C or higher and 400°C or lower, preferably 70°C or higher and 300°C or lower, and more preferably 100°C or higher and 200°C or lower. The curing time is 5 minutes to 5 hours, preferably 30 minutes to 3 hours. The heating method is not particularly limited, and may be carried out using, for example, an electric oven. The liquid resin before the curing reaction and the resin dissolved or dispersed in a solvent may be a thermosetting resin or a thermoplastic resin.

[0160] Alternatively, a resin molded body may be obtained by molding a liquid resin composition containing a thermoplastic resin as the resin and obtained by melting the resin into a desired shape by a known molding method such as extrusion molding, injection molding, press molding, or compression molding.

[0161] The shape of the resin molded body is not particularly limited and can be any moldable shape, such as a sheet, a film, a plate, a tube, or any other three-dimensional shape. When the resin molded body contains fibers, the fibers in the resin molded body may be in the form of a nonwoven fabric. When the resin molded body contains fibers, the resin molded body may be a molded body of a resin composition in which hollow resin particles obtained by the production method of the present invention are added to a fiber-reinforced plastic containing the resin and fibers as described above.

[0162] Examples of uses of the resin composition and rubber composition include the uses of the hollow resin particles obtained by the production method of the present invention described above, in which the resin composition and rubber composition can be used.

[0163] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. Note that "parts" and "%" are by weight unless otherwise specified. Various measurements were carried out according to the following methods.

[0164] <Method of Sampling from Upper Layer (A) and Lower Layer (B) After Floatation Separation> A preliminary test was conducted before floatation separation to estimate the height of the lower layer (B) after floatation separation. In the preliminary test, a small amount of sample was taken from the precursor composition that had been stirred and homogenized before floatation separation. The volume (V ０ After measuring the volume of the clarified liquid layer (V Ｂ The volume of the sample (V ０ The volume of the clear liquid layer (V Ｂ The ratio of V Ｂ ' / V ０ Separately, the height of the precursor composition before flotation separation (H ０ ) and measure the height (H ０ ) to the above ratio [V Ｂ ' / V ０ '] to obtain an estimate of the height of the lower layer (B) after flotation separation.

[0165] Next, after the flotation separation, samples were collected from inside the pressure vessel using a long-needle syringe. Samples were collected in small amounts at each of 10 equal height positions, dividing the water depth of the pressure vessel from the top to the bottom. Note that no samples were collected from the height position at the top of the pressure vessel where only gas was present. In addition, of the collected samples, three samples closest to the estimated height of the lower layer (B) obtained in the preliminary test were excluded. Then, of the remaining samples, the sum of the samples collected from the upper layer (A) side was used as the upper layer (A) sample, and the sum of the samples collected from the lower layer (B) was used as the lower layer (B) sample, and the following measurements were performed.

[0166] <Acquisition of Mass and Volume Data Groups for Upper Layer (A) and Lower Layer (B)> First, the sample of the upper layer (A) was divided into two, sample (A1) and sample (A2).

[0167] Mass (m Ａ ) was measured, and the volume of gas (V Ａｇ ), the volume of the aqueous medium (V Ａｗ ), and the volume of the precursor particles (V Ａｐ The bulk density of sample (A1), as well as the specific gravity of the aqueous medium, the specific gravity of the shell of the precursor particle, and the specific gravity of the hydrophobic organic solvent encapsulated in the precursor particle, were measured in advance.

[0168] First, sample (A1) was heated at 105°C for 2 hours to remove gas and volatile components, thereby obtaining the shell (solid content) of the precursor particle in sample (A1), and measuring its mass (m Ａｓ ) was measured. The mass (m Ａ -m Ａｓ ) was calculated, and the mass (m Ａｌ )

[0169] Next, the mass (m Ａｏ ) was determined by the following procedure. First, the mass of sample (A2) was measured. Next, 1 g of sample (A2) and 4 g of acetone as an extraction solvent were weighed into a 100 cc sample bottle, and ultrasonic treatment was performed for 5 minutes. The treated liquid was taken into a disposable syringe and filtered through a 0.50 μm membrane filter, and the filtered liquid was measured by gas chromatography. The mass of the hydrophobic organic solvent in sample (A2) was measured from a calibration curve of the hydrophobic organic solvent encapsulated in the precursor particles, which had been prepared in advance. Then, the ratio of the mass of the hydrophobic organic solvent in sample (A2) to the mass of sample (A2) was calculated, and this was taken as the mass ratio of the hydrophobic organic solvent in the upper layer (A). The mass ratio of the hydrophobic organic solvent in this upper layer (A) and the mass (m Ａ ) and the mass (m Ａｏ ) was calculated.

[0170] Furthermore, the mass (m Ａｌ) and the mass of the hydrophobic organic solvent (m Ａｏ ) and the mass (m Ａｗ From this and the specific gravity of the aqueous medium in sample (A1), the volume (V Ａｗ ) was calculated.

[0171] Furthermore, the mass (m Ａｏ ) and the specific gravity of the hydrophobic organic solvent measured separately, the volume (V Ａｏ In all examples, the specific gravity of the aqueous medium in sample (A1) was 1, and the specific gravity of the shell of the precursor particle was also about 1. Therefore, these specific gravities and the mass (m Ａ -m Ａｏ ) to calculate the volume (V Ａｗ ) and the shell volume of the precursor particle (V Ａｓ ) sum (V Ａｗ +V Ａｓ ) was calculated.

[0172] The sum of the volumes of the hydrophobic organic solvent, the aqueous medium, and the shells of the precursor particles in sample (A1) (V Ａｏ +V Ａｗ +V Ａｓ ) was calculated, and this was compared with the volume of the aqueous medium (V Ａｗ ) from the sum of the volumes of the hydrophobic organic solvent and the shells of the precursor particles in sample (A1) (V Ａｏ +V Ａｓ ), i.e., the volume (V) of the precursor particles in sample (A1) Ａｐ ) was calculated. In addition, the mass (m Ａ ) and the bulk density of sample (A1), the volume of sample (A1) was calculated, and the volume of the aqueous medium in sample (A1) (V Ａｗ ) and the volume of the precursor particles (V Ａｐ ) to find the volume of gas in the sample (A1) (V Ａｇ ) was calculated.

[0173] The sample of the lower layer (B) was also measured in the same manner as the upper layer (A). Based on the mass and volume data measured as above, the following values ​​were calculated: 1. Amount of gas in the upper layer (A) (volume of gas per 100 parts by volume of precursor particles in the upper layer (A)) 2. Volume of the aqueous medium in the upper layer (A) (V Ａｗ ) and the volume of the precursor particle (V Ａｐ ) to the total volume of the precursor particles (V Ａｐ ) ratio (particle ratio of upper layer (A)) [V Ａｐ / (V Ａｗ +V Ａｐ ) 3. The volume of the aqueous medium in the lower layer (B) (V Ｂｗ ) and the volume of the precursor particle (V Ｂｐ ) to the total volume of the precursor particles (V Ｂｐ ) ratio (particle ratio of lower layer (B)) [V Ｂｐ / (V Ｂｗ +V Ｂｐ ) 4. Average specific gravity of the lower layer (B) (D Ｂ ) the average specific gravity (D Ａ ) ratio (specific gravity ratio of upper layer (A) to lower layer (B)) [D Ａ / D Ｂ ]

[0174] In Example 1, the water depth of the pressure vessel was divided into 10 equal parts from the top to the bottom, and small amounts of samples were collected at each height. The ratio of the volume of the precursor particles to the total volume of the aqueous medium and the volume of the precursor particles was determined for each sample. All of the ratios were either sufficiently higher than 40% or sufficiently lower than 20%. Similarly, the ratios for each sample collected in Examples 2 to 12 were either sufficiently higher than 40% or sufficiently lower than 20%. From these results, it was determined that the volume ratio of the layer (boundary layer (C)) with a ratio greater than 20% and less than 40% was essentially 0% by volume in Examples 1 to 12.

[0175] <Volume Ratio of Upper Layer (A) and Lower Layer (B)> The volume ratio of the upper layer (A) and the lower layer (B) in the precursor composition after floatation separation was determined by the following method. First, the entire amount of the precursor composition after floatation separation was slowly poured into a measuring cylinder. Next, the volume of the precursor composition after floatation separation, the volume of the upper layer (A), and the volume of the lower layer (B) in the measuring cylinder were each visually read. Then, the volume ratio of the upper layer (A) and the lower layer (B) in the precursor composition after floatation separation was calculated.

[0176] <Foaming Suppression Level During Solvent Removal> The foaming suppression level during solvent removal was evaluated according to the following criteria. The smaller the amount of particles leaking outside the pressure vessel, the higher the foaming suppression level. Furthermore, the smaller the amount of particles leaking outside the pressure vessel, the higher the yield and productivity. Furthermore, from the viewpoint of being able to relatively increase the amount of hollow resin particles supplied to the solvent removal equipment and adopting removal conditions that promote the removal of the hydrophobic organic solvent, the smaller the amount of particles leaking outside the pressure vessel, the higher the productivity. A: Foaming was extremely well suppressed, and no particles leaked outside the pressure vessel during solvent removal. B: Foaming was well suppressed, and the amount of particles leaking outside the pressure vessel during solvent removal was more than 0 wt% and less than 0.1 wt% (in terms of the weight of the hollow resin particles obtained) of the total amount of particles in the system. C: Foaming was sufficiently suppressed, and the amount of particles leaking outside the pressure vessel during solvent removal was 0.1 wt% or more and less than 1 wt% (in terms of the weight of the hollow resin particles obtained) of the total amount of particles in the system. D: Foaming was not sufficiently suppressed, and the amount of particles leaking out of the pressure vessel during solvent removal was 1 wt% or more and less than 5 wt% (weight equivalent of the resulting hollow resin particles) of the total amount of particles in the system. E: Foaming was not sufficiently suppressed, and the amount of particles leaking out of the pressure vessel during solvent removal was 5 wt% or more and less than 10 wt% (weight equivalent of the resulting hollow resin particles) of the total amount of particles in the system. F: Foaming was not sufficiently suppressed, and the amount of particles leaking out of the pressure vessel during solvent removal was 10 wt% or more (weight equivalent of the resulting hollow resin particles) of the total amount of particles in the system.

[0177] <Level of Solvent Removal in Solvent Removal Step> At the end of the solvent removal step, a small amount of hollow resin particles floating on the lower layer (B) was collected, and the amount of solvent in the hollow resin particles was measured by headspace gas chromatography. The level of solvent removal in the solvent removal step was evaluated according to the following criteria: A: The amount of solvent was 0.01 g or less per 1 g of collected hollow resin particles. B: The amount of solvent was more than 0.01 g per 1 g of collected hollow resin particles.

[0178] <Level of residual monomer removal in solvent removal step> At the end of the solvent removal step, a small amount of hollow resin particles floating on the lower layer (B) was collected, and the amount of residual monomer in the hollow resin particles was measured by headspace gas chromatography. The level of residual monomer removal in the solvent removal step was evaluated according to the following criteria: A: The amount of residual monomer was 0.001 g or less per 1 g of collected hollow resin particles. B: The amount of residual monomer was more than 0.001 g per 1 g of collected hollow resin particles.

[0179] <Washing time in washing step> The number of times a series of treatments (addition of ion-exchanged water, elimination of agglomeration, and filtration) required until the electrical conductivity of the filtrate reached 10 μS / cm or less in the washing step was evaluated according to the following criteria. The fewer the number of times a series of treatments was required, the shorter the washing time in the washing step, and the better the productivity of hollow resin particles can be determined to be. A: 1 time or less. B: 2 to 4 times. C: 5 times or more.

[0180] <Residual Metal Amount in Hollow Resin Particles> 10 g of precisely weighed hollow resin particles were wet decomposed using a microwave (PerkinElmer, Multiwave 3000), and the resulting decomposition product was subjected to ICP emission analysis using an ICP emission analyzer (PerkinElmer, Optima 2100 DV) to measure the total mass of metals. The metal species were identified by elemental analysis using X-ray fluorescence analysis (XRF). The ratio of the total mass of metals in the decomposition product to the mass of the hollow resin particles was calculated, and this was taken as the metal content in the hollow resin particles.

[0181] <Moisture Content of Hollow Resin Particles> First, 10 μl of pure water was precisely weighed out using a microsyringe, and the amount of moisture (mg) per 1 ml of Karl Fischer reagent was calculated from the amount of reagent titration required to remove this water. Next, 100 to 200 mg of hollow resin particles were precisely weighed out and left in an environment of 30°C and 80% RH for 2 hours, after which they were thoroughly dispersed in a measuring flask using a magnetic stirrer for 5 minutes. Next, measurement was started using a Karl Fischer moisture meter (Kyoto Electronics Manufacturing Co., Ltd., MKA-3p), and the amount of Karl Fischer reagent titration required for titration (ml) was determined. The moisture amount and moisture content of the hollow resin particles were calculated using the following formula. The moisture content was then evaluated according to the following criteria: Moisture amount [mg] = Reagent consumption [ml] × Reagent titer [mgH ２ Water content [%] = (water amount [mg] / sample amount [mg]) x 100 A: The water content was less than 1%. B: The water content was 1% or more.

[0182] <Dielectric loss tangent (Df) of hollow resin particles> The dielectric loss tangent of the hollow resin particles was measured at a frequency of 1 GHz and room temperature (25° C.) using a measuring device (manufactured by AET Co., Ltd., model: ADMS01Nc). The lower the dielectric loss tangent, the better the electrical insulation properties.

[0183] <Porosity of hollow resin particles> Apparent density D of hollow resin particles １ and true density D ０ Then, the porosity (%) was calculated according to the following formula: Porosity (%) = 100 - (apparent density D １ / True density D ０ ) x 100

[0184] Example 1 (1) Mixture Preparation Step First, the following materials were mixed to prepare an oil phase: Divinylbenzene (crosslinkable hydrocarbon monomer) 37.5 parts Ethylvinylbenzene (monofunctional hydrocarbon monomer) 1.6 parts t-Butylperoxydiethyl acetate (oil-soluble polymerization initiator) 0.89 parts Heptane (hydrophobic organic solvent) 60.8 parts

[0185] Next, in a stirring tank, an aqueous solution of 11.0 parts of sodium hydroxide (alkali metal hydroxide) in 55 parts of ion-exchanged water was gradually added under stirring to an aqueous solution of 15.7 parts of magnesium chloride (a water-soluble polyvalent metal salt) in 225 parts of ion-exchanged water to prepare a magnesium hydroxide colloid (poorly water-soluble metal hydroxide colloid) dispersion (8 parts of magnesium hydroxide). The obtained aqueous dispersion of magnesium hydroxide was used as the aqueous phase. The obtained aqueous phase and oil phase were mixed to prepare a mixed liquid.

[0186] (2) Suspension step The mixture obtained in the mixture preparation step was subjected to a suspension treatment by stirring for 1 minute using an emulsifying disperser (manufactured by Primix Corporation, product name: Homomixer) at a rotation speed of 4,000 rpm, to prepare a suspension in which droplets of the polymerizable monomer composition containing a hydrophobic organic solvent were dispersed in water.

[0187] (3) Polymerization Step: The suspension obtained in the suspension step was supplied to a cylindrical pressure-resistant vessel. The temperature was then raised to 80°C in a nitrogen atmosphere, and the suspension was stirred for 24 hours at 80°C to carry out a polymerization reaction. This polymerization reaction yielded a precursor composition, which was a slurry liquid in which precursor particles encapsulating a hydrophobic organic solvent were dispersed in water. The polymerization conversion rate was approximately 100%, and the composition of the monomers used was roughly equivalent to the monomer composition of the formed resin. (This was also the case in Examples 2 to 12.)

[0188] (4) Flotation Separation Step The pressure vessel was cooled to 30°C. Next, compressed nitrogen was injected into the top of the pressure vessel to pressurize the pressure vessel to 0.3 MPa (gauge pressure), and this was maintained for 5 minutes. Thereafter, the gas at the top of the pressure vessel was discharged, and the pressure in the pressure vessel was reduced to 10 kPa (absolute pressure), and this was maintained for 5 minutes. This pressurization and decompression operation generated bubbles in the precursor composition, and the bubbles adhered to the outer surfaces of the precursor particles, and the bubbles and precursor particles floated together. As a result, the precursor composition separated into an upper layer (A) and a lower layer (B). The upper layer (A) was a foam layer containing water, precursor particles, and gas.

[0189] Various measurements were carried out according to the above methods. The amount of gas in the upper layer (A) was 41 parts by volume per 100 parts by volume of precursor particles in the upper layer (A). The particle ratio [V Ａｐ / (V Ａｗ +V Ａｐ The particle ratio of the lower layer (B) [V Ｂｐ / (V Ｂｗ +V Ｂｐ ) )] was 0 to 1%. (Note that in Examples 2 to 12, the particle ratio of the lower layer (B) [V Ｂｐ / (V Ｂｗ +V Ｂｐ ) The specific gravity ratio of the upper layer (A) to the lower layer (B) [D Ａ / D Ｂ The volume ratio of the upper layer (A) in the precursor composition after the floatation separation was 60 volume % and the volume ratio of the lower layer (B) in the precursor composition after the floatation separation was 40 volume %.

[0190] (5) Solvent Removal Process Compressed nitrogen is injected from the top of the pressure vessel to pressurize the pressure vessel, and the pressure vessel is heated to adjust the internal pressure of the pressure vessel to 0.9 MPa (gauge pressure). The temperature inside the pressure vessel is adjusted to the boiling point (T ｂ ) 2°C lower than the temperature (T ｂ The temperature was adjusted to −2°C. Next, while maintaining the internal pressure and temperature, nitrogen gas was injected into and discharged from the gas phase at the top of the pressure vessel, creating a nitrogen gas flow environment inside the pressure vessel. The flow rate of the nitrogen gas was adjusted to 1 / min of the volume (V ０ ) and equivalent (i.e., V ０ / min). Furthermore, a cleaning operation was performed in which ion-exchanged water (electrical conductivity: 1 μS / cm) was injected from the top of the pressure vessel and sprayed onto the upper layer (A), and the clear liquid was discharged from the bottom of the pressure vessel. The temperature of the ion-exchanged water was set to the same temperature as the temperature inside the pressure vessel. The injection rate of the ion-exchanged water and the discharge rate of the clear liquid were both set to the volume (V ０ ) and equivalent (i.e., V ０ / hour).

[0191] The above procedure was carried out continuously for 30 hours, yielding hollow resin particles from which the hydrophobic organic solvent had been removed, floating on the lower layer (B). At the end of the procedure, the clear liquid was collected from the bottom of the pressure vessel and its electrical conductivity was measured, revealing a value of 100 μS / cm. Using the above methods, the level of foam suppression during solvent removal, the level of removal of the hydrophobic organic solvent and residual monomers during the solvent removal process, and the yield of hollow resin particles at the end of the solvent removal process were evaluated. The results are shown in Table 1.

[0192] (6) Washing Process The pressure in the pressure vessel was released and the temperature was cooled to 25°C. Dilute sulfuric acid was added to the pressure vessel to adjust the pH to 5.5 or less, and washing was performed at 25°C for 10 minutes. All components in the pressure vessel were discharged, and water was removed by filtration to recover solids containing hollow resin particles. Ion-exchanged water was added to the recovered solids to form a reslurry. The slurry was stirred to disaggregate the hollow resin particles, and then filtered to separate the filtrate and the residue containing hollow resin particles. If the electrical conductivity of the filtrate exceeded 10 μS / cm, the series of processes consisting of adding ion-exchanged water, disaggregating the particles, and filtering was repeated. After each filtration, the electrical conductivity of the filtrate was measured, and the series of processes was terminated when the electrical conductivity of the filtrate reached 10 μS / cm or less. The washing time in the washing process was evaluated according to the method described above. The results are shown in Table 1.

[0193] (7) Drying Step: The filter cake containing hollow resin particles obtained in the washing step was pre-dried at 40°C. Next, the hollow resin particles were heated in a vacuum dryer at 200°C for 12 hours under vacuum conditions to obtain the hollow resin particles of Example 1. Observation of the obtained hollow resin particles using a scanning electron microscope confirmed that the hollow resin particles were spherical and had hollow portions. It was also confirmed that the shells of the hollow resin particles were not damaged. (Note that in each of the examples described below, it was also confirmed that the obtained hollow resin particles were spherical, had hollow portions, and had undamaged shells.) The residual metal content, moisture content, dielectric loss tangent (Df), and porosity of the hollow resin particles obtained in Example 1 were determined. The results are shown in Table 1.

[0194] <Examples 2 to 5> (4) Hollow resin particles were obtained in the same manner as in Example 1, except that the temperature, pressurization conditions, and decompression conditions in the flotation separation step were changed as shown in Table 1. In each step, measurements and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0195] Example 6 (5) Hollow resin particles were obtained in the same manner as in Example 1, except that the washing operation (spraying ion-exchanged water and discharging the clear liquid) was not performed in the solvent removal step. In each step, measurements and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0196] Example 7 (5) Hollow resin particles were obtained in the same manner as in Example 1, except that in the solvent removal step, nitrogen gas was not injected and the environment inside the pressure-resistant container was not made a nitrogen gas flow environment. Note that, in the solvent removal step, gas was discharged to maintain the internal pressure. Measurements and evaluations were carried out in each step in the same manner as in Example 1. The results are shown in Table 1.

[0197] Example 8 Hollow resin particles were obtained in the same manner as in Example 1, except that in the (5) solvent removal step, the pressure inside the pressure-resistant vessel was not pressurized (the gauge pressure was set to 0 MPa). In each step, measurements and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0198] Example 9: As in Example 1, (1) the mixed solution preparation step to (6) the washing step were carried out to obtain hollow resin particles as a filtrate. The residual metal amount and moisture content of the obtained hollow resin particles were evaluated according to the above methods. The results are shown in Table 1.

[0199] Example 10 Similar to Example 1, hollow resin particles from which the hydrophobic organic solvent had been removed were obtained in a floating state on the lower layer (B) by performing the steps (1) Mixture Preparation Step to (5) Solvent Removal Step. The hollow resin particles and the aqueous medium in the pressure-resistant container were stirred and mixed to obtain a slurry of hollow resin particles. The obtained slurry of hollow resin particles was then evaluated for moisture content according to the method described above. The obtained slurry of hollow resin particles was also dried at 105°C for 2 hours to obtain hollow resin particles after drying. The amount of residual metal was evaluated according to the method described above. The results are shown in Table 1.

[0200] Example 11 (1) Hollow resin particles were obtained in the same manner as in Example 1, except that in the mixed solution preparation step, the following materials were mixed to prepare an oil phase. In each step, measurements and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1. Divinylbenzene (crosslinkable hydrocarbon monomer) 24.6 parts Ethylvinylbenzene (monofunctional hydrocarbon monomer) 14.5 parts t-Butylperoxydiethyl acetate (oil-soluble polymerization initiator) 0.89 parts Heptane (hydrophobic organic solvent) 60.8 parts

[0201] Example 12 Hollow resin particles were obtained in the same manner as in Example 1, except that (1) in the mixed solution preparation step, the following materials were mixed to prepare an oil phase. In each step, measurements and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1. Ethylene glycol dimethacrylate (heteroatom-containing crosslinkable monomer) 25 parts Trimethylolpropane trimethacrylate (heteroatom-containing crosslinkable monomer) 30 parts Divinylbenzene (crosslinkable hydrocarbon monomer) 26 parts Ethylvinylbenzene (monofunctional hydrocarbon monomer) 19 parts 2,2'-Azobis(2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator) 3 parts Hexane (hydrophobic organic solvent) 100 parts

[0202] Comparative Example 1 A precursor composition, which was a slurry liquid in which precursor particles encapsulating a hydrophobic organic solvent were dispersed in water, was obtained by carrying out the steps (1) mixed solution preparation to (3) polymerization in the same manner as in Example 1. Next, the solvent removal step (5) was carried out without carrying out the floatation separation step (4).

[0203] (5) The solvent removal step was specifically carried out by the following procedure: Compressed nitrogen was injected from the top of the pressure vessel to pressurize the inside of the pressure vessel, and the inside of the pressure vessel was heated to adjust the internal pressure of the pressure vessel to 0.1 MPa (gauge pressure), and the temperature inside the pressure vessel was adjusted to the boiling point (T ｂ ) 2°C lower than the temperature (T ｂThe temperature was adjusted to −2°C. Next, while maintaining the internal pressure and temperature, nitrogen gas was injected into and discharged from the gas phase at the top of the pressure vessel, creating a nitrogen gas flow environment inside the pressure vessel. The flow rate of the nitrogen gas was adjusted to 1 / 2 sq. m / min of the precursor composition (volume: V ０ ) and the volume (i.e., V ０ The above operation was carried out continuously for 30 hours to obtain a slurry containing hollow resin particles from which the hydrophobic organic solvent had been removed.

[0204] Then, except for using the obtained slurry, (6) the washing step and (7) the washing step were carried out in the same manner as in Example 1. In each step, measurements and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0205] <Comparative Examples 2 and 3> (5) Hollow resin particles were obtained in the same manner as in Comparative Example 1, except that the pressurization conditions in the solvent removal step were changed as shown in Table 1. In each step, measurements and evaluations were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0206]

[0207] As is clear from Table 1, the production method including the solvent removal step of removing the hydrophobic organic solvent contained in the precursor particles while the precursor particles are floated and separated can suppress foaming during the removal of the hydrophobic organic solvent, and can produce hollow resin particles with high productivity (Examples 1 to 12).

[0208] On the other hand, when the hydrophobic organic solvent contained in the precursor particles was removed from a slurry liquid in which the precursor particles were dispersed in water without being floated and separated, excessive foaming occurred during the removal of the hydrophobic organic solvent, and hollow resin particles could not be produced with high productivity (Comparative Examples 1 to 3).

Claims

1. A method for producing hollow resin particles having a shell containing a resin and a hollow portion surrounded by the shell, comprising: a mixed solution preparation step of preparing a mixed solution containing a polymerizable monomer, a hydrophobic organic solvent, a polymerization initiator, and an aqueous medium; a suspension step of suspending the mixed solution to prepare a suspension in which droplets of a polymerizable monomer composition containing the polymerizable monomer, the hydrophobic organic solvent, and the polymerization initiator are dispersed in the aqueous medium; a polymerization step of subjecting the suspension to a polymerization reaction to prepare a precursor composition containing precursor particles having hollow portions and containing the hydrophobic organic solvent in the hollow portions; a floatation separation step of floating and separating the precursor particles in the precursor composition; and a solvent removal step of removing the hydrophobic organic solvent contained in the precursor particles while the precursor particles are in a floatation and separation state.

2. The method for producing hollow resin particles according to claim 1, further comprising a washing step of washing the hollow resin particles obtained in the solvent removal step from which the hydrophobic organic solvent contained in the precursor particles has been removed.

3. A method for producing hollow resin particles as described in claim 1 or 2, further comprising a drying step of performing a drying process on the hollow resin particles obtained by removing the hydrophobic organic solvent contained in the precursor particles obtained in the solvent removal step.

4. A method for producing hollow resin particles according to any one of claims 1 to 3, wherein in the floatation and separation step, the precursor composition is pressurized and then depressurized from the pressurized condition to float the precursor particles.

5. A method for producing hollow resin particles according to any one of claims 1 to 4, wherein in the solvent removal step, the hydrophobic organic solvent contained in the precursor particles is removed in a pressurized environment of 0.1 to 2.0 MPa in gauge pressure.

6. The method for producing hollow resin particles according to any one of claims 1 to 5, wherein in the solvent removal step, the hydrophobic organic solvent contained in the precursor particles is removed under a gas stream.

7. A method for producing hollow resin particles described in any one of claims 1 to 6, wherein the floatation separation process is a process for floating the precursor particles to separate them into an upper layer (A) and a lower layer (B) having a lower content of precursor particles than the upper layer (A).

8. The method for producing hollow resin particles according to claim 7, wherein the upper layer (A) contains 0.1 to 70 parts by volume of gas per 100 parts by volume of the precursor particles in the upper layer (A).

9. The volume (V Ａｗ ) and the volume of the precursor particle (V Ａｐ ) to the total volume of the precursor particles (V Ａｐ ) ratio [V Ａｐ / (V Ａｗ +V Ａｐ )] is 40% or more, and the volume (V Ｂｗ ) and the volume of the precursor particle (V Ｂｐ ) to the total volume of the precursor particles (V Ｂｐ ) ratio [V Ｂｐ / (V Ｂｗ +V Ｂｐ 9. The method for producing hollow resin particles according to claim 7, wherein the ratio of the total mass of the hollow resin particles to the total mass of the resin particles is 20% or less.

10. Average specific gravity (D Ｂ ) the average specific gravity (D Ａ ) ratio [D Ａ / D Ｂ 10. The method for producing hollow resin particles according to claim 7, wherein the value of is greater than 0.3 and less than 0.

97.

11. A method for producing hollow resin particles according to any one of claims 7 to 10, wherein in the solvent removal step, when removing the hydrophobic organic solvent contained in the precursor particles, an aqueous medium having an electrical conductivity of 2 μS / cm or less is added to the upper layer (A) while discharging the aqueous medium from the lower layer (B).

12. The method for producing hollow resin particles according to claim 4, wherein the pressure condition in the flotation separation step is a gauge pressure of 0.1 to 2.0 MPa.

13. A method for producing hollow resin particles according to any one of claims 1 to 12, wherein the proportion of the crosslinkable monomer in 100% by mass of the polymerizable monomer is 40% by mass or more and 100% by mass or less.

14. The method for producing hollow resin particles according to any one of claims 1 to 13, wherein the mixed liquid further contains a dispersion stabilizer.

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