Method for producing hollow particles

Abstract

Provided is a method for producing a hollow particle which has a high porosity and which is unlikely to collapse. The present invention is a method for producing a hollow particle which has a porosity of 50% or more and which comprises: a shell that contains a resin; and a hollow portion that is surrounded by the shell. This production method includes: a step for preparing a mixed liquid containing a first polymerizable monomer, a hydrocarbon-based solvent, a dispersion stabilizer and an aqueous medium; a step for suspending this mixed liquid so as to prepare a suspension in which droplets of a monomer composition containing the first polymerizable monomer and the hydrocarbon-based solvent are dispersed in the aqueous medium; and a step for subjecting the suspension to a polymerization reaction. The content of a crosslinkable monomer is 75-100 parts by mass relative to 100 parts by mass of the first polymerizable monomer. At a point where the polymerization conversion rate of the first polymerizable monomer reaches 93 mass% or more in the step for subjecting the suspension to a polymerization reaction, a second polymerizable monomer, which has a solubility in distilled water of 0.3 g / L or more at 20°C, is added and the suspension is subjected to a further polymerization reaction.

Description

[Technical Field]

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

[0002] Hollow particles (hollow resin particles) are particles that have a cavity inside them, and compared with solid particles whose interiors are substantially filled with resin, hollow particles scatter light well and have low light transmittance. Therefore, they are widely used as organic pigments or hiding agents with excellent optical properties such as opacity and whiteness in applications such as water-based paints and paper coating compositions, and are also used as weight-reducing agents and heat-insulating agents in resins and paints used in various fields such as automobiles, electricity, electronics, and construction.

[0003] In order to improve the effects of various compositions and molded articles containing hollow particles, such as weight reduction, heat insulation, opacity, and whitening, it is desirable for the hollow particles to maintain a high porosity during mixing with other materials and molding after mixing. However, increasing the porosity of hollow particles reduces the shell thickness of the hollow particles, making them more susceptible to crushing. Therefore, hollow particles that have a high porosity and are less likely to be crushed are desired.

[0004] Patent Document 1 discloses a method for producing hollow resin particles, which comprises dispersing a mixed solution containing a monomer mixture consisting of 20 to 70 parts by weight of a polyfunctional monomer having two or more ethylenically unsaturated groups and 80 to 30 parts by weight of a monofunctional monomer, a non-reactive organic solvent, and a non-crosslinkable polymer having a weight-average molecular weight of 10,000 to 1,000,000 in terms of polystyrene, in an aqueous solution containing a dispersion stabilizer or a surfactant, followed by polymerization. Patent Document 1 also describes that this production method provides small-sized hollow resin particles with few pinholes and little collapse.

[0005] Patent Document 2 discloses hollow resin particles having a single hollow surrounded by a shell, the hollow resin particles having a thermal decomposition onset temperature of 350°C or higher, the shell having fine throughholes with diameters in the range of 10 to 50 nm, and a thickness ratio of 0.03 to 0.25 relative to the average primary particle diameter of the hollow resin particles. Patent Document 2 also describes that the hollow resin particles are produced by dispersing a mixed solution containing a polyfunctional monomer and a non-reactive solvent in an aqueous solution, and then polymerizing the polyfunctional monomer.

[0006] Patent Document 3 discloses a method for producing hollow polymeric microparticles consisting of a single-layer shell and a hollow portion, which comprises dispersing a mixture consisting of (i) at least one crosslinkable monomer (B) or a mixture of at least one crosslinkable monomer (B) and at least one monofunctional monomer (B'), (ii) an initiator (C), and (iii) a poorly water-soluble solvent (D) that has low compatibility with a polymer or copolymer obtained from at least one crosslinkable monomer (B) or a copolymer of at least one crosslinkable monomer (B) and at least one monofunctional monomer (B') in an aqueous solution of a dispersion stabilizer (A), and then carrying out suspension polymerization. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-68037 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-190980 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-80503 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the hollow resin particles described in Patent Document 1 have the problem that they are easily crushed when mixing the hollow resin particles with a resin to prepare a paint or molding material, or when using a molding material containing the hollow resin particles to produce a molded product, because they cannot sufficiently withstand the shear and pressure that occur during twin-screw kneading or injection molding.

[0009] Like Patent Document 1, the hollow resin particles described in Patent Document 2 also have the problem of being easily crushed because they cannot sufficiently withstand the shear and pressure that occur during twin-screw kneading and injection molding. Furthermore, the hollow resin particles described in Patent Document 2 have fine through-holes, which causes a problem of resin penetrating into the interior of the particles during injection molding of a molding resin composition containing the hollow resin particles. While the fine through-holes of hollow resin particles can impart beneficial functions to the hollow resin particles, they also represent missing shells, which can reduce the strength of the hollow resin particles and make them more susceptible to crushing.

[0010] Although the hollow resin particles described in Patent Document 3 are less likely to be crushed than the hollow resin particles described in Patent Documents 1 and 2, they have the problem of being deformed by shear and pressure during twin-screw kneading and injection molding, resulting in a decrease in porosity.

[0011] An object of the present disclosure is to provide a method for producing hollow particles that have a high porosity and are resistant to crushing. [Means for solving the problem]

[0012] The present inventors have found that, in a method for obtaining hollow particles by suspension polymerization, in order to produce hollow particles having a high porosity and being resistant to crushing, it is effective to add a second polymerizable monomer, which is a hydrophilic monomer, and continue the polymerization reaction when the polymerization conversion rate of a first polymerizable monomer containing a large amount of a crosslinkable monomer reaches a specific value or more.

[0013] The present disclosure provides a method for producing hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, the hollow portion having a porosity of 50% or more, the method comprising: preparing a mixed liquid containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium; a step of suspending the mixed liquid to prepare a suspension in which droplets of a monomer composition containing the first polymerizable monomer and the hydrocarbon solvent are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction, the mixed solution contains a crosslinkable monomer as the first polymerizable monomer, and the content of the crosslinkable monomer in 100 parts by mass of the first polymerizable monomer is 75 to 100 parts by mass; In the step of subjecting the suspension to a polymerization reaction, when the polymerization conversion rate of the first polymerizable monomer reaches 93 mass% or more, a second polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C is added, and the suspension is further subjected to a polymerization reaction.

[0014] In the method for producing hollow particles according to the present disclosure, in the step of subjecting the suspension to a polymerization reaction, the amount of the second polymerizable monomer added is preferably 3 to 15 parts by mass relative to 100 parts by mass of the first polymerizable monomer.

[0015] In the method for producing hollow particles according to the present disclosure, the first polymerizable monomer preferably contains, as the crosslinkable monomer, a bifunctional crosslinkable monomer and a tri- or higher functional crosslinkable monomer.

[0016] In the method for producing hollow particles according to the present disclosure, it is preferable that the first polymerizable monomer contains, as the crosslinkable monomer, a trifunctional or higher crosslinkable monomer having three or more polymerizable functional groups, and that the content of the trifunctional or higher crosslinkable monomer in 100 parts by mass of the first polymerizable monomer is 5 to 50 parts by mass.

[0017] In the method for producing hollow particles according to the present disclosure, it is preferable that the first polymerizable monomer contains, as the crosslinkable monomer, at least one bifunctional crosslinkable monomer selected from the group consisting of divinylbenzene, ethylene glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate.

[0018] In the method for producing hollow particles according to the present disclosure, the first polymerizable monomer preferably contains, as the crosslinkable monomer, at least one trifunctional or higher crosslinkable monomer selected from the group consisting of pentaerythritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, and dipentaerythritol poly(meth)acrylate.

[0019] In the method for producing hollow particles according to the present disclosure, the mixed solution preferably contains at least one selected from the group consisting of rosin acid, higher fatty acid, and metal salts thereof.

[0020] In the method for producing hollow particles according to the present disclosure, the dispersion stabilizer is preferably an inorganic dispersion stabilizer, and more preferably a poorly water-soluble metal salt. [Effects of the Invention]

[0021] According to the manufacturing method of the present disclosure as described above, hollow particles having a high porosity and being resistant to crushing can be manufactured, and in particular, hollow particles that are resistant to crushing when kneaded with other materials such as resin can be manufactured. [Brief explanation of the drawings]

[0022] [Figure 1] 1A to 1C are diagrams illustrating an example of a manufacturing method according to the present disclosure. [Figure 2] FIG. 2 is a schematic diagram illustrating an embodiment of a suspension in a suspension step. DETAILED DESCRIPTION OF THE INVENTION

[0023] In the present disclosure, the use of "to" in a numerical range means that the numerical values ​​before and after it are included as the lower limit and upper limit. In the present disclosure, (meth)acrylate refers to each of acrylate and methacrylate, (meth)acrylic refers to each of acrylic and methacrylic, and (meth)acryloyl refers to each of acryloyl and methacryloyl. In the present disclosure, a polymerizable monomer is a compound having a functional group capable of addition polymerization (sometimes simply referred to as a polymerizable functional group in the present disclosure). In the present disclosure, a compound having an ethylenically unsaturated bond as the functional group capable of addition polymerization is generally used as the polymerizable monomer. Polymerizable monomers are classified into non-crosslinkable monomers and crosslinkable monomers. Non-crosslinkable monomers have only one polymerizable functional group, while crosslinkable monomers have two or more polymerizable functional groups and form crosslinks in the resin through a polymerization reaction. In addition, in the present disclosure, a polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C is referred to as a hydrophilic monomer, and a polymerizable monomer having a solubility of less than 0.3 g / L in distilled water at 20°C is referred to as a non-hydrophilic monomer.

[0024] The hollow particles obtained by the production method of the present disclosure are particles having a shell (outer shell) containing a resin and a hollow portion surrounded by the shell. In the present disclosure, the hollow portion is a hollow space that is clearly distinguishable from the shell of the hollow particle formed from a resin material. The shell of the hollow particle may have a porous structure, but in that case, the hollow portion has a size that allows it to be clearly distinguished from the numerous minute spaces uniformly dispersed within the porous structure. The hollow portion of the hollow particles can be confirmed by, for example, SEM observation of the cross section of the particles or TEM observation of the particles as they are. The hollow portion of the hollow particles may be filled with a gas such as air, may be in a vacuum or reduced pressure state, or may contain a solvent. Hereinafter, the method for producing hollow particles of the present disclosure and the hollow particles obtained by the production method of the present disclosure will be described in detail.

[0025] 1. Manufacturing method of hollow particles The method for producing hollow particles according to the present disclosure is a method for producing hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, the hollow particles having a porosity of 50% or more, the method comprising: preparing a mixed liquid containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium; a step of suspending the mixed liquid to prepare a suspension in which droplets of a monomer composition containing the first polymerizable monomer and the hydrocarbon solvent are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction, the mixed solution contains a crosslinkable monomer as the first polymerizable monomer, and the content of the crosslinkable monomer in 100 parts by mass of the first polymerizable monomer is 75 to 100 parts by mass; In the step of subjecting the suspension to a polymerization reaction, when the polymerization conversion rate of the first polymerizable monomer reaches 93 mass% or more, a second polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C is added, and the suspension is further subjected to a polymerization reaction.

[0026] The method for producing hollow particles according to the present disclosure follows a basic technique in which a mixed liquid containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium is suspended, whereby the first polymerizable monomer and the hydrocarbon solvent undergo phase separation, thereby preparing a suspension in which droplets having a distribution structure in which the first polymerizable monomer is unevenly distributed on the surface side and the hydrocarbon solvent is unevenly distributed in the center are dispersed in the aqueous medium, and this suspension is subjected to a polymerization reaction to harden the surfaces of the droplets, thereby forming hollow particles having hollow portions filled with the hydrocarbon solvent. In this basic technique, when the polymerization conversion rate of the first polymerizable monomer containing a specific amount or more of a crosslinkable monomer reaches 93 mass% or more in the step of subjecting the suspension to a polymerization reaction, a second polymerizable monomer, which is a hydrophilic monomer, is added and the suspension is further subjected to a polymerization reaction, thereby producing hollow particles that are resistant to crushing even when the porosity is high. By using a large amount of crosslinkable monomer as the polymerizable monomer used to form the shell of hollow particles, the content of crosslinkable monomer units in the shell increases, and a covalent bond network is densely spread throughout the shell, resulting in the formation of a shell that is strong, resistant to crushing, and resistant to deformation even when exposed to external heat, etc. However, using a large amount of crosslinkable monomer tends to leave unreacted polymerizable functional groups in the shell. The more unreacted polymerizable functional groups remain, the coarser the crosslinked structure of the shell becomes, and the lower the shell strength tends to be. Therefore, in hollow particles obtained by conventional manufacturing methods, the presence of unreacted polymerizable functional groups is thought to be one of the factors that lower the shell strength. In contrast, in the production method of the present disclosure, a suspension in which droplets of a monomer composition containing a first polymerizable monomer containing a large amount of a crosslinkable monomer are dispersed in an aqueous medium is subjected to a polymerization reaction, and the first polymerization reaction is carried out until the polymerization conversion rate of the first polymerizable monomer reaches 93 mass% or more. Thereafter, a second polymerizable monomer, which is a hydrophilic monomer, is added and a second polymerization reaction is further carried out, which is thought to improve the reaction rate of the entire polymerizable monomers including the first polymerizable monomer and the second polymerizable monomer. In the present disclosure, particles obtained by the first polymerization reaction and having a shell containing a polymer of a first polymerizable monomer and a hollow portion filled with a hydrocarbon-based solvent may be referred to as first precursor particles, and a composition containing the first precursor particles may be referred to as a first precursor composition. Furthermore, particles obtained by the second polymerization reaction and having a shell containing a polymer of a first polymerizable monomer and a second polymerizable monomer and a hollow portion filled with a hydrocarbon-based solvent may be considered as an intermediate of hollow particles whose hollow portions are filled with gas and may be referred to as second precursor particles, and a composition containing the second precursor particles may be referred to as a second precursor composition. In the manufacturing method of the present disclosure, the second polymerizable monomer has a solubility in distilled water at 20°C equal to or greater than the specific value, and thus is easily incorporated into the shell of the first precursor particles when added to the first precursor composition. The second polymerizable monomer, a hydrophilic monomer, has affinity for both the first polymerizable monomer and the aqueous medium. Therefore, when added to the first precursor composition, it is believed to be incorporated into the shell formed by the first polymerizable monomer and promote the thermal motion of the shell. During the second polymerization reaction, the second polymerizable monomer is incorporated into the shell formed by the first polymerizable monomer, and the thermal motion of the shell is promoted while the polymerization reaction proceeds. This results in a high reaction rate, and the polymerization reaction of the second polymerizable monomer incorporated into the shell and the polymerizable functional groups of the remaining unreacted first polymerizable monomer proceeds sufficiently, resulting in a dense crosslinked structure and the formation of a shell with excellent strength.

[0027] The method for producing hollow particles according to the present disclosure includes a step of preparing a mixed solution, a step of preparing a suspension, and a step of subjecting the suspension to a polymerization reaction, and may further include other steps. Furthermore, as far as technically possible, two or more of the above steps and other additional steps may be performed simultaneously as a single step, or the order of the steps may be reversed. For example, the preparation of the mixed solution and the suspension may be performed simultaneously in a single process, such as by adding the materials for preparing the mixed solution and suspending them at the same time.

[0028] A preferred example of the method for producing hollow particles according to the present disclosure includes the following steps. (1) Mixed liquid preparation process a step of preparing a mixed liquid containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium; (2) Suspension process a step of suspending the mixed liquid to prepare a suspension in which droplets of a monomer composition containing a first polymerizable monomer and a hydrocarbon solvent are dispersed in an aqueous medium; (3) Polymerization process (3-1) First polymerization step a step of performing a first polymerization reaction in which the suspension is subjected to a polymerization reaction until a polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, thereby preparing a first precursor composition containing first precursor particles having a shell containing a polymer of the first polymerizable monomer and a hollow portion filled with a hydrocarbon-based solvent. (3-2) Second polymerization step a step of adding a second polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C to the first precursor composition and carrying out a second polymerization reaction, thereby preparing a second precursor composition containing second precursor particles having a shell containing a polymer of the first polymerizable monomer and the second polymerizable monomer and a hollow portion filled with a hydrocarbon-based solvent; (4) Solid-liquid separation process A step of obtaining second precursor particles having hollow portions containing a hydrocarbon solvent by solid-liquid separation of the second precursor composition; and (5) Solvent removal process a step of removing the hydrocarbon solvent contained in the second precursor particles obtained by the solid-liquid separation step to obtain hollow particles;

[0029] FIG. 1 is a schematic diagram showing an example of the manufacturing method of the present disclosure. (1) to (5) in FIG. 1 correspond to the above-mentioned steps (1) to (5). The white arrows between the figures indicate the order of the steps. Note that FIG. 1 is merely a schematic diagram for explanatory purposes, and the manufacturing method of the present disclosure is not limited to those shown in the figure. Furthermore, the structure, dimensions, and shape of the materials used in each manufacturing method of the present disclosure are not limited to the structure, dimensions, and shape of the various materials in these figures. 1(1) is a cross-sectional schematic diagram showing one embodiment of a mixed solution in the mixed solution preparation step. As shown in this figure, the mixed solution contains an aqueous medium 1 and a low-polarity material 2 dispersed in the aqueous medium 1. Here, the low-polarity material 2 refers to a material that has low polarity and is difficult to mix with the aqueous medium 1. In the present disclosure, the low-polarity material 2 contains a first polymerizable monomer and a hydrocarbon solvent. FIG. 1 (2) is a cross-sectional schematic diagram showing one embodiment of a suspension in a suspending step. The suspension includes an aqueous medium 1 and droplets 10 of a monomer composition dispersed in the aqueous medium 1. The droplets 10 of the monomer composition contain a first polymerizable monomer and a hydrocarbon solvent, but the distribution within the droplets is not uniform. The droplets 10 of the monomer composition are phase-separated into a hydrocarbon solvent 4a and a material other than the hydrocarbon solvent, including the first polymerizable monomer, 4b, with the hydrocarbon solvent 4a unevenly distributed in the center and the material other than the hydrocarbon solvent 4b unevenly distributed on the surface, and a dispersion stabilizer (not shown) attached to the surface. 1(3) is a cross-sectional schematic diagram showing one embodiment of a composition (second precursor composition) containing hollow particles (second precursor particles) encapsulating a hydrocarbon solvent in their hollow spaces, obtained by a polymerization step. The composition contains an aqueous medium 1 and hollow particles (second precursor particles) 20 encapsulating a hydrocarbon solvent 4a in their hollow spaces, dispersed in the aqueous medium 1. A shell 6 forming the outer surface of the second precursor particle 20 is formed by polymerization of a first polymerizable monomer in droplets 10 of the monomer composition and polymerization of a second polymerizable monomer that is added later. Fig. 1(4) is a cross-sectional schematic diagram showing one embodiment of hollow particles (second precursor particles) containing a hydrocarbon solvent in the hollow portion after the solid-liquid separation step, which shows a state in which the aqueous medium 1 has been removed from the state shown in Fig. 1(3). Fig. 1 (5) is a cross-sectional schematic diagram showing one embodiment of hollow particles after the solvent removal step. Fig. 1 (5) shows a state in which the hydrocarbon solvent 4a has been removed from the state shown in Fig. 1 (4). By removing the hydrocarbon solvent from hollow particles (second precursor particles) containing the hydrocarbon solvent in their hollow portions, hollow particles 100 having gas-filled hollow portions 8 inside their shells 6 are obtained. The above five steps and other steps will be explained below in order.

[0030] (1) Mixed liquid preparation process This step is a step of preparing a mixed liquid containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium. The mixed solution preferably further contains a particle size control agent. The mixed solution preferably contains an oil-soluble polymerization initiator as a polymerization initiator. The mixed solution may further contain other materials such as a suspension stabilizer, as long as the effects of the present disclosure are not impaired. The materials of the mixed liquid will be explained in the following order: (A) first polymerizable monomer, (B) particle size control agent, (C) oil-soluble polymerization initiator, (D) hydrocarbon solvent, (E) dispersion stabilizer, and (F) aqueous medium.

[0031] (A) First Polymerizable Monomer The first polymerizable monomer includes at least a crosslinkable monomer, and may further include a non-crosslinkable monomer within a range that does not impair the effects of the present disclosure. As the first polymerizable monomer, a (meth)acrylic polymerizable monomer having a (meth)acryloyl group as a polymerizable functional group can be preferably used, since the polymerization reaction is likely to be stable and hollow particles having high heat resistance can be obtained.

[0032] [Crosslinking monomer] Since the crosslinkable monomer has a plurality of polymerizable functional groups, the monomers can be linked together, thereby increasing the crosslink density of the shell. Examples of the crosslinkable monomer include difunctional crosslinkable monomers having two polymerizable functional groups, such as divinylbenzene, divinyldiphenyl, divinylnaphthalene, diallyl phthalate, allyl (meth)acrylate, vinyl (meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, and 2-hydroxy-3-(meth)acrylpropyl (meth)acrylate; and trifunctional or higher crosslinkable monomers having three or more polymerizable functional groups, such as trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol poly(meth)acrylate, and ethoxylated versions of these. These crosslinkable monomers can be used either alone or in combination of two or more. Among these crosslinkable monomers, examples of hydrophilic crosslinkable monomers having a solubility of 0.3 g / L or more in distilled water at 20°C include ethylene glycol dimethacrylate, diethylene glycol diacrylate, allyl methacrylate, vinyl methacrylate, and 2-hydroxy-3-methacrylpropyl acrylate. The crosslinkable monomer contained in the first polymerizable monomer is not particularly limited and may be a hydrophilic crosslinkable monomer having a solubility in distilled water at 20°C of 0.3 g / L or more, or a non-hydrophilic crosslinkable monomer having a solubility of less than 0.3 g / L.

[0033] The first polymerizable monomer preferably contains at least a bifunctional crosslinkable monomer as a crosslinkable monomer, and more preferably contains a combination of a bifunctional crosslinkable monomer and a trifunctional or higher functional crosslinkable monomer. When the first polymerizable monomer contains a trifunctional or higher functional crosslinkable monomer, it is advantageous in that a covalent bond network can be more densely spread throughout the shell, but unreacted polymerizable functional groups tend to remain after the first polymerization reaction. In the production method of the present disclosure, even when the first polymerizable monomer contains a trifunctional or higher functional crosslinkable monomer, adding a hydrophilic monomer as the second polymerizable monomer and performing the second polymerization reaction facilitates the polymerization reaction of the unreacted polymerizable functional groups remaining after the first polymerization reaction. Therefore, when the first polymerizable monomer contains a trifunctional or higher functional crosslinkable monomer, the crosslinked structure of the shell becomes denser, improving the strength of the hollow particles and making them less susceptible to crushing. As the bifunctional crosslinkable monomer, divinylbenzene, ethylene glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate are preferred, and ethylene glycol di(meth)acrylate and pentaerythritol di(meth)acrylate are more preferred, since the polymerization reaction is likely to be stable and hollow particles having excellent strength and heat resistance can be obtained. From the same viewpoint, the trifunctional or higher crosslinkable monomer is preferably pentaerythritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, or dipentaerythritol poly(meth)acrylate, with pentaerythritol tetra(meth)acrylate being more preferred.

[0034] The content of the crosslinkable monomer is 75 to 100 parts by mass per 100 parts by mass of the first polymerizable monomer. When the content of the crosslinkable monomer is 75 parts by mass or more, the content of the crosslinkable monomer units in the shell of the hollow particle is sufficiently high, and a covalent bond network is densely spread throughout the shell. As a result, the hollow particle has excellent strength, is resistant to crushing, and is resistant to deformation even when exposed to heat or the like from the outside. The content of the crosslinkable monomer in 100 parts by mass of the first polymerizable monomer is preferably 80 to 100 parts by mass, more preferably 85 to 100 parts by mass, and even more preferably 90 to 100 parts by mass.

[0035] The content of the bifunctional crosslinkable monomer in 100 parts by mass of the first polymerizable monomer is not particularly limited, but the lower limit is preferably 50 parts by mass or more, more preferably 60 parts by mass or more, even more preferably 70 parts by mass or more, and still more preferably 75 parts by mass or more, and the upper limit is preferably 100 parts by mass or less, more preferably 95 parts by mass or less, and even more preferably 90 parts by mass or less.

[0036] When the first polymerizable monomer contains a trifunctional or higher crosslinkable monomer as a crosslinkable monomer, the content of the trifunctional or higher crosslinkable monomer in 100 parts by mass of the first polymerizable monomer is not particularly limited, but the lower limit is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 15 parts by mass or more, and the upper limit is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, even more preferably 30 parts by mass or less, and still more preferably 25 parts by mass or less.

[0037] [Non-crosslinkable monomer] The first polymerizable monomer may further include a non-crosslinkable monomer. As the non-crosslinkable monomer, a monovinyl monomer is preferably used. A monovinyl monomer is a compound having one polymerizable vinyl functional group. Examples of the monovinyl monomer include (meth)acrylic acid alkyl esters having an alkyl group of 6 or more carbon atoms, such as 2-ethylhexyl (meth)acrylate and lauryl (meth)acrylate; aromatic vinyl monomers such as styrene, vinyltoluene, α-methylstyrene, p-methylstyrene, and halogenated styrene; monoolefin monomers such as ethylene, propylene, and butylene; diene monomers such as butadiene and isoprene; vinyl carboxylic acid ester monomers such as vinyl acetate; halogenated vinyl monomers such as vinyl chloride; and vinylidene chloride. vinylpyridine monomers; and hydrophilic non-crosslinkable monomers such as (meth)acrylic acid alkyl esters having an alkyl group of 1 to 5 carbon atoms, such as methyl (meth)acrylate, ethyl (meth)acrylate, and butyl (meth)acrylate; (meth)acrylamides and derivatives thereof, such as (meth)acrylamide, N-methylol (meth)acrylamide, and N-butoxymethyl (meth)acrylamide; (meth)acrylic acid nitrile; and polar group-containing non-crosslinkable monomers. Preferred examples of the polar group-containing non-crosslinkable monomer include non-crosslinkable monomers containing a polar group selected from a carboxyl group, a hydroxyl group, a sulfonic acid group, an amino group, a polyoxyethylene group, and an epoxy group. More specifically, examples of the monomer include carboxyl group-containing monomers such as ethylenically unsaturated carboxylic acid monomers, such as (meth)acrylic acid, crotonic acid, cinnamic acid, itaconic acid, fumaric acid, maleic acid, and butenetricarboxylic acid; hydroxyl group-containing monomers, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; sulfonic acid group-containing monomers, such as styrenesulfonic acid; amino group-containing monomers, such as dimethylaminoethyl (meth)acrylate and diethylaminoethyl (meth)acrylate; polyoxyethylene group-containing monomers, such as methoxypolyethylene glycol (meth)acrylate; and epoxy group-containing monomers, such as glycidyl (meth)acrylate, allyl glycidyl ether, and 4-hydroxybutyl acrylate glycidyl ether. These non-crosslinkable monomers can be used either alone or in combination of two or more. As the non-crosslinkable monomer used as the first polymerizable monomer, hydrophilic non-crosslinkable monomers are preferred, from the viewpoint of obtaining hollow particles with excellent strength, and (meth)acrylic acid alkyl esters having an alkyl group with 1 to 5 carbon atoms are more preferred, (meth)acrylic acid alkyl esters having an alkyl group with 1 to 4 carbon atoms are more preferred, and methyl (meth)acrylate is even more preferred.

[0038] In the first polymerizable monomer, polymerizable monomers other than the crosslinkable monomer are non-crosslinkable monomers. The content of the non-crosslinkable monomer in the first polymerizable monomer is 0 to 25 parts by mass in 100 parts by mass of the first polymerizable monomer. From the viewpoint of suppressing a decrease in the reactivity of the first polymerizable monomer and improving the strength of the hollow particles to make them less likely to be crushed, the content of the non-crosslinkable monomer in the first polymerizable monomer is preferably 20 parts by mass or less, more preferably 15 parts by mass or less, and even more preferably 10 parts by mass or less, and it is particularly preferable that the first polymerizable monomer does not contain a non-crosslinkable monomer.

[0039] The content of the first polymerizable monomer in the mixed solution is not particularly limited, but from the viewpoint of the balance between the porosity, particle size, and mechanical strength of the hollow particles, it is usually 15 to 55 mass%, more preferably 25 to 40 mass%, relative to 100 mass% of the total mass of the components in the mixed solution excluding the aqueous medium.

[0040] (B) Particle size control agent The mixed solution preferably further contains a particle size control agent, which allows the particle size of the droplets of the monomer composition and the shell thickness of the resulting hollow particles to be appropriately adjusted, thereby producing hollow particles that are resistant to crushing even when they have a high porosity. Examples of particle size control agents include at least one selected from the group consisting of rosin acid, higher fatty acids, and metal salts thereof, or polar resins, as described below. These particle size control agents can appropriately adjust the particle size of droplets of a monomer composition containing a first polymerizable monomer and a hydrocarbon solvent in the suspension process described below. During the suspension process, droplets of the monomer composition are formed in an aqueous medium due to the action of a dispersion stabilizer. In the droplets of the monomer composition, phase separation occurs between the hydrocarbon solvent and materials other than the hydrocarbon solvent containing the first polymerizable monomer, resulting in the hydrocarbon solvent being concentrated in the center and the materials other than the hydrocarbon solvent being concentrated on the surface. When the mixed liquid contains a particle size control agent, it is estimated that the particle size control agent is concentrated near the surface of the monomer composition droplets, and the dispersion stabilizer is attached to the droplet surface. This distribution structure of materials is formed according to the difference in affinity of each material for the aqueous medium. It is believed that by including a particle size control agent in the mixed liquid, the droplets of the monomer composition in the suspension have the distribution structure of the materials described above, and an interaction occurs between the dispersion stabilizer and the particle size control agent on the droplet surface, which changes the dispersibility of the droplets due to the dispersion stabilizer, making it possible to appropriately adjust the particle size of the droplets of the monomer composition. Among the particle size control agents, at least one selected from the group consisting of rosin acid, higher fatty acids, and metal salts thereof is preferred, and at least one selected from rosin acid and alkali metal salts thereof is more preferred, since the particle size of the droplets can be appropriately adjusted with a small content.

[0041] Rosin acids preferably used as particle size control agents can be obtained from rosins such as gum rosin, tall rosin and wood rosin. Examples of components contained in rosin acids obtained from these rosins include abietic acid, dehydroabietic acid, palustric acid, isopimaric acid, pimaric acid, etc. The component ratios of rosin acids are not constant and vary depending on the type of rosin, the species of pine used as the raw material, the place of origin, etc. The rosin acid and metal salts thereof used in the present disclosure are preferably rosin acids containing 50 mass % or more of abietic acids such as abietic acid, dehydroabietic acid, palustric acid, and hydrogenated products thereof, and alkali metal salts thereof.

[0042] The higher fatty acid used as the particle size control agent is preferably a higher fatty acid having 10 to 25 carbon atoms, excluding the carbon atom in the carboxyl group. A preferred example of a higher fatty acid is lauric acid (CH3(CH2) 10 COOH), tridecanoic acid (CH3(CH2) 11 COOH), myristic acid (CH3(CH2) 12 COOH), pentadecanoic acid (CH3(CH2) 13 COOH), palmitic acid (CH3(CH2) 14 COOH), heptadecanoic acid (CH3(CH2) 15 COOH), stearic acid (CH3(CH2) 16 COOH), arachidic acid (CH3(CH2) 18 COOH), behenic acid (CH3(CH2) 20 COOH), and lignoceric acid (CH3(CH2) 22 COOH) etc.

[0043] Examples of metals used in the metal salts of rosin acid or higher fatty acids include alkali metals such as Li, Na, and K, and alkaline earth metals such as Mg and Ca. Of these, alkali metals are preferred, and at least one selected from Li, Na, and K is more preferred.

[0044] When at least one particle size control agent selected from the group consisting of rosin acid, higher fatty acid, and metal salts thereof is used as the particle size control agent, the total content of the rosin acid, higher fatty acid, and metal salt thereof is preferably 0.0001 to 0.1 parts by mass, more preferably 0.001 to 0.01 parts by mass, and even more preferably 0.0015 to 0.006 parts by mass, per 100 parts by mass of the first polymerizable monomer and hydrocarbon solvent combined. When the content is equal to or greater than the lower limit, the particle size and shell thickness of the hollow particles can be easily controlled, thereby improving the strength of the hollow particles. On the other hand, when the content is equal to or less than the upper limit, a decrease in the polymerizable monomer content can be suppressed, thereby suppressing a decrease in shell strength and further suppressing collapse of the hollow particles.

[0045] The polar resin preferably used as a particle size control agent is a polymer containing a repeating unit containing a heteroatom, and specific examples thereof include acrylic resins, polyester resins, and vinyl resins containing a heteroatom. The polar resin may be a homopolymer or copolymer of a heteroatom-containing monomer, or a copolymer of a heteroatom-containing monomer and a non-heteroatom-containing monomer. When the polar resin is a copolymer of a heteroatom-containing monomer and a non-heteroatom-containing monomer, the proportion of heteroatom-containing monomer units in 100% by mass of all repeating units constituting the copolymer is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more, in order to easily control the particle size of the hollow particles. Examples of heteroatom-containing monomers used in polar resins include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, acrylic acid, methacrylic acid, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, glycidyl (meth)acrylate, and 4-hydroxybutyl acrylate. Examples of suitable monomers include (meth)acrylic monovinyl monomers, which are monomers having a (meth)acryloyl group, such as acrylate glycidyl ether; aromatic vinyl monomers containing heteroatoms, such as halogenated styrenes and styrene sulfonic acids; vinyl carboxylic acid ester monomers, such as vinyl acetate; halogenated vinyl monomers, such as vinyl chloride; halogenated vinylidene monomers, such as vinylidene chloride; vinylpyridine monomers; carboxyl group-containing monomers, such as ethylenically unsaturated carboxylic acid monomers, such as crotonic acid, cinnamic acid, itaconic acid, fumaric acid, maleic acid, and butenetricarboxylic acid; and epoxy group-containing monomers, such as allyl glycidyl ether. These heteroatom-containing monomers can be used alone or in combination of two or more. Examples of heteroatom-free monomers used in polar resins include aromatic vinyl monomers that do not contain heteroatoms, such as styrene, vinyltoluene, α-methylstyrene, and p-methylstyrene; monoolefin monomers, such as ethylene, propylene, and butylene; and diene monomers, such as butadiene and isoprene. These heteroatom-free monomers can be used alone or in combination of two or more.

[0046] The polar resin is preferably an acrylic resin in which the total mass of (meth)acrylic monovinyl monomer units is preferably 50 mass% or more, more preferably 70 mass% or more, and even more preferably 90 mass% or more, of all repeating units constituting the resin (100 mass%), from the viewpoints of high compatibility with the first polymerizable monomer and ease of controlling the particle size of the hollow particles. In particular, the polar resin is preferably an acrylic resin in which all repeating units constituting the resin are (meth)acrylic monovinyl monomer units.

[0047] In particular, the polar resin preferably contains a polar group-containing monomer unit containing a polar group selected from a carboxyl group, a hydroxyl group, a sulfonic acid group, an amino group, a polyoxyethylene group, and an epoxy group, in order to facilitate control of the particle size of the hollow particles. Examples of polar group-containing monomers used in the polar resin include the same polar group-containing non-crosslinkable monomers that may be contained in the first polymerizable monomer described above. The polar group-containing monomers can be used alone or in combination of two or more. Carboxyl and hydroxyl groups are preferred as polar groups contained in the polar group-containing monomer units contained in the polar resin, in order to facilitate particle size control with a small amount of addition. When the polar resin contains a polar group-containing monomer unit, it is preferable that the polar group be located at the end of the main chain or side chain, or be attached in a pendant manner to the main chain or side chain, since this makes it easier for the polar resin to be positioned on the outer surface of the hollow particle and makes it easier to control the particle size of the hollow particle.

[0048] When the polar resin does not contain the polar group-containing monomer unit, the heteroatom-containing monomer unit contained in the polar resin preferably contains a monomer unit derived from a (meth)acrylic acid alkyl ester, from the viewpoints of high compatibility with the first polymerizable monomer and ease of control of the particle size of the hollow particles. In particular, from the viewpoint of high polarity, it is preferable to contain a monomer unit derived from a (meth)acrylic acid alkyl ester, preferably in which the alkyl group has 3 or less carbon atoms, more preferably in which the alkyl group is a methyl group or an ethyl group, and even more preferably in which the alkyl group is a methyl group.

[0049] The acrylic resin serving as the polar resin is preferably a polymer or copolymer of a polymerizable monomer for a polar resin containing 50% by mass or more of methyl methacrylate when the total mass of the polymerizable monomers for a polar resin is taken as 100% by mass, because it has high compatibility with the first polymerizable monomer and makes it easy to control the particle size of the hollow particles. In the present disclosure, the polymerizable monomer used in synthesizing the polar resin is referred to as the polymerizable monomer for a polar resin.

[0050] The polar resin can be obtained, for example, by polymerizing a polymerizable monomer for polar resins containing the heteroatom-containing monomer by a polymerization method such as solution polymerization or emulsion polymerization. Furthermore, when the polar resin is a copolymer, the copolymer may be any of a random copolymer, a block copolymer, or a graft copolymer, but is preferably a random copolymer. In addition, the polar resin is preferably pulverized as finely as possible in order to improve solubility.

[0051] The number average molecular weight (Mn) of the polar resin is not particularly limited, but is preferably in the range of 3,000 to 20,000, more preferably 4,000 to 17,000, and even more preferably 6,000 to 15,000, in terms of polystyrene, as measured by gel permeation chromatography (GPC) using tetrahydrofuran. When the number average molecular weight (Mn) of the polar resin is equal to or greater than the lower limit, the solubility of the polar resin is improved and the particle size of the hollow particles can be easily controlled. When the number average molecular weight (Mn) is equal to or less than the upper limit, a decrease in shell strength can be suppressed.

[0052] When a polar resin is used as the particle size control agent, the content of the polar resin is preferably 0.1 to 10.0 parts by mass, more preferably 0.3 to 8.0 parts by mass, and even more preferably 0.5 to 8.0 parts by mass, relative to 100 parts by mass of the first polymerizable monomer. When the content is equal to or greater than the lower limit, the particle size and shell thickness of the hollow particles can be easily controlled, and the strength of the hollow particles can be improved. On the other hand, when the content is equal to or less than the upper limit, a decrease in the polymerizable monomer content can be suppressed, thereby suppressing a decrease in shell strength and further suppressing collapse of the hollow particles.

[0053] (C) Oil-soluble polymerization initiator In the present disclosure, the mixed liquid preferably contains an oil-soluble polymerization initiator as a polymerization initiator. As a method for polymerizing droplets of the monomer composition after suspending the mixed liquid, there are an emulsion polymerization method using a water-soluble polymerization initiator and a suspension polymerization method using an oil-soluble polymerization initiator, and the suspension polymerization can be performed by using an oil-soluble polymerization initiator. 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. 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. When the total mass of the first polymerizable monomer in the mixed solution is 100 parts by mass, the content of the oil-soluble polymerization initiator is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 7 parts by mass, and even more preferably 1 to 5 parts by mass. By having the content of the oil-soluble polymerization initiator be 0.1 to 10 parts by mass, the polymerization reaction can be sufficiently progressed, and there is little risk of the oil-soluble polymerization initiator remaining after completion of the polymerization reaction, and there is also little risk of an unexpected side reaction proceeding.

[0054] (D) Hydrocarbon solvents In the present disclosure, a hydrocarbon-based solvent is used as a non-polymerizable, poorly water-soluble organic solvent. The hydrocarbon-based solvent acts as a spacer material that forms hollow spaces inside the particles. In the suspension process described below, a suspension is obtained in which droplets of a monomer composition containing a hydrocarbon-based solvent are dispersed in an aqueous medium. In the suspension process, phase separation occurs within the droplets of the monomer composition, and as a result, the hydrocarbon-based solvent, which has low polarity, tends to collect inside the polymerizable monomer droplets. Ultimately, the hydrocarbon-based solvent is distributed inside the droplets of the monomer composition, and other materials other than the hydrocarbon-based solvent are distributed around the periphery according to their respective polarities. Then, in the polymerization step described below, an aqueous dispersion containing hollow particles encapsulating the hydrocarbon solvent is obtained. That is, the hydrocarbon solvent collects inside the particles, and hollow portions filled with the hydrocarbon solvent are formed inside the obtained precursor particles.

[0055] The type of hydrocarbon solvent is not particularly limited, and examples of the hydrocarbon solvent include saturated hydrocarbon solvents such as butane, pentane, 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. The porosity of the hollow particles can be adjusted by changing the amount of hydrocarbon solvent in the mixture. In the suspension step described below, the polymerization reaction proceeds in a state in which the hydrocarbon solvent is encapsulated in oil droplets containing the crosslinkable monomer, etc., and therefore the porosity of the resulting hollow particles tends to increase as the hydrocarbon solvent content increases. The hydrocarbon solvent preferably contains saturated hydrocarbon solvents in a total amount of 100% by mass. This allows sufficient phase separation to occur within the droplets of the monomer composition, making it easier to obtain hollow particles having only one hollow portion and suppressing the generation of porous particles. The saturated hydrocarbon solvent preferably contains saturated hydrocarbon solvents in an amount of 60% by mass or more, more preferably 80% by mass or more, in order to further suppress the generation of porous particles and to make the hollow portions of each hollow particle uniform. Furthermore, the hydrocarbon solvent is preferably a hydrocarbon solvent having 4 to 7 carbon atoms. A hydrocarbon compound having 4 to 7 carbon atoms is easily encapsulated in the first precursor particles during the polymerization step, and can be easily removed from the second precursor particles during the solvent removal step. Among these, a hydrocarbon solvent having 5 or 6 carbon atoms is particularly preferred. Although not particularly limited, the hydrocarbon solvent preferably has a boiling point of 130° C. or less, more preferably 100° C. or less, because it is easily removed in the solvent removal step described below. Furthermore, the hydrocarbon solvent preferably has a boiling point of 50° C. or more, more preferably 60° C. or more, because it is easily encapsulated in the first precursor particles.

[0056] Furthermore, the hydrocarbon solvent preferably has a relative dielectric constant of 3 or less at 20°C. The relative dielectric constant is one of the indicators that indicates the polarity of a compound. When the relative dielectric constant of the hydrocarbon solvent is sufficiently small, 3 or less, it is thought that phase separation proceeds quickly in the droplets of the monomer composition, making it easy to form hollow spaces. Examples of solvents with a relative dielectric constant of 3 or less at 20°C are as follows. The value in parentheses is the relative dielectric constant. Heptane (1.9), cyclohexane (2.0), benzene (2.3), toluene (2.4). For the relative permittivity at 20°C, reference can be made to values ​​described in known literature (for example, "Chemical Handbook Basics" edited by the Chemical Society of Japan, 4th revised edition, Maruzen Co., Ltd., published September 30, 1993, pp. II-498 to II-503) and other technical information. Examples of methods for measuring the relative permittivity at 20°C include a relative permittivity test conducted in accordance with JISC 2101:1999, 23, at a measurement temperature of 20°C.

[0057] In the present disclosure, the content of the hydrocarbon solvent in the mixed solution is preferably 50 to 500 parts by mass relative to 100 parts by mass of the total mass of the first polymerizable monomer, because this makes it easier to control the particle size of the hollow particles, to increase the porosity while maintaining the strength of the hollow particles, and to reduce the amount of residual hydrocarbon solvent in the particles. The content of the hydrocarbon solvent in the mixed solution is preferably 60 to 400 parts by mass, more preferably 70 to 300 parts by mass, and even more preferably 80 to 200 parts by mass relative to 100 parts by mass of the total mass of the first polymerizable monomer.

[0058] (E) Dispersion stabilizer The dispersion stabilizer is an agent that disperses droplets of the monomer composition in an aqueous medium during the suspension process. In the present disclosure, it is preferable to use an inorganic dispersion stabilizer as the dispersion stabilizer because it makes it easy to control the particle size of the droplets in the suspension, narrows the particle size distribution of the resulting hollow particles, and prevents the shell from becoming too thin, thereby preventing a decrease in the strength of the hollow particles. Such effects of the inorganic dispersion stabilizer are particularly easily exhibited when the inorganic dispersion stabilizer is used in combination with the particle size control agent described above. Examples of inorganic dispersion stabilizers include inorganic compounds such as sulfates such as barium sulfate and calcium sulfate, carbonates such as barium carbonate, calcium carbonate and magnesium carbonate, phosphates such as calcium phosphate, metal oxides such as aluminum oxide and titanium oxide, and metal hydroxides such as aluminum hydroxide, magnesium hydroxide, calcium hydroxide, barium hydroxide and ferric hydroxide. These inorganic dispersion stabilizers can be used alone or in combination of two or more. Among the inorganic dispersion stabilizers, the above-mentioned poorly water-soluble inorganic metal salts such as sulfates, carbonates, phosphates and metal hydroxides are preferred, metal hydroxides are more preferred, and magnesium hydroxide is particularly preferred. In the present disclosure, the poorly water-soluble inorganic metal salt is preferably an inorganic metal salt having a solubility of 0.5 g or less in 100 g of water.

[0059] The content of the dispersion stabilizer is not particularly limited, but is preferably 0.5 to 10 parts by mass, more preferably 1.0 to 8.0 parts by mass, relative to 100 parts by mass of the total mass of the first polymerizable monomer and the hydrocarbon solvent. When the content of the dispersion stabilizer is equal to or greater than the lower limit, droplets of the monomer composition can be sufficiently dispersed so as not to coalesce in the suspension. On the other hand, when the content of the dispersion stabilizer is equal to or less than the upper limit, an increase in the viscosity of the suspension during granulation can be prevented, and the problem of the suspension being unable to pass through the granulator can be avoided. The content of the dispersion stabilizer is usually 2 parts by mass or more and 15 parts by mass or less, and preferably 3 parts by mass or more and 8 parts by mass or less, relative to 100 parts by mass of the aqueous medium.

[0060] (F)Aqueous medium In the present disclosure, the aqueous medium means a medium selected from the group consisting of water, a hydrophilic solvent, and a mixture of water and a hydrophilic solvent. The hydrophilic solvent in the present disclosure is not particularly limited as long as it is sufficiently miscible with water and does not cause phase separation. Examples of hydrophilic solvents include alcohols such as methanol and ethanol, tetrahydrofuran (THF), and dimethyl sulfoxide (DMSO). Among aqueous media, water is preferred due to its high polarity. When using a mixture of water and a hydrophilic solvent, it is important that the polarity of the entire mixture is not too low in order to form droplets of the monomer composition. In this case, for example, the mixing ratio (mass ratio) of water to hydrophilic solvent may be set to 99:1 to 50:50, etc.

[0061] A mixed solution is obtained by mixing the above-mentioned materials and other materials as needed, followed by appropriate stirring, etc. In this mixed solution, an oil phase containing the above-mentioned (A) first polymerizable monomer, (B) particle size control agent, (C) oil-soluble polymerization initiator, and (D) a lipophilic material such as a hydrocarbon solvent is dispersed in an aqueous phase containing (E) a dispersion stabilizer and (F) an aqueous medium, etc., with particle sizes of about several mm. The dispersion state of these materials in the mixed solution can be observed with the naked eye, depending on the type of material. In the mixed solution preparation step, the mixed solution may be obtained by simply mixing the above-mentioned materials and other materials as necessary and appropriately stirring the mixture. However, in terms of making the shell more uniform, it is preferable to prepare a mixed solution by separately preparing in advance an oil phase containing the first polymerizable monomer, the particle size control agent, and the hydrocarbon solvent, and an aqueous phase containing the dispersion stabilizer and the aqueous medium, and then mixing these to prepare the mixed solution. In this way, by preparing the oil phase and the water phase separately in advance and then mixing them, hollow particles having a uniform shell composition can be produced.

[0062] (2) Suspension process The suspending step is a step of preparing a suspension in which droplets of the monomer composition containing a hydrocarbon solvent are dispersed in an aqueous medium by suspending the mixed liquid described above. The method of suspending the monomer composition to form droplets is not particularly limited, but may be carried out using, for example, an apparatus capable of strong stirring, such as an (in-line type) emulsifying disperser (manufactured by Pacific Machinery Works, trade name: Milder) or a high-speed emulsifying disperser (manufactured by Primix Corporation, trade name: TK Homomixer MARK II type). In the suspension prepared in the suspending step, droplets of the monomer composition containing the lipophilic material and having a particle size of about 4 to 60 μm are uniformly dispersed in the aqueous medium. Such droplets of the monomer composition are difficult to observe with the naked eye, but can be observed using a known observation device such as an optical microscope. In the suspension step, phase separation occurs in the droplets of the monomer composition, and the hydrocarbon solvent with low polarity tends to collect inside the droplets. As a result, the obtained droplets have the hydrocarbon solvent distributed inside and materials other than the hydrocarbon solvent distributed around the periphery.

[0063] Fig. 2 is a schematic diagram showing one embodiment of the suspension in the suspension step. A droplet 10 of the monomer composition in Fig. 2 is intended to show a schematic cross section thereof. Note that Fig. 2 is merely a schematic diagram, and the suspension in the present disclosure is not necessarily limited to that shown in Fig. 2. A part of Fig. 2 corresponds to (2) in Fig. 1 described above. 2 shows droplets 10 of the monomer composition and a first polymerizable monomer 4c dispersed in the aqueous medium 1, which are dispersed in the aqueous medium 1. The droplets 10 are formed by surrounding an oil-soluble monomer composition 4 with a dispersion stabilizer 3. The monomer composition contains an oil-soluble polymerization initiator 5, as well as a first polymerizable monomer and a hydrocarbon solvent (none of which are shown). The droplets 10 are minute oil droplets containing the monomer composition 4, and the oil-soluble polymerization initiator 5 generates polymerization-initiating radicals inside the minute oil droplets. Therefore, precursor particles of the desired particle size can be produced without causing the minute oil droplets to grow too large. In the suspension polymerization method using such an oil-soluble polymerization initiator, there is no opportunity for the polymerization initiator to come into contact with the polymerizable monomer 4c dispersed in the aqueous medium 1. Therefore, by using an oil-soluble polymerization initiator, it is possible to prevent the generation of excess polymer particles such as dense particles having a relatively small particle size in addition to the desired resin particles having hollow portions.

[0064] (3) Polymerization process (3-1) First polymerization step In the manufacturing method of the present disclosure, the polymerization step is carried out in two stages. In the first polymerization step, the suspension is subjected to a first polymerization reaction until the polymerization conversion rate of the first polymerizable monomer reaches 93 mass% or more, thereby preparing a first precursor composition containing first precursor particles having a shell containing a polymer of the first polymerizable monomer and a hollow portion filled with a hydrocarbon solvent. During the first polymerization reaction, the droplets of the monomer composition are subjected to the polymerization reaction while encapsulating the hydrocarbon solvent, which allows the polymerization reaction to proceed while maintaining the shape. Therefore, during the first polymerization reaction, the size and porosity of the resulting hollow particles can be easily adjusted by adjusting the amount of hydrocarbon solvent, the amount of particle size control agent, the type of dispersion stabilizer, and the like. Furthermore, since the first polymerizable monomer and hydrocarbon solvent are used in combination, the polarity of the hydrocarbon solvent is low relative to the shell of the first precursor particle, making it difficult for the hydrocarbon solvent to blend with the shell. This leads to sufficient phase separation, resulting in only one hollow portion.

[0065] In the first polymerization reaction, the polymerization method is not particularly limited, and for example, a batch method, a semi-continuous method, a continuous method, etc. can be used. In the first polymerization reaction, the polymerization temperature is preferably 40 to 80°C, and more preferably 50 to 70°C. In the first polymerization reaction, the temperature rise rate when the temperature is raised to the polymerization temperature is preferably 10°C / h to 60°C / h, and more preferably 15°C / h to 55°C / h. The reaction time for the first polymerization reaction is preferably 0.5 to 5 hours, and more preferably 1 to 3 hours.

[0066] In the production method of the present disclosure, the first polymerization reaction is carried out until the polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, preferably 95% by mass or more, more preferably 98% by mass or more, and even more preferably 99% by mass or more. In the present disclosure, the polymerization conversion rate is calculated from the mass of the solid content of the first precursor particles obtained by the first polymerization reaction and the mass of the first polymerizable monomer remaining unreacted after the first polymerization reaction using the following formula (A). In the present disclosure, the solid content refers to all components excluding the solvent, and liquid polymerizable monomers and the like are considered to be included in the solid content. The mass of the unreacted first polymerizable monomer can be measured using gas chromatography (GC). Polymerization conversion rate (mass%)=100−(mass of unreacted first polymerizable monomer / mass of solid content of first precursor particles)×100 Formula (A)

[0067] (3-2) Second polymerization step In the second polymerization step, a second polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C is added to the first precursor composition obtained in the first polymerization step to carry out a second polymerization reaction, thereby preparing a second precursor composition including second precursor particles having a shell including a polymer of the first polymerizable monomer and the second polymerizable monomer, and a hollow portion filled with a hydrocarbon solvent. In the second polymerization reaction, the polymerization reaction proceeds in a state in which the second polymerizable monomer is incorporated into the shell of the first precursor particle. Since the thermal motion of the shell of the first precursor particle is promoted by the incorporation of the second polymerizable monomer, it is presumed that in the second polymerization reaction, the polymerization reaction of the polymerizable functional groups of the first polymerizable monomer remaining unreacted in the shell and the second polymerizable monomer proceeds, resulting in the formation of a dense crosslinked structure.

[0068] The second polymerizable monomer is not particularly limited as long as it has a solubility of 0.3 g / L or more in distilled water at 20°C. However, from the viewpoint of improving the strength of the hollow particles, a non-crosslinkable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C, i.e., a hydrophilic non-crosslinkable monomer, is preferred. Examples of hydrophilic non-crosslinkable monomers used as the second polymerizable monomer include those similar to those used as the first polymerizable monomer. Examples include (meth)acrylic acid alkyl esters having an alkyl group with 1 to 5 carbon atoms, (meth)acrylamides and their derivatives, (meth)acrylic nitrile, and polar group-containing non-crosslinkable monomers. The second polymerizable monomer preferably has a solubility in distilled water at 20° C. of 2 g / L or more, more preferably 10 g / L or more, even more preferably 15 g / L or more, still more preferably 20 g / L or more, and particularly preferably 50 g / L or more, so that the second polymerizable monomer is easily incorporated into the shell of the first precursor particle to promote thermal motion and improve the strength of the hollow particle. The upper limit of the solubility of the second polymerizable monomer in distilled water at 20° C. is not particularly limited, but is usually 80 g / L or less.

[0069] Furthermore, since the second polymerizable monomer is easily incorporated into the shell of the first precursor particle to promote thermal motion and improve the strength of the hollow particle, the molecular weight of the second polymerizable monomer is preferably 200 or less, more preferably 100 or less. The lower limit of the molecular weight of the second polymerizable monomer is not particularly limited, and is usually 50 or more.

[0070] In order to improve the strength of the hollow particles, the second polymerizable monomer is preferably at least one selected from the group consisting of (meth)acrylic acid alkyl esters having an alkyl group of 1 to 5 carbon atoms and (meth)acrylic acid nitrile, and more preferably at least one selected from the group consisting of methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, and acrylic acid nitrile.

[0071] The amount of the second polymerizable monomer added is preferably 3 to 15 parts by mass, more preferably 4 to 10 parts by mass, per 100 parts by mass of the first polymerizable monomer. When the amount of the second polymerizable monomer added is equal to or greater than the lower limit, the effect of accelerating the polymerization reaction by adding the second polymerizable monomer is improved, and the crosslinked structure of the hollow particle shell becomes denser, thereby improving the strength of the hollow particles and making them less susceptible to crushing. On the other hand, when the amount of the second polymerizable monomer added is equal to or less than the upper limit, a decrease in the content of the first polymerizable monomer relative to the total polymerizable monomer used to form the shell can be suppressed. Since the first polymerizable monomer contains a large amount of crosslinkable monomer, suppressing a decrease in the content of the first polymerizable monomer can result in hollow particles with excellent strength that contain a large amount of crosslinked structures formed by the crosslinkable monomer.

[0072] In the second polymerization reaction carried out after the addition of the second polymerizable monomer, the polymerization method is not particularly limited, and for example, the same polymerization method as that used in the first polymerization reaction can be adopted. In the second polymerization reaction, the polymerization temperature is preferably 40 to 80°C, and more preferably 50 to 70°C. The reaction time for the second polymerization reaction is preferably 1 to 6 hours, and more preferably 2 to 4 hours.

[0073] According to the production method of the present disclosure, the amount of unreacted polymerizable monomer remaining after the second polymerization reaction can be reduced to preferably 750 ppm or less, more preferably 500 ppm or less, and even more preferably 300 ppm or less. In the present disclosure, the amount of unreacted polymerizable monomer remaining after the second polymerization reaction is the ratio of the mass of the unreacted polymerizable monomer to the mass of the solid content of the hollow particles obtained by the second polymerization reaction. The mass of the unreacted polymerizable monomer can be measured using gas chromatography (GC).

[0074] (4) Solid-liquid separation process This step is a step of obtaining a solid content containing second precursor particles by solid-liquid separation of a second precursor composition containing hollow particles (second precursor particles) encapsulating a hydrocarbon-based solvent, which is obtained by the above-mentioned polymerization step.

[0075] The method for solid-liquid separation of the second precursor composition is not particularly limited, and any known method can be used. Examples of the solid-liquid separation method include centrifugation, filtration, and static separation. Among these, centrifugation or filtration can be used, and centrifugation may be used from the viewpoint of ease of operation. After the solid-liquid separation step, an optional step such as a pre-drying step may be carried out before the solvent removal step described below is carried out. Examples of the pre-drying step include a step of pre-drying the solid content obtained after the solid-liquid separation step using a drying device such as a dryer or a drying appliance such as a hand dryer.

[0076] (5) Solvent removal process This step is a step of removing the hydrocarbon solvent contained in the hollow particles (second precursor particles) obtained in the solid-liquid separation step. By removing the hydrocarbon solvent contained in the second precursor particles in the air, the hydrocarbon solvent inside the second precursor particles is replaced with air, and hollow particles filled with gas are obtained.

[0077] In this process, "in the air" strictly refers to an environment in which no liquid is present outside the second precursor particles, or an environment in which only a trace amount of liquid is present outside the second precursor particles, to the extent that it does not affect the removal of the hydrocarbon solvent. "In the air" can also be referred to as a state in which the second precursor particles are not present in a slurry, or a state in which the second precursor particles are present in a dry powder. In other words, in this process, it is important to remove the hydrocarbon solvent in an environment in which the second precursor particles are in direct contact with the external gas.

[0078] The method for removing the hydrocarbon solvent from the second precursor particles in air is not particularly limited, and any known method can be used, such as vacuum drying, heat drying, flash drying, or a combination of these methods. In particular, when a heat drying method is used, the heating temperature must be equal to or higher than the boiling point of the hydrocarbon solvent and equal to or lower than the maximum temperature at which the shell structure of the second precursor particles does not collapse. Therefore, depending on the shell composition of the second precursor particles and the type of hydrocarbon solvent, the heating temperature may be, for example, 50 to 200°C, 70 to 200°C, or 100 to 200°C. By the drying operation in air, the hydrocarbon solvent inside the second precursor particles is replaced by the external gas, resulting in hollow particles whose hollow portions are filled with gas.

[0079] The drying atmosphere is not particularly limited and can be appropriately selected depending on the application of the hollow particles. Examples of the drying atmosphere include air, oxygen, nitrogen, argon, etc. Hollow particles with a temporary vacuum inside can also be obtained by filling the inside of the hollow particles with a gas and then drying under reduced pressure.

[0080] As an alternative method, the hydrocarbon-based solvent may be removed by replacing the hydrocarbon-based solvent contained in the second precursor particles with the aqueous medium of the slurry containing the second precursor particles in a slurry containing the second precursor particles and an aqueous medium, without subjecting the slurry-like second precursor composition obtained in the polymerization step to solid-liquid separation. In this method, the hydrocarbon solvent contained in the second precursor particles can be removed by bubbling an inert gas through the second precursor composition at a temperature equal to or higher than the boiling point of the hydrocarbon solvent minus 35°C. Here, when the hydrocarbon solvent is a mixed solvent containing multiple types of hydrocarbon solvents and has multiple boiling points, the boiling point of the hydrocarbon solvent in the solvent removal step 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. The temperature at which the inert gas is bubbled into the second precursor composition is preferably at least 30°C below the boiling point of the hydrocarbon solvent, more preferably at least 20°C below that, in order to reduce the amount of hydrocarbon solvent remaining in the hollow particles. The bubbling temperature is usually set to a temperature equal to or higher than the polymerization temperature in the polymerization step. While not particularly limited, the bubbling temperature may be set to 50°C or higher and 100°C or lower. The inert gas to be bubbled is not particularly limited, but examples thereof include nitrogen and argon. The bubbling conditions are appropriately adjusted depending on the type and amount of the hydrocarbon solvent so as to remove the hydrocarbon solvent contained in the second precursor particles, and are not particularly limited, but may be, for example, bubbling an inert gas at a rate of 1 to 3 L / min for 1 to 10 hours. In this method, an aqueous slurry is obtained in which the aqueous medium is encapsulated in the second precursor particles, and the slurry is subjected to solid-liquid separation to obtain hollow particles, which are then dried to remove the aqueous medium from the hollow particles, thereby obtaining hollow particles whose hollow spaces are filled with gas.

[0081] Comparing a method of obtaining hollow particles having hollow portions filled with gas by performing solid-liquid separation on a slurry-like second precursor composition and then removing the hydrocarbon-based solvent from the second precursor particles in air, with a method of obtaining hollow particles having hollow portions filled with gas by replacing the hydrocarbon-based solvent contained in second precursor particles in a slurry containing the second precursor particles and an aqueous medium with the aqueous medium of the slurry, performing solid-liquid separation, and removing the aqueous medium from the second precursor particles in air, the former method has the advantage that the hollow particles are less likely to be crushed in the step of removing the hydrocarbon-based solvent, and the latter method has the advantage that the amount of residual hydrocarbon-based solvent is reduced by performing bubbling with an inert gas. When the hydrocarbon solvent contained in the second precursor particles is replaced with water, the resulting hollow resin particles will collapse unless the particles are filled with water in the same volume as the hydrocarbon solvent that has escaped from the particles. One way to prevent this is to adjust the pH of the slurry to 7 or higher, swell the particle shells with alkali, and then remove the hydrocarbon solvent. In this case, the particle shells acquire flexibility, allowing the hydrocarbon solvent inside the particles to be replaced with water more quickly.

[0082] (6) Other As steps other than the above steps (1) to (5), for example, the following step (6-a) of cleaning and the following step (6-b) of replacing the hollow portion again may be added. (6-a) Cleaning process The washing step is a step of adding an acid or alkali to wash the second precursor composition containing the second precursor particles before the solvent removal step in order to remove any dispersion stabilizer remaining in the second precursor composition. When the dispersion stabilizer used is an inorganic dispersion stabilizer soluble in acid, it is preferable to add an acid to the second precursor composition containing the second precursor particles to wash the second precursor composition. On the other hand, when the dispersion stabilizer used is an inorganic compound soluble in alkali, it is preferable to add an alkali to the second precursor composition containing the second precursor particles to wash the second precursor composition. Furthermore, when an acid-soluble inorganic dispersion stabilizer is used as the dispersion stabilizer, it is preferable to add an acid to the second precursor composition containing the second precursor particles to adjust the pH to preferably 6.5 or less, more preferably 6 or less. 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 preferable because it has a high efficiency in removing the dispersion stabilizer and places a small burden on the production equipment.

[0083] (6-b) Re-replacement process of hollow part The hollow re-substitution process is a process in which the gas or liquid inside the hollow particles is replaced with another gas or liquid. This substitution can change the environment inside the hollow particles, selectively confine molecules inside the hollow particles, or modify the chemical structure inside the hollow particles to suit the application.

[0084] 2.Hollow particles It is believed that the hollow particles obtained by the manufacturing method of the present disclosure are resistant to crushing even with a high porosity due to the dense crosslinked structure of the shell. As an indicator of the dense crosslinked structure of the shell, the hollow particles obtained by the manufacturing method of the present disclosure preferably have less than 5% by mass of hollow particles precipitated in acetone in an immersion test in which 0.1 mg of hollow particles are added to 4 mL of acetone in an environment of 25°C, shaken at a shaking speed of 100 rpm for 10 minutes, and then allowed to stand for 48 hours. In the immersion test, the fewer hollow particles precipitated in acetone, the denser the shell is considered to be, and the more resistant it is to acetone penetration.

[0085] Furthermore, the hollow particles obtained by the manufacturing method of the present disclosure have extremely few interconnected pores or shell defects in the shell, and SEM observation shows that 5 or fewer hollow particles out of 100 hollow particles have interconnected pores or shell defects. Generally, hollow particles are classified into those in which the shell does not have any interconnecting holes that connect the hollow portion to the external space of the particle, and those in which the shell has one or more interconnecting holes through which the hollow portion communicates with the outside of the particle. Although the diameter of the interconnecting holes varies depending on the size of the hollow particle, it is usually about 10 to 500 nm. While the interconnecting holes can impart beneficial functions to hollow particles, they can also reduce the strength of the hollow particles and make them more susceptible to crushing because they are missing portions of the shell. Furthermore, hollow particles may have shell defects in the form of cracks that are extremely large compared to the particle size. Although it depends on the size of the hollow particle, cracks with a length of 1 μm or more generally significantly reduce the strength of the hollow particle and are therefore recognized as shell defects. Furthermore, in the above-described hollow particle immersion test, if less than 5% by mass of hollow particles precipitate in acetone, it can be considered that 5 or fewer hollow particles have interconnected pores or shell defects out of 100 hollow particles. Even if the shell has no interconnected pores or shell defects, the above-described hollow particle immersion test may result in 5% or more by mass of the precipitated hollow particles. Therefore, in the above-described hollow particle immersion test, if less than 5% by mass of hollow particles precipitate, this is considered to indicate that the shell has very few interconnected pores and shell defects and a dense crosslinked structure.

[0086] The shape of the hollow particles obtained by the production method of the present disclosure is not particularly limited as long as a hollow portion is formed inside, and examples thereof include spherical, oval spherical, irregular, etc. Among these, spherical is preferred from the viewpoint of ease of production. The hollow particles may have one or more hollow portions. The shell of the hollow particles, and when the hollow particles have two or more hollow portions, the partition walls separating adjacent hollow portions, may be porous. The interior of the particle preferably has only one hollow portion in order to maintain a good balance between the high porosity of the hollow particles and the mechanical strength of the hollow particles. The hollow particles may have an average circularity of 0.950 to 0.995. An example of the shape of a hollow particle is a bag made of a thin membrane and inflated with gas, and its cross section is shown as hollow particle 100 in Figure 1 (5). In this example, a thin membrane is provided on the outside, and the inside is filled with gas. The particle shape can be confirmed by, for example, SEM or TEM. Furthermore, the internal shape of a particle can be confirmed by SEM or TEM after cutting the particle into cross sections by a known method.

[0087] The lower limit of the volume average particle diameter of the hollow particles is preferably 4.0 μm or more, more preferably 4.5 μm or more, and even more preferably 5.0 μm or more, while the upper limit of the volume average particle diameter of the hollow particles is preferably 60.0 μm or less, more preferably 55.0 μm or less, and even more preferably 50.0 μm or less. When the volume average particle diameter of the hollow particles is equal to or greater than the lower limit, the hollow particles are less likely to aggregate with each other, thereby exhibiting excellent dispersibility. When the volume average particle diameter of the hollow particles is equal to or less than the upper limit, the hollow particles are less likely to be crushed, thereby exhibiting high mechanical strength.

[0088] The particle size distribution of the hollow particles (volume average particle size (Dv) / number average particle size (Dn)) may be, for example, 1.1 or more and 2.5 or less. When the particle size distribution is 2.5 or less, particles with little variation in compressive strength characteristics and heat resistance among particles can be obtained. Furthermore, when producing a sheet-like molded product, for example, a product with a uniform thickness can be produced. The volume average particle size (Dv) and number average particle size (Dn) of the hollow particles can be determined by, for example, measuring the particle size of the hollow particles using a laser diffraction particle size distribution analyzer, calculating the number average and volume average, and using the resulting values ​​as the number average particle size (Dn) and volume average particle size (Dv) of the particles. The particle size distribution is calculated by dividing the volume average particle size by the number average particle size.

[0089] The hollow particles obtained by the production method of the present disclosure have a porosity of 50% or more, preferably 60% or more. When the porosity is equal to or greater than the above-mentioned lower limit, the hollow particles have excellent lightness, heat resistance, and heat insulation properties. The upper limit of the porosity of the hollow particles of the present disclosure is not particularly limited, but is preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less, in order to prevent a decrease in the strength of the hollow particles and make them less likely to be crushed.

[0090] The porosity of the hollow particles obtained by the manufacturing method of the present disclosure is calculated from the apparent density D1 and true density D0 of the hollow particles. The method for measuring the apparent density D1 of hollow particles is as follows. First, 3 Approximately 30 cm 3The volumetric flask is filled with hollow particles, and the mass of the filled hollow particles is accurately weighed. Next, the volumetric flask filled with hollow particles is accurately filled with isopropanol up to the marked line, taking care not to introduce air bubbles. The mass of isopropanol added to the volumetric flask is accurately weighed, and the apparent density D1 (g / cm) of the hollow particles is calculated based on the following formula (I): 3 ) is calculated. Formula (I) Apparent density D1 = [Mass of hollow particles] / (100 - [Mass of isopropanol] ÷ [Specific gravity of isopropanol at measurement temperature]) The apparent density D1 corresponds to the specific gravity of the entire hollow particle when the hollow portion is considered to be a part of the hollow particle.

[0091] The true density D0 of hollow particles is measured as follows: After crushing the hollow particles, 3 Approximately 10 g of crushed pieces of hollow particles are filled into the measuring flask, and the mass of the crushed pieces is accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol is added to the measuring flask, and the mass of the isopropanol is accurately weighed. The true density D0 (g / cm) of the hollow particles is calculated based on the following formula (II): 3 ) is calculated. Formula (II) True density D0 = [mass of crushed hollow particle fragments] / (100 - [mass of isopropanol] ÷ [specific gravity of isopropanol at measurement temperature]) The true density D0 corresponds to the specific gravity of only the shell portion of the hollow particle. As is clear from the above measurement method, the hollow portion is not considered to be part of the hollow particle when calculating the true density D0.

[0092] The porosity (%) of the hollow particles is calculated from the apparent density D1 and true density D0 of the hollow particles by the following formula (III). Formula (III) Porosity (%) = 100 - (apparent density D1 / true density D0) x 100 The porosity of a hollow particle can be expressed as the proportion of the hollow portion in the specific gravity of the hollow particle.

[0093] The shell thickness of the hollow particles obtained by the production method of the present disclosure is preferably 0.1 μm or more, more preferably 0.2 μm or more, and even more preferably 0.3 μm or more, as a lower limit, and preferably 6 μm or less, more preferably 5 μm or less, and even more preferably 4 μm or less, as an upper limit. Having a shell thickness of the hollow particles equal to or greater than the above lower limit improves the strength of the shell. Meanwhile, because the hollow particles obtained by the production method of the present disclosure have a dense shell structure, even if the shell thickness is thin and equal to or less than the above lower limit, the obtained hollow particles have excellent strength and are resistant to crushing. The shell thickness of the hollow particles can be calculated by calculating the inner diameter r of the hollow particles using the volume average particle diameter R and porosity of the hollow particles according to the following formula (1), and then using the inner diameter r and the volume average particle diameter R to calculate the shell thickness of the hollow particles according to the following formula (2). The porosity in the following formula (1) is a numerical value expressed as a percentage. 4 / 3π×(R / 2) 3 ×Porosity=4 / 3π×(r / 2) 3 Formula (1) Shell thickness = (Rr) / 2 Equation (2) The difference between the shell thickness calculated in this manner and the average thickness measured at 20 points on the shell is usually within ±10% of these average values, so the shell thickness calculated as above can be considered to be the shell thickness of the hollow particles. The thickness at each point of the shell of a hollow particle used to calculate the average thickness at 20 points on the shell can be measured, for example, by breaking the hollow particle and observing the shell fragments obtained with an SEM.

[0094] The hollow particles obtained by the production method of the present disclosure are particularly suitable as additives for molded bodies because they are resistant to crushing when kneaded with other materials and during molding after kneading, and when added to a molded body, they have excellent effects as a weight-saving material, heat insulating material, soundproofing material, vibration damping material, etc. The hollow particles of the present disclosure are particularly suitable as additives for resin molded bodies because they are resistant to crushing when kneaded with resin and during molding after kneading. Molded articles containing hollow particles of the present disclosure may contain, as a resin, a thermoplastic or thermosetting resin, such as polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyurethane, epoxy resin, acrylonitrile-butadiene-styrene (ABS) resin, acrylonitrile-styrene (AS) resin, poly(meth)acrylate, polycarbonate, polyamide, polyimide, polyphenylene ether, polyphenylene sulfide, polyester, or polytetrafluoroethylene. Molded articles containing hollow particles of the present disclosure may also contain organic or inorganic fibers, such as carbon fiber, glass fiber, aramid fiber, or polyethylene fiber. The hollow particles obtained by the manufacturing method of the present disclosure can also be incorporated as a filler in molded articles formed using a thermoplastic or thermosetting resin, and in molded articles formed using a thermoplastic or thermosetting resin and a material containing fiber. Examples of applications of resin molded articles containing the hollow particles of the present disclosure include light reflecting materials, heat insulating materials, sound insulating materials, and low dielectric 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, and tools. Furthermore, the hollow particles of the present disclosure have high porosity, are not easily crushed, and have excellent heat resistance, so they meet the heat insulation and shock-absorbing properties (cushioning properties) required for undercoating materials and also meet the heat resistance required for thermal paper applications.The hollow particles of the present disclosure are also useful as plastic pigments that are excellent in gloss, hiding power, etc. Furthermore, the hollow particles of the present disclosure 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 therein by means of immersion treatment, reduced pressure or pressure immersion treatment, or the like. [Example]

[0095] The present disclosure will be described in more detail below with reference to examples and comparative examples, but the present disclosure is not limited to these examples. Note that parts and percentages are by mass unless otherwise specified.

[0096] [Example 1] (1) Mixed liquid preparation process First, the following materials were mixed to form an oil phase. First polymerizable monomer: 80 parts ethylene glycol dimethacrylate and 20 parts pentaerythritol tetraacrylate 2,2'-Azobis(2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator, manufactured by Wako Pure Chemical Industries, Ltd., trade name: V-65) 3 parts Rosin acid (manufactured by Arakawa Chemical Co., Ltd., product name: Disproportionated Rosin Rhongis R-CH, softening point 150℃ or higher, acid value: 150~160mgKOH / g) 0.007 parts 187 parts cyclohexane Next, in a stirring tank, an aqueous solution of 12.1 parts of sodium hydroxide (alkali metal hydroxide) in 121 parts of ion-exchanged water was gradually added under stirring to an aqueous solution of 17.1 parts of magnesium chloride (a water-soluble polyvalent metal salt) in 494 parts of ion-exchanged water at room temperature to prepare a magnesium hydroxide colloid (poorly water-soluble metal hydroxide colloid) dispersion (4 parts magnesium hydroxide) as the aqueous phase. The resulting aqueous phase and oil phase were mixed to prepare a mixed liquid.

[0097] (2) Suspension process The mixture obtained in the mixture preparation step was stirred and suspended for 1 minute using a disperser (manufactured by Primix Corporation, trade name: Homomixer) at a rotation speed of 4,000 rpm, to prepare a suspension in which droplets of the monomer composition encapsulating cyclohexane were dispersed in water.

[0098] (3) Polymerization process The suspension obtained in the suspension step was heated in a nitrogen atmosphere from 40°C to 65°C over 30 minutes (heating rate: 50°C / hour), and stirred at 65°C for 1 hour and 30 minutes to carry out a first polymerization reaction, yielding a first precursor composition containing first precursor particles. The polymerization conversion rate at the end of the first polymerization reaction was 99.2% by mass. Subsequently, 5 parts of methyl acrylate as a second polymerizable monomer was added to the stirring tank, and the mixture was stirred under a nitrogen atmosphere at 65°C for 2 hours and 30 minutes to carry out a second polymerization reaction. This second polymerization reaction yielded a second precursor composition containing second precursor particles encapsulating cyclohexane.

[0099] (4) Washing process and solid-liquid separation process The second precursor composition was washed with dilute sulfuric acid (25°C, 10 minutes) to adjust the pH to 5.5 or less. Next, after separating the water by filtration, 200 parts of ion-exchanged water was added to re-slurry the mixture. The water washing process (washing, filtration, dehydration) was repeated several times at room temperature (25°C), and the mixture was filtered to obtain a solid fraction. The obtained solid fraction was dried in a dryer at 40°C to obtain second precursor particles containing cyclohexane.

[0100] (5) Solvent removal process The second precursor particles obtained in the solid-liquid separation step were heat-treated in a vacuum dryer at 200°C for 6 hours to remove the hydrocarbon solvent contained in the particles, thereby obtaining the hollow particles of Example 1. From the results of observation with a scanning electron microscope and the porosity value, it was confirmed that the obtained hollow particles were spherical and had hollow portions.

[0101] [Examples 2 to 5] Hollow particles of Examples 2 to 5 were produced in the same manner as in Example 1, except that the material of the second polymerizable monomer added in the above "(3) polymerization step" was as shown in Table 1.

[0102] [Example 6] The hollow particles of Example 6 were produced in the same manner as in Example 1, except that in the above "(1) Mixture preparation step", the material and amount of the first polymerizable monomer were changed as shown in Table 1.

[0103] [Examples 7 to 8, 10] The hollow particles of Examples 7 to 8 and 10 were produced in the same manner as in Example 1, except that the amount of methyl acrylate added as the second polymerizable monomer in the above "(3) polymerization step" was set as shown in Table 1.

[0104] [Example 9] The hollow particles of Example 9 were produced in the same manner as in Example 1, except that rosin acid was not added as a particle size regulator in the above "(1) mixed solution preparation step."

[0105] [Comparative Example 1] Hollow particles of Comparative Example 1 were produced in the same manner as in Example 1, except that in the above "(3) polymerization step", the second polymerizable monomer was not added and the second polymerization reaction was not carried out.

[0106] Comparative Example 2 Hollow particles of Comparative Example 2 were produced in the same manner as in Example 1, except that in the above "(3) polymerization step" in Example 1, 5 parts of styrene (solubility in distilled water at 20°C: 0.2 g / L) was added as the second polymerizable monomer instead of 5 parts of methyl acrylate.

[0107] Comparative Example 3 The hollow particles of Comparative Example 3 were produced in the same manner as in Example 1, except that in the above "(3) polymerization step", the reaction time of the first polymerization reaction was changed from 1 hour 30 minutes to 30 minutes, and when the total polymerization conversion rate of the first polymerizable monomers, ethylene glycol dimethacrylate and pentaerythritol tetraacrylate, reached 91.0 mass%, the second polymerizable monomer was added to carry out the second polymerization reaction.

[0108] Comparative Example 4 In Example 1, except that the material and amount of the first polymerizable monomer were as shown in Table 1 in the above “(1) Mixed solution preparation step”, hollow particles of Comparative Example 4 were produced in the same procedure as in Example 1.

[0109] [Comparative Example 5] In Example 1, except that the material and amount of the first polymerizable monomer were as shown in Table 1 in the above “(1) Mixed solution preparation step” and the second polymerizable monomer was not added and the second polymerization reaction was not carried out in the above “(3) Polymerization step”, hollow particles of Comparative Example 5 were produced in the same procedure as in Example 1.

[0110] [Table 1]

[0111] [Evaluation] 1. Polymerization conversion rate In the polymerization step of each example and each comparative example, 50 g of the first precursor composition produced by the first polymerization reaction was collected and pressure-filtered to obtain the first precursor particles (including moisture and hydrocarbon solvent) contained in the first precursor composition, and weighed accurately to the unit of 1 mg. To about 3 g of the accurately weighed first precursor particles, 27 g of ethyl acetate was added and stirred for 15 minutes, and then 13 g of methanol was added and stirred for another 10 minutes. The obtained solution was allowed to stand to precipitate insoluble matter, and the supernatant of this solution was collected as a measurement sample. 2 μL of the measurement sample was injected into a gas chromatograph, and the amount of the polymerizable monomer in the measurement sample was quantified by gas chromatography (GC) under the following conditions, and this was taken as the mass of the unreacted first polymerizable monomer. Further, the first precursor particles obtained by pressure filtration were dried at 200 °C for 2 hours to remove moisture and hydrocarbon solvent, and the mass of the solid content of the first precursor particles was determined. Then, the polymerization conversion rate was calculated by the following formula (A). Polymerization conversion rate (mass%) = 100 - (mass of unreacted first polymerizable monomer / mass of solid content of first precursor particles) × 100 Formula (A) [GC conditions] Column: TC-WAX (0.25 mm × 30 m) Column temperature: 80 °C Injection temperature: 200℃ FID detection temperature: 200°C

[0112] Table 2 shows the content (mass %) of each monomer unit in the polymer contained in the shell of the hollow particles obtained in each Example and Comparative Example. The hollow particles obtained in each example and comparative example were subjected to the following measurements and evaluations. The results are shown in Table 2.

[0113] 2. Volume average diameter of hollow particles The particle diameter of the hollow particles was measured using a laser diffraction particle size distribution analyzer (Shimadzu Corporation, trade name: SALD-2000), and the volume average was calculated to obtain the volume average particle diameter.

[0114] 3. Density and porosity of hollow particles 3-1. Measurement of apparent density of hollow particles First, a capacity of 100cm 3 Approximately 30 cm 3 The volumetric flask was filled with 10 ... 3 ) was calculated. Formula (I) Apparent density D1 = [Mass of hollow particles] / (100 - [Mass of isopropanol] ÷ [Specific gravity of isopropanol at measurement temperature])

[0115] 3-2. Measurement of true density of hollow particles After crushing the hollow particles in advance, 3 Approximately 10 g of crushed pieces of hollow particles was filled into a measuring flask, and the mass of the packed crushed pieces was accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol is added to the measuring flask, the mass of isopropanol is accurately weighed, and the true density D0 (g / cm) of the hollow particles is calculated based on the following formula (II).3 ) was calculated. Formula (II) True density D0 = [mass of crushed hollow particle fragments] / (100 - [mass of isopropanol] ÷ [specific gravity of isopropanol at measurement temperature])

[0116] 3-3. Calculation of void ratio The porosity of the hollow particles was calculated from the apparent density D1 and true density D0 of the hollow particles according to the following formula (III). Formula (III) Porosity (%) = 100 - (apparent density D1 / true density D0) x 100

[0117] 4. Shell thickness of hollow particles The inner diameter r of the hollow particles was calculated using the volume average particle diameter R and porosity of the hollow particles according to the following formula (1), and the shell thickness of the hollow particles was calculated using the inner diameter r and the volume average particle diameter R according to the following formula (2). 4 / 3π×(R / 2) 3 ×Porosity=4 / 3π×(r / 2) 3 Formula (1) Shell thickness = (Rr) / 2 Equation (2)

[0118] 5. Immersion test In an environment of 25°C, 0.1 mg of hollow particles were added to 4 mL of acetone, and the mixture was shaken for 10 minutes at a shaking speed of 100 rpm using a shaker. After leaving the mixture to stand for 48 hours, the percentage of hollow particles that precipitated was determined and evaluated according to the following evaluation criteria. The hollow particles that precipitated in the acetone were separated using a centrifuge, dried, and the mass of the hollow particles that precipitated in the acetone was measured. The percentage of the precipitated hollow particles was determined by calculating the ratio of the mass of the hollow particles that precipitated in the acetone to the total mass of the hollow particles immersed in the acetone. (Evaluation criteria for immersion test) ○: The precipitated hollow particles are less than 5% by mass ×: Precipitated hollow particles are 5% by mass or more

[0119] 6. Residual monomer amount Weigh 3 g of hollow particles to the precision of 1 mg, add 27 g of ethyl acetate, and stir for 15 minutes. Then add 13 g of methanol and stir for an additional 10 minutes. Let the resulting solution stand to precipitate the insoluble matter, and collect the supernatant of this solution as a measurement sample. Inject 2 μl of the measurement sample into a gas chromatograph and quantify the amount of unreacted polymerizable monomer in the measurement sample by gas chromatography (GC) under the following conditions. Calculate the content ratio of the unreacted polymerizable monomer contained in the hollow particles and use it as the residual monomer amount. <GC Conditions> Column: TC-WAX (0.25 mm × 30 m) Column temperature: 80 °C Injection temperature: 200 °C FID detection side temperature: 200 °C

[0120] 7. Void retention rate in the molded body 90 parts of polypropylene as a thermoplastic resin (manufactured by Mitsubishi Chemical, product name: MA1B, specific gravity 0.90 g / cm 3 ) and 10 parts of the hollow particles obtained in each example or each comparative example were mixed with a blender. Then, using a twin-screw kneader (manufactured by Toshiba Machine Co., Ltd., product name: TEM-35B), knead under the following kneading conditions, extrude, and pelletize to obtain pellets of the resin composition. <Kneading Conditions> Screw diameter 37 mm, L / D = 32 Screw rotation speed 250 rpm Resin temperature 190 °C Feed rate 20 kg / hour [[ID=2*]] The pellets of the obtained resin composition were heated and dried at 80 °C for 6 hours, and then using an injection molding device, molded under the following molding conditions to obtain a molded body with dimensions of 80 mm × 10 mm × thickness 4 mm. <Molding Conditions> Cylinder temperature: 230 °C Mold temperature: 40 °C Injection pressure: 70 MPa Using a: the specific gravity of the molded body after injection molding, b: the specific gravity of the molded body (calculated value) assuming that the voids were maintained, and c: the specific gravity of the molded body (calculated value) assuming that all the hollow particles were crushed, the void retention rate was calculated by the following formula (B). Porosity remaining rate (%)=[1-{(ca) / (cb)}]×100 Formula (B) The specific gravity of the injection-molded article was measured by the underwater displacement method in accordance with JIS K 7112. The specific gravity b of the molded body, assuming that voids were maintained, was calculated by the following formula (C). b=1 / {(P A / P G )+(R A / R G )} Formula (C) In the formula for calculating b above, P A is the amount of hollow particles added, P G is the specific gravity of the hollow particles, R A is the amount of thermoplastic resin added, R G represents the specific gravity of the thermoplastic resin. The specific gravity c of the molded body, assuming that all the hollow particles were crushed, was calculated by the following formula (D). c=[R G ×R A +{D0×P A ×(1-P V / 100)}] / {R A +P A ×(1-P V / 100)} Formula (D) In the formula for calculating c above, R A is the amount of thermoplastic resin added, R G is the specific gravity of the thermoplastic resin, D0 is the true density of the hollow particles, P A is the amount of hollow particles added, P V and represent the porosity (%) of the hollow particles.

[0121] [Table 2]

[0122] In Tables 1 and 2, the abbreviations have the following meanings: MMA: Methyl methacrylate MA: methyl acrylate EA: Ethyl acrylate BA: butyl acrylate AN: Acrylonitrile ST: styrene

[0123] [Consideration] As shown in Table 2 above, the hollow particles obtained in each comparative example had a high porosity of 65%, but the residual void ratio of the molded body of the resin composition containing the hollow particles was low, and it was revealed that the hollow particles were easily crushed. In Comparative Example 1, the second polymerizable monomer was not added, so the amount of residual monomer contained in the obtained hollow particles was large, and the amount of hollow particles that precipitated in acetone in the immersion test was 5 mass % or more. In Comparative Example 1, the polymerization reaction was carried out in one step, so unreacted polymerizable functional groups remained in the shell and the crosslinked structure of the shell was rough, which is presumably why the obtained hollow particles were easily penetrated by acetone and easily crushed. In Comparative Example 2, styrene, which has a solubility of 0.2 g / L in distilled water at 20°C, was used as the second polymerizable monomer instead of a hydrophilic monomer, which has a solubility of 0.3 g / L or more in distilled water at 20°C. As a result, the amount of residual monomer contained in the obtained hollow particles was large, and in the immersion test, 5 mass% or more of the hollow particles precipitated in acetone. In Comparative Example 2, it is presumed that the second polymerizable monomer was not easily incorporated into the shell, so unreacted polymerizable functional groups remained in the shell and the crosslinked structure of the shell was rough, which made the obtained hollow particles easily permeated with acetone and easily crushed. In Comparative Example 3, the second polymerizable monomer was added before the polymerization conversion rate of the first polymerizable monomer reached 93% by mass, so the amount of residual monomer contained in the obtained hollow particles was large, and 5% by mass or more of the hollow particles precipitated in acetone in the immersion test. In Comparative Example 3, it is presumed that the timing of adding the second polymerizable monomer was too early, so that unreacted polymerizable functional groups remained in the shell and the crosslinked structure of the shell was rough, which made the obtained hollow particles easily penetrated by acetone and easily crushed. In Comparative Example 4, the content of the crosslinkable monomer in 100 parts by mass of the first polymerizable monomer was less than 75 parts by mass, so the resulting hollow particles contained a large amount of residual monomer, and 5% by mass or more of the hollow particles precipitated in acetone in the immersion test. In Comparative Example 4, the content of the crosslinkable monomer in the first polymerizable monomer was low, and further, a large amount of unreacted non-crosslinkable monomer remained, so the crosslinked structure of the shell was coarse, and it is presumed that the resulting hollow particles were easily penetrated by acetone and easily crushed. In Comparative Example 5, methyl methacrylate was added to the mixed solution in one step together with the first polymerizable monomer, rather than as the second polymerizable monomer, so the amount of residual monomer contained in the obtained hollow particles was large, and in the immersion test, 5 mass% or more of the hollow particles precipitated in acetone. In Comparative Example 5, the polymerization reaction was carried out in one step, so unreacted polymerizable functional groups remained in the shell, and the crosslinked structure of the shell was rough. It is presumed that this was because the obtained hollow particles were easily penetrated by acetone and easily crushed.

[0124] In contrast, the hollow particles obtained in each example had a high porosity of 65%, and the molded articles of the resin compositions containing the hollow particles had a high void retention rate, demonstrating that they were resistant to crushing despite their high porosity. In Examples 1 to 10, the mixed solution contained 75 to 100 parts by weight of the first polymerizable monomer per 100 parts by weight of the first polymerizable monomer. When the polymerization conversion of the first polymerizable monomer reached 93% by weight or more, a second polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C was added for further polymerization. Therefore, the resulting hollow particles contained almost no unreacted polymerizable monomer, and less than 5% by weight of the hollow particles precipitated in acetone during the immersion test. In Examples 1 to 10, despite the use of a large amount of crosslinkable monomer, almost no unreacted polymerizable functional groups remained in the shell, resulting in a dense crosslinked shell structure. This is presumably why the resulting hollow particles were resistant to acetone penetration and were resistant to crushing even with a high porosity. [Explanation of symbols]

[0125] 1 Aqueous medium 2 Low polarity material 3. Dispersion stabilizer 4. Monomer composition 4a Hydrocarbon solvents 4b Materials other than hydrocarbon solvents 4c Polymerizable monomer dispersed in an aqueous medium 5. Oil-soluble polymerization initiator 6 shells 8 Hollow part 10 droplets 20 Hollow particles containing a hydrocarbon solvent in the hollow space (second precursor particles) 100 Hollow particles with the hollow space filled with gas

Claims

1. A method for producing hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, the hollow portion having a porosity of 50% or more, comprising the steps of: preparing a mixed liquid containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium; a step of suspending the mixed liquid to prepare a suspension in which droplets of a monomer composition containing the first polymerizable monomer and the hydrocarbon solvent are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction, the mixed solution contains a crosslinkable monomer as the first polymerizable monomer, and the content of the crosslinkable monomer in 100 parts by mass of the first polymerizable monomer is 75 to 100 parts by mass; in the step of subjecting the suspension to a polymerization reaction, when the polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, a second polymerizable monomer which is a non-crosslinkable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C is added, and the suspension is further subjected to a polymerization reaction; the second polymerizable monomer is at least one selected from the group consisting of (meth)acrylic acid alkyl esters having an alkyl group of 1 to 5 carbon atoms, (meth)acrylamides and derivatives thereof, (meth)acrylic nitrile, (meth)acrylic acid, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, glycidyl (meth)acrylate, and 4-hydroxybutyl acrylate glycidyl ether.

2. 2. The method for producing hollow particles according to claim 1, wherein in the step of subjecting the suspension to a polymerization reaction, an added amount of the second polymerizable monomer is 3 to 15 parts by mass per 100 parts by mass of the first polymerizable monomer.

3. 3. The method for producing hollow particles according to claim 1, wherein the first polymerizable monomer contains, as the crosslinkable monomer, a bifunctional crosslinkable monomer and a tri- or higher functional crosslinkable monomer.

4. 4. The method for producing hollow particles according to claim 1, wherein the first polymerizable monomer contains, as the crosslinkable monomer, a trifunctional or higher functional crosslinkable monomer having three or more polymerizable functional groups, and the content of the trifunctional or higher functional crosslinkable monomer in 100 parts by mass of the first polymerizable monomer is 5 to 50 parts by mass.

5. 5. The method for producing hollow particles according to claim 1, wherein the first polymerizable monomer contains, as the crosslinkable monomer, at least one bifunctional crosslinkable monomer selected from the group consisting of divinylbenzene, ethylene glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate.

6. 6. The method for producing hollow particles according to claim 1, wherein the first polymerizable monomer contains, as the crosslinkable monomer, at least one tri- or higher functional crosslinkable monomer selected from the group consisting of pentaerythritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, and dipentaerythritol poly(meth)acrylate.

7. The method for producing hollow particles according to any one of claims 1 to 6, wherein the mixed solution contains at least one selected from the group consisting of rosin acid, higher fatty acid, and metal salts thereof.

8. The method for producing hollow particles according to any one of claims 1 to 7, wherein the dispersion stabilizer is an inorganic dispersion stabilizer.

9. The method for producing hollow particles according to claim 8 , wherein the inorganic dispersion stabilizer is a poorly water-soluble metal salt.

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