Method for producing hollow particles and hollow particles

Abstract

The present invention provides a method for producing hollow particles, said method being capable of suppressing variation in shell thickness even in cases where hollow particles having a volume average particle diameter of 10 μm or more are produced. A method for producing hollow particles, each of which is provided with a shell that contains a resin and a hollow part that is surrounded by the shell, while having a void fraction of 50% or more, said method comprising: a step for preparing a mixed liquid that contains a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer and an aqueous medium; a step for preparing a suspension, in which droplets of a monomer composition that contains the polymerizable monomer, the hydrophobic solvent and the polymerization initiator are dispersed in the aqueous medium, by suspending the mixed liquid; and a step for preparing a precursor composition that contains precursor particles, each of which has a hollow part surrounded by a shell that contains a resin and internally contains the hydrophobic solvent in the hollow part, by subjecting the suspension to a polymerization reaction. With respect to this method for producing hollow particles, the polymerizable monomer comprises a crosslinkable monomer; and the HSP distance between the crosslinkable monomer and the hydrophobic solvent is from 5.80 to 6.50.

Description

[Technical Field]

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

[0002] Hollow particles (hollow resin particles) are particles with a cavity inside them, and compared with solid particles whose interiors are substantially filled with resin, they scatter light well and have low light transmittance, and therefore 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. In recent years, they have also been used as weight-reducing agents and heat-insulating agents for resins and paints used in various fields such as automobiles, electricity, electronics, and construction.

[0003] For example, Patent Document 1 discloses a method for producing hollow resin particles, comprising the steps of: mixing a crosslinkable monomer and a hydrophobic solvent with water to obtain crosslinkable monomer droplets containing the hydrophobic solvent; polymerizing the crosslinkable monomer in the crosslinkable monomer droplets to obtain resin particles containing a polymer of the crosslinkable monomer containing the hydrophobic solvent; and removing the hydrophobic solvent from the resin particles to obtain hollow resin particles containing the polymer of the crosslinkable monomer, wherein the crosslinkable monomer contains one or more polyfunctional monomers having three or more polymerizable double bonds. Patent Document 1 also discloses that a polyfunctional monomer and a monofunctional monomer are copolymerized to improve the strength of the hollow particles. Patent Document 1 also discloses that toluene and hexane are preferred hydrophobic solvents. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-33503 Summary of the Invention [Problem to be solved by the invention]

[0005] For hollow particles with the same porosity, the larger the particle size, the thicker the shell. The thicker the shell, the better the pressure resistance of hollow particles. Therefore, hollow particles with a large particle size are useful in that they have excellent pressure resistance while maintaining a high porosity. However, when hollow particles having a volume average particle size of 10 μm or more are produced by conventional production methods, porous particles are produced, or if hollow particles having a hollow portion are obtained, the shell thickness varies greatly. In Comparative Example 6 described below, the same combination of polymerizable monomer and hydrophobic solvent as in Example 1 of Patent Document 1 was used, but no hollow portion was formed inside the particles, and porous particles were produced. Hollow particles with large variations in shell thickness tend to have non-uniform particle properties, particularly non-uniform pressure resistance, so it is desirable for hollow particles to have small variations in shell thickness. An object of the present disclosure is to provide a method for producing hollow particles that can suppress variation in shell thickness even when producing hollow particles having a volume average particle size of 10 μm or more, and hollow particles obtained by the production method and having suppressed variation in shell thickness. [Means for solving the problem]

[0006] The present inventors have found that, when producing hollow particles having a volume average particle size of 10 μm or more by suspension polymerization, in order to suppress variations in shell thickness, it is effective to adjust the HSP distance between a crosslinkable monomer used as a polymerizable monomer and a hydrophobic solvent encapsulated in the particles during the production process within a specific range.

[0007] 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 polymerizable monomer, a hydrophobic solvent, a polymerization initiator, 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 polymerizable monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction to prepare a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a resin, and encapsulating the hydrophobic solvent in the hollow portion; the polymerizable monomer includes a crosslinkable monomer, The method for producing hollow particles is provided, wherein the HSP distance between the crosslinkable monomer and the hydrophobic solvent is 5.80 or more and 6.50 or less.

[0008] In the method for producing hollow particles according to the present disclosure, the hydrophobic solvent preferably contains two or more hydrophobic solvents.

[0009] In the method for producing hollow particles according to the present disclosure, it is preferable that the crosslinkable monomer includes a bifunctional crosslinkable monomer having two polymerizable functional groups, and that the content of the bifunctional crosslinkable monomer in 100 parts by mass of the polymerizable monomer is 70 parts by mass or more and 100 parts by mass or less.

[0010] In the method for producing hollow particles according to the present disclosure, the crosslinkable monomer preferably includes a bifunctional crosslinkable monomer having two polymerizable functional groups, and a trifunctional or higher crosslinkable monomer having three or more polymerizable functional groups.

[0011] 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.

[0012] 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.

[0013] In the method for producing hollow particles of the present disclosure, the volume average particle size of the hollow particles is preferably 10 μm or more and 50 μm or less.

[0014] The present disclosure provides a hollow particle having a shell containing a resin and a hollow portion surrounded by the shell, the hollow particle having a porosity of 50% or more, The volume average particle size is 10 μm or more and 50 μm or less, The ratio of the measured shell thickness to the theoretical shell thickness (measured shell thickness / theoretical shell thickness) is 0.80 or more and 1.00 or less, The theoretical shell thickness is a value calculated by calculating the inner diameter r of the hollow particles according to the following formula (1) using the volume average particle diameter R and porosity of the hollow particles, and then calculating the inner diameter r and volume average particle diameter R according to the following formula (2). 4 / 3π×(R / 2) 3 ×(porosity / 100)=4 / 3π×(r / 2) 3 Formula (1) Theoretical shell thickness = (Rr) / 2 Equation (2) [Effects of the Invention]

[0015] According to the manufacturing method of the present disclosure as described above, even when hollow particles having a volume average particle size of 10 μm or more are manufactured, hollow particles with reduced variations in shell thickness can be obtained. [Brief explanation of the drawings]

[0016] [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. [Figure 3] 1 is an SEM image obtained when observing the internal state of hollow particles obtained in Example 2. [Figure 4] 1 is an SEM image of the internal state of the particles obtained in Comparative Example 1. [Figure 5] 1 is an SEM image of the internal state of the particles obtained in Comparative Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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.

[0018] 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 is clearly distinguishable from the numerous minute spaces uniformly dispersed within the porous structure. According to the manufacturing method of the present disclosure, the shell of the hollow particle can be made solid. 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 of the present disclosure obtained by the production method of the present disclosure will be described in detail.

[0019] 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, comprising: preparing a mixed liquid containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, 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 polymerizable monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction to prepare a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a resin, and encapsulating the hydrophobic solvent in the hollow portion; the polymerizable monomer includes a crosslinkable monomer, The HSP distance between the crosslinkable monomer and the hydrophobic solvent is 5.80 or more and 6.50 or less.

[0020] The method for producing hollow particles disclosed herein follows a basic technique in which a mixed liquid containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium is suspended, whereby the polymerizable monomer and the hydrophobic solvent undergo phase separation, thereby preparing a suspension in which droplets having a distribution structure in which the polymerizable monomer is unevenly distributed on the surface side and the hydrophobic 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 hydrophobic solvent. The present inventors have found that when hollow particles having a volume average particle size of 10 μm or more are produced by a conventional method based on the above-mentioned basic technology, either no hollow portion is formed and the particles become porous, or even if a hollow portion is formed, the shell thickness varies greatly, and further, fine resin particles having a particle size much smaller than that of the hollow particles are generated inside the hollow portion. In the above-mentioned basic technology, a suspension is subjected to a polymerization reaction, which causes the polymerization reaction of the polymerizable monomer in the droplets to proceed, resulting in the precipitation of the resulting polymer, forming a shell. Using a polymerizable monomer containing a crosslinkable monomer increases the crosslink density of the resulting shell, resulting in a shell with excellent strength. On the other hand, polymers of polymerizable monomers containing a crosslinkable monomer tend to precipitate rapidly after the start of the polymerization reaction due to their high crosslink density. Therefore, when producing hollow particles with a volume average particle size of 10 μm or more, using a polymerizable monomer containing a crosslinkable monomer is likely to result in uneven precipitation of the polymer on the shell surface, resulting in large variations in shell thickness. Furthermore, the precipitation of the polymer in the form of fine particles within the hollows is likely to result in the generation of fine resin particles with a particle size significantly smaller than that of the hollow particles. Using a hydrophobic solvent with better compatibility with the polymer to slow the precipitation of the polymer is likely to result in the formation of porous particles due to insufficient phase separation between the polymerizable monomer and the hydrophobic solvent within the monomer composition droplets. In contrast, in the production method of the present disclosure, by selecting and using a crosslinkable monomer and a hydrophobic solvent so that the HSP distance between the crosslinkable monomer in the polymerizable monomer and the hydrophobic solvent is 5.80 to 6.50, hollow particles having a clearly distinguishable hollow portion from the shell, reduced shell thickness variation, and reduced generation of fine resin particles within the hollow portion can be obtained, even when producing hollow particles having a volume average particle size of 10 μm or more. In the production method of the present disclosure, the compatibility between the polymerizable monomer and the hydrophobic solvent is just right for producing hollow particles having a volume average particle size of 10 μm or more. This is thought to result in sufficient phase separation between the polymerizable monomer and the hydrophobic solvent in the monomer composition droplets, and when the suspension is subjected to a polymerization reaction, the polymer precipitates at an appropriate rate to achieve a uniform shell thickness and to suppress polymer precipitation within the hollow portion.

[0021] The HSP distance is an index that expresses the solubility between substances using the Hansen Solubility Parameter (HSP). The closer the HSP distance is to 0, the higher the compatibility between the substances. HSP is expressed as a vector in a three-dimensional space (Hansen space) with the dispersion term dD, the polar term dP, and the hydrogen bond term dH as its coordinate axes. The three parameters dD, dP, and dH have unique values ​​for each substance. The software developed by Hansen et al. (Hansen Solubility Parameter in Practice (HSPiP)) contains a database of dD, dP, and dH for various substances. HSPiP can also be used to calculate HSP based on the chemical structure of a substance. When calculating the HSP of a mixture containing multiple substances, the weighted averages of the dD, dP, and dH values ​​of each substance contained in the mixture and the proportion of each substance are calculated to determine the dispersion term dD, polarity term dP, and hydrogen bond term dH of the mixture, and the HSP is calculated. The HSP distance is the distance between vectors given by the HSP of two substances, and is calculated using the following formula (A) from the values ​​of the dispersion term dD1, polar term dP1, and hydrogen bond term dH1 of one substance and the values ​​of the dispersion term dD2, polar term dP2, and hydrogen bond term dH2 of the other substance. Formula (A) HSP distance={4(dD1-dD2) 2 +(dP1-dP2) 2 +(dH1-dH2) 2} 0.5 The distance from the origin of the Hansen space to the HSP is called the total HSP, and is calculated from the dispersion term dD, the polarity term dP, and the hydrogen bond term dH using the following formula (B): Formula (B) total HSP=(dD 2 +dP 2 +dH 2 ) 0.5 In this disclosure, the HSP distance and total HSP are values ​​calculated using HSPiP (Version 5.3.03). In HSPiP, the values ​​of dD, dP, and dH are expressed with significant figures up to one decimal place, and the values ​​of HSP distance and total HSP are expressed with significant figures up to two decimal places.

[0022] In the manufacturing method of the present disclosure, the crosslinkable monomer and the hydrophobic solvent are selected so that the HSP distance between the crosslinkable monomer and the hydrophobic solvent is 5.80 or more and 6.50 or less, preferably 5.85 or more and 6.40 or less, and more preferably 5.90 or more and 6.30 or less. The HSP distance between the crosslinkable monomer and the hydrophobic solvent can be adjusted within the above range by selecting the preferred crosslinkable monomer and hydrophobic solvent, as described below.

[0023] 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.

[0024] 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 polymerizable monomer, a hydrophobic solvent, a polymerization initiator, 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 polymerizable monomer, a hydrophobic solvent, and a polymerization initiator are dispersed in an aqueous medium. (3) Polymerization process a step of subjecting the suspension to a polymerization reaction to prepare a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a resin and encapsulating a hydrophobic solvent in the hollow portion; (4) Solid-liquid separation process A step of obtaining precursor particles having a hydrophobic solvent encapsulated in a hollow portion by performing solid-liquid separation of the precursor composition; and (5) Solvent removal process a step of removing the hydrophobic solvent contained in the precursor particles obtained by the solid-liquid separation step to obtain hollow particles; In the present disclosure, hollow particles whose hollow portions are filled with a hydrophobic solvent are considered to be intermediates of hollow particles whose hollow portions are filled with a gas, and may be referred to as "precursor particles." In the present disclosure, the term "precursor composition" refers to a composition containing precursor particles.

[0025] 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 explanation, 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 the 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 polymerizable monomer, a hydrophobic solvent, and a polymerization initiator. 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 polymerizable monomer, a hydrophobic solvent, and a polymerization initiator, but the distribution within the droplets is non-uniform. The droplets 10 of the monomer composition are phase-separated into a hydrophobic solvent 4a and a material other than the hydrophobic solvent, including the polymerizable monomer, a material 4b, with the hydrophobic solvent 4a unevenly distributed in the center and the material other than the hydrophobic solvent a material 4b unevenly distributed on the surface, with a dispersion stabilizer (not shown) attached to the surface. 1(3) is a cross-sectional schematic diagram showing one embodiment of a precursor composition containing precursor particles encapsulating a hydrophobic solvent in their hollow spaces, obtained by a polymerization step. The precursor composition contains an aqueous medium 1 and precursor particles 20 encapsulating a hydrophobic solvent 4a in their hollow spaces, dispersed in the aqueous medium 1. A shell 6 forming the outer surface of the precursor particles 20 is formed by polymerization of a polymerizable monomer in droplets 10 of the monomer composition, and contains a polymer of the polymerizable monomer as a resin. Fig. 1(4) is a cross-sectional schematic diagram showing one embodiment of precursor particles after the solid-liquid separation step, 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 the state after removing the hydrophobic solvent 4a from the state shown in Fig. 1 (4). By removing the hydrophobic solvent from the precursor particles, hollow particles 100 having gas-filled hollow portions 8 inside the shells 6 are obtained. The above five steps and other steps will be explained below in order.

[0026] (1) Mixed liquid preparation process This step is a step of preparing a mixed solution containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium. The mixed solution may further contain other materials 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) polymerizable monomer, (B) hydrophobic solvent, (C) polymerization initiator, (D) dispersion stabilizer, (E) aqueous medium, and (F) other materials.

[0027] (A) Polymerizable monomer In the production method of the present disclosure, the 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 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.

[0028] [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 crosslinkable monomers include difunctional crosslinkable monomers having two polymerizable functional groups, such as divinylbenzene, divinyldiphenyl, divinylnaphthalene, diallyl phthalate, allyl (meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, and pentaerythritol di(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 thereof. These crosslinkable monomers can be used alone or in combination of two or more. As the bifunctional crosslinkable monomer, divinylbenzene, ethylene glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate are preferred because they facilitate stable polymerization reactions and provide hollow particles with excellent strength and heat resistance. Among these, ethylene glycol di(meth)acrylate and pentaerythritol di(meth)acrylate are preferred because they facilitate suppression of variations in shell thickness. As the trifunctional or higher crosslinkable monomer, 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 are preferred, as they facilitate stable polymerization reactions and produce hollow particles with excellent strength and heat resistance. Among these, pentaerythritol tetra(meth)acrylate is preferred, as it facilitates suppression of variations in shell thickness.

[0029] Furthermore, although not particularly limited, the crosslinkable monomer used in the production method of the present disclosure has a total HSP of 17.50 to 18.50 MPa because the HSP distance between the crosslinkable monomer and the hydrophobic solvent is likely to be within the above range. 1 / 2 Preferably, the pressure is 17.60 to 18.20 MPa. 1 / 2 More preferably, it is 17.70 to 18.00 MPa. 1 / 2 It is more preferable that:

[0030] The content of the crosslinkable monomer per 100 parts by mass of the polymerizable monomer is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, and even more preferably 90 parts by mass or more, and the polymerizable monomer may consist of a crosslinkable monomer. When the content of the crosslinkable monomer is equal to or greater than the above-mentioned lower limit, the content of crosslinkable monomer units in the shell of the hollow particle is sufficiently high, resulting in a dense covalent bond network within the shell, thereby forming a shell that is strong, resistant to crushing, and resistant to deformation even when exposed to external heat, etc. Furthermore, when the content of the crosslinkable monomer is equal to or greater than the above-mentioned lower limit, phase separation between the polymerizable monomer and the hydrophobic solvent is facilitated in the droplets of the monomer composition, thereby further suppressing variation in shell thickness.

[0031] In the production method of the present disclosure, the polymerizable monomer preferably contains at least a bifunctional crosslinkable monomer as the crosslinkable monomer. This makes it easy to adjust the HSP distance between the crosslinkable monomer and the hydrophobic solvent within the above range. From the viewpoints of easily adjusting the HSP distance between the crosslinkable monomer and the hydrophobic solvent within the above range and further improving the strength of the shell by more densely spreading the covalent bond network in the shell, it is preferable to contain a combination of a bifunctional crosslinkable monomer and a trifunctional or higher functional crosslinkable monomer. The content of the bifunctional crosslinkable monomer in 100 parts by mass of the polymerizable monomer is preferably 70 to 100 parts by mass. When the polymerizable monomer contains a trifunctional or higher crosslinkable monomer as a crosslinkable monomer, the upper limit of the content of the bifunctional crosslinkable monomer in 100 parts by mass of the polymerizable monomer is preferably 95 parts by mass or less, more preferably 90 parts by mass or less.

[0032] When the 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 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 the upper limit is preferably 30 parts by mass or less.

[0033] [Non-crosslinkable monomer] The polymerizable monomer may further contain a non-crosslinkable monomer within a range that does not impair the effects of the present disclosure. As the non-crosslinkable monomer, a monovinyl monomer is preferably used, which is a compound having one polymerizable vinyl functional group. Examples of the monovinyl monomer include (meth)acrylic monovinyl monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, glycidyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, and (meth)acrylic acid; aromatic vinyl monomers such as styrene, vinyltoluene, α-methylstyrene, p-methylstyrene, and halogenated styrene; monoolefin monomers such as ethylene, propylene, and butylene; (meth)acrylamide monomers and derivatives thereof such as (meth)acrylamide, N-methylol (meth)acrylamide, and N-butoxymethyl (meth)acrylamide; diene monomers such as butadiene and isoprene; vinyl carboxylic acid ester monomers such as vinyl acetate; halogenated vinyl monomers such as vinyl chloride; halogenated vinylidene monomers such as vinylidene chloride; and vinylpyridine monomers. These non-crosslinkable monomers can be used alone or in combination of two or more. Among them, from the viewpoints of reactivity and heat resistance, (meth)acrylic monovinyl monomers are preferred, and at least one selected from butyl acrylate and methyl methacrylate is more preferred.

[0034] The content of the non-crosslinkable monomer in 100 parts by mass of the polymerizable monomer is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, even more preferably 10 parts by mass or less, and may be 0 part by mass. When the content of the non-crosslinkable monomer is equal to or less than the above upper limit, the content ratio of the crosslinkable monomer becomes sufficiently high, so that the strength and heat resistance of the shell can be improved, and further, phase separation between the polymerizable monomer and the hydrophobic solvent becomes easier in droplets of the monomer composition, so that variation in shell thickness can be further suppressed.

[0035] The content of the 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 preferably 15 to 50 mass%, more preferably 20 to 40 mass%, and even more preferably 20 to 30 mass%, relative to 100 mass% of the total mass of the components in the mixed solution excluding the aqueous medium. Furthermore, from the viewpoint of improving the mechanical strength of the hollow particles, the content of the polymerizable monomer relative to the total mass (100 mass%) of the solid content excluding the hydrophobic solvent among the materials that form the oil phase in the mixed liquid is preferably 90 mass% or more, more preferably 95 mass% or more, and even more preferably 99 mass% or more. 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.

[0036] (B) Hydrophobic solvent The hydrophobic solvent used in the production method of the present disclosure is a non-polymerizable and poorly water-soluble organic solvent. The hydrophobic 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 the monomer composition containing the hydrophobic solvent are dispersed in an aqueous medium. In the suspension process, phase separation occurs within the droplets of the monomer composition, and the hydrophobic solvent, which has low polarity, tends to collect inside the droplets of the monomer composition. Ultimately, the droplets of the monomer composition contain the hydrophobic solvent inside, and other materials other than the hydrophobic 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 hydrophobic solvent is obtained. That is, the hydrophobic solvent collects inside the particles, and hollow portions filled with the hydrophobic solvent are formed inside the obtained precursor particles.

[0037] The hydrophobic solvent is appropriately selected depending on the type of crosslinkable monomer so that the HSP distance between the solvent and the crosslinkable monomer is 5.80 or more and 6.50 or less. The hydrophobic solvent may be any known solvent, and is not particularly limited, but examples thereof include esters such as ethyl acetate and butyl acetate; ether esters such as propylene glycol monomethyl ether acetate and propylene glycol monoethyl ether acetate; aromatic hydrocarbons such as benzene, toluene, and xylene; aliphatic hydrocarbons such as hexane, methylhexane, heptane, octane, cyclohexane, and methylcyclohexane; etc. These hydrophobic solvents may be used alone or in combination of two or more. In particular, the hydrophobic solvent preferably contains at least one selected from the group consisting of esters, aromatic hydrocarbons, and aliphatic hydrocarbons, and more preferably contains at least one selected from the group consisting of aromatic hydrocarbons and aliphatic hydrocarbons, since the HSP distance between the crosslinkable monomer and the hydrophobic solvent is likely to fall within the above range.

[0038] Furthermore, since the HSP distance between the crosslinkable monomer and the hydrophobic solvent is likely to fall within the above range, it is preferable to contain two or more hydrophobic solvents. The combination of two or more hydrophobic solvents is appropriately selected so that the HSP distance falls within the above range, and is not particularly limited, but a combination of one or more selected from aliphatic hydrocarbons with one or more selected from esters and aromatic hydrocarbons is preferred, and among these, a combination of one or more selected from aliphatic hydrocarbons with one or more selected from esters, and a combination of one or more selected from aliphatic hydrocarbons with one or more selected from aromatic hydrocarbons are more preferred. In the above combination, the aliphatic hydrocarbon is preferably one or more selected from the group consisting of hexane, methylhexane, cyclohexane, and methylcyclohexane, more preferably one or more selected from hexane and cyclohexane, and even more preferably cyclohexane. In the above combination, the ester is preferably one or more selected from ethyl acetate and butyl acetate, more preferably ethyl acetate. In the above combination, the aromatic hydrocarbon is preferably one or more selected from benzene, toluene and xylene, more preferably toluene. Furthermore, the hydrophobic solvent used in the production method of the present disclosure is such that the total mass of the hydrophobic solvents in the preferred combination described above contained in 100 parts by mass of the total mass of the hydrophobic solvents is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, even more preferably 90 parts by mass or more, even more preferably 95 parts by mass or more, and particularly preferably 99 parts by mass or more. The hydrophobic solvent used in the production method of the present disclosure is most preferably composed of the hydrophobic solvents in the preferred combination described above.

[0039] In the hydrophobic solvent containing a combination of one or more aliphatic hydrocarbons and one or more esters, the mass ratio of the aliphatic hydrocarbons to the esters (aliphatic hydrocarbons:esters) is not particularly limited, but is preferably 70:30 to 95:5, and more preferably 80:20 to 95:5.

[0040] In a hydrophobic solvent containing a combination of one or more aliphatic hydrocarbons and one or more aromatic hydrocarbons, the mass ratio of the aliphatic hydrocarbons to the aromatic hydrocarbons (aliphatic hydrocarbons:aromatic hydrocarbons) is not particularly limited, but is preferably 20:80 to 80:20, and more preferably 30:70 to 70:30.

[0041] Furthermore, although not particularly limited, the hydrophobic solvent used in the production method of the present disclosure has a total HSP of 16.50 to 18.00 MPa because the HSP distance between the crosslinkable monomer and the hydrophobic solvent is likely to be within the above range. 1 / 2 Preferably, the pressure is 16.60 to 17.80 MPa. 1 / 2 More preferably, the pressure is 16.70 to 17.70 MPa. 1 / 2 It is more preferable that:

[0042] Furthermore, although not particularly limited, the boiling point of the hydrophobic solvent is preferably 130°C or lower, more preferably 120°C or lower, from the viewpoint of ease of removal in the solvent removal step described below, and on the other hand, is preferably 50°C or higher, more preferably 60°C or higher, from the viewpoint of ease of inclusion in the precursor particles. When the hydrophobic solvent is a mixed solvent containing multiple types of hydrophobic solvents and has multiple boiling points, it is preferable that the boiling point of the solvent with the highest boiling point among the solvents contained in the mixed solvent is not more than the above-mentioned upper limit value, and it is preferable that the boiling point of the solvent with the lowest boiling point among the solvents contained in the mixed solvent is not less than the above-mentioned lower limit value.

[0043] A preferred combination in which the HSP distance between the crosslinkable monomer and the hydrophobic solvent is 5.80 or more and 6.50 or less is, for example, a combination in which the crosslinkable monomer contains at least one selected from ethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, and pentaerythritol tetra(meth)acrylate, and the total content of ethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, and pentaerythritol tetra(meth)acrylate relative to 100 parts by mass of the crosslinkable monomer is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, even more preferably 90 parts by mass or more, and even more preferably 100 parts by mass, and the hydrophobic solvent includes a combination of one or more selected from hexane and cyclohexane and one or more selected from ethyl acetate and toluene, and the total content of hexane, cyclohexane, ethyl acetate, and toluene per 100 parts by mass of the hydrophobic solvent is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, even more preferably 90 parts by mass or more, and even more preferably 100 parts by mass. As the crosslinkable monomer, a crosslinkable monomer containing at least ethylene glycol di(meth)acrylate, in which the content of ethylene glycol di(meth)acrylate per 100 parts by mass of the crosslinkable monomer is 70 parts by mass or more, is more preferred. Among the hydrophobic solvents, those which include a combination of toluene with one or more selected from hexane and cyclohexane, wherein the total content of hexane, cyclohexane, and toluene per 100 parts by mass of the hydrophobic solvent is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, even more preferably 90 parts by mass or more, and still more preferably 100 parts by mass, and those which include a combination of ethyl acetate with one or more selected from hexane and cyclohexane, wherein the total content of hexane, cyclohexane, and ethyl acetate per 100 parts by mass of the hydrophobic solvent is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, even more preferably 90 parts by mass or more, and still more preferably 100 parts by mass, are more preferred.

[0044] Specific examples of hydrophobic solvents that have an HSP distance of 5.80 or more and 6.50 or less with ethylene glycol dimethacrylate, a preferred crosslinkable monomer, include a mixed solvent of cyclohexane and toluene in a cyclohexane:toluene mass ratio of 50:50 (HSP distance with ethylene glycol dimethacrylate: 6.04), a mixed solvent of cyclohexane and toluene in a cyclohexane:toluene mass ratio of 70:30 (HSP distance with ethylene glycol dimethacrylate: 6.40), and a mixed solvent of cyclohexane and toluene in a cyclohexane:toluene mass ratio of 40:60. Examples of suitable solvents include a solvent (HSP distance from ethylene glycol dimethacrylate: 5.91), a mixed solvent of cyclohexane and ethyl acetate in a cyclohexane:ethyl acetate mass ratio of 90:10 (HSP distance from ethylene glycol dimethacrylate: 6.01), a mixed solvent of hexane and toluene in a hexane:toluene mass ratio of 60:40 (HSP distance from ethylene glycol dimethacrylate: 6.03), and a mixed solvent of hexane and ethyl acetate in a hexane:ethyl acetate mass ratio of 80:20 (HSP distance from ethylene glycol dimethacrylate: 5.83). Furthermore, examples of hydrophobic solvents that have an HSP distance of 5.80 or more and 6.50 or less with a crosslinkable monomer that is a mixture of ethylene glycol dimethacrylate and pentaerythritol tetraacrylate, in which the mass ratio of ethylene glycol dimethacrylate:pentaerythritol tetraacrylate is 70:30 to 90:10, are the same as the hydrophobic solvents that have an HSP distance of 5.80 or more and 6.50 or less with ethylene glycol dimethacrylate.

[0045] The porosity of the hollow particles can be adjusted by changing the amount of hydrophobic solvent in the mixture. In the suspension step described below, the polymerization reaction proceeds in a state in which the oil droplets containing the crosslinkable monomer and the like encapsulate the hydrophobic solvent, and therefore the porosity of the resulting hollow particles tends to increase as the content of the hydrophobic solvent increases. In the present disclosure, the content of the hydrophobic solvent in the mixed solution is preferably 50 to 500 parts by mass relative to 100 parts by mass of the 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 hydrophobic solvent in the particles. The content of the hydrophobic solvent in the mixed solution is more preferably 60 to 400 parts by mass, and even more preferably 70 to 300 parts by mass relative to 100 parts by mass of the polymerizable monomer.

[0046] (C) Polymerization initiator In the production method of 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. The content of the oil-soluble polymerization initiator relative to 100 parts by mass of the polymerizable monomer in the mixed solution 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. When the content of the oil-soluble polymerization initiator is within the above range, the polymerization reaction proceeds sufficiently, 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.

[0047] (D) 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 a particle size control agent described below. 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 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 metal salt is preferably an inorganic metal salt having a solubility of 0.5 g or less in 100 g of water. In the present disclosure, it is particularly preferred to use the poorly water-soluble inorganic dispersion stabilizer in the form of colloidal particles dispersed in an aqueous medium, i.e., in the form of a colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles. By using the poorly water-soluble inorganic dispersion stabilizer in the form of a colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles, it is possible to narrow the particle size distribution of the droplets of the monomer composition, and also to easily reduce the amount of inorganic dispersion stabilizer remaining in the obtained hollow particles by washing. A colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles can be prepared, for example, by reacting at least one selected from alkali metal hydroxides and alkaline earth metal hydroxides with a water-soluble polyvalent metal salt (excluding alkaline earth metal hydroxides) in an aqueous medium. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, potassium hydroxide, etc. Examples of alkaline earth metal hydroxides include barium hydroxide, calcium hydroxide, etc. The water-soluble polyvalent metal salt may be any water-soluble polyvalent metal salt other than the compounds corresponding to the alkaline earth metal hydroxides, for example, magnesium metal salts such as magnesium chloride, magnesium phosphate, magnesium sulfate, etc.; calcium metal salts such as calcium chloride, calcium nitrate, calcium acetate, calcium sulfate, etc.; aluminum metal salts such as aluminum chloride, aluminum sulfate, etc.; barium salts such as barium chloride, barium nitrate, barium acetate, etc.; zinc salts such as zinc chloride, zinc nitrate, zinc acetate, etc. Among these, magnesium metal salts, calcium metal salts, and aluminum metal salts are preferred, magnesium metal salts are more preferred, and magnesium chloride is particularly preferred. The water-soluble polyvalent metal salts can be used alone or in combination of two or more. The method for reacting at least one selected from the alkali metal hydroxides and alkaline earth metal hydroxides with the water-soluble polyvalent metal salt in an aqueous medium is not particularly limited, but includes a method of mixing an aqueous solution of at least one selected from the alkali metal hydroxides and alkaline earth metal hydroxides with an aqueous solution of a water-soluble polyvalent metal salt. In this case, from the viewpoint of being able to suitably control the particle size of the poorly water-soluble metal hydroxide colloidal particles, a method of mixing is preferred in which the aqueous solution of the water-soluble polyvalent metal salt is stirred while the aqueous solution of at least one selected from the alkali metal hydroxides and alkaline earth metal hydroxides is gradually added to the aqueous solution. Furthermore, from the viewpoint of obtaining hollow particles having a volume average particle size of 10 μm or more and 50 μm or less, it is preferable to use a colloidal dispersion obtained by reacting at least one selected from alkali metal hydroxides and alkaline earth metal hydroxides with a water-soluble polyvalent metal salt in an aqueous medium at a temperature of 40°C or more and 90°C or less.

[0048] The content of the dispersion stabilizer is not particularly limited, but is preferably 0.5 to 10 parts by mass, and more preferably 1.0 to 8.0 parts by mass, per 100 parts by mass of the total mass of the polymerizable monomer and the hydrophobic solvent. By ensuring that the content of the dispersion stabilizer is equal to or greater than the lower limit, the droplets of the monomer composition can be sufficiently dispersed so as not to coalesce in the suspension. On the other hand, by ensuring that 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 clogging in 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.

[0049] (E)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 mass ratio of water to the hydrophilic solvent (water:hydrophilic solvent) may be 99:1 to 50:50.

[0050] (F) Other materials The mixed liquid may further contain other materials different from the above-mentioned materials (A) to (E) as long as the effects of the present disclosure are not impaired. The mixed solution preferably contains a particle size control agent as another material, 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. 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 polymerizable monomer and a hydrophobic 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 hydrophobic solvent and materials other than the hydrophobic solvent, resulting in the hydrophobic solvent being concentrated in the center and materials other than the hydrophobic 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 differences in the 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 these, the particle size control agent is preferably at least one selected from the group consisting of rosin acid, higher fatty acids, and metal salts thereof, and more preferably at least one selected from rosin acid and alkali metal salts thereof, because the particle size of the droplets of the monomer composition and the shell thickness of the hollow particles can be appropriately adjusted with a small amount.

[0051] 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 versions of these acids, and alkali metal salts thereof.

[0052] 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.

[0053] 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.

[0054] 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 total of the polymerizable monomer and the hydrophobic solvent. 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.

[0055] 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.

[0056] Among these, 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 polymerizable monomer and ease of controlling the particle size of the hollow particles. In particular, it is preferable that the polar resin is an acrylic resin in which all repeating units constituting the resin are (meth)acrylic monovinyl monomer units.

[0057] In particular, the polar resin preferably contains a polar group-containing monomer unit in which the heteroatom-containing monomer contains 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, from the viewpoint of facilitating control of the particle size of the hollow particles. Examples of polar group-containing monomers used in polar resins 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. The polar group-containing monomers can be used alone or in combination of two or more. As the polar group contained in the polar group-containing monomer unit contained in the polar resin, a carboxyl group and a hydroxyl group are preferred because they allow particle size control with a small addition amount. 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.

[0058] 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, because it has high compatibility with the polymerizable monomer and makes it easy to control the particle size of the hollow particles. In particular, it is preferable that the polar resin contains 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, because of its high polarity.

[0059] 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 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.

[0060] 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.

[0061] 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.

[0062] 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 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.

[0063] 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 lipophilic materials such as (A) the polymerizable monomer, (B) the hydrophobic solvent, and (C) the polymerization initiator is dispersed in an aqueous phase containing (D) the dispersion stabilizer and (E) the aqueous medium, with particles of about several mm in size. Depending on the type of material, the dispersion state of these materials in the mixed solution can be observed with the naked eye. 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, etc. However, in order to make the shell more uniform, it is preferable to prepare the mixed solution in advance separately from an oil phase containing a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator, and an aqueous phase containing a dispersion stabilizer and an aqueous medium, and then mixing these. In the present disclosure, a colloidal dispersion in which a poorly water-soluble inorganic dispersion stabilizer is dispersed in the form of colloidal particles in an aqueous medium can be preferably used as the aqueous phase. 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, and the particle size of the hollow particles can be easily controlled.

[0064] (2) Suspension process The suspending step is a step of preparing a suspension in which droplets of the monomer composition containing the hydrophobic solvent are dispersed in an aqueous medium by suspending the mixed liquid described above. The method of suspension for forming droplets of the monomer composition is not particularly limited, and may be carried out using a device capable of strong stirring, such as an (in-line type) emulsifying disperser (for example, horizontal in-line dispersers such as Milder, manufactured by Pacific Machinery Works, and Cavitron, manufactured by Eurotech Co., Ltd.; vertical in-line dispersers such as DRS 2000 / 5, manufactured by IKA), or a high-speed emulsifying disperser (for example, TK Homomixer MARK II, manufactured by Primix Corporation). In the suspension prepared in the suspending step, droplets of the monomer composition containing the lipophilic material and having a particle size of about 10 to 50 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 process, phase separation occurs in the droplets of the monomer composition, and the hydrophobic solvent with low polarity tends to collect inside the droplets. As a result, the obtained droplets have the hydrophobic solvent distributed inside and materials other than the hydrophobic solvent distributed around the periphery.

[0065] 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 a monomer composition and a 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 polymerizable monomer and a hydrophobic 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 by-production of excess resin particles such as dense particles having a relatively small particle size in addition to the desired resin particles having hollow portions.

[0066] (3) Polymerization process This step is a step of preparing a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a resin and encapsulating a hydrophobic solvent in the hollow portion by subjecting the suspension obtained by the above-mentioned suspension step to a polymerization reaction. The precursor particles are formed by polymerization of a polymerizable monomer contained in droplets of the monomer composition, and the shell of the precursor particles contains a polymer of the polymerizable monomer as a resin. The polymerization method is not particularly limited, and for example, a batch method, a semi-continuous method, a continuous method, etc. can be used. The polymerization temperature is preferably 40 to 80°C, more preferably 50 to 70°C. The temperature increase rate when increasing the temperature to the polymerization temperature is preferably 10°C / h to 60°C / h, and more preferably 15°C / h to 55°C / h. The polymerization reaction time is preferably 1 to 20 hours, more preferably 2 to 15 hours. In the polymerization process, the shell portion of the droplets of the monomer composition containing the hydrophobic solvent therein is polymerized, and as described above, a hollow portion filled with the hydrophobic solvent is formed inside the resulting precursor particles.

[0067] (4) Solid-liquid separation process This step is a step of obtaining a solid content containing precursor particles by solid-liquid separation of the precursor composition containing precursor particles obtained by the above-mentioned polymerization step.

[0068] The method for solid-liquid separation of the 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.

[0069] (5) Solvent removal process This step is a step for removing the hydrophobic solvent contained in the precursor particles obtained in the solid-liquid separation step. By removing the hydrophobic solvent contained in the precursor particles in the air, the hydrophobic solvent inside the precursor particles is replaced with air, and hollow particles filled with gas are obtained.

[0070] In this process, "in the air" strictly refers to an environment in which no liquid is present outside the precursor particles, or an environment in which only a trace amount of liquid is present outside the precursor particles, so that the removal of the hydrophobic solvent is not affected. "In the air" can also be referred to as a state in which the precursor particles are not present in a slurry, or a state in which the precursor particles are present in a dry powder. In other words, in this process, it is important to remove the hydrophobic solvent in an environment in which the precursor particles are in direct contact with the external gas.

[0071] The method for removing the hydrophobic solvent from the 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 hydrophobic solvent and equal to or lower than the maximum temperature at which the shell structure of the precursor particles does not collapse. Therefore, depending on the shell composition of the precursor particles and the type of hydrophobic 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 hydrophobic solvent inside the precursor particles is replaced by the external gas, resulting in hollow particles whose hollow spaces are filled with gas.

[0072] 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.

[0073] As an alternative method, the hydrophobic solvent may be removed by replacing the hydrophobic solvent contained in the precursor particles with the aqueous medium of the slurry containing the precursor particles in a slurry containing the precursor particles and the aqueous medium, without subjecting the slurry-like precursor composition obtained in the polymerization step to solid-liquid separation. In this method, the hydrophobic solvent contained in the precursor particles can be removed by bubbling an inert gas through the precursor composition at a temperature equal to or higher than the boiling point of the hydrophobic solvent minus 35°C. Here, when the hydrophobic solvent is a mixed solvent containing multiple types of hydrophobic solvents and has multiple boiling points, the boiling point of the hydrophobic 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 through the precursor composition is preferably at least 30°C below the boiling point of the hydrophobic solvent, more preferably at least 20°C below that, in order to reduce the amount of hydrophobic 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 hydrophobic solvent so as to remove the hydrophobic solvent contained in the precursor particles, and are not particularly limited. For example, an inert gas may be bubbled 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 precursor particles. 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.

[0074] Comparing a method of obtaining hollow particles having hollow portions filled with gas by performing solid-liquid separation on a slurry-like precursor composition and then removing the hydrophobic solvent in the precursor particles in an air atmosphere, and a method of obtaining hollow particles having hollow portions filled with gas by replacing the hydrophobic solvent contained in precursor particles in a slurry containing the precursor particles and an aqueous medium with the aqueous medium of the slurry, performing solid-liquid separation, and removing the aqueous medium in the precursor particles in an air atmosphere, the former method has the advantage that the hollow particles are less likely to be crushed in the step of removing the hydrophobic solvent, and the latter method has the advantage that the amount of residual hydrophobic solvent is reduced by performing bubbling with an inert gas. Alternatively, as a method for removing the hydrophobic organic solvent contained in the precursor particles after the polymerization step and before the solid-liquid separation step without performing solid-liquid separation on the slurry precursor composition obtained in the polymerization step, for example, a method of evaporating and distilling off the hydrophobic organic solvent contained in the precursor particles from the precursor composition under a predetermined pressure (high pressure, normal pressure, or reduced pressure); or a method of introducing an inert gas such as nitrogen, argon, or helium, or water vapor, into the precursor composition under a predetermined pressure (high pressure, normal pressure, or reduced pressure) and evaporating and distilling off the hydrophobic organic solvent may be used.

[0075] (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 precursor composition containing the precursor particles before the solvent removal step in order to remove any dispersion stabilizer remaining in the precursor composition containing the precursor particles. When the dispersion stabilizer used is an inorganic dispersion stabilizer soluble in acid, it is preferable to add an acid to the precursor composition containing the precursor particles to wash the 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 precursor composition containing the precursor particles to wash the 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 precursor composition containing the 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.

[0076] (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.

[0077] 2.Hollow particles The hollow particles of the present disclosure are hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, and having a porosity of 50% or more, The volume average particle size is 10 μm or more and 50 μm or less, The ratio of the measured shell thickness to the theoretical shell thickness (measured shell thickness / theoretical shell thickness) is 0.80 or more and 1.00 or less, The theoretical shell thickness is characterized in that it is a value calculated by calculating the inner diameter r of the hollow particles by the following formula (1) using the volume average particle diameter R and porosity of the hollow particles, and then calculating the inner diameter r and volume average particle diameter R by the following formula (2). 4 / 3π×(R / 2) 3 ×(porosity / 100)=4 / 3π×(r / 2) 3 Formula (1) Theoretical shell thickness = (Rr) / 2 Equation (2) The porosity in the above formula (1) is a numerical value expressed as a percentage.

[0078] The hollow particles of the present disclosure can be obtained by the manufacturing method of the present disclosure described above. Conventionally, it has been difficult to achieve a uniform shell thickness in hollow particles having a volume average particle size of 10 μm or more. In contrast, the hollow particles disclosed herein have a volume average particle size of 10 μm or more and 50 μm or less, and yet the ratio of the measured shell thickness to the theoretical shell thickness (measured shell thickness / theoretical shell thickness) is 0.80 to 1.00, resulting in small shell thickness variation. The closer this ratio (measured shell thickness / theoretical shell thickness) is to 1.00, the more uniform the shell thickness becomes. In a more preferred embodiment of the manufacturing method of the present disclosure, the ratio (measured shell thickness / theoretical shell thickness) can be 0.85 to 1.00, and in an even more preferred embodiment, it can be 0.90 to 1.00.

[0079] The hollow particles of the present disclosure preferably have a theoretical shell thickness and an actually measured shell thickness of 0.5 to 4 μm, more preferably 0.6 to 3 μm, and even more preferably 0.7 to 2 μm. When the shell thickness of the hollow particles is equal to or greater than the above lower limit, the strength of the shell is improved, and the pressure resistance of the hollow particles is improved. When the shell thickness of the hollow particles is equal to or less than the above upper limit, the porosity of the hollow particles can be increased. In this disclosure, the measured shell thickness is the average value of the shell thicknesses of 10 hollow particles actually measured. The shell thickness of each hollow particle can be measured by breaking the hollow particle and observing the shell fragments obtained with an SEM. The shell thickness of each hollow particle may be the shell thickness at any one point.

[0080] The hollow particles of the present disclosure have a volume average particle size of 10 μm or more and 50 μm or less. When the volume average particle size of the hollow particles is equal to or greater than the above-mentioned lower limit, the manufacturing method of the present disclosure is likely to exhibit the effect of suppressing variations in shell thickness. Furthermore, when the volume average particle size of the hollow particles is equal to or greater than the above-mentioned lower limit, excellent pressure resistance can be exhibited even when the porosity is high. Furthermore, when the volume average particle size of the hollow particles is equal to or greater than the above-mentioned lower limit, the tendency for the hollow particles to aggregate with each other is reduced, thereby exhibiting excellent dispersibility. When the volume average particle size of the hollow particles is equal to or less than the above-mentioned upper limit, particles with little variation in shell thickness can be obtained. The lower limit of the volume average particle diameter of the hollow particles of the present disclosure is preferably 13 μm or more, more preferably 15 μm or more, from the viewpoint of easily suppressing the effect of suppressing the variation in shell thickness and easily exhibiting the effect of improving pressure resistance due to the suppression of the variation in shell thickness.The upper limit of the volume average particle diameter of the hollow particles of the present disclosure is preferably 40 μm or less, more preferably 30 μm or less, from the viewpoint of easily suppressing the variation in shell thickness. In order to set the volume average particle diameter of the hollow particles within the above-mentioned preferred range, for example, it is preferable to use the above-mentioned preferred combination of dispersion stabilizer and particle size control agent in the mixed solution preparation step, and further to use the above-mentioned preferred hydrophobic solvent.

[0081] The shape of the hollow particles 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 of the present disclosure may have one or more hollow portions, but preferably have only one hollow portion in order to maintain a good balance between high porosity and mechanical strength and to suppress variations in shell thickness. The shell of the hollow particles of the present disclosure, and the partition walls separating adjacent hollow portions when the particles have two or more hollow portions, may be porous, but are preferably solid in order to suppress variations in shell thickness. The hollow particles of the present disclosure may have an average circularity of 0.950 to 0.995. An example of the shape of a hollow particle disclosed herein is a bag made of a thin membrane and inflated with gas, and its cross section is shown as hollow particle 100 in (5) of Figure 1. In this example, a single 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. The internal shape of the particle and the presence of fine resin particles inside the particle can be confirmed by SEM or TEM after slicing the particle into slices by a known method.

[0082] 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 diameter (Dv) and number average particle diameter (Dn) of the hollow particles can be determined by, for example, measuring the particle diameter of the hollow particles using a particle size distribution analyzer, calculating the number average and volume average, and using the resulting values ​​as the number average particle diameter (Dn) and volume average particle diameter (Dv) of the particles. The particle size distribution is calculated by dividing the volume average particle diameter by the number average particle diameter.

[0083] The hollow particles of the present disclosure have a porosity of 50% or more, preferably 60% or more, more preferably 70% or more, and even more preferably 75% 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, and the variation in shell thickness is easily suppressed. 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 suppress a decrease in the strength of the hollow particles and make them less likely to be crushed.

[0084] The porosity of the hollow particles of the present disclosure can be calculated from the amount and specific gravity of the shell-forming material used in producing the hollow particles and the amount and specific gravity of the hydrophobic solvent. Here, the shell-forming material is the solid material that forms the oil phase in the mixed liquid prepared in the mixed liquid preparation step, excluding the hydrophobic solvent. The hydrophobic solvent is the hydrophobic solvent in the mixed liquid. When the content of the polymerizable monomer is 99% by mass or more relative to 100% by mass of the material forming the shell, the shell can be considered to be made of a polymer of the polymerizable monomer, and the porosity of the hollow particle can be calculated using the following formula (C). Formula (C) Porosity (%) = 100 - [(amount of polymerizable monomer added / specific gravity of polymerizable monomer) / {(amount of polymerizable monomer added / specific gravity of polymerizable monomer) + (amount of hydrophobic solvent added / specific gravity of hydrophobic solvent)}] When the mixed solution contains multiple types of polymerizable monomers, the "amount of polymerizable monomer added / specific gravity of polymerizable monomer" in the above formula (C) is the sum of the "amount of polymerizable monomer added / specific gravity of polymerizable monomer" calculated for each polymerizable monomer. When the mixed solution contains multiple types of hydrophobic solvents, the "amount of hydrophobic solvent added / specific gravity of hydrophobic solvent" in the above formula (C) is the sum of the "amount of hydrophobic solvent added / specific gravity of hydrophobic solvent" calculated for each hydrophobic solvent.

[0085] The porosity of the hollow particles of the present disclosure can also be 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 3 The 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.

[0086] 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.

[0087] 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.

[0088] Furthermore, the hollow particles of the present disclosure obtained by the above-described manufacturing method of the present disclosure have suppressed generation of fine resin particles, which have a particle size significantly smaller than that of the hollow particles, inside the particles. Therefore, in the hollow particles of the present disclosure obtained by the manufacturing method of the present disclosure, the number of fine resin particles present in the hollow portion can be 3 particles / 1 particle or less. In a more preferred embodiment, the number of fine resin particles present in the hollow portion of the hollow particles of the present disclosure can be 1 particle / 1 particle or less, and in an even more preferred embodiment, the number can be 0 particles / 1 particle. The particle size of the fine resin particles is usually about 0.01 to 1 μm, which is 1 / 10 or less of the particle size of the hollow particles.

[0089] The hollow particles of the present disclosure have excellent strength due to the sufficient content of crosslinkable monomer units in the shell. Furthermore, the hollow particles of the present disclosure have excellent shell thickness uniformity even when the volume average particle size is 10 μm or more. Hollow particles with a volume average particle size of 10 μm or more are likely to exhibit excellent pressure resistance when the shell thickness is uniform. Because of these excellent strength and pressure resistance, the hollow particles of the present disclosure are resistant to crushing during kneading with other materials and during molding after kneading. When added to a molded body, they exhibit excellent effects as a lightweight material, heat insulating material, soundproofing material, vibration damping material, etc., and are therefore particularly suitable as additives for molded bodies. The hollow particles of the present disclosure are resistant to crushing during kneading with resin and during molding after kneading, and are therefore particularly suitable as additives for resin molded bodies. The molded article containing the hollow particles of the present disclosure may contain 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, polytetrafluoroethylene, maleimide resin, bismaleimide triazine resin, liquid crystalline polyester resin, phenolic resin, vinyl ester resin, unsaturated polyester resin, cyanate ester resin, polyether ketone ketone resin, or polyetherimide resin. When an epoxy resin is used as the resin component, it is preferable to mix a curing agent or catalyst such as an amine, acid anhydride, or imidazole with the epoxy resin. Furthermore, the molded article containing the hollow particles of the present disclosure may contain a thermoplastic elastomer as a resin. Examples of the thermoplastic elastomer include thermoplastic elastic polymers that have traditionally been used as molding resins, such as urethane elastomers, styrene elastomers, olefin elastomers, amide elastomers, and ester elastomers. Thermoplastic elastomers generally exhibit rubber elasticity at room temperature (25°C) and are plasticized and moldable at high temperatures. Furthermore, the molded article containing the hollow particles of the present disclosure is not limited to a resin molded article, but may be, for example, a rubber molded article, or may contain a mixture of resin and rubber. The molded article containing the hollow particles of the present disclosure may contain, for example, rubber such as natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene copolymer rubber (SBR), acrylonitrile-butadiene copolymer rubber (NBR), or ethylene-propylene-diene terpolymer (EPDM). The rubbers may be used alone or in combination of two or more. Furthermore, molded articles containing the hollow particles of the present disclosure may further contain organic or inorganic fibers such as carbon fibers, glass fibers, aramid fibers, polyethylene fibers, etc. The hollow particles of the present disclosure can also be contained as a filler in molded articles formed using a thermoplastic or thermosetting resin or rubber, and in molded articles formed using a material that includes a thermoplastic or thermosetting resin or rubber and further fibers. Examples of uses of the resin molded article or rubber molded article 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. The hollow particles of the present disclosure having a volume average particle size of 10 to 50 μm are particularly excellent in pressure resistance, and can therefore be suitably used as an additive for molded articles obtained through processes that apply external pressure, such as kneading and injection molding. 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]

[0090] 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.

[0091] [Example 1] (1) Mixed liquid preparation process First, the following materials were mixed to form an oil phase. Polymerizable monomer: ethylene glycol dimethacrylate 26.2 parts, 2,2'-Azobis(2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator, manufactured by Wako Pure Chemical Industries, Ltd., trade name: V-65) 0.61 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) 0.002 parts Hydrophobic solvent: 36.7 parts cyclohexane and 36.7 parts toluene Next, in a stirring tank, an aqueous solution of 5.5 parts of sodium hydroxide (alkali metal hydroxide) in 55 parts of ion-exchanged water was gradually added under stirring to an aqueous solution of 7.8 parts of magnesium chloride (a water-soluble polyvalent metal salt) in 225 parts of ion-exchanged water at a temperature of 80°C to prepare a magnesium hydroxide colloid (poorly water-soluble metal hydroxide colloid) dispersion, which was then cooled to 40°C and used as the aqueous phase. The resulting aqueous phase and oil phase were mixed to prepare a mixed liquid.

[0092] (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 the hydrophobic solvent were dispersed in water.

[0093] (3) Polymerization process The suspension obtained in the suspension step was heated in a nitrogen atmosphere from 40°C to 65°C over 1 hour and 30 minutes, and then stirred for 4 hours at 65°C to carry out a polymerization reaction. This polymerization reaction yielded a precursor composition, which was a slurry liquid in which precursor particles encapsulating a hydrophobic solvent were dispersed in water.

[0094] (4) Washing process and solid-liquid separation process The precursor composition obtained in the polymerization step 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 treatment (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 precursor particles encapsulating the hydrophobic solvent.

[0095] (5) Solvent removal process The 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 hydrophobic solvent contained in the particles, thereby obtaining 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.

[0096] [Examples 2 to 8, Comparative Examples 1 to 6] The hollow particles of Examples 2 to 8 and Comparative Examples 1 to 6 were produced in the same manner as in Example 1, except that the materials and amounts of the oil phase prepared in the above "(1) mixed solution preparation step" were as shown in Table 1, and the temperature during preparation of the magnesium hydroxide colloidal dispersion was changed from 80°C to 60°C.

[0097] [evaluation] 1. HSP distance between crosslinkable monomer and hydrophobic solvent The Hansen Solubility Parameter calculation software HSPiP (Version 5.3.03) was used to determine the dispersion term dD, polarity term dP, and hydrogen bond term dH of the crosslinkable monomer and hydrophobic solvent used in each Example and Comparative Example, and the HSP distance between the crosslinkable monomer and hydrophobic solvent was calculated using the following formula (A) using HSPiP. The total HSP between the crosslinkable monomer and hydrophobic solvent was also calculated using the following formula (B). The calculation results are shown in Table 1. Formula (A) HSP distance={4(dD1-dD2) 2 +(dP1-dP2) 2 +(dH1-dH2) 2} 0.5 Formula (B) total HSP=(dD 2 +dP 2 +dH 2 ) 0.5 The values ​​of dD, dP, and dH of the crosslinkable monomer and hydrophobic solvent were obtained from the HSPiP database, or for substances not in the database, the values ​​of dD, dP, and dH were calculated using HSPiP based on the chemical structure of the substance. When the crosslinkable monomer or hydrophobic solvent is a mixture, the dispersion term dD, polar term dP, and hydrogen bond term dH of the mixture are determined by calculating the weighted averages of the dD, dP, and dH values ​​of each substance contained in the mixture and the content ratio of each substance.

[0098] The hollow particles obtained in each example and comparative example were subjected to the following measurements and evaluations. The results are shown in Table 1. 2. Volume average diameter of hollow particles The volume average particle size of the hollow particles was measured using a particle size distribution analyzer (Beckman Coulter, product name: Multisizer 4e) under the following measurement conditions: aperture diameter: 280 μm, dispersion medium: Isoton II (product name), concentration: 10%, number of particles measured: 100,000. Specifically, 0.2 g of a particle sample was placed in a beaker, and a surfactant aqueous solution (manufactured by Fujifilm Corporation, product name: Drywell) was added as a dispersant. 2 ml of dispersion medium was then added to wet the particles, and 10 ml of dispersion medium was then added. The particles were dispersed in an ultrasonic disperser for 1 minute, and then measured using the particle size distribution analyzer.

[0099] 3.Porosity of hollow particles The porosity of the hollow particles was calculated from the amount and specific gravity of the polymerizable monomer added to the oil phase in the mixed liquid preparation step and the amount and specific gravity of the hydrophobic solvent added, using the following formula (C). Formula (C) Porosity (%) = 100 - [(amount of polymerizable monomer added / specific gravity of polymerizable monomer) / {(amount of polymerizable monomer added / specific gravity of polymerizable monomer) + (amount of hydrophobic solvent added / specific gravity of hydrophobic solvent)}]

[0100] 4. Theoretical 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 theoretical shell thickness of the hollow particles was calculated using the inner diameter r and volume average particle diameter R according to the following formula (2). 4 / 3π×(R / 2) 3 ×(porosity / 100)=4 / 3π×(r / 2) 3 Formula (1) Theoretical shell thickness = (Rr) / 2 Equation (2)

[0101] 5. Measured shell thickness of hollow particles The hollow particles were intentionally broken with a spatula, and the shell thickness was measured using a scanning electron microscope (manufactured by JEOL Ltd., product name: JSM7610F). The shell thicknesses of 10 hollow particles were measured, and the average value was taken as the actual shell thickness.

[0102] 6. Internal state of hollow particles The hollow particles were intentionally broken with a spatula, and the internal state of the particles was observed using a scanning electron microscope (manufactured by JEOL Ltd., product name: JSM7610F). If the number of fine resin particles present in the hollow portion was 0 per particle and no fine resin particles were observed inside the particle, it was evaluated as "no internal particles." If the number of fine resin particles present in the hollow portion was 1 per particle or more and fine resin particles were observed inside the particle, it was evaluated as "internal particles present." If no hollow portion was formed inside the particle and the entire inside of the particle was porous, it was evaluated as "porous."

[0103] [Table 1]

[0104] In Table 1, the abbreviations have the following meanings: EGDMA: Ethylene glycol dimethacrylate A-TMMT: Pentaerythritol tetraacrylate TMPTA: Trimethylolpropane triacrylate MAA: methacrylic acid V65: 2,2'-azobis(2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator, manufactured by Wako Pure Chemical Industries, Ltd., trade name: V-65)

[0105] [Consideration] In Comparative Examples 1 to 3, hollow particles having a volume average particle size of 22.0 to 33.5 μm and a porosity of 79% were obtained. However, because the HSP distance between the crosslinkable monomer and the hydrophobic solvent used exceeded 6.50, the measured shell thickness / theoretical shell thickness ratio was less than 0.80, resulting in a large variation in shell thickness and the presence of many fine resin particles within the hollow portions. Figure 4 shows an SEM image of the internal state of the particles obtained in Comparative Example 1. From the SEM image shown in Figure 4, it is clear that the shell thickness of the obtained particles varies widely and that many fine resin particles are present within the particles. Although not shown, SEM images of the internal state of the particles obtained in Comparative Examples 2 and 3 were similar to the SEM image shown in Figure 4. In Comparative Examples 1 to 3, it is presumed that the compatibility between the crosslinkable monomer and the hydrophobic solvent was too low, which resulted in rapid precipitation of the polymer after the start of the polymerization reaction, resulting in uneven formation of the shell and large variations in shell thickness, and that the polymer precipitated in the form of fine particles within the hollow portion, resulting in the generation of many fine resin particles. In Comparative Examples 4 to 6, particles with a volume average particle size of 16.0 to 23.0 μm and a porosity of 77 to 85% were obtained. However, because the HSP distance between the crosslinkable monomer and the hydrophobic solvent used was less than 5.80, the obtained particles were porous throughout, and no hollow portion clearly distinguishable from the shell was formed. Figure 5 shows an SEM image of the internal state of the particles obtained in Comparative Example 4. The SEM image in Figure 5 clearly shows that the obtained particles are porous throughout. SEM images of the internal state of the particles obtained in Comparative Examples 5 and 6 are not shown, but they were similar to the SEM image in Figure 5. In Comparative Examples 4 to 6, it is presumed that the compatibility between the crosslinkable monomer and the hydrophobic solvent was too high, resulting in insufficient phase separation between the polymerizable monomer and the hydrophobic solvent in the droplets of the monomer composition, resulting in the formation of porous particles. Note that in Comparative Example 6, the same combination of polymerizable monomer and hydrophobic solvent as in Example 1 of Patent Document 1 was used. In each example, hollow particles having a volume average particle size of 21.5 to 49.1 μm and a porosity of 78 to 79% were obtained. Furthermore, because the HSP distance between the crosslinkable monomer and the hydrophobic solvent used was 5.80 to 6.50, the measured shell thickness / theoretical shell thickness ratio was 0.80 to 1.00, with small shell thickness variations, and no fine resin particles were present within the hollow portions. Figure 3 shows an SEM image of the internal state of the hollow particles obtained in Example 2. From the SEM image shown in Figure 3, it is clear that the obtained particles have hollow portions clearly distinguishable from the shell, have small shell thickness variations, and are free of fine resin particles within the particles. SEM images of the internal state of the hollow particles obtained in other examples are not shown, but are similar to the SEM image shown in Figure 3. [Explanation of symbols]

[0106] 1 Aqueous medium 2 Low polarity material 3. Dispersion stabilizer 4. Monomer composition 4a Hydrophobic solvents 4b Materials other than hydrophobic solvents 4c Polymerizable monomer dispersed in an aqueous medium 5. Oil-soluble polymerization initiator 6 shells 8 Hollow part 10 droplets 20 precursor particles 100 Hollow particles with the hollow space filled with gas

Claims

1. 1. A method for producing hollow particles comprising 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 polymerizable monomer, a hydrophobic solvent, a polymerization initiator, 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 polymerizable monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction to prepare a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a resin, and encapsulating the hydrophobic solvent in the hollow portion; the polymerizable monomer includes a crosslinkable monomer, the HSP distance between the crosslinkable monomer and the hydrophobic solvent is 5.80 or more and 6.50 or less; the crosslinkable monomer includes a bifunctional crosslinkable monomer having two polymerizable functional groups capable of addition polymerization, and the content of the bifunctional crosslinkable monomer per 100 parts by mass of the polymerizable monomer is 70 parts by mass or more.

2. The method for producing hollow particles according to claim 1 , wherein the hydrophobic solvent comprises two or more hydrophobic solvents.

3. A method for producing hollow particles described in claim 1 or 2, wherein the bifunctional crosslinkable monomer is at least one selected from the group consisting of divinylbenzene, ethylene glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate.

4. A method for producing hollow particles described in claim 1 or 2, wherein the bifunctional crosslinkable monomer is a (meth)acrylic polymerizable monomer having a (meth)acryloyl group as the polymerizable functional group.

5. 5. The method for producing hollow particles according to claim 1, wherein the crosslinkable monomer further comprises a trifunctional or higher crosslinkable monomer having three or more polymerizable functional groups capable of addition polymerization.

6. The method for producing hollow particles according to any one of claims 1 to 5, wherein the mixed solution contains at least one selected from the group consisting of rosin acid, higher fatty acid, and metal salts thereof.

7. The method for producing hollow particles according to any one of claims 1 to 6, wherein the dispersion stabilizer is an inorganic dispersion stabilizer.

8. The method for producing hollow particles according to claim 7 , wherein the inorganic dispersion stabilizer is a poorly water-soluble metal salt.

9. The method for producing hollow particles according to any one of claims 1 to 8, wherein the hollow particles have a volume average particle size of 10 µm or more and 50 µm or less.

10. A hollow particle having a shell containing a resin and a hollow portion surrounded by the shell, the hollow particle having a porosity of 50% or more, The volume average particle size is 10 μm or more and 50 μm or less, The ratio of the measured shell thickness to the theoretical shell thickness (measured shell thickness / theoretical shell thickness) is 0.80 or more and 1.00 or less, The theoretical shell thickness is a value calculated by calculating the inner diameter r of the hollow particles by the following formula (1) using the volume average particle diameter R and porosity of the hollow particles, and then calculating the inner diameter r and the volume average particle diameter R by the following formula (2): The shell contains a polymer of a polymerizable monomer as the resin, and the content of a bifunctional crosslinkable monomer having two polymerizable functional groups capable of addition polymerization is 70 parts by mass or more per 100 parts by mass of the polymerizable monomer. 4 / 3π × (R / 2) ３ × (porosity / 100) = 4 / 3π × (r / 2) ３ Equation (1) Theoretical shell thickness = (R - r) / 2 Equation (2)

11. A hollow particle as described in claim 10, wherein the bifunctional crosslinkable monomer is at least one selected from the group consisting of divinylbenzene, ethylene glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate.

12. A hollow particle as described in claim 10, wherein the bifunctional crosslinkable monomer is a (meth)acrylic polymerizable monomer having a (meth)acryloyl group as the polymerizable functional group.

Images