Method for producing hollow particles and hollow particles

Abstract

The present invention provides a method for producing hollow particles, said method being capable of reducing the residual content of a hydrophobic solvent, while enabling the achievement of hollow particles that have excellent electrical insulation characteristics and excellent solvent resistance. 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 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 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 is a hydrocarbon monomer; the content of a crosslinkable monomer in 100% by mass of the polymerizable monomer is 70% by mass or more; and the hydrophobic solvent is a hydrocarbon-based solvent that has from 5 to 8 carbon atoms.

Description

[Technical Field]

[0001] This disclosure relates to a method for producing hollow particles and to hollow particles obtained by said production method. [Background technology]

[0002] Hollow particles (hollow resin particles) are particles with cavities inside. Compared to solid particles that are substantially filled with resin, they scatter light well and have low light transmittance. Therefore, they are widely used as organic pigments and opacities with excellent optical properties such as opacity and whiteness in water-based paints, paper coating compositions, and other applications. In recent years, they have also been used as lightweighting agents and heat insulating agents for resins and paints used in various fields such as automobiles, electrical and electronic equipment, and construction.

[0003] In electronic materials applications, for example, in electronic circuit boards, hollow particles are sometimes incorporated into the insulating resin layer to suppress crosstalk and increase transmission loss. Crosstalk and transmission loss in electronic circuit boards can be suppressed by reducing the dielectric constant and dielectric loss tangent of the insulating resin layer. Because hollow particles have a hollow interior, attempts have been made to lower the dielectric constant and dielectric loss tangent of the insulating resin layer by adding hollow particles.

[0004] For example, Patent Document 1 discloses hollow crosslinked resin particles used in low dielectric constant organic insulating materials, which are obtained by polymerizing 1 to 100% by weight of a crosslinkable monomer and 0 to 99% by weight of a non-crosslinked monomer (where the total of the crosslinkable and non-crosslinked monomers is 100% by weight), having an average particle diameter of 0.03 to 10 μm and an average metal ion concentration present in the particles of 50 ppm or less. Patent Document 1 states that it is preferable to use a crosslinkable monomer, a non-crosslinked hydrophilic monomer, and other non-crosslinked copolymerizable polymerizable monomers as polymerizable monomer components.

[0005] Furthermore, Patent Document 2 describes a method for producing hollow polymer fine particles by suspension polymerization, in which at least one crosslinkable monomer is used as a polymerizable component, and a water-poorly soluble solvent is used, which has low compatibility with the polymer or copolymer obtained from the at least one crosslinkable monomer, and the interfacial tension (Y) between the solvent and water is X ) and the interfacial tension (Y) between the polymer adsorption surface and water obtained by subjecting a solution of crosslinkable monomers dissolved in a solvent to suspension polymerization. P In relation to (mN / m), Y X ≧Y P A method using a solvent that satisfies such conditions has been disclosed. Patent Document 2 states that dipropylbenzene, divinylbiphenyl, etc. are used as crosslinkable monomers, and saturated hydrocarbons having 12 to 18 carbon atoms are used as water-insoluble solvents that satisfy the above conditions. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2000-313818 [Patent Document 2] Japanese Patent Publication No. 2004-190038 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the hollow crosslinked resin particles described in Patent Document 1 do not have sufficiently low dielectric constant and dielectric loss tangent, and therefore cannot be said to have excellent electrical insulation properties. Furthermore, the hollow crosslinked resin particles described in Patent Document 1 also have the problem of being permeated by polar organic solvents. Hollow particles are sometimes incorporated into epoxy resins, etc., using polar organic solvents such as methyl ethyl ketone as insulating materials for electronic materials. In such cases, if the polar organic solvent penetrates into the interior of the hollow particles, the effects of low dielectric constant and low dielectric loss tangent due to the hollow particles may be reduced. The method described in Patent Document 2 has the problem that poorly water-soluble solvents tend to remain embedded in hollow polymer microparticles. If a large amount of poorly water-soluble solvent remains in the hollow particles, for example, when the hollow particles are mixed with resin and kneaded in a twin-screw machine, the residual solvent may cause ignition or smoke. In addition, with the combination of crosslinkable monomer and poorly water-soluble solvent described in Patent Document 2, hollow portions may not be formed inside the particles.

[0008] The object of this disclosure is to provide a method for producing hollow particles that can reduce the amount of residual hydrophobic solvent used in the manufacturing process, have excellent electrical insulation properties, and have excellent solvent resistance to polar organic solvents, and to provide hollow particles obtained by this manufacturing method. [Means for solving the problem]

[0009] The inventors have found that, when producing hollow particles by suspension polymerization, in order to obtain hollow particles with excellent electrical insulation properties, excellent solvent resistance to polar organic solvents, and reduced residual hydrophobic solvents used in the manufacturing process, it is effective to use hydrocarbons as both the polymerizable monomer and the hydrophobic solvent, and furthermore, to ensure that the content of crosslinkable monomers in the polymerizable monomer is above a certain amount, and to use a hydrocarbon solvent having a specific number of carbon atoms.

[0010] This disclosure relates to a method for producing hollow particles comprising a resin-containing shell and a hollow portion surrounded by the shell, wherein the porosity is 50% or more. A step of preparing a mixture containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium, The steps include: preparing a suspension in which droplets of a monomer composition containing the polymerizable monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in an aqueous medium by suspending the aforementioned mixture; The process includes a step of subjecting the suspension to a polymerization reaction to prepare a precursor composition having a hollow portion surrounded by a resin-containing shell, and containing precursor particles that encapsulate the hydrophobic solvent within the hollow portion. The polymerizable monomer is a hydrocarbon monomer, and the content of the crosslinkable monomer containing two or more ethylenically unsaturated double bonds is 70% by mass or more in 100% by mass of the polymerizable monomer. Provided is a method for producing hollow particles, wherein the hydrophobic solvent is a hydrocarbon solvent having 5 to 8 carbon atoms.

[0011] In the method for producing hollow particles of the present disclosure, it is preferable that the mixed liquid 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 of the present disclosure, the dispersion stabilizer is preferably an inorganic dispersion stabilizer, and more preferably a hardly water-soluble metal salt.

[0013] In the method for producing hollow particles of the present disclosure, the volume average particle diameter of the hollow particles is preferably 1 μm or more and 10 μm or less.

[0014] The present disclosure provides hollow particles comprising a shell containing a resin and a hollow portion surrounded by the shell, having a porosity of 50% or more. The shell contains a hydrocarbon polymer as the resin. Provided are hollow particles having a relative permittivity of 1.5 or less at a frequency of 1 MHz.

[0015] In the hollow particles of the present disclosure, the dielectric loss tangent at a frequency of 1 MHz is preferably 0.010 or less. Further, in the hollow particles of the present disclosure, the relative permittivity at a frequency of 1 GHz is preferably 1.5 or less, and the dielectric loss tangent at a frequency of 1 GHz is preferably 0.010 or less.

[0016] In the hollow particles of the present disclosure, the porosity is preferably 60% or more.

[0017] In the hollow particles of the present disclosure, the volume average particle diameter is preferably 1 μm or more and 10 μm or less. [Effect of the Invention]

[0018] According to the manufacturing method of this disclosure as described above, hollow particles can be obtained that have excellent electrical insulation properties, excellent solvent resistance to polar organic solvents, and reduced residual amount of hydrophobic solvent used in the manufacturing process. [Brief explanation of the drawing]

[0019] [Figure 1] This figure illustrates an example of the manufacturing method described herein. [Figure 2] This is a schematic diagram showing one embodiment of a suspension in the suspension process. [Modes for carrying out the invention]

[0020] In this disclosure, the "~" in a numerical range means that the numbers before and after it are included as the lower and upper limits, respectively. Furthermore, in this disclosure, (meth)acrylate refers to acrylate and methacrylate respectively, and (meth)acrylic refers to acrylic and methacrylic respectively.

[0021] The hollow particles obtained by the manufacturing method of this disclosure are particles comprising a resin-containing shell (outer shell) and a hollow portion surrounded by the shell. In this disclosure, the hollow portion is a cavity-like space clearly distinguishable from the shell of the hollow particle formed from the resin material. The shell of the hollow particle may have a porous structure, in which case the hollow portion has a size that is clearly distinguishable from a multitude of minute spaces uniformly dispersed within the porous structure. According to the manufacturing method of this disclosure, the shell of the hollow particle can be made dense. The hollow portion of a hollow particle can be confirmed, for example, by SEM observation of the particle cross-section, or by TEM observation of the particle itself. Furthermore, in order to obtain the hollow particles obtained by the manufacturing method of this disclosure, it is preferable that the hollow portion of the hollow particles is filled with a gas such as air, or in a reduced pressure state close to a vacuum. The method for producing the hollow particles of this disclosure and the hollow particles obtained by the method for producing the hollow particles of this disclosure will be described in detail below.

[0022] 1. Method for producing hollow particles A method for producing hollow particles according to the present disclosure comprises a shell containing a resin and a hollow portion surrounded by the shell, wherein the porosity is 50% or more. A step of preparing a mixture containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium, The steps include: preparing a suspension in which droplets of a monomer composition containing the polymerizable monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in an aqueous medium by suspending the aforementioned mixture; The process includes a step of subjecting the suspension to a polymerization reaction to prepare a precursor composition having a hollow portion surrounded by a resin-containing shell, and containing precursor particles that encapsulate the hydrophobic solvent within the hollow portion. The polymerizable monomer is a hydrocarbon monomer, and the content of crosslinkable monomers containing two or more ethylenically unsaturated double bonds is 70% by mass or more of the polymerizable monomer by mass. The hydrophobic solvent is characterized by being a hydrocarbon solvent having 5 to 8 carbon atoms.

[0023] The method for producing hollow particles according to the present disclosure follows a basic technique in which a mixture containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium is suspended to prepare a suspension in which droplets having a distribution structure in which the polymerizable monomer and the hydrophobic solvent are phase-separated, 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 surface of the droplets and form hollow particles having a hollow portion filled with hydrophobic solvent. In the manufacturing method of this disclosure, by using a hydrocarbon monomer as the polymerizable monomer and a hydrocarbon solvent having 5 to 8 carbon atoms as the hydrophobic solvent, a hollow portion clearly distinguishable from the shell can be formed inside the particle, the amount of residual hydrophobic solvent inside the particle can be reduced, the relative permittivity and dielectric loss tangent of the hollow particle can be significantly reduced, and the solvent resistance of the hollow particle to polar organic solvents can be improved. In this disclosure, solvent resistance to polar organic solvents may be simply referred to as solvent resistance. The hollow crosslinked resin particles described in Patent Document 1 have poor electrical insulation properties and solvent resistance. The hollow crosslinked resin particles described in Patent Document 1 contain heteroatoms derived from methyl methacrylate, etc., in the shell, which prevents a sufficient reduction in the relative permittivity and dielectric loss tangent. Furthermore, the shell has good affinity with polar organic solvents, making it easy for polar organic solvents to penetrate the shell. The hollow polymer fine particles obtained by the method described in Patent Document 2 have a large amount of residual water-insoluble solvent used in the manufacturing process. In the method described in Patent Document 2, the above-mentioned Y X ≧Y P To satisfy these conditions, saturated hydrocarbons with 12 to 18 carbon atoms are used as poorly water-soluble solvents. Since saturated hydrocarbons with 12 to 18 carbon atoms have high boiling points, they tend to remain in the particles. In addition, the method described in Patent Document 2 does not easily form hollow spaces within the particles. In the combination of crosslinkable monomer and poorly water-soluble solvent described in Patent Document 2, the compatibility is good, so it is thought that the crosslinkable monomer and the poorly water-soluble solvent do not sufficiently phase separate in droplets dispersed in the suspension, and hollow spaces do not easily form inside the particles. In contrast, the manufacturing method of the present disclosure uses a hydrocarbon solvent having 5 to 8 carbon atoms as the hydrophobic solvent. This allows the polymerization reaction to proceed sufficiently because the hydrophobic solvent does not easily volatilize at the polymerization temperature, and the amount of residual hydrophobic solvent can be reduced because the hydrophobic solvent contained in the hollow portion is easily removed in the solvent removal step. Furthermore, the hollow particles obtained by the manufacturing method of this disclosure have a low relative permittivity and dielectric loss tangent, and excellent electrical insulation properties, because the polymer constituting the shell is a hydrocarbon polymer and does not contain heteroatoms. In this disclosure, the lower the relative permittivity and dielectric loss tangent, the better the electrical insulation properties. Furthermore, in the manufacturing method of this disclosure, by including a crosslinkable hydrocarbon monomer containing two or more ethylenically unsaturated double bonds in an amount of 70% by mass or more of polymerizable monomers per 100% by mass, hollow spaces are easily formed within the particles. When polymerizable monomers containing crosslinkable hydrocarbon monomers in the above proportion are polymerized, polymers with high crosslink density are produced. Polymers with high crosslink density are presumed to be more prone to phase separation with hydrophobic solvents than polymers with low crosslink density. In the manufacturing method of this disclosure, when the suspension is subjected to the polymerization reaction, the components constituting the shell formed in the droplet and the hydrophobic solvent, which is a hydrocarbon solvent with 5 to 8 carbon atoms, acquire appropriate compatibility. As a result, the components constituting the shell and the hydrophobic solvent undergo phase separation, forming hollow spaces within the particles, and forming shells that are clearly distinguishable from the hollow spaces. If the compatibility between the components constituting the shell and the hydrophobic solvent is too high within the droplet, porous particles will be produced. On the other hand, if their compatibility is too low, fine resin particles will be formed inside the hollow particles. Furthermore, in hollow particles, the relative permittivity and dielectric loss tangent tend to decrease as the size of the gas-filled space inside the particle increases. If the inside of the particle is porous or if fine resin particles are present inside the particle, even with the same porosity, the gas-filled space exists in a dispersed state, and the size of each individual space becomes smaller. Therefore, porous particles and hollow particles with many fine resin particles inside tend to have increased relative permittivity and dielectric loss tangent. In contrast, in the manufacturing method of this disclosure, a hollow portion that is clearly distinguishable from the shell is formed inside the particle at a porosity of 50% or more, and the generation of fine resin particles inside the particle is also suppressed. As a result, the increase in relative permittivity and dielectric loss tangent is suppressed, and hollow particles with further improved electrical insulation properties can be obtained. Furthermore, if the inside of the particle is porous, the solvent tends to penetrate the shell easily due to its structure, resulting in poor solvent resistance. In contrast, the manufacturing method of this disclosure forms a hollow portion that is clearly distinguishable from the shell, and the shell tends to be dense. When the shell is dense, the solvent does not easily penetrate it due to its structure. Moreover, in the manufacturing method of this disclosure, the polymerizable monomer contains 70% by mass or more of the crosslinkable monomer, so a dense covalent bond network is spread throughout the shell, and the high crosslink density of the shell also makes it difficult for the solvent to penetrate the shell. Furthermore, the hollow particles obtained by the manufacturing method of this disclosure have low affinity between the shell and polar organic solvents because the polymer constituting the shell is a hydrocarbon polymer, so from the viewpoint of affinity, polar solvents do not easily penetrate. For this reason, the hollow particles obtained by the manufacturing method of this disclosure have excellent solvent resistance to polar organic solvents.

[0024] The method for producing hollow particles according to this disclosure includes the steps of preparing a mixture, preparing a suspension, and subjecting the suspension to a polymerization reaction, and may also include other steps. Furthermore, to the extent that it is technically possible, two or more of the above steps and other additional steps may be performed simultaneously as a single step, or in any order. For example, the preparation of the mixture and the suspension may be performed simultaneously in a single process, such as by adding the materials for preparing the mixture while simultaneously performing the suspension.

[0025] A preferred example of a method for producing hollow particles according to this disclosure is a manufacturing method that includes the following steps. (1) Mixed liquid preparation process A process for preparing a mixture containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium. (2) Suspension process The process involves suspending the aforementioned mixture 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 process to prepare a precursor composition by subjecting the suspension to a polymerization reaction, wherein the precursor particles have a hollow portion surrounded by a resin-containing shell, and the hollow portion contains a hydrophobic solvent. (4) Solid-liquid separation process A step of obtaining precursor particles containing a hydrophobic solvent in a hollow portion by performing solid-liquid separation of the precursor composition, and (5) Solvent removal process A step to remove the hydrophobic solvent contained within the precursor particles obtained by the solid-liquid separation step to obtain hollow particles. In this disclosure, hollow particles in which the hollow portion is filled with a hydrophobic solvent may be considered an intermediate between hollow particles in which the hollow portion is filled with gas and referred to as "precursor particles." In this disclosure, "precursor composition" means a composition containing precursor particles.

[0026] Figure 1 is a schematic diagram showing an example of the manufacturing method of the present disclosure. (1) to (5) in Figure 1 correspond to the above steps (1) to (5). The white arrows between the figures indicate the order of each step. Note that Figure 1 is merely a schematic diagram for illustrative purposes, and the manufacturing method of the present disclosure is not limited to that 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 shown in these figures. Figure 1(1) is a schematic cross-sectional view showing one embodiment of the mixture in the mixture preparation process. As shown in this figure, the mixture includes an aqueous medium 1 and a low-polarity material 2 dispersed in the aqueous medium 1. Here, the low-polarity material 2 means a material that has low polarity and does not easily mix with the aqueous medium 1. In this disclosure, the low-polarity material 2 includes a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator. Figure 1(2) is a schematic cross-sectional view showing one embodiment of a suspension in the suspension process. The suspension comprises 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 have a structure in which the hydrophobic solvent 4a and the material other than the hydrophobic solvent containing the polymerizable monomer 4b are phase-separated, with the hydrophobic solvent 4a unevenly distributed in the center and the material other than the hydrophobic solvent 4b unevenly distributed on the surface side, and a dispersion stabilizer (not shown) attached to the surface. Figure 1(3) is a schematic cross-sectional view showing one embodiment of a precursor composition obtained by a polymerization process, which includes precursor particles containing a hydrophobic solvent in a hollow portion. The precursor composition includes an aqueous medium 1 and precursor particles 20 dispersed in the aqueous medium 1, each containing a hydrophobic solvent 4a in a hollow portion. The shell 6 forming the outer surface of the precursor particles 20 is formed by polymerization of polymerizable monomers in droplets 10 of the monomer composition, and contains the polymer of the polymerizable monomers as a resin. Figure 1(4) is a schematic cross-sectional view showing one embodiment of precursor particles after the solid-liquid separation process. Figure 1(4) shows the state after the aqueous medium 1 has been removed from the state shown in Figure 1(3). Figure 1(5) is a schematic cross-sectional view showing one embodiment of hollow particles after the solvent removal process. Figure 1(5) shows the state after the hydrophobic solvent 4a has been removed from the state shown in Figure 1(4). By removing the hydrophobic solvent from the precursor particles, hollow particles 100 are obtained that have a gas-filled hollow portion 8 inside the shell 6. The following describes the five processes mentioned above, as well as other processes, in order.

[0027] (1) Mixed liquid preparation process This process involves preparing a mixture containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium. The mixture may further contain other materials, to the extent that they do not impair the effects of the present disclosure. The materials of the mixed solution will be described in the following order: (A) polymerizable monomer, (B) hydrophobic solvent, (C) polymerization initiator, (D) dispersion stabilizer, (E) aqueous medium, and (F) other materials.

[0028] (A) Polymerizable monomers In the manufacturing method of this disclosure, the polymerizable monomer used is a hydrocarbon monomer consisting of carbon and hydrogen and containing one or more ethylenically unsaturated double bonds that are capable of addition polymerization. Polymerizable monomers include non-crosslinkable monomers containing only one ethylenically unsaturated double bond and crosslinkable monomers containing two or more ethylenically unsaturated double bonds. Crosslinkable monomers can form crosslinks in the resin through polymerization reactions. The polymerizable monomer used in the manufacturing method of this disclosure includes at least a crosslinkable monomer and may further include a non-crosslinkable monomer to the extent that it does not impair the effects of this disclosure.

[0029] [Cross-linkable monomers] Because crosslinkable monomers have multiple ethylenically unsaturated double bonds, they can be linked together, thereby increasing the crosslinking density of the shell. The crosslinkable monomers used in the manufacturing method of this disclosure are hydrocarbon monomers having two or more ethylenically unsaturated double bonds, such as divinylbenzene, divinylbiphenyl, and divinylnaphthalene. These crosslinkable monomers can be used individually or in combination of two or more. Among the crosslinkable monomers, divinylbenzene is preferred because the polymerization reaction is easily stabilized, and hollow particles with excellent strength and heat resistance can be obtained.

[0030] Furthermore, the molecular weight of the crosslinkable monomer is preferably 210 or less, more preferably 200 or less, and even more preferably 150 or less. When the molecular weight of the crosslinkable monomer is below the above upper limit, a polymer with high crosslink density is formed, and phase separation between the shell-constituting components and the hydrophobic solvent is promoted in the droplet of the monomer composition, making it easier for hollow parts to form within the particles. The lower limit of the molecular weight of the crosslinkable monomer is not particularly limited, but it is preferably 100 or more, and more preferably 120 or more, from the viewpoint of suppressing the volatilization of the crosslinkable monomer during polymerization.

[0031] In the manufacturing method of this disclosure, the content of crosslinkable monomers is 70% by mass or more in 100% by mass of the polymerizable monomer. A crosslinkable monomer content of 70% by mass or more allows sufficient phase separation between the shell-constituting components and the hydrophobic solvent within the droplet of the monomer composition, thus forming a hollow portion. Furthermore, a crosslinkable monomer content of 70% by mass or more results in a sufficiently high proportion of crosslinkable monomer units within the shell of the hollow particle, leading to a dense covalent network within the shell. This increases the crosslink density of the shell, improving solvent resistance, and furthermore, forming a shell that is strong, resistant to crushing, and less susceptible to deformation from external heat. The crosslinkable monomer content is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more.

[0032] If a commercially available product is used as the crosslinkable monomer as described above, and the purity of the crosslinkable monomer in the commercially available product is less than 100%, and it contains substances other than the crosslinkable monomer as impurities, then the crosslinkable monomer content mentioned above shall refer to the content of only the crosslinkable monomer, excluding the impurities, and not the content of the commercially available product. Commercially available crosslinkable monomers may contain impurities such as non-crosslinkable monomers in which some of the ethylenically unsaturated double bonds of the crosslinkable monomer have been converted into single bonds, resulting in a monomer with only one ethylenically unsaturated double bond, or non-crosslinkable monomers used as raw materials for the crosslinkable monomers. Specifically, commercially available divinylbenzene may contain ethylvinylbenzene, a non-crosslinkable monomer, as an impurity.

[0033] [Non-crosslinkable monomers] In the manufacturing method of this disclosure, the non-crosslinkable monomers that may be included in the polymerizable monomer are hydrocarbon monomers containing only one ethylenically unsaturated double bond, and examples include aromatic vinyl monomers such as styrene, vinyltoluene, α-methylstyrene, p-methylstyrene, ethyl vinylbenzene, ethyl vinyl biphenyl, and ethyl vinylnaphthalene; monoolefin monomers such as ethylene, propylene, and butylene; and diene monomers such as butadiene and isoprene. These non-crosslinkable monomers can be used individually or in combination of two or more. The non-crosslinkable monomers contained in the polymerizable monomer may be impurities present in commercially available crosslinkable monomers.

[0034] In the manufacturing method of the present disclosure, the content of non-crosslinkable monomers in 100% by mass of polymerizable monomer is 30% by mass or less, preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less.

[0035] The content of polymerizable monomers in the mixed solution is not particularly limited, but from the viewpoint of balancing the porosity, particle size, and mechanical strength of the hollow particles, it is preferably 15 to 50% by mass, more preferably 20 to 40% by mass, and even more preferably 20 to 30% by mass, based on 100% by 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 polymerizable monomers relative to 100% by mass of the total solid content of the material that becomes the oil phase in the mixed liquid, excluding the hydrophobic solvent, is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 99% by mass or more. In this disclosure, "solids" refers to all components excluding the solvent, and liquid polymerizable monomers, etc., are included in the solids.

[0036] (B) Hydrophobic solvents The hydrophobic solvent used in the manufacturing method of this disclosure is a nonpolymerizable and poorly water-soluble organic solvent. The hydrophobic solvent acts as a spacer material, forming hollow spaces within the particles. In the suspension step described later, a suspension is obtained in which droplets of the monomer composition containing the hydrophobic solvent are dispersed in an aqueous medium. In the suspension step, phase separation occurs within the monomer composition droplets, resulting in the less polar hydrophobic solvent tending to accumulate inside the monomer composition droplets. Ultimately, in the monomer composition droplets, the hydrophobic solvent is distributed inside, and other materials other than the hydrophobic solvent are distributed around its periphery according to their respective polarities. Then, in the polymerization process described later, an aqueous dispersion containing hollow particles encapsulating a hydrophobic solvent is obtained. That is, as the hydrophobic solvent accumulates inside the particles, a hollow space filled with the hydrophobic solvent is formed inside the resulting precursor particles.

[0037] In the manufacturing method of this disclosure, a hydrocarbon solvent having 5 to 8 carbon atoms, consisting of carbon and hydrogen, is used as the hydrophobic solvent. Preferably, at least one of the hydrocarbon solvents having 5 to 8 carbon atoms is selected from the group consisting of linear aliphatic hydrocarbon solvents, cyclic aliphatic hydrocarbon solvents, and aromatic hydrocarbon solvents having 5 to 8 carbon atoms. Examples of chain-like aliphatic hydrocarbon solvents having 5 to 8 carbon atoms include pentane, hexane, heptane, octane, 2-methylbutane, and 2-methylpentane. Examples of cyclic aliphatic hydrocarbon solvents having 5 to 8 carbon atoms include cyclohexane and cycloheptane. Examples of aromatic hydrocarbon solvents having 5 to 8 carbon atoms include benzene, toluene, and xylene. These hydrophobic solvents can be used individually or in combination of two or more. As the hydrophobic solvent used in the manufacturing method of this disclosure, it is preferable to use at least one selected from the group consisting of chain aliphatic hydrocarbon solvents having 5 to 8 carbon atoms and cyclic aliphatic hydrocarbon solvents, in order that hollow parts are easily formed, hollow particles with excellent electrical insulation properties and solvent resistance can be easily obtained, and the amount of residual hydrophobic solvent can be easily reduced. Chain aliphatic hydrocarbon solvents having 5 to 8 carbon atoms are more preferable, even more preferable are chain saturated hydrocarbon solvents having 5 to 8 carbon atoms such as pentane, hexane, heptane, and octane, and even more preferable is at least one selected from the group consisting of pentane, hexane, heptane, and octane. In this disclosure, when a hydrophobic solvent is defined as a hydrocarbon solvent having 5 to 8 carbon atoms, it means that hydrophobic solvents other than hydrocarbon solvents having 5 to 8 carbon atoms are not detected by gas chromatography (GC).

[0038] Furthermore, although not particularly limited, the boiling point of the hydrophobic solvent is preferably 130°C or lower, more preferably 100°C or lower, since it is easily removed in the solvent removal step described later. On the other hand, since it is easily encapsulated in precursor particles, it is preferably 50°C or higher, more preferably 60°C or higher. Furthermore, if 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 less than or equal to the above upper limit, 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 greater than or equal to the above lower limit.

[0039] Furthermore, the hydrophobic solvent used in the manufacturing method of this disclosure preferably has a relative permittivity of 2.0 or less at 20°C. Relative permittivity is one indicator of the polarity of a compound. When the relative permittivity of the hydrophobic solvent is sufficiently small, such as 2.0 or less, it is thought that phase separation proceeds rapidly in the polymerizable monomer droplets, and voids are easily formed. Examples of hydrophobic solvents with a relative permittivity of 2.0 or less at 20°C are as follows. (The value in parentheses is the relative permittivity.) Pentane (1.8), hexane (1.9), heptane (1.9), octane (1.9). Regarding the relative permittivity at 20°C, values ​​can be found in publicly available literature (for example, "Chemical Handbook: Basic Edition," edited by the Chemical Society of Japan, 4th revised edition, Maruzen Co., Ltd., published September 30, 1993, pp. II-498 to II-503), as well as other technical information. Methods for measuring the relative permittivity at 20°C include, for example, relative permittivity tests conducted in accordance with JIS C 2101:1999, item 23, with the measurement temperature set at 20°C.

[0040] The porosity of the hollow particles can be adjusted by changing the amount of hydrophobic solvent in the mixture. In the suspension step described later, the polymerization reaction proceeds with oil droplets containing crosslinkable monomers, etc., encapsulating the hydrophobic solvent. Therefore, the higher the hydrophobic solvent content, the higher the porosity of the resulting hollow particles tends to be. In this disclosure, the content of the hydrophobic solvent in the mixture is preferably 50 parts by mass or more and 500 parts by mass or less per 100 parts by mass of polymerizable monomer, as this makes it easier to control the particle size of the hollow particles, easier to increase the porosity while maintaining the strength of the hollow particles, and easier to reduce the amount of residual hydrophobic solvent in the particles. More preferably, the content of the hydrophobic solvent in the mixture is 60 parts by mass or more and 400 parts by mass or less per 100 parts by mass of polymerizable monomer, and even more preferably 70 parts by mass or more and 300 parts by mass or less.

[0041] (C) Polymerization initiator In the manufacturing method of this disclosure, it is preferable that the mixture contains an oil-soluble polymerization initiator as a polymerization initiator. As a method for polymerizing droplets of the monomer composition after suspending the mixture, there is an emulsion polymerization method using a water-soluble polymerization initiator and a suspension polymerization method using an oil-soluble polymerization initiator, and 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 oil-soluble polymerization initiators include benzoyl peroxide, lauroyl peroxide, t-butyl peroxide-2-ethylhexanoate, 2,2'-azobis(2,4-dimethylvaleronitrile), azobisisobutyronitrile, and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile). The content of the oil-soluble polymerization initiator in the mixture 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 per 100 parts by mass of polymerizable monomer. By keeping the content of the oil-soluble polymerization initiator within the above range, the polymerization reaction proceeds sufficiently, and there is little risk of residual oil-soluble polymerization initiator after the polymerization reaction is completed, and the risk of unexpected side reactions is also small.

[0042] (D) Dispersion stabilizer A dispersion stabilizer is an agent used in the suspension process to disperse droplets of a monomer composition in an aqueous medium. In this disclosure, it is preferable to use an inorganic dispersion stabilizer because it makes it easier to control the particle size of droplets in the suspension, narrows the particle size distribution of the resulting hollow particles, and suppresses a decrease in the strength of the hollow particles by preventing the shell from becoming too thin. These effects of inorganic dispersion stabilizers are particularly evident when used in combination with particle size control agents described later. Examples of inorganic dispersion stabilizers include 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 individually or in combination of two or more. Among the inorganic dispersion stabilizers mentioned above, 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 this 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 this disclosure, it is particularly preferable to use a poorly water-soluble inorganic dispersion stabilizer in the form of colloidal particles dispersed in an aqueous medium, that is, in the form of a colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles. By using a poorly water-soluble inorganic dispersion stabilizer in the form of a colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles, the particle size distribution of the monomer composition can be narrowed, and the amount of residual inorganic dispersion stabilizer in the resulting hollow particles can be easily kept low by washing. A colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloid 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, and potassium hydroxide. Examples of alkaline earth metal hydroxides include barium hydroxide and calcium hydroxide. The water-soluble polyvalent metal salt can be any water-soluble polyvalent metal salt other than the alkaline earth metal hydroxide compounds mentioned above. Examples include magnesium metal salts such as magnesium chloride, magnesium phosphate, and magnesium sulfate; calcium metal salts such as calcium chloride, calcium nitrate, calcium acetate, and calcium sulfate; aluminum metal salts such as aluminum chloride and aluminum sulfate; barium salts such as barium chloride, barium nitrate, and barium acetate; and zinc salts such as zinc chloride, zinc nitrate, and zinc acetate. 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 individually or in combination of two or more. The method for reacting at least one selected from the alkali metal hydroxide and alkaline earth metal hydroxide mentioned above with the water-soluble polyvalent metal salt mentioned above in an aqueous medium is not particularly limited, but one method is to mix an aqueous solution of at least one selected from the alkali metal hydroxide and alkaline earth metal hydroxide with an aqueous solution of the water-soluble polyvalent metal salt. In this case, from the viewpoint of being able to suitably control the particle size of poorly water-soluble metal hydroxide colloid particles, it is preferable to mix by gradually adding an aqueous solution of at least one selected from the alkali metal hydroxide and alkaline earth metal hydroxide to the aqueous solution while stirring the aqueous solution of the water-soluble polyvalent metal salt. Furthermore, from the viewpoint of obtaining hollow particles with a volume-average particle size of 1 μm or more and 10 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 20°C or more and 50°C or less.

[0043] 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 hydrophobic solvent. By having a dispersion stabilizer content above the lower limit, the monomer composition droplets can be sufficiently dispersed so that they do not coalesce in the suspension. On the other hand, by having a dispersion stabilizer content below the upper limit, it is possible to prevent the viscosity of the suspension from increasing during granulation and to avoid the problem of the suspension clogging the granulator. Furthermore, the content of the dispersion stabilizer is usually between 2 and 15 parts by mass, and preferably between 3 and 8 parts by mass, per 100 parts by mass of the aqueous medium.

[0044] (E)Aqueous medium In this disclosure, "aqueous medium" means a medium selected from the group consisting of water, hydrophilic solvents, and mixtures of water and hydrophilic solvents. The hydrophilic solvent in this disclosure is not particularly limited as long as it mixes well with water and does not undergo 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 overall polarity of the mixture does not become too low, from the viewpoint of forming droplets of the monomer composition. In this case, for example, the mass ratio of water to hydrophilic solvent (water:hydrophilic solvent) may be 99:1 to 50:50.

[0045] (F) Other materials The mixture may further contain other materials different from those described in (A) to (E) above, as long as it does not impair the effects of the present disclosure. The mixture preferably contains a particle size control agent as another material. By including a particle size control agent in the mixture, the particle size of the monomer composition droplets and the shell thickness of the resulting hollow particles can be appropriately adjusted. As the particle size control agent, at least one selected from the group consisting of rosinic acid, higher fatty acids, and metal salts thereof can preferably be used. These particle size control agents can appropriately adjust the particle size of droplets of the monomer composition containing a polymerizable monomer and a hydrophobic solvent in the suspension step described later. In the suspension step, droplets of the monomer composition are formed in an aqueous medium by the action of the dispersion stabilizer. In the droplets of the monomer composition, the materials other than the hydrophobic solvent containing the polymerizable monomer and the hydrophobic solvent undergo phase separation, with the hydrophobic solvent being concentrated in the center and the materials other than the hydrophobic solvent being concentrated on the surface. If the mixture contains a particle size control agent, it is presumed that the particle size control agent is concentrated near the surface of the droplets of the monomer composition, and the dispersion stabilizer adheres to the surface of the droplets. Such a distribution structure of materials is formed according to the differences in the affinity of each material to the aqueous medium. It is believed that by containing a particle size control agent in the mixture, the droplets of the monomer composition in the suspension adopt the material distribution structure described above, and interactions occur between the dispersion stabilizer and the particle size control agent on the droplet surface. This changes the dispersibility of the droplets due to the dispersion stabilizer, and thus the particle size of the monomer composition droplets can be appropriately adjusted. As for the particle size control agent, at least one selected from rosinic acid and its alkali metal salts is more preferred.

[0046] Rosin acid can be obtained from rosins such as gum rosin, tall rosin, and wood rosin. The components contained in the rosin acid obtained from these rosins include, for example, abietic acid, dehydroabietic acid, palastic acid, isopimal acid, and pima acid. The component ratio of rosin acid is not constant and varies depending on the type of rosin, the type of pine used as raw material, and the place of origin. The rosinic acid and its metal salts used in this disclosure are preferably rosinic acid and its alkali metal salts containing 50% by mass or more of abietic acids such as abietic acid, dehydroabietic acid, palastic acid, and their hydrides.

[0047] As the higher fatty acid, it is preferably a higher fatty acid having 10 to 25 carbon atoms excluding the carbon atom in the carboxyl group. Preferred higher fatty acids include, for example, 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.

[0048] Examples of the metal used for the rosin acid or the metal salt of the higher fatty acid include alkali metals such as Li, Na, K, and alkaline earth metals such as Mg, Ca, etc. Among them, alkali metals are preferred, and at least one selected from Li, Na, and K is more preferred.

[0049] When at least one selected from the group consisting of rosin acid, higher fatty acid, and their metal salts is used as the particle size control agent, the total content of rosin acid, higher fatty acid, and their metal salts is preferably 0.0001 part by mass or more and 0.1 part by mass or less, more preferably 0.001 part by mass or more and 0.01 part by mass or less, and even more preferably 0.0015 part by mass or more and 0.006 part by mass or less, based on 100 parts by mass in total of the polymerizable monomer and the hydrophobic solvent. When the above content is at least the above lower limit value, it is easy to control the particle diameter of the hollow particles and the thickness of the shell, and the strength of the hollow particles can be improved. On the other hand, when the above content is at most the above upper limit value, the decrease in the content ratio of the polymerizable monomer can be suppressed, so that the decrease in the strength of the shell can be suppressed, and the crushing of the hollow particles can be further suppressed.

[0050] A mixture is obtained by mixing the aforementioned materials and other materials as needed, and stirring as appropriate. In this mixture, the oil phase containing (A) polymerizable monomers, (B) hydrophobic solvents, and (C) lipophilic materials such as polymerization initiators is dispersed in the aqueous phase containing (D) dispersion stabilizers and (E) aqueous media, with particle sizes of several millimeters. Depending on the type of material, the dispersion state of these materials in the mixture can be observed with the naked eye. In the mixture preparation step, the mixture may be obtained by simply mixing the aforementioned materials and other materials as needed, and stirring as appropriate. However, in order to ensure a homogeneous shell, it is preferable to prepare the mixture by separately preparing 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 together. In this disclosure, a colloidal dispersion in which a poorly water-soluble inorganic dispersion stabilizer is dispersed in an aqueous medium in the form of colloidal particles can be preferably used as the aqueous phase. By preparing the oil phase and aqueous phase separately in advance and then mixing them, it is possible to produce hollow particles with a uniform shell composition, and it also becomes easier to control the particle size of the hollow particles.

[0051] (2) Suspension process The suspension step is a process of preparing a suspension in which droplets of a monomer composition containing a hydrophobic solvent are dispersed in an aqueous medium by suspending the above-mentioned mixture. The suspension method for forming droplets of monomer compositions is not particularly limited, but can be carried out using equipment capable of strong stirring, such as (in-line) emulsifiers / dispersers (horizontal in-line dispersers such as Taiheiyo Kiko Co., Ltd., product name: Milder, and Eurotech Co., Ltd., product name: Cavitron; vertical in-line dispersers such as IKA, product name: DRS 2000 / 5, etc.) or high-speed emulsifiers / dispersers (product name: TK Homomixer MARK II, etc., manufactured by Primix Co., Ltd.). In the suspension prepared in the suspension process, droplets of a monomer composition containing the above-mentioned lipophilic material and having a particle size of approximately 1 to 10 μm are uniformly dispersed in an aqueous medium. Such droplets of monomer composition are difficult to observe with the naked eye and can be observed using known observation equipment such as an optical microscope. During the suspension process, phase separation occurs within the monomer composition droplets, causing the less polar hydrophobic solvent to accumulate inside the droplets. As a result, the resulting droplets will have the hydrophobic solvent distributed inside and other materials distributed around their periphery.

[0052] Figure 2 is a schematic diagram showing one embodiment of a suspension in the suspension process. The droplet 10 of the monomer composition in Figure 2 is schematically shown as its cross-section. Note that Figure 2 is merely a schematic diagram, and the suspension in this disclosure is not necessarily limited to that shown in Figure 2. Part of Figure 2 corresponds to Figure 1(2) described above. Figure 2 shows how droplets 10 of the monomer composition and polymerizable monomers 4c dispersed in the aqueous medium 1 are dispersed in the aqueous medium 1. The droplets 10 are formed by a dispersion stabilizer 3 surrounding an oil-soluble monomer composition 4. The monomer composition contains an oil-soluble polymerization initiator 5, as well as a polymerizable monomer and a hydrophobic solvent (neither of which are shown). The droplet 10 is a micro-oil droplet containing the monomer composition 4, and the oil-soluble polymerization initiator 5 generates polymerization initiation radicals within the micro-oil droplet. Therefore, precursor particles of the desired particle size can be produced without over-growing the micro-oil droplets. In suspension polymerization 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 suppress the formation of extra resin particles, such as relatively small, dense particles, in addition to the desired hollow resin particles.

[0053] (3) Polymerization process This process involves subjecting the suspension obtained by the suspension process described above to a polymerization reaction to prepare a precursor composition containing precursor particles having a hollow portion surrounded by a resin-containing shell, and containing a hydrophobic solvent within the hollow portion. The precursor particles are formed by the polymerization of polymerizable monomers contained in droplets of the monomer composition, and the shell of the precursor particles contains the polymer of the polymerizable monomer as a resin. There are no particular limitations on the polymerization method; for example, batch, semi-continuous, and continuous methods can be used. The polymerization temperature is preferably 40 to 80°C, and more preferably 50 to 70°C. The heating rate when raising the temperature to the polymerization temperature is preferably 10°C / h to 60°C / h, more preferably 15°C / h to 55°C / h. The polymerization reaction time is preferably 1 to 48 hours, and more preferably 4 to 36 hours. During the polymerization process, the shell portion of the monomer composition droplet containing the hydrophobic solvent polymerizes, and as described above, a hollow portion filled with the hydrophobic solvent is formed inside the resulting precursor particles.

[0054] (4) Solid-liquid separation process This step involves obtaining a solid component containing precursor particles by performing solid-liquid separation of the precursor composition containing precursor particles obtained by the polymerization step described above.

[0055] The method for separating the precursor composition into solid and liquid is not particularly limited, and known methods can be used. Examples of solid-liquid separation methods 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 process, any optional steps such as a pre-drying process may be performed before carrying out the solvent removal process described later. An example of a pre-drying process is to pre-dry the solid obtained after the solid-liquid separation process using a drying device such as a dryer or a drying apparatus such as a hand dryer.

[0056] (5) Solvent removal process This step involves removing the hydrophobic solvent contained within the precursor particles obtained in the solid-liquid separation step. By removing the hydrophobic solvent encapsulated within the precursor particles in the air, the hydrophobic solvent inside the precursor particles is replaced with air, resulting in hollow particles filled with gas.

[0057] In this process, "in the air" strictly refers to an environment where there is absolutely no liquid outside the precursor particles, or an environment where there is only a very small amount of liquid outside the precursor particles that does not affect the removal of the hydrophobic solvent. "In the air" can also be rephrased as a state in which the precursor particles are not in a slurry, or a state in which the precursor particles are 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.

[0058] The method for removing the hydrophobic solvent from the precursor particles in air is not particularly limited, and known methods can be employed. Examples of such methods include vacuum drying, heat drying, airflow drying, or a combination of these methods. In particular, when using the heat drying method, the heating temperature must be above the boiling point of the hydrophobic solvent and below the maximum temperature at which the shell structure of the precursor particles does not collapse. Therefore, depending on the shell composition in the precursor particles and the type of hydrophobic solvent, the heating temperature may be, for example, 50-200°C, 70-200°C, or 100-200°C. Through a drying process in air, the hydrophobic solvent inside the precursor particles is replaced by the external gas, resulting in hollow particles in which the hollow portion is filled with gas.

[0059] The drying atmosphere is not particularly limited and can be appropriately selected depending on the application of the hollow particles. Examples of suitable drying atmospheres include air, oxygen, nitrogen, and argon. Furthermore, hollow particles with a temporarily vacuumed interior can be obtained by first filling the inside of the hollow particles with gas and then drying them under reduced pressure.

[0060] Alternatively, instead of performing solid-liquid separation of the slurry-like precursor composition obtained in the polymerization step, the hydrophobic solvent may be removed by replacing the hydrophobic solvent encapsulated in the precursor particles with the aqueous medium of the slurry while the slurry contains the precursor particles and an aqueous medium. In this method, the hydrophobic solvent encapsulated in the precursor particles can be removed by bubbling an inert gas through the precursor composition at a temperature at or above 35°C below the boiling point of the hydrophobic solvent. Here, if 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 bubbling through the precursor composition is preferably at or above the boiling point of the hydrophobic solvent minus 30°C, and more preferably at or above the boiling point of the hydrophobic solvent minus 20°C, in order to reduce the amount of residual hydrophobic solvent in the hollow particles. The bubbling temperature is usually at or above the polymerization temperature in the polymerization step. Although not particularly limited, the bubbling temperature may be between 50°C and 100°C. The inert gas used for bubbling is not particularly limited, but examples include nitrogen and argon. The bubbling conditions are adjusted as appropriate to remove the hydrophobic solvent encapsulated in the precursor particles, depending on the type and amount of hydrophobic solvent, 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 a water-based medium is encapsulated within precursor particles. By separating this slurry into solid and liquid phases to obtain hollow particles, and then drying the resulting hollow particles to remove the water-based medium from within the hollow particles, hollow particles in which gas occupies the hollow portion are obtained.

[0061] Comparing a method for obtaining hollow particles with gas-filled hollow portions by separating a slurry-like precursor composition into solid and liquid phases and then removing the hydrophobic solvent in the precursor particles in the air, with a method for obtaining hollow particles with gas-filled hollow portions by replacing the hydrophobic solvent contained in the precursor particles with the aqueous medium of the slurry in a slurry containing precursor particles and an aqueous medium, then separating the solid and liquid phases and removing the aqueous medium in the precursor particles in the air, the former method has the advantage that the hollow particles are less likely to be crushed in the step of removing the hydrophobic solvent, while the latter method has the advantage that less hydrophobic solvent remains due to bubbling with an inert gas. In addition, after the polymerization step and before the solid-liquid separation step, a method may be used to remove the hydrophobic organic solvent contained within the precursor particles without performing solid-liquid separation of the slurry-like precursor composition obtained in the polymerization step. For example, this could involve evaporating the hydrophobic organic solvent contained within the precursor particles from the precursor composition under a predetermined pressure (high pressure, atmospheric pressure, or reduced pressure); or introducing an inert gas such as nitrogen, argon, or helium, or water vapor, into the precursor composition under a predetermined pressure (high pressure, atmospheric pressure, or reduced pressure) and then evaporating the solvent.

[0062] (6) Others In addition to the above steps (1) to (5), other steps such as the cleaning step (6-a) and the re-replacement step (6-b) of the hollow section may be added. (6-a) Washing process The washing step is a step performed by adding an acid or alkali to wash the precursor composition containing the precursor particles in order to remove any remaining dispersion stabilizer before the solvent removal step. If the dispersion stabilizer used is an inorganic dispersion stabilizer soluble in acid, it is preferable to wash the precursor composition containing the precursor particles by adding an acid. On the other hand, if the dispersion stabilizer used is an inorganic compound soluble in alkali, it is preferable to wash the precursor composition containing the precursor particles by adding an alkali. 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 lower, more preferably 6 or lower. As the added acid, inorganic acids such as sulfuric acid, hydrochloric acid, and nitric acid, and organic acids such as formic acid and acetic acid can be used, but sulfuric acid is particularly preferred because it has a high efficiency in removing the dispersion stabilizer and places little burden on the manufacturing equipment.

[0063] (6-b) Replacement process for the hollow section The hollow portion re-substitution process is a process of replacing the gas or liquid inside a hollow particle with another gas or liquid. Such substitution can change the environment inside the hollow particle, selectively confine molecules inside the hollow particle, or modify the chemical structure inside the hollow particle to suit the application.

[0064] 2.Hollow particles The hollow particles of this disclosure are hollow particles comprising a resin-containing shell and a hollow portion surrounded by the shell, and having a porosity of 50% or more. The shell contains a hydrocarbon polymer as the resin, It is characterized by having a relative permittivity of 1.5 or less at a frequency of 1 MHz.

[0065] The hollow particles of this disclosure can be obtained by the manufacturing method of this disclosure described above. The hollow particles disclosed herein have low relative permittivity and dielectric loss tangent, and exhibit excellent electrical insulation properties. According to the manufacturing method of the present disclosure, the relative permittivity of the hollow particles at a frequency of 1 MHz can be set to 1.4 or less in a more preferred embodiment. The lower limit of the relative permittivity of the hollow particles of the present disclosure at a frequency of 1 MHz is not particularly limited, but is usually 1.0 or higher. Furthermore, according to the manufacturing method of the present disclosure, the dielectric loss tangent of the hollow particles at a frequency of 1 MHz can be made 0.010 or less, and in a more preferred embodiment, it can be made 0.007 or less. The lower limit of the dielectric loss tangent of the hollow particles of the present disclosure at a frequency of 1 MHz is not particularly limited, but is usually 0.0001 or more, and may be 0.001 or more. Furthermore, according to the manufacturing method of the present disclosure, the relative permittivity of the hollow particles at a frequency of 1 GHz can be set to 1.5 or less, and in a more preferred embodiment, it can be set to 1.4 or less. The lower limit of the relative permittivity of the hollow particles of the present disclosure at a frequency of 1 GHz is not particularly limited, but is usually 1.0 or higher. Furthermore, according to the manufacturing method of the present disclosure, the dielectric loss tangent of the hollow particles at a frequency of 1 GHz can be set to 0.010 or less, and in a more preferred embodiment, it can be set to 0.005 or less. The lower limit of the dielectric loss tangent of the hollow particles of the present disclosure at a frequency of 1 GHz is not particularly limited, but is usually 0.001 or more. In this disclosure, the relative permittivity and dielectric loss tangent of hollow particles are measured using a perturbation-type measuring device under the condition of a measurement frequency of 1 MHz or 1 GHz.

[0066] The hydrocarbon polymer contained as a resin in the shell of the hollow particles of this disclosure is a polymer of the polymerizable monomer used in the manufacturing method of this disclosure. The hydrocarbon polymer content is preferably 99% by mass or more, more preferably 99.5% by mass or more, and even more preferably 99.9% by mass or more, based on 100% by mass of the total solids content of the shell. By setting the hydrocarbon polymer content to above the above lower limit, the electrical insulation properties and solvent resistance of the hollow particles can be improved.

[0067] The shells of the hollow particles of this disclosure preferably do not contain heteroatoms in order to suppress a decrease in electrical insulation properties or solvent resistance, but may contain very small amounts of components other than hydrocarbons as long as it does not impair the effects of this disclosure. Examples of components other than hydrocarbons that the shells may contain include the particle size control agents described above. The content of components other than hydrocarbons in the shells is preferably 1% by mass or less, more preferably 0.5% by mass or less, and even more preferably 0.1% by mass or less, based on 100% by mass of the total solids content of the shells. By keeping the content of components other than hydrocarbons below the above upper limit, the electrical insulation properties and solvent resistance of the hollow particles can be improved.

[0068] The hollow particles of this disclosure preferably have a lower limit of volume-average particle size of 1 μm or more, more preferably 1.5 μm or more, and even more preferably 2 μm or more. On the other hand, the upper limit of volume-average particle size of the hollow particles is preferably 10 μm or less, more preferably 8 μm or less, and even more preferably 6 μm or less. When the volume-average particle size of the hollow particles is above the lower limit, the aggregation of the hollow particles is reduced, and excellent dispersibility can be achieved. When the volume-average particle size of the hollow particles is below the upper limit, variations in shell thickness are suppressed, a uniform shell is easily formed, and the hollow particles are less likely to be crushed, resulting in high mechanical strength. Furthermore, hollow particles with a volume-average particle size within the above range do not cause wiring problems even when incorporated into the insulating resin layer of an electronic circuit board, and are therefore suitably used as a material for electronic circuit boards. In order to keep the volume-average particle size of the hollow particles within the preferred range described above, it is preferable, for example, to use the preferred combination of dispersion stabilizer and particle size control agent described above, and further to use the preferred hydrophobic solvent described above, in the mixture preparation step.

[0069] The shape of the hollow particles in this disclosure is not particularly limited as long as a hollow portion is formed inside, and examples include spherical, ellipsoidal, and amorphous shapes. Among these, a spherical shape is preferred due to its ease of manufacture. The hollow particles of this disclosure may have one or more hollow sections, but it is preferable that they have only one hollow section in order to maintain a good balance between high porosity and mechanical strength, and to improve electrical insulation properties. Furthermore, the shell of the hollow particles of this disclosure, and the partition walls separating adjacent hollow sections when there are two or more hollow sections, may be porous, but it is preferable that they be dense in order to improve electrical insulation properties and solvent resistance. The hollow particles in this disclosure may have an average circularity of 0.950 to 0.995. An example of the shape of a hollow particle in this disclosure is a bag consisting of a thin film and inflated with gas, the cross-section of which is shown as hollow particle 100 in Figure 1(5). In this example, a thin film is provided on the outside, and the inside is filled with gas. The particle shape can be confirmed, for example, by SEM or TEM. Furthermore, the internal shape of the particles, and the presence of fine resin particles within them, can be confirmed by SEM or TEM after the particles have been cross-sectionally sliced ​​using a known method.

[0070] The particle size distribution (volume-average particle size (Dv) / number-average particle size (Dn)) of the hollow particles may be, for example, between 1.1 and 2.5. A particle size distribution of 2.5 or less allows for particles with less variation in compressive strength and heat resistance between particles. Furthermore, a particle size distribution of 2.5 or less enables the production of products with uniform thickness, for example, when manufacturing sheet-like molded articles. The volume-average particle size (Dv) and number-average particle size (Dn) of hollow particles can be determined, for example, by measuring the particle size of the hollow particles using a particle size distribution analyzer, calculating the number average and volume average, respectively, and using the obtained values ​​as the number-average particle size (Dn) and volume-average particle size (Dv) of those particles. The particle size distribution is defined as the volume-average particle size divided by the number-average particle size.

[0071] 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. Having a porosity above the above lower limit results in the hollow particles having excellent lightness, heat resistance, and thermal insulation properties, as well as excellent electrical insulation characteristics. While the upper limit of the porosity of the hollow particles of the present disclosure is not particularly limited, it is preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less, from the viewpoint of suppressing a decrease in the strength of the hollow particles and making them less susceptible to crushing.

[0072] The porosity of the hollow particles in this disclosure can be calculated from the amount and specific gravity of the shell-forming material used in the production of the hollow particles, and the amount and specific gravity of the hydrophobic solvent. Here, the shell-forming material is the solid component material obtained by excluding the hydrophobic solvent from the material that becomes the oil phase in the mixed liquid prepared in the above mixed liquid preparation step. The hydrophobic solvent is the hydrophobic solvent in the mixed liquid. If the polymerizable monomer content is 99% or more of the material forming the shell, the shell can be considered to be composed of a polymer of polymerizable monomers, and the porosity of the hollow particle can be calculated using the following formula (A). Formula (A) 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)}] If the mixture contains multiple types of polymerizable monomers, the "amount of polymerizable monomer added / specific gravity of polymerizable monomer" in formula (A) above shall be the sum of the "amount of polymerizable monomer added / specific gravity of polymerizable monomer" calculated for each type of polymerizable monomer. If the mixture contains multiple types of hydrophobic solvents, the "amount of hydrophobic solvent added / specific gravity of hydrophobic solvent" in formula (A) above shall be the sum of the "amount of hydrophobic solvent added / specific gravity of hydrophobic solvent" calculated for each type of hydrophobic solvent.

[0073] Furthermore, the porosity of the hollow particles in this 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, a volume of 100 cm³ is used. 3 Approximately 30 cm in a volumetric flask 3 Fill the volumetric flask with hollow particles and accurately weigh the mass of the filled hollow particles. Next, carefully fill the volumetric flask filled with hollow particles to the mark with isopropanol, taking care not to introduce air bubbles. Accurately weigh the mass of isopropanol added to the volumetric flask and calculate the apparent density D1 (g / cm³) of the hollow particles based on the following formula (I). 3 ) calculate. Equation (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, assuming that the hollow portion is considered part of the hollow particle.

[0074] The method for measuring the true density D0 of hollow particles is as follows: After pre-pulverizing the hollow particles, a volume of 100 cm³ is used.3 Fill a volumetric flask with approximately 10 g of crushed hollow particles and accurately weigh the mass of the crushed particles. Then, add isopropanol to the volumetric flask in the same manner as the apparent density measurement described above, accurately weigh the mass of isopropanol, and calculate the true density D0 (g / cm³) of the hollow particles based on the following formula (II). 3 ) calculate. Formula (II) True density D0 = [Mass of 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 measurement method described above, the hollow portion is not considered part of the hollow particle when calculating the true density D0.

[0075] The porosity (%) of a hollow particle is calculated using the following formula (III), given the apparent density D1 and true density D0 of the hollow particle. Formula (III) Porosity (%) = 100 - (Apparent density D1 / True density D0) × 100 The porosity of a hollow particle can be rephrased as the proportion of the particle's specific gravity that is occupied by the hollow portion.

[0076] Conventional hollow particles, such as the hollow particles obtained in Comparative Example 1 described later, contain extremely small fine resin particles within the hollow portion, which are smaller in diameter than the hollow particles themselves. In the hollow particles of this disclosure, from the standpoint of excellent electrical insulation properties, it is preferable that the number of such fine resin particles present in the hollow portion is 3 or less per particle, more preferably 1 or less per particle, and even more preferably 0 per particle. By keeping the number of such fine resin particles present in the hollow portion below the above upper limit, the proportion of gas occupying the hollow portion can be increased, thereby reducing the relative permittivity and dielectric loss tangent. The particle size of the fine resin particles mentioned above is typically around 0.01 to 0.5 μm, which is less than 1 / 10 the particle size of the hollow particles. The manufacturing method of the present disclosure described above makes it possible to keep the number of fine resin particles present in the hollow portion below the above upper limit.

[0077] The hollow particles disclosed herein have excellent strength due to the sufficient inclusion of crosslinkable monomer units in their shells, making them resistant to crushing during mixing with other materials and during molding after mixing. When added to molded articles, they exhibit excellent effects as lightweighting materials, heat insulating materials, sound insulating materials, vibration damping materials, etc. Furthermore, since the hollow particles disclosed herein have a reduced amount of residual hydrophobic solvent, there is no risk of ignition or smoke generation when mixed with other materials such as resins. For these reasons, the hollow particles disclosed herein are particularly suitable as additives for molded articles, and are especially suitable for use as additives for resin molded articles. The molded articles containing hollow particles in this disclosure may contain, as a resin, thermoplastic resins or thermosetting resins 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, polyetherketone ketone resin, and polyetherimide resin. When epoxy resin is used as the resin component, it is preferable to mix in a curing agent or catalyst such as amines, acid anhydrides, or imidazoles as appropriate. Furthermore, the 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, and polyethylene fibers. The hollow particles of the present disclosure can also be included as fillers in molded articles formed using thermoplastic or thermosetting resins, and in molded articles formed using materials that further contain fibers along with thermoplastic or thermosetting resins. Applications of resin molded articles containing the hollow particles of this disclosure include, for example, light-reflecting materials, heat-insulating materials, sound-insulating materials, and low-dielectric components used in various fields such as automobiles, electrical and electronic equipment, construction, aerospace, and space; food containers; footwear such as sports shoes and sandals; home appliance parts; bicycle parts; stationery; tools; and more. In particular, the hollow particles of this disclosure are suitable for use in the electrical or electronic fields as materials to achieve low dielectric constant or low transmission loss due to their excellent electrical insulation properties and solvent resistance. For example, the hollow particles of this disclosure are suitable for use as electronic circuit board materials. Specifically, by incorporating the hollow particles of this disclosure into the insulating resin layer of an electronic circuit board, the relative dielectric constant of the insulating resin layer can be reduced, thereby reducing the transmission loss of the electronic circuit board. Furthermore, the hollow particles of this disclosure are also suitable for semiconductor materials such as interlayer insulating materials, dry film resists, solder resists, bonding wires, magnet wires, semiconductor encapsulants, epoxy encapsulants, mold underfills, underfills, die bond pastes, buffer coat materials, copper-clad laminates, flexible substrates, high-frequency device modules, antenna modules, and automotive radar. Among these, they are particularly suitable for semiconductor materials such as interlayer insulating materials, solder resists, magnet wires, epoxy encapsulants, underfills, buffer coat materials, copper-clad laminates, flexible substrates, high-frequency device modules, antenna modules, and automotive radar. Furthermore, the hollow particles disclosed herein have a high porosity, are resistant to crushing, and have excellent heat resistance, thus meeting the thermal insulation and cushioning (cushioning) requirements for undercoat materials, as well as the heat resistance suitable for thermal paper applications. In addition, the hollow particles disclosed herein are also useful as plastic pigments with excellent gloss and opacity. Furthermore, the hollow particles of this disclosure can be used for various purposes depending on the components contained inside, as useful components such as fragrances, pharmaceuticals, pesticides, and ink components can be sealed inside by means of immersion treatment, reduced pressure treatment, or pressurized immersion treatment. [Examples]

[0078] The present disclosure will be further described below with reference to examples and comparative examples, but the present disclosure is not limited to these examples. Parts and percentages are by mass unless otherwise specified.

[0079] [Example 1] (1) Mixed liquid preparation process First, the following materials were mixed to form the oil phase. DVB960 (product name, manufactured by Nippon Steel Chemical & Material Co., Ltd., divinylbenzene purity: 96%, ethylvinylbenzene content: 4%) 26.2 parts 2,2'-Azobis(4-Methoxy-2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: V-70) 0.6 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: Hexane 73.4 parts Next, in a stirred tank, under a temperature of 40°C, an aqueous solution prepared by dissolving 7.8 parts magnesium chloride (water-soluble polyvalent metal salt) in 225 parts deionized water was gradually added under stirring to an aqueous solution prepared by dissolving 5.5 parts sodium hydroxide (alkali metal hydroxide) in 55 parts deionized water. A dispersion of magnesium hydroxide colloid (a poorly water-soluble metal hydroxide colloid) was prepared, and stirring was stopped 40 minutes after the addition to obtain the aqueous phase. A mixture was prepared by mixing the obtained aqueous phase and oil phase.

[0080] (2) Suspension process The mixture obtained in the above mixture preparation step was suspended by stirring it for 1 minute at a rotation speed of 4,000 rpm using a disperser (Primix Corporation, product name: Homomixer) to prepare a suspension in which droplets of monomer composition containing a hydrophobic solvent were dispersed in water.

[0081] (3) Polymerization process The suspension obtained in the above 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 under the temperature of 65°C to carry out the polymerization reaction. This polymerization reaction yielded a precursor composition, which is a slurry liquid in which precursor particles containing a hydrophobic solvent are dispersed in water.

[0082] (4) Washing process and solid-liquid separation process The precursor composition obtained in the polymerization process described above was washed with dilute sulfuric acid (25°C, 10 minutes) to reduce the pH to 5.5 or less. Next, after separating the water by filtration, 200 parts of freshly deionized water were added to re-form a slurry, and the water washing treatment (washing, filtration, dewatering) was repeated several times at room temperature (25°C), and the solid was separated by filtration. The obtained solid was dried in a dryer at a temperature of 40°C to obtain precursor particles containing a hydrophobic solvent.

[0083] (5) Solvent removal process The precursor particles obtained in the solid-liquid separation process described above were heat-treated in a vacuum dryer at 200°C for 6 hours to remove the hydrophobic solvent contained within the particles, thereby obtaining the hollow particles of Example 1. Based on observations using a scanning electron microscope and porosity values, it was confirmed that the obtained hollow particles were spherical and contained hollow portions.

[0084] [Examples 2-5, Comparative Examples 1-3] In Example 1, hollow particles were produced in the same procedure as in Example 1, except that the materials for the oil phase prepared in the "(1) Mixture Preparation Step" were as shown in Table 1.

[0085] [evaluation] 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.

[0086] 1. Volume-average particle size The volume-average particle size of hollow particles was measured using a particle size distribution analyzer (Beckman Coulter, product name: Multisizer 4e). The measurement conditions were: aperture diameter: 50 μm, dispersion medium: Isoton II (product name), concentration: 10%, and number of particles measured: 100,000. Specifically, 0.2 g of the particle sample was placed in a beaker, and an aqueous surfactant solution (manufactured by Fujifilm, product name: Drywell) was added as a dispersant. Then, 2 ml of dispersion medium was added to wet the particles, and after that, 10 ml of dispersion medium was added, and the particles were dispersed in an ultrasonic disperser for 1 minute before being measured using the particle size distribution analyzer mentioned above.

[0087] 2.Porosity The porosity of the hollow particles was calculated using the following formula (A) from the amount and specific gravity of the polymerizable monomer added to the oil phase during the mixture preparation process, and the amount and specific gravity of the hydrophobic solvent added. Formula (A) 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)}]

[0088] 3. Internal state of hollow particles Hollow particles were intentionally broken open with a spatula, and the internal state of the particles was observed using a scanning electron microscope (JEOL Ltd., product name: JSM7610F). If the number of fine resin particles in the hollow portion was 0 / 1 particle, and no fine resin particles were observed inside the particle, it was evaluated as "no internal particles". Hollow particles evaluated as "no internal particles" had a shell of uniform thickness and a hollow portion that was clearly distinguishable from the shell. If the number of fine resin particles in the hollow portion was 1 / 1 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".

[0089] 4. Solvent-removing properties Approximately 100 mg of hollow particles were placed in a 30 mL screw-top glass bottle and accurately weighed. Next, approximately 10 g of tetrahydrofuran (THF) was added and accurately weighed. The mixture in the glass bottle was stirred with a stirrer for 1 hour to extract the hydrophobic solvent contained in the hollow particles. After stopping the stirring, the resin components of the hollow particles, which were insoluble in THF, were allowed to precipitate. A filter (Advantec, product name: Membrane Filter 25JP020AN) was attached to a syringe to filter the precipitate and obtain a sample solution. This sample solution was injected into a gas chromatography (GC) system for analysis. The amount of hydrophobic solvent per unit mass (mass%) contained in the hollow particles was determined from the GC peak area and a pre-prepared calibration curve. Detailed analytical conditions are as follows. (Analysis conditions) Equipment: GC-2010 (manufactured by Shimadzu Corporation) Column: DB-5 (manufactured by Agilent Technologies, Inc.) Film thickness 0.25 μm, inner diameter 0.25 mm, length 30 m Detector: FID Carrier gas: Nitrogen (linear velocity: 28.8 cm / sec) Inlet temperature: 200℃ Detector temperature: 250℃ Oven temperature: Increase from 40°C to 230°C at a rate of 10°C / minute, then maintain at 230°C for 2 minutes. Sampling volume: 2 μL Based on the amount of hydrophobic solvent in the hollow particles determined above, the solvent removal performance was evaluated according to the following evaluation criteria. (Evaluation criteria for solvent removal properties) ◎: Amount of hydrophobic solvent per unit mass is less than 0.1% by mass. ○: The amount of hydrophobic solvent per unit mass is 0.1% by mass or more and 1% by mass or less. ×: The amount of hydrophobic solvent per unit mass exceeds 1% by mass.

[0090] 5. Solvent resistance Under conditions of 25°C, 200 mg of hollow particles were placed in a 5 mL glass bottle, followed by the addition of 4 mL of methyl ethyl ketone (MEK), and the bottle was sealed. The glass bottle was then shaken 10 times by hand and left at 25°C for 24 hours. The proportion of precipitated hollow particles was determined and evaluated according to the following evaluation criteria. The precipitated hollow particles in the MEK were separated using a centrifuge, dried, and their mass was measured. The proportion of precipitated hollow particles was determined by calculating the ratio of the mass of precipitated hollow particles in the MEK to the total mass of all hollow particles. (Evaluation criteria for solvent resistance) ○: Less than 5% by mass of precipitated hollow particles ×: Precipitated hollow particles make up 5% or more by mass.

[0091] 6. Measurement of relative permittivity and dielectric loss tangent The relative permittivity and dielectric loss tangent of hollow particles were measured using a perturbation-type measuring device (AET, model: ADMS01Nc) at a frequency of 1 MHz or 1 GHz and at room temperature (25°C).

[0092] [Table 1]

[0093] In Table 1, the meanings of the abbreviations are as follows: DVB960: Manufactured by Nippon Steel Chemical & Material Co., Ltd. Divinylbenzene purity: 96%, Ethyl vinylbenzene content: 4% DVB630: Manufactured by Nippon Steel Chemical & Material Co., Ltd. Divinylbenzene purity: 63%, Ethyl vinylbenzene content: 37% EGDMA: Ethylene glycol dimethacrylate V70: 2,2'-Azobis(4-Methoxy-2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: V-70)

[0094] [Consideration] In Comparative Examples 1 and 2, ethylene glycol dimethacrylate containing oxygen atoms was used as the crosslinkable monomer. Although hollow spaces were formed inside the particles, the particles exhibited poor solvent resistance and high dielectric constant and dielectric loss tangent. The particles obtained in Comparative Examples 1 and 2 contained oxygen atoms in their shells, which likely resulted in high affinity between the shell and polar solvents, allowing for easy penetration of the polar solvents. Furthermore, the presence of oxygen atoms likely contributed to the high dielectric constant and dielectric loss tangent. In addition, Comparative Example 1 contained many fine resin particles within the hollow spaces of the obtained particles. This is presumed to be due to the low compatibility between the crosslinkable monomer used in Comparative Example 1 and the hydrophobic solvent. The particles obtained in Comparative Example 1, with their high concentration of fine resin particles within the hollow spaces, likely exhibited even higher dielectric constant and dielectric loss tangent compared to Comparative Example 2. In Comparative Example 3, the commercially available divinylbenzene used contained 63% by mass of divinylbenzene and 37% by mass of ethylvinylbenzene as an impurity, and the content of crosslinkable monomers was less than 70% by mass of 100% by mass of the polymerizable monomer used. Therefore, in Comparative Example 3, the entire interior of the obtained particles was porous, and no hollow portion clearly distinguishable from the shell was formed. In addition, the particles obtained in Comparative Example 3 had poor solvent resistance and high dielectric constant and dielectric loss tangent. In Comparative Example 3, it is presumed that porous particles were formed because the components constituting the shell and the hydrophobic solvent did not sufficiently separate in the droplet of the monomer composition. Due to their structure, porous particles are easily permeated by solvents and tend to have high dielectric constant and dielectric loss tangent, so it is thought that the particles obtained in Comparative Example 3 had poor solvent resistance and electrical insulation properties. In contrast, in each example, a hydrocarbon monomer was used as the polymerizable monomer, with a crosslinkable monomer content of 70% or more of the polymerizable monomer by mass, and a hydrocarbon solvent having 5 to 8 carbon atoms was used as the hydrophobic solvent. As a result, the resulting particles were hollow particles having a hollow portion clearly distinguishable from the shell, exhibiting excellent solvent removal properties and solvent resistance, as well as excellent electrical insulation properties with low dielectric constant and dielectric loss tangent. [Explanation of Symbols]

[0095] 1 Aqueous medium 2 Low polarity material 3. Dispersion stabilizer 4. Monomer composition 4a Hydrophobic solvent 4b Materials other than hydrophobic solvents 4c Polymerizable monomers dispersed in an aqueous medium 5. Oil-soluble polymerization initiators 6 Shells 8 Hollow part 10 droplets 20 Precursor particles 100 Hollow particles with a hollow space filled with gas

Claims

1. A method for producing hollow particles comprising a resin-containing shell and a hollow portion surrounded by the shell, wherein the porosity is 50% or more, A step of preparing a mixture containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium, The steps include: preparing a suspension in which droplets of a monomer composition containing the polymerizable monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in an aqueous medium by suspending the aforementioned mixture; The process includes a step of subjecting the suspension to a polymerization reaction to prepare a precursor composition having a hollow portion surrounded by a resin-containing shell, and containing precursor particles that encapsulate the hydrophobic solvent within the hollow portion. The polymerizable monomer is a hydrocarbon monomer, and the content of crosslinkable monomers containing two or more ethylenically unsaturated double bonds is 90% by mass or more of the polymerizable monomer by mass. A method for producing hollow particles, wherein the hydrophobic solvent is a hydrocarbon solvent having 5 to 8 carbon atoms.

2. The method for producing hollow particles according to claim 1, wherein the mixture contains at least one selected from the group consisting of rosinic acid, higher fatty acids, and metal salts thereof.

3. The method for producing hollow particles according to claim 1 or 2, wherein the dispersion stabilizer is an inorganic dispersion stabilizer.

4. The method for producing hollow particles according to claim 3, wherein the inorganic dispersion stabilizer is a poorly water-soluble metal salt.

5. A method for producing hollow particles according to any one of claims 1 to 4, wherein the volume-average particle size of the hollow particles is 1 μm or more and 10 μm or less.

6. A hollow particle comprising a resin-containing shell and a hollow portion surrounded by the shell, having a porosity of 70% or more, The shell contains a hydrocarbon polymer as the resin, and the hydrocarbon polymer contains crosslinkable monomer units in a proportion of 90% by mass or more. The relative permittivity at a frequency of 1 MHz is 1.5 or less. Hollow particles having a hydrophobic solvent content of 1% by mass or less per unit mass.

7. The hollow particle according to claim 6, wherein the dielectric loss tangent at a frequency of 1 MHz is 0.010 or less.

8. The hollow particle according to claim 6 or 7, wherein the relative permittivity at a frequency of 1 GHz is 1.5 or less.

9. A hollow particle according to any one of claims 6 to 8, wherein the dielectric loss tangent at a frequency of 1 GHz is 0.010 or less.

10. A hollow particle according to any one of claims 6 to 9, wherein the volume-average particle size is 1 μm or more and 10 μm or less.

11. The hollow particles according to any one of claims 6 to 10, wherein when 200 mg of the hollow particles are placed in a 5 mL glass bottle at 25°C, then 4 mL of methyl ethyl ketone is added and the bottle is sealed, the glass bottle is shaken 10 times by hand, and then left at 25°C for 24 hours, the ratio of the mass of the hollow particles precipitated in the methyl ethyl ketone to the total mass of the hollow particles is less than 5% by mass.

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