Hollow particles and production method therefor

Abstract

Provided are hollow particles having an excellent balance of pressure resistance, dielectric properties, and a thermal expansion coefficient as a filler for electronic material applications. The hollow particles each comprising a shell and a hollow portion surrounded by the shell has a porosity of 50% or more, and a volume average particle diameter of 1.0-50.0 μm, wherein the shell contains an organic-inorganic composite material containing a resin and inorganic fine particles, and the content of the inorganic fine particles in the shell is 20-90 mass%.

Description

Hollow particles and their manufacturing method

[0001] The present disclosure relates to hollow particles with a shell of an organic-inorganic composite material and methods for making the same.

[0002] Hollow particles, which have a cavity inside the particle, are added to resins, paints, various molded products, etc. for the purposes of weight reduction, heat insulation, low dielectric constant, etc., and are used in a wide range of fields, such as automobiles, bicycles, aviation, electricity, electronics, construction, home appliances, containers, stationery, tools, and footwear.

[0003] In the field of electronic materials such as electronic circuit boards, attempts have been made to add hollow silica particles as a filler (e.g., Patent Documents 1 and 2) or hollow polymer particles (e.g., Patent Document 3) in order to lower the dielectric constant and dielectric loss tangent of resins.

[0004] Meanwhile, Patent Document 4 discloses a thermal conductivity modifier in the form of hollow particles having a shell made of an organic-inorganic hybrid vinyl resin containing a silicon component. In Patent Document 4, the shell is formed by crosslinking a copolymer of a radical-reactive monomer having an epoxy group or an oxetane group with a radical-reactive monomer having a silyl group using a crosslinkable monomer such as a polyamine compound. Furthermore, Patent Document 4 specifies that the average particle diameter of the hollow particles is 10 to 150 nm in order to prevent whitening of the coating film to which the hollow particles are added.

[0005] Japanese Patent Application Laid-Open No. 2012-136363 International Publication No. 2021 / 172294 International Publication No. 2004 / 067638 Japanese Patent Application Laid-Open No. 2017-066351

[0006] Furthermore, in metal-clad laminates, such as copper-clad laminates used in electronic circuit boards, it is necessary to reduce the coefficient of thermal expansion (CTE) of the resin layer to approximate that of a metal plate in order to improve dimensional stability. Furthermore, in order to obtain the effects of hollow particles, such as a low dielectric constant, the hollow particles must have good pressure resistance, allowing them to maintain a high porosity even after being added to various materials. However, while hollow silica particles have a low CTE, they are prone to crushing due to insufficient pressure resistance, and it is difficult to reduce their dielectric constant and dielectric loss tangent. Hollow particles having a shell made of an organic-inorganic hybrid vinyl resin, as described in Patent Document 4, also have insufficient pressure resistance and are prone to crushing. Furthermore, copolymerization of a radically reactive monomer having a silyl group tends to deteriorate their dielectric properties, making it difficult to reduce their dielectric constant and dielectric loss tangent. On the other hand, hollow polymer particles have good pressure resistance and dielectric properties, but it is difficult to sufficiently reduce their CTE. Therefore, there is a demand for a filler that has good pressure resistance and can sufficiently reduce the dielectric constant, dielectric loss tangent, and thermal expansion of resin materials for electronic materials.

[0007] An object of the present disclosure is to provide hollow particles that have an excellent balance of pressure resistance, dielectric properties, and thermal expansion coefficient as fillers for electronic materials.

[0008] The present inventors have found that by incorporating a specific amount of inorganic fine particles into the shell of hollow polymer particles, hollow particles having an excellent balance of pressure resistance, dielectric properties, and thermal expansion coefficient can be obtained as fillers for electronic materials.

[0009] The present disclosure provides hollow particles comprising a shell and a hollow portion surrounded by the shell, the hollow particles having a porosity of 50% or more and a volume average particle size of 1.0 μm or more and 50.0 μm or less, the shell comprising an organic-inorganic composite material containing a resin and inorganic fine particles, and the content of the inorganic fine particles in the shell being 20% ​​by mass or more and 90% by mass or less.

[0010] In the hollow particles of the present disclosure, the relative dielectric constant at a frequency of 1 GHz is preferably 2.00 or less.

[0011] The hollow particles of the present disclosure preferably have a thermal expansion coefficient at 23 to 100°C of 10 ppm / °C or more and 50 ppm / °C or less.

[0012] In the hollow particles of the present disclosure, the inorganic fine particles preferably have a relative dielectric constant of 5.0 or less at a frequency of 1 GHz and a thermal expansion coefficient of −5.0 ppm / ° C. or more and 10 ppm / ° C. or less at 23 to 100° C.

[0013] In the hollow particles of the present disclosure, the inorganic fine particles are preferably silica fine particles.

[0014] In the hollow particles of the present disclosure, the inorganic fine particles are preferably surface-treated with a surface treatment agent containing a radical-reactive functional group.

[0015] In the hollow particles of the present disclosure, the ratio of the volume average primary particle diameter of the inorganic fine particles to the shell thickness is preferably 0.90 or less.

[0016] The present disclosure provides a method for producing hollow particles of the present disclosure, the method comprising the steps of: preparing a mixed solution containing a polymerizable monomer, inorganic fine particles, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; suspending the mixed solution to prepare a suspension in which droplets of a monomer composition containing the polymerizable monomer, the inorganic fine particles, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; subjecting the suspension to a polymerization reaction to form precursor particles having a shell containing a polymer of the polymerizable monomer and the inorganic fine particles and a hollow portion surrounded by the shell, with the hydrophobic solvent encapsulated in the hollow portion, thereby obtaining a precursor composition in which the precursor particles are dispersed in the aqueous medium; and removing the hydrophobic solvent encapsulated in the precursor particles.

[0017] The present disclosure as described above provides hollow particles that have good pressure resistance and can reduce the dielectric constant, dielectric loss tangent, and thermal expansion of resin materials, and are suitable as fillers for electronic materials.

[0018] 1A and 1B are diagrams illustrating an example of a cross section of a hollow particle according to the present disclosure. 2A and 2B are diagrams illustrating an example of a method for producing hollow particles according to the present disclosure.

[0019] The hollow particles disclosed herein are hollow particles comprising a shell and a hollow portion surrounded by the shell, characterized in that they have a porosity of 50% or more and a volume average particle diameter of 1.0 μm or more and 50.0 μm or less, the shell comprises an organic-inorganic composite material containing a resin and inorganic fine particles, and the content of the inorganic fine particles in the shell is 20% by mass or more and 90% by mass or less.

[0020] FIG. 1 is a diagram schematically illustrating an example of a cross section of a hollow particle according to the present disclosure. The hollow particle 10 shown in FIG. 1 includes a shell 1 and a hollow portion 2 surrounded by the shell 1, and the shell 1 contains a resin 3 and inorganic fine particles 4. In the hollow particles according to the present disclosure, as shown in FIG. 1, the resin 3 forms the skeleton of the shell 1, and the resin 3 acts as a binder to hold the inorganic fine particles 4 within the shell. Note that FIG. 1 is merely a schematic diagram for explanatory purposes, and the hollow particles according to the present disclosure are not limited to those shown in the figure. Furthermore, the structure, dimensions, and shape of the hollow particles according to the present disclosure and the various materials contained therein are not limited to those shown in these figures.

[0021] In the hollow particles disclosed herein, the resin contained in the shell is typically a polymer. Polymers are materials with excellent strength and relatively low dielectric constants and dielectric dissipation factors, but have a high CTE. Therefore, hollow polymer particles with a polymer shell have excellent pressure resistance and dielectric properties, but a high CTE. In contrast, inorganic components such as silica are inferior in strength and dielectric properties, but have a low CTE. Furthermore, with hollow particles, the larger the particle size, the thicker the shell, which tends to improve pressure resistance. The higher the porosity, the greater the proportion of air spaces within the particle, which tends to improve dielectric properties. The hollow particles disclosed herein have a shell made of an organic-inorganic composite material in which a specific amount of inorganic fine particles is incorporated into a polymer, and by having the above-mentioned porosity and particle size, hollow particles with a reduced CTE are realized while maintaining good pressure resistance and dielectric properties. Furthermore, it is believed that the inorganic fine particles in the hollow particles disclosed herein have good dielectric properties due to their three-dimensional crystalline structure. It is presumed that the dielectric properties of hollow particles are improved when inorganic fine particles having a three-dimensional crystal structure are contained in the shell, because the dielectric loss tangent is reduced by suppressing the molecular motion of the shell, compared to when a shell is formed by copolymerizing a monomer containing an inorganic component in a polymer, as described in Patent Document 4. In this disclosure, good dielectric properties mean low relative permittivity and dielectric loss tangent, and the lower the relative permittivity and dielectric loss tangent, the better the dielectric properties.

[0022] Hereinafter, an example of a method for producing hollow particles according to the present disclosure will be described, followed by a detailed description of the hollow particles according to the present disclosure. Note that in this disclosure, the term "to" in a numerical range means that the numerical values ​​before and after the term are included as the lower and upper limits.

[0023] I. Method for Producing Hollow Particles The hollow particles of the present disclosure can be obtained by a method for producing hollow particles according to the present disclosure, the method including, for example, the steps of: preparing a mixed solution containing a polymerizable monomer, inorganic fine particles, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; suspending the mixed solution to prepare a suspension in which droplets of a monomer composition containing the polymerizable monomer, the inorganic fine particles, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction to form precursor particles having a shell containing a polymer of the polymerizable monomer and the inorganic fine particles and a hollow portion surrounded by the shell, with the hydrophobic solvent encapsulated in the hollow portion, thereby obtaining a precursor composition in which the precursor particles are dispersed in the aqueous medium.

[0024] The manufacturing method of the present disclosure follows the basic technique of forming hollow particles using suspension polymerization. In suspension polymerization, a mixture containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium is suspended, causing phase separation between the polymerizable monomer and the hydrophobic solvent, resulting in the preparation of a suspension in which droplets having a distribution structure in which the polymerizable monomer is unevenly distributed on the surface and the hydrophobic solvent is unevenly distributed in the center are dispersed in the aqueous medium. When this suspension is subjected to a polymerization reaction, polymers begin to precipitate on the surfaces of the droplets. As the polymerization reaction progresses, the droplet surfaces harden, forming shells, resulting in hollow particles with hollow spaces filled with the hydrophobic solvent. In the present disclosure, inorganic fine particles are further added to the mixture. By selecting appropriate inorganic fine particles depending on the type of polymerizable monomer, the inorganic fine particles can be unevenly distributed on the surface of the droplets together with the polymerizable monomer in the suspension. When this suspension is subjected to a polymerization reaction, the inorganic fine particles are retained within the shells formed by the polymer.

[0025] The method for producing hollow particles according to the present disclosure includes a step of preparing a mixed solution, a step of preparing a suspension, and a step of subjecting the suspension to a polymerization reaction, and may further include other steps. Furthermore, as far as technically possible, two or more of the above steps and other additional steps may be performed simultaneously as a single step, or the order of the steps may be reversed. For example, the preparation of the mixed solution and the suspension may be performed simultaneously in a single step, such as by adding the materials for preparing the mixed solution and suspending them at the same time.

[0026] A preferred example of the method for producing hollow particles of the present disclosure includes a first production method including the following steps: (1) mixed solution preparation step: preparing a mixed solution containing a polymerizable monomer, inorganic fine particles, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; (2) suspending step: suspending the mixed solution to prepare a suspension in which droplets of a monomer composition containing a polymerizable monomer, inorganic fine particles, a hydrophobic solvent, and a polymerization initiator are dispersed in an aqueous medium; (3) polymerization step: subjecting the suspension to a polymerization reaction to form precursor particles having a shell containing a polymer of the polymerizable monomer and inorganic fine particles and a hollow portion surrounded by the shell, the hollow portion encapsulating the hydrophobic solvent, thereby obtaining a precursor composition in which the precursor particles are dispersed in an aqueous medium; (4) solid-liquid separation step: performing solid-liquid separation of the precursor composition to obtain precursor particles encapsulating the hydrophobic solvent in the hollow portion; and (5) solvent removal step: removing the hydrophobic solvent encapsulated in the precursor particles obtained by the solid-liquid separation step to obtain hollow particles. In the present disclosure, hollow particles whose hollow portions are filled with a hydrophobic solvent are considered to be intermediates of hollow particles whose hollow portions are filled with a gas, and may be referred to as "precursor particles." In the present disclosure, the term "precursor composition" refers to a composition containing precursor particles.

[0027] FIG. 2 is a schematic diagram illustrating an example of the first manufacturing method of the present disclosure. (1) to (5) in FIG. 2 correspond to the above-described steps (1) to (5). The white arrows between the diagrams indicate the order of the steps. Note that FIG. 2 is merely a schematic diagram for explanatory purposes, and the manufacturing method of the present disclosure is not limited to those shown in the diagram. Furthermore, the structure, dimensions, and shape of the materials used in the manufacturing method of the present disclosure are not limited to those of the various materials shown in these diagrams. (1) in FIG. 2 is a cross-sectional schematic diagram illustrating one embodiment of a mixed solution in the mixed solution preparation step. As shown in this figure, the mixed solution contains an aqueous medium 11 and a low-polarity material 12 dispersed in the aqueous medium 11. Here, the low-polarity material 12 refers to a material that has low polarity and is difficult to mix with the aqueous medium 11. In the present disclosure, the low-polarity material 12 contains a polymerizable monomer, inorganic fine particles, a hydrophobic solvent, and a polymerization initiator. (2) in FIG. 2 is a cross-sectional schematic diagram illustrating one embodiment of a suspension in the suspension step. The suspension includes an aqueous medium 11 and droplets 20 of a monomer composition dispersed in the aqueous medium 11. The droplets 20 of the monomer composition contain a polymerizable monomer, inorganic fine particles, a hydrophobic solvent, and a polymerization initiator, but the distribution within the droplets is non-uniform. The droplets 20 of the monomer composition are phase-separated into a hydrophobic solvent 20a and a material other than the hydrophobic solvent 20b, including the polymerizable monomer and inorganic fine particles. The hydrophobic solvent 20a is concentrated in the center, while the material other than the hydrophobic solvent 20b is concentrated on the surface side. A dispersion stabilizer (not shown) is attached to the surface. (3) in FIG. 2 is a cross-sectional schematic diagram illustrating one embodiment of a precursor composition obtained by a polymerization process, including precursor particles encapsulating a hydrophobic solvent in hollow portions. The precursor composition includes an aqueous medium 11 and precursor particles 30 encapsulating a hydrophobic solvent 20a in hollow portions, dispersed in the aqueous medium 11. The shell 1 forming the outer surface of the precursor particle 30 is formed by polymerization of the polymerizable monomer in the droplet 20 of the monomer composition, and contains a polymer of the polymerizable monomer and inorganic fine particles. Figure 2 (4) is a cross-sectional schematic diagram showing one embodiment of the precursor particle after the solid-liquid separation step. Figure 2 (4) shows the state in which the aqueous medium 11 has been removed from the state shown in Figure 2 (3). Figure 2 (5) is a cross-sectional schematic diagram showing one embodiment of the hollow particle after the solvent removal step.2(5) shows the state after removing the hydrophobic solvent 20a from the state shown in FIG. 2(4). By removing the hydrophobic solvent from the precursor particles, hollow particles 10 having gas-filled hollow portions 2 inside shells 1 are obtained.

[0028] Another preferred example of the method for producing hollow particles according to the present disclosure is a second production method in which the steps up to the polymerization step are carried out in the same manner as in the first production method described above, and after the polymerization step, a solvent removal step is carried out without carrying out a solid-liquid separation step, and then a solid-liquid separation step is carried out after the solvent removal step. In this second production method, after the polymerization step, a solvent removal step is carried out in which the hydrophobic solvent is removed from the precursor particles in the slurry-like precursor composition to obtain a dispersion in which hollow particles are dispersed in an aqueous medium, and then a solid-liquid separation step is carried out on the aqueous dispersion to obtain hollow particles whose hollow portions are filled with gas.

[0029] The above five steps and other steps will be described below in order.

[0030] (1) Mixed Liquid Preparation Step This step is a step of preparing a mixed liquid containing a polymerizable monomer, inorganic fine particles, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium. The mixed liquid may further contain other materials as long as the effects of the present disclosure are not impaired. The materials of the mixed liquid will be described in the following order: (A) polymerizable monomer, (B) inorganic fine particles, (C) hydrophobic solvent, (D) polymerization initiator, (E) dispersion stabilizer, and (F) aqueous medium.

[0031] (A) Polymerizable Monomer In the present disclosure, a polymerizable monomer is a compound having a functional group capable of addition polymerization (sometimes simply referred to as a polymerizable functional group in the present disclosure). In the present disclosure, a compound having an ethylenically unsaturated bond as a functional group capable of addition polymerization is generally used as the polymerizable monomer. In the present disclosure, the polymerizable monomer can be any known polymerizable monomer conventionally used in the preparation of hollow polymer particles. While not particularly limited, it is preferable that the polymerizable monomer contains a crosslinkable monomer, and it may also contain a non-crosslinkable monomer. When the polymerizable monomer contains a crosslinkable monomer, the crosslink density of the polymer precipitated on the surface of the droplets increases when the suspension is subjected to a polymerization reaction. Furthermore, the precipitates are also crosslinked to each other, thereby increasing the crosslink density of the shell. This facilitates the formation of a shell with excellent strength. Furthermore, the hollow particles tend to become spherical, and hollow portions clearly distinguishable from the shell tend to form within the particles. In this disclosure, a polymerizable monomer having only one polymerizable functional group is referred to as a non-crosslinkable monomer, and a polymerizable monomer having two or more polymerizable functional groups is referred to as a crosslinkable monomer. A crosslinkable monomer is a polymerizable monomer that forms crosslinks in a resin by a polymerization reaction. In this disclosure, a polymerizable monomer consisting of carbon and hydrogen is referred to as a hydrocarbon monomer, a crosslinkable monomer consisting of carbon and hydrogen is referred to as a crosslinkable hydrocarbon monomer, and a non-crosslinkable monomer consisting of carbon and hydrogen is referred to as a non-crosslinkable hydrocarbon monomer. In addition, a polymerizable monomer having a (meth)acryloyl group as the polymerizable functional group is referred to as an acrylic monomer, a crosslinkable monomer having a (meth)acryloyl group as the polymerizable functional group is referred to as a crosslinkable acrylic monomer, and a non-crosslinkable monomer having a (meth)acryloyl group as the polymerizable functional group is referred to as a non-crosslinkable acrylic monomer. In the crosslinkable acrylic monomer, at least one polymerizable functional group may be a (meth)acryloyl group, but it is preferable that all polymerizable functional groups are (meth)acryloyl groups. In the present disclosure, "(meth)acrylate" refers to each of acrylate and methacrylate, "(meth)acrylic" refers to each of acrylic and methacrylic, and "(meth)acryloyl" refers to each of acryloyl and methacryloyl.

[0032] Examples of the crosslinkable monomer include aromatic divinyl monomers such as divinylbenzene, divinylbiphenyl, and divinylnaphthalene; diene monomers such as butadiene, isoprene, 2,3-dimethylbutadiene, pentadiene, and hexadiene; alicyclic olefins such as dicyclopentadiene, cyclopentadiene, and ethylidenetetracyclododecene; and other bifunctional crosslinkable hydrocarbon monomers; polymeric crosslinkable hydrocarbon monomers such as polybutadiene, polyisoprene, styrene-butadiene block copolymers (SBS), and styrene-isoprene block copolymers (SIS); allyl (meth)acrylate, vinyl (meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, tricyclo Examples of suitable crosslinkable monomers include difunctional crosslinkable acrylic monomers such as decanedimethanol di(meth)acrylate, 2-hydroxy-3-(meth)acryloyloxypropyl (meth)acrylate, and polyphenylene ether having both terminals (meth)acrylic-modified; trifunctional or higher crosslinkable acrylic monomers such as trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol poly(meth)acrylate, and ethoxylated versions thereof; crosslinkable allylic monomers such as diallyl phthalate; and polymeric crosslinkable monomers such as polyphenylene ether having both terminals vinyl-modified. These crosslinkable monomers can be used alone or in combination of two or more.

[0033] Examples of non-crosslinkable monomers include aromatic monovinyl monomers such as styrene, vinyltoluene, α-methylstyrene, p-methylstyrene, ethylvinylbenzene, ethylvinylbiphenyl, and ethylvinylnaphthalene; monoolefin monomers such as ethylene, propylene, and butylene; alicyclic monoolefins such as vinylcyclohexane, norbornene, and tricyclododecene; methyl(meth)acrylate, ethyl(meth)acrylate, butyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, and lauryl(meth)acrylate. acrylate, t-butylaminoethyl (meth)acrylate, glycidyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-aminoethyl (meth)acrylate, (meth)acrylic acid, (meth)acrylamide, N-methylol (meth)acrylamide, N-butoxymethyl (meth)acrylamide, methoxypolyethylene glycol (meth)acrylate, ethoxypolyethylene glycol (meth)acrylate, propoxypolyethylene glycol (meth)acrylate, butoxypolyethylene glycol non-crosslinkable acrylic monomers such as polyethylene glycol mono(meth)acrylate, hexaoxy polyethylene glycol (meth)acrylate, octoxy polyethylene glycol polypropylene glycol (meth)acrylate, lauroxy polyethylene glycol (meth)acrylate, stearoxy polyethylene glycol (meth)acrylate, phenoxy polyethylene glycol polypropylene glycol (meth)acrylate, polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, polyethylene glycol propylene glycol mono(meth)acrylate, polyethylene glycol tetramethylene glycol (meth)acrylate, propylene glycol polybutylene glycol mono(meth)acrylate, and monoethylene glycol mono(meth)acrylate; vinyl carboxylate vinyl ester monomers such as vinyl acetate; halogenated aromatic vinyl monomers such as halogenated styrene; halogenated vinyl monomers such as vinyl chloride; halogenated vinylidene monomers such as vinylidene chloride; vinylpyridine monomers; and the like.In addition, non-crosslinkable macromers such as (meth)acrylic-terminated polystyrene and (meth)acrylic-terminated polymethyl methacrylate can also be used. These non-crosslinkable monomers can be used alone or in combination of two or more.

[0034] The content of the crosslinkable monomer per 100 parts by mass of the polymerizable monomer is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, and even more preferably 90 parts by mass or more. When the content of the crosslinkable monomer is equal to or greater than the above-mentioned lower limit, the components constituting the shell and the hydrophobic solvent are easily phase-separated in droplets of the monomer composition dispersed in the suspension. Furthermore, the formation of a shell with excellent strength suppresses particle deformation, making it easier to form hollow particles having only one hollow portion clearly distinguishable from the shell. Furthermore, the formation of a shell with a high crosslinking density makes it easier to retain inorganic fine particles within the shell. The polymerizable monomer may also contain a non-crosslinkable monomer within a range that does not impair the effects of the present disclosure. When a crosslinkable monomer and a non-crosslinkable monomer are contained in combination, the strength of the shell may be increased in some cases. When the polymerizable monomer contains a crosslinkable monomer and a non-crosslinkable monomer, the content of the crosslinkable monomer may be, for example, 98 parts by mass or less, or 96 parts by mass or less, per 100 parts by mass of the polymerizable monomer.

[0035] The crosslinkable monomer preferably contains at least a bifunctional crosslinkable monomer in order to improve the pressure resistance of the hollow particles. Furthermore, when a bifunctional crosslinkable monomer and a trifunctional or higher crosslinkable monomer are combined, the pressure resistance may be further improved. When the polymerizable monomer contains a bifunctional crosslinkable monomer and a trifunctional or higher crosslinkable monomer, the content of the trifunctional or higher crosslinkable monomer per 100 parts by mass of the crosslinkable monomer is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 20 parts by mass or more, and is preferably 50 parts by mass or less, more preferably 40 parts by mass or less.

[0036] In addition, from the viewpoint of facilitating a stable polymerization reaction, the polymerizable monomer is preferably a polymerizable monomer whose polymerizable functional group is a (meth)acryloyl group or a vinyl group. In the present disclosure, "stable polymerization reaction" means that the reactivity of the polymerization reaction is good and the polymerization reaction proceeds uniformly. In the present disclosure, the content of the polymerizable monomer whose polymerizable functional group is a (meth)acryloyl group or a vinyl group is preferably 80 parts by mass or more, more preferably 90 parts by mass or more, and even more preferably 95 parts by mass or more, per 100 parts by mass of the polymerizable monomer.

[0037] Furthermore, it is preferable that the polymerizable monomer contains a hydrocarbon monomer, since the dielectric properties of the hollow particles are easily improved. From the viewpoint of improving the dielectric properties of the hollow particles, the content of the hydrocarbon monomer per 100 parts by mass of the polymerizable monomer is preferably 30 parts by mass or more, more preferably 50 parts by mass or more, even more preferably 70 parts by mass or more, and still more preferably 90 parts by mass or more.

[0038] The content of the polymerizable monomer in the mixed solution is not particularly limited, but from the viewpoint of the balance between the porosity, particle size, and mechanical strength of the hollow particles, the lower limit is preferably 5% by mass or more, more preferably 10% by mass or more, and the upper limit is preferably 50% by mass or less, more preferably 40% by mass or less, relative to 100% by mass of the total mass of the components in the mixed solution excluding the aqueous medium. Furthermore, the content of the polymerizable monomer relative to 100% by mass of the total mass of the solids of the materials that form the oil phase in the mixed solution excluding the hydrophobic solvent is preferably 10% by mass or more, more preferably 15% by mass or more, even more preferably 20% by mass or more, from the viewpoint of improving the mechanical strength of the hollow particles, and is preferably 80% by mass or less, more preferably 70% by mass or less, even more preferably 60% by mass or less, from the viewpoint of sufficiently incorporating inorganic fine particles. In the present disclosure, the solid content refers to all components excluding the solvent, and liquid polymerizable monomers and the like are considered to be included in the solid content.

[0039] (B) Inorganic Fine Particles From the viewpoint of obtaining hollow particles with an excellent balance between dielectric properties and CTE, inorganic fine particles having a relative dielectric constant of 5.0 or less at a frequency of 1 GHz and a thermal expansion coefficient of -5.0 ppm / °C or more and 10.0 ppm / °C or less at 23 to 100°C are preferred. The relative dielectric constant of the inorganic fine particles at a frequency of 1 GHz is more preferably 4.5 or less, even more preferably 4.0 or less, and even more preferably 3.0 or less. The lower limit of the relative dielectric constant of the inorganic fine particles is not particularly limited, but is usually 1.0 or more. The thermal expansion coefficient of the inorganic fine particles at 23 to 100°C is more preferably 7.5 ppm / °C or less, even more preferably 5.0 ppm / °C or less, and even more preferably 1.0 ppm / °C or less. The lower limit of the thermal expansion coefficient of the inorganic fine particles is not particularly limited, but may be, for example, -2.5 ppm / °C or more. In order to obtain hollow particles with good dielectric properties, the inorganic fine particles preferably have a dielectric loss tangent of 1.0×10 at a frequency of 1 GHz. －２ or less, more preferably 8.0 × 10 －３ The lower limit of the dielectric loss tangent of the inorganic fine particles is not particularly limited, but is usually 1.0 × 10 －４ That's all.

[0040] Examples of the main component of inorganic fine particles preferably used in the present disclosure include oxides and nitrides of at least one element selected from the group consisting of Group 2A and Group 3B elements in Periods 2 to 7 of the Periodic Table, and Group 4B, Group 5B, and Group 6B elements in Periods 3 to 7 of the Periodic Table. Here, the main component of inorganic fine particles refers to a component contained in the inorganic fine particles at a ratio of 50% by mass or more. In addition to the main component, the inorganic fine particles may also contain components derived from, for example, a surface treatment agent. In the present disclosure, inorganic fine particles containing at least one element selected from the group consisting of silicon dioxide, zinc oxide, and boron nitride as a main component are preferably used from the viewpoint of achieving a low CTE while suppressing deterioration of the dielectric properties of the hollow particles. These inorganic fine particles are also preferred because they typically satisfy the above-mentioned preferred relative dielectric constant, dielectric dissipation factor, and thermal expansion coefficient. Among these inorganic fine particles, silica fine particles containing silicon dioxide as a main component are particularly preferred.

[0041] Furthermore, the inorganic fine particles used in the present disclosure are preferably surface-treated with a surface treatment agent containing a radical-reactive functional group. By using such surface-treated inorganic fine particles, the inorganic fine particles are more easily retained within the shell. The surface-treated inorganic fine particles have good affinity with the polymerizable monomer, so they tend to be unevenly distributed on the surface side of the droplets together with the polymerizable monomer in the suspension. Furthermore, when the polymerizable monomer polymerizes, the radical-reactive functional groups on the surface of the inorganic fine particles react with the polymerizable functional groups of the polymerizable monomer, resulting in covalent bonding between the inorganic fine particles and the polymer. Furthermore, when the inorganic fine particles used in the present disclosure are surface-treated with a surface treatment agent containing a radical-reactive functional group, the hollow particles of the present disclosure tend to have improved pressure resistance. This is presumably due to the covalent bonding between the inorganic fine particles and the polymer within the shell of the hollow particle, thereby improving shell strength. The surface treatment agent containing a radical-reactive functional group is not particularly limited, but for example, a silane coupling agent having a radical-reactive functional group can be preferably used. Specific examples include a vinyl silane coupling agent, a methacryl silane coupling agent, an acrylic silane coupling agent, and a styryl silane coupling agent.

[0042] In addition, the inorganic fine particles used in the present disclosure may be surface-treated with a surface treatment agent that does not contain a radically reactive functional group. The surface treatment agent that does not contain a radically reactive functional group is not particularly limited, and for example, a known silane coupling agent can be used. Among them, a silane coupling agent that contains a hydrocarbon group is preferred in terms of improving the affinity between the inorganic fine particles and the polymerizable monomer. Examples of such preferred silane coupling agents include methyltrimethoxysilane, dimethyldimethoxysilane, phenyltrimethoxysilane, diphenyldimethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane, phenyltriethoxysilane, diphenyldiethoxysilane, n-butyltrimethoxysilane, isobutyltrimethoxysilane, trimethylmethoxysilane, triethylmethoxysilane, trimethylethoxysilane, triethylethoxysilane, n-hexyltrimethoxysilane, n-octyltrimethoxysilane, n-octyltriethoxysilane, n-decyltrimethoxysilane, n-hexadecyltrimethoxysilane, n-octadecyltrimethoxysilane, etc. Among these, trialkylalkoxysilanes such as trimethylmethoxysilane, triethylmethoxysilane, trimethylethoxysilane, and triethylethoxysilane are preferred.

[0043] Furthermore, it is preferable to select inorganic fine particles so that the ratio of the volume average primary particle diameter of the inorganic fine particles to the shell thickness of the hollow particles (volume average primary particle diameter of inorganic fine particles / shell thickness) is 0.90 or less. This makes it easier for the inorganic fine particles to be retained within the shell and improves the smoothness of the shell surface. Improved shell surface smoothness of hollow particles tends to improve pressure resistance by making it easier for external pressure to be applied uniformly. Furthermore, if the particle diameter of the inorganic fine particles is too large relative to the shell thickness, cracks are more likely to occur in the inorganic fine particles when external force is applied, which may deteriorate the pressure resistance of the hollow particles. By reducing this ratio, the occurrence of such cracks can be suppressed and pressure resistance can be improved. The ratio of the volume average primary particle diameter of the inorganic fine particles to the shell thickness of the hollow particles is more preferably 0.50 or less, even more preferably 0.30 or less, and even more preferably 0.20 or less. The lower limit is not particularly limited, but may be, for example, 0.001 or more, or 0.01 or more. The volume-average primary particle diameter of the inorganic fine particles may be, for example, 5 nm to 3,000 nm, preferably 10 nm to 2,000 nm, more preferably 15 nm to 1,000 nm, and even more preferably 15 nm to 500 nm. A volume-average primary particle diameter of the inorganic fine particles equal to or greater than the lower limit is preferred because it tends to improve the dispersibility of the inorganic fine particles in the mixed liquid. A volume-average primary particle diameter of the inorganic fine particles equal to or less than the upper limit can allow the inorganic fine particles to be uniformly contained in a sufficiently thin shell, making it easier to obtain hollow particles with an excellent balance between dielectric properties and thermal expansion coefficient. The volume-average primary particle diameter of the inorganic fine particles can be determined by measuring the primary particle diameters of 100 particles through image analysis of images taken by SEM observation and calculating the volume average.

[0044] The content of inorganic fine particles in the mixed liquid is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more, relative to 100% by mass of the total mass of solids of the materials that form the oil phase in the mixed liquid, excluding the hydrophobic solvent, in order to obtain hollow particles with a low CTE; on the other hand, in order to prevent deterioration of pressure resistance and dielectric properties, the content is preferably 90% by mass or less, more preferably 85% by mass or less, even more preferably 80% by mass or less, even more preferably 70% by mass or less, and particularly preferably 60% by mass or less.

[0045] (C) Hydrophobic Solvent The hydrophobic solvent used in the manufacturing method of the present disclosure is a non-polymerizable, poorly water-soluble organic solvent. The hydrophobic solvent acts as a spacer material that forms hollow spaces inside the particles. In the suspension process described below, a suspension is obtained in which droplets of a monomer composition containing a hydrophobic solvent are dispersed in an aqueous medium. In the suspension process, phase separation occurs within the droplets of the monomer composition, and the hydrophobic solvent, which has low polarity, tends to collect inside the droplets of the monomer composition. Ultimately, the droplets of the monomer composition contain the hydrophobic solvent inside, and other materials other than the hydrophobic solvent are distributed around the periphery according to their respective polarities. Then, in the polymerization process described below, an aqueous dispersion containing precursor particles encapsulating the hydrophobic solvent is obtained. In other words, as the hydrophobic solvent collects inside the particles, hollow spaces filled with the hydrophobic solvent are formed inside the resulting precursor particles.

[0046] The hydrophobic solvent can be appropriately selected from known hydrophobic solvents and is not particularly limited. Examples include esters such as ethyl acetate and butyl acetate; ether esters such as propylene glycol monomethyl ether acetate and propylene glycol monoethyl ether acetate; and hydrocarbon solvents, of which hydrocarbon solvents are preferred. Examples of hydrocarbon solvents include aliphatic hydrocarbons including chain hydrocarbon solvents such as pentane, hexane, heptane, octane, 2-methylbutane, 2-methylpentane, and paraffin solvents, and cyclic hydrocarbon solvents such as cyclohexane, methylcyclohexane, and cycloheptane; and aromatic hydrocarbons such as benzene, toluene, and xylene. These hydrophobic solvents can be used alone or in combination of two or more.

[0047] In the suspension step, because phase separation between the polymerizable monomer and the hydrophobic solvent is likely to occur within droplets of the monomer composition, it is preferable to select an organic solvent that has a lower solubility in water than the crosslinkable monomer contained in the polymerizable monomer as the hydrophobic solvent. Furthermore, when the polymerizable monomer contains a hydrocarbon monomer in a proportion of more than 50% by mass, it is preferable to use a chain hydrocarbon solvent as the hydrophobic solvent, more preferably a chain hydrocarbon solvent having 5 to 8 carbon atoms, and even more preferably at least one selected from the group consisting of pentane, hexane, heptane, and octane. On the other hand, when the polymerizable monomer contains an acrylic monomer and the proportion of the hydrocarbon monomer is 50% by mass or less, it is preferable to use a hydrocarbon solvent having 4 to 7 carbon atoms, more preferably a hydrocarbon solvent having 5 to 7 carbon atoms, as the hydrophobic solvent. Here, the hydrocarbon solvent may be either an aromatic hydrocarbon or an aliphatic hydrocarbon, but among these, an aliphatic hydrocarbon is preferred, a cyclic hydrocarbon solvent is more preferred, and at least one solvent selected from the group consisting of cyclohexane, cycloheptane, and methylcyclohexane is even more preferred.

[0048] Furthermore, although not particularly limited, the boiling point of the hydrophobic solvent is preferably 130° C. or lower, more preferably 100° C. or lower, from the viewpoint of ease of removal in the solvent removal step described below, and is preferably 50° C. or higher, more preferably 60° C. or higher, from the viewpoint of ease of inclusion in the precursor particles. When the hydrophobic solvent is a mixed solvent containing multiple types of hydrophobic solvents and has multiple boiling points, it is preferable that the boiling point of the solvent with the highest boiling point among the solvents contained in the mixed solvent is not higher than the above-mentioned 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 not lower than the above-mentioned lower limit.

[0049] Furthermore, the hydrophobic solvent used in the manufacturing method of the present disclosure preferably has a dielectric constant of 2.5 or less at 20°C, and more preferably 2.0 or less. The dielectric constant is one of the indicators of the polarity of a compound. When the dielectric constant of the hydrophobic solvent is sufficiently small, such as 2.5 or less, it is believed that phase separation proceeds rapidly in the droplets of the monomer composition, making it easy to form hollow spaces. Examples of hydrophobic solvents having a dielectric constant of 2.0 or less at 20°C are as follows. The values ​​in parentheses are the dielectric constant values: pentane (1.8), hexane (1.9), heptane (1.9), octane (1.9), and cyclohexane (2.0). Regarding the dielectric constant at 20°C, reference can be made to values ​​described in known literature (e.g., "Chemical Handbook: Basics," 4th Revised Edition, edited by the Chemical Society of Japan, Maruzen Co., Ltd., published September 30, 1993, pages II-498 to II-503) and other technical information. The method for measuring the relative dielectric constant at 20°C includes, for example, a relative dielectric constant test carried out in accordance with JIS C 2101:1999, 23, at a measurement temperature of 20°C.

[0050] The porosity of the hollow particles can be adjusted by changing the amount of hydrophobic solvent in the mixed solution. In the suspension process described below, the polymerization reaction proceeds with the hydrophobic solvent encapsulated in oil droplets containing the polymerizable monomer, etc., so the higher the hydrophobic solvent content, the higher the porosity of the resulting hollow particles tends to be. In the present disclosure, the content of the hydrophobic solvent in the mixed solution is preferably 50 to 500 parts by mass per 100 parts by mass of the polymerizable monomer and inorganic fine particles, as this facilitates control of the particle size of the hollow particles, increases the porosity while maintaining the strength of the hollow particles, and reduces the amount of residual hydrophobic solvent within the particles. The content of the hydrophobic solvent in the mixed solution is more preferably 70 to 300 parts by mass, and even more preferably 80 to 200 parts by mass per 100 parts by mass of the polymerizable monomer and inorganic fine particles.

[0051] (D) Polymerization Initiator In the production method of the present disclosure, the mixed liquid preferably contains an oil-soluble polymerization initiator as a polymerization initiator. The oil-soluble polymerization initiator is not particularly limited as long as it is an oil-philic initiator having a solubility in water of 0.2 mass% or less, and examples thereof include organic peroxides such as benzoyl peroxide, lauroyl peroxide, t-butylperoxy-2-ethylhexanoate, t-butylperoxydiethyl acetate, and t-butylperoxypivalate; and azo compounds such as 2,2'-azobis(2,4-dimethylvaleronitrile), azobisisobutyronitrile, and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile).

[0052] The content of the polymerization initiator relative to 100 parts by mass of the polymerizable monomer in the mixed solution is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 7 parts by mass, and even more preferably 1 to 5 parts by mass. When the content of the polymerization initiator is equal to or greater than the above-mentioned lower limit, the polymerization reaction can proceed sufficiently, while when the content is equal to or less than the above-mentioned upper limit, there is little risk of the oil-soluble polymerization initiator remaining after completion of the polymerization reaction, and there is also little risk of an unexpected side reaction proceeding.

[0053] (E) Dispersion Stabilizer The dispersion stabilizer is an agent that disperses droplets of the monomer composition in an aqueous medium during the suspension process. Examples of dispersion stabilizers include inorganic dispersion stabilizers, organic or inorganic water-soluble polymer stabilizers, and surfactants. In the present disclosure, inorganic dispersion stabilizers are preferably used as the dispersion stabilizer because they facilitate control of the particle size of the droplets in the suspension, narrow the particle size distribution of the resulting hollow particles, and prevent the shell from becoming too thin, thereby suppressing a decrease in the strength of the hollow particles. 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; metal hydroxides such as aluminum hydroxide, magnesium hydroxide, calcium hydroxide, barium hydroxide, and ferric hydroxide; and inorganic compounds such as silicon dioxide. These inorganic dispersion stabilizers can be used alone or in combination. Among these inorganic dispersion stabilizers, poorly water-soluble inorganic dispersion stabilizers are preferred. Here, poor water solubility preferably means a solubility of 0.5 g or less in 100 g of water.Among the inorganic dispersion stabilizers, metal hydroxides are preferred, with magnesium hydroxide being more preferred.

[0054] In the present disclosure, it is particularly preferable to use a poorly water-soluble inorganic dispersion stabilizer dispersed in an aqueous medium in the form of colloidal particles, i.e., in the form of a colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles. This not only narrows the particle size distribution of the droplets of the monomer composition, but also makes it easy to reduce the amount of inorganic dispersion stabilizer remaining in the resulting hollow particles by washing. A colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles can be prepared, for example, by reacting at least one selected from alkali metal hydroxides and alkaline earth metal hydroxides with a water-soluble polyvalent metal salt (excluding alkaline earth metal hydroxides) in an aqueous medium. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Examples of alkaline earth metal hydroxides include barium hydroxide and calcium hydroxide. The water-soluble polyvalent metal salt may be any water-soluble polyvalent metal salt other than the above-mentioned alkaline earth metal hydroxides, including, for example, 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 method for reacting at least one selected from the above-mentioned alkali metal hydroxides and alkaline earth metal hydroxides with the above-mentioned water-soluble polyvalent metal salt in an aqueous medium is not particularly limited, but includes, for example, mixing an aqueous solution of at least one selected from the alkali metal hydroxides and alkaline earth metal hydroxides with an aqueous solution of the water-soluble polyvalent metal salt. Furthermore, colloidal silica can also be used as a colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles.Examples of organic water-soluble polymer stabilizers include polyvinyl alcohol, polycarboxylic acids (such as polyacrylic acid), celluloses (such as hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, and ethyl cellulose), polyvinylpyrrolidone, polyacrylimide, polyethylene oxide, and poly(hydroxystearic acid-g-methyl methacrylate-co-methacrylic acid) copolymers. Examples of inorganic water-soluble polymer compounds include sodium tripolyphosphate. Surfactants are compounds that have both hydrophilic and hydrophobic groups in one molecule, and examples include known ionic surfactants such as anionic surfactants, cationic surfactants, and amphoteric surfactants, as well as nonionic surfactants.

[0055] The content of the dispersion stabilizer is not particularly limited, but is preferably 0.5 to 15 parts by mass, more preferably 1 to 10 parts by mass, per 100 parts by mass of the total mass of the polymerizable monomer, inorganic fine particles, and hydrophobic solvent. By having the content of the dispersion stabilizer at or above the lower limit, droplets of the monomer composition can be sufficiently dispersed so as not to coalesce in the suspension. On the other hand, by having the content of the dispersion stabilizer at or below the upper limit, an increase in the viscosity of the suspension during granulation can be prevented, and the problem of the suspension clogging in the granulator can be avoided. Furthermore, the content of the dispersion stabilizer is preferably 0.5 to 15 parts by mass, more preferably 0.5 to 10 parts by mass, per 100 parts by mass of the aqueous medium.

[0056] (F) Aqueous Medium In the present disclosure, the term "aqueous medium" refers to a medium selected from the group consisting of water, a hydrophilic solvent, and a mixture of water and a hydrophilic solvent. When using a mixture of water and a hydrophilic solvent, it is important that the polarity of the entire mixture is not too low in order to form droplets of the monomer composition. In this case, for example, the mass ratio of water to hydrophilic solvent (water:hydrophilic solvent) may be 99:1 to 50:50. The hydrophilic solvent in the present disclosure is not particularly limited as long as it is sufficiently miscible with water and does not cause phase separation. Examples of hydrophilic solvents include alcohols such as methanol and ethanol; tetrahydrofuran (THF); dimethyl sulfoxide (DMSO); and the like.

[0057] The content of the aqueous medium is not particularly limited, but from the viewpoint of setting the particle size and porosity of the hollow particles within the preferred ranges described below, the lower limit is preferably 200 parts by mass or more, more preferably 400 parts by mass or more, and even more preferably 600 parts by mass or more, relative to 100 parts by mass of the total mass of the polymerizable monomer and inorganic fine particles contained in the mixed liquid, and the upper limit is preferably 1,000 parts by mass or less, and more preferably 800 parts by mass or less.

[0058] The mixed liquid may further contain other materials different from the above-described materials (A) to (F) as long as the effects of the present disclosure are not impaired.

[0059] A mixed liquid is obtained by mixing the above-mentioned materials and other materials as needed, and stirring appropriately. In this mixed liquid, an oil phase containing lipophilic materials such as (A) polymerizable monomer, (B) inorganic fine particles, (C) hydrophobic solvent, and (D) polymerization initiator is dispersed with particle sizes of about several mm in an aqueous phase containing (E) dispersion stabilizer and (F) aqueous medium. The dispersion state of these materials in the mixed liquid can be observed with the naked eye, depending on the type of material. In the mixed liquid preparation step, the mixed liquid may be obtained by simply mixing the above-mentioned materials and other materials as needed, and stirring appropriately. However, in terms of making the shell more uniform, it is preferable to prepare a mixed liquid by separately preparing an oil phase containing polymerizable monomer, inorganic fine particles, hydrophobic solvent, and polymerization initiator, and an aqueous phase containing dispersion stabilizer and aqueous medium, and then mixing them. In the present 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 is preferably used as the aqueous phase. By preparing the oil phase and the water phase separately in advance and then mixing them, hollow particles having a uniform shell composition can be produced, and the particle size of the hollow particles can be easily controlled.

[0060] (2) Suspension Step The suspension step is a step of preparing a suspension in which droplets of a monomer composition containing a polymerizable monomer, inorganic fine particles, a hydrophobic solvent, and a polymerization initiator are dispersed in an aqueous medium by suspending the above-mentioned mixed liquid. The suspension method for forming droplets of the monomer composition is not particularly limited, and any known suspension method can be used. Examples of dispersers used in preparing the suspension include horizontal or vertical in-line dispersers such as Milder (product name) manufactured by Pacific Machinery Works, Ltd., Cavitron (product name) manufactured by Eurotec Co., Ltd., and in-line dispersers manufactured by IKA (e.g., DISPAX-REACTOR (registered trademark) DRS (product name)); and emulsifying dispersers such as the Homomixer MARK II series manufactured by Primix Corporation.

[0061] In the suspension prepared in the suspending step, droplets of the monomer composition containing the lipophilic material and having a particle size of approximately 1 to 50 μm are uniformly dispersed in the aqueous medium. Such droplets of the monomer composition are difficult to observe with the naked eye and can be observed using known observation equipment such as an optical microscope. In the suspending step, phase separation occurs in the droplets of the monomer composition, so that the hydrophobic solvent with low polarity tends to collect inside the droplets. As a result, the resulting droplets contain the hydrophobic solvent in their interiors and materials other than the hydrophobic solvent distributed around their peripheries.

[0062] The droplets of the monomer composition dispersed in an aqueous medium are formed by surrounding the lipophilic monomer composition with a dispersion stabilizer. The droplets of the monomer composition contain an oil-soluble polymerization initiator, a polymerizable monomer, inorganic fine particles, and a hydrophobic solvent. The droplets of the monomer composition are small oil droplets, and the polymerization initiator generates polymerization-initiating radicals inside the small oil droplets. Therefore, precursor particles of the desired particle size can be produced without excessive growth of the small oil droplets. In the suspension polymerization method using an oil-soluble polymerization initiator, there is no opportunity for the polymerization initiator to come into contact with the polymerizable monomer dispersed in the aqueous medium, so that in addition to the desired resin particles having hollow portions, the by-production of excess resin particles such as dense particles with a relatively small particle size can be suppressed.

[0063] (3) Polymerization Step This step is a step of forming precursor particles by subjecting the suspension obtained in the above-mentioned suspension step to a polymerization reaction, thereby obtaining a precursor composition in which the precursor particles are dispersed in an aqueous medium. In the polymerization step, a polymerization reaction of the polymerizable monomer contained in the droplets of the monomer composition progresses to form a polymer, thereby forming a shell containing the polymer and inorganic fine particles, and precursor particles having a hollow portion surrounded by the shell and encapsulating a hydrophobic solvent are obtained.

[0064] The polymerization method is not particularly limited, and for example, a batch method, a semi-continuous method, a continuous method, etc. can be used. The polymerization temperature is preferably 40 to 90°C, more preferably 50 to 80°C. The polymerization reaction time is preferably 1 to 48 hours, more preferably 1.5 to 36 hours. The polymerization reaction time is preferably adjusted so that the polymerization conversion rate of the polymerizable monomer becomes 100%.

[0065] (4) Solid-Liquid Separation Step In the first manufacturing method, this step is a step of obtaining precursor particles by solid-liquid separation of the precursor composition obtained by the above-mentioned polymerization step. The method of solid-liquid separation of the precursor composition 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 step, an optional step such as a pre-drying step may be performed before performing the solvent removal step described below. Examples of the pre-drying step include a step of pre-drying the solid content obtained after the solid-liquid separation step using a drying device such as a dryer or a drying appliance such as a hand dryer.

[0066] (5) Solvent Removal Step In the first production method, this step is a step of removing the hydrophobic solvent contained in the precursor particles obtained by the solid-liquid separation step. By removing the hydrophobic solvent contained in the precursor particles in air, the hydrophobic solvent inside the precursor particles is replaced with air, and hollow particles filled with gas are obtained.

[0067] In this process, "in the air" strictly refers to an environment in which no liquid is present outside the precursor particles, or an environment in which only a trace amount of liquid is present outside the precursor particles, so that the removal of the hydrophobic solvent is not affected. "In the air" can also be referred to as a state in which the precursor particles are not present in a slurry, or a state in which the precursor particles are present in a dry powder. In other words, in this process, it is important to remove the hydrophobic solvent in an environment in which the precursor particles are in direct contact with the external gas.

[0068] The method for removing the hydrophobic solvent from the precursor particles in air is not particularly limited, and known methods can be used. Examples of such methods include vacuum drying, heat drying, flash drying, or a combination of these methods. In particular, when using heat drying, the heating temperature must be equal to or higher than the boiling point of the hydrophobic solvent and equal to or lower than the maximum temperature at which the shell structure of the precursor particles does not collapse. Therefore, depending on the shell composition and the type of hydrophobic solvent in the precursor particles, the heating temperature may be, for example, 50 to 200°C, 70 to 200°C, or 100 to 200°C. The drying operation in air replaces the hydrophobic solvent inside the precursor particles with the external gas, resulting in hollow particles whose hollow portions are filled with gas.

[0069] The drying atmosphere is not particularly limited and can be appropriately selected depending on the application of the hollow particles. Examples of the drying atmosphere include air, oxygen, nitrogen, argon, etc. Hollow particles with a temporary vacuum inside can also be obtained by filling the inside of the hollow particles with a gas and then drying under reduced pressure.

[0070] Alternatively, the hydrophobic solvent encapsulated in the precursor particles may be removed from a slurry containing precursor particles and an aqueous medium without subjecting the slurry-like precursor composition obtained in the polymerization step to solid-liquid separation. This alternative method is the second manufacturing method described above. 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 equal to or higher than the boiling point of the hydrophobic solvent minus 35°C. When the hydrophobic solvent is a mixed solvent containing multiple hydrophobic solvents and has multiple boiling points, the boiling point of the hydrophobic solvent in the solvent removal step is the boiling point of the solvent with the highest boiling point among the solvents contained in the mixed solvent, i.e., the highest boiling point among the multiple boiling points. The temperature at which the inert gas is bubbled through the precursor composition is preferably equal to or higher than the boiling point of the hydrophobic solvent minus 30°C, more preferably equal to or higher than the boiling point of the hydrophobic solvent, in order to reduce the amount of hydrophobic solvent remaining in the hollow particles. The bubbling temperature is typically equal to or higher than the polymerization temperature in the polymerization step. Although not particularly limited, the temperature during bubbling may be 50°C or higher and 100°C or lower. The inert gas to be bubbling is not particularly limited, but examples thereof include nitrogen, argon, etc. The bubbling conditions are appropriately adjusted depending on the type and amount of hydrophobic solvent so as to remove the hydrophobic solvent encapsulated in the precursor particles. Although not particularly limited, for example, the inert gas may be bubbled at a rate of 1 to 3 L / min for 1 to 72 hours. This method produces a slurry of hollow particles encapsulating an inert gas. By drying the hollow particles obtained by solid-liquid separation of this slurry, hollow particles with a gas occupying the hollow portion are obtained. The solid-liquid separation method here is the same as the method described in the solid-liquid separation step above.

[0071] Comparing a method of obtaining hollow particles having hollow spaces filled with gas by performing solid-liquid separation on a slurry-like precursor composition and then removing the hydrophobic solvent in the precursor particles in air, with a method of obtaining hollow particles having hollow spaces filled with gas by removing the hydrophobic solvent encapsulated in precursor particles in a slurry containing the precursor particles and an aqueous medium and then performing solid-liquid separation, 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 the amount of residual hydrophobic solvent is reduced by performing bubbling with an inert gas. Alternatively, as a method for removing the hydrophobic solvent encapsulated in the precursor particles after the polymerization step and before the solid-liquid separation step without subjecting the slurry precursor composition obtained in the polymerization step to solid-liquid separation, for example, a method for evaporating and distilling off the hydrophobic solvent encapsulated in the precursor particles from the precursor composition under a predetermined pressure (high pressure, normal pressure, or reduced pressure); or a method for introducing an inert gas such as nitrogen, argon, or helium, or water vapor, into the precursor composition under a predetermined pressure (high pressure, normal pressure, or reduced pressure) and evaporating and distilling off the hydrophobic solvent may be used.

[0072] (6) Others Steps other than the above steps (1) to (5) may include, for example, the following (6-a) washing step and the following (6-b) particle interior substitution step. (6-a) Washing Step The washing step is a step of adding an acid or alkali to wash the precursor composition containing the precursor particles before the solvent removal step in order to remove any dispersion stabilizer remaining in the precursor composition. When the dispersion stabilizer used is an acid-soluble inorganic dispersion stabilizer, it is preferable to add an acid to the precursor composition containing the precursor particles to perform washing. On the other hand, when the dispersion stabilizer used is an alkali-soluble inorganic dispersion stabilizer, it is preferable to add an alkali to the precursor composition containing the precursor particles to perform washing. Furthermore, when an acid-soluble inorganic dispersion stabilizer is used as the dispersion stabilizer, it is preferable to add an acid to the precursor composition containing the precursor particles to adjust the pH to preferably 6.5 or less, more preferably 6 or less. The acid to be added may be an inorganic acid such as sulfuric acid, hydrochloric acid, or nitric acid, or an organic acid such as formic acid or acetic acid. However, sulfuric acid is particularly preferred because it has a high efficiency in removing the dispersion stabilizer and places a small burden on the production equipment.

[0073] (6-b) Particle Interior Substitution Process The particle interior substitution process is a process in which the gas or liquid inside the hollow particles is replaced with another gas or liquid. This substitution can change the environment inside the hollow particles, selectively confine molecules inside the hollow particles, or modify the chemical structure inside the hollow particles to suit the application.

[0074] II. Hollow Particles The hollow particles of the present disclosure are particles having a shell (outer shell) formed from an organic-inorganic composite material containing a resin and inorganic fine particles, and a hollow portion surrounded by the shell, wherein the resin contains a polymer of the polymerizable monomer. In the present disclosure, the hollow portion is a void space clearly distinguishable from the shell formed from the organic-inorganic composite material. The shell of the hollow particle may have a porous structure, and in such a case, the hollow portion has a size that allows it to be clearly distinguished from the numerous minute spaces uniformly dispersed within the porous structure. The hollow particles of the present disclosure may have one or more hollow portions, but are preferably those having only one hollow portion in order to maintain a good balance between high porosity and mechanical strength and to provide hollow particles with an excellent balance of pressure resistance, dielectric properties, and CTE. 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. The hollow particles of the present disclosure may contain a small amount of hollow particles having two or more hollow portions or particles having no hollow portion as impurities. In the hollow particles of the present disclosure, the proportion of particles having only one hollow portion is preferably 90% or more, more preferably 95% or more. Furthermore, the hollow portions of the hollow particles of the present disclosure may be filled with a gas such as air, may be in a vacuum or reduced pressure state, or may contain a solvent. When using the hollow particles of the present disclosure as a filler, it is preferable that the hollow portions be filled with a gas in order to obtain the effects of the hollow particles, such as a low dielectric constant, a low dielectric loss tangent, and a low thermal expansion.

[0075] In the hollow particles disclosed herein, the content of inorganic fine particles is 20% by mass or more and 90% by mass or less, based on 100% by mass of the solid content of the shell. When the content of inorganic fine particles is equal to or more than the above-mentioned lower limit, hollow particles with a low CTE are obtained. When the content of inorganic fine particles is equal to or less than the above-mentioned upper limit, deterioration of the pressure resistance and dielectric properties of the hollow particles can be suppressed. Therefore, by setting the content of inorganic fine particles within the above range, hollow particles with an excellent balance of pressure resistance, dielectric properties, and CTE can be obtained. The content of inorganic fine particles is preferably 30% by mass or more, more preferably 40% by mass or more, and is preferably 85% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less, and particularly preferably 60% by mass or less. From the same viewpoint, the content of inorganic fine particles, based on the volume of the shell as 100% by volume, is preferably 5% by volume or more, more preferably 10% by volume or more, even more preferably 20% by volume or more, and is preferably 80% by volume or less, more preferably 70% by volume or less, and even more preferably 60% by volume or less.

[0076] In the hollow particles of the present disclosure, the polymer content is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, relative to 100% by mass of the solid content of the shell, from the viewpoint of improving the pressure resistance and dielectric properties of the hollow particles, and is preferably 80% by mass or less, more preferably 70% by mass or less, and even more preferably 60% by mass or less, from the viewpoint of sufficiently incorporating inorganic fine particles to reduce the CTE of the hollow particles.

[0077] The shell of the hollow particle of the present disclosure may further contain other materials different from the inorganic fine particles and the polymer, as long as the effects of the present disclosure are not impaired. When the shell contains other materials, the total mass of the polymer and the inorganic fine particles relative to 100% by mass of the solid content of the shell is preferably 95% by mass or more, more preferably 97% by mass or more, and even more preferably 99% by mass or more.

[0078] The preferred composition of the polymer contained in the hollow particles of the present disclosure corresponds to the preferred composition of the polymerizable monomer described above. The content of the crosslinkable monomer unit per 100 parts by mass of all monomer units of the polymer is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, and even more preferably 90 parts by mass or more, from the viewpoint of improving the pressure resistance of the hollow particles and maintaining the inorganic fine particles within the shell. When the polymer contains a non-crosslinkable monomer unit, the content of the crosslinkable monomer unit may be, for example, 98 parts by mass or less, or 96 parts by mass or less, per 100 parts by mass of all monomer units of the polymer. From the viewpoint of improving the pressure resistance of the hollow particles, the crosslinkable monomer unit preferably contains at least a bifunctional crosslinkable monomer unit. Furthermore, a combination of a bifunctional crosslinkable monomer unit and a trifunctional or higher crosslinkable monomer unit may further improve pressure resistance. In the present disclosure, a crosslinkable monomer unit derived from a bifunctional crosslinkable monomer may be referred to as a "bifunctional crosslinkable monomer unit," and a crosslinkable monomer unit derived from a trifunctional or higher functional crosslinkable monomer may be referred to as a "trifunctional or higher functional crosslinkable monomer unit." When the crosslinkable monomer unit contains a bifunctional crosslinkable monomer unit and a trifunctional or higher functional crosslinkable monomer unit, the content of the trifunctional or higher functional crosslinkable monomer unit per 100 parts by mass of the crosslinkable monomer units is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 20 parts by mass or more, and is preferably 50 parts by mass or less, more preferably 40 parts by mass or less. Furthermore, it is preferable that the polymer contains a hydrocarbon monomer unit, as this tends to improve the dielectric properties of the hollow particles. From the viewpoint of improving the dielectric properties of the hollow particles, the content of hydrocarbon monomer units in 100 parts by mass of all monomer units of the polymer is preferably 30 parts by mass or more, more preferably 50 parts by mass or more, even more preferably 70 parts by mass or more, and still more preferably 90 parts by mass or more.

[0079] The mass content of each material in the shell and the composition of the polymer contained in the shell can be determined, for example, by elemental analysis such as X-ray fluorescence analysis (XRF) or simultaneous thermogravimetry-differential thermal analysis (TG-DTA). For example, the mass content of inorganic fine particles in the shell can be determined from the ash content determined by elemental analysis using XRF and TG-DTA. The volume content of each material contained in the shell can be calculated by dividing the mass content of each material by the specific gravity of each material. Furthermore, by using the above-mentioned preferred inorganic fine particles in the above-mentioned preferred amounts and performing a polymerization reaction of polymerizable monomers until the polymerization conversion rate reaches 100%, the mass content of each material in the shell and the composition of the polymer contained in the shell can be determined from the amount of raw materials charged during production, and the volume content of each material in the shell can be determined from the amount of raw materials charged during production and the specific gravity of each raw material.

[0080] In the hollow particles of the present disclosure, it is preferable that at least a portion of each inorganic particle is embedded in the resin in the shell. This improves the smoothness of the shell surface, thereby suppressing a decrease in the pressure resistance of the hollow particles and a decrease in adhesion between the hollow particles and the resin when the hollow particles are added to the resin, which are caused by unevenness on the shell surface. From the same perspective, it is more preferable that the inorganic particles are entirely embedded in the resin. Whether or not each inorganic particle is embedded in the resin in the shell of a hollow particle can be confirmed, for example, by confirming the inclusion of inorganic particles in the shell using XRF or TG-DTA, and then by observing the cross-section of the hollow particle using an SEM. For hollow particles containing inorganic particles in the shell, if the outer surface and cross-section are observed using an SEM and there are no inorganic particles merely adhering to the shell surface, it can be determined that at least a portion of each inorganic particle is embedded in the resin. Furthermore, if the smoothness of the shell surface is equivalent to that of hollow polymer particles that do not contain inorganic particles and no inorganic particles are observed on the inner or outer surfaces of the shell, it can be determined that the inorganic particles are entirely embedded in the resin. The hollow particles of the present disclosure may contain inorganic fine particles that are partially exposed from the shell surface and partially buried. To ensure that at least a portion of each inorganic fine particle is buried in the resin in the shell, it is effective to, for example, select inorganic fine particles of the preferred size described above, set the content of inorganic fine particles within the preferred range described above, and use inorganic fine particles that have been surface-treated with a surface treatment agent containing radical-reactive functional groups or other preferred surface treatment agents. It is presumed that the use of inorganic fine particles surface-treated with a surface treatment agent containing radical-reactive functional groups facilitates the polymerization reaction of polymerizable monomers to proceed with the inorganic fine particles as nuclei during the hollow particle production process, facilitating the formation of polymers that cover the inorganic fine particles. Whether the inorganic fine particles are surface-treated with a surface treatment agent containing radical-reactive functional groups can be confirmed by confirming the functional groups using infrared spectroscopy or by the degree of methanol hydrophobicity, etc.The radical-reactive functional groups of the surface-treated inorganic particles are expected to react with the polymerizable functional groups of the polymerizable monomer in the polymerization step, but may remain unreacted. Furthermore, the inorganic particles may be unevenly distributed near the outer surface of the shell. The location of the inorganic particles can be confirmed by elemental analysis in the depth direction using glow discharge optical emission spectroscopy or the like.

[0081] The hollow particles of the present disclosure have a porosity of 50% or more, preferably 60% or more, and more preferably 65% ​​or more. When the porosity is equal to or greater than the above-mentioned lower limit, the hollow particles have excellent dielectric properties, and are also excellent in terms of lightweight, heat resistance, and heat insulation. The upper limit of the porosity of the hollow particles is not particularly limited, but is preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less, in order to prevent a decrease in the pressure resistance of the hollow particles.

[0082] The porosity of the hollow particles is the apparent density D １ and true density D ０ The apparent density of the hollow particles D １ The measurement method is as follows: First, a volume of 100 cm ３ About 30 cm ３ The volumetric flask is filled with hollow particles, and the mass of the filled hollow particles is accurately weighed. Next, the volumetric flask filled with the hollow particles is accurately filled with isopropanol up to the marked line, taking care not to introduce air bubbles. The mass of isopropanol added to the volumetric flask is accurately weighed, and the apparent density D of the hollow particles is calculated based on the following formula (I): １ (g / cm ３ ) is calculated using the formula (I): Apparent density D １ Apparent density D = [Mass of hollow particles] / (100 - [Mass of isopropanol] / [Specific gravity of isopropanol at measurement temperature]) １ corresponds to the specific gravity of the entire hollow particle when the hollow portion is considered to be a part of the hollow particle.

[0083] True density D of hollow particles ０ The measurement method is as follows: After crushing the hollow particles in advance, ３Approximately 10 g of crushed pieces of hollow particles are filled into a measuring flask, and the mass of the crushed pieces is accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol is added to the measuring flask, and the mass of the isopropanol is accurately weighed. The true density D of the hollow particles is calculated based on the following formula (II): ０ (g / cm ３ ) is calculated using the formula (II): True density D ０ = [mass of crushed pieces of hollow particles] / (100 - [mass of isopropanol] / [specific gravity of isopropanol at measurement temperature]) True density D ０ As is clear from the above measurement method, the true density D ０ In calculating the particle diameter, the hollow portion is not considered to be part of the hollow particle.

[0084] The porosity (%) of the hollow particles is calculated by multiplying the apparent density D １ and true density D ０ The porosity (%) is calculated by the following formula (III): １ / True density D ０ ) x 100

[0085] The volume average particle diameter of the hollow particles disclosed herein is 1.0 μm or more and 50.0 μm or less. The lower limit of the volume average particle diameter of the hollow particles is preferably 1.5 μm or more, more preferably 2.0 μm or more, and the upper limit is preferably 40.0 μm or less, more preferably 30.0 μm or less, and even more preferably 20.0 μm or less. When the volume average particle diameter of the hollow particles is equal to or greater than the above lower limit, it is easy to achieve both high porosity and excellent pressure resistance, and the tendency for hollow particles to aggregate together is reduced, thereby enabling excellent dispersibility to be exhibited. When the volume average particle diameter of the hollow particles is equal to or less than the above upper limit, the CTE of the hollow particles is easily reduced, and the uniformity of the shell is easily improved, making it easy to obtain hollow particles with excellent pressure resistance. In the present disclosure, the volume average particle diameter (Dv) of the hollow particles can be determined by measuring the particle diameters of 100,000 hollow particles using a particle size distribution analyzer using the Coulter counter method and calculating the volume average. The particle size of the hollow particles of the present disclosure can be adjusted, for example, by adjusting the content of the dispersion stabilizer relative to the total mass of the polymerizable monomer, inorganic fine particles, and hydrophobic solvent.

[0086] The shell thickness of the hollow particles of the present disclosure is not particularly limited, but is preferably 0.10 μm or more, more preferably 0.12 μm or more, and even more preferably 0.15 μm or more from the viewpoint of improving pressure resistance, and is preferably 3.00 μm or less, more preferably 2.00 μm or less, and even more preferably 1.60 μm or less from the viewpoint of increasing porosity. In the present disclosure, the shell thickness of the hollow particles is a value calculated by calculating the inner diameter r of the hollow particles according to the following formula (1) using the volume average particle diameter R and porosity of the hollow particles, and then using the inner diameter r and volume average particle diameter R according to the following formula (2). Formula (1): 4 / 3π×(R / 2) ３ ×(porosity / 100)=4 / 3π×(r / 2) ３ Shell thickness=(R−r) / 2 Formula (2): Note that the porosity in the above formula (1) is a numerical value expressed as a percentage.

[0087] The shape of the hollow particles of the present disclosure is not particularly limited as long as a hollow portion is formed inside, and examples thereof include spherical, oval, and irregular shapes. Among these, a spherical shape is preferred from the viewpoints of ease of production and pressure resistance. The particle shape can be confirmed, for example, by SEM or TEM. The hollow particles of the present disclosure may have an average circularity of 0.950 to 0.995. One example of the shape of the hollow particles of the present disclosure is a bag made of a thin film and inflated with gas, and its cross-sectional view is shown as hollow particle 10 in (5) of Figure 2. In this example, a single thin film is provided on the outside, and the interior is filled with gas.

[0088] Furthermore, from the viewpoint of dielectric properties and pressure resistance, the hollow particles of the present disclosure preferably contain a small proportion of particles with a circularity of 0.85 or less. Particles with a circularity of 0.85 or less typically exhibit deformations such as dents or cracks, and are sometimes referred to as "irregularly shaped particles" in the present disclosure. These irregularly shaped hollow particles have inferior dielectric properties due to their lower porosity compared to spherical hollow particles. Therefore, reducing the proportion of irregularly shaped particles contained in the hollow particles can improve the dielectric properties of the hollow particles. Furthermore, irregularly shaped particles are more likely to aggregate when dispersed in a matrix resin than spherical particles, resulting in poor dispersibility. When irregularly shaped particles are dispersed in a matrix resin, aggregates are more likely to form, and the aggregates are more susceptible to external pressure, further reducing pressure resistance. Therefore, reducing the proportion of irregularly shaped particles contained in the hollow particles can improve the dispersibility and pressure resistance of the hollow particles. The hollow particles of the present disclosure may contain a small amount of impurities, such as particles with low circularity due to cracking or deformation. However, the proportion of particles with a circularity of 0.85 or less, based on 100% by mass of the hollow particles of the present disclosure, is preferably 10% by mass or less, more preferably 7% by mass or less, even more preferably 5% by mass or less, even more preferably 4% by mass or less, and particularly preferably 3% by mass or less. Circularity is defined as the value obtained by dividing the diameter of a circle having the same area as the projected image of the particle (equivalent circle area diameter) by the diameter of a circle having the same perimeter as the projected image of the particle (equivalent circumferential diameter). A perfectly spherical particle has a circularity of 1, and the more complex the particle's surface shape, the smaller the circularity value. In the present disclosure, circularity is measured using a flow-type particle image analyzer with an image resolution of 0.185 μm / pixel. A preferred example of a flow-type particle image analyzer is the "IF-3200" manufactured by Jasco International Co., Ltd. A measurement sample is prepared by dispersing a mixture of 0.10 to 0.12 g of hollow particles in an aqueous solution of linear alkylbenzenesulfonate (concentration: 0.3%) in an ultrasonic cleaner for 5 minutes. The average circularity is the average value of the circularity of 1,000 to 3,000 randomly selected particles.

[0089] From the viewpoint of achieving a low dielectric constant, the hollow particles of the present disclosure preferably have a relative dielectric constant of 2.00 or less, more preferably 1.80 or less, and even more preferably 1.60 or less at a frequency of 1 GHz. The lower limit is not particularly limited, and may be, for example, 1.00 or more. From the viewpoint of achieving a low dielectric loss tangent, the hollow particles of the present disclosure preferably have a dielectric loss tangent of 3.00×10 at a frequency of 1 GHz. －３ More preferably, it is 2.00 × 10 or less. －３ More preferably, 1.50×10 －３ The lower limit is not particularly limited, and is, for example, 1.00 × 10 －４ In the present disclosure, the relative permittivity and dielectric loss tangent of the particles are measured at room temperature and at a measurement frequency of 1 GHz using a perturbation type measuring device.

[0090] From the viewpoint of low thermal expansion, the hollow particles of the present disclosure have a thermal expansion coefficient at 23 to 100°C of preferably 50 ppm / °C or less, more preferably 45 ppm / °C or less, and even more preferably 40 ppm / °C or less. The lower limit is not particularly limited, but may be, for example, 10 ppm / °C or more, or 20 ppm / °C or more. In the present disclosure, the thermal expansion coefficient α of the particles ｐ is the thermal expansion coefficient α of the molded plate made of matrix resin and particles ｃ , the thermal expansion coefficient of the matrix resin alone α ｒ、 The volume ratio SG of the matrix resin in the molded plate ｒ , and the volume ratio W of particles in the molding plate ｐ From the above, it can be calculated by the following formula (A): Formula (A): α ｐ = (α ｃ -SG ｒ ×α ｒ ) / W ｐ The thermal expansion coefficient is measured within a predetermined temperature range in accordance with JIS K 7197. As the matrix resin, for example, an epoxy resin is used, and it may contain additives for curing the resin, such as a curing agent and a curing catalyst.

[0091] From the viewpoint of pressure resistance, the hollow particles according to the present disclosure have a compressive strength measured in accordance with JIS R1639-5 of preferably 2.0 MPa or more, more preferably 2.5 MPa or more, even more preferably 3.0 MPa or more, and even more preferably 3.5 MPa or more. The higher the compressive strength, the more excellent the pressure resistance of the hollow particles. The upper limit of the compressive strength of the hollow particles is not particularly limited, but may be, for example, 15.0 MPa or less, 12.0 MPa or less, or 10.0 MPa or less.

[0092] The hollow particles of the present disclosure have a thermal decomposition onset temperature of preferably 150 to 400°C, more preferably 200 to 350°C. Hollow particles with a thermal decomposition onset temperature within the above range have excellent heat resistance. In the present disclosure, the thermal decomposition onset temperature of the hollow particles is the temperature at which a weight loss of 5% occurs, and can be measured using a TG-DTA device in an air atmosphere under conditions of an air flow rate of 230 mL / min and a temperature rise rate of 10°C / min.

[0093] III. Uses of Hollow Particles As described above, the hollow particles of the present disclosure are suitable for use as fillers in electronic materials. For example, adding hollow particles of the present disclosure as a filler to a resin composition used in electronic materials can reduce the dielectric constant, dielectric dissipation factor, and thermal expansion coefficient of the resin composition, making it more suitable for electronic material applications. Resin compositions containing hollow particles of the present disclosure are suitable for use in semiconductor materials such as electronic circuit boards, prepregs, interlayer insulating materials, dry film resists, solder resists, bonding wires, magnet wires, semiconductor encapsulants, epoxy encapsulants, mold underfills, underfills, die bond pastes, buffer coating materials, copper-clad laminates, flexible substrates, high-frequency device modules, antenna modules, and automotive radars. Among these, the hollow particles of the present disclosure are suitable for use in interlayer insulating materials, solder resists, magnet wires, epoxy encapsulants, underfills, buffer coating materials, copper-clad laminates, flexible substrates, high-frequency device modules, antenna modules, and automotive radars. The hollow particles of the present disclosure are also useful as additives to insulating resin sheets used in the production of electronic components such as printed wiring boards. An insulating resin sheet containing the hollow particles of the present disclosure can be produced by, for example, preparing a resin composition by mixing a thermoplastic resin, a thermosetting resin, a thermoplastic elastomer, or a mixture thereof with the hollow particles of the present disclosure, and then applying the resin composition to one or both sides of a sheet-like substrate and drying, extrusion molding, transfer molding, or other methods to form a sheet. When the resin or elastomer contained in the insulating resin sheet has adhesive properties, the insulating resin sheet can be used as an adhesive sheet, specifically, for example, as a bonding sheet. A bonding sheet is an insulating adhesive layer-forming material used to bond a conductor layer and an organic insulating layer when producing a multilayer printed wiring board.In addition to electronic material applications, the hollow particles of the present disclosure can also be used as fillers in components such as light reflectors, heat insulators, sound insulators, and low dielectric materials used in various fields such as automobiles, construction, aviation, and spacecraft, as well as food containers, footwear such as sports shoes and sandals, home appliance parts, bicycle parts, stationery, tools, and 3D printer filaments. The hollow particles of the present disclosure can also be incorporated as fillers in molded articles formed using resin or rubber, or molded articles formed using a material containing resin or rubber and further reinforcing fibers. Examples of reinforcing fibers include organic or inorganic fibers such as carbon fiber, glass fiber, aramid fiber, polyethylene fiber, and cellulose nanofiber.

[0094] As described above, the hollow particles of the present disclosure can be used by mixing them with a resin or rubber. The resin is not particularly limited, but may be, for example, a thermoplastic resin, a thermosetting resin, a room-temperature curing resin, a thermoplastic elastomer, or a mixture thereof. Examples of thermosetting resins include phenolic resins, melamine resins, urea resins, unsaturated polyester resins, epoxy resins, polyurethane resins, silicone resins, alkyd resins, thermosetting modified polyphenylene ether resins, thermosetting polyimide resins, benzoxazine resins, allyl resins, aniline resins, maleimide resins, bismaleimide triazine resins, liquid crystalline polyester resins, vinyl ester resins, cyanate ester resins, and polyetherimide resins. Examples of room-temperature curing resins include adhesives that can be cured at room temperature by adding a catalyst, such as epoxy-based adhesives, silicone-based adhesives, and acrylic-based adhesives. Thermosetting resins and room temperature curing resins may be used in combination with curing agents or catalysts such as amines, acid anhydrides, imidazoles, thiols, phenols, naphthols, benzoxazines, cyanate esters, and carbodiimides, depending on the type of resin. Examples of thermoplastic resins include polyolefin resins such as polypropylene and polyethylene, polyamide resins such as PA6, PA66, and PA12, polyimide resins, polyamide-imide resins, polyetherimide resins, polyether ketone ketone resins, polyvinyl chloride resins, polystyrene resins, poly(meth)acrylate resins, polycarbonate resins, polyphenylene sulfide resins, polystyrene resins, polyphenylene oxide resins, liquid crystal polymers (LCPs), polyvinylidene fluoride resins, acrylonitrile-butadiene-styrene copolymer (ABS) resins, acrylonitrile-styrene copolymer (AS) resins, polyphenylene ether resins, polyester resins, and polytetrafluoroethylene resins. As the thermoplastic elastomer, a thermoplastic elastic polymer that has conventionally been used as a molding resin can be used, and examples thereof include urethane-based elastomers, styrene-based elastomers, olefin-based elastomers, amide-based elastomers, and ester-based elastomers.Thermoplastic elastomers generally exhibit rubber elasticity at room temperature (25°C) and have the property of being plasticized and moldable at high temperatures. The rubber is not particularly limited, but examples thereof include natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene copolymer rubber (SBR), acrylonitrile-butadiene copolymer rubber (NBR), and ethylene-propylene-diene terpolymer (EPDM).

[0095] Furthermore, the hollow particles of the present disclosure have high porosity, are resistant to crushing, and have excellent heat resistance, thereby meeting the insulation and shock-absorbing properties (cushioning) required for undercoating materials and also meeting the heat resistance required for thermal paper applications. The hollow particles of the present disclosure are also useful as plastic pigments with excellent gloss and hiding power. Furthermore, the hollow particles of the present disclosure can be encapsulated with useful ingredients such as fragrances, pharmaceuticals, pesticides, and ink components by means of immersion treatment, reduced pressure immersion treatment, or pressure immersion treatment, allowing them to be used for a variety of applications depending on the ingredients contained therein. Furthermore, the hollow particles of the present disclosure are also suitable for use as rust inhibitors. The hollow particles of the present disclosure are also useful as additives that reduce electrical conductivity. For example, paints containing the hollow particles of the present disclosure can be used as rust-preventive paints (paint primers, lubricating paints, etc.) to improve the corrosion and rust resistance of steel materials and the like. Rust-preventive additives can also be encapsulated in the hollow particles added to rust-preventive paints.

[0096] The present disclosure will be described in more detail below with reference to examples and comparative examples, but the present disclosure is not limited to these examples. Note that parts and percentages are by mass unless otherwise specified.

[0097] [Production Example 1] 10 parts of vinylmethoxysilane was diluted with 10 parts of ethanol as an organic solvent to prepare a vinylsilane solution. Next, 100 parts of silica powder (manufactured by Nippon Aerosil Co., Ltd., product name: AEROSIL (registered trademark) OX 130) having a primary average particle size of 18 nm was placed in a reaction vessel, and while stirring the silica powder with a rotating blade under a nitrogen atmosphere, 1 part of water and the vinylsilane solution prepared above were added to the reaction vessel. This was mixed under stirring at a temperature of 200 ° C. for 120 minutes under a nitrogen atmosphere, and then cooled with cooling water. Then, the mixture was heated to 80 ° C. under reduced pressure to completely remove the solvent, thereby obtaining silica microparticles A. The obtained silica microparticles A had a relative dielectric constant of 2.98 at 23 ° C. and 1 GHz, and a dielectric loss tangent of 6.2 × 10 －３ The CTE at 23 to 100°C was 0.5 ppm / °C, and the volume average primary particle size was 18 nm.

[0098] [Production Example 2] Silica fine particles B were obtained in the same manner as in Production Example 1, except that silica powder having an average primary particle diameter of 500 nm (manufactured by Admatechs Co., Ltd., product name: SO-C2) was used as the silica powder. The obtained silica fine particles B had a relative dielectric constant of 2.94 and a dielectric loss tangent of 6.9 × 10 at 23°C and 1 GHz. －４ The CTE at 23 to 100° C. was 0.5 ppm / ° C., and the volume average primary particle size was 500 nm.

[0099] [Production Example 3] Silica fine particles C were obtained in the same manner as in Production Example 1, except that a trimethylmethoxysilane solution prepared by diluting 10 parts of trimethylmethoxysilane with 10 parts of ethanol as an organic solvent was used instead of the vinylsilane solution in Production Example 1. The obtained silica fine particles C had a relative dielectric constant of 2.98 and a dielectric loss tangent of 6.2 × 10 at 23 ° C and 1 GHz. －３ The CTE at 23 to 100° C. was 0.5 ppm / ° C., and the volume average primary particle size was 18 nm.

[0100] [Production Example 4] Silica fine particles D were obtained in the same manner as in Production Example 1, except that a silica powder having an average primary particle diameter of 50 nm (manufactured by Nippon Aerosil Co., Ltd., product name: AEROSIL (registered trademark) OX 50) was used as the silica powder. The obtained silica fine particles D had a relative dielectric constant of 2.98 and a dielectric loss tangent of 6.2 × 10 at 23 ° C. and 1 GHz. －３ The CTE at 23 to 100° C. was 0.5 ppm / ° C., and the volume average primary particle size was 50 nm.

[0101] Y100SV (product name, manufactured by Admatechs Co., Ltd.), which is silica fine particles surface-treated with vinylmethoxysilane, was prepared as silica fine particles E. Silica fine particles E had a relative dielectric constant of 2.98 and a dielectric loss tangent of 6.2 × 10 at 23 °C and 1 GHz. －３ The CTE at 23 to 100° C. was 0.5 ppm / ° C., and the volume average primary particle size was 100 nm.

[0102] The relative permittivity and dielectric loss tangent of the silica fine particles A to E at 23°C and 1 GHz were measured by the same method as that for measuring the relative permittivity and dielectric loss tangent of hollow particles at 23°C and 1 GHz, which will be described later. The CTE of the silica fine particles A to E at 23 to 100°C was measured by the same method as that for measuring the coefficient of thermal expansion (CTE) of hollow particles at 23 to 100°C, which will be described later. The volume average primary particle diameter of the silica fine particles A to E was determined by measuring the primary particle diameter of 100 particles by SEM image analysis and calculating the volume average.

[0103] Example 1 (1) Mixed Liquid Preparation Step First, the following materials were mixed to form an oil phase: Divinylbenzene 32.06 parts Ethylvinylbenzene 1.34 parts Silica fine particles A 8.1 parts t-Butylperoxydiethyl acetate (oil-soluble polymerization initiator) 0.89 parts Hydrophobic solvent: Heptane 58.5 parts Meanwhile, in a stirring tank, at room temperature, an aqueous solution prepared by dissolving 19.59 parts of magnesium chloride (a water-soluble polyvalent metal salt) in 225 parts of ion-exchanged water was gradually added with stirring to prepare a magnesium hydroxide colloid (poorly water-soluble metal hydroxide colloid) dispersion (magnesium hydroxide 10 parts), which served as the aqueous phase. The resulting aqueous phase and oil phase were mixed to prepare a mixed liquid.

[0104] (2) Suspension step The mixture obtained in the mixture preparation step was stirred and suspended for 1 minute using a disperser (manufactured by Primix Corporation, product name: Homomixer) at a rotation speed of 4,000 rpm, to prepare a suspension in which droplets of the monomer composition encapsulating the hydrophobic solvent were dispersed in water.

[0105] (3) Polymerization step: The suspension obtained in the suspension step was stirred for 1 hour and 30 minutes under a nitrogen atmosphere at a temperature of 65°C to carry out a polymerization reaction until the polymerization conversion rate reached 100%, thereby obtaining a precursor composition in which precursor particles encapsulating a hydrophobic solvent were dispersed in water.

[0106] (4) Washing step and solid-liquid separation step The precursor composition was washed with dilute sulfuric acid (23°C, 10 minutes) to adjust the pH to 5.5 or less. Next, after separating the water by filtration, 200 parts of ion-exchanged water was added to re-slurry the mixture. The water washing treatment (washing, filtration, dehydration) was repeated several times at room temperature (23°C), and the mixture was filtered to obtain a solid. The obtained solid was dried in a dryer at 40°C to obtain precursor particles encapsulating the hydrophobic solvent.

[0107] (5) Solvent Removal Step The precursor particles obtained in the solid-liquid separation step were heat-treated in a vacuum dryer at 200°C in a nitrogen atmosphere for 12 hours to remove the hydrophobic solvent contained in the particles, thereby obtaining the hollow particles of Example 1.

[0108] [Examples 2 to 4] Hollow particles of Examples 2 to 4 were obtained in the same procedure as in Example 1, except that in the above "(1) mixed solution preparation step", the amounts of the polymerizable monomer, inorganic fine particles, and hydrophobic solvent were changed according to Table 1.

[0109] Examples 5 and 6 In Example 1, in the above "(1) mixed solution preparation step", the amount of magnesium chloride was changed from 19.59 parts to 7.83 parts, the amount of sodium hydroxide was changed from 13.72 parts to 5.49 parts, and the amount of magnesium hydroxide colloidal dispersion was changed to an amount equivalent to 4 parts in terms of magnesium hydroxide; in Example 5, the type of inorganic fine particles was further changed according to Table 1; and in Example 6, the type and amount of polymerizable monomer, the amount of inorganic fine particles, and the type and amount of hydrophobic solvent were further changed according to Table 1. Except for these, hollow particles of Examples 5 and 6 were obtained by the same procedure as in Example 1.

[0110] Comparative Example 1 Hollow particles (hollow polymer particles) of Comparative Example 1 were obtained in the same manner as in Example 1, except that in the above "(1) mixed solution preparation step", silica fine particles A were not added and the amounts of polymerizable monomer and hydrophobic solvent added were changed according to Table 1.

[0111] Comparative Example 2 In Example 1, in the above "(1) mixed solution preparation step", silica fine particles A was not added, the amount of magnesium chloride was changed from 19.59 parts to 11.75 parts, the amount of sodium hydroxide was changed from 13.72 parts to 8.23 ​​parts, the amount of magnesium hydroxide colloidal dispersion was changed to an amount equivalent to 6 parts in terms of magnesium hydroxide, and the type and amount of polymerizable monomer, and the type and amount of hydrophobic solvent were changed according to Table 1. Except for this, hollow particles (hollow polymer particles) of Comparative Example 2 were obtained in the same procedure as in Example 1.

[0112] Comparative Example 3 Hollow particles (hollow silica particles) of Comparative Example 3 were obtained by the same procedure as in Example 3 of Patent Document 2. That is, 42 parts of n-dodecane was added to an aqueous solution prepared by dissolving 7 parts of EO-PO-EO block copolymer (manufactured by ADEKA Corporation) in 1250 parts of pure water, and the mixture was stirred using a high-pressure emulsifier to prepare a fine emulsion. An aqueous sodium silicate solution (SiO ２ Concentration 10.4% by mass, Na ２ 41 parts of a dispersion having a 3.6% by mass O concentration and hydrochloric acid were added and stirred, and then an aqueous solution of sodium hydroxide was added dropwise to adjust the pH to 6. The resulting dispersion was heated to 70°C, and an aqueous solution of sodium hydroxide was added to adjust the pH to 9, and then an aqueous solution of sodium silicate (SiO ２ Concentration 10.4% by mass, Na ２ 460 parts of a sintered silica precursor (O concentration 3.6 mass%) was added together with 0.5 M hydrochloric acid to adjust the pH to 9, yielding a hollow silica precursor dispersion. The obtained hollow silica precursor dispersion was filtered and washed, then dried at 60°C for 1 hour under a nitrogen atmosphere, subsequently dried at 400°C for 4 hours, and further calcined at 800°C for 4 hours to obtain hollow calcined silica particles. 10 parts of the obtained hollow calcined silica particles, 120 ml of methyl ethyl ketone, and 0.8 parts of hexamethyldisilazane were mixed and stirred at room temperature for 3 hours. The mixture was then filtered under reduced pressure, washed with methyl ethyl ketone, and vacuum dried at 150°C for 2 hours in a vacuum dryer. The obtained solid was pulverized in an agate mortar to obtain surface-treated hollow silica particles of Comparative Example 3.

[0113] [Examples 7 to 9] Hollow particles of Examples 7 to 9 were obtained in the same procedure as in Example 1, except that in the above "(1) mixed solution preparation step", the type of inorganic fine particles was changed according to Table 2.

[0114] [Example 10] The hollow particles of Example 10 were obtained in the same manner as in Example 1, except that in the above "(1) mixed solution preparation step", the amount of magnesium chloride was changed from 19.59 parts to 7.83 parts, the amount of sodium hydroxide was changed from 13.72 parts to 5.49 parts, the amount of magnesium hydroxide colloidal dispersion was changed to an amount equivalent to 4 parts in terms of magnesium hydroxide, and the type of inorganic fine particles was changed according to Table 2.

[0115] Example 11 Hollow particles of Example 11 were obtained in the same manner as in Example 1, except that the type of inorganic fine particles was changed according to Table 2 in the "(1) mixed solution preparation step" in Example 1.

[0116] [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.

[0117] 1. Volume-average particle diameter (Dv) of hollow particles The volume-average particle diameter of hollow particles was measured using a particle size distribution analyzer (manufactured by Beckman Coulter, Inc., product name: Multisizer 4e) using the Coulter counter method. 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 particle sample was placed in a beaker, and a surfactant aqueous solution (manufactured by Fujifilm Corporation, product name: Drywell) was added as a dispersant. 2 ml of dispersion medium was added to wet the particles, and then 10 ml of dispersion medium was added. The mixture was dispersed in an ultrasonic disperser for 1 minute, and then measured using the particle size distribution analyzer.

[0118] 2. Density and porosity of hollow particles 2-1. Measurement of apparent density of hollow particles First, a 100 cm ３ About 30 cm ３ The hollow particles were packed into the measuring flask, and the mass of the packed hollow particles was accurately weighed. Next, the measuring flask packed with the hollow particles was accurately filled with isopropanol up to the marked line, taking care not to introduce air bubbles. The mass of the isopropanol added to the measuring flask was accurately weighed, and the apparent density D of the hollow particles was calculated based on the above formula (I). １ (g / cm ３ ) was calculated.

[0119] 2-2. Measurement of true density of hollow particles After crushing the hollow particles in advance, ３ Approximately 10 g of crushed pieces of hollow particles was filled into a measuring flask, and the mass of the crushed pieces was accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol was added to the measuring flask, and the mass of the isopropanol was accurately weighed. Based on the above formula (II), the true density D of the hollow particles was calculated. ０ (g / cm ３ ) was calculated.

[0120] 2-3. Calculation of porosity Apparent density D of hollow particles １ and true density D ０ The porosity of the hollow particles was calculated based on the above formula (III).

[0121] 3. Shell Thickness of Hollow Particles The inner diameter r of the hollow particles was calculated using the volume average particle diameter R and porosity of the hollow particles according to the following formula (1), and the shell thickness of the hollow particles was calculated using the inner diameter r and the volume average particle diameter R according to the following formula (2): Formula (1): 4 / 3π×(R / 2) ３ ×(porosity / 100)=4 / 3π×(r / 2) ３ Equation (2): Shell thickness = (R - r) / 2

[0122] 4. Coefficient of thermal expansion (CTE) 4-1. Preparation of hollow particle-containing epoxy resin plate A varnish was prepared by mixing 86 parts of epoxy resin (manufactured by Mitsubishi Chemical Corporation, product name: JER828), 10 parts of hollow particles, and 4 parts of 2-ethyl-4-methylimidazole as a curing agent using a planetary mixer. The resulting varnish was applied to a thickness of 3 mm and heated in an oven under a nitrogen atmosphere at 100°C for 30 minutes, then heated to 160°C at a heating rate of 2°C / min, and further heated at 160°C for 180 minutes to cure the coating, producing a hollow particle-containing epoxy resin plate.

[0123] 4-2. Measurement of the coefficient of thermal expansion (CTE) of hollow particles The coefficient of thermal expansion (CTE) of the hollow particle-containing epoxy resin plate prepared above was measured in accordance with JIS K 7197 using a TMA device (manufactured by Rigaku Corporation, model: TMA-8311) under the following measurement conditions. (CTE measurement conditions) Temperature range: 23 to 300°C Heating rate: 5°C / min Atmosphere: N ２ (100 mL / min) Mode: Compression (load 49 mN) Number of measurements: n=1 for each From the measurement results, the thermal expansion coefficient of the hollow particle-containing epoxy resin plate at 23 to 100°C was calculated. Thermal expansion coefficient α of hollow particle-containing epoxy resin plate at 23 to 100°C ｃ and the thermal expansion coefficient α of the epoxy resin alone ｒ and the volume ratio SG of the epoxy resin in the hollow particle-containing epoxy resin plate ｒ and the volume ratio W of hollow particles ｐ From this, the thermal expansion coefficient α of the hollow particles at 23 to 100°C is ｐ was calculated using the following formula (A): ｐ = (α ｃ -SG ｒ ×α ｒ ) / W ｐ

[0124] 5. Measurement of Relative Dielectric Constant (Dk) and Dielectric Loss Tangent (Df) The relative dielectric constant and dielectric loss tangent of the hollow particles were measured at a frequency of 1 GHz and room temperature (23°C) using a perturbation type measuring device (manufactured by AET Co., Ltd., model: ADMS01Nc).

[0125] 6. Microcompression Strength The microcompression strength of the hollow particles was measured in accordance with JIS R1639-5 using a microcompression testing machine (model: MCT-510) manufactured by Shimadzu Corporation, under the conditions of a measurement temperature of 23°C, a maximum test force of 2.000 mN, and a loading rate of 0.0466 mN / sec, where the compressive strength was measured when the particle diameter in the load direction changed by 10%.

[0126]

[0127]

[0128] The volumetric content (volume %) of inorganic fine particles shown in Tables 1 and 2 was calculated from the mass content (mass %) of each material in the shell and the specific gravity of each material. Furthermore, SEM observation of the particle surfaces and cross sections was performed on the hollow particles obtained in each Example and the hollow polymer particles obtained in Comparative Examples 1 and 2. The SEM observation results and porosity values ​​confirmed that the hollow particles obtained in each Example were spherical and had only one hollow portion. The hollow particles obtained in each Example contained trace amounts of hollow particles with two or more hollow portions or particles with no hollow portion as impurities, but the number ratio of particles having only one hollow portion was 90% or more in each Example. Furthermore, 3,000 randomly selected particles were examined for the hollow particles obtained in each Example. In all Examples, the proportion of particles with a circularity of 0.85 or less was 10% or less by mass. Furthermore, the hollow particles of each Example and the hollow polymer particles of Comparative Examples 1 and 2, which do not contain silica microparticles, all had similar shell surface smoothness and shell appearances. In the hollow particles of each Example, no silica-derived microparticles were observed on either the outer shell surface or the inside of the shell when the particles were broken and observed. Furthermore, elemental analysis by XRF and ash content determined by TG-DTA confirmed that the hollow particles obtained in each Example contained silica microparticles in the shell. Based on these results, it can be determined that the entire silica microparticles in the shell of the hollow particles of each Example are embedded in the resin.

[0129] [Discussion] The hollow polymer particles obtained in Comparative Examples 1 and 2 had a high CTE, while the hollow silica particles obtained in Comparative Example 3 had a low microcompressive strength, i.e., insufficient pressure resistance, and a high dielectric constant and dielectric dissipation factor. In contrast, the hollow particles obtained in each Example had a shell made of an organic-inorganic composite material containing inorganic fine particles in a proportion of 20% by mass to 90% by mass. Therefore, the microcompressive strength was not too low, i.e., the pressure resistance was good, and the CTE, dielectric constant, and dielectric dissipation factor were low, providing an excellent balance of pressure resistance, dielectric properties, and CTE as a filler for electronic materials. Furthermore, Examples 1 to 3 demonstrated that, in the hollow particles of the present disclosure, the CTE of the hollow particles decreased as the content of inorganic fine particles increased. It has also been shown that the higher the content of inorganic fine particles, the worse the pressure resistance and dielectric properties tend to be. However, the hollow particles of Examples 1 to 3, which contain inorganic fine particles in an amount of 20 to 81% by mass, maintain good pressure resistance and dielectric properties. Furthermore, based on the results of Examples 1 to 3, it can be inferred that even when the content of inorganic fine particles is 90% by mass, they still have superior pressure resistance and dielectric properties compared to hollow silica particles. A comparison of Examples 1 to 3 with Example 4 indicates that the higher the porosity of the hollow particles, the better the dielectric properties. A comparison of Example 1 with Example 5 indicates that the size of the inorganic fine particles has almost no effect on the CTE and dielectric properties of the hollow particles. The superior pressure resistance of the hollow particles of Example 5 is presumably due to the thicker shell. A comparison of Example 1 with Examples 8 to 11 also indicates that the size of the inorganic fine particles has almost no effect on the CTE and dielectric properties of the hollow particles. The hollow particles of Examples 8 and 9 were inferior in pressure resistance compared to Example 1, presumably because the hollow particles of Examples 8 and 9 had a relatively large ratio of the particle size of the inorganic fine particles to the shell thickness. The hollow particles of Example 10 were particularly excellent in pressure resistance, presumably because the particle size and shell thickness were large and the ratio of the particle size of the inorganic fine particles to the shell thickness was small. A comparison of Examples 1 to 5 with Example 6 showed that the dielectric properties of the hollow particles tend to be improved when the content of hydrocarbon monomer units in the polymer is high.The relatively high CTE of the hollow particles in Example 6 is presumably due to the large particle size of the droplets formed in the suspension process, which resulted in large polymer precipitates during shell formation in the polymerization process, making crosslinking between the precipitates difficult. A comparison of Examples 1 and 7 showed that the pressure resistance of hollow particles tends to improve when the surface treatment agent for inorganic fine particles contains radical-reactive functional groups. This is presumably because the inorganic fine particles have radical-reactive functional groups on their surfaces, which allow the inorganic fine particles and the polymer to be covalently bonded within the shell of the hollow particles, thereby improving shell strength.

[0130] REFERENCE SIGNS LIST 1 Shell 2 Hollow portion 3 Resin 4 Inorganic fine particles 10 Hollow particles 11 Aqueous medium 12 Low polarity material 20 Droplets of monomer composition 20a Hydrophobic solvent 20b Material other than hydrophobic solvent 30 Precursor particles

Claims

1. A hollow particle having a shell and a hollow portion surrounded by the shell, The porosity is 50% or more, The volume average particle size is 1.0 μm or more and 50.0 μm or less, The shell comprises an organic-inorganic composite material containing a resin and inorganic fine particles, and the content of the inorganic fine particles in the shell is 20% by mass or more and 90% by mass or less.

2. 2. The hollow particle according to claim 1, having a relative dielectric constant of 2.00 or less at a frequency of 1 GHz.

3. 3. The hollow particles according to claim 1, wherein the thermal expansion coefficient at 23 to 100°C is 10 ppm / °C or more and 50 ppm / °C or less.

4. A hollow particle as described in claim 1 or 2, wherein the resin in the shell comprises a polymer containing 70 parts by mass or more of crosslinkable monomer units per 100 parts by mass of total monomer units.

5. 3. The hollow particles according to claim 1, wherein the inorganic fine particles have a relative dielectric constant of 5.0 or less at a frequency of 1 GHz and a thermal expansion coefficient of −5.0 ppm / ° C. or more and 10 ppm / ° C. or less at 23 to 100° C.

6. The hollow particles according to claim 1 or 2, wherein the inorganic fine particles are silica fine particles.

7. 3. The hollow particles according to claim 1, wherein the inorganic fine particles are surface-treated with a surface treatment agent containing a radical-reactive functional group.

8. Hollow particles as described in claim 7, wherein the surface treatment agent is a vinyl silane coupling agent.

9. 3. The hollow particles according to claim 1, wherein the ratio of the volume average primary particle diameter of the inorganic fine particles to the shell thickness is 0.90 or less.

10. 3. The method for producing hollow particles according to claim 1 or 2, preparing a mixed liquid containing a polymerizable monomer, inorganic fine particles, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; a step of suspending the mixed liquid to prepare a suspension in which droplets of a monomer composition containing the polymerizable monomer, the inorganic fine particles, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; a step of subjecting the suspension to a polymerization reaction to form precursor particles each having a shell containing a polymer of the polymerizable monomer and the inorganic fine particles, and a hollow portion surrounded by the shell, the hollow portion containing the hydrophobic solvent, thereby obtaining a precursor composition in which the precursor particles are dispersed in the aqueous medium; and removing the hydrophobic solvent encapsulated in the precursor particles.