Hollow particles, resin composition, and molded body

Abstract

Provided are hollow particles having reduced dielectric loss tangent and improved performance stability. The hollow particles each comprise a shell containing at least one polymer and a hollow part surrounded by the shell. The contained amount of noncrosslinkable monomer units in 100 mass% of all monomer units forming the polymer contained in the shell is 15-60 mass%. The hollow particles exhibit a dielectric loss tangent of 5.00×10-4 or less at a frequency of 10 GHz.

Description

Hollow particles, resin composition, and molded body

[0001] The present disclosure relates to hollow particles, and to a resin composition and a molded article containing the hollow particles.

[0002] Hollow particles (hollow resin particles) have cavities inside the particles and are therefore added to resins, paints, various molded products, and the like for the purposes of weight reduction, heat insulation, low dielectric constant, and the like. Their applications extend to a wide range of fields, including automobiles, bicycles, aviation, electricity, electronics, construction, home appliances, containers, stationery, tools, and footwear.

[0003] In the fields of electricity and electronics, attempts have been made to add hollow particles to insulating materials in order to lower their dielectric constants and dielectric loss tangents. For example, Patent Document 1 discloses hollow crosslinked resin particles for use in low-dielectric-constant organic insulating materials, which are obtained by polymerizing 1 to 100% by weight of a crosslinkable monomer and 0 to 99% by weight of a non-crosslinkable monomer (here, the total of the crosslinkable monomer and the non-crosslinkable monomer is 100% by weight), have an average particle size of 0.03 to 10 μm, and have an average metal ion concentration of 50 ppm or less. The hollow crosslinked resin particles in Patent Document 1 are produced by dispersing monomers in water using an emulsifier (surfactant) and then performing seed polymerization.

[0004] Patent Document 2 discloses hollow particles obtainable by dispersing an organic mixed solution containing an aromatic crosslinkable monomer (a), an aromatic monofunctional monomer (b), a (meth)acrylic acid ester monomer (c) having a specific structure, a side-chain crystalline polyolefin, a polymerization initiator, and an organic solvent in an aqueous solution containing a surfactant, and then performing suspension polymerization.

[0005] Patent Document 3 discloses hollow particles obtained by dispersing an oil phase containing a hydrocarbon monomer having a crosslinkable monomer content of 70 mass% or more, a hydrocarbon solvent having 5 to 8 carbon atoms, and a polymerization initiator in an aqueous phase that is a poorly water-soluble metal hydroxide colloid, and then performing suspension polymerization.

[0006] JP 2000-313818 A International Publication No. 2021 / 085189 International Publication No. 2022 / 092076

[0007] Hollow particles added to various materials for the purpose of lowering their dielectric loss tangent are desirable. Hollow particles with a polymer shell have the problem that the dielectric properties of the hollow particles themselves tend to deteriorate in high-humidity environments or after long-term storage, and that electronic circuit boards containing hollow particles are prone to abnormalities such as ion migration. Therefore, hollow particles with excellent performance stability are desired.

[0008] An object of the present disclosure is to provide hollow particles having a reduced dielectric loss tangent and improved performance stability. Another object of the present disclosure is to provide a resin composition and a molded article containing the hollow particles, which have a reduced dielectric loss tangent while suppressing a decrease in performance stability.

[0009] The present inventors have discovered that, in hollow particles having a shell formed of a polymer, when the content of non-crosslinkable monomer units contained in the shell is within a specific range, the dielectric tangent can be reduced and performance stability can be improved, leading to the present disclosure.

[0010] The present disclosure provides hollow particles having a shell containing at least one polymer and a hollow portion surrounded by the shell, wherein a content of non-crosslinkable monomer units is 15% by mass or more and 60% by mass or less relative to 100% by mass of all monomer units constituting all polymers contained in the shell, and a dielectric loss tangent at a frequency of 10 GHz is 5.00 × 10 －４ Provided are hollow particles:

[0011] In the hollow particles of the present disclosure, it is preferable that the residual double bond ratio of all polymers contained in the shell is 15.0% or less.

[0012] In the hollow particles according to the present disclosure, the shell may contain a structure derived from a polymer of a non-crosslinkable monomer. The polymer of a non-crosslinkable monomer is preferably at least one polymer selected from the group consisting of a polymer composed of an aromatic monovinyl monomer unit and a polymer composed of an aromatic monovinyl monomer unit and a diene monomer unit.

[0013] In the hollow particles of the present disclosure, the relative dielectric constant at a frequency of 10 GHz is preferably 1.00 or more and 1.40 or less.

[0014] In the hollow particles of the present disclosure, it is preferable that all polymers contained in the shell are hydrocarbon polymers.

[0015] In the hollow particles of the present disclosure, the porosity is preferably 70% or more.

[0016] In the hollow particles of the present disclosure, the proportion of particles having a circularity of 0.85 or less is preferably 5% by mass or less.

[0017] The hollow particles of the present disclosure preferably have a volume average particle size of 1.0 μm or more and 10.0 μm or less.

[0018] In the hollow particles of the present disclosure, the particle size distribution (volume average particle size (Dv) / number average particle size (Dn)) is preferably 1.00 or more and 1.40 or less.

[0019] The present disclosure further provides a resin composition containing the hollow particles of the present disclosure and a matrix resin, and a molded article containing the hollow particles of the present disclosure and a solidified matrix resin.

[0020] According to the present disclosure as described above, hollow particles having a lower dielectric loss tangent and improved performance stability than conventional hollow particles can be provided. Furthermore, the present disclosure can provide a resin composition and a molded article containing the hollow particles, which have a reduced dielectric loss tangent while suppressing a decrease in performance stability.

[0021] FIG. 1 is a diagram illustrating an example of a method for producing hollow particles according to the present disclosure.

[0022] The hollow particles of the present disclosure are hollow particles having a shell containing at least one polymer and a hollow portion surrounded by the shell, wherein the content of non-crosslinkable monomer units is 15% by mass or more and 60% by mass or less relative to 100% by mass of all monomer units constituting all polymers contained in the shell, and the dielectric loss tangent at a frequency of 10 GHz is 5.00 × 10 －４ The present invention is characterized by the following:

[0023] The polymer contained in the shell of the hollow particles of the present disclosure is composed of multiple monomer units. In this disclosure, a "monomer unit" is a structural unit corresponding to a polymerizable monomer. A polymerizable monomer is a compound having a functional group capable of undergoing a polymerization reaction (sometimes simply referred to as a "polymerizable functional group" in this disclosure). The monomer units contained in the shell of the hollow particles of the present disclosure may be derived from a polymerizable monomer contained in the shell raw material, or may be derived from each monomer unit constituting a pre-prepared polymer contained in the shell raw material. In this disclosure, the raw material for the polymer contained in the shell is referred to as the "shell raw material," and the shell raw material does not include a polymerization initiator. In this disclosure, a "shell raw material composition" is a composition containing the shell raw material. The shell raw material used in producing the hollow particles of the present disclosure contains at least a polymerizable monomer and may also contain a polymer obtained by pre-polymerizing the polymerizable monomer together with the polymerizable monomer. The shell of the hollow particles of the present disclosure is formed by addition polymerization of the polymerizable monomer in the shell raw material. On the other hand, the pre-prepared polymer contained in the shell raw material may be a polymer obtained by known polymerization reactions such as addition polymerization, oxidative polymerization, polycondensation, or ring-opening polymerization, or a combination of these polymerization reactions. Therefore, the monomer units contained in the shell of the hollow particles of the present disclosure are typically structural units formed by addition polymerization reactions, but may also contain structural units formed by other polymerization reactions previously performed as monomer units. In the present disclosure, the functional group capable of addition polymerization is not particularly limited, but is typically a polymerizable functional group containing an ethylenically unsaturated bond. In the present 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. Here, only functional groups that are suitable for the polymerization reaction are considered as polymerizable functional groups. Crosslinkable monomers are capable of forming crosslinks in the polymer through polymerization reactions. Non-crosslinkable monomers typically do not form crosslinks in the polymer.

[0024] The hollow particles disclosed herein achieve a low dielectric dissipation factor by using shell materials containing a non-crosslinkable monomer unit within the above-described specific range and conducting a polymerization reaction during shell formation at a higher temperature and for a longer time than conventional methods. The dielectric dissipation factor (DTF) is the degree to which a portion of the energy of an applied electric field is converted into heat and lost. Suppressing molecular motion in the shell of hollow particles reduces energy loss and thus lowers the DTF. In the polymers that constitute the main component of the shell of hollow particles disclosed herein, molecular motion tends to be active at the ends of the main chain or side chains containing polymerizable unsaturated double bonds. Therefore, reducing the number of polymer terminals is an effective way to suppress molecular motion in the shell. Furthermore, reducing the number of polymer terminals in the shell is an effective way to suppress polymer branching and promote crosslinking of the polymerizable unsaturated double bonds at the ends of the polymer (hereinafter, sometimes simply referred to as "terminal crosslinking"). In the present disclosure, by using a shell raw material in which the content of non-crosslinkable monomer units contained in the shell falls within the above-mentioned specific range, it is believed that polymer branching can be suppressed and the crosslinking reaction at the terminals can be promoted. The reason for the promotion of the crosslinking reaction at the terminals is believed to be that suppressing polymer branching reduces steric hindrance, making it easier for polymers to react with each other. In the hollow particles of the present disclosure, suppressing polymer branching and reducing the number of polymer ends having polymerizable unsaturated double bonds suppresses molecular motion in the shell. Furthermore, entanglement between the polymers constituting the shell is believed to further suppress molecular motion in the shell. Furthermore, since the air layer has a dielectric loss tangent of zero, the greater the proportion of the air layer within the hollow particle, the lower the dielectric loss tangent of the hollow particle. However, when using a shell raw material in which the content of non-crosslinkable monomer units contained in the shell falls within the above-mentioned specific range, depressions are likely to occur in the resulting hollow particles, making it difficult to increase the porosity and the proportion of the air layer.In contrast, the present disclosure has found that even when a shell raw material is used in which the content of non-crosslinkable monomer units contained in the shell falls within the above-mentioned specific range, by carrying out the polymerization reaction for forming the shell at a higher temperature and for a longer period of time than conventionally, dents in the particles can be suppressed. This is presumably because, when a shell raw material in which the content of non-crosslinkable monomer units falls within the above-mentioned range is used, branching of the polymer is suppressed, even at a high polymerization temperature, so that the speed of polymer precipitation during shell formation is appropriately slowed down, and shell formation proceeds uniformly. In this way, the hollow particles of the present disclosure have suppressed molecular motion in the shell, and furthermore, particle dents are suppressed, allowing for sufficient porosity, resulting in a dielectric loss tangent of 5.0 × 10 at a frequency of 10 GHz. －４ A low dielectric loss tangent of 0.01 or less is achieved. Furthermore, since the hollow particles of the present disclosure have sufficient porosity, they have a low dielectric loss tangent as well as a low dielectric constant, and thus have excellent dielectric properties. In the present disclosure, the lower the dielectric constant and dielectric loss tangent, the better the dielectric properties.

[0025] Furthermore, because the hollow particles of the present disclosure have excellent performance stability, the dielectric properties of the hollow particles themselves are unlikely to deteriorate, even in high-humidity environments or after long-term storage, and electronic circuit boards containing hollow particles of the present disclosure are unlikely to experience ion migration. The insufficient performance stability of conventional hollow particles is presumably due to oxidative degradation of the polymer contained in the shell due to the influence of ozone in the air. Oxidative degradation of the polymer occurs due to ozone oxidation of the polymerizable unsaturated double bonds contained in the polymer. More specifically, oxygen in the air adds to carbon radicals generated by abstraction of hydrogen at the β-position of the unsaturated double bond, which has weak bonding strength, causing an oxidation reaction. It is believed that such oxidative degradation of the polymer increases the content of oxygen atoms in the shell, resulting in deterioration of the dielectric properties of the hollow particles. Furthermore, as the content of oxygen atoms in the shell increases, the shell becomes more susceptible to moisture absorption. Therefore, in electronic circuit boards containing hollow particles, the water in the shells of the hygroscopic hollow particles ionizes and dissolves the metal of the anode of the wiring pattern. The dissolved metal ions then migrate between electrodes or wirings, forming metal at the cathode or wiring, resulting in ion migration and causing short-circuit failures. Furthermore, because the polymerizable unsaturated double bonds contained in the polymer can be oxidized by ozone, the greater the amount of polymerizable unsaturated double bonds contained in the polymer, the more susceptible the shell is to oxidative degradation. Furthermore, the above-mentioned oxidative degradation of the shell is likely to progress in high-humidity environments with high air moisture content or during long-term storage, such as when exposed to air. Furthermore, the oxidation reaction is easily accelerated by heat. Therefore, the dielectric properties of hollow particles and abnormalities in electronic circuit boards containing hollow particles are likely to deteriorate in high-humidity or high-temperature environments or after long-term storage. In contrast, as described above, the hollow particles of the present disclosure have a reduced amount of polymerizable unsaturated double bonds due to the polymer contained in the shell having fewer main chain or side chain ends having polymerizable unsaturated double bonds, and as a result, the shell is less susceptible to oxidative degradation even in high-humidity or high-temperature environments, or after long-term storage, etc. As a result, the performance stability of the hollow particles is excellent. Note that, in the present disclosure, the more unlikely the hollow particles themselves are to undergo changes in properties, or the more unlikely the materials containing the hollow particles are to undergo changes in properties due to the hollow particles, the better the performance stability of the hollow particles.

[0026] Furthermore, hollow particles used in electronic materials such as printed circuit boards are exposed to high temperatures during component mounting processes, and therefore require heat resistance. However, conventional hollow particles have many polymer ends having polymerizable unsaturated double bonds remaining in the shell, which can easily accelerate decomposition at high temperatures and may result in insufficient heat resistance. In contrast, the hollow particles disclosed herein have few polymer ends having polymerizable unsaturated double bonds, resulting in excellent heat resistance.

[0027] Furthermore, in the hollow particle manufacturing process of the present disclosure, the formation of shells on each particle is likely to proceed uniformly, resulting in hollow particles with a low proportion of irregularly shaped particles. Here, irregularly shaped particles refer to particles with low circularity, typically particles with deformations such as dents or cracks. Such irregularly shaped hollow particles have lower porosity than spherical hollow particles, resulting in inferior dielectric properties. Therefore, reducing the proportion of irregularly shaped particles contained in hollow particles can improve the dielectric properties of hollow particles. Furthermore, irregularly shaped particles are more likely to aggregate when dispersed in a matrix resin than spherical particles, resulting in poor dispersibility. Furthermore, irregularly shaped particles are more likely to be subjected to localized external pressure, resulting in poorer pressure resistance than spherical particles. When irregularly shaped particles are dispersed in a matrix resin, aggregates are likely to form, and the aggregates are more likely to be subjected to external pressure, further worsening pressure resistance. Therefore, reducing the proportion of irregularly shaped particles contained in hollow particles can improve the dispersibility and pressure resistance of hollow particles. Furthermore, when a shell raw material is used in which the content of non-crosslinkable monomer units contained in the shell is equal to or greater than the above-mentioned lower limit, the resulting shell of the hollow particles is usually prone to swelling. However, by carrying out the polymerization reaction at a higher temperature and for a longer period than conventional methods, it is possible to form a shell that is less prone to swelling. When a shell raw material is used in which the content of non-crosslinkable monomer units contained in the shell is equal to or greater than the above-mentioned lower limit, the crosslinking becomes more sparse, making it easier to form a shell that is more prone to swelling. On the other hand, it is presumed that carrying out the polymerization reaction at a higher temperature and for a longer period than conventional methods results in denser crosslinking, forming a shell that is less prone to swelling. When hollow particles are dispersed in a resin varnish containing a matrix resin and a solvent, if the shell of the hollow particles swells, the matrix resin or solvent may penetrate into the interior of the hollow particles, reducing the internal air space and potentially making it difficult to maintain dielectric properties. In contrast, the hollow particles disclosed herein have suppressed shell swelling, making it difficult for the matrix resin or solvent to penetrate into the interior of the particles when dispersed in a resin varnish.

[0028] Hereinafter, an example of a method for producing hollow particles according to the present disclosure will be described, followed by a more 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 it are included as the lower and upper limits.

[0029] 1. Method for Producing Hollow Particles The hollow particles of the present disclosure are typically produced by a production method based on suspension polymerization, which will be described later. Examples of the method for producing hollow particles of the present disclosure include the steps of: preparing a mixture containing a shell raw material, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; suspending the mixture to prepare a suspension in which droplets of a shell raw material composition containing the shell raw material, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction to prepare a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a polymer and encapsulating the hydrophobic solvent in the hollow portion.

[0030] In the above-described manufacturing method, a mixture containing a shell material, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium is suspended, whereby the shell material and the hydrophobic solvent undergo phase separation, resulting in the preparation of a suspension in which droplets of the shell material composition are dispersed in the aqueous medium, with the shell material being unevenly distributed on the surface and the hydrophobic solvent being unevenly distributed in the center. When this suspension is subjected to a polymerization reaction, polymers begin to precipitate on the surfaces of the droplets of the shell material composition. As the polymerization reaction progresses, the surfaces of the droplets harden, forming shells, resulting in hollow particles with hollow spaces filled with the hydrophobic solvent. In the present disclosure, hollow particles with hollow spaces filled with the hydrophobic solvent are sometimes referred to as "precursor particles," as they are considered to be intermediates of hollow particles with hollow spaces filled with gas. In the present disclosure, "precursor composition" refers to a composition containing precursor particles.

[0031] The above-mentioned production method 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 carried out 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 carried out simultaneously in a single step, such as by simultaneously adding the materials for preparing the mixed solution and suspending them.

[0032] A preferred example of the method for producing hollow particles according to the present disclosure includes the following steps: (1) mixed solution preparation step: a step of preparing a mixed solution containing a shell raw material, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; (2) suspending step: a step of suspending the mixed solution to prepare a suspension in which droplets of a shell raw material composition containing the shell raw material, the hydrophobic solvent, and the polymerization initiator are dispersed in an aqueous medium; (3) polymerizing step: a step of subjecting the suspension to a polymerization reaction to prepare a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a polymer and encapsulating a hydrophobic solvent in the hollow portion; and (4) solvent removing step: a step of removing the hydrophobic solvent encapsulated in the precursor particles to obtain hollow particles.

[0033] FIG. 1 is a schematic diagram illustrating an example of the manufacturing method of the present disclosure. (1) to (4) in FIG. 1 correspond to the above-described steps (1) to (4). The white arrows between the diagrams indicate the order of the steps. Note that FIG. 1 is merely a schematic diagram for explanatory purposes, and the manufacturing method of the present disclosure is not limited to that 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. 1 is a cross-sectional schematic diagram illustrating one embodiment of the mixed solution used in the mixed solution preparation step. As shown in this figure, the mixed solution contains an aqueous medium 1 and a low-polarity material 2 dispersed in the aqueous medium 1. Here, the low-polarity material 2 refers to a material that has low polarity and is difficult to mix with the aqueous medium 1. In the present disclosure, the low-polarity material 2 contains a shell raw material, a hydrophobic solvent, and a polymerization initiator. (2) in FIG. 1 is a cross-sectional schematic diagram illustrating one embodiment of the suspension used in the suspension step. The suspension contains an aqueous medium 1 and droplets 3 of a shell raw material composition dispersed in the aqueous medium 1. The droplets 3 of the shell raw material composition contain a shell raw material, a hydrophobic solvent, and a polymerization initiator, but the distribution within the droplets is non-uniform. The droplets 3 of the shell raw material composition are phase-separated into a hydrophobic solvent 4a and materials other than the hydrophobic solvent, including the shell raw material, 4b, with the hydrophobic solvent 4a concentrated in the center and the materials other than the hydrophobic solvent 4b concentrated on the surface, and a dispersion stabilizer (not shown) attached to the surface. (3) of FIG. 1 is a cross-sectional schematic diagram illustrating one embodiment of a precursor composition containing precursor particles encapsulating a hydrophobic solvent in hollow portions obtained by a polymerization process. The precursor composition includes an aqueous medium 1 and precursor particles 5 dispersed in the aqueous medium 1 and encapsulating a hydrophobic solvent 4a in hollow portions. The shells 6 forming the outer surfaces of the precursor particles 5 are formed by polymerization of the polymerizable monomers contained in the droplets 3 of the shell raw material composition. (4) of FIG. 1 is a cross-sectional schematic diagram illustrating one embodiment of hollow particles after a solvent removal process. 1(4) shows a state in which the aqueous medium 1 and the hydrophobic solvent 4a have been removed from the state shown in FIG. 1(3). By removing the hydrophobic solvent contained in the precursor particles, hollow particles 10 having gas-filled hollow portions 7 inside the shells 6 are obtained. The above four steps and other steps will be described in order below.

[0034] (1) Mixed Liquid Preparation Step This step is a step of preparing a mixed liquid containing a shell raw material, 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) shell raw material, (B) hydrophobic solvent, (C) polymerization initiator, (D) dispersion stabilizer, and (E) aqueous medium.

[0035] (A) Shell Raw Material The shell raw material contains at least a polymerizable monomer and may further contain a pre-polymerized polymer. The polymer used as the shell raw material may or may not have a polymerizable functional group. The shell raw material preferably contains at least a crosslinkable monomer as a polymerizable monomer and further contains at least one selected from the group consisting of non-crosslinkable monomers and polymers of non-crosslinkable monomers, and more preferably contains a crosslinkable monomer and a non-crosslinkable monomer as a polymerizable monomer. The crosslinkable monomer contained in the shell raw material becomes a crosslinkable monomer unit in the shell, and the non-crosslinkable monomer contained in the shell raw material becomes a non-crosslinkable monomer unit in the shell. Furthermore, in the polymer contained in the shell raw material, the non-crosslinkable monomer units and crosslinkable monomer units constituting the polymer become the non-crosslinkable monomer units and crosslinkable monomer units in the shell, respectively.

[0036] In this disclosure, a polymerizable monomer consisting of carbon and hydrogen is referred to as a hydrocarbon monomer, and a polymerizable monomer having a (meth)acryloyl group as a polymerizable functional group is referred to as an acrylic monomer. In addition, in this 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.

[0037] As the crosslinkable monomer, a compound having two or more functional groups capable of addition polymerization is used. Examples of the crosslinkable monomer include aromatic divinyl monomers such as divinylbenzene, divinylbiphenyl, and divinylnaphthalene; crosslinkable hydrocarbon monomers such as diene monomers including linear or branched diolefins such as butadiene, isoprene, 2,3-dimethylbutadiene, pentadiene, and hexadiene, and alicyclic diolefins such as dicyclopentadiene, cyclopentadiene, and ethylidenetetracyclododecene; allyl (meth)acrylate, vinyl (meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, and the like. Examples of crosslinkable monomers include crosslinkable acrylic monomers such as hexa(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, 3-(meth)acryloyloxy-2-hydroxypropyl(meth)acrylate, 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; and crosslinkable allylic monomers such as diallyl phthalate. These crosslinkable monomers can be used alone or in combination of two or more. From the viewpoint of reactivity, the crosslinkable monomer is preferably at least one selected from the group consisting of crosslinkable hydrocarbon monomers, crosslinkable acrylic monomers, and crosslinkable allylic monomers. Among these, from the viewpoint of reducing the dielectric loss tangent of the hollow particles and improving the performance stability, crosslinkable hydrocarbon monomers are more preferred, aromatic divinyl monomers are even more preferred, and divinylbenzene is particularly preferred.

[0038] In the present disclosure, the content of the crosslinkable monomer in 100% by mass of the shell raw material is preferably 40% by mass or more, more preferably 45% by mass or more, even more preferably 50% by mass or more, and still more preferably 60% by mass or more, from the viewpoint of improving the strength of the shell and making it easier for the hollow particles to maintain a high porosity, while from the viewpoint of reducing the dielectric tangent of the hollow particles and improving performance stability, it is preferably 85% by mass or less, more preferably 80% by mass or less, even more preferably 75% by mass or less, and still more preferably 65% ​​by mass or less. Note that in the present disclosure, it is preferable to include the crosslinkable monomer as a raw material for the crosslinkable monomer units in the shell.

[0039] As the non-crosslinkable monomer, a compound having only one functional group capable of addition polymerization is used. Examples of the non-crosslinkable monomer include non-crosslinkable hydrocarbon monomers; methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)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 (meth)acrylate, hexaoxypolyethylene glycol (meth)acrylate, octoxypolyethylene glycol polypropylene glycol, non-crosslinkable acrylic monomers such as polyethylene glycol mono(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; and the like.Examples of the non-crosslinkable hydrocarbon monomer include aromatic monovinyl monomers such as styrene, vinyltoluene, α-methylstyrene, p-methylstyrene, ethylvinylbenzene, ethylvinylbiphenyl, and ethylvinylnaphthalene; linear or branched monoolefins such as ethylene, propylene, and butylene; and monoolefin monomers such as alicyclic monoolefins such as vinylcyclohexane, norbornene, and tricyclododecene. These non-crosslinkable monomers can be used alone or in combination of two or more. Among these, non-crosslinkable monomers are preferred from the viewpoint of reducing the dielectric dissipation factor of the hollow particles and improving performance stability. Among these, non-crosslinkable hydrocarbon monomers are more preferred, and at least one selected from aromatic monovinyl monomers and monoolefin monomers is more preferred, with aromatic monovinyl monomers being even more preferred, and at least one selected from styrene and ethylvinylbenzene being even more preferred.

[0040] The polymer used as the shell raw material is preferably a polymer of a non-crosslinkable monomer. In the present disclosure, the polymer of a non-crosslinkable monomer may be a polymer containing a non-crosslinkable monomer unit. For example, a polymer consisting of a non-crosslinkable monomer unit and a polymer consisting of a non-crosslinkable monomer unit and a diene-based monomer unit are preferably used. That is, the crosslinkable monomer used in the polymer of a non-crosslinkable monomer is preferably a diene-based monomer. Furthermore, when the polymer of a non-crosslinkable monomer contains a crosslinkable monomer unit, the content of the non-crosslinkable monomer unit contained in the polymer is preferably 20% by mass or more. The polymer of a non-crosslinkable monomer may be a polymer obtained by a known polymerization reaction. Examples of polymers of a non-crosslinkable monomer obtained by addition polymerization include the addition polymers of the non-crosslinkable monomers described above and addition polymers of the non-crosslinkable monomer and the crosslinkable monomer described above. Examples of polymers of non-crosslinkable monomers obtained by polymerization reactions other than addition polymerization include polyphenylene ether and modified polyphenylene ether. The above-mentioned polymers of non-crosslinkable monomers may be homopolymers or copolymers. From the viewpoint of reducing the dielectric loss tangent of hollow particles and improving performance stability, the polymer of non-crosslinkable monomers is preferably at least one polymer selected from a polymer composed of aromatic monovinyl monomer units and a polymer composed of aromatic monovinyl monomer units and diene monomer units. Among the polymers composed of aromatic monovinyl monomer units, polystyrene is particularly preferred. Examples of polymers comprising aromatic monovinyl monomer units and diene monomer units include block copolymers containing an aromatic monovinyl polymer block and a conjugated diene polymer block, such as block copolymers of styrene and butadiene (e.g., styrene-butadiene-styrene block copolymer (SBS)), block copolymers of styrene and isoprene (e.g., styrene-isoprene-styrene block copolymer (SIS)), and block copolymers of styrene and dicyclopentadiene, and among these, styrene-isoprene-styrene block copolymer (SIS) is particularly preferred.In the polymer of the non-crosslinkable monomer, at least a part of the unsaturated bonds may be subjected to a hydrogenation reaction. Furthermore, the polymer of the non-crosslinkable monomer may or may not contain a polymerizable functional group.

[0041] The weight-average molecular weight of the polymer of the non-crosslinkable monomer is not particularly limited, but is preferably 10,000 or more, more preferably 100,000 or more, from the viewpoint of reducing the dielectric tangent of the hollow particles and improving performance stability, and is preferably 1,000,000 or less, more preferably 500,000 or less. In the present disclosure, the weight-average molecular weight (Mw) is a polystyrene-equivalent value measured by gel permeation chromatography (GPC) using tetrahydrofuran.

[0042] As the polymer of a non-crosslinkable monomer, for example, commercially available products such as ZEONEX (registered trademark) and ZEONOR (registered trademark) (both cycloolefin polymers) manufactured by Zeon Corporation, and Quintac (registered trademark) (thermoplastic elastomer having a block structure of polystyrene and polyisoprene) manufactured by Zeon Corporation may be used.

[0043] In the present disclosure, the total content of the non-crosslinkable monomer and the polymer of the non-crosslinkable monomer in 100% by mass of the shell raw material is preferably 15% by mass or more, more preferably 20% by mass or more, even more preferably 25% by mass or more, and even more preferably 35% by mass or more, from the viewpoint of reducing the dielectric tangent of the hollow particles and improving performance stability, while from the viewpoint of improving the strength of the shell and making it easier for the hollow particles to maintain a high porosity, it is preferably 60% by mass or less, more preferably 55% by mass or less, even more preferably 50% by mass or less, and even more preferably 40% by mass or less. Note that in the present disclosure, the raw material for the non-crosslinkable monomer unit in the shell preferably includes at least one selected from a non-crosslinkable monomer and a polymer of a non-crosslinkable monomer, and more preferably includes at least a non-crosslinkable monomer.

[0044] In terms of reducing the dielectric loss tangent of hollow particles and improving performance stability, the preferred content of various non-crosslinkable monomers and polymers of non-crosslinkable monomers varies depending on the type of non-crosslinkable monomer or the type of shell raw material used in combination. When the shell raw material contains at least one non-crosslinkable monomer selected from an aromatic monovinyl monomer and a monoolefin monomer, the content of the non-crosslinkable monomer is preferably in the range of 15% by mass to 60% by mass, based on 100% by mass of the shell raw material, when the non-crosslinkable monomer is not used in combination with a polymer composed of aromatic monovinyl monomer units. The upper limit is preferably 55% by mass or less, more preferably 50% by mass or less, and even more preferably 40% by mass or less. On the other hand, when the non-crosslinkable monomer and a polymer composed of aromatic monovinyl monomer units are used in combination, the content of the non-crosslinkable monomer, relative to 100% by mass of the shell raw material, is preferably 1% by mass or more, more preferably 2% by mass or more, as a lower limit, and preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less, as an upper limit. When the shell raw material contains a polymer composed of aromatic monovinyl monomer units, the content of the polymer is preferably any amount up to 60% by mass relative to 100% by mass of the shell raw material. For example, it may be 30% by mass or more, 40% by mass or more, 50% by mass or more, or 60% by mass relative to 100% by mass of the shell raw material. This is because a polymer composed of aromatic monovinyl monomer units is thought to be less likely to cause a decrease in shell strength and porosity, and even if the content is increased, the shell strength is likely to be good and the hollow particles are likely to maintain a high porosity. It is preferable that the shell raw material contains a polymer composed of aromatic monovinyl monomer units, since this reduces the dielectric tangent of the shell while suppressing a decrease in the shell strength, making it easier to obtain hollow particles with high porosity. Polymers composed of aromatic monovinyl monomer units and diene monomer units are poorly soluble in hydrophobic solvents, as described below, so their content in 100% by mass of the shell raw material is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. The lower limit of this content is not particularly limited, but may be, for example, 1% by mass or more in 100% by mass of the shell raw material.In the present disclosure, it is preferable to adjust the content of each of the non-crosslinkable monomers and the polymers of non-crosslinkable monomers to preferred amounts depending on the type, so that the total content of the non-crosslinkable monomers and the polymers of non-crosslinkable monomers is within the range of 15% by mass to 60% by mass. Among these, it is more preferable to adjust the content of each of the aromatic monovinyl monomer, monoolefin monomer, polymer composed of aromatic monovinyl monomer units, and polymer composed of aromatic monovinyl monomer units and diene monomer units to preferred amounts, so that the total content is within the range of 15% by mass to 60% by mass. From the viewpoint of reducing the dielectric tangent of the hollow particles and improving performance stability, the total content is more preferably 20% by mass or more, even more preferably 25% by mass or more, and even more preferably 35% by mass or more. On the other hand, from the viewpoint of improving the strength of the shell and making it easier for the hollow particles to maintain a high porosity, the total content is more preferably 55% by mass or less, even more preferably 50% by mass or less, and even more preferably 40% by mass or less.

[0045] In the present disclosure, from the viewpoint of reducing the dielectric loss tangent of the hollow particles and improving performance stability, the total content of the non-crosslinkable monomer, the polymer of the non-crosslinkable monomer, and the crosslinkable monomer in 100% by mass of the shell raw material is preferably 95% by mass or more, more preferably 97% by mass or more, even more preferably 99% by mass or more, and most preferably 100% by mass.

[0046] The lower the content of heteroatoms in the shell, the lower the relative dielectric constant and dielectric loss tangent of the hollow particles, and the more likely they are to have improved performance stability and heat resistance. Therefore, from the viewpoint of improving the dielectric properties, performance stability, and heat resistance of the hollow particles, the content of the hydrocarbon compound is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and even more preferably 95% by mass or more, relative to 100% by mass of the shell raw material, and is most preferably 100% by mass. The hydrocarbon compound refers to a hydrocarbon monomer or a polymer thereof.

[0047] The content of the shell raw material in the mixed solution is not particularly limited, but from the viewpoint of the balance between the porosity, particle size, and mechanical strength of the hollow particles, it is preferably 15 to 50% by mass, more preferably 20 to 40% by mass, relative to 100% by mass of the total mass of the components in the mixed solution excluding the aqueous medium. Furthermore, in order to suppress a decrease in strength, deterioration of dielectric properties, and a decrease in performance stability in the resulting hollow particles, the content of the shell raw material 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 96% by mass or more, more preferably 97% by mass or more. In the present disclosure, the solids refer to all components excluding the solvent, and liquid polymerizable monomers and the like are considered to be included in the solids.

[0048] (B) 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 shell raw material composition containing a hydrophobic solvent are dispersed in an aqueous medium. In the suspension process, phase separation occurs within the droplets of the shell raw material composition, and the hydrophobic solvent, which has low polarity, tends to collect inside the droplets. Ultimately, the droplets of the shell raw material composition contain the hydrophobic solvent inside, and other materials other than the hydrophobic solvent are distributed around the droplets 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.

[0049] 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. Among these, hydrocarbon solvents are preferred, and hydrocarbon solvents having 5 to 8 carbon atoms are more 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.

[0050] In the suspension step, because phase separation between the shell raw material and the hydrophobic solvent is likely to occur within the droplets of the shell raw material composition, it is preferable to select an organic solvent that has a lower solubility in water than the crosslinkable monomer contained in the shell raw material. Furthermore, when the shell raw material contains a hydrocarbon compound in a proportion of more than 50 mass%, the hydrophobic solvent is preferably a hydrocarbon-based solvent, more preferably a chain hydrocarbon-based solvent, even more preferably a chain hydrocarbon-based 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.

[0051] 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.

[0052] Furthermore, the hydrophobic solvent preferably has a relative dielectric constant of 2.5 or less, more preferably 2.0 or less, at 20°C. The relative dielectric constant is one of the indicators of the polarity of a compound. When the relative dielectric constant of the hydrophobic solvent is sufficiently small, preferably 2.5 or less, more preferably 2.0 or less, it is believed that phase separation proceeds rapidly in the droplets of the shell raw material composition, making it easy to form hollow spaces. Examples of hydrophobic solvents with a relative dielectric constant of 2.0 or less at 20°C are as follows. The values ​​in parentheses are the relative dielectric constant values: pentane (1.8), hexane (1.9), heptane (1.9), octane (1.9), and cyclohexane (2.0). Regarding the relative 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.

[0053] 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 oil droplets containing the shell material and the like encapsulating the hydrophobic solvent. Therefore, the higher the content of the hydrophobic solvent, 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 shell material, as this makes it easier to control the particle size of the hollow particles, increase the porosity while maintaining the strength of the hollow particles, and reduce the amount of residual hydrophobic solvent in 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 shell material.

[0054] (C) Polymerization Initiator In the production method of the present disclosure, the mixed solution preferably contains an oil-soluble polymerization initiator as the polymerization initiator. The oil-soluble polymerization initiator is not particularly limited as long as it is lipophilic and has a solubility in water of 0.2% by mass or less. Examples 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). Among these, organic peroxides are preferably used as the oil-soluble polymerization initiator because they tend to improve the dielectric properties of the hollow particles. When an organic peroxide is used as the polymerization initiator, the amount of unreacted polymerizable unsaturated bonds remaining in the shell is easily reduced, thereby suppressing an increase in molecular motion in the shell. Furthermore, if decomposition products of the polymerization initiator remain in the shell, they tend to increase the molecular motion of the shell, but the decomposition products of the organic peroxide are easily removed. Therefore, when an organic peroxide is used as the polymerization initiator, the amount of decomposition products of the polymerization initiator remaining in the shell can be reduced, thereby suppressing the increase in molecular motion of the shell.

[0055] The content of the polymerization initiator is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 7 parts by mass, and even more preferably 1 to 5 parts by mass, relative to 100 parts by mass of the polymerizable monomer in the mixed solution. 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 polymerization initiator remaining after completion of the polymerization reaction, and there is also little risk of an unexpected side reaction proceeding.

[0056] (D) Dispersion Stabilizer The dispersion stabilizer disperses droplets of the shell raw material 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, facilitate removal of the dispersion stabilizer during the washing process, 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 less than 1 g / L in water at 25° C. Among these, poorly water-soluble inorganic dispersion stabilizers are preferably metal hydroxides, and more preferably magnesium hydroxide.

[0057] 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 allows the inorganic dispersion stabilizer to be easily removed by the washing step described below. 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. Examples include magnesium metal salts such as magnesium chloride, magnesium phosphate, and magnesium sulfate; calcium metal salts such as calcium chloride, calcium nitrate, calcium acetate, and calcium sulfate; aluminum metal salts such as aluminum chloride and aluminum sulfate; barium salts such as barium chloride, barium nitrate, and barium acetate; and zinc salts such as zinc chloride, zinc nitrate, and zinc acetate. Among these, magnesium metal salts, calcium metal salts, and aluminum metal salts are preferred, magnesium metal salts are more preferred, and magnesium chloride is particularly preferred. The 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. For example, an aqueous solution of at least one selected from the above-mentioned alkali metal hydroxides and alkaline earth metal hydroxides may be mixed with an aqueous solution of the water-soluble polyvalent metal salt. Furthermore, colloidal silica may be used as a colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles.

[0058] 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 stabilizers include sodium tripolyphosphate. Surfactants are compounds that have both hydrophilic and hydrophobic groups in one molecule, and include known ionic surfactants such as anionic surfactants, cationic surfactants, and amphoteric surfactants, as well as nonionic surfactants. Water-soluble polymer stabilizers and surfactants typically have a solubility of 1 g / L or more in water at 25°C.

[0059] 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 shell raw material and the hydrophobic solvent. By having the content of the dispersion stabilizer equal to or greater than the lower limit, droplets of the shell raw material 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 equal to or less than the upper limit, an increase in the viscosity of the suspension during granulation can be prevented, and the problem of the suspension clogging in the granulator can be avoided. 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.

[0060] In the hollow particles of the present disclosure, from the viewpoint of suppressing deterioration of dielectric properties and deterioration of performance stability, the smaller the residual amount of dispersion stabilizer, the more preferable, and it is most preferable that the hollow particles do not contain a dispersion stabilizer, and it is particularly preferable that the hollow particles do not contain an organic water-soluble polymer stabilizer, an inorganic water-soluble polymer stabilizer, or a surfactant. By using only an inorganic dispersion stabilizer as the dispersion stabilizer, hollow particles can be obtained that do not contain any of an organic water-soluble polymer stabilizer, an inorganic water-soluble polymer stabilizer, or a surfactant.

[0061] (E) 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, from the viewpoint of forming droplets of the shell raw material 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); and dimethyl sulfoxide (DMSO).

[0062] 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 shell raw material 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.

[0063] The mixed liquid may further contain other materials different from the above-described materials (A) to (E) as long as the effects of the present disclosure are not impaired.

[0064] A mixed solution is obtained by mixing the above-mentioned materials and other materials as needed, and stirring appropriately. In this mixed solution, an oil phase containing lipophilic materials such as (A) the shell raw material, (B) the hydrophobic solvent, and (C) the polymerization initiator is dispersed in an aqueous phase containing (D) the dispersion stabilizer and (E) the aqueous medium, with particles of about several mm in size. The dispersion state of these materials in the mixed solution can be observed with the naked eye, depending on the type of material. In the mixed solution preparation step, the mixed solution may be obtained by simply mixing the above-mentioned materials and other materials as needed, and stirring appropriately. However, in terms of making the shell more uniform, it is preferable to prepare a mixed solution by separately preparing an oil phase containing the shell raw material, the hydrophobic solvent, and the polymerization initiator, and an aqueous phase containing the dispersion stabilizer and the 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 the form of colloidal particles in an aqueous medium 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.

[0065] (2) Suspension Step The suspension step is a step of preparing a suspension in which droplets of a shell material composition containing a hydrophobic solvent are dispersed in an aqueous medium by suspending the above-mentioned mixed solution. The suspension method for forming droplets of the shell material composition is not particularly limited, and any known suspension method can be used. Examples of dispersers that can be used in preparing the suspension include horizontal or vertical in-line dispersers such as Milder manufactured by Pacific Machinery Works, Ltd., Cavitron manufactured by Eurotec Co., Ltd., and in-line dispersers manufactured by IKA (e.g., DISPAX-REACTOR (registered trademark) DRS); and emulsifying dispersers such as the Homomixer MARK II series manufactured by Primix Corporation.

[0066] In the suspension prepared in the suspending step, droplets of the shell raw material composition containing the lipophilic material and having a particle size of approximately 1 to 10 μm are uniformly dispersed in the aqueous medium. Such droplets of the shell raw material composition are difficult to observe with the naked eye, but can be observed using known observation equipment such as an optical microscope. During the suspending step, phase separation occurs in the droplets of the shell raw material composition, which makes it easier for the hydrophobic solvent, which has low polarity, 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.

[0067] The droplets of the shell raw material composition dispersed in an aqueous medium are formed by surrounding the oil-soluble shell raw material composition with a dispersion stabilizer. The droplets of the shell raw material composition contain an oil-soluble polymerization initiator, a shell raw material, and a hydrophobic solvent. The droplets of the shell raw material composition are small oil droplets, and the oil-soluble 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 suspension polymerization methods using such oil-soluble polymerization initiators, there is no opportunity for the polymerization initiator to come into contact with the polymerizable monomer dispersed in the aqueous medium. Therefore, by using an oil-soluble polymerization initiator, it is possible to suppress the by-production of excess resin particles, such as dense particles with a relatively small particle size, in addition to the desired resin particles having hollow portions.

[0068] (3) Polymerization Step This step involves subjecting the suspension obtained in the suspension step described above to a polymerization reaction to prepare a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a polymer and encapsulating a hydrophobic solvent in the hollow portion. The polymerization method is not particularly limited, and known polymerization methods such as batch, semi-continuous, and continuous polymerization methods can be employed. In the present disclosure, from the viewpoint of suppressing the occurrence of particle depressions and obtaining hollow particles with high porosity and excellent dielectric properties, the polymerization reaction conditions are preferably a polymerization temperature of 80°C or higher and a polymerization reaction time of 10 hours or longer. The polymerization reaction time is more preferably 20 hours or longer. The upper limit of the polymerization temperature is not particularly limited, but is preferably 95°C or lower in order to suppress evaporation of the aqueous medium. The upper limit of the polymerization reaction time is not particularly limited, but is preferably 36 hours or shorter in terms of ease of production.

[0069] (4) Solvent Removal Step This step is a step of removing the hydrophobic solvent contained in the precursor particles. In the present disclosure, a method may be adopted in which a slurry in which the precursor particles are dispersed in an aqueous medium is subjected to solid-liquid separation, and then the hydrophobic solvent contained in the precursor particles is removed in air, or a method may be adopted in which the hydrophobic solvent contained in the precursor particles is removed in the slurry.

[0070] The method of solid-liquid separation is not particularly limited, and examples thereof include centrifugation, filtration, and static separation. Among these, filtration is preferred because it is easy to operate and has a high efficiency of removing the dispersion stabilizer. The precursor particles obtained by solid-liquid separation may be pre-dried before removing the hydrophobic solvent. Pre-drying may be performed, for example, by drying the solid content containing the precursor particles obtained by solid-liquid separation using a drying device such as a dryer or a drying appliance such as a hand dryer.

[0071] 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, it is important to remove the hydrophobic solvent in an environment in which the precursor particles are in direct contact with the external gas.

[0072] 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 above the boiling point of the hydrophobic solvent and below the maximum temperature at which the shell structure of the precursor particles does not collapse. Therefore, depending on the shell composition 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. By drying in air, the hydrophobic solvent inside the precursor particles is replaced with the external gas, resulting in hollow particles with a gas-filled hollow space. The drying atmosphere is not particularly limited and can be selected appropriately depending on the application of the hollow particles. Examples of suitable drying atmospheres include air, oxygen, nitrogen, and argon. Alternatively, hollow particles with a temporary vacuum interior can be obtained by first filling the interior of the hollow particles with a gas and then drying under reduced pressure.

[0073] Examples of methods for removing the hydrophobic solvent contained in precursor particles from a slurry in which precursor particles are dispersed in an aqueous medium include evaporating and distilling off the hydrophobic solvent contained in the precursor particles in the slurry under a predetermined pressure. More specifically, examples include methods for evaporating and distilling off the hydrophobic solvent contained in the hollow particles by introducing an inert gas such as nitrogen, argon, or helium or water vapor into the slurry under a predetermined pressure selected from high pressure, normal pressure, and reduced pressure. Among these, methods for evaporating and distilling off the hydrophobic solvent contained in the hollow particles by introducing an inert gas into a slurry in which precursor particles are dispersed under a predetermined pressure are preferred because of their excellent hydrophobic solvent removal efficiency. Examples of methods for introducing an inert gas into the slurry include bubbling an inert gas into the slurry. Furthermore, the temperature when introducing the inert gas into the slurry is preferably a temperature equal to or higher than the boiling point of the hydrophobic solvent minus 35°C, from the viewpoint of reducing the residual amount of hydrophobic solvent, preferably a temperature equal to or higher than the boiling point of the hydrophobic solvent minus 30°C, and more preferably a temperature equal to or higher than the boiling point of the hydrophobic solvent minus 20°C. Here, when the hydrophobic solvent is a mixed solvent containing multiple types of hydrophobic solvents and has multiple boiling points, the boiling point of the hydrophobic solvent in the solvent removal step is the boiling point of the solvent with the highest boiling point among the solvents contained in the mixed solvent, that is, the highest boiling point among the multiple boiling points. Furthermore, the temperature when introducing the inert gas into the slurry is usually a temperature equal to or higher than the polymerization temperature in the polymerization step. The temperature when introducing the inert gas into the slurry is not particularly limited, but may be 50 to 100°C. The conditions for bubbling the inert gas into the slurry are appropriately adjusted depending on the type and amount of the hydrophobic solvent so as to remove the hydrophobic solvent encapsulated in the precursor particles, and are 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. By introducing the inert gas into the slurry, a slurry of hollow particles encapsulating the inert gas is obtained. The slurry is subjected to solid-liquid separation, and the remaining moisture in the obtained hollow particles is dried and removed, thereby obtaining hollow particles whose hollow portions are filled with gas.

[0074] (5) Washing Step The method for producing hollow particles according to the present disclosure preferably includes a washing step prior to the solvent removal step. The washing step may be a step of removing a dispersion stabilizer from precursor particles obtained by the polymerization step. The washing step is carried out, for example, by adding an acid or alkali to the precursor composition obtained by the polymerization step, dissolving the dispersion stabilizer contained in the precursor particles in an aqueous medium, and then separating the precursor particles from the aqueous medium. When the dispersion stabilizer used is an acid-soluble inorganic dispersion stabilizer, it is preferable to add an acid. When the dispersion stabilizer used is an alkali-soluble inorganic dispersion stabilizer, it is preferable to add an alkali. When the dispersion stabilizer used is an acid-soluble inorganic dispersion stabilizer, an acid is added to the precursor composition to adjust the pH to preferably 6.5 or less, more preferably 6 or less. Examples of the acid to be added include inorganic acids such as sulfuric acid, hydrochloric acid, and nitric acid, and organic acids such as formic acid and acetic acid. However, sulfuric acid is particularly preferred due to its high dispersion stabilizer removal efficiency and its low burden on production equipment. Since the dielectric loss tangent of the hollow particles is likely to decrease, it is preferable to repeat the procedure of washing the precursor particles separated after adding an acid or alkali with ion-exchanged water and dehydrating them several times in the washing step. In addition, in the washing step, a small amount of a nonionic surfactant may be added to the slurry to suppress aggregation of the precursor particles.

[0075] The method for separating the precursor particles from the aqueous medium is not particularly limited, and examples thereof include the same methods as the solid-liquid separation methods described above. Among these, filtration is preferred because it is easy to operate and has a high efficiency in removing the dispersion stabilizer. Furthermore, when separating the precursor particles from the aqueous medium, it is preferred to dehydrate the precursor particles obtained by filtration or the like using a known method.

[0076] (6) Sieving Step (Foreign Matter Removal Step) The method for producing hollow particles according to the present disclosure preferably includes a sieving step after the solvent removal step described above. By performing the sieving step, coarse powder and aggregates can be removed, and foreign matter can be easily removed. Any known sieving method can be used, and is not particularly limited. For example, sieving may be performed using a wire mesh made of stainless steel or the like. More specifically, the sieved hollow particles can be obtained by vibrating the wire mesh carrying the hollow particles and obtaining the hollow particles that have passed through the wire mesh. The mesh size of the wire mesh used in the sieving step is appropriately selected depending on the size of the hollow particles. It is preferable that the mesh size be such that the proportion of particles with a circularity of 0.85 or less is 5% by mass or less among the obtained hollow particles.

[0077] (7) Others: As a process other than the above (1) to (6), for example, a process of substituting the interior of a particle may be added. The process of substituting the interior of a particle is a process of substituting the gas or liquid inside the hollow particle with another gas or liquid. Such a substitution can change the environment inside the hollow particle, selectively confine molecules inside the hollow particle, or modify the chemical structure inside the hollow particle according to the application.

[0078] 2. Hollow Particles The hollow particles of the present disclosure are particles comprising a shell (outer shell) containing at least one polymer and a hollow portion surrounded by the shell. In the present disclosure, the hollow portion is a hollow space clearly distinguishable from the shell. The shell of the hollow particle may have a porous structure, but in that case, the hollow portion has a size clearly distinguishable from the numerous minute spaces uniformly dispersed within the porous structure. From the viewpoint of dielectric properties, etc., the hollow particles of the present disclosure preferably have a solid shell. The hollow portion of the hollow particle can be confirmed, for example, by SEM observation of the particle cross section or by TEM observation of the particle itself. Furthermore, in order to exhibit excellent dielectric properties, the hollow portion of the hollow particle of the present disclosure is preferably filled with a gas such as air.

[0079] The hollow particles of the present disclosure contain a polymer as the main component of the shell, and the polymer forms the skeleton of the shell of the hollow particles. The polymer contained in the shell of the hollow particles includes at least a polymer formed by polymerization of a polymerizable monomer contained in the shell raw material, and may further include a polymer that was contained in the shell raw material. The polymer contained in the shell raw material may or may not be bonded to other polymers in the shell. In order to suppress deterioration of dielectric properties, reduction in strength, and deterioration in performance stability, the content of the polymer in the shell of the hollow particles of the present disclosure is preferably 96% by mass or more, more preferably 97% by mass or more, even more preferably 98% by mass or more, and even more preferably 99% by mass or more, based on 100% by mass of the total solid content of the shell.

[0080] The hollow particles of the present disclosure are obtained by carrying out a polymerization reaction at a high temperature for a long period of time as described above, and therefore, substantially all of the shell raw materials form polymers. Therefore, in the hollow particles of the present disclosure, the composition of each monomer unit of the polymer contained in the shell can be determined from the composition of the shell raw materials used during production. In the hollow particles of the present disclosure, the content of non-crosslinkable monomer units is 15% by mass or more and 60% by mass or less, based on 100% by mass of all monomer units constituting all polymers contained in the shell. To reduce the dielectric tangent of the hollow particles and improve performance stability, the content of non-crosslinkable monomer units is preferably 20% by mass or more, more preferably 25% by mass or more, and even more preferably 35% by mass or more. On the other hand, to improve the strength of the shell and facilitate the hollow particles' maintenance of a high porosity, the content of non-crosslinkable monomer units is preferably 55% by mass or less, more preferably 50% by mass or less, and even more preferably 40% by mass or less. Furthermore, in order to reduce the dielectric tangent of the hollow particles of the present disclosure and improve performance stability, as well as to improve the strength of the shell and facilitate the hollow particles maintaining a high porosity, the total content of aromatic monovinyl monomer units and monoolefin monomer units relative to 100% by mass of the total amount of non-crosslinkable monomer units is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and may even be 100% by mass or more. From the same viewpoint, the content of aromatic monovinyl monomer units relative to 100% by mass of the total amount of non-crosslinkable monomer units is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and may even be 100% by mass or more. The shell of the hollow particles of the present disclosure may contain a structure derived from a polymer of a non-crosslinkable monomer. In the hollow particles of the present disclosure, the content of the structure derived from a polymer of a non-crosslinkable monomer relative to 100% by mass of all monomer units constituting all polymers contained in the shell corresponds to the content of the polymer of a non-crosslinkable monomer in 100% by mass of the shell raw material described above. The content of the structure derived from the polymer of the non-crosslinkable monomer is not particularly limited, and it is preferable to appropriately adjust the content of the non-crosslinkable monomer unit in the shell so that it falls within the above-mentioned range.Furthermore, the content of the structure derived from the polymer of the non-crosslinkable monomer is preferably adjusted so that the content of the crosslinkable monomer unit in the polymer is 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, based on 100% by mass of all monomer units constituting the entire polymer contained in the shell. In particular, it is preferable to adjust the content of the diene monomer unit in the polymer to be equal to or less than the above-mentioned upper limit. This makes it easier to obtain hollow particles having only one hollow portion. In the hollow particles of the present disclosure, the content of the crosslinkable monomer unit is 40% by mass or more and 85% by mass or less, based on 100% by mass of all monomer units constituting the entire polymer contained in the shell. The content of the crosslinkable monomer units is preferably 45% by mass or more, more preferably 50% by mass or more, and even more preferably 60% by mass or more, from the viewpoint of improving the strength of the shell and making it easier for the hollow particles to maintain a high porosity. On the other hand, from the viewpoint of reducing the dielectric tangent of the hollow particles and improving performance stability, the content is preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 65% ​​by mass or less. The shell of the hollow particles of the present disclosure preferably contains hydrocarbon monomer units. In the hollow particles of the present disclosure, the content of hydrocarbon monomer units in 100% by mass of all monomer units constituting all polymers contained in the shell corresponds to the content of hydrocarbon compounds in 100% by mass of the shell raw material described above. In the hollow particles of the present disclosure, it is most preferable that all polymers contained in the shell are hydrocarbon polymers.

[0081] Furthermore, in order to improve dielectric properties, performance stability, and heat resistance, the hollow particles of the present disclosure preferably have a heteroatom content of 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less, and most preferably 0% by mass, relative to 100% by mass of the total polymer contained in the shell.

[0082] In the hollow particles of the present disclosure, the residual double bond ratio in the entire polymer contained in the shell is preferably 15.0% or less, more preferably 13.5% or less, even more preferably 11.0% or less, still more preferably 10.0% or less, and particularly preferably 9.5% or less. The lower the residual double bond ratio, the lower the dielectric loss tangent of the hollow particles tends to be. Although a lower residual double bond ratio is preferable, from the viewpoint of ease of production, it may be 2.0% or more, 5.0% or more, or 7.0% or more.

[0083] The residual double bond ratio can be determined as follows. First, infrared absorption spectra, expressed as absorbance, are measured for the polymer contained in the hollow particles and the shell raw material used to produce the hollow particles. Meanwhile, the monomer unit with the highest content among all monomer units of the polymer contained in the hollow particles is identified as the reference monomer unit. If there are multiple monomer units with the highest content, one of them is identified as the reference monomer unit. From the structures of the reference monomer unit, one structure whose content does not change before and after the polymerization reaction is selected. As the structure that does not change before and after the polymerization reaction, it is preferable to select, from among structures of the reference monomer unit that are not involved in the polymerization reaction, a structure whose peak appearing in the infrared absorption spectrum can be clearly distinguished from the peak of the polymerizable unsaturated double bond (C═C) and has a strong intensity. The peak derived from the selected structure is used as the reference peak. Then, in each of the infrared absorption spectrum of the polymer and the infrared absorption spectrum of the shell raw material, the peak intensity of the reference peak is divided by the content ratio of the monomer containing the structure that exhibits the reference peak, and the calculated value is used as the reference peak intensity. Also, in each of the infrared absorption spectrum of the polymer and the infrared absorption spectrum of the shell raw material, the peak intensity of the peak derived from the polymerizable unsaturated double bond (C=C) is measured. The reference peak intensity (M ０ ) relative to the peak intensity (M １ ) ratio (M １ / M ０) is the peak intensity ratio when the residual double bond ratio is 100%. ０ ) relative to the peak intensity (P １ ) ratio (P １ / P ０ ) is also calculated in the same manner. Then, the peak intensity ratio (M １ / M ０ ) and the peak intensity ratio (P １ / P ０ ) can be calculated by the following formula (A): Residual double bond ratio (%) = {(P １ / P ０ ) / (M １ / M ０ )} × 100 The peak intensity can be quantified as the height from a baseline formed by placing base points outside both ends of the peak and connecting the base points with a straight line to the peak top. The values ​​of peak intensity and peak intensity ratio used in calculating the residual double bond ratio are rounded to three significant figures in accordance with Rule B of JIS Z8401:1999, and the residual double bond ratio is rounded to one decimal place. The infrared absorption spectrum can be measured, for example, by an attenuated total reflection method (ATR method). An example of an infrared absorption spectrum measuring device is Spectrum One, manufactured by Perkin Elmer.

[0084] The hollow particles of the present disclosure preferably have a porosity of 70% or more. When the porosity is 70% or more, the hollow particles have excellent dielectric properties and are also excellent in light weight, heat resistance, heat insulation, etc. 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, from the viewpoint of suppressing a decrease in the strength of the hollow particles and making them less likely to be crushed.

[0085] The porosity of the hollow particles is the apparent density D １ and true density D ０The 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 by 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.

[0086] True density D of hollow particles ０ The measurement method is as follows: After crushing the hollow particles in advance, ３ Approximately 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) 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.

[0087] 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

[0088] The hollow particles of the present disclosure preferably have a lower limit of the volume average particle diameter of 1.0 μm or more, more preferably 1.5 μm or more, and even more preferably 2.0 μm or more. On the other hand, the upper limit of the volume average particle diameter of the hollow particles is preferably 10.0 μm or less, more preferably 7.0 μm or less, even more preferably 5.0 μm or less, and even more preferably 4.0 μm or less. When the volume average particle diameter of the hollow particles is equal to or greater than the above lower limit, the dispersibility of the hollow particles is improved, a uniform shell is easily formed, and the variation in shell thickness is suppressed, which tends to improve the pressure resistance of the hollow particles. From the same viewpoint, the hollow particles of the present disclosure preferably have a volume ratio of particles having a particle diameter of less than 1.0 μm of 8 vol% or less, more preferably 5 vol% or less, and even more preferably 4 vol% or less. When the volume average particle diameter of the hollow particles is equal to or less than the above upper limit, the particle diameter is sufficiently small, making them suitable for use as substrate materials for electronic circuit boards and the like, and they can be added to thin, small substrates. From the same viewpoint, the hollow particles of the present disclosure preferably have a volume ratio of particles having a particle size exceeding 10.0 μm of 5 vol % or less, more preferably 2 vol % or less, and even more preferably 1 vol % or less.

[0089] The shape of the hollow particles of the present disclosure is not particularly limited as long as a hollow portion is formed inside. For example, they may be spherical, oval, or amorphous. However, spherical is preferred from the viewpoints of the dielectric properties, dispersibility, and pressure resistance of the hollow particles. In the method for producing hollow particles of the present disclosure described above, hollow particles with high circularity can be produced by using the above-described shell raw materials and subjecting the polymerization reaction to high temperature and long time conditions as described above. The hollow particles of the present disclosure may contain small amounts of impurities, such as hollow particles with low circularity due to particle cracking or deformation, or particles without hollow portions. However, the proportion of particles with a circularity of 0.85 or less in 100% by mass of the hollow particles of the present disclosure is preferably 5% by mass or less, more preferably 4% by mass or less, and even more preferably 3% by mass or less. By performing the sieving process described above, the proportion of particles with a circularity of 0.85 or less can be reduced to the above-described upper limit or less. Circularity is defined as the diameter of a circle having the same area as the projected image of a particle (equivalent circle area diameter) divided by the diameter of a circle having the same perimeter as the projected image of the particle (equivalent circumferential diameter). When a particle is a perfect sphere, the circularity is 1, and the more complex the particle's surface shape, the smaller the circularity value. The hollow particles disclosed herein may have an average circularity of 0.950 to 0.995. In this 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, for example, by dispersing 0.10 to 0.12 g of hollow particles in an aqueous solution of linear alkylbenzenesulfonate (0.3% concentration) in an ultrasonic cleaner for 5 minutes. The average circularity is the average value of the circularity of 1,000 to 3,000 arbitrarily selected particles.

[0090] The hollow particles of the present disclosure may have one or more hollow portions. However, from the viewpoint of maintaining a good balance between high porosity and mechanical strength and improving dielectric properties, those having only one hollow portion are preferred. When the hollow particles of the present disclosure have only one hollow portion, they may contain a small amount of hollow particles having two or more hollow portions or particles having no hollow portion as impurities. The proportion of particles having only one hollow portion in the hollow particles of the present disclosure is preferably 90% or more, more preferably 95% or more. Furthermore, the shell of the hollow particles of the present disclosure and the partition walls separating adjacent hollow portions when the hollow particles have two or more hollow portions may be porous, but are preferably solid from the viewpoint of improving dielectric properties. 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, the cross-sectional view of which is shown as hollow particle 10 in (4) of Figure 1. In this example, a thin film is provided on the outside, and the inside is filled with gas. The particle shape can be confirmed by, for example, SEM or TEM.

[0091] The particle size distribution (volume average particle size (Dv) / number average particle size (Dn)) of the hollow particles of the present disclosure is preferably 1.00 or more and 1.40 or less, and more preferably 1.00 or more and 1.35 or less. When the particle size distribution of the hollow particles is within the above range, particles with little variation in performance between particles can be obtained. Furthermore, for example, when manufacturing a sheet-shaped resin molding containing the hollow particles of the present disclosure, a product with a uniform thickness can be manufactured. The volume average particle size (Dv) and number average particle size (Dn) of the hollow particles can be determined, for example, by measuring the particle size of the hollow particles using a particle size distribution measuring instrument, calculating the number average and volume average, respectively, and using the obtained values ​​as the number average particle size (Dn) and volume average particle size (Dv) of the particles. The particle size distribution is the value obtained by dividing the volume average particle size by the number average particle size.

[0092] The hollow particles of the present disclosure have a dielectric loss tangent of 5.00×10 at a frequency of 10 GHz. －４ or less, preferably 4.00 × 10 －４ or less, more preferably 3.50 × 10 －４The lower limit of the dielectric loss tangent is not particularly limited, and is, for example, 1.00 × 10 －４ It may be more than that.

[0093] The hollow particles of the present disclosure have a relative dielectric constant at a frequency of 10 GHz of preferably 1.40 or less, more preferably 1.37 or less, and even more preferably 1.35 or less. The lower limit of the relative dielectric constant is not particularly limited, and may be, for example, 1.00 or more. In the present disclosure, the relative dielectric constant and dielectric loss tangent of the hollow particles are measured using a perturbation type measuring device.

[0094] The hollow particles of the present disclosure have excellent performance stability, and therefore the difference in dielectric loss tangent at a frequency of 10 GHz before and after a HAST test under conditions of 130°C, 85% RH, and 120 hours can be made less than 0.005, and in a more preferred embodiment, can be made less than 0.001.

[0095] In order to reduce the dielectric tangent of the hollow particles and improve the performance stability of the hollow particles, the hollow particles of the present disclosure preferably have a content of surfactants and organic or inorganic water-soluble polymer stabilizers (hereinafter simply referred to as "surfactants, etc.") present on the particle surface of 200 ppm or less, more preferably 100 ppm or less, and even more preferably 50 ppm or less. By using only inorganic dispersion stabilizers as dispersion stabilizers in the above-mentioned hollow particle production process, the content of surfactants, etc. present on the hollow particle surface can be made below the measurement limit. In this disclosure, the content of surfactants, etc. present on the hollow particle surface refers to the ratio of the mass of surfactants, etc. present on the hollow particle surface to the mass of the hollow particles. The surfactants, etc. present on the hollow particle surface can be extracted, for example, by ultrasonically treating the hollow particles in water. The type and mass of surfactants, etc. extracted into water can be determined as follows: １ It can be identified from the peak position and peak intensity of the H-NMR spectrum. In this method, the measurement limit of the amount of surfactant present on the hollow particle surface is usually 0.05 ppm.

[0096] In the present disclosure, the thermal decomposition onset temperature of the hollow particles is preferably 345°C or higher, more preferably 350°C or higher. Hollow particles having a thermal decomposition onset temperature equal to or higher than the above lower limit have excellent heat resistance. The upper limit of the thermal decomposition onset temperature of the hollow particles is not particularly limited, but may be, for example, 400°C or lower. In the present disclosure, the thermal decomposition onset temperature of the hollow particles is the temperature at which a 5% weight loss 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 heating rate of 10°C / min.

[0097] Examples of uses of the hollow particles of the present disclosure include additives in low-dielectric, heat-insulating, sound-insulating, and light-reflecting materials used in various fields such as automobiles, electrical appliances, electronics, architecture, aviation, and spacecraft; food containers; footwear such as sports shoes and sandals; home appliance parts; bicycle parts; stationery; tools; 3D printer filaments; and buoyancy materials such as syntactic foam. In particular, the hollow particles of the present disclosure have a low dielectric loss tangent, excellent dielectric properties, and excellent performance stability, making them suitable for use as highly reliable materials in the electrical or electronic fields. For example, the hollow particles of the present disclosure are suitable for use as materials for electronic circuit boards. Specifically, by incorporating the hollow particles of the present disclosure into the insulating resin layer of an electronic circuit board, the dielectric loss tangent of the insulating resin layer can be reduced while suppressing defects caused by the hollow particles. The hollow particles of the present disclosure are also suitable for use as additives in semiconductor materials such as interlayer insulating materials, dry film resists, solder resists, bonding wires, magnet wires, semiconductor encapsulants, epoxy encapsulants, mold underfills, underfills, die bond pastes, buffer coating materials, copper-clad laminates, and flexible substrates, or as additives in semiconductor materials used in high-frequency device modules, antenna modules, and automotive radars. Among these, the hollow particles of the present disclosure are particularly suitable as additives in semiconductor materials such as interlayer insulating materials, solder resists, magnet wires, semiconductor encapsulants, epoxy encapsulants, underfills, buffer coating materials, copper-clad laminates, and flexible substrates, or as additives in semiconductor materials used in high-frequency device modules, antenna modules, and automotive radars. The hollow particles of the present disclosure are not limited to semiconductor materials and can be used in various electronic materials. The hollow particles of the present disclosure are also useful as additives in insulating resin sheets used in the manufacture of electronic components such as printed wiring boards.An insulating resin sheet containing hollow particles according to 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 hollow particles according to 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, as a bonding sheet, for example. A bonding sheet is an insulating adhesive layer-forming material used to bond a conductor layer and an organic insulating layer when manufacturing a multilayer printed wiring board. Furthermore, the hollow particles according to the present disclosure have high porosity, are resistant to crushing, and have excellent heat resistance, thereby satisfying the heat insulation and shock-absorbing properties (cushioning properties) required for undercoating materials and also meeting the heat resistance required for thermal paper applications. The hollow particles according to the present disclosure are also useful as plastic pigments with excellent gloss, hiding power, etc. Furthermore, the hollow particles of the present disclosure can be encapsulated with useful ingredients such as fragrances, pharmaceuticals, pesticides, and ink components by immersion, vacuum immersion, or pressure immersion, and can be used for a variety of purposes 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, and therefore, 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. Furthermore, rust-preventive additives can also be encapsulated in the hollow particles added to rust-preventive paints.

[0098] 3. Resin Composition The resin composition of the present disclosure contains at least the hollow particles of the present disclosure and a matrix resin. It is typically liquid, but may be in pellet form or prepreg form. The resin composition of the present disclosure may be used as a molding material for the molded body described below. Examples of liquid resin compositions include those containing a liquid matrix resin before a curing reaction, and those obtained by dissolving or dispersing each component in a solvent. Here, the liquid matrix resin before a curing reaction and the matrix resin dissolved or dispersed in a solvent may be, for example, a thermosetting resin, a room-temperature curing resin, or a thermoplastic resin. Alternatively, the resin composition may be liquid due to the matrix resin being molten. Prepreg can be obtained, for example, by impregnating a substrate with the liquid resin composition and drying it. Examples of pellet-shaped resin compositions include those obtained by melt-kneading a resin composition containing a thermoplastic matrix resin and hollow particles, and then pelletizing the resulting composition by cooling and solidifying it. Conventional resin compositions and molded articles containing hollow particles tend to exhibit reduced performance stability in high-humidity environments or after long-term storage, compared to those containing no hollow particles. In contrast, the resin compositions and molded articles containing hollow particles of the present disclosure have a reduced dielectric tangent due to the inclusion of hollow particles, while suppressing the decline in performance stability. Examples of matrix resins include curable resins such as thermosetting resins, photocurable resins, and room-temperature curable resins, as well as thermoplastic resins. Among these, thermosetting resins, room-temperature curable resins, and thermoplastic resins are preferred. The matrix resin may be an unreacted monomer, prepolymer, or macromonomer; a polymer; or a precursor of a curable resin such as polyamic acid. The matrix resin contained in the resin composition of the present disclosure functions as a binder by, for example, heating, light irradiation, or curing with a curing agent, polymerization initiator, or catalyst.

[0099] Known thermosetting resins can be used, and are not particularly limited. Examples include phenolic resins, melamine resins, urea resins, unsaturated polyester resins, epoxy resins, polyurethane resins, silicon resins, alkyd resins, benzoxazine resins, allyl resins, aniline resins, modified polyphenylene ether resins, thermosetting polyimide resins, maleimide resins, bismaleimide triazine resins, liquid crystalline polyester resins, vinyl ester resins, cyanate ester resins, and precursors of these resins before curing. Examples of room-temperature curing resins include adhesives that can be cured at room temperature by adding a catalyst, such as epoxy adhesives, silicone adhesives, and acrylic adhesives. Examples of thermoplastic resins that are preferably used include polyamide resins such as PA6, PA66, and PA12, polyphenylene sulfide resins, liquid crystalline polymers (LCPs), polystyrene resins, and polyphenylene oxide resins. Further examples of thermoplastic resins include polyolefin resins such as polypropylene and polyethylene, polycarbonate resins, polyether ether ketone resins, polyether ketone ketone resins, polyimide resins, polyamide imide resins, polyether imide resins, polyvinyl chloride resins, poly(meth)acrylate resins, polyvinylidene fluoride resins, acrylonitrile-butadiene-styrene copolymer (ABS) resins, acrylonitrile-styrene copolymer (AS) resins, polyphenylene ether resins, polyester resins, polytetrafluoroethylene resins, and thermoplastic elastomers. Thermoplastic elastomers generally exhibit rubber elasticity at room temperature (25°C) and are plasticized and moldable at high temperatures. Examples of thermoplastic elastomers include urethane elastomers, styrene elastomers, olefin elastomers, amide elastomers, and ester elastomers.In applications requiring a low dielectric loss tangent, a thermosetting resin or a thermoplastic resin is preferably used as the matrix resin, and among these, thermosetting resins such as epoxy-based resins, thermosetting polyimide-based resins, modified polyphenylene ether-based resins, silicon-based resins, benzoxazine-based resins, and melamine-based resins are preferably used.

[0100] In the resin composition of the present disclosure, the content of the matrix resin is not particularly limited, but is typically preferably 40 to 95% by mass relative to 100% by mass of the total solids content of the resin composition. The lower limit is more preferably 50% by mass or more, even more preferably 70% by mass or more, and may even be 85% by mass or more. The upper limit is more preferably 90% by mass or less. When the content of the matrix resin is equal to or greater than the lower limit, the resin composition has excellent moldability and can exhibit improved mechanical strength when molded. When the content of the matrix resin is equal to or less than the upper limit, hollow particles can be sufficiently incorporated, resulting in excellent effects such as a low dielectric tangent, a low dielectric constant, and weight reduction of the resin composition due to the hollow particles. The matrix resins can be used alone or in combination of two or more.

[0101] [Hollow Particles] The hollow particles contained in the resin composition of the present disclosure are the hollow particles of the present disclosure described above. In the resin composition of the present disclosure, the content of the hollow particles is not particularly limited, but 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 30% by mass or less, and may be 20% by mass or less, or even 15% by mass or less, based on 100% by mass of the total solids content of the resin composition. When the content of the hollow particles is equal to or greater than the lower limit, the hollow particles have excellent effects such as lowering the dielectric loss tangent, lowering the dielectric constant, and reducing the weight of the resin composition. When the content of the hollow particles is equal to or less than the upper limit, the resin can be sufficiently contained in the resin composition, thereby suppressing deterioration in physical properties when formed into a molded product and improving mechanical strength.

[0102] [Solvent] The resin composition of the present disclosure may further contain a solvent for dissolving or dispersing each component. Known solvents can be used and are appropriately selected depending on the type of matrix resin.

[0103] [Other Additives] The resin composition of the present disclosure may further contain additives such as curing agents, curing accelerators, polymerization initiators, UV absorbers, colorants, heat stabilizers, and fillers, as necessary, within the scope that does not impair the effects of the present disclosure. The resin composition of the present disclosure may also contain organic or inorganic reinforcing fibers such as carbon fibers, glass fibers, aramid fibers, polyethylene fibers, cellulose nanofibers, and liquid crystal polymer (LCP) fibers. The reinforcing fibers contained in the resin composition of the present disclosure may be those used as a substrate for a prepreg, or may be contained as a filler.

[0104] As described above, the hollow particles contained in the resin composition of the present disclosure are resistant to the penetration of matrix resins or solvents into their interiors. Therefore, in the resin composition of the present disclosure, the period during which all hollow particles can maintain voids without the penetration of matrix resins or solvents into the interiors of the hollow particles can be set to 10 days or more, and in a more preferred embodiment, 30 days or more. For example, when a resin composition is centrifuged, hollow particles that float to the top of the resin composition can be determined to be free of the penetration of either matrix resins or solvents. Here, the matrix resin and solvent contained in the resin composition are as described above, and the matrix resin may be a resin commonly used in electronic materials, such as an epoxy resin, a thermosetting polyimide resin, or a modified polyphenylene ether resin.

[0105] 4. Molded Article The molded article of the present disclosure includes the hollow particles of the present disclosure and a solidified matrix resin. The hollow particles impart properties such as a low dielectric loss tangent, a low dielectric constant, and reduced weight. The molded article of the present disclosure may be a molded article made from the resin composition of the present disclosure, or may be a molded article made by melt-kneading the hollow particles of the present disclosure with a thermoplastic resin. In the present disclosure, the solidified matrix resin refers to a resin obtained by solidifying the matrix resin with or without a chemical reaction, such as a resin cured by a curing reaction, a resin solidified by drying, or a thermoplastic resin solidified by cooling. The molded article obtained using the resin composition of the present disclosure may contain a cured matrix resin obtained by curing using a curing agent, a polymerization initiator, a catalyst, or the like, as needed. In this case, the solidified matrix resin may contain a curing agent or the like. A molded article obtained by melt-kneading the hollow particles of the present disclosure with a thermoplastic resin and molding the mixture contains a solidified product of the thermoplastic resin that has been cooled and solidified as a solidified matrix resin.

[0106] The molded article of the resin composition of the present disclosure described above can be obtained, for example, by curing the curable resin described above. The curing method for the curable resin is not particularly limited, but examples include heating, irradiation with ultraviolet light or electron beams, etc. Alternatively, in the case of a curable resin that cures at room temperature with the addition of a catalyst or the like, curing may be achieved by adding a catalyst or the like and mixing at room temperature. The curing agent (crosslinking agent) or catalyst for curing the resin can be appropriately selected from known agents depending on the type of resin and is not particularly limited. Examples of curing agents for epoxy resins include amines, acid anhydrides, imidazoles, thiols, phenols, naphthols, benzoxazines, cyanate esters, and carbodiimides. The content of the curing agent is adjusted appropriately depending on the type of resin and is not particularly limited, but may be, for example, 5 to 120 parts by mass per 100 parts by mass of the resin (main component).

[0107] When curing the resin composition, for example, the resin composition may be applied to a support, dried as necessary, and then cured under curing conditions. Examples of materials for the support include resins such as polyethylene terephthalate and polyethylene naphthalate; and metals such as copper, aluminum, nickel, chromium, gold, and silver. The surface of the support may be coated with a release agent. Known methods for applying the resin composition can be used, including dip coating, roll coating, curtain coating, die coating, slit coating, and gravure coating. When the resin composition contains a solvent, it is preferable to dry the resin composition of the present disclosure after the application. The drying temperature is preferably set to a temperature at which the resin composition does not harden, from the viewpoint of removing the solvent while leaving the resin composition in an uncured or semi-cured state. The drying temperature is typically 20°C or higher and 200°C or lower, preferably 30°C or higher and 150°C or lower. The drying time is typically 30 seconds to 1 hour, preferably 1 minute to 30 minutes. The temperature for curing the resin composition is adjusted appropriately depending on the type of matrix resin and is not particularly limited. In the case of a thermosetting resin, the curing temperature is usually 30° C. or higher and 400° C. or lower, preferably 70° C. or higher and 300° C. or lower, and more preferably 100° C. or higher and 200° C. or lower. The heating method is not particularly limited, and may be performed using, for example, an electric oven. The curing time is usually 5 minutes to 5 hours, and preferably 30 minutes to 3 hours.

[0108] When the resin composition of the present disclosure is a prepreg, a molded article can be obtained by curing the prepreg. The preferred conditions for curing the prepreg are the same as the preferred conditions for curing the resin composition described above.

[0109] In the molded article of the present disclosure obtained by melt-kneading the hollow particles of the present disclosure with a thermoplastic resin and molding the mixture, any known thermoplastic resin can be used, and is not particularly limited. Examples include the same thermoplastic resins as those usable in the resin composition of the present disclosure described above. The molded article of the present disclosure containing a thermoplastic resin may further contain an additive, as needed. The additives usable in the resin composition of the present disclosure described above can be used in the same manner.

[0110] The temperature during the melt-kneading is not particularly limited as long as it can melt the thermoplastic resin used, but is preferably 250°C or less in order to prevent the hollow particles from being crushed. The kneading can be carried out by a known method, and is not particularly limited, but can be carried out using a kneading device such as a single-screw kneader or a twin-screw kneader. The molding method is not particularly limited, and can be, for example, a known molding method such as extrusion molding, injection molding, or press molding.

[0111] In the molded body of the present disclosure, the content of the solidified matrix resin in 100% by mass of the molded body is not particularly limited, but from the viewpoint of suppressing deterioration in the physical properties of the molded body and improving the mechanical strength, it is preferably 70% by mass or more, more preferably 80% by mass or more, and from the viewpoint of sufficiently containing hollow particles, it is preferably 95% by mass or less, more preferably 90% by mass or less.

[0112] The content of hollow particles contained in the molded body of the present disclosure is not particularly limited, but is preferably 5% by mass or more, more preferably 10% by mass or more, and preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 15% by mass or less, based on 100% by mass of the molded body. On the other hand, the content of hollow particles is preferably 10% by mass or more, more preferably 20% by mass or more, and preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or less, based on 100% by volume of the molded body. When the content of hollow particles is equal to or greater than the lower limit, the hollow particles can provide excellent effects such as a low dielectric loss tangent, a low dielectric constant, and a lightweight body. When the content of hollow particles is equal to or less than the upper limit, the molded body can be sufficiently filled with the matrix resin, thereby suppressing deterioration in the physical properties of the molded body and improving its mechanical strength.

[0113] The shape of the molded body is not particularly limited and may be any moldable shape, for example, a sheet, a film, or a plate. When the molded body contains fibers, the fibers in the molded body may be in the form of a nonwoven fabric, or the molded body may be a fiber-reinforced plastic to which the hollow particles of the present disclosure have been added.

[0114] The molded article of the present disclosure preferably has a dielectric loss tangent at a frequency of 10 GHz of 5.00 × 10 －３ or less, more preferably 4.50 × 10 －３ More preferably, 4.30 x 10 －３ The lower limit of the dielectric loss tangent is not particularly limited, and is, for example, 1.00 × 10 －３ The molded article of the present disclosure has a relative dielectric constant at a frequency of 10 GHz of preferably 2.50 or less, more preferably 2.30 or less, and even more preferably 2.20 or less. The lower limit of the relative dielectric constant is not particularly limited, and may be, for example, 1.00 or more. In the present disclosure, the relative dielectric constant and dielectric loss tangent of the molded article are measured using a perturbation type measuring device.

[0115] Examples of uses of the resin composition of the present disclosure and the molded article of the present disclosure include uses in which the resin composition or the molded article can be used, among the uses of the hollow particles of the present disclosure described above.

[0116] 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.

[0117] Example 1 1. Preparation of Hollow Particles (1) Mixture Preparation Step First, the following materials were mixed to form an oil phase: 31.4 parts divinylbenzene, 7.3 parts ethylvinylbenzene, 0.89 parts t-butylperoxydiethyl acetate, 61.3 parts hydrophobic solvent (heptane). Next, in a stirring vessel, an aqueous solution prepared by dissolving 19.6 parts magnesium chloride (a water-soluble polyvalent metal salt) in 211 parts ion-exchanged water was gradually added with stirring to prepare a magnesium hydroxide colloid (poorly water-soluble metal hydroxide colloid) dispersion (10 parts magnesium hydroxide), which served as the aqueous phase. The resulting aqueous phase and oil phase were mixed to prepare a mixture.

[0118] (2) Suspension Step The mixture obtained in the mixture preparation step was suspended by stirring for 1 minute using an emulsifying disperser (manufactured by Primix Corporation, product name: Homomixer) at a rotation speed of 4,000 rpm to prepare a suspension in which droplets of the shell raw material composition encapsulating the hydrophobic solvent were dispersed in water.

[0119] (3) Polymerization step: The suspension obtained in the suspension step was heated to 80° C. in a nitrogen atmosphere and stirred for 24 hours at a temperature of 80° C. to carry out a polymerization reaction. Through this polymerization reaction, a precursor composition was obtained, which was a slurry liquid in which precursor particles encapsulating a hydrophobic solvent were dispersed in water.

[0120] (4) Washing Step: At room temperature (25°C), diluted sulfuric acid was added to the precursor composition obtained in the polymerization step to adjust the pH to 5.5 or less, and the mixture was stirred for 10 minutes, followed by filtration and dehydration. Next, while still at room temperature (25°C), the precursor particles were washed with ion-exchanged water, filtered, and dehydrated. This series of steps was repeated several times.

[0121] (5) Solvent Removal Step The precursor particles obtained in the washing step were pre-dried by drying in a dryer at a temperature of 40° C. Next, the precursor particles were heat-treated in a vacuum dryer at 200° C. for 12 hours under vacuum conditions to remove the hydrophobic solvent contained in the particles, thereby obtaining hollow particles whose hollow portions were filled with air.

[0122] (6) Sieving Step The hollow particles obtained in the solvent removal step were sieved using a wire mesh with a mesh size of 45 μm, and the hollow particles that fell through the wire mesh were collected to obtain the hollow particles of Example 1.

[0123] 2. Preparation of Hollow Particle-Containing PPE Film 20 parts of a 65% toluene solution of polyphenylene ether (PPE) (manufactured by Mitsubishi Gas Chemical Company, Inc., product name: OPE-2200) was weighed into a cup, and 0.15 parts of hollow particles, Percumyl (registered trademark) D (manufactured by NOF Corporation), were added. The mixture was uniformly dispersed in a planetary stirring degassing device (manufactured by Kurabo Industries Co., Ltd., product name: Mazerustar) to obtain a hollow particle-containing resin composition. The amount of hollow particles added was adjusted so that the hollow particle content in the hollow particle-containing PPE film was 30% by volume. Aluminum foil was attached to a glass plate without wrinkles, and the obtained resin composition was coated on the aluminum foil using a bar coater No. 75 to form a coating film. The coating film was cured by heating under a nitrogen atmosphere at 80°C for 1 hour, 120°C for 30 minutes, and 160°C for 1 hour, and a hollow particle-containing PPE film was formed on the aluminum foil. The laminate of the film and aluminum foil was immersed in a 1N hydrochloric acid solution overnight to remove the aluminum foil, and the film alone was obtained. The obtained film was washed with ion-exchanged water and dried to obtain a hollow particle-containing PPE film.

[0124] 3. Preparation of hollow particle-containing double-sided copper-clad laminate (1) Preparation of hollow particle-containing resin composition 90 parts of a brominated epoxy resin (manufactured by Tohto Kasei Co., Ltd., product name: YDB-500EK75, epoxy equivalent: 500, solid content: 75% by mass) and 10 parts of a cresol novolac epoxy resin (manufactured by Tohto Kasei Co., Ltd., product name: YDCN220EK75, epoxy equivalent: 210, solid content: 75% by mass) were dissolved in a mixed solvent (room temperature) of 20 parts of dimethylformamide (DMF) and 6 parts of methyl ethyl ketone (MEK), and 2 parts of dicyandiamide (DICY) (manufactured by Nippon Carbide Corporation) and 0.1 parts of 2-ethyl-4-methylimidazole (product name: Curesol (registered trademark) 2E4MZ, manufactured by Shikoku Chemicals Corporation) were added thereto, and the mixture was stirred and mixed to prepare a resin varnish. Next, 95 parts of the resin varnish cooled to room temperature and 5 parts of the hollow particles were mixed by stirring using a disper at 3,000 rpm for 30 minutes to obtain a hollow particle-containing resin composition.

[0125] (2) Preparation of hollow particle-containing prepreg The obtained hollow particle-containing resin composition was impregnated into a glass cloth (manufactured by Nitto Boseki Co., Ltd., product name: WEA116E), and then the glass cloth was heated and dried at 150 to 170°C for 3 to 10 minutes to remove the solvent, thereby obtaining a hollow particle-containing prepreg.

[0126] (3) Preparation of hollow particle-containing double-sided copper-clad laminates. Copper foils with a thickness of 35 μm were placed on both sides of one of the obtained hollow particle-containing prepregs, and the laminates were heated at 180° C. for 2 hours under a pressure of 2.94 MPa (30 kg / cm ２ ) under the curing conditions of heating and pressing to obtain a hollow particle-containing double-sided copper-clad laminate having a thickness of 0.13 mm.

[0127] [Examples 2 to 8, Comparative Examples 1, 5, and 6] The hollow particles, hollow particle-containing PPE films, and hollow particle-containing double-sided copper-clad laminates of Examples 2 to 8 and Comparative Examples 1, 5, and 6 were produced in the same manner as in Example 1, except that in Example 1, the type and amount of shell raw material added to the oil phase in the above-mentioned "(1) mixed solution preparation step" were changed according to Table 1 or Table 2, and in Example 8, the amounts of hydrophobic solvent and polymerization initiator added were further changed according to Table 1.

[0128] Comparative Example 2 Hollow particles of Comparative Example 2 were produced in the same manner as in Example 1, except that in the above "(1) mixed solution preparation step", the types and amounts of the shell raw materials and polymerization initiator added to the oil phase were changed according to Table 2, and further, in the above "(3) polymerization step", the reaction temperature was changed from 80°C to 40°C and the reaction time was changed from 24 hours to 4 hours.

[0129] Comparative Example 3 The hollow particles of Comparative Example 3 were produced in the same manner as in Example 1, except that in the above "(1) mixed solution preparation step", the types and amounts of the shell raw materials and polymerization initiator added to the oil phase, and the amount of the hydrophobic solvent were changed according to Table 2, and further, in the above "(3) polymerization step", the reaction temperature was changed from 80°C to 65°C, and the reaction time was changed from 24 hours to 4 hours.

[0130] [Comparative Example 4] Hollow particles of Comparative Example 4 were produced as hollow particles corresponding to Patent Document 2 (WO 2021 / 085189). First, the following materials were mixed to form an oil phase. Divinylbenzene 17.5 parts Ethylvinylbenzene 4.1 parts Styrene 13.5 parts Polystyrene (weight average molecular weight 300,000) 2.1 parts Blemmer (registered trademark) 50PEP-300 (product name, polyethylene glycol propylene glycol monomethacrylate, manufactured by NOF Corporation) 3.5 parts t-Butyl peroxydiethyl acetate 1.16 parts Hydrophobic solvent (heptane) 28.1 parts Next, 34 parts of ion-exchanged water and 0.017 parts of Rapisol A-80 (surfactant, manufactured by NOF Corporation) were mixed to prepare an aqueous phase. The oil phase was added to the aqueous phase, and a suspension was prepared using an ultrasonic homogenizer. The resulting suspension was heated at 70 ° C. for 4 hours to polymerize, yielding a slurry. The obtained slurry was heated at 100° C. for 24 hours to obtain hollow particles of Comparative Example 4.

[0131] In addition, the percentage of the number of particles having only one hollow portion among the hollow particles obtained in each Example was 90% or more.

[0132] [Evaluation] The hollow particles, hollow particle-containing PPE films, and hollow particle-containing double-sided copper-clad laminates obtained in each Example and Comparative Example were subjected to the following measurements and evaluations. The results are shown in Tables 1 and 2.

[0133] <Measurement or Evaluation of Hollow Particles> 1. Particle Size and Particle Size Distribution The volume average particle size (Dv) and number average particle size (Dn) of hollow particles were 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 the beaker 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. The particle size distribution (Dv / Dn) was calculated from the measured volume average particle size (Dv) and number average particle size (Dn). Furthermore, the volumetric percentage (volume %) of particles having a particle size of less than 1.0 μm and the volumetric percentage (volume %) of particles having a particle size of more than 10.0 μm were calculated.

[0134] 2. Porosity 2-1. Measurement of apparent density of hollow particles First, a volume of 100 cm ３ About 30 cm ３ The volumetric flask was filled with hollow particles, and the mass of the filled hollow particles was accurately weighed. Next, the volumetric flask filled 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 volumetric flask was accurately weighed, and the apparent density D of the hollow particles was calculated based on the following formula (I): １ (g / cm ３ The apparent density D was calculated using the formula (I). １ = [Mass of hollow particles] / (100 - [Mass of isopropanol] / [Specific gravity of isopropanol at measurement temperature])

[0135] 2-2. Measurement of true density of hollow particles After crushing the hollow particles in advance, ３Approximately 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. The true density D of the hollow particles was calculated based on the following formula (II): ０ (g / cm ３ ) was calculated. ０ = [Mass of crushed hollow particle fragments] / (100 - [Mass of isopropanol] / [Specific gravity of isopropanol at measurement temperature])

[0136] 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 following formula (III): Formula (III) Porosity (%) = 100 - (apparent density D １ / True density D ０ ) x 100

[0137] 3. Residual Double Bond Ratio Using an ATR-IR (manufactured by PERKIN ELMER, product name: Spectrum One), infrared absorption spectra, displayed as absorbance, were measured for the polymer contained in the hollow particles and the shell raw material. Measurement of the shell raw material was performed by placing 0.1 g of the shell raw material used to prepare the hollow particles on the top of a cell and bringing the measurement sample into contact with a crystal. Measurement of the polymer contained in the hollow particles was performed by placing 0.1 g of hollow particles on the top of a cell and tightening the compression bearing from above to bring the crystal into contact with the hollow particles. From the obtained infrared absorption spectrum, the residual double bond ratio was calculated using the method described above.

[0138] For example, in Example 1, 31.4 parts (81% by mass) of divinylbenzene and 7.3 parts (19% by mass) of ethylvinylbenzene were used as the shell raw materials. The monomer unit derived from the highest content of divinylbenzene (81% by mass) was used as the reference monomer unit, and the C-H bond of the benzene ring contained in divinylbenzene was selected as the structure that does not increase or decrease before or after the polymerization reaction. In each of the infrared absorption spectrum of the shell raw material and the infrared absorption spectrum of the polymer, the peak derived from the C-H bond of the benzene ring contained in divinylbenzene was identified as the reference peak. Since the peak derived from the C-H bond of the benzene ring contained in divinylbenzene and the peak derived from the C-H bond of the benzene ring contained in ethylvinylbenzene appear at the same position, the peak intensity of the reference peak was determined by dividing the peak intensity by the sum (1.00) of the content percentage of divinylbenzene and the content percentage of ethylvinylbenzene. The reference peak intensity obtained from the infrared absorption spectrum of the shell raw material was 0.167, and the reference peak intensity obtained from the infrared absorption spectrum of the polymer was 0.0869. Next, in each infrared absorption spectrum, the peak intensities of the peaks of the polymerizable unsaturated double bonds (C=C) derived from divinylbenzene and ethylvinylbenzene were measured. The peak intensity of the peak derived from the polymerizable unsaturated double bond (C=C) obtained from the infrared absorption spectrum of the shell raw material was 0.0725, and the peak intensity of the peak derived from the polymerizable unsaturated double bond (C=C) obtained from the infrared absorption spectrum of the polymer was 0.00494. Therefore, the reference peak intensity (M ０ ) relative to the peak intensity (M １ ) ratio (M １ / M ０ The reference peak intensity (P) calculated from the infrared absorption spectrum of the polymer was 0.0725 / 0.167, which was 0.434. ０ ) relative to the peak intensity (P １ ) ratio (P １ / P ０) was calculated to be 0.00494 / 0.0869, which was 0.0568. The peak intensity ratio (M １ / M ０ ) and the peak intensity ratio (P １ / P ０ ) was applied to the following formula (A), and the residual double bond ratio was calculated to be 13.1% from the calculation formula (0.0568 / 0.434) × 100. Formula (A): Residual double bond ratio (%) = {(P １ / P ０ ) / (M １ / M ０ ) x 100

[0139] In Examples 2, 5, and 8 and Comparative Examples 1 to 3, the residual double bond ratio was determined using the same method as in Example 1. In Examples 3 and 4, in addition to divinylbenzene and ethylvinylbenzene, styrene was also used as the shell raw material. The peak derived from the C-H bond in the benzene ring contained in divinylbenzene, the peak derived from the C-H bond in the benzene ring contained in ethylvinylbenzene, and the peak derived from the C-H bond in the benzene ring contained in styrene appear at the same position. Therefore, when determining the peak intensity of the reference peak in each infrared absorption spectrum, the value obtained by dividing the reference peak by the sum of the content of divinylbenzene, the content of ethylvinylbenzene, and the content of styrene (1.00 in both Examples 3 and 4) was used as the reference peak intensity. Furthermore, in each infrared absorption spectrum, the peak intensity of the peak derived from the polymerizable unsaturated double bond (C═C) was measured by measuring the peak intensity of the polymerizable unsaturated double bond (C═C) derived from divinylbenzene, ethylvinylbenzene, and styrene. Otherwise, the residual double bond ratio was determined using the same method as in Example 1. In Example 6, in addition to divinylbenzene and ethylvinylbenzene, polystyrene was also used as the shell raw material. The peak derived from the C-H bond of the benzene ring contained in divinylbenzene, the peak derived from the C-H bond of the benzene ring contained in ethylvinylbenzene, and the peak derived from the C-H bond of the benzene ring contained in polystyrene appear at the same position. Therefore, when determining the peak intensity of the reference peak in each infrared absorption spectrum, the value obtained by dividing the reference peak by the sum (1.00) of the content percentage of divinylbenzene, the content percentage of ethylvinylbenzene, and the content percentage of polystyrene was used as the reference peak intensity. The residual double bond ratio was otherwise determined by the same method as in Example 1. In Example 7, in addition to divinylbenzene and ethylvinylbenzene, SIS was also used as the shell raw material.The peaks derived from the C-H bonds of the benzene rings contained in divinylbenzene, ethylvinylbenzene, and SIS appear at the same position. Therefore, when determining the peak intensity of the reference peak in each infrared absorption spectrum, the reference peak was divided by the sum (0.98) of the divinylbenzene content, the ethylvinylbenzene content, and the styrene unit content in SIS. Furthermore, in each infrared absorption spectrum, the peak intensities of the peaks derived from the polymerizable unsaturated double bonds (C=C) of divinylbenzene, ethylvinylbenzene, and SIS were measured. The residual double bond ratio was determined in the same manner as in Example 1. In Comparative Example 4, in addition to divinylbenzene and ethylvinylbenzene, styrene, polystyrene (weight average molecular weight 300,000), and Blemmer (registered trademark) 50PEP-300 (product name, polyethylene glycol propylene glycol monomethacrylate, manufactured by NOF Corporation) were used as shell raw materials. Therefore, when determining the peak intensity of the reference peak in each infrared absorption spectrum, the reference peak intensity was determined by dividing the reference peak by the sum (0.91) of the content ratio of divinylbenzene, the content ratio of ethylvinylbenzene, the content ratio of styrene, and the content ratio of polystyrene. Furthermore, in each infrared absorption spectrum, the peak intensity of the peak derived from the polymerizable unsaturated double bond (C=C) derived from divinylbenzene, ethylvinylbenzene, styrene, and polyethylene glycol propylene glycol monomethacrylate was measured. Otherwise, the residual double bond ratio was determined by the same method as in Example 1.

[0140] In Comparative Examples 5 and 6, in addition to divinylbenzene and ethylvinylbenzene, styrene was also used as the shell raw material. The monomer unit derived from the styrene with the highest content was used as the reference monomer unit, and the C-H bond of the benzene ring contained in styrene was selected as the structure that does not increase or decrease before or after the polymerization reaction. In each of the infrared absorption spectra of the shell raw material and the polymer, the peak derived from the C-H bond of the benzene ring contained in styrene was identified as the reference peak. When determining the peak intensity of the reference peak in each infrared absorption spectrum, the value obtained by dividing the value by the sum of the content percentage of divinylbenzene, the content percentage of ethylvinylbenzene, and the content percentage of styrene (1.00 for both Comparative Examples 5 and 6) was used as the reference peak intensity. Furthermore, in each infrared absorption spectrum, the peak intensity of the peak derived from the polymerizable unsaturated double bond (C=C) was measured by measuring the peak intensity of the polymerizable unsaturated double bond (C=C) derived from divinylbenzene, ethylvinylbenzene, and styrene. Otherwise, the residual double bond ratio was determined using the same method as in Example 1.

[0141] 4. Dielectric Constant (Dk) and Dielectric Loss Tangent (Df) The dielectric constant and dielectric loss tangent of the hollow particles were measured at a frequency of 10 GHz and room temperature (25°C) using a perturbation type measuring device (manufactured by AET Co., Ltd., model: ADMS01Nc).

[0142] 5. Performance Stability A HAST test was carried out on the hollow particles using a highly accelerated life tester (manufactured by Espec Corporation) at 130°C, 85% RH, and 120 hours, and the dielectric loss tangent (Df) after the HAST test was measured in the same manner as above. The dielectric loss tangent of the hollow particles before the HAST test was Df. １ and the dielectric loss tangent of the hollow particles after the HAST test is Df ２ When Df １ and Df ２ The difference δ (δ = Df ２ -Df １ The performance stability of the hollow particles was evaluated based on the above criteria. It is estimated that the larger δ is, the more advanced the oxidative degradation is. (Performance Stability Evaluation Criteria) A: δ is less than 0.001 B: δ is 0.001 or more and less than 0.005 C: δ is 0.005 or more and less than 0.01 D: δ is 0.01 or more

[0143] 6. Circularity A measurement sample was prepared by dispersing 0.10 to 0.12 g of hollow particles in an aqueous solution of linear alkylbenzenesulfonate (0.3% concentration) for 5 minutes in an ultrasonic cleaner. Using a flow-type particle image analyzer (manufactured by Jasco International Co., Ltd., product name: IF-3200), the circularity of each particle contained in the measurement sample was measured under the following measurement conditions. The mass ratio of particles with a circularity of 0.85 or less was calculated and evaluated according to the following evaluation criteria. The number of hollow particles contained in the measurement sample increased with decreasing particle diameter, but was within the range of 1,000 to 3,000 for all examples and comparative examples. (Measurement conditions) Flow cell spacer thickness: 50 μm Telecentric zoom lens magnification: 4.5x Total magnification: 9.0x Measurement volume: 0.5 mL Image resolution: 0.185 μm / pixel Detection algorithm: Ghost detection Threshold: 15% (Circularity evaluation criteria) A: Particles with a circularity of 0.85 or less are 3% by mass or less B: Particles with a circularity of 0.85 or less are more than 3% by mass and less than 5% by mass C: Particles with a circularity of 0.85 or less are more than 5% by mass and less than 10% by mass D: Particles with a circularity of 0.85 or less are more than 10% by mass

[0144] 7. Varnish Penetration 70 parts of epoxy resin jER (registered trademark) 828 (manufactured by Mitsubishi Chemical Corporation) and 25 parts of MEK (methyl ethyl ketone) were added to 8.4 parts of hollow particles and uniformly dispersed using a planetary stirring degassing device (manufactured by Kurabo Industries, Ltd., product name: Mazerustar) to obtain a hollow particle-containing resin composition (hollow particle-containing varnish). This was stirred in a shaker at 150 rpm. After shaking, 1.5 ml of the hollow particle-containing resin composition was sampled every 24 hours and centrifuged. The permeability was evaluated by the number of days until all hollow particles had risen to the top of the resin composition and no sediment was present. Note that when the hollow particle-containing resin composition was centrifuged, hollow particles floating to the top of the resin composition indicated that neither the epoxy resin nor MEK had penetrated into the interior of the resin composition. (Evaluation criteria for varnish penetration) A: Hollow particles float to the surface for 30 days or more B: Hollow particles float to the surface for 10 days or more but less than 30 days C: Hollow particles float to the surface for less than 10 days

[0145] <Measurement or Evaluation of Hollow Particle-Containing PPE Film> 8. Dielectric Constant (Dk) and Dielectric Loss Tangent (Df) The dielectric constant and dielectric loss tangent of the hollow particle-containing PPE film were measured at a frequency of 10 GHz and room temperature (25°C) using a perturbation type measuring device (manufactured by AET Co., Ltd., model: ADMS01Nc).

[0146] 9. Particle Dispersibility The hollow particle-containing PPE film was cut in the thickness direction using a cutter, and the cross section was observed using an SEM. 100 randomly selected hollow particles were checked for the presence or absence of particle aggregation, and the dispersibility of the hollow particles in the PPE film was evaluated based on the proportion of particles present alone. (Particle Dispersibility Evaluation Criteria) A: All particles are present alone in the film. B: The proportion of particles present alone in the film is 50% or more but less than 100%. C: The proportion of particles present alone in the film is more than 10% but less than 50%. D: The proportion of particles present alone in the film is 10% or less.

[0147] <Evaluation of hollow particle-containing double-sided copper-clad laminates> 10. Performance stability A HAST test was performed on the obtained double-sided copper-clad laminates using a highly accelerated life tester (manufactured by Espec Corporation) at 110°C, 85% RH, and 100 hours. After the HAST test, a voltage of 50 V was applied to the double-sided copper-clad laminate for a predetermined period of time, and the resistance value was measured. The presence or absence of abnormalities was confirmed and evaluated according to the following evaluation criteria. A change in resistance value was considered to be abnormal. The change in resistance value is believed to be due to ion migration. (Performance stability evaluation criteria) A: No abnormality even after 400 hours of voltage application. B: No abnormality even after 200 hours of voltage application, but abnormality occurs before the voltage application time reaches 400 hours. C: No abnormality even after 100 hours of voltage application, but abnormality occurs before the voltage application time reaches 200 hours. D: Abnormality occurs before the voltage application time reaches 100 hours.

[0148] [Reference Example 1] A PPE film containing no hollow particles was obtained in the same manner as in "2. Preparation of hollow particle-containing PPE film" in Example 1, except that hollow particles were not added. The dielectric constant (Dk) and dielectric loss tangent (Df) of the obtained PPE film were measured in the same manner as for the hollow particle-containing PPE film.

[0149]

[0150]

[0151] Tables 1 and 2 show the amount (parts by mass) of each material added and the results of each measurement or evaluation. In Tables 1 and 2, the dielectric loss tangent values ​​are expressed in exponential notation as specified in JIS X 0210 for simplification. For example, "3.64 x 10 －４ " is expressed as "3.64E-04". In Tables 1 and 2, the meanings of the abbreviations are as follows. DVB: divinylbenzene EVB: ethylvinylbenzene St: styrene PS: polystyrene (weight average molecular weight: 300,000) SIS: styrene-isoprene-styrene block copolymer (manufactured by Zeon Corporation, product name: Quintac (registered trademark), styrene content: 24 mass%, weight average molecular weight Mw: 106,000) 50PEP-300: polyethylene glycol propylene glycol monomethacrylate (manufactured by NOF Corporation, product name: Blenmar (registered trademark) 50PEP-300) A-80: Rapisol A-80 (surfactant, manufactured by NOF Corporation) The polystyrene (PS) used in the examples does not have a polymerizable unsaturated double bond. The styrene-isoprene-styrene block copolymer (SIS) used in the examples has not been subjected to a hydrogenation reaction and contains a polymerizable unsaturated double bond.

[0152] [Discussion] In Comparative Example 1, a shell raw material containing less than 15% by mass of non-crosslinkable monomer units was used. Therefore, even though the polymerization reaction conditions for forming the shell were the same as those in the Examples, the resulting hollow particles had a high residual double bond ratio, a high dielectric dissipation factor, and poor performance stability. Furthermore, the hollow particles obtained in Comparative Example 1 were inferior in their effect of lowering the dielectric dissipation factor of PPE films, and the hollow particle-containing double-sided copper-clad laminate also had poor performance stability. Comparative Example 2 is similar to Comparative Example 3 in Patent Document 3. In Comparative Example 2, a shell raw material containing 15% to 60% by mass of non-crosslinkable monomer units was used. However, the polymerization reaction for forming the shell was performed at a lower temperature and for a shorter time than in the Examples. As a result, the resulting hollow particles had a high residual double bond ratio, low porosity due to the presence of depressions, high relative dielectric constant and dielectric dissipation factor, and poor performance stability. The hollow particles obtained in Comparative Example 2 were inferior in their effect of lowering the dielectric loss tangent and dielectric constant of PPE film, and the hollow particle-containing double-sided copper-clad laminate also had poor performance stability. Furthermore, the hollow particles obtained in Comparative Example 2 contained a high proportion of irregularly shaped particles with low circularity, resulting in poor dispersibility in the resin composition. Furthermore, the hollow particles obtained in Comparative Example 2 were prone to the penetration of the matrix resin or solvent into the interior of the hollow particles in the varnish. This is presumably due to the low temperature and short time of the polymerization reaction, which resulted in many sparsely crosslinked areas and made the shell more susceptible to swelling. In Comparative Example 3, a shell raw material containing less than 15% non-crosslinkable monomer units was used, and the polymerization reaction for shell formation was performed at a lower temperature and for a shorter time than in the Examples. As a result, the resulting hollow particles had a higher residual double bond ratio, a higher dielectric loss tangent, and poorer performance stability than the hollow particles of Comparative Example 1. The hollow particles obtained in Comparative Example 3 were inferior in the effect of lowering the dielectric tangent of the PPE film, and the hollow particle-containing double-sided copper-clad laminate also had poor performance stability. Comparisons between Example 2 and Comparative Example 2, and between Comparative Example 1 and Comparative Example 3, show that when the content of non-crosslinkable monomer units was 15% by mass or more, the increase in the residual double bond ratio when the polymerization reaction was changed to a lower temperature and a shorter time was more significant than when the content was less than 15% by mass.

[0153] Comparative Example 4 is similar to Example 1 of Patent Document 2. In Comparative Example 4, a shell raw material containing 15% to 60% by mass of non-crosslinkable monomer units was used. However, the polymerization reaction for forming the shell was performed at a lower temperature and for a shorter time than in the Examples. As a result, the resulting hollow particles had a high residual double bond ratio, low porosity due to the presence of depressions, high relative dielectric constant and dielectric loss tangent, and poor performance stability. Furthermore, the hollow particles obtained in Comparative Example 4 were inferior in their effect of lowering the dielectric loss tangent and dielectric constant of PPE films, and the hollow particle-containing double-sided copper-clad laminate also had poor performance stability. Furthermore, the hollow particles obtained in Comparative Example 4 contained a high proportion of irregularly shaped particles with low circularity and a small volume average particle size, resulting in particularly poor dispersibility in the resin composition. In the hollow particle double-sided copper-clad laminate of Comparative Example 4, the poor dispersibility of the hollow particles resulted in the presence of hollow particle aggregates, which created gaps at the interface between the hollow particles and the matrix resin, allowing moisture to enter these gaps, presumably resulting in a significant deterioration in performance stability. Furthermore, since a surfactant was used as a dispersion stabilizer in Comparative Example 4, it is believed that the surfactant remained in the resulting hollow particles. It is presumed that the dielectric properties of the hollow particles obtained in Comparative Example 4 were also deteriorated due to the influence of the residual surfactant. It is also presumed that the dielectric properties and performance stability of the hollow particles obtained in Comparative Example 4 were deteriorated due to the relatively high heteroatom content in the shell caused by the use of 50PEP-300 as the shell raw material. In Comparative Examples 5 and 6, shell raw materials were used in which the content of non-crosslinkable monomer units exceeded 60% by mass. Therefore, even when the polymerization reaction conditions for forming the shell were the same as those in the Examples, the resulting hollow particles had low porosity due to the presence of indentations. Furthermore, the crosslinking reaction at the terminals was difficult to proceed, resulting in a high residual double bond ratio. This resulted in high relative dielectric constants and dielectric loss tangents, resulting in poor performance stability. Furthermore, the hollow particles obtained in Comparative Examples 5 and 6 were inferior in their effectiveness in reducing the dielectric loss tangent and dielectric constant of PPE films. Furthermore, the hollow particles obtained in Comparative Examples 5 and 6 contained a high proportion of irregularly shaped particles with low circularity, resulting in poor dispersibility in the resin composition.In the hollow particle double-sided copper-clad laminates of Comparative Examples 5 and 6, the poor dielectric properties of the hollow particles led to poor performance stability. In addition, the poor dispersibility of the hollow particles led to the presence of hollow particle aggregates, creating gaps at the interface between the hollow particles and the matrix resin, which allowed moisture to enter the gaps, further worsening performance stability. Furthermore, the hollow particles obtained in Comparative Examples 5 and 6 were prone to the penetration of the matrix resin or solvent into the interior of the hollow particles in the varnish. This is presumably due to the hollow particles having depressions and many sparse areas due to insufficient crosslinking, which led to the shells being prone to swelling.

[0154] In contrast, in each example, a shell raw material containing 15% to 60% by mass of non-crosslinkable monomer units was used, and the polymerization reaction for forming the shell was carried out at a high temperature and for a long time. As a result, the resulting hollow particles had a low residual double bond ratio, suppressed the occurrence of pitting, a high porosity, a low dielectric constant and dielectric dissipation factor, and excellent performance stability. The hollow particles obtained in each example were also effective in reducing the dielectric dissipation factor and dielectric constant of PPE films, and the hollow particle-containing double-sided copper-clad laminates also had excellent performance stability. In particular, the hollow particles of Examples 2, 4, 5, and 6, which contained 35% or more by mass of non-crosslinkable monomer units and 11.0% or less of residual double bonds, had a low dielectric dissipation factor and excellent performance stability, and the hollow particle-containing double-sided copper-clad laminates also had excellent performance stability. The hollow particles of Example 7 had a relatively high residual double bond ratio due to the use of SIS as the shell raw material, but the content of non-crosslinkable monomer units was 35% by mass or more, and the crosslinking reaction at the polymer terminals had progressed sufficiently, which is thought to have resulted in excellent performance. Furthermore, the hollow particles obtained in each Example had a low proportion of irregularly shaped particles with low circularity, and were therefore excellent in dispersibility in the resin composition. Furthermore, the shells of the hollow particles obtained in each Example were less likely to swell in the varnish, making it difficult for the matrix resin or solvent to penetrate into the interior of the hollow particles.

[0155] REFERENCE SIGNS LIST 1 aqueous medium 2 low-polarity material 3 droplets of shell raw material composition 4a hydrophobic solvent 4b material other than hydrophobic solvent 5 precursor particle 6 shell 7 hollow portion 10 hollow particle with hollow portion filled with gas

Claims

1. A hollow particle comprising a shell containing at least one polymer, and a hollow portion surrounded by the shell, The content of non-crosslinkable monomer units is 15% by mass or more and 60% by mass or less of the total monomer units constituting the total polymer contained in the shell, The dielectric loss tangent at a frequency of 10 GHz is 5.00 × 10⁻⁶. －４ The following are hollow particles.

2. The hollow particle according to claim 1, wherein the residual double bond rate of the total polymer contained in the shell is 15.0% or less.

3. The hollow particle according to claim 1 or 2, wherein the shell comprises a structure derived from a polymer of non-crosslinkable monomers.

4. The hollow particle according to claim 3, wherein the non-crosslinkable monomer polymer is at least one polymer selected from a polymer consisting of aromatic monovinyl monomer units and a polymer consisting of aromatic monovinyl monomer units and diene monomer units.

5. A hollow particle according to claim 1 or 2, wherein the relative permittivity at a frequency of 10 GHz is 1.00 or more and 1.40 or less.

6. The hollow particle according to claim 1 or 2, wherein the entire polymer contained in the shell is a hydrocarbon polymer.

7. A hollow particle according to claim 1 or 2, wherein the porosity is 70% or more.

8. The hollow particle according to claim 1 or 2, wherein the proportion of particles with a circularity of 0.85 or less is 5% by mass or less.

9. A hollow particle according to claim 1 or 2, wherein the volume-average particle size is 1.0 μm or more and 10.0 μm or less.

10. A hollow particle according to claim 1 or 2, wherein the particle size distribution (volume-average particle size (Dv) / number-average particle size (Dn)) is 1.00 or more and 1.40 or less.

11. A resin composition comprising hollow particles according to claim 1 or 2 and a matrix resin.

12. A molded article containing hollow particles according to claim 1 or 2 and a solidified matrix resin.