Hollow particles, method for producing hollow particles, and resin composition

Abstract

Provided are hollow particles capable of enhancing adhesion to a resin. The present invention provides hollow particles each comprising a shell containing a resin and a hollow portion surrounded by the shell, wherein: the shell contains, as the resin, a polymer containing at least 60 mass% of a crosslinkable monomer unit; the hollow particle has a carboxy group on the surface thereof; and the amount of carboxylic acid per unit area is 0.500 μmol / m2 or more, as determined from the acid value of the hollow particle and the specific surface area of the hollow particle by means of equation (A): amount of carboxylic acid (μmol / m2) = acid value (μmol / g) / specific surface area (m2 / g).

Description

Hollow particles, method for producing hollow particles, and resin composition

[0001] The present disclosure relates to hollow particles, a method for producing the same, and a resin composition 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 order to improve the effects of various compositions and molded articles containing hollow particles, such as weight reduction, heat insulation, and low dielectric constant, it is desirable for the hollow particles to maintain a high porosity during mixing with other materials and during molding after mixing. For example, Patent Document 1 discloses hollow particles with a shell containing 30 to 100 parts by mass of a resin containing a crosslinkable monomer unit, as hollow particles with little change in porosity during molding. Patent Document 1 also describes that in order to obtain particles with high heat resistance, the hollow particles may further contain a carboxyl group-containing monomer unit such as (meth)acrylic acid.

[0004] On the other hand, Patent Document 2 discloses a resin composition containing (A) hollow organic polymer particles, (B) an epoxy resin, and (C) a curing agent, for the purpose of improving the smear removal properties of the resin composition.

[0005] International Publication No. 2020 / 066704 Japanese Patent Application Laid-Open No. 2021-130780

[0006] However, conventional resin compositions containing hollow particles have the problem of lower physical properties related to strength against external forces, such as tensile strength and flexural strength, compared to resin compositions that do not contain hollow particles. Furthermore, when conventional resin compositions containing hollow particles are used as insulating resins or sealing resins for electronic components such as electronic circuit boards, there are problems such as insufficient copper foil peel strength due to poor adhesion between the resin layer containing the hollow particles and copper foil, and problems such as a tendency for malfunctions due to moisture absorption to occur in reliability tests such as PCT (pressure cooker test). These problems are thought to be caused by insufficient adhesion between the hollow particles in the resin composition and the resin.

[0007] An object of the present disclosure is to provide hollow particles that can improve adhesion to resins, a method for producing the hollow particles, and a resin composition containing the hollow particles.

[0008] The present inventors have found that when the amount of carboxylic acid per unit area on the surface of hollow particles is a specific amount or more, adhesion to resins can be improved.

[0009] The present disclosure provides hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, wherein the shell contains, as the resin, a polymer containing 60 mass % or more of crosslinkable monomer units, the hollow particles have carboxy groups on their surfaces, and the amount of carboxylic acid per unit area calculated from the acid value of the hollow particles and the specific surface area of ​​the hollow particles by the following formula (A) is 0.500 μmol / m ２ The present invention provides hollow particles having a carboxylic acid amount (μmol / m) expressed by the formula (A): ２ ) = acid value (μmol / g) / specific surface area (m ２ / g)

[0010] In the hollow particles of the present disclosure, it is preferred that the polymer contains a carboxy group-containing monomer unit derived from a carboxy group-introducing monomer containing a radical polymerizable group and a protected carboxy group. Alternatively, it is preferred that the polymer contains a carboxy group-containing monomer unit derived from a carboxy group-containing monomer containing a radical polymerizable group and a carboxy group.

[0011] In the hollow particles of the present disclosure, the carboxy group present on the surface of the hollow particle is preferably a carboxy group contained in the carboxy group-containing monomer unit.

[0012] In the hollow particles of the present disclosure, the content of hydrocarbon monomer units may be more than 50% by mass relative to 100% by mass of all monomer units contained in the polymer, or the content of acrylic monomer units may be more than 50% by mass relative to 100% by mass of all monomer units contained in the polymer.

[0013] In the hollow particles of the present disclosure, the porosity is preferably 50% or more.

[0014] The present disclosure provides a first manufacturing method for producing the hollow particles of the present disclosure, the method comprising the steps of: preparing a mixed solution containing a skeleton-forming monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; suspending the mixed solution to prepare a suspension in which droplets of a monomer composition containing the skeleton-forming monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction, and when a polymerization conversion rate of the skeleton-forming monomer reaches 1% by mass or more and 99% by mass or less, adding a carboxy group-introducing monomer containing a radical polymerizable group and a protected carboxy group to further polymerize the resulting mixture, followed by deprotection to prepare a precursor composition containing precursor particles having a shell containing a polymer of the skeleton-forming monomer and the deprotected carboxy group-introducing monomer and having a carboxy group on an outer surface, and a hollow portion surrounded by the shell, the hollow portion containing the hydrophobic solvent. The method for producing hollow particles is provided, wherein the solubility of the carboxy group-introducing monomer in water at 20°C is greater than that of the hydrophobic solvent and is 0.5 g / L to 1000 g / L, the skeleton-forming monomer includes a crosslinkable monomer, and the content of the crosslinkable monomer is 60 mass% or more relative to 100 mass% in total of the skeleton-forming monomer and the carboxy group-introducing monomer.

[0015] The present disclosure also provides a second method for producing the hollow particles of the present disclosure, the method comprising the steps of: preparing a mixed solution containing a skeleton-forming monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; suspending the mixed solution to prepare a suspension in which droplets of a monomer composition containing the skeleton-forming monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction, and when the polymerization conversion rate of the skeleton-forming monomer reaches 1% by mass or more and 99% by mass or less, adding a carboxy group-containing monomer containing a radical polymerizable group and a carboxy group, and further carrying out a polymerization reaction, thereby preparing a precursor composition containing precursor particles which include a shell that contains a polymer of the skeleton-forming monomer and the carboxy group-containing monomer and has a carboxy group on its outer surface, and a hollow portion surrounded by the shell, the hollow portion containing the hydrophobic solvent. The method for producing hollow particles further provides a method for producing hollow particles, wherein the solubility of the carboxyl group-containing monomer in water at 20°C is greater than that of the hydrophobic solvent and is 0.5 g / L to 1000 g / L, the skeleton-forming monomer includes a crosslinkable monomer, and the content of the crosslinkable monomer is 60 mass% or more relative to 100 mass% in total of the skeleton-forming monomer and the carboxyl group-containing monomer.

[0016] The present disclosure provides a resin composition containing the hollow particles of the present disclosure and a binder resin having a functional group capable of reacting with a carboxy group.

[0017] The present disclosure as described above provides hollow particles that can improve adhesion to resins, and further provides a method for producing the hollow particles and a resin composition containing the hollow particles.

[0018] 1A to 1C are diagrams illustrating an example of a method for producing hollow particles according to the present disclosure.

[0019] 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. 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. In this disclosure, a polymerizable monomer refers to a compound having a functional group capable of addition polymerization (sometimes simply referred to as a polymerizable functional group in this disclosure). In this disclosure, a compound having an ethylenically unsaturated bond as a functional group capable of addition polymerization is generally used as the polymerizable monomer. In this disclosure, a polymerizable monomer having only one polymerizable functional group is referred to as a non-crosslinkable monomer, and a polymerizable monomer having two or more polymerizable functional groups is referred to as a crosslinkable monomer. A crosslinkable monomer is a polymerizable monomer that forms crosslinks in a resin by a polymerization reaction. In addition, in the present disclosure, good dielectric properties means low relative permittivity and dielectric loss tangent, and the lower the relative permittivity and dielectric loss tangent, the better the dielectric properties.

[0020] The hollow particles of the present disclosure are hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, wherein the shell contains, as the resin, a polymer containing 60 mass% or more of crosslinkable monomer units, the hollow particles have carboxy groups on their surfaces, and the amount of carboxylic acid per unit area calculated from the acid value and specific surface area of ​​the hollow particles by the formula (A) described below is 0.500 μmol / m ２ The present invention is characterized in that:

[0021] When the hollow particles of the present disclosure are used in combination with a resin having functional groups reactive with carboxy groups, the carboxy groups on the surface of the hollow particles can react with the functional groups on the resin to form crosslinks, thereby improving the adhesion at the interface between the hollow particles and the resin. Here, the reaction between the carboxy groups on the hollow particles and the functional groups on the resin can be a reaction that forms a covalent bond. Furthermore, when hollow particles of the present disclosure having carboxy groups on their surface are dispersed in a resin having functional groups reactive with carboxy groups, the particles and the resin have similar polarities and high affinity, resulting in good dispersibility of the hollow particles. In a molded product containing hollow particles, if the hollow particles aggregate, pressure is likely to be applied to the aggregated areas when the molded product is pressed, causing the hollow particles to collapse. However, in a molded product obtained by incorporating the hollow particles of the present disclosure in a resin having functional groups, the hollow particles are more likely to be uniformly dispersed and aggregation of the hollow particles is suppressed, making it easier to apply pressure evenly when pressed. Furthermore, in a molded article obtained by incorporating the hollow particles of the present disclosure into a resin having functional groups, the carboxy groups of the hollow particles and the functional groups of the resin are crosslinked to form a three-dimensional crosslinked structure near the particle surface, resulting in improved pressure resistance and reduced crushing of the hollow particles.

[0022] The hollow particles of the present disclosure are particles having a shell (outer shell) containing a resin and a hollow portion surrounded by the shell. In the present disclosure, the hollow portion is a hollow space clearly distinguishable from the shell of the hollow particle formed from a resin material. The shell of the hollow particle may have a porous structure, but in that case, the hollow portion has a size clearly distinguishable from the numerous microscopic 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. Below, an example of a method for producing hollow particles of the present disclosure is described, followed by a detailed description of the hollow particles of the present disclosure, and further a description of a resin composition containing the hollow particles of the present disclosure.

[0023] 1. Method for producing hollow particles The method for producing hollow particles according to the present disclosure includes, for example, the steps of: preparing a mixed solution containing a skeleton-forming monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; suspending the mixed solution to prepare a suspension in which droplets of a monomer composition containing the skeleton-forming monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction, and when the polymerization conversion rate of the skeleton-forming monomer reaches 1% by mass or more and 99% by mass or less, adding a carboxy group-introducing monomer containing a radical polymerizable group and a protected carboxy group to further polymerize the resulting mixture, followed by deprotection to prepare a precursor composition containing precursor particles having a shell containing a polymer of the skeleton-forming monomer and the deprotected carboxy group-introducing monomer and having a carboxy group on the outer surface, and a hollow portion surrounded by the shell, the hollow portion containing the hydrophobic solvent. The hollow particles can be obtained by the first production method of the present disclosure, in which the solubility of the carboxy group-introducing monomer in water at 20°C is greater than that of the hydrophobic solvent and is 0.5 g / L to 1000 g / L, the skeleton-forming monomer includes a crosslinkable monomer, and the content of the crosslinkable monomer is 60 mass% or more relative to 100 mass% in total of the skeleton-forming monomer and the carboxy group-introducing monomer.

[0024] Alternatively, the hollow particles of the present disclosure may include, for example, the steps of: preparing a mixed solution containing a skeleton-forming monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; suspending the mixed solution to prepare a suspension in which droplets of a monomer composition containing the skeleton-forming monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction, and when the polymerization conversion rate of the skeleton-forming monomer reaches 1 mass % or more and 99 mass % or less, adding a carboxy group-containing monomer containing a radical polymerizable group and a carboxy group, and further carrying out a polymerization reaction, thereby preparing a precursor composition containing precursor particles comprising a shell containing a polymer of the skeleton-forming monomer and the carboxy group-containing monomer and having a carboxy group on an outer surface, and a hollow portion surrounded by the shell, the hollow portion containing the hydrophobic solvent; and wherein the solubility of the carboxy group-containing monomer in water at 20° C. is greater than that of the hydrophobic solvent and is 0.5 g / L to 1000 g / L, The hollow particles can be obtained by the second production method of the present disclosure, in which the skeleton-forming monomer includes a crosslinkable monomer, and the content of the crosslinkable monomer is 60 mass% or more relative to 100 mass% in total of the skeleton-forming monomer and the carboxy group-containing monomer.

[0025] The first and second manufacturing methods of the present disclosure are based on a basic technique of suspending a mixture containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium, thereby preparing a suspension in which the polymerizable monomer and the hydrophobic solvent phase-separate, forming droplets having a distribution structure in which the polymerizable monomer is concentrated on the surface and the hydrophobic solvent is concentrated in the center, and then subjecting this suspension to a polymerization reaction to harden the surfaces of the droplets and form hollow particles having hollows filled with the hydrophobic solvent. In the above basic technique, when a carboxyl group-containing monomer is added to the suspension in advance, it is difficult to position the carboxyl groups of the carboxyl group-containing monomer on the outer surface of the shell. Even if the content of the carboxyl group-containing monomer is increased, it is difficult to ensure that a sufficient number of carboxyl groups are present on the surface of the hollow particles. Furthermore, the dielectric properties of hollow particles tend to deteriorate as the amount of carboxyl groups in the shell increases, which presents a problem of difficulty in improving adhesion to resins while maintaining the dielectric properties of the hollow particles. In contrast, in the first production method of the present disclosure, a carboxyl group-introducing monomer containing a radically polymerizable group and a protected carboxyl group and having moderate water solubility is added to the suspension during the polymerization reaction, and the polymerization reaction is further carried out, thereby efficiently introducing monomer units derived from the carboxyl group-introducing monomer onto the outer surface of the shell. Then, by deprotecting the protected carboxyl group located on the outer surface of the shell, hollow particles having carboxyl groups on the outer surface of the shell can be obtained. In the first production method of the present disclosure, the carboxyl group-introducing monomer is added during the polymerization reaction, and because the carboxyl group-introducing monomer has moderate water solubility, it is presumed that the added carboxyl group-introducing monomer is likely to be located on the surface of the shell during the polymerization reaction. Furthermore, the carboxyl group-introducing monomer has a radically polymerizable group as a polymerizable functional group, and due to the high reactivity of the radically polymerizable group, it is presumed that it easily reacts with unreacted skeleton-forming monomer located on the surface of the shell.Therefore, in the first manufacturing method of the present disclosure, it is presumed that the carboxyl group-introducing monomer added during the polymerization reaction is bonded to the shell surface by addition polymerization, thereby efficiently introducing protected carboxyl groups onto the outer surface of the shell. In the first manufacturing method of the present disclosure, deprotection is performed after the polymerization reaction to convert the protected carboxyl groups introduced onto the particle surface into carboxyl groups, thereby efficiently introducing carboxyl groups onto the particle surface. In the first manufacturing method of the present disclosure, even if the polymerization conversion rate of the skeleton-forming monomer is relatively low when the carboxyl group-introducing monomer is added, a polymer composed of the skeleton-forming monomer reacts with the carboxyl group-introducing monomer in the oil droplets. It is presumed that the presence of this reactant on the outermost surface of the droplets allows the carboxyl group-introducing monomer to be introduced onto the shell surface. Subsequently, deprotection is performed as described above to introduce carboxyl groups onto the shell surface. On the other hand, the lower the polymerization conversion rate of the skeleton-forming monomer when the carboxyl group-introducing monomer is added, the more likely the reaction will proceed inside the shell. Therefore, in order to efficiently introduce the carboxyl group-introducing monomer onto the particle surface, it is preferable to add the carboxyl group-introducing monomer when the polymerization conversion rate of the backbone-forming monomer is relatively high. In the second production method of the present disclosure, a carboxyl group-containing monomer is added to the suspension during the polymerization reaction, and then the polymerization reaction is further carried out, thereby introducing monomer units derived from the carboxyl group-containing monomer onto the outer surface of the shell. In the second production method, a polymer composed of the backbone-forming monomer reacts with the carboxyl group-containing monomer in the oil droplets. It is presumed that the presence of this reaction product on the outermost surface of the droplets allows the carboxyl group-containing monomer to be introduced onto the shell surface. Since the carboxyl group-containing monomer ions in water, easily dissolving, and thus the reaction inside the shell is less likely to proceed, adding the carboxyl group-introducing monomer when the polymerization conversion rate of the backbone-forming monomer is low can effectively introduce carboxyl groups onto the shell surface, compared to adding a carboxyl group-introducing monomer having a protected carboxyl group.According to the manufacturing method of the present disclosure, carboxy groups can be efficiently introduced onto the surface of hollow particles. Therefore, the amount of carboxy group-introducing monomer or carboxy group-containing monomer added can be minimized, and the amount of carboxy groups necessary to improve adhesion to resins can be imparted to the surface of hollow particles. Therefore, the content of skeleton-forming monomer units can be relatively increased, and by adjusting the composition of the skeleton-forming monomer, hollow particles can be given desired physical properties. For example, hollow particles with excellent dielectric properties can be obtained by using a large amount of hydrocarbon monomer as the skeleton-forming monomer. Furthermore, in the first manufacturing method of the present disclosure, by setting the content of crosslinkable monomer to 60% by mass or more relative to 100% by mass of the total of the skeleton-forming monomer and the carboxy group-introducing monomer, and in the second manufacturing method of the present disclosure, by setting the content of crosslinkable monomer to 60% by mass or more relative to 100% by mass of the total of the skeleton-forming monomer and the carboxy group-containing monomer, spherical hollow particles having a hollow portion clearly distinguishable from the shell can be formed. This is presumably because the polymerizable monomer used to form the shell contains a sufficient amount of crosslinkable monomer, which makes it easier for the components that make up the shell and the hydrophobic solvent to phase separate in the droplets of the monomer composition dispersed in the suspension, and also because a shell with excellent strength is formed, which suppresses deformation of the particles.

[0026] The method for producing hollow particles according to the present disclosure includes a step of preparing a mixed solution, a step of preparing a suspension, and a step of preparing a precursor composition, and may further include other steps. Furthermore, as far as technically possible, two or more of the above steps and other additional steps may be performed simultaneously as a single step, or the order of the steps may be reversed. For example, the preparation of the mixed solution and the suspension may be performed simultaneously in a single step, such as by adding the materials for preparing the mixed solution and suspending them at the same time.

[0027] A preferred example of the method for producing hollow particles according to the present disclosure includes the following steps: (1) mixed solution preparation step: preparing a mixed solution containing a skeleton-forming monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; (2) suspending step: suspending the mixed solution to prepare a suspension in which droplets of a monomer composition containing the skeleton-forming monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in an aqueous medium; (3) precursor composition preparation step: subjecting the suspension to a polymerization reaction to prepare a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a resin and encapsulating a hydrophobic solvent in the hollow portion; (4) solid-liquid separation step: performing solid-liquid separation of the precursor composition to obtain precursor particles encapsulating a hydrophobic solvent in the hollow portion; and (5) solvent removal step: removing the hydrophobic solvent encapsulated in the precursor particles obtained by the solid-liquid separation step to obtain hollow particles. In the present disclosure, hollow particles whose hollow portions are filled with a hydrophobic solvent may be considered as intermediates of hollow particles whose hollow portions are filled with a gas and may be referred to as "precursor particles." In the present disclosure, a "precursor composition" refers to a composition containing precursor particles. In the first manufacturing method of the present disclosure, in the precursor composition preparation step (3), a carboxyl group-introducing monomer containing a radical polymerizable group and a protected carboxyl group and having suitable water solubility is added during the polymerization reaction of the suspension, followed by further polymerization and deprotection, thereby forming precursor particles having a shell containing a carboxyl group on the outer surface and a polymer of the skeleton-forming monomer and the deprotected carboxyl group-introducing monomer. In the second manufacturing method of the present disclosure, in the precursor composition preparation step (3), a carboxyl group-containing monomer containing a radical polymerizable group and a carboxyl group and having suitable water solubility is added during the polymerization reaction of the suspension, followed by further polymerization, thereby forming precursor particles having a shell containing a carboxyl group on the outer surface and a polymer of the skeleton-forming monomer and the carboxyl group-containing monomer.

[0028] FIG. 1 is a schematic diagram illustrating an example of the manufacturing method of the present disclosure. (1) to (5) in FIG. 1 correspond to the above-described steps (1) to (5). 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 a mixed solution in the mixed solution preparation step. As shown in this figure, the mixed solution contains an aqueous medium 1 and a low-polarity material 2 dispersed in the aqueous medium 1. Here, the low-polarity material 2 refers to a material that has low polarity and is difficult to mix with the aqueous medium 1. In the present disclosure, the low-polarity material 2 includes a skeleton-forming monomer, a hydrophobic solvent, and a polymerization initiator. (2) in FIG. 1 is a cross-sectional schematic diagram illustrating one embodiment of a suspension in the suspension step. The suspension includes an aqueous medium 1 and droplets 8 of a monomer composition dispersed in the aqueous medium 1. The droplets 8 of the monomer composition contain a skeleton-forming monomer, a hydrophobic solvent, and a polymerization initiator, but the distribution within the droplets is non-uniform. The droplets 8 of the monomer composition are phase-separated into a hydrophobic solvent 4a and materials other than the hydrophobic solvent, including the skeleton-forming monomer, 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) in FIG. 1 is a cross-sectional schematic diagram illustrating one embodiment of a precursor composition obtained by the precursor composition preparation step, including precursor particles encapsulating a hydrophobic solvent in hollow portions. The precursor composition includes an aqueous medium 1 and precursor particles 9 encapsulating a hydrophobic solvent 4a in hollow portions, dispersed in the aqueous medium 1. In the first manufacturing method of the present disclosure, the shell 6 that forms the outer surface of the precursor particle 9 is formed by polymerization of the skeleton-forming monomer in the droplets 8 of the monomer composition and the carboxyl group-introducing monomer added during the polymerization reaction, and by deprotection after the polymerization reaction, the protected carboxyl group in the carboxyl group-introducing monomer is deprotected and exists as a carboxyl group. The precursor particle 9 contains a polymer of the skeleton-forming monomer and the deprotected carboxyl group-introducing monomer as a resin.In the second manufacturing method of the present disclosure, the shell 6 forming the outer surface of the precursor particle 9 is formed by polymerization of the skeleton-forming monomer in the droplet 8 of the monomer composition and a carboxyl group-containing monomer added during the polymerization reaction. The precursor particle 9 contains a polymer of the skeleton-forming monomer and the carboxyl group-containing monomer as a resin. (4) in FIG. 1 is a cross-sectional schematic diagram showing one embodiment of a precursor particle after the solid-liquid separation step. (4) in FIG. 1 shows the state after the aqueous medium 1 has been removed from the state shown in (3) in FIG. 1. (5) in FIG. 1 is a cross-sectional schematic diagram showing one embodiment of a hollow particle after the solvent removal step. (5) in FIG. 1 shows the state after the hydrophobic solvent 4a has been removed from the state shown in (4) in FIG. 1. Removal of the hydrophobic solvent from the precursor particle results in a hollow particle 10 having a gas-filled hollow portion 7 inside the shell 6. The above five steps and other steps will be described in order below.

[0029] (1) Mixed Liquid Preparation Step This step is a step of preparing a mixed liquid containing a skeleton-forming monomer, 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) skeleton-forming monomer, (B) hydrophobic solvent, (C) polymerization initiator, (D) dispersion stabilizer, and (E) aqueous medium.

[0030] (A) Skeleton-Forming Monomer The skeleton-forming monomer may be any known polymerizable monomer conventionally used in the preparation of hollow particles. While not particularly limited, it is preferable for the skeleton-forming monomer to contain a crosslinkable monomer. When the skeleton-forming monomer contains a crosslinkable monomer, the resulting hollow particles tend to be spherical, and hollow portions that are clearly distinguishable from the shell tend to form within the particles. Furthermore, in order to obtain hollow particles with excellent dielectric properties, it is preferable for the skeleton-forming monomer to contain a hydrocarbon monomer. On the other hand, acrylic monomers are preferably used because they facilitate stable polymerization reactions. In the present disclosure, a polymerizable monomer consisting of carbon and hydrogen is referred to as a hydrocarbon monomer, a crosslinkable monomer consisting of carbon and hydrogen is referred to as a crosslinkable hydrocarbon monomer, and a non-crosslinkable monomer consisting of carbon and hydrogen is referred to as a non-crosslinkable hydrocarbon monomer. In the present disclosure, a polymerizable monomer having a (meth)acryloyl group as a polymerizable functional group is referred to as an acrylic monomer, a crosslinkable monomer having a (meth)acryloyl group as a polymerizable functional group is referred to as a crosslinkable acrylic monomer, and a non-crosslinkable monomer having a (meth)acryloyl group as a polymerizable functional group is referred to as a non-crosslinkable acrylic monomer. In the crosslinkable acrylic monomer, it is sufficient that at least one polymerizable functional group is a (meth)acryloyl group, but it is preferable that all of the polymerizable functional groups are (meth)acryloyl groups.

[0031] [Crosslinkable Monomer] Examples of crosslinkable hydrocarbon monomers include aromatic divinyl monomers such as divinylbenzene, divinylbiphenyl, and divinylnaphthalene; diene monomers such as linear or branched diolefins such as butadiene, isoprene, 2,3-dimethylbutadiene, pentadiene, and hexadiene; and alicyclic diolefins such as dicyclopentadiene, cyclopentadiene, and ethylidenetetracyclododecene. In addition, crosslinkable macromers such as polybutadiene, polyisoprene, styrene-butadiene block copolymers (SBS), and styrene-isoprene block copolymers (SIS) can also be used. Among these, aromatic divinyl monomers are preferred, and divinylbenzene is more preferred, because they facilitate stable polymerization reactions and enable hollow particles to be obtained that are excellent in dielectric properties, solvent resistance, strength, heat resistance, and the like. Examples of crosslinkable acrylic monomers include bifunctional ones such as allyl(meth)acrylate, vinyl(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, and 3-(meth)acryloyloxy-2-hydroxypropyl(meth)acrylate; and trifunctional or higher ones such as trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol poly(meth)acrylate, and ethoxylated versions of these. In addition, crosslinkable macromers such as polyphenylene ether, both ends of which are (meth)acrylic-modified, can also be used. Among these, ethylene glycol di(meth)acrylate and pentaerythritol di(meth)acrylate are preferred, and ethylene glycol di(meth)acrylate and pentaerythritol di(meth)acrylate are more preferred, since the polymerization reaction is likely to be stable and hollow particles having excellent strength can be obtained.Further examples of the crosslinkable monomer include crosslinkable allylic monomers such as diallyl phthalate, and crosslinkable macromers such as polyphenylene ether vinyl-modified at both ends. These crosslinkable monomers can be used alone or in combination of two or more. From the viewpoint of improving the strength of the hollow particles, it is also preferable to use a combination of a bifunctional crosslinkable monomer having only two polymerizable functional groups and a trifunctional or higher crosslinkable monomer having three or more polymerizable functional groups. Preferred bifunctional crosslinkable monomers are as described above. As the tri- or higher functional crosslinkable monomer, pentaerythritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, and dipentaerythritol poly(meth)acrylate are preferred, and pentaerythritol tetra(meth)acrylate and trimethylolpropane tri(meth)acrylate are more preferred.

[0032] The content of the crosslinkable monomer is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and even more preferably 90% by mass or more, relative to 100% by mass of the skeleton-forming monomer. When the content of the crosslinkable monomer is equal to or greater than the above-mentioned lower limit, hollow portions are easily formed within the particles, the particles are easily spherical, and the crosslinking density of the shell can be increased, which has the advantage of improving the solvent resistance, strength, pressure resistance, heat resistance, etc. of the hollow particles. When the skeleton-forming monomer contains a non-crosslinkable monomer, the content of the crosslinkable monomer may be, for example, 98% by mass or less, or 96% by mass or less, relative to 100% by mass of the skeleton-forming monomer.

[0033] When the crosslinkable monomer contained in the skeleton-forming monomer contains a combination of a bifunctional crosslinkable monomer and a tri- or higher functional crosslinkable monomer, the content of the trifunctional crosslinkable monomer is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, and even more preferably 30 parts by mass or more, relative to 100% by mass of the total mass of the crosslinkable monomers, and is preferably 70 parts by mass or less, more preferably 60 parts by mass or less, and even more preferably 50 parts by mass or less, in order to improve the strength of the hollow particles.

[0034] [Non-crosslinkable monomer] The skeleton-forming monomer may contain a non-crosslinkable monomer. Examples of non-crosslinkable hydrocarbon monomers include aromatic monovinyl monomers such as styrene, vinyltoluene, α-methylstyrene, p-methylstyrene, ethylvinylbenzene, ethylvinylbiphenyl, and ethylvinylnaphthalene; monoolefin monomers such as linear or branched monoolefins such as ethylene, propylene, and butylene, and alicyclic monoolefins such as vinylcyclohexane, norbornene, tricyclododecene, and 1,4-methano-1,4,4a,9a-tetrahydrofluorene; and the like. Among these, aromatic monovinyl monomers are preferred, and ethylvinylbenzene is particularly preferred, from the viewpoint of improving the dielectric properties of the hollow particles. Examples of non-crosslinkable acrylic monomers include (meth)acrylic acid alkyl esters such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and lauryl (meth)acrylate; glycidyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-aminoethyl (meth)acrylate, t-butylaminoethyl (meth)acrylate, (meth)acrylic acid, (meth)acrylamide, N-methylol (meth)acrylamide, and N-butoxymethyl (meth)acrylamide.Further examples include methoxypolyethylene glycol (meth)acrylate, ethoxypolyethylene glycol (meth)acrylate, propoxypolyethylene glycol (meth)acrylate, butoxypolyethylene glycol (meth)acrylate, hexaoxypolyethylene glycol (meth)acrylate, octoxypolyethylene glycol polypropylene glycol (meth)acrylate, lauroxypolyethylene glycol (meth)acrylate, stearoxypolyethylene glycol (meth)acrylate, phenoxypolyethylene 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, monoethylene glycol mono(meth)acrylate, etc. In addition, non-crosslinkable macromers such as (meth)acrylic-modified polystyrene and (meth)acrylic-modified polymethyl methacrylate can also be used. Among them, (meth)acrylic acid alkyl esters are preferred, and butyl acrylate and methyl methacrylate are more preferred, because they facilitate stable polymerization reactions.Furthermore, examples of non-crosslinkable monomers include carboxylic acid vinyl ester monomers such as vinyl acetate; halogenated aromatic vinyl monomers such as halogenated styrene; halogenated vinyl monomers such as vinyl chloride; halogenated vinylidene monomers such as vinylidene chloride; vinylpyridine monomers; and the like.These non-crosslinkable monomers can be used alone or in combination of two or more.

[0035] From the viewpoint of obtaining hollow particles with excellent dielectric properties, it is preferable to use a hydrocarbon monomer as the main component of the skeleton-forming monomer. In this case, the content of the hydrocarbon monomer is preferably more than 50% by mass, more preferably 60% by mass or more, even more preferably 80% by mass or more, and even more preferably 90% by mass or more, relative to 100% by mass of the skeleton-forming monomer. It is particularly preferable that the skeleton-forming monomer consists of a hydrocarbon monomer. Note that a polymerizable monomer containing a heteroatom may be included within a range that does not significantly impair the dielectric properties of the hollow particles. For example, the content of the hydrocarbon monomer may be 99% by mass or less, or 98% by mass or less, relative to 100% by mass of the skeleton-forming monomer.

[0036] From the viewpoint of the reactivity of the polymerization reaction, it is preferable to use an acrylic monomer as the main component of the skeleton-forming monomer. In this case, the content of the acrylic monomer relative to 100% by mass of the skeleton-forming monomer is preferably 40% by mass or more, more preferably more than 50% by mass, even more preferably 70% by mass or more, and even more preferably 90% by mass or more. It is particularly preferable that the skeleton-forming monomer consists of an acrylic monomer. Note that, as long as the stability of the polymerization reaction is not significantly impaired, a polymerizable monomer other than the acrylic monomer may be contained. For example, the content of the acrylic monomer relative to 100% by mass of the skeleton-forming monomer may be 99% by mass or less, or 98% by mass or less. In order to suppress deterioration of the dielectric properties, it may be 90% by mass or less, 80% by mass or less, 70% by mass or less, or 60% by mass or less.

[0037] The content of the skeleton-forming monomer in the mixed liquid 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 30 to 60% by mass, and more preferably 40 to 50% by mass, relative to 100% by mass of the total mass of the components in the mixed liquid excluding the aqueous medium. Furthermore, from the viewpoint of the mechanical strength of the hollow particles, the content of the skeleton-forming monomer relative to 100% by mass of the total mass of the solids, excluding the hydrophobic solvent, of the materials that form the oil phase in the mixed liquid is preferably 95% by mass or more, and 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.

[0038] (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 monomer composition containing a hydrophobic solvent are dispersed in an aqueous medium. In the suspension process, phase separation occurs within the droplets of the monomer composition, and the hydrophobic solvent, which has low polarity, tends to collect inside the droplets of the monomer composition. Ultimately, the droplets of the monomer composition contain the hydrophobic solvent inside, and other materials other than the hydrophobic solvent are distributed around the periphery according to their respective polarities. Then, in the precursor composition preparation 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.

[0039] As the hydrophobic solvent, it is preferable to select an organic solvent that has lower solubility in water than the crosslinkable monomer contained in the backbone-forming monomer. Known hydrophobic solvents can be appropriately selected depending on the type of crosslinkable monomer, and are not particularly limited. Examples of hydrophobic solvents include esters such as ethyl acetate and butyl acetate; ether esters such as propylene glycol monomethyl ether acetate and propylene glycol monoethyl ether acetate; and hydrocarbon solvents, of which hydrocarbon solvents are preferred. Examples of hydrocarbon solvents include aliphatic hydrocarbons, including chain hydrocarbon solvents such as pentane, hexane, heptane, octane, 2-methylbutane, and 2-methylpentane, 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. Among the hydrophobic solvents, when the skeleton-forming monomer contains more than 50% by mass of a hydrocarbon monomer, chain hydrocarbon solvents are preferred, chain hydrocarbon solvents having 5 to 8 carbon atoms are more preferred, and at least one selected from the group consisting of pentane, hexane, heptane, and octane is even more preferred, because they facilitate the formation of hollow portions and are easily removed, thereby reducing the amount of hydrophobic solvent remaining in the hollow particles. On the other hand, from the same viewpoint as above, when the skeleton-forming monomer contains more than 50% by mass of an acrylic monomer, hydrocarbon solvents having 4 to 7 carbon atoms are preferred, and hydrocarbon solvents having 5 or 6 carbon atoms are more preferred. Here, the hydrocarbon solvent may be either an aromatic hydrocarbon or an aliphatic hydrocarbon, with aliphatic hydrocarbons being preferred.

[0040] 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.

[0041] Furthermore, the hydrophobic solvent used in the manufacturing method of the present disclosure preferably has a dielectric constant of 2.0 or less at 20°C. The dielectric constant is one of the indicators of the polarity of a compound. When the dielectric constant of the hydrophobic solvent is sufficiently small, such as 2.0 or less, phase separation proceeds rapidly in the droplets of the monomer composition, and hollow spaces are likely to be formed. Examples of hydrophobic solvents having a dielectric constant of 2.0 or less at 20°C are as follows. The values ​​in parentheses are the dielectric constant values: pentane (1.8), hexane (1.9), heptane (1.9), octane (1.9), and cyclohexane (2.0). Regarding the dielectric constant at 20°C, reference can be made to values ​​described in known literature (e.g., "Chemical Handbook: Basics," edited by the Chemical Society of Japan, Revised 4th Edition, 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.

[0042] The porosity of the hollow particles can be adjusted by changing the amount of hydrophobic solvent in the mixed solution. In the suspension process described below, the polymerization reaction proceeds with the hydrophobic solvent encapsulated in oil droplets containing the skeleton-forming monomer and the like. Therefore, the higher the hydrophobic solvent content, the higher the porosity of the resulting hollow particles tends to be. In the present disclosure, the content of the hydrophobic solvent in the mixed solution is preferably 50 to 500 parts by mass per 100 parts by mass of the skeleton-forming monomer and the carboxyl group-introducing monomer or the carboxyl group-containing monomer, because 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 within the particles. In particular, from the viewpoint of achieving a porosity within the preferred range described below, the content of the hydrophobic solvent in the mixed solution is preferably 70 to 300 parts by mass, more preferably 90 to 200 parts by mass, per 100 parts by mass of the skeleton-forming monomer and the carboxyl group-introducing monomer or the carboxyl group-containing monomer.

[0043] (C) Polymerization Initiator In the production method of the present disclosure, the mixed liquid preferably contains an oil-soluble polymerization initiator as a polymerization initiator. Methods for polymerizing droplets of the monomer composition after suspending the mixed liquid include emulsion polymerization using a water-soluble polymerization initiator and suspension polymerization using an oil-soluble polymerization initiator. Suspension polymerization can be carried out using an oil-soluble polymerization initiator. The oil-soluble polymerization initiator is not particularly limited as long as it is lipophilic and has a solubility in water of 0.2% by mass or less. Examples of the oil-soluble polymerization initiator include organic peroxides such as benzoyl peroxide, lauroyl peroxide, t-butyl peroxide-2-ethylhexanoate, t-butyl peroxydiethyl acetate, and t-butyl peroxypivalate; and azo compounds such as 2,2'-azobis(2,4-dimethylvaleronitrile), azobisisobutyronitrile, and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile). Of these, in the production method of the present disclosure, it is preferable to use an organic peroxide as a polymerization initiator. Organic peroxides can easily accelerate the polymerization reaction, thereby reducing the amount of unreacted polymerizable functional groups, and are less likely to leave decomposition products after the polymerization reaction, thereby preventing deterioration of the dielectric properties of hollow particles. Unreacted polymerizable functional groups and decomposition products of the polymerization initiator remaining in the shell increase the molecular mobility of the shell, so if these remain in large amounts, the dielectric loss tangent of the hollow particles may increase.

[0044] The content of the polymerization initiator relative to 100 parts by mass of the skeleton-forming monomer in the mixed solution is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 7 parts by mass, and even more preferably 1 to 5 parts by mass. When the content of the polymerization initiator is equal to or greater than the above-mentioned lower limit, the polymerization reaction can proceed sufficiently, while when the content is equal to or less than the above-mentioned upper limit, there is little risk of the oil-soluble polymerization initiator remaining after completion of the polymerization reaction, and there is also little risk of an unexpected side reaction proceeding.

[0045] (D) Dispersion Stabilizer The dispersion stabilizer is an agent that disperses droplets of the monomer composition in an aqueous medium during the suspension process. Examples of dispersion stabilizers include inorganic dispersion stabilizers, organic or inorganic water-soluble polymer stabilizers, and surfactants. In the present disclosure, inorganic dispersion stabilizers are preferably used as the dispersion stabilizer because they facilitate control of the particle size of the droplets in the suspension, narrow the particle size distribution of the resulting hollow particles, and prevent the shell from becoming too thin, thereby suppressing a decrease in the strength of the hollow particles. Examples of inorganic dispersion stabilizers include sulfates such as barium sulfate and calcium sulfate; carbonates such as barium carbonate, calcium carbonate, and magnesium carbonate; phosphates such as calcium phosphate; metal oxides such as aluminum oxide and titanium oxide; metal hydroxides such as aluminum hydroxide, magnesium hydroxide, calcium hydroxide, barium hydroxide, and ferric hydroxide; and inorganic compounds such as silicon dioxide. These inorganic dispersion stabilizers can be used alone or in combination of two or more. Among the inorganic dispersion stabilizers, poorly water-soluble inorganic dispersion stabilizers are preferred, and poorly water-soluble metal salts such as the sulfates, carbonates, phosphates, and metal hydroxides mentioned above are more preferred, metal hydroxides are even more preferred, and magnesium hydroxide is particularly preferred. In the present disclosure, the poorly water-soluble inorganic dispersion stabilizer is preferably an inorganic compound having a solubility of 0.5 g or less in 100 g of water. The poorly water-soluble metal salt is preferably an inorganic metal salt having a solubility of 0.5 g or less in 100 g of water.

[0046] In the present disclosure, it is particularly preferable to use a poorly water-soluble inorganic dispersion stabilizer dispersed in an aqueous medium in the form of colloidal particles, i.e., in the form of a colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles. This not only narrows the particle size distribution of the droplets of the monomer composition, but also makes it easy to reduce the amount of inorganic dispersion stabilizer remaining in the resulting hollow particles by washing. A colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles can be prepared, for example, by reacting at least one selected from alkali metal hydroxides and alkaline earth metal hydroxides with a water-soluble polyvalent metal salt (excluding alkaline earth metal hydroxides) in an aqueous medium. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Examples of alkaline earth metal hydroxides include barium hydroxide and calcium hydroxide. The water-soluble polyvalent metal salt may be any water-soluble polyvalent metal salt other than the above-mentioned alkaline earth metal hydroxides. Examples include magnesium metal salts such as magnesium chloride, magnesium phosphate, and magnesium sulfate; calcium metal salts such as calcium chloride, calcium nitrate, calcium acetate, and calcium sulfate; aluminum metal salts such as aluminum chloride and aluminum sulfate; barium salts such as barium chloride, barium nitrate, and barium acetate; and zinc salts such as zinc chloride, zinc nitrate, and zinc acetate. Among these, magnesium metal salts, calcium metal salts, and aluminum metal salts are preferred, magnesium metal salts are more preferred, and magnesium chloride is particularly preferred. The water-soluble polyvalent metal salts can be used alone or in combination of two or more. 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, and examples include a method of mixing an aqueous solution of at least one selected from the alkali metal hydroxides and alkaline earth metal hydroxides with an aqueous solution of the water-soluble polyvalent metal salt. Furthermore, colloidal silica can also be used as the colloidal dispersion liquid containing poorly water-soluble inorganic dispersion stabilizer colloidal particles.Examples of organic water-soluble polymer stabilizers include polyvinyl alcohol, polycarboxylic acids (such as polyacrylic acid), celluloses (such as hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose, and ethyl cellulose), polyvinylpyrrolidone, polyacrylimide, polyethylene oxide, and poly(hydroxystearic acid-g-methyl methacrylate-co-methacrylic acid) copolymers. Examples of inorganic water-soluble polymer compounds include sodium tripolyphosphate. Surfactants are compounds that have both hydrophilic and hydrophobic groups in one molecule, and examples include known ionic surfactants such as anionic surfactants, cationic surfactants, and amphoteric surfactants, as well as nonionic surfactants.

[0047] The content of the dispersion stabilizer is not particularly limited, but is preferably 0.5 to 15 parts by mass, and more preferably 1 to 10 parts by mass, per 100 parts by mass of the total mass of the skeleton-forming monomer and hydrophobic solvent. By having the content of the dispersion stabilizer equal to or greater than the above-mentioned lower limit, droplets of the monomer composition can be sufficiently dispersed so as not to coalesce in the suspension. On the other hand, by having the content of the dispersion stabilizer equal to or less than the above-mentioned 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 typically 2 to 15 parts by mass, and preferably 3 to 8 parts by mass, per 100 parts by mass of the aqueous medium.

[0048] (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 in order to form droplets of the monomer composition. In this case, for example, the mass ratio of water to hydrophilic solvent (water:hydrophilic solvent) may be 99:1 to 50:50. The hydrophilic solvent in the present disclosure is not particularly limited as long as it is sufficiently miscible with water and does not cause phase separation. Examples of hydrophilic solvents include alcohols such as methanol and ethanol; tetrahydrofuran (THF); dimethyl sulfoxide (DMSO); and the like.

[0049] 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.

[0050] A mixed solution is obtained by mixing the above-mentioned materials and, if necessary, other materials and stirring them appropriately. In this mixed solution, an oil phase containing lipophilic materials such as (A) the skeleton-forming monomer, (B) the hydrophobic solvent, and (C) the polymerization initiator is dispersed in an aqueous phase containing, for example, (D) the dispersion stabilizer and (E) the aqueous medium, with particles of about several millimeters 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, if necessary, other materials and stirring appropriately. However, in terms of facilitating the formation of a uniform shell, it is preferable to prepare a mixed solution by separately preparing an oil phase containing the skeleton-forming monomer, the hydrophobic solvent, and the polymerization initiator and an aqueous phase containing the dispersion stabilizer and the aqueous medium in advance and 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.

[0051] (2) Suspension Step The suspension step is a step of preparing a suspension in which droplets of a monomer composition containing a hydrophobic solvent are dispersed in an aqueous medium by suspending the above-mentioned mixed liquid. The suspension method for forming droplets of the monomer composition is not particularly limited, and any known suspension method can be used. Examples of dispersers that can be used in preparing the suspension include horizontal or vertical in-line dispersers such as those manufactured by Pacific Machinery Works, Ltd. (product name: Milder), those manufactured by Eurotec Co., Ltd. (product name: Cavitron), and those manufactured by IKA (product name: DISPAX-REACTOR (registered trademark) DRS series); and emulsifying dispersers such as those manufactured by Primix Corporation (homomixer MARK II series).

[0052] In the suspension prepared in the suspending step, droplets of the monomer composition containing the lipophilic material and having a particle size of approximately 1 to 50 μm are uniformly dispersed in the aqueous medium. Such droplets of the monomer composition are difficult to observe with the naked eye and can be observed using known observation equipment such as an optical microscope. In the suspending step, phase separation occurs in the droplets of the monomer composition, so that the hydrophobic solvent with low polarity tends to collect inside the droplets. As a result, the resulting droplets contain the hydrophobic solvent in their interiors and materials other than the hydrophobic solvent distributed around their peripheries.

[0053] The droplets of the monomer composition dispersed in an aqueous medium are formed by surrounding the oil-soluble monomer composition with a dispersion stabilizer. The droplets of the monomer composition contain an oil-soluble polymerization initiator, a skeleton-forming monomer, and a hydrophobic solvent. The droplets of the monomer 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 skeleton-forming monomer dispersed in the aqueous medium. Therefore, the use of an oil-soluble polymerization initiator can prevent 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.

[0054] (3) Precursor Composition Preparation Step In the first manufacturing method, this step involves subjecting the suspension obtained in the above-described suspension step to a polymerization reaction, adding a carboxyl group-introducing monomer containing a protected carboxyl group during the polymerization reaction, further carrying out the polymerization reaction, and then deprotecting the resulting mixture to prepare a precursor composition containing precursor particles that include a shell containing a polymer of a backbone-forming monomer and the deprotected carboxyl group-introducing monomer, and that have a carboxyl group on the outer surface, and a hollow portion surrounded by the shell and that encapsulates a hydrophobic solvent in the hollow portion. The precursor particles are formed by polymerization of the backbone-forming monomer contained in droplets of the monomer composition and the carboxyl group-introducing monomer added during the polymerization reaction, followed by deprotection. In the second manufacturing method, this step involves subjecting the suspension obtained in the above-described suspension step to a polymerization reaction, adding a carboxyl group-containing monomer containing a carboxyl group during the polymerization reaction, and further conducting the polymerization reaction to prepare a precursor composition containing precursor particles having a shell containing a polymer of a backbone-forming monomer and a carboxyl group-containing monomer and having a carboxyl group on its outer surface, and a hollow portion surrounded by the shell and containing a hydrophobic solvent in the hollow portion. The precursor particles are formed by polymerization of the backbone-forming monomer contained in droplets of the monomer composition and the carboxyl group-containing monomer added during the polymerization reaction. Note that in the present disclosure, the polymerization reaction before the addition of the carboxyl group-introducing monomer or the carboxyl group-containing monomer may be referred to as the first polymerization reaction, and the polymerization reaction after the addition of the carboxyl group-introducing monomer or the carboxyl group-containing monomer may be referred to as the second polymerization reaction.

[0055] In the first production method of the present disclosure, a polymerizable monomer containing a radical polymerizable group and a protected carboxy group is used as the carboxy group-introducing monomer. Here, the radical polymerizable group is preferably at least one selected from a (meth)acryloyl group and a vinyl group, and more preferably a (meth)acryloyl group, in terms of excellent reactivity.

[0056] In the present disclosure, the protected carboxy group may be a known one and is not particularly limited. However, a group that is deprotected by hydrolysis under acidic or basic conditions to form a carboxy group is preferred. Preferred examples include a monovalent group represented by the following general formula (1) and a divalent group represented by the following general formula (2):

[0057] (In general formula (1), R １ represents a hydrocarbon group which may contain a heteroatom, and * represents a bond.

[0058] (In general formula (2), R ２ represents a hydrocarbon group which may contain a heteroatom, and * represents a bond.

[0059] R in general formula (1) １ and R in general formula (2). ２ The hydrocarbon group in may be a linear or cyclic saturated or unsaturated aliphatic hydrocarbon group, an aromatic hydrocarbon group, or a combination thereof, and is not particularly limited.

[0060] R in general formula (1) １ Examples of the hydrocarbon group which may contain a heteroatom in the general formula (1) include hydrocarbon groups such as alkyl groups having 1 to 6 carbon atoms such as methyl group, ethyl group, and tert-butyl group, aralkyl groups having 7 to 14 carbon atoms such as benzyl group, phenyl group, allyl group, and trityl group; alkoxyalkyl groups having 2 to 6 carbon atoms such as methoxymethyl group, 1-butoxyethyl group, 1-ethoxyethyl group, and 1-ethoxybutyl group, and alkoxyaralkyl groups having 7 to 14 carbon atoms such as methoxybenzyl group; and triorganosilyl groups such as trimethylsilyl group, triethylsilyl group, t-butyldimethylsilyl group, t-butyldiphenylsilyl group, triethylsilyl group, triphenylsilyl group, and triisopropylsilyl group. １ Among these, alkyl groups having 1 to 4 carbon atoms such as a methyl group, an ethyl group, or a tert-butyl group are preferred in terms of ease of deprotection.

[0061] R in general formula (2) ２ Examples of the hydrocarbon group which may contain a hetero atom include an alkylene group having 1 to 10 carbon atoms, -(C ２ H ４ O) ｎ -C ２ H ４ -(n is an integer from 1 to 8), -(C ３ H ６ O) ｍ -C ３ H ６ - (m is an integer of 1 to 6), etc. ２ Among these, an alkylene group having 2 to 4 carbon atoms is preferred from the viewpoint of ease of deprotection.

[0062] The carboxyl group-introducing monomer has a solubility in water at 20°C that is greater than that of the hydrophobic solvent and is 0.5 g / L to 1000 g / L, preferably 0.5 g / L to 80 g / L. This allows the carboxyl group-introducing monomer to be easily incorporated into the surface of the shell during the polymerization reaction. The solubility of the carboxyl group-introducing monomer in water at 20°C is more preferably 70 g / L or less, even more preferably 60 g / L or less, still more preferably 50 g / L or less, and particularly preferably 20 g / L or less, while more preferably 1.0 g / L or more, even more preferably 2.0 g / L or more.

[0063] In addition, the carboxyl group-introducing monomer has a molecular weight of preferably 300 or less, more preferably 200 or less, since it is easily incorporated into the surface of the shell during the polymerization reaction. The lower limit of the molecular weight of the carboxyl group-introducing monomer is not particularly limited, and is usually 50 or more.

[0064] Carboxy group-introducing monomers preferably used in the present disclosure include those having a protected carboxy group represented by the general formula (1) above, such as methyl(meth)acrylate, ethyl(meth)acrylate, and tert-butyl(meth)acrylate. Carboxy group-introducing monomers having a protected carboxy group represented by the general formula (2) above, such as ethylene glycol di(meth)acrylate. These carboxy group-introducing monomers may be used alone or in combination of two or more.

[0065] The amount of the carboxyl group-introducing monomer added is preferably 2% by mass or more, more preferably 5% by mass or more, and even more preferably 10% by mass or more relative to 100% by mass of the total of the backbone-forming monomer and the carboxyl group-introducing monomer contained in the mixed solution in order to introduce a sufficient amount of carboxyl groups onto the surface of the hollow particles from the viewpoint of adhesion to the resin. The upper limit is preferably 40% by mass or less, more preferably 30% by mass or less relative to 100% by mass of the total of the backbone-forming monomer and the carboxyl group-introducing monomer contained in the mixed solution in order to obtain performance such as dielectric properties attributable to the composition of the backbone-forming monomer.

[0066] Furthermore, in the first production method of the present disclosure, in order to obtain spherical hollow particles having hollow portions therein, the content of the crosslinkable monomer is set to 60% by mass or more relative to 100% by mass of the total of the skeleton-forming monomer and the carboxyl group-introducing monomer, and further, from the viewpoint of improving the solvent resistance, strength, pressure resistance, heat resistance, etc. of the hollow particles, the content is preferably 70% by mass or more, more preferably 80% by mass or more. The upper limit of the content of the crosslinkable monomer is not particularly limited, and when the skeleton-forming monomer or the carboxyl group-introducing monomer contains a non-crosslinkable monomer, the content may be 98% by mass or less, 95% by mass or less, 90% by mass or less, or 85% by mass or less relative to 100% by mass of the total of the skeleton-forming monomer and the carboxyl group-introducing monomer.

[0067] In the first production method of the present disclosure, the suspension obtained in the suspension step is subjected to a polymerization reaction, and when the polymerization conversion rate of the skeleton-forming monomer in the suspension reaches 1% by mass or more and 99% by mass or less, preferably 40% by mass or more and 99% by mass or less, the carboxyl group-introducing monomer is added to further carry out the polymerization reaction. The polymerization conversion rate of the skeleton-forming monomer in the suspension when the carboxyl group-introducing monomer is added is more preferably 50% by mass or more, even more preferably 60% by mass or more, and more preferably 90% by mass or less, even more preferably 80% by mass or less. In the present disclosure, the polymerization conversion rate can be calculated using the following formula (C) from the mass of the skeleton-forming monomer contained in the suspension before the addition of the carboxyl group-introducing monomer and the mass of the unreacted skeleton-forming monomer contained in the suspension immediately before the addition. Here, the mass of the skeleton-forming monomer contained in the suspension before the addition of the carboxyl group-introducing monomer is the total mass of the skeleton-forming monomer after the polymerization reaction and the unreacted skeleton-forming monomer. The mass of the unreacted skeleton-forming monomer can be measured using gas chromatography (GC) according to the following formula (C): Polymerization conversion rate (mass %)=100−(mass of unreacted skeleton-forming monomer / mass of skeleton-forming monomer contained in suspension)×100

[0068] In the second production method of the present disclosure, a polymerizable monomer containing a radical polymerizable group and a carboxy group is used as the carboxy group-containing monomer. Here, the radical polymerizable group is preferably at least one selected from a (meth)acryloyl group and a vinyl group, and more preferably a (meth)acryloyl group, in terms of excellent reactivity.

[0069] The carboxyl group-containing monomer used has a solubility in water at 20°C that is greater than that of the hydrophobic solvent and is 0.5 g / L to 1000 g / L. This allows the carboxyl group-containing monomer to be easily incorporated into the surface of the shell during the polymerization reaction. The solubility of the carboxyl group-containing monomer in water at 20°C is preferably 100 g / L or less, more preferably 90 g / L or less, and is preferably 10 g / L or more, more preferably 30 g / L or more, even more preferably 50 g / L or more, and even more preferably 70 g / L or more.

[0070] In addition, since the carboxyl group-containing monomer is easily incorporated into the surface of the shell during the polymerization reaction, the molecular weight of the carboxyl group-containing monomer is preferably not more than 300, more preferably not more than 200. The lower limit of the molecular weight of the carboxyl group-containing monomer is not particularly limited, and is usually 50 or more.

[0071] Examples of carboxyl group-containing monomers that can be preferably used in the present disclosure include (meth)acrylic acid, ethylacrylic acid, crotonic acid, maleic acid, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid, allylmalonic acid, and teraconic acid.

[0072] The amount of the carboxyl group-containing monomer added is preferably 2% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, and even more preferably 20% by mass or more relative to 100% by mass of the total of the skeleton-forming monomer and the carboxyl group-containing monomer contained in the mixed solution, in order to introduce a sufficient amount of carboxyl groups onto the surface of the hollow particles from the viewpoint of adhesion to the resin. The upper limit is preferably 40% by mass or less, more preferably 30% by mass or less, relative to 100% by mass of the total of the skeleton-forming monomer and the carboxyl group-containing monomer contained in the mixed solution, in order to obtain performance such as dielectric properties attributable to the composition of the skeleton-forming monomer.

[0073] Furthermore, in the second production method of the present disclosure, in order to obtain spherical hollow particles having hollow portions therein, the content of the crosslinkable monomer is set to 60% by mass or more relative to 100% by mass of the total of the skeleton-forming monomer and the carboxyl group-containing monomer, and further, from the viewpoint of improving the solvent resistance, strength, pressure resistance, heat resistance, etc. of the hollow particles, the content is preferably 70% by mass or more, more preferably 80% by mass or more. The upper limit of the content of the crosslinkable monomer is not particularly limited, and when the skeleton-forming monomer or the carboxyl group-containing monomer contains a non-crosslinkable monomer, the content may be 98% by mass or less, 95% by mass or less, 90% by mass or less, or 85% by mass or less relative to 100% by mass of the total of the skeleton-forming monomer and the carboxyl group-containing monomer.

[0074] In the second production method of the present disclosure, the suspension obtained in the suspension step is subjected to a polymerization reaction, and when the polymerization conversion rate of the skeleton-forming monomer in the suspension reaches 1% by mass or more and 99% by mass or less, preferably 1% by mass or more and 60% by mass or less, the carboxy group-containing monomer is added to further carry out the polymerization reaction. The polymerization conversion rate of the skeleton-forming monomer in the suspension when the carboxy group-containing monomer is added is more preferably 3% by mass or more, even more preferably 5% by mass or more, and more preferably 50% by mass or less, even more preferably 40% by mass or less.

[0075] In the production method of the present disclosure, the polymerization method for the polymerization reaction is not particularly limited, and for example, a batch method, a semi-continuous method, a continuous method, etc. can be employed. The polymerization temperature is preferably 40 to 90°C, more preferably 50 to 80°C. The temperature increase rate when raising the temperature to the polymerization temperature is preferably 10 to 60°C / h, more preferably 15 to 55°C / h. The polymerization reaction time, which is the total time of the first polymerization reaction carried out before adding the carboxy group-introducing monomer or the carboxy group-containing monomer, and the second polymerization reaction carried out after adding the carboxy group-introducing monomer or the carboxy group-containing monomer, is preferably 1 to 48 hours, more preferably 4 to 36 hours. The reaction times for the first polymerization reaction and the second polymerization reaction are not particularly limited, and may be appropriately adjusted so that the polymerization conversion rate of the backbone-forming monomer at the time of adding the carboxy group-introducing monomer or the carboxy group-containing monomer falls within the above-mentioned preferred range. For example, after the suspension is heated to the polymerization temperature, the first polymerization reaction is carried out preferably for 1 minute to 8 hours, more preferably for 20 minutes to 8 hours, and even more preferably for 30 minutes to 4 hours; and after the carboxy group-introducing monomer or the carboxy group-containing monomer is added, the second polymerization reaction is carried out preferably for 1 to 48 hours, and more preferably for 2 to 44 hours.

[0076] In the first production method of the present disclosure, after the polymerization reaction is completed, the protected carboxyl groups are deprotected. The deprotection method is not particularly limited, and can be performed, for example, by an existing method described in "Greene's Protective Groups in Organic Synthesis," 5th Edition, by Peter G. M. Wuts, published by Wiley-Interscience (2014). Among these, deprotection by hydrolysis under acidic or basic conditions is preferred in terms of ease of deprotection. In the precursor composition preparation step, the shell portion of the droplets of the monomer composition containing the hydrophobic solvent therein is polymerized, and thus, as described above, hollow portions filled with the hydrophobic solvent are formed inside the resulting precursor particles.

[0077] (4) Solid-Liquid Separation Step This step is a step of obtaining a solid content containing precursor particles by solid-liquid separation of the precursor composition containing precursor particles obtained in the precursor composition preparation step described above.

[0078] The method for solid-liquid separation of the precursor composition is not particularly limited, and known methods can be used. Examples of solid-liquid separation methods include centrifugation, filtration, and static separation. Among these, centrifugation or filtration can be used, and centrifugation may be used from the viewpoint of ease of operation. After the solid-liquid separation step, an optional step such as a pre-drying step may be performed before performing the solvent removal step described below. Examples of the pre-drying step include a step of pre-drying the solid content obtained after the solid-liquid separation step using a drying device such as a dryer or a drying appliance such as a hand dryer.

[0079] (5) Solvent Removal Step This step is a step of removing the hydrophobic solvent contained in the precursor particles obtained by the solid-liquid separation step. For example, by removing the hydrophobic solvent contained in the precursor particles in air, the hydrophobic solvent inside the precursor particles is replaced with air, and hollow particles filled with gas are obtained.

[0080] In this process, "in the air" strictly refers to an environment in which no liquid is present outside the precursor particles, or an environment in which only a trace amount of liquid is present outside the precursor particles, so that the removal of the hydrophobic solvent is not affected. "In the air" can also be referred to as a state in which the precursor particles are not present in a slurry, or a state in which the precursor particles are present in a dry powder. In other words, in this process, it is important to remove the hydrophobic solvent in an environment in which the precursor particles are in direct contact with the external gas.

[0081] The method for removing the hydrophobic solvent from the precursor particles in air is not particularly limited, and known methods can be used. Examples of such methods include vacuum drying, heat drying, flash drying, or a combination of these methods. In particular, when using heat drying, the heating temperature must be equal to or higher than the boiling point of the hydrophobic solvent and equal to or lower than the maximum temperature at which the shell structure of the precursor particles does not collapse. Therefore, depending on the shell composition and the type of hydrophobic solvent in the precursor particles, the heating temperature may be, for example, 50 to 200°C, 70 to 200°C, or 100 to 200°C. The drying operation in air replaces the hydrophobic solvent inside the precursor particles with the external gas, resulting in hollow particles whose hollow portions are filled with gas.

[0082] The drying atmosphere is not particularly limited and can be appropriately selected depending on the application of the hollow particles. Examples of the drying atmosphere include air, oxygen, nitrogen, argon, etc. Hollow particles with a temporary vacuum inside can also be obtained by filling the inside of the hollow particles with a gas and then drying under reduced pressure.

[0083] Alternatively, the hydrophobic solvent may be removed from the slurry containing precursor particles and an aqueous medium without solid-liquid separation of the precursor composition obtained in the precursor composition preparation step. In this method, for example, the hydrophobic solvent encapsulated in the precursor particles can be removed by bubbling an inert gas through the precursor composition at a temperature equal to or higher than the boiling point of the hydrophobic solvent minus 35°C. Here, when the hydrophobic solvent is a mixed solvent containing multiple hydrophobic solvents and has multiple boiling points, the boiling point of the hydrophobic solvent in the solvent removal step is the boiling point of the solvent with the highest boiling point among the solvents contained in the mixed solvent, i.e., the highest boiling point among the multiple boiling points. The temperature at which the inert gas is bubbled through the precursor composition is preferably equal to or higher than the boiling point of the hydrophobic solvent minus 30°C, more preferably equal to or higher than the boiling point of the hydrophobic solvent, in order to reduce the amount of hydrophobic solvent remaining in the hollow particles. The bubbling temperature is typically equal to or higher than the polymerization temperature of the polymerization reaction performed in the precursor composition preparation step. Although not particularly limited, the temperature during bubbling may be 50°C or higher and 100°C or lower. The inert gas to be bubbling is not particularly limited, but examples thereof include nitrogen, argon, etc. The bubbling conditions are appropriately adjusted depending on the type and amount of hydrophobic solvent so as to remove the hydrophobic solvent encapsulated in the precursor particles. Although not particularly limited, for example, the inert gas may be bubbled at a rate of 1 to 3 L / min for 1 to 10 hours. This method produces an aqueous slurry of hollow particles encapsulating an inert gas. The hollow particles obtained by solid-liquid separation of this slurry are dried, and the aqueous medium remaining in the hollow particles is removed, thereby producing hollow particles whose hollow portions are filled with gas.

[0084] Comparing a method of obtaining hollow particles having hollow spaces filled with gas by performing solid-liquid separation on a slurry-like precursor composition and then removing the hydrophobic solvent in the precursor particles in air, with a method of obtaining hollow particles having hollow spaces filled with gas by removing the hydrophobic solvent encapsulated in precursor particles in a slurry containing the precursor particles and an aqueous medium, performing solid-liquid separation, and removing the aqueous medium remaining in the particles in air, the former method has the advantage that the hollow particles are less likely to be crushed in the step of removing the hydrophobic solvent, while the latter method has the advantage that the amount of remaining hydrophobic solvent is reduced by performing bubbling with an inert gas. Alternatively, as a method for removing the hydrophobic solvent encapsulated in the precursor particles after the precursor composition preparation step and before the solid-liquid separation step, without performing solid-liquid separation on the slurry precursor composition obtained in the precursor composition preparation step, for example, a method for evaporating and distilling off the hydrophobic solvent encapsulated in the precursor particles from the precursor composition under a predetermined pressure (high pressure, normal pressure, or reduced pressure); or a method for introducing an inert gas such as nitrogen, argon, or helium, or water vapor, into the precursor composition under a predetermined pressure (high pressure, normal pressure, or reduced pressure), and evaporating and distilling off the hydrophobic solvent may be used.

[0085] (6) Others Steps other than the above steps (1) to (5) may include, for example, the following (6-a) washing step and the following (6-b) particle internal substitution step. (6-a) Washing Step The washing step is a step of adding an acid or alkali to wash the precursor composition containing precursor particles before the solvent removal step in order to remove any dispersion stabilizer remaining in the precursor composition. When the dispersion stabilizer used is an acid-soluble inorganic dispersion stabilizer, it is preferable to add an acid to the precursor composition containing precursor particles to perform washing. On the other hand, when the dispersion stabilizer used is an alkali-soluble inorganic compound, it is preferable to add an alkali to the precursor composition containing precursor particles to perform washing. Furthermore, when an acid-soluble inorganic dispersion stabilizer is used as the dispersion stabilizer, it is preferable to add an acid to the precursor composition containing precursor particles 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 small burden on production equipment.

[0086] (6-b) Particle Interior Substitution Process The particle interior substitution process is a process in which the gas or liquid inside the hollow particles is replaced with another gas or liquid. This substitution can change the environment inside the hollow particles, selectively confine molecules inside the hollow particles, or modify the chemical structure inside the hollow particles to suit the application.

[0087] 2. Hollow Particles The hollow particles disclosed herein are hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, wherein the shell contains, as the resin, a polymer containing 60% by mass or more of crosslinkable monomer units, the hollow particles have carboxy groups on their surfaces, and the amount of carboxylic acid per unit area calculated from the acid value of the hollow particles and the specific surface area of ​​the hollow particles using the following formula (A) is 0.500 μmol / m ２ Formula (A): Carboxylic acid amount (μmol / m ２ ) = acid value (μmol / g) / specific surface area (m ２ / g)

[0088] In the hollow particles of the present disclosure, the amount of carboxylic acid per unit area is 0.500 μmol / m ２ However, from the viewpoint of further improving the adhesion to the resin, it is preferably 0.550 μmol / m ２ or more, more preferably 0.600 μmol / m ２ More preferably, 0.650 μmol / m ２ The upper limit of the amount of carboxylic acid per unit area is not particularly limited, but from the viewpoint of obtaining performance such as dielectric properties resulting from the composition of the skeleton-forming monomer, it is preferably 20 μmol / m ２ or less, more preferably 10 μmol / m ２ In the present disclosure, the acid value of the hollow particles is measured by potentiometric titration in accordance with JIS K 0070. In addition, in the present disclosure, the specific surface area of ​​the hollow particles is determined by assuming that the hollow particles are spherical, and then calculating the volume average particle size and apparent density D of the hollow particles. １ The volume average particle diameter used in the following formula (B) is a value in units of "m", and the apparent density D １ The unit is "g / m ３ " Formula (B): Specific surface area (m ２ / g) = 6 / (volume average particle size (m) × apparent density D １ (g / m ３ ))

[0089] The hollow particles of the present disclosure typically contain, as the main component of the shell, a polymer of the above-described backbone-forming monomer and a deprotected carboxy group-introducing monomer, or a polymer of the backbone-forming monomer and a carboxy group-containing monomer, and since the polymer forms the skeleton of the shell of the hollow particle, the shell of the hollow particle of the present disclosure contains a carboxy group-containing monomer unit derived from the above-described carboxy group-introducing monomer, or a carboxy group-containing monomer unit derived from the above-described carboxy group-containing monomer. It is preferable that the carboxy group on the surface of the hollow particle of the present disclosure is a carboxy group contained in the carboxy group-containing monomer unit. When the hollow particles of the present disclosure contain carboxy group-containing monomer units derived from the carboxy group-introducing monomer, the content of the carboxy group-containing monomer units is preferably 2% by mass or more, more preferably 5% by mass or more, and even more preferably 10% by mass or more, relative to 100% by mass of all monomer units in the polymer contained in the shell, from the viewpoint of adhesion to resins. On the other hand, from the viewpoint of obtaining performance such as dielectric properties attributable to the composition of the skeleton-forming monomer units, the content of the carboxy group-containing monomer units is preferably 40% by mass or less, more preferably 30% by mass or less. When the hollow particles of the present disclosure contain carboxy group-containing monomer units derived from the carboxy group-containing monomer, the content of the carboxy group-containing monomer units is preferably 2% by mass or more, more preferably 5% by mass or more, even more preferably 10% by mass or more, relative to 100% by mass of all monomer units in the polymer contained in the shell, from the viewpoint of adhesion to resins. On the other hand, from the viewpoint of obtaining performance such as dielectric properties attributable to the composition of the skeleton-forming monomer units ...40% by mass or less, more preferably 30% by mass or less.

[0090] In the hollow particles of the present disclosure, the content of the crosslinkable monomer units relative to 100% by mass of all monomer units in the polymer contained in the shell is 60% by mass or more in order to make the hollow particles of the present disclosure spherical and hollow, and is preferably 70% by mass or more, more preferably 80% by mass or more in order to improve the solvent resistance, strength, pressure resistance, heat resistance, etc. of the hollow particles. The upper limit of the content of the crosslinkable monomer units is not particularly limited, and when the polymer contains non-crosslinkable monomer units, the content may be 98% by mass or less, 95% by mass or less, 90% by mass or less, or 85% by mass or less, relative to 100% by mass of all monomer units in the polymer.

[0091] In order to provide the hollow particles of the present disclosure with good dielectric properties, the content of hydrocarbon monomer units is preferably more than 50% by mass, more preferably 60% by mass or more, even more preferably 70% by mass or more, and even more preferably 80% by mass or more, based on 100 parts by mass of all monomer units in the polymer contained in the shell. In the hollow particles of the present disclosure containing hydrocarbon monomer units as the main component, the upper limit of the content of hydrocarbon monomer units is preferably 95% by mass or less, more preferably 90% by mass or less, based on 100 parts by mass of all monomer units in the polymer, in order to ensure a sufficient content of the carboxy group-containing monomer units.

[0092] In the hollow particles of the present disclosure obtained using an acrylic monomer as a skeleton-forming monomer, the content of the acrylic monomer units relative to 100 parts by mass of all monomer units in the polymer contained in the shell is not particularly limited, but is preferably more than 50% by mass, more preferably 60% by mass or more, even more preferably 70% by mass or more, and even more preferably 80% by mass or more. In this case, the upper limit of the content of the acrylic monomer units is not particularly limited, and the polymer may be composed of acrylic monomer units, may be 95% by mass or less, or may be 90% by mass or less. From the viewpoint of suppressing deterioration of dielectric properties, it may be 80% by mass or less, 70% by mass or less, or may be 60% by mass or less.

[0093] When the hollow particles of the present disclosure contain a polymer of the above-mentioned skeleton-forming monomer and the deprotected carboxyl group-introducing monomer, the content of the polymer 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 solids content of the shell. By ensuring that the content of the polymer is equal to or greater than the above-mentioned lower limit, the dielectric properties, strength, pressure resistance, etc. of the hollow particles can be improved. When the hollow particles of the present disclosure contain a polymer of the above-mentioned skeleton-forming monomer and the carboxyl group-containing monomer, the content of the polymer is the same as above.

[0094] Furthermore, in the hollow particles of the present disclosure, in order to prevent deterioration of dielectric properties, the content of components other than the polymer is preferably 4% by mass or less, more preferably 3% by mass or less, even more preferably 2% by mass or less, and even more preferably 1% by mass or less, based on 100% by mass of the total solids content of the shell. Examples of components other than the polymer contained in the hollow particles of the present disclosure include unreacted residual polymerizable monomers, polymers other than polymers of the polymerizable monomers, decomposition products of polymerization initiators, and low-molecular-weight compounds contained as impurities in raw materials for backbone-forming monomers, carboxyl-introducing monomers, or carboxyl-containing monomers. Low-boiling components (e.g., boiling points of 200°C or less) are typically removed during the production process of hollow particles, but high-boiling components (e.g., boiling points of 250°C or more) may remain unremoved.

[0095] The hollow particles of the present disclosure preferably have a relative dielectric constant of 2.00 or less, more preferably 1.50 or less, and even more preferably 1.40 or less at a frequency of 1 GHz, and the lower limit is not particularly limited, and may be, for example, 1.00 or more. The hollow particles of the present disclosure preferably have a dielectric loss tangent of 3.00×10 at a frequency of 1 GHz. －２ or less, more preferably 6.00 x 10 －３ More preferably, 4.00 x 10 －３ More preferably, 3.00 x 10 －３ The lower limit is not particularly limited, and is, for example, 1.00 × 10 －４In the present disclosure, the relative permittivity and dielectric loss tangent of hollow particles are measured using a perturbation type measuring device.

[0096] The hollow particles of the present disclosure preferably have a porosity of 50% or more, more preferably 60% or more, even more preferably 65% ​​or more, and even more preferably 70% or more. When the porosity is equal to or greater than the above-mentioned lower limit, the hollow particles have excellent dielectric properties, and are also excellent in terms of lightweightness and heat insulation. The upper limit of the porosity of the hollow particles is not particularly limited, but is preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less, in order to prevent a decrease in the strength of the hollow particles and make them less likely to be crushed.

[0097] The porosity of the hollow particles is the apparent density D １ and true density D ０ The apparent density of the hollow particles D １ The measurement method is as follows: First, a volume of 100 cm ３ About 30 cm ３ The volumetric flask is filled with hollow particles, and the mass of the filled hollow particles is accurately weighed. Next, the volumetric flask filled with the hollow particles is accurately filled with isopropanol up to the marked line, taking care not to introduce air bubbles. The mass of isopropanol added to the volumetric flask is accurately weighed, and the apparent density D of the hollow particles is calculated based on the following formula (I): １ (g / cm ３ ) is calculated using the formula (I): Apparent density D １ Apparent density D = [Mass of hollow particles] / (100 - [Mass of isopropanol] / [Specific gravity of isopropanol at measurement temperature]) １ corresponds to the specific gravity of the entire hollow particle when the hollow portion is considered to be a part of the hollow particle.

[0098] True density D of hollow particles ０ The measurement method is as follows: After crushing the hollow particles in advance, ３Approximately 10 g of crushed pieces of hollow particles are filled into a measuring flask, and the mass of the crushed pieces is accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol is added to the measuring flask, and the mass of the isopropanol is accurately weighed. The true density D of the hollow particles is calculated based on the following formula (II): ０ (g / cm ３ ) is calculated using the formula (II): True density D ０ = [mass of crushed pieces of hollow particles] / (100 - [mass of isopropanol] / [specific gravity of isopropanol at measurement temperature]) True density D ０ As is clear from the above measurement method, the true density D ０ In calculating the particle diameter, the hollow portion is not considered to be part of the hollow particle.

[0099] 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

[0100] The volume average particle diameter of the hollow particles of the present disclosure can be adjusted appropriately depending on the application and is not particularly limited. However, the lower limit is preferably 1.00 μm or more, more preferably 1.50 μm or more, and even more preferably 2.00 μm or more. When the volume average particle diameter of the hollow particles is equal to or greater than the above-mentioned lower limit, the tendency for the hollow particles to aggregate with each other is reduced, thereby enabling excellent dispersibility to be exhibited. The upper limit of the volume average particle diameter of the hollow particles of the present disclosure is preferably 10.00 μm or less, more preferably 8.00 μm or less, and even more preferably 5.00 μm or less, for hollow particles used to reduce the dielectric constant or dielectric loss tangent. Hollow particles having a volume average particle diameter equal to or less than the above-mentioned upper limit are sufficiently small that they are suitable for use as substrate materials for electronic circuit boards and the like, and can be added to thin, small substrates. On the other hand, from the viewpoint of a balance between pressure resistance and mechanical strength, the volume average particle diameter is preferably 50 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less.

[0101] The shape of the hollow particles of the present disclosure is not particularly limited as long as a hollow portion is formed inside, and examples thereof include spherical, oval, and amorphous shapes. Among these, spherical shapes are preferred from the viewpoints of ease of production, pressure resistance, and the like. The hollow particles of the present disclosure may have one or more hollow portions; however, from the viewpoints of maintaining a good balance between high porosity and mechanical strength and improving dielectric properties, those having only one hollow portion are preferred. The proportion of hollow particles of the present disclosure having only one or two hollow portions is preferably 90% by mass or more, more preferably 95% by mass or more. Furthermore, the proportion of particles having only one hollow portion is preferably 90% by mass or more, more preferably 95% by mass 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. The hollow particles of the present disclosure may have an average circularity of 0.950 to 0.995. One example of the shape of the hollow particles of the present disclosure is a bag made of a thin film and inflated with gas, the cross-sectional view of which is shown as hollow particle 10 in (5) of Figure 1. In this example, a single thin film is provided on the outside, and the interior is filled with gas. The particle shape can be confirmed, for example, by SEM or TEM.

[0102] The particle size distribution of the hollow particles (volume average particle size (Dv) / number average particle size (Dn)) may be, for example, 1.1 or more and 2.5 or less. When the particle size distribution is 2.5 or less, particles with little variation in compressive strength characteristics and heat resistance can be obtained. Furthermore, when manufacturing a sheet-shaped resin molded product 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 device, 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.

[0103] Furthermore, the hollow particles of the present disclosure have excellent dielectric properties due to the low proportion of particles with a circularity of 0.85 or less. Particles with a circularity of 0.85 or less typically have deformations such as dents or cracks, and are sometimes referred to as "irregularly shaped particles" in the present disclosure. These irregularly shaped hollow particles have inferior dielectric properties due to their lower porosity compared to spherical hollow particles. Therefore, reducing the proportion of irregularly shaped particles contained in the hollow particles can improve the dielectric properties of the hollow particles. Furthermore, irregularly shaped particles tend to aggregate more easily than spherical particles when dispersed in a binder resin, resulting in poor dispersibility. Furthermore, irregularly shaped particles are more susceptible to localized external pressure, resulting in poorer pressure resistance compared to spherical particles. When irregularly shaped particles are dispersed in a binder resin, aggregates tend to form, and the aggregates are more susceptible to external pressure, further worsening the pressure resistance. Therefore, reducing the proportion of irregularly shaped particles contained in the hollow particles can improve the dispersibility and pressure resistance of the hollow particles. The hollow particles of the present disclosure may contain a small amount of impurities, such as particles with low circularity due to cracking or deformation. However, the proportion of particles with a circularity of 0.85 or less, based on 100% by mass of the hollow particles of the present disclosure, is preferably 10% by mass or less, more preferably 7% by mass or less, even more preferably 5% by mass or less, even more preferably 4% by mass or less, and particularly preferably 3% by mass or less. Circularity is defined as the value obtained by dividing the diameter of a circle having the same area as the projected image of the particle (equivalent circle area diameter) by the diameter of a circle having the same perimeter as the projected image of the particle (equivalent circumferential diameter). A perfectly spherical particle has a circularity of 1, and the more complex the particle's surface shape, the smaller the circularity. In the present disclosure, circularity is measured using a flow-type particle image analyzer with an image resolution of 0.185 μm / pixel. A preferred example of a flow-type particle image analyzer is the "IF-3200" manufactured by Jasco International, Inc. A measurement sample is prepared by dispersing a mixture of 0.10 to 0.12 g of hollow particles in an aqueous solution of linear alkylbenzenesulfonate (concentration: 0.3%) in an ultrasonic cleaner for 5 minutes. The average circularity is the average value of the circularity of 1,000 to 3,000 randomly selected particles.

[0104] In the present disclosure, the thermal decomposition onset temperature of the hollow particles is preferably 150 to 400°C, more preferably 200 to 350°C. Hollow particles having a thermal decomposition onset temperature within the above range have excellent heat resistance. In the present disclosure, the thermal decomposition onset temperature of the hollow particles is the temperature at which a weight loss of 5% occurs, and can be measured using a TG-DTA device in an air atmosphere under conditions of an air flow rate of 230 mL / min and a temperature rise rate of 10°C / min.

[0105] Examples of uses of the hollow particles of the present disclosure include additives for low-dielectric, heat-insulating, sound-insulating, and light-reflecting components 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; and 3D printer filaments. The hollow particles of the present disclosure are resistant to crushing during mixing with other materials and molding after mixing, and when added to molded bodies, they exhibit excellent effects as lightweight materials, heat-insulating materials, soundproofing materials, and vibration-damping materials, making them suitable as additives for molded bodies. They are also resistant to crushing during mixing with resins and molding after mixing, and have excellent adhesion to resins, making them suitable as additives for resin molded bodies. The hollow particles of the present disclosure can also be incorporated as a filler in fiber-reinforced molded bodies formed using resin and reinforcing fibers. The hollow particles of the present disclosure, which have excellent dielectric properties, are suitable for use in electrical or electronic fields as additives for achieving low dielectric constants or low transmission losses, for example, as materials for electronic circuit boards. Specifically, by incorporating the hollow particles of the present disclosure into an insulating resin layer of an electronic circuit board, the relative dielectric constant of the insulating resin layer can be reduced, thereby reducing the transmission loss of the electronic circuit board. 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, flexible substrates, high-frequency device modules, antenna modules, and automotive radar. Among these, the hollow particles are particularly suitable as additives in semiconductor materials such as interlayer insulating materials, solder resists, magnet wires, epoxy encapsulants, underfills, buffer coating materials, copper-clad laminates, flexible substrates, high-frequency device modules, antenna modules, and automotive radar. Furthermore, the hollow particles of the present disclosure have high porosity, are difficult to crush, and have excellent heat resistance, so they meet the heat insulation and shock-absorbing properties (cushioning properties) required for undercoating materials and also meet the heat resistance required for thermal paper applications.The hollow particles of the present disclosure are also useful as plastic pigments that are excellent in gloss, hiding power, etc.Furthermore, the hollow particles of the present disclosure can be 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.

[0106] 3. Resin Composition The resin composition of the present disclosure contains the hollow particles of the present disclosure and a binder resin having a functional group capable of reacting with a carboxy group.

[0107] [Binder Resin] The binder resin having a functional group reactive with a carboxy group may have a functional group reactive with a carboxy group when mixed with the hollow particles of the present disclosure. After the carboxy group of the hollow particle reacts with the functional group of the binder resin to form a crosslinked bond, the binder resin does not need to have the functional group. In the present disclosure, the reaction between the carboxy group on the surface of the hollow particle and the functional group of the binder resin may be a reaction that forms a covalent bond, such as an addition reaction, a substitution reaction, or a dehydration condensation reaction. Furthermore, the binder resin having a functional group reactive with a carboxy group may be an unreacted monomer, prepolymer, or macromonomer, a polymer, or a precursor of a curable resin such as polyamic acid when mixed with the hollow particles of the present disclosure.

[0108] Examples of functional groups reactive with carboxy groups include epoxy groups, hydroxyl groups, amino groups, amide bonds, isocyanate groups, carboxy groups, and thiol groups. Among these, from the viewpoint of reactivity, at least one selected from the group consisting of epoxy groups, amino groups, isocyanate groups, hydroxyl groups, carboxy groups, and thiol groups is preferred, with epoxy groups being particularly preferred. Binder resins having these functional groups can be appropriately selected from known resins and are not particularly limited. For example, epoxy resins, polyimide resins, urethane resins, thiol resins, or precursors or raw material compounds thereof can be preferably used. Epoxy resins have epoxy groups at least before the curing reaction with a curing agent. Polyimide resins have amino groups or amide bonds and carboxy groups in the polyamic acid state before the curing reaction (imidization reaction). The diamine component serving as the raw material for the polyamic acid has amino groups, and the tetracarboxylic acid component has carboxy groups or carboxylic anhydride groups. Urethane-based resins are obtained by reacting a polyol component having a hydroxyl group with a polyisocyanate component having an isocyanate group, and therefore have hydroxyl groups and isocyanate groups before the curing reaction. Binder resins having these functional groups may be thermosetting resins, thermoplastic resins, adhesives that can be cured at room temperature, or photocurable resins. The binder resin contained in the resin composition of the present disclosure may function as a binder (binding agent) by being cured by heating, light irradiation, or using a curing agent, polymerization initiator, catalyst, or the like. Furthermore, epoxy-based resins, polyimide-based resins, and urethane-based resins have good dielectric properties and are therefore preferably used in applications requiring low dielectric constants and low dielectric loss tangents.

[0109] Examples of epoxy resins include bixylenol type epoxy resins, bisphenol A type epoxy resins, bisphenol F type epoxy resins, bisphenol S type epoxy resins, bisphenol AF type epoxy resins, dicyclopentadiene type epoxy resins, trisphenol type epoxy resins, naphthol novolac type epoxy resins, phenol novolac type epoxy resins, tert-butyl-catechol type epoxy resins, naphthalene type epoxy resins, naphthol type epoxy resins, anthracene type epoxy resins, glycidylamine type epoxy resins, and glycidyl ester type epoxy resins. , cresol novolac type epoxy resins, phenol aralkyl type epoxy resins, biphenyl type epoxy resins, linear aliphatic epoxy resins, epoxy resins having a butadiene structure, alicyclic epoxy resins, heterocyclic epoxy resins, spiro ring-containing epoxy resins, cyclohexane type epoxy resins, cyclohexane dimethanol type epoxy resins, naphthylene ether type epoxy resins, trimethylol type epoxy resins, tetraphenylethane type epoxy resins, isocyanurate type epoxy resins, phenolphthalimidine type epoxy resins, phenolphthalein type epoxy resins, etc. These epoxy resins can be used either alone or in combination of two or more.

[0110] Examples of polyol components used in urethane-based resins include ethylene glycol, propylene glycol, glycerin, trimethylolpropane, pentaerythritol, sorbitol, sucrose, polyoxypropylene triol, polyester polyol, etc. Examples of polyisocyanate components used in urethane-based resins include aliphatic polyisocyanates and aromatic polyisocyanates. Examples of the aliphatic polyisocyanate include polyisocyanates having a chain structure such as tetramethylene diisocyanate, hexamethylene diisocyanate, dodecamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, lysine diisocyanate, 2-methylpentane-1,5-diisocyanate, and 3-methylpentane-1,5-diisocyanate; and polyisocyanates having a cyclic structure such as isophorone diisocyanate, hydrogenated xylylene diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, 1,4-cyclohexane diisocyanate, methylcyclohexylene diisocyanate, and 1,3-bis(isocyanatemethyl)cyclohexane. Examples of aromatic polyisocyanates include tolylene diisocyanate, 2,2'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, 4,4'-diphenylmethane diisocyanate, 4,4'-dibenzyl diisocyanate, 1,5-naphthylene diisocyanate, xylylene diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, dialkyldiphenylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, and α,α,α,α-tetramethylxylylene diisocyanate.

[0111] Polyamic acid, a precursor of polyimide resins, is obtained by reacting a tetracarboxylic acid component, such as a tetracarboxylic acid dianhydride, a tetracarboxylic acid, or a tetracarboxylic acid derivative such as a tetracarboxylic acid diester, with a diamine component. Examples of the tetracarboxylic acid component include benzophenonetetracarboxylic acid dianhydride, 4,4'-oxydiphthalic anhydride, pyromellitic acid and its dianhydride, and 3,3',4,4'-biphenyltetracarboxylic acid and its dianhydride. Examples of the diamine component include paraphenylenediamine and 4,4'-diaminodiphenyl ether.

[0112] As the thiol resin, for example, polyphenylene sulfide resin is preferably used. Any known resin containing a polyphenylene sulfide skeleton can be used as the polyphenylene sulfide resin, and there is no particular limitation. Polymers homologous to polyphenylene sulfide (e.g., polyphenylene sulfide ketone PPSK, polyphenylene sulfide sulfone PPSS, polybiphenylene sulfide PBPS, etc.) are also included in the polyphenylene sulfide resin. Furthermore, as the thiol resin, thiol compounds such as trimethylolpropane tris(3-mercaptopropionate), pentaerythritol 3-mercaptopropionic acid ester, dipentaerythritol hexakis(3-mercaptopropionate), tris-[(3-mercaptopropionyloxy)-ethyl]-isocyanurate, polyethylene glycol bis(3-mercaptopropionate), and pentaerythritol tetrapropanethiol can also be used.

[0113] Furthermore, as the binder resin having a functional group, resins having a hydroxyl group such as phenolic resins, polyvinyl alcohol, and ethylene-vinyl alcohol copolymers; binder resins having an amino group such as melamine resins, etc. can also be used.

[0114] Furthermore, examples of binder resins having functional groups reactive with carboxy groups include modified resins modified with the functional groups. Examples of such modified resins include modified polyolefin resins such as epoxy-modified polyolefin resins and acid-modified polyolefin resins. For example, when the hollow particles of the present disclosure are mixed into a molten modified polyolefin resin, the mixing occurs at a high temperature, and the heat causes a reaction between the functional groups of the modified polyolefin resin and the carboxy groups of the hollow particles, resulting in the formation of crosslinks.

[0115] The resin composition of the present disclosure may further contain a binder resin not having the functional group in addition to the binder resin having the functional group, as long as the effects of the present disclosure are not impaired. The binder resin not having the functional group can be appropriately selected from, for example, known thermosetting resins, thermoplastic resins, thermoplastic elastomers, and the like. Specific examples include polyethylene, polypropylene, acrylonitrile-butadiene-styrene copolymers (ABS resins), styrene-isoprene-styrene block copolymers (SIS), styrene-butadiene-styrene block copolymers (SBS), styrene-ethylene-propylene-styrene block copolymers (SEPS), and styrene-ethylene-butylene-styrene block copolymers (SEBS). From the viewpoint of improving adhesion between the binder resin and the hollow particles, the content of the binder resin not having the functional group is preferably 90 parts by mass or less, more preferably 50 parts by mass or less, per 100 parts by mass of the binder resin having the functional group.

[0116] The content of the binder resin in 100% by mass of the total solid content of the resin composition is not particularly limited, but the lower limit is preferably 50% by mass or more, more preferably 60% by mass or more, from the viewpoint of improving mechanical strength, while the upper limit is preferably 95% by mass or less, more preferably 90% by mass or less, from the viewpoint of sufficiently containing hollow particles.

[0117] [Curing Agent] The resin composition of the present disclosure may further contain a curing agent for curing the binder resin. The curing agent can be appropriately selected from known curing agents depending on the type of binder resin, and is not particularly limited. However, 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 not particularly limited, and may be, for example, 5 to 120 parts by mass per 100 parts by mass of the binder resin.

[0118] [Catalyst] The resin composition of the present disclosure may further contain a catalyst for curing the binder resin. Examples of catalysts used for curing polyimide resins, i.e., for the imidization reaction, include a combination of an organic acid anhydride and an organic base. Examples of organic acid anhydrides include acetic anhydride, propionic anhydride, maleic anhydride, and phthalic anhydride. Examples of organic bases include heterocyclic compounds such as pyridine and picoline; and tertiary amines such as triethylamine and N,N-dimethylaniline. The content of the catalyst is adjusted appropriately depending on the type of resin, and is not particularly limited.

[0119] [Hollow Particles] The hollow particles contained in the resin composition of the present disclosure are the hollow particles described above. In the resin composition of the present disclosure, the content of the hollow particles is not particularly limited, but is preferably 5 to 50 mass%, more preferably 5 to 30 mass%, and even more preferably 5 to 15 mass%, based on 100 mass% of the total solids content of the resin composition. When the content of hollow particles is at or above the lower limit, the resin composition can achieve improved effects such as low dielectric constant, weight reduction, and thermal insulation. When the content of hollow particles is at or below the upper limit, the resin composition can be sufficiently filled with binder resin, thereby suppressing deterioration in physical properties when molded into a molded product and improving mechanical strength. Furthermore, although not particularly limited, from the viewpoint of improving adhesion between the binder resin and the hollow particles, the content of hollow particles per 100 parts by mass of binder resin having a functional group reactive with a carboxy group is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, and preferably 50 parts by mass or less, more preferably 40 parts by mass or less.

[0120] [Other Additives] The resin composition of the present disclosure may further contain additives such as a compatibilizer, an ultraviolet absorber, a colorant, a heat stabilizer, a filler, a flame retardant, and a solvent, as necessary, within a range that does not impair the effects of the present disclosure. Furthermore, when the resin composition of the present disclosure is formed into a resin molded product, it may contain organic or inorganic reinforcing fibers such as carbon fiber, glass fiber, aramid fiber, and polyethylene fiber.

[0121] The resin composition of the present disclosure can be obtained, for example, by mixing the hollow particles of the present disclosure, the binder resin, and further additives that are added as needed, a solvent, etc. When the binder resin in the resin composition of the present disclosure is a thermoplastic resin, the hollow particles of the present disclosure and further additives that are added as needed may be added to a molten thermoplastic resin and mixed by melt-kneading.

[0122] The resin composition of the present disclosure may be a resin composition before the carboxy groups on the surface of the hollow particles react with the functional groups on the binder resin to form crosslinks. In this case, during the process of curing or molding the resin composition of the present disclosure, the carboxy groups on the surface of the hollow particles react with the functional groups on the binder resin to form crosslinks, thereby achieving excellent adhesion at the interface between the binder resin and the hollow particles. Furthermore, even when the resin composition of the present disclosure contains components other than the hollow particles of the present disclosure and the binder resin having functional groups, the adhesiveness at the interface between the binder resin and the hollow particles can be excellent. This is presumably because the functional groups on the binder resin have a high affinity with the carboxy groups on the surface of the hollow particles, which allows the binder resin having functional groups to be unevenly distributed around the hollow particles in the resin composition, thereby allowing the functional groups to form crosslinks with the carboxy groups on the surface of the hollow particles. Examples of methods for forming crosslinks by reacting carboxy groups on the surface of hollow particles with functional groups on the binder resin include heating the resin composition of the present disclosure. Here, heating may be, for example, heating to reduce the melt viscosity of the resin for molding when the binder resin is a thermoplastic resin, heating to cure the resin when the binder resin is a thermosetting resin, or heating to dry the resin composition when the resin composition contains a solvent. The heating conditions are adjusted appropriately depending on the purpose of heating and are not particularly limited. However, from the perspective of reacting the carboxy groups on the hollow particles with the functional groups on the binder resin, heating is preferably performed at a temperature of 50 to 300°C for a total of 1 to 24 hours. Alternatively, the hollow particles and binder resin may be mixed and the binder resin cured at room temperature, whereby the carboxy groups on the surface of the hollow particles react with the functional groups on the binder resin to form crosslinks.

[0123] The resin composition of the present disclosure may be a liquid resin composition. The binder resin contained in the liquid resin composition may be a liquid binder resin before a curing reaction, may be dissolved or dispersed in a solvent, or may be a molten thermoplastic resin. The resin composition of the present disclosure may be a resin molded product obtained by forming the liquid resin composition into a resin molded product by a known method.

[0124] The resin molded product of the present disclosure comprises hollow particles derived from the hollow particles of the present disclosure and a binder resin, wherein the shells of the hollow particles are cross-linked to the binder resin. The binder resin contained in the resin molded product of the present disclosure is a solidified product. The solidified binder resin is a resin that has solidified with or without a chemical reaction, such as a resin that has hardened by a curing reaction, a resin that has solidified by drying, or a thermoplastic resin that has solidified by cooling. The resin molded product obtained using the above-described resin composition contains, as the binder resin, a cured product of a resin that has been cured using a curing agent, a polymerization initiator, a catalyst, or the like, as necessary. In this case, the binder resin may contain a curing agent, etc. The resin molded product of the present disclosure obtained by melt-kneading and molding the hollow particles of the present disclosure with a thermoplastic resin contains, as the binder resin, a solidified product of the thermoplastic resin that has solidified by cooling. In the resin molded article of the present disclosure, the hollow particles impart properties such as low dielectric constant, weight reduction, and heat insulation, and further, the excellent adhesion between the binder resin and the hollow particles suppresses deterioration of the resin properties.

[0125] The method for producing a resin molded product of the present disclosure includes, for example, a step of mixing the hollow particles of the present disclosure with a binder resin having a functional group reactive with a carboxy group (hereinafter referred to as step 1), a step of solidifying the binder resin (hereinafter referred to as step 2), and a step of reacting the carboxy group of the hollow particles with the functional group of the binder resin (hereinafter referred to as step 3). In the method for producing a resin molded product of the present disclosure, the order of steps 1, 2, and 3 is not particularly limited. For example, steps 2 and 3 may be performed after step 1. In this case, the order of steps 2 and 3 is not particularly limited. If technically possible, steps 2 and 3 may be performed simultaneously. Alternatively, steps 1 and 3 may be performed simultaneously before step 2.

[0126] In the first step, a mixture containing at least the hollow particles of the present disclosure and a binder resin having a functional group capable of reacting with a carboxy group is prepared. The first step may be a step of preparing the resin composition of the present disclosure.

[0127] In the second step, the mixture obtained in the first step is solidified with or without a chemical reaction, thereby solidifying the binder resin. When the binder resin is cured by a curing agent or a catalyst, the curing agent or catalyst may be added in the second step. When the binder resin is a curable resin, the curing method for the curable resin is not particularly limited, and examples include heating and irradiation with light such as ultraviolet light or electron beams. Alternatively, the binder resin may be cured by adding a catalyst and mixing at room temperature.

[0128] In the third step, the carboxyl groups of the hollow particles are reacted with the functional groups of the binder resin in the mixture obtained in the first step, or in a mixture obtained by further adding a curing agent or catalyst, or in the solidified product obtained in the second step, thereby crosslinking the shells of the hollow particles and the binder resin. The third step may be performed in conjunction with the curing reaction of the binder resin in the second step. For example, if the curing reaction of the binder resin and the reaction between the carboxyl groups of the hollow particles and the functional groups of the binder resin both proceed at room temperature by the addition of a curing agent or catalyst, the second and third steps can be performed simultaneously by adding a curing agent or catalyst to the resin composition of the present disclosure and then curing the resin composition. Alternatively, if the curing reaction of the binder resin and the reaction between the carboxyl groups of the hollow particles and the functional groups of the binder resin both proceed by heat, the second and third steps can be performed simultaneously by heating the resin composition of the present disclosure. When a thermoplastic resin is used as the binder resin, the kneading of the hollow particles with the thermoplastic resin and the subsequent molding are carried out at high temperatures. Therefore, when the binder resin is a thermoplastic resin and the reaction between the carboxyl groups of the hollow particles and the functional groups of the binder resin proceeds due to heat, the third step can be carried out simultaneously with the first or second step.

[0129] The resin molded article of the present disclosure may be, for example, a resin molded article obtained by forming the liquid resin composition of the present disclosure described above into a resin molded article using a known method. In the process of converting the liquid resin composition into a resin molded article, the aforementioned heating or room temperature curing reaction is typically carried out. In the resin molded article, the curing reaction involves the reaction of carboxyl groups present on the hollow particles with functional groups present on the binder resin to form crosslinks. The surfaces of the hollow particles are covalently bonded to the binder resin, resulting in excellent interfacial adhesion between the binder resin and the hollow particles. For example, a liquid resin composition containing hollow particles or the like in a liquid binder resin prior to the curing reaction, or a liquid resin composition obtained by dissolving or dispersing each component in a solvent, can be applied to a support, and then dried and cured as necessary to obtain the resin molded article of the present disclosure. 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. These supports may have a release agent applied to their surfaces. Known methods can be used to apply the liquid resin composition, including dip coating, roll coating, curtain coating, die coating, slit coating, and gravure coating. Alternatively, a resin molded product can be obtained by impregnating a substrate with the liquid resin composition and, if necessary, drying and curing it. Examples of the substrate include inorganic fibers such as carbon fiber, glass fiber, metal fiber, and ceramic fiber, and organic synthetic fibers such as polyamide fiber, polyester fiber, polyolefin fiber, and novoloid fiber. Glass fiber (glass cloth) is preferred. The substrate may be in any form, including woven fabrics and nonwoven fabrics. When the liquid resin composition contains a solvent, it is preferable to dry the liquid resin composition after application or impregnation. The drying temperature is preferably set to a temperature that does not harden the binder resin, typically 20°C to 200°C, preferably 30°C to 150°C. The drying time is typically 30 seconds to 1 hour, preferably 1 minute to 30 minutes. The curing reaction of the resin composition is carried out by a method that depends on the type of binder resin, and is not particularly limited.When a binder resin that cures upon heating is included, the heating temperature for the curing reaction is adjusted appropriately depending on the type of resin and is not particularly limited. However, it is typically 30°C to 400°C, preferably 70°C to 300°C, and more preferably 100°C to 200°C. The curing time is 5 minutes to 5 hours, preferably 30 minutes to 3 hours. The heating method is not particularly limited, and may be performed using, for example, an electric oven. The liquid binder resin before the curing reaction and the binder resin dissolved or dispersed in a solvent may be a thermosetting resin or a thermoplastic resin. Alternatively, a resin composition containing a thermoplastic resin as a binder resin may be molded into a desired shape using a known molding method such as extrusion molding, injection molding, press molding, or compression molding to obtain a resin molded product. The temperature during melt-kneading is not particularly limited, as long as it is a temperature at which the thermoplastic resin used can be melted. 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.

[0130] The shape of the resin molded body is not particularly limited and can be any moldable shape, such as a sheet, a film, a plate, a tube, or any other three-dimensional shape. When the resin molded body contains fibers, the fibers in the resin molded body may be in the form of a nonwoven fabric. When the resin molded body contains fibers, the resin molded body may be a molded body of a resin composition in which hollow particles of the present disclosure are added to a fiber-reinforced plastic containing the resin and fibers as described above.

[0131] Examples of uses of the resin composition and resin molded article of the present disclosure include uses in which the resin composition or resin molded article can be used, among the uses of the hollow particles of the present disclosure described above.

[0132] 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.

[0133] Example 1 (1) Mixed Liquid Preparation Step First, the following materials were mixed to form an oil phase: Divinylbenzene 83.7 parts Ethylvinylbenzene 3.5 parts t-Butylperoxydiethyl acetate (oil-soluble polymerization initiator) 2.3 parts Hydrophobic solvent: Heptane (solubility in water at 20°C: 2.2 mg / L, boiling point 98.4°C) 155.7 parts Next, in a stirring vessel, an aqueous solution prepared by dissolving 19.6 parts of magnesium chloride (a water-soluble polyvalent metal salt) in 225 parts of ion-exchanged water was gradually added with stirring to prepare a magnesium hydroxide colloid (poorly water-soluble metal hydroxide colloid) dispersion (magnesium hydroxide 10 parts), which served as the aqueous phase. The resulting aqueous phase and oil phase were mixed to prepare a mixed liquid.

[0134] (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 monomer composition encapsulating the hydrophobic solvent were dispersed in water.

[0135] (3) Precursor Composition Preparation Step: The suspension obtained in the suspension step was heated to 80°C in a nitrogen atmosphere and stirred at 80°C for 1 hour to carry out a first polymerization reaction. The polymerization conversion rate at the end of the first polymerization reaction was 50% by mass. Subsequently, 12.8 parts of ethylene glycol dimethacrylate (EGDMA) (solubility in water at 20°C: 0.6 g / L) was added to the stirring tank as a carboxyl group-introducing group-containing monomer, and a second polymerization reaction was carried out by stirring at 80°C for 23 hours under a nitrogen atmosphere. Next, an aqueous NaOH solution was added to the stirring tank to adjust the pH of the slurry to 13 or higher. The mixture was then stirred at 80°C for 24 hours to hydrolyze the monomer units derived from ethylene glycol dimethacrylate, thereby deprotecting the protected carboxyl groups. This resulted in a precursor composition that was a slurry liquid in which precursor particles containing a hydrophobic solvent and having carboxyl groups on their surfaces were dispersed in water.

[0136] (4) Washing Step and Solid-Liquid Separation Step The precursor composition obtained in the precursor composition preparation step was washed with dilute sulfuric acid (25°C, 10 minutes) to adjust the pH to 5.5 or less. Next, after separating the water by filtration, 200 parts of fresh ion-exchanged water was added to re-slurry the mixture. The water washing treatment (washing, filtration, dehydration) was repeated several times at room temperature (25°C), and the mixture was filtered to obtain a solid fraction. The obtained solid fraction was dried in a dryer at a temperature of 40°C to obtain precursor particles encapsulating the hydrophobic solvent.

[0137] (5) Solvent Removal Step The precursor particles obtained in the solid-liquid separation step were heat-treated in a vacuum dryer at 200°C for 12 hours to remove the hydrophobic solvent contained in the particles, thereby obtaining hollow particles of Example 1. From the results of observation with a scanning electron microscope and the porosity value, it was confirmed that the obtained hollow particles were spherical and had hollow portions.

[0138] [Example 2] Hollow particles of Example 2 were produced in the same manner as in Example 1, except that methyl methacrylate (MMA) (solubility in water at 20°C: 16 g / L) was added instead of EGDMA in the "(3) precursor composition preparation step" in Example 1.

[0139] Examples 3 to 5 Hollow particles of Examples 3 to 5 were produced in the same manner as in Example 1, except that in the "(1) mixed solution preparation step" in Example 1, the amount of divinylbenzene added was changed to 71.4 parts, and the amount of ethylvinylbenzene added was changed to 3.0 parts, and in the "(3) precursor composition preparation step" in Example 1, 25.6 parts of a carboxy group-introducing monomer shown in Table 1 was added instead of 12.8 parts of EGDMA. Note that the solubility of t-butyl acrylate used as the carboxy group-introducing monomer in Example 5 in water at 20°C is 2 g / L.

[0140] [Example 6] The hollow particles of Example 6 were produced in the same procedure as in Example 1, except that in the "(1) mixed solution preparation step", the amount of divinylbenzene added was changed to 91.2 parts, and the amount of ethylvinylbenzene added was changed to 3.8 parts, and in the "(3) precursor composition preparation step", 12.8 parts of EGDMA was replaced with 5.0 parts of methyl methacrylate (MMA), and the reaction time of the first polymerization reaction was changed from 1 hour to 0.5 hours, and the reaction time of the second polymerization reaction was changed from 23 hours to 23.5 hours.

[0141] [Example 7] The hollow particles of Example 7 were produced in the same manner as in Example 6, except that the reaction time of the first polymerization reaction was changed from 0.5 hours to 4 hours and the reaction time of the second polymerization reaction was changed from 23.5 hours to 20 hours.

[0142] [Example 8] The hollow particles of Example 8 were produced in the same procedure as in Example 1, except that the "(1) mixed solution preparation step" in Example 1 was changed as follows: the reaction time of the first polymerization reaction was changed from 1 hour to 0.5 hours, and the reaction time of the second polymerization reaction was changed from 23 hours to 23.5 hours. The mixed solution preparation step of Example 8 was carried out as follows. First, the following materials were mixed to form an oil phase. Divinylbenzene 25.7 parts Ethylvinylbenzene 18.0 parts Ethylene glycol dimethacrylate 22.2 parts Pentaerythritol tetraacrylate 20.0 parts t-Butylperoxydiethyl acetate (oil-soluble polymerization initiator) 2.0 parts Hydrophobic solvent: Hexane 92.3 parts Next, in a stirring tank, at room temperature, an aqueous solution obtained by dissolving 7.8 parts of magnesium chloride (a water-soluble polyvalent metal salt) in 225 parts of ion-exchanged water was gradually added with stirring to prepare a magnesium hydroxide colloid (poorly water-soluble metal hydroxide colloid) dispersion (magnesium hydroxide 4 parts), which served as the aqueous phase. The resulting aqueous phase and oil phase were mixed to prepare a mixed liquid.

[0143] [Example 9] The hollow particles of Example 9 were produced in the same procedure as in Example 8, except that in the above "(3) precursor composition preparation step", methyl methacrylate (MMA) was added instead of EGDMA, the reaction time of the first polymerization reaction was changed from 0.5 hours to 0.6 hours, and the reaction time of the second polymerization reaction was changed from 23.5 hours to 23.4 hours.

[0144] [Example 10] The hollow particles of Example 10 were produced in the same procedure as in Example 8, except that in the above "(1) mixed solution preparation step", the amount of skeleton-forming monomer added was changed according to Table 2, and in the above "(3) precursor composition preparation step", 5.0 parts of MMA was added instead of 12.8 parts of EGDMA, and the reaction time of the first polymerization reaction was changed from 0.5 hours to 4 hours, and the reaction time of the second polymerization reaction was changed from 23.5 hours to 20 hours.

[0145] [Example 11] The hollow particles of Example 11 were produced in the same procedure as in Example 8, except that in the above "(1) mixed solution preparation step", the amount of skeleton-forming monomer added was changed according to Table 2, the amount of EGDMA added was changed to 5.0 parts in the above "(3) precursor composition preparation step", the reaction time of the first polymerization reaction was changed from 0.5 hours to 0.1 hours, and the reaction time of the second polymerization reaction was changed from 23.5 hours to 23.9 hours.

[0146] [Example 12] The hollow particles of Example 12 were produced in the same procedure as in Example 1, except that in the above "(1) mixed solution preparation step", the amount of skeleton-forming monomer added was changed according to Table 2, the amount of heptane added was changed to 204.0 parts, and in the above "(3) precursor composition preparation step", 15.2 parts of MMA was added instead of 12.8 parts of EGDMA, and the reaction time of the first polymerization reaction was changed from 1 hour to 0.3 hours, and the reaction time of the second polymerization reaction was changed from 23 hours to 23.7 hours.

[0147] Example 13 The hollow particles of Example 13 were produced using the same procedure as in Example 1, except that in the "(1) mixed solution preparation step" above, the amount of skeleton-forming monomer added was changed according to Table 3, and the "(3) precursor composition preparation step" above was performed as follows. In the precursor composition preparation step of Example 13, the suspension obtained in the suspension step was heated to 80°C in a nitrogen atmosphere and stirred at 80°C for 0.2 hours to carry out a first polymerization reaction. 25.6 parts of methacrylic acid (MAA) (solubility in water at 20°C: 89 g / L) was added as a carboxyl group-containing monomer to a stirring tank, and the mixture was stirred at 80°C for 23.8 hours under a nitrogen atmosphere to carry out a second polymerization reaction. This resulted in a precursor composition that was a slurry liquid in which precursor particles containing a hydrophobic solvent and having carboxyl groups on their surfaces were dispersed in water.

[0148] Example 14 The hollow particles of Example 14 were produced in the same manner as in Example 13, except that in the above "(1) mixed solution preparation step", the types and amounts of the skeleton-forming monomer and hydrophobic solvent were changed according to Table 3, and in the above "(3) precursor composition preparation step", the amount of MAA added was changed to 22.0 parts, and the reaction time of the first polymerization reaction was changed to 0.1 hours, and the reaction time of the second polymerization reaction was changed to 23.9 hours.

[0149] [Example 15] Hollow particles of Example 15 were produced in the same manner as in Example 13, except that the reaction time of the first polymerization reaction was changed to 4 hours and the reaction time of the second polymerization reaction was changed to 20 hours.

[0150] [Example 16] Hollow particles of Example 16 were produced in the same manner as in Example 14, except that the reaction time of the first polymerization reaction was changed to 3 hours and the reaction time of the second polymerization reaction was changed to 21 hours.

[0151] Comparative Example 1 Hollow particles of Comparative Example 1 were produced in the same manner as in Example 1, except that in the above "(1) mixed solution preparation step", the amount of divinylbenzene added was changed to 96 parts and the amount of ethylvinylbenzene added was changed to 4.0 parts, in the above "(3) precursor composition preparation step", EGDMA was not added, and the suspension obtained in the suspension step was simply heated to 80°C in a nitrogen atmosphere and stirred at a temperature of 80°C for 24 hours.

[0152] Comparative Example 2 Hollow particles of Comparative Example 2 were produced in the same manner as in Example 2, except that the addition of an aqueous NaOH solution for deprotection and the subsequent stirring at 80°C for 24 hours were not performed in the above "(3) precursor composition preparation step".

[0153] Comparative Example 3 The hollow particles of Comparative Example 3 were produced using the same procedure as in Example 1, except that the "(1) mixed solution preparation step" in Example 1 was changed as follows: in the "(3) precursor composition preparation step," the suspension obtained in the suspension step was simply heated to 65°C in a nitrogen atmosphere and stirred at 65°C for 4 hours; and in the (5) solvent removal step, the heat treatment time was changed from 12 hours to 6 hours. The mixed solution preparation step of Comparative Example 3 was performed as follows. First, the following materials were mixed to form an oil phase: 60 parts ethylene glycol dimethacrylate, 40 parts methacrylic acid, 3 parts 2,2'-azobis(2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator), and 150 parts hydrophobic solvent: cyclohexane. Next, 4.0 parts of a surfactant was added to 800 parts of ion-exchanged water to prepare an aqueous solution, which was used as the water phase. The resulting water phase and oil phase were mixed to prepare a mixed solution.

[0154] <Measurement of Polymerization Conversion Rate> In the precursor composition preparation step of each Example and Comparative Example 2, 50 g of the suspension was sampled immediately before the addition of the carboxy group-introducing monomer or carboxy group-containing monomer. The pH of the suspension was adjusted to 5 to 6 and then pressure filtered to separate the polymer precipitate (including water and the hydrophobic solvent) contained in the suspension. After removing the water and hydrophobic solvent by drying at 200°C for 2 hours, the mass of the polymer precipitate was precisely weighed and used as the mass of the skeleton-forming monomer contained in the suspension (the total mass of the skeleton-forming monomer after the polymerization reaction and the unreacted skeleton-forming monomer). This polymer precipitate was dispersed in ethyl acetate, and 2 μL of the resulting dispersion was sampled as a measurement sample. The mass of the unreacted skeleton-forming monomer in the measurement sample was quantified by gas chromatography (GC) under the following conditions, and the mass of the unreacted skeleton-forming monomer contained in the suspension was calculated. The polymerization conversion rate was calculated using the mass of the skeleton-forming monomer contained in the suspension, i.e., the mass of the polymer precipitate, and the mass of the unreacted skeleton-forming monomer contained in the suspension, i.e., the mass of the unreacted skeleton-forming monomer measured by GC, using the following formula (C). Alternatively, if no polymer precipitate was formed, the suspension was adjusted to pH 5 to 6 and separated into two phases, and the oil phase was isolated. 2 μL of the resulting oil phase was collected as a measurement sample, and the amount of polymerized or unreacted skeleton-forming monomer in the measurement sample was quantified by gas chromatography (GC) under the following conditions, and the mass of polymerized or unreacted skeleton-forming monomer contained in the oil phase was calculated. At this time, the non-reactive encapsulating solvent (hydrophobic solvent) contained in the oil phase is divided (divided) to calculate the mass of the unreacted skeleton-forming monomer contained in the oil phase alone. The polymerization conversion rate was calculated from the mass of the skeleton-forming monomer contained in the suspension, i.e., the total mass of the skeleton-forming monomer after the polymerization reaction and the unreacted skeleton-forming monomer contained in the suspension, and the mass of the unreacted skeleton-forming monomer contained in the suspension, using the following formula (C): Formula (C): Polymerization conversion rate (mass %) = 100 - (mass of unreacted skeleton-forming monomer / mass of skeleton-forming monomer contained in suspension) x 100 (GC conditions) Column: TC-WAX (0.25 mm x 30 m) Column temperature: 80°C Injection temperature: 200°C FID detection side temperature: 200°C

[0155] [Evaluation] The hollow particles obtained in each Example and Comparative Example were subjected to the following measurements and evaluations. The results are shown in Tables 1 to 4.

[0156] 1. Density and porosity of hollow particles 1-1. Measurement of apparent density of hollow particles First, a 100 cm ３ About 30 cm ３ The hollow particles were packed into the measuring flask, and the mass of the packed hollow particles was accurately weighed. Next, the measuring flask packed with the hollow particles was accurately filled with isopropanol up to the marked line, taking care not to introduce air bubbles. The mass of the isopropanol added to the measuring flask was accurately weighed, and the apparent density D of the hollow particles was calculated based on the above formula (I). １ (g / cm ３ 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])

[0157] 1-2. Measurement of true density of hollow particles After crushing the hollow particles in advance, ３ Approximately 10 g of crushed pieces of hollow particles was filled into a measuring flask, and the mass of the crushed pieces was accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol was added to the measuring flask, and the mass of the isopropanol was accurately weighed. The true density D of the hollow particles was calculated based on the following formula (II): ０ (g / cm ３ ) was calculated using the formula (II): True density D ０ = [Mass of crushed hollow particle fragments] / (100 - [Mass of isopropanol] / [Specific gravity of isopropanol at measurement temperature])

[0158] 1-3. 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

[0159] 2. Measurement of Volume Average Particle Size (Dv) and Number Average Particle Size (Dn), and Calculation of Particle Size Distribution (Dv / Dn) 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), and the particle size distribution (Dv / Dn) was calculated. 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 further added to wet the particles, and then 10 ml of dispersion medium was added. The mixture was dispersed in an ultrasonic disperser for 1 minute, and then measured using the particle size distribution analyzer.

[0160] 3. Measurement of Carboxylic Acid Amount 3-1. Measurement of Acid Value The acid value was measured by potentiometric titration in accordance with JIS K 0070. The specific measurement method is as follows. 2 g of hollow particles were added to 50 mL of ethanol and mixed to obtain a sample solution containing hollow particles. The sample solution containing hollow particles was subjected to potentiometric titration using a potentiometer AT-710 (manufactured by Kyoto Electronics Manufacturing Co., Ltd.) and a 0.1 mol / L KOH alcohol solution as the titrant, and the obtained inflection point was taken as the endpoint. Separately from this main test, a blank test was conducted on a sample solution that did not contain hollow particles, and the acid value of the hollow particles was calculated using the following formula: Acid value (mmol / g) = (V １ -V ０ ) × N × f / S S: Mass of sample solution (g) V １ V: Volume of titrant in this test (mL) ０ : Volume of titrant in blank test (mL) N: Concentration of titrant (mol / L) f: Factor of titrant

[0161] 3-2. Measurement of specific surface area The volume average particle diameter and apparent density D of the hollow particles measured above １ From the above formula (B), the specific surface area (m ２ / g) was calculated.

[0162] 3-3. Calculation of the amount of carboxylic acid The amount of carboxylic acid per unit area of ​​the hollow particles (μmol / m) was calculated from the acid value of the hollow particles and the specific surface area of ​​the hollow particles using the above formula (A). ２ ) was calculated. In accordance with Rule B of JIS Z8401:1999, the acid value and specific surface area were rounded to three decimal places, and the carboxylic acid amount was rounded to three decimal places. The acid value used in the above formula (A) was a value converted into "μmol / g".

[0163] 4. Measurement of Dielectric Constant (Dk) and Dielectric Loss Tangent (Df) of Hollow Particles The dielectric constant and dielectric loss tangent of the hollow particles were measured at a frequency of 1 GHz and room temperature (25°C) using a measuring device (manufactured by AET Co., Ltd., model: ADMS01Nc).

[0164] 5. Adhesion to Resin 0.2 g of hollow particles was added to 0.8 g of Agent A (main component: 4,4'-isopropylidenediphenol and 1-chloro-2,3-epoxypropane polycondensate (bisphenol A liquid epoxy resin)) of an epoxy adhesive (product name: Quick 5, manufactured by Konishi Co., Ltd.) and mixed until uniform. 0.9 g of Agent B (curing agent: a mixture of polythiol (curing agent), polyamidoamine (curing agent), tertiary amine (curing agent), and silica) was then added and mixed to form a resin composition. The resulting resin composition was thinly spread to a thickness of approximately 2 mm and left at room temperature for 24 hours to harden and form a molded product. The resulting molded product was split, and the state of the interface between the hollow particles and the resin (epoxy resin) in the cross section was observed using an SEM. More than 100 hollow particles were observed, and the percentage of hollow particles that had undergone cohesive failure was calculated. Adhesion was evaluated according to the following evaluation criteria. Note that hollow particles that have undergone cohesive failure are those in which the shell has been broken, and the higher the adhesion at the interface between the hollow particles and the resin, the more likely cohesive failure occurs. In each example, the hollow particles that have undergone cohesive failure have had their shells broken, but the shape of the hollow portion was maintained. (Adhesion evaluation criteria) A: 80% or more of the hollow particles have undergone cohesive failure B: 60% or more but less than 80% of the hollow particles have undergone cohesive failure C: 40% or more but less than 60% of the hollow particles have undergone cohesive failure D: 10% or more but less than 40% of the hollow particles have undergone cohesive failure E: Less than 10% of the hollow particles have undergone cohesive failure

[0165]

[0166]

[0167]

[0168]

[0169] Tables 1 to 4 show the amount (parts by mass) of each material added and the results of each measurement or evaluation. In Tables 1 to 4, the dielectric loss tangent values ​​are expressed in exponential notation as specified in JIS X 0210 for simplicity. For example, "2.21 x 10 －３ " is expressed as "2.21E-03". In Tables 1 to 4, the abbreviations have the following meanings: EGDMA: ethylene glycol dimethacrylate A-TMMT: pentaerythritol tetraacrylate MMA: methyl methacrylate MAA: methacrylic acid

[0170] [Discussion] In Comparative Example 1, hollow particles were prepared without adding a carboxyl group-introducing monomer in the precursor composition preparation process. Therefore, the resulting hollow particles had no carboxyl groups introduced onto their surfaces, resulting in poor adhesion to the resin. In Comparative Example 2, a carboxyl group-introducing monomer was added in the precursor composition preparation process, but deprotection was not performed. Therefore, the resulting hollow particles had no carboxyl groups introduced onto their surfaces, resulting in poor adhesion to the resin. The reason why the carboxylic acid content of the hollow particles obtained in Comparative Examples 1 and 2 was not zero is presumably due to the detection of acids contained in residues of the decomposition products of the polymerization initiator. In Comparative Example 3, MAA was added as a backbone-forming monomer, resulting in an insufficient carboxylic acid content in the resulting hollow particles, resulting in poor adhesion to the resin. In Comparative Example 3, the amount of MAA added as a backbone-forming monomer was presumably insufficient because only a small amount of the MAA was located on the shell surface after the polymerization reaction. In Comparative Example 3, although the amount of carboxylic acid on the particle surface was insufficient, the shell contained a large amount of carboxylic acid, so the relative dielectric constant and dielectric loss tangent were high and the dielectric properties were poor.

[0171] In contrast, the hollow particles obtained in each example had a carboxylic acid amount per unit area of ​​0.500 μmol / m ２As described above, the particles had a sufficient amount of carboxy groups on their surfaces, resulting in excellent adhesion to resins. In Examples 1 to 12, in the precursor composition preparation step, when the polymerization conversion rate of the skeleton-forming monomer was 1 to 99% by mass, a sufficient amount of a carboxy group-introducing monomer containing a radical polymerizable group and a protected carboxy group and having moderate water solubility was added, followed by a further polymerization reaction, followed by deprotection. This is believed to have enabled the introduction of a sufficient amount of carboxy groups on the particle surfaces. In Examples 13 to 16, when the polymerization conversion rate of the skeleton-forming monomer was 1 to 99% by mass, a sufficient amount of a carboxy group-containing monomer containing a radical polymerizable group and a carboxy group and having moderate water solubility was added, followed by a further polymerization reaction, which is believed to have enabled the introduction of a sufficient amount of carboxy groups on the particle surfaces. Furthermore, the hollow particles obtained in each example were spherical and hollow, and had a high porosity. This is presumably because the crosslinkable monomer was 60% by mass or more relative to the total 100% by mass of the skeleton-forming monomer and the carboxyl group-introducing monomer used to prepare the hollow particles, or the crosslinkable monomer was 60% by mass or more relative to the total 100% by mass of the skeleton-forming monomer and the carboxyl group-containing monomer used to prepare the hollow particles. This resulted in sufficient phase separation between the shell-forming components and the hydrophobic solvent during the suspension process, resulting in the formation of a shell with excellent strength. Furthermore, the dielectric properties of hollow particles tend to deteriorate as the content of carboxyl group-containing monomer units increases, and this tendency is particularly pronounced when the polymer contains a large amount of hydrocarbon monomer units. In each example, the amount of carboxyl group-introducing monomer or carboxyl group-containing monomer was minimized while achieving a sufficient amount of carboxylic acid per unit area. Therefore, the hollow particles obtained in each example exhibited minimal deterioration in dielectric properties and an excellent balance between adhesion to resin and dielectric properties. In particular, the hollow particles of Examples 1 to 7, 12, 13 and 15, whose shells contained a large amount of hydrocarbon monomer units, had low relative permittivity and dielectric loss tangent, and were excellent in dielectric properties.In addition, when 3,000 randomly selected particles were examined from the hollow particles obtained in each Example, it was found that in all Examples, the proportion of particles with a circularity of 0.85 or less was 10 mass% or less, and the proportion of particles having only one or two hollow portions was 90 mass% or more.

[0172] REFERENCE SIGNS LIST 1 aqueous medium 2 low-polarity material 4a hydrophobic solvent 4b material other than hydrophobic solvent 6 shell 7 hollow portion 8 droplet 9 precursor particle 10 hollow particle with hollow portion filled with gas

Claims

1. A hollow particle having a shell containing a resin and a hollow portion surrounded by the shell, the shell contains, as the resin, a polymer containing 60% by mass or more of a crosslinkable monomer unit, The hollow particles have carboxy groups on their surfaces, The amount of carboxylic acid per unit area calculated from the acid value of the hollow particles and the specific surface area of ​​the hollow particles by the following formula (A) is 0.500 μmol / m ２ That's it, hollow particles. Formula (A): Amount of カルボン acid (μmol / m ２ = Acidity (μmol / g) / Specific surface area (m²) ２ / g)

2. 2. The hollow particle according to claim 1, wherein the polymer comprises a carboxy group-containing monomer unit, and the carboxy group-containing monomer unit is derived from a carboxy group-introducing monomer that comprises a radical polymerizable group and a protected carboxy group.

3. 3. The hollow particle according to claim 2, wherein the carboxy group present on the surface of the hollow particle is a carboxy group contained in the carboxy group-containing monomer unit.

4. 2. The hollow particle according to claim 1, wherein the polymer comprises a carboxyl group-containing monomer unit, and the carboxyl group-containing monomer unit is derived from a carboxyl group-containing monomer that contains a radically polymerizable group and a carboxyl group.

5. 5. The hollow particle according to claim 4, wherein the carboxy group present on the surface of the hollow particle is a carboxy group contained in the carboxy group-containing monomer unit.

6. 6. The hollow particle according to claim 1, wherein the content of hydrocarbon monomer units is more than 50% by mass relative to 100% by mass of all monomer units contained in the polymer.

7. 6. The hollow particle according to claim 1, wherein the content of acrylic monomer units is more than 50% by mass relative to 100% by mass of all monomer units contained in the polymer.

8. The hollow particle according to any one of claims 1 to 5, having a porosity of 50% or more.

9. A method for producing hollow particles according to any one of claims 1 to 3, comprising the steps of: A step of preparing a mixed liquid containing a skeleton-forming monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; a step of suspending the mixed liquid to prepare a suspension in which droplets of a monomer composition containing the skeleton-forming monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction, and when the polymerization conversion rate of the skeleton-forming monomer reaches 1% by mass or more and 99% by mass or less, adding a carboxyl group-introducing monomer containing a radical polymerizable group and a protected carboxyl group, and further carrying out a polymerization reaction, followed by deprotection, thereby preparing a precursor composition containing precursor particles which contain a polymer of the skeleton-forming monomer and the deprotected carboxyl group-introducing monomer, a shell having a carboxyl group on the outer surface, and a hollow portion surrounded by the shell, with the hydrophobic solvent encapsulated in the hollow portion, the solubility of the carboxyl group-introducing monomer in water at 20°C is greater than that of the hydrophobic solvent and is 0.5 g / L to 1000 g / L; the backbone-forming monomer includes a crosslinkable monomer, and the content of the crosslinkable monomer is 60% by mass or more relative to 100% by mass of the total of the backbone-forming monomer and the carboxyl group-introducing monomer.

10. A method for producing hollow particles according to any one of claims 1 to 3, comprising the steps of: A step of preparing a mixed liquid containing a skeleton-forming monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; a step of suspending the mixed liquid to prepare a suspension in which droplets of a monomer composition containing the skeleton-forming monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction, and when the polymerization conversion rate of the skeleton-forming monomer reaches 40% by mass or more and 99% by mass or less, adding a carboxyl group-introducing monomer containing a radical polymerizable group and a protected carboxyl group, and further carrying out a polymerization reaction, followed by deprotection, thereby preparing a precursor composition containing precursor particles which contain a polymer of the skeleton-forming monomer and the deprotected carboxyl group-introducing monomer, a shell having a carboxyl group on the outer surface, and a hollow portion surrounded by the shell, with the hydrophobic solvent encapsulated in the hollow portion, the solubility of the carboxyl group-introducing monomer in water at 20°C is greater than that of the hydrophobic solvent and is 0.5 g / L to 80 g / L; the backbone-forming monomer includes a crosslinkable monomer, and the content of the crosslinkable monomer is 60% by mass or more relative to 100% by mass of the total of the backbone-forming monomer and the carboxyl group-introducing monomer.

11. 6. A method for producing hollow particles according to claim 1, 4 or 5, comprising the steps of: A step of preparing a mixed liquid containing a skeleton-forming monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; a step of suspending the mixed liquid to prepare a suspension in which droplets of a monomer composition containing the skeleton-forming monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction, and when the polymerization conversion rate of the skeleton-forming monomer reaches 1% by mass or more and 99% by mass or less, adding a carboxyl group-containing monomer containing a radical polymerizable group and a carboxyl group, and further carrying out a polymerization reaction, thereby preparing a precursor composition containing precursor particles which contain a polymer of the skeleton-forming monomer and the carboxyl group-containing monomer, and which have a shell having a carboxyl group on the outer surface, and a hollow portion surrounded by the shell, and which encapsulate the hydrophobic solvent in the hollow portion, the solubility of the carboxy group-containing monomer in water at 20°C is greater than that of the hydrophobic solvent and is 0.5 g / L to 1000 g / L; the skeleton-forming monomer includes a crosslinkable monomer, and the content of the crosslinkable monomer is 60% by mass or more relative to 100% by mass of the total of the skeleton-forming monomer and the carboxy group-containing monomer.

12. A resin composition comprising the hollow particles according to any one of claims 1 to 5 and a binder resin having a functional group capable of reacting with a carboxy group.

13. The resin composition according to claim 12, wherein the functional group of the binder resin is at least one selected from the group consisting of an epoxy group, an amino group, an isocyanate group, a hydroxyl group, a carboxyl group, and a thiol group.