Hollow particle

Abstract

Provided is a hollow particle having exceptional pressure resistance. The present invention provides a hollow particle comprising a shell that contains a resin and a hollow portion that is surrounded by the shell, the hollow particle having a porosity of 50% or greater and a residual porosity measured according to a specific press test method of 80% or greater.

Description

hollow particles

[0001] The present disclosure relates to 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 when kneaded with other materials and when molded after kneading. For example, Patent Document 1 discloses a method for producing hollow particles having a high porosity and being resistant to crushing, in which a suspension polymerization method is used to produce hollow particles in which the total content of crosslinkable monomers is 80 to 98 parts by mass and the content of trifunctional or higher crosslinkable monomers is 10 to 98 parts by mass per 100 parts by mass of the total mass of polymerizable monomers, and a hydrocarbon solvent containing 50% by mass or more of a saturated hydrocarbon solvent is used as a hydrophobic solvent to be encapsulated in the particles during the production process. Furthermore, Patent Document 2 discloses hollow particles with high porosity and resistance to crushing, which have a shell thickness of 0.2 μm or more and 0.9 μm or less, and contain a polymer derived from a polymerizable monomer for the shell, which is composed of 70% by mass or more and 100% by mass or less of a crosslinkable monomer and 0% by mass or more and 30% by mass or less of a non-crosslinkable monomer, and a polar resin selected from the group consisting of polymers containing a repeating unit containing a heteroatom.

[0004] International Publication No. 2020 / 261926 International Publication No. 2021 / 112110

[0005] However, when the external pressure is increased in a process such as injection molding, conventional hollow particles may not be able to sufficiently maintain voids within the particles, and therefore, further improvement in the pressure resistance of hollow particles is required. An object of the present disclosure is to provide hollow particles with excellent pressure resistance.

[0006] In press tests to evaluate the pressure resistance of hollow particles, conventional methods have produced large variations in test results, making it difficult to accurately evaluate differences in pressure resistance. Through trial and error, the present inventors have perfected a high-precision press test method that produces small variations in test results. Furthermore, they have discovered that the void retention rate measured by this press test method can be used as a guide to the void retention rate of hollow particles after processes in which external pressure is applied, such as injection molding. Furthermore, the present inventors have succeeded in obtaining hollow particles with superior pressure resistance compared to conventional methods by adjusting the composition of the polymerizable monomer used to form the hollow particles, the type of hydrophobic solvent encapsulated in the particles during the manufacturing process, or the particle size of the hollow particles. They have also discovered that these hollow particles can be differentiated from conventional hollow particles by the void retention rate measured by the newly developed high-precision press test method.

[0007] The present disclosure provides hollow particles having a resin-containing shell and a hollow portion surrounded by the shell, the hollow particles having a porosity of 50% or more, and a residual void ratio of 80% or more as measured according to the following press test method: [Press test method] 0.1 g of hollow particles and 0.9 g of polypropylene resin are melted and mixed at 200°C to obtain a mixture, which is placed in a mold for a heat press and further heated at 200°C for 15 minutes and then stirred. The mold filled with the mixture is then placed in a heat press set at 80°C, an 80°C cylinder is placed in the mold and allowed to stand, and when the surface temperature of the mold reaches 140°C, a molded product of the mixture is obtained by pressing at 60 MPa, the molded product is removed from the mold and pressed at a pressure of 1 MPa or less to form the molded product into a sheet, and the residual void ratio of the hollow particles contained in the obtained sheet-like molded product is measured.

[0008] In the hollow particles of the present disclosure, the shell preferably contains, as the resin, a polymer containing 50 parts by mass or more of crosslinkable monomer units per 100 parts by mass of all monomer units.

[0009] In the hollow particles according to the present disclosure, it is preferred that the polymer contains, as the crosslinkable monomer units, trifunctional or higher crosslinkable monomer units derived from trifunctional or higher crosslinkable monomers, and that the content of the trifunctional or higher crosslinkable monomer units is 10 parts by mass or more and 50 parts by mass or less in 100 parts by mass of all monomer units of the polymer.

[0010] In the hollow particle according to the present disclosure, it is preferable that the polymer contains, as the crosslinkable monomer units, a bifunctional crosslinkable monomer unit derived from a bifunctional crosslinkable monomer and a trifunctional or higher crosslinkable monomer unit derived from a trifunctional or higher crosslinkable monomer.

[0011] In the hollow particles according to the present disclosure, the content of the trifunctional or higher functional crosslinkable monomer units is preferably 5 parts by mass or more and 40 parts by mass or less, based on a total of 100 parts by mass of the bifunctional crosslinkable monomer units and the trifunctional or higher functional crosslinkable monomer units.

[0012] In the hollow particles according to the present disclosure, it is preferred that the shell contains, as the resin, a polymer containing acrylic monomer units and hydrocarbon monomer units, and that the content of the hydrocarbon monomer units is 10 parts by mass or more and 80 parts by mass or less relative to 100 parts by mass in total of the acrylic monomer units and the hydrocarbon monomer units.

[0013] The hollow particles of the present disclosure preferably have a volume average particle size of 5.0 μm or more and 40.0 μm or less.

[0014] The present disclosure as described above provides hollow particles whose porosity is unlikely to decrease even after undergoing a process in which external pressure is applied, such as injection molding.

[0015] 1A to 1C are diagrams illustrating an example of a method for producing hollow particles according to the present disclosure.

[0016] 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.

[0017] The hollow particles disclosed herein are hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, and are characterized by having a porosity of 50% or more and a residual void ratio of 80% or more as measured according to the press test method described below.

[0018] 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 that is clearly distinguishable from the numerous microscopic spaces uniformly dispersed within the porous structure. From the viewpoint of pressure resistance, etc., the hollow portion of the hollow particle preferably has a solid shell. For example, the hollow portion of the hollow particle can be confirmed by SEM observation of the particle cross section or by TEM observation of the particle itself. Furthermore, the hollow portion of the hollow particle of the present disclosure may be filled with a gas such as air, may be in a vacuum or reduced pressure state, or may contain a solvent.

[0019] The press test method used in the present disclosure is as follows: 0.1 g of hollow particles and 0.9 g of polypropylene resin are melted and mixed at 200°C to obtain a mixture, which is then placed in a mold for a heat press and further heated at 200°C for 15 minutes, followed by stirring. The mold filled with the mixture is then placed in a heat press set at 80°C, an 80°C cylinder is placed in the mold and allowed to stand, and when the surface temperature of the mold reaches 140°C, a pressure of 60 MPa is applied to obtain a molded product of the mixture. The molded product is then removed from the mold and pressed at a pressure of 1 MPa or less to form the molded product into a sheet, and the residual void ratio of the hollow particles contained in the obtained sheet-like molded product is measured.

[0020] The void remaining rate of hollow particles contained in a molded body can be calculated by the following formula (A). Formula (A): Void remaining rate (%) = {(c-a) / (c-b)} x 100 In the above formula (A), a represents the specific gravity of the molded body, b represents the specific gravity (calculated value) of the molded body assuming that voids are maintained, and c represents the specific gravity (calculated value) of the molded body assuming that all hollow particles are crushed. The specific gravity of the molded body (a in the above formula (A)) can be measured by the underwater displacement method in accordance with JIS K 7112. The specific gravity of the molded body assuming that voids are maintained (b in the above formula (A)) can be calculated by the following formula (B). Formula (B): b = 1 / {(P Ａ / P Ｇ ) + (R Ａ / R Ｇ )} In the above formula (B), P Ａ is the amount of hollow particles added, P Ｇ is the specific gravity of the hollow particles, R Ａ is the amount of polypropylene resin added, R Ｇ and represent the specific gravity of the polypropylene resin. The specific gravity of the molded product (c in the above formula (A)) assuming that all the hollow particles are crushed can be calculated by the following formula (C). Formula (C): c = [R Ｇ ×R Ａ +{D ０ ×P Ａ × (1-P Ｖ / 100)}] / {R Ａ +P Ａ × (1-P Ｖ / 100)} In the above formula (C), R Ａ is the amount of polypropylene resin added, R Ｇ is the specific gravity of polypropylene resin, D ０ is the true density of the hollow particle, P Ａ is the amount of hollow particles added, P Ｖ and represent the porosity (%) of the hollow particles.

[0021] The void remaining rate measured by the above press test method is higher as the hollow particles have better pressure resistance. For the hollow particles of the present disclosure, it is sufficient to have a void remaining rate of 80% or more, preferably 85% or more, and more preferably 90% or more. The void remaining rate measured by the above press test method may be the average value of 10 samples (n = 10), i.e., the average void remaining rate measured for 10 sheet-like molded bodies prepared by the above method. The void remaining rate measured by the above press test method has low variability, for example, the coefficient of variation at n = 10 can be 0.010 or less. Because the above press test is so highly accurate, it can evaluate differences in pressure resistance due to shell composition or structure, or shell uniformity regarding shell composition and thickness, which are difficult to evaluate using conventional press tests. Specific procedures for the above press test method include, for example, the procedures for the press test method performed in the examples described below.

[0022] The polypropylene resin used in the press test may have an MFR (melt flow rate) of 10 to 30 g / min, preferably 15 to 25 g / min, at 230° C. Commercially available polypropylene resins include, for example, Novatec PP, Grade MA1B (MFR of 21 g / min at 230° C.), manufactured by Japan Polypropylene Corporation.

[0023] The hollow particles disclosed herein have a high porosity of 50% or more, yet exhibit a residual void ratio of 80% or more as measured according to the above-mentioned press test method, demonstrating excellent pressure resistance. Conventional hollow particles have insufficient pressure resistance, making it difficult to achieve a residual void ratio of 80% or more using the above-mentioned press test method, which involves applying a pressure of 60 MPa. To achieve a residual void ratio of 80% or more using the above-mentioned press test method, effective methods include, for example, improving the strength of the shell by adjusting the composition of the polymerizable monomer used to form the shell of the hollow particles, and further improving the uniformity of the shell, such as the shell composition and thickness, by adjusting the particle size of the hollow particles and the type of hydrophobic solvent encapsulated in the particles during the manufacturing process. It is believed that conventional hollow particles have insufficient pressure resistance, particularly due to insufficient shell uniformity. By improving the uniformity of the shell, stress is applied more uniformly to the hollow particles, improving the pressure resistance of the hollow particles and presumably enabling the previously unachievable residual void ratio of 80% or more using the above-mentioned press test method.

[0024] Hereinafter, an example of a method for producing hollow particles according to the present disclosure will be described, and then the hollow particles according to the present disclosure will be described in detail.

[0025] The hollow particles of the present disclosure can be obtained by a method for producing hollow particles of the present disclosure, the method including, for example, the steps of: preparing a mixed liquid containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; suspending the mixed liquid to prepare a suspension in which droplets of a monomer composition containing the polymerizable monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction to prepare a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a resin, and encapsulating the hydrophobic solvent in the hollow portion.

[0026] The manufacturing method of the present disclosure follows 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 causing phase separation between the polymerizable monomer and the hydrophobic solvent, thereby preparing a suspension in which droplets having a distribution structure in which the polymerizable monomer is unevenly distributed on the surface and the hydrophobic solvent is unevenly distributed in the center are dispersed in the aqueous medium, and the suspension is subjected to a polymerization reaction to harden the surfaces of the droplets and form hollow particles having hollows filled with the hydrophobic solvent. In the basic technique, by adjusting the composition of the polymerizable monomer and the type of hydrophobic solvent, etc., sufficient phase separation between the polymerizable monomer and the hydrophobic solvent is achieved in the droplets of the monomer composition dispersed in the suspension, and when the suspension is subjected to a polymerization reaction, it is presumed that the polymerization reaction of the polymerizable monomer proceeds uniformly, resulting in the formation of shells with excellent uniformity in composition, thickness, etc.

[0027] The method for producing hollow particles according to the present disclosure includes a step of preparing a mixed solution, a step of preparing a suspension, and a step of subjecting the suspension to a polymerization reaction, and may further include other steps. Furthermore, as far as technically possible, two or more of the above steps and other additional steps may be performed simultaneously as a single step, or the order of the steps may be reversed. For example, the preparation of the mixed solution and the suspension may be performed simultaneously in a single step, such as by adding the materials for preparing the mixed solution and suspending them at the same time.

[0028] 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 polymerizable 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 a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator are dispersed in an aqueous medium; (3) polymerizing 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 are considered to be intermediates of hollow particles whose hollow portions are filled with a gas, and may be referred to as "precursor particles." In the present disclosure, the term "precursor composition" refers to a composition containing precursor particles.

[0029] 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 each step. 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 contains a polymerizable 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 polymerizable 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 a material other than the hydrophobic solvent, including the polymerizable monomer, a material 4b, with the hydrophobic solvent 4a concentrated in the center and the material 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 a polymerization process, 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. The shell 6 forming the outer surface of the precursor particle 9 is formed by polymerization of the polymerizable monomer in the droplet 8 of the monomer composition, and contains a polymer of the polymerizable monomer as a resin. (4) in FIG. 1 is a cross-sectional schematic diagram showing one embodiment of the precursor particle after the solid-liquid separation step. (4) in FIG. 1 shows a state in which 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 the hollow particle after the solvent removal step. (5) in FIG. 1 shows a state in which the hydrophobic solvent 4a has been removed from the state shown in (4) in FIG.By removing the hydrophobic solvent from the precursor particles, hollow particles 10 are obtained, each having a gas-filled hollow portion 7 inside a shell 6. The above five steps and other steps will be described in order below.

[0030] (1) Mixed Liquid Preparation Step This step is a step of preparing a mixed liquid containing a polymerizable 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) polymerizable monomer, (B) hydrophobic solvent, (C) polymerization initiator, (D) dispersion stabilizer, and (E) aqueous medium.

[0031] (A) Polymerizable Monomer As the polymerizable monomer, any known polymerizable monomer conventionally used for producing hollow particles can be used, and although there are no particular limitations, it is preferable that the polymerizable monomer contains a crosslinkable monomer. When the polymerizable monomer contains a crosslinkable monomer, the crosslink density of the shell can be increased, so that a shell with excellent strength is easily formed, hollow particles are easily formed into a spherical shape, and hollow portions clearly distinguishable from the shell are easily formed within the particles.

[0032] Furthermore, from the viewpoint of facilitating a stable polymerization reaction, the polymerizable monomer is preferably a polymerizable monomer whose polymerizable functional group is a (meth)acryloyl group or a vinyl group, and more preferably an acrylic monomer containing a (meth)acryloyl group as the polymerizable functional group. In the present disclosure, "stable polymerization reaction" means that the reactivity of the polymerization reaction is good and the polymerization reaction proceeds uniformly. Furthermore, it is particularly preferable for the polymerizable monomer to contain an acrylic monomer and a hydrocarbon monomer, since this not only facilitates stability of the polymerization reaction but also improves the pressure resistance of the hollow particles. It is presumed that copolymerization of an acrylic monomer and a hydrocarbon monomer increases the reactivity of the hydrocarbon monomer, thereby improving the overall reactivity of the polymerizable monomers, thereby facilitating a stable polymerization reaction. Furthermore, when the polymerizable monomer contains an acrylic monomer and a hydrocarbon monomer, the compatibility with the hydrophobic solvent is appropriate, and therefore, when the suspension is subjected to a polymerization reaction, the polymerization reaction of the polymerizable monomer tends to proceed uniformly, and the formed shell tends to have excellent uniformity in composition, thickness, etc., which is presumably why the pressure resistance of the hollow particles is improved. As the hydrocarbon monomer, one whose polymerizable functional group is a vinyl group is preferred because it tends to stabilize the polymerization reaction. In this disclosure, a polymerizable monomer having a (meth)acryloyl group as the polymerizable functional group is referred to as an acrylic monomer, a crosslinkable monomer having a (meth)acryloyl group as the polymerizable functional group is referred to as a crosslinkable acrylic monomer, and a non-crosslinkable monomer having a (meth)acryloyl group as the polymerizable functional group is referred to as a non-crosslinkable acrylic monomer. In the crosslinkable acrylic monomer, at least one polymerizable functional group may be a (meth)acryloyl group, but it is preferable that all polymerizable functional groups are (meth)acryloyl groups. In addition, 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.

[0033] The crosslinkable monomer used in the manufacturing method of the present disclosure is preferably a crosslinkable acrylic monomer or a crosslinkable hydrocarbon monomer. Examples of the crosslinkable acrylic monomer include bifunctional crosslinkable acrylic monomers such as allyl(meth)acrylate, vinyl(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, 2-hydroxy-3-(meth)acryloyloxypropyl(meth)acrylate, and tricyclodecane dimethanol di(meth)acrylate; and trifunctional or higher crosslinkable acrylic monomers such as trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol poly(meth)acrylate, and ethoxylated versions thereof. In addition, crosslinkable macromers such as polyphenylene ether, both ends of which are (meth)acrylic-modified, can also be used. Examples of crosslinkable hydrocarbon monomers include aromatic divinyl monomers such as divinylbenzene, divinylbiphenyl, and divinylnaphthalene; and diene-based 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. Other examples of crosslinkable macromers that can be used include polybutadiene, polyisoprene, styrene-butadiene block copolymers (SBS), and styrene-isoprene block copolymers (SIS). Further examples of crosslinkable monomers include crosslinkable allylic monomers such as diallyl phthalate, and crosslinkable macromers such as polyphenylene ether, both ends of which are vinyl-modified. These crosslinkable monomers can be used either alone or in combination of two or more.

[0034] In order to improve the strength of the shell and obtain hollow particles having excellent pressure resistance, the crosslinkable monomer preferably contains a trifunctional or higher crosslinkable monomer having three or more polymerizable functional groups. As the trifunctional or higher crosslinkable monomer, the above-mentioned trifunctional or higher crosslinkable acrylic monomers are preferred, and among them, 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.

[0035] From the viewpoint of improving the shell strength and obtaining hollow particles with excellent pressure resistance, it is more preferable that the crosslinkable monomer contains a bifunctional crosslinkable monomer having only two polymerizable functional groups and a trifunctional or higher crosslinkable monomer having three or more polymerizable functional groups. The bifunctional crosslinkable monomer is preferably at least one selected from the group consisting of the above-mentioned bifunctional crosslinkable acrylic monomers and the above-mentioned bifunctional crosslinkable hydrocarbon monomers. Among the bifunctional crosslinkable acrylic monomers, ethylene glycol di(meth)acrylate and pentaerythritol di(meth)acrylate are preferred, with ethylene glycol di(meth)acrylate being more preferred. Among the bifunctional crosslinkable hydrocarbon monomers, divinylbenzene is preferred.

[0036] The content of the crosslinkable monomer is preferably 50 parts by mass or more, more preferably 60 parts by mass or more, even more preferably 70 parts by mass or more, and even more preferably 80 parts by mass or more, per 100 parts by mass of the polymerizable monomer, in order to improve the pressure resistance of the hollow particles. When the content of the crosslinkable monomer is equal to or greater than the above-mentioned lower limit, hollow portions are more likely to be formed within the particles, the particles are more likely to become spherical, and the crosslinking density of the shell can be increased, thereby improving the solvent resistance, strength, heat resistance, etc. of the hollow particles. On the other hand, the polymerizable monomer may contain a non-crosslinkable monomer, provided that the effects of the present disclosure are not impaired. In this case, the content of the crosslinkable monomer may be, for example, 95 parts by mass or less, or 90 parts by mass or less, per 100 parts by mass of the polymerizable monomer. The content of the crosslinkable monomer refers to the total content of the bifunctional crosslinkable monomer and the trifunctional or higher crosslinkable monomer.

[0037] When the crosslinkable monomer contains a trifunctional or higher functional crosslinkable monomer, in order to improve the pressure resistance of the hollow particles, the content of the trifunctional or higher functional crosslinkable monomer is, relative to 100 parts by mass of the polymerizable monomer, preferably 10 parts by mass or more as a lower limit, more preferably 20 parts by mass or more, and preferably 50 parts by mass or less as an upper limit, more preferably 40 parts by mass or less, and even more preferably 30 parts by mass or less.

[0038] When the crosslinkable monomer contains a bifunctional crosslinkable monomer and a trifunctional or higher functional crosslinkable monomer, in order to improve the pressure resistance of the hollow particles, the content of the trifunctional crosslinkable monomer relative to 100 parts by mass of the total mass of the bifunctional crosslinkable monomer and the trifunctional or higher functional crosslinkable monomer is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 20 parts by mass or more, and the upper limit is preferably 50 parts by mass or less, more preferably 40 parts by mass or less.

[0039] The polymerizable monomer may contain a non-crosslinkable monomer as long as the effects of the present disclosure are not impaired. Examples of the non-crosslinkable monomer 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, N-butoxymethyl (meth)acrylamide, methoxypolyethylene glycol (meth)acrylate, ethoxypolyethylene glycol (meth)acrylate, propoxypolyethylene glycol (meth)acrylate, butoxypolyethylene glycol (meth)acrylate, hexaoxypolyethylene glycol (meth)acrylate, octoxypolyethylene glycol polypropylene glycol (meth)acrylate, lauroxypolyethylene glycol (meth)acrylate, and stearoxypolyethylene glycol (meth)acrylate. non-crosslinkable acrylic monomers such as acrylate, phenoxy polyethylene glycol polypropylene glycol (meth)acrylate, polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, polyethylene glycol propylene glycol mono(meth)acrylate, polyethylene glycol tetramethylene glycol (meth)acrylate, propylene glycol polybutylene glycol mono(meth)acrylate, and monoethylene glycol mono(meth)acrylate; non-crosslinkable hydrocarbon monomers such as aromatic monovinyl monomers such as styrene, vinyl toluene, α-methylstyrene, p-methylstyrene, ethyl vinyl benzene, ethyl vinyl biphenyl, and ethyl vinyl naphthalene, as well as monoolefin monomers such as linear or branched monoolefins such as ethylene, propylene, and butylene, and alicyclic monoolefins such as vinyl cyclohexane, norbornene, tricyclododecene, and 1,4-methano-1,4,4a,9a-tetrahydrofluorene; carboxylic acid vinyl ester monomers such as vinyl acetate;Examples of suitable non-crosslinkable monomers include halogenated aromatic vinyl monomers such as halogenated styrene; halogenated vinyl monomers such as vinyl chloride; halogenated vinylidene monomers such as vinylidene chloride; and vinylpyridine monomers. Other examples include non-crosslinkable macromers such as (meth)acrylic-terminated polystyrene and (meth)acrylic-terminated polymethyl methacrylate. These non-crosslinkable monomers can be used alone or in combination of two or more. Among the non-crosslinkable monomers, (meth)acrylic acid alkyl esters and aromatic monovinyl monomers are preferred, with aromatic monovinyl monomers being more preferred, because they facilitate stable polymerization reactions and prevent a decrease in the pressure resistance of hollow particles. Among the (meth)acrylic acid alkyl esters, butyl acrylate and methyl methacrylate are preferred. Among the aromatic monovinyl monomers, ethylvinylbenzene is preferred.

[0040] In order to improve the stability of the polymerization reaction, the content of the acrylic monomer is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, per 100 parts by mass of the polymerizable monomer.

[0041] Furthermore, from the viewpoint of improving the stability of the polymerization reaction and the pressure resistance of the hollow particles, the content of the acrylic monomer and the hydrocarbon monomer per 100 parts by mass of the polymerizable monomer is preferably 80 parts by mass or more, more preferably 90 parts by mass or more, even more preferably 98 parts by mass or more, and still more preferably 99 parts by mass or more. When the polymerizable monomer contains an acrylic monomer and a hydrocarbon monomer, from the viewpoint of improving the pressure resistance of the hollow particles, the content of the hydrocarbon monomer per 100 parts by mass of the total of the acrylic monomer and the hydrocarbon 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, and the upper limit is preferably 90 parts by mass or less, more preferably 80 parts by mass or less.

[0042] The content of the polymerizable 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, the lower limit is preferably 30% by mass or more, more preferably 40% by mass or more, and the upper limit is preferably 60% by mass or less, more preferably 50% by mass or less, based on 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 polymerizable monomer in the mixed liquid based on 100% by mass of the total mass of the solids excluding the hydrophobic solvent, among the materials that form the oil phase, is preferably 95% by mass or more, more preferably 97% by mass or more. In the present disclosure, the solids refer to all components excluding the solvent, and liquid polymerizable monomers and the like are considered to be included in the solids.

[0043] (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 polymerization process described below, an aqueous dispersion containing precursor particles encapsulating the hydrophobic solvent is obtained. In other words, as the hydrophobic solvent collects inside the particles, hollow spaces filled with the hydrophobic solvent are formed inside the resulting precursor particles.

[0044] The hydrophobic solvent can be appropriately selected from known hydrophobic solvents and is not particularly limited. Examples include esters such as ethyl acetate and butyl acetate; ether esters such as propylene glycol monomethyl ether acetate and propylene glycol monoethyl ether acetate; and hydrocarbon solvents, of which hydrocarbon solvents are preferred. Examples of hydrocarbon solvents include aliphatic hydrocarbons including chain hydrocarbon solvents such as pentane, hexane, heptane, octane, 2-methylbutane, 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.

[0045] In the suspension step, because phase separation between the polymerizable monomer and the hydrophobic solvent is likely to occur within the droplets of the monomer composition, it is preferable to select an organic solvent that has a lower solubility in water than the crosslinkable monomer contained in the polymerizable monomer as the hydrophobic solvent. Furthermore, when the polymerizable monomer contains an acrylic monomer and a hydrocarbon monomer, it is preferable to use a chain hydrocarbon solvent as the hydrophobic solvent. Among the chain hydrocarbon solvents, chain hydrocarbon solvents having 5 to 8 carbon atoms are preferred, and at least one selected from the group consisting of pentane, hexane, heptane, and octane is more preferred. On the other hand, when the polymerizable monomer contains an acrylic monomer but no hydrocarbon monomer, it is preferable to use a hydrocarbon solvent having 4 to 7 carbon atoms as the hydrophobic solvent, and it is more preferable to use a hydrocarbon solvent having 5 to 7 carbon atoms. Here, the hydrocarbon solvent may be either an aromatic hydrocarbon or an aliphatic hydrocarbon, but among these, an aliphatic hydrocarbon is preferred, a cyclic hydrocarbon solvent is more preferred, and at least one selected from the group consisting of cyclohexane, cycloheptane, and methylcyclohexane is even more preferred. The use of a combination of the above-mentioned polymerizable monomer and hydrophobic solvent is preferred because it is easy to improve the pressure resistance of the hollow particles. In particular, the use of a combination of a polymerizable monomer containing an acrylic monomer and a hydrocarbon monomer with the above-mentioned preferred hydrophobic solvent is preferred because it improves the uniformity of the shell and thereby improves the pressure resistance of the hollow particles.

[0046] 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.

[0047] Furthermore, the hydrophobic solvent used in the manufacturing method of the present disclosure preferably has a dielectric constant of 2.5 or less at 20°C, and more preferably 2.0 or less. The dielectric constant is one of the indicators of the polarity of a compound. When the dielectric constant of the hydrophobic solvent is sufficiently small, such as 2.5 or less, it is believed that phase separation proceeds rapidly in the droplets of the monomer composition, making it easy to form hollow spaces. Examples of hydrophobic solvents having a dielectric constant of 2.0 or less at 20°C are as follows. The values ​​in parentheses are the dielectric constant values: pentane (1.8), hexane (1.9), heptane (1.9), octane (1.9), and cyclohexane (2.0). Regarding the dielectric constant at 20°C, reference can be made to values ​​described in known literature (e.g., "Chemical Handbook: Basics," 4th Revised Edition, edited by the Chemical Society of Japan, Maruzen Co., Ltd., published September 30, 1993, pages II-498 to II-503) and other technical information. The method for measuring the relative dielectric constant at 20°C includes, for example, a relative dielectric constant test carried out in accordance with JIS C 2101:1999, 23, at a measurement temperature of 20°C.

[0048] The porosity of the hollow particles can be adjusted by changing the amount of hydrophobic solvent in the mixed solution. In the suspension process described below, the polymerization reaction proceeds with the oil droplets containing the polymerizable monomer and the like encapsulating the hydrophobic solvent. 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 polymerizable monomer, as this facilitates control of the particle size of the hollow particles, increases the porosity while maintaining the strength of the hollow particles, and reduces the amount of residual hydrophobic solvent within the particles. The content of the hydrophobic solvent in the mixed solution is more preferably 70 to 300 parts by mass, and even more preferably 90 to 200 parts by mass per 100 parts by mass of the polymerizable monomer.

[0049] (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).

[0050] The content of the polymerization initiator relative to 100 parts by mass of the polymerizable monomer in the mixed solution is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 7 parts by mass, and even more preferably 1 to 5 parts by mass. When the content of the polymerization initiator is equal to or greater than the above-mentioned lower limit, the polymerization reaction can proceed sufficiently, while when the content is equal to or less than the above-mentioned upper limit, there is little risk of the oil-soluble polymerization initiator remaining after completion of the polymerization reaction, and there is also little risk of an unexpected side reaction proceeding.

[0051] (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, those having poor water solubility are preferred, and poorly water-soluble metal salts such as the sulfates, carbonates, phosphates, and metal hydroxides are more preferred, metal hydroxides are even more preferred, and magnesium hydroxide is particularly preferred. In the present disclosure, poorly water-soluble means that the solubility in 100 g of water is preferably 0.5 g or less.

[0052] 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.

[0053] The content of the dispersion stabilizer is not particularly limited, but is preferably 0.5 to 15 parts by mass, more preferably 1 to 10 parts by mass, per 100 parts by mass of the total mass of the polymerizable monomer and the hydrophobic solvent. By having the content of the dispersion stabilizer at or above the lower limit, droplets of the monomer composition can be sufficiently dispersed so as not to coalesce in the suspension. On the other hand, by having the content of the dispersion stabilizer at or below the upper limit, an increase in the viscosity of the suspension during granulation can be prevented, and the problem of the suspension clogging in the granulator can be avoided. Furthermore, the content of the dispersion stabilizer is preferably 0.5 to 15 parts by mass, more preferably 0.5 to 10 parts by mass, per 100 parts by mass of the aqueous medium.

[0054] (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.

[0055] The content of the aqueous medium is not particularly limited, but from the viewpoint of setting the particle size and porosity of the hollow particles within the preferred ranges described below, the lower limit is preferably 200 parts by mass or more, more preferably 400 parts by mass or more, and even more preferably 600 parts by mass or more, relative to 100 parts by mass of the polymerizable monomer contained in the mixed liquid, and the upper limit is preferably 1,000 parts by mass or less, and more preferably 800 parts by mass or less.

[0056] 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.

[0057] A mixed liquid is obtained by mixing the above-mentioned materials and other materials as needed, and stirring appropriately. In this mixed liquid, an oil phase containing lipophilic materials such as (A) the polymerizable monomer, (B) the hydrophobic solvent, and (C) the polymerization initiator is dispersed in an aqueous phase containing (D) the dispersion stabilizer and (E) the aqueous medium, with particles of about several mm in size. The dispersion state of these materials in the mixed liquid can be observed with the naked eye, depending on the type of material. In the mixed liquid preparation step, the mixed liquid may be obtained by simply mixing the above-mentioned materials and other materials as needed, and stirring appropriately. However, in terms of making the shell more uniform, it is preferable to prepare a mixed liquid by separately preparing an oil phase containing the polymerizable monomer, the hydrophobic solvent, and the polymerization initiator, and an aqueous phase containing the dispersion stabilizer and the aqueous medium, and then mixing them. In the present disclosure, a colloidal dispersion in which a poorly water-soluble inorganic dispersion stabilizer is dispersed in the form of colloidal particles in an aqueous medium is preferably used as the aqueous phase. By preparing the oil phase and the water phase separately in advance and then mixing them, hollow particles having a uniform shell composition can be produced, and the particle size of the hollow particles can be easily controlled.

[0058] (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 mixed liquid described above. The suspension method for forming droplets of the monomer composition is not particularly limited, and any known suspension method can be used. Examples of dispersers used in preparing the suspension include horizontal or vertical in-line dispersers such as Milder (product name) manufactured by Pacific Machinery Works, Ltd., Cavitron (product name) manufactured by Eurotec Co., Ltd., and in-line dispersers manufactured by IKA (e.g., DISPAX-REACTOR (registered trademark) DRS (product name)); and emulsifying dispersers such as the Homomixer MARK II series manufactured by Primix Corporation.

[0059] In the suspension prepared in the suspending step, droplets of the monomer composition containing the lipophilic material and having a particle size of approximately 5 to 40 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. During the suspending step, phase separation occurs in the droplets of the monomer composition, which makes it easier for the hydrophobic solvent, which has low polarity, to collect inside the droplets. As a result, the resulting droplets contain the hydrophobic solvent in their interiors and materials other than the hydrophobic solvent distributed around their peripheries.

[0060] 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 polymerizable 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 polymerizable 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.

[0061] (3) Polymerization Step This step is a step of preparing a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a resin and encapsulating a hydrophobic solvent in the hollow portion by subjecting the suspension obtained in the above-mentioned suspension step to a polymerization reaction. The precursor particles are formed by polymerization of a polymerizable monomer contained in droplets of the monomer composition, and the shell of the precursor particles contains a polymer of the polymerizable monomer as a resin.

[0062] The polymerization method is not particularly limited, and for example, a batch system, a semi-continuous system, a continuous system, etc. can be employed. The polymerization temperature is preferably 40 to 90°C, more preferably 50 to 80°C. The polymerization reaction time is preferably 1 to 48 hours, more preferably 4 to 36 hours. In the production method of the present disclosure, in the polymerization step, further polymerizable monomers may be added during the polymerization reaction of the polymerizable monomers in the suspension. By performing the polymerization reaction in two stages in this manner, the pressure resistance of the hollow particles may be improved. In the polymerization step, the shell portion of the droplets of the monomer composition containing the hydrophobic solvent therein is polymerized, and as described above, hollow portions filled with the hydrophobic solvent are formed inside the resulting precursor particles.

[0063] (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 by the above-mentioned polymerization step.

[0064] 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.

[0065] (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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] Alternatively, the hydrophobic solvent may be removed from the slurry containing precursor particles and an aqueous medium without solid-liquid separation of the slurry-like precursor composition obtained in the polymerization 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 in the polymerization step. Although not particularly limited, the temperature during bubbling may be 50°C or higher and 100°C or lower. The inert gas to be bubbling is not particularly limited, but examples thereof include nitrogen, argon, etc. The bubbling conditions are appropriately adjusted depending on the type and amount of hydrophobic solvent so as to remove the hydrophobic solvent encapsulated in the precursor particles. Although not particularly limited, for example, the inert gas may be bubbled at a rate of 1 to 3 L / min for 1 to 10 hours. This method produces a 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.

[0070] 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 an atmosphere of 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 then removing the aqueous medium remaining in the hollow particles in an atmosphere of 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 polymerization step and before the solid-liquid separation step without subjecting the slurry precursor composition obtained in the polymerization step to solid-liquid separation, for example, a method for evaporating and distilling off the hydrophobic solvent encapsulated in the precursor particles from the precursor composition under a predetermined pressure (high pressure, normal pressure, or reduced pressure); or a method for introducing an inert gas such as nitrogen, argon, or helium, or water vapor, into the precursor composition under a predetermined pressure (high pressure, normal pressure, or reduced pressure) and evaporating and distilling off the hydrophobic solvent may be used.

[0071] (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.

[0072] (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.

[0073] The hollow particles of the present disclosure contain a polymer of the above-described polymerizable monomer as the main component of the shell, and the polymer forms the skeleton of the shell of the hollow particles. In terms of improving pressure resistance, the hollow particles of the present disclosure preferably contain a crosslinkable monomer unit in the shell. The content of the crosslinkable monomer unit in 100 parts by mass of all monomer units of the polymer is preferably 50 parts by mass or more, more preferably 60 parts by mass or more, even more preferably 70 parts by mass or more, and even more preferably 80 parts by mass or more. On the other hand, the polymer may contain a non-crosslinkable monomer unit within a range that does not impair the effects of the present disclosure. In this case, the content of the crosslinkable monomer unit in 100 parts by mass of all monomer units of the polymer may be, for example, 95 parts by mass or less, or 90 parts by mass or less.

[0074] In order to improve pressure resistance, the hollow particles of the present disclosure preferably contain a trifunctional or higher crosslinkable monomer unit in the shell of the polymer. The content of the trifunctional or higher crosslinkable monomer unit in 100 parts by mass of all monomer units of the polymer is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, as a lower limit, and preferably 50 parts by mass or less, more preferably 40 parts by mass or less, and even more preferably 30 parts by mass or less, as an upper limit.

[0075] In order to improve pressure resistance, the hollow particles of the present disclosure preferably contain the polymer contained in the shell containing a bifunctional crosslinkable monomer unit and a trifunctional or higher functional crosslinkable monomer unit. When the polymer contains a bifunctional crosslinkable monomer unit and a trifunctional or higher functional crosslinkable monomer unit, the content of the trifunctional or higher functional crosslinkable monomer unit is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 20 parts by mass or more, per 100 parts by mass of the bifunctional crosslinkable monomer unit and the trifunctional or higher functional crosslinkable monomer unit combined, and is preferably 50 parts by mass or less, more preferably 40 parts by mass or less. In the present disclosure, a crosslinkable monomer unit derived from a bifunctional crosslinkable monomer may be referred to as a "bifunctional crosslinkable monomer unit," and a crosslinkable monomer unit derived from a trifunctional or higher functional crosslinkable monomer may be referred to as a "trifunctional or higher functional crosslinkable monomer unit."

[0076] In addition, in the hollow particles according to the present disclosure, the polymer contained in the shell preferably contains an acrylic monomer unit, since this facilitates improvement in the uniformity of the shell. The content of the acrylic monomer unit is not particularly limited, but is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, relative to 100% by mass of all monomer units.

[0077] Furthermore, in the hollow particles according to the present disclosure, the polymer contained in the shell preferably contains acrylic monomer units and hydrocarbon monomer units, from the viewpoint of improving the uniformity of the shell and improving the pressure resistance. The total content of the acrylic monomer units and hydrocarbon monomer units in 100 parts by mass of all the monomer units in the polymer is preferably 80 parts by mass or more, more preferably 90 parts by mass or more, even more preferably 98 parts by mass or more, and even more preferably 99 parts by mass or more. When the polymer contained in the shell contains acrylic monomer units and hydrocarbon monomer units, from the viewpoint of improving the pressure resistance of the hollow particles, the content of the hydrocarbon monomer units in 100 parts by mass of the total of the acrylic monomer units and the hydrocarbon monomer units 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, as a lower limit, and preferably 90 parts by mass or less, more preferably 80 parts by mass or less, as an upper limit.

[0078] In the hollow particles of the present disclosure, the content of the polymer of the polymerizable monomer is preferably 96% by mass or more, more preferably 97% by mass or more, based on 100% by mass of the total solids content of the shell. By setting the content of the polymer at or above the lower limit, a decrease in the pressure resistance of the hollow particles can be suppressed. That is, to suppress a decrease in the pressure resistance of the hollow particles, the content of components other than the polymer is preferably 4% by mass or less, more preferably 3% 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 polymerizable monomers, polymers other than the polymer of the polymerizable monomer, decomposition products of polymerization initiators, and low-molecular-weight compounds contained as impurities in the raw materials for the polymerizable monomers. Low-boiling components (e.g., boiling points of 200°C or less) are typically removed during the production process of hollow particles, while high-boiling components (e.g., boiling points of 250°C or more) may remain unreacted.

[0079] The hollow particles of the present disclosure have a porosity of 50% or more, preferably 60% or more, and more preferably 65% ​​or more. When the porosity is equal to or greater than the above-mentioned lower limit, the hollow particles also have excellent lightness, heat insulating properties, and dielectric properties. The upper limit of the porosity of the hollow particles is not particularly limited, but is preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less, in order to prevent a decrease in the pressure resistance of the hollow particles.

[0080] 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 D１ 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.

[0081] True density D of hollow particles ０ The measurement method is as follows: After crushing the hollow particles in advance, ３ Approximately 10 g of crushed pieces of hollow particles are filled into a measuring flask, and the mass of the crushed pieces is accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol is added to the measuring flask, and the mass of the isopropanol is accurately weighed. The true density D of the hollow particles is calculated based on the following formula (II): ０ (g / cm ３ ) is calculated using the formula (II): 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.

[0082] 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

[0083] The volume average particle diameter of the hollow particles of the present disclosure is preferably 5.0 μm or more, more preferably 6.0 μm or more, and even more preferably 7.0 μm or more, with the upper limit being preferably 40.0 μm or less, more preferably 30.0 μm or less, and even more preferably 20.0 μm or less. When the volume average particle diameter of the hollow particles is equal to or greater than the above-mentioned lower limit, it is easy to achieve both high porosity and excellent pressure resistance, and the tendency for hollow particles to aggregate with each other is reduced, thereby enabling excellent dispersibility to be exhibited. When the volume average particle diameter of the hollow particles is equal to or less than the above-mentioned upper limit, it is easy to improve the uniformity of the shell, making it easy to obtain hollow particles with excellent pressure resistance. The particle diameter of the hollow particles of the present disclosure can be adjusted, for example, by the content of the dispersion stabilizer relative to the total mass of the polymerizable monomer and the hydrophobic solvent.

[0084] The shell thickness of the hollow particles of the present disclosure is not particularly limited, but from the viewpoint of improving pressure resistance, it is preferably 0.30 μm or more, more preferably 0.40 μm or more, even more preferably 0.50 μm or more, and even more preferably 0.60 μm or more, and from the viewpoint of increasing porosity, it is preferably 3.00 μm or less, more preferably 2.00 μm or less, and even more preferably 1.50 μm or less. In the present disclosure, the shell thickness of the hollow particles is a value calculated by calculating the inner diameter r of the hollow particles according to the following formula (1) using the volume average particle diameter R and porosity of the hollow particles, and then using the inner diameter r and volume average particle diameter R according to the following formula (2). Formula (1): 4 / 3π×(R / 2) ３ ×(porosity / 100)=4 / 3π×(r / 2) ３ Shell thickness=(R−r) / 2 Formula (2): Note that the porosity in the above formula (1) is a numerical value expressed as a percentage.

[0085] 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 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.

[0086] 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, those having only one hollow portion are preferred in order to maintain a good balance between high porosity and mechanical strength. The proportion of hollow particles 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 in order to improve pressure resistance.

[0087] 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.

[0088] Furthermore, from the viewpoint of pressure resistance, it is preferable that the hollow particles of the present disclosure have a small 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. Such irregularly shaped particles are more susceptible to localized external pressure and therefore have inferior pressure resistance compared to spherical particles. Furthermore, irregularly shaped particles are more likely to aggregate when dispersed in a binder resin than spherical particles, resulting in poor dispersibility. When irregularly shaped particles are dispersed in a binder resin, aggregates are more likely to form, and the aggregates are more susceptible to external pressure, further reducing pressure resistance. Therefore, reducing the proportion of irregularly shaped particles contained in the hollow particles can improve the dispersibility and pressure resistance of the hollow particles. The hollow particles of the present disclosure may contain a small amount of impurities, such as particles with low circularity due to cracking or deformation. However, the proportion of particles with a circularity of 0.85 or less, based on 100% by mass of the hollow particles of the present disclosure, is preferably 10% by mass or less, more preferably 7% by mass or less, even more preferably 5% by mass or less, even more preferably 4% by mass or less, and particularly preferably 3% by mass or less. Circularity is defined as the value obtained by dividing the diameter of a circle having the same area as the projected image of the particle (equivalent circle area diameter) by the diameter of a circle having the same perimeter as the projected image of the particle (equivalent circumferential diameter). A perfectly spherical particle has a circularity of 1, and the more complex the particle's surface shape, the smaller the circularity. 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.

[0089] Furthermore, the hollow particles of the present disclosure have a thermal decomposition onset temperature of preferably 150 to 400°C, more preferably 200 to 350°C. Hollow particles 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.

[0090] The hollow particles of the present disclosure have excellent pressure resistance and are therefore resistant to crushing during kneading with other materials and during molding after kneading, and when added to a molded body, they exhibit excellent effects as a weight-saving material, heat insulating material, soundproofing material, vibration damping material, etc., making them suitable as additives for molded bodies, and are preferably used as additives for resin molded bodies, for example. Furthermore, the hollow particles of the present disclosure are resistant to crushing and are less likely to reduce porosity even after undergoing processes that apply external pressure, such as kneading and injection molding, and are therefore particularly suitable as additives for molded bodies obtained through processes that apply external pressure. The molded article containing the hollow particles of the present disclosure may contain, as a resin, for example, a thermoplastic resin, a thermosetting resin, or a room-temperature curable resin such as polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyurethane, epoxy resin, acrylonitrile-butadiene-styrene (ABS) resin, acrylonitrile-styrene (AS) resin, poly(meth)acrylate, polycarbonate, polyamide, polyimide, polyphenylene ether, polyphenylene sulfide, polyester, polytetrafluoroethylene, maleimide resin, bismaleimide triazine resin, liquid crystalline polyester resin, phenolic resin, vinyl ester resin, unsaturated polyester resin, cyanate ester resin, polyether ketone ketone resin, polyetherimide resin, polyphenylene oxide resin, melamine resin, urea resin, silicone resin, alkyd resin, benzoxazine resin, allyl resin, aniline resin, or acrylic resin. Depending on the type of resin, curing agents or catalysts such as amines, acid anhydrides, imidazoles, thiols, phenols, naphthols, benzoxazines, cyanate esters, and carbodiimides may be mixed in as appropriate. Furthermore, the molded article containing the hollow particles of the present disclosure may contain a thermoplastic elastomer as the resin. Examples of the thermoplastic elastomer include thermoplastic elastic polymers that have traditionally been used as molding resins, such as urethane-based elastomers, styrene-based elastomers, olefin-based elastomers, amide-based elastomers, and ester-based elastomers. Thermoplastic elastomers generally exhibit rubber elasticity at room temperature (25°C) and are plasticized and moldable at high temperatures.Furthermore, molded articles containing hollow particles of the present disclosure are not limited to resin molded articles, but may be, for example, rubber molded articles or articles containing a mixture of resin and rubber. Molded articles containing hollow particles of the present disclosure may contain rubbers such as natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene copolymer rubber (SBR), acrylonitrile-butadiene copolymer rubber (NBR), and ethylene-propylene-diene terpolymer (EPDM). The rubbers may be used alone or in combination of two or more. Molded articles containing hollow particles of the present disclosure may further contain organic or inorganic fibers such as carbon fiber, glass fiber, aramid fiber, and polyethylene fiber. The hollow particles of the present disclosure can also be incorporated as a filler in molded articles formed using thermoplastic or thermosetting resins or rubbers, and in molded articles formed using thermoplastic or thermosetting resins or rubbers and materials containing fibers. Examples of applications for resin or rubber molded articles containing hollow particles of the present disclosure include light reflectors, heat insulators, sound insulators, and low-dielectric materials used in various fields such as automobiles, electrical appliances, electronics, architecture, aviation, and spacecraft; food containers; footwear such as sports shoes and sandals; home appliance parts; bicycle parts; stationery; tools; and 3D printer filaments. Furthermore, because the hollow particles of the present disclosure also have excellent dielectric properties, they are also useful as additives to insulating resin sheets used in the manufacture of electronic components such as printed wiring boards. An insulating resin sheet containing hollow particles of the present disclosure can be produced by, for example, preparing a composition by mixing the hollow particles of the present disclosure with the thermoplastic resin, thermosetting resin, thermoplastic elastomer, or a mixture thereof described above, and then applying the composition to one or both sides of a sheet-like substrate and drying, extrusion molding, or transfer molding to form a sheet. When the resin or elastomer contained in the insulating resin sheet has adhesive properties, the insulating resin sheet can be used as an adhesive sheet, specifically, for example, as a bonding sheet.Bonding sheets are insulating adhesive layer-forming materials used to bond conductor layers and organic insulating layers when manufacturing multilayer printed wiring boards. Furthermore, the hollow particles of the present disclosure have high porosity, are resistant to crushing, and have excellent heat resistance, thereby satisfying the insulation and shock-absorbing properties (cushioning) required for undercoat materials and also meeting the heat resistance required for thermal paper applications. The hollow particles of the present disclosure are also useful as plastic pigments with excellent gloss and hiding power. Furthermore, the hollow particles of the present disclosure can be encapsulated with useful ingredients such as fragrances, pharmaceuticals, pesticides, and ink components by means of immersion treatment, reduced pressure immersion treatment, or pressure immersion treatment, allowing them to be used for various applications depending on the ingredients contained therein. Furthermore, the hollow particles of the present disclosure are also suitable for use as rust inhibitors. The hollow particles of the present disclosure are also useful as additives that reduce electrical conductivity. Therefore, paints containing the hollow particles of the present disclosure can be used, for example, as rust-preventive paints (paint primers, lubricating paints, etc.) to enhance the corrosion and rust resistance of steel materials and the like. Furthermore, the hollow particles added to the anti-rust paint can also contain anti-rust additives.

[0091] 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.

[0092] Example 1 (1) Mixed Liquid Preparation Step First, the following materials were mixed to form an oil phase: Ethylene glycol dimethacrylate 25 parts Trimethylolpropane trimethacrylate 30 parts Divinylbenzene 26 parts Ethylvinylbenzene 19 parts 2,2'-Azobis(2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator) 3 parts Hydrophobic solvent: Hexane 100 parts Separately, in a stirring tank, at room temperature, an aqueous solution prepared by dissolving 17.1 parts of magnesium chloride (a water-soluble polyvalent metal salt) in 494 parts of ion-exchanged water was gradually added with stirring to prepare a magnesium hydroxide colloid (poorly water-soluble metal hydroxide colloid) dispersion (4 parts magnesium hydroxide), which served as the aqueous phase. The resulting aqueous phase and oil phase were mixed to prepare a mixed liquid.

[0093] (2) Suspension step The mixture obtained in the mixture preparation step was stirred and suspended for 1 minute using a disperser (manufactured by Primix Corporation, product name: Homomixer) at a rotation speed of 4,000 rpm, to prepare a suspension in which droplets of the monomer composition encapsulating the hydrophobic solvent were dispersed in water.

[0094] (3) Polymerization step: The suspension obtained in the suspension step was stirred for 1 hour and 30 minutes under a nitrogen atmosphere at a temperature of 65°C to carry out a polymerization reaction, thereby obtaining a precursor composition containing precursor particles encapsulating a hydrophobic solvent.

[0095] (4) Washing step and solid-liquid separation step The precursor composition was washed with dilute sulfuric acid (25°C, 10 minutes) to adjust the pH to 5.5 or less. Next, after separating the water by filtration, 200 parts of ion-exchanged water was added to re-slurry the mixture. The water washing treatment (washing, filtration, dehydration) was repeated several times at room temperature (25°C), and the mixture was filtered to obtain a solid fraction. The obtained solid fraction was dried in a dryer at a temperature of 40°C to obtain precursor particles encapsulating the hydrophobic solvent.

[0096] (5) Solvent Removal Step The precursor particles obtained in the solid-liquid separation step were heat-treated in a vacuum dryer at 200°C in a nitrogen atmosphere for 12 hours to remove the hydrophobic solvent contained in the particles, thereby obtaining 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.

[0097] [Example 2] Hollow particles of Example 2 were obtained in the same procedure as in Example 1, except that the amount of hexane added in the "(1) mixed solution preparation step" was changed as shown in Table 1.

[0098] [Examples 3 and 4] Hollow particles of Examples 3 and 4 were obtained in the same manner as in Example 1, except that in the above "(1) mixed solution preparation step", the type and amount of polymerizable monomer added were changed as shown in Table 1.

[0099] [Example 5] The hollow particles of Example 5 were obtained in the same procedure as in Example 1, except that in the above "(1) mixed solution preparation step", the amount of magnesium hydroxide in the aqueous phase (magnesium hydroxide colloidal dispersion) was changed to 10 parts by adjusting the amounts of magnesium chloride and sodium hydroxide added.

[0100] [Example 6] The hollow particles of Example 6 were obtained in the same procedure as in Example 1, except that in the above "(1) mixed solution preparation step", the amount of ion-exchanged water in the aqueous phase was changed to a total of 780 parts.

[0101] [Examples 7 and 8] Hollow particles of Examples 7 and 8 were obtained in the same manner as in Example 1, except that in the above "(1) mixed solution preparation step", the type and amount of polymerizable monomer and the type and amount of hydrophobic solvent were changed as shown in Table 1.

[0102] Comparative Example 1 First, the following materials were mixed to form an oil phase: 5 parts methacrylic acid, 65 parts ethylene glycol dimethacrylate, 30 parts trimethylolpropane trimethacrylate, 3 parts 2,2'-azobis(2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator), 187 parts hydrophobic solvent: cyclohexane. Separately, a mixture of 615 parts ion-exchanged water and 3.0 parts surfactant was added and mixed to form an aqueous phase. A mixed solution was prepared by mixing the aqueous phase and oil phase obtained above. The mixed solution was stirred and suspended at a rotation speed of 15,000 rpm using an in-line emulsifying disperser (manufactured by Pacific Machinery Works, product name: Milder) to prepare a suspension in which polymerizable monomer droplets encapsulating the hydrophobic solvent were dispersed in water. The suspension was stirred for 4 hours in a nitrogen atmosphere at a temperature of 65°C to carry out a polymerization reaction, yielding a precursor composition containing precursor particles encapsulating the hydrophobic solvent. The precursor composition was bubbled with nitrogen at 2 L / min for 4 hours at 75°C to remove the hydrophobic solvent contained in the precursor particles, resulting in a slurry of hollow particles containing water. The water was then separated by filtration, and 200 parts of ion-exchanged water was added to re-slurry the mixture. A water washing treatment (washing, filtration, dehydration) was repeated several times at room temperature (25°C), followed by filtration to obtain a solid. The obtained solid was dried at 60°C for 5 hours to remove the water contained in the particles, resulting in the hollow particles of Comparative Example 1.

[0103] [Comparative Example 2] Hollow particles for Comparative Example 2 were obtained in the same procedure as in Example 1, except that the "(1) mixed solution preparation step" in Example 1 was changed as follows, the polymerization reaction time in the "(3) polymerization step" was changed to 4 hours, and the heat treatment time in the "(5) solvent removal step" was changed to 6 hours. In Comparative Example 2, the mixed solution preparation step was performed as follows. First, a mixture of the following materials was mixed to form an oil phase. Ethylene glycol dimethacrylate 100 parts Polar resin (MMA / AA / EA copolymer) 2 parts 2,2'-Azobis(2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator) 3 parts Hydrophobic solvent: Cyclohexane 207 parts Meanwhile, in a stirring tank, at room temperature, an aqueous solution of 5.5 parts of sodium hydroxide (alkali metal hydroxide) in 55 parts of ion-exchanged water was gradually added with stirring to an aqueous solution of 7.8 parts of magnesium chloride (water-soluble polyvalent metal salt) in 225 parts of ion-exchanged water 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.

[0104] [Comparative Example 3] The hollow particles of Comparative Example 3 were obtained in the same procedure as in Comparative Example 2, except that in the above "(1) mixed solution preparation step", the amount of cyclohexane added was changed to 187 parts and the amount of ion-exchanged water in the aqueous phase was changed to a total of 615 parts.

[0105] The polar resin (MMA / AA / EA copolymer) used in Comparative Examples 2 and 3 was synthesized by the following procedure. 200 parts of toluene was charged into a reaction vessel, and the atmosphere inside the reaction vessel was thoroughly replaced with nitrogen while stirring the toluene. The temperature was then raised to 90°C, and a mixed solution of 96.2 parts of methyl methacrylate (MMA), 0.3 parts of acrylic acid (AA), 3.5 parts of ethyl acrylate (EA), and 2.8 parts of t-butylperoxy-2-ethylhexanoate (manufactured by NOF Corporation, trade name: Perbutyl O) was added dropwise into the reaction vessel over 2 hours. The mixture was then held under toluene reflux for 10 hours to complete the polymerization, and the solvent was then distilled off under reduced pressure to obtain a polar resin (MMA / AA / EA copolymer, number average molecular weight 10,000).

[0106] [Evaluation] The hollow particles obtained in each Example and Comparative Example were subjected to the following measurements and evaluations. The results are shown in Table 1.

[0107] 1. Volume Average Particle Size of Hollow Particles The volume average particle size of hollow particles was measured using a particle size distribution analyzer (manufactured by Beckman Coulter, Inc., product name: Multisizer 4e). The measurement conditions were aperture diameter: 50 μm, dispersion medium: Isoton II (product name), concentration: 10%, and number of particles measured: 100,000. Specifically, 0.2 g of particle sample was placed in a beaker, and a surfactant aqueous solution (manufactured by Fujifilm Corporation, product name: Drywell) was added as a dispersant. 2 ml of dispersion medium was added to wet the particles, and then 10 ml of dispersion medium was added. The mixture was dispersed in an ultrasonic disperser for 1 minute, and then measured using the particle size distribution analyzer.

[0108] 2. Density and porosity of hollow particles 2-1. Measurement of apparent density of hollow particles First, a 100 cm ３ About 30 cm ３ The hollow particles were packed into the measuring flask, and the mass of the packed hollow particles was accurately weighed. Next, the measuring flask packed with the hollow particles was accurately filled with isopropanol up to the marked line, taking care not to introduce air bubbles. The mass of the isopropanol added to the measuring flask was accurately weighed, and the apparent density D of the hollow particles was calculated based on the above formula (I). １ (g / cm ３ ) was calculated.

[0109] 2-2. Measurement of true density of hollow particles After crushing the hollow particles in advance, ３ Approximately 10 g of crushed pieces of hollow particles was filled into a measuring flask, and the mass of the crushed pieces was accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol was added to the measuring flask, and the mass of the isopropanol was accurately weighed. Based on the above formula (II), the true density D of the hollow particles was calculated. ０ (g / cm ３ ) was calculated.

[0110] 2-3. Calculation of porosity Apparent density D of hollow particles １ and true density D ０The porosity of the hollow particles was calculated based on the above formula (III).

[0111] 3. Shell Thickness of Hollow Particles The inner diameter r of the hollow particles was calculated from the volume average particle diameter R and porosity of the hollow particles according to the above formula (1), and the shell thickness of the hollow particles was calculated from the inner diameter r and the volume average particle diameter R according to the above formula (2).

[0112] 4. Void Remaining Rate After Press Test The void remaining rate for hollow particles was measured according to the press test method described below. [Press Test Method] 0.1 g of hollow particles and 0.9 g of polypropylene resin (manufactured by Japan Polypropylene Corporation, product name: Novatec PP, grade: MA1B) were melted and mixed on a hot plate heated to 200°C to obtain a mixture. The resulting mixture was placed in a 14 mm diameter heat press mold (manufactured by AS ONE Corporation), heated together with the mold in a dryer at 200°C for 15 minutes, and then stirred with a spatula. Next, the mold filled with the stirred mixture was placed in a heat press (manufactured by AS ONE Corporation, model number: H300-15) set to 80°C, and an 80°C cylinder was placed in the mold and allowed to stand. When the mold surface temperature dropped to 140°C, a press pressure of 60 MPa (press cylinder pressure of the press machine was 4 MPa) was applied to obtain a disk-shaped molded product with a diameter of 14 mm and a thickness of 8 mm. The obtained disk-shaped molded body was removed from the mold and pressed at a pressure of 1 MPa or less using a press at 200 ° C. to form a sheet having a thickness of 0.3 mm, thereby obtaining a sheet-like molded body. The void remaining rate of the hollow particles contained in the obtained sheet-like molded body was calculated using the following formula (A) and used as the void remaining rate after the press test. Formula (A): Void remaining rate (%) = {(c - a) / (c - b)} x 100 In the above formula (A), a represents the specific gravity of the molded body, b represents the specific gravity (calculated value) of the molded body assuming that voids are maintained, and c represents the specific gravity (calculated value) of the molded body assuming that all hollow particles are crushed. The specific gravity of the sheet-like molded body used to calculate the void remaining rate (a in the above formula (A)) was measured by the underwater displacement method in accordance with JIS K 7112. The specific gravity of the molded body assuming that voids are maintained (b in the above formula (A)) was calculated using the following formula (B). Formula (B): b=1 / {(P Ａ / P Ｇ ) + (R Ａ / RＧ )} In the above formula (B), P Ａ is the amount of hollow particles added, P Ｇ is the specific gravity of the hollow particles (apparent density D １ ), R Ａ is the amount of polypropylene resin added, R Ｇ and represent the specific gravity of the polypropylene resin. The specific gravity of the molded product (c in the above formula (A)) assuming that all the hollow particles are crushed was calculated by the following formula (C). Formula (C): c = [R Ｇ ×R Ａ +{D ０ ×P Ａ × (1-P Ｖ / 100)}] / {R Ａ +P Ａ × (1-P Ｖ / 100)} In the above formula (C), R Ａ is the amount of polypropylene resin added, R Ｇ is the specific gravity of polypropylene resin, D ０ is the true density of the hollow particle D ０ , P Ａ is the amount of hollow particles added, P Ｖ and represent the porosity (%) of the hollow particles.

[0113] 5. Void Residual Rate After Injection Molding 10 parts of hollow particles and 90 parts of polypropylene resin (manufactured by Japan Polypropylene Corporation, product name: Novatec PP, grade: MA1B (MFR at 230°C: 21 g / min)) were mixed in a blender, and then kneaded using a twin-screw kneader (manufactured by Toshiba Machine Co., Ltd., product name: TEM-35B) under the following kneading conditions, extruded, and pelletized to obtain pellets of a resin composition. <Kneading Conditions> Screw diameter: 37 mm, L / D = 32 Screw rotation speed: 250 rpm Resin temperature: 190°C Feed rate: 6 kg / hour The obtained pellets of the resin composition were heated and dried at 80°C for 6 hours, and then molded using an injection molding machine under the following molding conditions to obtain a molded product having dimensions of 80 mm x 10 mm x 4 mm thick. <Molding conditions> Cylinder temperature: 200°C Mold temperature: 70°C Injection pressure: 70 MPa Holding pressure: 30 MPa The void remaining rate of hollow particles contained in the molded body obtained by injection molding was calculated using the above formula (A). The specific gravity of the molded body obtained by injection molding (a in the above formula (A)) was measured by the underwater displacement method in accordance with JIS K 7112. The specific gravity of the molded body assumed to maintain voids (b in the above formula (A)) and the specific gravity of the molded body assumed to have all hollow particles crushed (c in the above formula (A)) were the same values ​​as those used to calculate the void remaining rate after the press test.

[0114]

[0115] Table 1 shows the amount (parts by mass) of each material added and the results of each measurement or evaluation. The void remaining rate after the press test and the void remaining rate after injection molding shown in Table 1 are the average values ​​for 10 samples (n = 10). In each example and each comparative example, the coefficient of variation of the void remaining rate after the press test was 0.006 or less. This demonstrated that the above press test method is a highly accurate test method.

[0116] [Discussion] The hollow particles obtained in Comparative Examples 1 to 3 had a low void retention rate after a press test, poor pressure resistance, and also a low void retention rate after injection molding. In Comparative Examples 1 and 2, the composition of the polymerizable monomer used did not allow the formation of a strong shell, and the shell was thin, presumably contributing to the poor pressure resistance of the hollow particles. In Comparative Example 3, the same polymerizable monomer as in Comparative Example 2 was used, and the shell thickness was similar to that of the Examples. However, the resulting hollow particles had poorer pressure resistance than the hollow particles obtained in each Example. This demonstrates that in order to sufficiently improve the pressure resistance of hollow particles, it is necessary to adjust not only the shell thickness but also the composition of the polymerizable monomer used to form the shell.

[0117] On the other hand, the hollow particles obtained in each Example were spherical particles with a hollow portion. While possessing a high porosity of 50% or more, the void retention rate after a press test was as high as 80% or more, demonstrating excellent pressure resistance, and the void retention rate after injection molding was also high. In each Example, it is believed that hollow particles with excellent pressure resistance were obtained by adjusting the composition of the polymerizable monomer used to form the shell, the type of hydrophobic solvent encapsulated in the particles during the manufacturing process, and the particle size of the hollow particles. Among these, the hollow particles obtained in Examples 1 to 6 exhibited high void retention rates after a press test and after injection molding, demonstrating particularly excellent pressure resistance. In Examples 1 to 6, the use of a combination of an acrylic monomer and a hydrocarbon monomer as the polymerizable monomer and a chain hydrocarbon solvent as the hydrophobic solvent improved the uniformity of the shell, resulting in improved pressure resistance. 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.

[0118] 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 porosity is 50% or more, Hollow particles having a void remaining rate of 80% or more as measured according to the following press test method. [Press test method] A mixture obtained by melting and mixing 0.1 g of hollow particles and 0.9 g of polypropylene resin at 200°C is placed in a mold for a heat press and further heated at 200°C for 15 minutes, followed by stirring. The mold filled with the mixture is then placed in a heat press set at 80°C, an 80°C cylinder is placed in the mold and allowed to stand, and when the surface temperature of the mold reaches 140°C, a pressure of 60 MPa is applied to obtain a molded product of the mixture. The molded product is then removed from the mold and pressed at a pressure of 1 MPa or less to form the molded product into a sheet, and the residual void ratio of the hollow particles contained in the obtained sheet-like molded product is measured.

2. 2. The hollow particle according to claim 1, wherein the shell contains, as the resin, a polymer containing 50 parts by mass or more of crosslinkable monomer units per 100 parts by mass of all monomer units.

3. 3. The hollow particle according to claim 2, wherein the polymer contains, as the crosslinkable monomer unit, a tri- or higher functional crosslinkable monomer unit derived from a tri- or higher functional crosslinkable monomer, and the content of the tri- or higher functional crosslinkable monomer unit is 10 parts by mass or more and 50 parts by mass or less in 100 parts by mass of all monomer units of the polymer.

4. 3. The hollow particle according to claim 2, wherein the polymer contains, as the crosslinkable monomer units, a bifunctional crosslinkable monomer unit derived from a bifunctional crosslinkable monomer and a trifunctional or higher crosslinkable monomer unit derived from a trifunctional or higher crosslinkable monomer.

5. A hollow particle as described in claim 4, wherein the bifunctional crosslinkable monomer comprises divinylbenzene.

6. 6. The hollow particle according to claim 4, wherein the content of the tri- or higher functional crosslinkable monomer unit is 5 parts by mass or more and 40 parts by mass or less, based on a total of 100 parts by mass of the bifunctional crosslinkable monomer unit and the tri- or higher functional crosslinkable monomer unit.

7. 6. The hollow particle according to claim 1, wherein the shell contains, as the resin, a polymer containing an acrylic monomer unit and a hydrocarbon monomer unit, and a content of the hydrocarbon monomer unit is 10 parts by mass or more and 80 parts by mass or less relative to 100 parts by mass of the total of the acrylic monomer unit and the hydrocarbon monomer unit.

8. The hollow particles according to any one of claims 1 to 5, having a volume average particle size of 5.0 µm or more and 40.0 µm or less.