Hollow particles, hollow particle production method, coating composition, and coated article

Abstract

Provided are hollow particles which have excellent resiliency and from which a void-containing coating film having excellent adhesiveness can be formed. These hollow particles each comprise a shell containing a resin and a hollow part surrounded by the shell, wherein: the hollow particles have a porosity of 50-90% and a volume-average particle diameter of 100-300 μm; and the proportion of the number of particles having only one hollow part is at least 90%.

Description

Hollow particles, method for producing hollow particles, coating composition, and coated article

[0001] The present disclosure relates to hollow particles and a method for producing the same, a coating composition containing the hollow particles, and a coated article having a void-containing coating film formed using the coating composition.

[0002] Conventionally, a void-containing coating film has been formed on the surface of a substrate such as a metal plate for the purposes of weight reduction, imparting functionality, imparting design, etc. For example, Patent Document 1 discloses a precoated metal plate in which a void-containing coating film is formed on a metal plate, in which thermally expandable fine particles containing a foaming agent are contained in a paint, the paint is applied to a metal plate, and the metal plate is heated to expand the foaming agent in the thermally expandable fine particles, thereby forming a void-containing coating film on the metal plate.

[0003] On the other hand, monoporous hollow resin particles having voids already present inside the particles have been used as weight-reducing agents, heat insulating agents, light diffusing agents, etc. For example, Patent Document 2 discloses hollow particles having a porosity of 60% or more, a volume average particle size of 10 to 50 μm, a shell containing a polymer containing 70 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of all monomer units, the crosslinkable monomer units including monomer units derived from a difunctional crosslinkable monomer and monomer units derived from a trifunctional or higher functional crosslinkable monomer, and in which, when observed in a plan view, the proportion of particles having recesses each having a size of 5 to 50% of the particle diameter is 10% or less. Patent Document 3 discloses light-diffusing hollow particles derived from a monomer mixture containing 40 to 10% by weight of a hydrophilic (meth)acrylic monomer and 60 to 90% by weight of an aromatic monomer containing at least a crosslinkable styrene-based monomer, and containing 10% by weight or more of the crosslinkable styrene-based monomer, and having a porosity of 30% or more and an average particle size of 3 to 100 μm.

[0004] International Publication No. 2022 / 014498 International Publication No. 2022 / 181484 International Publication No. 2011 / 040376

[0005] However, both void-containing coating films having a large number of voids formed by the expansion of a blowing agent and void-containing coating films having a large number of voids due to the inclusion of monoporous hollow particles have the problem that when formed on the surface of a substrate that is prone to expansion and contraction, such as metal, they are prone to peeling off from the substrate when the substrate expands and contracts due to temperature changes. The present inventors have found that peeling of the void-containing coating film from the substrate is caused in part by strain occurring around the microparticles that form the voids. Therefore, the microparticles used to form the voids are required to have high elasticity so that they can absorb the strain occurring at the interface of the microparticles in the coating film.

[0006] An object of the present disclosure is to provide hollow particles having excellent elasticity that can form a void-containing coating film with excellent adhesion. Another object of the present disclosure is to provide a method for producing such hollow particles. Another object of the present disclosure is to provide a coating composition that can form a void-containing coating film with excellent adhesion to a substrate that is prone to expansion and contraction. Another object of the present disclosure is to provide a coated article having a void-containing coating film with excellent adhesion.

[0007] The present inventors have found that monoporous hollow resin particles, which already have voids inside the particles, have better adhesion to binder resins such as acrylic resins or epoxy resins, and to substrates such as metals, than thermally expandable microparticles containing a blowing agent. Furthermore, the present inventors have found that when the volume average particle size and porosity of monoporous hollow resin particles are each set within a specific range, they have high elasticity and can absorb strain generated around the particles by deforming and restoring in a coating film, and as a result, a void-containing coating film containing the particles is less likely to peel off from the substrate, leading to the present disclosure.

[0008] That is, the present disclosure provides the following hollow particles: [1] Hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, wherein the hollow particles have a porosity of 50% or more and 90% or less, a volume average particle size of 100 μm or more and 300 μm or less, and the proportion of particles having only one hollow portion is 90% or more.

[0009] [2] The hollow particles according to [1], wherein the proportion of particles having pores with a major axis of 1 μm or more is 5% or less. [3] The hollow particles according to [1] or [2], wherein the particle size distribution (volume average particle diameter (Dv) / number average particle diameter (Dp)) is 1.0 or more and 1.5 or less. [4] The hollow particles according to any one of [1] to [3], wherein the proportion of particles having a circularity of 0.93 or less is 10% by mass or less. [5] The hollow particles according to any one of [1] to [4], wherein the iodine value measured in accordance with JIS K 0070 is 5 g / 100 g or more and 50 g / 100 g or less.

[0010] The present disclosure further provides the following method for producing hollow particles. [6] A method for producing hollow particles according to any one of [1] to [5], comprising the steps of: preparing a mixed solution containing a polymerizable monomer, a hydrophobic solvent, a dispersion stabilizer, and an aqueous medium; suspending the mixed solution to prepare a suspension in which droplets of a monomer composition containing the polymerizable monomer and the hydrophobic solvent are dispersed in the aqueous medium; subjecting the suspension to a polymerization reaction to prepare a precursor composition in which precursor particles, each having a hollow portion surrounded by a shell containing a resin and the hollow portion filled with the hydrophobic solvent, are dispersed in the aqueous medium; and removing the hydrophobic solvent from the precursor particles, wherein the dispersion stabilizer is colloidal particles of a poorly water-soluble inorganic compound, and the colloidal particles have an average particle diameter of 100 nm to 500 nm, and are prepared by reacting raw material components in the aqueous medium while stirring them at a tip speed of a stirring blade of 0.2 m / s to 5.0 m / s. a content of the dispersion stabilizer is 0.1 parts by mass or more and 5.0 parts by mass or less relative to 100 parts by mass of the polymerizable monomer, the step of preparing the suspension is carried out by stirring the mixed liquid with a disperser, and a tip speed of a stirring blade provided in the disperser is 60 m / s or more and 120 m / s or less.

[0011] The present disclosure further provides the following coating composition. [7] A coating composition containing the hollow particles according to any one of [1] to [5]. The present disclosure further provides the following slurry composition. [8] A slurry composition containing the hollow particles according to any one of [1] to [5] and a solvent. The present disclosure further provides the following coated article. [9] A coated article having, at least in a part thereof, a void-containing coating film formed using the coating composition according to [7].

[0012] According to the present disclosure as described above, it is possible to provide hollow particles having excellent elasticity and capable of forming a void-containing coating film having excellent adhesion. Furthermore, according to the present disclosure as described above, it is possible to provide a method for producing such hollow particles. Furthermore, according to the present disclosure as described above, it is possible to provide a coating composition capable of forming a void-containing coating film having excellent adhesion even on a substrate that is prone to expansion and contraction. Furthermore, according to the present disclosure as described above, it is possible to provide a coated article having a void-containing coating film having excellent adhesion.

[0013] Fig. 1 is a diagram illustrating an example of a method for producing hollow particles according to the present disclosure, and Fig. 2 is a diagram illustrating an example of a stirring device used in the solvent removal step.

[0014] The hollow particles and their manufacturing method according to the present disclosure, a coating composition containing the hollow particles according to the present disclosure, and a coated article having a void-containing coating film formed using the coating composition are described in detail below. 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. Furthermore, among the numerical values ​​described to explain this disclosure, numerical values ​​that may contain decimal places are, unless otherwise specified, numerical values ​​obtained by rounding off the digit that is one place lower than the lowest digit included in the numerical value.

[0015] 1. Hollow Particles The hollow particles disclosed herein are hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, characterized in that the hollow particles have a porosity of 50% or more and 90% or less, a volume average particle size of 100 μm or more and 300 μm or less, and the proportion of particles having only one hollow portion is 90% or more.

[0016] In thermally expandable microparticles containing a blowing agent, the particle surface after expansion is usually porous, and the numerous pores present on the particle surface reduce the contact area between the particle surface and surrounding materials. Therefore, thermally expandable microparticles after expansion tend to have insufficient adhesion to the binder resin or substrate. On the other hand, smoothing the particle surface after expansion reduces the shell thickness, increasing the rate of plastic deformation or breakage of the particles. Particles with a high rate of plastic deformation or breakage also tend to have insufficient adhesion to the binder resin or substrate due to the reduced contact area between the particle surface and surrounding materials. The hollow particles disclosed herein have a resin-containing shell, and 90% or more of the particles have only one hollow portion, i.e., they are monoporous hollow particles with a resin-formed shell. Because the hollow portions of the hollow particles disclosed herein are formed without the use of a blowing agent, they have excellent particle surface smoothness even without smoothing that would thin the shell. Therefore, the hollow particles of the present disclosure can ensure a sufficient shell thickness, making them less susceptible to plastic deformation or breakage, thereby ensuring a sufficient contact area between the particle surface and the surrounding material. Furthermore, the hollow particles of the present disclosure have a porosity of 50% to 90% and a volume average particle size of 100 μm to 300 μm, which allows them to have moderate flexibility and be easily deformed. They also have excellent elasticity, so even if they are deformed by external force, they easily restore their particle shape when released from the external force. When a void-containing coating film is formed on the surface of a substrate that is prone to temperature changes, such as a metal substrate, using a coating composition containing hollow particles to form a laminate, heating and cooling the laminate causes the substrate to expand and contract with temperature changes, and the void-containing coating film containing hollow particles follows the expansion and contraction of the substrate. In conventional void-containing coating films containing hollow particles, distortion occurs at the interface of the hollow particles when the hollow particles follow the expansion and contraction of the substrate, which makes the interface of the hollow particles prone to peeling, and therefore the void-containing coating film is prone to peeling from the substrate. In contrast, in the void-containing coating film containing hollow particles of the present disclosure, the hollow particles have excellent elasticity, allowing the hollow particles to deform and restore in the coating film, absorbing strain generated at the interfaces of the hollow particles, thereby maintaining adhesion at the interfaces of the hollow particles and making the void-containing coating film less likely to peel off from the substrate.Furthermore, the void-containing coating film containing the hollow particles of the present disclosure has excellent adhesion to the substrate, and therefore can easily conform to the surface of a substrate with a complex shape.Furthermore, the void-containing coating film containing the hollow particles of the present disclosure also has excellent impact resistance because it easily absorbs distortions that occur at the interfaces of the hollow particles.

[0017] 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 hollow particles of the present disclosure, the hollow portion is a hollow space clearly distinguishable from the shell, and can be confirmed, for example, by SEM observation of the cross section of the hollow particle or by TEM observation of the hollow particle itself. From the viewpoints of light weight, heat insulation, etc., it is preferable that the hollow portion of the hollow particles of the present disclosure be filled with a gas such as air.

[0018] The hollow particles of the present disclosure have a number ratio of particles having only one hollow portion of 90% or more, preferably 92% or more, more preferably 93% or more, and even more preferably 95% or more. The hollow particles of the present disclosure have a number ratio of particles having only one hollow portion equal to or greater than the above-mentioned lower limit, i.e., a substantially single-pore structure, which provides moderate flexibility and ease of deformation, resulting in excellent breakage resistance. Furthermore, since hollow particles with a single-pore structure have excellent shell surface smoothness, when the number ratio of particles having only one hollow portion is equal to or greater than the above-mentioned lower limit, a sufficient contact area between the hollow particles and their surrounding materials in the coating film can be ensured, resulting in improved interfacial adhesion of the hollow particles. In this disclosure, "adhesion of hollow particles" or "interfacial adhesion of hollow particles" refers to hollow particles contained in a void-containing coating film.

[0019] The shape of the hollow particles of the present disclosure may be, for example, spherical, oval, or irregular, but is preferably spherical from the viewpoints of the adhesion, dispersibility, strength, and pressure resistance of the hollow particles. 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-section of which is shown as hollow particle 10 in FIG. 1 (4). In this example, a thin film is provided on the outside, and the interior is filled with gas. The shape of the hollow particles can be confirmed, for example, by observing the hollow particles using an SEM or TEM.

[0020] The volume average particle diameter of the hollow particles of the present disclosure is 100 μm or more and 300 μm or less, with the lower limit preferably being 120 μm or more and the upper limit preferably being 250 μm or less, more preferably 200 μm or less. When the volume average particle diameter is within the above range, the hollow particles have flexibility and elasticity and can absorb strain generated at the interfaces of the hollow particles, resulting in excellent adhesion. Furthermore, when the volume average particle diameter of the hollow particles of the present disclosure is equal to or greater than the above lower limit, the hollow particles have sufficient strength, breakage resistance, and impact resistance, and the voids inside the particles are easily maintained.

[0021] The particle size distribution (volume average particle size (Dv) / number average particle size (Dp)) of the hollow particles of the present disclosure is not particularly limited, but is preferably 1.0 or more and 1.5 or less. The larger the particle size distribution, the greater the amount of fine powder or coarse powder contained in the hollow particles. Fine powder is a particle with a particle size significantly smaller than the volume average particle size and tends to have small hollow portions. Coarse powder is a particle with a particle size significantly larger than the volume average particle size and tends to be easily plastically deformed. Therefore, the greater the content of fine powder in the hollow particles, the more difficult it is to increase the porosity of the entire hollow particle. The greater the amount of coarse powder, the more difficult it is to absorb strain generated at the interfaces of the hollow particles, resulting in a decrease in adhesion. When the particle size distribution of the hollow particles of the present disclosure is equal to or less than the above upper limit, a decrease in porosity and a decrease in adhesion are sufficiently suppressed. Furthermore, when the particle size distribution is equal to or less than the above upper limit, particles with little variation in performance among particles can be obtained, and when a sheet-shaped molded product containing the hollow particles of the present disclosure is produced, a product with a uniform thickness can be produced. From these viewpoints, the particle size distribution of the hollow particles of the present disclosure is more preferably less than 1.45, even more preferably less than 1.40, and even more preferably less than 1.30. From the viewpoint of ease of production, the particle size distribution of the hollow particles of the present disclosure may be 1.15 or more, 1.20 or more, or even 1.25 or more. The volume average particle size (Dv) and number average particle size (Dp) 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 based on the Coulter Counter method, calculating the number average and volume average, respectively, and using the resulting values ​​as the number average particle size (Dp) and volume average particle size (Dv) of the particles. The particle size distribution is defined as the value obtained by dividing the volume average particle size by the number average particle size. The Coulter Counter method is a method for measuring particle diameter using an electrical resistance method known as the Coulter principle.

[0022] The hollow particles of the present disclosure have a porosity of 50% or more and 90% or less. Because the hollow particles of the present disclosure have a porosity of 50% or more, they have flexibility and elasticity and can absorb strain generated at the interfaces of the hollow particles, resulting in excellent adhesion. Furthermore, because the hollow particles of the present disclosure have a porosity of 50% or more, they also have excellent lightweight properties and insulating properties. From these perspectives, the porosity of the hollow particles of the present disclosure is preferably 60% or more, more preferably 65% ​​or more. On the other hand, because the hollow particles of the present disclosure have a porosity of 90% or less, they have sufficient strength, breakage resistance, and impact resistance, thereby maintaining voids within the particles, and since a decrease in elasticity is suppressed, a decrease in adhesion is also suppressed. From these perspectives, the porosity of the hollow particles of the present disclosure is preferably 85% or less, more preferably 80% or less.

[0023] 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 by the formula (I) Apparent density D １ Apparent density D = [Mass of hollow particles] / (100 - [Mass of isopropanol] / [Specific gravity of isopropanol at measurement temperature]) １ corresponds to the specific gravity of the entire hollow particle when the hollow portion is considered to be a part of the hollow particle.

[0024] True density D of hollow particles ０ The measurement method is as follows: After crushing the hollow particles in advance, ３Approximately 10 g of crushed pieces of hollow particles are filled into a measuring flask, and the mass of the crushed pieces is accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol is added to the measuring flask, and the mass of the isopropanol is accurately weighed. The true density D of the hollow particles is calculated based on the following formula (II): ０ (g / cm ３ ) is calculated using the formula (II) 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.

[0025] 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

[0026] From the viewpoint of improving breakage resistance and elasticity, the hollow particles according to the present disclosure preferably have a shell thickness within the range of 2.5 to 8.0% of the volume average particle diameter, more preferably within the range of 2.5 to 5.5%, even more preferably within the range of 2.5 to 5.0%, and even more preferably within the range of 3.0 to 5.0%. The shell thickness of the hollow particles is defined as the average value of shell thicknesses measured at any 10 points on an SEM image of the cross section of the hollow particles.

[0027] The shell of the hollow particles of the present disclosure preferably has excellent smoothness on the outer surface in order to improve the adhesion of the hollow particles. In order to improve the smoothness of the outer surface of the shell, the shell is preferably solid, i.e., non-porous. The fact that the shell is solid can be confirmed, for example, by observing the cross section of the hollow particles using an SEM.

[0028] The hollow particles disclosed herein preferably have a number percentage of particles having irregularities of 10% or more of the volume average particle size or defects with a major axis of 3% or more of the volume average particle size of less than 20%. This improves the smoothness of the outer surface of the shell, thereby improving the adhesion of the hollow particles. The number percentage of particles having the irregularities or defects can be determined by photographing the hollow particles with an SEM (magnification: 1,000x), randomly selecting 100 hollow particles from the obtained SEM image, and observing the surfaces of the selected hollow particles to identify particles having the irregularities or defects.

[0029] The hollow particles of the present disclosure preferably have a percentage of particles having pores with a major axis of 1 μm or more in number of 5% or less, more preferably 4% or less, and even more preferably 3% or less. A pore is a minute hole with an opening penetrating the shell, and the major axis of a pore is the length of the longest line segment connecting two points on the contour of the pore opening. Pores with a major axis of 1 μm or more can be confirmed by SEM observation of hollow particles. Particles with pores with a major axis of 1 μm or more tend to undergo plastic deformation and become irregular. As a result, they may fill gaps between spherical particles, inhibiting the deformation or restoration of the spherical particles and reducing the adhesiveness of the hollow particles. Furthermore, the flexibility and elasticity of irregularly shaped hollow particles are reduced, which also reduces the adhesiveness of the hollow particles. Furthermore, particles with pores with a major axis of 1 μm or more tend to shrink the hollow portion due to deformation, thereby reducing the porosity of the entire hollow particle. On the other hand, if the proportion of particles having pores with a major axis of 1 μm or more is equal to or less than the upper limit, the decrease in adhesion of hollow particles and the decrease in porosity are sufficiently suppressed.

[0030] In the hollow particles of the present disclosure, the proportion of particles having a circularity of 0.93 or less is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 6% by mass or less. If the hollow particles contain a mixture of spherical particles and particles having a low circularity, when the hollow particles are incorporated into a coating film, the particles having a low circularity may fill the gaps between the spherical particles, inhibiting the deformation or restoration of the spherical particles and reducing the adhesion of the hollow particles. When the proportion of particles having a circularity of 0.93 or less is equal to or less than the upper limit, the reduction in the adhesion of the hollow particles is sufficiently suppressed. Furthermore, compared to spherical particles, particles having a low circularity tend to aggregate when dispersed in a binder resin, resulting in poor dispersibility. Therefore, reducing the proportion of particles having a circularity of 0.93 or less contained in the hollow particles of the present disclosure can improve the dispersibility of the hollow particles.

[0031] Circularity is defined as the diameter of a circle having the same area as the projected image of a particle (equivalent circle area diameter) divided by the diameter of a circle having the same perimeter as the projected image of the particle (equivalent circumferential diameter). When a particle is a perfect sphere, the circularity is 1, and the more complex the particle's surface shape, the smaller the circularity value. The hollow particles disclosed herein preferably have an average circularity of 0.950 to 0.995. In this disclosure, circularity is measured using a flow-type particle image analyzer with an image resolution of 0.185 μm / pixel. A preferred example of a flow-type particle image analyzer is the "IF-3200" manufactured by Jasco International Co., Ltd. A measurement sample is prepared, for example, by dispersing 0.10 to 0.12 g of hollow particles in an aqueous solution of linear alkylbenzenesulfonate (0.3% concentration) for 5 minutes in an ultrasonic cleaner. The average circularity is the average value of the circularity of 1,000 to 3,000 arbitrarily selected particles.

[0032] The hollow particles of the present disclosure preferably have an iodine value measured in accordance with JIS K 0070 of 5 g / 100 g or more and 50 g / 100 g or less, more preferably 8 g / 100 g or more and 45 g / 100 g or less, and even more preferably 10 g / 100 g or more and 40 g / 100 g or less. In the present disclosure, the iodine value measured in accordance with JIS K 0070 may be simply referred to as the "iodine value." The iodine value of hollow particles is usually an indicator of the amount of unsaturated bonds present on the outer surface of the hollow particles. However, if the shell has many pores, the reagent penetrates into the hollow particles in the measurement method in accordance with JIS K 0070, and unsaturated bonds present on the inner surface of the hollow particles are also detected. Therefore, if the shell has many pores, the iodine value of the hollow particles tends to be high. When the iodine value of the hollow particles is within the above range, the hollow particles have an appropriate amount of unsaturated bonds, which tends to improve affinity with the binder resin or substrate, and as a result, tends to improve adhesion of the hollow particles. On the other hand, when the iodine value of the hollow particles is equal to or less than the above upper limit, oxidation degradation of the shell due to reaction of the unsaturated bonds with oxygen is suppressed, so that deterioration of the heat resistance of the hollow particles is suppressed, and deterioration of the adhesion of the hollow particles due to oxidation degradation of the shell is also suppressed.

[0033] The hollow particles of the present disclosure are not particularly limited, but preferably have a thermal decomposition onset temperature of 330°C or higher, more preferably 335°C or higher. When the thermal decomposition onset temperature is equal to or higher than the above lower limit, the particles have excellent heat resistance. The upper limit of the thermal decomposition onset temperature of the hollow particles is not particularly limited, and may be, for example, 400°C or lower. In the present disclosure, the thermal decomposition onset temperature of the hollow particles is the temperature at which a 5% weight loss occurs, and can be measured using a TG-DTA device in an air atmosphere under conditions of an air flow rate of 230 mL / min and a temperature rise rate of 5°C / min.

[0034] The shell of the hollow particles of the present disclosure contains a resin. The resin contained in the hollow particles of the present disclosure is not particularly limited, but is typically a polymer of a polymerizable monomer used in the "method for producing hollow particles" described below. The shell of the hollow particles of the present disclosure may further contain an additive such as an inorganic filler, as long as the effects of the present disclosure are not impaired. In order to improve the smoothness of the shell surface and the flexibility and elasticity of the hollow particles, the content of the resin contained in the shell is preferably 96% by mass or more, more preferably 97% by mass or more, even more preferably 98% by mass or more, and even more preferably 99% by mass or more. The shell may consist solely of resin.

[0035] The hollow particles of the present disclosure are not particularly limited in their applications, but are used in various fields such as automobiles, bicycles, aviation, construction, space, and electronic materials. Preferred examples include weight-reducing agents, heat insulating agents, and sound-insulating agents added to interior materials or heat insulating materials. The hollow particles of the present disclosure can also be used as a light diffusing agent added to light diffusing materials such as light diffusing films or light diffusing plates.

[0036] 2. Method for Producing Hollow Particles The hollow particles of the present disclosure are produced, for example, by a production method described below using a suspension polymerization method. One embodiment of the method for producing hollow particles of the present disclosure includes the steps of: preparing a mixed solution containing a polymerizable monomer, a hydrophobic solvent, a dispersion stabilizer, and an aqueous medium; suspending the mixed solution to prepare a suspension in which droplets of a monomer composition containing the polymerizable monomer and the hydrophobic solvent are dispersed in the aqueous medium; subjecting the suspension to a polymerization reaction to prepare a precursor composition in which precursor particles having a hollow portion surrounded by a shell containing a resin and filled with the hydrophobic solvent are dispersed in the aqueous medium; and removing the hydrophobic solvent from the precursor particles.

[0037] 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.

[0038] In the above-mentioned manufacturing method, a mixture containing a polymerizable monomer, a hydrophobic solvent, a dispersion stabilizer, and an aqueous medium is suspended, whereby the polymerizable monomer and the hydrophobic solvent undergo phase separation, and a suspension is prepared in which droplets of a monomer composition having a distribution structure in which the polymerizable monomer is unevenly distributed on the surface side and the hydrophobic solvent is unevenly distributed in the center are dispersed in the aqueous medium. When this suspension is subjected to a polymerization reaction, a cured product of the polymerizable monomer begins to precipitate on the surface of the droplets of the monomer composition, and as the polymerization reaction progresses, the entire surface of the droplets hardens, forming a shell, resulting in hollow particles (precursor particles) having a hollow portion filled with the hydrophobic solvent.

[0039] The above-mentioned manufacturing method includes a step of preparing a mixed solution, a step of preparing a suspension, a step of subjecting the suspension to a polymerization reaction, and a step of removing the hydrophobic solvent from the precursor particles, 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 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 simultaneously adding the materials for preparing the mixed solution and suspending them.

[0040] A preferred example of the method for producing hollow particles according to the present disclosure includes a production method comprising the following steps: (1) mixed solution preparation step: a step of preparing a mixed solution containing a polymerizable monomer, a hydrophobic solvent, a dispersion stabilizer, and an aqueous medium, (2) suspending step: a step of suspending the mixed solution to prepare a suspension in which droplets of a monomer composition containing a polymerizable monomer and a hydrophobic solvent are dispersed in an aqueous medium, (3) polymerization step: a step of subjecting the suspension to a polymerization reaction to prepare a precursor composition in which precursor particles, each having a hollow portion surrounded by a shell containing a resin and filled with a hydrophobic solvent, are dispersed in an aqueous medium, and (4) solvent removal step: a step of removing the hydrophobic solvent from the precursor particles.

[0041] FIG. 1 is a schematic diagram illustrating an example of the manufacturing method of the present disclosure. (1) to (4) in FIG. 1 correspond to the above-described steps (1) to (4). The white arrows between the diagrams indicate the order of the steps. Note that FIG. 1 is merely a schematic diagram for explanatory purposes, and the manufacturing method of the present disclosure is not limited to that shown in the diagram. Furthermore, the structure, dimensions, and shape of the materials used in the manufacturing method of the present disclosure are not limited to those of the various materials shown in these diagrams. (1) in FIG. 1 is a cross-sectional schematic diagram illustrating an embodiment of a mixed solution in the mixed solution preparation step. As shown in this figure, the mixed solution contains an aqueous medium 1 and a low-polarity material 2 dispersed in the aqueous medium 1. Here, the low-polarity material 2 refers to a material that has low polarity and is difficult to mix with the aqueous medium 1. In the present disclosure, the low-polarity material 2 includes a polymerizable monomer, a hydrophobic solvent, and the like. (2) in FIG. 1 is a cross-sectional schematic diagram illustrating an embodiment of a suspension in the suspension step. The suspension contains an aqueous medium 1 and droplets 3 of a monomer composition dispersed in the aqueous medium 1. The monomer composition droplets 3 contain polymerizable monomers and hydrophobic solvents, but the distribution within the droplets is non-uniform. The monomer composition droplets 3 are phase-separated into a hydrophobic solvent 4a and a material other than the hydrophobic solvent, including the polymerizable monomer, 4b. The hydrophobic solvent 4a is concentrated in the center, while the material other than the hydrophobic solvent 4b is concentrated on the surface. A dispersion stabilizer (not shown) is attached to the surface. (3) in FIG. 1 is a cross-sectional schematic diagram illustrating one embodiment of a precursor composition obtained by the polymerization process, including precursor particles encapsulating a hydrophobic solvent in a hollow portion. The precursor particles 5 have a shell 6 containing a resin and a hollow portion filled with the hydrophobic solvent 4a. The shell 6 forming the outer surface of the precursor particles 5 is formed by polymerization of the polymerizable monomer contained in the monomer composition droplets 3. In (3) in FIG. 1, the precursor particles 5 are dispersed in an aqueous medium 1. (4) in FIG. 1 is a cross-sectional schematic diagram illustrating one embodiment of hollow particles obtained by the solvent removal process. In the solvent removal step, the hydrophobic solvent is removed from the precursor particles to obtain hollow particles 10 having a shell 6 containing a resin and a hollow portion 7 filled with gas. In (4) of Figure 1, the hollow particles 10 are separated from the aqueous medium 1 and exist in the air. The above four steps and other steps will be described in order below.

[0042] (1) Mixed Liquid Preparation Step This step is a step of preparing a mixed liquid containing a polymerizable monomer, a hydrophobic solvent, 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.

[0043] (A) Polymerizable Monomer In the present disclosure, a polymerizable monomer is a compound having a functional group capable of addition polymerization (sometimes simply referred to as a polymerizable functional group in the present disclosure). In the present disclosure, a compound having an ethylenically unsaturated bond as a functional group capable of addition polymerization is generally used as the polymerizable monomer. As the polymerizable functional group, a radically polymerizable group is preferred. In terms of excellent reactivity, at least one selected from the group consisting of a (meth)acryloyl group, a vinyl group, and an allyl group is preferred, and at least one selected from a (meth)acryloyl group and a vinyl group is more preferred. In the present disclosure, a polymerizable monomer having only one polymerizable functional group is referred to as a non-crosslinkable monomer, and a polymerizable monomer having two or more polymerizable functional groups is referred to as a crosslinkable monomer. The crosslinkable monomer can form a crosslinked bond in a polymer by a polymerization reaction. The crosslinkable monomer becomes a crosslinkable monomer unit in the shell, and the non-crosslinkable monomer becomes a non-crosslinkable monomer unit in the shell. 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. Furthermore, a polymerizable monomer having a (meth)acryloyl group as a polymerizable functional group is referred to as an acrylic monomer, a crosslinkable monomer having a (meth)acryloyl group as a polymerizable functional group is referred to as a crosslinkable acrylic monomer, and a non-crosslinkable monomer having a (meth)acryloyl group as a polymerizable functional group is referred to as a non-crosslinkable acrylic monomer. In a crosslinkable acrylic monomer, at least one polymerizable functional group may be a (meth)acryloyl group, but it is preferred that all polymerizable functional groups are (meth)acryloyl groups.

[0044] The polymerizable monomer may be any known polymerizable monomer conventionally used for producing hollow particles, and is not particularly limited. However, it is preferable to include a crosslinkable monomer as the polymerizable monomer, since this improves the breakage resistance and elasticity of the hollow particles, makes it easier to obtain hollow particles with only one hollow portion, and makes it easier to improve the smoothness of the shell surface.Examples of the crosslinkable monomer include crosslinkable hydrocarbon monomers such as aromatic divinyl monomers such as divinylbenzene, divinylbiphenyl, and divinylnaphthalene; straight-chain or branched diolefins such as butadiene, isoprene, 2,3-dimethylbutadiene, pentadiene, and hexadiene; and diene monomers such as alicyclic diolefins such as dicyclopentadiene, cyclopentadiene, and ethylidenetetracyclododecene; allyl (meth)acrylate, vinyl (meth)acrylate, ethylene glycol diolefins, and the like. (meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, 3-(meth)acryloyloxy-2-hydroxypropyl (meth)acrylate, 1,3-bis(methacryloyloxy)-2-hydroxypropane, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, 1,10-decanediol di(meth)acrylate, neopentyl glycol Difunctional crosslinkable acrylic monomers such as ricol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, bisphenol A di(meth)acrylate, and their ethoxylated derivatives, trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, crosslinkable acrylic monomers having three or more functionalities such as acrylate, dipentaerythritol poly(meth)acrylate, and their ethoxylated derivatives; crosslinkable allylic monomers such as diallyl phthalate; crosslinkable macromers such as polybutadiene, polyisoprene, styrene-butadiene block copolymer (SBS), styrene-isoprene block copolymer (SIS), polyphenylene ether vinyl-modified at both ends, and polyphenylene ether (meth)acrylic-modified at both ends. These crosslinkable monomers can be used alone or in combination of two or more.As the crosslinkable monomer, at least one selected from a crosslinkable hydrocarbon monomer and a crosslinkable acrylic monomer is preferred, from the viewpoints of improving the breakage resistance and elasticity of the hollow particles, increasing the proportion of particles having only one hollow portion, and improving the smoothness of the shell surface, and an aromatic divinyl monomer is more preferred, with divinylbenzene being particularly preferred.

[0045] The content of the crosslinkable monomer in 100% by mass of the polymerizable monomer is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 93% by mass or more, and even more preferably 94% by mass or more. When the content of the crosslinkable monomer is equal to or greater than the above-mentioned lower limit, the breakage resistance and elasticity of the hollow particles are improved, the proportion of particles having only one hollow portion increases, and the smoothness of the shell surface is improved. The polymerizable monomer contained in the mixed liquid may consist of a crosslinkable monomer, but may also contain a non-crosslinkable monomer described below. Therefore, the content of the crosslinkable monomer in 100% by mass of the polymerizable monomer may be, for example, 99% by mass or less, 98% by mass or less, or 97% by mass or less. In the present disclosure, the content of each monomer in 100% by mass of the polymerizable monomer contained in the mixed liquid corresponds to the content of each monomer unit in 100% by mass of all monomer units in the polymer of the polymerizable monomer contained in the shell.

[0046] The mixed solution may further contain a non-crosslinkable monomer as a polymerizable monomer. Examples of the non-crosslinkable monomer include aromatic monovinyl monomers such as styrene, vinyltoluene, α-methylstyrene, p-methylstyrene, ethylvinylbenzene, ethylvinylbiphenyl, and ethylvinylnaphthalene, linear or branched monoolefins such as ethylene, propylene, and butylene, and alicyclic monoolefins such as vinylcyclohexane, norbornene, tricyclododecene, and 1,4-methano-1,4,4a,9a-tetrahydrofluorene, and non-crosslinkable hydrocarbon monomers. acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, t-butylaminoethyl (meth)acrylate, glycidyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-aminoethyl (meth)acrylate, (meth)acrylic acid, (meth)acrylamide, N-methylol (meth)acrylamide, N-butoxymethyl (meth)acrylamide, methoxypolyethylene glycol (meth)acrylate, ethoxypolyethylene glycol non-crosslinkable acrylic monomers such as polyethylene glycol (meth)acrylate, propoxy polyethylene glycol (meth)acrylate, butoxy polyethylene glycol (meth)acrylate, hexaoxy polyethylene glycol (meth)acrylate, octoxy polyethylene glycol polypropylene glycol (meth)acrylate, lauroxy polyethylene glycol (meth)acrylate, stearoxy polyethylene glycol (meth)acrylate, phenoxy polyethylene glycol polypropylene glycol (meth)acrylate, polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, polyethylene glycol propylene glycol mono(meth)acrylate, polyethylene glycol tetramethylene glycol (meth)acrylate, propylene glycol polybutylene glycol mono(meth)acrylate, and monoethylene glycol mono(meth)acrylate; carboxylic acid vinyl ester monomers such as vinyl acetate; halogenated aromatic vinyl monomers such as halogenated styrene;Examples of suitable non-crosslinkable monomers include vinyl halide monomers such as vinyl chloride; vinylidene halide monomers such as vinylidene chloride; vinylpyridine; and non-crosslinkable macromers such as polystyrene having one end (meth)acrylic modified and polymethyl methacrylate having one end (meth)acrylic modified. These non-crosslinkable monomers can be used alone or in combination of two or more. Among these, non-crosslinkable monomers are preferred, and aromatic monovinyl monomers are more preferred, because they prevent the hollow particles from losing their breakage resistance and elasticity, prevent the formation of porous particles, and prevent the shell surface from losing its smoothness.

[0047] The content of the non-crosslinkable monomer in 100% by mass of the polymerizable monomer is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 7% by mass or less, still more preferably 6% by mass or less, even more preferably 5% by mass or less, particularly preferably 4.5% by mass or less, and most preferably 4% by mass or less, from the viewpoint of suppressing a decrease in the breakage resistance and elasticity of the hollow particles, suppressing the formation of porous particles, and suppressing a decrease in the smoothness of the shell surface. The lower limit of the content of the non-crosslinkable monomer in 100% by mass of the polymerizable monomer is not particularly limited, but may be, for example, 1% by mass or more, 2% by mass or more, or 3% by mass or more.

[0048] The content of the polymerizable monomer in the mixed solution is not particularly limited, but is preferably 15 to 60% by mass, more preferably 20 to 50% by mass, relative to 100% by mass of the total mass of the components in the mixed solution excluding the aqueous medium, from the viewpoints of easily controlling the porosity and volume average particle size of the hollow particles within the above-mentioned ranges and improving the breakage resistance and elasticity of the hollow particles. Furthermore, from the viewpoints of improving the breakage resistance and elasticity of the hollow particles and improving the smoothness of the shell surface, the content of the polymerizable monomer relative to 100% by mass of the total mass of the solids of the materials that form the oil phase in the mixed solution, excluding the hydrophobic solvent, is preferably 90% by mass or more, more preferably 94% by mass or more. In the present disclosure, the solid content refers to all components excluding the solvent, and liquid polymerizable monomers and the like are considered to be included in the solid content.

[0049] (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. 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.

[0050] The hydrophobic solvent can be appropriately selected from known hydrophobic solvents and is not particularly limited. Examples include esters such as ethyl acetate and butyl acetate; ether esters such as propylene glycol monomethyl ether acetate and propylene glycol monoethyl ether acetate; and hydrocarbon solvents. Among these, hydrocarbon solvents are preferred, and hydrocarbon solvents having 5 to 8 carbon atoms are more preferred. Examples of hydrocarbon solvents include aliphatic hydrocarbons including chain hydrocarbon solvents such as pentane, hexane, heptane, octane, 2-methylbutane, 2-methylpentane, and paraffin solvents, and cyclic hydrocarbon solvents such as cyclohexane, methylcyclohexane, and cycloheptane; and aromatic hydrocarbons such as benzene, toluene, and xylene. These hydrophobic solvents can be used alone or in combination of two or more.

[0051] Since phase separation between the polymerizable monomer and the hydrophobic solvent is likely to occur within the droplets of the monomer composition, making it easier to obtain hollow particles with only one hollow portion, it is preferable to select an organic solvent as the hydrophobic solvent that has a lower solubility in water than the crosslinkable monomer contained in the polymerizable monomer. Furthermore, since phase separation between the polymerizable monomer and the hydrophobic solvent is likely to occur within the droplets of the monomer composition, making it easier to obtain hollow particles with only one hollow portion, it is preferable to select a hydrophobic solvent so that the HSP distance between the crosslinkable monomer contained in the polymerizable monomer and the hydrophobic solvent is 5.00 or more and 8.00 or less. The HSP distance is more preferably 5.15 or more and 7.75 or less, and even more preferably 5.30 or more and 7.50 or less. The HSP distance is an index that represents the solubility between substances using the Hansen Solubility Parameter (HSP), and it can be determined that the closer the HSP distance is to 0, the higher the compatibility between the substances. HSP is expressed as a vector in a three-dimensional space (Hansen space) with the dispersion term dD, polar term dP, and hydrogen bonding term dH as coordinate axes. The three parameters dD, dP, and dH have unique values ​​for each substance. The software developed by Hansen et al. (Hansen Solubility Parameter in Practice (HSPiP)) contains a database of dD, dP, and dH for various substances. HSPiP can also be used to calculate HSP based on the chemical structure of a substance. To calculate the HSP of a mixture containing multiple substances, the weighted averages of the dD, dP, and dH values ​​of each substance contained in the mixture and their respective proportions are calculated to obtain the dispersion term dD, polar term dP, and hydrogen bonding term dH of the mixture. The HSP distance is the distance between vectors given by the HSPs of two substances, and the dispersion term dD of one substance is １ , polarity term dP １ , hydrogen bond term dH １ and the dispersion term dD of the other material ２ , polarity term dP ２ , hydrogen bond term dH ２ The HSP distance is calculated from the value of the HSP distance using the following formula (A): １ -dd ２ ) ２+ (dP １ -dP ２ ) ２ + (dH １ -dH ２ ) ２} ０．５ In the present disclosure, the HSP distance is a value calculated using HSPiP (Version 5.3.03), in which the values ​​of dD, dP, and dH are expressed with significant figures up to one decimal place, and the value of the HSP distance is expressed with significant figures up to two decimal places.

[0052] 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.

[0053] Furthermore, the hydrophobic solvent preferably has a dielectric constant of 2.5 or less at 20°C. The dielectric constant is one of the indicators of the polarity of a compound. When the dielectric constant of the hydrophobic solvent is sufficiently small, such as 2.5 or less, phase separation proceeds rapidly in the droplets of the monomer composition, and hollow spaces are likely to be formed. Examples of hydrophobic solvents having a dielectric constant of 2.5 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.

[0054] The porosity of the hollow particles can be adjusted by changing the amount of hydrophobic solvent in the mixed solution. In the suspension process described below, the polymerization reaction proceeds with the hydrophobic solvent encapsulated in the oil droplets containing the polymerizable monomer. Therefore, the higher the hydrophobic solvent content, the higher the porosity of the resulting hollow particles. 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, because it facilitates control of the particle size of the hollow particles, facilitates increasing the porosity while maintaining the strength of the hollow particles, facilitates the production of hollow particles with only one hollow portion, and facilitates reducing the amount of residual hydrophobic solvent within the particles. From these perspectives, the content of the hydrophobic solvent in the mixed solution is more preferably 60 to 400 parts by mass, and even more preferably 70 to 300 parts by mass per 100 parts by mass of the polymerizable monomer.

[0055] (C) Polymerization Initiator In the production method of the present disclosure, the mixed liquid preferably contains an oil-soluble polymerization initiator as the polymerization initiator. The oil-soluble polymerization initiator is not particularly limited as long as it is an oil-philic initiator having a solubility in water of 0.2 mass% or less, and examples thereof include organic peroxides such as benzoyl peroxide, lauroyl peroxide, t-butylperoxy-2-ethylhexanoate, t-butylperoxydiethyl acetate, and t-butylperoxypivalate; and azo compounds such as 2,2'-azobis(2,4-dimethylvaleronitrile), azobisisobutyronitrile, and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile).

[0056] The content of the polymerization initiator is preferably 0.1 to 10 parts by mass, more preferably 1 to 8 parts by mass, and even more preferably 3 to 6 parts by mass, per 100 parts by mass of the polymerizable monomer in the mixed solution. When the content of the polymerization initiator is equal to or greater than the above-mentioned lower limit, the polymerization reaction can proceed sufficiently. When the content is equal to or less than the above-mentioned upper limit, there is little risk of the polymerization initiator remaining after completion of the polymerization reaction, and there is also little risk of an unexpected side reaction proceeding. Furthermore, when the content of the polymerization initiator is within the above-mentioned range, hollow particles having only one hollow portion are likely to be obtained.

[0057] (D) Dispersion Stabilizer The dispersion stabilizer disperses droplets of the monomer composition in an aqueous medium during the suspension process. Because hollow particles having the above-mentioned volume average particle size and only one hollow portion are easily obtained, inorganic dispersion stabilizers are preferably used as the dispersion stabilizer. Inorganic dispersion stabilizers are also preferred because they are easily removed in the washing process and prevent the shell from becoming too thin, thereby suppressing a decrease in the strength of the hollow particles. Examples of inorganic dispersion stabilizers include sulfates such as barium sulfate and calcium sulfate; carbonates such as barium carbonate, calcium carbonate, and magnesium carbonate; phosphates such as calcium phosphate; metal oxides such as aluminum oxide and titanium oxide; metal hydroxides such as aluminum hydroxide, magnesium hydroxide, calcium hydroxide, barium hydroxide, and ferric hydroxide; and inorganic compounds such as silicon dioxide. These inorganic dispersion stabilizers can be used alone or in combination of two or more.

[0058] A poorly water-soluble inorganic compound is preferred as the inorganic dispersion stabilizer, since hollow particles having the above-mentioned volume average particle diameter and only one hollow portion are easily obtained. As the poorly water-soluble inorganic compound, a metal hydroxide is preferred, and magnesium hydroxide is particularly preferred. In the present disclosure, poorly water-soluble preferably means a solubility in water at 25°C of less than 1 g / L. The poorly water-soluble inorganic compound is preferably used in a colloidal state. That is, the dispersion stabilizer is preferably colloidal particles of a poorly water-soluble inorganic compound, more preferably colloidal particles of a poorly water-soluble metal hydroxide, and particularly preferably colloidal particles of magnesium hydroxide. Colloidal silica can also be used as the colloid of the poorly water-soluble inorganic compound. Using a poorly water-soluble inorganic compound in a colloidal state as the dispersion stabilizer makes it easier to obtain hollow particles having the above-mentioned volume average particle diameter and only one hollow portion, and also makes it easier to remove the dispersion stabilizer by washing.

[0059] The average particle size of the colloidal particles of the poorly water-soluble inorganic compound is not particularly limited, but is preferably 100 to 500 nm, more preferably 200 to 400 nm, and even more preferably 250 to 350 nm. When the average particle size of the colloidal particles used as a dispersion stabilizer is within the above range, the volume average particle size of the resulting hollow particles is likely to be 100 to 300 μm. The average particle size of the colloidal particles is measured using a laser diffraction particle size distribution analyzer (for example, manufactured by Shimadzu Corporation, product name: SALD-2300J) and is the 50% cumulative value of the number particle size distribution.

[0060] Colloid particles of a poorly water-soluble inorganic compound can be prepared, for example, by reacting raw materials of the poorly water-soluble inorganic compound in an aqueous medium while stirring them. During the stirring, the tip speed of the stirring blade is preferably 0.2 m / s or more and 5.0 m / s or less, more preferably 0.3 m / s or more and 4.0 m / s or less, and even more preferably 0.5 m / s or more and 3.5 m / s or less. When the tip speed of the stirring blade during preparation of the colloid particles is within the above range, the particle size distribution of the colloid particles can be narrowed.

[0061] Specifically, the raw material components of the poorly water-soluble inorganic compound include at least one selected from alkali metal hydroxides and alkaline earth metal hydroxides, and water-soluble polyvalent metal salts (excluding alkaline earth metal hydroxides). Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Examples of alkaline earth metal hydroxides include barium hydroxide and calcium hydroxide. The water-soluble polyvalent metal salt may be any water-soluble polyvalent metal salt other than the above-mentioned alkaline earth metal hydroxides. Examples include magnesium metal salts such as magnesium chloride, magnesium phosphate, and magnesium sulfate; calcium metal salts such as calcium chloride, calcium nitrate, calcium acetate, and calcium sulfate; aluminum metal salts such as aluminum chloride and aluminum sulfate; barium salts such as barium chloride, barium nitrate, and barium acetate; and zinc salts such as zinc chloride, zinc nitrate, and zinc acetate. Among these, magnesium metal salts, calcium metal salts, and aluminum metal salts are preferred, magnesium metal salts are more preferred, and magnesium chloride is particularly preferred. The method for reacting at least one selected from the alkali metal hydroxides and alkaline earth metal hydroxides with the water-soluble polyvalent metal salt in an aqueous medium is not particularly limited, but for example, an aqueous solution of at least one selected from the alkali metal hydroxides and alkaline earth metal hydroxides may be mixed with an aqueous solution of the water-soluble polyvalent metal salt.

[0062] The content of the dispersion stabilizer is not particularly limited, but is preferably 0.1 to 1.0 parts by mass, more preferably 0.2 to 0.8 parts by mass, even more preferably 0.3 to 0.6 parts by mass, and still more preferably 0.4 to 0.5 parts by mass, relative to 100 parts by mass of the total mass of the polymerizable monomer and the hydrophobic solvent. Furthermore, the content of the dispersion stabilizer relative to 100 parts by mass of the polymerizable monomer is preferably 0.1 to 5.0 parts by mass, more preferably 0.5 to 4.0 parts by mass, and even more preferably 1.0 to 2.0 parts by mass. When the content of the dispersion stabilizer is equal to or greater than the lower limit, the droplets of the monomer composition can be sufficiently dispersed in the suspension so as not to coalesce, and the size of the resulting hollow particles can be prevented from becoming excessively large. On the other hand, when the content of the dispersion stabilizer is equal to or less than the above upper limit, the volume average particle size of the obtained hollow particles can be set to 100 μm or more, and further, an increase in the viscosity of the suspension during granulation can be prevented, thereby avoiding the problem of the suspension clogging the granulator.

[0063] (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.

[0064] 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.

[0065] 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.

[0066] A mixed solution is obtained by mixing the above-mentioned materials and other materials as needed, followed by appropriate stirring. In this mixed solution, 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 particle sizes of approximately several millimeters. The dispersion state of these materials in the mixed solution can be observed with the naked eye, depending on the type of material. In the mixed solution preparation step, the mixed solution may be obtained by simply mixing the above-mentioned materials and other materials as needed, followed by appropriate stirring. However, in order to more easily obtain hollow particles having the above-mentioned volume average particle size and only one hollow portion, it is preferable to prepare a mixed solution by separately preparing an oil phase containing the polymerizable monomer, the hydrophobic solvent, the polymerization initiator, etc., and an aqueous phase containing the dispersion stabilizer and the aqueous medium, and then mixing them. In the present disclosure, a colloid in which colloidal particles of a poorly water-soluble inorganic compound are dispersed in an aqueous medium can preferably be used as the aqueous phase.

[0067] (2) Suspension Step The suspension step is a step of preparing a suspension in which droplets of a monomer composition containing a hydrophobic solvent are dispersed in an aqueous medium by suspending the above-described mixed solution. The suspension method for forming droplets of the monomer composition can be a known suspension method and is not particularly limited. For example, a method in which the above-described mixed solution is stirred using a disperser can be used. Examples of dispersers that can be used in preparing the suspension include horizontal or vertical in-line dispersers such as Milder manufactured by Pacific Machinery Works, Ltd., Cavitron manufactured by Eurotec Co., Ltd., and in-line dispersers manufactured by IKA (e.g., DISPAX-REACTOR (registered trademark) DRS); and emulsifying dispersers such as the Homomixer MARK II series manufactured by Primix Corporation. In the stirring using a disperser to prepare a suspension, the tip speed of the stirring blades provided in the disperser is preferably 60 m / s or more, more preferably 70 m / s or more, from the viewpoint of narrowing the particle size distribution of the hollow particles while keeping the volume average particle size of the hollow particles within the above-mentioned preferred range, and on the other hand, from the viewpoint of reducing the proportion of particles with low circularity and suppressing the generation of fine powder, the tip speed is preferably 120 m / s or less, more preferably 90 m / s or less.

[0068] In the suspension prepared in the suspending step, droplets of the monomer composition containing the lipophilic material and having a particle size of approximately 100 to 300 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.

[0069] The droplets of the monomer composition dispersed in an aqueous medium are formed by surrounding an oil phase containing a polymerizable monomer and a lipophilic material such as a hydrophobic solvent with a dispersion stabilizer. When an oil-soluble polymerization initiator is used as the polymerization initiator, the oil-soluble polymerization initiator generates polymerization initiation radicals inside the droplets of the monomer composition, which are 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.

[0070] (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 filled with the hydrophobic solvent by subjecting the suspension obtained in the above-mentioned suspension step to a polymerization reaction. In this step, the shell portion of the droplets of the monomer composition containing the hydrophobic solvent therein is polymerized, and therefore, a hollow portion filled with the hydrophobic solvent is formed inside the obtained precursor particles.

[0071] In the polymerization step, the polymerization method is not particularly limited, and known polymerization methods such as batch, semi-continuous, and continuous methods can be used. The polymerization temperature is not particularly limited, but is preferably 60°C or higher, more preferably 70°C or higher, and even more preferably 80°C or higher, from the viewpoint of allowing the polymerization reaction to proceed sufficiently and making it easier to obtain hollow particles having only one hollow portion. It is preferably 95°C or lower, from the viewpoint of suppressing evaporation of the aqueous medium. Furthermore, from the viewpoint of reducing irregularities and defects that occur in the shell of the hollow particles, the polymerization temperature is preferably 80 to 95°C, more preferably 80 to 90°C. The polymerization reaction time is not particularly limited, but is preferably 5 hours or higher, more preferably 10 hours or higher, from the viewpoint of allowing the polymerization reaction to proceed sufficiently. It is preferably 60 hours or lower, more preferably 48 hours or lower, and even more preferably 36 hours or lower, from the viewpoint of production efficiency.

[0072] The suspension during the polymerization reaction is usually stirred. The stirring power is not particularly limited, but is preferably 0.01 kW / m from the viewpoint of promoting the polymerization reaction. ３ More preferably, 0.02 kW / m ３ More preferably, 0.03 kW / m ３ From the viewpoint of reducing the ratio of particles with low circularity, it is preferable to set the power consumption at 0.20 kW / m or more. ３ Less than or equal to 0.10 kW / m ３ or less, more preferably 0.05 kW / m ３ The following is the result.

[0073] Furthermore, in the polymerization step, carrying out the polymerization reaction in one stage can improve the adhesion of the resulting hollow particles compared to carrying out the polymerization reaction in multiple stages, presumably because carrying out the polymerization reaction in one stage can reduce the shell thickness compared to carrying out the polymerization reaction in multiple stages, thereby improving the elasticity of the hollow particles.

[0074] (4) Solvent Removal Step This step removes the hydrophobic solvent from the precursor particles. The solvent removal step may involve removing the hydrophobic solvent contained in the precursor particles from a slurry precursor composition, or may involve subjecting the slurry precursor composition to solid-liquid separation and then removing the hydrophobic solvent from the precursor particles in air. For example, a method for removing the hydrophobic solvent contained in the precursor particles from a slurry precursor composition may involve evaporating and distilling off the hydrophobic solvent contained in the precursor particles from the slurry precursor composition under a predetermined pressure. More specifically, a method may involve introducing an inert gas such as nitrogen, argon, or helium, air, or water vapor into the slurry precursor composition under a predetermined pressure selected from high pressure, normal pressure, and reduced pressure, thereby evaporating and distilling off the hydrophobic solvent contained in the hollow particles. This method is preferred because it is highly efficient at removing the hydrophobic solvent and easily removes impurities along with the hydrophobic solvent. Among these, a method in which an inert gas is introduced into a slurry-like precursor composition under a predetermined pressure to evaporate and distill off the hydrophobic solvent inside the hollow particles is preferred because it has excellent efficiency in removing the hydrophobic solvent.

[0075] A preferred method for removing the hydrophobic solvent from precursor particles in a slurry is to remove the hydrophobic solvent from the precursor particles by bubbling a gas through the precursor composition while stirring the slurry precursor composition in a stirring tank. This method results in hollow particles whose hollow spaces are filled with the bubbled gas. Bubbling the gas into the precursor composition can be performed, for example, by directly introducing the gas into the precursor composition, i.e., by blowing the gas directly into the precursor composition. Alternatively, the precursor composition can be stirred in a stirring tank having a liquid phase containing the precursor composition and a gas phase, thereby bubbling the gas from the gas phase into the precursor composition. In particular, in order to efficiently remove the hydrophobic solvent contained in the precursor particles and impurities such as unreacted polymerizable monomers remaining in the precursor particles, bubbling the gas into the precursor composition by stirring the precursor composition in a stirring tank having a gas phase while directly introducing the gas into the precursor composition is preferred. It is preferable that the gas phase in the stirring tank is previously filled with the gas to be bubbled into the precursor composition.

[0076] The gas bubbled into the precursor composition is preferably at least one selected from an inert gas and air, more preferably an inert gas, such as nitrogen, argon, or helium, and among these, nitrogen is preferred.

[0077] The treatment time for bubbling a gas through the precursor composition is preferably 2 hours or more, more preferably 5 hours or more, even more preferably 10 hours or more, and even more preferably 15 hours or more from the viewpoint of efficiently removing the hydrophobic solvent contained in the precursor particles, and is preferably 72 hours or less, more preferably 60 hours or less from the viewpoint of production efficiency. On the other hand, when the polymerization reaction time in the above-mentioned polymerization step is 50 hours or more, the hydrophobic solvent contained in the precursor particles can be sufficiently removed even if the bubbling treatment time in the solvent removal step is less than 2 hours. When the polymerization reaction time in the above-mentioned polymerization step is 50 hours or more, the bubbling treatment time in the solvent removal step is preferably 20 minutes or more, more preferably 30 minutes or more.

[0078] The stirring power when stirring the precursor composition is not particularly limited, but is preferably 0.01 kW / m from the viewpoint of efficiently removing the hydrophobic solvent contained in the precursor particles. ３ More preferably, 0.02 kW / m ３ More preferably, 0.03 kW / m ３ From the viewpoint of reducing the proportion of particles with low circularity and suppressing foaming of the precursor composition, the power is preferably 0.60 kW / m ３ Less than or equal to 0.55 kW / m ３ or less, more preferably 0.50 kW / m ３ or less, and even more preferably 0.20 kW / m ３ Below 0.10 kW / m, particularly preferably ３ less than 0.05 kW / m ３ It may be the following:

[0079] The gas bubbling into the precursor composition is preferably carried out under a pressure of 20 kPa or more and 300 kPa or less, and more preferably under a pressure of 40 kPa or more and 200 kPa or less, in order to efficiently remove the hydrophobic solvent contained in the precursor particles. A pressure equal to or greater than the above-mentioned lower limit is preferred in terms of improving the yield, while a pressure equal to or less than the above-mentioned upper limit is preferred in terms of reducing the proportion of particles with low circularity. In particular, in terms of excellent hydrophobic solvent removal efficiency, the gas bubbling into the precursor composition is preferably carried out under a pressure equal to or less than atmospheric pressure, and more preferably under a reduced pressure of 60 kPa or less. On the other hand, in terms of suppressing foaming of the precursor composition, the gas is preferably carried out under a pressure of 150 kPa or more. By suppressing foaming of the precursor composition during bubbling, the yield of hollow particles is improved. The pressure may be the internal pressure of the stirring tank.

[0080] The flow rate of the gas introduced into the stirring tank is not particularly limited, but from the viewpoint of efficiently removing the hydrophobic solvent contained in the precursor particles, the flow rate per unit time is preferably 4 L / min or more, more preferably 8 L / min or more, and even more preferably 10 L / min or more. The flow rate per unit volume is preferably 200 L / (min m３ ) or more, more preferably 8,000 L / (min m ３ ) or more, more preferably 10,000 L / (min m ３ ) or more. From the viewpoint of cost reduction, the upper limit of the gas flow rate may be, for example, 10,000 L / min or less as the flow rate per unit time, and 500,000 L / (min m) as the flow rate per unit volume. ３ When a stirring vessel having a capacity of 3 to 15,000 L is used, the flow rate of the gas is preferably within the above range.

[0081] The stirring tank preferably has a liquid phase containing the precursor composition and a gas phase. The presence of the gas phase in the stirring tank allows the hydrophobic solvent encapsulated in the precursor particles to migrate to the gas phase, thereby improving the efficiency of removing the hydrophobic solvent. The ratio of the liquid phase to the entire stirring tank is preferably 30% by volume or more and less than 80% by volume, from the viewpoint of efficiently removing the hydrophobic solvent encapsulated in the precursor particles.

[0082] As a method for directly introducing a gas into the precursor composition, for example, it is preferable to directly introduce the gas into the precursor composition in the stirred tank from the side or bottom of the stirred tank, and it is more preferable to introduce the gas from the bottom of the stirred tank. Here, the bottom of the stirred tank refers to the portion that can be observed from the bottom of the stirred tank when in use. The stirred tank is typically cylindrical, and in a cylindrical stirred tank, the bottom surface located below the direction of gravity is the bottom, or the deepest part. When introducing a gas directly into the precursor composition, it is preferable to introduce the gas closer to the deepest part of the stirred tank, because this facilitates uniform bubbling of the gas into the precursor composition. When the gas is uniformly bubbling into the precursor composition, the hydrophobic solvent removed from the precursor particles is more likely to migrate to the upper gas phase, thereby improving removal efficiency. Furthermore, in order to facilitate uniform bubbling of the gas into the precursor composition, it is preferable to introduce the gas into the precursor composition toward the center of the region occupied by the precursor composition in the stirred tank. That is, when introducing the gas from the bottom of the stirred tank, it is preferable to introduce the gas in the direction opposite to the direction of gravity. In the present disclosure, the direction of gravity does not necessarily have to be collinear with the direction of gravity, and for example, an axial misalignment of within 30 degrees is permitted. On the other hand, the yield of hollow particles may be improved by not directly introducing the gas into the precursor composition, but by introducing the gas into a gas phase part in a stirring tank and simply stirring the precursor composition in the stirring tank, thereby bubbling the gas in the gas phase part into the precursor composition.

[0083] This step can be performed using, for example, a stirring device such as that shown in FIG. 2 . Note that FIG. 2 is merely a schematic diagram for illustrative purposes, and the stirring device used in the solvent removal step of the present disclosure is not limited to the one shown in the figure. Furthermore, the structure, dimensions, shape, etc. of the stirring device used in the solvent removal step of the present disclosure are not limited to the structure, dimensions, shape, etc. shown in FIG. 2 . In the stirring device shown in FIG. 2 , when a precursor composition in the form of a slurry liquid is injected into a supply tank 12, the precursor composition is supplied from the supply tank 12 to a stirring tank 11 via a supply line 16. The method for supplying the precursor composition is not particularly limited, and examples include a method using a liquid feed pump. The precursor composition is supplied into the stirring tank 11 through an inner nozzle (not shown). The precursor composition 20 in the stirring tank 11 is stirred by rotating a rotor (not shown) provided in the stirring tank 11. A gas can be introduced into the stirring tank 11 through a gas inlet 13a, 13b, or 13c provided in the stirring tank 11. The gas inlet 13a is provided at the bottom of the stirring tank 11, and the gas inlet 13b is provided at the side of the stirring tank 11. When a gas is introduced through the gas inlet 13a or 13b, the gas can be directly blown into the precursor composition 20 in the stirring tank 11 to bubble it. The gas inlet 13c is provided at the top of the stirring tank 11. When a gas is introduced through the gas inlet 13c, the gas is blown into a gas phase 21 in the stirring tank 11. By stirring the precursor composition 20 in the stirring tank 11, the gas in the gas phase 21 is bubbled into the precursor composition 20. In the stirring apparatus shown in FIG. 2, the precursor composition 20 stirred in the stirring tank 11 is returned from the outlet 14 through the circulation line 17a, the supply tank 12, and the supply line 16 to the stirring tank 11, thereby circulating the precursor composition 20 within the stirring apparatus. The stirring apparatus shown in FIG. 2 further includes a spraying mechanism 15. 2 , a portion of precursor composition 20 stirred in stirring tank 11 can be supplied from outlet 14 to spray mechanism 15 via circulation line 17b and sprayed onto liquid surface 20a of precursor composition 20 in stirring tank 11. Note that the internal pressure of stirring tank 11 can be adjusted by adjusting the flow rate of precursor composition discharged from stirring tank 11.

[0084] While the gas is being bubbled through the precursor composition, it is preferable that the precursor composition is circulated through the stirring device by repeatedly being supplied from a supply tank to a stirring tank, stirred in the stirring tank, and then discharged back to the supply tank. This allows the hydrophobic solvent encapsulated in the precursor particles to be efficiently removed. The flow rate of this circulation is not particularly limited.

[0085] When bubbling a gas into the precursor composition, it is preferable that a portion of the precursor composition is supplied from the stirring tank to a spray mechanism and sprayed onto the liquid surface of the precursor composition in the stirring tank. This allows the hydrophobic solvent encapsulated in the precursor particles to be efficiently removed, thereby shortening the bubbling treatment time. The precursor composition can be sprayed continuously or intermittently while the gas is being bubbling into the precursor composition, and continuous spraying is preferred. When the precursor composition is sprayed intermittently, it is preferable that when the precursor composition foams and the liquid level rises, the precursor composition is sprayed appropriately until the foaming disappears. Note that the amount of spray when spraying the precursor composition is not particularly limited.

[0086] Furthermore, in order to reduce the amount of residual hydrophobic solvent, the temperature when bubbling the gas into the precursor composition is preferably a temperature equal to or higher than the boiling point of the hydrophobic solvent minus 35°C, more preferably a temperature equal to or higher than the boiling point of the hydrophobic solvent minus 30°C, and even more preferably a temperature equal to or higher than the boiling point of the hydrophobic solvent minus 20°C. Here, when the hydrophobic solvent is a mixed solvent containing multiple types of hydrophobic solvents and has multiple boiling points, the boiling point of the hydrophobic solvent in the solvent removal step refers to 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. Furthermore, the temperature when bubbling the gas into the precursor composition is usually a temperature equal to or higher than the polymerization temperature in the polymerization step. The temperature when bubbling the gas into the precursor composition is not particularly limited, but may be 50 to 100°C.

[0087] A method of removing the hydrophobic solvent from the precursor particles in air after solid-liquid separation of the slurry precursor composition is also preferred because of its excellent hydrophobic solvent removal efficiency. In this method, after solid-liquid separation of the slurry precursor composition, the precursor particles are dried in air, and the hydrophobic solvent encapsulated in the precursor particles is removed in air. This replaces the hydrophobic solvent inside the precursor particles with gas, resulting in hollow particles filled with gas. In this process, "in 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 as not to affect the removal of the hydrophobic solvent. "In air" can also refer to a state in which the precursor particles are not present in a slurry or a state in which the precursor particles are present in a dry powder. In other words, it is important to remove the hydrophobic solvent in an environment in which the precursor particles are in direct contact with the external gas.

[0088] 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. Pre-drying may also be performed. Pre-drying may be performed, for example, using a drying device such as a dryer or a drying appliance such as a hand dryer. 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.

[0089] 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.

[0090] (5) Washing Step The above production method preferably further includes a washing step for removing the dispersion stabilizer. The washing step involves, for example, adding an acid or alkali to the slurry precursor composition or the aqueous dispersion of hollow particles after the solvent removal step, thereby dissolving the dispersion stabilizer contained in the hollow particles or precursor particles in the aqueous medium. The hollow particles or precursor particles are then separated from the aqueous medium, and the separated hollow particles or precursor particles are then dispersed in ion-exchanged water to form a reslurry, followed by separation of the hollow particles or precursor particles. This series of operations is preferably repeated at least twice. Performing the washing step on hollow particles after removing the hydrophobic solvent from the precursor particles is preferable because the particles are less likely to aggregate during washing and can be washed more evenly, resulting in improved washability, compared to performing the washing step on precursor particles before removing the hydrophobic solvent. In order to remove as much dispersion stabilizer remaining in the hollow particles or precursor particles as possible, the washing step preferably involves adding an acid or alkali, dispersing the separated hollow particles or precursor particles in ion-exchanged water to form a reslurry, and then separating the hollow particles or precursor particles. This series of operations is preferably repeated three or more times. The number of times this operation is performed is more preferably four or more, even more preferably five or more. There is no particular upper limit, but from the viewpoint of ease of production, it may be, for example, 10 or less, or 8 or less. When the dispersion stabilizer used is an acid-soluble inorganic dispersion stabilizer, it is preferable to add an acid. When the dispersion stabilizer used is an acid-soluble inorganic dispersion stabilizer, it is preferable to add an alkali. When the dispersion stabilizer used is an acid-soluble inorganic dispersion stabilizer, an acid is added to the slurry precursor composition or the aqueous dispersion of hollow particles after the solvent removal step to adjust the pH to preferably 6.5 or less, more preferably 6 or less. The acid to be added may be an inorganic acid such as sulfuric acid, hydrochloric acid, or nitric acid, or an organic acid such as formic acid or acetic acid. However, sulfuric acid is particularly preferred because it has a high efficiency in removing the dispersion stabilizer and places a small burden on the production equipment.

[0091] The method for separating the hollow particles or precursor particles from the aqueous medium, i.e., the solid-liquid separation method, is not particularly limited, and examples thereof include centrifugation, filtration, and static separation. Among these, filtration is preferred because it is simple to operate and has a high dispersion stabilizer removal efficiency. Furthermore, when performing solid-liquid separation, it is preferable to dehydrate the hollow particles or precursor particles obtained by filtration or the like using a known method. Any filtration method can be used, such as natural filtration (atmospheric pressure filtration), reduced pressure filtration, pressure filtration, or centrifugal filtration. Among these, pressure filtration is preferred because it efficiently removes the dispersion stabilizer remaining in the hollow particles or precursor particles. In the production method of the present disclosure, the operation of separating the hollow particles or precursor particles from the aqueous medium is performed multiple times in the washing step. It is preferable to perform pressure filtration at least once, and it is more preferable to always perform pressure filtration in the operation of separating the hollow particles or precursor particles from the aqueous medium.

[0092] (6) Drying Step The above production method may further include a drying step for removing moisture remaining in the hollow particles. The method for drying and removing moisture remaining in the hollow particles is not particularly limited, and the same method as the method for drying the precursor particles in air in the solvent step described above can be used. In the drying step, a dehydrated cake of hollow particles is typically dried. The moisture content of the hollow particles before drying used in the drying step is typically 40 to 80%. In the present disclosure, the moisture content can be calculated using the following formula (IV): Formula (IV) Moisture Content (%) = {(w1 - w2) / w1} × 100 In formula (IV), w1 represents the mass of the measurement sample, and w2 represents the mass of the measurement sample after drying at 200°C for 12 hours and then cooling to 25°C.

[0093] (7) Others: As a process other than the above (1) to (6), for example, a process of substituting the interior of a particle may be added. The process of substituting the interior of a particle is a process of substituting the gas or liquid inside the hollow particle with another gas or liquid. Such a substitution can change the environment inside the hollow particle, selectively confine molecules inside the hollow particle, or modify the chemical structure inside the hollow particle according to the application.

[0094] 3. Coating Composition The coating composition of the present disclosure contains at least the hollow particles of the present disclosure and typically further contains a binder resin, a curing agent, and a solvent. Coating compositions of the present disclosure that contain at least the hollow particles of the present disclosure and a solvent are sometimes referred to as "slurry compositions" in this disclosure. The coating composition of the present disclosure is used to form a void-containing coating film. A void-containing coating film formed using the coating composition of the present disclosure is able to easily absorb distortion at the interfaces of the hollow particles. Therefore, even when the void-containing coating film follows the expansion and contraction of the substrate, adhesion at the interfaces of the hollow particles is maintained, thereby suppressing peeling of the void-containing coating film from the substrate. Furthermore, a void-containing coating film formed using the coating composition of the present disclosure has excellent adhesion to the substrate, making it easy to conform to the surface of a substrate with a complex shape. Furthermore, since distortion occurring at the interfaces of the hollow particles is easily absorbed, it also has excellent impact resistance.

[0095] (Hollow Particles) The hollow particles contained in the coating composition of the present disclosure are the hollow particles of the present disclosure described above. In the coating composition of the present disclosure, the content of the hollow particles is not particularly limited, but from the viewpoint of forming sufficient voids in the coating film to improve the impact resistance of the coating film and from the viewpoint of easily exhibiting the adhesive effect of the hollow particles of the present disclosure, the lower limit is preferably 5 mass% or more, more preferably 7 mass% or more, and even more preferably 8 mass% or more, relative to 100 mass% of the total solids content of the coating composition. On the other hand, from the viewpoint of suppressing a decrease in the strength of the void-containing coating film formed from the coating composition, the upper limit is preferably 20 mass% or less, more preferably 13 mass% or less, and even more preferably 12 mass% or less.

[0096] (Binder Resin) The coating composition of the present disclosure preferably contains a binder resin. This improves the strength of the void-containing coating film formed using the coating composition of the present disclosure, and also effectively demonstrates the excellent adhesion of the hollow particles of the present disclosure. The binder resin is not particularly limited, but for example, a thermoplastic resin or a thermosetting resin can be used. Among these, a thermosetting resin is preferably used because it is likely to improve the adhesion of the hollow particles of the present disclosure and the adhesion of the void-containing coating film formed using the coating composition of the present disclosure. The type of thermosetting resin is not particularly limited, and known resins such as polyester resin, polyurethane resin, acrylic resin, and epoxy resin can be used. The binder resin can be used alone or in combination of two or more types.

[0097] The content of the binder resin is not particularly limited, but the lower limit is preferably 40% by mass or more, more preferably 50% by mass or more, and the upper limit is preferably 90% by mass or less, more preferably 80% by mass or less, based on 100% by mass of the total solids content of the coating composition. When the content of the binder resin is equal to or greater than the lower limit, the mechanical strength of the void-containing coating film can be improved. When the content of the binder resin is equal to or less than the upper limit, the coating composition can contain a sufficient amount of hollow particles, thereby improving the impact resistance of the void-containing coating film that is formed.

[0098] (Curing Agent) The curing agent is appropriately selected depending on the type of binder resin and is not particularly limited, and known curing agents such as melamine resins and isocyanates can be used. Examples of melamine resins include methyl group-type melamine, imino group-type melamine, and butyl group-type melamine. Melamine resins may be used in combination with an acidic catalyst as needed. Examples of isocyanates include blocked isocyanates. The curing agent and catalyst can each be used alone or in combination of two or more. The content of the curing agent is not particularly limited and may be, for example, 1 to 120 parts by mass per 100 parts by mass of the binder resin.

[0099] (Solvent) The coating composition of the present disclosure may contain a solvent as needed. The solvent may be any solvent capable of appropriately dissolving or dispersing the hollow particles of the present disclosure and the binder resin and other components further contained as needed, and may be appropriately selected from known solvents such as pure water or various organic solvents. The content of the solvent is appropriately adjusted depending on the application of the coating composition of the present disclosure and is not particularly limited.

[0100] (Additives) The coating composition of the present disclosure may further contain additives such as colorants, fillers, curing accelerators, coupling agents, stress reducing agents, antifoaming agents, leveling agents, UV absorbers, foaming agents, antioxidants, heat stabilizers, flame retardants, etc. Examples of fillers include inorganic or organic fine particles and fibers.

[0101] 4. Coated Articles The coated article of the present disclosure has at least a portion of a void-containing coating film formed using the coating composition of the present disclosure described above. The void-containing coating film of the coated article of the present disclosure is formed by the hollow particles of the present disclosure described above, and therefore easily absorbs strain that occurs at the interfaces of the hollow particles, making it easy to maintain adhesion at the interfaces of the hollow particles. Therefore, the void-containing coating film of the coated article of the present disclosure is resistant to peeling and has excellent adhesion. Furthermore, the coated article of the present disclosure has excellent adhesion of the void-containing coating film even when the void-containing coating film is disposed on the surface of a substrate with a complex shape. Furthermore, the void-containing coating film of the coated article of the present disclosure easily absorbs strain that occurs at the interfaces of the hollow particles, and therefore has excellent impact resistance. As described above, the void-containing coating film formed using the hollow particles of the present disclosure is particularly effectively prevented from peeling when following the expansion and contraction of the substrate. Therefore, as the coated article of the present disclosure, one having a void-containing coating film formed using the coating composition of the present disclosure on at least a portion of the surface of a substrate that is prone to expansion and contraction is preferred in that it effectively exhibits the effects of the hollow particles of the present disclosure. Specifically, a suitable example is a coated article having a void-containing coating film formed using the coating composition of the present disclosure on at least a portion of the surface of a metal article.

[0102] The average thickness of the void-containing coating film of the coated article of the present disclosure is not particularly limited, but is preferably 50 μm or more, more preferably 70 μm or more, from the viewpoint of exhibiting excellent impact resistance. On the other hand, from the viewpoint of suppressing a decrease in the adhesion of the void-containing coating film, the average thickness of the void-containing coating film is preferably 1000 μm or less, more preferably 700 μm or less. The average thickness of the void-containing coating film is determined by observing the vertical cross section of the void-containing coating film with a microscope in any 10 fields of view, determining the maximum thickness of the void-containing coating film in each field of view, and taking the average (arithmetic mean) of the maximum thicknesses at the 10 points as the average thickness.

[0103] The void-containing coating film possessed by the coated article of the present disclosure is formed, for example, by applying the coating composition of the present disclosure described above, drying or curing it by heating, and then cooling. The method for applying the coating composition of the present disclosure is not particularly limited, and various known methods such as roll coating, curtain coating, and spraying can be used. The heating and cooling of the coating composition of the present disclosure is not particularly limited, but it is preferable to heat the coating composition of the present disclosure at 100 to 250°C, and more preferably at 150 to 250°C, in order to more effectively exert the effects of the hollow particles of the present disclosure described above. The cooling method after heating is not particularly limited, and may be natural air cooling.

[0104] An embodiment of the coated article of the present disclosure can include, for example, a coated metal sheet having a metal sheet and a void-containing coating film formed on at least a portion of the surface of the metal sheet using the coating composition of the present disclosure. Examples of the metal sheet include various steel sheets, including alloys, and non-ferrous metal sheets, to which various known plating treatments have been applied as needed. Examples of such metal sheets include cold-rolled steel sheets, zinc-plated steel sheets, zinc-aluminum alloy-plated steel sheets, zinc-aluminum-magnesium alloy-plated steel sheets, aluminum-plated steel sheets, zinc-nickel alloy-plated steel sheets, zinc-iron alloy-plated steel sheets, zinc-vanadium-plated steel sheets, zinc-zirconium-plated steel sheets, aluminum sheets, aluminum alloy sheets, copper sheets, copper alloy sheets, magnesium sheets, magnesium alloy sheets, and stainless steel sheets. The thickness of the metal sheet can be appropriately set so as to satisfy the mechanical strength (e.g., tensile strength) required for the intended use of the coated metal sheet.

[0105] 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.

[0106] Example 1 1. Preparation of Hollow Particles (1) Mixture Preparation Step First, the following materials were mixed to form an oil phase. The HSP distance between divinylbenzene and heptane was in the range of 5.30 to 7.50. Divinylbenzene (DVB) 31.68 parts Ethylvinylbenzene (EVB) 1.32 parts t-Butylperoxydiethylacetate 0.75 parts t-Butylperoxypivalate (trade name: Perbutyl (registered trademark) PV, manufactured by NOF Corporation) 1.07 parts Hydrophobic solvent (heptane) 67.0 parts Next, an aqueous solution prepared by dissolving 0.98 parts of magnesium chloride (a water-soluble polyvalent metal salt) in 225 parts of ion-exchanged water was added to a stirring vessel. While stirring the aqueous solution at a tip speed of 1.53 m / s, an aqueous solution prepared by dissolving 0.69 parts of sodium hydroxide (alkali metal hydroxide) in 55 parts of ion-exchanged water was gradually added to the aqueous solution to prepare magnesium hydroxide colloid (0.5 parts of magnesium hydroxide, average particle size of colloid particles: 300 nm) as an aqueous phase. The obtained aqueous phase and oil phase were mixed to prepare a mixed liquid.

[0107] (2) Suspension step The mixture obtained in the mixture preparation step was stirred for 1 minute using an emulsifying disperser (manufactured by Primix Corporation, product name: Homomixer) at a stirring impeller tip speed of 80 m / s, thereby carrying out a suspension treatment, thereby preparing a suspension in which droplets of the monomer composition encapsulating the hydrophobic solvent were dispersed in water.

[0108] (3) Polymerization step: The suspension obtained in the suspension step was heated to 80°C in a nitrogen atmosphere, and the stirring power of the stirring blade was set to 0.04 kW / m ３ The polymerization reaction was carried out by stirring for 24 hours under the condition that the temperature was 100° C. or more, and ...

[0109] (4) Solvent Removal Step The solvent removal step was carried out using a stirring device as shown in FIG. 2 according to the following procedure. The stirring tank used had a capacity of 3.6 L, and the inside of the stirring tank was filled with nitrogen when the precursor composition was supplied. 2.6 L of the precursor composition obtained in the polymerization step was supplied from the supply tank to the stirring tank. The stirring power of the stirring blades installed in the stirring tank was 0.04 kW / m ３ While stirring the precursor composition in the stirring tank, the temperature of the precursor composition was raised to 95°C, the pressure inside the stirring tank was reduced to 40 kPa, and gas was introduced from a gas inlet (gas inlet 13a in Figure 2) at the bottom of the stirring tank in the direction opposite to the direction of gravity at a rate of 10 L / min (10,000 L / min m because the gas phase portion was 1 L). ３ A bubbling treatment was carried out for 15 hours by stirring the precursor composition in the stirring tank while blowing nitrogen directly into the precursor composition at a flow rate of 1000 kJ / min. During this treatment, the precursor composition placed in the stirring tank was stirred in the stirring tank, then discharged into the supply tank, and then repeatedly supplied from the supply tank to the stirring tank, circulating within the stirring device. During this treatment, a portion of the precursor composition placed in the stirring tank was supplied to a spraying mechanism and continuously sprayed onto the liquid surface of the precursor composition in the stirring tank. Through the above treatment, an aqueous dispersion of hollow particles whose hollow spaces were filled with nitrogen was obtained.

[0110] (5) Washing Step Dilute sulfuric acid was added to the aqueous dispersion of hollow particles obtained in the solvent removal step in an environment of 25°C, and the mixture was stirred for 10 minutes to obtain a pH-adjusted slurry having a pH of 5.5 or less. While maintaining the slurry at 25°C, the obtained pH-adjusted slurry was pressure filtered and dehydrated to obtain a dehydrated cake of hollow particles. The obtained dehydrated cake was washed with 400 parts of ion-exchanged water, pressure filtered, and dehydrated, and this series of steps was repeated six times while maintaining the temperature at 25°C.

[0111] (6) Drying Step The dehydrated cake (water content 67%, thickness 2 cm) after the washing step was pre-dried by drying in a dryer at a temperature of 40°C, and then heat-treated in a vacuum dryer at 200°C and 4 kPa for 12 hours to remove moisture remaining in the hollow particles, thereby obtaining hollow particles of Example 1.

[0112] 2. Preparation of Coating Composition (Slurry Composition) Acrylic resin (trade name: U-DOUBLE (registered trademark) S-2818, manufactured by Nippon Shokubai Co., Ltd.) was dissolved in an organic solvent (a mixture of cyclohexanone and Solvesso 150 (trade name, manufactured by ExxonMobil) at a mass ratio of 1:1 was used) to prepare an acrylic resin solution. Meanwhile, as a curing agent, a methyl group-type melamine resin (trade name: Cymel (registered trademark) 303, manufactured by Allnex Japan Co., Ltd.) and a butyl group-type melamine resin (trade name: Mycoat (registered trademark) 506, manufactured by Allnex Japan Co., Ltd.) were mixed in a mass ratio of 1:1 based on the solid content of the resin to prepare a mixed melamine resin. Thereafter, the acrylic resin solution and the mixed melamine resin were mixed so that the mass ratio of the solid content was 100:30 to prepare a mixed solution of acrylic resin and melamine resin. An acidic catalyst (product name: Catalyst 600, manufactured by Allnex Japan Co., Ltd.) was further added to this mixed solution of acrylic resin and melamine resin in an amount of 0.5% by mass. The resulting mixed solution was stirred to obtain a varnish paint. Hollow particles were added to the varnish paint in an amount of 30% by volume and stirred to obtain the coating paint of Example 1. The content of hollow particles was 19% by mass relative to 100% by mass of the total solids content of the coating paint.

[0113] 3. Preparation of painted metal sheet: Electrogalvanized steel sheet (manufactured by Nippon Steel Corporation, coating weight per side: 20 g / m ２ ) was prepared, and the coating composition (slurry composition) obtained above was applied to the surface of the substrate in an amount of 100 mg / cm. ２ The coating was then dried under reduced pressure at 25°C for 1 hour, and then further dried in a nitrogen stream at 250°C for 30 minutes to remove the solvent contained in the coating composition (slurry composition), thereby forming a void-containing coating film (average film thickness: 1000 µm) containing hollow particles on one side of the electrogalvanized steel sheet, and a coated metal sheet of Example 1 was obtained.

[0114] [Examples 2 to 5 and Comparative Examples 1 to 5] The hollow particles, coating compositions (slurry compositions), and coated metal plates of Examples 2 to 5 and Comparative Examples 1 to 5 were obtained in the same manner as in Example 1, except that at least one of the amount of divinylbenzene, the amount of ethylvinylbenzene, the amount of t-butylperoxydiethyl acetate, the amount of t-butylperoxypivalate, the amount of heptane, the amount of magnesium chloride, the amount of sodium hydroxide, and the tip speed of the stirring blade during preparation of the magnesium hydroxide colloid was changed according to Table 1.

[0115] SEM observation of the cross section of the hollow particles obtained in each Example revealed that the shells were dense. Furthermore, the hollow particles obtained in each Example were photographed with an SEM (magnification: 1,000x), and 100 hollow particles were randomly selected from the obtained SEM images. Observation of the surfaces of the selected hollow particles revealed that the percentage of particles having irregularities measuring 10% or more of the volume average particle size or defects with a major axis measuring 3% or more of the volume average particle size was less than 20%.

[0116] [Physical Properties] The hollow particles produced in each Example and Comparative Example were subjected to the following measurements. The measurement results are shown in Table 1.

[0117] 1. Porosity 1-1. Measurement of apparent density of hollow particles First, a volume of 100 cm ３ About 30 cm ３ The volumetric flask was filled with hollow particles, and the mass of the filled hollow particles was accurately weighed. Next, the volumetric flask filled with the hollow particles was accurately filled with isopropanol up to the marked line, taking care not to introduce air bubbles. The mass of isopropanol added to the volumetric flask was accurately weighed, and the apparent density D of the hollow particles was calculated based on the following formula (I): １ (g / cm ３ The apparent density D was calculated using the formula (I). １ = [Mass of hollow particles] / (100 - [Mass of isopropanol] / [Specific gravity of isopropanol at measurement temperature])

[0118] 1-2. Measurement of true density of hollow particles After crushing the hollow particles in advance, ３Approximately 10 g of crushed pieces of hollow particles was filled into a measuring flask, and the mass of the crushed pieces was accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol was added to the measuring flask, and the mass of the isopropanol was accurately weighed. The true density D of the hollow particles was calculated based on the following formula (II): ０ (g / cm ３ ) was calculated. ０ = [Mass of crushed hollow particle fragments] / (100 - [Mass of isopropanol] / [Specific gravity of isopropanol at measurement temperature])

[0119] 1-3. Calculation of porosity Apparent density D of hollow particles １ and true density D ０ The porosity of the hollow particles was calculated based on the following formula (III): Formula (III) Porosity (%) = 100 - (apparent density D １ / True density D ０ ) x 100

[0120] 2. Particle Size and Particle Size Distribution The particle size of hollow particles was measured using a particle size distribution analyzer (product name: Multisizer 4e, manufactured by Beckman Coulter, Inc.) using the Coulter counter method, and the number average and volume average were calculated to determine the number average particle size (Dp) and volume average particle size (Dv). The particle size distribution (Dv / Dp) was calculated by dividing the volume average particle size by the number average particle size. 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 (product name: Drywell, manufactured by Fujifilm Corporation) was added as a dispersant. 2 ml of dispersion medium was added to the beaker to wet the particles, and then 10 ml of dispersion medium was added. The mixture was dispersed in an ultrasonic disperser for 1 minute, and then measured using the particle size distribution analyzer.

[0121] 3. Shell Thickness / Dv Particles fixed on carbon tape were rubbed with a cotton swab to intentionally break the hollow particles. The cross sections of 100 broken hollow particles were observed with an SEM, and the shell thickness was measured at 10 points arbitrarily selected from the SEM image. The average value was taken as the shell thickness of the hollow particles, and the ratio (%) of the shell thickness of the hollow particles to the volume average particle diameter (Dv) of the hollow particles was calculated.

[0122] 4. Percentage of particles with only one hollow part Particles fixed on carbon tape were rubbed with a cotton swab to intentionally break the hollow particles. The interiors of 100 broken hollow particles were observed using an SEM to determine the number of hollow parts per hollow particle, and the percentage of particles with only one hollow part was calculated.

[0123] 5. Percentage of particles with pores One hundred hollow particles having a particle size of 50% to 150% of the volume average particle size were randomly selected and observed under an SEM to confirm the presence or absence of pores with a major axis of 1 μm or more in each hollow particle. The hollow particles with such pores were identified, and the percentage of particles having pores with a major axis of 1 μm or more in their shells was calculated.

[0124] 6. Mass Proportion of Particles with a Circularity of 0.93 or Less A measurement sample was prepared by dispersing 0.10 to 0.12 g of hollow particles in an aqueous solution of linear alkylbenzenesulfonate (0.3% concentration) for 5 minutes in an ultrasonic cleaner. Using a flow-type particle image analyzer (manufactured by Jasco International Inc., product name: IF-3200), the circularity of each hollow particle contained in the measurement sample was measured under the following measurement conditions. The mass proportion of hollow particles with a circularity of 0.93 or less was calculated and used as the proportion of particles with a low circularity. The number of hollow particles contained in the measurement sample increased with decreasing volume average particle size, but was within the range of 1,000 to 3,000 for all examples and comparative examples. (Measurement conditions) Flow cell spacer thickness: 50 μm Telecentric zoom lens magnification: 4.5x Total magnification: 9.0x Measurement volume: 0.5 mL Image resolution: 0.185 μm / pixel Detection algorithm: ghost detection Threshold: 15%

[0125] 7. Iodine Value Measurement The iodine value of hollow particles was measured in accordance with JIS K 0070. The specific measurement method is as follows: 0.7 to 2 g of hollow particles (sample) and 10 mL of chloroform were added to a 300 mL iodine flask, followed by the addition of 25 mL of Wiess's solution as a reaction solution, followed by gentle stirring and sealing. The flask was then left to stand in a dark place at 25°C for 30 minutes. Next, 20 mL of 100 g / L potassium iodide solution and 100 mL of purified water were added and stirred. Using a burette, titration was performed with a titrant (0.1 mol / L sodium thiosulfate solution). When the solution turned pale yellow, an indicator (1% starch solution) was added. The titration was continued until the blue color disappeared, marking the endpoint. Separately from this main test, a blank test was conducted using a solution without hollow particles, and the iodine value of the hollow particles was calculated using the following formula: The iodine value is the amount of halogen bonded to 100 g of a sample when the halogen is reacted with the sample, converted into grams of iodine. Iodine value (g / 100 g) = {(V ０ -V １ ) × f × 1.269} / S, S: sample mass (g), V １ V: Volume of titrant in this test (mL) ０ : Volume of titrant in blank test (mL) f: Factor of titrant

[0126] [Evaluation] The hollow particles produced in each Example and Comparative Example were evaluated as follows. The evaluation results are shown in Table 1.

[0127] 1. Breakage Resistance: Hollow particles were collected at a constant volume in each example and comparative example to prepare a measurement sample. One hundred hollow particles randomly selected from the measurement sample were observed using an optical microscope at 200x magnification to determine the number of broken particles among the 100 hollow particles, which was recorded as the number of broken particles before compression. Particles with cracks measuring 10 μm or more in major axis were recognized as broken particles. The measurement sample was placed in a circular mold with a diameter of 20 mm, and pressure was applied at a volume compression rate of 3% to compress the measurement sample for 10 minutes. The number of broken particles among the 100 hollow particles after compression at a volume compression rate of 3% was determined in the same manner as above, and recorded as the number of broken particles after compression at a volume compression rate of 3%. The increase in broken particles after compression at a volume compression rate of 3% was calculated using the number of broken particles before compression, the number of broken particles after compression at a volume compression rate of 3%, and the total number of particles (100) before compression, using the following formula (B): Formula (B) Increase in broken particles after compression at a volume compression rate of 3% (%) = {(number of broken particles after compression at a volume compression rate of 3% - number of broken particles before compression) / 100} x 100 In addition to the compression at a volume compression rate of 3%, a measurement sample was compressed for 10 minutes by applying pressure at a volume compression rate of 5%. For the measurement sample after compression, the number of broken particles out of 100 hollow particles was determined in the same manner as above, and this was taken as the number of broken particles after compression at a volume compression rate of 5%. The increase in broken particles after compression at a volume compression rate of 5% was calculated using the number of broken particles before compression, the number of broken particles after compression at a volume compression rate of 5%, and the total number of particles before compression (100) using the following formula (C): Formula (C) Percentage increase in broken particles after compression at a volume compression rate of 5% = {(number of broken particles after compression at a volume compression rate of 5% - number of broken particles before compression) / 100} x 100 Based on the percentage increase in broken particles after compression at a volume compression rate of 3% and the percentage increase in broken particles after compression at a volume compression rate of 5%, the flexibility and strength were evaluated according to the following evaluation criteria. Note that the higher the flexibility of the hollow particles, the less likely the number of broken particles to increase even if the volume compression rate is increased, and the higher the strength of the hollow particles, the less likely the number of broken particles to increase even if the volume compression rate is increased.(Evaluation criteria for breakage resistance) A: The increase in the number of broken particles after compression at a volume compression rate of 3% is less than 3%, and the increase in the number of broken particles after compression at a volume compression rate of 5% is less than 5%. B: The increase in the number of broken particles after compression at a volume compression rate of 3% is less than 3%, and the increase in the number of broken particles after compression at a volume compression rate of 5% is 5% or more but less than 10%. C: The increase in the number of broken particles after compression at a volume compression rate of 3% is less than 3%, and the increase in the number of broken particles after compression at a volume compression rate of 5% is 10% or more. D: The increase in the number of broken particles after compression at a volume compression rate of 3% is 3% or more.

[0128] 2. Heat Resistance Approximately 3 mg of hollow particles were placed in a sample pan, and measurements were performed in an air atmosphere using a TG-DTA device (manufactured by Rigaku Corporation, product name: Thermo Plus EVO2) at a heating rate of 5°C / min and an air flow rate of 230 mL to determine the temperature at which 5% weight loss (Td5). The higher the Td5, the better the heat resistance of the hollow particles. (Heat Resistance Evaluation Criteria) A: Td5 is 335°C or higher B: Td5 is 330°C or higher and lower than 335°C C: Td5 is 325°C or higher and lower than 330°C D: Td5 is lower than 325°C

[0129] 3. Elasticity An elasticity test was performed on each of the 20 undamaged hollow particles. The elasticity test was performed using a microcompression tester (product name: MCTM-500, manufactured by Shimadzu Corporation) under the following test conditions at 20°C. The hollow particles were compressed by 10% and held for 10 seconds. 30 minutes after the compression was released, the hollow particles were evaluated for their restoration. Specifically, the particle diameter of the hollow particles measured in the compression direction was measured before compression and 30 minutes after the compression was released (after compression). If the rate of change in particle diameter after compression was less than 5% compared to before compression, the hollow particles were determined to have restored their original size. The rate of change in particle diameter after compression was calculated using the following formula (D): Equation (D): Rate of change in particle diameter after compression (%) = {(particle diameter before compression - particle diameter after compression) / particle diameter before compression} × 100. Of the 20 hollow particles tested for elasticity, the elasticity was evaluated according to the following evaluation criteria based on the number of particles that restored their original size. <Elasticity test conditions> Indenter type: FLAT500 Objective lens magnification: 10 Load speed: 103.7 mN / sec (Elasticity evaluation criteria) A: 19-20 restored hollow particles B: 17-18 restored hollow particles C: 15-16 restored hollow particles D: 14 ​​or less restored hollow particles

[0130] 4. Adhesion A copper foil (thickness 18 μm) was placed on a coated metal sheet so that the void-containing coating film on the coated metal sheet was in contact with the drum surface (S surface, surface roughness Ra 0.30 μm) of the copper foil, and the copper foil was subjected to a pressure of 100 kg / cm ２The porous coating film was adhered to the copper foil by hot-press lamination under conditions of 170°C and 1 hour. A peel test was conducted to peel the laminated copper foil based on JIS C6481 to measure peel strength. In the peel test, a 25 mm x 200 mm test piece was cut out, and tape was attached to the copper foil at a temperature of 23°C, a peel rate of 50 mm / min, and a peel angle of 90°, and one edge was peeled off. Based on the peel strength value, the adhesion between the porous coating film and the copper foil was evaluated according to the following evaluation criteria. During the hot-press lamination, the porous coating film of the coated metal sheet is adhered to the copper foil, and at the same time, the electrogalvanized steel sheet and the copper foil expand and contract, and the porous coating film follows the expansion and contraction of the electrogalvanized steel sheet and the copper foil. During this process, the greater the strain generated at the interface of the hollow particles in the porous coating film, the more easily the porous coating film peels off from the copper foil. Therefore, the higher the peel strength of the copper foil, the easier it is to absorb the strain generated at the interface of the hollow particles in the void-containing coating film, and it can be determined that the adhesion at the interface of the hollow particles is high. (Adhesion evaluation criteria) A: Peel strength exceeds 0.7 kN / m B: Peel strength is greater than 0.6 kN / m and 0.7 kN / m or less C: Peel strength is greater than 0.4 kN / m and 0.6 kN / m or less D: Peel strength is greater than 0.1 kN / m and 0.4 kN / m or less

[0131] 5. Adhesion after heat history test A copper foil (thickness 18 μm) was placed on a coated metal plate so that the void-containing coating film on the coated metal plate was in contact with the drum surface (S surface, surface roughness Ra 0.30 μm) of the copper foil, and the copper foil was subjected to a pressure of 100 kg / cm ２The porous coating film was adhered to the copper foil by heating and pressurizing the laminate at 170°C for 1 hour, thereby obtaining a laminate. The resulting laminate was then subjected to a thermal history test. The thermal history test was performed by heating the laminate to 250°C, rapidly cooling it at -20°C for 10 minutes, and then returning the temperature of the laminate to room temperature, repeating this process twice. After the thermal history test, a peel test was performed in the same manner as in "4. Adhesion" above to measure the peel strength when peeling the copper foil. Based on the peel strength value, the adhesion between the porous coating film and the copper foil after the thermal history test was evaluated according to the following evaluation criteria. During the thermal history test, the electrogalvanized steel sheet and the copper foil expand and contract, and the porous coating film follows the expansion and contraction of the electrogalvanized steel sheet and the copper foil. The greater the strain generated at the interface between the hollow particles in the porous coating film, the more easily the porous coating film peels from the copper foil. Therefore, the higher the peel strength of the copper foil, the more easily the strain generated at the interface of the hollow particles in the void-containing coating film is absorbed, and it can be determined that the adhesion at the interface of the hollow particles is high. (Adhesion evaluation criteria after thermal history test) A: The peel strength is 0.5 kN / m or more, and less than 10% of the area of ​​the void-containing coating film peeled off from the electrogalvanized steel sheet side or the copper foil side before the peel test. B: The peel strength is 0.3 kN / m or more but less than 0.5 kN / m, and less than 10% of the area of ​​the void-containing coating film peeled off from the electrogalvanized steel sheet side or the copper foil side before the peel test. C: The peel strength is 0.1 kN / m or more but less than 0.3 kN / m, and less than 10% of the area of ​​the void-containing coating film peeled off from the electrogalvanized steel sheet side or the copper foil side before the peel test. D: Peel strength is less than 0.1 kN / m, or before the peel test, the porous coating film peels off from the electrogalvanized steel sheet side or copper foil side over a proportion of 10% or more of the area.

[0132]

[0133] [Discussion] The hollow particles obtained in Comparative Example 1 had insufficient flexibility due to a low porosity, leading to fragility and poor elasticity. Therefore, the void-containing coating film containing the hollow particles was prone to peeling from the copper foil, and the interfacial adhesion of the hollow particles was insufficient. The hollow particles obtained in Comparative Example 2 had a thin shell due to a high porosity, leading to insufficient shell strength and fragility, resulting in a particularly high proportion of broken particles. Furthermore, the hollow particles obtained in Comparative Example 2 had poor elasticity due to a too thin shell. Therefore, the void-containing coating film containing the hollow particles was prone to peeling from the copper foil and the interfacial adhesion of the hollow particles was insufficient. Furthermore, the hollow particles obtained in Comparative Example 2 were prone to breakage, leading to oxidation degradation from the inside of the particles due to frequent breakage, leading to poor heat resistance. The hollow particles obtained in Comparative Example 3 had poor elasticity due to a too small volume average particle size. Therefore, when a thermal history test was performed, the void-containing coating film containing the hollow particles was easily peeled off from the copper foil, and the adhesion at the interface of the hollow particles was insufficient. The hollow particles obtained in Comparative Example 4 had an excessively large volume average particle size, which made them prone to plastic deformation and poor in elasticity. Therefore, the void-containing coating film containing the hollow particles was easily peeled off from the copper foil, and the adhesion at the interface of the hollow particles was insufficient. Furthermore, the hollow particles obtained in Comparative Example 4 had many pores, which caused oxidation degradation to progress from the inside of the particles as well, resulting in poor heat resistance. The hollow particles obtained in Comparative Example 5 had a low proportion of particles having only one hollow portion, so the void-containing coating film containing the hollow particles was easily peeled off from the copper foil, and the adhesion at the interface of the hollow particles was insufficient. The insufficient interfacial adhesion of the hollow particles obtained in Comparative Example 5 is presumably due to the fact that the particles having multiple hollow portions had poor shell surface smoothness, resulting in a small contact area with the binder resin or substrate in the coating film, and insufficient flexibility, making them prone to breakage. Furthermore, the hollow particles obtained in Comparative Example 5 had poor shell surface smoothness, which made them prone to oxidative degradation, and the large number of pores meant that oxidative degradation also progressed from the inside of the particles, resulting in poor heat resistance.

[0134] In contrast, the hollow particles obtained in Examples 1 to 5 were hollow particles having a resin-containing shell and a hollow portion surrounded by the shell. They had a porosity of 50% to 90%, a volume-average particle size of 100 μm to 300 μm, and a proportion of particles having only one hollow portion of 90% or more. Therefore, they had a good balance of flexibility and strength, excellent breakage resistance, and excellent elasticity. Therefore, the void-containing coating film containing the hollow particles obtained in each Example was resistant to peeling from the copper foil, and the interfacial adhesion of the hollow particles was excellent. In particular, the hollow particles obtained in Examples 1 and 2 had a porosity of 60% to 80%, a volume-average particle size of 100 to 200 μm, and a proportion of particles having only one hollow portion of 92% or more. Therefore, the void-containing coating film containing the hollow particles was particularly resistant to peeling, and the interfacial adhesion of the hollow particles was particularly excellent. The particularly excellent adhesion of the hollow particles obtained in Examples 1 and 2 is presumably due to the fact that the good balance between breakage resistance and elasticity resulted in particularly excellent performance in absorbing strain generated at the interface of the hollow particles in the coating film. Furthermore, the hollow particles obtained in Examples 1, 2, 3, and 4 were particularly excellent in heat resistance. This is presumably due to the fact that the few pores or breakages in the shells suppressed oxidation degradation from the inside of the particles, and furthermore, the high proportion of particles having only one hollow portion resulted in excellent shell surface smoothness, making oxidation degradation of the outer surface of the shell less likely to progress.

[0135] REFERENCE SIGNS LIST 1 aqueous medium 2 low-polarity material 3 droplets of monomer composition 4a hydrophobic solvent 4b material other than hydrophobic solvent 5 precursor particle 6 shell 7 hollow portion 10 hollow particle 11 stirring tank 12 supply tank 13a, 13b, 13c gas inlet 14 outlet 15 spray mechanism 16 supply line 17a, 17b circulation line 20 precursor composition 20a liquid surface of precursor composition 21 gas phase portion

Claims

1. Hollow particles having a shell containing resin and a hollow portion surrounded by the shell, the porosity being 50% or more and 90% or less, the volume average particle size being 100 μm or more and 300 μm or less, and the proportion of particles having only one hollow portion being 90% or more.

2. The hollow particles according to claim 1, in which the percentage of particles having pores with a major axis of 1 μm or more is 5% or less.

3. The hollow particles according to claim 1 or 2, wherein the particle size distribution (volume average particle size (Dv) / number average particle size (Dp)) is 1.0 or more and 1.5 or less.

4. The hollow particles according to claim 1 or 2, wherein the ratio of particles having a circularity of 0.93 or less is 10 mass % or less.

5. The hollow particles according to claim 1 or 2, which have an iodine value measured in accordance with JIS K 0070 of 5 g / 100 g or more and 50 g / 100 g or less.

6. A method for producing the hollow particles according to claim 1 or 2, comprising the steps of: preparing a mixed solution containing a polymerizable monomer, a hydrophobic solvent, a dispersion stabilizer, and an aqueous medium; suspending the mixed solution to prepare a suspension in which droplets of a monomer composition containing the polymerizable monomer and the hydrophobic solvent are dispersed in the aqueous medium; subjecting the suspension to a polymerization reaction to prepare a precursor composition in which precursor particles having a hollow portion surrounded by a shell containing a resin and the hollow portion filled with the hydrophobic solvent are dispersed in the aqueous medium; and removing the hydrophobic solvent from the precursor particles, wherein the dispersion stabilizer is colloidal particles of a poorly water-soluble inorganic compound, and the colloidal particles are colloidal particles having an average particle size of 100 nm to 500 nm, which have been prepared by reacting raw material components in the aqueous medium while stirring them at a tip speed of a stirring blade of 0.2 m / s to 5.0 m / s, a content of the dispersion stabilizer is 0.1 parts by mass or more and 5.0 parts by mass or less with respect to 100 parts by mass of the polymerizable monomer, the step of preparing the suspension is performed by stirring the mixed liquid with a disperser, and a tip speed of a stirring blade of the disperser is 60 m / s or more and 120 m / s or less.

7. A coating composition comprising the hollow particles according to claim 1 or 2.

8. A slurry composition comprising the hollow particles according to claim 1 or 2 and a solvent.

9. A coated article having at least a portion thereof a void-containing coating film formed using the coating composition according to claim 7.

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