Hollow particles

Abstract

Provided is a method for producing hollow particles having a high porosity and being difficult to be crushed. The hollow particles comprising a shell containing a resin and a hollow section surrounded by the shell, and having a porosity of 50% or more, wherein the shell contains, as the resin, a polymer containing 70 to 100 parts by mass of a crosslinkable monomer unit in 100 parts by mass of all the monomer units, and in an immersion test of hollow particles, in which 0.1 mg of hollow particles is added to 4 mL of acetone in an environment of 25°C, shaken for 10 minutes under a condition of a shaking speed of 100 rpm, and allowed to stand for 48 hours, the amount of hollow particles that settle in acetone is less than 5% by mass.

Description

Technical Field

[0001] The present disclosure relates to hollow particles.

Background Art

[0002] Hollow particles (hollow resin particles) are particles having a cavity inside the particles. Compared with solid particles whose inside is substantially filled with resin, they can scatter light well and reduce the light transmittance. Therefore, they are widely used as organic pigments and hiding agents with excellent optical properties such as opacity and whiteness in applications such as water-based paints and paper coating compositions. Furthermore, they are also used as weight reducing agents, heat insulating agents, etc. for resins, paints, etc. used in various fields such as automobiles, electrics, electronics, and construction.

[0003] In order to improve the effects of weight reduction, heat insulation, opacity, whitening, etc. of various compositions and molded articles added with hollow particles, it is desirable that the hollow particles can maintain a high porosity during kneading with other materials and during molding after kneading. However, when the porosity of the hollow particles is increased, the shell thickness of the hollow particles becomes thin, so they are easily crushed. Therefore, there is a need for hollow particles having a high porosity and being difficult to crush.

[0004] Patent Document 1 discloses a method for producing hollow resin particles, which comprises dispersing a mixed solution containing 20 to 70 parts by weight of a polyfunctional monomer having two or more ethylenically unsaturated groups and 80 to 30 parts by weight of a monofunctional monomer, a non-reactive organic solvent, and a non-crosslinkable polymer having a weight average molecular weight of 10,000 to 1,000,000 in terms of polystyrene in an aqueous solution containing a dispersion stabilizer or a surfactant, and then polymerizing. Patent Document 1 describes that the production method provides small-sized hollow resin particles with few pinholes and little crushing.

[0005] Patent Document 2 discloses hollow resin particles having a single hollow surrounded by a shell, the hollow resin particles having a thermal decomposition onset temperature of 350°C or higher, the shell having fine throughholes with diameters in the range of 10 to 50 nm, and a thickness ratio of 0.03 to 0.25 relative to the average primary particle diameter of the hollow resin particles. Patent Document 2 also describes that the hollow resin particles are produced by dispersing a mixed solution containing a polyfunctional monomer and a non-reactive solvent in an aqueous solution, and then polymerizing the polyfunctional monomer.

[0006] Patent Document 3 discloses a method for producing hollow polymeric microparticles consisting of a single-layer shell and a hollow portion, which comprises dispersing a mixture consisting of (i) at least one crosslinkable monomer (B) or a mixture of at least one crosslinkable monomer (B) and at least one monofunctional monomer (B'), (ii) an initiator (C), and (iii) a poorly water-soluble solvent (D) that has low compatibility with a polymer or copolymer obtained from at least one crosslinkable monomer (B) or a copolymer of at least one crosslinkable monomer (B) and at least one monofunctional monomer (B') in an aqueous solution of a dispersion stabilizer (A), and then carrying out suspension polymerization. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-68037 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-190980 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-80503 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the hollow resin particles described in Patent Document 1 are liable to be crushed because they cannot sufficiently withstand the shear and pressure in biaxial kneading or injection molding when preparing paints or molding materials by mixing the hollow resin particles with a resin, or when manufacturing a molded body using a molding material containing the hollow resin particles.

[0009] The hollow resin particles described in Patent Document 2 also have the problem of being liable to be crushed because, similar to Patent Document 1, they cannot sufficiently withstand the shear and pressure in biaxial kneading or injection molding. Further, since the hollow resin particles described in Patent Document 2 have fine through-holes, there is a problem that resin intrudes into the inside of the particles during the injection molding of a molding resin composition containing the hollow resin particles. Although the fine through-holes of the hollow resin particles may impart a beneficial function to the hollow resin particles, on the other hand, since they are parts where the shell is missing, they also cause a decrease in the strength of the hollow resin particles and make them liable to be crushed.

[0010] The hollow resin particles described in Patent Document 3 are less liable to be crushed than the hollow resin particles described in Patent Documents 1 and 2, but have the problem of being deformed by the shear and pressure in biaxial kneading or injection molding and having a decrease in porosity.

[0011] An object of the present disclosure is to provide hollow particles having a high porosity and being difficult to be crushed.

Means for Solving the Problems

[0012] The present inventor focused on the permeability of the shell of the hollow particles to a polar solvent, and found that hollow particles having a shell that is difficult for acetone to penetrate by adjusting the composition and formation method of the shell are difficult to be crushed even if they have a high porosity.

[0013] The present disclosure is hollow particles comprising a shell containing a resin and a hollow portion surrounded by the shell, having a porosity of 50% or more, wherein the shell contains, as the resin, a polymer containing 70 to 100 parts by mass of a crosslinkable monomer unit per 100 parts by mass of all monomer units, In an immersion test of hollow particles, 0.1 mg of hollow particles are added to 4 mL of acetone, shaken for 10 minutes under the condition of a shaking speed of 100 rpm, and then allowed to stand for 48 hours. Hollow particles are provided in which the amount of hollow particles precipitating in acetone is less than 5% by mass.

[0014] In the hollow particles of the present disclosure, the polymer contained in the shell contains a hydrophilic non-crosslinking monomer unit derived from a hydrophilic non-crosslinking monomer having a solubility in distilled water at 20 °C of 0.3 g / L or more. In 100 parts by mass of all monomer units contained in the polymer, the content of the hydrophilic non-crosslinking monomer unit is preferably 2 to 15 parts by mass, and the content of the crosslinkable monomer unit is preferably 70 to 98 parts by mass.

[0015] In the hollow particles of the present disclosure, the polymer contained in the shell preferably contains, as the crosslinkable monomer unit, a crosslinkable monomer unit derived from a bifunctional crosslinkable monomer and a crosslinkable monomer unit derived from a trifunctional or higher-functional crosslinkable monomer.

[0016] In the hollow particles of the present disclosure, the polymer contained in the shell contains, as the crosslinkable monomer unit, a crosslinkable monomer unit derived from a trifunctional or higher-functional crosslinkable monomer. In 100 parts by mass of all monomer units contained in the polymer, the content of the crosslinkable monomer unit derived from the trifunctional or higher-functional crosslinkable monomer is preferably 5 to 50 parts by mass.

[0017] In the hollow particles of the present disclosure, the polymer contained in the shell preferably contains, as the crosslinkable monomer unit, a crosslinkable monomer unit derived from at least one bifunctional crosslinkable monomer selected from the group consisting of divinylbenzene, ethylene glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate.

[0018] In the hollow particles of the present disclosure, it is preferable that the polymer contained in the shell contains, as the crosslinkable monomer unit, a crosslinkable monomer unit derived from at least one trifunctional or higher-functional crosslinkable monomer selected from the group consisting of pentaerythritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, and dipentaerythritol poly(meth)acrylate.

[0019] In the hollow particles of the present disclosure, it is preferable that the shell contains at least one selected from the group consisting of rosin acid, higher fatty acids, and metal salts thereof.

Advantages of the Invention

[0020] According to the present disclosure as described above, it is possible to provide hollow particles having a high porosity and being difficult to collapse.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0022] In the present disclosure, "~" in a numerical range means including the numerical values described before and after it as the lower limit value and the upper limit value. In the present disclosure, (meth)acrylate represents each of acrylate and methacrylate, (meth)acrylic represents each of acrylic and methacrylic, and (meth)acryloyl represents each of acryloyl and methacryloyl. In the present disclosure, a polymerizable monomer is a compound having a functional group capable of addition polymerization (which may be simply referred to as a polymerizable functional group in the present disclosure). In the present disclosure, as the polymerizable monomer, a compound having an ethylenically unsaturated bond as the functional group capable of addition polymerization is generally used. The polymerizable monomers include non-crosslinkable monomers and crosslinkable monomers. A non-crosslinkable monomer is a polymerizable monomer having only one polymerizable functional group, and a crosslinkable monomer is a polymerizable monomer having two or more polymerizable functional groups and forming a crosslink bond in the resin by a polymerization reaction. In the present disclosure, a polymerizable monomer having a solubility in distilled water at 20 °C of 0.3 g / L or more is referred to as a hydrophilic monomer, and a polymerizable monomer having a solubility in distilled water at 20 °C of less than 0.3 g / L is referred to as a non-hydrophilic monomer.

[0023] The hollow particles of the present disclosure include a shell containing a resin and a hollow portion surrounded by the shell, and are hollow particles having a porosity of 50% or more. The shell contains, as the resin, a polymer containing 70 to 100 parts by mass of a crosslinkable monomer unit per 100 parts by mass of all monomer units. In the immersion test of the hollow particles, 0.1 mg of the hollow particles is added to 4 mL of acetone, shaken for 10 minutes under the condition of a shaking speed of 100 rpm, and then allowed to stand for 48 hours. The hollow particles precipitating in acetone are less than 5% by mass.

[0024] The hollow particles of the present disclosure are particles including a shell (outer shell) containing a resin and a hollow portion surrounded by the shell. In the present disclosure, the hollow portion is a hollow space clearly distinguished from the shell of the hollow particles formed of a resin material. The shell of the hollow particles may have a porous structure, but in that case, the hollow portion has a size that can be clearly distinguished from a large number of minute spaces uniformly dispersed in the porous structure. The hollow portion of the hollow particles can be confirmed, for example, by SEM observation of the particle cross-section or by directly observing the particles by TEM observation or the like. The hollow portion of the hollow particles may be filled with a gas such as air, may be in a vacuum or reduced pressure state, or may contain a solvent.

[0025] For hollow particles of the same particle size, the higher the porosity, the thinner the shell, which reduces the shell strength and tends to make the hollow particles more susceptible to crushing. In contrast, hollow particles of the present disclosure are resistant to crushing even at high porosity, even when kneaded with other materials such as resins. The polymer contained in the shell of the hollow particles of the present disclosure contains 70 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of total monomer units, which is presumed to result in a high content of crosslinkable monomer units in the shell and a dense covalent bond network within the shell. Furthermore, in the above-mentioned immersion test, less than 5% of the hollow particles precipitated in acetone, indicating that the shell has a structure that is resistant to acetone penetration. This is presumed to result in a denser crosslinked structure within the shell of the hollow particles of the present disclosure. As shown in Comparative Example 4 below, when the amount of crosslinkable monomer units in the shell is relatively small, 5% by mass or more of the hollow particles precipitate in acetone in the above-mentioned hollow particle immersion test, and the hollow particles are prone to crushing. On the other hand, as shown in Comparative Examples 1 to 3 and 5 below, even when the shell contains a large amount of crosslinkable monomer units, when 5% by mass or more of the hollow particles precipitate in acetone in the above-mentioned hollow particle immersion test, the hollow particles are prone to crushing. The shells of the hollow particles of the present disclosure have a denser structure than conventional hollow particles having shells containing a large amount of crosslinkable monomer units, and therefore are thought to have improved strength, making them less prone to crushing even with a high porosity. Hereinafter, an example of a method for producing hollow particles of the present disclosure will be described, followed by a detailed description of the hollow particles of the present disclosure, and further a description of a resin composition and a molded article containing the hollow particles of the present disclosure.

[0026] 1. Manufacturing method of hollow particles The hollow particles of the present disclosure may be, for example: preparing a mixed liquid containing a first polymerizable monomer including a crosslinkable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium; By suspending the mixed solution, a suspension in which droplets of the monomer composition containing the first polymerizable monomer and the hydrocarbon solvent are dispersed in the aqueous medium is prepared; including the step of subjecting the suspension to a polymerization reaction; In the step of subjecting the suspension to a polymerization reaction, when the polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, a second polymerizable monomer having a solubility in distilled water at 20 ° C of 0.3 g / L or more is added and further subjected to a polymerization reaction, and it can be obtained by a method for producing hollow particles.

[0027] The above method for producing hollow particles is based on the basic technique of suspending a mixed solution containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium, so that the first polymerizable monomer and the hydrocarbon solvent are phase-separated, and the first polymerizable monomer is unevenly distributed on the surface side, and the hydrocarbon solvent is unevenly distributed in the central part. A suspension in which droplets having a distribution structure are dispersed in the aqueous medium is prepared, and the surface of the droplets is cured by subjecting this suspension to a polymerization reaction to form hollow particles having a hollow portion filled with a hydrocarbon solvent. In such a basic technique, in the step of subjecting the suspension to a polymerization reaction, when the polymerization conversion rate of the first polymerizable monomer containing a crosslinkable monomer reaches 93% by mass or more, a second polymerizable monomer which is a hydrophilic monomer having a solubility in distilled water at 20 ° C equal to or higher than the above specific value is added and further subjected to a polymerization reaction, whereby in the immersion test of the above hollow particles, hollow particles having a content of less than 5% by mass of hollow particles precipitating in acetone can be produced. When a crosslinkable monomer is used as the polymerizable monomer used for forming the shell of the hollow particles, unreacted polymerizable functional groups tend to remain in the shell. The more unreacted polymerizable functional groups remain, the coarser the crosslinked structure of the shell becomes. Therefore, in the hollow particles obtained by the conventional production method, due to the remaining unreacted polymerizable functional groups, it is considered that in the immersion test of the above hollow particles, the content of hollow particles precipitating in acetone is 5% by mass or more. In the method for producing the hollow particles, a suspension in which droplets of a monomer composition containing a first polymerizable monomer containing a large amount of a crosslinkable monomer are dispersed in an aqueous medium is subjected to a polymerization reaction. After performing the first polymerization reaction until the polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, a second polymerizable monomer which is a hydrophilic monomer is added and the second polymerization reaction is further performed, whereby it is considered that the reaction rate of the entire polymerizable monomer containing the first polymerizable monomer and the second polymerizable monomer can be improved. In the present disclosure, particles having a shell containing a polymer of a first polymerizable monomer obtained by the first polymerization reaction and a hollow portion filled with a hydrocarbon-based solvent may be referred to as first precursor particles, and a composition containing the first precursor particles may be referred to as a first precursor composition. Further, particles having a shell containing a polymer of a first polymerizable monomer and a second polymerizable monomer obtained by the second polymerization reaction and a hollow portion filled with a hydrocarbon-based solvent are considered to be an intermediate of hollow particles in which the hollow portion is filled with a gas, and may be referred to as second precursor particles, and a composition containing the second precursor particles may be referred to as a second precursor composition. In the method for producing the hollow particles, since the second polymerizable monomer has a solubility in distilled water at 20 °C equal to or higher than the specific value, when added to the first precursor composition, it is easily incorporated into the shell of the first precursor particles. Since the second polymerizable monomer which is a hydrophilic monomer has an affinity for both the first polymerizable monomer and the aqueous medium, when added to the first precursor composition, it is incorporated into the shell formed by the first polymerizable monomer, and it is considered that the thermal motion of the shell is promoted. During the second polymerization reaction, the polymerization reaction proceeds while the thermal motion of the shell is promoted with the second polymerizable monomer incorporated into the shell formed by the first polymerizable monomer. Therefore, the reaction rate is high, and the polymerization reaction of the polymerizable functional groups of the second polymerizable monomer incorporated into the shell and the first polymerizable monomer remaining unreacted proceeds sufficiently, and the crosslinked structure becomes dense. It is presumed that a shell that is difficult for acetone to penetrate is formed.

[0028] The method for manufacturing the hollow particles includes a step of preparing a mixed solution, a step of preparing a suspension, and a step of subjecting the suspension to a polymerization reaction, and may further include steps other than these. Also, as long as technically possible, two or more of the above steps and other additional steps may be performed simultaneously as one step, or the order may be changed. For example, mixing and suspension may be performed simultaneously in one process, such as performing suspension while introducing the materials for preparing the mixed solution.

[0029] As a preferable example of the method for manufacturing the hollow particles, a manufacturing method including the following steps can be cited. (1) Mixed solution preparation step Step of preparing a mixed solution containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium (2) Suspension step Step of preparing a suspension in which droplets of a monomer composition containing the first polymerizable monomer and the hydrocarbon solvent are dispersed in the aqueous medium by suspending the mixed solution (3) Polymerization step (3-1) First polymerization step By performing a first polymerization reaction in which the suspension is subjected to a polymerization reaction until the polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, a shell containing a polymer of the first polymerizable monomer and a first precursor composition containing first precursor particles having a hollow portion filled with a hydrocarbon solvent are prepared. (3-2) Second polymerization step By adding a second polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C to the first precursor composition and performing a second polymerization reaction, a shell containing a polymer of the first polymerizable monomer and a second polymerizable monomer and a second precursor composition containing second precursor particles having a hollow portion filled with a hydrocarbon solvent are prepared. (4) Solid-liquid separation step Step of obtaining second precursor particles encapsulating a hydrocarbon solvent in the hollow portion by performing solid-liquid separation on the second precursor composition, and (5) Solvent removal step a step of removing the hydrocarbon solvent contained in the second precursor particles obtained by the solid-liquid separation step to obtain hollow particles;

[0030] FIG. 1 is a schematic diagram showing an example of the above-mentioned manufacturing method. (1) to (5) in FIG. 1 correspond to the above-mentioned steps (1) to (5). The white arrows between the figures indicate the order of the steps. Note that FIG. 1 is merely a schematic diagram for explanation, and the above-mentioned manufacturing method is not limited to those shown in the figure. Furthermore, the structure, dimensions, and shape of the materials used in each manufacturing method of the present disclosure are not limited to the structure, dimensions, and shape of the various materials in these figures. 1(1) is a cross-sectional schematic diagram showing one embodiment of a mixed solution in the mixed solution preparation step. As shown in this figure, the mixed solution contains an aqueous medium 1 and a low-polarity material 2 dispersed in the aqueous medium 1. Here, the low-polarity material 2 refers to a material that has low polarity and is difficult to mix with the aqueous medium 1. In the present disclosure, the low-polarity material 2 contains a first polymerizable monomer and a hydrocarbon solvent. FIG. 1 (2) is a cross-sectional schematic diagram showing one embodiment of a suspension in a suspending step. The suspension includes an aqueous medium 1 and droplets 10 of a monomer composition dispersed in the aqueous medium 1. The droplets 10 of the monomer composition contain a first polymerizable monomer and a hydrocarbon solvent, but the distribution within the droplets is not uniform. The droplets 10 of the monomer composition are phase-separated into a hydrocarbon solvent 4a and a material other than the hydrocarbon solvent, including the first polymerizable monomer, 4b, with the hydrocarbon solvent 4a unevenly distributed in the center and the material other than the hydrocarbon solvent 4b unevenly distributed on the surface, and a dispersion stabilizer (not shown) attached to the surface. 1(3) is a cross-sectional schematic diagram showing one embodiment of a composition (second precursor composition) containing hollow particles (second precursor particles) encapsulating a hydrocarbon solvent in their hollow spaces, obtained by a polymerization step. The composition contains an aqueous medium 1 and hollow particles (second precursor particles) 20 encapsulating a hydrocarbon solvent 4a in their hollow spaces, dispersed in the aqueous medium 1. A shell 6 forming the outer surface of the second precursor particle 20 is formed by polymerization of a first polymerizable monomer in droplets 10 of the monomer composition and polymerization of a second polymerizable monomer that is added later. Fig. 1(4) is a cross-sectional schematic diagram showing one embodiment of hollow particles (second precursor particles) containing a hydrocarbon solvent in the hollow portion after the solid-liquid separation step, which shows a state in which the aqueous medium 1 has been removed from the state shown in Fig. 1(3). Fig. 1 (5) is a cross-sectional schematic diagram showing one embodiment of hollow particles after the solvent removal step. Fig. 1 (5) shows a state in which the hydrocarbon solvent 4a has been removed from the state shown in Fig. 1 (4). By removing the hydrocarbon solvent from hollow particles (second precursor particles) containing the hydrocarbon solvent in their hollow portions, hollow particles 100 having gas-filled hollow portions 8 inside their shells 6 are obtained. The above five steps and other steps will be explained below in order.

[0031] (1) Mixed liquid preparation process This step is a step of preparing a mixed liquid containing a first polymerizable monomer, a hydrocarbon solvent, a dispersion stabilizer, and an aqueous medium. The mixed solution preferably further contains a particle size control agent. The mixed solution preferably contains an oil-soluble polymerization initiator as a polymerization initiator. The mixed solution may further contain other materials such as a suspension stabilizer, as long as the effects of the present disclosure are not impaired. The materials of the mixed liquid will be explained in the following order: (A) first polymerizable monomer, (B) particle size control agent, (C) oil-soluble polymerization initiator, (D) hydrocarbon solvent, (E) dispersion stabilizer, and (F) aqueous medium.

[0032] (A) First Polymerizable Monomer The first polymerizable monomer includes at least a crosslinkable monomer, and may further include a non-crosslinkable monomer within a range that does not impair the effects of the present disclosure. As the first polymerizable monomer, a (meth)acrylic polymerizable monomer having a (meth)acryloyl group as a polymerizable functional group can be preferably used, since the polymerization reaction is likely to be stable and hollow particles having high heat resistance can be obtained.

[0033] [Crosslinking monomer] Since the crosslinkable monomer has a plurality of polymerizable functional groups, the monomers can be linked together, thereby increasing the crosslink density of the shell. Examples of the crosslinkable monomer include difunctional crosslinkable monomers having two polymerizable functional groups, such as divinylbenzene, divinyldiphenyl, divinylnaphthalene, diallyl phthalate, allyl (meth)acrylate, vinyl (meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, and 2-hydroxy-3-(meth)acrylpropyl (meth)acrylate; and trifunctional or higher crosslinkable monomers having three or more polymerizable functional groups, such as trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol poly(meth)acrylate, and ethoxylated versions of these. These crosslinkable monomers can be used either alone or in combination of two or more. Among these crosslinkable monomers, examples of hydrophilic crosslinkable monomers having a solubility of 0.3 g / L or more in distilled water at 20°C include ethylene glycol dimethacrylate, diethylene glycol diacrylate, allyl methacrylate, vinyl methacrylate, and 2-hydroxy-3-methacrylpropyl acrylate. The crosslinkable monomer contained in the first polymerizable monomer is not particularly limited and may be a hydrophilic crosslinkable monomer having a solubility in distilled water at 20°C of 0.3 g / L or more, or a non-hydrophilic crosslinkable monomer having a solubility of less than 0.3 g / L.

[0034] The first polymerizable monomer preferably contains at least a bifunctional crosslinkable monomer as a crosslinkable monomer, and more preferably contains a combination of a bifunctional crosslinkable monomer and a crosslinkable monomer having three or more functional groups. When the first polymerizable monomer contains a crosslinkable monomer having three or more functional groups, it is excellent in that a covalent bond network can be stretched more densely in the shell. However, unreacted polymerizable functional groups tend to remain after the first polymerization reaction. In the above production method, even when the first polymerizable monomer contains a crosslinkable monomer having three or more functional groups, by adding a hydrophilic monomer as the second polymerizable monomer and performing the second polymerization reaction, the polymerization reaction of the unreacted polymerizable functional groups remaining after the first polymerization reaction easily proceeds. Therefore, when the first polymerizable monomer contains a crosslinkable monomer having three or more functional groups, the crosslinked structure of the shell can be made denser, the strength of the hollow particles can be improved, and they can be made less likely to collapse. From the viewpoint that the polymerization reaction is likely to be stable and hollow particles excellent in strength and heat resistance can be obtained, as the bifunctional crosslinkable monomer, divinylbenzene, ethylene glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate are preferable, and ethylene glycol di(meth)acrylate and pentaerythritol di(meth)acrylate are more preferable. From the same viewpoint, as the crosslinkable monomer having three or more functional groups, pentaerythritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol poly(meth)acrylate are preferable, and pentaerythritol tetra(meth)acrylate is more preferable.

[0035] In 100 parts by mass of the first polymerizable monomer, the content of the crosslinkable monomer is preferably 75 to 100 parts by mass, more preferably 80 to 100 parts by mass, still more preferably 85 to 100 parts by mass, and even more preferably 90 to 100 parts by mass. When the content of the crosslinkable monomer is at least the above lower limit value, the polymer contained in the formed shell tends to be a polymer containing 70 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of all monomer units. Further, since the content ratio of the crosslinkable monomer units in the shell of the hollow particles is sufficiently high, a covalent bond network is densely stretched in the shell, resulting in excellent strength, being difficult to collapse, and being difficult to deform even against heat or the like applied from the outside.

[0036] The content of the bifunctional crosslinkable monomer in 100 parts by mass of the first polymerizable monomer is not particularly limited, but the lower limit is preferably 50 parts by mass or more, more preferably 60 parts by mass or more, still more preferably 70 parts by mass or more, and even more preferably 75 parts by mass or more, and the upper limit is preferably 100 parts by mass or less, more preferably 95 parts by mass or less, still more preferably 90 parts by mass or less.

[0037] When the first polymerizable monomer contains a crosslinkable monomer having three or more functional groups as the crosslinkable monomer, the content of the crosslinkable monomer having three or more functional groups in 100 parts by mass of the first polymerizable monomer is not particularly limited, but the lower limit is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, still more preferably 15 parts by mass or more, and the upper limit is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, still more preferably 30 parts by mass or less, and even more preferably 25 parts by mass or less.

[0038] [Non-crosslinkable monomer] The first polymerizable monomer may further contain a non-crosslinkable monomer. As the non-crosslinkable monomer, a monovinyl monomer is preferably used. A monovinyl monomer is a compound having one polymerizable vinyl functional group. Examples of the monovinyl monomer include (meth)acrylic acid alkyl esters having an alkyl group of 6 or more carbon atoms, such as 2-ethylhexyl (meth)acrylate and lauryl (meth)acrylate; aromatic vinyl monomers such as styrene, vinyltoluene, α-methylstyrene, p-methylstyrene, and halogenated styrene; monoolefin monomers such as ethylene, propylene, and butylene; diene monomers such as butadiene and isoprene; vinyl carboxylic acid ester monomers such as vinyl acetate; halogenated vinyl monomers such as vinyl chloride; and vinylidene chloride. vinylpyridine monomers; and hydrophilic non-crosslinkable monomers such as (meth)acrylic acid alkyl esters having an alkyl group of 1 to 5 carbon atoms, such as methyl (meth)acrylate, ethyl (meth)acrylate, and butyl (meth)acrylate; (meth)acrylamides and derivatives thereof, such as (meth)acrylamide, N-methylol (meth)acrylamide, and N-butoxymethyl (meth)acrylamide; (meth)acrylic acid nitrile; and polar group-containing non-crosslinkable monomers. Examples of the non-crosslinkable monomer containing a polar group include non-crosslinkable monomers containing a polar group selected from, for example, a carboxyl group, a hydroxyl group, a sulfonic acid group, an amino group, a polyoxyethylene group, and an epoxy group. More specifically, carboxyl group-containing monomers such as ethylenically unsaturated carboxylic acid monomers such as (meth)acrylic acid, crotonic acid, cinnamic acid, itaconic acid, fumaric acid, maleic acid, and butenetricarboxylic acid; hydroxyl group-containing monomers such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; sulfonic acid group-containing monomers such as styrene sulfonic acid; amino group-containing monomers such as dimethylaminoethyl (meth)acrylate and diethylaminoethyl (meth)acrylate; polyoxyethylene group-containing monomers such as methoxypolyethylene glycol (meth)acrylate; epoxy group-containing monomers such as glycidyl (meth)acrylate, allyl glycidyl ether, and 4-hydroxybutyl acrylate glycidyl ether, etc. can be mentioned. These non-crosslinkable monomers can be used alone or in combination of two or more. Among the non-crosslinkable monomers used as the first polymerizable monomer, hydrophilic non-crosslinkable monomers are preferred from the viewpoint of obtaining hollow particles with excellent strength, and (meth)acrylic acid alkyl esters having an alkyl group with 1 to 5 carbon atoms are more preferred, (meth)acrylic acid alkyl esters having an alkyl group with 1 to 4 carbon atoms are more preferred, and methyl (meth)acrylate is even more preferred.

[0039] In the first polymerizable monomer, the polymerizable monomer other than the crosslinkable monomer is a non-crosslinkable monomer. The content of the non-crosslinkable monomer in the first polymerizable monomer is preferably 0 to 25 parts by mass in 100 parts by mass of the first polymerizable monomer. From the viewpoint of suppressing the decrease in the reactivity of the first polymerizable monomer and improving the strength of the hollow particles to make them less likely to collapse, the content of the non-crosslinkable monomer in the first polymerizable monomer is more preferably 20 parts by mass or less, still more preferably 15 parts by mass or less, even more preferably 10 parts by mass or less, and it is particularly preferred that the first polymerizable monomer does not contain a non-crosslinkable monomer.

[0040] The content of the first polymerizable monomer in the mixed solution is not particularly limited, but from the viewpoint of the balance between the porosity, particle size, and mechanical strength of the hollow particles, it is usually 15 to 55 mass%, more preferably 25 to 40 mass%, relative to 100 mass% of the total mass of the components in the mixed solution excluding the aqueous medium.

[0041] (B) Particle size control agent The mixed solution preferably further contains a particle size control agent, which allows the particle size of the droplets of the monomer composition and the shell thickness of the resulting hollow particles to be appropriately adjusted, thereby producing hollow particles that are resistant to crushing even when they have a high porosity. Examples of particle size control agents include at least one selected from the group consisting of rosin acid, higher fatty acids, and metal salts thereof, or polar resins, as described below. These particle size control agents can appropriately adjust the particle size of droplets of a monomer composition containing a first polymerizable monomer and a hydrocarbon solvent in the suspension process described below. During the suspension process, droplets of the monomer composition are formed in an aqueous medium due to the action of a dispersion stabilizer. In the droplets of the monomer composition, phase separation occurs between the hydrocarbon solvent and materials other than the hydrocarbon solvent containing the first polymerizable monomer, resulting in the hydrocarbon solvent being concentrated in the center and the materials other than the hydrocarbon solvent being concentrated on the surface. When the mixed liquid contains a particle size control agent, it is estimated that the particle size control agent is concentrated near the surface of the monomer composition droplets, and the dispersion stabilizer is attached to the droplet surface. This distribution structure of materials is formed according to the difference in affinity of each material for the aqueous medium. It is believed that by including a particle size control agent in the mixed liquid, the droplets of the monomer composition in the suspension have the distribution structure of the materials described above, and an interaction occurs between the dispersion stabilizer and the particle size control agent on the droplet surface, which changes the dispersibility of the droplets due to the dispersion stabilizer, making it possible to appropriately adjust the particle size of the droplets of the monomer composition. Among the particle size control agents, at least one selected from the group consisting of rosin acid, higher fatty acids, and metal salts thereof is preferred, and at least one selected from rosin acid and alkali metal salts thereof is more preferred, since the particle size of the droplets can be appropriately adjusted with a small content.

[0042] Rosin acids preferably used as particle size control agents can be obtained from rosins such as gum rosin, tall rosin and wood rosin. Examples of components contained in rosin acids obtained from these rosins include abietic acid, dehydroabietic acid, palustric acid, isopimaric acid, pimaric acid, etc. The component ratios of rosin acids are not constant and vary depending on the type of rosin, the species of pine used as the raw material, the place of origin, etc. The rosin acid and metal salts thereof used in the present disclosure are more preferably rosin acid and alkali metal salts thereof containing 50 mass % or more of abietic acids such as abietic acid, dehydroabietic acid, palustric acid, and hydrogenated versions thereof.

[0043] The higher fatty acid used as the particle size control agent is preferably a higher fatty acid having 10 to 25 carbon atoms, excluding the carbon atom in the carboxyl group. A preferred example of a higher fatty acid is lauric acid (CH3(CH2) 10 COOH), tridecanoic acid (CH3(CH2) 11 COOH), myristic acid (CH3(CH2) 12 COOH), pentadecanoic acid (CH3(CH2) 13 COOH), palmitic acid (CH3(CH2) 14 COOH), heptadecanoic acid (CH3(CH2) 15 COOH), stearic acid (CH3(CH2) 16 COOH), arachidic acid (CH3(CH2) 18 COOH), behenic acid (CH3(CH2) 20 COOH), and lignoceric acid (CH3(CH2) 22 COOH) etc.

[0044] Examples of the metal used for the metal salt of rosin acid or higher fatty acid include alkali metals such as Li, Na, K, and alkaline earth metals such as Mg, Ca, etc. Among them, alkali metals are preferred, and at least one selected from Li, Na, and K is more preferred.

[0045] When at least one selected from the group consisting of rosin acid, higher fatty acid, and their metal salts is used as the particle size controller, the total content of rosin acid, higher fatty acid, and their metal salts is preferably 0.0001 parts by mass or more and 0.1 parts by mass or less, more preferably 0.001 parts by mass or more and 0.01 parts by mass or less, and even more preferably 0.0015 parts by mass or more and 0.006 parts by mass or less with respect to 100 parts by mass in total of the first polymerizable monomer and the hydrocarbon solvent. When the above content is at least the above lower limit value, it is easy to control the particle size of the hollow particles and the thickness of the shell, and the strength of the hollow particles can be improved. On the other hand, when the above content is at most the above upper limit value, since the decrease in the content ratio of the polymerizable monomer can be suppressed, the decrease in the strength of the shell can be suppressed, and the crushing of the hollow particles can be further suppressed.

[0046] The polar resin preferably used as the particle size controller refers to a polymer containing a repeating unit containing a heteroatom. Specifically, acrylic resins, polyester resins, vinyl resins containing heteroatoms, etc. can be mentioned. The polar resin may be a homopolymer or copolymer of a heteroatom-containing monomer, or a copolymer of a heteroatom-containing monomer and a heteroatom-free monomer. When the polar resin is a copolymer of a heteroatom-containing monomer and a heteroatom-free monomer, from the viewpoint of easily controlling the particle size of the hollow particles, the ratio of the heteroatom-containing monomer unit in 100% by mass of all the repeating units constituting the copolymer is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more. Examples of the heteroatom-containing monomers used in the polar resin include (meth)acrylic monomers having a (meth)acryloyl group, such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, acrylic acid, methacrylic acid, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, glycidyl (meth)acrylate, and 4-hydroxybutyl acrylate glycidyl ether; aromatic vinyl monomers containing a heteroatom, such as halogenated styrene and styrene sulfonic acid; vinyl ester monomers of carboxylic acids, such as vinyl acetate; vinyl halide monomers, such as vinyl chloride; vinylidene halide monomers, such as vinylidene chloride; vinyl pyridine monomers; carboxyl group-containing monomers, such as ethylenically unsaturated carboxylic acid monomers, such as crotonic acid, cinnamic acid, itaconic acid, fumaric acid, maleic acid, and butene tricarboxylic acid; and epoxy group-containing monomers, such as allyl glycidyl ether. These heteroatom-containing monomers can be used alone or in combination of two or more thereof. Examples of the heteroatom-free monomers used in the polar resin include aromatic vinyl monomers not containing a heteroatom, such as styrene, vinyltoluene, α-methylstyrene, and p-methylstyrene; monoolefin monomers, such as ethylene, propylene, and butylene; and diene monomers, such as butadiene and isoprene. These heteroatom-free monomers can be used alone or in combination of two or more thereof.

[0047] Among them, the polar resin has high compatibility with the first polymerizable monomer and is easy to control the particle size of the hollow particles. Therefore, in 100% by mass of all the repeating units constituting the resin, the total mass of the (meth)acrylic monovinyl monomer units is preferably 50% by mass or more, more preferably 70% by mass or more, still more preferably 90% by mass or more, and it is preferably an acrylic resin. In particular, it is preferable that all the repeating units constituting the resin are acrylic resin units composed of (meth)acrylic monovinyl monomer units.

[0048] Among them, it is preferable that the heteroatom-containing monomer in the polar resin contains a polar group-containing monomer unit containing a polar group selected from a carboxyl group, a hydroxyl group, a sulfonic acid group, an amino group, a polyoxyethylene group, and an epoxy group, from the viewpoint of easy control of the particle size of the hollow particles. Examples of the polar group-containing monomer used in the polar resin include the same ones as the polar group-containing non-crosslinkable monomers that may be contained in the above-described first polymerizable monomer. The polar group-containing monomers can be used alone or in combination of two or more. As the polar groups contained in the polar group-containing monomer units contained in the polar resin, a carboxyl group and a hydroxyl group are preferable from the viewpoint that the particle size can be controlled with a small addition amount. When the polar resin contains a polar group-containing monomer unit, it is preferable that the polar group is located at the end of the main chain or side chain, or is pendantly bonded to the main chain or side chain, because the polar resin is likely to be disposed on the outer surface of the hollow particles and it is easy to control the particle size of the hollow particles.

[0049] When the polar resin does not contain the polar group-containing monomer unit, as the heteroatom-containing monomer unit contained in the polar resin, from the viewpoint of high compatibility with the first polymerizable monomer and easy control of the particle size of the hollow particles, it preferably contains a monomer unit derived from an alkyl (meth)acrylate. Among them, from the viewpoint of high polarity, it preferably contains a monomer unit derived from an alkyl (meth)acrylate in which the carbon number of the alkyl group is 3 or less, more preferably the alkyl group is a methyl group or an ethyl group, and still more preferably the alkyl group is a methyl group.

[0050] The acrylic resin serving as the polar resin is preferably a polymer or copolymer of a polymerizable monomer for a polar resin containing 50% by mass or more of methyl methacrylate when the total mass of the polymerizable monomers for a polar resin is taken as 100% by mass, because it has high compatibility with the first polymerizable monomer and makes it easy to control the particle size of the hollow particles. In the present disclosure, the polymerizable monomer used in synthesizing the polar resin is referred to as the polymerizable monomer for a polar resin.

[0051] The polar resin can be obtained, for example, by polymerizing a polymerizable monomer for polar resins containing the heteroatom-containing monomer by a polymerization method such as solution polymerization or emulsion polymerization. Furthermore, when the polar resin is a copolymer, the copolymer may be any of a random copolymer, a block copolymer, or a graft copolymer, but is preferably a random copolymer. In addition, the polar resin is preferably pulverized as finely as possible in order to improve solubility.

[0052] The number average molecular weight (Mn) of the polar resin is not particularly limited, but is preferably in the range of 3,000 to 20,000, more preferably 4,000 to 17,000, and even more preferably 6,000 to 15,000, in terms of polystyrene, as measured by gel permeation chromatography (GPC) using tetrahydrofuran. When the number average molecular weight (Mn) of the polar resin is equal to or greater than the lower limit, the solubility of the polar resin is improved and the particle size of the hollow particles can be easily controlled. When the number average molecular weight (Mn) is equal to or less than the upper limit, a decrease in shell strength can be suppressed.

[0053] When a polar resin is used as the particle size control agent, the content of the polar resin is preferably 0.1 part by mass or more and 10.0 parts by mass or less, more preferably 0.3 part by mass or more and 8.0 parts by mass or less, and even more preferably 0.5 part by mass or more and 8.0 parts by mass or less with respect to 100 parts by mass of the first polymerizable monomer. When the content is at least the above lower limit value, it is easy to control the particle diameter of the hollow particles and the thickness of the shell, and the strength of the hollow particles can be improved. On the other hand, when the content is at most the above upper limit value, since a decrease in the content ratio of the polymerizable monomer can be suppressed, a decrease in the strength of the shell can be suppressed, and crushing of the hollow particles can be further suppressed.

[0054] (C) Oil-soluble polymerization initiator In the present disclosure, it is preferable that the mixed solution contains an oil-soluble polymerization initiator as the polymerization initiator. As methods for polymerizing the droplets of the monomer composition after suspending the mixed solution, there are an emulsion polymerization method using a water-soluble polymerization initiator and a suspension polymerization method using an oil-soluble polymerization initiator. By using an oil-soluble polymerization initiator, suspension polymerization can be carried out. The oil-soluble polymerization initiator is not particularly limited as long as it is lipophilic with a solubility in water of 0.2% by mass or less. Examples of the oil-soluble polymerization initiator include benzoyl peroxide, lauroyl peroxide, t-butyl peroxide-2-ethylhexanoate, 2,2'-azobis(2,4-dimethylvaleronitrile), azobisisobutyronitrile, and the like. When the total mass of the first polymerizable monomer in the mixed solution is 100 parts by mass, the content of the oil-soluble polymerization initiator is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 7 parts by mass, and even more preferably 1 to 5 parts by mass. When the content of the oil-soluble polymerization initiator is 0.1 to 10 parts by mass, the polymerization reaction proceeds sufficiently, and there is little possibility that the oil-soluble polymerization initiator remains after the polymerization reaction is completed, and there is also little possibility that an unexpected side reaction proceeds.

[0055] (D) Hydrocarbon solvent In the present disclosure, a hydrocarbon solvent is used as a non-polymerizable and poorly water-soluble organic solvent. The hydrocarbon solvent serves as a spacer material that forms a hollow portion inside the particles. In the suspension step described later, a suspension in which droplets of the monomer composition containing the hydrocarbon solvent are dispersed in an aqueous medium is obtained. In the suspension step, as a result of phase separation occurring within the droplets of the monomer composition, the less polar hydrocarbon solvent tends to gather inside the polymerizable monomer droplets. Finally, in the droplets of the monomer composition, the hydrocarbon solvent is inside, and other materials other than the hydrocarbon solvent are distributed according to their respective polarities at the periphery. Then, in the polymerization step described later, an aqueous dispersion containing hollow particles encapsulating the hydrocarbon solvent is obtained. That is, when the hydrocarbon solvent gathers inside the particles, a hollow portion filled with the hydrocarbon solvent is formed inside the obtained precursor particles.

[0056] The type of the hydrocarbon solvent is not particularly limited. Examples of the hydrocarbon solvent include saturated hydrocarbon solvents such as butane, pentane, normal hexane, cyclohexane, heptane, and octane, aromatic hydrocarbon solvents such as benzene, toluene, and xylene, and relatively highly volatile solvents such as carbon disulfide and carbon tetrachloride. By changing the amount of the hydrocarbon solvent in the mixture, the porosity of the hollow particles can be adjusted. In the suspension step described later, since the polymerization reaction proceeds with the oil droplets containing a crosslinkable monomer or the like encapsulating the hydrocarbon solvent, the higher the content of the hydrocarbon solvent, the higher the porosity of the obtained hollow particles tends to be. Preferably, in the total amount of 100% by mass of the hydrocarbon solvent, the proportion of the saturated hydrocarbon solvent is 50% by mass or more. Thereby, phase separation sufficiently occurs within the droplets of the monomer composition, and hollow particles having only one hollow portion are easily obtained, and the generation of porous particles can be suppressed. The proportion of the saturated hydrocarbon solvent is preferably 60% by mass or more, more preferably 80% by mass or more, from the viewpoint of further suppressing the generation of porous particles and the tendency for the hollow portions of each hollow particle to be uniform. In addition, as the hydrocarbon solvent, a hydrocarbon solvent having 4 to 7 carbon atoms is preferable. Hydrocarbon compounds having 4 to 7 carbon atoms can be easily encapsulated in the first precursor particles during the polymerization step, and can be easily removed from the second precursor particles during the solvent removal step. Among them, a hydrocarbon solvent having 5 or 6 carbon atoms is particularly preferable. Also, although not particularly limited, as the hydrocarbon solvent, from the viewpoint of being easily removed in the solvent removal step described later, those having a boiling point of 130°C or lower are preferable, and those having a boiling point of 100°C or lower are more preferable. Also, from the viewpoint of being easily encapsulated in the first precursor particles, as the hydrocarbon solvent, those having a boiling point of 50°C or higher are preferable, and those having a boiling point of 60°C or higher are more preferable.

[0057] In addition, the hydrocarbon solvent preferably has a relative permittivity at 20°C of 3 or less. The relative permittivity is one of the indices indicating the polarity of a compound. When the relative permittivity of the hydrocarbon solvent is sufficiently small at 3 or less, it is considered that phase separation proceeds rapidly in the droplets of the monomer composition and a hollow is easily formed. Examples of solvents having a relative permittivity at 20°C of 3 or less are as follows. The values in parentheses are the relative permittivity values. Heptane (1.9), cyclohexane (2.0), benzene (2.3), toluene (2.4). Regarding the relative permittivity at 20°C, the values described in known literature (for example, "Chemical Handbook, Basic Edition" edited by the Chemical Society of Japan, Revised 4th Edition, Maruzen Co., Ltd., published on September 30, 1993, pages II-498 to II-503), and other technical information can be referred to. As a method for measuring the relative permittivity at 20°C, for example, a relative permittivity test conducted in accordance with 23 of JISC 2101:1999 and with the measurement temperature set at 20°C can be mentioned.

[0058] In the present disclosure, the content of the hydrocarbon solvent in the mixed solution is preferably 50 parts by mass or more and 500 parts by mass or less with respect to 100 parts by mass of the total mass of the first polymerizable monomer, because it is easy to control the particle diameter of the hollow particles, easy to increase the porosity while maintaining the strength of the hollow particles, and easy to reduce the amount of the residual hydrocarbon solvent in the particles. The content of the hydrocarbon solvent in the mixed solution is preferably 60 parts by mass or more and 400 parts by mass or less, more preferably 70 parts by mass or more and 300 parts by mass or less, and still more preferably 80 parts by mass or more and 200 parts by mass or less with respect to 100 parts by mass of the total mass of the first polymerizable monomer.

[0059] (E)Dispersion stabilizer The dispersion stabilizer is an agent for dispersing the droplets of the monomer composition in an aqueous medium in the suspension step. In the present disclosure, from the viewpoints of being able to easily control the particle diameter of the droplets in the suspension and being able to narrow the particle size distribution of the obtained hollow particles, and suppressing the shell from becoming too thin and suppressing the decrease in the strength of the hollow particles, it is preferable to use an inorganic dispersion stabilizer as the dispersion stabilizer. Such an effect of the inorganic dispersion stabilizer is particularly likely to be exhibited when the inorganic dispersion stabilizer is used in combination with the above-described particle diameter control agent. Examples of the inorganic dispersion stabilizer 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 the like. These inorganic dispersion stabilizers can be used alone or in combination of two or more. Among the above inorganic dispersion stabilizers, poorly water-soluble inorganic metal salts such as the above-described sulfates, carbonates, phosphates, and metal hydroxides are preferable, metal hydroxides are more preferable, and magnesium hydroxide is particularly preferable. In the present disclosure, the poorly water-soluble inorganic metal salt is preferably an inorganic metal salt having a solubility of 0.5 g or less in 100 g of water.

[0060] The content of the dispersion stabilizer is not particularly limited, but is preferably 0.5 to 10 parts by mass, more preferably 1.0 to 8.0 parts by mass, relative to 100 parts by mass of the total mass of the first polymerizable monomer and the hydrocarbon solvent. When the content of the dispersion stabilizer is equal to or greater than the lower limit, droplets of the monomer composition can be sufficiently dispersed so as not to coalesce in the suspension. On the other hand, when the content of the dispersion stabilizer is equal to or less than the upper limit, an increase in the viscosity of the suspension during granulation can be prevented, and the problem of the suspension being unable to pass through the granulator can be avoided. The content of the dispersion stabilizer is usually 2 parts by mass or more and 15 parts by mass or less, and preferably 3 parts by mass or more and 8 parts by mass or less, relative to 100 parts by mass of the aqueous medium.

[0061] (F)Aqueous medium In the present disclosure, the aqueous medium means a medium selected from the group consisting of water, a hydrophilic solvent, and a mixture of water and a hydrophilic solvent. The hydrophilic solvent in the present disclosure is not particularly limited as long as it is sufficiently miscible with water and does not cause phase separation. Examples of hydrophilic solvents include alcohols such as methanol and ethanol, tetrahydrofuran (THF), and dimethyl sulfoxide (DMSO). Among aqueous media, water is preferred due to its high polarity. 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 mixing ratio (mass ratio) of water to hydrophilic solvent may be set to 99:1 to 50:50, etc.

[0062] A mixed solution is obtained by mixing the above-mentioned materials and other materials as needed, followed by appropriate stirring, etc. In this mixed solution, an oil phase containing the above-mentioned (A) first polymerizable monomer, (B) particle size control agent, (C) oil-soluble polymerization initiator, and (D) a lipophilic material such as a hydrocarbon solvent is dispersed in an aqueous phase containing (E) a dispersion stabilizer and (F) an aqueous medium, etc., with particle sizes of about several mm. 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 above-mentioned respective materials and other materials as necessary may be simply mixed and appropriately stirred or the like to obtain a mixed solution. However, from the viewpoint that the shell is likely to become uniform, it is preferable to separately prepare in advance an oil phase containing a first polymerizable monomer, a particle size control agent, and a hydrocarbon solvent, and an aqueous phase containing a dispersion stabilizer and an aqueous medium, and mix these to prepare a mixed solution. By thus separately preparing the oil phase and the aqueous phase in advance and mixing these, hollow particles with a uniform composition in the shell portion can be produced.

[0063] (2) Suspension step The suspension step is a step of preparing a suspension in which droplets of a monomer composition containing a hydrocarbon solvent are dispersed in an aqueous medium by suspending the above-mentioned mixed solution. The suspension method for forming droplets of the monomer composition is not particularly limited. For example, it is carried out using a device capable of strong stirring such as an (in-line type) emulsifying disperser (manufactured by Taihei Kikai Kogyo Co., Ltd., trade name: Mildar), a high-speed emulsifying disperser (manufactured by Primix Corporation, trade name: T.K. Homomixer MARK II type), etc. In the suspension prepared in the suspension step, droplets of a monomer composition containing the above-mentioned lipophilic material and having a particle size of about 4 to 60 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 with a known observation device such as an optical microscope. In the suspension step, since phase separation occurs in the droplets of the monomer composition, a hydrocarbon solvent with low polarity tends to gather inside the droplets. As a result, in the obtained droplets, the hydrocarbon solvent is inside, and materials other than the hydrocarbon solvent are distributed on the periphery thereof.

[0064] Figure 2 is a schematic diagram showing an embodiment of the suspension in the suspension step. The droplet 10 of the monomer composition in Figure 2 schematically shows its cross-section. Note that Figure 2 is merely a schematic diagram, and the suspension in the present disclosure is not necessarily limited to that shown in Figure 2. A part of Figure 2 corresponds to (2) of Figure 1 described above. Figure 2 shows the state in which droplets 10 of the monomer composition and the first polymerizable monomer 4c dispersed in the aqueous medium 1 are dispersed in the aqueous medium 1. The droplet 10 is composed of the dispersion stabilizer 3 surrounding the oil-soluble monomer composition 4. The monomer composition contains an oil-soluble polymerization initiator 5, as well as a first polymerizable monomer and a hydrocarbon solvent (both not shown). The droplet 10 is a micro-oil droplet containing the monomer composition 4, and the oil-soluble polymerization initiator 5 generates polymerization-initiating radicals inside the micro-oil droplet. Therefore, precursor particles with a target particle size can be produced without overgrowing the micro-oil droplets. In such a suspension polymerization method using an oil-soluble polymerization initiator, there is no chance for the polymerization initiator to come into contact with the polymerizable monomer 4c dispersed in the aqueous medium 1. Therefore, by using an oil-soluble polymerization initiator, it is possible to suppress the generation of extra polymer particles such as relatively small-sized dense particles in addition to the resin particles having the target hollow portion.

[0065] (3) Polymerization step (3-1) First polymerization step In the above production method, the polymerization step is carried out in two stages. In the first polymerization step, a first polymerization reaction is carried out by subjecting the suspension to a polymerization reaction until the polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, thereby preparing a first precursor composition containing a shell containing a polymer of the first polymerizable monomer and first precursor particles having a hollow portion filled with a hydrocarbon solvent. During the first polymerization reaction, the droplets of the monomer composition are subjected to the polymerization reaction while encapsulating the hydrocarbon solvent, which allows the polymerization reaction to proceed while maintaining the shape. Therefore, during the first polymerization reaction, the size and porosity of the resulting hollow particles can be easily adjusted by adjusting the amount of hydrocarbon solvent, the amount of particle size control agent, the type of dispersion stabilizer, and the like. Furthermore, since the first polymerizable monomer and hydrocarbon solvent are used in combination, the polarity of the hydrocarbon solvent is low relative to the shell of the first precursor particle, making it difficult for the hydrocarbon solvent to blend with the shell. This leads to sufficient phase separation, resulting in only one hollow portion.

[0066] In the first polymerization reaction, the polymerization method is not particularly limited, and for example, a batch method, a semi-continuous method, a continuous method, etc. can be used. In the first polymerization reaction, the polymerization temperature is preferably 40 to 80°C, and more preferably 50 to 70°C. In the first polymerization reaction, the temperature rise rate when the temperature is raised to the polymerization temperature is preferably 10°C / h to 60°C / h, and more preferably 15°C / h to 55°C / h. The reaction time for the first polymerization reaction is preferably 0.5 to 5 hours, and more preferably 1 to 3 hours.

[0067] In the above production method, the first polymerization reaction is carried out until the polymerization conversion rate of the first polymerizable monomer reaches 93% by mass or more, preferably 95% by mass or more, more preferably 98% by mass or more, and even more preferably 99% by mass or more. In the present disclosure, the polymerization conversion rate is calculated from the mass of the solid content of the first precursor particles obtained by the first polymerization reaction and the mass of the first polymerizable monomer remaining unreacted after the first polymerization reaction using the following formula (A). 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. The mass of the unreacted first polymerizable monomer can be measured using gas chromatography (GC). Coincidence conversion rate (mass %) = 100 - (mass of unreacted first polymerizable monomer / mass of solid content of first precursor particles) × 100, Formula (A)

[0068] (3-2) Second polymerization step In the second polymerization step, a second polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C is added to the first precursor composition obtained in the first polymerization step, and a second polymerization reaction is carried out to prepare a second precursor composition containing a shell containing a polymer of the first polymerizable monomer and the second polymerizable monomer and second precursor particles having a hollow portion filled with a hydrocarbon solvent. In the second polymerization reaction, the polymerization reaction proceeds with the second polymerizable monomer incorporated into the shell of the first precursor particles. Since the shell of the first precursor particles promotes thermal motion when the second polymerizable monomer is incorporated, in the second polymerization reaction, the polymerizable functional groups of the first polymerizable monomer remaining unreacted in the shell and the polymerization reaction of the second polymerizable monomer proceed, and it is presumed that a dense crosslinked structure is formed.

[0069] The second polymerizable monomer is not particularly limited as long as it is a polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C. However, from the viewpoint of improving the strength of the hollow particles, a non-crosslinkable monomer having a solubility of 0.3 g / L or more in distilled water at 20°C, that is, a hydrophilic non-crosslinkable monomer is preferable. Examples of the hydrophilic non-crosslinkable monomer used as the second polymerizable monomer include the same ones as those used as the first polymerizable monomer. For example, alkyl (meth)acrylate esters having an alkyl group with 1 to 5 carbon atoms, (meth)acrylamides and their derivatives, (meth)acrylonitrile, and polar group-containing non-crosslinkable monomers can be mentioned. In addition, as the second polymerizable monomer, from the viewpoint that the second polymerizable monomer is incorporated into the shell of the first precursor particles to facilitate thermal motion and improve the strength of the hollow particles, the solubility in distilled water at 20 °C is preferably 2 g / L or more, more preferably 10 g / L or more, still more preferably 15 g / L or more, even more preferably 20 g / L or more, and particularly preferably 50 g / L or more. The upper limit of the solubility of the second polymerizable monomer in distilled water at 20 °C is not particularly limited, but is usually 80 g / L or less.

[0070] In addition, from the viewpoint that the second polymerizable monomer is incorporated into the shell of the first precursor particles to facilitate thermal motion and improve the strength of the hollow particles, the molecular weight of the second polymerizable monomer is preferably 200 or less, more preferably 100 or less. The lower limit of the molecular weight of the second polymerizable monomer is not particularly limited and is usually 50 or more.

[0071] From the viewpoint of improving the strength of the hollow particles, as the second polymerizable monomer, at least one selected from the group consisting of (meth)acrylic acid alkyl esters having an alkyl group with 1 to 5 carbon atoms and (meth)acrylonitrile is preferable, and at least one selected from the group consisting of methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, and acrylonitrile is more preferable.

[0072] The addition amount of the second polymerizable monomer is preferably 3 to 15 parts by mass, more preferably 4 to 10 parts by mass, based on 100 parts by mass of the first polymerizable monomer. When the addition amount of the second polymerizable monomer is equal to or more than the above lower limit value, the effect of promoting the polymerization reaction by the addition of the second polymerizable monomer is improved, and the cross-linked structure of the shell of the hollow particles becomes denser, so that the strength of the hollow particles is improved and they are less likely to be crushed. On the other hand, when the addition amount of the second polymerizable monomer is equal to or less than the above upper limit value, a decrease in the content ratio of the first polymerizable monomer with respect to the total polymerizable monomer used for shell formation can be suppressed. Since the first polymerizable monomer contains a large amount of cross-linkable monomers, by suppressing a decrease in the content ratio of the first polymerizable monomer, it is possible to obtain hollow particles having excellent strength and containing a large amount of cross-linked structures formed by the cross-linkable monomers.

[0073] In the second polymerization reaction performed after adding the second polymerizable monomer, the polymerization method is not particularly limited. For example, the same polymerization method as that used in the first polymerization reaction can be adopted. In the second polymerization reaction, the polymerization temperature is preferably 40 to 80°C, more preferably 50 to 70°C. The reaction time of the second polymerization reaction is preferably 1 to 6 hours, more preferably 2 to 4 hours.

[0074] According to the above production method, the residual amount of the unreacted polymerizable monomer after the second polymerization reaction can be preferably 750 ppm or less, more preferably 500 ppm or less, and even more preferably 300 ppm or less. In the present disclosure, the residual amount of the unreacted polymerizable monomer after the second polymerization reaction is the ratio of the mass of the polymerizable monomer remaining unreacted to the solid content mass of the hollow particles obtained by the second polymerization reaction. The mass of the unreacted polymerizable monomer can be measured using gas chromatography (GC).

[0075] (4) Solid-liquid separation step This step involves obtaining a solid fraction containing the second precursor particles by solid-liquid separation of a second precursor composition containing hollow particles (second precursor particles) encapsulating a hydrocarbon solvent obtained by the polymerization step described above.

[0076] The method for solid-liquid separation of the second precursor composition is not particularly limited, and known methods can be used. Examples of the solid-liquid separation method include centrifugation, filtration, sedimentation separation, etc. Among these, centrifugation or filtration can be adopted, and centrifugation may be adopted from the viewpoint of ease of operation. After the solid-liquid separation step and before performing the solvent removal step described below, an optional step such as a preliminary drying step may be carried out. Examples of the preliminary drying step include a step of preliminarily drying the solid fraction obtained after the solid-liquid separation step using a drying device such as a dryer or a drying tool such as a hand dryer.

[0077] (5) Solvent removal step This step is a step of removing the hydrocarbon solvent encapsulated in the hollow particles (second precursor particles) obtained by the solid-liquid separation step. By removing the hydrocarbon solvent encapsulated in the second precursor particles in the air, the hydrocarbon solvent inside the second precursor particles is replaced with air, and hollow particles filled with gas are obtained.

[0078] Strictly speaking, "in the air" in this step means an environment where there is no liquid component at all outside the second precursor particles, and an environment where there is only a very small amount of liquid component outside the second precursor particles that does not affect the removal of the hydrocarbon solvent. "In the air" can also be rephrased as a state where the second precursor particles do not exist in a slurry, or a state where the second precursor particles exist in dry powder. That is, in this step, it is important to remove the hydrocarbon solvent in an environment where the second precursor particles are in direct contact with the external gas.

[0079] The method for removing the hydrocarbon solvent from the second precursor particles in air is not particularly limited, and any known method can be used, such as vacuum drying, heat drying, flash drying, or a combination of these methods. In particular, when a heat drying method is used, the heating temperature must be equal to or higher than the boiling point of the hydrocarbon solvent and equal to or lower than the maximum temperature at which the shell structure of the second precursor particles does not collapse. Therefore, depending on the shell composition of the second precursor particles and the type of hydrocarbon solvent, the heating temperature may be, for example, 50 to 200°C, 70 to 200°C, or 100 to 200°C. By the drying operation in air, the hydrocarbon solvent inside the second precursor particles is replaced by the external gas, resulting in hollow particles whose hollow portions are filled with gas.

[0080] 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.

[0081] As an alternative method, the hydrocarbon-based solvent may be removed by replacing the hydrocarbon-based solvent contained in the second precursor particles with the aqueous medium of the slurry containing the second precursor particles in a slurry containing the second precursor particles and an aqueous medium, without subjecting the slurry-like second precursor composition obtained in the polymerization step to solid-liquid separation. In this method, the hydrocarbon solvent contained in the second precursor particles can be removed by bubbling an inert gas through the second precursor composition at a temperature equal to or higher than the boiling point of the hydrocarbon solvent minus 35°C. Here, when the hydrocarbon solvent is a mixed solvent containing multiple types of hydrocarbon solvents and has multiple boiling points, the boiling point of the hydrocarbon solvent in the solvent removal step is the boiling point of the solvent with the highest boiling point among the solvents contained in the mixed solvent, i.e., the highest boiling point among the multiple boiling points. When bubbling an inert gas into the second precursor composition, the temperature is preferably a temperature equal to or higher than the boiling point of the hydrocarbon-based solvent minus 30°C, more preferably a temperature equal to or higher than the boiling point of the hydrocarbon-based solvent minus 20°C, from the viewpoint of reducing the residual amount of the hydrocarbon-based solvent in the hollow particles. Note that the temperature during bubbling is usually set to a temperature equal to or higher than the polymerization temperature in the polymerization step. Although not particularly limited, the temperature during bubbling may be set to 50°C or higher and 100°C or lower. The inert gas to be bubbled is not particularly limited, and examples thereof include nitrogen, argon, and the like. The bubbling conditions are appropriately adjusted according to the type and amount of the hydrocarbon-based solvent so that the hydrocarbon-based solvent encapsulated in the second precursor particles can be removed. Although not particularly limited, for example, the inert gas may be bubbled at a rate of 1 to 3 L / min for 1 to 10 hours. In this method, an aqueous slurry in which the second precursor particles encapsulate an aqueous medium can be obtained. By subjecting the hollow particles obtained by solid-liquid separation of this slurry to drying to remove the aqueous medium in the hollow particles, hollow particles in which the hollow portion is occupied by a gas can be obtained.

[0082] When comparing the method of obtaining hollow particles in which the hollow portion is filled with a gas by removing the hydrocarbon-based solvent in the second precursor particles in the gas phase after solid-liquid separation of the slurry-like second precursor composition with the method of obtaining hollow particles in which the hollow portion is filled with a gas by substituting the hydrocarbon-based solvent encapsulated in the second precursor particles with the aqueous medium of the slurry in a slurry containing the second precursor particles and the aqueous medium, followed by solid-liquid separation and removing the aqueous medium in the second precursor particles in the gas phase, the former method has the advantage that the hollow particles are less likely to be crushed in the step of removing the hydrocarbon-based solvent, and the latter method has the advantage that the residual amount of the hydrocarbon-based solvent is reduced by performing bubbling using an inert gas. When replacing the hydrocarbon solvent encapsulated in the second precursor particles with water, if the same volume of water as the hydrocarbon solvent that has escaped from the particles does not enter the particles, there is a problem that the resulting hollow resin particles collapse. As a means of preventing this, for example, it is conceivable to make the pH of the slurry 7 or higher and then remove the hydrocarbon solvent after swelling the shell of the particles with an alkali. In this case, since the shell of the particles acquires flexibility, the replacement of the hydrocarbon solvent inside the particles with water proceeds rapidly.

[0083] (6) Others As steps other than the above (1) to (5), for example, the following (6-a) washing step or the following (6-b) re-replacement step of the hollow part may be added. (6-a) Washing step The washing step is a step of adding an acid or an alkali for washing in order to remove the dispersion stabilizer remaining in the second precursor composition containing the second precursor particles before the solvent removal step. When the used dispersion stabilizer is an inorganic dispersion stabilizer soluble in an acid, it is preferable to add an acid to the second precursor composition containing the second precursor particles for washing. On the other hand, when the used dispersion stabilizer is an inorganic compound soluble in an alkali, it is preferable to add an alkali to the second precursor composition containing the second precursor particles for washing. Further, when an inorganic dispersion stabilizer soluble in an acid is used as the dispersion stabilizer, it is preferable to add an acid to the second precursor composition containing the second precursor particles and adjust the pH to preferably 6.5 or lower, more preferably 6 or lower. As the acid to be added, inorganic acids such as sulfuric acid, hydrochloric acid, and nitric acid, and organic acids such as formic acid and acetic acid can be used. However, sulfuric acid is particularly suitable because of its high removal efficiency of the dispersion stabilizer and the small burden on the manufacturing equipment.

[0084] (6-b) Re-replacement step of the hollow part The re-replacement step of the hollow part is a step of replacing the gas or liquid inside the hollow particles with another gas or liquid. By such replacement, the environment inside the hollow particles can be changed, molecules can be selectively confined inside the hollow particles, or the chemical structure inside the hollow particles can be modified according to the application.

[0085] 2.Hollow particles The hollow particles of the present disclosure are hollow particles having a shell containing a resin and a hollow portion surrounded by the shell, and having a porosity of 50% or more, the shell contains, as the resin, a polymer containing 70 to 100 parts by mass of crosslinkable monomer units in 100 parts by mass of all monomer units, In a hollow particle immersion test in which 0.1 mg of hollow particles are added to 4 mL of acetone in a 25°C environment, shaken at 100 rpm for 10 minutes, and then left to stand for 48 hours, less than 5% by mass of hollow particles precipitate in the acetone. In the above immersion test, it is believed that the fewer hollow particles precipitated in acetone, the denser the shell structure is, which makes it more difficult for acetone to penetrate.

[0086] Furthermore, in SEM observation of the hollow particles of the present disclosure, it is preferable that 5 or less of 100 hollow particles have interconnected pores or shell defects. Generally, hollow particles are classified into those in which the shell does not have any interconnecting holes that connect the hollow portion to the external space of the particle, and those in which the shell has one or more interconnecting holes through which the hollow portion communicates with the outside of the particle. Although the diameter of the interconnecting holes varies depending on the size of the hollow particle, it is usually about 10 to 500 nm. While the interconnecting holes can impart beneficial functions to hollow particles, they can also reduce the strength of the hollow particles and make them more susceptible to crushing because they are missing portions of the shell. Furthermore, hollow particles may have shell defects in the form of cracks that are extremely large compared to the particle size. Although it depends on the size of the hollow particle, cracks with a length of 1 μm or more generally significantly reduce the strength of the hollow particle and are therefore recognized as shell defects. Furthermore, in the above-described hollow particle immersion test, if less than 5% by mass of hollow particles precipitate in acetone, it can be considered that 5 or fewer hollow particles have interconnected pores or shell defects out of 100 hollow particles. Even if the shell has no interconnected pores or shell defects, the above-described hollow particle immersion test may result in 5% or more by mass of the precipitated hollow particles. Therefore, in the above-described hollow particle immersion test, if less than 5% by mass of hollow particles precipitate, this is considered to indicate that the shell has very few interconnected pores and shell defects and a dense crosslinked structure.

[0087] The hollow particles of the present disclosure contain, as a resin in the shell, a polymer containing 70 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of all monomer units. The polymer forms the skeleton of the shell of the hollow particle. When the content of the crosslinkable monomer units in the polymer is less than 100 parts by mass, the monomer units other than the crosslinkable monomer units are non-crosslinkable monomer units. In the hollow particles of the present disclosure obtained by the above-described hollow particle production method, the polymer is a polymer of a first polymerizable monomer and a second polymerizable monomer obtained by a first polymerization reaction and a second polymerization reaction. In the hollow particles of the present disclosure, the crosslinkable monomer units and non-crosslinkable monomer units contained in the polymer are usually derived from the first polymerizable monomer and the second polymerizable monomer. In the above polymer, the content of the crosslinkable monomer unit in 100 parts by mass of all monomer units is preferably 75 parts by mass or more, more preferably 85 parts by mass or more, from the viewpoint of improving the strength of the hollow particles and making them less likely to be crushed. On the other hand, when a hydrophilic non-crosslinkable monomer is added as the second polymerizable monomer, the content of the crosslinkable monomer unit in 100 parts by mass of all monomer units is preferably 98 parts by mass or less, more preferably 96 parts by mass or less, from the viewpoint of adding a sufficient amount of the second polymerizable monomer.

[0088] The above polymer preferably contains, as a crosslinkable monomer unit, a crosslinkable monomer unit derived from at least a bifunctional crosslinkable monomer, and more preferably contains a combination of a crosslinkable monomer unit derived from a bifunctional crosslinkable monomer and a crosslinkable monomer unit derived from a crosslinkable monomer having a functionality of 3 or more. In the present disclosure, the crosslinkable monomer unit derived from a bifunctional crosslinkable monomer may be referred to as a "bifunctional crosslinkable monomer unit", and the crosslinkable monomer unit derived from a crosslinkable monomer having a functionality of 3 or more may be referred to as a "crosslinkable monomer unit having a functionality of 3 or more". In 100 parts by mass of all the monomer units of the above polymer, the content of the bifunctional crosslinkable monomer unit is not particularly limited, but the lower limit is preferably 50 parts by mass or more, more preferably 60 parts by mass or more, still more preferably 70 parts by mass or more, and even more preferably 75 parts by mass or more, and the upper limit is preferably 100 parts by mass or less, more preferably 98 parts by mass or less, still more preferably 95 parts by mass or less, and even more preferably 90 parts by mass or less. When the above polymer contains a crosslinkable monomer unit having a functionality of 3 or more, in 100 parts by mass of all the monomer units of the above polymer, the content of the crosslinkable monomer unit having a functionality of 3 or more is not particularly limited, but the lower limit is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, still more preferably 15 parts by mass or more, and the upper limit is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, still more preferably 30 parts by mass or less, and even more preferably 25 parts by mass or less.

[0089] The above polymer preferably further contains a non-crosslinkable monomer unit, more preferably contains a hydrophilic non-crosslinkable monomer unit having a solubility in distilled water at 20 °C of 0.3 g / L or more, and particularly preferably contains a hydrophilic non-crosslinkable monomer unit derived from the above second polymerizable monomer. By the above polymer containing a combination of a crosslinkable monomer unit and a non-crosslinkable monomer unit, the mechanical properties of the shell of the hollow particles are improved. In the above polymer, the content of the non-crosslinkable monomer unit in 100 parts by mass of all the monomer units is 0 to 30 parts by mass, and from the viewpoint of improving the strength of the hollow particles and making them less likely to collapse, it is preferably 2 to 25 parts by mass, more preferably 4 to 15 parts by mass. In addition, in the above polymer, the content of the hydrophilic non-crosslinkable monomer units in 100 parts by mass of all monomer units is preferably 2 to 15 parts by mass, more preferably 3 to 13 parts by mass, and even more preferably 4 to 10 parts by mass, in order to improve the strength of the hollow particles and make them less likely to be crushed.

[0090] The shell of the hollow particles according to the present disclosure preferably further contains a particle size control agent, which preferably contains at least one selected from the group consisting of rosin acid, higher fatty acids, and metal salts thereof, or the polar resin, more preferably contains at least one selected from the group consisting of rosin acid, higher fatty acids, and metal salts thereof, and even more preferably contains at least one selected from rosin acid and alkali metal salts thereof. The presence of the particle size control agent in the shell of the hollow particles and the amount of the particle size control agent contained therein can be confirmed by, for example, pyrolysis gas chromatography.

[0091] When the shell of the hollow particles according to the present disclosure contains, as a particle size control agent, at least one selected from the group consisting of rosin acid, higher fatty acid, and metal salts thereof, the total content of the rosin acid, higher fatty acid, and metal salt thereof in the shell is preferably 0.0001 to 0.1% by mass, and more preferably 0.001 to 0.01% by mass. On the other hand, when the shell of the hollow particles of the present disclosure contains the above polar resin as a particle size control agent, the content of the above polar resin in the shell is preferably 0.1 to 10.0 mass %, more preferably 0.3 to 8.0 mass %. When the content of the particle size control agent is equal to or greater than the lower limit, the particle size and shell thickness of the hollow particles can be easily controlled, and the strength of the hollow particles can be improved. On the other hand, when the content of the particle size control agent is equal to or less than the upper limit, a decrease in the polymer content can be suppressed, which in turn suppresses a decrease in shell strength and further suppresses collapse of the hollow particles.

[0092] The shape of the hollow particles of the present disclosure is not particularly limited as long as a hollow portion is formed inside, and examples thereof include spherical, ellipsoidal, and amorphous shapes. Among these, a spherical shape is preferable from the viewpoint of ease of production. The hollow particles may have one or two or more hollow portions. Further, the shell of the hollow particles and the partition walls that partition adjacent hollow portions when there are two or more hollow portions may be porous. In order to maintain a good balance between the high porosity of the hollow particles and the mechanical strength of the hollow particles, it is preferable that the hollow particles have only one hollow portion. The hollow particles may have an average circularity of 0.950 to 0.995. An example of the image of the shape of the hollow particles is a bag made of a thin film and inflated with gas, and its cross-sectional view is as shown by the hollow particle 100 in (5) of FIG. 1. In this example, a single thin film is provided on the outside, and its interior is filled with gas. The particle shape can be confirmed by, for example, SEM or TEM. Further, the shape inside the particles can be confirmed by SEM or TEM after cutting the particles into slices by a known method.

[0093] The lower limit of the volume average particle diameter of the hollow particles is preferably 4.0 μm or more, more preferably 4.5 μm or more, and even more preferably 5.0 μm or more. On the other hand, the upper limit of the volume average particle diameter of the hollow particles is preferably 60.0 μm or less, more preferably 55.0 μm or less, and even more preferably 50.0 μm or less. When the volume average particle diameter of the hollow particles is equal to or greater than the above lower limit value, the cohesiveness between the hollow particles becomes small, so that excellent dispersibility can be exhibited. Further, when the volume average particle diameter of the hollow particles is equal to or less than the above upper limit value, the hollow particles are less likely to be crushed, so that they have high mechanical strength.

[0094] The particle size distribution of the hollow particles (volume average particle size (Dv) / number average particle size (Dn)) may be, for example, 1.1 or more and 2.5 or less. When the particle size distribution is 2.5 or less, particles with little variation in compressive strength characteristics and heat resistance among particles can be obtained. Furthermore, when producing a sheet-like molded product, for example, a product with a uniform thickness can be produced. The volume average particle size (Dv) and number average particle size (Dn) of the hollow particles can be determined by, for example, measuring the particle size of the hollow particles using a laser diffraction particle size distribution analyzer, calculating the number average and volume average, and using the resulting values as the number average particle size (Dn) and volume average particle size (Dv) of the particles. The particle size distribution is calculated by dividing the volume average particle size by the number average particle size.

[0095] The hollow particles of the present disclosure have a porosity of 50% or more, preferably 60% or more. When the porosity is equal to or greater than the above lower limit, the hollow particles have excellent lightness, heat resistance, and heat insulation properties. The upper limit of the porosity of the hollow particles of the present disclosure is not particularly limited, but is preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less, in order to prevent a decrease in the strength of the hollow particles and make them less likely to be crushed.

[0096] The porosity of the hollow particles of the present disclosure is calculated from the apparent density D1 and true density D0 of the hollow particles. The method for measuring the apparent density D1 of hollow particles is as follows. First, 3 Approximately 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 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 D1 (g / cm) of the hollow particles is calculated based on the following formula (I): 3 ) is calculated. Formula (I) Apparent density D1 = [Mass of hollow particles] / (100 - [Mass of isopropanol] ÷ [Specific gravity of isopropanol at measurement temperature]) The apparent density D1 corresponds to the specific gravity of the entire hollow particle when the hollow part is regarded as a part of the hollow particle.

[0097] The method for measuring the true density D0 of the hollow particle is as follows. After previously pulverizing the hollow particle, about 10 g of the pulverized pieces of the hollow particle are filled into a graduated flask with a volume of 100 cm 3 . The mass of the filled pulverized pieces is accurately weighed. Then, similar to the measurement of the above apparent density, isopropanol is added to the graduated flask, the mass of the isopropanol is accurately weighed, and based on the following formula (II), the true density D0 (g / cm 3 ) of the hollow particle is calculated. Formula (II) True density D0 = [mass of the pulverized pieces of the hollow particle] / (100 - [mass of the isopropanol] ÷ [specific gravity of the isopropanol at the measurement temperature]) The true density D0 corresponds to the specific gravity of only the shell part of the hollow particle. As is clear from the above measurement method, in calculating the true density D0, the hollow part is not regarded as a part of the hollow particle.

[0098] The porosity (%) of the hollow particle is calculated by the following formula (III) based on the apparent density D1 and the true density D0 of the hollow particle. Formula (III) Porosity (%) = 100 - (apparent density D1 / true density D0) × 100 The porosity of the hollow particle can be rephrased as the ratio occupied by the hollow part in the specific gravity of the hollow particle.

[0099] The thickness of the shell of the hollow particle of the present disclosure is preferably 0.1 μm or more, more preferably 0.2 μm or more, still more preferably 0.3 μm or more as the lower limit, and preferably 6 μm or less, more preferably 5 μm or less, still more preferably 4 μm or less as the upper limit. When the thickness of the shell of the hollow particle is at least the above lower limit value, the strength of the shell is improved. On the other hand, since the hollow particle of the present disclosure has a dense structure in which acetone hardly penetrates the shell, even if the thickness of the shell is thin and less than the above lower limit value, the hollow particle has excellent strength and is difficult to be crushed. Incidentally, the thickness of the shell of the hollow particles can be calculated by the following formula (1) using the volume average particle diameter R and the porosity of the hollow particles to calculate the inner diameter r of the hollow particles, and can be calculated by the following formula (2) using the inner diameter r and the volume average particle diameter R. Incidentally, the porosity in the following formula (1) is a numerical value when expressed as a ratio. 4 / 3π×(R / 2) 3 ×porosity = 4 / 3π×(r / 2) 3 Formula (1) Shell thickness = (R - r) / 2 Formula (2) The difference between the thickness of the shell calculated in this way and the average value of the thicknesses at 20 points of the actually measured shell is usually within ±10% of these average values. Therefore, the thickness of the shell calculated as described above can be regarded as the thickness of the shell of the hollow particles. The thickness at each point of the shell of the hollow particles used when obtaining the average value of the thicknesses at 20 points of the shell can be measured, for example, by observing the fragments of the shell obtained by splitting the hollow particles with SEM.

[0100] The hollow particles of the present disclosure are difficult to be crushed during kneading with other materials and during molding after kneading, and when added to a molded body, they are excellent in effects as a weight reducing material, a heat insulating material, a sound insulating material, a vibration damping material, etc., and are particularly suitable as an additive for a molded body. The hollow particles of the present disclosure are difficult to be crushed even during kneading with a resin and during molding after kneading, and are particularly preferably used as an additive for a resin molded body. Further, the molded body containing the hollow particles of the present disclosure may further contain organic or inorganic fibers such as carbon fiber, glass fiber, aramid fiber, and polyethylene fiber. The hollow particles of the present disclosure can also be contained as a filler in a molded body formed using a thermoplastic or thermosetting resin, and in a molded body formed using a material containing a thermoplastic or thermosetting resin and further containing fibers. Incidentally, the uses of the resin molded body containing the hollow particles of the present disclosure will be described in detail later. Furthermore, the hollow particles of the present disclosure have high porosity, are not easily crushed, and have excellent heat resistance, so they meet the heat insulation and shock-absorbing properties (cushioning properties) required for undercoating materials and also meet the heat resistance required for thermal paper applications.The hollow particles of the present disclosure are also useful as plastic pigments that are excellent in gloss, hiding power, etc. Furthermore, the hollow particles of the present disclosure can be used for various purposes depending on the components contained therein, since useful components such as fragrances, medicines, agricultural chemicals, and ink components can be encapsulated therein by means of immersion treatment, reduced pressure or pressure immersion treatment, or the like.

[0101] 3.Resin composition A resin composition containing hollow particles of the present disclosure contains at least the hollow particles of the present disclosure and a resin. The resin composition is typically obtained by kneading the hollow particles of the present disclosure, the resin, and additives, etc., which are added as needed, and may be in the form of pellets, for example. In a resin composition containing hollow particles of the present disclosure, the hollow particles of the present disclosure are resistant to crushing during kneading and molding after kneading, thereby providing effects such as weight reduction due to the hollow particles.

[0102] The resin used in the resin composition is not particularly limited, but is preferably a thermoplastic resin or a thermosetting resin. The thermoplastic resin may be any known one, and is not particularly limited. Examples of the thermoplastic resin include polyolefins such as polypropylene and polyethylene; polyamides such as PA6, PA66, and PA12; polyimide, polyamideimide, polyvinyl chloride, polystyrene, poly(meth)acrylate, polycarbonate, polyvinylidene fluoride, acrylonitrile-butadiene-styrene copolymer (ABS), acrylonitrile-styrene copolymer (AS), polyphenylene ether, polyphenylene sulfide, polyester, polytetrafluoroethylene, and thermoplastic elastomers. These thermoplastic resins can be used either alone or in combination of two or more. As the thermosetting resin, known resins can be used without particular limitation. For example, phenolic resins, melamine resins, urea resins, unsaturated polyester resins, epoxy resins, polyurethane resins, silicone resins, alkyd resins, etc. can be mentioned. These thermosetting resins can be used individually or in combination of two or more. When the resin composition contains a thermosetting resin, it may further contain a crosslinking agent for crosslinking the thermosetting resin by heat, a solvent for dissolving or dispersing each component, etc. as necessary. As the crosslinking agent, known ones can be used and are appropriately selected according to the type of thermosetting resin.

[0103] In 100% by mass of the total mass of the above resin composition, the content of the resin is not particularly limited, but is usually 70% by mass or more and 99% by mass or less. When the content of the resin is at least the lower limit value, the moldability is excellent when the resin composition is made into a molded body, and the mechanical strength of the obtained molded body is excellent. On the other hand, when the content of the resin is at most the upper limit value, the hollow particles of the present disclosure can be sufficiently contained, so that the resin composition can be lightened. In 100% by mass of the total mass of the above resin composition, the content of the hollow particles of the present disclosure is not particularly limited, but is usually 1% by mass or more and 30% by mass or less. When the content of the hollow particles is at least the lower limit value, the resin composition can be sufficiently lightened. On the other hand, when the content of the hollow particles is at most the upper limit value, the resin can be sufficiently contained, so that the moldability can be improved.

[0104] In addition to the hollow particles and resin of the present disclosure, the above resin composition may further contain additives such as ultraviolet absorbers, colorants, heat stabilizers, fillers, etc. as necessary within a range not impairing the effects of the present disclosure. Also, the above resin composition may further contain organic or inorganic fibers such as carbon fibers, glass fibers, aramid fibers, polyethylene fibers, etc. Examples of uses of the resin composition containing the hollow particles of the present disclosure include uses similar to those of molded articles described below.

[0105] The resin composition can be obtained, for example, by mixing the hollow particles of the present disclosure, the resin, and additives that are further added as needed, followed by kneading. The kneading can be carried out by a known method, and is not particularly limited, but can be carried out using a kneading device such as a single-screw kneader or a twin-screw kneader. When the resin in the resin composition is a thermoplastic resin, the kneading is melt-kneading, which is performed by heating the resin composition to melt the thermoplastic resin. The temperature during the melt-kneading is not particularly limited as long as it is a temperature at which the thermoplastic resin used can be melted, but is preferably 250°C or less in order to prevent the hollow particles from being crushed. When the resin composition is made into pellets, for example, this is usually the case when the resin in the resin composition is a thermoplastic resin, and after the melt-kneading, the resin composition can be molded into pellets by a known molding method such as extrusion molding or injection molding. On the other hand, when the resin in the resin composition is a thermosetting resin, the kneading may be carried out in a temperature environment below the curing temperature of the thermosetting resin, and is not particularly limited, but is usually carried out in a temperature environment of 180°C or higher and 240°C or lower.

[0106] 4. Molded body Examples of molded articles containing the hollow particles of the present disclosure include articles molded from the resin composition described above. The molded article of the resin composition contains the hollow particles of the present disclosure, which are not easily crushed, and thus the effects of the hollow particles, such as weight reduction, can be effectively exhibited.

[0107] When the resin composition contains the thermoplastic resin, the molded article of the resin composition can be obtained, for example, by melt-kneading the resin composition and then molding it into a desired shape by a known molding method such as extrusion molding, injection molding, press molding, compression molding, etc. Note that the melt-kneading method performed when obtaining the molded article may be the same as the melt-kneading method performed when obtaining the resin composition. Since the hollow particles in the resin composition are difficult to be crushed, even when a molding method performed under heating and pressurizing conditions such as injection molding and compression molding is used, a molded article with the crushing of the hollow particles suppressed can be obtained.

[0108] On the other hand, when the resin composition contains the thermosetting resin, for example, a molded article can be obtained by applying the resin composition to a support, drying it if necessary, and then curing it by heating. Examples of the material of the support include resins such as polyethylene terephthalate and polyethylene naphthalate; metals such as copper, aluminum, nickel, chromium, gold, and silver. As a method for applying the resin composition containing the thermosetting resin, a known method can be used, and examples thereof include dip coating, roll coating, curtain coating, die coating, slit coating, and gravure coating. When the resin composition contains a solvent, it is preferable to dry the resin composition after the application. The drying temperature is preferably set to a temperature at which the resin composition does not cure from the viewpoint of removing the solvent while keeping the resin composition in an uncured or semi-cured state, and is usually 20°C or higher and 200°C or lower, preferably 30°C or higher and 150°C or lower. Also, the drying time is usually 30 seconds or longer and 1 hour or shorter, preferably 1 minute or longer and 30 minutes or shorter. The heating temperature for curing the resin composition is appropriately adjusted according to the type of the thermosetting resin and is not particularly limited, but is usually 30°C or higher and 400°C or lower, preferably 70°C or higher and 300°C or lower, more preferably 100°C or higher and 200°C or lower. Also, the curing time is 5 minutes or longer and 5 hours or shorter, preferably 30 minutes or longer and 3 hours or shorter. The heating method is not particularly limited, and for example, an electric oven can be used.

[0109] The shape of the molded body is not particularly limited and can be various shapes that can be molded using the above resin composition. For example, it can be any shape such as sheet-like, film-like, plate-like, tube-like, and other various three-dimensional shapes. Further, when the molded body contains fibers, the fibers in the molded body may be in a non-woven fabric form. Further, when the molded body contains fibers, it may be a molded body of a resin composition in which the hollow particles of the present disclosure are added to a fiber-reinforced plastic containing the resin and fibers as described above. Examples of the uses of the molded body containing the hollow particles of the present disclosure include members such as light reflectors, heat insulators, sound insulators, and low dielectrics used in various fields such as automobiles, electrics, electronics, architecture, aviation, and space, food containers, footwear such as sports shoes and sandals, home appliance parts, bicycle parts, stationery, tools, and the like.

Examples

[0110] Hereinafter, the present disclosure will be described more specifically with reference to Examples and Comparative Examples, but the present disclosure is not limited to only these Examples. In addition, parts and % are based on mass unless otherwise specified.

[0111] [Example 1] (1) Mixed solution preparation step First, the following materials were mixed to form an oil phase. First polymerizable monomer: 80 parts of ethylene glycol dimethacrylate and 20 parts of pentaerythritol tetraacrylate [[ID=2,4]]2,2'-azobis(2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator, manufactured by Wako Pure Chemical Industries, Ltd., trade name: V-65) 3 parts Rosin acid (manufactured by Arakawa Chemical Industries, Ltd., trade name: disproportionated rosin R-CH, softening point 150 °C or higher, acid value: 150-160 mgKOH / g) 0.007 part Cyclohexane 187 parts Next, in a stirring tank, at room temperature, an aqueous solution prepared by dissolving 17.1 parts of magnesium chloride (water-soluble polyvalent metal salt) in 494 parts of ion-exchanged water was gradually added with stirring to an aqueous solution prepared by dissolving 12.1 parts of sodium hydroxide (alkali metal hydroxide) in 121 parts of ion-exchanged water to prepare a magnesium hydroxide colloid (water-insoluble metal hydroxide colloid) dispersion (4 parts of magnesium hydroxide), which was used as the aqueous phase. A mixed solution was prepared by mixing the obtained aqueous phase and oil phase.

[0112] (2) Suspension step The mixed solution obtained in the above mixed solution preparation step was stirred for 1 minute at a rotation speed of 4,000 rpm by a disperser (manufactured by Primix Corporation, trade name: Homomixer) to be suspended, and a suspension in which droplets of the monomer composition containing cyclohexane were dispersed in water was prepared.

[0113] (3) Polymerization step The suspension obtained in the above suspension step was heated from 40°C to 65°C over 30 minutes in a nitrogen atmosphere (heating rate: 50°C / hour), and stirred at a temperature of 65°C for 1 hour and 30 minutes to carry out the first polymerization reaction, obtaining a first precursor composition containing first precursor particles. The polymerization conversion rate at the end of the first polymerization reaction was 99.2% by mass. Subsequently, 5 parts of methyl acrylate was added to the stirring tank as the second polymerizable monomer, and the second polymerization reaction was carried out by stirring at a temperature of 65°C for 2 hours and 30 minutes under a nitrogen atmosphere. By this second polymerization reaction, a second precursor composition containing second precursor particles encapsulating cyclohexane was obtained.

[0114] (4) Washing step and solid-liquid separation step The above second precursor composition was washed with dilute sulfuric acid (25°C, 10 minutes) to adjust the pH to 5.5 or less. Then, after separating water by filtration, 200 parts of fresh ion-exchanged water was added for reslurrying, and the water washing treatment (washing, filtration, dehydration) was repeated several times at room temperature (25°C), followed by filtration separation to obtain a solid content. The obtained solid content was dried at a temperature of 40°C by a dryer to obtain second precursor particles encapsulating cyclohexane.

[0115] (5) Solvent removal step The second precursor particles obtained in the solid-liquid separation step were heat-treated in a vacuum dryer at 200 °C under vacuum conditions for 6 hours to remove the hydrocarbon solvent encapsulated in the particles, and the hollow particles of Example 1 were obtained. From the observation results of a scanning electron microscope and the value of the porosity, it was confirmed that these particles were spherical and had a hollow portion.

[0116] [Examples 2-5] In Example 1, except that the material of the second polymerizable monomer added in the above “(3) Polymerization step” was as shown in Table 1, hollow particles of Examples 2-5 were produced in the same procedure as in Example 1.

[0117] [Example 6] In Example 1, except that the material and amount of the first polymerizable monomer in the above “(1) Mixed solution preparation step” were as shown in Table 1, hollow particles of Example 6 were produced in the same procedure as in Example 1.

[0118] [Examples 7-8, 10] In Example 1, except that the addition amount of methyl acrylate added as the second polymerizable monomer in the above “(3) Polymerization step” was as shown in Table 1, hollow particles of Examples 7-8, 10 were produced in the same procedure as in Example 1.

[0119] [Example 9] In Example 1, except that rosin acid as a particle size control agent was not added in the above “(1) Mixed solution preparation step”, hollow particles of Example 9 were produced in the same procedure as in Example 1.

[0120] [Comparative Example 1] In Example 1, except that the second polymerizable monomer was not added and the second polymerization reaction was not carried out in the above “(3) Polymerization step”, hollow particles of Comparative Example 1 were produced in the same procedure as in Example 1.

[0121] [Comparative Example 2] In Example 1, in the above “(3) Polymerization step”, except that 5 parts of styrene (solubility in distilled water at 20°C is 0.2 g / L) were added instead of 5 parts of methyl acrylate as the second polymerizable monomer, hollow particles of Comparative Example 2 were produced in the same procedure as in Example 1.

[0122] [Comparative Example 3] In Example 1, in the above “(3) Polymerization step”, the reaction time of the first polymerization reaction was changed from 1 hour 30 minutes to 30 minutes, and when the total polymerization conversion rate of ethylene glycol dimethacrylate and pentaerythritol tetraacrylate as the first polymerizable monomers reached 91.0% by mass, hollow particles of Comparative Example 3 were produced in the same procedure as in Example 1, except that the second polymerizable monomer was added to carry out the second polymerization reaction.

[0123] [Comparative Example 4] In Example 1, in the above “(1) Mixed solution preparation step”, hollow particles of Comparative Example 4 were produced in the same procedure as in Example 1, except that the materials and amounts of the first polymerizable monomers were as shown in Table 1.

[0124] [Comparative Example 5] In Example 1, in the above “(1) Mixed solution preparation step”, the materials and amounts of the first polymerizable monomers were as shown in Table 1, and in the above “(3) Polymerization step”, hollow particles of Comparative Example 5 were produced in the same procedure as in Example 1, except that the second polymerizable monomer was not added and the second polymerization reaction was not carried out.

[0125]

Table 1

[0126] [Evaluation] 1. Polymerization conversion rate In the polymerization process of each Example and Comparative Example, 50 g of the first precursor composition produced in the first polymerization reaction was collected and pressure-filtered to obtain the first precursor particles (containing water and a hydrocarbon solvent) contained in the first precursor composition, which were then weighed to the nearest 1 mg. Approximately 3 g of the precisely weighed first precursor particles were added with 27 g of ethyl acetate and stirred for 15 minutes, followed by the addition of 13 g of methanol and stirring for an additional 10 minutes. The resulting solution was allowed to stand to precipitate the insoluble matter, and the supernatant of this solution was collected as a measurement sample. 2 μL of the measurement sample was injected into a gas chromatograph, and the amount of polymerizable monomer in the measurement sample was quantified by gas chromatography (GC) under the following conditions. This was taken as the mass of the unreacted first polymerizable monomer. The first precursor particles obtained by pressure filtration were then dried at 200°C for 2 hours to remove water and the hydrocarbon solvent, and the mass of the solids content of the first precursor particles was determined. The polymerization conversion rate was then calculated using the following formula (A): Polymerization conversion rate (mass%)=100−(mass of unreacted first polymerizable monomer / mass of solid content of first precursor particles)×100 Formula (A) <GCの条件> Column: TC-WAX (0.25 mm x 30 m) Column temperature: 80℃ Injection temperature: 200℃ FID detection temperature: 200°C

[0127] Table 2 also shows the content (mass %) of each monomer unit in the polymer contained in the shell of the hollow particles obtained in each Example and Comparative Example. The hollow particles obtained in each example and comparative example were subjected to the following measurements and evaluations. The results are shown in Table 2.

[0128] 2. Volume average diameter of hollow particles The particle diameter of the hollow particles was measured using a laser diffraction particle size distribution analyzer (Shimadzu Corporation, trade name: SALD-2000), and the volume average was calculated to obtain the volume average particle diameter.

[0129] 3. Density and porosity of hollow particles 3-1. Measurement of apparent density of hollow particles First, about 30 cm 3 of hollow particles were filled into a graduated flask with a capacity of 100 cm 3 , and the mass of the filled hollow particles was accurately weighed. Next, while taking care not to let air bubbles in, the graduated flask filled with hollow particles was accurately filled with isopropanol up to the calibration mark. The mass of the isopropanol added to the graduated flask was accurately weighed, and based on the following formula (I), the apparent density D1 (g / cm 3 ) of the hollow particles was calculated. Formula (I) Apparent density D1 = [mass of hollow particles] / (100 - [mass of isopropanol] ÷ [specific gravity of isopropanol at the measurement temperature])

[0130] 3-2. Measurement of the true density of hollow particles After previously pulverizing the hollow particles, about 10 g of the pulverized pieces of the hollow particles were filled into a graduated flask with a capacity of 100 cm 3 , and the mass of the filled pulverized pieces was accurately weighed. After that, isopropanol was added to the graduated flask in the same manner as the measurement of the apparent density above, the mass of the isopropanol was accurately weighed, and based on the following formula (II), the true density D0 (g / cm 3 ) of the hollow particles was calculated. Formula (II) True density D0 = [mass of the pulverized pieces of hollow particles] / (100 - [mass of isopropanol] ÷ [specific gravity of isopropanol at the measurement temperature])

[0131] 3-3. Calculation of the porosity Based on the apparent density D1 and the true density D0 of the hollow particles, the porosity of the hollow particles was calculated according to the following formula (III). Formula (III) Porosity (%) = 100 - (apparent density D1 / true density D0) × 100

[0132] 4. Thickness of the shell of hollow particles Using the volume average particle diameter R and the porosity of the hollow particles, the inner diameter r of the hollow particles was calculated from the following formula (1), and using the inner diameter r and the volume average particle diameter R, the thickness of the shell of the hollow particles was calculated from the following formula (2). 4 / 3π×(R / 2) 3 × void fraction = 4 / 3π×(r / 2) 3 Equation (1) Shell thickness = (R - r) / 2 Equation (2)

[0133] 5. Immersion test Under the environment of 25°C, 0.1 mg of hollow particles was added to 4 mL of acetone, shaken for 10 minutes at a shaking speed of 100 rpm using a shaker, left standing for 48 hours, the ratio of the precipitated hollow particles was determined, and the evaluation was carried out according to the following evaluation criteria. In addition, the hollow particles precipitated in acetone were separated by a centrifuge, dried, and the mass of the hollow particles precipitated in acetone was measured. The ratio of the mass of the hollow particles precipitated in acetone to the mass of all the hollow particles immersed in acetone was calculated to determine the ratio of the precipitated hollow particles. (Evaluation criteria for immersion test) 〇: The precipitated hollow particles are less than 5% by mass ×: The precipitated hollow particles are 5% by mass or more

[0134] 6. Residual monomer amount 3 g of hollow particles were accurately weighed to the unit of 1 mg, 27 g of ethyl acetate was added and stirred for 15 minutes, then 13 g of methanol was added and stirred for another 10 minutes. The obtained solution was left standing to precipitate the insoluble matter, and the supernatant of this solution was collected as a measurement sample. 2 μl of the measurement sample was injected into a gas chromatograph, and the amount of unreacted polymerizable monomer in the measurement sample was quantified by gas chromatography (GC) under the following conditions. The content ratio of the unreacted polymerizable monomer contained in the hollow particles was calculated and used as the residual monomer amount. <GC conditions> Column: TC-WAX (0.25 mm × 30 m) Column temperature: 80°C Injection temperature: 200°C FID detection side temperature: 200°C

[0135] 7. Void remaining ratio in the molded body Polypropylene as a thermoplastic resin (manufactured by Mitsubishi Chemical, product name: MA1B, specific gravity 0.90 g / cm 3)90 parts and 10 parts of the hollow particles obtained in each example or each comparative example were mixed with a blender. Next, using a twin-screw kneader (manufactured by Toshiba Machine Co., Ltd., product name: TEM-35B), kneading was performed under the following kneading conditions, followed by extrusion and pelletization to obtain pellets of the resin composition. <Kneading conditions> Screw diameter 37 mm, L / D = 32 Screw rotation speed 250 rpm Resin temperature 190 °C Feed rate 20 kg / hour The obtained pellets of the resin composition were heated and dried at 80 °C for 6 hours, and then using an injection molding apparatus, molding was performed under the following molding conditions to obtain a molded body having dimensions of 80 mm × 10 mm × thickness 4 mm. <Molding conditions> Cylinder temperature: 230 °C Mold temperature: 40 °C Injection pressure: 70 MPa Using a: the specific gravity of the molded body after injection molding, b: the specific gravity (calculated value) of the molded body assuming that the voids were maintained, and c: the specific gravity (calculated value) of the molded body assuming that all the hollow particles were crushed, the void remaining ratio was calculated by the following formula (B). Void remaining ratio (%) = [1 - {(c - a) / (c - b)}] × 100 Formula (B) Note that the specific gravity of the molded body after injection molding was measured by the water displacement method in accordance with JIS K 7112. The specific gravity b of the molded body assuming that the voids were maintained was calculated by the following formula (C). b = 1 / {(P A / P G ) + (R A / R G )} Formula (C) In the calculation formula for obtaining b above, P A is the addition amount of the hollow particles, P G is the specific gravity of the hollow particles, R A is the addition amount of the thermoplastic resin, and R G is the specific gravity of the thermoplastic resin, respectively. The specific gravity c of the molded body assuming that all the hollow particles were crushed was calculated by the following formula (D). c = [R G × R A+{D0×P A ×(1 - P V / 100)}] / {R A +P A ×(1 - P V / 100)} Equation (D) In the calculation formula for obtaining the above c, R A is the addition amount of the thermoplastic resin, R G is the specific gravity of the thermoplastic resin, D0 is the true density of the hollow particles, P A is the addition amount of the hollow particles, P V is the porosity (%) of the hollow particles, respectively.

[0136]

Table 2

[0137] In Tables 1 and 2, the meanings of the abbreviations are as follows. MMA: Methyl methacrylate MA: Methyl acrylate EA: Ethyl acrylate BA: Butyl acrylate AN: Acrylonitrile ST: Styrene

[0138] [Discussion] As shown in Table 2 above, the hollow particles obtained in each comparative example had a high porosity of 65%, but the void remaining rate of the molded body of the resin composition containing the hollow particles was low, and the hollow particles were easily crushed. In Comparative Examples 1 to 5, since the hollow particles precipitated in acetone in the above immersion test were 5% by mass or more, it is presumed that the shell was insufficiently dense, and thus the hollow particles were easily crushed.

[0139] On the other hand, the hollow particles obtained in each example had a high porosity of 65%, and the void remaining rate of the molded body of the resin composition containing the hollow particles was high. The hollow particles had a high porosity and were difficult to crush. The hollow particles obtained in Examples 1 to 10 had a shell containing a polymer containing 70 to 100 parts by mass of a crosslinkable monomer unit per 100 parts by mass of all monomer units, and had a dense structure in which the hollow particles precipitating in acetone in the immersion test were less than 5% by mass. Therefore, even though they had a high porosity, they were presumed to be difficult to collapse.

Explanation of Signs

[0140] 1 Aqueous medium 2 Low-polarity material 3 Dispersion stabilizer 4 Monomer composition 4a Hydrocarbon solvent 4b Material other than hydrocarbon solvent 4c Polymerizable monomer dispersed in aqueous medium 5 Oil-soluble polymerization initiator 6 Shell 8 Hollow part 10 Droplet 20 Hollow particle encapsulating a hydrocarbon solvent in the hollow part (second precursor particle) 100 Hollow particle with the hollow part filled with gas

Claims

1. Hollow particles comprising a shell containing a resin and a hollow portion surrounded by the shell, having a porosity of 50% or more, wherein the shell contains, as the resin, 70 to 98 parts by mass of crosslinkable monomer units derived from a crosslinkable monomer and 2 to 15 parts by mass of hydrophilic non-crosslinkable monomer units derived from a hydrophilic non-crosslinkable monomer having a solubility in distilled water at 20 °C of 0.3 g / L or more, in 100 parts by mass of all monomer units, the crosslinkable monomer is a (meth)acrylic polymerizable monomer, the hydrophilic non-crosslinkable monomer is at least one selected from the group consisting of (meth)acrylic acid alkyl esters having an alkyl group having 1 to 5 carbon atoms, (meth)acrylamides and their derivatives, (meth)acrylonitrile, and non-crosslinkable monomers containing a polar group selected from a carboxyl group, a hydroxyl group, a sulfonic acid group, an amino group, a polyoxyethylene group, and an epoxy group, hollow particles in which, in an immersion test of hollow particles where 0.1 mg of the hollow particles is added to 4 mL of acetone, shaken at a shaking speed of 100 rpm for 10 minutes, and then allowed to stand for 48 hours, the hollow particles precipitating in the acetone are less than 5% by mass.

2. The hollow particles according to Claim 1, wherein the hydrophilic non-crosslinkable monomer is at least one selected from the group consisting of (meth)acrylic acid alkyl esters having an alkyl group having 1 to 5 carbon atoms and (meth)acrylonitrile.

3. The hollow particles according to Claim 1 or 2, wherein the crosslinkable monomer contains a hydrophilic crosslinkable monomer having a solubility in distilled water at 20 °C of 0.3 g / L or more.

4. The crosslinkable monomer consists of only a bifunctional crosslinkable monomer or contains a bifunctional crosslinkable monomer and a crosslinkable monomer having three or more functional groups, the bifunctional crosslinkable monomer is at least one selected from the group consisting of ethylene glycol di(meth)acrylate and pentaerythritol di(meth)acrylate, The hollow particles according to any one of claims 1 to 3, wherein the crosslinkable monomer having three or more functional groups is at least one selected from the group consisting of pentaerythritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, and dipentaerythritol poly(meth)acrylate.

5. The hollow particles according to claim 4, wherein the crosslinkable monomer having three or more functional groups is at least one selected from the group consisting of pentaerythritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, and ethoxylated pentaerythritol tetra(meth)acrylate.

6. The hollow particles according to claim 4 or 5, wherein the polymer contained in the shell contains, as the crosslinkable monomer unit, a crosslinkable monomer unit derived from a bifunctional crosslinkable monomer and a crosslinkable monomer unit derived from a crosslinkable monomer having three or more functional groups.

7. The hollow particles according to any one of claims 4 to 6, wherein the polymer contained in the shell contains, as the crosslinkable monomer unit, a crosslinkable monomer unit derived from a crosslinkable monomer having three or more functional groups, and the content of the crosslinkable monomer unit derived from the crosslinkable monomer having three or more functional groups is 5 to 50 parts by mass per 100 parts by mass of all monomer units contained in the polymer.

8. The hollow particles according to any one of claims 1 to 7, wherein the shell contains at least one selected from the group consisting of rosin acid, higher fatty acids, and metal salts thereof.

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