hollow particles

Abstract

Provided is a hollow particle having high porosity and excellent pressure resistance. A hollow particle including a shell containing a resin and a hollow section surrounded by the shell, wherein: the porosity is not less than 60%; the volume average particle diameter is 10 to 50 μm; the shell contains, as the resin, a polymer including 70 to 100 parts by mass of a crosslinkable monomer unit in 100 parts by mass of the total monomer units; the crosslinkable monomer unit includes a bifunctional crosslinkable monomer unit derived from a bifunctional crosslinkable monomer and a tri- or higher functional crosslinkable monomer unit derived from a tri- or higher functional crosslinkable monomer; and the ratio of particles having a recess having a size of 5 to 50% to the particle diameter is not more than 10%, when observed in plan view.

Description

[Technical Field]

[0001] The present disclosure relates to hollow particles. [Background technology]

[0002] Hollow particles (hollow resin particles) are particles with a cavity inside them, and compared with solid particles whose interiors are substantially filled with resin, they scatter light well and have low light transmittance, and therefore are widely used as organic pigments or hiding agents with excellent optical properties such as opacity and whiteness in applications such as water-based paints and paper coating compositions. In recent years, they have also been used as weight-reducing agents and heat-insulating agents for resins and paints used in various fields such as automobiles, electricity, electronics, and construction.

[0003] In order to improve the effects of various compositions and molded articles containing hollow particles, such as weight reduction, heat insulation, opacity, and whitening, it is desirable for the hollow particles to maintain a high porosity during mixing with other materials and molding after mixing. However, when the porosity of hollow particles is increased, the shell thickness becomes thin, making the particles more susceptible to crushing. Therefore, hollow particles that have a high porosity and are not easily crushed are desired.

[0004] Furthermore, the hollow particles are preferably spherical. Patent Documents 1 to 4 describe methods for obtaining spherical hollow particles using a polymerization method. Patent Document 1 describes that spherical hollow resin microparticles can be produced by a method that uses a reactive monomer having a radically polymerizable functional group such as a vinyl group and a crosslinkable functional group such as an epoxy group to prepare microcapsule-type polymer particles encapsulating an organic solvent by radical polymerization, followed by a step of crosslinking the resins that make up the microcapsule-type polymer particles with a crosslinking agent.The hollow resin microparticles described in Patent Document 1 are added to antireflection films used in displays, and have an average particle diameter of 100 nm or less for use in antireflection films.

[0005] Patent Document 2 describes that by using a combination of at least one monomer selected from the group consisting of monovinyl monomers and hydrophilic monomers and a crosslinkable monomer in a specific ratio, and further using an oil-soluble polymerization initiator as a polymerization initiator, the obtained hollow resin particles are not likely to be dented, and therefore the porosity of the hollow resin particles can be maintained high. Patent Document 2 describes a bifunctional crosslinkable monomer as a preferred crosslinkable monomer.

[0006] Patent Document 3 describes that by using a polyfunctional monomer having three or more polymerizable double bonds as a crosslinkable monomer, hollow resin particles with a high percentage of true spheres and a high hollowness can be obtained while maintaining strength. Patent Document 3 also describes that the crosslinkable monomer preferably does not contain a bifunctional monomer and is composed of a trifunctional or higher functional monomer.

[0007] Patent Document 4 describes that suspension polymerization is performed using a monomer composition containing a large proportion of a crosslinkable monomer, and the aqueous medium contained in the suspension after the polymerization reaction is replaced with a polar solvent, thereby removing the hydrocarbon solvent contained in the precursor particles in a short period of time and obtaining hollow resin particles with good porosity and compressive strength. Patent Document 4 describes a bifunctional crosslinkable monomer as a preferred crosslinkable monomer. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-89648 [Patent Document 2] International Publication No. 2019 / 26899 [Patent Document 3] Japanese Patent Publication No. 2020-33503 [Patent Document 4] Japanese Patent Publication No. 2020-132820 Summary of the Invention [Problem to be solved by the invention]

[0009] Increasing the porosity of hollow particles tends to reduce the shell thickness, resulting in a decrease in pressure resistance. However, for hollow particles with a large particle size, the shell thickness does not become too thin even when the porosity is increased, so it is expected that they will exhibit excellent pressure resistance while maintaining a high porosity. However, with hollow particles with a volume average particle size of 10 μm or more, pressure resistance may decrease even if the shell thickness is sufficient. An object of the present disclosure is to provide hollow particles having high porosity and excellent pressure resistance. [Means for solving the problem]

[0010] The present inventors have found that in hollow particles having a volume average particle size of 10 μm or more, the polymer forming the shell contains a large amount of crosslinkable monomer units, and the crosslinkable monomer units are derived from a bifunctional crosslinkable monomer and a trifunctional or higher crosslinkable monomer, and further the shell surface is smooth, so that the particles exhibit excellent pressure resistance even when the porosity is increased to 60% or more.

[0011] The present disclosure provides a hollow particle having a shell containing a resin and a hollow portion surrounded by the shell, The porosity is 60% or more, The volume average particle size is 10 to 50 μm, 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, the crosslinkable monomer units including bifunctional crosslinkable monomer units derived from a bifunctional crosslinkable monomer and trifunctional or higher crosslinkable monomer units derived from a trifunctional or higher crosslinkable monomer, The present invention provides hollow particles in which, when observed in a plan view, the ratio of particles having recesses each having a size that is 5 to 50% of the particle diameter is 10% or less.

[0012] In the hollow particles of the present disclosure, the content of the bifunctional crosslinkable monomer units is preferably 60 to 95 parts by mass based on 100 parts by mass of all monomer units of the polymer contained in the shell.

[0013] In the hollow particles of the present disclosure, the content of the tri- or higher functional crosslinkable monomer units is preferably 5 to 40 parts by mass based on 100 parts by mass of all monomer units of the polymer contained in the shell.

[0014] In the hollow particles of the present disclosure, the tri- or higher functional crosslinkable monomer unit may be a crosslinkable monomer unit derived from a methacrylic crosslinkable monomer having a methacryloyl group as a polymerizable functional group.

[0015] In the hollow particles of the present disclosure, the shell thickness is preferably 0.20 to 4.00 μm. [Effects of the Invention]

[0016] According to the present disclosure as described above, hollow particles having high porosity and excellent pressure resistance can be provided. [Brief explanation of the drawings]

[0017] [Figure 1] 1A to 1C are diagrams illustrating an example of a method for producing hollow particles according to the present disclosure. [Figure 2] FIG. 2 is a schematic diagram illustrating an embodiment of a suspension in a suspension step. DETAILED DESCRIPTION OF THE INVENTION

[0018] In the present disclosure, the use of "to" in a numerical range means that the numerical values ​​before and after it are included as the lower limit and upper limit. In the present disclosure, (meth)acrylate refers to each of acrylate and methacrylate, (meth)acrylic refers to each of acrylic and methacrylic, and (meth)acryloyl refers to each of acryloyl and methacryloyl. In the present disclosure, a polymerizable monomer is a compound having a functional group capable of addition polymerization (sometimes simply referred to as a polymerizable functional group in the present disclosure). In the present disclosure, a compound having an ethylenically unsaturated bond as the functional group capable of addition polymerization is generally used as the polymerizable monomer. Polymerizable monomers are classified into non-crosslinkable monomers and crosslinkable monomers. Non-crosslinkable monomers have only one polymerizable functional group, while crosslinkable monomers have two or more polymerizable functional groups and form crosslinks in the resin through a polymerization reaction.

[0019] 1.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, The porosity is 60% or more, The volume average particle size is 10 to 50 μm, 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, the crosslinkable monomer units including bifunctional crosslinkable monomer units derived from a bifunctional crosslinkable monomer and trifunctional or higher crosslinkable monomer units derived from a trifunctional or higher crosslinkable monomer, When observed in a plan view, the ratio of particles having recesses each having a size that is 5 to 50% of the particle diameter is 10% or less.

[0020] The hollow particles of the present disclosure are particles having a shell (outer shell) containing a resin and a hollow portion surrounded by the shell. In the present disclosure, the hollow portion is a hollow space that is clearly distinguishable from the shell of the hollow particle formed from a resin material. The shell of the hollow particle may have a porous structure, but in that case, the hollow portion has a size that allows it to be clearly distinguished from the numerous minute spaces uniformly dispersed within the porous structure. The hollow portion of the hollow particles can be confirmed by, for example, SEM observation of the cross section of the particles or TEM observation of the particles as they are. The hollow portion of the hollow particles may be filled with a gas such as air, may be in a reduced pressure state close to a vacuum, or may contain a solvent.

[0021] Because the thicker the shell of hollow particles, the greater their pressure resistance, hollow particles with a large particle size are expected to achieve both high porosity and excellent pressure resistance. However, conventional hollow particles with a volume average particle size of 10 μm or more sometimes exhibit reduced pressure resistance even when the shell is sufficiently thick. For example, as described in Patent Documents 2 and 4, when hollow particles having a volume average particle size of 10 μm or more are produced using only a bifunctional crosslinkable monomer as the crosslinkable monomer, there is a problem that depressions occur in the particles, resulting in a significant decrease in pressure resistance, as shown in Comparative Example 6 described below. On the other hand, as described in Patent Document 3, when hollow particles having a volume average particle size of 10 μm or more are produced using only a trifunctional crosslinkable monomer as the crosslinkable monomer, the resulting particles have a shell with pores or interconnected pores, as shown in Comparative Example 5 described below. As a result, when a resin composition containing the hollow particles is kneaded or injection-molded, the resin penetrates into the particles, which results in the problem that the weight of the resin composition or its molded article cannot be reduced. Furthermore, when hollow particles having a volume average particle size of 10 μm or more are produced by the method described in Patent Document 1, the crosslinking reaction in the shell does not proceed sufficiently because the crosslinking agent is water-soluble, resulting in a problem that the pressure resistance of the hollow particles is not sufficiently improved. The present inventors have found that hollow particles with a volume average particle size of 10 μm or more often have recesses in their shells, and that the presence of recesses in the shell significantly reduces pressure resistance. It is presumed that when hollow particles have recesses in their shells, pressure is concentrated in the recesses when external pressure is applied to the hollow particles, making the shell more likely to crack from the recesses. In contrast, the hollow particles of the present disclosure exhibit excellent pressure resistance despite having a volume average particle diameter of 10 to 50 μm. Because the hollow particles of the present disclosure have a volume average particle diameter of 10 to 50 μm, they have a sufficient shell thickness even when the porosity is as high as 60% or more. Furthermore, the polymer forming 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 all monomer units, and the crosslinkable monomer units contain difunctional crosslinkable monomer units and trifunctional or higher crosslinkable monomer units, thereby resulting in a crosslinked structure with excellent strength. Furthermore, when observed in a plan view, the hollow particles of the present disclosure have 10% or less of particles with recesses that are 5 to 50% of the particle diameter, and the shell surface is smooth, thereby suppressing a decrease in pressure resistance due to the recesses in the shell. Therefore, the hollow particles of the present disclosure have a shell with excellent strength and sufficient thickness, and furthermore, reduction in pressure resistance due to recesses is suppressed, so that they can exhibit excellent pressure resistance and are hollow particles that achieve both high porosity and excellent pressure resistance.

[0022] The hollow particles of the present disclosure have a porosity of 60% or more, preferably 65% ​​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, from the viewpoint of suppressing a decrease in the strength of the hollow particles and improving their pressure resistance.

[0023] The porosity of the hollow particles of the present disclosure can be 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 30cm into a measuring flask 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 portion is considered to be a part of the hollow particle.

[0024] The true density D0 of hollow particles is measured as follows: After crushing the hollow particles, 3 Approximately 10 g of crushed pieces of hollow particles are filled into the measuring flask, and the mass of the crushed pieces is accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol is added to the measuring flask, and the mass of the isopropanol is accurately weighed. The true density D0 (g / cm) of the hollow particles is calculated based on the following formula (II): 3 ) is calculated. Formula (II) True density D0 = [mass of crushed hollow particle fragments] / (100 - [mass of isopropanol] ÷ [specific gravity of isopropanol at measurement temperature]) The true density D0 corresponds to the specific gravity of only the shell portion of the hollow particle. As is clear from the above measurement method, the hollow portion is not considered to be part of the hollow particle when calculating the true density D0.

[0025] The porosity (%) of the hollow particles is calculated from the apparent density D1 and true density D0 of the hollow particles by the following formula (III). Formula (III) Porosity (%) = 100 - (apparent density D1 / true density D0) x 100

[0026] The hollow particles of the present disclosure have a volume average particle size of 10 μm or more and 50 μm or less, with a lower limit of preferably 13 μm or more, more preferably 15 μm or more, and an upper limit of preferably 40 μm or less, more preferably 30 μm or less. When the volume average particle size of the hollow particles is equal to or greater than the lower limit, excellent pressure resistance can be exhibited even with a high porosity. Furthermore, the tendency for the hollow particles to aggregate together is reduced, resulting in excellent dispersibility. When the volume average particle size of the hollow particles is equal to or less than the upper limit, variation in shell thickness can be suppressed.

[0027] The shell thickness of the hollow particles of the present disclosure is not particularly limited, but is preferably 0.20 μm or more, more preferably 0.50 μm or more, from the viewpoint of improving pressure resistance, and is preferably 4.00 μm or less, more preferably 2.00 μm or less, from the viewpoint of increasing porosity. In the present disclosure, the shell thickness of a hollow particle is a value calculated by calculating the inner diameter r of the hollow particle according to the following formula (1) using the volume average particle diameter R and porosity of the hollow particle, and then calculating the inner diameter r and volume average particle diameter R according to the following formula (2). 4 / 3π×(R / 2) 3 ×(porosity / 100)=4 / 3π×(r / 2) 3 Formula (1) Shell thickness = (Rr) / 2 Equation (2) The porosity in the above formula (1) is a numerical value expressed as a percentage.

[0028] 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 diameter (Dv) and number average particle diameter (Dn) of the hollow particles can be determined by, for example, measuring the particle diameter of the hollow particles using a particle size distribution analyzer, calculating the number average and volume average, and using the resulting values ​​as the number average particle diameter (Dn) and volume average particle diameter (Dv) of the particles. The particle size distribution is calculated by dividing the volume average particle diameter by the number average particle diameter.

[0029] The shape of the hollow particles of the present disclosure is not particularly limited as long as a hollow portion is formed inside, but from the viewpoint of pressure resistance, a spherical shape is preferred. The hollow particles of the present disclosure may have one or more hollow portions, but preferably have only one hollow portion in order to maintain a good balance between high porosity and mechanical strength. The shells of the hollow particles of the present disclosure, and the partition walls separating adjacent hollow portions when the particles have two or more hollow portions, may be porous, but are preferably solid in order to prevent a decrease in pressure resistance.

[0030] The hollow particles of the present disclosure preferably have an average circularity of 0.950 to 1.000, more preferably 0.970 to 1.000. The circularity is defined as the circumferential length of a circle having the same projected area as the particle image divided by the circumferential length of the projected image of the particle. The average circularity in the present invention is used as a simple method for quantitatively expressing the particle shape; the average circularity is 1 when the hollow particles are perfectly spherical, and the more complex the surface shape of the hollow particles, the smaller the value. An example of the shape of a hollow particle disclosed herein is a bag made of a thin membrane and inflated with gas, and its cross section is shown as hollow particle 100 in (5) of Figure 1. In this example, a single thin membrane is provided on the outside, and the inside is filled with gas. The particle shape can be confirmed by, for example, SEM or TEM. The internal shape of the particle and the presence of fine resin particles inside the particle can be confirmed by SEM or TEM after slicing the particle into slices by a known method.

[0031] When the hollow particles of the present disclosure are observed in a plan view, the proportion of particles having recesses each having a size that is 5 to 50% of the particle diameter is 10% or less, and preferably less than 5%. When observed in a plan view, hollow particles having recesses that are 5% or more of the particle diameter tend to have reduced pressure resistance due to the recesses, while hollow particles having recesses that are more than 50% of the particle diameter typically have a porosity of less than 60%, resulting in an insufficient porosity. Therefore, for hollow particles of the present disclosure having a porosity of 60% or more, by setting the proportion of particles having recesses that are 5 to 50% of the particle diameter when observed in a plan view to the above-mentioned upper limit or less, the reduction in pressure resistance due to the recesses is suppressed, and the hollow particles of the present disclosure can achieve both high porosity and excellent pressure resistance. It is believed that the recesses in hollow particles are likely to be formed after the polymerization reaction for forming the shell. To ensure that hollow particles have no recesses, it is effective to set, for example, the shell composition, the particle diameter of hollow particles, the type of hydrophobic solvent to be encapsulated in the particles when producing the hollow particles, the stirring speed of the suspension during the polymerization reaction when producing the hollow particles, the temperature rise rate, etc., to the preferred values ​​set forth in the present disclosure. It is believed that adjusting these factors can suppress distortion in the shell, thereby suppressing the formation of recesses.

[0032] The proportion of particles having recesses of 5 to 50% of the particle diameter when observed in a plan view can be determined by observing the hollow particles of the present disclosure after solvent removal in a plan view from any one direction. Planar observation is performed, for example, using a field emission scanning electron microscope (FE-SEM). When hollow particles of the present disclosure are obtained by the production method described below, the hollow particles after solvent removal refer to hollow particles obtained after the solvent removal step, i.e., hollow particles in a state in which no liquid is present either outside or within the hollow portion. The absence of liquid outside the hollow particles means that the hollow particles are a dry powder, as opposed to hollow particles present in a slurry. The absence of liquid within the hollow portion of the hollow particles means that the amount of liquid within the hollow portion is 0.5 parts by mass or less per 100 parts by mass of the hollow particles. The particle diameter of a hollow particle is the diameter of a circle having the same projected area as the particle image, and can be measured from an FE-SEM image of the hollow particle. The size of the recesses of a hollow particle is the maximum width of the recesses in a planar view, and can be measured from an FE-SEM image of the hollow particle. The percentage of hollow particles that have recesses that are 5 to 50% of the particle diameter when observed in a planar view can be measured by counting the number of particles that have recesses that are 5 to 50% of the particle diameter for 100 hollow particles randomly selected from an FE-SEM image of the hollow particles.

[0033] The hollow particles according to the present disclosure contain, as the 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, and the crosslinkable monomer units include bifunctional crosslinkable monomer units derived from bifunctional crosslinkable monomers and trifunctional or higher crosslinkable monomer units derived from trifunctional or higher functional crosslinkable monomers. The above polymer forms the skeleton of the shell of the hollow particle. By containing the crosslinkable monomer units in the above ratio, the shell of the hollow particle of the present disclosure has a densely packed covalent bond network. Furthermore, by containing the crosslinkable monomer units in combination with bifunctional crosslinkable monomer units and trifunctional or higher crosslinkable monomer units, the crosslinked structure becomes even denser, thereby improving the strength of the shell and its pressure resistance. In the above polymer, the content of the crosslinkable monomer units in 100 parts by mass of all monomer units is preferably 75 to 98 parts by mass, more preferably 85 to 96 parts by mass, from the viewpoint of improving the strength and pressure resistance of the hollow particles. In the present disclosure, a crosslinkable monomer unit is a monomer unit derived from a crosslinkable monomer, a bifunctional crosslinkable monomer unit is a monomer unit derived from a bifunctional crosslinkable monomer, and a trifunctional or higher functional crosslinkable monomer unit is a monomer unit derived from a trifunctional or higher functional crosslinkable monomer. In the above polymer, when the content of the crosslinkable monomer units is less than 100 parts by mass, the monomer units other than the crosslinkable monomer units are non-crosslinkable monomer units derived from non-crosslinkable monomers. The polymer is typically a polymer of a polymerizable monomer used in the method for producing hollow particles of the present disclosure, which will be described later. Specific details of the crosslinkable monomer and non-crosslinkable monomer used in the synthesis of the polymer are as described in the method for producing hollow particles of the present disclosure, which will be described later.

[0034] The content of the bifunctional crosslinkable monomer units in 100 parts by mass of all the monomer units in the polymer is not particularly limited, but the lower limit is preferably 60 parts by mass or more, more preferably 65 parts by mass or more, and may be 70 parts by mass or more, from the viewpoint of improving the strength and pressure resistance of the hollow particles, and the upper limit is preferably 95 parts by mass or less, and may be 90 parts by mass or less, from the viewpoint of sufficiently containing trifunctional or higher crosslinkable monomer units. The content of trifunctional or higher crosslinkable monomer units in 100 parts by mass of all monomer units of the polymer is not particularly limited, but the lower limit is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 15 parts by mass or more, from the viewpoint of improving the strength and pressure resistance of the hollow particles, and the upper limit is preferably 40 parts by mass or less, more preferably 30 parts by mass or less, and may be 20 parts by mass or less, from the viewpoint of sufficiently containing bifunctional crosslinkable monomer units.

[0035] The crosslinkable monomer units contained in the polymer preferably contain crosslinkable monomer units derived from a (meth)acrylic crosslinkable monomer having a (meth)acryloyl group as a polymerizable functional group, which can improve the strength, pressure resistance, and heat resistance of the hollow particles of the present disclosure. When the polymer contains crosslinkable monomer units derived from a (meth)acrylic crosslinkable monomer, the content of the crosslinkable monomer units derived from the (meth)acrylic crosslinkable monomer is preferably 50 parts by mass or more, more preferably 70 parts by mass or more, and even more preferably 90 parts by mass or more, per 100 parts by mass of the crosslinkable monomer units, and the crosslinkable monomer units may be composed of (meth)acrylic crosslinkable monomer units.

[0036] When the volume average particle size of the hollow particles is set to 10 μm or more and 20 μm or less, the trifunctional or higher crosslinkable monomer units contained in the polymer are preferably crosslinkable monomer units derived from a trifunctional or higher methacrylic crosslinkable monomer, and more preferably crosslinkable monomer units derived from a trifunctional methacrylic crosslinkable monomer. On the other hand, when the volume average particle size of the hollow particles is set to be more than 20 μm and not more than 50 μm, the tri- or higher functional crosslinkable monomer units contained in the polymer are not particularly limited, and may be crosslinkable monomer units derived from a tri- or higher functional (meth)acrylic crosslinkable monomer, may be crosslinkable monomer units derived from a tri- or higher functional acrylic crosslinkable monomer, or may be crosslinkable monomer units derived from a tetra- or higher functional acrylic crosslinkable monomer. The volume-average particle size of hollow particles can be adjusted by the type of crosslinkable monomer used in producing the hollow particles, the type of hydrophobic solvent encapsulated in the particles during the production process, and whether or not a second polymerization reaction is performed. To obtain hollow particles with a volume-average particle size of 10 μm to 20 μm, it is preferable to use a trifunctional or higher methacrylic crosslinkable monomer as the crosslinkable monomer and select a hydrophobic solvent appropriate for the particle size. It is also preferable to add an additional polymerizable monomer and perform a second polymerization reaction. To obtain hollow particles with a volume-average particle size of more than 20 μm to 50 μm, it is preferable to use a trifunctional or higher acrylic crosslinkable monomer as the crosslinkable monomer, or to use a trifunctional or higher methacrylic crosslinkable monomer without performing a second polymerization reaction. Specific details of the (meth)acrylic crosslinkable monomer and the methacrylic crosslinkable monomer are as described later in the method for producing hollow particles of the present disclosure.

[0037] The polymer may further contain a non-crosslinkable monomer unit. When the polymer contains a combination of a crosslinkable monomer unit and a non-crosslinkable monomer unit, the mechanical properties of the shell of the hollow particle may be improved in some cases. In the above polymer, the content of the non-crosslinkable monomer units in 100 parts by mass of all monomer units is 0 to 30 parts by mass, preferably 25 parts by mass or less, and more preferably 15 parts by mass or less, in order to ensure that the crosslinkable monomer units are contained sufficiently. On the other hand, in order to obtain the effect of improving the strength of the hollow particles by combining the crosslinkable monomer units and the non-crosslinkable monomer units, the content of the non-crosslinkable monomer units may be 2 parts by mass or more, or may be 4 parts by mass or more.

[0038] In the hollow particles of the present disclosure, the content of the polymer is preferably 90% by mass or more, more preferably 95% by mass or more, based on 100% by mass of the total solid content of the shell. By making the content of the polymer equal to or greater than the lower limit, the strength of the hollow particles can be improved.

[0039] The shell of the hollow particles of the present disclosure may further contain a polar component. Examples of the polar component include an organic acid or a metal salt thereof, and a polar resin. Specific details of the polar component are as described later in the method for producing hollow particles of the present disclosure. Furthermore, the presence of a polar component in the shell of the hollow particle and the content thereof can be confirmed by, for example, pyrolysis gas chromatography.

[0040] When the shell of the hollow particles of the present disclosure contains an organic acid or a metal salt thereof as a polar component, the total content of the organic acid or the metal salt thereof in the shell is preferably 0.0001 to 0.1 mass%, more preferably 0.001 to 0.01 mass%. On the other hand, when the shell of the hollow particles of the present disclosure contains a polar resin as a polar component, the content of the polar resin in the shell is preferably 0.1 to 10.0 mass %, more preferably 0.3 to 8.0 mass %. When the shell contains a polar component, a decrease in the strength of the shell can be suppressed, and a decrease in the pressure resistance of the hollow particles can be suppressed.

[0041] Furthermore, in the hollow particles of the present disclosure, it is preferable that, when observed under an SEM, 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 are generally recognized as shell defects because they significantly reduce the strength of the hollow particle.

[0042] 2. Manufacturing method of hollow particles The hollow particles of the present disclosure may be, for example: preparing a mixed liquid containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium; a step of suspending the mixed liquid to prepare a suspension in which droplets of a monomer composition containing the polymerizable monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in the aqueous medium; and subjecting the suspension to a polymerization reaction to prepare a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a resin, and encapsulating the hydrophobic solvent in the hollow portion.

[0043] The method for producing the hollow particles follows a basic technique in which a mixed liquid containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium is suspended, whereby the polymerizable monomer and the hydrophobic solvent undergo phase separation, thereby preparing a suspension in which droplets having a distribution structure in which the polymerizable monomer is unevenly distributed on the surface side and the hydrophobic solvent is unevenly distributed in the center are dispersed in the aqueous medium, and this suspension is subjected to a polymerization reaction to harden the surfaces of the droplets, thereby forming hollow particles having hollow portions filled with the hydrophobic solvent. The method for producing 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 other steps. Furthermore, as far as technically possible, two or more of the above steps and other additional steps may be performed simultaneously as a single step, or the order may be reversed. For example, the preparation of the mixed solution and the suspension may be performed simultaneously in a single process, such as by adding the materials for preparing the mixed solution and suspending them at the same time.

[0044] A preferred example of the method for producing the hollow particles includes the following steps. (1) Mixed liquid preparation process A step of preparing a mixed liquid containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium. (2) Suspension process a step of suspending the mixed liquid to prepare a suspension in which droplets of a monomer composition containing a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator are dispersed in an aqueous medium. (3) Polymerization process a step of subjecting the suspension to a polymerization reaction to prepare a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a resin and encapsulating a hydrophobic solvent in the hollow portion; (4) Solid-liquid separation process A step of obtaining precursor particles having a hydrophobic solvent encapsulated in a hollow portion by solid-liquid separation of the precursor composition; and (5) Solvent removal process A step of removing the hydrophobic solvent contained in the precursor particles obtained in the solid-liquid separation step to obtain hollow particles. In the present disclosure, hollow particles whose hollow portions are filled with a hydrophobic solvent are considered to be intermediates of hollow particles whose hollow portions are filled with a gas, and may be referred to as "precursor particles." In the present disclosure, the term "precursor composition" refers to a composition containing precursor particles.

[0045] FIG. 1 is a schematic diagram showing an example of a method for producing hollow particles according to the present disclosure. (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 explanatory purposes, and the above-mentioned production method is not limited to that shown in the figure. Furthermore, the structure, dimensions, and shape of the materials used in the production method according to the present disclosure are not limited to those of the various materials shown 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 polymerizable monomer, a hydrophobic solvent, and a polymerization initiator. 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 polymerizable monomer, a hydrophobic solvent, and a polymerization initiator, but the distribution within the droplets is non-uniform. The droplets 10 of the monomer composition are phase-separated into a hydrophobic solvent 4a and a material other than the hydrophobic solvent, including the polymerizable monomer, a material 4b, with the hydrophobic solvent 4a unevenly distributed in the center and the material other than the hydrophobic solvent a material 4b unevenly distributed on the surface, with a dispersion stabilizer (not shown) attached to the surface. 1(3) is a cross-sectional schematic diagram showing one embodiment of a precursor composition containing precursor particles encapsulating a hydrophobic solvent in their hollow spaces, obtained by a polymerization step. The precursor composition contains an aqueous medium 1 and precursor particles 20 encapsulating a hydrophobic solvent 4a in their hollow spaces, dispersed in the aqueous medium 1. A shell 6 forming the outer surface of the precursor particles 20 is formed by polymerization of a polymerizable monomer in droplets 10 of the monomer composition, and contains a polymer of the polymerizable monomer as a resin. Fig. 1(4) is a cross-sectional schematic diagram showing one embodiment of precursor particles after the solid-liquid separation step, 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 the state after removing the hydrophobic solvent 4a from the state shown in Fig. 1 (4). By removing the hydrophobic solvent from the precursor particles, hollow particles 100 having gas-filled hollow portions 8 inside the shells 6 are obtained. The above five steps and other steps will be explained below in order.

[0046] (1) Mixed liquid preparation process This step is a step of preparing a mixed solution containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium. The mixed solution may further contain other materials as long as the effects of the present disclosure are not impaired. The materials of the mixed liquid will be explained in the following order: (A) polymerizable monomer, (B) hydrophobic solvent, (C) polymerization initiator, (D) dispersion stabilizer, (E) aqueous medium, and (F) other materials.

[0047] (A) Polymerizable monomer In the above production method, the polymerizable monomer in the mixed solution contains at least a crosslinkable monomer, and may further contain a non-crosslinkable monomer within a range that does not impair the effects of the present disclosure. From the viewpoint that the polymerization reaction is likely to be stable and hollow particles having excellent strength and heat resistance can be obtained, a (meth)acrylic polymerizable monomer having a (meth)acryloyl group as the polymerizable functional group can be preferably used as the polymerizable monomer.

[0048] [Crosslinking monomer] The crosslinkable monomer has a plurality of ethylenically unsaturated double bonds, and therefore, the monomers can be linked together, thereby increasing the crosslink density of the shell. Examples of the crosslinkable monomer include a bifunctional crosslinkable monomer having two polymerizable functional groups, and a trifunctional or higher crosslinkable monomer having three or more polymerizable functional groups. Examples of bifunctional crosslinkable monomers include divinylbenzene, divinyldiphenyl, divinylnaphthalene, diallyl phthalate, allyl (meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, polyphenylene ether vinyl-modified at both ends, and polyphenylene ether (meth)acrylic-modified at both ends, and other polymer-type crosslinkable monomers. Among these, bifunctional (meth)acrylic crosslinkable monomers such as allyl (meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate are preferred, and ethylene glycol di(meth)acrylate and pentaerythritol di(meth)acrylate are more preferred, as they facilitate stable polymerization reactions and enable the production of hollow particles with excellent strength and heat resistance. Among these difunctional (meth)acrylic crosslinkable monomers, difunctional methacrylic crosslinkable monomers in which the polymerizable functional group is a methacryloyl group are more preferred. In the present disclosure, the (meth)acrylic crosslinkable monomer may be a crosslinkable monomer having at least one methacryloyl group or acryloyl group as a polymerizable functional group, but it is preferable that all of the polymerizable functional groups are methacryloyl groups or acryloyl groups. On the other hand, it is preferable that the methacrylic crosslinkable monomer is a crosslinkable monomer having at least one methacryloyl group as a polymerizable functional group and no acryloyl group, and it is preferable that all of the polymerizable functional groups are methacryloyl groups.

[0049] When the polymerizable monomer in the mixed solution contains a (meth)acrylic crosslinkable monomer, the content of the (meth)acrylic crosslinkable monomer is preferably 50 parts by mass or more, more preferably 70 parts by mass or more, and even more preferably 90 parts by mass or more, per 100 parts by mass of the crosslinkable monomer contained in the polymerizable monomer, and the crosslinkable monomer contained in the polymerizable monomer may consist of a (meth)acrylic crosslinkable monomer.

[0050] As the tri- or higher functional crosslinkable monomer, tri- or higher functional (meth)acrylic crosslinkable monomers such as trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol poly(meth)acrylate, and their ethoxylated derivatives are preferred, as they tend to stabilize the polymerization reaction and produce hollow particles with excellent strength and heat resistance. Monomers are preferred, and among them, pentaerythritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, and dipentaerythritol poly(meth)acrylate are more preferred, and trimethylolpropane tri(meth)acrylate and pentaerythritol tetra(meth)acrylate are even more preferred. Other cross-linkable polymeric hydrocarbon monomers can also be used, such as polybutadiene, polyisoprene, styrene-butadiene block copolymers (SBS), and styrene-isoprene block copolymers (SIS).

[0051] From the viewpoint of making the volume average particle diameter of the hollow particles 10 μm or more and 20 μm or less, the trifunctional or more crosslinkable monomer is preferably a trifunctional or more methacrylic crosslinkable monomer, more preferably a trifunctional methacrylic crosslinkable monomer. Examples of the trifunctional methacrylic crosslinkable monomer include trimethylolpropane trimethacrylate, ethoxylated trimethylolpropane trimethacrylate, and pentaerythritol trimethacrylate, and among these, trimethylolpropane trimethacrylate is preferred. On the other hand, from the viewpoint of setting the volume average particle size of the hollow particles to more than 20 μm and not more than 50 μm, the trifunctional or higher crosslinkable monomer is not particularly limited and may be a trifunctional or higher (meth)acrylic crosslinkable monomer, a trifunctional or higher acrylic crosslinkable monomer, or a tetrafunctional or higher acrylic crosslinkable monomer. In the present disclosure, the acrylic crosslinkable monomer is a crosslinkable monomer having at least one acryloyl group as a polymerizable functional group and no methacryloyl groups, and it is preferable that all of the polymerizable functional groups are acryloyl groups. Examples of the trifunctional acrylic crosslinkable monomer include trimethylolpropane triacrylate, ethoxylated trimethylolpropane triacrylate, and pentaerythritol triacrylate. Examples of the tetrafunctional or higher acrylic crosslinkable monomer include ditrimethylolpropane tetraacrylate, pentaerythritol tetraacrylate, dipentaerythritol hexaacrylate, dipentaerythritol polyacrylate, and ethoxylated versions of these. Of these, pentaerythritol tetraacrylate, ethoxylated pentaerythritol tetraacrylate, and dipentaerythritol polyacrylate are preferred, with pentaerythritol tetraacrylate being more preferred. These crosslinkable monomers can be used either alone or in combination of two or more.

[0052] The content of the crosslinkable monomer is preferably 70 to 100 parts by mass per 100 parts by mass of the total mass of the polymerizable monomers in the mixed solution. When the content of the crosslinkable monomer is 70 parts by mass or more, the content of crosslinkable monomer units in the shell of the hollow particles is sufficiently high, and a covalent bond network is densely spread throughout the shell, resulting in the formation of a shell that is excellent in strength, resistant to crushing, and resistant to deformation even when exposed to heat or the like from an external source. The content of the crosslinkable monomer is preferably 80 parts by mass or more, more preferably 90 parts by mass or more. When the polymerizable monomers in the mixed solution contain a non-crosslinkable monomer, the content of the crosslinkable monomer may be, for example, 98 parts by mass or less, or may be 96 parts by mass or less.

[0053] The polymerizable monomers in the mixed solution contain a difunctional crosslinkable monomer and a trifunctional or higher crosslinkable monomer as crosslinkable monomers, which facilitates the formation of hollow spaces inside the particles and further improves the strength of the shell. The content of the bifunctional crosslinkable monomer in 100 parts by mass of the polymerizable monomer in the mixed solution is not particularly limited, but the lower limit is preferably 60 parts by mass or more, more preferably 65 parts by mass or more, and may be 70 parts by mass or more, from the viewpoint of improving the strength and pressure resistance of the hollow particles, and the upper limit is preferably 95 parts by mass or less, and may be 90 parts by mass or less, from the viewpoint of sufficiently containing trifunctional or higher functional crosslinkable monomer units. The content of the trifunctional or higher crosslinkable monomer in 100 parts by mass of the polymerizable monomer in the mixed solution is not particularly limited, but the lower limit is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 15 parts by mass or more, from the viewpoint of improving the strength and pressure resistance of the hollow particles, and the upper limit is preferably 40 parts by mass or less, more preferably 30 parts by mass or less, and may be 20 parts by mass or less, from the viewpoint of sufficiently containing bifunctional crosslinkable monomer units.

[0054] Furthermore, when the volume average particle diameter of the hollow particles is set to 10 μm or more and 20 μm or less, the trifunctional or higher crosslinkable monomer in the mixed solution is preferably a trifunctional crosslinkable monomer. On the other hand, when the volume average particle diameter of the hollow particles is set to more than 20 μm and 50 μm or less, the trifunctional or higher crosslinkable monomer in the mixed solution is preferably a tetrafunctional or higher crosslinkable monomer.

[0055] [Non-crosslinkable monomer] The polymerizable monomer in the mixed solution may further contain a non-crosslinkable monomer within a range that does not impair the effects of the present disclosure. When the polymerizable monomer in the mixed solution contains a crosslinkable monomer and a non-crosslinkable monomer in combination, the mechanical properties of the shell of the obtained hollow particle may be improved in some cases. 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 monovinyl monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, glycidyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, and (meth)acrylic acid; styrene, vinyltoluene, α-methylstyrene, p-methylstyrene, ethylvinylbenzene, ethylvinylbiphenyl, ethylvinylnaphthalene; Examples of suitable non-crosslinkable monomers include aromatic vinyl monomers such as halogenated styrene, monoolefin monomers such as ethylene, propylene, and butylene, (meth)acrylamide monomers and derivatives thereof such as (meth)acrylamide, N-methylol (meth)acrylamide, and N-butoxymethyl (meth)acrylamide, diene monomers such as butadiene and isoprene, vinyl carboxylic acid ester monomers such as vinyl acetate, vinyl halide monomers such as vinyl chloride, vinylidene halide monomers such as vinylidene chloride, and vinylpyridine monomers. These non-crosslinkable monomers can be used alone or in combination of two or more. Among these, from the viewpoints of reactivity and heat resistance, (meth)acrylic monovinyl monomers are preferred, and at least one selected from butyl acrylate and methyl methacrylate is more preferred.

[0056] The content of the non-crosslinkable monomer relative to 100 parts by mass of the total mass of the polymerizable monomers in the mixed solution is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and even more preferably 10 parts by mass or less, in order to ensure that the crosslinkable monomer is contained sufficiently. On the other hand, in order to obtain the effect of improving the strength of the hollow particles by combining the crosslinkable monomer and the non-crosslinkable monomer, the content of the non-crosslinkable monomer may be 2 parts by mass or more, or may be 4 parts by mass or more.

[0057] The content of the polymerizable monomer in the mixed solution is preferably 15 to 50 parts by mass, more preferably 20 to 40 parts by mass, and even more preferably 20 to 30 parts by mass, relative to 100 parts by mass of the total of the polymerizable monomer and the hydrophobic solvent in the mixed solution. When the content of the polymerizable monomer is within the above range, the hollow particles have a good balance of porosity, particle size, and mechanical strength. Furthermore, from the viewpoint of improving the mechanical strength of the hollow particles, the content of the polymerizable monomer relative to the total mass of the solids excluding the hydrophobic solvent, which constitutes the oil phase in the mixed liquid, is preferably 90 mass% or more, and more preferably 95 mass% or more. In the present disclosure, the solid content refers to all components excluding the solvent, and liquid polymerizable monomers and the like are considered to be included in the solid content.

[0058] (B) Hydrophobic solvent The hydrophobic solvent used in the above production method is a non-polymerizable and poorly water-soluble organic solvent. The hydrophobic solvent acts as a spacer material that forms hollow spaces inside the particles. In the suspension process described below, a suspension is obtained in which droplets of the monomer composition containing the hydrophobic solvent are dispersed in an aqueous medium. In the suspension process, phase separation occurs within the droplets of the monomer composition, and the hydrophobic solvent, which has low polarity, tends to collect inside the droplets of the monomer composition. Ultimately, the droplets of the monomer composition contain the hydrophobic solvent inside, and other materials other than the hydrophobic solvent are distributed around the periphery according to their respective polarities. Then, in the polymerization step described below, an aqueous dispersion containing hollow particles encapsulating the hydrophobic solvent is obtained. That is, the hydrophobic solvent collects inside the particles, and hollow portions filled with the hydrophobic solvent are formed inside the obtained precursor particles.

[0059] In the above-mentioned production method, the hydrophobic solvent is appropriately selected depending on the type of polymerizable monomer, and is not particularly limited. Known hydrophobic solvents can be used, including, for example, esters such as ethyl acetate and butyl acetate; ether esters such as propylene glycol monomethyl ether acetate and propylene glycol monoethyl ether acetate; aromatic hydrocarbons such as benzene, toluene, and xylene; and aliphatic hydrocarbons such as hexane, methylhexane, heptane, octane, cyclohexane, and methylcyclohexane. These hydrophobic solvents can be used alone or in combination of two or more. In particular, it is preferable to select a hydrophobic solvent so that the HSP distance between the crosslinkable monomer in the polymerizable monomer and the hydrophobic solvent is 5.40 or more and 6.50 or less. The HSP distance is more preferably 5.80 or more and 6.40 or less, and even more preferably 5.90 or more and 6.30 or less. When the HSP distance between the crosslinkable monomer and the hydrophobic solvent is within the above range, the polymerizable monomer and the hydrophobic solvent are sufficiently phase-separated in the droplets of the monomer composition, making it easy to form hollow portions within the particles, and also making it easy to make the thickness of the shell formed by the polymerization reaction uniform.

[0060] The HSP distance is an index that expresses the solubility between substances using the Hansen Solubility Parameter (HSP). The closer the HSP distance is to 0, the higher the compatibility between the substances. HSP is expressed as a vector in a three-dimensional space (Hansen space) with the dispersion term dD, the polar term dP, and the hydrogen bond term dH as its coordinate axes. The three parameters dD, dP, and dH have unique values ​​for each substance. The software developed by Hansen et al. (Hansen Solubility Parameter in Practice (HSPiP)) contains a database of dD, dP, and dH for various substances. HSPiP can also be used to calculate HSP based on the chemical structure of a substance. When calculating the HSP of a mixture containing multiple substances, the weighted averages of the dD, dP, and dH values ​​of each substance contained in the mixture and the proportion of each substance are calculated to determine the dispersion term dD, polarity term dP, and hydrogen bond term dH of the mixture, and the HSP is calculated. The HSP distance is the distance between vectors given by the HSP of two substances, and is calculated using the following formula (A) from the values ​​of the dispersion term dD1, polar term dP1, and hydrogen bond term dH1 of one substance and the values ​​of the dispersion term dD2, polar term dP2, and hydrogen bond term dH2 of the other substance. Formula (A) HSP distance={4(dD1-dD2) 2 +(dP1-dP2) 2 +(dH1-dH2) 2} 0.5 The distance from the origin of the Hansen space to the HSP is called the total HSP, and is calculated from the dispersion term dD, the polarity term dP, and the hydrogen bond term dH using the following formula (B): Formula (B) total HSP=(dD 2 +dP 2 +dH 2 ) 0.5 In this disclosure, the HSP distance and total HSP are values ​​calculated using HSPiP (Version 5.3.03). In HSPiP, the values ​​of dD, dP, and dH are expressed with significant figures up to one decimal place, and the values ​​of HSP distance and total HSP are expressed with significant figures up to two decimal places.

[0061] Since the HSP distance between the crosslinkable monomer and the hydrophobic solvent is likely to be within the above range, the hydrophobic solvent preferably contains at least one selected from the group consisting of esters, aromatic hydrocarbons, and aliphatic hydrocarbons, and more preferably contains at least one selected from the group consisting of aromatic hydrocarbons and aliphatic hydrocarbons.

[0062] Furthermore, since the HSP distance between the crosslinkable monomer and the hydrophobic solvent is likely to fall within the above range, it is preferable to contain two or more hydrophobic solvents. The combination of two or more hydrophobic solvents is appropriately selected so that the HSP distance falls within the above range, and is not particularly limited, but a combination of one or more selected from aliphatic hydrocarbons with one or more selected from esters and aromatic hydrocarbons is preferred, and among these, a combination of one or more selected from aliphatic hydrocarbons with one or more selected from esters, and a combination of one or more selected from aliphatic hydrocarbons with one or more selected from aromatic hydrocarbons are more preferred. In the above combination, the aliphatic hydrocarbon is preferably one or more selected from the group consisting of hexane, methylhexane, cyclohexane, and methylcyclohexane, more preferably one or more selected from hexane and cyclohexane, and even more preferably cyclohexane. In the above combination, the ester is preferably one or more selected from ethyl acetate and butyl acetate, more preferably ethyl acetate. In the above combination, the aromatic hydrocarbon is preferably one or more selected from benzene, toluene and xylene, more preferably toluene. Furthermore, the hydrophobic solvent used in the production method of the present disclosure is such that the total mass of the hydrophobic solvents in the preferred combination described above contained in 100 parts by mass of the total mass of the hydrophobic solvents is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, even more preferably 90 parts by mass or more, even more preferably 95 parts by mass or more, and particularly preferably 99 parts by mass or more. The hydrophobic solvent used in the production method of the present disclosure is most preferably composed of the hydrophobic solvents in the preferred combination described above.

[0063] Furthermore, a combination of one or more aliphatic hydrocarbons and one or more aromatic hydrocarbons as the hydrophobic solvent is preferred because it facilitates the production of hollow particles with reduced pitting. In particular, when the volume average particle diameter of the hollow particles is greater than 10 μm and less than 50 μm, particularly when the volume average particle diameter of the hollow particles is 20 μm or more and less than 50 μm, a combination of one or more aliphatic hydrocarbons and one or more aromatic hydrocarbons as the hydrophobic solvent is more likely to prevent pitting. It is believed that when the hydrophobic solvent contains aromatic hydrocarbons such as toluene, the polymer is more likely to precipitate in a high molecular weight state in the oil phase during polymerization. Precipitation of the polymer in a high molecular weight state reduces reactivity during shell formation, thereby suppressing shell distortion and resulting in particles without pitting.

[0064] In the hydrophobic solvent containing a combination of one or more aliphatic hydrocarbons and one or more esters, the mass ratio of the aliphatic hydrocarbons to the esters (aliphatic hydrocarbons:esters) is not particularly limited, but is preferably 70:30 to 95:5, and more preferably 80:20 to 95:5.

[0065] In a hydrophobic solvent containing a combination of one or more aliphatic hydrocarbons and one or more aromatic hydrocarbons, the mass ratio of the aliphatic hydrocarbons to the aromatic hydrocarbons (aliphatic hydrocarbons:aromatic hydrocarbons) is not particularly limited, but is preferably 20:80 to 80:20, and more preferably 30:70 to 70:30.

[0066] Furthermore, although not particularly limited, the hydrophobic solvent used in the production method of the present disclosure has a total HSP of 16.50 to 18.00 MPa because the HSP distance between the crosslinkable monomer and the hydrophobic solvent is likely to be within the above range. 1 / 2 Preferably, the pressure is 16.60 to 17.80 MPa. 1 / 2 More preferably, the pressure is 16.70 to 17.70 MPa. 1 / 2 It is more preferable that:

[0067] A preferred combination in which the HSP distance between the crosslinkable monomer and the hydrophobic solvent is 5.40 or more and 6.50 or less is, for example, a combination in which the crosslinkable monomer contains ethylene glycol di(meth)acrylate and at least one selected from pentaerythritol di(meth)acrylate and pentaerythritol tetra(meth)acrylate, and the total content of ethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate and pentaerythritol tetra(meth)acrylate relative to 100 parts by mass of the crosslinkable monomer is preferably 7. The total amount of hexane, cyclohexane, ethyl acetate, and toluene per 100 parts by mass of the hydrophobic solvent is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, even more preferably 90 parts by mass or more, and even more preferably 100 parts by mass, and the hydrophobic solvent includes a combination of one or more selected from hexane and cyclohexane and one or more selected from ethyl acetate and toluene, and the total amount of hexane, cyclohexane, ethyl acetate, and toluene per 100 parts by mass of the hydrophobic solvent is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, even more preferably 90 parts by mass or more, and even more preferably 100 parts by mass. As the crosslinkable monomer used in the above combination, a crosslinkable monomer in which the content of ethylene glycol di(meth)acrylate per 100 parts by mass of the crosslinkable monomer is 70 parts by mass or more is more preferable. The hydrophobic solvents used in the above combination are preferably a combination of toluene with one or more selected from hexane and cyclohexane, wherein the total content of hexane, cyclohexane, and toluene per 100 parts by mass of the hydrophobic solvent is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, even more preferably 90 parts by mass or more, and still more preferably 100 parts by mass, and a combination of ethyl acetate with one or more selected from hexane and cyclohexane, wherein the total content of hexane, cyclohexane, and ethyl acetate per 100 parts by mass of the hydrophobic solvent is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, even more preferably 90 parts by mass or more, and still more preferably 100 parts by mass.

[0068] Other preferred combinations in which the HSP distance between the crosslinkable monomer and the hydrophobic solvent is 5.40 or more and 6.50 or less include, for example, a combination in which the crosslinkable monomer includes ethylene glycol di(meth)acrylate and trimethylolpropane tri(meth)acrylate, and the total content of ethylene glycol di(meth)acrylate and trimethylolpropane tri(meth)acrylate per 100 parts by mass of the crosslinkable monomer is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, even more preferably 90 parts by mass or more, and still more preferably 100 parts by mass, and the hydrophobic solvent includes cyclohexane, and the content of cyclohexane per 100 parts by mass of the hydrophobic solvent is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, even more preferably 90 parts by mass or more, and still more preferably 100 parts by mass. As the crosslinkable monomer used in the above combination, a crosslinkable monomer in which the content of ethylene glycol di(meth)acrylate per 100 parts by mass of the crosslinkable monomer is 70 parts by mass or more is preferred. When the hydrophobic solvent used in the above combination contains a hydrophobic solvent other than cyclohexane, the hydrophobic solvent is preferably toluene.

[0069] The preferred crosslinkable monomer is a mixture of ethylene glycol dimethacrylate and pentaerythritol tetraacrylate, and the mass ratio of ethylene glycol dimethacrylate to pentaerythritol tetraacrylate is 70:30 to 90:10. Examples of hydrophobic solvents that have an HSP distance of 5.40 or more and 6.50 or less with the crosslinkable monomer include a mixed solvent of cyclohexane and toluene in a mass ratio of cyclohexane to toluene of 50:50 (HSP distance with ethylene glycol dimethacrylate: 6.04), a mixed solvent of cyclohexane and toluene in a mass ratio of cyclohexane to toluene of 70:30 (HSP distance with ethylene glycol dimethacrylate: 6.04), and a mixed solvent of cyclohexane and toluene in a mass ratio of 70:30 (HSP distance with ethylene glycol dimethacrylate: 6.04). Examples of such a solvent include a mixed solvent of cyclohexane and toluene in a cyclohexane:toluene mass ratio of 40:60 (HSP distance from ethylene glycol dimethacrylate: 5.91), a mixed solvent of cyclohexane and ethyl acetate in a cyclohexane:ethyl acetate mass ratio of 90:10 (HSP distance from ethylene glycol dimethacrylate: 6.01), a mixed solvent of hexane and toluene in a hexane:toluene mass ratio of 60:40 (HSP distance from ethylene glycol dimethacrylate: 6.03), and a mixed solvent of hexane and ethyl acetate in a hexane:ethyl acetate mass ratio of 80:20 (HSP distance from ethylene glycol dimethacrylate: 5.83).

[0070] Furthermore, specific examples of hydrophobic solvents that have an HSP distance of 5.40 or more and 6.50 or less with the crosslinkable monomer, which is a mixture of ethylene glycol dimethacrylate and trimethylolpropane trimethacrylate, in which the mass ratio of ethylene glycol dimethacrylate:trimethylolpropane trimethacrylate is 60:40 to 80:20, include cyclohexane and a mixed solvent of cyclohexane and toluene in which the mass ratio of cyclohexane:toluene is 40:60 to 90:10.

[0071] Furthermore, although not particularly limited, the boiling point of the hydrophobic solvent is preferably 130°C or lower, more preferably 120°C or lower, from the viewpoint of ease of removal in the solvent removal step described below, while it is preferably 50°C or higher, more preferably 60°C or higher, from the viewpoint of ease of inclusion in the precursor particles. When the hydrophobic solvent is a mixed solvent containing multiple types of hydrophobic solvents and has multiple boiling points, it is preferable that the boiling point of the solvent with the highest boiling point among the solvents contained in the mixed solvent is not more than the above-mentioned upper limit value, and it is preferable that the boiling point of the solvent with the lowest boiling point among the solvents contained in the mixed solvent is not less than the above-mentioned lower limit value.

[0072] Furthermore, the hydrophobic solvent used in the above production method preferably has a relative dielectric constant of 3 or less at 20°C. The relative dielectric constant is one of the indicators that indicates the polarity of a compound. When the relative dielectric constant of the hydrophobic solvent is sufficiently small, 3 or less, it is thought that phase separation proceeds rapidly in the polymerizable monomer droplets, and hollow spaces are easily formed. Examples of hydrophobic solvents with a relative dielectric constant of 3 or less at 20°C are as follows. The values ​​in parentheses are the relative dielectric constant values. Pentane (1.8), hexane (1.9), heptane (1.9), octane (1.9), cyclohexane (2.0), benzene (2.3), toluene (2.4). For the relative permittivity at 20°C, reference can be made to values ​​described in known literature (for example, "Chemical Handbook Basics" edited by the Chemical Society of Japan, 4th revised edition, Maruzen Co., Ltd., published September 30, 1993, pp. II-498 to II-503) and other technical information. Examples of methods for measuring the relative permittivity at 20°C include a relative permittivity test conducted in accordance with JIS C 2101:1999, 23, at a measurement temperature of 20°C.

[0073] The porosity of the hollow particles can be adjusted by changing the amount of hydrophobic solvent in the mixture. In the suspension step described below, the polymerization reaction proceeds in a state in which the oil droplets containing the crosslinkable monomer and the like encapsulate the hydrophobic solvent, and therefore the porosity of the resulting hollow particles tends to increase as the content of the hydrophobic solvent increases. In the present disclosure, the content of the hydrophobic solvent in the mixed solution is preferably 50 to 500 parts by mass relative to 100 parts by mass of the polymerizable monomer, because this makes it easier to control the particle size of the hollow particles, to increase the porosity while maintaining the strength of the hollow particles, and to reduce the amount of residual hydrophobic solvent in the particles. The content of the hydrophobic solvent in the mixed solution is more preferably 60 to 400 parts by mass, and even more preferably 70 to 300 parts by mass relative to 100 parts by mass of the polymerizable monomer.

[0074] (C) Polymerization initiator In the above-mentioned production method, the mixed liquid preferably contains an oil-soluble polymerization initiator as a polymerization initiator. As a method for polymerizing droplets of the monomer composition after suspending the mixed liquid, there are an emulsion polymerization method using a water-soluble polymerization initiator and a suspension polymerization method using an oil-soluble polymerization initiator, and the suspension polymerization can be carried out by using an oil-soluble polymerization initiator. The oil-soluble polymerization initiator is not particularly limited as long as it is lipophilic and has a solubility of 2 g / L or less in water at 25° C. Examples of oil-soluble polymerization initiators include benzoyl peroxide, lauroyl peroxide, t-butyl peroxide-2-ethylhexanoate, 2,2′-azobis(2,4-dimethylvaleronitrile), azobisisobutyronitrile, and 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile). The content of the oil-soluble polymerization initiator relative to 100 parts by mass of the polymerizable monomer in the mixed solution is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 7 parts by mass, and even more preferably 1 to 5 parts by mass. When the content of the oil-soluble polymerization initiator is within the above range, the polymerization reaction proceeds sufficiently, and there is little risk of the oil-soluble polymerization initiator remaining after completion of the polymerization reaction, and there is also little risk of an unexpected side reaction proceeding.

[0075] (D) Dispersion stabilizer The dispersion stabilizer is an agent for dispersing droplets of the monomer composition in an aqueous medium in the suspension step. In the present disclosure, it is preferable to use an inorganic dispersion stabilizer as the dispersion stabilizer, because it is easy to control the particle size of the droplets in the suspension, it is possible to narrow the particle size distribution of the obtained hollow particles, and it is possible to prevent the shell from becoming too thin, thereby preventing a decrease in the strength of the hollow particles. Examples of inorganic dispersion stabilizers include inorganic compounds such as 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, and metal hydroxides such as aluminum hydroxide, magnesium hydroxide, calcium hydroxide, barium hydroxide and ferric hydroxide. These inorganic dispersion stabilizers can be used alone or in combination of two or more. Among the inorganic dispersion stabilizers, the above-mentioned poorly water-soluble metal salts such as sulfates, carbonates, phosphates and metal hydroxides are preferred, metal hydroxides are more preferred, and magnesium hydroxide is particularly preferred. In the present disclosure, the poorly water-soluble metal salt is preferably an inorganic metal salt having a solubility of 0.5 g or less in 100 g of water. In the present disclosure, it is particularly preferred to use the poorly water-soluble inorganic dispersion stabilizer in a state in which it is dispersed in the form of colloidal particles in an aqueous medium, i.e., in the state of a colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles, which not only makes it possible to narrow the particle size distribution of the droplets of the monomer composition but also makes it possible to easily reduce the amount of inorganic dispersion stabilizer remaining in the obtained hollow particles by washing. A colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles can be prepared, for example, by reacting at least one selected from alkali metal hydroxides and alkaline earth metal hydroxides with a water-soluble polyvalent metal salt (excluding alkaline earth metal hydroxides) in an aqueous medium. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, potassium hydroxide, etc. Examples of alkaline earth metal hydroxides include barium hydroxide, calcium hydroxide, etc. The water-soluble polyvalent metal salt may be any water-soluble polyvalent metal salt other than the above-mentioned alkaline earth metal hydroxides, and examples thereof include magnesium metal salts such as magnesium chloride, magnesium phosphate, magnesium sulfate, etc.; calcium metal salts such as calcium chloride, calcium nitrate, calcium acetate, calcium sulfate, etc.; aluminum metal salts such as aluminum chloride, aluminum sulfate, etc.; barium salts such as barium chloride, barium nitrate, barium acetate, etc.; zinc salts such as zinc chloride, zinc nitrate, zinc acetate, etc. Among these, magnesium metal salts, calcium metal salts, and aluminum metal salts are preferred, magnesium metal salts are more preferred, and magnesium chloride is particularly preferred. The method for reacting at least one selected from the alkali metal hydroxides and alkaline earth metal hydroxides with the water-soluble polyvalent metal salt in an aqueous medium is not particularly limited, but examples include a method of mixing an aqueous solution of at least one selected from the alkali metal hydroxides and alkaline earth metal hydroxides with an aqueous solution of the water-soluble polyvalent metal salt.

[0076] The content of the dispersion stabilizer is not particularly limited, but is preferably 0.5 to 10 parts by mass, and more preferably 1.0 to 8.0 parts by mass, per 100 parts by mass of the total mass of the polymerizable monomer and the hydrophobic solvent. By ensuring that the content of the dispersion stabilizer is equal to or greater than the lower limit, the droplets of the monomer composition can be sufficiently dispersed so as not to coalesce in the suspension. On the other hand, by ensuring that 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 clogging in 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.

[0077] (E)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, and ion-exchanged water is preferred as the water to reduce the content of metals contained in the shell. When a mixture of water and a hydrophilic solvent is used, it is important that the polarity of the entire mixture is not too low from the viewpoint of forming droplets of the monomer composition. In this case, for example, the mass ratio of water to the hydrophilic solvent (water:hydrophilic solvent) may be 99:1 to 50:50.

[0078] (F) Other materials The mixed liquid may further contain other materials different from the above-mentioned materials (A) to (E) as long as the effects of the present disclosure are not impaired. The mixed liquid may contain a polar component as another material. When the mixed liquid contains a polar component, hollow particles having a high porosity and excellent pressure resistance are easily obtained. As the polar component, for example, a polar resin, which will be described later, or an organic acid or a metal salt thereof can be used.

[0079] In the present disclosure, a polar resin refers to a polymer containing a repeating unit containing a heteroatom, and specific examples thereof include acrylic resins, polyester resins, and vinyl resins containing heteroatoms. Polar resins typically have a solubility in water of less than 1 g / L. In this disclosure, polar resins are distinguished from surfactants in that they are insoluble in water.

[0080] The polar resin may be a homopolymer or copolymer of a heteroatom-containing monomer, or a copolymer of a heteroatom-containing monomer and a non-heteroatom-containing monomer. When the polar resin is a copolymer of a heteroatom-containing monomer and a non-heteroatom-containing monomer, the proportion of heteroatom-containing monomer units in 100% by mass of all 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, in order to easily control the particle size of the hollow particles.

[0081] 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.

[0082] When a polar resin is used as the polar component, the content of the polar resin is preferably 0.1 to 10.0 parts by mass, more preferably 0.3 to 8.0 parts by mass, and even more preferably 0.5 to 8.0 parts by mass, relative to 100 parts by mass of the polymerizable monomer. When the content of the polar resin is equal to or greater than the lower limit, the shell thickness of the hollow particles is easily controlled. When the content of the polar resin is equal to or less than the upper limit, a decrease in the content ratio of the polymerizable monomer can be suppressed, thereby suppressing a decrease in the shell strength.

[0083] Examples of organic acids include rosin acid and higher fatty acids, such as higher fatty acids having 10 to 25 carbon atoms and excluding the carbon atom in the carboxyl group.

[0084] Examples of metals used in metal salts of organic acids include alkali metals such as Li, Na, and K, and alkaline earth metals such as Mg and Ca. Of these, alkali metals are preferred, and at least one selected from Li, Na, and K is more preferred.

[0085] When an organic acid or a metal salt thereof is used as the polar component, the total content of the organic acid or the metal salt thereof is preferably 0.0001 to 0.02 parts by mass, more preferably 0.001 to 0.01 parts by mass, and even more preferably 0.0015 to 0.006 parts by mass, per 100 parts by mass of the total of the polymerizable monomer and the hydrophobic solvent. When the content of the organic acid or the metal salt thereof is equal to or greater than the lower limit, the shell thickness of the hollow particles is easily controlled. When the content of the organic acid or the metal salt thereof is equal to or less than the upper limit, a decrease in the polymerizable monomer content can be suppressed, thereby suppressing a decrease in shell strength and a decrease in the pressure resistance of the hollow particles.

[0086] 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 lipophilic materials such as (A) the polymerizable monomer, (B) the hydrophobic solvent, and (C) the polymerization initiator is dispersed in an aqueous phase containing (D) the dispersion stabilizer and (E) the aqueous medium, with particles of about several mm in size. Depending on the type of material, the dispersion state of these materials in the mixed solution can be observed with the naked eye. In the mixed solution preparation step, the mixed solution may be obtained by simply mixing the above-mentioned materials and other materials as necessary and appropriately stirring, etc. However, in order to make the shell more uniform, it is preferable to prepare the mixed solution in advance separately from an oil phase containing a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator, and an aqueous phase containing a dispersion stabilizer and an aqueous medium, and then mixing these. In the present disclosure, a colloidal dispersion in which a poorly water-soluble inorganic dispersion stabilizer is dispersed in the form of colloidal particles in an aqueous medium can be preferably used as the aqueous phase. In this way, by preparing the oil phase and the water phase separately in advance and then mixing them, hollow particles having a uniform shell composition can be produced.

[0087] (2) Suspension process The suspending step is a step of preparing a suspension in which droplets of the monomer composition containing the hydrophobic solvent are dispersed in an aqueous medium by suspending the mixed liquid described above. The method for suspending the monomer composition to form droplets is not particularly limited, but for example, The mixture is stirred using a device capable of strong stirring, such as an (in-line type) emulsifying disperser (horizontal in-line dispersers such as Pacific Machinery Works' trade name: Milder and Eurotech's trade name: Cavitron; or vertical in-line dispersers such as IKA's trade name: DRS 2000 / 5), or a high-speed emulsifying disperser (Primix Corporation's trade name: TK Homomixer MARK II type). In the suspension prepared in the suspending step, droplets of the monomer composition containing the lipophilic material and having a particle size of about 10 to 50 μm are uniformly dispersed in the aqueous medium. Such droplets of the monomer composition are difficult to observe with the naked eye, but can be observed using a known observation device such as an optical microscope. In the suspension process, phase separation occurs in the droplets of the monomer composition, and the hydrophobic solvent with low polarity tends to collect inside the droplets. As a result, the obtained droplets have the hydrophobic solvent distributed inside and materials other than the hydrophobic solvent, such as polymerizable monomers, distributed around the periphery.

[0088] Fig. 2 is a schematic diagram showing one embodiment of the suspension in the suspension step. A droplet 10 of the monomer composition in Fig. 2 is intended to show a schematic cross section thereof. Note that Fig. 2 is merely a schematic diagram, and the suspension in the present disclosure is not necessarily limited to that shown in Fig. 2. A part of Fig. 2 corresponds to (2) in Fig. 1 described above. 2 shows droplets 10 of a monomer composition and a polymerizable monomer 4c dispersed in the aqueous medium 1, which are dispersed in the aqueous medium 1. The droplets 10 are formed by surrounding an oil-soluble monomer composition 4 with a dispersion stabilizer 3. The monomer composition 4 contains an oil-soluble polymerization initiator 5, as well as a polymerizable monomer and a hydrophobic solvent (none of which are shown). The droplets 10 are minute oil droplets containing the monomer composition 4, and the oil-soluble polymerization initiator 5 generates polymerization-initiating radicals inside the minute oil droplets. Therefore, precursor particles of the desired particle size can be produced without causing the minute oil droplets to grow too large. In the suspension polymerization method using such an oil-soluble polymerization initiator, there is no opportunity 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 prevent the by-production of excess resin particles such as dense particles having a relatively small particle size in addition to the desired resin particles having hollow portions.

[0089] (3) Polymerization process This step is a step of preparing a precursor composition containing precursor particles having a hollow portion surrounded by a shell containing a resin and encapsulating a hydrophobic solvent in the hollow portion by subjecting the suspension obtained by the above-mentioned suspension step to a polymerization reaction. The precursor particles are formed by polymerization of a polymerizable monomer contained in droplets of the monomer composition, and the shell of the precursor particles contains a polymer of the polymerizable monomer as a resin. The polymerization method is not particularly limited, and for example, a batch method, a semi-continuous method, a continuous method, etc. can be used. The polymerization temperature is preferably 40 to 80°C, more preferably 50 to 70°C. The rate of temperature increase when the temperature is increased to the polymerization temperature is preferably 20°C / h or more, more preferably 45°C / h or more, from the viewpoint of suppressing dents in the hollow particles. The rate of temperature increase is usually 100°C / h or less. The polymerization reaction time is preferably 1 to 48 hours, more preferably 4 to 36 hours. The stirring speed of the suspension during the polymerization reaction is preferably 20 to 200 rpm, more preferably 20 to 100 rpm, from the viewpoint of suppressing dents in the hollow particles. When a tetrafunctional or higher crosslinkable monomer is contained as the crosslinkable monomer, the stirring speed is particularly preferably 20 to 100 rpm. In the polymerization process, the shell portion of the droplets of the monomer composition containing the hydrophobic solvent therein is polymerized, and as described above, a hollow portion filled with the hydrophobic solvent is formed inside the resulting precursor particles.

[0090] In this step, after a first polymerization reaction in which the suspension is subjected to a polymerization reaction, a polymerizable monomer may be further added to the precursor composition obtained by the first polymerization reaction, and a second polymerization reaction may be carried out. By carrying out the polymerization reaction in two stages in this manner in the polymerization step, the solvent resistance of the hollow particles can be improved.

[0091] The first polymerization reaction is preferably carried out until the polymerization conversion rate of the polymerizable monomer in the suspension reaches preferably 93% by mass or more, more preferably 95% by mass or more, even more preferably 98% by mass or more, and still 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 particles produced by the first polymerization reaction contained in the reaction solution immediately after the first polymerization reaction and the mass of the polymerizable monomer remaining unreacted after the first polymerization reaction, using the following formula (C): The mass of the unreacted polymerizable monomer can be measured using gas chromatography (GC). Polymerization conversion rate (mass%)=100−(mass of unreacted polymerizable monomer / mass of solid content of particles produced by first polymerization reaction)×100 (Equation (C)) The reaction time for the first polymerization reaction is preferably 0.5 to 5 hours, and more preferably 1 to 3 hours.

[0092] The polymerizable monomer added during the second polymerization reaction is not particularly limited, but from the viewpoint of improving the solvent resistance and strength of the hollow particles, a polymerizable monomer having a solubility of 0.3 g / L or more in distilled water at 20° C. is preferred, and a non-crosslinkable monomer having a solubility of 0.3 g / L or more in distilled water at 20° C. is more preferred. Preferred polymerizable monomers added during the second polymerization reaction include (meth)acrylic acid alkyl esters having an alkyl group of 1 to 5 carbon atoms, (meth)acrylamides and derivatives thereof, (meth)acrylic acid nitrile, and polar group-containing non-crosslinkable monomers. Among these, at least one selected from the group consisting of (meth)acrylic acid alkyl esters having an alkyl group of 1 to 5 carbon atoms and (meth)acrylic acid nitrile is preferred, at least one selected from the group consisting of (meth)acrylic acid alkyl esters having an alkyl group of 1 to 4 carbon atoms and acrylic acid nitrile is more preferred, and (meth)acrylic acid alkyl esters having an alkyl group of 1 to 4 carbon atoms are even more preferred. Preferred examples of the polar group-containing non-crosslinkable monomer include 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. More specifically, examples of the monomer include 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 styrenesulfonic acid; amino group-containing monomers, such as dimethylaminoethyl (meth)acrylate and diethylaminoethyl (meth)acrylate; polyoxyethylene group-containing monomers, such as methoxypolyethylene glycol (meth)acrylate; and epoxy group-containing monomers, such as glycidyl (meth)acrylate, allyl glycidyl ether, and 4-hydroxybutyl acrylate glycidyl ether. The polymerizable monomer added during the second polymerization reaction preferably has a solubility in distilled water at 20° C. of 2 g / L or more, more preferably 10 g / L or more, and even more preferably 15 g / L or more, so that it is easily incorporated into the shell and promotes thermal motion, thereby improving the strength of the hollow particles. The upper limit of the solubility of the polymerizable monomer added during the second polymerization reaction in distilled water at 20° C. is not particularly limited, but is usually 80 g / L or less.

[0093] The molecular weight of the polymerizable monomer added during the second polymerization reaction is not particularly limited, but from the viewpoint of improving the solvent resistance and strength of the hollow particles, it is preferably 200 or less, more preferably 100 or less. The lower limit of the molecular weight is not particularly limited, and is usually 50 or more.

[0094] The amount of the polymerizable monomer added during the second polymerization reaction is preferably 3 to 15 parts by mass, more preferably 4 to 10 parts by mass, per 100 parts by mass of the polymerizable monomer in the mixed solution, in order to improve the solvent resistance and strength of the hollow particles. The reaction time for the second polymerization reaction is preferably 1 to 6 hours, and more preferably 2 to 4 hours.

[0095] (4) Solid-liquid separation process This step is a step of obtaining a solid content containing precursor particles by solid-liquid separation of the precursor composition containing precursor particles obtained by the above-mentioned polymerization step.

[0096] The method for solid-liquid separation of the precursor composition is not particularly limited, and any known method can be used. Examples of the solid-liquid separation method include centrifugation, filtration, and static separation. Among these, centrifugation or filtration can be used, and centrifugation may be used from the viewpoint of ease of operation. After the solid-liquid separation step, an optional step such as a pre-drying step may be carried out before the solvent removal step described below is carried out. Examples of the pre-drying step include a step of pre-drying the solid content obtained after the solid-liquid separation step using a drying device such as a dryer or a drying appliance such as a hand dryer.

[0097] (5) Solvent removal process This step is a step for removing the hydrophobic solvent contained in the precursor particles obtained in the solid-liquid separation step. By removing the hydrophobic solvent contained in the precursor particles in the air, the hydrophobic solvent inside the precursor particles is replaced with air, and hollow particles filled with gas are obtained.

[0098] In this process, "in the air" strictly refers to an environment in which no liquid is present outside the precursor particles, or an environment in which only a trace amount of liquid is present outside the precursor particles, so that the removal of the hydrophobic solvent is not affected. "In the air" can also be referred to as a state in which the precursor particles are not present in a slurry, or a state in which the precursor particles are present in a dry powder. In other words, in this process, it is important to remove the hydrophobic solvent in an environment in which the precursor particles are in direct contact with the external gas.

[0099] The method for removing the hydrophobic solvent from the 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 hydrophobic solvent and equal to or lower than the maximum temperature at which the shell structure of the precursor particles does not collapse. Therefore, depending on the shell composition of the precursor particles and the type of hydrophobic 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 hydrophobic solvent inside the precursor particles is replaced by the external gas, resulting in hollow particles whose hollow spaces are filled with gas.

[0100] 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.

[0101] As an alternative method, the hydrophobic solvent may be removed by replacing the hydrophobic solvent contained in the precursor particles with the aqueous medium of the slurry containing the precursor particles in a slurry containing the precursor particles and the aqueous medium, without subjecting the slurry-like precursor composition obtained in the polymerization step to solid-liquid separation. In this method, the hydrophobic solvent contained in the precursor particles can be removed by bubbling an inert gas through the precursor composition at a temperature equal to or higher than the boiling point of the hydrophobic solvent minus 35°C. Here, when the hydrophobic solvent is a mixed solvent containing multiple types of hydrophobic solvents and has multiple boiling points, the boiling point of the hydrophobic solvent in the solvent removal step is the boiling point of the solvent with the highest boiling point among the solvents contained in the mixed solvent, i.e., the highest boiling point among the multiple boiling points. The temperature at which the inert gas is bubbled through the precursor composition is preferably at least 30°C below the boiling point of the hydrophobic solvent, more preferably at least 20°C below that, in order to reduce the amount of hydrophobic solvent remaining in the hollow particles. The bubbling temperature is usually set to a temperature equal to or higher than the polymerization temperature in the polymerization step. While not particularly limited, the bubbling temperature may be set to 50°C or higher and 100°C or lower. The inert gas to be bubbled is not particularly limited, but examples thereof include nitrogen and argon. The bubbling conditions are appropriately adjusted depending on the type and amount of the hydrophobic solvent so as to remove the hydrophobic solvent contained in the precursor particles, and are not particularly limited. For example, an 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 is obtained in which the aqueous medium is encapsulated in the precursor particles. The slurry is subjected to solid-liquid separation to obtain hollow particles, which are then dried to remove the aqueous medium from the hollow particles, thereby obtaining hollow particles whose hollow spaces are filled with gas.

[0102] Comparing a method of obtaining hollow particles having hollow portions filled with gas by performing solid-liquid separation on a slurry-like precursor composition and then removing the hydrophobic solvent in the precursor particles in an air atmosphere, and a method of obtaining hollow particles having hollow portions filled with gas by replacing the hydrophobic solvent contained in precursor particles in a slurry containing the precursor particles and an aqueous medium with the aqueous medium of the slurry, performing solid-liquid separation, and removing the aqueous medium in the precursor particles in an air atmosphere, the former method has the advantage that the hollow particles are less likely to be crushed in the step of removing the hydrophobic solvent, and the latter method has the advantage that the amount of residual hydrophobic solvent is reduced by performing bubbling with an inert gas.

[0103] Alternatively, as a method for removing the hydrophobic organic solvent contained in the precursor particles after the polymerization step and before the solid-liquid separation step without performing solid-liquid separation on the slurry precursor composition obtained in the polymerization step, for example, a method of evaporating and distilling off the hydrophobic organic solvent contained in the precursor particles from the precursor composition under a predetermined pressure (high pressure, normal pressure, or reduced pressure); or a method of introducing an inert gas such as nitrogen, argon, or helium, or water vapor, into the precursor composition under a predetermined pressure (high pressure, normal pressure, or reduced pressure) and evaporating and distilling off the hydrophobic organic solvent may be used.

[0104] (6) Other As steps other than the above steps (1) to (5), for example, the following step (6-a) of cleaning and the following step (6-b) of replacing the hollow portion again may be added. (6-a) Cleaning process The washing step is a step of adding an acid or alkali to wash the precursor composition containing the precursor particles before the solid-liquid separation step in order to remove any dispersion stabilizer remaining in the precursor composition containing the precursor particles. When the dispersion stabilizer used is an inorganic dispersion stabilizer soluble in acid, it is preferable to add an acid to the precursor composition containing the precursor particles to wash the precursor composition. On the other hand, when the dispersion stabilizer used is an inorganic compound soluble in alkali, it is preferable to add an alkali to the precursor composition containing the precursor particles to wash the precursor composition. Furthermore, when an acid-soluble inorganic dispersion stabilizer is used as the dispersion stabilizer, it is preferable to add an acid to the precursor composition containing the precursor particles to adjust the pH to preferably 6.5 or less, more preferably 6 or less. The acid to be added may be an inorganic acid such as sulfuric acid, hydrochloric acid, or nitric acid, or an organic acid such as formic acid or acetic acid, but sulfuric acid is particularly preferable because it has a high efficiency in removing the dispersion stabilizer and places a small burden on the production equipment.

[0105] (6-b) Re-replacement process of hollow part The hollow re-substitution process is a process in which the gas or liquid inside the hollow particles is replaced with another gas or liquid. This substitution can change the environment inside the hollow particles, selectively confine molecules inside the hollow particles, or modify the chemical structure inside the hollow particles to suit the application.

[0106] 3. Uses of hollow particles The hollow particles of the present disclosure have excellent pressure resistance and are therefore resistant to crushing when kneaded with other materials and during molding after kneading, and when added to a molded body, they are particularly suitable as additives for molded bodies because they have excellent effects as a weight-saving material, heat insulating material, soundproofing material, vibration damping material, etc. The hollow particles of the present disclosure are particularly suitable as additives for resin molded bodies because they are resistant to crushing when kneaded with resin and during molding after kneading. The molded article containing the hollow particles of the present disclosure may contain a thermoplastic resin, such as polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyurethane, epoxy resin, acrylonitrile-butadiene-styrene (ABS) resin, acrylonitrile-styrene (AS) resin, poly(meth)acrylate, polycarbonate, polyamide, polyimide, polyphenylene ether, polyphenylene sulfide, polyester, polytetrafluoroethylene, maleimide resin, bismaleimide triazine resin, liquid crystalline polyester resin, phenolic resin, vinyl ester resin, unsaturated polyester resin, cyanate ester resin, polyether ketone ketone resin, or polyetherimide resin, a thermosetting resin, or a room-temperature curing resin. The resin may be used together with an additive for curing the resin, such as a curing agent or a curing catalyst. The curing agent and curing catalyst may be appropriately selected from known curing agents and curing catalysts depending on the type of resin, and examples thereof include amines, acid anhydrides, imidazoles, thiols, phenols, naphthols, benzoxazines, cyanate esters, and carbodiimides. Furthermore, the molded article containing the hollow particles of the present disclosure may contain a thermoplastic elastomer as a resin. Examples of the thermoplastic elastomer include thermoplastic elastic polymers that have traditionally been used as molding resins, such as urethane elastomers, styrene elastomers, olefin elastomers, amide elastomers, and ester elastomers. Thermoplastic elastomers generally exhibit rubber elasticity at room temperature (25°C) and are plasticized and moldable at high temperatures. Furthermore, the molded article containing the hollow particles of the present disclosure is not limited to a resin molded article, but may be, for example, a rubber molded article, or may contain a mixture of resin and rubber. The molded article containing the hollow particles of the present disclosure may contain, for example, rubber such as natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene copolymer rubber (SBR), acrylonitrile-butadiene copolymer rubber (NBR), or ethylene-propylene-diene terpolymer (EPDM). The rubbers may be used alone or in combination of two or more. Furthermore, molded articles containing the hollow particles of the present disclosure may further contain organic or inorganic reinforcing fibers such as carbon fibers, glass fibers, aramid fibers, polyethylene fibers, etc. The hollow particles of the present disclosure can also be contained as a filler in molded articles formed using a thermoplastic or thermosetting resin or rubber, and in fiber-reinforced molded articles formed using a thermoplastic or thermosetting resin or rubber and further reinforcing fibers. Examples of uses of resin molded articles or rubber molded articles containing the hollow particles of the present disclosure include light-reflecting materials, heat insulating materials, sound insulating materials, and low-dielectric materials used in various fields such as automobiles, electricity, electronics, construction, aviation, and space, food containers, footwear such as sports shoes and sandals, home appliance parts, bicycle parts, stationery, tools, and 3D printer filaments. The hollow particles of the present disclosure having a volume average particle size of 10 to 50 μm are particularly excellent in pressure resistance, and can therefore be suitably used as an additive for molded articles obtained through processes that apply external pressure, such as kneading and injection molding. 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. [Example]

[0107] The present disclosure will be described in more detail below with reference to examples and comparative examples, but the present disclosure is not limited to these examples. Note that parts and percentages are by mass unless otherwise specified.

[0108] [Example 1] (1) Mixed liquid preparation process First, the following materials were mixed to form an oil phase. Ethylene glycol dimethacrylate 70 parts Trimethylolpropane trimethacrylate 30 parts 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 0.007 parts 131 parts cyclohexane and 56 parts toluene (hydrophobic solvent) Next, in a stirring tank, an aqueous solution of 12.1 parts of sodium hydroxide (alkali metal hydroxide) in 121 parts of ion-exchanged water was gradually added under stirring to an aqueous solution of 17.1 parts of magnesium chloride (a water-soluble polyvalent metal salt) in 494 parts of ion-exchanged water at room temperature to prepare a magnesium hydroxide colloid (poorly water-soluble metal hydroxide colloid) dispersion (4 parts magnesium hydroxide) as the aqueous phase. The resulting aqueous phase and oil phase were mixed to prepare a mixed liquid.

[0109] (2) Suspension process The mixture obtained in the mixture preparation step was stirred and suspended for 1 minute using a disperser (manufactured by Primix Corporation, trade name: Homomixer) at a rotation speed of 4,000 rpm, to prepare a suspension in which droplets of the monomer composition encapsulating the hydrophobic solvent were dispersed in water.

[0110] (3) Polymerization process The suspension obtained in the suspension step was heated in a nitrogen atmosphere from 40°C to 65°C over 30 minutes (heating rate: 50°C / h), and stirred at 65°C for 1 hour and 30 minutes at a stirring speed of 100 rpm to carry out a first polymerization reaction. The polymerization conversion rate at the end of the first polymerization reaction was 99.2 mass%. Subsequently, 5 parts of methyl acrylate was added to the stirring tank, and the mixture was stirred in a nitrogen atmosphere at 65°C for 2 hours and 30 minutes at a stirring speed of 100 rpm to carry out a second polymerization reaction. This second polymerization reaction yielded a precursor composition containing precursor particles encapsulating a hydrophobic solvent.

[0111] (4) Washing process and solid-liquid separation process The precursor composition was washed with dilute sulfuric acid (25°C, 10 minutes) to adjust the pH to 5.5 or less. Next, after separating the water by filtration, 200 parts of ion-exchanged water was added to re-slurry the mixture. The water washing process (washing, filtration, dehydration) was repeated several times at room temperature (25°C), and the mixture was filtered to obtain a solid fraction. The obtained solid fraction was dried in a dryer at 40°C to obtain precursor particles encapsulating the hydrophobic solvent.

[0112] (5) Solvent removal process The precursor particles obtained in the solid-liquid separation step were heat-treated in a vacuum dryer at 200°C for 6 hours to remove the hydrophobic solvent contained in the particles, thereby obtaining hollow particles of Example 1. From the results of observation with a scanning electron microscope and the porosity value, it was confirmed that the obtained hollow particles were spherical and had hollow portions.

[0113] [Example 2] The hollow particles of Example 2 were obtained in the same manner as in Example 1, except that in the above "(1) Mixture preparation step", the materials and amounts of the polymerizable monomers added to the oil phase were as shown in Table 1.

[0114] [Example 3] The hollow particles of Example 3 were obtained in the same manner as in Example 1, except that in the above "(1) mixed solution preparation step", the type and amount of the hydrophobic solvent were changed as shown in Table 1.

[0115] [Example 4] The hollow particles of Example 4 were produced in the same manner as in Example 2, except that in the above "(3) polymerization step", only the first polymerization reaction was carried out, and the addition of methyl acrylate and the second polymerization reaction were not carried out.

[0116] [Comparative Examples 1, 3 to 7] The hollow particles of Comparative Examples 1 and 3 to 7 were produced in the same manner as in Example 1, except that in the above "(1) mixed solution preparation step," the materials and amounts of the polymerizable monomers added to the oil phase were as shown in Table 1, and further, the type and amount of the hydrophobic solvent were as shown in Table 1.

[0117] Comparative Example 2 (1) Mixed liquid preparation process First, the following materials were mixed to form an oil phase. Methacrylic acid 5 parts Ethylene glycol dimethacrylate 65 parts Trimethylolpropane trimethacrylate 30 parts 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 0.007 parts 187 parts cyclohexane Next, in a stirring tank, 3 parts of a surfactant (sodium dodecylbenzenesulfonate) was added to 1160 parts of ion-exchanged water at room temperature to form an aqueous phase. The resulting aqueous phase and oil phase were mixed to prepare a mixed liquid.

[0118] (2) Suspension process The mixture obtained in the mixture preparation step was stirred and suspended for 1 minute using a disperser (manufactured by Primix Corporation, trade name: Homomixer) at a rotation speed of 4,000 rpm, to prepare a suspension in which droplets of the monomer composition encapsulating cyclohexane were dispersed in water.

[0119] (3) Polymerization process The suspension obtained in the above suspension step was heated in a nitrogen atmosphere from 40°C to 65°C over 30 minutes (heating rate: 50°C / h), and then stirred at a temperature of 65°C at a stirring speed of 100 rpm for 4 hours to carry out a polymerization reaction, thereby obtaining a precursor composition containing precursor particles encapsulating cyclohexane.

[0120] (4) Washing process and solid-liquid separation process After filtering the precursor composition to separate the water, 200 parts of ion-exchanged water was added to re-slurry the mixture, and the water washing process (washing, filtration, dehydration) was repeated several times at room temperature (25°C), followed by filtering to obtain a solid fraction. The obtained solid fraction was dried in a dryer at 40°C to obtain precursor particles containing cyclohexane.

[0121] (5) Solvent removal process The precursor particles obtained in the solid-liquid separation step were heat-treated in a vacuum dryer at 200°C for 6 hours to remove the cyclohexane contained in the particles, thereby obtaining hollow particles of Comparative Example 2. From the results of observation with a scanning electron microscope and the porosity value, it was confirmed that the obtained hollow particles were spherical and had hollow portions.

[0122] [Comparative Example 8] The hollow particles of Comparative Example 8 were produced in the same manner as in Example 1, except that in the above "(1) mixed solution preparation step", the material and amount of the polymerizable monomer to be added to the oil phase were as shown in Table 1, the type and amount of the hydrophobic solvent were as shown in Table 1, and in the above "(3) polymerization step", the temperature increase rate when increasing the temperature from 40°C to 65°C and the stirring speed during the first polymerization reaction and the second polymerization reaction were as shown in Table 1.

[0123] [Table 1]

[0124] [evaluation] 1. Polymerization conversion rate In the polymerization process of each example and each comparative example, 50 g of the reaction solution immediately after the first polymerization reaction was collected and pressure-filtered to obtain the particles (containing moisture and a hydrophobic solvent) generated by the first polymerization reaction contained in the reaction solution, and they were accurately weighed to the unit of 1 mg. To about 3 g of the accurately weighed particles, 27 g of ethyl acetate was added and stirred for 15 minutes, and then 13 g of methanol was added and stirred for another 10 minutes. The obtained 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 the polymerizable monomer in the measurement sample was quantified by gas chromatography (GC) under the following conditions, and this was taken as the mass of the unreacted polymerizable monomer. Further, the particles generated by the first polymerization reaction obtained by pressure filtration were dried at 200 °C for 2 hours to remove moisture and the hydrophobic solvent, and the mass of the solid content of the particles was determined. Then, the polymerization conversion rate was calculated by the following formula (C). Polymerization conversion rate (mass %) = 100 - (mass of unreacted polymerizable monomer / mass of solid content of particles generated by the first polymerization reaction) × 100 Formula (C) <GC conditions> Column: TC-WAX (0.25 mm × 30 m) Column temperature: 80 °C Injection temperature: 200 °C FID detection side temperature: 200 °C

[0125] Regarding the hollow particles obtained in each example and each comparative example, the content ratio (mass %) of each monomer unit in the polymer contained in the shell is shown in Table 2. In addition, the following measurements and evaluations were performed on the hollow particles obtained in each example and each comparative example. The results are shown in Table 2.

[0126] 2. Volume average particle diameter of hollow particles Using a laser diffraction particle size distribution analyzer (manufactured by Shimadzu Corporation, product name: SALD-2000), the particle diameter of the hollow particles was measured, and the volume average was calculated to obtain the volume average particle diameter.

[0127] 3. Average circularity A container was pre-filled with 10 mL of ion-exchanged water, to which 0.02 g of a surfactant (alkylbenzene sulfonic acid) was added as a dispersant. Then, 0.02 g of hollow particles were added and dispersed using an ultrasonic disperser at 60 W (Watt) for 3 minutes. The amount of ion-exchanged water was adjusted to a hollow particle concentration of 3,000 to 10,000 particles / μL to obtain a measurement sample. Using the resulting measurement sample, 5,000 hollow particles with a circle-equivalent diameter of 0.4 μm or greater were measured using a flow particle image analyzer (manufactured by CIMEX Corporation, product name: FPIA-2100) to measure the perimeter of a circle equal to the projected area of ​​the particle and the perimeter of the projected image of the particle. The circularity was calculated using the following formula 1. The average circularity of the 5,000 hollow particles was taken as the average circularity. Calculation formula 1: (Circularity) = (perimeter of a circle equal to the projected area of ​​the particle) / (perimeter of the projected image of the particle)

[0128] 4. Density and porosity of hollow particles 4-1. Measurement of apparent density of hollow particles First, a capacity of 100cm 3 30cm into a measuring flask 3 The volumetric flask was filled with 10 ... 3 ) was calculated. Formula (I) Apparent density D1 = [Mass of hollow particles] / (100 - [Mass of isopropanol] ÷ [Specific gravity of isopropanol at measurement temperature])

[0129] 4-2. Measurement of true density of hollow particles After crushing the hollow particles in advance, 3 Approximately 10 g of crushed pieces of hollow particles was filled into a measuring flask, and the mass of the packed crushed pieces was accurately weighed. Then, in the same manner as in the measurement of the apparent density, isopropanol is added to the measuring flask, the mass of isopropanol is accurately weighed, and the true density D0 (g / cm) of the hollow particles is calculated based on the following formula (II). 3 ) was calculated. Formula (II) True density D0 = [mass of crushed hollow particle fragments] / (100 - [mass of isopropanol] ÷ [specific gravity of isopropanol at measurement temperature])

[0130] 4-3. Calculation of void ratio The porosity of the hollow particles was calculated from the apparent density D1 and true density D0 of the hollow particles according to the following formula (III). Formula (III) Porosity (%) = 100 - (apparent density D1 / true density D0) x 100

[0131] 5. Shell thickness of hollow particles The inner diameter r of the hollow particles was calculated using the volume average particle diameter R and porosity of the hollow particles according to the following formula (1), and the shell thickness of the hollow particles was calculated using the inner diameter r and the volume average particle diameter R according to the following formula (2). 4 / 3π×(R / 2) 3 ×(porosity / 100)=4 / 3π×(r / 2) 3 Formula (1) Shell thickness = (Rr) / 2 Equation (2)

[0132] 6. Percentage of particles with recesses The hollow particles were photographed using a field emission scanning electron microscope (FE-SEM), and 100 hollow particles were randomly selected from the photographs. The diameter of a circle having the same projected area as the particle image was measured as the particle diameter. If the hollow particles had recesses, the maximum width of the recesses in the FE-SEM image was measured as the size of the recesses. The number of particles having recesses that were 5 to 50% of the particle diameter was counted and evaluated according to the following evaluation criteria. (Evaluation criteria) ◎: The percentage of particles with recesses that are 5 to 50% of the particle diameter is less than 5% Good: The percentage of particles with recesses that are 5 to 50% of the particle diameter is 5% or more and 10% or less ×: The ratio of particles having recesses of 5 to 50% of the particle diameter exceeds 10%

[0133] 7. Void Remaining Rate in Molded Product Polypropylene as a thermoplastic resin (Mitsubishi Chemical, product name: MA1B, specific gravity 0.90 g / cm 3 90 parts of the cellulose acetate copolymer (C100) and 10 parts of the hollow particles obtained in each Example or Comparative Example were mixed in a blender. The mixture was then kneaded under the following kneading conditions using a twin-screw kneader (manufactured by Toshiba Machine Co., Ltd., product name: TEM-35B), extruded, and pelletized to obtain pellets of the resin composition. <Mixing conditions> Screw diameter 37mm, L / D=32 Screw rotation speed: 250 rpm Resin temperature 190℃ Feed rate 20kg / hour The resulting resin composition pellets were dried by heating at 80°C for 6 hours, and then molded using an injection molding machine under the following molding conditions to obtain a molded product measuring 80 mm x 10 mm x 4 mm thick. <Molding conditions> Cylinder temperature: 230℃ Mold temperature: 40℃ Injection pressure: 70MPa The void remaining rate was calculated using the following formula (D) using a: specific gravity of the molded body after injection molding, b: specific gravity (calculated value) of the molded body assuming that the voids are maintained, and c: specific gravity (calculated value) of the molded body assuming that all hollow particles are crushed. Void survival rate (%)={(ca) / (cb)}×100 Formula (D) The specific gravity of the injection-molded article was measured by the underwater displacement method in accordance with JIS K 7112. The specific gravity b of the molded body, assuming that voids were maintained, was calculated by the following formula (E). b=1 / {(P A / P G )+(R A / R G )} Formula (E) In the formula for calculating b above, P A is the amount of hollow particles added, P Gis the specific gravity of the hollow particles, R A is the amount of thermoplastic resin added, R G represents the specific gravity of the thermoplastic resin. The specific gravity c of the molded body, assuming that all the hollow particles were crushed, was calculated by the following formula (F). c=[R G ×R A +{D0×P A ×(1-P V / 100)}] / {R A +P A ×(1-P V / 100)} Formula (F) In the formula for calculating c above, R A is the amount of thermoplastic resin added, R G is the specific gravity of the thermoplastic resin, D0 is the true density of the hollow particles, P A is the amount of hollow particles added, P V and represent the porosity (%) of the hollow particles.

[0134] [Table 2]

[0135] [Consideration] The hollow particles obtained in Comparative Examples 1, 4, 7, and 8 had a ratio of particles having recesses of 5 to 50% of the particle diameter when observed in a plan view of more than 10%, and therefore had a low residual void rate in the molded product and poor pressure resistance. It is presumed that the recesses caused the reduced pressure resistance of the hollow particles obtained in Comparative Examples 1, 4, 7, and 8. Furthermore, when Comparative Example 1 and Comparative Example 4, which have the same shell composition, are compared, the void remaining rate in Comparative Example 1 was lower than that in Comparative Example 4. This is presumably because, in particles having recesses that are 5 to 50% of the particle diameter, differences in shell thickness are likely to affect pressure resistance. The hollow particles obtained in Comparative Example 1 had a thinner shell thickness than the hollow particles obtained in Comparative Example 4, and therefore, it is presumed that the pressure resistance of the particles having recesses that are 5 to 50% of the particle diameter was further reduced. In addition, in Comparative Example 7, the shell composition was the same as in Example 2, but the type of hydrophobic solvent used in producing the hollow particles was different, which is presumably why recesses were formed on the surface of the particles. Comparative Example 8 also had the same shell composition as Example 2. However, like Comparative Example 7, the type of hydrophobic solvent used in producing the hollow particles was different, and the heating rate and stirring speed during the polymerization reaction were also different. Therefore, it is estimated that more recesses were generated on the particle surface in Comparative Example 8 than in Comparative Example 7, which is thought to have resulted in a further decrease in the residual void rate in the molded body. The hollow particles obtained in Comparative Example 2 had a volume average particle size of 3 μm, and therefore had a low void remaining rate in the molded body and poor pressure resistance. It is presumed that the hollow particles obtained in Comparative Example 2 had poor pressure resistance because of their small volume average particle size and thin shell thickness. The hollow particles obtained in Comparative Example 3 had a content of crosslinkable monomer units of less than 70 parts by mass per 100 parts by mass of all monomer units of the polymer contained in the shell, and when observed in a plan view, the proportion of particles having recesses 5 to 50% of the particle diameter exceeded 10%, resulting in a low residual void rate in the molded product and poor pressure resistance.It is presumed that the hollow particles obtained in Comparative Example 3 had poor pressure resistance because the shell strength was insufficient and the recesses further reduced pressure resistance. The hollow particles obtained in Comparative Example 5 had a low residual void rate in the molded product because the crosslinkable monomer units in the polymer contained in the shell did not contain any bifunctional crosslinkable monomer units. In Comparative Example 5, the shell was formed using only trifunctional or higher crosslinkable monomers as the crosslinkable monomer, so the obtained hollow particles had interconnected pores, and it is presumed that the resin penetrated into the particles through the interconnected pores, resulting in a low residual void rate. The hollow particles obtained in Comparative Example 6 did not contain any tri- or higher functional crosslinkable monomer units in the polymer contained in the shell, and when observed in a plan view, the proportion of particles having recesses of 5 to 50% of the particle diameter exceeded 10%, resulting in a low residual void rate in the molded product and poor pressure resistance. In Comparative Example 6, the shell was formed using only bifunctional crosslinkable monomers, which is thought to be why recesses were likely to occur on the particle surface and reduced pressure resistance.

[0136] The hollow particles obtained in each example had a volume average particle diameter of 10 to 50 μm, a content of crosslinkable monomer units of 70 parts by mass or more per 100 parts by mass of all monomer units of the polymer contained in the shell, and the crosslinkable monomer units included difunctional crosslinkable monomer units and trifunctional or higher crosslinkable monomer units. When observed in a plan view, the proportion of particles having recesses with a size of 5 to 50% of the particle diameter was 10% or less. Therefore, even though the porosity was as high as 60% or more, the residual void rate in the molded product was high and the pressure resistance was excellent. [Explanation of symbols]

[0137] 1 Aqueous medium 2 Low polarity material 3. Dispersion stabilizer 4. Monomer composition 4a Hydrophobic solvents 4b Materials other than hydrophobic solvents 4c Polymerizable monomer dispersed in an aqueous medium 5. Oil-soluble polymerization initiator 6 shells 8 Hollow part 10 droplets 20 precursor particles 100 Hollow particles with the hollow space filled with gas

Claims

1. A hollow particle having a shell containing a resin and a hollow portion surrounded by the shell, The porosity is 60% or more, The volume average particle size is 10 to 50 μm, 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, the crosslinkable monomer units including bifunctional crosslinkable monomer units derived from bifunctional crosslinkable monomers and trifunctional or higher crosslinkable monomer units derived from trifunctional or higher crosslinkable monomers; Hollow particles, wherein the proportion of particles having recesses whose size is 5 to 50% of the particle diameter when observed in a plan view is 10% or less, and the proportion of particles is measured using 100 hollow particles selected from an FE-SEM image of the hollow particles taken with an FE-SEM (field emission scanning electron microscope), measuring the diameter of a circle having the same projected area as the particle image in the FE-SEM image as the particle diameter, and if the hollow particles have recesses, measuring the distance in the maximum width direction of the recesses in the FE-SEM image as the size of the recesses, and counting the number of particles having the recesses whose size is 5 to 50% of the particle diameter.

2. 2. The hollow particle according to claim 1, wherein the content of the bifunctional crosslinkable monomer unit is 60 to 95 parts by mass based on 100 parts by mass of all monomer units of the polymer contained in the shell.

3. 3. The hollow particle according to claim 1, wherein the content of the tri- or higher functional crosslinkable monomer units is 5 to 40 parts by mass relative to 100 parts by mass of all monomer units of the polymer contained in the shell.

4. The hollow particle according to any one of claims 1 to 3, wherein the tri- or higher functional crosslinkable monomer unit is a crosslinkable monomer unit derived from a methacrylic crosslinkable monomer having a methacryloyl group as a polymerizable functional group.

5. The hollow particle according to any one of claims 1 to 4, wherein the shell has a thickness of 0.20 to 4.00 µm.

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