Hollow particles

Abstract

The present invention provides hollow particles having high porosity and excellent pressure resistance. The hollow particles have a shell composed of a resin and a hollow portion surrounded by the shell, have a porosity of 60% or greater, and a volume average particle size of 10 to 50 μm. The shell comprises, as the resin, a polymer containing 70 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of all monomer units, wherein the crosslinkable monomer units include 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 viewed from above, the proportion of the particles having recesses having a size of 5 to 50% of the particle size is 10% or less.

Description

Technical Field

[0001] This invention relates to hollow particles. Background Technology

[0002] Hollow granules (hollow resin granules) are granules with cavities inside. Compared to solid granules that are essentially filled with resin, they can scatter light effectively and reduce light transmittance. Therefore, they are widely used as organic pigments and masking agents with excellent optical properties such as opacity and whiteness in water-based coatings and paper coating compositions. In recent years, they have also been further used as lightweighting agents and heat-insulating agents in resins and coatings used in various fields such as automobiles, electrical appliances, electronics, and construction.

[0003] To improve the lightweighting, heat insulation, opacity, and whitening effects of various compositions and molded articles incorporating hollow particles, it is desirable for the hollow particles to maintain high porosity during both compounding with other materials and during molding after compounding. However, for hollow particles, increasing porosity leads to a thinner shell, making them more prone to breakage. Therefore, there is a need for hollow particles with high porosity and resistance to breakage.

[0004] Furthermore, the hollow particles are preferably spherical. Patent documents 1 to 4 describe methods for obtaining spherical hollow particles using a polymerization method.

[0005] Patent Document 1 describes a method for manufacturing perfectly spherical hollow resin microparticles: After a step of producing microcapsule-type polymer particles containing an organic solvent via free radical polymerization using a reactive monomer having functional groups such as vinyl groups capable of free radical polymerization and functional groups such as epoxy groups capable of crosslinking, a step of crosslinking the resins constituting the microcapsule-type polymer particles using a crosslinking agent is performed. The hollow resin microparticles described in Patent Document 1 are added to an anti-reflective film used in displays for this purpose, and have an average particle size of 100 nm or less.

[0006] Patent Document 2 describes a method in which hollow resin particles are obtained by combining at least one monomer selected from monovinyl monomers and hydrophilic monomers and a crosslinking monomer in a specific ratio, and further using an oil-soluble polymerization initiator as a polymerization initiator. This method avoids the risk of pitting and thus maintains a high porosity in the hollow resin particles. In Patent Document 2, a bifunctional crosslinking monomer is described as a preferred crosslinking monomer.

[0007] Patent Document 3 describes a method for obtaining hollow resin particles with a high ratio of perfectly spherical shapes and a high hollowness while ensuring strength, by using a polyfunctional monomer having three or more polymerizable double bonds as a crosslinking monomer. Patent Document 3 also describes that the crosslinking monomer does not contain a bifunctional monomer, and is preferably formed from a trifunctional or higher monomer.

[0008] Patent Document 4 describes a suspension polymerization method using a monomer composition containing a high proportion of crosslinking monomers. The aqueous medium in the resulting suspension is replaced with a polar solvent, thereby removing the hydrocarbon solvent contained in the precursor particles in a short time, resulting in hollow resin particles with good porosity and compressive strength. Patent Document 4 describes a bifunctional crosslinking monomer as a preferred crosslinking monomer.

[0009] Existing technical documents

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application Publication No. 2006-89648;

[0012] Patent Document 2: International Publication No. 2019 / 26899;

[0013] Patent Document 3: Japanese Patent Application Publication No. 2020-33503;

[0014] Patent document 4: Japanese Patent Application Publication No. 2020-132820. Summary of the Invention

[0015] The problem the invention aims to solve

[0016] For hollow particles, increasing the porosity leads to a thinner shell, which tends to reduce pressure resistance. However, for large-diameter hollow particles, even with increased porosity, the shell thickness does not become excessively thin, thus allowing for excellent pressure resistance while maintaining high porosity. Nevertheless, for hollow particles with a volume average diameter of 10 μm or more, sometimes even sufficient shell thickness does not guarantee adequate pressure resistance.

[0017] The purpose of this invention is to provide a hollow particle with high porosity and excellent pressure resistance.

[0018] Solution for solving the problem

[0019] The inventors have discovered that in hollow particles with a volume average particle size of 10 μm or more, the polymer forming the shell contains a large number of crosslinking monomer units, and these crosslinking monomer units are crosslinking monomer units derived from bifunctional crosslinking monomers and trifunctional or higher crosslinking monomers. As a result, the surface of the shell is smooth, thus exhibiting excellent pressure resistance even with a porosity of 60% or more.

[0020] This invention provides a hollow particle having a shell containing resin and a hollow portion surrounded by the shell.

[0021] The porosity of the hollow particles is over 60%.

[0022] The volume average particle size of the hollow particles is 10–50 μm.

[0023] The aforementioned shell contains a polymer comprising 70 to 100 parts by mass of crosslinking monomer units in 100 parts by mass of all monomer units as the aforementioned resin. The crosslinking monomer units include bifunctional crosslinking monomer units derived from bifunctional crosslinking monomers and trifunctional or higher crosslinking monomer units derived from trifunctional or higher crosslinking monomers.

[0024] When viewed from above, the proportion of particles with concave portions that are 5 to 50% of the particle size is less than 10%.

[0025] In the hollow particles of the present invention, the content of the bifunctional crosslinkable monomer unit in 100 parts by mass of all monomer units of the polymer contained in the shell is preferably 60 to 95 parts by mass.

[0026] In the hollow particles of the present invention, the content of the trifunctional or higher crosslinked monomer units in 100 parts by mass of all monomer units of the polymer contained in the shell is preferably 5 to 40 parts by mass.

[0027] In the hollow particles of the present invention, the aforementioned trifunctional or higher crosslinking monomer units can be crosslinking monomer units derived from methacrylic acid-based crosslinking monomers having methacryloyl groups as polymerizable functional groups.

[0028] In the hollow particles of the present invention, the thickness of the shell is preferably 0.20 to 4.00 μm.

[0029] Invention Effects

[0030] As described above, according to the present invention, it is possible to provide hollow particles with high porosity and excellent pressure resistance. Attached Figure Description

[0031] Figure 1 A figure illustrating an example of a method for manufacturing hollow particles according to the present invention.

[0032] Figure 2 This is a schematic diagram illustrating one embodiment of the suspension in the suspension process. Detailed Implementation

[0033] It should be noted that in this invention, the “~” in the numerical range refers to the values ​​recorded before and after it as the lower limit and upper limit.

[0034] Furthermore, in this invention, (meth)acrylate represents each of acrylate and methacrylate, (meth)acrylic acid represents each of acrylic acid and methacrylic acid, and (meth)acryloyl group represents each of acryloyl group and methacryloyl group.

[0035] Furthermore, in this invention, a polymerizable monomer refers to a compound having a functional group capable of addition polymerization (sometimes simply referred to as a polymerizable functional group in this invention). In this invention, compounds having an olefinic unsaturated bond as a functional group capable of addition polymerization are generally used as polymerizable monomers.

[0036] As polymerizable monomers, there exist non-crosslinked monomers and crosslinked monomers. Non-crosslinked monomers are polymerizable monomers with only one polymerizable functional group, while crosslinked monomers are polymerizable monomers with two or more polymerizable functional groups that form crosslinks in the resin through polymerization.

[0037] 1. Hollow particles

[0038] The hollow particles of the present invention are characterized in that they have a shell containing resin and a hollow portion surrounded by the shell.

[0039] With a porosity of over 60%,

[0040] The volume average particle size is 10–50 μm.

[0041] The aforementioned shell contains a polymer comprising 70 to 100 parts by mass of crosslinking monomer units in 100 parts by mass of all monomer units as the aforementioned resin. The crosslinking monomer units include bifunctional crosslinking monomer units derived from bifunctional crosslinking monomers and trifunctional or higher crosslinking monomer units derived from trifunctional or higher crosslinking monomers.

[0042] When viewed from above, the proportion of particles with concave portions that are 5 to 50% of the particle size is less than 10%.

[0043] The hollow particles of the present invention are particles having a shell (outer shell) containing resin and a hollow portion surrounded by the shell.

[0044] In this invention, the hollow portion is a void-like space that is clearly distinguishable from the shell of a hollow particle formed of resin material. The shell of the hollow particle may have a porous structure, in which case the hollow portion has a size that is clearly distinguishable from the multiple tiny spaces uniformly dispersed within the porous structure.

[0045] The hollow portion of hollow particles can be confirmed, for example, by SEM observation of particle profiles or by TEM observation of the particles directly.

[0046] In addition, the hollow part of hollow particles can be filled with gases such as air, or it can be in a near-vacuum depressurization state, or it can contain solvents.

[0047] The thicker the shell of hollow particles, the higher their pressure resistance. Therefore, large-diameter hollow particles can be expected to balance high porosity and excellent pressure resistance. However, conventional hollow particles with a volume average particle size of 10 μm or more sometimes lack sufficient pressure resistance even with a sufficiently thick shell.

[0048] For example, as described in Patent Documents 2 and 4, when hollow particles with a volume average particle size of 10 μm or more are manufactured using only bifunctional crosslinking monomers as crosslinking monomers, as in Comparative Example 6 described later, there is a problem that the particles develop indentations and the pressure resistance is significantly reduced.

[0049] On the other hand, as described in Patent Document 3, when only a trifunctional crosslinking monomer is used as the crosslinking monomer to manufacture hollow particles with a volume average particle size of 10 μm or more, as in Comparative Example 5 described later, since the shell has fine pores or interconnected pores, the resin is impregnated into the particles during the mixing of the resin composition containing the hollow particles or during injection molding, resulting in the problem that the resin composition or its molded body cannot be made lightweight.

[0050] Furthermore, when hollow particles with a volume average particle size of 10 μm or more are manufactured using the method described in Patent Document 1, since the crosslinking agent is water-soluble, there is a problem that the crosslinking reaction in the shell is not fully carried out, resulting in insufficient improvement in the pressure resistance of the hollow particles.

[0051] The inventors have discovered that hollow particles with a volume average particle size of 10 μm or more mostly have a concave portion at the shell. When a concave portion is present at the shell, the pressure resistance is significantly reduced. It can be inferred that when hollow particles have a concave portion at the shell, when external pressure is applied to the hollow particles, the pressure is concentrated at the concave portion, therefore, the shell is prone to rupture from the concave portion.

[0052] In contrast, the hollow particles of the present invention exhibit excellent pressure resistance while maintaining a volume average particle size of 10–50 μm. The hollow particles of the present invention have a volume average particle size of 10–50 μm, thus possessing sufficient shell thickness even with a porosity exceeding 60%. Furthermore, the hollow particles of the present invention possess a cross-linked structure with excellent strength because the polymer forming the shell contains 70–100 parts by mass of cross-linking monomer units in all monomer units per 100 parts by mass, and these cross-linking monomer units include bifunctional and trifunctional or higher cross-linking monomer units. Moreover, when viewed from above, the hollow particles of the present invention have less than 10% of particles with recesses that are 5–50% of the particle size, resulting in a smooth shell surface and suppressing the reduction in pressure resistance caused by the recesses in the shell. Therefore, the hollow particles of the present invention possess a shell with excellent strength at a sufficient thickness, thereby suppressing the reduction in pressure resistance caused by the recesses, thus achieving excellent pressure resistance and balancing high porosity with excellent pressure resistance.

[0053] The hollow particles of the present invention have a porosity of 60% or more, preferably 65% ​​or more. With a porosity of 60% or more, the hollow particles exhibit excellent lightweight, heat resistance, and thermal insulation properties. The upper limit of the porosity of the hollow particles of the present invention is not particularly limited, but from the perspective of suppressing the reduction in strength of the hollow particles and improving their pressure resistance, it is preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less.

[0054] The porosity of the hollow particles of the present invention can be calculated based on the apparent density D1 and true density D0 of the hollow particles.

[0055] The method for determining the apparent density D1 of hollow particles is as follows. First, approximately 30 cm³ of... 3 Hollow particles are filled in a volume of 100 cm³ 3 In a volumetric flask, accurately weigh the mass of the hollow granules used to fill it. Next, carefully fill the volumetric flask with isopropanol to the mark, taking care to avoid introducing air bubbles. Accurately weigh the mass of isopropanol added to the volumetric flask, and calculate the apparent density D1 (g / cm³) of the hollow granules based on the following formula (I). 3 ).

[0056] Formula (I)

[0057] Apparent density D1 = [mass of hollow particles] / (100 - [mass of isopropanol] ÷ [mass of the particles in the determination])

[0058] The specific gravity of isopropanol at temperature

[0059] Apparent density D1 is equivalent to the total density of the hollow particle when the hollow part is considered as part of the hollow particle.

[0060] The method for determining the true density D0 of hollow granules is as follows. After pre-crushing the hollow granules, approximately 10g of the hollow granule fragments are filled into a 100cm³ container. 3 In a volumetric flask, the mass of the filled fragments was accurately weighed. Then, isopropanol was added to the volumetric flask in the same manner as the apparent density determination described above, and the mass of the isopropanol was accurately weighed. Based on the following formula (II), the true density D0 (g / cm³) of the hollow particles was calculated. 3 ).

[0061] Equation (II)

[0062] True density D0 = [fragment mass of hollow particles] / (100 - [mass of isopropanol] ÷ [in the determination])

[0063] The specific gravity of isopropanol at temperature

[0064] True density D0 corresponds to the specific gravity of the shell portion only within a hollow particle. As clearly demonstrated by the aforementioned measurement method, the hollow portion is not considered part of the hollow particle when calculating true density D0.

[0065] The porosity (%) of the hollow particles is calculated using the following formula (III) based on the apparent density D1 and true density D0 of the hollow particles.

[0066] Equation (III)

[0067] Porosity (%) = 100 - (apparent density D1 / true density D0) × 100

[0068] The hollow particles of the present invention have a volume average particle size of 10 μm or more and 50 μm or less, preferably 13 μm or more, more preferably 15 μm or more, and preferably 40 μm or less, more preferably 30 μm or less, as the lower limit. By having a volume average particle size of the hollow particles that is at or above the aforementioned lower limit, excellent pressure resistance can be achieved even with high porosity, and excellent dispersibility can be achieved because the agglomeration of the hollow particles is reduced. When the volume average particle size of the hollow particles is below the aforementioned upper limit, uneven shell thickness can be suppressed.

[0069] The thickness of the shell of the hollow particles of the present invention is not particularly limited, but from the perspective of improving pressure resistance, it is preferably 0.20 μm or more, more preferably 0.50 μm or more, and from the perspective of improving porosity, it is preferably 4.00 μm or less, more preferably 2.00 μm or less.

[0070] In addition, in this invention, the thickness of the shell of the hollow particle is the inner diameter r of the hollow particle calculated by the following formula (1) using the volume average particle size R and porosity of the hollow particle, and the value calculated by the following formula (2) using the inner diameter r and the volume average particle size R.

[0071] 4 / 3π×(R / 2) 3 ×(porosity / 100)=4 / 3π×(r / 2) 3

[0072] Equation (1)

[0073] Shell thickness = (Rr) / 2 Equation (2)

[0074] In addition, the porosity in the above formula (1) is a value expressed as a percentage.

[0075] The particle size distribution (volume average particle size (Dv) / number average particle size (Dn)) of the hollow particles can be, for example, 1.1 or more and 2.5 or less. With a particle size distribution of 2.5 or less, particles with small non-uniformity in compressive strength and heat resistance can be obtained. Furthermore, with a particle size distribution of 2.5 or less, for example, when manufacturing sheet-like molded bodies, products with uniform thickness can be manufactured.

[0076] The volume-average particle size (Dv) and number-average particle size (Dn) of hollow particles can be obtained, for example, by measuring the particle size of the hollow particles using a particle size distribution measuring device, calculating their number-average and volume-average values, and using these values ​​as the number-average particle size (Dn) and volume-average particle size (Dv) of the particle. Particle size distribution is the value of volume-average particle size divided by number-average particle size.

[0077] The shape of the hollow particles of the present invention 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.

[0078] The hollow particles of the present invention may have one or more hollow portions, but from the perspective of maintaining a good balance between high porosity and mechanical strength, it is preferable to have only one hollow portion. Furthermore, the shell of the hollow particles of the present invention and the partitions separating adjacent hollow portions when there are two or more hollow portions may be porous, and from the perspective of suppressing a decrease in pressure resistance, they are preferably dense.

[0079] The average roundness of the hollow particles of the present invention is preferably 0.950 to 1.000, more preferably 0.970 to 1.000. Roundness is defined as the circumference of a circle having the same projected area as the particle image divided by the circumference of the particle's projected image. Furthermore, the average roundness in the present invention is used as a simple method to quantitatively represent the shape of the particles; when the hollow particles are perfectly spherical, the average roundness is 1; the more complex the surface shape of the hollow particles, the smaller the value.

[0080] An example of the shape of the hollow particles of the present invention is a bag formed by a thin membrane and filled with gas, the cross-sectional view of which is shown below. Figure 1 Like the hollow particle 100 in (5). In this example, a thin membrane is provided on the outside, which is filled with gas.

[0081] Furthermore, the particle shape can be confirmed, for example, by SEM or TEM. In addition, the internal shape of the particles and the presence of tiny resin particles within the particles can be confirmed by SEM or TEM after the particles are cut into discs using known methods.

[0082] When viewed from above, the hollow particles of the present invention have a concave portion that is 5 to 50% of the particle size, and preferably less than 5%.

[0083] For hollow particles that have a concave portion larger than 5% of their particle size when viewed from above, the compressive strength is easily reduced due to this concave portion. On the other hand, for hollow particles that have a concave portion larger than 50% of their particle size when viewed from above, the porosity is typically less than 60%, which is insufficient. Therefore, in the hollow particles of the present invention with a porosity of 60% or more, the proportion of particles having a concave portion larger than 5% to 50% of their particle size when viewed from above is kept below the aforementioned upper limit, thereby suppressing the reduction in compressive strength caused by the concave portion. Thus, the hollow particles of the present invention can achieve both high porosity and excellent compressive strength.

[0084] It can be considered that the recesses in hollow particles are easily formed after the polymerization reaction used to form the shell. To prevent the hollow particles from having recesses, it is effective to optimize aspects such as the shell composition, the particle size of the hollow particles, the type of hydrophobic solvent contained within the particles during manufacturing, the stirring speed of the suspension during the polymerization reaction, and the heating rate, as preferred in this invention. It can be inferred that by adjusting these factors, deformation occurring within the shell can be suppressed, and the formation of recesses can be prevented.

[0085] When observing the hollow particles from above, the proportion of particles having a concave portion with a size relative to 50% of the particle size can be determined by observing the hollow particles of the present invention after solvent removal from any direction. The top-view observation is performed using, for example, a FE-SEM (field emission scanning electron microscope). Hollow particles after solvent removal refer to hollow particles obtained after the solvent removal process when the hollow particles of the present invention are obtained by the manufacturing method described later; that is, hollow particles in a state where no liquid component exists on the outside or inside the hollow portion. Furthermore, the absence of liquid component on the outside of the hollow particles means that, unlike when the hollow particles are present in a slurry, the hollow particles are dry powder. Moreover, the absence of liquid component inside the hollow portion of the hollow particles means that, relative to the mass of 100 parts by mass of hollow particles, the amount of liquid component inside the hollow portion is 0.5 parts by mass or less.

[0086] Furthermore, the particle size of hollow particles refers to the diameter of a circle with the same projected area as the particle image, which can be determined from FE-SEM images of the hollow particles. The size of the concave portion of the hollow particles is the distance along the maximum width direction of the concave portion's top-view shape, which can also be determined from FE-SEM images of the hollow particles.

[0087] In hollow particles, the proportion of particles with a concave portion that is 5 to 50% of the particle size when viewed from above can be determined by counting the number of particles with a concave portion that is 5 to 50% of the particle size among 100 hollow particles randomly selected from FE-SEM images of hollow particles.

[0088] In the hollow particles of the present invention, a polymer containing 70 to 100 parts by mass of crosslinking monomer units in 100 parts by mass of all monomer units is used as a resin in the shell. The crosslinking monomer units include bifunctional crosslinking monomer units derived from bifunctional crosslinking monomers and trifunctional or more crosslinking monomer units derived from trifunctional or more crosslinking monomers.

[0089] The aforementioned polymer forms the skeleton of the shell of the hollow particles. By including crosslinking monomer units in the above proportion, the hollow particles of the present invention have a shell densely distributed with a covalent bond network. Furthermore, by combining crosslinking monomer units including bifunctional crosslinking monomer units and trifunctional or higher crosslinking monomer units, the crosslinking structure becomes more compact, thereby improving the strength and pressure resistance of the shell.

[0090] In the above polymer, from the perspective of improving the strength and pressure resistance of hollow particles, the content of 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.

[0091] Furthermore, in this invention, the crosslinking monomer unit is a monomer unit derived from a crosslinking monomer, the bifunctional crosslinking monomer unit is a monomer unit derived from a bifunctional crosslinking monomer, and the trifunctional or higher crosslinking monomer unit is a monomer unit derived from a trifunctional or higher crosslinking monomer.

[0092] In the above polymers, when the content of crosslinking monomer units is less than 100 parts by mass, the monomer units other than the crosslinking monomer units are non-crosslinking monomer units derived from non-crosslinking monomers.

[0093] The polymers described above are typically polymers of polymeric monomers used in the method for manufacturing hollow particles of the present invention, which will be described later.

[0094] Furthermore, the specific details of the crosslinked and non-crosslinked monomers used to synthesize the above-mentioned polymers are as described later in the method for manufacturing hollow particles of the present invention.

[0095] In all monomer units of 100 parts by mass of the above polymer, the content of bifunctional crosslinkable monomer units is not particularly limited. As a lower limit, from the perspective of improving the strength and pressure resistance of hollow particles, it is preferably 60 parts by mass or more, more preferably 65 parts by mass or more, and may also be 70 parts by mass or more. As an upper limit, from the perspective of ensuring that the polymer contains sufficient crosslinkable monomer units with trifunctionality or above, it is preferably 95 parts by mass or less, and may also be 90 parts by mass or less.

[0096] In all monomer units of 100 parts by mass of the above polymer, the content of trifunctional or higher crosslinking monomer units is not particularly limited. As a lower limit, from the perspective of improving the strength and pressure resistance of hollow particles, it 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. As an upper limit, from the perspective of ensuring that the polymer fully contains bifunctional crosslinking monomer units, it is preferably 40 parts by mass or less, more preferably 30 parts by mass or less, and may also be 20 parts by mass or less.

[0097] The crosslinking monomer units contained in the above-mentioned polymer preferably comprise crosslinking monomer units derived from (meth)acryloyl-based crosslinking monomers having (meth)acryloyl groups as polymerizable functional groups. This improves the strength, pressure resistance, and heat resistance of the hollow particles of the present invention.

[0098] When the polymer contains crosslinking monomer units derived from (meth)acrylic acid-based crosslinking monomers, the content of crosslinking monomer units derived from the (meth)acrylic acid-based crosslinking monomers in 100 parts by mass 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. The crosslinking monomer units may be composed of (meth)acrylic acid-based crosslinking monomer units.

[0099] When the volume average particle size of the hollow particles is 10 μm or more and 20 μm or less, the crosslinking monomer unit containing the above polymer with trifunctional or higher crosslinking properties is preferably a crosslinking monomer unit derived from a trifunctional or higher methacrylic crosslinking monomer, and more preferably a crosslinking monomer unit derived from a trifunctional methacrylic crosslinking monomer.

[0100] On the other hand, when the volume average particle size of the hollow particles is greater than 20 μm and less than 50 μm, the crosslinking monomer unit containing the above polymer is not particularly limited. It can be a crosslinking monomer unit from a trifunctional or higher (meth)acrylic crosslinking monomer, a crosslinking monomer unit from a trifunctional or higher acrylic crosslinking monomer, or a crosslinking monomer unit from a tetrafunctional or higher acrylic crosslinking monomer.

[0101] The volume average particle size of the hollow particles can be adjusted by the type of crosslinking monomer used in manufacturing the hollow particles, the type of hydrophobic solvent contained within the particles during manufacturing, and the presence or absence of a second polymerization reaction. When obtaining hollow particles with a volume average particle size of 10 μm or more and 20 μm or less, it is preferable to use a trifunctional or higher-grade methacrylic acid-based crosslinking monomer as the crosslinking monomer, select a hydrophobic solvent corresponding to the particle size, and preferably add a further polymerizable monomer to carry out a second polymerization reaction. When obtaining hollow particles with a volume average particle size greater than 20 μm and less than 50 μm, it is preferable to use a trifunctional or higher-grade acrylic acid-based crosslinking monomer as the crosslinking monomer, or to use a trifunctional or higher-grade methacrylic acid-based crosslinking monomer without carrying out a second polymerization reaction.

[0102] Furthermore, the specific details regarding (meth)acrylic acid-based crosslinking monomers and methacrylic acid-based crosslinking monomers are as described later in the method for manufacturing hollow particles of the present invention.

[0103] The polymers described above may further comprise non-crosslinked monomer units. By including both crosslinked and non-crosslinked monomer units in the polymer combination, the mechanical properties of the shell of the hollow particles can sometimes be improved.

[0104] In the aforementioned polymer, from the perspective of ensuring sufficient crosslinkable monomer units, the content of non-crosslinkable monomer units in 100 parts by mass is 0 to 30 parts by mass, preferably 25 parts by mass or less, and more preferably 15 parts by mass or less. On the other hand, from the perspective of obtaining the effect of improved strength of hollow particles by combining crosslinkable and non-crosslinkable monomer units, the content of the aforementioned non-crosslinkable monomer units can be 2 parts by mass or more, or 4 parts by mass or more.

[0105] In the hollow particles of the present invention, the content of the aforementioned polymer is preferably 90% by mass or more, more preferably 95% by mass or more, in 100% by mass of all solid components of the shell. By setting the content of the aforementioned polymer to the aforementioned lower limit or above, the strength of the hollow particles can be improved.

[0106] The shell of the hollow particles of the present invention may further contain a polar component. Examples of polar components include organic acids or their metal salts, and polar resins. Furthermore, the specific details of the polar component are described later in the method for manufacturing the hollow particles of the present invention.

[0107] Furthermore, the presence and content of polar components in the shell of hollow particles can be confirmed by, for example, pyrolysis gas chromatography.

[0108] When the shell of the hollow particles of the present invention contains an organic acid or its metal salt as a polar component, the total content of the organic acid or its metal salt in the shell is preferably 0.0001 to 0.1% by mass, more preferably 0.001 to 0.01% by mass.

[0109] On the other hand, when the shell of the hollow particles of the present invention contains a polar resin as a polar component, the content of the polar resin in the shell is preferably 0.1 to 10.0% by mass, more preferably 0.3 to 8.0% by mass.

[0110] By including polar components in the shell, the reduction in shell strength can be suppressed, as can the reduction in the pressure resistance of the hollow particles.

[0111] Furthermore, the hollow particles of the present invention are preferably such that, in SEM observation, there are 5 or fewer hollow particles with connecting holes or shell defects out of 100 hollow particles.

[0112] Hollow particles typically include those whose shells lack connecting holes that link the hollow portion to the outside space of the particle, and those whose shells have one or more connecting holes through which the hollow portion communicates with the outside of the particle. While the diameter of the connecting holes typically varies depending on the size of the hollow particle, it generally ranges from 10 to 500 nm. Connecting holes can sometimes provide beneficial functions to hollow particles; however, because they are defects in the shell, they can also reduce the strength of the hollow particles and make them more prone to breakage.

[0113] Furthermore, hollow particles sometimes exhibit crack-like shell defects that are very large relative to their size. Although this also depends on the size of the hollow particle, cracks with a length of 1 μm or more typically significantly degrade the strength of the hollow particle and are therefore considered shell defects.

[0114] 2. Manufacturing method of hollow particles

[0115] The hollow particles of the present invention can be obtained by, for example, a hollow particle manufacturing method comprising the following steps:

[0116] The process of preparing a mixture comprising a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersing stabilizer, and an aqueous medium;

[0117] The process of preparing a suspension in an aqueous medium by suspending the above-mentioned mixture, wherein droplets of a monomer composition containing the above-mentioned polymerizable monomer, the above-mentioned hydrophobic solvent, and the above-mentioned polymerization initiator are dispersed in the above-mentioned aqueous medium; and

[0118] The process of preparing a precursor composition comprising precursor particles by supplying the above suspension to a polymerization reaction, the precursor particles having a hollow portion surrounded by a shell containing resin and containing the above hydrophobic solvent within the hollow portion.

[0119] The manufacturing method of the above-mentioned hollow particles follows the following basic technology: by suspending a mixture containing polymerizable monomers, hydrophobic solvents, polymerization initiators, dispersing stabilizers and an aqueous medium, a suspension is prepared in which droplets with a distribution structure in which polymerizable monomers and hydrophobic solvents are separated, polymerizable monomers are concentrated on the surface side and hydrophobic solvents are concentrated in the center are dispersed in an aqueous medium. By supplying the suspension to a polymerization reaction, the surface of the droplets is solidified to form hollow particles with hollow parts filled with hydrophobic solvent.

[0120] The aforementioned method for manufacturing hollow particles includes a step of preparing a mixture, a step of preparing a suspension, and a step of supplying the suspension to the polymerization reaction, and may further include steps other than these. Furthermore, provided it is technically feasible, two or more of the aforementioned steps and other additional steps can be performed simultaneously as a single step, or their order can be reversed. For example, the preparation and suspension of the mixture can be carried out simultaneously in one cycle by adding the material for preparing the mixture while simultaneously suspending it.

[0121] As a preferred example of the manufacturing method of the above-mentioned hollow particles, a manufacturing method including the following steps can be cited.

[0122] (1) Mixture preparation process

[0123] The process of preparing a mixture comprising a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersing stabilizer, and an aqueous medium;

[0124] (2) Suspension process

[0125] The process of preparing a suspension in an aqueous medium by suspending the above mixture, wherein droplets of a monomer composition containing a polymerizable monomer, a hydrophobic solvent and a polymerization initiator are dispersed in the mixture.

[0126] (3) Polymerization process

[0127] The process of preparing a precursor composition comprising precursor particles by supplying the above suspension to a polymerization reaction, wherein the precursor particles have a hollow portion surrounded by a shell containing resin and contain a hydrophobic solvent within the hollow portion.

[0128] (4) Solid-liquid separation process

[0129] A process of obtaining precursor particles containing a hydrophobic solvent within a hollow portion by solid-liquid separation of the precursor composition; and

[0130] (5) Solvent removal process

[0131] The process of removing the hydrophobic solvent contained in the precursor particles obtained through the above solid-liquid separation process to obtain hollow particles.

[0132] Furthermore, in this invention, hollow particles whose hollow portion is filled with a hydrophobic solvent are considered intermediates of hollow particles whose hollow portion is filled with gas, and are sometimes referred to as "precursor particles." In this invention, "precursor composition" refers to a composition containing precursor particles.

[0133] Figure 1 This is a schematic diagram illustrating an example of a method for manufacturing hollow particles according to the present invention. Figure 1 Steps (1) to (5) in the diagram correspond to the steps (1) to (5) mentioned above. The white arrows between the diagrams indicate the sequence of each step. Additionally, Figure 1 These are merely illustrative diagrams, and the manufacturing method described above is not limited to the method shown in the figures. Furthermore, the structure, size, and shape of the materials used in the manufacturing method of this invention are not limited to the structures, sizes, and shapes of the various materials shown in these figures.

[0134] Figure 1 Figure (1) is a cross-sectional schematic diagram showing one embodiment of the mixture in the mixture preparation process. As shown in the figure, the mixture includes 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 with low polarity that is not easily mixed with the aqueous medium 1. In this invention, the low-polarity material 2 includes a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator.

[0135] Figure 1 (2) is a cross-sectional schematic diagram showing one embodiment of the suspension in the suspension process. 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 include a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator, but the distribution within the droplets is not uniform. The droplets 10 of the monomer composition have the following structure: the hydrophobic solvent 4a and the material 4b containing the polymerizable monomer other than the hydrophobic solvent are phase-separated, the hydrophobic solvent 4a is concentrated in the central part, the material 4b other than the hydrophobic solvent is concentrated on the surface side, and the dispersion stabilizer (not shown) is attached to the surface.

[0136] Figure 1 (3) is a cross-sectional schematic diagram showing an embodiment of a precursor composition containing precursor particles with a hydrophobic solvent in a hollow portion, obtained by a polymerization process. The precursor composition includes an aqueous medium 1 and precursor particles 20 containing a hydrophobic solvent 4a in a hollow portion dispersed in the aqueous medium 1. The shell 6 forming the outer surface of the precursor particles 20 is formed by polymerization of a polymerizable monomer in a droplet 10 of the monomer composition described above, wherein the polymer containing the polymerizable monomer is a resin.

[0137] Figure 1 (4) is a cross-sectional schematic diagram showing one embodiment of the precursor particles after the solid-liquid separation process. Figure 1 (4) shows the above Figure 1 The state of (3) is the state of water medium 1.

[0138] Figure 1 (5) is a cross-sectional schematic diagram showing one embodiment of hollow particles after the solvent removal process. Figure 1 (5) shows the above Figure 1 The state of (4) is the state in which the hydrophobic solvent 4a has been removed. By removing the hydrophobic solvent from the precursor particles, hollow particles 100 with a gas-filled hollow portion 8 inside the shell 6 can be obtained.

[0139] The following sections will describe the five processes mentioned above and the other processes in turn.

[0140] (1) Mixture preparation process

[0141] This step involves preparing a mixture comprising a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersant stabilizer, and an aqueous medium. Without impairing the effects of this invention, the mixture may further contain other materials.

[0142] For the materials in the mixture, describe them in the following order: (A) polymerizable monomers, (B) hydrophobic solvents, (C) polymerization initiators, (D) dispersion stabilizers, (E) aqueous media, and (F) other materials.

[0143] (A) Polymerizing monomers

[0144] In the above manufacturing method, the polymerizable monomers in the mixture include at least crosslinking monomers, and may further include non-crosslinking monomers without impairing the effects of the present invention.

[0145] As a polymerizable monomer, from the perspective of easy and stable polymerization reaction and the ability to obtain hollow particles with excellent strength and heat resistance, (meth)acrylic acid-based polymerizable monomers with (meth)acryloyl groups as polymerizable functional groups can be preferred.

[0146] [Crosslinking monomers]

[0147] Crosslinkable monomers have multiple olefinic unsaturated double bonds, which enable them to link monomers together and increase the crosslinking density of the shell.

[0148] Examples of crosslinking monomers include bifunctional crosslinking monomers having two polymerizable functional groups and trifunctional or higher crosslinking monomers having three or more polymerizable functional groups.

[0149] Examples of bifunctional crosslinking monomers include divinylbenzene, divinylbiphenyl, divinylnaphthalene, diallyl phthalate, allyl acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, tricyclodecanediethanol di(meth)acrylate, polyphenylene ether with vinyl-modified ends, and polyphenylene ether with (meth)acrylate-modified ends. Among these, bifunctional (meth)acrylate-based crosslinking monomers such as allyl acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate are preferred from the perspective of easy and stable polymerization reaction and the ability to obtain hollow particles with excellent strength and heat resistance. Ethylene glycol di(meth)acrylate and pentaerythritol di(meth)acrylate are more preferred. Furthermore, among these bifunctional (meth)acrylic acid crosslinking monomers, bifunctional methacrylic acid crosslinking monomers with a methacryl group as the polymerizable functional group are even more preferred.

[0150] Furthermore, in this invention, the (meth)acrylic acid-based crosslinking monomer is any crosslinking monomer having at least one methacryloyl group or acryloyl group as a polymerizable functional group; preferably, all polymerizable functional groups are methacryloyl groups or acryloyl groups. On the other hand, the methacrylic acid-based crosslinking monomer is a crosslinking monomer having at least one methacryloyl group as a polymerizable functional group but not an acryloyl group; preferably, all polymerizable functional groups are methacryloyl groups.

[0151] When the polymerizable monomer in the mixture includes a (meth)acrylic acid-based crosslinking monomer, the content of the (meth)acrylic acid-based crosslinking monomer in 100 parts by mass of the polymerizable 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. The crosslinking monomer contained in the polymerizable monomer may be composed of (meth)acrylic acid-based crosslinking monomers.

[0152] As trifunctional or higher crosslinking monomers, from the perspective of easy and stable polymerization reaction and the ability to obtain hollow particles with excellent strength and heat resistance, trimethylolpropane tri(meth)acrylate, bis(trimethylolpropane)tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol poly(meth)acrylate and their ethoxides are preferred (meth)acrylate-based crosslinking monomers. 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.

[0153] In addition, high-molecular-weight cross-linked hydrocarbon monomers can also be used. Examples include polybutadiene, polyisoprene, styrene-butadiene block copolymers (SBS), and styrene-isoprene block copolymers (SIS).

[0154] From the viewpoint of having a volume average particle size of hollow particles of 10 μm or more and 20 μm or less, trifunctional or higher crosslinking monomers are preferred, and trifunctional methacrylate crosslinking monomers are more preferred. Examples of trifunctional methacrylate crosslinking monomers include trimethylolpropane trimethacrylate, ethoxylated trimethylolpropane trimethacrylate, and pentaerythritol trimethacrylate, among which trimethylolpropane trimethacrylate is preferred.

[0155] On the other hand, from the viewpoint of having a volume average particle size of hollow particles greater than 20 μm and less than 50 μm, there is no particular limitation on the crosslinking monomer that is trifunctional or higher. It can be a trifunctional or higher (meth)acrylic acid crosslinking monomer, a trifunctional or higher acrylic acid crosslinking monomer, or a tetrafunctional or higher acrylic acid crosslinking monomer. In addition, in this invention, the acrylic acid crosslinking monomer is a crosslinking monomer that has at least one acryloyl group as a polymerizable functional group but does not have a methacryloyl group, and preferably all polymerizable functional groups are acryloyl groups.

[0156] Examples of trifunctional acrylic crosslinking monomers include trimethylolpropane triacrylate, ethoxylated trimethylolpropane triacrylate, and pentaerythritol triacrylate.

[0157] Examples of acrylic crosslinking monomers with four or more functionalities include bis(trimethylolpropane)tetraacrylate, pentaerythritol tetraacrylate, dipentaerythritol hexaacrylate, dipentaerythritol polyacrylate, and their ethoxides, among which pentaerythritol tetraacrylate, pentaerythritol tetraacrylate ethoxide, and dipentaerythritol polyacrylate are preferred, and pentaerythritol tetraacrylate is more preferred.

[0158] In addition, these crosslinking monomers can be used individually or in combination of two or more.

[0159] The content of crosslinking monomers is preferably 70 to 100 parts by mass relative to 100 parts by mass of the total mass of polymerizable monomers in the mixture. By making the content of crosslinking monomers 70 parts by mass or more, the proportion of crosslinking monomer units in the shell of the hollow particles is sufficiently large, so that the covalent bond network is densely distributed in the shell. As a result, a shell with excellent strength, not easily broken, and not easily deformed even by externally applied heat can be formed. The content of crosslinking monomers is preferably 80 parts by mass or more, more preferably 90 parts by mass or more. When the polymerizable monomers in the mixture include non-crosslinking monomers, the content of crosslinking monomers can be, for example, 98 parts by mass or less, or 96 parts by mass or less.

[0160] Furthermore, the polymerizable monomers in the mixture include bifunctional crosslinking monomers and trifunctional or higher crosslinking monomers. This facilitates the formation of hollow spaces within the particles, further enhancing the shell strength.

[0161] The content of bifunctional crosslinking monomers in 100 parts by mass of polymerizable monomers in the mixture is not particularly limited. As a lower limit, from the perspective of improving the strength and pressure resistance of hollow particles, it is preferably 60 parts by mass or more, more preferably 65 parts by mass or more, and can be 70 parts by mass or more. As an upper limit, from the perspective of fully containing crosslinking monomer units with trifunctionality or above, it is preferably 95 parts by mass or less, and can be 90 parts by mass or less.

[0162] The content of trifunctional or higher crosslinking monomers in 100 parts by mass of polymerizable monomers in the mixture is not particularly limited. As a lower limit, from the perspective of improving the strength and pressure resistance of hollow particles, it 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. As an upper limit, from the perspective of fully containing bifunctional crosslinking monomer units, it is preferably 40 parts by mass or less, more preferably 30 parts by mass or less, and can be 20 parts by mass or less.

[0163] Furthermore, when the volume average particle size of the hollow particles is 10 μm or more and 20 μm or less, the trifunctional or higher crosslinking monomer in the mixture is preferably a trifunctional or higher crosslinking monomer. On the other hand, when the volume average particle size of the hollow particles is greater than 20 μm and less than 50 μm, the trifunctional or higher crosslinking monomer in the mixture is preferably a tetrafunctional or higher crosslinking monomer.

[0164] [Non-crosslinked monomer]

[0165] Without impairing the effects of the invention, the polymerizable monomers in the mixture may further comprise non-crosslinked monomers. By including both crosslinked and non-crosslinked monomers in the polymerizable monomer combination of the mixture, it is sometimes possible to improve the mechanical properties of the shell of the resulting hollow particles.

[0166] Monovinyl monomers are preferred as non-crosslinking monomers. A monovinyl monomer is a compound having a single polymerizable vinyl functional group. Examples of monovinyl monomers include: methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, glycidyl methacrylate, 2-hydroxyethyl methacrylate, and other methacrylic acid-based monovinyl monomers; aromatic vinyl monomers such as styrene, vinyltoluene, α-methylstyrene, p-methylstyrene, ethylvinylbenzene, ethylvinylbiphenyl, ethylvinylnaphthalene, and halostyrene; monoolefin monomers such as ethylene, propylene, and butene; methacrylamide monomers and their derivatives such as (meth)acrylamide, N-hydroxymethyl(meth)acrylamide, and N-butoxymethyl(meth)acrylamide; diene monomers such as butadiene and isoprene; vinyl acetate and other vinyl carboxylic acid ester monomers; halovinyl monomers such as vinyl chloride; vinylidene dihaloethylene monomers such as vinylidene chloride; and vinylpyridine monomers. These non-crosslinked monomers can be used alone or in combination of two or more.

[0167] From the perspectives of reactivity and heat resistance, (meth)acrylic acid-based monovinyl monomers are preferred, and at least one selected from butyl acrylate and methyl methacrylate is more preferred.

[0168] Relative to 100 parts by mass of the total polymerizable monomers in the mixture, from the perspective of sufficiently containing crosslinking monomers, the content of non-crosslinking monomers 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. On the other hand, from the perspective of obtaining the effect of improving the strength of hollow particles by combining crosslinking monomers and non-crosslinking monomers, the content of the aforementioned non-crosslinking monomers can be 2 parts by mass or more, or 4 parts by mass or more.

[0169] Relative to the total 100 parts by mass of polymerizable monomers and hydrophobic solvents in the mixture, the content of polymerizable monomers 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. When the content of polymerizable monomers is within the above range, the balance between porosity, particle size, and mechanical strength of the hollow particles becomes good.

[0170] Furthermore, from the viewpoint of improving the mechanical strength of hollow particles, the content of polymerizable monomers is preferably 90% by mass or more, and more preferably 95% by mass or more, relative to 100% by mass of the total mass of solid components other than hydrophobic solvents in the material that becomes the oil phase in the mixture.

[0171] Furthermore, in this invention, solid components refer to all components except solvents, and liquid polymerizable monomers are included in solid components.

[0172] (B) Hydrophobic solvents

[0173] The hydrophobic solvent used in the above manufacturing method is a non-polymerizable organic solvent that is poorly soluble in water.

[0174] The hydrophobic solvent acts as a spacer material forming hollow spaces within the particles. In the suspension process described later, a suspension of droplets containing the hydrophobic solvent is obtained, dispersed in an aqueous medium. During the suspension process, phase separation occurs within the droplets of the monomer composition, resulting in the low-polarity hydrophobic solvent easily accumulating inside the droplets. Ultimately, within the droplets of the monomer composition, the hydrophobic solvent is distributed within, while other materials, according to their respective polarities, are distributed around the periphery.

[0175] Furthermore, in the polymerization process described later, an aqueous dispersion containing hollow particles containing a hydrophobic solvent can be obtained. That is, by causing the hydrophobic solvent to aggregate inside the particles, a hollow portion filled with hydrophobic solvent is formed inside the obtained precursor particles.

[0176] In the above manufacturing method, the hydrophobic solvent is selected appropriately according to the type of polymerizable monomer, and there is no particular limitation. Known hydrophobic solvents can be used, such as esters like ethyl acetate and butyl acetate; ether esters like propylene glycol monomethyl ether acetate and propylene glycol monoethyl ether acetate; aromatic hydrocarbons like benzene, toluene, and xylene; and aliphatic hydrocarbons like hexane, methylhexane, heptane, octane, cyclohexane, and methylcyclohexane. These hydrophobic solvents can be used individually or in combination of two or more.

[0177] Preferably, the hydrophobic solvent is selected such that the HSP distance between the crosslinking monomer in the polymerizable monomer and the hydrophobic solvent is 5.40 or more and 6.50 or less. More preferably, the HSP distance is 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 crosslinking monomer and the hydrophobic solvent is within the above range, the polymerizable monomer and the hydrophobic solvent are sufficiently separated in the droplets of the monomer composition, making it easier to form hollow portions within the particles, and the thickness of the shell formed by the polymerization reaction tends to become uniform.

[0178] The HSP distance is an indicator of the solubility between substances using the Hansen Solubility Parameter (HSP). A closer HSP distance to 0 indicates higher miscibility between substances. HSP is represented as a vector in three-dimensional space (Hansen space) with the dispersion term dD, polarity term dP, and hydrogen bonding term dH as coordinate axes. These three parameters represent the inherent values ​​of each substance. The software developed by Hansen et al. (software name: Hansen Solubility Parameters in Practice (HSPiP)) contains a database of dD, dP, and dH values ​​for various substances. Furthermore, HSP can also be calculated based on the chemical structure of substances using HSP.

[0179] When the HSP of a mixture of multiple substances is determined, the HSP is calculated by taking the weighted average of the dD, dP, and dH values ​​of each substance in the mixture and the proportion of each substance, as the dispersion term dD, polarity term dP, and hydrogen bonding term dH of the mixture.

[0180] The HSP distance is the vector distance between two substances given by their HSPs. It is calculated using the following formula (A) based on the values ​​of the dispersion term dD1, polarity term dP1, and hydrogen bond term dH1 of one substance and the values ​​of the dispersion term dD2, polarity term dP2, and hydrogen bond term dH2 of the other substance.

[0181] Formula (A)

[0182] HSP distance = {4(dD1-dD2)} 2 +(dP1-dP2) 2 +(dH1-dH2) 2} 0.5

[0183] Furthermore, the distance from the origin of Hansen space to the HSP is called total HSP, which is calculated using the following equation (B) based on the dispersion term dD, the polarity term dP, and the hydrogen bond term dH.

[0184] Formula (B)

[0185] total HSP=(dD 2 +dP 2 +dH 2 ) 0.5

[0186] Furthermore, in this invention, the HSP distance and total HSP are set to values ​​calculated using HSPiP (version 5.3.03). In the aforementioned HSPiP, the values ​​of dD, dP, and dH are represented using significant digits up to the first decimal place, while the values ​​of the HSP distance and total HSP are represented using significant digits up to the second decimal place.

[0187] From the perspective of making it easy for the HSP distance between the crosslinking monomer and the hydrophobic solvent to be within the above-mentioned range, the hydrophobic solvent preferably includes at least one selected from esters, aromatic hydrocarbons and aliphatic hydrocarbons, and more preferably includes at least one selected from aromatic hydrocarbons and aliphatic hydrocarbons.

[0188] Furthermore, from the perspective of easily making the HSP distance between the crosslinking monomer and the hydrophobic solvent within the aforementioned range, it is preferable to include two or more hydrophobic solvents. The combination of two or more hydrophobic solvents is appropriately selected in such a way that the HSP distance falls within the aforementioned range, without particular limitation. Preferably, it is a combination of one or more aliphatic hydrocarbons and one or more esters and aromatic hydrocarbons. More preferably, it is a combination of one or more aliphatic hydrocarbons and one or more esters, and a combination of one or more aliphatic hydrocarbons and one or more aromatic hydrocarbons.

[0189] In the above combination, the aliphatic hydrocarbon is preferably one or more selected from hexane, methylhexane, cyclohexane and methylcyclohexane, more preferably one or more selected from hexane and cyclohexane, and even more preferably cyclohexane.

[0190] In the above combination, the ester is preferably selected from one or more of ethyl acetate and butyl acetate, more preferably ethyl acetate.

[0191] In the above combination, the aromatic hydrocarbons are preferably selected from one or more of benzene, toluene, and xylene, and more preferably toluene.

[0192] Furthermore, for the hydrophobic solvent used in the manufacturing method of the present invention, the total mass of the hydrophobic solvent comprising the above-mentioned preferred combination of hydrophobic solvents in 100 parts by mass is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, further 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 manufacturing method of the present invention is most preferably composed of the above-mentioned preferred combination of hydrophobic solvents.

[0193] Furthermore, from the perspective of easily obtaining hollow particles that suppress the formation of depressions, it is also preferable that the hydrophobic solvent is a combination of one or more aliphatic hydrocarbons and one or more aromatic hydrocarbons. Specifically, when the volume average particle size of the hollow particles is greater than 10 μm and less than 50 μm, and particularly when the volume average particle size of the hollow particles is 20 μm or more and less than 50 μm, the formation of depressions in the hollow particles is easily suppressed when the hydrophobic solvent is a combination of one or more aliphatic hydrocarbons and one or more aromatic hydrocarbons. It can be inferred 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. When the polymer precipitates in a high molecular weight state, the reactivity during shell formation decreases, thus suppressing shell deformation and resulting in particles without depressions.

[0194] In a hydrophobic solvent comprising a combination of one or more aliphatic hydrocarbons and one or more esters, the mass ratio of aliphatic hydrocarbons to esters (aliphatic hydrocarbons: esters) is not particularly limited, but is preferably 70:30 to 95:5, more preferably 80:20 to 95:5.

[0195] In a hydrophobic solvent comprising a combination of one or more aliphatic hydrocarbons and one or more aromatic hydrocarbons, the mass ratio of aliphatic hydrocarbons to 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.

[0196] Furthermore, although not particularly limited, from the perspective of easily making the HSP distance between the crosslinking monomer and the hydrophobic solvent within the above-mentioned range, the total HSP of the hydrophobic solvent used in the manufacturing method of the present invention is preferably 16.50 to 18.00 MPa. 1 / 2 More preferably, it is 16.60–17.80 MPa. 1 / 2 More preferably, it is 16.70–17.70 MPa. 1 / 2 .

[0197] As a preferred combination where the HSP distance between the crosslinking monomer and the hydrophobic solvent is 5.40 or more and 6.50 or less, examples include the following combination: the crosslinking monomer includes at least one selected from ethylene glycol di(meth)acrylate, 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 crosslinking monomer is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, further preferably 90 parts by mass or more, and even more preferably 100 parts by mass; 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 content of hexane, cyclohexane, ethyl acetate, and toluene relative to 100 parts by mass of the hydrophobic solvent is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, further preferably 90 parts by mass or more, and even more preferably 100 parts by mass.

[0198] As the crosslinking monomer used in the above combination, more preferably, the crosslinking monomer has an ethylene glycol di(meth)acrylate content of 70 parts by mass or more relative to 100 parts by mass of the crosslinking monomer.

[0199] The hydrophobic solvent used in the above combinations is more preferably: a combination of hexane and cyclohexane selected from toluene, wherein the total content of hexane, cyclohexane and toluene is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, further preferably 90 parts by mass or more, and even more preferably 100 parts by mass of the hydrophobic solvent; and a combination of hexane and cyclohexane selected from toluene and ethyl acetate, wherein the total content of hexane, cyclohexane and ethyl acetate is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, further preferably 90 parts by mass or more, and even more preferably 100 parts by mass of the hydrophobic solvent.

[0200] Other preferred combinations in which the HSP distance between the crosslinking monomer and the hydrophobic solvent is 5.40 or more and 6.50 or less include, for example: the crosslinking monomer comprises 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 relative to 100 parts by mass of the crosslinking monomer is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, further preferably 90 parts by mass or more, and even more preferably 100 parts by mass; the hydrophobic solvent comprises cyclohexane, and the content of cyclohexane relative to 100 parts by mass of the hydrophobic solvent is preferably 70 parts by mass or more, more preferably 80 parts by mass or more, further preferably 90 parts by mass or more, and even more preferably 100 parts by mass.

[0201] As the crosslinking monomer used in the above combination, the crosslinking monomer preferably has an ethylene glycol di(meth)acrylate content of 70 parts by mass or more relative to 100 parts by mass of the crosslinking monomer.

[0202] When using the above-mentioned combination of hydrophobic solvents, in the case of other hydrophobic solvents different from cyclohexane, the preferred hydrophobic solvent is toluene.

[0203] Specifically, examples of hydrophobic solvents with an HSP distance of 5.40 or more and 6.50 or less with respect to the following crosslinking monomers include: a cyclohexane-toluene mixture with a mass ratio of 50:50 (HSP distance with ethylene glycol dimethacrylate: 6.04); a cyclohexane-toluene mixture with a mass ratio of 70:30 (HSP distance with ethylene glycol dimethacrylate: 6.40); a cyclohexane-toluene mixture with a mass ratio of 40:60 (HSP distance with ethylene glycol dimethacrylate: 5.91); and a cyclohexane-ethyl acetate mixture with a mass ratio of 90:10. The preferred crosslinking monomers are mixtures of ethylene glycol dimethacrylate and pentaerythritol tetraacrylate, such as a mixed solvent of ethyl acetate (HSP distance to ethylene glycol dimethacrylate: 6.01), a mixed solvent of hexane and toluene with a mass ratio of hexane to toluene of 60:40 (HSP distance to ethylene glycol dimethacrylate: 6.03), and a mixed solvent of hexane and ethyl acetate with a mass ratio of hexane to ethyl acetate of 80:20 (HSP distance to ethylene glycol dimethacrylate: 5.83).

[0204] Furthermore, as a hydrophobic solvent with an HSP distance of 5.40 or more and 6.50 or less from the crosslinking monomer described above, examples include cyclohexane and a mixed solvent of cyclohexane and toluene in a mass ratio of 40:60 to 90:10. The crosslinking monomer is preferably a mixture formed from ethylene glycol dimethacrylate and trimethylolpropane trimethacrylate, wherein the mass ratio of ethylene glycol dimethacrylate to trimethylolpropane trimethacrylate is 60:40 to 80:20.

[0205] Furthermore, although there are no particular limitations, from the perspective of ease of removal in the solvent removal process described later, the boiling point of the hydrophobic solvent is preferably 130°C or less, more preferably 120°C or less, and from the perspective of ease of inclusion in the precursor particles, it is preferably 50°C or more, more preferably 60°C or more.

[0206] Furthermore, when the hydrophobic solvent is a mixture of multiple 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 mixture is below the aforementioned 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 mixture is above the aforementioned lower limit value.

[0207] Furthermore, the hydrophobic solvent used in the above manufacturing method preferably has a relative permittivity of 3 or less at 20°C. The relative permittivity is one of the indicators of the polarity of a compound. It can be considered that when the relative permittivity of the hydrophobic solvent is sufficiently small, such as 3 or less, phase separation in the polymerizable monomer droplets proceeds rapidly, easily forming hollow structures.

[0208] Examples of hydrophobic solvents with a relative permittivity of 3 or less at 20°C are as follows. The values ​​in parentheses are the relative permittivity values.

[0209] Pentane (1.8), Hexane (1.9), Heptane (1.9), Octane (1.9), Cyclohexane (2.0), Benzene (2.3), Toluene (2.4).

[0210] Regarding the relative permittivity at 20°C, one can refer to the values ​​recorded in well-known literature (e.g., Chemical Society of Japan, *Basic Handbook of Chemistry*, revised 4th edition, Maruzen Co., Ltd., published September 30, 1993, pp. II-498 to II-503), as well as other technical information. As a method for determining the relative permittivity at 20°C, examples include the relative permittivity test performed according to section 23 of JIS C2101:1999 with the measurement temperature set to 20°C.

[0211] The porosity of hollow particles can be adjusted by changing the amount of hydrophobic solvent in the mixture. In the suspension process described later, the polymerization reaction is carried out in the state of oil droplets containing crosslinking monomers and the like containing hydrophobic solvent. Therefore, there is a tendency that the higher the content of hydrophobic solvent, the higher the porosity of the resulting hollow particles.

[0212] In this invention, from the perspectives of easily controlling the particle size of hollow particles, easily increasing porosity while maintaining the strength of hollow particles, and easily reducing the amount of residual hydrophobic solvent within the particles, the content of hydrophobic solvent in the mixture is preferably 50 parts by mass and 500 parts by mass or less relative to 100 parts by mass of polymerizable monomer. More preferably, the content of hydrophobic solvent in the mixture is 60 parts by mass and 400 parts by mass or less relative to 100 parts by mass of polymerizable monomer, and even more preferably 70 parts by mass and 300 parts by mass or less.

[0213] (C) Polymerization initiator

[0214] In the above manufacturing method, it is preferable that the mixture contains an oil-soluble polymerization initiator as a polymerization initiator. As a method for polymerizing droplets of monomer composition after suspending the mixture, there are emulsion polymerization methods using water-soluble polymerization initiators and suspension polymerization methods using oil-soluble polymerization initiators, which can be carried out by using oil-soluble polymerization initiators.

[0215] There are no particular restrictions on oil-soluble polymerization initiators as long as they are lipophilic polymerization initiators with a solubility of less than 2 g / L in water at 25°C. Examples of oil-soluble polymerization initiators include benzoyl peroxide, lauroyl peroxide, tert-butylperoxy-2-ethylhexanoate, 2,2'-azobis(2,4-dimethylpentanonitrile), azobisisobutyronitrile, and 2,2'-azobis(4-methoxy-2,4-dimethylpentanonitrile).

[0216] The content of the oil-soluble polymerization initiator relative to 100 parts by mass of polymerizable monomer in the mixture 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. By keeping the content of the oil-soluble polymerization initiator within the above range, the polymerization reaction proceeds fully, and after the polymerization reaction is terminated, the possibility of residual oil-soluble polymerization initiator is small, and the possibility of unintended side reactions is also small.

[0217] (D) Dispersant stabilizer

[0218] A dispersion stabilizer is a reagent used in a suspension process to disperse droplets of a monomer composition in an aqueous medium. In this invention, from the perspective of easily controlling the droplet size in the suspension to narrow the particle size distribution of the resulting hollow particles, and from the perspective of preventing the shell from becoming too thin and thus preventing a decrease in the strength of the hollow particles, an inorganic dispersion stabilizer is preferably used as the dispersion stabilizer.

[0219] Examples of inorganic dispersion stabilizers include: sulfates such as barium sulfate and calcium sulfate; carbonates such as barium carbonate, calcium carbonate, and magnesium carbonate; phosphates such as calcium phosphate; metal oxides such as alumina and titanium oxide; and inorganic compounds such as metal hydroxides such as aluminum hydroxide, magnesium hydroxide, calcium hydroxide, barium hydroxide, and iron hydroxide. One or more of these inorganic dispersion stabilizers can be used.

[0220] Among the above-mentioned inorganic dispersing stabilizers, the preferred ones are water-insoluble metal salts such as sulfates, carbonates, phosphates, and metal hydroxides, with metal hydroxides being more preferred, and magnesium hydroxide being particularly preferred.

[0221] Furthermore, in this invention, the metal salt that is sparingly soluble in water is preferably an inorganic metal salt with a solubility of 0.5g or less in 100g of water.

[0222] In this invention, it is particularly preferred to use the water-insoluble inorganic dispersion stabilizer in the form of colloidal particles in an aqueous medium, that is, in the form of a colloidal dispersion containing the water-insoluble inorganic dispersion stabilizer colloidal particles. This not only narrows the droplet size distribution of the monomer composition, but also allows for easy suppression of the residual amount of inorganic dispersion stabilizer in the resulting hollow particles through washing.

[0223] Colloidal dispersions containing inorganic dispersant stabilizer particles that are poorly soluble in water can be prepared, for example, by reacting at least one of alkali metal hydroxides and alkaline earth metal hydroxides with a water-soluble polyvalent metal salt (other than alkaline earth metal hydroxides) in an aqueous medium.

[0224] Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Examples of alkaline earth metal hydroxides include barium hydroxide and calcium hydroxide.

[0225] As a water-soluble polyvalent metal salt, any polyvalent metal salt exhibiting water solubility other than those belonging to the aforementioned alkaline earth metal hydroxides is acceptable. Examples include: magnesium metal salts such as magnesium chloride, magnesium phosphate, and magnesium sulfate; calcium metal salts such as calcium chloride, calcium nitrate, calcium acetate, and calcium sulfate; aluminum metal salts such as aluminum chloride and aluminum sulfate; barium salts such as barium chloride, barium nitrate, and barium acetate; and zinc salts such as zinc chloride, zinc nitrate, and zinc acetate. Among these, magnesium metal salts, calcium metal salts, and aluminum metal salts are preferred, magnesium metal salts are more preferred, and magnesium chloride is particularly preferred.

[0226] The method for reacting at least one of the above-mentioned alkali metal hydroxide and alkaline earth metal hydroxide with the above-mentioned water-soluble polyvalent metal salt in an aqueous medium is not particularly limited. A method of mixing an aqueous solution of at least one of the above-mentioned alkali metal hydroxide and alkaline earth metal hydroxide with an aqueous solution of the water-soluble polyvalent metal salt can be cited.

[0227] The content of the dispersant stabilizer is not particularly limited, but is preferably 0.5 to 10 parts by mass relative to the total mass of 100 parts by mass of the polymerizable monomer and the hydrophobic solvent, more preferably 1.0 to 8.0 parts by mass. By keeping the content of the dispersant stabilizer at or above the lower limit mentioned above, the droplets of the monomer composition can be sufficiently dispersed in the suspension without coalescing. On the other hand, by keeping the content of the dispersant stabilizer at or below the upper limit mentioned above, the viscosity of the suspension can be prevented from increasing during granulation, thus avoiding malfunctions such as clogging of the granulator.

[0228] Furthermore, relative to 100 parts by weight of the aqueous medium, the content of the dispersant stabilizer is typically 2 parts by weight or more and 15 parts by weight or less, preferably 3 parts by weight or more and 8 parts by weight or less.

[0229] (E) Aquatic media

[0230] In this invention, an aqueous medium refers to a medium selected from water, hydrophilic solvents, and mixtures of water and hydrophilic solvents.

[0231] Regarding the hydrophilic solvent used in this invention, there are no particular limitations as long as the solvent can be thoroughly mixed with water without phase separation. Examples of hydrophilic solvents include: alcohols such as methanol and ethanol; tetrahydrofuran (THF); and dimethyl sulfoxide (DMSO).

[0232] In an aqueous medium, water is preferred due to its high polarity. Furthermore, ion-exchanged water is preferred to reduce the metal content contained in the shell.

[0233] When using a mixture of water and a hydrophilic solvent, it is important to prevent the overall polarity of the mixture from becoming too low from the viewpoint of forming droplets of the monomer composition. In this case, for example, the mass ratio of water to hydrophilic solvent (water: hydrophilic solvent) can be 99:1 to 50:50.

[0234] (F) Other materials

[0235] Without impairing the effects of the present invention, the mixture may further contain other materials different from those in (A) to (E) described above.

[0236] As other materials, the mixture can contain polar components. By including polar components in the mixture, hollow particles with high porosity and excellent pressure resistance can be easily obtained.

[0237] As a polar component, polar resins, such as those described later, or organic acids or their metal salts, can be used.

[0238] In this invention, polar resin refers to a polymer containing repeating units comprising heteroatoms. Specifically, examples include acrylic resins, polyester resins, and vinyl resins containing heteroatoms.

[0239] Polar resins typically have a solubility of less than 1 g / L in water. In this invention, polar resins are distinguished from surfactants in that they are insoluble in water.

[0240] The aforementioned polar resin can be a homopolymer or copolymer of heteroatom-containing monomers, or a copolymer of heteroatom-containing monomers and heteroatom-free monomers. When the aforementioned polar resin is a copolymer of heteroatom-containing monomers and heteroatom-free monomers, from the perspective of easily controlling the particle size of the hollow particles, 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.

[0241] The number-average molecular weight (Mn) of the aforementioned polar resin is not particularly limited, but is preferably in the range of 3,000 or more and 20,000 or less, more preferably in the range of 4,000 or more and 17,000 or less, and even more preferably in the range of 6,000 or more and 15,000 or less, based on the polystyrene equivalent value determined by gel permeation chromatography (GPC) using tetrahydrofuran. By ensuring that the number-average molecular weight (Mn) of the aforementioned polar resin is at or above the aforementioned lower limit, the solubility of the polar resin is improved, making it easier to control the particle size of the hollow particles. By ensuring that the number-average molecular weight (Mn) of the aforementioned polar resin is below the aforementioned upper limit, the reduction in shell strength can be suppressed.

[0242] When using a polar resin as the polar component, the content of the polar resin is preferably 0.1 parts by mass or more and 10.0 parts by mass or less, more preferably 0.3 parts by mass or more and 8.0 parts by mass or less, and even more preferably 0.5 parts by mass or more and 8.0 parts by mass or less, relative to 100 parts by mass of the polymerizable monomer. When the content of the polar resin is at or above the aforementioned lower limit, it is easy to control the thickness of the shell of the hollow particles. When the content of the polar resin is at or below the aforementioned upper limit, it is possible to suppress the decrease in the content ratio of the polymerizable monomer, thereby suppressing the decrease in shell strength.

[0243] Examples of organic acids include abietic acid and higher fatty acids. Examples of higher fatty acids include higher fatty acids with 10 to 25 carbon atoms that do not contain a carbon atom in the carboxyl group.

[0244] Examples of metals that can be used as metal salts for organic acids include alkali metals such as Li, Na, and K, and alkaline earth metals such as Mg and Ca. Alkali metals are preferred, and at least one selected from Li, Na, and K is more preferred.

[0245] When using an organic acid or its metal salt as a polar component, the total content of the organic acid or its metal salt, relative to 100 parts by mass of the polymerizable monomer and the hydrophobic solvent, is preferably 0.0001 parts by mass or more and 0.02 parts by mass or less, more preferably 0.001 parts by mass or more and 0.01 parts by mass or less, and even more preferably 0.0015 parts by mass or more and 0.006 parts by mass or less. When the content of the organic acid or its metal salt is at or above the aforementioned lower limit, it is easy to control the shell thickness of the hollow particles. When the content of the organic acid or its metal salt is at or below the aforementioned upper limit, it is possible to suppress the decrease in the proportion of polymerizable monomer, and therefore, it is possible to suppress the decrease in shell strength and the decrease in the pressure resistance of the hollow particles.

[0246] A mixture can be obtained by mixing the above-mentioned materials with other required materials and stirring appropriately. In this mixture, the oil phase, containing the above-mentioned (A) polymerizable monomer, (B) hydrophobic solvent, and (C) polymerization initiator, and other lipophilic materials, is dispersed in an aqueous phase containing (D) dispersion stabilizer and (E) aqueous medium, with a particle size of about several millimeters. Depending on the type of materials, the dispersion state of these materials in the mixture can be observed even with the naked eye.

[0247] In the preparation of the mixture, although a mixture can be obtained simply by mixing the aforementioned materials with other required materials and stirring appropriately, it is preferable to prepare an oil phase containing a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator, and an aqueous phase containing a dispersing stabilizer and an aqueous medium separately beforehand, and then mix them to prepare the mixture. In this invention, a colloidal dispersion is preferably used as the aqueous phase, wherein the colloidal dispersion is obtained by dispersing a poorly water-soluble inorganic dispersing stabilizer in the form of colloidal particles in an aqueous medium.

[0248] By preparing the oil phase and water phase separately in advance and then mixing them, it is possible to manufacture hollow particles with a uniform composition of shell portion.

[0249] (2) Suspension process

[0250] The suspension process is a process of preparing a suspension in which droplets of a monomer composition containing a hydrophobic solvent are dispersed in an aqueous medium by suspending the above mixture.

[0251] There are no particular limitations on the suspension method used to form droplets of monomer compositions. Devices capable of strong stirring, such as (tandem type) emulsifying dispersers (such as the horizontal tandem disperser Milder manufactured by Tairayo Kiko Co., Ltd., and the horizontal tandem disperser Cavitron manufactured by Eurotec Co., Ltd.; and the vertical tandem disperser DRS2000 / 5 manufactured by IKA Co., Ltd.), or high-speed emulsifying dispersers (such as the TKHomo Mixer MARK II manufactured by PRIMIX Corporation), can be used.

[0252] In the suspension prepared by the suspension process, droplets of the monomer composition containing the aforementioned oleophilic material and having a particle size of approximately 10–50 μm are uniformly dispersed in an aqueous medium. These droplets of monomer composition are not easily observed with the naked eye, but can be observed using known observation equipment such as an optical microscope.

[0253] During the suspension process, phase separation occurs within the droplets of the monomer composition, thus making it easier for the low-polarity hydrophobic solvent to aggregate inside the droplets. As a result, in the resulting droplets, the hydrophobic solvent is distributed inside, while materials other than the hydrophobic solvent, such as polymerizable monomers, are distributed around the periphery.

[0254] Figure 2 This is a schematic diagram illustrating one embodiment of the suspension in the suspension process. Figure 2 The droplet 10 of the monomer composition is shown schematically as a droplet with its cross-section. Additionally, Figure 2 This is for illustrative purposes only; the suspension in this invention is not necessarily limited to... Figure 2The suspension shown. Figure 2 Part of the above Figure 1 (2) Corresponds to.

[0255] exist Figure 2 The diagram shows the dispersion of droplets 10 of the monomer composition in an aqueous medium 1 and the dispersion of polymerizable monomer 4c in the aqueous medium 1. The droplets 10 are formed by surrounding the oil-soluble monomer composition 4 with a dispersion stabilizer 3.

[0256] 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).

[0257] Droplet 10 is a tiny oil droplet containing monomer composition 4, and oil-soluble polymerization initiator 5 generates polymerization initiation free radicals inside the tiny oil droplet. Therefore, precursor particles of the target particle size can be manufactured without causing the tiny oil droplets to grow excessively.

[0258] In this suspension polymerization method using 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 suppress the generation of excess resin particles, such as small, dense particles other than the target hollow resin particles.

[0259] (3) Polymerization process

[0260] This process involves preparing a precursor composition containing precursor particles by supplying a suspension obtained using the aforementioned suspension process to a polymerization reaction. The precursor particles have a hollow portion surrounded by a shell containing a resin, and the hollow portion contains a hydrophobic solvent. The precursor particles are formed by the polymerization of polymerizable monomers contained in droplets of a monomer composition, and the shell of the precursor particles contains a polymer of the aforementioned polymerizable monomers as a resin.

[0261] There are no particular restrictions on the aggregation method; it can be, for example, batch aggregation, semi-continuous aggregation, or continuous aggregation.

[0262] The polymerization temperature is preferably 40–80°C, and more preferably 50–70°C.

[0263] From the perspective of suppressing the depression of hollow particles, the heating rate when heating to the polymerization temperature is preferably 20°C / hour or more, more preferably 45°C / hour or more. In addition, the above heating rate is usually 100°C / hour or less.

[0264] The polymerization reaction time is preferably 1 to 48 hours, more preferably 4 to 36 hours.

[0265] Furthermore, from the perspective of suppressing the depression of hollow particles, the stirring speed of the suspension during the polymerization reaction is preferably 20 to 200 rpm, more preferably 20 to 100 rpm. When the crosslinking monomer includes a tetrafunctional or higher crosslinking monomer, the above-mentioned stirring speed of 20 to 100 rpm is particularly preferred.

[0266] In the polymerization process, polymerization is carried out on the shell portion of a droplet containing a monomer composition with a hydrophobic solvent inside, thus forming a hollow portion filled with a hydrophobic solvent inside the resulting precursor particles, as described above.

[0267] In this process, after the first polymerization reaction in which the suspension is supplied to the polymerization reaction, a polymerizable monomer can be further added to the precursor composition obtained by the first polymerization reaction to carry out a second polymerization reaction. By carrying out the polymerization reaction in two stages in this way in the polymerization process, the solvent resistance of the hollow particles can be improved.

[0268] The polymerization conversion rate of the polymerizable monomer in the suspension of the first polymerization reaction is preferably 93% by mass or more, more preferably 95% by mass or more, even more preferably 98% by mass or more, and even more preferably 99% by mass or more.

[0269] Furthermore, in this invention, the polymerization conversion rate is calculated using the following formula (C) based on the mass of the solid component of the particles generated by the first polymerization reaction contained in the reaction liquid immediately after the first polymerization reaction and the mass of the polymerizable monomers remaining in an unreacted state after the first polymerization reaction. Moreover, the mass of the unreacted polymerizable monomers can be determined using gas chromatography (GC).

[0270] Polymerization conversion rate (mass%) = 100 - (mass of unreacted polymerizable monomers / mass of monomers passing through the first polymerization stage)

[0271] (Mass of the solid component of the particles produced by the polymerization reaction) × 100 Equation (C)

[0272] The reaction time for the first polymerization reaction is preferably 0.5 to 5 hours, more preferably 1 to 3 hours.

[0273] The polymerizable monomer added during the second polymerization reaction is not particularly limited. However, from the perspective of improving the solvent resistance and strength of the hollow particles, a polymerizable monomer with a solubility of 0.3 g / L or more in distilled water at 20°C is preferred, and a non-crosslinking monomer with a solubility of 0.3 g / L or more in distilled water at 20°C is more preferred. Examples of preferred polymerizable monomers added during the second polymerization reaction include, for example, alkyl (meth)acrylates having 1 to 5 carbon atoms, (meth)acrylamides and their derivatives, (meth)acrylonitrs, and non-crosslinking monomers containing polar groups. Preferably, at least one of alkyl (meth)acrylates having 1 to 5 carbon atoms and (meth)acrylonitrs is selected, more preferably, at least one of alkyl (meth)acrylates having 1 to 4 carbon atoms and acrylonitriles is selected, and even more preferably, alkyl (meth)acrylates having 1 to 4 carbon atoms are selected.

[0274] As non-crosslinking monomers containing polar groups, examples of preferred non-crosslinking monomers include those containing polar groups selected from carboxyl, hydroxyl, sulfonic acid, amino, polyoxyethylene, and epoxy groups. More specifically, examples include: olefinic unsaturated carboxylic acid monomers such as (meth)acrylic acid, crotonic acid, cinnamic acid, itaconic acid, fumaric acid, maleic acid, and butene tricarboxylic acid; hydroxyl monomers such as (meth)acrylic acid-2-hydroxyethyl ester, (meth)acrylic acid-2-hydroxypropyl ester, and (meth)acrylic acid-4-hydroxybutyl ester; sulfonic acid monomers such as styrene sulfonic acid; amino monomers such as dimethylaminoethyl (meth)acrylic acid and diethylaminoethyl (meth)acrylic acid; polyoxyethylene monomers such as methoxy polyethylene glycol (meth)acrylic acid; and epoxy monomers such as (meth)acrylic acid glycidyl ether, allyl glycidyl ether, and 4-hydroxybutyl acrylate glycidyl ether.

[0275] Furthermore, the polymerizable monomer added during the second polymerization reaction is preferably 2 g / L or more, more preferably 10 g / L or more, and even more preferably 15 g / L or more, in distilled water at 20°C, from the perspective of being easily absorbed into the shell to promote thermal motion and improve the strength of the hollow particles. Additionally, there is no particular upper limit to the solubility of the polymerizable monomer added during the second polymerization reaction in distilled water at 20°C, and it is generally 80 g / L or less.

[0276] Furthermore, the molecular weight of the polymerizable monomer added in the second polymerization reaction is not particularly limited, but from the perspective 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 above molecular weight is not particularly limited, and is generally 50 or more.

[0277] From the perspective of improving the solvent resistance and strength of hollow particles, the amount of polymerizable monomer added during the second polymerization reaction is preferably 3 to 15 parts by mass, more preferably 4 to 10 parts by mass, relative to 100 parts by mass of polymerizable monomer in the mixture.

[0278] The reaction time for the second polymerization reaction is preferably 1 to 6 hours, and more preferably 2 to 4 hours.

[0279] (4) Solid-liquid separation process

[0280] This process involves obtaining a solid component containing precursor particles by separating the precursor composition containing precursor particles obtained through the above polymerization process into a solid and liquid phase.

[0281] There is no particular limitation on the method for solid-liquid separation of the precursor composition, and known methods can be used. Examples of solid-liquid separation methods include centrifugation, filtration, and settling. Among these, centrifugation or filtration can be used, and from the viewpoint of ease of operation, centrifugation can be used.

[0282] After the solid-liquid separation process and before the solvent removal process described later, any process such as a pre-drying process can be performed. Examples of pre-drying processes include pre-drying the solid components obtained after the solid-liquid separation process using a drying device such as a dryer or a hand dryer.

[0283] (5) Solvent removal process

[0284] This process is for removing the hydrophobic solvent contained in the precursor particles obtained by the above solid-liquid separation process.

[0285] By removing the hydrophobic solvent contained in the precursor particles in the gas, and replacing the hydrophobic solvent inside the precursor particles with air, hollow particles filled with gas can be obtained.

[0286] In this process, "in the gas" strictly refers to an environment where there is absolutely no liquid component outside the precursor particles, or an environment where only a trace amount of liquid component exists outside the precursor particles, to a degree that does not affect the removal of hydrophobic solvents. "In the gas" can also mean a state where the precursor particles are not present in the slurry, or a state where the precursor particles are present in the dry powder. That is, in this process, removing hydrophobic solvents in an environment where the precursor particles are in direct contact with the external gas is particularly important.

[0287] There is no particular limitation on the method for removing hydrophobic solvents from precursor particles in a gas, and known methods can be used. Examples of such methods include vacuum drying, heating drying, airflow drying, or a combination of these methods.

[0288] Especially when using the heat drying method, the heating temperature needs to be set above the boiling point of the hydrophobic solvent and below the highest temperature at which the shell structure of the precursor particles will not be damaged. Therefore, although it also depends on the composition of the shell in the precursor particles and the type of hydrophobic solvent, for example, the heating temperature can be set to 50–200°C, 70–200°C, or 100–200°C.

[0289] By performing a drying process in a gas, the hydrophobic solvent inside the precursor particles is replaced by the external gas, resulting in hollow particles with the gas occupying the hollow part.

[0290] There are no particular limitations on the drying atmosphere; it can be appropriately selected based on the intended use of the hollow particles. Suitable drying atmospheres include, for example, air, oxygen, nitrogen, and argon. Alternatively, by temporarily filling the interior of the hollow particles with gas and then drying under reduced pressure, hollow particles with a temporarily vacuum interior can also be obtained.

[0291] Alternatively, instead of separating the solid and liquid components of the slurry-like precursor composition obtained in the polymerization process, the hydrophobic solvent contained in the precursor particles can be replaced with the aqueous medium of the slurry in the slurry containing the precursor particles and the aqueous medium, thereby removing the hydrophobic solvent.

[0292] In this method, an inactive gas is bubbled in the precursor composition at a temperature above 35°C below the boiling point of the hydrophobic solvent, thereby removing the hydrophobic solvent contained in the precursor particles.

[0293] Here, the aforementioned hydrophobic solvent is a mixed solvent containing multiple hydrophobic solvents. In the case of multiple boiling points, the boiling point of the hydrophobic solvent in the solvent removal process refers to the boiling point of the solvent with the highest boiling point among the solvents contained in the mixed solvent, that is, the highest boiling point among multiple boiling points.

[0294] From the perspective of reducing the residual amount of hydrophobic solvent in hollow particles, the temperature at which the inactive gas is foamed in the precursor composition is preferably a temperature of 30°C or higher than the boiling point of the hydrophobic solvent, and more preferably a temperature of 20°C or higher than the boiling point of the hydrophobic solvent. Furthermore, the foaming temperature is generally set to a temperature higher than the polymerization temperature in the aforementioned polymerization step. Although not particularly limited, the foaming temperature can be set to 50°C or higher and 100°C or lower.

[0295] There are no particular limitations on the inactive gas used for foaming; examples include nitrogen and argon.

[0296] The foaming conditions can be appropriately adjusted according to the type and amount of hydrophobic solvent in order to remove the hydrophobic solvent contained in the precursor particles. There are no particular limitations. For example, the inactive gas can be foamed at a rate of 1 to 3 L / min for 1 to 10 hours.

[0297] In this method, an aqueous slurry containing an aqueous medium is obtained from precursor particles. The hollow particles obtained by solid-liquid separation of the slurry are dried to remove the aqueous medium from the hollow particles, thereby obtaining hollow particles with gas occupying the hollow part.

[0298] The method of obtaining hollow particles with gas-filled hollow sections by separating the solid and liquid components of a slurry-like precursor composition and removing the hydrophobic solvent from the precursor particles in a gas is compared with the method of obtaining hollow particles with gas-filled hollow sections by replacing the hydrophobic solvent contained in the precursor particles with the aqueous medium of the slurry in a slurry containing precursor particles and an aqueous medium, performing solid-liquid separation, and removing the aqueous medium from the precursor particles in a gas. The former method has the advantage that the hollow particles are less likely to break during the process of removing the hydrophobic solvent, while the latter method has the advantage of reducing the residue of hydrophobic solvent by using inactive gas for foaming.

[0299] In addition, as a method to remove the hydrophobic organic solvent contained in the precursor particles without solid-liquid separation of the slurry-like precursor composition obtained in the polymerization process and before the solid-liquid separation process, for example, the following methods can be used: evaporating the hydrophobic organic solvent contained in the precursor particles from the precursor composition under a specified pressure (high pressure, normal pressure, or reduced pressure); and evaporating the solvent by introducing inactive gases such as nitrogen, argon, or helium, or water vapor into the precursor composition under a specified pressure (high pressure, normal pressure, or reduced pressure).

[0300] (6) Other

[0301] In addition to the processes described in (1) to (5) above, a cleaning process (6-a) and a hollow part replacement process (6-b) below may be added.

[0302] (6-a) Cleaning process

[0303] The cleaning process refers to the process of adding acid or alkali to remove residual dispersant stabilizers in the precursor composition containing precursor particles before the aforementioned solid-liquid separation process. When the dispersant stabilizer used is an inorganic dispersant stabilizer soluble in acid, it is preferable to add acid to the precursor composition containing precursor particles for cleaning. On the other hand, when the dispersant stabilizer used is an inorganic compound soluble in alkali, it is preferable to add alkali to the precursor composition containing precursor particles for cleaning.

[0304] Furthermore, when using an acid-soluble inorganic dispersion stabilizer as the dispersion stabilizer, it is preferable to add an acid to the precursor composition containing the precursor particles, and the pH is preferably adjusted to 6.5 or less, more preferably to 6 or less. As the added acid, inorganic acids such as sulfuric acid, hydrochloric acid, and nitric acid can be used; as well as organic acids such as formic acid and acetic acid. From the perspective of high removal efficiency of the dispersion stabilizer and low burden on the manufacturing equipment, sulfuric acid is particularly preferred.

[0305] (6-b) Re-displacement process of the hollow section

[0306] The hollow particle re-displacement process refers to the process of replacing the gas or liquid inside the hollow particles with other gases or liquids. Through this displacement, the internal environment of the hollow particles can be changed, molecules can be selectively encapsulated inside the hollow particles, and the internal chemical structure of the hollow particles can be modified according to the intended use.

[0307] 3. Applications of hollow granules

[0308] The hollow particles of this invention exhibit excellent pressure resistance, thus they are not easily broken during mixing with other materials or during molding after mixing. When added to molded bodies, they perform excellently as lightweight materials, heat insulation materials, sound insulation materials, and shock-absorbing materials, and are therefore particularly preferred as additives for molded bodies. The hollow particles of this invention are also not easily broken during mixing with resin or during molding after mixing, and are therefore particularly preferred as additives for resin-molded bodies.

[0309] The molded body containing the hollow particles of the present invention may contain, for example, thermoplastic resins, thermosetting resins, or room-temperature curing resins 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 crystal polyester resin, phenolic resin, vinyl ester resin, unsaturated polyester resin, cyanate ester resin, polyetherketoneketone resin, and polyetherimide resin. Furthermore, the above-mentioned resins may be used with additives such as curing agents and curing catalysts for curing the resin. As curing agents and curing catalysts, they can be appropriately selected from known curing agents and curing catalysts according to the type of resin; examples include amines, acid anhydrides, imidazoles, thiols, phenols, naphthols, and benzo[a]benzene. Azides, cyanates, and carbodiimides, etc.

[0310] Furthermore, the molded article containing the hollow particles of the present invention may contain a thermoplastic elastomer as a resin. As a thermoplastic elastomer, thermoplastic elastic polymers that have traditionally been used as molding resins can be used, such as polyurethane-based elastomers, styrene-based elastomers, olefin-based elastomers, amide-based elastomers, and ester-based elastomers. A thermoplastic elastomer is a substance that typically exhibits rubber-like elasticity at room temperature (25°C) and can be plasticized and molded at high temperatures.

[0311] Furthermore, the molded article containing the hollow particles of the present invention is not limited to a resin-based molded article; for example, it can be a rubber-based molded article, or it can contain a mixture of resin and rubber. The molded article containing the hollow particles of the present invention may contain rubbers such as natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene copolymer rubber (SBR), acrylonitrile-butadiene copolymer rubber (NBR), and ethylene-propylene-diene terpolymer (EPDM). The above-mentioned rubbers can be used alone or in combination of two or more.

[0312] Furthermore, the molded articles containing the hollow particles of the present invention may further contain organic or inorganic reinforcing fibers such as carbon fiber, glass fiber, aromatic polyamide fiber, and polyethylene fiber. The hollow particles of the present invention can also be contained as fillers in molded articles formed using thermoplastic or thermosetting resins or rubber; and in fiber-reinforced molded articles formed using thermoplastic or thermosetting resins or rubber and further using reinforcing fibers.

[0313] Applications of resin molds or rubber molds containing the hollow particles of the present invention include components such as light-reflecting materials, heat-insulating materials, sound-insulating materials, and low-dielectric materials used in various fields such as automobiles, electrical, electronics, construction, aviation, and aerospace; food containers; footwear such as sports shoes and sandals; appliance parts; bicycle parts; stationery; tools; and fiber filaments for 3D printers.

[0314] The hollow particles of the present invention, with a volume average particle size of 10 to 50 μm, have particularly excellent pressure resistance and are therefore preferred as additives for molded articles obtained by processes involving the application of external pressure, such as mixing and injection molding.

[0315] Furthermore, the hollow particles of this invention have high porosity, are not easily broken, and exhibit excellent heat resistance, thus meeting the requirements for heat insulation and cushioning (shock absorption) of inner coating materials, as well as the heat resistance suitable for thermal paper applications. In addition, the hollow particles of this invention are also useful as plastic pigments with excellent gloss and opacity.

[0316] Furthermore, the hollow particles of the present invention can be filled with useful ingredients such as fragrances, pharmaceuticals, pesticides, and ink components through impregnation, depressurization, or pressurization impregnation, thus enabling them to be used for various purposes depending on the contents contained therein.

[0317] Example

[0318] The present invention will be further described below with examples and comparative examples, but the present invention is not limited to these examples. Furthermore, unless otherwise specified, parts and percentages are based on mass.

[0319] [Example 1]

[0320] (1) Mixture preparation process

[0321] First, mix the following materials to form an oil phase.

[0322] 70 parts of ethylene glycol dimethacrylate

[0323] 30 parts of trimethylolpropane trimethacrylate

[0324] 3 parts of 2,2'-azobis(2,4-dimethylvalerate) (oil-soluble polymerization initiator, manufactured by Wako Pure Chemical Industries, Ltd., Japan, trade name: V-65)

[0325] 0.007 parts of rosin acid

[0326] 131 parts cyclohexane and 56 parts toluene (hydrophobic solvent)

[0327] Next, in a stirred tank at room temperature, under stirring, an aqueous solution of sodium hydroxide (alkali metal hydroxide) dissolved in 121 parts of ion-exchanged water was slowly added to an aqueous solution of magnesium chloride (a water-soluble polyvalent metal salt) dissolved in 494 parts of ion-exchanged water, to prepare a magnesium hydroxide colloidal (water-insoluble metal hydroxide colloidal) dispersion (4 parts magnesium hydroxide), thus forming an aqueous phase.

[0328] A mixture is prepared by mixing the aqueous and oil phases.

[0329] (2) Suspension process

[0330] The mixture obtained in the above-mentioned mixture preparation process is suspended by stirring in a disperser (manufactured by PRIMIX Corporation, trade name: HOMOMIXER) at a speed of 4000 rpm for 1 minute to prepare a suspension in which droplets of the monomer composition containing a hydrophobic solvent are dispersed in water.

[0331] (3) Polymerization process

[0332] The suspension obtained in the above suspension process was heated from 40°C to 65°C over a nitrogen atmosphere for 30 minutes (heating rate: 50°C / hour), and then stirred at 100 rpm for 1 hour and 30 minutes at 65°C to carry out the first polymerization reaction. The polymerization conversion rate at the end of the first polymerization reaction was 99.2% by mass. Next, 5 parts of methyl acrylate were added to the stirred tank, and the mixture was stirred at 100 rpm for 2 hours and 30 minutes at 65°C under a nitrogen atmosphere to carry out the second polymerization reaction. Through this second polymerization reaction, a precursor composition containing precursor particles containing a hydrophobic solvent was obtained.

[0333] (4) Cleaning process and solid-liquid separation process

[0334] The above precursor composition was washed with dilute sulfuric acid (25°C, 10 minutes) to bring the pH to below 5.5. Next, after separating the water by filtration, 200 parts of ion-exchanged water were added to re-slurry the mixture, and the process was repeated multiple times with water washing (washing, filtration, dehydration) at room temperature (25°C). Filtration was then performed to obtain the solid component. The obtained solid component was dried in a dryer at 40°C to obtain precursor particles containing a hydrophobic solvent.

[0335] (5) Solvent removal process

[0336] The precursor particles obtained in the above solid-liquid separation process were heated in a vacuum dryer at 200°C for 6 hours to remove the hydrophobic solvent contained in the particles, thereby obtaining the hollow particles of Example 1. Based on the observation results of a scanning electron microscope and the porosity value, the obtained hollow particles were confirmed to be spherical and have a hollow portion.

[0337] [Example 2]

[0338] In Example 1, in the above "(1) Mixture Preparation Step", the materials and amounts of polymerizable monomers added to the oil phase are as shown in Table 1. Otherwise, the hollow particles of Example 2 are obtained by the same steps as in Example 1.

[0339] [Example 3]

[0340] In Example 1, in the above "(1) Mixture Preparation Step", the type and amount of hydrophobic solvent are as shown in Table 1. Otherwise, the hollow particles of Example 3 are obtained by the same steps as in Example 1.

[0341] [Example 4]

[0342] In Example 2, in the above "(3) Polymerization step", only the first polymerization reaction is carried out, and the addition of methyl acrylate and the second polymerization reaction are not carried out. Otherwise, the hollow particles of Example 4 are manufactured using the same steps as in Example 2.

[0343] [Comparative Examples 1, 3-7]

[0344] In Example 1, in the above-mentioned "(1) Mixture Preparation Step", the materials and amounts of polymerizable monomers added to the oil phase are as shown in Table 1, and the types and amounts of hydrophobic solvents are as shown in Table 1. Otherwise, the hollow particles of Comparative Examples 1, 3 to 7 are manufactured using the same steps as in Example 1.

[0345] [Comparative Example 2]

[0346] (1) Mixture preparation process

[0347] First, mix the following materials to form an oil phase.

[0348] 5 parts methacrylic acid

[0349] 65 parts of ethylene glycol dimethacrylate

[0350] 30 parts of trimethylolpropane trimethacrylate

[0351] 3 parts of 2,2'-azobis(2,4-dimethylvalerate) (oil-soluble polymerization initiator, manufactured by Wako Pure Chemical Industries, Ltd., Japan, trade name: V-65)

[0352] 0.007 parts of rosin acid

[0353] 187 parts of cyclohexane

[0354] Next, in a stirred tank at room temperature, 3 parts of surfactant (sodium dodecylbenzenesulfonate) were added to 1160 parts of ion-exchanged water to prepare an aqueous phase.

[0355] A mixture is prepared by mixing the aqueous and oil phases.

[0356] (2) Suspension process

[0357] The mixture obtained in the above-mentioned mixture preparation process is suspended by stirring in a disperser (manufactured by PRIMIX Corporation, trade name: HOMOMIXER) at a speed of 4000 rpm for 1 minute to prepare a suspension of droplets of the monomer composition containing cyclohexane dispersed in water.

[0358] (3) Polymerization process

[0359] The suspension obtained in the above suspension process is heated from 40°C to 65°C in a nitrogen atmosphere for 30 minutes (heating rate: 50°C / hour), and stirred at 100 rpm for 4 hours at 65°C to carry out the polymerization reaction, thereby obtaining a precursor composition containing precursor particles containing cyclohexane.

[0360] (4) Cleaning process and solid-liquid separation process

[0361] After separating the water from the above precursor composition by filtration, 200 parts of ion-exchanged water were added to re-slurry the mixture. The mixture was then subjected to repeated water washing (washing, filtration, dehydration) at room temperature (25°C), followed by filtration to obtain the solid component. The obtained solid component was dried in a dryer at 40°C to obtain precursor particles containing cyclohexane.

[0362] (5) Solvent removal process

[0363] The precursor particles obtained in the above solid-liquid separation process were heated in a vacuum dryer at 200°C for 6 hours to remove the cyclohexane contained in the particles, thereby obtaining the hollow particles of Comparative Example 2. Based on the observation results of scanning electron microscopy and the porosity value, the obtained hollow particles were confirmed to be spherical and have a hollow part.

[0364] [Comparative Example 8]

[0365] In Example 1, in the above-mentioned "(1) Mixture Preparation Step", the materials and amounts of polymerizable monomers added to the oil phase are as shown in Table 1. Furthermore, the types and amounts of hydrophobic solvents are as shown in Table 1. In the above-mentioned "(3) Polymerization Step", the heating rate from 40°C to 65°C and the stirring speed during the first polymerization reaction and the second polymerization reaction are as shown in Table 1. Otherwise, the hollow particles of Comparative Example 8 were manufactured using the same steps as in Example 1.

[0366] [Table 1]

[0367]

[0368] [evaluate]

[0369] 1. Aggregation conversion rate

[0370] 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. Through pressure filtration, the particles (containing moisture and hydrophobic solvent) generated by the first polymerization reaction contained in the reaction solution were obtained and accurately weighed to the unit of 1 mg. In about 3 g of the accurately weighed particles, 27 g of ethyl acetate was added. After stirring for 15 minutes, 13 g of methanol was added and stirring was continued for another 10 minutes. The resulting solution was allowed to stand to precipitate the insoluble components, and the supernatant of this solution was collected as a measurement sample. 2 μL of the measurement sample was injected into a gas chromatograph, and the amount of polymerizable monomer in the sample was quantitatively determined by gas chromatography (GC) under the following conditions, and this was taken as the mass of the unreacted polymerizable monomer. In addition, the particles generated by the first polymerization reaction obtained by pressure filtration were dried at 200 °C for 2 hours to remove moisture and hydrophobic solvent, and the mass of the solid component of the particles was determined. Then, the polymerization conversion rate was calculated by the following formula (C).

[0371] Polymerization conversion rate (mass %) = 100 - (mass of unreacted polymerizable monomer / mass of solid component of particles generated by the first

[0372] polymerization reaction) × 100 Formula (C)

[0373] <GC conditions>

[0374] Column: TC-WAX (0.25 mm × 30 m)

[0375] Column temperature: 80 °C

[0376] Injection temperature: 200 °C

[0377] FID detection side temperature: 200 °C

[0378] For 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.

[0379] In addition, for the hollow particles obtained in each example and each comparative example, the following measurements and evaluations were carried out. The results are shown in Table 2.

[0380] 2. Volume average particle diameter of hollow particles

[0381] 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 its volume average was calculated as the volume average particle diameter.

[0382] 3. Average roundness

[0383] 10 mL of ion-exchanged water was added to a container, followed by 0.02 g of surfactant (alkylbenzene sulfonic acid) as a dispersant. Then, 0.02 g of hollow particles were added, and the mixture was dispersed using an ultrasonic disperser at 60 W for 3 minutes. The amount of ion-exchanged water was adjusted to achieve a hollow particle concentration of 3000–10000 particles / μL, yielding the test sample. Using the obtained test sample, 5000 hollow particles with an equivalent circular diameter of 0.4 μm or greater were analyzed using a flow cytometry particle image analyzer (Sysmex Corporation, trade name: FPIA-2100) to measure the circumference of a circle equal to the projected area of ​​the particle and the circumference of the particle's projected image. The roundness was calculated using the following formula 1. The average roundness of the 5000 hollow particles was taken as the average roundness.

[0384] Formula 1: (Roundness) = (Circumference of a circle with the same projected area as the particle) / (Particle)

[0385] (Perimeter of the projected image)

[0386] 4. Density and porosity of hollow particles

[0387] 4-1. Determination of the apparent density of hollow particles

[0388] First, about 30cm 3 Hollow particles are filled in a volume of 100 cm³ 3 In a volumetric flask, accurately weigh the mass of the hollow granules used to fill it. Next, carefully fill the volumetric flask with isopropanol to the mark, taking care to avoid introducing air bubbles. Accurately weigh the mass of isopropanol added to the volumetric flask, and calculate the apparent density D1 (g / cm³) of the hollow granules according to the following formula (I). 3 ).

[0389] Formula (I)

[0390] Apparent density D1 = [mass of hollow particles] / (100 - [mass of isopropanol] ÷ [mass of the particles in the determination])

[0391] The specific gravity of isopropanol at temperature

[0392] 4-2. Determination of the true density of hollow particles

[0393] After pre-crushing the hollow granules, fill a 100cm³ container with approximately 10g of hollow granule fragments. 3 In a volumetric flask, the mass of the filling fragments is precisely weighed.

[0394] Then, in the same manner as the apparent density determination described above, isopropanol is added to the volumetric flask, the mass of isopropanol is accurately weighed, and the true density D0 (g / cm³) of the hollow particles is calculated according to the following formula (II). 3 ).

[0395] Equation (II)

[0396] True density D0 = [fragmented mass of hollow particles] / (100 - [mass of isopropanol] ÷ [specific gravity of isopropanol at the measurement temperature])

[0397] 4-3. Calculation of Porosity

[0398] Based on the apparent density D1 and true density D0 of the hollow particles, the porosity of the hollow particles is calculated according to the following formula (III).

[0399] Equation (III)

[0400] Porosity (%) = 100 - (apparent density D1 / true density D0) × 100

[0401] 5. The thickness of the shell of hollow particles

[0402] Using the volume average particle size R and porosity of the hollow particles, the inner diameter r of the hollow particles is calculated by the following formula (1). Using the inner diameter r and the volume average particle size R, the thickness of the shell of the hollow particles is calculated by the following formula (2).

[0403] 4 / 3π×(R / 2) 3 ×(porosity / 100)=4 / 3π×(r / 2) 3

[0404] Equation (1)

[0405] Shell thickness = (Rr) / 2 Equation (2)

[0406] 6. The proportion of particles with concave portions

[0407] Hollow particles were imaged using FE-SEM (Field Emission Scanning Electron Microscopy). For 100 randomly selected hollow particles, the diameter of a circle with the same projected area as the particle image was measured as the particle size. In the case of hollow particles with concave areas, the distance in the direction of the maximum width of the concave area in the FE-SEM image was measured as the size of the concave area. The number of particles with concave areas that are 5% to 50% of the particle size was counted and evaluated according to the following evaluation criteria.

[0408] (Evaluation Criteria)

[0409] ◎: The proportion of particles having a concave portion relative to their diameter of 5-50% is less than 5%.

[0410] ○: The proportion of particles having a concave portion that is 5% to 50% of the particle size is 5% to 10%.

[0411] ×: The proportion of particles having a concave portion relative to 5-50% of the particle size is greater than 10%.

[0412] 7. Pore retention rate in the molded body

[0413] 90 parts of polypropylene (manufactured by Mitsubishi Chemical Corporation, product name: MA1B, specific gravity 0.90 g / cm³) as a thermoplastic resin were mixed using a mixer. 3 The mixture was then mixed with 10 parts of the hollow granules obtained in each of the examples or comparative examples. Next, the mixture was kneaded, extruded, and granulated using a twin-shaft mixer (manufactured by Toshiba Machine Co., Ltd., product name: TEM-35B) under the following kneading conditions to obtain granules of the resin composition.

[0414] <Mixing Conditions>

[0415] Screw diameter 37mm, L / D = 32

[0416] Screw speed 250 rpm

[0417] Resin temperature 190℃

[0418] Feed rate 20kg / hour

[0419] The obtained resin composition particles were dried by heating at 80°C for 6 hours, and then molded using an injection molding device under the following molding conditions to obtain a molded body with dimensions of 80mm × 10mm × 4mm thickness.

[0420] Molding conditions

[0421] Barrel temperature: 230℃

[0422] Mold temperature: 40℃

[0423] Injection pressure: 70 MPa

[0424] Using a: the specific gravity of the injection-molded part, b: the specific gravity of the molded part assuming porosity is maintained (calculated value), and c: the specific gravity of the molded part assuming all hollow particles are broken (calculated value), the porosity is calculated by the following formula (D).

[0425] Pore ​​retention rate (%) = {(ca) / (cb)} × 100 (Equation D)

[0426] In addition, the specific gravity of the injection-molded part was determined according to the water displacement method in JIS K 7112.

[0427] The specific gravity b of the molded body that maintains porosity is calculated by the following formula (E).

[0428] b = 1 / {(P A / P G )+(R A / R G Equation (E)

[0429] In the formula for calculating b above, P A P represents the amount of hollow particles added. G R represents the specific gravity of hollow particles. A R indicates the amount of thermoplastic resin added. G This indicates the specific gravity of thermoplastic resins.

[0430] The specific gravity c of the molded body, assuming all hollow particles are broken, is calculated by the following formula (F).

[0431] c = [R] G ×R A +{D0×P A ×(1-P V / 100)}] / {R A +P A ×(1-P v / 100)} Formula (F)

[0432] In the formula for calculating c above, R A R indicates the amount of thermoplastic resin added. G D0 represents the specific gravity of the thermoplastic resin, and P represents the true density of the hollow particles. A P represents the amount of hollow particles added. v This indicates the porosity (%) of the hollow particles.

[0433] [Table 2]

[0434]

[0435] [Inspection]

[0436] When the hollow particles obtained in Comparative Examples 1, 4, 7, and 8 are observed from a top view, the proportion of particles with concave portions that are 5 to 50% of the particle size is greater than 10%. Therefore, the porosity in the molded body is low, resulting in poor pressure resistance. It can be inferred that the hollow particles obtained in Comparative Examples 1, 4, 7, and 8 have reduced pressure resistance due to the concave portions.

[0437] Furthermore, when Comparative Example 1 and Comparative Example 4, which have the same shell composition, were compared, the porosity retention rate of Comparative Example 1 was lower than that of Comparative Example 4. It can be inferred that this is because in particles having recesses with a size relative to 50% of the particle size, differences in shell thickness easily affect pressure resistance. It can be inferred that the hollow particles obtained in Comparative Example 1 have a thinner shell thickness compared to the hollow particles obtained in Comparative Example 4; therefore, the pressure resistance of particles having recesses with a size relative to 50% of the particle size is further reduced.

[0438] Furthermore, it can be inferred that although the composition of the shell of Comparative Example 7 is the same as that of Example 2, the surface of the particles is concave due to the different types of hydrophobic solvents used in manufacturing the hollow particles.

[0439] It can be inferred that although the composition of the shell of Comparative Example 8 is the same as that of Example 2, the type of hydrophobic solvent used in manufacturing the hollow particles is different, as is the case with Comparative Example 7, and the heating rate and stirring rate during the polymerization reaction are also different. Therefore, in Comparative Example 8, more depressions are generated on the surface of the particles than in Comparative Example 7, which can be considered as a further reduction in the porosity of the molded body.

[0440] The hollow particles obtained in Comparative Example 2 had a volume average particle size of 3 μm, resulting in a low porosity and poor pressure resistance in the molded body. It can be inferred that the hollow particles obtained in Comparative Example 2 had poor pressure resistance due to their small volume average particle size and thin shell.

[0441] Regarding the hollow particles obtained in Comparative Example 3, the content of crosslinked monomer units in all monomer units of 100 parts by mass of the polymer contained in the shell is less than 70 parts by mass. When viewed from above, the proportion of particles having recesses with a size relative to 50% of the particle size is greater than 10%. Therefore, the porosity in the molded body is low, and the pressure resistance is poor. It can be inferred that the shell strength of the hollow particles obtained in Comparative Example 3 is insufficient, and thus the pressure resistance is reduced due to the recesses, resulting in poor pressure resistance.

[0442] Regarding the hollow particles obtained in Comparative Example 5, the crosslinking monomer units in the polymer contained in the shell do not include bifunctional crosslinking monomer units; therefore, the porosity retention rate in the molded body is low. It can be inferred that in Comparative Example 5, since only trifunctional or higher crosslinking monomers were used as crosslinking monomers to form the shell, the resulting hollow particles have interconnected pores, and the resin penetrates into the particles through these interconnected pores, resulting in a low porosity retention rate.

[0443] Regarding the hollow particles obtained in Comparative Example 6, the crosslinking monomer units in the polymer contained in the shell do not include trifunctional or higher crosslinking monomer units. When viewed from above, the proportion of particles with recesses of 5-50% of their particle size is greater than 10%. Therefore, the porosity in the molded body is low, resulting in poor pressure resistance. It can be inferred that in Comparative Example 6, since only bifunctional crosslinking monomers were used as crosslinking monomers to form the shell, recesses are easily generated on the particle surface, which reduces the pressure resistance.

[0444] The hollow particles obtained in each embodiment have a volume average particle size of 10 to 50 μm. The content of crosslinking monomer units in all monomer units of 100 parts by mass of the polymer contained in the shell is more than 70 parts by mass. The crosslinking monomer units include bifunctional crosslinking monomer units and trifunctional or more crosslinking monomer units. When viewed from above, the proportion of particles with concave portions that are 5 to 50% of the particle size is less than 10%. Therefore, even if the porosity is as high as 60% or more, the porosity retention rate in the molded body is high and the pressure resistance is excellent.

[0445] Description of Reference Numerals

[0446] 1: Aquatic medium;

[0447] 2: Low polarity materials;

[0448] 3: Dispersing stabilizer;

[0449] 4: Monomer composition;

[0450] 4a: Hydrophobic solvent;

[0451] 4b: Materials other than hydrophobic solvents;

[0452] 4c: Polymerizable monomers dispersed in aqueous media;

[0453] 5: Oil-soluble polymerization initiators;

[0454] 6: Shell;

[0455] 8: Hollow section;

[0456] 10: Droplets;

[0457] 20: Precursor particles;

[0458] 100: Hollow particles whose hollow core is filled with gas.

Claims

1. A hollow particle having a shell containing resin and a hollow portion surrounded by the shell, The hollow particles have a porosity of over 60%. The hollow particles have a volume average particle size of 10–50 μm. The shell contains a polymer comprising 70 to 100 parts by weight of crosslinking monomer units in 100 parts by weight of all monomer units as the resin, wherein the crosslinking monomer units include bifunctional crosslinking monomer units derived from bifunctional crosslinking monomers and trifunctional or higher crosslinking monomer units derived from trifunctional or higher crosslinking monomers. The trifunctional or higher crosslinking monomer unit is a crosslinking monomer unit derived from a methacryloyl-based crosslinking monomer having a methacryloyl group as a polymerizable functional group. When viewed from above, the proportion of particles having a concave portion that is 5 to 50% of the particle size is less than 10%. The proportion of these particles is determined as follows: the hollow particles in their dry powder state are photographed using a field emission scanning electron microscope. For 100 randomly selected hollow particles, the diameter of a circle with the same projected area as the particle image is measured as the particle size. In the case where the hollow particles have a concave portion, the distance in the direction of the maximum width of the concave portion in the field emission scanning electron microscope image is measured as the size of the concave portion. The number of particles having a concave portion that is 5 to 50% of the particle size is counted.

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

3. The hollow particles according to claim 1 or 2, wherein, Of all 100 parts by weight of the polymer contained in the shell, the content of the trifunctional or higher crosslinkable monomer units is 5 to 40 parts by weight.

4. The hollow particles according to claim 1 or 2, wherein, The thickness of the shell is 0.20 to 4.00 μm.

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