hollow particles

Abstract

This invention provides hollow particles with excellent performance stability under high humidity conditions and low dielectric constant. [Solution] A hollow particle comprising a shell containing a resin and a hollow portion surrounded by the shell, wherein the shell contains a polymer as the resin, with 70 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of total monomer units, a porosity of 60% or more, a surfactant content of 200 ppm or less on the surface of the hollow particle, and a relative permittivity of 1.6 or less at a frequency of 1 MHz.

Description

[Technical Field]

[0001] This disclosure relates to hollow particles. [Background technology]

[0002] Hollow particles (hollow resin particles) are particles with cavities inside. Compared to solid particles that are substantially filled with resin, they scatter light well and have low light transmittance. Therefore, they are widely used as organic pigments and opacities with excellent optical properties such as opacity and whiteness in water-based paints, paper coating compositions, and other applications. In recent years, they have also been used as lightweighting agents and heat insulating agents for resins and paints used in various fields such as automobiles, electrical and electronic equipment, and construction.

[0003] In electronic materials applications, for example, in electronic circuit boards, hollow particles are sometimes incorporated into the insulating resin layer to suppress crosstalk and increase transmission loss. Crosstalk and transmission loss in electronic circuit boards can be suppressed by reducing the dielectric constant and dielectric loss tangent of the insulating resin layer. Because hollow particles have a hollow interior, attempts have been made to lower the dielectric constant and dielectric loss tangent of the insulating resin layer by adding hollow particles.

[0004] For example, Patent Document 1 discloses hollow crosslinked resin particles used in low dielectric constant organic insulating materials, which are obtained by polymerizing 1 to 100% by weight of crosslinkable monomers and 0 to 99% by weight of non-crosslinkable monomers (where the total of crosslinkable and non-crosslinkable monomers is 100% by weight), having an average particle diameter of 0.03 to 10 μm and an average metal ion concentration present in the particles of 50 ppm or less. The hollow crosslinked resin particles of Patent Document 1 are manufactured by seed polymerization, in which monomers are dispersed in water using an emulsifier (surfactant). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2000-313818 [Overview of the project] [Problems that the invention aims to solve]

[0006] Furthermore, electronic circuit boards require stable performance in high-humidity environments; for example, they must not experience problems such as migration in high-humidity environments. However, electronic circuit boards containing hollow cross-linked resin particles described in Patent Document 1 may experience migration in high-humidity environments. In addition, the hollow cross-linked resin particles described in Patent Document 1 do not have a sufficiently low dielectric constant, and further reduction of the dielectric constant is required.

[0007] The objective of this disclosure is to provide hollow particles that exhibit excellent performance stability in high-humidity environments and have a low dielectric constant. [Means for solving the problem]

[0008] The inventors have discovered that one of the causes of migration of electronic circuit boards in high-humidity environments is the presence of surfactant residue on the surface of hollow particles contained in the electronic circuit board.

[0009] This disclosure relates to a hollow particle comprising a resin-containing shell and a hollow portion surrounded by the shell, The shell contains a polymer as the resin, in which 70 to 100 parts by mass of crosslinkable monomer units are present in 100 parts by mass of total monomer units. The porosity is 60% or more. The surfactant content present on the surface of the hollow particles is 200 ppm or less. The present invention provides hollow particles having a relative permittivity of 1.6 or less at a frequency of 1 MHz.

[0010] In the hollow particles of this disclosure, the volume-average particle size is preferably 1 to 10 μm.

[0011] In the hollow particles of the present disclosure, the porosity is preferably 90% or less.

[0012] In the hollow particles of the present disclosure, the metal content is preferably 100 ppm or less.

Advantages of the Invention

[0013] According to the present disclosure as described above, it is possible to provide hollow particles that are excellent in performance stability in a high-humidity environment and have a low relative permittivity.

Brief Description of the Drawings

[0014] [Figure 1] It is a diagram for explaining an example of a method for producing the hollow particles of the present disclosure. [Figure 2] It is a schematic diagram showing an embodiment of the suspension in the suspension step.

Embodiments for Carrying Out the Invention

[0015] In the present disclosure, "~" in a numerical range means that the numerical values described before and after it are included as the lower limit value and the upper limit value. In the present disclosure, (meth)acrylate represents each of acrylate and methacrylate, (meth)acrylic represents each of acrylic and methacrylic, and (meth)acryloyl represents each of acryloyl and methacryloyl. In the present disclosure, a polymerizable monomer is a compound having a functional group capable of addition polymerization (which may be simply referred to as a polymerizable functional group in the present disclosure). In the present disclosure, as the polymerizable monomer, a compound having an ethylenically unsaturated bond as the functional group capable of addition polymerization is generally used. The polymerizable monomers include non-crosslinkable monomers and crosslinkable monomers. A non-crosslinkable monomer is a polymerizable monomer having only one polymerizable functional group, and a crosslinkable monomer is a polymerizable monomer having two or more polymerizable functional groups and forming a crosslink bond in the resin by a polymerization reaction.

[0016] The hollow particles of this disclosure are hollow particles comprising a resin-containing shell and a hollow portion surrounded by the shell, The shell contains a polymer as the resin, in which 70 to 100 parts by mass of crosslinkable monomer units are present in 100 parts by mass of total monomer units. The porosity is 60% or more. The surfactant content present on the surface of the hollow particles is 200 ppm or less. It is characterized by having a relative permittivity of 1.6 or less at a frequency of 1 MHz.

[0017] The hollow particles in this disclosure are particles comprising a resin-containing shell (outer shell) and a hollow portion surrounded by the shell. In this disclosure, the hollow portion is a cavity-like space clearly distinguishable from the shell of the hollow particle formed from the resin material. The shell of the hollow particle may have a porous structure, in which case the hollow portion is of a size clearly distinguishable from a multitude of minute spaces uniformly dispersed within the porous structure. The hollow portion of a hollow particle can be confirmed, for example, by SEM observation of the particle cross-section, or by TEM observation of the particle itself. Furthermore, from the viewpoint of reducing dielectric constant, it is preferable that the hollow portion of the hollow particles in this disclosure is filled with air or a gas such as nitrogen, or is in a reduced-pressure state close to a vacuum.

[0018] Because hollow particles have a hollow portion inside, materials containing hollow particles are expected to have properties such as lighter weight, better heat insulation, and lower dielectric constant. However, if the amount of surfactant present on the surface of hollow particles is high, in high-humidity environments, the surfactant on the particle surface may adsorb moisture, imparting undesirable properties to the hollow particles. For example, if hollow particles incorporated into the insulating resin layer of an electronic circuit board contain a large amount of surfactant on the particle surface, migration is likely to occur in high-humidity environments due to the adsorption of moisture by the surfactant on the particle surface. In contrast, the hollow particles disclosed in this disclosure have a sufficiently reduced amount of surfactant present on the particle surface, so they do not impart undesirable properties even in high-humidity environments and exhibit excellent performance stability. In this disclosure, a surfactant is defined as a compound having both a hydrophilic group and a hydrophobic group in one molecule, and includes compounds commonly used as surfactants. Surfactants typically have a solubility of 1 g / L or more in water at 25°C. Furthermore, the hollow particles of this disclosure have a porosity of 60% or more, and the polymer contained in the shell contains 70 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of total monomer units, thus enabling a low dielectric constant. In hollow particles, the larger the space within the hollow portion, the lower the relative dielectric constant tends to be. The hollow particles of this disclosure have a high porosity of 60% or more, thus enabling a low dielectric constant. When the space within the hollow portion becomes smaller due to deformation or crushing of the hollow particles, the relative dielectric constant of the hollow particles tends to increase. However, the hollow particles of this disclosure have a high proportion of crosslinkable monomer units in the shell, and a dense covalent network is spread throughout the shell, resulting in excellent strength and resistance to deformation. Therefore, the shape of the hollow portion is easily maintained, and a low dielectric constant can be maintained. The following describes an example of a method for producing the hollow particles of this disclosure, followed by a detailed description of the hollow particles of this disclosure.

[0019] 1. Method for producing hollow particles The hollow particles of this disclosure are, for example, A step of preparing a mixture containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium, The steps include: preparing a suspension in which droplets of a monomer composition containing the polymerizable monomer, the hydrophobic solvent, and the polymerization initiator are dispersed in an aqueous medium by suspending the aforementioned mixture; The hollow particles can be obtained by a method for producing hollow particles, which includes the step of preparing a precursor composition containing precursor particles having a hollow portion surrounded by a resin-containing shell, and the hydrophobic solvent encapsulated in the hollow portion, by subjecting the suspension to a polymerization reaction.

[0020] The above method for producing hollow particles includes the steps of preparing a mixture, preparing a suspension, and subjecting the suspension to a polymerization reaction, and may also include other steps. Furthermore, to the extent that it is technically possible, two or more of the above steps and other additional steps may be performed simultaneously as a single step, or in any order. For example, the preparation of the mixture and the suspension may be performed simultaneously in a single process, such as adding the materials for preparing the mixture while simultaneously performing the suspension.

[0021] A preferred example of a method for producing the above-mentioned hollow particles is a manufacturing method that includes the following steps. (1) Mixed liquid preparation process A process for preparing a mixture containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium. (2) Suspension process The process involves suspending the aforementioned mixture to prepare a suspension in which droplets of a monomer composition containing a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator are dispersed in an aqueous medium. (3) Polymerization process A process to prepare a precursor composition by subjecting the suspension to a polymerization reaction, wherein the precursor particles have a hollow portion surrounded by a resin-containing shell, and the hollow portion contains a hydrophobic solvent. (4) Washing and solid-liquid separation process The process involves washing the precursor composition to remove any remaining dispersion stabilizers, then separating the precursor composition from its solid-liquid state to obtain precursor particles containing a hydrophobic solvent in their hollow portions, and (5) Solvent removal process A step to remove the hydrophobic solvent contained within the precursor particles obtained by the solid-liquid separation step to obtain hollow particles. In this disclosure, hollow particles in which the hollow portion is filled with a hydrophobic solvent may be considered an intermediate between hollow particles in which the hollow portion is filled with gas and referred to as "precursor particles." In this disclosure, "precursor composition" means a composition containing precursor particles.

[0022] Figure 1 is a schematic diagram showing an example of a method for manufacturing hollow particles according to the present disclosure. (1) to (5) in Figure 1 correspond to the above steps (1) to (5). The white arrows between the figures indicate the order of each step. Note that Figure 1 is merely a schematic diagram for illustrative purposes, and the manufacturing method is not limited to that shown in the figure. Furthermore, the structure, dimensions, and shape of the materials used in the manufacturing method according to the present disclosure are not limited to the structure, dimensions, and shape of the various materials shown in these figures. Figure 1(1) is a schematic cross-sectional view showing one embodiment of the mixture in the mixture preparation process. As shown in this 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 means a material that has low polarity and does not easily mix with the aqueous medium 1. In this disclosure, the low-polarity material 2 includes a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator. Figure 1(2) is a schematic cross-sectional view showing one embodiment of a suspension in the suspension process. The suspension comprises an aqueous medium 1 and droplets 10 of a monomer composition dispersed in the aqueous medium 1. The droplets 10 of the monomer composition contain a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator, but the distribution within the droplets is non-uniform. The droplets 10 of the monomer composition have a structure in which the hydrophobic solvent 4a and the material other than the hydrophobic solvent containing the polymerizable monomer 4b are phase-separated, with the hydrophobic solvent 4a unevenly distributed in the center and the material other than the hydrophobic solvent 4b unevenly distributed on the surface side, and a dispersion stabilizer (not shown) attached to the surface. Figure 1(3) is a schematic cross-sectional view showing one embodiment of a precursor composition obtained by a polymerization process, which includes precursor particles containing a hydrophobic solvent in a hollow portion. The precursor composition includes an aqueous medium 1 and precursor particles 20 dispersed in the aqueous medium 1, each containing a hydrophobic solvent 4a in a hollow portion. The shell 6 forming the outer surface of the precursor particles 20 is formed by polymerization of polymerizable monomers in droplets 10 of the monomer composition, and contains the polymer of the polymerizable monomers as a resin. Figure 1(4) is a schematic cross-sectional view showing one embodiment of precursor particles after the solid-liquid separation process. Figure 1(4) shows the state after the aqueous medium 1 has been removed from the state shown in Figure 1(3). Figure 1(5) is a schematic cross-sectional view showing one embodiment of hollow particles after the solvent removal process. Figure 1(5) shows the state after the hydrophobic solvent 4a has been removed from the state shown in Figure 1(4). By removing the hydrophobic solvent from the precursor particles, hollow particles 100 are obtained that have a gas-filled hollow portion 8 inside the shell 6. The following describes the five processes mentioned above, as well as other processes, in order.

[0023] (1) Mixed liquid preparation process This process involves preparing a mixture containing a polymerizable monomer, a hydrophobic solvent, a polymerization initiator, a dispersion stabilizer, and an aqueous medium. The mixture may further contain other materials, to the extent that they do not impair the effects of the present disclosure. The materials of the mixed solution will be described in the following order: (A) polymerizable monomer, (B) hydrophobic solvent, (C) polymerization initiator, (D) dispersion stabilizer, (E) aqueous medium, and (F) other materials.

[0024] (A) Polymerizable monomers In the above manufacturing method, the polymerizable monomer in the mixture contains at least a crosslinkable monomer and may further contain non-crosslinkable monomers to the extent that it does not impair the effects of the present disclosure. From the standpoint of easily stabilizing the polymerization reaction and obtaining hollow particles with excellent strength and heat resistance, (meth)acrylic polymerizable monomers having (meth)acryloyl groups as polymerizable functional groups can preferably be used. On the other hand, from the viewpoint of reducing the relative permittivity and dielectric loss tangent of hollow particles, hydrocarbon monomers consisting of carbon and hydrogen can be preferably used.

[0025] [Cross-linkable monomers] Because crosslinkable monomers have multiple ethylenically unsaturated double bonds, they can be linked together, thereby increasing the crosslinking density of the shell. Examples of crosslinkable monomers include difunctional crosslinkable monomers having two polymerizable functional groups, such as divinylbenzene, divinyldiphenyl, divinylnaphthalene, diallyl phthalate, allyl (meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate; and trifunctional or more crosslinkable monomers having three or more polymerizable functional groups, such as trimethylolpropane tri(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol poly(meth)acrylate, and their ethoxylated derivatives. These crosslinkable monomers can be used individually or in combination of two or more types. Divinylbenzene, ethylene glycol di(meth)acrylate, and pentaerythritol di(meth)acrylate are preferred as bifunctional crosslinkable monomers because the polymerization reaction is easily stabilized, hollow particles with excellent strength and heat resistance can be obtained, and the dielectric constant of the hollow particles can be reduced. Among these, ethylene glycol di(meth)acrylate and pentaerythritol di(meth)acrylate are preferred in terms of improving the strength and heat resistance of the hollow particles. Divinylbenzene, a hydrocarbon monomer, is preferred in terms of further reducing the dielectric constant and dielectric loss tangent of the hollow particles. As crosslinkable monomers with three or more functions, pentaerythritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, and dipentaerythritol poly(meth)acrylate are preferred, with trimethylolpropane tri(meth)acrylate and pentaerythritol tetra(meth)acrylate being more preferred, due to their stable polymerization reaction, excellent strength and heat resistance, and reduced dielectric constant of the hollow particles.

[0026] The crosslinkable monomer content is preferably 70 to 100 parts by mass per 100 parts by mass of the total polymerizable monomers in the mixture. A crosslinkable monomer content of 70 parts by mass or more ensures a sufficiently high proportion of crosslinkable monomer units within the hollow particle shell, resulting in a densely packed covalent network within the shell. This creates a shell with excellent strength, resistance to crushing, and resistance to deformation from external heat. Furthermore, a crosslinkable monomer content of 70 parts by mass or more improves the strength of the hollow particles, thereby suppressing the increase in dielectric constant due to crushing or deformation of the hollow particles. The crosslinkable monomer content is preferably 80 parts by mass or more, more preferably 90 parts by mass or more.

[0027] The polymerizable monomer in the mixture preferably contains at least a bifunctional crosslinkable monomer as a crosslinkable monomer. This facilitates the formation of hollow spaces within the particles. From the viewpoint of facilitating the formation of hollow spaces within the particles and further improving the strength of the shell, it is preferable to include a combination of a bifunctional crosslinkable monomer and a trifunctional or more crosslinkable monomer. The content of a bifunctional crosslinkable monomer in 100 parts by mass of polymerizable monomer in the mixture is preferably 50 parts by mass or more, more preferably 60 parts by mass or more. If the polymerizable monomer contains a trifunctional or more crosslinkable monomer as a crosslinkable monomer, the upper limit of the content of a bifunctional crosslinkable monomer in 100 parts by mass of polymerizable monomer in the mixture is preferably 95 parts by mass or less, more preferably 90 parts by mass or less, and even more preferably 80 parts by mass or less. When the polymerizable monomer in the mixture contains a crosslinkable monomer with three or more functions, the content of the crosslinkable monomer with three or more functions in 100 parts by mass of polymerizable monomer in the mixture is not particularly limited, but the lower limit is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 20 parts by mass or more, and the upper limit is preferably 50 parts by mass or less, and more preferably 40 parts by mass or less.

[0028] [Non-crosslinkable monomers] The polymerizable monomer in the mixture may further contain non-crosslinkable monomers, to the extent that it does not impair the effects of the present disclosure. As non-crosslinkable monomers, monovinyl monomers are preferably used. A monovinyl monomer is a compound having one polymerizable vinyl functional group. Examples of monovinyl monomers include (meth)acrylic monovinyl monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, glycidyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, (meth)acrylic acid, etc.; styrene, vinyltoluene, α-methylstyrene, p-methylstyrene, ethyl vinylbenzene, ethyl vinyl biphenyl, ethyl vinyl naphthalene, Examples include aromatic vinyl monomers such as styrene halogens; monoolefin monomers such as ethylene, propylene, and butylene; (meth)acrylamide monomers and their derivatives such as (meth)acrylamide, N-methylol(meth)acrylamide, and N-butoxymethyl(meth)acrylamide; diene monomers such as butadiene and isoprene; vinyl carboxylate monomers such as vinyl acetate; vinyl halogenated monomers such as vinyl chloride; vinylidene halogenated monomers such as vinylidene chloride; vinylpyridine monomers; and others. These non-crosslinkable monomers can be used individually or in combination of two or more. Among these, (meth)acrylic monovinyl monomers are preferred from the viewpoint of reactivity and heat resistance, and at least one selected from butyl acrylate and methyl methacrylate is more preferred. From the viewpoint of reducing the relative permittivity and dielectric loss tangent of hollow particles, hydrocarbon monomers such as styrene, vinyltoluene, α-methylstyrene, p-methylstyrene, ethyl vinylbenzene, ethyl vinyl biphenyl, and ethyl vinyl naphthalene are preferred.

[0029] The content of non-crosslinkable monomers in the mixed solution is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, even more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less, per 100 parts by mass of the total mass of polymerizable monomers.

[0030] In the mixed solution, the content of polymerizable monomer 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, per 100 parts by mass of the total of polymerizable monomer and hydrophobic solvent. When the content of polymerizable monomer is within the above range, a good balance is achieved between the porosity, particle size, and mechanical strength of the hollow particles. Furthermore, from the viewpoint of improving the mechanical strength of the hollow particles, the content of polymerizable monomers relative to 100% by mass of the total solid content of the material that forms the oil phase in the mixed liquid, excluding the hydrophobic solvent, is preferably 90% by mass or more, more preferably 95% by mass or more. In this disclosure, "solids" refers to all components excluding the solvent, and liquid polymerizable monomers, etc., are included in the solids.

[0031] (B) Hydrophobic solvents The hydrophobic solvent used in the above manufacturing method is a nonpolymerizable and poorly water-soluble organic solvent. The hydrophobic solvent acts as a spacer material, forming hollow spaces within the particles. In the suspension step described later, a suspension is obtained in which droplets of the monomer composition containing the hydrophobic solvent are dispersed in an aqueous medium. In the suspension step, phase separation occurs within the monomer composition droplets, resulting in the less polar hydrophobic solvent tending to accumulate inside the monomer composition droplets. Ultimately, in the monomer composition droplets, the hydrophobic solvent is distributed inside, and other materials other than the hydrophobic solvent are distributed around its periphery according to their respective polarities. Then, in the polymerization process described later, an aqueous dispersion containing hollow particles encapsulating a hydrophobic solvent is obtained. That is, as the hydrophobic solvent accumulates inside the particles, a hollow space filled with the hydrophobic solvent is formed inside the resulting precursor particles.

[0032] In the above manufacturing method, the hydrophobic solvent is appropriately selected according to the type of polymerizable monomer and is not particularly limited. Known hydrophobic solvents can be used, for example, esters such as ethyl acetate and butyl acetate; ether esters such as propylene glycol monomethyl ether acetate and propylene glycol monoethyl ether acetate; aromatic hydrocarbons such as benzene, toluene, and xylene; aliphatic hydrocarbons such as hexane, methyl hexane, cyclohexane, and methylcyclohexane; and so on. These hydrophobic solvents can be used individually or in combination of two or more. In particular, it is preferable to select a hydrophobic solvent such that the HSP distance between the crosslinkable monomer in the polymerizable monomer and the hydrophobic solvent is 5.80 to 6.50. More preferably, the HSP distance is 5.85 to 6.40, and even more preferably 5.90 to 6.30. When the HSP distance between the crosslinkable monomer and the hydrophobic solvent is within the above range, the polymerizable monomer and the hydrophobic solvent separate sufficiently in the droplet of the monomer composition, making it easier to form hollow parts within the particles, and also making it easier to achieve a uniform thickness in the shell formed by the polymerization reaction. The HSP distance is an index representing the solubility between substances using the Hansen solubility parameter (HSP). The closer the HSP distance is to 0, the higher the compatibility between the substances. HSP is represented as a vector in a three-dimensional space (Hansen space) with the dispersion term dD, polarity term dP, and hydrogen bonding term dH as coordinate axes. The three parameters dD, dP, and dH have unique values ​​for each substance. The software developed by Hansen et al. (software name: Hansen Solubility Parameter in Practice (HSPiP)) contains a database of dD, dP, and dH for various substances. Furthermore, HSPiP can also be used to calculate HSP based on the chemical structure of a substance. When determining the HSP of a mixture of multiple substances, the weighted average of the dD, dP, and dH values ​​of each substance in the mixture, as well as the proportion of each substance, is calculated to determine the dispersion term dD, polarity term dP, and hydrogen bonding term dH of the mixture, and the HSP is then determined. The HSP distance is the vector distance between two substances given by their HSPs, and is calculated using the following formula (A) from the values ​​of the dispersion term dD1, polarity term dP1, and hydrogen bonding term dH1 of one substance and the values ​​of the dispersion term dD2, polarity term dP2, and hydrogen bonding term dH2 of the other substance. Formula (A) HSP distance={4(dD1-dD2) 2 +(dP1-dP2) 2 +(dH1-dH2) 2} 0.5 In this disclosure, the HSP distance is defined as a value calculated using HSPiP (Version 5.3.03). In HSPiP, the values ​​of dD, dP, and dH are expressed with significant figures up to one decimal place, and the HSP distance value is expressed with significant figures up to two decimal places.

[0033] Furthermore, although not particularly limited, the boiling point of the hydrophobic solvent is preferably 130°C or lower, more preferably 100°C or lower, from the viewpoint of ease of removal in the solvent removal process described later. On the other hand, from the viewpoint of ease of encapsulation into precursor particles, it is preferably 50°C or higher, more preferably 60°C or higher. Furthermore, if the hydrophobic solvent is a mixed solvent containing multiple types of hydrophobic solvents and has multiple boiling points, it is preferable that the boiling point of the solvent with the highest boiling point among the solvents contained in the mixed solvent is less than or equal to the above upper limit, and it is preferable that the boiling point of the solvent with the lowest boiling point among the solvents contained in the mixed solvent is greater than or equal to the above lower limit.

[0034] Furthermore, the hydrophobic solvent used in the above manufacturing method preferably has a dielectric constant of 3 or less at 20°C. Relative dielectric constant is one indicator of the polarity of a compound. When the dielectric constant of the hydrophobic solvent is sufficiently small, such as 3 or less, it is thought that phase separation proceeds rapidly in the polymerizable monomer droplets, and voids are easily formed. Examples of hydrophobic solvents with a relative permittivity of 3 or less at 20°C are as follows. (The value in parentheses is the relative permittivity.) Pentane (1.8), hexane (1.9), heptane (1.9), octane (1.9), cyclohexane (2.0), benzene (2.3), toluene (2.4). Regarding the relative permittivity at 20°C, values ​​can be found in publicly available literature (for example, "Chemical Handbook: Basic Edition," edited by the Chemical Society of Japan, 4th revised edition, Maruzen Co., Ltd., published September 30, 1993, pp. II-498 to II-503), as well as other technical information. Methods for measuring the relative permittivity at 20°C include, for example, relative permittivity tests conducted in accordance with JIS C 2101:1999, item 23, with the measurement temperature set at 20°C.

[0035] The porosity of the hollow particles can be adjusted by changing the amount of hydrophobic solvent in the mixture. In the suspension step described later, the polymerization reaction proceeds with oil droplets containing polymerizable monomers, etc., encapsulating the hydrophobic solvent. Therefore, the higher the hydrophobic solvent content, the higher the porosity of the resulting hollow particles tends to be. In this disclosure, the content of the hydrophobic solvent in the mixture is preferably 50 parts by mass or more and 500 parts by mass or less per 100 parts by mass of polymerizable monomer, as this makes it easier to control the particle size of the hollow particles, easier to increase the porosity while maintaining the strength of the hollow particles, and easier to reduce the amount of residual hydrophobic solvent in the particles. More preferably, the content of the hydrophobic solvent in the mixture is 60 parts by mass or more and 400 parts by mass or less per 100 parts by mass of polymerizable monomer, and even more preferably 70 parts by mass or more and 300 parts by mass or less.

[0036] (C) Polymerization initiator 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 the monomer composition after suspending the mixture, there are emulsion polymerization using a water-soluble polymerization initiator and suspension polymerization using an oil-soluble polymerization initiator, and suspension polymerization can be performed by using an oil-soluble polymerization initiator. The oil-soluble polymerization initiator is not particularly limited as long as it is lipophilic and has a solubility in water at 25°C of 2 g / L or less. Examples of oil-soluble polymerization initiators include benzoyl peroxide, lauroyl peroxide, t-butyl peroxide-2-ethylhexanoate, 2,2'-azobis(2,4-dimethylvaleronitrile), azobisisobutyronitrile, and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile). The content of the oil-soluble polymerization initiator 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 per 100 parts by mass of polymerizable monomer. By keeping the content of the oil-soluble polymerization initiator within the above range, the polymerization reaction proceeds sufficiently, and there is little risk of residual oil-soluble polymerization initiator after the polymerization reaction is completed, and the risk of unexpected side reactions is also small. Furthermore, in order to reduce the metal content in the shell, it is preferable to use a polymerization initiator that does not contain metal.

[0037] (D) Dispersion stabilizer A dispersion stabilizer is an agent used in the suspension process to disperse droplets of a monomer composition in an aqueous medium. In this disclosure, it is preferable not to use a surfactant as a dispersion stabilizer, from the viewpoint of keeping the surfactant content on the surface of the hollow particles to 200 ppm or less. In this disclosure, it is preferable to use an inorganic dispersion stabilizer. By using an inorganic dispersion stabilizer, it is possible to easily control the particle size of droplets in the suspension, narrow the particle size distribution of the resulting hollow particles, and further suppress the reduction in strength of the hollow particles by preventing the shell from becoming too thin. Such effects of inorganic dispersion stabilizers are particularly likely to be exhibited when the inorganic dispersion stabilizer is used in combination with a particle size control agent described later. Examples of inorganic dispersion stabilizers include sulfates such as barium sulfate and calcium sulfate; carbonates such as barium carbonate, calcium carbonate, and magnesium carbonate; phosphates such as calcium phosphate; metal oxides such as aluminum oxide and titanium oxide; and metal hydroxides such as aluminum hydroxide, magnesium hydroxide, calcium hydroxide, barium hydroxide, and ferric hydroxide. These inorganic dispersion stabilizers can be used individually or in combination of two or more. Among the inorganic dispersion stabilizers mentioned above, poorly water-soluble metal salts such as sulfates, carbonates, phosphates, and metal hydroxides are preferred, metal hydroxides are more preferred, and magnesium hydroxide is particularly preferred. In this disclosure, the poorly water-soluble metal salt is preferably an inorganic metal salt having a solubility of 5 g / L or less in water at 25°C. In this disclosure, it is particularly preferable to use a poorly water-soluble inorganic dispersion stabilizer in the form of colloidal particles dispersed in an aqueous medium, that is, in the form of a colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles. By using a poorly water-soluble inorganic dispersion stabilizer in the form of a colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloidal particles, the particle size distribution of the monomer composition can be narrowed, and the amount of residual inorganic dispersion stabilizer in the resulting hollow particles can be easily kept low by washing. A colloidal dispersion containing poorly water-soluble inorganic dispersion stabilizer colloid particles can be prepared, for example, by reacting at least one selected from alkali metal hydroxides and alkaline earth metal hydroxides with a water-soluble polyvalent metal salt (excluding alkaline earth metal hydroxides) in an aqueous medium. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Examples of alkaline earth metal hydroxides include barium hydroxide and calcium hydroxide. The water-soluble polyvalent metal salt can be any water-soluble polyvalent metal salt other than the alkaline earth metal hydroxide compounds mentioned above. Examples include magnesium metal salts such as magnesium chloride, magnesium phosphate, and magnesium sulfate; calcium metal salts such as calcium chloride, calcium nitrate, calcium acetate, and calcium sulfate; aluminum metal salts such as aluminum chloride and aluminum sulfate; barium salts such as barium chloride, barium nitrate, and barium acetate; and zinc salts such as zinc chloride, zinc nitrate, and zinc acetate. Among these, magnesium metal salts, calcium metal salts, and aluminum metal salts are preferred, magnesium metal salts are more preferred, and magnesium chloride is particularly preferred. The water-soluble polyvalent metal salts can be used individually or in combination of two or more. The method for reacting at least one selected from the alkali metal hydroxide and alkaline earth metal hydroxide mentioned above with the water-soluble polyvalent metal salt mentioned above in an aqueous medium is not particularly limited, but one method is to mix an aqueous solution of at least one selected from the alkali metal hydroxide and alkaline earth metal hydroxide with an aqueous solution of the water-soluble polyvalent metal salt. In this case, from the viewpoint of being able to suitably control the particle size of poorly water-soluble metal hydroxide colloid particles, it is preferable to mix by gradually adding an aqueous solution of at least one selected from the alkali metal hydroxide and alkaline earth metal hydroxide to the aqueous solution while stirring the aqueous solution of the water-soluble polyvalent metal salt. Furthermore, from the viewpoint of obtaining hollow particles with a volume-average particle size of 1 μm or more and 10 μm or less, it is preferable to use a colloidal dispersion obtained by reacting at least one selected from alkali metal hydroxides and alkaline earth metal hydroxides with a water-soluble polyvalent metal salt in an aqueous medium at a temperature of 20°C or more and 50°C or less.

[0038] The content of the dispersion stabilizer is not particularly limited, but is preferably 0.5 to 10 parts by mass, and more preferably 1.0 to 8.0 parts by mass, per 100 parts by mass of the total mass of the polymerizable monomer and hydrophobic solvent. By having a dispersion stabilizer content above the lower limit, the monomer composition droplets can be sufficiently dispersed so that they do not coalesce in the suspension. On the other hand, by having a dispersion stabilizer content below the upper limit, it is possible to prevent the viscosity of the suspension from increasing during granulation and to avoid the problem of the suspension clogging the granulator. Furthermore, the content of the dispersion stabilizer is usually between 2 and 15 parts by mass, and preferably between 3 and 8 parts by mass, per 100 parts by mass of the aqueous medium. From the viewpoint of achieving a volume-average particle size of 1 to 10 μm for the hollow particles, it is preferable to keep the content of the dispersion stabilizer within the above range.

[0039] (E)Aqueous medium In this disclosure, "aqueous medium" means a medium selected from the group consisting of water, hydrophilic solvents, and mixtures of water and hydrophilic solvents. The hydrophilic solvent in this disclosure is not particularly limited as long as it mixes well with water and does not undergo phase separation. Examples of hydrophilic solvents include alcohols such as methanol and ethanol; tetrahydrofuran (THF); and dimethyl sulfoxide (DMSO). Among aqueous media, water is preferred due to its high polarity. Furthermore, ion-exchanged water is preferred as it reduces the metal content contained in the shell. When using a mixture of water and a hydrophilic solvent, it is important that the overall polarity of the mixture does not become 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) may be 99:1 to 50:50.

[0040] (F) Other materials The mixture may further contain other materials different from those described in (A) to (E) above, as long as it does not impair the effects of the present disclosure. The mixture preferably contains a particle size control agent as another material. By including a particle size control agent in the mixture, the particle size of the monomer composition droplets and the shell thickness of the resulting hollow particles can be appropriately adjusted. As particle size control agents, for example, at least one selected from the group consisting of polar resins (described later), rosinic acid, higher fatty acids, and metal salts thereof can be used. These particle size control agents can appropriately adjust the particle size of droplets of the monomer composition containing a polymerizable monomer and a hydrophobic solvent in the suspension step described later. In the suspension step, droplets of the monomer composition are formed in an aqueous medium by the action of the dispersion stabilizer. In these droplets of the monomer composition, the materials other than the hydrophobic solvent containing the polymerizable monomer and the hydrophobic solvent undergo phase separation, with the hydrophobic solvent being concentrated in the center and the materials other than the hydrophobic solvent being concentrated on the surface. If the mixture contains a particle size control agent, it is presumed that the particle size control agent is concentrated near the surface of the droplets of the monomer composition, and the dispersion stabilizer adheres to the surface of the droplets. Such a distribution structure of materials is formed according to the differences in the affinity of each material to the aqueous medium. Because the mixture contains a particle size control agent, the droplets of the monomer composition in the suspension adopt the material distribution structure described above, and interactions occur between the dispersion stabilizer and the particle size control agent on the droplet surface. As a result, the dispersibility of the droplets by the dispersion stabilizer changes, and it is thought that the particle size of the monomer composition droplets can be appropriately adjusted.

[0041] In this disclosure, polar resins refer to polymers containing repeating units that include heteroatoms. Specifically, examples include acrylic resins, polyester resins, and vinyl resins containing heteroatoms. Polar resins typically have a solubility in water of less than 1 g / L. In this disclosure, polar resins are distinguished from surfactants in that they are insoluble in water.

[0042] The polar resin may be a homopolymer or copolymer of heteroatom-containing monomers, or a copolymer of heteroatom-containing monomers and heteroatom-non-containing monomers. When the polar resin is a copolymer of heteroatom-containing monomers and heteroatom-non-containing monomers, the proportion of heteroatom-containing monomer units in 100% by mass of all repeating units constituting the copolymer is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more, in order to easily control the particle size of the hollow particles. Examples of heteroatom-containing monomers used in polar resins include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, acrylic acid, methacrylic acid, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, glycidyl (meth)acrylate, and 4-hydroxybutyl Examples include monomers having a (meth)acryloyl group, such as acrylate glycidyl ethers; (meth)acrylic monovinyl monomers; aromatic vinyl monomers containing heteroatoms, such as styrene halogens and styrene sulfonic acid; vinyl carboxylate monomers such as vinyl acetate; vinyl halogenated monomers such as vinyl chloride; vinylidene halogenated monomers such as vinylidene chloride; vinylpyridine monomers; carboxyl group-containing monomers such as ethylenically unsaturated carboxylic acid monomers such as crotonic acid, cinnamic acid, itaconic acid, fumaric acid, maleic acid, and butentricarboxylic acid; and epoxy group-containing monomers such as allyl glycidyl ether. These heteroatom-containing monomers can be used individually or in combination of two or more. Examples of heteroatom-free monomers used in polar resins include heteroatom-free aromatic vinyl monomers such as styrene, vinyltoluene, α-methylstyrene, and p-methylstyrene; monoolefin monomers such as ethylene, propylene, and butylene; and diene monomers such as butadiene and isoprene. These heteroatom-free monomers can be used individually or in combination of two or more.

[0043] In particular, the polar resin is preferably an acrylic resin in which the total mass of (meth)acrylic monovinyl monomer units is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more, out of 100% by mass of all repeating units constituting the resin, due to its high compatibility with the polymerizable monomer and ease of controlling the particle size of hollow particles. In particular, it is preferable that the acrylic resin is composed of (meth)acrylic monovinyl monomer units as the total repeating units constituting the resin.

[0044] In particular, the polar resin is preferable if the heteroatom-containing monomer contains polar group-containing monomer units that include polar groups selected from carboxyl groups, hydroxyl groups, sulfonic acid groups, amino groups, polyoxyethylene groups, and epoxy groups, as this allows for easier control of the particle size of hollow particles. Examples of polar group-containing monomers used in the polar resin include those similar to the polar group-containing non-crosslinkable monomers described later. The polar group-containing monomers can be used individually or in combination of two or more. Among the polar groups contained in the polar group-containing monomer units of the polar resin, carboxyl groups and hydroxyl groups are preferred because they allow for particle size control with small amounts of additive. When the polar resin contains monomer units containing polar groups, it is preferable that the polar groups are located at the ends of the main chain or side chains, or are attached to the main chain or side chains in a pendant-like manner, as this makes it easier for the polar resin to be arranged on the outer surface of the hollow particles and easier to control the particle size of the hollow particles.

[0045] When the polar resin does not contain the polar group-containing monomer units, the heteroatom-containing monomer units contained in the polar resin preferably include monomer units derived from alkyl (meth)acrylates, due to their high compatibility with the polymerizable monomers and ease of controlling the particle size of hollow particles. In particular, due to their high polarity, monomer units derived from alkyl (meth)acrylates, where the alkyl group has 3 or fewer carbon atoms, more preferably the alkyl group is a methyl or ethyl group, and even more preferably the alkyl group is a methyl group, are preferred.

[0046] Among the acrylic resins that are polar resins, it is preferable that the polymer or copolymer of the polymerizable monomer for polar resins contains 50.0% by mass or more of methyl methacrylate when the total mass of the polymerizable monomer for polar resins is 100% by mass, due to its high compatibility with the polymerizable monomer and ease of controlling the particle size of the hollow particles. In this disclosure, the polymerizable monomer used in the synthesis of polar resins is referred to as the polymerizable monomer for polar resins. The acrylic resin, which is the polar resin, is more preferably a copolymer of polymerizable monomers for polar resins containing 50.0% to 99.9% by mass of methyl methacrylate and 0.1% to 5.0% by mass of the polar group-containing monomer, from the viewpoint of being able to more easily control the particle size of hollow particles, even more preferably a copolymer of polymerizable monomers for polar resins containing 50.0% to 99.0% by mass of methyl methacrylate and 0.1% to 5.0% by mass of the polar group-containing monomer, and even more preferably a copolymer of polymerizable monomers for polar resins containing 50.0% to 98.0% by mass of methyl methacrylate and methyl A copolymer of polymerizable monomers for polar resins comprising 1.0% to 5.0% by mass of a (meth)acrylic monovinyl monomer that is different from methyl methacrylate and does not contain the aforementioned polar group, and 0.1% to 5.0% by mass of the aforementioned polar group-containing monomer, and particularly preferably a copolymer of polymerizable monomers for polar resins comprising 50.0% to 98.0% by mass of methyl methacrylate, 1.0% to 5.0% by mass of a (meth)acrylic monovinyl monomer that is different from methyl methacrylate and does not contain the aforementioned polar group, and 0.2% to 3.0% by mass of the aforementioned polar group-containing monomer. Unlike methyl methacrylate and lacking the aforementioned polar group, the (meth)acrylic monovinyl monomer is preferably at least one selected from ethyl acrylate and butyl acrylate, with ethyl acrylate being particularly preferred, in order to allow control of the glass transition temperature. As the polar group-containing monomer, a (meth)acrylic monovinyl monomer containing the polar group is preferred from the viewpoint of compatibility with polymerizable monomers in the mixture, and a (meth)acrylic monovinyl monomer containing a carboxyl group or a hydroxyl group is even more preferred because particle size can be controlled with a smaller amount of additive.

[0047] The polar resin can be obtained, for example, by polymerizing a polymerizable monomer for polar resins containing the heteroatom-containing monomer using polymerization methods such as solution polymerization or emulsion polymerization. Furthermore, if the polar resin is a copolymer, the copolymer may be a random copolymer, a block copolymer, or a graft copolymer, but a random copolymer is preferred. Furthermore, it is preferable that the polar resin is finely ground in order to improve its solubility.

[0048] The number-average molecular weight (Mn) of the polar resin is not particularly limited, but is preferably in the range of 3,000 to 20,000, more preferably in the range of 4,000 to 17,000, and even more preferably in the range of 6,000 to 15,000, as measured by gel permeation chromatography (GPC) using tetrahydrofuran, in polystyrene equivalent. When the number-average molecular weight (Mn) of the polar resin is above the lower limit, the solubility of the polar resin is improved and the particle size of the hollow particles can be easily controlled, and when it is below the upper limit, a decrease in the strength of the shell can be suppressed.

[0049] When a polar resin is used as a particle size control agent, the content of the polar resin is preferably 0.1 parts by mass or more, more preferably 0.3 parts by mass or more, even more preferably 0.4 parts by mass or more, and even more preferably 0.5 parts by mass or more, per 100 parts by mass of polymerizable monomer in the mixture, while preferably 10.0 parts by mass or less, more preferably 8.0 parts by mass or less, even more preferably 5.0 parts by mass or less, and even more preferably 2.0 parts by mass or less. If the content of the polar resin is above the lower limit, it is easier to control the particle size of the hollow particles and the thickness of the shell, and if the content of the polar resin is below the upper limit, a decrease in the content ratio of polymerizable monomer can be suppressed, and thus a decrease in the strength of the shell can be suppressed.

[0050] Rosin acid can be obtained from rosins such as gum rosin, tall rosin, and wood rosin. Examples of the components contained in rosin acids obtained from these rosins include abietic acid, dehydroabietic acid, palustric acid, isopimaric acid, pimaric acid, and the like. The component ratio of rosin acid is not constant and varies depending on the type of rosin, the pine species of the raw material, the production area, and the like. As the rosin acid and its metal salts, rosin acids containing 50% by mass or more of abietic acids such as abietic acid, dehydroabietic acid, palustric acid and their hydrides, and their alkali metal salts are preferable.

[0051] The higher fatty acid is preferably a higher fatty acid having 10 to 25 carbon atoms excluding the carbon atom in the carboxyl group. Preferred higher fatty acids include, for example, lauric acid (CH3(CH2) 10 COOH), tridecanoic acid (CH3(CH2) 11 COOH), myristic acid (CH3(CH2) 12 COOH), pentadecanoic acid (CH3(CH2) 13 COOH), palmitic acid (CH3(CH2) 14 COOH), heptadecanoic acid (CH3(CH2) 15 COOH), stearic acid (CH3(CH2) 16 COOH), arachidic acid (CH3(CH2) 18 COOH), behenic acid (CH3(CH2) 20 COOH), and lignoceric acid (CH3(CH2) 22 COOH), and the like.

[0052] Examples of the metal used for the metal salt of rosin acid or higher fatty acid include alkali metals such as Li, Na, K, and alkaline earth metals such as Mg, Ca, etc. Among them, alkali metals are preferable, and at least one selected from Li, Na, and K is more preferable.

[0053] When at least one selected from the group consisting of rosin acid, higher fatty acid, and their metal salts is used as the particle size control agent, it is adjusted so that the total content of rosin acid, higher fatty acid, and their metal salts on the surface of the hollow particles is 200 ppm or less. The total content of rosinic acid, higher fatty acids, and their metal salts 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, based on 100 parts by mass of the total of polymerizable monomers and hydrophobic solvent in the mixture. When the above content is above the above lower limit, it is easier to control the particle size of the hollow particles and the shell thickness.

[0054] A mixture is obtained by mixing the aforementioned materials and other materials as needed, and stirring as appropriate. In this mixture, the oil phase containing (A) polymerizable monomers, (B) hydrophobic solvents, and (C) lipophilic materials such as polymerization initiators is dispersed in the aqueous phase containing (D) dispersion stabilizers and (E) aqueous media, with particle sizes of several millimeters. Depending on the type of material, the dispersion state of these materials in the mixture can be observed with the naked eye. In the mixture preparation step, the mixture may be obtained by simply mixing the aforementioned materials and other materials as needed, and stirring as appropriate. However, in order to ensure a homogeneous shell, it is preferable to prepare the mixture by separately preparing an oil phase containing a polymerizable monomer, a hydrophobic solvent, and a polymerization initiator, and an aqueous phase containing a dispersion stabilizer and an aqueous medium, and then mixing these together. In this disclosure, a colloidal dispersion in which a poorly water-soluble inorganic dispersion stabilizer is dispersed in an aqueous medium in the form of colloidal particles can be preferably used as the aqueous phase. By preparing the oil phase and aqueous phase separately in advance and then mixing them, it is possible to produce hollow particles with a uniform shell composition, and it also becomes easier to control the particle size of the hollow particles.

[0055] Furthermore, in the method for producing hollow particles according to this disclosure, it is preferable that the amount of surfactant in the total solid content of the mixed liquid is 200 ppm or less. Examples of surfactants include anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants, and any known surfactant may be used. Examples of anionic surfactants include carboxylates such as alkali metal salts of higher fatty acids; sulfates such as higher alcohol sulfates and higher alkyl ether sulfates; sulfonates such as alkylbenzene sulfonates, alkyl sulfonates, and paraffin sulfonates; and phosphates such as higher alcohol phosphates. Examples of nonionic surfactants include polyethylene glycol-type nonionic surfactants such as higher alcohol ethylene oxide adducts, fatty acid ethylene oxide adducts, higher alkylamine ethylene oxide adducts, and polypropylene glycol ethylene oxide adducts; and polyhydric alcohol-type nonionic surfactants such as fatty acid esters of polyethylene oxide and glycerin, fatty acid esters of pentaerythritol, fatty acid esters of sorbitol or sorbitan, alkyl ethers of polyhydric alcohols, and aliphatic amides of alkanolamines. Examples of cationic surfactants include quaternary ammonium salts such as alkyltrimethylammonium salts. Examples of amphoteric surfactants include amino acid-type amphoteric surfactants such as higher alkylaminopropionates, and betaine-type amphoteric surfactants such as higher alkyldimethyl betaine and higher alkyldihydroxyethyl betaine. Furthermore, in this disclosure, polymer compounds having both hydrophilic and hydrophobic groups, such as polyvinyl alcohol, methylcellulose, ethylcellulose, polyacrylic acid, polyacrylimide, polyethylene oxide, and poly(hydrooxystearate-g-methyl methacrylate-co-methacrylic acid) copolymer, are also included as surfactants. Furthermore, although not specifically limited, the molecular weight of surfactants is usually less than 3000.

[0056] (2) Suspension process The suspension step 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-mentioned mixture. The suspension method for forming droplets of monomer compositions is not particularly limited, but can be carried out using equipment capable of strong stirring, such as in-line emulsifiers (horizontal in-line dispersers such as Taiheiyo Kiko Co., Ltd., product name: Milder, and Eurotech Co., Ltd., product name: Cavitron; vertical in-line dispersers such as IKA, product name: DRS 2000 / 5, etc.) or high-speed emulsifiers (product name: TK Homomixer MARK II, etc., manufactured by Primix Co., Ltd.). In the suspension prepared in the suspension process, droplets of a monomer composition containing the above-mentioned lipophilic material and having a particle size of approximately 1 to 10 μm are uniformly dispersed in an aqueous medium. Such droplets of monomer composition are difficult to observe with the naked eye and can be observed using known observation equipment such as an optical microscope. During the suspension process, phase separation occurs within the monomer composition droplets, causing the less polar hydrophobic solvent to accumulate inside the droplets. As a result, the resulting droplets will have the hydrophobic solvent distributed inside, while materials other than the hydrophobic solvent, such as polymerizable monomers, will be distributed around their periphery.

[0057] Figure 2 is a schematic diagram showing one embodiment of a suspension in the suspension process. The droplet 10 of the monomer composition in Figure 2 is schematically shown as its cross-section. Note that Figure 2 is merely a schematic diagram, and the suspension in this disclosure is not necessarily limited to that shown in Figure 2. Part of Figure 2 corresponds to Figure 1(2) described above. Figure 2 shows how droplets 10 of the monomer composition and polymerizable monomers 4c dispersed in the aqueous medium 1 are dispersed in the aqueous medium 1. The droplets 10 are formed by surrounding an oil-soluble monomer composition 4 with a dispersion stabilizer (not shown). The monomer composition 4 contains an oil-soluble polymerization initiator 5, as well as a polymerizable monomer and a hydrophobic solvent (neither of which are shown). The droplet 10 is a micro-oil droplet containing the monomer composition 4, and the oil-soluble polymerization initiator 5 generates polymerization initiation radicals within the micro-oil droplet. Therefore, precursor particles of the desired particle size can be produced without over-growing the micro-oil droplets. In suspension polymerization using such an oil-soluble polymerization initiator, there is no opportunity for the polymerization initiator to come into contact with the polymerizable monomer 4c dispersed in the aqueous medium 1. Therefore, by using an oil-soluble polymerization initiator, it is possible to suppress the formation of extra resin particles, such as relatively small, dense particles, in addition to the desired hollow resin particles.

[0058] (3) Polymerization process This process involves subjecting the suspension obtained by the suspension process described above to a polymerization reaction to prepare a precursor composition containing precursor particles having a hollow portion surrounded by a resin-containing shell, and containing a hydrophobic solvent within the hollow portion. The precursor particles are formed by the polymerization of polymerizable monomers contained in droplets of the monomer composition, and the shell of the precursor particles contains the polymer of the polymerizable monomer as a resin. There are no particular limitations on the polymerization method; for example, batch, semi-continuous, and continuous methods can be used. The polymerization temperature is preferably 40 to 80°C, and more preferably 50 to 70°C. The heating rate when raising the temperature to the polymerization temperature is preferably 10 to 60°C / h, more preferably 15 to 55°C / h. The polymerization reaction time is preferably 1 to 20 hours, and more preferably 2 to 15 hours. During the polymerization process, the shell portion of the monomer composition droplet containing the hydrophobic solvent polymerizes, and as described above, a hollow portion filled with the hydrophobic solvent is formed inside the resulting precursor particles.

[0059] In this process, after performing a first polymerization reaction in which the suspension is subjected to a polymerization reaction, a polymerizable monomer may be added to the precursor composition obtained by the first polymerization reaction to perform a second polymerization reaction. By performing the polymerization reaction in two stages in this way, the solvent resistance of the hollow particles can be improved.

[0060] The first polymerization reaction described above is preferably carried out until the polymerization conversion rate of the polymerizable monomer in the suspension 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. In this disclosure, the polymerization conversion rate is determined by formula (B) below, using the mass of the solid content of the precursor particles in the precursor composition obtained by the first polymerization reaction and the mass of the polymerizable monomer remaining unreacted after the first polymerization reaction. The mass of the unreacted polymerizable monomer can be measured using gas chromatography (GC). Polymerization conversion rate (mass%) = 100 - (mass of unreacted polymerizable monomers / mass of solid content of precursor particles) × 100 Equation (B) The reaction time for the first polymerization reaction described above is preferably 0.5 to 5 hours, and more preferably 1 to 3 hours.

[0061] The polymerizable monomer added during the second polymerization reaction described above is not particularly limited, but from the viewpoint of improving the solvent resistance and strength of the hollow particles, polymerizable monomers having a solubility of 0.3 g / L or more in distilled water at 20°C are preferred, and non-crosslinked monomers having a solubility of 0.3 g / L or more in distilled water at 20°C are more preferred. Examples of preferred polymerizable monomers added during the second polymerization reaction described above include alkyl (meth)acrylates having alkyl groups with 1 to 5 carbon atoms, (meth)acrylamides and their derivatives, nitrile (meth)acrylates, and non-crosslinked monomers containing polar groups. Among these, at least one selected from the group consisting of alkyl (meth)acrylates having alkyl groups with 1 to 5 carbon atoms and nitrile (meth)acrylates is preferred, and at least one selected from the group consisting of methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, and nitrile acrylates is more preferred. Examples of non-crosslinkable monomers containing polar groups include, for example, non-crosslinkable monomers containing polar groups selected from carboxyl groups, hydroxyl groups, sulfonic acid groups, amino groups, polyoxyethylene groups, and epoxy groups. More specifically, examples include carboxyl group-containing monomers such as ethylenically unsaturated carboxylic acid monomers like (meth)acrylic acid, crotonic acid, cinnamic acid, itaconic acid, fumaric acid, maleic acid, and butentricarboxylic acid; hydroxyl group-containing monomers such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; sulfonic acid group-containing monomers such as styrene sulfonic acid; amino group-containing monomers such as dimethylaminoethyl (meth)acrylate and diethylaminoethyl (meth)acrylate; polyoxyethylene group-containing monomers such as methoxypolyethylene glycol (meth)acrylate; and epoxy group-containing monomers such as glycidyl (meth)acrylate, allyl glycidyl ether, and 4-hydroxybutyl acrylate glycidyl ether.

[0062] Furthermore, the molecular weight of the polymerizable monomer added during the second polymerization reaction described above is not particularly limited, but is preferably 200 or less, more preferably 100 or less, from the viewpoint of improving the solvent resistance and strength of the hollow particles. The lower limit of the above molecular weight is not particularly limited, but is usually 50 or more.

[0063] The amount of polymerizable monomer added during the second polymerization reaction described above is preferably 3 to 15 parts by mass, and more preferably 4 to 10 parts by mass, per 100 parts by mass of polymerizable monomer in the mixture, in order to improve the solvent resistance and strength of the hollow particles. The reaction time for the second polymerization reaction described above is preferably 1 to 6 hours, and more preferably 2 to 4 hours.

[0064] By carrying out the two-stage polymerization reaction described above in the polymerization process, the amount of unreacted polymerizable monomers remaining in the resulting hollow particles can be preferably 750 ppm or less, more preferably 500 ppm or less, and even more preferably 300 ppm or less. In this disclosure, the amount of unreacted polymerizable monomers remaining is the ratio of the mass of unreacted polymerizable monomers to the mass of solids of the hollow particles. The mass of unreacted polymerizable monomers can be measured using gas chromatography (GC).

[0065] (4) Washing and solid-liquid separation process This step involves washing the precursor composition containing precursor particles, which is obtained by the polymerization step described above, to remove any remaining dispersion stabilizers, and then separating the precursor composition into solid and liquid components to obtain a solid component containing precursor particles.

[0066] Washing to remove residual dispersion stabilizers from the precursor composition can be performed, for example, by adding an acid or alkali to the precursor composition. If the dispersion stabilizer used is an inorganic compound soluble in acid, it is preferable to wash the precursor composition containing the precursor particles by adding an acid. On the other hand, if the dispersion stabilizer used is an inorganic compound soluble in alkali, it is preferable to wash the precursor composition containing the precursor particles by adding an alkali. Furthermore, when an acid-soluble inorganic compound is used as a dispersion stabilizer, it is preferable to add an acid to the precursor composition containing the precursor particles to adjust the pH to preferably 6.5 or lower, more preferably 6 or lower. As the added acid, inorganic acids such as sulfuric acid, hydrochloric acid, and nitric acid, and organic acids such as formic acid and acetic acid can be used, but sulfuric acid is particularly preferred because it has a high efficiency in removing the dispersion stabilizer and places little burden on the manufacturing equipment. By performing the above cleaning, the amount of metal contained in the shell can be reduced.

[0067] The method for separating the precursor composition into solid and liquid is not particularly limited, and known methods can be used. Examples of solid-liquid separation methods include centrifugation, filtration, and static separation. Among these, centrifugation or filtration can be used, and centrifugation may be used from the viewpoint of ease of operation. After the solid-liquid separation process, any optional steps such as a pre-drying process may be performed before carrying out the solvent removal process described later. An example of a pre-drying process is to pre-dry the solid obtained after the solid-liquid separation process using a drying device such as a dryer or a drying apparatus such as a hand dryer.

[0068] (5) Solvent removal process This step involves removing the hydrophobic solvent contained within the precursor particles obtained in the solid-liquid separation step. By removing the hydrophobic solvent encapsulated within the precursor particles in the air, the hydrophobic solvent inside the precursor particles is replaced with air, resulting in hollow particles filled with gas.

[0069] In this process, "in the air" strictly refers to an environment where there is absolutely no liquid outside the precursor particles, or an environment where there is only a very small amount of liquid outside the precursor particles that does not affect the removal of the hydrophobic solvent. "In the air" can also be rephrased as a state in which the precursor particles are not in a slurry, or a state in which the precursor particles are in a dry powder. In other words, in this process, it is important to remove the hydrophobic solvent in an environment in which the precursor particles are in direct contact with the external gas.

[0070] The method for removing the hydrophobic solvent from the precursor particles in air is not particularly limited, and known methods can be employed. Examples of such methods include vacuum drying, heat drying, airflow drying, or a combination of these methods. In particular, when using the heat drying method, the heating temperature must be above the boiling point of the hydrophobic solvent and below the maximum temperature at which the shell structure of the precursor particles does not collapse. Therefore, depending on the shell composition in the precursor particles and the type of hydrophobic solvent, the heating temperature may be, for example, 50-200°C, 70-200°C, or 100-200°C. Through a drying process in air, the hydrophobic solvent inside the precursor particles is replaced by the external gas, resulting in hollow particles in which the hollow portion is filled with gas.

[0071] The dry atmosphere is not particularly limited and can be appropriately selected depending on the application of the hollow particles. Examples of dry atmospheres include air, oxygen, nitrogen, and argon. Furthermore, the hydrophobic solvent encapsulated in the precursor particles can be removed, for example, by replacing the hydrophobic solvent encapsulated in the precursor particles with the aqueous medium of the slurry in a slurry containing the precursor particles and an aqueous medium, without performing solid-liquid separation of the precursor composition. However, the method of removing the hydrophobic solvent encapsulated in the precursor particles in the air is preferred because it can reduce the metal content contained in the shell.

[0072] (6) Others In addition to the above steps (1) to (5), the process may further include, for example, a step of replacing the hollow portion. The hollow portion re-substitution process is a process of replacing the gas or liquid inside a hollow particle with another gas or liquid. Such substitution can change the environment inside the hollow particle, selectively confine molecules inside the hollow particle, or modify the chemical structure inside the hollow particle to suit the application.

[0073] 2.Hollow particles The hollow particles of this disclosure are hollow particles comprising a resin-containing shell and a hollow portion surrounded by the shell, The shell contains a polymer as the resin, in which 70 to 100 parts by mass of crosslinkable monomer units are present in 100 parts by mass of total monomer units. The porosity is 60% or more. The surfactant content present on the surface of the hollow particles is 200 ppm or less. It is characterized by having a relative permittivity of 1.6 or less at a frequency of 1 MHz.

[0074] The hollow particles of the present disclosure have a relative permittivity of 1.6 or less at a frequency of 1 MHz, and preferably 1.5 or less from the viewpoint of lowering the dielectric constant. The lower limit of the relative permittivity of the hollow particles of the present disclosure is not particularly limited, but is usually 1.0 or higher. In this disclosure, the relative permittivity of the hollow particle is measured using a perturbation-type measuring device under the condition of a measurement frequency of 1 MHz.

[0075] The hollow particles of this disclosure have a surfactant content of 200 ppm or less on the surface of the hollow particles, preferably 100 ppm or less, and more preferably 50 ppm or less. When a surfactant is used in the manufacturing process of the hollow particles, for example, when a surfactant is included in the mixed solution, the surfactant may remain on the surface of the resulting hollow particles. However, by not using a surfactant as a dispersion stabilizer in the manufacturing process of the hollow particles, the surfactant content on the surface of the hollow particles can be reduced to 200 ppm or less. In this disclosure, the surfactant content on the hollow particle surface refers to the ratio of the mass of the surfactant present on the hollow particle surface to the mass of the hollow particle. The surfactant present on the hollow particle surface can be extracted, for example, by ultrasonically treating the hollow particle in water. The type and mass of the surfactant extracted into the water are as follows: 1 It can be identified from the peak position and peak intensity of the 1H-NMR spectrum.

[0076] Furthermore, the hollow particles of this disclosure preferably have a metal content of 100 ppm or less, more preferably 80 ppm or less, and even more preferably 70 ppm or less. Here, the metal includes metal ions. By reducing the metal content, the performance stability of the hollow particles in a high-humidity environment can be improved. In addition, if both the surfactant content and the metal content of the hollow particles of this disclosure are reduced to or below the above-mentioned upper limits, the performance stability can be improved by exceeding the sum of the effects of reducing the surfactant amount and the effects of reducing the metal content. In this disclosure, the metal content in hollow particles refers to the ratio of the total mass of the metal components contained in the hollow particles to the mass of the hollow particles. The metal content in hollow particles can be measured by ICP emission spectrometry. The type of metal can be identified by X-ray fluorescence analysis (XRF).

[0077] The hollow particles of this disclosure contain a polymer in the shell as a resin, with 70 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of total monomer units. As a result, the hollow particles of this disclosure have excellent strength, are resistant to crushing, are resistant to deformation from heat applied from the outside, and the increase in dielectric constant due to crushing or deformation of the hollow particles is suppressed. The above polymer forms the shell skeleton of hollow particles. In the above polymer, if the content of crosslinkable monomer units is less than 100 parts by mass, monomer units other than crosslinkable monomer units are non-crosslinkable monomer units. In the hollow particles of this disclosure obtained by the method for producing hollow particles described above, the polymer contained in the shell is obtained by polymerization of the polymerizable monomers described above, and the crosslinkable monomer units or non-crosslinkable monomer units contained in the polymer originate from the polymerizable monomers described above. Therefore, the content of each monomer unit in the polymer can be calculated from the amount of each polymerizable monomer subjected to the polymerization reaction. The above polymer may consist entirely of crosslinkable monomer units, but if it contains a combination of crosslinkable and non-crosslinkable monomer units, the content of crosslinkable monomer units in 100 parts by mass of total monomer units is preferably 80 parts by mass or more, more preferably 90 parts by mass or more, and preferably 99 parts by mass or less, more preferably 97 parts by mass or less, as a lower limit.

[0078] The above polymer preferably contains at least two-functional crosslinkable monomer units as crosslinkable monomer units, and more preferably contains a combination of two-functional crosslinkable monomer units and three-functional or more crosslinkable monomer units in order to further improve the strength of the hollow particles. The content of bifunctional crosslinkable monomer units in 100 parts by mass of the total monomer units of the above polymer is not particularly limited, but the lower limit is preferably 50 parts by mass or more, more preferably 60 parts by mass or more, and the upper limit is preferably 90 parts by mass or less, more preferably 80 parts by mass or less. When the above polymer contains three or more crosslinkable monomer units, the content of three or more crosslinkable monomer units in 100 parts by mass of the total monomer units of the above polymer is not particularly limited, but the lower limit is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 20 parts by mass or more, and the upper limit is preferably 40 parts by mass or less, and more preferably 30 parts by mass or less.

[0079] In the above polymer, the content of non-crosslinkable monomer units in 100 parts by mass of total monomer units is 30 parts by mass or less. The upper limit is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, in order to improve the strength of the hollow particles and suppress the increase in dielectric constant due to crushing or deformation of the hollow particles. The lower limit is preferably 1 part by mass or more, more preferably 3 parts by mass or more, in order to improve solvent resistance.

[0080] In the hollow particles of this disclosure, the content of the polymer is preferably 90% by mass or more, more preferably 95% by mass or more, of 100% by mass of the total solids content of the shell. By setting the content of the polymer to above the lower limit, the strength of the hollow particles can be improved, and the increase in dielectric constant due to crushing or deformation of the hollow particles can be suppressed.

[0081] The shell of the hollow particles of this disclosure may further contain, as a particle size control agent, the polar resin described above, or at least one selected from the group consisting of rosinic acid, higher fatty acids, and metal salts thereof. When the shell of the hollow particles of the present disclosure contains the polar resin as a particle size control agent, the content of the polar resin in 100% by mass of the total solids of the shell is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, even more preferably 0.4% by mass or more, and even more preferably 0.5% by mass or more, while preferably 10.0% by mass or less, more preferably 8.0% by mass or less, even more preferably 5.0% by mass or even more preferably 2.0% by mass or less. When the shell of the hollow particles of the present disclosure contains at least one selected from the group consisting of rosinic acid, higher fatty acids, and metal salts thereof as a particle size control agent, the total content of rosinic acid, higher fatty acids, and metal salts thereof in 100% by mass of the total solid content of the shell is preferably 0.0001 to 0.02% by mass, more preferably 0.0010 to 0.01% by mass, and even more preferably 0.0015 to 0.006% by mass. Furthermore, the presence of a particle size control agent within the shell of the hollow particles, and its content, can be confirmed, for example, by pyrolysis gas chromatography.

[0082] The hollow particles of this disclosure preferably have a lower limit of volume-average particle size of 1 μm or more, more preferably 1.5 μm or more, and even more preferably 2 μm or more. On the other hand, the upper limit of volume-average particle size of the hollow particles is preferably 10 μm or less, more preferably 8 μm or less, and even more preferably 6 μm or less. When the volume-average particle size of the hollow particles is above the lower limit, the aggregation of the hollow particles is reduced, and excellent dispersibility can be achieved. When the volume-average particle size of the hollow particles is below the upper limit, variations in shell thickness are suppressed, a uniform shell is easily formed, and the hollow particles have high mechanical strength and are less likely to be crushed, thus suppressing the increase in dielectric constant due to crushing or deformation of the hollow particles. Furthermore, hollow particles with a volume-average particle size within the above range do not cause wiring problems even when incorporated into the insulating resin layer of an electronic circuit board, and are therefore suitably used as a material for electronic circuit boards. In order to keep the volume-average particle size of the hollow particles within the preferred range described above, it is preferable, for example, to use the preferred combination of dispersion stabilizer and particle size control agent described above, and further to use the preferred hydrophobic solvent described above, in the mixture preparation step.

[0083] The shape of the hollow particles in this disclosure is not particularly limited as long as a hollow portion is formed inside, and examples include spherical, ellipsoidal, and amorphous shapes. Among these, a spherical shape is preferred due to its ease of manufacture. The hollow particles of this disclosure may have one or more hollow portions, but it is preferable that they have only one hollow portion in order to maintain a good balance between high porosity and mechanical strength, and to reduce dielectric constant. The hollow particles of this disclosure may have an average circularity of 0.950 to 0.995. An example of the shape of a hollow particle in this disclosure is a bag consisting of a thin film and inflated with gas, the cross-section of which is shown as hollow particle 100 in Figure 1(5). In this example, a thin film is provided on the outside, and the inside is filled with gas. The particle shape can be confirmed, for example, by SEM or TEM. Furthermore, the internal shape of the particle can be confirmed by SEM or TEM after the particle has been sliced ​​crosswise using a known method.

[0084] The particle size distribution (volume-average particle size (Dv) / number-average particle size (Dn)) of the hollow particles may be, for example, between 1.05 and 2.5. A particle size distribution of 2.5 or less allows for particles with less variation in compressive strength and heat resistance between particles. Furthermore, a particle size distribution of 2.5 or less enables the production of products with uniform thickness, for example, when manufacturing sheet-like molded articles. The volume-average particle size (Dv) and number-average particle size (Dn) of hollow particles can be determined, for example, by measuring the particle size of the hollow particles using a particle size distribution analyzer, calculating the number average and volume average, respectively, and using the obtained values ​​as the number-average particle size (Dn) and volume-average particle size (Dv) of those particles. The particle size distribution is defined as the volume-average particle size divided by the number-average particle size.

[0085] The hollow particles of this disclosure have a porosity of 60% or more, preferably 65% ​​or more. By having a porosity above the above lower limit, the relative permittivity of the hollow particles is sufficiently reduced, and the particles also have excellent lightness, heat resistance, and heat insulation properties. The upper limit of the porosity of the hollow particles of this disclosure is not particularly limited, but from the viewpoint of suppressing a decrease in the strength of the hollow particles, it is preferably 90% or less, more preferably 85% or less.

[0086] The porosity of the hollow particles in this disclosure can be calculated from the apparent density D1 and true density D0 of the hollow particles. The method for measuring the apparent density D1 of hollow particles is as follows. First, a volume of 100 cm³ is used. 3 Approximately 30 cm in a volumetric flask 3 Fill the volumetric flask with hollow particles and accurately weigh the mass of the filled hollow particles. Next, carefully fill the volumetric flask filled with hollow particles to the mark with isopropanol, taking care not to introduce air bubbles. Accurately weigh the mass of isopropanol added to the volumetric flask and calculate the apparent density D1 (g / cm³) of the hollow particles based on the following formula (I). 3 Calculate ). Equation (I) Apparent density D1 = [Mass of hollow particles] / (100 - [Mass of isopropanol] ÷ [Specific gravity of isopropanol at measurement temperature]) The apparent density D1 corresponds to the specific gravity of the entire hollow particle, assuming that the hollow portion is considered part of the hollow particle.

[0087] The method for measuring the true density D0 of hollow particles is as follows: After pre-pulverizing the hollow particles, a volume of 100 cm³ is used. 3 Fill a volumetric flask with approximately 10 g of crushed hollow particles and accurately weigh the mass of the crushed particles. Then, add isopropanol to the volumetric flask in the same manner as the apparent density measurement described above, accurately weigh the mass of isopropanol, and calculate the true density D0 (g / cm³) of the hollow particles based on the following formula (II). 3 Calculate ). Formula (II) True density D0 = [Mass of hollow particle fragments] / (100 - [Mass of isopropanol] ÷ [Specific gravity of isopropanol at measurement temperature]) The true density D0 corresponds to the specific gravity of only the shell portion of the hollow particle. As is clear from the measurement method described above, the hollow portion is not considered part of the hollow particle when calculating the true density D0.

[0088] The porosity (%) of a hollow particle is calculated using the following formula (III), given the apparent density D1 and true density D0 of the hollow particle. Formula (III) Porosity (%) = 100 - (Apparent density D1 / True density D0) × 100 The porosity of a hollow particle can be rephrased as the proportion of the particle's specific gravity that is occupied by the hollow portion.

[0089] The hollow particles disclosed herein have excellent strength due to the sufficient inclusion of crosslinkable monomer units in their shells, making them resistant to crushing during mixing with other materials and during molding after mixing. When added to molded articles, they exhibit excellent effects as lightweighting materials, heat insulating materials, sound insulating materials, vibration damping materials, etc. Furthermore, since the hollow particles disclosed herein have a reduced amount of residual hydrophobic solvent, there is no risk of ignition or smoke generation when mixed with other materials such as resins. For these reasons, the hollow particles disclosed herein are particularly suitable as additives for molded articles, and are especially suitable for use as additives for resin molded articles. The molded articles containing hollow particles of the present disclosure may contain, as a resin, thermoplastic resins or thermosetting 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 crystalline polyester resin, phenolic resin, vinyl ester resin, unsaturated polyester resin, cyanate ester resin, polyetherketone ketone resin, and polyetherimide resin. When epoxy resin is used as the resin component, it is preferable to mix in a curing agent or catalyst such as amines, acid anhydrides, or imidazoles as appropriate. Furthermore, the molded articles containing hollow particles of the present disclosure may further contain organic or inorganic fibers such as carbon fibers, glass fibers, aramid fibers, and polyethylene fibers. The hollow particles of this disclosure can also be included as fillers in molded articles formed using thermoplastic or thermosetting resins, and in molded articles formed using materials that further include thermoplastic or thermosetting resins and fibers. Applications of resin molded articles containing the hollow particles of this disclosure include, for example, light-reflecting materials, heat-insulating materials, sound-insulating materials, and low-dielectric components used in various fields such as automobiles, electrical and electronic equipment, construction, aerospace, and space; food containers; footwear such as sports shoes and sandals; home appliance parts; bicycle parts; stationery; and tools. In particular, the hollow particles of this disclosure exhibit excellent performance stability in high-humidity environments and have a low dielectric constant, making them suitable for use in the electrical or electronic fields as a low-dielectric and highly reliable material. For example, the hollow particles of this disclosure are suitable for use as an electronic circuit board material. Specifically, by incorporating the hollow particles of this disclosure into the insulating resin layer of an electronic circuit board, the dielectric constant of the insulating resin layer can be reduced, thereby suppressing problems such as migration in high-humidity environments. Furthermore, the hollow particles of this disclosure are also suitable for semiconductor materials such as interlayer insulating materials, dry film resists, solder resists, bonding wires, magnet wires, semiconductor encapsulants, epoxy encapsulants, mold underfills, underfills, die bond pastes, buffer coat materials, copper-clad laminates, flexible substrates, high-frequency device modules, antenna modules, and automotive radar. Among these, they are particularly suitable for semiconductor materials such as interlayer insulating materials, solder resists, magnet wires, epoxy encapsulants, underfills, buffer coat materials, copper-clad laminates, flexible substrates, high-frequency device modules, antenna modules, and automotive radar. Furthermore, the hollow particles disclosed herein have a high porosity, are resistant to crushing, and have excellent heat resistance, thus meeting the thermal insulation and cushioning (cushioning) requirements for undercoat materials, as well as the heat resistance suitable for thermal paper applications. In addition, the hollow particles disclosed herein are also useful as plastic pigments with excellent gloss and opacity. Furthermore, the hollow particles of this disclosure can be used for various purposes depending on the components contained inside, as useful components such as fragrances, pharmaceuticals, pesticides, and ink components can be sealed inside by means of immersion treatment, reduced pressure treatment, or pressurized immersion treatment. [Examples]

[0090] The present disclosure will be further described below with reference to examples and comparative examples, but the present disclosure is not limited to these examples. Parts and percentages are by mass unless otherwise specified.

[0091] [Manufacturing Example 1: Manufacturing of Polar Resin A (MMA / AA / EA Copolymer)] 200 parts of toluene were added to the reaction vessel, and the vessel was thoroughly replaced with nitrogen while stirring the toluene. The temperature was then raised to 90°C, and a mixed solution of 96.2 parts of methyl methacrylate (MMA), 0.3 parts of acrylic acid (AA), 3.5 parts of ethyl acrylate (EA), and 2.8 parts of t-butyl peroxy-2-ethylhexanoate (manufactured by NOF Corporation, trade name: Perbutyl O) was added dropwise to the reaction vessel over 2 hours. The mixture was then maintained under reflux of toluene for 10 hours to complete the polymerization, and the solvent was subsequently removed by distillation under reduced pressure to obtain polar resin A (MMA / AA / EA copolymer). Of the total mass of repeating units constituting the obtained polar resin A (MMA / AA / EA copolymer), 96.2% were derived from MMA, 0.3% from AA, and 3.5% from EA. Furthermore, the obtained polar resin A was insoluble in water, and its number-average molecular weight was 10,000. The number-average molecular weight was determined as polystyrene-equivalent molecular weight by gel permeation chromatography (GPC) using tetrahydrofuran as a carrier at a flow rate of 0.35 ml / min. The apparatus used was a Tosoh HLC8220, with three Shodex® KF-404HQ columns linked together (column temperature 40°C), and a differential refractometer and ultraviolet detector. Molecular weight calibration was performed at 12 points using standard polystyrene (5 million to 3 million) from Polymer Laboratory.

[0092] [Example 1] (1) Mixed liquid preparation process First, the following mixture was prepared as the oil phase. Ethylene glycol dimethacrylate 31.9 parts Trimethylolpropane triacrylate 13.7 parts Polar resin A (MMA / AA / EA copolymer) 0.2 parts 2,2'-Azobis(2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: V-65) 1.04 parts Hydrophobic solvent: Cyclohexane 54.5 parts Meanwhile, in a stirred tank, at room temperature, an aqueous solution prepared by dissolving 7.8 parts of magnesium chloride (water-soluble polyvalent metal salt) in 225 parts of deionized water was gradually added under stirring to an aqueous solution prepared by dissolving 5.5 parts of sodium hydroxide (alkali metal hydroxide) in 55 parts of deionized water to prepare a dispersion of magnesium hydroxide colloid (poorly water-soluble metal hydroxide colloid) (4.0 parts magnesium hydroxide), which was then used as the aqueous phase. A mixture was prepared by mixing the aqueous phase and the oil phase.

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

[0094] (3) Polymerization process The suspension obtained in the above suspension step was heated in a nitrogen atmosphere from 40°C to 65°C over 30 minutes (heating rate: 50°C / hour), and stirred for 1 hour and 30 minutes under the temperature of 65°C to carry out the first polymerization reaction. Furthermore, 2.3 parts of methyl acrylate were added to the stirring tank, and a second polymerization reaction was carried out by stirring for 2 hours and 30 minutes under a nitrogen atmosphere and a temperature of 65°C. Through the first and second polymerization reactions, a precursor composition was prepared, which is a slurry liquid in which precursor particles containing a hydrophobic solvent are dispersed in water.

[0095] (4) Washing and solid-liquid separation process The precursor composition obtained in the polymerization process described above was washed with dilute sulfuric acid at 25°C for 10 minutes to reduce the pH to 5.5 or less. Next, after separating the water by filtration, 200 parts of freshly deionized water were added to re-form a slurry, and the water washing treatment (washing, filtration, dewatering) was repeated several times at 25°C, followed by filtration separation to obtain the solid. The obtained solid was dried in a dryer at a temperature of 40°C to obtain precursor particles containing a hydrophobic solvent.

[0096] (5) Solvent removal process The precursor particles obtained in the solid-liquid separation process described above were heat-treated in a vacuum dryer at 200°C for 6 hours, then cooled to room temperature under atmospheric pressure using nitrogen to obtain the hollow particles of Example 1. Observation results using a scanning electron microscope and porosity values ​​confirmed that these particles were spherical and contained hollow portions.

[0097] [Example 2] In Example 1, the hollow particles of Example 2 were produced using the same procedure as in Example 1, except that the amounts of polymerizable monomer and hydrophobic solvent added during the preparation of the oil phase in the (1) mixed liquid preparation step were changed according to Table 1.

[0098] [Example 3] In Example 1, when preparing the aqueous phase in step (1) above, the amount of magnesium chloride (water-soluble polyvalent metal salt) added was changed from 7.8 parts to 15.7 parts, the amount of sodium hydroxide (alkali metal hydroxide) added was changed from 5.5 parts to 11.0 parts, and the amount of magnesium hydroxide in the magnesium hydroxide colloid was set to 8.0 parts. Otherwise, the hollow particles of Example 3 were produced in the same procedure as in Example 1.

[0099] [Comparative Example 1] Hollow particles for Comparative Example 1 were manufactured in the same manner as in Manufacturing Example 1 of Japanese Patent Publication No. 2000-313818. Specifically, polymer particles were obtained by polymerizing an aqueous solution prepared by dissolving 70 parts styrene, 27 parts butadiene, 3 parts itaconic acid, and 12 parts t-dodecyl mercaptan in 200 parts distilled water with 0.5 parts reactive emulsifier SE10N (manufactured by Adeka) and 1.0 part ammonium persulfate, while stirring at 75°C for 8 hours. Next, polymer particles were carried out using these polymer particles as a seed polymer. Specifically, 10 parts of these polymer particles, along with 0.1 parts polyoxyethylene nonylphenyl ether (a surfactant), 0.4 parts ammonium lauryl sulfate, and 0.5 parts ammonium persulfate, were dispersed in 900 parts distilled water. A mixture of 50 parts methyl methacrylate, 40 parts divinylbenzene, 10 parts α-methylstyrene, and 20 parts toluene was added to this mixture and polymerized at 75°C for 5 hours, obtaining a dispersion of precursor particles containing toluene inside the particles. The obtained precursor particles were spray-dried to obtain the hollow particles of Comparative Example 1.

[0100] [Comparative Example 2] In Comparative Example 2, hollow particles were produced using the same procedure as in Comparative Example 1, except that the amount of polyoxyethylene nonylphenyl ether added was changed from 0.1 parts to 0.3 parts, and sodium lauryl sulfate was used instead of 0.4 parts ammonium lauryl sulfate.

[0101] [evaluation] The hollow particles obtained in each example and comparative example were subjected to the following measurements and evaluations. The results are shown in Table 1.

[0102] 1. Volume-average particle size The volume-average particle size of hollow particles was measured using a particle size distribution analyzer (Beckman Coulter, product name: Multisizer 4e). The measurement conditions were: aperture diameter: 50 μm, dispersion medium: Isoton II (product name), concentration: 10%, and number of particles measured: 100,000. Specifically, 0.2 g of the particle sample was placed in a beaker, and an aqueous surfactant solution (manufactured by Fujifilm, product name: Drywell) was added as a dispersant. Then, 2 ml of dispersion medium was added to wet the particles, and after that, 10 ml of dispersion medium was added, and the particles were dispersed in an ultrasonic disperser for 1 minute before being measured using the particle size distribution analyzer mentioned above.

[0103] 2.Porosity 2-1. Measurement of the apparent density of hollow particles First, a capacity of 100cm 3 Approximately 30 cm in a volumetric flask 3 The hollow particles were packed into the volumetric flask, and the mass of the packed hollow particles was accurately weighed. Next, isopropanol was accurately filled to the mark into the volumetric flask filled with hollow particles, taking care not to introduce air bubbles. The mass of isopropanol added to the volumetric flask was accurately weighed, and the apparent density D1 (g / cm³) of the hollow particles was calculated based on the following formula (I). 3 ) was calculated. Equation (I) Apparent density D1 = [Mass of hollow particles] / (100 - [Mass of isopropanol] ÷ [Specific gravity of isopropanol at measurement temperature])

[0104] 2-2. Measurement of the true density of hollow particles After crushing the hollow particles in advance, a volume of 100 cm³ 3 Approximately 10 g of hollow particle fragments were packed into a volumetric flask, and the mass of the packed fragments was accurately weighed. Next, as with the measurement of apparent density above, isopropanol is added to a volumetric flask, the mass of isopropanol is accurately weighed, and the true density D0 (g / cm³) of the hollow particles is calculated based on the following formula (II). 3 ) was calculated. Formula (II) True density D0 = [Mass of hollow particle fragments] / (100 - [Mass of isopropanol] ÷ [Specific gravity of isopropanol at measurement temperature])

[0105] 2-3. Calculation of void ratio The porosity of the hollow particles was calculated from the apparent density D1 and true density D0 of the hollow particles based on the following equation (III). Formula (III) Porosity (%) = 100 - (Apparent density D1 / True density D0) × 100

[0106] 3. Measurement of relative permittivity The relative permittivity of hollow particles was measured at a frequency of 1 MHz and room temperature (25°C) using a perturbation-type measuring device (AET, model: ADMS01Nc).

[0107] 4. Surfactant content on the particle surface 50 ml of ultrapure water and 5 g of hollow particles were accurately weighed and thoroughly mixed. The mixture was irradiated with ultrasound for 30 minutes and filtered through a 0.45 μm diameter syringe membrane filter. The filtrate was freeze-dried, and 1 g of tetramethylsilane (TMS) was added to the residue to dissolve it. Under the following conditions, 1 1H-NMR measurements were performed. 1 For surfactants identified from 1H-NMR spectra, a calibration curve based on TMS intensity was created, and the amount of surfactant extracted from the hollow particle surface was calculated. The calibration curve was created from the ratio of TMS intensity to the peak intensity derived from the surfactant. The ratio of the amount of surfactant extracted from the hollow particle surface to the mass of the hollow particle was calculated and used as the surfactant content present on the hollow particle surface. In addition, 1 Particles in which no surfactant was detected from the 1H-NMR spectrum were classified as not detected (ND). < 1 H-NMR measurement conditions> Equipment name: FT-NMR equipment Resonance frequency: 400MHz Measurement mode: 1H-NMR Pulse width: 5.0 μs (Pulse angle: 90°) Measurement range: 26 ppm (Frequency range: 10500 Hz) Total number of times: 1024 Measurement temperature: 40℃ Solvent: Deuterated chloroform (TMS (tetramethylsilane) 1%) Reference substance: Tetramethylsilane-derived peak: 0.00 ppm (internal standard method)

[0108] 5. Metal content in particles Wet decomposition of 10 g of precisely weighed hollow particles was performed using a microwave (PerkinElmer Multiwave 3000). The resulting decomposition products were subjected to ICP emission spectrometry (PerkinElmer Optima 2100 DV) to measure the total mass of metals. The metal species were identified by elemental analysis using X-ray fluorescence (XRF). The ratio of the total mass of metals in the decomposition products to the mass of the hollow particles was calculated and used as the metal content in the hollow particles.

[0109] 6. Reliability testing under high humidity conditions <Preparation of hollow particle-containing resin varnish> First, 90 parts of brominated epoxy resin (manufactured by Toto Kasei Co., Ltd., trade name: YDB-500EK75, epoxy equivalent 500, solids content 75% by mass) and 10 parts of cresol novolac type epoxy resin (manufactured by Toto Kasei Co., Ltd., trade name: YDCN220EK75, epoxy equivalent 210, solids content 75% by mass) were dissolved in a mixed solvent (room temperature) of 20 parts dimethylformamide (DMF) and 6 parts methyl ethyl ketone (MEK). Then, 2 parts of dicyandiamide (DICY) (manufactured by Nippon Carbide Co., Ltd.) and 0.1 parts of 2-ethyl-4-methylimidazole (2E4MZ) (manufactured by Shikoku Kasei Kogyo Co., Ltd.) were added, and the mixture was stirred to prepare a resin varnish. Next, 95 parts of resin varnish cooled to room temperature and 5 parts of hollow particles were mixed using a disperser at 3000 rpm for 30 minutes to obtain a resin varnish containing hollow particles.

[0110] <Prepreg fabrication> The obtained hollow particle-containing resin varnish was impregnated into glass cloth (manufactured by Nitto Boseki, product name: WEA116E), and then the solvent was removed by heating and drying at 150-170°C for 3-10 minutes to obtain a prepreg.

[0111] <Fabrication of double-sided copper-clad laminated boards> A 35 μm thick copper foil (ST foil) was placed on both sides of the resulting prepreg sheet, and it was subjected to a temperature of 180°C for 2 hours at a pressure of 2.94 MPa (30 kg / cm²). 2 A double-sided copper-clad laminate with a thickness of 0.13 mm was obtained by heating and pressurizing under the curing conditions specified above.

[0112] <Reliability Testing> The obtained double-sided copper-clad laminates were subjected to pressure cooker treatment (110°C, 85% RH humidity, 100 hours). After this treatment, a voltage of 50V was applied to the double-sided copper-clad laminates for a predetermined time, and the resistance value was measured to check for any abnormalities and evaluate them according to the evaluation criteria below. An abnormality was indicated if there was a change in the resistance value. The change in resistance value is thought to be due to corrosion of the copper substrate. (Reliability Testing and Evaluation Criteria) ◎: No abnormalities even after applying voltage for 300 hours. ○: No abnormalities were found even after applying voltage for 100 hours. ×: An abnormality occurs before the voltage application time reaches 100 hours.

[0113] [Table 1]

[0114] In Table 1, the meanings of the abbreviations are as follows: EGDMA: Ethylene glycol dimethacrylate TMPT: Trimethylolpropane triacrylate MA: Methyl acrylate MMA: Methyl methacrylate DVB: Divinylbenzene α-MSt: α-methylstyrene V-65: 2,2'-Azobis(2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name: V-65)

[0115] [Consideration] The hollow particles obtained in Comparative Examples 1 and 2 had a ratio of less than 70 parts by mass of crosslinkable monomer units per 100 parts by mass of total monomer units of the polymer contained in the shell, a porosity of less than 60%, and a surfactant content exceeding 200 ppm on the particle surface. As a result, the hollow particles obtained in Comparative Examples 1 and 2 showed abnormalities in reliability tests before the voltage application time reached 100 hours, exhibiting poor performance stability under high humidity conditions, and also had a high dielectric constant exceeding 1.6. The surfactants detected on the surface of the hollow particles obtained in Comparative Example 1 were polyoxyethylene nonylphenyl ether (solubility in water at 25°C: 1 g / L or more) and ammonium lauryl sulfate (solubility in water at 25°C: 100 g / L). The surfactants detected on the surface of the hollow particles obtained in Comparative Example 2 were polyoxyethylene nonylphenyl ether (solubility in water at 25°C: 1 g / L or more) and sodium lauryl sulfate (solubility in water at 25°C: 100 g / L). In contrast, the hollow particles obtained in each example had a polymer contained in the shell that included 70 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of total monomer units, a porosity of 60% or more, and a surfactant content of 200 ppm or less on the particle surface. As a result, no abnormalities were observed even after applying voltage for 300 hours in reliability tests, demonstrating excellent performance stability under high humidity environments. Furthermore, the dielectric constant was 1.6 or less, indicating that these were hollow particles with a low dielectric constant. Notably, no surfactant was detected on the surface of the hollow particles obtained in each example. [Explanation of symbols]

[0116] 1 Aqueous medium 2 Low polarity material 4. Monomeric compositions 4a Hydrophobic solvent 4b Materials other than hydrophobic solvents 4c Polymerizable monomers dispersed in an aqueous medium 5. Oil-soluble polymerization initiators 6 Shells 8 Hollow part 10 droplets 20 Precursor particles 100 Hollow particles with a hollow space filled with gas

Claims

1. A hollow particle comprising a resin-containing shell and a hollow portion surrounded by the shell, wherein the shell contains a polymer as the resin, with 70 to 100 parts by mass of crosslinkable monomer units per 100 parts by mass of total monomer units. The porosity is 60% or more. The surfactant content present on the surface of the hollow particles is 200 ppm or less. A hollow particle having a relative permittivity of 1.6 or less at a frequency of 1 MHz.

2. Hollow particle according to claim 1, wherein the volume average particle size is 1 to 10 μm

3. A hollow particle according to claim 1 or 2, wherein the porosity is 90% or less.

4. A hollow particle according to any one of claims 1 to 3, wherein the metal content is 100 ppm or less.

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