JP7528520B2 - Method for producing hollow particles - Google Patents

Description

The present invention relates to a method for producing hollow particles.

Hollow resin particles scatter light well and have low light transmittance compared to resin particles that have virtually no internal voids, and are therefore widely used in applications such as water-based paints and paper coating compositions as organic pigments with excellent optical properties such as opacity and whiteness, and as hiding agents. In particular, hollow polymer particles are used in applications such as paper coating compositions for thermal paper, as they can increase heat insulation.

For example, Patent Document 1 discloses the following method for producing such hollow resin particles. That is, the method includes a step of preparing a mixed liquid containing at least one monomer selected from the group consisting of monovinyl monomers and hydrophilic monomers, a crosslinkable monomer, an oil-soluble polymerization initiator, a hydrocarbon solvent, a suspension stabilizer, and an aqueous medium, and the content of the crosslinkable monomer is within a specific range (mixture preparation step), a step of preparing a suspension in which monomer droplets containing a hydrocarbon solvent are dispersed in an aqueous medium by suspending the turbid liquid (suspension preparation step), a step of preparing a precursor composition containing precursor particles having hollow portions and containing a hydrocarbon solvent in the hollow portions by subjecting the suspension to a polymerization reaction (polymerization step), a step of obtaining the precursor particles by performing solid-liquid separation of the precursor composition (solid-liquid separation step), and a step of removing the hydrocarbon solvent contained in the precursor particles in air (solvent removal step).

Patent Document 2 discloses a method for producing hollow resin particles, which is characterized by dispersing a mixed solution containing a polyfunctional monomer and a non-reactive solvent in an aqueous solution, and then polymerizing the polyfunctional monomer to obtain hollow resin particles.

On the other hand, it is desirable to increase the porosity and increase the particle size of the resulting hollow resin particles in order to further improve the heat insulation. For example, in the manufacturing methods disclosed in Patent Documents 1 and 2, if the porosity and particle size of the hollow resin particles to be obtained are increased, the ease of removing the solvent in the solvent removal step is not sufficient, and therefore there are problems in that the resulting hollow resin particles have a large amount of solvent remaining in the hollow space and the hollow resin particles are crushed in the solvent removal step.

International Publication No. 2019/026899 JP 2016-190980 A

The present invention was made in consideration of these circumstances, and aims to provide a method for producing hollow particles in which the amount of residual solvent in the hollow particles is effectively reduced, even when the porosity is high and the particle diameter is large.

The present inventors have conducted research to achieve the above object, and have found that the above object can be achieved by using a suspension stabilizer containing a specific amount of a nonionic anionic surfactant when producing hollow particles having through holes by polymerizing an oil phase containing a monomer, an oil-soluble polymerization initiator, and a hydrocarbon solvent suspended in an aqueous medium, and then using the monomer as a suspension stabilizer. This has led to the completion of the present invention.

That is, according to the present invention, there is provided a method for producing hollow particles having through holes, comprising: a suspending step of preparing a suspension in which an oil phase containing the monomer (A), the oil-soluble polymerization initiator (B), and the hydrocarbon solvent (C) is dispersed in the aqueous medium (E) by suspending a mixed liquid containing the monomer (A), the oil-soluble polymerization initiator (B), and the hydrocarbon solvent (C); a polymerization step of preparing a precursor dispersion containing precursor particles having through holes and containing the hydrocarbon solvent (C) by polymerizing the monomer (A) in the suspension; and a removal step of removing the hydrocarbon solvent (C) from the precursor dispersion to obtain hollow particles containing hollow particles having through holes, in which the suspension stabilizer (D) contains a nonionic anionic surfactant, and the amount of the suspension stabilizer used is 0.03 to 3.0 parts by mass relative to 100 parts by mass of all components constituting the oil phase.

In the method for producing hollow particles of the present invention, it is preferable to use a polyoxyethylene alkyl ether sulfate as the nonionic anionic surfactant.
In the method for producing hollow particles of the present invention, the monomer (A) preferably contains a hydrophilic group-containing monomer.
In the method for producing hollow particles of the present invention, the monomer (A) preferably contains an acid group-containing monomer as a hydrophilic group-containing monomer.
In the method for producing hollow particles of the present invention, it is preferable that the monomer (A) contains a (meth)acrylic acid monomer as the acid group-containing monomer.
In the method for producing hollow particles of the present invention, the amount of the acid group-containing monomer is preferably 0.5 to 15% by mass based on 100% by mass of the monomer (A).
In the method for producing hollow particles of the present invention, the hollow particles preferably have a number average particle size of 1 to 12 μm.

The present invention provides a method for producing hollow particles that has a high porosity and effectively reduces the amount of solvent remaining in the hollow particles, even when the particle size is large.

FIG. 1 is a scanning electron microscope photograph of the hollow particles obtained in Example 1. FIG. 2 is a scanning electron microscope photograph of the hollow particles obtained in Comparative Example 2.

The method for producing hollow particles of the present invention comprises the steps of:
A method for producing hollow particles having through holes, comprising the steps of:
a suspending step of suspending a mixed liquid containing a monomer (A), an oil-soluble polymerization initiator (B), a hydrocarbon solvent (C), a suspension stabilizer (D), and an aqueous medium (E) to prepare a suspension in which an oil phase containing the monomer (A), the oil-soluble polymerization initiator (B), and the hydrocarbon solvent (C) is dispersed in the aqueous medium (E);
a polymerization step of polymerizing the monomer (A) in the suspension to prepare a precursor aqueous solution containing the hydrocarbon solvent (C) and precursor particles having through holes;
a removing step of removing the hydrocarbon-based solvent (C) from the precursor aqueous solution to obtain hollow particles having through holes,
The suspension stabilizer (D) contains a nonionic anionic surfactant, and the amount of the suspension stabilizer (D) used is 0.03 to 3.0 parts by mass per 100 parts by mass of all components constituting the oil phase.
Each step of the production method of the present invention will be described below.

[Suspension step]
The suspension step is a step of preparing a suspension in which an oil phase containing the monomer (A), the oil-soluble polymerization initiator (B), the hydrocarbon solvent (C), the suspension stabilizer (D), and the aqueous medium (E) is dispersed in the aqueous medium (E) by suspending a mixed liquid containing the monomer (A), the oil-soluble polymerization initiator (B), and the hydrocarbon solvent (C).

(Monomer (A))
The monomer (A) used in the present invention is not particularly limited, but examples thereof include a monovinyl monomer, which is a compound having one vinyl functional group, and a crosslinkable monomer having two or more polymerizable functional groups.

Examples of monovinyl monomers include ethylenically unsaturated carboxylic acid monomers such as acrylic acid, methacrylic acid, itaconic acid, and butene tricarboxylic acid; carboxyl group-containing monomers such as monoalkyl esters of unsaturated dicarboxylic acids such as monoethyl itaconate, and sulfonic acid group-containing monomers such as styrene sulfonic acid; at least one acrylic monovinyl monomer selected from the group consisting of acrylates and methacrylates; aromatic vinyl monomers such as styrene, vinyl toluene, α-methylstyrene, p-methylstyrene, and halogenated styrenes; Examples of such monomers include monoolefin monomers such as ethylene, propylene, and butylene; (meth)acrylamide (meaning acrylamide and/or methacrylamide; the same applies below); (meth)acrylamide monomers and derivatives thereof such as (meth)acrylamide (meaning acrylamide and/or methacrylamide), N-methylol (meth)acrylamide, and N-butoxymethyl (meth)acrylamide; diene monomers such as butadiene and isoprene; vinyl carboxylate ester monomers such as vinyl acetate; vinyl halide monomers such as vinyl chloride; vinylidene halide monomers such as vinylidene chloride; vinylpyridine monomers; and the like. These monovinyl monomers can be used alone or in combination of two or more.

Among monovinyl monomers, from the viewpoint of obtaining hollow particles with high heat resistance, it is preferable to use a monomer containing a hydrophilic group (hereinafter, hydrophilic group-containing monomer), and it is more preferable to use a monomer containing an acid group (hereinafter, acid group-containing monomer). The acid group here includes both a proton-donating group (Bronsted acid group) and an electron pair-accepting group (Lewis acid group).

The acid group-containing monomer is not particularly limited as long as it has one vinyl functional group and an acid group, and examples thereof include, among the above examples, ethylenically unsaturated carboxylic acid monomers such as acrylic acid, methacrylic acid, itaconic acid, and butene tricarboxylic acid; carboxyl group-containing monomers such as monoalkyl esters of unsaturated dicarboxylic acids such as monoethyl itaconate; and sulfonic acid group-containing monomers such as styrene sulfonic acid. Among the acid group-containing monomers, ethylenically unsaturated carboxylic acid monomers are preferred, acrylic acid and methacrylic acid are more preferred, and methacrylic acid is even more preferred.

In addition, hydrophilic group-containing monomers other than acid group-containing monomers may be used, such as hydroxyl group-containing monomers, amide group-containing monomers, polyoxyethylene group-containing monomers, etc.

Examples of hydroxyl group-containing monomers include 2-hydroxyethyl acrylate monomer, 2-hydroxyethyl methacrylate monomer, 2-hydroxypropyl acrylate monomer, 2-hydroxypropyl methacrylate monomer, and 4-hydroxybutyl acrylate monomer.

Examples of amide group-containing monomers include acrylamide monomers and dimethylacrylamide monomers.

Examples of polyoxyethylene group-containing monomers include methoxypolyethylene glycol acrylate monomers, methoxypolyethylene glycol methacrylate monomers, etc.

These hydrophilic group-containing monomers can be used alone or in combination of two or more.

The monovinyl monomer may be at least one acrylic monovinyl monomer selected from the group consisting of acrylates and methacrylates. Examples of acrylic monovinyl monomers include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, glycidyl (meth)acrylate, and 2-hydroxyethyl (meth)acrylate.

In this way, by using a monomer that is relatively resistant to high temperature conditions, such as a hydrophilic group-containing monomer or an acrylic monovinyl monomer, as the monovinyl monomer, the heat resistance of the resulting hollow particles can be improved compared to the use of a monomer having, for example, a nitrile group.

When a hydrophilic group-containing monomer and an acrylic monovinyl monomer are used in combination as the monovinyl monomer, the mass ratio of the hydrophilic group-containing monomer:acrylic monovinyl monomer is preferably 100:0 to 30:70, and more preferably 95:5 to 35:65.

The amount of monovinyl monomer used in 100% by mass of monomer (A) is preferably 0.5 to 15% by mass, more preferably 0.5 to 12% by mass, and even more preferably 2.0 to 10% by mass. By using the amount of monovinyl monomer within the above range, the obtained hollow particles can have high latex stability.

The crosslinkable monomer is not particularly limited as long as it is a compound having two or more polymerizable functional groups. Specific examples of the crosslinkable monomer include aromatic divinyl compounds such as divinylbenzene, divinylnaphthalene, diallyl phthalate, and derivatives thereof; ester compounds in which a compound having two or more hydroxyl groups or carboxyl groups is ester-bonded to a compound having two or more carbon-carbon double bonds, such as allyl (meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, and dipentaerythritol hexa(meth)acrylate; other divinyl compounds such as N,N-divinylaniline and divinyl ether; and the like. Among these, divinylbenzene and ethylene glycol di(meth)acrylate are preferred, and ethylene glycol di(meth)acrylate is more preferred. These can be used alone or in combination of two or more.

The amount of the crosslinkable monomer used in 100% by mass of monomer (A) is preferably 85 to 99.5% by mass, more preferably 90 to 99.5% by mass, and even more preferably 90 to 98% by mass. By using the amount of the crosslinkable monomer within the above range, it is possible to suppress the occurrence of particle crushing due to removal of the solvent when producing hollow particles.

(Oil-soluble polymerization initiator (B))
The oil-soluble polymerization initiator (B) used in the present invention is not particularly limited as long as it is lipophilic and has a solubility in water of 0.2 mass% or less. Examples of the oil-soluble polymerization initiator (B) include organic peroxides such as benzoyl peroxide, lauroyl peroxide, t-butyl peroxide-2-ethylhexanoate, orthochlorobenzoyl peroxide, orthomethoxybenzoyl peroxide, 3,5,5-trimethylhexanoyl peroxide, and di-t-butyl peroxide; and azo compounds such as 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobisisobutyronitrile, and 1,1'-azobiscyclohexanecarbonitrile.

The amount of oil-soluble polymerization initiator (B) used 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 the total amount of monomer (A). By setting the amount of oil-soluble polymerization initiator (B) used within the above range, the polymerization reaction can be sufficiently progressed, and there is little risk of oil-soluble polymerization initiator (B) remaining after completion of the polymerization reaction, and there is also little risk of unexpected side reactions proceeding.

(Hydrocarbon Solvent (C))
The hydrocarbon solvent (C) used in the present invention is used to form a hollow portion inside the particle. The hydrocarbon solvent (C) is not particularly limited, but examples thereof include relatively volatile solvents such as benzene, toluene, xylene, butane, pentane, hexane, cyclohexane, carbon disulfide, and carbon tetrachloride.

In addition, the hydrocarbon solvent (C) used in the present invention preferably has a relative dielectric constant of 3 or less at 20°C. The relative dielectric constant is one of the indicators that show the polarity of a compound. When the relative dielectric constant of the hydrocarbon solvent (C) is sufficiently small, such as 3 or less, it is believed that phase separation proceeds quickly in the monomer droplets, and hollow spaces are easily formed.

Examples of solvents with a relative dielectric constant of 3 or less at 20° C. are as follows. The value in parentheses is the relative dielectric constant.
Heptane (1.9), cyclohexane (2.0), benzene (2.3), toluene (2.4).

For the dielectric constant at 20°C, values described in known documents (for example, "Chemical Handbook Basics" edited by the Chemical Society of Japan, Revised 4th Edition, Maruzen Co., Ltd., published on September 30, 1993, pages II-498 to II-503) and other technical information can be referenced. Examples of methods for measuring the dielectric constant at 20°C include a dielectric constant test that conforms to JIS C 2101:1999-23 and is performed at a measurement temperature of 20°C.

The hydrocarbon-based solvent (C) used in the present invention may be a hydrocarbon compound having 5 to 7 carbon atoms. A hydrocarbon compound having 5 to 7 carbon atoms is easily encapsulated in the precursor particles during the polymerization step, and can be easily removed from the precursor particles during the removal step. Of these, it is preferable that the hydrocarbon-based solvent (C) is a hydrocarbon compound having 6 carbon atoms.

The amount of the hydrocarbon solvent (C) used is preferably 100 to 900 parts by mass, more preferably 150 to 750 parts by mass, and even more preferably 200 to 600 parts by mass, per 100 parts by mass of the total amount of the monomer (A). By adjusting the amount of the hydrocarbon solvent (C) used, it is possible to control the porosity of the resulting hollow particles, and by setting the amount of the hydrocarbon solvent (C) used within the above range, it is possible to suitably control the porosity of the hollow particles without impairing the mechanical properties of the hollow particles.

(Suspension Stabilizer (D))
The suspension stabilizer (D) has the effect of stabilizing the suspension state in the suspension in the suspension polymerization method described below. In the present invention, the suspension stabilizer (D) may contain a nonionic anionic surfactant, and the action of such a suspension stabilizer (D) can form micelles encapsulating the monomer (A), the oil-soluble polymerization initiator (B), and the hydrocarbon solvent (C) in the suspension polymerization method described below.

According to the present invention, by using a nonionic anionic surfactant as the suspension stabilizer (D), when the monomer (A) is polymerized to obtain precursor particles containing a hydrocarbon solvent (C), the action of the nonionic anionic surfactant can provide such precursor particles with through holes having a relatively large pore size. By forming such through holes having a relatively large pore size, the hydrocarbon solvent (C) contained in the precursor particles can be easily removed even when the obtained hollow particles have a high porosity and a large particle size, and as a result, the amount of solvent remaining in the hollow particles can be effectively reduced.

The nonionic anionic surfactant is not particularly limited as long as it is an anionic surfactant (i.e., a substance that ionizes in an aqueous solution and the anionic portion exhibits surface activity) and has a segment in its molecular main chain that acts as a nonionic surfactant, such as a polyalkylene oxide chain.

Examples of such nonionic anionic surfactants include compounds represented by the following general formula (1).
R ¹ -O-(CR ² R ³ CR ⁴ R ⁵ ) _n -SO ₃ M (1)
(In the above general formula (1), R ¹ is an alkyl group having 6 to 16 carbon atoms or an aryl group having 6 to 14 carbon atoms which may be substituted with an alkyl group having 1 to 25 carbon atoms, R ² to R ⁵ are each a group independently selected from the group consisting of hydrogen and a methyl group, M is an alkali metal atom or an ammonium ion, and n is an integer from 3 to 40.)

Specific examples of the compound represented by the above general formula (1) include polyoxyethylene alkyl ether sulfates such as polyoxyethylene lauryl ether sulfate, polyoxyethylene cetyl ether sulfate, polyoxyethylene stearyl ether sulfate, and polyoxyethylene oleyl ether sulfate; and polyoxyethylene aryl ether sulfates such as polyoxyethylene nonylphenyl ether sulfate, polyoxyethylene octylphenyl ether sulfate, and polyoxyethylene distyryl ether sulfate.

Among nonionic anionic surfactants, nonionic anionic surfactants having a polyoxyalkylene structure are preferred, and nonionic anionic surfactants having a polyoxyethylene structure are more preferred. The nonionic anionic surfactants may be used alone or in combination of two or more kinds.

In addition to the nonionic anionic surfactant, a surfactant other than the nonionic anionic surfactant, a poorly water-soluble inorganic compound, a water-soluble polymer, etc. may be used as the suspension stabilizer (D). The surfactant other than the nonionic anionic surfactant is not particularly limited, but for example, a cationic surfactant other than the nonionic anionic surfactant, an anionic surfactant, a nonionic surfactant, etc. may be used. The surfactant other than the nonionic anionic surfactant, the poorly water-soluble inorganic compound, the water-soluble polymer, etc. may also be used in combination. It is preferable not to use any surfactant other than the nonionic anionic surfactant as the suspension stabilizer (D).

The amount of suspension stabilizer (D) used may be 0.03 to 3.00 parts by mass, preferably 0.03 to 2.80 parts by mass, more preferably 0.03 to 2.50 parts by mass, and even more preferably 0.045 to 2.50 parts by mass, relative to 100 parts by mass of all components constituting the oil phase. If the amount of suspension stabilizer (D) used is too small, when the monomer (A) is polymerized to obtain precursor particles containing the hydrocarbon solvent (C), the pore size of the through holes formed in the precursor particles cannot be made sufficiently large, making it difficult to remove the hydrocarbon solvent (C). On the other hand, if the amount of suspension stabilizer (D) used is too large, the obtained hollow particles will not be able to maintain their hollow shape.

(Aqueous medium (E))
Examples of the aqueous medium (E) include water, a hydrophilic solvent, or a mixture of water and a hydrophilic solvent. The hydrophilic solvent is not particularly limited as long as it is sufficiently mixed with water and does not cause phase separation. Examples of the hydrophilic solvent include alcohols such as methanol and ethanol; tetrahydrofuran (THF); dimethyl sulfoxide (DMSO); and the like. Among the aqueous media, water is preferably used because of its high polarity. When using a mixture of water and a hydrophilic solvent, it is important that the polarity of the entire mixture is not too low from the viewpoint of forming monomer droplets. For example, the mixing ratio (mass ratio) of water to hydrophilic solvent may be water:hydrophilic solvent=99:1 to 50:50, etc.

In the suspension process, first, the monomer (A), oil-soluble polymerization initiator (B), hydrocarbon solvent (C), suspension stabilizer (D), and aqueous medium (E) are simply mixed and appropriately stirred to obtain a mixed liquid. In this mixed liquid, the oil phase containing the monomer (A), oil-soluble polymerization initiator (B), and hydrocarbon solvent (C) is usually dispersed in the aqueous medium (E) with particle diameters of about several mm. Depending on the type of material, the dispersion state of these materials in the mixed liquid can be observed with the naked eye.

When preparing the mixed liquid, an oil phase mixed liquid containing the monomer (A), oil-soluble polymerization initiator (B), and hydrocarbon solvent (C) constituting the oil phase, and an aqueous phase mixed liquid containing the suspension stabilizer (D) and aqueous medium (E) constituting the aqueous phase may be separately prepared in advance, and the mixed liquid may be obtained by mixing the prepared oil phase mixed liquid and aqueous phase mixed liquid. In this way, hollow particles having a uniform composition can be produced by preparing the oil phase mixed liquid and the aqueous phase mixed liquid separately in advance and then mixing them.

In the suspension process, the mixture prepared as described above is subjected to a suspension treatment to prepare a suspension in which monomer droplets containing the monomer (A), the oil-soluble polymerization initiator (B), and the hydrocarbon solvent (C) are finely dispersed in the aqueous medium (E). There are no particular limitations on the method for carrying out the suspension treatment on the mixture, but a treatment in which the mixture is suspended by vigorous stirring is preferred.

In the suspension step, the oil phase containing the monomer (A), the oil-soluble polymerization initiator (B), and the hydrocarbon solvent (C) is uniformly dispersed in the aqueous medium as monomer droplets having a particle diameter of preferably 1.0 to 20 μm. The particle diameter of the monomer droplets formed in the suspension step is preferably 2.0 to 15 μm, more preferably 2.0 to 12 μm. The particle diameter of the monomer droplets may be determined according to the particle diameter of the hollow particles to be obtained, and the particle diameter of the monomer droplets can be adjusted, for example, by the apparatus used for the suspension process, the conditions of the suspension process, etc. The formed monomer droplets are usually difficult to observe with the naked eye, but can be observed with a known observation device such as an optical microscope.

Here, the manufacturing method according to the present invention employs a suspension polymerization method using an oil-soluble polymerization initiator, rather than an emulsion polymerization method. The suspension polymerization method using an oil-soluble polymerization initiator will be explained below in comparison with the emulsion polymerization method.

In emulsion polymerization, a water-soluble polymerization initiator is usually used as the polymerization initiator. In emulsion polymerization, micelles, micelle precursors, monomers dissolved in the solvent, and a water-soluble polymerization initiator are first dispersed (emulsified) in an aqueous medium. Micelles are formed when a surfactant surrounds an oil-soluble monomer composition. The monomer composition contains monomers that are the raw materials for the polymer, but does not contain a polymerization initiator.

On the other hand, although the micelle precursor is an aggregate of surfactants, it does not contain a sufficient amount of monomer composition inside. The micelle precursor grows into a micelle by incorporating monomers dissolved in the solvent inside, or by procuring a part of the monomer composition from other micelles, etc. The water-soluble polymerization initiator penetrates into the inside of the micelles and micelle precursors while diffusing in the aqueous medium, and promotes the growth of oil droplets inside them. Therefore, in the emulsion polymerization method, although each micelle is monodispersed in the aqueous medium, it is expected that the particle diameter of the micelle will grow to several hundred nm. Therefore, when the emulsion polymerization method is used, the obtained hollow particle precursor has poor uniformity in particle diameter, and there is a possibility that many hollow particles having particle diameters other than the target particle diameter are obtained.

In contrast, in the suspension polymerization method using an oil-soluble polymerization initiator, micelles and monomers dispersed in the aqueous medium are first dispersed in an aqueous medium. Micelles are formed by surrounding an oil-soluble monomer composition with a surfactant. The monomer composition contains an oil-soluble polymerization initiator, as well as a monomer and a hydrocarbon solvent.

In the suspension process according to the present invention, fine oil droplets containing a monomer composition are formed in advance inside the micelles, and then polymerization initiation radicals are generated in the fine oil droplets by an oil-soluble polymerization initiator. Therefore, hollow particle precursors with the desired particle size can be produced without causing the fine oil droplets to grow too much.

Also, as is clear from a comparison between suspension polymerization and emulsion polymerization, in suspension polymerization, there is no opportunity for the oil-soluble polymerization initiator to come into contact with the monomer dispersed in the aqueous medium. Therefore, by using an oil-soluble polymerization initiator, it is possible to prevent the production of excess polymer particles in addition to the desired hollow particles.

In the suspension process, the method of vigorously stirring the mixture to form the monomer droplets is not particularly limited, but may be performed using a device capable of vigorously stirring, such as an (in-line type) emulsifying disperser (product name "Milder", manufactured by Pacific Machinery Works, Ltd.) or a high-speed emulsifying disperser (product name "T.K. Homomixer MARK II type", manufactured by Primix Corporation). In particular, in the suspension process according to the present invention, phase separation occurs in the monomer droplets, so that the hydrocarbon solvent (C) with low polarity tends to collect inside the monomer droplets. As a result, the obtained monomer droplets have the hydrocarbon solvent (C) localized inside them, and materials other than the hydrocarbon solvent (C) are distributed around the periphery.

[Polymerization step]
The polymerization step is a step of preparing a precursor dispersion liquid containing precursor particles encapsulating the hydrocarbon solvent (C) by polymerizing the suspension obtained in the above-mentioned suspension step. Here, the precursor particles are particles formed by polymerizing the monomer (A).

In the present invention, since a suspension stabilizer (D) containing a nonionic anionic surfactant is used, the precursor particles obtained by the polymerization step can have through holes. The size of the through hole diameter can be adjusted by adjusting the type and amount of the nonionic anionic surfactant used, but from the viewpoint of being able to more appropriately remove the hydrocarbon solvent (C) in the removal step described below, it is preferably more than 30 nm, more preferably more than 50 nm, and even more preferably 80 nm or more. In addition, from the viewpoint of making the effect of forming a hollow structure, such as the effect of improving heat insulation, sufficient, the upper limit of the pore diameter of the through hole possessed by the precursor particle is not particularly limited, but is preferably 1000 nm or less, more preferably 800 nm or less.

The polymerization method is not particularly limited, but for example, a batch system, a semi-continuous system, a continuous system, etc. can be adopted. The polymerization temperature is preferably 40 to 80°C, more preferably 50 to 70°C. The polymerization reaction time is preferably 1 to 20 hours, more preferably 2 to 15 hours. In the present invention, in the precursor dispersion obtained, the precursor particles obtained by polymerization form a shell by polymerization of the monomer (A), and the hydrocarbon solvent (C) is enclosed inside the shell.

[Removal process]
The removal step is a step of obtaining hollow particles having through holes by removing the hydrocarbon solvent (C) contained in the precursor particles.

The manufacturing method of the present invention uses a suspension stabilizer (D) containing a nonionic anionic surfactant, so the precursor particles obtained in the polymerization step described above can have through holes, which makes it easy to remove the hydrocarbon solvent (C) contained in the precursor particles.

In particular, in the present invention, from the viewpoint of being able to obtain hollow particles with superior heat insulation, it is preferable that the porosity is 70 to 98% and the number average particle diameter is preferably 1 to 12 μm. That is, it is preferable that the porosity is high and the particle diameter is large, and according to the present invention, even if the obtained hollow particles have a high porosity and a large particle diameter, the hydrocarbon solvent (C) can be effectively removed. In particular, when producing hollow particles with a high porosity and a large particle diameter, the amount of solvent contained in the hollow particles increases, so there is a problem that the solvent is not easily removed in the removal step. In contrast, according to the present invention, by using a suspension stabilizer (D) containing a nonionic anionic surfactant, the precursor particles obtained in the above-mentioned polymerization step can be made to have through holes with a relatively large pore diameter, and therefore, in the removal step, the hydrocarbon solvent (C) contained in the precursor particles can be easily removed. As a result, even if the porosity of the resulting hollow particles is high and the particle size is large, the amount of solvent remaining in the hollow particles can be effectively reduced.

The porosity of the obtained hollow particles is preferably 70 to 98%, more preferably 75 to 98%, and even more preferably 80 to 98%. By setting the porosity of the hollow particles in the above range, the heat insulating properties of the hollow particles can be further improved. The porosity of the hollow particles can be calculated, for example, from the volume of the monomer and the volume of the hydrocarbon solvent used when producing the hollow particles, using the following formula.
Porosity (%)=[Volume of hydrocarbon solvent used (ml)/{Volume of monomer used (ml)+Volume of hydrocarbon solvent used (ml)}]×100
The porosity of the hollow particles can be adjusted, for example, by adjusting the amount of the hydrocarbon solvent (C) used.

The number average particle diameter of the hollow particles obtained is preferably 1 to 12 μm, more preferably 1 to 11 μm, even more preferably 1 to 10 μm, even more preferably 1.5 to 10 μm, and particularly preferably 2.0 to 10 μm. By setting the number average particle diameter of the hollow particles in the above range, the obtained hollow particles can be made to have better heat insulation properties. The number average particle diameter of the hollow particles can be determined, for example, by measuring the particle size distribution using a laser diffraction particle size distribution measuring device and calculating the number average. The number average particle diameter of the hollow particles can be adjusted, for example, by adjusting the amount of monomer (A) used or by adjusting the suspension conditions in the suspension process.

In the removal step, for example, a method of removing the hydrocarbon-based solvent (C) contained in the precursor particles from the precursor dispersion liquid obtained by the polymerization step, and then removing the liquid content containing the aqueous medium (E), or a method of simultaneously removing the liquid content containing the aqueous medium (E) and the hydrocarbon-based solvent (C) contained in the precursor particles, etc. can be adopted. The boiling point of the hydrocarbon-based solvent (C) is often lower than the boiling point of the aqueous medium (E), and therefore, from the viewpoint that the hydrocarbon-based solvent (C) can be more easily removed than the aqueous medium (E), it is preferable to adopt a method of removing the hydrocarbon-based solvent (C) contained in the precursor particles from the precursor dispersion liquid obtained by the polymerization step, and then removing the liquid content containing the aqueous medium (E).

In the removal step, a method for removing the liquid content including the aqueous medium (E) from the precursor dispersion obtained by the polymerization step can be, for example, a method for obtaining precursor particles by performing solid-liquid separation of the precursor dispersion.

The method for solid-liquid separation of the precursor dispersion is not particularly limited, and any known method can be used as long as it separates the solid content containing the precursor particles and the liquid content containing the aqueous medium (E) without substantially removing the hydrocarbon solvent (C) contained in the precursor particles. Examples of the solid-liquid separation method include centrifugation, filtration, and static separation. Of these, either centrifugation or filtration may be used, but it is preferable to use centrifugation from the viewpoint of ease of operation.

After solid-liquid separation, any step such as a pre-drying step may be carried out before removing the hydrocarbon solvent (C) contained in the precursor particles. An example of the pre-drying step is a step of pre-drying the solid content obtained after solid-liquid separation using a drying device such as a dryer or a drying tool such as a hand dryer.

The method for removing the hydrocarbon solvent (C) contained in the precursor particles is not particularly limited, and may be performed in air or in liquid. In the following, an example of a method for removing the hydrocarbon solvent (C) in air will be described. Note that "in air" in the removal process means an environment in which no liquid exists outside the precursor particles, or an environment in which only a very small amount of liquid exists outside the precursor particles that does not affect the removal of the hydrocarbon solvent (C). "In air" can also be expressed as a state in which the precursor particles are not present in a slurry, or a state in which the precursor particles are present in a dry powder.

The method for removing the hydrocarbon solvent (C) in the precursor particles in air is not particularly limited, and known methods can be used. Examples of such methods include vacuum drying, heat drying, air flow drying, or a combination of these methods. In particular, when using heat drying, the heating temperature must be equal to or higher than the boiling point of the hydrocarbon solvent and equal to or lower than the maximum temperature at which the shell structure of the hollow particles does not collapse. Therefore, depending on the composition of the shell in the precursor particles and the type of hydrocarbon solvent (C), the heating temperature may be, for example, 50 to 150°C, 60 to 130°C, or 70 to 100°C. By the drying operation in air, the hydrocarbon solvent (C) inside the precursor particles is replaced by the external gas, resulting in hollow particles in which the hollow portion is occupied by gas.

The drying atmosphere is not particularly limited and can be appropriately selected depending on the application of the hollow particles. Examples of the drying atmosphere include air, oxygen, nitrogen, argon, etc. In addition, hollow particles with a temporary vacuum inside can be obtained by filling the inside of the hollow particles with a gas once and then drying under reduced pressure.

Methods for simultaneously removing the liquid component containing the aqueous medium (E) and the hydrocarbon solvent (C) contained in the precursor particles include, for example, reduced pressure drying, heat drying, air flow drying, or a combination of these methods.

The hollow particles thus obtained have through holes. The diameter of the through holes is usually the same as that of the precursor particles obtained in the polymerization process described above, and is preferably more than 30 nm, more preferably more than 50 nm, and even more preferably 80 nm or more. In addition, from the viewpoint of making the effect of making the hollow structure such as the heat insulation improvement effect sufficient, the upper limit of the diameter of the through holes is not particularly limited, but is preferably 1000 nm or less, more preferably 800 nm or less. It is considered that the diameter of the through holes of the precursor particles is almost the same as the diameter of the through holes of the obtained hollow particles. Therefore, the diameter of the through holes of the precursor particles can be determined by measuring the diameter of the through holes of the obtained hollow particles. The diameter of the through holes of the hollow particles can be determined, for example, by observing 100 randomly selected hollow particles with a scanning electron microscope, measuring the diameter of the largest through hole in each hollow particle, and calculating the number average of the diameter of the largest through hole in each hollow particle. The pore size of the through holes in the precursor particles can be adjusted, for example, by adjusting the type of nonionic anionic surfactant used as the suspension stabilizer (D) or the amount of the suspension stabilizer (D) used.

After the polymerization step and before the removal step, the pH of the precursor dispersion obtained by the polymerization step may be adjusted. For example, a base may be added to the precursor dispersion obtained by the polymerization step to adjust the pH of the precursor dispersion to, for example, 6.0 or more. On the other hand, according to the manufacturing method of the present invention, the desired hollow particles can be obtained without pH adjustment, so from the viewpoint of simplifying the manufacturing process, it is preferable not to perform such pH adjustment in the manufacturing method of the present invention.

The shape of the hollow particles is not particularly limited as long as they have a through hole and a hollow space formed inside, and examples include a spherical shape, an oval spherical shape, and an irregular shape. Among these, a spherical shape is preferred because of ease of production.

The interior of the hollow particles may have one or more hollow spaces, and may be porous. In order to maintain a good balance between the high porosity of the hollow particles and the mechanical strength of the hollow particles, it is preferable that the interior of the particle has only one hollow space.

According to the manufacturing method of the present invention, it is possible to obtain hollow particles in which the amount of residual solvent is effectively reduced, even when the particles have a high porosity and a large particle size. In addition, by making the hollow particles have a high porosity and a large particle size, the particles can be provided with excellent heat insulation properties, and therefore the hollow particles obtained by the manufacturing method of the present invention can be suitably used in applications requiring excellent heat insulation properties, such as undercoating agents for thermal paper.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. Note that the "parts" below are based on mass unless otherwise specified. Note that various physical properties were measured as follows.

[Porosity of hollow particles]
The porosity of the hollow particles was calculated from the volume of the monomer and the volume of the hydrocarbon solvent used in producing the hollow particles, using the following formula.
Porosity (%)=[Volume of hydrocarbon solvent used (ml)/(Volume of monomer used (ml)+Volume of hydrocarbon solvent used (ml)]×100
(The volume of cyclohexane used as the hydrocarbon solvent was calculated assuming a specific gravity of cyclohexane of 0.779 g/ml, and the volume of the monomer used was calculated assuming a specific gravity of the monomer of 1.0 g/ml.)

[Number average particle size of hollow particles]
The particle size of each hollow particle was measured using a laser diffraction particle size distribution analyzer (product name "SALD-2000", manufactured by Shimadzu Corporation), and the number average was calculated. The obtained value was regarded as the number average particle size of the hollow particles.

[Shell thickness of hollow particles]
The shell thickness of the hollow particles was calculated from the porosity of the hollow particles calculated above and the number average particle size.

[Diameter of through holes in hollow particles]
The diameter of the through holes was determined as follows: First, 100 randomly selected hollow particles were observed with a scanning electron microscope, and the diameter of the largest through hole in each hollow particle was measured. Then, the number average of the diameters of the largest through holes in each hollow particle was calculated, and the obtained value was regarded as the diameter of the through holes of the hollow particles.

[Residual Solvent Amount in Hollow Particles]
The amount of residual solvent in the hollow particles was calculated by the following formula. It can be determined that the smaller the amount of residual solvent, the more effectively the amount of solvent remaining in the hollow particles has been reduced.
Residual solvent amount (%)=100×(total solid content−theoretical solid content)/total solid content (the total solid content is the actual measured value (g) of the total amount of solids after heating the hollow particles at 105° C. for 2 hours, and the theoretical solid content is the theoretical value (g) of the mass of the hollow particles not including the solvent).

[Stability of hollow particles during polymerization]
The polymerization stability of the hollow particles was evaluated by filtering the precursor dispersion after polymerization through a 80-mesh wire screen, checking the presence or absence of aggregates obtained as the filtration residue, and rating the particles as ◯ when no aggregates were observed and as × when aggregates were observed. If no aggregates were observed, it could be determined that the polymerization stability was excellent.

[Hollow particle crushing ratio]
The crushed percentage of hollow particles was determined by observing 100 hollow particles with a scanning electron microscope and calculating the percentage of crushed and broken hollow particles among the observed hollow particles.

Example 1
(Suspension process)
First, 14 parts of methacrylic acid, 86 parts of ethylene glycol dimethacrylate, 3.33 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator, product name "V-65", manufactured by Wako Pure Chemical Industries, Ltd.), and 556.7 parts of cyclohexane were mixed to form an oil phase.
Next, sodium polyoxyethylene alkyl sulfate (nonionic anionic surfactant, product name "Latemul E-118B", manufactured by Kao Corporation) was mixed with 1,300 parts of ion-exchanged water so that the amount of active ingredient was 0.075 parts per 100 parts of the oil phase (0.5 parts per 100 parts of the monomer used), and this was used as the aqueous phase.
Then, the aqueous phase and the oil phase were mixed to prepare a mixed liquid.

The prepared mixture was stirred and suspended for 5 minutes at a rotation speed of 15,000 rpm using an in-line emulsifier disperser (product name "Milder", manufactured by Pacific Machinery Works, Ltd.), to obtain a suspension in which monomer droplets containing cyclohexane were dispersed in water.

(Polymerization process)
The suspension obtained in the suspension step was heated to 65° C., and then the temperature condition of 65° C. was maintained for 3 hours to carry out a polymerization reaction. A precursor dispersion containing precursor particles encapsulating cyclohexane was obtained by this polymerization reaction. The obtained precursor dispersion was evaluated for its stability during polymerization. The results are shown in Table 1.

(Removal process)
First, the precursor composition obtained in the polymerization step was centrifuged using a cooled high-speed centrifuge (product name "H-9R", manufactured by Kokusan Co., Ltd.) under conditions of rotor MN1, rotation speed 3,000 rpm, and centrifugation time 20 minutes to dehydrate the solid content (solid-liquid separation). The dehydrated solid content was dried in a dryer at a temperature of 40°C to obtain precursor particles encapsulating cyclohexane.

The obtained precursor particles were then heat-treated in air at 80°C for 15 hours in a vacuum dryer to obtain hollow particles (1). The obtained hollow particles (1) were measured for porosity, particle size, shell thickness, through-hole pore size, residual solvent amount, and crushing ratio according to the above-mentioned method. The results are shown in Table 1. It was confirmed from the results of observation of the obtained hollow particles (1) with a scanning electron microscope that these particles were spherical and had only one hollow portion. An image obtained by observing the hollow particles (1) with a scanning electron microscope is shown in Figure 1. The ratio of the monomers constituting the resin portion of the hollow particles (1) was almost the same as the amount charged (the same applies to Example 2 and Comparative Examples 1 to 4 described below).

Example 2
The amount of methacrylic acid was changed from 14 parts to 5 parts, the amount of ethylene glycol dimethacrylate was changed from 86 parts to 95 parts, and the amount of sodium polyoxyethylene alkyl sulfate was changed from 0.075 parts to 1.5 parts (10.0 parts per 100 parts of the monomer) in terms of active ingredient amount per 100 parts of the oil phase, in the same manner as in Example 1, and the hollow particles (2) were obtained and evaluated in the same manner. The results are shown in Table 1.

Comparative Example 1
Hollow particles (3) were obtained in the same manner as in Example 1, except that 0.5 parts of sodium dodecylbenzenesulfonate (product name "Neopelex G-15", manufactured by Kao Corporation) was used in place of 0.075 parts of sodium polyoxyethylene alkyl sulfate, in terms of the amount of active ingredient, per 100 parts of the oil phase, and were evaluated in the same manner. Note that, since the hollow particles (3) were broken during the removal step, it was not possible to measure the porosity, particle size, pore size of the through holes, and amount of residual solvent of the hollow particles (3). The results are shown in Table 1.

Comparative Example 2
Hollow particles (4) were obtained and evaluated in the same manner as in Example 2, except that the amount of sodium polyoxyethylene alkyl sulfate used was changed from 1.5 parts to 0.015 parts (0.10 parts per 100 parts of the monomer) in terms of the amount of active ingredient per 100 parts of the oil phase. The results are shown in Table 1. Also, an image of hollow particles (4) obtained by observing them with a scanning electron microscope is shown in FIG. 2.

Comparative Example 3
Hollow particles (5) were obtained in the same manner as in Example 2, except that 0.5 parts of polyoxyethylene lauryl ether (product name "EMULGEN 120", manufactured by Kao Corporation) was used in place of 1.5 parts of sodium polyoxyethylene alkyl sulfate in terms of the amount of active ingredient per 100 parts of the oil phase, and were evaluated in the same manner. Note that since hollow particles (5) were broken during the removal process, the porosity, particle size, pore size of the through holes, and amount of residual solvent of hollow particles (5) could not be measured. The results are shown in Table 1.

Comparative Example 4
(Suspension process)
First, 41 parts of methacrylic acid, 59 parts of ethylene glycol dimethacrylate, 3.33 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (oil-soluble polymerization initiator, product name "V-65", manufactured by Wako Pure Chemical Industries, Ltd.), 3 parts of linoleic acid, and 553.7 parts of cyclohexane were mixed to form an oil phase.
Next, 1,300 parts of ion-exchanged water was mixed with sodium dodecylbenzenesulfonate (product name "Neopelex G-15", manufactured by Kao Corporation) in an amount of 0.5 part in terms of active ingredient per 100 parts of the oil phase to form an aqueous phase.
Then, the aqueous phase and the oil phase were mixed to prepare a mixed liquid.

The prepared mixture was stirred and suspended for 5 minutes at a rotation speed of 15,000 rpm using an in-line emulsifier disperser (product name "Milder", manufactured by Pacific Machinery Works, Ltd.), to obtain a suspension in which monomer droplets containing cyclohexane were dispersed in water.

(Polymerization process)
The suspension obtained in the suspension step was heated to 65° C., and then the temperature condition of 65° C. was maintained for 3 hours to carry out a polymerization reaction. A precursor dispersion containing precursor particles encapsulating cyclohexane was obtained by this polymerization reaction. The obtained precursor dispersion was evaluated for its stability during polymerization. The results are shown in Table 1.

(pH adjustment step)
A 10% aqueous sodium hydroxide solution was added to the cooled precursor dispersion obtained in the polymerization step until the pH of the mixture in the reaction vessel reached 7.0. The aqueous sodium hydroxide solution was added while the precursor dispersion was being stirred by an in-line emulsifying disperser.

(Removal process)
To 100 parts of the precursor dispersion liquid that had been subjected to the base addition step, 0.1 to 0.5 parts by mass of an antifoaming agent was further added, and the mixture was maintained at 70° C. for 6 hours while blowing in nitrogen at a flow rate of 6 min/L to remove cyclohexane from the precursor particles, thereby obtaining hollow particles (6). The obtained hollow particles (6) were evaluated in the same manner as in Example 1. The results are shown in Table 1.

As can be seen from Table 1, when an oil phase containing a monomer, an oil-soluble polymerization initiator, and a hydrocarbon solvent is suspended in an aqueous medium and then a process of polymerizing the monomer is carried out to produce hollow particles having through holes, the hollow particles obtained by the hollow particle production method using a suspension stabilizer containing a specific amount of a nonionic anionic surfactant have a relatively high porosity of 87.9% and a relatively large number average particle diameter of 4 μm, and even when the hollow particles have a relatively high porosity of 87.9% and a relatively large number average particle diameter of 4 μm, the amount of solvent remaining in the hollow particles is effectively reduced and the rate of crushing or cracking of particles due to removal of the solvent is low, making it possible to produce hollow particles well (Examples 1 and 2).
On the other hand, when a nonionic anionic surfactant was not used as a suspension stabilizer, even if hollow particles having the same porosity and particle size as those in Examples 1 and 2 were to be produced, the through holes were not properly formed, so that the particles were cracked during the removal of the hydrocarbon solvent (C) in the removal step, and hollow particles could not be obtained (Comparative Examples 1 and 3). In addition, in Comparative Example 3, the stability during polymerization was also poor.
Furthermore, when the amount of the nonionic anionic surfactant used was too small, the through holes were not properly formed, resulting in a large amount of the hydrocarbon solvent (C) remaining. Furthermore, when the hydrocarbon solvent (C) was removed, significant crushing of the particles occurred, and good hollow particles could not be obtained (Comparative Example 2, FIG. 2).
Furthermore, when a nonionic anionic surfactant was not used as a suspension stabilizer, but linoleic acid was used instead, and the pH was adjusted, through pores were formed, but the pore size was small, resulting in a relatively large amount of hydrocarbon solvent (C) remaining. Furthermore, when the hydrocarbon solvent (C) was removed, many particles were crushed, and good hollow particles could not be obtained (Comparative Example 4).

Claims (7)

A method for producing hollow particles having through holes, comprising the steps of:
a suspending step of suspending a mixed liquid containing a monomer (A), an oil-soluble polymerization initiator (B), a hydrocarbon solvent (C), a suspension stabilizer (D), and an aqueous medium (E) to prepare a suspension in which an oil phase containing the monomer (A), the oil-soluble polymerization initiator (B), and the hydrocarbon solvent (C) is dispersed in the aqueous medium (E);
a polymerization step of polymerizing the monomer (A) in the suspension to prepare a precursor dispersion liquid containing the hydrocarbon solvent (C) and precursor particles having through holes;
a removing step of removing the hydrocarbon-based solvent (C) from the precursor dispersion liquid to obtain hollow particles having through holes,
The monomer (A) includes an acid group-containing monomer ,
The method for producing hollow particles, wherein the suspension stabilizer (D) contains an anionic surfactant having a polyalkylene oxide chain in its molecular main chain, and the amount of the suspension stabilizer (D) used is 0.03 to 3.0 parts by mass per 100 parts by mass of all components constituting the oil phase. The method for producing hollow particles according to claim 1, wherein polyoxyethylene alkyl ether sulfate is used as the anionic surfactant. The method for producing hollow particles according to claim 1 or 2, wherein the amount of monovinyl monomer in 100% by mass of the monomer (A) is 0.5 to 15% by mass. 3. The method for producing hollow particles according to claim 1, wherein the monomer (A) contains an ethylenically unsaturated carboxylic acid monomer as the acid group-containing monomer . The method for producing hollow particles according to claim 4, wherein the monomer (A) contains a (meth)acrylic acid monomer as the acid group-containing monomer. The method for producing hollow particles according to claim 4 or 5, wherein the amount of the acid group-containing monomer in 100% by mass of the monomer (A) is 0.5 to 15% by mass. 7. The method for producing hollow particles according to claim 1, wherein the hollow particles have a number average particle size of 1 to 12 μm.