Core-shell type particles

Abstract

Provided are core-shell type microparticles from which the core material is easily released in response to temperature changes. [Solution] The core-shell type microparticles have a shell portion and a core portion encapsulated in the shell portion, wherein the shell portion contains a side-chain crystalline polymer and the core portion contains a core material that can be released from the core-shell type microparticles.

Description

[Technical Field]

[0001] The present invention relates to core-shell type microparticles. [Background technology]

[0002] Core-shell microparticles, which have a shell and a core enclosed therein, can be provided with a variety of properties depending on the combination of materials for the core and shell, and are therefore used in a variety of applications.

[0003] Examples of such applications include those related to the release of core materials contained in the core from microparticles due to changes in the properties of the shell under specific conditions, such as high temperatures. For example, Patent Document 1 discloses a particulate flame retardant in which a component that provides flame retardancy is microencapsulated in a resin, and the component that provides flame retardancy is released during kneading and molding or in the event of a fire. Patent Document 2 also discloses a water-based ink containing core-shell resin particles having a core containing an ultraviolet absorber and a shell. Patent Document 3 also discloses thermally expandable microcapsules consisting of a polymer outer shell and a blowing agent encapsulated within the shell. The shell softens at high temperatures and the blowing agent vaporizes, causing the microcapsules to expand. During this process, at least a portion of the blowing agent is released from the microcapsules. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 55-118988 [Patent Document 2] Japanese Patent Publication No. 2023-074770 [Patent Document 3] Japanese Patent Application Laid-Open No. 2013-028818 Summary of the Invention [Problem to be solved by the invention]

[0005] Conventional core-shell microparticles have the problem that the core material is difficult to release because the properties of the shell change slowly with temperature changes.

[0006] The present invention has been made in view of the above-mentioned problems, and has as its object to provide core-shell type microparticles from which the core material is easily released in response to temperature changes. [Means for solving the problem]

[0007] A first aspect of the present invention is a core-shell microparticle having a shell portion and a core portion encapsulated in the shell portion, the shell portion contains a side chain crystalline polymer, The core is a core-shell type microparticle, the core portion of which contains a core material that can be released from the core-shell type microparticle. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide core-shell type microparticles that easily release a core material in response to a temperature change. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a graph showing the results of a thermogravimetric analysis test of an example. DETAILED DESCRIPTION OF THE INVENTION

[0010] <Core-shell type particles> The core-shell microparticles of the first embodiment have a shell portion and a core portion encapsulated in the shell portion. The shell portion contains a side-chain crystalline polymer. The core portion contains a core material that can be released from the core-shell microparticles.

[0011] In core-shell microparticles, the shell contains a side-chain crystalline polymer, which makes it easier for the core material to be released in response to temperature changes. The reason for this effect is thought to be that the shell contains a side-chain crystalline polymer, which causes abrupt changes in the properties of the shell in response to temperature changes, softening the shell and making it more susceptible to reduced gas barrier properties or collapse. One specific reason is presumed to be as follows. In conventional core-shell microparticles such as those disclosed in Patent Document 1, the softening of the shell and the deterioration of the gas barrier properties are controlled based on the glass transition point of the shell, so that in conventional core-shell microparticles, the properties of the shell change slowly with temperature changes, and the release rate is also slow. On the other hand, the shell containing the side-chain crystalline polymer is thought to soften and lose its gas barrier properties due to melting of the crystalline portion, which results in a rapid change in the properties of the shell in response to temperature changes, resulting in a fast release rate of the core material.

[0012] <Shell part> [Side chain crystalline polymer] The shell portion includes a side-chain crystalline polymer. In this specification, a side-chain crystalline polymer is a polymer containing a structural unit having a linear alkyl group having 14 or more carbon atoms in the side chain. The linear alkyl groups in the side chain are aligned in an orderly arrangement by intermolecular forces or the like, thereby exhibiting crystallinity. The number of carbon atoms in the linear alkyl group is preferably 50 or less, more preferably 30 or less, and even more preferably 25 or less. The number of carbon atoms in the linear alkyl group is preferably 16 or more, more preferably 18 or more.

[0013] The ratio of the mass of the structural unit having a linear alkyl group having 14 or more carbon atoms to the mass of the side-chain crystalline polymer is, for example, 5% by mass or more and 50% by mass or less, 5% by mass or more and 45% by mass or less, or 10% by mass or more and 40% by mass or less.

[0014] ((Meth)acrylic acid ester monomer) The side-chain crystalline polymer preferably contains, as a structural unit having a linear alkyl group having 14 or more carbon atoms, a structural unit derived from a (meth)acrylic acid ester monomer having a linear alkyl group having 14 or more carbon atoms. The linear alkyl group preferably has 50 or fewer carbon atoms, more preferably 30 or fewer carbon atoms, and even more preferably 25 or fewer carbon atoms. The linear alkyl group preferably has 16 or more carbon atoms, more preferably 18 or more carbon atoms. Examples of (meth)acrylic monomers having a linear alkyl group having 14 or more carbon atoms include cetyl (meth)acrylate, stearyl (meth)acrylate, eicosyl (meth)acrylate, and behenyl (meth)acrylate. These may be used alone or in combination of two or more.

[0015] (nitrile monomer) The side-chain crystalline polymer may contain a structural unit derived from a nitrile monomer, which is likely to improve the gas barrier property before the properties of the shell portion change. The nitrile monomer is a compound having a carbon-carbon unsaturated bond and a cyano group. Examples of the nitrile monomer include (meth)acrylonitrile, α-chloroacrylonitrile, α-ethoxyacrylonitrile, and fumaronitrile. These may be used alone or in combination of two or more.

[0016] The ratio of the mass of the structural unit derived from a nitrile monomer to the mass of the side chain crystalline polymer is, for example, 10% by mass to 70% by mass, 20% by mass to 60% by mass, or 25% by mass to 50% by mass.

[0017] (polyfunctional monomer) The side chain crystalline polymer may contain a structural unit derived from a polyfunctional monomer having two or more polymerizable functional groups. Examples of the polymerizable functional group include a group containing an ethylenically unsaturated double bond. Examples of the ethylenically unsaturated group include, but are not limited to, alkenyl groups such as vinyl, 1-propenyl, 2-n-propenyl (allyl), 1-n-butenyl, 2-n-butenyl, and 3-n-butenyl; and (meth)acryloyl group-containing groups such as (meth)acryloyl, (meth)acryloyloxy, and (meth)acryloylamino.

[0018] Examples of polyfunctional monomers include aromatic divinyl compounds such as divinylbenzene and divinylnaphthalene, allyl (meth)acrylate, tri(meth)acrylic formal, triallyl isocyanate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, 1,10-decanediol di(meth)acrylate, PEG#200 di(meth)acrylate, and the like. ) acrylate, PEG#400 di(meth)acrylate, PEG#600 di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, EO-modified trimethylolpropane tri(meth)acrylate, glycerin di(meth)acrylate, dimethylol-tricyclodecane di(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipe Pentaerythritol hexa(meth)acrylate, neopentyl glycol acrylic acid benzoate, trimethylolpropane acrylic acid benzoate, 2-hydroxy-3-(meth)acryloyloxypropyl(meth)acrylate, hydroxypivalic acid neopentyl glycol di(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, 2-butyl-2-ethyl-1,3-propanediol di(meth)acrylate, polytetramethylene glycol di(meth)acrylate, phenylglycol Examples of such urethane prepolymers include phenyl glycidyl ether (meth)acrylate hexamethylene diisocyanate urethane prepolymer, phenyl glycidyl ether (meth)acrylate toluene diisocyanate urethane prepolymer, pentaerythritol tri(meth)acrylate hexamethylene diisocyanate urethane prepolymer, pentaerythritol tri(meth)acrylate toluene diisocyanate urethane prepolymer, pentaerythritol tri(meth)acrylate isophorone diisocyanate urethane prepolymer, etc. These may be used alone or in combination of two or more.

[0019] The ratio of the mass of the structural unit derived from the polyfunctional monomer to the mass of the side chain crystalline polymer is, for example, 10% by mass to 70% by mass, 20% by mass to 60% by mass, or 25% by mass to 50% by mass.

[0020] (Other monomers) The side chain crystalline polymer may contain structural units derived from monomers other than the above-mentioned monomers.

[0021] Examples of other monomers include (meth)acrylic acid ester monomers other than (meth)acrylic acid ester monomers having a linear alkyl group having 14 or more carbon atoms, halogen-containing vinylidene monomers, halogen-containing vinyl monomers, vinyl ester monomers, carboxyl group-containing monomers, (meth)acrylamide monomers, maleimide monomers, styrene monomers, ethylenically unsaturated monoolefin monomers, vinyl ether monomers, vinyl ketone monomers, and N-vinyl monomers.

[0022] Examples of (meth)acrylic acid ester monomers other than (meth)acrylic acid ester monomers having a linear alkyl group having 14 or more carbon atoms include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, n-pentyl (meth)acrylate, isopentyl (meth)acrylate, n-hexyl (meth)acrylate, and isopropyl (meth)acrylate. Hexyl (meth)acrylate, n-heptyl (meth)acrylate, isoheptyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-nonyl (meth)acrylate, isononyl (meth)acrylate, n-decyl (meth)acrylate, isodecyl (meth)acrylate, n-undecyl (meth)acrylate, isoundecyl (meth)acrylate, n-dodecyl (meth)acrylate, isododecyl (meth)acrylate Acrylate, n-tridecyl (meth)acrylate, isotridecyl (meth)acrylate, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, phenyl (meth)acrylate, benzyl (meth)acrylate, toluyl (meth)acrylate, 2-methoxyethyl (meth)acrylate, 2-methoxybutyl (meth)acrylate, glycidyl (meth)acrylate, methylglycidyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate N,N-dimethylaminoethyl (meth)acrylate, 3-(meth)acryloyloxypropyltrimethoxysilane, 3-(meth)acryloyloxypropyltriethoxysilane, alkylene oxide adducts of (meth)acrylic acid, and the like.

[0023] Examples of halogen-containing vinylidene monomers include vinylidene chloride, vinylidene bromide, and vinylidene fluoride. Examples of halogen-containing vinyl monomers include vinyl chloride, vinyl bromide, and vinyl fluoride. Examples of vinyl ester monomers include vinyl acetate, vinyl propionate, and vinyl butyrate. Examples of carboxyl group-containing monomers include acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, citraconic acid, etc. Some or all of the carboxyl groups in the carboxyl group-containing monomers may be neutralized during polymerization.

[0024] Examples of the (meth)acrylamide-based monomer include (meth)acrylamide and substituted (meth)acrylamide. Examples of the maleimide monomer include N-phenylmaleimide, N-(2-chlorophenyl)maleimide, N-cyclohexylmaleimide, N-laurylmaleimide, etc. The maleimide monomer is preferably an N-substituted maleimide monomer having a structure in which a nitrogen atom has a substituent.

[0025] Examples of styrene-based monomers include styrene, α-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, p-ethylstyrene, 2,4-dimethylstyrene, pn-butylstyrene, pt-butylstyrene, pn-hexylstyrene, pn-octylstyrene, pn-nonylstyrene, pn-decylstyrene, pn-dodecylstyrene, n-methoxystyrene, p-phenylstyrene, p-chlorostyrene, and 3,4-dichlorostyrene. Examples of the ethylenically unsaturated monoolefin monomer include ethylene, propylene, butylene, and isobutylene.

[0026] Examples of vinyl ether monomers include vinyl methyl ether, vinyl ethyl ether, and vinyl isobutyl ether. Examples of vinyl ketone monomers include vinyl methyl ketone, vinyl hexyl ketone, and methyl isopropenyl ketone. Examples of N-vinyl monomers include N-vinylpyrrole, N-vinylcarbazole, N-vinylindole, and N-vinylpyrrolidone. The other monomers may be used alone or in combination of two or more.

[0027] <Core> [Core material] The core portion includes a core material that can be released from the core-shell microparticle. The core material is not particularly limited as long as it is a substance that has been used in the core portion of conventional core-shell microparticles and can be released from the core-shell microparticles, and may be appropriately selected depending on the intended use of the core-shell microparticles. The state of the core material at atmospheric pressure and 25°C may be any of gas, liquid, and solid, but is preferably gas or liquid, and more preferably liquid. The state of the core material at the time of release may be any of gas, liquid, and solid, but is preferably gas or liquid, and more preferably gas.

[0028] Core materials include linear hydrocarbons such as propane, butane, pentane, hexane, heptane, octane, nonane, decane, dodecane, undecane, tridecane, tetradecane, pentadecane, and hexadecane, as well as isobutane, isopentane, neopentane, isohexane, isoheptane, isooctane, isononane, isodecane, isododecane, 3-methylundecane, isotridecane, 4-methyldodecane, isotetradecane, isopentadecane, isohexadecane, and 2,2,4, Examples of suitable amines include aliphatic hydrocarbons such as branched hydrocarbons (e.g., 4,6,8,8-heptamethylnonane, isoheptadecane, and isooctadecane), and alicyclic hydrocarbons (e.g., cyclobutane, cyclopentane, cyclohexane, cycloheptane, cyclooctane, cyclononane, cyclodecane, and cyclododecane); petroleum ether; halides of aliphatic hydrocarbons and petroleum ether; fluorine-containing compounds (e.g., hydrofluoroether); tetraalkylsilane; and compounds that undergo thermal decomposition upon heating to produce gas. These may be used alone or in combination of two or more. Among these, aliphatic hydrocarbons are preferred.

[0029] The core material may also be an antioxidant, a chemical (medicine, agricultural chemical, etc.), an ultraviolet absorber, a coloring matter, a fragrance, a resin modifier, an enzyme, or the like.

[0030] The core-shell fine particles preferably have a spherical or nearly spherical shape. The average particle diameter of the core-shell microparticles is not particularly limited, but is, for example, 0.1 μm to 100 μm. In this specification, the average particle diameter is defined as the volume-based cumulative 50% particle diameter (D50) in the particle size distribution measured by a laser diffraction / scattering particle size distribution analyzer.

[0031] Core-shell type microparticles can be used as foaming agents, DDS (drug delivery system) preparations, sustained release agents for drugs, sustained release agents for antioxidants, sustained release agents for UV absorbers, etc. In particular, when used as a foaming agent, the foam expands more rapidly than conventional core-shell type microparticles, making it less likely that uneven foaming will occur and more likely to form a uniform foam.

[0032] The core-shell type fine particles may be used in combination with various base components.

[0033] <Method of manufacturing core-shell type microparticles> The method for producing core-shell microparticles is not particularly limited, but examples thereof include a method (hereinafter also referred to as "production method A") that includes a step of dispersing an oily mixture containing a monomer used in a side-chain crystalline polymer and a core material in an aqueous dispersion medium and polymerizing the monomer (hereinafter also referred to as "polymerization step"). In the polymerization step, it is preferable to use an oily mixture containing a polymerization initiator to polymerize the monomer in the presence of the polymerization initiator.

[0034] The polymerization initiator is not particularly limited, but examples thereof include commonly used peroxides and azo compounds. Examples of peroxides include peroxydicarbonates such as diisopropyl peroxydicarbonate, di-sec-butyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and dibenzyl peroxydicarbonate; diacyl peroxides such as lauroyl peroxide and benzoyl peroxide; ketone peroxides such as methyl ethyl ketone peroxide and cyclohexanone peroxide; 2,2- Examples include peroxyketals such as bis(t-butylperoxy)butane; hydroperoxides such as cumene hydroperoxide and t-butyl hydroperoxide; dialkyl peroxides such as dicumyl peroxide and di-t-butyl peroxide; and peroxyesters such as t-hexyl peroxypivalate and t-butylperoxyisobutyrate.

[0035] Examples of azo compounds include 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2-methylpropionate), 2,2'-azobis(2-methylbutyronitrile), and 1,1'-azobis(cyclohexane-1-carbonitrile). These polymerization initiators may be used alone or in combination of two or more. As the polymerization initiator, an oil-soluble polymerization initiator that is soluble in the monomer is preferred. The polymerization initiator may be added to the oil mixture before dispersing the oil mixture in an aqueous dispersion medium, or may be added to the aqueous suspension after dispersing the oil mixture in an aqueous dispersion medium.

[0036] The amount of the polymerization initiator to be added is not particularly limited, but is preferably 0.05 parts by mass or more and 10 parts by mass or less, more preferably 0.1 parts by mass or more and 8 parts by mass or less, and even more preferably 0.2 parts by mass or more and 5 parts by mass or less, relative to 100 parts by mass of the monomer.

[0037] In the polymerization step, the oily mixture may further contain a chain transfer agent or the like. In production method A, an oily mixture is dispersed in an aqueous dispersion medium to prepare an aqueous suspension, and the monomer is polymerized. The aqueous dispersion medium is a medium containing water, such as ion-exchanged water, as a main component for dispersing the oily mixture, and may further contain alcohols, such as methanol, ethanol, and propanol, or hydrophilic organic solvents, such as acetone. In this specification, "hydrophilic" means a state in which the aqueous dispersion medium can be arbitrarily mixed with water. The amount of the aqueous dispersion medium used is not particularly limited, but it is preferable to use 100 to 5,000 parts by mass of the aqueous dispersion medium per 100 parts by mass of the monomer.

[0038] The aqueous dispersion medium may further contain an electrolyte. Examples of the electrolyte include sodium chloride, magnesium chloride, calcium chloride, sodium sulfate, magnesium sulfate, ammonium sulfate, and sodium carbonate. These electrolytes may be used alone or in combination. The content of the electrolyte is not particularly limited, but it is preferably 0.1 to 50 parts by mass per 100 parts by mass of the aqueous dispersion medium.

[0039] The aqueous dispersion medium may contain at least one water-soluble compound selected from the group consisting of polyalkyleneimines with a molecular weight of 1000 or more, each having at least one structure in which an alkyl group substituted with a hydrophilic functional group selected from a carboxylic acid (salt) group and a phosphonic acid (salt) group is bonded to a nitrogen atom, water-soluble 1,1-substituted compounds in which a hydrophilic functional group selected from a hydroxyl group, a carboxylic acid (salt) group, and a phosphonic acid (salt) group and a heteroatom are bonded to the same carbon atom, potassium dichromate, alkali metal nitrite, metal (III) halides, boric acid, water-soluble ascorbic acids, water-soluble polyphenols, water-soluble B vitamins, and water-soluble phosphonic acids (salts). In this specification, "water-soluble" means that 1 g or more of the compound dissolves in 100 g of water.

[0040] The amount of the water-soluble compound contained in the aqueous dispersion medium is not particularly limited, but is preferably 0.0001 part by mass or more and 1.0 part by mass or less, more preferably 0.0003 part by mass or more and 0.1 part by mass or less, and even more preferably 0.001 part by mass or more and 0.05 part by mass or less, relative to 100 parts by mass of the monomer.

[0041] The aqueous dispersion medium may contain a dispersion stabilizer or a dispersion stabilization aid in addition to the electrolyte and the water-soluble compound. The dispersion stabilizer is not particularly limited, and examples thereof include dispersion stabilizers of poorly water-soluble inorganic compounds such as colloidal silica, colloidal calcium carbonate, magnesium hydroxide, calcium hydroxide, aluminum hydroxide, ferric hydroxide, calcium sulfate, barium sulfate, calcium oxalate, calcium metasilicate, calcium carbonate, barium carbonate, magnesium carbonate, phosphates such as calcium phosphate, magnesium phosphate, aluminum phosphate, and zinc phosphate, pyrophosphates such as calcium pyrophosphate, aluminum pyrophosphate, and zinc pyrophosphate, and alumina sol. These dispersion stabilizers may be used alone or in combination, and the type is appropriately selected taking into consideration the particle size of the resulting core-shell microparticles, dispersion stability during polymerization, and the like.

[0042] The amount of the dispersion stabilizer to be added is preferably 0.1 parts by mass or more and 30 parts by mass or less, more preferably 0.5 parts by mass or more and 20 parts by mass or less, relative to 100 parts by mass of the monomer. The dispersion stabilization aid is not particularly limited, and examples thereof include surfactants such as polymer-type dispersion stabilization aids, cationic surfactants, anionic surfactants, zwitterionic surfactants, nonionic surfactants, etc. These dispersion stabilization aids may be used alone or in combination of two or more, and are appropriately selected taking into consideration the particle size of the resulting core-shell microparticles, dispersion stability during polymerization, etc.

[0043] Examples of polymer-type dispersion stabilizing aids include condensation products of diethanolamine and aliphatic dicarboxylic acids, gelatin, polyvinylpyrrolidone, methylcellulose, polyethylene oxide, and polyvinyl alcohol. The amount of the dispersion stabilizing aid is not particularly limited, but is preferably 0.0001 to 100 parts by mass, more preferably 0.0003 to 50 parts by mass, per 100 parts by mass of the monomer.

[0044] The aqueous dispersion medium is prepared by, for example, blending water such as ion-exchanged water with electrolytes, water-soluble compounds, dispersion stabilizers, dispersion stabilization aids, etc. as needed. The pH of the aqueous dispersion medium during polymerization is appropriately determined depending on the types of water-soluble compounds, dispersion stabilizers, and dispersion stabilization aids. When colloidal silica is used as the dispersion stabilizer, the pH of the aqueous dispersion medium during polymerization is preferably 2 to 7, more preferably 2 to 6.5, even more preferably 2 to 6, and particularly preferably 2 to 4.

[0045] In production method A, polymerization may be carried out in the presence of sodium hydroxide, or sodium hydroxide and zinc chloride. In production method A, an oily mixture is suspended and dispersed in an aqueous dispersion medium so as to prepare spherical oil droplets of a predetermined particle size.

[0046] Examples of methods for suspending and dispersing an oily mixture include general dispersion methods such as stirring with a homomixer, homogenizer, etc., methods using a static dispersion device such as a static mixer, membrane suspension methods, and ultrasonic dispersion methods. Next, the dispersion in which the oily mixture is dispersed as spherical oil droplets in the aqueous dispersion medium is heated to initiate suspension polymerization. During the polymerization reaction, the dispersion is preferably stirred, and the stirring may be carried out gently, for example, to a degree sufficient to prevent floating of the monomer and settling of the core-shell type microparticles after polymerization.

[0047] The polymerization temperature can be freely set depending on the type of polymerization initiator, but is preferably controlled within a range of 20°C to 100°C, more preferably 20°C to 90°C. During the polymerization reaction, the reaction temperature may be changed stepwise or in a single step. The reaction temperature is preferably maintained for approximately 1 hour to 30 hours. The initial polymerization pressure is not particularly limited, but is preferably in the range of 0 MPa to 5 MPa, more preferably 0.1 MPa to 3 MPa, in gauge pressure.

[0048] After the polymerization reaction is completed, if desired, the dispersion stabilizer is decomposed with hydrochloric acid or the like, and the resulting product (core-shell type microparticles) is isolated from the dispersion by suction filtration, centrifugation, centrifugal filtration, etc. Furthermore, the resulting water-containing cake of core-shell type microparticles is washed with water and dried to obtain core-shell type microparticles. [Example]

[0049] The present invention will be explained in more detail below by showing examples, but the scope of the present invention is not limited to these examples.

[0050] <Production of core-shell type microparticles> Example 1 An aqueous dispersion medium was prepared by dissolving 32 parts by mass of polyvinyl alcohol (average degree of polymerization: 1,500, degree of saponification: 78 to 82 mol %) in 3,000 parts by mass of ion-exchanged water. 37.5 parts by mass of acrylonitrile, 37.5 parts by mass of ethylene glycol dimethacrylate, 25 parts by mass of behenyl acrylate, and 30 parts by mass of isooctane were mixed. The mixture was stirred at around 30°C while bubbling with nitrogen until the behenyl acrylate was dissolved. After confirming dissolution, 2 parts by mass of 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) was dissolved in the mixture to prepare an oily mixture. The aqueous dispersion medium and the oil mixture were mixed and then mixed for 15 minutes at 3,500 rpm using a homogenizer to prepare a suspension. A nitrogen flow was applied to the suspension, and the mixture was reacted at 30°C under atmospheric pressure in a sealed state for 24 hours to obtain a dispersion of core-shell microparticles. The dispersion was suction filtered at room temperature, washed with ion-exchanged water, and then air-dried to obtain core-shell microparticles.

[0051] Example 2 Core-shell type microparticles were obtained in the same manner as in Example 1, except that the amounts of acrylonitrile, ethylene glycol dimethacrylate, and behenyl acrylate used were changed to 30 parts by mass, 30 parts by mass, and 40 parts by mass, respectively.

[0052] (Comparative Example 1) Core-shell type microparticles were obtained in the same manner as in Example 1, except that behenyl acrylate was not used and the amounts of acrylonitrile and ethylene glycol dimethacrylate used were changed to 50 parts by mass and 50 parts by mass, respectively.

[0053] The amounts (parts by mass) of the compounding ingredients added in each example are shown in Table 1. [Table 1]

[0054] <Thermogravimetric analysis test> The resulting core-shell microparticles were evaluated by thermogravimetric analysis (TGA). Specifically, a differential thermogravimetric simultaneous analyzer (STA7200RV, Hitachi High-Tech Corporation) was used to measure the mass change of the core-shell microparticles during heating from 25°C to 600°C (10°C / min) under a nitrogen gas atmosphere. Figure 1 shows the mass change plotted against the mass at 25°C (100%).

[0055] Two stages of mass loss are observed in all graphs in Figure 1. The first stage of mass loss is presumably due to the evaporation of isooctane, the core material, and its release from the core-shell microparticles.

[0056] In the first stage of mass loss in Figure 1, the temperature at which the mass loss reached 0.5% / 5°C or more was defined as the core material release start temperature T1 (°C), and the temperature at which the mass loss subsequently reached 0.5% / 5°C or less was defined as the core material release end temperature T2 (°C). T1, T2, and T2-T1 for each example are shown in Table 2. The smaller the difference between T1 and T2 (T2-T1), the faster (suddener) the core material is released.

[0057] [Table 2]

[0058] As shown in Figure 1 and Table 2, the Examples prepared using behenyl acrylate and containing a side-chain crystalline polymer in the shell portion have a narrower temperature range for the first mass loss compared to the Comparative Examples that do not contain a side-chain crystalline polymer in the shell portion. This shows that the inclusion of a side-chain crystalline polymer in the shell portion causes a rapid change in the properties of the shell portion.

Claims

1. A core-shell type microparticle having a shell portion and a core portion encapsulated in the shell portion, the shell portion contains a side chain crystalline polymer, Core-shell type microparticles, wherein the core portion comprises a core material that can be released from the core-shell type microparticles.

2. 2. The core-shell type fine particle according to claim 1, wherein the side chain crystalline polymer contains a structural unit derived from a (meth)acrylic acid ester monomer having a linear alkyl group having 14 or more carbon atoms.

Images