Polymer-grafted barium titanate

Abstract

The purpose of the present invention is to provide: a polymer-grafted barium titanate having high dispersion ability in rubber and a solvent serving as a matrix base material; and a polymer-grafted barium titanate-containing composition and dielectric sheet in both of which the polymer-grafted barium titanate is used. By introducing a radical polymerization initiation group into the surface of barium titanate and forming a polymer graft chain having a glass transition temperature of at most 30°C from the radical polymerization initiation group as a starting point, it becomes possible to impart flexibility to the polymer-grafted barium titanate. Thus, dispersibility of the same in rubber and a solvent becomes high. Furthermore, it was found that a polymer-grafted barium titanate-containing composition containing the polymer-grafted barium titanate has high dielectric properties even when the added amount of the same is relatively small compared to the normal added amount of barium titanate.

Description

Polymer-grafted barium titanate

[0001] This invention relates to a rubber composition that has improved dispersibility in solvents and rubbers by grafting a low glass transition temperature polymer onto the surface of barium titanate, which is used as a dielectric material, and that has a high dielectric constant even with a small blending amount.

[0002] Titanates, typified by barium titanate, have traditionally been widely used as dielectric materials in multilayer ceramic capacitors, etc., due to their extremely high dielectric constants. In recent years, as such electronic materials have become increasingly miniaturized and highly integrated, there has been a corresponding demand for smaller, more functional dielectric materials.

[0003] In response to these problems, Patent Document 1 discloses that by blending 300 to 500 parts by weight of barium titanate powder having a relative dielectric constant of 2000 or more at room temperature to 90°C with 100 parts by weight of rubber such as peroxide-crosslinked ethylene propylene rubber, and setting the relative dielectric constant to 10 or more, preferably 20 or more, it is possible to further suppress the deterioration of electrical properties such as dielectric loss tangent, breakdown voltage, and insulation resistance. However, the amount of inorganic material filled is three to five times the amount of the rubber base material, and concerns remain regarding the dispersibility of the inorganic material in the base material, moldability during processing, and the strength of the molded product.

[0004] On the other hand, with regard to moldability, Patent Document 2 discloses that a titanate compound chemically modified with a polymer chain obtained by polymerizing methyl methacrylate starting from a radical polymerization initiating group introduced to the surface of the titanate via a chemical bond has good moldability and excellent mechanical strength even when highly filled into a resin, but does not disclose any description of the dielectric properties when highly filled into a resin.

[0005] JP 2003-138067 A JP 2010-090272 A

[0006] In view of the above problems, an object of the present invention is to provide a polymer-grafted barium titanate that has high dispersibility in solvents and rubbers, and a polymer-grafted barium titanate-containing composition and a dielectric sheet that use the same.

[0007] The present inventors conducted extensive research to achieve the above-mentioned object, and as a result, they discovered that by introducing radical polymerization initiator groups onto the surface of barium titanate and using these as the starting point to form polymer graft chains with a glass transition temperature of 30°C or lower, it is possible to impart flexibility to the barium titanate, thereby improving its dispersibility in solvents and rubber. Furthermore, they discovered that a polymer-grafted barium titanate-containing composition containing polymer-grafted barium titanate has high dielectric properties even at a relatively small amount compared to the amount of ordinary barium titanate blended, and thus completed the present invention.

[0008] That is, the present invention provides the following features. Item 1: Polymer-grafted barium titanate in which the surfaces of barium titanate particles are modified with polymer graft chains, and the glass transition temperature of the polymer constituting the polymer graft chains is 30°C or lower. Item 2: Polymer-grafted barium titanate according to Item 1, in which the polymer constituting the polymer graft chains is a polymer containing a structural unit derived from an acrylic acid ester. Item 3: Polymer-grafted barium titanate-containing composition comprising at least the polymer-grafted barium titanate according to Item 1 or 2 and a matrix material, in which the content of the polymer-grafted barium titanate is 1 to 50 parts by mass per 100 parts by mass of the matrix material. Item 4: Polymer-grafted barium titanate-containing composition according to Item 3, in which the matrix material is a (meth)acrylic copolymer. Item 5: A dielectric sheet obtained by molding the polymer-grafted barium titanate-containing composition according to Item 3 or 4. Item 6: A dielectric sheet obtained by molding the polymer-grafted barium titanate according to Item 1 or 2, in which the content of the polymer graft chains is 5% by mass or more.

[0009] The polymer-grafted barium titanate of the present invention has high dispersibility in solvents and matrix materials, and has high dielectric properties even when incorporated in a relatively small amount.

[0010] The polymer-grafted barium titanate of the present invention is a polymer-grafted barium titanate in which the surfaces of barium titanate particles are modified with polymer graft chains, and the polymer constituting the polymer graft chains has a glass transition temperature of 30° C. or lower. This allows the polymer to have high dispersibility in solvents and matrix materials, and high dielectric properties even when incorporated in a relatively small amount.

[0011] While the reason for the above-mentioned effects is not entirely clear, it is believed to be due to the following mechanism. The polymer-grafted barium titanate of the present invention has polymer graft chains with a glass transition temperature of 30°C or lower. The presence of polymer graft chains with a relatively low glass transition temperature can impart flexibility to the barium titanate, thereby increasing the dispersibility of the polymer-grafted barium titanate in matrix materials such as solvents and rubber. Furthermore, because the polymer-grafted barium titanate of the present invention has high dispersibility in solvents and matrix materials, it exhibits high dielectric properties even when incorporated in relatively small amounts. On the other hand, if the polymer-grafted barium titanate has polymer graft chains with a glass transition temperature exceeding 30°C, the flexibility of the polymer graft chains decreases, resulting in a resin (plastic) rather than an elastomer, which reduces the dispersibility of the polymer-grafted barium titanate in matrix materials such as solvents and rubber.

[0012] Furthermore, by increasing the content of polymer graft chains, the polymer-grafted barium titanate of the present invention can be used alone to form a dielectric sheet without the need for a matrix material. This is presumably because the polymer graft chains, which have a relatively low glass transition temperature, function like a matrix material. Furthermore, since no matrix material is used in this case, the problem of poor dispersion in the matrix material does not occur, and a dielectric sheet with a high dielectric constant is obtained. Even with a high content of polymer graft chains, the polymer-grafted barium titanate of the present invention remains highly dispersible in solvents and matrix materials, and maintains high dielectric properties even at relatively small blend amounts. On the other hand, when polymer-grafted barium titanate has polymer graft chains with a glass transition temperature exceeding 30°C, the flexibility of the polymer graft chains decreases, resulting in a resin (plastic) rather than an elastomer, making it difficult to form into a sheet.

[0013] <Polymer-grafted barium titanate> The polymer-grafted barium titanate of the present invention has a core-shell structure with barium titanate as the core and polymer graft chains as the shell. The core-shell structure may be one in which the shell covers the core without being directly chemically bonded to the core, or one in which the barium titanate particles as the core are surface-treated to enable chemical bonding with the polymer graft chains that form the shell.

[0014] <Barium titanate> Barium titanate can be used without any particular limitation as long as it is obtained by a known manufacturing method. In the present invention, it is preferable to use barium titanate produced by the oxalic acid method. Barium titanate obtained by the oxalic acid method is finely divided, and since the particle size is small, the specific surface area per unit mass is large, which tends to make it easier to introduce polymer graft chains onto the barium titanate surface.

[0015] The particle size of the barium titanate used in the present invention is preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 30 nm or more as a lower limit, and is preferably 2000 nm or less, more preferably 1000 nm or less, even more preferably 500 nm or less, and particularly preferably 300 nm or less as an upper limit.

[0016] The specific surface area per unit mass of the barium titanate used in the present invention is 0.5 m as a lower limit. ２ / g or more, and ２ / g or more, and 0.7m ２ / g or more is more preferable. ２ / g or less, and ２ / g or less is more preferable, and 20m ２ It is more preferable that the SiO2 content is 1 / g or less.

[0017] <Polymer Graft Chain> The polymer constituting the polymer graft chain of the present invention (also simply referred to as polymer graft chain) is characterized in that it has a glass transition temperature of 30° C. or lower. Only one type of polymer graft chain may be used, or two or more types may be used in combination.

[0018] Glass Transition Temperature of Polymer Graft Chains The glass transition temperature of the polymer graft chains of the present invention is preferably 30°C or lower, more preferably 15°C or lower, even more preferably 10°C or lower, particularly preferably 0°C or lower, most preferably -10°C or lower, and most preferably -20°C or lower. The lower limit of the glass transition temperature is not particularly limited, but is, for example, -60°C or higher. Polymer-grafted barium titanate having polymer graft chains at or below these glass transition temperatures has high dispersion stability, without sedimentation of the barium titanate when dispersed in a solvent or the like. Furthermore, flexibility can be imparted to the barium titanate, thereby improving the dispersibility of the polymer-grafted barium titanate in matrix materials such as solvents and rubber.

[0019] In the present invention, the glass transition temperature of the polymer graft chain was calculated using the Fox formula, which is as follows: Glass transition temperature = ΣCi / ΣTgi, where Ci is the weight ratio of the copolymer component and Tgi is the glass transition temperature of the copolymer component.

[0020] Constituent Units of Polymer Graft Chains As described above, the polymer graft chains formed in the polymer-grafted barium titanate of the present invention are not particularly limited as constituent units as long as they are composed of polymers with a glass transition temperature of 30°C or less, and may be homopolymers composed of one type of constituent unit or copolymers using two or more types of constituent units. In the case of copolymers, they may be alternating copolymers, random copolymers, or block copolymers. It is also preferable that the polymers constituting the polymer graft chains are elastomers.

[0021] The structural unit that forms the main chain of the polymer graft chain of the present invention (the structural unit of the polymer that constitutes the polymer graft chain) is preferably a structural unit derived from an acrylic acid ester, and may optionally contain a structural unit derived from an ethylenically unsaturated dicarboxylic acid dialkyl ester, a structural unit derived from an ethylenically unsaturated nitrile, a structural unit derived from a methacrylic acid ester, or a structural unit derived from a crosslinkable monomer. That is, the polymer that constitutes the polymer graft chain is preferably a polymer composed of one or more structural units derived from an acrylic acid ester, or a polymer that, in addition to the structural units, contains one or more copolymerization components selected from the group consisting of a structural unit derived from an ethylenically unsaturated dicarboxylic acid dialkyl ester, a structural unit derived from an ethylenically unsaturated nitrile, a structural unit derived from a methacrylic acid ester, and a structural unit derived from a crosslinkable monomer.

[0022] Here, when a polymer-grafted barium titanate-containing composition containing at least polymer-grafted barium titanate and a matrix material is prepared, the polymer graft chain preferably contains a structural unit derived from a crosslinkable monomer, and more preferably contains a structural unit derived from an acrylic acid ester and a structural unit derived from a crosslinkable monomer, because this improves the dispersibility of the polymer-grafted barium titanate in the matrix material. On the other hand, when the polymer-grafted barium titanate of the present invention is used alone to prepare a dielectric sheet, it is not necessary to mix it with a matrix material, and therefore the polymer graft chain does not need to contain a structural unit derived from a crosslinkable monomer. Therefore, it is preferable that the polymer graft chain contains a structural unit derived from an acrylic acid ester. Of course, even when the polymer-grafted barium titanate of the present invention is used alone to prepare a dielectric sheet, the polymer graft chain may contain a structural unit derived from a crosslinkable monomer.

[0023] The structural unit derived from an acrylic acid ester is preferably a structural unit derived from an alkyl acrylate ester or a structural unit derived from an alkoxyalkyl acrylate ester, and more preferably a structural unit derived from an alkyl acrylate ester. These structural units may be used alone or in combination.

[0024] Examples of structural units derived from alkyl acrylates include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-pentyl acrylate, n-hexyl acrylate, n-heptyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, and cyclohexyl acrylate, with structural units derived from ethyl acrylate and n-butyl acrylate being preferred. These may be used alone or in combination.

[0025] Examples of structural units derived from alkoxyalkyl acrylates include structural units derived from acrylate esters such as methoxymethyl acrylate, ethoxymethyl acrylate, 2-methoxyethyl acrylate, 2-ethoxyethyl acrylate, 2-propoxyethyl acrylate, 2-butoxyethyl acrylate, 2-methoxypropyl acrylate, 2-ethoxypropyl acrylate, 3-methoxypropyl acrylate, 3-ethoxypropyl acrylate, 4-methoxybutyl acrylate, and 4-ethoxybutyl acrylate, with 2-methoxyethyl acrylate being preferred. These may be used alone or in combination.

[0026] In the present invention, the content of structural units derived from acrylic esters in 100% by mass of the polymer graft chains is preferably 50% by mass to 100% by mass. More specifically, the lower limit is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, particularly preferably 80% by mass or more, and most preferably 85% by mass or more. The upper limit is preferably 100% by mass or less, more preferably 95% by mass or less, and even more preferably 90% by mass or less. By being in the above range, the glass transition temperature of the polymer graft chains can be set to 30°C or less.

[0027] Furthermore, the polymer graft chain of the present invention may contain, as other copolymerization components, a structural unit derived from an ethylenically unsaturated dicarboxylic acid dialkyl ester, a structural unit derived from an ethylenically unsaturated nitrile, a structural unit derived from a methacrylic acid ester, or a structural unit derived from a compound having a crosslinking group (a structural unit derived from a crosslinkable monomer), as necessary.

[0028] Structural Units Derived from Ethylenically Unsaturated Dicarboxylic Acid Dialkyl Esters Examples of structural units derived from ethylenically unsaturated dicarboxylic acid dialkyl esters include fumaric acid dialkyl esters such as dimethyl fumarate, diethyl fumarate, dipropyl fumarate, dibutyl fumarate, dihexyl fumarate, and dictyl fumarate, and maleic acid dialkyl esters such as dimethyl maleate, diethyl maleate, dipropyl maleate, dibutyl maleate, dipentyl maleate, and didecyl maleate. These can be used alone or in combination of two or more.

[0029] The content of the structural units derived from the ethylenically unsaturated dicarboxylic acid dialkyl ester, based on 100% by mass of the polymer graft chains, is preferably 0% by mass or more, more preferably 1% by mass or more, and even more preferably 5% by mass or more. The content is preferably 20% by mass or less, more preferably 18% by mass or less, and even more preferably 15% by mass or less. A content within these ranges has a positive effect on the polarity of the polymer, thereby increasing the dielectric constant of the resulting dielectric sheet.

[0030] Structural units derived from ethylenically unsaturated nitriles Examples of structural units derived from ethylenically unsaturated nitriles include structural units derived from compounds such as acrylonitrile, methacrylonitrile, α-methoxyacrylonitrile, vinylidene cyanide, etc. These may be used alone or in combination.

[0031] The content of the structural units derived from ethylenically unsaturated nitrile is, based on 100% by mass of the polymer graft chain, preferably 0% by mass or more, more preferably 0.5% by mass or more, and even more preferably 0.7% by mass or more. The upper limit is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less. A content within these ranges has a positive effect on the polarity of the polymer, thereby increasing the dielectric constant of the resulting dielectric sheet.

[0032] Structural units derived from methacrylic acid esters are preferably structural units derived from methacrylic acid alkyl esters or structural units derived from methacrylic acid alkoxyalkyl esters, and more preferably structural units derived from methacrylic acid alkyl esters. These structural units may be used alone or in combination.

[0033] Examples of structural units derived from alkyl methacrylate include methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-pentyl methacrylate, n-hexyl methacrylate, n-heptyl methacrylate, n-octyl methacrylate, 2-ethylhexyl methacrylate, and cyclohexyl methacrylate. Structural units derived from methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, and 2-ethylhexyl methacrylate are preferred. These may be used alone or in combination. The longer the alkyl chain, the lower the glass transition temperature tends to be. Furthermore, when using methyl methacrylate, which exhibits a high glass transition temperature, it is preferable to reduce the ratio.

[0034] Examples of structural units derived from alkoxyalkyl methacrylate include structural units derived from acrylic esters such as methoxymethyl methacrylate, ethoxymethyl methacrylate, 2-methoxyethyl methacrylate, 2-ethoxyethyl methacrylate, 2-propoxyethyl methacrylate, 2-butoxyethyl methacrylate, 2-methoxypropyl methacrylate, 2-ethoxypropyl methacrylate, 3-methoxypropyl methacrylate, 3-ethoxypropyl methacrylate, 4-methoxybutyl methacrylate, and 4-ethoxybutyl methacrylate, with 2-methoxyethyl methacrylate being preferred. These may be used alone or in combination.

[0035] The content of the structural unit derived from methacrylic acid ester, based on 100% by mass of the polymer graft chain, is preferably 0% by mass or more, more preferably 5% by mass or more, and even more preferably 10% by mass or more. The upper limit is preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less. If the content is outside the upper limit range, the glass transition temperature exceeds 30°C, and dispersibility in solvents and the like tends to decrease.

[0036] Constituent units derived from crosslinkable monomers having a crosslinking group Concerning the constituent units derived from compounds having a crosslinking group, types of crosslinking groups include carboxy groups, epoxy groups, and halogen groups. Among these, constituent units derived from crosslinkable monomers having an epoxy group as a crosslinking group are preferred.

[0037] Examples of structural units derived from crosslinkable monomers having an epoxy group as a crosslinking group include structural units derived from epoxy group-containing (meth)acrylic acid esters such as glycidyl (meth)acrylate, structural units derived from epoxy group-containing styrenes such as p-vinylbenzyl glycidyl ether, structural units derived from epoxy group-containing ethers such as allyl glycidyl ether and vinyl glycidyl ether, 3,4-epoxy-1-pentene, 3,4-epoxy-1-butene, 4,5-epoxy-2-pentene, 4-vinylcyclohexyl glycidyl ether, cyclohexenylmethyl glycidyl ether, 3,4-epoxy-1-vinylcyclohexene, and allylphenyl glycidyl ether, and the like. Of these, structural units derived from epoxy group-containing (meth)acrylic acid esters such as glycidyl (meth)acrylate are preferred. These can be used alone or in combination of two or more.

[0038] Examples of structural units derived from crosslinkable monomers having a carboxy group as a crosslinking group include structural units derived from ethylenically unsaturated monocarboxylic acids such as methacrylic acid, acrylic acid, crotonic acid, 2-pentenoic acid, and cinnamic acid; structural units derived from ethylenically unsaturated dicarboxylic acids such as fumaric acid, maleic acid, and itaconic acid; and structural units derived from ethylenically unsaturated dicarboxylic acid monoesters such as monoalkyl fumarates such as monomethyl fumarate, monoethyl fumarate, monopropyl fumarate, monobutyl fumarate, monohexyl fumarate, and monooctyl fumarate; and monoalkyl maleates such as monomethyl maleate, monoethyl maleate, monopropyl maleate, monobutyl maleate, monopentyl maleate, and monodecyl maleate. These may be used alone or in combination of two or more.

[0039] Examples of constituent units derived from crosslinkable monomers having a halogen group as a crosslinking group include constituent units derived from 2-chloroethyl vinyl ether, 2-chloroethyl acrylate, vinylbenzyl chloride, vinyl monochloroacetate, allyl chloroacetate, etc. These may be used alone or in combination of two or more.

[0040] The content of the structural unit derived from the crosslinkable monomer is, relative to 100% by mass of the polymer graft chain, preferably 0% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more. The upper limit is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less. If the content is outside the upper range, the glass transition temperature will be 30°C or more, and dispersibility in solvents and the like will tend to decrease.

[0041] <Method for producing polymer-grafted barium titanate> As described above, the polymer-grafted barium titanate of the present invention has a core-shell structure in which barium titanate forms the core and polymer graft chains form the shell. The core-shell structure may be one in which the shell covers the core without direct chemical bonding to the core, or one in which the barium titanate particles forming the core are surface-treated to enable chemical bonding with the polymer graft chains that form the shell. In the present invention, it is preferable to adopt a core-shell structure in which the core and shell can be directly chemically bonded.

[0042] The method for obtaining the polymer-grafted barium titanate of the present invention includes at least a surface treatment step of introducing radical polymerization initiating groups onto the surface of barium titanate, and a grafting step of forming polymer graft chains.

[0043] Surface Treatment Step: To obtain the polymer-grafted barium titanate of the present invention, it is necessary to introduce radical polymerization initiator groups onto the barium titanate particle surfaces. To introduce radical polymerization initiator groups, three steps are required: a step of introducing hydroxyl groups onto the barium titanate particle surfaces, a step of introducing organic functional groups into the hydroxyl-group-introduced barium titanate, and a step of introducing radical polymerization initiator groups.

[0044] Methods for introducing hydroxyl groups onto the barium titanate particle surface include oxidizing agent treatment using an oxidizing agent such as hydrogen peroxide, such as reflux, immersion, or spraying; alkali treatment using alkali metal hydroxides such as sodium hydroxide or potassium hydroxide, alkali metal alkoxides, amines, pyridine derivatives, or salts of weak acids and strong bases; ultraviolet irradiation treatment, corona discharge treatment, or plasma treatment; and in the present invention, oxidizing agent treatment or alkali treatment is preferred, with oxidizing agent treatment being more preferred. Furthermore, if hydroxyl groups have already been introduced onto the barium titanate particle surface, this step may be omitted.

[0045] As a method for introducing organic functional groups into hydroxyl-introduced barium titanate, a silane coupling agent having an organic functional group can be used to introduce the organic functional group onto the surface of barium titanate particles. Specifically, known methods such as a combination of hydrolysis by heating and dehydration condensation can be used. Silane coupling agents having an alkoxy group, acetoxy group, or halogen group capable of hydrolysis at one terminal functional group are preferred. Silane coupling agents having an amino group, epoxy group, vinyl group, or mercapto group as the organic functional group are preferred, with silane coupling agents having an amino group being more preferred.

[0046] Examples of silane coupling agents that can be used in the present invention include γ-aminopropyltriethoxysilane, vinyltriethoxysilane, γ-glycidoxypropyltriethoxysilane, and γ-mercaptopropyltriethoxysilane. γ-aminopropyltriethoxysilane, vinyltriethoxysilane, and γ-glycidoxypropyltriethoxysilane are preferred, and γ-aminopropyltriethoxysilane is more preferred.

[0047] The method for introducing the radical polymerization initiating group is not particularly limited, and any known method can be used as long as it can cause a substitution reaction between the organic functional group introduced into barium titanate and a compound having a radical polymerization initiating group. The compound having a radical polymerization initiating group is preferably a compound in which a halogen group is the radical polymerization initiating group.

[0048] Examples of compounds having a radical polymerization initiating group that can be used in the present invention include 2-bromoisobutyryl bromide, 2-chloroisobutyryl bromide, 2-bromoisobutyryl chloride, and 2-chloroisobutyryl chloride.

[0049] Grafting Step Polymer graft chains are formed starting from the radical polymerization initiator groups introduced onto the surface of the barium titanate particles. As a method for forming the graft chains, known radical polymerization methods can be used. In the present invention, it is preferable to adopt a living radical polymerization method. Among living radical polymerization methods, it is more preferable to adopt atom transfer radical polymerization (ATRP) or reversible irreversible chain transfer polymerization (RAFT polymerization), and it is even more preferable to adopt ATRP.

[0050] ATRP is a polymerization method that uses a transition metal complex as a catalyst and an organic halogen compound as a polymerization initiator. During polymerization, the growing end of the polymer is in equilibrium between an active species having a radical and a dormant species in which the radical is capped with a halogen atom, and this equilibrium is heavily biased toward the dormant species, keeping the radical concentration in the reaction system low. Therefore, bimolecular termination reactions in which radicals react with each other are suppressed, and this polymerization method produces polymers with high molecular weights and narrow molecular weight distributions.

[0051] The transition metal complex used in ATRP is preferably a salt of any of rubidium, copper, iron, titanium, nickel, rhodium, and palladium, more preferably a salt of any of rubidium, copper, iron, and nickel, and even more preferably copper chloride(I), copper chloride(II), copper bromide(II), copper bromide(I), copper iodide(I), dichlorobis(triphenylphosphine)nickel(II), cyclopentadienyliron dicarbonyl (dimer), dichlorotris(triphenylphosphine)ruthenium(II), or chlorocyclopentadienylbis(triphenylphosphine)ruthenium(II).

[0052] In addition, examples of the ligand for the central metal include bipyridyl compounds such as 2,2'-bipyridyl, 4,4'-dimethyl-2,2'-bipyridyl, 4,4'-di-tert-butyl-2,2'-bipyridyl, 4,4'-dimethoxy-2,2'-bipyridyl, and 4,4'-dinonyl-2,2'-bipyridyl; N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA), N,N,N',N'',N''',N'''-hexamethyltriethylenetriamine (PMDETA), and N,N,N',N'',N''',N'''-hexamethyltriethylenetriamine (N,N,N',N'', ... aliphatic amine compounds such as tris(2-pyridylmethyl)amine (HMTETA), tris(2-pyridylmethyl)amine (TPMA), tris[2-(dimethylamino)ethyl]amine (Me6TREN), N,N,N',N'-tetraethylethylenediamine, and N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine; 1,4,7-trimethyl-1,4,7-triazacyclononane, 1,4,8,11-tetraazacyclotetradecane, and 1,4,8,11-tetramethyl cyclic amine compounds such as 1,4,8,11-tetraazacyclotetradecane and 1,4,7-triazacyclononane; and phosphorus compounds such as tetrabutylphosphonium bromide, 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene, 2-(diphenylphosphino)benzoic acid, diphenyl-2-pyridylphosphine, tris(4-methoxyphenyl)phosphine, tributylphosphine, triphenylphosphine, and trimethyl phosphite. Of these, aliphatic amine compounds and cyclic amine compounds are preferred, and N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA), N,N,N',N'',N''',N'''-hexamethyltriethylenetetramine (HMTETA), tris(2-pyridylmethyl)amine (TPMA), tris[2-(dimethylamino)ethyl]amine (Me6TREN), and 1,4,8,11-tetraazacyclotetradecane are more preferred.

[0053] The polymerization can be carried out, for example, by introducing nitrogen at room temperature to deoxygenate the mixture, then raising the temperature in the system to 40 to 100°C with stirring, and maintaining the temperature at 40 to 100°C for 3 to 48 hours.

[0054] The polymerization method for the grafting step is not particularly limited, and may be solution polymerization or bulk polymerization. To increase the content of polymer graft chains in the polymer-grafted barium titanate, bulk polymerization is preferred. To increase the content of polymer graft chains in the polymer-grafted barium titanate, a larger amount of monomer component may be used relative to the polymer-grafted barium titanate during polymerization.

[0055] The lower the content of the polymer graft chains in 100% by mass of the polymer-grafted barium titanate of the present invention, the higher the dielectric constant, while the higher the content, the easier it is to form a dielectric sheet using the polymer-grafted barium titanate of the present invention alone. Therefore, the content of the polymer graft chains in 100% by mass of the polymer-grafted barium titanate of the present invention is preferably 5 to 60% by mass, more preferably 7 to 55% by mass, even more preferably 10 to 50% by mass, and particularly preferably 15 to 40% by mass, because this more suitably achieves both good dispersibility in solvents and matrix materials and high dielectric properties. On the other hand, because this more suitably enables the polymer-grafted barium titanate of the present invention to be used alone to form a dielectric sheet, the lower limit of the content of the polymer graft chains in 100% by mass of the polymer-grafted barium titanate of the present invention is preferably 5% by mass or more, more preferably 7% by mass or more, even more preferably 10% by mass or more, and particularly preferably 20% by mass or more. On the other hand, the upper limit is preferably 60% by mass or less, more preferably 55% by mass or less, and even more preferably 50% by mass or less. If the content is less than the lower limit, it will not be possible to directly form a sheet, and if the content exceeds the upper limit, it will tend not to be possible to obtain a dielectric sheet with a high relative permittivity.

[0056] The total content of the polymer graft chains and barium titanate in 100% by mass of the polymer-grafted barium titanate of the present invention is preferably 95% by mass or more, more preferably 98% by mass or more, even more preferably 99% by mass or more, and may be 100% by mass.

[0057] <Polymer-grafted barium titanate-containing composition> The polymer-grafted barium titanate-containing composition of the present invention contains at least the polymer-grafted barium titanate of the present invention and a matrix material.

[0058] Matrix Material The matrix material used in the present invention is not particularly limited as long as it can disperse polymer-grafted barium titanate, such as thermosetting resins, thermoplastic resins, natural rubber, or synthetic rubber. These may be used alone or in combination. From the viewpoint of compatibility with the polymer graft chain, it is preferable to use a matrix material having an sp value similar to that of the polymer graft chain. In the present invention, since the sp value of the polymer graft chain is approximately 9 when the structural unit derived from an acrylic acid ester is used, it is preferable to use a matrix material having an sp value of 8 to 11. In this specification, the "solubility parameter value (SP value)" refers to the value δ calculated based on the following formula using the Fedors method [Robert F. Fedors, Polymer Engineering and Science, 14, 147-154 (1974)]. Fedors' formula: δ = (ΣΔei / ΣΔvi)^1 / 2 [unit: (cal / cm ３ )^1 / 2] [where Δei: evaporation energy of atoms and atomic groups (cal / mol), Δvi: molar volume (cm ３ / mol).

[0059] Examples of matrix materials having an sp value in the range of 8 to 11 include thermosetting resins such as epoxy resins; thermoplastic resins such as polyethylene, polystyrene, acrylic resins, methacrylic resins, polyvinyl chloride, vinyl acetate, PUR (polyurethane) resin, EVA (ethylene / vinyl acetate) resin, and AS (styrene / acrylonitrile) resin; natural rubber; and synthetic rubbers such as butadiene rubber, styrene-butadiene rubber, isoprene rubber, acrylonitrile-butadiene rubber, acrylonitrile-butadiene-isoprene rubber, (meth)acrylic rubber ((meth)acrylic copolymer), ethylene-acrylic rubber, ethylene-propylene-diene rubber, and epichlorohydrin rubber. Among these, acrylic resins, methacrylic resins, (meth)acrylic rubbers ((meth)acrylic copolymers), and ethylene-acrylic rubbers, which are similar to the structural units of the polymer graft chain, are more preferred, and (meth)acrylic rubbers ((meth)acrylic copolymers) are even more preferred. Furthermore, rubbers (natural rubbers and synthetic rubbers), which are elastomer components, are also preferred as matrix materials.

[0060] In the polymer-grafted barium titanate-containing composition of the present invention, the content of the (meth)acrylic copolymer in 100% by mass of the matrix material is preferably 95% by mass or more, more preferably 98% by mass or more, even more preferably 99% by mass or more, and may be 100% by mass.

[0061] In the polymer-grafted barium titanate-containing composition of the present invention, the content of the polymer-grafted barium titanate of the present invention is preferably 1 to 50 parts by mass per 100 parts by mass of matrix material. The lower limit is preferably 1 part by mass or more, more preferably 2 parts by mass or more, and even more preferably 5 parts by mass or more. The upper limit is preferably 50 parts by mass or less, more preferably 45 parts by mass or less, and even more preferably 40 parts by mass or less. By setting the content within these ranges, aggregation of the polymer-grafted barium titanate can be more effectively prevented, and poor dispersion in the matrix material tends to be more effectively suppressed. Furthermore, since the polymer-grafted barium titanate of the present invention has high dispersibility, it tends to exhibit sufficient dielectric effect even with a small amount added.

[0062] In the polymer-grafted barium titanate-containing composition of the present invention, barium titanates other than the polymer-grafted barium titanate of the present invention may be used as long as the objectives of the present invention are not impaired. Examples of barium titanates other than the polymer-grafted barium titanate of the present invention include barium titanate without a polymer graft chain and polymer-grafted barium titanate with a polymer graft chain having a glass transition temperature exceeding 30°C. These may be used alone or in combination. In the polymer-grafted barium titanate-containing composition of the present invention, the content of the polymer-grafted barium titanate of the present invention in 100% by mass of barium titanate is preferably 95% by mass or more, more preferably 98% by mass or more, even more preferably 99% by mass or more, and may even be 100% by mass.

[0063] <(Meth)acrylic Copolymer> The (meth)acrylic copolymer used in the present invention contains at least a structural unit derived from a (meth)acrylic acid ester. In this specification, "(meth)acrylic" is a general term for acrylic and methacrylic, and means either or both of them.

[0064] The (meth)acrylic copolymer used in the present invention preferably contains one or more structural units derived from an alkyl acrylate ester, a structural unit derived from an alkoxyalkyl acrylate ester, and a structural unit derived from an alkyl methacrylate ester, and more preferably contains one or more structural units derived from an alkyl acrylate ester.

[0065] Structural Units Derived from Alkyl Acrylate Esters The (meth)acrylic copolymer used in the present invention preferably contains structural units derived from alkyl acrylate esters, more preferably structural units derived from alkyl acrylate esters having an alkyl group containing 1 to 8 carbon atoms, even more preferably structural units derived from alkyl acrylate esters having an alkyl group containing 2 to 8 carbon atoms, and particularly preferably structural units derived from alkyl acrylate esters having an alkyl group containing 2 to 6 carbon atoms.

[0066] Examples of structural units derived from alkyl acrylate esters having an alkyl group of 1 to 8 carbon atoms include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-pentyl acrylate, n-hexyl acrylate, n-heptyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, and cyclohexyl acrylate, with structural units derived from ethyl acrylate and n-butyl acrylate being preferred. These may be used alone or in combination.

[0067] The content of the alkyl acrylate ester in 100% by mass of the (meth)acrylic copolymer used in the present invention is preferably 50% by mass to 99% by mass. More specifically, the lower limit is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, particularly preferably 80% by mass or more, and most preferably 90% by mass or more. The lower limit is preferably 99% by mass or less, more preferably 95% by mass or less, and even more preferably 90% by mass or less. By being in the above range, good physical properties can be obtained in terms of the cold resistance and oil resistance of the (meth)acrylic copolymer.

[0068] Structural Units Derived from Alkoxyalkyl Acrylate Esters The (meth)acrylic copolymer used in the present invention may contain structural units derived from alkoxyalkyl acrylate esters, and in this case, it is preferable that the structural units are derived from alkoxyalkyl acrylate esters having an alkoxyalkyl group having 2 to 6 carbon atoms, more preferably structural units are derived from alkoxyalkyl acrylate esters having an alkoxyalkyl group having 2 to 5 carbon atoms, and even more preferably structural units are derived from alkoxyalkyl acrylate esters having an alkoxyalkyl group having 2 to 4 carbon atoms. These structural units may be derived from a single type of alkoxyalkyl acrylate ester, or two or more types of alkoxyalkyl acrylate esters may be used.

[0069] Examples of structural units derived from alkoxyalkyl acrylate esters having an alkoxyalkyl group having 2 to 8 carbon atoms include structural units derived from acrylate esters such as methoxymethyl acrylate, ethoxymethyl acrylate, 2-methoxyethyl acrylate, 2-ethoxyethyl acrylate, 2-propoxyethyl acrylate, 2-butoxyethyl acrylate, 2-methoxypropyl acrylate, 2-ethoxypropyl acrylate, 3-methoxypropyl acrylate, 3-ethoxypropyl acrylate, 4-methoxybutyl acrylate, and 4-ethoxybutyl acrylate, with 2-methoxyethyl acrylate being preferred. These may be used alone or in combination.

[0070] In 100% by mass of the (meth)acrylic copolymer used in the present invention, the content of structural units derived from an alkoxyalkyl acrylate ester is preferably 0% by mass to 30% by mass. More specifically, the lower limit is preferably 0% by mass or more, more preferably 2% by mass or more, even more preferably 3% by mass or more, and particularly preferably 4% by mass or more. The upper limit is preferably 30% by mass or less, more preferably 25% by mass or less, even more preferably 20% by mass or less, particularly preferably 15% by mass or less, and most preferably 10% by mass or less. A content within the above range allows the (meth)acrylic copolymer to obtain good physical properties in terms of mechanical strength, and is also preferred in terms of cold resistance and oil resistance.

[0071] Structural Units Derived from Alkyl Methacrylate Esters The (meth)acrylic copolymer used in the present invention may contain structural units derived from alkyl methacrylate esters, and in such cases, the structural units are preferably structural units derived from alkyl methacrylate esters having an alkyl group of 1 to 8 carbon atoms, more preferably structural units derived from alkyl methacrylate esters having an alkyl group of 1 to 7 carbon atoms, even more preferably structural units derived from alkyl methacrylate esters having an alkyl group of 1 to 6 carbon atoms, and preferably structural units derived from alkyl methacrylate esters having an alkyl group of 1 to 4 carbon atoms.

[0072] Examples of structural units derived from alkyl methacrylate esters having an alkyl group of 1 to 8 carbon atoms include methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-pentyl methacrylate, n-hexyl methacrylate, n-heptyl methacrylate, n-octyl methacrylate, 2-ethylhexyl methacrylate, and cyclohexyl methacrylate, and structural units derived from methyl methacrylate, ethyl methacrylate, and n-butyl methacrylate are preferred. These may be used alone or in combination.

[0073] In 100% by mass of the (meth)acrylic copolymer used in the present invention, the content of structural units derived from alkyl methacrylate esters is preferably 0% by mass to 30% by mass. More specifically, the lower limit is preferably 0% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more. The upper limit is preferably 30% by mass or less, more preferably 25% by mass or less, even more preferably 20% by mass or less, and particularly preferably 15% by mass or less. By being in the above range, good physical properties can be obtained in terms of the mechanical strength of the (meth)acrylic copolymer.

[0074] Structural Units Derived from Alkoxyalkyl Methacrylate Esters The (meth)acrylic copolymer used in the present invention may contain structural units derived from alkoxyalkyl methacrylate esters, and in this case, it is preferable that the structural units be structural units derived from alkoxyalkyl methacrylate esters having an alkoxyalkyl group having 2 to 6 carbon atoms, more preferably structural units derived from alkoxyalkyl methacrylate esters having an alkoxyalkyl group having 2 to 5 carbon atoms, and even more preferably structural units derived from alkoxyalkyl methacrylate esters having an alkoxyalkyl group having 2 to 4 carbon atoms. These structural units may be derived from a single type of alkoxyalkyl methacrylate ester, or two or more types of alkoxyalkyl methacrylate esters may be used.

[0075] Examples of structural units derived from alkoxyalkyl methacrylate esters having an alkoxyalkyl group having 2 to 8 carbon atoms include structural units derived from methacrylic acid esters such as methoxymethyl methacrylate, ethoxymethyl methacrylate, 2-methoxyethyl methacrylate, 2-ethoxyethyl methacrylate, 2-propoxyethyl methacrylate, 2-butoxyethyl methacrylate, 2-methoxypropyl methacrylate, 2-ethoxypropyl methacrylate, 3-methoxypropyl methacrylate, 3-ethoxypropyl methacrylate, 4-methoxybutyl methacrylate, and 4-ethoxybutyl methacrylate, with 2-methoxyethyl methacrylate being preferred. These may be used alone or in combination.

[0076] In 100% by mass of the (meth)acrylic copolymer used in the present invention, the content of structural units derived from alkoxyalkyl methacrylate ester is preferably 0% by mass to 30% by mass. More specifically, the lower limit is preferably 0% by mass or more, more preferably 2% by mass or more, even more preferably 3% by mass or more, and particularly preferably 4% by mass or more. The upper limit is preferably 30% by mass or less, more preferably 25% by mass or less, even more preferably 20% by mass or less, particularly preferably 15% by mass or less, and most preferably 10% by mass or less. A content within the above range provides good physical properties in terms of the mechanical strength of the (meth)acrylic copolymer, and is also preferred in terms of cold resistance and oil resistance.

[0077] Crosslinkable Monomer Having a Crosslinking Group The (meth)acrylic copolymer used in the present invention may contain a structural unit derived from a crosslinkable monomer having a crosslinking group, and examples of the crosslinkable group include a carboxy group, an epoxy group, and a halogen group. Among these, a structural unit derived from a crosslinkable monomer having an epoxy group as a crosslinking group is preferred.

[0078] Examples of structural units derived from crosslinkable monomers having a carboxy group as a crosslinking group include structural units derived from ethylenically unsaturated monocarboxylic acids such as methacrylic acid, acrylic acid, crotonic acid, 2-pentenoic acid, and cinnamic acid; structural units derived from ethylenically unsaturated dicarboxylic acids such as fumaric acid, maleic acid, and itaconic acid; and structural units derived from ethylenically unsaturated dicarboxylic acid monoesters such as monoalkyl fumarates such as monomethyl fumarate, monoethyl fumarate, monopropyl fumarate, monobutyl fumarate, monohexyl fumarate, and monooctyl fumarate; monoalkyl maleates such as monomethyl maleate, monoethyl maleate, monopropyl maleate, monobutyl maleate, monopentyl maleate, and monodecyl maleate; and monoalkyl itaconic acid esters such as monomethyl itaconate, monoethyl itaconate, monopropyl itaconate, and monobutyl itaconate. These may be used alone or in combination of two or more types. A structural unit derived from an ethylenically unsaturated dicarboxylic acid monoester is preferred, a structural unit derived from a fumaric acid monoalkyl ester is more preferred, and a structural unit derived from a fumaric acid monoalkyl ester having an alkyl group having 1 to 4 carbon atoms is most preferred.

[0079] Examples of structural units derived from crosslinkable monomers having an epoxy group as a crosslinking group include structural units derived from epoxy group-containing (meth)acrylic acid esters such as glycidyl (meth)acrylate, structural units derived from epoxy group-containing styrenes such as p-vinylbenzyl glycidyl ether, and structural units derived from epoxy group-containing ethers such as allyl glycidyl ether and vinyl glycidyl ether, 3,4-epoxy-1-pentene, 3,4-epoxy-1-butene, 4,5-epoxy-2-pentene, 4-vinylcyclohexyl glycidyl ether, cyclohexenylmethyl glycidyl ether, 3,4-epoxy-1-vinylcyclohexene, and allylphenyl glycidyl ether. These may be used alone or in combination of two or more.

[0080] Examples of constituent units derived from crosslinkable monomers having a halogen group as a crosslinking group include constituent units derived from 2-chloroethyl vinyl ether, 2-chloroethyl acrylate, vinylbenzyl chloride, vinyl monochloroacetate, allyl chloroacetate, etc. These may be used alone or in combination of two or more.

[0081] The content of the structural unit derived from the crosslinkable monomer having a crosslinking group in the (meth)acrylic copolymer used in the present invention is, in all structural units, preferably 0% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.4% by mass or more as a lower limit.The upper limit is preferably 4% by mass or less, more preferably 3% by mass or less, and even more preferably 1.5% by mass or less.The content of the structural unit derived from the crosslinkable monomer having a crosslinking group in the above range is preferable in terms of physical properties such as strength and compression set, and processability.

[0082] Furthermore, the (meth)acrylic copolymer used in the present invention may contain, in addition to the above-mentioned structural units, structural units derived from other monomers copolymerizable therewith. Examples of other structural units include structural units derived from ethylenically unsaturated nitriles, structural units derived from acrylamide monomers, structural units derived from aromatic vinyl monomers, structural units derived from conjugated diene monomers, structural units derived from non-conjugated dienes, and structural units derived from other olefins. These may be used alone or in combination.

[0083] Examples of constituent units derived from ethylenically unsaturated nitriles include constituent units derived from compounds such as acrylonitrile, methacrylonitrile, α-methoxyacrylonitrile, and vinylidene cyanide.

[0084] Examples of structural units derived from acrylamide monomers include structural units derived from compounds such as acrylamide, methacrylamide, diacetone acrylamide, diacetone methacrylamide, N-butoxymethyl acrylamide, N-butoxymethyl methacrylamide, N-butoxyethyl acrylamide, N-butoxyethyl methacrylamide, N-methoxymethyl acrylamide, N-methoxymethyl methacrylamide, N-propioxymethyl acrylamide, N-propioxymethyl methacrylamide, N-methyl acrylamide, N-methyl methacrylamide, N,N-dimethyl acrylamide, N,N-dimethyl methacrylamide, N,N-diethyl acrylamide, N,N-diethyl methacrylamide, N-methylolacrylamide, N-methylol methacrylamide, ethacrylamide, crotonamide, cinnamic acid amide, maleindiamide, itacondiamide, methylmaleamide, methylitaconamide, maleimide, and itaconimide.

[0085] Examples of structural units derived from aromatic vinyl monomers include structural units derived from compounds such as styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene, o-ethylstyrene, p-ethylstyrene, α-fluorostyrene, p-trifluoromethylstyrene, p-methoxystyrene, p-aminostyrene, p-dimethylaminostyrene, p-acetoxystyrene, styrenesulfonic acid or a salt thereof, α-vinylnaphthalene, 1-vinylnaphthalene-4-sulfonic acid or a salt thereof, 2-vinylfluorene, 2-vinylpyridine, 4-vinylpyridine, divinylbenzene, diisopropenylbenzene, and vinylbenzyl chloride.

[0086] Examples of structural units derived from conjugated diene monomers include structural units derived from compounds such as 1,3-butadiene, 2-methyl-1,3-butadiene, 2-chloro-1,3-butadiene, 1,2-dichloro-1,3-butadiene, 2,3-dichloro-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-neopentyl-1,3-butadiene, 2-bromo-1,3-butadiene, 2-cyano-1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene, chloroprene, and piperylene.

[0087] Furthermore, examples of constituent units derived from non-conjugated dienes include constituent units derived from non-conjugated diene compounds such as 1,4-pentadiene, 1,4-hexadiene, ethylidenenorbornene, norbornadiene, and dicyclopentadiene.

[0088] Examples of structural units derived from other olefin-based monomers include esters such as dicyclopentadienyl acrylate, dicyclopentadienyl methacrylate, dicyclopentadienyl ethyl acrylate, and dicyclopentadienyl ethyl methacrylate, and structural units derived from compounds such as ethylene, propylene, vinyl chloride, vinylidene chloride, 1,2-dichloroethylene, vinyl acetate, vinyl fluoride, vinylidene fluoride, 1,2-difluoroethylene, vinyl bromide, vinylidene bromide, 1,2-dibromoethylene, ethyl vinyl ether, and butyl vinyl ether.

[0089] When the (meth)acrylic copolymer used in the present invention contains structural units derived from these other copolymerizable monomers, the content of these structural units in all structural units may be 0 to 15% by mass, 0 to 10% by mass, or 0 to 5% by mass.

[0090] In the (meth)acrylic copolymer used in the present invention, the content of the structural units can be determined by nuclear magnetic resonance spectroscopy of the obtained polymer.

[0091] <Method for producing (meth)acrylic copolymer> The (meth)acrylic copolymer used in the present invention can be obtained by polymerizing various monomers. The monomers used may be commercially available products and are not particularly limited.

[0092] The polymerization reaction may be carried out by any of emulsion polymerization, suspension polymerization, bulk polymerization, and solution polymerization. From the viewpoint of ease of control of the polymerization reaction, however, it is preferable to use suspension polymerization or emulsion polymerization, which are generally used as conventional methods for producing (meth)acrylic copolymers.

[0093] In the suspension polymerization method or emulsion polymerization method, the polymerization initiator, chain transfer agent, polymerization terminator, etc., which are commonly used, may be any of the conventionally known and commonly used agents.

[0094] The polymerization initiator is not particularly limited, and polymerization initiators generally used in suspension polymerization or emulsion polymerization can be used. Specific examples include inorganic polymerization initiators typified by persulfates such as potassium persulfate, sodium persulfate, and ammonium persulfate, organic peroxide polymerization initiators, and azo initiators. These polymerization initiators may be used alone or in combination of two or more.

[0095] The amount of polymerization initiator used is preferably in the range of 0.0001 to 0.1 parts by mass relative to 100 parts by mass of charged monomer. More specifically, the lower limit is preferably 0.0001 parts by mass or more, more preferably 0.0005 parts by mass or more, and even more preferably 0.001 parts by mass or more. The upper limit is preferably 0.1 parts by mass or less, more preferably 0.05 parts by mass or less, and even more preferably 0.01 parts by mass or less.

[0096] The chain transfer agent may be any of those commonly used in suspension polymerization or emulsion polymerization. Examples include alkyl mercaptans, xanthogen compounds, thiuram compounds, phenolic compounds, allyl compounds, halogenated hydrocarbon compounds, and vinyl ethers, and one or more of these may be used. The amount of these chain transfer agents is not particularly limited, but they are typically used in an amount of 0 to 0.1 parts by mass, and may be used in an amount of 0.01 to 0.05 parts by mass, per 100 parts by mass of the charged monomer.

[0097] The polymerization terminator may be any of those commonly used in suspension polymerization or emulsion polymerization. Examples include hydroxylamine, hydroxyamine sulfate, diethylhydroxyamine, hydroxyamine sulfonic acid and its alkali metal salts, sodium dimethyldithiocarbamate, and quinone compounds such as hydroquinone. The amount of the polymerization terminator used is not particularly limited, but is usually 0 to 2 parts by mass per 100 parts by mass of the charged monomer.

[0098] The emulsifier used in emulsion polymerization is not particularly limited, and commonly used nonionic emulsifiers and anionic emulsifiers can be used. Nonionic emulsifiers include, for example, polyoxyethylene alkyl ethers, polyoxyethylene alcohol ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene polycyclic phenyl ethers, polyoxyalkylene alkyl ethers, sorbitan fatty acid esters, polyoxyethylene fatty acid esters, and polyoxyethylene sorbitan fatty acid esters. Anionic emulsifiers include alkylbenzenesulfonates, alkyl sulfates, polyoxyethylene alkyl ether sulfates, polyoxyalkylene alkyl ether phosphates or their salts, fatty acid salts, and the like, and these may be used alone or in combination of two or more. Typical examples of anionic emulsifiers include sodium dodecyl sulfate, sodium dodecylbenzenesulfonate, and triethanolamine dodecyl sulfate.

[0099] The amount of emulsifier used in the present invention may be an amount generally used in emulsion polymerization. Specifically, it is preferably 0.01 to 10% by mass relative to the amount of charged monomer. More specifically, the lower limit is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and even more preferably 0.05% by mass or more. The upper limit is preferably 10% by mass or less, more preferably 7% by mass or less, and even more preferably 5% by mass or less. When a reactive surfactant is used as a monomer component, the addition of an emulsifier is not necessarily required.

[0100] The dispersant used in the suspension polymerization method is not particularly limited, and a commonly used dispersant can be used. For example, the emulsifiers used in emulsion polymerization can also be used as the dispersant, and examples thereof include nonionic polymer compounds such as polyvinyl alcohol, polyvinylpyrrolidone, polyethylene oxide, and cellulose derivatives, anionic polymer compounds such as polyacrylic acid and its salts, polymethacrylic acid and its salts, and rubbers of methacrylic acid esters and methacrylic acid and / or its salts, and poorly water-soluble inorganic compounds such as calcium phosphate, calcium carbonate, and aluminum hydroxide.

[0101] These dispersants can be used alone or in combination of two or more. The amount of dispersant used in the present invention may be any amount generally used in suspension polymerization. Specifically, the amount is preferably in the range of 0.01 to 10% by mass relative to the amount of charged monomer. More specifically, the amount is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and even more preferably 0.05% by mass or more. The upper limit is preferably 10% by mass or less, more preferably 7% by mass or less, and even more preferably 5% by mass or less.

[0102] Furthermore, the pH of the polymer obtained by the above method can be adjusted, if necessary, by using a base as a pH adjuster. Specific examples of bases include sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonia, inorganic ammonium compounds, and organic amine compounds. The pH range is preferably 1 to 11, more preferably 1.5 or higher, even more preferably 2 or higher, and more preferably 10.5 or lower, even more preferably 10 or lower.

[0103] In addition, if necessary, polymerization secondary materials such as particle size adjusters, chelating agents, and oxygen scavengers can be used.

[0104] Suspension polymerization and emulsion polymerization may be carried out in a batch, semi-batch, or continuous manner. The polymerization time and temperature are not particularly limited. They can be appropriately selected depending on the type of polymerization initiator used, etc., but generally, the polymerization temperature is 10 to 100°C, and the polymerization time is 0.5 to 100 hours.

[0105] The method for recovering the polymer obtained by the above method is not particularly limited, and any commonly used method can be used. One example of such a method is to continuously or batchwise supply the polymerization solution to an aqueous solution containing a coagulant, thereby obtaining a coagulated slurry. The temperature of the aqueous solution containing the coagulant is affected by coagulation conditions such as the type and amount of monomer used, and shear force due to stirring, and therefore cannot be uniformly specified, but is generally 50°C or higher, preferably in the range of 60°C to 100°C.

[0106] The coagulated slurry obtained by the above method is preferably washed with water to remove the coagulant. If washing with water is not performed at all or washing is insufficient, ion residues derived from the coagulant may be precipitated during the molding process.

[0107] The (meth)acrylic copolymer can be obtained by removing water from the coagulated slurry after washing with water and drying it. The drying method is not particularly limited, but is generally carried out using a flash dryer, a fluidized bed dryer, or the like. Furthermore, a dehydration step using a centrifuge or the like may be carried out before the drying step.

[0108] The polymer-grafted barium titanate-containing composition of the present invention may contain any of the following additives, so long as they do not impair the object of the present invention: crosslinking agents, crosslinking aids, lubricants, antioxidants, light stabilizers, fillers, reinforcing agents, plasticizers, processing aids, pigments, colorants, crosslinking accelerators, crosslinking retarders, antistatic agents, foaming agents, etc. These may be used alone or in combination.

[0109] As the crosslinking agent, conventionally known crosslinking agents that are usually used for crosslinking rubber, such as organic peroxides, polyamine compounds, polyepoxy compounds, polyisocyanate compounds, aziridine compounds, sulfur compounds, basic metal oxides, and organometallic halides, can be used.

[0110] In the polymer-grafted barium titanate-containing composition of the present invention, the amount of the crosslinking agent is 0.05 to 20 parts by mass, preferably 0.1 to 10 parts by mass, per 100 parts by mass of the matrix material. These crosslinking agents can be used alone or in combination of two or more.

[0111] Examples of reinforcing agents include carbon black, and in the polymer-grafted barium titanate-containing composition of the present invention, the content thereof is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 30 parts by mass or more, per 100 parts by mass of matrix material, and is preferably 120 parts by mass or less, and more preferably 100 parts by mass or less.

[0112] Examples of lubricants include metal soaps such as zinc stearate, calcium stearate, and magnesium stearate, higher fatty acids such as lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, and oleic acid, higher fatty acid esters such as methyl esters, isopropyl esters, butyl esters, and octyl esters of higher fatty acids, higher alcohols such as myristyl alcohol, cetyl alcohol, and stearyl alcohol, and hydrocarbon lubricants such as liquid paraffin, paraffin wax, and synthetic polyethylene wax. Among these, higher fatty acids are preferred in the present invention, and stearic acid is more preferred.

[0113] In the polymer-grafted barium titanate-containing composition of the present invention, the amount of lubricant blended is preferably 0.1 to 10 parts by mass per 100 parts by mass of matrix material. More specifically, the lower limit is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, and even more preferably 1 part by mass or more. The upper limit is preferably 10 parts by mass or less, more preferably 7 parts by mass or less, even more preferably 5 parts by mass or less, and particularly preferably 3 parts by mass or less.

[0114] Examples of antioxidants include amine-based, phosphate-based, quinoline-based, cresol-based, phenol-based, and dithiocarbamate metal salts. In the present invention, it is preferable to use amine-based and phenol-based antioxidants. These antioxidants may be used alone or in combination of two or more.

[0115] Examples of amine-based antiaging agents include phenyl-α-naphthylamine, phenyl-β-naphthylamine, p-(p-toluenesulfonylamido)-diphenylamine, 4,4′-bis(α,α-dimethylbenzyl)diphenylamine, N,N-diphenyl-p-phenylenediamine, N-isopropyl-N′-phenyl-p-phenylenediamine, and butyraldehyde-aniline condensates.

[0116] Examples of phenolic antioxidants include 2,6-di-t-butyl-4-methylphenol, 2,6-di-t-butylphenol, butylhydroxyanisole, 2,6-di-t-butyl-α-dimethylamino-p-cresol, octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, styrenated phenol, 2,2'-methylene-bis(6-α-methyl-benzyl-p-cresol), 4,4'-methylenebis( 2,6-di-t-butylphenol), 2,2'-methylene-bis(4-methyl-6-t-butylphenol), 2,4-bis[(octylthio)methyl]-6-methylphenol, 2,2'-thiobis-(4-methyl-6-t-butylphenol), 4,4'-thiobis-(6-t-butyl-o-cresol), 2,6-di-t-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenol, and the like.

[0117] In the polymer-grafted barium titanate-containing composition of the present invention, the content of the antioxidant is preferably 0.01 to 10 parts by mass, more preferably 0.01 to 5 parts by mass, and particularly preferably 0.03 to 3 parts by mass, per 100 parts by mass of the matrix material.

[0118] <Method for producing polymer-grafted barium titanate-containing composition> The method for producing the polymer-grafted barium titanate-containing composition of the present invention can be any means conventionally used in the field of polymer processing, such as a kneading method using an open roll, a Banbury mixer, various kneaders, etc., or a casting method using a dispersion in which rubber, resin, or polymer-grafted barium titanate is dispersed in a solvent, etc., and it is preferable to use a casting method in the present invention.

[0119] The blending procedure in the kneading method can be the same as that commonly used in the field of polymer processing. For example, first, the polymer alone is blended, and then the polymer-grafted barium titanate and other blending ingredients required depending on the application are added to obtain a polymer-grafted barium titanate-containing composition.

[0120] On the other hand, the compounding procedure in the casting method involves dispersing polymeric barium titanate and a matrix material (e.g., a (meth)acrylic copolymer) in a solvent to obtain a dispersion of a polymer-grafted barium titanate-containing composition, which is then poured into any molding frame or mold and the solvent is evaporated to obtain a polymer-grafted barium titanate-containing composition. When preparing the dispersion, any solvent can be used as long as it can dissolve or disperse the polymer-grafted barium titanate and the matrix material (e.g., a (meth)acrylic copolymer). The dispersion can be prepared by either mixing all the necessary ingredients, such as the polymer-grafted barium titanate and the matrix material (e.g., a (meth)acrylic copolymer), in a single container, or by preparing them in separate containers and then mixing them.

[0121] The dispersion method can be any method commonly used in the field of dispersion processing. Since it is a dispersion method using a dispersion liquid, there is no limitation as long as a wet dispersion method is used, and examples include a ball mill, a sand mill, a bead mill, a colloid mill, a jet mill, an ultrasonic homogenizer, a high-pressure homogenizer, and a vibration stirrer.

[0122] Furthermore, a crosslinking agent can be added to the polymer-grafted barium titanate-containing composition of the present invention to obtain a polymer-grafted barium titanate-containing composition for crosslinking. In the kneading method, a polymer-grafted barium titanate-containing composition for crosslinking can be obtained by adding a crosslinking agent, a crosslinking aid, etc., and kneading the resulting composition. In the casting method, a crosslinking agent with high solubility in a solvent can be directly dissolved in the solvent, or a crosslinking agent with low solubility in a solvent can be used, for example, by kneading a matrix substrate and then dissolving it in a solvent.

[0123] <Method for Producing Dielectric Sheet> The dielectric sheet of the present invention can be obtained by forming a polymer-grafted barium titanate-containing composition into a sheet.

[0124] In the case of the rubber kneading method, after obtaining a polymer-grafted barium titanate-containing composition, kneading may be carried out using an open roll or the like until a predetermined thickness is reached, or it may be carried out by any means used in the processing field, such as a molding method using a mold or the like.

[0125] In the case of the casting method, when the polymer-grafted barium titanate-containing composition is produced, any desired dielectric sheet can be formed by using a mold or die.

[0126] The crosslinking time for producing a crosslinked dielectric sheet from a crosslinking polymer-grafted barium titanate-containing composition varies depending on the temperature, but is typically between 0.5 and 300 minutes. Crosslinking molding may be performed in one step, or a previously molded polymer-grafted barium titanate-containing composition may be heated again to form a crosslinked product, or a crosslinked product may be heated first and then processed for molding. Specific methods for crosslinking molding include compression molding using a mold, injection molding, steam can, air bath, infrared radiation, or microwave heating, among other methods.

[0127] The dielectric sheet of the present invention can also be obtained by sheet-molding the polymer-grafted barium titanate of the present invention. That is, the dielectric sheet of the present invention can also be obtained by sheet-molding the polymer-grafted barium titanate of the present invention alone. In this case, as described above, the lower limit of the content of the polymer graft chains in 100% by mass of the polymer-grafted barium titanate of the present invention is preferably 5% by mass or more, more preferably 7% by mass or more, even more preferably 10% by mass or more, and particularly preferably 20% by mass or more. On the other hand, the upper limit is preferably 60% by mass or less, more preferably 55% by mass or less, and even more preferably 50% by mass or less. A content below the lower limit tends to make it impossible to directly form the sheet, while a content above the upper limit tends to make it difficult to obtain a dielectric sheet with a high dielectric constant. In this case, barium titanate other than the polymer-grafted barium titanate of the present invention may be used within a range that does not impede the objectives of the present invention. However, the content of the polymer-grafted barium titanate of the present invention in 100% by mass of the dielectric sheet of the present invention is preferably 95% by mass or more, more preferably 98% by mass or more, even more preferably 99% by mass or more, and may even be 100% by mass. Similarly, the content of the polymer-grafted barium titanate of the present invention in 100% by mass of the material to be formed into a sheet (the material before being formed into the dielectric sheet of the present invention) is preferably 95% by mass or more, more preferably 98% by mass or more, and even more preferably 99% by mass or more, and may be 100% by mass.

[0128] The polymer-grafted barium titanate of the present invention can be molded into a sheet in the same manner as in the rubber kneading method described above, thereby obtaining the dielectric sheet of the present invention obtained by molding the polymer-grafted barium titanate of the present invention.

[0129] Since the dielectric sheet of the present invention has high dielectric properties, it is possible to provide a material that has high piezoelectric and dielectric properties and excellent flexibility, and it can be suitably used as electroacoustic conversion devices such as speakers and buzzers, actuators, tactile displays, sensors, power generation devices, etc.

[0130] The present invention will be specifically explained by way of examples and comparative examples, but the present invention is not limited to these.

[0131] [Production Example 1] (Production of Radical Polymerization Initiator Group-Introduced Barium Titanate) Synthesis of Hydroxyl Group-Introduced Barium Titanate: 100 parts by weight of barium titanate (product name: Parceram BT, manufactured by Nippon Chemical Industry Co., Ltd.) and 500 parts by weight of 30% by weight hydrogen peroxide solution were added to a flask and the mixture was refluxed at 105°C for 4 hours. After the reaction was completed, the mixture was repeatedly washed with ion-exchanged water and centrifuged, and then dried under reduced pressure at 80°C for 12 hours to obtain hydroxyl group-introduced barium titanate. Reaction of Hydroxyl Group-Introduced Barium Titanate with Silane Coupling Agent: 100 parts by weight of hydroxyl group-introduced barium titanate, 50 parts by weight of γ-aminopropyltriethoxysilane, and 800 parts by weight of toluene were added to a flask and heated to 80°C for 24 hours. After the reaction was completed, the mixture was repeatedly washed with toluene and centrifuged, and then dried under reduced pressure at 80°C for 12 hours to obtain amino group-bonded barium titanate (amino group-containing barium titanate). Synthesis of radical polymerization initiator group-introduced barium titanate: 240 parts by mass of dichloromethane as a solvent, 1.4 parts by mass of triethylamine, and 48 parts by mass of amino group-containing barium titanate were added to a flask, and while maintaining the reaction solution at 0°C, 2.7 parts by mass of 2-bromoisobutyryl bromide as a radical polymerization initiator group-introducing compound was added dropwise over 1 hour. After the dropwise addition, the reaction was carried out at 0°C for 3 hours. The reaction was then carried out at room temperature for 20 hours. After completion of the reaction, the mixture was repeatedly washed with dichloromethane and centrifuged, and then dried under reduced pressure at 80°C for 12 hours to obtain radical polymerization initiator group-introduced barium titanate.

[0132] [Example 1] (Production of polymer-grafted barium titanate A) A polymerization reactor equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a pressure reducing device was charged with 100 parts by mass of barium titanate having radical polymerization initiator groups introduced therein, 2500 parts by mass of DMF as a solvent, 12.5 parts by mass of copper (I) bromide, 15 parts by mass of N,N,N',N'',N''-pentamethyldiethylenetriamine, 195 parts by mass (65% by mass) of ethyl acrylate so that the total amount of monomers was three times the amount by mass of barium titanate, 90 parts by mass (30% by mass) of n-butyl acrylate, and 15 parts by mass (5% by mass) of glycidyl methacrylate. After sufficient removal of oxygen by repeated degassing and nitrogen substitution under reduced pressure, the temperature inside the polymerization reactor was raised to 60 ° C., and the polymerization reaction was carried out for 24 hours. After completion of the reaction, washing with acetone and centrifugation were repeated, and the mixture was dried under reduced pressure at 80 ° C. for 12 hours to obtain polymer-grafted barium titanate A. The glass transition temperature of the polymer graft chain was −29° C. according to the Fox equation. Furthermore, the following evaluations were carried out, and the results are shown in Table 1. The mass % in parentheses indicates the composition ratio of each monomer (the content of the constituent units derived from each monomer in 100 mass % of the polymer graft chain) when the composition of the polymer graft chain is taken as 100 mass %.

[0133] <Measurement of polymer content> For the polymer-grafted barium titanate A obtained in Example 1, a thermogravimetric (TG) analyzer (product name: TG / DTA6300, manufacturer: Hitachi High-Tech Corporation) was used to measure the mass change of the polymer-grafted barium titanate A at 30 to 500°C while supplying nitrogen gas at 200 mL / min, thereby measuring the polymer content (mass%).

[0134] <Evaluation of Dispersibility> Polymer-grafted barium titanate A was mixed in a toluene solvent at a solid content of 10 wt %, dispersed for 5 minutes using an ultrasonic cleaner, and then left to stand for 24 hours, after which the state was visually observed. ◯: No sedimentation of barium titanate was observed, and the dispersibility was good. ×: Sedimentation of barium titanate was observed, and the mixture was separated into a supernatant and a sediment.

[0135] Example 2 (Production of Polymer-Grafted Barium Titanate B) A polymerization reactor equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a pressure reducing device was charged with 100 parts by mass of barium titanate to which a radical polymerization initiator group had been introduced, 2500 parts by mass of DMF as a solvent, 12.5 parts by mass of copper (I) bromide, 15 parts by mass of N,N,N',N",N"-pentamethyldiethylenetriamine, 150 parts by mass (50% by mass) of ethyl acrylate and 150 parts by mass (50% by mass) of n-butyl acrylate so that the total amount of monomers was three times the mass ratio of the barium titanate. Degassing and nitrogen substitution were repeated under reduced pressure to thoroughly remove oxygen, and the temperature inside the polymerization reactor was then raised to 60°C, and the polymerization reaction was carried out for 24 hours. After completion of the reaction, the mixture was repeatedly washed with acetone and centrifuged, and then dried under reduced pressure at 80°C for 12 hours to obtain polymer-grafted barium titanate B. The glass transition temperature of the polymer graft chain was determined to be -39°C according to the Fox equation. The above evaluations were performed, and the results are shown in Table 1.

[0136] Example 3 (Production of Polymer-Grafted Barium Titanate C) A polymerization reactor equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a pressure reducing device was charged with 100 parts by mass of barium titanate to which a radical polymerization initiator group had been introduced, 2500 parts by mass of DMF as a solvent, 12.5 parts by mass of copper (I) bromide, 15 parts by mass of N,N,N',N",N"-pentamethyldiethylenetriamine, and 300 parts by mass (100% by mass) of ethyl acrylate so that the total amount of monomers was three times the mass of the barium titanate. Degassing and nitrogen substitution were repeated under reduced pressure to thoroughly remove oxygen, and the temperature inside the polymerization reactor was then raised to 60°C, and the polymerization reaction was carried out for 24 hours. After completion of the reaction, the mixture was repeatedly washed with acetone and centrifuged, and then dried under reduced pressure at 80°C for 12 hours to obtain polymer-grafted barium titanate C. The glass transition temperature of the polymer graft chain was determined to be -20°C according to the Fox equation. The above evaluations were performed, and the results are shown in Table 1.

[0137] Example 4 (Production of Polymer-Grafted Barium Titanate D) A polymerization reactor equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a pressure reducing device was charged with 100 parts by mass of barium titanate to which a radical polymerization initiator group had been introduced, 2500 parts by mass of DMF as a solvent, 12.5 parts by mass of copper (I) bromide, 15 parts by mass of N,N,N',N",N"-pentamethyldiethylenetriamine, 210 parts by mass (70% by mass) of ethyl acrylate and 90 parts by mass (30% by mass) of methyl methacrylate so that the total amount of monomers was three times the mass ratio of barium titanate. The polymerization reactor was then repeatedly degassed and substituted with nitrogen under reduced pressure to thoroughly remove oxygen, and the temperature inside the polymerization reactor was raised to 60°C, and the polymerization reaction was carried out for 24 hours. After completion of the reaction, the mixture was repeatedly washed with acetone and centrifuged, and then dried under reduced pressure at 80°C for 12 hours to obtain polymer-grafted barium titanate D. The glass transition temperature of the polymer graft chain was 8°C according to the Fox equation. The above evaluations were performed, and the results are shown in Table 1.

[0138] Comparative Example 1 (Production of Polymer-Grafted Barium Titanate E) A polymerization reactor equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a pressure reducing device was charged with 100 parts by mass of barium titanate to which a radical polymerization initiator group had been introduced, 2500 parts by mass of DMF as a solvent, 12.5 parts by mass of copper (I) bromide, 15 parts by mass of N,N,N',N",N"-pentamethyldiethylenetriamine, and 300 parts by mass (100% by mass) of methyl methacrylate so that the total amount of monomers was three times the mass of the barium titanate. The polymerization reactor was then repeatedly degassed under reduced pressure and substituted with nitrogen to thoroughly remove oxygen, after which the temperature inside the polymerization reactor was raised to 60°C and the polymerization reaction was carried out for 24 hours. After completion of the reaction, the mixture was repeatedly washed with acetone and centrifuged, and then dried under reduced pressure at 80°C for 12 hours to obtain polymer-grafted barium titanate E. The glass transition temperature of the polymer graft chain was 105°C according to the Fox equation. The above evaluations were carried out, and the results are shown in Table 1.

[0139]

[0140] The polymer-grafted barium titanate obtained in Examples 1 to 4 had high dispersibility in solvents because the glass transition temperature of the polymer graft chains was 30°C or lower, and the barium titanate did not settle even after 24 hours of standing, maintaining sufficient dispersibility. On the other hand, when the glass transition temperature of the polymer graft chains was high, as in Comparative Example 1, the dispersion ability in solvents was low, and after 24 hours of standing, the barium titanate clearly separated into two layers: barium titanate and solvent. From this, it can be said that the polymer-grafted barium titanate of the present invention, which has polymer graft chains with low glass transition temperatures, has good dispersibility.

[0141] Example 5: 100 parts by weight of a (meth)acrylic copolymer (Lacrestar EC, manufactured by Osaka Soda Co., Ltd., crosslinking group: epoxy group, sp value: 10.1) containing 1.2 parts by weight of ammonium benzoate as a vulcanizing agent was dissolved in 775 parts by weight of toluene to prepare an acrylic copolymer varnish. Furthermore, 11.63 parts by weight of the polymer-grafted barium titanate A obtained in Example 1 (10 parts by weight of barium titanate contained in polymer-grafted barium titanate A) was added to 125 parts by weight of toluene and ultrasonicated (at room temperature for 5 minutes) to prepare a polymer-grafted barium titanate dispersion. The polymer-grafted barium titanate dispersion was mixed with the acrylic copolymer varnish and stirred to prepare a polymer-grafted barium titanate-containing composition dispersion. The polymer-grafted barium titanate-containing composition dispersion was then poured into a Petri dish and allowed to air-dry to volatilize the toluene, yielding a dielectric sheet. The resulting dielectric sheet was evaluated as follows, and the results are shown in Table 2.

[0142] <Measurement of relative permittivity> The dielectric sheet obtained above was heated in an oven at 180°C for 15 minutes to obtain a crosslinked dielectric sheet. Using the crosslinked dielectric sheet, a relative permittivity measurement test was performed by the automatic balancing bridge method described in JIS C 2138;2007 using a chemical impedance analyzer IM3590 manufactured by Hioki E.E. Corporation and a dielectric test fixture 16451B manufactured by Keysight Technologies.

[0143] [Example 6] A dielectric sheet was produced in the same manner as in Example 5, except that the amount of polymer-grafted barium titanate A added was changed to 34.9 parts by mass (30 parts by mass of barium titanate contained in polymer-grafted barium titanate A). The above evaluations were carried out, and the results are shown in Table 2.

[0144] Comparative Example 2 A dielectric sheet was produced using only the acrylic copolymer in the same manner as in Example 5. The above evaluations were carried out, and the results are shown in Table 2.

[0145] Comparative Example 3 A dielectric sheet was produced in the same manner as in Example 5, except that the polymer-grafted barium titanate A was replaced with 10 parts by mass of untreated barium titanate. The above evaluations were carried out, and the results are shown in Table 2.

[0146] Comparative Example 4 A dielectric sheet was produced in the same manner as in Example 5, except that polymer-grafted barium titanate A was changed to polymer-grafted barium titanate E and the amount charged was changed to 15.2 parts by mass (10 parts by mass of barium titanate contained in polymer-grafted barium titanate E). The above evaluations were carried out, and the results are shown in Table 2.

[0147]

[0148] It can be seen that the dielectric sheet containing the polymer-grafted barium titanate of the present invention has high dielectric properties even with a small amount compared to Comparative Example 2, which does not contain barium titanate. On the other hand, Comparative Example 3, which used untreated barium titanate, showed the same dielectric constant as Comparative Example 2. Furthermore, Comparative Example 4, which introduced a high-glass-transition-temperature polymer graft chain, was difficult to mold into a uniform sheet, making it impossible to measure the dielectric constant. Comparative Example 3, which used untreated barium titanate, showed the same dielectric constant as Comparative Example 2, which is presumably due to the poor dispersibility of untreated barium titanate in rubber. On the other hand, the polymer-grafted barium titanate of the present invention has high dispersibility in solvents and rubber, so an increase in the dielectric constant was observed even with the addition of a small amount.

[0149] Example 7 The polymer-grafted barium titanate C obtained in Example 3 was directly formed into a sheet without using a matrix material, and the sheet-forming evaluation was carried out. The results are shown in Table 3.

[0150] <Sheet Formation Evaluation> The polymer-grafted barium titanate C obtained in Example 3 was pressed at 180°C for 5 minutes to produce a sheet measuring 6 cm x 6 cm and 1 mm thick. The evaluation was carried out as follows: ∘: Flexible, the sheet did not break when bent. ×: Not flexible, the sheet broke when bent.

[0151] Example 8 The polymer-grafted barium titanate D obtained in Example 4 was directly formed into a sheet without using a matrix material, and the sheet-forming evaluation was carried out. The results are shown in Table 3.

[0152] Example 9 (Production of Polymer-Grafted Barium Titanate F) A polymerization reactor equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a pressure reducing device was charged with 100 parts by mass of barium titanate to which a radical polymerization initiator group had been introduced, 2500 parts by mass of DMF as a solvent, 12.5 parts by mass of copper (I) bromide, 15 parts by mass of N,N,N',N'',N''-pentamethyldiethylenetriamine, 150 parts by mass (50% by mass) of ethyl acrylate so that the total amount of monomers was three times the mass of barium titanate, and 150 parts by mass (50% by mass) of 2-methoxyethyl acrylate. After sufficient removal of oxygen by repeated degassing and nitrogen substitution under reduced pressure, the polymerization reactor was heated to 60°C, and the polymerization reaction was carried out for 96 hours. After completion of the reaction, washing with acetone and centrifugation were repeated, and the mixture was dried under reduced pressure at 80°C for 12 hours to obtain polymer-grafted barium titanate E. The glass transition temperature of the polymer graft chain was -36°C according to the Fox equation. The polymer content was measured and the sheet formation was evaluated. The results are shown in Table 3.

[0153] [Example 10] (Production of polymer-grafted barium titanate G) A polymerization reactor equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a pressure reducing device was charged with 100 parts by mass of barium titanate to which a radical polymerization initiator group had been introduced, 2500 parts by mass of DMF as a solvent, 12.5 parts by mass of copper (I) bromide, 15 parts by mass of N,N,N',N'',N''-pentamethyldiethylenetriamine, 270 parts by mass (90% by mass) of ethyl acrylate so that the total amount of monomers was three times the amount by mass of barium titanate, and 30 parts by mass (10% by mass) of dibutyl maleate. After sufficient removal of oxygen by repeated degassing and nitrogen substitution under reduced pressure, the polymerization reactor was heated to 60 ° C., and the polymerization reaction was carried out for 96 hours. After completion of the reaction, washing with acetone and centrifugation were repeated, and the mixture was dried under reduced pressure at 80 ° C. for 12 hours to obtain polymer-grafted barium titanate F. The glass transition temperature of the polymer graft chain was -20 ° C. according to the Fox equation. The polymer content was measured and the sheet formation was evaluated. The results are shown in Table 3.

[0154] [Example 11] (Production of polymer-grafted barium titanate H) A polymerization reactor equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a pressure reducing device was charged with 100 parts by mass of barium titanate to which a radical polymerization initiator group had been introduced, 2500 parts by mass of DMF as a solvent, 12.5 parts by mass of copper (I) bromide, 15 parts by mass of N,N,N',N'',N''-pentamethyldiethylenetriamine, 297 parts by mass (99% by mass) of ethyl acrylate so that the total amount of monomers was three times the amount by mass of barium titanate, and 3 parts by mass (1% by mass) of glycidyl methacrylate. After sufficient removal of oxygen by repeated degassing and nitrogen substitution under reduced pressure, the polymerization reactor was heated to 60 ° C., and the polymerization reaction was carried out for 24 hours. After completion of the reaction, washing with acetone and centrifugation were repeated, and the mixture was dried under reduced pressure at 80 ° C. for 12 hours to obtain polymer-grafted barium titanate G. The glass transition temperature of the polymer graft chain was -20 ° C. according to the Fox equation. The polymer content was measured and the sheet formation was evaluated. The results are shown in Table 3.

[0155] Comparative Example 5 (Production of Polymer-Grafted Barium Titanate I) A polymerization reactor equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a pressure reducing device was charged with 100 parts by mass of barium titanate to which a radical polymerization initiator group had been introduced, 2500 parts by mass of DMF as a solvent, 12.5 parts by mass of copper (I) bromide, 15 parts by mass of N,N,N',N'',N''-pentamethyldiethylenetriamine, 120 parts by mass (40% by mass) of ethyl acrylate so that the total amount of monomers was three times the amount by mass of barium titanate, and 180 parts by mass (60% by mass) of methyl methacrylate. After sufficient removal of oxygen by repeated degassing and nitrogen substitution under reduced pressure, the temperature inside the polymerization reactor was raised to 60 ° C., and the polymerization reaction was carried out for 24 hours. After completion of the reaction, washing with acetone and centrifugation were repeated, and the mixture was dried under reduced pressure at 80 ° C. for 12 hours to obtain polymer-grafted barium titanate I. The glass transition temperature of the polymer graft chain was 43 ° C. according to the Fox equation. The polymer content was measured and the sheet formation was evaluated. The results are shown in Table 3.

[0156]

[0157] It was found that the polymer-grafted barium titanate of the present invention can be used to directly produce dielectric sheets without using a matrix material, provided that the polymer constituting the polymer graft chain has a glass transition temperature of 30°C or less and the polymer graft chain content is 5% by mass or more. Using the polymer-grafted barium titanate of the present invention eliminates the need for a dispersion during kneading, and also eliminates the need for a matrix material. This suggests that the elimination of processes can help reduce manufacturing costs and mitigate the risk of poor dispersion during dielectric sheet production.

[0158] Since the dielectric sheet of the present invention has high dielectric properties, it is possible to provide a material that has high piezoelectric and dielectric properties and excellent flexibility, and it can be suitably used as electroacoustic conversion devices such as speakers and buzzers, actuators, tactile displays, sensors, power generation devices, etc.

Claims

1. Polymer-grafted barium titanate, in which the surface of a barium titanate particle is modified with a polymer graft chain, and the glass transition temperature of the polymer constituting the polymer graft chain is 30°C or lower.

2. The polymer-grafted barium titanate according to claim 1, wherein the polymer constituting the polymer graft chain is a polymer containing a structural unit derived from an acrylic ester.

3. A polymer-grafted barium titanate-containing composition comprising at least the polymer-grafted barium titanate according to claim 1 and a matrix material, the content of the polymer-grafted barium titanate being 1 to 50 parts by mass per 100 parts by mass of the matrix material.

4. The polymer-grafted barium titanate-containing composition according to claim 3, wherein said matrix material is a (meth)acrylic copolymer.

5. A dielectric sheet obtained by molding the polymer-grafted barium titanate-containing composition according to claim 3 or 4.

6. A dielectric sheet obtained by molding the polymer-grafted barium titanate according to claim 1 or 2, in which the content of the polymer graft chains is 5 mass % or more.

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