JP2006257414A - Magnetic particulate for use in magnetic heat-sensitive electronic paper - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic particulate for use in magnetic heat-sensitive electronic paper which gives electronic paper with good contrast and a high response speed. <P>SOLUTION: The magnetic particulate, composed of a hollow particulate consisting of a shell and a hollow part which includes a magnetic material, is characterized in that the shell contains a layer which is constituted of a polymer from a crosslinkable monomer, a copolymer from two or more kinds of crosslinkable monomers, or a copolymer from a crosslinkable monomer and a monofunctional monomer, and has the following characteristics: (a) a specific gravity of the particulate: 0.5-1.4, (b) a magnetic material content: 2-35% by weight, (c) an average particle diameter: 0.1-50 μm, and (d) a variation coefficient of an average particle diameter: 20 or less. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to magnetic fine particles used for display in magnetic thermal electronic paper, electronic paper, and a method for displaying information using the electronic paper.

  Electronic paper has the advantages of paper that can continue to be displayed without power supply, can be read freely, has a wide viewing angle, is flexible and can be scribbled, and can be easily written down. It has the advantage of electronic display that information can be processed and erased freely.

  As such electronic paper, a magnetic heat-sensitive electronic paper has been proposed (Patent Documents 1 and 2). Magnetic heat-sensitive electronic paper is obtained by sandwiching a normal white wax or resin having a low melting point mixed with magnetic fine particles between a pair of substrates at least one of which is transparent. To display information using this, a magnetic head is applied in a direction substantially perpendicular to the substrate to form a magnetic field, and then partially heated using a thermal head from the lower substrate side. To display information. That is, when the wax or resin is melted by bringing the thermal head into contact with the substrate in the magnetic field, the magnetic fine particles in the wax or resin can move freely and move to the upper substrate side according to the magnetic field. Since white pigment such as titanium oxide is usually added to the low melting point wax or resin, the magnetic particles do not appear on the upper substrate side but appear white in the portion where they are in a solid state, but the magnetic fine particles are on the upper substrate side. The part moved to is displayed in black.

  This magnetic thermosensitive electronic paper has an advantage that a high resolution corresponding to the thickness of the thermal head can be obtained and the used material can be recycled.

  As magnetic fine particles for magnetic thermosensitive electronic paper, solid fine particles comprising a core made of powdered ferrite or spherical sintered ferrite solidified with a binder and a resin coating layer covering the core have been proposed (Patent Document 1, 2).

However, this magnetic fine particle has a drawback that the contrast of the recording portion and the response speed are poor.
JP 2003-302660 A JP 2004-4412 A

  It is an object of the present invention to provide magnetic fine particles that provide an electronic paper with good contrast and high response speed when used in magnetic recording type electronic paper.

In order to solve the above-mentioned problems, the present inventors have conducted research and obtained the following findings (i) to (iii).
(i) When the magnetic fine particles used in the magnetic thermosensitive electronic paper have the following characteristics (a) to (d), an electronic paper having good contrast and high response speed can be obtained.

(a) Specific gravity of fine particles: 0.5 to 1.4
(b) Magnetic substance content: 2 to 35% by weight
(c) Average particle diameter: 0.1 to 50 μm
(d) Coefficient of variation of average particle size: 20% or less
(ii) Since the magnetic fine particles have a shell and a hollow portion, and the magnetic material is included in the hollow portion, the specific gravity can be easily adjusted.
(iii) By including a crosslinkable monomer in the shell, a hollow structure can be obtained, and the particle shape can be maintained in the wax or resin.

  The present invention has been completed based on the above findings, and provides the following magnetic fine particles.

Item 1. Magnetic fine particles in which a magnetic substance is encapsulated in a hollow part of a hollow fine particle comprising a shell and a hollow part, wherein the shell is a polymer of a crosslinkable monomer, a copolymer of two or more crosslinkable monomers, or a crosslinkable monomer A magnetic fine particle comprising a layer formed of a copolymer of a monofunctional monomer and having the following characteristics (a) to (d).
(a) Specific gravity of fine particles: 0.5 to 1.4
(b) Magnetic substance content: 2 to 35% by weight
(c) Average particle diameter: 0.1 to 50 μm
(d) Coefficient of variation of average particle diameter: 20% or less Item 2. Item 2. The magnetic fine particle according to Item 1, wherein the shell comprises a layer composed of a polymer of a crosslinkable monomer or a copolymer of two or more crosslinkable monomers.

  Item 3. Item 3. The magnetic fine particle according to Item 1 or 2, wherein the magnetic material is at least one selected from the group consisting of iron simple substance, iron compound, cobalt simple substance, cobalt compound, nickel simple substance, and nickel compound.

  Item 4. Item 4. The magnetic fine particle according to any one of Items 1 to 3, wherein a pigment is encapsulated in a hollow portion of the hollow fine particle.

  Item 5. Item 5. The magnetic fine particle according to any one of Items 1 to 4, wherein the average value of the volume ratio R of the hollow portion of the magnetic fine particle represented by the following formula is 10 to 80%.

R (%) = (rh / rp) ³ × 100
(In the formula, rh is the radius of the hollow portion of the magnetic fine particles, and rp is the radius of the magnetic fine particles.)
Item 6. Item 6. The magnetic fine particles according to any one of Items 1 to 5 for magnetic thermal electronic paper.

  Item 7. An electronic paper in which a composition containing the magnetic fine particles according to any one of Items 1 to 5 and a low melting point medium is sandwiched between a pair of substrates at least one of which is transparent.

  Item 8. With respect to the electronic paper of Item 7, in a state where a magnetic field is formed in the substrate thickness direction with respect to the substrate pair, the electronic paper is partially heated to melt the low-melting-point medium in the portion, and in accordance with the magnetic field in the portion An information display method for displaying information by moving magnetic fine particles to the transparent substrate side.

Hereinafter, the present invention will be described in detail.
(I) Magnetic fine particles
Basic Structure The magnetic fine particle of the present invention is a magnetic fine particle in which a magnetic substance is encapsulated in a hollow part of a hollow fine particle comprising a shell and a hollow part, and the shell is a polymer of a crosslinkable monomer or two or more kinds of crosslinkable substances. It includes a layer composed of a copolymer of monomers or a copolymer of a crosslinkable monomer and a monofunctional monomer, and has the following characteristics (a) to (d).
(a) Specific gravity of fine particles: 0.5 to 1.4
(b) Magnetic substance content: 2 to 35 wt%
(c) Average particle diameter: 0.1 to 50 μm
(d) Variation coefficient of average particle diameter: 20% or less In the magnetic fine particles of the present invention, it is important that the specific gravity, magnetic substance content, average particle diameter, and variation coefficient of average particle diameter are in the above ranges. Only when these are in the above range, practical electronic paper can be obtained while achieving both contrast and responsiveness.
Specific gravity The specific gravity referred to in the present invention is an apparent specific gravity, and is a value that can be measured by the floatation method, although it is calculated from the ratio of monomers, magnetic material, and solvent. The specific gravity is more preferably about 0.6 to 0.9.

  When a portion to be displayed of a low melting point wax or resin (hereinafter also abbreviated as “wax or the like”) is melted by heat, a portion adjacent to the portion is weakly heated and softened. Therefore, if the specific gravity is too small, the magnetic fine particles move in accordance with the magnetic field in the softened wax in the adjacent portion, resulting in poor print contrast. On the other hand, if the specific gravity is too large, the magnetic fine particles cannot rapidly move (usually move upward) in accordance with the magnetic field in the display portion, resulting in poor responsiveness. Within the above specific gravity range, the electronic paper has good contrast and high response speed.

The specific gravity can be adjusted to the above range by adjusting the ratio of the hollow portion, that is, the porosity, the type of the shell constituent material, and the magnetic substance content. What is necessary is just to set the optimal value of specific gravity suitably in the said range according to specific gravity of the wax etc. to be used.
Magnetic magnetism is determined by the type of magnetic material and its content. In the present invention, the content of the magnetic material is set to 2 to 35% by weight with respect to the total amount of the capsule, and this content is a value measured by thermogravimetric analysis. The content of the magnetic material is more preferably about 5 to 20% by weight.

If the content of the magnetic substance is too small, even if the wax or the like melts, it cannot move according to the magnetic field. On the other hand, if the content of the magnetic material is too large, the weight of the magnetic material increases and the specific gravity cannot be made 0.5 to 1.4. Such a problem does not occur within the above range of the magnetic substance content.
In an average particle size present invention, the average particle size using an optical microscope or a scanning microscope to measure the particle diameters of 100 particles, the average value. The average particle size is preferably about 1 to 20 μm.

  If the average particle size of the fine particles is too small, the amount of magnetic substance held per fine particle decreases, so that it is necessary to increase the number of magnetic fine particles in order to display at a constant concentration. As a result, the viscosity of wax or the like containing magnetic fine particles becomes high, making it difficult for the fine particles to move, resulting in poor responsiveness. On the contrary, if the average particle size is too large, the magnetic fine particles are difficult to move in the molten wax or the like, and the response speed becomes slow. If it is the said range, such a problem will not arise.

A method for adjusting the average particle diameter will be described later.
Variation coefficient of average particle diameter The variation coefficient of average particle diameter is a value obtained by dividing the standard deviation of the average particle diameter by the average value, and represents a particle size distribution. The coefficient of variation of the average particle diameter is preferably 15% or less.

If the coefficient of variation is too large, the display portion is blurred and the contrast is deteriorated. However, if it is in the above range, a good contrast can be obtained. A method for making the particle size as uniform as possible and setting the coefficient of variation within the above range will be described later.
The shell constituting material shell includes a layer composed of a polymer or copolymer of a crosslinkable monomer, or a copolymer of a crosslinkable monomer and a monofunctional monomer. The shell is mainly composed of this layer, but depending on the method for producing fine particles, an auxiliary polymer may adhere to the shell inner surface or a thin layer of the auxiliary polymer may be formed as described later. . In some cases, the shell consists of only the above layers.

By using a crosslinkable monomer, the fine particles can have a hollow structure and can maintain the shape of the fine particles in a wax or the like. Although all the constituent monomers of the polymer may be crosslinkable monomers, the cost increases accordingly. When a monofunctional monomer and a crosslinkable monomer are used, the ratio of the crosslinkable monomer is preferably 10% or more, more preferably 30% or more in terms of molar ratio.
<Crosslinking monomer>
Examples of the crosslinkable monomer include polymerizable functional groups, particularly polyfunctional monomers having two or more polymerizable double bonds (particularly 2 to 4). In particular, a polyfunctional monomer having two or more polymerizable C═C double bonds (particularly 2 to 4) is preferred. Examples thereof include divinylbenzene, divinylbiphenyl, divinylnaphthalene, diallyl phthalate, triallyl cyanurate, ethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, and the like. Particularly preferred are divinylbenzene and ethylene glycol dimethacrylate, and most preferred is ethylene glycol dimethacrylate. A crosslinkable monomer can be used individually by 1 type or in mixture of 2 or more types.
<Monofunctional monomer>
Examples of monofunctional monomers include monovinyl aromatic monomers, acrylic monomers, vinyl ester monomers, vinyl ether monomers, monoolefin monomers, halogenated olefin monomers, A diolefin etc. are mentioned.

  Examples of the monovinyl aromatic monomer include a monovinyl aromatic hydrocarbon represented by the following general formula (1), a vinylbiphenyl optionally substituted with a lower (1 to 4 carbon) alkyl group, and a lower (carbon number). 1-4) Vinyl naphthalene which may be substituted with an alkyl group.

[Wherein, R ¹ represents a hydrogen atom, a lower (1 to 4 carbon atoms) alkyl group or a halogen atom, and R ² represents a hydrogen atom, a lower (1 to 4 carbon atoms) alkyl group, a halogen atom, —SO 2 _{3 represents a} Na group, a lower (C1-4) alkoxy group, an amino group or a carboxyl group. ]
In the general formula (1), R ¹ is preferably a hydrogen atom, a methyl group or a chlorine atom, and R ² is preferably a hydrogen atom, a chlorine atom, a methyl group or a —SO ₃ Na group.

  Specific examples of the monovinyl aromatic hydrocarbon represented by the general formula (1) include styrene, α-methylstyrene, vinyltoluene, α-chlorostyrene, o-chlorostyrene, m-chlorostyrene, p-chlorostyrene, Examples include sodium styrene sulfonate.

  Furthermore, vinyl biphenyl which may be substituted with a lower alkyl group, and vinyl naphthalene which may be substituted with a lower alkyl group include vinyl biphenyl substituted with a lower alkyl group such as vinyl biphenyl, methyl group and ethyl group. And vinylnaphthalene substituted with a lower alkyl group such as vinylnaphthalene, methyl group, and ethyl group.

  Moreover, as said acrylic monomer, the acrylic monomer represented by following General formula (2) is mentioned.

[Wherein R ³ represents a hydrogen atom or a lower (1 to 4 carbon atoms) alkyl group, and R ⁴ represents a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, a phenyl group, or a hydroxy group having 1 to 6 carbon atoms. An alkyl group, a lower (C 1-4) aminoalkyl group or a di (C ₁ -C ₄ alkyl) amino- (C ₁ -C ₄ ) alkyl group is shown. ]
In the general formula (2), R ³ is preferably a hydrogen atom or a methyl group, and R ⁴ is a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, a phenyl group, or lower (1 to 4 carbon atoms) hydroxy. An alkyl group and a lower (C1-4) aminoalkyl group are preferred.

  Specific examples of the acrylic monomer include acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, cyclohexyl acrylate, phenyl acrylate, methyl methacrylate, and methacrylic acid. Hexyl, 2-ethylhexyl methacrylate, β-hydroxyethyl acrylate, γ-hydroxybutyl acrylate, δ-hydroxybutyl acrylate, β-hydroxyethyl methacrylate, γ-aminopropyl acrylate, γ-N, N acrylate -Diethylaminopropyl and the like.

  Examples of the vinyl ester monomer include those represented by the following general formula (3).

[Wherein, R ⁵ represents a hydrogen atom or a lower (1 to 4 carbon atoms) alkyl group. ]
Specific examples of the vinyl ester monomer include vinyl formate, vinyl acetate, vinyl propionate and the like.

  Examples of the vinyl ether monomer include vinyl ether monomers represented by the following general formula (4).

[R ⁶ represents an alkyl group having 1 to 12 carbon atoms, a phenyl group, or a cyclohexyl group. ]
Specific examples of the vinyl ether monomer include vinyl methyl ether, vinyl ethyl ether, vinyl n-butyl ether, vinyl phenyl ether, vinyl cyclohexyl ether and the like.

  Examples of the monoolefin monomer include those represented by the following general formula (5).

[In formula, R < ⁷ > and R < ⁸ > is a hydrogen atom or a lower (C1-C4) alkyl group, and may differ or may be the same respectively. ]
Specific examples of the monoolefin monomer include ethylene, propylene, butene-1, pentene-1, 4-methylpentene-1.

  Examples of the halogenated olefin monomer include vinyl chloride and vinylidene chloride.

  Furthermore, diolefins such as butadiene, isoprene and chloroprene can also be included in the monofunctional monomer.

  As the monofunctional monomer to be copolymerized with the crosslinkable monomer, a monovinyl aromatic monomer, an acrylic monomer, a vinyl ester monomer, a vinyl ether monomer and the like are preferable. Particularly preferred are styrene, methyl methacrylate and butyl methacrylate.

A monofunctional monomer can be used individually by 1 type or in combination of 2 or more types.
<Combination of crosslinkable monomer and monofunctional monomer>
In the case of using both a crosslinkable monomer and a monofunctional monomer, a suitable combination of both is ethylene glycol dimethacrylate, which is a crosslinkable monomer, and styrene, monoacrylate, methacrylic ester, monofunctional monomer. A single combination of styrene and acrylic acid ester, styrene and methacrylic acid ester, acrylic acid ester and methacrylic acid ester, styrene, acrylic acid ester and methacrylic acid ester, or the like.
A magnetic substance is contained in the magnetic substance hollow part. The type of the magnetic material is not particularly limited, and a known magnetic material can be used. As such a known magnetic substance, for example, a simple substance or a compound of iron, nickel or cobalt can be mentioned. In the case of a compound of these elements, an oxide is preferable. Examples of the iron oxide include triiron tetroxide (Fe ₃ O ₄ ), iron monoxide (FeO), and ferric trioxide (Fe ₂ o ₃ ). Examples of the nickel oxide include nickel oxide (NiO). Examples of the cobalt oxide include cobalt oxide (CoO), dicobalt trioxide (Co ₂ O ₃ ), dicobalt oxide (III) cobalt (II) (Co ₃ O ₄ ), and the like. In particular, iron alone or an iron compound is preferable, and iron oxide is more preferable because it does not cause toxicity and is low in cost.

A magnetic fluid can also be preferably used.
In addition, the thickness of the shell of the fine particles of the present invention is usually about 0.01 to 5 μm, preferably about 0.1 to 3 μm. By being this thickness, the hollow fine particle shape can be maintained and the specific gravity can be in the above range.

  Further, the average value (n = 100) of the volume ratio of the hollow portion of the magnetic fine particles may be about 10 to 80%, preferably about 30 to 60%. The volume ratio R is a value represented by the following formula.

R (%) = (rh / rp) ³ × 100
(In the formula, rh is the radius of the hollow part measured excluding the magnetic part of the magnetic fine particles, and rp is the radius of the magnetic fine particles.)
The fine particles can contain a pigment in the hollow portion. For example, by including a black pigment such as carbon black, when the display density is insufficient with only the magnetic material, this can be compensated.

  The pigment may be a color pigment, which enables color display. As the color pigment, a known color pigment used for a toner or the like can be used. In such a color pigment, a color index No. C.I. I. 21090 (Pigment Yellow 12, KET Yellow 406, Dainippon Ink and Chemicals), C.I. I. 21095 (Pigment Yellow 14, KET Yellow 404, Dainippon Ink and Chemicals), C.I. I. 21100 (Pigment Yellow 13, KET Yellow 405, Dainippon Ink and Chemicals) and the like. In addition, as a magenta pigment, C.I. I. 73916 (Pigment Red 122, KET Red 309, Dainippon Ink and Chemicals), color index No. C.I. I. 45160 (Pigment Red 81, Ultra Rose R, manufactured by Toyo Ink), and the like. In addition, as a cyan pigment, C.I. I. 74160 (Pigment Blue 15, KET Blue 102, KET Blue 103, KET Blue 104, KET Blue 105, KET Blue 106, KET Blue 111, Dainippon Ink and Chemicals), C.I. I. 74260 (Pigment Green 7, KET Green 201, Dainippon Ink and Chemicals) and the like.

  The amount used when the pigment is contained may be about 10 to 100% by weight, particularly about 20 to 60% by weight, based on the monomer. If it is the said range, sufficient color or black display can be performed. Moreover, if it is the said range, the uptake | capture amount of an auxiliary | assistant polymer or an alternative solvent will be sufficient at the time of magnetic particle manufacture, and a hollow part will be formed.

  Further, the magnetic fine particles can cover the above shell with a protective layer. The protective layer is a layer for preventing wax or the like from entering the inside of the fine particles, and examples thereof include a layer made of a resin such as polyacrylonitrile. Such a protective layer can be formed by adding a monomer at the same time when forming the hollow particles, or can be formed by seed polymerization (see JP-A-8-20604).

Furthermore, a color pigment can also be added to such a protective layer, and a color display can also be performed by this.
Method for producing magnetic fine particles The method for producing the magnetic fine particles is not particularly limited. Known ones can be used without limitation. For example,
A first step in which a mixture containing a magnetic material, a monomer, an auxiliary polymer, and an initiator is dispersed in an aqueous dispersion stabilizer solution, and suspension polymerization is performed; and a second step in which the resulting hollow fine particles are magnetized. It can manufacture by the method containing. The second step may be performed as necessary.

In this method, the auxiliary polymer is low in compatibility with a polymer (PA) obtained by polymerizing or copolymerizing monomers, and has an interfacial tension (γ ^x ) (mN) between the auxiliary polymer and water. / m) and a polymer satisfying the condition of γ ^x ≧ γ ^{y in} relation to the interfacial tension (γ ^y ) (mN / m) between the polymer (PA) and water.
<Dispersion stabilizer>
As a dispersion stabilizer, use a wide range of dispersion stabilizers that prevent liquid droplets formed by dispersing a homogeneous solution consisting of magnetic material, monomer, auxiliary polymer and initiator in water. it can.

  For example, polymer dispersion stabilizers such as polyvinyl alcohol, methyl cellulose, ethyl cellulose, polyacrylic acid, polyacrylimide, polyethylene oxide, poly (hydroxystearic acid-g-methyl methacrylate-co-methacrylic acid) copolymer, nonion -Based surfactants, anionic surfactants, amphoteric surfactants and the like. Among these, polymer dispersion stabilizers such as polyvinyl alcohol are preferable.

  The amount of these dispersion stabilizers can be selected from a wide range, but is generally about 0.005 to 1 part by weight with respect to 1 part by weight of the mixture comprising the magnetic material, monomer, auxiliary polymer, and initiator. Is preferable.

In addition, in the dispersion stabilizer aqueous solution, the concentration of the dispersion stabilizer may be appropriately selected so that the droplets do not coalesce. In general, the concentration of the aqueous dispersion stabilizer solution is preferably adjusted to a range of about 0.05 to 5% by weight.
<Magnetic material>
As the magnetic material, a material that is liquid at room temperature and changes to a magnetic material by heat treatment is used. It is preferable to use a raw material such that the magnetic material obtained by the heat treatment becomes a compound containing a ferromagnetic material such as iron, nickel, or cobalt.

Specifically, carbonyl complexes such as pentacarbonyl iron (Fe (CO) ₅ ), tetracarbonyl nickel (Ni (CO) ₄ ), tetracarbonyl cobalt (Co ₂ (CO) ₈ ); iron alkoxide, magnetic fluid, etc. It can be illustrated. These may be used by diluting in an appropriate organic solvent. The magnetic fluid is obtained by dispersing a magnetic material such as ultrafine magnetite (Fe ₃ O ₄ ) in a medium at a high density. In the method of the present invention, any known magnetic fluid can be used without limitation as long as magnetic ultrafine particles containing iron, nickel, or cobalt are dispersed in an oily medium. Examples of such known magnetic fluid include those in which Fe ₃ O ₄ is dispersed in silicone oil, isoparaffin, hexane, kerosene, and the like. These can be purchased from Sigma High Chemical Co., Ferrotec Co., Ltd., Taiho Kogyo Co., etc.

If attention is paid to the ligand, a carbonyl complex is preferable in terms of good miscibility with the monomer and the solvent. If attention is paid to metals, iron complexes are preferred because they are non-toxic and low in cost. Most preferred is pentacarbonyliron (Fe (CO) ₅ ).

The amount of the magnetic material used varies depending on the type, but may be 5 to 70% by weight with respect to the total amount of fine particles before heating. By using the magnetic material in this range, the magnetic material is usually contained in an amount of about 2 to 35% by weight with respect to the entire fine particles obtained.
<Auxiliary polymer>
The auxiliary polymer has low compatibility with a polymer (PA) obtained by polymerizing or copolymerizing monomer components, and has an interfacial tension (γ ^x ) (mN / m) between the auxiliary polymer and water. ) And the polymer (PA) and the interfacial tension between the water (γ ^y ) (mN / m), a polymer satisfying the condition of γ ^x ≧ γ ^y is used.

  Specifically, a polymer having a lower polarity than the polymer (PA) can be used as the auxiliary polymer.

  In the present specification, the compatibility between the auxiliary polymer and the polymer (PA) is measured by the following method. That is, an initiator (2% by weight based on the monomer component) is added to a monomer solution containing a monomer (auxiliary polymer), an auxiliary polymer, and, if necessary, toluene in an appropriate weight ratio, and 30 ° C. In the nitrogen gas atmosphere, a monomer polymerization reaction is caused. This reaction is carried out in a quartz glass cell having an optical path length of 1 cm, and the light transmittance when irradiating light with a wavelength of 550 nm is measured over time. As the concentration of the auxiliary polymer is increased, the permeation drops to near 0%, which was about 100% at the beginning. In this case, the compatibility between the auxiliary polymer and the polymer (PA) is reduced to near 0%, but when the compatibility between the auxiliary polymer and the polymer (PA) is high, the transmittance hardly decreases. Further, the lower the compatibility between the auxiliary polymer and the polymer (PA), the shorter the time from the start of polymerization until the decrease in transmittance occurs.

  As an auxiliary polymer having low compatibility with the polymer (PA), when the transmittance is measured by the above method, the polymerization rate of the monomer is about 1 to 10%, preferably about 1 to 5%. The target component which a fall occurs is mentioned.

  In the present specification, the interfacial tension is a value measured by Dunui's platinum ring method defined in ASTM-971-50.

  The auxiliary polymer is desirably soluble in the monomer, but this requirement is usually satisfied.

  Auxiliary polymers that meet these requirements promote phase separation between the monomer and the polymer (PA) obtained by polymerization or copolymerization thereof.

  Furthermore, in a homogeneous solution of magnetic material, monomer, auxiliary polymer, and initiator, the monomer is polymerized or copolymerized to become a polymer (PA), and when the polymer (PA) is adsorbed at the interface with water, The polymer (PA) is more easily adsorbed at the interface with water than the auxiliary polymer. As a result, fine particles in which the magnetic material is encapsulated inside the shell made of the polymer (PA) are obtained.

  As such an auxiliary polymer, for example, polystyrene, polymethyl methacrylate, polybutyl methacrylate and the like can be used.

  The combination of the monomer and the auxiliary polymer that satisfies the above requirements can be easily selected by the method described above, and examples thereof include the combinations shown in the following table.

Monomer Auxiliary polymer Ethylene glycol dimethacrylate Polystyrene Ethylene glycol dimethacrylate Polymethyl methacrylate
Or
Polybutyl methacrylate Divinylbenzene Polystyrene Divinylbenzene Polybutyl methacrylate

Although the usage-amount of an auxiliary polymer can be suitably selected from a wide range, generally it is preferable to set it as about 0.05-0.4 weight part with respect to 1 weight part of monomers.

The molecular weight of the auxiliary polymer can usually be about several hundred thousand. The auxiliary polymer can be produced by a known method such as solution polymerization or bulk polymerization. For example, polystyrene having a molecular weight of about several hundred thousand can be obtained by solution polymerization in which 18 g of styrene as a monomer, 12 g of toluene as a solvent, and 54 mg of AIBN as an initiator are reacted at 60 ° C. for 24 hours.
<Solvents to replace auxiliary polymers>
Instead of the auxiliary polymer, it is also possible to use a solvent which serves the same function. Such a solvent is highly compatible with the monomer, has a low compatibility with the polymer (PA), and has an interfacial tension (γ ^z ) (mN / m) between the solvent and water and the polymer. It is a solvent that satisfies the condition of γ ^z ≧ γ ^{y in} relation to the interfacial tension (γ ^y ) (mN / m) between (PA) and water.

Such polymer (PA) and solvent combinations include, for example, polydivinylbenzene and hexadecane, polydivinylbenzene and dodecane, polystyrene and hexadecane, polystyrene and octane, and polystyrene. And a combination of dodecane and the like.
<Initiator>
The initiator used in the present invention starts polymerization of the monomer component in the droplet, and oil-soluble polymerization initiators can be widely used. For example, azo compounds such as azobisisobutyronitrile which is a radical polymerization initiator, cumene hydroperoxide, t-butyl hydroperoxide, dicumyl peroxide, di-t-butyl peroxide, benzoyl peroxide, lauroyl peroxide, etc. Those soluble in monomers such as peroxides. Moreover, you may use the photoinitiator which starts superposition | polymerization by light, such as an ultraviolet-ray. Such a photopolymerization initiator is not particularly limited as long as it is oil-soluble, and includes those conventionally used.

The amount of the initiator used is preferably about 0.005 to 0.1 parts by weight with respect to 1 part by weight of the monomer.
<Dispersing process>
Suspension polymerization is carried out by dispersing a mixture containing the magnetic raw material, monomer, initiator, auxiliary polymer or solvent in place of the above in the above-mentioned use ratio in an aqueous solution of the dispersion stabilizer. When a pigment is used supplementarily, it may be contained in the above mixture. The amount of the pigment used may be about 10 to 100 parts by weight, particularly about 20 to 60% by weight, based on the monomer.

  It is preferable that the magnetic material, the initiator, and the auxiliary polymer or the solvent instead thereof are dissolved in the monomer to form a uniform solution. There is no limitation in particular as temperature at the time of mixing, For example, what is necessary is just to mix at about 0-30 degreeC.

  The mixture thus obtained is dispersed in an aqueous solution of the dispersion stabilizer. This mixture is preferably used in an amount of about 1 to 200 parts by weight per 100 parts by weight of the aqueous dispersion stabilizer solution, but is not particularly limited to this range. Although the temperature at the time of dispersion | distribution is not limited to it, What is necessary is just to be about 0-30 degreeC.

  Generally, as a dispersion method, a dispersion method using mechanical shearing force such as a homogenizer or an ultrasonic method is employed. However, in these dispersion methods, the size of droplets formed by dispersing a mixture of magnetic material, monomer, initiator, and auxiliary polymer or solvent is not monodispersed, but generally liquids of various different particle sizes are used. Drops are mixed. Therefore, the finally obtained magnetic fine particles also have different particle sizes. In this case, particles having a uniform particle diameter can be obtained by classifying the final product.

  In the method of the present invention, in addition to the above classification, for example, monodispersed droplets are prepared by a membrane emulsification method using porous glass (SPG), or a seed swelling method (method described in JP-A-8-20604). By producing the above-described droplets, monodispersed droplets can be obtained with uniform droplet sizes. Specifically, monodispersed droplets are produced by, for example, extruding the above mixture into an aqueous solution containing a dispersion stabilizer through an SPG film having a very uniform porous structure by gas pressure. When such monodispersed droplets are prepared, the coefficient of variation of the finally obtained magnetic particles can be in the range of 5 to 20%. Further, by using the seed swelling method, magnetic particles having an average particle size of 0.1 to 50 μm and a coefficient of variation of 1 to 10% can be obtained.

The average particle size of the droplets is usually about 0.1 to 50 μm, and thereby magnetic fine particles having an average particle size of about 0.1 to 50 can be obtained. By appropriately setting the viscosity of the mixture comprising the magnetic material raw material, the monomer, the initiator, and the auxiliary polymer or the solvent instead thereof, the amount of the dispersion stabilizer used, the viscosity of the aqueous dispersion stabilizer solution, and the dispersion method / dispersion conditions, the above-mentioned range. The average particle size of the droplets is obtained.
Suspension polymerization In order to use in suspension polymerization an aqueous solution of a dispersion stabilizer in which a magnetic material raw material, a monomer component, an initiator, and an auxiliary polymer or a mixture of an alternative solvent are dispersed, the aqueous solution is stirred. It is only necessary to heat while.

  The heating temperature is not particularly limited as long as it is a temperature at which the monomer can be polymerized by the initiator in the droplets of the mixture, but generally about 30 to 90 ° C is preferable.

  Suspension polymerization is performed until fine particles enclosing the magnetic material are obtained. The time required for suspension polymerization varies depending on the kind of magnetic material, monomer, and initiator used, but is generally about 3 to 48 hours.

  The suspension polymerization is preferably performed in an inert gas atmosphere such as nitrogen gas or argon gas.

  By performing suspension polymerization in this way, the monomer is polymerized in the droplets of the mixture.

  The resulting monomer polymer or copolymer (PA) is phase-separated by the presence of an auxiliary polymer or an alternative solvent, resulting in a monolayer shell, i.e. a shell made of polymer (PA). Is formed. On the other hand, the core part is in a state in which the magnetic material and the auxiliary polymer are included.

The volume ratio of the hollow portion can be adjusted by mixing a volatile solvent that does not affect the polymerization, such as toluene or hexadecane, during the polymerization.
<Magnetic treatment>
Next, the fine particles enclosing the magnetic material are magnetized. When a magnetic material such as a magnetic fluid is used from the beginning as a magnetic material, no magnetic treatment is required.
(i) Heat treatment When a carbonyl complex is used as the magnetic material, the magnetic material can be magnetized by heat-treating the hollow fine particles enclosing the magnetic material.

  The heat treatment may be performed on the magnetic substance-encapsulated fine particles usually at about 150 to 300 ° C. for about 1 to 10 hours, preferably at about 180 to 260 ° C. for about 2 to 6 hours. The heat treatment can be performed in an air atmosphere or a nitrogen atmosphere. By this heat treatment, the carbonyl complex is oxidized or the like in the hollow portion of the fine particles, and a magnetic material is generated.

By adjusting the treatment temperature and / or atmosphere, the composition of the obtained magnetic material can be changed to, for example, Fe ₃ O ₄ , FeO, or Fe ₂ O ₃ . Moreover, a simple substance can be obtained as a magnetic substance by processing at a relatively high temperature in a nitrogen atmosphere.
(ii) Hydrolysis treatment When alkoxide is used as the magnetic material, the hollow fine particles enclosing the magnetic material are immersed in an acid aqueous solution or an alkali aqueous solution, and the acidic condition (pH 1 to 4) or the alkaline condition (pH 9). To 12), it is usually about 5 to 50 ° C. for about 1 to 10 hours, preferably about 10 to 40 ° C. for about 2 to 5 hours. In this case, the solvent in the hollow may be evaporated by performing a short-time steam treatment so that the acid or alkali penetrates into the hollow.

  By adjusting the treatment temperature, the composition of the obtained magnetic material can be adjusted.

In both the case where magnetic treatment is not performed and the case where magnetic treatment is performed, the magnetic substance is usually present in the shell inner surface region in the obtained magnetic hollow fine particles. Specifically, the magnetic body is mainly attached to the inner surface of the shell, and a part thereof is buried in the shell or may exist in a hollow portion away from the inner surface of the shell.
<Drying>
The magnetic fine particles thus obtained may be dried under conditions of, for example, a temperature of about 0 to 50 ° C. and a pressure of about 10 ³ to 10 ⁵ Pa when water or a solvent is still contained therein. The fine particles can also be dried by natural evaporation, reduced pressure treatment, or the use of a desiccant such as silica gel.

The magnetic fine particles obtained in this way may be used after further forming a protective layer if necessary and then adding to a low melting point wax or resin.
(II) Electronic Paper The electronic paper of the present invention is obtained by sandwiching the above-described composition containing the magnetic fine particles of the present invention and a low-melting-point medium between a pair of transparent substrates.

  The substrate material is not particularly limited, but is preferably a film made of a transparent resin. Thereby, it becomes a flexible electronic paper. The transparent resin preferably has a heat resistance of about 130 ° C., and examples of such a resin include methacrylic acid resin, polychlorotrifluoroethylene, polyvinylidene fluoride, cellulose acetate, polyester, and vinylidene chloride.

  In use, this electronic paper is partially heated from one substrate side. For example, carbon particles are dispersed in the substrate to be heated so as to have a predetermined resistance value, and further the electrodes are arranged in a matrix. The predetermined pixel formed by the intersection of the electrodes can be configured to generate heat when energized.

  The pair of substrates may be disposed at a distance of usually about 5 to 15 μm from each other, for example, by sandwiching beads as spacers. In the meantime, the composition containing the fine particles of the present invention and the low melting point medium is filled. As a low melting point medium, for example, about 30 to 50% by weight of a white pigment such as titanium oxide, alumina, silica, or calcium carbonate is mixed with petroleum-based paraffin that melts at a temperature of about 60 to 120 ° C. White medium.

The amount of the magnetic fine particles used in the present invention may be usually about 1 to 20% by weight with respect to the low melting point medium.
(III) Information Display Method The information display method of the present invention is the same as the electronic paper of the present invention described above in the thickness direction of the substrate pair (a direction perpendicular or substantially perpendicular to the substrate pair or a surface direction of the substrate pair). In a state where a magnetic field is formed), the electronic paper is partially heated to melt the low melting point medium in that part, and the magnetic fine particles are moved to the transparent substrate side according to the magnetic field in that part. This is a method of displaying information by moving it. By this method, information such as images and characters can be displayed or rewritten on the electronic paper.

  Specifically, for example, a magnetic head and a heating unit such as an energizing unit are opposed to each other with a substrate pair of electronic paper interposed therebetween, and a portion of the substrate to be displayed in the thickness direction of the substrate pair is to be displayed. In a state where a magnetic field is formed, the portion can be heated by energization. Thereby, the low melting point medium in the heated part is melted and the magnetic particles move to the transparent substrate side, and information is displayed in the part.

  For example, by using cyan magnetic fine particles, magenta magnetic fine particles, yellow magnetic fine particles having different magnetic substance contents and specific gravity, and black magnetic fine particles as necessary, a pixel containing each color fine particle is formed in one electronic paper. Full color display can also be achieved by displaying each pixel separately by controlling the magnetic strength and temperature.

The electronic paper and the information display method of the present invention are not limited to this, and the magnetic fine particles of the present invention can be applied to the electronic paper and the image display system described in Patent Documents 1 and 2, for example.
EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples.
<Apparent specific gravity>
The apparent specific gravity was evaluated by the ups and downs of particles by dispersing dry particles in paraffin having a density of 0.7 (g / cm ³ ) using the ups and downs method.
<Magnetic content>
The magnetic substance content was determined by decomposing the polymer portion by raising the temperature to 500 ° C. using a thermogravimetric analyzer and measuring the remaining weight of the magnetic substance.
<Average particle size>
100 fine particles were observed using an optical microscope, and an average value was obtained.
<Suction test with magnet>
Responsiveness and contrast when applying the magnetic fine particles of the present invention to electronic paper were evaluated by the following methods.

  A mixture of paraffin (Parffin Block, mp 60-62 ° C, manufactured by Nacalai Tesque) with 30 wt% magnetic particles is poured into an aluminum vat so that the liquid height is 2 mm, at a temperature above the melting point of paraffin. A measurement sample was prepared by cooling the magnetic particles below the melting point in a state where all the magnetic particles were once attracted downward by a 1650 gauss magnet from below and fixing the magnetic particles downward.

  A 1 mm thick glass plate was placed on the solidified paraffin layer, and a 100 V, 20 W soldering iron (tip diameter: 2 mmφ) with a 1200 gauss magnet was scanned linearly at a speed of 1 cm / sec. The contrast and blur of the obtained image were visually observed and evaluated according to the following criteria.

○: Excellent, △: Acceptable, ×: Impossible
Example 1
As a dispersion stabilizer, 0.85 g of polyvinyl alcohol (polymerization degree 1700, saponification degree 88%) in an aqueous solution obtained by dissolving 85 g of water, 2.89 g of divinylbenzene as a monomer, ferricolloid as a magnetic material (Taiho Industries) Monodisperse by adding 2.89 g, further 2.0 g of hexadecane, and a homogeneous solution in which 51 mg of V-65 (Wako Pure Chemical Industries, Ltd.) is uniformly mixed as an initiator, and emulsifying with an SPG membrane emulsifier at room temperature. The suspension was suspended to obtain an O / W type suspension. The obtained suspension droplets had an average particle diameter of 10 μm and a coefficient of variation of about 15%.

  Next, the suspension was heated at 50 ° C. for 24 hours with stirring in a nitrogen gas atmosphere to perform polymerization.

The obtained magnetic hollow fine particles had an average particle size of 10 μm, a shell thickness of about 1 μm, and a volume ratio of the hollow portion of about 50%. The apparent specific gravity of the obtained particles was about 0.7. When dispersed in paraffin, neither floating nor settling occurred. Further, the magnetic substance retention was 35% by weight. It was contrast ○ and blur ○.
Example 2
First, seed polymer particles were prepared. That is, first, a mixed solution of 20 mg of styrene, 36 mg of ion-exchanged water, 144 mg of ethanol, 284 mg of azobisisobutyronitrile and 2 g of polyacrylic acid was put into a three-necked separable flask equipped with a refluxer, and 100 rpm under a nitrogen gas stream. The mixture was reacted at 70 ° C. for 12 hours with stirring for dispersion polymerization to obtain seed polymer particles. When the seed polymer particles were observed with an optical microscope, they were monodisperse particles having a particle size (diameter) of about 1.5 μm.

  Next, a mixed solution of 5 g of ethanol, 15 g of ion-exchanged water, 0.3 g of divinylbenzene, 0.3 g of pentacarbonyl iron, 0.05 g of polyvinyl alcohol and 0.006 g of V-65 (initiator) was sonicated to obtain an emulsion. Prepared.

  This emulsion was added to the seed particle dispersion and absorbed by the seed particles (dynamic swelling method). Thereafter, 40 g of a conical solution was dropped, and the emulsion was observed with an optical microscope in this state. As a result, the particles were spherical and swollen to a particle size of about 5 μm.

  The emulsion was allowed to react at 70 ° C. for 24 hours while stirring in a seal to perform seed polymerization.

After filtering the polymer obtained by the seed polymerization reaction, magnetic particles were obtained by heat treatment at 250 ° C. in an autoclave. The average particle diameter of the obtained particles was 5 μm and the coefficient of variation was about 6%. Apparent specific gravity was about 0.7, and when dispersed in paraffin, neither floating nor sinking occurred. The magnetic substance retention was 12% by weight. It was contrast ○ and blur ○.
Example 3
In an aqueous solution obtained by dissolving 0.28 g of polyvinyl alcohol (polymerization degree 1700, saponification degree 88%) as a dispersion stabilizer in 28 g of water, 0.77 g of divinylbenzene as a monomer, 0.39 g of acrylonitrile, and ferriyl as a magnetic material Add a uniform solution of 0.67 g of colloid (Taio Industries), 0.72 g of dodecane, 0.39 g of blue pigment (BASF, BlueL7080), and 17 mg of V-65 (Wako Pure Chemical) as an initiator, Under the emulsification using a homogenizer, an O / W type suspension was obtained.

  Next, the suspension was heated at 50 ° C. for 24 hours with stirring in a nitrogen gas atmosphere to perform polymerization. The obtained particles were polydispersed, but after removing particles with a large particle size and small particles with a centrifuge and classifying them, an average particle size of 10 μm and a coefficient of variation of about 19% were obtained.

The thickness of the shell was about 1 μm, and the volume ratio of the hollow portion was about 50%. The whole particle was blue, and the apparent specific gravity of the obtained particle was about 0.7. When dispersed in paraffin, neither floated nor settled. Moreover, the magnetic substance retention rate was 19 weight%. It was contrast ○ and blur ○.
Example 4
In an aqueous solution obtained by dissolving 0.28 g of polyvinyl alcohol (polymerization degree 1700, saponification degree 88%) as a dispersion stabilizer in 28 g of water, 0.77 g of divinylbenzene as a monomer, 0.39 g of acrylonitrile, and ferriyl as a magnetic material Add a uniform solution of 0.67 g of colloid (Taio Industries), 0.72 g of dodecane, 0.39 g of yellow pigment (Yellow 4517), and 17 mg of V-65 (Wako Pure Chemical) as an initiator at room temperature. Then, an O / W suspension was obtained by emulsification using a homogenizer.

  Next, the suspension was heated at 50 ° C. for 24 hours with stirring in a nitrogen gas atmosphere to perform polymerization. The obtained particles were polydispersed, but after removing large particles and small particles with a centrifuge and classifying them, particles having an average particle size of 10 μm and a coefficient of variation of about 20% were obtained.

The thickness of the shell was about 1 μm, and the volume ratio of the hollow portion was about 50%. The whole particles were yellow, and the apparent specific gravity of the obtained particles was about 0.7. When dispersed in paraffin, neither floated nor settled. Moreover, the magnetic substance retention rate was 19 weight%. It was contrast ○ and blur ○.
Example 5
In an aqueous solution obtained by dissolving 0.28 g of polyvinyl alcohol (polymerization degree 1700, saponification degree 88%) as a dispersion stabilizer in 28 g of water, 0.77 g of divinylbenzene as a monomer, 0.39 g of acrylonitrile, and ferriyl as a magnetic material Uniform solution in which 0.67 g of colloid (Taio Corporation), 0.72 g of dodecane, 0.39 g of black pigment (Mitsubishi Chemical, Carbon Black MA-100), and 17 mg of V-65 (Wako Pure Chemical Industries) as an initiator were uniformly mixed. Was added and emulsified with a homogenizer at room temperature to obtain an O / W type suspension.

  Next, the suspension was heated at 50 ° C. for 24 hours with stirring in a nitrogen gas atmosphere to perform polymerization. The obtained particles were polydispersed, but after removing particles with a large particle size and small particles by a centrifugal separator and classifying them, an average particle size of 10 μm and a coefficient of variation of about 18% were obtained.

The thickness of the shell was about 1 μm, and the volume ratio of the hollow portion was about 50%. The whole particles were black, and the apparent specific gravity of the obtained particles was about 0.7. When dispersed in paraffin, neither floated nor settled. Moreover, the magnetic substance retention rate was 19 weight%. It was contrast ○ and blur ○.
Example 6
In an aqueous solution obtained by dissolving 0.3 g of polyvinyl alcohol (polymerization degree 1700, saponification degree 88%) as a dispersion stabilizer in 30 g of water, 0.35 g of divinylbenzene as a monomer, 0.437 g of toluene, and ferriyl as a magnetic material 0.3 g of colloid (TAIHO KOGYO), 0.072 g of polystyrene (molecular weight 160,000) as an auxiliary polymer, 0.18 g of blue pigment (BASF, BlueL7080), and 15 mg of V-65 (manufactured by Wako Pure Chemical Industries, Ltd.) as an initiator A uniformly mixed solution was added and emulsified with a homogenizer at room temperature to obtain an O / W type suspension.

  Next, the suspension was heated at 50 ° C. for 24 hours with stirring in a nitrogen gas atmosphere to perform polymerization. Although the obtained particles were polydispersed, particles having a large particle diameter and small particles were removed by classification using a centrifugal separator, and after classification, particles having an average particle diameter of 10 μm and a coefficient of variation of about 20% were obtained.

The thickness of the shell was about 1 μm, and the volume ratio of the hollow portion was about 50%. The whole particle was blue, and the apparent specific gravity of the obtained particle was about 0.7. When dispersed in paraffin, neither floated nor settled. Moreover, the magnetic substance retention was 21% by weight. It was contrast ○ and blur ○.
Example 7
In an aqueous solution obtained by dissolving 0.3 g of polyvinyl alcohol (polymerization degree 1700, saponification degree 88%) as a dispersion stabilizer in 30 g of water, 0.35 g of divinylbenzene as a monomer, 0.437 g of toluene, and ferriyl as a magnetic material Uniformity of 0.3 g of colloid (Taiho Kogyo), 0.072 g of polystyrene (molecular weight 160,000) as auxiliary polymer, 0.18 g of yellow pigment (Yellow 4517), and 15 mg of V-65 (manufactured by Wako Pure Chemical Industries) as initiator The mixed homogeneous solution was added and emulsified with a homogenizer at room temperature to obtain an O / W type suspension.

  Next, the suspension was heated at 50 ° C. for 24 hours with stirring in a nitrogen gas atmosphere to perform polymerization. Although the obtained particles were polydispersed, particles having a large particle diameter and small particles were removed by classification using a centrifugal separator, and after classification, particles having an average particle diameter of 10 μm and a coefficient of variation of about 20% were obtained.

The thickness of the shell was about 1 μm, and the volume ratio of the hollow portion was about 50%. The whole particles were yellow, and the apparent specific gravity of the obtained particles was about 0.7. When dispersed in paraffin, neither floated nor settled. Moreover, the magnetic substance retention was 21% by weight. It was contrast ○ and blur ○.
Example 8
In an aqueous solution obtained by dissolving 0.3 g of polyvinyl alcohol (polymerization degree 1700, saponification degree 88%) as a dispersion stabilizer in 30 g of water, 0.35 g of divinylbenzene as a monomer, 0.437 g of toluene, and ferriyl as a magnetic material 0.3 g of colloid (Taiho Kogyo), 0.072 g of polystyrene (molecular weight 160,000) as an auxiliary polymer, 0.18 g of black pigment (Mitsubishi Chemical, Carbon Black MA-100), and V-65 (Wako Pure Chemical Industries) as an initiator A uniform solution in which 15 mg was uniformly mixed was added and emulsified with a homogenizer at room temperature to obtain an O / W type suspension.

  Next, the suspension was heated at 50 ° C. for 24 hours with stirring in a nitrogen gas atmosphere to perform polymerization. Although the obtained particles were polydispersed, particles having a large particle diameter and small particles were removed by classification using a centrifugal separator, and after classification, particles having an average particle diameter of 10 μm and a coefficient of variation of about 20% were obtained.

The thickness of the shell was about 1 μm, and the volume ratio of the hollow portion was about 50%. The whole particles were black, and the apparent specific gravity of the obtained particles was about 0.7. When dispersed in paraffin, neither floated nor settled. Moreover, the magnetic substance retention was 21% by weight. It was contrast ○ and blur ○.
Comparative example 1 (when only specific gravity is too high)
As a dispersion stabilizer, 0.85 g of polyvinyl alcohol (polymerization degree 1700, saponification degree 88%) in an aqueous solution obtained by dissolving 85 g of water, 2.89 g of divinylbenzene as a monomer, ferricolloid as a magnetic material (Taiho Industries) By adding 2.89 g, further 0.578 g of hexadecane, and a uniform solution in which 51 mg of V-65 (Wako Pure Chemical Industries, Ltd.) is uniformly mixed as an initiator, and emulsifying using an SPG membrane emulsifier at room temperature, O / W type suspension was obtained by suspending in monodisperse. The obtained suspension droplets had an average particle size of about 10 μm.

  Next, the suspension was heated at 50 ° C. for 24 hours with stirring in a nitrogen gas atmosphere to perform polymerization.

The obtained magnetic hollow fine particles had an average particle size of 10 μm, a coefficient of variation of 17%, a shell thickness of about 1 μm, and a volume ratio of the hollow portion of about 50%. The apparent specific gravity of the obtained particles was about 0.85, and settled when dispersed in paraffin. The magnetic substance retention was 35% by weight. Contrast ○ and blur △.
Comparative Example 2 (when specific gravity is too low)
As a dispersion stabilizer, 0.85 g of polyvinyl alcohol (polymerization degree 1700, saponification degree 88%) in an aqueous solution obtained by dissolving 85 g of water, 2.89 g of divinylbenzene as a monomer, ferricolloid as a magnetic material (Taiho Industries) By adding 2.89 g, further hexadecane 2.89 g, and a homogeneous solution in which 51 mg of V-65 (Wako Pure Chemical Industries, Ltd.) is uniformly mixed as an initiator, the mixture is monodispersed by emulsification using an SPG membrane emulsifier at room temperature. To obtain an O / W type suspension. The obtained suspension droplets had an average particle diameter of 10 μm and a coefficient of variation of about 15%.

  Next, the suspension was heated at 50 ° C. for 24 hours with stirring in a nitrogen gas atmosphere to perform polymerization.

The obtained magnetic hollow fine particles had an average particle size of 10 μm, a shell thickness of about 1 μm, and a volume ratio of the hollow portion of about 50%. The apparent specific gravity of the obtained particles was about 0.55, and floated when dispersed in paraffin. The magnetic substance retention was 35% by weight. Contrast ○ and blur ×.
Comparative Example 3 (when only the coefficient of variation is too high)
As a dispersion stabilizer, 0.85 g of polyvinyl alcohol (polymerization degree 1700, saponification degree 88%) in an aqueous solution obtained by dissolving 85 g of water, 2.89 g of divinylbenzene as a monomer, ferricolloid as a magnetic material (Taiho Industries) Add a homogeneous solution in which 2.89 g, 2.0 g of hexadecane and 51 mg of V-65 (Wako Pure Chemical Industries) as an initiator are uniformly mixed, and emulsify with a homogenizer at room temperature to obtain an O / W type suspension. A turbid liquid was obtained. The obtained suspension droplets had an average particle size of 20 μm and a coefficient of variation of about 35%.

  Next, the suspension was heated at 50 ° C. for 24 hours with stirring in a nitrogen gas atmosphere to perform polymerization.

The obtained magnetic hollow fine particles had an average particle size of 20 μm, a shell thickness of about 2 μm, and a volume ratio of the hollow portion of about 50%. The apparent specific gravity of the obtained particles was about 0.7. When dispersed in paraffin, neither floated nor settled, but because it was polydispersed, there was a variation in the suction speed of suction with a magnet, contrast Δ, blur × Met. The magnetic substance retention was 35% by weight.
Comparative Example 4 (when the average particle size is too small)
As a dispersion stabilizer, 0.85 g of polyvinyl alcohol (polymerization degree 1700, saponification degree 88%) in an aqueous solution obtained by dissolving 85 g of water, 2.89 g of divinylbenzene as a monomer, ferricolloid as a magnetic material (Taiho Industries) 2.89 g, hexadecane 2.0 g, and a homogeneous solution in which 51 mg of V-65 (Wako Pure Chemical Industries) was uniformly mixed as an initiator were added and emulsified using an SPG membrane emulsifier at room temperature. O / W type suspension was obtained by suspending in dispersion. The obtained suspension droplets had an average particle size of 0.4 μm and a coefficient of variation of about 20%.

  Next, the suspension was heated at 50 ° C. for 24 hours with stirring in a nitrogen gas atmosphere to perform polymerization.

The average particle diameter of the obtained magnetic hollow fine particles was 0.4 μm, the shell thickness was about 0.04 μm, and the volume ratio of the hollow portion was about 50%. The magnetic particle retention rate of the obtained particles was 35% by weight. Apparent specific gravity was about 0.7, and when dispersed in paraffin, neither floating nor sedimentation occurred. However, the response speed to the magnet was slow due to the small particle size. The magnetic substance retention was 35% by weight. Contrast ×, blur △.
Comparative Example 5 (when the average particle size is too large)
A dispersion stabilizer of 0.85 g of polyvinyl alcohol (polymerization degree 1700, saponification degree 88%) dissolved in 85 g of water was dissolved in 2.89 g of divinylbenzene as a monomer and ferricolloid as a magnetic material (Taiho Industries). 2.89 g, hexadecane 2.0 g, and a homogeneous solution in which 51 mg of V-65 (Wako Pure Chemical Industries) was uniformly mixed as an initiator were added and emulsified using an SPG membrane emulsifier at room temperature. O / W type suspension was obtained by suspending in dispersion. The obtained suspension droplets had an average particle size of 100 μm and a coefficient of variation of about 20%.

  Next, the suspension was heated at 50 ° C. for 24 hours with stirring in a nitrogen gas atmosphere to perform polymerization.

The obtained magnetic hollow fine particles had an average particle size of 100 μm, and no shell layer was formed. The magnetic particle retention rate of the obtained particles was 35% by weight. Therefore, the apparent specific gravity was about 1.5, and when dispersed in paraffin, it settled and the response speed to the magnet was slow. Contrast Δ and blur Δ.
Comparative Example 6 (when magnetic retention is too large)
Dispersion stabilizer polyvinyl alcohol (polymerization degree 1700, saponification degree 88%) 0.85g dissolved in water 85g, divinylbenzene 1.45g as a monomer, ferricolloid as a magnetic material (Taiho Industries) 2.89 g and a homogeneous solution in which 25 mg of V-65 (Wako Pure Chemical Industries, Ltd.) was uniformly mixed as an initiator were added and emulsified with an SPG membrane emulsifier at room temperature to be suspended in a monodisperse form. A / W type suspension was obtained. The obtained suspension droplets had an average particle diameter of 10 μm and a coefficient of variation of about 20%.

  Next, the suspension was heated at 50 ° C. for 24 hours with stirring in a nitrogen gas atmosphere to perform polymerization.

The obtained magnetic hollow fine particles had an average particle size of 10 μm, a shell thickness of about 1 μm, and a volume ratio of the hollow portion of about 50%. The magnetic particle retention rate of the obtained particles was 50% by weight. The apparent specific gravity was about 1.5, and when dispersed in paraffin, it settled and the response speed to the magnet was slow. Contrast Δ and blurring ○.
Comparative Example 7 (when the magnetic retention is too small)
2.89 g of divinylbenzene as a monomer and ferricolloid as a magnetic material (Taiho Industries) in an aqueous solution obtained by dissolving 0.85 g of polyvinyl alcohol (polymerization degree 1700, saponification degree 88%) as a dispersion stabilizer in 85 g of water 0.05 g, 1.0 g of hexadecane, and a uniform solution in which 25 mg of V-65 (Wako Pure Chemical Industries, Ltd.) was uniformly mixed were added and emulsified using an SPG membrane emulsifier at room temperature. O / W type suspension was obtained by suspending in dispersion. The obtained suspension droplets had an average particle diameter of 10 μm and a coefficient of variation of about 20%.

  Next, the suspension was heated at 50 ° C. for 24 hours with stirring in a nitrogen gas atmosphere to perform polymerization.

  The obtained magnetic hollow fine particles had an average particle size of 10 μm, a shell thickness of about 1 μm, and a volume ratio of the hollow portion of about 50%. The magnetic particle retention of the obtained particles was 1.5% by weight. The apparent specific gravity was about 0.7, and when dispersed in paraffin, it settled and the response speed to the magnet was slow. Contrast x and blur x.

  As a result, the specific gravity of the fine particles is 0.5 to 1.4, the magnetic substance content is 2 to 35% by weight, the average particle size is 0.1 to 50 μm, and the variation coefficient of the average particle size is 20% or less. In this case, for the first time, it can be seen that an electronic paper with good contrast and no blur is obtained.

Claims (8)

Magnetic fine particles in which a magnetic substance is encapsulated in a hollow part of a hollow fine particle comprising a shell and a hollow part, wherein the shell is a polymer of a crosslinkable monomer, a copolymer of two or more crosslinkable monomers, or a crosslinkable monomer A magnetic fine particle comprising a layer formed of a copolymer of a monofunctional monomer and having the following characteristics (a) to (d).
(a) Specific gravity of fine particles: 0.5 to 1.4
(b) Magnetic substance content: 2 to 35% by weight
(c) Average particle diameter: 0.1 to 50 μm
(d) Coefficient of variation of average particle size: 20% or less The magnetic fine particle according to claim 1, wherein the shell includes a layer composed of a polymer of a crosslinkable monomer or a copolymer of two or more kinds of crosslinkable monomers. The magnetic fine particles according to claim 1 or 2, wherein the magnetic substance is at least one selected from the group consisting of iron simple substance, iron compound, cobalt simple substance, cobalt compound, nickel simple substance, and nickel compound. The magnetic fine particle according to any one of claims 1 to 3, wherein a pigment is further encapsulated in a hollow portion of the hollow fine particle. The magnetic fine particle according to any one of claims 1 to 4, wherein an average value of a volume ratio R of a hollow portion of the magnetic fine particle represented by the following formula is 10 to 80%.
R (%) = (rh / rp) ³ × 100
(In the formula, rh is the radius of the hollow portion of the magnetic fine particles, and rp is the radius of the magnetic fine particles.) Magnetic fine particles according to any one of claims 1 to 5 for magnetic thermal electronic paper. An electronic paper in which a composition containing the magnetic fine particles according to claim 1 and a low-melting-point medium is sandwiched between a pair of substrates at least one of which is transparent. The electronic paper according to claim 7, in a state where a magnetic field is formed in the substrate thickness direction with respect to the substrate pair, the electronic paper is partially heated to melt the low-melting-point medium in the portion, and the magnetic field in the portion The information display method of displaying information by moving magnetic fine particles to the transparent substrate side according to the above.