JP2010002586A - Electrophoretic particle and manufacturing method thereof, electrophoretic dispersion containing electrophoretic particle, image display medium, and image display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide electrophoretic particles having excellent memory property and a manufacturing method thereof, electrophoretic dispersion containing the electrophoretic particles, an image display medium storing the electrophoretic dispersion, and an image display. <P>SOLUTION: The electrophoretic particles are formed by modifying the surface of pigment particles by a manufacturing method having at least processes for: [I] bonding charged organic groups having a cationic or anionic functional group onto the pigment particle surface by using a sulfonium compound expressed by a specific structural formula; and [II] bonding a graft polymer chain including a molecular structure having affinity to a dispersion medium onto the pigment particle surface by using a reactive polymer. The electrophoretic particles (11a) are dispersed into a nonpolar solvent (11c) to obtain the electrophoretic dispersion (11), and the dispersion is enclosed between transparent conductive layers (12, 13) to obtain the image display medium (10). The image display is constituted by using the image display medium. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing electrophoretic particles having excellent memory properties, electrophoretic particles obtained by this production method, electrophoretic dispersion containing electrophoretic particles, image display medium containing electrophoretic dispersion, image The present invention relates to an image display device using a display medium.

  Conventionally, cathode ray tubes (CRT), liquid crystal displays, and the like are used as image display devices that display images such as characters, still images, and moving images. These image display devices can display images instantly and can be rewritten, but they are difficult to carry around, eyes are easily fatigued, and images cannot be displayed when the power is turned off. There are problems such as. On the other hand, images such as characters and still images are distributed or stored after being recorded on a paper medium using a printer. A paper medium on which such an image is recorded is widely used as a hard copy. The hard copy has features such as easy-to-read characters, less fatigue of eyes, reading in a free posture, lightweight and free to carry than a display. However, the hard copy is discarded or recycled after use, but it requires a lot of labor and cost, which is problematic in terms of resource saving.

  For this reason, the need for a rewritable paper-like image display medium that has the advantages of both the above-mentioned display and hard copy has increased, and so far, for example, polymer dispersed liquid crystal elements, bistable cholesteric liquid crystal elements Various techniques for making an image display medium using an electrochromic element, an electrophoretic element, and the like have been studied. Since these image display media are of a reflective type, they can display bright images and have a memory property, and thus attract attention. Among these, an image display medium using an electrophoretic element is excellent in terms of display quality, power consumption during display operation, and the like.

  In such an image display medium, for example, a colored dispersion medium is sealed between a pair of transparent electrodes, and electrophoretic particles having different colors and charged surfaces are contained in the colored dispersion medium. It is distributed and configured. For this reason, when a voltage opposite to the charged charge of the electrophoretic particles is applied to one of the transparent electrodes, the electrophoretic particles are deposited on the one transparent electrode side, and the color of the deposited electrophoretic particles is the transparent electrode. Observed through the side. In addition, when the same voltage as the charged charge of the electrophoretic particles is applied to one of the transparent electrodes, the electrophoretic particles move to the other electrode side of the transparent electrode, and the color of the dispersion medium from one transparent electrode side Is observed. In the case of an image display medium using an electrophoretic element in which electrophoretic particles are dispersed, an image is displayed based on such a principle.

  The stability of the electrophoretic particles in the dispersion medium is generally obtained by an electrostatic effect or a steric effect (also called an adsorption layer effect). Regarding the electrostatic effect, the DLVO theory is known, and the spread of the electric double layer and the interface potential, so-called ζ potential, are important factors. In order to form these, the presence of ions is required. On the other hand, regarding the steric effect, Non-Patent Document 1 describes a method for producing a stable non-aqueous solvent dispersion. That is, a block copolymer or a graft copolymer including a component having compatibility with particles dispersed in a solvent and a component having solubility in a solvent is produced and used.

On the other hand, Patent Document 1 describes that a diazonium group is preferable for reacting carbon black with a compound. In addition, a series of patents by Cabot Corporation, Boston, Massachusetts (see, for example, US Pat. Nos. 5,037,036) describe the use of diazonium chemistry to attach a wide range of functional groups to carbon black. Yes. However, it is not intended for electrophoretic dispersions, but describes the diazonium reaction of carbon black as a dispersion used in inkjet inks.
On the other hand, Patent Document 5 proposes that a polymer particle bonded to pigment particles suspended in an electrophoresis medium (1 to 15% by weight of the pigment) is used. In order to improve the memory property of the conventional image display medium, it is described that polyisobutylene is blended. Further, Patent Document 6 discloses that image stability is improved by dissolving or dispersing a polymer (number average molecular weight exceeding 20,000) in which an electrophoretic medium containing a plurality of particles is non-absorbable in the particles. Improvements have been proposed and the conditions for controlling the optical stability of electrophoretic particles (compatible and incompatible in charged fluid suspensions) are described.
However, none of the above prior arts has sufficient memory properties for image display based on electrophoresis of electrophoretic particles, and further improvements have been desired.
The present applicant has previously proposed an image display medium containing white or colored particles, a hydrocarbon solvent, a resin soluble in the hydrocarbon solvent, and a nonionic compound (see Patent Document 7). In this proposal, white or colored particles have an acidic group or basic group on the surface, and a resin soluble in a hydrocarbon solvent has a basic group or acidic group opposite to that of the particle, and is reversible. Display is possible.

Special table 2004-526210 gazette US Pat. No. 5,554 US Pat. No. 6,068,688 International application WO96 / 18695 JP-T-2004-526199 Special table 2007-508588 JP 2002-62545 A F. A. Waite, J .; Oil Col. Chem. Assoc. , 54, 342 (1971)

  The present invention has been made in view of the above prior art, and a method for producing electrophoretic particles having excellent memory properties, electrophoretic particles obtained by the production method, and electrophoretic dispersion liquid containing electrophoretic particles An object of the present invention is to provide an image display medium containing an electrophoretic dispersion and an image display device using the image display medium.

  As a result of intensive studies, the present inventors have found that the above problems can be solved by the inventions described in the following [1] to [10], and have reached the present invention. Hereinafter, the present invention will be specifically described.

[1]: The above problem is a method for producing electrophoretic particles comprising charged pigment particles that can migrate in a dispersion medium by applying a voltage,
This is solved by a method for producing electrophoretic particles, wherein the surface of the pigment particles is modified by at least the following steps [I] and [II].
[I] A step of bonding a charged organic group having a cationic or anionic functional group to the surface of the pigment particle using a sulfonium compound represented by the following general formula (1) [II] Using a reactive monomer And bonding a graft polymer chain having a molecular structure having an affinity for the dispersion medium to the pigment particle surface

[In the general formula (1), R ¹ and R ² represent a hydrogen atom, a linear or branched alkyl group having 1 to 6 carbon atoms which may have a substituent, and R ¹ And R ² may be the same or different. R ³ is a charged organic group and represents an aryl group having a cationic or anionic functional group or a linear or branched alkyl group having 1 to 12 carbon atoms, and Z ⁻ represents a hydroxyl ion Represents an organic anion derived from an inorganic anion or an organic acid. ]

  [2]: The method for producing electrophoretic particles according to [1] above, wherein the pigment particles are carbon black.

  [3]: The method for producing electrophoretic particles according to [1] or [2] above, wherein the aryl group is a phenyl group.

  [4]: In the method for producing electrophoretic particles according to any one of [1] to [3], the inorganic anion is a halogen ion, nitrate ion, nitrite ion, acetate ion, phosphate ion, or An anion selected from sulfate ions, and the organic anion derived from the organic acid is an anion selected from formic acid, acetic acid, propionic acid, butyric acid, valeric acid, glycolic acid, gluconic acid, or lactic acid. Features.

  [5]: In the method for producing electrophoretic particles according to any one of [1] to [4], the dispersion medium is a nonpolar hydrocarbon.

  [6]: In the method for producing electrophoretic particles according to any one of [1] to [5], the reactive monomer is methacrylate or / and acrylate.

  [7]: The above problem is solved by electrophoretic particles obtained by the production method according to any one of [1] to [6].

  [8]: The above-described problem is solved by an electrophoretic dispersion characterized by containing at least a dispersion medium and the electrophoretic particles according to [7].

  [9]: The above problem is solved by an image display medium characterized in that the electrophoretic dispersion liquid according to [8] is sealed between a pair of opposed conductive layers, at least one of which is light transmissive. The

  [10] The above problem is solved by an image display device using the image display medium described in [9].

According to the method for producing electrophoretic particles of the present invention, the charged organic group having a cationic or anionic functional group on the surface of the pigment particle can be efficiently produced by the steps [I] and [II]. Ratio, etc.), and electrophoretic particles in which graft polymer chains having a molecular structure having an affinity for the dispersion medium are bonded together are obtained.
According to the electrophoretic particle of the present invention, since the graft polymer is bonded to the particle surface together with the chargeable group (charged organic group), the particles migrated in the vicinity of the electrode by voltage application are deposited and aggregated in the vicinity of the electrode. Even after the voltage application is stopped, it is difficult to change over time and has excellent memory properties. In addition, other display characteristics such as response speed, contrast, and good reflectance in display (for example, white / black reflectance) are also excellent.
According to the electrophoretic dispersion liquid containing the electrophoretic particles of the present invention, the image display medium containing the electrophoretic dispersion liquid, and the image display apparatus using the image display medium, display quality and low power consumption (during display operation) It is possible to display an image with excellent memory characteristics while maintaining the above.

As described above, the method for producing electrophoretic particles in the present invention is a method for producing electrophoretic particles composed of charged pigment particles that can migrate in a dispersion medium by applying a voltage,
The surface of the pigment particles is modified by at least the following steps [I] and [II].
[I] A step of bonding a charged organic group having a cationic or anionic functional group to the surface of the pigment particle using a sulfonium compound represented by the following general formula (1) [II] Using a reactive monomer And bonding a graft polymer chain having a molecular structure having an affinity for the dispersion medium to the pigment particle surface

[In the general formula (1), R ¹ and R ² represent a hydrogen atom, a linear or branched alkyl group having 1 to 6 carbon atoms which may have a substituent, and R ¹ And R ² may be the same or different. R ³ is a charged organic group and represents an aryl group having a cationic or anionic functional group or a linear or branched alkyl group having 1 to 12 carbon atoms, and Z ⁻ represents a hydroxyl ion Represents an organic anion derived from an inorganic anion or an organic acid. ]

Examples of the linear or branched alkyl group having 1 to 6 carbon atoms that may have a substituent include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, Examples include pentyl group, isopentyl group, neopentyl group, hexyl group and the like.
The “charged organic group” in the present invention refers to the entire monovalent charged organic group having at least a cationic or anionic functional group. Hereinafter, the “charged organic group” may be referred to as “charged group”.
Examples of the aryl group having a cationic or anionic functional group include a phenyl group, a diphenyl group, and a naphthyl group, and these groups may have a substituent. Examples of the substituent include linear or branched alkyl groups having 1 to 6 carbon atoms. The alkyl group having a cationic or anionic functional group is preferably a linear or branched one having 1 to 12 carbon atoms. For example, in addition to the alkyl group, heptyl group, octyl group , Nonyl group, decyl group, undecyl group, dodecyl group and the like. An alkyl group having an alicyclic structure can also be used.
In the nucleophilic substitution reaction between sulfonium ions and carbon black, when considering solubility in a solvent such as water, the total carbon of R ¹ , R ² and R ³ of the sulfonium compound represented by the general formula (1) The number is preferably 16 or less, more preferably 12 or less. When the number of carbon atoms exceeds 12, the solubility in a solvent such as water tends to decrease. When the number of carbon atoms exceeds 16, the decrease in solubility tends to increase, resulting in a decrease in reactivity in the substitution reaction with carbon black. I have a negative impact on rates.

  Examples of the cationic or anionic functional group bonded to the aryl group or a linear or branched alkyl group having 1 to 12 carbon atoms include, for example, cationic functions such as amino group, methoxy group, and methyl group And anionic functional groups such as a chlorine atom and a nitro group. Alternatively, a cationic functional group as shown in the following formulas (a) to (d) or an anionic functional group as shown in the following formulas (e) to (h) can be mentioned.

  [In the formulas (b) and (d), R represents an alkyl group having 1 to 12 carbon atoms, a substituted or unsubstituted phenyl group, or a naphthyl group. ]

[In the formulas (e) to (h), M ² represents a hydrogen atom, an alkali metal, ammonium or organic ammonium. ]
Among those expressed as M ² , specific examples of the alkali metal include Li, Na, K, Rb and Cs. Specific examples of organic ammonium include methyl ammonium, dimethyl ammonium, trimethyl ammonium, ethyl ammonium, diethyl ammonium, triethyl ammonium, methanol ammonium, dimethanol ammonium, and trimethanol ammonium.

A cationic or anionic functional group as described above bonded to an aryl group or a linear or branched alkyl group having 1 to 12 carbon atoms is a so-called charged organic group. A charged organic group composed of a phenyl group is preferably used. Such a phenyl group can be synthesized by using a halogenated phenyl derivative (for example, a chlorobenzene derivative or the like) as described later. In the present invention, the aryl group also includes a heterocycle in which the carbon atom of the aryl group is replaced with a heteroatom (for example, a nitrogen atom).
Hereinafter, the description will focus on the charged organic group in which an amino group, a methoxy group, a nitro group, and the like are bonded to the phenyl group, but the present invention is not limited thereto, and other charged organic groups such as the following formulas (I) to Examples shown in (IX) are exemplified.

Z ⁻ includes hydroxyl ions, inorganic anions such as halogen ions, nitrate ions, nitrite ions, acetate ions, phosphate ions, sulfate ions, or formic acid, acetic acid, propionic acid, butyric acid, valeric acid, glycolic acid. , Organic anions derived from organic acids such as gluconic acid and lactic acid. These are examples and are not limited to these.
In the reaction with carbon black, it is preferably ionically dissociated at a high rate in a solvent. From this point, monovalent anions such as hydroxyl ion, halogen ion, nitrate ion, nitrite ion, and acetate ion are preferable. Used.

  That is, the method for producing electrophoretic particles of the present invention uses at least a sulfonium compound represented by the general formula (1), and performs cationic or anion by a nucleophilic substitution reaction using the sulfonium ion of the compound as a counter ion. In addition to introducing (bonding) a charged organic group having a functional group to the surface of the pigment particles, graft polymerization is performed using a reactive monomer, and the surface of the pigment particles includes a molecular structure having an affinity for the dispersion medium. It consists of a step of introducing (bonding) polymer chains. That is, it is desirable that a reaction site (active species: functional group) having nucleophilic reactivity exists on the surface of the pigment particle, and in particular, carbon black is preferably used in the present invention.

As for the surface treatment of fine particles, various methods including chemical surface treatment are known. In the case of organic pigments such as carbon black, diazonium reaction, Friedel-Crafts alkylation reaction or Friedel-Crafts acyl is used. A surface treatment method using a chemical reaction or the like is known. In the case of inorganic pigments, for example, there is a method of surface treatment using a silane coupling reaction. As a surface treatment of carbon black or the like (using a substitution reaction), a diazonium reaction has been generally used conventionally.
On the other hand, regarding the method using a sulfonium compound applied to the present invention, since the reaction of the sulfonium compound itself belongs to a new field and is difficult to obtain, there is a problem that it is necessary to newly synthesize from the starting material, so the diazonium reaction The current situation is not generally used. However, as a result of experiments by the present inventors, the sulfonium compound (organic sulfur compound) is preferably used for the nucleophilic substitution reaction because of its high reactivity and reaction selectivity compared to a diazonium salt which is a nitrogen compound, In particular, the reaction using a sulfonium compound was found to be optimal in terms of easiness of reaction with carbon black, safety, reaction time, yield, etc., and the present invention was achieved.
That is, using the sulfonium compound represented by the general formula (1), a step of bonding a charged organic group having a cationic or anionic functional group to the pigment particle surface, and using a reactive monomer, It has been found that electrophoretic particles having excellent memory properties can be obtained by producing electrophoretic particles by a process of bonding a graft polymer chain having a molecular structure having an affinity for a dispersion medium to the pigment particle surface.

  The memory property, which is the object of the present invention, is that when a voltage is applied between a pair of oppositely disposed conductive layers (at least one of which is transparent) at least one of which is light transmissive, In the encapsulated (or contained) electrophoretic dispersion, the electrophoretic particles migrate according to the polarity of the electrodes, but the phenomenon that the image is maintained without being erased even if the voltage application is stopped and both electrodes are short-circuited. .

As a result of examining the memory property, the polymer chain is simply pigmented by graft polymerization without using the sulfonium compound of the present invention to bind the charged organic group having a cationic or anionic functional group to the pigment particle surface. It has been confirmed that there is no memory property when it is only bonded to the particle surface.
Although it is not clear about the mechanism in which memory property appears when electrophoretic particles are bonded to the surface of pigment particles by using charged organic groups (“charged groups”), the electrophoretic particles are applied to an electrode to which a positive or negative voltage is applied. When migrating, it is presumed that pigment particles having a charged group are more likely to be adsorbed to the electrode or more likely to be deposited or aggregated near the electrode than pigment particles having no charged group. In fact, when the electrophoretic particles migrated near the electrode are observed with a microscope, it is observed that pigment particles having a charged group are accumulated or aggregated near the electrode, and it can be confirmed that they do not change over time. . On the other hand, it is observed that pigment particles not having a charged group are diffused over time without accumulation or aggregation. From these observation results, it is apparent that the memory property appears by having charged groups on the pigment particle surface.

Among the pigment particles having a charged group, those in which a charged organic group having a cationic or anionic functional group is introduced using a sulfonium compound are charged by other reactions such as a conventionally known diazonium reaction. High memory performance compared to the case with.
The reason for this is considered to be the specificity of the molecular structure and molecular form of the charged group when bound to the pigment particles by a reaction using a sulfonium compound (substantially, “sulfonium reaction”).
The sulfonium reaction is a so-called nucleophilic substitution reaction that takes place between a so-called sulfonium ion as a counter ion and a nucleophile [having a reaction site (active species: functional group) having nucleophilic reactivity (for example, carbon black)]. It is. It is considered that when the nucleophile reacts with the sulfonium compound, there is a difference in reactivity and selectivity compared to other reactions such as diazonium reaction. Here, the reactivity means the reaction rate and the reaction amount of the charged group when the charged group is chemically bonded to the pigment particle surface. The reaction selectivity means compatibility and compatibility with each other when the sulfonium ion of the sulfonium compound reacts with the reaction site on the pigment particle surface. In other words, the reactivity in the sulfonium reaction is higher than that in other reactivities such as the diazonium reaction, and in terms of reaction selectivity, the sulfonium reaction is more effective in other reactions such as the diazonium reaction in terms of the reaction yield of the charged organic group. It is better than that.

  That is, from the reactivity and reaction selectivity, it is presumed that in the reaction using the sulfonium compound, the molecular structure of the charged group (charged organic group) bonded to the pigment particle surface is arranged precisely and orderly. Therefore, it is considered that the charge amount expressed in a state where the charged group is bonded to the pigment particle surface is higher than the charge amount of the functional group bonded by other reaction such as diazonium reaction. When charged groups are attached to the surface of pigment particles by other synthesis methods other than reactions using sulfonium compounds, the memory state is not so high. ) Is considered to be related. This is also supported by the experimental reaction yield. These are considered to be related to high memory characteristics.

Below, a sulfonium compound and reaction (sulfonium reaction) using the same are demonstrated in detail.
In general, sulfonium refers to a group of compounds having ^a general structural formula represented by R ^a (R ^b ) (R ^c ) S ⁺ (R ^a , R ^b and R ^c are hydrogen atoms or organic compounds). Represents a group).
(The cation H ₃ S ⁺ is also sulfonium in a broad sense.) In addition, the monovalent substituent represented by R ^a (R ^b ) S ⁺ — is a sulfonio group.
Called (sulfonio group). The sulfonium is obtained by allowing an alkyl halide to act on a sulfide compound. The sulfonium compound in the present invention is a sulfide compound (substantially, “sulfide” or “dialkyl sulfide”) represented by the following reaction formula (A), and a cationic or anion. It is obtained by the reaction of an aryl compound having a functional group and a halogen atom as a substituent (substantially, “halogenated aryl compound”).

[Sulphonium compounds]
R ¹ −S−R ² + R ³ Z → R ¹ (R ² ) (R ³ ) S ⁺ · Z ⁻
... (A)
[In Formula (A), R ¹ and R ² represent a hydrogen atom, a linear or branched alkyl group having 1 to 6 carbon atoms which may have a substituent, and R ¹ and R ² May be the same or different. R ³ is a charged organic group and represents an aryl group having a cationic or anionic functional group or a linear or branched alkyl group having 1 to 12 carbon atoms, and Z ⁻ represents a hydroxyl ion Represents an organic anion derived from an inorganic anion or an organic acid. ]

[Sulphonium reaction]
As described above, the sulfonium reaction is a so-called nucleophile [Nu-] having an active species (reaction site) having a nucleophilic reactivity with a sulfonium ion as a counter ion as shown in the following reaction formula (B). Nucleophilic substitution reaction that occurs between
R ¹ (R ² ) (R ³ ) S ⁺ + Nu ⁻ → R ¹ (R ² ) S + R ³ −Nu (B)
[In the formula (B), R ¹ and R ² represent a hydrogen atom, a linear or branched alkyl group having 1 to 6 carbon atoms which may have a substituent, and R ¹ and R ² May be the same or different. R ³ is a charged organic group, and represents an aryl group having at least a cationic or anionic functional group or a linear or branched alkyl group having 1 to 12 carbon atoms, Nu ⁻ is a nucleophile Represents (nucleophile). ]

Hereinafter, the synthesis of the sulfonium compound will be described more specifically.
[Synthesis of a sulfonium compound having a cationic or anionic functional group]
As shown in the following reaction formula (C), the sulfonium compound includes a sulfide compound (sulfur compound) and a halogenated aryl derivative (halogenated benzene: in the present invention, having a cationic or anionic functional group), etc. It can be synthesized by reacting while heating in a solvent. The counter ion (Y ⁻ ) of the obtained product can be subjected to a salt exchange reaction to a desired counter ion (Z ⁻ ) by a salt exchange reaction.

[In Reaction Formula (C), R ¹ and R ² represent a hydrogen atom, a linear or branched alkyl group having 1 to 6 carbon atoms which may have a substituent, R ¹ and R ² may be the same or different. R ³ is a charged organic group and represents at least an aryl group having a cationic or anionic functional group or a linear or branched alkyl group having 1 to 12 carbon atoms. A represents a cation, Z ⁻ represents a hydroxyl ion, an inorganic anion or an organic anion derived from an organic acid, and Y ⁻ represents an inorganic anion or an organic anion derived from an organic acid. ]
AZ represents a compound used for salt exchange reaction with a desired counter ion (Z ⁻ ). A is a cation such as sodium or potassium.

  For example, when dimethyl sulfide is reacted with chlorobenzene having a cationic or anionic functional group in acetonitrile, a sulfonium compound having a chloride ion as a counter ion is obtained as shown in the following reaction formula (D). When the sulfonium compound is subjected to salt exchange using sodium hydroxide, a sulfonium compound substituted with hydroxyl ions is synthesized. The reaction formula (D) shows a basic synthesis process for obtaining a sulfonium compound, and is an example of one having no cationic or anionic functional group. The corresponding sulfonium compound can be obtained in the same manner also when it has a cationic or anionic functional group.

  Reaction in the case of using chlorobenzene having a cationic or anionic functional group (X) in the reaction formula (C) is shown in the following reaction formula (E).

[In Reaction Formula (E), R ¹ and R ² represent a hydrogen atom, a linear or branched alkyl group having 1 to 6 carbon atoms which may have a substituent, R ¹ and R ² may be the same or different. X represents a cationic or anionic functional group. A represents a cation, and Z ⁻ represents a hydroxyl ion, an inorganic anion, or an organic anion derived from an organic acid. ]

In addition, as X, cationic functional groups, such as an amino group, a methoxy group, and a methyl group, and anionic functional groups, such as a chlorine atom and a nitro group, are mentioned. AZ represents a compound used for salt exchange reaction of chloride ion (Cl ⁻ ) to a desired counter ion (Z ⁻ ). A is a cation such as sodium or potassium.

When chlorobenzene having a cationic or anionic functional group is used as the substituent (X), a sulfonium compound to which a corresponding charged organic group is bonded can be synthesized.
(X) can be selected in various ways. At that time, according to Hammett's rule, as X goes to the left, the influence of the electron-withdrawing group increases and the anionicity becomes stronger. Can be selected.
X: p-NO ₂ > m-NO ₂ >p-Cl> -H >> p-CH ₃ > m-NH ₂ > p-OCH ₃ > p-NH ₂
For example, when X is p-NH ₂ , a cationic sulfonium compound is obtained, and using this, a phenyl group (charged organic) having p-NH ₂ as a functional group on the surface of the pigment particles (for example, carbon black) is used. If the group is introduced, the carbon black surface can be imparted with a cationic property. On the other hand, if X is p-NO ₂ , an anionic sulfonium compound can be obtained, and anionicity can be imparted to the carbon black surface. When used in an electrophoretic dispersion, the polarities are different, but in both cases, the adsorptive power to the electrode is increased and an improvement in memory performance can be expected.

Specific examples of the basic synthesis process (sulfonium compound having no cationic or anionic functional group) for obtaining the sulfonium compound represented by the reaction formula (D) are shown below. In addition, about what has a cationic or anionic functional group, it shows in the below-mentioned Example.
[Method of synthesizing sulfonium compound]
A 1-liter four-necked flask equipped with a sulfonium compound synthesis stirrer, reflux condenser and thermometer was purged with nitrogen, and then 12.4 g (0.2 mol) of dimethyl sulfide and 200 ml of acetonitrile were added and mixed to dissolve. Then, 22.5 g (0.2 mol) of chlorobenzene dissolved in 100 ml of acetonitrile was added dropwise over 1 hour under stirring under heating and refluxing conditions, and the reaction was continued under heating and refluxing conditions while stirring for another 5 hours. A white crystal was precipitated. Subsequently, the mixture was cooled to room temperature, and the precipitated crystals were separated by filtration and dried to obtain 30.2 g (yield 86.7%) of phenyldimethylsulfonium chloride. 8.7 g (0.05 mol) of phenyldimethylsulfonium chloride thus obtained was dissolved in 200 ml of methanol-water (1/1 (volume ratio)) and placed in a flask of the same volume. When 2.0 g (0.05 mol) of sodium hydroxide dissolved in 300 ml of water was added dropwise with stirring, precipitation occurred and the mixture became cloudy and became a slurry.
After completion of the dropwise addition, stirring was continued at room temperature for 1 hour, and then the crystals were filtered off and recrystallized from methanol to obtain 6.5 g (yield 83.0%) of phenyldimethylsulfonium hydroxide. The results of elemental analysis of this phenyldimethylsulfonium hydroxide are shown in Table 1 below.

  In the present invention, first, a charged organic group having a cationic or anionic functional group is introduced into the surface of pigment particles (for example, carbon black) by the step [I] using the sulfonium compound to improve the memory property. In addition, a graft polymer chain containing a molecular structure having an affinity for the dispersion medium is further introduced into the dispersion medium (for example, a nonpolar solvent such as isoparaffin hydrocarbon) by the step [II] using a reactive monomer. Easy to disperse. For example, when a graft polymer chain is introduced to the surface of carbon black, it is necessary to perform surface treatment by previously attaching a vinyl group to the surface of carbon black. Can be carried out by a diazonium reaction.

In the present invention, the reactive monomer used in the step [II] includes methacrylate or acrylate, and they may be used in combination. Examples of the methacrylate include 2-ethylhexyl (meth) acrylate, octyl (meth) acrylate, nonyl (meth) acrylate, decyl (meth) acrylate, lauryl (meth) acrylate, stearyl (meth) acrylate, methyl (meth) acrylate, Ethyl (meth) acrylate, butyl (meth) acrylate, hexyl (meth) acrylate, cyclohexyl (meth) acrylate, benzyl (meth) acrylate, phenyl (meth) acrylate, ethylene glycol di (meth) acrylate, diethylene glycol di (meth) acrylate , Triethylene glycol tri (meth) acrylate, butanediol di (meth) acrylate, 1,6-hexanediol di (meth) acrylate, trimethylolpro Pantri (meth) acrylate, tetramethylol methane tri (meth) acrylate, tetramethylol methane tetra (meth) acrylate, dipropylene glycol di (meth) acrylate, trimethylol hexane tri (meth) acrylate, pentaerylit tetra (meth) Examples thereof include acrylate, 1,3-dibutylene glycol di (meth) acrylate, trimethylolethane tri (meth) acrylate, and the like.
Moreover, vinyl laurate, styrene, vinyl toluene, vinyl acetate, divinylbenzene, etc. are also used as a reactive monomer, and may be used by mixing with the above methacrylates or acrylates.

Hereinafter, the case where carbon black is used as the electrophoretic particles of the electrophoretic dispersion will be described as an example. The important factors when using carbon black as an electrophoretic particle (electrophoretic dispersion) are the particle size, structure and physicochemical properties of the particle surface, which are usually the three basic characteristics of carbon black as described below. It is called.
<The three basic characteristics of carbon black>
(I) Particle size: Particle size and specific surface area (II) Structure: DBP oil supply (ml / 100 g) and structure index (III) Surface chemical properties: Volatile content (%) and pH

  The particle diameter and the specific surface area are opposite to each other, and the specific surface area of the particles having a large particle diameter is small. The electrophoretic dispersion requires a certain particle size from the viewpoint of obtaining blackness and memory properties, and preferably has a primary particle size of 20 nm or more. The structure development can be estimated from the DBP oil supply amount, and a moderate structure development is preferable. If the structure is developed too much, it will have a negative effect on migration and response speed.

  The volatile content and pH defined by the chemical properties of the surface are not particularly defined because the surface modification of the present invention is performed by a substitution reaction using a sulfonium compound. Since it can be considered that pH and volatile matter indirectly represent the active species (reaction sites) on the surface of the carbon black particles, so-called functional groups, for example, a nonpolar hydrocarbon solvent (isoparaffinic hydrocarbon as a dispersion medium) Etc.) is considered to have some effect.

<PH of carbon black>
The pH of carbon black can be measured by the following measurement method.
Weigh 1 to 10 g of carbon black sample in a beaker, add water at a rate of 10 ml per 1 g of sample, cover with a watch glass and boil for 15 minutes (a few drops of ethanol may be added to make the sample easy to wet). After boiling, the mixture is cooled to room temperature, and the supernatant is removed by a gradient method or a centrifugal separation method to leave a mud. An electrode of a glass electrode pH meter is placed in this mud, and the pH is measured by JISZ8802 (pH measurement method). In this case, the measurement value may change depending on the electrode insertion position. Therefore, move the beaker to change the electrode position and measure carefully so that the muddy surface of the electrode is in full contact with the pH value. Read the value when it becomes.
<Carbon black volatiles>
The volatile content of carbon black is obtained by the following measuring method.
A dry sample of carbon black is packed into a platinum crucible or a porcelain crucible with the same shape and capacity as a drop lid by shaking it to a level not exceeding 2 mm under the lid, and its mass is measured. Cover it, put it in an electric furnace, heat it at 950 ± 25 ° C for exactly 7 minutes, take it out, let it cool to room temperature in a desiccator, weigh the mass after heating, and volatilize by the following formula (2) Calculate minutes. In formula (I), V: volatile content (%), WD: mass (g) of dried sample, WR: mass (g) of sample after heating.
V = [(WD−WR) / WD] × 100 (2)

  As the carbon black used in the present invention, a single type or a plurality of types of carbon blacks may be used in combination. Such carbon blacks include Degussa Color Black FW200, Color Black FW2, Color Black FW2V, Color Black FW1, Color Black FW18, Special Black 6, Color Black S170, Color Black S160, Special Black 5, Special Black 4, Special Black 4A, Printex 150T, Printex U, Printex V, Printex 140U, Printex 140V, Special Black 550, Special Black 350, Special Black 250, Special Black 100; Mitsubishi Chemical Corporation MA7, MA77 , MA8, MA11, MA100, MA100R, MA230, MA220, # 2200B; MONARCH700 manufactured by Cabot Corporation MONARCH800, MONARCH880, MONARCH900, MONARCH1000, MONARCH1300, MONARCH1400, MOGUL-L, REGAL400R, VULCAN XC-72R; Columbia-made Raven1255; Commercial products such as Color Black S170, Color Black S150, and Printex U can be used, and even those newly manufactured for this purpose can be used.

The electrophoretic dispersion of the present invention contains electrophoretic particles composed of charged pigment particles (for example, carbon black) modified in the steps [I] and [II] in a dispersion medium. A dye may be further contained in the electrophoretic dispersion. Although it does not specifically limit as dye, Macrolex blue RR (made by Bayer) etc. are mentioned.
The dispersion medium used in the electrophoretic dispersion of the present invention is not particularly limited, but nonpolar solvents such as isoparaffin hydrocarbons (Isoper manufactured by Exxon Chemical Co., Ltd.) and modified silicone oils (produced by Toray Dow Chisso) are preferred. Can be used.

  The electrophoretic dispersion of the present invention may further contain a dispersant in order to further improve the storage stability of the electrophoretic particles. The dispersant is not particularly limited as long as it is compatible with the dispersion medium and can stably disperse the electrophoretic particles, and conventionally known pigment dispersants can be used.

  Examples of the dispersant include sorbitan fatty acid esters (for example, sorbitan monooleate, sorbitan monolaurate, sorbitan sesquioleate, sorbitan trioleate), polyoxyethylene sorbitan fatty acid esters (for example, polyoxyethylene sorbitan monostearate, Polyoxyethylene sorbitan monooleate, etc.), polyethylene glycol fatty acid esters (eg, polyoxyethylene monostearate, polyethylene glycol diisostearate, etc.), polyoxyethylene alkyl phenyl ethers (eg, polyoxyethylene nonylphenyl ether, poly Nonionic surfactants such as oxyethylene octyl phenyl ether and aliphatic diethanolamides can be used.

  Examples of the polymer dispersant include, for example, styrene-maleic acid resin, styrene-acrylic resin, rosin, urethane polymer compound BYK-160, 162, 164, 182 (manufactured by Big Chemie), urethane dispersant EFKA. -47, LP-4050 (manufactured by EFKA), polyester polymer compound Solsperse 24000 (manufactured by Zeneca), aliphatic diethanolamide polymer compound Solsperse 17000 (Zeneca), and the like. The polymer dispersant preferably has a number average molecular weight of 1000 or more.

  Other polymeric dispersants include monomers such as lauryl methacrylate, stearyl methacrylate, 2-ethylhexyl methacrylate, and cetyl methacrylate that can form a solvated part in the dispersion medium, and a part that is difficult to solvate in the dispersion medium. Random copolymers of monomers such as methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, styrene, vinyltoluene and monomers having polar functional groups that can be formed, and graft copolymers disclosed in JP-A-3-188469. A polymer etc. are mentioned.

  Monomers having a polar functional group include monomers having an acidic functional group such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, maleic acid, styrene sulfonic acid; dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, vinylpyridine, Monomers having basic functional groups such as vinylpyrrolidine, vinylpiperidine, vinyllactam, salts thereof, styrene-butadiene copolymers, styrene and long-chain alkyl methacrylates disclosed in JP-A-60-10263 Examples thereof include block copolymers. Of these, the graft copolymers disclosed in JP-A-3-188469 are preferred.

  The weight ratio of the dispersant to the electrophoretic particles when dispersing the electrophoretic particles in the dispersion medium in the present invention is preferably 0.05 to 5%. By selecting the weight ratio of the dispersant within this range, the electrophoretic particles can be stably dispersed.

As described above, in the image display medium of the present invention, the electrophoretic particles are dispersed in a dispersion medium between a pair of opposed conductive layers, at least one of which is light transmissive, that is, between at least one of transparent electrodes. The electrophoretic dispersion prepared as described above is enclosed (or accommodated). The image display medium of the present invention will be described below with reference to the drawings.
FIG. 1 is a schematic cross-sectional view showing a configuration example of an image display medium according to the present invention.
In FIG. 1, an image display medium 10 includes an electrophoretic dispersion liquid 11 of the present invention sandwiched between conductive layers 12 and 13. The electrophoretic dispersion 11 has black particles 11a and white particles 11b dispersed in a nonpolar solvent (dispersion medium) 11c, and has both an electrophoretic effect and a memory effect. The black particles 11a are positively charged. As the black particles 11a, for example, those in which a charged organic group having a cationic functional group is bonded to the surface of carbon black are used (of course, a graft polymer having a molecular structure having an affinity for a dispersion medium). The chains are also bound to the surface).
The conductive layer 12 has a light transmitting property, and the conductive layer 13 may or may not have a light transmitting property. Although not shown, the conductive layers 12 and 13 are divided so as to be divided into a right half and a left half on the paper surface so that a voltage can be applied, and the conductive layers 12 and 13 can function as an image display medium.

Next, the operation of displaying an image on the image display medium 10 will be described with reference to the schematic diagram of FIG.
First, negative and positive voltages are respectively applied to the right half of the conductive layers 12 and 13 in FIG. 2 using an external voltage applying means (not shown) [see FIG. 2 (a)]. As a result, the black particles 11a move upward due to electrostatic attraction (see FIG. 2B) but gradually reach the conductive layer 12 (see FIG. 2C) and adhere to the conductive layer 12. [Refer FIG.2 (d)]. At this time, when the image display medium 10 is viewed from above, the color of the white particles 11b can be seen on the left half of the paper and the color of the black particles 11b can be seen on the right half of the paper.
Even if the voltage application is stopped, the image is continuously displayed because of the memory property. Further, since the image can be displayed reversibly on the image display medium 10 by changing the polarity of voltage application, the image display medium 10 can be used repeatedly.

FIG. 3 is a schematic sectional view showing another configuration example of the image display medium according to the present invention. In FIG. 3, the same components as those in FIG. 1 are denoted by the same reference numerals and description thereof is omitted.
In the image display medium 20, the microcapsule 21 is sandwiched between the conductive layers 12 and 13. Further, as shown in FIG. 4, the microcapsule 21 includes the electrophoretic dispersion 11 of the present invention in a microcapsule film 21a.
The image display medium 20 is manufactured by arranging the microcapsules 21 on the conductive layer 12 or 13 so as to form a single layer by electrodeposition coating or the like. The microcapsule 21 is formed by, for example, dispersing the electrophoretic dispersion 11 in water in which gelatin is dissolved, encapsulating with gelatin and gum arabic coacervate, and then crosslinking with glutaraldehyde to form a microcapsule film 21a. It can produce by doing.

FIG. 5 is a schematic cross-sectional view showing still another configuration example of the image display medium according to the present invention.
In FIG. 5, an image display medium 30 has transparent electrodes 32A and 32B patterned on transparent substrates 31A and 31B, respectively, and a spacer 33 is interposed between the transparent electrodes 32A and 32B arranged to face each other. The electrophoretic dispersion liquid 34 of the present invention is enclosed. With such a configuration, occurrence of display unevenness due to aggregation and adhesion of electrophoretic particles can be suppressed.
Although it does not specifically limit as transparent substrate 31A and 31B, A film etc. can be used. Also, in the electrophoretic dispersion liquid 34, white and black particles are dispersed in a nonpolar solvent as described above. The spacer 33 is not particularly limited, but a mesh or porous perforated spacer can be used.

Next, an operation for displaying an image on the image display medium 30 will be described.
First, when positive and negative voltages are applied to the transparent electrodes 32A and 32B, black particles adhere to the transparent electrode 32B due to electrostatic attraction, and white particles adhere to the transparent electrode 32A due to electrostatic attraction, When the image display medium 30 is viewed from above, white is observed. On the other hand, when negative and positive voltages are applied to the transparent electrodes 32A and 32B, white particles adhere to the transparent electrode 32B due to electrostatic attraction, and black particles adhere to the transparent electrode 32A due to electrostatic attraction. When the image display medium 30 is viewed from above, black is observed. As described above, since the image can be displayed reversibly on the image display medium 30, the image display medium 30 can be used repeatedly.
FIG. 6 is a schematic sectional view showing still another configuration example of the image display medium according to the present invention.
In FIG. 6, the image display medium 40 can be folded in a sheet shape, and thus can be carried in a folded state.

An image display device can be configured by using the image display medium of the present invention.
FIG. 7 is an external perspective view showing an example of an image display device according to the present invention.
In FIG. 7, the image display device 50 uses the image display medium 10 on a flat screen, and can display an image by inputting image information from the input unit 51.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited by a following example. “Parts” means all parts by weight.

(Example 1) to (Example 3)
Steps [I] and [II] of the present invention are carried out under the following conditions, respectively, and the electrophoretic particles of [Example 1] to (Example 3) [grafted carbon black having a charged group (charged organic group)] Were manufactured and evaluated.

[Synthesis Example 1]
<Synthesis of sulfonium compound (X = p-NH ₂ in reaction formula (E))>
In the reaction formula (E), a sulfonium compound in which the substituent X is a p-amino group was synthesized. That is, using dimethyl sulfide as the sulfide compound (Compound 1) and p-aminochlorobenzene as the chlorobenzene derivative (Compound 2), a sulfonium having an amino group in the same manner as described in the above-mentioned “Method of synthesizing a sulfonium compound”. The compound was synthesized.

<Cationization treatment of carbon black>
Using the sulfonium compound having an amino group obtained above, a charged organic group having a cationic functional group was introduced onto the surface of the graft carbon black under the following conditions.
In the sulfonium compound, the cation of the sulfonium salt is cleaved by heating, and the S ⁺ cation is activated. That is, by mixing a sulfonium compound and carbon black in toluene and raising the temperature to 80 ° C., a predetermined charged organic group can be bonded to the surface of the graft carbon black.
Specifically, carbon black (Printex L (Degussa): specific surface area 150 m ² / g, DBP oil absorption 116 ml / 100 g) was used in an atmosphere of N ₂ gas purge using a glass flask equipped with a ₂ L stirring device. 20 g and 6.12 g of the previously synthesized sulfonium compound having a p-amino group were mixed well with 500 ml of toluene, and then the temperature was raised to 60 ° C. and stirred for 30 minutes. Further, the temperature was raised to 80 ° C. and stirred at 250 rpm for 1 hour. The obtained slurry was filtered with a filter paper (trade name: Toyo Filter Paper No. 2; manufactured by Advantis), and the carbon black particles were sufficiently washed with toluene and dried in an oven at 110 ° C. [Particle S1] in which a p-aminophenyl group was introduced on the surface of carbon black was obtained by the above method.

[Synthesis Example 2]
<Synthesis of sulfonium compound (X = p-NO ₂ in reaction formula (E))>
In the reaction formula (E), a sulfonium compound in which the substituent X is a p-nitro group was synthesized. That is, dimethyl sulfide is used as the sulfide compound (compound 1), p-nitrochlorobenzene is used as the chlorobenzene derivative (compound 2), and the above-mentioned “method of synthesizing sulfonium compound”
A sulfonium compound having a nitro group was synthesized in the same manner as described above.

<Anionization treatment of carbon black>
Using the sulfonium compound having a nitro group obtained above, a charged organic group having an anionic functional group was introduced onto the surface of the graft carbon black under the following conditions.
Using a glass flask equipped with a ₂ L stirrer, carbon black [MA100 (Mitsubishi Chemical): specific surface area 110 m ² / g, DBP oil absorption 100 ml / 100 g] 20 g was synthesized first in an N ₂ gas purge atmosphere. After 5.18 g of the sulfonium compound having a p-nitro group was well mixed with 500 ml of toluene, the temperature was raised to 70 ° C. and stirred for 30 minutes. Further, the temperature was raised to 80 ° C. and stirred at 250 rpm for 1 hour. The obtained slurry was filtered with a filter paper (trade name: Toyo Filter Paper No. 2; manufactured by Advantis), and the carbon black particles were sufficiently washed with toluene and dried in an oven at 110 ° C. [Particle S2] in which a p-nitrophenyl group was introduced on the surface of carbon black was obtained by the above method.

[Synthesis Example 3]
<Synthesis of sulfonium compound (X = p-OCH ₃ in reaction formula (E))>
In the reaction formula (E), a sulfonium compound in which the substituent X is a p-methoxy group was synthesized. That is, dimethylsulfide is used as the sulfide compound (compound 1) and p-methoxychlorobenzene is used as the chlorobenzene derivative (compound 2).
A sulfonium compound having a methoxy group was synthesized in the same manner as described above.

<Cationization treatment of carbon black>
Using the sulfonium compound having a methoxy group obtained above, a charged organic group having a cationic functional group was introduced onto the surface of the graft carbon black under the following conditions.
Using a glass flask equipped with a ₂ L stirrer, carbon black (BLACK PEARLS L (CABOT): specific surface area 138 m ² / g, DBP oil absorption 62 ml / 100 g) and 20 g in an N ₂ gas purge atmosphere Then, 7.86 g of the sulfonium compound having a p-methoxy group synthesized in the above was mixed well with 500 ml of toluene, and then the temperature was raised to 70 ° C. and stirred for 30 minutes. Further, the temperature was raised to 80 ° C. and stirred at 250 rpm for 1 hour. The obtained slurry was filtered with a filter paper (trade name: Toyo Filter Paper No. 2; manufactured by Advantis), and the carbon black particles were sufficiently washed with toluene and dried in an oven at 110 ° C. [Particle S3] having a p-methoxyphenyl group introduced on the surface of carbon black was obtained by the above method.

<Production of grafted carbon black having a charged group>
Using the particles S1 to S3 obtained above, a graft polymer chain containing a molecular structure having an affinity for a dispersion medium is bonded to the surface of each particle under the following conditions, and a graft having a charged group (charged organic group) Carbon black [particles G1] to [particles G3] were produced.

While adding 3.0 L of deionized water to a 4 L glass reaction vessel and stirring at 250 rpm, 115 g of carbon black (particles S1) was added. Next, after adding 3.0 mL of 37% hydrochloric acid, 2.5 g of 4-vinylaniline was added and stirred at 65 ° C. for 30 minutes or more. Further, a solution obtained by dissolving 1.43 g of sodium nitrite in 10 mL of deionized water was dropped in about 1 hour, and stirred at 65 ° C. for 3 hours. Subsequently, the dispersion obtained by returning to room temperature and stirring overnight was centrifuged at 30000 rpm for 20 minutes and decanted. To the resulting black powder, 500 mL of deionized water was added and stirred to redisperse. Further, the obtained dispersion was centrifuged at 30000 rpm for 20 minutes and decanted. The obtained black powder was left overnight, dried and then vacuum dried at 40 ° C. for 4 hours to obtain surface-treated carbon black.
Next, 50 g of surface-treated carbon black, 100 mL of toluene, 100 mL of 2-ethylhexyl methacrylate, and AIBN (0.65 g) were charged into a 1 L glass reaction vessel. Next, the mixture was purged with nitrogen for 20 minutes while stirring at 250 rpm, heated to 70 ° C. in about one hour, stirred for 7 hours, and cooled to room temperature. Further, 500 mL of THF was added and stirred, and then reprecipitated in 3 L of methanol, and suction filtered. The obtained residue was stirred and redispersed in 1.5 L of THF, then cooled to 10 ° C., centrifuged at 30000 rpm for 20 minutes, and decanted three times in total. Furthermore, it was vacuum-dried at 70 ° C. for 4 hours to obtain a grafted carbon black [particle G1] having a charged group into which a graft polymer chain was introduced.
In the same manner as described above, [Particle G2] was synthesized using [Particle S2], and [Particle G3] was synthesized using [Particle S3].
When the obtained graft carbon black [particles G1] to [particles G3] were subjected to thermogravimetric analysis, the weight reductions were 9.76%, 10.5%, and 9.85%, respectively.

(Comparative Example 1)
<Production of grafted carbon black without charged groups>
For use in a comparative example, carbon black [particle G0] having no charged group was produced under the following conditions.
Carbon black (Printex A (manufactured by Degussa): specific surface area 45 m ² / g, DBP oil supply 118 ml / 100 g) while charging 3.0 L of deionized water into a 4 L glass reaction vessel and stirring at 250 rpm 115 g was charged. Next, after adding 3.0 mL of 37% hydrochloric acid, 2.5 g of 4-vinylaniline was added and stirred at 65 ° C. for 30 minutes or more. Further, a solution obtained by dissolving 1.43 g of sodium nitrite in 10 mL of deionized water was dropped in about 1 hour, and stirred at 65 ° C. for 3 hours. Subsequently, the dispersion obtained by returning to room temperature and stirring overnight was centrifuged at 30000 rpm for 20 minutes and decanted. To the resulting black powder, 500 mL of deionized water was added and stirred to redisperse. Further, the obtained dispersion was centrifuged at 30000 rpm for 20 minutes and decanted. The obtained black powder was allowed to stand overnight, dried, and then vacuum dried at 40 ° C. for 4 hours to obtain surface-treated carbon black.
Next, 50 g of surface-treated carbon black, 100 mL of toluene, 100 mL of 2-ethylhexyl methacrylate, and AIBN (0.65 g) were charged into a 1 L glass reaction vessel. Next, the mixture was purged with nitrogen for 20 minutes while stirring at 250 rpm, heated to 70 ° C. in about one hour, stirred for 7 hours, and cooled to room temperature. Further, 500 mL of THF was added and stirred, and then reprecipitated in 3 L of methanol, and suction filtered. The obtained residue was stirred and redispersed in 1.5 L of THF, then cooled to 10 ° C., centrifuged at 30000 rpm for 20 minutes, and decanted three times in total. Furthermore, it was vacuum-dried at 70 ° C. for 4 hours to obtain grafted carbon black [particles G0] having no charged groups and having graft polymer chains introduced therein. When the graft carbon black was subjected to thermogravimetric analysis, the weight loss was 12.3%.

In order to obtain an electrophoretic dispersion in later-described evaluation, titanium oxide [particle T] having no charged group, which is used together with the above [particle G1] to [particle G3] and [particle G0], was produced under the following conditions.
<Production of titanium oxide without charged groups>
Into a 4 L glass reaction vessel, 930.7 g of ethanol and 69.3 g of deionized water were added, and while stirring at 150 rpm, acetic acid was added dropwise to adjust the pH to 4.5. Next, 160 g of 3- (trimethoxysilyl) propyl methacrylate was added and stirred at 150 rpm for 5 minutes, and then 250 rpm was achieved. Furthermore, 1000 g of titanium oxide [R-960 (manufactured by Dupont)] was added and stirred at 250 rpm for 10 minutes, and then stirred at 200 rpm. Next, 1826.6 g of methanol was added and stirred at 200 rpm for 1 minute, and then centrifuged at 3000 rpm for 20 minutes and decanted. The obtained white powder was allowed to stand overnight and dried, followed by vacuum drying at 70 ° C. for 4 hours to obtain surface-treated titanium oxide.
Next, 960 g of lauryl methacrylate and 1386 g of toluene were charged into a 4 L glass reaction vessel and stirred at 200 rpm, and then stirred at 300 rpm at 50 ° C. Next, 750 g of surface-treated titanium oxide pulverized in advance was added, and the atmosphere was substituted with nitrogen for 20 minutes while stirring. Further, a solution obtained by dissolving AIBN (5.64 g) in 500 g of toluene is dropped in about 1 hour, heated to 70 ° C. in about 1 hour, stirred overnight at 70 ° C., and then centrifuged at 10,000 rpm for 30 minutes. Decanted. The obtained white powder was charged with 1000 g of toluene, stirred and redispersed, and then the operation of centrifuging at 3000 rpm for 30 minutes and decanting was repeated twice. Furthermore, after leaving it to stand overnight and drying, it was vacuum-dried at 70 ° C. for 4 hours to obtain grafted titanium oxide having no charged group. As a result of thermogravimetric analysis of the grafted titanium oxide, the weight loss was 8.3%.

[Thermogravimetric analysis] was carried out by the following method.
Thermogravimetric analysis was started using TGA-50H (manufactured by Shimadzu Corporation) by putting about 20 mg of the dried sample into a platinum cell. Next, after raising the temperature from room temperature to 600 ° C. at 20 ° C./minute in a nitrogen atmosphere, the temperature is held at 600 ° C. for 10 minutes, and the atmosphere is switched to air 3 minutes after reaching 600 ° C. The temperature was increased from 600 ° C. to 1000 ° C. at a rate of 1 minute. At this time, the polymer is thermally decomposed in a nitrogen atmosphere, and carbon black burns in an air atmosphere.

Using each of [Particle G1] to [Particle G3] and [Particle G0] produced in (Example 1) to (Example 3) and (Comparative Example 1), respective electrophoretic dispersions were obtained. [Particle T] is a particle commonly used in each dispersion.
<Electrophoretic dispersion using [particle G1]: electrophoretic dispersion of (Example 1)>
1.7 parts of grafted carbon black (particles G1), 40 parts of grafted titanium oxide (particles T), dispersant Solsperse 17000 (manufactured by Avicia) 0.47 parts, nonionic surfactant sorbitan trioleate (Wako Pure Chemical) 0.53 part) and Isopar G (ExxonMobil Corporation) 57.26 parts were mixed and dispersed with ultrasound for 1 hour to obtain an electrophoretic dispersion.
<Electrophoretic Dispersion Using [Particle G2]: Electrophoretic Dispersion of (Example 2)>
An electrophoretic dispersion using [Particle G2] was obtained in the same manner except that (Particle G1) was replaced with (Particle G2) in the preparation of the electrophoretic dispersion using [Particle G1].
<Electrophoretic Dispersion Using [Particle G3]: Electrophoretic Dispersion of (Example 3)>
An electrophoretic dispersion using [Particle G3] was obtained in the same manner except that (Particle G1) was replaced with (Particle G3) in the preparation of the electrophoretic dispersion using [Particle G1].
<Electrophoretic Dispersion Using [Particle G0]: Electrophoretic Dispersion of (Comparative Example 1)>
An electrophoretic dispersion using [particle G0] was obtained in the same manner except that (particle G1) was replaced by (particle G0) in the preparation of the electrophoretic dispersion using [particle G1].

An image display cell was produced using each of the electrophoretic dispersions obtained above.
[Production of image display cell]
Each of the two glass substrates with ITO electrodes (manufactured by Geomat Co., Ltd.) can be separated by a capillary phenomenon using a microsyringe in a space provided by sandwiching a 50 μm thick polyester film with a 1 cm ² opening. The electrophoretic dispersion was sealed, and the image display cell of Example 1 to the image display cell of Example 3 and the image display cell of Comparative Example 1 were produced.
The glass substrate with an ITO electrode is made of soda glass, the dimensions are 29.8 ± 0.1 mm × 39.8 ± 0.1 mm, the outer periphery is chamfered, and the surface of the ITO electrode The resistance is 50 ± 10Ω / □.

Display characteristics (white reflectance, black reflectance, contrast, response speed, memory property) using the image display cell of Example 1 to the image display cell of Example 3 and the image display cell of Comparative Example 1 produced above. Evaluated.
[Evaluation method of image display cell]
Change the polarity of the applied voltage to the polarity of the ITO electrode of the image display cell (15V, -15V), the LCD EVALUATION to reflect the reflectivity, contrast, response speed, and 1.5hr (90 minutes) memory performance of the image display cell The measurement was performed using a SYSTEM LCD-5000 (manufactured by Otsuka Electronics Co., Ltd.). In addition, switching of voltage application was performed about 100 times at a frequency of 0.1 Hz, and the memory property was evaluated. The results are shown in Table 2 below.
Note that the presence / absence of the memory property was determined by the ratio of change in black and white reflectance during 90 minutes. As a criterion for determining the presence or absence of memory, in principle, if the white reflectance is 40% or more and the black reflectance is 5% or less during the 90-minute holding time, it is judged that there is memory property when both do not change significantly. Is done. However, even if one of the black and white changes slightly during this holding time, it may be determined that there is a memory property. When it is determined that there is no memory property, both black and white change greatly.

[Results of image display cell evaluation]
When 15 V is applied to one (for example, the upper) ITO electrode of the image display cell, the black electrophoretic particles (grafted carbon black having charged groups) in Examples 1 and 3 are positively charged. It moves quickly to the ITO electrode (for example, the lower part). White electrophoretic particles (titanium oxide having no charged group) are slightly negatively charged and can move depending on the polarity of the ITO electrode. Is observed. Next, when −15 V is applied to the upper ITO electrode, the black electrophoretic particles of Example 1 and Example 3 quickly move to the upper ITO electrode, and similarly white is observed if observed from the lower part. . On the contrary, in Example 2, the black electrophoretic particles are negatively charged, and the white electrophoretic particles are also slightly negatively charged. In this case, the negative charge amount of the black electrophoretic particles is stronger than the charge amount of the white electrophoretic particles. Therefore, when 15 V is applied to the upper ITO electrode, the black electrophoretic particles are Immediately move to the ITO electrode. The white electrophoretic particles can be observed when viewed from below. Further, when −15 V is applied to the upper ITO electrode, the black electrophoretic particles move quickly to the lower portion. When observed from above, white is observed.
From Table 2, by using the electrophoretic particles of the present invention, the white reflectance, black reflectance contrast, response speed, etc. were all good. In addition, although switching of voltage application was performed about 100 times at a frequency of 0.1 Hz, it could be repeated stably. Moreover, even if the voltage was removed, the electrodeposited state (memory property) was maintained. That is, the white reflectance was 40% or more, the black reflectance was about 3% to 10%, and both were judged to have a memory property with no significant change for 90 minutes. FIG. 8 shows changes in white reflectance and black reflectance over time of the image display cell of Example 1.
On the other hand, in the case of the comparative example, since the carbon black and titanium oxide pigments were charged slightly positively and negatively, respectively, the response speed was slow and there was no memory property. If the response speed is not taken into consideration, electrophoresis can be performed according to the polarity of the ITO electrode. That is, when 15 V is applied to the upper ITO electrode, the black electrophoretic particles move to the lower part, and the white electrophoretic particles move to the upper part. On the other hand, when -15 V is applied to the upper ITO electrode, the black electrophoretic particles move to the upper part and the white electrophoretic particles move to the lower part. However, it does not have memory characteristics. FIG. 9 shows changes in white reflectance and black reflectance over time of the image display cell of Comparative Example 1.
8 and 9, in the two reflectance curves, the low reflectance curve indicates black reflectance (carbon black), and the high reflectance curve indicates white reflectance (titanium oxide). .

  As described above, the memory property is excellent by introducing a charged organic group using a sulfonium compound on the surface of the pigment particle, and introducing a graft polymer chain having a molecular structure having an affinity for the dispersion medium on the surface of the pigment particle. When electrophoretic particles are produced and an electrophoretic dispersion liquid containing the electrophoretic particles is used, an image display medium and an image display device can be configured.

It is a schematic sectional drawing which shows one structural example of the image display medium which concerns on this invention. It is a schematic diagram for demonstrating the operation | movement which displays an image on the image display medium 10 of FIG. It is a schematic sectional drawing which shows another structural example of the image display medium which concerns on this invention. It is an enlarged view of the microcapsule 21 of FIG. It is a schematic sectional drawing which shows another structural example of the image display medium based on this invention. It is a schematic sectional drawing which shows another example of a structure of the image display medium based on this invention. 1 is an external perspective view showing an example of an image display device according to the present invention. It is a figure which shows the change of the white reflectance and black reflectance with the passage of time of the image display cell of Example 1. FIG. It is a figure which shows the change of the white reflectance and black reflectance with the time passage of the image display cell of the comparative example 1. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Image display medium 11 Electrophoresis dispersion liquid 11a Black particle | grains 11b White particle | grains 11c Nonpolar solvent 12, 13 Conductive layer 20 Image display medium 21 Microcapsule 21a Microcapsule film 30 Image display medium 31A, 31B Transparent substrate 32A, 32B Transparent Electrode 33 Spacer 34 Electrophoretic dispersion liquid 40 Image display medium 50 Image display device 51 Input section

Claims (10)

A method for producing electrophoretic particles comprising charged pigment particles capable of migrating in a dispersion medium by applying a voltage,
The method for producing electrophoretic particles, wherein the surface of the pigment particles is modified by at least the following steps [I] and [II].
[I] A step of bonding a charged organic group having a cationic or anionic functional group to the surface of the pigment particle using a sulfonium compound represented by the following general formula (1) [II] Using a reactive monomer And bonding a graft polymer chain having a molecular structure having an affinity for the dispersion medium to the pigment particle surface
[In the general formula (1), R ¹ and R ² represent a hydrogen atom, a linear or branched alkyl group having 1 to 6 carbon atoms which may have a substituent, and R ¹ And R ² may be the same or different. R ³ is a charged organic group and represents an aryl group having a cationic or anionic functional group or a linear or branched alkyl group having 1 to 12 carbon atoms, and Z ⁻ represents a hydroxyl ion Represents an organic anion derived from an inorganic anion or an organic acid. ]   The method for producing electrophoretic particles according to claim 1, wherein the pigment particles are carbon black.   The method for producing electrophoretic particles according to claim 1, wherein the aryl group is a phenyl group.   The inorganic anion is an anion selected from halogen ion, nitrate ion, nitrite ion, acetate ion, phosphate ion, or sulfate ion, and the organic anion derived from the organic acid is formic acid, acetic acid, propion The method for producing electrophoretic particles according to any one of claims 1 to 3, which is an anion selected from acid, butyric acid, valeric acid, glycolic acid, gluconic acid, or lactic acid.   The method for producing electrophoretic particles according to any one of claims 1 to 4, wherein the dispersion medium is a nonpolar hydrocarbon.   The method for producing electrophoretic particles according to claim 1, wherein the reactive monomer is methacrylate or / and acrylate.   Electrophoretic particles obtained by the production method according to claim 1.   An electrophoretic dispersion liquid comprising at least a dispersion medium and the electrophoretic particles according to claim 7.   9. An image display medium, wherein the electrophoretic dispersion according to claim 8 is sealed between a pair of opposed conductive layers, at least one of which is light transmissive.   An image display device using the image display medium according to claim 9.