JP2007206570A - Particle for image display medium, manufacturing method thereof, image display medium, and image display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide particles for an image display medium which are capable of maintaining a stable image display state regardless of long-time preservation or use of the image display medium, a manufacturing method thereof, and the image display medium and an image display device which use the particles for the image display medium. <P>SOLUTION: Particles for the image display medium are used in the image display medium wherein at least one kind of particles different by at least color or charge polarity are enclosed between two substrates facing each other. The particles are obtained by; discharging a liquid containing at least a resin and a coloring agent from an oscillating nozzle; applying a DC potential to an electrode facing the nozzle; charging liquid drops sprayed from the nozzle, with a positive or negative polarity by an induction phenomenon; and drying and solidifying the liquid drops on which at least charges remain. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a particle for an image display medium used for a rewritable and paper-like image display medium having the advantages of both a display and a hard copy, a manufacturing method thereof, an image display medium, and an image display apparatus.

In recent years, with the heightened environmental awareness of paperless, research on paper-like displays that can display a desired image on a display substrate using electric power and can be rewritten has been advanced. As such a paper-like display, liquid phase type image display media such as an electrophoretic type and a thermal rewritable type are known (for example, see Patent Documents 1 to 3).
However, in such a liquid phase type image display medium, since particles migrate in the liquid, there is a problem that the response speed becomes slow due to the viscous resistance of the liquid. For this reason, recently, a dry type in which colored particles are sealed between two opposing substrates, or a type in which easily movable insulating colored particles or powdered fluid is sealed are attracting attention. Moreover, as the colored particles used, for example, a technique relating to electret particles (permanently charged particles) obtained by a pulverization method is disclosed (see Patent Document 1).

  In such a dry image display medium, the basis of the display mechanism is the surface charge of the particles, and in order to obtain a stable high-definition display, it is necessary to keep the surface charge of the particles stable. . However, the surface charge is very unstable and easily attenuates due to the surrounding environment such as humidity. In addition, changes easily occur due to contact or friction with other substances. Further, if any mass transfer or deformation occurs in the particles, the surface charge changes by itself. Such a change in surface charge leads to a change in display state, and there is a problem that the display state becomes unstable due to long-term storage or long-term use of the image display medium.

  In order to solve this point, Patent Document 4 proposes to use an electret material produced by heating and melting with a high voltage applied to both surfaces of a sample, cooling and solidifying the voltage while maintaining the voltage. Has been. However, in this proposal, the charge amount and particle size of each particle are not uniform, and the shape is an irregular shape that is crushed. Since the characteristics change, there is a problem that a stable image display state cannot always be maintained.

Japanese Patent No. 2551783 Japanese Patent Publication No.51-46396 JP 2001-98206 A JP 2005-31189 A

  An object of the present invention is to solve various problems in the prior art and achieve the following objects. That is, the present invention provides particles for an image display medium that can maintain a stable image display state even when the image display medium is stored and used for a long period of time, a method for manufacturing the same, an image display medium, and an image display device. The purpose is to provide.

Means for solving the problems are as follows. That is,
<1> Used for an image display medium in which at least one kind of particles different in at least one of color and charge polarity is encapsulated between two opposing substrates,
The particles are discharged from a nozzle that vibrates at least a resin and a colorant, and a direct current potential is applied to an electrode opposed to the nozzle, and droplets ejected from the nozzle are positively and negatively induced by an induction phenomenon. It is a particle for an image display medium, which is obtained by drying and solidifying a liquid droplet that is charged to any negative polarity and at least the charge remains.
<2> The particles for an image display medium according to <1>, wherein the particles are electretized.
<3> any surface charge density of the particles is the 5~20μC / ^{m 2,} and the surface charge density of 75% or more of the total is 7~15μC / ^{m 2} a number basis of <1> to <2> The particles for an image display medium according to claim 1.
<4> The image display according to any one of <1> to <3>, wherein the average circularity of the particles is 0.98 or more, and the coefficient of variation in the number distribution of the particle diameters of the particles is 20% or less. Medium particles.
<5> The image display medium particle according to any one of <1> to <4>, wherein an average particle diameter of the particles is 2 to 40 μm.
<6> A liquid containing at least a resin and a colorant is discharged from a vibrated nozzle, a direct current potential is applied to the electrode facing the nozzle, and droplets ejected from the nozzle are positively and negatively induced by an induction phenomenon. A method for producing particles for an image display medium, comprising: electrifying particles by drying and solidifying at least the droplets with electric charge remaining after being charged to any polarity of It is.
<7> In an image display medium in which at least one kind of particles having at least one of color and charging polarity is sealed between two opposing substrates,
The particle is an image display medium particle according to any one of <1> to <5>.
<8> An image display device using the image display medium according to <7> as a display unit.

  The particles for an image display medium of the present invention are droplets ejected from a nozzle which is made to eject a liquid containing at least a resin and a colorant from a vibrating nozzle, and is applied with a direct current potential to the electrode opposed to the nozzle. This is an electret particle obtained by charging a droplet with positive or negative polarity by an induction phenomenon and drying and solidifying at least a droplet with the charge remaining. The image display medium can maintain a stable image display state even when stored or used for a long period of time.

  The image display device of the present invention has an image display medium using the image display medium particles of the present invention composed of electrets (permanently charged bodies) having an internal space charge, so that the charge amount of the particles can be stabilized. Even if the image display medium is stored or used for a long period of time, a stable image display state can be maintained. For example, a display unit of a mobile device such as a notebook computer, a PDA or a mobile phone, an electronic book , Electronic newspapers such as electronic newspapers, bulletin boards such as signboards, posters, blackboards, photocopiers, rewritable paper for printer paper replacement, calculators, display units for home appliances, card display units such as point cards, electronic advertisements, electronic POPs, etc. It is suitably used for various image display devices.

  According to the present invention, the conventional problems can be solved, and the image display medium particles capable of maintaining a stable image display state even when the image display medium is stored or used for a long period of time, and the image display medium It is possible to provide an image display medium and an image display apparatus using the particles for an image display, and the particles for an image display medium.

(Image display medium particles and method for producing the same)
The particles for an image display medium of the present invention are used for an image display medium in which at least one kind of particles having at least one of color and charge polarity is sealed between two opposing substrates,
The particles are discharged from a nozzle that vibrates at least a resin and a colorant, and a direct current potential is applied to an electrode opposed to the nozzle, and droplets ejected from the nozzle are positively and negatively induced by an induction phenomenon. It can be obtained by charging the liquid to any negative polarity and drying and solidifying at least the droplets with the charge remaining.
In the method for producing particles for an image display medium of the present invention, a liquid containing at least a resin and a colorant is discharged from a vibrating nozzle, a direct current potential is applied to the electrode facing the nozzle, and the liquid is ejected from the nozzle. The droplets are charged to either positive or negative polarity by an induction phenomenon, and at least the droplets with the charge remaining are dried and solidified.
The particles for image display medium of the present invention are produced by the method for producing particles for image display medium of the present invention.
The details of the image display medium particles and the method for producing the image display medium particles of the present invention will be described below.

First, a general electret (permanent charged body) means that an insulating material, particularly a polymer material, is melted, and a dipole is oriented by lowering the temperature and freezing it while applying a high voltage thereto. It means the one that is permanently polarized.
Conventionally, electrets have been industrially applied to film capacitors and electret filters. However, electretization of resin fine particles can be achieved by arranging fine particles on an electrode when applying a high voltage, or for insulating materials with a large surface area. Since it is not easy to apply a high voltage effectively, there has been virtually no electretization method applicable to resin fine particles such as particles for image display media.

As a method for giving a high charge to the insulating material, for example, a corona charging method, an electron beam method, and a method using radiation such as soft X-rays are generally known. When charges start to accumulate on the insulator surface, there is a common problem that the shielding effect works by the electric field created by itself and only the surface layer portion of the fine particles can be charged.
When charging fine particles, this phenomenon is particularly likely to occur. For example, even if a fine particle layer is formed on a flat plate electrode and charged by the method described above, only one layer of particles on the surface can be charged. Can not. This is because the surface area is greatly increased due to the fine particles.

Therefore, as a method of charging the fine particles effectively and uniformly, there is a method of charging by using an induction phenomenon at the time of discharging the droplet.
Specifically, in a droplet generator such as a vibrating orifice, a counter electrode provided at a position facing the nozzle of the orifice is superimposed with a direct current opposite to the charging polarity to be applied to the droplet side or an alternating current on a direct current. Apply potential. By connecting the nozzle plate to the ground, an induction phenomenon occurs in the droplet ejected from the nozzle, and the droplet has a charge opposite to that of the counter electrode, and gives an electrostatic charge amount corresponding to the potential of the counter electrode. It is possible.
This method is excellent in controlling the charge amount, and can always give a constant amount of electrostatic charge to the droplets, and has an advantage that the so-called charge amount distribution is extremely sharp.
For example, as a method of corona charging the floating solid particles from multiple directions, a method by switching the electric field like a box charger has been tried, but when comparing the charge amount distribution of the particles after actually charging, In the charged fine particles of this method, the charge amount exists in the range of 101 ± 2 fC / particle and there is almost no change, while the one using the box charger has a very wide distribution, 12 ± 12 fC / This is a particle for an image display medium in a range of one particle.
However, if an attempt is made to give more charge to the fine particles, it is necessary to apply a very high potential corona, and in the case of combustible particles such as resin, the particles are burned out and charged immediately after the treatment. It was impossible to give.

In the production method of the present invention, charging can be performed up to the maximum charge amount that the droplet can hold in the atmosphere, that is, the charge amount at which the droplet is split by Rayleigh splitting.
In addition, it was confirmed that all the charges applied to the solid particles obtained by drying and solidifying the highly charged droplets as they were remained.
If the maximum charge amount has already been reached at the time of droplets, the amount of charge is reduced by the Rayleigh splitting due to the decrease in the surface area due to drying, but it can be confirmed that the maximum charge amount corresponding to the surface area also remains in the solid particles. It was.
Considering the example described above, the charge amount of 101 ± 2 fC / particle given at the time of droplets was exactly the same 101 ± 2 fC / particle even when solidified.
In this case, the charge amount at the time of droplets was adjusted so that the maximum charge amount was obtained when solid particles were formed. That is, as a means for producing highly charged particles, it has been found that the method of charging particles by inductive charging at the time of droplets according to the present invention is the most excellent method in terms of both charge controllability and high charge.

Next, a method for electretizing highly charged particles will be described.
In the process of inductive charging at the time of droplets to obtain a highly charged state, and further drying and solidifying this, if radiation such as soft X-rays is applied to the droplets during drying, they are dissolved or deposited in the droplets. Charge (carrier) separation occurs in the image display medium particles.
Among the charges generated here, the charge of the opposite polarity to the charge existing on the droplet surface is immediately attracted to the very high electrostatic charge existing on the droplet surface and neutralized with the charge on the droplet surface. The polar charge is electrostatically repelled by the charge on the droplet surface and moves inside the particle.
During the drying process, by continuing irradiation with radiation, it is possible to create a state in which charges having the same polarity as the charges given to the droplets are confined inside the particles when completely dried and solidified.
As a result, resin fine particles having extremely good charging stability, that is, excellent electret characteristics were obtained. This is because electric charges exist on the particle surface in an unstable state, but move to the inside of the resin fine particles.
The high electret characteristics are not sufficiently exhibited by the surface charge, and are stable when they are located inside the particles, and the temperature of the TSC peak observed by thermally stimulated current measurement shifts to a higher temperature side. Taken together, it is determined that the charge trap energy is deeper and more stable.
In the case of a conventional film-like electret, the charge stability is obtained by inducing hetero charges on the back surface of the film and causing permanent polarization with respect to the homo charge on the treated surface.

When a true charge is given to fine particles as in the method for producing particles for an image display medium of the present invention, since a stabilizing effect due to such polarization cannot be expected, complete electretization is achieved in addition to confining charges inside the particles. There seems to be no means to do this.
It is considered that electretization of particles for image display media by the production method of the present invention is the only processing method that can highly electretize particles.
In addition, even when drying is performed without irradiating radiation such as soft X-rays in the drying step, the amount of charge does not reach the same level as when irradiating with radiation, but the charge may partially move inside the particles and be electretized. confirmed. This is because the electrostatic charge applied to the droplet surface is extremely high, so when drying and solidifying, the surface charge density increases rapidly due to the reduction in the surface area of the particles, and the electrostatic discharge occurs before the droplet starts Rayleigh splitting. It is expected that this phenomenon occurs when electric charges are diffused inside the particles due to electric repulsion and are dried as they are.
As described above, when electretization is performed in the process of obtaining solid particles from droplets, unlike the case of electretization of solid resin fine particles, it is not necessary to melt them. This is due to the highly conductive ions present in the droplets, and the charge can move freely in the dissolved or semi-dissolved state. This is because it can be charged.
Thus, the production method of the present invention can be said to be a production method with a high degree of freedom in that it can be treated at room temperature and a solvent suitable for the material constituting the particles can be used.

Next, the means for producing microdroplets will be described in detail.
Conventionally, methods for producing several microdroplets are known. For example, (1) when a solution is jetted from a nozzle, a shearing is applied by applying mechanical vibration to the nozzle itself to generate micro droplets having a very uniform particle size. (2) ) Various ink jet methods that apply pressure pulses by piezoelectric elements such as PZT or eject fine droplets using gas expansion by thermal energy applied to the nozzles, and (3) ultrasonic vibration of a plate having a plurality of nozzles In addition, there is a mesh vibration method for producing atomized droplets by utilizing the capillary effect of liquid. In the present invention, in consideration of the above-described principle, any one of these methods can be applied, and electretization can be performed.

The resin fine particles having electret properties obtained by the above-described method cause electrostatic repulsion between the particles. Therefore, even when used for applications such as particles for image display media, external additives are added for the purpose of improving fluidity. No agent is required.
In addition, it was confirmed that even when an external additive was added for the purpose of reducing the adhesive force, the effect was exhibited in a very small amount. Furthermore, considering the deterioration of the external additive due to stress, it is desirable that the external additive is not used as much as possible. Therefore, the production method of the present invention is excellent in this respect.
The particles for an image display medium having electret properties obtained by the production method of the present invention can be easily redispersed in an air stream, that is, suspended by an electrostatic repulsion effect. As a result, high fluidity can be obtained, and display with extremely fast response speed can be performed.

Here, the manufacturing method of the particle | grains for image display media of this invention is demonstrated with reference to drawings.
In the method for producing particles for an image display medium of the present invention, a liquid containing at least a resin and a colorant is discharged from a vibrating nozzle, a direct current potential is applied to the electrode facing the nozzle, and the liquid is ejected from the nozzle. The charged droplets are charged to either positive or negative polarity by an induction phenomenon, and at least the droplets with the charge remaining are dried and solidified to produce electret particles.

-Device-
As an apparatus used in the method for producing particles for an image display medium of the present invention (hereinafter sometimes referred to as “particle production apparatus for image display medium”), the particles for an image display medium are produced by the production method of the present invention. There is no particular limitation as long as it is an apparatus capable of producing the same, and it can be appropriately selected and used. However, a solution or dispersion of a particle material for an image display medium containing at least a resin and a colorant is fixed. The liquid droplets are ejected from a nozzle oscillated at a frequency to form droplets, and the droplets are charged by induction charging by removing the solvent contained in the droplets, and then dried. An image display medium particle manufacturing apparatus having image display medium particle forming means for forming image display medium particles is preferable.

  Here, FIG. 4 shows an example of an apparatus for producing particles for an image display medium, and FIG. 5 is an enlarged view of the nozzle part of FIG. As the image display medium particle manufacturing apparatus, the nozzle 1 and the electrode 2 as the droplet forming means, the solvent removal equipment 3 as the image display medium particle forming means, the static eliminator 4, A particle collection unit 12 for image display medium, 5 a particle collection unit, 6 a dry particle, 7 a vortex, 8 an electric field curtain, 9 a liquid transport tube, 10 a drying container, 11 is a droplet, 13 is a dry gas supply tube, 14 is a metering pump, 15 is a slurry dispersion reservoir, 16 is a nozzle surface, 17 is a substrate, 18 is a liquid supply channel, 19 is a DC high-voltage power supply, and 20 is O A ring, 21 represents dispersed air, and 22 represents a piezoelectric body.

In the particle display device for an image display medium shown in FIG. 4, the dissolved or dispersed liquid is discharged from the nozzle 1 as droplets 11, the droplets 11 are charged by the electrode 2, and then the solvent is removed by the solvent removal equipment 3. Is removed to obtain an image display medium particle 6, and the image display medium particle 6 is collected by the eddy current 7 in the image display medium particle collecting unit 12 after being neutralized by the static eliminator 4. It is designed to be transported to a particle reservoir.
Hereinafter, the particle production apparatus for an image display medium will be described in detail for each member.

−−Nozzle−−
As described above, the nozzle 1 is a member that discharges a solution or dispersion of a particle material for an image display medium into droplets.
There is no restriction | limiting in particular as a material and shape of the said nozzle, Although it can be set as the shape selected suitably, For example, an ejection hole is formed with a metal plate with a thickness of 5-50 micrometers, and the opening diameter is 3-3. When the solution is ejected from the nozzle 1, the liquid droplets of 35 μm cause vibration of the nozzle 1 itself, thereby causing a constriction that assists in forming droplets in the liquid column, so that micro droplets having an extremely uniform particle diameter are formed. It is preferable from the viewpoint of generating. The opening diameter means a diameter if it is a perfect circle, and a minor diameter if it is an ellipse.
The means for applying vibration to the nozzle 1 is not particularly limited as long as it can provide reliable vibration at a constant frequency, and can be appropriately selected and used. From the above viewpoint, for example, The nozzle 1 is preferably vibrated at a constant frequency by expansion and contraction of the piezoelectric body.
The piezoelectric body has a function of converting electrical energy into mechanical energy. Specifically, it is expanded and contracted by applying a voltage, and the nozzle can be vibrated by the expansion and contraction.
Examples of the piezoelectric body include piezoelectric ceramics such as lead zirconate titanate (PZT). Generally, since the amount of displacement is small, the piezoelectric body is often used by being laminated. In addition, piezoelectric polymers such as polyvinylidene fluoride (PVDF); single crystals such as quartz, LiNbO ₃ , LiTaO ₃ , KNbO ₃ , and the like can be given.
The fixed frequency is not particularly limited and may be appropriately selected according to the purpose. However, 50 kHz to 50 MHz is preferable, and from the viewpoint of generating micro droplets having a very uniform particle size, 100 kHz to 10 MHz is preferable. More preferred.
Although only one nozzle 1 may be provided, from the viewpoint of generating minute droplets having a very uniform particle diameter, a plurality of nozzles 1 are provided, and one droplet 11 discharged from each nozzle is used as one solvent removal facility. In the illustrated example, it is preferable that the solvent is removed by the solvent removal equipment 3.
From the same viewpoint, it is preferable to vibrate the discharge hole with one piezoelectric body as the piezoelectric body.
The number of ejection holes of the nozzle vibrated by the one piezoelectric body is not particularly limited, but is preferably 1 to 300 in order to more surely generate micro droplets having a very uniform particle diameter. Therefore, the number of the nozzles 1 is preferably 1 to 15, and the number of ejection holes of the nozzle 1 is preferably 1 to 20 per nozzle.

--- Electrode--
The electrode 2 is a member for charging the droplet 11 discharged from the nozzle into monodisperse particles.
The electrode 2 is a pair of members installed facing the nozzle 1, and the shape thereof is not particularly limited and may be appropriately selected. For example, the electrode 2 is preferably formed in a ring shape.
The charging method by the electrode 2 is not particularly limited, but a constant charge amount can always be given to the droplet 11 discharged from the nozzle. It is preferable to give a positive charge or a negative charge by charging. More specifically, the dielectric charging is preferably performed by passing the droplet through a ring electrode to which a DC voltage is applied. Since the amount of charge induced in the droplet is determined in proportion to the DC voltage, the voltage is set according to the desired amount of charge. However, if the droplet is made too high, Rayleigh splitting may occur. is there. When reaching the Rayleigh splitting generation region, even if a DC potential for induction charging is further applied, the droplets are only split and the charge amount cannot be increased. Furthermore, since the particle size distribution of the particles for image display media obtained when the droplet breakup frequently occurs, it is desirable to keep the upper limit value of the voltage below the Rayleigh breakup. The region where Rayleigh splitting occurs varies depending on the material and solvent used, and thus cannot be determined uniquely. However, the DC voltage is preferably 10 kV or less, more preferably 8 kV or less, in a generated droplet of about 20 μm. Further, in the method of induction charging, it is also possible to charge a droplet by directly applying a direct current voltage to the nozzle and providing a potential difference with respect to the ground arranged at the bottom of the drying equipment. In this case, an electric potential can be applied via the conductive image display medium particle solution or dispersion in the image display medium particle reservoir. If the liquid supply to the reservoir is insulated by using air pressure or the like, induction charging can be achieved relatively easily. In addition, the formation of droplets in the air stream has already been demonstrated in electrospray methods and microparticle production by electrostatic spraying. In this case, it is possible in principle to maintain a charge amount higher than the charge to the solid from the effect of reducing the surface area of the droplet by evaporation of the volatile component, and it is possible to obtain solid particles with higher charge.

--Solvent removal equipment--
The solvent removal equipment 3 is not particularly limited as long as the solvent of the droplet 11 can be removed, but an air flow is generated by flowing a dry gas in the same direction as the discharge direction of the droplet 11, It is preferable to form the image display medium particles 6 by transporting the droplets 11 in the solvent removal equipment 3 and removing the solvent in the droplets 11 during the transport. Here, “dry gas” means a gas having a dew point temperature of −10 ° C. or lower under atmospheric pressure. The dry gas is not particularly limited as long as it is a gas that can dry the droplets 11, and examples thereof include air and nitrogen gas.

The temperature of the drying gas is preferably higher in terms of drying efficiency, and even if a drying gas having a boiling point higher than the boiling point of the solvent used is used due to the characteristics of spray drying, in the constant rate drying region during drying, The droplet temperature does not rise above the boiling point of the solvent, and the resulting image display medium particles are not thermally damaged. However, since the main constituent material of the particles for the image display medium is a resin, the particles for the image display medium are thermally melted when they are exposed to a dry gas having a temperature equal to or higher than the boiling point of the resin to be used after drying. There is a risk that it tends to occur and the monodispersity is impaired. Therefore, the temperature of the dry gas is preferably 40 to 200 ° C, more preferably 60 to 150 ° C.
In addition, as shown in the example, from the viewpoint of preventing the droplet 11 from adhering to the inner wall surface of the solvent removal equipment 3 from the wall surface of the solvent removal equipment 3, the polarity is opposite to the charge of the droplet. It is preferable that a charged electric field curtain 8 is provided, a transport path whose periphery is covered with the electric field curtain is formed, and liquid droplets are allowed to pass through the transport path.

--- Static eliminator--
The static eliminator 4 temporarily neutralizes the charge of the image display medium particles 6 formed by passing the droplets 11 through the transport path, and then displays the image display medium particles 6 as an image. It is a member for accommodating in the particle collection part 5 for media.
There is no particular limitation on the method of static elimination by the static eliminator 4, and a conventionally known method can be appropriately selected and used. However, since neutralization can be performed efficiently, for example, soft X-rays can be used. It is preferable to carry out by irradiation, plasma irradiation or the like.

--Particle collection part for image display medium--
The image display medium particle collecting unit 5 is a member provided at the bottom of the image display medium particle manufacturing apparatus from the viewpoint of efficiently collecting and transporting the image display medium particles.
The structure of the image display medium particle collecting unit 5 is not particularly limited as long as the image display medium particles can be collected, and can be selected as appropriate. The particle 6 for the image display medium is formed using a dry gas from the outlet portion having a tapered surface whose diameter is gradually reduced, and the opening diameter is smaller than that of the inlet portion. It is preferable that the particles for the image display medium are transferred to the particle storage container for the image display medium by a gas flow.
As the transfer method, as shown in the illustrated example, the particles 7 by the dry gas through the pressure feeding tube 13 may be pumped to the particle storage container for the image display medium, or from the image storage medium particle storage container side. The image display medium particles 6 may be sucked.
Although there is no restriction | limiting in particular as a flow of the said dry gas, From a viewpoint that the particle | grains 6 for image display media can be reliably conveyed by generating a centrifugal force, it is preferable that it is a vortex | eddy_current.
Further, from the viewpoint of more efficiently transporting the image display medium particles 6, the image display medium particle collection unit 5, the pressure feed tube 13, and the image display medium particle collection container are made of a conductive material. More preferably, they are formed and connected to ground. Moreover, it is preferable that the particle | grain manufacturing apparatus for image display media is an exposure protection specification.

--- Droplet--
As described above, the droplet 11 is generated by discharging a dissolved or dispersed liquid of a particle material for an image display medium containing a specific substance from a nozzle 1 oscillated at a constant frequency. The particle material for image display medium will be described separately.
The dissolution or dispersion liquid for the image display medium particle material is not particularly limited as long as at least one of dissolution and dispersion is performed, and the image display medium particle material is appropriately selected and used. However, from the viewpoint of maintaining a high charge amount, the electrolytic conductivity is preferably 1.0 × 10 ⁻⁷ S / m or more.
From the same viewpoint, the electrolytic conductivity as a solvent of the dissolved or dispersed liquid is preferably 1.0 × 10 ⁻⁷ S / m or more.
There is no restriction | limiting in particular as a method to melt | dissolve thru | or disperse | distribute the said particle | grain material for image display media, The method used normally can be selected suitably. Specifically, a particle binder for an image display medium such as a styrene acrylic resin, a polyester resin, a polyol resin, and an epoxy resin may be melt-kneaded together with a colorant and finely pulverized. The kneaded product obtained in (1) may be once dissolved in an organic solvent in which the resin component is soluble, and this may be processed as fine droplets.

-Action-
According to the method for producing particles for an image display medium of the present invention described in detail above, the number of droplets generated from one ejection hole of the nozzle is very high, from tens of thousands to several millions per second. It is easy to increase the number of discharge holes. Moreover, it can be said that it is the most suitable method for producing particles for an image display medium from the viewpoint of obtaining a very uniform droplet diameter and sufficient productivity. Furthermore, in this production method, the particle diameter of the image display medium particles finally obtained can be accurately determined by the following calculation formula (1), and there is almost no change in the particle diameter depending on the material used.
〔a formula〕
Dp = (6QC / πf) ^(1/3) (1)
However, Dp: solid particle diameter, Q: liquid flow rate (determined by pump flow rate and nozzle diameter), f: vibration frequency, C: volume concentration of solid content.
The particle diameter for the image display medium can be accurately calculated only by the above calculation formula (1), but more simply can be obtained by the following calculation formula (2).
〔a formula〕
Solid content volume concentration (volume%) = (solid particle diameter / droplet diameter) ³ (2)
That is, the diameter of the particles for an image display medium obtained by the present invention is twice the opening diameter of the nozzle regardless of the vibration frequency at which the droplet is ejected. Therefore, the target solid particle diameter can be obtained by obtaining and adjusting the solid content concentration in advance from the relationship of the above formula (2). For example, when the nozzle diameter is 7.5 μm, the droplet diameter is 15 μm. Therefore, if the solid content volume concentration is 6.40% by volume, 6.0 μm solid particles can be obtained. In this case, the vibration frequency is preferably higher from the viewpoint of productivity, but Q (liquid flow rate) is determined from the calculation formula (1) according to the vibration frequency determined here.
In the conventional manufacturing method, the particle size often varies greatly depending on the material used. In this manufacturing method, the particle size as set is controlled by managing the droplet diameter and solid content concentration at the time of discharge. It becomes possible to continuously obtain particles having a diameter.

-Resin-
The resin is not particularly limited, and any resin can be used as long as it is a known binder resin. For example, vinyl polymers such as styrene monomers, acrylic monomers, and methacrylic monomers can be used. Or a copolymer comprising two or more of these monomers, a polyester polymer, a polyol resin, a phenol resin, a silicone resin, a polyurethane resin, a polyamide resin, a furan resin, an epoxy resin, a xylene resin, a terpene resin, Examples include malon indene resin, polycarbonate resin, and petroleum resin. Among these, vinyl polymers and polyester polymers are particularly preferable.

Examples of the styrenic monomer, acrylic monomer, and methacrylic monomer that form the vinyl polymer or copolymer are shown below, but are not limited thereto.
Examples of the styrene monomer include styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, p-phenylstyrene, p-ethylstyrene, 2,4-dimethylstyrene, and pn-amyl. Styrene, p-tert-butyl styrene, pn-hexyl styrene, pn-octyl styrene, pn-nonyl styrene, pn-decyl styrene, pn-dodecyl styrene, p-methoxy styrene, Examples thereof include styrene such as p-chlorostyrene, 3,4-dichlorostyrene, m-nitrostyrene, o-nitrostyrene, and p-nitrostyrene, or derivatives thereof.
Examples of the acrylic monomer include acrylic acid, methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, n-octyl acrylate, n-dodecyl acrylate, and acrylic acid 2 -Acrylic acid such as ethylhexyl, stearyl acrylate, 2-chloroethyl acrylate, and phenyl acrylate, or esters thereof.
Examples of the methacrylic monomer include methacrylic acid, methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-octyl methacrylate, n-dodecyl methacrylate, and methacrylic acid 2 -Methacrylic acid such as ethylhexyl, stearyl methacrylate, phenyl methacrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, or esters thereof.

The following (1) to (18) may be mentioned as other monomers that form the vinyl polymer or copolymer.
(1) Monoolefins such as ethylene, propylene, butylene and isobutylene, (2) Polyenes such as butadiene and isoprene, (3) Vinyl halides such as vinyl chloride, vinylidene chloride, vinyl bromide and vinyl fluoride, (4) Vinyl esters such as vinyl acetate, vinyl propionate and vinyl benzoate, (5) Vinyl ethers such as vinyl methyl ether, vinyl ethyl ether and vinyl isobutyl ether, (6) Vinyl methyl ketone, vinyl hexyl ketone and methyl Vinyl ketones such as isopropenyl ketone, (7) N-vinyl compounds such as N-vinyl pyrrole, N-vinyl carbazole, N-vinyl indole, N-vinyl pyrrolidone, (8) vinyl naphthalenes, (9) acrylonitrile, methacrylate. Ronitrile, acrylamide, etc. (10) unsaturated dibasic acids such as maleic acid, citraconic acid, itaconic acid, alkenyl succinic acid, fumaric acid, mesaconic acid, (11) maleic anhydride, citraconic anhydride, itaconic Unsaturated anhydrides such as acid anhydrides and alkenyl succinic anhydrides, (12) maleic acid monomethyl ester, maleic acid monoethyl ester, maleic acid monobutyl ester, citraconic acid monomethyl ester, citraconic acid monoethyl ester, Monoesters of unsaturated dibasic acids such as citraconic acid monobutyl ester, itaconic acid monomethyl ester, alkenyl succinic acid monomethyl ester, fumaric acid monomethyl ester, mesaconic acid monomethyl ester, (13) dimethyl maleic acid, dimethyl fumaric acid, etc. Saturated salt Acid esters, (14) α, β-unsaturated acids such as crotonic acid and cinnamic acid, (15) α, β-unsaturated acid anhydrides such as crotonic acid anhydride and cinnamic anhydride, (16) the α , Β-unsaturated acid and lower fatty acid anhydrides, alkenylmalonic acid, alkenylglutaric acid, alkenyladipic acid, these acid anhydrides and monomers having a carboxyl group such as monoesters, (17) 2-hydroxy Acrylic acid or methacrylic acid hydroxyalkyl esters such as ethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, (18) 4- (1-hydroxy-1-methylbutyl) styrene, 4- (1-hydroxy-1) -Methylhexyl) Monomers having a hydroxy group such as styrene.

In the image display medium particles of the present invention, the vinyl polymer or copolymer in the binder resin may have a crosslinked structure crosslinked with a crosslinking agent having two or more vinyl groups. Examples of the cross-linking agent include aromatic divinyl compounds, diacrylate compounds linked by alkyl chains, diacrylate compounds linked by alkyl chains containing an ether bond, and other compounds.
Examples of the aromatic divinyl compound include divinylbenzene and divinylnaphthalene.
Examples of the diacrylate compound linked by the alkyl chain include ethylene glycol diacrylate, 1,3-butylene glycol diacrylate, 1,4-butanediol diacrylate, 1,5-pentanediol diacrylate, and 1,6. Examples include xanthdiol diacrylate, neopentyl glycol diacrylate, or those obtained by replacing the acrylate of the above compound with methacrylate.
Examples of the diacrylate compound linked by an alkyl chain containing an ether bond include diethylene glycol diacrylate, triethylene glycol diacrylate, tetraethylene glycol diacrylate, polyethylene glycol # 400 diacrylate, polyethylene glycol # 600 diacrylate, and dipropylene. Examples include glycol diacrylate or those obtained by replacing acrylate of the above compound with methacrylate.
Examples of the other compounds include diacrylate compounds or dimethacrylate compounds linked by a chain containing an aromatic group and an ether bond.
Moreover, as a polyester type diacrylate, brand name: MANDA (made by Nippon Kayaku Co., Ltd.) etc. are mentioned, for example.
In addition, as the polyfunctional crosslinking agent, for example, pentaerythritol triacrylate, trimethylolethane triacrylate, trimethylolpropane triacrylate, tetramethylolmethane tetraacrylate, oligoester acrylate, or those obtained by replacing acrylates of these compounds with methacrylates , Triallyl cyanurate, and triallyl trimellitate.

  Among these crosslinkable monomers, aromatic divinyl compounds (particularly divinylbenzene), aromatic groups and ether bonds are preferably used as resin for image display medium particles from the viewpoint of fixability and offset resistance. And diacrylate compounds linked by a linking chain containing two. Among these, a combination of monomers that becomes a styrene copolymer or a styrene-acrylic copolymer is preferable.

  The addition amount of these crosslinking agents is preferably 0.01 to 10 parts by mass, more preferably 0.03 to 5 parts by mass with respect to 100 parts by mass of the other monomer components.

  Examples of the polymerization initiator used in the production of the vinyl polymer or copolymer include 2,2′-azobisisobutyronitrile and 2,2′-azobis (4-methoxy-2,4-dimethylvaleronitrile). ), 2,2′-azobis (2,4-dimethylvaleronitrile), 2,2′-azobis (2-methylbutyronitrile), dimethyl-2,2′-azobisisobutyrate, 1,1 ′ -Azobis (1-cyclohexanecarbonitrile), 2- (carbamoylazo) -isobutyronitrile, 2,2'-azobis (2,4,4-trimethylpentane), 2-phenylazo-2 ', 4' -Dimethyl-4'-methoxyvaleronitrile, 2,2'-azobis (2-methylpropane), methyl ethyl ketone peroxide, acetylacetone peroxide, cyclohexanone peroxide Ketone peroxides such as side; 2,2-bis (tert-butylperoxy) butane, tert-butyl hydroperoxide, cumene hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, di -Tert-butyl peroxide, tert-butyl cumyl peroxide, dicumyl peroxide, α- (tert-butylperoxy) isopropylbenzene, isobutyl peroxide, octanoyl peroxide, decanoyl peroxide, lauroyl peroxide, 3,5,5-trimethylhexanoyl peroxide, benzoyl peroxide, m-tolyl peroxide, di-isopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, -N-propyl peroxydicarbonate, di-2-ethoxyethyl peroxycarbonate, di-ethoxyisopropyl peroxydicarbonate, di (3-methyl-3-methoxybutyl) peroxycarbonate, acetylcyclohexylsulfonyl peroxide Tert-butyl peroxyacetate, tert-butyl peroxyisobutyrate, tert-butyl peroxy-2-ethylhexarate, tert-butyl peroxylaurate, tert-butyl-oxybenzoate, tert-butyl peroxy Isopropyl carbonate, di-tert-butyl peroxyisophthalate, tert-butyl peroxyallyl carbonate, isoamyl peroxy-2-ethylhexanoate, di-tert-butyl peroxy Kisa Hydro terephthalate to oxy, tert- butylperoxy azelate, and the like.

The following are mentioned as a monomer which comprises the said polyester-type polymer.
Examples of the divalent alcohol component include ethylene glycol, propylene glycol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, diethylene glycol, triethylene glycol, 1,5-pentanediol, , 6-hexanediol, neopentyl glycol, 2-ethyl-1,3-hexanediol, hydrogenated bisphenol A, or diol obtained by polymerizing cyclic ether such as ethylene oxide or propylene oxide on bisphenol A It is done.
In addition, it is preferable to use a trivalent or higher alcohol in order to crosslink the polyester resin. Examples of the trihydric or higher polyhydric alcohol include sorbitol, 1,2,3,6-hexanetetrol, 1,4-sorbitan, pentaerythritol, dipentaerythritol, tripentaerythritol, 1,2,4-butanetriol. 1,2,5-pentatriol, glycerol, 2-methylpropanetriol, 2-methyl-1,2,4-butanetriol, trimethylolethane, trimethylolpropane, 1,3,5-trihydroxybenzene, etc. Can be mentioned.
Examples of the acid component that forms the polyester polymer include benzene dicarboxylic acids such as phthalic acid, isophthalic acid, and terephthalic acid, or anhydrides thereof; succinic acid, adipic acid, sebacic acid, alkyldicarboxylic acids such as azelaic acid, or the like. Unsaturated dibasic acids such as maleic acid, citraconic acid, itaconic acid, alkenyl succinic acid, fumaric acid, mesaconic acid; maleic anhydride, citraconic acid anhydride, itaconic acid anhydride, alkenyl succinic acid anhydride And unsaturated dibasic acid anhydrides.
Examples of the trivalent or higher polyvalent carboxylic acid component include trimethic acid, pyromethic acid, 1,2,4-benzenetricarboxylic acid, 1,2,5-benzenetricarboxylic acid, and 2,5,7-naphthalenetricarboxylic acid. 1,2,4-naphthalenetricarboxylic acid, 1,2,4-butanetricarboxylic acid, 1,2,5-hexanetricarboxylic acid, 1,3-dicarboxy-2-methyl-2-methylenecarboxypropane, tetra ( Methylenecarboxy) methane, 1,2,7,8-octanetetracarboxylic acid, emporic trimer acid, their anhydrides, or their partially lower alkyl esters.

  As the binder resin that can be used in the particles for an image display medium of the present invention, a resin containing a monomer component capable of reacting with both of these resin components in the vinyl polymer and / or polyester resin can also be used. Examples of monomers that can react with the vinyl polymer among the monomers constituting the polyester-based resin include unsaturated dicarboxylic acids such as phthalic acid, maleic acid, citraconic acid, and itaconic acid, or anhydrides thereof. As a monomer which comprises the said vinyl polymer, what has a carboxyl group or a hydroxy group, acrylic acid or methacrylic acid ester, etc. are mentioned.

-Colorant-
The colorant is not particularly limited, and all known dyes and pigments can be used. For example, carbon black, nigrosine dye, iron black, naphthol yellow S, Hansa yellow (10G, 5G, G), cadmium yellow, yellow Iron oxide, ocher, yellow lead, titanium yellow, polyazo yellow, oil yellow, Hansa yellow (GR, A, RN, R), pigment yellow L, benzidine yellow (G, GR), permanent yellow (NCG), Vulcan fast Yellow (5G, R), Tartrazine Lake, Quinoline Yellow Lake, Anthrazan Yellow BGL, Isoindolinone Yellow, Bengala, Red Dan, Lead Zhu, Cadmium Red, Cadmium Mercury Red, Antimon Zhu, Permanent Red 4R, Para Red, Phi Sared, parac Luort Nitroaniline Red, Resol Fast Scarlet G, Brilliant Fast Scarlet, Brilliant Carnmin BS, Permanent Red (F2R, F4R, FRL, FRLL, F4RH), Fast Scarlet VD, Belkan Fast Rubin B, Brilliant Scarlet G, Resol Rubin GX, Permanent Red F5R, Brilliant Carmine 6B, Pigment Scarlet 3B, Bordeaux 5B, Tolujing Maroon, Permanent Bordeaux F2K, Helio Bordeaux BL, Bordeaux 10B, Bon Maroon Light, Bon Maroon Medium, Eosin Lake, Rhodamine Lake B, Rhodamine Lake Y, Alizarin Lake, Thioindigo Red B, Thioindigo Maroon, Oil Red, Quinacridone Red, Pi Zoron Red, Polyazo Red, Chrome Vermilion, Benzidine Orange, Perinone Orange, Oil Orange, Cobalt Blue, Cerulean Blue, Alkaline Blue Lake, Peacock Blue Lake, Victoria Blue Lake, Metal Free Phthalocyanine Blue, Phthalocyanine Blue, Fast Sky Blue, Indance Ren Blue (RS, BC), Indigo, Ultramarine Blue, Bituminous Blue, Anthraquinone Blue, Fast Violet B, Methyl Violet Lake, Cobalt Purple, Manganese Purple, Dioxane Violet, Anthraquinone Violet, Chrome Green, Zinc Green, Chrome Oxide, Pyridian, Emerald Green , Pigment Green B, Naphthol Green B, Green Gold, Acid Green Lake, Malachite Gree Lake, phthalocyanine green, anthraquinone green, titanium oxide, zinc white, ritbon, or mixtures thereof can be used. Among these, carbon black is preferable as the black colorant, and titanium oxide is preferable as the white colorant.
The content of the colorant is preferably 1 to 15% by mass and more preferably 3 to 10% by mass with respect to the image display medium particles.

The surface charge density of the image for display media particles (charge amount) is preferably preferably 5~20μC / ^{m 2,} and the entire 75% or more of the surface charge density a number basis is 7~15μC / ^{m 2.}
Here, the surface charge density (charge amount) of the particles can be determined by, for example, leaving the obtained particles in an environment of 25 ° C. and 50% RH for one week, and keeping the Faraday cage carefully without applying friction. The charge amount is measured by using a charge amount distribution measuring device (model Sopart Analyzer EST-3, manufactured by Hosokawa Micron Corporation). An amount capable of measuring about 30 particles per second is fed at a field voltage of 100 V, 2800 to 3000 particles are measured, and an arithmetic calculation is performed.

The average circularity of the particles is 0.98 or more, and the coefficient of variation in the number distribution of the particle diameters of the particles is 20% or less.
The particles for an image display medium are at least one kind of colored particles different in at least one of color and charge polarity, are movable by Coulomb force, have an average circularity of 0.98 or more, and the particles As long as the coefficient of variation of the number distribution is 20% or less, there is no particular limitation, and it can be appropriately selected according to the purpose, but particles having a low specific gravity are preferred.
The color is preferably a single color, and white or black particles are suitably used, and white particles are particularly preferred.

  The average circularity of the particles for image display medium is preferably 0.98 or more. The closer the average circularity is to 1, the closer to a true sphere, and the lower the average circularity, the more the shape becomes distorted. As a general particle property, the fluidity is better as the circularity of the particle is closer to 1 and closer to a true sphere. In the present invention, it has been found that the influence of the average circularity of the particles on the fluidity of the particles is extremely large. However, an image contrast that is considered to be generated due to the fluidity, particularly with an average circularity of 0.98 as a threshold. It changes a lot. That is, if the average circularity of the particles is 0.98 or more, excellent contrast characteristics can be obtained, and the electric field threshold necessary for obtaining the same contrast value for the same reason can be reduced. It was. Lowering the electric field threshold is advantageous because it can reduce power consumption required for driving.

Here, as a method for measuring the average circularity of the particles, for example, an optical system in which a suspension containing particles is passed through an imaging unit detection zone on a flat plate, and a particle image is optically detected and analyzed by a CCD camera. The method of target detection zone is appropriate. With this method, circularity, which is a value obtained by dividing the perimeter of an equivalent circle having the same projected area by the perimeter of a real particle, is obtained. The average circularity and the circularity distribution can be calculated from the circularity of each obtained particle. These values were measured by a flow type particle image analyzer FPIA-2100 (manufactured by Toa Medical Electronics Co., Ltd.).
As a specific measurement method, 0.1 to 0.5 ml of a surfactant, preferably an alkylbenzene sulfonate, is added as a dispersant to 100 to 150 ml of water from which impure solids have been removed in advance. About 0.1 to 0.5 g. The suspension in which the sample is dispersed is obtained by performing dispersion treatment for about 1 to 3 minutes with an ultrasonic disperser and measuring the shape and distribution of particles for an image display medium at a dispersion concentration of 3000 to 10,000 / μl. It is done.

The coefficient of variation of the particle size distribution of the particles for image display medium needs to be 20% or less. If the coefficient of variation in the number distribution exceeds 20%, it means that the particle size distribution is wide. The wider the particle size distribution, the worse the fluidity as particles, and the uneven charge amount of one particle. For this reason, the moving speed of particles due to the electric field becomes uneven, and the contrast may be lowered.
Here, the variation coefficient of the number distribution is a value obtained by the following mathematical formula 1.
<Formula 1>
Coefficient of variation = {(standard deviation of number distribution) / (number average particle diameter of particles for image display medium)} × 100

  The average particle diameter of the particles for image display medium is preferably 2 to 40 μm, and more preferably 4 to 20 μm. If the average particle diameter is smaller than this range, the charge density of the particles is too large and the mirror image force to the electrode or substrate is too strong, and the memory performance is good, but the followability when the electric field is reversed may deteriorate. On the contrary, if the average particle diameter is larger than this range, the followability is good, but the memory property may be deteriorated.

The average particle size and particle size distribution of the particles for image display media can be measured by various methods such as Coulter Counter TA-II type or Coulter Multisizer (manufactured by Coulter Co.). For example, using a Coulter Multisizer (manufactured by Coulter), an interface (manufactured by Nikkaki Co., Ltd.) that outputs the number distribution and volume distribution is connected to a PC 9801 personal computer (manufactured by NEC), and the electrolyte is first grade sodium chloride. 1 mass% NaCl aqueous solution (for example, ISOTON R-II, manufactured by Coulter Scientific Japan) can be used.
Specifically, 0.1 to 5 ml of a surfactant, preferably alkylbenzene sulfonate, is added as a dispersant to 100 to 150 ml of the electrolytic aqueous solution, and 2 to 20 mg of a measurement sample is further added. The electrolytic solution in which the sample is suspended is subjected to a dispersion treatment for 1 to 3 minutes with an ultrasonic disperser, and the number of particles for image display media of 2 μm or more is measured using the 100 μm aperture as an aperture by the Coulter Multisizer. Calculate the distribution. Next, the number-based length average particle diameter obtained from the number distribution, that is, the number average particle diameter and the standard deviation of the number distribution are obtained.

(Image display medium)
The image display medium of the present invention is formed by enclosing at least one kind of particles having different colors and charging characteristics between two opposing substrates, and the particles are particles for the image display medium of the present invention. Furthermore, it has other members as needed.

-Board-
As the substrate, a transparent substrate capable of confirming the color of particles from the outside of the image display medium is preferable, and a material having high visible light transmittance and good heat resistance is preferable. The presence or absence of flexibility is appropriately selected depending on the application. For example, the material is flexible for applications such as electronic paper, and is not flexible for applications such as mobile phones, PDAs, and notebook PCs. Material is used.
Examples of the substrate material include polymer sheets such as polyethylene terephthalate, polyethersulfone, polyethylene, and polycarbonate; and inorganic sheets such as glass and quartz.

  The substrate thickness is preferably 2 to 5,000 μm, and more preferably 5 to 1,000 μm. If the thickness is too thin, it will be difficult to maintain strength and uniformity between the substrates, and if it is too thick, the display function will have sharpness and a decrease in contrast, especially in the case of electronic paper applications. Lack. In addition, you may provide an electrode in a board | substrate as needed.

When the substrate is not provided with an electrode, an electrostatic latent image is provided on the outer surface of the substrate, and colored particles charged with a predetermined characteristic are applied to the substrate by an electric field generated according to the electrostatic latent image. By attracting or repelling, the particles arranged corresponding to the electrostatic latent image are visually recognized from the outside of the display device through a transparent substrate. This electrostatic latent image is formed by transferring an electrostatic latent image formed by an ordinary electrophotographic system using an electrophotographic photosensitive member onto the substrate of the image display device of the present invention, or by ion flow. An electrostatic latent image can be formed directly on the substrate.
On the other hand, when the electrode is provided on the substrate, the color particles charged to a predetermined characteristic are attracted or repelled by an electric field generated at each electrode position on the substrate by external voltage input to the electrode portion, In this method, particles arranged in accordance with the electric potential are visually recognized from the outside of the display device through a transparent substrate.

The electrode provided on the transparent substrate side is formed of a conductive material that is transparent and can be patterned. Examples of the material include metals such as indium oxide and aluminum; conductive polymers such as polyaniline, polypyrrole, and polythiophene. Examples of the forming method such as vacuum deposition and coating can be given. The electrode thickness is preferably 3 to 1,000 nm, more preferably 5 to 400 nm, as long as electrical conductivity can be secured and light transmittance is not affected.
The electrode provided on the back substrate side is formed of a conductive material that does not need to be transparent and can be patterned. Examples of the material include metals such as indium oxide, aluminum, gold, silver, and copper; polyaniline, polypyrrole And conductive polymers such as polythiophene, and examples of the forming method such as vacuum deposition and coating. The thickness of the electrode is not particularly limited as long as the conductivity can be ensured and the light transmittance is not impaired, and can be appropriately selected according to the purpose, but is preferably 3 to 1,000 nm, more preferably 5 to 400 nm. . In this case, the external voltage input may be superimposed with direct current or alternating current.

The shape of the partition wall is appropriately set appropriately depending on the size of the particles involved in the display, and is not generally limited. However, the width of the partition wall is preferably 2 to 100 μm, more preferably 3 to 50 μm. Moreover, 10-5000 micrometers is preferable and, as for the height of a partition, 10-500 micrometers is more preferable.
Further, when forming the partition walls, even if the rib formation by the both-rib method in which ribs (partition walls) are formed on each of the opposing substrates is bonded, the ribs are bonded to one of the opposing substrates after the ribs are formed. Partition formation by a single rib method may be used.
The display cells formed by the partition walls made of these ribs are exemplified by a square shape, a triangular shape, a line shape, a circular shape, and a hexagonal shape (honeycomb structure) when viewed from the substrate plane direction. It is better to make the portion corresponding to the partition wall cross-section visible from the display side (the area of the frame portion of the display cell) as small as possible, and the sharpness of the image display increases.
The distance between the transparent substrate and the counter substrate in the image display medium of the present invention is not limited as long as particles can move and maintain contrast, but is preferably 10 to 5,000 μm, more preferably 30 to 500 μm.
Further, the particle filling amount (volume occupancy) is preferably 10 to 80%, more preferably 10 to 70% with respect to the space volume between the substrates.

-Other components-
Examples of the other members include a metal reflection plate, a light diffusion plate, and an antireflection layer. These may be used alone or in combination of two or more.
Note that a TFT or the like may be further attached to the electrode substrate (lower substrate).

  In the image display medium of the present invention, each charged particle moves in the gas, so that the response speed of image display is high and the response speed can be 1 msec or less. Further, unlike the liquid crystal display element, an alignment film, a polarizing plate, and the like are unnecessary, the structure is simple, and the cost and the large area are possible. It is stable against temperature changes and can be used from low to high temperatures. Furthermore, there is no viewing angle, high reflectivity, reflection type, easy to see even in bright places, and low power consumption. It also has memory characteristics and does not consume power when holding images. The force applied to the particles may be an electric image force applied between the electrodes, an intermolecular force, a liquid cross-linking force, gravity, and the like in addition to the above-described force attracted by the Coulomb force between the particles.

Here, FIG. 1A to FIG. 1C are diagrams respectively showing the configuration of the image display medium of the present invention and the display driving principle thereof.
1A to 1C, 101 is a transparent substrate, 102 is a counter substrate, 103 is a display electrode, 104 is a counter electrode, 105 is a negatively chargeable particle, 106 is a positively chargeable particle, and 107 is a partition wall. In these examples, the particles for the image display medium are composed of negatively chargeable particles 105 and positively chargeable particles 106.

  FIG. 1A shows a state in which negatively chargeable particles 105 and positively chargeable particles 106 are arranged between opposing substrates (transparent substrate 101 and counter substrate 102). In this state, when a voltage is applied so that the display electrode 103 side is at a low potential and the counter electrode 104 side is at a high potential, the positively charged particles 106 are moved to the display electrode 103 side by Coulomb force as shown in FIG. 1B. The negatively chargeable particles 105 move to the counter electrode 104 side. In this case, the display surface viewed from the transparent substrate 101 side looks like the color of the positively charged particles 106. Next, when the potential is switched and a voltage is applied to the particles for the image display medium having a high potential on the display electrode 103 side and a low potential on the counter electrode 104 side, the negatively charged particles 105 are displayed by Coulomb force as shown in FIG. 1C. Moving to the electrode 103 side, the positively chargeable particles 106 move to the counter electrode 104 side. In this case, the display surface viewed from the transparent substrate 101 side looks like the color of the negatively charged particles 106.

  1B and 1C can be repeatedly displayed simply by reversing the potential of the power source, and the color can be reversibly changed by reversing the potential of the power source in this way. The color of the particles can be selected at will. For example, if the negatively charged particles 105 are white and the positively charged particles 106 are black, or the negatively charged particles 105 are black and the positively charged particles 106 are white, the display is a reversible display between white and black. Become. In this method, since each particle is once attached to the electrode by mirror image force, the display image is retained for a long time even after the power is turned off, and the memory retainability is good.

Next, FIGS. 2A to 2C are diagrams showing another configuration of the image display medium of the present invention and the display driving principle thereof, respectively. 2A to 2C, 101 is a transparent substrate, 102 is a counter substrate, 103 is a display electrode, 104 is a counter electrode, 105 is a negatively chargeable particle, 107 is a partition, and 108 is a color plate. In these examples, the image display medium particles are composed of negatively chargeable particles 105.
FIG. 2A shows a state in which the negatively charged particles 105 are arranged between the opposing substrates. In this state, when a voltage is applied to the particles for the image display medium at the display electrode 103 side and at the counter electrode 104 side by the power source, the negatively charged particles 105 are transparent due to the Coulomb force as shown in FIG. 2B. Move to the substrate 101 side. In this case, the display surface viewed from the transparent substrate 101 side looks like the color of the negatively charged particles 105. Next, when the potential of the power source is switched and a voltage is applied so that the display electrode side 103 is at a low potential and the counter electrode 104 side is at a high potential, the negatively charged particles 105 are opposed to each other by Coulomb force as shown in FIG. 2C. Move to the substrate 102 side. In this case, the display surface viewed from the transparent substrate 101 side looks like the color of the color plate 108.

  Among the examples described above, in the example shown in FIGS. 1A to 1C, two types of particles having different colors and charging characteristics are used, and in the example shown in FIGS. 2A to 2C, one kind of single color and charging characteristics is used. Using particles, in each case each particle was a single color particle charged to a single property. In the particles for an image display medium of the present invention, in addition to these single color particles charged to a single characteristic, particles each having a portion having a different color and charging characteristic can also be used. That is, for example, the present invention can be applied to an example in which half of each particle is made of materials having different colors and charging characteristics, and the particles are rotated by reversal of the electric field to form a display body.

  The image display medium of the present invention includes a display unit of a mobile device such as a notebook computer, a PDA, or a mobile phone, an electronic paper such as an electronic book or an electronic newspaper, a bulletin board such as a signboard, a poster, or a blackboard, a copying machine, and a rewritable substitute for printer paper. It is used for paper, calculators, home appliance display units, card display units such as point cards, electronic advertisements, electronic POPs, and the like. Specifically, it is suitable for the following image display devices.

(Image display device)
The image display apparatus of the present invention uses the image display medium of the present invention as a display means, and includes a drive circuit, an arithmetic circuit, an internal memory, a power source, and other means as necessary.
The image display device of the present invention uses particles for an image display medium that can maintain a stable image display state even if the image display medium is stored or used for a long period of time, so that long-term stability and display response are achieved. It is possible to provide an image display device excellent in the above.
Here, FIG. 3 is a schematic view showing an example of the image display apparatus of the present invention. As shown in FIG. 3, the image display device 110 includes an image display medium 111, a housing 112, an information input unit 113, a drive circuit, an arithmetic circuit, an internal memory, and a power source that are not shown. The electrodes in the image display medium 111 in FIG. 3 form a dot matrix, and can display an image as a whole by displaying ON a designated dot.

  Next, the present invention will be described with specific examples, but the present invention is not limited to the following examples.

(Example 1 and Example 2)
-Preparation of colorant dispersion-
First, a dispersion of carbon black as a colorant was prepared as follows.
15 parts by mass of carbon black, 3 parts by mass of a pigment dispersant (Ajisper PB821, Ajinomoto Fine Techno Co., Ltd.) and 82 parts by mass of ethyl acetate were primarily dispersed using a mixer having stirring blades.
Next, the obtained primary dispersion was finely dispersed by a strong shearing force using a dyno mill, and aggregates were completely removed to prepare a dispersion. Then, the carbon black dispersion liquid which passed the PTFE filter which has a 0.45 micrometer pore, and was disperse | distributed to the submicron area | region was prepared.
On the other hand, 30 parts by mass of titanium oxide was used as a colorant instead of carbon black, and a titanium oxide dispersion was prepared in the same manner.

-Preparation of resin dispersion-
100 parts by mass of styrene resin as a binder resin, 30 parts by mass of the carbon black dispersion, 700 parts by mass of ethyl acetate, and 300 parts by mass of methyl alcohol are dispersed by stirring for 10 minutes using a mixer having stirring blades. Thus, a carbon black-containing resin dispersion was prepared. It was confirmed that the pigments could be completely prevented from aggregating due to solvent dilution.
When the dispersion liquid at this stage was filtered with a 0.45 μm PTFE filter in the same manner as in the preparation of the colorant dispersion liquid, it was confirmed that there was no clogging and all of the dispersion liquid passed therethrough.
In addition, the electrolytic conductivity of this dispersion liquid was 2.0 × 10 ⁻⁴ S / m.
On the other hand, a titanium oxide-containing resin dispersion was similarly prepared using 30 parts by mass of a titanium oxide dispersion instead of the carbon black dispersion.

-Production of particles for image display media-
The particle for image display media was produced using the manufacturing apparatus of the particle for image display media shown in FIG. 5 which expanded the nozzle part of FIG.4 and FIG.4.
First, each obtained resin dispersion was put into the reservoir 15, and the pump 14 was operated to supply the nozzle 1 through the liquid transport tube 9.
As shown in FIG. 5, the used nozzle 1 is a nickel plate 17 having a thickness of 20 μm, in which a perfectly circular discharge hole having a diameter of 15 μm is formed by processing with a femtosecond laser. The through-hole has a straight structure in which the diameter of the hole does not change between the front and back sides, and the ring electrode 2 that generates inductive charge is installed at a position opposite to the nozzle surface 16 at a distance of 2 mm from the nozzle. The voltage was applied by The voltage polarity here applies a negative voltage to obtain positively charged particles, and a positive voltage to obtain negatively charged particles.
Furthermore, an irradiation head part of a photoionizer (manufactured by Hamamatsu Photonics) is installed as a static eliminator 4 so that soft X-rays can be irradiated 20-40 cm below the nozzle discharge position, and soft X-rays are widely applied to charged droplets. Irradiated. The particles were collected by suction with a filter having 1 μm pores.

The detailed numerical conditions for the production of particles for image display media are described below.
・ Dispersion density: ρ = 1.1888 g / cm ³
・ Dry air flow rate: Sheath air 7L / min, In-apparatus air 30L / min ・ Dry air (dew point temperature): -20 ° C
・ Internal temperature: Adjusted to 60 to 62 ° C. ・ Orifice frequency: 480 kHz
・ Counter electrode voltage: DC ± 5kV
・ Radiation irradiation: Photoionizer (soft X-ray irradiator)

By the above method, a total of four types of carbon particles containing positive and negative electrets containing two types of black particles and two types of positive and negative electret containing white particles containing titanium oxide were prepared. did.
Various physical properties of the obtained particles for image display media were measured as follows. The results are shown in Table 1.

<Particle size distribution>
Using a Coulter Multisizer (manufactured by Coulter), the particle size distribution of the obtained particles was connected to an interface (manufactured by Nikka Machine Co., Ltd.) for outputting the number distribution and volume distribution, and a PC 9801 personal computer (manufactured by NEC Corporation). As the liquid, 1% NaCl aqueous solution (ISOTON R-II, manufactured by Coulter Scientific Japan) can be used using primary sodium chloride.
Specifically, 0.1 to 5 ml of a surfactant (alkylbenzene sulfonate) is added as a dispersant to 100 to 150 ml of the electrolytic aqueous solution, and 2 to 20 mg of a measurement sample is further added. The electrolytic solution in which the sample is suspended is subjected to a dispersion treatment for 1 to 3 minutes with an ultrasonic disperser, and the number of particles for image display media of 2 μm or more is measured using the 100 μm aperture as an aperture by the Coulter Multisizer. Calculate the distribution. Then, the number average length average particle diameter obtained from the number distribution, that is, the number average particle diameter and the standard deviation of the number distribution were determined, and all four types had a mass average particle diameter of 10.0 μm and a number average particle diameter. The diameter was 10.0 μm, and completely monodispersed particles for an image display medium were obtained.

<Average circularity>
In addition, the average circularity of the particles is projected by a method of optical detection zone in which the suspension containing the particles is passed through the imaging zone detection zone on the flat plate, and the particle image is optically detected and analyzed by the CCD camera. The circularity, which is a value obtained by dividing the perimeter of an equivalent circle having the same area by the perimeter of the actual particle, is obtained. The average circularity and the circularity distribution can be calculated from the circularity of each obtained particle. These values were measured by a flow type particle image analyzer FPIA-2100 (manufactured by Toa Medical Electronics Co., Ltd.).
Specifically, 0.1 to 0.5 ml of a surfactant (alkyl benzene sulfonate) is added as a dispersant to 100 to 150 ml of water from which impure solids have been removed in advance, and a measurement sample is added to 0.1 to 0.5 ml. Add ~ 0.5g. The suspension in which the sample is dispersed is obtained by carrying out a dispersion treatment for about 1 to 3 minutes with an ultrasonic disperser and measuring the shape and distribution of the particles for the image display medium at a dispersion concentration of 3000 to 10,000 / μl. .

<Average charge amount>
For each of the obtained negatively chargeable particles and positively chargeable particles, the average charge amount q0 at the time of particle production, the average charge amount q1 after 100 times of operation, and the average charge amount q2 after 1 million times of operation were obtained.
Here, the amount of charge (surface charge density) at the time of particle preparation was measured by leaving the particles obtained after classification for 1 week in an environment of 25 ° C. and 50% RH and taking care not to apply friction as it is. It put in the cage and measured using the charge amount distribution measuring apparatus (Hosokawa Micron Corporation make, model espart analyzer EST-3). An amount capable of measuring about 30 particles per second was fed at a field voltage of 100 V, 2800 to 3000 particles were measured, and arithmetic calculation was performed.
In addition, the measurement of the amount of charge after 100 times or 1 million times of operation is performed by producing two exactly the same image display media, repeating the display 100 times or 1 million times simultaneously, and disassembling one of the two panels, The charge amount of white particles and black particles was measured with a suction type Faraday cage.

<Electret characteristic evaluation>
The produced particles for image display media have a very high charge retention capacity, and in normal conditions (room temperature and humidity), the charge amount does not change for several months. It takes.
Therefore, electret properties were evaluated by a thermally stimulated surface potential decay method as an accelerated evaluation method.
This is a method of determining electret properties by placing electret toner on a hot plate that is heated at a constant temperature and measuring the sample temperature and surface potential.
The rate of temperature increase was 4 ° C./min, and a sample mass of 1 g was spread in a circle on a smooth aluminum plate so as to have a thickness of 2 mm, and measurement was performed with a vibration type surface potential measuring device. The aluminum plate is connected to earth.
As the electret performance is higher, the surface charge is not attenuated to higher temperatures. Therefore, as an indicator of the degree of electretization, evaluation was performed using the onset temperature of potential decay with temperature rise and the decay rate at 100 ° C. It was. Those having sufficient electret properties have a charge decay rate at 100 ° C. of 75% or more.
It should be noted that immediately after the electret treatment of the present invention, many excessive ions exist on the toner surface, and immediately after the treatment, surface potential attenuation due to the removal of the excessive ions is observed. Therefore, in order to judge true electret property, this measurement was performed after leaving for one day.

-Production of image display media-
The image display media of Example 1 and Example 2 were produced by combining particles having different colors or charged polarities among the four types of particles as shown in Table 1 below.
Specifically, the first black particles and the second white particles were mixed at a mass ratio of 2: 1 to obtain mixed particles.
A glass substrate (thickness 1.1 mm) was used as the transparent substrate, a glass substrate (thickness 1.1 mm) was used as the counter substrate, the display electrode was an ITO electrode, and the counter electrode was a copper electrode formed by vapor deposition. The surface of each electrode was coated with a silicone resin with a thickness of about 3 μm to prevent particle adhesion. Each cell was provided with a partition wall having a height (distance between substrates) of 50 μm and a width of 20 μm. The total amount of the mixed particles was sealed so as to be 30% of the space volume. The space surrounded by the partition walls was 350 μm × 350 μm × 50 μm (inter-substrate distance).
An image display device is assembled using the produced image display medium, and after calculating 100 times and after 1 million operations, the image reflectivity during full white display and the image reflectivity during full black display are obtained, respectively, and the contrast is obtained therefrom. Asked. The results are shown in Table 1.

(Comparative Example 1 and Comparative Example 2)
-Production and evaluation of particles for image display media-
A polypropylene for general film was added with 0.3% by mass of a polar nucleating agent paraditrilidenesorbitol and 50% by mass of titanium oxide, and molded into a film having a thickness of 50 μm at a molding temperature of 240 ° C. Electrodes were provided on both sides of the film, and the temperature was raised to 200 ° C. and a voltage of 10 kV was applied, and held for 30 minutes. Then, it cooled to room temperature, applying a voltage, and canceled the voltage when it fully solidified. The obtained film was pulverized and classified to produce particles having an average particle diameter of 10 μm.
The obtained particles were uniformly dispersed on the electrode surface connected to the GND electrode. A metal electrode was placed at a position 200 μm away from the electrode, and +1 kV was applied to this electrode to cause the particles to fly from the GND electrode, and the adhered particles were collected. These white particles exhibited negative chargeability, and the surface charge density measured by a method using a Faraday cage was −15 μC / m ² .
Similarly, a metal electrode was disposed, and particles flying by applying -1 kV were collected. The surface charge density of the particles was +12 μC / m ² and white positively charged particles were obtained. In the above resin, 5% by mass of carbon was mixed in place of titanium oxide, and positive and negative charging black particles were obtained by the same operation. Surface charge density, respectively + 10μC ^/ m 2, was -12μC / ^{m 2.}
Of these four types of particles, particles having at least one of different colors and charging polarities were combined as shown in Table 1 below to produce image display media of Comparative Example 1 and Comparative Example 2.

<Evaluation>
For each of the negatively charged particles and the positively charged particles, the particle diameter is obtained, the average charge amount q0 at the time of particle production, the average charge amount q1 after 100 times of operation, and the average charge amount q2 after 1 million times of operation. Asked.
Here, the amount of charge during particle preparation was measured by leaving the particles obtained after classification for 1 week in an environment of 25 ° C. and 50% RH, and placing them in the Faraday cage with care so as not to add friction. It was measured. In addition, the measurement of the charge amount after 100 times or 1 million operations is performed by producing two completely the same display bodies and simultaneously repeating the display 100 times or 1 million times. One of the two image display media was disassembled, and the charge amount of white particles and black particles was measured with a suction type Faraday cage.
The obtained particles for an image display medium were evaluated for electret characteristics in the same manner as in Example 1.
In addition, for the image display medium, the image reflectance at the time of full white display and the image reflectivity at the time of full black display after 100 and 100 million operations were obtained, respectively, and the contrast was obtained therefrom. These results are shown in Table 1.

(Comparative Example 3 and Comparative Example 4)
-Production and evaluation of particles for image display media-
It was obtained by adding 0.3% by mass of a polar nucleating agent (paradyllidene sorbitol) and 50% by mass of titanium oxide to ordinary extrusion grade polypropylene, kneading with a twin-screw kneader, and performing extrusion. The mixture was pulverized and classified to produce particles having a particle size of 10 μm.
The obtained particles were uniformly deposited on a 5 mm thick polypropylene plate. Discharge was performed on the particles under conditions of a pulse discharge voltage (discharge rising 50 nS, maximum pulse voltage about 50 kV, 200 pulses / second) on the sawtooth electrode, and continued until the surface charge density reached 30 μC / m ² . The obtained particles showed positive chargeability, and when the amount of charge was measured by the Faraday cage method, the surface charge density was +20 μC / m ² . The same operation was performed by inverting the polarity of the pulse discharge voltage to obtain negatively chargeable particles having a surface charge density of about −22 μC / m ² .
A metal electrode was placed, and particles flying by applying -1 kV were collected. The surface charge density of the particles was +12 μC / m ² and white positively charged particles were obtained. In the above resin, 5% by mass of carbon was mixed in place of titanium oxide, and positive and negatively chargeable black particles were produced by the same operation.
Each resulting surface charge density of the particles + 10μC ^/ m 2, was -12μC / ^{m 2.} Of these four types of particles, particles having at least different colors and charge polarities were combined as shown in Table 1, and image display media of Comparative Example 3 and Comparative Example 4 were produced in the same manner as in Example 1.
In the same manner as in Example 1, various characteristics were obtained for the negatively chargeable particles, the positively chargeable particles, and the image display medium. The results are shown in Table 1.

Surface charge density distribution in the description of Table 1 (*) indicates the percentage (%) in the range surface charge density a number basis of 7 .mu.C / m ² to 15 .mu.C / m ^2.

  From the results of Table 1, it can be seen that Example 1 and Example 2 can maintain a high contrast even after operating 1 million times as compared with Comparative Examples 1 to 4, and the image display medium is stored and used for a long period of time. It was also confirmed that a stable image display state can be maintained.

  The image display medium of the present invention uses the image display medium including the image display medium particles of the present invention composed of electrets (permanently charged bodies) having an internal space charge, and stably holds the charge amount of the particles. Even if the image display medium is stored or used for a long period of time, a stable image display state can be maintained. For example, display units of mobile devices such as notebook computers, PDAs, and mobile phones, electronic books, electronic Electronic paper such as newspapers, signboards, posters, bulletin boards such as blackboards, photocopiers, rewritable paper for printer paper replacement, calculators, display units for home appliances, card display units such as point cards, various images such as electronic advertisements and electronic POPs It is suitably used for a display device.

FIG. 1A is a diagram showing the configuration of the image display medium of the present invention and its display driving principle. FIG. 1B is a diagram showing the configuration of the image display medium of the present invention and its display driving principle. FIG. 1C is a diagram showing the configuration of the image display medium of the present invention and its display driving principle. FIG. 2A is a diagram showing another configuration of the image display medium of the present invention and its display driving principle. FIG. 2B is a diagram showing another configuration of the image display medium of the present invention and its display driving principle. FIG. 2C is a diagram showing another configuration of the image display medium of the present invention and its display driving principle. FIG. 3 is a diagram showing an example of an image display apparatus using the image display medium of the present invention as a display means. FIG. 4 is a diagram illustrating an example of an apparatus for producing particles for an image display medium used in the examples. FIG. 5 is an enlarged view of the nozzle portion of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Nozzle 2 Electrode 3 Solvent removal (drying) equipment 4 Electric discharger 5 Particle collection part 6 Dry particle 7 Eddy current 8 Electric field curtain 9 Liquid conveyance tube 10 Drying container 11 Droplet 12 Particle collection container 13 Drying gas supply tube 14 Metering pump DESCRIPTION OF SYMBOLS 15 Slurry dispersion | distribution liquid storage device 16 Nozzle surface 17 Insulating substrate 18 Liquid supply flow path 19 DC high voltage power supply 20 O-ring 21 Dispersed air 22 Piezoelectric body 101 Transparent substrate 102 Opposite substrate 103 Display electrode 104 Counter electrode 105 Negatively charged particle 106 Positive Chargeable particle 107 Bulkhead 108 Color plate 110 Image display device 111 Image display medium 112 Housing 113 Information input means

Claims (8)

Used in an image display medium in which at least one kind of particles different in at least one of color and charge polarity is encapsulated between two opposing substrates,
The particles are discharged from a nozzle that vibrates at least a resin and a colorant, and a direct current potential is applied to an electrode opposed to the nozzle, and droplets ejected from the nozzle are positively and negatively induced by an induction phenomenon. A particle for an image display medium, which is obtained by drying and solidifying a liquid droplet charged with any negative polarity and at least the charge remaining.   The particles for an image display medium according to claim 1, wherein the particles are electretized. Is the surface charge density 5~20μC / ^{m 2} of particles, and the image display according to any one of claims 1 2 surface charge density of 75% or more of the total is 7~15μC / ^{m 2} on a particle number basis Medium particles.   The particle for an image display medium according to any one of claims 1 to 3, wherein the average circularity of the particle is 0.98 or more, and the coefficient of variation in the number distribution of the particle diameter of the particle is 20% or less.   The particles for an image display medium according to any one of claims 1 to 4, wherein the average particle diameter of the particles is 2 to 40 µm.   A liquid containing at least a resin and a colorant is ejected from a vibrating nozzle, a direct current potential is applied to an electrode facing the nozzle, and a droplet ejected from the nozzle is either positive or negative by an induction phenomenon. A method for producing particles for an image display medium, comprising electrifying particles by drying and solidifying at least the droplets with the charge remaining after charging to a polarity of. In an image display medium formed by enclosing at least one kind of particles different in at least one of color and charge polarity between two opposing substrates,
An image display medium, wherein the particles are particles for an image display medium according to any one of claims 1 to 5. An image display device using the image display medium according to claim 7 as display means.