HK1244545B - Electrophoretic display fluid - Google Patents

Description

Electrophoretic display of fluids

Field of the Invention

The present invention relates to an electrophoretic display fluid and an electrophoretic device using the same.

Background of the Invention

An electrophoretic display (EPD) is a non-radiative device based on the electrophoretic phenomenon that affects charged pigment particles dispersed in a dielectric solvent. An EPD typically comprises a pair of spaced-apart plate-shaped electrodes. At least one of the electrode plates is typically transparent on the viewing side. An electrophoretic fluid, consisting of a dielectric solvent in which the charged pigment particles are dispersed, is enclosed between the two electrode plates.

Electrophoretic fluids can include one or more types of charged particles. For color electrophoretic display devices, the fluid typically includes at least three types of charged particles. These include white particles, black particles, and non-white and non-black particles. For non-white and non-black particles, organic pigments are often used due to their excellent tinting strength.

In practice, however, only a few types of surface modification techniques are available for preparing organic pigment particles to function in electrophoretic displays. If organic pigment particles of different colors coexist in a fluid and have the same surface chemistry, these particles can be difficult to separate at a given drive voltage. This is due to the fact that because these particles have the same surface chemistry, it is difficult to prepare them to have the desired charge polarity and different levels of charge potential.

Furthermore, if a charge control agent (or multiple charge control agents) is used in the fluid, differently colored organic pigment particles with the same surface chemistry will compete for the same charge control agent, resulting in charge instability during driving. All of these factors can lead to poor color performance of the display device, such as coloration and poor color contrast.

Detailed Description of the Invention

In the present invention, an inorganic color pigment is used instead of at least one organic color pigment in an electrophoretic fluid including two or more types of non-white and non-black particles.

A first aspect of the present invention relates to an electrophoretic fluid comprising at least two types of charged particles dispersed in a solvent or a solvent mixture, wherein:

(i) one type of particles is non-white and non-black and is formed from an inorganic pigment,

(ii) another type of particle is non-white and non-black and is formed from an organic pigment, and

The two types of particles have different colors and carry the same charge polarity.

In one embodiment, the fluid comprises one additional type of differently colored particles (i.e., three types in total). In one embodiment, the fluid comprises two additional types of differently colored particles (i.e., four types in total). In one embodiment, the fluid comprises three additional types of differently colored particles (i.e., five types in total). In one embodiment, the fluid comprises more than five types of differently colored particles.

In one embodiment, both types of particles (i) and (ii) are positively charged. In one embodiment, the two types of particles have different types of surface chemistries.

In one embodiment, particles of type (i) carry a higher charge than particles of type (ii).

In one embodiment, the fluid includes additional types of identically charged particles that are black or white. In this embodiment, the three types of particles are positively charged. In one embodiment, the additional types of identically charged particles are black. In another embodiment, the three types of particles carry different levels of charge potential. In yet another embodiment, the amount of type (i) particles and the amount of type (ii) particles are progressively less than the amount of the additional types of identically charged particles. In other words, the additional types of identically charged particles carry the highest charge potential; the types of particles (ii) carry the lowest charge potential; and the charge of the types of particles (i) is between the charge of the additional types of identically charged particles and the charge of the types of particles (ii).

In one embodiment, the fluid further comprises other types of particles having an opposite charge. In one embodiment, the other types of particles are negatively charged.

A second aspect of the present invention relates to an electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles, a fourth type of particles, and a fifth type of particles, wherein the first, second, third, fourth, and fifth types of particles are all dispersed in a solvent or solvent mixture, wherein:

(a) the five types of particles have colors different from each other;

(b) the particles of the first, second and third types carry the same charge polarity,

(c) the second type of particles are non-white and non-black and are formed of an inorganic pigment, and the third type of particles are non-white and non-black and are formed of an organic pigment;

(d) The fourth and fifth types of particles carry charges opposite to the charges carried by the first, second and third types of particles.

In one embodiment, the first, second, and third types of particles are positively charged. In one embodiment, the first type of particles are black. In one embodiment, the second and third types of particles are present in progressively smaller amounts than the first type of particles. In other words, the first type of particles are the most highly charged, the third type of particles are the least highly charged, and the second type of particles have a charge intermediate between the first and third types of particles.

In one embodiment, the second type of particles carries a higher charge than the third type of particles.

In one embodiment, the fourth and fifth types of particles are negatively charged. In one embodiment, the amount of the fifth type of particles is less than the amount of the fourth type of particles.

In addition to color, it is possible for the multiple types of particles to have other different optical properties, such as light transmission, reflectivity, luminescence, or, in the case of displays intended for machine reading, false color in the sense of varying reflectivity for electromagnetic wavelengths outside the visible range.

The charge potential of the particles can be measured according to the zeta potential. In one embodiment, the zeta potential is determined by a Colloidal Dynamics AcoustoSizer IIM with a CSPU-100 signal processing unit and an ESA EN#Attn flow cell (K:127). Instrument constants such as the density of the solvent used in the sample, the dielectric constant of the solvent, the speed of sound in the solvent, and the viscosity of the solvent are input before the test, all of which are constants at the test temperature (25°C). The pigment sample is dispersed in a solvent (usually a hydrocarbon fluid having a carbon atom number less than 12) and diluted to 5-10% by weight. The sample also contains a charge control agent (Solsperse available from Lubrizol Corporation, Berkshire Hathaway; "Solsperse" is a registered trademark) with a weight ratio of charge control agent to particles of 1:10. The mass of the diluted sample is determined and then the sample is loaded into the flow cell for determining the zeta potential.

As for non-white and non-black organic pigments, they may include, but are not limited to, CI pigments PR254, PR122, PR149, PG36, PG58, PG7, PB15:3, PY83, PY138, PY150, PY155, or PY20. These are commonly used organic pigments described in the color index manuals "New Pigment Application Technology" (CMC Publishing Co. Ltd, 1986) and "Printing Ink Technology" (CMC Publishing Co., Ltd, 1984). Specific examples include Hostaperm Red D3G 70-EDS, Hostaperm Pink E-EDS, PV Fast Red D3G, Hostaperm Red D3G 70, Hostaperm Blue B2G-EDS, Hostaperm Yellow H4G-EDS, F2G-EDS, Novoperm Yellow HR-70-EDS, Hostaperm Green GNX, BASF Irgazine Red L 3630, Cinquasia Red L 4100HD, and Irgazin Red L 3660HD; and Sun Chemical's Phthalocyanine Blue, Phthalocyanine Green, Diaryl Yellow, or Diaryl AAOT Yellow.

For non-white and non-black inorganic pigments, they may include but are not limited to mixed metal oxide pigments typically prepared by a high temperature calcination process, such as CI (Color Index) Pigment Blue 36 or 28 (PB36 or PB28), CI Pigment Yellow 227 or 53, CI Pigment Green 50 or 26, CI Pigment Red 102, etc.

The solvent in which the pigment particles are dispersed has a dielectric constant of about 2 to about 30, preferably about 2 to about 15 for high particle mobility. Examples of suitable dielectric solvents include hydrocarbons such as Isopar, decaline (DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oils; silicone fluids; aromatic hydrocarbons such as toluene, xylene, phenyldimethylethane, dodecylbenzene, and alkylnaphthalenes; halogenated solvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene, dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride, chloropentafluorobenzene, dichlorononane, pentachlorobenzene; and perfluorinated solvents such as FC-43, FC-70, and FC-5060 from 3M Company in St. Paul, MN, low molecular weight halogen-containing polymers such as poly(perfluoropropylene oxide) from TCI in Portland, Oregon, USA, poly(chlorotrifluoroethylene) such as halocarbon oils from Halocarbon Product Corp. in River Edge, New Jersey, Krytox Oils and Greases from Galden in Ausimont or DuPont in Delaware. Perfluoropolyalkyl ether of the K-Fluid Series, a polydimethylsiloxane-based silicone oil (DC-200) from Dow-corning.

Particles (organic and inorganic) may exhibit an intrinsic charge, or may be explicitly charged using a charge control agent, or may acquire a charge when suspended in a solvent or solvent mixture.

Suitable charge control agents are well known in the art; they can be polymeric or non-polymeric in nature, or ionic or non-ionic. Examples of charge control agents may include, but are not limited to: Solsperse 17000 (reactive polymeric dispersant), Solsperse 19000, Solsperse 9000 (reactive polymeric dispersant), OLOA 11000 (succinimide ashless dispersant), Unithox 750 (ethoxylate), Span 85 (sorbitan trioleate), Petronate L (sodium sulfonate), Alcolec LV30 (soy lecithin), Petrostep B100 (petroleum sulfonate), B70 (barium sulfonate), Aerosol OT, polyisobutylene derivatives, poly(ethylene-co-butylene) derivatives, and the like.

Several methods that can be used to modify the surface of the particles of the present invention are provided below.

I. Surface modification methods:

One type of surface modification method is surface grafting. For pigments with hydroxyl (-OH) functional groups on the surface, an organosilane coupling agent can be used to react with the hydroxyl functional groups, thereby chemically bonding molecules with polymerizable functional groups to the pigment surface. Surface polymerization is then performed to graft the polymer onto the surface of the pigment particles.

Inorganic pigments are usually treated by this method because of the inherent presence of hydroxyl (-OH) functional groups on the metal oxide surface. However, this method can also be applied to organic pigments if they have hydroxyl (-OH) functional groups.

Chemistry and methods of this type are described in US Patent No. 6,822,782, the contents of which are incorporated herein by reference in their entirety.

I(a) Radical Random Graft Polymerization (RGP) Method

This method is more suitable for inorganic pigments. Therein, the particles are first reacted with a reagent having a functional group and a polymerizable group, wherein the functional group is capable of reacting with and bonding to the particle surface. The functional group reacts with the particle surface, leaving the polymerizable group covalently bonded to the particle surface and free to participate in subsequent polymerization reactions. The particles carrying the polymerizable group are then treated with one or more polymerizable monomers or oligomers under conditions effective to induce a reaction between the polymerizable group on the particle and the monomer(s) or oligomer(s); such conditions typically include the presence of a polymerization initiator, but in some cases polymerization can be thermally initiated without the presence of an initiator.

The polymerization reaction produces a polymer chain that includes at least one residue from a polymerizable group previously attached to the particle. If multiple polymerizable groups are attached to the particle in the first stage of the process, the residues of two or more of these polymerizable groups can be incorporated into the same polymer chain, which will then be attached to the particle surface at two or more points.

It is believed that the presence of multiple attached polymer chains is particularly advantageous for stabilizing particles used in electrophoretic fluids. The polymer chains do not completely cover the surface of the particles. Incomplete coverage of the surface of the pigment particles by the polymer chains is important in providing good electrophoretic properties to the particles.

I(b) Ionic Random Graft Polymerization (Ionic RGP) Method

Alternatively, the polymerizable group can be attached to the particle via an ionic bond. Depending on the chemical properties of the particle, in some cases, it is possible to simply react the monomer with the particle to form the desired ionic bond. However, in most cases, it will be necessary to pre-treat the particle with a bifunctional reagent having one functional group capable of reacting with and bonding to the particle and a second functional group capable of forming the necessary ionic bond. The resulting particle is then reacted with a monomer having a polymerizable group and a third functional group capable of reacting with the second functional group to form the desired ionic bond. The final polymerization step of the RGP process is then carried out as described above to prepare the product. The ionic bond forming reaction is typically an acid-base reaction; for example, the second functional group may be an ammonium group, such as an alkyl-substituted ammonium group, and the third functional group may be a sulfonic acid, or vice versa.

The ion-RGP process, which is also more suitable for inorganic pigments, has the following advantages: some of the ionically bonded polymer chains in the final particles can separate and disperse in the suspension of the electrophoretic fluid, thus providing stabilizing counterions to the charged particles. In effect, the ionically bonded polymer acts as both a stabilizing polymer and a charge control agent for the particles.

I(c) Atom Transfer Radical Polymerization (ATRP) Method

Alternatively, a group capable of initiating polymerization can first be attached to the pigment particles and a polymer can be formed by the initiating group. The initiating group can be attached to the polymer surface in any of the aforementioned ways by covalent or ionic bonds. In the first stage of the process, a bifunctional reagent having a group capable of reacting with the particle surface and a second group providing an initiation site for atom transfer radical polymerization (ATRP) is used to treat the surface of the particles. The ATRP initiator site can be, for example, benzyl chloride or other halogen atoms. The resulting particles are then treated with an atom transfer radical polymerizable monomer (e.g., methyl methacrylate) to form a polymer on the particle surface. The advantage of ATRP is that the polymerization reaction with the first monomer can be stopped by cooling the reaction mixture, the first monomer can be replaced by the second monomer, and then the reaction can be restarted by raising the temperature of the reaction mixture so that the second monomer is polymerized to the end of the polymer previously formed of the first monomer. When a third monomer is introduced, these steps can be repeated. The process forms a block copolymer of two (or more) monomers on the particles.

The method is not limited to the use of ATRP initiation sites on the particles, but also includes the use of other types of initiation sites, such as ionic or free radical initiation sites. Furthermore, the above-mentioned bifunctional reagents need not be single monomeric reagents, but can themselves be polymeric.

The ATRP method is also more suitable for inorganic pigments.

The method in this section can include more than one stage and/or more than one type of polymerization. For example, in addition to using a monomer mixture containing at least one monomer (e.g., chloromethylstyrene) that provides a group for ATRP initiation sites, the particles are first subjected to the above-mentioned free radical polymerization method. Therefore, polymer chains containing ATRP initiation sites are formed on the particles. After the free radical polymerization is completed, the particles are then subjected to ATRP so that polymer side chains are formed by the ATRP initiation sites, thereby producing a "hyperbranched" polymer with a main chain formed by the RGP process and a side chain formed by ATRP. It has been found that this type of polymer structure is very advantageous in stabilizing the suspension of charged particles in a non-ionic fluid medium that is commonly used as a suspending fluid in an electrophoretic display. Similar types of hyperbranched polymers can be prepared by including a monomer containing an initiation group for stable free radical polymerization (SFRP) in the monomer mixture used in the RGP step, the SFRP initiation group being selected so that it does not substantially initiate polymerization under the conditions used in the RGP step. After the RGP step is completed, the particles are then subjected to SFRP to prepare hyperbranched polymers.

Furthermore, any bifunctional reagent having one group capable of covalently or ionically bonding to the surface and a second group providing the required polymerizable or initiating functionality can be used to attach the polymerizable group and initiator to the surface of the particle. The independent functionality of the two groups has the advantage of providing great flexibility in adapting the method to different types of particles while maintaining the same polymerizable or initiating functionality, so that later stages of the process will require only minimal changes (if necessary) due to changing the type of particle being coated.

Where a reagent for providing the desired polymerizable or initiating functional group is described as "bifunctional", the reagent may include more than one group of each type, and indeed in some cases it may be desirable to provide more than one group of one or both types. For example, polymerization initiators having more than one ionic site are known (such as 4,4'-azobis(4-cyanovaleric acid)), and such initiators may be used in the method. Furthermore, the bifunctional reagent may be in the form of a polymer containing repeating units capable of bonding to the particle surface and other repeating units having the desired polymerizable or initiating functional group, and such polymeric bifunctional reagents will typically include multiple repeating units of both types.

The polymerizable and initiating groups used may be any of those known in the art, provided that the groups in question are compatible with the reaction used to attach them to the particle surface. Many examples are given in US Patent No. 6,822,782.

Typically, particles prepared by the surface grafting method described above have from about 1 to about 15% by weight of polymer chemically bonded to or cross-linked around the particle surface. The pigment surface has hairy polymer chains attached. The polymer chains are partially or completely dissolved in the electrophoretic fluid.

II. Alternative surface modification methods:

Another type of surface modification method produces particles with one or more core particles encapsulated within a polymer shell or matrix. The polymer shell or matrix is insoluble in the electrophoretic fluid. The encapsulated particles have polymer chains attached as stabilizers to help disperse the particles in the electrophoretic fluid. The polymer chains are soluble in the fluid.

The polymer matrix and stabilizer are not chemically bonded to the original pigment surface. They can be washed off with a suitable solvent. The polymer content can range from 10-80% by weight.

Organic pigments are often modified by encapsulation methods because they typically have a crystalline structure with a surface that is very difficult to chemically bond additional molecules to. Surface grafting is a major challenge for organic pigments because it can destroy the chromophore and change the pigment's color.

These methods are described in U.S. Publication Nos. 2012-0199798 and 2013-0175479, the contents of which are incorporated herein by reference in their entireties.

The core particle may be any of those organic or inorganic pigments described above.

The core particles may optionally be surface treated. When the final particles are formed, the surface treatment improves the compatibility of the core pigment particles with the monomers in the reaction medium or improves chemical bonding with the monomers. For example, the surface treatment can be performed using organosilanes having functional groups such as acrylates, vinyl, _-NH2 , -NCO, -OH, etc. These functional groups can chemically react with the monomers. Other organic materials can be used to pretreat the pigment, including polymers or oligomers that act as dispersants, such as polyacrylates, polyurethanes, polyureas, polyethylene, polyesters, polysiloxanes, etc.

The surface treatment may also be from inorganic materials including silica, alumina, zinc oxide, etc. or combinations thereof. Sodium silicate or tetraethoxysilane may be used as common precursors for silica coatings.

Additionally, the surface treatment may optionally have functional groups that would enable charge generation or interaction with charge control agents.

The core particle(s) and the surface treatment material should behave as a single unit. The core particles are then encapsulated with a polymer through the following process.

II(a) Dispersion Polymerization Method

In either case, the final particles can be formed by this method, whether inorganic or organic core particles. During dispersion polymerization, monomers polymerize around the core pigment particles. The solvent chosen as the reaction medium must be a good solvent for both the monomers and the forming polymer chains, but a non-solvent for the forming polymer shell. For example, in the aliphatic hydrocarbon solvent of Isopar, the monomer methyl methacrylate is soluble; however, after polymerization, the resulting polymethyl methacrylate is insoluble.

The polymer shell must be completely incompatible or relatively incompatible with the solvent in which the final particles are dispersed. Suitable monomers can be those mentioned above, such as styrene, methyl acrylate, methyl methacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl acrylate, t-butyl methacrylate, vinyl pyridine, N-vinyl pyrrolidone, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, and the like.

In order to incorporate functional groups for charge generation, comonomers can be added to the reaction medium. The comonomers can directly charge the composite pigment particles or interact with the charge control agent in the display fluid to impart the desired charge polarity and charge density to the composite pigment particles. Suitable comonomers can include vinylbenzylaminoethylaminopropyltrimethoxysilane, methacryloxypropyltrimethoxysilane, acrylic acid, methacrylic acid, vinylphosphoric acid, 2-acrylamido-2-methylpropanesulfonic acid, 2-(dimethylamino)ethyl methacrylate, N-[3-(dimethylamino)propyl]methacrylamide, and the like.

The polymer chains on the surface of the particles are typically formed from high molecular weight polymers such as polyethylene, polypropylene, polyester, polysiloxane or mixtures thereof. The polymer chains facilitate and stabilize the dispersion of the particles in the solvent.

The polymer chains can be reactive and polymerizable macromonomers that adsorb, bind, or chemically bond to the surface of the forming polymer shell. The macromonomers act as polymer chains, determining the particle size and colloidal stability of the system.

The macromonomers may be acrylate terminated or vinyl terminated macromonomers, which are suitable because the acrylate or vinyl groups can copolymerize with the monomers in the reaction medium.

The macromonomer preferably has a long terminal R, which can stabilize the final particles in hydrocarbon solvents.

One type of macromonomer is an acrylate-terminated polysiloxane (Gelest, MCR-M11, MCR-M17, MCR-M22), as shown below:

Another type of macromonomer suitable for this process is the PE-PEO macromonomer, as shown below:

R _m O-[-CH ₂ CH ₂ O-] _n -CH ₂ -phenyl-CH=CH ₂

Or R _m O-[-CH ₂ CH ₂ O-] _n -C(=O)-C(CH ₃ )=CH ₂

The substituent R may be a polyethylene chain, n is 1 to 60, and m is 1 to 500. The synthesis of these compounds can be found in Dongri Chao et al., Polymer Journal, Vol. 23, no. 9, 1045 (1991) and Koichi Ito et al., Macromolecules, 1991, 24, 2348.

Another type of suitable macromonomer is the PE macromonomer, as shown below:

CH ₃ -[-CH ₂ -]n-CH ₂ OC(=O)-C(CH ₃ )=CH ₂

In this case, n is between 30 and 100. The synthesis of macromonomers of this type can be found in Seigou Kawaguchi et al., Designed Monomers and Polymers, 2000, 3, 263.

II(b) Living Radical Dispersion Polymerization Method

Alternatively, the particles can be prepared by living free radical dispersion polymerization. This method is applicable to both organic and inorganic pigments; but may be more suitable for organic pigments.

Living free radical dispersion polymerization techniques are similar to the dispersion polymerization described above by starting the process with pigment particles and monomers dispersed in a reaction medium.

The monomers used in the process of forming the shell may include styrene, methyl acrylate, methyl methacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl acrylate, t-butyl methacrylate, vinyl pyridine, N-vinyl pyrrolidone, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, etc.

However, in this alternative process, multiple active ends are formed on the surface of the shell. Active ends can be generated by adding reagents such as TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) or RAFT (reversible addition-fragmentation chain transfer) reagents to the reaction medium for living free radical polymerization.

In a further step, a second monomer is added to the reaction medium to react the active end with the second monomer to form a polymer chain. The second monomer can be lauryl acrylate, lauryl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, hexyl acrylate, hexyl methacrylate, n-octyl acrylate, n-octyl methacrylate, n-octadecyl acrylate, n-octadecyl methacrylate, or the like.

The polymer chains should be compatible with the solvent in which the particles are dispersed to facilitate dispersion of the particles in the solvent.

The monomers used for the polymer chain can be a mixture of hydroxyethyl methacrylate and other acrylates compatible with non-polar solvents, such as lauryl acrylate, lauryl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, hexyl acrylate, hexyl methacrylate, n-octyl acrylate, n-octyl methacrylate, n-octadecyl acrylate, n-octadecyl methacrylate, etc.

Advantages of the present invention include color saturation and color brightness, which can be measured using the L*a*b* color system. Surprisingly, it has been discovered that an electrophoretic fluid comprising organic colored (non-white and non-black) particles carrying a certain level of charge potential and inorganic colored (also non-white and non-black) particles carrying the same charge polarity can exhibit significantly better color performance than an electrophoretic fluid in which both types of similarly charged organic colored particles coexist in the electrophoretic fluid. Furthermore, the system of the present invention allows for switching between two color states with shorter waveforms, resulting in higher switching speeds.

Because the inorganic and organic pigments in the system of the present invention have different surface modification chemistries, the separation of the organic color pigments from the inorganic color pigments is much easier. This allows for more saturated color states and higher contrast.

Example:

In the experiments conducted by the present inventors, the electrophoretic fluid had multiple types of charged particles of different colors. Among the multiple types of charged particles, red and blue particles were positively charged.

In Sample A, both the red and blue particles were formed from organic pigments (ie, PR254 and PB15, respectively) and were surface treated using the same method, namely, dispersion polymerization.

In sample B, the red particles were formed of an organic pigment (ie, PR254) and surface-treated by dispersion polymerization, while the blue particles were formed of an inorganic pigment (ie, PB28) and surface-treated by a free radical polymerization method.

When the two samples were driven to different color states, it was found that sample A was unable to display the red state because the organic red and organic blue particles competed for the same charge control agent, Solsperse 17K, in the fluid, and the two types of particles were not sufficiently separated. In contrast, sample B did not have this problem and was able to display red and blue with good color saturation and brightness.

The electrophoretic fluid of the present invention is filled in a display unit. The display unit can be a cup-shaped microunit as described in U.S. Patent No. 6,930,818, the contents of which are incorporated herein by reference in their entirety. The display unit can also be other types of microcontainers, such as microcapsules, microchannels, or equivalents, regardless of their shape or size. All of these are within the scope of the present application.

Although the present invention has been described with reference to specific embodiments thereof, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation, material, composition, process, process step or steps to the purpose, spirit and scope of the present invention. All such modifications are intended to be within the scope of the appended claims.

Claims (14)

1. An electrophoretic fluid comprising three types of charged particles dispersed in a solvent or solvent mixture, wherein: (i) the first type of particles are non-white and non-black and are formed from inorganic pigments, which are surface-treated by free radical polymerization, (ii) the second type of particles are non-white and non-black and are formed of an organic pigment, and are surface treated by dispersion polymerization, and (iii) The third type of particles is black in color, The first, second and third types of particles have different colors, different surface chemistries, carry the same charge polarity, and carry different charge amounts from each other. 2. The fluid of claim 1, comprising four, five or more types of differently colored particles. 3. The fluid of claim 1, wherein the first type of particles and the second type of particles are both positively charged. 4. The fluid of claim 1, wherein the first type of particles carries a higher charge than the second type of particles. 5. The fluid of claim 1, wherein the three types of particles are positively charged. 6. A fluid according to claim 1, wherein the third type of equally charged particles have the highest charge, the second type of particles have the lowest charge, and the first type of particles have a charge amount that is between the charge amount of the third type of equally charged particles and the charge amount of the second type of particles. 7. The fluid of claim 1, further comprising other types of particles carrying a charge polarity opposite to the charge polarity carried by the first, second, and third types of particles. 8. The fluid of claim 7, wherein the other type of particles are negatively charged. 9. An electrophoretic fluid comprising a first type of particles, a second type of particles, a third type of particles, a fourth type of particles, and a fifth type of particles, wherein the first, second, third, fourth, and fifth types of particles are all dispersed in a solvent or solvent mixture, wherein: (a) Five types of particles have different colors from each other; (b) the first, second, and third types of particles carry the same charge polarity but different charge magnitudes from one another; (c) the first type of particles are black in color, the second type of particles are non-white and non-black and are formed of an inorganic pigment, and are surface-treated by free radical polymerization, and the third type of particles are non-white and non-black and are formed of an organic pigment, and are surface-treated by dispersion polymerization; (d) The fourth and fifth types of particles carry charges opposite to the charges carried by the first, second and third types of particles. 10. The fluid of claim 9, wherein the first, second, and third types of particles are positively charged. 11. The fluid of claim 9, wherein the second type of particles carries a higher charge than the third type of particles. 12. The fluid of claim 9, wherein the first type of particles have the highest charge, the third type of particles have the lowest charge, and the second type of particles have a charge intermediate between the first and third types of particles. 13. The fluid of claim 9, wherein the fourth and fifth types of particles are negatively charged. 14. The fluid of claim 9, wherein the amount of the fifth type of particles is less than the amount of the fourth type of particles.