HK1235868B - Electrophoretic fluid - Google Patents

Description

electrophoresis fluid

This application is a divisional application of the patent application with application number 201210083230.X and invention name “Electrophoretic Fluid” filed on January 31, 2012.

Technical Field

The present invention relates to the preparation of composite pigment particles that can be used to form electrophoretic fluids and synthetic display fluids.

Background Art

An electrophoretic display (EPD) is a non-emissive device based on the electrophoretic phenomenon affecting 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 (typically on the viewing side) is transparent. An electrophoretic fluid, consisting of a dielectric solvent in which the charged pigment particles are dispersed, is enclosed between the two electrode plates.

The electrophoretic fluid may contain one type of charged pigment particles dispersed in a solvent or solvent mixture of a contrasting color. In this case, when a voltage difference is applied across the two electrode plates, the pigment particles migrate to the electrode plate with the opposite polarity to the pigment particles due to attraction. Therefore, the color displayed on the transparent plate is either the color of the solvent or the color of the pigment particles. Reversing the plate polarity will cause the particles to migrate back to the opposite plate, thus reversing the color.

Alternatively, the electrophoretic fluid may have two types of pigment particles having contrasting colors and carrying opposite charges, and the two types of pigment particles may be dispersed in a transparent solvent or solvent mixture. In this case, when a voltage difference is applied to the two electrode plates, the two types of pigment particles will move to opposite ends (top or bottom) of the display cell. As a result, one of the colors of the two types of pigment particles will be displayed on the viewing side of the display cell.

For all types of electrophoretic displays, the liquid contained within the display's individual cells is undoubtedly one of the most critical components of the device. The composition of this liquid largely determines the device's lifetime, contrast, switching rate, and bistability.

In an ideal liquid, the charged pigment particles should remain separate and not clump or stick to each other or the electrodes under all operating conditions. In addition, all components in the liquid must be chemically stable and compatible with the other materials present in the electrophoretic display.

Currently, pigment particles in electrophoretic fluids typically have a density much higher than the density of the solvent in which the particles are dispersed, thus leading to performance issues such as poor grayscale bistability, homeotropic driving, and sedimentation phenomena.

Summary of the Invention

The present invention relates to a display liquid comprising charged composite pigment particles dispersed in a solvent, wherein each of the composite pigment particles comprises at least one core pigment particle, a shell encapsulating the core pigment particle, and three-dimensional stabilizer molecules on the surface of the composite pigment particle.

In one embodiment, the density of the composite pigment particles substantially matches the density of the solvent.

In one embodiment, the difference between the density of the composite pigment particles and the density of the solvent is less than 2 g/cm ³ .

In one embodiment, the core pigment particles are inorganic pigment particles. In one embodiment, the core pigment particles are surface treated.

In one embodiment, described shell is made of inorganic material.For example, this shell can be made of silicon dioxide, aluminum oxide, zinc oxide or their combination.In one embodiment, if this shell is made of inorganic material, then this shell is porous.In one embodiment, the organic matter content of this composite pigment particle is in the scope of about 10wt% (weight percent) to about 50wt%, more preferably be greater than about 15wt% and go up to about 30wt%.

In one embodiment, the shell is made of an organic material. For example, the shell can be made of polyacrylate, polyurethane, polyurea, polyester, or polysiloxane. In one embodiment, the composite pigment particles have an organic content of at least about 20 wt %, preferably about 20 wt % to about 70 wt %, and more preferably about 20 wt % to about 40 wt %.

In one embodiment, the shell is completely incompatible or relatively incompatible with the solvent.

In one embodiment, the steric stabilizer molecules are made of polyacrylate, polyethylene, polypropylene, polyester, polysiloxane or a mixture thereof.

In one embodiment, the surface of the shell includes functional groups that can generate charge or interact with a charge control agent.

In one embodiment, the liquid further includes a second type of charged pigment particles. In one embodiment, the second type of charged pigment particles are composite pigment particles, each comprising at least one core pigment particle, a shell surrounding the core pigment particle, and steric stabilizer molecules on the surface of the composite pigment particle. The two types of composite pigment particles in the liquid have contrasting colors.

In one embodiment, the solvent in which the composite pigment particles are dispersed is a hydrocarbon solvent or a mixture of a hydrocarbon solvent and another solvent, such as a halogenated solvent or a silicone oil solvent.

In one embodiment, the composite pigment particles are prepared by dispersion polymerization or living free radical polymerization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 shows the composite pigment particles of the present invention.

FIG2 shows the reaction steps of a process suitable for preparing the composite pigment particles of the present invention.

DETAILED DESCRIPTION

As shown in Figure 1, the first aspect of the present invention is a composite pigment particle having a density that closely matches the density of the solvent in which it is dispersed, particularly a hydrocarbon solvent.

The composite pigment particle (10) may have one or more core pigment particles (11). The core particle (11) is surrounded by a shell (12). A three-dimensional stabilizer molecule (13) is present on the surface of the composite pigment particle.

The core pigment particles are preferably inorganic, such as _TiO2 , _BaSO4 , ZnO, metal oxides, manganese iron black spinel, copper chromium black spinel, carbon black or zinc sulfide pigment particles. They may be black, white or another color.

The core particles may optionally be surface treated. This surface treatment improves the compatibility of the core pigment particles with the monomers in the reaction medium or their chemical bonding with the monomers when forming the composite pigment particles. For example, the surface treatment may be performed using organosilanes having functional groups such as acrylate, vinyl, _-NH₂ , -NCO, -OH, or the like. These functional groups are capable of chemically reacting with the monomers.

The shell may be made of inorganic or organic material.

Inorganic shell materials may include silica, alumina, zinc oxide and the like or combinations thereof. Sodium silicate or tetraethoxysilane may be used as common precursors for the silica coating.

The organic shell can be made of an organic polymer such as polyacrylate, polyurethane, polyurea, polyethylene, polyester, polysiloxane or the like. For example, the polyacrylate shell can be made of a monomer such as styrene, methyl acrylate, methyl methacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl acrylate, t-butyl methacrylate, vinylpyridine, n-vinylpyrrolidone, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, dimethylaminoethyl methacrylate or the like. The polyurethane shell can be made of a monomer or oligomer such as a multifunctional isocyanate or thioisocyanate, a primary alcohol or the like. The polyurea shell can be made of a monomer comprising a reactive group such as amino/isocyanate, amino/thioisocyanate or the like. Based on the main idea of the present invention, those skilled in the art will be able to select suitable monomers or oligomers and their variants.

If the shell is inorganic, the shell structure can be porous to reduce density, and the "organic content" of the synthesized composite pigment particles is in the range of about 10 wt% to about 50 wt%, more preferably greater than about 15 wt% up to about 30 wt%.

If the shell is organic, the "organic content" of the synthesized composite pigment particle is at least about 20 wt%, preferably about 20 wt% to about 70 wt%, and more preferably about 20 wt% to about 40 wt%.

The term "organic content" is determined by dividing the total weight of the shell (12) and the steric stabilizer (13) by the total weight of the core pigment particle (11), the shell (12) and the steric stabilizer (13).

In any case, the density of the resultant shell is preferably low, less than 2 g/cm ³ , more preferably less than about 1 g/cm ³ .The thickness of the shell can be controlled based on the density of the shell material and the desired final particle density.

The shell material is either completely incompatible or relatively incompatible with the display fluid in which the composite pigment particles are dispersed. In other words, no more than about 5% (preferably no more than 1%) of the shell material is soluble in the display fluid.

To achieve this complete or relative incompatibility, the shell polymer material may have polar functional groups on its main chain or side chains. Examples of such polar functional groups may include -COOH, -OH, NH ₂ , ROR, R-NH-R, and the like (wherein R is a hydrocarbon group or an aryl group). In this case, each of the side chains preferably has fewer than 6 carbon atoms. In one embodiment, the main chain or side chain may include a portion of an aromatic molecule.

In addition, the core pigment particle and shell should function as a single unit. As described below, this can be achieved by cross-linking or encapsulation techniques.

The steric stabilizer (13) in Figure 1 is usually composed of a high molecular weight polymer, such as polyethylene, polypropylene, polyester, polysiloxane or a mixture thereof. The steric stabilizer promotes and stabilizes the dispersion of the composite pigment particles in the solvent.

Furthermore, the surface of the shell may optionally have functional groups capable of generating charge or interacting with a charge control agent.

The second aspect of the present invention is the preparation of the composite pigment particles of the present invention, which may involve a variety of techniques.

For example, they can be formed by dispersion polymerization. During dispersion polymerization, monomers polymerize around the core pigment particle in the presence of a steric stabilizer polymer that is soluble in the reaction medium. The solvent chosen as the reaction medium must be soluble in both the monomers and the steric stabilizer polymer, but not in the resulting polymer shell. For example, the monomer methyl methacrylate is soluble in isoparaffinic aliphatic hydrocarbon solvents; however, after polymerization, the resulting polymethyl methacrylate is insoluble.

The polymer shell formed from the monomers must be completely incompatible or relatively incompatible with the solvent in which the composite pigment particles are dispersed. Suitable monomers may be those mentioned above, for example, styrene, methyl acrylate, methyl methacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl acrylate, t-butyl methacrylate, vinylpyridine, n-vinylpyrrolidone, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, dimethylaminoethyl methacrylate or the like.

The steric stabilizer polymer may be a reactive and polymerizable macromonomer that is adsorbed, bound or chemically bonded to the surface of the formed polymer shell. The macromonomer as a steric stabilizer determines the particle size and colloidal stability of the system.

The macromonomer may be a suitable acrylate-terminated or vinyl-terminated macromer, since the acrylate or vinyl groups are capable of copolymerizing with the monomers in the reaction medium.

The macromonomer preferably has a long tail R, which can stabilize the composite pigment particles in hydrocarbon solvents.

One type of macromonomer suitable for the 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 1, Vol. 23, no. 9, 1045 (1991) and Koichi Ito et al., Macromolecules, 1991, 24, 2348.

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

CH ₃ -[-CH ₂ -] _n -CH ₂ OC(CH ₃ )=CH ₂

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

To incorporate functional groups to generate a charge, a comonomer may be added to the reaction medium. The comonomer either directly imparts a charge to the composite pigment particles or interacts with a charge control agent in the display fluid to impart the desired charge polarity and charge density to the composite pigment particles. Suitable comonomers may include vinylbenzylaminoethylamine-propyl-trimethylsiloxane, methacryloxypropyl trimethylsiloxane, acrylic acid, methacrylic acid, vinylphosphonic acid, and the like.

Alternatively, the composite pigment particles can be prepared by living radical dispersion polymerization, as shown in FIG2 .

The living radical dispersion polymerization technique is similar to the dispersion polymerization described above by initiating the process with inorganic pigment particles (21) and monomers dispersed in a reaction medium.

The monomers used in the process to form the shell (22) 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 and the like.

However, in this alternative embodiment, a plurality of active ends (24) are formed on the surface of the shell (22). The active ends can be generated by adding additives such as TEMPO (2,2,6,6-tetramethyl-1-piperidinyl oxide), RAFT (reversible addition-fragmentation transfer) agents or the like to the reaction medium for living free radical polymerization.

In the next step, a second monomer is added to the reaction medium so that the active end (24) reacts with the second monomer to form a steric stabilizer (23). 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 steric stabilizer should be compatible with the solvent in which the composite pigment particles are dispersed to facilitate dispersion of the composite pigment particles in the solvent.

The steric stabilizer can also be prepared by living free radical polymerization.

Comonomers may also be added to impart charge. Suitable comonomers may include vinylbenzylaminoethylamine-propyl-trimethylsiloxane, methacryloxypropyl trimethylsiloxane, acrylic acid, methacrylic acid, vinylphosphonic acid, and the like.

Further alternatively, the composite pigment particles can be prepared by coating core pigment particles with polyurethane and/or polyurea.

Polyurethanes and polyureas are generally incompatible with non-polar hydrocarbon solvents, and their hardness and elastic properties can be adjusted by this monomer component.

In the composite pigment particles of the present invention, the shell can be a polyurethane or polyurea material. The steric stabilizer can be a non-polar long-chain hydrocarbon molecule.

The synthesis method is similar to emulsion polymerization or dispersion polymerization, except that polycondensation occurs between the polyurethane monomer and the inorganic core pigment particle inside the micelle.

This polyurethane or polyurea wrapping system can be considered as a kind of oil-in-oil emulsion, and it comprises two kinds of incompatible solvents, one of which is a non-polar organic solvent and another is a polar organic solvent.This system also can be called non-aqueous emulsion polycondensation, and wherein this non-polar solvent is that continuous phase and this polar solvent are discontinuous phase.Monomer and inorganic pigment particle are in discontinuous phase.Suitable non-polar solvent can comprise the solvent in the isoparaffin series, hexamethylenetetradecane, n-tetradecane, hexane or analogue.This polar solvent can comprise acetonitrile, DMF and analogue.

Emulsifiers or dispersants are key to this two-phase organic system. Their molecular structure can contain a portion that is soluble in the nonpolar solvent and another portion that binds to the polar phase. This stabilizes the micelles/droplets, which contain the monomer and inorganic pigment particles and serve as microreactors for particle formation via polycondensation.

Suitable emulsifiers or dispersants may include di-block copolymers such as poly(isoprene)-b-poly(methyl methacrylate), polystyrene-b-poly(ethylene-alt-propylene) (Kraton), or the like.

Additionally, a co-emulsifier may be added to form a chemical bond with the particles. For example, an amino-terminated hydrocarbon molecule may react with the particles during polycondensation and bond to the surface as a robust steric stabilizer. Suitable co-emulsifiers may include SURFONAMINE (B-60, B-100, or B-200) as shown below:

CH ₃ -[-OCH ₂ CH ₂ -]x-[-OCH ₂ CH(CH ₃ )-]y-NH ₂

Where x is 5-40 and y is 1-40.

An alternative approach is to continue growing the polyacrylate steric stabilizer after the polycondensation reaction in the microreactor is complete. In this case, while the steric stabilizer may be a polyacrylate chain, the shell is composed of a polyurethane. After the emulsifier or dispersant used in the process has been washed from the particle surface, the composite pigment particles are stable in a nonpolar solvent (i.e., a display solution) with the polyacrylate stabilizer. Some materials that can initiate acrylate polymerization include ethyl isocyanate acrylate, styrene isocyanate, or the like.

The monomer used for the steric stabilizer 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 or the like.

In any of the above processes, the amounts of reagents used (eg, inorganic core pigment particles, shell materials, and materials for forming steric stabilizers) can be adjusted and controlled to achieve a desired organic content in the resulting composite pigment particles.

A third aspect of the present invention is a display fluid comprising the composite pigment particles of the present invention, the composite pigment particles being dispersed in a solvent. Preferred solvents have a low dielectric constant (preferably about 2-3), a high volume resistivity (preferably about 1015 ohm-cm or greater), and a low water solubility (preferably less than 10 units per million). Suitable hydrocarbon solvents may include, but are not limited to, dodecane, n-tetradecane, aliphatic hydrocarbons of the isoparaffin series (Exxon, Houston, Tex), and the like. The solvent may also be a hydrocarbon and a halocarbon or silicone oil-based material.

The present invention can be used in a single-particle or double-particle electrophoresis display liquid system.

In other words, the present invention may relate to a display fluid comprising only the composite pigment particles prepared according to the present invention dispersed in a hydrocarbon solvent, the composite pigment particles and the solvent having contrasting colors.

Alternatively, the present invention can be about the display liquid of two kinds of pigment particles that are included in the organic solvent dispersion, and at least one of these two kinds of pigment particles prepare according to the present invention.These two kinds of pigment particles carry opposite charge polarity and have contrasting colors.For example, these two kinds of pigment particles can be respectively black and white.In this case, this black particle can be prepared according to the present invention, or this white particle can be prepared according to the present invention, or white particle and black particle both can be prepared according to the present invention.

The composite pigment particles prepared according to the present invention, when dispersed in an organic solvent, offer numerous advantages. For example, the density of the composite pigment particles can be substantially matched to that of the organic solvent, thereby improving the performance of the display device. In other words, the difference between the density of the composite pigment particles and the density of the solvent is less than 2 g/ ^cm³ , more preferably less than 1.5 g/ ^cm³ , and most preferably less than 1 g/ ^cm³ .

In a dual particle system, if only one pigment particle is prepared according to the present invention, the other pigment particle can be prepared according to any other method. For example, the particle can be a polymer-embedded pigment particle. The microencapsulation of the pigment particle can be accomplished chemically or physically. Typical microencapsulation processes include interfacial polymerization/crosslinking, in-situ polymerization/crosslinking, phase separation, simple or complex coagulation, electrostatic encapsulation, dry spraying, fluidized bed coating, and solvent evaporation.

Pigment particles prepared by known techniques may also exhibit a neutral charge, or may be charged explicitly using a charge control agent, or may acquire a charge when suspended in an organic solvent. Suitable charge control agents are well known in the art; they may be polymeric or non-polymeric and may be ionic or non-ionic, and include ionic surfactants such as dye materials, sodium dodecylbenzenesulfonate, metallic soaps, polybutylene succinimide, maleic anhydride copolymers, vinyl pyridine copolymers, vinyl pyrrolidone copolymers, (meth)acrylic acid copolymers, N,N-dimethylaminoethyl (meth)acrylic acid copolymers, or the like.

Example

Step A: Deposition of Vinylbenzylaminoethylamine-propyl-trimethicone on black pigment particles

To a 1 L reactor, black pigment 444 (Shepherd, 40 g), isopropyl alcohol (320 g), DI water (12 g), ammonium hydroxide (28%, 0.4 g), and Z-6032 (Dow Corning, 16 g, 40% in methanol) were added. The reactor was heated to 65°C with mechanical stirring in an ultrasonic bath. After 5 hours, the mixture was centrifuged at 6000 rpm for 10 minutes. The solids were redispersed in isopropyl alcohol (300 g), centrifuged, and dried under vacuum at 50°C overnight to produce 38 g of the desired pigment granules.

Step B: Preparation of a polymer coating on the pigment particles by dispersion polymerization

Two (2) grams of polyvinylpyrrolidone (PVPK30) was dissolved in a mixed solvent of 94.5 grams of water and 5.5 grams of ethanol. The solution was purged with nitrogen for 20 minutes and heated to 65°C. The pigment particles (4 grams) prepared in step A were dispersed in a mixture of 3.0 grams of lauryl acrylate, 0.2 grams of divinylbenzene, and 0.03 grams of AIBN (azobisisobutyronitrile) to form a balanced suspension. The suspension was added to the PVP solution at 65°C. The polymerization reaction was continued for approximately 12 hours with stirring.

Then a mixture of 3.0 g of octadecyl acrylate and 0.03 g of AIBN was added to the above reaction flask and the reaction was continued for 12 hours.

The solid product was separated from the liquid by centrifugation and then washed with isopropyl alcohol and methyl ethyl ketone to remove PVP K30 and other chemicals not bound to the pigment particle surface. The solid was dried at 50°C under vacuum to produce the final composite black particles. The organic content of the resulting particles was approximately 34 wt% as determined by TGA (thermogravimetric analysis).

While 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 which fall within the scope of the present invention.

Claims (12)

1. A display liquid, comprising: Charge control agent; Charged composite pigment particles comprise nucleochrome particles, an organic polymer shell, and a stereochemical stabilizer polymer selected from the group consisting of polyethylene, polypropylene, polyester, polysiloxane, and mixtures thereof. The stereochemical stabilizer polymer is formed by active radical dispersion polymerization, thereby chemically bonding the stereochemical stabilizer polymer to the surface of the organic polymer shell. A hydrocarbon solvent, wherein the charge control agent and the charged composite pigment particles are dispersed in the hydrocarbon solvent. 2. The display liquid of claim 1, wherein the density of the composite pigment particles is substantially matched to the density of the solvent. 3. The display liquid according to claim 1, wherein the nucleochrome particles are inorganic pigment particles. 4. The display liquid according to claim 1, wherein the nucleochrome particles are surface-treated. 5. The display liquid of claim 1, wherein the organic polymer shell is made of polyacrylate, polyurethane, polyurea, polyethylene, polyester or polysiloxane. 6. The display liquid of claim 1, wherein the organic polymer shell is completely incompatible or relatively incompatible with the solvent. 7. The display liquid of claim 1, wherein the surface of the organic polymer shell includes functional groups capable of generating charge or interacting with the charge control agent. 8. The display liquid as claimed in claim 1, further comprising a second type of charged pigment particles, wherein the second type of charged pigment particles is a contrasting color. 9. The display liquid of claim 8, wherein the second charged pigment particle is composed of nucleochrome particles, an organic polymer shell, and a three-dimensional stabilizer polymer, wherein the three-dimensional stabilizer polymer is chemically bonded to the surface of the organic polymer shell. 10. The display liquid of claim 9, wherein the second type of charged pigment particles are prepared by conventional microencapsulation technology. 11. The display liquid of claim 1, wherein the composite pigment particles are prepared by dispersion polymerization. 12. The display liquid according to claim 1, wherein the composite pigment particles are prepared by living free radical polymerization.