HK1051409A - An improved electrophoretic display with sub relief structure for high contrast ratio and improved shear and/or compression resistance - Google Patents

Description

Improved electrophoretic display with high contrast and improved shear and/or compression resistance and local raised structure

The technical field to which the invention belongs

The present invention relates to an improved electrophoretic display comprising a plurality of isolated cells of well-defined shape, size and aspect ratio and having an internal partially raised structure and filled with charged particles dispersed in a dielectric solvent.

The display may have a conventional up/down switching mode, an in-plane switching mode or a dual switching mode.

Background of the invention

Electrophoretic display (EPD) is a non-emissive device made from electrophoretic phenomena affecting charged pigment particles suspended in a colored dielectric solvent. This general type of display was first proposed in 1969. A typical electrophoretic display comprises a pair of oppositely disposed and spaced apart plate-like electrodes, the distance between the electrodes being predetermined by spacers. At least one of the electrodes (typically on the viewing side) is transparent. For passive electrophoretic displays, row and column electrodes on the top (viewing side) and bottom plates, respectively, are required to drive the display. In contrast, active electrophoretic displays require an array of thin film transistors on a backplane and a common unpatterned transparent conductor plate on a top-view substrate. An electrophoretic fluid consisting of a colored dielectric solvent and charged pigment particles dispersed therein is enclosed between two electrodes.

When a voltage difference is applied between the two electrodes, the pigment particles are attracted to migrate to the electrode plate of opposite polarity to the pigment particles. Thus, the color displayed on the transparent electrode plate (determined by selectively charging the electrode plate) may be the color of the solvent or the color of the pigment particles. The polarity reversal of the plates causes the particles to migrate back to the opposite plate, thereby switching the color. Due to the intermediate pigment density at the transparent plate, an intermediate color density (or grey scale) can be obtained by controlling the charge of the electrode plates with a range of voltages.

Electrophoretic displays of different pixel or cell structures have been reported in the prior art, such as the divided electrophoretic display (m.a. hopper and v.novotny, IEEE trans.electric.dev., 26 (8): 1148-1152(1979)) and the microencapsulated electrophoretic display (U.S. Pat. nos. 5,961,804 and 5,930,026). However, these displays have respective problems as described below.

In a divided electrophoretic display, a plurality of spacers are present between two electrodes, dividing the space into relatively small cells, in order to avoid unwanted particle movement, such as sedimentation. However, difficulties are encountered in forming the partitions, filling the display with fluid, encapsulating the fluid in the display, and keeping suspensions of different colors separate from each other.

Microencapsulated electrophoretic displays have microcapsules arranged in substantially two dimensions, each having therein an electrophoretic composition consisting of a dielectric fluid and a suspension of charged pigment particles that are visually contrasting with the dielectric fluid. Microcapsules are typically prepared in aqueous solution to achieve useful contrast with large average particle sizes (50-150 microns). Large microcapsule size results in poor scratch resistance and slow reaction times at a given voltage because large capsules require a large gap between two opposing electrodes. Meanwhile, the hydrophilic shell of microcapsules prepared in aqueous solution generally results in sensitivity to conditions of high temperature and high humidity. If the microcapsules are embedded in a large number of polymeric substrates to eliminate these drawbacks, the use of substrates may result in slower reaction times and/or lower contrast. In order to improve the switching rate, a charge control agent is often required in such electrophoretic displays. However, microencapsulation in aqueous solutions places limitations on the types of charge control agents that can be used. Other disadvantages associated with microcapsule systems include poor resolution and poor addressability for color applications.

An improved electrophoretic display technology has recently been disclosed in several pending applications, U.S. patent application serial No. 09/518,488, filed 2000, 3.3.3; U.S. patent application serial No. 09/759,212, filing date 2001, month 1, day 11; U.S. patent application Ser. No. 09/606,654, filed 2000 and 28, and U.S. patent application Ser. No. 09/784,972, filed 2001 and 2, 15, are incorporated herein by reference. An improved electrophoretic display comprises closed isolated cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent. The electrophoretic fluid is isolated and sealed in each microcup.

The microcup structure enables the preparation of electrophoretic displays in a flexible and efficient roll-to-roll continuous process. The display may be fabricated on a continuous conductive film (e.g., ITO/PET) support web by: (1) coating a radiation-curable composition on an ITO/PET film, (2) fabricating microcups in a micro-embossing or photo-etching process, (3) filling the microcups with an electrophoretic fluid and sealing the microcups, (4) laminating another conductor film on the sealed microcups, and (5) cutting the display to a desired size or format for assembly.

One advantage of this electrophoretic display design is that: the microcup walls are in fact built-in spacers that separate the top and bottom substrates into a fixed distance. The mechanical properties and structural integrity of the microcup display are significantly better than any prior art display including that made using spacer particles. In addition, displays containing microcups have desirable mechanical properties, including reliable display performance when the display is bent, rolled or under compressive pressure in, for example, touch screen applications. The use of microcup technology eliminates the need for edge sealing adhesives, which limit and predefine the size of the display panel and define the display fluid within a predefined area. If a conventional display prepared by the edge-sealing adhesive method is cut in any way, or if the display is drilled with a hole, the display fluid therein will leak out entirely. The damaged display will not be functional. In contrast, the display fluid in the display prepared in the microcup technology is sealed and isolated in each cell. The microcup display can be cut to almost any size without risk of compromising the display function due to leakage of display fluid from the active area. In other words, the microcup structure enables a flexible form of display manufacturing process to be performed that produces displays that are continuously output in large sheets and can be cut into any desired form. Isolated microcup or cassette structures are particularly important when the cassette is filled with fluids of different specific properties (e.g., color and switching rate). Without such a microcup structure, it is difficult to avoid intermixing of fluids in adjacent regions or to avoid interference during operation.

To achieve a relatively high contrast ratio, a cell formed of relatively wide microcups and having relatively narrow dividing walls is preferred because it allows a relatively high proportion of the cell open area (i.e., the ratio of the cell open area to the total area) so that relatively little light leaks out of the inactive walls. Although the resolution of the display may decrease as the proportion of the cell opening increases, the use of relatively wide cells (up to about 300 microns) remains one of the lowest cost efficient ways to achieve high contrast for low resolution and monochrome applications. However, as the proportion of the open area increases, the resistance to compression and/or shear forces (e.g., exerted by a stylus for a touch screen panel) also decreases significantly. Moreover, as the aperture ratio increases, unwanted particle movement (e.g., convection) within the cell becomes more pronounced and the contrast of the display may actually decrease. The image uniformity of the display may also deteriorate if there is excessive unwanted movement.

Brief description of the invention

The present invention relates to an improved electrophoretic display using microcup technology. The display includes isolated cells formed from microcups of well-defined shape, size and aspect ratio. The cartridge has an internal partially raised structure and is filled with charged pigment particles dispersed in a dielectric solvent.

The display may have a conventional up/down switching mode, an in-plane switching mode or a dual switching mode. In displays having a conventional up/down switching mode or dual switching mode, there is one top transparent electrode plate, one bottom electrode plate, and a plurality of isolated cells enclosed between the two electrode plates. In a display with in-plane switching mode, the cell is sandwiched between a top electrode plate and a bottom insulating layer. Among these boxes are partially raised structures raised from the bottom layer. They may be discrete structures such as columns, cylinders, wedges, crosses, or continuous structures such as walls and grids. However, the partial structure does not touch the top transparent layer. In other words, there is a gap between the top of the structure and the top transparent layer. The distance between the top of the partial protrusion structure and the top transparent layer is typically about 3 to 50 microns, preferably about 5 to 30 microns, more preferably about 10 to 20 microns. The top surface of the continuous partially raised structure may be of any shape, preferably flat and no larger than the bottom of the structure. The cross-section of the local raised structures may be any shape, including circular, square, rectangular, oval, and other shapes.

Brief description of the drawings

Figure 1 is a schematic view of an electrophoresis cassette of the present invention.

Fig. 1a is an electrophoretic display with an in-plane switching mode.

Fig. 1b is an electrophoretic display with a dual switching mode.

Fig. 2 is a flow chart for fabricating a black/white electrophoretic display or other monochrome electrophoretic display.

Fig. 3a to 3h are a flow chart for manufacturing a full-color electrophoretic display.

Fig. 4A-4F illustrate a display having an in-plane switching mode.

Fig. 5A-5C illustrate a display having dual switching modes.

It is noted that the figures are not drawn to scale.

Detailed description of the invention definitions

Unless otherwise defined in this specification, all technical terms herein are used in accordance with conventional definitions commonly used and understood by those of ordinary skill in the art.

The term "microcups" refers to cup-shaped indentations made by micromolding or pattern exposure.

The term "cassette" as used in the description of the present invention is intended to mean a unit formed by a sealed microcup. The cell is filled with charged pigment particles dispersed in a solvent or solvent mixture.

The term "well-defined" when referring to the microcups or cassettes means that the microcups or cassettes have a well-defined shape, size, and aspect ratio that are predetermined according to the specific parameters of the fabrication process.

The term "aspect ratio" is a term commonly known in the art of electrophoretic displays. In this application, it refers to the ratio of the depth to the width or depth to the length of the microcups.

The term "isolated" refers to electrophoretic cells that are individually sealed with a sealing layer such that electrophoretic components encapsulated in one cell cannot be transferred to other cells. PREFERRED EMBODIMENTS

As shown in fig. 1, an electrophoretic display 100 of the present invention comprises a top transparent layer 101, a bottom layer 102, and a layer of isolated cells 103 encapsulated between the two layers. The top transparent layer 101 is a transparent conductor film such as ITO on PET. The cells 103a, 103b, 103c have well-defined shapes, sizes, and aspect ratios, and are filled with charged particles 104 dispersed in a dielectric solvent 105. The isolated cartridges are sealed with a sealing layer 106. The top transparent layer is typically laminated to the sealed box with an adhesive layer.

In a display with in-plane switching mode (fig. 1A), the bottom layer 102 is an insulator substrate, and the top electrode plate 101 includes in-plane electrodes 110a and 110b and a top electrode 111 between the two in-plane electrodes separated by a gap 112. Alternatively, the top layer may have only one in-plane switching electrode and one top electrode with a gap between each other. In a display with dual switching mode (FIG. 1B), the bottom layer 102 includes one column electrode 102 a. The top layer 101 contains one in-plane electrode 113a on the left, one top electrode 114, and another in-plane electrode 113b on the right. A gap 115 separates the in-plane electrode and the top electrode. Alternatively, the top electrode plate may have only one in-plane electrode and one top electrode with a gap therebetween (not shown).

The cassettes have internal partial raised structures 116. The partial structure may be of any shape rising from the bottom layer. The structure may be a discrete structure such as a cylinder, wedge, cross, or a continuous structure such as a wall and grid. However, the partial structure does not touch the top transparent layer. In other words, there is a gap 117 between the top of the partial structure and the top transparent layer. The gap is typically between about 3 and 50 microns, preferably between about 5 and 30 microns, and most preferably between 10 and 20 microns. For discrete structures, the cross-section of the structure can be any shape, including circular, square, rectangular, oval, and other shapes. The diameter or width of the discrete local structures is typically from 3 to 50 microns, preferably from 5 to 30 microns, most preferably from 8 to 20 microns. For a continuous partial structure, the top surface of the structure may be of any shape, preferably flat and no larger than the bottom of the structure. The length of the continuous partial structure may be as long as the box wall.

Although some of the cells may have no local structures, it is preferred that there is on average at least one local raised structure per cell. Depending on the height h and width w of the box 103, the best contrast and the optimal number of local structures resistant to shear and compression forces may vary. The shortest distance between the local structure to any box wall may also vary. It may be more than three times the height of the cassette. However, the distance is preferably less than three times the height of the cassette, and more preferably less than two times the height of the cassette.

The display of the present invention exhibits very good screen panel resistance. The locally raised structure effectively limits the degree of deformation or dishing that can be caused by a stylus and greatly reduces the chance of damage to a fragile top electrode layer (e.g., an ITO conductor film of a display made of wide microcups).

The presence of the local structure also significantly reduces unwanted particle movement (e.g. convection within the box). Furthermore, the gap 117 between the top of the partially raised structures and the top transparent layer allows the electrophoretic fluid to cover these structures and significantly reduces the loss of contrast due to the reduced light leakage from the structures. As a result, the contrast of the display of the invention with a partial structure within the cell is significantly improved. Optionally, the support may be colored (e.g., blackened) to further improve contrast. I. Preparation of the microcups

The microcups may be generally fabricated by micro-molding or photo-etching as disclosed in U.S. patent application No. 09/518,488 (3/2000 application) and U.S. patent application No. 09/784,972 (2/15/2001 application). I (a) preparation of the male mould for the preparation of the microcups by moulding

The positive mold may be prepared by any suitable method, such as a diamond turning (diamond turning) process, or a photoresist process followed by etching or plating after developing the photoresist. For large microcups, a diamond turning process is preferred. The master template of the male mould may be made by any suitable method, for example by electroplating. A thin layer (typically 3000 angstroms) of a seed metal, such as Chrome nickel (Chrome Inconel), is sputtered onto a glass substrate using an electroplating process. A photoresist film is then applied and exposed to radiation, such as Ultraviolet (UV). A photomask is placed between the UV and photoresist layers. The exposed areas of the photoresist film harden. The unexposed areas are then removed by rinsing with a suitable solvent. The remaining hardened photoresist is dried and a thin layer of seed metal is sputtered again. The master template is then ready for electroforming. A typical material for electroforming is nickel-cobalt. Alternatively, the master template may be made of nickel by electroforming or electroless nickel deposition, such as continuous fabrication of Thin Cover Sheet Optical Media ("continuous manufacturing of Thin Cover Sheet Optical Media" SPIE Proc).1663: 324 (1992)). The die floor is typically about 50 to 400 microns thick. The master template may also be fabricated using other micromachining Techniques, including e-beam writing, dry etching, chemical etching, laser writing, or laser interference, such as "Replication technologies for Micro-Optics" SPIE Proc.3099: 76-82 (1997)). Alternatively still, the mold may be fabricated by photo-processing using plastic, ceramic, or metal.

The male mold thus prepared typically has protrusions of between about 5 and 200 microns, preferably between about 10 and 100 microns, and most preferably between about 10 and 50 microns. The local structure dimensions of the mold are adjusted so that the difference between the height of the local structure and the height of the walls (i.e., the distance between the top of the structure and the transparent top viewing layer) is typically between about 3 and 50 microns, preferably between about 5 and 30 microns, and most preferably between 10 and 20 microns after the microcups are released from the mold. The male mold may be in the form of a belt, roll or sheet. For continuous production, a belt-like or roll-like mold is preferred. Prior to application of the uv curable resin composition, the mold may be treated with a release agent to aid in the release process. Formation of microcups

Microcups with internal localized raised structures can be formed in a batch process or in a continuous roll-to-roll process as described in serial pending application, U.S. patent application No. 09/784,972, filed 2001, 2/15. The latter provides a continuous, low cost, high throughput manufacturing technique for fabricating separations used in electrophoretic or liquid crystal displays. Prior to application of the uv curable resin composition, the mold may be treated with a release agent to aid in the release process. To further improve the demolding process, the conductor film may be pre-coated with a primer or adhesion-promoting layer to improve adhesion between the conductor film and the microcups.

The ultraviolet curable resin may be degassed before use, and may optionally contain a solvent. The solvent, if any, should evaporate easily. The uv curable resin is applied to the male mold in any suitable manner, such as coating, dropping, pouring, and the like. The applicator may be moving or stationary. For manufacturing a display having a conventional up/down switching mode or a dual switching mode, an ultraviolet curable resin is coated on the conductor film. Examples of suitable conductive films include transparent conductive ITO on plastic substrates such as polyethylene terephthalate, polyethylene naphthalate, aromatic polyamides, polyimides, polycycloolefins, polysulfones, and polycarbonates. Pressure may be applied if necessary to ensure proper bonding between the resin and the plastic and to control the thickness of the base plate of the microcups. The pressure may be applied using a laminating roller, vacuum molding, a press, or any other similar mechanism. If the male mold is metallic and opaque, the plastic substrate is typically transparent to actinic radiation used to harden the resin. Conversely, the male mold may be transparent while the plastic substrate is opaque to actinic radiation. In order to transfer the molded features well to the transfer sheet, the conductor film needs to have good adhesion to the ultraviolet curable resin, which should have good release properties to the mold surface.

For the manufacture of displays with in-plane switching mode, a transparent insulator substrate may be used in the embossing step instead of the conductor film layer. Suitable transparent insulator substrates include polyethylene terephthalate, polyethylene naphthalate, aromatic polyamide, polyimide, polycycloolefin, polysulfone, and polycarbonate.

The UV curable components used to prepare the microcups may include multivalent acrylates or methacrylates, multivalent vinyl compounds (including vinylbenzenes, vinylsilanes, vinyl ethers), multivalent epoxides, multivalent acryl compounds, oligomers or polymers containing crosslinkable functional groups, and the like. Multifunctional acrylates and oligomers thereof are preferred. Combinations of multifunctional epoxides with multifunctional acrylates are also very useful to achieve the desired physicomechanical properties. Crosslinkable oligomers imparting flexibility, such as urethane acrylates or polyester acrylates, are also typically added to improve the bending resistance of the molded microcups. The component may comprise oligomers, monomers, additives, and optionally polymers. The glass transition temperature (or Tg) of such materials typically ranges from about-70 deg.C to 150 deg.C, preferably from about-20 deg.C to 50 deg.C. The process of micro-molding is typically carried out at a temperature above its glass transition temperature. A heated stamper or a heated indwelling substrate (housingsubstrate) pressed by a mold may be used to control the temperature and pressure of the micro-molding. The male mold may be released from the formed microcups during or after the uv curable resin is uv cured. I (b) preparation of the microcups by photolithography

Alternatively, the microcups of the display may be prepared by a photolithography process. For example, an ultraviolet curable microcup composition is coated on an ITO film on a transparent substrate (e.g., PET) and exposed through a photomask from the uncoated PET side. The height of the internal local structure can be controlled by the optical density of the photomask. The higher the optical density in the photomask, the lower the height of the internal raised structures after the exposure and development steps.

Suitable transparent substrates include polyethylene terephthalate, polyethylene naphthalate, aromatic polyamides, polyimides, polycycloolefins, polysulfones, polycarbonates, and the like.

In general, the microcups may be of any shape and may vary in size and shape. In one system, the microcups may be approximately the same size and shape. However, in order to maximize the optical effect, mixed microcups of different shapes and sizes may be manufactured. For example, microcups filled with a red suspension may have a shape or size different from that of a green microcup or a blue microcup. Furthermore, a pixel may be composed of a different number of microcups of different colors. For example, a pixel may be composed of a plurality of small green microcups, a plurality of large red microcups, and a plurality of small blue microcups. The three colors need not be the same shape and number.

The openings of the microcups may be circular, square, rectangular, hexagonal, or any other shape. The separation area between the openings is preferably kept small in order to achieve high color saturation and contrast while maintaining the desired mechanical properties. Thus, for example, honeycomb openings are preferred over circular openings.

The size of each individual microcup may be about 1X 10²To 1X 10⁶The square micron range is preferably from about 1X 10³To 1X 10⁵Square micron. The depth of the microcups is in the range of about 5 to 200 microns, preferably from about 10 to 100 microns. The ratio of openings to total area is in the range of about 0.05 to 0.95, preferably from about 0.4 to 0.9. Preparation of suspensions/dispersions

The suspension filled in the microcups includes a dielectric solvent in which charged pigment particles are dispersed, and the particles migrate under the influence of an electric field. The suspension may optionally contain additional colorants that do not migrate in the electric field. Dispersions can be prepared according to methods known in the art, for example, U.S. Pat. Nos. 6,017,584, 5,914,806, 5,573,711, 5,403,518, 5,380,362, 4,680,103, 4,285,801, 4,093,534, 4,071,430 and 3,668,106, as described in the institute of Electrical and electronics Engineers transport electronics (IEEE transactions, electronic devices, 24: 827, (1977)) and in the applications Physics (J.Appl. Phys.49 (9): 4820 (1978)).

The suspending fluid medium is a dielectric solvent preferably having a low viscosity and a dielectric constant in the range 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 DECALIN (DECALIN), 5-ethylidene-2-norbornene, fatty oils, undersea oils; aromatic hydrocarbons such as toluene, xylene, phenylxylylethane, dodecylbenzene and alkylnaphthalenes; halogenated solvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene, trifluorotoluene dichloride (dichlorobenzotrifluoride), 3, 4, 5-trichlorobenzotrifluoride (3, 4, 5-trichlorobenzotrifluoride), chloropentafluorobenzene (chlorotetrafluoro-benzene), dichlorononane, pentachlorobenzene; perfluorinated solvents such as FC-43, FC-70 and FC-5060 from 3M company of St.Paul, Minnesota; low molecular weight halogen-containing polymers such as poly (perfluoropropylene oxide) from TCI America corporation of Portland, ruslerian; polychlorotrifluoroethylene (poly (chlorotrifluoroethylene)), such as Halocarbon oil (Halocarbon oil) from Halocarbon products, Inc. of River Edge, N.J.; perfluorinated polyalkyl ethers (perfluoropolyalkylethers), such as Galden, HT-200 from Ausimont, and Fluorolink or Krytox Oils and Greases K-Fluid series from DuPont, Delaware. In a preferred embodiment, polychlorotrifluoroethylene is used as the dielectric solvent. In another preferred embodiment, polyperfluoropropylene ether is used as the dielectric solvent.

The contrasting colorant may be a dye or a pigment. Nonionic azo and anthraquinone dyes are particularly useful. Examples of useful dyes include, but are not limited to: oil-soluble Red EGN (Oil Red EGN), Sudan Red (Sudan Red), Sudan Blue (Sudan Blue), Oil-soluble Blue (Oil Blue), Macrolex Blue, Solvent Blue 35(Solvent Blue 35), Pylam Spirit Black and Fast Spirit Black, Thermoplastic Black X-70 from BASF, and anthraquinone Blue, anthraquinone yellow 114, anthraquinone Red 111 and 135, anthraquinone Green 28 and Sudan Black B (Sudan Black B), from Aldrich. Fluorinated dyes are particularly useful when perfluorinated solvents are used. In the case of a contrast color pigment, colored pigment particles may also be dispersed in the dielectric medium, and the colored particles are preferably uncharged. If the pigment particles used to produce the contrasting color are charged, they are preferably of opposite charge to the charged pigment particles. If the two pigment particles carry the same charge, they should have different charge densities or different electrophoretic mobility rates. Dyes or pigments used in electrophoretic displays must be chemically stable and compatible with the other components of the suspension system.

The charged primary pigment particles are preferably white and may be organic or inorganic pigments, such as TiO₂。

If colored pigment particles are used, they may be formed from phthalocyanine blue from SUN Chemical, phthalocyanine green, diarylide Yellow, diarylide AAOT Yellow, quinacridone, azo, azo, rhodamine, perylene pigments, Hansa Yellow G (Hansa Yellow G) particles from Kanto Chemical, and Carbon black (Carbon Black) from Fisher. The particle size is preferably in the range of 0.01 to 5 μm, more preferably in the range of 0.05 to 2 μm. These particles should have acceptable optical properties, should not be swollen or softened by the dielectric solvent, and should be chemically stable. The resulting suspension must also be stable against settling, emulsification or coagulation under normal operating conditions.

The pigment particles may themselves be charged, or may be significantly charged using charge control agents, or acquire a charge when suspended in a dielectric solvent. Suitable charge control agents are known in the art; they may be of polymeric or non-polymeric nature, and may be ionized or non-ionized, including ionic surfactants such as Aerosol ortho-toluidine (Aerosol OT), sodium dodecyl benzene sulfonate, metal soaps, polybutylene succinimide, maleic anhydride copolymers, vinyl pyridine copolymers, vinyl pyrrolidone copolymers (such as Ganex from International specialty Products), (meth) acrylic acid copolymers, N-dimethylaminoethyl (meth) acrylate (N, N-dimethylallyl) copolymers. Fluorinated surfactants are particularly useful as charge control agents in perfluorocarbon solvents. These include FC fluorinated surfactants such as FC-170C, FC-171, FC-176, FC430, FC431 and FC-740 from 3M company, and Zonyl fluorinated surfactants such as Zonyl FSA, FSE, FSN-100, FSO-100, FSD and UR from Dupont company.

Suitable charged pigment dispersions can be made by any existing method, including milling, grinding, ball milling, microfluidization (microfluidization), and ultrasonic techniques. For example, pigment particles in the form of a fine powder are added to the suspending solvent, and the resulting mixture is ball milled or ground for several hours to break up the highly agglomerated dry pigment powder into primary particles. Although not a preferred method, dyes or pigments for producing the color of the non-migrating fluid may be added to the suspension during ball milling.

The microparticles can be microencapsulated by using an appropriate polymer to eliminate precipitation or emulsification of the pigment microparticles to match the specific gravity of the dielectric solvent. Microencapsulation of the pigment particles can be accomplished by chemical or physical means. Typical microencapsulation processes include interfacial polymerization, in situ polymerization, phase separation, coacervation, electrostatic coating, spray drying, fluidized bed coating, and solvent evaporation. Filling and sealing of microcups

Methods of filling and sealing are described in co-pending U.S. patent application Ser. No. 09/518,488 and U.S. patent application Ser. No. 09/784,972, of the same series, see previous sections, the disclosures of which are incorporated herein by reference in their entirety.

The sealing of the microcups may be accomplished by a variety of methods. One preferred method is to disperse a UV curable component comprising a multifunctional acrylate, an acrylic oligomer, and a photoinitiator in an electrophoretic fluid comprising charged pigment particles and a colored dielectric solvent. The UV curable component is immiscible with the dielectric solvent and has a specific gravity lower than that of the dielectric solvent and the pigment particles. The two components (uv curable component and electrophoretic fluid) are thoroughly mixed in a radial mixer and immediately applied to the microcups using precision coating machinery such as a Myrad bar, gravure plate, doctor blade, slot coating, or slot coating. Excess fluid is scraped off with a scraper or similar device. A small amount of a weak solvent or solvent mixture, such as isopropanol, methanol, or other aqueous solution mixture, may be used to wash the electrophoretic fluid remaining on the top surfaces of the partition walls of the microcups. Volatile organic solvents may be used to control the viscosity and coverage of the electrophoretic fluid. The filled microcups are then dried and the uv curable component floats to the top of the electrophoretic fluid. The microcups are sealed by curing the uv curable layer floating to the surface during or after it floats to the top. Ultraviolet or visible light, infrared, and electron beam radiation may be used to cure and seal the microcups. In addition, heat or moisture may also be used to cure and seal the microcups, if appropriate, heat or moisture curable components may be used.

A preferred group of dielectric solvents having desirable densities and desirable solubility differences for acrylate monomers and oligomers are halocarbons and derivatives thereof. Surfactants may be used to improve the viscosity and wettability of the interface between the electrophoretic fluid and the encapsulant. Useful surfactants include FC surfactants from 3M, Zonyl fluorinated surfactants from DuPont, fluorinated acrylates, fluorinated methacrylates, fluoro-substituted long chain alcohols, perfluoro-substituted long chain carboxylic acids and derivatives thereof.

Alternatively, where the sealing precursor is at least partially compatible with the dielectric solvent, then the electrophoretic fluid and the sealing precursor may be sequentially coated into the microcups. Sealing of the microcups may thus be accomplished by applying a thin layer of a thermosetting precursor (which may harden by radiation, heat, solvent evaporation, moisture or interfacial reaction) to the surface of the filled microcups.

Volatile organic solvents can be used to adjust the viscosity and thickness of the coating. When a volatile solvent is used in the coating, it is preferably immiscible with the dielectric solvent. Thermoplastic elastomers have been disclosed as preferred sealing materials in co-pending patent application, U.S. patent application No. 09/874,391 (6/4, 2001).

Examples of useful thermoplastic elastomers include two, three or more stage copolymers represented by the formula ABA or (AB) n, wherein A is styrene, alpha-methylstyrene, ethylene, propylene or norbornene; b is butadiene, isoprene, ethylene, propylene, butylene, dimethylsiloxane or propylene sulfide; in the formula, A and B cannot be the same. The number n is 1 or more, preferably 1 to 10. Representative copolymers include poly (styrene-b-butadiene), poly (styrene-b-butadiene-b-styrene), poly (styrene-b-isoprene-b-styrene), poly (styrene-b-ethylene/butylene-b-styrene), poly (styrene-b-dimethylsiloxane-b-styrene), poly (alpha-methylstyrene-b-isoprene-b-alpha-methylstyrene), poly (alpha-methylstyrene-b-propylene sulfide-b-alpha-methylstyrene), and poly (alpha-methylstyrene-b-dimethylsiloxane-b-alpha-methylstyrene). Additives such as silica gel particles and surfactants may also be used to improve the integrity of the film and the coating quality.

The interfacial polymerization followed by uv curing is very advantageous for the sealing process. Intermixing between the electrophoretic layer and the coating layer is significantly suppressed by interfacial polymerization to form a thin separation layer. Sealing is then accomplished by a post-curing step, preferably ultraviolet or other light radiation. To further reduce the degree of intermixing, it is preferred that the specific gravity of the coating layer be significantly lower than the specific gravity of the electrophoretic fluid. This two-step coating method is particularly suitable for the case where the dye used is at least partially soluble in the sealing material. Preparation of monochrome electrophoretic displays

The process is illustrated by the flow diagram shown in figure 2. All microcups were filled with a suspension of the same color component. The process may be a continuous roll-to-roll process comprising the steps of:

1. a layer of uv curable composition 20, optionally with a solvent, is coated onto a continuous support web 21. The solvent (if any) can be easily evaporated. The continuous support screen 21 may be a plastic substrate, a patterned or unpatterned conductor film on a plastic substrate, depending on the application and display switching mode.

2. The uv curable composition 20 is embossed with a pre-patterned male mold 22 at a temperature above its glass transition temperature.

3. The mold is preferably released from the ultraviolet-curable layer 20 during or after the ultraviolet exposure curing.

4. The array of microcups 23 thus formed having discrete localized raised structures 25 is filled with a charged pigment dispersion 24 dispersed in a colored dielectric solution.

5. The microcups are sealed using the methods described in co-pending patent applications, U.S. patent application No. 09/518,488 (3.3.2000), U.S. patent application No. 09/759,212 (11.1.2001), U.S. patent application No. 09/606,654 (28.6.2000), U.S. patent application No. 09/784,972 (15.2.2001), and U.S. patent application No. 09/874,391 (4.6.2001), which are co-pending patent applications, thus forming a closed electrophoresis cassette containing an electrophoresis fluid.

The sealing method comprises the following steps: at least one thermosetting precursor which is incompatible with the solvent and has a lower specific gravity than the solvent and the pigment particles (i.e., the electrophoretic fluid) is added to the dielectric solvent, and then the thermosetting precursor is cured, optionally with radiation such as ultraviolet light, or with heat or moisture, during or after separation of the thermosetting precursor. Alternatively, the sealing component may be applied and cured directly on the surface of the electrophoretic fluid to complete the sealing of the microcups.

6. The sealed array of electrophoretic cells is laminated to another layer of continuous support screen 26 comprising electrodes or conductor wires pre-coated with an adhesive layer 27, which may be a pressure sensitive adhesive, a hot melt adhesive or a heat, moisture or radiation curable adhesive.

The laminating adhesive may be post-cured by, for example, heat or ultraviolet light 28 through either side of the support web. The finished product may be cut 29 after the lamination step. Further alternatively, the sealed microcups may be cut to size prior to the lamination step.

The preparation of the microcups described above can be conveniently replaced by an alternative method by pattern-wise exposing the thermosetting precursor-coated conductor film followed by removal of the unexposed areas with a suitable solvent.

For the manufacture of displays with in-plane switching mode, the thermoplastic or thermoset precursor may be coated on a transparent insulator substrate, rather than a conductive film, prior to micro-molding or pattern exposure. Preparation of V. multicolor electrophoretic display

Sealed microcups containing electrophoretic fluids of different colors can be prepared using the methods described in co-pending applications, U.S. patent application No. 09/518,488 (3.3.2000 application) and U.S. patent application No. 09/879,408 (11.6.2001 application). The process comprises the following steps: (1) in the already formedLaminating a positive working dry film photoresist on the microcups, the photoresist consisting of at least a removable support (e.g., PET-4851 from Saint-Gobain corporation of Worcester, Mass.), a phenolic positive photoresist (e.g., Microposition S1818 from Shipley), and an alkali developable adhesive layer (e.g., a mixture of Nacor 72-8685 from National Starch and Carboset 515 from BF Goodrich); (2) pattern-exposing the photoresist film to selectively open a number of microcups, removing the removable support film, and developing the positive photoresist film with a developer (e.g., Microposit 351 from Shipley diluted developer); (3) the open microcups were filled to contain charged white pigment (TiO)₂) An electrophoretic fluid of microparticles and a dye or pigment of a first primary color; and (4) sealing the filled microcups together as described in the preparation of the monochrome display. These additional steps may be repeated to produce microcups filled with electrophoretic fluid in the second and third primary colors.

In more detail, a multicolor electrophoretic display may be prepared according to the steps shown in fig. 3:

1. a layer of thermoset precursor 30 is applied to the conductor film 31.

2. The thermoplastic or thermoset precursor layer is embossed with a pre-patterned male mold at a temperature above the glass transition temperature of the thermoplastic or thermoset precursor (not shown).

3. The mold is preferably released from the layer of thermoplastic or thermosetting precursor during or after the hardening of the thermoplastic or thermosetting precursor by cooling or crosslinking by radiation, heat or moisture.

4. A positive dry film photoresist comprising at least one adhesive layer 33, a positive photoresist 34, and a removable plastic cover sheet (not shown) is laminated over the array of microcups 32 thus formed having discrete localized raised structures 32 a.

5. The positive photoresist is pattern exposed with ultraviolet, visible or other radiation (figure 3c), the cover slip is removed, the microcups in the exposed areas are developed and opened. The purpose of steps 4 and 5 is to selectively open the microcups in a predetermined area (figure 3 d).

6. The open microcups are filled with a dispersion 35 of charged white pigment dispersed in a dielectric solvent containing at least one dye or pigment of a first primary color, and a thermoset precursor 36 that is incompatible with the solvent and has a lower specific gravity than the solvent and pigment particles.

7. During or after the separation of the thermosetting precursor and the formation of a supernatant on top of the liquid phase, the thermosetting precursor is cured (preferably by radiation, e.g. uv, by heat or moisture) to seal the microcups to form a closed electrophoretic cell containing electrophoretic fluid of the first primary color (figure 3 e).

8. The above steps 5-7 can be repeated to produce well-defined cassettes containing electrophoretic fluids of different colors in different regions (fig. 3e, 3f and 3 g).

9. The sealed electrophoretic cell array is laminated in register with a pre-patterned transparent second conductor film 37, which is pre-coated with an adhesive layer 38, which may be a pressure sensitive adhesive, a hot melt adhesive, or a heat, moisture, radiation curable adhesive.

10. Curing the adhesive.

In step 4 above, the lamination of the positive dry film photoresist on the microcups may be replaced by coating the positive photoresist directly on the microcups. The microcups may be filled with a removable filler before the photoresist is coated or laminated on the microcups. In this case, no cover slip is required. This method is disclosed in co-pending application, U.S. patent application No. 09/879,408 (application date 2001, 6/11).

Suitable materials for use as fillers include inorganic, organic, organometallic, and polymeric materials, or particles thereof. The filler should be soluble or dispersible in the scavenging solution. More preferred filler materials are non-film forming microparticles such as PMMA, polystyrene, polyethylene and its carbonylated copolymers and their corresponding salt latexes, wax latexes, colloidal silica, titania, calcium carbonate dispersions, and mixtures thereof. Particularly preferred filler materials include aqueous dispersions of plasmons of ethylene copolymers, such as ACqua220, ACqua240, and ACqua250 from Honeywell, new jersey.

Multicolor displays with in-plane switching modes can be similarly prepared, except that the thermosetting precursor layer in step 1 can be coated on a transparent insulator substrate instead of a conductor film.

The preparation of the microcups described in the above process may be conveniently replaced by an alternative method of pattern-wise exposing the thermosetting precursor-coated conductor film followed by removal of the unexposed areas with a suitable solvent.

Alternatively, the sealing of the microcups may be accomplished by directly coating a layer of the thermoset precursor material onto the surface of the liquid phase.

Alternatively, the color electrophoretic display of the present invention may be completed using a color filter on the top of the display as disclosed in co-pending application, U.S. patent application No. 60/308,437 (7.27.2001), or a colored background on the bottom of the display as disclosed in another co-pending application, U.S. patent application No. 60/306,312 (7.17.2001).

The thickness of the display manufactured by the method can be as thin as a piece of paper. The width of the display is the width of the coated support screen (typically 1-90 inches). The length of the display can range from a few inches to thousands of feet, depending on the size of the roll.

An optional background layer may be added to the bottom of the display by painting, printing, coating or laminating a color layer. For increased contrast, a black or gray background layer is preferred.

An active matrix electrophoretic display may also be prepared using Thin Film Transistors (TFTs) on the bottom electrode plate of the display. VI, display VI (a) display of the invention with up/down switching mode

When there is a potential difference between the top and bottom electrode plates, the charged particles migrate to the top or bottom of the cartridge. When the particles migrate to and rest on the top of the cell, the color of the particles is seen through the top transparent layer. When the particles migrate to and settle at the bottom of the cell, the color of the dielectric solvent is seen through the top transparent layer. VI (b) display of in-plane switching mode

For a monochrome display, in the cell shown in fig. 4A, white particles are dispersed in a clear, colorless dielectric solvent. The background of all the cartridges is the same color (black, blue, cyan, red, magenta, etc.). When there is a potential difference between the top electrode (not shown) and the two in-plane switching electrodes (not shown), the white particles migrate to the sides of the cell, resulting in the color of the background being seen through the top transparent opening. When there is no potential difference between the top electrode and the two in-plane electrodes, the white particles are distributed in the dielectric solvent, with the result that the color of the particles (i.e., white) is seen through the top transparent insulator layer.

Alternatively, as shown in fig. 4B, the same color particles in all of the cartridges are dispersed in a clear, colorless dielectric solvent, and the background of the cartridges is white. When there is a potential difference between the top electrode (not shown) and the two in-plane switching electrodes (not shown), the colored particles migrate to the sides of the cell, resulting in the color of the background (i.e., white) being seen through the top transparent opening. When there is no potential difference between the two in-plane electrodes and the top electrode, the coloured particles are distributed in the dielectric solvent, with the result that the colour of the particles is seen through the top transparent layer.

Fig. 4C-4F illustrate a multi-color display with in-plane switching modes.

In fig. 4C, the cartridge is filled with a colorless dielectric solvent having white charged particles dispersed therein, and the cartridge has a different background color (i.e., red, green, or blue). When there is a potential difference between the in-plane electrode and the top electrode (not shown), the white particles migrate to the sides of the cell, resulting in the color of the background (i.e., red, green, or blue) being seen through the top transparent opening. When there is no potential difference between the in-plane electrode and the top electrode, the particles are distributed in the dielectric solvent, resulting in a white color (i.e., the color of the particles) being seen through the top transparent opening.

In fig. 4D, the cell is filled with a colorless dielectric solvent having black particles dispersed therein, and the cell has a different background color (i.e., red, green, or blue). When there is a potential difference between the in-plane electrode and the top electrode (not shown), the particles migrate to the sides of the cell, resulting in the color of the background (i.e., red, green, or blue) being seen through the top transparent opening. When there is no potential difference between the in-plane electrode and the top electrode, the particles are distributed in the dielectric solvent, resulting in a black color (i.e., the color of the particles) being seen through the top transparent opening.

The cell shown in fig. 4E is filled with a colorless dielectric solvent having particles of different colors (i.e., red, green, or blue) dispersed therein. The background of the box is black. When there is a potential difference between the in-plane electrode and the top electrode (not shown), the colored charged particles migrate to the sides of the cell, resulting in the color of the background (i.e., black) being seen through the top transparent opening. When there is no potential difference between the in-plane electrode and the top electrode, the colored particles are distributed in the dielectric solvent, resulting in the color of the particles (i.e., red, green, or blue) being visible through the top transparent opening. In this design, the black state is of high quality.

In fig. 4F, the cell is filled with a colorless dielectric solvent having particles of different colors (i.e., red, green, or blue) dispersed therein. The background of the box is white. When there is a potential difference between the in-plane electrode (not shown) and the top electrode, the particles migrate to the sides of the cell, and the color of the background (i.e., white) is seen through the top transparent opening, resulting in a high quality white state. When there is no potential difference between the in-plane electrode and the top electrode, the particles are distributed in the dielectric solvent, resulting in the color of the particles (i.e., red, green, or blue) being visible through the top transparent opening.

As shown in these figures, the in-plane switching mode allows the particles to move in the direction of the plane (left/right) and different color combinations of particles, background and fluid (each of which is white, black, red, green or blue, respectively) can be used to produce different multi-color electrophoretic displays.

Furthermore, the particles in the dielectric solvent may be of mixed colors, and the cassettes have the same background color.

The transparent top viewing layer of the display may be colored or developed with the addition of color filters. In this case, the cartridge is filled with an electrophoretic composition comprising white charged particles in a clear colorless or colored dielectric solvent, and the background of the cartridge may be black. In a monochrome display, the transparent viewing layer on each pixel is the same color (e.g., black, red, green, blue, yellow, cyan, magenta, etc.). In a multi-color display, the transparent viewing layers may be different colors. VI (c) display with double switching modes

For the purpose of illustration, it is assumed that positively charged white particles are used throughout this application. As shown in fig. 5A-5C, the dual switching mode allows the particles to move in either a vertical direction (up/down) or a planar direction (left/right). For example, in fig. 5A, the voltage of the bottom electrode is set high and the voltages of the top and in-plane electrodes are set low. The white particles migrate to and cluster in the transparent top conductor film and the viewer sees a white color (i.e., the color of the particles).

In fig. 5B, the in-plane electrodes are set at a low voltage, and the top and bottom electrodes are set at a high voltage. In this case, the white particles migrate to several sides of the cell, so that the color seen through the transparent top conductor film is that of the background (i.e., black).

In fig. 5C, when the top electrode is set at a high voltage, the bottom electrode is set at a low voltage, and the in-plane electrode is set at a high voltage, the white particles migrate to the bottom of the cell. In this case, the viewer sees the color of the fluid (i.e., red, green, or blue) through the transparent top conductor film, as shown by the red box of fig. 5C. To represent a red pixel in a full color display, the white particles in the green and blue cells may be attracted to several sides as shown in fig. 5C, or to the top (not shown). The former is preferred as it typically exhibits better color saturation than the latter. Thus, the dual switching mode technique gives the first full color electrophoretic display in which all high quality colors (including red, green, blue, black and white) are available in the same device.

Further, the background color may be any color, such as cyan, yellow, or magenta, rather than the commonly used black. For example, the cartridge may be filled with a red clear dielectric solvent having white positively charged particles dispersed therein, and the background color of the cartridge may be yellow. In this case, when the particles migrate to the top, the viewer sees a white color (i.e., the color of the particles), and when the particles migrate to cover the bottom of the cell, the color of the medium (i.e., red) is seen through the transparent conductor. However, when the particles migrate to several sides of the cell, the color seen through the transparent top conductor film will be a certain shade of orange.

Other shades or hues may be achieved using different particle/medium/background color combinations, such as white/red/cyan, white/red/magenta, white/blue/yellow, white/blue/cyan, white/blue/magenta, white/green/yellow, white/green/cyan, white/blue/magenta, and the like.

The preferred combination to achieve a full color display is a white particle, a black background, and a fluid colored in the additive primary color (i.e., red, green, or blue), respectively.

Another aspect of the invention is a monochrome display with highlighted selection. In this case, all cells in the display have the same background color and are filled with the same electrophoretic fluid (i.e., have the same particle/solvent color combination). For example, the display may have white particles, the solvent is one of the primary colors (red, green or blue), and the background color is a color that contrasts with the solvent color. This arrangement is useful for a relatively simple two-color device with a color highlighting option. For example, an electrophoretic display with white particles, a yellow dielectric solvent, and a black background may display at least three different colors in each pixel. When the white particles are all attracted to the top viewing row electrode, the pixel is seen as white. When the white particles are attracted evenly to the bottom column electrode, the pixel is seen to be yellow. When the white particles are attracted to the in-plane electrodes on either side of the cell, the pixel is seen to be black. Intermediate colors are also possible if the particles are in an intermediate state.

While the 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 true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, composition, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be included within the scope of the claims of this application.

Claims (31)

1. An electrophoretic display comprising:

a) a top transparent layer;

b) a plurality of isolated cells having well-defined size, shape and aspect ratio, said plurality of cells having internal localized raised structures and filled with charged pigment particles dispersed in a dielectric solvent or solvent mixture; and

c) a bottom layer.

2. The electrophoretic display of claim 1 wherein the partially raised structures are discontinuous structures.

3. The electrophoretic display of claim 2 wherein the non-continuous structure is cylindrical, wedge-shaped, or criss-cross shaped.

4. The electrophoretic display of claim 1 wherein the locally convex structures are continuous structures.

5. An electrophoretic display according to claim 4 wherein the continuous structure is a wall or a grid.

6. An electrophoretic display according to claim 1 wherein there is a gap between the top of the partially raised structures and the top transparent layer.

7. The electrophoretic display of claim 6 wherein the gap is about 3 to 50 microns.

8. The electrophoretic display of claim 7 wherein the gap is about 5 to 30 microns.

9. The electrophoretic display of claim 8 wherein the gap is about 10 to 20 microns.

10. The electrophoretic display of claim 1 wherein the local structures have a circular, square, rectangular or elliptical cross-section.

11. The electrophoretic display of claim 3 wherein the diameter or width of the localized raised structures is in the range of about 3 to 50 microns.

12. The electrophoretic display of claim 11 wherein the diameter or width of the localized raised structures is in the range of about 5 to 30 microns.

13. The electrophoretic display of claim 12 wherein the diameter or width of the localized raised structures is in the range of about 8 to 20 microns.

14. An electrophoretic display according to claim 1 wherein each cell of the display has on average at least one locally convex structure.

15. An electrophoretic display according to claim 1 wherein said cells are free of internal local structures.

16. An electrophoretic display according to claim 1 wherein the top surface of the partial structures may be any shape and not larger than the bottom surface of the structures.

17. The electrophoretic display of claim 1 wherein the shortest distance between the local relief structures to any cell wall is less than three times the cell height.

18. The electrophoretic display of claim 17 wherein the shortest distance between the locally convex structures to any cell wall is less than twice the cell height.

19. The electrophoretic display of claim 1 wherein the plurality of cells comprise cells of different sizes and shapes.

20. An electrophoretic display according to claim 1 wherein said cells are non-spherical.

21. The electrophoretic display of claim 1 wherein said cells are defined by an open area ranging from about 1 x 10²To 1X 10⁶Square micron microcups.

22. The electrophoretic display of claim 1 wherein said cells are defined by an open area ranging from about 1 x 10³To 1X 10⁵Square micron microcups.

23. The electrophoretic display of claim 1, wherein the cells are formed by microcups having openings in the shape of circles, polygons, hexagons, rectangles or squares.

24. The electrophoretic display of claim 1 wherein the cells have a depth ranging from about 5 to 200 microns.

25. The electrophoretic display of claim 1 wherein the cells have a depth ranging from about 10 to 100 microns.

26. The electrophoretic display of claim 1 wherein the ratio of the cell openings to the total area is in the range of about 0.05 to 0.95.

27. The electrophoretic display of claim 1 wherein the ratio of the cell openings to the total area is in the range of about 0.4 to 0.9.

28. An electrophoretic display according to claim 1 wherein the top and bottom layers are electrode plates and the display has an up/down switching mode.

29. An electrophoretic display according to claim 1 wherein the top transparent layer is an electrode plate, the bottom layer is an insulator layer, and the display has an in-plane switching mode.

30. An electrophoretic display according to claim 1 wherein both the top and bottom layers are electrode plates and the display has a dual switching mode.

31. The electrophoretic display of claim 1 wherein the partially raised structures are colored.