Patent Description:
Electro-optic displays comprise a layer of electro-optic material. The term "electro-optic", as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence, or in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.

Some electro-optic materials are solid in the sense that the materials have solid external surfaces, although the materials may, and often do, have internal liquid- or gas-filled spaces. Such displays using solid electro-optic materials may hereinafter for convenience be referred to as "solid electro-optic displays". Thus, the term "solid electro-optic displays" includes rotating bichromal member displays, encapsulated electrophoretic displays, microcell electrophoretic displays and encapsulated liquid crystal displays.

Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type as described, for example, in <CIT>; <CIT>; <CIT>; <CIT> <CIT>; <CIT>;<CIT>; <CIT>; and <CIT> (although this type of display is often referred to as a "rotating bichromal ball" display, the term "rotating bichromal member" is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed by applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface. This type of electro-optic medium is typically bistable.

Another type of electro-optic display uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example<NPL>; and <NPL>). See also <NPL>. Nanochromic films of this type are also described, for example, in <CIT>; <CIT>; and <CIT>. This type of medium is also typically bistable.

Another type of electro-optic display is an electro-wetting display developed by Philips and described in <NPL>). It is shown in <CIT> that such electro-wetting displays can be made bistable.

One type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle-based electrophoretic display, in which a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.

As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be produced using gaseous fluids; see, for example,<NPL>, and <NPL>). See also <CIT> and <CIT>. Such gas-based electrophoretic media appear to be susceptible to the same types of problems due to particle settling as liquid-based electrophoretic media, when the media are used in an orientation which permits such settling, for example in a sign where the medium is disposed in a vertical plane. Indeed, particle settling appears to be a more serious problem in gas-based electrophoretic media than in liquid-based ones, since the lower viscosity of gaseous suspending fluids as compared with liquid ones allows more rapid settling of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT), E Ink Corporation, E Ink California, LLC. and related companies describe various technologies used in encapsulated and microcell electrophoretic and other electro-optic media. Encapsulated electrophoretic media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. In a microcell electrophoretic display, the charged particles and the fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. The technologies described in these patents and applications include:.

Relevant background art useful for understanding the invention, according to Rule <NUM>(<NUM>)(b) EPC includes: <CIT> and <CIT>. <CIT> discloses an electro-optic display having a layer of electro-optic material, at least one conductor, and an adhesive material between the layer of electro-optic material and the at least one conductor. The layer of electro-optic material or the adhesive material comprises a composite material, wherein the composite material includes a polymer phase and a filler phase. <CIT> discloses an electro-optic display having a layer of electro-optic material, at least one conductor, and an adhesive material between the layer of electro-optic material and the at least one conductor. The layer of electro-optic material or the adhesive material comprises a composite material, wherein the composite material includes a polymer phase and a filler phase.

Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned <CIT>. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.

Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called "shutter mode" in which one display state is substantially opaque and one is light-transmissive. See, for example, <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; and <CIT>. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see <CIT>. Other types of electro-optic displays may also be capable of operating in shutter mode. Electro-optic media operating in shutter mode may be useful in multi-layer structures for full color displays; in such structures, at least one layer adjacent the viewing surface of the display operates in shutter mode to expose or conceal a second layer more distant from the viewing surface.

An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word "printing" is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; electrophoretic deposition (See <CIT>); and other similar techniques. ) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.

An electrophoretic display normally comprises a layer of electrophoretic material and at least two other layers disposed on opposed sides of the electrophoretic material, one of these two layers being an electrode layer. In most such displays both the layers are electrode layers, and one or both of the electrode layers are patterned to define the pixels of the display. For example, one electrode layer may be patterned into elongate row electrodes and the other into elongate column electrodes running at right angles to the row electrodes, the pixels being defined by the intersections of the row and column electrodes. Alternatively, and more commonly, one electrode layer has the form of a single continuous electrode and the other electrode layer is patterned into a matrix of pixel electrodes, each of which defines one pixel of the display. In another type of electrophoretic display, which is intended for use with a stylus, print head or similar movable electrode separate from the display, only one of the layers adjacent the electrophoretic layer comprises an electrode, the layer on the opposed side of the electrophoretic layer typically being a protective layer intended to prevent the movable electrode damaging the electrophoretic layer.

The manufacture of a three-layer electrophoretic display normally involves at least one lamination operation. For example, in several of the aforementioned MIT and E Ink patents and applications, there is described a process for manufacturing an encapsulated electrophoretic display in which an encapsulated electrophoretic medium comprising capsules in a binder is coated on to a flexible substrate comprising indium-tin-oxide (ITO) or a similar conductive coating (which acts as one electrode of the final display) on a plastic film, the capsules/binder coating being dried to form a coherent layer of the electrophoretic medium firmly adhered to the substrate. Separately, a backplane, containing an array of pixel electrodes and an appropriate arrangement of conductors to connect the pixel electrodes to drive circuitry, is prepared. To form the final display, the substrate having the capsule/binder layer thereon is laminated to the backplane using a lamination adhesive. (A very similar process can be used to prepare an electrophoretic display usable with a stylus or similar movable electrode by replacing the backplane with a simple protective layer, such as a plastic film, over which the stylus or other movable electrode can slide. ) In one preferred form of such a process, the backplane is itself flexible and is prepared by printing the pixel electrodes and conductors on a plastic film or other flexible substrate. The obvious lamination technique for mass production of displays by this process is roll lamination using a lamination adhesive.

Electro-optic displays manufactured using the aforementioned front plane laminates or double release films normally have a layer of lamination adhesive between the electro-optic layer itself and the backplane, and the presence of this lamination adhesive layer affects the electro-optic characteristics of the displays. In particular, the electrical conductivity of the lamination adhesive layer affects both the low temperature performance and the resolution of the display. The low temperature performance of the display can (it has been found empirically) be improved by increasing the conductivity of the lamination adhesive layer, for example by doping the layer with tetrabutylammonium hexafluorophosphate or other materials as described in the aforementioned <CIT> and <CIT>. However, increasing the conductivity of the lamination adhesive layer in this manner tends to increase pixel blooming (a phenomenon whereby the area of the electro-optic layer which changes optical state in response to change of voltage at a pixel electrode is larger than the pixel electrode itself), and this blooming tends to reduce the resolution of the display. Hence, this type of display apparently intrinsically requires a compromise between low temperature performance and display resolution, and in practice it is usually the low temperature performance which is sacrificed. Moreover, because conductivity of the lamination adhesive is temperature dependent, the temperature range at which a display is able to acceptably function may be limited to a narrow range of temperatures. This limits the potential applications of the display to environments that do not experience a wide temperature swing.

Thus, there is a need for improved lamination adhesive compositions and other composite materials that provide electrophoretic displays with improved performance across a wider range of ambient temperatures.

According to a first embodiment of the present invention, an electro-optic display is provided comprising a layer of electro-optic material, at least one conductor, and an adhesive material between the layer of electro-optic material and the at least one conductor. At least one of the electro-optic material and adhesive material comprises a composite material containing a polymer phase and a filler phase, such that a volume fraction of the filler phase is about the percolation threshold. Furthermore, the filler phase may have a conductivity greater than or equal to <NUM> X <NUM><NUM> S/m, a ratio of the coefficient of thermal expansion of the filler to the polymer may be less than or equal to <NUM>, and a concentration of the filler phase in the composite material may be greater than or equal to a filler concentration corresponding to a conductivity transition point of the composite material.

The composite materials according to the various embodiments of the present invention are well suited for particle-based electrophoretic displays. However, the composite materials may also be used in other types of electro-optic displays, such as displays utilizing polymer-dispersed liquid crystal.

These and other aspects of the present invention will be apparent in view of the following description.

The drawing Figures depict one or more implementations in accord with the present concepts, by way of example only, not by way of limitations.

In the following detailed description, numerous specific details are set forth by way of examples in order to provide a thorough understanding of the relevant teachings. However, it should be apparent to those skilled in the art that the present teachings may be practiced without such details.

In a typical process for manufacturing an electro-optic display, two subassemblies are first manufactured, one subassembly comprising an electro-optic layer and a first substrate, and the second comprising a second substrate; at least one of the subassemblies, and typically both, comprise an electrode. Also, in one common form of such a process, used for manufacturing an active matrix display, one subassembly comprises a substrate, a single continuous ("common") electrode which extends across multiple pixels, typically the whole of the display and the electro-optic layer, while the second assembly (usually referred to as the "backplane") comprises a substrate carrying a matrix of pixel electrodes, which define the individual pixels of the display, and non-linear devices (typically thin film transistors) and other circuitry used to produce on the pixel electrodes the potentials needed to drive the display (i.e., to switch the various pixels to the optical states necessary to provide a desired image on the display). The lamination adhesive is provided between the first and second subassemblies and adheres them together to form the final display.

A preferred lamination process of the present invention will now be described, though by way of illustration only, with reference to <FIG> of the accompanying drawings, which is a schematic section through one subassembly (a front plane laminate, or FPL) used in a process of the present invention, this subassembly comprising a substrate, a conductive layer, an electro-optic layer and an adhesive layer, the subassembly being illustrated at an intermediate stage of the process before this subassembly is laminated to a second subassembly.

The front plane laminate (generally designated <NUM>) shown in <FIG> comprises a light-transmissive substrate <NUM>, a light-transmissive electrode layer <NUM> (note that this is not the electrode lying on the opposed side of the lamination adhesive from the electro-optic layer in the final electro-optic display), an electro-optic layer <NUM>, a lamination adhesive layer <NUM> and a release sheet <NUM>; the release sheet is illustrated in the process of being removed from the lamination adhesive layer <NUM> preparatory to lamination of the FPL <NUM> to a backplane.

The substrate <NUM> is typically a transparent plastic film, such as a <NUM> mil (<NUM>) polyethylene terephthalate) (PET) sheet. The lower surface (in <FIG>) of substrate <NUM>, which forms the viewing surface of the final display, may have one or more additional layers (not shown), for example a protective layer to absorb ultra-violet radiation, barrier layers to prevent ingress of oxygen or moisture into the final display, and anti-reflection coatings to improve the optical properties of the display. Coated on to the upper surface of substrate <NUM> is the thin light-transmissive electrically conductive layer <NUM>, preferably of ITO, which acts as the common front electrode in the final display. PET films coated with ITO are available commercially.

The electro-optic layer <NUM> may be deposited on the conductive layer <NUM>, typically by slot coating, the two layers being in electrical contact. The electro-optic layer <NUM> shown in <FIG> is an encapsulated electrophoretic medium and comprises microcapsules <NUM>, each of which comprises negatively charged white particles <NUM> and positively charged black particles <NUM> in a hydrocarbon-based fluid <NUM>. The microcapsules <NUM> are held retained within a polymeric binder <NUM>. Upon application of an electrical field across electro-optic layer <NUM>, white particles <NUM> move to the positive electrode and black particles <NUM> move to the negative electrode, so that electro-optic layer <NUM> appears, to an observer viewing the display through substrate <NUM>, white or black depending on whether conductive layer <NUM> is positive or negative relative to the adjacent pixel electrode in the backplane. As would be understood by one of skill in the art, the charged particles (<NUM>, <NUM>) are not limited to black and white and may be of any color.

The FPL <NUM> is desirably prepared by coating the lamination adhesive <NUM>, in liquid form, conveniently by slot coating, on to release sheet <NUM>, drying (or otherwise curing) the adhesive to form a solid layer and then laminating the adhesive and release sheet to the electro-optic layer <NUM>, which has previously been coated on to the substrate <NUM> bearing the conductive layer <NUM>; this lamination may conveniently be effected using hot roll lamination. (Alternatively, but less desirably, the lamination adhesive may be applied over the electro-optic layer <NUM> and there dried or otherwise cured before being covered with the release sheet <NUM>. ) The release sheet <NUM> is conveniently a <NUM> mil (<NUM>) film; depending upon the nature of the electro-optic medium used, it may be desirable to coat this film with a release agent, for example a silicone. As illustrated in <FIG>, the release sheet <NUM> is peeled or otherwise removed from the lamination adhesive <NUM> before the FPL <NUM> is laminated to a backplane (not shown) to form the final display.

The front plane laminate <NUM> is illustrated in <FIG> in a general form which may be used in all areas of the present invention. The adhesive layer <NUM> incorporated in the FPL <NUM> may comprise a composite material having low thermal sensitivity, as will be described in greater detail below.

A double release sheet (generally designated <NUM>) according to another embodiment of the present invention is shown in <FIG> of the accompanying drawings. The double release sheet <NUM> comprises a central layer <NUM> of electro-optic material, specifically in <FIG> a layer comprising capsules <NUM> in a polymeric binder <NUM>. The capsules <NUM> may be similar to those described above with reference to <FIG>. The sheet <NUM> further comprises a first adhesive layer <NUM>, a first release sheet <NUM> covering the first adhesive layer <NUM>, a second adhesive layer <NUM> disposed on the opposed side of the layer <NUM> from the first adhesive layer <NUM>, and a second release sheet <NUM> covering the second adhesive layer <NUM>.

The sheet <NUM> may be formed by first coating a the release sheet <NUM> with a layer of adhesive which is then dried or cured to form the first adhesive layer <NUM>. Next, a mixture of the capsules <NUM> and binder <NUM> is printed or otherwise deposited on the first adhesive layer <NUM>, and then the mixture is dried or cured to form a coherent layer <NUM>. Finally, a layer of adhesive is deposited over the layer <NUM>, dried or cured to form the second adhesive layer <NUM>, and covered by the second release sheet <NUM>.

It will be apparent to those skilled in coating technology that this sequence of operations used to form the sheet <NUM> is well adapted for continuous production and that, by careful choice of materials and process conditions, it may be possible to carry out the entire sequence of operations in a single pass through conventional roll-to-roll coating apparatus.

To assemble a display using a double release film such as the film <NUM>, one release sheet (typically that on to which the electro-optic material was coated) is peeled away, and the remaining layers of the double release film are attached to a front substrate using, for example a thermal, radiation, or chemically based lamination process. Typically, the front substrate will include a conductive layer which will form the front electrode of the final display. The front substrate may include additional layers, such as an ultra-violet filter or a protective layer intended to protect the conductive layer from mechanical damage. Thereafter, the other release sheet is peeled away, thereby exposing the second adhesive layer, which is used to attach the electro-optic material coating assembly to a backplane. Again, a thermal, radiation, or chemically based lamination process may be used. It will be appreciated that the order of the two laminations described is essentially arbitrary and could be reversed, although it practice it is almost always more convenient to laminate the double release film to the front substrate first, and thereafter to laminate the resulting front subassembly to the backplane.

Either or both of the adhesive layers <NUM> and <NUM> may comprise a composite material containing a polymer phase and filler phase. The concentration of the filler phase is selected such that the composite system exhibits a lower temperature dependence of its conductivity over a temperature range that is relevant to display operation.

The filler phase added to the polymeric phase of the composite system preferably has a greater conductivity than the polymeric phase, so that adding more filler to the polymeric phase increases the conductivity of the composite. Furthermore, it is preferred that the filler has a thermal coefficient of expansion (CTE) that is substantially less than that of the polymer composite. For example, a composite material containing a polyurethane for the polymer phase and metal needles or flake, such as aluminum or nickel, may be provided. The CTE of polyurethanes may be in the range of <NUM> to <NUM> ppm/K. The CTE of aluminum is about <NUM> ppm/K. Nickel has a CTE of about <NUM> ppm/K. A ratio of the CTE of the filler phase to the CTE of the polymeric phase is preferably <NUM> or less, more preferably <NUM> or less, and most preferably <NUM> or less.

In order to suppress the temperature effects on the conductivity of the composite, filler is added to the composite in an amount about the percolation threshold. As used herein throughout the specification and the claims, "percolation threshold" means the filler volume fraction in a composite material containing a polymer phase and filler phase below which the composite material exhibits a conductivity whose logarithm is closer to the logarithm of the conductivity of the polymer phase than the filler phase and above which the composite material exhibits a conductivity whose logarithm is closer to the logarithm conductivity of the filler phase than the polymer phase.

By ensuring that the volume fraction of filler in the polymeric matrix is about the percolation threshold, the conductivity of the composite is restrained despite large temperature swings. Referring to <FIG>, a representative plot is provided of the conductivity of a polymeric composite (y-axis) versus the volume fraction of filler in the polymeric composite (x-axis). The point <NUM> on the plot is the percolation threshold.

If the coefficient of thermal expansion of the polymeric phase in the composite is much greater than the coefficient of thermal expansion of the filler, the volume fraction of the filler phase in the composite material will decrease with increasing temperature. For example, if the starting volume fraction φ of filler in the composite is about the percolation threshold (<NUM>), the conductivity of the composite will substantially decrease with increasing temperature because the filler concentration will decrease. This can combat the natural tendency for increased conduction at elevated temperature of the unfilled polymer matrix. As the temperature cools, the polymeric phase will contract more than the filler causing the volume fraction of the filler to increase. Thus, if the starting volume fraction of the filler in the composite is about the percolation threshold (<NUM>), the conductivity of the composite will increase with decreasing temperature.

Thus, by incorporating a filler having a substantially smaller coefficient of thermal expansion than a polymeric phase in a composite material and loading the composite with a filler about the percolation threshold, one may reduce the fractional change in conductivity with temperature effects that would have been expressed by the unfilled polymeric material. In other words, the tendency toward increased electrical conductivity with increased temperature and decreased conductivity with decreased temperature is reduced. The composite material according to the various embodiments of the present invention may provide materials having much less variation in the fractional change of the electrical conductivity over a temperature range of interest than one may find from using an unfilled polymer.

As used herein throughout the specification and the claims, the phrase "about the percolation threshold" means a filler concentration (volume percent) at or above the conductivity transition point. The conductivity transition point may be calculated from the conductivity curve of the composite material, such as the plot provided in <FIG> and <FIG>. Referring to <FIG>, a first line y1 is drawn through the point on the conductivity curve for the composite material at a filler concentration of zero, the line y1 having a slope m1 equal to the slope of the conductivity curve at a filler concentration of zero. A second line y2 is drawn through the point on the conductivity curve corresponding to the percolation threshold, the line y2 having a slope m2 equal to the slope of the conductivity curve at the percolation threshold. The intersection <NUM> on <FIG> of the two lines, y1 and y2, provides the conductivity transition point. The filler concentration at the conductivity transition point may be calculated, for example, using equations (<NUM>) and (<NUM>) below for y1 and y2: <MAT> <MAT> At the conductivity transition point, y1 equals y2, therefore: <MAT> Solving for "φ" provides equation (<NUM>): <MAT> Thus, the filler concentration at the conductivity transition point is equal to the ratio of the difference between the y-intercept of y2 and y1 to the difference between the slope of y1 and y2.

It is preferred that the composite material used in the various embodiments of the present invention exhibits a change in conductivity with temperature, i.e. (<NUM>/s)*ds/dT, that is less than or equal to <NUM>% of the change in conductivity with temperature of the unfilled polymer, more preferably less than or equal to <NUM>%, most preferably less than or equal to <NUM>%.

When incorporating the composite in the adhesive layer of an electro-optic display, the polymeric phase of the composite material according to the various embodiments of the present invention is preferably an anisotropic lamination adhesive having a conductivity which is greater in the direction from the electrode to the electro-optic material than in the plane of this layer. Such an anisotropic adhesive produces only a small voltage drop between the electrode and the electro-optic material (thus allowing as large an electric field as possible across the layer of electro-optic material) while presenting high resistance to current flow between adjacent electrodes, and thus minimizing cross-talk between adjacent pixels of a display.

The lamination adhesive may be a hot melt adhesive, but can also be a thermosetting, radiation curable, or pressure sensitive adhesive. The adhesive may be based upon ethylene vinyl acetate, acrylic, polyolefin, polyamide, polyester, polyurethane, silicone, epoxy, polyvinyl butyrate, polystyrene-butadiene or vinyl monomers or oligomers. To provide the necessary anisotropic conductivity, the adhesive may be loaded with conductive particles, for example carbon particles, silver particles, plated polymer spheres, plated glass spheres, indium tin oxide particles, or nano-phase indium tin oxide particles. Alternatively, conductive polymers such as polyacetylene, polyaniline, polypyrrole, poly(<NUM>,<NUM>-ethylenedioxythiophene) (PEDOT), or polythiophene can be used to dope the polymer and cause it to conduct well in the z-axis direction (perpendicular to the thickness of the layer of adhesive) but not in the plane of this layer. In order to make these films, the adhesive sheet can be cast and then stretched in one or both axes to introduce the necessary anisotropic conductivity. Various types of anisotropic adhesives are described in <CIT>; <CIT>; and <CIT>, and anisotropic adhesives are available commercially, for example from Minnesota Mining and Manufacturing Corporation ("<NUM>"), Henkel Loctite Corporation, <NUM> Trout Brook Crossing, Rocky Hill CT <NUM>, Btech Corporation, <NUM> Greenwood Drive, Longmont CO <NUM>, and Dana Enterprises International, <NUM> Osgood Road, Fremont CA <NUM>.

It is generally preferred that the anisotropic adhesive used in the various embodiments of the present invention have a conductivity of less than about <NUM>-<NUM> S/cm. in the plane of the adhesive layer and a conductivity greater than about <NUM>-<NUM> S/cm. in the z-axis direction.

A process for forming a composite material according to one embodiment of the present invention is also provided. The composite material may be provided in the form of a layer of an anisotropic adhesive having greater conductivity perpendicular to the plane of the layer than in a parallel direction. The process comprises dispersing a plurality of conductive filler particles in an adhesive matrix, the particles having a conductivity differing from that of the matrix; applying to the particles/matrix mixture an electric or magnetic field effective to cause the particles to form conductive strands extending substantially perpendicular to the plane of the layer; and increasing the viscosity of the matrix (typically by gelling or curing the matrix) to prevent the particles moving out of the strands.

The use of electrorheological and magnetorheological effects to produce anisotropically conductive films by the process of the present invention is illustrated in a highly schematic manner in <FIG> of the accompanying drawings. The electrorheological (ER) effect is an effect by which application of an electrical field across a particulate dispersion, e.g., a dispersion contained between parallel plate electrodes, causes the particles composing the dispersion to form into strands or acicular agglomerates. As shown in <FIG>, the process begins by forming a layer of conductive particles <NUM> dispersed in a matrix (continuous phase) of a lamination adhesive <NUM>. An electric or magnetic field is applied perpendicular to the plane of the layer, thereby causing the particles <NUM> to form strands <NUM> extending through the thickness of the layer, as illustrated in <FIG>. Finally, the viscosity of the matrix <NUM> is greatly increased, typically by gelling or curing the matrix, to prevent further movement of the particles <NUM> through the matrix <NUM>, and thus lock the strands <NUM> in place.

Formation of strands parallel to the electric field lines occurs when the complex conductivity (k*) of the particles is significantly higher than that of the matrix. In a preferred process of the present invention, the precursor adhesive (as shown in <FIG>) consists of relatively high conductivity particles, kp > <NUM>-<NUM> S/cm, dispersed in a low conductivity matrix, km < <NUM>-<NUM> S/cm. The particles are aligned in the z-axis direction by application of an electric field across the dispersion, and the resultant z-axis strands are locked in place by curing or gelling the matrix. The final adhesive film then contains conductive strands spanning or percolating only in the z-axis direction because strands are spaced laterally and therefore separated by the low conductivity matrix. Z-axis conduction will be dominated by the conductivity of the strands and therefore by the particle conductivity, while lateral conductivity will be dominated by that of the continuous low conductivity matrix.

This method is particularly well-suited for preparing continuous thin films of a z-axis lamination adhesive. For example, continuous thin film z-axis conductive adhesives can be prepared by feeding a well-mixed dispersion of conductive particles in a low conductivity adhesive matrix into a coating die, and coating the dispersion into a thin film, typically <NUM>-<NUM> thick, followed by application of the electric field and curing or gelling.

A magnetorheological process for producing a z-axis conductive adhesive is similar to the electrorheological process; magnetic particles are dispersed in a non-magnetic adhesive matrix, the particles are aligned into strands in the z-axis direction by application of a magnetic field, and the strands are locked in by curing or gelling of the matrix. In this case, the particles need to be both magnetically polarizable and to have a conductivity in the range described above, typically greater than <NUM>-<NUM> S/cm. The range of particles meeting these requirements for the magnetorheological process is perhaps more limited than the range of particles suitable for the electrorheological process, but a magnetorheological process is preferred because application of the magnetic field across the layer does not require making electrical contact with the surface of the layer, and thus an aqueous based adhesive matrix can be used, which is typically not the case with the electrorheological process.

In an electrorheological process to produce a composite material containing lamination adhesive compositions as the polymeric matrix, the conductive particles used as the filler may preferably have a conductivity, with increasing preference in the order given, not less than <NUM> X <NUM>-<NUM> S/cm, <NUM> X <NUM>-<NUM> S/m, <NUM> X <NUM>-<NUM> S/m, <NUM> X <NUM>-<NUM> S/m, <NUM> X <NUM>-<NUM> S/m, <NUM>/m, <NUM> X <NUM><NUM> S/m, <NUM> X <NUM><NUM> S/m, <NUM> X <NUM><NUM> S/m, <NUM> X <NUM><NUM> S/m, and <NUM> X <NUM><NUM> S/m and a diameter not greater than about <NUM>/<NUM> of the thickness of the final film. (The term "diameter" is used herein to include what is usually known as the "equivalent diameter" of a non-spherical particle, namely the diameter of a spherical particle having the same volume as the non-spherical one. ) The particles can be formed from semiconducting polymers, for example acidically doped polyanilines, polythiophenes, and pyrolyzed polyacrylonitriles. The particles may alternatively be formed from low k* materials that are "activated" (raised) to higher k* (relative to that of the matrix) by addition of trace amount of a polar material, for example water or ethylene glycol, that adsorbs primarily on the surface of the particles. Suitable low k* materials include cellulosic materials, and various aluminas, silicates, and zeolites. The matrix (continuous phase) should have a low conductivity relative to that of the particles, this low conductivity preferably being less than <NUM>-<NUM> S/cm. Many low-conductivity, low-viscosity oils, such as the hydrocarbons used as the fluids in many encapsulated electrophoretic media may be suitable; however the matrix must also behave as an adhesive, and be gellable or curable to lock in the z-axis strands formed in the process. For example thermally reversible gelling materials such as Kraton (Registered Trade Mark) rubbers (block copolymers) or polyurethanes may be particularly advantageous as a continuous phase, alone or in combination with a diluent to lower viscosity for coating and particle alignment. Typical non-aqueous pressure sensitive adhesives like polyacrylate solutions, or succinate-functionalized hydrocarbon polymers, for example ethylene propylene copolymers, or silicone rubber type adhesives, may also be used. For gelling type matrices, coating and particle alignment occurs under conditions where the material is not gelled, and then after formation of the strands, the matrix is gelled, for example by lowering the temperature or cross-linking the matrix by addition or removal of a chemical component or reactant or exposure to an ultra-violet light source. For solvent based non-aqueous adhesives, the strands can be locked in position by rapidly evaporating the solvent.

The magnetorheological process of the present invention may use any of the types of matrices discussed above, since the magnetorheological process differs from the electrorheological process only in the formation of the strands by magnetic field alignment instead of electric field alignment. However, since the magnetorheological process can make use of aqueous matrices as well as non-aqueous ones, polyurethane adhesives, gelatin, or other aqueous continuous phases may also be used. Particles suitable for use in the magnetorheological process include iron and other magnetizable materials, such as nickel and carbonyl iron; these materials are generally supplied in particle sizes greater than <NUM>-<NUM>, which may not be optimal for preparing thin films of adhesive, but they may be milled to smaller sizes for use in the present process. Iron oxides, such as γ-Fe<NUM>O<NUM> materials used in the magnetic recording industry, are typically supplied in much smaller particle sizes, around <NUM>-<NUM>, and may therefore be used as supplied in the preparation of thin films.

The criteria for particles to be used in the magnetorheological process of the present invention differ somewhat from most other applications of magnetorheological particles. The present process does not require strong saturation magnetization, a typical criterion for choosing particles for a magnetorheological fluid, but in the present process the particles should be conductive enough to satisfy the z-axis conductivity range outlined above.

The lamination adhesive film containing the composite material described above can then be laminated to the electro-optic material and ultimately to an active matrix backplane, under conditions where the anisotropic conductivity of the adhesive is maintained because the film never undergoes large scale flow (with respect to strand length) during the lamination processes.

Any anisotropic z-axis conductive adhesive based upon conductive strands or similar conductive regions within an essentially non-conductive matrix should have sufficient strands or conductive regions per unit area for each pixel of the electro-optic material with which the adhesive is used to experience a substantially uniform or homogeneous electric field over its entire area, or undesirable optical effects may occur. The strand density per unit area may be adjusted, for example, by altering conductive particle size, volume fraction of conductive particles, and the intensity and duration of the aligning field, thereby allowing the fine structure of the anisotropic adhesive to be varied.

Several types of lamination processes may be in the fabrication of electrophoretic devices, including batch unit operations. A preferred process of this type is illustrated in a highly schematic manner and in side elevation in <FIG> of the accompanying drawings. As shown in this Figure, the process forms a display by the convergence of two webs <NUM> and <NUM>. The web <NUM>, comprises a rear electrode assembly on a flexible substrate, although the individual components are not shown in <FIG>. Similarly, the web <NUM> comprises a flexible substrate, a transparent electrode layer, for example an ITO layer, and a dried film of capsules and binder, although again the individual components are not shown in <FIG>. As shown in <FIG>, the web <NUM> is unwound from a feed spool <NUM> and brought, electrode side up, beneath a die <NUM> for coating with a thin layer of a radiation curable lamination adhesive <NUM>. The adhesive <NUM> may for example be curable by visible, ultra-violet or electron beam radiation. The web <NUM> bearing adhesive <NUM> passes a radiation source <NUM>, the intensity of which is adjusted having regard to both the catalyst concentration (and hence curing speed) of the adhesive <NUM> and the speed of the web <NUM>. (If an anisotropic adhesive is being used, an electrical or magnetic head may be provided between the die <NUM> and the radiation source <NUM> to form the desired strands of conductive particles before the matrix is cured or gelled by the radiation source <NUM>.

The web <NUM> is unwound from a feed spool <NUM> and carries the dried capsule-containing layer to a convergence point <NUM>, at which the webs <NUM> and <NUM> are brought together. At this convergence point <NUM>, the radiation curable resin <NUM> is still in liquid form, and easily fills voids on the surface of the capsule-containing layer. The coordination of web speed, catalyst concentration and radiation intensity is adjusted to provide a cure rate such that hardening occurs after the convergence point <NUM>, while the two webs <NUM> and <NUM> are being held together with rollers <NUM>. Finally, the laminated web <NUM> produced is wound on a take-up spool 222a.

It will be seen that the foregoing process allows the lamination process to operate in-line, and thus enables higher production rates than the batch unit processes previously discussed.

In another embodiment of the present invention, the composite materials may be incorporated in the electro-optic material layers of the display. Specifically, the composite material may be incorporated in either the binder of encapsulated media or the polymeric film used to form microcells. Some binder or polymeric film materials exhibit changes in volume resistivity of more than two orders of magnitude when the temperature and humidity of their environment is varied within the ranges of <NUM>-<NUM> and <NUM>-<NUM> per cent relative humidity (RH). For satisfactory performance, it has been found that the volume resistivity of the binder/film material should not vary by a factor of more than about <NUM> within the ranges of <NUM> to <NUM> per cent RH and <NUM> to <NUM>, or within any broader RH and temperature ranges within which the display is intended to operate. Desirably, the volume resistivity does not change by a factor of more than about <NUM>, and preferably not more than about <NUM> within the specified RH and temperature ranges.

In order to limit the thermal sensitivity of the binder/film material to temperature and RH, a filler may be incorporated in the binder/film material to provide a composite similar to the above described composite materials. Incorporating a conductive filler about the percolation threshold in a polymeric matrix having a greater coefficient of thermal expansion than the filler will similarly limit the temperature effects on the resistivity of the binder/film. For example, with increasing temperature the polymeric phase will expand at a greater rate than the filler phase. If the volume fraction of the filler phase is about the percolation threshold, the resistivity of the composite material will decrease with increasing temperature. When the temperature decreases, the polymeric phase will contract at a higher rate causing an increase in the volume fraction of filler and in turn a decrease in the resistivity of the composite. Thus, a composite material according to the various embodiments of the present invention may provide a binder/film having reduced thermal sensitivity as compared to a binder/film lacking any filler.

Examples of acceptable binder materials include, but are not limited to, aliphatic polyurethanes free from aromatic materials, such as NeoResin R <NUM>, NeoResin R <NUM>, NeoResin R <NUM>, NeoResin R <NUM>, NeoResin <NUM>, and mixtures thereof. It is preferred that the binder should have a volume resistivity, measured at <NUM>, which does not change by a factor of more than about <NUM> after being held at <NUM> and <NUM> per cent relative humidity for <NUM> hours. The phrase "being held" is deliberately used to stress that, in testing binders to determine whether they conform to the requirements of this aspect of the present invention, care should be taken to ensure that the binder material does equilibrate with the specified atmosphere within a reasonable time. If a binder material is tested in thick layers, it may not equilibrate with the specified atmosphere for a considerable period and misleading results may be obtained. Such misleading results can be avoided by testing successively thinner layers of the binder material and checking that the results are consistent. For sufficiently thin layers, merely storing the binder material under the specified conditions for the specified period will suffice.

When testing materials for changes in volume resistivity with RH and temperature, the same precautions as discussed above should be observed to ensure that the samples tested are truly in equilibrium with the atmosphere at the desired RH and temperature before the volume resistivities are measured.

The polymeric material that may be used to form microcells include thermoplastics, thermosets, or precursors thereof. Examples of thermoplastic or thermoset precursors may include, but are not limited to multifunctional acrylate or methacrylate, multifunctional vinylether, multifunctional epoxide and oligomers or polymers thereof. A crosslinkable oligomer imparting flexibility, such as urethane acrylate or polyester acrylate, may also be added to improve the flexure resistance of the embossed microcells.

Further embossable compositions for microcells may comprise a polar oligomeric or polymeric material. Such a polar oligomeric or polymeric material may be selected from the group consisting of oligomers or polymers having at least one of the groups, such as nitro (-NO<NUM>), hydroxyl (--OH), carboxyl (--COO), alkoxy (--OR wherein R is an alkyl group), halo (e.g., fluoro, chloro, bromo or iodo), cyano (--CN), sulfonate (--SO<NUM>) and the like. The glass transition temperature of the polar polymer material is preferably below about <NUM>° C. and more preferably below about <NUM>° C. Specific examples of suitable polar oligomeric or polymeric materials may include, but are not limited to, polyhydroxy functionalized polyester acrylates (such as BDE <NUM>, Bomar Specialties Co, Winsted, CT) or alkoxylated acrylates, such as ethoxylated nonyl phenol acrylate (e.g., SR504, Sartomer Company), ethoxylated trimethylolpropane triacrylate (e.g., SR9035, Sartomer Company) or ethoxylated pentaerythritol tetraacrylate (e.g., SR494, from Sartomer Company).

Another type of embossable composition for forming microcell comprises (a) at least one difunctional UV curable component, (b) at least one photoinitiator, and (c) at least one mold release agent. Suitable difunctional components may have a molecular weight higher than about <NUM>. Difunctional acrylates are preferred and difunctional acrylates having a urethane or ethoxylated backbone are particularly preferred. More specifically, suitable difunctional components may include, but are not limited to, diethylene glycol diacrylate (e.g., SR230 from Sartomer), triethylene glycol diacrylate (e.g., SR272 from Sartomer), tetraethylene glycol diacrylate (e.g., SR268 from Sartomer), polyethylene glycol diacrylate (e.g., SR295, SR344 or SR610 from Sartomer), polyethylene glycol dimethacrylate (e.g., SR603, SR644, SR252 or SR740 from Sartomer), ethoxylated bisphenol A diacrylate (e.g., CD9038, SR349, SR601 or SR602 from Sartomer), ethoxylated bisphenol A dimethacrylate (e.g., CD540, CD542, SR101, SR150, SR348, SR480 or SR541 from Sartomer), and urethane diacrylate (e.g., CN959, CN961, CN964, CN965, CN980 or CN981 from Sartomer; Ebecryl <NUM>, Ebecryl <NUM>, Ebecryl <NUM>, Ebecryl <NUM>, Ebecryl <NUM> or Ebecryl <NUM> from Cytec). Suitable photoinitiators may include, but are not limited to, bis-acyl-phosphine oxide, <NUM>-benzyl-<NUM>-(dimethylamino)-<NUM>-[<NUM>-(<NUM>-morpholinyl)phenyl]-<NUM>-butanone, <NUM>,<NUM>,<NUM>-trimethylbenzoyl diphenyl phosphine oxide, <NUM>-isopropyl-<NUM>-thioxanthen-<NUM>-one, <NUM>-benzoyl-<NUM>'-methyldiphenylsulphide and <NUM>-hydroxy-cyclohexyl-phenyl-ketone, <NUM>-hydroxy-<NUM>-methyl-<NUM>-phenyl-propan-<NUM> -one, <NUM>-[<NUM>-(<NUM>-hydroxyethoxy)-phenyl]-<NUM>-hydroxy-<NUM>-methyl-<NUM>-propane-<NUM>-one, <NUM>,<NUM>-dimethoxy-<NUM>,<NUM>-diphenylethan-<NUM>-one or <NUM>-methyl-<NUM>[<NUM>-(methylthio)phenyl]-<NUM>-morpholinopropan-<NUM>-one. Suitable mold release agents may include, but are not limited to, organomodified silicone copolymers such as silicone acrylates (e.g., Ebercryl <NUM> or Ebercyl <NUM> from Cytec), silicone polyethers (e.g., Silwet <NUM>, Silwet <NUM>, Silwet <NUM>, Silwet <NUM>, Silwet <NUM>, Silwet <NUM> or Silwet <NUM> from Momentive). The composition may further optionally comprise one or more of the following components, a co-initiator, monofunctional UV curable component, multifunctional UV curable component or stabilizer.

The materials used as the filler phase in the binder materials may be the same as those previously listed for the composites incorporated in the lamination adhesive compositions described above.

The electro-optic medium present in the displays made according to the various embodiments of the present invention may be of any of the types previously discussed. Thus, the electro-optic medium within the binder may comprise, for example, droplets of a dispersion of colored, charged particles that are optionally encapsulated in microcapsules or microcells, droplets containing a rotating multichromal or bichromal particles encapsulated within a dispersion fluid, or an electrochromic medium. However, it is generally preferred that the electro-optic medium be an electrophoretic medium comprising a plurality of capsules, each capsule comprising a capsule wall and an internal phase comprising electrically charged particles in a fluid and capable of moving through the fluid on application of an electric field to the electrophoretic medium, the electrophoretic medium further comprising a polymeric binder within which the capsules are held.

Apart from the inclusion of the composite materials of the present invention, the electrophoretic media and displays of the present invention may employ the same components and manufacturing techniques as in the aforementioned E Ink and MIT patents and applications. The reader is referred to these patent and applications, and especially to the aforementioned <CIT>, for details of preferred materials and processes for the production of encapsulated electrophoretic displays.

Claim 1:
An electro-optic display comprising:
a layer of electro-optic material (<NUM>, <NUM>),
at least one conductor, and
an adhesive material (<NUM>, <NUM>, <NUM>) between the layer of electro-optic
material (<NUM>, <NUM>) and the at least one conductor,
wherein the layer of electro-optic material (<NUM>, <NUM>) or the adhesive material (<NUM>, <NUM>, <NUM>) comprises a composite material, wherein the composite material includes a polymer phase and a filler phase,
the filler phase having a conductivity greater than or equal to <NUM> X <NUM><NUM> S/m,
a ratio of the coefficient of thermal expansion of the filler phase to the polymer phase being less than or equal to <NUM>, and
a concentration of the filler phase in the composite material being greater than or equal to a filler concentration corresponding to a conductivity transition point (<NUM>) of the composite material.