HK1215075B - Colored electrophoretic displays - Google Patents

Description

Color electrophoretic display

Related applications

All of the below-referenced US patents and publications and co-pending applications are incorporated herein by reference in their entirety.

Technical Field

The present invention relates to color electrophoretic displays, and more particularly to electrophoretic displays capable of rendering more than two colors using a single layer of electrophoretic material containing a plurality of colored particles.

Background Art

The term color as used herein includes black and white.White particles are generally of the light scattering type.

The term grayscale state as used herein has a traditional meaning in the field of imaging technology to refer to a state between the two extreme optical states of a pixel, but does not necessarily mean a black and white transition between the two extreme states. For example, many of the E Ink patents and published applications referenced below describe electrophoretic displays in which the extreme states are white and dark blue, so that the intermediate grayscale state is actually light blue. In fact, as already mentioned, the change in optical state may not be a color change at all. The terms black and white used below refer to the two extreme optical states of the display and should be understood to generally include extreme optical states (such as the white and dark blue states mentioned above), which are not strictly black and white.

As used herein, the terms bistable and bistability are used in their conventional sense in the art to refer to a display comprising a display element having first and second display states, wherein the first and second display states differ in at least one optical property such that after any given element is driven to assume its first or second display state by an addressing pulse of finite duration, that state persists after termination of the addressing pulse for a time at least several times (e.g., at least four times) the minimum duration of the addressing pulse required to change the state of the display element. U.S. Patent No. 7,170,670 shows that some particle-based electrophoretic displays capable of displaying grayscale can be stable not only in their extreme black and white states but also in intermediate gray states, as can some other types of electro-optical displays. Such displays are properly referred to as multistable rather than bistable, although for convenience, the term bistable is used herein to cover both bistable and multistable displays.

In this document, the term pulse when used in connection with driving an electrophoretic display refers to the integral of the applied voltage with respect to time during which the display is driven.

Particles that absorb, scatter, or reflect light over a broad range or at selected wavelengths are referred to herein as colored or pigment particles. In addition to pigments (strictly a term for insoluble colored materials), a variety of materials that absorb or reflect light, such as dyes or photonic crystals, can also be used in the electrophoretic media and displays of the present invention.

For many years, particle-based electrophoretic displays (EPDs) have been the subject of intensive research and development. In such displays, multiple charged particles (sometimes referred to as pigment particles) are moved through a fluid under the influence of an electric field. Compared to liquid crystal displays (LCDs), EPDs offer good brightness and contrast, wide viewing angles, a bi-stable state, and low power consumption. However, issues with the long-term image quality of these displays have hindered their widespread use. For example, the particles that make up EPDs tend to settle, resulting in an insufficient service life for these displays.

As mentioned above, electrophoretic media require the presence of a fluid. In most existing electrophoretic media, this fluid is a liquid, but electrophoretic media can be generated using a gaseous fluid; see, for example, Kitamura, T. et al., "Electrical toner movement for electronic paper-like display," IDW Japan, 2001, Paper HCS1-1 and Yamaguchi, Y. et al., "Toner display using insulative particles charged triboelectrically," IDW Japan, 2001, Paper AMD4-4. See also U.S. Patents Nos. 7,321,459 and 7,236,291. When the medium is used in an orientation that allows particles to deposit in this way, such as when the medium is arranged in a vertical plane, such gas-based electrophoretic media appear to be susceptible to the same types of problems caused by particle deposition as liquid-based electrophoretic media. In fact, particle deposition appears to be a more serious problem in gas-based electrophoretic media than in liquid-based electrophoretic media because the lower viscosity of gas suspensions allows for faster deposition of electrophoretic particles than liquid suspensions.

Numerous patents and applications assigned to or filed in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various technologies for encapsulated electrophoretic and other electro-optical media. Such encapsulated media comprise a plurality of 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 themselves are held in a polymer binder to form a coherent layer between two electrodes. The technologies described in these patents and applications include:

(a) electrophoretic particles, fluids, and fluid additives; see, for example, U.S. Patent Nos. 7,002,728 and 7,679,814;

(b) capsules, adhesives, and encapsulation processes; see, for example, U.S. Patent Nos. 6,922,276 and 7,411,719;

(c) Films and subassemblies containing electro-optical materials; see, for example, U.S. Patent Nos. 6,982,178 and 7,839,564;

(d) Backplanes, adhesive layers, and other auxiliary layers and methods for displays; see, for example, U.S. Patent Nos. 7,116,318 and 7,535,624;

(e) Color composition and color adjustment; see, for example, U.S. Patent Nos. 6,017,584, 6,664,944, 6,864,875, 7,075,502, 7,167,155, 7,667,684, 7,791,789, 7,956,841, 8,040,594, 8,054,526, 8,098,418, 8,213,076, and 8,363 ,299 and U.S. Patent Application Publication Nos. 2004/0263947, 2007/0109219, 2007/0223079, 2008/0023332, 2008/0043318, 2008/0048970, 2009/0004442, 2009/0225398, 2010/0103502, 2010/0156780, 201 1/0164307, 2011/0195629, 2011/0310461, 2012/0008188, 2012/0019898, 2012/0075687, 2012/0081779, 2012/0134009, 2012/0182597, 2012/0212462, 2012/0157269, and 2012/0326957;

(f) Methods for driving displays; see, e.g., U.S. Patent Nos. 5,930,026, 6,445,489, 6,504,524, 6,512,354, 6,531,997, 6,753,999, 6,825,970, 6,900,851, 6,995,550, 7,012,600, 7,023,420, 7,034,783, 7,116,466, 7,119,772, 7,193,625, 7,202,847, 7,304,784, 7,492,444, etc. 7, 7,312,794, 7,327,511, 7,453,445, 7,492,339, 7,528,822, 7,545,358, 7,583,251, 7,602,374, 7,612,760, 7,679,599, 7,688,297, 7,729,039, 7,733,311, 7,733,335, 7,787,169, 7,952,557, 7,956,841, 7,999,787, 8,077,141, 8,125,501, 39,050, 8,174,490, 8,289,250, 8,300,006, and 8,314,784 and U.S. Patent Application Publication Nos. 2003/0102858, 2005/0122284, 2005/0179642, 2005/0253777, 2007/0091418, 2007/0103427, 2008/0024429, 2008/0024482, 2008/0136774, 2008/0150888, 2008/0291129, 2008 09/0174651, 2009/0179923, 2009/0195568, 2009/0322721, 2010/0045592, 2010/0220121, 2010/0220122, 2010/0265561, 2011/0187684, 2011/0193840, 2011/0193841, 2011/0199671, and 2011/0285754 (these patents and applications are hereinafter referred to as MEDEOD (Method for Driving an Electro-Optical Display) applications);

(g) Display applications; see, for example, U.S. Patent Nos. 7,312,784 and 8,009,348; and

(h) Non-electrophoretic displays, such as those described in U.S. Patent Nos. 6,241,921, 6,950,220, 7,420,549, and 8,319,759, and U.S. Patent Application Publication No. 2012/0293858.

Many of the aforementioned patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thereby producing a so-called polymer-dispersed electrophoretic display, wherein the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymer material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display can be considered capsules or microcapsules, even though there is no discrete capsule membrane associated with each individual droplet; see, e.g., U.S. Patent No. 6,866,760. Therefore, for the purposes of this application, such polymer-dispersed electrophoretic media are considered a subclass of encapsulated electrophoretic media.

A related type of electrophoretic display is the so-called microcell electrophoretic display. In a microcell electrophoretic display, charged particles and fluids are not encapsulated in microcapsules, but rather are held within multiple cavities within a carrier medium (usually a polymer film). See, for example, U.S. Patent Nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging, Inc.

Although electrophoretic media are typically opaque (because, for example, in many electrophoretic media, the particles substantially block visible light from being transmitted 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 display state is light-transmissive. See, for example, U.S. Patents Nos. 5,872,552, 6,130,774, 6,144,361, 6,172,798, 6,271,823, 6,225,971, and 6,184,856. Dielectrophoretic displays are similar to electrophoretic displays, but rely on variations in electric field strength. Dielectrophoretic displays can operate in a similar mode; see U.S. Patent No. 4,418,346. Other types of electro-optical displays can also operate in a shutter mode. Electro-optical media operating in a shutter mode can be used in a multilayer structure for a full-color display; in such a structure, at least one layer adjacent to the viewing surface of the display is operated in a shutter mode to expose or conceal a second layer located away from the viewing surface.

Encapsulated electrophoretic displays are generally not subject to the aggregation and deposition failure modes of conventional electrophoretic devices and offer additional benefits, such as the ability to print or coat the display on a variety of flexible and rigid substrates. (The use of the word printing is intended to include all forms of printing and coating, including but not limited to: pre-metered coating such as patch die coating, slot or extrusion coating, slide or laminate coating, curtain coating, roll coating such as blade over roller, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brush coating, air knife coating, screen printing processes, electrostatic printing processes, thermal printing processes, inkjet printing processes, electrophoretic deposition (see U.S. Patent No. 7,339,715), and other similar techniques.) Thus, the resulting display can be flexible. Additionally, because the display medium can be printed (using a variety of methods), the display itself can be inexpensively manufactured.

The aforementioned U.S. Patent No. 6,982,178 describes a method for assembling solid-state electro-optical displays (including encapsulated electrophoretic displays) that is well suited for mass production. In essence, the patent describes a so-called front plane laminate (FPL) comprising, in order: a light-transmitting conductive layer, a layer of a solid electro-optic medium in electrical contact with the conductive layer, an adhesive layer, and a release sheet. Typically, the light-transmitting conductive layer is carried on a light-transmitting substrate, which is preferably flexible in the sense that the substrate can be manually wound around a cylinder having a diameter of, say, 10 inches (254 mm) without permanent deformation. The term light-transmitting, as used in the patent and herein, means that the layer so designated transmits sufficient light to enable an observer to see through that layer to observe changes in the display state of the electro-optic medium, typically by viewing through the conductive layer and the adjacent substrate (if present); if the electro-optic medium displays changes in reflectivity at non-visible wavelengths, the term light-transmitting should of course be interpreted as referring to transmission at the relevant non-visible wavelengths.

Assembly of an electro-optical display using such a front plane laminate can be accomplished by removing the release sheet from the front plane laminate and contacting the adhesive layer with the backplane in a manner effective to bond the adhesive layer to the backplane, thereby securing the adhesive layer, the layer of electro-optic medium, and the conductive layer to the backplane. This process is well suited for mass production because the front plane laminate can typically be mass-produced using roll-to-roll coating techniques and cut into sheets of any size required for use with a particular backplane.

U.S. Patent No. 7,561,324 describes a so-called double release sheet, which is essentially a simplified version of the front plane lamination of the aforementioned U.S. Patent No. 6,982,178. One form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two adhesive layers, one or both of which are covered by a release sheet. Another form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two release sheets. Both forms of double release film are intended to be used in a process essentially similar to the process already described for assembling an electro-optic display from front plane lamination, but involving two separate laminates; typically, in a first laminate, the double release sheet is laminated to the front electrode to form the front subassembly, and then in a second laminate, the front subassembly is laminated to the backplane to form the final display, although the order of the two laminates can be reversed if desired.

U.S. Patent No. 7,839,564 describes a so-called reverse front plane lamination, which is a variation of the front plane lamination described in the aforementioned U.S. Patent No. 6,982,178. The reverse front plane lamination comprises, in order: at least one of a light-transmitting protective layer and a light-transmitting conductive layer, an adhesive layer, a layer of a solid electro-optic medium, and a release sheet. The reverse front plane lamination is used to form an electro-optic display having a layer of laminating adhesive between the electro-optic layer and the front electrode or front substrate; a second layer of adhesive, which is typically thinner, may or may not be present between the electro-optic layer and the backplane. Such an electro-optic display can combine good resolution with good low-temperature performance.

As mentioned above, most simple prior art electrophoretic media essentially display only two colors. Such electrophoretic media either use a single type of electrophoretic particles having a first color in a colored fluid with a different second color (in which case, the first color is displayed when the particles are close to the viewing surface of the display, and the second color is displayed when the particles are away from the viewing surface); or use first and second types of electrophoretic particles having different first and second colors in a colorless fluid (in which case, the first color is displayed when the first type of particles are close to the viewing surface of the display, and the second color is displayed when the second type of particles are close to the viewing surface). The two colors are typically black and white. If a full color display is required, a color filter array can be deposited on the viewing surface of a monochrome (black and white) display. Displays with color filter arrays rely on area sharing and color mixing to produce color stimulation. The available display area is shared between three or four primary colors, such as red/green/blue (RGB) or red/green/blue/white (RGBW), and the filters can be arranged in a one-dimensional (strip) or two-dimensional (2×2) repeating pattern. Other choices of primary colors or beyond are also known in the art. The three (in the case of an RGB display) or four (in the case of an RGBW display) subpixels are chosen to be small enough so that, at the intended viewing distance, they visually blend together into a single pixel with uniform color stimulation ('color mixing'). The inherent drawback of area sharing is that the pigments are always present, and color is adjusted only by converting the corresponding pixels of the underlying monochrome display to white or black (turning the corresponding primary colors on or off). For example, in an ideal RGBW display, each of the red, green, blue, and white primary colors occupies one-quarter of the display area (one of four subpixels), with the white subpixel being as bright as the underlying monochrome white, and each colored subpixel being no lighter than one-third the monochrome white. The brightness of white shown on the display is generally no greater than one-half the brightness of the white subpixels (a white area on the display is created by displaying one white subpixel for every four subpixels, plus each colored subpixel being equal to one-third the white subpixel in its color form, so that the three colored subpixels together contribute no more than one white subpixel). By sharing area with color pixels converted to black, the brightness and saturation of the color are reduced. Area sharing is particularly problematic when mixing yellow because it is lighter than any other color of equal brightness, and saturated yellow is almost as bright as white. Converting blue pixels (one-quarter of the display area) to black makes yellow too dark.

Multilayer stacked electrophoretic displays are well known in the art; see J. Heikenfeld, P. Drzaic, J.S. Yeo, and T. Koch, Journal of the SID, Vol. 19, No. 2, pp. 129-156, 2011. In such displays, ambient light is passed through the image in each of the three subtractive primary colors, much like conventional color printing. U.S. Patent No. 6,727,873 describes a stacked electrophoretic display in which three layers of switchable cells are placed on a reflective background. Similar displays are known in which the colored particles are moved laterally (see International Application No. WO 2008/065605) or in which the colored particles are isolated within micropits using a combination of longitudinal and lateral motion. In both cases, each layer has an electrode for collecting or dispersing the colored particles on a pixel-by-pixel basis, so each of the three layers requires a thin-film transistor (TFT) layer (two of the three TFT layers must be substantially transparent) and a light-transmitting counterelectrode. This complex configuration of electrodes is expensive to produce, and in the current state of technology, it is difficult to provide a sufficiently transparent plane for the pixel electrodes, especially since the white state of the display must be viewed through multiple electrode layers. Multilayer displays also suffer from parallax problems because the thickness of the display stack approaches or exceeds the pixel size.

U.S. Patent Publication Nos. 2012/0008188 and 2012/0134009 describe a multicolor electrophoretic display having a single backplane, which includes independently addressable pixel electrodes and a common light-transmitting front electrode. Multiple electrophoretic layers are arranged between the backplane and the front electrode. The displays described in these applications are capable of displaying any primary color (red, green, blue, cyan, magenta, yellow, white, and black) at any pixel location. However, the use of multiple electrophoretic layers located between a single set of addressing electrodes has disadvantages. The electric field experienced by the particles in a particular layer is lower than that experienced by a single electrophoretic layer addressed with the same voltage. In addition, light losses in the electrophoretic layer closest to the viewing surface (e.g., caused by light scattering or unwanted absorption) may affect the appearance of the image formed in the underlying electrophoretic layer.

Attempts have been made to provide full-color electrophoretic displays using a single electrophoretic layer. See, for example, U.S. Patent Application Publication No. 2011/0134506. However, in the current state of the art, such displays often suffer from issues such as slow switching speeds (several seconds long) or high addressing voltages.

Summary of the Invention

The present invention seeks to provide a color display using only a single electrophoretic layer but capable of displaying more than two, preferably all, colors at each location in the display area of the display, and a method of driving such an electrophoretic display.

Therefore, the present invention provides an electrophoretic medium comprising a fluid and at least a first type of particles arranged in the fluid, the first type of particles being such that when a first addressing pulse is applied to the medium, the first type of particles move in one direction relative to the electric field, and when a second addressing pulse greater than the first addressing pulse but having the same polarity as the first addressing pulse is applied to the medium, the first type of particles move in an opposite direction relative to the electric field.

The first address pulse and the second address pulse may differ in field strength, duration, or both. Furthermore, while the first address pulse includes applying a first electric field having a first period length to the medium, and the second address pulse includes applying a second electric field having a second period length to the medium, this does not necessarily mean that the first and second electric fields must be constant during the first and second periods, respectively, nor should it be understood that the first and second electric fields must differ in magnitude or in duration. The only requirement is that the second address pulse (i.e., the time integral of the voltage used to generate the second electric field applied during the second period) be greater than the first address pulse.

The present invention also provides a method for driving an electrophoretic medium comprising a fluid and at least a first type of particles arranged in the fluid, the method comprising:

(a) applying a first addressing pulse to the medium, thereby causing a first type of particle to move in a first direction relative to the electric field; and

(b) applying a second address pulse to the medium that is greater than the first address pulse but has the same polarity as the first address pulse, thereby causing the first species of particles to move in an opposite direction relative to the electric field,

The present invention also provides an electrophoretic display capable of presenting multiple different colors, the display comprising an electrophoretic medium having a fluid and a plurality of particles arranged in the fluid, the display further comprising first and second electrodes arranged on opposite sides of the electrophoretic medium, wherein when a first addressing pulse is applied to the electrophoretic medium, the particles move toward the first electrode, and when a second addressing pulse greater than the first addressing pulse but having the same polarity as the first addressing pulse is applied, the particles move toward the second electrode.

In one form of such an electrophoretic display, when a first addressing pulse is applied, the particles move toward the more positive electrode, but when a second addressing pulse is applied, the particles move toward the more negative electrode. In such an electrophoretic display, the particles generally have a negative charge when no electric field is applied to the particles. The display may also include a second type of particle that is a different color than the first type and that moves toward the more negative electrode regardless of whether the first or second addressing pulse is applied.

For convenience, the aforementioned media and display of the present invention are hereinafter referred to as charge-switching particles or CSP media and display of the present invention.

In another aspect, the present invention provides an electrophoretic medium comprising a fluid and first, second, and third classes of particles disposed in the fluid. The first class of particles carries an electric charge of one polarity, while the second and third classes of particles carry an electric charge of an opposite polarity. The characteristics of the first, second, and third classes of particles are such that the particle-particle interaction between the first class of particles and the second class of particles is less than the particle-particle interaction between the first class of particles and the third class of particles. When a first addressing pulse is applied to the electrophoretic medium, the first and third classes of particles move in one direction relative to the electric field, while the second class of particles moves in an opposite direction relative to the electric field. When a second addressing pulse that is greater than the first addressing pulse but has the same polarity as the first addressing pulse is applied to the electrophoretic medium, the first class of particles moves in the one direction relative to the electric field, while the second and third classes of particles move in the opposite direction relative to the electric field.

In such an electrophoretic medium, one way to control the interactions between the first, second, and third classes of particles is by controlling the type, amount, and thickness of the polymer coating on the particles. For example, in order to control the particle properties so that the particle-particle interaction between the first and second classes of particles is less than the particle-particle interaction between the first and third classes of particles, the second class of particles may be subjected to a polymeric surface treatment, while the third class of particles may not be subjected to a polymeric surface treatment or may be subjected to a polymeric surface treatment that has a lower material coverage per unit area of the particles than the second class of particles. More generally, the Hamaker constant (a measure of the strength of the van der Waals interaction between two particles, the potential being proportional to the Hamaker constant and inversely proportional to the sixth power of the distance between the two particles) and/or the inter-particle spacing may need to be adjusted by appropriate selection of the polymer coatings for the three classes of particles.

On the other hand, the present invention provides an electrophoretic display capable of presenting a variety of different colors, the display comprising an electrophoretic medium and a first and second electrode arranged on opposite sides of the electrophoretic medium. The electrophoretic medium contains a fluid and a plurality of first class particles having a negative charge, a plurality of second class particles having a positive charge, and a plurality of third class particles having a positive charge. The particle pair interactions (Coulomb and attractive non-Coulomb) between the first class particles and the second class particles are less than the particle pair interactions (Coulomb and attractive non-Coulomb) between the first class particles and the third class particles. With a first addressing pulse, the first and third class particles move toward the more positive electrode, while the second class particles move toward the more negative electrode. However, with a second addressing pulse greater than the first addressing pulse, the first class particles move toward the more positive electrode or remain near the more positive electrode, while the third class particles move toward the more negative electrode and the second class particles remain near the more negative electrode.

For reasons that will appear below, for convenience, these electrophoretic media and displays of the present invention are referred to as spot color or SC media and displays of the present invention.

In another aspect, the present invention provides an electrophoretic medium comprising a fluid and first, second, and third types of particles arranged in the fluid. The fluid is dyed a first color. The first type of particles are light-scattering and carry an electric charge of one polarity, while the second and third types of particles are non-light-scattering and have second and third colors, respectively, that are different from the first color and from each other, and carry electric charges of opposite polarities. The properties of the first, second, and third types of particles are such that the particle-particle interaction between the first type of particles and the second type of particles is less than the particle-particle interaction between the first type of particles and the third type of particles. When a first addressing pulse is applied to the electrophoretic medium, the first and third types of particles move in one direction relative to the electric field, while the second type of particles move in an opposite direction relative to the electric field. When a second addressing pulse that is greater than the first addressing pulse but has the same polarity as the first addressing pulse is applied to the electrophoretic medium, the first type of particles move in the one direction relative to the electric field, while the second and third types of particles move in the opposite direction relative to the electric field. When a third addressing pulse greater than the second addressing pulse but having the same polarity as the second addressing pulse is applied to the electrophoretic medium, the first type of particles moves in the opposite direction relative to the electric field, while the second and third types of particles continue to move in the opposite direction relative to the electric field.

The present invention also provides an electrophoretic display capable of displaying multiple different colors, the display comprising an electrophoretic medium and first and second electrodes disposed on opposite sides of the electrophoretic medium. The electrophoretic medium comprises a fluid dyed with a first color; a plurality of light-scattering first type particles having a negative charge; a plurality of non-light-scattering second type particles having a second color and a positive charge; and a plurality of non-light-scattering third type particles having a third color and a positive charge. The particle pair interactions (Coulombic and attractive non-Coulombic, which can be adjusted as described above with respect to the SC medium and display of the present invention) between the first and second types of particles are less than the particle pair interactions between the first and third types of particles. When a first addressing pulse is applied to the display, the first and third type of particles move toward the more positive electrode, while the second type of pigment particles move toward the more negative electrode. When a second addressing pulse, greater than the first addressing pulse, is applied to the display, the first type of particles move toward or remain near the more positive electrode, and the third type of particles move toward the more negative electrode, while the second type of particles remain near the more negative electrode. When a third addressing pulse, which is greater than the second addressing pulse, is applied to the display, the first type of particles moves towards the more negative electrode.

For reasons that will appear below, for convenience, these electrophoretic media and displays of the present invention may be referred to hereinafter as full color or FC media or displays of the present invention.

Finally, the present invention provides an electrophoretic medium comprising a fluid and at least one type of charged particles disposed in the fluid and capable of moving through the fluid when an electric field is applied to the medium, the medium further comprising a charge control adjuvant capable of transferring a relatively positive charge to the charged particles, wherein the charge control adjuvant is a metal salt of a carboxylic acid, wherein the metal is selected from the group consisting of lithium, magnesium, calcium, strontium, rubidium, barium, zinc, copper, tin, titanium, manganese, iron, vanadium and aluminum.

The present invention extends to front-plane laminates, reverse front-plane laminates with dual release sheets, or electrophoretic displays comprising the electrophoretic medium of the present invention. The displays of the present invention can be used in applications where prior art electro-optical displays are used. Thus, for example, the present invention can be used in e-book readers, portable computers, tablet computers, mobile phones, smart cards, signage, watches, shelf labels, and flash drives.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 of the accompanying drawings is a schematic cross-sectional view of an electrophoretic display device according to the present invention.

2 is a highly schematic cross-sectional view of an electrophoretic display with two unobstructed electrodes illustrating the displacement of charged species within the electrophoretic fluid during switching.

3A and 3B are plots showing the electric potential within an electrophoretic medium as a function of distance in the direction of an applied electric field.

4 is a highly schematic cross-sectional view similar to FIG. 2 of an electrophoretic display with two unobstructed electrodes illustrating the displacement of charged species and the additional flow of electrochemically generated ions during switching.

5A and 5B are schematic cross-sectional views (not to scale) of a negatively charged electrophoretic particle and surrounding fluid, and illustrate hypotheses regarding the charge transport pattern near the particle.

FIG6 is a micrograph showing the movement of negatively charged white pigments in some of the experiments described below.

7 is a highly schematic cross-sectional view of an electrophoretic display similar to FIGs. 2 and 4 and illustrating the displacement of charged species and the additional flow of electrochemically generated ions during switching.

8A and 8B are schematic cross-sectional views of a packaged CSP electrophoretic display of the present invention showing various optical states of the display under first and second addressing pulses.

FIG. 9 is a graph illustrating colors that can be produced by a CSP electrophoretic display of the present invention (such as that shown in FIGs. 8A and 8B ).

10A and 10B are schematic cross-sectional views of a packaged SC electrophoretic display of the present invention showing various optical states of the display under first and second addressing pulses.

11A-11D are schematic cross-sectional views of a packaged FC electrophoretic display of the present invention showing various optical states of the display under addressing pulses of first, second, third, and opposite polarity.

FIG12 is a graph showing the effect of adding a small proportion of aluminum di(tert-butyl)salicylate as a charge control agent on controlling the charging of the pigment in the electrophoretic fluid of the present invention; and

13A and 13B are L*b* plots of colors obtained from certain displays of the present invention, as described in Example 2 below.

FIG. 14 is a graph showing driving waveforms applied to a display of the present invention in Example 3 below, and the resulting L* and b* values.

15A-15J are plots showing various waveforms used for the experiments described in Example 6 below.

16A-16J are plots of L*, a*, and b* values as a function of time obtained using the waveforms of FIGs. 15A-15J, respectively.

FIG. 17 is a plot of the a*b* plane of a conventional L*a*b* color space, illustrating the gamut of colors obtained in an experiment as described in Example 6 below.

18 and 19 are plots of the a*b* plane similar to FIG. 17 , but showing the gamut of colors obtained in experiments as described in Examples 7 and 8, respectively, below.

20 is a plot similar to FIG. 14 , but showing the results obtained with the modified waveform applied to the display in Example 9 below.

21 is a graph of the L*b* plane of the CIE L*a*b* color space showing the improved color obtained using the improved waveform shown in FIG. 20 .

FIG22 is a schematic diagram of a DC balanced drive scheme as described in Example 9 below.

FIG23 is a plot showing a simple square wave waveform and the resulting pulse potential as described in Example 9 below;

24 is a plot of voltage versus time for the waveform used for the black to yellow transition in Example 9 below.

FIG. 25 is a graph showing batch variations in optimal driving voltage obtained in the experiment described in Example 9 below.

FIG26 shows a plot of voltage versus time for several picket fence waveforms used in Example 9 below.

FIG. 27 is a graph showing L* and b* values for a white state obtained using the picket fence waveform shown in FIG. 26 .

DETAILED DESCRIPTION

As described above, the present invention provides a variety of types of color electrophoretic media and displays. However, all of these types of electrophoretic media and displays rely on colored particles that move in one direction of the electric field when the medium or display is driven by low addressing pulses, and move in the opposite direction of the electric field when the medium or display is driven by higher addressing pulses. The reversal of the direction of movement of the colored particles with increasing addressing pulses may result from an actual reversal of the polarity of the charge on the colored particles (as in the CSP media and displays of the present invention), or from colored particles that form part of an aggregate with a second particle at low addressing pulses but detach from the aggregate at high addressing pulses (as in the SC and FC media and displays of the present invention).

It should be noted that when using subtractive primary color materials (i.e., cyan, magenta, and yellow) to present a mixed color, regardless of whether the three colored materials are present in the same stacked layer of the electrophoretic display or in different stacked layers, the light must selectively pass through at least two of the colored materials before being reflected back to the observer through the white light reflector (if all three materials are light-transmissive) or through a third backscattering material. The third colored material can be light-transmissive or light-reflective, as described in more detail below. Therefore, when the subtractive primary color materials are used in the media or displays of the present invention, at least two of the colored materials must be light-transmissive and substantially free of backscattering. Thus, for example, in order to present a color such as red or blue, the magenta pigment intended to absorb green light must propagate the blue and red light to the underlying colored materials before it is scattered back to the observer.

In areas where (for example) green light will not be absorbed, the magenta material that absorbs green light must be removed from the light path, which extends from the viewing surface of the display to the position where the light is scattered back to the observer. The removal of this colored material can be achieved by concentrating the colored material in only a portion of each pixel area when it is not desired to see the colored material (thereby reducing its hiding power), and spreading the colored material throughout the entire pixel area when it is desired to absorb the maximum amount of light. Hereinafter, spatially concentrating the colored material to reduce its area hiding power is referred to as closing the shutter on the material. In the media and displays of the present invention, when viewed from the viewing surface of the display, instead of removing the unwanted pigment particles from the light path by closing the shutter, the unwanted pigment particles are hidden behind light scattering particles.

The display of the present invention can reproduce the appearance of high-quality color printing in this way. This high-quality printing is usually achieved using at least three colorants in a subtractive primary color system, usually cyan/magenta/yellow (CMY) and optionally black. It is not generally recognized that the so-called three-color CMY printing system is actually a four-color system, the fourth color being a white background provided by the surface of a substrate (paper or the like) to which the colorant is applied and which performs the function of reflecting the light filtered by the subtractive colorant back to the observer. Because there is no similar background color in the essentially opaque electrophoretic medium (unless used in shutter mode), the non-shutter mode electrophoretic medium should be able to adjust four colors (white and the three primary colors, the three primary colors are usually cyan, magenta and yellow, or red, green and blue). Optionally, black material can also be included, but it is also possible to present black by a combination of cyan, magenta and yellow colors.

Before describing in detail the preferred electrophoretic media and displays of the present invention, some general guidance is given as to the materials used in such media and displays and their preferred methods of preparation.

The materials and methods used to prepare the media and displays of the present invention are generally similar to those used to prepare similar prior art media and displays. For example, as described in U.S. Patent No. 6,822,782 having the same assignee, a typical electrophoretic medium includes a fluid, a plurality of electrophoretic particles disposed in the fluid and capable of moving through the fluid (i.e., translating, and not simply rotating) when an electric field is applied to the fluid. The fluid also typically contains at least one charge control agent (CCA), a charging adjuvant, and a polymeric rheology modifier. These various components will now be described individually.

A: Fluid

The fluid contains charged electrophoretic particles that move through the fluid under the influence of an electric field. Preferred suspending fluids have a low dielectric constant (approximately 2), a high volume resistivity (approximately 10 ¹⁵ Ohm.cm), a low viscosity (less than 5 mPas), low toxicity and environmental impact, a low water solubility (less than 10 parts per million (ppm) if conventional aqueous encapsulation methods are used; however, note that this requirement can be reduced for unencapsulated or certain micro-cell displays), a high boiling point (greater than approximately 90°C), and a low refractive index (less than 1.5). The last requirement arises from the use of a high refractive index scattering (usually white) pigment, whose scattering efficiency depends on the refractive index mismatch between the particles and the fluid.

Organic solvents such as saturated linear or branched hydrocarbons, silicone oils, halogenated organic solvents, and low molecular weight halogen-containing polymers are some useful fluids. In order to adjust its chemical and physical properties, the fluid can contain a single component or can be a mixture of more than one component. Reactants or solvents such as oil-soluble monomers used in the microencapsulation process (if used) can also be included in the fluid.

Useful organic fluids include, but are not limited to, aliphatic hydrocarbons such as, but not limited to, saturated or unsaturated hydrocarbons (such as, but not limited to, dodecane, tetradecane, the Isopar (registered trademark) series (of Exxon, Houston, Texas), Norpar (registered trademark) (a series of normal paraffin liquids), Shell-Sol (registered trademark) (of Shell, Houston, Texas), and Sol-Trol (registered trademark) (of Shell), naphtha, and other petroleum solvents; silicone oils (such as, but not limited to, octamethylcyclosiloxane and higher molecular weight cyclosiloxanes, poly(methylphenylsiloxane), hexamethyldisiloxane, and polydimethylsiloxane; vinyl ethers such as cyclohexyl vinyl ether and Decave (registered trademark of International Flavors & Fragrances, Inc., New York, NY); aromatic hydrocarbons such as toluene; and halogenated materials including, but not limited to, tetrafluorodibromoethylene, tetrachloroethylene, chlorotrifluoroethylene, 1,2,4-trichlorobenzene and carbon tetrachloride as well as perfluorocarbons or partially fluorinated hydrocarbons.

In some electrophoretic media of the present invention, it is beneficial for the fluid to contain a light absorbing dye. The dye must be soluble or dispersible in the fluid, but generally cannot be dissolved in the other components of the microcapsule. The choice of dye material is very flexible. The dye can be a pure compound, or a mixture of dyes can be used to obtain a specific color, including black. The dye can be fluorescent, which can form a display whose fluorescent properties depend on the position of the particles. The dye can be photosensitive, which changes to another color or becomes colorless under irradiation with visible light or ultraviolet light, which provides another way to obtain a light response. The dye can also be polymerizable by, for example, thermal, photochemical or chemical diffusion processes to form a solid absorbing polymer in the boundary shell.

Many dyes can be used in electrophoretic media. Important dye properties include light resistance, solubility or dispersibility in fluids, color, and cost. Dyes are generally selected from nitrogenous, azomethine, fluorescein, anthraquinone, and triphenylmethane dye classes and can be chemically modified to increase their solubility in fluids and reduce their adsorption to particle surfaces.

B: Electrophoretic particles

The electrophoretic particles used in the media and displays of the present invention are preferably white, black, yellow, magenta, cyan, red, green or blue, but other (special) colors may also be used. The choice of such particles is very flexible. For the purposes of the present invention, electrophoretic particles are any particles that are insoluble in the fluid and are charged or capable of acquiring an electric charge (i.e., have or are capable of acquiring an electrophoretic mobility). In some cases, this mobility is zero or close to zero (i.e., the particles do not move). The particles may be, for example, non-derivatized pigments or dyed (laked) pigments, or any other components that are charged or capable of acquiring an electric charge. What generally needs to be considered for electrophoretic particles are their optical properties, electrical properties, and surface chemistry. The particles may be organic or inorganic compounds, and they may absorb light or scatter light, i.e., the particles used in the present invention may include scattering pigments, absorbing pigments, and luminescent particles. The particles may be retroreflective or they may be electroluminescent, such as zinc sulfide particles, or they may be photoluminescent.

Electrophoretic particles can have any shape, i.e., spherical, flat-like, or needle-shaped. Scattering particles typically have a high refractive index, a high scattering coefficient, and a low absorption coefficient, and can be composed of inorganic materials such as rutile (titanium dioxide), anatase (titanium dioxide), barium sulfate, zirconium oxide, kaolin, or zinc oxide. Other particles are absorptive, such as carbon black or colored organic or inorganic pigments used in paints or inks. Reflective materials, such as metal particles, can also be used. Useful particle diameters can range from 10 nm to about 10 μm, but for light scattering particles, a particle diameter of not less than about 200 nm is preferred.

Useful raw pigments for electrophoretic particles include, but are not limited to, PbCrO ₄ , Cyan Blue GT55-3295 (American Cyanamide Corporation, Wayne, New Jersey), Cibacron Black BG (Ciba Corporation, Newport, Delaware), Cibacron Turquoise Blue G (Ciba), Cibalon Black BGL (Ciba), Orasol Black BRG (Ciba), Orasol Black RBL (Ciba), Acetamine Black, CBS (EI du Pont de Nemours and Company, Wilmington, Delaware, hereinafter referred to as DuPont), Crocein Scarlet N Ex (DuPont) (27290), Fiber Black VF (DuPont) (30235), Luxol Fast Black L (DuPont) (Solv. Black 17), Nirosine Base No. 424 (DuPont) (50415B), Ink Black BG (DuPont) (Solv. Black 16), Rotalin Black RM (DuPont), Sevron Brilliant Red 3B (DuPont); Basic Black DSC (Dye Specialties), Hectolene Black (Dye Specialties), Azosol Brilliant Blue B (GAF, Dyes and Chemical Division, Wayne, New Jersey) (Solv. Blue 9), Azosol Brilliant Green BA (GAF) (Solv. Green 2), Azosol Fast Brilliant Red B (GAF), Azosol Fast Orange RAConc. (GAF) (Solv. Orange 20), Azosol Fast Yellow GRA Conc.(GAF)(13900A), Basic Black KMPA(GAF), Benzofix Black CW-CF(GAF)(35435), Cellitazol BNFV Ex Soluble CF(GAF)(Disp. Black 9), Celliton Fast Blue AF Ex Conc.(GAF)(Disp. Blue 9), Indigo Black IA(GAF)(Basic Black 3), Diamine Black CAP Ex Conc.(GAF)(30235), Diamond Black EAN Hi Con.CF(GAF)(15710), Diamond Black PBBA Ex(GAF)(16505); Direct Deep Black EA Ex CF(GAF)(30235), Hansa Yellow G(GAF)(11680); Indanthrene Black BBK Powd.(GAF)(59850), Indocarbon CLGS Conc.CF(GAF)(53295), Katigen Deep Black NND Hi Conc.CF (GAF) (15711), Rapidogen Black 3G (GAF) (Azo Black 4); Sulfone Blue Black BA-CF (GAF) (26370), Zambezi Black VD ExConc. (GAF) (30015); Rubanox Red CP-1495 (Sherwin-Williams Company, Cleveland, Ohio) (15630); Raven 11 (Columbian Carbon Company, Atlanta), (carbon black aggregates with a particle size of approximately 25 μm), Statex B-12 (Columbian Carbon Company) (furnace black with an average particle size of 33 μm), Green 223 and 425 (Shepherd Color Company, Cincinnati, Ohio, 45246); Black 1, 1G and 430 (Shepherd); Yellow 14 (Shepherd); Krolor Yellow KO-788-D (Dominion Color Company, North York, Ontario Color Company; KROLOR is a registered trademark); Red Synthesis 930 and 944 (Alabama Pigment Company, Green Pond, Alabama 35074), Krolor Orange KO-786-D and KO-906-D (Dominion Color Company); Green GX (Bayer); Green 56 (Bayer); Light Blue ZR (Bayer); Fast Black 100 (Bayer); Bayferrox 130M (Bayer, BAYFERROX is a registered trademark); Black 444 (Shepherd); Light Blue 100 (Bayer); Light Blue 16 (Bayer); Yellow 6000 (First Color Co., Ltd., 1236-1, Jeongneung-dong, Siheung, Gyeonggi-do, Korea 429-450), Blue 214 and 385 (Shepherd); Blue Violet 92 (Shepherd); and Chrome Green.

Electrophoretic particles can also include laked or dyed pigments. Laked pigments are particles on which dyes are deposited or dyed. Lake dyes are metal salts of readily soluble anionic dyes. These are nitrogen-containing triphenylmethane or anthraquinone dyes containing one or more sulfonic acid or carboxylic acid groups. They are usually precipitated on the substrate by calcium, barium or aluminum salts. Typical examples are peacock blue lake dye (CI Pigment Blue 24) and Persian Orange (CI Acid Orange 7 lake dye), Black M Toner (GAF) (a mixture of carbon black and black dye precipitated on the lake dye).

Preferably, the pigments of the three subtractive primary colors (yellow, magenta, and cyan) have high extinction coefficients and sufficiently small particle sizes so that incident light is not substantially scattered.

Particularly preferred raw pigment particles of the present invention are the black spinel described in U.S. Patent No. 8,270,064; titanium dioxide, preferably with a silica, alumina or zirconium oxide coating; red: Pigment Red 112, Pigment Red 179, Pigment Red 188 and Pigment Red 254; green: Pigment Green 7; blue: Pigment Blue Violet 23; yellow: Pigment Yellow 74, Pigment Yellow 120, Pigment Yellow 138, Pigment Yellow 139, Pigment Yellow 151, Pigment Yellow 155, and Pigment Yellow 180; magenta: Pigment Blue Violet 19, Pigment Red 52:2 and Pigment Red 122; cyan: Pigment Blue 15:2, Pigment Blue 15:3, Pigment Blue 15:4 and Pigment Blue 15:6.

Additional pigment properties that may be relevant are particle size distribution and light fastness. Composite particles (ie polymeric particles containing smaller pigment particles or dyes) may be used with the present invention. The pigments may be surface functionalized as described below, or may be used without functionalization.

It is well known that the physical characteristics and surface properties of electrophoretic particles can be modified by adsorbing different materials on the surface of the particles, or chemically tethering different materials to these surfaces; see U.S. Patent No. 6,822,782, particularly at column 4, line 27 to column 5, line 32. This same U.S. patent indicates that there is an optimal amount of polymer that should be precipitated (too large a proportion of polymer in the modified particles causes an undesirable reduction in the electrophoretic mobility of the particles) and that the structure of the polymer used to form the particle coating is important.

C: Charge control agent

The electrophoretic medium of the present invention typically comprises a charge control agent (CCA) and may comprise a charge director. These electrophoretic medium components typically comprise a low molecular weight surfactant, a polymerizing agent, or a mixture of one or more components and are used to stabilize or otherwise change the sign and/or magnitude of the charge on the electrophoretic particles. CCA is typically a molecule comprising ionic or other polar groups (hereinafter referred to as head groups). At least one of the positive or negative ionic head groups is preferably connected to a non-polar chain (typically a hydrocarbon chain), which is hereinafter referred to as a tail group. It is believed that CCA forms reverse micelles in the internal phase and that it is a small part of the charged reverse micelles, producing conductivity in the very non-polar fluids typically used as electrophoretic fluids.

Reverse micelles contain a highly polar core (typically containing water) whose size varies from 1 nm to tens of nanometers (and can have a spherical, cylindrical or other geometric shape), which is surrounded by the non-polar tail groups of the CCA molecules. Reverse micelles have been widely studied, particularly in ternary mixtures such as oil/water/surfactant mixtures. An example is the isooctane/water/AOT mixture described by Fayer et al. in 2009, No. 131, p. 14704, in J.Chem.Phys. In an electrophoretic medium, three phases can be typically distinguished: the solid particles have a surface, the highly polar phase is distributed in the form of extremely small droplets (reverse micelles), and the continuous phase contains a fluid. When an electric field is applied, both the charged particles and the charged reverse micelles can move through the fluid, so there are two parallel channels for conduction through the fluid (the fluid itself typically has extremely small conductivity).

The polar core of the CCA is believed to influence the charge on the surface through surface adsorption. In electrophoretic displays, this adsorption can be on the surface of the electrophoretic particles, or on the inner wall of the microcapsule (or other solid phase, such as the wall of the microcell), to form structures similar to reverse micelles, which are referred to as hemi-micelles below. When one ion in an ion pair is more strongly attached to the surface than the other (e.g., by covalent bonds), ion exchange between the hemi-micelle and the unbound reverse micelle causes charge separation, where the more strongly bound ion remains associated with the particle, while the weaker bound ion is incorporated into the core of the free reverse micelle.

It is also possible that the ionic materials forming the head groups of CCAs can induce ion pair formation at the particle (or other) surface. Thus, CCAs can perform two basic functions: charge generation at the surface and charge separation from the surface. Charge generation can result from acid-base or ion exchange reactions between a portion of the CCA molecule present in or incorporated into the reverse micelle core or fluid and the particle surface. Therefore, useful CCA materials are materials that can participate in such reactions, or any other charging reactions known in the art.

Non-limiting classes of charge control agents useful in the media of the present invention include organic sulfates or sulfonates, metal soaps, block or comb copolymers, organic amides, organic zwitterions, and organic phosphates and phosphate esters. Useful organic sulfates and sulfonates include, but are not limited to, sodium di(2-ethylhexyl)sulfosuccinate, calcium dodecylbenzenesulfonate, calcium petroleum sulfonate, neutral or basic barium dinonylnaphthalenesulfonate, neutral or basic calcium dinonylnaphthalenesulfonate, sodium dodecylbenzenesulfonate, and ammonium lauryl sulfate. Useful metal soaps include, but are not limited to, basic or neutral barium sulfonated naphthenate, calcium sulfonated naphthenate, salts of carboxylic acids (e.g., naphthenic acid, octanoic acid, oleic acid, oleic acid, stearic acid, and tetradecanoic acid, etc.) of cobalt, calcium, copper, manganese, magnesium, nickel, zinc, aluminum, and iron. Useful block or comb copolymers include, but are not limited to, AB diblock copolymers of (A) 2-(N,N-dimethylamino)ethyl methacrylate quaternized with methyl p-toluenesulfonate and (B) poly(2-ethylhexyl methacrylate), and comb graft copolymers having an oil-soluble tail of poly(12-carboxystearic acid) and a molecular weight of approximately 1800, pendant from an oil-soluble anchoring group of poly(methyl methacrylate-methacrylic acid). Useful organic amides/amines include, but are not limited to, polyisobutylene succinimides such as OLOA371 or 1200 (available from Chevron Oronite, Houston, Texas), or Solsperse 17000 (available from Lubrizol, Wickliffe, Ohio; Solsperse is a registered trademark), and N-vinylpyrrolidone polymers. Useful organic zwitterions include, but are not limited to, lecithin. Useful organic phosphates and phosphate esters include, but are not limited to, sodium salts of mono- and diglycerol phosphates with saturated and unsaturated acid substituents. Useful CCA tail groups include olefin polymers such as poly(isobutylene) with a molecular weight in the range of 200-10,000. The head group can be a sulfonic acid, phosphoric acid, or carboxylic acid or amide, or alternatively an amino group, such as the first, second, third, or fourth ammonium group.

The charge adjuvant used in the media of the present invention will bias the charge on the surface of the electrophoretic particles, as described in more detail below. Such a charge adjuvant may be a Bronsted or Lewis acid or base.

Particle dispersion stabilizers may be added to prevent particle flocculation or adhesion to the capsule or other walls or surfaces. For typical high-resistivity liquids used as fluids in electrophoretic displays, anhydrous surfactants may be used. These include, but are not limited to, glycol ethers, acetylenic glycols, alkanolamides, sorbitol derivatives, alkylamines, quaternary amines, imidazolines, dialkyl oxides, and sulfosuccinates.

D: Polymerization additives

As described in US Pat. No. 7,170,670, the bistability of electrophoretic media can be enhanced by including in the fluid a polymer having an average molecular weight exceeding about 20,000 that is substantially non-absorbent on the electrophoretic particles; poly(isobutylene) is a preferred polymer for this purpose.

In addition, as described in, for example, U.S. Patent No. 6,693,620, particles with fixed charges on their surfaces establish a double layer of opposite charges in the surrounding fluid. The ionic head group of CCA can be an ion pair with the charged groups on the surface of the electrophoretic particle, forming a layer of fixed or partially fixed charged species. Outside this layer is a diffusion layer containing charged (reverse) micelles, which contains CCA molecules in the fluid. In traditional DC electrophoresis, the applied electric field applies a force to the fixed surface charge and an opposite force to the mobile countercharge, so that sliding occurs in the diffusion layer and the particles move relative to the fluid. The potential at the sliding plane is called the zeta potential.

The electrophoretic motion of charged particles in a fluid is described in most textbooks on colloid science. See, for example, Hiemenz, P.C. and Rajagopalan, R., Principles of Colloid and Surface Chemistry, 3rd edition (Marcel Dekker, NY, 1997). In the systems of interest for electrophoretic displays, the dielectric constant is typically low (in the range of 2-10) and the number of ions is small. In this case, the following formula applies:

where ζ is the Zed potential; q is the electrostatic charge on the particle; ε ₀ is the vacuum permittivity; ε _r is the dielectric constant; and a is the particle radius. Note that the particle has a Zed potential of ~50 mV and a radius of ~150 nm, so the electrostatic charge in a medium with a dielectric constant of 2 is only about 10 charge units.

This concludes the general discussion of the composition of the electrophoretic medium and display.Preferred electrophoretic media and displays of the present invention will now be described with reference to the accompanying drawings.

FIG1 of the accompanying drawings is a schematic cross-sectional view of an electrophoretic display (generally designated 100) of the present invention including an encapsulated electrophoretic medium; such a display and its method of manufacture are described in U.S. Patent No. 6,982,178. Display 100 includes a light-transmissive substrate 102, typically a transparent plastic film, such as a sheet of polyethylene terephthalate (PET) having a thickness of about 25 to 200 μm. Although not shown in FIG1 , substrate 102 (whose upper surface forms the viewing surface of the display, as shown in FIG1 ) may include one or more additional layers, such as a protective layer to absorb ultraviolet radiation, a barrier layer to prevent oxygen or moisture from entering the display, and an antireflective coating to enhance the optical properties of the display.

Substrate 102 carries a thin, light-transmitting, conductive layer 104 that serves as the front electrode for the display. Layer 104 can comprise a continuous coating of a conductive material that has minimal intrinsic absorption of electromagnetic radiation in the visible spectrum, such as indium tin oxide (ITO), poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), graphene, or the like, or it can be a discontinuous layer of a material such as silver (e.g., in the form of nanowires or printed grids) or carbon (e.g., in the form of nanotubes), which absorbs or reflects visible light and is present at a surface coverage such that the layer as a whole is effectively transparent.

A layer of electrophoretic medium (generally designated 108) is in electrical contact with the conductive layer 104 via one or more layers 106 of an optional polymer, as described in greater detail below. The electrophoretic medium 108 is shown as an encapsulated electrophoretic medium comprising a plurality of microcapsules. The microcapsules may be held within a polymer binder. When an electric field is applied to the layer 108, negatively charged particles therein migrate toward the positive electrode, while positively charged particles migrate toward the negative electrode, thereby causing the layer 108 to change color to an observer viewing the display through the substrate 102.

While display 100 is shown with an encapsulated electrophoretic layer 108, this is not a necessary feature of the present invention. Layer 108 can be encapsulated or contain sealed or unsealed microcells or microcups, or it can be unencapsulated. When the layer is unencapsulated, the electrophoretic internal phase (electrophoretic particles and fluid) can be located between two planar electrodes, at least one of which is light-transmissive. The spacing between the electrodes can be controlled by using spacers, which can be in the form of ribs or beads. Alternatively, the spacing can be controlled by using microcapsules containing the internal phase; the internal phase can be located both inside and outside the capsules. The internal phase inside and outside the microcapsules does not have to be the same, but in some cases this is preferred. For example, if a capsule containing the same internal phase as the internal phase outside the capsule is used as a spacer, then an observer of the display will not easily discern the presence of the spacer (because the internal phase and the external internal phase will convert to at least approximately the same color).

As described in U.S. Patent Nos. 6,982,178 and 7,012,735, the display 100 also includes a layer 110 of laminating adhesive covering the electrophoretic layer 108. The laminating adhesive makes it possible to construct an electro-optical display by combining two subassemblies, namely a backplane 118 and a front panel 116. The backplane 118 includes an array 112 of pixel electrodes and conductors 114 connected to the pixel electrodes to drive the circuit, while the front panel 116 includes a substrate 102 carrying transparent electrodes 104, an electrophoretic layer 108, a laminating adhesive 110, and optional additional components such as one or more layers 106 of a polymer. To form the final display, the front panel 116 is laminated to the backplane 118 via the laminating adhesive 110. The laminating adhesive can be cured thermally or by actinic radiation (e.g., by UV curing), or can be uncured.

Because the laminating adhesive 110 is in the circuit path from the backplane electrode 112 to the front electrode 104, its electrical properties must be carefully designed. As described in U.S. Patent No. 7,012,735, in addition to the polymer material, the laminating adhesive may contain an ionic dopant, which may be an additive selected from the group consisting of salts, polyelectrolytes, polymer electrolytes, solid electrolytes, conductive metal powders, ferrofluids, non-reactive solvents, conductive organic compounds, and combinations thereof. The volume resistivity of the encapsulated electrophoretic electrolyte of the present invention is typically about 10 ¹⁰ Ohm.cm, and the resistivity of other electro-optical media is generally of the same order of magnitude. Therefore, the volume resistivity of the laminating adhesive is generally about 10 ⁸ to 10 ¹² Ohm.cm at the operating temperature of the display, which is typically about 20°C.

The polymer layer 106 may be a laminating adhesive layer having properties similar to those of the laminating adhesive layer 110 (see, for example, U.S. Patent No. 7,839,564), but because the polymer layer 106 is adjacent to the non-pixelated light-transmissive common electrode 104, its conductivity may be higher than the conductivity of the laminating adhesive layer 110, which is adjacent to the pixelated backplane electrode 112 and is not so conductive as to produce an appreciable current flow from one backplane electrode to its neighboring electrode when the backplane electrode and its neighboring electrode are held at different potentials during display switching. When the polymer layer 106 is a laminating adhesive, it may be used to adhere the electrophoretic layer 108 to the front electrode 104 during manufacture of the front panel, as described in detail in the aforementioned U.S. Patent No. 6,982,178.

Figure 2 schematically illustrates the flow of charged material, which can occur within the electrophoretic medium of the present invention contained in a cell (generally designated 200) in response to an electric field applied by electrodes 202 and 204. The mobile charged species in the electrophoretic fluid are generally shown as charged particles P+ and P- and charged (reverse) micelles RM+ and RM-. When an electric field is applied, the charged species move and, as they move, shield the field inside the cell. If the electrodes are blocked (i.e., the electrodes do not allow electrochemical current to pass), the charged species accumulate at the electrode interface until the field at the midpoint between the electrodes drops to zero. This polarization process can be considered as charging of the interfacial capacitance by conduction through the electrophoretic internal phase (although, as will be appreciated by those skilled in the art, the situation is more complex than the basic situation presented, as the decomposition of neutral reverse micelles into charged micelles will cause charge generation within the internal phase).

The movement of charged species in a configuration such as that shown in FIG2 with blocked electrodes can be simulated using a Poisson-Nernst-Planck system of partial differential equations. Using the Butler-Volmer-Frumkin equations, the effects of electrochemical reactions at the electrodes can be taken into account in this model. FIG3A and FIG3B show the results of this simulation and depict the potential within the electrophoretic inner phase separating the two electrodes as a function of distance from the first electrode. FIG3A shows the evolution of this potential over time when the electrodes are blocked; a large potential drop occurs at the electrode interface, while the potential gradient at the center of the cell becomes zero. Therefore, after the cell is polarized, the net flow of electrophoretic particles (and reverse micelles) ceases as drift and thermal diffusion balance each other. Note that if, after polarization is completed, the electrodes are both connected to ground (or to a common potential), the inner phase will experience an electric field equal to and opposite to the initially applied field. This will cause the erasure of any image on the medium (the so-called kick-back problem).

FIG3B illustrates a case where an electrochemical reaction (charge injection) can occur at the electrode interface. In this case, after initial polarization, the electric field near the electrode becomes high enough to transfer electrons in both directions between the molecules in the internal phase and the electrode. The material is oxidized at the anode and reduced at the cathode. Assuming there is an ample supply of redox material near the electrodes, a steady-state current flows within the cell 200 (and the potential gradient at the center of the cell is non-zero).

FIG4 is a schematic cross-sectional view similar to FIG2 , but illustrating the case where an electrochemical reaction occurs at the electrode interface (discussed just with reference to FIG3B ). FIG4 illustrates a unipolar electrochemical current; that is, ions are generated at one electrode and consumed at the other. Thus, at anode 204, species A loses electrons to form protons, which migrate through the internal phase to cathode 202 as shown by arrows 206 and are reduced to neutral hydrogen species (shown as H in FIG4 , which may be hydrogen gas) at cathode 202.

It is believed (although the present invention is not limited in any way by this view) that one of the electrochemical reactions occurring in the electrophoretic medium of the present invention is water electrolysis, where water is oxidized at the anode to produce protons in pure water, thus:

Anode (oxidation): H ₂ O→1/2O ₂ +2H ⁺ +2e ^- (2)

At the cathode, protons are reduced to produce hydrogen (or other hydrogen radical products):

Cathode (reduction): 2H ⁺ +2e ^- →H ₂ (3)

The net effect of these electrochemical reactions is the transfer of protons from the anode to the cathode with the consumption of water, as shown by the bold arrow 206 in Figure 4. Note the unipolar (one direction) transfer of protons in the opposite direction to the travel of the negatively charged reverse micelles and pigment.

As described above, the present invention provides an electrophoretic medium comprising a fluid and at least a first type of particles disposed in the fluid, wherein the first type of particles are such that when a first electric field is applied to the medium for a first period, thereby applying a first addressing pulse to the medium, the first type of particles move in one direction relative to the electric field, but when a second electric field having the same polarity as the first electric field is applied to the medium for a second period, thereby applying a second addressing pulse greater than the first addressing pulse to the medium, the first type of particles move in an opposite direction relative to the electric field. For a better understanding of the present invention, it is assumed below how pigment particles move in a first direction (i.e., behave as if the particles carry a negative charge) in response to a first addressing pulse and move in a second direction (i.e., behave as if the particles carry a positive charge) in response to a higher second addressing pulse, but the present invention is not limited in any way to this assumption.

FIG5A shows a negatively charged particle 500 having an electrostatic charge of four charge units. In the absence of an electrochemical current, the particle 500 would move in the direction of the dashed arrow in FIG5A . Adsorbed onto the particle 500 is a layer 502 of a high dielectric constant semi-micellar material. (FIGS. 5A and 5B are not drawn to scale, and the shapes of the particle and adsorbed layer are shown as idealized concentric ellipses. In reality, the adsorbed layer is likely polarized and may not actually be a continuous layer or have a constant thickness.)

The material of layer 502 is likely to have similar composition with the core material of reverse micelle, and comprises water.The negative charge 504 of four surface constraints (or surface adsorption) is shown, is contained in the layer 502 of high dielectric constant.The counterion of these four electric charges forms non-connected, micellar diffusion layer (not shown), and it can move in the direction opposite to the direction that particle moves in electric field, because these electric charges are positioned at outside the liquid dynamic sliding envelope around particle.This is the normal state of electrophoretic motion in the suspended liquid of low dielectric constant that comprises reverse micelle charged species.

As shown in Figure 5B, when the current mainly carried by reverse micelles flows through the internal phase, the situation is considered to be different. This electric current can originate from the electrochemical reaction at the electrode or from the displacement of the layer of charge from the adjacent internal phase. Electric current can be considered to be mainly carried by a charge carrier of a sign; This can originate from a specific electrochemical reaction (as described above) or from the different mobility of charge carriers of opposite polarity. In Figure 5B, it is assumed that the charge carrier is mainly positively charged, as in the case of the electrolysis of water at neutral or acidic pH.

As previously mentioned, conduction in highly non-polar fluids is likely to be mediated by charged reverse micelles or charged electrophoretic particles. Therefore, any electrochemically generated protons (or other ions) are likely to be transported by the fluid into the micelle core or adsorbed on the electrophoretic particles. In Figure 5B, positively charged reverse micelles 506 are shown approaching particle 500, and during polarization of the internal phase, positively charged reverse micelles 506 travel in a direction opposite to the direction of travel of particle 500. Reverse micelles approaching much larger particles can travel past the particles without interacting, or can be contained to the double layer surrounding the positively charged particles. The double layer includes a charge diffusion layer with an increased counterion concentration and a semi-micelle surface adsorption coating on the particle; in the latter case, the reverse micelle charge will be associated with the particle within a sliding envelope that defines the boundary potential of the particle as described above. In the embodiment of the present invention, the electrochemical current of positively charged ions is used to determine the electronegative particles. For example, the electrochemical current of positively charged ions is used to determine the charge bias of negatively charged particles. This is due to the type of ion exchange driven by the transmission of ions (such as ions 508 in Fig. 5 B) to micelle cores at the particle surface. It is also assumed that reverse micelles may be sent from particles, as shown in the reverse micelles 510 of Fig. 5 B. Therefore, the static charge on the particle is the size of the electrochemical current and the function of the holdup time near the positive charge of the particle surface. This holdup time is likely to be affected by the chemical property of the micelle in particle size, its surface chemistry and fluid and the semi-micelle adsorbed on the particle surface.

FIG6 is a micrograph showing the results of an experiment intended to demonstrate the above hypothesis by tracking particle motion. FIG6 shows interdigitated electrodes arranged on a glass surface, separated by a 25 μm gap. The cell was constructed with the interdigitated electrodes as one boundary and a flat piece of glass as a second boundary. Between the two boundaries was placed an internal phase comprising a fluid (Isopar V), CCA (Solsperse 17000), and a plurality of white (light-scattering) polymer-coated titanium dioxide particles. The polymer-coated titanium dioxide particles were negatively charged and had a barrier potential of approximately -40 mV; the polymer-coated titanium dioxide was prepared generally as described in Example 28 of U.S. Patent No. 6,822,782. When one electrode was grounded and its neighboring electrode was at +40 V, the white particles accumulated at the more positive electrode. As the voltage increased (and the current accordingly increased), the white particles moved toward the less positive electrode, until at +160 V, the majority of the white particles accumulated around the more negative electrode. Due to the thickness of the experimental cells used, the voltages used for this experiment were much greater than those required for commercially available electrophoretic displays, which use much thinner electrophoretic medium layers.

FIG7 is a schematic cross-sectional view of an electrophoretic display (generally designated 700) similar to FIG4 embodying the above principles. An electrophoretic medium layer 706 is sandwiched between polymeric laminating adhesive layers 708 and 710; if the (encapsulated) internal phase fluid is applied directly to one of the electrodes 704 and 712, only a single laminating adhesive layer needs to be used. The electrochemical current flowing through the device is shown with bold arrows. As described above, the laminating adhesive can be doped with mobile ionic species, and the (unipolar) current flowing between the anode 712 and the cathode 704 is not necessarily carried by common ions. Thus, cationic species B+ are shown to span the boundary between the laminating adhesive layer 710 and the electrophoretic layer 706, where B+ may be a proton or other positive ion. Similarly, particles C+ are shown to span the boundary between the electrophoretic layer 706 and the laminating adhesive layer 708. In addition, C+ may be a proton or other cation. At the cathode 704 itself, protons are shown to be reduced, but this is not a necessary feature of the present invention.

As described above, in the present invention, the current flowing in the device is not necessarily electrochemical in origin; provided that this displacement current is unipolar, a displacement current containing the ions B+ and C+ is sufficient to induce the reverse motion of negatively charged particles. The above argument assumes that the electrochemical current or displacement current is unipolar in positively charged ions and thus induces a reversal of direction in negatively charged particles. This argument also applies to unipolar electrochemical currents in negatively charged ions, in which case it is the positively charged particles that reverse direction. However, in practice, it has been found that it is easier to induce a reversal of direction in negatively charged particles.

Furthermore, the unipolar nature of the current flow is not a requirement of the present invention, although the observed phenomena (eg, the behavior recorded in FIG. 6 ) are readily understood if unipolar current flow is assumed.

Various embodiments of the electrophoretic media and displays of the present invention and their use in forming color images will now be described in more detail. In these embodiments, the following general conversion mechanism is used:

(A) Conventional electrophoretic motion, in which particles with associated charges (surface-bound or adsorbed) move in an electric field;

(B) Traditional racing particles, in which particles with higher zeta potentials move faster than particles with lower zeta potentials (e.g., as described in U.S. Pat. No. 8,441,714 and earlier patents cited therein)

(C) Coulombic aggregation between particles of opposite sign such that the aggregates move in an electric field according to their net charge in the absence of an electrochemical (or displacement) current, but wherein the aggregates are separated by modulation of the charge on at least one particle by means of an electrochemical (or displacement) current;

(D) The direction of motion of at least one type of particle is reversed due to an electrochemical (or displacement) current.

The waveform used to drive the display of the present invention can adjust the electrical pulses provided to the display using any one or more of at least four different methods:

(i) Pulse width modulation, in which the duration of a specific voltage pulse is varied;

(ii) duty cycle modulation, wherein a pulse train is provided whose duty cycle varies with the desired pulse;

(iii) voltage modulation, where the supplied voltage is varied to match the desired pulse; and (iv) a DC voltage offset applied to an AC waveform (the DC voltage offset itself has a zero pulse)

Which method is used is selected according to the specific form of the intended application and the display used. As mentioned above, the term pulse in this article is used to represent the integral of the voltage applied to the time during the process of the medium or display being addressed. As also mentioned above, for the change of direction of a type of particles (typically negatively charged) or the decomposition of Coulomb aggregates, a certain electrochemical or displacement current is required, so when a high pulse is applied to the medium of the display, the addressing voltage must be sufficient to provide this current. Lower pulses can be provided by lower addressing voltages or by reducing the addressing time at the same higher voltage. As mentioned above, there is such a polarization phase during which the electrochemical current is not at its maximum value, and the particles move according to their original charge (i.e., the charge carried before any addressing voltage is applied to the medium or display). Therefore, for example, polarizing the electrophoretic medium but not causing the duration of high steady-state current, low pulse addressing at high voltage is ideal.

Figures 8A and 8B are schematic cross-sections illustrating various possible states of a single microcapsule 800 (sealed or unsealed microcells or other similar enclosures may alternatively be used) containing a fluid 806 dyed with a yellow dye (uncharged yellow particles may be substituted for the yellow dye). Disposed within the fluid 806 are positively charged light-transmitting magenta particles 802 and negatively charged white particles 804. As shown in Figures 8A and 8B, on the upper side of the microcapsule 800 is a generally transparent front electrode 810, the upper surface of which (as shown) forms the viewing surface of the display, while on the opposite side of the microcapsule 800 is a pixel electrode or back electrode 812. In Figures 8A and 8B, and in subsequent similar figures, it is assumed that the front electrode 810 is maintained at ground potential (although this is not a necessary feature of the present invention; in some cases, a change in the potential of this electrode is desirable, for example, to provide a higher electric field), and that the electric field across the microcapsule 800 can be controlled by varying the voltage of the back electrode 812.

FIG8A illustrates two possible states of microcapsule 800 when driven by a low pulse. Under this low pulse, particles 802 and 804 undergo conventional electrophoretic motion. As shown on the left-hand side of FIG8A , when the rear electrode 812 is at a positive voltage, white particles 804 move toward the rear electrode 812, while magenta particles 802 are located near the front electrode 810. As a result, microcapsule 800 appears red when viewed against a white background, with the red color resulting from the combination of the magenta particles and the yellow dye, while the white background is provided by the white particles. As shown on the right-hand side of FIG8A , when the rear electrode 812 is at a negative voltage, white particles 804 move closer to the front electrode 810, and microcapsule 800 appears white (both the yellow fluid 806 and the magenta particles 802 are obscured by the white particles 804).

FIG8B illustrates two possible states of microcapsule 800 when driven by a high pulse. Under this high pulse, magenta particles 802 continue to experience conventional electrophoretic motion. However, white particles 804 experience charge reversal and behave as if they are positively charged. Therefore, as shown in the left-hand side of FIG8B , when the rear electrode 812 is at a positive voltage, the magenta particles 802 are located near the front electrode 810, while the white particles 804 also move toward the front electrode and are arranged to be deposited directly below the magenta particles, so that the microcapsule 800 displays a magenta color (light passing through the magenta particles 802 from the viewing surface is reflected back from the white particles 804, passes through the magenta particles 802 and outwards through the viewing surface; the white particles 804 obscure the yellow fluid 806). As shown on the right-hand side of FIG8B , when the back electrode 812 is at a negative voltage, the white particles 804 move toward the back electrode 812 and are positioned above the magenta particles 802, so that the microcapsule appears yellow (light transmitted from the viewing surface is filtered by the yellow fluid 806 and reflected back from the white particles 804, passes through the yellow fluid 806, and out through the viewing surface; the white particles 804 obscure the magenta particles 802). Thus, the complementary color pair white/red is produced at low address pulses, while the color pair yellow/magenta is produced at high address pulses.

Obviously, other combinations of colored particles and dyes can also replace the white and magenta particles used in Figures 8A and 8B, as well as the yellow dye. A particularly preferred embodiment of the present invention is one in which one of the dyes or particles has one of the additive primary colors, while the other is a complementary subtractive primary color. Thus, for example, the dye can be cyan and the two particles are white and red. The four states provided by this combination are white and black (at low pulse drive) and red and cyan (at high pulse drive). Similarly, green/magenta and blue/yellow combinations of dyes and particles can be used together with white particles.

Figures 9, 10A, and 10B illustrate another embodiment of the present invention, which is intended to provide black, white, and a spot color. It is desirable to be able to provide intermediate gray levels between white and black, between white and the spot color, and between black and the spot color in such a display. Figures 10A and 10B illustrate a display in which the spot color is yellow.

FIG9 is a graph showing the CIE L* (lightness) and CIE C* (chromaticity) values achievable from this type of display. The display can be written in black, white, or a spot color (assuming yellow), and intermediate states of gray (arrow 904), black/yellow (arrow 906), and white/yellow (arrow 902) can be achieved. For applications such as e-book readers with spot (highlight) colors in addition to black and white, being able to achieve these intermediate states is important for text and image presentation.

Figures 10A and 10B schematically illustrate a variety of possible states of a display of the present invention. The display comprises a microcapsule 1000 having a grounded front electrode 1010 (as shown, its upper surface provides the viewing surface of the display) and a rear electrode 1012. All of these integers are substantially the same as the corresponding integers in Figures 8A and 8B. However, the fluid in the microcapsule 1000 is not dyed, but is provided with three types of particles, i.e., positively charged black particles 1008, positively charged colored particles 1002 (shown as yellow), and negatively charged white particles 1004. The yellow particles 1002 are light-transmissive and preferably substantially free of scattering. The charges shown on particles 1002, 1004, and 1008 are labeled as +2, -3, and +8, respectively, but these are merely for illustrative purposes and do not limit the scope of the present invention.

Black particles 1008 carry a polymer coating (as shown in the bold outlines in Figures 10A and 10B). Yellow particles 1002 do not carry a polymer coating, or the polymer coating carried by yellow particles 1002 has a lower coverage per unit area of the particles than black particles 1008, and white particles 1004 do not carry a polymer coating, or the polymer coating carried by white particles 1004 has a lower coverage per unit area of the particles than black particles 1008. The polymer coating on black particles 1008 ensures that spacing is maintained between black particles 1008 and white particles 1004 so that any Coulomb aggregates formed between particles 1004 and 1008 are weak enough to be separated by low addressing pulses. On the other hand, the absence or minimal presence of polymer on yellow particles 1002 and white particles 1004 enables stronger aggregation between these two types of particles so that the aggregates cannot be separated by low addressing pulses, but can be separated by high addressing pulses, as described in more detail below.

More generally, the Hamaker constant (a measure of the strength of the van der Waals interaction between two particles, the pair potential is proportional to the Hamaker constant and inversely proportional to the sixth power of the distance between the two particles) and/or the inter-particle spacing needs to be adjusted by appropriate choice of polymer coating so that the particle pair interactions (Coulombic and attractive non-Coulombic) are smaller between white and black particles than between white and yellow particles.

The effect of these inter-particle interactions is that, in the absence of addressing pulses, or in the presence of low addressing pulses, Coulombic aggregates form between the yellow particles 1002 and the white particles 1004, and the two particles travel together as a weakly negatively charged unit. Thus, in the absence of electrochemical or displacement ion current, the internal phase fluid within the microcapsule 1000 behaves as if it contains positive black particles and negative yellow particles. On the other hand, as described above, under high-pulse drive conditions, the white particles 1004 and (possibly) the yellow particles 1002 move to a more positively charged state through the flow of electrochemically generated or displacement-generated positive ions, and the strength of the Coulombic aggregates formed between the white and yellow particles weakens. The electric field is now sufficient to separate the two types of particles, so that the yellow particles 1002 now move toward the more negative electrode along with the black particles 1008, while the white particles 1004 (although more weakly negatively charged than in the absence of current through the internal phase) still migrate toward the more positive electrode.

Figures 10A and 10B illustrate the effects of these changes. The left-hand side of Figure 10A shows a microcapsule 1000 under low pulse drive conditions and with its rear electrode 1012 positive relative to the front electrode 1010. Black particles 1008 migrate to the front electrode 1010, while white particle/yellow particle aggregates (which remain intact under low pulse drive) migrate to the rear electrode 1012. The black particles mask the white and yellow particles, so the microcapsule appears black. The right-hand side of Figure 10A shows a microcapsule 1000 under low pulse drive conditions and with its rear electrode 1012 negative relative to the front electrode 1010. Black particles 1008 migrate to the rear electrode 1010, while white particle/yellow particle aggregates migrate to the front electrode 1010. Therefore, the microcapsule 1000 appears yellow.

On the other hand, the left-hand side of FIG. 10B shows a microcapsule 1000 under high-pulse drive conditions and with its rear electrode 1012 positive relative to the front electrode 1010. High-pulse drive conditions disrupt the white/yellow particle aggregates, so the white, yellow, and black particles all move independently of each other. Accordingly, the black and yellow particles move closer to the front electrode 1010, while the white particles move closer to the rear electrode 1012, and the microcapsule appears black; the light-transmitting yellow particles do not affect the black color in this state because the light-absorbing black particles absorb all light incident on the viewing surface and also mask the white particles. The right-hand side of FIG. 10B shows a microcapsule 1000 under high-pulse drive conditions, but with its rear electrode 1012 negative relative to the front electrode 1010. The black and yellow particles move closer to the rear electrode 1012, while the white particles move closer to the front electrode 1010 and mask the black and yellow particles. As a result, the microcapsule appears white.

Therefore, under low pulse driving conditions, the microcapsule 1000 can switch between black and yellow states, while under high pulse driving conditions, the microcapsule can switch between black and white states.

The drive sequence for displaying a spot color (yellow in FIG. 10A ) and grayscale is as follows. Using a high-pulse drive condition with the backplane positive (left-hand side of FIG. 10B ), the display is driven to black. Then, using a low-pulse drive condition with the backplane negative (right-hand side of FIG. 10A ), the display is driven to yellow (along arrow 906 in FIG. 9 ). A high-pulse drive condition with the backplane negative (right-hand side of FIG. 10B ) produces a white state from the yellow state (along arrow 902 in FIG. 9 ). Finally, a low-pulse drive condition with the backplane positive (left-hand side of FIG. 10A ) drives from white to black (along arrow 904 in FIG. 9 ) to provide grayscale.

11A-11D of the accompanying drawings illustrate various states of a display that utilizes the principles of the display shown in FIG8A, 8B, 10A, and 10B and provides the display with three different levels of driving pulses, wherein each microcapsule is capable of displaying black and white and additive primary colors as well as subtractive primary colors (red, green, blue, cyan, magenta, and yellow). In FIG11A-11D, the particle charges are shown for illustrative purposes only and do not limit the scope of the present invention in any way.

11A-11D illustrate a microcapsule having a grounded front electrode 1110 (as shown, the upper surface of the grounded front electrode 1110 provides the viewing surface of the display) and a rear electrode 1112. All of these integers are substantially the same as the corresponding integers in FIG8A, 8B, 10A, and 10B. Microcapsule 1100 contains a fluid 1108 dyed with a cyan dye. However, within fluid 1108 are disposed three types of particles, namely, positively charged light-transmitting magenta particles 1102, positively charged light-transmitting yellow particles 1104, and negatively charged light-scattering white particles 1106. Magenta particles 1102 bear a polymer coating, whereas white and yellow particles 1104 and 1106 bear no polymer coating or only a thin polymer coating. Thus, under low-pulse drive conditions, as shown in FIG11A , the microcapsule 1100 behaves in a manner completely similar to the microcapsule 1000 shown in FIG10A and 10B , with the white and yellow particles traveling together as a negatively charged aggregate, and the microcapsule 1100 can switch between dark blue (see the left-hand side of FIG11A ) and yellow (see the right-hand side of FIG11A ) states. (The dark blue state results from light entering from the viewing surface, passing through the cyan fluid, being reflected by the white particles, and returning through the cyan fluid and magenta particles.) Because the white and yellow particles aggregate together and provide a weaker yellow color than would be achieved if the yellow particles were between the observer and the white particles, a very short pulse of a higher pulse (not sufficient to reverse the positions of the magenta and white particles) can be used to separate the white particles from the yellow particles to achieve a better yellow color (or, in the state shown on the left-hand side of FIG11A , a better blue color). In the embodiment of the invention shown in Figures 11A-11D, the weakest color may be the complementary color of the particles with a lower positive charge (in Figures 11A-11D, these particles with a lower positive charge are yellow particles and therefore the weakest color is blue).

Under the medium pulse drive condition (see FIG11B ), the behavior of the microcapsule is also completely similar to the behavior under the high pulse drive condition shown in FIG10B ; the aggregation between the white and yellow particles breaks up, and all three types of particles travel independently, so that the microcapsule switches between the black (see the left-hand side of FIG11B ) and white (see the right-hand side of FIG11B ) states. The only difference between FIG10B and FIG11B is that in the latter, the black state is caused by both the magenta and yellow particles arranged near the front electrode 1110, and the light passes through these particles and through the cyan fluid 1108.

When the drive pulse is increased still further (see FIG11C ), the white particles behave as if they are positively charged, and all three pigments migrate toward the more negative electrode, so that when read outward, a continuous magenta, yellow, and white layer is formed from the more negative electrode, see FIG8B . The resulting displayed colors are red (see the left-hand side of FIG11C ; this color is produced by light passing through the magenta and yellow particles, reflecting from the white particles, and returning through the magenta and yellow particles) and cyan (see the right-hand side of FIG11C ; this color is produced by light passing through the cyan fluid 1108, reflecting from the white particles, and returning through the cyan fluid).

The last two colors of microcapsule 1100 are produced by the so-called polarity-reversed state shown in FIG11D . To produce the green state shown on the left-hand side of FIG11D , the microcapsule is first driven with a mid-level pulse and the back electrode 1112 is positive to produce the state shown on the left-hand side of FIG11B . The back electrode is then reversed to negative polarity and the mid-level pulse is still applied for a sufficient time to cause the highly charged magenta particles to move past the yellow and white particles until they are near the back electrode and the microcapsule assumes the state shown on the left-hand side of FIG11D . In this state, light entering the viewing surface passes through the cyan fluid and the yellow particles, reflects from the white particles (which mask the magenta particles) and returns through the yellow particles and cyan fluid, so that the microcapsule appears green.

Similarly, the magenta state shown on the right hand side of Figure 11D is produced by first driving the microcapsule with the mid-level pulse and the back electrode 1112 negative to produce the state shown on the right hand side of Figure 11B, then reversing the back electrode to positive polarity and still applying positive polarity with the mid-level pulse for a sufficient time to cause the highly charged magenta particles to move past the yellow and white particles until they are near the front electrode, and the microcapsule assumes the state shown on the right hand side of Figure 11D. In this state, light entering the viewing surface passes through the magenta particles, reflects from the white particles (which mask the yellow particles) and returns through the magenta particles, so the microcapsule appears magenta.

The electrophoretic medium of the present invention comprises a fluid and at least the following additional components:

(a) first and second particles carrying oppositely charged polarities; typically at least one of the particles, and usually both, carry a polymer surface coating, but as previously described, the possibility of controlling particle-particle interactions in other ways is not excluded. For example, the microcapsule 1100 shown in Figures 11A-11D comprises negatively charged white particles and positively charged magenta particles. The non-white particles are preferably substantially non-scattering (i.e., light-transmitting) and have one of the three subtractive primary colors (yellow, magenta, and cyan);

(b) third particles that may or may not carry a polymer surface coating (or otherwise treated to control particle-particle interactions), the polymer surface coating of the third particles having a lower material coverage per unit area of the particle than the polymer surface coatings of the first and second particles. More generally, the Hamaker constant and/or interparticle spacing can be adjusted by judicious selection of the polymer coating so that the particle pair interactions (Coulombic and attractive non-Coulombic) between particles of the first type and the second type are less than between particles of the first type and the third type. For example, the microcapsule 1100 shown in Figures 11A-11D comprises positively charged yellow particles. The third particles are preferably substantially non-scattering (i.e., light transmissive) and have one of the subtractive primary colors that is different from the first and second pigments;

(c) a dye that is soluble or dispersible in the fluid and has a third subtractive primary color; for example, the microcapsule 1100 shown in Figures 11A-11D contains a cyan dye;

(d) at least one charge control agent;

(e) charging aids; and

(f) Polymerization stabilizers.

In a preferred embodiment of the invention, the first (white) particles are silanol-functionalized scattering materials such as titanium dioxide to which a polymeric material is attached; the second particles are positively charged magenta materials such as dimethylquinacridone coated as described below, and as for the third pigment, if it is cyan, it is a copper phthalocyanine material such as Heliogen (registered trademark) Blue D7110F available from BASF used without coating, or if it is yellow, it is an organic pigment such as Pigment Yellow 180 also used without coating.

The dye in this preferred embodiment is a hydrocarbon (Isopar E) soluble material, which can be an azo dye such as Sudan I or Sudan II or a derivative thereof. Other hydrocarbon soluble dyes such as azomethine (yellow and cyan are readily available) or other materials known in the art can also be used in the following examples. A particularly preferred cyan dye for use in the present invention is represented by the following structure:

Wherein R is the branch or the unbranched hydrocarbon chain that comprises at least six carbon atoms, and it can be saturated or unsaturated.Expectation uses the mixture of multiple dyestuff, for example has the mixture of two or more dyestuffs of the above-mentioned molecular formula of different R groups.The use of this mixture can be provided in the good solubility in the hydrocarbon stream, still allows the single dye molecule to be purified by recrystallization simultaneously.The preparation of these dyestuffs is described in following example 5.

The Zed potential of each particle in the presence of a single CCA (e.g., Solsperse 17000) cannot be ideally configured to switch as described above. A second (or common) CCA can be added to the electrophoretic medium to adjust the Zed potential of each particle. Careful selection of the common CCA can allow the Zed potential of one particle to be changed while the Zed potentials of other particles remain essentially unchanged, which allows both the electrophoretic velocity of each particle and the inter-particle interactions during the switching process to be tightly controlled. Figure 12 is a graph showing the change in the Zed potential of four particles of Solsperse 17000 in Isopar E with the addition of a small proportion of an acidic material (Bontron E-88, available from Orient, Kenilworth, New Jersey, and stated by the manufacturer to be aluminum di-tert-butyl salicylate). The addition of the acidic material shifts the Zed potential of many particles (but not all) to a more positive value. As can be seen from Figure 12, the use of 1% acidic material and 99% Solsperse 17000 (based on the total weight of the two materials) shifts the Zheda potential of Pigment Yellow 180 from approximately -5 mV to approximately +20 mV. The addition of the same proportion of acidic material changes the Zheda potential of polymer-coated white particles (prepared as described in U.S. Patent No. 7,002,728) from approximately -45 mV to approximately -20 mV. However, the Zheda potential of the magenta pigment is not significantly altered by the addition of an aluminum salt. Whether the Zheda potential of a particular particle will be altered by a Lewis acidic material like an aluminum salt will depend on the details of the particle's surface chemistry.

The addition of a basic co-CCA (such as OLOA 371 available from Chevron Corporation, Sam Ramon, Canada) shifts the Zyder potential of the various pigments to more negative values.

The following examples are now given, by way of example only, showing details of particularly preferred materials, methods, conditions and techniques for preparing the media and electrophoretic displays of the present invention.

Example 1

This example illustrates the preparation of a two-particle colored fluid electrophoretic display of the type shown in Figures 8A and 8B of the accompanying drawings.

Part A: Preparation of Magenta Pigment Dispersion

Inkjet fuchsin E 02 VP2621 (15 g), available from Clariant of Basel, Switzerland, was dispersed in toluene. The resulting dispersion was transferred to a 500 mL round-bottom flask and degassed with nitrogen. The reaction mixture was then warmed to 42° C., and when the temperature equilibrated, 4-vinylbenzyl chloride was added and the reaction was allowed to stir overnight at 42° C. under a nitrogen atmosphere. The resulting reaction mixture was allowed to cool to room temperature and then centrifuged to isolate the functionalized pigment. The centrifuge cake was washed with toluene (3 x 250 mL) to produce 14.76 g of fuchsin pigment functionalized with vinyl groups, to which polymer chains could be attached.

To this end, the dry pigment was dispersed in toluene using sonication and rolled on a roller mill. The resulting dispersion was transferred to a 500 mL two-necked round-bottom flask equipped with a large magnetic stir bar and placed in a preheated silicone oil bath maintained at 65°C. Lauryl methacrylate was added to the flask, a Vickers column was attached to serve as an air condenser, and the second neck of the flask was sealed with a rubber septum. The system was purged with nitrogen for at least one hour, and then a solution of AIBN (2,2'-azobis(2-methylpropionitrile)) in toluene was simultaneously injected into the reaction flask. The reaction mixture was stirred vigorously at 65°C overnight, then poured into a 1 L plastic centrifuge bottle, diluted with toluene, and centrifuged at 4500 RPM for 30 minutes. The centrifuge cake was rinsed once with toluene, and the mixture was centrifuged again at 4500 RPM for 30 minutes. The supernatant was decanted and the resulting pigment dried overnight in vacuo at 70°C, then ground using a mortar and pestle and dispersed in Isopar E to form a 20 wt% dispersion, which was sonicated and rolled on a roller mill for at least 24 hours (or longer if necessary). The resulting dispersion was filtered through a fabric mesh to remove any large particles, and a sample was removed and its solids content measured.

Part B: Preparation of the internal phase

The magenta pigment dispersion prepared in Part A above (13.92 g of a 14% w/w dispersion in Isopar E) was mixed with 83.07 g of a 60% w/w dispersion of titanium dioxide in Isopar E (a polymer coated as described in the aforementioned U.S. Patent No. 7,002,728), 7.76 g of a 20% w/w solution of Solsperse 17000 in Isopar E, a 15% w/w solution of poly(isobutylene) having a molecular weight of 1,270,000 in Isopar E (the poly(isobutylene) is used as an image stabilizer; see U.S. Patent No. 7,170,670), 0.575 g of Sudan 1 (available from Acros Organics of New Jersey) of the formula:

and 5.82 g of Isopar E. The resulting mixture was dispersed on a mechanical roller overnight to produce an internal phase that was easily encapsulated and had a conductivity of 304.7 pS/cm.

Part C: Microencapsulation

The internal phase prepared in Part B was encapsulated according to the process described in U.S. Patent No. 7,002,728. The resulting encapsulated material was isolated by precipitation, washed with deionized water, and size-separated by sieving. Capsule size analysis using a Coulter particle size analyzer showed that the average size of the resulting capsules was 40 μm, and more than 85% of the total capsule volume was in capsules with a desired size between 20 and 60 μm.

Part D: Display Preparation

The sieved capsules produced in Part C above were adjusted to a pH of 9 as the ammonium hydroxide solution and excess water were removed. The capsules were then concentrated and the supernatant liquid was discarded. The concentrated capsules were mixed with an aqueous polyurethane binder (prepared in a manner similar to that described in U.S. Patent Application Publication No. 2005/0124751) at a binder to capsule weight ratio of 1:15. Subsequently, Triton X-100 surfactant and hydroxypropyl methylcellulose were added and mixed thoroughly to provide a suspension.

The capsule suspension thus prepared was coated onto the indium tin oxide (ITO) coated surface of a 125 μm thick polyethylene terephthalate (PET)/ITO film using a bar coater, and the coated film was dried at 60° C. Separately, a polyurethane adhesive layer doped with tetraethylammonium hexafluorophosphate as a conductive dopant was coated onto a release sheet, and the resulting PET film/adhesive subassembly was laminated to the top of the coated capsules as described in the aforementioned U.S. Patent No. 7,002,728. The release sheet was removed and the resulting multilayer structure was laminated to a graphite back electrode to produce an experimental single-pixel display, which included a PET film, an ITO layer, a capsule layer, a laminating adhesive layer, and a graphite back electrode to form its viewing surface.

Part E: Electro-optical Testing

The resulting display was converted using a 50 Hz square wave AC waveform applied to a ±30 V graphite back electrode (front ITO electrode grounded), with the square wave AC waveform offset from zero as listed below (e.g., a 5 V offset would provide a 50 Hz square wave oscillation of +35/-25 V). Table 1 below shows the reflectivity (in percentage) at various wavelengths obtained from the indicated color states of the display.

Table 1

red Magenta White yellow 450nm 13.4 16.8 31.5 13.5 550nm 8.8 9.7 44.9 35.2 650nm 60.8 55.0 60 54.4

The DC offset for red/white conversion is ±10V. In this case, white and magenta pigments move through a yellow dye fluid. The red state results from viewing a magenta (green absorbing) pigment and a yellow (blue absorbing) dye on a white background. The DC offset for magenta/yellow conversion is ±60V. A yellow color is obtained when the white pigment moves away from the viewing side of the display toward the negatively charged back electrode, as described above with reference to Figures 8A and 8B.

In summary, under low applied fields, the white pigment behaves as if it is negatively charged, is driven toward the front electrode when the rear electrode is at a relatively low negative voltage, and provides high reflectivity at 450 nm (the wavelength absorbed by the dye); under a more negative voltage applied to the rear electrode, the white pigment moves toward the rear electrode, behaves as if it is positively charged, exposes the dye and reduces the reflectivity at 450 nm.

Example 2

This example illustrates the preparation of a three-particle undyed fluidic electrophoretic display of the type shown in Figures 10A and 10B of the accompanying drawings.

Part A: Preparation of Yellow Pigment Dispersion

Novoperm Yellow P-HG yellow pigment available from Clariant of Basel, Switzerland, was mixed with a solution of Isopar E and Solsperse 17000 in Isopar E and the mixture was dispersed using a Szegvari Attritor (registered trademark) type 01-HD, glass beads of size 01 having a size of 0.4-0.6 mm at 650 rpm for 1 hour to provide a yellow pigment dispersion.

Part B: Preparation of white pigment dispersion

Titanium dioxide was treated with silanes as described in the aforementioned US Patent No. 7,002,728. The resulting silane-treated white pigment was treated with monomers and polymerization initiators as described in US Patent Application Publication No. 2011/0012825 to produce a polymer-coated white pigment, which was mixed with Isopar E to produce a white pigment dispersion.

Part C: Preparation of Black Pigment Dispersion

Black pigment (BK444 or BK20C920, available from Shepherd Color, Inc., Cincinnati, Ohio) was ground in water to a particle size of approximately 300 nm for BK444 and 500 nm for BK20C920. The surface of the ground pigment was functionalized using N-[3-(trimethoxysilyl)propyl]-N'-(4-vinylbenzyl)ethylenediamine dihydrochloride (available from United Chemical Technologies) in a manner similar to that described in U.S. Patent No. 6,822,782. Thermogravimetric analysis (TGA) indicated the presence of 4-10% volatile (organic) material for BK444 and 1.1-1.3% volatile material for BK20C920. The pigment was then provided with a lauryl methacrylate coating as described in U.S. Patent No. 6,822,782. The final pigments showed by TGA to have 15-25% volatile material present for BK444 and 4-6% volatile material present for BK20C920.

Part D: Preparation of electrophoresis medium

The yellow pigment dispersion prepared in step A above (1.91 g) was mixed with the black dispersion made from BK444 in step C above (0.92 g), the white dispersion prepared in step B above (4.95 g), butyl aluminum 3,5-di-tert-butylsalicylate available from Esprix Technologies of Sarasota, Florida, (0.1 g of a 1% w/w solution of Isopar E), the same poly(isobutylene) as in Example 1 above, 0.46 g of a 15% w/w solution of Isopar E), and 1.66 g of Isopar E. The resulting mixture was sonicated and heated to 42° C. for 30 minutes to produce an electrophoretic medium having a conductivity of 240 pS/cm containing the three pigments in a hydrocarbon fluid.

Part E: Electro-optical Testing

Cell (a): A parallel-plate cell was prepared consisting of two 50 mm x 55 mm glass plates, each coated with a transparent ITO conductive coating. The electrophoretic medium (15 μL) prepared in Section D above was dispensed onto the ITO-coated surface of the lower glass plate. The upper glass plate was then placed on the electrophoretic medium so that the ITO coating was in contact with the fluid. Electrical connections to the cell were then made using conductive copper tape attached to the ITO-coated sides of the upper and lower glass plates.

Unit (b): Unit (b) was prepared as described above for unit (a), except that the conductive ITO coating on each glass plate was blocked by applying a polymer protective layer (a solution of poly(methyl methacrylate) (PMMA) in acetone was bar coated using a #7 Mayer rod to provide a dry coating approximately 0.5 μm thick).

Units (a) and (b) were electrically driven under the following conditions: a waveform consisting of a square wave with a frequency of 10 Hz was used, voltages of ±30, 15, and 7.5 V were applied to the lower electrode, the upper electrode was grounded, and a duty cycle sequence of 0.05, 0.1, 0.2, 0.4, 0.8, and 1 as shown in Table 2 below was used, preceded by a set of conditioning pulse trains of ±30 V, 10 Hz, and 6 x 1 second duration.

Table 2

Time (seconds) Duty cycle 0 0.05 1 .1 2 .2 3 .4 4 .8 5 1 6 .8 7 .4 8 .2 9 .1 10 .05

Reflectance spectra were obtained when cells (a) and (b) were electrically driven, and the results are shown in Figures 13A and 13B, respectively. As can be seen in these figures, in cell (a), where the electrode is not blocked, the cell is able to exhibit black, white, and yellow states (in Figures 13A and 13B, a more positive b* value indicates increased yellow coloration, while a more positive L* value indicates increased brightness). In contrast, in cell (b), where the electrode is blocked and minimal current is passed, no white state is seen (no high L* and low b* states); the cell simply switches between black and yellow states.

Example 3

This example illustrates the preparation of another three-particle undyed fluidic electrophoretic display shown in Figures 10A and 10B of the accompanying drawings.

The internal phase was prepared from the following components (by weight):

The prepared internal phase was then encapsulated as described in U.S. Patent No. 7,002,728. The resulting capsules were isolated by precipitation, washed with deionized water, and size-separated by sieving. Capsule size analysis using a Coulter particle size analyzer showed that the average size of the resulting capsules was 40 μm and that more than 85% of the total capsule volume was in capsules with a desired size between 20 and 60 μm. The capsules were then converted into a single-pixel experimental display with a graphite back electrode, as described in Section D of Example 1 above.

The display thus constructed was driven using the waveform shown in FIG14 (the back electrode voltage relative to the grounded front electrode is shown). The waveform consisted of a 1-second pulse of -15V, followed by a 1-second pulse of +15V, followed by a -15V test pulse, with the length of the test pulse varying from 50ms to 400ms in 50ms increments. FIG14 also shows the L* and b* values measured while the display was driven. During the test pulse, the display transitioned from black to white via yellow (as shown by arrows 906 and 902 in FIG9 ), while during the +15V pulse device, the display transitioned from white to black (as shown by arrow 904 in FIG9 ).

Example 4

This example illustrates the preparation of a three-particle dye-fluid electrophoretic display as shown in Figures 11A-11D of the accompanying drawings.

Part A: Preparation of Cyan Pigment Dispersion

Irgalite Blue GLVO cyan pigment available from BASF, Ludwigshafen, Germany, was mixed with Isopar E and Solsperse 17000 solutions and the resulting mixture was dispersed by rubbing with 0.4-0.6 mm glass beads at 650 rpm for one hour to provide a cyan pigment dispersion.

Part B: Preparation of electrophoresis medium

The electrophoretic medium was prepared from the following components (by weight):

The resulting fluid was loaded into cell (a) in Example 2 above and processed as follows: a waveform comprising a square wave with a frequency of 30 Hz, voltages of ±10, 15, 20 and 40 V applied to the rear electrode while the front electrode was grounded, using a duty cycle sequence as shown in Table 2 above.

The light reflected from the test cell was analyzed spectrophotometrically, and the CIE L*a*b* values closest to the SNAP color standard were recorded. These values (a* and b*) are shown in Table 3 below. As can be seen, the electrophoretic fluid is able to distinguish all primary colors (CMYRGBKW).

Table 3

L* a* b* C 49.5 -23.4 -15.7 G 61.6 -18.4 1.9 Y 43.9 -5 20 R 27.6 22.9 12.2 M 33.6 30.8 -7.9 B 38.1 6.5 -21.4 K 28.1 1.4 3.3 W 63.8 -9 4.3

Example 5

This example illustrates the preparation of a set of cyan dyes for use in the electrophoretic media and displays of the present invention.

Part A: Preparation of the first cyan dye

This part of the example shows the preparation of a cyan dye by the reaction

wherein R represents a C ₁₂ H ²³ alkyl group. This reaction is according to Example 3 in U.S. Patent No. 5,122,611.

In the two-necked round-bottom flask of 500mL that is equipped with reflux exchanger and magnetic stirring bar, add 4-bromo-N-dodecyl-1-hydroxy-2-naphthamide, methylene dichloride (DCM) and ethanol.N in the deionized water, N-diethyl-p-phenylenediamine sulfate and the salt of wormwood in the deionized water, the ammonium persulfate in the deionized water are added into resulting reaction mixture.Reaction mixture was at room temperature stirred 30 minutes, then poured into large separating funnel and separated.Use DCM to extract water-bearing stratum, and use deionized water to clean organic layer.Resulting organic phase is concentrated under reduced pressure, and uses DCM and methyl alcohol to purify the raw material that is produced by recrystallization.

The λ _max of the dye in Isopar E solution is 648 nm, ε=28,100 Lmol ^-1 cm ^-1 , and the solubility of the dye in Isopar E at 4°C is 1.2 wt %.

Part B: Preparation of the Second Cyan Dye

This portion of the example shows the preparation of a cyan dye via a three-step reaction sequence:

The first step of the reaction sequence is based on the content published by Huang, Y., Luedtke, R.R., Freeman, R.A., Wu, L., Mach, R.H. in J. Med. Chem., Vol. 44, No. 1815-1826, 2001, and the third step is based on U.S. Patent No. 5,122,611.

Step 1:

In a 1 L round-bottom flask equipped with an overhead stirrer, 1-hydroxy-2-naphthoic acid, pyridinium bromide, and acetic acid were added. The resulting reaction mixture was stirred overnight at room temperature and then filtered. The resulting solid was washed with deionized water, dried under vacuum, and used without further purification.

Step 2:

4-bromo-1-hydroxy-2-naphthoic acid and N,N-dimethylformamide (DFM) are added to a 250mL round-bottom flask. Once acid is dissolved, 1-hydroxybenzotriazole hydrate and N-(3-dimethylaminophenyl)-N'-ethyl hydrochloride carbodiimide are added to the flask. Finally, oleamide is added to the flask by a syringe. The resulting reaction mixture was stirred at room temperature for 5 days, then injected into deionized water and extracted with dichloromethane (DCM, 3x 100mL aliquots). The organic phase is mixed and cleaned with a 10wt% hydrochloric acid solution (4x 100mL). Solid is formed and filtered from the organic layer. Organic layer is filtered through a silicon plug and the product is concentrated under reduced pressure (48% yield).

Step 3:

4-Bromo-1-hydroxy-N-oleyl-2-naphthamide, DCM, and ethanol were added to a 500 mL double-necked round-bottom flask equipped with a reflux condenser and a magnetic stir bar. N,N-diethyl-p-phenylenediamine sulfate in deionized water, potassium carbonate in deionized water, and ammonium persulfate in deionized water were added to the resulting reaction mixture. The reaction mixture was stirred at room temperature for 30 minutes, then poured into a large separatory funnel and separated. The aqueous layer was extracted with DCM, and the combined organic layers were washed with deionized water. The resulting organic phase was concentrated under reduced pressure to give the raw material, which was purified by silica gel chromatography using DCM as the eluent.

The obtained dye had a λ _max of 622 nm and ε of 25,800 Lmol ^-1 cm ^-1 in Isopar E solution. The solubility of the dye in Isopar E at 4°C was 3.9 wt%.

Example 6

This example illustrates the preparation of a three-particle dye-fluid electrophoretic display of the type shown in Figures 11A-11D of the accompanying drawings.

Part A: Preparation of Yellow Pigment Dispersion

Novoperm Yellow P-HG yellow pigment available from Clariant of Basel, Switzerland, was mixed with a solution of Isopar E and Solsperse 17000 in Isopar E and the mixture was rubbed at full speed with a Szegvari Attritor Type 01-HD, Size 01 at 650 rpm for one hour using 0.4-0.6 mm glass beads to provide a yellow pigment dispersion.

Part B: Preparation of Magenta Pigment Dispersion

Inkjet fuchsin E 02 VP2621 available from Clariant, Basel, Switzerland, was dispersed at 10% w/w in toluene. The pigment dispersion was transferred to a 500 mL round-bottom flask and the flask was degassed with nitrogen, and the solution was warmed to 42° C. Upon reaching this temperature, 4-vinylbenzyl chloride was added and the reaction mixture was stirred overnight at 42° C. under a nitrogen atmosphere. The resulting product was allowed to cool to room temperature and then centrifuged to isolate the functionalized pigment. The centrifuge cake was washed with toluene (3 x 250 mL) to produce the functionalized fuchsin pigment.

The magenta pigment thus prepared was coated with lauryl methacrylate as described in the aforementioned U.S. Patent No. 7,002,728. The final pigment was then mixed with Isopar E to produce a magenta pigment dispersion which was filtered through a 200 micron mesh filter and whose solids content was determined to be 15.9%.

Part C: Preparation of white pigment dispersion

A titanium dioxide dispersion was prepared as in part B of Example 2 above.

Part D: Preparation of electrophoretic media and electro-optical testing

The yellow pigment dispersion prepared in Part A above (0.65 g), the magenta dispersion prepared in Part B above (0.83 g), the white dispersion prepared in Part C above (3.22 g), the cyan dye prepared in Part A of Example 5 above (0.10 g), butyl 3,5-di-tert-butylsalicylate aluminum (0.07 g of a 1% w/w solution in Isopar E), poly(isobutylene) of molecular weight 600,000 (0.31 g of a 15% w/w solution in Isopar E), and 1.26 g of additional Isopar E were mixed. The resulting mixture was sonicated and heated to 42° C. for 30 minutes to produce an electrophoretic medium having a conductivity of 74 pS/cm.

This electrophoretic medium fluid was loaded into the first test cell described in Example 2 above. Reflectance spectra were obtained when the rear electrode was driven with the waveforms shown in Figures 15A-15J and the front electrode was grounded. Figures 16A-16J show the optical states (plotted as L*, a*, and b* versus time) obtained using the waveforms of Figures 15A-15J, respectively. The sampling rate of the spectrometer was 20 Hz, so optical transients during the reset pulse at the beginning of each waveform were not recorded.

In Figure 15 A-15F, the waveform used includes the reset pulse of the 30V or ± 15V of a series of rapid oscillations, followed by a constant rear electrode drive of -30V (Figure 15A), -15V (Figure 15B), -7.5V (Figure 15C), +30V (Figure 15D), +15V (Figure 15E) or +7.5V (Figure 15F) for 1.5 seconds. The waveform shown in Figure 15G-15J has different types, including the reset pulse of ± 30V or ± 15V of the same series of rapid oscillations, but using the driving portion in which the rear electrode voltage of the waveform changes between a positive voltage and a negative voltage, the remaining zero voltage between the positive pulse and the negative pulse. The pulses are ± 30V (Figure 15G) and ± 15V (Figure 15H). The waveforms in Figures 15I and 15J are basically inverse forms of the waveforms in Figures 15G and 15H, respectively. In the sense of Figures 15G and 15H, the drive sequence is positive-zero-negative-zero-positive-zero-negative-zero, etc., whereas in Figures 15I and 15J, the drive sequence is negative-zero-positive-zero-negative-zero-positive-zero, etc.

As can be seen in Figures 16A-16C, the result of the waveforms of Figures 15A-15C is that b* starts at a positive value in all three cases and then moves to a negative value as the pulse continues, while a* remains quite stable; thus, the display changes from a (slightly greenish) yellow to cyan, crossing zero at the slightly white cyan. The waveform of Figure 15C produces an almost white state. These results are consistent with the transition mechanism described above with reference to the right-hand side of Figures 11A-11C. Conversely, using the positive drive waveforms of Figures 15D-15F, Figures 16D-16F show the display transitioning from a dark blue state (b* approximately -10) to a red state (a* of +22; b* approximately +10), while the waveform of Figure 15F produces an almost black state. These results are consistent with the transition mechanism described above with reference to the left-hand side of Figures 11A-11C.

Figures 16G and 16H show that the inverted waveforms of Figures 15G and 15H cause the display to oscillate between a magenta state (back electrode positive) and a green state (back electrode negative); the best magenta color is achieved using the waveform of Figure 15H. Similarly, Figures 16I and 16J show that the inverted waveforms of Figures 15I and 15J cause the display to oscillate between a green state (back electrode negative) and a red/magenta state (back electrode positive); the best green color is achieved using the waveform of Figure 15J. The difference between the waveforms of Figures 15G and 15H and the waveforms of Figures 15I and 15J (each of which alternates between positive and negative drive pulses) is that in Figures 15G and 15H, the waveform of the reset train begins and ends with a negative pulse, while in Figures 15I and 15J, the waveform of the reset train begins and ends with a positive pulse. Therefore, in the waveforms of Figures 15G and 15H, the starting point has a net negative pulse, while in the waveforms of Figures 15I and 15J, the starting point has a net positive pulse. An initial net negative pulse favors magenta over green, while an initial net positive pulse favors green over magenta.

FIG. 17 is a plot of all colors obtained by the waveforms of FIG. 15A to FIG. 15J on the a*/b* plane, and it can be seen from the figure that all primary colors can be provided by the electrophoretic display of the present invention.

Example 7

This example illustrates the preparation of a second three-particle dye-fluid electrophoretic display of the type shown in Figures 11A-11D of the accompanying drawings.

Part A: Preparation of Cyan Pigment Dispersion

Hostaperm Blue BT-617-D cyan pigment (26 g), available from Clariant of Basel, Switzerland, was mixed with Isopar E (70 g) and Solsperse 17000 solution (70 g of a 20% w/w solution in Isopar E), and the resulting mixture was dispersed by rubbing at full force using 0.4-0.6 mm glass beads at 650 rpm for one hour to provide a cyan pigment dispersion.

Part B: Preparation of electrophoretic media and electro-optical testing

The electrophoretic medium was prepared from the following components (by weight):

The resulting fluid was placed in cell (a) described in Example 2 above and driven using the waveforms shown in Figures 15A-15J. Figure 18 is a plot of all the resulting colors on the a*b* plane, similar to Figure 17. As can be seen from Figure 18, this electrophoretic medium can provide all primary colors except red.

Example 8

This example illustrates the preparation of a third three-particle dye-fluid electrophoretic display of the type shown in Figures 11A-11D of the accompanying drawings.

The internal phase was prepared from the following components (by weight):

The thus prepared inner phase was encapsulated according to the process described in U.S. Patent No. 7,002,728. The resulting encapsulated material was isolated by precipitation, washed with deionized water, and size-separated by sieving. Capsule size analysis using a Coulter particle size analyzer indicated an average capsule size of 74 μm, with over 85% of the total capsule volume contained within capsules of the desired size between 20 and 60 μm. The capsules were then converted into experimental single-pixel displays in the same manner as in Part D of Example 1 above.

The displays were then driven using the waveforms shown in Figures 15A-15J. Figure 19 is a plot of all the colors obtained on the a*b* plane similar to the plots of Figures 17 and 18. As can be seen from Figure 19, this electrophoretic medium can provide all the primary colors.

Example 9 - Waveform Optimization for Spot Colors

Following the experiments described in Example 3 above, it was discovered that a waveform of the type shown in FIG14 is not, in fact, the optimal waveform for achieving good spot colors of the type shown in FIG10A and 10B with a three-particle black/white/spot color electrophoretic medium. (The spot color shown in FIG10A and 10B is yellow, and the discussion below assumes the same spot color, but this is for illustrative purposes only; of course, any spot color other than white and black could be used.) It was discovered that better saturation (i.e., increased b* values for the yellow spot color and increased a* values for the other spot colors) can be achieved using a square wave with a suitably selected frequency and duty cycle.

As shown in Figures 10A and 10B, when the rear electrode 1012 is positive relative to the front electrode 1010, the positively charged black pigment moves toward the viewing (top) surface of the display, while when the rear electrode 1012 is negative relative to the front electrode 1010, the positively charged black pigment moves toward the back of the display. On the other hand, when the rear electrode is negative, the negatively charged white pigment moves toward the viewing surface of the display, while when the rear electrode is positive, the negatively charged white pigment moves toward the back of the display. The third (yellow) pigment is initially negatively charged and under low pulses, when the rear electrode is negative, the yellow pigment first moves toward the viewing surface of the display (Figure 10A, right side - the display looks yellow), but when the voltage is applied long enough to provide a high addressing pulse, the yellow pigment disappears behind the white pigment and the display changes from yellow to white (Figure 10B, right side).

Figure 14 shows these color changes. Consider the first cycle shown in Figure 14. Initially, the display is in its black state (Figure 10A, left). When applying-15V pulses, the black pigment moves to the rear surface and the white and yellow pigments move to the viewing surface (Figure 10A, right). Initially, as the display becomes yellow, b* and L* increase. After a period of time, as the arrival of the high addressing pulse and the display become white (Figure 10B, right), b* (" yellowness ") increases to its maximum value, then reduces, and L* keeps increasing.

It has been found that a very simple waveform using alternating negative and positive pulses can achieve maximum b* values that are higher than the values obtainable using a single positive-negative cycle as shown in Figure 14. This waveform is shown in Figure 20, from which it can be seen that the waveform comprises a series of short (approximately 0.5 second) negative pulses separated by longer (1 second) positive pulses and terminating in one of the negative pulses. The optimum duration of the positive and negative transitions depends to some extent on the composition of the electrophoretic medium and is typically several hundred milliseconds long. The key factor in determining the required negative pulse length is how long the negative drive needs to be applied from (black) before the yellow color starts to decrease. The positive pulse should be longer than the negative pulse and needs to be long enough to drive the display back to the extreme optical state of black. As shown in Figure 20, the waveform used increases b* by a value of approximately 55, while the waveform shown in Figure 14 is approximately 41.

FIG21 shows a plot of L* versus b* for the first cycle shown in FIG14 and the waveform shown in FIG20 . It can be seen that the b* value for the extreme yellow state (labeled “Y” in FIG21 ) of the waveform of FIG20 is significantly higher than the b* value for the extreme yellow state of the waveform of FIG14 . One possible explanation for this improvement in b* is that some of the yellow pigment begins to “spin” (i.e., move away from the viewing surface of the display) before the best yellow state is achieved. By briefly reversing the polarity of the drive pulse, the charge on the yellow pigment is reset and when the voltage switches back to negative, the yellow pigment continues to travel to the viewing surface of the display, and most, if not all, of the yellow pigment reaches the surface, resulting in a better yellow state and improved maximum b*.

While the preceding discussion has focused on an improved yellow state, waveforms of the type shown in FIG20 have other advantages. By varying the length of the drive pulses, improved intermediate optical states can be achieved. As shown in FIG21 , a "gray" state with variable L* and very low b* can be achieved by following the W-K path of FIG21 , while a "Y-gray" state with variable L* and variable b* can be achieved by following the K-Y-W path of FIG21 .

Waveforms of the type shown in FIG20 can be incorporated into an overall DC-balanced drive scheme (see the aforementioned MEDEOD application, which is important for maintaining overall DC balance in drive schemes for bi-stable electro-optical displays) in various ways:

(a) Self-Balanced Transition: As described in U.S. Patent No. 7,119,772, in this drive scheme, each waveform defining a transition between two optical states has zero net pulse. Since waveforms of the type of FIG. 20 that are optimal for a transition to yellow can have a net pulse, this net pulse must be canceled, for example by a pre-pulse with an opposite pulse. The waveform shown in FIG. 20 transitioning to yellow has a net positive pulse, so the overall transition to yellow should first have an equal negative pulse (towards white) preceding the waveform of FIG. 20 ; and

(b) Round-trip DC Balance: As described in U.S. Patent No. 7,012,600, in many prior art drive schemes, individual waveforms are not DC balanced. Instead, the entire drive scheme is designed so that each closed loop of transitions (i.e., each set of transitions starting and ending at the same grayscale) has zero net pulse. To achieve this, each optical state is assigned a "pulse potential," and the net pulse of the waveform used for any transition between two different optical states must equal the difference in pulse potential between those two optical states. Figure 22 illustrates such a drive scheme. For example, the ellipses represent optical states with assigned pulse potentials. The indicator arrows show the net pulse between the two optical states represented by the arrow ends; this pulse must equal the difference in pulse potential between the two optical states. A yellow transition of the type shown in Figure 20 can be incorporated into this drive scheme in various ways. For example, the transition to yellow can be hypothetically considered a two-part transition, one from the current state to black and another from black to yellow. Because the yellow waveform of the type shown in Figure 20 has a net positive pulse, it can be considered part of the first transition to black (which also has a net positive pulse). In addition, the second portion of the waveform has a set of negative pulses toward yellow that are equal to the difference between the black and selected yellow pulse potentials. Figures 23 and 24 illustrate this approach. Figure 23 shows a simple square wave drive and pulsed potential. The highlighted area shows the black-to-yellow transition. The yellow portion of a waveform similar to that of Figure 20 can be considered to be a component of the black-to-white portion of the transition shown in Figure 24, which shows the actual voltage pulses used for the black-to-yellow transition.

It has been found that the optimum waveform of the type shown in Figure 20 is voltage dependent in a manner that may vary from batch to batch of electrophoretic material, even if the batches have the same nominal composition. Figure 25 shows an example of a batch where the optimum voltage for driving to yellow is in the range of 20-29V, as opposed to the 15V used above.

One of the factors that limits the performance of displays of the type shown in Figures 10A and 10B is the duration of the waveform required to go from black or yellow to white. As shown in Figure 14, even without any reverse polarity pulses, it may take a full two seconds to transition from black to white. This is even more pronounced when the waveform includes multiple cycles of reverse polarity and/or when higher drive voltages are used. To address these performance limitations, a "picket fence" type waveform can be used, as shown in Figure 26, which shows four (simplified) examples of this type of waveform. Figure 26 shows, from top to bottom, the original waveform, a waveform with extended negative pulses, a waveform with several cycles of zero voltage inserted, and a waveform with several cycles of reverse (positive) voltage added. Adding closely spaced zeros, or positive voltages, to the waveform in this way allows for faster removal of the yellow from the white state, thereby achieving a shorter waveform, and/or improving the white state by reducing the amount of yellow. Figure 27 shows the improvement in the L* and b* values of the white optical state, which can be achieved by using a picket fence waveform. In Figure 27, the picket gap symbol indicates the type of picket fence used; a value of 1 means extended drive, a value of 0 means adding several cycles of zero voltage, and a value of -1 means adding reverse (positive) voltage. The total gap time indicates the total amount of drive time increased in this manner.

Those skilled in the art of electrophoretic displays will readily appreciate that the use of waveforms such as those discussed above can result in noticeable flicker during transitions. This flicker can be reduced by using different waveforms for two or more subsets of pixels so that some pixels are in a bright optical state and others in a dark optical state; the average optical state of such a display viewed from a distance will slowly change shades of gray. This flicker reduction technique is most effective when applied to a clearing signal having drive pulses of both polarities, because all pixels experience equal duty cycles of periodic drive to black and white, and thus can be easily divided into groups using different waveforms. These techniques have been previously disclosed in some of the aforementioned MEDEOD applications (for black and white electrophoretic displays).

As can be seen from the foregoing, the present invention provides a full-color display capable of providing all primary colors across the entire area of the display. If desired, in addition to the color modulation provided by the present invention, regional modulation can also be used to enable the display to show the full range of saturation in each color. The present invention can also provide a display capable of producing spot colors across the entire area of the display.

Claims (26)

1. An electrophoretic medium comprising a fluid (806) and at least a first type of particles (804; 1004; 1104) disposed in the fluid (806), the first type of particles (804; 1004; 1104) moving in one direction when a first addressing pulse is applied to the electrophoretic medium, characterized in that: when a second addressing pulse greater than the first addressing pulse and having the same polarity as the first addressing pulse is applied to the electrophoretic medium, the first type of particles (804; 1004; 1104) moving in the opposite direction, wherein the first addressing pulse is a voltage-time integral for generating a first electric field employed in a first period, and the second addressing pulse is a voltage-time integral for generating a second electric field employed in a second period. 2. The electrophoretic medium according to claim 1, wherein the fluid (806) is colored and the electrophoretic medium contains a second type of particles (802; 1102; 1106), the second type of particles (802; 1102; 1106) being different in color from the first type of particles (804; 1104), and moving in the same direction under both the first and second addressing pulses. 3. The electrophoretic medium according to claim 1, wherein the fluid (806) is colored and the electrophoretic medium comprises a second type of particles (802; 1102; 1106), the second type of particles (802; 1102; 1106) being a different color from the first type of particles (804; 1104), wherein under the first addressing pulse, the second type of particles (802; 1102) move in the opposite direction to the first type of particles (804; 1104), and under the second addressing pulse, the second type of particles (802; 1102) move in the same direction as the first type of particles (804; 1104). 4. The electrophoretic medium according to claim 2, wherein the staining fluid and one of the two types of particles have one of the additive primary colors, while the staining fluid and the other of the two types of particles have complementary subtractive primary colors. 5. The electrophoretic medium according to claim 4, wherein the first type of particles is white, one of the staining fluid and the second type of particles has one of the additive primary colors, and the other of the staining fluid and the second type of particles has a complementary subtractive primary color. 6. The electrophoretic medium according to claim 1, wherein the electrophoretic medium comprises a second type of particles (1102) and a third type of particles (1106) that are different in color from and different from each other than the first type of particles (1104), the second type of particles (1102) and the third type of particles (1106) carrying charges of opposite polarities, and the first type of particles (1104) and the third type of particles (1106) carrying charges of the same polarity, such that when the first addressing pulse is applied in one direction, a surface of the electrophoretic medium displays the color of the third type of particles; when the first addressing pulse is applied in the opposite direction, the surface of the display displays a mixture of the colors of the first and second particles; when the second addressing pulse is applied in the one direction, the surface displays a mixture of the colors of the first and third particles; and when the second addressing pulse is applied in the opposite direction, the surface displays the color of the second particles. 7. The electrophoretic medium according to claim 6, wherein the fluid is colorless. 8. The electrophoretic medium according to claim 7, wherein the second type of particles and the third type of particles are white and black, respectively. 9. The electrophoretic medium according to claim 6, wherein when a third addressing pulse greater than the second addressing pulse is applied, the first particle, the second particle, and the third particle all travel in the same direction. 10. The electrophoretic medium according to claim 9, wherein the color of the fluid is different from the colors of the first type of particles, the second type of particles, and the third type of particles. 11. The electrophoretic medium according to claim 10, wherein one class of particles is white, and the other two classes of particles and the color of the fluid are selected in any order from yellow, cyan and magenta. 12. The electrophoretic medium according to claim 1, wherein the particles and the fluid are confined within a plurality of capsules or microunits. 13. The electrophoretic medium of claim 1, wherein the particles and the fluid exist in the form of a plurality of discrete droplets surrounded by a continuous phase comprising a polymer material. 14. A front-plane lamination, a dual-release sheet, a reverse front-plane lamination, or an electrophoretic display comprising the electrophoretic medium according to claim 1. 15. An electrophoretic display comprising an electrophoretic medium according to claim 1 and at least one electrode arranged to apply an electric field to the electrophoretic medium. 16. An e-book reader, portable computer, tablet computer, mobile phone, smart card, sign, watch, shelf label, or flash drive comprising the electrophoretic medium of claim 15. 17. A method for driving an electrophoretic medium comprising a fluid and at least a first type of particles disposed in the fluid, the method comprising: a. Applying a first addressing pulse to the electrophoretic medium, thereby causing the first type of particles to move in one direction; and b. Apply a second addressing pulse, greater than the first addressing pulse and having the same polarity as the first addressing pulse, to the electrophoretic medium, thereby causing the first type of particles to move in the opposite direction. Wherein, the first addressing pulse is used to generate the voltage of the first electric field used in the first cycle as an integral of time, and the second addressing pulse is used to generate the voltage of the second electric field used in the second cycle as an integral of time. 18. The method of claim 17, wherein the fluid is colored and the electrophoretic medium contains a second type of particles of a different color from the first type of particles, and wherein, under the first addressing pulse and the second addressing pulse, the second type of particles move in the same direction. 19. The method of claim 17, wherein the electrophoretic medium comprises a second type of particles and a third type of particles that are different in color from and different from each other than the first type of particles, the second type of particles and the third type of particles carrying charges of opposite polarities, while the first type of particles and the third type of particles carrying charges of the same polarity, the method comprising: a. Apply the first addressing pulse in one direction, thereby causing one surface of the electrophoretic medium to display the color of the third type of particles; b. Apply a first addressing pulse in the opposite direction, thereby causing the surface of the display to display a mixture of the colors of the first and second particles; c. Applying the second addressing pulse in one direction, thereby causing the surface to display a mixture of the colors of the first and third particles; and d. Apply the second addressing pulse in the opposite direction, thereby causing the surface to display the color of the second particle. 20. The method of claim 19 further comprises applying a third addressing pulse greater than the second addressing pulse to the electrophoretic medium, such that the first particle, the second particle, and the third particle all travel in the same direction. 21. The method of claim 19, wherein step b is implemented by applying a waveform of a series of short voltage pulses of one polarity to the electrophoretic medium, each short voltage pulse except the last being followed by a longer voltage pulse of the opposite polarity. 22. An electrophoretic display capable of displaying multiple different colors, the display comprising an electrophoretic medium containing a fluid and a plurality of particles disposed in the fluid, the display further comprising a first electrode and a second electrode disposed on opposite sides of the electrophoretic medium, wherein when a first addressing pulse is applied to the electrophoretic medium, the particles move toward the first electrode, and when a second addressing pulse greater than the first addressing pulse but having the same polarity as the first addressing pulse is applied, the particles move toward the second electrode, wherein the first addressing pulse is a voltage-time integral for generating a first electric field employed in a first period, and the second addressing pulse is a voltage-time integral for generating a second electric field employed in a second period. 23. The electrophoretic display of claim 22, wherein when the first addressing pulse is applied, the particles move toward a more positive electrode, and when the second addressing pulse is applied, the particles move toward a more negative electrode. 24. The electrophoretic display of claim 23 further comprises a second type of particles that are different in color from the first type of particles and move toward the more negative electrode when the first addressing pulse or the second addressing pulse is applied. 25. An electrophoretic medium comprising a fluid and first, second, and third type particles disposed in the fluid, the first type particles carrying a charge of one polarity, and the second and third type particles carrying charges of opposite polarities, such that when a first addressing pulse is applied to the electrophoretic medium, the first and third type particles move in one direction while the second type particles move in the opposite direction; however, when a second addressing pulse greater than the first addressing pulse and having the same polarity as the first addressing pulse is applied to the electrophoretic medium, the first type particles move in the one direction while the second and third type particles move in the opposite direction, wherein the first addressing pulse is a voltage-time integral used to generate a first electric field employed in a first period, and the second addressing pulse is a voltage-time integral used to generate a second electric field employed in a second period. 26. The electrophoretic medium of claim 25, wherein the second type of particles carries a polymer surface treatment, while the third type of particles does not carry a polymer surface treatment or, compared to the second type of particles, the polymer surface treatment carried by the third type of particles has a lower material coverage per unit area of the particle surface.