Electrophoretic media comprising electrophoretic particles and combination of charge control agents

Abstract

To provide an electrophoretic medium comprising four types of particles.SOLUTION: The first type of particles has a first charge polarity. The second and third types of particles have a second charge polarity that is opposite to the first charge polarity. The electrophoretic medium further comprises a first charge control agent and a second charge control agent. The molecular structure of the first charge control agent includes a quaternary ammonium group and a non-polar tail, and the molecular structure of the second charge control agent includes two or more polar groups and a non-polar tail. In an aspect, the present invention discloses an improved electrophoretic medium for color electrophoretic displays.SELECTED DRAWING: None

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application No. 63 / 296,179, filed January 4, 2022, which is incorporated by reference in its entirety, along with all other patents and patent applications disclosed herein. [Background technology]

[0002] background Electrophoretic displays change color by modifying the position of charged colored particles relative to a light-transmitting viewing surface. Such electrophoretic displays are typically called "electronic paper" or "ePaper" because the resulting display has high contrast, similar to ink on paper, and is sunlight-readable. Electrophoretic displays have been widely adopted in eReaders such as the AMAZON Kindle® because they provide a book-like reading experience, use little power, and allow users to have a library of hundreds of books on a lightweight, portable device.

[0003] For many years, electrophoretic displays included only two types of charged colored particles: black and white. As used herein, the term "color" includes both black and white. White particles are often light-scattering, such as titanium dioxide, while black particles are light-absorbing across the visible range and may include carbon black or light-absorbing metal oxides, such as copper chromite. In its simplest form, a black-and-white electrophoretic display requires only a light-transmitting electrode layer at the viewing surface, a second electrode layer (also called the back electrode or bottom electrode), and an electrophoretic medium containing white and black particles of opposite charge. When a voltage of one polarity is applied across the electrophoretic medium, the white particles migrate to the viewing surface, and when a voltage of the opposite polarity is applied, the black particles migrate to the viewing surface. If the second electrode layer (back electrode) contains controllable areas (pixels), which are either segmented electrodes or an active matrix of pixel electrodes controlled by transistors, patterns can be electronically generated on the viewing surface. The pattern can be, for example, the text of a book.

[0004] More recently, various color options have become commercially available for electrophoretic displays, including three-color displays (black, white, red, and black, white, yellow) and four-color displays (black, white, red, yellow). Similar to the operation of black-and-white electrophoretic displays, electrophoretic displays using three or four reflective particles operate similarly to simple black-and-white displays, as the desired color particles are driven to the viewing surface. While the driving scheme is much more complex than black-and-white displays, the optical function of the particles is ultimately the same.

[0005] Advanced Color Electronic Paper (ACeP™) also contains four particles, but the cyan, yellow, and magenta particles are subtractive rather than reflective, allowing thousands of colors to be produced per pixel. The color production process is functionally equivalent to the printing method long used in offset and inkjet printers. A given color is produced by using the appropriate ratios of cyan, yellow, and magenta on a bright white paper background. In the ACeP example, the relative positions of the cyan, yellow, magenta, and white particles with respect to the viewing surface determine the color per pixel. While this type of electrophoretic display allows for thousands of colors per pixel, careful control of the position of each pigment (50 to 500 nanometers in size) within a working space approximately 10 to 20 micrometers thick is crucial. Needless to say, variations in particle positioning will result in an inaccurate color being displayed at a given pixel. Therefore, such systems require sophisticated voltage control. Further details of this system are available in the following U.S. patents, all of which are incorporated by reference in their entirety: U.S. Patent No. 9,361,836, U.S. Patent No. 9,921,451, U.S. Patent No. 10,276,109, U.S. Patent No. 10,353,266, U.S. Patent No. 10,467,984, and U.S. Patent No. 10,593,272.

[0006] The term gray state is used herein in its conventional sense in the imaging arts to refer to an intermediate state between two extreme optical states of a pixel, without necessarily implying a black-to-white transition between these two extreme states. For example, several of the E Ink patents and published applications referenced below describe electrophoretic displays whose extreme states are white and deep blue, and thus the intermediate gray state is actually light blue. Indeed, as already mentioned, a change in optical state may not be a color change at all. Hereinafter, the terms black and white may be used to refer to the two extreme optical states of a display, and should generally be understood to include extreme optical states that are not strictly black and white, such as the white and dark blue states mentioned above.

[0007] The terms "bistable" and "bistable" are used herein in their conventional sense in the art to refer to displays having display elements with first and second display states that differ in at least one optical property, such that any given element exhibits either its first or second display state after being driven with an address pulse of finite duration. After the address pulse is terminated, that state persists for at least several times, e.g., at least four times, the minimum duration of the address pulse required to change the state of the display element. U.S. Pat. No. 7,170,670 shows that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in intermediate gray states, as well as for several other types of electro-optic displays. Displays of this type are properly called multistable, not bistable, but for convenience, the term bistable may be used herein to encompass both bistable and multistable displays.

[0008] The term impulse, when used herein to refer to driving an electrophoretic display, refers to the integral of the applied voltage with respect to time over the period that the display is driven.

[0009] Particles that absorb, scatter, or reflect light, either broadly or at selected wavelengths, are referred to herein as colored or pigment particles. A variety of light-absorbing or reflecting materials other than pigments (in the strict sense of the term, meaning insoluble colored materials), such as dyes or photonic crystals, can also be used in the electrophoretic media and displays of the present invention.

[0010] Particle-based electrophoretic displays have been the subject of intensive research and development for many years. In such displays, a plurality of charged particles, sometimes called pigment particles, move through a fluid under the influence of an electric field. Compared to liquid crystal displays, electrophoretic displays can have attributes such as good brightness and contrast, wide viewing angles, state bistability, and low power consumption. Nevertheless, issues with the long-term image quality of these displays have prevented their widespread use. For example, the particles that make up electrophoretic displays tend to settle, resulting in insufficient service life for these displays.

[0011] As mentioned above, an electrophoretic medium requires the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but an electrophoretic medium may also be generated using a gaseous fluid, 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. Patent Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic media are likely to be susceptible to the same types of problems as liquid-based electrophoretic media due to particle settling when used in an orientation that permits such settling, e.g., in a code where the medium is positioned in a vertical plane. Indeed, particle settling is likely to be a more severe problem in gas-based electrophoretic media than in liquid-based electrophoretic media, because the lower viscosity of gaseous suspending fluids compared to liquid suspending fluids allows electrophoretic particles to settle more rapidly.

[0012] Numerous patents and patent applications assigned to or in the name of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various techniques used in encapsulating electrophoretic and other electro-optic media. Such encapsulated media include numerous microcapsules, each of which comprises an internal phase containing electrophoretically mobile particles in a fluid, and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held in a polymer binder, forming a coherent layer positioned between two electrode layers. Techniques described in these patents and patent applications include: (a) Electrophoretic particles, fluids, and fluid additives, see, e.g., U.S. Pat. Nos. 7,002,728, 7,679,814, 10,214,647, and 11,098,206, and U.S. Patent Application Publication No. 2020 / 0355978; (b) capsules, binders and encapsulation processes, see, e.g., U.S. Pat. No. 6,922,276 and U.S. Pat. No. 7,411,719; (c) Microcell structures, wall materials, and methods of forming microcells, see, e.g., U.S. Pat. No. 7,072,095 and U.S. Pat. No. 9,279,906; (d) methods for filling and sealing microcells, see, e.g., U.S. Pat. Nos. 7,144,942 and 7,715,088; (e) films and subassemblies containing electro-optical materials, see, e.g., U.S. Pat. Nos. 6,982,178 and 7,839,564; (f) backplanes, adhesive layers and other auxiliary layers and methods used in displays, see, e.g., U.S. Pat. Nos. 7,116,318 and 7,535,624; (g) Color forming color adjustment, e.g., U.S. Pat. No. 6,017,584, U.S. Pat. No. 6,545,797, U.S. Pat. No. 6,664,944, U.S. Pat. No. 6,788,452, U.S. Pat. No. 6,864,875, U.S. Pat. No. 6,914,714, U.S. Pat. No. 6,972,893, U.S. Pat. No. 7,038,656, U.S. Pat. No. 7,038,670, U.S. Pat. No. 7,046,228, U.S. Pat. No. 7,052,571, U.S. Pat. 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Nos. 7,312,784 and 8,009,348; and (j) Non-electrophoretic displays, such as those described in U.S. Patent No. 6,241,921, and U.S. Patent Application Publication Nos. 2015 / 0277160 and 2015 / 0005720 and 2016 / 0012710.

[0013] Many of the above-mentioned patents and patent applications recognize that the walls surrounding individual microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, where the electrophoretic medium comprises a plurality of individual droplets of fluid and a continuous phase of polymer material, and that the individual droplets of electrophoretic medium in such a polymer-dispersed electrophoretic display can be considered capsules or microcapsules, even though a separate capsule membrane is not associated with each individual droplet. See, e.g., U.S. Pat. No. 6,866,760. Therefore, for purposes of this application, such polymer-dispersed electrophoretic media are considered a subspecies of encapsulated electrophoretic media.

[0014] A related type of electrophoretic display is the so-called microcell electrophoretic display, in which the charged particles and fluid are not encapsulated in microcapsules but instead are held within a plurality of voids formed in a carrier medium, typically a polymer film. See, e.g., U.S. Pat. Nos. 6,672,921 and 6,788,449.

[0015] Although electrophoretic media are often opaque (e.g., because in many electrophoretic media, the particles substantially block the transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called shutter mode, in which one display state is substantially opaque and one is light-transmitting. See, e.g., U.S. Pat. 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, which are similar to electrophoretic displays but rely on variations in electric field strength, can operate in a similar mode. See U.S. Pat. No. 4,418,346. Other types of electro-optic displays may also be capable of operating in a shutter mode. Electro-optic media operating in shutter mode may be used in multi-layer structures for full-color displays, in which at least one layer adjacent to the viewing surface of the display operates in shutter mode to expose or conceal a second layer further from the viewing surface.

[0016] Encapsulated electrophoretic displays typically do not suffer from the clustering and settling failure modes of conventional electrophoretic devices and offer additional advantages, such as the ability to print or coat displays on a wide variety of flexible and rigid substrates. The use of the term "printing" is intended to encompass all forms of printing and coating, including, but not limited to, pre-metered coating, e.g., patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating, roll coating, e.g., knife-over-roll coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brushing, air knife coating, silk screen printing processes, electrostatic printing processes, thermal printing processes, inkjet printing processes, electrophoretic deposition (see U.S. Pat. No. 7,339,715), and other similar techniques. Thus, the resulting display can be flexible. Furthermore, because the display medium can be printed (using a variety of methods), the display itself can be inexpensively manufactured.

[0017] As noted above, most simple prior art electrophoretic media inherently display only two colors. Such electrophoretic media either use a single type of electrophoretic particles with a first color in a colored fluid with a second, different color, or use first and second types of electrophoretic particles with different first and second colors in an uncolored fluid. In the first case, when the particles are adjacent to the viewing surface of the display, the first color is displayed, and when the particles are spaced from the viewing surface, the second color is displayed. In the second case, when the first type of particles are adjacent to the viewing surface of the display, the first color is displayed, and when the second type of particles are adjacent to the viewing surface, the second color is displayed. Typically, the two colors are black and white. If a full-color display is desired, a color filter array may be placed over the viewing surface of a monochrome (black and white) display. Displays with color filter arrays rely on area sharing and color blending to create color stimuli. The available display area is shared among three or four primary colors, e.g., red / green / blue (RGB) or red / green / blue / white (RGBW), and the filters may be arranged in a one-dimensional (striped) or two-dimensional (2x2) repeating pattern. Other primary color choices, or more than three primary colors, are also known in the art. In the case of an RGB display, the three partial pixels, or in the case of an RGBW display, the four partial pixels, are selected to be small enough so that they visually blend into one another ("color blending") into a single pixel with a uniform color stimulus at the intended viewing distance. An inherent disadvantage of area sharing is that the colorant is always present, and the color can only be adjusted by switching the corresponding pixel of the monochrome display below it white or black and turning the corresponding primary color 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 sub-pixels), the white sub-pixel is as bright as the white of the monochrome display below it, and each of the colored sub-pixels is one-third less bright than the white of the monochrome display.The brightness of white shown by the display as a whole cannot be higher than half the brightness of the white partial pixel (the white area of ​​the display is created by displaying one white partial pixel out of every four partial pixels, and because each colored partial pixel in its colored form is equal to one-third of a white partial pixel, three colored partial pixels combined only contribute to one white partial pixel). Color brightness and saturation are reduced by area sharing with color pixels switched to black. Area sharing is particularly problematic when mixing yellow, because yellow is brighter than any other color of equal brightness, and saturated yellow has nearly the same brightness as white. Switching the blue pixel (one-quarter of the display area) to black makes the yellow excessively dark.

[0018] U.S. Patent Nos. 8,576,476 and 8,797,634 describe multicolor electrophoretic displays having a single backplane with independently addressable pixel electrodes and a common light-transmitting first electrode layer (also called a front electrode or top electrode). Multiple electrophoretic layers are disposed between the backplane and the first light-transmitting electrode layer. The displays described in these applications can render any of the primary colors (red, green, blue, cyan, magenta, yellow, white, and black) at any pixel location. However, the use of multiple electrophoretic layers positioned between a single set of address electrodes has drawbacks. Particles in a particular layer experience a lower electric field than in a single electrophoretic layer addressed with the same voltage. Furthermore, optical losses (e.g., caused by light scattering or unwanted light absorption) in the electrophoretic layer closest to the viewing surface can affect the appearance of the image formed in the underlying electrophoretic layer.

[0019] Another type of electrophoretic display system provides a single electrophoretic medium capable of rendering color at any pixel location. Specifically, U.S. Pat. No. 9,697,778 describes a display in which a dyed solvent is combined with white (light-scattering) particles that move in one direction when addressed with a low applied voltage and in the opposite direction when addressed with a higher voltage. When the white particles and dyed solvent are combined with two additional particles of opposite charge to the white particles, it is possible to render a full-color display. However, the color states of the '778 patent are unacceptable for applications such as text readers. In particular, there is always some dyed fluid separating the white scattering particles from the viewing surface, which results in a tint in the white state of the display.

[0020] U.S. Patent No. 10,475,399 describes an electrophoretic medium capable of rendering a variety of colors at any pixel location. In this case, the electrophoretic medium contains white pigment particles having a first charge polarity, two types of colored pigment particles having a second charge polarity opposite to the first charge polarity, and a dye that is soluble in the fluid and provides the color to the electrophoretic medium.

[0021] Yet another form of electrophoretic medium capable of rendering any color at any pixel location is described in U.S. Patents 9,921,451 and 10,678,111. The electrophoretic media disclosed in these patents, which can be conveniently referred to as "Type I" electrophoretic media, contain four types of particles: white, cyan, magenta, and yellow; two of the particle types have a positive charge and two have a negative charge. However, the corresponding display suffers from color mixing with the white state. Because one type of particle has the same charge as the white particles, a certain amount of particles of the same charge will move toward the viewing surface along with the white particles when the white state is desired. This means that it is difficult to separate the white pigment from one type of colored pigment particle that has the same charge polarity as the white pigment particles. While it is possible to overcome this undesirable color shift with complex waveforms, such waveforms significantly increase the display's update time and, in some instances, result in unacceptable "flicker" between images. For example, switching between white and black optical states can be slow with Type I electrophoretic media. On the other hand, it has been observed that the color gamut achievable with Type I electrophoretic media is wide. Another form of electrophoretic medium capable of rendering any color at any pixel location is described in U.S. Patent Application Publication No. 2022 / 0082896 (Serial No. 17 / 474,582). This form of electrophoretic medium, which can be conveniently referred to as a "Type II" electrophoretic medium, contains four types of particles: white, cyan, magenta, and yellow. The white particles have a negative charge, while the cyan, magenta, and yellow particles have a positive charge. Equivalently, the white particles can have a positive charge, and the cyan, magenta, and yellow particles can have a negative charge. Having three colored pigments with polarities opposite to the white pigment in the Type II medium enables an uncontaminated white state. Furthermore, it has been observed that electrophoretic displays with Type II electrophoretic media solve the problem of slow switching between different optical states, such as between a white optical state and a black optical state. However, the present inventors have found it difficult to separate three types of colored particles of the same charge polarity into the combinations required to render various colors. Thus, it has been observed that the color gamut achieved with Type II electrophoretic media is narrower than that of Type I electrophoretic media. The above indicates the need to develop an electrophoretic medium that can achieve a good color gamut and rapid switching between different optical states of the corresponding electrophoretic display. Surprisingly, the inventors of the present invention have found that an electrophoretic medium containing four types of particles (Type I or Type II) and further containing a combination of a first charge control agent and a second charge control agent enables the construction of a corresponding display having both a wide color gamut and rapid switching. The molecular structure of the first charge control agent includes a quaternary ammonium group and a non-polar tail. The molecular structure of the second charge control agent includes two or more polar groups and a non-polar tail. [Prior art documents] [Patent documents]

[0022] [Patent Document 1] U.S. Patent No. 9,361,836 [Patent Document 2] U.S. Patent No. 9,921,451 [Patent Document 3] U.S. Patent No. 10,276,109 [Patent Document 4] U.S. Patent No. 10,353,266 [Patent Document 5] U.S. Patent No. 10,467,984 [Patent Document 6] U.S. Patent No. 10,593,272 [Patent Document 7] U.S. Patent No. 7,170,670 [Patent Document 8] U.S. Patent No. 7,321,459 [Patent Document 9] U.S. Patent No. 7,236,291 [Non-patent literature]

[0023] [Non-Patent Document 1] Kitamura, T., et al., Electrical toner movement for electronic paper-like display, IDW Japan, 2001, Paper HCS1-1 [Non-patent document 2] Yamaguchi, Y., et al., Toner display using insulative particles charged triboelectrically, IDW Japan, 2001, Paper AMD4-4 Summary of the Invention [Means for solving the problem]

[0024] overview In one aspect, the present invention discloses an improved electrophoretic medium for color electrophoretic displays. The electrophoretic medium includes a non-polar fluid, four types of particles: a first type of particles, a second type of particles, a third type of particles, and a fourth type of particles; a first charge control agent; and a second type of charge control agent. The first type of particles includes a first type of pigment, the first type of pigment being inorganic and having a first color, the first type of particles having a first charge polarity. The second type of particles includes a second type of pigment, the second type of pigment having a second color, the second color being different from the first and second colors, the second type of particles having a second charge polarity, the second charge polarity being opposite to the first charge polarity. The third type of particles includes a third type of pigment having a third color, the third color being different from the first and second colors, and the third type of particles having a second charge polarity. The fourth type of particles includes a fourth type of pigment having a fourth color, the fourth color being different from the first, second, and third colors, and the fourth type of particles having a first charge polarity or a second charge polarity. The second, third, and fourth types of pigments may be light-absorbing organic pigments. The fourth type of particles may include pigment particles and a polymer, the polymer being present in an amount of less than 35 weight percent of the weight of the particles. The first color may be white, the second color may be cyan, the third color may be magenta, and the fourth color may be yellow. The yellow, magenta, and cyan pigments may exhibit diffuse reflectance at 650, 550, and 450 nm, respectively, when each particle is approximately isotropically distributed at 15 volume percent (particle volume relative to fluid volume) in a nonpolar fluid having a refractive index less than 1.55.

[0025] The electrophoretic medium of the present invention can be used in a color electrophoretic display. The color electrophoretic display may comprise a first light-transmitting electrode layer, a second electrode layer, and an electro-optical material layer on the viewing surface. The second electrode layer includes an array of thin film transistors coupled to pixel electrodes. The electro-optical material layer containing the electrophoretic medium of the present invention is disposed between the first light-transmitting electrode layer and the second electrode layer.

[0026] The first charge control agent has a molecular structure. The molecular structure of the first charge control agent includes at least one quaternary ammonium group and a non-polar tail. The second charge control agent has a molecular structure. The molecular structure of the second charge control agent includes two or more polar groups and a non-polar tail. The two or more polar groups are selected from the group consisting of amino groups, sulfonate groups, sulfate groups, sulfinate groups, carboxylic acid groups, phosphonic acid groups, phosphinate groups, phosphate groups, hydroxyl groups, thiol groups, alpha-diketone groups, beta-diketone groups, ethylene oxide groups, and propylene oxide groups. The polar group of the second charge control agent may be a primary or secondary amino group. The polar group may also be a tertiary amino group, but is not a quaternary ammonium. The nitrogen of the amino group may be part of a heterocycle, which may be aromatic or non-aromatic. The molecular structure of the first charge control agent (and the second charge control agent) may include at least one amide group. The amide group may be part of a non-polar tail.

[0027] The non-polar tail comprises a polymeric group formed by monomers containing alkyl or alkenyl groups having at least 10 carbon atoms.

[0028] The molecular structure of the monomer used to form the polymeric group of the non-polar tail of the second charge control agent (as well as the non-polar tail of the first charge control agent) may include a carboxylic acid, a carboxylic acid anhydride, or a carboxylic acid halide. The carboxylic acid and the carboxylic acid halide may contain 10 to 22 carbon atoms. The carboxylic acid anhydride may contain 20 to 44 carbon atoms. The molecular structure of the monomer forming the polymeric tail of the second charge control agent may further include a hydroxyl group or an amine group. The monomer used to form the polymeric tail of the second charge control agent (and the first charge control agent) may be selected from the group consisting of ricinoleic acid, linoleic acid, oleic acid, linoleic acid, acyl halides of ricinoleic acid, acyl halides of linoleic acid, acyl halides of linolenic acid, ricinoleic anhydride, linoleic anhydride, oleic anhydride, and linolenic anhydride.

[0029] The molecular structure of the second charge control agent may comprise a comb polymer having alkyl or alkenyl branches and two terminal polar functional groups.

[0030] The molecular structure of the second charge control agent may include two or more polar functional groups, none of which are quaternary ammonium groups. That is, the molecular structure of the second charge control agent may include one or more quaternary ammonium functional groups. Alternatively, the molecular structure of the second charge control agent may include two or more polar groups, one or more of which may be quaternary ammonium functional groups.

[0031] The electrophoretic medium may further comprise a water-soluble ether. The water-soluble ether has a molecular weight of 75 to 5,000 daltons. The water-soluble ether can be represented by Formula I, Formula II, or Formula III, where n is 1 to 145; R1 is hydrogen, a methyl, or an ethyl group; and R2, R3, R4, R5, R6, and R7 are independently selected from the group consisting of hydrogen, a linear or branched alkyl group containing 1 to 6 carbon atoms, phenyl, and a benzyl group. Each of Formulas I, II, and III contains at least one ether functional group. For Formula I, n may also be 1 to 10. [ka] [ka]

[0032] The first, second, third, and fourth types of particles may have a polymer layer. The polymer may be complexed, adsorbed, or covalently bonded to the first, second, third, and fourth types of pigments. The first type of particles may be light-scattering particles. The first type of particles may include inorganic pigment particles, such as titanium dioxide, treated with a silane and a monomer or a combination of monomers. The monomer may be an alkyl methyl methacrylate, such as lauryl methacrylate, or an alkyl acrylate, such as lauryl acrylate. The combination of monomers may include 2,2,2-trifluoroethyl methacrylate. The first type of particles may include inorganic pigment particles, such as titanium dioxide, treated with a monomer, such as alkyl methyl methacrylate and 2,2,2-trifluoroethyl methacrylate. The second type of particles may include a second type of pigment particle, such as phthalocyanine blue (Pigment Blue 15:3), and a polymer formed from a monomer including methyl methacrylate and dimethylsiloxane, such as monomethyl methacrylate poly(dimethylsiloxane). The third type of particles may include a third type of pigment particle and a polymer formed from a monomer including methyl methacrylate and dimethylsiloxane, such as monomethyl methacrylate poly(dimethylsiloxane). The fourth type of particles may include a fourth type of pigment particle and a polymer formed from a monomer including methyl methacrylate and dimethylsiloxane, such as monomethyl methacrylate poly(dimethylsiloxane). The third type of particles may include a third type of pigment particle, such as Pigment Red 122, treated with a monomer such as vinylbenzyl chloride and methyl methacrylate. The fourth type of particles may include fourth type particles such as Pigment Yellow 155 that are treated with a monomer comprising methyl methacrylate and dimethyl siloxane, for example, monomethyl methacrylate poly(dimethyl siloxane).

[0033] The polarity of the second and third types of particles may be a second polarity, such as positive, and the first and fourth types of particles may be a first polarity, such as negative. The second, third, and fourth types of particles may all have a second polarity opposite to the first polarity. That is, the first type of particles may have a positive polarity, and the second, third, and fourth types of particles may have a negative polarity.

[0034] Alternatively, the first type of particles may have a negative polarity, and the second, third, and fourth types of particles may have a positive polarity. The second type of particles may have a second zeta potential, the third type of particles may have a third zeta potential, and the fourth type of particles may have a fourth zeta potential. The second, third, and fourth zeta potentials may all be positive. The second zeta potential may be greater than the third and fourth zeta potentials, and the fourth zeta potential may be less than the third zeta potential.

[0035] In another aspect, the present invention provides a color electrophoretic display comprising: (a) a first light-transmitting electrode layer; (b) a microcell layer comprising a plurality of microcells; (c) a sealing layer; and (d) a second electrode layer. Each microcell of the plurality of microcells has an opening. The sealing layer spans the opening of each microcell of the plurality of microcells. Each microcell of the plurality of microcells comprises an electrophoretic medium. The electrophoretic medium comprises a non-polar fluid, four types of particles: a first type of particles, a second type of particles, a third type of particles, and a fourth type of particles; and a first type of charge control agent. The first type of particles comprises a first type of pigment, the first type of pigment being inorganic and having a first color, and the first type of particles having a first charge polarity. The second type of particles may include a second type of pigment, the second type of pigment having a second color, the second color being different from the first and second colors, and the second type of particles having a second charge polarity, the second charge polarity being opposite to the first charge polarity. The third type of particles may include a third type of pigment having a third color, the third color being different from the first and second colors, and the third type of particles having a second charge polarity. The fourth type of particles may include a fourth type of pigment having a fourth color, the fourth color being different from the first, second, and third colors, and the fourth type of particles having either the first charge polarity or the second charge polarity. The fourth type of particles may include pigment particles and a polymer. The polymer may be present in an amount of less than 35 weight percent of the weight of the particles. The first color may be white, the second color may be cyan, the third color may be magenta, and the fourth color may be yellow.

[0036] The first charge control agent has a molecular structure, and the molecular structure of the first charge control agent includes a quaternary ammonium group and a non-polar tail.

[0037] The color electrophoretic display also includes a water-soluble ether. The water-soluble ether has a molecular weight of 75 to 5,000 daltons. The water-soluble ether is present in the sealing layer or in the electrophoretic layer of the color electrophoretic display. The water-soluble ether may be present in the sealing layer and the electrophoretic layer of the color electrophoretic display. The water-soluble ether can be represented by Formula I, Formula II, or Formula III, where n is 1 to 145; R1 is hydrogen, a methyl, or an ethyl group; and R2, R3, R4, R5, R6, and R7 are independently selected from the group consisting of hydrogen, a linear or branched alkyl group containing 1 to 6 carbon atoms, phenyl, and a benzyl group. Each of Formulas I, II, and III includes at least one ether functional group. For Formula I, n may also be 1 to 10. Water-soluble ethers include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-propyl ether, ethylene glycol monoisopropyl ether, ethylene glycol n-monobutyl ether, ethylene glycol monoisobutyl ether, ethylene glycol mono-t-butyl ether, ethylene glycol monobenzyl ether, ethylene glycol monophenyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol di-n-propyl ether, ethylene glycol diisopropyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol mono-n-propyl ether, diethylene glycol monoisopropyl ether, diethylene glycol n-monobutyl ether, diethylene glycol monoisobutyl ether, diethylene glycol mono-t-butyl ether, diethylene glycol monobenzyl ether, diethylene glycol monophenyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol ethyl methyl ether, diethylene glycol di-n-propyl ether, diethylene glycol disopropyl ether, diethylene glycol di-n-butyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether,Triethylene glycol mono-n-propyl ether, triethylene glycol monoisopropyl ether, triethylene glycol n-monobutyl ether, triethylene glycol monoisobutyl ether, triethylene glycol mono-t-butyl ether, triethylene glycol monobenzyl ether, triethylene glycol monophenyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol di-n-propyl ether, triethylene glycol diisopropyl ether, tetraethylene glycol monomethyl ether, tetraethylene glycol monoethyl ether, triethylene glycol monophenyl ether, tetraethylene glycol dimethyl ether, polyethylene glycol monomethyl ether, polyethylene glycol monoethyl ether, polyethylene glycol monophenyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol mono-n-propyl ether, propylene glycol monoisopropyl ether, The propylene glycol mono-n-butyl ether, propylene glycol monoisobutyl ether, propylene glycol monophenyl ether, propylene glycol dimethyl ether, propylene glycol diethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol mono-n-propyl ether, dipropylene glycol monoisopropyl ether, dipropylene glycol mono-n-butyl ether, dipropylene glycol monoisobutyl ether, dipropylene glycol dimethyl ether, dipropylene glycol diethyl ether, dipropylene glycol di-n-propyl ether, dipropylene glycol diisopropyl ether, tripropylene glycol monomethyl ether, tripropylene glycol monoethyl ether, tripropylene glycol mono-n-propyl ether, tripropylene glycol monoisopropyl ether, tripropylene glycol mono-n-butyl ether, and tripropylene glycol monoisobutyl ether.

[0038] The electrophoretic medium may further include a second charge control agent having a molecular structure. The molecular structure of the second charge control agent may include two or more polar groups and a non-polar tail. The two or more polar groups may be selected from the group consisting of amino groups, sulfonate groups, sulfate groups, sulfinate groups, carboxylic acid groups, phosphonic acid groups, phosphinate groups, phosphate groups, hydroxyl groups, thiol groups, alpha-diketone groups, beta-diketone groups, ethylene oxide groups, and propylene oxide groups. The non-polar tail includes a polymeric group. The polymeric group may be formed from a monomer containing an alkyl or alkenyl group having at least 10 carbon atoms. The molecular structure of the monomer used to form the polymeric group of the non-polar tail of the second charge control agent may include a carboxylic acid, a carboxylic acid anhydride, or a carboxylic acid halide. The carboxylic acid and the carboxylic acid halide may contain 10 to 22 carbon atoms. The carboxylic acid anhydride may contain 20 to 44 carbon atoms. The molecular structure of the monomer forming the polymer tail of the second charge control agent may further include a hydroxyl group or an amine group. The monomer used to form the polymer tail of the second charge control agent may be selected from the group consisting of ricinoleic acid, linoleic acid, oleic acid, linoleic acid, acid halides of ricinoleic acid, acid halides of linoleic acid, acid halides of linolenic acid, ricinoleic anhydride, linoleic anhydride, oleic anhydride, and linolenic anhydride. The molecular structure of the second charge control agent may include a comb polymer having alkyl or alkenyl branches and two terminal polar functional groups.

[0039] The molecular structure of the second charge control agent may include two or more polar functional groups, none of which are quaternary ammonium groups. That is, the molecular structure of the second charge control agent may include one or more quaternary ammonium functional groups. Alternatively, the molecular structure of the second charge control agent may include two or more polar groups, one or more of which may be quaternary ammonium functional groups. In an embodiment of the present invention, for example, the following items are provided: (Item 1) a non-polar fluid; a first type of particles comprising a first type of pigment, the first type of pigment being inorganic and having a first color, and the first type of particles having a first charge polarity; a second type of particles comprising a second type of pigment, the second type of pigment having a second color, the second color being different from the first color, the second type of particles having a second charge polarity, the second charge polarity being opposite to the first charge polarity; a third type of particles including a third type of pigment having a third color, the third color being different from the first and second colors, and the third type of particles having the second charge polarity; a fourth type of particles including a fourth type of pigment having a fourth color, the fourth color being different from the first, second, and third colors, and the fourth type of particles having the first charge polarity or the second charge polarity; a first charge control agent having a molecular structure comprising at least one quaternary ammonium group and a non-polar tail; and a second charge control agent having a molecular structure comprising two or more polar groups and a non-polar tail, wherein the two or more polar groups are selected from the group consisting of amino groups, sulfonate groups, sulfate groups, sulfinate groups, carboxylic acid groups, phosphonic acid groups, phosphinate groups, phosphate groups, hydroxyl groups, thiol groups, alpha diketone groups, beta diketone groups, ethylene oxide groups, and propylene oxide groups, and wherein the non-polar tail comprises a polymeric group formed by a monomer comprising an alkyl or alkenyl group having at least 10 carbon atoms. 1. An electrophoretic medium comprising: (Item 2) 2. The electrophoretic medium according to item 1, wherein the molecular structure of the monomer used to form the polymer group of the non-polar tail of the second charge control agent comprises a carboxylic acid, a carboxylic acid anhydride, or a carboxylic acid halide, wherein the carboxylic acid and the carboxylic acid halide comprise 10 to 22 carbon atoms, and the carboxylic acid anhydride comprises 20 to 44 carbon atoms. (Item 3) 3. The electrophoretic medium of claim 2, wherein the molecular structure of the monomer forming the polymer tail of the second charge control agent further comprises a hydroxyl group or an amine group. (Item 4) 2. The electrophoretic medium of claim 1, wherein the monomer used to form the polymer tail of the second charge control agent is selected from the group consisting of ricinoleic acid, linoleic acid, oleic acid, linoleic acid, acid halides of ricinoleic acid, acid halides of linoleic acid, acid halides of linolenic acid, ricinoleic anhydride, linoleic anhydride, oleic anhydride, and linolenic anhydride. (Item 5) Item 2. An electrophoretic medium according to item 1, wherein the molecular structure of the second charge control agent comprises a comb polymer having alkyl or alkenyl branches and two terminal polar functional groups. (Item 6) 2. The electrophoretic medium according to item 1, further comprising a water-soluble ether, the water-soluble ether having a molecular weight of 75 to 5,000 daltons. (Item 7) the first, second, third and fourth types of particles have a layer of a polymer; is complexed, adsorbed, or covalently bonded to said first, second, third, and fourth types of pigments. (Item 8) 8. The electrophoretic medium of claim 7, wherein the second and fourth types of particles comprise a polymer formed from a monomer comprising methyl methacrylate and dimethylsiloxane. (Item 9) 8. The electrophoretic medium of claim 7, wherein the third type of particles is formed by treatment of pigment particles with (a) methyl methacrylate and a monomer comprising dimethylsiloxane, or (b) vinylbenzyl chloride and an acrylate or methacrylate monomer. (Item 10) 2. The electrophoretic medium according to item 1, further comprising a water-soluble ether, the water-soluble ether having a molecular weight of 75 to 5,000 daltons. (Item 11) 2. The electrophoretic medium of claim 1, wherein the polarities of the second, third, and fourth types of particles are all positive, the second type of particles have a second zeta potential, the third type of particles have a third zeta potential, and the fourth type of particles have a fourth zeta potential, and the first zeta potential is greater than the second and third zeta potentials and the fourth zeta potential is less than the third zeta potential. (Item 12) Item 1. An electrophoretic display according to item 1, wherein the first color is white, the second color is cyan, the third color is magenta, and the fourth color is yellow. (Item 13) a first optically transparent first electrode layer at the viewing surface; a second electrode layer including an array of thin film transistors coupled to the pixel electrodes; An electro-optical material layer including the electrophoretic medium according to item 1, the electro-optical material layer being disposed between the first light-transmitting electrode layer and the second electrode layer; A color electrophoretic display comprising: (Item 14) a first light-transmitting electrode layer; a microcell layer comprising a plurality of microcells, each microcell of the plurality of microcells having an opening, each microcell of the plurality of microcells containing an electrophoretic medium; a sealing layer with a sealing membrane spanning the opening of each microcell; a second electrode layer; A color electrophoretic display comprising: The electrophoretic medium is a non-polar fluid; a first type of particles comprising a first type of pigment, the first type of pigment being inorganic and having a first color, and the first type of particles having a first charge polarity; a second type of particles comprising a second type of pigment, the second type of pigment having a second color, the second color being different from the first color, the second type of particles having a second charge polarity, the second charge polarity being opposite to the first charge polarity; a third type of particles including a third type of pigment having a third color, the third color being different from the first and second colors, and the third type of particles having the second charge polarity; a fourth type of particles including a fourth type of pigment having a fourth color, said fourth color being different from said first, second and third colors, said fourth type of particles having said first charge polarity; or a fourth type of particles having the second charge polarity; a first charge control agent having a molecular structure, the molecular structure of the first charge control agent including a quaternary ammonium group and a non-polar tail; Including, The color electrophoretic display comprises a water-soluble ether, the water-soluble ether having a molecular weight of 75 to 5,000 daltons, and the water-soluble ether is present in the sealing layer or in the electrophoretic layer of the color electrophoretic display. (Item 15) Item 15. The color electrophoretic display of item 14, wherein the electrophoretic medium further comprises a second charge control agent having a molecular structure comprising two or more polar groups and a non-polar tail, the two or more polar groups being selected from the group consisting of amino groups, sulfonate groups, sulfate groups, sulfinate groups, carboxylic acid groups, phosphonic acid groups, phosphinate groups, phosphate groups, hydroxyl groups, thiol groups, alpha diketone groups, beta diketone groups, ethylene oxide groups, and propylene oxide groups, and the non-polar tail comprises a polymeric group formed by a monomer comprising an alkyl or alkenyl group having at least 10 carbon atoms. (Item 16) 15. The color electrophoretic display of item 14, wherein the second type of particles has a second zeta potential, the third type of particles has a third zeta potential, and the fourth type of particles has a fourth zeta potential, the second, third, and fourth zeta potentials are positive, the second zeta potential is greater than the third and fourth zeta potentials, and the fourth zeta potential is less than the third zeta potential. (Item 17) Item 15. The color electrophoretic display of item 14, wherein the fourth type of particles comprises pigment particles and a polymer, the polymer having a content of less than 35 weight percent of the weight of the particles. (Item 18) Item 15. The color electrophoretic display of item 14, wherein the first color is white, the second color is cyan, the third color is magenta, and the fourth color is yellow. (Item 19) Item 15. The color electrophoretic display of item 14, wherein the second and fourth types of particles comprise a polymer formed from a monomer comprising methyl methacrylate and dimethyl siloxane. (Item 20) Item 15. The color electrophoretic display of item 14, wherein the third type of particles is formed by treatment of pigment particles with (a) methyl methacrylate and a monomer comprising dimethylsiloxane, or (b) vinylbenzyl chloride and an acrylate or methacrylate monomer. [Brief explanation of the drawings]

[0040] [Figure 1] FIG. 1 is a schematic cross-sectional view showing the positions of various types of particles in an electrophoretic medium of the present invention when displaying black, white, three subtractive primary colors, and three additive primary colors.

[0041] [Figure 2] FIG. 2 is a schematic illustration of an electrophoretic display having a Type I electrophoretic medium with four types of particles in a non-polar fluid.

[0042] [Figure 3A] FIG. 3A is a schematic illustration of an electrophoretic display having a Type II electrophoretic medium with four types of particles in a non-polar fluid.

[0043] [Figure 3B] FIG. 3B shows a transition between a first optical state in which all particles of a first charge polarity are present at the observation surface, and a second optical state in which particles with a second (opposite) polarity are present at the observation surface.

[0044] [Figure 3C] FIG. 3C shows a transition between a first optical state, in which all of the particles of a first charge polarity are at the observation surface, and a third optical state, in which intermediate charged particles of the first polarity are located at the observation surface followed by particles of a second (opposite) polarity.

[0045] [Figure 3D]FIG. 3D shows a transition between a first optical state, in which all of the particles of a first charge polarity are at the observation surface, and a fourth optical state, in which less charged particles of the first polarity are located at the observation surface followed by particles of a second (opposite) polarity.

[0046] [Figure 3E] FIG. 3E shows a transition between a first optical state, in which all of the particles of a first charge polarity are at the observation surface, and a fifth optical state, in which a combination of low- and medium-charge particles of the first polarity are located at the observation surface followed by particles with a second (opposite) polarity.

[0047] [Figure 4] FIG. 4 shows an exemplary push-pull driving scheme for addressing an electrophoretic medium containing three subtractive particles and a scattering (white) particle.

[0048] [Figure 5] FIG. 5 shows an exemplary equivalent circuit of a single pixel of an electrophoretic display.

[0049] [Figure 6] FIG. 6 shows the layers of an exemplary microcell electrophoretic color display.

[0050] [Figure 7] FIG. 7 shows a graph of zeta potential versus weight ratio of charge control agent in an electrophoretic medium containing a white pigment with a polymer surface treatment.

[0051] [Figure 8] FIG. 8 shows a graph of color versus applied voltage for the white optical state of an electrophoretic display comprising an electrophoretic medium having white, yellow, cyan, and magenta particles and three different ratios of two charge control agents.

[0052] [Figure 9]FIG. 9 shows a graph for determining the time required to switch from a black state to a white state in three electrophoretic compositions having different ratios of two charge control agents.

[0053] [Figure 10] FIG. 10 shows the test waveforms used to determine the color gamut of the inventive and comparative electro-optical devices.

[0054] [Figure 11] FIG. 11 shows a graph for determining the color gamut of an electrophoretic display comprising an electrophoretic medium having white, yellow, cyan, and magenta particles and three different ratios of two charge control agents. DETAILED DESCRIPTION OF THE INVENTION

[0055] Detailed Description The term "polyamine," as used herein, refers to an organic compound having a molecular structure containing two or more amino groups, which may be primary, secondary, or tertiary amino groups.

[0056] The term "carboxylic acid derivative" refers to a compound having a molecular structure that includes a carboxylic acid halide or a carboxylic acid anhydride.

[0057] The term "condensation polymerization" is a form of polymerization in which monomers and / or oligomers react with each other to form larger structures while releasing smaller molecules as by-products, such as water, hydrochloric acid, methanol, etc.

[0058] As used herein, "molecular weight" refers to weight average molecular weight unless otherwise specified. Molecular weight is measured using industry standard size exclusion column chromatography.

[0059] The terms "amine," "amino group," and "amino functional group" include primary, secondary, and tertiary amines, and primary, secondary, and tertiary amine functional groups. The terms do not include "quaternary amines" and "quaternary amine functional groups."

[0060] The terms "quaternary amine" or "quaternary ammonium" or "quaternary ammonium salt" or "quaternary amine functional group" include functional groups in which the nitrogen atom has four substituents, none of which is hydrogen. That is, the nitrogen atom of a quaternary amine has no hydrogen atoms directly attached to it. The term "cationic charge control agent" means that the charge control agent comprises a "quaternary amine."

[0061] The terms "pigment" and "pigment particles" are synonymous in this application.

[0062] The present invention provides an improved four-particle electrophoretic medium comprising four types of particles, a first type, a second type, a third type, and a fourth type, in a non-polar fluid. The first type of particles comprises a first type of pigment having a first color, the first type of pigment being inorganic. The first type of particles have a first charge polarity. The second type of particles comprise a second type of pigment having a second color different from the first color. The second type of particles have a second charge polarity, the second charge polarity being opposite to the first charge polarity. The third type of particles comprise a third type of pigment having a third color different from the first and second colors. The third type of particles have a second charge polarity. The fourth type of particles comprise a fourth type of pigment having a fourth color different from the first, second, and third colors. The fourth type of particles have either the first or second charge polarity.

[0063] Charge Control Agent

[0064] Charge control agents (CCAs) are used in the electrophoretic medium of electrophoretic displays to control the charge on electrophoretic particles. Typically, CCAs are surfactant-like molecules with an ionic or other polar group, hereafter referred to as the head group, and a nonpolar chain (typically a hydrocarbon chain), hereafter referred to as the tail. CCAs may be complexed with charged particles or absorbed into the particles. That is, the electrophoretic particles and CCA may exist as charge complexes or may be loosely associated through van der Waals forces. It is believed that CCAs form reverse micelles in the electrophoretic medium, and that it is the small populations of charged reverse micelles that provide electrical conduction in the medium. Reverse micelles comprise a polar core, which may vary in size from 1 nm to tens of nanometers and may have a spherical, cylindrical, or other geometric structure, surrounded by the nonpolar tail groups of CCA molecules. In an electrophoretic medium, three phases can typically be distinguished: solid particles with surfaces, a highly polar phase distributed in the form of very small droplets (reverse micelles), and a continuous phase containing a nonpolar fluid. Both the electrophoretic particles and the charged reverse micelles can move through the fluid upon application of an electric field; therefore, there are two parallel paths for electrical conduction through the fluid (which itself typically has negligible electrical conductivity).

[0065] The polar core of the reverse micelle is thought to affect the charge on the surface by adsorption to the surface. In electrophoretic displays, such adsorption can occur on the surface of electrophoretic particles or on the inner walls of microcapsules (or other solid phases, e.g., microcell walls) to form structures similar to reverse micelles; these structures are hereafter referred to as semi-micelles. If one ion of an ion pair is more strongly bound to the surface than the other ion, ion exchange between the semi-micelle and the unbound reverse micelle can result in charge separation, with the more strongly bound ion remaining associated with the particle and the more weakly bound ion becoming incorporated into the core of the free reverse micelle.

[0066] It is also possible that the ionic substance of the head group of CCA induces ion pair formation on particle (or other) surfaces. Thus, CCA can perform two basic functions: charge generation on the surface and charge separation from the surface. Charge generation can be the result of an acid-base or ion exchange reaction between some moieties present in the CCA molecule (or otherwise incorporated into the reverse micelle core or fluid) and the particle surface. Therefore, useful CCA substances can participate in such interactions or any other charge interactions known in the art.

[0067] The mechanism of particle control in electrophoretic media is not fully understood. Uncontrolled processes can lead to undesirably high conductivity of the electrophoretic medium. Furthermore, if the charge control agent is only physically adsorbed on the particles, changes in conditions can cause partial or complete desorption of the charge control agent from the particles, resulting in undesirable changes in the particles' electrophoretic properties. The desorbed charge control agent can re-adsorb onto other surfaces in the electrophoretic medium, which can potentially cause additional problems. The effects of charge control agents are particularly difficult to predict in electrophoretic media containing multiple types of electrophoretic particles, and charge control agents may adsorb onto the surfaces of different types of particles. In the case of encapsulated electrophoretic media, it is also possible for the charge control agent to adsorb onto the capsule walls.

[0068] The charge control agent may have an average molecular weight of greater than 500 grams / mole, or greater than 1,000 grams / mole, or greater than 1,500 grams / mole, or greater than 3,000, or greater than 5,000, or greater than 10,000. For example, the average molecular weight of the charge control agent may be 500 grams / mole to 12,000 grams / mole, 1,000 to 10,000 grams / mole, 2,000 to 8,000 grams / mole, 600 grams / mole to 2,000 grams / mole, or 2,000 to 11,000 grams / mole.

[0069] The electrophoretic medium of the present invention comprises a combination of first and second charge control agents, the first and second charge control agents being soluble in the non-polar fluid of the electrophoretic medium.

[0070] The molecular structure of the first charge control agent includes a quaternary ammonium group and a non-polar tail.

[0071] The non-polar tail of the first charge control agent may comprise a polymeric group formed by a monomer comprising at least 10 carbon atoms. The non-polar tail of the first charge control agent may comprise a polymeric group formed by a monomer, the molecular structure of which comprises at least 10 carbon atoms.

[0072] The molecular structure of the monomer used to form the non-polar tail of the first charge control agent may include a carboxylic acid, a carboxylic acid anhydride, or a carboxylic acid halide, where the carboxylic acid and the carboxylic acid halide contain 10 to 22 carbon atoms, and the carboxylic acid anhydride contains 20 to 44 carbon atoms.

[0073] The molecular structure of the monomer forming the non-polar tail of the first charge control agent may include at least one carbon-carbon double bond. The molecular structure of the monomer used to form the polymeric tail of the first charge control agent may further include a hydroxyl group. The monomer used to form the polymeric tail of the first charge control agent may be selected from the group consisting of 8-hydroxystearic acid, ricinoleic acid, linoleic acid, oleic acid, linoleic acid, acid halides of 8-hydroxystearic acid, acid halides of ricinoleic acid, acid halides of linoleic acid, acid halides of linolenic acid, 8-hydroxystearic acid anhydride, ricinoleic acid anhydride, linoleic acid anhydride, oleic acid anhydride, and linolenic acid anhydride.

[0074] The molecular structure of the first charge control agent may include one or more quaternary ammonium groups. The molecular structure of the first charge control agent may include 1 to 12 quaternary ammonium groups. The quaternary ammonium groups have the structure [NR1R2R3R4] +where examples of R1, R2, R3, and R4 may include alkyl and aryl groups. The molecular structure of the first charge control agent includes a non-polar tail and at least one quaternary ammonium group. Reagents useful for preparing the first charge control agent are a class of fatty acid quaternary ammonium compounds offered by Akzo Nobel under the trade name ARQUAD™.

[0075] Non-limiting examples of charge control agents that meet the structural criteria of the first charge control agent include commercially available polymeric materials such as Solsperse™ 17000, Solsperse™ 16000, and Solsperse™ 19000, supplied by Lubrizol Corporation. Solsperse™ 17000 is a reaction product of 12-hydroxyoctadecanoic acid homopolymer with N,N-dimethyl-1,3-propanediamine and methyl bisulfate. Solsperse™ 16000 and Solsperse™ 19000 have carbon-carbon bonds in their nonpolar tails. When unsaturated quaternary ammonium charge control agents are included in electrophoretic media, the media have been observed to have improved switching speeds at low temperatures. The nonpolar tails of quaternary ammonium charge control agents can be formed from polymeric or oligomeric compounds, such as polyesters. In some examples, polyesters can be formed by the condensation reaction of carboxylic acids, such as fatty acids, with secondary amines. In such instances, the condensation reaction results in repeating units having at least one carbon-carbon double bond (i.e., unsaturated) with repeating pendant carbon chains that may be saturated or unsaturated, and the overall length of the monomers from which the tail is formed is at least 10 carbon atoms in length, e.g., 14 carbon atoms in length, e.g., 18 carbon atoms in length.

[0076] Other compounds meeting the structural criteria for the first charge control agent are disclosed in U.S. Patent Application Publication No. 2020 / 0355978, which is incorporated herein by reference in its entirety. Non-limiting examples of first charge control agents are provided in the structures of Formulae IV-X below, where R1 is polyricinoleic acid and R2 is polyisobutylene. These compounds can be prepared by reacting a molecule containing a primary amino group (or hydroxyl group) and a tertiary amine with a monomer (e.g., ricinoleic acid or isobutylene), followed by reacting the formed polymer with dimethyl sulfate to quaternize the tertiary amine. The amine reagent can be selected from, for example, 3-(dimethylamino)-1-propylamine, 3-dimethylamino-1-propanol, 1,4-bis(3-aminopropyl)piperazine, and tris(3-aminopropyl)amine. [ka] [ka] [ka]

[0077] The first charge control agent may be added to the electrophoretic medium at a concentration of greater than 0.2 g of charge control agent per 100 g of electrophoretic particles, or greater than 0.5 g of charge control agent per 100 g of electrophoretic particles, or greater than 1.0 g of charge control agent per 100 g of electrophoretic particles. The first charge control agent may be added to the electrophoretic medium at a concentration of greater than 1 g of charge control agent per 100 g of electrophoretic particles. For example, the ratio of the first charge control agent to the electrophoretic particles may be 1:30 (wt / wt), such as 1:25 (wt / wt), or 1:20 (wt / wt).

[0078] The molecular structure of the second charge control agent includes two or more polar groups and a non-polar tail. The polar groups are selected from the group consisting of amino groups, carboxylic acid groups, phosphonic acid groups, hydroxyl groups, thiol groups, alpha-diketone groups, beta-diketone groups, ethylene oxide groups, and propylene oxide groups. The amino groups may be primary, secondary, or tertiary. The nitrogen atom of the amino group may be part of a heterocycle, which may be aromatic or non-aromatic. Non-limiting examples of such heterocycles include acridine, benzimidazole, pyrazole, imidazole, piperazine, pyrazine, pyrimidine, pyrrole, quinazoline, triazine, azaindole, pyridine, bipyridine, indole, piperidine, pyridazine. Examples of suitable alpha-diketone groups include benzophenone, pyrrolidine, quinoxaline, triazole, azetidine, carbazole, imidazole, imidazoline, indoline, isoindoline, piperidone, pyrazoline, pyrazolidine, aziridine, isoquinoline, purine, pyrazolo[1,5,α]pyrimidine, quinazoline, and derivatives thereof. The alpha-diketone group may be part of a ring, such as in squaric acid and derivatives, or croconic acid and derivatives.

[0079] The non-polar tail of the second charge control agent may include a polymeric group having one or more saturated alkyl functional groups. The non-polar tail of the second charge control agent may include a polymeric group having at least one carbon-carbon double bond. The polymeric group may be formed by a monomer containing at least 10 carbon atoms. The molecular structure of the monomer may include a carboxylic acid, a carboxylic acid anhydride, or a carboxylic acid halide, where the carboxylic acid and carboxylic acid halide contain 10 to 22 carbon atoms and a carbon-carbon double bond, and the carboxylic acid anhydride contains 20 to 44 carbon atoms and two carbon-carbon double bonds. The molecular structure of the monomer may also include a hydroxyl group. Non-limiting examples of monomers used to form the polymeric tail of the second charge control agent may be selected from the group consisting of ricinoleic acid, linoleic acid, oleic acid, linoleic acid, acyl halides of ricinoleic acid, acyl halides of linoleic acid, acyl halides of linolenic acid, ricinoleic anhydride, linoleic anhydride, oleic anhydride, and linolenic anhydride. The second charge control agent may be a brush-like polymer with a long nonpolar (hydrophobic) chain attached to two or more polar groups (head groups) via a linking group. The head groups of the second charge control agent may independently interact with and adsorb to the surface of electrophoretic particles. The presence of multiple head groups may introduce strong adsorption of the second charge control agent onto certain electrophoretic particles. As a result, a first charge control agent with an affinity for the particle surface containing the second charge control agent may strongly adsorb onto the particle and modify the particle's zeta potential. Surface modification of particles (pigments and other particles) is common in many industries. In many cases, this is achieved by modifying the particle's synthetic route to form customized particles. This method can be expensive, and the synthesized product may only be used for specific systems. On the other hand, the method of adjusting the particle's zeta potential using a combination of two charge control agents can be carried out directly with the composition of the present application. Specifically, one or more charge control agents can be added directly before or during milling of the particle dispersion used to prepare the electrophoretic medium. In other words, commercially available pigment products can be used without the need to pre-synthesize customized pigments.Furthermore, by adjusting the properties and weight ratio of the corresponding charge control agent, the zeta potential of the particles can be adjusted to the desired level. Therefore, the same pigment can be used for various electrophoretic media, and the particles will have different zeta potentials in each medium. It should be noted that when the term "adsorption of a polymer (or other material) onto particles in a liquid carrier" is used in reference to a polymer that is soluble in the carrier, it is recognized that a dynamic equilibrium of the polymer exists between the particle surface and the soluble polymer in the carrier. Stronger adsorption of the polymer onto the particles means an increase in the amount of polymer on the particles relative to the total amount of polymer present.

[0080] A non-limiting example of a commercially available polymeric material that can be used as the second charge control agent is Solsperse™ 8000 supplied by Lubrizol Corporation.

[0081] The second charge control agent can be formed by a condensation reaction of a hydroxycarboxylic acid (or a hydroxycarboxylic acid derivative) in the presence of a polyamine. The polymerization reaction is a polycondensation between the reagents. The condensation reaction forms (a) an ester bond between the hydroxyl group of the hydroxycarboxylic acid (or a hydroxycarboxylic acid derivative) and the carboxyl group (or a carboxylic acid halide or a carboxylic acid anhydride) of another molecule of the hydroxycarboxylic acid (or a hydroxycarboxylic acid derivative), and (b) an amide bond between the amine group of the polyamine and the carboxyl group (or a carboxylic acid halide or a carboxylic acid anhydride) of the hydroxycarboxylic acid (or a hydroxycarboxylic acid derivative).

[0082] The second charge control agent may be added to the electrophoretic medium at a concentration of greater than 0.2 g of charge control agent per 100 g of electrophoretic particles, or greater than 0.5 g of charge control agent per 100 g of electrophoretic particles, or greater than 1.0 g of charge control agent per 100 g of electrophoretic particles. For example, the ratio of first charge control agent to electrophoretic particles may be 1:30 (wt / wt), such as 1:25 (wt / wt), or 1:20 (wt / wt).

[0083] The weight ratio of the second charge control agent to the first charge control agent may be about 1:10 to about 10:1, or about 1:9 to about 10:9, or about 1:8 to about 8:1, or about 1:6 to about 6:1, or about 1:5 to about 5:1, or about 1:3 to about 3:1, or about 1:2 to about 2:1, or about 1:1:5 to 1.5:1.

[0084] Electrophoretic particles

[0085] In one embodiment, the electrophoretic medium of the present invention comprises four types of particles: first, second, third, and fourth types of particles, where the first and fourth types of particles have a first charge polarity and the second and third types of particles have a second polarity opposite to the first polarity. This represents a Type I electrophoretic medium. Typically, such a system comprises negatively charged white particles, negatively charged yellow particles, positively charged cyan particles, and positively charged magenta particles. Yellow, cyan, and magenta represent subtractive primary colors.

[0086] In another embodiment, the electrophoretic medium of the present invention comprises four types of particles: first, second, third, and fourth types of particles, where the first type of particles has a first charge polarity and the second, third, and fourth types of particles have a second polarity opposite to the first polarity. This represents a Type II electrophoretic medium. Typically, such a system comprises negative white particles and positively charged particles of the subtractive primary colors yellow, magenta, and cyan.

[0087] Furthermore, one or more types of particles (in both Type I and Type II electrophoretic media) may be designed so that their electrophoretic mobility is nonlinear with the strength of the applied electric field. Thus, application of a high electric field (e.g., 20 V or higher) of the correct polarity will cause one or more types of particles to experience a decrease in electrophoretic mobility. The various optical states of such a four-particle system (Type I and Type II) are shown schematically in Figure 1. A corresponding display can provide white, yellow, red, magenta, blue, cyan, green, and black color states at each pixel.

[0088] As shown in Figure 1, each of the eight primary colors (red, green, blue, cyan, magenta, yellow, black, and white) corresponds to a different arrangement of four types of particles so that the viewer sees only the colored particles on the viewing side of the white-type particles (i.e., the only type of particle that scatters light). To achieve a wide range of colors, additional voltage levels must be used for finer control of particle type. In the described formulation, the first type of particle is reflective (typically white), and the other three types of particles include three substantially non-light-scattering ("SNLS") particles. As mentioned above, the use of SNLS particles allows for color mixing, providing more color results than can be achieved with the same number of scattering-type particles. These thresholds must be sufficiently separated to avoid crosstalk, which necessitates the use of high address voltages for some colors. The disclosed four-particle electrophoretic media can also be rapidly updated, requiring "flicker-resistant" transitions, and produce a spectrum of colors that are pleasing to the viewer (and therefore commercially valuable). Additionally, the disclosed formulation provides rapid (e.g., less than 500 ms, e.g., less than 300 ms, e.g., less than 200 ms, e.g., less than 100 ms) updates between black and white pixels, thereby enabling rapid page turns in black against white text.

[0089] In FIG. 1 , the viewing surface of the display is at the top (as shown), i.e., the user views the display from this direction, and light is assumed to be incident from this direction. As previously mentioned, in a preferred embodiment, only one of the four types of particles used in the electrophoretic medium of the present invention substantially scatters light, and in FIG. 1 , this type of particle is assumed to be a white pigment. The light-scattering white-type particles form a white reflector against which any particles above them are viewed (as shown in FIG. 1 ). Light entering the viewing surface of the display passes through these types of particles, reflects off the white-type particles, and passes through these types of particles again before exiting the display. Thus, the particles above the white-type particles may absorb various colors, and the color seen by the user is the color resulting from the combination of the particles above them. Any particles located below the white-type particles (behind them from the user's perspective) are masked by the white-type particles and do not affect the displayed color. Because the second, third and fourth types of particles are substantially non-light scattering, their order or placement relative to each other is not important, but for reasons previously discussed, their order or placement relative to the white (light scattering) particles is very important.

[0090] More specifically, when cyan, magenta, and yellow particles are located beneath the white particles (situation [A] in Figure 1), no particles are located above the white particles, and the pixel simply displays white. When a single type of particle is located above the white particles, the color of that single particle is displayed—yellow, magenta, and cyan, respectively, in situations [B], [D], and [F] in Figure 1. When two types of particles are located above the white particles, the displayed color is a combination of the colors of these two particles: red, magenta, and yellow particles in situation [C]; blue, cyan, and magenta particles in situation [E]; and green, yellow, and cyan particles in situation [G]. Finally, when all three types of colored particles are located above the white particles (situation [H] in Figure 1), all of the incident light is absorbed by the three subtractive primary color particles, and the pixel displays black.

[0091] A subtractive primary color can be rendered by a type of light-scattering particle, such that the display includes two types of light-scattering particles, one white and the other colored. However, in this case, the location of the light-scattering colored particles relative to the other colored particles that cover the white particles is important. For example, when rendering black (when all three colored particles are on top of the white particles), the scattering colored particles cannot be on top of the non-scattering colored particles (otherwise they will be partially or completely hidden behind the scattering particles, and the color rendered will be the color of the scattering colored particles, not black).

[0092] FIG. 1 illustrates an ideal situation where there is no color contamination (i.e., the light-scattering white particles completely mask any particle types behind them). In reality, masking by the white particles may be imperfect, and thus some slight light absorption may occur by particle types that would ideally be completely masked. Such contamination typically reduces both the brightness and saturation of the colors being rendered. In the electrophoretic media of the present invention, such color contamination should be minimized to the point where the colors produced meet industry standards for color rendering. A particularly preferred standard is SNAP (Standard for Newspaper Advertising Production), which provides L for each of the eight primary colors mentioned above. * , a * and b * Hereafter, "primary colors" will be used to refer to the eight colors shown in Figure 1: black, white, the three subtractive primaries, and the three additive primaries.

[0093] FIG. 2 shows a schematic cross-sectional view of four types of particles of a Type I electrophoretic medium used in the present invention.

[0094] A display layer utilizing the improved electrophoretic medium includes a first (viewing) surface 23 on the viewing side and a second surface 24 opposite the first surface 23. The electrophoretic medium is disposed between the two surfaces. Each space between two vertical dotted lines represents a pixel. Within each pixel, the electrophoretic medium can be addressed, and each pixel's viewing surface 23 can achieve the color states shown in FIG. 1 without the need for additional layers and without a color filter array.

[0095] As is standard for electrophoretic displays, the first surface 23 includes a first light-transmitting electrode layer 21 constructed from a sheet of PET with indium tin oxide (ITO) disposed thereon. On the second surface (24) is a second electrode layer 22 including a plurality of pixel electrodes 25. Such pixel electrodes are described in U.S. Pat. No. 7,046,228, the contents of which are incorporated herein by reference in their entirety. It is noted that while the active matrix driven by a thin-film transistor (TFT) backplane is referred to in terms of a layer of pixel electrodes, the scope of the present invention encompasses other types of electrode addresses, so long as the electrodes perform the desired function. For example, the first and second electrode layers (or top and bottom electrodes) may be continuous. Furthermore, pixel electrode backplanes different from those described in the '228 patent are also suitable, including active matrix backplanes capable of providing higher drive voltages than those typically found with amorphous silicon thin-film transistor backplanes.

[0096] Newly developed active matrix backplanes may include thin-film transistors incorporating metal oxide materials, such as tungsten oxide, tin oxide, indium oxide, zinc oxide, or more complex metal oxides, such as indium gallium zirconium oxide. In these applications, such metal oxide materials are used to form the channel-forming region for each transistor, allowing for faster switching of higher voltages. Such metal oxide transistors also enable lower leakage in the "off" state of the thin-film transistor (TFT) than can be achieved with, for example, amorphous silicon TFTs. In a typical scanning TFT backplane containing n lines, the transistors are in the "off" state for a period approximately equal to (n-1) / n of the time required to refresh every line of the display. Any leakage of charge from the storage capacitor associated with each pixel results in degradation of the electro-optical performance of the display. TFTs typically include a gate electrode, a gate insulating film (typically SiO2), a metal source electrode, a metal drain electrode, and a metal oxide semiconductor film on the gate insulating film that at least partially covers the gate electrode, source electrode, and drain electrode. Such backplanes are available from manufacturers such as Sharp / Foxconn, LG, and BOE. Such backplanes can provide drive voltages of ±30V (or greater). In some embodiments, intermediate voltage drivers are included so that the resulting drive waveform can include five levels, or seven levels, or nine levels, or more levels.

[0097] One preferred metal oxide for such applications is indium gallium zinc oxide (IGZO). IGZO TFTs have electron mobilities 20 to 50 times greater than those of amorphous silicon. Using IGZO TFTs in an active matrix backplane allows for voltages in excess of 30 V via a suitable display driver. Furthermore, a source driver capable of providing at least five, and preferably seven, levels provides different driving paradigms for four-particle electrophoretic display systems. In one embodiment, there are two positive voltages, two negative voltages, and zero volts. In another embodiment, there are three positive voltages, three negative voltages, and zero volts. In one embodiment, there are four positive voltages, four negative voltages, and zero volts. These levels can be selected within the range of approximately -27 V to +27 V, without the limitations imposed by the top-plane switching discussed above.

[0098] The electrophoretic display of Figure 2 comprises a Type I electrophoretic medium of the present invention, which contains four types of electrophoretic particles in a non-polar fluid 27. The first type of particles (W- * The first type of particle (the negatively charged particle) may be surface treated so that its electrophoretic mobility depends on the strength of the driving electric field (discussed in detail below). In such instances, the electrophoretic mobility of this type of particle actually decreases in the presence of a stronger electric field, which is somewhat counterintuitive.

[0099] The second type of particles (C++; gray circles) may have a higher positive charge than the third type of particles. They may also have a surface treatment. As shown in Figure 2, the colors of this type of particle are nominally white, magenta, yellow, and cyan, producing the colors shown in Figure 1. However, the present invention is not limited to this particular color set, nor is it limited to one reflective particle and three absorbing particles. For example, a system may include one black absorbing particle and three reflective particles in red, yellow, and blue with appropriately matched reflectance spectra, producing a process white state when all three reflective particles are mixed and observable on the surface.

[0100] The third type of particle (M+ * ; dark circles) have a positive charge and may be surface treated (or may be intentionally untreated) such that the electrophoretic mobility of the third type of particles depends on the strength of the driving electric field, or the release rate of the population of second and third types of particles after being driven to one side of the void containing that type of particle upon reversal of the electric field direction is slower than the release rate of the population of second type of particles.

[0101] The fourth type of particles (Y-; checkered circles) have a negative charge. This charge may be higher or lower than the charge of the first type of particles. Furthermore, the fourth type of particles may be surface-treated. The electrophoretic mobility of the fourth type of particles may or may not depend on the strength of the driving electric field. That is, the fourth type of particles may have a surface treatment, but such surface treatment may not result in the aforementioned decrease in electrophoretic mobility with increasing electric field.

[0102] Figure 3A shows a schematic cross-sectional view of four types of particles of a Type II electrophoretic medium used in the present invention. The display shown in Figure 3A is similar to the display shown in Figure 2, but has a different electrophoretic medium (Type II in Figure 3A versus Type I in Figure 2).

[0103] As shown in Figures 3A to 3E, the type II electrophoretic medium of the present invention contains four types of electrophoretic particles in a non-polar fluid 27. The first type of particles (W- * The first type of particle (the negatively charged particle) may be surface treated so that its electrophoretic mobility depends on the strength of the driving electric field (discussed in detail below). In such instances, the electrophoretic mobility of this type of particle actually decreases in the presence of a stronger electric field, which is somewhat counterintuitive.

[0104] The second type of particles (C+++; gray circles) have the highest magnitude positive charge and the same type of surface treatment as the third and fourth types of particles. As shown in Figure 3A, the colors of this type of particle are nominally white, magenta, yellow, and cyan, producing the colors shown in Figure 1. However, the present invention is not limited to this particular color set, nor is it limited to one reflective particle and three absorbing particles. For example, a system may include one black absorbing particle and three reflective particles in red, yellow, and blue with appropriately matched reflectance spectra, producing a process white state when all three reflective particles are mixed and observable on the surface.

[0105] The third kind of particle (M++ * ; dark circles) have a positive charge and may be surface treated (or may be intentionally untreated) so that the electrophoretic mobility of the third type of particles depends on the strength of the driving electric field, or the release rate of the third particle population after being driven to one side of the void containing that type of particle upon reversal of the electric field direction is slower than the release rates of the second and fourth type particle populations.

[0106] The fourth type of particles (Y+; checkered circles) are positive but have a smaller charge magnitude than the third type of particles. Furthermore, the fourth type of particles may be surface-treated, but not in a manner that makes the electrophoretic mobility of the fourth type of particles dependent on the strength of the driving electric field. That is, the fourth type of particles may have a surface treatment, but such surface treatment does not result in the aforementioned decrease in electrophoretic mobility with increasing electric field.

[0107] 3B-3E show schematic cross-sectional views of different optical states of a display pixel having four types of particles (type II) used in the present invention.

[0108] In one embodiment, the first type of particles (negative) are white and scattering; the second type of particles (positive, high charge magnitude) are cyan and absorbing; the third type of particles (positive, medium charge magnitude) are magenta and absorbing; and the fourth type of particles (positive, low charge magnitude) are yellow and absorbing. In another embodiment, the first type of particles (negative) are white and scattering; the second type of particles (positive, high charge magnitude) are cyan and absorbing; the third type of particles (positive, medium charge magnitude) are magenta and absorbing; and the fourth type of particles (negative) are yellow and absorbing. Table 1 below shows the diffuse reflectance of exemplary yellow, magenta, cyan, and white particles useful in the electrophoretic media of the present invention when dispersed in a poly(isobutylene) matrix, along with the ratios of their absorption and scattering coefficients according to Kubelka-Munk analysis.

[0109] [Table 1]

[0110] The electrophoretic medium of the present invention may be in any of the forms discussed above. Thus, the electrophoretic medium may be unencapsulated, encapsulated in individual capsules surrounded by a capsule wall, encapsulated in sealed microcells, or in the form of a polymer dispersion medium. Pigments are described in detail elsewhere, for example, in U.S. Pat. Nos. 9,697,778 and 9,921,451. Briefly, white-type particles W1 are silanol-functionalized light-scattering pigments (titanium dioxide) to which a polymeric material containing lauryl methacrylate (LMA) monomer is attached, as described in U.S. Pat. No. 7,002,728. White-type particles W2 are polymer-coated titania prepared substantially as described in Example 1 of U.S. Pat. No. 5,852,196, where the polymeric coating comprises lauryl methacrylate and 2,2,2-trifluoroethyl methacrylate in a ratio of approximately 99:1. Yellow type particles Y1 are CI Pigment Yellow 180 used without coating and dispersed by milling in the presence of Solsperse™ 19000, as outlined in U.S. Pat. No. 9,697,778. Yellow type particles Y2 are CI Pigment Yellow 155 used without coating and dispersed by milling in the presence of Solsperse™ 19000, as outlined in U.S. Pat. No. 9,697,778. Yellow type particles Y3 are CI Pigment Yellow 139 used without coating and dispersed by milling in the presence of Solsperse™ 19000, as outlined in U.S. Pat. No. 9,697,778. Yellow type particles Y4 are CI Pigment Yellow 139 coated by dispersion polymerization, incorporating trifluoroethyl methacrylate, methyl methacrylate, and dimethylsiloxane-containing monomers, as described in Example 4 of US Pat. No. 9,921,451.Magenta particles M1 are positively charged magenta materials (dimethylquinacridone, CI Pigment Red 122) coated using vinylbenzyl chloride and LMA as described in Example 5 of U.S. Pat. Nos. 9,697,778 and 9,921,451.

[0111] Magenta-type particles M2 are CI Pigment Red 122 coated with dispersion polymerization, methyl methacrylate, and dimethylsiloxane-containing monomers, as described in Example 6 of U.S. Pat. No. 9,921,451. Cyan-type particles C1 are copper phthalocyanine materials (CI Pigment Blue 15:3) coated with dispersion polymerization, methyl methacrylate, and dimethylsiloxane-containing monomers, as described in Example 7 of U.S. Pat. No. 9,921,451. In some embodiments, the color gamut has been found to be improved by using Inkjet Yellow 4GC (Clariant) as the core yellow pigment and incorporating a methyl methacrylate surface polymer. The zeta potential of this yellow pigment can be adjusted by adding 2,2,2-trifluoroethyl methacrylate (TFEM) monomer and monomethacrylate-terminated poly(dimethylsiloxane).

[0112] Electrophoretic medium additives and surface treatments for promoting differences in electrophoretic mobility, as well as proposed mechanisms for interactions between the surface treatment and surrounding charge control agents and / or free polymers, are discussed in detail in U.S. Pat. No. 9,697,778, which is incorporated by reference in its entirety. In such electrophoretic media, one approach to controlling interactions between various types of particles is by controlling the type, amount, and thickness of the polymer coating on the particles. For example, to control particle properties such that particle-particle interactions are lower between the second type of particles and the third and fourth types of particles than between the third type of particles and the fourth type of particles, for example, the second type of particles may have a polymer surface treatment, while the third and fourth types of particles may have no polymer surface treatment or a polymer surface treatment with a lower mass coverage per unit area of ​​particle surface than the second type of particles. More generally, the Hammerker constant (a measure of the strength of the van der Waals interaction between two particles; the pair potential is proportional to the Hammerker constant and inversely proportional to the sixth power of the distance between the two particles) and / or the interparticle spacing need to be adjusted by judicious selection of the polymer coating(s) on the three particles.

[0113] As discussed in U.S. Patent No. 9,921,451, different types of polymers may contain different types of polymer surface treatments. For example, Coulombic interactions can be weakened when the closest distance between oppositely charged particles is maximized by steric hindrance (typically a polymer grafted or adsorbed to the surface of one or both particles). The polymer shell may be a covalently bonded polymer created by grafting processes or chemisorption, as known in the art, or it may be physically adsorbed onto the particle surface. For example, the polymer may be a block copolymer containing insoluble and soluble segments. Alternatively, the polymer shell may be dynamic, in that it is a loose network of free polymer from the electrophoretic medium that complexes with the pigment particle in the presence of an electric field and a sufficient amount and type of charge control agent (CCA—discussed below). Thus, depending on the strength and polarity of the electric field, the particle may have more associated polymer, which causes the particle to differentially interact with the container (e.g., microcapsule or microcell) and other particles. The extent of the polymer shell is conveniently assessed by thermogravimetric analysis (TGA), a technique in which the temperature of a dried sample of a particle is increased and the mass loss due to thermal decomposition is measured as a function of temperature. Using TGA, the percentage of the particle's mass that is polymer can be measured, which can be converted to a volume fraction using the known densities of the core pigment and its attached polymer. Conditions can be found under which the polymer coating is lost but the core pigment remains (these conditions depend on the exact core pigment particle used). Various polymer combinations can be engineered to work as described below with respect to Figures 3B-3E. For example, in some embodiments, particles (typically the first and / or second particles) may have a covalently attached polymer shell that interacts strongly with the container (e.g., microcell or microcapsule), while other particles of the same charge either lack a polymer coating or are complexed with free polymer in solution, thus having little interaction with the container.In other embodiments, the particles (typically the first and / or second particles) do not have a surface coating, and therefore the particles more readily form a charge double layer and have reduced electrophoretic mobility in the presence of a strong field.

[0114] The fluid 27 in which the four particles are dispersed is clear and colorless. The fluid contains charged electrophoretic particles that move through the fluid under the influence of an electric field. A preferred suspending fluid has a low dielectric constant (about 2), a high volume resistivity (about 10 15 They have a low viscosity (less than 5 mPas), low toxicity and environmental impact, low water solubility (less than 10 parts per million (ppm) when traditional aqueous encapsulation methods are used; however, note that this requirement may be relaxed for non-encapsulated or certain microcell displays), a high boiling point (above about 90°C), and a low refractive index (less than 1.5). The last requirement is due to the use of high refractive index scattering (typically white) pigments, whose scattering efficiency depends on the refractive index mismatch between the particle and the fluid.

[0115] Some useful fluids include organic solvents such as saturated linear or branched hydrocarbons, silicone oils, halogenated organic solvents, and low molecular weight halogen-containing polymers. The fluid may contain a single component or may be a blend of more than one component to tailor its chemical and physical properties. Reactants for the microencapsulation process, such as oil-soluble monomers or solvents (if used), may also be contained in the fluid.

[0116] The fluid preferably has a low viscosity and a dielectric constant in the range of about 2 to about 30, preferably about 2 to about 15, for high particle mobility. Examples of suitable dielectric fluids are hydrocarbons such as Isopar®, decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oils, silicone fluids, aromatic hydrocarbons such as toluene, xylene, phenylxylylethane, dodecylbenzene or alkylnaphthalenes, halogenated solvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene, dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride, chloropentafluoro-benzene, dichlorononane or pentachlorobenzene, and perfluorinated solvents such as FC-43, FC-70 or FC-5060 from 3M Company, St. Paul, MN, low molecular weight halogen-containing polymers such as poly(perfluoropropylene oxide) from TCI America, Portland, Oregon, poly(chlorotrifluoro-ethylene) from Halocarbon Product Corp., River Halocarbon oils from Edge, NJ, perfluoropolyalkyl ethers such as Galden from Ausimont or Krytox oils and greases K-Fluid series from DuPont, Delaware, polydimethylsiloxane-based silicone oil (DC-200) from Dow-corning.

[0117] As described in U.S. Patent No. 7,170,670, the bistability of electrophoretic media can be improved by including in the fluid a polymer with a number-average molecular weight greater than approximately 20,000. This polymer is essentially non-absorbent on the electrophoretic particles; poly(isobutylene) or polydimethylsiloxane can be used for this purpose. Furthermore, as described, for example, in U.S. Patent No. 6,693,620, particles with fixed charges on their surfaces establish an electric double layer of opposite charges in the surrounding fluid. The ionic head groups of CCA can ion-pair with charged groups on the electrophoretic particle surface, forming a layer of fixed or partially fixed charged species. Outside this layer, there is a diffusion layer containing charged (reverse) micelles containing CCA molecules in the medium. In conventional DC electrophoresis, the applied electric field exerts a force on the fixed surface charges and an opposing force on the mobile countercharges, resulting in slip within the diffusion layer and movement of the particles relative to the fluid. The potential at the slip surface is known as the zeta potential.

[0118] As a result, some types of particles in an electrophoretic medium have different electrophoretic mobilities depending on the strength of the electric field across the electrophoretic medium. For example, when a first (low-intensity, i.e., about ±10 V or less) electric field is applied to the electrophoretic medium, the first type of particles move in a first direction relative to the electric field, but when a second (high-intensity, i.e., about ±20 V or more) electric field is applied, even though it has the same polarity as the first electric field, the first type of particles begin to move in the opposite direction relative to the electric field. This behavior is theorized to result from conduction in highly nonpolar fluids being mediated by charged reverse micelles or electrophoretic particles with opposite charges. Therefore, any electrochemically generated protons (or other ions) are likely transported through the nonpolar fluid within the micelle core or adsorbed onto the electrophoretic particles. For example, as shown in Figure 5B of U.S. Patent No. 9,697,778, a positively charged reverse micelle can approach a negatively moving electrophoretic particle, incorporating the reverse micelle into an electric double layer around the negatively charged particle. The electric double layer includes both a diffuse layer of charge with an increased counterion concentration and a semimicellar surface adsorbed coating on the particle. In the latter case, the reverse micelle charge associates with the particle within a slip envelope that defines the particle's zeta potential, as described above. Through this mechanism, an electrochemical flow of positively charged ions flows through the electrophoretic fluid, biasing the negatively charged particle toward a more positive charge. As a result, the electrophoretic mobility of, for example, a first negatively charged particle is a function of the magnitude of the electrochemical flow and the residence time of the positive charge near the particle surface, which is a function of the strength of the electric field.

[0119] Furthermore, as also described in U.S. Patent No. 9,697,778, positively charged particles can be prepared that can also exhibit different electrophoretic mobilities depending on the applied electric field. In the present invention, a combination of charge control agents can be used in the electrophoretic medium to adjust the zeta potential of various particles.

[0120] In some embodiments, a portion of the charge control agent intended for the final formulation is added during electrophoretic particle synthesis to engineer the desired zeta potential and affect the reduction in electrophoretic mobility due to strong electric fields. For example, adding a charge control agent during polymer grafting has been observed to complex a certain amount of CCA to the particles. This can be confirmed by removing the particles from the electrophoretic medium and then stripping the pigment of surface species with THF to remove all adsorbed species. H NMR analysis of the THF extract reveals that a significant amount of CCA has adsorbed to the pigment particles or complexed with the surface polymer. Experiments suggest that a high CCA loading in the particle's surface polymer promotes the formation of a charge double layer around the particles in the presence of a strong electric field. For example, magenta particles with more than 200 mg of charge control agent (CCA) per gram of final magenta particles have excellent retention properties in the presence of high positive electric fields. (See, e.g., Figure 3C and the discussion above.)

[0121] Table 2 shows exemplary relative zeta potentials for three types of colored particles and a single white particle in a preferred embodiment.

[0122] [Table 2]

[0123] In one embodiment, the negative (white) particles have a zeta potential of -30 mV, and the remaining three particles are all positive relative to the white particles. Thus, a display containing positive cyan, magenta, and yellow particles can switch between a black state (where all colored particles are in front of the white particles relative to the viewing surface) and a white state where the white particles are closest to the viewer and block the viewer from perceiving the remaining three particles. In contrast, if the white particles have a zeta potential of 0 V, the negatively charged yellow particles are the most negative of all particles, and thus a display containing these particles will switch between a yellow and blue state. This would also occur if the white particles were positively charged. However, the positively charged yellow particles will be more positive than the white particles unless their zeta potential is more than +20 mV.

[0124] The behavior of the electrophoretic medium of the present invention is consistent with the mobility of the white particles (represented as zeta potential in Table 2) depending on the applied electric field. Thus, in the example shown in Table 2, when addressed at a low voltage, the white particles may behave as if their zeta potential were -30 mV, but when addressed at a higher voltage, they may behave as if their zeta potential were more positive, possibly as high as +20 mV (matching the zeta potential of the yellow particles). Thus, when addressed at a low voltage, the display switches between a black and white state, but when addressed at a higher voltage, it switches between a blue and yellow state.

[0125] The motion of various particles in the presence of high (e.g., "±H," e.g., ±20 V, e.g., ±25 V) and low (e.g., "±L," e.g., ±5 V, e.g., ±10 V) electric fields is shown in Figures 3B-3E. For illustrative purposes, each box defined by a dashed line represents a pixel defined by a first light-transmitting electrode layer 21 (front electrode) and a second electrode layer 22 (back electrode), which may comprise a pixel electrode of the active matrix, but which may also be a light-transmitting electrode, a segmented electrode, or the like. Starting from a first state (nominally black) in which all of the positive particles are present at the viewing surface, the electrophoretic medium can be driven to four different optical states, as shown in Figures 3B-3E. In a preferred embodiment, this results in a white optical state (Figure 3B), a magenta optical state (Figure 3C), a yellow optical state (Figure 3D), and a red optical state (Figure 3E). It will be apparent that the remaining four optical states of FIG. 1 can be achieved by reversing the sequence of the initial states and the driving electric fields, as shown schematically in FIG.

[0126] When addressed with a low voltage, as in Figure 3B, the particles behave according to their relative zeta potentials, with relative velocities indicated by the arrows when a negative voltage is applied to the backplane. Thus, in this example, the cyan particles move more quickly than the magenta particles, which move more quickly than the yellow particles. Because the particles are already constrained by the enclosure walls, the first (positive) pulse does not change their position. The second (negative) pulse swaps the positions of the colored and white particles, so the display switches between black and white states, but with a transient color reflecting the relative mobility of the colored particles. Reversing the starting position and polarity of the pulses allows for a white-to-black transition. This embodiment therefore provides a black-to-white refresh that requires a lower voltage (and consumes less power) than other black-and-white schemes achieved with process black or multiple colors using process white.

[0127] In Figure 3C, the first (positive) pulse is a pulse of sufficiently high positive voltage to reduce the mobility of the magenta particles (i.e., particles with intermediate mobility among the three positively charged colored particles). Due to their reduced mobility, the magenta particles are essentially fixed in place, and a subsequent pulse of lower voltage in the opposite direction moves the cyan, white, and yellow particles more than the magenta particles, thereby producing a magenta color at the viewing surface, with the negative white particles behind the magenta particles. Importantly, if the starting position and polarity of the pulses are reversed (equivalent to viewing the display from the opposite side of the viewing surface, i.e., through the second electrode layer 22), this pulse sequence produces a green color (i.e., a mixture of yellow and cyan particles).

[0128] In Figure 3D, the first pulse is a low-voltage pulse that does not significantly reduce the mobility of either the magenta or white particles. However, the second pulse is a high-negative voltage pulse that reduces the mobility of the white particles. This allows for more effective competition between the three positive particles, so the slowest type of particle (yellow in this example) remains visible before the white particles, whose mobility has been reduced by the previous negative pulse. Notably, the yellow particles do not reach the upper surface of the particle-containing cavity. Importantly, if the starting position and polarity of the pulses are reversed (equivalent to viewing the display from the opposite side of the viewing surface, i.e., through the second electrode layer 22), this pulse sequence produces a blue color (i.e., a mixture of magenta and cyan particles).

[0129] Finally, Figure 3E shows that when both pulses are high voltage, the mobility of magenta particles is reduced by the first, highly positive pulse, and the reduction in white mobility caused by the second, highly negative pulse enhances the competition between cyan and yellow, thereby producing red. Importantly, if the starting position and polarity of the pulses are reversed (equivalent to viewing the display from the opposite side of the viewing surface, i.e., through the second electrode layer 22), this pulse sequence produces cyan.

[0130] To achieve high-resolution displays, individual pixels of a display must be addressable without interference from neighboring pixels. One approach to this goal is to provide an array of nonlinear elements, such as transistors or diodes, with at least one nonlinear element associated with each pixel, creating an "active matrix" display. An address electrode, or pixel electrode, that addresses a pixel is connected to an appropriate voltage source through the associated nonlinear element. Typically, when the nonlinear element is a transistor, the pixel electrode is connected to the drain of the transistor. While this arrangement will be assumed in the following discussion, this is essentially arbitrary, and the pixel electrode may also be connected to the source of the transistor. Conventionally, in high-resolution arrays, pixels are arranged as a two-dimensional array of rows and columns, such that any particular pixel is uniquely defined by the intersection of a designated row and a designated column. The sources of all transistors in each column are connected to a single column electrode, while the gates of all transistors in each row are connected to a single row electrode. Again, the source-to-row and gate-to-column designations, while conventional, are essentially arbitrary and may be reversed if desired. The row electrodes are connected to a row driver, which essentially ensures that only one row is selected at a given moment—i.e., a selected voltage is applied to the selected row electrode, e.g., to ensure that all transistors in the selected row are conductive, while a non-select voltage is applied to all other rows, e.g., to ensure that all transistors in the unselected rows remain non-conductive. The column electrodes are connected to a column driver, which applies selected voltages to the various column electrodes to drive the pixels in the selected row to the desired optical state. These voltages are conventionally provided on the opposite side of the electro-optic medium from the nonlinear array and are relative to a common front electrode (first light-transmitting electrode layer) that extends across the entire display. After a preselected interval known as the “line address time,” the selected row is deselected, the next row is selected, and the voltages to the column drivers are changed so that the next line of the display is written.This process is repeated until the entire display is written row by row.

[0131] Conventionally, each pixel electrode is associated with a capacitor electrode such that the pixel electrode and the capacitor electrode form a capacitor. See, for example, International Patent Application WO 01 / 07961. In some embodiments, an N-type semiconductor (e.g., amorphous silicon) may be used to form the transistor, and the "select" and "unselect" voltages applied to the gate electrode may be positive and negative, respectively.

[0132] FIG. 5 of the accompanying drawings shows an exemplary equivalent circuit of a single pixel of an electrophoretic display. As shown, the circuit includes a capacitor 10 formed between a pixel electrode and a capacitor electrode. An electrophoretic medium 20 is represented as a parallel capacitor and resistor. In some examples, direct or indirect coupling capacitance 30 (commonly referred to as "parasitic capacitance") between the gate electrode of a transistor associated with a pixel and the pixel electrode can create undesirable noise in the display. Typically, the parasitic capacitance 30 is much smaller than the capacitance of the storage capacitor 10, and when a pixel row of the display is selected or deselected, the parasitic capacitance 30 can introduce a slight negative offset voltage, also known as a "kickback voltage," on the pixel electrode, which is typically less than 2 volts. In some embodiments, to compensate for the undesirable "kickback voltage," a common potential V is applied to the first electrode layer (front electrode) and the capacitor electrode associated with each pixel. com may be supplied, and therefore V com is the kickback voltage (V KB ), any voltage supplied to the display can be offset by the same amount and no net DC imbalance will be experienced.

[0133] However, V comProblems can arise if V is set to a voltage that is not compensated for kickback voltage. This can occur when it is desired to apply a higher voltage to the display than is available from the backplane alone. In the art, it is often said that the maximum voltage applied to a display is, for example, V com It is well known that this can be doubled if the top plane is supplied with -V while the backplane is selectively supplied with, for example, nominally +V, 0, or -V. In this case the maximum voltage experienced is +2V (i.e., at the backplane relative to the top plane), while the minimum is zero. If a negative voltage is required, V com The potential must be raised to at least zero. Therefore, the waveforms used to address displays with positive and negative voltages using top-plane switching must be higher than one V com Each voltage setting must have a specific frame assigned to it.

[0134] A set of waveforms for driving a color electrophoretic display having four particles is described in U.S. Pat. No. 9,921,451, which is incorporated herein by reference. In U.S. Pat. No. 9,921,451, seven different voltages are applied to the pixel electrodes: three positive voltages, three negative voltages, and zero. However, in some embodiments, the maximum voltages used in these waveforms are higher than can be accommodated by amorphous silicon thin film transistors. In such instances, suitable high voltages can be obtained through the use of top-plane switching. (As discussed above) V com intentionally V KB However, if top-plane switching is used, V com Using as many separate power supplies as there are settings is expensive and inconvenient. Additionally, top-plane switching is known to increase kickback, thereby reducing color state stability.

[0135] Display devices can be constructed using the electrophoretic medium of the present invention in several ways known in the art. The electrophoretic medium may be encapsulated in microcapsules or incorporated into microcell structures that are then sealed with a polymer layer. The microcapsule or microcell layer can be coated or embossed onto a plastic substrate or film with a transparent coating of electrically conductive material. This assembly can be laminated to a backplane with pixel electrodes using an electrically conductive adhesive. Alternatively, the electrophoretic medium can be dispensed directly onto a thin open-cell grid disposed on a backplane containing the active matrix of pixel electrodes. The filled grid can then be top-sealed with an integrated protective sheet / light-transmitting electrode.

[0136] FIG. 6 shows a schematic cross-sectional view (not to scale) of a display structure 600 suitable for use with the present invention. In display 600, the electrophoretic medium is shown confined in microcells, although equivalent structures incorporating microcapsules could also be used. A substrate 602, which can be glass or plastic, bears a second electrode layer comprising pixel electrodes 604, either individually addressed segments or associated with thin film transistors in an active matrix arrangement. The combination of substrate 602 and second electrode layer comprising pixel electrodes 604 is conventionally referred to as the backplane of the display. Layer 606 is an optional dielectric layer according to the present invention that is applied to the backplane. Methods for depositing suitable dielectric layers are described in U.S. patent application Ser. No. 16 / 862,750, which is incorporated by reference. The front surface of the display comprises a transparent substrate 622 bearing a first light-transmitting electrode layer 620, which can be formed by an electrically conductive coating. Overlying first light-transmitting layer 620 is optional dielectric layer 618. Layer(s) 616 are polymer layers that may include a primer layer for adhesion of the microcell to the first light-transmitting electrode layer 620 and some residual polymer comprising the bottom of the microcell. The walls of the microcell 612 are used to contain the electrophoretic medium 614. The microcell is sealed with a sealing layer 610, and the entire front structure may be adhered to a backplane using an electrically conductive adhesive layer 608. Processes for forming microcells are described in the prior art, for example, in U.S. Pat. No. 6,930,818. In some examples, the microcells are less than 20 μm deep, for example less than 15 μm deep, for example less than 12 μm deep, for example about 10 μm deep, for example about 8 μm deep.

[0137] Most commercially available electrophoretic displays use amorphous silicon-based thin film transistors (TFTs) in the construction of the active matrix backplane due to the wider availability of manufacturing facilities and the cost of various starting materials. Unfortunately, amorphous silicon thin film transistors are unstable when supplied with gate voltages that allow switching voltages greater than approximately ±15 V. Nevertheless, as discussed below, ACeP performance is improved when high positive and negative voltages are allowed to exceed ±15 V. Therefore, as explained in previous disclosures, improved performance is achieved by further varying the bias of the first light-transmitting electrode relative to the bias of the backplane pixel electrodes (also known as top-plane switching). Thus, if a voltage of +30 V (relative to the backplane) is required, the appropriate backplane pixels can be switched to +15 V, while the top plane can be switched to −15 V. Methods for driving four-particle electrophoretic systems using top-plane switching are described in more detail, for example, in U.S. Pat. No. 9,921,451.

[0138] These waveforms are shown with each pixel of the display being exemplified as 30V, 15V, 0, -15V and -30V. 高 , +V 低 , 0, -V 低 and -V 高 In practice, it may be preferable to use a larger number of address voltages. 高 , 0 and -V 高 ) is available, the voltage V 高 By addressing the pulses with a duty cycle of 1 / n, a lower voltage (e.g., V 高 / n, where n is a positive integer greater than 1), the same results may be achievable.

[0139] Figure 4 shows a typical waveform (in simplified form) used to drive the four-particle color electrophoretic display system described above. Such waveforms have a "push-pull" structure; that is, they consist of a dipole containing two pulses of opposite polarity. The magnitude and length of these pulses determine the resulting color. At a minimum, five such voltage levels should be present. Figure 4 shows high and low, positive and negative voltages, as well as zero volts. Typically, "low" (L) refers to a range of approximately 5 to 15 volts, while "high" (H) refers to a range of approximately 15 to 30 volts. Generally, the higher the magnitude of the "high" voltage, the better the color gamut achieved by the display. In some embodiments, an additional "medium" (M) level is used, which is typically approximately 15 volts, although the value of M depends somewhat on the particle composition and the environment of the electrophoretic medium.

[0140] Figure 4 shows the simplest dipoles needed to form colors, but the actual waveform may be multiple repetitions of these patterns, or other patterns that are aperiodic and use more than five voltage levels.

[0141] Of course, achieving the desired color with the drive pulses in Figure 4 relies on particles starting the process from a known state, which is unlikely to be the final color displayed on the pixel. Therefore, a series of reset pulses precedes the drive pulse, which increases the amount of time required to update the pixel from a first color to a second color. Reset pulses are described in detail in U.S. Patent No. 10,593,272, which is incorporated by reference. The lengths of these pulses (refresh and address), as well as the lengths of any pauses (i.e., periods of zero voltage between them), can be selected so that the entire waveform (i.e., the integral of voltage with respect to time over the entire waveform) is DC-balanced (i.e., the integral of voltage over time is substantially zero). DC balance can be achieved by adjusting the lengths of the pulses and pauses in the reset phase so that the net impulse delivered in the address phase is equal in magnitude and opposite in sign (during the address phase, the display is switched to a particular desired color). However, as shown in Figures 3B-3E, the starting states of the eight primary colors are black or white, which can be achieved with sustained low-voltage drive pulses. The simplicity of achieving this starting state further reduces the update time between states, which is more pleasant for the user, and also reduces power consumption (thus increasing battery life).

[0142] Furthermore, the above waveform discussion, and especially the DC balance discussion, ignores the issue of kickback voltage. In reality, as mentioned above, any backplane voltage will vary from the voltage provided by the power supply by a kickback voltage V KB Therefore, if the power supply used provides three voltages +V, 0, and -V, the backplane will actually be offset by an amount equal to the voltage V+V KB , V KB and -V +V KB (V for amorphous silicon TFTs) KB(Note that V is usually a negative number.) However, the same power supply supplies +V, 0, and -V to the first electrode (front electrode) without any kickback voltage cancellation. So, for example, if the first electrode (front electrode) is supplied with -V, the display will KB Maximum voltage and V KB The first electrode (front electrode) receives a minimum voltage of V KB Instead of using a separate power supply to supply V (which can be expensive and inconvenient), the waveform is split into sections, supplying a positive voltage, a negative voltage, and V to the first electrode (front electrode). KB may be provided.

[0143] Microcell electrophoretic display

[0144] As shown in FIG. 6, a typical microcell electrophoretic display includes a first light-transmitting electrode layer, a microcell layer including a plurality of microcells, and a second electrode layer including pixel electrodes. Each of the plurality of microcells has an opening. The plurality of microcells contains an electrophoretic medium. A sealing layer spans the openings of the plurality of microcells. The sealing layer may be formed from an aqueous sealing composition. As described in U.S. Patent Application Serial No. 18 / 055,072, filed November 14, 2022, the aqueous sealing composition affects the volume resistivity of the sealing layer. This reference is incorporated herein by reference in its entirety. Specifically, it has been found that an aqueous sealing composition including a water-soluble ether reduces the volume resistivity of the sealing layer, thereby affecting display performance. The water-soluble ether may have a weight-average molecular weight of 75 to 5,000 daltons. The water-soluble ether may be represented by Formula I, Formula II, or Formula III shown above. Water-soluble ethers include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-propyl ether, ethylene glycol monoisopropyl ether, ethylene glycol n-monobutyl ether, ethylene glycol monoisobutyl ether, ethylene glycol mono-t-butyl ether, ethylene glycol monobenzyl ether, ethylene glycol monophenyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol di-n-propyl ether, ethylene glycol diisopropyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol mono-n-propyl ether, diethylene glycol monoisopropyl ether, diethylene glycol n-monobutyl ether, diethylene glycol monoisobutyl ether, diethylene glycol mono-t-butyl ether, diethylene glycol monobenzyl ether, diethylene glycol monophenyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol ethyl methyl ether, diethylene glycol di-n-propyl ether, diethylene glycol diisopropyl ether,Diethylene glycol di-n-butyl, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol mono-n-propyl ether, triethylene glycol monoisopropyl ether, triethylene glycol n-monobutyl ether, triethylene glycol monoisobutyl ether, triethylene glycol mono-t-butyl ether, triethylene glycol monobenzyl ether, triethylene glycol monophenyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol di-n-propyl ether, triethylene glycol diisopropyl ether, tetraethylene glycol monomethyl ether, tetraethylene glycol monoethyl ether, triethylene glycol monophenyl ether, tetraethylene glycol dimethyl ether, polyethylene glycol monomethyl ether, polyethylene glycol monoethyl ether, polyethylene glycol monophenyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether ether, propylene glycol mono-n-propyl ether, propylene glycol monoisopropyl ether, propylene glycol mono-n-butyl ether, propylene glycol monoisobutyl ether, propylene glycol monophenyl ether, propylene glycol dimethyl ether, propylene glycol diethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol mono-n-propyl ether, dipropylene glycol monoisopropyl ether, dipropylene glycol mono-n-butyl ether, dipropylene glycol monoisobutyl ether, dipropylene glycol dimethyl ether, dipropylene glycol diethyl ether, dipropylene glycol di-n-propyl ether, dipropylene glycol diisopropyl ether, tripropylene glycol monomethyl ether, tripropylene glycol monoethyl ether, tripropylene glycol mono-n-propyl ether, tripropylene glycol monoisopropyl ether, tripropylene glycol mono-n-butyl ether,The water-soluble ether may be selected from the group consisting of tripropylene glycol monoisobutyl ether, tripropylene glycol monoisobutyl ether, and mixtures thereof. The aqueous sealing composition may contain 1.0% to 40% by weight of the water-soluble ether based on the weight of the aqueous sealing composition excluding water. The sealing layer may contain 0.5% to 25% by weight of the water-soluble ether based on the weight of the sealing film. The water-soluble ether may optionally contain a hydroxyl group.

[0145] The inventors of the present invention have observed that the inclusion of a water-soluble ether in the electrophoretic medium improves the color gamut achievable by the electro-optical performance of the display, as demonstrated in the Examples section below. [Example]

[0146] Example 1 Preparation of white particle dispersion.

[0147] Dispersions of titanium dioxide pigment were prepared as described in U.S. Patent No. 7,002,728, which involves silanizing titanium dioxide particles followed by covalent bonding of poly(lauryl methacrylate) to the silanized pigment.

[0148] Example 2 Zeta potential determination of white particles in various electrophoretic media.

[0149] Mixtures of this dispersion from Example 1 with various combinations of charge control agents (Solsperse™ 19000 and Solsperse™ 8000) were prepared. The zeta potential of the white particles in each mixture was determined using a Colloidal Dynamics AcoustoSizer II and ZetaProbe on samples dispersed in Isopar G. A graph of the zeta potential of the particles versus the weight fraction of Solsperse™ 8000 in the total weight of charge control agents (Solsperse™ 8000 + Solsperse™ 19000) is shown in Figure 7. Figure 7 shows that the zeta potential of the white particles in an electrophoretic medium containing only Solsperse™ 19000 (no Solsperse™ 8000) is negative. With the addition of Solsperse™ 8000, the zeta potential becomes progressively less negative, and at a weight fraction of Solsperse™ 8000 of about 0.4, the zeta potential of the white particles becomes positive. Presumably, when a sufficient amount of Solsperse™ 8000 is adsorbed onto the particles, the amount of positively charged Solsperse™ 19000 adsorbed increases, altering the charge on the particle surface. The data in Figure 7 demonstrate that the surface charge of electrophoretic particles can be controlled by varying the nature and amount of charge control agent used.

[0150] Example 3 Preparation of Type I electrophoresis medium.

[0151] Three different electrophoretic media, A, B, and C, were prepared according to Example 11 of U.S. Pat. No. 10,678,111, except that the charge control agent was a compound of Formula IV (R1 is polyricinoleic acid; MW 9,000) instead of Solsperse™ 19000 used in U.S. Pat. No. 10,678,111. The electrophoretic media comprised white particles (W1), cyan particles (C1), and magenta particles (M1) according to U.S. Pat. No. 10,678,111. The electrophoretic media further comprised a yellow pigment (Pigment Yellow 155; Inkjet Yellow 4GC supplied by Clariant, Basel, Switzerland) dispersed as described in Example 11 of U.S. Pat. No. 10,678,111. All three electrophoretic media A, B, and C also contained polydimethylsiloxane (PDMS DMS-T72 having a molecular weight of approximately 700,000, available from Gelest Corporation) at a concentration of 0.9 weight percent of the weight of the electrophoretic medium composition. The polydimethylsiloxane was added to act as an image stabilizer. Electrophoretic medium A did not contain Solsperse™ 8000. Electrophoretic medium B contained Solsperse™ 8000 at a concentration of 140 mg of charge control agent per gram of yellow pigment. Electrophoretic medium C contained Solsperse™ 8000 at a concentration of 280 mg of charge control agent per gram of yellow pigment. Table 3 provides the concentrations of particles in the electrophoretic media as weight percent of each type of particle by weight of the electrophoretic medium composition.

[0152] [Table 3]

[0153] Example 4 Preparation of electrophoretic displays A, B, and C.

[0154] Electrophoretic media A, B, and C from Example 3 were used to prepare electrophoretic displays A, B, and C, respectively.

[0155] Electrophoretic displays A, B, and C from Example 4 (prepared from electrophoretic media A, B, and C, respectively) were addressed with square-wave pulses of 500 ms duration having voltages varying between +24 V and -24 V in 1 volt intervals at 25°C. The rate of change of density was evaluated using an electro-optical measurement bench including a spectrophotometer. D. Hertel, "Optical measurement See "Standards for reflective e-paper to predict colors displayed in ambient illumination environments," Color Research & Application, 43, 6, (907-921), (2018).

[0156] The rate of change was higher in electrophoretic displays B and C compared to electrophoretic display A, as shown in Figures 8A-8C. Figures 8A, 8B, and 8C show the optical appearance of displays containing electrophoretic media A, B, and C, respectively, with voltage on the x-axis and time on the y-axis.

[0157] The color of the white state of each of electrophoretic displays A, B, and C was determined at the end of 500 ms of driving at various negative voltages. This study demonstrated that the separation of white and yellow particles was more complete in displays B and C than in display A (display A did not contain Solsperse™ 8000). The voltage window available for creating the transition between the white and black states is preferably as wide as possible with a simple voltage pulse. The study showed that in display A, which contained electrophoretic medium A without Solsperse™ 8000, the white state became contaminated with yellow at voltages more negative than about -8 V. In display B, which contained electrophoretic medium B containing 140 mg of Solsperse™ 8000 per gram of yellow pigment, the white state became contaminated at voltages more negative than about -10 V, while in the case of electrophoretic medium C, there was almost no yellow contamination even when addressed at -13 V. It was also found that the white / yellow boundary was sharper in displays B and C with electrophoretic media B and C containing Solsperse™ 8000 than in display A, which had an electrophoretic medium without additives.

[0158] The ability to use a more negative voltage to achieve the white state allows for faster switching from black to white. Figure 9 shows black-to-white switching traces for three electrophoretic media A, B, and C at -8V, -10V, and -13V, respectively. These are the most negative voltages possible without producing excessive white-state contamination by the yellow pigment.

[0159] The color gamuts of electrophoretic displays A, B, and C at 25° C. were determined using the color gamut measurement method described below. The results are summarized in FIG. 11 and Table 4.

[0160] [Table 4]

[0161] The data in Table 4 show that displays using electrophoretic media B and C, which contain Solsperse™ 8000 and a first charge control agent, have a higher color gamut than the color gamut of the display using electrophoretic media A, which does not contain Solsperse™ 8000.

[0162] Color gamut measurement method.

[0163] Electrophoretic displays A, B and C from Example 4 were electrically driven to produce eight optical states (white, yellow, red, magenta, blue, cyan, green and black). * , a * and b *Measurements were performed using a color computer at 100 kHz. A sequence of electrical pulses was used to address the electrophoretic device (such a sequence is called a "waveform"). In the following description, it is assumed that the voltages used in the waveforms are supplied to the second electrode layer (back electrode) of the display, and that the first, light-transmitting electrode on the front (viewing) surface of the display is the common electrode for all pixels and is connected to ground. The test waveforms include a sequence of "dipoles" as shown in Figure 10. Each dipole consists of two monopoles, each a pulse of length t and magnitude V. The two monopoles in each dipole have opposite polarities. The voltages used at the test wavelengths were ±24 V, ±18 V, ±15 V, and ±10 V. Time was discretized into 11.74 ms units called "frames." Each frame corresponds to one scan of a thin-film transistor array backplane refreshed at a frequency of 85 Hz, although in the described tests, the backplane was segmented and directly driven. Two test waveforms were used to evaluate the electro-optical performance of the device. The waveform used in the first test was 18 frames long, while the waveform used in the second test was 42 frames long. In each case, the waveform had as many identical dipoles as would fit within the allowed number of frames. The color gamut of the display was measured by calculating the volume of a convex hull that contained each colored state produced by the set of test waveforms. The color gamut was calculated using the DE 3 A wider color gamut, i.e., a larger space, means better electro-optical performance of the electrophoretic display.

[0164] Example 5 Preparation of Type II electrophoretic media and corresponding electrophoretic displays.

[0165] Type II electrophoretic media were prepared similar to those prepared in Example 1, but using different yellow particles. The yellow particles and their preparation are described below in Example 6. Using the yellow particles from Example 6 and the control yellow particles from Example 7, various electrophoretic media (both inventive and control) were prepared containing various contents of a second charge control agent (e.g., Solsperse™ 8000) and a first charge control agent (Formula IV).

[0166] Example 6 Example 5 Preparation of yellow pigment for Type II electrophoretic medium.

[0167] To a 1-liter plastic bottle were added 72.0 g of Pigment Yellow 155 (Inkjet Yellow 4GC, supplied by Clariant Corporation), 28.8 g of an Isopar E solution of a charge control agent represented by Formula IV (containing 21.6 g of charge control agent and 7.2 g of Isopar E), and 349.2 g of Isopar E. The dispersion was roll milled for 16 hours using Zirconox beads (1.7-2.4 mm). A 450 g quantity of the resulting dispersion was mixed in a reactor with 1.94 g of 2,2,2-trifluoroethyl methacrylate, 27.68 g of methyl methacrylate, 52.36 g of monomethacrylate-terminated poly(dimethylsiloxane) (Gelest MCR-M22), and 65.38 g of Isopar E. The reactor was equipped with a nitrogen-impregnated tube, an overhead stirring impeller, and an air condenser. Set the overhead stirring to 250 rpm and purify the reaction mixture with nitrogen for 65 seconds. oThe mixture was purged with 60°C for 60 minutes, after which the impregnation tube was removed and the nitrogen level was set on a rotameter. In a small vial, 0.187 g of 2,2'-azobis(2-methylpropionitrile) (AIBN) was dissolved in 2.24 g of ethyl acetate and added to a syringe. The resulting AIBN solution was injected into the reactor over 5-10 minutes, and the reaction mixture was heated at 65°C for 16 hours. The reaction mixture was dispensed into two 1-liter centrifuge bottles and centrifuged. The supernatant was decanted, and the remaining pigment was washed with Isopar E and centrifuged. The washing process was repeated two more times. The remaining pigment was dried in a vacuum oven at 40°C. The dried pigment was dispersed in Isopar E to 25 weight percent using ultrasonication. The resulting dispersion was filtered through a 200 μm mesh and used to prepare electrophoretic media. The yellow particles contained 31 weight percent polymer based on the particle weight. The zeta potential of the yellow particles in the final dispersion was determined on the sample using a Colloidal Dynamics AcoustoSizer II and ZetaProbe and was found to be +6 mV, i.e., the yellow particles have a slightly positive surface.

[0168] Example 7 The process of Example 6 was repeated, but without the charge control agent. This is the control yellow particles for Type II media.

[0169] Using the color gamut measurement method described above, the color gamuts of various inventive electrophoretic displays from Example 5 and control displays were determined. The results of this evaluation are summarized in Table 5. For each display, the upper number in the corresponding cell corresponds to the color gamut measurement using a waveform 18 frames long. The lower number in the corresponding cell corresponds to the color gamut measurement using a waveform 42 frames long. The color gamut volume of Type II electrophoretic media is somewhat smaller than that achieved with Type I media.

[0170] [Table 5]

[0171] The same display with a Type II electrophoretic medium was evaluated for the time required to switch from a white to a black state. The determination was made by applying a +16V pulse and a delta L of 30° between the initial and final states. * This was done by measuring the time required to reach . The times were measured and are reported in milliseconds in Table 6.

[0172] [Table 6]

[0173] The data show that Type II electrophoretic media switches much more rapidly than Type I media.

[0174] The data presented above demonstrate that electrophoretic medium compositions of the present invention containing a combination of first and second charge control agents perform better in terms of color gamut and switching speed than media not according to the present invention, for both Type I and Type II electrophoretic media.

[0175] Example 8 Preparation of charge control agent A (hydrogenated polyfarnesene with two terminal sulfate functional groups and H+ as counter ions).

[0176] A 9.8 g quantity of hydrogenated hydroxyl-terminated polyfarnesene (Krasol F3100, having a number-average molecular weight of 3100 g / mol; supplied by Cray Valley) was dissolved in 65 mL of ethyl ether. A 0.66 mL quantity of 6 mL of chlorosulfonic acid was slowly added to the solution under nitrogen gas at 0 °C with constant stirring. The reaction was allowed to warm to room temperature with constant stirring for 18 h. The ethyl ether layer was washed once with an equal volume of water and then dried over sodium sulfate. The solvent was removed under reduced pressure, and the material was purified by column chromatography (0–10% methanol / dichloromethane). This afforded the desired product in 76% yield.

[0177] Example 9 Preparation of charge control agent B (hydrogenated polyfarnesene with two terminal sulfate functional groups and Na+ as counter ions).

[0178] A quantity of hydrogenated hydroxyl-terminated polyfarnesene (Krasol F3100, having a number-average molecular weight of 3100 g / mol; supplied by Cray Valley) was dissolved in 62 mL of ethyl ether. A 0.4 mL portion of 6 mL of chlorosulfonic acid was slowly added to the solution under constant stirring under nitrogen gas. The reaction was stirred for 48 hours at room temperature. 0.75 g of NaOH dissolved in 50 mL of water was then added to the reaction and stirred for an additional hour. The organic layer was then separated from the aqueous layer and dried over sodium sulfate. The solvent was removed under reduced pressure to give the desired product in 91% yield.

[0179] Example 10 Preparation of various electrophoretic displays using charge control agent A and charge control agent B.

[0180] The charge control agent in Examples 8 and 9 is a comb polymer with a hydrogenated farnesyl group and two terminal sulfate groups. Various electrophoretic displays include the above-described Type II electrophoretic medium. The electrophoretic medium pigments (white, cyan, magenta, and yellow particles) are shown. The white particles have a negative charge, while the cyan, magenta, and yellow particles have a positive charge. The particle weight ratio is white:cyan:magenta:yellow 72:9:11:9. The total charge control agent content is 42 mg of charge control agent per gram of pigment. When two charge control agents are present, the weight ratio of cationic Solsperse™ 8000 charge control agent to non-cationic Solsperse™ 8000 charge control agent is 1:8. The electrophoretic medium composition also includes a charge control agent or a combination of two charge control agents, a hydrocarbon solvent, and a polydimethylsiloxane fluid, as shown in Table 7. The displays were exposed to a driving voltage sweep of -11 V to -16 V for 500 ms at 25°C, which resulted in a change in the color state of the display. At 0 s and approximately every 80 ms during the voltage sweep, the color (L * , a *, and b * ) was measured by a color computer. Examples 10B, 10C, and 10D of the present invention exhibited significantly faster color switching than Comparative Example 10A. The fastest switching was observed for the display of Example 10C, followed by the display of Example 10D, and then the display of Example 10B. The display of Comparative Example 10A did not provide a stable color (steady state) within the 500 millisecond test interval. Evaluation of the displays of the four examples was repeated at 0°C for a voltage sweep of -18V to -22V over 500 milliseconds. Results were similar to those observed in the evaluation at 25°C.

[0181] [Table 7-1]

[0182] Example 11 An aqueous sealing composition for forming a sealing layer.

[0183] Evaluation of sealing layers in microcell electrophoretic displays. Various microcell electrophoretic displays were prepared using a type II electrophoretic medium. The microcell electrophoretic displays included a first light-transmitting electrode layer, a microcell layer, and a second electrode layer. The microcell layer included a plurality of microcells, each with an opening, and a sealing layer spanning the openings of the microcells. The sealing layer was formed by coating an aqueous composition as described in U.S. Patent Application Publication No. 2022 / 0251364A1 (Application Serial No. 17 / 590,705), U.S. Patent Application Publication No. 2022 / 0244612A1 (Application Serial No. 17 / 590,835), and U.S. Patent Application Serial No. 18 / 055,072. To evaluate the effectiveness of the sealing layer, two different aqueous sealing compositions were prepared and used to form corresponding sealing layers (Examples 11A and 11B in Table 7). Example 1 contains a water-soluble ether, while Example 12 does not contain a water-soluble ether.

[0184] [Table 7-2]

[0185] [1] Poly(vinyl alcohol-co-ethylene) copolymer; Exceval™ RS-1717, supplied by Kuraray; [2] Polyurethane aqueous dispersion; L3838 aqueous dispersion, supplied by Hauthaway as a 35% dispersion in water; [3] Carbon black; Nerox® 3500, supplied by Orion Engineered Carbon; [4] Polycarbodiimide (multifunctional polycarbodiimide-aqueous solution); CARBODILITE® V-02-L2, supplied by Nisshinbo Chemical Inc. as a 40% aqueous solution; [5] Hydrophobically modified alkali swellable acrylic emulsion; Solthix™ A-100, supplied by Lubrizol; [6] Siloxane polyalkylene oxide copolymer; Silwet® L-7607 copolymer, supplied by Momentive.

[0186] Example 12 Preparation and evaluation of electrophoretic displays having an electrophoretic medium containing a charge control agent or combination of charge control agents.

[0187] A series of microcell electrophoretic displays with different Type II electrophoretic media were prepared and evaluated. The microcell electrophoretic displays included a first light-transmitting electrode layer, a microcell layer, and a second electrode layer. The microcell layer included a plurality of microcells, each with an opening, and a sealing layer spanning the openings of the microcells. The sealing layer was formed by coating the aqueous composition of Example 11B, which did not contain the water-soluble ether dipropylene glycol dimethyl ether. The electrophoretic media included negatively charged white particles and positively charged cyan, magenta, and yellow particles. The white particles were based on titanium dioxide pigment surface-treated with a polymer formed by copolymerization of methyl methacrylate monomer and 2,2,2-trifluoroethyl methacrylate. The cyan particles were based on copper phthalocyanine pigment (PB15:3) surface-treated with a polymer formed from methyl methacrylate and dimethylsiloxane monomer as described in Example 7 of U.S. Pat. No. 9,921,451. The magenta particles were based on dimethylquinacridone (PR122) surface-treated with a polymer formed from vinylbenzyl chloride and laurylmethylacrylate, as described in Example 5 of U.S. Patent Nos. 9,697,778 and 9,921,451. The yellow particles were based on Pigment Yellow 155 surface-treated with a polymer formed from methylmethacrylate and dimethylsiloxane. All electrophoretic media also contained the cationic charge control agent from Example 1 of CCA111 of U.S. Patent No. 2020 / 0355978. Two of the inventive examples (12B and 12E) that contained an electrophoretic medium also contained another charge control agent (Solsperse™ 8000). The molecular structure of the second charge control agent included two or more polar groups (amino groups) and a nonpolar tail. All compositions and evaluation data are described in Table 8, Examples 12A through 12E. The symbol X in the table means the presence of the corresponding component in the composition. The color gamut of the electrophoretic display was measured by the color gamut measurement method described above. The C of each color state in the table *The saturation was measured by a color computer. The zeta potential was measured by the method disclosed in Example 6.

[0188] [Table 8-1] [Table 8-2]

[0189] The data in Table 8 demonstrate that electrophoretic displays using electrophoretic media containing a combination of charge control agents (charge control agent from Example 1 CCA111 of US 2020 / 0355978 and Solsperse™ 8000) exhibited improved electro-optical performance. Specifically, the color gamut of an electrophoretic display medium containing Solsperse™ 8000 in combination with a cationic charge control agent (and yellow particles having a zeta potential of 7) was wider than that of an electrophoretic display having an electrophoretic medium containing the same yellow particles but without Solsperse™ 8000 (Example 12B vs. Comparative Example 12A). Similarly, the color gamut of the display of Example 12E was wider than that of Example 12D.

[0190] Example 13 Compositions and evaluation data for electrophoretic displays having (a) an electrophoretic medium comprising a charge control agent or combination of charge control agents, and (b) a sealing layer comprising a water-soluble ether.

[0191] Another series of microcell electrophoretic displays with different Type II electrophoretic media were prepared and evaluated. The display structure is described above in Example 12. The properties of the electrophoretic particles are also described in Example 12. The aqueous sealing compositions of Example 13 (from Example 11A) forming the sealing layers of the electrophoretic displays of Example 12 all contained a water-soluble ether. All electrophoretic media also contained the cationic charge control agent from Example 1 of CCA111 in US 2020 / 0355978. Different yellow particles with various polymer contents and zeta potentials were prepared, as shown in Table 9. The symbol X in the table indicates the presence of the corresponding component in the composition. The color gamut of the electrophoretic displays was measured using the color gamut measurement method described above. The C of each color state in the table is * The saturation was measured by a color computer. The zeta potential was measured by the method disclosed in Example 6.

[0192] [Table 9-1] [Table 9-2]

[0193] The data in Table 9 demonstrate that the inclusion of a water-soluble ether in the sealing composition forming the sealing layer significantly improves the electro-optical performance of the corresponding electrophoretic display (color gamut of Example 13A vs. Comparative Example 12A, Example 13B vs. Example 12B, Example 13C vs. Comparative Example 12C, Example 13E vs. Comparative Example 12D, Example 13F vs. Example 12E). Furthermore, the data in Table 9 demonstrate that improved electro-optical performance is observed in displays in which a combination of charge control agents is present in an electrophoretic medium having similar yellow particles (color gamut of Example 13B vs. Example 13A, Example 13D vs. Example 13C, Example 13F vs. Example 13D, etc.). Finally, the data in Table 9 demonstrate that an electrophoretic medium having yellow particles with a lower zeta potential exhibits better performance (color gamut of Example 13H vs. Example 13G). The improved color performance is due to increased C in the yellow and green states. * This can be attributed to the better of these two states, as shown by the lower dark state C of Comparative Example 13B compared to Comparative Example 13A. * As shown by (and from the series of examples in Table 8), a more neutral dark state (black) can also be obtained. Thus, both (a) the combination of charge control agents (the cationic charge control agent from Example 1 CCA111 of US 2020 / 0355978 and Solsperse™ 8000) and (b) the use of a water-soluble ether in the aqueous sealing composition of an electrophoretic display improve the yellow, green, and dark states, as well as the overall color gamut of the corresponding electrophoretic display. This improvement is observed in all cases, regardless of the yellow particles used (see Example 13D vs. Example 13C, Example 12E vs. Comparative Example 12D, and Example 13F vs. Example 13E). However, the data in Table 1 show that the highest color gamut is achieved in examples where the zeta potential of the yellow particles is in the range of 6 to 11 and the polymer content of the yellow particles is relatively low, such as in Examples 13F and 13H.

[0194] Thus, in general, electrophoretic displays having an electrophoretic medium (Examples 13G and 13H) containing (a) an electrophoretic medium containing a combination of charge control agents, (b) a water-soluble ether in the aqueous sealing composition forming the sealing layer, and (c) yellow particles having a relatively low polymer content while maintaining a low zeta potential exhibited higher chroma (C * ), and higher saturation in the green state (C * ), and a lower C in the dark state * The present inventors have also observed that displays having the provided features (a), (b) and (c) exhibit significantly faster switching speeds from dark to white states (when driven with a voltage sweep from +24V to -24V) than displays without these features. The most significant improvement in switching speed was observed in displays having electrophoretic media containing yellow particles with a relatively low polymer content, for example, Example 13F, in which the polymer content was 32.7% by weight of the yellow particles.

[0195] Example 14 Compositions and evaluation data for electrophoretic displays comprising (a) an electrophoretic medium containing a charge control agent or combination of charge control agents, and (b) a water-soluble ether.

[0196] Another series of microcell electrophoretic displays with different Type II electrophoretic media were prepared and evaluated. The display construction is described above in Example 12. The properties of the electrophoretic particles are also described in Example 12.

[0197] Two of the aqueous sealing compositions of Example 14 (from Example 11A) that formed the sealing layer of the electrophoretic display of Example 14 contained a water-soluble ether (Example 14D and Example 14E). The remaining aqueous sealing compositions of Example 14 (from Example 11B) that formed the sealing layer of the electrophoretic display of Example 14 did not contain a water-soluble ether (Comparative Example 14A, Example 14B, Example 14C, and Example 14F). All of the electrophoretic media contained the cationic charge control agent from Example 1 of CCA111 in US 2020 / 0355978. Three of the electrophoretic medium compositions of Example 14 (Example 14B, Example 14E, and Example 14F) also contained a second charge control agent (Solsperse™ 8000). The molecular structure of the second charge control agent contained two or more polar groups (amino groups) and a non-polar tail. All of the yellow particles in the examples contain the same polymer content of 33.3 weight percent of the particle weight. The symbol X in the table indicates the presence of the corresponding component in the composition. The color gamut of the electrophoretic display was measured using the color gamut measurement method described above. The C of each color state in the table * The (chroma) measurements were measured by a color computer. The compositions and evaluation data are listed in Table 10.

[0198] The combined data in Tables 8 and 9, as can be concluded by comparing Examples 12A, 12B, 13A, and 13B, indicated a synergistic effect on the elements of (a) Solsperse™ 8000 in the electrophoretic medium and (b) the presence of a water-soluble ether in the sealing layer. That is, the color gamut-expanding effect of (a) Solsperse™ 8000 in the electrophoretic medium and (b) the inclusion of a water-soluble ether in the sealing layer is greater than the additive effect of each of the elements (a) and (b) independently.

[0199] The combined data in Tables 8 and 9 also indicated that there was a synergistic effect of the elements (b) the presence of the water-soluble ether in the sealing layer and (c) the lower polymer content on the yellow particles, as can be concluded by comparing Examples 12A, 12C, 13A, and 13C. That is, the color gamut-expanding effect of (c) the yellow particles having a lower polymer content in the electrophoretic medium and (b) the inclusion of the water-soluble ether in the sealing layer was greater than the additive effect of each of the independent elements (c) and (b).

[0200] [Table 10-1] [Table 10-2]

[0201] The present inventors have surprisingly found that the effect on the color performance of a microcell electrophoretic display of having a water-soluble ether in the aqueous sealing composition forming the sealing layer can be achieved in part by incorporating the water-soluble ether in the electrophoretic medium. This is shown in the data in Table 10. The data in Example 14C indicate that the presence of the charge control agent from Example 1 of CCA111 of US 2020 / 0355978 in combination with dipropylene glycol dimethyl ether in the electrophoretic medium improves the electro-optical performance of the corresponding display compared to the control Comparative Example 14A. The further addition of a second charge control agent (Solsperse™ 8000) in the electrophoretic medium of Comparative Example 14C resulted in further improvement, as shown in Example 14F, although the effect was not as pronounced as when a water-soluble ether was included in the aqueous sealing composition forming the sealing layer (Example 14E).

[0202] Microcell electrophoretic displays were constructed using various electrophoretic media (Type II). The displays included a first light-transmitting electrode layer, a microcell layer comprising a plurality of microcells, each microcell having an opening, a sealing layer spanning the openings of the plurality of microcells, and a second electrode layer (bottom electrode).

[0203] While several aspects and embodiments of the technology of the present application have been described, it should be understood that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the technology described herein. For example, those skilled in the art will readily envision various other means and / or structures for performing the functions and / or obtaining one or more of the results and / or advantages described herein, and each such variation and / or modification is deemed to be within the scope of the embodiments described herein.

Claims

[Claim 1] The invention described in this specification.

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