This application claims priority from U.S. patent application No.62/846,342, filed on 2019, 5/10, the entire contents of which are incorporated herein by reference.
Background
The term color as used herein includes black and white. The white particles are typically of the light scattering type.
The term "gray state" is used herein in its conventional sense in the imaging art to refer to a state intermediate two extreme optical states of a pixel, but does not necessarily imply a black-and-white transition between the two extreme states. For example, several patents and published applications by the incorporated of lngk referred to below describe electrophoretic displays in which the extreme states are white and dark blue, so that the intermediate gray state is effectively pale blue. In fact, as already mentioned, the change in optical state may not be a color change at all. The terms "black" and "white" may be used hereinafter to refer to the two extreme optical states of the display and should be understood to generally include extreme optical states that are not strictly black and white, such as the white and deep blue states mentioned above.
The terms "bistable" and "bistability" are used herein in their conventional sense in the art to refer to displays comprising display elements having first and second display states differing in at least one optical characteristic such that, after any given element is driven to assume its first or second display state using an addressing pulse of finite duration, that state will last at least several times (e.g. at least 4 times) the minimum duration of the addressing pulse required to change the state of that display element after the addressing pulse has terminated. It is shown in U.S. patent No.7,170,670 that some particle-based electrophoretic displays that support gray scale can be stabilized not only in their extreme black and white states, but also in their intermediate gray states, as well as some other types of electro-optic displays. This type of display is properly referred to as multi-stable rather than bi-stable, but for convenience the term "bi-stable" may be used herein to cover both bi-stable and multi-stable displays.
The term "impulse", when used in relation to driving an electrophoretic display, is used herein to refer to the integral of the applied voltage with respect to time during the period in which the display is driven.
Particles that absorb, scatter or reflect light in a broad band or selected wavelengths are referred to herein as colored or pigmented particles. Various materials that absorb or reflect light, such as dyes or photonic crystals, other than pigments (which term is meant to be an insoluble color material in the strict sense) may also be used in the electrophoretic media and displays of the present invention.
Particle-based electrophoretic displays have been the subject of intensive development for many years. In such displays, a plurality of charged particles (sometimes referred to as pigment particles) move through a fluid under the influence of an electric field. Electrophoretic displays may have the following properties compared to liquid crystal displays: good brightness and contrast, wide viewing angle, state bistability, and low power consumption. However, problems 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 lifetime of these displays.
As mentioned above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, the fluid is a liquid, but the electrophoretic medium can be produced using a gaseous fluid; see, e.g., Kitamura, T.et al, Electrical tuner movement for electronic Paper-like display, IDW Japan,2001, Paper HCS1-1, and Yamaguchi, Y.et al, tuner display using insulating substrates charged switchgear, IDW Japan,2001, Paper AMD 4-4. See also U.S. patent nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic media appear to be susceptible to the same types of problems due to particle settling as liquid-based electrophoretic media when used in a direction that allows particle settling, such as in signs where the media are arranged in a vertical plane. In fact, the problem of particle settling is more severe in gas-based electrophoretic media than in liquid-based electrophoretic media, because the lower viscosity of gaseous suspending fluids allows faster settling of electrophoretic particles than liquids.
A number of patents and applications assigned to or in the name of the Massachusetts Institute of Technology (MIT) and yingke corporation describe various techniques for encapsulating electrophoretic and other electro-optic media. Such encapsulated media comprise a plurality of microcapsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held in a polymeric binder to form a coherent layer between two electrodes. The techniques described in these patents and applications include:
(a) electrophoretic particles, fluids, and fluid additives; see, e.g., U.S. Pat. Nos. 7,002,728 and 7,679,814;
(b) capsule, adhesive and packaging process; see, e.g., U.S. patent nos. 6,922,276 and 7,411,719;
(c) films and sub-assemblies comprising electro-optic material; see, e.g., U.S. Pat. Nos. 6,982,178 and 7,839,564;
(d) backsheets, adhesive layers, and other auxiliary layers and methods for use in displays; see, e.g., U.S. patent nos. 7,116,318 and 7,535,624;
(e) color formation and color adjustment; see, e.g., U.S. Pat. Nos. 6,017,584; 6,664,944, respectively; 6,864,875, respectively; 7,075,502, respectively; 7,167,155, respectively; 7,667,684, respectively; 7,791,789, respectively; 7,839,564, respectively; 7,956,841, respectively; 8,040,594, respectively; 8,054,526, respectively; 8,098,418, respectively; 8,213,076 and 8,363,299; and U.S. patent application publication No. 2004/0263947; 2007/0223079, respectively; 2008/0023332, respectively; 2008/0043318, respectively; 2008/0048970, respectively; 2009/0004442, respectively; 2009/0225398, respectively; 2010/0103502, respectively; 2010/0156780, respectively; 2011/0164307, respectively; 2011/0195629, respectively; 2011/0310461, respectively; 2012/0008188, respectively; 2012/0019898, respectively; 2012/0075687, respectively; 2012/0081779, respectively; 2012/0134009, respectively; 2012/0182597, respectively; 2012/0212462, respectively; 2012/0157269, respectively; and 2012/0326957;
(f) A method for driving a display; see, e.g., U.S. Pat. Nos. 5,930,026; 6,445,489, respectively; 6,504,524; 6,512,354, respectively; 6,531,997, respectively; 6,753,999, respectively; 6,825,970, respectively; 6,900,851, respectively; 6,995,550, respectively; 7,012,600; 7,023,420, respectively; 7,034,783, respectively; 7,116,466, respectively; 7,119,772; 7,193,625, respectively; 7,202,847, respectively; 7,259,744; 7,304,787, respectively; 7,312,794, respectively; 7,327,511, respectively; 7,453,445, respectively; 7,492,339, respectively; 7,528,822, respectively; 7,545,358, respectively; 7,583,251, respectively; 7,602,374, respectively; 7,612,760, respectively; 7,679,599, respectively; 7,688,297, respectively; 7,729,039, respectively; 7,733,311, respectively; 7,733,335, respectively; 7,787,169, respectively; 7,952,557, respectively; 7,956,841, respectively; 7,999,787, respectively; 8,077,141, respectively; 8,125,501, respectively; 8,139,050, respectively; 8,174,490, respectively; 8,289,250, respectively; 8,300,006, respectively; 8,305,341, respectively; 8,314,784, respectively; 8,384,658, respectively; 8,558,783, respectively; and 8,558,785; and U.S. patent application publication No. 2003/0102858; 2005/0122284, respectively; 2005/0253777, respectively; 2007/0091418, respectively; 2007/0103427, respectively; 2008/0024429, respectively; 2008/0024482, respectively; 2008/0136774, respectively; 2008/0291129, respectively; 2009/0174651, respectively; 2009/0179923, respectively; 2009/0195568, respectively; 2009/0322721, respectively; 2010/0220121, respectively; 2010/0265561, respectively; 2011/0193840, respectively; 2011/0193841, respectively; 2011/0199671, respectively; 2011/0285754, respectively; and 2013/0194250 (these patents and applications may be referred to hereinafter as MEDEOD (means for Driving Electro-optical Displays) applications);
(g) An application for a display; see, e.g., U.S. patent nos. 7,312,784 and 8,009,348; and
(h) non-electrophoretic displays, such as those described in U.S. patent nos. 6,241,921; 6,950,220, respectively; 7,420,549 and 8,319,759; and U.S. patent application publication No. 2012/0293858.
Many of the aforementioned patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thereby creating a so-called polymer-dispersed electrophoretic display, wherein the electrophoretic medium comprises a plurality of discrete droplets of electrophoretic fluid and a continuous phase of polymeric material, and the discrete droplets of electrophoretic fluid within such polymer-dispersed electrophoretic display can be considered as capsules or microcapsules, even if no discrete capsule film is associated with each individual droplet; see, for example, U.S. patent No.6,866,760. Accordingly, for the purposes of this application, such polymer-dispersed electrophoretic media are considered to be a subclass of encapsulated electrophoretic media.
A related type of electrophoretic display is the so-called microcell electrophoretic display. In microcell electrophoretic displays, the charged particles and fluid are not encapsulated within microcapsules, but rather are held within a plurality of cavities formed within a carrier medium, typically a polymer film. See, for example, U.S. patent nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging, inc.
Although electrophoretic media are typically opaque (because, for example, in many electrophoretic media, the particles substantially block 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 display state is light-transmissive. See, for example, U.S. patent nos. 5,872,552, 6,130,774, 6,144,361, 6,172,798, 6,271,823, 6,225,971, and 6,184,856. A dielectrophoretic display similar to an electrophoretic display but relying on a change in electric field strength may operate in a similar mode; see U.S. patent No.4,418,346. Other types of electro-optic displays can also operate in the shutter mode. Electro-optic media operating in shutter mode may be used in the multilayer structure of a full color display; in this configuration, at least one layer adjacent to the viewing surface of the display operates in a shutter mode to expose or hide a second layer further from the viewing surface.
Encapsulated electrophoretic displays are generally not plagued by the aggregation and settling failure modes of conventional electrophoretic devices and provide further benefits such as the ability to print or coat the display on a variety of flexible and rigid substrates. (use of the term "printing" is intended to include all forms of printing and coating including, but not limited to, premetered coating such as patchwork die coating, slot or extrusion coating, slide or stack coating, curtain coating, roll coating such as knife over roll coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brush coating, air knife coating, screen printing processes, electrostatic printing processes, thermal printing processes, ink jet printing processes, electrophoretic deposition (see U.S. patent No.7,339,715), and other similar techniques.) thus, the resulting display may be flexible. In addition, since the display media can be printed (using a variety of methods), the display itself can be inexpensively manufactured.
The aforementioned U.S. patent No.6,982,178 describes a method of assembling solid state electro-optic displays, including encapsulated electrophoretic displays, that is well suited for mass production. Essentially, this patent describes a so-called Front Plane Laminate (FPL) comprising, in order, a light-transmissive electrically conductive layer, a layer of a solid electro-optic medium in electrical contact with the electrically conductive layer, a layer of adhesive, and a release sheet. Typically, the light-transmissive electrically-conductive layer will be carried on a light-transmissive substrate, which is preferably flexible, in the sense that the substrate can be manually wound on a drum of, for example, 10 inches (254mm) in diameter without permanent deformation. The term "light transmissive" is used in this patent and refers herein to the layer designated thereby transmitting sufficient light to enable a viewer looking through the layer to observe a change in the display state of the electro-optic medium, typically through the conductive layer and adjacent substrate (if present); where the electro-optic medium exhibits a change in reflectivity at non-visible wavelengths, the term "light transmissive" should of course be understood to refer to transmission at the relevant non-visible wavelengths. The substrate is typically a polymeric film and typically has a thickness in the range of about 1 to about 25 mils (25-634 μm), preferably about 2 to about 10 mils (51-254 μm). Conveniently, the conductive layer is a thin metal layer or metal oxide layer, for example of aluminium or Indium Tin Oxide (ITO), or may be a conductive polymer. Poly (ethylene terephthalate) (PET) films coated with aluminum or ITO are commercially available, for example aluminized Mylar (Mylar is a registered trademark) from dupont, wilmington, tera, and this commercial material can be used with good effect in front plane laminates.
Assembly of an electro-optic display using such a front plane laminate may be achieved by: the release sheet is removed from the front plane laminate and the adhesive layer is contacted with the backplane under conditions effective to cause the adhesive layer to adhere to the backplane, thereby securing the adhesive layer, the layer of electro-optic medium, and the conductive layer to the backplane. This process is well suited for mass production, as the front plane laminate can be mass produced, typically using roll-to-roll coating techniques, and then cut into pieces of any size needed for a particular back sheet.
U.S. patent No.7,561,324 describes a so-called double release tab, which is essentially a simplified version of the front plane laminate of the aforementioned U.S. patent No.6,982,178. One form of dual release sheet comprises a layer of a solid electro-optic medium sandwiched between two adhesive layers, one or both of which are covered by a release sheet. Another form of dual release sheet comprises a layer of a solid electro-optic medium sandwiched between two release sheets. Both forms of dual release film are intended for use in a process generally similar to that of assembling an electro-optic display from the aforementioned front plane laminate but involving two separate laminations; typically, in a first lamination, the dual release sheet is laminated to the front electrode to form a front sub-assembly, and then in a second lamination, the front sub-assembly is laminated to the backplane to form the final display, although the order of the two laminations can be reversed as desired.
U.S. patent No.7,839,564 describes a so-called inverted front plane laminate which is a variation of the front plane laminate described in the aforementioned U.S. patent No.6,982,178. The inverted front plane laminate comprises, in order, at least one of a light-transmissive protective layer and a light-transmissive electrically-conductive layer, an adhesive layer, a solid electro-optic dielectric layer, and a release sheet. The inverted front plane laminate is used to form an electro-optic display having a laminating adhesive layer between the electro-optic layer and a front electrode or front substrate, and a second typically thin adhesive layer may or may not be present between the electro-optic layer and the backplane. Such an electro-optic display may combine good resolution with good low temperature performance.
As mentioned above, the simplest prior art electrophoretic media display substantially only two colors. Such electrophoretic media use one type of electrophoretic particles having a first color in a colored fluid having a different second color (in which case the first color is displayed when the particles are adjacent to the viewing surface of the display and the second color is displayed when the particles are spaced from the viewing surface), or first and second types of electrophoretic particles having different first and second colors in a colorless fluid (in which case the first color is displayed when the first type of particles are adjacent to the viewing surface of the display and the second color is displayed when the second type of particles are adjacent to the viewing surface). Typically, the two colors are black and white. If a full color display is desired, a color filter array may be placed on the viewing surface of the monochrome (black and white) display. Displays with color filter arrays rely on region sharing and color mixing to create color stimuli. The available display area is shared between three or four primary colors, such as red/green/blue (RGB) or red/green/blue/white (RGBW), and the filters may be arranged in a one-dimensional (stripe) or two-dimensional (2 × 2) repeating pattern. Other choices of primary colors or more than three primary colors are also known in the art. Three (in the case of an RGB display) or four (in the case of an RGBW display) sub-pixels are chosen small enough so that at the desired viewing distance they visually blend together into a single pixel with a uniform color stimulus ("color blending"). An inherent drawback of area sharing is that the colorant is always present and the color can only be modulated by switching the corresponding pixel of the underlying monochrome display to white or black (turning the corresponding primary color on or off). For example, in an ideal RGBW display, each of the red, green, blue and white primaries occupy a quarter (a quarter of a sub-pixel) of the display area, where the white sub-pixel is as bright as the underlying monochrome display white, and each of the color sub-pixels is no brighter than one third of the monochrome display white. The luminance of the white color shown by the display as a whole cannot be greater than half the luminance of the white sub-pixel (the white area of the display is produced by displaying one white sub-pixel every four sub-pixels plus each colored sub-pixel in colored form is equivalent to one third of the white sub-pixel so that the combined three colored sub-pixels contribute no more than one white sub-pixel). In the case where the color pixel is switched to black, the luminance and saturation of the color are reduced by the region sharing. Region sharing is particularly problematic when mixing yellow because it is brighter than any other color of equal brightness, and saturated yellow is almost as bright as white. Switching the blue pixels (one quarter of the display area) to black makes the yellow too dark.
Multilayer stacked electrophoretic displays are known in the art; see, e.g., J.Heikenfeld, P.Drzaic, J-S Yeo and T.Koch, Journal of the SID,19(2),2011, pp.129-156. In such displays, ambient light passes through the image in each of the three subtractive primary colors, much like conventional color printing. Us patent No.6,727,873 describes a stacked electrophoretic display in which three layers of switchable cells are placed on a reflective background. Similar displays are known in which the colour particles are laterally displaced (see international application No. wo 2008/065605), or isolated into the micro-pits using a combination of vertical and lateral movements. In both cases, each layer is provided with electrodes for concentrating or dispersing the color particles on a pixel-by-pixel basis, so that each of the three layers requires a Thin Film Transistor (TFT), of which two of the three layers must be substantially transparent, and a light-transmissive counter electrode. Such a complex arrangement of electrodes is expensive to manufacture and in the prior art it is difficult to provide a sufficiently transparent plane of the pixel electrodes, especially when the white state of the display has to be viewed through several layers of electrodes. Multi-layer displays also suffer from parallax problems because the thickness of the display stack approaches or exceeds the pixel size.
U.S. application publication nos. 2012/0008188 and 2012/0134009 describe a multicolor electrophoretic display having a single backplane that includes independently addressable pixel electrodes and a common light-transmissive front electrode. A plurality of electrophoretic layers are disposed between the back plate and the front electrode. The displays described in these applications are capable of rendering any of the primary colors (red, green, blue, cyan, magenta, yellow, white and black) at any pixel location. However, there are disadvantages to using multiple electrophoretic layers between a single set of address electrodes. The particles in a particular layer experience a lower electric field than in the case of a single electrophoretic layer addressed with the same voltage. In addition, optical losses (e.g., caused by light scattering or unwanted absorption) in the electrophoretic layer closest to the viewing surface may affect the appearance of the image formed in the underlying electrophoretic layer.
Attempts have been made to provide full-color electrophoretic displays using a single electrophoretic layer. For example, U.S. patent application publication No.2013/0208338 describes a color display that includes an electrophoretic fluid that includes one or both types of pigment particles dispersed in a transparent and colorless or colored solvent, the electrophoretic fluid being sandwiched between a common electrode and a plurality of drive electrodes. The drive electrode is held at a certain distance to expose the background layer. U.S. patent application publication No.2014/0177031 describes a method for driving a display cell filled with an electrophoretic fluid comprising two types of charged particles carrying opposite charge polarities and having two contrasting colors. Both types of pigment particles are dispersed in a colored solvent or a solvent in which uncharged or slightly charged colored particles are dispersed. The method includes driving a display cell to display a color of a solvent or a color of uncharged or slightly charged color particles by applying a driving voltage, wherein the driving voltage is about 1 to about 20% of a full driving voltage. U.S. patent application publication nos. 2014/0092465 and 2014/0092466 describe electrophoretic fluids, and methods for driving electrophoretic displays. The fluid includes a first type, a second type, and a third type of pigment particles, all dispersed in a solvent or solvent mixture. The first and second types of pigment particles carry opposite charge polarities, and the third type of pigment particles have a charge level that is less than about 50% of the charge level of the first or second type. The three types of pigment particles have different levels of threshold voltage, or different levels of mobility, or both. None of these patent applications disclose a full-color display in the sense that the term "full-color display" is used hereinafter.
U.S. patent application publication No.2007/0031031 describes an image processing apparatus for processing image data to display an image on a display medium, where each pixel is capable of displaying white, black, and one of the other colors. U.S. patent application publication No. 2008/0151355; 2010/0188732 and 2011/0279885 describe color displays in which moving particles move through a porous structure. U.S. patent application publication nos. 2008/0303779 and 2010/0020384 describe display media that include first, second, and third particles of different colors. The first and second particles may form an aggregate, and the smaller third particles may move through pores left between the aggregated first and second particles. U.S. patent application publication No.2011/0134506 describes a display device comprising an electrophoretic display element comprising: a plurality of types of particles encapsulated between a pair of substrates, at least one of the substrates being translucent and each of the respective plurality of types of particles differing in charge of the same polarity, optical characteristics, and in a migration speed and/or an electric field threshold value for movement, a translucent display-side electrode being provided on a substrate side on which the translucent substrate is arranged, a first backside electrode being provided on a side of the other substrate facing the display-side electrode, and a second backside electrode being provided on a side of the other substrate facing the display-side electrode; and a voltage control section that controls voltages applied to the display-side electrode, the first back-side electrode, and the second back-side electrode such that a type of particle having a fastest migration speed among the plurality of types of particles, or a type of particle having a lowest threshold among the plurality of types of particles, sequentially passes through each of the different types of particles, moves to the first back-side electrode or moves to the second back-side electrode The back side electrode, and then the particles moved to the first back side electrode move to the display side electrode. U.S. patent application publication No. 2011/0175939; 2011/0298835, respectively; 2012/0327504 and 2012/0139966 describe color displays that rely on the aggregation and threshold voltage of multiple particles. U.S. patent application publication No.2013/0222884 describes electrophoretic particles comprising: colored particles comprising a polymer containing a charged group and a colorant; and a branched silicone-based polymer attached to the colored particles and comprising a reactive monomer and at least one monomer selected from a specific set of monomers as a copolymerization component. U.S. patent application publication No.2013/0222885 describes a dispersion for an electrophoretic display, which comprises a dispersion medium, a colored electrophoretic particle group dispersed in the dispersion medium and migrating in an electric field, a non-electrophoretic particle group not migrating and having a color different from that of the electrophoretic particle group, and a compound having a neutral polar group and a hydrophobic group, the compound being contained in the dispersion medium in a proportion of about 0.01 to about 1 mass% based on the entire dispersion. U.S. patent application publication No.2013/0222886 describes a dispersion for a display comprising floating particles comprising: a core particle including a colorant and a hydrophilic resin; and covering a surface of each of the core particles and comprising a particle having a particle size of 7.95 (J/cm)3)1/2Or a shell of a larger hydrophobic resin with a poor solubility parameter. U.S. patent application publication nos. 2013/0222887 and 2013/0222888 describe electrophoretic particles having a specified chemical composition. Finally, U.S. patent application publication No.2014/0104675 describes particle dispersion including first color particles and second color particles that move in response to an electric field, and a dispersion medium, the second color particles having a larger diameter than the first color particles and the same charging characteristics as those of the first color particles, wherein a ratio (Cs/Cl) of a charge amount Cs of the first color particles to a charge amount Cl of the second color particles per unit area of the display is less than or equal to 5. Some of the aforementioned displays offer full color but at the cost of requiring long and cumbersome addressing methods.
U.S. patent application publication nos. 2012/0314273 and 2014/0002889 describe electrophoretic devices including a plurality of first electrophoretic particles and second electrophoretic particles contained in an insulating liquid, the first particles and the second particles having different charging characteristics from each other; the device further comprises a porous layer which is comprised in the insulating liquid and which is formed by the fibrous structure. These patent applications are not full color displays in the sense that the term "full color display" is used hereinafter.
See also U.S. patent application publication No.2011/0134506 and U.S. patent No.9,697,778, which describe full color displays using three different types of particles in a colored fluid, but the presence of the colored fluid limits the quality of the white state that the display can achieve.
In summary, prior art full color displays typically involve compromises such as slow switching speeds (up to several seconds), high addressing voltages or compromises with respect to color quality. Accordingly, there is a need for improved full color electrophoretic displays.
Detailed Description
Definition of
Unless otherwise specified herein, the following terms and phrases have the meanings indicated below. The present disclosure may employ other terms and phrases not expressly defined herein. Such other terms and phrases should have the meaning that they would have to those of ordinary skill in the art within the context of this disclosure. In some instances, terms or phrases may be defined in singular or plural forms. In this case, it should be understood that any term in the singular may include its plural counterparts and vice versa, unless explicitly stated to the contrary.
As used herein, the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise. For example, reference to "a substituent" includes a single substituent as well as two or more substituents and the like.
As used herein, "for example," "for instance," "such as," or "including" is intended to introduce examples that further illustrate more general subject matter. Such examples are provided merely as an aid in understanding the embodiments illustrated in the present disclosure and are not intended to be limiting in any way unless explicitly stated otherwise. Nor do these phrases indicate any kind of preference for the disclosed embodiments.
As used herein, the term "polymer" refers to a polymeric compound prepared by polymerizing monomers (whether of the same or two or more types). Thus, the generic name "polymer" is intended to encompass the term "homopolymer" and the term "interpolymer," as defined below. Minor amounts of impurities may be incorporated into and/or within the polymer structure.
As used herein, the term "interpolymer" refers to a polymer prepared by the polymerization of at least two different monomers. The generic name "interpolymer" includes copolymers (used to refer to polymers prepared from two different types of monomers) and polymers prepared from more than two different types of monomers. Thus, "a polymer derived from one or more monomers" refers to a homopolymer where the monomers are one, a copolymer where the monomers are two, and other types of interpolymers where the monomers are three or more.
The terms "unit of a monomer", "monomer unit", "monomer residue" or "residue of a monomer" are to be understood as referring to a residue resulting from the polymerization of the corresponding monomer. For example, a polymer derived from the polymerization of styrene monomer will provide a polymer segment comprising repeating styrene monomer units, i.e.,
“—CH(C6H5)CH2—”。
as used herein, the term "functional group" refers to a linked set of multiple atoms or a single atom within a molecular entity, wherein a molecular entity is any structurally or isotopically distinct atom, molecule, ion pair, radical ion, complex, conformer, etc., identifiable as an individually distinguishable entity. Unless otherwise specified, a description of a group as "formed" from a particular chemical transformation does not imply that the chemical transformation involves the manufacture of a molecular entity that comprises the group.
As used herein, the various functional groups represented will be understood to have attachment points at functional groups with hyphens or dashes (-) or asterisks (#). In other words in-CH2CH2CH3In the case of (c), it will be understood that the attachment point is the leftmost CH2A group. If a group is described as having no asterisk or dash, the point of attachment is indicated by the general and ordinary meaning of the group recited.
As used herein, polyatomic divalent functionality is read from left to right. For example, if the specification or claims recite a-D-E and D is defined as-oc (o) -, then substitution of D results in the following groups: A-OC (O) -E instead of A-C (O) O-E.
As used herein, the term "alkyl" refers to a straight or branched chain saturated hydrocarbon having from 1 to 30 carbon atoms, which may be optionally substituted, as further described herein, allowing for multiple degrees of substitution. Examples of "alkyl" as used herein include, but are not limited to, methyl, ethyl, n-propyl, isopropyl, isobutyl, n-butyl, sec-butyl, tert-butyl, isopentyl, n-pentyl, neopentyl, n-hexyl, and 2-ethylhexyl. The number of carbon atoms in the alkyl group is defined by the phrase "Cx-yAlkyl "refers to an alkyl group as defined herein containing x to y (inclusive) carbon atoms. Thus, "C1-6Alkyl "denotes a group havingAlkyl chains of 1 to 6 carbon atoms and include, for example, but are not limited to, methyl, ethyl, n-propyl, isopropyl, isobutyl, n-butyl, sec-butyl, tert-butyl, isopentyl, n-pentyl, neopentyl, and n-hexyl. In some cases, an "alkyl" group may be divalent, in which case the group may alternatively be referred to as an "alkylene" group. Further, in some cases, one or more carbon atoms in an alkyl or alkylene group can be replaced by a heteroatom (e.g., selected from nitrogen, oxygen, or sulfur, including nitrogen oxides, sulfur oxides, and sulfur dioxide, where feasible), and is referred to as a "heteroalkyl" or "heteroalkylene" group.
As used herein, "cycloalkyl" refers to a 3 to 24 membered cyclic hydrocarbon ring, which may be optionally substituted as further described herein, and allows for multiple degrees of substitution. Such "cycloalkyl" groups are monocyclic or polycyclic. As used herein, the term "cycloalkyl" does not include ring systems containing aromatic rings, but does include ring systems that may have one or more degrees of unsaturation. As used herein, examples of "cycloalkyl" groups include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, 1-norbornyl, 2-norbornyl, 7-norbornyl, 1-adamantyl, and 2-adamantyl. In some cases, a "cycloalkyl" group may be divalent, in which case the group may alternatively be referred to as a "cycloalkylene" group. Further, in some cases, one or more carbon atoms in a cycloalkyl or cycloalkylene group may be replaced by a heteroatom (e.g., selected from nitrogen, oxygen, or sulfur, including nitrogen oxides, sulfur oxides, and sulfur dioxide, where feasible), and is referred to as a "heterocycloalkyl" or "heterocycloalkylene" group.
As used herein, "aryl" refers to a 6 to 30 membered cyclic aromatic hydrocarbon, which may be optionally substituted as further described herein, and allows for multiple degrees of substitution. Examples of "aryl" as used herein include, but are not limited to, phenyl and naphthyl. As used herein, the term "aryl" also includes ring systems in which a phenyl or naphthyl group is optionally fused to one to three non-aromatic, saturated or unsaturated carbocyclic rings. For example, "aryl" would include a ring system such as indene, which may be attached to an aromatic or non-aromatic ring. In some cases, an "aryl" group may be divalent, in which case the group may alternatively be referred to as an "arylene" group. Furthermore, as used herein, "arylalkyl" refers to an alkyl substituent (as defined above) that is further substituted with one or more (e.g., one to three) aryl groups (as defined herein). Similarly, "alkylaryl" refers to an aryl substituent that is further substituted with one or more (e.g., one to five) alkyl groups.
As used herein, the term "heteroaryl" refers to a 5 to 30 membered monocyclic or polycyclic ring system comprising at least one aromatic ring and further comprising one or more heteroatoms. Such "heteroaryl" groups may be optionally substituted as further described herein, and allow for multiple degrees of substitution. In polycyclic "heteroaryl" groups containing at least one aromatic ring and at least one non-aromatic ring, the aromatic ring need not contain heteroatoms. Thus, for example, as used herein, "heteroaryl" will include indolyl. Furthermore, the attachment point may be any ring within the ring system, regardless of whether the ring containing the attachment point is aromatic or contains a heteroatom. Thus, for example, as used herein, "heteroaryl" shall include indol-1-yl, indol-3-yl and indol-5-yl. Examples of heteroatoms, where feasible, include nitrogen, oxygen, or sulfur atoms, including nitrogen oxides, sulfur oxides, and sulfur dioxide. As used herein, examples of "heteroaryl" groups include, but are not limited to, furyl, thienyl, pyrrolyl, oxazolyl, thiazolyl, imidazolyl, isoxazolyl, isothiazolyl, 1,2, 4-triazolyl, pyrazolyl, pyridyl, pyridazinyl, pyrimidinyl, indolyl, isoindolyl, benzo [ b ] thienyl, benzimidazolyl, benzothiazolyl, pteridinyl, and phenazinyl, wherein attachment may occur at any point on the ring, so long as attachment is chemically feasible. Thus, for example, "thiazolyl" refers to thiazol-2-yl, thiazol-4-yl and thiazol-5-yl. In some cases, a "heteroaryl" group may be divalent, in which case the group may alternatively be referred to as a "heteroarylene" group. Furthermore, as used herein, "heteroarylalkyl" refers to an alkyl substituent (as defined above) which is further substituted with one or more (e.g., one to three) heteroaryl groups (as defined herein). Similarly, "alkylheteroaryl" refers to an aryl substituent that is further substituted with one or more (e.g., one to five) alkyl groups.
As used herein, "alkoxy" refers to — OR, wherein R is alkyl (as defined above). The number of carbon atoms in an alkyl group is represented by the phrase "Cx-y alkoxy," which refers to an alkoxy group having an alkyl group as defined herein containing x to y (inclusive) carbon atoms.
As used herein, "halogen" or "halo" refers to fluorine, chlorine, bromine, and/or iodine. In some embodiments, the term refers to fluorine and/or chlorine. As used herein, "haloalkyl" or "haloalkoxy" refers to alkyl or alkoxy groups, respectively, that are substituted with one or more halogen atoms. The terms "perfluoroalkyl" or "perfluoroalkoxy" refer to alkyl and alkoxy groups, respectively, in which each available hydrogen is replaced by fluorine.
In some cases, the disclosure can refer to "a combination" or "combinations" of particular groups, meaning that two or more of the foregoing groups can be combined to form new groups. For example, the phrase "R is alkylene, arylene, or a combination thereof" means that R can be a group that includes both an alkylene group and an arylene group, such as- (alkylene) - (arylene) -, - (arylene) - (alkylene) -, - (alkylene) (arylene) (alkylene) -and the like.
As used herein, "substituted" means that one or more hydrogens of the designated moiety are replaced with the named substituent or substituents, unless otherwise indicated, multiple degrees of substitution are allowed, provided that the substitution results in a stable or chemically feasible compound. A stable compound or chemically feasible compound is one whose chemical structure remains substantially unchanged at a temperature of about-80 ℃ to about +40 ℃ for at least one week in the absence of moisture or other chemical reaction conditions, or one whose integrity is maintained long enough for electrophoretic applications. As used herein, the phrase "substituted with one or more … …" or "substituted one or more times …" refers to the number of substituents which is equal to one to the maximum number of substituents possible based on the number of available bonding sites, provided that the conditions of stability and chemical feasibility described above are met.
As used herein, "optionally" means that the subsequently described event may or may not occur. In some embodiments, the optional event does not occur. In some other embodiments, the optional event does occur one or more times.
As used herein, "comprising" or "including" or "containing" or "including" refers to an open group, meaning that the group may include additional ingredients in addition to those explicitly recited. For example, the phrase "comprising a" means that a must be present, but that other components may also be present. The terms "comprising," "having," and "including" have the same meaning, along with grammatical variants thereof. Conversely, "consisting of … …" or "consisting of … …" or "consisting of … …" refers to a closed group. For example, the phrase "consisting of a" means that a is present and only a is present.
As used herein, "or" is to be given its broadest reasonable interpretation and is not limited to "not that of another". Thus, the phrase "comprising a or B" means that a may be present and B is absent, or B is present and a is absent, or both a and B are present. Further, for example, if A defines a class that may have multiple components (e.g., A1 and A2), one or more components of the class may exist simultaneously.
As used herein, "wt%" is an abbreviation for the mass percentage of a given component of an article. It is a way to express the composition of a mixture or product in dimensionless dimensions; mole fraction (mol percent, mol%) and volume fraction (vol%) are other ways.
Detailed Description
As described above, the present invention provides in one aspect an electrophoretic medium comprising one light-scattering particle (typically white) and three other particles typically providing the three primary colors of subtraction. When the mobility of one of the particles of the subtractive primary colors (measured, for example, according to Zeta potential) is less than or equal to half the mobility of the other particles in the electrophoretic medium, an improved switching time can be achieved without sacrificing the overall color gamut.
The three particles providing the subtractive primary colors may be substantially non-light scattering ("SNLS"). The use of SNLS particles allows for the mixing of colors and provides more color results than can be achieved with the same number of scattering particles. The aforementioned US 2012/0327504 uses particles with subtractive primary colors, but requires two different voltage thresholds for independent addressing of non-white particles (i.e. addressing the display with three positive and three negative voltages). These thresholds must be sufficiently separated to avoid cross talk and this separation requires the use of high addressing voltages for some colors. In addition, addressing the color particle with the highest threshold also moves all other color particles, and these other particles must then be switched to their desired positions with a lower voltage. This stepwise color addressing scheme produces flickering and long transition times of undesired colors. Certain embodiments of the present invention do not require the use of such stepped waveforms and addressing of all colors can be accomplished with only two positive voltages and two negative voltages (i.e., only five different voltages are needed in the display, two positive voltages, two negative voltages and zero, but in other embodiments, it is preferable that more different voltages can be used to address the display).
Fig. 8 of the accompanying drawings is a schematic cross-sectional view showing the position of individual particles in the electrophoretic medium of the color display described in us patent 9,921,451 when displaying black, white, three primary colors subtracted and three primary colors added. In fig. 8, it is assumed that 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 incident from this direction. As previously mentioned, in a preferred embodiment, only one of the four particles used in the electrophoretic medium of the present invention substantially scatters light, and in fig. 8, the particle is assumed to be a white pigment. Basically, the white light-scattering particles form a white reflector against which any particles above the white particles are observed (as shown in fig. 8). Light entering the viewing surface of the display passes through the particles, reflects from the white particles, passes back through the particles and is presented from the display. Thereby, the particles above the white particles may absorb various colors, and the color appearing to the user results from the combination of the particles above the white particles. Any particles disposed below the white particles (behind from the user's perspective) are masked by the white particles and do not affect the displayed color. Since the second, third and fourth particles are substantially non-light scattering, their order or arrangement with respect to each other is unimportant, but for the reasons already stated their order or arrangement with respect to the white (light scattering) particles is critical.
More specifically, when cyan, magenta, and yellow particles are located below the white particles (case [ a ] in fig. 8), no particles are present above the white particles, and the pixel simply displays white. When a single particle is on top of a white particle, the color of the single particle is displayed, in fig. 8 yellow, magenta and cyan are displayed in cases [ B ], [ D ] and [ F ], respectively. When two kinds of particles are located on top of a white particle, the displayed color is a combination of the colors of the two kinds of particles; in fig. 8, in case [ C ], magenta and yellow particles show red, in case [ E ], cyan and magenta particles show blue, and in case [ G ], yellow and cyan particles show green. Finally, when all three color particles are located above the white particles (case [ H ] in fig. 8), all incident light is absorbed by the particles of the subtractive primary colors, and the pixel displays black.
It is possible that one subtractive primary color may be present by the light scattering particles, so that the display will comprise two types of light scattering particles, one of which is white and the other of which is colored. However, in this case the position of the colored light scattering particles with respect to the other colored particles covering the white particles will be important. For example, when appearing black (when all three colored particles are on top of a white particle), the colored scattering particles cannot be on top of the colored non-scattering particles (otherwise they will be partially or completely hidden behind the scattering particles and the color appearing will be that of the colored scattering particles, not black).
If more than one type of colored particles scatter light, it will not be easy to appear black.
Fig. 8 shows an idealized situation in which the color is uncontaminated (i.e. the white light-scattering particles completely mask any particles located behind the white particles). In practice, the masking with white particles may be imperfect so that there may be some small absorption of light by the particles that would ideally be completely masked. Such contamination typically reduces both the luminance and the chrominance of the color being rendered. In the electrophoretic media of the present invention, this color contamination should be minimized to the extent that the color formed is commensurate with industry standards for color reduction. A particularly preferred criterion is SNAP (a criterion for newspaper advertisement production) which specifies values of L, a and b for each of the eight primary colors referred to above. (hereinafter, "primary colors" will be used to refer to eight colors: black, white, three subtractive primary colors, and three additive primary colors, as shown in FIG. 8.)
Methods for electrophoretically arranging a plurality of differently colored particles in a "layer" as shown in fig. 8 have been described in the prior art. The simplest of these methods involves "racing" pigments with different electrophoretic mobilities; see, for example, U.S. patent No.8,040,594. This competition is more complex than initially understood, since the motion of the charged pigments themselves alters the electric field experienced locally within the electrophoretic fluid. For example, as positively charged particles move towards the cathode and negatively charged particles move towards the anode, their charge screens the electric field experienced by the charged particles intermediate the two electrodes. It is envisaged that although the electrophoresis of the present invention involves pigment competition, this is not the only phenomenon that causes the arrangement of particles shown in figure 8.
In a color display, as described in us patent 9,921,451, one of the color pigments has the same charge polarity (usually negatively charged) as the white pigment. Negatively charged color pigments and white pigments move in the same direction in an electric field, and therefore the creation of pure white and yellow states requires some means to selectively retard or enhance the motion of one of these pigments relative to the other. In practice, complex waveforms are used to ensure that the white pigment covers the negatively charged color pigment to an extent exceeding the uncontaminated white state for the viewer. Such complex waveforms may not allow for a fast transition from another color to the white state. For example, for some displays, this transition may last about 5-10 seconds. In the case where the white and yellow bands are negatively charged and the cyan and magenta colors are positively charged and the cyan pigment forms a weaker aggregate with the yellow than the magenta pigment, the cyan color must be formed from the previous white state, followed by a second stage of bringing the cyan pigment to the viewing surface, as described in U.S. patent 9,921,451. Therefore, forming cyan from another color requires a longer update time than forming the white state. However, in many applications of full color electrophoretic displays, it is preferable to have much shorter image transitions, on the order of three seconds or less.
As mentioned above, it has now been found that the switching time can be improved when the mobility of one of the particles of the subtractive primary colors is less than or equal to half the mobility of the other particles in the electrophoretic medium. In a broad sense, mobility can be expressed asWhere v is the speed of the electrophoresis,is an applied electric field. The measurement of the mobility of the colored particles can be obtained in one of a number of ways. Although mobility can be measured directly, for non-polar media, direct measurement may not be simple. For example, the mobility can be calculated from a measurement of the Zeta potential ζ or charge-to-mass ratio Q/M of the particles, or from the conductivity λ of the dispersion and of the slurry thereof0Is calculated from the measured value of the difference of (c). Mobility is discussed in relation to Zeta potential, charge to mass ratio and conductivity in Morrison, i.d. and Tarnawskyj, c.j., Langmuir 1991,7, 2358.
In one example, the electrophoretic medium may include a low dielectric constant solvent, a white pigment having a silane surface treatment and a polymeric coating and a Zeta potential of < -60mV, a first color pigment comprising a polymeric coating and a Zeta potential of >30mV, a second color pigment which may or may not comprise a polymeric coating and a Zeta potential of >20mV, the polymeric coating providing less spatial stability than the polymeric coating on the first and third color pigments, and third color particles having a Zeta potential in the range of-20 mV to +20 mV. More generally, formulations according to various embodiments of the present invention may include a white scattering pigment comprising a polymer coating and having a first polarity, two color pigments having a second polarity opposite the first polarity (at least one of the two pigments having a polymer coating), and a third color pigment comprising a polymer coating, wherein the mobility of the third color pigment is less than half the mobility of any other pigment. The polymeric coating of the third colored pigment is preferably insoluble in the electrophoretic solvent, but can be removed by a more polar solvent. In some formulations, the white pigment, the two color pigments having the second polarity, and the third color pigment all comprise a polymeric coating. Each coating may have a different composition than all of the other coatings. Alternatively, two or more pigments may have the same coating.
Assuming that the core pigments comprising the particles are approximately the same size, and assuming that the Zeta potential of each uncoated particle is approximately the same, the magnitude of the Zeta potential of the composite particles depends on the polymeric shell surrounding each core pigment. According to one method for manufacturing pigment particles for inclusion in an electrophoretic medium according to various embodiments of the present invention, a dispersion polymerization operation may be used to provide a polymeric coating on a core pigment particle. In a preferred method, the dispersion of core pigment particles is provided in a solution of suitable monomers in a solvent that also contains a polymerization initiator. Homopolymers formed from at least one monomer are soluble in the solvent, while homopolymers formed from at least another monomer are insoluble at a sufficiently high molecular weight. When a copolymer is produced as the polymerization of the monomer mixture proceeds, the polymer begins to deposit onto the core pigment particles, becoming insoluble in the solvent as its molecular weight increases. However, a sufficient amount of the more soluble monomer is present to provide a solvent soluble copolymer portion. These segments provide steric stability to the coated core pigment particles.
A schematic of the polymer-coated pigment particles is provided in fig. 1. The core pigment particles 102 are modified with smaller polymer particles 104 that include solvent-soluble segments 106. As more polymer particles are deposited on the core particles, their surfaces become increasingly covered. Without wishing to be bound by theory, a simple model of this process will now be described. Many assumptions are made in this model; however, this model has some value in aiding understanding of various embodiments of the invention.
If it is assumed that the polymer particles 104 are attached at random locations on the surface of the modified pigment particle 102 and that the polymer particles, once attached, do not spread over the surface of the particle, the probability of the additional polymer particles striking a portion of the core pigment particle 102 that is not yet covered by polymer depends on the amount of polymer already present. This yields an expression for the proportion P of the surface of the pigment particles covered by the following types of polymers:
P＝1–exp(-kQ) (1)
where Q is the relative mass of the polymer particles added to the pigment particles and k is a constant that depends on the radii of the polymer and pigment particles and their relative densities.
If it is further assumed that the Zeta potential of the modified particles is the surface area average Zeta potential of the uncoated pigment surface and the polymer and that the Zeta potential of the polymer is the mass ratio average of the Zeta potentials of the pure polymers made from each of the constituent monomers, the Zeta potential of the composite particles is given by the following formula:
wherein m isiIs the relative mass, ζ, of the monomer i in the polymer comprising the polymer particlesiZeta potential of particles made of pure polymer made from monomer i, and ζpigIs the Zeta potential of the unmodified pigment particles. Equation (2) allows a rough prediction of the Zeta potential of a particular composite particle given its composition.
Fig. 2 shows a comparison between the predicted and measured Zeta potentials of a number of different composite pigment particles of the present invention, all of which are derivatives of the same base pigment particle (pigment yellow 155). Note that in the preparation of pigment particles (e.g., the examples detailed in example 1 below), only about one-third of the mass of monomer added to the polymerization reaction actually deposits on the surface of the isolated pigment particles. As a result, the composition of the deposited polymer is not necessarily the same as the composition of the monomer added to the reaction vessel. Therefore, the Zeta potential estimated by equation (2) is only an approximation.
Generally, the monomers used to prepare the particles incorporated into the electrophoretic medium according to various embodiments of the present invention should be soluble in the polymerization solvent. At least one monomer should form a polymer that becomes insoluble as polymerization proceeds.
Monomers that impart a more positive charge to the product polymer include esters and amides of vinyl acids, such as those of formula (1):
CH2＝C(R1a)C(O)R2formula (1)
Functional group R1aIs usually-H or-CH3. In one embodiment, the group R2Having the formula-OR3To form an acrylate or methacrylate. When the group R2Having the formula-NHR3When the monomer is acrylamide. Also includes wherein R2Having the formula-NR32In this case, the monomer is N, N-bisacrylamide. In non-limiting embodiments, the group R3May be C1-6Alkyl radical, C1-6Heteroalkyl group, C3-10Cycloalkyl radical, C3-10Heterocycloalkyl radical, C6-14Aryl radical, C5-14One or a combination of heteroaryl groups. Furthermore, the radical R3May be independently selected from R4Substituted one or more times.In one representative embodiment, R4May be C1-6Alkyl, -OH, C1-6Alkoxy, -NH2、-NH(C1-6Alkyl), -N (C)1-6Alkyl radical)2、C1-6Haloalkyl, or C1-6A haloalkoxy group. Representative individual monomers of formula (1) include Methyl Methacrylate (MMA), methoxyphenyl methacrylate, and N, N-diisopropylacrylamide.
Monomers imparting a more negative charge include styrene and substituted styrenes. Non-limiting examples of substituted styrenes include styrene which is independently selected from C1-6Alkyl radical, C1-6Alkoxy, -NH (C)1-6Alkyl), -N (C)1-6Alkyl radical)2And those in which the radical of halogen is substituted one or more times. Another class of monomers that introduce more negative charge is provided by molecules of formula (2):
CH2＝C(R1b)C(O)R5formula (2)
Functional group R1bIs usually-H or-CH3. In one embodiment, the group R5Having the formula-OR6To form an acrylate or methacrylate. When the group R5Having the formula-NHR6When the monomer is acrylamide. Also includes wherein R5Having the formula-NR62The monomer was changed to N, N-bisacrylamide. In non-limiting embodiments, the group R6May be C1-6Alkyl radical, C1-6Heteroalkyl group, C3-10Cycloalkyl radical, C3-10Heterocycloalkyl radical, C6-14Aryl radical, C5-14One or a combination of heteroaryl groups. Furthermore, the radical R6Is independently selected from R7Substituted one or more times. In representative embodiments, R7Is halogen, -CN, -NO2-S (O) -or-S (O)2-. Representative individual monomers of formula (2) include fluorinated or partially fluorinated esters of ethylene acids, such as trifluoromethyl, difluoromethyl, monofluoromethyl, pentafluoroethyl, tetrafluoroethyl, trifluoroethyl, difluoroethyl, monofluoroethyl acrylates or methacrylates. In one exemplary embodiment, the monomer of formula (2) is trifluoro methacrylateEthyl ester (TFEM).
The monomer providing the soluble homopolymer may be a derivative of Polydimethylsiloxane (PDMS), such as an acrylate terminated polydimethylsiloxane, or a long chain or branched acrylate, such as lauryl methacrylate or 2-ethylhexyl methacrylate.
As described above, in a preferred embodiment of the present invention, a set of composite particles in an electrophoretic medium has a Zeta potential in the range of-20 mV to +20 mV. As is well known in the art, a population of pigment particles will typically exhibit a range of mobilities depending on the distribution of the particles within the population. Thus, the Zeta potential value represents the average value of the mobility of the entire collection of particles.
The extent of the polymeric shell is conveniently assessed by thermogravimetric analysis (TGA), a technique in which the temperature of a dried sample of particles is increased and the mass loss due to pyrolysis is measured as a function of temperature. The conditions under which the polymer coating is lost but the core pigment is retained can be found (these conditions depend on the precise core pigment particles used). By using TGA, the mass fraction of the polymer particles, i.e. the mass fraction of the polymer shell in the composite particle, can be measured and converted to volume fraction using the known density of the core pigment and the polymer attached to the core pigment.
A method of estimating the polymer coverage of the above pigment particles by gravimetric analysis is also provided in example 2 below. The coverage can be adjusted by varying the mass ratio of the monomers and particles in the reaction mixture used to prepare the composite pigment particles. According to an exemplary embodiment, the mass fraction of the polymer shell in the composite particle is at least 25 wt% and at most 75 wt%. In further embodiments, the mass fraction is at least 25 wt% to at most 70 wt%, at least 25 wt% to at most 60 wt%, at least 25 wt% to at most 50 wt%, or at least 25 wt% to at most 40 wt%. In further embodiments, the mass fraction is at least 20 wt% to at most 70 wt%, at least 20 wt% to at most 60 wt%, at least 20 wt% to at most 50 wt%, or at least 20 wt% to at most 40 wt%.
A wide variety of forms can be used for the core pigment: spherical, needle-shaped, or otherwise anisometric, aggregates of smaller particles (i.e., "grape clusters"), composite particles comprising small pigment particles or dyes dispersed in a binder, and the like, as is well known in the art. The polymer shell may be a covalently bonded polymer made by modification processes or chemisorption as is well known in the art, or may be physisorbed onto the particle surface.
In this analysis, it is assumed that the polymer shell uniformly encapsulates the entire surface of the core pigment. However, this is difficult to guarantee. (see, e.g., the aforementioned U.S. Pat. No.6,822,782, FIG. 6, and related descriptions at columns 16-17.) the attachment method of the polymer may be advantageous to crystallize one side of the core pigment rather than the other, and some areas of the core pigment may be covered by the polymer, with other areas not covered by the polymer or minimally covered by the polymer. Moreover, particularly when grafting techniques are used to attach the polymer to the pigment surface, the growth of the polymer may be patchy, leaving large areas of the core pigment uncovered even though the mass of grafted polymer is large.
As already mentioned, in a preferred embodiment, the invention entails the use of generally white light-scattering particles, and three substantially non-light-scattering particles. There are of course no totally light-scattering particles or totally non-light-scattering particles present, and the minimum degree of light scattering of the light-scattering particles used in the electrophoresis of the present invention, and the maximum degree of permissible light scattering in the substantially non-light-scattering particles, may vary somewhat depending on factors such as the exact pigments used, their color, and the ability of the user or application to tolerate a slight deviation from the ideal desired color. The scattering and absorption properties of pigments can be assessed by measuring the diffuse reflectance of a sample of the pigment against a white and black background, dispersed in a suitable matrix or liquid. The results from these measurements can be interpreted according to a variety of models well known in the art, for example, the one-dimensional Kubelka-Munk process. In the present invention, it is preferable that the white pigment exhibits a diffuse reflectance of at least 5% measured on a black background at 550nm when the pigment is approximately isotropically distributed at a volume of 15% in a layer having a thickness of 1 μm including the pigment and a liquid having a refractive index of less than 1.55. Under the same conditions, the yellow, magenta and cyan pigments preferably exhibit diffuse reflectance of less than 2.5% measured on a black background at 650, 550 and 450nm, respectively. (the wavelengths selected above for the measurements of yellow, magenta, and cyan pigments correspond to the spectral regions of minimum absorption by these pigments.) color pigments meeting these criteria are referred to hereinafter as "non-scattering" or "substantially non-light-scattering".
Table 1 below shows the diffuse reflectance of the preferably yellow, magenta, cyan and white pigments (Y1, M1, C1 and W1, described in more detail below) used in the electrophoretic media of the present invention, as well as their ratio of absorption and scattering coefficients according to the Kubelka-Munk analysis of these materials dispersed in a polyisobutylene matrix.
TABLE 1
The core pigment used in the white particles is typically a high refractive index metal oxide, such as titanium dioxide, which is well known in the art of electrophoretic displays. Core pigments for providing the subtractive primary colors, i.e., cyan, magenta, and yellow, include, but are not limited to, the following.
Suitable yellow core pigments include c.i. pigment yellow 1,3,12,13,14,16,17,73,74,81,83,97,111,120,126,137,139,150,151,155,174,175,176,180,181,191,194,213, and 214. Preferred yellow core pigments include c.i. pigment yellow 139,155 and 180.
Suitable magenta core pigments include c.i. pigment reds 12,14,48:2,48:3,48:4,57:1,112,122,146,147,176,184,185,209,257 and 262; and c.i. pigment violet 19 and 32. One preferred magenta core pigment is c.i. pigment red 122.
Suitable cyan core pigments include c.i. pigment blue 15:1,15:2,15:3,15:4, and 79; and c.i. solvent blue 70.
The display device may be constructed using the electrophoretic fluid of the present invention in several ways known in the art. The electrophoretic fluid may be encapsulated in microcapsules or incorporated into a microcell structure that is subsequently sealed with a polymer layer. The microcapsules or microcell layers may be coated or embossed onto a plastic substrate or film carrying a transparent coating of conductive material. The assembly may be laminated to a backplane carrying the pixel electrodes using a conductive adhesive.
Examples of the invention
Examples are now given, but by way of illustration only, to show details of preferred electrophoretic media of the present invention and processes for driving these preferred electrophoretic media.
EXAMPLE 1 preparation of yellow pigment
Step 1: and (4) preparing the grinding material.
A mixture of pigment Yellow 155 (available as Ink Jet Yellow 4GC from Clariant corporation, 1670g) and isopar-E (9440g) was charged to the storage tank. The mixture was circulated through a LabStar horizontal stirred bead mill (Netzsch Premier Technologies) charged with 0.7 to 1.2mm spherical grinding media (ceria stabilized zirconia, available from Jyoti, 1840 g). The milling was carried out at a stirrer speed of 1000rpm and a run time corresponding to 375 minutes per kg of pigment.
Step 2: and (4) polymerizing.
The millbase prepared as described above (183.18g, 13.67% based on the pigment content determined, i.e. 25.04g) was charged to a 250mL polypropylene bottle, sonicated for 75 minutes, and then transferred to a 500mL three-neck round-bottom flask fitted with a mechanical stirrer, a rubber septum, and a subsurface nitrogen gas delivery tube. Wash with Isopar E (27mL) was added. The suspension was stirred rapidly under a nitrogen sparge for 15 minutes at which time a mixture of monomethacryloxypropyl terminated polydimethylsiloxane macromer (12.90g) available from Gelest as MCR-M22, having a molecular weight of about 10000, methyl methacrylate (8.40g), and trifluoroethyl methacrylate (2.70g), and additional Isopar E wash (5mL) were added.
The mixture was stirred under a continuous nitrogen sparge and heated in an oil bath to 55 ℃ after one hour. At this point the nitrogen delivery was changed to above the surface, a thermometer was introduced into the flask, and a solution of azobisisobutyronitrile ((92mg) in ethyl acetate (0.72g) was added by syringe, after 75 minutes again, the batch (batch) temperature was 66 ℃ after 15.5 hours again, the temperature was 63 ℃, the heating bath was removed, stirring was continued until the temperature dropped to 40 ℃, at which time the batch was diluted with Isopar E (60mL), and stirred with slow cooling to 33 ℃, at which time the contents were transferred to two 250mL polypropylene bottles while the total dispersion volume reached 500mL with enough Isopar E wash, the batch was centrifuged at 3500rpm for 30 minutes, a sample of supernatant was retained, the solids were suspended in Isopar E to a total volume of 250mL and roller milled for 4 hours, then centrifuged at 3440rpm for 30 minutes, the supernatant was resuspended in Isopar E to a total volume of 250mL, centrifuge again for 30 minutes. This process was repeated 3 more times (5 total centrifugations). The wet cake at the end of the process was suspended in hexane to 250mL and centrifuged at 3500rpm for 30 minutes. The solid was air dried for 3 days and then placed in a vacuum oven at 50 ℃ for 24 hours to provide a yellow solid weighing 34.42 g.
And step 3: and (4) dispersing the composite pigment.
To a 125mL polypropylene bottle was added dry pigment (10.00g) prepared as described above and Isopar E (40.00 g). During the course of six days, the mixture was subjected to eight 90 minute ultrasonic baths, during which the mixture was rotated on a roll mill. The resulting dispersion was filtered through a 200 micron fabric mesh to provide a mobile dispersion and no residual solids were left. The weighed sample was dried in a convection oven overnight at 170 ° F, leaving a residual solids weight of 20.26%.
Example 2-estimation of mass fraction relative to weight of composite particles
To a scintillation vial were added the pigment to be tested (0.5899g), tetrahydrofuran (7.05g) and a small magnetic stir bar. The bottle was capped and placed on a stirrer/hot plate and heated to stir. A 250mL glass jar was inverted on the bottle to prevent air flow in and to provide safety protection in the event of a bottle burst. The temperature of the heating plate was adjusted so that the batch temperature was 60 ℃ (measured periodically using a pyrometer). After two hours the heating was stopped and the contents were stirred for another two hours and then transferred to a 15mL Nalgene centrifuge cone along with about 2mL of THF wash. The dispersion was centrifuged at 3070rpm for 30 minutes. The supernatant was transferred to a scintillation vial weighing 14.0412g and placed in a convection oven at 170 ° F overnight. The following day THF (6.5g) was added to the centrifuged pellet. The cone was capped, shaken and sonicated to achieve dispersion, and then centrifuged at 3070rpm for 30 minutes. The supernatant was added to the bottle containing the residue of the first supernatant and the solution was again dried overnight in a convection oven at 170 ° F. The centrifuged precipitate was air dried overnight and then both components were vacuum dried at 70 ℃ for 8 hours. The gross weight of the bottle plus contents was 14.1873g, representing a dry weight of 0.1461g of polymer (24.77% of the original sample weight). The residual pigment dried in the centrifuge cone was transferred to a scintillation vial to give a net weight of 0.4532g (76.83% of the original sample weight) and thus a mass balance of 101.60%.
Table 2 (fig. 9) shows the physical properties of pigments prepared and analyzed as described in examples 1 and 2 above. The particle sizes quoted are measured in solution, where the polymer shell (if present) is swollen by the solvent.
EXAMPLE 3 measurement of the electro-optical Properties of the formulations
Step 1: preparation of exemplary electrophoretic fluids
Fluid (i): a white particle dispersion similar to that described in example 12, part a of us patent 9,921,451 (15.53g) was combined with a cyan particle dispersion similar to that prepared as described in example 7 of us patent 9,921,451 (1.93g), a magenta particle dispersion similar to that prepared as described in example 5 of us patent 9,921,451 (2.29g), a yellow pigment dispersion similar to that described in step 3 of example 1 above (2.30g), a surfactant similar to Solsperse 19000 available from Lubrizol corporation of vickiv, ohio (1.16g of a 50% w/w Isopar E solution), and polyisobutylene with a molecular weight of 850000 (1.06g of a 15% w/w Isopar E solution). The resulting mixture was thoroughly mixed overnight and sonicated for 90 minutes to produce an electrophoretic fluid.
Step 2: preparation of display device
The microcell array imprinted on the polyethylene terephthalate film with a transparent conductor (indium tin oxide, ITO) coating was filled with the electrophoretic fluid prepared as described in step 1 above. The microcells are hexagonal in shape, and have a depth of 14 or 17 microns measured from side to side and a width of 130 microns. Excess electrophoretic fluid is removed from the microcells with a spatula and they are sealed with a composite polymer coating as described in U.S. patent No.9,759,978. The assembly was laminated to a glass backplane with ITO electrodes using a doped thermal adhesive of thickness 3 μm substantially as described in U.S. patent No.7,012,735 to produce a display device.
The devices were electro-optically tested in a manner similar to that described in section D of example 11 of us patent 9,921,451. The waveforms used are shown in fig. 3 and 4. The waveform of fig. 3 is similar to that shown in fig. 7B of U.S. patent 9,921,451 for producing cyan. The voltages shown in fig. 3 refer to the voltage of the back plate of the device relative to the front plane (viewing surface).
After applying the waveforms of fig. 3, the reflection spectrum of the display device was measured. The recorded optical density is converted to an "analytical density": i.e. the contribution to the absorption spectrum observed for each individual colour pigment. The analytical density is determined after baseline correction to compensate for optical losses in the display device. Then, the mass of cyan is estimated as the greater of the analytical density corresponding to the light absorption of the cyan pigment minus the analytical density corresponding to the light absorption of the magenta and yellow pigments. The larger the value, the more desirable the cyan color is considered.
The waveforms of fig. 4 (again showing the voltages applied to the backplane relative to the front plane) are intended to probe the total color gamut (i.e., the volume of all colors addressable by the device). The waveform consists of "dipoles", i.e., pairs of pulses of opposite polarity, whose duration and amplitude vary systematically, as shown by the dark envelope in the figure. Voltages were explored to +/-3.5, 6.1, 9.4, 13.4, 18.2, 23.7 and 30V, with pulse length durations of 50, 80, 120, 190 and 300 milliseconds. For each voltage pair (+, -), each pulse duration pair is accessed once. This is done in such a way That is, the pulse duration in one dipole varies for exactly one value in the next dipole, and that value is adjacent in the ordered list of pulse duration values. The voltage was explored in a similar manner. In this way, the change in the waveform is as smooth as possible, since successive dipoles are as similar to each other as possible. The reflectance spectrum is obtained throughout the waveform (not only at its end) and converted to CIEL a b units. In three-dimensional color space (in Δ E)3In units) of the convex hull surrounding this point cloud is considered to be the total color gamut available for that particular display device.
Fig. 5 and 6 show the cyan quality obtained by applying the waveform of fig. 3 and the total color gamut obtained by applying the waveform of fig. 4 as a function of the Zeta potential of the yellow pigment. As described above, the point corresponding to a Zeta potential of-55 mV is from the non-functionalized control yellow pigment.
As can be seen in fig. 5, an excellent cyan mass fraction can be obtained when the magnitude of the Zeta potential of the yellow pigment is less than about 20 mV. The cyan quality score was also higher with thinner microcups (14 microns) compared to thicker microcups (17 microns). When the magnitude of the Zeta potential of the yellow pigment is above about 20mV, the cyan quality score is very poor with thicker microcups.
Without wishing to be bound by theory, it is believed that the improved cyan quality scores achieved with the yellow pigments made according to the examples of the present invention are attributed to their low mobility. When the yellow pigment is positively charged and has high mobility, its properties are very similar to those of the cyan pigment. Therefore, it is difficult to distinguish the two colors with any applied waveform. On the other hand, when the yellow pigment is negatively charged and has high mobility, it becomes difficult to separate from the white pigment, at least in the case of a short application waveform.
As is apparent from fig. 6, when a long waveform (as shown in fig. 4) is used, a high color gamut can be obtained for all yellow pigments, and the color gamut may be slightly reduced when a positively charged yellow pigment with high mobility is used. Fig. 7 shows that a high cyan quality score can be obtained using the fast waveform of fig. 3 without sacrificing the total color gamut measured using the waveform of fig. 4.
It will be apparent to those skilled in the art that many changes and modifications can be made to the specific embodiments of the invention described above without departing from the scope of the invention. Accordingly, the entire foregoing description is to be construed in an illustrative and not a restrictive sense.
The electrophoretic medium of the present application may contain any of the additives used in conventional electrophoretic media as described, for example, in the above-mentioned E Ink and MIT patents and applications. Thus, for example, the electrophoretic medium of the present application will typically include at least one Charge Control Agent (CCA) to control the charge on the various particles, and a polymer having a number average molecular weight in excess of about 20000 and substantially non-absorbing particles may be dissolved or dispersed in the fluid to improve bistability of the display, such as described in U.S. Pat. No.7,170,670.
All of the above patents and patent application publications are incorporated herein by reference in their entirety. If there is any inconsistency between the contents of the present application and any of the patents and applications incorporated herein by reference, the contents of the present application shall control to the extent necessary to resolve such inconsistency.