TWI867636B - Electro-optic assemblies and materials for use therein - Google Patents

Abstract

An electro-optic medium including an encapsulated material and a binder, the encapsulated material being configured to switch optical states upon application of an electric field, and the binder including a polymer having a plurality of side chains, wherein at least a portion of the side chains have an ionic moiety.

Description

Electro-optical components and materials used therein

The present invention relates to an electro-optical assembly and the materials used therein.

The present invention discloses an electro-optical assembly useful in the manufacture of electro-optical displays and materials used in the assembly. More particularly, the present invention discloses an adhesive composition having controlled volume resistivity and an electro-optical assembly incorporating the material.

The term "electro-optical" as applied to a material or display is used herein in its well-known sense in the imaging arts to refer to a material having first and second distinct display states in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically a color discernible by the human eye, it may be another optical property such as light transmittance, reflectivity, brightness; or in the case of a display intended for machine reading, such as false color in reflectivity variations at electromagnetic wavelengths outside the visible light range.

Some electro-optic materials are solid, in the sense that they have a solid outer surface, but they can and often do have spaces inside that are filled with a liquid or a gas. For convenience, such displays using solid electro-optic materials may be referred to hereinafter as "solid-state electro-optic displays." Thus, the term "solid-state electro-optic display" includes rotating bichromal member displays, encapsulated electrophoretic displays, micelle electrophoretic displays, and encapsulated liquid crystal displays.

The terms "bi-stable" and "bi-stable" are used herein in their sense as known in the art to refer to a display including display elements having first and second display states that differ in at least one optical property, and such that after any provided element has been driven for a finite period of time by an address pulse to assume its first or second display state, that state will continue for a period of time that is at least several times, for example, at least four times, the minimum duration required for the address pulse to change the state of the display element after the address pulse has terminated. It has been shown in U.S. Patent No. 7,170,670 that certain particle-based electrophoretic displays capable of grayscale are stable not only in their extreme black and white states, but also in their intermediate gray states, and the same applies to certain other types of electro-optical displays. Such displays are properly called "multistable" rather than bistable, although for convenience the term "bistable" may be used herein to cover both bistable and multistable displays.

One type of electro-optical display that has been the subject of intense research and development for many years is the particle-based electrophoretic display, in which a plurality of charged particles move through a fluid under the influence of an electric field. When compared to liquid crystal displays, electrophoretic displays may have the attributes of good brightness and contrast, wide viewing angles, state bi-stability, and low power consumption. However, problems with the long-term image quality of these displays have hampered their widespread use. For example, the particles that make up electrophoretic displays tend to settle and produce an undesirable lifespan for these displays.

As mentioned above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but the electrophoretic media can be made using a gaseous fluid; see, for example, Kitamura, T. et al., "Electrical toner movement for electronic paper-like display", IDW Japan, 2001, Paper HCS1-1; and Yamaguchi, Y. et al., "Toner display using insulative particles charged triboelectrically", IDW Japan, 2001, Paper AMD4-4. See also U.S. Patent Nos. 7,321,459 and 7,236,291. When the medium is used in an orientation that permits such settling, for example, in a sign where the medium is arranged in a vertical plane, the gaseous electrophoretic media appear to be subject to the same types of problems due to particle settling as liquid electrophoretic media. More specifically, particle sedimentation presents a more serious problem in gas-based electrophoretic media than in liquid-based systems because the lower viscosity of the gas suspension fluid compared to liquids allows the electrophoretic particles to sediment more rapidly.

Numerous patents and applications belonging to or in the name of the Massachusetts Institute of Technology (MIT), E Ink Corporation, E Ink California, LLC., and related companies describe various technologies used in encapsulated and micellar electrophoresis and other electro-optical media. Encapsulated electrophoretic media include a plurality of small capsules, each of which itself includes an inner phase, which includes electrophoretically mobile particles in a fluid medium; and a capsule wall surrounding the inner phase. Typically, the capsules themselves are held in a polymer binder to form a coherence layer placed between two electrodes. The technologies described in these patents and applications include: (a) electrophoretic particles, fluids, and fluid additives; see, for example, U.S. Patent Nos. 5,961,804; 6,017,584; 6,120,588; 6,120,839; 6,262,706; 6,262,833; 6,300,932; 6,323,989; 6,377,387; 6,515,649; 6,538,801; 6,580,545; 6,652,075; 6,693,620; 6,7 21,083; 6,727,881; 6,822,782; 6,831,771; 6,870,661; 6,927,892; 6,956,690; 6,958,849; 7,002,72 8; 7,038,655; 7,052,766; 7,110,162; 7,113,323; 7,141,688; 7,142,351; 7,170,670; 7,180,649; 7,22 6,550; 7,230,750; 7,230,751; 7,236,290; 7,247,379; 7,277,218; 7,286,279; 7,312,916; 7,375,875 ;7,382,514;7,390,901;7,411,720;7,473,782;7,532,388;7,532,389;7,572,394;7,576,904;7,58 0,180; 7,679,814; 7,746,544; 7,767,112; 7,848,006; 7,903,319; 7,951,938; 8,018,640; 8,115,729 ;8,199,395;8,257,614;8,270,064;8,305,341;8,361,620;8,363,306;8,390,918;8,582,196;8,593 ,718; 8,654,436; 8,902,491; 8,961,831; 9,052,564; 9,114,663; 9,158,174; 9,341,915; 9,348,193; 9,361,836; 9,366,935; 9,372,380; 9,382,427; and 9,423,666; and U.S. Patent Application Publication Nos. 2003/0048522; 2003/01510 29;2003/0164480;2003/0169227;2003/0197916;2004/0030125;2005/0012980;2005/0136347;2006 /0132896; 2006/0281924; 2007/0268567; 2009/0009852; 2009/0206499; 2009/0225398; 2010/0148385 ;2011/0217639;2012/0049125;2012/0112131;2013/0161565;2013/0193385;2013/0244149;2014/0 011913; 2014/0078024; 2014/0078573; 2014/0078576; 2014/0078857; 2014/0104674; 2014/0231728; 2014/0339481; 2014/0347718; 2015/0015932; 2015/0177589; 2015/0177590; 2015/0185509; 2015/0218384; 2015/0241754; 2015/0248045; 2015/0301425; 2015/0378236; 2016/0139483; and 2016/0170106; (b) Capsules, adhesives, and encapsulation methods; see, e.g., U.S. Patent Nos. 5,930,026; 6,067,185; 6,130,774; 6,172,798; 6,249,271; 6,327,072; 6,392,785; 6,392,786; 6,459,418; 6,839,158; 6,866,760; 6,922,276; 6,958,848; 6,987,603; 7,061,663; 7,071,913; 7,079,305; 7,109,968; 7,110,164; 7,184,197; 7,202,9 91; 7,242,513; 7,304,634; 7,339,715; 7,391,555; 7,411,719; 7,477,444; 7,561,324; 7,848,007; 7,910,175; 7,952,790; 7,955,532; 8,035,886; 8,129,655; 8,446,664; and 9,005,494; and U.S. Patent Application Publication Nos. 2005/0156340; 2007/0091417; 2008/0130092; 2009/0122389; and 2011/0286081; (c) Films and subassemblies comprising electro-optical materials; see, e.g., U.S. Patent Nos. 6,982,178 and 7,839,564; (d) Backplanes, 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. Patent Nos. 7,075,502 and 7,839,564; (f) Methods for driving displays; see, e.g., U.S. Patent Nos. 7,012,600 and 7,453,445; (g) Applications for displays; see, e.g., U.S. Patent Nos. 7,312,784 and 8,009,348; and (h) Non-electrophoretic displays, such as described in U.S. Patent No. 6,241,921 and U.S. Patent Application Publication No. 2015/0277160; and applications of encapsulation and micelle technology other than displays, see, for example, U.S. Patent Application Publication Nos. 2015/0005720 and 2016/0012710.

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

Although electrophoretic media are often opaque (because, for example, in many electrophoretic media, the particles substantially block visible light from passing through the display) and are operated in a reflective mode, many electrophoretic displays can be made to operate in a so-called "shutter mode," in which one display state is substantially opaque and one is light transmissive. See, for example, 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 similar mode can be used to operate a dielectrophoretic display that is similar to an electrophoretic display but relies on changes in electric field strength, see U.S. Patent No. 4,418,346. Other types of electro-optical displays can also be operated in a shutter mode. Electro-optical media operating in a shutter mode may be useful in a multi-layer structure for a full-color display in which at least one layer adjacent to a viewing surface of the display is operated in a shutter mode to expose or conceal a second layer further from the viewing surface.

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

Other types of electro-optical media may also be used in the display of the present invention. One type of electro-optical display is a rotating dichroic element type, such as described, for example, in U.S. Patent Nos. 5,808,783, 5,777,782, 5,760,761, 6,054,071, 6,055,091, 6,097,531, 6,128,124, 6,137,467, and 6,147,791 (although this type of display is often referred to as a "rotating dichroic sphere" display, more accurately the term "rotating dichroic element" is preferred because in some of the above-mentioned patents, the rotating element is not spherical). This display uses a large number of small bodies (typically spherical or cylindrical) having two or more portions with different optical characteristics and internal dipole distances. The subjects are encapsulated and suspended in liquid-filled bubbles within a matrix, which are filled with liquid so that the subjects are free to rotate. The appearance of the display is changed by applying an electric field thereto, thereby rotating the subjects to a variety of positions to change the portion of the subject seen through the viewing surface. This type of electro-optic medium is typically bi-stable.

Another type of electro-optical display uses an electrochromic medium, for example, in the form of a nanochromic film, which includes an electrode formed at least in part from a semiconductor metal oxide and a plurality of dye molecules attached to the electrode that can reversibly change color; see, for example, O'Regan, B. et al., Nature 1991, 353 , 737; and Wood, D., Information Display, 18(3) , 24 (March 2002). See also Bach, U. et al., Adv. Mater., 2002, 14(11) , 845. Nanochromic films of this type are also described, for example, in U.S. Patent Nos. 6,301,038, 6,870,657, and 6,950,220. This type of medium is also typically bi-stable.

Another type of electro-optical display is the electrowetting display developed by Philips and described in Hayes, R.A. et al., "Video-Speed Electronic Paper Based on Electrowetting", Nature, 425, 383-385 (2003). It was shown in U.S. Patent No. 7,420,549 that this electrowetting display can be made bi-stable.

Electro-optical displays manufactured using a laminate process normally have a laminate adhesive layer between the electro-optic layer itself and the backplane, and the presence of this laminate adhesive layer affects the electro-optical characteristics of the display. In particular, the conductivity of the laminate adhesive layer affects both the low temperature performance and the resolution of the display. The low temperature performance of the display can (and has been found empirically) be improved by increasing the conductivity of the laminate adhesive layer, for example, by doping the binder and/or adhesive with inorganic or organic salts such as potassium acetate, tetrabutylammonium chloride, tetrabutylammonium hexafluorophosphate, or other materials as described in the aforementioned U.S. Patent Nos. 7,012,735 and 7,173,752. However, increasing the conductivity of the laminate adhesive tends to increase pixel blooming (which is the phenomenon that the area of the electro-optic layer that changes optical state in response to a change in voltage at the pixel electrode is larger than the pixel electrode itself), and this blooming tends to reduce the resolution of the display.

In order to prevent blooming without negatively impacting temperature performance, dopants have been incorporated into the binder system of the electro-optic medium rather than the laminate binder. For example, color-encapsulated electrophoretic media, particularly multicolor capsules containing four pigments, a highly conductive binder, have been demonstrated to have improved performance in electro-optical states, color gamut and temperature performance, and blooming. However, there are difficulties in achieving sufficiently high conductivity in the binder to obtain these results using small molecule additives such as tetrabutylammonium hexafluorophosphate. High dopant concentrations are required, and these high concentration levels will significantly change the mechanical properties of the adhesive, often resulting in difficult layering and coating or poor adhesion. Furthermore, small molecule dopants can diffuse out of the adhesive and into the layered adhesive.

Therefore, there is a need for improved adhesive compositions for electro-optical displays and assemblies.

One aspect of the invention is an electro-optic medium comprising an encapsulating material configured to switch an optical state upon application of an electric field and a binder comprising a polymer having a plurality of side chains, wherein at least a portion of the side chains comprise an ionic moiety.

These and other aspects of the present invention will become apparent in light of the following description.

Generally speaking, multiple specific examples of the present invention provide an electro-optical medium, which includes an encapsulation material configured to switch optical states upon application of an electric field; and a binder comprising a polymer having a plurality of side chains, wherein at least a portion of the side chains comprises an ionic portion. As used herein throughout the present patent specification and the scope of the application, "polymer" includes both homopolymers and copolymers. The polymer is preferably a polyion with a high molecular weight (about 100,000 g/mol to a maximum of 1,000,000 g/mol), more preferably a polycation, and most preferably a polycation comprising a quaternary amine. The polymer is preferably a good film-forming polymer. When mixed into the binder and applied in a layer, the resulting film preferably exhibits consistent mechanical properties throughout the entire area of the film and, when incorporated into a display, is chemically compatible with the other materials in the display stack, thus minimizing or eliminating phase separation.

The conductivity of the polymer in the adhesive can be controlled by varying the molecular weight of the polymer or by varying the number of ionic side chains along the polymer backbone of the polymer. Blends of polymers may also be used in adhesive compositions according to various embodiments of the present invention. The polymer containing ionic side chains is preferably light in color, e.g., pale yellow, but more preferably colorless. The film comprising the polymer preferably has substantially uniform conductivity over the area of the film, and preferably maintains this property over time to improve the operating life of the display.

As previously mentioned, the binder composition prepared according to various embodiments of the present invention may be included in various types of encapsulated electro-optical materials such as rotational dichroic components, electro-wetting materials, or electrophoretic media. The binders included in various embodiments of the present invention are particularly well suited for encapsulated electrophoretic media comprising a plurality of charged particles and a fluid, wherein the charged particles are movable upon application of an electric field. The plurality of charged particles may include particles of at least two different colors, such as red, green, blue, cyan, magenta, yellow, black, and white.

The one or more polymers including ionic side chains that may be included in the adhesive composition according to various embodiments of the present invention are preferably highly conductive. The one or more polymers may have a conductivity of about ^10-10 to ^10-1 Siemens/cm, preferably about ^10-9 to about ^10-2 Siemens/cm, more preferably about ^10-8 to about ^10-4 Siemens/cm, and most preferably about ^10-7 to ^10-6 Siemens/cm.

The ionic functionality of the polymer may be covalently bonded to the polymer backbone of the polymer. The ionic functionality present in the side chain may be provided by anionic moieties, cationic moieties, zwitterionic moieties, and combinations thereof. When the functionality of the side chain is anionic, it is preferred that the functionality is provided by a pH-insensitive moiety, such as a sulfonic acid or phosphonic acid group. Of the total number of side chains bonded to the polymer backbone, the ionic side chains may constitute at least 5%, 10%, 15%, 20%, 25%, 30%, 35% and 40% of the total number in the order of increasing preference provided; and no more than 100%, 95%, 90%, 85%, 80%, 75%, 70%, 65%, 60%, 55%, 50% and 45% in the order of increasing preference provided. As previously mentioned, the conductivity of the polymer can be controlled not only by the number of the ionic side chains, but also by the molecular weight of the polymer. In increasing order of preference provided, the ionic polymers included in various embodiments of the present invention may have a number average molecular weight of at least 20,000, 50,000, 100,000, 150,000, 200,000, 250,000, 300,000, 350,000, and 400,000 g/mole; and in increasing order of preference provided, no more than 1,000,000, 950,000, 900,000, 850,000, 800,000, 750,000, 700,000, 650,000, 600,000, 550,000, and 500,000 g/mole.

The molecular weight of the polymer and its glass transition temperature (Tg) can be selected based on the desired mechanical properties of the final film comprising the polymer, and the desired physical properties of the material during processing such as layering when making a display.

The ionic polymer may be a linear or branched polymer, for example, a comb polymer, a brush polymer, a tree polymer, a star polymer, etc. The polymer backbone may be derived from one or more polymers, including but not limited to polyvinyl alcohol, polyvinyl pyrrolidone, polyvinyl acetate, polyacrylate, polymethacrylate, polyurethane, polysaccharide, polyolefin, polyether, polyester, polypeptide, protein, and combinations thereof. In order to provide ionic side chains to the polymer backbone of the one or more polymers, the one or more polymers may be functionalized by reacting the one or more polymers with one or more compounds, including but not limited to sulfonic acids, phosphonic acids, quaternary amines, pyridines, imidazoles, carboxylic acids, salts of the foregoing compounds, and combinations thereof.

In addition to the polyion, the adhesive composition may further include one or more polymers and/or oligomers that are not ionically side-functionalized, such as water-soluble polymers, waterborne polymers, oil-soluble polymers, thermosetting (e.g., epoxides) and thermoplastic polymers (e.g., polyesters), and radiation-curable polymers. Optional additives may include additives such as small molecule coating aids or tackifiers to modify the viscosity of the adhesive.

Among the water-soluble polymers, there may be various polysaccharides, polyvinyl alcohols, N-methylpyrrolidone, N-vinylpyrrolidone, various Carbowax® series (Union Carbide, Danbury, Conn) and poly(2-hydroxyethyl acrylate).

The water-dispersible or water-based systems are usually emulsion compositions, represented by Neorez® and Neocryl® resins (Zeneca Resins, Wilmington, Mass.), Acrysol® (Dow, Philadelphia, Pa.), Bayhydrol® (Bayer, Pittsburgh, Pa.), and the HP line of Cytec Industries (West Paterson, NJ). These are usually lattices of polyurethanes, occasionally compounded with one or more acrylics, polyesters, polycarbonates, or polysilicones, each of which adds a specific set of properties to the final cured resin defined by glass transition temperature, degree of "tack," softness, clarity, flexibility, water permeability and solvent resistance, modulus of elongation and tensile strength, thermoplastic flow, and solids level. Some aqueous systems can be mixed with reactive monomers and catalyzed to form more complex resins. Some can be further crosslinked using crosslinking agents such as acrylamides that react with carboxyl groups. .

Electro-optical media according to various embodiments of the present invention comprise a blend of the aforementioned binder and encapsulating material. The internal phase of the electro-optical medium preferably comprises charged pigment particles in a suspended fluid. The fluid may be of low dielectric constant (preferably less than 10 and desirably less than 3). Particularly preferred solvents include aliphatic hydrocarbons such as heptane, octane and petroleum distillates such as Isopar® (Exxon Mobil) or Isane® (Total); terpenes such as limonene, for example, l-limonene; and aromatic hydrocarbons such as toluene. A particularly preferred solvent is limonene because it combines a low dielectric constant (2.3) with a relatively high refractive index (1.47). The refractive index of the internal phase can be modified with the addition of a refractive index matching agent, such as Cargille® Refractive Index Matching Fluids available from Cargille-Sacher Laboratories Inc. (Cedar Grove, NJ). In the encapsulation medium of the present invention, it is preferred that the refractive index of the dispersion of the particles be matched as closely as possible to that of the encapsulating material.

The charged pigment particles can be a variety of colors and compositions. In addition, the charged pigment particles can be functionalized with surface polymers to improve state stability. Such pigments are described in U.S. Patent Publication No. 2016/0085132, which is incorporated herein by reference in its entirety. For example, if the charged particles are white, they can be formed from inorganic pigments such as TiO ₂ , ZrO ₂ , ZnO, Al ₂ O ₃ , Sb ₂ O ₃ , BaSO ₄ , PbSO ₄ or the like. They can also be polymer particles with a high refractive index (>1.5) and a certain size (>100 nm) to appear white, or composite particles designed to have a desired refractive index. Black charged particles, which may be formed from CI Pigment Black 26 or 28 or its analogs (e.g., manganese ferrite black spinel or copper chromite black spinel) or carbon black. Other colors (non-white and non-black) may be formed from organic pigments, such as CI Pigments PR254, PR122, PR149, PG36, PG58, PG7, PB28, PB15:3, PY83, PY138, PY150, PY155 or PY20. Other examples include Clariant Hostaperm Red D3G 70-EDS, Hostaperm Pink E-EDS, PV Fast Red D3G, Hostaperm Red D3G 70, Hostaperm Blue B2G-EDS, Hostaperm Yellow H4G-EDS, Novoperm Yellow HR-70-EDS, Hostaperm Green GNX, BASF Irgazine Red L 3630, Cinquasia Red L 4100 HD, and Irgazin Red L 3660 HD; Sun Chemical Phthalocyanine Blue, Phthalocyanine Green, Diaryl Yellow, or Diaryl AAOT Yellow. Color particles may also be formed from inorganic pigments, such as CI Pigment Blue 28, CI Pigment Green 50, CI Pigment Yellow 227, and the like. The surface of the charged particles can be modified based on the desired particle charge polarity and charge level by known techniques, such as described in U.S. Patent Nos. 6,822,782, 7,002,728, 9,366,935 and 9,372,380 and U.S. Publication No. 2014-0011913, the contents of which are incorporated herein by reference in their entirety.

The particles may have a natural charge, or may be explicitly charged using a charge control agent, or may acquire a charge when suspended in a solvent or solvent mixture. Suitable charge control agents are well known in the art and may be polymeric or non-polymeric or ionic or non-ionic in nature. Examples of the charge control agent may include, but are not limited to, Solsperse 17000 (active polymer dispersant), Solsperse 9000 (active polymer dispersant), OLOA 11000 (succinimide ashless dispersant), Unithox 750 (ethoxylate), Span 85 (sorbitan trioleate), Petronate L (sodium sulfonate), Alcolec LV30 (soy lecithin), Petroste PB100 (petroleum sulfonate) or B70 (barium sulfonate), Aerosol OT, polyisobutylene derivatives or poly(ethylene co-butylene) derivatives, and the like. In addition to the suspension fluid and charged pigment particles, the internal phase may include a stabilizer, a surfactant, and a charge control agent. The stabilizing material can be adsorbed on the charged pigment particles when dispersed in a solvent. The stabilizing material keeps the particles separated from each other so that the variable transmittance medium is substantially non-transmissive when the particles are in their dispersed state. As is known in the art, the dispersion of charged particles (typically carbon black, as described above) in a low dielectric constant solvent can be assisted by the use of a surfactant. The surfactant typically includes a polar "head end group" and a non-polar "tail end group", which is compatible with or soluble in the solvent. In the present invention, it is preferred that the non-polar tail end group is a saturated or unsaturated hydrocarbon moiety, or other group that is soluble in a hydrocarbon solvent, such as, for example, a poly(dialkylsiloxane). The polar group can be any polar organic functional group, including ionic materials, such as ammonium, sulfonates or phosphonates, or acidic or basic groups. Particularly preferred head groups are carboxylic acid or carboxylate groups. Stabilizers suitable for use with the present invention include polyisobutylene and polystyrene. In certain specific examples, dispersants such as polyisobutylene succinimide, and/or sorbitan trioleate, and/or 2-hexyldecanoic acid are added.

The use of gelatin-based capsule walls in the variable permeability device has been described in many of the E Ink and MIT patents and applications mentioned above. The gelatin is available from multiple commercial suppliers, such as Sigma Aldrich or Gelitia USA. It can be obtained in a variety of grades and purities depending on the application requirements. Gelatin mainly consists of collagen that has been collected and hydrolyzed from animal products (cattle, pigs, poultry, fish). It contains a mixture of peptides and proteins. In many of the specific examples described herein, the gelatin is combined with acacia gum (gum arabic) derived from the hardened sap of the acacia tree. Acacia gum is a complex mixture of glycoproteins and polysaccharides, and is often used in food as a stabilizer. Aqueous solutions of acacia gum and gelatin can be coacervated with a non-polar internal phase, as described below, to produce transparent and flexible capsules comprising the internal phase.

The gelatin/acacia gum-incorporated capsules can be prepared as follows; see, e.g., U.S. Patent No. 7,170,670, which is incorporated herein by reference in its entirety. In this method, an aqueous mixture of gelatin and acacia gum is emulsified with a hydrocarbon internal phase (or other water-immiscible phase) to encapsulate the internal phase. The mixture is heated and the pH is lowered to form a gelatin/acacia gum coacervate, thereby forming a capsule. The temperature of the resulting mixture is then lowered and an aqueous solution of glutaraldehyde (agent for crosslinking the capsule wall) is added. The resulting mixture is then heated and vigorously stirred. A finishing step (maintaining the capsule mixture at 50°C for about one hour) is used to deactivate residual glutaraldehyde, thereby ensuring that the capsules will separate during screening. The method produces capsules in the 20-100 micron range and often incorporates more than 50 percent of the starting material into usable capsules. The produced capsules are then separated by size screening or other particle size screening classification. After particle size classification, the capsules are mixed with the binder to produce a slurry for coating, for example, using a slit coater, a scraper coater, a rotary coater, etc. The weight ratio of the capsule to the binder in the electro-optical medium may be 4:1 to 50:1, more preferably 10:1 to 30:1, and most preferably about 15:1.

Electro-optical media made according to a number of specific examples of the present invention may be incorporated into electro-optical displays and assemblies. Electro-optical displays normally include a layer of electro-optical material and at least two other layers disposed on opposite sides of the electro-optical material, one of the two layers being an electrode layer. In most such displays, both layers are electrode layers, and one or both of the electrode layers are patterned to define pixels of the display. For example, one electrode layer may be patterned into an elongated column electrode and the other patterned into an elongated row electrode arranged at right angles to the column electrode, the pixel being defined by the intersection of the column electrode and the row electrode. Optionally and more commonly, one electrode layer has a single continuous electrode form and the other electrode layer is patterned into a matrix of pixel electrodes, each of which defines a pixel of the display. In another type of electro-optical display intended to use a stylus, print head or similar removable electrode separate from the display, only one of the layers adjacent to the electro-optical layer includes electrodes, and the layer on the opposite side of the electro-optical layer is typically a protective layer intended to prevent the removable electrode from damaging the electro-optical layer.

The manufacture of the three-layer electro-optical display normally includes at least one lamination operation. For example, in several of the aforementioned MIT and E Ink patents and applications, there is described a method for manufacturing an encapsulated electrophoretic display in which an encapsulated electrophoretic medium comprising capsules in an adhesive is coated on a flexible substrate comprising indium tin oxide (ITO) or a similar conductive coating on a plastic film (which serves as an electrode of the final display), and the capsule/adhesive coating is dried to form a coherent layer of electrophoretic medium firmly adhered to the substrate. Separately, a backplane is prepared that includes an array of pixel electrodes and conductors appropriately arranged to connect the pixel electrodes to the drive circuit system. To form the final display, the substrate with the capsule/adhesive layer on top is laminated to the backplane using a lamination adhesive. (A very similar approach can be used to prepare electrophoretic displays using a stylus or similar removable electrode by replacing the backplane with a simple protective layer such as a plastic film over which the stylus or other removable electrode can slide.) In a preferred form of this method, the backplane itself is flexible and is prepared by printing the pixel electrodes and conductors on a plastic film or other flexible substrate. The obvious lamination technique for mass manufacturing displays by this method is roller lamination using a lamination adhesive. Similar fabrication techniques can be used for other types of electro-optical displays. For example, micelle electrophoretic media or rotational dichroic component media can be laminated to a backplane in substantially the same manner as encapsulated electrophoretic media.

The aforementioned U.S. Patent No. 6,982,178 describes a method of assembling solid-state electro-optical displays (including encapsulated electrophoretic displays) that is well suited for mass production. Basically, this patent describes a so-called "front panel layer" ("FPL") that includes, in order, a light-transmitting conductive layer; a solid electro-optic medium layer in electrical contact with the conductive layer; an adhesive layer; and a release sheet. Typically, the light-transmitting conductive layer is supported on a light-transmitting substrate that is preferably flexible, in the sense that the substrate can be manually wrapped around a roller having a diameter of (approximately) 10 inches (254 mm) without permanent deformation. In this patent and herein, the term "light-transmitting" is used to mean that the layer so designated transmits sufficient light for a viewer to see through the layer in order to observe the change in the displayed state of the electro-optical medium, which normally will be viewed through the conductive layer and the adjacent substrate (if present); in the case where the electro-optical medium displays a change in reflectivity at non-visible wavelengths, the term "light-transmitting" should of course be interpreted as referring to the penetration of the associated non-visible wavelengths. The substrate will typically be a polymer film, and will normally have a thickness ranging from about 1 to about 25 mils (25 to 634 microns), preferably about 2 to about 10 mils (51 to 254 microns). The conductive layer is conveniently a thin metal or metal oxide layer, such as aluminum or ITO; or it may be a conductive polymer. Poly(ethylene terephthalate) (PET) films coated with aluminum or ITO are commercially available, for example, as "Aluminized Mylar" ("Mylar" is a registered trademark) from E.I. du Pont de Nemours & Company, Wilmington DE, and this commercial material can be used in the front laminate layer with good results.

Assembly of an electro-optical display using this front laminate can be achieved by removing the release sheet from the front laminate and contacting the adhesive layer with the backplane under conditions effective to cause the adhesive layer to adhere to the backplane, thereby securing the adhesive layer, electro-optic medium layer and conductive layer to the backplane. This method is well suited for mass production because the front laminate can typically be mass produced using roll-to-roll coating technology and then cut into sheets of any size required for use with a particular backplane.

U.S. Patent No. 7,561,324 describes a so-called "double release sheet", which is essentially a simplified version of the laminated body of the aforementioned U.S. Patent No. 6,982,178. One form of the double release sheet includes a solid electro-optic medium layer sandwiched between two adhesive layers, one or both of which are covered by a release sheet. Another form of the double release sheet includes a solid electro-optic medium layer sandwiched between two release sheets. Both forms of the double release film are intended to be used in a method generally similar to the method of assembling an electro-optical display from a front plate lamination already described, but involving two separate laminations; typically, in a first lamination, the double release film is laminated to a front electrode to form a front subassembly, and then in a second lamination, the front terminal subassembly is laminated to a back plate to form the final display, although the order of the two laminations may be reversed if necessary.

U.S. Patent No. 7,839,564 describes a so-called "transverse front plate laminate," which is a variation of the front plate laminate described in the aforementioned U.S. Patent No. 6,982,178. This transverse front plate laminate comprises, in sequence, at least one of a light-transmitting protective layer and a light-transmitting conductive layer; an adhesive layer; a solid electro-optic medium layer; and a release sheet. This transverse front plate laminate is used to form an electro-optical display having a laminated adhesive layer between the electro-optical layer and the front electrode or front substrate; a typical second thin adhesive layer may or may not be present between the electro-optical layer and the backplane. This electro-optical display can combine good resolution with good low temperature performance.

Aqueous capsule slurries of gelatin/acacia microcapsules containing dispersions of white and three different colored pigments (cyan, magenta, and yellow) having an average size of about 40 microns in a nonpolar solvent were mixed into four different binders: A) poly(vinyl alcohol) containing cationic side chains, B) low molecular weight polyvinyl pyrrolidone containing cationic side chains, C) high molecular weight polyvinyl pyrrolidone containing cationic side chains, and D) a polyurethane dispersion control. Each binder was mixed at a ratio of 1 part binder to 15 parts capsules by weight. The polyurethane adhesive is also mixed with a tackifier, a small molecule ion dopant and a coating aid. The resulting mixture is rod coated on a polyester film coated with 125 microns of indium tin oxide. The coated film is dried to produce an electrophoretic medium approximately 21 microns thick (18-25 g/m2, depending on density) essentially comprising a single layer of capsules. Each dried film is layered with a polyurethane adhesive doped with a conductive salt and then layered onto a screen printed backplane assembly approximately 2 inches square to produce an electrophoretic display test module. The module is driven using the waveforms summarized in Table 1. The basic waveform is divided into six sections, each 20.5 seconds long. During each section, a square wave AC fundamental with a frequency of 30 Hz is biased by a DC voltage as shown in the table (each bias is not shown, but the sequence should be apparent from the table entry). The duty cycle of the square wave AC is varied as shown in the table (i.e., the ratio of time during which the positive voltage is applied during a positive and negative voltage cycle). The entire test consists of three repetitions of the basic waveform, each time with a different sequence of voltage biases, shown as "High V Bias", "Mid V Bias", and "Low V Bias". Thus, for example, the initial "High V Bias" is -15 volts. For the "high V bias" sequence, the magnitude of the square wave AC is +/-30 volts; for the "medium V bias" sequence, +/-20 volts; and for the "low V bias" sequence, +/-10 volts. Table 1 part High V Square Wave AC Magnitude (+/- Volts) High V Bias (Volts) Medium V square wave AC magnitude (+/- volts) Mid V Bias (Volts) Low V Square Wave AC Level (+/- Volts) Low V Bias (Volts) Square wave AC frequency (Hz) Square wave AC load cycle (%) Cycle time (milliseconds) 1 30 -15 20 -10 10 -5 30 30 500 30 -14.25 20 -9.5 10 -4.75 30 30 500 30 … 20 … 10 … 30 30 500 30 14.25 20 9.5 10 4.75 30 30 500 30 15 20 10 10 5 30 30 500 2 30 -15 20 -10 10 -5 30 50 500 30 -14.25 20 -9.5 10 -4.75 30 50 500 30 … 20 … 10 … 30 50 500 30 14.25 20 9.5 10 4.75 30 50 500 30 15 20 10 10 5 30 50 500 3 30 -15 20 -10 10 -5 30 70 500 30 -14.25 20 -9.5 10 -4.75 30 70 500 30 … 20 … 10 … 30 70 500 30 14.25 20 9.5 10 4.75 30 70 500 30 15 20 10 10 5 30 70 500 4 30 15 20 10 10 5 30 30 500 30 14.25 20 9.5 10 4.75 30 30 500 30 … 20 … 10 … 30 30 500 30 -14.25 20 -9.5 10 -4.75 30 30 500 30 15 20 -10 10 -5 30 30 500 5 30 15 20 10 10 5 30 50 500 30 14.25 20 9.5 10 4.75 30 50 500 30 … 20 … 10 … 30 50 500 30 -14.25 20 -9.5 10 -4.75 30 50 500 30 15 20 -10 10 -5 30 50 500 6 30 15 20 10 10 5 30 70 500 30 14.25 20 9.5 10 4.75 30 70 500 30 … 20 … 10 … 30 70 500 30 -14.25 20 -9.5 10 -4.75 30 70 500 30 15 20 -10 10 -5 30 70 500

When the display is electrically driven, a variety of optical states are produced and reflected spectra are obtained. These are used to calculate the CIE L*, a*, and b* values of the light reflected from the display when the waveform is applied. For each spectrum sample, the distance in L*a*b* space of the color of the display and each of the eight SNAP (Specifications for Newsprint Advertising Production) color standard primaries is calculated in units of ΔE*. For each electrophoretic display tested, the minimum distance of the displayed color from the SNAP primary is recorded. The primary colors are red, green, blue, yellow, cyan, magenta, white and black (R, G, B, Y, C, M, W and K). The shorter the distance, the closer the performance of the electrophoretic display is to the SNAP target, indicating that the optical state of the display has better color saturation. Compared with the primary colors of the control electrophoretic display, the primary colors of the display of the present invention are significantly closer to the SNAP color values. The results are summarized in the table. Table 2 ΔΕ* of the original color display state to the SNAP standard Binder polymer R G B Y C M W K Average ΔE* vs. SNAP Standard Color gamut capacity A) Cationic PVOH 9 17 1 3 14 7 3 1 6.9 110148 B) Cationic PVP 100 kg/mol 11 11 5 0 16 16 6 1 8.3 79410 C) Cationic PVP 1000 kg/mol 9 13 0 2 16 15 5 0 7.5 85567 D) Doped polyurethane control 18 twenty three 9 9 20 19 3 0 12.6 48805

Color Gamut Results : In addition to the primary color measurements provided in Table 2, full color gamut capacity from all measurement points was determined for each electrophoretic display. A greater color gamut capacity indicates a higher color capability of the electrophoretic device. The results of this evaluation are provided in Figures 1 to 4. Referring to these figures, the size of the overall color gamut of the electrophoretic display made with a binder comprising a polymer with ionic side chains (electrophoretic medium of the present invention) is approximately twice that of the control display made with a control binder comprising a polyurethane dispersion doped with small molecules. More specifically, for the comparative display having a color gamut capacity of 49,000 (corresponding to FIG. 4 ), the electrophoretic displays including binders A, B and C of the present invention (corresponding to FIGS. 1 , 2 and 3 ) have color gamut capacities of approximately 110,000, 79,000 and 86,000.

Evaluations of electrophoretic displays comprising control and inventive electrophoretic media were also performed at different temperatures (0°C, 25°C and 50°C). They show that displays comprising inventive electrophoretic media with binders comprising polymers having cationic side chains provide improved color performance, i.e., more saturated primary colors and greater color gamut capacity, compared to displays comprising the control electrophoretic medium having a polyurethane dispersion comprising small molecule dopants. Bloom results : Bloom evaluations were performed using a Mikrotron K/W camera at 50x magnification at 25°C and 35°C, switching from both K (black) and W (white) backgrounds, using EVK3 with a TPC 28V controller and 1000 ms pulses at 65 Hz. The samples were switched from the previous state to a checkerboard pattern and the optical shift was normalized to the dynamic range. The electrophoretic display comprising (a) an electrophoretic medium of a poly(vinyl alcohol) polymer binder (binder A) having cationic side chains of the present invention and (b) an adhesive layer having an ionic dopant (tetrabutylammonium hexafluorophosphate) at a concentration of 180 ppm had very slight halo at both temperatures. Figures 5A and 5B show the results of halo evaluation at 25 °C. On the other hand, an electrophoretic display comprising (a) an electrophoretic medium having a polyurethane dispersion control adhesive (Adhesive D) and (b) an adhesive layer having a high concentration of 1000 ppm of an ionic dopant (tetrabutylammonium hexafluorophosphate) has extensive blooming even at 25°C (see FIGS. 6A and 6B ), which loses resolution in the display. The resolution loss increases at higher temperatures (35°C). As mentioned above, increasing the conductivity of the laminate adhesive increases blooming. That is, the higher the dopant concentration in the adhesive layer of the electro-optical device, the worse the blooming. By using the adhesive material of the present invention comprising a polymer having ionic side chains, a lower dopant concentration can be used in the laminate adhesive layer to reduce blooming at higher temperatures while maintaining good electro-optical performance at low temperatures.

It will be apparent to those skilled in the art that many changes and modifications may be made to the specific embodiments of the present invention described above without departing from the scope of the present invention. In addition, the foregoing description is intended to be interpreted as illustrative rather than limiting. The aforementioned published patents and pending patent applications are all incorporated herein by reference in their entirety.

without.

The drawings depict one or more embodiments according to the present concept, which are illustrated by way of example only and are not limiting. In the drawings, similar element symbols refer to the same or similar elements.

1-3 are graphs illustrating the color gamut resulting from measured colors of display samples containing various electro-optical media having a binder according to embodiments of the present invention.

FIG. 4 is a graph illustrating the color gamut resulting from measured colors of a sample display including an electro-optic medium having a binder not in accordance with an embodiment of the present invention.

5A and 5B are photographs of bloom results for display samples including an adhesive layer having a low ionic dopant concentration and an electro-optic medium having an adhesive according to an embodiment of the present invention.

6A and 6B are photographs of bloom results for display samples including an adhesive layer having a high ionic dopant concentration and an electro-optic medium having an adhesive not according to an embodiment of the present invention.

without.

Claims (16)

An electro-optical medium comprising an encapsulating material and a binder, wherein the encapsulating material is configured to switch an optical state upon application of an electric field, and the binder comprises a polymer having a plurality of side chains, wherein 5 to 100% of the side chains comprise ionic moieties; and the side chains are derived from one or more compounds selected from the group consisting of sulfonic acids, phosphonic acids, quaternary amines, pyridines, imidazoles, carboxylic acids, salts thereof, and combinations thereof. The electro-optical medium of claim 1, wherein the encapsulating material comprises a plurality of charged particles and a fluid, and the charged particles are movable after an electric field is applied. As in claim 2, the electro-optical medium, wherein the plurality of charged particles include at least two particles of different colors. As in claim 3, the electro-optical medium, wherein the color of the particles of different colors is selected from the group consisting of red, green, blue, cyan, magenta, yellow, black and white. An electro-optical medium as claimed in claim 1, wherein the ionic portion is anionic. An electro-optical medium as claimed in claim 1, wherein the ionic portion is cationic. The electro-optical medium of claim 1, wherein the ionic portion is zwitterionic. The electro-optical medium of claim 1, wherein the polymer has a conductivity of 10 ^-10 to 10 ^-1 Siemens/cm. The electro-optical medium of claim 1, wherein the polymer has a conductivity of 10 ^-6 to 10 ^-7 Siemens/cm. An electro-optical medium as claimed in claim 1, wherein the polymer has a number average molecular weight of at least 20,000 g/mol. The electro-optical medium of claim 1, wherein the polymer has a number average molecular weight of 100,000 to 1,000,000 g/mol. The electro-optical medium of claim 1, wherein the polymer is a linear or branched polymer. The electro-optical medium of claim 1, wherein the polymer has a polymer backbone derived from one or more polymers selected from the group consisting of: polyvinyl alcohol, polyvinyl pyrrolidone, polyvinyl acetate, polyacrylate, polymethacrylate, polyurethane, polysaccharide, polyolefin, polyether, polyester, polypeptide, protein and combinations thereof. An electro-optical film, which sequentially comprises a light-transmitting substrate, a light-transmitting conductive material layer, and an electro-optical medium layer as claimed in claim 1. An electro-optical display, which sequentially comprises a light-transmitting substrate, a light-transmitting conductive material layer, an electro-optical medium layer as claimed in claim 1, and a rear substrate. An electro-optical display as claimed in claim 15, wherein the rear substrate comprises a plurality of electrodes.