HK40011929A - Electro-optic media including encapsulated pigments in gelatin binder - Google Patents

Description

Electro-optic medium comprising encapsulated pigments in a gelatin binder

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit and priority to U.S. provisional patent application serial No. 62/520, 731, filed on 16/6/2018. This application, and all other U.S. patents listed below and applications in published and concurrent applications, are incorporated by reference in their entirety.

Background

Light modulators represent a potentially important market for electro-optic media. As building and vehicle energy performance becomes increasingly important, electro-optic media may be used as coatings on windows, including building rooflights and vehicle rooflights, to enable electrical control of the proportion of incident radiation that passes through the window by changing the optical state of the electro-optic medium. Efficient implementation of this "variable transmittance" ("VT") is expected in buildings to provide (1) a reduction in unwanted thermal effects on hot days, thus reducing energy required for cooling, air conditioning size, and peak power requirements; (2) increased use of natural sunlight, thus reducing energy usage and peak power requirements for lighting; and (3) user comfort by increasing thermal and visual comfort. It is expected that greater benefits will be obtained in automobiles where the ratio of light transmitting surface to enclosed space is much greater than in ordinary buildings. The implementation of efficient VT technology in automobiles is particularly expected to not only provide the aforementioned advantages, but also (1) increase safety of motion, (2) reduce glare, (3) improve reflection performance (using electro-optic coatings on mirrors), and (4) increase the ability to use heads-up displays. Other potential VT technology applications include privacy glass and glare protection for electronic devices.

U.S. patent No. 7, 327, 511 describes a variable transmission device comprising charged pigment particles distributed in a non-polar solvent and encapsulated. These variable transmission devices can be driven to an on state by an AC drive voltage, thereby driving the charged pigment particles to the capsule wall. Therefore, in the case where it is desired to change the transmittance at will as required, this variable transmission device can be used for viewing surfaces such as privacy glass, vehicle sunroofs, and windows on buildings.

The us patent No. 7,327,511 describes various important factors for adjusting the optimum performance of an electrophoretic medium in a variable transmission device. One important factor is to minimize haze. In this application, "haze" refers to the percentage of diffuse transmitted light (light scattered in transmission) compared to the total transmitted light. When using an encapsulated electro-optic medium, haze can be reduced by matching the refractive index of the binder to that of the capsules holding the switching medium as much as possible. As described in the' 511 patent, it may be advantageous to use a polymeric binder that has a refractive index difference from the capsules of no greater than 0.07. As recognized by the' 511 patent, however, attempts to closely match the refractive indices of the capsules and the adhesive often fail because the adhesive material with the correct refractive index typically has other properties such as electrical conductivity or workability that make the adhesive material unsuitable for use in electro-optic displays. The' 511 patent teaches that gelatin, polyvinylpyrrolidone, cellulose and polymethacrylamide can be used as binder materials, but all suffer from their disadvantages. In particular, these binder materials do not lend themselves to well performing coacervate encapsulation slurries that can be coated onto sheets and subsequently laminated. Therefore, most electro-optic media (e.g., Amazon) used to produce electrophoretic displays) All rely on polyurethane binders.

Another disadvantage of "heterogeneous" binder materials is the "kick-back" (kick-back) or self-erase (self-erasing) of the optical display state due to impedance (or other electrical) mismatch between the internal phase, microcapsule wall and the surrounding binder, where the impedance can be characterized as a series of resistances. The impedance mismatch allows charge domains to be established between the materials and affects the electrophoretic particle position in the internal phase, resulting in a degradation of the optical state. Self-erasing is obviously highly undesirable because it can reverse the desired optical state of the display (or distort a gray-scale display) or allow the transmissive device to decay from on to off. It has been found that self-erasure is a particular problem in polymer dispersed electrophoretic media and displays where the capsules are substantially removed from the electro-optic medium, leaving only the internal phase foam in the polymer binder.

Disclosure of Invention

Although comparable to earlier work, it has been found that certain gelatin formulations, especially mixtures of fish gelatin and gum arabic, are suitable as binders for encapsulating electro-optic media. Furthermore, these gelatin binders provide excellent refractive index matching and thus low haze when used in combination with porcine gelatin/gum arabic coacervates to encapsulate an internal phase, such as when incorporated into a transmissive device. Furthermore, electro-optical media incorporating mixtures of fish gelatin and gum arabic as binders do not suffer from kickback observed only in compositions containing gelatin binders.

The invention thus relates to an electro-optical medium comprising a plurality of capsules in a binder comprising a mixture of fish gelatin and gum arabic. The capsules are typically formed from an agglomerate of gelatin and gum arabic and encapsulate an internal phase comprising a mixture of a non-polar solvent and charged pigment particles. In some embodiments, the binding agent comprises a weight ratio of fish gelatin to gum arabic of 0.5 to 2.0, or more preferably the weight ratio of fish gelatin to gum arabic is approximately equal. In some embodiments, the capsules additionally encapsulate more than one type of charged pigment particles, such as second, third, etc. charged pigment particles, and each group of charged particles may have a color selected from one of a plurality of colors, such as white, black, red, green, blue, magenta, cyan, and yellow. In one embodiment, the capsule may comprise second charged pigment particles that are oppositely charged and of a different color than the first charged pigment particles. In some embodiments, the binder additionally comprises a pigment or dye. The mixture of non-polar solvent and first charged pigment particles may additionally include a charge control agent, and the non-polar solvent may be a hydrocarbon or a mixture of limonene (e.g., 1-limonene). The refractive index of the binder of the present invention may be between 1.47 and 1.57 at 550 nm.

The electro-optic medium of the present invention may be incorporated into a variety of electro-optic devices. For example, a Front Panel Laminate (FPL) may comprise a light-transmissive electrode layer, an adhesive layer, and the electro-optic medium of the present invention. In some embodiments, the front panel laminate also includes a release sheet or an adhesive layer or both. The electro-optic medium of the present invention may also be incorporated into an electro-optic display comprising a light-transmissive electrode layer, an adhesive layer, and the electro-optic medium of the present invention and an array of pixel electrodes. Alternatively, the variable transmission device may be manufactured by combining a first light-transmissive electrode layer, an adhesive layer, the electro-optic medium of the invention and a second light-transmissive electrode layer. In some variable transmission devices, the charged pigment particles will contain carbon black.

Drawings

FIG. 1A is an illustration of an electro-optic display containing two types of charged particles. The particles can move toward (away from) a viewer under the application of an electric field;

FIG. 1B is an illustration of a variable transmission device comprising first and second light-transmissive electrode layers with an electro-optic medium disposed therebetween. The particles can move adjacent to the capsule wall under the application of an electric field, thereby allowing light to pass through the medium;

fig. 2A shows haze measurements for variable transmission samples of various fish gelatin binder compositions. The left panel sample used a binder without gum arabic, the middle panel sample used a binder with 33% (wt./wt.) gum arabic, and the right panel sample used a binder with equal amounts of fish gelatin and gum arabic;

fig. 2B shows contrast measurements of variable transmission samples of various fish gelatin binder compositions. The left panel sample used a binder without gum arabic, the middle panel sample used a binder with 33% (wt./wt.) gum arabic, and the right panel sample used a binder with equal amounts of fish gelatin and gum arabic;

figure 2C shows kickback measurements of variable transmission samples of various fish gelatin binder compositions. The left panel sample used a binder without gum arabic, the middle panel sample used a binder with 33% (wt./wt.) gum arabic, and the right panel sample used a binder with equal amounts of fish gelatin and gum arabic;

figure 3 shows the decay of the off-state of a sample with a binder of fish gelatin only compared to a sample with equal amounts of fish gelatin and gum arabic. In the test, the 1: 1 the fish gelatin mixed with gum arabic has substantially less kickback, resulting in open and closed states that are stable for weeks.

Detailed Description

The present invention provides improved electro-optic media comprising encapsulated pigment particles. In particular, it has been found that a mixture of approximately equal amounts of fish gelatin and gum arabic is an excellent binder for use with capsules formed by coacervation of (porcine) gelatin and gum arabic. This result is surprising because neither fish gelatin nor gum arabic alone is a suitable binder material for electro-optic media. As described below, fish gelatin alone has unacceptable kickback, while gum arabic alone will shrink and break upon post-coating treatment. The resulting electro-optic medium has low haze and less kickback when used in a transmissive device, and thus has higher long-term state stability. It has been observed that color electrophoretic displays incorporating electro-optic media according to various embodiments of the present invention can exhibit a wide range of operating temperatures. Electro-optic media can be coated on large surfaces and laminated with electrodes and the like to produce a variety of electro-optic devices, including displays readable in sunlight and smart windows.

Electrophoretic displays (such as edreaders) are generally not transmissive and operate in a reflective mode. This functionality is illustrated in fig. 1A, where the reflectivity of light striking a surface is modulated by moving black or white charged particles with an appropriate voltage towards the viewing surface. It is also possible to have the electrophoretic device operate in a so-called "shutter mode", in which one operational state is substantially light-tight and the other operational state is light-transmissive, as shown in fig. 1B. When such "shutter mode" electrophoretic devices are constructed on transparent substrates, the transmission of light through the device can be adjusted. One potential use for shutter mode electrophoretic media is a window with variable light transmission.

The device of figures 1A and 1B comprises an electro-optic medium consisting of capsules in a polymer binder. The capsules contain charged pigment particles that move in response to an electric field. The capsule is generally formed from a gelatin material as described in more detail below. The electro-optic medium is distributed between first and second electrode layers, which may be made of known materials such as polyethylene terephthalate (PET) coated with Indium Tin Oxide (ITO). Alternatively, an electrode layer may include a metal electrode, which may be provided as a pixel. The pixels may be controlled as an active matrix, thereby allowing text and images to be displayed. An additional adhesive layer is typically present between the electro-optic medium and one of said electrode layers. The adhesive layer may be UV curable and may generally improve the flatness of the final device by "filling in" the differences created by the capsules. Suitable adhesive formulations are described in us 2017/0022403, which is incorporated herein by reference.

When a DC electric field is applied to the device of fig. 1A, dark or bright particles move towards the viewing surface, thereby changing the optical state from dark to bright. In fig. 1B when an alternating electric field is applied to one of the electrodes, the charged pigment particles are driven to the wall of the capsule, resulting in an aperture of the capsule for light transmission, i.e. in an open state. In both embodiments, the optical state (black/white; on/off) is maintained for a long period (weeks) without the need to maintain an electric field, since the solvent is non-polar and includes charge control agents and/or stabilizers. As a result, devices consume very little power when they are only "switched" several times a day.

An electrophoretic display generally comprises a layer of electrophoretic material and at least two other layers disposed on opposite sides of the electrophoretic 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 as extended row electrodes and the other layer may be patterned as column electrodes extending at right angles to the row electrodes, with pixels being defined by the intersections of the row and column electrodes. Alternatively, and more typically, one electrode layer is in the form of a single continuous electrode, and the other electrode layer is patterned as a matrix of pixel electrodes, each defining one pixel of the display. In another type of electrophoretic display, intended for use with a stylus, print head or similar movable electrode independent of the display, only one of the layers adjacent to the electrophoretic layer comprises an electrode, the layer on the opposite side of the electrophoretic layer generally being a protective layer, intended to avoid the movable electrode damaging the electrophoretic layer.

The term "gray-scale state," as used herein in its conventional sense in imaging technology, generally refers to a state intermediate the two extreme optical states of a pixel, and does not necessarily refer to a black-and-white transition between the two extreme states. It is known, for example, to use electro-optic displays as variable transmission windows in which the extreme states are substantially light transmissive and substantially opaque, so that intermediate "grey-scale states" may be partially light transmissive and may not be grey in nature. In practice, if the particles employed are light scattering, the partially transmitted "grey state" may be white in nature. The term "monochrome" may be used hereinafter to refer to a driving scheme that drives a pixel only to its two extreme optical states, with no intervening gray scale states.

The terms "bistable" and "bistability" are used herein in their conventional sense in the art to mean that the display comprises display elements having first and second display states differing in at least one optical property and such that, after any given element has been driven by an addressing pulse of finite duration, it assumes its first or second display state which, after the addressing pulse has terminated, will continue to change the state of the display element by at least a multiple, for example at least 4 times, the duration of the minimum addressing pulse required to change the state of the display element. Some particle-based electrophoretic displays with gray levels are shown in U.S. patent No. 7,170,670 and are stable not only in their extreme black and white states, but also in their intermediate gray-level states, and also for some other types of electro-optic displays. Such displays are suitably referred to as "multi-stable" rather than bistable, but for convenience, "bistable" is used herein to include both bistable and multi-stable displays.

Various techniques employed in encapsulated electrophoretic and other electro-optic media are described in a number of patents and applications assigned to or under the names of the institute of technology and technology (MIT) and E Ink corporation. Such encapsulated electrophoretic media include a plurality of microcapsules, each of which itself includes an internal phase containing electrophoretically-mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Some of the materials and techniques described in the following patents and applications relate to the fabrication of variable transmission devices as described herein, including:

(a) electrophoretic particles, fluids, and fluid additives; see, e.g., 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; 6721, 083; 6,727, 881; 6,822, 782; 6,870, 661; 7,002,728; 7,038, 655; 7, 170, 670; 7, 180, 649; 7, 230, 750; 7, 230, 751; 7, 236, 290; 7, 247, 379; 7,312, 916; 7,375, 875; 7, 411, 720; 7,532, 388; 7, 679, 814; 7, 746, 544; 7,848,006; 7,903, 319; 8, 018, 640; 8,115, 729; 8,199,395; 8,270, 064; and 8, 305, 341; and U.S. patent application publication No. 2005/0012980; 2008/0266245, respectively; 2009/0009852, respectively; 2009/0206499, respectively; 2009/0225398, respectively; 2010/0148385, respectively; 2010/0207073, respectively; and 2011/0012825;

(b) capsule, adhesive and encapsulation treatment; see, e.g., U.S. patent nos. 6, 922, 276 and 7, 411, 719;

(c) films and sub-assemblies containing electro-optic 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) a method for driving a display; see, e.g., U.S. patent nos. 7, 012, 600 and 7, 453, 445;

(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, described in U.S. patent nos. 6, 241, 921; 6,950,220; 7, 420, 549 and 8, 319, 759; and U.S. patent application publication No. 2012/0293858.

The internal phase of the electro-optic medium comprises a charged pigment in a suspending fluidAnd (3) granules. The fluid used in the variable transmission medium of the present invention will generally have a low dielectric constant (preferably below 10 and desirably below 3). Particularly preferred solvents include aliphatic hydrocarbons such as heptane, octane, and petroleum distillates such as(ExxonMobil) or(Total); terpenes such as limonene, e.g., 1-limonene; and aromatic hydrocarbons such as toluene. It is particularly preferred that the solvent is limonene, as it combines a low dielectric constant (2.3) with a relatively high refractive index (1.47). The refractive index of the internal phase may be adjusted by the addition of a refractive index matching agent such as those available from Cargille-Sacher Laboratories Inc. (Cedar Grove, NJ), Todar, N.J.)The index of refraction is modified to match the fluid. In the encapsulation medium of the present invention, it is preferable that the refractive index of the dispersed particles match the refractive index of the encapsulation material as much as possible to reduce haze. This index matching is best achieved when the refractive index of the solvent is close to that of the encapsulant (when commonly available polymeric encapsulants are employed). In most cases, it is advantageous for the refractive index of the internal phase at 550nm to be between 1.51 and 1.57, preferably about 1.54 at 550 nm.

The charged pigment particles can have a variety of colors and compositions. In addition, the charged pigment particles may 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 may be formed of inorganic pigments such as TiO2, ZrO2, ZnO, Al2O3, Sb2O3, BaSO4, PbSO4, and the like. It can also be a polymer particle with a high refractive index (>1.5) and a specific size (>100nm) to appear white, or a composite particle engineered to have the required refractive index. The black charged particles can be formed from CI pigment black 26 or 28, etc. (e.g., manganese ferrite black spinel or copper chromite black spinel) or carbon black. Other colors (non-white and non-black) may be represented by organic pigments such as CI pigments PR254, PR122, PR149, PG36, PG58, PG7, PB28, PB 15: 3. PY83, PY138, PY150, PY155, or PY 20. Other examples include Clariant Hostaperm Red D3G70-EDS, Hostaperm Pink E-EDS, PV fast Red D3G, Hostaperm Red D3G70, Hostaperm blue B2G-EDS, Hostaperm Yellow H4G-EDS, Novoperm Yellow HR-70-EDS, Hostaperm GreenGNX, BASF Irgazine Red L3630, Cinqasia Red L4100 HD, and Irgazin Red L3660 HD; sun chemical phthalocyanine blue, phthalocyanine green, diarylide yellow, or diarylide AAO T yellow. The colored particles can also be formed from inorganic pigments such as CI pigment blue 28, CI pigment green 50, CI pigment yellow 227, and the like. The charged particle surface may be modified by known techniques based on the charge polarity and the desired charge level of the particles, 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 entire contents of which are incorporated herein by reference in their entirety.

The particles may exhibit an innate charge or may be explicitly charged by means of charge control agents, 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 in nature, or may be ionic or non-ionic. Examples of charge control agents may include, but are not limited to, Solsperse 17000 (active polymeric dispersant), Solsperse 9000 (active polymeric dispersant), OLOA11000 (succinimide ashless dispersant), unitox 750 (ethoxylate), Span 85 (sorbitan trioleate), Petronate L (sodium sulfonate), Alcolec LV30 (soy lecithin), Petrostep B100 (petroleum sulfonate) or B70 (barium sulfonate), aerosol OT, polyisobutylene derivatives or poly (ethylene-co-butylene) derivatives, and the like. In addition to the suspending fluid and the charged pigment particles, the internal phase may comprise stabilizers, surfactants, and charge control agents. The stabilizing material may be adsorbed onto the charged pigment particles when dispersed in the solvent. The stabilizing material maintains the particles separated from each other such that the variable transmission medium is substantially non-transmissive when the particles are in a 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 may be aided by the use of a surfactant. Such surfactants typically comprise a polar "head group" and a non-polar "tail group" that is compatible with or soluble in a solvent. In the present invention, preferred nonpolar tail groups are saturated or unsaturated hydrocarbon moieties, or another group soluble in a hydrocarbon solvent, such as a poly (dialkylsiloxane). The polar group may be any polar organic functional group, including ionic materials such as ammonium salts, sulfonates or phosphates, or acidic or basic groups. Particularly preferred head groups are carboxylic acid or carboxylate groups. Suitable stabilizers for use in the present invention include polyisobutylene and polystyrene. In some embodiments, a dispersant such as polyisobutylene succinimide and/or sorbitan trioleate and/or 2-hexyldecanoic acid is added.

Gelatin-based capsule walls for use in variable transmission devices have been described in many of the above-mentioned E Ink and MIT patents and applications. Gelatin is available from various suppliers, such as Sigma Aldrich or Gelitia USA. Gelatin of various grades and purities can be obtained according to application requirements. Gelatin mainly comprises collagen collected and hydrolyzed from animal products (cattle, pigs, poultry, fish). Which includes a mixture of peptides and proteins. In many of the embodiments described herein, gelatin is combined with acacia gum (gum arabic) derived from acacia senegal sclerosant sap. Gum arabic is a complex mixture of glycoproteins and polysaccharides, and it is often used as a stabilizer in food products. Aqueous solutions of gum arabic and gelatin may be agglomerated with the non-polar internal phase, as described below, to produce clear and soft capsules containing the internal phase.

Capsules incorporating gelatin/gum arabic were prepared as follows: see, e.g., U.S. patent No. 7,170,670, which is incorporated by reference in its entirety. In this process, an aqueous mixture of gelatin and gum arabic is emulsified through a hydrocarbon internal phase (or other water-immiscible phase to be encapsulated) to encapsulate the internal phase. The mixture was raised to 40 ℃ and the pH of the mixture was lowered to about 4.9, resulting in the formation of gelatin/gum arabic agglomerates, thereby forming capsules. The resulting mixture was then cooled to 10 ℃ and aqueous glutaraldehyde solution (agent for cross-linking the capsule walls) was added. The resulting mixture was then warmed to 25 ℃ and stirred vigorously for an additional 12 hours. The remaining glutaraldehyde was deactivated using a final step (keeping the capsule mixture at 50 ℃ for one hour), thereby ensuring that the capsules separated during sieving. This treatment allows the capsules to be in the range of 20-100 μm and typically results in over 50% of the starting material being incorporated into the available capsules. The resulting capsules are then differentiated by size by sieving or other size exclusion classification. Capsules exceeding about 120 μm are difficult to use because they are easily broken by shear forces during handling. Further, capsules larger than 100 μm are visible to the naked eye, so that their presence is considered as ripples in the variable transmission film.

After size classification, the capsules are mixed with a binder to produce a slurry for coating, for example, using slot coating, knife coating, spin coating, and the like. In an embodiment of the invention, the binder comprises gelatin, typically fish gelatin. In a preferred embodiment, gelatin is combined with gum arabic, but it has been found that the mixture should not coagulate, as the coagulated mixture will not produce a suitably homogeneous slurry. Furthermore, it has been found that the haze of the transmission medium can be improved by varying the amount of gum arabic added to the binder mixture.

To improve off-axis transparency, it may be advantageous to keep the electrophoretic layer as thin as possible, thus reducing the structural size of any particles that penetrate the thickness of the electrophoretic layer, but as mentioned above, a thin electrophoretic layer correspondingly increases the volume fraction of electrophoretic particles to achieve sufficient opacity in the off-state of the display. Thus, for any given material selection used in the light modulator, there may be an optimal electrophoretic layer thickness. Off-axis transparency can also be improved by controlling the particle structure so that it does not occupy the entire sidewall of the droplet. In particular, it is advantageous to concentrate the particles such that the particle structure occupies only a portion of the sidewalls adjacent to the major surface of the electrophoretic medium layer. This particle structure can be manufactured according to the DC/AC driving method of the present invention: all particles are first brought into the droplet adjacent to one major surface of the electrophoretic layer by applying a DC electric field to the layer, and then driven to the side walls using an AC electric field of appropriate frequency.

Various waveforms have been described for controlling electrophoretic particles in a "shutter mode", such as 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. Dielectrophoretic displays are similar to electrophoretic displays and can operate in a similar mode, but depend on changes in electric field strength, as seen in U.S. patent No. 4,418,346. "shutter mode" is shorthand for a display having a first state that is substantially opaque and a second state that is substantially transparent.

Examples of the invention

Example 1

The nonaqueous internal phase is formed by combining OLOA11000, l-limonene,5040 soaking liquid,L (Cabot Corp.), polystyrene (Sigma-Aldrich 331651), and 2-hexyldecanoic acid (Aldrich). The internal phase mixture was then encapsulated by adding the mixture to an aqueous solution of dry porcine gelatin (GelitaUSA, Inc) and gum arabic (AEP Colloids). After the addition of the internal phase is complete, the mixture is stirred and heated to emulsify the internal phase into droplets having an average diameter of about 40-45 μm. To prevent further droplet break down, water and a dispersion of 10 wt.% of Emperor 2000 carbon black (CabotCorp.) and 5 wt.% of Kolliphor P188 (Aldrich 15759) in water were added.

After mixing and pH adjustment, the temperature of the mixture was reduced and 50 wt.% glutaraldehyde was added with continued vigorous stirring. After this addition, the mixture was gradually warmed and vigorously stirred, and then cooled. The resulting capsules were classified to produce a mixture of capsules ranging in size from 15-50 μm with an average size of about 30 μm.

The resulting aqueous capsule slurry was centrifuged and then mixed into three different aqueous fish gelatin based binders; A) without gum arabic, B) gum arabic to fish gelatin is 1: 3, and C) gum arabic to fish gelatin is 1: 1. Fish Gelatin is HiPure Liquid Gelatin from Norland, and gum arabic is from AEP colloid. Each gelatin binder was mixed with a solution of 10 wt.% Emperor 2000 carbon black and 5 wt.% Kolliphor P188 colorant in a ratio of 1 part binder to 7 parts capsule by weight, and 1 part carbon black colorant to 49 parts binder in water. The resulting mixture was bar coated onto a 125mm thick indium tin oxide coated polyester film. The coated film was allowed to dry to produce an approximately 25 μm thick electrophoretic medium containing essentially a monolayer of capsules.

And then coated on the capsule coating surface of the coating film with a urethane acrylate based binder. With the addition of an adhesive layer, a 125mm thick screen printed sheet of indium tin oxide coated polyester film was applied. The resulting assembly is then cured by exposure to UV light from a CSun UV lamp.

Several samples of variable transmission test films were prepared for each adhesive formulation. These samples were then evaluated for on and off transmission and haze using the optical evaluation settings described in U.S. patent No. 7, 679, 814. Briefly, each sample is placed in front of a calibrated light source, and there is an integrating detector on the opposite side of the sample. Each sample is driven to an on and off state and evaluated for transmission. In addition, the diffuse versus transmitted light was measured using a calibrated chopper wheel to assess haze. The amount of kickback can also be estimated by comparing the decay in the on state versus time (see fig. 3). The data obtained are shown in FIGS. 2A-2C.

Evaluating the differences between the three binder formulations, it is clear that 1: the fish gelatin of 1 produced an electro-optic medium with good contrast (difference between on and off states; fig. 2B) and very low haze (fig. 2A) to the gum arabic mixture. Furthermore, the two-binder mixture containing gum arabic has little kickback, resulting in extremely long-term stability in the open and closed states. See fig. 3.

Example 2

The non-aqueous internal phase is prepared by combining a white pigment dispersion with a first color pigment dispersion, a second color pigment dispersion, and a third color pigment dispersion. It is mixed withMixed with Isopar E solvent mixture and further mixed with polyisobutylene solution. The internal phase mixture was then encapsulated by adding the mixture to an aqueous solution of dried porcine gelatin and gum arabic. After the addition of the internal phase is complete, the mixture is emulsified by adjusting the pH while mixing and heating. The mixture temperature was then reduced and 50 wt.% glutaraldehyde was added with continued vigorous stirring. After this addition, the mixture was gradually warmed and vigorously stirred and then cooled. The resulting capsules were sorted by sieving to produce a mixture of capsules ranging in size from 15-50 μm with an average size of about 40 μm.

The resulting aqueous capsule slurry was then mixed into three different binders, a) charged polyvinyl alcohol, B) gum arabic to fish gelatin 1: 1, and C) a standard polyurethane dispersion. Fish gelatin is HiPure LiquidGelatin from Norland and acacia gum is from AEP colloid. Each gelatin binder was mixed in a ratio of 1 part binder to 7 parts capsule by weight. The resulting mixture was bar coated onto a 125mm thick indium tin oxide coated polyester film. The coated film was allowed to dry to produce an approximately 21 μm thick electrophoretic medium containing essentially a monolayer of capsules.

The coated film was then laminated to a capsule coated surface with a polyurethane adhesive doped with a conductive salt, which was then laminated to an approximately 2 square inch screen printed backplane assembly. The phase color states and gamut quantities are then measured at multiple temperatures and the resulting gamut is taken to cover the maximum number of states possible by the experimental system without exceeding the drive condition limits for the maximum voltage. For these three samples, the films containing the fish gelatin/gum arabic binder exhibited the greatest and most consistent amount of color gamut over the temperature range. The results are shown in Table 1 below.

Table 1:

from the foregoing, it can be seen that the present invention provides an electro-optic medium suitable for use in a variable transmission device having high contrast between on and off states while having very low haze; and attractive for commercial use (e.g., variable transmission windows).

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 foregoing description as a whole should be considered illustrative and not restrictive.

Claims (9)

1. An electro-optic medium comprising a plurality of capsules, each capsule containing a plurality of electrically charged particles and a fluid, the electrically charged particles being movable by the application of an electric field, and a binder comprising fish gelatin and gum arabic.

2. The electro-optic medium of claim 1, wherein the capsule comprises gelatin and gum arabic.

3. The electro-optic medium of claim 1, wherein the binder has a mass ratio of fish gelatin to gum arabic of 0.5 to 2.0.

4. The electro-optic medium of claim 1, wherein the binder has a refractive index between 1.47 and 1.57 at 550nm at 50% RH.

5. The electro-optic medium of claim 1, wherein application of the electric field switches the charged particles between an on state and an off state, wherein the capsule has a higher rate of light absorption in the off state than in the on state.

6. The electro-optic medium of claim 1, wherein the charged pigment particles comprise a first type of particles having a first color and a second type of particles having a second color,

wherein the first color is different from the second color and the first type of particles have an opposite charge to the second type of particles.

7. The electro-optic medium of claim 1, wherein the charged pigment particles comprise at least three groups of particles, each group of particles having a color different from the colors of the other two groups of particles, wherein the color of each group of particles is selected from the group consisting of white, black, red, green-blue-fast, magenta, cyan, and yellow.

8. A device comprising a layer of the variable transmission electrophoretic medium of claim 1, disposed between two electrodes, at least one of which is light transmissive.

9. The device of claim 8, wherein the two electrodes are light transmissive.