Variable transmittance electrophoresis device

Abstract

A variable transmission film can include an electrophoretic medium having a plurality of capsules and a binder, each capsule containing a plurality of charged particles and a fluid, the charged particles being movable by application of an electric field and capable of switching between an open state and a closed state. The film can include at least one of: a binder including fish gelatin and a polyanion; a binder containing one or more colorants; a capsule containing a charge control agent, such as an oligoamine terminated polyolefin and a branched aliphatic acid containing at least 8 carbon atoms; a selection of capsules in which at least 60% have a diameter between 50 μm and 90 μm and at least 15% have a diameter between 20 μm and 49 μm; a pigmented binder layer; and a fluid selected from one or more non-conjugated olefins.

Description

[0001] This application is a divisional application of application number 201880039519.7, filed on June 14, 2018, with the title "Variable Transmission Electrophoretic Device".

[0002] Cross Reference to Related Applications

[0003] This application claims the benefit of and priority to U.S. Provisional Patent Application Serial No. 62 / 520,629, filed on June 16, 2017, U.S. Provisional Patent Application Serial No. 62 / 520,600, filed on June 16, 2017, U.S. Provisional Patent Application Serial No. 62 / 520,731, filed on June 16, 2017, U.S. Provisional Patent Application Serial No. 62 / 520,699, filed on June 16, 2017, U.S. Provisional Patent Application Serial No. 62 / 563,137, filed on September 26, 2017, and U.S. Provisional Patent Application Serial No. 62 / 673,743, filed on May 18, 2018. This application is also related to U.S. Patent Nos. 7,256,766; 7,327,511; 7,679,814; and 7,999,787.

[0004] The contents of these applications, patents, and all other U.S. patents and published and co-pending applications referred to below are incorporated herein by reference. TECHNICAL FIELD

[0005] The present invention relates to variable transmission devices. More particularly, the present invention relates to variable transmission devices comprising an electrophoretic medium including a plurality of capsules in a binder and adhesive layer, which can improve the optical performance of the variable transmission device. BACKGROUND

[0006] Optical modulators represent a potentially important market for electro-optic media. As the energy performance of buildings and vehicles becomes more important, electro-optic media can be used as coatings on windows (including glazing and sunroofs) to achieve a proportion of incident radiation that is transmitted via the window that is electronically controlled by changing the optical state of the electro-optic media. Effective implementation of such "variable transmission" ("VT") technology in buildings is expected to provide (1) reduced effects of unwanted heat during hot weather, thereby reducing the energy required for cooling, the size of air conditioning equipment, and peak electrical demand; (2) increased utilization of natural light, thereby reducing the energy and peak electrical demand for lighting; and (3) increased occupant comfort by increasing thermal and visual comfort. Greater benefits are expected in automobiles, where the ratio of glass surface to enclosed volume is much greater than in a typical building. In particular, effective implementation of VT technology in automobiles is expected to provide not only the above benefits, but also (1) enhanced driving safety, (2) reduced glare, (3) enhanced rearview mirror performance (by using electro-optic coatings on rearview mirrors), and (4) enhanced capability for use of head-up displays. Other potential applications for VT technology include privacy glass and anti-glare covers for electronic devices.

[0007] U.S. Patent No. 7,327,511 describes variable transmission devices that include charged pigment particles that are distributed in a non-polar solvent and encapsulated. These variable transmission devices can be driven to an open state with an alternating current driving voltage, thereby driving the charged pigment particles to the capsule walls. Such variable transmission devices are useful for viewing surfaces where it is desirable to change the transmission at will, such as privacy glass, sunroofs, and windows on buildings.

[0008] U.S. Patent No. 7,327,511 also describes various factors that are important for adapting electrophoretic media for optimal performance in optical modulators. One important factor is minimization of haze. In this application, "haze" refers to the percentage of diffused transmitted light (light that is scattered when transmitted) to total transmitted light. When designing optical modulators that can be electrically switched from an open, transparent state to a closed, opaque state, it is desirable for the haze in the open state to be less than 10%, more preferably less than 2%. Another important factor is contrast ratio. Another problem with variable transmission devices that include capsules is grain. In this application, "grain" refers to visual non-uniformities caused by a variety of factors, such as clumps or layers of colored binder or capsules, variability in capsule packing, voids, thickness variations, and coating defects including pinholes. These non-uniformities reduce visibility when a user views the device in the open state. The term "grain" has its origins in film photography, where clumped silver was known in early silver gelatin film to make the developed photograph look "grainy." Efforts to reduce grain in variable transmission devices have generally focused on reducing variations in capsule packing density.

[0009] One method for reducing the amount of particles in a encapsulated electrophoretic medium is to use more capsules, which are smaller in size, for example, between 20 and 50 μιη in diameter. This small size allows for tighter packing of the capsules, and due to the microscopic size of the capsules, individual capsules are less likely to be discerned. As a result, a user of a viewing device will see fewer particles. However, it has been observed that variable transmission devices including a single layer of "small" capsules (i.e., in the range of 20 μιη to 50 μιη in diameter) have less contrast between the open and closed states due to the thinner electro-optic layer. That is, less pigment interferes with transmission through the layer, so the transmission in the closed state is higher. Additionally, the presence of many similarly sized capsules in a tight packing in a single layer results in "halos" or "starbursts" when viewing a light source through the variable transmission device. These interference patterns can be distracting to the observer and reduce the viewing experience in the open state.

[0010] Another drawback of the variable transmission devices described in U.S. Patent No. 7,327,511 is "recoil," or the self-erasure of the optical display state due to impedance mismatch between the electrophoretic internal phase and the binder surrounding the electrophoretic internal phase. The impedance mismatch can cause charge regions to build up between the various materials and affect the position of the electrophoretic particles in the internal phase, resulting in decay of the optical state. Obviously, self-erasure is highly undesirable because it can reverse the desired optical state of the display (or otherwise distort it in the case of a gray scale display), or cause the transmission device to change from open to closed. It has been found that self-erasure is a particular problem in polymer-dispersed electrophoretic media and displays in which the capsules are substantially removed from the electro-optic medium, leaving only bubbles of the internal phase in the polymer binder.

[0011] Efforts to reduce the impedance mismatch between the electrophoretic internal phase and the binder present new challenges. For example, a binder formulation that is resistively matched to the electrophoretic internal phase (and the encapsulation layer, if present) can produce an undesirable color, or introduce "haze." In this case, when the variable transmission film is in its highest transmission (i.e., "open") state, the observer perceives the film as having a "color" and / or "spots," which is detrimental to viewing. While color and spots are annoying in most applications, they can present a safety issue when used in automotive or aircraft glazing.

[0012] Accordingly, there is a need for improved electro-optic media that can be incorporated in variable transmission devices. SUMMARY

[0013] In one aspect, the present application provides a variable transmission electrophoretic medium comprising a plurality of capsules in a binder, each capsule having an internal phase comprising a plurality of charged particles in a fluid, the charged particles being movable between an open state in which the capsule has a lower absorbance of light passing through the medium and a closed state in which the capsule has a higher absorbance of light passing through the medium, and wherein the absorbance of the binder is 0.5 to 2.0 times the absorbance of the capsules in the closed state. For example, the absorbance of the binder can be 0.75 to 1.25 times, or 0.9 to 1.1 times, the absorbance of the capsules in the closed state. Since the binder used is typically a polymer and is substantially transparent at least in the thickness typically used in electrophoretic media, which is of the order of 50 μιη, to produce the necessary absorbance in the binder, it is usually necessary to color the binder with a dye or pigment, and when the variable transmission medium is to be used in a place where it will be subjected to a lot of ultraviolet radiation (such as in a sunroof or window of a variable transmission vehicle or a house window), the latter is usually preferred because dyes tend to fade on prolonged exposure to ultraviolet radiation. One preferred pigment for this purpose is carbon black, which should have a very small particle size (primary aggregates less than 100 nm) to minimize undesirable light scattering. In other embodiments, a color can be obtained with a plurality of pigments (e.g., a combination of cyan, yellow and magenta pigments), which can provide a medium with a relatively low haze.

[0014] For the reasons discussed below, it is desirable that the proportion of binder in the electrophoretic medium be higher than in most prior art encapsulated electrophoretic media; there should be at least 1 part by weight of binder for every 15 parts by weight of capsules, and optionally at least 1 part by weight of binder for every 4 parts by weight of capsules.

[0015] In another aspect, the present application provides an electrophoretic medium comprising a plurality of charged particles, a liquid in which the particles are dispersed, and a charge control agent ("CCA") comprising an oligoamine-terminated polyolefin and a branched aliphatic acid comprising at least about 8 carbon atoms.

[0016] The polyolefin used as part of the charge control agent in the electrophoretic medium of the present application can be an oligoamine-terminated polyisobutylene. Such copolymers are commercially available as 11000 (manufactured by Chevron Oronite Company LLC, San Ramon, CA) can be purchased. It is desirable that the branched aliphatic acid used be a 2-alkyl fatty acid, which can contain 12 or more carbon atoms, with the particularly preferred acid being 2-hexyldecanoic acid ("2-HDA"). The fatty acid used should be one that is readily soluble in the liquid used to disperse the particles and is resistant to crystallization.

[0017] In another aspect, the present application provides an electro-optic medium comprising a plurality of capsules in a binder, the binder comprising a mixture of fish gelatin and a polyanion. The capsules are typically formed of a coacervate of gelatin and gum arabic, and they encapsulate an internal phase comprising a mixture of a non-polar solvent and charged pigment particles. The gelatin formulations according to various embodiments of the present application, particularly the fish gelatin and gum arabic mixture, are suitable as binders for encapsulated electro-optic media. Moreover, these gelatin binders, when used with a pig gelatin / gum arabic coacervate to encapsulate an internal phase, provide excellent refractive index matching, so that haze is low, for example when incorporated into a transmissive device. In addition, electro-optic media incorporating a mixture of fish gelatin and polyanion as a binder do not suffer from kickback observed in binder compositions with gelatin alone.

[0018] In some embodiments, the binder comprises fish gelatin and polyanion in a weight ratio of 0.5 to 2.0 or more preferably, about equal parts by weight of fish gelatin and polyanion. In some embodiments, the capsules additionally encapsulate a second charged pigment particle. The second charged pigment particle can be oppositely charged from the first charged pigment particle and have a different color. In some embodiments, the binder additionally comprises a pigment or dye. The mixture of non-polar solvent and first charged pigment particle can additionally comprise a charge control agent, and the non-polar solvent can be a mixture of hydrocarbons or limonene (e.g., 1-limonene). The binder of the present application can have a refractive index of 1.47 to 1.57 at 550 nm and 50% relative humidity (RH).

[0019] In another aspect, an electro-optic medium is provided comprising a plurality of capsules in a polymeric binder, each capsule encapsulating a charged pigment particle in a non-polar solvent, wherein the plurality of capsules comprises at least 60% in a size range of 50 pm to 90 pm in diameter, and at least 15% in a size range of 20 pm to 49 pm in diameter. In another aspect, an electro-optic medium is provided comprising a plurality of capsules and a binder, each capsule encapsulating a charged pigment particle in a non-polar solvent, wherein the plurality of capsules comprises at least 90% in a size range of 5 pm to 50 pm in diameter, and the plurality of capsules has an average number diameter of 20 pm to 30 pm. In another aspect, an electro-optic medium is provided comprising a plurality of capsules and a binder, each capsule encapsulating a charged pigment particle in a non-polar solvent, wherein the plurality of capsules comprises less than 90% in a size range of 5 pm to 50 pm in diameter, and the plurality of capsules has an average number diameter of 25 pm to 35 pm.

[0020] In another aspect, a method of forming an electro-optic medium can include providing an internal phase mixture of a non-polar solvent and charged pigment particles, encapsulating a portion of the internal phase mixture in a plurality of capsules, size fractionating the plurality of capsules to separate at least two size distributions, a first size distribution having a diameter between 50 pm and 90 pm, and a second portion having a diameter between 20 pm and 49 pm, and mixing 2 to 5 parts by weight of the first size distribution with 1 part by weight of the second size distribution along with a polymeric binder.

[0021] In another aspect, a binder formulation for a variable transmission film is provided that can include a blend of colored particles to achieve a colorless variable transmission film (i.e., a neutral density film). Generally, the variable transmission film of the present invention includes a first and second light transmissive electrode, an electrophoretic layer, and a polyurethane acrylate based binder disposed between the first and second light transmissive electrodes. The electrophoretic layer can include an encapsulated internal phase that includes charged pigments in a non-polar liquid and a binder that includes a blend of colored particles. The blend of colored particles includes black, cyan, and magenta particles. In most embodiments, the average diameter of the particles in the blend of colored particles is 20 to 100 nm. In some embodiments, the ratio of black to cyan particles can be 10: 1 to 3:2 (black:cyan). In some embodiments, the ratio of black to magenta particles can be 10: 1 to 3:2 (black:magenta). In some embodiments, the blend of colored particles can be 0.1% and 3% (by weight of the binder).

[0022] In another aspect, the present invention provides a variable transmission electrophoretic medium that includes a plurality of capsules in a binder, each capsule having an internal phase that includes a plurality of charged particles in a fluid, the charged particles being movable between an open state and a closed state by application of an electric field, wherein the fluid comprises one or more non-conjugated olefins.

[0023] In another aspect, the present invention provides a variable transmission electrophoretic device that includes a layer of the above-described variable transmission electrophoretic medium disposed between two light transmissive electrodes. Such a variable transmission electrophoretic device can also include at least one light transmissive substrate on the opposite side of one of the light transmissive electrodes from the electro-optic medium; obviously, such a substrate can be provided for each of the light transmissive electrodes. The electro-optic medium according to various embodiments of the present invention can also be incorporated into other kinds of electro-optic devices. For example, a front plane laminate (FPL) can include a light transmissive electrode layer, a binder layer, and the electro-optic medium of the present invention. In some embodiments, the front plane laminate will also include a release sheet or a layer of adhesive or both. The electro-optic medium of the present invention can also be incorporated into an electro-optic display that includes a light transmissive electrode layer, a binder layer, the electro-optic medium of the present invention, and an array of pixel electrodes.

[0024] These and other aspects of the application will become apparent from the following description. BRIEF DESCRIPTION OF DRAWINGS

[0025] The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this application, illustrate embodiments in accordance with the concepts of the present application. In the drawings:

[0026] The drawings of the Figures 1A-1F are schematic cross-sectional views of: (i) a prior art variable transmission display without colorant in the adhesive Figure 1A schematically showing the closed state, Figure 1B schematically showing the open state); (ii) a modification of the prior art display using a very thin adhesive with high coloration Figure 1A and 1B ; and (iii) a variable transmission display of the present application with a relatively thicker colored adhesive Figure 1C and 1D . Figure 1E and 1F .

[0027] Figure 2 is a photomicrograph of the closed state of a prior art display as shown in Figure 1A and shows the poor contrast of such a display.

[0028] Figure 3 is a photomicrograph of the open state of a display as shown in Figure 1D and shows the dark multilayer present and the resulting graininess.

[0029] Figure 4 is a photomicrograph similar to Figure 3 but shows the open state of a display as shown in Figure 1E .

[0030] Figure 3 is a graph showing the amount of light transmitted in the closed state Figure 1F , the amount of light transmitted in the open state Figures 5A-5C , and the resulting contrast Figure 1A for displays with colorless adhesive (as shown in Figure 1E / 1B) and colored adhesive Figure 5A / 1F.

[0031] Figure 5B and 6B are graphs showing the amount of light transmitted in the closed state Figure 5C / 1D Figure 6A and the amount of light transmitted in the open state Figure 1C / 1F Figure 6Aphotographs of the off and on states of a single pixel display of the present application and show the improvement in particles provided by the present application.

[0032] The accompanying Figure 1E is a graph showing the contrast achieved using encapsulated electrophoretic media with OLOA and various other charge control agents.

[0033] Figure 6B is similar to Figure 7 but shows the contrast achieved using OLOA and different ratios of 2-HDA or Control CCA.

[0034] Figure 8 is similar to Figure 7 but shows the minimum haze values rather than contrast.

[0035] Figure 9 is a graph showing the minimum on state transmittance, minimum off state transmittance, minimum on state haze, and contrast of encapsulated electrophoretic media using OLOA / 2-HDA and OLOA / Span 85 charge control agents at varying drive voltages, respectively.

[0036] Figure 8 is similar to Figures 10A-10D but shows the contrast achieved by encapsulated electrophoretic media using OLOA / 2-HDA and OLOA / Oleic Acid charge control agents at varying ratios and at various drive voltages.

[0037] Figure 11 is an illustration of an electro-optic display comprising two types of charged particles. The particles can be moved toward (away from) the viewer by the application of an electric field.

[0038] Figure 10D is an illustration of a variable transmission device comprising a first light-transmissive electrode layer and a second light-transmissive electrode layer and an electro-optic medium disposed between the layers. The particles can be moved against the capsule wall by the application of an electric field, thereby allowing light to pass through the medium.

[0039] Figure 12A shows the haze measurements of variable transmission samples of various fish gelatin binder compositions. The left panel samples contain a binder without gum arabic, the middle panel samples contain a binder with 33% (wt. / wt.) gum arabic, and the right panel samples contain a binder with an equal portion of fish gelatin and gum arabic.

[0040] Figure 12BContrast measurements of variable transmission samples of various fish gelatin binder compositions are shown. The left panel sample contains a binder without gum arabic, the middle panel sample contains a binder with 33% (wt. / wt.) gum arabic; and the right panel sample contains a binder of equal parts fish gelatin and gum arabic.

[0041] Figure 13A Recoil measurements of variable transmission samples of various fish gelatin binder compositions are shown. The left panel sample contains a binder without gum arabic, the middle panel sample contains a binder with 33% (wt. / wt.) gum arabic; and the right panel sample contains a binder of equal parts fish gelatin and gum arabic.

[0042] Figure 13B Attenuation of the off state for samples with fish gelatin as the binder only compared to samples with equal parts fish gelatin and gum arabic is shown. In testing, the 1 : 1 mixture of fish gelatin and gum arabic had a significantly reduced recoil, resulting in stable on and off states for weeks.

[0043] Figure 13C is a flow chart depicting a method for making electro-optic media having a predetermined distribution of cell sizes.

[0044] Figure 14 Differences in on state transmittance for a sample of a variable transmission device with only small cells (left) and for a variable transmission device with a blend of large and small cells (right) are shown.

[0045] Figure 15 Differences in off state transmittance for a sample of a variable transmission device with only small cells (left) and for a variable transmission device with a blend of large and small cells (right) are shown.

[0046] Figure 16A Differences in on state haze for a sample of a variable transmission device with only small cells (left) and for a variable transmission device with a blend of large and small cells (right) are shown.

[0047] Figure 16B Contrast (on transmittance / off transmittance) for a sample of a variable transmission device with only small cells (left) and for a variable transmission device with a blend of large and small cells (right) is shown. The blend of large and small cells has a much greater contrast between the on and off states.

[0048] Figure 16C Images of variable transmission devices including only small cells (left) and a blend of large and small cells (right) are shown. Figure 16D It is shown that there is little change in the particles between the two different cell size distributions.

[0049] Figure 17 The visible absorption spectrum of various binder-pigment formulations is shown. As Figure 17 shown, a combination of 3 parts cyan, 3 parts magenta, and 14 parts black produces a relatively uniform visible light absorption spectrum.

[0050] Figure 18 Contrast measurements are shown for a binder with 3 parts cyan, 3 parts magenta, and 14 parts black (left) and a binder with only carbon black colorant (right). Although the black-only binder has better contrast, the contrast of the colored blend binder is sufficient for most variable transmission applications.

[0051] Figure 18 Haze measurements are shown for a binder with 3 parts cyan, 3 parts magenta, and 13 parts black (left) and a binder with only carbon black colorant (right). The haze of both formulations is very low.

[0052] Figure 19 is a cross-sectional side view of a variable transmission film with a transparent binder layer.

[0053] Figure 20 and 22B are cross-sectional side views of an electrophoretic medium in combination with a transparent binder in the closed and open states, respectively.

[0054] Figure 21 and 23B are cross-sectional side views of an electrophoretic medium in combination with a colored binder in the closed and open states, respectively.

[0055] Figure 22A is a photograph of a variable transmission assembly including a transparent binder layer in the open state.

[0056] Figure 23A is a photograph of a variable transmission assembly including a colored binder layer in the open state according to an embodiment of the present application.

[0057] Figure 24A is a graph showing the minimum closed state transmittance, minimum open state transmittance, contrast, and minimum open state haze of encapsulated electrophoretic media according to various embodiments of the present application.

[0058] Figure 24B is a graph showing the minimum closed state transmittance, minimum open state transmittance, contrast, and minimum open state haze of encapsulated electrophoretic media containing capsules of various sizes according to various embodiments of the present application. DETAILED DESCRIPTION

[0059] Electrophoretic displays, such as e-readers, are typically opaque and operate in a reflective mode. This functionality is shown in Figures 25A to 25D , where the reflectivity of light impinging on the viewing surface is modulated by moving black or white charged particles toward the viewing surface with a suitable voltage. However, electrophoretic devices can also be operated in a so-called "shutter mode" as shown in Figures 26A to 26L , where one operating state is substantially opaque and the other is transmissive to light. When this "shutter mode" electrophoretic device is constructed on a transparent substrate, the transmission of light through the device can be modulated. One potential use of a shutter mode electrophoretic medium is a window with variable light transmission.

[0060] Figure 12A and 12B Devices include an electro-optic medium composed of capsules in a polymeric binder. The capsules contain charged pigment particles that move in response to an electric field. The capsules are typically formed of a gelatin material described in more detail below. The electro-optic medium is distributed between first and second electrode layers, which can be made of known materials such as indium tin oxide (ITO) coated polyethylene terephthalate (PET). Alternatively, the electrode layers can include metal electrodes, which can be arranged as pixels. The pixels can be controllable as an active matrix, allowing text and pictures to be displayed. There is typically an additional adhesive layer between the electro-optic medium and one of the electrode layers. The adhesive layer can be ultraviolet curable and typically improves the flatness of the final device by "filling" deviations created by the capsules. Suitable adhesive formulations are described in U.S. Patent 2017 / 0022403, which is incorporated herein by reference.

[0061] When a DC field is applied to the device of Figure 12B , the dark or light particles move toward the viewing surface, changing the optical state from dark to light. In Figure 12A , when an alternating electric field is applied to one of the electrodes, the charged pigment particles are driven to the wall of the capsule, creating an aperture through the capsule for light transmission, i.e., an open state. In both embodiments, because the solvent is non-polar and contains a charge control agent and / or a stabilizer, the optical state (black / white; open / closed) can be maintained for long periods of time (weeks) without the need to maintain an electric field. As a result, the device can be "switched" only a few times a day and consumes very little power.

[0062] The term "gray state" is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optically different states, but not necessarily equally intermediate the optically bright and dark states. For example, gray states can be intermediate white and black, white and light gray, black and dark gray, or black and white. The term "gray state" can be used in this document to refer to a state intermediate two extreme optical states of a display, even if the display is monochrome. For example, a display having a gray state between "on" and "off" is discussed below. It should be understood that an optical state different from the two extreme states of the display is addressed. The term "black" and "white" or "off and "on" can be used herein to refer to the two extreme optical states (also called the "state contrast") of a display even where these states are not literally black and white. For example, a display that is monochrome black and white can have a state contrast of a dark shade of gray to a light shade of gray. Similarly, a display that is monochrome yellow and blue can have a state contrast of a dark shade of yellow to a light shade of blue. The terms "color" and "monochrome" can be used herein to refer to the number of extreme optical states of a display. For example, a display that is monochrome black and white has two extreme optical states and is therefore a monochrome display. A display that is monochrome yellow and blue has two extreme optical states and is therefore a monochrome display. A display that is monochrome red, green, and blue has three extreme optical states and is therefore a monochrome display. A display that is monochrome red, green, blue, yellow, cyan, and magenta has six extreme optical states and is therefore a monochrome display. A display that is monochrome red, green, blue, yellow, cyan, magenta, white, and black has eight extreme optical states and is therefore a monochrome display. A display that is monochrome red, green, blue, yellow, cyan, magenta, white, black, orange, and purple has ten extreme optical states and is therefore a monochrome display. A display that is monochrome red, green, blue, yellow, cyan, magenta, white, black, orange, purple, pink, and brown has twelve extreme optical states and is therefore a monochrome display. A display that is monochrome red, green, blue, yellow, cyan, magenta, white, black, orange, purple, pink, brown, olive, and navy has fourteen extreme optical states and is therefore a monochrome display. A display that is monochrome red, green, blue, yellow, cyan, magenta, white, black, orange, purple, pink, brown, olive, navy, lime, and maroon has sixteen extreme optical states and is therefore a monochrome display. A display that is monochrome red, green, blue, yellow, cyan, magenta, white, black, orange, purple, pink, brown, olive, navy, lime, maroon, gray, and aqua has eighteen extreme optical states and is therefore a monochrome display. A display that is monochrome red, green, blue, yellow, cyan, magenta, white, black, orange, purple, pink, brown, olive, navy, lime, maroon, gray, aqua, peach, and lavender has twenty extreme optical states and is therefore a monochrome display. A display that is monochrome red, green, blue, yellow, cyan, magenta, white, black, orange, purple, pink, brown, olive, navy, lime, maroon, gray, aqua, peach, lavender, mint, and plum has twenty-two extreme optical states and is therefore a monochrome display. A display that is monochrome red, green, blue, yellow, cyan, magenta, white, black, orange, purple, pink, brown, olive, navy, lime, maroon, gray, aqua, peach, lavender, mint, plum, salmon, and

[0063] The terms "bistable" and "bistability" are used herein in their conventional meaning in the art to refer to displays comprising 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 to assume its first or second display state using an addressing pulse of finite duration followed by a non-addressing period, the state will persist indefinitely. Some particle-based electrophoretic displays are capable of stable gray states in which the display element is between its first and second states, and the display element is stable in this gray state as long as it is not addressed selected for a change. An example of such a display is given in U.S. Pat. No. 7,170,670, which is incorporated by reference. Such a bistable display is described generally in U.S. Pat. No. 6,130,773 issued to Sheridon, et al. on Oct. 17, 2000, and U.S. Pat. No. 6,525,355 issued to Johnson on May 6, 2003, both of which are incorporated by reference. The term "bistable" and "bistability" can be used herein to refer to displays that are not only stable in their extreme states but also stable in intermediate states, as is the case for some electrophoretic displays capable of true gray states.

[0064] One type of electro-optic display that has been the subject of intense research and development for many years is the electrophoretic display, in which a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. However, these displays are typically considered to be slow, as they can take a long time to change state. For the reasons discussed above, the use of electrophoretic displays in applications such as flat-panel displays, e-book readers and electronic shelf labels has been the subject of intense research and development for many years. Such displays are often driven by a method known as "binary drive", in which the display is switched between only two states, namely a first state in which the display is white or light-colored and a second state in which the display is dark or black. The display is switched between these two states by applying a first voltage between the electrodes of the display, which causes the charged particles to move to one of the electrodes, and a second voltage, which causes the charged particles to move to the other of the electrodes. The display is then left in one of these states until it is desired to change the state of the display, at which point the process is repeated. The time taken for the charged particles to move to the desired electrode is typically in the order of seconds, and the time taken for the charged particles to move to the other electrode is also typically in the order of seconds. Thus, the time taken to change the state of the display is typically in the order of 10 seconds. This is a relatively long time, and it is desirable to reduce this time.

[0065] As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be produced using gaseous fluids; see for example Kitamura, T. et al., "Electronic paper based on electrophoretic absorption of titanium beads", Nature, 402, 29 August 2001, and Yamaguchi, Y. et al., "Toner display using insulative particles charged triboelectrically", IDW Japan, 2001, Paper AMD4-4). See also US Patents Nos. 7,321,459 and 7,236,291. When such gas-based electrophoretic media are used in orientations that allow sedimentation of the particles, such as in signs where the medium is arranged in a vertical plane, such gas-based electrophoretic media suffer from the same problems as liquid-based electrophoretic media due to the same particle sedimentation. In fact, the problem of particle sedimentation is more severe in gas-based electrophoretic media than in liquid-based electrophoretic media, because the viscosity of gaseous suspending fluids is lower than that of liquids, thus allowing faster sedimentation of the electrophoretic particles.

[0066] Many patents and applications assigned to, or in the names of, the Massachusetts Institute of Technology (MIT), E Ink Corporation, E Ink California, LLC, and related companies describe various technologies for encapsulated and microencapsulated electrophoretic and other electroluminescent media. Encapsulated electrophoretic media comprise a plurality of small capsules, each of which itself comprises an inner phase and a capsule wall surrounding the inner phase, wherein the inner phase comprises electrophoretically mobile particles in a fluid medium. Typically, the capsules themselves are held in a polymer binder to form a coherent layer positioned between two electrodes. In microencapsulated electrophoretic displays, the charged particles and fluid are not encapsulated within microcapsules, but are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. The technologies described in these patents and applications include:

[0067] (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; 6,721,083; 6,727,881; 6,822,782; 6,831,771; 6,870,661; 6,927,892; 6,956,690; 6,958,849; 7,002,728; 7,038,655; 7,052,766; 7,110,162; 7,113,323; 7,141,688; 7,142,351; 7,170,670; 7,180,649; 7,226,550; 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; 7,580,180; 7,679,814; 7,746,544; 7,767,112; 7,848,006; 7,903,319; 7,951,938; 8,018,640; 8,115,729; 8,119,802; 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 / 0151029; 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 / 0011913; 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.

[0068] (b) Encapsulants, binders and packaging processes; see for example U.S. Patents 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; 7,202,991; 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;

[0069] (c) Microcell structures, wall materials, and methods of forming microcells; see for example U.S. Patents Nos. 7,072,095 and 9,279,906;

[0070] (d) Methods for filling and sealing microcells; see for example U.S. Patents Nos. 7,144,942 and 7,715,088;

[0071] (e) Films and sub-assemblies containing electro-optic materials; see for example U.S. Patents Nos. 6,982,178 and 7,839,564;

[0072] (f) Backlights, adhesive layers and other auxiliary layers and methods for use in displays; see for example U.S. Patents Nos. 7,116,318 and 7,535,624;

[0073] (g) Color formation and color adjustment; see for example U.S. Patents Nos. 7,075,502 and 7,839,564;

[0074] (h) Methods for driving displays; see for example U.S. Patents Nos. 7,012,600 and 7,453,445; and

[0075] (i) Applications of displays; see for example U.S. Patents Nos. 7,312,784 and 8,009,348.

[0076] Many of the foregoing patents and applications recognize that the walls surrounding discrete microcapsules in encapsulated electrophoretic media can be replaced by a continuous phase, thereby creating so-called "polymer-dispersed electrophoretic displays," in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymer material, and the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display can be considered capsules or microcapsules, even though no discrete capsule membranes are associated with each individual droplet; see for example the aforementioned U.S. Patents Nos. 6,866,760 and 7,079,305. Accordingly, for the purposes of the present application, such a polymer-dispersed electrophoretic medium is considered to be a sub-class of an encapsulated electrophoretic medium.

[0077] Although electrophoretic media are often opaque (as, for example, in many electrophoretic media, the particles block most or almost all of the visible wavelengths of radiation), and operate in a reflective mode, many electrophoretic displays can be made to operate in so-called "shutter mode," in which one display state is substantially opaque and the other is light-transmissive. See for example U.S. Patents Nos. 5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, similar to electrophoretic displays but relying on changes in the electrical field strength rather than particle charge, can operate in similar modes; see U.S. Patent No. 4,418,346. Other types of electro-optic displays are also capable of operating in shutter mode. An electro-optic medium operating in shutter mode can be used in a multilayer construction for a full-color display; in such a construction, at least one layer, adjacent the viewing surface of the display, operates in shutter mode to expose or hide a second layer further from the viewing surface.

[0078] Charged pigment particles can have a variety of colors and compositions. Additionally, 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 by reference herein in its entirety. For example, if the charged particles are white, they can be formed from inorganic pigments such as Ti02, Zr02, ZnO, AI2O3, Sb203, BaS04, PbS04, etc. They can also be polymeric particles with high refractive index (>1.5) and of a certain size (>100 nm) to show white color, or composite particles engineered to have a desired refractive index. Black charged particles can be formed from CI Pigment Black 26 or 28 or similar (e.g., iron manganese black or copper chromium black) or carbon black. Other colors (non-white and non-black) can be formed from organic pigments such as CI Pigment PR 254, PR 122, PR 149, PG 36, PG 58, PG 7, PB 28, PB 15:3, PY 83, PY 138, PY 150, PY 155, or PY 20. 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 4100HD, and Irgazin Red L 3660HD; Sun Chemical phthalocyanine blue, phthalocyanine green, aniline yellow, or AAOT aniline yellow. Colored particles can also be formed from inorganic pigments such as CI Pigment Blue 28, CI Pigment Green 50, CI Pigment Yellow 227, etc. The surface of the charged particles can be modified by known techniques based on the charge polarity and charge level of the desired particle, 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 by reference herein in their entirety.

[0079] The particles can exhibit a natural charge, or can be explicitly charged using a charge control agent, or can acquire a charge when suspended in a solvent or solvent mixture. Suitable charge control agents are well known in the art; they can be polymeric or non-polymeric in nature, or can be ionic or non-ionic. Examples of charge control agents can include, but are not limited to, Solsperse 17000 (active polymeric dispersant), Solsperse 9000 (active polymeric dispersant), (amberlyst® succinimide ashless dispersant), Unithox 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 suspension and the charged pigment particles, the internal phase can also include stabilizing agents, surfactants, and charge control agents. When the charged pigment particles are dispersed in a solvent, a stabilizing material can be adsorbed onto the charged pigment particles. This stabilizing material separates the particles from one another, such that the variable transmission medium is substantially non-transmissive when the particles are in their dispersed state.

[0080] 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 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 the solvent. In the present invention, it is preferred that the non-polar tail group be a saturated or unsaturated hydrocarbon moiety, or another group that is soluble in a hydrocarbon solvent, such as a poly(dialkylsiloxane). The polar group can be any polar organic functionality, including ionic materials such as ammonium, sulfonate, or phosphonate salts, or acidic or basic groups. A particularly preferred head group is a carboxylic acid or carboxylate group. Suitable stabilizing agents for use in the present invention include polyisobutylene and polystyrene. In some embodiments, a dispersant is added, such as polyisobutylene succinimide and / or sorbitan trioleate and / or 2-hexyldecyl acid.

[0081] The fluid used in the variable transmission medium of the present invention will generally have a low dielectric constant (preferably less than 10, and desirably less than 3). The fluid is preferably a solvent with low viscosity, relatively high refractive index, low cost, low reactivity, and low vapor pressure / high boiling point. Examples of solvents include, but are not limited to, aliphatic hydrocarbons such as heptane, octane, and petroleum distillates such as (Exxon Mobil) or (Total); terpenes, such as limonene, e.g., d-limonene; and aromatic hydrocarbons, e.g., toluene. A particularly preferred solvent is limonene because it combines a low dielectric constant (2.3) and a relatively high refractive index (1.47). The refractive index of the internal phase can be modified by the addition of a refractive index matching agent. For example, U.S. Patent No. 7,679,814, described above, describes an electrophoretic medium suitable for use in a variable transmission device in which the fluid surrounding the electrophoretic particles comprises a mixture of a partially hydrogenated aromatic hydrocarbon and a terpene, with a preferred mixture being d-limonene and partially hydrogenated terphenyl, available from Cargille-Sacher Laboratories, 55 Commerce Rd, Cedar Grove N.J. 07009 under the trade designation "Cargille 1.54". The partially hydrogenated terphenyl is a mixture of 2,2',3,3'-tetrahydroterphenyl and 2,3,2',3'-tetrahydroterphenyl. The partially hydrogenated terphenyl has a refractive index of 1.54 at 25°C and a dielectric constant of 2.3 at 25°C. The partially hydrogenated terphenyl is available from Cargille-Sacher Laboratories, 55 Commerce Rd, Cedar Grove N.J. 07009 under the trade designation "Cargille 1.54". The partially hydrogenated terphenyl is a mixture of 2,2',3,3'-tetrahydroterphenyl and 2,3,2',3'-tetrahydroterphenyl. The partially hydrogenated terphenyl has a refractive index of 1.54 at 25°C and a dielectric constant of 2.3 at 25°C. 5040available from Cargille-Sacher Laboratories, 55 Commerce Rd, Cedar Grove N.J. 07009 under the trade designation "Cargille 1.54". In the encapsulated media prepared according to various embodiments of the present application, it is preferred that the refractive index of the encapsulated dispersion be as closely matched as possible to the encapsulating material to reduce haze. In most cases, it is advantageous to have an internal phase with a refractive index at 550 nm between 1.51 and 1.57, preferably about 1.54 refractive index at 550 nm.

[0082] In preferred embodiments of the present application, the encapsulated fluid can comprise one or more non-conjugated olefins, preferably cyclic hydrocarbons. Examples of non-conjugated olefins include, but are not limited to, terpenes, e.g., limonene; phenylcyclohexanes; hexyl benzoate; cyclododecatriene; 1,5-dimethyltetralin; partially hydrogenated terphenyls, e.g., phenylmethylsiloxane oligomers; and combinations thereof. The most preferred composition for the encapsulating fluid according to embodiments of the present application comprises cyclododecatriene and partially hydrogenated terphenyl.

[0083] It has been previously suggested that the best approach for a variable transmission electrophoretic medium is a polymer dispersed electrophoretic medium with a wide size distribution of droplets; such a wide size distribution should result in low granularity due to the good uniformity of the electrophoretic medium layer and the presence of very small droplets (often referred to as "fines") to fill the gaps between the larger droplets. Previous attempts to use microcapsule media in variable transmission devices have suffered from excessive granularity (local variations in optical density) and higher minimum transmission values (i.e., relatively leaky blackout states) making the media useless in certain commercial applications such as automotive sunroofs. However, polymer dispersed electrophoretic media have their own problems, including unpredictable gelation of the continuous phase with parameters such as shear and temperature (which greatly complicates the production process of the medium), and the use of a skim coat to reduce the likelihood of droplet breakage. Capsules are more resistant to breakage than droplets in a polymer dispersed medium, which allows a capsule-based medium to use certain coating techniques (e.g., spray coating) that are not possible with a polymer dispersed medium. A capsule-based medium does not require a skim coat to prevent capsule breakage, and the capsules can be size separated by techniques such as sieving, allowing the production of a medium with a restricted range of capsule sizes in a way that is difficult or impossible with a polymer dispersed medium.

[0084] However, as previously mentioned, capsule-based electrophoretic media tend to suffer from undesirably high transmission in the off state and / or excessive granularity. For example, at low coating weights, a capsule-based medium has a high transmission through pinholes or voids (the gaps between adjacent capsules), while at higher coating weights, excessive granularity and poor contrast can become a serious problem. Even with non-sized capsules, the capsule walls (especially those of the small capsules) increase the transmission in the off state and reduce the possible contrast in a capsule-based variable transmission device. Even at high operating voltages (e.g., 120 V), a capsule-based medium can suffer from insufficient contrast (the ratio of maximum on transmission to minimum off transmission). It is now found that the above-mentioned disadvantages of a capsule-based variable transmission device can be reduced, even substantially eliminated, by using a variable transmission electrophoretic medium made in accordance with various embodiments of the present application.

[0085] According to a first embodiment of the present application, by providing a variable transmission electrophoretic medium comprising a plurality of capsules and a binder, in which the light absorbance of the binder (continuous phase) is approximately equal to the light absorbance of the internal phase within the capsules in the off state, the undesirable high transmission and / or excessive granularity can be reduced or substantially eliminated. The binder used in the capsule-based electrophoretic medium is typically polymeric and substantially transparent, and the thickness typically used in such media is on the order of at least 50 μιη. In order to produce the necessary light absorbance in the binder, a dye or pigment (collectively referred to as "colorant") can be used to add color to the binder. In order to minimize the haze, a dye or a pigment that is finely dispersed should be used as the colorant. However, since variable transmission devices such as windows and sunroofs are exposed to sunlight for long periods of time, which has a significant proportion of near-ultraviolet radiation, pigments are generally preferred as the colorant since dyes are susceptible to photobleaching or heat bleaching. The presently preferred pigment is carbon black, which has a desirable neutral color tone and excellent light stability. However, carbon black does have a strong tendency to agglomerate (almost all commercial carbon black is an agglomeration of small particles), so it is necessary to add a dispersant or surfactant to the carbon black to keep it at a particle size that does not excessively scatter light. Furthermore, it is critical to maintain good dispersion of the carbon black in the electrophoretic medium, since carbon black is highly conductive, and the formation of agglomerates or filaments of the particles can cause electrical shorts between the electrodes or at least increase the electrical conductivity of the binder to an undesirable degree. Finally, the concentration of carbon black in the binder must be kept at a low level to prevent the electric field between most of the electrodes from passing through the binder and around the capsules, thereby degrading the electro-optical performance of the capsules. Although a low molecular weight surfactant can be used to assist in the dispersion of the carbon black, such a surfactant tends to migrate away from the interface between the carbon black and the surrounding phase, and the migration of such a surfactant can cause various problems. Therefore, it is preferred to use a polymeric surfactant, such as a poloxamer dispersant such as Kolliphor P188 (available from BASF) has been found to work well with the preferred carbon black sold by Cabot Corporation as Emperor 2000 carbon black in a liquid gelatin such as Hipure liquid gelatin sold by Norland Products or a blend of liquid gelatin and gum arabic.

[0086] In preferred embodiments of the application, the colorant can comprise a blend of colored particles. The blend of colored particles can include black, cyan and magenta particles. In most embodiments, the average diameter of the particles in the blend of colored particles can be between 20 and 100 nm. In some embodiments, the ratio of black to cyan particles can be between 10:1 and 3:2 (black:cyan). In some embodiments, the ratio of black to magenta particles can be between 10:1 and 3:2 (black:magenta). In some embodiments, the blend of colored particles can be present at 0.1% to 3% (by weight of the binder). In some embodiments, the binder comprises about 3 parts cyan, 3 parts magenta, 14 parts carbon black, for a total color mixture of about 2% of the mass of the binder. This binder mixture has been found to work well when the internal phase is encapsulated in a coacervate of (porcine) gelatin and gum arabic. The resulting electro-optic medium has low haze when used in a transmissive device, and less backflash, and thus higher long-term state stability.

[0087] For the reasons explained below with reference to the accompanying drawings, it is not desirable for the variable transmission medium of the present application to comprise a thin layer of a heavily colored binder; a relatively thicker layer of a less heavily colored binder achieves the best results. Indeed, it has been found desirable for the medium of the present application to comprise a higher proportion of binder than most prior art encapsulated electrophoretic media; there should be at least 1 part by weight of binder for every 15 parts by weight of capsules, and optionally at most 1 part by weight of binder for every 4 parts by weight of capsules. This relatively high proportion of binder minimizes transmission of light through pinholes and voids in the capsule layer. Furthermore, the coating weight used should be such that at least a single layer of capsules, or slightly more, is deposited on the substrate used for coating.

[0088] Figure 12A and 1B are schematic cross-sections through the closed and open states, respectively, of a prior art capsule-based electrophoretic medium in which the binder is not colored. It can be seen that, due to the presence of pinholes (as shown) or voids, the transmission of the medium in the closed state is increased, here to substantially no blocking of transmission of light through the medium even in the closed state. The "leakage" of light through the closed state of the medium greatly reduces its contrast. In practice, even if the coating weight is increased from the standard 20 g / m 2 to about 24 g / m 2 , the presence of such voids and pinholes is problematic because they limit the minimum transmission in the closed state to more than 3%, which is too high for an automotive sunroof.

[0089] Figure 12B and 1D are schematic cross-sections of a capsule-based electrophoretic medium, similar to Figure 1A and 1B, the capsule-based electrophoretic medium includes a small amount of a heavily colored binder. The presence of this colored binder greatly reduces the impact of pinholes and voids in the off state Figure 1C ), thus improving the contrast of the medium compared to the media of Figure 1A and 1B . However, in the on state Figure 1C ), the heavily colored binder causes undesirably high granularity because the heavily colored binder tends to over-emphasize the capsule walls in the multi-layered portion of the display, with reference to Figure 1A .

[0090] Figure 1D and 1F are schematic cross-sectional views of preferred capsule-based electrophoretic media of the present application, which are similar to Figure 1D and 1B , respectively, using a less colored but more abundant binder than in Figure 1E and 1D , so that in the off state the absorbance of the binder substantially matches the absorbance of the internal phase. As can be seen, the presence of this less colored binder has substantially the same effect Figure 1A ) in reducing the impact of pinholes and voids in the off state, thus improving the contrast of the medium compared to the media of Figure 1C and Figure 1E , as in Figure 1A the heavily colored but thinner binder. However, compared to Figure 1B , the granularity Figure 1C ) in the on state is greatly reduced because there is no over-emphasis of the capsule walls in the multi-layered portion of the display with reference to Figure 1D .

[0091] According to a second embodiment of the present application, the contrast deficiency of a variable transmission electrophoretic medium can be improved by including a charge control agent comprising a branched aliphatic acid having at least about 8 carbon atoms and an oligoamine-terminated polyolefin, such as Encapsulation of the electrophoretic medium containing the charge control agent results in a capsule-based medium with significantly improved contrast, even at reduced operating voltages (and thus reduced power consumption), and low haze in the on state, which is of course important for variable transmission devices such as windows and sunroofs. The aliphatic acid used is preferably a water-insoluble, oil-soluble acid.

[0092] According to a third embodiment of the present application, an improved electro-optic medium is provided that includes encapsulated pigment particles and a binder. In particular, it has been discovered that a mixture of fish gelatin and a polyanion, such as gum arabic, is an excellent binder to use with capsules formed from a coacervate of (pig) gelatin and gum arabic. Polyanions that can be included with the fish gelatin in the binder include, but are not limited to, carbohydrate polymers such as starches and cellulose derivatives, plant extracts such as gum arabic, and polysaccharides such as alginate; proteins such as gelatin or whey proteins; lipids such as waxes or phospholipids; and combinations thereof. This result is surprising because neither fish gelatin nor a polyanion such as gum arabic can be used alone as a binder material for electro-optic media. As described below, fish gelatin alone has unacceptable recoil, while a polyanion such as gum arabic shrinks and cracks when conditioned after coating. The resulting electro-optic medium has low haze when used in a transmissive device and less recoil, and thus has higher long-term state stability. The electro-optic medium can be coated on a larger surface and laminated with electrodes and the like to create various electro-optic devices, including daylight readable displays and smart windows.

[0093] Gelatin-based capsule walls for use in variable transmission devices have been described in many of the E Ink and MIT patents and applications described above. Gelatin is available from various commercial suppliers, such as Sigma Aldrich or Gelitia USA. It is available in various grades and purities as needed for the application. Gelatin primarily comprises collagen that has been collected from animal products (cattle, swine, poultry, fish) and hydrolyzed. It comprises a mixture of peptides and proteins. In many of the embodiments described herein, gelatin is combined with gum arabic (acacia gum) that is derived from hardened sap of the acacia tree. Gum arabic is a complex mixture of glycoproteins and polysaccharides, and is often used as a stabilizer in foods. The pH of aqueous solutions of gum arabic and gelatin can be adjusted to form a polymer-rich coacervate phase that can encapsulate droplets of a non-polar internal phase, as described below.

[0094] Gelatin / gum arabic containing capsules can be prepared as follows; see, for example, U.S. Patent No. 7,170,670, which is incorporated by reference in its entirety. In this process, a water soluble mixture of gelatin and / or gum arabic is emulsified with a hydrocarbon internal phase (or other water immiscible phase desired to be encapsulated) to encapsulate the internal phase. The solution can be heated to 40°C prior to emulsification to dissolve the gelatin. After the desired droplet size distribution is achieved, the pH is typically lowered to form a coacervate. The capsules are formed under controlled cooling and mixing (typically to room temperature or lower) of the emulsion. If the wetting and diffusion conditions are correct (which is largely dependent on the internal phase composition), proper mixing and certain encapsulation formulations (e.g. gelatin, gum arabic concentration and pH) can be achieved to discretely gel the coacervate around the internal phase droplets in a uniform manner. The capsules produced by this process are 20-100 μιη, typically more than 50% of the original material is incorporated into the usable capsules. The resulting capsules are then separated by size by sieving or other size exclusion sorting. Capsules larger than 100 μιη are typically excluded as they are visible to the naked eye, and larger capsules increase the gap between electrodes, thereby increasing the necessary drive voltage.

[0095] Surprisingly, according to the fourth embodiment of the present application, it has been found that the addition of larger capsules can improve the viewing experience through the open state of the variable transmission device. This result is unexpected as traditionally, variable transmission devices with a dominant population of capsules in the 50 μιη and 90 μιη range are "grainy" due to the inconsistent packing of the capsules. It has been found that by intentionally sizing the capsules during the manufacture of the electro-optic medium, and then intentionally combining the selected size distributions to obtain a particular size distribution ratio, a variable transmission device with good contrast and low grain can be obtained. When viewing a light source through the device, the resulting variable transmission device has low grain, good contrast, and a reduced diffraction pattern.

[0096] The process for preparing the electro-optic medium of the present application according to the fourth embodiment of the present application can be seen in Figure 1F where the process begins with the preparation of capsules comprising charged pigment particles, as described above. The resulting encapsulated electrophoretic medium is then size fractionated according to size using a two sieve process (typically Sieve A and Sieve B in Figure 1F Although three different size exclusions are shown, it will be appreciated that additional size exclusions can be performed, however, it can only be necessary to separate the capsules into two size distributions. The size distributions are then recombined with the desired ratio (by weight) to a polymeric binder (e.g. gelatin) to form an electro-optic medium that can be coated onto a light transmissive electrode layer. The device of the present application can include between two and five parts by weight of larger capsules for each part by weight of smaller capsules. In some embodiments, there can be about three parts by weight of larger capsules for each part by weight of smaller capsules.

[0097] After size sorting, the capsules are mixed with a binder to produce a slurry for coating (e.g., using slot coating, doctor blade coating, spin coating, etc.). In embodiments of the present application, the binder comprises gelatin, typically fish gelatin. In preferred embodiments, the gelatin is mixed with gum arabic, but it has been found that the mixture should not be complexed into a coacervate, as it is more difficult to maintain slurry uniformity. In addition, it has been found that the haze of the transmissive medium can be improved by varying the amount of gum arabic added to the binder mixture.

[0098] According to various embodiments of the present application, in order to improve the off-axis transparency of a display comprising an electrophoretic medium layer, it can be advantageous to keep the layer as thin as possible, thereby reducing the size of any particle structures that traverse the thickness of the electrophoretic layer; however, as noted above, a thin electrophoretic layer requires a corresponding increase in the volume fraction of electrophoretic particles to achieve sufficient opacity in the off state of the display. Thus, for any given selection of materials for the light modulator, the electrophoretic layer can have an optimal 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 so that the particle structure occupies only a portion of the sidewall adjacent one major surface of the electrophoretic medium layer. Such a particle structure can be produced according to the DC / AC driving method of the present application by first positioning all of the particles within the droplet adjacent one major surface of the electrophoretic layer by applying a DC field to the electrophoretic layer, and then driving the particles to the sidewall using an AC field of appropriate frequency.

[0099] As previously noted, the variable transmission film of the present application can include a first light transmissive electrode and a second light transmissive electrode, with an electrophoretic layer and a UV-cured binder (e.g., a polyurethane acrylate blend) disposed between the first and second light transmissive electrodes, such as the variable transmission film shown in Figure 15 According to yet another embodiment of the present application, a non-scattering colorant can be included in the binder layer of the variable transmission film. Coloring the binder of the film can further reduce particulates and pinholes. Adding a non-scattering colorant (e.g., a dye or a very small (<100 nm) well-dispersed pigment) to the UV-cured binder can fill pinholes in the capsule layer, with the result being a reduction in light transmission through the pinholes. The colored UV binder can also significantly improve the particulates of the variable transmission film. Examples of colorants include, but are not limited to, Macrolex Black 2B, Keyplast Black AN, Macrolex Violet 3R, Macrolex Blue 3R, Orasol Black X51, and Orasol Black X55.

[0100] The adhesive composition according to various embodiments of the present application preferably comprises 0.25 to 0.5 phr of one or more colorants. The adhesive is not limited to being colored with a single material. Blends of pigments and / or dyes can be used to improve lightfastness and high temperature resistance. Depending on the application of the variable transmission film, the color of the adhesive can be adjusted by selecting a colored dye / pigment without having to change the underlying capsule layer.

[0101] While not wishing to be bound by theory, it is possible that certain speckles are caused by variations in the thickness of the capsule layer and the use of a UV-cured adhesive with 100% solids content and effective planarization. The variable thickness of the colored adhesive layer can effectively complement the shape of the capsule layer. For example, the thickest portion of the colored adhesive will be located near the location of the pinholes in the capsule layer, providing maximum coloration where it is most needed (see Figures 23A and 23B). For example, the addition of a black dye, such as Keyplast Black AN, which is soluble in the adhesive, can reduce the speckles (cqi score) in the open state and the pinholes in the closed state without increasing the haze in the open state. Because the UV-curable adhesive layer cannot change its transmission during switching of the device, the colorant loading is preferably selected so that the transmission of the adhesive layer is between the transmission of the capsule layer in the open and closed states. For example, if a device with a capsule layer of about 1 mil has a closed state transmission of about 1% and an open state transmission of about 30%, the adhesive layer should be colored to a thickness of about 1 mil to have a transmission between 1 and 30% to complement the active capsule layer. Figure 15 、 22B

[0102] Example

[0103] An example is now presented, although only by way of illustration, to show the details of an electrophoretic medium prepared according to various embodiments of the present application.

[0104] Figure 21

[0105] Example 1

[0106] By combining 1 - limonene, The non-aqueous internal phase was prepared by combining the immersion liquid, carbon black, polystyrene, and sorbitan trioleate. The resulting mixture was encapsulated by adding the mixture to the gelatin / gum arabic water solution with stirring. After the internal phase addition was complete, the mixture was emulsified. After a period of mixing, heating, and pH adjustment, the mixture was cooled and the resulting capsules were sieved to a range of 20-60 μm, with an average size of 30-40 μm.

[0107] ​The resulting capsules were then mixed with an aqueous binder of fish gelatin (Norland HiPure liquid gelatin) at a ratio of 1 part binder to 7 parts capsules by weight, and mixed with an aqueous colorant dispersion comprising 10% by weight of Emperor 2000 carbon black and 5% by weight of Kolliphor P188 at a ratio of 1 part colorant dispersion to 49 parts binder. The resulting mixture was bar coated onto a 125 mm thick indium tin oxide coated polyester film (with the capsules deposited on the ITO coated surface), and the coated film was dried to produce an electrophoretic medium of about 25 μm thickness, which comprised substantially a single layer of capsules.

[0108] The exposed surface of the electrophoretic medium was then coated with a radiation curable polyurethane acrylate based adhesive. When the adhesive layer was applied, a screen printed piece of 125 mm thick indium tin oxide coated polyester film was applied. The resulting assembly was then cured by exposure to ultraviolet light.

[0109] A second capsule based electrophoretic display was prepared in a similar manner, except that the colorant dispersion was omitted.

[0110] Figure 22A is a micrograph of a capsule based electrophoretic medium similar to that shown in I. Variable Transmission Media Containing Colored Binder and 1B , where the uncolored binder is in its off state (in effect, Figure 2 is a top view of the medium of Figure 1A . It can be seen that in various regions of the medium (see the circled regions in Figure 2 and the center left) there are large numbers of pinholes and voids, and even in the off state these regions are much lighter in color than the rest of the medium, resulting in a high minimum transmittance and poor contrast.

[0111] Figure 1A is a micrograph of a capsule based electrophoretic medium similar to that shown in Figure 2 and 1D , but with a higher proportion of binder than Figure 3 , which has a more heavily colored binder when in the on state (in effect, Figure 1C is a top view of the medium of . It can be seen that in various regions of the medium (especially the circled regions) there are multiple layers of capsules, and these regions appear darker than the surrounding single layer regions, resulting in excessive granularity.

[0112] Figure 3 is a micrograph of a capsule based electrophoretic medium similar to that shown in Figure 1D but with a higher proportion of binder than Figure 4 and 1F , where the proportion of binder is less colored than Figure 3 , the medium is shown in its on state (in effect,Figure 1E is Figure 3 a top view of the medium). It can be seen that there are multiple layers of capsules in each region of the medium (especially the circled regions), but these regions do not have much contrast with the surrounding single layer regions, resulting in a much lower particle ratio Figure 3 than the medium of

[0113] Figure 1F , 5B and 5C show the minimum transmittance in the off state, the minimum transmittance in the on state, and the contrast ratio of a single pixel display made using a capsule-based electrophoretic medium with an uncolored binder (as shown in Figure 3 and 1B ) and a similar medium with an optimally colored binder (as shown in Figure 5A and 1F ). It can be seen from Figure 1A that the coloring of the binder reduces the minimum transmittance in the off state from about 7.5% to about 2%, while the transmittance in the on state is reduced only from about 21% to about 19%, so the contrast ratio increases from about 9 to about 23.

[0114] Figure 1E and 6B are photographs showing the on and off states of a capsule-based electrophoretic medium with a highly colored binder (as shown in Figures 5A-5C and 1D — Figure 6A ) and a similar medium with an optimally colored binder (as shown in Figure 1C and 1F — Figure 6A It can be seen that the highly colored binder medium has more particles than the optimally colored binder medium.

[0115] Example 2

[0116] A non-aqueous internal phase was prepared by combining 1 -limonene, an impregnation solution, carbon black, polystyrene, and 2-hexyldecanoic acid. The internal phase thus prepared was then encapsulated by adding the mixture to an aqueous solution of porcine gelatin and gum arabic, followed by the addition of Emperor 2000 carbon black with 5 wt% Kolliphor P188. After heating, mixing, and pH adjustment, the resulting capsules were cooled and then sorted to form a mixture of capsules having a size distribution of 20 to 90 μιη in diameter, with an average diameter of 50-70 μιη.

[0117] The capsule slurry was centrifuged and then mixed with an aqueous binder of 50:50 fish gelatin (Norland HiPure liquid gelatin): gum arabic (AEP colloids) at a ratio of 1 part binder to 4 parts capsules by weight. A solution of colorant (7 wt% Emperor 2000 carbon black with 3.5 wt% Kolliphor P188 (Sigma-Aldrich 15759), 1.5 wt% Cab-o-jet 265M (Cabot Corp) and 1.5 wt% Cab-o-jet 250C (Cabot Corp)) was prepared in water and then added to the aqueous binder at a ratio of 1 part colorant to 52.3 parts binder. The resulting mixture of binder and encapsulated internal phase was bar coated onto a 125 μιη thick indium tin oxide coated polyester film. The coated film was dried to produce an electrophoretic medium of about 33 μιη thick, which comprised substantially a single layer of capsules.

[0118] The coated film was then coated with a polyurethane acrylate based adhesive on the capsule coated surface of the film. When the adhesive layer was added, a silk screen printed piece of 125 mm thick indium tin oxide coated polyester film was applied. The resulting assembly was then cured by exposure to UV light from a CSun UV lamp.

[0119] By using the above technique, a neutral density window pixel (i.e. top and bottom light transmitting electrodes) was established with a binder that was colored with a mixture of Emperor 2000 carbon black, Cab-o-jet 265M magenta pigment and Cab-o-jet 250C cyan pigment. As a control, a similar window pixel was constructed using the same technique, except that the equivalent weight of the colorant mixture was replaced with Emperor 2000 carbon black. Using an optical evaluation test bench, the contrast ratio and haze of the samples were evaluated. As shown in Table 1, the color mixture had almost as good contrast ratio and haze as the control. Figure 1E and 20

[0120] While the optical performance was almost equivalent, the color difference between the two samples was readily apparent to the naked eye in the open state. To quantify the color difference, reflectance measurements were made on the dual transmission electrode test cell against a white print paper background. As shown in Table 1, the sample with the pigment blend in its binder had a more neutral color (a* and b* values closer to zero) and a greater total reflectance.

[0121] ​Table 1. Reflectance measurements of dual window test pixels by the same internal phase, where the binder included: A) only Emperor 2000 carbon black, colorant weight approximately 2%; and B) a mixture of colorants (14 parts Emperor 2000 carbon black, 3 parts Cab-o-jet 265M magenta, and 3 parts Cab-o-jet 250C cyan) at a colorant weight of approximately 2%.

[0122] Figure 6B L* a* b* Figure 19 94.6 3.99 -11.45 Sample 37.5 1.64 6.38 White Paper 40.75 -0.05 3.38

[0123] A) Carbon Black Only Window

[0124] By combining 1 -limonene, An internal phase was prepared with an impregnation solution, carbon black, polystyrene, and 2-hexyldecanoic acid. The mixture was then emulsified by adding the mixture to an aqueous gelatin / arabic gum solution, and a dispersion of Emperor 2000 carbon black with 5 weight percent Kolliphor P 188 was added, and the resulting mixture was encapsulated. After mixing, heating, and pH adjustment, the resulting capsules were cooled and sieved to a range of 20-60 μιη, with an average size of 30-40 μιη.

[0125] The capsules were centrifuged, then mixed with an aqueous binder of fish gelatin (Norland HiPure liquid gelatin) at a ratio of 1 part binder to 7 parts capsules by weight, and an aqueous colorant dispersion was mixed in, including 10 weight percent Emperor 2000 carbon black with 5 weight percent Kolliphor P 188, at a ratio of 1 part colorant dispersion to 49 parts binder. The resulting mixture was bar coated onto a 125 mm thick indium tin oxide coated polyester film (with the capsules deposited on the ITO coated surface), and the coated film was dried to produce an electrophoretic medium of approximately 25 μιη thickness, which comprised essentially a single layer of capsules.

[0126] The exposed surface of the electrophoretic medium was then coated with a radiation curable polyurethane acrylate based adhesive. When the adhesive layer was applied, a screen printed piece of 125 mm thick indium tin oxide coated polyester film was applied. The resulting assembly was then cured by exposure to ultraviolet light.

[0127] The capsule based electrophoretic medium produced as described above was compared to other similar capsule based electrophoretic media, the difference being that the charge control agent used in the comparative sample was alone or in combination with Pluronic L31 (two samples), Span 65 and Span 85 (two samples). For each medium, coating weight, internal phase conductivity and viscosity were measured, as well as contrast ratio at drive voltages of 72, 90 and 120 volts and relative humidities of 50% and 60%, with results as shown in B) Neutral Color Window

[0128] As can be seen from II. Variable Transmission Media Containing CCA Blends , the medium containing only has an undesirably low contrast ratio, and the addition of Pluronic L31 produces little change. While the measured contrast ratio is very susceptible to changes in humidity, the addition of Span 65 produces some improvement in contrast ratio, especially at 120V. The addition of Span 85 produces better results than the addition of Span 65, although the measured contrast ratio is still very susceptible to changes in humidity and requires a high drive voltage of 120V to achieve consistently high contrast ratios above 30. The results achieved by the addition of 2-HAD are the best of the test compositions, achieving high contrast ratios even at 72V, while reducing the humidity dependence.

[0129] Example 2

[0130] Various experimental displays were prepared in the same manner as described above in Example 1, but with 2-HAD: at weight ratios of 0.025:1, 0.05:1, 0.1:1 and 0.2:1, respectively. To provide a control, a similar display was prepared with Span 85 and at a weight ratio of 0.825:1 (the larger molecular weight of Span 85 makes this ratio approximately equal to a 2-HAD: ratio of 0.2:1 on a molar basis). Coating weight, internal phase conductivity, contrast ratio and minimum haze value were measured at the same drive voltages and relative humidities as described above in Example 1, and results are shown in Figure 7 and 9

[0131] As can be seen from Figure 7 and 9 , the contrast ratio of the 2-HAD displays monotonically increases with the proportion of 2-HAD and drive voltage, reaching values well over 30 at a 2-HAD ratio of 0.2:1 and a drive voltage of 90V, which are substantially greater than the corresponding values for the Span 85 display. The minimum haze value does not vary greatly with the proportion of 2-HAD, but decreases with increasing drive voltage. Thus, the display with a 2-HAD: weight ratio of 0.2:1 has the best overall performance.

[0132] Example 3​​

[0133] The displays prepared in Example 2 above containing 2-HDA and Span 85 in a weight ratio of 0.2: 1 and Figure 8 The maximum open transmittance, minimum closed transmittance, minimum open haze and contrast ratio determined in these tests are shown respectively.

[0134] It can be seen from Figure 8 that the displays containing 2-HDA show a higher maximum open transmittance, a lower minimum closed transmittance, a lower open haze and substantially higher contrast ratio under the same conditions as the displays containing Span 85. In particular, the Span 85 displays fail to reach the ideal contrast ratio of 30 under any of the test conditions, whereas the 2-HDA displays consistently reach this contrast ratio at a driving voltage of at least 60 V.

[0135] Example 4

[0136] Displays were prepared as described in Example 1 above containing 2-HDA and oleic acid in a weight ratio of 0.2: 1 and These displays were tested at driving voltages of 72, 90 and 120 V. Figure 5 shows the contrast ratio determined in these tests.

[0137] It can be seen from Figures 10A-10D that the displays containing 2-HDA consistently exhibit a significantly higher contrast ratio under the same conditions as the displays containing oleic acid.

[0138] Figures 10A-10D

[0139] A non-aqueous internal phase was prepared by combining 1 -limonene, an impregnation liquid, carbon black, polystyrene and 2-hexyl decanoic acid. The internal phase mixture was then encapsulated by adding the mixture to an aqueous gelatin / gum arabic solution, emulsifying the mixture and adding a dispersion of 10 wt% Emperor 2000 carbon black with 5 wt% Kolliphor P188. After mixing, heating and pH adjustment, the resulting capsules were cooled and sorted using sieving to produce a capsule mixture having a size range of 15-50 pm, with an average size of about 30 pm.

[0140] The resulting aqueous capsule slurry was centrifuged and then mixed into three different fish gelatin based aqueous binders; A) no gum arabic, B) a 1 :2 mixture of gum arabic and fish gelatin, and C) a 1 : 1 mixture of gum arabic and fish gelatin. The fish gelatin was HiPure liquid gelatin purchased from Norland and the gum arabic was obtained from AEP Colloids. Each gelatin binder was mixed at a ratio of 1 part binder to 7 parts capsules by weight and mixed with a colorant solution that included 10% by weight of Emperor 2000 carbon black and 5% by weight of Kolliphor P188 (Aldrich 15759) in water at a ratio of 1 part carbon black colorant to 49 parts binder. The resulting mixture was bar coated onto a 125 mm thick indium tin oxide coated polyester film. The coated film was dried to produce an electrophoretic medium of approximately 25 μιη thickness that consisted essentially of a single layer of capsules.

[0141] The coated film was then coated with a polyurethane acrylate based adhesive on the capsule coated surface of the film. When the adhesive layer was added, a silk screen printed piece of 125 mm thick indium tin oxide coated polyester film was applied. The resulting assembly was then cured by exposure to UV light from a CSun UV lamp.

[0142] Several samples of variable transmission test films were prepared for each binder formulation. The samples were then evaluated for open and closed transmission and haze using the optical evaluation apparatus described in U.S. Patent No. 7,679,814. Briefly, each sample was placed in front of a calibrated light source with an integrated detector on the other side of the sample. Each sample was driven to the open and closed states and its transmission was evaluated. Additionally, a calibrated chopper wheel was used to measure the relationship between diffuse and transmitted light to evaluate the haze. The amount of kickback was also evaluated by comparing the decay over time in the open state (see Figure 11 ). The resulting data is shown in III. Variable Transmission Media Containing Fish Gelatin: Gum Arabic Binder .

[0143] By evaluating the differences between the three binder formulations, it is clear that the 1 : 1 mixture of fish gelatin and gum arabic produced an electro-optic medium with good contrast (difference between the open and closed states; Figure 14 ) and low haze ( Figures 13A-13C ). Additionally, the two binder mixtures containing gum arabic had little kickback resulting in very long stability in the open and closed states. See Figure 13B .

[0144] Figure 13A

[0145] Example 1

[0146] By combining 1 -limonene, An internal non-aqueous phase was prepared using the impregnation solution, carbon black, polystyrene, and 2-hexyl decanoic acid. The internal phase mixture was then encapsulated by adding the mixture to an aqueous gelatin / acacia solution, emulsifying the mixture, and adding a 10 wt% dispersion of Emperor 2000 carbon black with 5 wt% Kolliphor P 188.

[0147] After mixing, heating, and pH adjustment, the resulting capsules were cooled and sorted into two size distributions, one ranging from about 20 μιη to about 50 μιη with an average size of about 35 μιη, and the other ranging from about 50 μιη to about 90 μιη with an average size of about 60 μιη. For some of the experiments detailed below, three parts by weight of the second portion were combined with one part by weight of the first portion.

[0148] The resulting capsule slurry was centrifuged, then mixed with an aqueous binder of 50:50 fish gelatin (Norland HiPure liquid gelatin): acacia (AEP colloids) at a ratio of 1 part binder to 7 parts capsules, and mixed with a colorant solution comprising 10 wt% Emperor 2000 carbon black with 5 wt% Kolliphor P 188 in water at a ratio of 1 part carbon black colorant to 49 parts binder. The resulting mixture was bar coated onto a 125 mm thick indium tin oxide coated polyester film. The coated film was allowed to oven dry to produce an electrophoretic medium of about 25 μιη thickness that comprised substantially a single layer of capsules.

[0149] The coated film was then coated with a polyurethane acrylate based adhesive on the capsule coated surface of the film. A silk screen printed piece of 125 mm thick indium tin oxide coated polyester film was applied when the adhesive layer was added. The resulting assembly was then cured by exposure to UV light from a CSun UV lamp.

[0150] Two sets of samples were prepared. The first set of samples ("Small Only Samples" in Figure 14 ) was prepared by using only capsules of about 20 μιη to about 50 μιη. The second set of samples ("Mix" in IV. Variable Transmission Media Containing Capsules ) was prepared by mixing three parts by weight of capsules of about 50 μιη to about 90 μιη with one part by weight of capsules of 20 μιη to about 50 μιη.

[0151] The samples were then evaluated for open and closed transmittance and haze using the optical evaluation apparatus described in U.S. Patent No. 7,679,814. Briefly, each sample was placed in front of a calibrated light source with an integrated detector on the other side of the sample. Each sample was driven to the open and closed states and its transmittance was evaluated. Additionally, a calibrated chopper wheel was used to measure the relationship between diffuse and transmitted light to evaluate the haze. The resulting data is summarized in Figures 16A-16Dare shown in FIG. 6. Figures 16A-16D Side-by-side images of samples of only small sample and mixed capsule sample in the open state are shown. From Figures 16A-16D As can be seen in FIG. 6, there is little difference in the particles between the only small capsule formulation and the mixed capsule formulation.

[0152] By evaluating the differences between the only small sample and the mixed sample, it is clear that the mixture of large and small capsules produces an electro-optic medium with excellent contrast (difference between open and closed states) and lower haze. Also observed in the mixed sample is significantly less halo (interference pattern) compared to the only small sample.

[0153] Example 2

[0154] Capsules were prepared similarly to the procedure in Example 1 above, except that the capsules were sieved into three size categories and two blends. The blends included a small group with a size distribution of about 5 pm to about 50 pm and an average number diameter of about 20 pm; a medium group with a size distribution of about 20 pm to about 90 pm and an average number diameter of about 35 pm; and a large group with a size distribution of about 20 pm to about 90 pm and an average number diameter of about 40 pm. Two blends included a medium blend and a large blend, the medium size capsules in the medium blend were in a 2: 1 weight ratio to the small size capsules, resulting in an average number diameter of about 25 pm, and the large size capsules in the large blend were in a 7: 1 weight ratio to the small size capsules, resulting in an average number diameter of about 30 pm.

[0155] The electro-optic properties of the small, medium, large, medium blend, and large blend capsules were tested to determine the appearance of the particles, haze, and transmittance. The results are provided in FIG. 7. Figure 17 As can be seen from the results, the addition of small capsules reduced the particles in the closed state, while the large capsules better reduced the particles in the open state and reduced the haze. Thus, the data indicates that the electro-optic properties can be tuned depending on which property is most important for a particular application.

[0156] Figure 17

[0157] Sample 1 : A non-aqueous internal phase was prepared by combining 1 -limonene, A non-aqueous internal phase was prepared by combining

[0158] The resulting capsules were centrifuged and then mixed with a 50:50 mixture of fish gelatin (Norland HiPure liquid gelatin): gum arabic aqueous binder at a ratio of 1 part by weight of binder to 7 parts by weight of capsules. An aqueous colorant dispersion comprising 10% by weight of Emperor 2000 carbon black and 5% by weight of Kolliphor P188 was also mixed at a ratio of 1 part by weight of colorant dispersion to 54 parts by weight of binder. The resulting mixture was rod-coated onto a 125 μm thick indium tin oxide (ITO) coated polyester film (capsules deposited on the ITO-coated surface), and the coated film was oven-dried to produce an electrophoretic medium approximately 27 μm thick, which essentially consisted of a monolayer of capsules.

[0159] Then, a radiation-curable polyurethane acrylate-based adhesive composition is applied to the exposed surface of the electrophoretic medium. While the adhesive layer is being applied, a 125 μm thick indium tin oxide-coated polyester film is applied via screen printing. The resulting assembly is then cured by exposure to ultraviolet light.

[0160] Sample 2: Performed the same procedure as Sample 1, but the UV-curable adhesive also contained 0.5 phr of Keyplast Black AN. The 1 mil unit cured with the colored adhesive blend had a transmittance of 21.5% and a haze of 0.7%. Table 2 provides the electro-optic properties of Sample 1 and Sample 2.

[0161] Table 2. EO properties of electrophoretic media with and without coloring binders.

[0162]

[0163]

[0164] Based on the comparison results of Sample 1 and Sample 2, the variable transmission film containing a colored UV-curable adhesive provides fewer particles and reduced pinholes, and the use of dyes soluble in the adhesive does not increase haze. Figures 26A to 26L The image provided shows a photograph of sample 1 in its open state, and... V. Variable Transmission Films Containing Colored Binder The image provided shows sample 2 in an open state.

[0165] Figure 24A

[0166] Example 1

[0167] By combination A non-aqueous internal phase was prepared from an impregnation solution, trans, trans, cis 1,5,9-cyclododecatriene (CDT), carbon black, polystyrene and 2-hexyldecanoic acid. The mixture was then emulsified by adding the mixture to an aqueous gelatin / acacia solution and adding a dispersion of 10 wt% Emperor 2000 carbon black with 5 wt% Kolliphor P188. The resulting mixture was encapsulated. After mixing, heating and pH adjustment, the resulting capsules were cooled and sieved to a range of 20-60 pm, with an average size of 30-40 pm.

[0168] The resulting capsules were centrifuged and then mixed with an aqueous binder of fish gelatin (Norland HiPure liquid gelatin) at a ratio of 1 part binder to 7 parts capsules by weight and mixed with an aqueous colorant dispersion comprising 10 wt% Emperor 2000 carbon black with 5 wt% Kolliphor P188 at a ratio of 1 part colorant dispersion to 49 parts binder. The resulting mixture was bar coated onto a 125 pm thick indium tin oxide coated polyester film (with the capsules deposited on the ITO coated surface) and the coated film was dried to produce an electrophoretic medium of approximately 22 pm thick which comprised substantially a single layer of capsules.

[0169] The exposed surface of the electrophoretic medium was then coated with a radiation curable polyurethane acrylate based adhesive. When the adhesive layer was applied, a screen printed piece of 125 mm thick indium tin oxide coated polyester film was applied. The resulting assembly was then cured by exposure to ultraviolet light.

[0170] Example 2

[0171] A comparative assembly comprising encapsulated 1 -limonene and is prepared according to the procedure provided in Section III of the above example. The electro-optical performance of the assemblies of Example 1 and Example 2 are provided in Figure 24B VI. Variable Transmission Films Containing Encapsulated Non- Conjugated Olefins Figures 25A to 25D

[0172] As described above, the present application provides an improved variable transmission electrophoretic medium which is well suited for use in, for example, variable transmission windows and vehicle sunroofs. The medium of the present application can be readily produced using conventional processes and can be more easily deposited on glass or other rigid substrates than slit-type extrusion coating.

[0173] It will be apparent to those skilled in the art that numerous changes and modifications can be made to the specific embodiments of the application described above without departing from the scope of the application. Accordingly, the whole of the foregoing description is to be interpreted in an illustrative rather than a limiting sense.​

Claims

1. An electro-optic medium comprising a plurality of capsules and a binder, each capsule encapsulating charged pigment particles in a nonpolar solvent, wherein the plurality of capsules comprises at least 60% in a size range of 50 μm to 90 μm in diameter and at least 15% in a size range of 20 µm to 49 µm in diameter.

2. A method for forming an electro-optic dielectric, comprising: It provides an internal phase mixture of nonpolar solvents and charged pigment particles. A portion of the internal phase mixture is encapsulated in multiple capsules. The plurality of capsules are sieved into at least two parts, the first part comprising capsules with a size distribution having a diameter between 50 μm and 90 μm, and the second part comprising capsules with a size distribution having a diameter between 20 μm and 49 μm. The polymer binder is mixed with two to five parts by weight of the first part and one part by weight of the second part.

Images