This application relates to U.S. patent No. 7,012,735 and U.S. patent application publication No. 2009/0122389.
Background
The terms "bistable" and "bistability" are used herein in their conventional sense in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property such that, after any given element is driven to assume either its first or second display state by an addressing pulse of finite duration, that state will last for a time at least several times (e.g. at least 4 times) the minimum duration of the addressing pulse required to change the state of that display element after the addressing pulse has terminated. U.S. patent No. 7,170,670 shows that certain particle-based electrophoretic displays have stable gray levels not only in their extreme black and white states, but also in their intermediate gray states, as well as some other types of electro-optic displays. Although for convenience the term "bistable" is used herein to cover both bistable and multi-stable displays, displays of the type described above are more suitably referred to as "multi-stable" rather than bistable.
Electrophoretic displays have been the subject of extensive research and development for many years. In a microparticle based electrophoretic display, a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays have good brightness and contrast, wide viewing angles, bistability of states, and low power consumption properties compared to liquid crystal displays. However, the widespread use of electrophoretic displays has been hampered by the long-standing image quality problems. For example, particles that make up electrophoretic displays tend to settle, which results in inadequate service life of such displays.
As noted above, electrophoretic media require the presence of a fluid. In most existing electrophoretic media, the fluid is a liquid, but the electrophoretic medium can also be made of a gaseous fluid; see, for example, Kitamura, T.et al, "electric tuner movement for electronic Paper-like display," IDWJapan,2001, Paper HCSl-1, and Yamaguchi, Y.et al, "tuner display using insulating particles charged triboelectric, IDW Japan,2001, Paper AMD 4-4). See also U.S. patent nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic media are susceptible to the same problems when the media is used in an orientation that allows settling, such as in a sign in which the media is placed in a vertical plane, due to the same particle settling phenomenon as liquid-based electrophoretic media. In fact, the problem of particle settling is more severe in gas-based electrophoretic media than in liquid-based electrophoretic media, because the viscosity of the gaseous suspending fluid is lower compared to that of the liquid, thereby allowing faster settling of the electrophoretic particles.
A number of patents and applications assigned to or entitled to the institute of technology and technology (MIT) and yingke (E Ink) corporation describe various techniques for use in encapsulated electrophoretic and other electro-optic media. Such encapsulated media comprise a plurality of capsules, each of which itself comprises an internal phase comprising electrophoretically mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules themselves are held within a polymeric binder to form a coherent layer between two electrodes. The techniques described in these patents and applications include:
(a) electrophoretic particles, fluids, and fluid additives; see, e.g., U.S. Pat. nos. 7,002,728 and 7,679,814;
(b) bladders, adhesives, and encapsulation methods; see, e.g., U.S. Pat. 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, 7,202,991, 7,242,513, 7,304,634, 7,339,715, 7,391,555, 7,411,719, 7,477,444, and 7,561,324, and U.S. patent application publication Nos. 2004/0112750, 2005/0156340, 2007/0057908, 2007/0091417, 2007/0223079, 2008/0023332, 2008/0130092, 2008/0264791, 2009/0122389, and 2010/0044894;
(c) films and sub-assemblies comprising electro-optic material; see, e.g., U.S. Pat. nos. 6,982,178 and 7,839,564;
(d) a backplane, adhesive layer and other auxiliary layers and methods for use in a display; see, e.g., U.S. Pat. Nos. 7,116,318 and 7,535,624;
(e) color formation and color adjustment, see, e.g., U.S. patent No. 7,075,502, and U.S. patent application publication No. 2007/0109219;
(f) a method for driving a display; see, for example, U.S. Pat. nos. 7,012,600 and 7,453,445;
(g) application for a display; see, e.g., U.S. patent No. 7,312,784; and U.S. patent application publication No. 2006/0279527, and
(h) non-electrophoretic displays such as U.S. patent nos. 6,241,921, 6,950,220, and 7,420,549, and U.S. patent application publication No. 2009/0046082.
Many of the foregoing patents and applications recognize that the walls surrounding discrete microcapsules in an encapsulated electrophoretic medium can be replaced by a continuous phase, thus creating a so-called polymer dispersed electrophoretic display, wherein the electrophoretic medium contains a plurality of discrete droplets of an electrophoretic fluid and a continuous phase polymeric material, and the discrete particles of the electrophoretic fluid in such a polymer dispersed electrophoretic display can be considered as capsules or microcapsules, although there is no discrete capsule associated with each individual droplet; see, for example, the aforementioned U.S. patent No. 6,866,760. Such polymer-dispersed electrophoretic media are therefore considered, for the purposes of this application, to be a subclass of encapsulated electrophoretic media.
The type of electrophoretic display associated with this is known as a "microcell electrophoretic display". In microcell electrophoretic displays, the charged particles and the fluid are not encapsulated in microcapsules, but are held within a plurality of cavities formed in a carrier medium, typically a polymer film. See, for example, U.S. patent nos. 6,672,921 and 6,788,449, assigned to SipixImaging, inc.
Although electrophoretic media are typically opaque (e.g., because in many electrophoretic media the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be operated in a so-called "shutter mode" in which one display state is substantially opaque and one display state is light-transmissive. See, for example, the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat. Nos. 5,872,552, 6,144,361, 6,271,823, 6,225,971, and 6,184,856. A dielectrophoretic display similar to the electrophoretic display but relying on variations in electric field strength may operate in a similar mode; see U.S. patent No. 4,418,346. Other types of electro-optic displays can also operate in the shutter mode.
Encapsulated electrophoretic displays are generally immune to the aggregation and settling failure modes of conventional electrophoretic devices and have additional advantages, such as the ability to print or coat the display on a variety of different flexible and rigid substrates. (the use of the term "printing" is intended to include all forms of printing and coating including, but not limited to, premetered coating such as slot die coating, slot or die coating, slide or cascade coating, curtain coating, roll coating such as knife-over-roll coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brushing, air knife coating, screen printing processes, electrostatic printing processes, thermal printing processes, ink jet printing processes, electrophoretic deposition (see U.S. Pat. No. 7,339,715), and other similar techniques). Thus, the resulting display may be flexible. In addition, since the display media can be printed (using various methods), the display itself can be made inexpensively.
An electrophoretic display typically comprises a layer of electrophoretic material and at least two further layers on opposite sides of the electrophoretic material, one of the layers acting as an electrode layer. In most such displays, both layers are electrode layers, with one or both electrode layers being patterned to define the pixels of the display. For example, one electrode layer may be patterned into elongate row electrodes and the other into elongate column electrodes at right angles to the row electrodes, with pixels being defined at the intersections of the row and column electrodes. Alternatively, or more commonly, one electrode layer is in the form of a single continuous electrode and the other electrode layer is patterned into a matrix of pixel electrodes, each pixel electrode defining one pixel of the display. In another type of electrophoretic display intended for use with a stylus, a print head or similar movable electrode separate from the display, only one of the layers adjacent to the electrophoretic layer contains the electrode, the layer on the opposite side of the electrophoretic layer typically being a protective layer, which is intended to prevent the movable electrode from damaging the electrophoretic layer.
The manufacture of a three-layer electrophoretic display usually comprises at least one lamination operation. For example, several patents and applications by the Massachusetts Institute of Technology (MIT) and Eink corporation, as noted above, describe a process for the manufacture of encapsulated electrophoretic displays in which an encapsulated electrophoretic medium comprising capsules in a binder is coated onto a flexible substrate comprising Indium Tin Oxide (ITO) or onto a similar conductive coating on a plastic film as one electrode of the final display, the capsule/binder coating drying to form a coherent layer of electrophoretic medium firmly bonded to the substrate. Separately, a backplane is prepared, which contains an array of pixel electrodes and wires with appropriate layout to connect the pixel electrodes to the drive circuitry. To form the final display, the substrate with the bladder/adhesive layer thereon is laminated to a backplane using a laminating adhesive. (very similar processes can be used to prepare electrophoretic displays that can be used with a stylus or other similar moving electrode that can slide over a protective layer by replacing the backplane with a simple protective layer (e.g., a plastic film). In a preferred form of this process, the backplane is itself flexible and is prepared by printing the pixel electrodes and the leads on a plastic film or other flexible substrate. One known lamination technique for mass production of displays by this process is roll lamination using a lamination adhesive. Similar manufacturing techniques may be used to produce other types of displays. For example, microcell electrophoretic media may also be laminated to a backplane in substantially the same manner as encapsulated electrophoretic media.
As discussed in the aforementioned U.S. patent No. 6,982,178 (see column 3, line 63 through column 5, line 46), many of the components used in electrophoretic displays and methods for making such displays have been derived from the technology used in Liquid Crystal Displays (LCDs), which are, of course, electro-optic displays, although they use a liquid rather than a solid medium. For example, an electrophoretic display may use an active matrix backplane comprising an array of transistors or diodes and a corresponding array of pixel electrodes, and a "continuous" front electrode on a transparent substrate (in the sense that the electrodes extend across a plurality of pixels, typically the entire display), these components being substantially the same as in an LCD. However, the method of assembling the LCD cannot be used for an electrophoretic display. The assembly method of the LCD is generally to form the back plate and the front electrode on separate glass substrates, then to firmly adhere these components together with a small gap left therebetween, to place the resulting assembly under vacuum, and to immerse the assembly in liquid crystal, whereby the liquid crystal flows through the gap between the back plate and the front electrode. Finally, the liquid crystal is put in place and the gap is sealed to provide the final display.
The assembly process of such LCDs cannot be easily transferred to electrophoretic displays. Since the electrophoretic material is in a solid state, it must be present between the back plate and the front electrode before they are fixed to each other. In addition, the liquid crystal material only needs to be simply placed between the back plate and the front electrode, and does not need to be attached to either of the back plate and the front electrode, and unlike the liquid crystal material, the electrophoretic medium generally needs to be fixed together with the back plate and the front electrode; in most cases, the electrophoretic layer is formed on the front electrode because it is easier than forming the electrophoretic layer on the back sheet containing the circuitry, and then, the entire surface of the electrophoretic layer is usually covered with an adhesive and lamination is performed under heat, pressure and possibly vacuum conditions to laminate the front electrode/electrophoretic layer combination to the back sheet as a whole. Thus, most of the existing final lamination processes for electrophoretic displays are essentially batch processes, in which the (usually) electrophoretic medium, the lamination adhesive and the backplane are brought together between final assembly, and there is a need to provide a process that is better suited for mass production.
Electro-optic displays tend to be expensive; for example, the cost of a color LCD in a portable computer typically accounts for a large portion of the overall cost of the entire computer. As the use of electro-optic displays expands to devices such as mobile phones and Personal Digital Assistants (PDAs) that are much less costly than portable computers, there is pressure to greatly reduce the cost of such displays. The ability to form layers of solid electro-optic media on flexible substrates by printing techniques, as described above, opens the possibility of reducing the cost of display electro-optic assemblies by mass production techniques, such as roll coating using commercial equipment for producing coated paper, polymer films and similar media.
The aforementioned U.S. patent No. 6,982,178 describes a method of assembling a solid state electro-optic display, including an electrophoretic display, which is suitable for large scale production. Basically, this patent describes a so-called "front panel lamination assembly" ("FPL") which comprises, in order: a transparent conductive layer; a layer of solid electro-optic medium in electrical contact with the light-transmissive electrically-conductive layer; an adhesive layer; and a release plate. Typically, the transparent conductive layer is carried on a light-transmissive substrate, preferably flexible, in the sense that the substrate can be manually wound into a 10 inch (254 mm) diameter drum (for example) without permanent deformation. The term "light transmissive" is used in this patent and herein to mean that the layer in question is sufficiently light transmissive to allow a viewer to see through the layer, typically through the conductive layer and the adjacent substrate (if present), the change in display state of the electro-optic medium; where the electro-optic medium exhibits a change in reflectivity at non-visible wavelengths, the term "light transmissive" should be interpreted to mean transmissive to the relevant non-visible wavelengths. The substrate is typically a polymeric film, typically having a thickness in the range of 1 to 25 mils (25 to 634 micrometers), preferably about 2 to 10 mils (51 to 254 micrometers). The conductive layer is typically a thin layer of metal or metal oxide, such as aluminum or ITO, and may also be a conductive polymer. Aluminum or ITO coated poly (ethylene terephthalate) film (PET) is commercially available, for example, as "aluminum coated mylar" from dupont of wilmington, terawal (the "mylar" is a registered trademark), and such commercial materials are well suited for use in front panel laminates.
The aforementioned U.S. patent No. 6,982,178 also describes a method of testing the electro-optic medium in the front panel laminate assembly before the front panel laminate assembly is incorporated into a display. In this test method a conductive layer is provided on a release plate and a voltage sufficient to change the optical state of the electro-optical medium is applied between the conductive layer and the conductive layer on the opposite side of the electro-optical medium. The electro-optic medium is observed for any defects in the medium, thereby avoiding the rejection of the entire display by laminating a defective electro-optic medium to the display, rather than just the rejection of a defective front panel laminate assembly.
A second method of testing electro-optic media in a front panel laminate assembly by placing an electrostatic charge on a release sheet to form an image on the electro-optic medium is also described in the aforementioned U.S. Pat. No. 6,982,178. This image is observed by the same method as described above to detect any defects in the electro-optic medium.
An electro-optic display is assembled using such a front plane laminate assembly by removing the release sheet from the front plane laminate assembly and contacting the backplane with the adhesive layer under conditions effective to cause the adhesive layer to adhere to the backplane, thereby securing the adhesive layer, the layer of electro-optic medium and the conductive layer to the backplane. This process is well suited for high volume production, as the front panel laminate assembly can be mass produced, typically using roll-to-roll coating techniques, and then cut into sheets of any size required for a particular back panel.
The so-called "double release sheet" technique is described in the aforementioned U.S. patent No. 7,561,324, and is essentially a simplification of the front panel laminate assembly technique of the aforementioned U.S. patent No. 6,982, 178. One form of dual release sheet comprises a layer of solid electro-optic medium sandwiched between two adhesive layers, one or both of which is covered by the release sheet. Another form of dual release sheet comprises a layer of solid electro-optic medium sandwiched between two release sheets. Both forms of the dual release film are intended for use in a process generally similar to the assembly of an electro-optic display from a front panel laminate assembly, as has been previously discussed, but involving two separate laminations; typically, in a first lamination, the dual release sheet is laminated to the front electrode to form a front subassembly, which is then laminated to the backplane in a second lamination to form the final display, although these two laminations can be reversed in order if desired.
U.S. patent No. 7,839,564 describes a so-called "reverse front panel lamination assembly" technique that is a variation of the front panel lamination assembly technique in the aforementioned U.S. patent No. 6,982,178. This reverse front panel lamination assembly comprises in sequence: at least one of a light-transmitting protective layer and a light-transmitting conductive layer; an adhesive layer; a layer of solid electro-optic medium; and a release sheet. Such a reverse front plane lamination assembly is used to form an electro-optic display comprising a lamination adhesive layer disposed between an electro-optic layer and a front electrode or substrate; a second adhesive layer, which is typically a thin layer, may or may not be present between the electro-optic layer and the backplane. The photoelectric display can combine good resolution and good low-temperature performance.
It is apparent from the above discussion that electrophoretic displays are complex systems that may include (1) the electrophoretic particles themselves, which may be composite particles having a core/shell structure and/or a polymer coating; (2) a fluid surrounding the electrophoretic particles; (3) additives to the fluid, such as charge control agents, surfactants, and dispersed polymers, etc.; (4) a capsule wall surrounding the electrophoretic particles and the fluid; an adhesive or other continuous phase around the capsules, microdroplets or microcells; (5) one or more adhesive layers; and (6) one or more electrode layers. Not surprisingly, in view of the complexity of such a system, the exact relationship between the electro-optical properties of such a display and the mechanical, physico-chemical and electrical properties of the various materials used to make up the display is only partially understood. For example, it is known that the electro-optic performance of a display may be adversely affected if the electrical conductivity of the adhesive used in the front panel laminate assembly is insufficient. The aforementioned U.S. Pat. No. 7,012,735 describes the benefits obtained by doping a salt or other material into a binder to improve its electrical conductivity, and a preferred dopant for this purpose is tetrabutylammonium hexafluorophosphate (hereinafter "TBAHFP"). The same patent also describes similar advantages of doping the display adhesive with salts, including, for example, TBAHFP.
One of the problems to be reduced or overcome by binder doping is the so-called "white state degradation" or "WSD". WSD manifests itself as a decrease in reflectance (typically measured in CIE L a b color space L) of the extreme white optical state during operation of the display. It has been found empirically that the amount of WSD present depends on the operating duty cycle, temperature, and the internal phase composition of the electrophoretic medium (electrophoretic particles and fluid). Electrophoretic media of the type described in U.S. patent application publication No. 2010/0289736, lacking ionic dopants in the binder, have been found to exhibit WSD of 2-9L units during the first 48 hours of operation at 25 ℃ with a 20% duty cycle of operation. This level of WSD is unacceptable for most applications requiring that the WSD be no greater than (and preferably much less than) about 3L x units after 240 hours of operation under these conditions.
While the incorporation of an ionic dopant (typically a salt) into the binder can reduce WSD, there are considerable difficulties in selecting a suitable salt. In practice, the fluid used in the internal phase of electrophoretic displays is an organic material, usually a low molecular weight hydrocarbon, the binder is a polymer formed from an aqueous solution or dispersion, the most common type of binder being polyurethane added as an aqueous emulsion. The latex is mixed with the capsules (when such capsules are present), or the internal phase is emulsified in the latex (in the case of polymer-dispersed electrophoretic media). A slurry of latex and capsules or droplets is then applied to the substrate and the slurry layer is dried to form a coherent electrophoretic layer. At first sight it appears that the ionic dopant should be a water-soluble salt that can be easily added to the aqueous polymer latex in the form of an aqueous solution. However, it was found that common water-soluble salts such as sodium chloride are not effective in mitigating WSD; for example, by adding a molar amount of sodium chloride corresponding to 250-350ppm TBAHPF to the binder of an electrophoretic medium of the type described in 2010/0289736 above, the WSD was about 7.5L units after 240 hours at a temperature of 25 ℃ and an operating duty cycle of 20%. Some hydrophobic (water insoluble) salts can achieve better results; for example, under the same conditions, TBAHPF, as mentioned in U.S. Pat. No. 7,012,735 at 250-350ppm, is added with a WSD of about 1L. The effect of these salts on WSD is concentration dependent, and their effect on a wider range of EO properties, including for example Thin Film Transistor (TFT) performance, image stability and dwell dependence, is sensitive to the concentration of other components in the slurry. Other components that apparently interact with the slurry dopant to affect EO performance include triton x100 (polyethylene glycol octyl phenyl ether) and the solvent used to disperse the salt into solution.
Unfortunately, water-insoluble salts present other problems. It is critical that the salt should be uniformly dispersed throughout the binder and simply dispersing the salt in water is not sufficient to produce such uniform dispersion. In practice, it is necessary to dissolve such salts in a suitable water-soluble organic solvent and to add the salt solution dissolved in this solvent to the latex forming the binder. Of course, the solvent chosen must not have any adverse effect on the properties of the capsules or microdroplets, the binder itself or the final dried electrophoretic layer. In practice, suitable solvents are limited to N-methylpyrrolidone (NMP), tetrahydrofuran and acetone. It is difficult to remove all traces of organic solvent (trace) during drying of the electrophoretic layer because the drying conditions are limited to those that can be tolerated by capsules or droplets containing volatile organic liquids. Traces of organic solvents remaining in the dried electrophoretic layer or adhesive layer are known to cause serious problems; see, for example, 2009/0122389 supra, which describes that traces of NMP remaining in the dried adhesive layer can cause damage to the back sheet containing the organic semiconductor. Traces of organic solvent remaining in the dried adhesive have also been found to negatively affect the electro-optic performance of the display; for example, such organic solvents affect the white state edge and the blackness of the black state of the display.
Accordingly, there is a need for a method of adding an ionic dopant to a binder for an electrophoretic medium without introducing an organic solvent into the binder, and the present invention provides such a method.