HK1171266B - Electrophoretic particles - Google Patents

Description

Electrophoretic particles

Technical Field

The present invention relates to electrophoretic particles (i.e., particles for use in an electrophoretic medium) and methods for making such electrophoretic particles. The invention also relates to electrophoretic media and displays using such particles. More particularly, the present invention relates to electrophoretic particles having surfaces modified with polymers.

This application is related to the applicant's international patent application No. wo 02/093246, the reader being referred to by way of background information.

Background

Electrophoretic displays have been the subject of extensive research and development for many years. Such displays have the advantage over liquid crystal displays of good brightness and contrast, a wide viewing angle, bistability of the states, and low power consumption (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 which differ in at least one optical property such that, after any given element is driven to assume its first or second display state using an addressing pulse of finite duration, that state will last for 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). However, long-term image quality issues of such displays have prevented their widespread use. For example, particles that make up electrophoretic displays tend to settle, which results in inadequate service life of such displays.

A number of patents and applications recently published, assigned to or entitled to the institute of technology and technology (MIT) and yingke (E Ink) corporation, describe various techniques for use in encapsulated electrophoresis and media. Such encapsulated media comprise a plurality of capsules (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 an adhesive layer (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. 5,961,804; 6,017,584; 6,120,588; 6,120,839, respectively; 6,262,706, respectively; 6,262,833; 6,300,932, respectively; 6,323,989, respectively; 6,377,387, respectively; 6,515,649, respectively; 6,538,801, respectively; 6,580,545, respectively; 6,652,075, respectively; 6,693,620, respectively; 6,721,083, respectively; 6,727,881, respectively; 6,822,782; 6,870,661, respectively; 7,002,728; 7,038,655, respectively; 7,170,670; 7,180,649, respectively; 7,230,750, respectively; 7,230,751, respectively; 7,236,290, respectively; 7,247,379, respectively; 7,312,916, respectively; 7,375,875, respectively; 7,411,720, respectively; 7,532,388 and 7,679,814; and U.S. patent application publication No. 2005/0012980; 2006/0202949, respectively; 2008/0013155, respectively; 2008/0013156, respectively; 2008/02662452008/0266246, respectively; 2009/0009852, respectively; 2009/0027762, respectively; 2009/0206499, respectively; 2009/0225398 and 2010/0045592;

(b) bladders, adhesives, and encapsulation methods; see, e.g., U.S. patent nos. 6,922,276 and 7,411,719;

(c) films and sub-assemblies comprising electro-optic material; see, e.g., U.S. patent No.6,982,178 and U.S. patent application publication No. 2007/0109219;

(d) a backplane, adhesive layer and other auxiliary layers and methods for use in a display; see, e.g., U.S. patent nos. 7,116,318 and 7,535,624;

(e) color formation and color adjustment; see, e.g., U.S. patent 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; and

(g) application for a display; see, e.g., U.S. patent No. 7,312,784; and U.S. patent application publication No. 2006/0279527.

Known encapsulated and non-encapsulated electrophoretic media can be divided into two main types, referred to hereinafter for convenience as "single particles" and "double particles", respectively, as discussed, for example, in U.S. patent No.6,870,661. A single particle medium has only one type of electrophoretic particle suspended in a colored suspension medium having at least one optical property different from that of the electrophoretic particle. A dual particle medium has two different types of particles that differ in at least one optical characteristic, and a suspension that may be colorless or colored, but is generally colorless. The electrophoretic mobility of the two types of particles is different; such differences in mobility can be in polarity (hereinafter this type of medium is referred to as "pair of charge double particles") and/or in size. Both single particle electrophoretic displays and dual particle electrophoretic displays can have intermediate gray states having optical properties intermediate the two extreme optical states described above.

In some of the aforementioned patents and published applications, it is disclosed to have three or more different types of particles in each capsule of an encapsulated electrophoretic medium. For the purposes of this application, these multi-particulate media are considered to be a sub-class of dual particulate media.

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 comprises a plurality of discrete electrophoretic fluid droplets and a continuous phase of polymeric material, and the discrete electrophoretic fluid droplets 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.

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 be made of a gaseous fluid; see, for example, Kitamura, T.et al, "electric tuner movement for electronic Paper-like display", IDW Japan, 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 gaseous-based electrophoretic media suffer from the same problems, for example, when used in an orientation that allows particle settling (e.g., when the media is used in a sign placed along a vertical plane), as liquid-based electrophoretic media suffer from the same particle settling. In fact, the problem of particle settling is more severe in gaseous-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.

The type of electrophoretic display associated with this is known as a "microcell electrophoretic display". In microcell electrophoretic displays, the charged particles and 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.

It has been known to alter the physical properties and surface characteristics of electrophoretic particles by absorbing different materials onto the surface of the particles or chemically bonding different materials to these surfaces. However, it has been found that such simple coatings of modifying materials on electrophoretic particles are not satisfactory because changes in operating conditions can cause some or all of the modifying material to detach from the surface of the particle, thereby causing the particle to undergo an undesirable change in electrophoretic properties; the modification material may also be deposited onto other surfaces within the electrophoretic display, which may cause other problems. Therefore, techniques for fixing the modifying material to the surface of the particle are still being studied.

The aforementioned WO 02/093246 describes a number of electrophoretic particles coated with a polymer. The monomers taught therein for use in such polymer-coated particles are some fluorinated monomers, and in example 20 of this patent application, a polymer coating is formed with about 10 mole percent of fluorinated monomers and about 90 mole percent of non-fluorinated monomers combined.

It has now been found that the use of a relatively low mole percentage (not higher than about 5 mole percentage) of fluorinated acrylate or methacrylate monomers (especially 2,2, 2-trifluoroethyl methacrylate, hereinafter abbreviated to "TFEM") in the polymeric shell of the electrophoretic particles can bring significant advantages, which are not mentioned in the aforementioned WO 02/093246. In particular, the use of such fluorinated monomers enables the charge on the pigment particles to be adjusted.

Disclosure of Invention

In one aspect, the present invention provides an electrophoretic medium comprising a plurality of pigment particles in a fluid, the pigment particles having a polymer chemically bonded to the pigment particles, wherein the polymer comprises from about 0.1 to about 5 mole percent of repeating units derived from a fluorinated acrylate monomer or a fluorinated methacrylate monomer.

The electrophoretic medium of the present invention may comprise any of the optional features of the polymeric shell described in the aforementioned WO 02/093246. The preferred proportion of polymer in the coated particles is substantially as described in the aforementioned WO 02/093246, i.e., the particles have from about 4 to about 15 percent, more desirably from about 8 to about 12 percent, by weight of the particles of polymer bonded to the particles. The particles may comprise a metal oxide or hydroxide, such as titanium dioxide. The polymer may comprise charged or chargeable groups, such as amino groups or carboxylic acid groups. The polymer can include a main chain and a plurality of side chains extending from the main chain, each side chain including at least about 4 carbon atoms. Generally, the polymer is formed from two or more acrylate monomers and/or methacrylate monomers.

Generally, fluorinated monomers are used in combination with non-fluorinated acrylate monomers or methacrylate monomers (i.e., the polymer may include residues derived from fluorinated and non-fluorinated acrylate monomers and/or methacrylate monomers), for which reason, lauryl methacrylate is a preferred monomer. The molar ratio of fluorinated monomer to non-fluorinated monomer can vary, but the fluorinated monomer typically comprises about 1 to 5 mole percent of all monomers in the polymer. Highly fluorinated monomers containing at least three fluorine atoms are preferred. A particularly preferred fluorinated monomer is 2,2, 2-trifluoroethyl methacrylate, but other fluorinated monomers such as 2,2,3,4,4, 4-hexafluorobutyl acrylate and 3,3,4,4,5,5,6,6,7,7,8,8, 8-tridecafluorooctyl acrylate can also be used.

The present invention relates to an electrophoretic display comprising an electrophoretic medium of the invention and at least one electrode arranged to apply an electric field to the electrophoretic medium, and to an electronic book reader, portable computer, tablet computer, cell phone, smart card, sign, watch, shelf label or flash drive comprising such a display.

Drawings

FIGS. 1A and 1B are schematic cross-sectional views through a first electrophoretic display according to the invention, wherein the electrophoretic medium comprises a single type of particles in a colored suspension;

FIGS. 2A and 2B are schematic cross-sectional views, similar to FIGS. 1A and 1B, respectively, through a second electrophoretic display in accordance with the invention, wherein the electrophoretic medium comprises two different types of particles of opposite charge in a colorless suspension;

FIGS. 3A and 3B are schematic cross-sectional views, similar to FIGS. 2A and 2B, respectively, through a third electrophoretic display in accordance with the invention, wherein the electrophoretic medium comprises two different types of particles of the same polarity of charge but having different electrophoretic mobilities in a colorless suspension;

FIGS. 4A and 4B illustrate polymer-dispersed electrophoretic media of the present invention and methods for making such electrophoretic media;

FIG. 5 is a bar graph showing the change in zeta potential with the proportion of fluorinated monomer in the polymer shell in the experiment of example 1 below;

FIG. 6 is a bar graph showing the instability of the dark and white states as a function of the proportion of fluorinated monomer within the polymer shell in the experiment of example 3 below;

FIG. 7 is a bar graph showing the maximum white state, minimum dark state and total dynamic range as a function of the proportion of fluorinated monomer in the polymer shell in the experiment of example 3 below.

Detailed Description

Before discussing in detail the electrophoretic media and methods of the present invention, a brief description of some types of electrophoretic displays in which these electrophoretic media may be used will be required.

The electrophoretic medium of the present invention may be any of the types of electrophoretic media described in the aforementioned patents and patent applications for E Ink and MIT, preferred embodiments of which are now described with reference to fig. 1 to 4.

Fig. 1A and 1B illustrate a first electrophoretic display (generally designated 100) of the present invention comprising an encapsulated electrophoretic medium (generally designated 102) comprising a plurality of capsules 104 (only one of which is shown in fig. 1A and 1B), each capsule containing a suspension 106 and a plurality of single type particles 108 dispersed therein, which for purposes of illustration is assumed to be black. The particles 108 are electrophoretically mobile and may be formed of carbon black. In the following description, it is assumed that particles 108 are positively charged, and if desired, negatively charged particles may also be used (triangular particles 108, and other square and round particles discussed below are for illustration only, to facilitate distinguishing between different types of particles in the figures, and thus never correspond to the physical form of actual particles (generally substantially spherical). The display 100 further comprises a common transparent front electrode 110 and a plurality of discrete back electrodes 102, wherein the common transparent front electrode 110 is used to form a viewing surface through which a viewer can see the display 100, and each back electrode 112 defines one pixel of the display 100 (only one back electrode 112 is shown in fig. 1A and 1B). Although in practice there are typically a large number (20 or more) of microcapsules for each pixel, for ease of illustration and understanding, fig. 1A and 1B show only one microcapsule for forming a pixel defined by the back electrode 112. The back electrode 112 is mounted on a substrate 114.

The suspension 106 is coloured such that the particles 108 located near the back electrode 112 as shown in fig. 1A are not visible to an observer viewing the display 100 through the front electrode 110. The desired color in suspension 106 may be provided by dissolving a dye in the liquid. Since the colored suspension 106 and particles 108 make the electrophoretic medium 102 opaque, the back electrode 112 and substrate 114 may be transparent or opaque because they are not visible through the opaque electrophoretic medium 102.

A larger size range of capsules 104 and particles 108 may be produced. In general, however, the thickness of the capsule (measured perpendicular to the electrodes) is preferably about 15 to 500 μm, and the diameter of the particles 108 is typically about 0.25 to about 2 μm.

Fig. 1A shows a display 100 having a negatively charged back electrode 112 and a positively charged front electrode 110. In this case, the positively charged particles 108 are attracted to the negative back electrode 112 and are positioned near the back electrode 112 so that they are hidden there by the coloured liquid 106 when the viewer views the display 100 through the front electrode 110. Thus, the pixel shown in FIG. 1A shows the color of the liquid 106 to the viewer, which is assumed to be white for ease of illustration (although FIG. 1A and FIG. 1B illustrate the display 100 with the back electrode 112 at the bottom, in practice, both the front and back electrodes are generally vertically disposed to maximize the visibility of the display 100. generally, the media and displays of the invention described herein do not control the movement of particles in any manner that relies on gravity, and in fact, the movement under gravity is much slower and therefore cannot be used to control the movement of particles).

FIG. 1B shows the display 100 having the front electrode 110 negative relative to the back electrode 112. As the particles 108 are positively charged they are attracted to the negatively charged front electrode 110 and the particles 108 move to the vicinity of the front electrode 110 and the pixel appears black to the particles 108.

In fig. 1A and 1B, the capsule 104 is illustrated in the general form of a prism (prism) whose width (parallel to the plane of the electrodes) is significantly greater than its height (perpendicular to the plane of the electrodes). This prismatic shape of the balloon 104 is intentional. Because the capsule, if generally spherical, in the black state shown in FIG. 1B, the particles 108 will tend to collect in the highest portion of the capsule within a limited area directly above the center of the capsule. The color seen by the viewer is then substantially the average of the black area at the center and the white ring (where the white liquid 106 is visible) surrounding the center area. Thus, even in the assumed black state, the viewer will see a grey colour instead of a pure black colour, and the contrast between the two extreme optical states of the pixel will be correspondingly limited. In contrast, in the prismatic form of the microcapsules shown in fig. 1A and 1B, the particles 108 cover substantially the entire cross-section of the capsule, and therefore, no or at least very little white liquid is visible, thereby enhancing the contrast between the two extreme optical states of the capsule. In this regard, and for further discussion of the desire to encapsulate the capsules in the electrophoretic layer, the reader is referred to the aforementioned U.S. patent No.6,067,185 and its corresponding international patent application publication WO 99/10767. In addition, as described in the aforementioned E Ink and MIT patents and applications, microcapsules are typically embedded in a solid binder for the purpose of mechanically integrating the electrophoretic medium, but the adhesive is omitted from fig. 1-3 for ease of illustration.

Fig. 2A and 2B illustrate a second electrophoretic display of the invention, generally indicated at 200, comprising an encapsulated electrophoretic medium, generally indicated at 202, comprising a plurality of capsules 204, each capsule containing a suspension 206 and a plurality of positively charged black particles 108 (the same as those discussed above in the first display 100) dispersed therein. The display 200 further comprises a front electrode 110, a back electrode 112 and a substrate 114, which are identical to the corresponding components in the first display 100. However, in addition to the black particles 108, there are a plurality of negatively charged particles 218, assumed to be white for the purposes of this application, suspended in the liquid 206.

Although the liquid 206 may have some color to adjust the optical performance of the display in various states, the liquid 206 is typically colorless (i.e., substantially transparent). Fig. 2A shows a display 200 having a front electrode 110 that is positively charged relative to the back electrode 112 of the pixel shown. The positively charged particles 108 are electrostatically close to the back electrode 112 and the negatively charged particles 208 are electrostatically close to the front electrode 110. A viewer looking at the display 200 through the front electrode sees a white pixel, since the white particles 218 are visible, while the black particles 108 are hidden.

Fig. 2B shows a display 200 having a front electrode 110 that is negatively charged relative to the back electrode 112 of the pixel shown. When in the corresponding optical state shown in fig. 1B, the positively charged particles 108 are electrostatically attracted to the negative front electrode 110 and the negatively charged particles 218 are electrostatically attracted to the positive back electrode 112. Thus, the particles 108 move to the vicinity of the front electrode 110 and the pixel shows the black color of the particles 108, while the white particles 218 are hidden.

Fig. 3A and 3B illustrate a third electrophoretic display of the invention, generally indicated at 300, comprising an encapsulated electrophoretic medium, generally indicated at 302, comprising a plurality of capsules 304. The display 300 further includes a front electrode 110, a back electrode 112, and a substrate 114, which are identical to the corresponding components in the first display 100 and the second display 200 described previously. The display 300 is similar to the display 200 described above in that the liquid 306 is colorless and has negatively charged white particles 218 suspended therein. However, the display 300 differs from the display 200 in that the former has negatively charged red particles 320, which have a much lower electrophoretic mobility than the white particles 218.

Fig. 3A shows a display 300 having a front electrode 110 that is positively charged relative to the back electrode 112 of the pixel shown. The negatively charged white particles 218 and the negatively charged red particles 320 are both attracted to the front electrode 110, but due to the much higher electrophoretic mobility of the white particles 218 they reach the front electrode 110 first (note that the optical state shown in fig. 3A is typically due to the polarity on the electrodes being abruptly reversed in the optical state shown in fig. 3B, thus forcing the white particles 218 and the red particles 320 to flow through the thickness of the capsules 304 and allowing the white particles 218 with the greater mobility to reach a position close to the front electrode 110 before the red particles 320). Thus, the white particles 218 form a continuous layer next to the front electrode 110, thereby hiding the red particles 320. A viewer looking at the display 300 through the front electrode 110 sees a white pixel, since the white particles 218 are visible and the red particles 320 are hidden.

Fig. 3B shows a display 300 having a front electrode 110 that is negatively charged relative to the back electrode 112 of the pixel shown. Both the negatively charged white particles 218 and the negatively charged red particles 320 are attracted to the back electrode 112, but due to the higher electrophoretic mobility of the white particles 218, when the polarity of the electrodes in the optical state shown in fig. 3A is reversed to produce the optical state shown in fig. 3B, the white particles 218 reach the back electrode 112 faster than the red particles 320, so that the white particles 218 form a continuous layer near the electrode 112, while the red particles 320 form a continuous layer facing the front electrode 110. Thus, a viewer looking at the display 300 through the front electrode 110 sees a red pixel, since the red particles 320 are visible, while the white particles 218 are hidden.

Fig. 4A and 4B illustrate a polymer-dispersed electrophoretic medium of the present invention and a method for manufacturing such a medium. The polymeric dispersion medium comprises droplets that are non-spherical and is prepared using a film-forming material, wherein a film formed from the film-forming material shrinks significantly after being formed. For this purpose, the preferred discontinuous phase is gelatin, however other proteinaceous materials and crosslinkable polymers may be employed instead. The liquid material (which will eventually form the continuous phase) and droplets are formed into a mixture and the mixture is coated onto a substrate to form the structure illustrated in fig. 4A. Fig. 4A shows a layer 410 during film formation, comprising droplets 412 dispersed in a liquid medium 414, the layer 410 being coated onto a substrate 416 (preferably a flexible polymer film, such as a polyester film) having a layer 418 of transparent conductive material (such as indium tin oxide) prior to the substrate 416. A relatively thick layer 410 of liquid material contains generally spherical droplets 412, as shown in fig. 4A. After layer 410 forms the solid continuous phase, the layer is dried, preferably at about room temperature, for a period of time sufficient to dehydrate the gelatin (although the layer may be heated if desired) to provide a significant reduction in layer thickness and form the structure shown in fig. 4B, where the dried and shrunk layer is indicated at 410'. Vertical contraction of the layers (i.e., contraction perpendicular to the surface of the substrate 416) causes the formerly spherical droplets to be compressed into oblate ellipsoids whose thickness in the direction perpendicular to the surface is significantly less than their size in the transverse direction parallel to the surface. In practice, the droplets will typically be packed very tightly (pack) so that the lateral edges of adjacent droplets touch each other and the final shape of the droplets is closer to an irregular prism than a flat ellipsoid. Furthermore, as shown in FIG. 4B, there may be more than one layer of droplets in the resulting medium. In the type of media shown in fig. 4B, where the droplets are polydisperse (i.e., there is a large range of droplet sizes), the benefit of such a multilayer droplet is to reduce the chance that tiny areas of the substrate will not be covered by any droplet when the media is of the above type; such a multilayer thus helps to ensure that the electrophoretic medium is completely opaque and that no part of the substrate is visible in a display formed from the medium. However, in media that employ substantially monodisperse droplets (i.e., all droplets are substantially the same size), it is generally recommended to coat a layer of media that can form a close packed monolayer of droplets upon contraction of the layer of media, see U.S. patent No.6,839,158. Because of the lack of relatively rigid microcapsule walls in microencapsulated electrophoretic media, droplets in the polymeric dispersion media of the present invention tend to pack more tightly into a tightly packed monolayer than microcapsules.

Contrary to what was expected, experiments found that the droplets did not bind when the medium was dry. However, we do not exclude this possibility: in some embodiments of the invention, the walls of adjacent capsules may rupture, forming a partial connection between droplets.

During the drying step, the droplets are deformed, and the degree of deformation of the droplets can be varied by controlling the proportion of water in the gelatin solution and the ratio of the solution to the droplets, thereby varying the final form of the droplets. For example, experiments were conducted using 2 to 15 weight percent gelatin solution, taking 200 grams of each gelatin solution and 50 grams of the inner non-aqueous phase forming the droplets. In order to produce a final thickness of 30 μm for the electrophoretic medium layer, a 139 μm thick mixture of 2% gelatin solution/inner phase was applied; after drying, the layer formed a 30 μm thick electrophoretic medium containing 92.6 volume percent droplets. On the other hand, in order to produce an electrophoretic medium having the same final thickness, it is necessary to coat a layer of a mixture having a thickness of 93 μm, wherein the mixture is a 15% gelatin solution/internal phase; after drying, an electrophoretic medium was formed containing 62.5 volume percent droplets. Media made from 2% gelatin solutions are more breakable than desired when subjected to strong external forces; while media made from solutions containing 5 to 15 weight percent gelatin have satisfactory mechanical properties.

The degree of deformation of a droplet in the final electrophoretic medium is also affected by the original size of the droplet and the relationship between this original size and the thickness of the final layer of the electrophoretic medium. Experiments have shown that the larger the average original size of a droplet and/or the larger the ratio of this average original size to the thickness of the final layer, the larger the deformation of the droplet from a spherical shape into a shape in the final layer. In general, the preferred average original size of the droplets is from about 25 to about 400 percent of the final layer thickness. For example, in the experiments described above with a final layer thickness of 30 μm, good results were obtained with droplets having an average original size of 10 to 100 μm.

Gelatin forms a film by sol/gel transition, but the present invention is not limited to a film-forming material that forms a film by such sol/gel transition. Film formation may be achieved, for example, by polymerization of monomers or oligomers, crosslinking of polymers or oligomers, radiation curing of polymers, or other known film forming methods. Similarly, in a preferred variation of the invention, the film is formed first and then shrinkage in thickness is induced, but this shrinkage need not be achieved in the same manner as the gelatin film shrinkage (dehydration mechanism), which can be achieved by removing the aqueous or non-aqueous solvent, the crosslinked polymer film, or any other conventional process from the film.

In the polymer-dispersed electrophoretic medium of the present invention, it is desirable to include droplets in an amount of at least about 40 percent, and preferably from about 50 to about 80 percent, by volume of the electrophoretic medium; see U.S. patent No.6,866,760. It should be emphasized that the droplets used in the polymer-dispersed electrophoretic medium of the present invention may have any combination of particles and suspensions as shown in fig. 1 to 3.

The invention is applicable to any form of encapsulated electrophoretic medium shown in figures 1 to 4. However, the invention is not limited to encapsulated and polymer dispersed electrophoretic media and may also be applied to microcell and non-encapsulated media.

As is evident from the following example, controlling the amount of fluorinated monomer used in a polymer shell of particles for use in an electrophoretic display increases the zeta potential of negatively charged particles, which in the usual case are white particles such as titanium dioxide, and the resulting increased negative zeta potential causes themselves to exhibit an improved (more reflective) white state. The zeta potential becomes more negative as the proportion of fluorinated monomer in the polymer shell increases. However, some defects are also evident when the mole percentage of zeta fluorinated monomer is above about 5. The image loss in the dark state (measured as the change in the dark state of the display after, say, 2 minutes without the display being driven) starts to increase and the dark state itself becomes lighter, thereby adversely affecting the dynamic range of the display (the difference between the dark and white states of the display, in units of L)^*Is represented by (wherein L^*The general CIE definition of (a) is:

L^*＝116(R/Ro)^1/3-16，

wherein R is the reflectance, R₀Standard reflectance value)). Therefore, the molar ratio of fluorinated monomer in the polymer shell is preferably maintained in the range of about 0.1 to about 5 mole percent, and more desirably in the range of about 1 to about 5 mole percent. It will be appreciated that the optimum ratio of fluorinated monomersExamples may vary somewhat depending on the particular fluorinated monomer used (and in particular the degree of fluorination of the fluorinated monomer), other monomers being used and other factors including other particles present in the electrophoretic medium. Generally, the optimum ratio of fluorinated monomer is about 1 mole percent, since the level of fluorinated monomer allows for a significant increase in the zeta potential while avoiding the aforementioned drawbacks associated with higher ratios of fluorinated monomer.

The polymer-coated particles used in the electrophoretic medium of the present invention may be manufactured using any of the methods mentioned in the aforementioned WO 02/093246. In such a process, the particles on which the polymeric coating is formed are reacted with a bifunctional reagent having functional groups capable of reacting with and bonding to the particles and having polymerizable groups, such as pendant double bonds or other ethylenically unsaturated groups.

Some examples are given below by way of illustration only to show specific details of particularly preferred reagents, conditions and techniques for use in the present invention.

Example 1: preparation of white titanium dioxide pigment having 2,2, 2-trifluoroethyl methacrylate and lauryl methacrylate in the polymer shell

DuPont R-794 titanium dioxide surface functionalized with 3- (trimethoxysilyl) propyl methacrylate (3- (trimethyoxysilyl) propyl methacrylate) was prepared essentially as described in the aforementioned PCEP application. In a 1L plastic bottle, 500 g of the pigment was dispersed by sonication in 426 g (500mL) of toluene. A 1L glass sleeve reactor was filled with 1.7158 moles of monomer, wherein 1.7158 moles of monomer were allocated to lauryl methacrylate and TFEM to obtain the desired molar concentration of each monomer. The molar ratios of TFEM were 0.1, 1, 5, 10, 25 and 50 mol%, the remainder being lauryl methacrylate. The pigment dispersant was added to the reactor, which was purged with nitrogen and heated to 65 ℃. A radical initiator (5.0 g of 2, 2-Azobisisobutyronitrile (AIBN)) dissolved in advance in 110mL of toluene was added dropwise over 60 minutes. The vessel was heated overnight under a nitrogen atmosphere with constant stirring at 65 ℃ and then exposed to air. Thereafter, the mixture was divided into 4 1L plastic bottles, and about 500mL of toluene was added to each bottle. The bottles were stirred vigorously. The pigment was isolated by centrifugation at 3500rpm for 20 minutes. The upper liquid was decanted off and the pigment was washed twice by adding approximately 700mL of toluene to each bottle, vigorously stirring until the pigment was dispersed, and then centrifuging at 3500rpm for 20 minutes. The pigment was air dried overnight and then dried under vacuum at 65 ℃ overnight. Thermogravimetric analysis (TGA) was performed, yielding a concentration of polymer between 6.7% and 9.7% by weight. Samples dispersed in Isopar E with surfactant (Solsperse 17K) were subjected to Zeta potential testing using a colloid Dynamics Zeta potentiometer. Fig. 5 shows the zeta potential number.

As can be seen from the data in FIG. 5, the magnitude of the zeta potential increases with increasing TFEM in the polymer shell.

Example 2: manufacture of displays using the electrophoretic media of the invention

The polymer-coated titanium dioxide particles prepared in example 1 above were converted into an electrophoretic medium in the following manner

Part A: preparation of capsules

Gelatin-gum arabic microcapsules were prepared using the pigment prepared in example 1 and following procedure. The internal phase was prepared by adding the following to a 250mL plastic bottle.

The resulting mixture is then converted into gelatin-gum arabic microcapsules substantially as described in examples 27-29 of the aforementioned U.S. patent No.6,822,782.

And part B: manufacturing a display

The microcapsules prepared above in part a were allowed to stand and excess water was decanted off. Thereafter, the capsules and the polymer binder were mixed in a weight ratio of 8 parts to 1 part to prepare a slurry. The resulting slurry was coated by bar coating onto a polymer film coated with Indium Tin Oxide (ITO) with a coating gap of 4 mils (101 μm) to a final thickness of 18 μm, and then dried in a conveying oven at 60 ℃ for about 2 minutes, and the resulting sheet was cut into a plurality of cut pieces.

Separately, the custom polyurethane laminate adhesive described in U.S. Pat. No. 7,012,735 was doped with 180ppm tetrabutylammonium hexafluorophosphate (tetrabutylammonium hexafluorophosphate) and coated onto a release sheet to a thickness of 25 μm and then cut to a size slightly smaller than the cut pieces of the microcapsule/polymer film. After passing through a hot roll laminator with the upper and lower roll temperatures set to 120 ℃, the two films were laminated to the coating layer, and the resulting combined film was cut to the desired size. The release sheet was removed and the laminating adhesive layer was laminated to a 2 inch (51mm) square polymer film comprising an additional graphite layer laminated through a laminator with upper and lower roll temperatures of 93 ℃. Individual pixel displays were cut out of the resulting laminate and electrically connected to produce experimental individual pixel displays which were conditioned at a relative humidity of 50% for 5 days.

Example 3: electro-optical testing

Electro-optical testing was performed on the single pixel display prepared in example 2 using a PR-650 SpectraScan colorimeter. In conducting the test, the display was repeatedly driven to the black and white extreme optical states and then to one of the black or white extreme optical states by a 250 millisecond, 15V pulse. The reflectance of the optical state was tested at about 3 seconds after the final drive pulse (to stagger some transient effects) and 2 minutes after the final drive pulse, and the two test results were compared to monitor for any image instability (i.e., lack of bistability in the image).

Fig. 6 shows the test results (where "DS" indicates a dark state and "WS" indicates a white state — the image instability value in the white state is negative because the image instability in the white state results in lower reflectance), from which it can be seen that the image instability increases significantly when the content of TFEM in the polymer shell layer exceeds 1 mole percent. When the mole percentage of TFEM was 0.1 and 1, the image stability was equal to or slightly better than the control. FIG. 7 (where "DR" denotes dynamic range) shows the maximum white state, minimum dark state and total dynamic range for each display after accounting for image instability as shown in FIG. 6. It can be seen that as the TFEM content increases to 10 mole percent (but not 10 mole percent), the optical state tends to improve over the control. However, the degradation of the optical state at higher TFEM contents during the improvement can be attributed to the reduced image stability shown in fig. 6. As can be seen from fig. 6 and 7, the addition of TFEM in the polymer shell layer may improve the optical states, especially the final dynamic range, but TFEM has a window content within which TFEM can improve the optical states without loss of image bistability, a major advantage of electrophoretic displays.

It has been confirmed by other experiments that other fluorinated monomers (i.e. 2,2,3,4,4, 4-hexafluorobutyl acrylate and 3,3,4,4,5,5,6,6,7,7,8,8, 8-tridecafluorooctyl acrylate) can also adjust the zeta potential of the white pigment in a similar manner to TFEM and can also improve the optical state. The exact mechanism by which these curing monomers cause a change in zeta potential and a change in optical state is not known to date.

Claims (16)

1. An electrophoretic medium comprising a plurality of pigment particles suspended in a fluid, the pigment particles having a polymer chemically bonded to the pigment particles, wherein 0.1 to 5 mole percent of the polymer comprises repeating units derived from a fluorinated acrylate monomer or a fluorinated methacrylate monomer.

2. An electrophoretic medium according to claim 1 wherein 1 to 5 mole percent of the polymer comprises repeat units derived from fluorinated acrylate monomers or fluorinated methacrylate monomers.

3. An electrophoretic medium according to claim 1 wherein the particles have 4 to 15 percent by weight of the pigment of the polymer chemically bonded to the pigment particles.

4. An electrophoretic medium according to claim 3 wherein the particles have from 8 to 12 percent by weight of the pigment of the polymer chemically bonded to the pigment particles.

5. An electrophoretic medium according to claim 1 wherein the polymer comprises a main chain and a plurality of side chains extending from the main chain, each side chain comprising at least 4 carbon atoms.

6. An electrophoretic medium according to claim 1 wherein the polymer further comprises residues derived from non-fluorinated acrylate monomers and/or non-fluorinated methacrylate monomers.

7. An electrophoretic medium according to claim 6 wherein the non-fluorinated methacrylate monomer comprises lauryl methacrylate.

8. An electrophoretic medium according to claim 1 wherein the fluorinated methacrylate monomer comprises 2,2, 2-trifluoroethyl methacrylate and the fluorinated acrylate monomer comprises at least one of 2,2,3,4,4, 4-hexafluorobutyl acrylate and 3,3,4,4,5,5,6,6,7,7,8,8, 8-tridecafluorooctyl acrylate.

9. An electrophoretic medium according to claim 1 having two types of particles which differ in at least one optical property and have different electrophoretic mobilities.

10. An electrophoretic medium according to claim 9 wherein the two types of particles carry charges of opposite polarity.

11. An electrophoretic medium according to claim 1 wherein the pigment particles and the fluid are encapsulated in a plurality of capsules or microcells.

12. An electrophoretic medium according to claim 11 wherein the capsules are held in a polymeric binder.

13. An electrophoretic medium according to claim 1 wherein the pigment particles and the fluid are embodied as a plurality of discrete droplets surrounded by a continuous phase comprising a polymeric material.

14. An electrophoretic medium according to claim 1 wherein the fluid is gaseous.

15. An electrophoretic display comprising an electrophoretic medium according to claim 1 and at least one electrode arranged to apply an electric field to the electrophoretic medium.

16. An electronic book reader, portable computer, tablet computer, cell phone, smart card, sign, watch, shelf label or flash drive comprising an electrophoretic display according to claim 15.