JP2012530283A - Electrophoretic particles - Google Patents

Abstract

In electrophoretic media, it is advantageous to use pigment particles having a polymer shell containing 0.1 to 5 mole percent of repeating units derived from a fluorinated acrylate monomer or a fluorinated methacrylate monomer. The polymer desirably has a branched structure having side chains extending from the main chain. The present invention relates to an electrophoretic medium and a display comprising such particles. More particularly, the present invention relates to electrophoretic particles whose surface is modified with a polymer.

Description

  The present invention relates to electrophoretic particles (ie, particles for use in electrophoretic media) and processes for making such electrophoretic particles. The invention also relates to electrophoretic media and displays comprising such particles. More specifically, the present invention relates to an electrophoretic particle whose surface is modified with a polymer.

  This application is related to Applicant's International Application No. 02/093246, and the reader is referred to this International Application for background information.

  Electrophoretic displays have been the subject of intense research and development for many years. Such displays may have the characteristics of better brightness and contrast, wider viewing angles, higher state bistability, and lower power consumption than liquid crystal displays. (As used herein, the terms “bistable” and “bistable” are displays having at least one display element having first and second display states with different optical properties, given either After the display element is driven to take these first or second display states using an address pulse of finite duration, the state is determined after the address pulse has ended. (Although used in the normal sense of the art to refer to a display configured to last at least several times, eg at least 4 times, the minimum duration of the address pulse required to change state). First, these electrophoretic displays have been hindered from widespread use due to the problems associated with long-term image quality. For example, the particles that make up electrophoretic displays tend to settle, and as a result, they do not have a sufficient useful life for these displays.

Recently, Massachusetts Institute of Technology and E Ink Corporation, which disclosed various techniques used in encapsulated electrophoresis and media, have been published or published numerous patents and patent applications in the name of these. Such an encapsulating medium consists of a number of small capsules with an internal phase which itself contains electrophoretic mobile particles in a liquid medium and a capsule wall surrounding the internal phase. Usually, such capsules are themselves held in a polymer binder to form a coherent layer located between the two electrodes. The technologies disclosed in these patents and patent applications include the following:
(A) Electrophoretic particles, fluid and fluid additive; for example, Patent Literature 1, Patent Literature 2, Patent Literature 3, Patent Literature 4, Patent Literature 5, Patent Literature 6, Patent Literature 7, Patent Literature 8, Patent Literature 9 , Patent Literature 10, Patent Literature 11, Patent Literature 12, Patent Literature 13, Patent Literature 14, Patent Literature 15, Patent Literature 16, Patent Literature 17, Patent Literature 18, Patent Literature 19, Patent Literature 20, Patent Literature 21, Patent Literature 22, Patent Literature 23, Patent Literature 24, Patent Literature 25, Patent Literature 26, Patent Literature 27, Patent Literature 28, Patent Literature 29, Patent Literature 30 and Patent Literature 31, and Patent Literature 32, Patent Literature 33, Patent Literature 34 , Patent Literature 35, Patent Literature 36, Patent Literature 37, Patent Literature 38, Patent Literature 39, Patent Literature 40, Patent Literature 41, and Patent Literature 42;
(B) capsules, binders and encapsulation processes; see, for example, US Pat.
(C) a subassembly containing a film and an electro-optic material; see, for example, Patent Document 45 and Patent Document 46;
(D) Backplane, adhesive layer and other auxiliary layers and methods used in displays; see, for example, Patent Document 47 and Patent Document 48;
(E) Color formation and color adjustment; for example, Patent Document 49 and Patent Document 46;
(F) Display driving method; for example, Patent Document 50 and Patent Document 51; and (g) Display application; for example, Patent Document 52 and Patent Document 53.

  For example, as described in Patent Document 18, known electrophoretic media can be mainly divided into two types of encapsulation and non-encapsulation. This is referred to as “two particles”. A single particle medium has only one type of electrophoretic particles suspended in a colored suspending medium, and at least one optical property of the medium is different from the optical properties of the particles. A two-particulate medium has two types of particles that differ in at least one optical property and a suspension fluid that may or may not be colored, but is usually not colored. These two types of particles differ in electrophoretic mobility, and the difference in mobility is in polarity (this type may hereinafter be referred to as “differently charged two-particle” medium) and / or in size. is there. It is believed that both single particle electrophoretic displays and two particle electrophoretic displays can assume an intermediate gray state with optical properties that are intermediate between the two extreme optical states already described.

  Some of the above patents and published applications disclose encapsulated electrophoretic media having more than two types of particles within each capsule. In this application, such multiparticulate media are considered a subclass of two-particle media.

  In many of the above patents and patent applications, the walls surrounding the discrete microcapsules in the encapsulated electrophoretic medium can be replaced by a continuous phase, so that the electrophoretic medium is a plurality of discrete electrophoretic fluids. A so-called polymer-dispersed electrophoretic display comprising a droplet and a continuous phase of polymer material, and the discrete droplets of electrophoretic fluid in such a polymer-dispersed electrophoretic display, each individual droplet Are not accompanied by a separate capsule membrane, but they can be considered as capsules or microcapsules; see, for example, US Pat. Therefore, in this application, such polymer-dispersed electrophoretic media are regarded as a subclass of encapsulated electrophoretic media.

  As mentioned above, the electrophoretic medium requires the presence of a fluid. In many prior art electrophoretic media, this fluid is a liquid, but gaseous fluids can be used to make electrophoretic media; see, for example, Non-Patent Document 1 and Non-Patent Document 2. See also Patent Literature 55 and Patent Literature 56. Such gas-based electrophoretic media are liquid-based electrophoretic media when the media is used in a label that allows particles to settle, for example, when the media is used as a vertical surface. It seems likely to cause problems due to the same kind of particle sedimentation. In fact, the problem of particle settling is more serious in gas-based electrophoretic media than in liquid-based electrophoretic media. This is because the electrophoretic particles can settle more rapidly because the viscosity of the gaseous suspension fluid is lower than that of the liquid suspension.

  A related type of electrophoretic display is the so-called “microcell electrophoretic display”. In microcell electrophoretic displays, charged particles and fluid are not encapsulated in microcapsules but are held in a plurality of cavities formed in a carrier medium, usually a polymer film. For example, see Patent Document 57 and Patent Document 58 (both assigned to Sipix Imaging, Inc.).

  The physical properties and surface properties of electrophoretic particles can be significantly modified by adsorbing various materials on the surface of the particles or by chemically bonding various materials to such surfaces. Known since before. After that, simply covering the electrophoretic particles with the modifying material causes part or all of the modifying material to detach from the particle surface when the operating conditions change, thereby causing an undesirable change in the electrophoretic properties of the particles. It has been found that this is not very satisfactory and the modifier may be deposited on other surfaces in the electrophoretic display, which may cause further problems. . Therefore, a technique for fixing the modifying material to the particle surface has been developed.

  Patent Document 59 describes a wide variety of polymer-coated electrophoretic particles. There are a number of fluorinated monomers that are taught to be useful in such polymer-coated particles, and the present patent application includes about 10 mole percent fluorinated monomer to about 90 mole percent non-fluorine. Example 20 is included which is used in combination with a fluorinated monomer to form a polymer coating.

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Kitamura, T .; “Electrical toner movement for electrical paper-like display”, IDW Japan, 2001, Paper HCS1-1 Yamaguchi, Y .; “Toner display using insulative particles charged triboelectrically”, IDW Japan, 2001, Paper AMD4-4

  At present, electrophoresis of fluorinated acrylate monomer or fluorinated methacrylate monomer (especially 2,2,2-trifluoroethyl methacrylate, hereinafter abbreviated as “TFEM”) with a relatively small molar ratio (about 5 mole percent or less). Use in the polymer outer shell of the particles provides significant advantages not described in WO02 / 093246 above. Specifically, when such a fluorinated monomer is used, the charge of the pigment particles can be adjusted.

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

  The electrophoretic medium of the present invention can comprise any of the optional features of the polymer shell described in WO02 / 093246 above. The preferred proportion of polymer in the coated particles will usually be substantially as described in WO 02/093246, i.e. such particles are about 4 to about 15 percent by weight of the polymer particles, desirably about Eight to about 12 percent are bound to the particles. The particles can include metal oxides or hydroxides such as titania. The polymer can contain charged or charged groups, such as amino or carboxylic acid groups. The polymer includes a main chain and a plurality of side chains extending from the main chain, each of which can include at least about 4 carbon atoms. Usually the polymer is formed from two or more acrylate and / or methacrylate monomers.

  Typically, the fluorinated monomer will be used in combination with a non-fluorinated acrylate monomer or a non-fluorinated methacrylate monomer (ie, the polymer is a fluorinated acrylate monomer and / or methacrylate monomer and a non-fluorinated acrylate monomer and / or Residues derived from both of the methacrylate monomers can be included), lauryl methacrylate is a preferred monomer for this purpose. Although the molar ratio of fluorinated monomer to non-fluorinated monomer can vary, typically the fluorinated monomer will comprise from about 1 to about 5 mole percent of the total monomer in the polymer. Highly fluorinated monomers containing at least 3 fluorine atoms are preferred. A particularly preferred fluorinated monomer is 2,2,2-trifluoroethyl methacrylate, although 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.

  The present invention includes an electrophoretic display according to the present invention, an electrophoretic display including at least one electrode arranged to apply an electric field to the electrophoretic medium, an electronic book reader including such a display, and a portable computer. , Tablet computers, mobile phones, smart cards, signs, watches, shelf labels or flash drives.

FIG. 1A is a schematic cross-sectional view of a first electrophoretic display of the present invention in which the electrophoretic medium includes a single type of particles in a colored suspending fluid. FIG. 1B is a schematic cross-sectional view of a first electrophoretic display of the present invention in which the electrophoretic medium includes a single type of particles in a colored suspending fluid. FIG. 2A is a schematic cross-sectional view, generally similar to FIG. 1A, of a second electrophoretic display of the present invention in which the electrophoretic medium includes two different types of particles having different polar charges in a non-colored suspending fluid. It is. FIG. 2B is a schematic cross-sectional view, generally similar to FIG. 1B, of a second electrophoretic display of the present invention in which the electrophoretic medium includes two different types of particles having different polar charges in a non-colored suspending fluid. It is. FIG. 3A shows a third electrophoretic display of the present invention in which the electrophoretic medium includes two different types of particles having the same polarity of charge in a non-colored suspending fluid but different electrophoretic mobility. FIG. 2 is a schematic cross-sectional view that is substantially similar. FIG. 3B shows a third electrophoretic display of the present invention in which the electrophoretic medium includes two different types of particles having the same polarity of charge in the non-colored suspending fluid but differing in electrophoretic mobility. FIG. 2 is a schematic cross-sectional view that is substantially similar. FIG. 4A is a diagram illustrating the polymer-dispersed electrophoretic medium of the present invention and the process used to manufacture the medium. FIG. 4B is a diagram illustrating the polymer dispersed electrophoretic medium of the present invention and the process used to manufacture the medium. FIG. 5 is a bar graph showing the zeta potential varying with the proportion of fluorinated monomer in the polymer shell in the experiment reported in Example 1 below. FIG. 6 is a bar graph showing the dark and white state instabilities varying with the proportion of fluorinated monomer in the polymer shell in the experiment reported in Example 3 below. FIG. 7 is a bar graph showing the highest white state, lowest dark state, and total dynamic range that vary with the proportion of fluorinated monomer in the polymer shell in the experiment reported in Example 3 below.

  Before describing in detail the electrophoretic medium and process of the present invention, it may be desirable to briefly describe some types of electrophoretic displays that are intended to use the medium.

  The electrophoretic medium of the present invention can be of any of the types described in the above E Ink and MIT patents and patent applications, wherein a preferred embodiment of such a medium is illustrated in FIG. 1 of the accompanying drawings. It demonstrates based on thru | or FIG.

  The first electrophoretic display of the present invention shown in FIGS. 1A and 1B (generally indicated by reference numeral 100) has a plurality of capsules 104 (only one of them is shown in FIGS. 1A and 1B). Encapsulated electrophoretic medium (denoted as a whole by reference numeral 102), each of the plurality of capsules including a suspension 106, and a plurality of a single type of black inside the capsule for purposes of illustration. The particles 108 are dispersed. The particles 108 have electrophoretic mobility and can be formed of carbon black. In the following description, it is assumed that the particles 108 are positively charged, but of course, negatively charged particles can be used if desired. (The triangles of the particles 108 and the squares and circles of the other particles described below are used merely to illustrate the various particle types in the accompanying figures and are generally substantially spherical. However, it does not preclude the use of non-spherical particles in the display.) In addition, the display 100 includes a transparent common front electrode 110 and Including a plurality of spaced back electrodes 112 (only one back electrode 112 is shown in FIGS. 1A and 1B), the transparent common front electrode forming a viewing surface, Viewed through the display surface, each of the plurality of spaced back electrodes defines a pixel of the display 100. For ease of explanation and understanding, FIGS. 1A and 1B show only a single microcapsule that forms a pixel defined by the back electrode 112, but in practice a large number (20 Or more) microcapsules are usually used. The back electrode 112 is attached to the base material 114.

  The suspension 106 is colored so that the particles 108 in the position shown in FIG. 1A adjacent to the back electrode 112 are not visible to the viewer viewing the display 100 through the front electrode 110. The color necessary for the suspension 106 can be obtained by dissolving the dye in this liquid. Since the colored suspension 106 and the particles 108 make the electrophoretic medium 102 opaque, the back electrode 112 and the substrate 114 are not visible through the opaque electrophoretic medium 102 and may be transparent or opaque.

  Capsule 104 and particles 108 can be of various sizes. In general, however, it is preferred that the capsule thickness, measured at right angles to the electrodes, is in the range of about 15 to about 500 μm, and the particles 108 have a diameter typically in the range of about 0.25 to about 2 μm.

FIG. 1A shows a display 100 in which the back electrode 112 is negatively charged and the front electrode 110 is positively charged. Under this condition, the positively charged particles 108 are attracted to the negative back electrode 112, so that they are adjacent to the back electrode 112, in which case the particles are from an observer viewing the display 100 through the front electrode 110. Concealed by the colored liquid 106. Accordingly, the pixel shown in FIG. 1A displays the color of the liquid 106 to the observer, and this liquid 106 is white for the sake of illustration. (In FIGS. 1A and 1B, the display 100 is shown with the back electrode 112 down, but in practice, both the front and back electrodes are vertically positioned so that the display 100 is best seen. In general, the media and displays of the present invention described herein do not rely on gravity to control the movement of particles; such movement by gravity is actually very slow. So it is not useful to control the movement of particles.)
FIG. 1B shows the display 100 with the front electrode 110 negative relative to the back electrode 112. Since the particle 108 is positively charged, it is attracted to the negatively charged front electrode 110, so that the particle 108 moves and is adjacent to the front electrode 110, and the pixel displays the black color of the particle 108.

  In FIGS. 1A and 1B, the capsule 104 is illustrated as being substantially prismatic and having a width (parallel to the face of the electrode) that is significantly greater than a height (perpendicular to such face). . This prism shape of the capsule 104 is intentional. If the capsule 104 is essentially spherical, the black-state particles 108 shown in FIG. 1B will tend to collect in the highest part of the capsule in a limited area centered just above the center of the capsule. . At that time, the color visible to the observer is the average of the central black region and the white annular portion surrounding the central region, and in this case, the white liquid 106 is visible. Therefore, even in this black state, the observer will see a grayish color instead of pure black, and the contrast between the extreme optical states of the pixel will be limited accordingly. become. In contrast, in the case of the prism-shaped microcapsules shown in FIGS. 1A and 1B, the particles 108 basically cover the entire cross section of the capsule, so that no or at least almost no white liquid is visible and the extremes of the capsule are The contrast of the optical state is amplified. In order to further discuss this point and the desirability of tightly packing such capsules in the electrophoretic layer, the reader is referred to the above-mentioned US Pat. No. 6,067,185 and the corresponding published international application WO 99/10767. See Also, as described in the above E Ink and MIT patents and patent applications, in order to provide mechanical integrity to the electrophoretic medium, the microcapsules are usually embedded in a solid binder. The agent is omitted from FIGS. 1 to 3 for easy understanding.

  The second electrophoretic display of the present invention shown in FIGS. 2A and 2B (generally designated 200) includes an encapsulated electrophoretic medium (generally designated 202) comprising a plurality of capsules 204. Each of the plurality of capsules includes a suspension body 206, and a plurality of positively charged black particles 108, which are also studied, are dispersed inside the capsule in the capsule of the first display 100 studied above. The display 200 further includes a front electrode 110, a back electrode 112, and a substrate 114, all of which are the same as the corresponding integers in the first display 100. However, a plurality of negatively charged particles 218 are suspended in the liquid 206 in addition to the black particles 108, and the negatively charged particles 218 are white for this purpose.

  Normally, liquid 206 is not colored (ie, essentially transparent), but some color can be present therein to adjust the optical properties of the various states of the display. FIG. 2A shows a display 200 in which the front electrode 110 is positively charged with respect to the back electrode 112 of the exemplary pixel. Positively charged particles 108 are electrostatically held adjacent to the back electrode 112, while negatively charged particles 218 are electrostatically held relative to the front electrode 110. Therefore, the white particles 218 are visible to the observer viewing the display 200 through the front electrode 110 and the black particles 108 are hidden, so that white pixels are visible.

  FIG. 2B shows the display 200 in which the front electrode 110 is negatively charged with respect to the back electrode 112 of the exemplary pixel. As in the corresponding optical state shown in FIG. 1B, positively charged particles 108 are then electrostatically attracted to the negative front electrode 110, while negatively charged particles 218 are positively charged. The electrode 112 is electrostatically attracted. Thus, the particle 108 moves and is adjacent to the front electrode 110 and the pixel displays the black color of the particle 108, which hides the white particle 218.

  The third electrophoretic display of the present invention shown in FIGS. 3A and 3B (generally designated 300) includes an encapsulated electrophoretic medium (generally designated 302) comprising a plurality of capsules 304. . The display 300 further includes a front electrode 110, a back electrode 112, and a substrate 114, all of which are identical to the corresponding integers in the displays 100 and 200 described above. Display 300 is similar to display 200 described above in that liquid 306 is not colored and negatively charged white particles 218 are suspended therein. However, the display 300 differs from the display 200 in that there are negatively charged red particles 320 that have substantially lower electrophoretic mobility than the white particles 218.

  FIG. 3A shows a display 300 in which the front electrode 110 is positively charged with respect to the back electrode 112 of the exemplary pixel. Both the negatively charged white particles 218 and the negatively charged red particles 320 are attracted to the front electrode 110, but the white particles 218 have a substantially higher electrophoretic mobility and this is the first to the front electrode 110. (The optical state shown in FIG. 3A typically abruptly reverses the polarity from the electrode in the optical state shown in FIG. 3B, so that both the white particle 218 and the red particle 320 have a thick portion of the capsule 304. Note that as a result of the crossing, the higher mobility of the white particles 218 results in reaching a position adjacent to the front electrode 110 before the red particles 320). Thus, the white particles 218 form a continuous layer in the immediate vicinity of the front electrode 110, thereby hiding the red particles 320. Therefore, a white pixel is visible to an observer viewing the display 300 through the front electrode 110 because the white particles 218 are visible and hide the red particles 320.

  FIG. 3B shows a display 300 in which the front electrode 110 is negatively charged with respect to the back electrode 112 of the exemplary pixel. Both the negatively charged white particles 218 and the negatively charged red particles 320 are attracted to the back electrode 112, but since the white particles have higher electrophoretic mobility, the optical state shown in FIG. The white particles 218 reach the back electrode 112 more quickly than the red particles 320, resulting in the white particles 218 adjoining the electrode 112 and forming a continuous layer. Form and leave a continuous layer of red particles 320 facing the front electrode 110. Accordingly, the red pixel 320 is visible to the viewer viewing the display 300 through the front electrode 110 because the red particles 320 are visible and hide the white particles 218.

  4A and 4B illustrate the polymer dispersed electrophoretic medium of the present invention and the process used to make the medium. This polymer-dispersed electrophoretic medium contains non-spherical droplets and is prepared by using a film-forming material that generates a film that can be sufficiently contracted after film formation. A preferred discontinuous phase for this purpose is gelatin, although other proteinaceous materials and possibly crosslinkable polymers can be used instead. A mixture of this liquid material (which will eventually form a continuous phase) and droplets is formed and applied to the substrate to form the structure shown in FIG. 4A. FIG. 4A is a layer 410 containing drops 412 dispersed in a liquid medium 414 in the process of forming a film, with a substrate 416 previously provided with a layer 418 of a transparent conductive material such as indium-tin oxide. A layer 410 applied to (preferably a flexible polymer film such as a polyester film) is shown. This liquid material forms a relatively thick layer 410 containing essentially spherical drops 412 as shown in FIG. 4A. After layer 410 has formed a solid continuous phase, the layer is preferably dried at about room temperature (which may be heated if desired) for a period sufficient to dehydrate the gelatin, so that The thickness is substantially reduced to obtain a structure of the type shown in FIG. 4B, and this dried and shrunk layer is designated 410 ′ in FIG. 4B. The vertical shrinkage of this layer (ie, the shrinkage perpendicular to the surface of the substrate 416) results in the compression of the original spherical drop into a flat ellipsoid, resulting in a flat ellipsoid perpendicular to the surface. Is substantially smaller than the lateral dimension parallel to the surface. In practice, the drops are usually packed tight enough that the outer edges of adjacent drops touch each other, so that the final form of the drops is more like an irregular prism than a flat ellipsoid . Also, as shown in FIG. 4B, the final medium can have more than one layer of drops. If the medium is of the type shown in FIG. 4B where the droplets are polydisperse (ie, there is a wide range of droplet sizes), the presence of such multiple layers is due to the small area of the substrate being This is advantageous in that it reduces the possibility of being covered by a drop, so such multiple layers ensure that the electrophoretic medium is completely opaque and that no part of the substrate is visible to the display formed from this medium. Help to ensure. However, in media using essentially monodisperse drops (ie, drops of all substantially the same size), the media is generally as a layer that, after shrinking, will result in a single layer of closely packed drops. Is better applied (see US Pat. No. 6,839,158). The droplets in the polymer-dispersed medium of the present invention do not have the relatively hard microcapsule wall found in microencapsulated electrophoretic media, and are thus more likely to be packed in a tightly packed single layer than microcapsules.

  Contrary to what is expected, it has been experimentally found that the drops do not coalesce during the drying of the medium. However, it does not exclude the possibility that in some embodiments of the present invention a rupture occurs in the wall between adjacent capsules, thereby resulting in partial bonding between the drops.

  The degree of drop deformation that occurs during the drying process, and thus the final form of the drop, can be varied by controlling the proportion of water in the gelatin solution and the ratio of this solution to the drop. For example, experiments were performed using 2 to 15 weight percent gelatin solution and 200 grams of each gelatin solution and 50 grams of the internal non-aqueous phase forming drops. To make the final layer of electrophoretic media 30 μm thick, a layer of 139 μm thick 2 percent gelatin solution / internal phase mixture needs to be applied, which upon drying is 92.6 volume percent An electrophoretic medium with a thickness of 30 μm containing droplets was produced. On the other hand, to make an electrophoretic medium of the same final thickness, a 15 percent gelatin solution / internal phase mixture is applied to a thickness of 93 μm and dried to obtain an electrophoretic medium containing 62.5 volume percent drops. It was. The media made from the 2 percent gelatin solution was softer than desired to withstand rough handling, and the media made from the gelatin solution containing 5 to 15 weight percent gelatin had satisfactory mechanical properties. .

  The degree of deformation of the drops in the final electrophoretic medium is also affected by the initial size of the drops and the relationship between the initial size and the thickness of the final layer of the electrophoretic medium. Experiments show that the larger the average initial size of the drops and / or the greater the ratio of this average initial size to the thickness of the final layer, the greater the deformation of the drops from the sphere in the final layer. In general, it is preferred that the average initial size of the droplets is from about 25 percent to about 400 percent of the final layer thickness. For example, in the experiments described above where the final layer thickness was 30 μm, good results were obtained with an initial average droplet size of 10 to 100 μm.

  Gelatin forms a film by sol / gel conversion, but the present invention is not limited to a film forming material that forms a film by such sol / gel conversion. For example, film formation can be achieved by polymerization of monomers or oligomers, cross-linking of polymers or oligomers, radiation curing of polymers or any other known film formation process. Similarly, in a preferred variant of the invention where the membrane is first formed and then the thickness of the membrane is reduced, this reduction need not be achieved with the same type of dewatering mechanism that the gelatin membrane is reduced, but from the membrane. Removal of aqueous or non-aqueous solvents, cross-linking of polymer membranes or any other conventional method.

  In the polymer-dispersed electrophoretic medium of the present invention, it is desirable that the droplets comprise at least about 40 volume percent, preferably about 50 to about 80 volume percent of the electrophoretic medium (see US Pat. No. 6,866,760). . It should be emphasized that the drops used in the polymer-dispersed electrophoretic medium of the present invention can have any of the particle and suspension fluid combinations shown in FIGS.

  The present invention can be applied to any of the forms of the encapsulated electrophoretic medium shown in FIGS. However, the present invention is not limited to encapsulated electrophoretic media and polymer-dispersed electrophoretic media, and can also be applied to microcells and non-encapsulated media.

As is apparent from the examples below, the controlled use of the amount of fluorinated monomer in the polymer shell of particles used in electrophoretic displays increases the zeta potential of negatively charged particles, as is generally the case. When the negative particles are white particles such as titania, the increase in the negative zeta potential obtained appears as an improvement in white state (reflection increase). As the proportion of fluorinated monomer in the polymer shell increases, this zeta potential becomes increasingly negative. However, certain disadvantages become apparent when the fluorinated monomer exceeds about 5 percent. The dark state image attenuation (measured as a change in the dark state of the display after 2 minutes (for example) when the display is not driven) begins to increase, and the darkness of the dark state itself decreases, The dynamic range of the display (the difference between the dark state and the white state of the display measured in L ^* units (L ^* is the usual CIE definition:
L ^* = 116 (R / R ₀ ) ^1/3 -16,
Where R is the reflectivity and R ₀ is the standard reflectivity value)). Accordingly, it is generally preferred to maintain the molar ratio of fluorinated monomer in the polymer shell in the range of about 0.1 to about 5 mole percent, desirably about 1 to about 5 mole percent. The optimal proportion of fluorinated monomers will vary somewhat with other factors including the particular fluorinated monomer used (and in particular its degree of fluorination), other monomers used and other particles present in the electrophoretic medium What can be done is clearly understood. In general, the optimum proportion of fluorinated monomer appears to be about 1 mole percent. This is because at this level of fluorinated monomer, a substantial increase in the magnitude of the zeta potential can be obtained while avoiding the above disadvantages associated with a high proportion of fluorinated monomers.

  The polymer-coated particles used in the electrophoretic medium of the present invention can be produced by any of the processes described in WO02 / 093246 described above. In one such process, a particle to be formed with a polymer coating is treated with a bifunctional reagent having a functional group capable of reacting and binding with the particle and a polymerizable group such as pendant vinyl or other React with ethylenically unsaturated groups.

  Next, in order to illustrate the details of particularly preferred reagents, conditions and techniques used in the present invention, the following examples are given solely by way of illustration.

Example 1
Preparation of white titania pigments containing 2,2,2-trifluoroethyl methacrylate and lauryl methacrylate in the polymer shell. DuPont R-794 titania surface functionalized with 3- (trimethoxysilyl) propyl methacrylate is substantially as described above. Prepared as described in PCEP applications. In a 1 L plastic bottle, 500 g of this pigment was dispersed in 426 g (500 mL) of toluene by sonication. A 1 L jacketed glass reactor was charged with 1.7158 moles of monomer divided between lauryl methacrylate and TFEM to give the desired molar concentration of each monomer. The molar ratio of TFEM was 0.1, 1, 5, 10, 25, and 50 mol%, and the remainder was lauryl methacrylate. This pigment dispersion was added to the reactor, and the reactor was purged with nitrogen and heated at 65 ° C. A free radical initiator (5.0 g of 2,2′-azobis (2-methylpropionitrile (AIBN)) previously dissolved in 110 mL of toluene was added dropwise over 60 minutes. Heated with continuous stirring at 0 ° C. and then exposed to air, then the mixture was divided into four 1 L plastic bottles, and about 500 mL of toluene was added to each bottle. The pigment was isolated by centrifugation at 3500 rpm for 20 minutes, the supernatant was discarded, and about 700 mL of toluene was added to each bottle, stirred vigorously to disperse the pigment, and centrifuged at 3,500 rpm for 20 minutes. The pigment was air dried overnight and then vacuum dried overnight at 65 ° C. Thermogravimetric analysis (TGA) was performed to determine a polymer concentration of 6.7 wt% to 9.7 wt%. The zeta potential was measured using Colloidal Dynamics ZetaProbe with respect to the sample dispersed in Isopar E with a surfactant (Solsperse 17K), and the numerical value of the zeta potential is shown in FIG.

  From the data in FIG. 5, it can be seen that the magnitude of the zeta potential increased with increasing TFEM in the polymer shell.

Example 2
Preparation of display using electrophoretic particles of the present invention The polymer-coated titania particles prepared in Example 1 were converted into an electrophoretic display as follows.

Part A: Preparation of Capsule Using the pigment prepared in Example 1, gelatin-acacia microcapsules were prepared by the following procedure. The internal phase was prepared by mixing the following in a 250 mL plastic bottle.

次いで、得られた混合物を実質的に上記米国特許第６，８２２，７８２号中の実施例２７乃至２９に記載されているようにしてゼラチン−アカシアマイクロカプセルに変換した。

The resulting mixture was then converted to gelatin-acacia microcapsules substantially as described in Examples 27-29 in US Pat. No. 6,822,782.

Part B: Display Preparation After the microcapsules prepared in Part A above were allowed to settle, excess water was decanted. These capsules were then mixed with a polymer binder in a weight ratio of 8 parts capsules to 1 part binder to produce a slurry. This slurry was bar coated onto an indium tin oxide (ITO) coated polymer film with a target coating thickness of 18 μm using a 4 mil (101 μm) gap and dried in a conveyor oven at 60 ° C. for about 2 minutes. The sheet was cut into small pieces.

  Separately, the release sheet was coated with a 25 μm layer custom polyurethane laminate adhesive as described in US Pat. No. 7,012,735, doped with 180 ppm tetrabutylammonium hexafluorophosphate, and the microcapsule / polymer membrane Cut to a size slightly smaller than one piece. These two kinds of films were passed through a hot roll laminator having a top roller and a bottom roller set at 120 ° C. and laminated on the coating film, and the resulting composite film was cut into a desired size. The release sheet was removed, and the adhesive layer was laminated on a 2 inch (51 mm) square polymer film having a graphite layer by separately passing a laminator at a top roller and bottom roller temperature of 93 ° C. Single pixel displays were cut from the resulting laminate, electrically connected, and the experimental single pixel display thus produced was conditioned for 5 days at 50% relative humidity.

Example 3
Electro-Optical Test Electro-optical measurements were performed on the single pixel display prepared in Example 2 using a PR-650 SpectraScan Colorimeter. In this test, the display was repeatedly driven to the black and white extreme optical state using a 250 millisecond 15 V pulse and then to either the black or white extreme optical state. The reflectivity of the optical state is measured approximately 3 seconds after the last drive pulse (which allows certain transient effects to disappear) and then 2 minutes after the last drive pulse, comparing the two measurements Each image instability (ie lack of bistability in the image) was detected.

  The results are shown in FIG. 6 (in the figure, “DS” indicates the dark state, “WS” indicates the white state—the white state image instability results in a decrease in reflectance, so the white state image instability value is negative). As shown, it can be seen that image instability increases significantly when the level of TFEM in the polymer shell exceeds 1 mole percent. At 0.1 and 1 mole percent levels of TFEM, the image stability is the same or slightly better than the control. FIG. 7 (“DR” in the figure indicates the dynamic range) shows the highest white state, the lowest dark state, and the entire dynamic range of each display after taking the image instability shown in FIG. 6 into account. As the level of TFEM increases up to 10 mole percent (but not including 10 mole percent), the optical state tends to improve over the control. The reduction in optical state improvement at higher TFEM levels can be attributed to the decrease in image stability shown in FIG. From FIGS. 6 and 7, it can be seen that the incorporation of TFEM into the polymer shell can improve the optical state, in particular the final dynamic range, and the image bistability that is the main advantage of the electrophoretic display. It can be seen that there is a TFEM level frame that provides an improved optical state without degradation.

  In other experiments, other fluorinated monomers (ie 2,2,3,4,4,4-hexafluorobutyl acrylate and 3,3,4,4,5,5,6,6,7,7, It has been shown that 8,8,8-tridecafluorooctyl acrylate) can adjust the zeta potential of the white pigment, similar to TFEM, resulting in a similar improvement in optical state. The exact mechanism by which these fluorinated monomers cause zeta potential changes and optical state changes is currently unknown.

Claims (16)

  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 is fluorinated An electrophoretic medium comprising repeating units derived from acrylate monomers or fluorinated methacrylate monomers.   The electrophoretic medium of claim 2, wherein 1 to 5 mole percent of the monomer in the polymer comprises a fluorinated monomer.   The electrophoretic medium of claim 1, wherein the particles have 4 to 15 weight percent of the pigment of the polymer chemically bonded to the pigment particles.   The electrophoretic medium of claim 3, wherein the particles have 8 to 12 weight percent of the pigment of the polymer chemically bonded to the pigment particles.   The 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 of the side chains comprising at least 4 carbon atoms.   The electrophoretic medium of claim 1, wherein the polymer further comprises residues derived from non-fluorinated acrylate monomers and / or methacrylate monomers.   The electrophoretic medium of claim 1, wherein the non-fluorinated methacrylate monomer comprises lauryl methacrylate.   The fluorinated monomer is 2,2,2-trifluoroethyl methacrylate, 2,2,3,4,4,4-hexafluorobutyl acrylate and 3,3,4,4,5,5,6,6,7 , 7, 8, 8, 8-tridecafluorooctyl acrylate. The electrophoretic medium according to claim 2.   The electrophoretic medium according to claim 1, comprising two types of particles having at least one optical characteristic and different electrophoretic mobilities.   The electrophoretic medium according to claim 9, wherein the two types of particles have different polar charges.   The electrophoretic medium according to claim 1, wherein the pigment particles and the fluid are enclosed in a plurality of capsules or microcells.   The electrophoretic medium according to claim 11, wherein the capsule is held in a polymer binder.   The electrophoretic medium of claim 1, wherein the pigment particles and the fluid are present as a plurality of discrete droplets surrounded by a continuous phase comprising a polymeric material.   The electrophoretic medium according to claim 1, wherein the fluid is gaseous.   An electrophoretic display comprising the electrophoretic medium according to claim 1 and at least one electrode arranged to apply an electric field to the electrophoretic medium.   An electronic book reader, a portable computer, a tablet computer, a mobile phone, a smart card, a sign, a wristwatch, a shelf label, or a flash drive including the display according to claim 15.

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