KR20140101879A - Electrophoretic media - Google Patents

Abstract

The first electrophoretic medium comprises electrically charged particles (700) suspended in a suspension fluid, wherein the particles (700) comprise repeating units derived from one or more monomers, wherein the homopolymer of the monomers is non-compatible with the suspension fluid Has a polymeric shell. The second, pseudo-electrophoretic medium comprises a suspension fluid, first electrically charged first and second type particles suspended in a suspension fluid, the two types of particles having different optical characteristics, but both polymeric shells . The polymeric shell is arranged such that homologous aggregation of the two types of particles is thermodynamically more advantageous than heterogeneous aggregation.

Description

ELECTROPHORETIC MEDIA

The present invention relates to an electrophoretic medium. The medium is particularly, but not exclusively, for use in capsule and microcell electrophoretic displays. The invention also relates to electrophoretic particles for use in such media, and displays incorporating such media. Certain aspects of the invention extend beyond electrophoretic displays to electro-optical displays. The electrophoretic particles of the present invention are modified with a polymer. The electro-optic display of the present invention uses an electro-optic medium having a threshold voltage.

In the display of the present invention, the electro-optic medium (in the case of a non-electrophoretic electro-optical medium) will typically be solid in terms of the solid outer surface of the electro- Quot; solid electro-optic display "), the inner space of the medium can be filled with liquid or gas, and often so, and uses such an electro-optic medium for the display assembly method. Thus, the term "solid electro-optic display" includes a capsule electrophoretic display, a capsule liquid crystal display, and other types of displays discussed below.

The term "electro-optic" as used herein, when applied to a material or display, refers to a material having a first and a second display state in which at least one optical characteristic is different, Quot; refers to a material that changes from a first display state to a second display state. The optical properties are typically visually recognizable hues, but in terms of other optical properties such as light transmittance, reflectivity, luminescence, or in the case of a machine readable display, in terms of reflectivity changes of electromagnetic wavelengths outside the visible range, Lt; / RTI >

The term "gray state " is used herein in its ordinary sense in the field of imaging, which refers to the intermediate state of the optical states of the two extremes of a pixel, and the term " gray state" Do not. For example, the various patents and published applications cited below describe electrophoretic displays in which the extreme states are white and dark blue, so that the intermediate "gray state" is effectively light blue. In fact, as already mentioned, the transition between the two extreme states may not be a change in color at all.

The terms "bistable" and "bistable" in this context include display elements having first and second display states different in at least one optical characteristic, and using an addressing pulse of limited duration, For example, four times or more times the minimum duration of the addressing pulse required to change the state of the display element after the addressing pulse is terminated Quot; is used in the ordinary sense of the field to refer to the display in which the state will last. Published US Patent Application 2002/0180687 states that some particle-based electrophoretic displays capable of gray tones are stable in their extreme black and white states as well as their mid-gray states, It also shows that it fits. This type of display is called precisely "multi-stable" rather than bistable, but the term "bistable" can be used herein for convenience in dealing with both bistable and multi-stable displays.

Various types of electro-optic displays are known. One type of electro-optic display is disclosed, for example, in U.S. Patent Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071; 6,055,091; 6,097,531; 6,128,124; 6,137,467; And 6,147,791 (this type of display is often referred to as a "rotating dichroism ball" display, but in some of the above-mentioned patents the rotating element is not spherical, Is preferred because the term is more accurate). Such a display employs a plurality of small bodies (typically spherical or cylindrical) having two or more compartments with different optical characteristics and internal dipoles. These bodies are suspended in liquid-filled vacuoles inside the matrix, which are filled with liquid so that the rotation of the bodies is free. The appearance of the display changes by applying an electric field to rotate the bodies to various positions and changing the compartments of the bodies seen through the viewing surface. This type of electro-optic medium is typically bistable.

Another type of electro-optic display is an electrochromic medium, for example, an electrode formed from at least a part of a semi-conductive metal oxide, and a nano-discolorable film form comprising a plurality of dye molecules attached to the electrode and capable of reversible color change Use an electrochromic medium; For example, O'Regan, B., et al., Nature 1991, 353 , 737; And Wood, D., Information Display, 18 (3) , 24 (March 2002). Bach, U., et al., Adv. Mater., 2002, 14 (11) , 845. Nano-discoloration films of this type are described, for example, in U.S. Patent 6,301,038, published international application WO 01/27690, and U.S. patent application 2003/0214695. This type of medium is also typically bistable.

Another type of electro-optic display, which has been the subject of intensive research and development for many years, is a particle-based electrophoretic display in which a plurality of charged particles move under a suspension fluid under the influence of an electric field. Electrophoretic displays can have properties such as good luminance and contrast, wide viewing angle, state bistability, and low power consumption compared to liquid crystal displays. Nonetheless, problems associated with the long-term image quality of such displays have hindered their adoption. For example, particles that make up an electrophoretic display tend to settle, which makes the use time of such a display insufficient.

A number of patents and applications filed by or assigned to the Massachusetts Institute of Technology (MIT) and E Ink Corporation describing encapsulated electrophoretic media have recently been disclosed. Such a capsule medium comprises a plurality of small capsules, each of which includes an internal phase, which itself comprises particles that migrate in electrophoresis, suspended in a liquid suspending medium, and a capsule wall surrounding the internal phase thereof. Typically, the capsule is itself fixed with a polymeric binder to form an adhesion layer that is positioned between the two electrodes. Encapsulated media of this type are described, for example, in U.S. Patent Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,721; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413, 790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545; 6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,704,133; 6,710,540; 6,721,083; 6,727,881; 6,738,050; 6,750,473; And 6,753,999; And Published U.S. Patent Application 2002/0019081; 2002/0021270; 2002/0060321; 2002/0060321; 2002/0063661; 2002/0090980; 2002/0113770; 2002/0130832; 2002/0131147; 2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378; 2003/0011560; 2003/0020844; 2003/0025855; 2003/0038755; 2003/0053189; 2003/0102858; 2003/0132908; 2003/0137521; 2003/0137717; 2003/0151702; 2003/0214695; 2003/0214697; 2003/0222315; 2004/0008398; 2004/0012839; 2004/0014265; 2004/0027327; 2004/0075634; 2004/0094422; 2004/0105036; 2004/0112750; And 2004/0119681; And Published International Application WO 99/67678; WO 00/05704; WO 00/38000; WO 00/38001; WO 00/36560; WO 00/67110; WO 00/67327; WO 01/07961; WO 01/08241; WO 03 / 107,315; WO 2004/023195; And WO 2004/049045.

Known electrophoretic media can be divided into two main types, both encapsulated and non-encapsulated, hereinafter referred to for convenience as "single particle" and "double particle", respectively. A single particle medium has only a single type of electrophoretic particle suspended in the suspended medium (at least one optical characteristic of which is different from the optical characteristic of the particle). (With respect to a single type of particle, it does not mean that all particles of that type are exactly the same.) For example, if all particles of that type have the same polarity of charge, For example, two particles of the same color but different charge can be mixed in a single capsule with a single pigment (or multiple pigments) of opposite charge, resulting in a change in color, size and electrophoretic mobility, By appropriately selecting the hue of these pigments, it is possible to obtain any desired intermediate color hue of either or both of the optical states.) When placing such a medium between a pair of electrodes, at least one of which is transparent, The medium can display the optical characteristics of the particles (hereinafter referred to as "front" electrodes, If it if the particle adjacent the near electrode) or may display the optical characteristic of the suspending medium (since the particles are adjacent the electrode remote from the "back" electrode called, the viewer, the particles are obscured by the suspending medium).

The dual particle medium has two different types of particles differing in at least one optical characteristic and an undyed or colored, but typically not colored, suspension fluid. The two types of particles differ in electrophoretic mobility; This difference in mobility may be in polarity (this type may hereinafter be referred to as the "counter charge double particle" medium) and / or scale. When such a dual particle medium is placed between the above-mentioned pair of electrodes, depending on the relative potentials of the two electrodes, the medium can display the optical characteristics of either set of particles, but the exact way to achieve this is It depends on whether the difference is in polarity or scale only. To put it easily, consider an electrophoretic medium in which one type of particle is black and the other type is white. If the two types of particles differ in polarity (for example, if the black particles are positively charged and the white particles are negatively charged) then the particles will be attracted to two different electrodes so that, for example, If the front electrode is negative compared to the back electrode, the black particles will be drawn to the front electrode and the white particles will be drawn to the rear electrode, so that the medium will appear black to the observer. Conversely, if the front electrode is positive relative to the back electrode, the white particles will be drawn to the front electrode and the black particles will be drawn to the back electrode, so that the medium will appear white to the observer.

If both types of particles have the same polarity of charge but different electrophoretic mobility (this type of medium can be referred to as the "same polar double particle" medium hereinafter), the two types of particles are attracted to the same electrode However, one type will reach the electrode before the other, so the type that will face the observer will depend on the electrode that the particle is attracted to. For example, suppose that the black and white particles are positively charged in the previous description, but the electrophoretic mobility of the black particles is higher. If the front electrode is now negative compared to the back electrode, both the black and white particles will be attracted to the front electrode, but the black particles will first arrive due to their higher mobility, so that the black particle layer will coat the front electrode, Will appear black to the observer. Conversely, if the front electrode is positive relative to the back electrode, both the black and white particles will be attracted to the back electrode, but the black particles will first reach due to their higher mobility, so that the black particle layer will coat the back electrode , The white particulate layer will fall off the back electrode and face the observer so that the medium will appear white to the observer. This type of dual particulate media is a type of double particulate medium in which the suspended particle is separated from the back electrode Note that you need something. Typically, the suspension fluid in such a display is not tinted at all, but it can be used to calibrate any undesirable tint of white particles seen through it, or to adjust the color tint of any color . ≪ / RTI >

Both single and dual particle electrophoretic displays may be capable of an intermediate gray state with optical features that are intermediate of the two extreme optical states already described.

Some of the aforementioned patents and published applications disclose encapsulated electrophoretic media having three or more different types of particles in each capsule. For purposes of the present application, such a multi-particle medium is considered a subspecies of the dual particle medium.

In addition, many of the aforementioned patents and applications disclose that by replacing the walls surrounding the individual microcapsules with a continuous phase in a capsule type electrophoretic medium, the electrophoretic medium is separated into a plurality of individual droplets of electrophoretic fluid and a continuous phase of polymeric material Called electrophoretic display, and although no separate encapsulating membrane is associated with each individual droplet, the electrophoretic fluid in such a polymer-dispersed electrophoretic display It is acknowledged that individual droplets can be regarded as capsules or microcapsules; See, for example, 2002/0131147 mentioned earlier. Thus, for purposes of this application, such a polymer-dispersed electrophoretic medium is considered a subspecies of the encapsulated electrophoretic medium.

Related types of electrophoretic displays are so-called "microcell electrophoretic displays ". In a microcell electrophoretic display, charged particles and suspension fluids are not encapsulated within microcapsules, but instead are retained in a plurality of cavities, typically in a polymeric film, formed in the carrier medium. See, for example, published international application WO 02/01281, and published US application 2002/0075556 (both assigned to Sipix Imaging, Inc.).

Although the electrophoretic medium is often opaque (e.g., in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in reflective mode, many electrophoretic displays have a single display State can be made to operate in a so-called "shutter mode ", which is substantially opaque and one is light-transmissive. See, for example, the aforementioned U.S. Patent Nos. 6,130,774 and 6,172,798, and U.S. Patent Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; And 6,184,856. Dielectrophoretic displays similar to electrophoretic displays but depending on variations in field strength can operate in a similar mode; See U.S. Patent 4,418,346.

Capsule or microcell electrophoretic displays typically do not suffer from the clustering and sedimentation defect modes of conventional electrophoretic devices and offer additional advantages such as display printability or coating ability on a variety of flexible and rigid substrates. (The use of the word "printing" is not meant to be limiting, but is intended to include all forms of printing and coating, including: pre-metering coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, ; Roll coating such as knife over roll coating, forward and reverse roll coating, gravure coating, dip coating, spray coating, meniscus coating, spin coating, brush coating, air knife coating, electrostatic printing process, Process; ink jet printing process; and other similar techniques.) Thus, the resulting display may be flexible. In addition, since the display medium can be printed (using a variety of methods), the display itself can be made at low cost.

However, the use life of capsular electrophoretic displays in both single and dual particle types is much lower than desirable. (Although the invention is by no means limited to such theory in any way), the tendency of the electrophoretic particles to stick to the capsule wall, and the tendency of the particles to aggregate into clusters (which is necessary for the display to switch between its optical states) Which interferes with the movement of the particles). In this respect, counter-charge dual-particle electrophoretic displays have particularly difficult problems because the particles which are inversely charged by themselves will tend to electrostatically attract each other and form a stable aggregate. Experimentally, when trying to manufacture this type of black / white capsule display using untreated titania and carbon black pigments, it is found that the display does not switch at all or its useful life is too short to be desirable for commercial purposes .

It has long been known that the physical and surface characteristics of electrophoretic particles can be modified by adsorbing various substances to the surface of the particles or by chemically bonding various materials to these surfaces. To review the record in detail, reference is made to the international application WO 02/093246, specifically its 6, 3, 7, 26 line.

The aforementioned WO 02/093246 describes the advantages of using pigment particles in the electrophoretic medium in which the polymer is chemically bonded thereto or is crosslinked around it. This application includes techniques for controlling the amount of polymer deposited on the particles, the structure of the polymer, techniques for forming a polymeric coating on the electrophoretic particles, and pretreatment of the particles prior to forming the polymeric coating on the electrophoretic particles , Various improvements in such polymer-coated particles are also described. The application also describes a process for the preparation of such polymer-coated pigment particles, which comprises: (a) a functional group capable of reacting with and bonding to the pigment particles, and also a polymerizable or polymerizable Reacting the pigment particles with a reagent having a period of time, reacting the functional group with the particle surface and bonding the polymerizable group thereto; And (b) reacting the product of step (a) with one or more monomers or oligomers, under conditions effective to cause a reaction between the polymerizable group or polymerization initiator phase on the particle and the one or more monomers or oligomers, ≪ / RTI >

The above-mentioned 2002/0180687 includes a plurality of particles suspended in a hydrocarbon suspension fluid, the particles being able to move between fluids when applying an electric field to the medium, the fluid having a viscosity average molecular weight of about 400,000 to 1,200,000 g / Wherein the molar polyisobutylene is dissolved or dispersed therein and the polyisobutylene is contained in an amount of about 0.25 to about 2.5 wt% of the suspension fluid. Also included in the same application are a plurality of particles suspended in a suspension fluid, which particles can move between fluids when applying an electric field to the medium, the fluid having intrinsic viscosity in the suspension fluid η and ions An electrophoretic medium is described wherein the polymer substantially free of groups or ionizable groups is dissolved or dispersed therein and the polymer is present in the suspension fluid at a concentration of from about 0.5 [?] ^-1 to about 2.0 [?] ^-1 have. The presence of polyisobutylene (PIB) or other polymers in the suspension fluid significantly increases the bistability of the display.

As already indicated, the electrophoretic display requires only a low power to switch from one state to another. In bistable displays, this low power required for switching is directly transformed into the overall low power required to operate the display. However, electrophoretic displays do not have unlimited image stability. Brownian diffusion and gravitational settling of pigment particles can degrade the quality of the optical state resulting from switching of the displays altogether, together with the motion driven by small residual voltages and other factors induced by the applied switching pulses. In the absence of such a mechanism to prevent optical state decay, the optical state must be periodically refreshed. Display reproduction consumes power, impairing the utility of the display. In addition, in a particular application (particularly an active matrix driven display), a single pixel is reproduced without a blanking pulse (i.e., a pulse driving the pixel to one of the extreme optical states before driving to the desired optical state) It is difficult or impossible (see above, 2003/0137521). For this reason, improvement of the image stability of the electrophoretic medium is still highly required.

Also, as already discussed, the aforementioned 2002/0180687 describes the incorporation of high molecular weight polymers, such as PIB, which have good solubility in the suspending medium (typically an aliphatic hydrocarbon such as Isopar G) but which are not adsorbed to the electrophoretic particles Thereby realizing good image stability. If such a polymer is present in solution (although the invention is by no means limited by the following) it is believed to cause weak aggregation of the pigment by a mechanism known in the colloidal science as "depletion agglomeration ". Polymers other than PIB can be used for the same purpose. An example of a second polymer that appears to be useful for this purpose is Kraton G, a block copolymer comprising a polystyrene block and a hydrogenated polyisobutylene block, which forms a cohesive structure in the suspension medium. In this case, the aggregate is a species that causes depletion aggregation rather than the monomeric block copolymer itself.

Whichever polymer is used to cause depletion, incorporation of soluble high molecular weight material into the suspension medium will increase the viscosity of the medium. This approach to image stability, because of the response time of the display (the time required to change the display at a given operating voltage, or any given pixel of it, between the optical states of its two extremes) is proportional to the viscosity of the medium Will reduce the switching speed of the display. Furthermore, since the depletion agglomeration mechanism is only effective at polymer concentrations exceeding the superposition concentration (which can be defined as the concentration of the polymer that doubles the viscosity of the medium in practice), all the polymers moving by this mechanism have a switching speed Which is expected to result in a similar decrease in In fact, the switching speed is reduced by about two to three times when using polymers sufficient to achieve adequate image stability. Other means of achieving image stability without interfering with response times are desirable.

As discussed above, PIB and other polymers improve image stability by adjusting the colloidal stability of the pigment particles. The preferred polymer coated particles described in the aforementioned WO 02/093246 are colloidally stable in the suspending medium, which means that the polymer (typically) of the poly (lauryl methacrylate) Because of the shell. The composition of the polymer shell is suitably adjusted so that particles with colloidal stability of the same order of magnitude as provided by PIB, Kraton, and other polymers dispersed in the suspension medium in principle (without the addition of PIB in the suspension medium) Display with image stability). Such a display would be significantly faster than a display containing PIB, or similarly, it would operate at the same rate at a lower applied voltage.

In one aspect, the present invention seeks to provide a method of providing electrophoretic particles having a modified polymer shell that allows for the production of fast, image-stable displays.

In another aspect, the present invention seeks to provide an improved form of the two-step process for preparing polymer-coated electrophoretic particles as described in paragraph 21 above. In a preferred form of this method, the titania (or similar metal oxide pigment) is first coated with silica and the silica-coated titania is treated with a silane containing an ethylene group. The resulting silane-treated titania is then reacted with various unsaturated monomers such as 2-ethylhexyl acrylate or lauryl methacrylate in the presence of a free-radical polymerization initiator to form the desired polymer-coated titania can do. Treatment of the carbon black with a diazotizing agent comprising an ethylene group, for example, the reaction product of 4-vinyl aniline and nitrous acid, bonds the ethylene group to the carbon black surface and then, in substantially the same manner as described for titania, Can be reacted with an unsaturated monomer.

In the specific process illustrated in the aforementioned WO 02/093246 embodiment, the final polymerization step (so-called "graft polymerization step") is carried out in toluene and above all has good properties for use in such free radical polymerization, ≪ / RTI > However, it is considerably inconvenient for use as a solvent in the process for producing polymer-coated electrophoretic pigment particles. Due to various reasons discussed in detail in the aforementioned E Ink and MIT patents and applications, the suspending fluid actually used in electrophoretic displays is an aliphatic hydrocarbon (either alone or in combination with a halo carbon). Thus, since polymer-coated pigment particles will eventually be dispersed in aliphatic hydrocarbons and it is necessary to prevent the aliphatic hydrocarbons from being contaminated with toluene (in some cases, the behavior of the electrophoretic medium may be a small change in the composition of the suspension fluid , It is necessary to remove all of the toluene residue after the polymerization in toluene has ended and the polymer-coated pigment has been separated from the toluene and the polymer-coated pigment particles have been suspended in the final suspension fluid. In practice, the toluene-containing pigment particles from the graft polymerization step are washed one or more times with tetrahydrofuran (THF), washed and centrifuged to separate the pigment from the THF and finally dried in the oven to the final residual THF You need to remove it. All these processes must be performed separately for the two pigments used in the dual particle electrophoresis medium.

These washing, centrifuging and drying steps are labor intensive and costly. Additional costs arise due to the need to redisperse the dry pigment in the final suspension fluid. Also, due to the presence of toluene and THF, the cleaning, centrifugation and drying steps tend to be hazardous, and commercial scale production of polymer-coated pigments has been found to be beneficial in explosion-proof ovens, mixers and centrifuges, and explosion- The use of panels is required, which significantly increases the cost of production equipment. In addition, despite the use of protective devices or exposure protection methods, the device operator may be significantly exposed to steam during processing. Finally, the performance of the pigment in the final electrophoretic medium can be detrimental to the drying step. It is therefore desirable to find an alternative solvent which is capable of carrying out the polymerization reaction and, if possible, eliminating the need for redispersing the dry and dry pigment.

Finally, the present invention seeks to provide an electro-optic display that can be driven in a simple manner. In order to obtain a high-resolution display, whether the display is reflective or transmissive, and whether the electro-optic medium used is binocular or not, each pixel of the display must be addressable without being disturbed from adjacent pixels. One approach to achieving this goal is to provide an "active matrix" display by providing an array of non-linear elements such as transistors or diodes (one or more non-linear elements connected to each pixel). An addressing or pixel electrode addressing one pixel is connected to a suitable voltage source through a connected non-linear element. Typically, when the non-linear element is a transistor, the pixel electrode is connected to the drain of the transistor, this point is essentially random and the pixel electrode may be connected to the source of the transistor, but in the following description, I will assume. Typically, in a high resolution array, the pixels are arranged in a two-dimensional array of transverse lines and vertical lines, and any particular pixel is uniquely defined by the intersection of one specific transverse line and one particular vertical line. While the sources of all the transistors in each vertical row are connected to a single vertical row electrode, the gates of all the transistors in each horizontal row are connected to a single horizontal row electrode; The designation of the gate for the source and the vertical lines for the horizontal line also has a certain frame, but is essentially random and may be conflicting if desired. The transverse electrodes are connected to a single transverse driver, which essentially allows only one transverse line to be selected at any given moment, i.e. the selected transverse electrode has a voltage that allows all transistors in the selected transverse line to be conductive While all remaining transverse lines allow all transistors in those unselected horizontal lines to be energized with this non-conducting voltage. The column electrodes are connected to a column driver, which applies a selected voltage to the various column electrodes to drive the pixels in the selected row to their desired optical state. (The above-mentioned voltages are related to the common front electrode that is normally provided on the opposite side of the non-linear array of the electro-optic medium to the entire display.) After a predetermined interval known as "line address time & The horizontal line is deselected, the next horizontal line is selected, and the voltage in the vertical line driver is changed to create the next line of the display. This process is repeated so that the entire display is created in a row-by-row manner. Thus, in a display with N horizontal lines, any given pixel can be addressed for a time of 1 / N only.

A manufacturing method of an active matrix display has been established. For example, thin film transistors can be assembled using various deposition and photolithography techniques. The transistor includes a gate electrode, an insulating dielectric layer, a semiconductor layer, and source and drain electrodes. Applying a voltage to the gate electrode provides an electric field throughout the dielectric layer, which dramatically increases the conductivity from the source to the drain of the semiconductor layer. This change allows electrical conduction between the source and drain electrodes. Typically, the gate electrode, the source electrode, and the drain electrode are patterned. In general, the semiconductor layer is also patterned to minimize floating conduction (i. E., Cross-talk) between neighboring circuit elements.

Liquid crystal displays typically use amorphous silicon ("a-Si") thin film transistors ("TFTs ") as switching devices for display pixels. Such a TFT typically has a bottom-gate arrangement. Within one pixel, the thin film capacitor typically retains the charge moved by the switching TFT. Electrophoretic displays can use similar TFTs with batteries, but the function of the batteries is somewhat different from that of liquid crystal displays; WO 00/67327, and 2002/0106847 and 2002/0060321 mentioned above. Thin film transistors can be assembled to provide high performance. However, the assembly process can be costly.

In the TFT addressing array, the pixel electrodes are charged through the TFT during the line address time. During the line address time, the applied gate voltage is switched to switch the TFT to a conducting state. For example, in an n-type TFT, the gate voltage is switched to a " high "state to switch the TFT to a conduction state.

Undesirably, the pixel electrode typically exhibits a voltage shift when it depletes the TFT channel by varying the selected line voltage. The pixel electrode voltage shift is caused by the capacitance between the pixel electrode and the TFT gate electrode. The voltage transfer can be designed as follows:

? V _p = G _gp ? / (C _gp + C _p + C _s )

Where C _gp is the gate-pixel capacitance, C _p is the pixel capacitance, C _s is the storage capacitance, and? Is the gate voltage shift when the TFT is effectively depleted. This voltage transfer is often referred to as "gate feedthrough ".

Gate feedthrough can be compensated for by moving as a (voltage to be applied to the front electrode of the common) △ V _p both the top surface voltage. However, it is complicated because? V _p changes for each pixel due to the C _gp change amount for each pixel. Therefore, the voltage bias can be maintained even when the upper surface is moved to compensate for the average pixel voltage shift. Voltage bias can not only cause errors in the optical state of the pixel but also degrade the quality of the electro-optical medium.

For example, the change in C _gp is caused by the displacement between the two conductive layers used to form the gate and source-drain levels of the TFT; Change in gate dielectric thickness; And line etch, i.e., a change in line width error.

By using a gate electrode which completely overlaps the drain electrode, a certain tolerance can be obtained in the mismatched conductive layer. However, this technique can result in large gate-pixel capacitance. The large gate-pixel capacitance is undesirable because it can cause the need for large compensation in one of the selected line voltage levels. In addition, existing addressing structures can produce unintended bias voltages, for example, due to pixel-to-pixel variations in gate-pixel capacitance. Such voltages can have detrimental effects on certain electro-optical media, especially when present for extended periods of time.

These problems make it difficult to design a bistable electro-optic display using an electro-optic medium without a threshold voltage. Since some pixels on the display may be updated occasionally, if possible, the optical state of the pixel should be kept as unobtrusive as possible. In practice, this means minimizing the amount and amplitude of the vortex voltage peak applied to the pixel.

As an example, consider the voltage applied to the source (data) line of an active matrix display scanned in the conventional manner described above. In the encapsulated electrophoretic display, these lines frequently switch between +15 V and -15 V for each electrode address of the display (time to select a predetermined transverse line of the active matrix display) for the common electrode. These voltages are capacitively coupled directly to the pixel electrodes of the display, and this coupling can be quite robust in the field-shielded pixel design. Even if these coupling voltage peaks are forced to be DC balanced for a long time, the optical state of the pixel can be changed if such a voltage peak is continuously applied.

It is known that such voltage peaks, and problems arising therefrom, can be reduced by coupling pixel storage cells to each pixel electrode; See, for example, 2002/0106847 mentioned earlier. In the prior art, essentially the only viable way to minimize or eliminate the effects of these voltage peaks is to increase the size of the pixel storage battery, which significantly increases the power consumption of the display. In addition, the large size of the storage battery can limit the maximum achievable resolution, and the metal-metal overlap area can be increased to reduce the panel yield.

It is now known that in an active matrix electro-optic display, the above discussed problems can be reduced or eliminated using an electro-optic medium representing the threshold voltage, i.e. a medium that does not essentially switch at a low but non-zero voltage. .

SUMMARY OF THE INVENTION In one aspect, the present invention provides a polymeric shell comprising a plurality of electrically charged particles suspended in a suspension fluid, wherein the particles comprise a polymeric shell having a repeating unit derived from one or more monomers of which the homopolymer of the monomer is non- The electrophoretic medium according to claim 1,

This aspect of the invention may hereinafter be referred to as "incompatible monomeric medium " for convenience. In such a medium, the polymeric shell preferably further comprises a repeating unit derived from one or more monomers in which the homopolymer of the monomers is compatible with the suspension fluid. Monomers or monomers that form compatible homopolymers (these monomers may hereinafter be referred to as "compatible monomers" for convenience) comprise from about 15 to about 99 weight percent, preferably from about 50 to about 99 weight percent, of the polymer shell ≪ / RTI > Suspension fluids are typically hydrocarbons, but hydrocarbons and other compatible solvents such as a mixture of halocarbons can be used. Alternatively, silicone fluid or fluorocarbon can be used as the suspension fluid.

Monomers that form an incompatible homopolymer (such monomers may hereinafter be referred to as "incompatible monomers for convenience "). Whether a particular monomer can be considered an incompatible monomer will depend on the particular suspension fluid used and one particular monomer will be an incompatible monomer in one suspension fluid and a compatible monomer in another suspension fluid. For example, lauryl methacrylate is a compatible monomer in aliphatic hydrocarbon suspension fluids, but is usually an incompatible monomer in silicone suspension fluids.

In a medium comprising hydrocarbon alone or predominantly of hydrocarbon, the incompatible monomers comprise up to about 8 carbon atoms and optionally contain hydroxyl or halogen or other polar substituents such as carboxyl, cyano, ketone or aldehydes Acrylates and methacrylates formed from an alcohol comprising; Acrylamide and methacrylamide; N, N-dialkyl acrylamide; N-vinylpyrrolidone; Styrene and its derivatives; Vinyl esters; Vinyl halide. ≪ / RTI > Specific examples of the incompatible monomers include methyl methacrylate, ethyl methacrylate, butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, 2- Acrylic acid, acrylonitrile, methyl vinyl ketone, methacrylamide, N-vinyl pyrrolidone, styrene, vinyl acetate, vinyl chloride, and vinylidene chloride. Further examples of incompatible monomers include fluorine-containing esters of acrylic acid and methacrylic acid, such as trifluoroethyl methacrylate, hexafluorobutyl acrylate, or other fluorinated monomers such as pentafluorostyrene or polymerized And other polyfluoroaromatic molecules including possible functional groups. Another class of incompatible monomers in the hydrocarbon medium includes silicon-containing molecules containing polymerizable vinyl groups in their structure. In the specific hydrocarbon medium of the present invention described in the following examples, the compatible monomers include lauryl methacrylate and the incompatible monomers include trifluoroethyl methacrylate, hexafluorobutyl acrylate, styrene, t-butyl Methacrylate, and N-vinylpyrrolidone.

Many types of the incompatible monomers can be used, with the majority of fluorocarbon monomers being compatible monomers, with other types of suspension fluids, such as fluorocarbon media, as compatible monomers. Similarly, for silicone suspension fluids, the compatible monomers may comprise a majority of silicon groups, and the incompatible monomers may include any of the monomers listed above, except those containing a majority of the silicon functionality.

The incompatible monomeric medium may further comprise an electrically charged second type of particles having at least one different optical characteristic from the other (first) particles or particles that are electrically charged, and the electrically charged second type Of the particles have a polymeric shell. In one form of such a two-particle system, the electrically charged first particles comprise titania and the electrically charged second type particles comprise carbon black or copper chromite.

The present invention also provides electrophoretic particles for use in an incompatible monomeric medium of the present invention. In an electrophoretic medium using hydrocarbons or halocarbons as the suspension fluid, such electrophoretic particles (hereinafter referred to as "incompatible monomer particles " for convenience) may be prepared by reacting a homopolymer of a monomer with one or more monomers And pigment particles having a polymeric shell having a repeating unit. (As will be apparent to those skilled in the art of electrophoretic media technology, hydrocarbon suspension fluids typically used in such media include various mixtures of low molecular weight aliphatic hydrocarbons and, to be sure, a single test compound to test compatibility with such hydrocarbon mixtures N-hexane can be used.)

In the incompatible monomer particles of the present invention, the polymeric shell may further comprise repeating units derived from one or more monomers wherein the homopolymer of the monomers is compatible with n-hexane. Compatible monomers can comprise from about 15 to about 99 weight percent, preferably from about 50 to about 99 weight percent, of the polymer shell. The incompatible monomers may comprise any one or more of those listed in the 56th paragraph above. In the specific particles of the invention described in the following examples, the compatible monomers include lauryl methacrylate and the incompatible monomers include any one or more of styrene, t-butyl methacrylate, and N-vinyl pyrrolidone . The pigments used to form the particles of the present invention may be any one or more of, for example, titania, carbon black and copper chromite.

In another aspect, the present invention provides similar electrophoretic particles for use in fluorinated and silicone-based suspension fluids. Accordingly, the present invention provides electrophoretic particles comprising pigment particles, wherein the homopolymer of the monomer has a polymeric shell having repeating units derived from at least one monomer that is incompatible with perfluorodecalin. The present invention also provides electrophoretic particles comprising a pigment particle wherein the homopolymer of the monomer has a polymeric shell having a polydimethylsiloxane 200 (viscosity 0.65 centistokes) and repeating units derived from one or more monomers that are non-flammable.

In another aspect, the invention provides an electrophoretic medium comprising:

Suspension fluid;

An electrically charged first type of particles suspended in a suspension fluid, the first type of particles having a first optical characteristic and a polymeric shell; And

A second type of particles (having a second optical feature and a polymeric shell different from the first optical feature);

Wherein the polymeric shell is arranged such that homologous aggregation of the first and second types of particles is thermodynamically more advantageous than heterogeneous aggregation.

Such a medium may hereinafter be referred to for convenience as the " homogenous flocculating medium "of the present invention. Between the homogenous flocculating medium and the binary particle incompatible monomeric material of the present invention, it will be appreciated that there is considerable overlap in that many of the media can simultaneously satisfy both definitions.

In the homogeneous coagulation medium of the present invention, the polymeric shells of the first and second types of particles may each comprise a repeating unit derived from one or more monomers, wherein the homopolymer of the monomers is non-compatible with the suspension fluid. Each polymeric shell may further comprise repeating units derived from one or more monomers wherein the homopolymer of the monomers is compatible with the suspension fluid. Compatible monomers can comprise from about 15 to about 99 weight percent, preferably from about 50 to about 99 weight percent, of the polymer shell. The suspension fluid may have a dielectric constant of less than about 5 and may include hydrocarbons, preferably aliphatic hydrocarbons. Alternatively, the suspension fluid may comprise an aryl-alkane or dodecylbenzene.

In addition, the incompatible monomers may comprise any one or more of those listed in the 46th paragraph above. In a preferred form of homogeneous flocculating media, the compatible monomers include lauryl methacrylate and the incompatible monomers include any one or more of styrene, t-butyl methacrylate, and N-vinyl pyrrolidone.

For reasons explained in detail below, the homogeneous flocculating media of the present invention may have an operating threshold voltage. The medium can be encapsulated, i. E. The suspension fluid and particles can be contained in a plurality of capsules or cells.

The present invention also provides an electrophoretic display comprising one of the electrophoretic media of the present invention and one or more electrodes disposed adjacent the electrophoretic medium and arranged to apply an electric field thereto.

The invention also relates to an electro-optical medium layer; And a plurality of pixel electrodes disposed adjacent the electro-optic medium layer and arranged to apply an electric field thereto, the electro-optic medium exhibiting a threshold voltage.

Such an electro-optic display may hereinafter be referred to as the "threshold voltage display" In such a display, a battery can be connected to each pixel electrode. The electro-optical medium may comprise a plurality of charged particles suspended in a suspension fluid and movable therebetween when applying an electric field to the electro-optical medium. These charged particles may have polymeric shells in which the homopolymer of the monomer has repeating units derived from one or more monomers that are non-compatible with the suspension fluid. The electro-optical medium may also be an homogeneous agglomerating medium of the present invention.

Finally, the present invention provides a process for the preparation of polymer-coated pigment particles, the process comprising: (a) carrying out in an aliphatic hydrocarbon;

(a) reacting a pigment particle with a reagent having a functional group capable of reacting with the particle and capable of bonding to the particle and having a polymerizable group or a polymerization initiator, reacting the functional group with the particle surface, Attaching a group; And

(b) reacting the product of step (a) with one or more monomers or oligomers under conditions effective to react the polymerizable or polymeric initiator phase on the particle with one or more monomers or oligomers to form a polymer bound to the pigment particles Forming step.

Such a process may hereinafter be referred to as the "aliphatic polymerization method" of the present invention for convenience.

지수 α 는 용매의 질에 따라 0.5 내지 약 0.8 (유연성 중합체의 경우) 의 범위이고, 그 값이 클수록 더 양호한 용매임을 나타낸다. α = 0.5 인 경우, 유체를 "세타 (theta) 용매" 라고 칭하고 중합체의 형상은 중합체 용융물과 유사하여 (즉, 다른 중합체 부분으로만 둘러싸임), 사실상 중합체는 유체와의 중립적 상용성을 가진다. 이러한 조건 하에서, 중합체 사슬의 형상은 랜덤 워크 (random walk) 이다.
단독중합체 및 공중합체 시스템 모두에 있어서, 용매의 질 (즉, 용매와의 중합체 상용성) 을 나타내는 지표로서 허긴즈 (Huggins) 계수를 사용할 수 있다. 허긴즈 상수는 중합체의 희석 용액의 상대 점도에 대한 하기 식에서 [η]²k' 항이다:

이 식에서 η_S 는 용매의 점도이고, [η] 는 고유 점도이고, c 는 유체 중 중합체의 농도이다. 상용성 유체에 있어서, k' 는 0.30 내지 0.40 의 범위인 반면, 덜 상용성인 유체에서는, k' 는 더 크다 (0.50 내지 0.80). (C.W. Macosko, Rheology Principles, Methods, and Applications, VCH Publishers, 1994, p. 481 참고).
정적 광 산란 또는 삼투압 측정으로 측정한, 주어진 유체-중합체 용액에서의 2차 비리알 (virial) 계수의 값으로부터 유사한 정보를 모을 수 있다. 일반적으로, 큰 2차 비리알 계수는 더 상용성인 유체를 의미한다. 용해된 중합체 강의 관계에 있어서, 사실상 비상용성인 중합체-용매 상호작용에 해당하는, 2차 비리알 계수는 - 가 될 수도 있다 (C. Tanford, Physical Chemistry of Macromolecules, John Wiley and Sons, New York, 1961, pp. 293-296 참고).
중합체가 유체에 불용성인 경우, 즉, 비상용성인 경우, 중합체는 때때로 유체에 의해 팽창될 수도 있다. 팽창도 (유체 존재 하 중합체 샘플 대 건조 중합체 부피 비) 는 이런 상황에서 유체 및 중합체의 상용성 정도의 척도로서 사용할 수 있다. 팽창도가 클수록, 상용성도 커진다.
여러 방식으로, 중합체 쉘을 현탁 유체와 덜 상용성이 되도록 만들 수 있다. 우선, 중합체 쉘은, 본 발명의 비상용성 단량체 전기영동 매질 측면에 따르면, 단량체의 단독중합체가 현탁 유체와 비상용성인 하나 이상의 단량체로부터 유도된 반복 단위체를 가질 수 있다. 전형적으로, 그러한 중합체 쉘은 상용성 단량체, 즉 단량체의 단독중합체가 현탁 유체와 상용성인 단량체도 포함할 것이고, 중합체 쉘과 현탁 유체의 상용성은 그 두 단량체의 비를 변형시켜 조절할 수 있다.
비상용성 및 상용성 단량체 모두를 포함하는 중합체 쉘 (명백하게는, 존재하는 각각의 유형의 단량체가 하나를 초과할 수 있음) 은 마지막 중합 단계 (단독 중합 단계일 수 있음) 에서 중합가능 단량체의 혼합물을 사용하여 형성되는 랜덤 공중합체 쉘일 수 있다. 단량체 중 하나가 현탁 유체와 상용성이고, 하나는 아닐 경우, 쉘 내의 단량체의 비에 따라 어느 정도의 콜로이드성 안정성을 입자에 부여할 수 있다. 또한, 상이한 단량체 종이 상이한 비상용성 정도를 부여할 수 있기 때문에, 비상용성 단량체를 바꿔서, 비상용성 정도를 더욱 변형시킬 수 있다. 예를 들어, 중합체 쉘 내에 단독 단량체로서 사용하는 경우, 약 8 개 이하의 탄소 원자를 포함하는 짧은 측 사슬을 가지는 아크릴레이트 에스테르 (예를 들어, 부틸 메타크릴레이트) 가 Isopar G 현탁 유체 내에서 콜로이드적으로 안정하지 않은 입자를 산출한다는 것이 관찰되었다. 반면에, 라우릴 메타크릴레이트는 Isopar G 내에서 콜로이드성 안정성이 우수한 전기영동 입자를 산출한다. 따라서, 라우릴 메타크릴레이트 및 부틸 메타크릴레이트의 공중합체는, 중합 혼합물 중 부틸 메타크릴레이트 대 라우릴 메타크릴레이트의 어떤 몰 비에서, Isopar G 와 한계 상용성인 중합체 쉘 및 콜로이드적으로 한계 안정한 입자를 제공할 것이다.
전기영동 입자 주변의 중합체 쉘을 개질하는 제 2 의 방법은 제 2 단계 중합에 의한 것이다. 상기 화학식 (I) 및 (II) 의 시약에 의해 제공되는 표면 작용성은 단일 그래프트 중합 단계 도중에 완전히 소모되지는 않는다는 것이 실험으로 나타났다. 예를 들어, 표면 작용화되고 중합체 코팅된 티타니아를 중합 매질 (전형적으로는 중합 개시제로서 톨루엔, 라우릴 메타크릴레이트 및 아조비스이소부티로니트릴 (AIBN) 포함) 에 단순히 재현탁시키는 경우, 16 시간 동안 가열한 후 반응 혼합물로부터 단리한 안료의 열중량 분석은 결합된 중합체의 양이 증가했음을 나타낸다 (열중량 분석 (TGA) 으로 측정: 제 1 중합 후: 7.3 %; 제 2 중합 후: 8.9 % 및 9.9 % (두 번의 상이한 실행)). 그러한 이중 그래프트 중합 방법에서, 제 2 중합에 사용하는 단량체는 제 1 중합에서 사용한 것과 상이할 수 있어서, 최종 중합체 쉘에 있어서 상이한 단량체로부터 구축된 중합체 사슬을 혼입시키는 것이 가능하다. 또한, 제 1 중합 단계의 조건을 변형시켜, 사슬의 초기 그래프팅 밀도뿐만 아니라 제 2 단계 중합에 있어서 표면 비닐 작용 가능성 (및 접근성) 을 조절할 수 있다. 따라서, 이런 이중 중합 방법은 중합체 안정화 전기영동 입자의 구축에 제 2 의 유연도를 제공한다. 사슬 이동제를 중합 혼합물에 혼입시켜, 또는 중합체 분야에 주지된 방식으로 단량체 또는 라디칼 개시제의 농도를 조절하여, 제 1 또는 제 2 중합 단계에서 중합체 사슬의 분자량을 조절하는 것이 이로울 수 있다.
현재로서는 지방족 탄화수소를 선호하는 현탁 유체가 다수의 단독중합체와의 상용성이 불량하다고 일반적으로 간주하기 때문에, 사용이 용이한, 매우 큰 범위의 단량체를 상기 기재한 두 가지 방법 중 어느 하나로 제조되는 중합체 쉘에서의 비상용성 단량체로서 사용할 수 있다. 그러한 비상용성 단량체에는 사슬 길이 및 분지 구조가 변하지만 전형적으로 8 개 이하의 탄소 원자를 가지는 알킬기가 있는 아크릴레이트 에스테르 (예를 들어, 메틸 메타크릴레이트, 에틸 메타크릴레이트, 부틸 메타크릴레이트, 이소부틸 메타크릴레이트, t-부틸 메타크릴레이트, 옥틸 메타크릴레이트, 2-에틸헥실 메타크릴레이트), 뿐만 아니라 상응하는 아크릴산의 에스테르, 플루오로카본 에스테르 측 사슬이 있는 아크릴레이트, 작용성 알코올의 아크릴레이트 에스테르, 예를 들어, 2-히드록시에틸 메타크릴레이트, 아크릴아미드류, 예컨대 아크릴아미드, 메타크릴아미드, N-모노알킬 아크릴아미드 및 메타크릴아미드, N,N-디알킬아크릴아미드, N-비닐피롤리돈, 및 작용성 아크릴아미드 유도체, 스티렌, 치환 스티렌 유도체, 비닐 아세테이트 및 기타 비닐 에스테르, 할로겐화 비닐 유도체 (비닐 클로라이드, 비닐리덴 클로라이드, 등) 및 기타 중합가능 단량체 종이 포함된다.
본 발명은 이론적 사항으로는 결코 제한되지 않지만, 입자 주변의 중합체 쉘이 비상용성 단량체를 포함하는 전기영동 매질의 개선된 특성을 논리적으로 설명하는 것은 가능하다. 전적으로 상용성 단량체로부터 형성된 중합체 쉘은, 다른 중합체 부분에 의해서보다 현탁 유체를 둘러싸는 쉘을 형성하는 중합체 사슬에 있어서 열역학적으로 유리하기 때문에, 전기영동 입자에 입체 안정성을 제공한다. 따라서, 전적으로 상용성 단량체로부터 형성된 중합체 쉘, 예를 들어 지방족 탄화수소에 분산된 순수 폴리(라우릴 메타크릴레이트) 중합체 쉘은 매우 팽창된다. 쉘이 인접 입자에 상호침투하면 더욱 적합한 중합체-용매 접촉에 비해 중합체-중합체 접촉의 개수가 증가하기 때문에, 입자들 사이에 유효 반발력이 있고, 이들은 서로로부터 공간을 둔 채로 있으려는 경향이 있어서, 현탁 유체에 분산된다. 완전히 비상용성인 단량체로부터 형성되는 중합체 쉘은, 중합체 사슬이, 현탁 유체에 의해서보다는 다른 중합체 부분으로 둘러싸이는 쉘을 형성하는 것이 열역학적으로 유리해서 중합체 쉘이 붕괴되고, 현탁 유체를 배척하고 전기영동 입자가 서로 끌려가려는 경향이 있고 그리하여 응집하려는 경향이 있기 때문에, 그러한 입체 안정성을 제공하지 않는다. 즉, 매우 상용성인 중합체 쉘은 현탁 유체 여기저기에 전기영동 입자를 분산시키려고 하는 반면, 비상용성 중합체 쉘은 전기영동 입자를 응집시키려 한다. 전기영동 매질의 영상 안정성은, 도 1 내지 3 에 나타내는 전기영동 매질의 각각의 유형에 있어서, 일단 작성된 영상의 안정성은 영상의 원인이 되는 입자의 동종응집체 (층) 에서 전기영동 입자가 유사한 전기적 입자와 응집한 채로 남아 있으려는 능력에 따르기 때문에, 전기영동 입자가 응집하려는 경향에 의해 촉진된다. (도 1 내지 3 은 간략화한 것임을 주의한다; 실제로, 영상화 도중 형성되는 입자 응집체에는 입자층이 보통 하나를 초과할 것이다.) 상용성 및 비상용성 단량체 모두를 중합체 쉘에 혼입시켜 중합체 쉘 및 현탁 유체의 상용성을 조절하여, 중합체 쉘의 현탁 유체와의 전체 상용성, 및 그러한 입자 응집체의 안정성, 및 그리하여 생성되는 디스플레이의 영상 안정성을 조절하는 것이 가능하다.
본 발명의 바람직한 구현예의 추가의 한 가지 장점은 전기영동 매질의 스위칭에 문턱을 도입하는 것이다. 전기영동 입자의 서로에 대한 인력, 및 그리하여 이들이 안정한 동종응집체로 남으려는 경향이 충분히 강하다면, 동종응집체로부터 입자를 끌어당겨서 전기장에서 이동할 수 있도록 해 주는 상당한 전기장이 필요할 것이다. 전기영동 매질을 스위칭하는데 있어서 상당한 문턱 전압이 있으면 여러 장점이 있다. 문턱이 충분히 크면, 고해상도 디스플레이의 수동형 매트릭스 어드레싱을 사용할 수 있다. 더 작은 문턱은 와류 전압 및 화소간 전압 누설에 대한 능동형 매트릭스 디스플레이의 감도를 감소시키는데 유용할 수 있다.
본 발명은 또한 이중 입자 전기영동 매질에서 영상 안정성을 개선하려는 또다른 접근법을 제공한다. 이중 입자 전기영동 매질에서, 두 가지 유형의 전기영동 입자 모두는 전체 영상 안정성에 기여할 필요가 있다. 제 2 유형의 입자가 현탁 유체 도처에 침강 또는 확산되기 쉽다면, 한 가지 유형의 입자가 "영상 안정"한 (즉, 스위칭될 위치에 동종응집체로 남는) 것은 충분하지 않다. 그러한 경우라면, 매질의 하나의 극단의 광학 상태가 상당히 양호한 영상 안정성을 나타낼지도 모르지만, 나머지는 불량한 안정성을 보일 것이다. 또한 입자가 확산할 수 있다면, 한 입자의 불량한 영상 안정성이 더구나 제 2 극단의 광학 상태의 광학적 퇴보의 원인이 되기 쉽다. 따라서 두 가지 유형의 입자 모두가 양호한 영상 안정성을 제공하도록 적당히 조절된 입체 안정성을 가지는 것이 바람직하다. 현탁 유체에 중합체를 포함시켜 양호한 영상 안정성을 달성하려고 "고갈 응집" 법을 사용하려 한다면 (앞서 언급한 2002/0180687 참고), 두 가지 유형의 입자에 적용하는 고갈 메카니즘의 정도는 입자 크기 및 입자 형상에 따라 상이하며, 독립적으로 조절할 수 없는데, 이는 오직 한 가지의 중합체만을 현탁 유체에 첨가하고, 두 가지 유형의 입자에 영상 안정성을 부여하기 위해서 사용하기 때문이다. 그러나, 본 발명의 비상용성 단량체 전기영동 매질에서, 각각의 유형의 입자의 콜로이드성 안정성은 독립적으로 조절할 수 있다. 또한, 고갈 응집법에 따라 현탁 유체에 중합체를 첨가하여 두 가지 별개의 유형의 입자를 서로에 대하여 뿐만 아니라 각자에 대해서도 콜로이드적으로 불안정하게 만들 수 있고, 그리하여 두 가지 유형의 입자의 이종응집을 조장할 수 있다. 이종응집은 이중 입자 전기영동 디스플레이에 있어서 응답 시간 지연 및 더 높은 인가 전압을 사용할 필요성 (이종응집체를 분리시키는데 더 높은 전기장 세기가 필요할 것이기 때문) 뿐만 아니라 불량한 광학 상태 (그 안에 한 가지 유형의 입자가 대부분인 경우 대전된 이종응집체는 적용된 전기장에 의해 완전히 분리될 수 없기 때문) 를 포함하여 다수의 바람직하지 못한 효과를 야기할 수 있다.
본 발명의 동조응집 매질의 측면에 따르면, 두 가지 유형의 입자의 동종응집이 이종응집보다 열역학적으로 유리한 이중 입자 전기영동 매질을 제공함으로써 이러한 문제들을 제거하거나 적어도 감소시킨다. 이는 현탁 유체와의 그들의 상용성을 동종응집이 유리하도록 조절한 중합체 쉘을 가지는 두 가지 유형의 입자를 제공하지만, 또한 두 가지 유형의 중합체 쉘의 상용성을 이종응집이 불리하도록 조절하여 달성할 수 있다. 두 가지 유형의 입자의 중합체 쉘에 상이한 비상용성 단량체를 사용하여 이를 달성하는 것은 비교적 쉽다; 전형적으로는, 두 가지 유형의 입자 상의 중합체 쉘에 동일한 상용성 단량체를 사용할 수 있다. 두 가지 비상용성 단량체가 서로 상용성이 아니라면, 그 때는 두 가지 유형의 입자가 이종응집하려는 경향이 최소화될 것이고, 동종응집만이 촉진되어, 응답 시간이 더욱 빨라지고 전기영동 매질의 광학 상태가 더 순수해진다.
상기 이미 언급한 바와 같이, 본 발명은 현탁 유체로서 지방족 탄화수소를 사용하도록 제한되지는 않는다. 플루오로- 및 할로탄소 오일, 실리콘 오일, 아르알킬 용매 (예를 들어, 톨루엔, 도데실벤젠 등) 또는 이들의 혼합물이 본 발명에서 유용할 수 있고, 이는 이들이 상기 기재한 바와 같이, 중합체 쉘의 용매화에 관하여 현탁 유체의 특성을 더욱 개질시키기 때문이다.
본 발명은 모든 유형의 전기영동 매질에 적용할 수 있지만, 사전제조한 캡슐을 사용하던 마이크로셀을 사용하던 간에, 특히 캡슐형 전기영동 매질에 유용할 수 있다. 본 발명은 또한 전송 및 후송 둘 다, 뿐만 아니라 좌우 (side-to-side) 및 셔터 모드 스위칭을 포함하여, 모든 유형의 스위칭 형태의 디스플레이에 적용할 수 있다.
첨부한 도면의 도 1 은, 하기 실시예에 사용하는 바와 같이, 본 발명에 따른 전기영동 입자의 바람직한 제조 방법을 도식적으로 나타내고, 이 방법은 본질적으로는 앞서 언급한 WO 02/093246 에 기재한 것과 유사하다. 제 1 단계 방법에서, 기본 안료 입자 700 을 실리카로 코팅하여 실리카화 안료 702 를 제조한다; 이 단계는 앞서 언급한 WO 02/093246 에 전부 기재되어 있다. 다음으로, 실리카화 안료 702 를 실리카 표면과 반응하는 하나의 작용기 및 제 2 의 전하 조절기를 가지는 2작용성 시약으로 처리하여 그 표면에 전하 조절기를 보유하는 표면 작용화 안료 704 를 제조한다. 2작용성 시약은 또한 안료 입자 상에 중합체 형성 위치를 제공한다. 마지막으로, 도 1 에 나타내는 바와 같이, 표면 작용화 안료 704 를 전하 조절기에 부착된 중합체를 형성하기에 효과적인 조건 하에 하나 이상의 단량체 또는 올리고머와 접촉시켜 중합체-코팅 작용성 안료 706 을 제조하고, 이것을 전기영동 매질에 사용한다.BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagrammatic representation of the method used to prepare the incompatible monomeric electrophoretic particles of the present invention.
Figure 2 is a graph showing changes in the dynamic range of the iso-aggregated electrophoretic medium of the present invention as a function of applied voltage for various pulse lengths, as described in the following examples.
As already indicated, the present invention provides an electrophoretic medium using particles having a polymeric shell, electrophoretic particles for use in such a medium, an electrophoretic display comprising such a medium or a similar electro-optical medium, To a process for producing a polymeric shell. For background information on electrophoretic particles, methods of forming polymer shells thereon, electrophoretic media and displays incorporating such particles, see WO 02/093246 mentioned above, in particular on pages 12 to 41 thereof. See also paragraphs [0124] to [0165], 2002/0185378 mentioned above. As such information is readily available in this published application, it is not recited herein, except as necessary to describe how the electrophoretic particles, media, displays and methods of the present invention differ from analogues described in such international application I will not.
The polymer shell present on the electrophoretic particles of the present invention may not completely cover the particle surface such that no further polymer can form on the polymer surface; Indeed, as will be discussed below, it is often found that the second polymerization step will often form additional polymers on the particle surface. Thus, the use of the term "polymer shell" herein does not imply that the possibility of the polymer coating forming additional polymer on the particle by a further polymerization step is precluded.
A preferred class of functional groups that bind to a number of ceramic oxide pigments, but especially to silica- and / or alumina-coated titania and like silica-coated pigments, are silane coupling groups, especially trialkoxysilane coupling groups. One preferred reagent for attaching polymerizable groups to titania and similar pigments is the 3- (trimethoxysilyl) propyl methacrylate mentioned above:
H₂C = C (CH₃) CO₂(CH₂)₃Si (OCH₃)₃. (I)
This material is available from Dow Chemical Company, Wilmington, Delaware under the trade designation Z6030. The corresponding acrylates can also be used. Another useful reagent is an aminosilyl derivative of the formula:
H₂C = CHC₆H₄CH₂NHCH₂CH₂NH (CH₂)₃Si (OCH₃)₃. HCl. (II)
To provide a "anchor" to the polymer shell that is to be formed later around the electrophoretic particles, these silylmethacrylates can be added to the final electrophoretic particles in the presence of suitable charge or charge agents, And a charge-regulator that imparts a charge. When positively charged particles are desired, reagents of formula (II) are used, but the reagents of formula (I) provide negatively charged particles. Both reagents include, of course, polymerizable vinyls that allow grafting of polymeric radicals produced in solution.
Indeed, the suspension fluid in the electrophoretic medium is typically a low dielectric constant liquid, such as a hydrocarbon, halocarbon (especially fluorocarbon), or silicon. The incompatible monomeric medium of the present invention can use any of these types of suspension fluids. Modifying the medium for use with other types of suspension fluids will be readily apparent to those skilled in the art of colloidal chemistry in such suspension fluids, as will be discussed below with particular reference to media comprising a hydrocarbon suspension fluid. The polymer shell itself typically contains most of the hydrocarbon chain (i. E., A chain that forms a homopolymer highly compatible with the suspending medium); As discussed below, the majority of strong polar or ionic groups are undesirable, except for the groups provided to control compatibility with suspension fluids and to charge. Also, in the case where the medium in which particles are to be used, at least, comprises an aliphatic hydrocarbon suspension fluid (usually in such a case), the polymer may be branched or "comb ", having a plurality of side chains extending from the main chain and its main chain, Structure. Each of these side chains should have at least about 4, preferably at least about 6, carbon atoms. Virtual side chains can be advantageous longer; For example, lauryl (C₁₂Side chain. The side chain may itself be branched; For example, each side chain may be a branched alkyl group, such as a 2-ethylhexyl group. Because of the high affinity of the hydrocarbon chains for hydrocarbon-based suspension fluids (although the invention is not limited in any way by the following), the branches of the polymer can spread from one another in a large volume of liquid in a brush or wood- And it is believed that the particles become colloidally stable in the suspension fluid.
As described in the above-mentioned WO 02/093246, there is an optimum range of the amount of polymer formed on the electrophoretic particles, and if the polymers are overfilled or insufficiently formed on the particles, the quality of their electrophoretic characteristics deteriorates . The optimum range will vary depending upon a number of factors including the density and size of the particles to be coated, the nature of the suspending medium in which the particles are to be used, and the nature of the polymer being formed on the particle, and any particular particles, For the medium, the optimal range is best determined empirically. However, as a general indication, it should be noted that as the particles become richer, the optimum ratio of the polymer to the weight of the particles is lowered, and the finer the particles are divided (the smaller the particle size) do. In the aforementioned WO 02/093246, the particles should be coated with at least 2, preferably at least 4% by weight of polymer, and in most cases the optimum proportion of polymer is from 4 to 15% by weight of the particles, typically from 6 To 15% by weight, and most preferably from 8 to 12% by weight. More specifically, in the case of titania particles, in WO 02/093246 mentioned above, the preferred range of polymer is indicated as 8-12 wt% of titania.
However, in order to facilitate the application of the present invention to particles having a wide range of particle size and density, the amount of polymer can be adjusted by changing the surface density of the polymer (i.e., the weight of the polymer per unit area of the particle surface, Milligram of sugar polymer) may be advantageously described. The surface density of the polymer can be calculated from the following formula:
Γ = WρD / 6
Where W is the surface density of the polymer, W is the weight of polymer per gram of sample (obtained from thermogravimetric analysis), p is the density of the base particles and D is the diameter of the base particles. In copper chromate, it has been found that the useful range of surface density is 2 to 40 mg / g, preferably 14 to 24 mg / g, and particularly preferably 18 to 22 mg / g.
The polymer-coated particles of the present invention may also be useful for applications other than electrophoretic displays. For example, the controlled affinity for the hydrocarbon material, provided by the polymer coating on the current pigment, is such that the pigment is more easily dispersed in the polymeric and rubber matrices than the similar but uncoated pigment, so that the polymeric and rubber matrix Which is advantageous for use in the present invention. The flexibility of the polymeric coating chemistry allows the coating to be "tuned" for controlled dispersion in any particular matrix. Therefore, the present pigment can be used as a dispersing pigment or a reactive extrusion compound. In addition, the polymer coating on the particle may be useful in improving the physical properties of such a blend by reducing the tendency of such pigment / polymer or rubber blend to tend to shear or crack at the interface between the particle and the matrix material. In the case of preparing polymer-coated particles (as discussed above) by a process wherein the polymer-coated particles are prepared as an additive mixture with a "free" polymer that is not attached to the particles, in many cases, There will be no need to separate the coated particles from the glass polymer before dispersing the particles into the polymeric or rubber matrix.
In the medium of the present invention, a suspension fluid is prepared on the basis of chemical inertness, density matching to electrophoretic particles, or chemical compatibility with both electrophoretic particles and capsules or microcell walls (in the case of encapsulated electrophoretic displays) You can choose. If particle motion is desired, the viscosity of the fluid should be low. The refractive index of the suspension fluid can also substantially match the refractive index of the particles. As used herein, the refractive index of a suspended fluid is "substantially matched" to the refractive index of the particles when their individual refractive index difference is from about 0 to about 0.3, preferably from about 0.05 to about 0.2. However, in the electrophoretic display in which the optical state is partially determined by the scattering efficiency, it is preferable that the refractive index difference between the scattering medium and the medium is large. Titania particles are typically used as scattering particles to create a white state in a dual particle electrophoretic display, and the refractive index of such materials is high (about 2.7), and the refractive index of the suspended medium is preferably low.
Useful organic solvents include epoxides such as decane epoxide and dodecane epoxide; Vinyl ethers such as cyclohexyl vinyl ether and Decave (registered trademark of International Flavors & Fragrances, Inc., New York, NY); And aromatic hydrocarbons such as toluene and other alkylbenzene derivatives such as dodecylbenzene and naphthalene and alkylnaphthalene derivatives. Useful halogenated organic solvents include, but are not limited to, tetrafluoro dibromoethylene, tetrachlorethylene, trifluorochloroethylene, 1,2,4-trichlorobenzene, and carbon tetrachloride. These materials are high in density. Useful hydrocarbons include dodecane, tetradecane, Isopar TM series (Exxon, Houston, TX), Norpar TM (a series of normal paraffinic liquids), Shell-Sol TM (Shell, ), And Sol-Trol (Shell) aliphatic hydrocarbons, naphthas, and other petroleum solvents. These materials are usually of low density. Useful examples of silicone oils include, but are not limited to, octamethylcyclosiloxane and high molecular weight cyclic siloxane, poly (methylphenylsiloxane), hexamethyldisiloxane, and polydimethylsiloxane. These materials are usually of low density. Useful low molecular weight halogen-containing polymers include poly (chlorotrifluoroethylene) polymers (Halogenated Hydrocarbon Inc., River Edge, NJ), Galden TM (perfluorinated ethers of Ausimont, Morristown, NJ) But are not limited to, Krytox (R) of Wilmington, DE. A number of such materials are available in the range of viscosity, density, and melting point.
Display with controlled electrophoretic particle, medium and particle / suspension fluid compatibility
It has heretofore been evident that it is desirable to form a polymer coating or shell around the electrophoretic particles from a polymer that is highly compatible with the suspension fluid surrounding the particles in an electrophoretic medium (typically an aliphatic hydrocarbon, such as Isopar G). It has been considered desirable to use such highly compatible polymeric shells to provide good steric stability. Thus, in most of the above-mentioned examples of WO 02/093426, only a single monomer and a single polymerization step were required to provide a sterically stabilized polymeric shell on the functional pigment. The monomer used to form such a polymeric shell is typically lauryl methacrylate, but other monomers are also used in the aforementioned WO 02/093426.
It is now known that polymer shells can be modified such that the polymer shells are less compatible with the suspension fluid, that is, by making some portions of the polymer shell incompatible with the suspending fluid, certain significant advantages, particularly improved image stability, can be obtained .
The terms "compatible" and "incompatible" are used herein in their sense of meaning in the polymer art. To the simplest extent, the polymer compatibility with the suspension fluid means that the polymer (when separated from the bound basic electrophoretic particles) is soluble in the solvent. Whether the polymer is soluble can usually be determined by simple visual inspection; The solution is generally optically clear, while the non-solution (mixture or dispersion) is opaque or has two distinct phases.
However, compatibility is not a binary phenomenon, i.e. the polymer is not necessarily completely compatible or completely non-compatible with a given suspension fluid. Instead, there is a degree of compatibility, depending on the interaction between the fluid and the polymer moiety (approximately, the monomers making up the polymer) and the relative intensity of interaction of the polymer moieties. When the polymer is highly soluble in a fluid, the polymer-fluid interaction is more energy-efficient than the polymer-polymer interaction. In such a case, as a polymer that tends to form a coil, the polymer coils in solution will be stretched relative to the polymer coils in the polymer melt. Such elongation can be measured using a capillary viscometer or light scattering technique. For example, when measuring the intrinsic viscosity [[eta]] of a series of polymer samples with different molecular weights (M), we usually know that there is a power law relationship between the two quantities:

The index a ranges from 0.5 to about 0.8 (for a flexible polymer), depending on the quality of the solvent, and the larger the value, the better the solvent. When alpha = 0.5, the fluid is referred to as the " theta solvent "and the shape of the polymer is similar to that of the polymer melt (i.e., surrounded by other polymer moieties), in effect the polymer has neutral compatibility with the fluid. Under these conditions, the shape of the polymer chain is a random walk.
In both the homopolymer and copolymer systems, the Huggins coefficient can be used as an indicator of the solvent quality (i.e., polymer compatibility with the solvent). The Huggins constant is the [η] in the following equation for the relative viscosity of a dilute solution of polymer:²k 'term:

In this equation, η_S Is the viscosity of the solvent, [eta] is the intrinsic viscosity, and c is the concentration of the polymer in the fluid. For compatible fluids, k 'ranges from 0.30 to 0.40, while for less compatible fluids, k' is larger (0.50 to 0.80). (C. W. Macosko,Rheology Principles, Methods, and Applications, VCH Publishers, 1994, p. 481).
Similar information can be collected from the value of the second virial coefficient in a given fluid-polymer solution, measured by static light scattering or osmometry measurements. Generally, a large secondary virgin coefficient means a more compatible fluid. In the relation of the dissolved polymer steels, the second virial coefficient, which corresponds to a substantially non-compatible polymer-solvent interaction, may be - (C. Tanford,Physical Chemistry of Macromolecules, ≪ / RTI > John Wiley and Sons, New York, 1961, pp. 293-296).
When the polymer is insoluble in the fluid, i. E. When it is non-flammable, the polymer may sometimes be swollen by the fluid. The degree of swelling (polymer sample in the presence of fluid versus dry polymer volume ratio) can be used as a measure of the degree of compatibility of fluids and polymers in this situation. The larger the expansion degree, the greater the compatibility.
In many ways, the polymer shell can be made less compatible with the suspension fluid. First of all, according to the non-compatible monomeric electrophoretic medium aspect of the present invention, the polymer shell may have a repeating unit derived from at least one monomer in which the homopolymer of the monomer is non-compatible with the suspension fluid. Typically, such polymeric shells will also contain compatible monomers, i.e., homopolymers of monomers that are compatible with the suspending fluid, and compatibility of the polymer shell with the suspension fluid can be controlled by modifying the ratio of the two monomers.
The polymer shell containing both the incompatible and compatible monomers (apparently, one or more monomers of each type present) may comprise a mixture of polymerizable monomers in a final polymerization step (which may be a homopolymerization step) ≪ / RTI > One of the monomers is compatible with the suspension fluid, and if not one can impart some degree of colloidal stability to the particles, depending on the ratio of monomers in the shell. In addition, since different monomer species can impart different degree of incompatibility, the incompatibility can be further modified by changing the incompatible monomers. For example, when used as the sole monomers in polymer shells, acrylate esters (e.g., butyl methacrylate) having short side chains containing up to about 8 carbon atoms are used in the Isopar G suspension fluid, It was observed that it produced unstable particles. On the other hand, lauryl methacrylate yields electrophoretic particles with excellent colloidal stability in Isopar G. Thus, copolymers of lauryl methacrylate and butyl methacrylate can be prepared from polymeric mixtures in a molar ratio of butyl methacrylate to lauryl methacrylate, a polymer shell that is marginally compatible with Isopar G, and a colloidally marginally stable Lt; / RTI >
The second method of modifying the polymer shell around the electrophoretic particles is by the second stage polymerization. It has been experimentally demonstrated that the surface functionalities provided by the reagents of the above formulas (I) and (II) are not completely consumed during the single graft polymerization step. For example, if the surface functionalized, polymer coated titania is simply resuspended in a polymerization medium (typically including toluene, lauryl methacrylate and azobisisobutyronitrile (AIBN) as polymerization initiator) The thermogravimetric analysis of the pigments isolated from the reaction mixture after heating indicates an increase in the amount of bound polymer (measured by thermogravimetric analysis (TGA): 7.3% after the first polymerization: 8.9% after the second polymerization and 9.9% (two different runs)). In such a double graft polymerization process, the monomers used in the second polymerization can be different from those used in the first polymerization so that it is possible to incorporate polymer chains built up from different monomers in the final polymer shell. In addition, the conditions of the first polymerization step can be modified to control the initial grafting density of the chain as well as the possibility of surface vinyl action (and accessibility) in the second step polymerization. Thus, this double polymerization method provides a second degree of flexibility in the construction of the polymer-stabilized electrophoretic particles. It may be advantageous to adjust the molecular weight of the polymer chain in the first or second polymerization step, by incorporating the chain transfer agent into the polymerization mixture, or by adjusting the concentration of the monomer or radical initiator in a manner known in the polymer art.
At present, it is generally assumed that the suspension fluid favoring aliphatic hydrocarbons is poorly compatible with a large number of homopolymers, so that a very large range of monomers which are easy to use can be obtained by polymerizing a polymer prepared by either of the two methods described above It can be used as an incompatible monomer in the shell. Such incompatible monomers include but are not limited to acrylate esters (for example, methyl methacrylate, ethyl methacrylate, butyl methacrylate, iso-propyl methacrylate, Butyl methacrylate, t-butyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate), as well as esters of the corresponding acrylic acids, acrylates with fluorocarbon ester side chains, acrylates of functional alcohols Acrylates, such as acrylamide, methacrylamide, N-monoalkyl acrylamide and methacrylamide, N, N-dialkyl acrylamide, N- Vinyl pyrrolidone, and functional acrylamide derivatives, styrene, substituted styrene derivatives, vinyl acetate, and others Carbonyl esters, and halogenated vinyl derivatives include poly (vinyl chloride, vinylidene chloride, etc.), and other polymerizable monomer paper.
Although the present invention is by no means limited in theory, it is possible to logically describe the improved properties of an electrophoretic medium in which the polymer shells around the particles comprise an incompatible monomer. Polymer shells formed entirely from compatible monomers provide steric stability to the electrophoretic particles because they are thermodynamically advantageous in the polymer chain forming shells surrounding the suspension fluid than by other polymeric moieties. Thus, pure polymer (lauryl methacrylate) polymer shells dispersed in polymer shells formed entirely from compatible monomers, such as aliphatic hydrocarbons, are highly swollen. Intercellular penetration of the shell into adjacent particles increases the number of polymer-polymer contacts as compared to more suitable polymer-solvent contacts, so there is an effective repulsion between the particles and they tend to remain spaced from each other, Dispersed in the fluid. Polymer shells formed from totally incompatible monomers have the disadvantage that it is thermodynamically advantageous for the polymer chain to form a shell that is surrounded by other polymeric moieties than by a suspending fluid such that the polymer shells disintegrate and reject the suspension fluid and electrophoretic particles They do not provide such steric stability because they tend to attract each other and thus tend to flocculate. That is, the highly compatible polymer shell tries to disperse the electrophoretic particles in and out of the suspension fluid, while the incompatible polymer shell tends to aggregate the electrophoretic particles. The image stability of the electrophoretic medium is such that, in each type of electrophoretic medium shown in Figs. 1 to 3, the stability of the image once created is due to the fact that electrophoretic particles in the same aggregate (layer) And due to the ability to remain coagulated, it is promoted by the tendency of the electrophoretic particles to agglomerate. (Note that Figures 1 to 3 are simplified; in fact, particle agglomerates formed during imaging will usually have more than one particle layer.) Both compatible and incompatible monomers are incorporated into polymer shells to form polymeric shells and suspension fluids By adjusting the compatibility, it is possible to control the overall compatibility of the polymer shell with the suspension fluid, and the stability of such particle agglomerates, and thus the image stability of the resulting display.
A further advantage of the preferred embodiment of the invention is the introduction of a threshold for switching of the electrophoretic medium. If the attraction of the electrophoretic particles to each other, and thus of their tendency to remain a stable homogeneous aggregate, is strong enough, then a considerable electric field will be needed to pull the particles from the homogeneous aggregates and move them in the electric field. There are several advantages if there is a significant threshold voltage in switching the electrophoretic medium. If the threshold is large enough, passive matrix addressing of the high resolution display can be used. A smaller threshold may be useful to reduce the sensitivity of the active matrix display to vortex voltage and inter-pixel voltage leakage.
The present invention also provides another approach to improve image stability in a dual particle electrophoretic medium. In the dual particle electrophoresis medium, both types of electrophoretic particles need to contribute to overall image stability. If the second type of particles is susceptible to settling or diffusing all over the suspended fluid, it is not enough that one type of particle is "image stabilized" (i.e., remains as homogeneous aggregates at the location to be switched). In such a case, the optical state of one extremum of the medium may exhibit fairly good image stability, but the remainder will exhibit poor stability. Also, if particles can diffuse, poor image stability of one particle is likely to cause optical degradation of the optical state of the second extreme. It is therefore desirable that both types of particles have moderately controlled steric stability to provide good image stability. The extent of the depletion mechanism applied to both types of particles is dependent upon the particle size and particle shape (see, for example, < RTI ID = 0.0 > And can not be independently controlled because only one polymer is added to the suspension fluid and used to impart image stability to both types of particles. However, in the incompatible monomeric electrophoretic medium of the present invention, the colloidal stability of each type of particle can be independently controlled. It is also possible to add a polymer to the suspension fluid according to the depletion agglomeration method to make the two distinct types of particles unstable both colloquially as well as individually with respect to each other and thus promote the heterogeneous aggregation of the two types of particles . The heterogeneous agglomeration results in a response time delay in the dual particle electrophoretic display and the need to use a higher applied voltage (because higher field intensities will be needed to separate the heterogeneous aggregates) as well as poor optical state Since most charged aggregates can not be completely separated by the applied electric field).
According to aspects of the tuned coagulation medium of the present invention, such problems are eliminated or at least reduced by providing a dual particle electrophoretic medium in which the homogeneous aggregation of the two types of particles is thermodynamically more favorable than the heterogeneous aggregation. This provides for two types of particles with polymer shells adjusted to favor homogeneous agglomeration of their compatibility with the suspension fluid, but also the compatibility of the two types of polymer shells can be achieved by controlling the heterogeneous agglomeration to be disadvantageous have. Achieving this by using different incompatible monomers in the polymer shell of both types of particles is relatively easy; Typically, the same compatible monomers can be used in polymer shells on two types of particles. If the two incompatible monomers are not compatible with each other, then the two types of particles will be minimally prone to heterogeneous aggregation, only homologous aggregation will be promoted, resulting in a faster response time and an optical state of the electrophoretic medium becoming more pure It becomes.
As already mentioned above, the present invention is not limited to using aliphatic hydrocarbons as a suspension fluid. (E.g., toluene, dodecylbenzene, etc.), or mixtures thereof, may be useful in the present invention, as they are described above, in the polymer shells This is because it further improves the properties of the suspended fluid with respect to solvation.
Although the present invention is applicable to all types of electrophoretic media, it may be useful for encapsulated electrophoretic media, whether using microcells that used pre-manufactured capsules. The present invention is also applicable to displays of all types of switching, including both transmission and forwarding as well as side-to-side and shutter mode switching.
Figure 1 of the accompanying drawings schematically illustrates a preferred method of preparing the electrophoretic particles according to the invention, as used in the following examples, which essentially consists of those described in the aforementioned WO 02/093246 similar. In the first step process, base pigment particle 700 is coated with silica to produce silica pigment 702; This step is fully described in the aforementioned WO 02/093246. Next, a surface functionalized pigment 704 is prepared that is treated with a bifunctional reagent having a functional group that reacts with the silica surface and a second charge control agent to provide a charge controller on the surface of the silica pigment 702. The bifunctional reagent also provides a polymer forming site on the pigment particles. Finally, as shown in FIG. 1, the surface-functionalizing pigment 704 is contacted with one or more monomers or oligomers under conditions effective to form the polymer attached to the charge controller to produce a polymer-coated functional pigment 706, It is used for the migration of the medium.

The following examples are provided by way of example only to illustrate the synthesis of several different electrophoretic particles of the present invention having a polymer shell with controlled suspension compatibility with a suspension fluid and illustrate the advantages achieved by such electrophoretic particles .

Experiments in these examples will be described using the symbols outlined below. These experiments used white titania pigments based on du Pont Ti-Pure R960 and black copper chromate pigments based on Shepherd Black 1G. These particles were surface-functionalized with Z6030 to a white pigment and a reagent of the above formula (I) in black according to the reaction formula of Fig. 7, and then a polymer shell was formed thereon in the manner described below. When the thus prepared white particles were incorporated into an Isopar G suspension fluid containing Solsperse 17K and Span as an antistatic agent, the negative charge was negatively charged and the black particles were positively charged. As a control experiment, particles having lauryl methacrylate homopolymer shells were prepared, and the white particles and the black particles were referred to as J and D, respectively. The modification of the polymer shell is described as follows: For the particles prepared using the random copolymer, one-step polymerization, the comonomer is, in parentheses, the number representing the moles of the comonomer used in the polymerization reaction It is distinguished by the letter code shown in Table I below, after the pigment indicator. Remaining in each case was lauryl methacrylate. Thus, the symbol J (BMA15) represents a white pigment made from a polymerization mixture comprising 15 mol% t-butyl methacrylate and 85 mol% lauryl methacrylate. The two-step polymerization is indicated by the + sign; In general, the molar ratio of the polymer in these particles is not so known, and the composition is not shown. Thus, J (+ St) represents a white pigment prepared by two-stage polymerization. The starting material is in this case the control pigment and 100 mol% lauryl methacrylate was used in the first stage polymerization; The second stage polymerization was carried out using only styrene as the polymerizable monomer.

Monomer Simplified symbol Styrene St t-butyl methacrylate TBMA Vinylpyrrolidone VP

The procedure used to form the polymer shell, and incorporation of the resulting polymer-coated particles into the electrophoretic medium and display was as follows. All centrifugations mentioned were performed on a Beckman GS-6 or Allegra 6 centrifuge (purchased from Beckman Coulter, Inc., Fullerton, CA 92834).

1. Standard Lauryl Methacrylate Polymerization

A single-neck, 250 mL round bottom flask was equipped with magnetic stir bar, reflux condenser and argon / or nitrogen inlet and placed in a silicone oil bath. 60 g of silane-coated pigment milled to be a fine powder using a bowl and mortar were added 60 ml of lauryl methacrylate (LMA, Aldrich) and 60 ml of toluene to the flask. The reaction mixture was then rapidly stirred and the flask was purged with argon or nitrogen for 1 hour. During this time the silicon bath was heated to 50 < 0 > C. During the purge, 0.6 g of AIBN (2,2'-azobisisobutyronitrile, Aldrich) was dissolved in 13 ml of toluene. At the end of the 1 hour purge, the AIBN / toluene solution was added quickly with a glass pipette. The reaction vessel was sealed, heated to 65 < 0 > C and stirred overnight. At the end of the polymerization, 100 mL of ethyl acetate was added to the viscous reaction mixture and the mixture was stirred for an additional 10 minutes. The mixture was poured into a plastic bottle and centrifuged at 3600 rpm for 15-20 minutes and the supernatant was drained. New ethyl acetate was added to the centrifuged pigment, which was stirred with a stainless steel spatula and ultrasonicated for 10 minutes. The pigment was further washed twice with ethyl acetate according to the above procedure. The pigment was air dried overnight and then dried under high vacuum for 24 hours. The free polymer in the bulk solution was precipitated in methanol and dried under vacuum. The molecular weight of the free polymer in solution was determined by gas phase chromatography (GPC). The polymerization bound to the pigment was measured by TGA.

2. Copolymerization in titania

A single-neck, 250 mL round bottom flask was equipped with magnetic stir bar, reflux condenser and argon / or nitrogen inlet and placed in a silicone oil bath. To the flask was charged 60 ml of toluene, 60 g of titania (Z6030 coated Dupont R960), lauryl methacrylate (LMA) and a second monomer such as styrene, t-butyl methacrylate, 1-vinyl- Dimethylaniline, dinonon, hexafluorobutyl acrylate and methacrylate, N-isopropylacrylamide or acrylonitrile (Aldrich) are added and the amount of LMA and second monomer is in accordance with the desired monomer ratio. The ratio of LMA / second monomer was usually 95/5, 85/15 and 75/25. The reaction mixture was then rapidly stirred and the flask was purged with argon or nitrogen for 1 hour. During this time the silicon bath was heated to 50 < 0 > C. During the purge, 0.6 g of AIBN was dissolved or partially dissolved in 13 ml of toluene. At the end of the 1 hour purge, the AIBN / toluene solution was added quickly with a glass pipette. The reaction vessel was sealed, heated to 65 < 0 > C and stirred overnight. 100 mL of ethyl acetate was added to the viscous reaction mixture and the resulting mixture was stirred for an additional 10 minutes. The mixture was poured into a plastic bottle and centrifuged at 3600 rpm for 15-20 minutes and the supernatant was drained. New ethyl acetate was added to the centrifuged pigment and the resulting mixture was stirred with a stainless steel spatula and sonicated for 10 minutes. The pigment was washed two more times with ethyl acetate, centrifuged and the supernatant was drained. The pigment was air dried overnight and then dried under high vacuum for 24 hours. The free polymer in the bulk solution was precipitated in methanol and dried under vacuum. The molecular weight of the free polymer in solution was measured by GPC. The polymer bound to the pigment was measured by TGA.

2.1 LMA and 1-vinyl-2-pyrrolidinone copolymerization in titania

A single-neck, 250 mL round bottom flask was equipped with magnetic stir bar, reflux condenser and argon / or nitrogen inlet and placed in a silicone oil bath. To the flask was added 60 mL of toluene, 60 g of titania (Z6030 coated Dupont R960), 51 mL of lauryl methacrylate (LMA) and 3.3 mL of 1-vinyl-2-pyrrolidinone. The molar ratio of LMA / 1-vinyl-2-pyrrolidone is usually 85/15. The reaction mixture was then rapidly stirred and the flask was purged with argon or nitrogen for 1 hour. During this time the silicon bath was heated to 50 < 0 > C. During the purge, 0.6 g of AIBN was dissolved or partially dissolved in 13 ml of toluene. At the end of the 1 hour purge, the AIBN / toluene solution was added quickly with a glass pipette. The reaction vessel was sealed, heated to 65 < 0 > C and stirred overnight. 100 mL of ethyl acetate was added to the viscous reaction mixture and the resulting mixture was stirred for an additional 10 minutes. The mixture was poured into a plastic bottle and centrifuged at 3600 rpm for 15-20 minutes and the supernatant was drained. New ethyl acetate was added to the centrifuged pigment and the resulting mixture was stirred with a stainless steel spatula and sonicated for 10 minutes. The pigment was washed two more times with ethyl acetate, centrifuged and the supernatant was drained. The pigment was air dried overnight and then dried under high vacuum for 24 hours. The free polymer in the bulk solution was precipitated in methanol and dried under vacuum. The molecular weight of the free polymer in solution was measured by GPC. The polymer bound to the pigment was measured by TGA.

3. Two-step polymerization using LMA-coated white pigment

A single-neck, 250 mL round bottom flask was equipped with magnetic stir bar, reflux condenser and argon / or nitrogen inlet and placed in a silicone oil bath. 60 grams of the silane-coated pigment milled to a fine powder using a bowl and a mortar followed by 60 milliliters of lauryl methacrylate and 60 milliliters of toluene was added to the flask. The reaction mixture was then rapidly stirred and the flask was purged with argon or nitrogen for 1 hour. During this time the silicon bath was heated to 50 < 0 > C. During the purge, 0.6 g of AIBN was dissolved in 13 ml of toluene. At the end of the 1 hour purge, the AIBN / toluene solution was added quickly with a glass pipette. The reaction vessel was sealed, heated to 65 < 0 > C and stirred overnight. At the end of the polymerization, 100 mL of ethyl acetate was added to the viscous reaction mixture and the resulting mixture was stirred for an additional 10 minutes. The mixture was poured into a plastic bottle and centrifuged at 3600 rpm for 15-20 minutes and the supernatant was drained. New ethyl acetate was added to the centrifuged pigment and the resulting mixture was stirred with a stainless steel spatula. The pigment was further washed twice with ethyl acetate according to the above procedure. The pigment was air dried overnight and then dried under high vacuum for 24 hours. The free polymer in the bulk solution was precipitated in methanol and dried under vacuum. The molecular weight of the free polymer in solution was measured by GPC. The polymer bound to the pigment was measured by TGA.

Note) In the above procedure, the ultrasonic treatment used in the procedure described above is omitted in order to avoid any damage to the ethylene groups remaining on the pigment surface after the first stage polymerization described above.

For the two-stage polymerization, another single-neck, 250 ml round bottom flask was equipped with magnetic stir bar, reflux condenser and argon / or nitrogen inlet and placed in a silicone oil bath. Fifty grams of LMA-coated pigment, prepared as described above and ground to a fine powder using a bowl and mortar, followed by 85 mL of toluene and 16 grams of styrene was added to the flask. The reaction mixture was then rapidly stirred and the flask was purged with argon or nitrogen for 1 hour. During this time the silicon bath was heated to 50 < 0 > C. During the purge, 0.4 g of AIBN was dissolved in 10 ml of toluene. At the end of the 1 hour purge, the AIBN / toluene solution was added quickly with a glass pipette. The reaction vessel was sealed, heated to 65 < 0 > C and stirred overnight. At the end of the polymerization, 100 mL of ethyl acetate was added to the viscous reaction mixture and the resulting mixture was stirred for an additional 10 minutes. The mixture was poured into a plastic bottle and centrifuged at 3600 rpm for 15-20 minutes and the supernatant was drained. New ethyl acetate was added to the centrifuged pigment and the resulting mixture was stirred with a stainless steel spatula and sonicated for 10 minutes. The pigment was further washed twice with ethyl acetate according to the above procedure. The pigment was air dried overnight and then dried under high vacuum for 24 hours. The free polymer in the bulk solution was precipitated in methanol and dried under vacuum. The molecular weight of the free polymer in solution was measured by GPC. The polymer bound to the pigment was measured by TGA.

4. Copolymerization in coated copper chromite

A single-neck, 250 mL round bottom flask was fitted with a magnetic stir bar, reflux condenser and argon inlet and placed in a silicone oil bath. 60 g of copper chromate (CuCr ₂ O ₄ , Shepherd Black 1G), which was coated with the silane of the above formula (I) and pulverized to be a fine powder in a mortar and pestle, was charged with 1.2 ml of styrene, 57 ml of La Urea methacrylate and 60 ml of toluene were added. The reaction mixture was then rapidly stirred and the flask was purged with argon or nitrogen for 1 hour and the silicon bath was heated to 50 < 0 > C. During the purge, 0.6 g of AIBN was dissolved or partially dissolved in 13 ml of toluene. At the end of the 1 hour purge, the AIBN / toluene solution was added quickly with a glass pipette. The reaction vessel was sealed, heated to 65 < 0 > C and stirred overnight. 100 mL of ethyl acetate was added to the viscous reaction mixture and the resulting mixture was stirred for an additional 10 minutes. The mixture was poured into a plastic bottle and centrifuged at 3600 rpm for 15-20 minutes and the supernatant was drained. New ethyl acetate was added to the centrifuged pigment and the resulting mixture was stirred with a stainless steel spatula and sonicated for 10 minutes. The pigment was washed two more times with ethyl acetate, centrifuged and the supernatant was drained. The pigment was air dried overnight and then dried under high vacuum for 24 hours. The free polymer in the bulk solution was precipitated in methanol and dried under vacuum. The molecular weight of the free polymer in solution was measured by GPC. The polymer bound to the pigment was measured by TGA.

Preparation of In-Control (Electrophoretic Particle + Suspension Fluid) in Control

(a) 85 g of a stock solution of J pigment containing 60% by weight of LMA-coated titania in Isopar G; (b) 42.5 g stock solution of D pigment containing 60% by weight of LMA-coated copper chromate in Isopar G; (c) 10.71 g of a stock solution containing 10% by weight of Solsperse 17000 in Isopar G; (d) 31.03 g of Isopar G; And (e) 0.77 g Span 85 (non-ionic surfactant).

J and D stock solutions were added to a 250 ml plastic bottle, followed by Solsperse 17000 solution and Span 85, and finally the remaining solvent. The resulting injuries were shaken vigorously for approximately 5 minutes and then placed in a roll mill overnight (over 12 hours).

Production of the internal phase using the white pigment of the present invention and the prior art black pigment

(a) 40 g stock solution of a modified J pigment containing 60% by weight of LMA / TBMA-coated titania (molar ratio 85/15) in Isopar G; (b) 20 g stock solution of D pigment containing 60% by weight of LMA-coated copper chromate in Isopar G; (c) 5.04 g of a stock solution containing 10% by weight of Solsperse 17000 in Isopar G; (d) 14.60 g of Isopar G; And (e) 0.36 g Span 85 (non-ionic surfactant).

The internal phase was mixed and stored in the same manner as the previous internal phase described above.

The preparation of the internal phase using the prior art white pigment and the black pigment of the present invention

(a) 40 g of a stock solution of the same J pigment as in the control, which contains 60% by weight of LMA-coated titania in Isopar G; (b) 20 g stock solution of D pigment containing 60% by weight of LMA / St-coated copper chromate (85/15 or 95/5 monomer ratio) in Isopar G; (c) 5.04 g of a stock solution containing 10% by weight of Solsperse 17000 in Isopar G; (d) 14.60 g of Isopar G; And (e) 0.36 g Span 85 (non-ionic surfactant).

The internal phase was mixed and stored in the same manner as the previous internal phase described above.

The inner wound thus produced was encapsulated (separately) in gelatin / acacia microcapsules substantially as described in paragraphs [0069] to [0074] of 2002/0180687 mentioned above. The resulting microcapsules were separated by size and capsules having an average particle size of about 35 占 퐉 were used in the following experiments. In practice, as described in paragraphs [0075] and [0076] of 2002/0180687 mentioned above, the microcapsules are mixed with the polyurethane binder to be a slurry, and a layer of indium tin oxide (ITO) (177 μm) poly (ethylene terephthalate) (PET) film having a dry coating weight of 18 gm ^{-2 by} a roll-to-roll process, and the microcapsules were coated with ITO Lt; / RTI > A capsule-retaining film was then laminated to the polyurethane laminated adhesive layer associated with the release sheet to form a front plane laminate, which laminated at 65 psig (0.51 mPa) to 6 inches / minute (2.5 mm / sec ) Using a Western Magnum twin roll laminator, and the two rolls were maintained at 120 < 0 > C. In order to provide an experimental single-pixel display suitable for use in these experiments, the release sheet of the resulting front laminated piece was removed and laminated at 75 ° C to be a 5 cm x 5 cm PET film covered with a carbon black layer, And used as a back electrode of a single pixel display.

Image stability measurement

The thus fabricated single pixel display was switched to a grounded back electrode using a 500 msec square wave pulse at an alternating current of 10 V applied to the upper surface (ITO layer). A two second pause between pulses was used for switching reorganization. The image stability was measured by switching the pixels to the appropriate optical state (white or black), grounding the top surface, and measuring the light reflectance continuously for 10 minutes. It is assumed that the optical kickback resulting from the residual voltage of the binder and laminated adhesive layer is lost within 5-10 seconds. The difference in optical state (measured in L * units) between 5 seconds and 10 minutes was taken as a measure of image stability. The results shown in Table 2 below were obtained in selected pixels.

Pigment Example No. White state IS (dL *) The black state IS (dL *) J / D (control group) One -6.3 2.7 J (TBMA5) / D 2 -2.2 2.1 J (TBMA15) / D 3 -4.2 1.1 J / D (St5) 4 -4.7 2.0 J / D (St15) 5 -4.7 1.6 J (TBMA5) / D (St15) 6 -3.3 1.7 J (TMBA15) / D (St5) 7 -1.6 3.0 J (+ St) / D 8 -2.2 0.3

Control pixels show relatively good black state stability, but white state stability is somewhat poor (similar displays using carbon black instead of copper chromate as black pigment and no PIB in suspension fluid show 10- < RTI ID = 0.0 > Indicating a much poorer stability of the order of 15 L *). Any of the electrophoretic pigments of the present invention improves image stability in either or both of the white or black states as compared to the control. In only one case, the image stability of the pixel of the present invention is less than that of the control (J (TBMA15) / D (St5)), where the difference is probably within experimental pixel-to-pixel tolerances. A display made of white pigment using styrene as the second monomer and synthesized using the two-step process provides the best overall image stability. Such a display has good image stability in a white state and excellent image stability in a black state.

Response time

The response time of the electrophoretic display shown in Table 2 was measured by measuring the electro-optic response as a function of pulse length at an operating voltage of 10 V. [ A pause length of 2000 msec was used for all measurements. By measuring the L * difference between the white and dark states, it has been found that the electro-optic response saturates at a certain pulse length and then decays slightly at longer pulse lengths. The control sample was a pixel of the same formulation as the control in Table 2 and a pseudo pixel containing 0.3 to 0.9 weight percent high molecular weight PIB as a means for achieving good image stability. Table 3 shows the pulse lengths for which the electro-optic response achieves 90% of its value with a 1 second pulse length in the display of Table 2.

Pigment Example No. Time to 90% saturation (msec) J / D (control - without PIB) One 294 J (TBMA5) / D 2 173 J (TBMA15) / D 3 281 J / D (St5) 4 325 J / D (St15) 5 375 J (+ St) / D 8 380 J / D + 0.9% PIB (control group) 9 740 J / D + 0.3% PIB (control group) 10 575

The response time of the control sample containing PIB was significantly longer than the response time of the sample according to the present invention. To obtain adequate image stability in this system, at least 0.3% PIB is required and 0.9% PIB is preferred. Thus, the method of the present invention achieves good image stability with a response time 1.5 to 4 times faster than the control sample.

Threshold voltage

The above-described pixels using a white pigment having 5 mol% TBMA and a black pigment having 15 mol% styrene in the polymer shell exhibit a large operating threshold in addition to a relatively good image stability. The threshold voltage is shown in Fig. 2 which shows the dynamic range of the pixel as a function of the applied voltage for several pulse lengths. The dynamic range is very small (on the order of 1 L *), but good dynamic range (above 30 L *) for impulses exceeding 10 V and 600 msec, at an applied voltage of less than 4-5 volts, regardless of pulse length.

From the above it can be seen that the non-compatible monomers, particles, media and displays of the present invention, particles and displays, and homogeneous flocculation media provide good image stability, and in particular the present invention does not sacrifice response time to achieve good image stability, Will have a number of advantages over similar media containing polymer additives. The advantage of this response time is that it can be used to operate the display at low voltages. The display written in Table 2 is switched to be nearly saturated within 300 msec at 7.5 V and achieves an optimal contrast ratio at 10 V and 250-300 msec. At 15 volts, a switching time of about 100 msec can be achieved.

Aliphatic polymerization method

As already mentioned, it has been found that the above-mentioned WO 02/093246 and the graft polymerization step described above can be carried out in aliphatic hydrocarbons, in particular in isoparaffinic solvents; A preferred solvent for this purpose is Isopar G, available from Exxon Mobil Corporation, Houston TX. Because such aliphatic hydrocarbons are the same as the materials typically used as suspension fluids in electrophoretic media, the coated pigments can remain in essentially the same environment until incorporated into the final display in the graft polymerization step.

At the end of the graft polymerization step, the aliphatic hydrocarbon solvent may comprise any one or more of an excess polymerization initiator, an excess monomer or oligomer, and a free polymer not attached to the pigment particles. These materials need to be substantially removed from the pigment particles before the pigment particles are incorporated into the final electrophoretic medium, because the materials can adversely affect the electro-optical properties and / or operating lifetime of the medium. Thus, it will also be necessary to separate the polymer-coated pigment from the graft polymerization solvent, typically by centrifugation, for example, and to wash the pigment particles to remove to the last remaining amount of the above-mentioned material. However, there is no need to dry the polymer-coated pigment to remove from the polymer-coated pigment to the last residue of the hydrocarbon solvent (thus eliminating the contamination control problems associated with solvent vapors), re-dispersing the dry pigment in the aliphatic halocarbon solvent You do not have to; This is because, at the end of the washing step, the pigment is already wetted with the aliphatic hydrocarbon solvent and there will be no difficulty in redispersing the pigment in a large amount of solvent required for the final electrophoretic medium. Obviously, separation and washing of the polymer-coated pigment could in principle be removed if the polymerization can be carried out in such a way that only a minimal amount of the abovementioned substances remain in the graft polymerization solvent after the polymerization is completed.

Although it is not believed that there is any inherent obvious difference between the polymer-coated pigment prepared in the aliphatic hydrocarbon solvent according to the present invention and those prepared in toluene according to the prior art process, some experiments have shown that the pigment prepared in an aliphatic hydrocarbon solvent ≪ / RTI > tend to have a higher polymer content.

As mentioned above, the suspension fluid in the electrophoretic medium is usually an aliphatic hydrocarbon either alone or in combination with a halocarbon. When the suspension fluid is a mixture of aliphatic hydrocarbons and halocarbons, graft polymerization is carried out at the aliphatic hydrocarbons as a sole because there is no difficulty in redispersing the pigment that has been wetted with the aliphatic hydrocarbons to the mixture of aliphatic hydrocarbons and halo carbons, . However, it does not exclude the possibility that the graft polymerization itself can be carried out in a mixture of aliphatic hydrocarbon and halocarbon.

When the dual particle electrophoretic medium is prepared (for example, a medium containing white titania particles and carbon black particles), the method of the present invention can be separately applied to two types of particles. However, if two types of particles are provided with a polymer coating of a similar type, it is preferred to first provide the polymerizable groups on the two types of pigments, then blend the pigments and then perform the graft polymerization step on the pigment mixture It can also be executed. Thus, the two pigments move together during the remaining steps required to form the electrophoretic medium. In fact, the polymer-coated mixed pigment produced in this way can be regarded as a pigment storage solution, which may require little or no additional processing to remove residual impurities after washing. Thus, this modification of the method further reduces the number of individual steps that need to be performed to convert the raw pigment into a final electrophoretic medium.

The process of the present invention reduces the safety hazards associated with prior art processes because the aliphatic hydrocarbon solvent used in the process is less hazardous than toluene and THF. As already mentioned, the present process reduces solvent use and also improves the overall quality of the final dispersion of the pigment in the aliphatic hydrocarbon solvent by reducing the pre-drying and pigment redispersion steps to reduce the dispersion treatment time, Thereby reducing the total number of process steps for electrophoretic media production.

Another aspect of the invention relates to controlling the relative size and mass of particles in the electrophoretic medium to reduce the tendency of the electrophoretic particles to agglomerate.

It is known that in electrophoretic media, especially in dual particle electrophoretic media, the particles tend to aggregate. This tendency may be particularly problematic in a dual particle electrophoretic medium in which both types of particles possess opposite polarity charges because it is natural that any particles bearing an electrically opposite charge will stick to one another to be. Such particle agglomeration reduces the contrast of the display; For example, when black and white particles aggregate and form non-separating gray particles when applying an electric field to change the optical state of the display, presumably the pure black or white optical state of the display is contaminated with gray aggregates Thereby reducing the contrast ratio. In addition, severe and persistent agglomeration can eventually hinder the display from functioning, thereby reducing the operational life of the display.

It has now been found that the relative movement of two types of particles in a dual particle electrophoretic medium can be used to separate aggregates and thus reduce problems associated with such aggregates. When a second type of particles moving in the opposite direction to the first type of particles under a given electric field is introduced into the electrophoretic medium, the two types of particles move between each other to separate the particles and reduce particle agglomeration through physical means will be.

In a preferred embodiment, the electrophoretic medium contains one or more particles of one type whose average diameter is about 0.25 to about 4 times the average particle diameter of the second type particles bearing opposite charge; Preferably, the average particle mass ratio between the two types of particles is from about 0.25 to about 4.00.

Also in a preferred embodiment, the electrophoretic medium comprises at least two types of electrophoretic media, each having an opposite polarity charge, which is capable of reaching a velocity of V1 and V2, respectively, Contains particles:

V1 + V2 > f (F, G)

Where f is a function that reduces or eliminates particle aggregation if the inequality is satisfied.

Threshold voltage display

As already indicated, the present invention provides an active matrix electro-optic display using an electro-optic medium with a threshold voltage. Using this type of electro-optic medium reduces design constraints on the backplane of electro-optic displays.

As an example, consider an electro-optic medium that exhibits a strong threshold voltage at 5V. The term "strong threshold voltage" is used to mean that the medium does not switch at all at voltages below the threshold, no matter how long the pulse is. At voltages above the threshold, the medium usually switches. The use of such a medium alleviates the data peak problem caused by coupling the data line to the pixel electrode (see paragraphs 42-52 above); If the voltage peak magnitude for a pixel is less than 5 volts, such a strong threshold voltage medium will not respond. The pixel storage batteries used in such a display need only be large enough to keep the voltage peak experienced by the electro-optic medium below the threshold voltage, which is a problem with prior art electro-optic displays in which the electro-optic medium does not exhibit a threshold voltage Compared with the required battery, the size of the storage battery can be significantly reduced.

Although the present invention is not limited to the use of an electro-optic medium exhibiting a strong threshold voltage, it does respond to a voltage below the threshold applied for a very long period of time, but such a voltage applied for a short period of time, To the use of an electro-optic medium having only a "weak threshold voltage" as used herein to refer to a non-responsive medium. For example, the period of interest may be a single scan frame (typically about 20 msec), or a full image update (typically about 1000 msec).

The present invention may utilize any of the types of electro-optic media discussed above in which the electro-optic medium exhibits a strong or weak threshold voltage. As described above, an incompatible monomeric electrophoretic medium having a threshold voltage is generally preferred.

The present invention relaxes the design rules of an active matrix array, typically a thin film transistor (TFT) array. More specifically, the present invention makes it possible to manufacture TFT backplanes using lower resolution technologies such as printing, using smaller storage cells per unit area of the pixel electrodes. In addition, the size of the storage battery can be reduced, and the size of the transistor can be reduced. Reducing the size of these two elements will significantly reduce line capacitance, and thus power consumption, in the backplane. Thus, the electro-optic display of the present invention will have improved performance over conventional designs using prior art amorphous silicon structures.

Claims (1)

All inventions described in the Detailed Description of the Invention.

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