KR101197503B1 - Electrophoretic media - Google Patents

Abstract

The first electrophoretic medium comprises electrically charged particles 700 suspended in a suspension fluid, which particles 700 contain repeating units derived from one or more monomers whose homopolymers are incompatible with the suspension fluid. Branch has a polymeric shell. The second, pseudoelectrophoretic medium comprises a suspension fluid, electrically charged first and second types of particles suspended in the suspension fluid, both types of particles having different optical characteristics but both of which are polymeric shells. Has The polymeric shells are arranged such that homogeneous aggregation of the two types of particles is thermodynamically advantageous over heteroaggregation.

Description

Electrophoretic medium {ELECTROPHORETIC MEDIA}

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

In the display of the present invention, the electro-optic medium (if it is a non-electrophoretic electro-optic medium) will typically be solid, in view of the fact that the outer surface of the electro-optical medium is solid (hereinafter such display is for convenience). The internal space of the medium can be filled with liquid or gas, and often so, use such an electro-optical medium in the display assembly method. Thus, the term "solid electro-optical display" includes capsule electrophoretic displays, capsule liquid crystal displays, and other types of displays discussed below.

As used herein, the term “electro-optical” refers to a material having at least one optical property that has different first and second display states when applied to a material or display, the application of which results in its first display state. It is used in the conventional sense in the field of imaging, indicating a substance that changes from to a second display state. Optical properties are typically visually recognizable colors, but virtually color, in terms of other optical properties, such as light transmittance, reflectance, luminescence, or reflectance change of electromagnetic wavelengths outside the visible range for machine-readable displays. Can be.

The term "gray state" is used herein in the conventional sense in the field of imaging, referring to the intermediate state of the two extreme optical states of a pixel, and does not necessarily mean a black-white transition between these two extreme states. Do not. For example, several patents and published applications cited below describe electrophoretic displays in which the extreme states are white and dark blue, so that the intermediate "grey state" is actually light blue. Indeed, as already mentioned, the transition between the two extreme states may not be a change in color at all.

The terms "bistable" and "bistable" herein include any display device having a first and a second display state in which at least one optical property is different, and using any addressing pulse of limited duration, After driving, the first or second display state is taken, and after the addressing pulse is terminated, for several times or more, for example, four times or more the minimum duration of the addressing pulses required to change the state of the display element. It is used in the conventional sense in the art to indicate the display whose state is to be sustained. In published US patent application 2002/0180687 some of the particle-based electrophoretic displays capable of gray gradation are stable not only in their extreme black and white states, but also in their intermediate gray states, which lead to some other types of electro-optical displays. It also appears to fit. Although this type of display is more precisely referred to as "multi-stable" rather than bistable, the term "bistable" may be used herein for convenience in dealing with both bistable and multi-stable displays.

Several types of electro-optic displays are known. One type of electro-optic display is described, for example, in US Pat. No. 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 a rotating dichroic element type as described in 6,147,791 (this type of display is often referred to as a "rotating dichroic ball" display, but in some of the patents mentioned above the rotating element is not spherical, therefore, the "rotating dichroic element" Is preferred because the term is more accurate). Such displays use two or more compartments with different optical characteristics, and a number of small bodies (typically spherical or cylindrical) having internal dipoles. These bodies are suspended in vacuoles filled with liquid inside the matrix, which is 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. Electro-optic media of this type are typically bistable.

Another type of electro-optical display is in the form of a nanochromic film comprising an electrochromic medium, for example an electrode formed from a semi-conductive metal oxide and a plurality of dye molecules attached to the electrode and capable of reversible color change. Electrochromic medium is used; See, eg, O'Regan, B., et al., Nature 1991, 353 , 737; And Wood, D., Information Display, 18 (3) , 24 (March 2002). See also Bach, U., et al., Adv. See Mater., 2002, 14 (11) , 845. Nanochromic films of this type are also described, for example, in US Pat. No. 6,301,038, published international application WO 01/27690, and US patent application 2003/0214695. This type of medium is also typically bistable.

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

Numerous patents and applications have been published recently, assigned or assigned to the Massachusetts Institute of Technology (MIT) and E Ink Corporation, which describe encapsulated electrophoretic media. Such encapsulated media include a number of small capsules, each of which includes an interior phase that itself contains particles suspended in electrolytic suspension and electrophoretically moved, and a capsule wall surrounding the interior phase. Typically, the capsule itself is fixed with a polymeric binder to form an adhesive layer positioned between the two electrodes. Encapsulated media of this type are described, for example, in US Pat. No. 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 US 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 as "single particles" and "double particles", respectively, for convenience. The single particle medium has only a single type of electrophoretic particle suspended in a suspending medium, at least one of which the optical characteristics differ from the optical characteristics of the particles. (For a single type of particle, this does not mean that all particles of that type are exactly the same. For example, if all particles of that type carry the same polarity of charge, they do not affect the use of the medium, Variables such as color, size and electrophoretic mobility can be significantly modified, for example, two particles of different colors but with the same charge are mixed together in a single capsule with a single pigment (or multiple pigment) of opposite charge, By appropriately selecting the color of these pigments, it is possible to obtain the color of any desired halftone of either or both of the optical states.) If at least one is such a medium between a pair of electrodes, the two electrodes Depending on the relative potential of the medium, the medium can display the optical characteristics of the particle (hereinafter referred to as the "front" electrode), the observer 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).

Dual particle media have two different types of particles that differ in at least one optical characteristic and an uncolored or colored but typically uncolored suspension fluid. The two types of particles differ in electrophoretic mobility; This difference in mobility may be in polarity (this type may be hereinafter referred to as "anti-charge double particle" medium) and / or scale. When such a dual particle medium is placed between the aforementioned pair of electrodes, depending on the relative potential of the two electrodes, the medium can display the optical characteristics of either set of particles, but the rigorous way to achieve this is to It depends on whether the difference is in polarity or only in scale. For simplicity, consider an electrophoretic medium where one type of particle is black and the other type is white. If the two types of particles differ in polarity (eg, black particles are positively charged and white particles are negatively charged), the particles will be attracted to two different electrodes, and so, for example, If the front electrode is negative compared to the back electrode, the black particles will be attracted to the front electrode and the white particles to the back electrode, so the medium will appear black to the viewer. Conversely, if the front electrode is positive compared to the back electrode, the white particles will be attracted to the front electrode and the black particles to the back electrode, so the medium will appear white to the viewer.

If two types of particles have the same polarity charge but differ in electrophoretic mobility (this type of medium may be referred to as "the same polar double particle" medium), the two types of particles are attracted to the same electrode. Although one type will reach the electrode before the other, the type encountered by the observer will depend on the electrode from which the particle is attracted. For example, suppose that in the previous description both black and white particles were positively charged, the electrophoretic mobility of the black particles is higher. Now if the front electrode is negative compared to the back electrode, both black and white particles will be attracted to the front electrode, but the black particles will reach first because of their higher mobility, so the black particle layer will coat the front electrode and the medium Will appear black to the viewer. Conversely, if the front electrode is positive relative to the back electrode, both black and white particles will be attracted to the back electrode, but the black particles will reach first because of their higher mobility, and thus the black particle layer will coat the back electrode. The white particle layer will remain away from the back electrode and face the observer so that the medium will appear white to the observer: this type of dual particle medium will have a suspension fluid that is sufficiently transparent so that the white particle layer away from the back electrode is easy to gather to the observer. Note that it requires. Typically, the suspension fluid in such a display is not colored at all, but any color to correct any undesirable tint of white particles seen therethrough, or to produce a desirable tint of color in the gray state. Can be incorporated.

Both single and dual particle electrophoretic displays may be capable of a medium gray state with optical characteristics that are intermediate to 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 multi-particle media are considered to be subspecies of dual particle media.

In addition, many of the aforementioned patents and applications replace continuous walls of individual microcapsules in a capsular electrophoretic medium such that the electrophoretic medium is a plurality of individual droplets of electrophoretic fluid and a continuous phase of polymeric material. So-called polymer-dispersible electrophoretic displays can be prepared, and although no separate capsule membrane is associated with each individual drop, the electrophoretic fluid in such polymer-disperse electrophoretic displays It is recognized that individual drops can be considered capsules or microcapsules; See, for example, 2002/0131147 mentioned earlier. Thus, for the purposes of the present application, such polymer-disperse electrophoretic media are considered to be subspecies of the encapsulated electrophoretic medium.

A related type of electrophoretic display is the so-called "microcell electrophoretic display". In microcell electrophoretic displays, charged particles and suspension fluid are not encapsulated in microcapsules but instead remain in a plurality of cavities, typically polymeric films, formed in a carrier medium. See, eg, published international application WO 02/01281, and published US application 2002/0075556, both assigned to Sipix Imaging, Inc.

Electrophoretic media are often opaque (eg, in many electrophoretic media, because the particles substantially block the transmission of visible light through the display), but many electrophoretic displays, one display The state can be made to operate in a so-called "shutter mode" which is substantially opaque and one which is light-transparent. For example, see US Pat. Nos. 6,130,774 and 6,172,798, and US Pat. No. 5,872,552; 6,144,361; 6,271,823; 6,225,971; And 6,184,856. Dielectrophoretic displays that are similar to electrophoretic displays but rely on variations in field strength can operate in a similar mode; See U.S. Patent 4,418,346.

Encapsulated or microcell electrophoretic displays typically do not suffer from the clustering and settling fault modes of conventional electrophoretic devices and provide additional advantages such as display printing or coating capability for a variety of flexible and rigid substrates. (The use of the word "printing" is not intended to be limiting, but is intended to include all forms of printing and coating, including: pre-measure coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating Roll coatings 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; silk screen printing process; electrostatic printing process; thermal printing Process; ink jet printing process; and other similar techniques.) Thus, the resulting display can be flexible. In addition, since the printing medium can be printed (using various methods), the display itself can be made at no cost.

However, the service life of encapsulated electrophoretic displays of both single and dual particle types is much lower than very desirable. The electrophoretic particles stick to the capsule wall, and the particles tend to aggregate and cluster (although the present invention is by no means limited to such a problem), which is necessary for the display to switch between its optical states. This age seems to be limited by factors such as disturbing particle movement. In this regard, counter-charge double particle electrophoretic displays have a particularly difficult problem since particles that are in close proximity to one another inherently oppositely charged will be electrostatically attracted to one another and will have a strong tendency to form stable aggregates. Experimentally, when attempting to produce this type of black / white encapsulated display using untreated commercial titania and carbon black pigments, we find that the display does not switch at all or its age is too short to be undesirable for commercial purposes. It became.

The properties and surface characteristics of electrophoretic particles have long been known to be able to modify them by adsorbing various materials on the surface of the particles or by chemically bonding the various materials to these surfaces, and the For a detailed review of the record, reference is made to the international application WO 02/093246, specifically its verses 6, 3 to 7, 26.

The aforementioned WO 02/093246 describes the advantages of the use of pigment particles in an electrophoretic medium in which the polymer is chemically bonded thereto or crosslinked around it. This application includes controlling the amount of polymer deposited on the particles, the structure of the polymer, the technique of forming a polymeric coating on the electrophoretic particles, and the technique of pretreating the particles prior to forming the polymer coating on the electrophoretic particles. Thus, various improvements in such polymer-coated particles are also described. The application also describes a process for preparing 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 group or polymerized group Reacting the pigmented particles with the timing reagent, reacting the functional group with the particle surface and binding 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 initiator on the particle and the one or more monomers or oligomers, thereby binding to the pigment particles. Forming the polymer.

The aforementioned 2002/0180687 includes a plurality of particles suspended in a hydrocarbon suspending fluid, which particles can move between fluids when an electric field is applied to the medium, the fluid having a viscosity average molecular weight of about 400,000 to 1,200,000 g / An electrophoretic medium is described in which a molar polyisobutylene is dissolved or dispersed therein, the polyisobutylene being comprised between about 0.25 and about 2.5 weight percent of the suspension fluid. The same application also includes a plurality of particles suspended in a suspension fluid, which particles can move between fluids when an electric field is applied to the medium, the fluid having an intrinsic viscosity η in the suspension fluid and ions in the suspension fluid. A polymer substantially free of groups or ionizable groups is dissolved or dispersed therein, the polymer being described in an electrophoretic medium in which the concentration is present in a suspension fluid at a concentration of about 0.5 [η] ⁻¹ to about 2.0 [η] ^{− 1} have. The presence of polyisobutylene (PIB) or other polymers in the suspension fluid significantly increases the bistable stability of the display.

As already indicated, electrophoretic displays only require 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 infinite image stability. Brown diffusion and gravitational sedimentation of the pigment particles, together with the movements driven by small residual voltages and other factors induced by the applied switching pulses, can all degrade the quality of the optical state resulting from the switching of the display. If there is no mechanism to prevent this kind of optical state decline, the optical state should be refreshed periodically. Display playback consumes power, impairing the utility of the display. Also, in certain applications (especially active matrix drive displays), a single pixel can be reproduced without blanking pulses (ie pulses that drive the pixel into one of the extreme optical states before finally driving to the desired optical state). This is difficult or impossible (see 2003/0137521 mentioned above). For this reason, there is still a great need for improvement of image stability of electrophoretic media.

As already discussed, the aforementioned 2002/0180687 incorporates high molecular weight polymers, such as PIB, which have good solubility in suspension media (typically aliphatic hydrocarbons such as Isopar G) but are not adsorbed onto electrophoretic particles. An electrophoretic medium is described that achieves good image stability. It is believed that the presence of such polymers in solution (although the invention is in no way limited by the following) leads to a weak aggregation of the pigment by a mechanism known in the art of colloidal science as "depletion flocculation". Polymers other than PIB can be used for the same purpose. An example of a second polymer that is shown to be useful for this purpose is Kraton G, which is a block copolymer comprising polystyrene blocks and hydrogenated polyisobutylene blocks, which form aggregated structures in a suspension medium. In this case, the aggregate is a species that causes depletion rather than the monomeric block copolymer itself.

Whatever polymer is used to cause depletion, the incorporation of soluble high molecular weight materials into the suspension medium will increase the viscosity of the medium. Since the response time of the display (the time required to change the display, or any given pixel thereof, between its two extreme optical states at a given operating voltage) is proportional to the viscosity of the medium, this approach to image stability Will reduce the switching speed of the display. In addition, since the depletion flocculation mechanism is only effective at polymer concentrations above the superimposition concentration (which can be defined as the concentration of the polymer that doubles the viscosity of the medium in practice), all polymers driven by this mechanism are required to achieve a switching speed. It can be expected to result in a similar reduction of. In practice, the switching speed is reduced by approximately two to three times when using enough polymer to achieve adequate image stability. Other means of achieving image stability without compromising response time are desirable.

As discussed above, PIB and other polymers improve image stability by adjusting the colloidal stability of pigment particles. Preferred polymer coated particles described in WO 02/093246 mentioned above are colloidally stable in a suspension medium, which is a polymer of (typically) poly (lauryl methacrylate) that occurs on the surface of the particles during their preparation. Because of the shell By appropriately adjusting the composition of the polymer shell, particles having the same degree of colloidal stability as provided by PIB, Kraton, and other polymers dispersed in the suspension medium in principle without additions such as PIB in the suspension medium (and thus the same Display with image stability). Such a display will be considerably faster than a display containing PIB, or likewise operate at the same speed at a lower applied voltage.

In one aspect, the present invention seeks to provide a method for providing electrophoretic particles with modified polymer shells that allow for the manufacture of fast, image-stable displays.

In another aspect, the present invention seeks to provide an improved form of a two-step process for producing the polymer-coated electrophoretic particles described in paragraph 21 above. In a preferred form of this method, titania (or similar metal oxide pigment) is first coated with silica and the silica-coated titania is treated with a silane comprising ethylene groups. 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 carbon black with a reaction product of an ethylene group including a ethylene group, for example 4-vinylaniline and nitrous acid, binds the ethylene group to the carbon black surface and then variously in substantially the same manner as described for titania. It may be reacted with an unsaturated monomer.

In the specific process shown in the examples of WO 02/093246 mentioned above, the final polymerization stage (so-called "graft polymerization stage") is carried out in toluene, among other things that it has good properties for use in such free radical polymerization. This is because it is a known solvent. However, it is quite inconvenient to use as a solvent in the process for producing polymer-coated electrophoretic pigment particles. For the various reasons discussed in detail in the aforementioned E Ink and MIT patents and applications, the suspension fluids actually used in electrophoretic displays are aliphatic hydrocarbons (alone or in combination with halocarbons). Thus, the polymer-coated pigment particles will eventually disperse in aliphatic hydrocarbons and it is necessary to prevent the aliphatic hydrocarbons from contaminating with toluene (in some cases, the behavior of the electrophoretic medium is a small change in the composition of the suspension fluid. After the polymerization in toluene is finished and the polymer-coated pigment is separated from toluene, it is necessary to remove all the toluene residue before the polymer-coated pigment particles are suspended in the final suspension fluid. Indeed, the toluene-containing pigment particles from the graft polymerization stage are washed one or more times with tetrahydrofuran (THF), centrifuged after washing to separate the pigment from THF and finally dry the pigment in an oven to the final THF balance. Need to be removed. All two processes must be performed separately for the two pigments used in the dual particle electrophoretic medium.

These washing, centrifugation and drying steps are labor intensive and costly. The additional cost arises from the need to redisperse the dry pigment in the final suspension fluid. In addition, due to the presence of toluene and THF, washing, centrifugation and drying steps tend to be dangerous, and commercial scale production of polymer-coated pigments has led to explosion-proof ovens, mixers and centrifuges, and explosion-proof electrical control. The use of panels is required, which significantly increases the cost of production equipment. In addition, despite the use of protective devices or methods of exposure prevention, device operators may be exposed to steam significantly during the process. Finally, the drying step can be detrimental to the performance of the pigment in the final electrophoretic medium. It is therefore desirable to find alternative solvents which can carry out the polymerization reaction and possibly eliminate the need for drying and redispersion of the dry pigment.

Finally, the present invention seeks to provide an electro-optical display that can be driven in a simple manner. Whether the display is reflective or transmissive, and whether the electro-optical medium used is bistable or not, to obtain a high-resolution display, individual pixels of the display must be addressable without being disturbed from adjacent pixels. One way to achieve this goal is to provide an array of non-linear elements, such as transistors or diodes, where one or more non-linear elements are associated with each pixel to produce an "active matrix" display. The addressing or pixel electrode addressing one pixel is connected to a suitable voltage source via 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, which is essentially random and the pixel electrode may be connected to the source of the transistor, but the following description will illustrate this arrangement. Will assume. Typically, in a high resolution array, the pixels are arranged in a two dimensional array of rows and columns, such that any particular pixel is uniquely defined by the intersection of one particular row and one particular row. While the sources of all transistors in each column are connected to a single column electrode, the gates of all transistors in each row are connected to a single row electrode; The designation of the source for the horizontal line and the gate for the vertical line is also fixed, but is essentially random and may be contradictory if desired. The row electrodes are connected to one row driver, which essentially applies a voltage that allows only one row to be selected at any given moment, i.e., the selected row electrodes to conduct all transistors in the selected row. On the other hand, all of the transistors in these rows, which are not selected for all remaining rows, will apply a voltage that remains non-conductive. The column electrodes are connected to a column driver which provides the selected voltages to the various column electrodes to drive the pixels in the selected row to their desired optical state. (The aforementioned voltage is related to a common front electrode that is typically provided opposite the non-linear array of electro-optical media and extends across 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 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 only be addressed for a time of 1 / N.

Methods of manufacturing active matrix displays have 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 for electrical conduction between the source and drain electrodes. Typically, the gate electrode, source electrode, and drain electrode are patterned. In general, the semiconductor layer is also patterned to minimize floating conduction (ie 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 TFTs typically have a bottom-gate arrangement. Within one pixel, thin film capacitors typically retain charges moved by the switching TFTs. Electrophoretic displays can use similar TFTs with accumulators, but the functions of the accumulators are somewhat different than in liquid crystal displays; See 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 electrode is charged through the TFT during the line address time. During the line address time, the applied gate voltage is changed to switch the TFT to the conducting state. For example, in an n-type TFT, the gate voltage is switched to the "high" state in order to switch the TFT to the conductive state.

Undesirably, the pixel electrode typically exhibits a voltage shift when changing the selected line voltage to deplete the TFT channel. The pixel electrode voltage shift occurs due to the capacitance between the pixel electrode and the TFT gate electrode. The voltage shift can be designed as follows:

ΔV _p = G _gp Δ / (C _gp + C _p + C _s )

[ _Wherein C _gp is the gate-pixel capacitance, C _p is the pixel capacitance, C _s is the storage capacitance and Δ is the gate voltage shift when the TFT is effectively depleted]. This voltage shift is often referred to as "gate feedthrough."

The gate feedthrough can be compensated by shifting the upper surface voltage (voltage applied to the common front electrode) by the amount ΔV _p . However, it is complicated because ΔV _p changes from pixel to pixel due to the amount of change in C _{gp for} each pixel. Therefore, the voltage bias can be maintained even when the top surface is moved to compensate for the average pixel voltage shift. Voltage bias not only introduces errors in the optical state of the pixel but can also degrade the quality of the electro-optic 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, ie change in line width error.

By using a gate electrode that 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. Large gate-pixel capacitance is undesirable because it can cause the need for large compensation at one of the selected line voltage levels. In addition, existing addressing structures can produce unintentional bias voltages, for example, due to pixel-to-pixel variation in gate-pixel capacitance. Such voltages can have a detrimental effect on certain electro-optical media, especially if they are present for a long time.

These problems make it difficult to design bistable electro-optic displays using electro-optic media without threshold voltages. Since some pixels on the display may be updated inadvertently, if possible, the optical state of the pixels should be as disturbed as possible. In practice, this means minimizing the amount and amplitude of eddy current peaks 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 encapsulated electrophoretic displays, these lines frequently switch between + 15V and -15V for each common line address of the display (time to select a predetermined row of active matrix displays) for the common electrode. These voltages are capacitively coupled directly to the pixel electrodes of the display, which can be quite strong in field-shielded pixel designs. Even if these coupling voltage peaks are inevitably made DC balanced for a long time, if such voltage peaks are continuously applied, the optical state of the pixel can be changed.

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

In an active matrix electro-optical display, it is now known that the problems discussed above can be reduced or eliminated using an electro-optical medium representing the threshold voltage, i.e., a medium that is essentially free of switching at low but nonzero voltages. It became.

In one aspect, the present invention includes a polymeric shell comprising electrically charged particles suspended in a suspension fluid, the particles having repeating units derived from at least one monomer whose homopolymer is incompatible with the suspension fluid. It provides an electrophoretic medium having a.

This aspect of the invention may hereinafter be referred to as "incompatible monomer medium" for convenience. In such media, the polymeric shell preferably further contains repeating units derived from one or more monomers whose homopolymers are compatible with the suspension fluid. The monomer or monomers that form the compatible homopolymer (these monomers may hereinafter be referred to as "compatibility monomers" for convenience) comprise about 15 to about 99 weight percent, preferably about 50 to about 99 weight percent of the polymer shell. It may contain. Suspension fluids are typically hydrocarbons, but a mixture of hydrocarbons and another compatible solvent such as halocarbons can be used. Alternatively, silicone fluid or fluorocarbons can be used as the suspension fluid.

Monomers that form incompatible homopolymers (such monomers may hereinafter be referred to as "incompatible monomers" for convenience). Whether a particular monomer can be considered as an incompatible monomer depends on the particular suspension fluid used, and it will be appreciated that one particular monomer may 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 alone or mainly comprising hydrocarbons, the incompatible monomer comprises up to about 8 carbon atoms and optionally comprises hydroxyl or halogen or other polar substituents such as carboxyl, cyano, ketone or aldehydes. Acrylates and methacrylates formed from alcohols, including; Acrylamide and methacrylamide; N, N-dialkylacrylamide; N-vinylpyrrolidone; Styrene and its derivatives; Vinyl esters; May be any one or more of vinyl halides. Specific examples of incompatible monomers include methyl methacrylate, ethyl methacrylate, butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, 2- Hydroxyethyl methacrylate, acrylamide, acrylic acid, acrylonitrile, methyl vinyl ketone, methacrylamide, N-vinylpyrrolidone, 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 types of fluorinated monomers such as pentafluorostyrene or polymerization. Other polyfluoroaromatic molecules containing possible functional groups are included. Another class of incompatible monomers in hydrocarbon media include silicon-containing molecules that include polymerizable vinyl groups in their structure. In certain hydrocarbon media of the invention described in the examples below, the compatible monomers comprise lauryl methacrylate and the incompatible monomers are trifluoroethyl methacrylate, hexafluorobutyl acrylate, styrene, t-butyl Any one or more of methacrylate and N-vinylpyrrolidone.

Many other types of incompatible monomers can be used, in combination with other types of suspension fluids, such as fluorocarbon media, where the proportion of fluorocarbon monomers is largely the compatible monomer. Similarly, in silicone suspension fluids, compatible monomers may comprise a majority of silicone groups, and incompatible monomers may include any of the monomers listed above except those comprising a majority of silicone functional groups.

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 second type of electrically charged particles comprise carbon black or copper chromite.

The present invention also provides electrophoretic particles for use in the incompatible monomer medium of the present invention. In electrophoretic media using hydrocarbons or halocarbons as suspension fluids, these electrophoretic particles (hereinafter referred to as " incompatible monomer particles " for convenience) are derived from one or more monomers whose homopolymers are incompatible with n-hexane. Pigment particles having a polymeric shell having repeating units. (As obvious to those skilled in the art of electrophoretic media, hydrocarbon suspension fluids typically used in such media include a variety of mixtures of low molecular weight aliphatic hydrocarbons, to be clear, a single test compound for testing 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 whose homopolymers are compatible with n-hexane. The compatible monomer may comprise about 15 to about 99 weight percent of the polymer shell, preferably about 50 to about 99 weight percent. Incompatible monomers may include any one or more of those listed in the 56th paragraph above. In certain particles of the invention described in the Examples below, the compatible monomers comprise lauryl methacrylate and the incompatible monomers comprise any one or more of styrene, t-butyl methacrylate and N-vinylpyrrolidone. Include. The pigment used to form the particles of the present invention can be, for example, any one or more of titania, carbon black and copper chromite.

In another aspect, the present invention provides analogous electrophoretic particles for use in fluorinated and silicon-based suspension fluids. Accordingly, the present invention provides electrophoretic particles comprising pigment particles having a polymeric shell whose repeating polymer is derived from at least one monomer incompatible with perfluorodecalin. The present invention also provides electrophoretic particles comprising pigment particles having a polymeric shell whose repeating polymer is derived from at least one monomer incompatible with polydimethylsiloxane 200 (viscosity 0.65 centistoke).

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

Suspension fluids;

Electrically charged first type of particles suspended in suspension fluid, the first type of particles having a first optical feature and a polymeric shell; And

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

Wherein the polymeric shell is arranged such that the homogeneous aggregation of the first and second types of particles is thermodynamically advantageous over heteroaggregation.

Such a medium may hereinafter be referred to as the "homogeneous medium" of the present invention for convenience. It will be appreciated that there is a significant overlap between the coagulation medium and the dual particle incompatible monomer medium of the present invention, in that many media can satisfy both definitions at the same time.

In the coagulation media of the invention, the polymeric shells of the particles of the first and second types may each comprise repeating units derived from one or more monomers whose homopolymers are incompatible with the suspension fluid. Each polymeric shell may further comprise repeating units derived from one or more monomers whose homopolymers are compatible with the suspension fluid. The compatible monomer may comprise about 15 to about 99 weight percent of the polymer shell, preferably about 50 to about 99 weight percent. Suspension fluids may have a dielectric constant of less than about 5 and may include hydrocarbons, preferably aliphatic hydrocarbons. Alternatively, the suspension fluid may comprise aryl-alkanes or dodecylbenzene.

Incompatible monomers may also include any one or more of those listed in the 46th paragraph above. In a preferred form of homocoagulation medium, the compatible monomer comprises lauryl methacrylate and the incompatible monomer comprises any one or more of styrene, t-butyl methacrylate and N-vinylpyrrolidone.

For reasons detailed in the following, the coagulating medium of the present invention may have an operating threshold voltage. The medium can be encapsulated, that is, the suspension fluid and particles can be contained in a plurality of capsules or cells.

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

The invention also provides 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 providing an active matrix electro-optic display indicative of a threshold voltage.

Such an electro-optic display may hereinafter be referred to as the "threshold voltage display" In such a display, a storage battery can be connected with each pixel electrode. The electro-optic medium may comprise a plurality of charged particles suspended in a suspension fluid and capable of moving in between when applying an electric field to the electro-optic medium. These charged particles may have a polymeric shell having repeating units derived from one or more monomers whose homopolymers are incompatible with the suspension fluid. The electro-optic medium may also be homogenous in this subject.

Finally, the present invention provides a process for preparing polymer-coated pigment particles, the process comprising: step (b) is carried out in aliphatic hydrocarbons:

(a) a pigment particle is reacted with a reagent having a functional group capable of reacting with and binding to the particle and having a polymerizable group or a polymerization initiator to react the functional group with the particle surface and polymerize there Attaching the group; And

(b) reacting the product of step (a) under conditions effective to react the at least one monomer or oligomer with the polymerizable group or initiator on the particle and the at least one monomer or oligomer to react the polymer bound to the pigment particles. Forming step.

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

As already indicated, the present invention relates to electrophoretic media using particles having a polymeric shell, electrophoretic particles for use in such media, electrophoretic displays comprising such media or similar electro-optic media, and the aforementioned It relates to a method for producing a polymeric shell. For background information on electrophoretic particles, a method of forming a polymer shell thereon, an electrophoretic medium and a display incorporating such particles, reference is made to the aforementioned WO 02/093246, in particular pages 12 to 41 thereof. See also the aforementioned 2002/0185378, paragraphs [0124] to [0165]. Since such information is readily available in this published application, it is not repeated herein except to the extent necessary to describe how the electrophoretic particles, media, displays and methods of the invention differ from the analogs described in these international applications. Will not.

The polymer shell present on the electrophoretic particles of the present invention may not completely cover the particle surface such that no additional polymer can be formed on the polymer surface; Indeed, as 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 mean that the polymer coating is excluded from the possibility of forming additional polymer on the particles by an additional polymerization step.

Preferred classes of functional groups which bind to many ceramic oxide pigments, but in particular silica- and / or alumina-coated titania and similar silica-coated pigments, are silane coupling groups, in particular trialkoxy silane coupling groups. One preferred reagent for attaching polymerizable groups to titania and similar pigments is the aforementioned 3- (trimethoxysilyl) propyl methacrylate:

H ₂ C═C (CH ₃ ) CO ₂ (CH ₂ ) ₃ Si (OCH ₃ ) ₃ . (I)

This material is sold under the trade name Z6030 from Dow Chemical Company, Wilmington, Delaware. The acrylate can also be used. Another useful reagent is an aminosilyl derivative of the formula:

H ₂ C═CHC ₆ H ₄ CH ₂ NHCH ₂ CH ₂ NH (CH ₂ ) ₃ Si (OCH ₃ ) ₃ .HCl. (II)

In order to provide an "anchor" to the polymer shell which will later be formed around the electrophoretic particles, these silyl meths (acrylates) have the ability to obtain the desired charge in the presence of a suitable charge or charge agent to the final electrophoretic particles. Imparting charge-regulators. Where positively charged particles are desired, the reagents of formula (II) are used, but the reagents of formula (I) provide negatively charged particles. Both reagents, of course, include polymerizable vinyls that allow for the grafting of polymeric radicals produced in solution.

In practice, suspension fluids in electrophoretic media are typically low dielectric constant liquids such as hydrocarbons, halocarbons (particularly fluorocarbons), or silicon. The incompatible monomer medium of the present invention may use any of these types of suspension fluids. Although the present invention will be described below in relation to a medium comprising a hydrocarbon suspension fluid above all, it will be readily apparent to those skilled in the art in colloidal chemistry in such suspension fluids to modify the medium to use other types of suspension fluids. The polymer shell itself typically contains at most proportions of hydrocarbon chains (ie, chains forming homopolymers that are highly compatible with the suspending medium); As discussed below, most of the strong polar or ionic groups are undesirable except for the groups provided to control compatibility with the suspension fluid and to charge. Further, if at least the medium in which the particles are to be used contains an aliphatic hydrocarbon suspension fluid, which is usually the case, the polymer is branched or "combed", having a main chain and a plurality of side chains extending from the main chain. It is advantageous to have a structure. Each of these side chains should have at least about 4, preferably at least about 6 carbon atoms. In fact the side chains can be advantageous longer; For example, lauryl (C ₁₂ ) side chain. The side chains can themselves be branched; For example, each side chain can 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 present invention is in no way limited to the following, branches of the polymer are separated from one another in a large volume of liquid in a brush or wood-like structure, thus causing other similar particles. It is believed that the particles become colloidally stable in the suspension fluid, because they do not connect closely with each other.

As described above in WO 02/093246, there is an optimal range of polymer amounts to be formed on electrophoretic particles, and the formation of overflowing or insufficient amounts of polymer on the particles will degrade their electrophoretic characteristics. It was found that it can. The optimal range will vary depending on a number of factors including the density and size of the particles to be coated, the nature of the suspension medium in which the particles will be used, and the nature of the polymers formed on the particles, and any particular particle, polymer and suspension In the medium, the optimum range is determined empirically best. However, as a general indication, it should be noted that the thicker the particles, the lower the optimal ratio of the polymer to the weight of the particles, and the finer the particles are divided (the smaller the particle size), the higher the optimal ratio of the polymer. do. In the aforementioned WO 02/093246, the particles must be coated with at least 2, preferably at least 4% by weight of polymer, and in most cases the optimal proportion of the polymer is from 4 to 15% by weight of the particles, typically 6 It is indicated to be in the range from 15% by weight to 15% by weight, most preferably from 8 to 12% by weight. More specifically, for titania particles, WO 02/093246 mentioned above indicates that the preferred range of polymer is 8 to 12% by weight of titania.

However, in order to facilitate the application of the present invention to particles having a wide range of particle sizes and densities, the amount of polymer is determined by the surface density of the polymer (ie, the weight of the polymer per unit area of the particle surface, e. Milligrams of sugar polymer). The surface density of the polymer can be calculated from the following formula:

Γ = WρD / 6

[Wherein Γ is the surface density of the polymer, W is the weight of the polymer per gram sample (obtained from thermogravimetric analysis), ρ is the density of the base particles, and D is the diameter of the base particles. For copper chromates, it has been found that a useful range of surface densities is 2 to 40 mg / g, preferred ranges are 14 to 24 mg / g, particularly preferred ranges are 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 hydrocarbon materials, provided by polymer coatings on current pigments, allows the pigments to disperse more readily in polymeric and rubber matrices than similar but uncoated pigments, thereby making the polymeric and rubber matrices Makes it easier to use. Flexibility in the chemical nature of the polymer coating allows the coating to be "tuned" for controlled dispersibility in any particular matrix. Thus, the present pigments can be used as dispersible pigments or reactive extrusion compounds. In addition, the polymer coating on the particles may be useful for improving the properties of such formulations by reducing the tendency of such pigment / polymer or rubber formulations to shear or crack at the interface between the particles and the matrix material. When polymer-coated particles are prepared (as discussed above) by the process of preparing the polymer-coated particles as an admixture with a "glass" polymer that is not attached to the particles (as discussed above), in many cases the glass polymer is detrimental to the matrix. Since the particles will be dispersed without squeezing, there will be no need to separate the coated particles from the glass polymer before the particles are dispersed in the polymeric or rubber matrix.

In the media of the present invention, suspension fluids may be prepared on the basis of chemical inertness, density matching to electrophoretic particles, or chemical compatibility with both electrophoretic particles and capsule or microcell walls (for capsule type electrophoretic displays). You can choose. If particle movement is desired, the viscosity of the fluid should be low. The refractive index of the suspension fluid can also be substantially matched to the refractive index of the particles. As used herein, when their individual refractive index differences are about 0 to about 0.3, preferably about 0.05 to about 0.2, the refractive index of the suspension fluid is "virtually matched" to the refractive index of the particles. However, in electrophoretic displays in which the optical state is determined in part by scattering efficiency, it is desirable for the difference in refractive index between the scatterer and the medium to be large. Titania particles are typically used as scattering particles to create a white state in dual particle electrophoretic displays, and the refractive index of such materials is high (about 2.7), so that the refractive index of the suspension medium is 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 alkyl benzene derivatives such as dodecylbenzene and naphthalene and alkyl naphthalene derivatives. Useful halogenated organic solvents include, but are not limited to, tetrafluorodibromoethylene, tetrachloroethylene, trifluorochloroethylene, 1,2,4-trichlorobenzene and carbon tetrachloride. These materials are dense. Useful hydrocarbons include dodecane, tetradecane, Isopar (registered trademark) series (Exxon, Houston, TX), Norpar (registered trademark) (series of normal paraffinic liquids), Shell-Sol (registered trademark) (Shell, Houston, TX ), And Sol-Trol (registered trademark) (Shell) aliphatic hydrocarbons, naphtha, and other petroleum solvents. These materials are usually of low density. Useful examples of silicone oils include, but are not limited to, octamethyl cyclosiloxanes and high molecular weight cyclic siloxanes, poly (methyl phenyl siloxanes), hexamethyldisiloxanes, and polydimethylsiloxanes. 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. Many of these materials are available in a range of viscosities, densities, and melting points.

Electrophoretic particles, media and particles / suspension  Display with fluid compatibility

To date, it has been evident that it is desirable to form polymer coatings or shells around electrophoretic particles from polymers that are highly compatible with the suspension fluid surrounding the particles in an electrophoretic medium (typically aliphatic hydrocarbons such as Isopar G). It was considered desirable to use such highly compatible polymer shells to provide good steric stability. Thus, in most of the examples of WO 02/093426 mentioned above, only a single monomer and a single polymerization step were required to provide a steric stabilized polymer shell on a colloidally stable functional pigment. The monomers used to form such polymeric shells are typically lauryl methacrylate, but other monomers are also used in the aforementioned WO 02/093426.

It is now known that the polymer shell can be modified such that the polymer shell is somewhat less compatible with the suspension fluid, i.e., some parts of the polymer shell are incompatible with the suspension fluid, thereby obtaining certain important advantages, in particular improved image stability. It became.

The terms "compatibility" and "incompatibility" are used herein in their meaning in the polymer field. To the simplest extent, the polymer compatibility with the suspension fluid means that the polymer (if 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; Solutions are generally optically clear, while non-solutions (mixtures or dispersions) are opaque or have two prominent phases.

However, compatibility is not a binary phenomenon, that is, the polymer is not necessarily completely compatible or completely incompatible with a given suspension fluid. Instead, there is a degree of compatibility, depending on the relative strength of the interaction between the fluid and the polymer moiety (approximately, the monomers constituting the polymer) and the interaction between the polymer moieties. If the polymer is highly soluble in the fluid, the polymer-fluid interaction is more advantageous in energy than the polymer-polymer interaction. In such cases, with polymers that tend to form coils, the polymer coils in solution will stretch relative to the polymer coils in the polymer melt. This elongation can be measured using capillary viscometer or light scattering techniques. For example, when measuring [η], the intrinsic viscosity of a series of polymer samples with different molecular weights (M), it is usually found that there is a power law relationship between the two quantities:

The index α ranges from 0.5 to about 0.8 (for flexible polymers) depending on the quality of the solvent, with larger values indicating better solvent. When α = 0.5, the fluid is referred to as a “theta solvent” and the shape of the polymer is similar to the polymer melt (ie surrounded only by other polymer moieties), so that the polymer is in fact neutrally compatible with the fluid. Under these conditions, the shape of the polymer chain is a random walk.

For both homopolymer and copolymer systems, the Huggins coefficient can be used as an indicator of the solvent quality (ie, polymer compatibility with the solvent). The Huggins constant is the [η] ² k 'term in the formula for the relative viscosity of the dilute solution of the polymer:

Where η _S is the viscosity of the solvent, [η] 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). (CW Macosko, Rheology Principles , Methods , and Applications , VCH Publishers, 1994, p. 481).

Similar information can be gathered from the values of the secondary virial coefficients in a given fluid-polymer solution, measured by static light scattering or osmotic pressure measurements. In general, large secondary virial coefficients mean more compatible fluids. In the relationship of dissolved polymer steels, the secondary virial coefficient, corresponding to a substantially incompatible polymer-solvent interaction, may be-(C. Tanford, Physical Chemistry of Macromolecules , John Wiley and Sons, New York, 1961, pp. 293-296).

If the polymer is insoluble in the fluid, ie, incompatible, the polymer may sometimes be expanded by the fluid. The degree of swelling (polymer sample to dry polymer volume ratio in the presence of fluid) can be used as a measure of the degree of compatibility of the fluid and the polymer in this situation. The greater the degree of expansion, the greater the compatibility.

In many ways, the polymer shell can be made less compatible with the suspension fluid. First, the polymer shell may have repeating units derived from one or more monomers whose homopolymer is incompatible with the suspension fluid, according to the incompatible monomer electrophoretic medium aspect of the present invention. Typically, such polymer shells will also include compatible monomers, ie monomers whose homopolymers are compatible with the suspension fluid, and the compatibility of the polymer shell with the suspension fluid can be controlled by modifying the ratio of the two monomers.

Polymer shells comprising both incompatible and compatible monomers (obviously, each type of monomer present may be in excess of one) may undergo a mixture of polymerizable monomers in the last polymerization stage (which may be a single polymerization stage). It may be a random copolymer shell formed using. If one of the monomers is compatible with the suspension fluid and not one, it may 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 degrees of incompatibility, the degree of incompatibility can be further modified by changing the incompatible monomer. For example, when used as the sole monomer in a polymer shell, acrylate esters (eg butyl methacrylate) with short side chains containing up to about 8 carbon atoms are colloidal in Isopar G suspension fluids. It was observed to yield particles that are not stable as a rule. On the other hand, lauryl methacrylate yields electrophoretic particles with good colloidal stability in Isopar G. Thus, copolymers of lauryl methacrylate and butyl methacrylate are colloidally marginally stable and polymer shells that are marginally compatible with Isopar G at any molar ratio of butyl methacrylate to lauryl methacrylate in the polymerization mixture. Will provide particles.

The second method of modifying the polymer shell around the electrophoretic particles is by second stage polymerization. Experiments have shown that the surface functionality provided by the reagents of Formulas (I) and (II) is not completely consumed during a single graft polymerization step. For example, when surface functionalized and polymer coated titania is simply resuspended in a polymerization medium (typically including toluene, lauryl methacrylate and azobisisobutyronitrile (AIBN) as polymerization initiator), Thermogravimetric analysis of the pigments isolated from the reaction mixture after heating for a period of time indicated an increase in the amount of polymer bound (as determined by thermogravimetric analysis (TGA): after first polymerization: 7.3%; after second polymerization: 8.9% and 9.9% (two different runs)). In such double graft polymerization methods, 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 constructed from different monomers in the final polymer shell. In addition, the conditions of the first polymerization stage can be modified to control not only the initial grafting density of the chain but also the likelihood (and accessibility) of the surface vinyl in the second stage polymerization. Thus, this dual polymerization method provides a second degree of flexibility in the construction of polymer stabilized electrophoretic particles. It may be advantageous to adjust the molecular weight of the polymer chain in the first or second polymerization stage by incorporating a chain transfer agent into the polymerization mixture or by adjusting the concentration of monomer or radical initiator in a manner well known in the polymer art.

Polymers prepared by either of the two methods described above for a very large range of monomers that are easy to use, because at present the suspension fluids that prefer aliphatic hydrocarbons are generally considered to be incompatible with many homopolymers. It can be used as an incompatible monomer in a shell. Such incompatible monomers have acrylate esters that vary in chain length and branching structure, but typically have alkyl groups having up to 8 carbon atoms (eg, methyl methacrylate, ethyl methacrylate, butyl methacrylate, iso Butyl methacrylate, t-butyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate), as well as the corresponding acrylic acid esters, fluorocarbon ester side chain acrylates, acrylics of functional alcohols Esters such as 2-hydroxyethyl methacrylate, acrylamides such as acrylamide, methacrylamide, N-monoalkyl acrylamide and methacrylamide, N, N-dialkylacrylamide, N- Vinylpyrrolidone, 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.

The invention is by no means limited in theory, but it is possible to logically explain the improved properties of the electrophoretic medium in which the polymer shell around the particles comprises incompatible monomers. Polymer shells formed entirely from compatible monomers provide stereostability to electrophoretic particles because they are thermodynamically advantageous in the polymer chain forming the shell surrounding the suspension fluid rather than by other polymer moieties. Thus, polymer shells formed entirely from compatible monomers, for example pure poly (lauryl methacrylate) polymer shells dispersed in aliphatic hydrocarbons, are highly expanded. 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 fully incompatible monomers are thermodynamically advantageous in that the polymer chains form shells that are surrounded by other polymer portions rather than by the suspension fluid such that the polymer shell collapses, rejects the suspension fluid and the electrophoretic particles It does not provide such steric stability because it tends to be attracted to each other and hence tends to aggregate. That is, highly compatible polymer shells attempt to disperse the electrophoretic particles throughout the suspension fluid, while incompatible polymer shells attempt to agglomerate the electrophoretic particles. The image stability of an electrophoretic medium is such that the electrophoretic particles are dependent on their ability to remain coagulated with similar electrical particles in the homogenous aggregates (layers) of the particles causing the image. Promoted by a tendency to agglomerate. Incorporation of both compatible and incompatible monomers into the polymer shell to control the compatibility of the polymer shell and the suspension fluid, such that the overall compatibility of the polymer shell with the suspension fluid, and the stability of such particle aggregates, and thus images of the resulting display It is possible to adjust the stability.

A further advantage of the preferred embodiment of the invention is the introduction of a threshold in the switching of the electrophoretic medium. If the attraction of the electrophoretic particles to each other, and thus their tendency to remain stable homogenous aggregates, a significant electric field will be required to attract the particles from the homoaggregates and allow them to move in the electric field. There are several advantages to having a significant threshold voltage in switching electrophoretic media. If the threshold is large enough, passive matrix addressing of high resolution displays can be used. Smaller thresholds may be useful for reducing the sensitivity of active matrix displays to eddy voltages and inter-pixel voltage leakage.

The present invention also provides another approach to improve image stability in dual particle electrophoretic media. In dual particle electrophoretic media, both types of electrophoretic particles need to contribute to overall image stability. If particles of the second type are susceptible to settling or spreading throughout the suspension fluid, it is not sufficient for one type of particles to be " image stable " If so, the optical state of one extreme of the medium may exhibit fairly good image stability, while the rest will exhibit poor stability. In addition, if the particles can diffuse, poor image stability of one particle is more likely to cause optical degeneration of the optical state of the second extreme. It is therefore desirable for both types of particles to have suitably adjusted steric stability to provide good image stability. If you intend to use the "depleted flocculation" method to incorporate polymers into the suspension fluid to achieve good image stability (see 2002/0180687, mentioned earlier), the degree of depletion mechanism applied to both types of particles depends on particle size and particle shape. It is different from, and cannot be controlled independently, 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 monomer electrophoretic medium of the present invention, the colloidal stability of each type of particle can be controlled independently. In addition, polymers can be added to the suspension fluid according to the depletion flocculation method to render the two distinct types of particles colloidally unstable, not only against each other, but also against each other, thereby promoting heterogeneous aggregation of the two types of particles. Can be. Heteroaggregation can be attributed to poor optical conditions (one type of particle in it), as well as the need for response time delays and higher applied voltages in dual particle electrophoretic displays (because higher field strengths will be required to separate heteroaggregates). In most cases, the charged heteroaggregates cannot be completely separated by the applied electric field), which can lead to a number of undesirable effects.

According to aspects of the coagulation media of the present invention, homogeneous aggregation of the two types of particles eliminates or at least reduces these problems by providing a dual particle electrophoretic medium that is thermodynamically advantageous over heteroaggregation. This can be achieved by providing two types of particles with polymer shells whose homogeneous coagulation is adapted to their compatibility with the suspension fluid, but also by controlling the compatibility of the two types of polymer shells against heteroaggregation. have. It is relatively easy to achieve this by using different incompatible monomers in the polymer shell of the two types of particles; Typically, the same compatible monomer can be used for the polymer shell on both types of particles. If the two incompatible monomers are not compatible with each other, then the tendency of the two types of particles to disaggregate will be minimized, and only homogeneous aggregation is promoted, resulting in faster response times and a more pure optical state of the electrophoretic medium. Become.

As already mentioned above, the present invention is not limited to using aliphatic hydrocarbons as suspension fluid. Fluoro- and halocarbon oils, silicone oils, aralkyl solvents (e.g., toluene, dodecylbenzene, etc.) or mixtures thereof may be useful in the present invention, as they described above, This is because the properties of the suspension fluid are further modified with regard to solvation.

The present invention is applicable to all types of electrophoretic media, but may be particularly useful for encapsulated electrophoretic media, whether using prefabricated capsules or microcells. The invention is also applicable to displays of all types of switching forms, including both transmission and transport, as well as side-to-side and shutter mode switching.

1 of the accompanying drawings schematically shows a preferred method for producing electrophoretic particles according to the invention, as used in the following examples, which essentially consists of the aforementioned WO 02/093246 similar. In a first step method, silicate pigment 702 is prepared by coating the base pigment particle 700 with silica; This step is described in full in the aforementioned WO 02/093246. Next, the silicated pigment 702 is treated with a bifunctional reagent having one functional group and a second charge regulator reacting with the silica surface to produce a surface functionalized pigment 704 having a charge regulator on its surface. The bifunctional reagent also provides a polymer formation site on the pigment particles. Finally, as shown in FIG. 1, the surface functionalized pigment 704 is contacted with one or more monomers or oligomers under conditions effective to form a polymer attached to the charge controller to produce a polymer-coated functional pigment 706, which It is used for the electrophoretic medium.

1 is a schematic diagram illustrating a method used to prepare incompatible monomeric electrophoretic particles of the present invention.
FIG. 2 is a graph showing the change in dynamic range of the homogeneous electrophoretic medium of the present invention as a function of applied voltage, for various pulse lengths, as described in the Examples below.

Example

The following examples are provided by way of illustration only to illustrate the synthesis of several different electrophoretic particles of the present invention having polymer shells with controlled compatibility with suspension fluids, and illustrate the advantages obtained 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 chromite pigments based on Shepherd Black 1G. After surface functionalization of these particles using Z6030 on a white pigment and reagents of formula (I) above on black, a polymer shell was formed thereon in the manner described below. The white particles thus prepared were negatively charged when incorporated into an Isopar G suspension fluid containing Solsperse 17K and Span as charging agents, and the black particles were positively charged. Particles with lauryl methacrylate homopolymer shells were prepared as a control experiment, and the white and black particles were named J and D, respectively. The modification of the polymer shell is described as follows: For particles prepared using random copolymers, one-step polymerization, the comonomer is indicated in parentheses with a number indicating the mole fraction of comonomer used in the polymerization reaction. It is distinguished by the letter code shown in following Table I after a pigment indicator. What remained in each case was lauryl methacrylate. Thus, the symbol J (BMA15) denotes a white pigment prepared from a polymerization mixture comprising 15 mol% t-butyl methacrylate and 85 mol% lauryl methacrylate. Two-step polymerization is indicated by the + sign; Generally, the polymer molar ratio in these particles is not very well known, and the composition is not shown. J (+ St) thus represents a white pigment prepared by two-step polymerization. The starting material was in this case a 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 polymerizable monomer.

Monomer A simplified symbol Styrene St t-butyl methacrylate TBMA Vinyl pyrrolidone VP

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

1. Standard Lauryl Methacrylate  polymerization

The single-neck, 250 ml round bottom flask was equipped with a magnetic stir bar, reflux condenser and argon / or nitrogen inlet and placed in a silicone oil bath. Following 60 g of silane-coated pigment ground to a fine powder using a bowl and pestle, 60 ml of lauryl methacrylate (LMA, Aldrich) and 60 ml of toluene were added to the flask. The reaction mixture was then stirred rapidly 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 quickly added with a glass pipette. The reaction vessel was sealed, heated to 65 ° 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 decanted. Fresh ethyl acetate was added to the centrifuged pigment, which was stirred with a stainless steel spatula and sonicated for 10 minutes. The pigment was washed twice more 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 the solution was measured by gas phase chromatography (GPC). The polymerization bound to the pigment was measured by TGA.

2. In titania  Copolymerization

The single-neck, 250 ml round bottom flask was equipped with a magnetic stir bar, reflux condenser and argon / or nitrogen inlet and placed in a silicone oil bath. In a flask 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-2-pyrroli Dinon, hexafluorobutyl acrylate and methacrylate, N-isopropylacrylamide or acrylonitrile (Aldrich) were added and the amount of LMA and the second monomer depend on the desired monomer ratio. The ratio of LMA / second monomer was usually 95/5, 85/15 and 75/25. The reaction mixture was then stirred rapidly 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 quickly added with a glass pipette. The reaction vessel was sealed, heated to 65 ° C. and stirred overnight. 100 mL of ethyl acetate was added to the viscous reaction mixture and the resulting mixture was stirred for a further 10 minutes. The mixture was poured into a plastic bottle and centrifuged at 3600 rpm for 15-20 minutes and the supernatant drained. Fresh 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 twice more with ethyl acetate, centrifuged and decanted. 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 the solution was measured by GPC. The polymer bound to the pigment was measured by TGA.

2.1 In titania LMA  And 1-vinyl-2- Pyrrolidinone  Copolymerization

The single-neck, 250 ml round bottom flask was equipped with a 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-pyrrolidinone is usually 85/15. The reaction mixture was then stirred rapidly 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 quickly added with a glass pipette. The reaction vessel was sealed, heated to 65 ° C. and stirred overnight. 100 mL of ethyl acetate was added to the viscous reaction mixture and the resulting mixture was stirred for a further 10 minutes. The mixture was poured into a plastic bottle and centrifuged at 3600 rpm for 15-20 minutes and the supernatant drained. Fresh 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 twice more with ethyl acetate, centrifuged and decanted. 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 the solution was measured by GPC. The polymer bound to the pigment was measured by TGA.

3. LMA Two-stage polymerization with coated white pigment

The single-neck, 250 ml round bottom flask was equipped with a magnetic stir bar, reflux condenser and argon / or nitrogen inlet and placed in a silicone oil bath. 60 g of silane-coated pigment ground to a fine powder using a bowl and pestle were added to the flask followed by 60 ml of lauryl methacrylate and 60 ml of toluene. The reaction mixture was then stirred rapidly 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 quickly added with a glass pipette. The reaction vessel was sealed, heated to 65 ° 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 a further 10 minutes. The mixture was poured into a plastic bottle and centrifuged at 3600 rpm for 15-20 minutes and the supernatant drained. Fresh ethyl acetate was added to the centrifuged pigment and the resulting mixture was stirred with a stainless steel spatula. The pigment was washed twice more 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 the solution was measured by GPC. The polymer bound to the pigment was measured by TGA.

Caution) In the above procedure, the sonication used in the above described procedure was omitted to avoid any damage that may occur to the ethylene groups remaining on the pigment surface after the one-step polymerization already described.

For two stage polymerization, another single-neck, 250 ml round bottom flask was equipped with a magnetic stir bar, reflux condenser and argon / or nitrogen inlet and placed in a silicone oil bath. 50 g of LMA-coated pigment prepared as described above and ground to a fine powder using a bowl and pestle, followed by 85 ml of toluene and 16 g of styrene were added to the flask. The reaction mixture was then stirred rapidly 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 quickly added with a glass pipette. The reaction vessel was sealed, heated to 65 ° 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 a further 10 minutes. The mixture was poured into a plastic bottle and centrifuged at 3600 rpm for 15-20 minutes and the supernatant drained. Fresh 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 twice more 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 the solution was measured by GPC. The polymer bound to the pigment was measured by TGA.

4. Coated Copper At chromite  Copolymerization

A single-neck, 250 ml round bottom flask was equipped with a magnetic stir bar, reflux condenser and argon inlet and placed in a silicone oil bath. 60 g of copper chromite (CuCr ₂ 0 ₄ , Shepherd Black 1G) coated on the flask with silane of formula (I) and ground to a fine powder with a bowl and pestle, followed by 1.2 ml of styrene, 57 ml of la Uryl methacrylate and 60 ml of toluene were added. The reaction mixture was then stirred rapidly and the flask was purged with argon or nitrogen for 1 hour and the silicon bath was heated to 50 ° 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 quickly added with a glass pipette. The reaction vessel was sealed, heated to 65 ° C. and stirred overnight. 100 mL of ethyl acetate was added to the viscous reaction mixture and the resulting mixture was stirred for a further 10 minutes. The mixture was poured into a plastic bottle and centrifuged at 3600 rpm for 15-20 minutes and the supernatant drained. Fresh 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 decanted. 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 the solution was measured by GPC. The polymer bound to the pigment was measured by TGA.

Control group Internal image  (Electrophoretic particles + suspension  Fluid)

(a) 85 g 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 chromite in Isopar G; (c) 10.71 g stock solution containing 10% by weight Solsperse 17000 in Isopar G; (d) 31.03 g of Isopar G; And (e) a control internal phase from 0.77 g Span 85 (non-ionic surfactant).

After addition of the J and D stock solutions to a 250 ml plastic bottle, Solsperse 17000 solution and Span 85 were added, and finally the remaining solvent. The resulting internal phase was shaken vigorously for approximately 5 minutes and then placed on a roll mill overnight (12 hours or more).

Using the white pigment of the present invention and the black pigment of the prior art Internal  Produce

(a) 40 g stock solution of 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 chromite in Isopar G; (c) 5.04 g stock solution containing 10% by weight Solsperse 17000 in Isopar G; (d) 14.60 g of Isopar G; And (e) the internal phase was formulated from 0.36 g Span 85 (non-ionic surfactant).

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

Using the white pigment of the prior art and the black pigment of the present invention Internal  Produce

(a) 40 g stock solution of the same J pigment as on the control interior, the stock solution containing 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 chromite (85/15 or 95/5 monomer ratio) in Isopar G; (c) 5.04 g stock solution containing 10% by weight Solsperse 17000 in Isopar G; (d) 14.60 g of Isopar G; And (e) the internal phase was formulated from 0.36 g Span 85 (non-ionic surfactant).

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

The internal phase thus prepared was substantially encapsulated (separately) in gelatin / cacacia microcapsules as described in paragraphs [0069] to [0074] of 2002/0180687 mentioned above. The resulting microcapsules were separated by size and capsules with an average particle size of about 35 μm were used in the following experiment. Substantially as described in paragraphs [0075] and [0076] of 2002/0180687 mentioned above, the microcapsules are mixed together with the polyurethane binder to be a slurry and carry an indium tin oxide (ITO) layer on one surface. 7 mil (177 um) poly (ethylene terephthalate) (PET) on the surface of the film 18 gm ^to-roll on a dry coating weight of ^2-to-roll coating (roll-to-roll) process, and microcapsules ITO Placed on the surface covered with. The capsule-bearing film is then laminated to a polyurethane laminated adhesive layer accompanying the release sheet to form a front plane laminate, which is 6 inches / minute (2.5 mm / sec.) At 65 psig (0.51 mPa). ), Using a Western Magnum twin roll laminator and the two rolls were kept at 120 ° C. In order to provide an experimental single-pixel display suitable for use in this experiment, the release sheet of the resulting front-laminated piece was removed and then laminated at 75 ° C. to a 5 cm × 5 cm PET film covered with a carbon black layer. It was used as the back electrode of a single pixel display.

Image Stability Measurement

The single pixel display thus manufactured was switched using a 500 msec square wave pulse at an alternating current 10 V applied to the upper surface (ITO layer) with respect to the grounded rear electrode. A two second pause between pulses was used for the switching reorganization. Image stability was measured by switching the pixels to the appropriate optical state (white or black), grounding the top surface, and measuring light reflectance continuously for 10 minutes. It was assumed that the optical kickback resulting from the residual voltage of the binder and laminated adhesive layer disappeared 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 with selected pixels.

Pigment Example  number White state IS  ( dL *) Black state IS  ( dL *) J / D (Control) 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 as a black pigment instead of copper chromite and without PIB in the suspension fluid are 10- in both white and dark states). Much worse state stability, on the order of 15 L *). Any electrophoretic pigment of the present invention improves image stability in either or both white or black states as compared to the control. In only one case, the image stability of the pixels of the present invention is less favorable than the control (J (TBMA15) / D (St5)), and the difference at this time is probably within experimental pixel-to-pixel tolerance. Displays made of white pigments using styrene as the second monomer and synthesized using a two-step method provide 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-optical response as a function of pulse length at an operating voltage of 10 V. A rest 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-optical response saturates at a particular pulse length and then slightly decays at longer pulse lengths. The control sample was a pixel of the same formulation as the control in Table 2, and a similar pixel containing 0.3 to 0.9 weight percent high molecular weight PIB as a means to obtain good image stability. Table 3 shows the pulse lengths for which the electro-optical response achieves 90% of its value with a pulse length of 1 second in the display of Table 2.

Pigment Example  number Time to 90% saturation ( msec ) J / D (Control-No 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) 9 740 J / D + 0.3% PIB (Control) 10 575

The response time of the control sample comprising the PIB was significantly longer than the response time of the sample according to the invention. At least 0.3% PIB is required to achieve adequate image stability in this system, with 0.9% PIB being preferred. The method of the present invention thus achieves good image stability with a response time of 1.5 to 4 times faster than the control sample.

Threshold voltage

The above-described pixel using a white pigment having 5 mol% TBMA and a black pigment having 15 mol% styrene in the polymer shell exhibits a large operating threshold in addition to 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 at various pulse lengths. The dynamic range is very small (approximately 1 L *) at applied voltages less than 4-5 volts, regardless of pulse length, but exhibits good dynamic range (greater than 30 L *) at impulses exceeding 10 V and 600 msec.

From the above, the incompatible monomers, particles, media and displays, and homocoagulation media of the present invention provide good image stability, and in particular, because the present invention does not sacrifice response time to achieve good image stability, It will be appreciated that it has several advantages over similar media containing polymer additives. This response time advantage can be used to operate the display at low voltages. The display shown in Table 2 is switched to nearly saturation within 300 msec at 7.5 V and achieves an optimum 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 aforementioned WO 02/093246 and the graft polymerization step described above can be carried out in aliphatic hydrocarbons, in particular isoparaffin solvent; A preferred solvent for this purpose is Isopar G, sold by Exxon Mobil Corporation, Houston TX. Since such aliphatic hydrocarbons are the same materials typically used as suspension fluids in electrophoretic media, the coated pigments may stay 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 excess polymerization initiator, excess monomers or oligomers, and free polymer that is not attached to the pigment particles. Such materials need to be substantially removed from the pigment particles before they can be incorporated into the final electrophoretic medium because they can adversely affect the electro-optical properties and / or operating life of the medium. Thus, it will typically also be necessary to separate the polymer-coated pigment from the graft polymerization solvent, for example by centrifugation, and wash the pigment particles to remove to the last remaining amount of the abovementioned materials. However, it is not necessary to dry the polymer-coated pigment to remove the last remaining amount of the hydrocarbon solvent from the polymer-coated pigment (thus eliminating the problem of contamination control associated with solvent vapor), and redispersing the dry pigment in the aliphatic halocarbon solvent. You don't have to; This is because at the end of the washing step, the pigment is already wetted with aliphatic hydrocarbon solvent and there will be no difficulty in redispersing the pigment in the large amount of solvent required for the final electrophoretic medium. Obviously, if the polymerization can be carried out in a manner that leaves only a minimum amount of the above-mentioned substances in the graft polymerization solvent after the polymerization is completed, separation and washing of the polymer-coated pigment may in principle be eliminated.

Although no inherent apparent difference appears between polymer-coated pigments prepared in aliphatic hydrocarbon solvents according to the invention and those made in toluene according to the prior art methods, some experiments have shown that pigments prepared in aliphatic hydrocarbon solvents Shows a tendency to have a higher polymer content.

As mentioned above, the suspension fluid in the electrophoretic medium is an aliphatic hydrocarbon, usually alone or in combination with halocarbons. If the suspension fluid is a mixture of aliphatic hydrocarbons and halocarbons, graft polymerization is carried out on aliphatic hydrocarbons alone, since at present there is no difficulty in redispersing the pigment wetted with aliphatic hydrocarbons into the mixture of aliphatic hydrocarbons and halocarbons after washing. It is desirable to. However, it does not exclude the possibility that the graft polymerization itself can be carried out in a mixture of aliphatic hydrocarbons and halocarbons.

Where a dual particle electrophoretic medium is prepared (for example a medium comprising white titania particles and carbon black particles), the method of the invention can be applied separately to the two types of particles. However, if two types of particles are provided with a similar type of polymer coating, the present invention may be prepared by first providing a polymerizable group on the two types of pigments, then blending the pigments and then performing a graft polymerization step on the pigment mixture. You can also run Thus, the two pigments move together during the remaining steps required to form the electrophoretic medium. Indeed, the polymer-coated mixed pigments produced in this way can be regarded as pigment stock solutions in which little or no further processing to remove residual impurities after washing may be required. This modification of the process thus further reduces the number of individual steps that need to be carried out to convert the raw pigment into the final electrophoretic medium.

The process of the present invention reduces the safety risks associated with the prior art methods because the aliphatic hydrocarbon solvents used in the process are less dangerous than toluene and THF. As already mentioned, the process also reduces solvent usage, as it eliminates predrying and pigment redispersion steps, reducing dispersion processing time and possibly improving the overall quality of the final dispersion of pigments in aliphatic hydrocarbon solvents, Reduce the total number of process steps for producing electrophoretic media.

Another aspect of the present invention relates to reducing the tendency of electrophoretic particles to aggregate by controlling the relative size and mass of the particles in the electrophoretic medium.

It is known that in electrophoretic media, in particular double particle electrophoretic media, the particles tend to aggregate. This tendency can be particularly problematic in dual particle electrophoretic media in which the two types of particles carry opposite polarity charges, since it is natural for any particles that carry electrically opposite charges to stick together. to be. Such particle agglomeration reduces the contrast of the display; For example, if black and white particles aggregate to form gray particles that do not separate when an electric field is applied to change the optical state of the display, then the pure black or white optical state of the display is contaminated with gray aggregates. Will reduce the contrast ratio. In addition, extreme and persistent cohesion may eventually interfere with the display's functioning, reducing the operating life of the display.

It has now been found that the relative motion of two types of particles in dual particle electrophoretic media can be used to separate aggregates and thus reduce the problems associated with such aggregates. The introduction of a second type of particle into the electrophoretic medium under a given electric field, which moves in the opposite direction to the first type of particle, causes the two types of particles to move between each other to separate the particles and reduce particle aggregation by 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 having opposite charges; Preferably, the average particle mass ratio between the two types of particles is about 0.25 to about 4.00.

In a further preferred embodiment, the electrophoretic medium has at least two types of charges of opposite polarity, which can reach the following speeds of V1 and V2, respectively, by a distance G under an applied field of size F: Contains Particles:

V1 + V2> f (F, G)

[Wherein f is a function of decreasing or eliminating 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. The use of this type of electro-optic medium reduces the design constraints of the backplane of the electro-optical display.

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 no matter how long the pulse is, the medium does not switch at all at sub-threshold voltages. At voltages above the threshold, the medium usually switches. Using 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 the pixel is less than 5 V, such a strong threshold voltage medium will not respond. The pixel storage capacitors used in such displays need to be large enough to maintain the voltage peak experienced by the electro-optic medium below the threshold voltage, which is useful for prior art electro-optic displays in which the electro-optic medium does not exhibit threshold voltages. Compared with the required storage 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 can utilize any type of electro-optic medium discussed above where the electro-optic medium exhibits a strong or weak threshold voltage. As described above, incompatible monomer electrophoretic media having threshold voltages are generally preferred.

The present invention relaxes the design rules of active matrix arrays, typically thin film transistor (TFT) arrays. More specifically, the present invention enables the production of TFT backplanes using low resolution techniques such as printing, using smaller storage capacitors per unit area of pixel electrodes. In addition, by reducing the size of the storage battery, the size of the transistor can be reduced. Reducing the size of these two elements will significantly reduce the line capacitance, and thus power consumption, in the backplane. Thus, the electro-optical display of the present invention will have improved performance compared to conventional designs using prior art amorphous silicon structures.

Claims (6)

Suspended fluid, particles of a first electrically charged type suspended in the suspension fluid, the first type of particles having a first optical feature and a polymeric shell, and electrically charged agents suspended in the suspended fluid Comprises two types of particles, the second type of particles having a second optical feature different from the first optical feature, and a polymeric shell,
An electrophoretic medium, characterized in that the polymeric shell is arranged such that homogeneous aggregation of the first and second types of particles is thermodynamically advantageous over heteroaggregation. The electrophoretic medium of claim 1, wherein each of the polymeric shells of the first and second types of particles comprises repeating units derived from one or more monomers in which the homopolymer of monomer is incompatible with the suspension fluid. The electrophoretic medium of claim 2, wherein each polymeric shell further comprises repeating units derived from one or more monomers wherein the homopolymer of monomer is compatible with the suspension fluid. The electrophoretic medium of claim 1, having an operating threshold voltage. The electrophoretic medium of claim 1, wherein the suspension fluid and particles are in a plurality of capsules or cells. An electrophoretic display comprising an electrophoretic medium according to claim 1 and at least one electrode arranged adjacent to and arranged to apply an electric field thereto.

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