Abstract:
An electrophoretic medium comprises two different types of electrically charged particles in a fluid. One type of electrically charged particles comprises a dark colored particle and a second type of electrically-charged particles comprises titania having a zirconia surface treatment.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of copending Application Ser. No. 61/256,376, filed Oct. 30, 2009. 
     This application is also related to:
         (a) U.S. Pat. No. 6,822,782;   (b) U.S. Pat. No. 7,411,720;   (c) U.S. Pat. No. 7,230,750;   (d) U.S. Pat. No. 7,375,875;   (e) U.S. Pat. No. 7,532,388;   (f) U.S. Pat. No. 7,002,728;   (g) U.S. Pat. No. 7,247,379;   (h) U.S. Pat. No. 7,679,814;   (i) copending application Ser. No. 12/188,648, filed Aug. 8, 2008 (Publication No. 2009/0009852);   (j) copending application Ser. No. 12/121,211, filed May 15, 2008 (Publication No. 2008/0266245) and   (k) International Application Publication No. WO 2010/091938       

     The entire contents of these patents and copending applications, and of all other U.S. patents and published and copending applications mentioned below, are herein incorporated by reference. 
    
    
     BACKGROUND OF INVENTION 
     This invention relates to electrophoretic particles (i.e., particles for use in an electrophoretic medium) and processes for the production of such electrophoretic particles. This invention also relates to electrophoretic media and displays incorporating such particles. More specifically, this invention relates to white, titania-based electrophoretic particles. 
     Particle-based electrophoretic displays, in which a plurality of charged particles move through a suspending fluid under the influence of an electric field, have been the subject of intense research and development for a number of years. Such displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. 
     The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in U.S. Pat. No. 7,170,670 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays. 
     Nevertheless, problems with the long-term image quality of electrophoretic displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays. 
     Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various technologies used in encapsulated electrophoretic and other electro-optic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. The technologies described in the these patents and applications include:
         (a) Electrophoretic particles, fluids and fluid additives; see for example U.S. Pat. Nos. 5,961,804; 6,017,584; 6,120,588; 6,120,839; 6,262,706; 6,262,833; 6,300,932; 6,323,989; 6,377,387; 6,515,649; 6,538,801; 6,580,545; 6,652,075; 6,693,620; 6,721,083; 6,727,881; 6,822,782; 6,870,661; 7,002,728; 7,038,655; 7,170,670; 7,180,649; 7,230,750; 7,230,751; 7,236,290; 7,247,379; 7,312,916; 7,375,875; 7,411,720; 7,532,388; and 7,679,814; and 7,746,544; and U.S. Patent Applications Publication Nos. 2005/0012980; 2006/0202949; 2008/0013155; 2008/0013156; 2008/0266245 2008/0266246; 2009/0009852; 2009/0206499; 2009/0225398; 2010/0045592; 2010/0148385; and 2010/0207073;   (b) Capsules, binders and encapsulation processes; see for example U.S. Pat. Nos. 6,922,276; and 7,411,719;   (c) Films and sub-assemblies containing electro-optic materials; see for example U.S. Pat. No. 6,982,178; and U.S. Patent Application Publication No. 2007/0109219;   (d) Backplanes, adhesive layers and other auxiliary layers and methods used in displays; see for example U.S. Pat. Nos. 7,116,318; and 7,535,624;   (e) Color formation and color adjustment; see for example U.S. Pat. No. 7,075,502; and U.S. Patent Application Publication No. 2007/0109219;   (f) Methods for driving displays; see for example U.S. Pat. Nos. 7,012,600; and 7,453,445;   (g) Applications of displays; see for example U.S. Pat. No. 7,312,784; and U.S. Patent Application Publication No. 2006/0279527; and   (h) Non-electrophoretic displays, as described in U.S. Pat. Nos. 6,241,921; 6,950,220; and 7,420,549; and U.S. Patent Application Publication No. 2009/0046082.       

     Known electrophoretic media, both encapsulated and unencapsulated, can be divided into two main types, referred to hereinafter for convenience as “single particle” and “dual particle” respectively. A single particle medium has only a single type of electrophoretic particle suspended in a suspending medium, at least one optical characteristic of which differs from that of the particles. (In referring to a single type of particle, we do not imply that all particles of the type are absolutely identical. For example, provided that all particles of the type possess substantially the same optical characteristic and a charge of the same polarity, considerable variation in parameters such as particle size and electrophoretic mobility can be tolerated without affecting the utility of the medium.) When such a medium is placed between a pair of electrodes, at least one of which is transparent, depending upon the relative potentials of the two electrodes, the medium can display the optical characteristic of the particles (when the particles are adjacent the electrode closer to the observer, hereinafter called the “front” electrode) or the optical characteristic of the suspending medium (when the particles are adjacent the electrode remote from the observer, hereinafter called the “rear” electrode (so that the particles are hidden by the suspending medium). 
     A dual particle medium has two different types of particles differing in at least one optical characteristic and a suspending fluid which may be uncolored or colored, but which is typically uncolored. The two types of particles differ in electrophoretic mobility; this difference in mobility may be in polarity (this type may hereinafter be referred to as an “opposite charge dual particle” medium) and/or magnitude. When such a dual particle medium is placed between the aforementioned pair of electrodes, depending upon the relative potentials of the two electrodes, the medium can display the optical characteristic of either set of particles, although the exact manner in which this is achieved differs depending upon whether the difference in mobility is in polarity or only in magnitude. For ease of illustration, consider an electrophoretic medium in which one type of particles is black and the other type white. If the two types of particles differ in polarity (if, for example, the black particles are positively charged and the white particles negatively charged), the particles will be attracted to the two different electrodes, so that if, for example, the front electrode is negative relative to the rear electrode, the black particles will be attracted to the front electrode and the white particles to the rear electrode, so that the medium will appear black to the observer. Conversely, if the front electrode is positive relative to the rear electrode, the white particles will be attracted to the front electrode and the black particles to the rear electrode, so that the medium will appear white to the observer. 
     If the two types of particles have charges of the same polarity, but differ in electrophoretic mobility (this type of medium may hereinafter to referred to as a “same polarity dual particle” medium), both types of particles will be attracted to the same electrode, but one type will reach the electrode before the other, so that the type facing the observer differs depending upon the electrode to which the particles are attracted. For example suppose the previous illustration is modified so that both the black and white particles are positively charged, but the black particles have the higher electrophoretic mobility. If now the front electrode is negative relative to the rear electrode, both the black and white particles will be attracted to the front electrode, but the black particles, because of their higher mobility will reach it first, so that a layer of black particles will coat the front electrode and the medium will appear black to the observer. Conversely, if the front electrode is positive relative to the rear electrode, both the black and white particles will be attracted to the rear electrode, but the black particles, because of their higher mobility will reach it first, so that a layer of black particles will coat the rear electrode, leaving a layer of white particles remote from the rear electrode and facing the observer, so that the medium will appear white to the observer: note that this type of dual particle medium requires that the suspending fluid be sufficiently transparent to allow the layer of white particles remote from the rear electrode to be readily visible to the observer. Typically, the suspending fluid in such a display is not colored at all, but some color may be incorporated for the purpose of correcting any undesirable tint in the white particles seen therethrough. 
     Both single and dual particle electrophoretic displays may be capable of intermediate gray states having optical characteristics intermediate 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 within each capsule. For purposes of the present application, such multi-particle media are regarded as sub-species of dual particle media. 
     Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media. 
     A related type of electrophoretic display is a so-called “microcell electrophoretic display”. In a microcell electrophoretic display, the charged particles and the fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging, Inc. 
     Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, U.S. Pat. Nos. 5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346. Electrophoretic media operating in shutter mode may be useful in multi-layer structures for full color displays; in such structures, at least one layer adjacent the viewing surface of the display operates in shutter mode to expose or conceal a second layer more distant from the viewing surface. 
     An encapsulated or microcell electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; electrophoretic deposition (See U.S. Pat. No. 7,339,715); and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively. 
     However, the electro-optical properties of electrophoretic displays could still be improved. Typically, an electrophoretic display is designed to have black and white extreme optical states; color can then be produced by providing color filters adjacent the electrophoretic medium. One of the main advantages of electrophoretic displays is that they can closely mimic the appearance of printing on paper; hence such displays are often used in electronic document readers (usually called “E-book readers” but the term “electronic document readers” is preferred as such devices are often used for reading electronic versions of printed publications other than books, for example maps, newspapers and magazines). It is desirable that electrophoretic displays used in such readers be able to imitate accurately the appearance of all types of books, including the dense black printing on very white paper used in expensive books, and although current electrophoretic displays provide an easily readable text display, they have not quite reached the high standard of the best printed books. 
     Some prior art commercial dual particle electrophoretic displays used polymer coated titania as the white particle and polymer coated carbon black as the dark particle; see for example, U.S. Pat. No. 6,822,782, especially Examples 27-30. Such displays did not provide a dark state as dark as is desirable. Black pigments capable of providing darker dark states have been developed; see for example, U.S. Pat. No. 7,002,728 (copper chromite based particles) and International Application Publication No. WO 2010/091398 (particles based upon high surface area metal oxides, especially certain mixed metal oxides). However, there is still room for improvement in the dark state of electrophoretic displays, especially since it transpires that the dark state of a display is of critical importance in obtaining optimum color performance when a color filter is used with a display, and that the dark states of prior art electrophoretic displays still adversely affect the color gamut of such displays. 
     It has now surprising been found that the dark state of a dual particle electrophoretic can be significantly improved by the choice of the white pigment used in the display, and the present invention relates to preferred white pigments and electrophoretic media and displays using these preferred white pigments. 
     SUMMARY OF INVENTION 
     Accordingly, this invention provides an electrophoretic medium comprising at least two different types of electrically charged particles in a fluid and capable of moving through the fluid on application of an electrical field to the fluid, wherein one type of electrically charged particles comprises a dark colored particle and a second type of electrically-charged particles comprises titania having a zirconia surface treatment. 
     In such an electrophoretic medium, the titania particles may have a zirconia alumina surface treatment. The titania particles may have an average diameter of from about 0.2 to about 0.5 μm, and a surface area of from about 8 to about 24 m 2 /g by BET. The titania particles may have a polymer chemically bonded to, or cross-linked around, them. It is generally preferred that the polymer be chemically bonded to the titania particle. The polymer may comprise from about 1 to about 15 percent by weight, preferably from about 4 to about 14 percent by weight, of the titania particle. The polymer may comprise charged or chargeable groups, for example amino groups. The polymer may also comprise a main chain and a plurality of side chains extending from the main chain, each of the side chains comprising at least about four carbon atoms. The polymer may be formed from an acrylate or a methacrylate. 
     The dark colored particles used in the present process may be as described in the aforementioned U.S. Pat. No. 7,002,728 or WO 2010/091398. 
     In the electrophoretic medium of the invention, the fluid may be gaseous or liquid, and in the latter case may comprise a hydrocarbon, or a mixture of a hydrocarbon and a chlorinated hydrocarbon. 
     The electrophoretic medium of the present invention may be of the encapsulated type and comprise a capsule wall within which the fluid and the electrically charged particles are retained. Such an encapsulated medium may comprise a plurality of capsules each comprising a capsule wall, with the fluid and electrically charged particle retained therein, the medium further comprising a polymeric binder surrounding the capsules. Alternatively, the medium may be of the microcell or polymer-dispersed types discussed above. 
     This invention also provides an electrophoretic medium comprising electrically charged particles in a fluid and capable of moving through the fluid on application of an electrical field to the fluid, wherein at least one electrically charged particle comprises titania having a zirconia surface treatment. 
     This invention extends to an electrophoretic display comprising an electrophoretic medium of the present invention and at least one electrode disposed adjacent the electrophoretic medium for applying an electric field to the medium. In such an electrophoretic display, the electrophoretic medium may comprises a plurality of capsules. Alternatively, the electrophoretic medium may be of the polymer-dispersed type and comprise a plurality of droplets comprising the fluid and the electrically charged particles, and a continuous phase of a polymeric material surrounding the droplets. The electrophoretic display may also be of the microcell type and comprise a substrate having a plurality of sealed cavities formed therein, with the suspending fluid and the electrically charged particles retained within the sealed cavities. 
     The displays of the present invention may be used in any application in which prior art electro-optic displays have been used. Thus, for example, the present displays may be used in electronic book readers, portable computers, tablet computers, cellular telephones, smart cards, signs, watches, shelf labels and flash drives. 
     In another aspect, this invention provides a titania particle having a zirconia surface treatment and a polymer chemically bonded to the particle. The polymer may comprise from 1 to about 15 percent by weight, preferably from about 4 to about 14 percent by weight, of the particle. The polymer may comprise charged or chargeable groups, for example amino groups. The polymer may comprise a main chain and a plurality of side chains extending from the main chain, each of the side chains comprising at least about four carbon atoms. The polymer may be formed from an acrylate or a methacrylate. The titania particles may have a zirconia alumina surface treatment, and may have an average diameter of from about 0.2 to about 0.5 μm (towards the lower end of the size range of titania particles conventionally used in electrophoretic displays), and a surface area of from about 8 to about 24 m 2 /g by BET. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  of the accompanying drawings is a graph showing the zeta potentials of a white pigment of the present invention and a similar prior art pigment, as measured in experiments described in Example 4 below. 
         FIG. 2  is a graph showing the particle sizes of the same pigments as in  FIG. 1  above. 
         FIGS. 3 and 4  are graphs showing respectively the dark state and light state L* values of experimental dual particle displays produced using the same pigments as in  FIG. 1  above. 
         FIG. 5  is a graph comparing the switching speeds of the dual particle displays used to generate the data shown in  FIGS. 3 and 4 . 
     
    
    
     DETAILED DESCRIPTION 
     As indicated above, the present invention provides an electrophoretic medium comprising a fluid, and titania particles having a zirconia, and preferably a zirconia alumina, surface treatment. The electrophoretic medium may also comprise dark electrophoretic particles. 
     The dark particles used in the electrophoretic medium of the present invention may be any of those known in the art. Preferred dark particles are copper chromite or an inorganic black pigment having a surface area of at least about 7 m 2 /g. In the latter case, preferred black pigments are metal oxides, for example magnetite, (Fe 3 O 4 ), and mixed metal oxides containing two or more of iron, chromium, nickel, manganese, copper, cobalt, and possibly other metals. Two specific useful pigments are copper iron manganese oxide spinel and copper chromium manganese oxide spinel (available from the same company as Black 20C920). The dark particles are preferably polymer coated using the techniques described in U.S. Pat. Nos. 6,822,782; 7,230,750; 7,375,875; and 7,532,388. 
     Titania having a zirconia alumina surface treatment (hereinafter called “ZA titania”) suitable for use in the present electrophoretic medium is available commercially from various sources. One commercial pigment which has been found to give good results is Tronox CR-828, available from Tronox Incorporated, One Leadership Square, Suite 300, 211 N. Robinson Avenue, Oklahoma City, Okla. 73102-7109. This pigment is a zirconia alumina pacified titanium dioxide with weight mean average size of about 0.350 μm, specific gravity of 4.1, and surface area of 16 m 2 /g. For optimum results, the titania particles should be polymer coated using the techniques described in U.S. Pat. Nos. 6,822,782; 7,230,750; 7,375,875; and 7,532,388. Typically, such a polymer coating is produced by first treating the ZA titania with a silane having one functional group capable of reacting with the ZA titania surface and a second functional group capable of undergoing polymerization. The silane-treated titania is then treated with a polymerizable monomer or oligomer in the presence of a polymerization catalyst so as to cause formation of polymer chemically bonded to the titania particle. 
     In addition to producing darker dark states than similar electrophoretic media using prior art titania-based pigment particles, preferred electrophoretic media of the present invention also display improved charging of the white pigment and increased black-to-white switching speed, as illustrated in the Examples below. 
     The following Examples are now given, though by way of illustration only, to show details of preferred reagents, conditions and techniques used in the present invention. 
     Example 1 
     Preparation of Polymer-Coated ZA Titania Particles 
     Part A: Preparation of Silane-Treated Titania 
     ZA titania (Tronox CR-828) was surface-functionalized using N-3-(trimethoxysilyl)-propyl]methacrylate (available from Dow Chemical Company under the trade name Z-6030) substantially as described in the aforementioned U.S. Pat. No. 6,822,782, Example 28, Part A. The amount of surface functionalization was estimated by thermogravimetric analysis (TGA), which indicated the presence of approximately 2.2% of volatile (organic) material, representing substantially more than a monolayer of the silane. 
     Part B: Polymer Coating of Silane-Treated Titania 
     The silanized pigment produced in Part A above was treated to produce a surface-grafted polymer layer substantially as described in the aforementioned U.S. Pat. No. 6,822,782, Example 28, Part B. This process resulted in a final pigment containing 8-10% volatile material by TGA. 
     Example 2 
     Preparation of Capsules Containing Polymer-Coated ZA Titania Particles 
     Gelatin-acacia capsules containing the polymer-coated ZA titania particles prepared in Example 1 above, and similar capsules containing a prior art polymer-coated titania with a silica alumina surface treatment (hereinafter “SA titania”), were prepared substantially as described in Example 2 of application Ser. No. 12/702,566, filed Feb. 19, 2010 (see the same Example in the corresponding International Application WO 2010/091398). 
     Example 3 
     Preparation of Experimental Electrophoretic Displays 
     The capsules were allowed to settle and excess water was decanted. The resulting capsule slurry was adjusted to pH 9-9.5 with 10 weight percent ammonium hydroxide solution. The capsules were then concentrated by centrifugation and the supernatant liquid discarded. The capsules were mixed with an aqueous urethane binder at a ratio of 1 part by weight binder to 8 parts by weight of capsules and surfactant and hydroxypropylmethyl cellulose were added and mixed thoroughly. 
     The mixture thus produced was bar coated, using a 4 mil (101 μm) coating slot on to an indium-tin oxide (ITO) coated polyester film, the capsules being deposited on the ITO-coated surface of the film at a target coating thickness of 18 μm, and the resultant coated film was dried in a conveyor oven at 60° C. for approximately 2 minutes. 
     Separately, a polyurethane lamination adhesive was coated on the a release sheet to form a dried adhesive layer 25 μm thick, and the resultant coated sheet was cut to a size slightly smaller than that of a capsule-coated film piece. The two sheets were then laminated together (with the lamination adhesive in contact with the capsule layer) by running them through a hot roll laminator with both rolls set to 120° C. to form a front plane laminate as described in U.S. Pat. No. 6,982,178. The front plane laminate was then cut to the desired size, the release sheet removed, and the lamination adhesive layer thereof laminated to a 2 inch (51 mm) square backplane comprising a polymeric film covered with a graphite layer, the lamination adhesive being contacted with the graphite layer. This second lamination was effected using the same laminator but with both rolls at 93° C. The laminated pixels were cut out using a laser cutter, and electrical connections applied to produce experimental single-pixel displays suitable for use in the electro-optic tests described below. 
     Example 4 
     Electro-Optic Tests 
     The following test results are based upon measurements taken from seven single-pixel displays using the white pigment of the invention and seven single-pixel displays using the prior art white pigment, all being prepared as described in Example 3 above. In each of the accompanying drawings, “ZAT” denote the medium of the invention using ZA titania, while “SAT” denoted the medium using the prior art SA titania. 
     Firstly, the zeta potentials of the white pigments were measured by standard methods, and the results are shown in  FIG. 1  of the accompanying drawings, from which it will be seen that the ZA titania used in the present invention charged substantially more negatively than the prior art white pigment. (It will be appreciated that the greater zeta potential provided by the ZA titania particles of the present invention will be useful in electrophoretic media in which no dark particles are present, for example an electrophoretic medium in which the fluid is dyed and the ZA titania particles are the only particles present. The higher zeta potential provided by the ZA titania will tend to result in faster switching of such displays.) 
     The particle size of the white pigments were also measured by standard methods, and the results for the mode of particle size (in microns) are shown in  FIG. 2 . From this Figure it will be seen that the ZA titania is generally smaller than the SA titania, which according to accepted scattering theory should improve the white state of the electrophoretic medium, although no such improved white state was observed experimentally. 
     Next, the extreme (black and white) optical states of the experimental displays produced as described above were tested by switching them through several transitions between their extreme black and white optical states. The display was driven to both its extreme optical states using, then 15 V 240 millisecond drive pulses. The reflectance of the optical state produced by the driving pulse was measured 30 seconds after the end of the drive pulse; this delay in measuring allows certain short term effects which occur after the drive pulse ends to dissipate and provides a value representative of that perceived by, for example, a user of a E-booker reader reading in a normal manner. The measured reflectances were converted to L* units, where L* has the usual CIE definition.  FIG. 3  of the accompanying drawings shows the dark state L* values obtained, while  FIG. 4  shows the light state L* values, in both cases for both the displays of the present invention and the prior art displays. 
     From  FIG. 3 , it will be seen that the ZA titania particles of the present invention provided displays with substantially darker dark states than the prior art SA titania particles. The mean dark state L* for the displays of the invention was 19.56, and for the prior art displays 22.28; in both cases the standard error was less than 0.35, so that the difference between the two means exceeded seven standard errors. From  FIG. 4 , it will be seen that there was no substantial difference between the light states of the two types of displays. The mean light state L* for the displays of the invention was 70.04±0.33, and for the prior art displays 70.15±0.36, so that the difference between the two displays was less than half the standard error. 
     The improved dark state of the displays of the present invention resulted in a contrast ratio (after the aforementioned 30 second delay) of 14, significantly better than the contrast ratio of 11 for the prior art displays. 
     During the foregoing experiments, it was observed that the display of the present invention appeared to switch more rapidly than the prior art displays. To quantify this quicker switching, a final series of experiments were conducted, in which the displays were repeatedly switched between their extreme black and white optical states as before, driven to their extreme black optical state using a 15 V 240 millisecond drive pulse, and then a 15 V 60 millisecond drive pulse was applied to drive the displays part way toward their white state. The displays were then allowed to stand undriven for 30 seconds after the end of the 15 V 60 millisecond drive pulse and their reflectances were measured and converted to L* units. The results are shown in  FIG. 5 , which plots the L* change achieved by the 15 V 60 millisecond drive pulse as a fraction of the total L* change achieved by the 15 V 240 millisecond drive pulse. From this Figure, it will be seen that the displays of the present invention did indeed achieve a greater fraction of the black-white transition as a result of the 60 millisecond drive pulse than did the prior art displays. The mean values are 45.06±1.23% for the displays of the invention and 40.47±1.13% for the prior art displays, so that the difference is significant to a high level of confidence. 
     From the foregoing, it will be seen that the present invention can provide displays having darker dark states, higher contrasts ratios and more rapid switching speeds than the prior art displays. The present invention also provides white electrophoretic particles having larger zeta potentials than in prior art displays. These advantages can be achieved without major changes to the manufacturing processes for the displays, or major capital investment, since the present invention can be implemented by simply replacing one commercial titania starting material with another and minor variations in the subsequent manufacturing steps. 
     It will be apparent to those skilled in the art that numerous changes and modifications can be made in the specific embodiments of the invention described above without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be interpreted in an illustrative and not in a limitative sense.