CN100412936C - Electrophoretic displays - Google Patents

Abstract

Abstract An electrophoretic medium has walls defining a microcavity containing an internal phase. This internal phase comprises electrophoretic particles suspended in a suspending fluid and capable of moving therethrough upon application of an electric field to the electrophoretic medium. The average height of the microcavity differs by not more than 5mm from the saturated particle thickness of the electrophoretic particle divided by the volume fraction of the electrophoretic particles in the internal phase.

Description

Electrophoretic medium and the electrophoretic display device (EPD) that comprises electrophoretic medium

The present invention relates to contain the electrophoretic display device (EPD) of controlled in a large number pigment.

For many years, the electrophoretic display device (EPD) based on particulate is the focus of research and development always.In such display, be subjected to electric field effects, a large amount of charged corpuscles moves in suspending liquid.Compare with liquid crystal display, electrophoresis showed can provide qualities such as the bistability of gratifying brightness and contrast, wide visual angle, state and low-power consumption.(term used herein " bistable " and " bistability " refer generally in the art: display comprises the display element with first and second show states, it is different having a kind of optical characteristics under first and second show states at least, like this, after any point element is driven, addressing pulse by a finite duration, present its first show state or second show state, after addressing pulse finishes, it is several times of needed minimum addressing pulse duration of state that change display element at least that this state will continue, for example at least four times).Yet the long-term image quality problem of these displays has hindered them and has been widely used.For example, the particulate that forms electrophoresis showed is tending towards precipitation, causes the deficiency in serviceable life of these displays.

Recently, manyly be authorized to or patent and application under Massachusetts Institute Technology (MIT) and E Ink company name, that describe encapsulated electrophoretic media (encapsulated electrophoreticmedia) come forth.This encapsulated electrophoretic media comprises many Caplets, and each Caplet itself constitutes an interior phase (internal phase), contains the electrophoresis movable corpuscle that is suspended in the liquid suspending medium mutually in described; This encapsulated electrophoretic media also comprises the cyst wall around interior phase.Usually, capsule itself is maintained in the polymer adhesive, to form a bonding coat between two electrodes.This packing medium was described in following patent: for example United States Patent(USP) Nos. 5,930, and 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; 6,724,519; With 6,727,881; And U.S. Patent Application Publication Nos.2002/0019081; 2002/0021270; 2002/0060321; 2002/0063661; 2002/0090980; 2002/0113770; 2002/0130832; 2002/0131147; 2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378; 2003/0011560; 2003/0011868; 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; With 2004/0027327; And international application published Nos.WO99/67678; WO 00/05704; WO 00/26761; WO 00/36560; WO 00/38000; WO 00/38001; WO 00/67110; WO 00/67327; WO 01/07961; WO 01/08241; WO 03/107315; WO 04/017135; WO 04/023195; With WO 04/023202.

Many above-mentioned patents and application are thought: in encapsulated electrophoretic media, wall around discrete microcapsules can be substituted by external phase, thereby produce one so-called " polymer-dispersed electrophoretic display ", electrophoretic medium wherein comprises the droplet of a large amount of electrophoresis liquid that disperse and the external phase of polymeric material; And these patents and application are thought: but regard the droplet of electrophoresis liquid discrete in this polymer dispersed electrophoresis showed as capsule or microcapsules, though not have the cyst membrane that disperses with each independently droplet be associated; Can be referring to such as aforesaid 2002/0131147.Therefore, with regard to the application, this polymer-dispersed electrophoretic media is considered to the subspecies of encapsulated electrophoretic media.

The packing electrophoretic display device (EPD) can not suffer the cohesion of conventional electrophoretic device usually and precipitate fault and more benefit is provided, and for example can print or apply display surface on the substrate of wide variety of flexible and rigidity.(employed word " printing " will comprise the printing and the coating of form of ownership, include but not limited to: pre-metering coating, as sticking patch coating, slit or extrusion coating, slip or lamination coating, curtain coating; Roller coat is as scraper roller coat, roller coat forward and backward; Gravure coating; Dip coating; Spraying; The meniscus coating; Spin coated; Brush; The air knife coating; Screen printing process; The xeroprinting process; The temperature-sensitive printing process; The ink jet printing process; And other similar techniques.) therefore, the display surface of Sheng Chenging can have flexibility thus.In addition, because show media can be printed (make and in all sorts of ways), so the manufacturing cost of display itself can be very cheap.

A kind of electrophoresis showed of correlation type is so-called " micella electrophoresis showed ", is also sometimes referred to as " little glass of electrophoresis showed ".In microcell electrophoretic display, charged corpuscle and suspending liquid are non-encapsulated to be advanced in the capsule, but is maintained in a plurality of cavitys, and these cavitys form in mounting medium (being generally polymer thin film).Referring to as international application published WO02/01281 and U.S. Patent Application Publication 2002/0075556, all transferred SipixImaging company limited.

Hereinafter, with term " microcavity electrophoretic displays " cover both encapsulated electrophoresis showed and micella electrophoresis showed.

Known microcavity electrophoretic displays can be divided into two kinds of main types, hereinafter, for simplicity, can respectively these two kinds of main types be called " single particulate " and " two particulate ".Single particulate media only is suspended in the electrophoretic particle of the single type in the colored medium, and at least a optical characteristics of this colored medium is different with particulate.(when mentioning the single type particulate, and do not mean that such all particulates all are absolute identical.For example, if such all particulates have the identical optical characteristics electric charge identical with polarity, but under the condition that does not influence medium effectiveness, sizable variation in the parameter of tolerable as particle size and electrophoresis movability).Optical characteristics is often referred to the color that human eyes can be seen, but, absorb under the non-visible wavelength of expression or the more wide in range meaning of reflection differences on, the optical characteristics that can select else or increase can be in reflectivity, retroreflectance, luminous, fluorescence, phosphorescence or the color etc. any one or a plurality of.This medium is placed between the pair of electrodes, have at least one to be transparent in these two electrodes, depend on the relative current potential of these two electrodes, this medium can show optical signature (the approaching electrode nearer apart from the observer of particulate this moment of these particulates, this electrode is known as " preceding " electrode hereinafter) or show the optical signature (this moment particulate near the electrode (hereinafter be called " back " electrode) far away, particulate is hidden by colored suspending medium) of suspending medium apart from the observer.

Two particulates have two kinds of dissimilar particulates, and at least a optical characteristics in these two kinds of dissimilar particulates is different with suspending liquid, and described suspending liquid can be for coloured or colourless, but normally colourless.The electrophoresis movability of these two types of particulates is different; This movability difference can show as polarity difference (this type is called as " the two particulates of the charges of different polarity " medium hereinafter) and/or magnitude difference.When this pair of particulate media is placed between the aforesaid electrode pair, depend on the relative current potential of these two electrodes, medium can show the optical characteristics of arbitrary group of particulate, though the appropriate mode that achieves and inequality, this depend on its movability difference be on the polarity on magnitude.For be easy to the explanation, consider a kind of electrophoretic medium, wherein one type particulate be black and the particulate of another kind of type is white.If there is polarity difference (for example black particle positively charged and white particles is electronegative) in this particulate of two types, then particulate will be adsorbed on two different electrodes, like this, if for example with respect to rear electrode, preceding electrode is for negative, black particle on the electrode and white particles will be adsorbed on the rear electrode, therefore will manifest black before then will be adsorbed at observer's medium.On the contrary, if with respect to rear electrode, preceding electrode is being for just, on the electrode and black particle will be adsorbed on the rear electrode, therefore will manifest white at observer's medium before then white particles will be adsorbed to.

, these two types of particulates have electrophoresis movability difference (this hereinafter type media is called as " the two particulates of same sex electric charge " medium) if having the electric charge of identical polar, then two types of particulates will be adsorbed on the same electrode, but one type will arrive electrode prior to another kind of type, like this, depending on particulate and be adsorbed to which electrode, is different towards observer's type.For example, with illustrated different in front, the black and white particulate is all positively charged, but black particle has higher electrophoresis movability.If with respect to rear electrode, electrode is for negative now, and then the black and white particulate all will be adsorbed on the preceding electrode, but black particle will be because its higher movability will at first arrive preceding electrode, the black particle layer will be revealed as black with electrode before coated at observer's medium like this.On the contrary, if with respect to rear electrode, before electrode for just, then the black and white particulate will be adsorbed on the rear electrode, but black particle will be because its higher movability will at first arrive rear electrode, like this black particle layer with coated rear electrode and layer of white particles away from rear electrode and towards the observer, the result will be revealed as white at observer's medium.It should be noted that such pair of particulate media requires suspending liquid enough transparent, sees so that layer of white particles is easy to the observed person away from rear electrode.Usually, the suspending liquid in this demonstration is colourless fully, but capable of being combinedly advances some color and correct any color that does not meet the requirements in the white particle of seeing through suspending liquid.

Some have described the electrophoretic medium that more than two types of electrophoretic particle are arranged in aforesaid E Ink and MIT patent and the application certain in single capsule.At present, this multiparticulates medium is regarded as the subclass of two particulate media.

Single particulate and two particle electrophoretic displays device can both show the Intermediate grey states that has between the optical characteristics of two extreme optical state having described.

The microcavity electrophoretic displays device can comprise arbitrarily the suitably microcavity of shape; For example, some aforesaid E Ink and MIT patent and application (are especially seen U.S. Patent application 6,067,185 and 6,392,785) the packing electrophoresis showed has been described, in the packing electrophoresis showed, be initially spherical capsule and evened up, make capsule be essentially the oblate ellipsoid shape.When a large amount of this oblate ellipsoid shape capsules were deposited upon on the substrate, cyst wall can contact with each other, and near the packing state, under this state, adjacent cyst wall is evened up against each other, so capsule is rendered as the multiedge cylinder form fully until capsule.In theory, in close-packed layer of capsules, single capsule will be rendered as the hexagonal prism form, and in fact, the microphoto of some encapsulated electrophoretic media shows as very near this state.Yet more typical is that single capsule is rendered as irregular multiedge cylinder form fully.In the encapsulated electrophoretic media of polymer dispersed, do not have independently capsule certainly, but the droplet of interior phase can present and the similar form of having discussed of capsule.

So the microcavity in the microcavity electrophoretic displays device can be irregular.Below discussion will consider that the microcavity of sheet film form, described microcavity have planar dimension and littler, and this plane size perpendicular, that be represented as the Z axle of band X and Y-axis substantially.To be expressed as " the interior phase height " or " IP height " of microcavity along the microcavity average internal height of Z axle.With the XY plane parallel of microcavity and do not comprise that the average area (average along Z-direction) of capsule or cavity wall will be represented as " IP area ", and comprise that the average area of the correspondence of capsule or cavity wall will be represented as " capsule area ".With the XY plane parallel of microcavity on the arbitrary height and do not comprise that the maximum gauge of capsule or cavity wall will be represented as " IP diameter ", and comprised that the corresponding mean diameter of capsule or cavity wall will be expressed as " capsule diameter ".

As everyone knows, the ideal style of the optical property of optimization electrophoresis and the demonstration of other electric light is active part (just, can change the viewing area part of optical states when electric field the is applied in electro-optical medium) maximization that makes the viewing area.When being applied in electric field, the non-active region that shows (zone that is occupied by cyst wall or microcavity walls in black matrix of for example usually using in LCD and the microcavity electrophoretic displays) does not change optical states, thereby has reduced the contrast between the extreme optical state that shows.Yet, relevant other parameter that influences the electrophoresis showed optical property of less consideration in the document of having announced, needed amount of pigment in the especially less consideration electrophoretic medium.This may part because of the following fact: most of electrophoretic display device (EPD)s of being discussed in the document are single particle electrophoretic displays devices, and in this display, the limiting factor of electrophoretic medium thickness is the optical density (OD) of dyestuff rather than the amount of pigment that presents in the suspending liquid normally.The situation of two particle electrophoretic displays is not such, and uses the situation of single particulate demonstration of the ratio dyestuff that used optical density (OD) is also high in the most prior art electrophoresis showed to do like this.

The optical property of having found at present electrophoresis showed is subjected to the IP height and interior mutually the influence of pigment filling (ratio that promptly comprises the interior phase volume of pigment) of variation, the medium of the amount of pigment that presents in the electrophoretic medium in fact, and the present invention relates to electrophoretic medium and display, wherein, concern Be Controlled between these different parameters, to improve the optical property of (preferably optimizing) medium and display.

Therefore, the invention provides a kind of electrophoretic medium, described electrophoretic medium has qualification, and at least one comprises the wall of the microcavity of an interior phase, in case should in comprise mutually a plurality of at least a types be suspended in the suspending liquid and electrophoretic medium be coupled with the electrophoretic particle that electric field just can move in suspending liquid, the average height of at least one microcavity and the saturated particle thickness of electrophoretic particle differ divided by the interior income value of the volume ratio (volume fraction) of middle electrophoretic particle mutually and are no more than 5 microns.

The thickness of the interior particulate layer that forms that has comprised just enough electrophoretic particles represented to utilize in " saturated particle thickness " this term of electrophoretic particle in the microcavity used herein on the IP of microcavity area, be enough to make electrophoretic medium between its extreme optical state, to switch if in time T, specific electric field is put on medium, then the changes in optical properties that this time that applies specific electric field changes arbitrary extremity that will can not make electrophoretic medium in the scope of 0.95T to 1.05T surpasses 2 L* units, and wherein L* has common CIE definition.When calculating, this saturated particle thickness do not consider the packing factor; In other words, suppose that electrophoretic particle constitutes a void-free fully closely knit layer on the IP area, saturated particle thickness is the supposition thickness of the layer that constitutes on this zone.For example, if the IP of electrophoretic medium highly is 50 microns and has held the electrophoretic particle volume of 1O% that then its saturated particle thickness is 5 microns.As conspicuous to the people who is familiar with the packing of multiparticulates layer, the get along well actual (real) thickness of the particulate layer that constituted when all particulates are driven to be an end surfaces of microcavity of this thickness is corresponding, because this stratum granulosum will comprise the volume part in the space of a great deal of inevitably.For simplicity, suppose that electrophoretic medium comprises the essentially identical spherical particle of diameter, these spherical particles have constituted a dense layer.Because the packing rate of closely knit uniform spheroid is approximately 0.64, so the actual (real) thickness of the layer that constitutes at microcavity one end surfaces is approximately 5/0.64 or 7.8 micron.

In a preferred form of the invention, in the scope of 0.9T to 1.1T, the changes in optical properties that the time that this specific electric field applies changes under arbitrary extremity that will can not make electrophoretic medium surpasses 2 L* units, and in the particularly preferred form of the present invention, in the scope of 0.8T to 1.2T, the changes in optical properties that the time that this specific electric field applies changes under arbitrary extremity that will can not make electrophoretic medium surpasses 2 L* units.

Saturated particle thickness is usually between about 1 micron to about 5 microns, and preferably between about 1.5 microns to about 2.5 microns.In mutually in the volume ratio (being the volume ratio of the occupied interior phase of electrophoretic particle) of electrophoretic particle usually between 3% to 40%, and preferably between 6% to 18%.

Electrophoretic medium of the present invention can be any one in the above-mentioned type.So electrophoretic medium can be the single particulate media that comprises the single type electrophoretic particle in colored suspension liquid.Perhaps, electrophoretic medium can be the two particulate media that comprised the first kind electrophoretic particle and the second type electrophoretic particle, wherein first kind electrophoretic particle has first optical characteristics and the first electrophoresis movability, and the second type electrophoretic particle has second optical characteristics different with first optical characteristics and the second electrophoresis movability different with the first electrophoresis movability.In this pair of particulate media, suspending liquid can be colourless.Electrophoretic medium can belong to the micella type, and electrophoretic particle wherein and suspending liquid are maintained in a plurality of cavitys that are formed in the mounting medium.Perhaps, electrophoretic medium also can be an encapsulated electrophoretic media, and electrophoretic particle wherein and suspending liquid are maintained in a plurality of capsules.

The class that the present invention especially is suitable for shows it is that so-called " venetian blind type " microcavity shows.Shutter mode microcavity shows it is that a kind of like this microcavity shows: have " opaque " optical states, other optical signature of (or wherein any known pixels) Show Color of display under this state or electrophoretic particle; Have second optical states, electrophoretic medium or pixel wherein are light-permeables under this state.It can be single particulate or two particle type that this venetian blind type shows, and can have more than two kinds of specific optical states; For example, use two particulate venetian blind types of black and white electrophoretic particle to show to have black non transparent state, White-opalescent state and light transmission state.Usually can produce the light transmission state that venetian blind type shows on the smaller portions of the cross-sectional area by electrophoretic particle being limited in each microcavity, light beam can pass freely through the major part of this cross-sectional area like this.The smaller portions that electrophoretic particle are limited in cross-sectional area can realize with following manner: the microcavity by using typing (referring to, United States Patent(USP) Nos. 6,130 as the aforementioned, and 774 and 6,172,798); By electrode is placed ad-hoc location with respect to microcavity (referring to, as the aforementioned 2002/0180688 and Japan published Nos.2002-174828 and 2001-356374); Or the electrophoresis by electrophoretic particle drive (referring to, as the copending application No.10/687 that submitted on October 16th, 2003,166).

Though second optical states that venetian blind type shows above is being considered to " printing opacity ", but the venetian blind type display can comprise coloured or colourless reverberator, the contiguous microcavity medium of described coloured or colourless reverberator and be positioned at usually In the view of the observer be with the opposed side of microcavity medium (for simplicity, this counter surface is called microcavity medium " rear surface " hereinafter), in fact the light-transmissive optical state of (as aforesaid copending application Nos.10/687,166 is described) demonstration has shown the color (if any) of reverberator like this.Particularly, can constitute favourable colored microcavity display format: a base plate that has a plurality of pixel electrodes is set, on base plate, constitutes color filter or reverberator, then on color filter or reverberator, form one deck shutter mode microcavity medium by following manner.Can form the micella medium by photoetching technique: on color filter, form one deck photoresist, and expose in a conventional manner and develop, form the cell wall that branch is isolated a plurality of micellas with this.In addition, can on color filter and the microcavity that constitutes mechanically wherein, provide a layer (being generally polymeric layer), the preformed layer that contains microcavity perhaps is provided on color filter.Under arbitrary situation, formed microcavity is filled and sealed to available electrophoresis potpourri (electrophoretic particle adds suspending liquid).

No matter how are the exact method of making and the definite type of used electrophoretic medium, such colored venetian blind type display has following advantage: because pixel electrode is in sight easily when forming and attach color filter, so can simplify the operation that color filter is set with respect to pixel electrode, the more important thing is, color filter is arranged on avoids the visible artifact (artifact) that produces owing to parallax during from axle at the color filter of isolating fully with base plate near the pixel electrode.

The problem that this shutter mode microcavity shows is to guarantee satisfactory contrast (contrast ratio), even because under the light-transmissive optical state that this venetian blind type shows, the color that the smaller portions of each microcavity that electrophoretic particle occupies still show these particulates (perhaps, what show under the situation that two particulates show is the mixing of relevant colors), and the continuous demonstration of this electrophoretic particle color has reduced contrast.The quantity of required electrophoretic particle during may command microcavity of the present invention shows, thereby make the area of each microcavity that under the light transmission state of this demonstration electrophoretic particle is occupied for minimum and make contrast reach maximum, still provide enough electrophoretic particle to guarantee the good optical characteristic under first optical states of display simultaneously.

The preferred white electrophoretic particle that is used for electrophoretic medium of the present invention comprises titania (TiO ₂).If electrophoretic medium is two particle type, then electrophoretic medium also may comprise the black particle that is made of carbon black or copper chromite, and the electrophoresis movability that the black particle that is made of carbon black or copper chromite has is different from the electrophoresis movability of titanium dioxide fine particles.

IP height among the useful embodiment of the present invention is between about 10 to about 30 microns, and the volume ratio of electrophoretic particle is between about 3% to about 15%.The IP height is between about 12 to about 25 microns in the preferred embodiment, and the volume ratio of electrophoretic particle is between about 5% to about 12%.The viscosity of interior phase is usually less than about 5mPasec and usually greater than 1mPasec.

The present invention relates to a kind of electrophoretic display device (EPD), described electrophoretic display device (EPD) comprises electrophoretic medium of the present invention and at least one and is arranged on and is used for applying to this place the electrode of electric field near the electrophoretic medium.Usually, this electrophoretic display device (EPD) is provided with a rear electrode structure, and this structure has a plurality of electrodes that are used for applying to electrophoretic medium electric field.

On the other hand, the invention provides a kind of electrophoresis suspensioning liquid that is used for electrophoresis showed, described electrophoresis suspensioning liquid comprises the viscosity of the white particles that surpasses 5% weight and this suspending liquid between about 2 to about 7mPa.s.

Just as has already been described, this electrophoresis suspensioning liquid of the present invention can comprise any in the preferred feature of electrophoretic medium of the present invention.

As pointing out, find: in microcavity electrophoretic displays, there is a preferred IP height, this preferred IP height is relevant with two key variables, the saturated particle thickness (promptly obtaining the minimum thickness of each required pigment of suitable optical states) that variable is an electrophoretic particle, another variable be display interior mutually in the volume ratio of this pigment.

At first sight, in electrophoresis showed, obtain " suitable optical states " and as if with the optical characteristics that requires of the pigment that is used for any given application relation is arranged uniquely.Yet find: if electrophoretic medium does not comprise enough pigment, the optical characteristics of medium also may be adversely affected; For example, if the Chinese white deficiency that electrophoretic medium comprises, then the reflectivity of this medium under white states may be lower than the reflectivity of similar mediums under same state that contains more Chinese whites.

Because various technical reasons are preferably under the condition of coordinating mutually with the good optical characteristic usually, keep electrophoretic medium thin as far as possible.Because the rate travel of electrophoretic particle is determined by electric field intensity, and because the electric field intensity in (all other factorses are impartial) electrophoresis showed is directly proportional divided by distance between electrodes gained result with the voltage that is applied between the electrode, so in order to make operating voltage keep lowly as far as possible, usually preferably make this apart from keeping minimum value (just making electrophoretic medium keep thinly as far as possible), general low-work voltage can reduce the energy consumption of display (especially portable, under the situation of battery powered device) and make the complexity of required cost of driving display and electronic circuit reach minimum, this is desirable.In addition, electrophoretic medium keep the thin as far as possible display that reduces in the distance that electrophoretic particle need move between transfer period between its extreme optical state, thereby under the steady electric field effect, increased the demonstration switch speed.Similarly, in some applications, electrophoretic display device (EPD) seems noticeable because of its dirigibility, and is easy to make the flexible display with thin electrophoretic medium.Therefore, be not difficult to find out that electrophoresis showed should possess minimum IP height and the higher interior volume ratio of middle pigment mutually, so that enough pigment to be provided, thus the preferred optical states when guaranteeing that this pigment is visible.

Yet, the factor that also exists some to cancel out each other.The filling that increases pigment can cause higher interior phase viscosity usually, and this higher viscosity has reduced the speed of electrophoretic particle and slowed down demonstration switch speed under the given electric field action.

So, utilize pigment, suspending liquid, operating voltage and expectation switching time random specific combination and the preferred structure that obtains electrophoretic medium is the problem of a complexity.Relation complicated between the voltage that applies and the optical states of electrophoretic medium makes this situation become more complicated.As aforesaid 2003/0137521 and some other patent of aforesaid E Ink and MIT and application in discussed, electrophoretic medium no longer only is used as simple voltage switch (as doing) and has served as the pulse switch in the first approximation scope in liquid crystal, the end-state of pixel not only depends on the electric field that is applied and also depends on the electric field state of pixel before that applies like this.Generate on being desirably in display when being considered to uniformly color region, this class behavior can cause serious complicacy.Such as, imagine a preparation is used for reading black literal (with or without illustrations) under white background black and white display.When this demonstration is rewritten (when new page is shown), unless the structure of electrophoretic medium and used drive scheme are selected through careful, roughly uniformly can there be fine distinction between the optical states of numerous pixels in the white background, and human eye is very responsive for the nuance that is considered to optical states in the zone uniformly, especially because the reader has been accustomed on the printer page white background highly uniformly.

Find that in the present invention: in order to be obtained the good optical performance by the microcavity electrophoretic medium, it is very important that the average height of microcavity is associated with the interior middle mutually electrophoretic particle volume ratio of saturated particle thickness (as top defined) and electrophoretic medium.Also find: have a preferred IP height concerning the microcavity electrophoretic medium.If the saturation thickness of pigment be T and in mutually in the pigment volume concentration ratio be F, then preferred IP highly be T/F, and in fact preferably actual IP highly differs not above about 5 microns with this optimal value.When electrophoretic medium comprises two or more pigment, should calculate the T/F value separately at each pigment, and preferred IP highly is set to maximal value in the value that the result obtains.

Obviously can see: in the electrophoretic medium that comprises more than a kind of type electrophoretic particle, every type particulate all has the saturated particle thickness of oneself.Single particle electrophoretic media generally includes the Chinese white in the colored suspension liquid, and two particle electrophoretic media generally includes white and black particle in the colourless suspending liquid.Yet under these two kinds of situations, critical saturated particle thickness is often referred to the saturated particle thickness of white particles, the black particle absorbing light because white particles makes light scattering, and make the required pigment thickness of light scattering be greater than the required pigment thickness of absorbing light.

Find: if the IP of microcavity is highly obviously greater than this preferred T/F value in the electrophoretic medium, then display performance can reduce.A reason is: it is bigger that pigment arrives the distance that microcavity walls institute must be mobile.Second reason is: with respect to the given voltage field at phase two ends in being added in, field intensity descends.Lower field intensity has reduced the speed of particulate.In addition, in many pigment system, charges of different polarity particulate is tending towards cohesion, has reduced the number of separated aggregation than low field intensity.On the other hand, if the IP of microcavity highly is significantly less than this preferred value, then can't reach desired optical states because the optical density (OD) of pigment is not enough.

In order to assess various electrophoretic mediums, can measure total pigment " saturation thickness ", described total pigment saturation thickness was obtained with the electric field level that is added in interior two ends mutually by the duration of pulse, will make the optical characteristics of pigment change the critical value that is no more than desired visible artefacts in its variation that acts on the following recurrence interval.In a typical white/black switch speed is approximately 300 milliseconds system, at a saturation thickness place, the variation in the duration of pulse that 15 volts of voltage is following 50 milliseconds will make optical characteristics change and be no more than 2L*.In using the system of faster switching electrophoretic medium, if the change of optical characteristics is no more than 2L* when the duration of pulse is changed to the 5-20% of white/black switch speed of typical medium, then saturation thickness may be suitable.As above-mentioned, for clearly definition, " saturated particle thickness " of electrophoretic particle expression in the terminology used here microcavity: with the thickness (supposing 100% packing) of the interior particulate layer that on the IP of microcavity area, constitutes that comprises just enough electrophoretic particle, if the specific electric field that is applied in time T on the medium is enough to make electrophoretic medium to switch between its extreme optical states, then the variation of (promptly ± 5% time change) this specific electric field action time will can not make any the changes in optical properties in two kinds of extremities of electrophoretic medium surpass 2 L* units in the scope of 0.95T to 1.05T.Desirable situation is, system should stand ± time of 10% changes, and preferably stand ± time of 20% changes, and change of optical property is no more than 2L* simultaneously.

The preferred white pigment that is used for electrophoretic medium is a titania.The relevant U. S. application No.60/481 of pending trial when submitting to as 28,2002/0185378 or 2003 on the October of mentioning in front, described in 572, titania preferably has the surface of silicon dioxide and/or aluminum oxide coating layer, and preferably also scribbles one layer of polymeric.Known as pigment and paint field, desirable situation is, the diameter of this titanium dioxide fine particles should be between 0.1 micron and 0.5 micron, and in order to obtain maximum scattering efficient with minimum thickness, desirable diameter is between 0.2 micron and 0.4 micron.Yet, also can use the synthetic particulate that has comprised multiple pigment particle.What depend on used addressing waveforms, titanium dioxide fine particles is (especially aforesaid 2002/0185378 and 60/481, those particulates of describing in 572) can have scope in about 1 to 10 micron saturated particle thickness, and preferably about 1 to 5 micron scope.The inventor finds: in the two particle electrophoretic media of a kind of preferred titania/carbon black of the present invention, use 15 volts preferred addressing pulse and the duration of pulse between 200 to 500 milliseconds, titania provides the suitable overlayer of thickness between 1.5 and 2.5 microns.Having than low viscosity, in the duration of pulse at another kind is under 100 milliseconds 15 deep-sited pulses dash to drive in the very fast interior preferred electrophoretic medium of switching of switching mutually, and titania also provides the suitable covering of thickness between 1.5 and 2.5 microns.

Available copper chromite particulate replaces carbon black particle as the black particle (perhaps, also needing black particle certainly in single particulate media) in the two particulate media of the present invention.The preparation of copper chromite particulate and use the relevant U. S. application No.10/708 of pending trial when on February 11st, 2004 submitted in the electrophoretic medium is fully described in 142.

As discussed above, as if preferably use higher pigment loadings to come formulate thin electrophoretic media at first view, but implement the limited ability of this way in the increase of the viscosity relevant with high pigment loadings.Compare with the pigment loadings of the electrophoresis showed that is used for prior art, a little increase of pigment loadings may be favourable.For example, in order to obtain white system, it may be that appropriateness is useful that the titania loadings is increased to higher level (for example thickness of 5-7 micron).In addition, although some electrophoresis suspensioning liquid well known in the art once used the pigment filling of less than 2% weight, can have the viscosity that the electric field intensity effect that particulate is used in commercial device is issued to suitable speed but can find to include, be generally 15 volts or littler driving voltage so that device uses up to 45% weight or up to the interior phase of the titania of 15% volume.Preferable medium of the present invention has the titania loadings of 5% to 15% volume usually, and phase viscosity is between 1 to 6mPasec in it.Given 1.5 to 2.5 microns saturated particle thickness and reach the titania loadings of 10% volume, can find: when the IP of microcavity height between 15 microns and 25 microns, when preferred value is essentially 20 microns, visible artefacts reduces.

The interior phase of other type can allow the viscosity that reduces, thereby allows higher pigment loadings.For example, the interior phase of use gaseous suspension liquid (referring to, such as the copending application No.10/605 that submitted on September 3rd, 2003,039 and aforesaid corresponding International Application No. WO 2004/023202) can support higher pigment loadings and corresponding therewith lower IP highly.This will when the particulate filling reaches 90% volume, play a role mutually based on gaseous state.

Can think (although the present invention never is not subject to this view) that one of reason of the improvement optical states that the present invention realized is: in microcavity electrophoretic displays,, then show to be easy to present afterimage (image ghosting) if pigment is thick inadequately.Cause the reason of this afterimage can comprise in the microcavity of change in voltage trickle in the addressing system, display slowly the residual voltage of decay or polarization, in time and incorrect mixing and the RC time constant of interior phase and the difference between its external environment condition of the pigment that produces precipitation, various pigment, comprising the existence of any bonding agent.The effect of all these can cause the visible amount of pigment of observer to change, and this variation is superimposed in the variation that the addressing by display causes.

Can measure " visual artifact level " of display (being generally high resolution display) by any suitable mode.In one approach, allow a plurality of pixels stand to represent the different switching course that uses a model of intending usefulness separately.Greatest optical difference between any two pixels is " maximum visual artifact level ".Perhaps, also can allow single pixel stand a plurality of different switching courses, then be applied in a uniformity test addressing pulse.Greatest optical difference between the consequent optical characteristics, this is the another kind of method of measuring " maximum visual artifact level ".

In order to obtain the coherent image quality of band minimal visual artifact, the way that meets the requirements is that electrophoretic medium comprises minimum suitable pigment thickness, so that the slight change of pigment content only can bring minimum optical effect.Use for portable, high resolution display, under the typical change condition in given actual pigment packing thickness, this optical effect preferably is no more than 1 to 2 L* unit.

Following example has illustrated the details (only as an example) of preferred material, condition and technology that the present invention uses.

Example

The prepared interior white particles of 10% volume in the hydrocarbon suspending liquid and the black particle (carbon black) of 1% volume of comprising mutually; The viscosity of phase is 4.75mPasec in being somebody's turn to do.White particles comprises titania, and its average-size is about 0.6 micron and saturated particle thickness and is estimated as 1.5 to 2.5 microns.As described in aforesaid 2002/0180687 paragraph [0069] and [0074], be completely wrapped in mutually in this in gel/gum arabic micro-capsule.The microcapsules that the result obtains are divided into three groups different on wet capsule diameter Size Distribution.Basically as described in aforesaid 2002/0180687 paragraph [0075] and [0076], each group is mixed into the slurry of band polymer adhesive, and is coated and the formation electrophoretic film rolls on the rear electrode then to constitute switchable display pixel.In coating process, obtain a series of wet film coating amount with proper device setting (as speed, pressure and Mo Gao), this a series of wet film coating then is dried and becomes the capsule with different IP height, the different IP height of described capsule, part is owing to the evaporation and the surface tension of bonding agent cause.In first group, dried capsule is substantially spherical; In second group, dried capsule is substantially the oblate spheroid bodily form; In the 3rd group, dried capsule is essentially the prolate spherical of height greater than diameter.Under every kind of situation, the circle under the XY projection is to hexagon, and spheroidite changes with the variation of packing density in the film of electrophoretic medium.

As shown in following table, under 350 milliseconds, 15 volts pulse, be switched, three kinds of electrophoretic mediums that the result obtains are being estimated the IP height and are being different aspect the optical characteristics typically.

Form

Estimation IP height (micron) in the microcavity	White states L*	Black state L*	Contrast	Maximum visual artifact L*
					18	62	21	9.4∶1	1.5
44	59	27	5.3∶1	3.7
					55	60	24	6.9∶1	5.5

The saturated particle thickness of supposing titania is 2 microns, and preferred IP of the present invention highly should be 20 microns; The IP of the black pigment that calculates is less highly basically, and like this, Here it is for the preferred IP height of the important titania of this medium.As can be seen from the above table data: compare with other the two kinds media with big IP height, estimation IP highly is that the capsule of 18 microns (near 20 microns of calculated values) has optical characteristics (contrast that comprises improvement) preferably basically.

Claims (21)

1. electrophoretic medium, at least one holds the wall of the microcavity of an interior phase to have qualification, the electrophoresis particle that comprises a plurality of at least a types in being somebody's turn to do mutually, they are suspended in the suspending liquid and can move under effect of electric field and pass through electrophoretic medium, described medium is characterised in that: the saturated particle thickness of the average height of at least one microcavity and electrophoresis particle divided by interior mutually in the volume ratio income value of electrophoresis particle differ and be no more than 5 microns, wherein said saturated particle thickness represent to utilize comprised just enough electrophoretic particles interior microcavity with microcavity XY plane parallel and do not comprise the thickness of the particulate layer that forms on the average average area along Z-direction of capsule or cavity wall.

2. electrophoretic medium as claimed in claim 1, wherein, described saturated particle thickness is between 1 and 5 micron.

3. electrophoretic medium as claimed in claim 2, wherein, described saturated particle thickness is between 1.5 and 2.5 microns.

4. electrophoretic medium as claimed in claim 1, be enough to make this medium to switch between its extreme optical state if apply specific electric field in time T in medium, then the variation of the driving time of this specific electric field in the scope of 0.9T to 1.1T will can not make the changes in optical properties of arbitrary extremity of described electrophoretic medium surpass 2 L* units.

5. electrophoretic medium as claimed in claim 4, the variation of the application time of described specific electric field in the scope of 0.8T to 1.2T will can not make the changes in optical properties of arbitrary extremity of electrophoretic medium surpass 2 L* units.

6. electrophoretic medium as claimed in claim 1 comprises a kind of single type electrophoresis particle in colored suspension liquid.

7. electrophoretic medium as claimed in claim 1 comprises and has first optical signature and the ambulant first type electrophoresis particle of first electrophoresis and have second optical signature that is different from first optical signature and be different from the ambulant second type electrophoresis particle of ambulant second electrophoresis of first electrophoresis.

8. electrophoretic medium as claimed in claim 7, wherein, described suspending liquid is colourless.

9. electrophoretic medium as claimed in claim 1, wherein, described electrophoresis particle and described suspending liquid are maintained in a plurality of cavitys that are formed in the mounting medium.

10. electrophoretic medium as claimed in claim 1, wherein, described electrophoresis particle and described suspending liquid are maintained in a plurality of capsules.

11. electrophoretic medium as claimed in claim 1, wherein, described electrophoresis particle comprises titania.

12. electrophoretic medium as claimed in claim 11, wherein, described electrophoresis particle also comprises the black particles that is formed by carbon black or copper chromite, and described black particles is formed by carbon black or copper chromite and its electrophoresis movability is different from the electrophoresis movability of TiO 2 particles.

13. electrophoretic medium as claimed in claim 1, wherein, described in mutually in the volume ratio of electrophoresis particle be 3% to 40%.

14. electrophoretic medium as claimed in claim 13, wherein, described in mutually in the volume ratio of electrophoresis particle be 6% to 18%.

15. remove to require 1 described electrophoretic medium as right, in it mutually height between between 10 and 30 microns and the volume ratio of its electrophoresis particle between 3% and 15%.

16. remove to require 15 described electrophoretic mediums as right, in it mutually height between between 12 and 25 microns and its electrophoresis particle volume ratio between 5% and 12%.

17. electrophoretic medium as claimed in claim 1, wherein, the viscosity of described interior phase is in 1mPasec arrives the scope of 5mPasec.

18. an electrophoretic display device (EPD) comprises a kind of electrophoretic medium as claimed in claim 1 and is located at and is used for applying to this medium at least one electrode of electric field near the described electrophoretic medium.

19. electrophoretic display device (EPD) as claimed in claim 18 has first optical states of the optical signature that wherein said display shows a kind of type electrophoresis particle and second optical states that wherein said electrophoretic medium is printing opacity.

20. electrophoretic display device (EPD) as claimed in claim 19, wherein, under described light-transmissive optical state, described electrophoresis particle is limited in the smaller portions of each microcavity cross-sectional area.

21. electrophoretic display device (EPD) as claimed in claim 19 comprises a base plate that is provided with a plurality of pixel electrodes and color filter or reverberator, described color filter or reverberator are located between described base plate and the described electrophoretic medium.