Bichromal beads having charge adjuvants therein

Details Bichromal beads having charge adjuvants therein Bichromal beads having charge adjuvants therein

1999-10-26

2002-12-17

Lester, Evelyn A. (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C428S402210, C428S407000, C523S207000, C523S210000, C345S107000, C345S085000

Reexamination Certificate

active

06496298

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
The following U.S. Pat. Nos. 6,335,818 and 6,362,915, are assigned to the same assignee of the present application. The entire disclosures of these patents are totally incorporated herein by reference in their entirety.
BACKGROUND OF THE INVENTION
This invention generally relates to display media, and the preparation of display media, and to devices using such display media. In particular, this invention relates to display media and displays for which the image remains in view after the field and/or power used to form the image is eliminated (completely reduced to zero), or reduced (decreased to a level below normally required to form the image). The image is formed by switching materials in the pixels between two states (such as, for example, black and white). In embodiments, the display media comprise bichromal beads, and in preferred embodiments, Gyricon beads. In embodiments, the bichromal beads comprise charge adjuvants dispersed or contained therein. In embodiments, the charge adjuvants are polymers, and in preferred embodiments, polymers containing a relatively high level of ethylene oxide groups. The display media made with Gyricon beads are useful in generating images which can be stored or erased, and function by rotating a bichromal sphere by an external field to create the image.
Display media, such as Electric Paper or twisted ball panel display devices, are known and are described, for example, in U.S. Pat. Nos. 4,126,854; 4,143,103; 4,261,653; 4,438,160; 5,389,945. The media generally are comprised of an encapsulant medium material, for example, an elastomer, such as a cured polysiloxane, sandwiched between two indium tin oxide coated substrates, such as glass or MYLAR™. Generally, the elastomer layer has closely packed cavities, each containing a bichromal sphere suspended in a dielectric liquid. The dielectric liquid may also be present in substantial amounts in the elastomer matrix. In media that are active in an electric field, the bichromal spheres have a net dipole due to different levels of charge on the two sides of the sphere. An image is formed by the application of an electric field to each pixel of the display, which rotates the bichromal spheres to expose one color or the other to the viewing surface of the media. The spheres may also have a net charge, in which case they will translate in the electric field as well as rotate. When the electric field is reduced or eliminated, the spheres ideally do not rotate further; hence, both colors of the image remain intact. This image bistability is one feature of display media made with bichromal Gyricon beads.
The fabrication of certain bichromal spheres is known, for example, as set forth in the above mentioned U.S. Pat. No. 4,143,103 patent, wherein the sphere is comprised of black polyethylene with a light reflective material, for example, titanium oxide, sputtered on hemisphere. Also in U.S. Pat. No. 4,438,160, a rotary ball is prepared by coating white glass balls of about 50 microns in diameter, with an inorganic coloring layer such as indium by evaporation. In a similar process, there is disclosed in an article entitled “The Gyricon—A twisting Ball Display”, published in the proceedings of the S.I.D., Vol. 18/3 and 4 (1977), a method for fabricating bichromal balls by first heavily loading glass balls with a white pigment such as titanium oxide, followed by coating from one direction in a vacuum evaporation chamber with a dense layer of nonconductive black material which coats only one hemisphere.
Also in U.S. Pat. No. 4,810,431 by Leidner, there is disclosed a process for generating spherical particles by (a) coextruding a fiber of a semi-circular layer of a polyethylene pigmented white and a semi-circular black layer of polyethylene containing magnetite, (b) chopping the resultant fiber into fine particles ranging from 10 microns to about 10 millimeters, (c) mixing the particles with clay or anti-agglomeration materials, and (d) heating the mixture with a liquid at about 120° C. to spherodize the particles, followed by cooling to allow for solidification.
Reference is made to U.S. Pat. No. 5,262,098, and in co-pending patent applications U.S. application Ser. No. 09/360,088, filed Jul. 23, 1999, entitled “Method and Apparatus for Fabricating Bichromal Elements”, and U.S. application Ser. No. 09/360,052, filed Jul. 23, 1999, entitled “Method and Apparatus for Fabricating Bichromal Elements”, now abandoned. These applications disclose apparatuses for fabricating hemispherically bichromal balls comprising a separator member having opposing first and second surfaces and an edge region in contact with both surfaces, and delivery means for flowing first and second colored hardenable liquid material over the first and second surfaces, respectively, so that the liquid materials arrive at the edge, usually at substantially the same flow rate, and form a reservoir outboard of the edge region. The reservoir comprises side-by-side regions of different colors, which in a preferred embodiment, do not intermix. Further means are provided for propelling the first and second liquid materials away from the separator member and out of the reservoir into a fluid medium. As this occurs, a plurality of forward ends of side-by-side bichromal streams become unstable and break up into droplets. The droplets form into spherical balls, each of the balls approximately comprising hemispheres of differently colored hardenable liquids. These bichromal balls are from about 5 to about 200 microns in diameter.
The aforementioned display media can suffer from drawbacks caused by incomplete or lack of rotation of the bichromal beads. When the beads do not rotate close to 180°, the switching from one color to the other is not complete. As a result, image quality suffers. In some cases, increasing the strength of the electric field used to rotate the spheres can help in achieving more complete rotation, but in other cases sufficient rotation cannot be attained, even at higher fields. In the latter cases, it is believed that the dipole strength of the sphere relative to the monopole strength is too small, rendering it difficult to get sufficient rotation before the sphere translates across its cavity in the elastomer matrix. Many of the beads lack sufficient monopole and dipole strengths to dislodge them from the cavity walls. Furthermore, it is usually preferable to produce media requiring an electric field that is not too high in magnitude, since the cost of display products made from media that switch at lower electric fields can be advantaged.
Another drawback of the aforementioned display media can be the lack of a sharp voltage threshold, depending upon the formulation. Consider a media that requires a voltage ±V to get sufficient sphere rotation and hence switching between colors. There is a sharp voltage threshold above a magnitude of ½V when the spheres do not rotate at voltages of magnitudes less than or equal to ½V. During the course of writing an image on a media with a sharp voltage threshold above a magnitude of ½V, pixels that experience voltages between +½V and 0 will not change their color. For certain applications this property is desired. An example application is a display device with passive matrix addressing. In the case of passive matrix addressing, a pixel of the display is addressed by applying half of the required voltage to both the row and column of that pixel, the two half voltages having opposite polarities to yield a total voltage across the pixel equal to the switching voltage of ±V. At the same time, however, other pixels in the same row but in other columns, or in the same column but in other rows, experience a voltage of ±½V. Thus, it is desired that the spheres in those pixels do not rotate at the voltage ±½V. A sharp voltage threshold above ±½V gives the desired behavior, whereas a media that lacks a sharp voltage threshold does not. Since the display media described in th

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