Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2002-03-06
2004-12-14
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C345S107000, C430S032000
Reexamination Certificate
active
06831770
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
An invention directed to an electrophoretic display comprising isolated cells filled with charged pigment particles dispersed in a dielectric solvent. The filled cells are individually sealed with a polymeric sealing layer.
b) Description of Related Art
The electrophoretic display is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by using spacers. One of the electrodes is usually transparent. A suspension composed of a colored solvent and charged pigment particles is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side and then either the color of the pigment or the color of the solvent can be seen according to the polarity of the voltage difference.
In order to prevent undesired movement of the particles, such as sedimentation, partitions between the two electrodes were proposed for dividing the space into smaller cells. However, in the case of partition-type electrophoretic displays, some difficulties were encountered in the formation of the partitions and the process of enclosing the suspension. Furthermore, it was also difficult to keep different colors of suspensions separate from each other in the partition-type electrophoretic display.
Subsequently, attempts were made to enclose the suspension in microcapsules. U.S. Pat. Nos. 5,961,804 and 5,930,026 describe microencapsulated electrophoretic displays. The reference display has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a suspension of charged pigment particles that visually contrast with the dielectric solvent. The microcapsules can be formed by interfacial polymerization, in-situ polymerization or other known methods such as physical processes, in-liquid curing or simple/complex coacervation. The microcapsules, after their formation, may be injected into a cell housing two spaced-apart electrodes, or “printed” into or coated on a transparent conductor film. The microcapsules may also be immobilized within a transparent matrix or binder that is itself sandwiched between the two electrodes.
The electrophoretic displays prepared by these prior art processes, in particular the microencapsulation process as disclosed in U.S. Pat. Nos. 5,930,026, 5,961,804, and 6,017,584, have many shortcomings. For example, the electrophoretic display manufactured by the microencapsulation process suffers from sensitivity to environmental changes (in particular sensitivity to moisture and temperature) due to the wall chemistry of the microcapsules. Secondly the electrophoretic display based on the microcapsules has poor scratch resistance due to the thin wall and large particle size of the microcapsules. To improve the handleability of the display, microcapsules are embedded in a large quantity of polymer matrix which results in a slow response time due to the large distance between the two electrodes and a low contrast ratio due to the low payload of pigment particles. It is also difficult to increase the surface charge density on the pigment particles because charge-controlling agents tend to diffuse to the water/oil interface during the microencapsulation process. The low charge density or zeta potential of the pigment particles in the microcapsules also results in a slow response rate. Furthermore, because of the large particle size and broad size distribution of the microcapsules, the prior art electrophoretic display of this type has poor resolution and addressability for color applications.
SUMMARY OF THE INVENTION
The first aspect of the present invention is directed to an electrophoretic display comprising cells of well-defined shape, size and aspect ratio. The cells are filled with an electrophoretic fluid comprising charged particles dispersed in a dielectric solvent and are individually sealed with a polymeric sealing layer. The polymeric sealing is preferably formed from a composition comprising a thermoset or thermoplastic precursor. In one embodiment of the invention, the cells are partially filled with electrophoretic fluid above which the sealing layer forms a contiguous film and is in intimate contact with both the fluid and the peripheral of the cell walls that are not covered by the fluid. In one of the preferred embodiments of the invention, the sealing layer further extends over the top surface of the cell side walls.
In another preferred embodiment of the invention, the top surface of the cell walls is at least 0.01 micrometer (&mgr;) above the top surface of electrophoretic fluid. More preferably, the top surface of the cell walls is about 0.02&mgr; to 15&mgr; above the top surface of electrophoretic fluid. Most preferably, the top surface of the cell walls is about 0.1&mgr; to 4&mgr; above the top surface of electrophoretic fluid.
In another preferred embodiment of the invention, the top surface of the polymeric sealing layer is at least 0.01&mgr; above the top surface of the cell walls to improve the adhesion between the sealing layer and the cells. More preferably, the top surface of the polymeric sealing layer is about 0.01&mgr; to 50&mgr; above the top surface of the cell walls. Even more preferably, the top surface of the polymeric sealing layer is about 0.5&mgr; to 8&mgr; above the top surface of the cell walls. The total thickness of the sealing layer is about 0.1&mgr; to 50&mgr;, preferably about 0.5 to 15&mgr;, more preferably 1&mgr; to 8&mgr;. Most preferably, the sealing layer forms a contiguous film above the cell walls and the electrophoretic fluid.
Another aspect of the invention relates to a novel process for the manufacture of such an electrophoretic display.
A further aspect of the invention relates to the preparation of cells of well-defined shape, size and aspect ratio. The cells enclose a suspension of charged pigment particles dispersed in a dielectric solvent and are formed from microcups prepared according to the present invention. Briefly, the process for the preparation of the microcups involves embossing a thermoplastic or thermoset precursor layer coated on a conductor film with a pre-patterned male mold, followed by releasing the mold during or after the thermoplastic or thermoset precursor layer is hardened by radiation, cooling, solvent evaporation, or other means. Alternatively, the microcups may be formed from imagewise exposure of the conductor film coated with a radiation curable layer followed by removing the unexposed areas after the exposed areas have become hardened.
Solvent-resistant, thermomechanically stable microcups having a wide range of size, shape, and opening ratio can be prepared by either one of the aforesaid methods. The microcups are then filled with a suspension of charged pigment particles in a dielectric solvent, and sealed.
Yet another aspect of the present invention relates to the sealing of the microcups filled with the electrophoretic fluid containing a dispersion of charged pigment particles in a dielectric fluid. Sealing can be accomplished by a variety of ways. One of the preferred embodiments, is accomplished by dispersing a sealing composition comprising a thermoplastic, thermoset, or their precursors in the electrophoretic fluid before the filling step. The sealing composition is immiscible with the dielectric solvent and has a specific gravity lower than that of the solvent and the pigment particles. After filling, the sealing composition phase separates from the electrophoretic fluid and forms a supernatant layer at the top of the fluid. The sealing of the microcups is then conveniently accomplished by hardening the sealing layer by solvent evaporation, interfacial reaction, moisture, heat, or radiation. UV radiation is the preferred method to harden the sealing layer, although a combination of two or more curing mechanisms as d
Liang Rong-Chang
Tseng Scott C-J
Wu Zarng-Arh George
Zang Hongmei
Dang Hung Xuan
Halluin Albert P.
Howrey Simon Arnold & White , LLP
Kung Viola T.
SiPix Imaging Inc.
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