Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-03-12
2004-03-09
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06704073
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the coating of a polymer dispersed electro-optical material. In a preferred embodiment, the electro-optical material is a cholesteric liquid-crystal material.
BACKGROUND OF THE INVENTION
Currently, information is displayed on sheets using permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically writable areas to carry ticketing or financial information, however magnetically written data is not visible.
World patent application PCT/WO 97/04398, entitled “Electronic Book With Multiple Display Pages”, is a thorough recitation of the art of thin, electronically written sheet display technologies. Disclosed is the assembling of multiple display sheets that are bound into a “book”, each sheet provided with means to individually address each page. The patent recites prior art in forming thin, electronically written pages, including flexible sheets, image modulating material formed from a bi-stable liquid crystal system, and thin metallic conductor lines on each page.
An early patent, U.S. Pat. No. 3,578,844, discloses a light modulating structure suitable for a display device. In the patent, cholesteric liquid crystal material is encapsulated by light penetrable gelatin and gum arabic capsules that are coated on a screen. The capsules were formed by emulsifying the cholesteric material in a gelatin solution using a blender to form droplets between 10 and 30 microns in diameter. The pH of the emulsion was changed to precipitate a gelatin coating over each droplet of cholesteric material. The gelatin was hardened and the capsules sieved from the solution. The capsules are then coated over a field-carrying surface to provide an electrically switchable image.
U.S. Pat. No. 3,600,060 to Churchill et al. discloses another process for providing cholesteric liquid crystals in a polymer matrix. The patent discloses emulsifying droplets of liquid crystal in a solution having a dissolved film-forming polymer. The patent further discloses coatings or films having droplets of cholesteric liquid crystal material between 1 and 50 microns in diameter. Suitable binders mentioned in the paper include gelatin, gum arabic, and other water-soluble polymers. Churchill et al. disclose the emulsion can be coated on a substrate, e.g., by means of a draw down applicator to a wet thickness of about 10 mils and air dried at about 25° C. Churchill et al. state that the layers can be dried to touch. In Example 6, 60 grams of cholesteric liquid crystal material is disposed in 100 cubic centimeters of an aqueous polymer solution, polyvinyl alcohol or gelatin, and heated in a WARING blender to 70° C. by a heating jacket to form a desired emulsion, after which the emulsions were coated onto glass previously coated with tin oxide.
Another technique for providing liquid crystal domains in a coating is disclosed in U.S. Pat. No. 4,673,255. A resin polymer is dissolved into a liquid crystal. The resulting solution is induced into a cavity between two conductors. The resin polymer phase separated from the liquid crystal to form microdroplets of the liquid crystal in a polymeric matrix. The phase separation can be thermally induced, solvent induced or polymerization induced to create domains of liquid crystal. These processes, however, require lengthy periods of time to polymerize and phase separate the polymer binder. Organic solvents used in such systems are of environmental concern and are sensitive to processing conditions.
U.S. Pat. No. 6,061,107 reiterates the phase separation technique to form polymer dispersed liquid crystals found in U.S. Pat. No. 4,673,255. The patent discloses that controlling the shape of domains of liquid crystal material in a polymer binder can improve light scattering properties. The patent discloses the use of temperature, solvent and polymer induced phase separation techniques to provide flattened domains of liquid crystal. Such processes are disadvantageous and often require lengthy time periods. When solvents are used, there may be significant environmental concerns.
Published application EP 1 116 771 A2 to Stephenson et al. discloses, in one embodiment, dispersing a liquid crystal material in an aqueous bath containing a water-soluble binder material such as gelatin, along with a quantity of colloidal particles wherein the colloidal particles limit coalescence. The limited coalescent materials were coated over a substrate and dried, wherein the coated material formed a set of uniform limited-coalescence domains having a plurality of electrically responsive optical states.
All of the above-mentioned processes require the manufacture of a liquid crystal display in individual units or on non-flexible substrates or in an environmentally unfriendly manner. This results in liquid crystal displays that are difficult to manufacture or insufficiently economical for wide spread use. There is a need, therefore, for an improved method for making a liquid crystal display or other electro-optical display involving the coating of a dispersed electro-optical fluid on a substrate.
SUMMARY OF THE INVENTION
Accordingly, the need is met according to the present invention by providing a method of making a display having a polymer-dispersed electro-optical fluid, which method includes the steps of:
(a) providing an emulsion comprising an electro-optical fluid, having a plurality of optical states responsive to electric fields, dispersed in a gelatin solution;
(b) heating the emulsion to reduce the viscosity of the emulsion;
(c) forming a coating of the heated emulsion on a substrate having a field carrying layer on its surface;
(d) lowering the temperature of the coating to change the state of the dispersing phase in the coating from a liquid to a gel state characterized by a corresponding increased-viscosity state; and
(e) drying the coating, while maintaining it in the increased-viscosity state, to form a coating in which domains of said electro-optical fluid are dispersed in a dried gelatin-containing matrix.
The electro-optical fluid can be a liquid crystal or an electrophoretic material. In a particularly preferred embodiment, the invention relates to the coating of polymer-dispersed liquid crystals comprising the steps of:
(a) providing an emulsion having cholesteric liquid crystal material in a gelatin solution;
(b) heating the emulsion to reduce the viscosity of the emulsion
(c) coating the heated emulsion on a substrate;
(d) lowering the temperature of the coated emulsion to change the state of the coated emulsion from a liquid to a gel state, thereby forming a coating characterized by a corresponding increased-viscosity state; and
(e) drying the coating, while maintaining it in the high viscosity-state, to form a coating in which domains of polymer dispersed cholesteric liquid crystals are dispersed in a gelatin-containing matrix.
In a preferred embodiment of the invention, the coating is dried, and water is removed, at a rate sufficiently fast to form domains that are flattened to improve reflectance in the reflective optical state of a display in which it is used. Flexible sheets can be made efficiently in accordance with the present invention. High speed, environmentally friendly processes are used. By changing the field applied across the layer, information can be written in the sheet.
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Rankin Charles M.
Stephenson Stanley W.
Eastman Kodak Company
Kim Richard
Konkol Chris P.
Ton Toan
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