Abstract:
A method of making a liquid crystal display, comprising the steps of: providing a substrate; providing a first electrode over the substrate; coating the first electrode with aqueous dispersed material which when dried provides a dielectric layer over the first electrode; coating the dielectric layer with liquid crystal bearing material and drying such liquid crystal bearing material and providing a second electrode in contact with the dried liquid crystal bearing material.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly assigned U.S. patent application Ser. No. 09/379,776, filed Aug. 24, 1999 entitled “Forming A Display Having Conductive Image Areas Over A Light Modulating Layer” by Dwight J. Petruchik et al., U.S. patent application Ser. No. 09/723,389, filed Nov. 28, 2000, entitled “Unipolar Drive for Cholesteric Liquid Crystal Displays” by David M. Johnson et al., U.S. patent application Ser. No. 09/915,831, filed Jul. 26, 2001, entitled “Method of Making Liquid Crystal Display Having a Dielectric Adhesive Layer for Laminating a Liquid Crystal Layer” by Smith et al and U.S. patent application Ser. No. 09/915,614, filed Jul. 26, 2001, entitled “Making a Liquid Crystal Display Using Heat and Pressure Lamination of Liquid Crystal Coating” by Smith et al, the disclosures of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to providing a dielectric layer for polymer dispersed liquid crystal displays. 
     BACKGROUND OF THE INVENTION 
     Currently, information is displayed using assembled sheets of paper carrying 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. 
     A structure is disclosed in PCT/WO 97/04398, entitled “Electronic Book With Multiple Display Pages” which is a thorough recitation of the art of thin, electronically written display technologies. Disclosed is the assembling of multiple display sheets that are bound into a “book”, each sheet is arranged to be individually addressed. 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. 
     Fabrication of flexible, electronically written display sheets are disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent ITO conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be glass, but in practice have been formed of Mylar polyester. A dispersion of liquid crystal material in a binder is coated on the first sheet, and the second sheet is bonded to the liquid crystal material. Electrical potential applied to opposing conductive areas operate on the liquid crystal material to expose display areas. The display uses nematic liquid crystal material which ceases to present an image when de-energized. 
     U.S. Pat. No. 5,223,959 discloses a plurality of polymer dispersed liquid crystal material, each having a different dye material of red, green or blue dye material. Differing electrical signals to common electrodes operate on each of the materials to control the state of each type of dyed liquid crystal material. The patent requires the use of conventional nematic liquid crystals with a dye to absorb light. The droplets are chemically treated to be stable in either a clear or a light absorbing state. The invention also requires materials having different response times to electrical signals. The device must be continually driven so that the human eye perceives complementary colors. This arrangement has the disadvantage of requiring continuous, high speed electrical drive because the materials do not maintain their state. The material must be driven to achieve a neutral color density. 
     U.S. Pat. No. 5,437,811 discloses a light-modulating cell having a polymer dispersed chiral nematic liquid crystal. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal-conic state. Said structure has the capacity of maintaining one of the given states in the absence of an electric field. 
     U.S. Pat. No. 3,816,786 discloses droplets of cholesteric liquid crystal in a polymer matrix responsive to an electric field. The electrodes in the patent can be transparent or non-transparent and formed of various metals or graphite. It is disclosed that one electrode must be light absorbing and it is suggested that the light absorbing electrode be prepared from paints contains conductive material such as carbon. 
     U.S. Pat. No. 5,289,300 discusses forming a conductive layer over a liquid crystal coating to form a second conductor. The description of the preferred embodiment discloses Indium-Tin-Oxide (ITO) over a liquid crystal dispersion to create a transparent electrode. 
     Prior art discloses the use of dielectric barrier layers formed over ITO conductors. The dielectric layer protects the ITO transparent conductor from damage from electrochemical interaction with the light modulating material. The protective layers are typically formed by vacuum sputtering silicon dioxide over the ITO conductors. The vacuum forming process is slow and expensive. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a highly effective dielectric coating over electrodes used in polymer dispersed liquid crystal displays. 
     This object is achieved in a method of making a liquid crystal display, comprising the steps of: 
     (a) providing a substrate; 
     (b) providing a first electrode over the substrate; 
     (c) coating the first electrode with aqueous dispersed material which when dried provides a dielectric layer over the first electrode; 
     (d) coating the dielectric layer with liquid crystal bearing material and drying such liquid crystal bearing material; and 
     (e) providing a second electrode in contact with the dried liquid crystal bearing material. 
     The invention provides an inexpensive dielectric layer between field carrying electrodes in displays that are aqueous coated. Such dielectric layers provide a good bond between the aqueous suspension and the ITO on a flexible substrate. 
     The present invention provides a dielectric layer over a conductive layer using simple, inexpensive aqueous coatings. Such coatings permit the fabrication of electronic privacy screens having long life and durability. The aqueous dielectric coating significantly improves yields of cholesteric memory displays having electrical electrodes applied over a polymer dispersed liquid crystal layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a sheet having a coated liquid crystal in accordance with the present invention; 
     FIG. 2 is a plot of a distribution of domain size for aqueous dispersed liquid crystal; 
     FIG. 3A is a sectional view a sheet having a coated emulsion before drying; 
     FIG. 3B is a sectional view of a sheet having a coated emulsion after drying; 
     FIG. 4A is a sectional view of a nematic liquid crystal without an applied electric field; 
     FIG. 4B is a sectional view of a nematic liquid crystal with an applied electric field; 
     FIG. 5A is a display sheet having a laminated second electrode in accordance with prior art; 
     FIG. 5B is a display sheet having a laminated second electrode in accordance with a first embodiment of this invention; 
     FIG. 5C is a display sheet having a laminated second electrode in accordance with a second embodiment of this invention; 
     FIG. 5D is a display sheet having a laminated second electrode in accordance with a third embodiment of this invention; 
     FIG. 6A is a view of the optical characteristics of a chiral nematic material in a planar state reflecting light; 
     FIG. 6B is a view of the optical characteristics of a chiral nematic material in a focal-conic light diffusing state; 
     FIG. 7 is a plot of the response of a cholesteric to an electrical field of varying strength; 
     FIG. 8A is a display sheet having a coated second electrode in accordance with prior art; and 
     FIG. 8B is a display sheet having a coated second electrode in accordance with this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is an isometric partial view of a new structure for a medium shown as a sheet  10  made in accordance with the invention. It will be understood that other forms of media such as a more permanent display can also be used in accordance with the present invention. Sheet  10  includes a flexible substrate  15 , which is a thin transparent polymeric material, such as Kodak Estar film base formed of polyester plastic that has a thickness of between 20 and 200 microns. In an exemplary embodiment, substrate  15  can be a 125-micron thick sheet of polyester film base. Other polymers, such as transparent polycarbonate, can also be used. 
     First electrode  20  is formed over substrate  15 . First electrode  20  can be Tin-Oxide or Indium-Tin-Oxide (ITO), with ITO being the preferred material. Typically the material of first electrode  20  is sputtered as a layer over substrate  15  having a resistance of less than 250 ohms per square. In certain applications, the sputtered layer is patterned in any well known manner. Alternatively, first electrode  20  can be an opaque electrical electrode material such as copper, aluminum or nickel. If first electrode  20  is an opaque metal, the metal can be a metal oxide to create light absorbing first electrode  20 . First electrode  20  can be patterned by conventional lithographic or laser etching means. 
     A light modulating layer  30  which preferably is a polymer dispersed liquid crystal layer overlays first patterned electrodes  20 . In a first case, the liquid crystal material is a nematic liquid crystal. Cholesteric liquid crystal materials can be Merck BL12, BL48, available from EM Industries of Hawthorne, N.Y. Such materials have high anisotropy indices of diffraction, which can act as a light diffusing surface in the absence of an electric field and as a transparent sheet  10  in the presence of an electric field. 
     In a second case the liquid crystal is a cholesteric liquid crystal, having peak reflection from the infrared through the visible spectrum. Application of electrical fields of various intensities and duration can drive a chiral nematic material (cholesteric) into a reflective, a transmissive state or an intermediate state. These materials have the advantage of maintaining a given state indefinitely after the field is removed. Cholesteric liquid crystal materials can be Merck BL112, BL118 or BL126, available from EM Industries of Hawthorne, N.Y. 
     Second electrode  40  is formed over light modulating layer  30 . Second electrode  40  should have sufficient conductivity to carry a field light modulating layer  30 . Second electrode  40  can be formed in a vacuum environment using materials such as Aluminum, Tin, Silver, Platinum, carbon, Tungsten, Molybdenum, Tin or Indium or combinations thereof. Oxides of said metals can be used provide a dark second electrode  40 . The metal material can be excited by energy from resistance heating, cathode arc, electron beam, sputtering or magnetron excitation. Tin-Oxide or Indium-Tin Oxide coatings permit second electrode  40  to be transparent. 
     Alternatively, second electrode  40  can be printed conductive ink such as Electrodag 423SS screen printable electrical conductive material from Acheson Corporation. Such printed materials are finely divided graphite particles in a thermoplastic resin. In the preferred embodiment, second electrode  40  is formed using printable ink electrodes to produce a low cost display. The use of a flexible support for substrate  15 , laser etching to pattern first electrode  20 , machine coating light modulating layer  30  and printing second electrode  40  permits the fabrication of very low cost display sheets having memory. 
     The dispersion of liquid crystals in aqueous suspension is done in any conventional manner. One method is to disperse liquid crystal oils in deionized water containing dissolved gelatin. Other water soluble binders such as polyvinyl alcohol (PVA) or polyethylene oxide (PEO) can be used. Such compounds are machine coatable on equipment associated with photographic films. FIG. 2 is a plot of the dispersions of domain size for a liquid crystal oil in aqueous suspension. The oil domains have a size distribution around a mean diameter. A certain number are above a certain diameter D, and are called oversized domains  31 . 
     FIG. 3A is a section view of a typical liquid crystal oil dispersed in water coated over first electrode  20  and containing oversized liquid crystal oil domains  31 . Such coatings are dried to remove water from the suspension. FIG. 3B is a section view of the dried coating. The liquid crystal material is encapsulated by the water-soluble binder to create a pressure light modulating layer  30 . Oversize oil domains  31  can be significantly larger in diameter than the dry thickness of polymer dispersed liquid crystal layer  30 . Oversized oil domains  31  create coating defects  33  in the dried light modulating layer  30 . 
     FIG. 4A is a sectional view of a first, privacy light modulating layer  30 , which is a nematic liquid crystal material having high optical anisotropy. It has been found that 2-micron diameter domains of the liquid crystal in aqueous suspension converts incident light  54  into scattered light  58  in the absence of an electric field. In this case, polymer dispersed liquid crystal layer  30  within sheet  10  can be used as a privacy screen. The material is further provided with first electrode  20  and second electrode  40  on either side of polymer dispersed liquid crystal layer  30  so that an electrical field can be applied across the material. FIG. 4B is a sectional view of polymer dispersed liquid crystal layer  30  with an electrical field applied. Liquid crystal material within each domain is aligned by the electrical field, and sheet  10  will becomes transparent. Electrically switching between the light scattering and transparent state using an electric field provides an electrically switched privacy screen. 
     FIG. 5A is a sectional view of a privacy screen sheet  10  built in accordance with prior art. A second substrate  16 , having second electrode  40  is bonded to a substrate  15  having a first electrode  20  and a light modulating layer  30 . One method of bonding the two sheets of the privacy screens is to provide heat and pressure to bond second electrode  40  to light modulating layer  30 . Coating defect  33  creates an air filled cavity in sheet  10 . When sheets  10 , formulated for privacy screen window application, are manufactured and a field is applied, liquid crystals in light modulating layer  30  begins to permanently align in the transparent state, even in the absence of a field. 
     FIG. 5B is a sectional view of a sheet  10  built in accordance with the current invention. A protective layer  35  is aqueous coated over first electrode  20 . The dielectric layer was created by coating a 1.3% deionized gelatin solution at a rate of 0.38 cc per square meter. The resulting coating was about 0.5 microns thick. An emulsion of high anisotropy liquid crystal in a gel-water solution was coated over an ITO coated sheet of polyester. A second polyester sheet, also having an ITO coated surface was heat bonded over the dried light modulating layer  30 . The liquid crystal material in experimental sheet  10  did not begin to align in the direction of the electrical field after several weeks of application of an electrical field. It is believed that the gelatin dielectric layer acts to prevent alignment of the liquid crystal material with the gelatin encapsulated domain. 
     Dielectric protective layers can be built in to sheet  10  in a variety of ways. FIG. 5C is a sectional view of sheet  10  fabricated by coating a solution of gelatin over second electrode  40  to create protective layer  35  instead of between first electrode  20  and light modulating layer  30 . FIG. 5D is a sectional view of third embodiment. Protective layers  35  are coated over both first electrode  20  and second electrode  40  before assembly to provide a dielectric layer between both electrodes and light modulating layer  30 . These configurations are effective in preventing liquid crystal material in polymer dispersion from taking permanent alignment to long term electrical fields. 
     FIG.  6 A and FIG. 6B show two stable states of cholesteric liquid crystals. In FIG. 6A, a high voltage field has been applied and quickly switched to zero potential, which converts cholesteric liquid crystal to a planar state  50 . Incident light  54  striking cholesteric liquid crystal in planar state  50  is reflected as reflected light  56  to create a bright image. In FIG. 6B, application of a lower voltage field pulse leaves cholesteric liquid crystals in a transparent focal conic state  52 . Incident light  54  passing through a cholesteric liquid crystal in focal conic state  52  is transmitted. Second patterned electrodes  40  can be black which will absorb incident light  54  to create a dark image when the liquid crystal material is in focal conic state  52 . As a result, a viewer perceives a bright or dark image depending on if the cholesteric material is in planar state  50  or focal conic state  52 , respectively. 
     FIG. 7 is a plot of the response of a cholesteric material to a pulsed electrical field. Such curves can be found in U.S. Pat. Nos. 5,453,863 and 5,695,682. For a given pulse time, typically between 5 and 200 milliseconds, a pulse at a given voltage can change the optical state of a cholesteric liquid crystal. Voltage below disturbance voltage V 1  can be applied without changing the state of the cholesteric material. A higher voltage pulse at a focal-conic voltage V 2  will force a cholesteric material into the focal conic state  52 . A voltage pulse at planar voltage V 3  will force the cholesteric material into the planar state  50 . The curve characteristic of cholesteric liquid crystal permits passive matrix writing of cholesteric displays. 
     FIG. 8A is a sectional view of a coated cholesteric display sheet  10 . Substrate  15  supports a plurality of first electrodes  20 . A cholesteric liquid is has been dispersed in a gelatin solution, coated and dried to create light modulating layer  30  over first electrodes  20 . Second electrodes  40  are printed over light modulating layer  30  to provide a black, electrically electrodes that can be selectively energized to apply fields to the cholesteric liquid crystal in light modulating layer  30 . Coating defects  33  in light modulating layer  30  causes electrical shorting between first electrodes  20  and second electrodes  40 . Material adjacent to coating defect  33  cannot be switched between planar state  50  and focal-conic state  52 . 
     FIG. 8B is a sectional view of an experimental sheet  10  formed in accordance with the present invention. A protective layer  35  is aqueous coated and dried over first electrode  20  prior to application of the aqueous light modulating layer  30 . The dielectric layer was created by coating a 1.3% deionized gelatin solution at a rate of 0.38 cc per square meter. The resulting coating was about 0.5 microns thick. Sheet  10  assembled, incorporating protective layer  35  and electrically tested. The 0.5 micron thick protective layer was effective in preventing image defects due to coating and provides effective insulation between electrodes used in changing the state of cholesteric liquid crystals. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
       10  sheet 
       15  substrate 
       16  second substrate 
       20  first electrode 
       30  light modulating layer 
       31  oversized domains 
       33  coating defect 
       35  protective layer 
       40  second electrode 
       50  planar liquid crystals 
       52  focal-conic liquid crystals 
       54  incident light 
       56  reflected light 
       58  scattered light 
     V 1  disturbance voltage 
     V 2  focal-conic voltage 
     V 3  planar voltage