Patent Publication Number: US-6707517-B2

Title: Transparent field spreading layer for dispersed liquid crystal coatings

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly assigned U.S. patent application Ser. No. 09/146,656, filed Sep. 3, 1998 by Stanley W. Stephenson et al.; U.S. patent application Ser. No. 09/336,93, filed Sep. 14, 2001 by Stanley W. Stephenson; U.S. patent application Ser. No. 09/764,015, filed Jan. 17, 2001 by Stanley W. Stephenson; and U.S. patent application Ser. No. 10/036,149, filed concurrently herewith entitled “Field Spreading Layer for Dispersed Liquid Crystal Coatings” by Stanley W. Stephenson; the disclosures of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a display sheet that can change states to provide a viewable image. 
     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 written 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 can 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 is 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 operates on the liquid crystal material to expose display areas. The display uses nematic liquid crystal material that ceases to present an image when de-energized. 
     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 a layer of encapsulated cholesteric liquid crystal responsive to an electric field. The conductors in the patent can be transparent or non-transparent and formed of various metals or graphite. It is disclosed that one conductor must be light absorbing and it is suggested that the light absorbing conductor be prepared from paints containing conductive material such as carbon. 
     U.S. Pat. No. 5,289,301 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 conductor. 
     Current state of the art discloses the need for a second conductor over a polymer dispersed liquid crystal material. In particular, cholesteric materials require one of the two conductors to be light absorbing and conductive. Materials have been proposed for the application including carbon or metal oxides to create a black and conductive surface for polymer dispersed cholesteric liquid crystal materials. Because there is inactive material between the conductors, it would be desirable to maximize the use of the inactive material. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method of increasing the active area driven by two crossing electrodes. 
     This object is achieved in a display sheet having polymer dispersed liquid crystals, comprising: 
     a) a substrate; 
     b) a state changing layer disposed over the substrate and defining first and second surfaces, such state changing layer having the polymer dispersed liquid crystals having first and second optical states, which can change state; 
     c) a first conductor disposed over the first surface of the state changing layer; 
     d) a second conductor on the second surface of the state changing layer so that when a field is applied between the first and second conductors, the liquid crystals change state; and 
     e) a nonconductive, field spreading layer having a transparent electrically conductive polymer dispersed sub-micron particles disposed between the state changing layer and the first conductor to provide a change of state in the state changing layer outside of areas between both conductors in response to a field applied between the first and second conductors which changes the state of the liquid crystals. 
     The present invention uses a transparent field spreading layer to improve the active areas driven by crossed electrodes. The structure of the transparent field spreading layer minimizes additional voltage required for a thicker active materials coating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a sectional view of a sheet having a polymer dispersed cholesteric liquid crystal in accordance with prior art; 
     FIG. 1B is a sectional view of a sheet having a polymer dispersed cholesteric liquid crystal in accordance with the present invention; 
     FIG. 2 is a sectional view of a domain of cholesteric liquid crystal in a polymer matrix; 
     FIG. 3 is a view of the optical characteristics of cholesteric liquid crystal in each of two stable states; 
     FIG. 4A is a sectional view of a sheet coated with a polymer dispersed cholesteric liquid crystal and a transparent field spreading layer in accordance with the present invention; 
     FIG. 4B is as sectional view of the sheet of FIG. 4A receiving an evaporative coating; 
     FIG. 4C is a sectional view of the sheet of FIG. 4B being laser etched; 
     FIG. 5A is a sectional view light passing through the sheet of FIG. 1A in accordance with prior art; 
     FIG. 5B is a sectional view light passing through the sheet of FIG. 1B in accordance with the current invention; 
     FIG. 6 is the spectral reflection of sheets in accordance with FIG.  1 A and FIG. 1B; 
     FIG. 7A is as sectional view of light passing through the sheet in FIG. 1A; 
     FIG. 7B is as sectional view of light passing through the sheet in FIG. 1B; 
     FIG. 8A is a top view of a written sheet in FIG. 1A; and 
     FIG. 8B is a top view of a written sheet in FIG.  1 B. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1A is a sectional view of a display sheet  10  having a polymer dispersed cholesteric liquid crystal made in accordance prior art. 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. Alternatively, substrate  15  can be a glass sheet. 
     First conductors  20  are formed over substrate  15 . First conductors  20  can be Tin-Oxide or Indium-Tin-Oxide (ITO), with ITO being the preferred material. Typically the ITO comprising first conductors  20  is sputtered as a layer over substrate  15  to form a layer having a sheet resistance of less than 250 ohms per square. First conductors  20  can be patterned by conventional lithographic or laser etching means. 
     A state-changing layer is formed by coating a polymer dispersed cholesteric layer  30  onto first patterned conductors  20 . The polymer dispersed cholesteric layer  30  defines first and second surfaces. Cholesteric materials can be created that have peak reflectance from the infrared through the visible spectrum by varying the concentration of chiral dopant in a nematic liquid crystal. Application of electrical fields of various intensities and duration can drive a chiral nematic material (cholesteric) into a reflective state, a transmissive state or an intermediate state. These materials have the advantage of maintaining a given state indefinitely after the field is removed. Such materials can be cholesteric liquid crystal materials such as Merck BL112, BL118 or BL126, available from EM Industries of Hawthorne, N.Y. 
     FIG. 2 shows a portion of a polymer dispersed cholesteric layer  30 , which can be cholesteric material dispersed in deionized photographic gelatin. A liquid crystal material can be dispersed at 8% concentration in a 5% deionized gelatin aqueous solution. It has been found that 10 micron diameter domains of the cholesteric liquid crystal in aqueous suspension optimize the electrooptical properties of the cholesteric material. The first surface of polymer dispersed cholesteric layer  30  is coated over first conductors  20  to provide a 10 micron thick polymer dispersed cholesteric coating. Other organic binders such as polyvinyl alcohol (PVA) or polyethylene oxide (PEO) can be used as the polymeric agent. Such compounds are can be coated on equipment associated with photographic films. 
     FIG. 3 shows two stable states of cholesteric liquid crystals. On the left, a high voltage field has been applied and quickly switched to zero potential, which converts cholesteric liquid crystal to planar liquid crystal  50 . Portions of incident light  54  striking planar liquid crystal  50  becomes reflected light  56  to create a bright image. On the right, application of a lower voltage field converts cholesteric liquid crystal to a transparent focal conic liquid crystal  52 . Incident light  54  striking focal conic liquid crystal  52  is transmitted. A light absorber  58  will absorb incident light  54  to create a dark image in areas having focal conic liquid crystal  52 . As a result, a viewer perceives an image having bright and dark areas depending on if the cholesteric material is planar liquid crystal  50  or focal conic liquid crystal  52 , respectively. A sheet  10  having polymer dispersed cholesteric layer  30  must have a transparent conductor and one light absorbing conductor. In the first exemplary embodiment, first conductor  20  is transparent ITO. 
     In FIG. 1A, a second conductors  40  opposed to the first conductors  20  need to be light absorbing to act as light absorber  58 . Second conductors  40  should have sufficient conductivity to carry an electric field across polymer dispersed cholesteric layer  30 . Second conductors  40  have been characterized by prior art by being a conductive material such as aluminum, tin, silver, platinum, carbon, tungsten, molybdenum, tin or indium or combinations thereof. It is also well known that oxides of many of these metals are light absorbing to provide light absorber  58 . The prior art teaches that second conductors  40  can be printed conductors. First conductors  20  and second conductors  40  can be a pattern of orthogonal conductors forming an addressable matrix of pixels. In a preferred embodiment, the first conductor is transparent and includes indium-tin-oxide and the second conductor is substantially opaque. Alternatively, the first conductor can be opaque and the second conductor transparent. 
     FIG. 1B is a sectional view of a display sheet having polymer dispersed cholesteric layer  30  in accordance with the present invention. A transparent field spreading layer  34  is disposed between first conductors  20  and polymer dispersed cholesteric layer  30 . The transparent organic conductor can be Baytron B polythiophene suspension from Agfa-Gevaert N.V. of Morsel, Belgium. For an experimental coating, field spreading layer  32  can be 1.0 weight percent deionized gelatin and 1.0 weight percent sub-micron (nanoparticle) polythiophene coated over first conductors  20  at a 25 micron wet thickness. The dried transparent field spreading layer  34  will be approximately 0.4 microns thick. The resulting transparent field spreading layer  34  is functionally transparent and has a sheet resistance of over one mega-ohm. The resulting coating is functionally nonconductive compared to adjacent first conductors  20 . Transparent field spreading layer  34  can activate cholesteric liquid crystal material past the edge of a field carrying electrode. In making sheet  10 , the sheet can be in the form of a web that is sequentially moved through one or more stations which sequentially or simultaneously deposits the state changing layer  30  or transparent field spreading layer  32 . 
     Second conductors  40  overlay polymer dispersed cholesteric layer  30 . Second conductor  40  has sufficient conductivity to support a field across polymer dispersed cholesteric layer  30 . Second conductor  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 conductor  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 conductor  40  to be transparent. 
     Alternatively, second conductor  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. Printed conductors require at least 125 microns between adjacent conductive conductors. Material between conductors is typically inactive. Printed conductors are applicable to coarse displays having large intra-conductor spacing, such as matrix displays with a pitch of over 1 millimeter, and vacuum evaporated metals are most applicable to sub-millimeter pitch displays. Field spreading layer  32  is useful in applications using either etched evaporated metal or printed second conductors  40 . 
     The voltage required to change the optical state the polymer dispersed cholesteric layer  30  is proportional to the distance between the opposing conductors. Polymer dispersed cholesteric layer  30  must be at least 4 microns thick to have high reflectivity. The disclosed transparent field spreading layer  32  transmits an applied voltage sufficiently so that the thickness of transparent field spreading layer  32  does not require substantial increases in drive voltages. 
     FIG. 4A is a sectional view of a sheet coated with a polymer dispersed cholesteric liquid crystal and a transparent field spreading layer in accordance with the present invention. Polymer dispersed cholesteric layer  30  is codeposited in accordance with the exemplary embodiment over transparent field spreading layer  32 . The cholesteric material has a peak reflectance of 550 nanometers. In FIG. 4B, vacuum evaporated chrome D 1  is deposited over the polymer dispersed cholesteric layer  30  as evaporated metal  38 . FIG. 4C is a sectional view of evaporated metal  38  being etched using a YAG laser having a wavelength of 1064 nanometers to create second conductors  40 . Laser energy hυ is used to etch second conductors  40  without penetrating through polymer dispersed cholesteric liquid crystal  30  and vaporizing first conductor  20 . Alternatively, second conductors  40  can be conductive material screen printed over polymer dispersed cholesteric liquid crystal  30 . 
     FIG. 5A is a sectional view of light passing through the sheet of FIG.  1 A. Incident light  54  passing through polymer dispersed cholesteric layer  30  in the focal conic state is nominally absorbed by second conductors  40 . Areas between first conductors  20  are inactive areas  70 . After coating, inactive areas  70  are in an inactive, semi-reflecting state and some incident light  54  is reflected back as back scatter light  57  when polymer dispersed cholesteric layer  30  is in the focal conic state. Back scatter light  57  reduces light absorbency, creating a gray image instead of a black image. Therefore it is highly desirable to make material in inactive area  70  responsive to fields applied to adjacent first conductors  20 . FIG. 5B is a sectional view of a sheet  10  in accordance with the present invention. The presence of transparent field spreading layer  32  causes material between adjacent first conductors  20  to become active areas  72  which eliminates back scatter light  57 . 
     FIG. 6 is a plot of light reflected from planar reflection  60 , prior art focal conic reflection  62  and improved focal conic reflection  64 . Back scatter light  57  creates a lighter, gray reflection for prior art focal conic reflection  62 . The elimination of back scatter light  57  in sheet  10  with a transparent field spreading layer  32  lowers the darkness of sheet  10  and improves contrast ratio between the planar reflection  60  and improved focal conic reflection  64 . 
     FIG. 7A is a sectional view of an experimental sheet  10  having green reflective cholesteric liquid crystal of conventional design. A high voltage pulse has been applied to the first three of first conductors  20  to convert cholesteric material the planar, reflective state  50 . A low voltage pulse has been applied to the third first conductor  20  to convert cholesteric material to transparent focal conic liquid crystals  52 . It was observed that cholesteric material not having both conductors was inactive material  70 . Inactive material  70  provides a constant back scatter light  57  regardless of the state of electrically active pixels. 
     FIG. 7B is the same sheet in FIG. 7A having field spreading layer  32 . The cholesteric material between adjacent second conductors  40  having a common potential becomes active material  72 . Active area  72  nominally spreads out a millimeter. In the case of a matrix display, adjacent second conductors  40  with different electrical potential limits the spread of the electrical field present on adjacent conductors. Typically, the field spreads half-way between conductors of varying potential. 
     FIG. 8A is a top view of a sheet  10  without a transparent field spreading layer  32 . Four pixels are written into the focal conic state. Inactive material  70  exists horizontally and vertically in areas not covered by both first conductors  20  and second conductors  40 . FIG. 8B is a top view of a display  10  having a transparent field spreading layer  32  disposed between polymer dispersed cholesteric layer  30  and vertical first conductors  20 . Active material  72  exists between adjacent first conductors  20  at a common potential half way between adjacent second conductors  40  at different potential. The resulting sheet  10  has improved contrast. 
     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 
       20  first conductors 
       30  polymer dispersed cholesteric layer 
       32  transparent field spreading layer 
       38  evaporated metal 
       40  second conductors 
       50  planar liquid crystals 
       52  focal conic liquid crystals 
       54  incident light 
       56  reflected light 
       57  back scatter light 
       58  light absorber 
       60  planar reflection 
       62  prior art focal conic reflection 
       64  improved focal conic reflection 
       70  inactive material 
       72  active material 
     D 1  vacuum evaporated chrome 
     hυ laser energy