U.S. patent application number 10/445761 was filed with the patent office on 2005-01-06 for polymer-dispersed liquid-crystal display comprising an ultraviolet blocking layer and methods for making the same.
Invention is credited to Brick, Mary C., Stephenson, Stanley W..
Application Number | 20050001954 10/445761 |
Document ID | / |
Family ID | 33489377 |
Filed Date | 2005-01-06 |
United States Patent
Application |
20050001954 |
Kind Code |
A1 |
Stephenson, Stanley W. ; et
al. |
January 6, 2005 |
POLYMER-DISPERSED LIQUID-CRYSTAL DISPLAY COMPRISING AN ULTRAVIOLET
BLOCKING LAYER AND METHODS FOR MAKING THE SAME
Abstract
A display comprising a substrate, transparent first conductors,
second conductors, a layer of polymer-dispersed liquid-crystal
material disposed between the first and second conductors, and at
least one layer of ultraviolet blocking material in a binder
disposed to block ultraviolet radiation from striking said
polymer-dispersed liquid-crystal material is disclosed. Also
disclosed is a method for making such a display.
Inventors: |
Stephenson, Stanley W.;
(Spencerport, NY) ; Brick, Mary C.; (Webster,
NY) |
Correspondence
Address: |
Paul A. Leipold
Patent Legal Staff
Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Family ID: |
33489377 |
Appl. No.: |
10/445761 |
Filed: |
May 27, 2003 |
Current U.S.
Class: |
349/86 |
Current CPC
Class: |
G02F 1/1334 20130101;
G02F 1/133345 20130101; G02F 1/13718 20130101; G02F 2201/086
20130101 |
Class at
Publication: |
349/086 |
International
Class: |
G02F 001/1333 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 16, 2001 |
EP |
01200984.1 |
Claims
What is claimed is:
1. A display comprising: a) a substrate; b) transparent first
conductors; c) second conductors; d) a layer comprising
polymer-dispersed liquid-crystal material disposed between the
first and second conductors; e) at least one barrier layer
comprising a polymeric binder and water-dispersible or
water-soluble ultraviolet blocking material, said barrier layer
disposed to block ultraviolet radiation from striking said
polymer-dispersed liquid-crystal material.
2. The display of claim 1 wherein the ultraviolet blocking material
comprises a dissolved compound and/or dispersed particles.
3. The display of claim 1 wherein the ultraviolet blocking material
is in the form of dispersed particles comprising a UV-absorbing
organic compound, and wherein said polymeric binder is gelatin.
4. The display of claim 3 wherein the UV-absorbing organic compound
is dissolved in an oil that is dispersed in said gelatin.
5. The display of claim 3 wherein the dispersed particle
essentially consists of the UV-absorbing organic compound which
forms a discrete hydrophobic phase in the barrier layer.
6. The display of claim 2 wherein the ultraviolet blocking material
comprises particles selected from the group consisting of
TiO.sub.2, ZnO, iron oxide, and combinations thereof.
7. The display of claim 1 wherein the barrier layer is less than or
about 1 micrometer thick.
8. The display of claim 7 wherein the ultraviolet blocking material
comprises particles having an average diameter less than the
thickness of the barrier layer.
9. The display of claim 1 wherein the barrier layer is also an
insulating material separating said first conductors.
10. A display comprising; (a) a flexible transparent support; b) a
patterned first conductor layer comprising transparent first
conductors; c) a patterned second conductor layer comprising second
optionally transparent conductors; d) an imaging layer comprising
polymer-dispersed liquid-crystal material disposed between the
first and second conductors; and e) at least one barrier layer,
less than about one micrometer thick, disposed between the two
conductor layers, said barrier layer comprising water-dispersible
or water-soluble ultraviolet blocking material with a hydrophilic
polymeric binder, said barrier layer disposed to block ultraviolet
radiation from striking said polymer-dispersed liquid-crystal
material.
11. The display of claim 10 wherein the barrier layer is less than
1 micrometer thick.
12. The display of claim 10 wherein the barrier layer is less than
0.7 micrometer thick.
13. The display of claim 10 wherein the barrier layer is disposed
between the first conductor layer and the imaging layer.
14. The display of claim 10 wherein the barrier layer comprises
gelatin as the binder.
15. A method for fabricating a display comprising: a) providing a
substrate; b) forming transparent first conductors over the
substrate; c) coating a layer of polymer-dispersed liquid-crystal
material disposed over the first conductors; d) forming at least
one barrier layer comprising water-dispersible or water-soluble
ultraviolet blocking material and a hydrophilic polymeric binder,
said barrier layer disposed to block ultraviolet radiation from
striking said polymer dispersed liquid crystal material; e) forming
second conductors over said polymer dispersed liquid crystal
material.
16. The method of claim 15 wherein the barrier layer is disposed
between the transparent first conductors and the layer of
polymer-dispersed liquid-crystal material.
17. The method of claim 16 wherein the barrier layer is also
incorporated within the transparent first conductor layer
separating conductors in said layer.
18. The method of claim 15 wherein the hydrophilic polymeric binder
is gelatin.
19. The method of claim 15 wherein the ultraviolet blocking
material comprises a dissolved organic compound and/or dispersed
organic or inorganic particles.
20. A method of fabricating a display comprising a) providing a
substrate; b) forming transparent first conductors; c) forming
second conductors; d) forming a layer comprising polymer-dispersed
liquid-crystal material disposed between the first and second
conductors; e) forming at least one barrier layer comprising
water-dispersible or water-soluble ultraviolet blocking material
and a hydrophilic polymeric binder, said barrier layer disposed to
block ultraviolet radiation from striking said polymer dispersed
liquid crystal material; f) subsequently etching the second
conductors with a ultraviolet laser beam while the ultraviolet
blocking material absorbs substantial ultraviolet radiation from
the ultraviolet laser beam before the radiation reaches the first
conductors.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] Reference is made to commonly assigned U.S. Pat. No.
6,394,870 filed Aug. 24, 1999 by Dwight J. Petruchik et al., and
U.S. patent application Ser. No. 09/723,389, filed Nov. 28, 2000 by
David M. Johnson et al., the disclosures of which are incorporated
herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to providing an ultraviolet
blocking layer for polymer-dispersed liquid-crystal displays.
BACKGROUND OF THE INVENTION
[0003] Currently, information is commonly 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. Magnetically written data, however, is not
visible.
[0004] 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 visibly written
display technologies. Disclosed is the assembling of multiple
display sheets that are bound into a "book," each sheet arranged to
be individually addressed. The patent recites prior art disclosing
thin, electronically written pages in the form of flexible sheets,
including image-modulating material formed from a bistable liquid
crystal system and thin metallic conductor lines on each page.
[0005] Fabrication of flexible, electronically written display
sheets are also 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.RTM.
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
selectively expose display areas. The display uses nematic
liquid-crystal material which ceases to present an image when
de-energized.
[0006] U.S. Pat. No. 5,437,811 discloses a light-modulating cell
having a polymer-dispersed chiral-nematic liquid crystal, also
referred to as polymer-dispersed cholesteric 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 either one of the given states in the
absence of an electric field.
[0007] U.S. Pat. No. 5,539,552 recites failures of liquid-crystal
displays due to ultraviolet light affecting the liquid-crystal
material, seals and thin films in displays. The patent discloses a
solution for these problems by incorporating a glass plate that is
compatible with the display, approximately 36 mils thick. The
resulting display blocks 99 percent of ultraviolet light energy
with wavelength less than 400.+-.10 nanometers. Such plates,
however, are inflexible, heavy and expensive.
[0008] It would be desirable to provide a method of providing a
polymer-dispersed cholesteric display with protection from
ultraviolet ("UV") radiation without requiring the presence of
glass plates. It would be desirable to provide UV protection for a
display having a thin flexible substrate.
[0009] It is known that fine particles of titanium dioxide in a
binder can provide a visually transparent ultraviolet blocking
layer that blocks ultraviolet radiation. U.S. Pat. No. 5,736,308
discloses a method in the photographic arts to produce titanium
dioxide particles that are transparent to visible light and
absorbent to UV light of a broad spectrum. Titanium dioxide
(TiO.sub.2) pigment having an average size from 0.02 to 0.1 microns
is formed, which exhibits good absorption from 290 to 400
nanometers while being functionally transparent in the visible
region. U.S. Pat. No. 5,736,308 discloses a multilayer photographic
film with a UV protective layer using fine titanium dioxide
particles. The TiO.sub.2 particles are dispersed in "gelatine"
(gelatin). A considerably thinner layer is achieved than with
customary organic UV blocker dyes that are dispersed in oil
droplets.
[0010] Another problem in the prior art relates to the manufacture
of displays made by coating layers of materials to form
polymer-dispersed liquid-crystal display sheets. Polymer-dispersed
liquid-crystal layers can be vacuum coated with conductive metal
which is then laser etched to form an image-forming electrode. The
use of laser etching can be problematic in that the UV laser that
produces etching in one layer of the structure has the potential to
damage the other layers, for example another conductor layer, in
the structure, typically an intermediate, on an assembly line,
formed in the manufacture of a display. This potential problem can
limit, render difficult, or even prevent the use of desired or
optimal laser-etching techniques
[0011] Thus, it would be useful and advantageous to impart UV
blocking, in this case blocking of a UV laser, into the
intermediate structures used to make display sheets. For example,
it would be advantageous to provide UV blocking to protect a first
conductors not intended to be etched during a laser etching process
involving a second or different conductor.
[0012] In view of the above, it would be desirable to either
protect a polymer-dispersed liquid-crystal display from UV
radiation during use and/or to protect a polymer-dispersed
liquid-crystal display, or intermediate thereof, from UV radiation
during manufacture.
SUMMARY OF THE INVENTION
[0013] It is the main object of this invention to protect
polymer-dispersed liquid-crystal displays from ultraviolet light
during use. In another aspect of this invention, it is possible to
protect the same or manufacturing intermediates thereof from
ultraviolet radiation from an ultraviolet laser used for etching of
conductors during manufacture. This invention can be used to
protect second conductors in the display from ultraviolet radiation
during laser etching of first conductors. It is possible for the
same means that provides a polymer-dispersed liquid-crystal display
with ultraviolet protection during use to also function to protect
the display, or manufacturing intermediate thereof, from
ultraviolet radiation during manufacture.
[0014] These objects are achieved by a display comprising:
[0015] a) a substrate;
[0016] b) transparent first conductors;
[0017] c) second conductors;
[0018] d) a layer comprising polymer-dispersed liquid-crystal
material disposed between the first and second conductors;
[0019] e) at least one barrier layer comprising a polymeric binder
and water-dispersible or water-soluble ultraviolet blocking
compounds or particles, which barrier layer is disposed to block
ultraviolet radiation from striking said polymer-dispersed
liquid-crystal material.
[0020] In one embodiment of a display according to the present
invention, the barrier layer comprises ultraviolet-blocking
inorganic particles in a binder. Fine particles such as titanium
dioxide and/or other inorganic ultraviolet-absorbing particles can
be incorporated in the barrier layer of the display. For example, a
low concentration of TiO.sub.2 particles can block UV radiation.
Alternatively, a ultraviolet-blocking dye can be used, preferably
contained in a discrete organic phase within a continuous aqueous
phase in the barrier layer. Such UV-blocking materials are
chemically and electrically inert. Thus, the barrier layer can be
located between the electrodes of the display without affecting
display performance.
[0021] The invention is also directed to a method of manufacturing
the above display.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a sectional view of a sheet having a coated
liquid-crystal material in accordance with the present
invention;
[0023] FIG. 2 is a sectional view showing a display with
cholesteric material in two stable optical states;
[0024] FIG. 3 is a plot of the response of a cholesteric to an
electrical field of varying strength;
[0025] FIG. 4 is a sectional view of the metallic layer of FIG. 7
being laser etched.
[0026] FIG. 5 shows a transmission spectroscopy of a polyester
support, the support coated with the organic near ultraviolet
blocking material, and the support coated with a titanium dioxide
coating.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0027] As indicated above, the present invention involves a display
comprising a substrate, transparent first conductors, second
conductors, a layer of polymer-dispersed liquid-crystal material
disposed between the first and second conductors, and at least one
layer comprising water-dispersible or water-soluble ultraviolet
blocking compounds or particles in a polymeric binder, which layer
is disposed to block ultraviolet radiation from striking said
polymer dispersed liquid crystal.
[0028] The ultraviolet-blocking materials that can be used in the
present invention are diverse so long as they are water-dispersible
or water-soluble and can absorb or otherwise block ultraviolet
radiation without adversely affecting the desired reflection of
visible light in the display. Ultraviolet blocking compounds
conventionally used to absorb UV light in other contexts can be
used, for example, aryl-substituted benzotriazole compounds (U.S.
Pat. No. 3,533,794, DE 42 29 233), 4-thiazolidone compounds (U.S.
Pat. No. 3,314,794, U.S. Pat. No. 3,352,681), benzophenone
compounds (JP-A-2784/71), cinnamic acid esters (U.S. Pat. No.
3,705,805, U.S. Pat. No. 3,707,375), butadiene compounds (U.S. Pat.
No. 4,045,229), benzoxazole compounds (U.S. Pat. No. 3,700,455),
arylsubstituted triazine compounds (DE 21 13 833, EP 520 938, EP
530 135, EP 531 258) and benzoylthiophene compounds (GB 973 919, EP
521 823). UV absorbing photographic couplers or polymers can also
used. UV absorbing polymers can be also be employed. Non-water
soluble dyes can be dissolved in an hydrophobic phase or themselves
form a hydrophobic phase dispersed in an aqueous continuous phase.
Thus, UV blockers can be in the form of dispersed particles, either
organic or inorganic and either solid or liquid. Certain UV
absorbing dyes are water soluble. Mixtures of UV blocking materials
can be used.
[0029] TiO.sub.2 particles including both anatase and rutile forms
and zinc oxide (ZnO) can be used to block UV radiation. One
preferred embodiment of a UV blocker is TiO.sub.2 pigment in the
form of particles having an average primary particle diameter of 1
to 100 nm, preferably of 5 to 50 nm. These TiO.sub.2 pigments are
transparent and, unlike conventional TiO.sub.2-based white pigments
(rutile and anatase) with an optimum particle size of approximately
0.2 .mu.m, they have virtually no light-scattering characteristics.
They are moreover colorless. Transparent TiO.sub.2 in rutile form
is particularly advantageous as a UV absorber in the present
invention. The TiO.sub.2 pigments according to the invention are
particularly advantageous where more than 90% of the primary
particles have a diameter of less than 100 nm. Transparent
TiO.sub.2 pigments having the stated characteristics are known, for
example, see Gunter Buxbaum, Industrial Inorganic Pigments, VCH
Weinheim, New York, Basel, Cambridge, Tokyo (1993), pages 227 to
228, and also U.S. Pat. No. 5,736,308, hereby incorporated by
reference. It is known from DE 43 02 896 that TiO.sub.2 pigments
containing iron oxide have an overall higher absorbance in the UV
range than corresponding TiO.sub.2 pigments containing no iron
oxide. TiO.sub.2 pigments containing iron oxide having an average
primary particle diameter of 1 to 100 nm, preferably of 5 to 50 nm
and an iron oxide content of 0.01 to 20 wt. %, preferably of 0.05
to 10 wt. %, particularly preferably of 0.5 to 5 wt. % are a
preferred UV absorber in the present invention for use in a UV
protective layer. Fe.sub.2O.sub.3 may primarily be considered as
the iron oxide, and TiO.sub.2 of Futile structure is preferably
used. The TiO.sub.2 pigments containing iron oxide are preferably
coated on the surface with SiO.sub.2 or Al..sub.2O.sub.3. It is
particularly advantageous for the TiO.sub.2 pigments containing
iron oxide to be dispersed in a gelatin solution so that they may
be cast into a layer. In this manner, a thin layer is achieved.
[0030] One particle dispersion for a UV protective layer comprises
2% by weight gelatin and 0.5% by weight TiO.sub.2. Either deionized
or undeionized gel can be used. Preferably, the coated layer, after
drying is less than 1 micron thick, preferably less than 0.2 micron
thick.
[0031] In another embodiment of the invention, water-insoluble
UV-absorbing compounds such as are sold under the brand name
Tinuvin.RTM. UV absorbers can be dissolved in oil to form particles
that can be dispersed in a gelatin binder to form an emulsion that
can be coated onto the display structure during manufacture.
Water--insoluble UV absorbing compounds in liquid form can also be
used to form dispersed particles without requiring a solvent.
[0032] In one preferred embodiment of the invention, the display
comprises (a) a flexible transparent support; (b) transparent first
conductors; (c) second optionally transparent conductors (for
example, in a privacy screen; (d) an imaging layer comprises of
polymer dispersed liquid crystal material disposed between the
first and second conductors; and (e) at least one coated barrier
layer, preferably less than 1 micron thick, disposed between the
two conductors, which coated layer comprises ultraviolet blocking
compounds or particles (either solid or liquid, organic or
inorganic) mixed with a hydrophilic polymeric binder, preferably
gelatin, and which coated layer is disposed to block ultraviolet
radiation from striking said polymer dispersed liquid crystal. In
one particularly preferred embodiment, the coated layer is disposed
between the transparent first conductors and the layer of
polymer-dispersed liquid-crystal material.
[0033] Another aspect of the present invention relates to the
fabrication or manufacture of a display or intermediate component
thereof, which process comprises providing a substrate, applying
transparent first conductors over the substrate, coating a layer of
polymer-dispersed liquid-crystal material disposed over the first
conductors, and forming second conductors over said polymer
dispersed liquid crystal material, wherein the process further
comprises providing at least one layer comprising ultraviolet
blocking material dispersed in the layer with a binder, which layer
is disposed to block ultraviolet radiation from striking said
polymer-dispersed liquid crystal. The barrier layer is preferably
disposed between the first and second conductors, more preferably
between the first conductors and the imaging layer. Optionally the
barrier layer and the imaging layer can be coated simultaneously in
a single composite film.
[0034] Referring now to the figures, FIG. 1 is an isometric view of
a display 10 in accordance with one embodiment of the present
invention. Flexible substrate 15 can be a thin transparent
polymeric material such as Kodak Estar.RTM. film base formed of
polyester plastic that has a thickness of between 20 and 200
micrometers. In an exemplary embodiment, substrate 15 can be a
125-micrometer-thick sheet of polyester film base. Other polymers,
such as transparent polycarbonate, can also be used.
[0035] In FIG. 1, first conductors 20 are formed over substrate 15.
First conductors 20 can be, for example, tin-oxide or
indium-tin-oxide (ITO), with ITO being the preferred material.
Typically, the material of first conductors 20 is sputtered as a
layer, over substrate 15, having a resistance of less than 500 ohms
per square [units?]. The layer is then patterned to form first
conductors 20 in any well-known manner.
[0036] Alternatively, transparent first conductors 20 can also be
formed by printing a transparent organic conductor such as
PEDT/PSS, PEDOT/PSS polymer, which materials are sold as
Baytron.RTM. P by Bayer AG Electronic Chemicals. Alternatively
again, first conductors 20 can be an opaque electrical conductor
material such as copper, aluminum or nickel. If first conductors 20
are an opaque metal, the metal can have an oxidized surface to
provide a light-absorbing surface. First conductors 20 can be
formed in a conductive coating by conventional lithographic or
laser etching means.
[0037] Barrier layer or coating 17, also referred to as the
"ultraviolet-radiation blocking layer," "UV protective layer," or
the like, is applied over first conductors 20. In the embodiment of
FIG. 1, barrier layer 17 is a thin coating that electrically
isolates first conductors 20 as well as provides UV protection. In
the preferred embodiment, barrier layer 17 is a coated and dried
layer comprising a UV blocking agent and gelatin that is
approximately 0.50 micron thick. In one example, barrier layer 17
includes a concentration of fine TiO.sub.2 particles to block
ultraviolet radiation.
[0038] As used herein, when referring to a barrier layer being
"disposed between" two other layers it means, unless otherwise
indicated, that the barrier layer is at least partially between the
other two layers or, in other words, at least forms a surface area
or sub-layer between the other layers. However, it is possible for
the barrier layer to extend into another layer (that is, is not
necessarily exclusively between the other layers) as shown in FIG.
1 wherein the barrier layer 17 extends both over the layer formed
by conductors 20 but is also present within the layer formed by
conductors 20.
[0039] To illustrate one method of making such a display 10,
barrier layer 17 is aqueous coated and dried over first conductor
20 prior to application of an aqueous polymer-dispersed
liquid-crystal layer 30. Barrier layer 17 can be, for example, a
dried coating of a 1.3% deionized gelatin solution coated at a rate
of 0.38 cc per square meter. The resulting dried coating forms a
barrier coating that is about 0.5 microns thick. A display in
accordance with FIG. 1, assembled and incorporating protective or
barrier layer 17 can serve a plurality of functions. For example,
such a barrier layer can also function to effectively prevent image
defects due to defects in the coating of the polymer-dispersed
liquid-crystal layer. In addition, the same barrier layer can be
used to provide effective insulation, as confirmed by electrical
testing, between conductors in the same layer used in changing the
state of the cholesteric liquid-crystals material in the imaging or
polymer-dispersed cholesteric layer 30.
[0040] In one embodiment of the invention, polymer-dispersed
cholesteric layer 30 covering barrier layer 17 includes a
polymeric-dispersed cholesteric liquid-crystal material such as
those disclosed in U.S. Pat. No. 5,695,682, the disclosure of which
is incorporated by reference. Application of electrical fields of
various intensity and duration can drive a chiral-nematic
(cholesteric) material into a reflective state, to a transmissive
state, or to 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, for
example, Merck.RTM. BL112, BL118 or BL126, available from E.M.
Industries of Hawthorne, N.Y.
[0041] In a preferred embodiment, polymer-dispersed cholesteric
layer 30 comprises E.M. Industries' cholesteric material BL-118
that is subsequently dispersed in deionized photographic gelatin to
form an emulsion. For example, the liquid-crystal material is
dispersed at 8% concentration in a 5% deionized-gelatin aqueous
solution. The mixture is dispersed to provide, on average, 10
micrometer-diameter domains, on average, of the liquid crystal in
aqueous suspension. The material is coated over patterned ITO first
conductors 20 to provide a 9-micron-thick polymer-dispersed
cholesteric coating. Other organic binders such as polyvinyl
alcohol (PVA) or polyethylene oxide (PEO) can be used. Such
compounds are machine coatable on various equipment, including but
not limited to equipment of the type associated with the making of
photographic films. A conventional surfactant can be added to the
emulsion to improve adhesion to the underlying layer.
Conventionally known surfactants can be employed and provided at a
concentration that corresponds to the critical micelle
concentration (CMC) of the solution.
[0042] The barrier layer 17 can be applied over the first
conductors 20 prior to applying a coating of the polymer-dispersed
cholesteric layer 30. The barrier coating can be a gel coating
containing dispersed UV blocking materials. A gel coating in which
UV blocking material can be included is disclosed in U.S. Patent
U.S. Ser. No. 09/915,441 (docket 83142) filed Jul. 26, 2001 by
Stephenson et al., hereby incorporated by reference.
[0043] In the embodiment of FIG. 1, second conductors 40 overlay
polymer-dispersed cholesteric layer 30. Second conductors 40 should
have sufficient conductivity to carry a field across the
polymer-dispersed cholesteric layer 30. Second conductors 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. The metal material can be excited by
energy from resistance heating, cathodic arc, electron beam,
sputtering, or magnetron excitation. Oxides of said metals could be
used to darken second conductors 40. Tin-oxide or indium-tin oxide
coatings can permit second conductors 40 to be transparent to
operate in conjunction with opaque first conductors 20. Vacuum
deposited second conductors 40 can be areas delimited by etched
areas in a conductive coating.
[0044] In a preferred embodiment, second conductors 40 are printed
using a conductive ink such as Electrodag.RTM. 423SS
screen-printable electrical conductive material from Acheson
Corporation. Such printable materials are finely divided graphite
particles in a thermoplastic resin. The second conductors 40 are
formed using printed inks to reduce cost display. The use of a
flexible support for substrate 15, laser etched first conductors
20, machine coated polymer-dispersed cholesteric layer 30, and
printed second conductors 40 permit the fabrication of very low
cost memory displays.
[0045] FIG. 2 is a sectional view showing a portion of a display
with cholesteric material in two stable optical states in adjacent
areas of the display. On the left, a higher voltage field has been
applied and quickly switched to zero potential, which causes the
liquid crystal molecules in domains to become planar liquid
crystals 72. On the right, application of a lower voltage field has
caused molecules of the cholesteric liquid crystal in the domains
to break into transparent tilted cells that are known as
focal-conic liquid crystals 74. Varying electrical field pulses can
progressively change the molecular orientation from planar state 72
to a fully evolved and transparent focal conic state 74.
[0046] Light-absorbing second conductors 40 are positioned on the
side opposing the incident light 60. A thin layer of
light-absorbing submicron carbon in a gel binder can be disposed
between second conductors 40 and polymer-dispersed cholesteric
layer 30 as disclosed in copending U.S. Ser. No. 10/036,149 filed
Dec. 26, 2001 by Stephenson, hereby incorporated by reference.
Focal-conic liquid crystals 74 are transparent (transmissive and
light scattering), passing incident light 60, which is absorbed by
second conductors 40 to provide a black image. Progressive
evolution from planar to focal-conic state causes a viewer to see
an initial bright reflected light 62 that transitions to black as
the cholesteric material changes from planar state 72 to a fully
evolved focal-conic state 74. The transition to the
light-transmitting state is progressive, and varying the
low-voltage time permits variable levels of reflection. These
variable levels can be mapped out to corresponding gray levels, and
when the field is removed, polymer dispersed cholesteric layer 30
maintains a given optical state indefinitely. The states are more
fully discussed in U.S. Pat. No. 5,437,811.
[0047] FIG. 3 is a plot of the response of a cholesteric material
to a pulsed electrical field using one possible type of display
driver mechanism. 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 V1 can be applied without changing the state of
the cholesteric material. A higher voltage pulse at a focal-conic
voltage V3 will force a cholesteric material into the focal conic
state 52. A voltage pulse at planar voltage V4 will force the
cholesteric material into the planar state 50. The curve
characteristic of cholesteric liquid crystal permits passive matrix
writing of cholesteric displays.
[0048] FIG. 4 illustrates a method of fabricating a display in
accordance with one aspect of the present invention. This method
comprises: (a) providing a substrate 15; (b) forming transparent
first conductors 20; (c) forming second conductors 40; (d) forming
a layer of polymer-dispersed liquid-crystal material 30 disposed
between the first and second conductors; (e) forming at least one
barrier layer 17 comprising ultraviolet blocking material and a
binder, which layer is disposed to block ultraviolet radiation from
striking said polymer-dispersed liquid-crystal material, and (f)
subsequently etching the second conductors with a UV laser beam 80
while the ultraviolet blocking material effectively absorbs UV
radiation from the UV laser beam before the radiation reaches the
first conductors, thereby effectively preventing damage to the
first conductors. Methods of etching are disclosed in U.S. Pat. No.
6,236,442 to Stephenson et al., hereby incorporated by
reference.
EXAMPLE
[0049] A coating according to the present invention was made to
determine if UV absorbers were effective in accordance with the
present invention. In a first test, a coating contained a
dispersion containing 2 wt. % photographic-grade gelatin and 3 wt.
% organic near ultraviolet blocking material. The ultraviolet
blocking material is a mixture of
2-(2'-hydroxy-3',5'-di-tert-amylphenyl)benzotriazole (CAS
025973-555-1) and
2-(3'-tert-butyl-2'-hydroxy-5'-methylphenyl)-5-chloro-benzotriazole
(CAS 003896-11-5), each compound available under the brand name
Tinuvin.RTM. from Ciba Corporation. The dispersion was coated at a
rate of 11.6 cc per square meter onto a polyester support. The
resulting layer was 0.70 microns thick. In a second test, TiO.sub.2
particles were coated in gelatin onto the polyester support. The
TiO.sub.2 particles had an average particle size of 60 nm, and
originated from a dispersion comprising (by weight percent) 10.9%
TiO.sub.2, 1.4% amino-trimethyl phosphonic acid dispersant CAS
002235-43-0, and 87.7% water. The resulting layer was 0.5 microns
thick.
[0050] FIG. 5 shows the transmission spectroscopy of a 125-micron
polyester support alone, the support coated with the organic near
ultraviolet blocking material, and the support coated with the
titanium dioxide coating that was 0.5 microns thick. The
titanium-dioxide coating provides UV protection in the spectral
region between 350 and 400 nanometers. The coating with the organic
near ultraviolet blocking material provides almost complete
blocking of the region. The coating with the organic near
ultraviolet blocking material was, therefore, found to be more
effective in protecting cholesteric liquid crystal than the
TiO.sub.2 blocking material. The coating with the ultraviolet
blocking materials were thin enough to add the UV protection
without significantly increasing in driving voltage.
[0051] 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
[0052] 10 display
[0053] 15 display substrate
[0054] 17 barrier coating
[0055] 20 first conductors
[0056] 30 polymer-dispersed cholesteric layer
[0057] 40 second conductors
[0058] 60 incident light
[0059] 62 reflected light
[0060] 72 planar liquid crystal
[0061] 74 focal-conic liquid crystal
[0062] 80 laser beam
[0063] VI disturbance voltage
[0064] V2 focal-conic voltage
[0065] V3 planar voltage
* * * * *