U.S. patent application number 10/211467 was filed with the patent office on 2004-02-05 for liquid crystal cell with compensator layer and process.
This patent application is currently assigned to Eastman Kodak Company. Invention is credited to Elman, James F..
Application Number | 20040021815 10/211467 |
Document ID | / |
Family ID | 31187580 |
Filed Date | 2004-02-05 |
United States Patent
Application |
20040021815 |
Kind Code |
A1 |
Elman, James F. |
February 5, 2004 |
Liquid crystal cell with compensator layer and process
Abstract
Disclosed is a liquid crystal cell having contiguous to a
surface of a constraint thereof one or more compensator layers,
each containing a transparent amorphous polymeric birefringent
material having an out-of plane birefringence more negative than
-0.005. The invention also provides a liquid crystal display and a
process for making such a cell.
Inventors: |
Elman, James F.; (Fairport,
NY) |
Correspondence
Address: |
Paul A. Leipold
Patent Legal Staff
Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Assignee: |
Eastman Kodak Company
|
Family ID: |
31187580 |
Appl. No.: |
10/211467 |
Filed: |
August 2, 2002 |
Current U.S.
Class: |
349/117 |
Current CPC
Class: |
G02F 1/133634 20130101;
C08G 63/193 20130101; C08L 67/03 20130101 |
Class at
Publication: |
349/117 |
International
Class: |
G02F 001/1335 |
Claims
What is claimed is:
1. A liquid crystal cell having contiguous to a surface of a
constraint thereof one or more compensator layers, each containing
a transparent amorphous polymeric birefringent material having an
out-of plane birefringence more negative than -0.005.
2. The cell of claim 1 bearing more than one of said polymeric
compensator layers.
3. The cell of claim 1 comprising compensator layers containing
selected polymeric materials having sufficient thickness so that
the overall in-plane retardation (Re) of all of the layers of the
liquid crystal cell is from +20 to -20 nm and the out-of-plane
retardation (Rth) of at least one of the compensator layers is more
negative than -20 nm.
4. The cell of claim 1 comprising at least two compensator
layers.
5. The cell of claim 1 wherein the one or more compensator layers
have a combined thickness of less than 30 micrometers.
6. The cell of claim 5 wherein the one or more compensator layers
have a combined thickness of from 0.1 to 20 micrometers.
7. The cell of claim 6 wherein the one or more compensator layers
have a combined thickness of from 1.0 to 10 micrometers.
8. The cell of claim 7 wherein the one or more compensator layers
have a combined thickness of from 2 to 8 micrometers.
9. The cell of claim 1 wherein the Rth of the one or more
compensator layers is -60 nm or more negative.
10. The cell of claim 1 wherein the Rth of the one or more
compensator layers is from -60 to -600 nm.
11. The cell of claim 1 wherein the Rth of the one or more
compensator layers is from -150 to -500 nm.
12. The cell of claim 1 wherein the one or more compensator layers
comprise a polymer containing in the backbone a non-visible
chromophore group and having a Tg above 180.degree. C.
13. The cell of claim 12 wherein the compensator layer comprises a
polymer containing in the backbone a carbonyl or an aromatic
group.
14. The cell of claim 12 wherein the non-visible chromophore group
includes a carbonyl, amide, imide, ester, carbonate, phenyl,
naphthyl, biphenyl, bisphenol, or thiophene group.
15. The cell of claim 12 wherein the compensator layer comprises
1)poly(4,4'-hexaflouroisopropylidene-bisphenol)
terephthalate-co-isophtha- late,
2)poly(4,4'-hexahydro-4,7-methanoinden-5-ylidene bisphenol)
terephthalate, or 3)
Poly(4,4'-isopropylidene-2,2'6,6'-tetrachlorobisphen- ol)
terephthalate-co-isophthalate, or copolymers thereof.
16. The cell of claim 12 wherein the compensator layer comprises a
polymer containing in the backbone a non-visible chromopore group
that does not contain a chromophore off of the backbone.
17. The cell of claim 1 wherein the liquid crystal cell is a
vertically aligned or a twisted nematic cell.
18. The cell of claim 1 employing in-plane switching.
19. The cell of claim 1 wherein the constraints are glass.
20. The cell of claim 1 wherein the surface is an exterior surface
of the cell constraint.
21. A liquid crystal display comprising the liquid crystal cell of
claim 1.
22. A process for forming a liquid crystal display comprising
coating the polymer of claim 1 in a solvent onto a surface of a
constraint of the cell.
23. The process of claim 22 wherein the constraint is glass.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a liquid crystal cell
having contiguous to an exterior surface of a constraint thereof a
compensator layer containing an amorphous polymeric birefringent
material having an out-of plane birefringence more negative than
-0.005. The invention also provides an LC display and a process for
making such a cell.
BACKGROUND OF THE INVENTION
[0002] Triacetylellulose (TAC, also called cellulose triacetate)
film has traditionally been used by the photographic industry due
to its unique physical properties and flame retardance. TAC film is
also the preferred polymer film for use as a cover sheet for the
polarizers used in liquid crystal displays. It is the preferred
material for this use because of its extremely low in-plane
birefringence. Its out of plane birefringence is also small (but
not zero), and is useful in providing some optical compensation to
the LCD.
[0003] Intrinsic birefringence describes the fundamental
orientation of a material at a molecular level. It is directly
related to the molecular structure (bond angles, rotational
freedom, presence of aromatic groups, etc.) of the material. The
intrinsic birefringence is not affected by process conditions
(temperature, stresses, pressures) used to make a macroscopic
object.
[0004] Crystalline and liquid crystalline materials have the
convenient property that their intrinsic birefringence manifests
itself almost perfectly when they are assembled into a macroscopic
article. Layers of crystalline and liquid crystalline molecules
often can be manufactured such that all the molecules in the
article are in registry with each other and thus preserve their
fundamental orientation. The same is not true when making layers of
an amorphous polymeric material. Their intrinsic birefringence can
be highly modified by the manufacturing process. Thus, the measured
birefringence of an actual article will be a resultant of its
intrinsic birefringence and the manufacturing process. Because we
are dealing with such amorphous polymeric materials, the following
definitions refer to this measured birefringence and not intrinsic
birefringence.
[0005] In-plane birefringence means the difference between n.sub.x
and n.sub.y, where x and y lie in the plane of the layer. n.sub.x
will be defined as being parallel to the casting direction of the
polymer, and n.sub.y being perpendicular to the casting direction
of the polymer film. The sign convention used will be
n.sub.x-n.sub.y.
[0006] Out of-plane birefringence means the difference between
n.sub.z and the average of n.sub.x and n.sub.y, where x and y lie
in the plane of the layer and z lies in the plane normal to the
layer. The sign convention used will be:
n.sub.z-[(n.sub.x+n.sub.y)/2]. TAC typically has a negative out of
plane birefringence as its n.sub.z is smaller than its n.sub.x and
n.sub.y.
[0007] In-plane retardation (Re) means the product of in-plane
birefringence and layer thickness (t). Thus
Re=t(n.sub.x-n.sub.y)
[0008] Out-of-plane retardation (Rth) means the product of
out-of-plane birefringence and layer thickness (t). Thus
Rth=t(n.sub.z-[(n.sub.x+n.sub- .y)/2]).
[0009] Synthetic polymer films (such as polycarbonate or
polysulfone) are often used to enhance the minimal optical
compensation that TAC provides. These synthetic polymers films are
attached to the rest of the display by adhesive lamination.
[0010] Generally in the field of optical materials, the synthetic
polymer film is used as an optically anisotropic film (having a
high retardation value), while a TAC film is used as an optical
isotropic film (having a low retardation value).
[0011] Japanese Published Patent Application JP1999-95208 describes
a liquid crystal display having an optical compensator (having high
retardation) prepared by uniaxial stretching of a high polymer
film. Such polymers include polyesters, polycarbonate, polyarylate
or polysulfone. This stretching step is essential to obtain the
desired optical properties. This stretching affects both inand
out-of-plane retardation simultaneously. These two orthogonal
retardations cannot be independently controlled by this method.
Also, producing uniform optical compensators by this method is
described as being difficult.
[0012] This application also describes a compensator where the
inventor uses an exfoliated inorganic clay material in a polymeric
binder coated on top of a TAC support. The exfoliated inorganic
clay material in this layer is the optically active material, not
the polymeric binder.
[0013] World patent WO 01/31394 A2 discusses the use of the color
filter array layer as a source of additional out-of-plane
retardation for a liquid crystal display. The color filter array is
located within the constraints of the liquid crystal cell. The use
of an aromatic polyimide binder rather than a polyacrylate binder
for the color filter array dyes provides enhanced retardation. The
overall retardation is achieved with the combination of the color
filter array retarder plus optional additional out-of-plane
retardation from the TAC used as a supporting member for the
polarizers.
[0014] The proposal to select the binder for the color filter array
with retardation in mind has an advantage versus polarizer-based
retarders that are laminated to the liquid crystal cell: mechanical
stresses to the display induced by room condition changes or
perhaps direct shock can cause polarizer-based retarders to move
relative to the liquid crystal cell. Retarders coated directly on
the glass substrate are more rigidly held in registry with the
cell, and thus do not suffer this problem. However the requirement
that this color filter array be also a retarder means that this
layer must serve two purposes: color filtering and adding
retardation. This limits the potential thickness to be considered
for this layer. This layer must also be pixilated, and this adds
additional complications. Finally it is taught on the internal
surface of the constraint only, where the color filter array is
located.
[0015] It is a problem to be solved to provide a liquid crystal
cell that is readily manufactured and that readily provides the
required degree of in-plane and out-of-plane compensation while
reducing the problems associated with a laminated compensator.
SUMMARY OF THE INVENTION
[0016] The invention provides liquid crystal cell having contiguous
to a surface of a constraint thereof one or more compensator
layers, each containing a transparent amorphous polymeric
birefringent material having an out-of plane birefringence more
negative than -0.005. The invention also provides a liquid crystal
display and a process for preparing a compensator of the
invention.
[0017] The invention cell is readily manufactured and provides the
required degree of in-plane and out-of-plane compensation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1A is a cross-sectional schematic of an embodiment of
the invention with the amorphous polymeric compensator layer on the
side of the constraint opposite the liquid crystal
[0019] FIG. 1B is a cross-sectional schematic of an embodiment of
the invention with the amorphous polymeric compensator layer on the
side of the constraint adjacent to the liquid crystal
[0020] FIG. 2A is an exploded view schematic of a liquid crystal
display with one amorphous polymeric compensator layer of the
invention
[0021] FIG. 2B is an exploded view schematic of a liquid crystal
display with two amorphous polymeric compensator layers of the
invention
DETAILED DESCRIPTION OF THE INVENTION
[0022] The invention is summarized above.
[0023] The present invention is not limited by the requirements of
WO 01/31394 mentioned above.
[0024] The present invention provides a liquid crystal cell having
contiguous to at least one surface of a constraint thereof a
compensator layer containing a birefringent amorphous polymeric
material having an out-of plane birefringence more negative than
-0.005.
[0025] As used herein, constaints are the two principal supporting
members of the LC cell (typically glass) that sandwich the
switchable liquid crystal layer (and typically the color filter
array, black matrix, and thin film transistor, alignment and
electrode layers, and other optional layers) and are normally at
least 10 micrometers thick. The term "transparent" is used in its
normal sense to mean a layer that absorbs little or no visible
light.
[0026] The liquid crystal cell referred to herein extends from the
external surface of one constraint to the other, including any
compensator layer on the constraint surfaces.
[0027] Amorphous polymeric materials are used for this optical
compensator. In this case amorphous means that the optical
compensator would not produce any sharp diffraction peaks when
exposed to X-ray diffraction analysis. Crystalline polymers, liquid
crystal molecules and crystalline inorganic materials would produce
such sharp peaks when subjected to such X-ray diffraction analysis.
Such materials are desirably suitable to be solvent cast or coated
such as TAC, polycarbonate, cyclic polyolefins, and
polyarylate.
[0028] Typical lamination uses pressure sensitive adhesive layers
of greater than 4 micrometers in thickness. As used herein, the
term contiguous means without the use of any intervening laminating
adhesive layer and therefore contemplates the possible use of only
a very thin (0.2 .mu.m or less) adhesion promoting layer or an
adhesion promoting surface treatment such as corona discharge,
plasma glow discharge, or flame treatment. Other adhesion enhancing
methods could be employed as known to those skilled in the art.
[0029] The compensator layer will typically be solvent coated onto
the constraint exterior surface. This solvent coating could be
accomplished by spin coating, hopper coating, gravure coating, wire
bar coating, spray coating, or other coating methods known to those
skilled in the art.
[0030] The compensator layer is coated from a solution containing a
polymer that yields high negative birefringence upon solvent
coating. To produce negative birefringence (negative retardation),
polymers that contain non-visible chromopore groups such as vinyl,
carbonyl, amide, imide, ester, carbonate, sulfone, azo, and
aromatic groups (i.e. benzene, naphthalate, biphenyl, bisphenol A)
in the polymer backbone will be used, such as polyesters,
polycarbonates, polyimides, polyetherimides, and polythiophenes.
One could also add fillers and non-polymeric molecules to these
polymers for this contiguous layer.
[0031] A chromophore is defined as an atom or group of atoms that
serve as a unit in light adsorption. (Modern Molecular
Photochemistry Nicholas J. Turro Editor, Benjamin/Cummings
Publishing Co., Menlo Park, Calif. (1978) Pg 77). Typical
chromophore groups include vinyl, carbonyl, amide, imide, ester,
carbonate, aromatic (i.e. phenyl, naphthyl, biphenyl, thiophene,
bisphenol), sulfone, and azo or combinations of these chromophores.
A non-visible chromophore is one that has an absorption maximum
outside the range of 400-700 nm.
[0032] Desirably, polymers to be used in the compensator layer will
not have chromophores off of the backbone. An example of such an
undesirable polymer with chromophores in and off the backbone would
be polyarylates possessing the fluorene group.
[0033] The glass transition temperature (Tg) of the polymers used
in the compensator layer is significant. It should be above
180.degree. C. to achieve the desired results.
[0034] The polymers used in the contiguous compensator layer can be
synthesized by a variety of techniques: condensation, addition,
anionic, cationic or other common methods of synthesis can be
employed.
[0035] The thickness of this layer should be less than 30 .mu.m.
Typically it should be from 0.1 .mu.m to 20 .mu.m. Conveniently it
should be from 1.0 .mu.m to 10 .mu.m. Desirably it should be from 2
.mu.m to 8 .mu.m.
[0036] The compensator layer should be of sufficient thickness so
that the out-of-plane retardation of the second layer is more
negative than -20 nm. Typically it should be from -600 to -60.
Conveniently it should be from -500 to -100. Desirably it should be
from -400 to -150.
[0037] Compared to WO 01/31394, the compensator layer(s) can be
applied onto any and/or all of the four surfaces provided by the
two liquid crystal constraints (be they glass or some other very
low birefringence material). Further, the applied layers are not
limited in thickness due to their coloration since they are
transparent. Finally the materials suitable for these contiguous
amorphous polymeric compensator layers are much broader than the
aromatic polyimides of '394. A wide variety of amorphous, high
glass transition temperature, polymeric materials are identified
that will serve this purpose.
[0038] The invention is described in more detail by referring to
the drawings as follows.
[0039] FIG. 1A shows a cross-sectional schematic of part of a
liquid crystal display 5 including an amorphous polymeric
compensator layer 50 and constraint 40 in accordance with the
present invention. Also shown are the switchable liquid crystal 10,
an alignment layer 20, a TFT (thin film transistor) layer 30, and a
color filter array 35. The said amorphous polymeric compensator
layer 50 having an out-of-plane birefringence more negative than
-0.005, and the combined in-plane retardation (Re) of layers 20,
30, 35, 40 and 50 is from +20 to -20 nm and the out-of-plane
retardation (Rth) of layer 50 is more negative than -20 nm.
[0040] FIG. 1B shows a cross-sectional schematic of part of another
liquid crystal display 6 including an amorphous polymeric
compensator layer 50 and constraint 40 in accordance with the
present invention. Also shown are the switchable liquid crystal 10,
an alignment layer 20, a TFT (thin film transistor) layer 30, and a
color filter array 35. The said amorphous polymeric compensator
layer 50 having an out-of-plane birefringence more negative than
-0.005, and the combined in-plane retardation (Re) of layers 20,
30, 35, 40 and 50 is from +20 to -20 nm and the out-of-plane
retardation (Rth) of layer 50 is more negative than -20 nm. In this
embodiment layer 50 is on the other side of the constraint as
compared to FIG. 1A.
[0041] FIG. 2A shows a liquid crystal display 700 having an
amorphous polymeric compensator layer 200, a constraint 300 with
alignment layer/TFT layer/color filter array on one side of the
electrically switchable liquid crystal 600, a second constraint 400
with alignment layer/TFT layer which is on the other side of the
electrically switchable liquid crystal 600, and polarizers 500 and
550. The transmission axes of polarizers 500 and 550 form a
90.degree..+-.10.degree. angle relative to each other. The angles
of their transmission axes are denoted as 45.degree. and
135.degree. relative to the liquid crystal display 700. However,
other angles are possible depending on the kind of liquid crystal
display 700 and this is obvious to those who skilled in the
art.
[0042] FIG. 2B shows another liquid crystal display 700 having two
amorphous polymeric compensator layers 200, a constraint 300 with
alignment layer/TFT layer/color filter array on one side of the
electrically switchable liquid crystal 600, a second constraint 400
with alignment layer/TFT layer which is on the other side of the
electrically switchable liquid crystal 600, and polarizers 500 and
550. The transmission axes of polarizers 500 and 550 form a
90.degree..+-.10.degree. angle relative to each other. The angles
of their transmission axes are denoted as 45.degree. and
135.degree. relative to the liquid crystal display 700. However,
other angles are possible depending on the kind of liquid crystal
display 700 and this is obvious to those who skilled in the
art.
[0043] The present invention is further illustrated by the
following non-limiting examples of its practice.
EXAMPLES
[0044] The aromatic polyesters used herein can be prepared using
any suitable or conventional procedure. The procedure used herein
followed that outlined by P. W. Morgan in Condensation Polymers: By
Interfacial and Solution Methods, Interscience, New York City, N.Y.
(1965).
Example 1
[0045] Polymer I (Synthesis):
[0046] To a stirred mixture of
4,4'-hexaflouroisopropylidenediphenol (33.62 g, 0.1 mole) and
triethylamine (22.3 g, 0.22 mole) in methylene chloride (200 mL) at
10.degree. C. was added a solution of terephthaloyl chloride (10.15
g, 0.05 mole) and isophthaloyl chloride (10.15 g, 0.05 mole) in
methylene chloride (100 mL). After the addition, the temperature
was allowed to rise to room temperature and the solution was
stirred under nitrogen for 4 hours, during which time triethylamine
hydrochloride precipitated in a gelatinous form and the solution
became viscous. The solution was then filtered and washed with
dilute hydrochloric acid, (100 mL of 2% acid) followed three times
by water (200 mL). The solution was then poured into methanol with
vigorous stirring, and a white fibrous polymer precipitated. The
glass transition temperature of this polymer was measured by
differential scanning calorimetry to be 199.degree. C. 1
[0047] A solution of the polyester (polymer I, 10% solids, 45%
methylethylketone, 45% toluene) was coated onto a TAC web. This
included the steps of unrolling the TAC web, coating the polymer
solution (using a slot hopper), and applying sufficient drying
(85.degree. C.) to remove the majority of the solvents. These steps
occurred in a roll to roll, continuous process. Spin coating and
other coating methods such as spray application could also be used.
Optically clear films of the TAC/polyester structure were produced
with the following optical properties. Re, Rth and the second layer
thickness were measured with an ellipsometer (model M2000V, J. A.
Woollam Co.) at 550 nm wavelength.
1TABLE I Second Layer: Combined Re, In- Combined Rth, Out- Polymer
I Layer Plane Retardation of-Plane Retardation First Layer
thickness (.mu.m) (nm) (nm) 80 .mu.m TAC 0 3 -58 80 .mu.m TAC 2.8 3
-84 80 .mu.m TAC 5.6 3 -104
Example 2
[0048] Polymer II was similarly prepared using terephthaloyl
chloride and 4,4'(hexahydro-4,7-methanoinden-5-ylidene) bisphenol.
The glass transition temperature of this polymer was measured by
differential scanning calorimetry to be 289.degree. C. 2
[0049] When polymer II is spun cast onto a glass substrate (10%
solids in dichloroethane), it shows the following optical
retardations. Re, Rth and the polymer II layer thickness are
measured with an ellipsometer (model M2000V, J. A. Woollam Co.) at
550 nm wavelength.
2TABLE II Polymer II Layer Re, In-Plane Rth, Out-of-Plane thickness
(.mu.m) retardation (nm) Retardation (nm) 3.4 0.2 -74
Example 3
[0050] Polymer III was similarly prepared using terephtaloyl
chloride, isophthaloyl chloride and
4,4'-isopropylidene-2,2',6,6'-tetrachlorobisphe- nol. The glass
transition temperature of this polymer was measured by differential
scanning calorimetry to be 250.degree. C. 3
[0051] Polymer III
[0052] When polymer III is spun cast onto glass (10% solids in
dichloroethane), it shows the following optical retardations. Re,
Rth and the polymer III layer thickness are measured with an
ellipsometer (model M2000V, J. A. Woollam Co.) at 550 nm
wavelength.
3TABLE III Polymer III Layer Re, In Plane Rth, Out of Plane
thickness (.mu.m) Retardation (nm) Retardation (nm) 2.8 0.8 -66
[0053] A series of polymers were analyzed for their glass
transition temperatures and out of plane birefringence values. It
was found that the more desirable polymers for this invention had
glass transition temperatures above 180.degree. C. Those with lower
glass transition temperatures were found to generally have
birefringence values less negative than -0.005.
[0054] 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 scope of the invention.
PARTS LIST
[0055] 5 compensator/constraint according to the present
invention
[0056] 10 liquid crystal
[0057] 20 alignment layer
[0058] 30 transparent conductive layer
[0059] 40 constraint
[0060] 50 polymeric layer having high birefringence
[0061] 200 polymeric layer having high birefringence
[0062] 300 constraint with transparent conductive layer/alignment
layer
[0063] 500 polarizer
[0064] 550 polarizer
[0065] 600 liquid crystal
[0066] 700 liquid crystal display
* * * * *