U.S. patent application number 12/018580 was filed with the patent office on 2008-06-05 for optical compensation film, polarizing plate and liquid crystal display.
This patent application is currently assigned to KONICA MINOLTA OPTO, INC.. Invention is credited to Noriyasu KUZUHARA, Koji TASAKA, Hiroki UMEDA, Takatoshi YAJIMA.
Application Number | 20080129934 12/018580 |
Document ID | / |
Family ID | 34055830 |
Filed Date | 2008-06-05 |
United States Patent
Application |
20080129934 |
Kind Code |
A1 |
UMEDA; Hiroki ; et
al. |
June 5, 2008 |
OPTICAL COMPENSATION FILM, POLARIZING PLATE AND LIQUID CRYSTAL
DISPLAY
Abstract
An optical compensation film wherein: (i) molecules of a liquid
crystalline compound are oriented in a manner that an average tilt
angle of the compounds is not more than 4 degrees; (ii) the
orientation is fixed; (iii) Ro(.lamda.) values are: at wavelength
589 nm: 40-130 nm; at 600 nm: 35-125 nm; and at 630 nm: 35-120 nm;
(iv) Rt(.lamda.) valuea are 120-400 nm in .lamda. range of 500-630
cm; and (v) Values of Ro(589), Ro(600) and Ro(630) satisfy the
following formulae: 1.0<B<7.0
B=(Ro(589)-Ro(630))/(Ro(600)-Ro(630)).
Inventors: |
UMEDA; Hiroki; (Tokyo,
JP) ; YAJIMA; Takatoshi; (Tokyo, JP) ;
KUZUHARA; Noriyasu; (Tokyo, JP) ; TASAKA; Koji;
(Tokyo, JP) |
Correspondence
Address: |
LUCAS & MERCANTI, LLP
475 PARK AVENUE SOUTH, 15TH FLOOR
NEW YORK
NY
10016
US
|
Assignee: |
KONICA MINOLTA OPTO, INC.
Tokyo
JP
|
Family ID: |
34055830 |
Appl. No.: |
12/018580 |
Filed: |
January 23, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10886434 |
Jul 7, 2004 |
7345725 |
|
|
12018580 |
|
|
|
|
Current U.S.
Class: |
349/96 ;
359/487.02; 359/487.06; 359/489.05 |
Current CPC
Class: |
G02B 5/3016 20130101;
G02F 1/133634 20130101 |
Class at
Publication: |
349/96 ;
359/499 |
International
Class: |
G02F 1/13363 20060101
G02F001/13363; G02B 5/30 20060101 G02B005/30 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 15, 2003 |
JP |
2003197052 |
Claims
1. A polarizing plate comprising: (i) an optical: compensation film
and (ii) a polarizing film or a polarizing element, wherein an
angle between an in-plane delayed phase axis of the optical
compensation film and an absorbing axis of the polarizing film or
of the polarizing element is 85-95 degrees, wherein the optical
compensation film comprises a support exhibiting optical
biaxiality, on which at least one optically anisotropic layer is
provided, wherein: (i) the optically anisotropic layer is formed by
orienting molecules of a liquid crystalline compound in a manner
that an average tilt angle of the molecules is not more than 4
degrees followed by fixing the orientation of the liquid crystal
molecules; (ii) in-plane retardation values of the optical
compensation film Ro(.lamda.) given by Formula (1) are as follows,
Ro(589): 40-130 nm Ro(600): 35-125 nm Ro(630): 35-120 nm; (iii) an
out-of-plane retardation value in a thickness direction of the
optical compensation film Rt(.lamda.) represented by Formula (2) is
in the range of 120-400 nm when measured at wavelengths of 500-630
nm; and (iv) Ro(589), Ro(600) and Ro(630) satisfy Formula (3),
Ro.lamda.(.lamda.)=(nx(.lamda.)-ny(.lamda.)).times.d Formula (1)
Rt(.alpha.)=((nx(.lamda.)+ny(.lamda.))/2-nz(.lamda.)).times.d
Formula (2) 1.0<B<7.0 Formula (3)
B=(Ro(589)-Ro(630))/(Ro(600)-Ro(630)) nx(.lamda.) representing a
refractive index in a x direction which gives a maximum refractive
index in the plane of the optical compensation film; ny(.lamda.)
representing a refractive index in a y direction which is
orthogonal to the x direction in the plane of the optical
compensation film; nz(.lamda.) representing a refractive index in
the thickness direction of the optical compensation film; .lamda.
representing a wavelength (nm) at which each measurement is carried
out; and d representing a thickness (nm) of the optical
compensation film.
2. A vertically aligned ECB type liquid crystal display having the
polarizing plate of claim 1 on one surface or on both surfaces of a
liquid crystal cell of the liquid crystal display.
3. A vertically aligned ECB type liquid crystal display having the
polarizing plate of claim 1 on one surface or on both surfaces of a
liquid crystal cell of the liquid crystal display wherein
So(.lamda.) and C(.lamda.) satisfy Formulae (9)-(12):
0.21.times.C(589)<So(589)<0.66.times.C(589) Formula (9)
0.22<Ps(.lamda.)/Pc(.lamda.)<1.75 Formula (10)
Ps(.lamda.)=(So(.lamda.)/So(589)1).times.100 Formula (11)
Pc(.lamda.)=C(.lamda.)/C(589)-1).times.100 Formula (12) So(.lamda.)
representing a sum of in-plane retardation values (Ro(.lamda.)) of:
(i) the polarizing films or the polarizing elements of the
polarizing plates of claim 1 provided on one surface or both
surfaces of the said liquid crystal cell; (ii) the optical
compensation films in the polarizing plates; and (iii) the liquid
crystal cell, C(.lamda.) representing a product of a birefringence
.DELTA.n(.lamda.) and a thickness of the liquid crystal cell,
.DELTA.n(.lamda.).times.d; and .lamda. representing a wavelength in
a range of 500-670 nm.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a Divisional of U.S. patent application Ser. No.
10/886,434 filed Jul. 7, 2004, now U.S. Pat. No. ______, which, in
turn, claimed the priority of Japanese Patent Application
2003-197052 filed Jul. 15, 2003, the priority of both applications
claimed and both applications are incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present invention relates to an optical compensation
film, a polarizing plate and a liquid crystal display.
BACKGROUND
[0003] According to the current expansion of the personal computers
used for multimedia information, liquid crystal displays (LCDs)
have become larger, and TFT type liquid crystal displays providing
high quality images have become the main current in the field of
liquid crystal displays. For these displays, a major improvement in
viewing angle characteristics is strongly demanded.
[0004] In order to meet this demand, In-Plane Switching (IPS) mode
and Vertically Aligned (VA) mode LCDs have been developed in
addition to conventional Twisted Nematic (TN) type LCDS, and have
already been put into practical use.
[0005] Furthermore, Optically Compensatory Bend (OCB) mode LCDs
which enable high speed switching of the image and exhibit
superiority in showing motion pictures are also being
commercialized. However, these types of displays, except for IPS
type, have problems in viewing angle characteristics and trials to
improve the viewing angle characteristics by using an optical
compensation sheet (hence, also referred to as an optical
compensation film) are being carried out.
[0006] Descriptions regarding optical compensation sheets, to be
used for the following modes of liquid crystal cells, are found in
the following patent documents, for example: for TN (Twisted
Nematic) mode LCDs, Patent Documents 1-4; for IPS (In-Plane
Switching) mode and FLC (Ferroelectric Liquid crystal) mode LCDs,
Patent Document 5; for OCB (Optically Compensatory Bend) mode and
HAN (Hybrid Aligned Nematic) mode LCDs, Patent Documents 6 and 7;
for STN (Super Twisted Nematic) mode LCDs, Patent Document 8; and
for VA (Vertically Aligned) mode LCDs, Patent Document 9.
[0007] The above mentioned optical compensation sheet (also
referred to as an optical compensation film) is usually adhered to
a polarizing plate and used as a monolithic elliptic polarizing
plate.
[0008] Known as one of the examples of optical compensation sheets
usable for this purpose is an optical compensation sheet for a TN
mode liquid crystal display, which comprises stacked layers of a
coated and immobilized discotic liquid crystalline compound formed
on a cellulose acetate support.
[0009] Another example is an optical compensation sheet for a TN
mode liquid crystal display, which comprises stacked layers of an
optically positive and uniaxial liquid crystalline compound formed
on a cellulose acetate support.
[0010] Each of these optical compensation sheets has been prepared
so as to exhibit desired optical compensation properties under a
specified condition by controlling the in-plane retardation value
(Ro) and the out-of-plane retardation value in the thickness
direction (Rt) of the sheet by suitably selecting a cellulose ester
support and liquid crystal layers.
[0011] Vertically Aligned mode liquid crystal displays (such as VA,
MVA (multidomained VA) and PVA (patterned VA)) are of "normally
black mode displays", and enable higher contrast. These displays
also enable higher switching speed of images compared to TN or IPS
mode displays. Accordingly, these displays are widely used for
monitors and TVs. Since VA mode LCDs by nature have narrow viewing
angles in oblique directions (or being 45 degrees to the absorbing
axis or a transmission axis of a polarizing plate), various types
of optical compensation sheets or wide viewing angle films have
been proposed to improve viewing angle characteristics, some of
which have already been marketed.
[0012] For example, trials to prevent leakage of light in any
direction using in combination of various .lamda./2 and .lamda./4
retardation plates have been proposed (for example, Patent
Documents 10-14). However, the retardation plate to be used for
this purpose requires having very high uniformity in retardation as
is easily deduced from the very sensitive alignment angle to the
polarizing plate, which results in a considerably low productivity.
A very precise technique in adjusting the angle between the optical
compensation sheet and the polarizing plate is also required, which
also causes lowering of productivity. Accordingly, an easier method
to effectively improve the viewing angle characteristics has been
searched for.
[0013] In order to widen the viewing angle of a Vertically Aligned
mode LCD, a method to use a retardation sheet, of which the
in-plane delayed phase axis is placed orthogonal to the absorbing
axis of a polarizing film, has been proposed.
[0014] This method was proposed in the early stages of the
development of a Vertically Aligned ECB mode LCD (for example, in
Patent Document 15), and a drastic improvement has been attained in
viewing angle characteristics in oblique directions. The above
improvement has been due to a design specifically focusing on
broadening viewing angles.
[0015] A method to use a stacked layer type wide viewing angle film
using a cholesterick liquid crystal compound has been proposed to
widen the viewing angle while suppressing light leakage not only in
the frontal direction but also in oblique directions (for example,
in Patent Document 16).
[0016] As described above, a viewing angle of over 80 degrees at a
wavelength of, for example, around 550 nm has been attained by use
of various retardation films.
[0017] However, as the viewing angle in an oblique direction has
exceeded 80 degrees, red hue in black areas in the display has
become a pronounced problem, which had been tolerable at lesser
viewing angles. The improvement in viewing angle characteristics
has lead to the commercialization of large size liquid crystal
displays, which resulted in a strong need for solving the red hue
problem in the field of LCDs. The red hue problem has been
difficult to solve merely by optimizing the retardation value of
optical film at a wavelength of 550 nm, or by adjusting the
wavelength dispersion in the frontal viewing angle.
[0018] (Patent Document 1) [0019] Japanese Patent Publication Open
to Public Inspection (hereinafter referred to as JP-A) No. 6-214,
116.
[0020] (Patent Document 2) [0021] U.S. Pat. No. 5,583,679
[0022] (Patent Document 3) [0023] U.S. Pat. No. 5,646,703
[0024] (Patent Document 4) [0025] DE No. 3,911,620(A1)
[0026] (Patent Document 5) [0027] JP-A 10-54, 982
[0028] (Patent Document 6) [0029] U.S. Pat. No. 5,805,253
[0030] (Patent Document 7) [0031] WO 96/37804
[0032] (Patent Document 8) [0033] JP-A 9-26, 572
[0034] (Patent Document 9) [0035] Japanese Pat. No. 2,866,372
[0036] (Patent Document 10) [0037] JP-A 5-11, 356
[0038] (Patent Document 11) [0039] JP-A 6-14, 842
[0040] (Patent Document 12) [0041] JP-A 2002-174, 727
[0042] (Patent Document 13) [0043] JP-A 2002-372, 622
[0044] (Patent Document 14) [0045] JP-A 2003-43, 262
[0046] (Patent Document 15) [0047] Examined Japanese Patent
Publication No. 7-69536
[0048] (Patent Document 16) [0049] JP-A 2002-182, 212
OBJECT & SUMMARY
[0050] An object of the present invention is to provide an optical
compensation film, a polarizing plate using the optical
compensation film and a liquid crystal display which enable a
viewing angle over 80 degrees while red hue in black areas of the
display is suppressed.
[0051] An embodiment of the present invention includes an optical
compensation film having a specific optically anisotropic layer.
The anisotropic layer satisfies a predetermined formula containing
in-plane retardation values and out-of-plane retardation
values.
BRIEF DESCRIPTION OF DRAWING
[0052] FIG. 1 is a schematic illustration of the measurement of
retardation values.
DESCRIPTION of PREFERRED EMBODIMENTS
[0053] The object of the present invention are achieved by the
following structures:
1. An optical compensation film containing a support exhibiting
optical biaxiality, on which at least one optically anisotropic
layer is provided,
[0054] wherein: [0055] (i) the optically anisotropic layer is
formed by orienting molecules of a liquid crystalline compound in a
manner that an average tilt angle of the molecules is not more than
4 degrees followed by fixing the orientation of the liquid crystal
molecules; [0056] (ii) in-plane retardation values of the optical
compensation film Ro(.lamda.) given by Formula (1) are as
follows,
[0056] Ro(589): 40-130 nm
Ro(600): 35-125 nm
Ro(630): 35-120 nm [0057] (iii) an out-of-plane retardation value
in a thickness direction of the optical compensation film
Rt(.lamda.) represented by Formula (2) is in the range of 120-400
nm when measured at wavelengths of 500-630 nm; and [0058] (iv)
Ro(589), Ro(600) and Ro(630) satisfy Formula (3),
[0058] Ro(.lamda.)=(nx(.lamda.)-ny(.lamda.)).times.d Formula
(1)
Rt(.lamda.)=((nx(.lamda.)+ny(.lamda.))/2-nz(.lamda.)).times.d
Formula (2)
1.0<B<7.0
B=(Ro(589)-Ro(630))/(Ro(600)-Ro(630)) Formula (3) [0059]
nx(.lamda.) representing a refractive index in a x direction which
gives a maximum refractive index in the plane of the optical
compensation film; [0060] ny(.lamda.) representing a refractive
index in a y direction [0061] which is orthogonal to the x
direction in the plane of the optical compensation film; [0062]
nz(.lamda.) representing a refractive index in the thickness
direction of the optical compensation film; [0063] .lamda.
representing a wavelength (nm) at which each measurement is carried
out; and [0064] d representing a thickness (nm) of the optical
compensation film. 2. The optical compensation film of Item 1
wherein a ratio of R.sub.70(589)/Ro(589) is 1.25-1.40 and a ratio
of R.sub.50(589)/Ro(589) is 1.10-1.25: [0065] R.sub.70(589) being a
retardation value of the optical compensation film measured by a
method of: [0066] (i) placing the film parallel to a reference
plane; [0067] (ii) rotating the film 70 degrees on an in-plane
delayed phase axis of the film; and [0068] (iii) measuring the
retardation value from a direction perpendicular to the reference
plane at a wavelength of 589 nm. [0069] R.sub.50(589) being a
retardation value of the optical compensation film measured by the
same method as R.sub.70(589) except for that the film being rotated
50 degrees on the in-plane delayed phase axis of the film; and the
in-plane delayed phase axis representing an axis of a direction
giving a maximum in-plane retardation value. 3. The optical
compensation film of Item 1 or Item 2 wherein a wavelength
dispersion of the retardation values of the film satisfies Formula
(4):
[0069] 0<C/D<7.0 Formula (4) [0070]
C=(R.sub.50'(589)-R.sub.50' (630)) [0071] D=(R.sub.70'
(589)-R.sub.70' (630)) [0072]
R.sub.70'(.lamda.)=(nx.sub.70'(.lamda.)-ny.sub.70'(.lamda.)).times.d
[0073]
R.sub.50'(.lamda.)=(nx.sub.50'(.lamda.)-ny.sub.50'(.lamda.).times.-
d [0074] R.sub.70' (589) and R.sub.70' (630) each being a
retardation value of the optical compensation film measured by a
method of: [0075] (i) placing the film on a plane; [0076] (ii)
rotating the film 70 degrees on an in-plane leading phase axis of
the film; and [0077] (iii) measuring the retardation values from a
direction perpendicular to the original plane at wavelengths 589
and 630 nm, respectively. [0078] R.sub.50' (589) and R.sub.50'
(630) each being a retardation value of the optical compensation
film measured by the same method as R.sub.70' (589) and
R.sub.70'(630) except for that the film being rotated 50 degrees on
the in-plane leading phase axis of the film; [0079] the in-plane
leading phase axis representing an axis in the plane of the film
being orthogonal to the in-plane delayed phase axis. 4. The optical
compensation film of any one of Items 1 to 3 wherein retardation
values of the optically biaxial support and the optically
anisotropic layer measured at a wavelength of 589 nm satisfy
Formula (5),
[0079] 0.44<((R(b)o+R(b)t)/R(e)o)<2.05 Formula (5) [0080]
R(b)o representing an in-plane retardation value of the support
defined by Formula (6); [0081] R(b).sub.t representing an out-of
plane retardation value of the support in a thickness direction
defined by Formula (7); and [0082] R(e)o representing an in-plane
retardation value of the optically anisotropic layer defined by
Formula (8),
[0082] R(b)o=(n(b)x-n(b)y).times.d(b) Formula (6)
R(b)t=((n(b)x+n(b)y)/2-n(b)z).times.d(b) Formula (7)
R(e)o=(n(e)x-n(e)y).times.d(e) Formula (8) [0083] n(b).sub.x
representing a refractive index in a x direction which gives a
maximum refractive index in the plane of the support; [0084]
n(b).sub.y representing a refractive index in a y direction which
is orthogonal to the said x direction in the plane of the support;
[0085] n(b).sub.z representing a refractive index in a thickness
direction of the support; [0086] d(b) representing a thickness (nm)
of the support; n(e)x representing a refractive index in a x
direction which gives a maximum refractive index in the plane of
the optically anisotropic layer; [0087] n(e)y representing a
refractive index in a y direction which is orthogonal to the x
direction in the plane of the optically anisotropic layer; and
[0088] d(e) representing a thickness (nm) of the optically
anisotropic layer. 5. The optical compensation film of any one of
Items 1 to 4 wherein the liquid crystalline compound comprises a
nematic liquid crystalline compound. 6. A polarizing plate
comprising: [0089] (i) the optical compensation film of any one of
Items 1 to 5; and [0090] (ii) a polarizing film or a polarizing
element, wherein an angle between an in-plane delayed phase axis of
the optical compensation film and an absorbing axis of the
polarizing film or of the polarizing element is 85-95 degrees. 7. A
vertically aligned ECB type liquid crystal display having the
polarizing plate of Item 6 on one surface or on both surfaces of a
liquid crystal cell of the liquid crystal display. 8. A vertically
aligned ECB type liquid crystal display having the polarizing plate
of Item 6 on one surface or on both surfaces of a liquid crystal
cell of the liquid crystal display wherein So(.lamda.) and
C(.lamda.) satisfy Formulae (9)-(12):
[0090] 0.21.times.C(589).ltoreq.So(589).ltoreq.0.66.times.C(589)
Formula (9)
0.22.ltoreq.Ps(.lamda.)/Pc(.lamda.).ltoreq.1.75 Formula (10)
Ps(.lamda.)=(So(.lamda.)/So(589)-1).times.100 Formula (11)
Pc(.lamda.)=(C(.lamda.)/C(589)-1).times.100 Formula (12) [0091]
So(.lamda.) representing a sum of in-lane retardation values
(Ro(.lamda.)) of [0092] (i) the polarizing films or the polarizing
elements of the polarizing plates of claim 6 provided on one
surface or both surfaces of the said liquid crystal cell; [0093]
(ii) the optical compensation films in the polarizing plates; and
[0094] (iii) the liquid crystal cell, [0095] C(.lamda.)
representing a product of a birefringence .DELTA.n(.lamda.) and a
thickness of the liquid crystal cell, .DELTA.n(.lamda.).times.d;
and [0096] .lamda. representing a wavelength in a range of 500-670
nm.
[0097] FIG. 1 gives a schematic illustration of the measurements of
R.sub.50(589), R.sub.70(589), R.sub.50' (.lamda.) and R.sub.70'
(.lamda.) described above. Each number in FIG. 1 represents below:
1: reference plane; 2: optical compensation film; 3: in-plane
delayed phase axis or in-plane leading phase axis; 4: rotating
angle (50 or 70 degrees); and 5: measuring direction.
[0098] Details of the present invention will be described
below:
[0099] The process in which the present inventors have developed
the optical compensation film of the present invention will be
described.
[0100] In the field of liquid crystal displays, the shift to large
size displays was achieved after the viewing angle problem in
oblique directions was solved by the use of retardation film in VA
mode LCDs. The result of hue tests carried out for VA mode LCDs in
a normal color display mode (not in a black display mode) was
evaluated highly in comparison to other monitors.
[0101] However, as a result of: (i) the broadening of a viewing
angle in oblique directions over 80 degrees; (ii) the observation
from a wide angle (which had been impossible so far); and (iii) the
shift to large size displays, red hue in black areas of a display
has become a definite problem which had been tolerable in lesser
viewing angles. The red hue problem has been difficult to solve
merely by optimizing the retardation value of optical film at a
wavelength of 550 nm or by adjusting a wavelength dispersion in a
frontal direction.
[0102] The present inventors have widely examined the red hue
problem in a black area of a display in VA mode LCD and have found
that the optical compensation film notably reduces the generation
of red hue in a black areas of the display even under observation
from a viewing angle over 80 degrees in any directions, namely: The
optical compensation film wherein: (i) molecules of a liquid
crystalline compound are oriented in a manner that an average tilt
angle of the molecules is not more than 4 degrees; (ii) the said
orientation is fixed; (iii) in-plane retardation values Ro(.lamda.)
(Formula (1)) are: at wavelength 589 nm: 40-130 nm; at 600 nm:
35-125 nm; and at 630 nm: 35-120 nm; (iv) out-of-plane retardation
values in the thickness direction Rt(.lamda.) (Formula (2)) are
120-400 nm in .lamda. range of 500-630 nm; and (v) Values of
Ro(589), Ro(600) and Ro(630) satisfy Formula (3) as follows:
1.0<B<7.0
B=(Ro(589)-Ro(630))/(Ro(600)-Ro(630)).
<Optically Biaxial Support>
[0103] The optical compensation film of the present invention is
characterized by an optically biaxial support having at least one
optically anisotropic layer on it.
[0104] The optically biaxial support is characterized in that nx,
ny and nz, which represent three dimensional components of the
refractive index of the optically biaxial support, satisfy Formula
(1) (This state is referred to as exhibiting "a biaxial plate
characteristic").
nx>ny>nz Formula (1)
[0105] In addition, the in-plane retardation value and the
out-of-plane (thickness direction) retardation value are
different.
[0106] Herein, the in-plane retardation value (Ro) and the
out-of-plane (thickness direction) retardation value (Rt) are
defined as follows:
[0107] (Thickness direction retardation value (Rt))
Rt=((nx+ny)/2-nz).times.d [0108] (An in-plane retardation value
(Ro))
[0108] Ro=(nx-ny).times.d
[0109] In case of nx>ny, the in-plane retardation value
obviously never becomes 0.
[0110] In the present invention, the expression "optically biaxial"
is defined as (i) the three dimensional components of the
refractive index satisfy Formula (1) (this state is said to as
exhibit "a biaxial plate characteristic") or (ii) the in-plane
retardation value is not 0.
[0111] Herein, nx represents the refractive index in the x
direction which gives the maximum in-plane refractive index, ny
represents a refractive index in the y direction which is
orthogonal to the x direction, and d represents the thickness of
the support (nm). Retardation values Ro and Rt are calculated from
nx, ny, nz values which are determined by means of an automatic
birefringence meter, for example, KOBRA-21ADH (manufactured by Oji
Keisokukiki Co., Ltd.) at 23.degree. C. and 55% RH employing a 590
nm wavelength light.
[0112] An optically biaxial support comprises a material exhibiting
a biaxial plate characteristic (to be explained later) as a major
component, a major component being a component whose content is
more than 50 percent by weight. Specific examples of a material
used as a major component of an optically biaxial support include,
for example: stretched cellulose ester (such as stretched cellulose
acetate propionate (CAP) and stretched cellulose triacetate (TAC)
and cycloolefin polymers (to which optically biaxial properties are
provided by biaxial stretching).
[0113] The optically biaxial properties in the films of cellulose
ester derivatives such as cellulose acetate propionate and
cellulose triacetate are provided when these films are biaxially
stretched.
[0114] Examples of cycloolefin polymer include olefin type
transparent plastics, for example: ZEONEX (manufactured by ZEON
Corp.), ARTON (manufactured by JSR Corp.) and APEL (manufactured by
Mitsui Chemicals).
[0115] The above mentioned optically biaxial support materials of
the present invention may contain other polymers such as those
listed below within a compositional range not deteriorating
optically biaxial properties. The examples include, foe example:
polycarbonate, polysulfone, polyetersulfone, polyacrylate and
polymethacrylate.
[0116] Various methods are available to provide an optical
anisotropy to a cycloolefin polymer based film (support) and above
mentioned synthesized polymer films. A film having a desired
optical anisotropy is formed by adjusting a stretching direction or
a stretching magnification during film forming process.
<Optically Anisotropic Layer>
[0117] On the above mentioned optically biaxial support, optically
anisotropic layers are formed. In the present invention, at least
one of the optically anisotropic layers has following properties
(optical property, thickness and orientation of a liquid
crystalline compound):
[0118] An optically anisotropic layer is formed on a support by the
following method: (i) coating a liquid crystalline compound,
examples of which will be listed below, (ii) orienting the liquid
crystalline molecules in a manner that the average tilt angle of
the molecules is not more than 4 degrees by using a rubbing method
or an orienting treatment using light both of which are known in
the art and (iii) fixing the orientation of the liquid crystalline
molecules. Thickness of the anisotropic layer is preferably 0.1-10
.mu.m and more preferably 0.2-5 .mu.m.
<Measurement of Average Tilt Angle>
[0119] The average tilt angle is defined as the average angle
between directors of liquid crystalline molecules and the plane of
the optically anisotropic layer. Measurement of the average tilt
angle is carried out by the well known method in the LCD industry,
for example, using automatic birefringence analyzer KOBRA21-ADH.
(Oji Scientific Instruments). When an optically positive liquid
crystalline molecule is used, an average tilt angle is calculated
from (i) the angle which gives the maximum retardation value when a
film is rotated on a leading phase axis of the film, and (ii) the
refractive index of the optically anisotropic layer, based on
Snell's Law. When an optically negative liquid crystalline compound
is used, an average tilt angle is determined in a same manner as
above, except for using an angle which gives a minimum retardation
value when the film is rotated on a delayed phase axis of the
film.
<Optical Properties of Optically Biaxial Support and Optically
Anisotropic Layer>
[0120] In the present invention, the retardation values of the
support and of one of the optical compensation layers of the
present optical compensation film, at a wavelength of 589 nm,
preferably satisfy Formula (5).
0.44<((R(b)o+R(b)t)/R(e)o)<2.05 Formula (5)
R(b)o representing an in-plane retardation value of the support
described by above mentioned Formula (6); R(b).sub.t representing
an out-of-plane retardation value of the support in a thickness
direction described in above mentioned Formula (7); and R(e)o
representing an in-plane retardation value of the optically
anisotropic layer described in above Formula (8).
[0121] The optical compensation film of the present invention may
include a plurality of optically anisotropic layers. In such case,
d(e) in Formula (8) indicates the total thickness of the plurality
of optically anisotropic layers.
<Liquid Crystalline Compound>
[0122] Liquid crystalline compounds used in the present invention
will be explained.
[0123] A liquid crystalline compound used in the present invention
may either be a low molecular weight liquid crystalline compound or
a high molecular weight liquid crystalline compound. With respect
to optical properties, a positive uniaxial rod-shaped liquid
crystalline compound or a biaxial liquid crystalline compound is
preferably used. A negative uniaxial compound such as a discotic
liquid crystalline compound may also be used. As for optically
biaxial liquid crystalline compounds, a rod-shaped compound is
preferably used, while a discotic liquid crystalline compound
having a broadened shape may also be used.
[0124] Optically negative uniaxial liquid crystal compounds
specifically include, for example: discotic liquid crystalline
compounds such as described in "Ekisho no Kagaku": Quarterly
Publication, Kagaku Sosetsu, No. 22 (Chemistry of Liquid Crystal
Compounds), pp 60-72 (1994) published by the Chemical Society of
Japan, more specifically, liquid crystalline compounds having
molecular structures 1 through 46 shown on p62 of the same
publication, and liquid crystalline compounds disclosed in Japanese
Patent No. 2,587,398, 2,640,083, 2,641,086, 2,692,033, 2,692,035,
2,767,382 and 2,747,789.
[0125] A rod-shaped liquid crystalline compound exhibiting positive
uniaxial properties (also referred to as having positive
uniaxiality) and an optically biaxial liquid crystalline compound
exhibiting an optical property similar to that of a rod-shaped
liquid crystalline compound may be treated as a rod-shaped liquid
crystalline compound. Herein, "positive uniaxiality (being
optically uniaxial)" indicates the following. In an anisotropic
element having optical anisotropy, among the refractive values of
nx, ny, nz for three dimensional axes, only two of them are the
same, and further, two refractive values are smaller than the other
refractive value. "Having biaxiality" indicates that each of the
refractive values nx, ny, nz has a different value.
[0126] The positive uniaxial rod-shaped liquid crystalline compound
of the present invention may be a compound having a positive or
negative dielectric constant anisotropy, but is preferably a
compound having a positive dielectric constant anisotropy in view
of easiness in controlling inclination of optical axes of the
crystal liquid molecules in the thickness direction of the
sheet.
[0127] Dielectric constant anisotropy (.DELTA..di-elect cons.) of
the rod-shaped liquid crystalline compound is represented by the
difference .DELTA..di-elect cons.(.DELTA..di-elect cons.=.di-elect
cons..parallel.-.di-elect cons..perp..noteq.0) between dielectric
constant (.di-elect cons..parallel.) in a state in which the major
axis of the molecule is oriented parallel to the electric field
direction, and dielectric constant (.di-elect cons..perp.) in a
state in which the minor axis of the molecule is oriented parallel
to the electric field direction. Dielectric constant anisotropy
(.DELTA..di-elect cons.) directly influences anisotropic property
of refractive index of light transmitting liquid crystal molecules.
The relationship among .DELTA..di-elect cons. and anisotropic
refractive indices is represented by the following formula:
.DELTA..di-elect cons.=(n.parallel.).sup.2-(n.perp.).sup.2
wherein n.parallel. represents a refractive index to light in the
orientation vector direction of liquid crystal molecules, and n
.perp. represents a refractive index to light in the direction
perpendicular to the orientation vector of liquid crystal
molecules.
[0128] Herein, .DELTA..di-elect cons. or .DELTA.n is a positive
value in the liquid crystalline compound used for driving an
ordinary TN liquid crystal cell.
[0129] In the low molecular weight liquid crystalline compound the
whole molecule determines the optically anisotropic property
(typically, refractive index anisotropy) of the liquid crystalline
compound. A polymer liquid crystalline compound is divided into two
kinds, and the one is a main chain type polymer liquid crystalline
compound and the other a side chain type polymer liquid crystalline
compound. In both types of polymer liquid crystalline compounds, a
structure containing a mesogenic group determines the optically
anisotropic property in a similar manner as in the low molecular
weight compound.
[0130] The mesogenic group (or a mesogenic unit) as described above
refers to a part necessary for developing a liquid crystal property
in a liquid crystalline compound, and the mesogenic group
ordinarily comprises a core group being a rigid part, a spacer
group being a flexible part, and an end group positioned at the
molecular end. However, the mesogenic group does not require all of
the above three groups as long as a liquid crystalline compound has
a group developing a liquid crystal property.
[0131] Examples of the positive uniaxial liquid crystalline
compounds will be shown below, but they are not limited
thereto.
##STR00001## ##STR00002##
[0132] In addition to the above exemplified compounds, compounds
described in "Ekisho no Kagaku": Quarterly Publication, Kagaku
Sosetsu, No. 22 (1994), pp. 42, 44 edited by Nihon Kagakukai
(Gakkai Shuppan Center)" can be used. The above described
rod-shaped liquid crystalline compounds having a positive
uniaxiality can be suitably used as ordinary rod-shaped nematic
liquid crystalline compounds used in TN cells.
[0133] Compounds developing a nematic liquid crystal phase are
preferably used as the rod-shaped liquid crystalline compounds in
the present invention.
[0134] The biaxial liquid crystalline compounds include compounds
described in Yukigoseikagaku, Vol. 49, No. 5 (1991), pp. 124-143,
compounds described in D. W. Bruce et al., AN Eu-Sponsored' Oxford
Woershop On Biaxial Nematics' (St. Benet's Hall, University of
Oxford 20-22 Dec., 1996), p. 157-293, compounds described in S.
Chandrasekhar et al., A Thermotropic Biaxial Nematic Liquid
Crystal, Mol. Cryst. Liq. Cryst., 1988, Vol. 165, pp. 123-130, and
compounds described in D. Demus, J. Goodby et al., Handbook of
Liquid Crystals, Vol. 2B, Low Molecular Weight Liquid Crystals II,
pp. 933-943, published by WILEY VCH Co.
[0135] The liquid crystal polymers of the present invention are not
specifically limited, but are preferably ones having a positive or
negative specific double refractive index, which are detailed in
"Liquid Crystals, 1989, Vol. 5, No. 1, pp. 159-170".
[0136] The polymer liquid crystalline compounds having a mesogenic
group used in the present invention are divided into two kinds: one
is a polymer having a mesogenic group in the main chain; and the
other a polymer having a mesogenic group in the side chain. The
polymer liquid crystalline compounds used in the present invention
are also divided into different two kinds, namely, thermotropic and
liotropic polymer liquid crystalline compounds.
[0137] The liquid crystal polymers in the invention are not
specifically limited, but are preferably ones forming a nematic
liquid crystal. The polymer having a mesogenic group in the side
chain is preferable in orientation property, and a thermotropic
liquid crystal polymer is preferable in orientation fixation. The
main chain used in the above side chain type liquid crystal
polymers is preferably that of a vinyl type polymer, polysiloxane,
polypeptide, polyphosphazene, polyethylene imine, and
cellulose.
[0138] In the present invention, molecules of an optically
anisotropic compound are preferably oriented in the form of a
monodomain or in the form of multi-domains, each domain of which is
smaller than 0.1 .mu.M. Herein, "monodomain" means having no
disclination, however, in the present invention, disclinations may
be basically allowable when the domain size is less than 0.1
.mu.m.
[0139] Above mentioned disclination means an existence of minute
portions in which optically anisotropic molecules are not oriented.
Existence of disclinations may cause various problems such as a
decline of contrast of the display or lower tilt angles compared to
designed values.
<Birefringent Behavior of Optical Compensation Layer>
[0140] Following aspects of the birefringent anisotropic layer of
the present invention will be explained, namely: C plate (C plate
compensation); A plate (A plate compensation); biaxial plate
(biaxial plate compensation); and O plate (O plate
compensation).
<C Plate>
[0141] The C plate of the present invention will be explained.
[0142] The relationships among the three-dimensional components nx,
ny and nz composing a refractive index of a layer are as
follows:
nx=ny>nz
[0143] Herein, the in-plane retardation in the C plane: Ro=0.
[0144] Materials (optical materials, resin films) exhibiting the C
plate properties include, for example: discotic liquid crystal
compounds, non-stretched cellulose ester films (such as
non-stretched cellulose triacetate (TAC) and non-stretched
cellulose acetate propionate (CAP)) and biaxially stretched
norbornene resin films. The C plate is also produced by a
controlled biaxial stretching of the above mentioned resins.
<A Plate>
[0145] The A plate of the present invention will be explained.
[0146] The relationships among the three-dimensional components nx,
ny and nz composing a refractive index of a layer are as
follows:
nx>ny=nz
[0147] Herein, the in-plane retardation in the A plane Ro almost
equals to 2.times.Rt.
[0148] Materials (optical materials, resin films) exhibiting the A
plate properties include, for example: rod-shaped liquid
crystalline compounds and uniaxially stretched polymer (such as
polycarbonates).
<Biaxial Plate>
[0149] The biaxial plate of the present invention will be
explained.
[0150] The relationships among the three-dimensional components nx,
ny and nz composing a refractive index of a layer are as
follows:
nx>ny>nz
[0151] Herein, the in-plane retardation and the out-of-plane
retardation in the thickness direction are different.
[0152] Materials (optical materials, resin films) exhibiting the
biaxial plate properties include, for example: stretched cellulose
ester (such as stretched cellulose acetate propionate (CAP) and
stretched cellulose triacetate (TAC)).
<O Plate>
[0153] In the O plate of the present invention, utilized is a
positive birefringent compound the main optical axis of which is
oriented substantially in an oblique direction to the display
surface (this is the reason why it is called an O plate).
[0154] "Substantially in an oblique direction" means that the angle
is more than 0 degree and less than 90 degrees.
[0155] The 0 plate of the present invention may contain a compound
which is either uniaxial or biaxial.
<Optical Compensation Film>
[0156] The optical compensation films of the present invention will
be explained.
<Retardation Properties of Optical Compensation Films>
[0157] The optical compensation film of the present invention
contains a support exhibiting optical biaxiality, on which at least
one optically anisotropic layer is provided,
[0158] wherein: [0159] (i) the optically anisotropic layer is
formed by orienting molecules of a liquid crystalline compound in a
manner that an average tilt angle of the molecules is not more than
4 degrees followed by fixing the orientation of the liquid crystal
molecules; [0160] (ii) in-plane retardation values of the optical
compensation film Ro(.lamda.) given by above mentioned [0161]
Formula (1) are as follows,
[0161] Ro(589): 40-130 nm
Ro(600): 35-125 nm
Ro(630): 35-120 nm [0162] .lamda. representing a wavelength (nm) at
which a measurement is carried out; [0163] (iii) an out-of-plane
retardation value in a thickness direction of the optical
compensation film Rt(.lamda.) represented by above mentioned
Formula (2) is in the range of 120-400 nm when measured at
wavelengths of 500-630 nm; and [0164] (iv) Ro(589), Ro(600) and
Ro(630) satisfy above mentioned Formula (3).
[0165] The B value of Formula (3) is necessarily more than 1 and
less than 7.0 to attain the properties described in the present
invention, however, it is preferably 1.3-6.0 and more preferably
1.4-5.0.
<Optical Properties of Film Rotated on In-Plane Delayed Phase
Axis>
[0166] Optical properties of an optical compensation film which is
rotated on a delayed phase axis of the film:
[0167] (1) Rotating 70 degrees:
[0168] The (R.sub.70(589)/Ro(589)) value is preferably 1.25-1.40
and more preferably 1.3-1.4,
[0169] wherein, R.sub.70(589) representing a retardation value of
the optical compensation film measured via a method of: [0170] (i)
placing the film parallel to a reference plane; [0171] (ii)
rotating the film 70 degrees on an in-plane delayed phase axis of
the film; and [0172] (iii) measuring the retardation value from a
direction perpendicular to the reference plane at wave length 589
nm, and
[0173] Ro(589) representing an in-plane retardation of the film
measured at 589 nm.
[0174] (2) Rotating 50 degrees: [0175] The (R.sub.50(589)/Ro(589))
value is preferably 1.10-1.25 and more preferably 1.11-1.24, [0176]
wherein R.sub.50(589) representing a retardation value of the film
determined by 50 degrees rotating the film on the delayed phase
axis of the film and measuring from a direction perpendicular to
the reference plane.
[0177] R.sub.70(589) and R.sub.50(589) are determined in the same
manner as Ro(.lamda.) given by Formula (1) except that the film is
rotated, respectively, 70 and 50 degrees on the in-plane delayed
phase axis of the film and measured from a direction perpendicular
to the reference plane.
<Optical Properties of Film Rotated On In-Plane Leading Phase
Axis>: Wavelength Dispersion
[0178] The optical properties of the optical compensation film
rotated 70 and 50 degrees on the in-plane leading phase axis of the
film preferably satisfy Formula (4), namely, the C/D value in
Formula (4) is preferably lager than 0 and smaller than 7.0, more
preferably 0.4-6.0 and specifically preferably 0.4-5.0.
<Retardation-Enhancing Agent>
[0179] Retardation-enhancing agents used in the present invention
will be explained.
[0180] Retardation-enhancing agents of the present invention are
preferably added in a support such as a cellulose triacetate film
(also described as a polymer film) in order to optimize retardation
values at various wavelengths. The preferable amount of
retardation-enhancing agent is 0.05-20 weight parts in 100 weight
parts of a polymer film, more preferably 0.1-10 weight parts,
further more preferably 0.2-5 weight parts and specifically more
preferably 0.5-2 weight parts.
[0181] A retardation-enhancing agent may be used alone or in
combination with other retardation-enhancing agents. The
retardation-enhancing agent may preferably have maximum absorption
in a wavelength range of 250-400 nm. Moreover, it may preferably
have substantially no absorption in the visible region.
[0182] Specific examples of retardation-enhancing agents include
compounds disclosed in JP-A 2000-111,914,2000-275,434 2002-62,477
and European Patent No. 0,911,656A2.
<Orientation Layers (Orientation Films)>
[0183] Orientation films use in the present invention will be
described:
[0184] Orientation films used in the present invention may be
prepared by, for example: (i) a rubbing treatment of a film of an
organic compound (preferably a polymer film); (ii) oblique
evaporation of an inorganic material; (iii) formation of a layer
having microgrooves; or (iv) formation of stacked layers of an
organic compound (such as, .omega.-tricosanoic acid, dioctadecyl
methylammonium chloride and methyl stearate) using
Langmuir-Blodgett technique. Orientation films of which orientation
is generated by applying an electric or a magnetic field or by
irradiating light, are also known.
[0185] An orientation film prepared by rubbing a polymer film is
specifically preferable. The rubbing treatment is carried out by
rubbing the surface of a polymer film several times in one
direction with paper or cloth. Types of polymer used in orientation
films are described in the literature on optical compensation films
using discotic type liquid crystalline compounds applied for
various modes of displays described above.
[0186] The thickness of an orientation film used in the present
invention is preferably 0.01-5 .mu.m and more preferably 0.05-1
.mu.m. The following preparation method of an orientation film is
also available, in which a birefringent layer (optically
anisotropic layer) is oriented using a pre-oriented layer
(pre-oriented film) followed by transferring the oriented optical
anisotropic layer onto a transparent support.
[0187] In order to prepare an optically anisotropic layer on an
above mentioned orientation film (orientation layer), a monodamain
layer may be formed by applying a liquid crystalline compound or a
solution containing liquid crystalline compound on the orientation
film (orientation layer) followed by drying and heat treating (heat
orientation), thus, an oriented layer of liquid crystalline
compound is obtained.
[0188] Orientation of a liquid crystalline compound in a film or in
a layer may be fixed by the following methods: In a case when a
UV-curable liquid crystalline compound is used, an coating solution
of a liquid crystalline compound including a polymerization
initiator is applied and then the liquid crystalline compound is
oriented. Under a controlled temperature at which a liquid
crystallinity is maintained, the orientation is fixed via
irradiation of UV rays. A method of forming chemical bonds such as
thermal polymerization may also be used to fix an orientation. When
a polymer liquid crystalline compound is used, an orientation may
be fixed by heat treating an orientation layer within a temperature
range where liquid crystallinity is maintained, followed by
quenching the layer below the glass-transition temperature of the
polymer.
[0189] Liquid crystal molecules the orientation of which is fixed
may retain the oriented state even after the pre-oriented film is
removed.
<Polarizing Plate>
[0190] The polarizing plates used in the present invention will be
explained.
[0191] A polarizing plate of the present invention is prepared by
adhering at least one surface of a commonly used polarizing film
with an optical compensation film of the present invention. It may
also be prepared merely by forming a polarizing film or a
polarizing layer on one surface of an optical compensation film of
the present invention.
[0192] In order to prepare a polarizing film, an easily
stretched-orienting film such as a conventional polyvinyl alcohol
film is treated with a dichromatic dye such as iodine, followed by
stretching the film in the longitudinal direction. Since a
polarizing film itself is not sufficiently self-supporting nor has
enough durability, optically isotropic cellulose triacetate films
are generally adhered on both sides of the polarizing film as
protective films to form a polarizing plate.
[0193] The polarizing plate of the present invention may either be
prepared by adhering an optical compensation film of the present
invention onto the above mentioned polarizing plate or by directly
adhering optical compensation film of the present invention as a
protective film with a polarizing film. The method of adhering
films (also referred to as an adhering method) is not specifically
limited, and a glue comprising a solution of water-soluble polymer
may be used. An aqueous solution of fully saponified polyvinyl
alcohol is preferably used as a water-soluble polymer adhesive. A
long size polarizing plate may be prepared by adhering a long size
optical compensation film of the present invention with the above
briefly described long size polarizing film, which is stretched in
a longitudinal direction and treated with a dichromatic dye.
<Polarizing Layer (Polarizing Film)>
[0194] The polarizing layer of the present invention will be
described.
[0195] A polarizing film includes: an iodine type polarizing film,
a dye type polarizing film using a dichromatic dye and a polyene
type polarizing film. An iodine type polarizing film and a dye type
polarizing film are generally prepared from a polyvinyl alcohol
type film. The polarizing axis of a polarizing plate is preferably
orthogonal to the stretching direction of the film.
<Vertically Aligned ECB Mode Liquid Crystal Display>
[0196] The vertically aligned ECB mode liquid crystal display of
the present invention will be described.
[0197] In order to reduce red hue in black areas of a wide viewing
angle display, the polarizing plate of the present invention is
preferably used in a display in which vertically aligned liquid
crystals are employed, examples of which include: a VA (Vertically
Aligned) mode display and a MVA (Multi-domain Vertically Aligned)
mode display.
[0198] One of the examples of a construction of a single polarizing
plate-reflective type liquid crystal display may be shown as:
[protective film/polarizing film/optical compensation film (present
invention)/glass substrate/ITO transparent electrode/orientation
film/VA mode liquid crystal/orientation film/metal electrode also
working as a reflector of light/glass substrate]
[0199] Optical compensation films (also referred to as optical
compensation sheets) prepared so far have not sufficiently reduced
red hue which notably appears in large size liquid crystal
displays, however, by using the optical compensation sheet of the
present invention, red hue in black areas in the display is
drastically reduced even in a wide viewing angle.
[0200] A reflection type polarizing element comprising an optical
compensation film of the present invention, and a cholesteric mode
liquid crystal may be structured as follows: [back
light/cholesteric liquid crystal layer/optical compensation film of
the present invention/polarizing film/protective film].
[0201] In order to reduce red hue in black areas of a wide viewing
angle display, a liquid crystal display to be used with the optical
compensation film of the present invention is required to be of a
vertically aligned ECB mode. Examples of a liquid crystal cell to
be used in a vertically aligned ECB mode liquid crystal display
include, for example: a VA (Vertically Aligned) mode cell and a MVA
(Multi domained VA) mode cell.
[0202] In the vertically aligned ECB mode liquid crystal display of
the present invention, Formula (9) is preferably satisfied and the
Ps(.lamda.)/Pc(.lamda.) value of Formulae (10), (11) and (12) is
preferably 0.22-1.75 and more preferably 0.23-1.50.
EXAMPLES
[0203] The present invention will be explained according to
examples as follows, however, embodiments of the present invention
are not limited thereto.
Example 1
[0204] Optically biaxial Supports A-G, optical compensation films
of the present invention A-G, and optical compensation films for
comparison H and I were prepared as follows:
<Preparation of Optically Biaxial Support A>
[0205] Support A having optical biaxiality and exhibiting the
retardation values (Ro, Rt) shown in Table 1 was prepared as
follows: On a 80 .lamda.m thick triacetyl cellulose film
(manufactured by Konica Corp.), (i) a 0.1 .lamda.m thick gelatin
film was formed, (ii) Solution 1, composition of which was shown
below, was coated at a thickness of 0.2 .mu.m, (iii) the film was
dried, and (iv) the resulting film was subjected to a rubbing
treatment.
(Composition of Solution 1)
TABLE-US-00001 [0206] Compound 1 1 percent by weight Ion exchanged
water 98 percent by weight Methanol 1 percent by weight Compound 1
##STR00003##
<Preparation of Optically Biaxial Support B>
[0207] Optically biaxial Support B having a thickness of 80
.lamda.m and exhibiting the retardation values (Ro, Rt) shown in
Table 1 was produced using the same preparation method as Support A
except that a CAP (cellulose acetate propionate) solution was used
instead of a triacetyl cellulose solution.
[0208] The specific procedure of coating the CAP solution is as
follows: (i) a coating solution is uniformly flow-casted on an
endless 6 m long stainless-steel belt (effective length is 5.5 m);
(ii) the film is dried for 2 minutes by applying water of
35.degree. C. to the rear side of the stainless-steel belt; (iii)
water of 15.degree. C. is applied onto the rear side of the belt;
(iv) the film is dried until the residual solvent content is
reduced to 20%; (v) the film is peeled from the support at a
tension of 150 N/m; and (vi) the film is further dried.
Preparation of Cellulose Acetate Propionate Solution)
[0209] Following compositions were charged into a sealed and
pressurised vessel, and stirred at 80.degree. C. to form a
homogeneous solution of cellulose acetate propionate.
[0210] Cellulose acetate propionate (CAP) 120 weight parts [0211]
(substitution degrees of CAP were as follows:
TABLE-US-00002 [0211] Acetyl substitution degree: 1.9; propionyl
substitution degree: 0.75; and total substitution degree: 2.66)
2-(2'-hydroxy-3',5'-di-t-butylphenyl) benzotriazole 1 weight part
(UV absorbing agent) Ethylphthalylethyl glycolate (plasticizer) 4
weight parts Silica microparticles (Aerosil200 from Nippon 0.1
weight parts Aerosil Co., Ltd., mean particle diameter: 0.016
.mu.m) Methyl acetate 300 weight parts Ethanol 45 weight parts
Preparation of Optically Biaxial Supports C and D>
[0212] Optically biaxial Supports C and D were prepared in the same
manner as Support B except that, after the flow-casted CAP film was
dried and peeled from the support when the content of residual
solvent became lower than 45 percent, the web (film) was stretched
in the machine direction (MD) to form Support C and in the
transverse direction (TD) to form Support D by clipping both sides
of the web using tenter hooks. Each of the stretching
magnifications in both MD and TD directions was in the range of
1.1-1.5. The difference in the optical property of Supports C and D
is shown in Table 1.
<Preparation of Optically Biaxial Support E>
[0213] Support E exhibiting the retardation values (Ro, Rt) shown
in Table 1 was prepared in the manner described below. In the
present invention, support E is also referred to as cycloolefin
polymer film E.
[0214] Process 1: As a polymerization catalyst, 10 weight parts of
15 percent solution of triethylaluminum in cyclohexane, 5 weight
parts of triethylamine and 10 weight parts of titanium
tetrachloride were added to
6-methyll,4,5,8-dimethanol-1,4,4a,5,6,7,8,8a-octahydronaphthalene
for a ring-opening polymerization in cyclohexane. A polymer was
obtained by hydrogenating the product. The polymer was coagulated
in isopropyl alcohol and dried to form resin powder. The resin
showed the following characteristics: the number average mean
molecular weight was 40,000; the degree of hydrogenation was not
less than 99.3 percent; and Tg was 142.degree. C.
[0215] Process 2: The resin powder obtained in process 1 was
pelletized by melting at 250.degree. C. A sheet was prepared by
melt-extruding the pellets through a T-die of 300 mm width using a
uniaxial extruder having a 40 mm fullflight screw. The obtained
film was cooled and wound using three rollers, each of which had a
diameter of 300 mm. The temperature of the film at the die was
270.degree. C., and the temperatures of the 1st, 2nd and the 3rd
rollers were, respectively, 120, 120 and 100.degree. C.
[0216] Process 3: The sheet was stretched at a controlled
temperature of 140.+-.2.degree. C. to form a 51 .mu.m film.
[0217] Process 4: 2 g of PMMA (polymethyl methacrylate) was
dissolved in a mixed solvent of 45 g of MEK (methyl ethyl ketone)
and 5 g of cyclopentanon and coated on the stretched and oriented
film. The film was then treated with a rubbing instrument to form
optically biaxial Support E. The direction of rubbing was parallel
to the delayed phase axis of the film.
<Preparation of Optically Biaxial Support F>
[0218] Support F exhibiting the retardation values (Ro, Rt) shown
in Table 1 was prepared in the manner described below. In the
present invention, Support F is also referred to as cycloolefin
polymer film F.
[0219] A 100 .mu.m polynorbornene film (trade name "ARTON FILM"
manufacture by JSR Corp.) was biaxially stretched at 180.degree. C.
to a longitudinal stretching magnification of 1.10 and a lateral
stretching magnification of 1.15 using a simultaneous biaxial
stretcher. The thickness of the central part of the film was 81
.mu.m, the width of the film was 345 mm.
[0220] 2 g of PMMA (polymethyl methacrylate) was dissolved in a
mixed solvent of 45 g of MEK (methyl ethyl ketone) and 5 g of
cyclopentanon, and then coated onto the above mentioned oriented
film by stretching. The film was then treated in a rubbing
instrument to form a optically biaxial Support F. The rubbing
direction was parallel to the delayed phase axis of the film.
<Preparation of Optically Biaxial Support G>
[0221] Support G exhibiting retardation values (Ro, Rt) shown in
Table 1 was prepared in the manner described below. In the present
invention, Support G is also referred to as cycloolefin polymer
film G.
[0222] A 100 .mu.m thick polynorbornene type film (trade name
"ARTON FILM" manufactured by JSR Corp.) was biaxially stretched at
180.degree. C. to a longitudinal stretching magnification of 1.10
and a lateral stretching magnification of 1.15 using a simultaneous
biaxial stretcher. The film was then left at 180.degree. C. for 60
seconds to reduce the film width down to 98%. The thickness of a
central part of the film was 81 .mu.m, and the width of the film
was 345 mm.
[0223] 2 g of PMMA (polymethyl methacrylate) was dissolved in a
mixed solvent of 45 g of MEK (methyl ethyl ketone) and 5 g of
cyclopentanon, and subsequently coated on the stretched and
oriented film. The film was then treated with a rubbing instrument
to form an optically biaxial Support F. The rubbing direction was
parallel to the delayed phase axis of the film.
<Preparation of Support H for Comparison>: A support Which
Does Not Show Optical Biaxiality.
[0224] Support H for comparison was prepared as follows: 100 g of
polycarbonate was dissolved in 500 g of methylene chloride and
flow-casted on a stainless-steel belt. After the resulting film was
peeled, the film was biaxially stretched.
<Preparation of Support I for Comparison>: A support Which
Does not show optical biaxiality.
[0225] A 100 .mu.m thick TAC film was used as Support I for
comparison.
[0226] Retardation values (Ro, Rt) of optically biaxial Supports
A-G and Supports H and I for comparison are given in Table 1.
TABLE-US-00003 TABLE 1 Support Optical No. Ro(589 nm)) Rt(589 nm)
biaxiality A 1.7 54 Yes B 2.2 108 Yes C 15 116 Yes D 30 206 Yes E
1.1 30 Yes F 2.2 45 Yes G 15 230 Yes H 125 62.5 No I 0 71 No
[0227] Table 1 reveals that Supports A-G, the in-plane retardation
values Ro of which are not 0, are optically biaxial supports of the
present invention and Supports H and I do not show optical
biaxiality.
<Preparation of Optical Compensation Films A-G> of the
Present Invention:
[0228] Optical compensation films A-G of the present invention
having optically anisotropic layers were prepared as follows: A
solution having the following composition was coated on the
optically biaxial Supports A-G obtained as above and dried at
100.degree. C. for 15 seconds. The resulting films were gradually
cooled and UV-cured at 45.degree. C.
(Composition of Coating Solution of Optically Anisotropic
Layer)
TABLE-US-00004 [0229] MEK 86 weight parts Compound 2 3 weight parts
Compound 3 2 weight parts Compound 4 3 weight parts Compound 5 3
weight part Irgacure 369 (CIBA SPECIALTY CHEMICALS) 1 weight part
Compound 2 ##STR00004## Compound 3 ##STR00005## Compound 4
##STR00006## Compound 5 ##STR00007##
[0230] Each hardened layer containing one of the liquid crystalline
compounds 2-5 was an optically anisotropic layer of a thickness of
1.4 .mu.m. The average tilt angle of each anisotropic layer was
measured by KOBPA-21ADH (manufactured by Oji Keisokukiki Co., Ltd.)
and was found to be no greater than 4 degrees.
<Preparation of Optical Compensation Film H for
Comparison>
[0231] Support H was coated with Solution 1 which had been used for
the preparation of optically biaxial Support A. The resulting film
was dried, subjected to a rubbing treatment and was coated with the
above described coating solution of an optically anisotropic layer.
After that, optical compensation film H, for comparison, was
prepared through the same treatment as that of optical compensation
films A-G.
<Preparation of Optical Compensation Film I for
comparison>
[0232] Optical compensation film I was prepared as follows: (i) A
gelatin layer was formed on Support I prepared for comparison; (ii)
a PVA solution was coated, and then dried; (iii) a coating solution
was applied; (iv) heat treated for drying; and (v) after confirming
that a monodomain was formed, the film was hardened by UV
irradiation. Formation of monodomain was confirmed by observing a
small portion of the sample with a polarizing microscope.
[0233] Optical properties of optical compensation films A-G
obtained as above and those of optical compensation films H and I
for comparison are given in Table 2.
TABLE-US-00005 TABLE 2 Optical compensation R.sub.0(.lamda.nm)
R.sub.t(.lamda.nm) film No. 589 600 630 589 B* Remarks A 40 39.6
38.4 150 1.33 Inv. B 83 81.3 78.3 266 1.57 Inv. C 107.9 106 102.2
308 1.50 Inv. D 98.2 97.4 96.3 288 1.73 Inv. E 125 123 120 134 1.67
Inv. F 125 123 117 169 1.33 Inv. G 110 107.8 107.1 281 4.14 Inv. H
125.0 122.0 121.0 120.5 7.00 Comp. I 0 0 1.1 1.2 0.00 Comp. B* =
(R.sub.0(589) - R.sub.0(630))/(R.sub.0(600) - R.sub.0(630)) Inv.:
Present Invention Comp.: Comparison
[0234] The optical properties of the optically biaxial supports,
optically anisotropic layers and optical compensation films
(refractive indices, in-plane retardation values, and out-of-plane
retardation values in the thickness direction) were calculated from
the three dimensional refractive indices nx, ny, nz which were
determined via three dimensional measurement of refractive indices
using an automatic birefringence analyzer KOBPA21-ADH. (Oji
Scientific Instruments) at wavelengths of 589, 600, and 630 nm at
23.degree. C. under 55% RH (relative humidity).
<Preparation of Polarizing Plates A-G of the Present Invention
and Polarizing Plates H and I for Comparison>
[0235] Polarizing plates A-G of the present invention, and
polarizing plates H and I for comparison were prepared as follows:
Each of the polarizing plates was prepared by adhering (directly
glued or using adhesive sheets) optical compensation films of one
of A-I on both sides of a polarizing film, of which the preparation
method is described below.
[0236] A commercially available PVA type glue was used for adhering
a polarizing film with optical compensation films to form
polarizing plates A-D, while 25 .mu.m adhesive sheets was used to
prepare polarizing plates E-G of the present invention and H and I
for comparison.
(Preparation of Polarizing Film)
[0237] A polarizing plate was prepared as follows: (i) a
non-stretched PVA film of a thickness of 80 .mu.m, a width of 800
mm, and a polymerization degree of 2400 was stained with iodine in
an aqueous solution of 0.3 percent by weight of iodine; (ii) the
film was stretched to a factor of 6.8 in an aqueous solution of 4
percent by weight of boric acid and 3 percent by weight of
potassium iodide; (iii) the film was dried at 40.degree. C. for 4
minutes; and (iv) the film was wound onto a roll.
<Preparation of Liquid Crystal Displays LCDa-LCDi>
[0238] LCDa-LCDi were prepared by: (i) removing polarizing plates
of commercially available MVA mode LCDs (LCDAD19H manufactured by
IO DATA Corp.); (ii) on each LCD, a pair of the polarizing plates
of any one of A-G of the present invention and H and I for
comparison were adhered so as to match the delayed phase axes and
leading phase axes of the polarizing plate with the directions of
those of the original polarizing plate.
[0239] However, for polarizing plates D and G, a polarizing plate
was adhered on only one side of a LCD, and on the other side a TAC
film (Ro(589)=0.2 nm, Rt(589)=31 nm) manufactured by Konica Corp.
was adhered so as to match the delayed phase axis and leading phase
axis of the TAC film with the directions of those of the original
polarizing plate.
<Evaluation of Red Hue in Black Areas of the Display>
[0240] Evaluation of red hue in black areas of the display for
liquid crystal displays LCDa-LCDi was carried out by visual
observations (sensory observations) of randomly selected 30
subjects under conditions of (i) applying no voltage to the
display; and (ii) at a viewing angle of 80 degrees. The results of
a majority of these observations are given below:
A: Substantially no red hue is observed.
[0241] Slight red hue is observed but it is not a practical
problem.
C: Appreciable red hue is observed so that the display is not
suitable for practical use.
TABLE-US-00006 LCD No. Evaluation of red hue LCDa B LCDb B LCDb B
LCDc B LCDd B LCDe B LCDf B LCDg B LCDh C LCDi C
[0242] The above results revealed that, in liquid crystal displays
a-g using optical compensation films A-G of the present invention,
occurrence of red hue was only slight and was not a practical
problem, while in liquid crystal displays h and i in which
polarizing plates having respectively optical compensation films H
and I for comparison, were adhered on liquid crystal cells,
occurrence of noticeable red hue was observed and the LCDs were not
practically suitable.
Example 2
Preparation of Optical Compensation Films A1-G1 and I1
[0243] Optical compensation films A1-G1 (of the present invention)
and I1 (for comparison) were prepared in the same manner as
described in Example 1 for the preparation of optical compensation
films A-G (of the present invention) and I (for comparison) except
that the optical compensation films were prepared to exhibit the
following properties: for each of the optical compensation films
A1-G1 and I1, the ratio of retardation values A
(R.sub.70(589)/Ro(589)) is in the range of 1.25-1.40 and the ratio
of retardation values B (R.sub.50(589)/Ro(589)) is in the range of
1.10-1.25.
TABLE-US-00007 TABLE 3 Optical compensation film No.
R.sub.50(589)/R.sub.0(589) R.sub.70(589)/R.sub.0(589) Remarks A1
1.12 1.26 Inv. B1 1.24 1.38 Inv. C1 1.15 1.30 Inv. D1 1.11 1.26
Inv. E1 1.07 1.18 Inv. F1 1.20 1.31 Inv. G1 1.27 1.44 Inv. I1 0.04
0.02 Comp. Inv.: Present Invention Comp.: Comparison
[0244] The in-plane retardation values and the wavelength
dispersions of the obtained films were evaluated as follows:
<In-Plane Retardation Values and Wavelength Dispersions>
[0245] Retardation ratio A is defined as (R.sub.70(589)/Ro(589)),
wherein, R.sub.70(589) representing a retardation value of the
optical compensation film measured by a method of: (i) placing the
film on a plane; (ii) rotating the film 70 degrees on a delayed
phase axis of the film; and (iii) measuring the retardation value
from a direction perpendicular to the original plane at a
wavelength of 589 nm, and Ro(589) representing an in-plane
retardation of the film measured at 589 nm. Retardation ratio B is
defined as (R.sub.50(589)/Ro(589)), wherein, R.sub.50(589)
representing a retardation value of the film determined in the same
manner as R.sub.70(589) except that the film is rotated 50 degrees
on the delayed phase axis of the film and Ro(589) represents the
same as described above. Retardation ratios A and B were determined
as follows:
(1) Each In-Plane Retardation Values and Wavelength dispersion.
[0246] KOBRA-21ADH (manufactured by Oji Keisokukiki Co., Ltd.) was
used for a measurement of a rotating angle of 50 degrees or
less.
[0247] For a measurement of a rotating angle of more than 50
degrees, a spectroellipsometer DVA36VW (manufactured by Mizojiri
Optical Co., Ltd.) was used.
(2) Refractive indices (nx, ny and nz) at various wavelengths.
[0248] Refractive indices at various wavelengths were determined by
KOBRA-21ADH. An Abbe refractometer and a spectrometric light source
were also used for the determination of wavelength dispersion.
[0249] In a liquid crystal display employing one of the optical
compensation films of the present invention, of which the
retardation ratio A (R.sub.70(589)/Ro(589)) is in the range of
1.25-1.40 and the retardation ratio B (R.sub.50(589)/Ro(589)) is in
the range of 1.10-1.25, the red hue problem was improved to a level
in which substantially no red hue was observed, while in a liquid
crystal display employing optical compensation film I for
comparison, a noticeable red hue was observed and the display was
not in a level of commercialization.
Example 3
Preparation of Optical Compensation Films A2-G2
[0250] Optical compensation films A2-G2 were prepared in the same
manners as those of A-G of Example 1 except for that the films were
prepared to exhibit the ((R(b)o+R(b).sub.t/R(e)o) values in the
range of 0.44-2.05.
<Optical Properties of Optically Biaxial Support and Optically
Anisotropic Layers>
[0251] In-plane retardation values of the optical compensation
films of the present invention were determined and the values of
the ratio of Formula (5) were calculated:
TABLE-US-00008 Optical Compensation Film ((R(b)o + R(b)t/R(e)o) A2
1.83 B2 1.67 C2 2.11 D2 2.00 E2 0.32 F2 0.47 G2 1.68
[0252] In the liquid crystal displays employing optical
compensation films A2-G2 of the present invention which were
prepared to exhibit the ((R(b)o+R(b)t/R(e)o) values of not less
than 0.44 and less than 2.05, it was found that the appearance of
red hue in the black area of the display was reduced to a level in
which substantially no red hue was observed.
Example 4
[0253] Liquid crystal displays LCDa1-LCDh1 were prepared in the
same manner as that of LCDa-LCDh described in Example 1 except for
that the liquid crystal displays were prepared to exhibit the
(Ps(.lamda.)/Pc(.lamda.)) values of Formula (10) (described in
claim 8) as shown below, wherein .lamda. represents a wavelength in
the range of 500-670 mm.
[0254] The sum So(.lamda.) of the in-plane retardation values
(Ro(.lamda.)) at a wavelength .lamda. was assigned in Formulae (11)
and(12) to obtain the Ps(.lamda.)/Pc(.lamda.) value:
TABLE-US-00009 Liquid Crystal Display(*) Ps(.lamda.)/Pc(.lamda.)
LCDa1 0.47-0.52 LCDb1 0.64-1.74 LCDc1 0.67-0.82 LCDd1 0.23-1.11
LCDe1 0.50-0.75 LCDf1 0.84-1.51 LCDg1 0.32-0.52 (*)Verticaly
Aligned ECB mode Liquid Crystal Display
In the liquid crystal displays of the present invention which were
prepared to exhibit the Ps(.lamda.)/Pc(.lamda.) values in the range
of 0.22-1.75, the appearance of red hue in the black area of the
display was further reduced and substantially no red hue was
observed, while in the liquid crystal displays described in Example
1, slight red hue was observed.
[0255] In the present invention, the following are provided: (i) an
optical compensation film which enables a viewing angle of not less
than 80 degrees in all directions and notably reduces a red hue in
a black area of the display, when used in a liquid crystal display,
and (ii) a polarizing plate and a liquid crystal display employing
the above mentioned optical compensation film.
* * * * *