U.S. patent application number 12/461240 was filed with the patent office on 2009-12-03 for polarizing plate, and liquid crystal display.
This patent application is currently assigned to FUJIFILM Corporation. Invention is credited to Yoji Ito.
Application Number | 20090296213 12/461240 |
Document ID | / |
Family ID | 28449276 |
Filed Date | 2009-12-03 |
United States Patent
Application |
20090296213 |
Kind Code |
A1 |
Ito; Yoji |
December 3, 2009 |
Polarizing plate, and liquid crystal display
Abstract
A polarizing plate including: a protective film; a polarizer;
and a polymer substrate, laminated in this order, wherein the
polarizer has a thickness of 10 to 25 .mu.m and a width of 1700 to
2300 mm, and the polymer substrate has a thickness of 30 to 70
.mu.m.
Inventors: |
Ito; Yoji; (Kanagawa,
JP) |
Correspondence
Address: |
BUCHANAN, INGERSOLL & ROONEY PC
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
FUJIFILM Corporation
Minato-ku
JP
|
Family ID: |
28449276 |
Appl. No.: |
12/461240 |
Filed: |
August 5, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11716048 |
Mar 9, 2007 |
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12461240 |
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10508853 |
Sep 24, 2004 |
7202922 |
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PCT/JP03/03358 |
Mar 19, 2003 |
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11716048 |
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Current U.S.
Class: |
359/485.01 |
Current CPC
Class: |
G02F 1/1393 20130101;
G02F 1/133528 20130101; G02B 5/3033 20130101; G02B 5/3016 20130101;
G02F 2413/105 20130101 |
Class at
Publication: |
359/485 |
International
Class: |
G02B 1/08 20060101
G02B001/08; G02B 5/30 20060101 G02B005/30 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 26, 2002 |
JP |
2002-085868 |
Claims
1. A polarizing plate comprising: a protective film; a polarizer;
and a polymer substrate, laminated in this order, wherein the
polarizer has a thickness of 10 to 25 .mu.m and a width of 1700 to
2300 mm, and the polymer substrate has a thickness of 30 to 70
.mu.m.
2. The polarizing plate as claimed in claim 1, wherein the
polarizer has a width of 1900 mm.
3. The polarizing plate as claimed in claim 1, wherein the polymer
substrate comprises a norbornene type polymer.
4. The polarizing plate as claimed in claim 1, wherein the polymer
substrate comprises cellulose acetate propionate.
5. The polarizing plate as claimed in claim 1, further comprising a
polyimide layer.
6. The polarizing plate as claimed in claim 5, comprising: the
protective film; the polarizer; the polymer substrate; and the
polyimide layer, laminated in this order.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S.
application Ser. No. 11/716,048, filed Mar. 9, 2007, which is a
continuation application of U.S. application Ser. No. 10/508,853,
filed Sep. 24, 2004, now U.S. Pat. No. 7,202,922, the contents of
which are incorporated herein by reference, which was the National
Stage filing under .sctn.371 of International Application No.
PCT/JP03/03358 filed Mar. 19, 2003, which in turn claims priority
to Japanese Application No. 2002-085868, filed Mar. 26, 2002.
TECHNICAL FIELD
[0002] The present invention relates to a polarizing plate, and a
liquid crystal display using the polarizing plate.
BACKGROUND ART
[0003] Liquid crystal display is constituted of a polarizing plate
and a liquid crystal cell.
[0004] The most widely used TFT liquid crystal display of TN-mode
has an optical compensation film inserted between the polarizing
plate and the liquid crystal cell to realize a high display quality
of the liquid crystal display as described in JP-A-8-50206.
However, such a display has the disadvantage that the thickness of
the liquid crystal display itself increases.
[0005] JP-A-1-68940 describes that use of an elliptic polarizing
plate having a retardation film on one side of a polarizer and a
protective film on the other side thereof improves front-face
contrast of the liquid display without increasing the thickness
thereof. However, it was apparent that the retardation film
(optical compensation film) of the invention disclosed in
JP-A-1-68940 is liable to cause retardation by thermal strain or
other causes, and thus has the problem on durability. This
retardation causes framelike light leakage (increase of
transmittance) to deteriorate the display quality of the liquid
crystal display.
[0006] To suppress occurrence of the retardation by strain,
JP-A-7-191217 and EP-0911656A2 directly use an optical compensation
film formed by applying an optical anisotropic layer comprising a
discotic compound on a transparent support as a protective film for
the polarizing plate, thereby solving the above-described problems
on the durability without increasing thickness of the liquid
crystal display.
[0007] However, it was found that when the polarizing plate using
the optical compensation film as the protective film was set on a
large panel of 17-inch or larger, it is impossible to completely
prevent the light leakage caused by the thermal strain. The optical
compensation film should have not only the function of compensating
optically the liquid cell but also sufficient durability against
change of service environments.
DISCLOSURE OF THE INVENTION
[0008] One object of the present invention is to optically
compensate a liquid crystal cell by using an optical compensation
sheet.
[0009] Another object of the present invention is to provide a
liquid crystal display giving high quality display without light
leakage by using an optical compensation sheet placed on one side
of a polarizer.
[0010] Still another object of the present invention is to
remarkably improve the production yield of the polarizing
plate.
[0011] An optical anisotropic layer formed from a liquid crystal
compound is used for optically compensating a liquid crystal cell.
Generally, the optical anisotropic layer is provided on a polymer
substrate (optical compensation film), and a polarizer is
interposed between the optical compensation film and a
triacetylcellulose film as a protective film.
[0012] When the optical compensation sheet is used in a liquid
crystal display, the optical compensation sheet used in a liquid
crystal display is usually fixed to a liquid crystal cell or the
like using a pressure-sensitive adhesive or the like. Therefore,
the strain produced by expansion or contraction of a polymer film
of the optical compensation sheet is retained in the entire of the
optical compensation sheet, changing the optical properties of the
polymer film.
[0013] Changes of the optical properties have conventionally been
considered to be mainly resulted from the following causes. One
cause is variation of humidity and temperature conditions in the
service environment of the liquid crystal display, resulting in
expansion or contraction of the polymer film to give change of the
optical properties of the optical compensation sheet. Another cause
is nonuniform temperature distribution in the plane of the optical
compensation sheet resulting from backlight illumination or the
like of the liquid crystal display, resulting in thermal strain to
give change of the optical properties of the optical compensation
sheet.
[0014] In particular, it is known that the polymers having hydroxyl
groups such as cellulose esters are greatly influenced by
environmental conditions.
[0015] Therefore, it has hitherto been believed that the light
leakage can be prevented by suppressing the variation of the
optical properties of the optical compensation sheet under the
environmental conditions, and uniformizing the temperature
distribution in the optical compensation sheet.
[0016] As a result of extensive investigations by the present
inventors, an important cause has been found on the variation of
the optical properties of the optical compensation sheet under
environmental conditions.
[0017] Generally, a polarizing plate comprises a pair of protective
films and a polarizer comprising PVA as a main component. It has
been found that PVA used in the polarizer causes the largest
dimensional change due to variation of humidity and temperature in
the service environment of the liquid crystal display. In
particular, in a practical liquid crystal display, since the
polarizing plate is bonded to a liquid crystal cell through a
pressure-sensitive adhesive, the dimensional change caused by the
environment is transmitted as deformation stress to the protective
film (i.e., optical compensation sheet). This stress will cause
variation of the optical properties of the protective film.
[0018] Accordingly, it has been found that the strain caused by
environment can be reduced by decreasing the stress
((strain).times.(sectional area).times.(elasticity modulus)) due to
the dimensional change of the polarizer, specifically by decreasing
the thickness, and that the light leakage can remarkably be reduced
by decreasing the elastic modulus.
[0019] The objects of the present invention have been achieved by
the polarizer and the liquid crystal display described below.
(1) A polarizing plate comprising a polymer film, a polarizer, a
polymer substrate, and an optically anisotropic layer formed of a
liquid crystal compound, laminated in this order, wherein the
polarizer has a thickness of 10 to 25 .mu.m. (2) The polarizing
plate described in the above item (1), wherein the polymer
substrate has a thickness of 30 to 70 (3) The polarizing plate
described in the above item (1) or (2), wherein the polymer film
comprises cellulose acetate. (4) The polarizing plate described in
the above items (1) to (3), wherein the polymer substrate comprises
cellulose acetate. (5) The polarizing plate described in the above
items (1) to (4), wherein the liquid crystal compound used in the
optical anisotropic layer is a discotic liquid crystal compound,
the plane of the discotic structural units is inclined relative to
the surface of the polymer substrate, and the angle between the
plane of the discotic structural units and the surface of the
polymer substrate changes in the direction of the depth of the
optically anisotropic layer. (6) A liquid crystal display
comprising a liquid crystal cell, and two polarizing plates placed
on both sides of the liquid crystal cell, wherein at least one
polarizing plate is the polarizing plate described in items (1) to
(5). (7) The liquid crystal display described in the above item
(6), wherein the liquid crystal cell is of an OCB node, a VA mode,
or a TN mode.
BEST MODE FOR CARRYING OUT THE INVENTION
[0020] The polarizing plate of the present invention has one
characteristic in that a polarizer used therein has a thickness of
10 to 25 .mu.m.
Polarizer:
[0021] The polarizer of the present invention comprises a binder,
and iodine or a dichroic dyestuff.
[0022] The iodine and the dichroic dyestuff in the present
invention are oriented in the binder, whereby the polarizing plate
of the present invention exhibits the polarizing performance. The
iodine and the dichroic dyestuff are oriented along the binder
molecules.
[0023] The polarizer is prepared by immersing a stretched polymer
in a bath containing iodine or a dichroic dyestuff to permeate the
iodine or the dichroic in the binder.
[0024] In a commercially available polarizer, the iodine or the
dichroic dyestuff is distributed in the regions of about 4 .mu.m
inside from the polymer surfaces (about 8 .mu.m in total on the
both sides). Therefore, a thickness of at least 10 .mu.m is
necessary to obtain a sufficient polarizing performance. The degree
of permeation can be controlled by the concentration of the
dissolved iodine or dichroic dyestuff, the bath temperature, and
the immersion time.
[0025] The lower limit of the thickness of the binder in the
present invention is preferably at least 10 .mu.m from the
above-described reasons. The upper limit of the thickness is
preferably as small as possible in view of the light leakage of the
liquid crystal display, and the thickness should be not more than
that of the commercially available polarizing plate (about 30
.mu.m), preferably 25 .mu.m or less. The light leakage can be
prevented in a 17-inch liquid crystal display by decreasing the
thickness to 20 .mu.m or less.
[0026] The binder used is not particularly limited, and may be a
self-crosslinkable polymer, or may be a binder crosslinkable by a
crosslinking agent. The binder layer can be formed by reaction
between binder molecules having a functional group or having an
introduced functional group, under action of light, heat, pH
change, or the like; or by introducing bonding groups between
binders using a crosslinking agent that is a compound having high
reactivity, and crosslinking the binders.
[0027] The crosslinking can be conducted by applying a coating
liquid containing the binder or a mixture of the binder and a
crosslinking agent to a transparent support, and then heating or
the like. The crosslinking treatment may be conducted in any stage
until obtaining the final polarizer so long as the durability can
be secured at the stage of final commercial article.
[0028] The binder used in the present invention can be either a
self-crosslinkable polymer or a polymer crosslinkable by a
crosslinking agent. Of course, there are binders self-crosslinkable
and also crosslinkable by a crosslinking agent. Examples of the
binders include polymers such as polymethyl methacrylate, acrylic
acid/methacrylic acid copolymers, styrene/maleimide copolymers,
polyvinyl alcohols, modified polyvinyl alcohols, poly
(N-methylol-acrylamide), styrene/vinyltoluene copolymers,
chlorosulfonated polyethylene, nitrocellulose, polyvinyl chloride,
chlorinated polyolefins, polyesters, polyimides, vinyl
acetate/vinyl chloride copolymers, ethylene/vinyl acetate
copolymers, carboxymethylcellulose, polyethylene, polypropylene,
and polycarbonate; and compounds such as silane coupling agents.
Examples of the polymers preferably are water-soluble polymers such
as poly(N-methylolacrylamide), carboxymethylcellulose, gelatin,
polyvinyl alcohol, and modified polyvinyl alcohol. Of those,
gelatin, polyvinyl alcohol, and modified polyvinyl alcohol are more
preferable, and polyvinyl alcohol and modified polyvinyl alcohol
are especially preferable.
[0029] Of the above binders, polyvinyl alcohols and modified
polyvinyl alcohols are preferable. The polyvinyl alcohol has, for
example, a degree of saponification of 70 to 100%, preferably 80 to
100%, and more preferably 95% or more, and has a degree of
polymerization in range of preferably 100 to 5,000. Examples of the
modified polyvinyl alcohol include the ones modified by
copolymerization (the modifying groups including COONa,
Si(OH).sub.3, N(CH.sub.3).sub.3Cl, C.sub.9H.sub.19COO, SO.sub.3Na,
and C.sub.12H.sub.25), the ones modified by chain transfer (the
modifying groups including COONa, SH, and C.sub.12H.sub.25), and
the ones modified by block polymerization (the modifying groups
including COOH, CONH.sub.2, COOR (alkyl), and C.sub.6H.sub.5). The
modified polyvinyl alcohol preferably has a degree of
polymerization of 100-3,000. Of those, unmodified and modified
polyvinyl alcohols having a degree of saponification of 80 to 100%
are preferable, and unmodified and alkylthio-modified polyvinyl
alcohols having a degree of saponification of 85 to 95% are more
preferable.
[0030] The polyvinyl alcohol or the modified polyvinyl alcohols
used in the binder layer may be used alone or in combination of two
or more thereof.
[0031] Compounds disclosed in JP-A-8-338913, JP-A-9-152509, and
JP-A-9-316127 are particularly preferably used as the modified
polyvinyl alcohol.
[0032] The crosslinking agent for the binder is not particularly
limited. A larger addition amount of the crosslinking agent tends
to give more improvement in resistance to high humidity and high
temperature. However, addition of the crosslinking agent in an
amount of 50 mass % or more, based on the binder, lowers the
orientation property of the iodine or the dichroic dyestuff.
Therefore, the addition amount is preferably 0.1 to 20 mass %, more
preferably 0.5 to 15 mass %. The alignment film of the present
invention contains unreacted crosslinking agent to some extent
after completion of the crosslinking reaction. The amount of the
remaining crosslinking agent is preferably 1.0 mass % or less, more
preferably 0.5 mass % or less. If the crosslinking agent is
contained in an amount of more than 1.0 mass % in the binder layer,
sufficient durability cannot be achieved. Specifically, when such
an alignment film is used in a liquid crystal display,
deterioration of the polarization performance may occur during
long-term service, or long-term storage under high temperature and
high humidity atmosphere.
[0033] The specific examples of the crosslinking agent are
disclosed in U.S. Reissued Pat. 23,297. Of those, boric acids
(boron, and borax) are preferably used.
Iodine and Dichroic Dyestuff:
[0034] The dichroic molecule includes dyestuff compounds such as
azo dyestuffs, stilbene dyestuffs, pyrazolone dyestuffs,
triphenylmethane dyestuffs, quinoline dyestuffs, oxazine dyestuffs,
thiazine dyestuffs, and anthraquinone dyestuffs. The dyestuff is
preferably water-soluble, but is not limited thereto. These
dichroic molecules have preferably a hydrophilic substituent
introduced thereto, such as a sulfonic acid group, an amino group,
and a hydroxyl group. Examples of the dichroic molecule include
C.I. Direct Yellow 12, C.I. Direct Orange 39, C.I. Direct Orange
72, C.I. Direct Red 39, C.I. Direct Red 79, C.I. Direct Red 81,
C.I. Direct 83, C.I. Direct 89, C.I. Direct Violet 48, C.I. Direct
Blue 67, C.I. Direct Blue 90, C.I. Direct Green 59, and C.I. Acid
Red 37; and dyestuffs disclosed in JP-A-1-161202, JP-A-1-172906,
JP-A-1-172907, JP-A-1-183602, JP-A-1-248105, JP-A-1-265205, and
JP-A-7-261024. Such dichroic molecules are used in a form of a free
acid, or a salt such as an alkali metal salt, an ammonium salt, and
an amine salt. Polarizers having various color tones can be
prepared by blending two or more of the dichroic molecules. The
compounds (dyestuffs) which give a black color when the
polarization axises of polarizing elements or polarizing plates are
placed perpendicularly, and blends of two or more kinds of dichroic
molecules which give black color are preferred because of excellent
single plate high transmittance and degree of polarization.
[0035] From the standpoint of increasing the contrast ratio of the
liquid crystal display, the transmittance of the polarizing plate
is preferably higher, and the degree of polarization thereof is
preferably higher. The transmittance of the polarizing plate is in
a range of preferably 30 to 50%, more preferably 35 to 50%, most
preferably 40 to 50%, at a light wavelength of 550 nm. The degree
of polarization is in a range of preferably 90 to 100%, more
preferably 95 to 100%, most preferably 99 to 100%, at a light
wavelength of 550 nm.
[0036] The transmittance of the polarizing plate can be increased
by increasing the transmittance of the polymer film as described
later, or by adjusting the refractive index of the adhesive bonding
the polarizer and the polymer film.
[0037] The transmittance of the polymer film as described later can
be increased by decreasing the film thickness or lowering the haze
of the film.
[0038] The adhesive for bonding the polarizer and the polymer film
together, or the polarizer and the optically anisotropic layer,
together, is not particularly limited. Examples of the adhesive
include PVA type resins (including PVA modified by a group of
acetoacetyl, sulfo, carboxyl, oxyalkylene, etc.) and aqueous
solution of boron compounds. Of those, PVA type resins are
preferred. The thickness of the adhesive is in a range of
preferably 0.01 to 10 .mu.m, more preferably 0.05 to 5 .mu.m, after
drying.
[0039] The refractive index of the adhesive is preferably close to
that of cellulose acetate film. The difference in the refractive
index between the adhesive and the cellulose acetate film is
preferably 0.1 or smaller, more preferably 0.05 or smaller, most
preferably 0.01 or smaller.
[0040] The polymer film and the polymer substrate for interposing
the polarizer of the present invention therebetween are explained
below.
Polymer Film and Polymer Substrate:
[0041] The polymer film used preferably has light transmittance of
80% or more. Examples of the polymer for constituting the film
include cellulose esters (e.g., cellulose acetate, and cellulose
diacetate), norbornene type polymers, and polymethyl methacrylate.
Commercially available polymers may also be used (Artone, and
Zeonex as the norbornene type polymer). Cellulose esters are
preferable, and lower fatty acid esters of cellulose are more
preferable. The term "lower fatty acid" used herein means a fatty
acid having 6 or less carbon atoms. Of those, the carbon atom
number is preferably 2 (cellulose acetate), 3 (cellulose
propionate), or 4 (cellulose butyrate). Of those, cellulose acetate
is particularly preferred. Mixed fatty acid esters such as
cellulose acetate propionate, and cellulose acetate butyrate may be
used.
[0042] Even conventional polymers that are liable to develop
birefringence, such as polycarbonate or polysulfone, can be used by
suppressing the tendency by modifying the molecule as described in
the patent specification of WO 00/26705.
[0043] Cellulose acetate having an acetic acid content of 55.0 to
62.5%, preferably 57.0 to 62.0% is preferably used as the polymer
film.
[0044] The term "acetic acid content" used herein means an amount
of bonded acetic acid to a unit mass of cellulose. The acetic acid
content is measured and calculated according to ASTM: D-817-91
(Method of Testing Cellulose Acetate).
[0045] The viscosity-average degree of polymerization (DP) of the
cellulose ester is preferably 250 or more, more preferably 290 or
more. The cellulose ester used in the present invention has
preferably narrower molecular weight distribution, Mw/Mn (Mw:
mass-average molecular weights Mn: number-average molecular
weight), as measured by gel permeation chromatography.
Specifically, the value of Mw/Mn is in a range of preferably 1.0 to
1.7, more preferably 1.3 to 1.65, most preferably 1.4 to 1.6.
[0046] In the cellulose ester, hydroxyl groups at 2-, 3-, and
6-positions are not uniformly esterified respectively at 1/3 of the
entire substitution. The degree of substitution tends to decrease
at the 6-position. In the present invention, the degree of
substitution is preferably higher at the 6-position than at the 2-
and 3-positions.
[0047] Specifically, the proportion of substitution of the hydroxyl
at 6-position accounts for preferably 30-40% of the entire
substitution; more preferably 31% or more, most preferably 32% or
more. The degree of substitution at the 6-position is preferably
0.88 or more.
[0048] The hydroxyl at the 6-position may be substituted by an acyl
group having 3 or more carbon numbers other than acetyl (e.g.,
propionyl, butyryl, valeryl, benzoyl, and acryloyl). The degree of
substitution of the respective positions can be measured by
NMR.
[0049] The cellulose ester having hydroxyl groups at 6-position
substituted at a higher degree of substitution can be synthesized
by reference to the methods described in JP-A-11-5851, paragraphs
0043-0044, Synthesis Example 1; paragraphs 0048-0049, Synthesis
Example 2; and paragraphs 0051-0052, Synthesis Example 3.
[0050] The cellulose acetate film can be produced from the prepared
cellulose acetate solution (dope) by a solvent cast method. The
dope contains preferably a retardation-increasing agent.
[0051] The film is formed by casting the dope onto a drum or a band
and evaporating the solvent. The dope for the casting is preferably
adjusted to have a solid concentration of 18-35%. The surface of
the drum or band is preferably finished in a mirror state. The
casting and drying methods in the solvent casting method are
disclosed in U.S. Pat. Nos. 2,336,310, 2,367,603, 2,492,078,
2,492,977, 2,492,978, 2,607,704, 2,739,069, and 2,739,070; British
Patents 640731, and 736892; JP-B 45-4554, and 49-5614 and JP-A
60-176834, 60-203430, and 62-115035.
[0052] The dope is cast preferably onto a drum or band having a
surface temperature of preferably 10.degree. C. or lower. After
casting, the dope thus cast is dried by air stream preferably for 2
seconds or longer. The obtained film is stripped off from the drum
or band. The film may be further dried by a hot air stream at
successively raised temperatures of from 100 to 160.degree. C. to
evaporate the remaining solvent. The above method is described in
JP-B-5-17844. By this method, the time from the casting to the
peeling can be shortened. For conducting this method, the dope
should gel at the surface temperature of the drum or band during
casting.
[0053] The film may be formed by casting a prepared cellulose
acetate solution (dope) in two or more layers. The dope is cast
onto a drum or band and the solvent is evaporated to form a film.
The dope for the casting is preferably adjusted to have a solid
matter concentration of 10-40%. The surface of the drum or band is
preferably finished in a mirror state.
[0054] In the case where two or more cellulose acetate solutions
are cast, the cellulose acetate-containing solutions may be cast
respectively through plural casting dies placed at intervals along
the support movement direction to form layers. Such a lamination
can be conducted according the method disclosed in, for example,
JP-A-61-158414, JP-A-1-122419, and JP-A-11-198285. The cellulose
acetate solution may be cast through two casting dies to form a
film by a method shown in, for example, JP-B-60-27562; and
JP-A-61-947244, 61-947245, 61-104813, 61-158413, and 6-134933.
Another casting method of the cellulose acetate film may be used in
which a flow of a high-viscosity cellulose acetate solution is
enveloped with a low-viscosity cellulose acetate solution and the
cellulose acetate solutions are simultaneously extruded.
[0055] In forming an optically anisotropic layer comprising a
liquid crystal compound on a polymer film, it is preferable to
control the retardation of the polymer film (herein, a polymer
substrate).
Retardation of Polymer Substrate:
[0056] The preferred range of the retardation of the polymer
substrate depends on the kind of the liquid crystal cell using the
optical compensation film and the use manner thereof. It is
preferable in the present invention that the Re retardation value
is controlled to a range of 20 to 70 nm, and the Rth retardation
value is controlled to a range of 70 to 400 nm.
[0057] In the case using two optically anisotropic layers in a
liquid crystal display, it is preferable that the Rth retardation
value of the substrate is controlled to a range of 70 to 250
nm.
[0058] In the case of using one optically anisotropic layer in a
liquid crystal display, it is preferable that the Rth retardation
value of the substrate is controlled to a range of 150 to 400
nm.
[0059] The birefringent index (.DELTA.n: nx-ny) of the substrate is
preferably in a range of 0.00028 to 0.020. The birefringent index
in the thickness direction {(nx+ny)/2-nz} of the cellulose acetate
film is preferably in a range of 0.001 to 0.04.
Retardation-Increasing Agent:
[0060] For adjusting the retardation of the polymer film, an
aromatic compound having at least two aromatic rings is preferably
used as a retardation-increasing agent.
[0061] Hereinafter, cellulose acetate, as one example, is explained
in detail which is the best embodiment of the polymer film.
[0062] The aromatic compound is used in an amount of 0.01-20 parts
by mass, preferably 0.05 to 15 parts by mass, and more preferably
0.1 to 10 parts by mass, per 100 parts by mass of cellulose
acetate. Two or more aromatic compounds may be used in
combination.
[0063] The aromatic ring of the aromatic compound includes aromatic
hydrocarbon rings as well as aromatic heterocyclic rings.
[0064] Six-membered rings (i.e., benzene ring) are particularly
preferred as the aromatic hydrocarbon ring.
[0065] The aromatic heterocyclic rings are generally unsaturated
rings. The aromatic heterocyclic ring is preferably a five-membered
ring, a six-membered ring, or a seven-membered ring, more
preferably a five-membered ring or a six-membered ring. The
aromatic heterocyclic ring has generally the maximum number of
double bonds. The heteroatom is preferably nitrogen atom, oxygen
atom, or sulfur atom; particularly nitrogen atom. The aromatic
heterocylic ring includes furan rings, thiophene rings, pyrrole
rings, oxazole rings, isoxazole rings, thiazole rings, isothiazole
rings, imidazole rings, pyrazole rings, furazan rings, triazole
rings, pyran rings, pyridine rings, pyridazine rings, pyrimidine
rings, pyrazine rings, and 1,3,5-triazine rings.
[0066] Preferable aromatic rings are benzene rings, furan rings,
thiophene rings, pyrrole rings, oxazole rings, thiazole rings,
imidazole rings, triazole rings, pyridine rings, pyrimidine rings,
pyrazine rings, and 1,3,5-trizine rings: particularly preferred are
benzene rings and 1,3,5-triazine rings.
[0067] The aromatic compound has preferably at least one
1,3,5-triazine ring.
[0068] The aromatic compound has preferably 2-20 aromatic rings,
more preferably 2-12 aromatic rings, further preferably 2-8
aromatic rings, most preferably 2-6 aromatic rings.
[0069] Two aromatic rings are linkable by (a) formation of a
condensed ring, (b) direct bonding by a single bond, or (c) linkage
by a linking group (spiro bonding cannot be formed due to aromatic
rings). The rings may be linked in any of the above manners (a) to
(c).
[0070] Such retardation-increasing agents are described in, for
example, WO 01/88574A1, WO 00/2619A1, JP-A-2000-111914, and
JP-A-2000-275434.
[0071] The retardation of the cellulose acetate film can also be
adjusted by stretching treatment. The stretch ratio is preferably
in a range of 3 to 100%. The stretching of the cellulose acetate in
the present invention is preferably conducted by tenter stretching.
For fine control of the phase retarding axis, the difference in the
tenter-clipping rates at the both sides and the release timing is
preferably minimized. The stretching treatment can be conducted
according to WO-01/88574A1, page 37, line 8 to page 38, line 8.
Surface Treatment of Cellulose Acetate Film:
[0072] The cellulose acetate film is preferably subjected to a
surface treatment. The treatment specifically includes corona
discharge treatment, glow discharge treatment, flame treatment,
acid treatment, alkali treatment, and UV irradiation treatment. An
under-coating layer is preferably provided as described in
JP-A-7-333433.
[0073] In the above treatment, the temperature of the cellulose
acetate film should be Tg (glass transition temperature) or lower,
specifically 150.degree. C., from the standpoint of maintaining the
flatness of the film.
[0074] The surface treatment of the cellulose acetate is preferable
an acid treatment or an alkali treatment, namely saponification
treatment, from the standpoint of adhesiveness to the
polarizer.
[0075] The surface energy is preferably 55 mN/m or more, more
preferably in a range of 60 to 75 mN/m. The alkali saponification
treatment is specifically explained below as an example.
[0076] The alkali saponification treatment is preferably conducted
by the cycle of immersion of the film surface into an alkali
solution, neutralization with an acid solution, washing with water,
and drying.
[0077] The alkali solution includes a potassium hydroxide solution,
and a sodium hydroxide solution. The hydroxyl ion concentration
thereof is in a range of preferably 0.1 to 3.0N, more preferably
0.5 to 2.0N. The temperature of the alkali solution is in a range
of preferably from room temperature to 90%, more preferably 40 to
70.degree. C.
[0078] The surface energy of a solid can be measured by a contact
angle method, a wetting heat method, or an adsorption method as
described in the literature "Nure no Kiso to Ouyou (Wetting:
Elements and Applications)" (Riaraizu K.K., Dec. 10, 1989.). The
surface energy of the cellulose acetate film of the present
invention is preferably measured by the contact angle method.
[0079] Specifically, two kinds of liquids having respectively a
known surface energy are dropped onto the cellulose acetate film,
and the contact angles are measured. The contact angle is an angle
between the surface of the film and the tangent to the liquid drop
at the crossing point of the drop surface and the film surface at
the liquid drop side. The surface energy of the film is calculated
from the measured contact angles.
Optically Anisotropic Layer Comprising Liquid Crystalline
Compound:
[0080] The surface of the support of the polarizing plate of the
present invention can be provided with a functional layer such as
an optically anisotropic layer for compensation of the visual field
angle of LCD as described in JP-A-4-229828, JP-A-6-75115,
JP-A-8-50206, etc.; a glare-preventing or reflection-preventing
layer for visibility improvement of display; a PS wave-separating
layer for LCD luminance improvement against anisotropic scattering
and anisotropically optical interference (polymer dispersion liquid
crystal layer, cholesteric liquid crystal layer, etc.); a hard coat
layer for scratch resistance of the polarizing plate; a gas barrier
layer for preventing diffusion of moisture or oxygen; an
adhesion-promoting layer for strengthening the contact of the
polarizer with an adhesive or a pressure-sensitive adhesive;
slipping-imparting layer; and so forth.
[0081] The functional layer may be provided on the side of the
polarizer, or may be provided on the side opposite the polarizer,
depending on the object.
[0082] Various functional films as a protective film can directly
be bonded to one or both faces of the polarizer of the present
invention. Examples of the functional film include retardation
films such as a .lamda./4 plate, and .lamda./2 plate; a
light-diffusing film; a plastic cell having an electroconductive
layer on the face opposite the polarizing plate; a
luminance-improving film having a function of anisotropic
scattering or anisotropically optical interference; a reflective
film; and a semi-transparent reflective plate.
[0083] The polarizing plate of the present invention exhibits its
function more effectively by combining with a coat type of optical
member (e.g., an optical compensation film, and a
luminance-improving film), thereby controlling the transmission
axis of the polarizing plate and the retardation axis of the
respective optical members. As specific examples of the coat type
optical member, optical compensating sheets using discotic liquid
crystal molecules are described in, for example, JP-A-6-214116,
U.S. Pat. No. 5,583,679, U.S. Pat. No. 5,646,703, and German Patent
3911620A1; optical compensating sheet using rod-shaped liquid
crystalline molecules are described in, for example, JP-A-7-35924;
and a luminance-improving film is described in, for example,
JP-A-11-149015.
[0084] A preferable embodiment of the anisotropically optical layer
comprising the liquid crystal compound of the present invention is
described in detail below.
[0085] An alignment film is preferably provided between the polymer
substrate and the optically anisotropic layer. The alignment film
functions to align the liquid crystal compound of the present
invention in a constant direction. The alignment film is essential
to realize the preferred embodiment of the present invention.
However, after orientation of the liquid crystal compound and
fixation of the aligned state, the alignment film is not essential
as the structural element of the present invention because the
alignment film plays its role. Therefore, the polarizing plate of
the present invention can be prepared by transferring only the
optically anisotropic layer on the alignment film having the fixed
aligning state to the polarizer.
Alignment Film:
[0086] The alignment film has a function to define the orienting
direction of the liquid crystal compound. The alignment film can be
formed in various methods such as a rubbing treatment of an organic
compound (preferably a polymer); oblique deposition of an inorganic
compound; formation of a layer containing microgrooves; and
building-up of an organic compound (e.g., .omega.-tricosanoic acid,
dioctadecylmethylammonium chloride, and methyl stearate) by a
Langmuir-Blodgett technique (LB film). Other alignment films are
known in which an orienting function is imparted by application of
an electric field, application of a magnetic field, or irradiation
of light. The alignment film is preferably formed by rubbing
treatment of a polymer.
[0087] The alignment film is preferably formed by rubbing treatment
of a polymer. Polyvinyl alcohol is preferred as the polymer.
Modified polyvinyl alcohols having hydrophobic groups bonded
thereto are particularly preferred.
[0088] The alignment film is described in, for example, WO
01/88574A1, page 43 line 24 to page 49, line 8.
Optically Anisotropic Layer:
[0089] The optically anisotropic layer formed from a liquid crystal
compound is preferably prepared on an alignment film provided on a
polymer substrate in the present invention.
[0090] The liquid crystal compound used for the optically
anisotropic layer includes rod-like liquid crystal compounds and
discotic liquid crystal compounds. The rod-like liquid crystal
compounds and discotic liquid crystal compounds may be a polymeric
liquid crystal or a low-molecular liquid crystal. The liquid
crystal compound also includes low-molecular liquid crystals in
which the liquid crystallinity has been lost by crosslinking.
[0091] The optically anisotropic layer can be formed by applying a
coating liquid containing a liquid crystal compound and optionally
a polymerization initiator or another component to an alignment
film.
[0092] In the case of using a discotic liquid crystal compound, it
is preferable that the plane of the discotic structural units is
inclined relative to the polymer substrate, and the angle between
the plane of the discotic structural unit and the surface of the
polymer substrate varies in the depth direction of the optically
anisotropic layer.
[0093] The angle (inclined angle) of the plane of the discotic
structural units is generally increased or decreased depending on
the distance from the bottom face of the optically anisotropic
layer in the depth direction of the optically anisotropic layer.
The inclined angle is preferably increased with increase of the
distance. The change of the inclined angle may be continuous
increase, continuous decrease, intermittent increase, intermittent
decrease, combination of continuous increase and continuous
decrease, or intermittent change including increase and decrease.
The intermittent change includes a region in which the inclined
angle does not change in the course of the thickness direction.
Even in the case where no angle-change region is contained in the
layer, the inclined angle is preferably increased or decreased as a
whole. More preferably, the inclined angle is increased as a whole,
and still more preferably the change is continuous.
[0094] The optically anisotropic layer is described in WO
01/88574A1, page 49, line 10 to page 67, line 20 as the
reference.
[0095] Polarizer Having No Transmission Axis in Length Direction
and in Width Direction:
[0096] The polarizer of the present invention is prepared by
stretching a raw film in a direction inclined to the length
direction at an angle of 10 to 80.degree. (stretching method), or
rubbing the film (rubbing method), and then dyeing the film with
iodine or a dichroic dyestuff. Stretching is conducted such that
this inclined angle is made equal to the angle between the
transmission axis of the two polarizing plates bonded to a liquid
crystal cell of LCD and the vertical or lateral direction of the
liquid crystal cell.
[0097] Usually, this angle is 45.degree.. However, in new models of
the transmission type LCD, the reflection type LCD, the
semi-transmission type LCD, and the like, the angle is not
necessarily 45.degree.. The stretch direction is adjusted suitably
in accordance with the design of the LCD.
[0098] In the stretching method, the stretch ratio is preferably
2.5 to 30.0, more preferably 3.0 to 10.0. The stretching may be
conducted by dry stretching in the air or by wet stretching in
water. The stretch ratio is about 2.5 to 5.0 in dry stretching, and
about 3.0 to 10.0 in wet etching. This oblique stretching may be
conducted fractionally in plural times. The fractional stretching
enables uniform stretching even in a high stretch ratio. Before the
oblique stretching, slight stretching may be conducted in the width
direction or the length direction (to an extent to prevent the
contraction in the width direction).
[0099] In the stretching, the tenter stretching for biaxial
stretching in usual film molding may be conducted differently in
the right side and the left side. That is, the stretching is
conducted at different speeds at the left side and the right side.
For such differential stretching, the thickness of the binder film
before the stretching is necessarily made different between the
right side and the left side of the film. In casting film
formation, for example, the flow rate of the binder solution can be
differentiated between the right side and the left side by, for
example, providing tapers to the die.
[0100] In such a process, the polarizer of the present invention is
produced with the stretching direction oblique by 10 to 80.degree.
to the length direction.
[0101] The rubbing method can be conducted according to a usual
rubbing treatment method employed widely in a liquid crystal
orienting process of LCD. Specifically, the surface of the
alignment film is rubbed in one direction with paper, gauze, felt,
rubber, fiber of nylon or polyester, or a like material to impart
orienting property. Generally, the treatment is conducted by
rubbing the film several times with a cloth having fibers of
uniform length and diameter transplanted uniformly, or a like
material. In the rubbing treatment employed preferably in the
present invention, a rubbing roll is used which has circularity,
cylindricity, and eccentricity of the roll of 30 .mu.m or less. The
wrapping angle of the film to the rubbing roll is preferably in a
range of 0.1 to 90.degree.. However, stable rubbing treatment can
be achieved by winding the film at an angle of 360.degree. or more
as disclosed in JP-A-8-160430. For rubbing of a long film, the film
is preferably delivered at a constant tensile force at a rate of 1
to 100 m/min. The rubbing roll is preferably turnable horizontally
relative to the film delivery direction for setting a desired
rubbing angle, the angle being suitably selected from a range of 0
to 60.degree., with 45.degree. being preferable. For use as the
liquid crystal display, the angle is preferably in a range of 40 to
50.degree..
Liquid Crystal Display:
[0102] The polarizing plate using the cellulose acetate film can
advantageously be used for liquid crystal displays, particularly
for transmission type liquid crystal displays.
[0103] The transmission type liquid crystal display comprises a
liquid crystal cell and two polarizing plate placed on the both
sides thereof. The liquid crystal cell holds a liquid crystal
between two electrode base plates.
[0104] The polarizing plate of the present invention can be used as
one or both of the above polarizing plates. In this case, the
(optically anisotropic) cellulose acetate film of the polarizing
plate is placed so as to face the liquid crystal cell.
[0105] The liquid crystal cell is preferably of an OCB mode, a VA
mode, an ECB mode, or a TN mode.
[0106] The liquid crystal cell of the OCB mode is a liquid crystal
cell of a bend orientation mode in which the rod-like liquid
crystal molecules are oriented to become reverse substantially
(symmetrically) in the orientation direction between the top and
the bottom of the liquid crystal cell. A liquid crystal display
using the liquid crystal cell of the bend orientation mode is
disclosed in U.S. Pat. Nos. 4,583,825, and 5,410,422. The liquid
crystal cell of the bend orientation mode has self optical
compensation function due to the symmetric orientation of the
rod-like liquid crystal molecules between the top and the bottom of
the liquid cell. Therefore, this liquid crystal mode is called an
OCB (optically compensatory bend) crystal liquid mode. The liquid
crystal display of the bend orientation mode has an advantage of
high response speed.
[0107] The polarizing plate of the present invention, which is used
in a liquid crystal display of an OCB mode, may have an optically
anisotropic layer containing a discotic compound or a rod-like
compound on the cellulose acetate film used as the polarizing
plate. The optical anisotropic layer is formed by orienting the
discotic compound (or the rod-like liquid crystal compound) and
fixing the oriented state.
[0108] Discotic compounds have generally a higher birefringent
index. The discotic compound can take various oriented states.
Therefore, by using the discotic compound, a polymer film (optical
compensation film) can be produced which has optical properties
that cannot be obtained from a conventional stretched birefringent
film. The polymer film using the discotic compound is described in
JP-A-6-214116, U.S. Pat. Nos. 5,583,679 and 5,646,703, and German
Patent 3911620A1.
[0109] In the liquid crystal cell of the VA mode, the rod-like
liquid crystal molecules orients substantially vertically when not
applying voltage.
[0110] The Liquid crystal cell in the VA mode includes (1) liquid
crystal cells in a narrow sense in which rod-like liquid crystal
molecules are aligned substantially vertically when not applying
voltage and are oriented substantially horizontally when applying
voltage (JP-A-2-176625), (2) liquid crystal cells (MVA mode) in
which VA mode is formed in multiple domains for visual field angle
expansion (SID97, Digest of tech. Papers (Preprint) 28 (1997) 845),
(3) liquid crystal cells (n-ASM mode) in which rod-like liquid
crystal molecules are oriented substantially vertically when not
applying voltage and are oriented in twisted multiple domains when
applying voltage (Nippon Ekisho Toronkai (Japan Liquid Crystal
Symposium), Preprint 58-59 (1998)), and (4) liquid crystal cells of
SURVIVAL mode (presented at LCD International 98).
[0111] The ECB mode of the liquid crystal is the oldest of the
liquid crystal modes, and is described in many documents.
[0112] In the liquid crystal cell of the TN mode, the rod-like
liquid crystal molecules are oriented substantially vertically when
not applying voltage in a state twisted at an angle of 60 to
120.degree..
[0113] TN mode liquid crystal cells are used most widely, and are
described in many documents.
EXAMPLES
[0114] The present invention is explained in detail by reference to
the following Examples, but it should be understood that the
invention is not construed as being limited thereto.
Example 1
Preparation of Polarizer
[0115] A PVA film having an average degree of polymerization of
1700 and a degree of saponification of 99.5 mol % (thickness, 80
.mu.m; width, 2,500 mm) was stretched uniaxially in the vertical
direction in warm water at 40.degree. C. at a stretch ratio of 8.
The film in this state was immersed in an aqueous solution
containing 0.2 g/l of iodine and 60 g/l of potassium iodide at
30.degree. C. for 5 minutes, and then immersed in an aqueous
solution containing 100 g/l of boric acid and 30 g/l of potassium
iodide. The film in this state had a width of 1,300 mm and a
thickness of 17 .mu.m.
[0116] The film was then immersed in a water-washing vessel at
20.degree. C. for 10 seconds, and further immersed in an aqueous
solution containing 0.1 g/l of iodine and 20 g/l of potassium
iodide at 30.degree. C. for 15 seconds. The film was dried at room
temperature for 24 hours to obtain an iodine type polarizer
(HF-1).
Preparation of Polymer Substrate
[0117] The composition shown below was put into a mixing tank, and
the components were dissolved by stirring with heating to prepare a
cellulose acetate solution.
Composition of Cellulose Acetate Solution
TABLE-US-00001 [0118] Parts by mass Cellulose acetate 100 (acetic
acid content: 60.9%) Triphenyl phosphate (plasticizer) 7.8 Biphenyl
diphenyl phosphate (plasticizer) 3.9 Methylene chloride (first
solvent) 300 Methanol (second solvent) 54 1-Butanol (third solvent)
11
[0119] 16 Parts by mass of the retardation-increasing agent shown
below, 80 parts by mass of methylene chloride, and 20 parts by mass
of methanol were introduced into another mixing tank. The resulting
mixture was stirred with heating to prepare a solution of the
retardation-increasing agent.
[0120] Parts by mass of the cellulose acetate solution and 36 parts
by mass of the retardation-increasing agent solution were mixed and
stirred sufficiently to prepare a dope. The amount of the
retardation-increasing agent added was 5.0 parts by mass per 100
parts by mass of cellulose acetate.
Retardation-Increasing Agent
##STR00001##
[0122] The dope obtained was cast using a band-casting machine.
When the film surface temperature had reached 40.degree. C. on the
band, the film formed was dried for 1 minute. The film was stripped
off from the band, and dried by drying air stream at 140.degree. C.
The film was stretched by 28% in the width direction by a tenter,
and then dried with a drying air stream at 135.degree. C. for 20
minutes. Thus, a polymer substrate (PK-1) was prepared which
contained the residual solvent at a content of 0.3 mass %.
[0123] The thus-obtained polymer substrate (PK-1) had a thickness
of 92 Sn. The retardation value (Re) at 590 nm of the substrate was
43 nm, and the retardation value (Rth) at 590 nm was 175 nm as
measured by an ellipsometer (M-150, manufactured by Nippon Bunko
K.K.).
[0124] The thus-prepared polymer substrate (PK-1) was immersed in a
2.0N potassium hydroxide solution (25.degree. C.) for 2 minutes,
then neutralized with sulfuric acid, washed with pure water, and
dried. This PK-1 had a surface energy of 63 mN/m, as measured by a
contact angle method.
[0125] This PK-1 was coated with an alignment film-coating liquid
having the composition shown below in an application amount of 28
ml/m.sup.2 with a #16 wire bar coater. The coated film was dried by
warm air stream at 60.degree. C. for 60 seconds and further at
90.degree. C. for 150 seconds.
Composition of Alignment Film Coating Liquid
TABLE-US-00002 [0126] Parts by mass Modified polyvinyl alcohol
shown below 10 Water 371 Methanol 119 Glutaraldehyde (crosslinking
agent) 0.5 Modified Polyvinyl Alcohol ##STR00002##
[0127] The formed alignment film was subjected to rubbing treatment
in a direction of 45.degree. to the phase retardation axis
(measured at 632.8 nm) of the polymer substrate (PK-1).
Formation of Optical Anisotropic Layer
[0128] The alignment film was coated with a coating liquid
containing 41.01 g of the discotic liquid crystal compound shown
below, 4.06 g of ethylene oxide-modified trimethylolpropane
triacrylate (V#360, produced by Osaka Yuki Kagaku K.K.), 0.35 g of
cellulose acetate butyrate (CAB531-1, produced by Eastman Chemical
Co.), 1.35 g of a photopolymerization initiator (Irgacure 907,
produced by Ciba Geigy Co.), and 0.45 g of a sensitizer (Kayacure
DETX, produced by Nippon Kayaku Co.) dissolved in 102 g of methyl
ethyl ketone, by means of a #3 wire bar. The coated film was fixed
to a metal frame, and was heated at 130.degree. C. for 2 minutes in
a thermostatic oven to orient the discotic liquid crystal compound.
The treated film was irradiated with UV for 1 minute at 130.degree.
C. by a high pressure mercury lamp of 120 W/cm to polymerize the
discotic liquid crystal compound, and was allowed to cool to room
temperature. Thus, an optical compensation sheet (KH-1) having an
optical anisotropic layer was prepared.
[0129] The optical anisotropic layer had a Re retardation value of
38 nm at 546 nm. The angle (inclined angle) between the discotic
plane and the first transparent supporting member face was 400 in
average.
Discotic Liquid Crystal Compound
##STR00003##
[0130] Preparation of Polarizing Plate
[0131] The optical compensation sheet (KH-1) was bonded at the face
of the polymer substrate (PK-1) to the one face of the polarizer
(HF-1) using a polyvinyl alcohol type adhesive. Separately, 80
.mu.m thick triacetylcellulose film (TD-80U, produced by Fuji Photo
Film Co.) was treated for saponification, and this film was bonded
to the reverse face of the polarizer.
[0132] The transmission axis of the polarizer and the phase
retardation axis of the polymer substrate (PK-1) were placed
parallel to each other, whereas the transmission axis of the
polarizer and the phase retardation axis of the commercial
triacetylcellulose film were placed perpendicularly to each other.
Thus, a polarizer (HB-1) was prepared.
Example 2
Preparation of Polymer Substrate
[0133] 16 parts by mass of the retardation-increasing agent used in
Example 1, 80 parts by mass of methylene chloride, and 20 parts by
mass of methanol were placed in a mixing tank. The resulting
mixture was stirred with heating to prepare a solution of the
retardation-increasing agent solution.
[0134] 474 parts by mass of the cellulose acetate solution prepared
in Example 1 and 25 parts by mass of the retardation-increasing
agent solution were mixed and stirred sufficiently to prepare a
dope. The amount of the retardation-increasing agent added was 3.5
parts by mass per 100 parts by mass of cellulose acetate.
[0135] The thus-obtained dope was cast using a band-casting
machine. When the film surface temperature had reached 40.degree.
C., the formed film was dried for 1 minute. The formed film was
stripped off, dried by a drying air stream at 140.degree. C. Thus,
a polymer substrate (PK-2) was prepared which contained the
residual solvent at a content of 0.3 mass %.
[0136] The obtained polymer substrate (PK-2) had a thickness of 65
.mu.m. The retardation value (Re) at 590 nm of the polymer
substrate was 8 nm, and the retardation value (Rth) at 590 nm was
78 nm as measured by an ellipsometer (M-150, manufactured by Nippon
Bunko K.K.).
Preparation of Optical Compensation Sheet Having Optically
Anisotropic Layer
[0137] The polymer substrate (PK-2) was immersed in a 2.0N
potassium hydroxide solution (25.degree. C.) for 2 minutes,
neutralized with sulfuric acid, washed with pure water, and dried.
This PK-2 had a surface energy of 63 mN/m, as measured by a contact
angle method.
Formation of Alignment Film
[0138] The prepared PK-2 was coated with the alignment film-coating
liquid having the composition shown below in an application amount
of 28 mL/m.sup.2 with a #16 wire bar coater. The coated film was
dried by a warm air stream at 60.degree. C. for 60 seconds and
further at 90.degree. C. for 150 seconds.
Composition of Aligning Film Coating Liquid
TABLE-US-00003 [0139] Parts by mass Modified polyvinyl alcohol of
Example 1 10 Water 371 Methanol 119 Glutaraldehyde (crosslinking
agent) 0.5
[0140] The formed film was subjected to rubbing treatment in a
direction parallel to the length direction of PK-2.
Formation of Optically Anisotropic Layer
[0141] The alignment film was coated with a coating liquid
containing 41.01 g of the discotic liquid crystal compound used in
Example 1, 4.06 g of ethylene oxide-modified trimethylolpropane
triacrylate (V#360, produced by Osaka Yuki Yagaku K.K.), 0.90 g of
cellulose acetate butyrate (CAB551-0.2, produced by Eastman
Chemical Co.), 0.23 g of cellulose acetate butyrate (CAB531-1,
produced by Eastman Chemical Co.), 1.35 g of a photopolymerization
initiator (Irgacure 907, produced by Ciba Geigy Co.), and 0.45 g of
a sensitizer (Kayacure DETX, produced by Nippon Kayaku Co.)
dissolved in 102 g of methyl ethyl ketone, by means of #3.6 wire
bar. The coated film was heated in a thermostatic zone at
130.degree. C. for 2 minutes to orient the discotic liquid crystal
compound. The film thus treated was irradiated with UV for 1 minute
at 60.degree. C. by a high pressure mercury lamp of 120 W/cm to
polymerize the discotic liquid crystal compound. The film was
allowed to cool to room temperature. Thus, an optical compensation
sheet (KH-2) having an optically anisotropic layer was
prepared.
[0142] The optically anisotropic layer had a Re retardation value
of 43 nm measured at 546 nm. The angle (inclination angle) between
the discotic plane and the first transparent supporting member face
was 42.degree. in average.
Preparation of Polarizing Plate
[0143] The optical compensation sheet (KH-2) was bonded to the one
face of the polarizer (HF-1) using a polyvinyl alcohol type
adhesive. Separately, 80 .mu.m thick triacetylcellulose film
(TD-80U, produced by Fuji Photo Film Co.) was treated for
saponification, and this film was bonded to the reverse face of the
polarizer.
[0144] The transmission axis of the polarizer and the phase
retardation axis of the polymer substrate (PK-2) were placed
parallel to each other, whereas the transmission axis of the
polarizer and the phase retardation axis of the commercial
triacetylcellulose film were placed perpendicularly to each other.
Thus, a polarizer (HB-2) was prepared.
Example 3
Preparation of Bend-Alignment Liquid Crystal Cell
[0145] A polyimide film was provided as an alignment film on two
glass substrates having an ITO electrode, respectively. The
alignment films were subjected to rubbing treatment. Two glass
substrates formed in duplication were placed in opposition with the
cell gap of 6 .mu.m with the rubbing treatment directions thereof
parallel to each other. A liquid crystal compound (ZLI 1132,
produced by Merck Co.) having .DELTA.n of 0.1396 was injected into
the cell gap. Thereby, a bend-alignment liquid crystal cell was
produced. The liquid cell had a size of 20 inches.
[0146] Two polarizing plates (HB-1) prepared in Example 1 were
bonded to the both faces of the above produced bend-alignment cell.
In the bonding, the polarizing plates were placed with the
optically anisotropic layers facing respectively to the cell
substrates with the rubbing-treated directions of the liquid
crystal cell and the elliptic polarizing plate kept
antiparallel.
[0147] A rectangular wave voltage of 55 Hz was applied to the
liquid crystal cell in a normally-white mode of white display of 2V
and black display of 5V. Taking the transmittance ratio (white
display/black display) as the contrast ratio, the visual field
angle was measured at 8 steps from black display (L1) to white
display (L8) by a tester (EZ-Contrast 160D, manufactured by ELDIM
Co.).
[0148] The results obtained are shown in Table 1 below.
TABLE-US-00004 TABLE 1 Liquid Visual field angle (Range of contrast
ratio of 10 Display or more, and no gradation reversal in black
side) Display Top Bottom Right and Left Example 3 80.degree.
80.degree. 80.degree. (Note) Gradation reversal in balk side:
Reversal between L1 and L2
Evaluation of Light Leakage
[0149] The backlight was turned on continuously for 5 hours under
the environmental conditions of temperature 25.degree. C. and
relative humidity 60%. The whole-area black display state was
visually examined in a dark room to evaluate the light leakage. As
a result, no light leakage was found in the display screen of the
liquid crystal display.
Example 4
[0150] A pair of polarizing plates were stripped off from a liquid
crystal display using TN type liquid crystal cells (AQUOS LC20C1S,
manufactured by Sharp Corp.), and instead thereof, the polarizing
plates (HB-2) prepared in Example 2 were bonded thereto, one on the
observer's side and another one on the backlight side with the
optical compensation sheets (KH-2) facing the liquid cell with an
adhesive. The transmission axis of the polarizing plate on the
observer's side and the transmission axis of the polarizing plate
on the backlight side are placed in an O-node.
[0151] The prepared liquid crystal display was tested for the
visual field angle in 8 steps from black display (L1) to white
display (L8) with a tester (EZ-Contrast 160D, manufactured by ELDIM
Co.). The results obtained are shown in Table 2 below.
Comparative Example 1
[0152] A liquid crystal display using TN type liquid crystal cells
(AQUOS LC20C1S, manufactured by Sharp Corp.) was tested for the
visual field angle in 8 steps from black display (L1) to white
display (L8) with a tester (EZ-Contrast 160D, manufactured by ELDIM
Co.).
[0153] The results obtained are shown in Table 2 below.
TABLE-US-00005 TABLE 2 Liquid Visual field angle (Range of contrast
ratio of 10 Crystal or more, and no gradation reversal in black
side) Display Top Bottom Right and Left Example 4 75.degree.
43.degree. 80.degree. Comparative 70.degree. 42.degree. 80.degree.
Example 1 (Note) Gradation reversal in balk side: Reversal between
L1 and L2
Evaluation of Light Leakage
[0154] The backlight was turned on continuously for 5 hours under
the environmental conditions of temperature 25.degree. C. and
relative humidity 60%. The whole-area black display state was
visually examined in a dark room to evaluate the light leakage. As
the results, no light leakage was found in the display screen of
the liquid crystal display of Example 4. However, framelike light
leakage was observed in the display screen of Comparative Example
1.
Example 5
Preparation of Polymer Film
[0155] 16 parts by mass of the retardation-increasing agent used in
Example 1, 80 parts by mass of methylene chloride, and 20 parts by
mass of methanol were placed in a mixing tank. The mixture was
stirred with heating to prepare a retardation-increasing agent
solution.
[0156] 464 parts by mass of the cellulose acetate solution prepared
in Example 1 and 36 parts by mass of the retardation-increasing
agent solution were mixed and stirred sufficiently to prepare a
dope. The amount of the retardation-increasing agent added was 5.0
parts by mass per 100 parts by mass of cellulose acetate.
[0157] The obtained dope was cast with a band-casting machine. When
the film surface temperature had reached 40.degree. C., the formed
film was dried for 1 minute. The formed film was stripped off, and
dried by drying air stream at 140.degree. C. The film was stretched
by 30% in the width direction with a tenter, and then dried with a
drying air stream at 135.degree. C. for about 20 minutes. Thus, a
polymer substrate (PK-3) was prepared which contained the residual
solvent at a content of 0.3 mass %.
[0158] The obtained polymer substrate (PK-3) had a thickness of 102
.mu.m. The retardation value (Re) at 590 nm of the substrate was 47
nm, and the retardation value (Rth) at 590 nm was 153 nm as
measured by an ellipsometer (M-150, manufactured by Nippon Bunko
K.K.).
Preparation of Polarizing Plate
[0159] The polymer substrate (PK-3) was bonded to one face of the
polarizer (HF-1) using a polyvinyl alcohol type adhesive.
Separately, 80 .mu.m thick triacetylcellulose film (TD-80U,
produced by Fuji Photo Film Co.) was treated for saponification,
and this film was bonded to the reverse face of the polarizer.
[0160] The transmission axis of the polarizer (HF-1) and the phase
retardation axis of the polymer substrate (PK-3) were placed
parallel to each other, whereas the transmission axis of the
polarizer and the phase retardation axis of the commercial
triacetylcellulose film were placed perpendicularly to each other.
Thus, a polarizer (HB-3) was prepared.
Vertical Alignment Liquid Crystal Cell
[0161] A pair of polarizing plates and a pair of retardation films
were stripped off from a liquid crystal display using a vertical
alignment liquid crystal cells (VL-1530S, manufactured by Fujitsu
Ltd.), and instead thereof, the respective polarizing plates (HB-3)
were bonded thereto with the polymer substrate (PK-3) facing the
liquid cell side with interposition of an adhesive. The
transmission axis of the polarizing plate on the observer's side is
directed vertically and the transmission axis of the polarizing
plate on the backlight side is placed laterally in a cross-nicol
arrangement.
[0162] The prepared liquid crystal display was tested for the
visual field angles for 8 steps from black display (L1) to white
display (L8) with a tester (EZ-Contrast 160D, manufactured by ELDIM
Co.). The results obtained are shown in Table 3 below.
Comparative Example 2
[0163] The liquid crystal display using a vertical alignment liquid
crystal cells (VL-1530S, manufactured by Fujitsu Ltd.) was tested
for the visual field angles for 8 steps from black display (L1) to
white display (L8) with a tester (EZ-Contrast 160D, manufactured by
ELDIM Co.). The results obtained are shown in Table 3 below.
TABLE-US-00006 TABLE 3 Visual field angle (Range of contrast ratio
of 10 or more, and no gradation reversal in black side) Liquid
45.degree. from Crystal Transmission transmission Display axis
direction axis direction Example 4 >80.degree. >80.degree.
Comparative >80.degree. 44.degree. Example 1 (Note) Gradation
reversal in balk side: Reversal between L1 and L2
Evaluation of Light Leakage
[0164] The backlight was turned on continuously for 5 hours under
the environmental conditions of temperature 25.degree. C. and
relative humidity 60%. Thereafter the whole-area black display
state was visually examined in a dark room to evaluate the light
leakage. As a result, no light leakage was found in the display
screen of the liquid crystal display of Example 5. However,
framelike light leakage was observed in the display screen of
Comparative Example 2.
Example 6
[0165] PVA having an average degree of polymerization of 4000 and a
degree of saponification of 99.8% was dissolved in water to obtain
a 4.0% aqueous solution thereof. This solution was cast through a
tapered die onto a casting band, and dried to obtain a film having
a width of 110 mm, and a thickness of 120 .mu.m at the left end and
135 .mu.m at the right end before stretching.
[0166] This film was stripped off from the casting band, and was
stretched obliquely at an angle of 45.degree. in dry conditions.
The film in this state was immersed in an aqueous solution
containing iodine (0.5 g/l) and potassium iodide (50 g/l) at
30.degree. C. for 1 minute, and then immersed in an aqueous
solution containing boric acid (100 g/l) and potassium iodide (60
g/l) at 70.degree. C. for 5 minutes. The film was washed with water
in a water washing vessel at 20.degree. C. for 10 seconds, and
dried at 80.degree. C. for 5 minutes to obtain an iodine type
polarizer (HF-4). The polarizer had a width of 660 mm and a
thickness of 20 .mu.m at both of the right and left edges.
[0167] A polarizing plate (HB-4) was prepared in the same manner as
in the preparation of the polarizing plate in Example 2 except that
the polarizer (HF-4) was used in place of the polarizer (HF-1).
Example 7
[0168] PVA having an average degree of polymerization of 2500 and a
degree of saponification of 99.5% was dissolved in water to obtain
a 50% aqueous solution. This solution was cast through a die with a
taper on a casting band, and dried to obtain a film having a width
of 300 mm, and a thickness of 100 .mu.m at the left end and 115
.mu.m at the right end before stretching.
[0169] This film was stripped off from the casting band, and the
film in this state was immersed in an aqueous solution containing
iodine (0.2 g/l) and potassium iodide (60 g/l) at 30.degree. C. for
5 minutes. The film was immersed in an aqueous solution containing
boric acid (100 g/l) and potassium iodide (30 g/l) with stretching
in an oblique direction at an angle of 45.degree. at 60.degree. C.
for 10 minutes. The film had a width of 1,900 mm and a thickness of
15 .mu.m at both of the right and left edges.
[0170] The film was immersed in a water washing vessel at
20.degree. C. for 10 seconds, and further immersed in an aqueous
solution containing iodine (0.1 g/l) and potassium iodide (30 g/l)
at 30.degree. C. for 15 seconds. The film was dried at room
temperature for 24 hours to obtain an iodine type polarizer
(HF-5).
[0171] A polarizing plate (HB-5) was prepared in the same manner as
in the polarizing plate in Example 2 except that the polarizer
(HF-5) was used in place of the polarizer (HF-1).
Evaluation of Production Yield of Polarizing Plate
[0172] The number of sheets having a size of 219.0.times.291.4 mm
which could be punched out from the polarizing sheet was measured.
The size of the polarizing plate was 650 mm in width and 1000 mm in
length in correspondence with Comparative Example 1.
[0173] From the polarizing plates of Examples 6 and 7, nine sheets
could be punched out for 14.1-inch LCD. This yield is much higher
than the yield of six sheets of commercial polarizing plates.
Examples 8 and 9
[0174] The polarizing plates of Examples 6 and 7 were evaluated for
the visual field angle and the marginal irregularity in the same
manner as in Example 4 except that the polarizing plate of example
6 or 7 (HB-4 or HB-5) was used in place of the polarizing plate
(HB-2). Both of the polarizing plates were excellent.
Example 10
[0175] A polarizing plate (HB-6) was prepared in the same manner as
in Example 2 except that the polymer substrate (PK-2) was replaced
by a polymer substrate (PK-4) having a thickness of 80 .mu.m and
adjusted to have the same retardation value.
[0176] The polarizing plate was evaluated for the visual field
angle and the marginal irregularity in the same manner as in
Example 4 except that the polarizing plate (HB-6) was used in place
of the polarizing plate (HB-2). The polarizing plate (HB-6) caused
much less marginal irregularity than that of Comparative Example 1
and was excellent, but caused slight marginal irregularity in
comparison with the polarizing plate (HB-2). Thereby, it was
confirmed that the polymer substrate is preferably thinner. The
visual field angle was excellent.
INDUSTRIAL APPLICABILITY
[0177] The present invention enables optical compensation of an
optical cell without adverse effect, and suppression of marginal
increase of transmission, by the use of a polarizing plate
comprising a polarizer having a thickness of 10-25 .mu.m and an
optically anisotropic layer comprising a liquid crystal
compound.
[0178] Further, the production yield of the polarizing plate could
be remarkably improved by providing a large polarizing plate in
which a transmission axis is present neither in the length nor in
the breadth direction.
* * * * *