Title: Polarizing plate having protective films with plasticizer, liquid crystal display using the same, and methods of making the same
Abstract: A polarizing plate and a liquid crystal display using the same are disclosed. The polarizing plate includes a polarizer made of a synthetic resin and protective films. The same protective films are attached to both sides of the polarizer. When a FTIR-ATR method is carried out with respect to the both sides of the protective film and a peak intensity (A) in the wavelength range around 1488 cm.sup.-1 of one side, a peak intensity (B) in the wavelength range around 1365 cm.sup.-1 of one side, a peak intensity (A') in the wavelength range around 1488 cm.sup.-1 of another side and a peak intensity (B') in the wavelength range around 1365 cm.sup.-1 of another side are measured, and (C) and (C') are represented by the relationships: (A)/(B)=(C) and (A')/(B')=(C'), (C)/(C').gtoreq.1.2 is satisfied. The same sides of the protective films having the (C) and (C') are adhered to both sides of the polarizer. In the polarizing plates, in accordance with the invention, even at the exposure to heat and humidity, advantageously occurrence of curling (warping) is reproduced.
Patent Number: 6,882,385 Issued on 04/19/2005 to Kusumoto, et al.
| Inventors:
|
Kusumoto; Seiichi (Ibaraki, JP);
Mihara; Hisashi (Ibaraki, JP);
Kitagawa; Atsushi (Ibaraki, JP);
Hamamoto; Eiji (Ibaraki, JP)
|
| Assignee:
|
Nitto Denko Corporation (Ibaraki, JP)
|
| Appl. No.:
|
745624 |
| Filed:
|
December 29, 2003 |
Foreign Application Priority Data
| Mar 05, 2001[JP] | 2001-060284 |
| Current U.S. Class: |
349/96; 428/1.31 |
| Intern'l Class: |
G02F 001//13.35; C09K 019//00 |
| Field of Search: |
349/96,122
428/1.31-1.33
|
References Cited [Referenced By]
U.S. Patent Documents
| 4025688 | May., 1977 | Nagy et al. | 428/350.
|
| 5286418 | Feb., 1994 | Nakamura et al. | 252/585.
|
| 5354513 | Oct., 1994 | Nakamura et al. | 252/585.
|
| 5753140 | May., 1998 | Shigemura | 252/299.
|
| 5818559 | Oct., 1998 | Yoshida | 349/122.
|
| 6147738 | Nov., 2000 | Okamoto | 349/122.
|
| 6512562 | Jan., 2003 | Kobayashi et al. | 349/122.
|
| 6731357 | May., 2004 | Tachibana et al. | 349/96.
|
| Foreign Patent Documents |
| 07-020317 | Jan., 1995 | JP.
| |
| 2001-343528 | Dec., 2001 | JP.
| |
Primary Examiner: Chowdhury; Tarifur R.
Attorney, Agent or Firm: Westerman, Hattori, Daniels & Adrian LLP
Parent Case Text
This application is a continuation of U.S. Ser. No. 10/071,217 filed on
Feb. 8, 2002.
Claims
What is claimed is:
1. A polarizing plate comprising:
a polarizer made of a synthetic resin and protective films attached to both
sides of the polarizer, each of said protective films having an amount of
plasticizer higher on one side than on an opposite side, wherein sides of
the protective films having a same amount of plasticizer are adhered to
both sides of the polarizer.
2. A liquid crystal display comprising a liquid crystal cell and a
polarizing plate on at least one side of the liquid crystal cell, the
polarizing plate comprising a polarizer made of a synthetic resin and
protective films, the same protective films being attached to both sides
of the polarizer, each protective film having a difference in an amount of
plasticizer between one side and another side, wherein sides of the
protective films having a same amount of plasticizer are adhered to both
sides of the polarizer.
3. The liquid crystal display of claim 2, wherein each of said protective
films has an amount of plasticizer higher on one side than on an opposite
side.
4. A method of making a polarizing plate comprising attaching protective
films to both sides of a polarizer, each protective film having a
difference in an amount of plasticizer between one side and another side,
wherein sides of the protective films having a same amount of plasticizer
are adhered to both sides of the polarizer.
5. The method of claim 4, wherein each of said protective films has an
amount of plasticizer higher on one side than on an opposite side.
6. A method of making a liquid crystal display comprising attaching
protective films to both sides of a polarizer to form a polarizing plate,
each protective film having a difference in an amount of plasticizer
between one side and another side, wherein sides of the protective films
having a same amount of plasticizer are adhered to both sides of the
polarizer, and disposing the polarizing plate on at least one side of a
liquid crystal cell.
7. The method of claim 6, wherein each of said protective films has an
amount of plasticizer higher on one side than on an opposite side.
8. A polarizing plate comprising:
a polarizer made of a synthetic resin and protective films attached to both
sides of the polarizer, each protective film having a difference in an
amount of plasticizer between one side and another side, wherein sides of
the protective films having a same amount of plasticizer are adhered to
both sides of the polarizer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a polarizing plate used for a liquid
crystal display (LCD) and a liquid crystal display using such a polarizing
plate.
2. Description of the Prior Art
Recently, demand for LCDs used for, for example, personal computers has
increased sharply. Application for LCDs has also broadened. Recently, such
LCDs are used for monitoring as well.
A polarizing plate used for a LCD is manufactured, for example, by a method
including steps of: dyeing a polyvinyl alcohol (PVA) film with dichroic
iodine or a dichroic dyestuff; crosslinking the film with boric acid,
borax, or the like; stretching the film uniaxially, followed by drying the
film and sticking it to a protective layer such as a triacetylcellulose
(TAC) film. The respective steps of dyeing, crosslinking and stretching
are not necessarily carried out separately and can be carried out
simultaneously. Furthermore, there is no limitation on the order of the
steps.
However, there is a problem in that when a TAC film is simply attached to a
PVA-based polarizer to produce a polarizing plate, curling occurs in the
polarizing plate due to the difference in shrinkage between the film and
the polarizer. Furthermore, there is also a problem in that when the
polarizing plate is left under a heating and humid condition, the curling
occurs more significantly.
SUMMARY OF THE INVENTION
In one aspect, the present invention relates to a polarizing plate in which
the occurrence of curling (warping) is prevented even if a polarizer and a
protective film are attached to each other. In another aspect, the present
invention relates to a liquid crystal display using such a polarizing
plate.
In some embodiments of the present invention, a polarizing plate includes a
polarizer made of a synthetic resin and protective films. The protective
films are attached to both sides of the polarizer, wherein when a FTIR-ATR
(Fourier transform infrared radiation-attenuated total reflection) method
is carried out with respect to both sides of the protective film and a
peak intensity (A) in the wavelength range around 1488 cm.sup.-1 of one
side, a peak intensity (B) in the wavelength range around 1365 cm.sup.-1
of one side, a peak intensity (A') in the wavelength range around 1488
cm.sup.-1 of another side and a peak intensity (B') in the wavelength
range around 1365 cm.sup.-1 of another side are measured, and (C) and (C')
are represented by the relationships: (A)/(B)=(C) and (A')/(B')=(C'),
(C)/(C').gtoreq.1.2 is satisfied, and the same sides of the protective
films having the (C) and (C') are adhered to both sides of the polarizer.
In some embodiments of the present invention, the synthetic resin film
comprises a polyvinyl alcohol film and the protective film is a
triacetylcellulose film.
In some embodiments, the polarizing plate of the present invention includes
a pressure-sensitive adhesive layer.
In some embodiments, the polarizing plate of the present invention includes
an anti-glare layer.
In some embodiments, the polarizing plate of the present invention includes
a reflector or a transreflector is attached to the polarizing plate.
In some embodiments, the polarizing plate of the present invention includes
a retardation plate or a .lambda. plate is attached to the polarizing
plate.
In some embodiments of the present invention, a viewing angle compensating
film is attached to the polarizing plate.
In some embodiments of the present invention, a brightness enhanced film is
attached to the polarizing plate.
In some embodiments of the invention, a liquid crystal display uses a
polarizing plate in accordance with an embodiment as described above on at
least one side of a liquid crystal cell.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A TAC film includes a plasticizer. It is found that curling (warping)
occurs in a polarizing plate frequently when the amount of the plasticizer
is different between one side and another side of the TAC film. The
inventors have found that the curling (warping) of the polarizing plate is
caused by the difference in shrinkage between one side and another side of
the TAC film due to the difference in the amount of the plasticizer
between one side and another side of the TAC film. In some embodiments of
the present invention, TAC films are adhered to both sides of a PVA film
so that the same sides (sides having the same amount of plasticizer) of
the TAC film are brought into contact with the PVA film. The amount of the
plasticizer on the sides of the TAC film may be measured by the FTIR-ATR
method. Thereby, the shrinking power of the TAC film on both sides of the
PVA film are offset, and thus the occurrence of curling (warping) in the
polarizing plate may be reduced. In some embodiments of a polarizing plate
of the present invention, it may be possible to reduce curling (warping)
even under a heating and humid condition. When the polarizing plate is
attached to a LCD panel, it may be possible to prevent foams from
entering, thus improving the efficiency in manufacturing liquid crystal
displays.
Specifically, the present invention provides a polarizing plate including a
polarizer made of a synthetic resin and protective films. In the
polarizing plate, the same protective films are adhered to both sides of
the polarizer. When a FTIR-ATR method is carried out with respect to both
sides of the protective film and a peak intensity (A) in the wavelength
range around 1488 cm.sup.-1 of one side, a peak intensity (B) in the
wavelength range around 1365 cm.sup.-1 of one side, a peak intensity (A')
in the wavelength range around 1488 cm.sup.-1 of another side and a peak
intensity (B') in the wavelength range around 1365 cm.sup.-1 of another
side are measured, and (C) and (C') are represented by the relationships:
(A)/(B)=(C) and (A')/(B')=(C'), (C)/(C').gtoreq.1.2 is satisfied. The same
sides of the protective films having the (C) and (C') are adhered to both
sides of the polarizer.
The range: (C)/(C').gtoreq.1.2 is defined from the following reason. That
is, if the difference in the amount of plasticizer between one side and
another side of the protective film is beyond the difference satisfying
the relationship: (C)/(C').gtoreq.1.2, no curling (warping) occurs from
the beginning. However, in fact, since it is difficult to equalize the
amount of the plasticizer on one side and the another side of the
protective film, most of TAC films are within the range satisfying the
relationship: (C)/(C').gtoreq.1.2.
In a basic configuration of a polarizing plate in accordance with the
present invention, a transparent protective film as a protective layer may
be adhered to one side or both sides of the polarizer made of a polyvinyl
alcohol-based polarizing film containing dichroic substance, and the like,
via an appropriate adhesive layer, for example, a layer of adhesive made
of a vinyl alcohol-based polymer.
A polarizer (polarizing film) made of an appropriate vinyl alcohol-polymer
film that is known in the art, such as polyvinyl alcohol film, a partially
formalized polyvinyl alcohol film, or the like, is subjected to
appropriate treatment such as dyeing with dichroic substances such as
iodine and a dichroic dyestuff, stretching, and crosslinking into any
suitable orders and manners. Any polarizer can be used, as long as it
allows linearly polarized light to pass through the film when natural
light enters. In some embodiments, a polarizer with an excellent light
transmittance and a polarization degree may be preferred.
As a material for the protective film forming a transparent protective
layer provided on one side or both sides of the polarizer (polarizing
film), an appropriate transparent film can be used. As the polymer, for
example, an acetate-based resin such as triacetylcellulose may be used.
However, the polymer is not necessarily limited thereto.
When factors such as polarizing property and durability are taken into
consideration, a preferred transparent protective film may be a
triacetylcellulose film having a surface saponified with alkali or the
like.
In some embodiments, the transparent protective film used for the
protective layer may be subject to treatment for providing properties such
as hard coating, anti-reflection, anti-sticking, dispersion, or
anti-glaring. Hard coating treatment may be carried out to prevent
scratches on the surfaces of the polarizing plate by, for example,
applying a surface of the transparent protective film with a coating film
of a hardening resin (e.g., a silicon-based ultraviolet hardening resin)
having excellent hardness and smoothness, etc.
Anti-reflection treatment may be carried out to prevent reflection of
outdoor daylight on the surface of the polarizing plate by, for example,
forming an anti-reflection film in a conventional manner. Anti-sticking
treatment may be carried out to prevent adjacent layers from sticking to
each other. Anti-glaring treatment may be carried out to prevent
visibility of light passing through the polarizing plate from being
hindered by outdoor daylight reflected on the surface of the polarizing
plate. The anti-glaring treatment can be carried out by providing
microscopic asperities on a surface of a transparent protective film in an
appropriate manner, for example, by roughening the surface by
sand-blasting or embossing, by blending transparent particles, or the
like.
An example of the above-mentioned transparent fine particles includes
silica, alumina, titania, zirconia, stannic oxide, indium oxide, cadmium
oxide, antimony oxide or the like, which have an average particle diameter
ranging from 0.5 .mu.m to 20 .mu.m. Inorganic fine particles having
electroconductivity may also be used. Alternatively, organic fine
particles including, for example, crosslinked or uncrosslinked polymer
particles, etc. can be used. The amount of the transparent fine particles
may range generally from 2 parts by weight to 70 parts by weight, and
particularly from 5 parts by weight to 50 parts by weight for 100 parts by
weight of the transparent resin.
An anti-glare layer including transparent fine particles can be provided as
the transparent protective layer or a coating layer applied onto the
surface of the transparent protective layer. The anti-glare layer may a
function as a diffusion layer to diffuse light passing through the
polarizing plate to enlarge the viewing angle (this function is referred
to as a viewing angle compensating function). The above-mentioned layers
such as the anti-reflection layer, the anti-sticking layer, the diffusion
layer, and the anti-glare layer can be provided separately from the
transparent protective layer as an optical layer, for example, in sheet
form including the above-mentioned layers.
There is no specific limitation on treatment for adhering the polarizer
(polarizing film) to the transparent protective film that is a protective
layer. Adhesion can be carried out, for example, by using an adhesive such
as an adhesive including a vinyl alcohol-based polymer, or an adhesive
including at least a water-soluble crosslinking agent of vinyl
alcohol-based polymer such as boric acid, borax, glutaraldehyde, melamine
and oxalic acid. A layer of such an adhesive can be formed by, for
example, applying and drying an aqueous solution. In preparation of the
aqueous solution, other additives, or a catalyst such as an acid can be
blended if necessary.
A polarizer can be used as an optical member that is laminated onto another
optical layer. Although there is no specific limitation on the optical
layer, one or two or more of appropriate optical layer(s) applicable for
formation of a liquid crystal display, etc. can be used. Examples of an
optical layer include, for example, a reflector, a transreflector, a
retardation plate (such as a .lambda. plate like a half wavelength plate
and a quarter wavelength plate), a viewing angle compensating film, a
brightness enhanced film, and the like. Examples of a polarizing plate
include a reflective polarizing plate or a semitransparent polarizing
plate formed by laminating a reflector or a transreflector on the
above-mentioned polarizing plate including a polarizer and a protective
layer according to the present invention; an elliptical polarizing plate
or a circular polarizing plate formed by laminating a retardation plate on
the above-mentioned polarizing plate including a polarizer and a
protective layer; a polarizing plate formed by laminating a viewing angle
compensating film on the above-mentioned polarizing plate including a
polarizer and a protective layer; and a polarizing plate formed by
laminating a brightness enhanced film on the above-mentioned polarizing
plate including a polarizer and a protective layer.
A reflector may be provided on a polarizing plate to form a reflective
polarizing plate. In general, such a reflective polarizing plate is
provided on the backside of a liquid crystal cell in order to make a
liquid crystal display, etc. to display by reflecting incident light from
a visible side (display side). The reflective polarizing plate has some
advantages, for example, light sources such as backlight need not be built
in, and thus the liquid crystal display can be thinner.
The reflective polarizing plate can be formed in an appropriate manner such
as by attaching a reflecting layer made of, for example, metal on one
surface of the polarizing plate via, for example, the above-mentioned
transparent protective film as required. As a specific example, a
reflecting layer may be formed by attaching a foil of a reflective metal
such as aluminum or a deposition film on one surface of the transparent
protective film that has been subjected to matting treatment as required.
Another example of a reflective polarizing plate includes the
above-mentioned transparent protective film having a surface of
microscopic asperities due to fine particles contained and a reflecting
layer corresponding to the microscopic asperities. The reflecting layer
having a surface with microscopic asperities diffuses incident light
irregularly, so that directivity and glare can be prevented and
irregularity in color tones can be controlled. This transparent protective
film can be formed by attaching a metal directly onto a surface of a
transparent protective film using any appropriate methods including
deposition, such as vacuum deposition, and plating, such as ion plating
and sputtering.
Furthermore, the reflector can be used as, for example, a reflecting sheet
formed by providing a reflecting layer onto an appropriate film similar to
the transparent protective film, instead of the above-mentioned method of
producing a reflector directly on the transparent protective film of the
polarizing plate. The reflecting layer of the reflector, which generally
is made of metal, may be preferably used with its surface covered with a
film, a polarizing plate or the like, because the reduction of reflectance
due to oxidation can be prevented, the initial reflectance can be
maintained for a long time, an additional protective layer need not be
formed, or the like.
A semitransparent polarizing plate can be obtained by using the method for
forming the reflective polarizing plate described above, except that a
semitransparent reflecting layer such as a half mirror, which reflects
light and transmits light, is used instead of the reflecting layer. In
general, the semitransparent polarizing plate may be provided on the
backside of a liquid crystal cell. When a liquid crystal display is used
in a relatively bright atmosphere, the semitransparent polarizing plate
allows an incident light from the visible side (display side) to be
reflected to display an image, while in a relatively dark atmosphere, an
image is displayed by using a built-in light source such as a backlight
behind the semitransparent polarizing plate. In other words, the
semitransparent polarizing plate can be used to form a liquid crystal
display that can save energy for a light source such as a backlight under
a bright atmosphere, while a built-in light source can be used under a
relatively dark atmosphere.
An elliptical polarizing plate or a circular polarizing plate in which a
retardation plate is additionally laminated on the above-mentioned
polarizing plate including a polarizer and a protective layer will now be
explained.
A retardation plate is typically used for modifying linearly polarized
light to either elliptical polarized light or circular polarized light,
modifying elliptical polarized light or circular polarized light to
linearly polarized light, or modifying a polarization direction of
linearly polarized light. In particular, a retardation plate called a
quarter wavelength plate (.lambda./4 plate) is generally used for
modifying linearly polarized light to circular polarized light, and for
modifying circular polarized light to linearly polarized light. A half
wavelength plate (.lambda./2 plate) is generally used for modifying a
polarization direction of linearly polarized light.
An elliptical polarizing plate can be effective in compensating
(preventing) colors (blue or yellow) generated due to birefringence in a
liquid crystal layer of a super twist nematic (STN) liquid crystal display
so as to provide a black-and-white display free from the above-mentioned
colors. Controlling three-dimensional refractive index may be further
preferred since an elliptical polarizing plate can compensate (prevent)
colors observed when looking a screen of the liquid crystal display from
an oblique direction. A circular polarizing plate is effective, for
example, in adjusting color tones of an image of a reflective liquid
crystal display that has a color image display, and it also serves to
prevent reflection as well.
Examples of the retardation plate include, for example, a birefringent film
prepared by stretching an appropriate polymer film, an oriented film of a
liquid crystal polymer, and an oriented layer of a liquid crystal polymer
that is supported by a film, and the like. Examples of the polymer
include, polycarbonate, polyvinyl alcohol, polystyrene, polymethyl
methacrylate, polyolefins such as polypropylene, polyalylate, and
polyamide. The incline-oriented film may be prepared by, for example,
bonding a heat shrinkable film to a polymer film and subjecting the
polymer film to stretching treatment and/or shrinking treatment under the
influence of a shrinkage force by heat, or by orienting obliquely a liquid
crystal polymer.
A polarizing plate in which a viewing angle compensating film is
additionally laminated on the above-mentioned polarizing plate including a
polarizer and a protective layer will now be explained.
The viewing angle compensating film is typically used for widening a
viewing angle so that an image can be seen relatively dearly even when a
screen of a liquid crystal display is viewed from a slightly oblique
direction.
As the viewing angle compensating film, a triacetylcellulose film etc.
coated with a discotic liquid crystal, or a retardation plate can be used.
While an ordinary retardation plate is a birefringent polymer film that is
stretched uniaxially in the face direction, a retardation plate used as
the viewing angle compensating film is a two-way stretched film such as a
birefringent polymer film stretched biaxially in the face direction, or an
incline-oriented polymer film with a controlled refractive index in the
thickness direction that is stretched uniaxially in the face direction and
stretched also in the thickness direction. The incline-oriented film is
prepared by, for example, bonding a heat shrinkable film to a polymer film
and subjecting the polymer film to stretching treatment and/or shrinking
treatment under the influence of a shrinkage force by heat, or by
obliquely orienting a liquid crystal polymer. A polymer as a material of
the retardation plate is similar to the polymer used for the
above-mentioned retardation plate.
A polarizing plate in which a brightness enhanced film is attached to the
above-mentioned polarizing plate including a polarizer and a protective
layer is generally arranged on the backside of a liquid crystal cell. When
natural light enters by the backlight of the liquid crystal display etc.
and reflection from the backside and the like, the brightness enhanced
film reflects linearly polarized light of a predetermined polarizing axis
or circularly polarized light in a predetermined direction, while
transmitting other light. The polarizing plate in which the brightness
enhanced film is laminated on the above-mentioned polarizing plate
including a polarizer and a protective layer allows entrance of light from
a light source such as a backlight to obtain transmitted light in a
predetermined polarization state, while reflecting light other than light
in the predetermined polarization state. Light reflecting by the
brightness enhanced film is reversed through a reflecting layer or the
like arranged additionally behind the brightness enhanced film. The
reversed light is allowed to re-enter the brightness enhanced plate. The
re-entering light is transmitted partly or entirely as light in a
predetermined polarization state so as to increase the amount of light
passing through the brightness enhanced film and polarized light that is
hardly absorbed in the polarizer is supplied so as to increase the amount
of light available for the liquid crystal display, etc. Thus, the
brightness can be improved. When light enters through a polarizer from the
backside of the liquid crystal cell by using a backlight or the like
without using any brightness enhanced films, most of the light having a
polarization direction inconsistent with the polarization axis of the
polarizer is absorbed in the polarizer but not transmitted by the
polarizer. Depending on the characteristics of the polarizer, about 50% of
light is absorbed in the polarizer, which decreases the quantity of light
available in the liquid crystal display or the like and makes the image
dark. The brightness enhanced film repeatedly prevents light having a
polarization direction to be absorbed in the polarizer from entering the
polarizer to reflect the light on the brightness enhanced film, and
reverses the light through a reflecting layer or the like provided behind
the brightness enhanced film to make the light re-enter the brightness
enhanced plate. Since the brightness enhanced film transmits the polarized
light that is reflected and reversed between the brightness enhanced film
and the reflecting layer only if the polarized light has a polarization
direction to pass the polarizer, light from a backlight or the like can be
used efficiently for displaying images of a liquid crystal display to
provide a bright screen.
Examples of the brightness enhanced film include, for example, a film which
transmits a linearly polarized light having a predetermined polarization
axis and reflects other light, for example, a multilayer thin film of a
dielectric or a multilayer laminate of thin films with varied refraction
aeolotropy; a film that reflects either clockwise or counterclockwise
circular polarized light while transmitting other light, for example, a
cholesteric liquid crystal layer, more specifically, an oriented film of a
cholesteric liquid crystal polymer or an oriented liquid crystal layer
supported on a supportive substrate, or the like.
Therefore, with the brightness enhanced film transmitting a linearly
polarized light having a predetermined polarization axis, the transmitted
light directly enters the polarizing plate with the polarization axes
matched, so that absorption loss due to the polarizing plate is controlled
and the light can be transmitted efficiently. On the other hand, with the
brightness enhanced film transmitting a circular polarized light, such as
a cholesteric liquid crystal layer, preferably, the transmission circular
polarized light is converted into linearly polarized light before entering
the polarizing plate in an aspect of controlling of the absorption loss,
although the circular polarized light can enter the polarizer directly.
Circular polarized light can be converted into linearly polarized light by
using a quarter wavelength plate as a retardation plate.
A retardation plate having a function as a quarter wavelength plate in a
wide wavelength range of a visible light region can be obtained, for
example, by overlapping a retardation layer functioning as a quarter
wavelength plate for monochromatic light such as light having 550 nm
wavelength, and another retardation plate showing a separate optical
retardation property, for example, a retardation plate functioning as a
half wavelength plate. Therefore, a retardation plate arranged between a
polarizing plate and a brightness enhanced film can include a single layer
or at least two layers of retardation layers.
A cholesteric liquid crystal layer also can be provided by combining layers
different in the reflection wavelength and it can be configured by
overlapping two or at least three layers. As a result, the obtained
retardation plate can reflect circular polarized light in a wide
wavelength region of a visible light region, thus providing transmission
circular polarized light in a wide wavelength region.
Furthermore, a polarizing plate can be formed by laminating a polarizing
plate and two or at least three optical layers like the above-mentioned
polarization separating type polarizing plate. Therefore, the polarizing
plate can be a reflective elliptical polarizing plate, a semitransparent
elliptical polarizing plate or the like, which is prepared by combining
the above-mentioned reflective polarizing plate or a semitransparent
polarizing plate with a retardation plate. An optical member including a
lamination of two or at least three optical layers can be formed in a
method of laminating layers separately in a certain order for
manufacturing a liquid crystal display etc. or in a method for preliminary
lamination. Because an optical member that has been laminated previously
has excellent stability in quality and assembling operability, efficiency
in manufacturing a liquid crystal display can be improved. Any appropriate
adhesion means, such as a pressure sensitive adhesive layer, can be used
for lamination.
The pressure sensitive adhesive layer can be provided on a polarizing plate
or on an optical member for adhesion with other members such as a liquid
crystal cell. The adhesive layer can be formed by the conventional
appropriate pressure sensitive adhesives, such as an acrylic pressure
sensitive adhesive. Pressure sensitive adhesives having a low moisture
absorption coefficient and an excellent heat resistance may be preferred
due to aspects of prevention of foaming or peeling caused by moisture
absorption, prevention of decrease in the optical properties and warping
of a liquid crystal cell caused by difference in thermal expansion
coefficients, formation of a high quality liquid crystal display having
excellent durability, etc. The pressure sensitive adhesive layer can
contain fine particles to obtain optical diffusivity. Pressure sensitive
adhesive layers can be provided on necessary surfaces if required. For
example, the polarizing plate including a polarizer and a protective layer
can be provided with a pressure sensitive adhesive layer on at least one
surface of the protective layer as required.
When a pressure sensitive adhesive layer provided on the polarizing plate
or the optical member is exposed on the surface, preferably, the pressure
sensitive adhesive layer is temporarily covered with a separator for
preventing contamination by the time the pressure sensitive adhesive layer
is used. The separator can be made of an appropriate thin sheet by coating
a peeling agent if required. Examples of a peeling agent include, for
example, a silicone-based peeling agent, a long-chain alkyl-based peeling
agent, a fluorine-based peeling agent, a peeling agent including
molybdenum sulfide or the like.
The above-described members forming a polarizing plate and an optical
member, such as a polarizing film, a transparent protective film, an
optical layer, and a pressure sensitive adhesive layer can have
ultraviolet absorption power by treating with an ultraviolet absorber such
as, for example, an ester salicylate compound, a benzophenone compound, a
benzotriazole compound, a cyanoacrylate compound, a nickel complex salt
compound, or the like.
The above-mentioned polarizing plate can be used for formation of various
apparatus such as a liquid crystal display. The liquid crystal display can
be produced as conventionally known structures, such as transmission type,
reflection type, or a transmission-reflection type. A liquid crystal cell
forming the liquid crystal display can be selected arbitrarily from
appropriate cells such as active matrix driving type represented by a thin
film transistor, a simple matrix driving type represented by a twist
nematic type and a super twist nematic type.
When polarizing plates or optical members are provided on both sides of a
liquid crystal cell, the polarizing plates or the optical members on both
sides can be the same or different. Moreover, for forming a liquid crystal
display, one or at least two layers of appropriate members such as a prism
array sheet, a lens array sheet, an optical diffuser, or a backlight can
be arranged at appropriate positions.
Hereinafter, the present invention will be explained with reference to
Examples and Comparative Examples.
EXAMPLE 1
A non-stretched PVA film having a polymerization degree of 2400, a
thickness in a non-stretched state of 75 .mu.m and a width in a
non-stretched state of 800 mm was prepared as a synthetic film. The PVA
film was stretched to three times an original length in a first bath (a
bath 1) including water as a main component; then stretched to 1.1 times
an original length in a dyeing bath including an aqueous solution
dissolving iodine and potassium iodide; thereafter immersed in a
crosslinking bath containing boric acid and potassium iodide, and
stretched to 1.8 times an original length in a washing bath including
water. Thereafter, the film was dried and was rolled up as a polarizer.
Next, a TAC film was prepared as a protective film. In the following
explanation, one side of this TAC film is referred to as a side 1 and
another side of this TAC film is referred to as a side 2. The FTIR-ATR
method was carried out with respect to the both sides of the protective
film. A peak intensity (A) in the wavelength range around 1488 cm.sup.-1
of the side 1, a peak intensity (B) in the wavelength range around 1365
cm.sup.-1 of the side 1, a peak intensity (A') in the wavelength range
around 1488 cm.sup.-1 of the side 2 and a peak intensity (B') in the
wavelength range around 1365 cm.sup.-1 of the side 2 were measured; and
(A)/(B)=(C), (A')/(B')=(C') and (C)/(C') were calculated, respectively.
Table 1 shows the results.
Then, the polarizer was attached to the TAC film so that the side 1 of the
TAC film was brought into contact with both sides of the polarizer. This
polarizing plate was cut out into a size of 12.1 inches at the absorption
axis of 45.degree. or 135.degree.. The initial curling (warping) amount
right after it was cut out and the curling (warping) amount of the
polarizing plate after left at a temperature of 23.degree. C. and humidity
of 60% for one hour were measured respectively. Table 1 shows the results.
The measurement of the curling (warping) amount was carried out by
measuring the distance from the horizontal level in the portion where the
curling (warping) is at the maximum when the curled polarizing plate was
put on the horizontal level in a form of concave shape.
EXAMPLE 2
A polarizing plate was prepared by the same method as in Example 1 except
that a different TAC film was used. The same measurement results as in
Example 1 are shown in Table 1.
EXAMPLE 3
Furthermore, a polarizing plate was prepared by the same method as in
Example 1 except that a different TAC film was used. The same measurement
results as in Example 1 are shown in Table 1.
COMPARATIVE EXAMPLE 1
A polarizing plate was prepared by the same method as in Example 1 except
that the polarizer and the TAC film were attached to each other so that
the side 1 of the TAC film was brought into contact with one side of the
polarizer and the side 2 of the TAC film was brought into contact with
another side of the polarizer. The same measurement results as in Example
1 are shown in Table 1.
COMPARATIVE EXAMPLE 2
A polarizing plate was prepared by the same method as in Comparative
Example 1 except that the same TAC film as in Example 2 was used. The same
measurement results as in Example 1 are shown in Table 1.
COMPARATIVE EXAMPLE 3
A polarizing plate was prepared by the same method as in Comparative
Example 1 except that the same TAC film as in Example 3 was used. The same
measurement results as in Example 1 are shown in Table 1.
TABLE 1
(A) (B) (C)
(A') (B') (C') (C)/(C') curling*1 curling*2
Ex. 1 side 1 0.234 0.640 0.366 1.22 15 mm 15 mm
side 2 0.132 0.442 0.299 or less or less
Ex. 2 side 1 0.195 0.564 0.346 1.37 15 mm 15 mm
side 2 0.155 0.616 0.252 or less or less
Ex. 3 side 1 0.244 0.616 0.396 1.48 15 mm 15 mm
side 2 0.170 0.637 0.267 or less or less
Co. 1 side 1 0.234 0.640 0.366 1.22 25-50 mm 25-80 mm
side 2 0.132 0.442 0.299
Co. 2 side 1 0.195 0.564 0.346 1.37 25-50 mm 25-80 mm
side 2 0.155 0.616 0.252
Co. 3 side 1 0.244 0.616 0.396 1.48 25-50 mm 25-80 mm
side 2 0.170 0.637 0.267
Ex. = Example
Co. = Comparative Example
curling*1 = amount of initial curling
curling*2 = amount of curling after subjected to a heating and humid
condition
As is apparent from Table 1, in the polarizing plate of Examples 1, 2 and
3, the initial curling amount and the curling amount after it was
subjected to a heating and humid condition were less than 15 mm. On the
other hand, in the polarizing plate of the Comparative Examples 1, 2 and
3, the initial curling amount was 25 to 50 mm and the curling amount after
subjected to a heating and humid condition were 25 to 80 mm. Thus, the
curling (warping) occurred significantly.
As mentioned above, according to the present invention, when the amount of
plasticizer in the vicinity of the surface of the protective film is
different between one side and another side, by attaching the protective
film to the polarizer so that one side to protective film is attached to
one side of the polarizer and another side of the protective film is
attached to another side of the polarizer, the shrinking power of the
protective film is allowed to be balanced, thereby preventing the
polarizing plate from curling (warping). Thus, a polarizing plate
according to the present invention may provide an excellent industrial
value.
The invention may be embodied in other forms without departing from the
spirit or essential characteristics thereof. The embodiments disclosed in
this application are to be considered in all respects as illustrative and
not limitative. The scope of the invention is indicated by the appended
claims rather than by the foregoing description, and all changes which
come within the meaning and range of equivalency of the claims are
intended to be embraced therein.
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