Polarizing plate

Stock material or miscellaneous articles – Composite – Of polyamidoester

Reexamination Certificate

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C359S483010, C428S424200, C428S424800, C428S520000, C428S521000

Reexamination Certificate

active

06726995

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a polarizing plate, particularly to having high durability and high polarization efficiency that is useful for liquid crystal displays (LCDs) and more specifically, to an improvement in a polarizing plate for substantially preventing mainly drop in polarization efficiency, hue shift, and light leakage even under a high humidity/high temperature environment.
BACKGROUND ART
A conventional polarizing plate with high polarization efficiency is generally such that a cellulose-triacetate-based (hereinafter, referred to as TAC) film, which serves as a protective film, is laminated on a polarizing film in which iodine or a dichroic dye is adsorbed and oriented in a polyvinyl-alcohol-based (hereinafter, referred to as PVA) film with an aqueous solution of PVA resin, which serves as an adhesive, in a state of wet or semidry flowability.
However, since the water absorption and water vapor permeability of TAC is high in a polarizing plate using TAC for the protective film, deterioration in polarization performance under a high humidity/high temperature environment, specifically, drop in polarization efficiency, hue shift, and light leakage under crossed nicols, has been significant.
In order to overcome these problems, polarizing plates that use a film made of a resin with low water absorption and low water vapor permeability for the protective film have been proposed.
For example, Japanese Unexamined Patent Publication No. 7-77608 discloses a polarizing plate such that a film serving as a protective film and made of a thermoplastic saturated norbornene-based resin is adhered to a PVA-based polarizing film using an acrylic-based adhesive or a polyester-isocyanate-based adhesive. After such a polarizing plate is subjected to an environment of 80° C. and 90%RH for 500 hours, the polarization efficiency is 95% or higher and the single transmissivity is 38% or higher.
In addition, Japanese Unexamined Patent Publication 7-294732 discloses a polarizing plate such that a film having a photoelastic coefficient of 25.0×10
−13
cm
2
/dyne or less, for example, a film made of an amorphous polyolefin as Zeonex, or polymethyl methacrylate such, serves as a support for a polarizing element film, and the support is adhered to the polarizing element film using an acrylic-based adhesive. After such a polarizing plate is subjected to an environment of 60° C. and 90%RH for 100 hours, the pyschometric lightness is small.
However, although these polarizing plates are able to suppress a drop in polarization efficiency under a wet heat environment, it cannot be said that suppression of hue shift and light leakage is sufficiently realized.
DISCLOSURE OF THE INVENTION
In the view of the foregoing and other problems, it is an object of the present invention to provide a polarizing plate with which, not only suppression of a drop in polarization efficiency is achieved, but also with which hue shift and light leakage do not substantially arise, under a high temperature/high humidity environment.
A polarizing plate of the present invention comprises a polyvinyl-alcohol-based polarizing film, a protective film, mainly composed of a cyclic-olefin-based resin having thereon two anchor coat agent layers, laminated on at least one side of the polarizing film with an adhesive.
The amount of change in optical in-plane retardation of the protective film is 5 nm or less after 24 hours in an atmosphere of 80° C., and the wetting tension of the laminated surface of the protective film is 500 &mgr;N/cm (23° C.) or more.
A first anchor coat agent layer is made of polyisocyanate and polyester polyol and/or polyether polyol. A second anchor coat agent layer is made of polyvinyl alcohol.
The adhesive is made of polyvinyl alcohol.
The polarizing film of the present invention is produced by uniaxially stretching and orienting a film made of PVA or a derivative thereof, and subsequently, adsorbing iodine, carrying out a boric acid solution treatment, and drying the film while under tension. Such a film also may be produced by immersing a film made of PVA or a derivative thereof in an aqueous solution of iodine such that the iodine is adsorbed, and subsequently, uniaxially stretching and orienting the film in a boric acid solution and drying the film while under tension. Polarizing films that utilize dichroic dyes, such as those that are azo-based, anthraquinone-based, and tetrazine-based, instead of iodine are fabricated in the same manner as well.
The polarization efficiency of a polarizing film obtained in such a manner is preferably, 95.0% or higher, more preferably, 99.0% or higher, and even more preferably, 99.7% or higher.
Hue shift as referred to in the present invention denotes a phenomenon such that when a single polarizing plate or crossed nicols is placed in a wet heat atmosphere, hue shift occurs with the single polarizing plate or the crossed nicols.
When a liquid crystal display employing polarizing plates with which hue shift arises is used for a long period, the hue of the display changes and contrast deteriorates, becoming one cause of deterioration in the performance of the liquid crystal display.
Light leakage as referred to in the present invention denotes a phenomenon such that in-plane luminance changes when two polarizing plates arranged to have a crossed nicols relation are placed in a wet heat environment.
When a liquid crystal display that employs polarizing plates that generate light leakage is used for a long period, light leaks at the edges of the display when black is displayed, and thereby display contrast deteriorates, becoming one cause of deterioration in the performance of a liquid crystal display.
Having fully considered how to provide a polarizing plate with which, not only suppression of a drop in polarization efficiency is achieved, but also with which hue shift and light leakage do not substantially arise, under a high temperature/high humidity environment, the present inventors came to the following conclusion, by which the present invention was achieved.
First, suppression of a drop in polarization efficiency under a high temperature/high humidity environment can be achieved by using a film with low water absorption and low water vapor permeability for the protective film of a polarizing plate. Suppression of hue shift under a high temperature/high humidity environment can be realized by sufficiently adhering a polarizing film and a protective film and by suppressing reversion in the alignment of the polarizing film. Suppression of light leakage under a high temperature/high humidity environment can be realized by using a film having a small amount of change in optical in-plane retardation for the protective film of a polarizing plate.
The present inventors then fully considered how to substantiate these inferences.
For the present invention, a film mainly composed of cyclic-olefin-based resin was employed for the protective film of the polarizing plate, because such a film has low water absorption and low water vapor permeability, and various physical properties required of a protective film for a polarizing plate, such as light transmissivity. (In addition, because cyclic-olefin-based resin has a small photoelastic coefficient, it was conjectured to be useful in preventing light leakage.)
In the present invention, cyclic-olefin-based resin is used as a general term, specific examples (a) to (d) being shown below.
(a) polymers that are ring-opening (co-)polymers of cyclic olefin with hydrogen added as needed
(b) (co-)polymers with cyclic olefin attached
(c) random copolymers of cyclic olefin and an &agr;-olefin such as ethylene or propylene
(d) graft modified substances that result when the above (a) to (c) are modified with unsaturated carboxylic acid or derivatives thereof.
The cyclic olefin is not particularly limited, examples including norbornene, tetracyclododecene, and derivatives thereof (for example, substances containing a carboxyl group or an ester group).
Known additives such as ultraviolet absorbers, organic or inorgani

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