Polarizer protection film

Details Polarizer protection film Polarizer protection film

2002-07-31

2003-04-22

Teskin, Fred (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C428S001100, C428S001310, C428S523000, C359S506000

Reexamination Certificate

active

06552145

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a polarizer protection film for protecting a polarizer having a polarization function of a polarizing film used in a liquid crystal display.
BACKGROUND OF THE INVENTION
Liquid crystal displays which have recently been used for many applications, such as personal computers and word processors, exhibit a basic displaying performance by liquid crystal cells having the function of changing a plane of polarization or a phase of light by ON-OFF switching of an electric field and polarizing films having an optical shutter function varying a transmission state of light with a change in the plane of polarization or the phase.
This polarizing film is a laminated product of a polarizer having a polarization function and a protective film for securing its durability or mechanical strength with the interposition of an adhesive.
The polarization function of the polarizer is obtained by orienting a light anisotropic absorber in a transparent polymer film. There are, for example, a PVA-based polarizer obtained by allowing iodine or a dichromatic dye to be adsorbed on polyvinyl alcohol (hereinafter referred to as “PVA”) film, followed by uniaxial orientation by stretching, and a polyene-based polarizer to which the polyene structure is given by dehydration or dehydrochlorination of a stretched PVA or vinyl chloride film.
In the polarizer made of such a material, a problem of deteriorated optical characteristics or dimensional shrinkage caused by disorder of orientation in reliability test of such as heat resistance is easily encountered, because PVA itself is a hydrophilic polymer as for the PVA-based film, and because the polarizer is wholly oriented by stretching, including the polyene-based film. Accordingly, the polarizer made of the above-mentioned material is provided with protective films on both surfaces thereof. These protective films are required to have high light transmittance, no birefringence, excellent heat resistance, moisture resistance, cold resistance and light resistance, and to be optically uniform, that is to say, to have no foreign matter or no streaky visual defects. Saponified solvent-cast films of triacetyl cellulose (hereinafter referred to as “TAC”) have hitherto been used from the viewpoints of transparency, appearance and easy adhesion to the polarizers.
However, the TAC film has a moisture permeability of about 300 g/m
2
/24 hours/100 &mgr;m, so that the ability of it to protect the polarizing film from the circumstances of high temperature and humidity such as 60° C. and 90% RH is insufficient. Further, the TAC film has the problem that a phase difference develops by a shrinkage stress of the polarizer in reliability test of the polarizing film because of its high photoelastic constant, which causes deterioration of a polarizing performance.
As means for solving this problem, it has been proposed that a norbornene resin sheet is used as a protective film for a polarizer (refer to Japanese Patent Laid-open Publication (Hei) 06-51117). The norbornene resin is lower in the moisture permeability and photoelastic constant than that of the TAC resin, so that the norbornene resin is excellent in ability to protect the polarizer under circumstances of high temperature and humidity or in the reliability test.
The norbornene resin is largely restricted by a solvent when a film thereof is formed by solvent casting. Accordingly, melt film formation is preferred, considering economy. However, its molecular skeleton contains many tertiary carbon atoms, so that it is easily deteriorated by heat or oxygen.
Usually, as means for preventing deterioration of resins due to heat or oxygen, various antioxidants such as a phenol antioxidant and a phosphate antioxidant have been used. However, in the case of the norbornene resin, it is very difficult to prevent deterioration of the resin only by addition of an additive, because of brittleness of the resin itself, and it is also difficult to prevent dot-like defects such as fish eyes or appearance defects such as die lines incident to local changes in melt viscosity.
SUMMARY OF THE INVENTION
An object of the present invention is to obtain a film having optical uniformity and being sufficient to protect a polarizer, by melt film formation of the above-mentioned norbornene resin, thereby providing a highly reliable polarizing film and a liquid crystal display using the same.
The present inventors have conducted intensive investigation on the above-mentioned problems. As a result, the inventors have discovered that the problems can be solved by using the following film for protection of the polarizer. According to the invention, there is provided a polarizer protection film obtained by melt film formation of a norbornene resin having (1) a glass transition temperature Tg (measured in accordance with JIS K 7121) of 135° C. to 180° C., and (2) an inherent viscosity (&eegr;
inh
) of 0.4 or more, which is calculated by the following equation (1) from the drop time T (sec.) between marked lines measured at 30° C. with an Ubbelohde viscometer for a solution of the resin in chloroform having a resin concentration of C (g/dl) and the drop time B (sec.) between marked lines measured at 30° C. with an Ubbelohde viscometer for chloroform, the solvent, wherein (3) the above-mentioned Tg and &eegr;
inh
are within the range indicated by the following equation (2):
&eegr;
inh
={ln(
T/B
)}/
C
  (1)
T: the drop time (sec.) between marked lines of a sample solution
B: the drop time (sec.) between marked lines of a solvent alone
C: the concentration (g/dl) of the sample solution
45&eegr;
inh
≦58.5−0.2Tg  (2)
DESCRIPTION OF THE PREFERRED EMBODIMENTS
When the glass transition temperature (Tg) of the norbornene resin used in the invention is lower than 135° C., the strength of the protective film is decreased under circumstances of a temperature as high as 90° C. or more under which the polarizing film is used, and the function sufficient for protecting the polarizer can not be exhibited. On the other hand, a norbornene resin having a Tg exceeding 180° C. brings about poor appearance such as a reduction in surface smoothness caused by gelation or non-uniform viscosity, because the molding temperature exceeds the decomposition temperature of the resin itself. The above-mentioned glass transition temperature is preferably from 140° C. to 160° C.
When the inherent viscosity (&eegr;
inh
) of the norbornene resin used in the invention is less than 0.4, the strength is insufficient for handling as a film, resulting in the occurrence of the problem of breakage or cracks while the film runs in a production line. The above-mentioned inherent viscosity is preferably from 0.43 to 0.60.
Further, when Tg and &eegr;
inh
do not satisfy equation (2), poor appearance such as a reduction in surface smoothness caused by gelation or non-uniform viscosity is caused, similarly to the case that Tg exceeds 180° C.
The weight average molecular weight of the norbornene resin used in the invention is usually 45,000 or more, and preferably from 50,000 to 90,000.
The norbornene resins used in the invention include the following copolymers:
(1) A ring-opening copolymer of monomer A represented by the following general formula (I) and monomer B represented by the following general formula (II) (each of monomer A and monomer B is hereinafter also referred to as a “specific monomer”);
(2) A ring-opening copolymer of monomer A represented by the following general formula (I), monomer B represented by the following general formula (II) and a copolymerizable monomer; and
(3) A saturated copolymer obtained by addition copolymerization of monomer A represented by the following general formula (I), monomer B represented by the following general formula (II) and an unsaturated a polymerizable double bond-containing compound.
In general formulas (I) and (II), R
1
to R
8
each independently represent an atom or a group selected from the group consisting of hydrogen atom, a hydrocarbon group having

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