JP2023053968A - Polarizing plate, liquid crystal display device, method for producing the same, and polyvinyl alcohol-based polarizing film - Google Patents

Abstract

[Problem] To provide a high-quality polyvinyl alcohol-based polarizing plate having excellent balance between heat resistance and polarizing performance, a liquid crystal display device including the polarizing plate, a method for manufacturing the same, and a polyvinyl alcohol-based polarizing film used for the polarizing plate. .
The polarizing plate includes a polyvinyl alcohol polarizing film having a glass transition temperature of 95° C. or higher, and a protective film provided on at least one side of the polyvinyl alcohol polarizing film. In the liquid crystal display device, the polarizing plate and the image display device are laminated. In the method for manufacturing the liquid crystal display device, the polarizing plate is formed by performing a drying treatment on a laminate of a polarizing plate having a polyvinyl alcohol-based polarizing film having a glass transition temperature of less than 95° C. and an image display device. The glass transition temperature of the polyvinyl alcohol-based polarizing film is controlled to 95° C. or higher.
[Selection figure] None

Description

TECHNICAL FIELD The present invention relates to a polarizing plate, a liquid crystal display device and its manufacturing method, and a polyvinyl alcohol-based polarizing film. More specifically, the present invention relates to a polarizing plate (polyvinyl alcohol polarizing plate) having a polyvinyl alcohol polarizing film with well-balanced optical properties and heat resistance.

In recent years, the development of liquid crystal display devices has been remarkable, and they are widely used in smartphones, tablets, personal computers, liquid crystal televisions, projectors, vehicle-mounted panels, and the like. A polarizing plate is used in the above-described liquid crystal display device, and as the polarizing plate, a polarizing film obtained by adsorbing and aligning iodine or a dichroic dye to a polyvinyl alcohol film and a protective film are laminated is mainly used. ing. In recent years, with the expansion of the use of liquid crystal display devices, the operating environment has expanded both in terms of temperature range and humidity range. There is a need for polarizers that exhibit a high degree of polarization.

As a method for improving the heat resistance of the polarizing plate, for example, a polarizing film using a specific azo dye is used (see, for example, Patent Document 1), iodine is adsorbed and oriented, and a dichroic organic A method of using a polarizing film containing a dye (see, for example, Patent Document 2) has been proposed.
In addition, a polarizing plate having specific optical characteristics, including a polarizer in which iodine is adsorbed and oriented, and having a retardation value Ri of iodine of 160 nm or more (see, for example, Patent Document 3), and a weight ratio of iodine and potassium and a polarizing plate having a defined amount of boron (see, for example, Patent Document 4) has been proposed as a polarizing plate having excellent heat resistance and durability.

WO2016/186196 Japanese Unexamined Patent Application Publication No. 2012-3172 JP 2016-139151 A WO2016/060087

However, in the technique disclosed in Patent Document 1, although a polarizing film having high heat resistance can be obtained by using a specific dye, the degree of polarization is as low as 99.9%, and the contrast is insufficient. There is a problem.

In Patent Document 2, although a polarizing film having high polarizing performance is obtained, in the manufacturing process, it is necessary to provide two or more dyeing tanks, a dyeing tank for dyeing with a dichroic dye and a dyeing tank for dyeing with iodine. Furthermore, since fine concentration control of two or more dye tanks is required to adjust the color tone of the polarizing film, there is a problem of low productivity.

Furthermore, the technique disclosed in Patent Document 3 above can suppress the deterioration of optical properties in a heat resistance test at about 85° C., but has the problem that heat resistance under severer temperature conditions cannot be improved.
Even with the technology disclosed in Patent Document 4, the absorbance at 700 nm when left at 105° C. for 30 minutes can be maintained at 2.3 or more, but the heat resistance when exposed to a high temperature environment for a long time cannot be improved. There is a problem.

Therefore, in view of this background, the present invention provides a high-quality polyvinyl alcohol-based polarizing plate that is excellent in well-balanced heat resistance and polarizing performance, a liquid crystal display device equipped with the polarizing plate, a method for manufacturing the same, and the above polarizing plate. A polyvinyl alcohol-based polarizing film to be used is provided.

The inventors of the present invention have conducted intensive research in view of such circumstances, and found that by using a polarizing plate having a polyvinyl alcohol-based polarizing film having a glass transition temperature higher than that of a conventional polyvinyl alcohol-based polarizing plate, heat durability can be improved. It was found that a polyvinyl alcohol-based polarizing plate having a well-balanced and excellent polarizing performance can be obtained.

That is, the gist of the present invention is the following [1] to [6].
[1] A polarizing plate comprising a polyvinyl alcohol-based polarizing film and a protective film provided on at least one side of the polyvinyl alcohol-based polarizing film, wherein the polyvinyl alcohol-based polarizing film has a glass transition temperature of 95°C or higher. A polarizing plate.
[2] The polarizing plate of [1] above, which has a moisture content of 2% by weight or less.
[3] The polarizing plate of [1] or [2] above, wherein the polyvinyl alcohol-based polarizing film has a boric acid content of 20% by weight or more.
[4] A liquid crystal display device comprising a laminate of the polarizing plate according to any one of [1] to [3] above and an image display device.
[5] The method for manufacturing a liquid crystal display device according to [4] above, wherein a laminate of a polarizing plate having a polyvinyl alcohol-based polarizing film having a glass transition temperature of less than 95°C and an image display device is dried. A method for manufacturing a liquid crystal display device, wherein the glass transition temperature of the polyvinyl alcohol-based polarizing film constituting the polarizing plate is controlled to 95° C. or higher by performing.
[6] A polyvinyl alcohol-based polarizing film having a glass transition temperature of 95°C or higher.

Since the polarizing plate of the present invention has a polyvinyl alcohol-based polarizing film with a glass transition temperature of 95°C or higher, it has excellent heat resistance and polarizing performance in a well-balanced manner. Also, it can stably exhibit its performance.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention are described in detail below. However, the invention is not limited to these embodiments.

One embodiment of the polarizing plate of the present invention has a polyvinyl alcohol polarizing film having a glass transition temperature (Tg) of 95° C. or higher, and a protective film provided on at least one side of the polyvinyl alcohol polarizing film. ing. This is the greatest feature of the present invention.

The glass transition temperature (Tg) of the polyvinyl alcohol-based polarizing film is required to be 95° C. or higher, and from the viewpoint of heat resistance durability, is preferably 100° C. or higher, particularly preferably 105° C. or higher, and the upper limit is Usually 125°C. That is, if the glass transition temperature (Tg) is less than the lower limit, the heat resistance of the polarizing plate becomes insufficient and the object of the present invention cannot be achieved.

The glass transition temperature (Tg) was obtained by using a polyvinyl alcohol polarizing film (polarizing plate) with DSC (Q2000 manufactured by TA instruments, sample 5 mg) from -80 ° C. to 155 ° C. at a heating rate of 5 ° C./min. It is a value measured warmed up.

The polyvinyl alcohol polarizing film is produced by first obtaining a polyvinyl alcohol film using a polyvinyl alcohol resin as a raw material, then using the polyvinyl alcohol film as a raw material, swelling, dyeing, cross-linking with boric acid, stretching, washing, It is carried out through processes such as drying.
Here, an example of the method for producing the polyvinyl alcohol-based polarizing film will be described in detail in order of steps.

[Material for Forming Polyvinyl Alcohol Film]
First, the polyvinyl alcohol-based resin, which is the material for forming the polyvinyl alcohol-based film, will be described.
As the polyvinyl alcohol-based resin, an unmodified polyvinyl alcohol-based resin, that is, a resin produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate, is usually used. If necessary, a resin obtained by saponifying a copolymer of vinyl acetate and a small amount (usually 10 mol % or less, preferably 5 mol % or less) of a component copolymerizable with vinyl acetate may be used. can. Components copolymerizable with vinyl acetate include, for example, unsaturated carboxylic acids (including salts, esters, amides, nitriles, etc.), olefins having 2 to 30 carbon atoms (eg, ethylene, propylene, n-butene, , isobutene, etc.), vinyl ethers, unsaturated sulfonates, and the like. Modified polyvinyl alcohol-based resins obtained by chemically modifying hydroxyl groups after saponification can also be used.

As the polyvinyl alcohol-based resin, a polyvinyl alcohol-based resin having a 1,2-diol structure in a side chain can also be used. Polyvinyl alcohol resins having 1,2-diol structures in the side chains can be prepared, for example, by (i) saponifying a copolymer of vinyl acetate and 3,4-diacetoxy-1-butene, (ii) acetic acid (iii) saponifying and decarboxylating a copolymer of vinyl and vinyl ethylene carbonate, (iii) saponifying and decarboxylating a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane (iv) a method of saponifying a copolymer of vinyl acetate and glycerin monoallyl ether; and the like.

The weight average molecular weight of the polyvinyl alcohol-based resin is preferably from 100,000 to 300,000, particularly preferably from 110,000 to 280,000, more preferably from 120,000 to 120,000, from the viewpoint of production of the polyvinyl alcohol-based polarizing film. 260,000. That is, if the weight average molecular weight is too small, sufficient optical performance tends to be difficult to obtain when the polyvinyl alcohol resin is used as an optical film. Stretching during production tends to be difficult. The weight average molecular weight of the polyvinyl alcohol-based resin is the weight average molecular weight measured by the GPC-MALS method.

From the viewpoint of the optical performance of the polyvinyl alcohol-based polarizing film, the average degree of saponification of the polyvinyl alcohol-based resin is usually preferably 98 mol% or more, particularly preferably 99 mol% or more, and further preferably 99.5. mol % or more, particularly preferably 99.8 mol % or more. That is, if the average degree of saponification is too small, the polyvinyl alcohol-based polarizing film produced from the polyvinyl alcohol-based film tends not to have sufficient optical performance.
Here, the average degree of saponification is measured according to JIS K 6726.

As the polyvinyl alcohol-based resin, two or more different modified species, modified amount, weight average molecular weight, average saponification degree, etc. may be used in combination.

Next, a polyvinyl alcohol-based resin aqueous solution is prepared using the polyvinyl alcohol-based resin. The polyvinyl alcohol-based resin aqueous solution will be described.

The polyvinyl alcohol-based resin aqueous solution is obtained by dissolving the polyvinyl alcohol-based resin described above in a solvent such as water. As a solvent, in addition to water, for example, dimethyl sulfoxide (DMSO); N-methylpyrrolidone; polyhydric alcohols such as glycerin, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, and trimethylolpropane; Amines such as diethylenetriamine; and mixtures thereof may also be used.

In the polyvinyl alcohol resin aqueous solution, other than the polyvinyl alcohol resin, if necessary, glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, polyethylene glycol, trimethylolpropane, etc. are generally used. A plasticizer and at least one of nonionic, anionic, and cationic surfactants can be contained, and containing these is preferable from the viewpoint of film-forming properties. One selected from these may be used alone, or two or more may be used in combination.

From the viewpoint of productivity, the resin concentration of the polyvinyl alcohol resin aqueous solution thus obtained is preferably 15 to 60% by weight, particularly preferably 17 to 55% by weight, and further preferably 20 to 50% by weight. is. That is, if the resin concentration of the aqueous solution is too low, the drying load tends to increase and productivity tends to decrease.

The obtained polyvinyl alcohol-based resin aqueous solution is usually defoamed. Examples of defoaming methods include static defoaming and defoaming using a multi-screw extruder. As the multi-screw extruder, any multi-screw extruder having a vent may be used, and a twin-screw extruder having a vent is usually used.

[Method for producing polyvinyl alcohol-based film]
After the defoaming treatment, the polyvinyl alcohol-based resin aqueous solution is introduced into a T-shaped slit die in fixed amounts, discharged and cast onto a rotating cast drum, and formed into a film by a continuous casting method. Then, a polyvinyl alcohol-based film is obtained by drying the formed film. A method for producing this polyvinyl alcohol film will be described.

The continuous casting method is, for example, a method in which a polyvinyl alcohol-based resin aqueous solution is discharged from a T-shaped slit die onto a casting mold such as a rotating casting drum, an endless belt, or a resin film to form a film.
The surface temperature of the cast mold such as the cast drum is preferably 40 to 99°C, particularly preferably 60 to 95°C. Drying of the film formed by the cast type is carried out by conveying the film while alternately contacting the front surface and the back surface of the film with the outer peripheral portions of a plurality of heat rolls. After drying with this hot roll, the film may be heat-treated. The heat treatment is preferably carried out at 60 to 150°C, particularly preferably 80 to 130°C. Both side edges of the film, which has been dried and optionally heat-treated, are cut off (slitted) to form the polyvinyl alcohol-based film. Then, the polyvinyl alcohol-based film is wound around a core tube to form a film roll.

The thickness of the polyvinyl alcohol film is preferably 5 to 75 μm, particularly preferably 10 to 60 μm from the viewpoint of thinning the polarizing film, and further preferably 15 to 60 μm from the viewpoint of durability. be.

The width of the polyvinyl alcohol-based film is preferably 4 m or more, more preferably 4.5 m or more from the viewpoint of increasing the surface area, and particularly preferably from 4.5 to 6 m from the viewpoint of avoiding breakage.

The length of the polyvinyl alcohol-based film is preferably 4 km or more, more preferably 4.5 km or more from the viewpoint of increasing the area, and particularly preferably from 4.5 to 50 km from the viewpoint of transportation weight.

[Method for producing polyvinyl alcohol-based polarizing film]
A method for producing one embodiment of the polyvinyl alcohol-based polarizing film of the present invention using the polyvinyl alcohol-based film will be described below.

The polyvinyl alcohol-based polarizing film is produced by unrolling the polyvinyl alcohol-based film from the film roll, transferring it in the horizontal direction, and going through steps such as swelling, dyeing, cross-linking with boric acid, stretching, washing, and drying. These steps will be explained.

The swelling step is performed before the dyeing step. The swelling step is usually performed by immersing the polyvinyl alcohol-based film in a treatment liquid in a water tank. By the swelling process, stains on the surface of the polyvinyl alcohol film can be cleaned, and the swelling of the polyvinyl alcohol film also has the effect of preventing uneven dyeing. Water is usually used as the treatment liquid. The treatment liquid may contain a small amount of an iodide compound, an additive such as a surfactant, alcohol, etc., as long as the main component is water. The temperature of the treatment liquid is usually about 10 to 45° C., and the immersion time in the treatment liquid is usually about 0.1 to 10 minutes.

The dyeing step is usually carried out by immersing the polyvinyl alcohol film in a liquid containing iodine or a dichroic dye. As the liquid, an iodine-potassium iodide aqueous solution is usually used, and the appropriate iodine concentration is 0.1 to 2 g/L and the potassium iodide concentration is 1 to 100 g/L. A dyeing time of about 30 to 500 seconds is practical. The temperature of the liquid is preferably 5 to 50°C. The aqueous solution may contain a small amount of an organic solvent compatible with water in addition to the water solvent.

The boric acid cross-linking step is usually carried out by immersing the polyvinyl alcohol-based film in a liquid containing a boron compound such as boric acid or borax. The liquid is an aqueous solution or a water-organic solvent mixture, and the concentration of the boron compound in the liquid is about 10 to 100 g/L. It is preferable to allow potassium iodide to coexist in the liquid in terms of stabilizing the polarizing performance. The temperature of the liquid is preferably about 30 to 70° C., and the immersion time in the liquid is preferably about 0.1 to 20 minutes.

The stretching step may be performed independently, or may be performed during at least part of the swelling step, dyeing step, and boric acid cross-linking step. It is preferable that the total draw ratio is 3 to 10 times, preferably 3.5 to 6 times, in the uniaxial direction. At this time, the film may be slightly stretched in the direction perpendicular to the stretching direction (stretching to the extent that shrinkage in the width direction is prevented or more). During stretching, the temperature around the polyvinyl alcohol film to be stretched is preferably 40 to 170°C.

The washing step is performed, for example, by immersing the polyvinyl alcohol film in water or an aqueous solution of iodide such as potassium iodide. Precipitates generated on the surface of the polyvinyl alcohol-based film can be removed by the washing step. When an aqueous potassium iodide solution is used, the concentration of potassium iodide is usually about 1 to 80 g/L. The temperature of the iodide aqueous solution during washing is usually 5 to 50°C, preferably 10 to 45°C. The immersion time in the water or iodide aqueous solution is usually 1 to 300 seconds, preferably 10 to 240 seconds. The washing with water and the washing with an aqueous solution of potassium iodide may be appropriately combined.

In the drying step, the polyvinyl alcohol film is usually dried in the air at an atmospheric temperature of 40 to 100°C, preferably 70 to 98°C, for 0.5 to 20 minutes.

Here, as a method for controlling the glass transition temperature of the polyvinyl alcohol-based polarizing film to be produced to be high, (1) in the boric acid crosslinking step, the boric acid concentration of the liquid in which the polyvinyl alcohol-based film is immersed is increased. (2) a method of increasing the cross-linking density of the polyvinyl alcohol-based polarizing film; (3) adding a drying agent such as diphosphorus pentoxide; and (4) adjusting the atmospheric temperature and drying time in the drying step.
Among these, the above methods (1) and (4) are preferable in terms of ease of adjustment.

In the case of the above method (1), the boric acid concentration of the liquid is preferably 20 to 90 g / L from the viewpoint of making the glass transition temperature 95 ° C. or higher. More preferably 30 to 80 g/L.
By increasing the boric acid concentration of the liquid in this way, the boric acid content in the polyvinyl alcohol-based polarizing film to be produced can be increased. The boric acid content is preferably 20% by weight or more, particularly preferably 20 to 30% by weight, more preferably 21 to 29% by weight.
If the boric acid content is too low, the polyvinyl alcohol polarizing film tends to have insufficient heat durability. It tends to break easily.

In the case of the above method (4), from the viewpoint of making the glass transition temperature 95° C. or higher, the ambient temperature in the drying step is preferably 50 to 99° C., and particularly preferably 60 to 98° C. from the viewpoint of heat resistance durability. The drying time is preferably 1 to 15 minutes, particularly preferably 1.5 to 10 minutes.
By this drying process, the moisture content of the polyvinyl alcohol-based polarizing film can be reduced to 2.0% by weight or less.

Thus, a polyvinyl alcohol-based polarizing film having a glass transition temperature of 95° C. or higher is obtained.

The degree of polarization of the polyvinyl alcohol-based polarizing film is preferably 99.9% or more, particularly preferably 99.92% or more. If the degree of polarization is too low, there is a tendency that the contrast in a liquid crystal display device such as a liquid crystal display cannot be ensured.

Furthermore, the single transmittance of the polyvinyl alcohol-based polarizing film is preferably 42% or more. If the single transmittance is too low, there is a tendency that a liquid crystal display device such as a liquid crystal display cannot achieve high brightness.
Single transmittance is a value obtained by measuring the light transmittance of a single polyvinyl alcohol polarizing film using a spectrophotometer.

[Method for producing polarizing plate]
Then, by bonding an optically isotropic protective film to one or both surfaces of the polyvinyl alcohol-based polarizing film thus obtained via an adhesive, a polarizing plate having a protective film on at least one surface is obtained. can get.

Examples of the protective film include cellulose triacetate, cellulose diacetate, polycarbonate, polymethyl methacrylate, cycloolefin polymer, cycloolefin copolymer, polystyrene, polyethersulfone, polyarylene ester, poly-4-methylpentene, polyphenylene oxide, and the like. A resin film or a resin sheet can be mentioned.

The bonding of the polyvinyl alcohol-based polarizing film and the protective film is performed by a known method. For example, a liquid adhesive composition is uniformly applied to the polyvinyl alcohol-based polarizing film, the protective film, or both. After that, the two are adhered together and pressure-bonded, and heated or irradiated with an active energy ray.

The polarizing plate preferably has a moisture content of 2% by weight or less, particularly preferably 1.5% by weight or less, and further preferably 1% by weight or less, from the viewpoint of enhancing the heat resistance of the polyvinyl alcohol-based polarizing film. is. The lower limit of this moisture content is usually 0.1% by weight.
If the moisture content is too high, the polyvinyl alcohol-based polarizing film tends to have insufficient heat resistance.

Methods for adjusting the moisture content to a low level include (a) a method in which the polarizing plate is continuously dried, (b) a method in which the polarizing plate is once wound and then heat-treated in a high-temperature storage, and (c) polarized light. A method of vacuum-drying after chip-cutting the plate may be mentioned.
Among these methods, the above method (a) is preferable in order to uniformly adjust the moisture content in the width direction and the length direction.

In the case of method (a) above, the ambient temperature in the drying treatment is usually 60 to 100° C., preferably 70 to 95° C. in terms of optical properties. Also, the drying time is usually 1 to 10 minutes, preferably 2 to 8 minutes. In this drying treatment, it is preferable to perform two or more stages of drying treatment with different atmospheric temperatures and drying times, and it is particularly preferable to increase the atmospheric temperature in the subsequent drying treatment from the viewpoint of the appearance of the polarizing plate. Specifically, for example, it is preferable to set the atmospheric temperature in the first stage drying treatment to 60 to 80°C, and to set the atmospheric temperature in the second stage drying treatment to 80°C to 100°C.

In the case of the above method (c), vacuum drying after chip cutting of the polarizing plate is preferably carried out at an ambient temperature of 40 to 60° C. for about 15 minutes to 36 hours.

The polarization degree of the polarizing plate is preferably 99.9% or more, particularly preferably 99.92% or more. If the degree of polarization is too low, there is a tendency that the contrast in a liquid crystal display device such as a liquid crystal display cannot be ensured.
In addition, the degree of polarization is generally measured in a state in which two polarizing plates are superimposed so that their orientation directions are in the same direction, and the light transmittance (H ₁₁ ) measured at the wavelength λ and the two polarized light It is calculated according to the following formula from the light transmittance (H ₁ ) measured at the wavelength λ in a state in which the plates are superimposed so that their orientation directions are perpendicular to each other.
Degree of polarization=[(H ₁₁ −H ₁ )/(H ₁₁ +H ₁ )] ^1/2

Furthermore, the single transmittance of the polarizing plate is preferably 42% or more. If the single transmittance is too low, there is a tendency that a liquid crystal display device such as a liquid crystal display cannot achieve high brightness.
Single transmittance is a value obtained by measuring the light transmittance of a single polyvinyl alcohol polarizing plate using a spectrophotometer.

[Liquid crystal display device]
An embodiment of the liquid crystal display device of the present invention includes the above polarizing plate.
That is, the liquid crystal display device is formed by laminating the polarizing plate and an image display device such as a liquid crystal cell with an adhesive layer interposed therebetween.
Components such as an illumination system may be added to the liquid crystal display device as required.
Examples of the liquid crystal cell include TN type, STN type, VA type, and IPS type.

Since the liquid crystal display device includes the polarizing plate having a polyvinyl alcohol-based polarizing film with a glass transition temperature of 95° C. or higher, it stably displays images even under extremely high temperature conditions of 95° C. or higher. be able to.

[Method for manufacturing liquid crystal display device]
In the liquid crystal display device, when the glass transition temperature of the polyvinyl alcohol-based polarizing film constituting the polarizing plate is less than 95° C., the liquid crystal display device can be manufactured as follows.
That is, a polarizing plate having a polyvinyl alcohol-based polarizing film having a glass transition temperature of less than 95° C. and the liquid crystal cell are laminated via an adhesive layer, and the laminate is subjected to drying treatment or vacuum drying treatment. Thereby, the glass transition temperature of the polyvinyl alcohol-based polarizing film of the polarizing plate is set to 95° C. or higher.
In this case, the ambient temperature in the drying treatment is preferably 60 to 100° C., and the drying time is preferably 0.5 to 10 minutes. The atmospheric temperature in the vacuum drying treatment is preferably 50 to 80° C., and the drying time is preferably 10 to 120 minutes.
The vacuum drying treatment is a drying treatment in a vacuum (absolute pressure of 1 kPa or less).

The above polarizing plate has excellent heat resistance and polarizing performance, and can be Meters, audio equipment, liquid crystal display devices for automobiles and machinery, sunglasses, anti-glare glasses, 3D glasses, wearable displays, antireflection layers for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication It is preferably used for equipment, medical equipment, building materials, toys and the like.

EXAMPLES The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded.
Each physical property was measured as follows.

<Measurement conditions>
(1) Moisture content (% by weight)
100 mg of the resulting polarizing plate was weighed and placed in a 2 ml vial, followed by adding anhydrous ethanol, sealing the vial, and heating at 60° C. for 24 hours. After cooling to room temperature (25° C.), the moisture content of the polarizing plate was measured using a trace moisture analyzer CA-200 (manufactured by Mitsubishi Chemical Analytic).

(2) Glass transition temperature of polyvinyl alcohol polarizing film (°C)
The resulting polyvinyl alcohol polarizing film was heated from −80° C. to 155° C. using a DSC (TA instruments Q2000, sample 5 mg) at a heating rate of 5° C./min to obtain a polyvinyl alcohol polarizing film. Glass transition temperatures were measured.

(3) boric acid content (% by weight)
After drying the obtained polyvinyl alcohol-based polarizing film at 105° C. for 2 hours, 100 mg was weighed, and 30 ml of water was added, followed by heating and dissolving. After cooling the obtained solution to 25° C., 5 ml of glycerin was added and stirred, then titrated with a 0.1 mol/L sodium hydroxide aqueous solution, and calculated according to the following formula.
Boric acid content (% by weight) = 61.83 x 0.1 x f x V/weight of polyvinyl alcohol-based polarizing film x 100
f: factor V of 0.1 mol / L sodium hydroxide aqueous solution: titration volume (ml)

(4) Degree of polarization (%), single transmittance (%)
A test piece having a length of 4 cm and a width of 4 cm is cut out from the central portion of the obtained polarizing plate in the width direction, and an automatic polarizing film measuring device (manufactured by JASCO Corporation: VAP7070) is used to measure the degree of polarization (%) and the single transmission. The rate (%) was measured.

(5) Heat resistance endurance test The obtained polarizing plate was sandwiched between glass on both sides and placed in a drier at 105°C. After 150 hours, the appearance was visually observed and evaluated according to the following criteria.
(Evaluation criteria)
○: No discoloration at all △: Slight discoloration ×: Clear discoloration

<Example 1>
(Preparation of polyvinyl alcohol-based polarizing film and polarizing plate)
A polyvinyl alcohol-based film with a thickness of 45 μm was unwound from a film roll and stretched 1.7 times in the machine direction (MD) while being transported in the horizontal direction and immersed in a water tank containing water at a water temperature of 25 ° C. to swell. . Then, the sheet was immersed in an aqueous solution containing 1.2 g/L of iodine and 30 g/L of potassium iodide at 30° C. and stretched 1.6 times in the machine direction (MD) while being dyed. The film was immersed in an aqueous solution (55° C.) having a composition of L and 30 g/L of potassium iodide and uniaxially stretched 2.1 times in the machine direction (MD) while cross-linking with boric acid. Then, the film was washed with an aqueous potassium iodide solution and dried at an atmospheric temperature of 60° C. for 2 minutes to obtain a polyvinyl alcohol-based polarizing film with a total draw ratio of 5.7 times. Table 1 shows the properties of the obtained polyvinyl alcohol-based polarizing film.
Subsequently, on both sides of the obtained polyvinyl alcohol-based polarizing film, using an aqueous polyvinyl alcohol solution as an adhesive, a triacetyl cellulose film having a film thickness of 40 μm was laminated, dried at an atmospheric temperature of 95° C. for 2 minutes, and a single transparent film was obtained. A polarizing plate with a ratio of 42.8% and a degree of polarization of 99.95% was obtained. Table 1 shows the properties of the polyvinyl alcohol-based polarizing film and the polarizing plate.

<Example 2>
A polyvinyl alcohol-based film with a thickness of 45 μm was unwound from a film roll and stretched 1.7 times in the machine direction (MD) while being transported in the horizontal direction and immersed in a water tank containing water at a water temperature of 25 ° C. to swell. . Then, the sheet was immersed in an aqueous solution containing 1.2 g/L of iodine and 30 g/L of potassium iodide at 30° C. and stretched 1.6 times in the machine direction (MD) while being dyed. The film was immersed in an aqueous solution (55° C.) having a composition of L and 30 g/L of potassium iodide and uniaxially stretched 2.2 times in the machine direction (MD) while cross-linking with boric acid. Then, the film was washed with an aqueous solution of potassium iodide and dried at an atmospheric temperature of 60° C. for 2 minutes to obtain a polyvinyl alcohol-based polarizing film with a total draw ratio of 6.0 times. Table 1 shows the properties of the obtained polyvinyl alcohol-based polarizing film.
Subsequently, on both sides of the obtained polyvinyl alcohol polarizing film, a 40 μm-thick triacetyl cellulose film was laminated using an aqueous polyvinyl alcohol solution as an adhesive, dried at an ambient temperature of 80° C. for 5 minutes, and further continued. Then, it was dried at an atmospheric temperature of 95° C. for 1 minute to obtain a polarizing plate having a single transmittance of 42.9% and a degree of polarization of 99.96%. Table 1 shows the properties of the polyvinyl alcohol-based polarizing film and the polarizing plate.

<Example 3>
A polyvinyl alcohol-based polarizing plate was obtained in the same manner as in Example 2, except that the drying after bonding the triacetyl cellulose film was performed at an atmospheric temperature of 80° C. for 2 minutes.
After providing an adhesive layer on one side of the obtained polarizing plate, chip cutting was performed to obtain a polarizing plate chip having a length of 10 cm×5 cm. The polarizing plate chip was vacuum-dried at an atmospheric temperature of 40° C. for 16 hours. The properties of the obtained polarizing plate are as shown in Table 1.

<Comparative Example 1>
A polarizing plate was obtained in the same manner as in Example 2, except that drying after bonding the triacetyl cellulose film was performed at an ambient temperature of 80° C. for 2 minutes. Table 1 shows the properties of the polyvinyl alcohol-based polarizing film and the polarizing plate.

In the polarizing plates of Examples 1 to 3, the glass transition temperature of the polyvinyl alcohol polarizing film contained is 95° C. or higher, which is higher than usual, so that there is no discoloration during the heat resistance test, and the heat resistance is excellent. I understand.
On the other hand, in the polarizing plate of Comparative Example 1, since the polyvinyl alcohol-based polarizing film contained therein has a glass transition temperature of less than 95° C., discoloration occurs during the heat resistance test.

<Example 4>
On both sides of the polarizing film obtained in Example 1, a 40 μm-thick triacetyl cellulose film was laminated using an aqueous polyvinyl alcohol solution as an adhesive, and dried at an ambient temperature of 80° C. for 2 minutes to obtain a single transmittance of 42. A polarizing plate with a degree of polarization of 99.94% was obtained.
After providing an adhesive layer on one side of the obtained polarizing plate, it was attached to a liquid crystal display device to form a laminate of the polarizing plate and the liquid crystal display device. Subsequently, the laminate was vacuum-dried at an atmospheric temperature of 60° C. for 45 minutes using a vacuum dryer. After that, a glass substrate was laminated on the polarizing plate of the laminate, and a heat resistance durability test was performed in a dryer at an atmospheric temperature of 105° C. for 150 hours. Table 2 shows the results.
In addition, the polarizing plate was peeled off from the separately prepared laminate after vacuum drying treatment, and the moisture content and the glass transition temperature were measured. there were.

<Example 5>
The laminate obtained in Example 4 was vacuum-dried at an ambient temperature of 70° C. for 60 minutes using a vacuum dryer. After that, a glass substrate was laminated on the polarizing plate of the laminate, and a heat resistance durability test was performed in a dryer at an atmospheric temperature of 105° C. for 150 hours. Table 2 shows the results.
In addition, the polarizing plate was peeled off from the separately prepared laminate after the vacuum drying treatment, and the moisture content and the glass transition temperature were measured. rice field.

<Comparative Example 2>
The laminate obtained in Example 4 was vacuum-dried at an ambient temperature of 70° C. for 15 minutes using a vacuum dryer. After that, a glass substrate was laminated on the polarizing plate of the laminate, and a heat resistance durability test was performed in a dryer at an atmospheric temperature of 105° C. for 150 hours. Table 2 shows the results.
In addition, the polarizing plate was peeled off from the separately prepared laminate after the vacuum drying treatment, and the moisture content and the glass transition temperature were measured. rice field.

The evaluation criteria for the heat resistance durability test of the laminate in Examples 4 and 5 and Comparative Example 2 are as follows.
(Evaluation criteria)
○: No discoloration at all △: Slight discoloration ×: Clear discoloration

In Examples 4 and 5, the glass transition temperature of the polyvinyl alcohol-based polarizing film in the polarizing plate was 95° C. or higher. It turns out that it is excellent in the property.
On the other hand, in Comparative Example 2, since the glass transition temperature of the polyvinyl alcohol-based polarizing film in the polarizing plate is less than 95°C, the laminate using the polarizing plate undergoes discoloration during the heat resistance test. I understand.

Although specific embodiments of the present invention have been described in the above examples, the above examples are merely illustrative and should not be construed as limiting. Various modifications apparent to those skilled in the art are intended to be within the scope of the invention.

The polarizing plate of the present invention is excellent in well-balanced heat resistance and polarizing performance, and is suitable for various liquid crystal display devices, automotive applications requiring high durability and polarizing performance, and display applications for industrial instruments required in various environments. It is preferably used.

Claims (6)

A polarizing plate comprising a polyvinyl alcohol polarizing film and a protective film provided on at least one side of the polyvinyl alcohol polarizing film, wherein the polyvinyl alcohol polarizing film has a glass transition temperature of 95° C. or higher. A polarizing plate characterized by: 2. The polarizing plate according to claim 1, wherein the moisture content is 2% by weight or less. 3. The polarizing plate according to claim 1, wherein the polyvinyl alcohol polarizing film has a boric acid content of 20% by weight or more. A liquid crystal display device comprising a polarizing plate according to any one of claims 1 to 3 and an image display device laminated together. 5. The method for manufacturing a liquid crystal display device according to claim 4, wherein a laminate of a polarizing plate having a polyvinyl alcohol-based polarizing film having a glass transition temperature of less than 95° C. and an image display device is dried, A method for manufacturing a liquid crystal display device, wherein the glass transition temperature of the polyvinyl alcohol-based polarizing film constituting the polarizing plate is controlled to 95° C. or higher. A polyvinyl alcohol-based polarizing film having a glass transition temperature of 95°C or higher.