JP2024133756A - Polyvinyl alcohol polarizer and polarizing plate - Google Patents

Abstract

To provide a polyvinyl alcohol-based polarizer which is usable in a polarizing plate capable of obtaining a display that is excellent in a balance between a contrast and clearness even under a severe environmental condition such as high temperature and high humidity.SOLUTION: A polarizer contains a polyvinyl alcohol-based resin, and has a rate of esterification formed of a hydroxyl group of the polyvinyl alcohol-based resin and a boric acid calculated by the following expression (1) of 60% or more. Expression (1): A rate of esterification (unit%)=100×(1-a)/((1-a)+b). In expression (1), when methyl of heavy dimethyl sulfoxide is set at 2.5 ppm and an integral value of main chain methylene (CH2) of 2.2 to 1.9 ppm is set at 2.00, in NMR measurement of a polarizer using a 4 mm HRMAS probe using the heavy dimethyl sulfoxide as a medium, an integral value of 5.0 to 4.2 ppm is represented by a, and an integral value of 7.5 to 6.8 ppm is represented by b.SELECTED DRAWING: None

Description

The present invention relates to a polyvinyl alcohol-based polarizer and a polarizing plate, and more particularly to a polyvinyl alcohol-based polarizer which, when made into a polarizing plate, results in a polarizing plate with little change in optical performance even when exposed to harsh environments such as high temperature and high humidity.

Polyvinyl alcohol films have been used in many applications as films with excellent transparency, and one of their useful applications is as a polarizer. Such polarizers are used as a basic component of liquid crystal displays as a member constituting a polarizing plate, and in recent years, their use has been expanded to devices that require high quality and high reliability.

Under these circumstances, polarizing plates used in displays of liquid crystal televisions, multi-function mobile terminals and the like are required to meet the demands of higher brightness, higher definition, larger area and thinner profile, and furthermore, are required to have high durability with little change in optical performance even in harsh environments such as high temperature and high humidity.

As such polarizing plates with excellent durability, polarizing plates manufactured using dye-based polarizing films (see Patent Documents 1 and 2) have been used, and a method has been used in which a polarizer with low transmittance is used to manufacture a polarizing plate that maintains a high degree of polarization even when used under harsh conditions.

JP 2013-57909 A JP 2021-101237 A

However, the techniques disclosed in the above Patent Documents 1 and 2 have a problem that the polarizing plate produced has a low degree of polarization and cannot achieve high brightness when used in a display, although it has excellent durability under high temperature and high humidity. Also, a polarizing plate using a polarizer with low transmittance has excellent durability under high temperature and high humidity, but is inferior in contrast and clarity when used in a display.

Under such circumstances, an object of the present invention is to provide a polyvinyl alcohol-based polarizer that can be used in a polarizing plate that can provide a display having a good balance between contrast and clarity even under harsh environmental conditions of high temperature and high humidity.

However, the present inventors focused on the esterification rate formed by the hydroxyl groups of the polyvinyl alcohol-based resin in a polyvinyl alcohol-based polarizer and boric acid, and discovered that when this value is within a specific range that is higher than conventional values, the above-mentioned problem is solved and a display having an excellent balance between contrast and clarity can be obtained when the polarizer is used as a polarizing plate, thereby completing the present invention.

That is, the gist of the present invention is as follows.
[1] A polyvinyl alcohol-based polarizer comprising a polyvinyl alcohol-based resin, wherein an esterification rate formed between a hydroxyl group of the polyvinyl alcohol-based resin and boric acid, as calculated by the following formula (1), is 60% or more:
Equation (1): Esterification rate (unit %) = 100 × (1-a) / ((1-a) + b)
(In the formula, a and b represent the ratio of the methyl group of deuterated dimethyl sulfoxide to 2.5 in NMR measurement using a 4 mm HRMAS probe and a polarizer with deuterated dimethyl sulfoxide as a medium.)
When the integral value of the main chain methylene (CH ₂ ) from 2.2 to 1.9 ppm is taken as 2.00, the integral value from 5.0 to 4.2 ppm is taken as a, and the integral value from 7.5 to 6.8 ppm is taken as b.
[2] The weight average molecular weight of the polyvinyl alcohol resin is 100,000 to 300,000
0. The polyvinyl alcohol-based polarizer according to [1].
[3] A polarizing plate comprising the polyvinyl alcohol-based polarizer according to [1] or [2].

The polyvinyl alcohol-based polarizer of the present invention is a polarizer that can provide a polarizing plate having excellent durability performance under high temperature and high humidity conditions, and when used in a display as a polarizing plate, can provide a display having a good balance between contrast and clarity.

The present invention will be described in detail below.
The polyvinyl alcohol-based polarizer of the present invention is a polarizer containing a polyvinyl alcohol-based resin, and has an "esterification rate formed by hydroxyl groups of the polyvinyl alcohol-based resin and boric acid, calculated by the following formula (1)" (hereinafter, sometimes simply referred to as "esterification rate") of 60%.
The present invention is characterized in that:
Equation (1): Esterification rate (unit %) = 100 × (1-a) / ((1-a) + b)
(In the formula, a and b represent the ratio of the methyl group of deuterated dimethyl sulfoxide to 2.5 in NMR measurement using a 4 mm HRMAS probe and a polarizer with deuterated dimethyl sulfoxide as a medium.)
When the integral value of the main chain methylene (CH ₂ ) from 2.2 to 1.9 ppm is taken as 2.00, the integral value from 5.0 to 4.2 ppm is taken as a, and the integral value from 7.5 to 6.8 ppm is taken as b.

The esterification rate of the polyvinyl alcohol-based polarizer of the present invention must be 60% or more, preferably 62% or more, and particularly preferably 65% or more.
By making the esterification rate 60% or more, the polarizing plate can be manufactured with a 60° C./90R
A polarizing plate having small changes in cross transmittance at wavelengths of 700 nm and 430 nm under high temperature and humidity conditions of H % can be obtained, making it possible to reduce leakage of both red and blue light.

Examples of the method for controlling the esterification rate within a predetermined range include a method for using a polyvinyl alcohol-based resin having a weight average molecular weight of 150,000 or more, a method for controlling the moisture content of a film at the time of peeling off from a cast drum to 10% or less when producing a polyvinyl alcohol-based polarizer using a polyvinyl alcohol-based resin, and a method for controlling the evaporation rate of moisture on a cast mold to 10% or less.
Examples of suitable methods include a method of setting the esterification rate of the polarizer at 85°C or higher during polarizer production, and a method of setting the drying temperature of the polarizer at 85°C or higher during polarizer production. Among these, in terms of improving the esterification rate of the polarizer and producing a polarizing plate that can provide a display with a good balance of contrast and clarity even under harsh environmental conditions of high temperature and high humidity, it is particularly preferable to combine a method of setting the drying temperature of the polarizer at 85°C or higher during polarizer production with a method of setting the evaporation rate of water on the cast mold to 1.6%/second or higher.

The polyvinyl alcohol polarizer of the present invention is produced by first producing a polyvinyl alcohol film as a raw material, and then using the raw material film.
First, the method for producing the raw polyvinyl alcohol film will be described in more detail in the order of each step, but the polyvinyl alcohol film of the present invention is not limited to these embodiments.

The polyvinyl alcohol film of the present invention is preferably produced through the following steps (A) to (C), and preferably also through step (D) as necessary.
Step (A) is a step of preparing an aqueous solution of a polyvinyl alcohol-based resin.
Step (B) A step of casting an aqueous solution of a polyvinyl alcohol-based resin into a cast mold to form a membrane.
Step (C) A step of heating and drying the formed film by contacting it with a plurality of heated rolls.
Step (D) A step of heat treating the obtained film using hot air.

<Step (A)>
Step (A) is a step of preparing an aqueous solution of a polyvinyl alcohol-based resin.
First, the polyvinyl alcohol resin, which is the material of the polyvinyl alcohol film, and the aqueous solution of the polyvinyl alcohol resin will be described.
In the present invention, the polyvinyl alcohol resin constituting the polyvinyl alcohol film is usually an unmodified polyvinyl alcohol resin, i.e., a resin produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate. 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 can also be used. Examples of the component copolymerizable with vinyl acetate include unsaturated carboxylic acids (e.g., salts, esters,
Examples of the polyvinyl alcohol resin include olefins having 2 to 30 carbon atoms (e.g., ethylene, propylene, n-butene, isobutene, etc.), vinyl ethers, and unsaturated sulfonates. Modified polyvinyl alcohol resins obtained by chemically modifying hydroxyl groups after saponification can also be used. These resins can be used alone or in combination of two or more.

In addition, a polyvinyl alcohol resin having a 1,2-diol structure in a side chain can also be used as the polyvinyl alcohol resin. Such a polyvinyl alcohol resin having a 1,2-diol structure in a side chain can be prepared, for example, by (i) saponifying a copolymer of vinyl acetate and 3,4-diacetoxy-1-butene, (ii) saponifying and decarbonating a copolymer of vinyl acetate and vinyl ethylene carbonate, or (iii) saponifying a copolymer of vinyl acetate and 2,2-
(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 resin is preferably 100,000 to 300,000, particularly preferably 110,000 to 280,000, further preferably 120,000 to 260,000, and particularly preferably 150,000 to 260,000.
The weight average molecular weight is 0. If the weight average molecular weight is too small, it is difficult to obtain sufficient optical performance when the polyvinyl alcohol-based resin is used as an optical film, and the durability performance under high temperature and high humidity conditions is insufficient, and the contrast and clarity when used as a display tend to be poor, while if the weight average molecular weight is too large, it tends to be difficult to stretch the polyvinyl alcohol-based film when a polarizer is produced using the polyvinyl alcohol-based film. The weight average molecular weight of the polyvinyl alcohol-based resin is a weight average molecular weight measured by a GPC-MALS method.

The average saponification degree of the polyvinyl alcohol resin used in the present invention is preferably 98 mol% or more, particularly preferably 99 mol% or more, further preferably 99.5 mol% or more, and particularly preferably 99.8 mol% or more. If the average saponification degree is too small, sufficient optical performance tends not to be obtained when the polyvinyl alcohol film is used as a polarizer.
Here, the average degree of saponification in the present invention is measured in accordance with JIS K6726.

As the polyvinyl alcohol-based resin used in the present invention, two or more kinds of resins differing in the modified species, the modified amount, the weight average molecular weight, the average saponification degree, etc. may be used in combination.

In terms of film-forming properties, it is more preferable that the aqueous polyvinyl alcohol resin solution contains, in addition to the polyvinyl alcohol resin, a commonly used plasticizer such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, polyethylene glycol, trimethylolpropane, etc., and at least one nonionic, anionic, or cationic surfactant, as necessary. These may be used alone or in combination of two or more kinds.

The resin concentration of the aqueous polyvinyl alcohol resin solution thus obtained is 15 to 60
% by weight, particularly preferably 17 to 55% by weight, and further preferably 20 to 55% by weight.
If the resin concentration in the aqueous solution is too low, the drying load increases, and the productivity tends to decrease, whereas if the resin concentration is too high, the viscosity becomes too high, and it tends to be difficult to dissolve the resin uniformly.

Next, the obtained aqueous polyvinyl alcohol resin solution is subjected to a defoaming treatment. Examples of the defoaming method include static defoaming and defoaming using a multi-screw extruder. The multi-screw extruder may be any multi-screw extruder having a vent, and typically a twin-screw extruder having a vent is used.

<Step (B)>
Step (B) is a step of casting an aqueous solution of a polyvinyl alcohol-based resin into a cast mold to form a membrane.
After the degassing treatment, the aqueous polyvinyl alcohol resin solution is introduced into a T-shaped slit die in a constant amount, and then discharged and cast onto a rotating casting drum to form a film by a continuous casting method.

The resin temperature of the aqueous polyvinyl alcohol resin solution at the outlet of the T-type slit die is 80 to 10
The temperature is preferably 0°C, and particularly preferably 85 to 98°C.
If the resin temperature of the aqueous polyvinyl alcohol resin solution is too low, the flow tends to be poor, whereas if it is too high, the solution tends to foam.

The viscosity of the aqueous polyvinyl alcohol resin solution during ejection is 50 to 200 Pa·s.
It is preferable that the viscosity is within a range from 70 to 150 Pa·s.
If the viscosity of the aqueous solution is too high, the flowability tends to be poor, whereas if the viscosity is too low, casting of the aqueous solution tends to be difficult.

The discharge speed of the aqueous polyvinyl alcohol resin solution discharged from the T-shaped slit die onto the casting drum is preferably 0.2 to 5 m/min, and more preferably 0.4 to 4 m/min.
The speed is preferably 0.6 to 3 m/min.
If the discharge speed is too slow, the productivity tends to decrease, whereas if it is too fast, casting tends to become difficult.

The diameter of such a casting drum is preferably 2 to 5 m, particularly preferably 2.4 to 4.5
m, and more preferably 2.8 to 4 m.
If the diameter is too small, the drying zone on the cast drum becomes short, making it difficult to increase the speed, whereas if the diameter is too large, transportability tends to decrease.

The width of such a casting drum is preferably 4 m or more, particularly preferably 4.5 m or more, further preferably 5 m or more, and particularly preferably 5 to 8 m.
If the width of the casting drum is too small, productivity tends to decrease.

The rotation speed of the cast drum is preferably from 3 to 50 m/min, particularly preferably from 7 to 40 m/min, and further preferably from 10 to 35 m/min.
If the rotation speed is too slow, the productivity tends to decrease, whereas if it is too fast, the drying tends to be insufficient.

The upper limit of the surface temperature of the cast drum is preferably 100° C. or less, since foaming during drying can be suppressed and a film with excellent appearance can be obtained. Particularly preferably, the upper limit of the surface temperature of the cast drum is 97° C. or less.
It is more preferably 95° C. or lower, and particularly preferably 92° C. or lower.
The lower limit is preferably 60° C. or higher, in terms of excellent releasability when the formed film is peeled off from the cast mold, and more preferably 65° C. or higher, and even more preferably 70° C.
It is preferably 75° C. or higher, more preferably 80° C. or higher.

The moisture content of the film immediately after peeling from the cast mold is preferably 23% or less, particularly preferably 21% or less, further preferably 20% or less, and particularly preferably 10% or less. If the moisture content is too high, the residual crystallinity tends to be large, and high tension is applied during the production of the polarizer, which tends to cause breakage easily, resulting in low stretchability, which in turn reduces crosslinking with boric acid and increases the change in transmittance and the transmittance at a wavelength of 430 nm and the transmittance at a wavelength of 730 nm after durability testing.
The above-mentioned "undissolved crystallinity" refers to the ratio of crystals remaining when the polyvinyl alcohol film is immersed in water for 30 seconds.
The moisture content immediately after peeling is preferably 3% or more, particularly preferably 5% or more, and even more preferably 7% or more. If the moisture content is too low, the film tends to curl.

The evaporation rate of water on the cast mold is preferably 1.4%/sec or more at the lower limit,
Particularly preferably, the rate is 1.5%/sec or more, further preferably, 1.6%/sec or more, and particularly preferably, 1
.7%/sec or more.
The upper limit is preferably 3.0%/sec or less, particularly preferably 2.7%/sec or less, and further preferably 2.6%/sec or less.
If the evaporation rate of water on the cast mold is too slow, the residual crystallinity tends to be large, resulting in high tension during production of the polarizer, making it more likely to break, and also reducing extensibility and crosslinking with boric acid, which tends to result in large changes in transmittance and transmittance at wavelengths of 430 nm and 730 nm after durability testing. If the evaporation rate is too fast, this tends to result in poor appearance, such as waviness.

<Step (C)>
Step (C) is a step of heating and drying the formed film.

The film (the film formed above) peeled off from the cast drum is transported in the machine direction (MD direction) using nip rolls or the like, and the front and back sides of the film are alternately brought into contact with multiple heated rolls to dry. The heated rolls are, for example, rolls with a diameter of 0.2 to 2 m whose surfaces have been hard chrome plated or mirror-finished, and it is preferable to carry out drying using usually 2 to 30 rolls, preferably 10 to 25 rolls.

The surface temperature of the heat roll is not particularly limited, but is usually 60 to 150° C., and more preferably 70
It is preferable that the surface temperature is not more than 140° C. If the surface temperature is too low, drying will tend to be insufficient, whereas if the surface temperature is too high, drying will tend to be excessive, which will tend to result in poor appearance such as undulation.

The upper limit of the heat roll drying time is preferably 60 seconds or less, particularly preferably 55 seconds or less, further preferably 50 seconds or less, and particularly preferably 45 seconds or less, and the lower limit is preferably 20 seconds or more, particularly preferably 25 seconds or more, further preferably 27 seconds or more, and particularly preferably 30 seconds or more.
If the hot roll drying time is too long, the dyeability during polarizer production tends to decrease, whereas if the time is too fast, the film becomes prone to wrinkles or folds during the swelling step during polarizer production, tending to deteriorate the appearance of the polarizer.

<Step (D)>
Step (D) is a step of heat treating the obtained film with hot air.
The film that has been subjected to the above step (C) may be subjected to a heat treatment, for example, using a floating dryer.
The upper limit of the heat treatment temperature is preferably 140° C. or less, and particularly preferably 1
It is preferably 37° C. or lower, more preferably 135° C. or lower, and particularly preferably 130° C. or lower. The lower limit is preferably 100° C. or higher, particularly preferably 103° C. or higher, more preferably 105° C. or higher, and particularly preferably 110° C. or higher.
If the heat treatment temperature is too high, the dyeability during the production of the polarizer tends to decrease, whereas if the heat treatment temperature is too low, the film is likely to wrinkle or fold during the swelling step during the production of the polarizer, tending to deteriorate the appearance of the polarizer.
The heat treatment time is preferably 20 to 100 seconds, and more preferably 40 to 7
It is 0 seconds.

[Polyvinyl alcohol film]
Thus, a polyvinyl alcohol film is obtained through the above steps (A) to (D), and is finally wound up into a roll to become a finished product.

The thickness of the polyvinyl alcohol-based film thus obtained is preferably 10 μm or more in terms of production stability of the polarizer, more preferably 15 μm or more, even more preferably 20 μm or more, and particularly preferably 25 μm or more. The upper limit is preferably 70 μm or less in terms of reducing warping of a liquid crystal display using a polarizer produced from the polyvinyl alcohol-based film, more preferably 60 μm or less, even more preferably 50 μm or less.
Particularly preferably, the thickness is 45 μm or less.

The length of the polyvinyl alcohol film is preferably 4 km or more from the viewpoint of increasing the area of the polarizer, and particularly preferably 5 to 50 km from the viewpoint of the transport weight.

The width of the polyvinyl alcohol film is preferably 4 m or more in terms of widening the polarizer, particularly preferably 5 m or more, and further preferably 5 to 6 m in terms of avoiding breakage during production of the polarizer.
The polyvinyl alcohol-based film obtained by the above-mentioned production method is useful for optical purposes, and is particularly useful as a raw film for a polarizer. Hereinafter, a method for producing a polarizer and a polarizing plate made of the polyvinyl alcohol-based film will be described.

[Method for producing polyvinyl alcohol-based polarizer]
The polyvinyl alcohol-based polarizer of the present invention is produced by unrolling the polyvinyl alcohol-based film obtained by the above-mentioned production method from a roll and transporting it in the horizontal direction, and then passing through steps such as swelling, dyeing, boric acid crosslinking, stretching, washing, and drying.

The swelling step is carried out before the dyeing step. The swelling step can clean dirt on the surface of the polyvinyl alcohol-based film, and also has the effect of preventing uneven dyeing by swelling the polyvinyl alcohol-based film. In the swelling step, water is usually used as the treatment liquid. As long as the treatment liquid is mainly composed of water, it may contain an iodide compound, an additive such as a surfactant, alcohol, etc. The temperature of the swelling bath is usually about 10 to 45°C, and the immersion time in the swelling bath is usually about 0.1 to 10 minutes.

The dyeing process is carried out by contacting the film with a liquid containing iodine or a dichroic dye. Usually, an aqueous solution of iodine-potassium iodide is used, with an appropriate iodine concentration of 0.1 to 2 g/L and an appropriate potassium iodide concentration of 1 to 100 g/L. A practical dyeing time is about 30 to 500 seconds. The temperature of the treatment bath 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 crosslinking step is carried out using a boron compound such as boric acid or borax. The boron compound is used in the form of an aqueous solution or an aqueous-organic solvent mixture at a concentration of about 10 to 100 g/L, and it is preferable to have potassium iodide coexist in the solution from the viewpoint of stabilizing the polarizing performance. The treatment temperature is preferably about 30 to 70°C, and the treatment time is preferably about 0.1 to 20 minutes, and a stretching operation may be carried out during the treatment as necessary.

In the stretching step, the film is stretched uniaxially by 3 to 10 times, preferably 3.5 to 7 times. At this time, slight stretching (stretching to prevent shrinkage in the width direction or more) may be performed in the direction perpendicular to the stretching direction. The temperature during stretching is 40 to 170° C.
Furthermore, the stretching ratio may be set to be within the above range, and the stretching operation may be carried out not only in one stage but also multiple times in the production process.

The washing step is carried out, for example, by immersing the film in water or an aqueous solution of an iodide such as potassium iodide, and can remove precipitates generated on the surface of the film. When an aqueous solution of potassium iodide is used, the potassium iodide concentration may be about 10 to 1000 g/L. The temperature during the washing treatment is usually 5 to 50° C., preferably 10 to 45° C. The treatment time is usually 1 to 300 seconds, preferably 10 to 240 seconds. Note that washing with water and washing with an aqueous solution of potassium iodide may be appropriately combined.

The drying step is typically performed using a dryer at 40 to 100° C. for 0.1 to 10 minutes. The drying temperature is preferably 70° C. or higher, more preferably 75° C. or higher, particularly preferably 80° C. or higher, and especially preferably 85° C. or higher.

A polarizer is thus obtained, and the degree of polarization of such a polarizer is preferably 99.90% or more, more preferably 99.99% or more. If the degree of polarization is too low, the contrast in a liquid crystal display tends to decrease.
In general, the degree of polarization is calculated according to the following formula (1) from the light transmittance (H ₁₁ ) measured at a wavelength λ when two polarizers are stacked such that their orientation directions are in the same direction, and the light transmittance (H ₁ ) measured at a wavelength λ when two polarizers are stacked such that their orientation directions are perpendicular to each other.
Degree of polarization = [(H ₁₁ - _{H 1} )/(H ₁₁ + H ₁ )] ^1/2 ...(1)

Furthermore, the single transmittance of the polarizer of the present invention is preferably 42% or more. If the single transmittance is too low, it tends not to be possible to achieve high brightness of the liquid crystal display.
The single transmittance is a value obtained by measuring the light transmittance of a single polarizer using a spectrophotometer.

Next, a method for producing a polarizing plate of the present invention using the polarizer of the present invention will be described.
The polarizer of the present invention is suitable for producing a polarizing plate having little color unevenness and excellent polarizing performance.

[Method of manufacturing polarizing plate]
The polarizing plate of the present invention is produced by laminating an optically isotropic resin film as a protective film on one or both sides of the polarizer of the present invention via an adhesive. Examples of the protective film include a film made of an acetyl cellulose-based resin such as triacetyl cellulose or diacetyl cellulose, a film made of a polyester-based resin such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate,
Examples of the resin include films made of linear olefin resins such as polycarbonate resin films, cycloolefin resin films, acrylic resin films, and polypropylene resin films.

The bonding method is carried out by a known method, for example, by uniformly applying a liquid adhesive composition to the polarizer, the protective film, or both, and then bonding the two together under pressure, followed by heating or irradiating with active energy rays.

A polarizing plate can also be produced by applying a curable resin such as a urethane resin, an acrylic resin, or a urea resin to one or both sides of a polarizer and curing the resin to form a cured layer. In this way, the cured layer serves as a substitute for the protective film, making it possible to reduce the thickness of the polarizing plate.

The polarizer and polarizing plate obtained using the polyvinyl alcohol-based film of the present invention have excellent polarization performance, and can be used in a wide range of applications, including liquid crystal display devices such as portable information terminals, personal computers, televisions, projectors, signage, electronic desk calculators, electronic clocks, word processors, electronic paper, game machines, videos, cameras, photo albums, thermometers, audio equipment, and instruments for automobiles and machinery, sunglasses, anti-glare glasses, stereoscopic glasses, wearable displays, foldable displays, anti-reflection layers for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication devices,
It is preferably used in medical equipment, building materials, toys, etc.

The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples as long as it does not depart from the gist of the present invention.
In the examples, "parts" and "%" are based on mass.

<Measurement conditions>
<Esterification Ratio of Polyvinyl Alcohol-Based Polarizer>
For the NMR measurement, the polarizer was cut into 2 mm × 2 mm pieces, 20 mg of which was weighed out and diluted with dehydrated DMSO (DMSO-d
6) was added and heated at 90°C for 10 minutes, and then the mixture was cooled using AVANCE NEO40 manufactured by Bruker.
0 spectrometer, equipped with a 4 mm MAS probe, 90° pulse width 5 μs, 23° C.
The measurement was performed under high-speed magic angle spinning at 10,000 Hz with a single pulse, a pulse interval of 10 seconds, and an accumulation count of 64.
Regarding the obtained NMR measurement results, assuming that the methyl of deuterated dimethyl sulfoxide is 2.5 ppm and the integral value of the main chain methylene (CH ₂ ) from 2.2 to 1.9 ppm is 2.00,
The integral value from 5.0 to 4.2 ppm is a, and the integral value from 7.5 to 6.8 ppm is b, and the following formula (
The esterification rate was calculated from 1).
Equation (1): Esterification rate (unit %) = 100 × (1-a) / ((1-a) + b)

<Durability under high temperature and humidity conditions>
The prepared polarizing plate was cut into a size of 3 cm x 3 cm, and a test piece was prepared by attaching the polarizing plate and glass with a double-sided adhesive film, and the initial cross transmittance was measured.
After a 500-hour durability test in a thermo-hygrostat set at 0°C and 90% RH, the product was placed in a 24°C/5°C/1-hour
The cross transmittance after storage was measured under a 0% RH condition. Using the above measured values, the change in cross transmittance at wavelengths of 700 nm and 430 nm before and after the durability test (cross transmittance after durability test - initial cross transmittance) was calculated.
<Polarizing plate with initial transmittance of 44%>
(evaluation)
A: The change in cross transmittance at TC 430 nm and at TC 700 nm is 3 or less.
x: Either or both of the changes in cross transmittance at TC 430 nm and TC 700 nm are greater than 3.
<Polarizing plate with initial transmittance of 42.5%>
(evaluation)
A: The change in cross transmittance at TC 430 nm and TC 700 nm was 0.2 or less.
x: Either or both of the changes in cross transmittance at TC 430 nm and TC 700 nm are 0.
Greater than 2.

Example 1
(Preparation of polyvinyl alcohol film)
In a 500-L dissolving vessel, 100 kg of polyvinyl alcohol resin having a weight-average molecular weight of 123,000 and a degree of saponification of 99.8 mol%, 300 kg of water, and 11.5 kg of glycerin as a plasticizer were added.
The mixture was heated to 150° C. with stirring and dissolved under pressure to obtain an aqueous polyvinyl alcohol resin solution with a resin concentration of 29.0% by mass.
The aqueous polyvinyl alcohol resin solution was then fed to a twin-screw extruder with a vent and degassed, after which the temperature of the aqueous solution was adjusted to 95°C, and the solution was extruded and cast from the outlet of a T-shaped slit die at a temperature of 84°C into a rotating cast mold, and a film was formed at a water evaporation rate of 1.8%/s. The moisture content of the film immediately after peeling from the cast mold was 10% by mass. Next, the front and back sides of the film were dried while alternately contacting them with a total of 10 heated rolls, and further heat-treated by blowing hot air at 121°C onto both sides of the film, after which it was slit and wound up to obtain a rolled polyvinyl alcohol film (film thickness 30 μm,
Width 0.6m, length 1km).
(Preparation of polyvinyl alcohol-based polarizer)
The obtained polyvinyl alcohol-based film was unwound from the roll and conveyed horizontally while being immersed in a water tank at 25°C to swell and stretched in the machine direction (MD) to 1.6 times the size of the original roll. No folds or wrinkles were generated in the film during this swelling process. Next, the amount of iodine was adjusted so that the single transmittance of the final polarizer was 43.6%,
The fabric was immersed in an aqueous solution (28°C) containing 30 g/L potassium iodide, and while dyeing, it was stretched in the machine direction (MD) to 2.3 times the original size. Then, it was diluted with 30 g/L boric acid.
The film was immersed in an aqueous solution (57°C) of 30 g/L potassium iodide and 30 g/L potassium iodide, and uniaxially stretched in the machine direction (MD) to 6.00 times the original size while crosslinking with boric acid. Finally, the film was washed with an aqueous potassium iodide solution and dried at 80°C to obtain a polyvinyl alcohol polarizer. The polarization performance of the polarizer was as shown in Table 1.
(Preparation of Polarizing Plate)
A water-based adhesive was applied to the upper and lower sides of the obtained polyvinyl alcohol-based polarizer, and then a 40 μm TAC was attached as a protective film, followed by drying in a dryer at 80° C. to obtain a polarizing plate with a single transmittance of 44%. The obtained polarizing plate was used to evaluate durability. The results are shown in Table 2.

Example 2
(Preparation of polyvinyl alcohol film)
In a 500-L dissolving vessel, 100 kg of polyvinyl alcohol resin having a weight-average molecular weight of 156,000 and a saponification degree of 99.8 mol%, 300 kg of water, and 11.5 kg of glycerin as a plasticizer were added.
The mixture was heated to 150° C. with stirring and dissolved under pressure to obtain an aqueous polyvinyl alcohol resin solution with a resin concentration of 26.0% by mass.
The aqueous polyvinyl alcohol resin solution was then fed to a twin-screw extruder with a vent and degassed, after which the temperature of the aqueous solution was adjusted to 95°C, and the solution was extruded and cast from the outlet of a T-shaped slit die at a temperature of 69°C into a rotating cast mold, and a film was formed at a water evaporation rate of 1.6%/s. The moisture content of the film immediately after peeling from the cast mold was 20% by mass. Next, the front and back sides of the film were dried while alternately contacting them with a total of 10 heated rolls, and further heat-treated by blowing hot air at 127°C onto both sides of the film, after which it was slit and wound up to obtain a rolled polyvinyl alcohol film (film thickness 30 μm,
Width 0.6m, length 1km).
(Preparation of Polarizer)
The obtained polyvinyl alcohol-based film was unwound from the roll and conveyed horizontally while being immersed in a water tank at 25°C to swell and stretched in the machine direction (MD) to 1.6 times the size of the original roll. No folds or wrinkles were generated in the film during this swelling process. Next, the amount of iodine was adjusted so that the single transmittance of the final polarizer was 43.6%,
The fabric was immersed in an aqueous solution (28°C) containing 30 g/L potassium iodide, and while dyeing, it was stretched in the machine direction (MD) to 2.3 times the original size. Then, it was diluted with 30 g/L boric acid.
The film was immersed in an aqueous solution (61°C) of 30 g/L potassium iodide and 30 g/L potassium iodide, and uniaxially stretched in the machine direction (MD) to 5.70 times the original size while crosslinking with boric acid. Finally, the film was washed with an aqueous potassium iodide solution and dried at 80°C to obtain a polarizer. The polarizing performance of the polarizer was as shown in Table 1.
(Preparation of Polarizing Plate)
A water-based adhesive was applied to the upper and lower sides of the obtained polarizer, and then a 40 μm thick protective film was applied.
After the TAC was attached, the polarizing plate was dried in a dryer at 80° C. to obtain a polarizing plate having a single transmittance of 44%. The durability of the obtained polarizing plate was evaluated. The results are shown in Table 2.

<Comparative Example 1>
(Preparation of polyvinyl alcohol film)
In a 500 L dissolving vessel, 100 kg of polyvinyl alcohol resin having a weight average molecular weight of 123,000 and a saponification degree of 99.8 mol %, 300 kg of water, and 11.5 kg of glycerin as a plasticizer were added.
The mixture was heated to 150° C. with stirring and dissolved under pressure to obtain an aqueous polyvinyl alcohol resin solution with a resin concentration of 29.0% by mass.
The aqueous polyvinyl alcohol resin solution was then fed to a twin-screw extruder with a vent and degassed, after which the temperature of the aqueous solution was adjusted to 95°C, and the solution was extruded and cast from the outlet of a T-shaped slit die at a temperature of 65°C into a rotating cast mold, and a film was formed at a water evaporation rate of 1.5%/s. The moisture content of the film immediately after peeling from the cast mold was 21% by mass. Next, the front and back sides of the film were dried while alternately contacting them with a total of 10 heated rolls, and further heat-treated by blowing hot air at 127°C onto both sides of the film, after which it was slit and wound up to obtain a rolled polyvinyl alcohol film (film thickness 30 μm,
Width 0.6m, length 1km).
(Preparation of Polarizer)
The obtained polyvinyl alcohol-based film was unwound from the roll and conveyed horizontally while being immersed in a water tank at 25°C to swell and stretched in the machine direction (MD) to 1.6 times the size of the original roll. No folds or wrinkles were generated in the film during this swelling process. Next, the amount of iodine was adjusted so that the single transmittance of the final polarizer was 43.6%,
The fabric was immersed in an aqueous solution (28°C) containing 30 g/L potassium iodide, and while dyeing, it was stretched in the machine direction (MD) to 2.3 times the original size. Then, it was diluted with 30 g/L boric acid.
The film was immersed in an aqueous solution (57°C) containing 30 g/L potassium iodide and 30 g/L potassium iodide, and uniaxially stretched in the machine direction (MD) to 5.70 times the original size while crosslinking with boric acid. Finally, the film was washed with an aqueous potassium iodide solution and dried at 80°C to obtain a polarizer. The polarizing performance of the polarizer was as shown in Table 1.
(Preparation of Polarizing Plate)
A water-based adhesive was applied to the upper and lower sides of the obtained polarizer, and then a 40 μm thick protective film was applied.
After the TAC was attached, the polarizing plate was dried in a dryer at 80° C. to obtain a polarizing plate having a single transmittance of 44%. The durability of the obtained polarizing plate was evaluated. The results are shown in Table 2.

Example 3
A polyvinyl alcohol-based polarizer and polarizing plate were obtained in the same manner as in Comparative Example 1, except that the stretching ratio during preparation of the polarizer was changed to 5.85 times, the drying temperature was changed to 90° C., and the transmittance of the polarizing plate was changed to 42.5%. The polarization performance of the obtained polarizer is as shown in Table 1, and the results of durability evaluation using the obtained polarizing plate are as shown in Table 2.

<Comparative Example 2>
A polyvinyl alcohol-based polarizer and a polarizing plate were obtained in the same manner as in Example 3, except that the drying temperature during the preparation of the polarizer was changed to 80° C. The polarizing performance of the obtained polarizer is as shown in Table 1, and the results of durability evaluation using the obtained polarizing plate are as shown in Table 2.

<Comparative Example 3>
A polyvinyl alcohol-based polarizer and polarizing plate were obtained in the same manner as in Example 3, except that the stretching temperature, stretch ratio, and drying temperature during preparation of the polarizer were changed to 58.5° C., 6.0 times, and 80° C. The polarizing performance of the obtained polarizer was as shown in Table 1, and the results of durability evaluation using the obtained polarizing plate were as shown in Table 2.

Comparing Examples 1 and 2, in which the polarizing plate has an initial transmittance of 44%, with Comparative Example 1, it can be seen that by using the polyvinyl alcohol-based polarizers of Examples 1 and 2, in which the esterification rates are within the range specified in the present application, a polarizing plate can be obtained that has little change in the transmittance at wavelengths of 430 nm and 700 nm, even under harsh environmental conditions of high temperature and high humidity, and that can provide a display with an excellent balance between contrast and clarity.
Similarly, in comparison between Example 3, in which the initial transmittance of the polarizing plate is 42.5%, and Comparative Examples 2 and 3, when the polyvinyl alcohol polarizer of Example 3, in which the esterification rate is within the range specified in the present application, is used, the transmittance at wavelengths of 430 nm and 700 nm is 42.5% even under harsh environmental conditions of high temperature and high humidity.
It is understood that a polarizing plate can be obtained which has little change in transmittance and provides a display with a good balance between contrast and clarity.

The polyvinyl alcohol-based polarizer of the present invention and a polarizing plate using the same have excellent polarization performance, and are preferably used in liquid crystal display devices such as portable information terminals, personal computers, televisions, projectors, signage, electronic desk calculators, electronic clocks, word processors, electronic paper, game machines, videos, cameras, photo albums, thermometers, audio equipment, instruments for automobiles and machines, sunglasses, anti-glare glasses, three-dimensional glasses, wearable displays, foldable displays, rollable televisions, rollable displays, anti-reflection films for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication devices, medical devices, building materials, toys, and the like.

Claims (3)

A polyvinyl alcohol-based polarizer comprising a polyvinyl alcohol-based resin, the polyvinyl alcohol-based polarizer having an esterification rate formed by a hydroxyl group of the polyvinyl alcohol-based resin and boric acid, as calculated by the following formula (1), of 60% or more:
Equation (1): Esterification rate (unit %) = 100 × (1-a) / ((1-a) + b)
(In the formula, a and b represent the ratio of the methyl group of deuterated dimethyl sulfoxide to 2.5 in NMR measurement using a 4 mm HRMAS probe and a polarizer with deuterated dimethyl sulfoxide as a medium.)
When the integral value of the main chain methylene (CH ₂ ) from 2.2 to 1.9 ppm is taken as 2.00, the integral value from 5.0 to 4.2 ppm is taken as a, and the integral value from 7.5 to 6.8 ppm is taken as b.) 2. The polyvinyl alcohol polarizer according to claim 1, wherein the weight average molecular weight of the polyvinyl alcohol resin is from 100,000 to 300,000. A polarizing plate comprising the polyvinyl alcohol-based polarizer according to claim 1 or 2.