KR102669613B1 - Polyvinyl alcohol-based film, polarizing film and polarizing plate, and method for producing polyvinyl alcohol-based film - Google Patents

Abstract

A polyvinyl alcohol-based film that has an excellent balance between swelling and stretching properties during the production of a polarizing film, does not cause breakage even when manufacturing a thin polarizing film, exhibits high polarization performance, and can produce a polarizing film with less color unevenness, the polyvinyl alcohol-based film. A polarizing film and polarizing plate using a film, and a method for manufacturing the polyvinyl alcohol-based film are provided.
The polyvinyl alcohol-based film of the present invention is a long polyvinyl alcohol-based film, and is characterized by satisfying the following formulas (A) and (B).
Δn(MD) Ave≥2.2×10 ^-3 ···(A)
Δn(TD) Ave≥2.0×10 ^-3 ···(B)
The Δn(MD) Ave represents the value obtained by averaging the birefringence in the longitudinal direction in the thickness direction in the polyvinyl alcohol-based film, and the Δn(TD) Ave represents the value obtained by averaging the birefringence in the width direction in the thickness direction. indicates.

Description

Polyvinyl alcohol-based film, polarizing film and polarizing plate, and method for producing polyvinyl alcohol-based film

The present invention relates to a polyvinyl alcohol-based film that is a material for forming a polarizing film with excellent dyeability, high polarization degree, and less color staining, a polarizing film and polarizing plate using the polyvinyl alcohol-based film, and a method for producing the polyvinyl alcohol-based film. It's about.

Conventionally, polyvinyl alcohol-based films are films with excellent transparency and are used for many purposes, and one of their useful uses is a polarizing film. The polarizing film is used as a basic component of liquid crystal displays, and in recent years, its use has been expanded to devices requiring high quality and high reliability.

Among these, polarizing films with excellent optical properties are required as screens such as liquid crystal TVs and multi-functional portable terminals become brighter, more precise, larger in area, and thinner. The specific request is further improvement in polarization degree and further elimination of color unevenness.

Generally, polyvinyl alcohol-based films are manufactured by a continuous casting method using an aqueous solution of polyvinyl alcohol-based resin as a material. Specifically, first, an aqueous solution of polyvinyl alcohol-based resin is flexible and formed into a film on a cast form such as a cast drum or endless belt, and then the formed film is peeled from the cast form and then film is formed using a nip roll or the like. It is manufactured by drying using a heat roll, floating dryer, etc. while conveying in the flow direction (MD). In the conveyance process, the formed film is pulled in the flow direction (MD), so the polyvinyl alcohol-based polymer is likely to be oriented in the flow direction (MD).

Meanwhile, a polarizing film is generally manufactured by first swelling the original polyvinyl alcohol-based film with water (including hot water), then dyeing it with a dichroic dye such as iodine, and then stretching it.

In addition, what is important in the swelling process is to quickly swell the polyvinyl alcohol-based film in the thickness direction, and to uniformly swell the polyvinyl alcohol-based film so that the dye can smoothly penetrate into the film in the dyeing process. .

In addition, the stretching process is a process in which the dyed film is stretched in the flow direction (MD) to highly orient the dichroic dye in the polyvinyl alcohol-based film. In order to improve the polarization performance of the polarizing film, the disk used in this stretching process is It is important that the polyvinyl alcohol-based film exhibits good stretchability in the flow direction (MD).

In addition, cases in which the order of the stretching process and the dyeing process are reversed to the above in the production of the polarizing film are also being implemented. That is, this is a case in which a raw polyvinyl alcohol-based film is first swollen with water (including hot water), then stretched, and then dyed with a dichroic dye such as iodine. In this case as well, what is important for improving the polarization performance of the polarizing film is that the original polyvinyl alcohol-based film exhibits good swelling properties in the thickness direction and also exhibits good stretchability in the flow direction (MD).

In addition, in order to recently reduce the thickness of the polarizing film, the original polyvinyl alcohol-based film has also been reduced in thickness. However, the thin polyvinyl alcohol-based film has productivity problems, such as breaking during stretching when manufacturing the polarizing film.

As a method of improving the swelling property of a polyvinyl alcohol-based film, for example, a method of adding a polyhydric alcohol as a water swelling aid to a polyvinyl alcohol-based resin (see, for example, Patent Document 1) has been proposed.

Additionally, as a method of improving the stretchability of a polyvinyl alcohol-based film, for example, a method of specifying the ratio between the speed of the cast drum when forming the film and the winding speed of the final polyvinyl alcohol-based film (e.g. For example, see Patent Document 2), a method of floating and drying the formed film after forming it with a cast drum (for example, see Patent Document 3), a method of controlling the tensile state in the drying process of the formed film (for example, see Patent Document 3). For example, see Patent Document 4), the birefringence in the flow direction (MD) of the polyvinyl alcohol-based polymer film is averaged in the thickness direction of the polyvinyl alcohol-based polymer film [Δn(MD) Ave], and poly A method of adjusting the birefringence in the width direction (TD) of the vinyl alcohol-based polymer film so that the averaged value [Δn(TD) Ave] in the thickness direction of the polyvinyl alcohol-based polymer film satisfies a specific relationship (for example, (see Patent Document 5 and Patent Document 6) has been proposed.

Patent Document 1 JP 2001-302867 A Patent Document 2 JP 2001-315141 A Patent Document 3 JP 2001-315142 A Patent Document 4 JP 2002-79531 A Patent Document 5 WO 2012/132984 A Patent Document 6 WO 2016/084836 A

However, for the thin polyvinyl alcohol-based film, the method of Patent Document 1 is insufficient to improve the swelling properties, and the methods of Patent Documents 2 to 6 are insufficient to improve stretchability during production of the polarizing film. .

That is, in the technology specified in Patent Document 1, even if the swelling property of the entire polyvinyl alcohol-based film can be improved, the orientation state of the polyvinyl alcohol-based polymer is not considered, and the direction of flow (MD) of the polarizing film manufacturing time is not considered. It is difficult to efficiently improve stretchability. Conversely, the addition of a water swelling aid tends to disrupt the orientation of the polymer, making uniform stretching in the flow direction (MD) difficult.

The technology specified in Patent Document 2 specifies the degree of stretching (tensile state) in the flow direction (MD) when manufacturing a polyvinyl alcohol-based film, but if stretching in the width direction (TD) is not also considered, the polarizing film It is insufficient to improve stretchability during manufacturing.

In the technology specified in Patent Document 3, the formed film can be dried uniformly, but the orientation of the polymer cannot be controlled, so it is insufficient to improve swelling and stretchability during production of the polarizing film.

In addition, in the technology specified in Patent Document 4, the film thickness of the polyvinyl alcohol-based film can be made uniform, but the orientation of the polymer cannot be controlled, so it is insufficient to improve swelling and stretchability when manufacturing a polarizing film.

In the technology specified in Patent Documents 5 and 6, high stretchability can be achieved in the polyvinyl alcohol-based film with a thickness of about 60 ㎛ used in the examples, but the specification of Δn(MD) Ave and Δn(TD) Ave is difficult. Since the above is for a thick polyvinyl alcohol-based film, it is difficult to cope with the new thinning of the polarizing film, and in a thin polyvinyl alcohol-based film with a film thickness of 50 ㎛ or less, swelling and stretchability during production of the polarizing film are difficult. It is insufficient to improve.

Therefore, under this background, the present invention is very excellent in the balance between swelling and stretchability when manufacturing a polarizing film, does not break even when manufacturing a thin polarizing film, exhibits high polarization performance, and can obtain a polarizing film with less color unevenness. A polyvinyl alcohol-based film, a polarizing film and polarizing plate using the polyvinyl alcohol-based film, and a method for manufacturing the polyvinyl alcohol-based film are provided.

Taking these circumstances into account, the present inventors conducted extensive research and found that, in a polyvinyl alcohol-based film manufactured by a continuous casting method, the birefringence in the flow direction (MD) of the film was averaged in the thickness direction of the film. If the birefringence in the width direction (TD) of the film and the averaged value in the thickness direction of the film are set to be larger than those of the polyvinyl alcohol-based film of the conventional polarizing film production raw material, the swelling property during production of the polarizing film is It was discovered that a polarizing film with excellent balance between stretchability and stretchability could be obtained without breaking even during the manufacture of a thin polarizing film, showing high polarization performance, and with less color unevenness.

That is, the first point of the present invention is a long polyvinyl alcohol-based film, which satisfies the following formulas (A) and (B).

Δn(MD) Ave≥2.2×10 ^-3 ···(A)

Δn(TD) Ave≥2.0×10 ^-3 ···(B)

[Δn(MD) Ave in the above formula (A) represents the value obtained by averaging the birefringence in the longitudinal direction (MD) of the polyvinyl alcohol-based film in the thickness direction of the polyvinyl alcohol-based film. In addition, Δn(TD) A v e in the formula (B) represents the value obtained by averaging the birefringence in the width direction (TD) of the polyvinyl alcohol-based film in the thickness direction of the polyvinyl alcohol-based film.〕

In addition, the second point of the present invention is a polarizing film characterized in that the polyvinyl alcohol-based film is used. In addition, the third point is a polarizing plate characterized by being provided with the polarizing film and a protective film provided on at least one side of the polarizing film.

Then, a film forming process of forming an aqueous solution of polyvinyl alcohol resin into a film by a continuous casting method, and a drying process of continuously drying and stretching the film while transporting the formed film in the flow direction (MD). · Production of a polyvinyl alcohol-based film comprising a method for producing a polyvinyl alcohol-based film including a stretching process, wherein the polyvinyl alcohol-based film to be produced satisfies the following formulas (A) and (B): The fourth point is the method.

Δn(MD) Ave≥2.2×10 ^-3 ···(A)

Δn(TD) Ave≥2.0×10 ^-3 ···(B)

[Δn(MD) Ave in the above formula (A) represents the value obtained by averaging the birefringence in the flow direction (MD) of the polyvinyl alcohol-based film in the thickness direction of the polyvinyl alcohol-based film. In addition, Δn(TD) A v e in the formula (B) represents the value obtained by averaging the birefringence in the width direction (TD) of the polyvinyl alcohol-based film in the thickness direction of the polyvinyl alcohol-based film.〕

Since the polyvinyl alcohol-based film of the present invention satisfies the above formulas (A) and (B), it has excellent swelling and stretchability when manufacturing a polarizing film, and can be used to manufacture a thin polarizing film by making itself thin. This can prevent fractures from occurring. In addition, by using the polyvinyl alcohol-based film, a polarizing film that exhibits high polarization performance and less color unevenness can be obtained.

In particular, when the thickness of the polyvinyl alcohol-based film is 5 to 50 ㎛, a polarizing film with superior swelling and stretchability during production of the polarizing film and superior performance can be obtained.

In addition, since the above-mentioned polyvinyl alcohol-based film is used in the polarizing film of the present invention, it exhibits high polarization performance and has little color unevenness.

And, since the polarizing film described above is used in the polarizing plate of the present invention, it exhibits high polarization performance and has less color unevenness.

In addition, the method for producing a polyvinyl alcohol-based film of the present invention includes a film forming process according to a continuous cast method and carrying out continuous drying and continuous stretching of the film while conveying the formed film in the flow direction (MD). Since the drying and stretching processes are provided, the manufacturing conditions in each of these processes are aligned, and the polyvinyl alcohol-based film of the present invention having a specific birefringence can be obtained.

In particular, when the formed film is stretched 1.05 to 1.5 times in the width direction (TD) in the drying/stretching process, the birefringence becomes suitable, and polyvinyl alcohol has superior swelling and stretching properties when manufacturing a polarizing film. A film can be obtained.

Next, the present invention will be described in detail.

The polyvinyl alcohol-based film of the present invention is a long polyvinyl alcohol-based film characterized by satisfying the following formulas (A) and (B).

Δn(MD) Ave≥2.2×10 ^-3 ···(A)

Δn(TD) Ave≥2.0×10 ^-3 ···(B)

[Δn(MD) Ave in the formula (A) represents the value obtained by averaging the birefringence in the longitudinal direction (MD) of the polyvinyl alcohol-based film in the thickness direction of the polyvinyl alcohol-based film. In addition, Δn(TD) A v e in the formula (B) represents the value obtained by averaging the birefringence in the width direction (TD) of the polyvinyl alcohol-based film in the thickness direction of the polyvinyl alcohol-based film.]

The birefringence in the longitudinal direction (MD) of the polyvinyl alcohol-based film averaged in the thickness direction of the polyvinyl alcohol-based film [Δn(MD) Ave] is Δn(MD) Ave≥2.2×10 ^-3 . is required, preferably Δn(MD) Ave≥2.5×10 ^-3 , particularly preferably Δn(MD) Ave≥3.0×10 ^-3 , more preferably Δn(MD) Ave≥3.5×10 ^-3 am.

If the value of Δn(MD) Ave is too low, wrinkles will occur during the swelling process during the production of the polarizing film, which will be described later, and color unevenness will occur in the polarizing film, making it impossible to achieve the purpose of the present invention.

In addition, the upper limit of Δn(MD) Ave is usually 1.5 × 10 ^-2 (preferably 1.0 × 10 ^-2 ), and even if the value of Δn(MD) Ave is too large, color unevenness tends to occur in the polarizing film. there is.

The birefringence in the width direction (TD) of the polyvinyl alcohol-based film averaged in the thickness direction of the polyvinyl alcohol-based film [Δn(TD) Ave] is Δn(TD) Ave≥2.0×10 ^-3 . is required, preferably Δn(TD) Ave≥2.5×10 ^-3 , particularly preferably Δn(TD) Ave≥3.0×10 ^-3, more preferably Δn(TD) Ave≥3.5×10 ^-3 am.

If the value of Δn(TD) Ave is too low, wrinkles are generated during the swelling process during the production of the polarizing film, which will be described later, and color unevenness is likely to occur in the polarizing film, making it impossible to achieve the purpose of the present invention. In addition, the upper limit of Δn(TD) Ave is usually 1.5×10 ^-2 (preferably 1.0×10 ^-2 ), and even if the value of Δn(TD) Ave is too large, color unevenness tends to occur in the polarizing film. There is.

In the present invention, as a method of controlling the Δn(MD) Ave and Δn(TD) Ave, a film formed in a cast form is used for the production method of the polyvinyl alcohol-based film according to the continuous casting method described later. A method of stretching in the width direction (TD) after peeling from the cast mold is preferred. In this case, the conditions for other processes are appropriately adjusted depending on the stretching conditions (stretch ratio, atmospheric temperature during stretching, stretching time, etc.) in the width direction (TD). The conditions include, for example, the chemical structure of the polyvinyl alcohol-based resin, which is the forming material of the polyvinyl alcohol-based film, the film forming conditions of the film (cast type temperature, etc.), and the drying conditions for drying the formed film ( temperature, time), conveyance speed in the flow direction (MD) of the formed film, etc. By matching at least one of these conditions with the stretching condition in the width direction (TD), the Δn(MD) Ave and n(TD) Ave are controlled.

In addition, in the polyvinyl alcohol-based film, there is often a change in at least one value of Δn(MD) Ave and Δn(TD) Ave in the width direction (TD), especially at both ends in the width direction (TD). ) Although Ave tends to be high, equations (A) and (B) can be satisfied at least in the center of the width direction (TD) of the polyvinyl alcohol-based film, and centered on the center of the width direction (TD) of the polyvinyl alcohol-based film. It is preferable that equations (A) and (B) are satisfied in an area of more than 80% of the width direction (TD). Both ends in the width direction (TD) of the polyvinyl alcohol-based film that do not satisfy formulas (A) and (B) can be cut and removed (cut) before stretching the polyvinyl alcohol-based film in the flow direction (MD). You can.

The Δn(MD) Ave and Δn(TD) Ave in the present invention are measured, for example, by the following method. In addition, the measurement positions of these Δn(MD) Ave and Δn(TD) Ave are within an area of 50 mm x 50 mm of the polyvinyl alcohol-based film.

[Method of measuring Δn(MD) Ave]

(1) At an arbitrary position in the flow direction (MD) of the polyvinyl alcohol-based film, three pieces with a size of MD x TD = 5 mm x 10 mm are cut from the central portion in the width direction (TD) of the polyvinyl alcohol-based film. Next, the three pieces are sandwiched between both sides by a PET (polyethylene terephthalate) film with a thickness of 100 μm, and then placed in a wooden frame and mounted on a microtome device.

(2) Next, the three pieces cut in (1) above are sliced at 10㎛ intervals parallel to the flow direction (MD) of the three pieces to produce an observation sheet (MD×TD=5 mm×10μm).

(3) Next, in order to observe the slice surface, the slice piece was flipped over, placed with the slice surface facing upward, placed on a slide glass, sealed with a cover glass and tricredyl phosphate (refractive index 1.557), and used with the two-dimensional photoelasticity evaluation system “PA-micro.” 」(manufactured by Photonic Lattice) was used to measure the retardation of three slice pieces.

(4) With the retardation distribution of the slice piece displayed on the measurement screen of “PA-micro,” a line segment Line analysis is performed to acquire retardation distribution data in the thickness direction of the slice piece. In addition, observation is performed using a 40x objective lens, and the average value of the retardation is adopted with a line width of 3 pixels.

(5) The obtained retardation distribution data in the thickness direction of the slice piece is removed to a thickness of 10 μm, the birefringence Δn (MD) distribution in the thickness direction of the slice piece is obtained, and the birefringence Δn (MD) distribution in the thickness direction of the slice piece is obtained. Take the average value of The average value of the birefringence Δn(MD) distribution in the thickness direction of each slice obtained for three slices is averaged again and referred to as “Δn(MD) Ave” of the polyvinyl alcohol-based film.

[Method for measuring Δn(TD) Ave]

(1) At an arbitrary position in the flow direction (MD) of the polyvinyl alcohol-based film, a small piece with a size of MD x TD = 10 mm x 5 mm is cut from the central portion in the width direction (TD) of the polyvinyl alcohol-based film. Then, the three pieces are placed on both sides with a 100 ㎛ thick PET film, placed again in a wooden frame, and mounted on a microtome device.

(2) Next, the three pieces cut in (1) above are sliced at 10㎛ intervals parallel to the width direction (TD) of the three pieces, and slice pieces (MD×TD=10μm×5mm) for observation are produced.

(3) Next, in order to observe the slice surface, the slice piece was flipped over, placed on a slide glass with the slice surface facing upward, sealed with a cover glass and tricredyl phosphate (refractive index 1.557), and used as a two-dimensional photoelasticity evaluation system “PA”. The retardation of three slices was measured using “-micro” (manufactured by Photonic Lattice).

(4) With the retardation distribution of the slice piece displayed on the measurement screen of “PA-micro,” a line segment Line analysis is performed to acquire retardation distribution data in the thickness direction of the slice piece. In addition, observation is performed using a 40x objective lens, and the average value of the retardation is adopted with a line width of 3 pixels.

(5) The obtained retardation distribution data in the thickness direction of the slice piece was removed to a thickness of 10 μm, the birefringence Δn(TD) distribution in the thickness direction of the slice piece was obtained, and the birefringence Δn(TD) distribution in the thickness direction of the slice piece was calculated. Take the average value. The average value of the birefringence Δn(TD) distribution in the thickness direction of each slice obtained for the three slice pieces is averaged again and referred to as “Δn(TD) Ave” of the polyvinyl alcohol-based film.

Here, the manufacturing method of the polyvinyl alcohol-based film of the present invention is explained in process order.

〔Film Material〕

First, the polyvinyl alcohol-based resin used in the present invention and the polyvinyl alcohol-based resin aqueous solution are explained.

As the polyvinyl alcohol-based resin constituting the polyvinyl alcohol-based film in the present invention, 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. Components that can be copolymerized with vinyl acetate include, for example, unsaturated carboxylic acids (including salts, esters, amides, nitriles, etc.), olefins having 2 to 30 carbon atoms (e.g., ethylene, propylene, n -butene, isobutene, etc.), vinyl ethers, unsaturated sulfonates, etc. These can be used individually or in combination of two or more types. Additionally, a modified polyvinyl alcohol-based resin obtained by chemically modifying the hydroxyl group after saponification can also be used.

Additionally, as the polyvinyl alcohol-based resin, a polyvinyl alcohol-based resin having a 1,2-diol structure in the side chain can also be used. The polyvinyl alcohol-based resin having a 1,2-diol structure in the side chain is, for example, (1) saponifying a copolymer of vinyl acetate and 3,4-diacetoxy-1-butene, (2) ) Method of saponifying and decarboxylating a copolymer of vinyl acetate and vinylethylene carbonate; (3) Saponification and decarboxylation of a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane; It can be obtained by a method of deoxidation, (4) a method of saponifying a copolymer of vinyl acetate and glycerin monoaryl ether, etc.

The weight average molecular weight of the polyvinyl alcohol-based resin is preferably 100,000 to 300,000, particularly preferably 110,000 to 280,000, and even more preferably 120,000 to 260,000. If the weight average molecular weight is too small, it tends to be difficult to obtain sufficient optical performance when using polyvinyl alcohol-based resin as an optical film, and if it is too large, stretching of the polyvinyl alcohol-based film during production of the polarizing film tends to become difficult. There is. In addition, the weight average molecular weight of the polyvinyl alcohol-based resin is the weight average molecular weight measured by the GPC-MALS method.

The average degree of saponification of the polyvinyl alcohol-based resin used in the present invention is generally preferably 98 mol% or more, particularly preferably 99 mol% or more, more preferably 99.5 mol% or more, particularly preferably 99.8 mol% or more. am. If the average degree of saponification is too small, sufficient optical performance tends not to be obtained when a polyvinyl alcohol-based film is used as a polarizing film.

Here, the average degree of saponification in the present invention is measured according to JIS K 6726.

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

In the polyvinyl alcohol-based resin aqueous solution, in addition to the polyvinyl alcohol-based resin, if necessary, commonly used plasticizers such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, polyethylene glycol, and trimethylene propane, It is preferable from the viewpoint of film forming properties to contain at least one nonionic, anionic, and cationic surfactant. These can be used individually or in combination of two or more types.

The resin concentration of the polyvinyl alcohol-based resin aqueous solution thus obtained is preferably 15 to 60% by weight, particularly preferably 17 to 55% by weight, and still more preferably 20 to 50% by weight. If the resin concentration of the aqueous solution is too low, the drying load increases, so production capacity tends to decrease, and if it is too high, the viscosity tends to become too high, making uniform dissolution impossible.

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

〔Film unveiling process〕

The polyvinyl alcohol-based film of the present invention is manufactured by a casting method or a melt extrusion method, but in the present invention, the casting method is preferable from the viewpoints of transparency, thickness, surface smoothness, etc., and particularly preferably, the continuous casting method is preferable from the viewpoint of productivity. It's the law.

The continuous casting method is, for example, a method of forming a film by continuously discharging and casting an aqueous solution of the polyvinyl alcohol-based resin from a T-shaped slit die onto a rotating cast drum, endless belt, or resin film. .

Here, the film forming process when the cast type is a cast drum will be described.

The temperature of the polyvinyl alcohol-based resin aqueous solution at the outlet of the T-slit die is preferably 80 to 100°C, and particularly preferably 85 to 98°C.

If the temperature of the polyvinyl alcohol-based resin aqueous solution is too low, it tends to flow poorly, and if it is too high, it tends to foam.

The viscosity of the polyvinyl alcohol-based resin aqueous solution is preferably 50 to 200 Pa·s (relative to the above preferred temperature of 80 to 100°C) at the time of discharge, and particularly preferably (relative to the above particularly preferred temperature of 85 to 98°C). It is 70∼150Pa·s. If the viscosity of the polyvinyl alcohol-based resin aqueous solution is too low, it tends to flow poorly, and if it is too high, softening tends to be difficult.

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

The diameter of the cast 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 of the cast drum is too small, the drying length tends to be insufficient and speed tends to be difficult to achieve, and if it is too large, transportability tends to decrease.

The width of the cast drum is preferably 4 m or more, particularly preferably 4.5 m or more, more preferably 5 m or more, and particularly preferably 5 to 7 m.

If the width of the cast drum is too small, productivity tends to decrease.

The rotation speed of the cast drum is preferably 5 to 50 m/min, particularly preferably 6 to 40 m/min, and more preferably 7 to 35 m/min. If the rotation speed of the cast drum is too slow, productivity tends to decrease, and if it is too fast, drying tends to become insufficient.

The surface temperature of the cast drum is preferably 40 to 99°C, and particularly preferably 60 to 95°C. If the surface temperature of the cast drum is too low, it tends to dry poorly, and if it is too high, it tends to foam.

In this way, the film forming process is performed. Then, the formed film is peeled from the cast drum and conveyed in the flow direction (MD). The moisture content of the formed film is preferably 0.5 to 15% by weight, particularly preferably 1 to 13% by weight, and more preferably 2 to 12% by weight. Even if the water content is too low or too high, it tends to be difficult to achieve the desired swelling properties and stretchability.

[Drying/stretching process]

When the moisture content of the film before stretching in the transverse direction (TD) is too high, it is preferable to dry the film before stretching in the transverse direction (TD). Conversely, the moisture content of the film before stretching in the transverse direction (TD) is preferably adjusted. If the water content is too low, it is preferable to control the humidity before stretching in the transverse direction (TD). Particularly preferably, the conditions of the drying process are adjusted so that the moisture content is within the above range.

The drying is carried out continuously. This continuous drying can be carried out by a known method using a heating roll or an infrared heater, but in the present invention, it is preferably carried out with a plurality of heating rolls, and particularly preferably, the temperature of the heating rolls is 40 to 150 ° C. , and more preferably 50 to 140°C. Additionally, in order to adjust the moisture content, a humidity control area may be provided before stretching in the width direction (TD).

There is no need to particularly stretch the film formed in the present invention in the flow direction (MD), and it is sufficient to convey it with a tensile tension that does not cause the film to bend. Naturally, due to stretching in the transverse direction (TD), necking depending on Poisson's ratio occurs in the flow direction (MD), and dehydration shrinkage also occurs in the flow direction (MD) during the drying. Because of these contractions, even if the rotational speed of the conveyance roll or heating roll is constant, an appropriate tension is obtained in the flow direction (MD), and complicated rotational speed control as in Patent Document 2 above is unnecessary. From a manufacturing standpoint, it is preferable that the dimensions of the film in the flow direction (MD) are constant, and particularly preferably, the dimensional change rate in the flow direction (MD) before and after stretching in the width direction (TD) is 0.8 to 1.2, and particularly preferably is 0.9 to 1.1.

The conveyance speed of the formed film in the flow direction (MD) is preferably 5 to 30 m/min, particularly preferably 7 to 25 m/min, and even more preferably 8 to 20 m/min. If this conveyance speed is too slow, productivity tends to decrease, and if it is too fast, the in-plane variation of Δn(MD) Ave and Δn(TD) Ave tends to increase.

The method of simultaneously conveying the formed film in the flow direction (MD) and stretching it in the width direction (TD) is not particularly limited, but for example, clamping both ends of the film in the width direction with a plurality of clips. It is desirable to carry out conveyance and stretching simultaneously. In this case, the arrangement of the clips at each end is preferably at a pitch of 200 mm or less, particularly preferably at a pitch of 100 mm or less, and more preferably at a pitch of 50 mm or less.

If the pitch of the clip is too wide, the film after stretching may be curved or the variation in Δn(MD) Ave and Δn(TD) Ave at both ends in the width direction of the resulting polyvinyl alcohol-based film tends to increase. . In addition, the clamping position of the clip (the tip of the clip) is preferably 100 mm or less from both ends in the width direction of the formed film. If the clamping position (tip) of the clip is located too close to the center in the width direction of the film, the edge of the film to be broken increases and the product width tends to narrow.

The draw ratio in the width direction (TD) in the present invention is preferably 1.05 to 1.5 times, particularly preferably 1.05 to 1.4 times, and further preferably 1.1 to 1.3 times. If the stretching ratio in the width direction (TD) is too high, the in-plane variation of Δn(MD) Ave and Δn(TD) Ave tends to increase, and if it is too low, wrinkles tend to easily occur during the production of the polarizing film. there is.

Stretching in the width direction (TD) is performed continuously. This continuous stretching may be one step (one time), or may be multiple steps (several times) so that the total stretching ratio is within the range of the stretching ratio (also called sequential stretching). For example, after the first step of continuous stretching, simple conveyance with the width direction (TD) fixed may be performed, and then the second step and subsequent continuous stretching may be performed. In particular, in the case of a thin film, by inserting a simple width-fixing conveyance step after performing continuous stretching in the first step, stress in the film is relieved and fracture can be avoided.

When inserting a conveyance process of fixed width, it is also possible to narrow the fixed width than the width after continuous stretching in the first step. Since the film immediately after stretching tends to shrink for stress relief, and shrinkage occurs due to dehydration, it is possible to narrow the fixed width to this shrinkage width. However, if it is narrowed beyond the shrinkage width, it is not preferable because the film becomes curved.

The continuous stretching is preferably carried out after the film drying process as described above, but is carried out at least before the film drying process, during the drying process, and after the drying process.

In a preferred form of the present invention, a method is used in which the formed film is temporarily stretched beyond 1.5 times in the width direction (TD) and then dimensionally shrunk so that the final stretch ratio in the width direction (TD) is 1.05 to 1.5 times. You can.

In this case, after temporarily stretching beyond 1.5 times, the film can be simply conveyed at a fixed width with a stretching ratio of 1.05 to 1.5. By this method, stress relaxation of the film is achieved, especially in the case of thin films. It becomes possible to avoid fracture.

In the present invention, stretching in the width direction (TD) of the film formed into a film is preferably carried out at an ambient temperature of 50 to 150°C. The ambient temperature during this stretching is particularly preferably 60 to 140°C, and even more preferably 70 to 130°C. Even if the ambient temperature during stretching is too low or too high, the in-plane fluctuations in Δn(MD) Ave and Δn(TD) Ave tend to increase. When sequential stretching is performed, the ambient temperature during stretching can be changed at each stretching step.

The stretching time when stretching the film formed in the present invention in the width direction (TD) is preferably 2 to 60 seconds, particularly preferably 5 to 45 seconds, and more preferably 10 to 30 seconds. If this stretching time is too short, fracture tends to occur in the film, and conversely, if it is too long, the equipment load tends to increase. When performing sequential stretching, the stretching time may be changed at each stretching step.

In the present invention, after stretching the formed film in the width direction (TD), heat treatment can be performed on both sides of the film using a floating dryer or the like, if necessary. This heat treatment temperature is preferably 60 to 200°C, and particularly preferably 70 to 150°C. In addition, the heat treatment using the floating dryer is a treatment that emits hot air, and the heat treatment temperature refers to the temperature of the emitted hot air.

If the heat treatment temperature is too low, dimensional stability tends to decrease, and conversely, if it is too high, stretchability during production of the polarizing film tends to decrease.

Moreover, the heat treatment time is preferably 1 to 60 seconds, and particularly preferably 5 to 30 seconds. If the heat treatment time is too short, dimensional stability tends to decrease, and conversely, if it is too long, stretchability during production of the polarizing film tends to decrease.

[Polyvinyl alcohol-based film]

In this way, the polyvinyl alcohol-based film of the present invention is obtained. This polyvinyl alcohol-based film is long in the flow direction (MD), is wound around a core pipe in roll form, and is produced into a film roll.

The thickness of the polyvinyl alcohol-based film of the present invention is usually 5 to 50 ㎛ from the viewpoint of in-plane retardation, preferably 5 to 45 ㎛ from the viewpoint of thinning the polarizing film, and particularly preferably 10 ㎛ from the viewpoint of fracture avoidance. ~40㎛, more preferably 10~30㎛.

The width of the polyvinyl alcohol-based film of the present invention is preferably 2 m or more, and particularly preferably 2 to 6 m from the viewpoint of avoiding breakage.

The length of the polyvinyl alcohol-based film of the present invention is preferably 2 km or more, particularly preferably 3 km or more from the viewpoint of increasing the area, and more preferably 3 to 50 km from the viewpoint of transport weight.

Since the polyvinyl alcohol-based film of the present invention has excellent stretchability, it is particularly preferably used as a disk for a polarizing film.

Next, a method for producing a polarizing film obtained using the polyvinyl alcohol-based film of the present invention will be described.

[Method for manufacturing polarizing film]

The polarizing film of the present invention is manufactured by unwinding the polyvinyl alcohol-based film from the film roll and conveying it in the horizontal direction, and going through processes such as swelling, dyeing, boric acid crosslinking, stretching, washing, and drying.

The swelling process is carried out before the dyeing process. In addition to being able to clean contaminants on the surface of the polyvinyl alcohol-based film through the swelling process, swelling the polyvinyl alcohol-based film also has the effect of preventing staining, etc. Water is usually used as a treatment liquid in the swelling process. If the main component of the treatment liquid is water, it may contain small amounts of additives such as iodinated compounds, surfactants, and alcohol. 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 bringing a polyvinyl alcohol-based film into contact with a liquid containing iodine or a dichroic dye. Usually, an aqueous solution of iodine-potassium iodide is used, and the appropriate concentration of iodine is 0.1 to 2 g/L and the concentration of potassium iodide is 1 to 100 g/L. The 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 process is carried out using boron compounds such as boric acid or borax. The boron compound is used in the form of an aqueous solution or a water-organic solvent mixture at a concentration of about 10 to 100 g/L, and it is preferable to coexist potassium iodide in the solution from the viewpoint of stabilizing polarization performance. The temperature during treatment is preferably about 30 to 70°C, and the treatment time is preferably about 0.1 to 20 minutes, and stretching may be performed during treatment as needed.

In the stretching process, the polyvinyl alcohol-based film is preferably stretched 3 to 10 times, preferably 3.5 to 6 times, in the uniaxial direction (flow direction (MD)). At this time, a little stretching (stretching to the extent of preventing shrinkage in the width direction (TD) or more) may be performed in a direction perpendicular to the stretching direction. The temperature during stretching is preferably 40 to 70°C. In addition, the stretching ratio may be finally set to the above range, and the stretching operation may be performed not only in one step (once) but also several times in the polarizing film manufacturing process.

The cleaning process is performed, for example, by immersing the polyvinyl alcohol-based film in water or an iodide aqueous solution such as potassium iodide, and precipitates occurring on the surface of the polyvinyl alcohol-based film can be removed. When using an aqueous potassium iodide solution, the potassium iodide concentration is about 1 to 80 g/L. The temperature during washing treatment is usually 5 to 50°C, preferably 10 to 45°C. The processing time is usually 1 to 300 seconds, preferably 10 to 240 seconds. Additionally, washing with water and washing with an aqueous potassium iodide solution may be performed in appropriate combination.

The drying process is performed, for example, by drying the polyvinyl alcohol-based film at 40 to 80° C. for 1 to 10 minutes in the air.

Moreover, the polarization degree of the polarizing film is preferably 99.5% or more, and more preferably 99.8% or more. If the degree of polarization is too low, there is a tendency that contrast in the liquid crystal display cannot be secured.

In addition, the degree of polarization is generally determined by the light transmittance (H ₁₁ ) measured for the wavelength λ in a state in which two polarizing films are overlapped so that their orientation directions are in the same direction, and the orientation directions of the two polarizing films are orthogonal to each other. It is calculated according to the following equation from the light transmittance (H ₁ ) measured for the wavelength λ in a state of overlapping as much as possible.

Polarization degree (%) = [(H ₁₁ -H ₁ )/(H ₁₁ + H ₁ )] ^1/2

Additionally, the single transmittance of the polarizing film of the present invention is preferably 42% or more. If this single transmittance is too low, there is a tendency that high brightness of the liquid crystal display cannot be achieved.

The single transmittance is a value obtained by measuring the light transmittance of the polarizing film alone using a spectrophotometer.

Next, a method for manufacturing the polarizing plate of the present invention using the polarizing film of the present invention will be described.

The polarizing film of the present invention is very suitable for producing a polarizing plate with little color unevenness and excellent polarization performance.

[Method of manufacturing polarizer]

The polarizing plate of the present invention is produced by bonding an optically isotropic resin film as a protective film to one or both sides of the polarizing film of the present invention via an adhesive. As a protective film, for example, cellulose triacetate, cellulose diacetate, polycarbonate, polymethyl methacrylate, cycloolefin polymer, cycloolefin copolymer, polystyrene, polyether sulfone, polyarylene ester, poly-4-methyl. Films or sheets of pentene, polyphenylene oxide, etc. can be mentioned.

The bonding method is carried out by a known method. For example, a liquid adhesive composition is uniformly applied to a polarizing film or a protective film, or both, then both are bonded and pressed, and then heated or irradiated with active energy rays. It is carried out by doing.

Additionally, a curable resin such as urethane resin, acrylic resin, or urea resin can be applied to one or both sides of the polarizing film and cured to form a cured layer to form a polarizing plate. In this way, the cured layer takes the place of the protective film, and thinning can be achieved.

The polarizing film and polarizing plate using the polyvinyl alcohol-based film of the present invention have excellent polarization performance and can be used in portable information terminals, PCs, televisions, projectors, signage, electronic desk calculators, electronic watches, word processors, electronic papers, game machines, and video devices. , reflection reduction layer for liquid crystal displays such as cameras, photo albums, thermometers, audio, automobile and machinery instruments, sunglasses, anti-glare glasses, 3D glasses, wearable displays, display elements (CRT, LCD, organic EL, electronic paper, etc.) , is preferably used in optical communication devices, medical devices, building materials, toys, etc.

(Example)

Next, the present invention will be described in more detail by way of examples, but the present invention is not limited to the later examples as long as the gist thereof is not exceeded.

In addition, measurement and evaluation of the properties of the polyvinyl alcohol-based film [Δn(MD) Ave, Δn(TD) Ave] and the properties of the polarizing film (polarization degree, single transmittance, color unevenness) in the later examples and comparative examples. was carried out as follows.

[Method for measuring Δn(MD) Ave of polyvinyl alcohol-based film]

(1) At an arbitrary position in the flow direction (MD) of the obtained polyvinyl alcohol-based film, three pieces of size MD × TD = 5 mm × 10 mm were cut from the central portion in the width direction (TD) of the polyvinyl alcohol-based film. . Next, the three pieces were placed on both sides with a 100-μm-thick PET film, placed again in a wooden frame, and mounted on a microtome device.

(2) Next, the three pieces cut in (1) above were sliced at 10 μm intervals parallel to the flow direction (MD) of the three pieces, and slice pieces (MD × TD = 5 mm × 10 μm) for observation were produced.

(3) Next, so that the slice surface can be observed, the slice piece is flipped over, the slice surface is turned upward, placed on a slide glass, sealed with a cover glass and tricredyl phosphate (refractive index 1.557), and used as a two-dimensional photoelasticity evaluation system “PA-” micro" (manufactured by Photonic Lattice) was used to measure the retardation of three slices.

(4) With the retardation distribution of the slice piece displayed on the measurement screen of “PA-micro,” a line segment Line analysis was performed to obtain retardation distribution data in the thickness direction of the slice piece. In addition, observation was performed using a 40x objective lens, and the average value of the retardation was adopted with the line width being 3 pixels.

(5) The obtained retardation distribution data in the thickness direction of the slice piece is removed to a slice thickness of 10 ㎛, the birefringence Δn (MD) distribution in the thickness direction of the slice piece is obtained, and the average value of the birefringence Δn (MD) distribution in the thickness direction of the slice piece is obtained. took. The average value of the birefringence Δn(MD) distribution in the thickness direction of each slice obtained for three slices was averaged again and designated as “Δn(MD) Ave” of the polyvinyl alcohol-based film.

[Method for measuring Δn(TD) Ave of polyvinyl alcohol-based film]

(1) At an arbitrary position in the flow direction (MD) of the obtained polyvinyl alcohol-based film, three pieces of size MD × TD = 10 mm × 5 mm were cut from the central portion in the width direction (TD) of the polyvinyl alcohol-based film. . Next, the three pieces were placed on both sides with a 100-μm-thick PET film, placed again in a wooden frame, and mounted on a microtome device.

(2) Next, the three pieces cut in (1) above were sliced at 10 μm intervals parallel to the width direction (TD) of the pieces, and slice pieces (MD x TD = 10 μm x 5 mm) for observation were produced.

(3) Next, in order to observe the slice surface, the slice piece is flipped over, placed on a glass slide with the slice surface facing upward, sealed with a cover glass and tricredyl phosphate (refractive index 1.557), and used as a two-dimensional photoelasticity evaluation system “PA-micro”. "(manufactured by Photonic Lattice) was used to measure the retardation of three slices.

(4) With the retardation distribution of the slice piece displayed on the measurement screen of “PA-micro,” a line segment Line analysis was performed to obtain retardation distribution data in the thickness direction of the slice piece. In addition, the observation was performed using a 40x objective lens, and the average value of the retardation was adopted with a line width of 3 pixels.

(5) The obtained retardation distribution data in the thickness direction of the slice piece is removed to 10 μm of the thickness of the slice piece, the birefringence Δn(TD) distribution in the thickness direction of the slice piece is obtained, and the birefringence Δn(TD) distribution in the thickness direction of the slice piece is obtained. The average value was taken. The average value of the birefringence Δn(TD) distribution in the thickness direction of each slice piece determined for the three slice pieces was averaged again and designated as “Δn(TD) Ave” of the polyvinyl alcohol-based film.

[Polarization degree (%), group transmittance (%)]

From the center of the obtained polarizing film in the width direction (TD), a test piece of 4 cm in length Measured.

[Color stains]

From the center of the obtained polarizing film in the width direction (TD), a test piece measuring 30 cm long After placement, optical color unevenness was observed in transmission mode using a light box with a surface illumination of 14,000 lx and evaluated based on the following criteria.

(Evaluation standard)

○···There was no color staining

△···There was a faint color stain

×···There was a clear color stain

<Example 1>

(Production of polyvinyl alcohol-based film)

In a 5,000 L dissolution can, add 1,000 kg of polyvinyl alcohol-based resin with a weight average molecular weight of 142,000 and a degree of saponification of 99.8 mol%, 2,500 kg of water, 105 kg of glycerin as a plasticizer, and 0.25 kg of polyoxyethylene laurylamine as a surfactant and stir. While heating, the temperature was raised to 150°C and dissolution was performed under pressure, and an aqueous solution of polyvinyl alcohol-based resin with a resin concentration of 25% by weight was obtained by adjusting the concentration. Next, the polyvinyl alcohol-based resin aqueous solution is supplied to a twin-screw extruder and degassed, the aqueous solution temperature is set to 95°C, and the aqueous solution is discharged from the T-slit die discharge port onto a cast drum with a surface temperature of 80°C (discharge speed: 1.3 m). /min) and flexible. The formed film was peeled from the cast drum and dried while conveyed in the flow direction (MD) while the front and back surfaces of the film were alternately brought into contact with a total of 10 heat rolls. As a result, a film (width 2 m, thickness 30 μm) with a moisture content of 7% by weight was obtained. Next, both left and right ends of the film were clamped with clips with a clip pitch of 45 mm, and the film was stretched 1.2 times in the width direction (TD) at 80°C using a stretching machine while conveying the film in the flow direction (MD) at a speed of 8 m/min. Afterwards, the film was transported into a dryer at 130°C with a fixed width of 2.4 m to obtain a polyvinyl alcohol-based film (width 2.4 m, thickness 25 μm, length 2 km). The properties of the obtained polyvinyl alcohol-based film are as shown in Table 1 below. Finally, the polyvinyl alcohol-based film was wound into a roll around the core pipe to obtain a film roll.

(Production of polarizing film and polarizing plate)

The obtained polyvinyl alcohol-based film was unwound from the film roll, transported in the horizontal direction, immersed in a water bath at a temperature of 30°C, swelled, and stretched 1.7 times in the flow direction (MD). No folds or wrinkles occurred in the film during the swelling process. Next, it was immersed in an aqueous solution at 30°C consisting of 0.5 g/L of iodine and 30 g/L of potassium iodide, and stretched 1.6 times in the direction of flow (MD) while dyeing, and then the product had the composition of 40 g/L of boric acid and 30 g/L of potassium iodide. It was uniaxially stretched 2.1 times in the flow direction (MD) while immersed in an aqueous solution (50°C) and crosslinked with boric acid. Finally, it was washed with an aqueous potassium iodide solution and dried at 50°C for 2 minutes to obtain a polarizing film with a total stretching ratio of 5.8 times. No breakage occurred during the production of this polarizing film. In addition, the properties of the obtained polarizing film are as shown in Table 1 later.

A polyvinyl alcohol aqueous solution was used as an adhesive on both sides of the polarizing film obtained above, and a triacetylcellulose film with a film thickness of 40 μm was bonded and dried at 70° C. to obtain a polarizing plate.

<Example 2>

In Example 1, the polyvinyl alcohol-based resin aqueous solution was discharged onto a cast drum with a surface temperature of 88°C (discharge speed of 1.9 m/min) and formed into a flexible film, thereby producing a film (width of 2 m) with a moisture content of 10% by weight. , thickness 45㎛) was obtained. Next, in the same manner as in Example 1, the film was stretched 1.2 times in the width direction (TD) at 80°C using a stretching machine, and then transported into a dryer at 135°C at a fixed width of 2.4 m, forming a polyvinyl alcohol-based film. (width 2.4m, thickness 35㎛, length 2km) was obtained. The properties of the obtained polyvinyl alcohol-based film are as shown in Table 1 below.

Additionally, a polarizing film and a polarizing plate were obtained in the same manner as in Example 1 using the polyvinyl alcohol-based film. During the swelling process during the production of the polarizing film, no folds or wrinkles occurred in the polyvinyl alcohol-based film, and no breakage occurred. The properties of the obtained polarizing film are as shown in Table 1 below.

<Example 3>

In Example 1, the same procedure was performed except that the film was discharged at a discharge speed of 0.8 m/min and the film was formed to be flexible, thereby obtaining a film (width 2 m, thickness 20 ㎛) with a moisture content of 5% by weight. Next, in the same manner as in Example 1, the film was stretched 1.4 times in the width direction (TD) at 80°C using a stretching machine, and then returned into a dryer at 130°C at a fixed width of 2.8 m to form a polyvinyl alcohol-based film (width 2.8m, thickness 14㎛, length 2km) was obtained. The properties of the obtained polyvinyl alcohol-based film are as shown in Table 1 below.

Additionally, a polarizing film and a polarizing plate were obtained in the same manner as in Example 1 using the polyvinyl alcohol-based film. During the swelling process during the production of the polarizing film, no folds or wrinkles occurred in the polyvinyl alcohol-based film, and no breakage occurred. The properties of the obtained polarizing film are as shown in Table 1 below.

<Comparative Example 1>

In Example 1, an aqueous polyvinyl alcohol-based resin solution was discharged (discharge speed of 2.5 m/min) onto a cast drum with a surface temperature of 90°C and softened to form a film. The formed film was prepared in the same manner as in Example 1 except that it was heat treated at 110°C using a floating dryer instead of being stretched in the width direction (TD) using a stretching machine, and a polyester with a moisture content of 2.5% by weight was obtained. A vinyl alcohol-based film (width 2 m, thickness 60 μm, length 2 km) was obtained. The properties of the obtained polyvinyl alcohol-based film are as shown in Table 1 below.

In addition, when a polarizing film and a polarizing plate were manufactured in the same manner as in Example 1 using the polyvinyl alcohol-based film, folds and wrinkles occurred in the polyvinyl alcohol-based film during the swelling process. The properties of the obtained polarizing film are as shown in Table 1 below.

<Comparative Example 2>

In Example 1, an aqueous polyvinyl alcohol-based resin solution was discharged (discharge speed of 1.9 m/min) onto a cast drum with a surface temperature of 88°C and flexible to form a film. The formed film was prepared in the same manner as in Example 1 except that it was not stretched in the width direction (TD) using a stretching machine and was heat treated using a heat treatment roll with a surface temperature of 105°C, and polyvinyl film with a moisture content of 2.0% by weight was prepared. An alcohol-based film (width 2 m, thickness 45 μm, length 2 km) was obtained. The properties of the obtained polyvinyl alcohol-based film are shown in Table 1 below.

In addition, when a polarizing film and a polarizing plate were manufactured in the same manner as in Example 1 using the polyvinyl alcohol-based film, folds or wrinkles occurred in the polyvinyl alcohol-based film during the swelling process. The properties of the obtained polarizing film are as shown in Table 1 below.

From the results of the above examples and comparative examples, the polarizing film obtained from the polyvinyl alcohol-based films of Examples 1 to 3 in which Δn(MD) Ave and Δn(TD) Ave satisfy both formulas (A) and (B) has a high It can be seen that it has polarization characteristics and is uniform without color stains.

On the other hand, the polarizing film obtained from the polyvinyl alcohol-based films of Comparative Examples 1 and 2, where the values of Δn(MD) Ave and Δn(TD) Ave are smaller than the range specified by formulas (A) and (B), is polarized. It can be seen that the characteristics are poor and color unevenness is also observed.

Although the above examples show specific forms of the present invention, the examples are merely examples and are not to be construed as limiting. Various modifications apparent to those skilled in the art are expected to fall within the scope of the present invention.

The polarizing film made of the polyvinyl alcohol-based film of the present invention has excellent polarization performance and can be used in portable information terminals, PCs, televisions, projectors, signage, electronic desk calculators, electronic watches, word processors, electronic papers, game machines, videos, cameras, and photo processors. Liquid crystal displays such as albums, thermometers, audio, automobile and machinery instruments, sunglasses, anti-glare glasses, 3D glasses, wearable displays, reflection reduction layers for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication devices, It is preferably used in medical devices, building materials, toys, etc.

Claims (6)

A long polyvinyl alcohol-based film that satisfies the following formulas (A) and (B) and has a thickness of 5 to 50 μm.
1.5×10 ^-2 ≥Δn(MD) Ave>3.0×10 ^-3 ···(A)
1.5×10 ^-2 ≥Δn(TD) Ave≥2.0×10 ^-3 ···(B)
Δn(MD) Ave in the formula (A) represents the value obtained by averaging the birefringence in the longitudinal direction of the polyvinyl alcohol-based film in the thickness direction of the polyvinyl alcohol-based film. In addition, Δn(TD) Ave in the formula (B) represents the value obtained by averaging the birefringence in the width direction of the polyvinyl alcohol-based film in the thickness direction of the polyvinyl alcohol-based film. delete A polarizing film characterized in that the polyvinyl alcohol-based film according to claim 1 is used. A polarizing plate comprising the polarizing film according to claim 3 and a protective film provided on at least one side of the polarizing film. It includes a film forming process of forming an aqueous solution of polyvinyl alcohol resin into a film by a continuous casting method, and a drying/stretching process of continuously drying and continuously stretching the film while transporting the formed film in the flow direction. In the method for producing a polyvinyl alcohol-based film, the polyvinyl alcohol-based film produced satisfies the following formulas (A) and (B), and the polyvinyl alcohol-based film has a thickness of 5 to 50 μm. A method for producing a polyvinyl alcohol-based film.
1.5×10 ^-2 ≥Δn(MD) Ave>3.0×10 ^-3 ···(A)
1.5×10 ^-2 ≥Δn(TD) Ave≥2.0×10 ^-3 ···(B)
Δn(MD) Ave in the formula (A) represents a value obtained by averaging the birefringence in the flow direction of the polyvinyl alcohol-based film in the thickness direction of the polyvinyl alcohol-based film. In addition, Δn(TD) Ave in the formula (B) represents the value obtained by averaging the birefringence in the width direction of the polyvinyl alcohol-based film in the thickness direction of the polyvinyl alcohol-based film. In claim 5,
A method for producing a polyvinyl alcohol-based film, characterized in that the film formed in the drying/stretching process is stretched 1.05 to 1.5 times in the width direction.