JP7335697B2 - Polyvinyl alcohol film, polarizing film, polarizing plate, and method for producing polyvinyl alcohol film - Google Patents

Description

The present invention provides a polyvinyl alcohol-based film that has excellent dyeability and serves as a material for forming a polarizing film with a high degree of polarization and little color unevenness, a polarizing film and a polarizing plate using the polyvinyl alcohol-based film, and the polyvinyl alcohol The present invention relates to a method for producing an alcohol-based film.

BACKGROUND ART Conventionally, polyvinyl alcohol-based films have been used in many applications as films with excellent transparency, and one of the useful applications thereof is a polarizing film. The polarizing film is used as a basic component of a liquid crystal display, and in recent years, its use has expanded to equipment that requires high quality and high reliability.

Under these circumstances, a polarizing film with excellent optical properties is required as the screens of liquid crystal televisions, multifunctional mobile terminals, etc. become brighter, have higher definition, have a larger area, and are thinner. The specific requirements are further improvement of the degree of polarization and elimination of color unevenness.

In general, a polyvinyl alcohol film is produced by a continuous casting method using an aqueous solution of a polyvinyl alcohol resin as a material. Specifically, first, an aqueous solution of a polyvinyl alcohol-based resin is cast onto a cast mold such as a cast drum or an endless belt to form a film, and then the film formed is peeled off from the cast mold. , while conveying in the machine direction (MD) using nip rolls or the like, and drying using hot rolls, a floating dryer, or the like. In the conveying step, the formed film is pulled in the machine direction (MD), so the polyvinyl alcohol-based polymer tends to be oriented in the machine direction (MD).

On the other hand, in general, a polarizing film is made by first swelling a polyvinyl alcohol-based film, which is the original film, with water (including hot water), then dyeing it with a dichroic dye such as iodine, and then stretching it. Manufactured by
What is important in the swelling step is to quickly swell the polyvinyl alcohol film in the thickness direction, and to uniformly swell the polyvinyl alcohol film so that the dye can smoothly enter the inside of the film in the dyeing step. That is.
In addition, the stretching step is a step of stretching the dyed film in the machine direction (MD) to highly orient the dichroic dye in the polyvinyl alcohol film. Therefore, in this stretching step, it is important that the polyvinyl alcohol-based film, which is the original fabric, exhibits good stretchability in the machine direction (MD).

In the production of the polarizing film, the order of the stretching process and the dyeing process is sometimes reversed. That is, in this case, the original polyvinyl alcohol film is first swelled with water (including warm water), then stretched, and then dyed with a dichroic dye such as iodine. In this case as well, what is important for improving the polarizing performance of the polarizing film is that the raw polyvinyl alcohol film exhibits good swelling properties in the thickness direction and good stretchability in the machine direction (MD). is to show

Furthermore, in recent years, in order to make the polarizing film thinner, the original polyvinyl alcohol-based film has also been made thinner. However, the thin film has productivity problems such as breakage due to stretching during production of the polarizing film.

As a method for improving the swelling properties of polyvinyl alcohol-based films, for example, a method of adding a polyhydric alcohol as a water swelling aid to a polyvinyl alcohol-based resin has been proposed (see, for example, Patent Document 1).
In addition, as a method of improving the stretchability of the polyvinyl alcohol film, for example, a method of specifying the ratio of the speed of the cast drum when forming the film and the final winding speed of the polyvinyl alcohol film (for example, patent see Document 2), a method of floating and drying the film after film formation on a cast drum (see, for example, Patent Document 3), and a method of controlling the degree of tension in the drying process of the formed film (see, for example, Patent Document 4). ), a film having a thickness of 40 μm or less and having a degree of swelling before and after being stretched in water at 30° C. within a specific range (see, for example, Patent Document 5).

JP-A-2001-302867 Japanese Patent Application Laid-Open No. 2001-315141 Japanese Patent Application Laid-Open No. 2001-315142 Japanese Unexamined Patent Application Publication No. 2002-79531 WO2014/050697

However, for the thin polyvinyl alcohol-based film, the method of Patent Document 1 is insufficient to improve the swelling property, and the methods of Patent Documents 2 to 5 do not improve the stretchability during the production of the polarizing film. Insufficient to improve.

That is, in the technique disclosed 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 taken into consideration. It is difficult to efficiently improve drawability in the MD). Conversely, the addition of the water-swelling aid tends to disturb the orientation of the polymer, making uniform stretching in the machine direction (MD) difficult.

The technique disclosed in Patent Document 2 above specifies the degree of stretching (pulling condition) in the machine direction (MD) when producing a polyvinyl alcohol film, but also considers stretching in the width direction (TD). Otherwise, it is insufficient to improve the stretchability during the production of the polarizing film.

With the technique disclosed in Patent Document 3, although the formed film can be dried uniformly, it is not possible to control the orientation of the polymer, and it is not sufficient to improve the swelling property and stretchability during the production of the polarizing film. be.
Further, in the technique disclosed in Patent Document 4, although the film thickness of the polyvinyl alcohol film can be made uniform, it is not possible to control the orientation of the polymer. inadequate.

In the technique disclosed in Patent Document 5, by forming a polyvinyl alcohol film with a specific degree of swelling before and after stretching in water at 30°C, wrinkles during stretching are prevented and a polyvinyl alcohol film with high stretchability is obtained. However, wrinkles at the time of swelling cannot be controlled, and are insufficient for producing a polarizing film with excellent uniformity. Moreover, since wet heat treatment is required for a relatively long time, the productivity of the polyvinyl alcohol film is insufficient.

In view of this background, the present invention provides an excellent balance between swelling property and stretchability during the production of a polarizing film, no breakage during the production of a thin polarizing film, high polarizing performance, and excellent color polarizing performance. Provided are a polyvinyl alcohol-based film from which a polarizing film with little unevenness can be obtained, a polarizing film and a polarizing plate using the polyvinyl alcohol-based film, and a method for producing the polyvinyl alcohol-based film.

The present inventors have conducted intensive research in view of such circumstances, and found that in a long polyvinyl alcohol film having a thickness of 40 μm or less and a mass-based swelling degree of 190 to 230%, the length direction (MD) By adjusting the range of the ratio of the degree of swelling based on the dimensions of the width direction (TD) to the degree of swelling based on the dimensions of the width direction (TD) to a specific range, and adjusting the swelling degree based on mass after stretching in water to a specific range It has been found that a polarizing film can be obtained that has a good balance between swelling property and stretchability during the production of the polarizing film, does not break even during the production of a thin polarizing film, exhibits high polarizing performance, and has little color unevenness.

That is, the first gist of the present invention is a polyvinyl alcohol film having a thickness of 40 μm or less, characterized by satisfying the following formulas (1) to (3). .
190%≦S _A ≦230% (1)
220%≦S _B ≦310% (2)
1.01≦S _Y /S _X ≦1.20 (3)
[In the above formula (1), S _A represents the degree of swelling on a mass basis when the polyvinyl alcohol film is immersed in water at 30°C for 5 minutes,
In the above formula (2), _SB is the mass-based swelling degree when the polyvinyl alcohol film is uniaxially stretched in water at 30 ° C. at a stretching speed of 0.12 m / min so that the stretching ratio is 3 times. represents
In the above formula (3), _S represents the degree of swelling of the width direction (TD) when the polyvinyl alcohol film is immersed in water at 30 ° C. for 5 minutes, and S _Y represents the polyvinyl alcohol. Shows the degree of swelling in the machine direction (MD) when the system film is immersed in water at 30°C for 5 minutes. ]

A second aspect of the present invention is a polarizing film using the polyvinyl alcohol film. Further, a third aspect is a polarizing plate comprising the polarizing film and a protective film provided on at least one side of the polarizing film.

Then, a film-forming step of forming a film from an aqueous solution of a polyvinyl alcohol-based resin by a continuous casting method, and continuously drying and stretching the film while conveying the film in the flow direction. A polyvinyl alcohol-based film manufacturing method comprising drying and stretching steps, wherein the polyvinyl alcohol-based film to be manufactured satisfies the following formulas (1) to (3): A fourth gist is a method for producing a system film.
190%≦S _A ≦230% (1)
220%≦S _B ≦310% (2)
1.01≦S _Y /S _X ≦1.20 (3)
[In the above formula (1), S _A represents the degree of swelling on a mass basis when the polyvinyl alcohol film is immersed in water at 30°C for 5 minutes,
In the above formula (2), _SB is the mass-based swelling degree when the polyvinyl alcohol film is uniaxially stretched in water at 30 ° C. at a stretching speed of 0.12 m / min so that the stretching ratio is 3 times. represents
In the above formula (3), _S represents the degree of swelling of the width direction (TD) when the polyvinyl alcohol film is immersed in water at 30 ° C. for 5 minutes, and S _Y represents the polyvinyl alcohol. Shows the degree of swelling in the machine direction (MD) when the system film is immersed in water at 30°C for 5 minutes. ]

The polyvinyl alcohol film of the present invention is a long polyvinyl alcohol film having a thickness of 40 μm or less and a mass-based swelling degree (S _A ) of 190 to 230%. The ratio (S _Y /S X ) to the degree of swelling (S _X ) based on the dimension in the width direction (TD) of the degree (S Y ) is within a specific range, and the ratio (S _Y /S _X ) on the basis of mass after stretching in water Since the degree of swelling (S _B ) is within a specific range, it has excellent swelling and stretchability during the production of the polarizing film, and can be made thin (thickness of 40 μm or less) to produce a thin polarizing film. Even if it is used, breakage can be prevented from occurring. Furthermore, by using the polyvinyl alcohol film, it is possible to obtain a polarizing film exhibiting high polarizing performance and little color unevenness.

Further, the polarizing film of the present invention uses the polyvinyl alcohol-based film, so that it exhibits high polarizing performance and little color unevenness.

Furthermore, since the polarizing film of the present invention is used, the polarizing plate of the present invention exhibits high polarizing performance and little color unevenness.

The method for producing a polyvinyl alcohol-based film of the present invention includes a film forming step by a continuous casting method, and continuous drying and continuous stretching of the film while conveying the formed film in the machine direction. The polyvinyl alcohol film of the present invention, which satisfies the above formulas (1) to (3), can be obtained by combining the production conditions in each step.

In particular, in the drying/stretching step, when the film formed above is stretched 1.05 to 1.3 times in the width direction (TD), S _A and S _B in the formulas (1) to (3) , S _Y /S _X values are suitable, and a polyvinyl alcohol film having excellent swelling properties and stretching properties during the production of the polarizing film can be obtained.

Further, in the drying/stretching step, the film formed above is temporarily stretched by more than 1.3 times in the width direction (TD), and then the final stretch ratio in the width direction (TD) is 1.3 times. When the dimension is shrunk to 3 times or less, the stress applied to the formed film can be relaxed. Therefore, even if the formed film is thin, breakage of the film can be avoided.

The present invention will now be described in detail.
The polyvinyl alcohol film of the present invention is a long polyvinyl alcohol film having a thickness of 40 μm or less, and is characterized by satisfying the following formulas (1) to (3).
190%≦S _A ≦230% (1)
220%≦S _B ≦310% (2)
1.01≦S _Y /S _X ≦1.20 (3)
[In the above formula (1), S _A represents the degree of swelling on a mass basis when the polyvinyl alcohol film is immersed in water at 30°C for 5 minutes,
In the above formula (2), S _B is when the polyvinyl alcohol film is uniaxially stretched in water at 30° C. at a stretching speed of 0.12 m/min (240%/min) so that the stretch ratio is 3 times. represents the degree of swelling based on the mass of
In the above formula (3), _S represents the degree of swelling of the width direction (TD) when the polyvinyl alcohol film is immersed in water at 30 ° C. for 5 minutes, and S _Y represents the polyvinyl alcohol. Shows the degree of swelling in the machine direction (MD) when the system film is immersed in water at 30°C for 5 minutes. ]

The thickness of the polyvinyl alcohol-based film of the present invention must be as thin as 40 μm or less, preferably 5 to 40 μm from the viewpoint of thinning the polarizing film, and particularly preferably to avoid breakage. The point is 10 to 40 μm, more preferably 10 to 30 μm.
If the thickness of the polyvinyl alcohol-based film is too thick, the tension increases during stretching in the process of manufacturing the polarizing film, which will be described later, which is not preferable. If the thickness is too thin, it tends to be easily broken during the production of the polarizing film.

That is, the polyvinyl alcohol-based film of the present invention is controlled so that the specific degree of swelling when immersed in water at 30° C. falls within a specific range, as described above. This controlled degree of swelling allows the polyvinyl alcohol film to have excellent performance as a very thin film having a thickness of 40 μm or less.

As described above, the polyvinyl alcohol film of the present invention has a mass-based swelling degree (S _A ) of 190%≦S _A ≦230% when immersed in water at 30° C. for 5 minutes. controlled to be The degree of swelling (S _A ) based on mass is preferably 195%≦S _A ≦225%, particularly preferably 195%≦S _A ≦220%.
If the value of the degree of swelling (S _A ) is too small, the stretchability during the production of the polarizing film, which will be described later, is lowered, so that the object of the present invention cannot be achieved, and the value of the degree of swelling (S _A ) is low. If it is too large, wrinkles will occur in the swelling process during the production of the polarizing film, and the polarizing film will tend to have color unevenness, making it impossible to achieve the object of the present invention.

The degree of swelling (S _A ) is an index showing the water retention capacity when the polyvinyl alcohol film is immersed in water, and the polyvinyl alcohol film was immersed in water at 30° C. as it is (without stretching) for 5 minutes. It can be determined as a percentage by dividing the mass after immersion by the mass after drying at 105° C. for 16 hours after immersion, and can be measured, for example, by the method described later in the Examples.

In addition, as described above, the polyvinyl alcohol film of the present invention was uniaxially stretched in water at 30° C. at a stretching rate of 0.12 m/min (240%/min) so that the stretch ratio was 3 times. The degree of swelling (S _B ) based on mass at the time is controlled so as to satisfy 220%≦S _B ≦310%. The mass-based swelling degree (S _B ) is preferably 225%≦S _B ≦300%, particularly preferably 230%≦S _B ≦280%, and most preferably 230%≦S _B ≦260%.
If the value of the degree of swelling (S _B ) is too small _, the stretchability during the production of the polarizing film, which will be described later, is deteriorated. If it is too large, wrinkles will occur in the swelling and dyeing steps during the production of the polarizing film, and the polarizing film will tend to have color unevenness, making it impossible to achieve the object of the present invention.

The degree of swelling (S _B ) is an index showing the water retention capacity of a polyvinyl alcohol film after being stretched in water at 30°C to a draw ratio of 3 times. Calculated as a percentage by dividing the mass after stretching to a stretch ratio of 3 times at a stretching speed of 0.12 m / min (240% / min) in water by the mass after drying at 105 ° C. for 16 hours after stretching For example, it can be measured by the method described later in Examples. Generally, when a polarizing film is produced from a polyvinyl alcohol-based film, the film is swollen and dyed at 30° C. and then introduced into a high-temperature stretching bath.
Since the polyvinyl alcohol film is stretched to a little less than 3 times in this swelling and dyeing process, the degree of swelling (S _B ) when stretched to a stretch ratio of 3 times in water is the stretchability of the film in a high-temperature stretching bath. It is an index.

Furthermore, as described above, the polyvinyl alcohol film of the present invention has a degree of swelling (S _x ) based on the dimension in the width direction (TD) when immersed in water at 30°C for 5 minutes, and The degree of swelling (S _Y ) of the dimension standard in the length direction (MD) when immersed in water for 5 minutes is controlled so as to satisfy the relationship of 1.01≦S _Y /S _X ≦1.20. there is This ratio (S _Y /S _X ) is preferably 1.03≦S _Y /S _X ≦1.18, particularly preferably 1.05≦S _Y /S _X ≦1.15.
If the value of the ratio (S _Y /S _X ) is too small, the stretchability during the production of the polarizing film described later deteriorates, so the object of the present invention cannot be achieved, and the ratio (S _Y /S _X ) is too large, wrinkles are likely to occur in the swelling process during the production of the polarizing film, and the polarizing film tends to have color unevenness, making it impossible to achieve the object of the present invention.

The degree of swelling (S _x and S _Y ) is an index showing the spread when the polyvinyl alcohol film is immersed in water. It can be obtained as a percentage by dividing the length direction (MD) and width direction (TD) dimensions after immersion by the dimensions before immersion in water, for example, by the method described later in Examples. can do.

In the present invention, as a method for controlling the formulas (1) to (3), in the method for producing the polyvinyl alcohol film by the continuous casting method described later, the film formed in the cast mold is peeled from the cast mold. A method of stretching in the width direction (TD) after stretching is preferred. In this case, the conditions in other steps are appropriately adjusted according to the stretching conditions in the width direction (TD) (stretch ratio, ambient temperature during stretching, stretching time, etc.). The conditions include, for example, the chemical structure of the polyvinyl alcohol-based resin that is the material for forming the polyvinyl alcohol-based film, the type and amount of the plasticizer added, the film-forming conditions (cast mold temperature, etc.), and the film-forming conditions. Drying conditions (temperature, time) for drying the formed film, transport speed of the formed film in the machine direction (MD), and the like can be mentioned. Formulas (1) to (3) are controlled by combining at least one of these conditions with the stretching condition in the width direction (TD).

The measurement positions of the degree of swelling (S _A , S _B , S _X , S _Y ) in the formulas (1) to (3) are within a 500 mm×500 mm area of the polyvinyl alcohol film.

Here, the method for producing the polyvinyl alcohol film of the present invention will be described in order of steps.

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

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

The weight average molecular weight of the polyvinyl alcohol-based resin is preferably 100,000 to 300,000, particularly preferably 110,000 to 280,000, further 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 the polyvinyl alcohol-based resin is used as an optical film. tend to become 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 resin used in the present invention is generally preferably 98 mol% or more, particularly preferably 99 mol% or more, still more preferably 99.5 mol% or more, particularly preferably 99.5 mol% or more. 8 mol % or more. If the average degree of saponification is too small, there is a tendency that sufficient optical performance cannot be obtained when the polyvinyl alcohol film is used as a polarizing film.
Here, the average degree of saponification in the present invention is measured according to JIS K6726.

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

In addition to the polyvinyl alcohol resin, the polyvinyl alcohol resin aqueous solution may contain commonly used plasticizing agents such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, polyethylene glycol, trimethylolpropane, etc., if necessary. From the viewpoint of film-forming properties, it is preferable to contain a surfactant or at least one of nonionic, anionic, and cationic surfactants. These can be used alone or in combination of two or more.

The resin concentration of the polyvinyl alcohol resin aqueous solution thus obtained is preferably 15 to 60% by weight, particularly preferably 17 to 55% by weight, further preferably 20 to 50% by weight. If the resin concentration of the aqueous solution is too low, the drying load tends to increase, resulting in a decrease in production capacity.

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

[Film forming process]
The polyvinyl alcohol film of the present invention is produced by a casting method or a melt extrusion method. It is a continuous casting method from the point of view of quality.

The continuous casting method is, for example, a film formation by continuously discharging and casting the aqueous solution of the polyvinyl alcohol resin from a T-shaped slit die to a casting mold such as a rotating cast drum, endless belt, or resin film. It is a way to
Here, the film forming method when the cast mold is a cast drum will be described.

The temperature of the polyvinyl alcohol resin aqueous solution at the exit of the T-shaped slit die is preferably 80 to 100°C, particularly preferably 85 to 98°C.
If the temperature of the polyvinyl alcohol 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 at the time of ejection (at the preferred temperature of 80 to 100 ° C.), particularly preferably 70 to 150 Pa (at the particularly preferred temperature of 85 to 98 ° C.). · 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, casting tends to be difficult.

The discharge speed of the polyvinyl alcohol resin aqueous solution discharged from the T-type slit die onto the cast drum is preferably 0.2 to 5 m/min, particularly preferably 0.4 to 4 m/min, and more preferably 0.4 m/min. 6 to 3 m/min.
If the ejection speed is too slow, productivity tends to decrease, and if it is too fast, casting tends to be difficult.

The diameter of the casting drum is preferably 2-5 m, particularly preferably 2.4-4.5 m, more preferably 2.8-4 m.
If the diameter is too small, the dry length tends to be insufficient and the speed tends to be difficult to achieve.

The width of the casting drum is preferably 4 m or more, particularly preferably 4.5 m or more, still 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 rotational speed of the cast drum is preferably 5 to 50 m/min, particularly preferably 6 to 40 m/min, further preferably 7 to 35 m/min.
If the rotational speed is too slow, the productivity tends to decrease, and if it is too fast, the drying tends to be insufficient.

The surface temperature of the cast drum is preferably 40 to 99°C, particularly preferably 60 to 95°C.
If the surface temperature is too low, there is a tendency to cause poor drying, and if it is too high, there is a tendency for foaming to occur.

Thus, the film forming process is performed. Then, the formed film is separated from the cast drum and transported in the machine direction (MD).
The water content of the formed film is preferably 0.5 to 15% by weight, particularly preferably 1 to 13% by weight, further preferably 2 to 12% by weight. If the moisture content is too low or too high, it tends to be difficult to achieve the desired swelling properties and stretchability.

[Drying/stretching process]
For adjusting the moisture content, if the moisture content of the film before stretching in the width direction (TD) is too high, it is preferable to dry the film before stretching in the width direction (TD). If the moisture content of the film before stretching in the TD) is too low, it is preferable to condition the film before stretching in the width direction (TD). More preferably, the conditions of the drying process are adjusted so that the moisture content falls within the above range.

The drying is performed continuously. This continuous drying can be carried out by a known method using a heating roll, an infrared heater, or the like. to 150°C, more preferably 50 to 140°C. In addition, a humidity conditioning area may be provided before stretching in the width direction (TD) to adjust the moisture content.

In the present invention, it is not necessary to particularly stretch the formed film in the machine direction (MD), and it is sufficient to convey the film with a tensile tension that does not bend the film. As a matter of course, stretching in the width direction (TD) causes neck-in depending on the Poisson's ratio in the machine direction (MD), and dehydration shrinkage also occurs in the machine direction (MD) during drying. Due to these shrinkages, even if the rotational speeds of the transport roll and the heating roll are constant, an appropriate tension can be obtained in the flow direction (MD), and the complicated control of the rotational speed as in Patent Document 2 is unnecessary. From the manufacturing point of view, it is preferable that the dimension of the film in the machine direction (MD) is constant, and particularly preferably, the dimensional change rate in the machine direction (MD) is 0.8 before and after stretching in the width direction (TD). to 1.2, particularly preferably 0.9 to 1.1.

The conveying speed of the formed film in the machine direction (MD) is preferably 5 to 30 m/min, particularly preferably 7 to 25 m/min, further preferably 8 to 20 m/min. If the transport speed is too slow, the productivity tends to decrease, and if it is too fast, the in-plane uniformity tends to decrease.

The method of conveying the formed film in the machine direction (MD) and stretching it in the width direction (TD) at the same time is not particularly limited. Therefore, it is preferable to carry out the transport and the stretching at the same time. In the above case, the arrangement of the clips at each end preferably has a pitch of 200 mm or less, particularly preferably a pitch of 100 mm or less, and even more preferably a pitch of 50 mm or less.
If the pitch of the clips is too wide, the stretched film tends to flex, and the in-plane uniformity of the resulting polyvinyl alcohol film tends to deteriorate. Moreover, the clamping position of the clip (tip of the clip) is preferably 100 mm or less from both edges 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 film ends to be discarded increase and the width of the product tends to become narrow.

The draw ratio in the width direction (TD) in the present invention is preferably 1.05 to 1.3 times, particularly preferably 1.05 to 1.25 times, more preferably 1.1 to 1.2 times. is. If the draw ratio in the width direction (TD) is too high or too low, the in-plane uniformity tends to decrease.

The stretching in the width direction (TD) is performed continuously. This continuous drawing may be performed in one step (once) or in multiple steps (multiple times) so that the total draw ratio is within the range of the above draw ratio (also called sequential drawing). For example, after performing the first stage of continuous stretching, simple transport with a fixed width direction (TD) may be performed, and then the second and subsequent stages of continuous stretching may be performed. In particular, in the case of a thin film, by inserting a simple width-fixed conveying process after the first stage of continuous stretching, the stress of the film can be relaxed and breakage can be avoided. .
When inserting a width-fixed conveying step, the fixed width can be narrower than the width after the first stage of continuous stretching. Since the film immediately after being stretched tends to shrink due to stress relaxation and also shrinks due to dehydration, it is possible to narrow the fixed width to the shrinkage width. However, if the width is narrowed beyond the shrinkage width, the film will flex, which is not preferable.
As described above, the continuous stretching is preferably performed after the film drying process, but it is performed at least one of before the film drying process, during the drying process, and after the drying process.

As a preferred embodiment of the present invention, after temporarily stretching the film in the width direction (TD) by more than 1.3 times, the final stretch ratio in the width direction (TD) is 1.05. A method of dimensional shrinkage to 1.3 times can be used.
In this case, the film may be temporarily stretched over 1.3 times and then simply conveyed at a fixed width of 1.05 to 1.3 stretch ratio. This method provides stress relaxation of the film and makes it possible to avoid breakage, especially in the case of thin films.

In the present invention, the transverse direction (TD) stretching of the formed film is preferably carried out at an ambient temperature of 50 to 150°C. The ambient temperature during the stretching is particularly preferably 60 to 140°C, more preferably 70 to 130°C. If the ambient temperature during the stretching is too low or too high, the in-plane uniformity tends to decrease. In the case of sequential stretching, the ambient temperature during the stretching may be changed at each stage of stretching.

In the present invention, the stretching time in the width direction (TD) of the formed film is preferably 2 to 60 seconds, particularly preferably 5 to 45 seconds, and still more preferably 10 to 30 seconds. If the stretching time is too short, the film tends to break, and if it is too long, the equipment load tends to increase. In the case of 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), if necessary, both sides of the film may be heat-treated with a floating dryer or the like. The temperature of this heat treatment is preferably 60 to 200°C, particularly preferably 70 to 150°C. The heat treatment by the floating dryer is a process of blowing hot air, and the heat treatment temperature means the temperature of the hot air.
If the heat treatment temperature is too low, the dimensional stability tends to decrease, and if it is too high, stretchability tends to decrease during the production of the polarizing film.
Also, the heat treatment time is preferably 1 to 60 seconds, particularly preferably 5 to 30 seconds. If the heat treatment time is too short, the dimensional stability tends to decrease, and if it is too long, the stretchability tends to decrease during the production of the polarizing film.

[Polyvinyl alcohol film]
Thus, the polyvinyl alcohol film of the present invention is obtained. This polyvinyl alcohol-based film is long in the machine direction (MD), and is produced as a film roll by being wound around a core tube in the form of a roll.
As described above, the thickness of the polyvinyl alcohol film of the present invention must be very thin, 40 μm or less, and preferably 5 to 40 μm from the viewpoint of thinning the polarizing film. Particularly preferably, it is 10 to 40 μm, more preferably 10 to 38 μm in terms of avoiding breakage.

The width of the polyvinyl alcohol 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 in terms of increasing the area, and further preferably 3 to 50 km in terms of transportation weight.

The polyvinyl alcohol-based film of the present invention is particularly preferably used as a raw material for a polarizing film because of its excellent stretchability.

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

[Method for producing polarizing film]
The polarizing film of the present invention is produced by unwinding the polyvinyl alcohol film from the film roll, conveying it in the horizontal direction, and subjecting it to swelling, dyeing, cross-linking with boric acid, stretching, washing, drying, and the like.

The swelling step is performed before the dyeing step. By the swelling process, stains on the surface of the polyvinyl alcohol film can be cleaned, and the swelling of the polyvinyl alcohol film also has the effect of preventing uneven dyeing. In the swelling step, water is usually used as the treatment liquid. The treatment liquid may contain a small amount of an iodide compound, an additive such as a surfactant, alcohol, etc., as long as the main component is water. The temperature of the 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 the polyvinyl alcohol film into contact with a liquid containing iodine or a dichroic dye. Usually, an iodine-potassium iodide aqueous solution is used, and an iodine concentration of 0.1 to 2 g/L and a potassium iodide concentration of 1 to 100 g/L are suitable. A dyeing time of about 30 to 500 seconds is practical. The temperature of the treatment bath is preferably 5-50°C. The aqueous solution may contain a small amount of an organic solvent compatible with water in addition to the water solvent.

The boric acid cross-linking step is performed using a boron compound 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 potassium iodide is preferably coexistent in the liquid 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. If necessary, a stretching operation may be performed during the treatment.

In the stretching step, the polyvinyl alcohol film is preferably stretched 3 to 10 times, preferably 3.5 to 6 times in the uniaxial direction [machine direction (MD)]. At this time, the film may be slightly stretched in the direction perpendicular to the stretching direction [stretching to such an extent that shrinkage in the width direction (TD) is prevented or more]. The temperature during stretching is preferably 40 to 70°C. Furthermore, the stretching ratio may be finally set within the above range, and the stretching operation may be performed not only in one step (once) but also multiple times in the polarizing film manufacturing process.

The washing step is carried out by, for example, immersing the polyvinyl alcohol film in water or an aqueous solution of iodide such as potassium iodide to remove deposits generated on the surface of the polyvinyl alcohol film. When a potassium iodide aqueous solution is used, the potassium iodide concentration is about 1 to 80 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.

In the drying step, for example, the polyvinyl alcohol film is dried in the atmosphere at 40 to 80° C. for 1 to 10 minutes.

Also, the degree of polarization of the polarizing film is preferably 99.5% or more, more preferably 99.8% or more. If the degree of polarization is too low, there is a tendency that the contrast in the liquid crystal display cannot be ensured.
In addition, the degree of polarization is generally measured in a state where two polarizing films are superimposed so that their orientation directions are in the same direction, and the light transmittance (H ₁₁ ) measured at the wavelength λ, and the two polarized light It is calculated according to the following formula from the light transmittance (H ₁ ) measured at the wavelength λ in a state in which the films are stacked so that the orientation directions are perpendicular to each other.
Degree of polarization (%)=[(H ₁₁ −H ₁ )/(H ₁₁ +H ₁ )] ^1/2

Furthermore, the single transmittance of the polarizing film of the present invention is preferably 42% or more. If the single transmittance is too low, there is a tendency that a liquid crystal display with high brightness cannot be achieved.
Single transmittance is a value obtained by measuring the light transmittance of a single polarizing film using a spectrophotometer.

A method for manufacturing the polarizing plate of the present invention using the polarizing film of the present invention will be described below.
The polarizing film of the present invention is suitable for producing a polarizing plate with little color unevenness and excellent polarizing performance.

[Method for producing 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 polarizing film of the present invention via an adhesive. Examples of protective films include films of cellulose triacetate, cellulose diacetate, polycarbonate, polymethyl methacrylate, cycloolefin polymer, cycloolefin copolymer, polystyrene, polyethersulfone, polyarylene ester, poly-4-methylpentene, polyphenylene oxide, and the like. Or you can get a sheet.

The lamination method is performed by a known method. For example, a liquid adhesive composition is uniformly applied to the polarizing film, the protective film, or both, and then the two are laminated and pressure-bonded, followed by heating or It is performed by irradiating with active energy rays.

A curable resin such as a urethane resin, an acrylic resin, or a urea resin may be applied to one or both sides of the polarizing film and cured to form a cured layer, thereby forming a polarizing plate. By doing so, the hardening layer serves as a substitute for the protective film, and a thin film can be achieved.

The polarizing film and polarizing plate using the polyvinyl alcohol-based film of the present invention have excellent polarizing performance, and are used in personal digital assistants, personal computers, televisions, projectors, signage, electronic desk calculators, electronic clocks, word processors, electronic paper, and game machines. , videos, cameras, photo albums, thermometers, audio, liquid crystal display devices such as automobiles and machinery instruments, sunglasses, anti-glare glasses, stereoscopic glasses, wearable displays, display elements (CRT, LCD, organic EL, electronic paper etc.), optical communication equipment, medical equipment, building materials, toys and the like.

EXAMPLES The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the examples described below as long as the gist of the present invention is not exceeded.

Then, the properties of the polyvinyl alcohol film in Examples and Comparative Examples described later [degree of swelling (S _A , S _B , S _X , S _Y ) and properties of the polarizing film (polarization degree, single transmittance, color unevenness) were measured. And evaluation was performed as follows.

[Method for measuring the degree of swelling (S _A ) of polyvinyl alcohol-based film]
A rectangular film-like sample of 10 cm in the width direction (TD) and 20 cm in the machine direction (MD) is cut out from the central part of the polyvinyl alcohol film in the width direction, and this film-like sample is further cut into a width of 3 mm and a length of 20 cm. Cut into strips. After that, all of these strip-shaped first samples were immersed in 1,000 g of water at 30° C. as they were. After immersion for 30 minutes, the strip-shaped first sample was taken out, centrifuged at 3,000 rpm for 5 minutes using a centrifuge, and the mass "N" after dehydration (total of all strip-shaped samples) was measured. Subsequently, after drying the strip-shaped first sample in a dryer at 105 ° C. for 16 hours, the mass “M” (sum of all strip-shaped first samples) was measured, and the degree of swelling ( _SA ) was calculated. The same measurement was performed three times, and the average value was adopted.
Swelling degree (S _A ) (%) = 100 x N/M

[Method for measuring the degree of swelling (S _B ) of polyvinyl alcohol-based film]
A rectangular second sample of 5 cm in the width direction (TD) and 15 cm in the flow direction (MD) is cut out from the width direction central part of the polyvinyl alcohol film, and the distance between the chucks is 5 cm. After sandwiching both ends in the direction (MD) with a chuck, it was immersed in water at 30 ° C. Immediately in the water, the stretching rate was 3 times (after stretching) at a stretching speed of 0.12 m / min (240% / min). The second sample was uniaxially stretched so that the chuck-to-chuck distance was 15 cm. After stretching, the second sample is taken out of water, and within 1 minute in an environment with a temperature of 20 ° C. and a relative humidity of 65% RH, the moisture on the surface of the second sample after stretching is wiped off using filter paper, and the second sample is was measured. Subsequently, the second sample was dried in a drier at 105°C for 16 hours, then the mass "K" was measured, and the degree of swelling (S _B ) was calculated. The same measurement was performed 5 times, and the average value was adopted.
Degree of swelling ( _SB ) (%) = 100 x L/K

[Method for measuring the degree of swelling (S _x and S _Y ) of polyvinyl alcohol-based film]
A third square sample with a width direction (TD) of 10 cm and a machine direction (MD) of 10 cm is cut out from the width direction central part of the polyvinyl alcohol film, and this third sample is placed on a flat glass plate. The dimensions in the width direction (TD) and the machine direction (MD) were each measured with vernier calipers. Next, the third sample was immersed in an ion exchange water bath adjusted to 30 ° C. for 5 minutes, then taken out, immediately placed on a flat glass plate, and the width direction (TD) and the flow direction of the third sample The dimensions (MD) were each measured with vernier calipers, and the degree of swelling (S _X and S _Y ) was calculated according to the following formula. The above operation was performed in an environment of 23° C. and 50% RH.
Swelling degree (S _x ) (%) = (TD dimension after immersion/TD dimension before immersion) x 100
Degree of swelling ( _SY ) (%) = (MD dimension after immersion/MD dimension before immersion) x 100

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

〔Color unevenness〕
A test piece having a length of 30 cm and a width of 30 cm was cut out from the central portion in the width direction of the obtained polarizing film, and two polarizing plates in a crossed Nicol state (single transmittance 43.5%, degree of polarization 99.9%). After sandwiching it between the two at an angle of 45°, using a light box with a surface illuminance of 14,000 lx, optical color unevenness was observed in transmission mode and evaluated according to the following criteria.
(Evaluation criteria)
A: There was no color unevenness.
Δ: Slight color unevenness was observed.
x: Distinct color unevenness was observed.

<Example 1>
(Preparation of polyvinyl alcohol film)
In a 5,000 L dissolving can, 1,000 kg of polyvinyl alcohol resin having 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 polyoxyl as a surfactant. 0.25 kg of ethylene lauryl amine was added, and the temperature was raised to 150° C. while stirring to perform pressure dissolution. Next, the polyvinyl alcohol-based resin aqueous solution is supplied to a twin-screw extruder and defoamed, then the temperature of the aqueous solution is set to 95 ° C., and the surface temperature is discharged from the T-shaped slit die outlet to a cast drum having a surface temperature of 80 ° C. (discharge speed 1.3 m/min) and casting to form a film. The formed film was peeled off from the cast drum, and dried while being transported in the machine direction (MD) while bringing the front and back surfaces of the film into contact with a total of 10 hot rolls alternately. As a result, a film (width 2 m, thickness 30 μm) with a water content of 7% by weight was obtained. Next, the left and right ends of the film are clamped by clips with a clip pitch of 45 mm, and the film is transported in the machine direction (MD) at a speed of 8 m / min while being stretched in the width direction (TD) at 80 ° C. using a stretching machine. After being stretched 1.2 times, the film was transported in a dryer at a fixed width of 2.4 m and at 130° C. to obtain a polyvinyl alcohol film (width 2.4 m, thickness 25 μm, length 2 km). The properties of the obtained polyvinyl alcohol film were as shown in Table 1 below. Finally, the polyvinyl alcohol-based film was wound around a core tube in the form of a roll to obtain a film-wound body.

(Preparation of polarizing film and polarizing plate)
The obtained polyvinyl alcohol film was unwound from the film roll and stretched 1.7 times in the machine direction (MD) while being transported in the horizontal direction and immersed in a water tank at a water temperature of 30° C. to swell. During the swelling process, no creases or wrinkles occurred in the film. Then, the sheet was immersed in an aqueous solution containing 0.5 g/L of iodine and 30 g/L of potassium iodide at 30° C. and stretched 1.6 times in the machine direction (MD) while being dyed. The film was immersed in an aqueous solution (50° C.) having a composition of L and 30 g/L of potassium iodide and uniaxially stretched 2.1 times in the machine direction (MD) while cross-linking with boric acid. Finally, the film was washed with an aqueous solution of potassium iodide and dried at 50° C. for 2 minutes to obtain a polarizing film with a total draw ratio of 5.8. No breakage occurred during the production of this polarizing film, and the properties of the obtained polarizing film were as shown in Table 1 below.
A 40 μm-thick triacetyl cellulose film was attached to both surfaces of the polarizing film obtained above using an aqueous polyvinyl alcohol solution as an adhesive, and dried at 70° C. to obtain a polarizing plate.

<Example 2>
In Example 1, after stretching 1.4 times in the width direction (TD) at 80 ° C. using a stretching machine, it was shrunk by stress relaxation to a fixed width of 2.4 m (equivalent to 1.2 times stretching) at 130 ° C. and conveyed. A polyvinyl alcohol film (width 2.4 m, thickness 25 μm, length 2 km) was obtained in the same manner as in Example 1 except that The properties of the obtained polyvinyl alcohol film were as shown in Table 1 below.
Furthermore, a polarizing film and a polarizing plate were obtained in the same manner as in Example 1 using the polyvinyl alcohol film. In the swelling process during the production of the polarizing film, the polyvinyl alcohol-based film was neither bent nor wrinkled, nor was it broken. The properties of the obtained polarizing film were as shown in Table 1 below.

<Example 3>
In Example 1, the procedure was repeated except that the polyvinyl alcohol resin aqueous solution was discharged onto a cast drum having a surface temperature of 88° C. (discharging speed: 1.9 m/min) and cast to form a film. A film (width 2 m, thickness 45 μm) was formed. Then, in the same manner as in Example 1, after stretching 1.2 times in the width direction (TD) at 80°C using a stretching machine, the film formed above was dried in a dryer at 135°C with a fixed width of 2.4 m. It was transported to obtain a polyvinyl alcohol film (2.4 m wide, 35 µm thick, 2 km long). The properties of the obtained polyvinyl alcohol film were as shown in Table 1 below.
Furthermore, a polarizing film and a polarizing plate were obtained in the same manner as in Example 1 using the polyvinyl alcohol film. In the swelling process during the production of the polarizing film, the polyvinyl alcohol-based film was neither bent nor wrinkled, nor was it broken. The properties of the obtained polarizing film were as shown in Table 1 below.

<Example 4>
In Example 1, the procedure was repeated except that the polyvinyl alcohol resin aqueous solution was discharged onto a cast drum having a surface temperature of 88° C. (discharging speed: 1.9 m/min) and cast to form a film. A film (width 2 m, thickness 45 μm) was formed. Then, in the same manner as in Example 1, after stretching 1.4 times in the width direction (TD) at 80 ° C. using a stretching machine, stress was applied to a fixed width of 2.4 m (equivalent to 1.2 times stretching) at 135 ° C. It was shrunk by relaxation to obtain a polyvinyl alcohol-based film (width 2.4 m, thickness 34 μm, length 2 km). The properties of the obtained polyvinyl alcohol film were as shown in Table 1 below.
Furthermore, a polarizing film and a polarizing plate were obtained in the same manner as in Example 1 using the polyvinyl alcohol film. In the swelling process during the production of the polarizing film, the polyvinyl alcohol-based film was neither bent nor wrinkled, nor was it broken. The properties of the obtained polarizing film were as shown in Table 1 below.

<Comparative Example 1>
In Example 1, the polyvinyl alcohol-based resin aqueous solution was ejected (discharge speed: 1.3 m/min) and cast onto a cast drum having a surface temperature of 80°C to form a film. Polyvinyl alcohol A system film (width 2 m, thickness 30 μm, length 2 km) was obtained. The properties of the obtained polyvinyl alcohol film were as shown in Table 1 below.
Furthermore, when a polarizing film and a polarizing plate were produced in the same manner as in Example 1 using the polyvinyl alcohol film, folds and wrinkles occurred in the polyvinyl alcohol film during the swelling process. The properties of the obtained polarizing film were as shown in Table 1 below.

<Comparative Example 2>
In Example 1, the polyvinyl alcohol-based resin aqueous solution was ejected (discharge speed: 1.3 m/min) and cast onto a cast drum having a surface temperature of 80°C to form a film. Polyvinyl alcohol A system film (width 2 m, thickness 30 μm, length 2 km) was obtained. The properties of the obtained polyvinyl alcohol film were as shown in Table 1 below.
Furthermore, when a polarizing film and a polarizing plate were produced in the same manner as in Example 1 using the polyvinyl alcohol film, folds and wrinkles occurred in the polyvinyl alcohol film during the swelling process. The properties of the obtained polarizing film were as shown in Table 1 below.

From the results of the above Examples and Comparative Examples, the degree of swelling (S _A ) when the polyvinyl alcohol film was immersed in water at 30° C., the degree of swelling (S _B ) when uniaxially stretched 3 times at 30° C., 30 Polarized light obtained from the polyvinyl alcohol-based films of Examples 1 to 4 in which the degree of swelling (S _x and S _Y ) indicating the spread of the film after being immersed in water at 10° C. satisfies all of the above formulas (1) to (3). It can be seen that the film has high polarizing properties and is uniform without color unevenness.
On the other hand, it can be seen that the polyvinyl alcohol films of Comparative Examples 1 and 2, which do not satisfy the formula (3), are inferior in polarizing properties and color unevenness is observed.

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

The polarizing film made of the polyvinyl alcohol-based film of the present invention has excellent polarizing performance, and is used for personal digital assistants, personal computers, televisions, projectors, signage, electronic desk calculators, electronic clocks, word processors, electronic paper, game machines, videos, Cameras, photo albums, thermometers, audio equipment, liquid crystal display devices for automobiles and machinery instruments, sunglasses, anti-glare glasses, stereoscopic glasses, wearable displays, display elements (CRT, LCD, organic EL, electronic paper, etc.) It is preferably used for reflection reducing layers, optical communication equipment, medical equipment, building materials, toys and the like.

Claims (3)

A film-forming process in which an aqueous solution of a polyvinyl alcohol-based resin is formed into a film by a continuous casting method, and a drying and continuous stretching of the film while conveying the film in the flow direction. and a stretching step , and in the drying and stretching step, the film formed above is stretched 1.1 to 1.3 times in the width direction, and has a thickness of 40 μm or less, which is used as a raw film for manufacturing a polarizing film. A method for producing a long polyvinyl alcohol-based film, wherein the polyvinyl alcohol-based film to be produced satisfies the following formulas (1) to (3). A method for producing a polyvinyl alcohol film used as an anti-film.
190%≦S _A ≦230% (1)
220%≦S _B ≦310% (2)
1.03≦S _Y /S _X ≦1.20 (3)
In the above formula (1), S _A is a rectangular film sample of 10 cm in the width direction (TD) and 20 cm in the machine direction (MD) from the width direction center of the polyvinyl alcohol film. is further cut into strips with a width of 3 mm and a length of 20 cm, and the strip-shaped sample is immersed in water at 30 ° C. for 5 minutes.
In the above formula (2), S _B is a rectangular film-like sample of 5 cm in the width direction (TD) and 15 cm in the machine direction (MD) from the width direction center of the polyvinyl alcohol film. ° C. in water at a stretching speed of 0.12 m / min, and uniaxially stretched so that the stretch ratio is 3 times.
In the above formula (3), _S is a square film-like sample of 10 cm in the width direction (TD) and 10 cm in the machine direction (MD) from the width direction center of the polyvinyl alcohol film. Represents the degree of swelling in the width direction when immersed in water for 5 minutes, and S _Y is a square 10 cm in the width direction (TD) and 10 cm in the machine direction (MD) from the width direction center of the polyvinyl alcohol film. is cut out from a film-like sample, and this sample is immersed in water at 30°C for 5 minutes. In the drying/stretching step, the formed film is temporarily stretched in the width direction by more than 1.3 times, and then dimensionally contracted so that the final stretch ratio in the width direction is 1.3 times or less. 2. The method for producing a polyvinyl alcohol film used as a raw film for producing a polarizing film according to claim 1 , wherein 3. A method for producing a polarizing film, which comprises producing a polarizing film using a polyvinyl alcohol-based film obtained by the production method according to claim 1 or 2 and used as a raw film for producing a polarizing film.