JP2022100289A - Polyvinyl alcohol film for producing polarizing film, manufacturing method therefor, and polarizing film - Google Patents

Abstract

To provide a polyvinyl alcohol film for producing a polarizing film, which features a less amount of a polyvinyl alcohol resin eluted in water during manufacture of a polarizing film and superior stretchability, and allows for highly productively producing polarizing films with little foreign matter defects.SOLUTION: A polyvinyl alcohol film for producing polarizing films is provided, featuring an elasticity modulus of 0.7-2 MPa in water at 50°C.SELECTED DRAWING: None

Description

The present invention relates to a polyvinyl alcohol-based film for producing a polarizing film. More specifically, the present invention provides a polarizing film with few foreign matter defects with good productivity because the elution of impurities into water is small and the stretchability is excellent during the production of the polarizing film. It relates to a polyvinyl alcohol-based film useful as a raw film for producing a polarizing film.

In recent years, the development of liquid crystal display devices has been remarkable, and they are widely used in smartphones, tablets, personal computers, liquid crystal televisions, projectors, in-vehicle panels, and the like. A polarizing film is used in such a liquid crystal display device, and as the polarizing film, a polyvinyl alcohol-based film in which iodine is adsorbed and oriented is mainly used. In recent years, with the increase in definition, brightness, size, and thinning of screens, there is a need for a wide and long polarizing film having better polarization performance than conventional products and having no color unevenness.

Generally, the polarizing film is formed by unwinding a polyvinyl alcohol-based film, which is a raw material, from a roll, swelling it with water (including warm water) while transporting it in the length direction (MD direction), and then dyeing it with iodine. It is manufactured through steps such as stretching for orienting iodine and boric acid cross-linking for fixing the oriented state. The defects that occur during these steps greatly reduce the productivity of the polarizing film. For example, in the swelling step, if impurities are eluted from the polyvinyl alcohol-based film and the swelling tank is contaminated, the contamination will diffuse to the entire post-step. Even in the dyeing step and the boric acid cross-linking step, if impurities are eluted from the polyvinyl alcohol-based film, not only the polarizing performance of the obtained polarizing film deteriorates, but also a great deal of labor is required for filtration and replacement of the chemical solution used in each step. It will be. Examples of such impurities include low molecular weight polyvinyl alcohol resins (including oligomers) present in polyvinyl alcohol films, and in particular, low molecular weight substances having a molecular weight of 50,000 or less are easily eluted in water and have a degree of polarization. There was a tendency to easily form a low molecular weight iodine complex.
Further, the eluted low molecular weight polyvinyl alcohol-based resin can form an aggregate with iodine and boric acid in the polarizing film manufacturing process and become foreign substances. When this foreign matter adheres to the surface of the polarizing film being processed, it becomes a foreign matter defect of the finally obtained polarizing film.

On the other hand, the polyvinyl alcohol-based film, which is the raw material, is generally produced by dissolving a polyvinyl alcohol-based resin as a raw material in water and using such an aqueous solution (film-forming stock solution) by a continuous casting method. Specifically, an aqueous solution of a polyvinyl alcohol-based resin is discharged and cast on a cast mold such as a cast drum or an endless belt to form a film, and the obtained film is peeled from the cast mold and then in the length direction (MD direction). ), While being dried and heat-treated. Among these steps, the drying step is an important step for controlling the elution component from the polyvinyl alcohol-based film, and if the drying method in the drying step is not appropriate, the polyvinyl alcohol-based step is eluted during the production of the polarizing film. The resin tends to increase.

As a method for improving the above-mentioned problems, for example, a polyvinyl alcohol-based film in which the amount of polyvinyl alcohol-based resin eluted in water is reduced (see Patent Document 1) or a polyvinyl alcohol-based film in which polyvinyl alcohol-based resins having different molecular weights are mixed (Patent). Reference 2) has been proposed.

Japanese Unexamined Patent Publication No. 2009-22246 Japanese Unexamined Patent Publication No. 2018-135426

However, in the disclosed technique of Patent Document 1, the eluent concentration of the polyvinyl alcohol-based resin when a 10 cm square film is immersed in 1 liter of water at 50 ° C. for 4 hours is 10 to 50 ppm, but per film m ² . ^In terms of Was needed.
Further, in the technique disclosed in Patent Document 2, although a polyvinyl alcohol-based film having a high critical stretch ratio and good stretchability can be obtained, a mixed low-molecular-weight polyvinyl alcohol-based resin component is eluted during the polarizing film manufacturing process. It was difficult to improve the stretchability and reduce foreign matter defects at the same time because it causes contamination of the chemical solution and an increase in foreign matter defects.

Therefore, in the present invention, under such a background, it is possible to obtain a polarizing film having a small amount of polyvinyl alcohol-based resin eluted into water at the time of manufacturing the polarizing film, excellent stretchability, and few foreign matter defects with good productivity. It is an object of the present invention to provide a polyvinyl alcohol-based film for producing a polarizing film.

However, the present inventors have focused on the relationship between the elastic modulus of the polyvinyl alcohol-based film in water at 50 ° C. and the elution amount and stretchability of the polyvinyl alcohol-based resin with respect to water, and have focused on the relationship between the polyvinyl alcohol-based film in water at 50 ° C. and the like. We have found that when the elastic modulus is within a specific range, the above-mentioned problems can be solved and a polarizing film having high productivity and few foreign matter defects can be produced, and the present invention has been completed.

That is, the gist of the present invention is the following [1] to [4].
[1] A polyvinyl alcohol-based film for producing a polarizing film having an elastic modulus of 0.7 to 2 MPa in water at 50 ° C.
[2] The polyvinyl alcohol-based film for producing a polarizing film according to [1], which has a thickness of 5 to 50 μm.
[3] The method for producing a polyvinyl alcohol-based film for producing a polarizing film according to [1] or [2], wherein an aqueous solution of a polyvinyl alcohol-based resin is continuously discharged and cast in a cast mold to form a film. The film-forming step of obtaining the film, the drying step of drying the film-formed film, and the heat treatment step of heat-treating the dried film are provided.
In the film forming step, the moisture content of the film immediately after being peeled from the cast mold is A [mass%], and in the drying step, the film moisture content 35 seconds after being peeled from the cast mold is B [mass%]. A method for producing a polyvinyl alcohol-based film for producing a polarizing film, which is dried so as to satisfy the following formula (1).
0.23 ≤ B / A ≤ 0.45 ... (1)
[4] A polarizing film obtained by using the polyvinyl alcohol-based film for producing a polarizing film according to [1] or [2].

The polyvinyl alcohol-based film for manufacturing a polarizing film of the present invention has a small amount of polyvinyl alcohol-based resin eluted during manufacturing of the polarizing film and is excellent in stretchability. Therefore, a polarizing film having few foreign matter defects can be produced with high productivity. It is useful as a raw film that can be manufactured.

Hereinafter, the present invention will be described in detail. However, the present invention is not limited to the following embodiments.
In the present invention, the "main component" means a component that has a great influence on the characteristics of the object, and the content of the component is usually 40% by mass or more in the object, preferably 45. By mass or more, more preferably 50% by mass or more. In addition, it is often the component that occupies the largest mass ratio in the object, and when it occupies 50% by mass or more, it is 55% by mass or more, especially 60% by mass or more, and 70% by mass among them. As mentioned above, it is assumed that 80% by mass or more, and 90% by mass or more (including 100% by mass) is occupied among them.
Further, in the present invention, "X and / or Y (where X and Y are arbitrary configurations)" means at least one of X and Y, and 3 of X only, Y only, and X and Y. It means a street.

The polyvinyl alcohol-based film for producing a polarizing film of the present invention (hereinafter, may be simply referred to as “polyvinyl alcohol-based film”) is a polyvinyl alcohol-based film having an elasticity of 0.7 to 2 MPa in water at 50 ° C. The biggest feature is that there is.

The elastic modulus of the polyvinyl alcohol-based film for producing a polarizing film in water at 50 ° C. is preferably 0.74 to 1.90 MPa, more preferably 0.78 to 1.80 MPa, still more preferably 0.82 to 1.75 MPa. It is particularly preferably 0.85 to 1.70 MPa, and particularly preferably 0.87 to 1.60 MPa.
When the elastic modulus in water at 50 ° C. satisfies the above range, the elution amount can be reduced and the stretchability can be improved at the same time, so that a polarizing film having few foreign matter defects can be obtained with good productivity.
The elastic modulus of the polyvinyl alcohol-based film for producing a polarizing film produced by a conventional general production method in water at 50 ° C. is outside the above range, and is usually 2.10 to 2.65.

When the elastic modulus in water at 50 ° C. is less than the lower limit, the stretchability is improved, but the tension at the time of stretching is lowered, the foreign matter defects of the polarizing film are increased, and the productivity is lowered as a result. Further, when it becomes larger than the upper limit, the elution amount decreases and the foreign matter defect of the polarizing film decreases, but the tension at the time of stretching becomes too high, the productivity decreases due to the decrease in stretchability, and the production loss due to fracture occurs. Occurs.

In the present invention, the elastic modulus in the flow direction (MD) in water at 50 ° C. and the elastic modulus in the width direction (TD) in water at 50 ° C. are obtained according to the following method, and then obtained by the square root of the square root of the elastic modulus. The value is the elastic modulus in water at 50 ° C.

(Measurement of elastic modulus of MD in water at 50 ° C)
A test piece is cut out from a polyvinyl alcohol-based film stored at 23 ° C. and 50% RH for 24 hours so that the central portion in the film width direction has a size of TD (5 mm) × MD (25 mm). The measurement width of the test piece is measured with a digital microscope. The thickness of the test piece is measured at any three points with a contact film thickness meter, and the average value thereof is taken as the measured thickness.
Next, after fixing the test piece to the viscoelasticity measuring device (manufactured by IT Measurement Control Co., Ltd., "DVA-225") so that the gripping distance between both ends on the short side (TD side) is 15 mm, the measurement conditions are set. , Set the dynamic strain to 0.3%, the frequency to 10 Hz, and the upper limit elongation rate to 80%, and start the measurement of the elastic modulus. In the measurement, the elastic modulus in the atmosphere is first measured for 100 seconds, and then the elastic modulus in water at 50 ° C. is measured for 600 seconds.
For the measurement results, the average value of the elastic modulus for 200 to 600 seconds from the start of measurement in water at 50 ° C. is obtained, and the value corrected by the thickness after measurement is taken as the elastic modulus of MD in water at 50 ° C. ( The same measurement is performed 3 times and the average value is adopted.)
The thickness after measurement is an average value when the thickness of the test piece after viscoelasticity measurement in water is measured at three points with a contact film thickness meter.

(Measurement of elastic modulus of TD in water at 50 ° C)
The test piece is cut out so as to have a size of TD (25 mm) × MD (5 mm), and the measurement is performed in the same manner as the elastic modulus measurement of MD, and the elastic modulus of TD in water at 50 ° C. is obtained.

As a method for controlling the elasticity in water at 50 ° C. in a specific range, a method for adjusting the molecular weight and the degree of saponification of the polyvinyl alcohol-based resin as a raw material, a method for introducing a trace amount of a modifying group into the polyvinyl alcohol-based resin, and a method for introducing a trace amount of a modifying group into the polyvinyl alcohol-based resin. A method of cleaning a polyvinyl alcohol-based resin to improve its purity, a method of controlling the degree of crystallization and orientation of a polymer in the process of forming a film of a polyvinyl alcohol-based resin aqueous solution, and a method of drying the film during and after film formation. Examples thereof include a method of controlling, and the elastic coefficient can be controlled in a specific range by using these alone or in combination of two or more kinds. Among these, a method of controlling the dry state of the film during and after film formation is preferable, and a method of controlling the dry state of the film by using specific drying conditions described later is particularly preferable.

The polyvinyl alcohol-based film for producing a polarizing film of the present invention contains a polyvinyl alcohol-based resin as a main component. Hereinafter, a method for producing a polyvinyl alcohol-based film for producing a polarizing film of the present invention will be described.
Generally, a polyvinyl alcohol-based film includes a film-forming step of continuously ejecting and casting an aqueous solution of a polyvinyl alcohol-based resin into a cast mold to form a film, and a drying step of drying the formed film. The dried film is manufactured through a heat treatment step of heat-treating the dried film. As will be described later, the manufacturing method of the present invention is the moisture content of the film immediately after being peeled from the cast mold in the film forming step. It is characterized in that B / A is set to a specific range when the ratio is A [mass%] and the moisture content 35 seconds after being peeled from the cast mold in the drying step is B [mass%]. It is a thing.

<Film formation process>
As the polyvinyl alcohol-based resin used 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 vinyl acetate and a copolymerizable component may be used. can. Examples of the component copolymerizable with vinyl acetate include unsaturated carboxylic acids (including, for example, salts, esters, amides, nitriles, etc.) and olefins having 2 to 30 carbon atoms (for example, ethylene, propylene, n-butene). , Isobutene, etc.), vinyl ethers, unsaturated sulfonates, etc. Further, a modified polyvinyl alcohol-based resin obtained by chemically modifying the hydroxyl group after saponification can also be used.

Further, 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, a method for saponifying a copolymer of vinyl acetate and 3,4-diacetoxy-1-butene, vinyl acetate and vinyl ethylene carbonate. Method of saponifying and decarbonizing the copolymer of vinyl acetate, method of saponifying and decarbonizing a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane, vinyl acetate and glycerin. It can be obtained by a method of saponifying a copolymer with monoallyl ether, or 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, and even more preferably 120,000 to 260,000. If the weight average molecular weight is too small, the degree of polarization of the polarizing film tends to decrease, and if it is too large, stretching during manufacturing of the polarizing film tends to be difficult. The weight average molecular weight of the polyvinyl alcohol-based resin is the weight average molecular weight measured by the GPC-MALS method.

The average degree of saponification of the polyvinyl alcohol-based resin used in the present invention is usually preferably 98 mol% or more, particularly preferably 99 mol% or more, still more preferably 99.5 mol% or more, and particularly preferably 99 mol% or more. It is 8 mol% or more. If the average saponification degree is too small, the degree of polarization of the polarizing film tends to decrease.
Here, the average saponification degree 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 kinds having different modified species, modified amounts, weight average molecular weight, average saponification degree and the like may be used in combination.

Using the above polyvinyl alcohol-based resin, an aqueous solution to be a film-forming stock solution is prepared.
First, it is preferable that the above-mentioned polyvinyl alcohol-based resin is washed with water and dehydrated using a centrifuge or the like to obtain a polyvinyl alcohol-based resin wet cake having a water content of 50% by mass or less. If the water content is too high, it tends to be difficult to obtain the desired aqueous solution concentration.
The polyvinyl alcohol-based resin wet cake is dissolved in warm water or hot water to prepare a polyvinyl alcohol-based resin aqueous solution.

The method for preparing the polyvinyl alcohol-based resin aqueous solution is not particularly limited, and for example, it may be prepared using a heated multi-screw extruder, and the melting can provided with the vertical circulation flow generation type stirring blade is described above. It is also possible to put the obtained polyvinyl alcohol-based resin wet cake into the can and blow steam into the can to dissolve and prepare an aqueous solution having a desired concentration.

In addition to the polyvinyl alcohol-based resin, the aqueous solution of the polyvinyl alcohol-based resin includes commonly used plasticizers such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, polyethylene glycol, and trimethylolpropane, and nonions. It is preferable to contain a sexic, anionic, and / or cationic surfactant from the viewpoint of film-forming property of the polyvinyl alcohol-based film. These may be used alone or in combination of two or more.

The resin concentration of the polyvinyl alcohol-based resin aqueous solution thus obtained is preferably 15 to 60% by mass, particularly preferably 17 to 55% by mass, and further preferably 20 to 50% by mass. If the resin concentration of the aqueous solution is too low, the drying load tends to be large and the production capacity tends to decrease, and if it is too high, the viscosity tends to be too high and uniform dissolution tends to be difficult.

In the film-forming step, the polyvinyl alcohol-based resin aqueous solution is discharged and cast into a rotating cast mold to form a film, and then the obtained film is peeled off from the cast mold.

Here, examples of the cast type include a cast drum (drum type roll), an endless belt, a resin film, and the like, but the cast drum is used because of its excellent width, lengthening, and film thickness uniformity. Is preferable.
Hereinafter, the case where the cast type is a cast drum will be described as an example.

First, the obtained polyvinyl alcohol-based resin aqueous solution is defoamed. Examples of the defoaming method include static defoaming and defoaming with a multi-screw extruder having a vent. As the multi-screw extruder having a vent, a twin-screw extruder having a vent is usually used.

After the defoaming treatment, the polyvinyl alcohol-based resin aqueous solution is introduced into the T-shaped slit die in a fixed amount, discharged and cast on a rotating cast drum, and formed into a film by a continuous casting method.

The temperature of the polyvinyl alcohol-based resin aqueous solution at the outlet 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-based resin aqueous solution is too low, the flow tends to be poor, and if the temperature is too high, foaming tends to occur.

The viscosity of the polyvinyl alcohol-based resin aqueous solution is preferably 50 to 200 Pa · s at the time of ejection, and particularly preferably 70 to 150 Pa · s. If the viscosity of the aqueous solution is too low, it tends to cause poor flow, and if it is too high, it tends to be difficult to spread.

The discharge rate of the polyvinyl alcohol-based resin aqueous solution discharged from the T-shaped slit die to the cast drum is preferably 0.1 to 5 m / min, particularly preferably 0.2 to 4 m / min, and even more preferably 0. It is 3 to 3 m / min. If the discharge rate is too slow, the productivity tends to decrease, and if it is too fast, the flow tends to be difficult.

As the cast drum, a drum made of stainless steel (SUS) containing iron as a main component and having a metal plating applied to prevent scratches is usually used. Examples of the metal plating include chrome plating, nickel plating, zinc plating and the like, and these are used alone or in two or more layers. Among these, it is preferable that the outermost surface is chrome-plated from the viewpoint of durability of the drum surface.

The diameter of the cast drum is preferably 2 to 5 m, particularly preferably 2.4 to 4.5 m, and even more preferably 2.8 to 4 m. If the diameter is too small, the drying length tends to be insufficient and the speed tends to be difficult to obtain, and if it is too large, the transportability tends to decrease.

The width of the cast drum is preferably 4 to 7 m, particularly preferably 5 to 6 m. If the width of the cast drum is too narrow, the productivity tends to decrease, and if the width is too wide, the equipment load tends to increase.

The rotation speed of the cast drum is preferably 3 to 50 m / min, particularly preferably 4 to 40 m / min, and even more preferably 5 to 35 m / min. If the rotation speed is too slow, the productivity tends to decrease, and if it is too fast, the peelability when the film obtained by forming the film is peeled from the cast mold tends to decrease.

The surface temperature of the cast drum is preferably 40 to 99 ° C, more preferably 50 to 98 ° C, still more preferably 60 to 97 ° C, particularly preferably 70 to 96 ° C, and particularly preferably 80 to 95 ° C. Is. If the surface temperature is too low, the peelability when peeling the film obtained by forming the film from the cast mold tends to decrease, and if it is too high, foaming tends to occur.

Thus, the film is formed, and the obtained film is peeled off from the cast drum. The film moisture content at this time is defined as A [mass%].

The method for adjusting the water content A [mass%] of the film immediately after being peeled from the cast mold is not particularly limited to the following specific examples, but for example, an aqueous solution of a polyvinyl alcohol-based resin and an organic solvent can be used. A method of adding a plasticizer or a surfactant, a method of adjusting the resin concentration of a polyvinyl alcohol-based resin aqueous solution, a method of adjusting the temperature of a polyvinyl alcohol-based resin aqueous solution, a method of adjusting the discharge rate of a polyvinyl alcohol-based resin aqueous solution, a cast Method to adjust the rotation speed and surface temperature of the drum, method to irradiate infrared rays, method to process under reduced pressure atmosphere, method to blow air, method to blow hot air, method to adjust hot air temperature and hot air velocity, under pressurized atmosphere A method of treating with a humidified atmosphere, a method of spraying water vapor, and the like can be mentioned.
The water content A [mass%] is measured by a method described later.

<Drying process>
Next, a drying step of drying the obtained film will be described.

In the present invention, the following formula is used when the moisture content of the film immediately after being peeled from the cast mold is A [mass%] and the moisture content of the film 35 seconds after being peeled from the cast mold is B [mass%]. Drying so as to satisfy 1) is preferable in that the elastic modulus in water at 50 ° C. can be adjusted in a suitable range, and the B / A value is particularly preferably the following formula (1'), more preferably. Is to satisfy the following equation (1 ″).
0.23 ≤ B / A ≤ 0.45 ... (1)
0.235 ≤ B / A ≤ 0.40 ... (1')
0.24 ≤ B / A ≤ 0.35 ... (1 ")

If the B / A value is too low, the elastic modulus in water at 50 ° C. tends to decrease too much, and if it is too high, the elastic modulus in water at 50 ° C. tends to increase too much.
In a general method for producing a polyvinyl alcohol-based film for producing a polarizing film, mild drying conditions are preferred to the production method of the present invention from the viewpoint of production stability, and the B / A is usually 0.49. It is about 0.55 and does not satisfy the above formula (1).

The method for controlling the film moisture content B [mass%] 35 seconds after the film is peeled from the cast mold (cast drum) is not limited to the following specific examples, but is, for example, a polyvinyl alcohol type. A method of adding an organic solvent, a plasticizer or a surfactant to an aqueous solution of a resin, a method of contacting a film with multiple metal heating rolls, a method of introducing the film into a heating oven, a method of irradiating infrared rays, a method of treating under a reduced pressure atmosphere. , A method of blowing air, a method of blowing hot air, a method of adjusting hot air temperature and hot air velocity, a method of treating in a pressurized atmosphere, a method of treating in a humid atmosphere, a method of blowing steam, and the like.

<Heat treatment process>
In the present invention, it is preferable to heat-treat the dried film, and it is particularly preferable to heat-treat from both sides of the film in that the dried state of both sides of the film can be made uniform. Examples of such a heat treatment method include a method of blowing hot air on both sides of a film using a floating dryer and a method of irradiating both sides of a film with near infrared rays using an infrared lamp.
The heat treatment temperature is preferably 50 to 150 ° C, particularly preferably 70 to 120 ° C. The heat treatment time is not particularly limited, but when a floating dryer is used, it is preferably 10 to 100 seconds, and particularly preferably 20 to 80 seconds. The heat treatment method in the present invention is a method that does not use a heat roll.

The water content of the film after the heat treatment step is preferably 5.0% by mass or less, particularly preferably 0.1 to 4.5% by mass, still more preferably 0.2 to 4.0% by mass, and particularly preferably 0. .3 to 3.5% by mass.
If the water content is too high, the finally obtained polyvinyl alcohol-based film tends to be poorly dried.

After drying, the film that has been heat-treated as necessary is slit at both ends in the width direction and wound up on a roll to form a product (polyvinyl alcohol-based film).

The length of the polyvinyl alcohol-based film thus obtained is preferably 5 km or more from the viewpoint of increasing the area of the polarizing film, and particularly preferably 5 to 50 km from the viewpoint of the transport mass.

The width of the polyvinyl alcohol-based film is preferably 4 m or more in terms of widening the polarizing film, particularly preferably 5 m or more, and further preferably 5 to 6 m in terms of avoiding breakage during manufacturing of the polarizing film.

The polyvinyl alcohol-based film preferably has a thickness of 5 to 50 μm, and is particularly preferably 10 to 50 μm, more preferably 15 to 45 μm in terms of thinning.

Up to this point, the method of producing a polyvinyl alcohol-based film has been described by taking the case of using a cast drum (drum type roll) as a cast type as an example, but the method of producing a polyvinyl alcohol-based film has been described. Is also possible.

Thus, the polyvinyl alcohol-based film for producing a polarizing film of the present invention can be obtained.
The polyvinyl alcohol-based film for producing a polarizing film of the present invention is particularly preferably used as a raw fabric for a polarizing film because the polyvinyl alcohol-based resin is less eluted when immersed in water and has excellent stretchability.

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

The polarizing film of the present invention is produced by unwinding the polyvinyl alcohol-based film for manufacturing a polarizing film from a roll, transferring it in the horizontal direction, and performing steps such as swelling, dyeing, boric acid cross-linking, stretching, washing, and drying. ..

The swelling step is performed before the dyeing step. In addition to being able to clean the surface of the polyvinyl alcohol-based film for manufacturing a polarizing film by the swelling step, there is also an effect of preventing uneven dyeing by swelling the polyvinyl alcohol-based film for manufacturing a polarizing film. In the swelling step, water is usually used as the treatment liquid. If the main component of the treatment liquid is water, the treatment liquid may contain a small amount of an iodide compound, additives such as a surfactant, alcohol and the like. 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 step is performed by contacting the film with a liquid containing iodine or a dichroic dye. Usually, an aqueous solution of iodine-potassium iodide 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. It is practical that the 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 cross-linking step is carried out using a boron compound such as boric acid or borax. The boron compound is used in the form of an aqueous solution or a water-organic solvent mixed solution at a concentration of about 10 to 100 g / L, and it is preferable that potassium iodide coexists in the solution from the viewpoint of stabilizing the polarization performance. The temperature at the time of treatment is preferably about 30 to 70 ° C., the treatment time is preferably about 0.1 to 20 minutes, and if necessary, a stretching operation may be performed during the treatment.

In the stretching step, it is preferable to stretch 3 to 10 times, preferably 3.5 to 6 times in the uniaxial direction. At this time, some stretching (stretching to the extent of preventing shrinkage in the width direction or more) may be performed in the direction perpendicular to the stretching direction. The temperature at the time of stretching is preferably 30 to 170 ° C. Further, the stretching ratio may be finally set in the above range, and the stretching operation may be performed not only in one step but also in any range of the manufacturing process.

The washing step is performed, for example, by immersing a polyvinyl alcohol-based film for producing a polarizing film in an iodide aqueous solution such as water or potassium iodide, and deposits generated on the surface of the film can be removed. When an aqueous potassium iodide solution is used, the potassium iodide concentration may be about 1 to 80 g / L. The temperature during the cleaning 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. In addition, washing with water and washing with an aqueous solution of potassium iodide may be performed in an appropriate combination.

The drying step may be carried out in the air at 40 to 80 ° C. for 1 to 10 minutes.

The degree of polarization of the polarizing film is preferably 99.8% or more, more preferably 99.9% or more. If the degree of polarization is too low, it tends to be impossible to secure the contrast in the liquid crystal display.
The degree of polarization is generally the light transmittance (H ₁₁ ) measured at the wavelength λ and the two polarizations in a state where two polarizing films are superposed so that their orientation directions are in the same direction. It is calculated according to the following equation from the light transmittance (H ₁ ) measured at the wavelength λ in a state where the films are superposed so that the orientation directions are orthogonal to each other.
[(H ₁₁ -H ₁ ) / (H ₁₁ + H ₁ )] ^1/2

Further, the simple substance transmittance of the polarizing film of the present invention is preferably 42% or more, more preferably 43% or more. If the single transmittance is too low, it tends to be impossible to achieve high brightness of the liquid crystal display.
The simple substance transmittance is a value obtained by measuring the light transmittance of a single polarizing film using a spectrophotometer.

Thus, the polarizing film of the present invention can be obtained, and the polarizing film of the present invention is suitable for producing a polarizing plate having few foreign matter defects and no color unevenness with good productivity.
Hereinafter, the method for manufacturing the polarizing plate of the present invention will be described.

The polarizing film of the present invention is formed by laminating an optically isotropic resin film as a protective film on one side or both sides thereof via an adhesive to form a polarizing plate. Examples of the protective film include films such as cellulose triacetate, cellulose diacetate, polycarbonate, polymethylmethacrylate, cycloolefin polymer, cycloolefin copolymer, polystyrene, polyether sulfone, polyarylene ester, poly-4-methylpentene, and polyphenylene oxide. Or a sheet.

The bonding method is a known method. For example, a liquid adhesive composition is uniformly applied to a polarizing film, a protective film, or both, and then the two are bonded and pressure-bonded to be heated or activated. It is performed by irradiating with energy rays.

Further, for the purpose of thinning the polarizing film, a curable resin such as a urethane resin, an acrylic resin, or a urea resin is applied to one or both sides of the protective film instead of the protective film, and the film is cured to form a polarizing plate. You can also do it.

The polarizing film or polarizing plate made of the polyvinyl alcohol-based film for manufacturing the polarizing film of the present invention has excellent polarizing performance, and is used for portable information terminals, personal computers, televisions, projectors, signage, electronic desk computers, electronic clocks, word processors, and electronics. Liquid crystal displays such as paper, game machines, videos, cameras, photo albums, thermometers, audio, automobile and mechanical instruments, sunglasses, anti-glare glasses, stereoscopic glasses, wearable displays, display elements (CRT, LCD, organic) It is preferably used for antireflection layers for (EL, electronic paper, etc.), optical communication equipment, medical equipment, building materials, toys, and the like.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples as long as the gist of the present invention is not exceeded.
In the example, "part" means a mass reference.

<Example 1>
(Preparation of polyvinyl alcohol film)
Add 2000 kg of polyvinyl alcohol resin having an average molecular weight of 132000 and an average degree of saponification of 99.8 mol%, 5000 kg of water, and 220 kg of glycerin as a plasticizing agent. The concentration was adjusted to 25% by mass to obtain a uniformly dissolved polyvinyl alcohol-based resin aqueous solution. Next, the polyvinyl alcohol-based resin aqueous solution is supplied to a twin-screw extruder having a vent to defoam, then the aqueous solution temperature is set to 95 ° C., and the aqueous solution is discharged from a T-shaped slit die discharge port to a rotating cast drum (discharge speed). 2.5 m / min) and cast to form a film. Then, the B / A value obtained from the moisture content A [mass%] of the film immediately after being peeled from the cast mold and the moisture content B [mass%] of the film 35 seconds after being peeled from the cast drum is 0. After drying while adjusting the moisture content of the film so as to be .248, hot air at 90 ° C. was blown from both sides of the film using a floating dryer to perform heat treatment. Finally, both ends were slit and wound around a roll to obtain a polyvinyl alcohol-based film having a thickness of 45 μm, a width of 5 m, and a length of 5 km. The characteristics of the obtained polyvinyl alcohol-based film are shown in Table 1.

(Manufacturing of polarizing film)
The obtained polyvinyl alcohol-based film is unwound from the roll, transported in the horizontal direction, immersed in a water tank having a water temperature of 30 ° C. and swollen, and stretched 1.7 times in the flow direction (MD) based on the original fabric. did. Next, while immersing in an aqueous solution (liquid temperature 30 ° C.) containing 0.5 g / L of iodine and 30 g / L of potassium iodide and dyeing, the original fabric is used as a reference in the flow direction (MD direction). It is stretched to a ratio of 0.7 times, and further immersed in an aqueous solution having a composition of 40 g / L of boric acid and 30 g / L of potassium iodide (liquid temperature 55 ° C., the treatment tank is called a stretching tank) for boric acid cross-linking. However, it was uniaxially stretched in the flow direction (MD direction) until it became 5.7 times based on the original original fabric. Finally, it was washed with an aqueous potassium iodide solution and dried at 70 ° C. for 2 minutes to obtain a polarizing film having a total draw ratio of 5.7 times. The characteristics of the obtained polarizing film are shown in Table 1.

<Examples 2 to 3>
Table 1 shows the B / A values obtained from the moisture content A [mass%] of the film immediately after being peeled from the cast mold and the moisture content B [mass%] of the film 35 seconds after being peeled from the cast mold. A polyvinyl alcohol-based film was obtained in the same manner as in Example 1 except that the film was changed as shown in. Table 1 shows the characteristics of the obtained polyvinyl alcohol-based film and the characteristics of the polarizing film obtained in the same manner as in Example 1.

<Example 4>
A polyvinyl alcohol-based resin having a weight average molecular weight of 156000 was used, the water content of the film immediately after being peeled from the cast mold A [mass%], and the water content of the film 35 seconds after being peeled from the cast mold. A polyvinyl alcohol-based film was obtained in the same manner as in Example 1 except that the value of B / A obtained from B [% by mass] was changed as shown in Table 1. Table 1 shows the characteristics of the obtained polyvinyl alcohol-based film and the characteristics of the polarizing film obtained in the same manner as in Example 1.

<Comparative Examples 1 and 2>
Table 1 shows the B / A values obtained from the moisture content A [mass%] of the film immediately after being peeled from the cast mold and the moisture content B [mass%] of the film 35 seconds after being peeled from the cast mold. A polyvinyl alcohol-based film was obtained in the same manner as in Example 1 except that the changes were shown in 1. Table 1 shows the characteristics of the obtained polyvinyl alcohol-based film and the characteristics of the polarizing film obtained in the same manner as in Example 1.

<Elastic modulus in water at 50 ° C. (MPa)>
According to the following method, the elastic modulus in the flow direction (MD) in water at 50 ° C. and the elastic modulus in the width direction (TD) in water at 50 ° C. are obtained, and then the value obtained by the square root of the square of them is calculated at 50 ° C. The elastic modulus in water was used.
(Measurement of elastic modulus of MD in water at 50 ° C)
A test piece was cut out from a polyvinyl alcohol-based film stored at 23 ° C. and 50% RH for 24 hours so that the central portion in the film width direction had a size of TD (5 mm) × MD (25 mm). The measurement width of the test piece was measured with a digital microscope. The thickness of the test piece was measured at any three points with a contact film thickness meter, and the average value was taken as the measured thickness.
Next, after fixing the test piece to the viscoelasticity measuring device (manufactured by IT Measurement Control Co., Ltd., "DVA-225") so that the gripping distance between both ends on the short side (TD side) is 15 mm, the measurement conditions are set. , The dynamic strain was set to 0.3%, the frequency was set to 10 Hz, and the upper limit elongation rate was set to 80%, and the measurement of the elastic modulus was started. In the measurement, the elastic modulus in the atmosphere was first measured for 100 seconds, and then the elastic modulus in water at 50 ° C. was measured for 600 seconds.
Regarding the measurement results, the average value of the elastic modulus for 200 to 600 seconds from the start of measurement in water at 50 ° C. was obtained, and the value corrected by the thickness after measurement was taken as the elastic modulus of MD in water at 50 ° C. ( The same measurement was performed 3 times and the average value was adopted.)
The thickness after measurement was taken as an average value when the thickness of the test piece after viscoelasticity measurement in water was measured at three points with a contact film thickness meter.

(Measurement of elastic modulus of TD in water at 50 ° C)
The test piece was cut out so as to have a size of TD (25 mm) × MD (5 mm), and the measurement was carried out in the same manner as the elastic modulus measurement of MD, and the elastic modulus of TD in water at 50 ° C. was determined.

<Moisture ratio B / A>
[Moisture content A [mass%] of the film immediately after being peeled from the cast mold]
About the film immediately after being peeled from the cast mold, 20% when one end is 0%, the center is 50%, and the other end is 100% with respect to the total width of the film in three places in the width direction. , 50% (center), 80% positions), and three films were sampled, and the initial mass A ₀ (g) was measured for each. Next, the moisture content A ₂ [%] was calculated from the mass A ₁ (g) after drying these films in a vacuum dryer at 83 ° C. for 20 minutes by the following formula (i).
Moisture content A ₂ [mass%] = 100 x (A ₀ -A ₁ ) / A ₀ ... (i)
The average value of the moisture content of a total of 9 films (3 locations x 3 films) obtained by the above formula (i) was determined as the moisture content A [mass%] of the film immediately after being peeled from the cast mold. ..

[Moisture content B [mass%] of the film 35 seconds after being peeled from the cast mold]
For the film at the point 35 seconds after being peeled from the cast mold, 3 points in the width direction (0% at one end, 50% at the center, and 100% at the other end with respect to the total width of the film). Occasionally, three films were sampled from 20%, 50% (center), and 80% positions), and the initial mass B ₀ (g) was measured for each. Next, the moisture content B ₂ [mass%] was calculated from the mass B ₁ (g) after drying these films in a vacuum dryer at 83 ° C. for 20 minutes by the following formula (ii).
Moisture content B ₂ [mass%] = 100 x (B ₀ -B ₁ ) / B ₀ ... (ii)
The average value of the moisture content of a total of 9 films (3 locations x 3 films) obtained by the above formula (ii) was taken as the moisture content B [mass%] of the film 35 seconds after being peeled from the cast mold. I asked.
The time 35 seconds after peeling from the cast mold is the time calculated with 0 seconds as the point where the film was sampled when determining the moisture content A [mass%] of the film immediately after being peeled from the cast mold. The point located 35 seconds after being peeled off from the cast mold can be easily identified by marking the film surface in advance.

<Resin elution amount (ppm / m ² )>
The obtained polyvinyl alcohol-based film was humidified at 23 ° C. and 50% RH for 24 hours, then five 100 mm × 100 mm (0.01 m ² ) test pieces were cut out, and the five test pieces were cut out one by one. An operation of immersing in 1 L of ion-exchanged water at 50 ° C. for 1 minute was performed to obtain an eluate. To 10 mL of the eluate, 10 mL of a color-developing reagent (a reagent consisting of 500 g of ion-exchanged water, 7.4 g of potassium iodide, 0.65 g of iodine, and 10.6 g of boric acid) was added, mixed at room temperature (23 ° C.), and then spectroscopically analyzed. The absorbance at a wavelength of 690 nm is measured using a photometer (manufactured by Shimadzu Corporation, "UV-3600Plus"), the concentration (ppm) of the polyvinyl alcohol-based resin is calculated from a calibration curve prepared in advance, and elution is performed by area conversion. The amount was (ppm / m ² ).
(Evaluation criteria)
○ ・ ・ ・ 900ppm / m ² or less × ・ ・ ・ Larger than 900ppm / m ²

<Stretchability>
The obtained polyvinyl alcohol-based film is unwound from the roll, transported in the horizontal direction, immersed in a water tank having a water temperature of 30 ° C. and swollen, and stretched 1.7 times in the flow direction (MD) based on the original fabric. did. Next, while immersing in an aqueous solution (liquid temperature 30 ° C.) containing 0.5 g / L of iodine and 30 g / L of potassium iodide and dyeing, the original fabric is used as a reference in the flow direction (MD direction). It is stretched to a ratio of 0.7 times, and further immersed in an aqueous solution having a composition of 40 g / L of boric acid and 30 g / L of potassium iodide (liquid temperature 55 ° C., the treatment tank is called a stretching tank) for boric acid cross-linking. However, the tension (N) when uniaxially stretched to 5.7 times with respect to the original original fabric in the flow direction (MD direction) was measured. The value obtained by dividing the tension by the width (mm) of the polarizing film being processed at the outlet of the stretching tank was defined as the 5.7-fold stretching tension (N / mm).
The stretchability of the 5.7-fold stretching tension (N / mm) was evaluated according to the following evaluation criteria.
(Evaluation criteria)
○ ・ ・ ・ 0.70N / mm or less × ・ ・ ・ Larger than 0.70N / mm

<Contaminability of treatment tank>
Collect 10 mL each of the chemical solution in the stretching tank before processing the polarizing film and the chemical solution in the stretching tank after processing the obtained polyvinyl alcohol-based film to 5000 m, and use a coloring reagent (ion-exchanged water 500 g, potassium iodide) for each chemical solution sample. Add 10 mL of a reagent consisting of 7.4 g, 0.65 g of iodine, and 10.6 g of boric acid), mix at room temperature, and then use a spectrophotometer (“UV-3600Plus” manufactured by Shimadzu Corporation) to a wavelength of 690 nm. Absorbance was obtained.
The difference in the absorbance of the chemical solution before and after the polarizing film processing is defined as ΔI, and the larger ΔI is, the more the chemical solution is contaminated in the stretching tank, while the smaller the ΔI is, the less the chemical solution is contaminated. Evaluated according to the criteria.
(Evaluation criteria)
〇 ・ ・ ・ 0 ≦ ΔI <0.040
Δ ・ ・ ・ 0.040 ≦ ΔI <0.060
× ・ ・ ・ 0.060 ≦ ΔI

<Number of foreign matter defects in the polarizing film>
A test piece having a length of 30 cm and a width of 30 cm was cut out from the obtained polarizing film and fixed at a position 10 cm in front of the screen. Light was applied to the polarizing film in a dark room using a floodlight, and the number of defects projected on the screen was counted.
(Evaluation criteria)
〇 ・ ・ ・ 0 to 2 pieces △ ・ ・ ・ 3 to 4 pieces × ・ ・ ・ 5 pieces or more

<Color unevenness>
A test piece having a length of 30 cm and a width of 30 cm was cut out from the obtained polarizing film, and 45 ° was formed between two polarizing plates in a cross-nicoled state (single transmittance 43.5%, polarization degree 99.9%). After sandwiching the particles at an angle, optical color unevenness was observed in a transmission mode using a light box having a surface illuminance of 14,000 lp, and evaluation was performed according to the following criteria.
(Evaluation criteria)
○ ・ ・ ・ No color unevenness × ・ ・ ・ Color unevenness

The polyvinyl alcohol-based films of Examples 1 to 4 have an elastic coefficient in water at 50 ° C. within the range specified in the present invention, a small amount of polyvinyl alcohol-based resin eluted during the production of the polarizing film, and excellent stretchability. It can be seen that the productivity is good because the processing tank is not contaminated in the polarizing film manufacturing process, there are few foreign matter defects in the obtained polarizing film, and there is no color unevenness.
On the other hand, the polyvinyl alcohol-based film of Comparative Example 1 has an elasticity in water at 50 ° C. of less than the lower limit specified in the present invention, so that the treatment tank is contaminated in the polarizing film manufacturing process, and the obtained polarizing film is contaminated. Productivity is poor due to many foreign matter defects and uneven color. Further, in the polyvinyl alcohol-based film of Comparative Example 2, since the elastic modulus in water at 50 ° C. exceeds the upper limit value specified in the present invention, the stretchability is remarkably deteriorated, and the yield of the polarizing film is lowered. Productivity is poor because color unevenness, which is considered to be caused by non-uniform strain during stretching, also occurs.

The polarizing film or polarizing plate made of the polyvinyl alcohol-based film for manufacturing the polarizing film of the present invention has excellent polarizing performance, and is used for portable information terminals, personal computers, televisions, projectors, signage, electronic desk computers, electronic clocks, word processors, and electronics. Liquid crystal displays such as paper, game machines, videos, cameras, photo albums, thermometers, audio, automobile and mechanical instruments, sunglasses, anti-glare glasses, stereoscopic glasses, wearable displays, display elements (CRT, LCD, organic) It is preferably used for antireflection layers for (EL, electronic paper, etc.), optical communication equipment, medical equipment, building materials, toys, and the like.

Claims (4)

A polyvinyl alcohol-based film for producing a polarizing film, which has an elastic modulus of 0.7 to 2 MPa in water at 50 ° C. The polyvinyl alcohol-based film for producing a polarizing film according to claim 1, wherein the film has a thickness of 5 to 50 μm. The method for producing a polyvinyl alcohol-based film for producing a polarizing film according to claim 1 or 2, wherein an aqueous solution of the polyvinyl alcohol-based resin is continuously discharged and cast in a cast mold to form a film to obtain a film. The film is provided with a drying step of drying the formed film and a heat treatment step of heat-treating the dried film.
In the film forming step, the moisture content of the film immediately after being peeled from the cast mold is A [mass%], and in the drying step, the film moisture content 35 seconds after being peeled from the cast mold is B [mass%]. A method for producing a polyvinyl alcohol-based film for producing a polarizing film, which comprises drying the film so as to satisfy the following formula (1).
0.23 ≤ B / A ≤ 0.45 ... (1) A polarizing film obtained by using the polyvinyl alcohol-based film for producing a polarizing film according to claim 1 or 2.