WO2016148160A1 - Polyvinyl alcohol film, method for producing polyvinyl alcohol film, polarizing film, and polarizing plate - Google Patents

Abstract

Provided are: a polyvinyl alcohol film having few variations in thickness, with which it is possible to obtain a polarizing film having exceptional polarization properties; and a polarizing film. A polyvinyl alcohol film having a thickness of 5-60 μm, a width of 2 m or greater, and a length of 2 km or greater, wherein the polyvinyl alcohol film is characterized by having a coefficient of variation in thickness over the entire film surface of 1% or less.

Description

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

The present invention relates to a polyvinyl alcohol film. More specifically, the present invention relates to a polyvinyl alcohol film capable of obtaining a polarizing film having little thickness variation and excellent polarization characteristics, a method for producing such a polyvinyl alcohol film, a polarizing film, and a polarizing plate.

Conventionally, a polyvinyl alcohol film is prepared by dissolving a polyvinyl alcohol resin in a solvent such as water to prepare a stock solution, forming a film by a solution casting method (casting method), and drying using a metal heating roll or the like. It is manufactured by doing. The polyvinyl alcohol film thus obtained is used in many applications as a film excellent in transparency and dyeability, and one of its useful applications is a polarizing film. Such a polarizing film is used as a basic constituent element of a liquid crystal display, and in recent years, its use has been expanded to equipment requiring high quality and high reliability.

Under such circumstances, with the enlargement of the screen of a liquid crystal television or the like, there is a need for a polarizing film that is more flat and wider than conventional products, and a polyvinyl alcohol film that is a raw material. . If the polyvinyl alcohol film is not flat due to thickness variation, the thickness variation of the polarizing film also increases, resulting in insufficient in-plane uniformity of polarization performance or stress in the assembled liquid crystal cell. This causes deformation of the liquid crystal cell itself and deterioration of durability. As a countermeasure against such a problem, for example, it has been proposed to use a polyvinyl alcohol film having a thickness variation in the TD direction (width direction) of 0.5 μm / mm or less for manufacturing a polarizing film (for example, Patent Documents). 1). Moreover, in order to suppress thickness fluctuation | variation, the method of forming into a film using the die | dye and metal roll which have a specific positional relationship in the polyvinyl alcohol-type film is proposed (for example, refer patent document 2).

JP 2002-31720 A JP 2002-144355 A

However, in the above-mentioned Patent Document 1, the thickness variation in the TD direction is 0.5 μm or less per 1 mm length, but it is a variation value within a length of 1 mm, and especially the total width is 2 m or more. In the case of a wide film, such a fluctuation value is considered to increase greatly. Further, in the case of a long film having a total length of 2 km or more, the thickness variation in the TD direction is further increased.
Moreover, even if only the thickness variation in the TD direction is reduced, when the film is unwound from the polyvinyl alcohol film roll to produce a polarizing film, because of the thickness variation in the MD direction (flow direction: longitudinal direction), There is a problem that uniform treatment becomes difficult in each step such as dyeing, stretching, and boric acid treatment, and the degree of polarization becomes uneven in the plane of the polarizing film.

In the example of Patent Document 2, the thickness variation in the TD direction of the film is 1.5 μm. However, the thickness of the film is as large as 75 μm, and it is difficult to cope with the thinning of the polarizing film, which is 60 μm or less. However, when the thickness is reduced, there is a problem that sufficient thickness accuracy cannot be secured. When the film thickness is 30 μm, the thickness variation coefficient is about 0.8% when the film thickness variation is 1.5 μm.
Further, as in the case of Patent Document 1 described above, even when only the thickness variation in the TD direction is reduced, when the film is unwound from the polyvinyl alcohol film roll to produce a polarizing film, the thickness in the MD direction is reduced. Due to the fluctuations, there is a problem that uniform treatment becomes difficult in each step such as dyeing, stretching, and boric acid treatment, and the degree of polarization becomes uneven in the plane of the polarizing film.

In addition, when the thickness variation of the polyvinyl alcohol film is large and uneven, it is difficult to wind it on a roll, and when the film absorbs moisture during storage and transportation, the swell increases and the film is produced before the polarizing film is produced. There was a problem that most of it had to be destroyed.

Therefore, in the present invention, under such a background, a polyvinyl alcohol film capable of obtaining a polarizing film with little thickness variation and excellent polarization characteristics, a method for producing such a polyvinyl alcohol film, a polarizing film, and a polarizing plate are provided. It is intended to provide.

However, as a result of intensive studies in view of such circumstances, the present inventors have focused on the film thickness of the polyvinyl alcohol film and controlled the coefficient of variation so that a polarizing film having excellent polarization characteristics can be obtained. As a result, the present invention was completed.

That is, the first gist of the present invention is a polyvinyl alcohol film having a thickness of 5 to 60 μm, a width of 2 m or more, and a length of 2 km or more, wherein the variation coefficient of the thickness of the entire film surface is 1% or less. It relates to a polyvinyl alcohol film.

The second gist of the present invention is that a polyvinyl alcohol film obtained by discharging an aqueous solution of a polyvinyl alcohol resin onto a cast drum rotating from a T-shaped slit die and continuously drying the film. The present invention relates to a method for producing a polyvinyl alcohol film, characterized in that the coefficient of variation in thickness over the entire film surface is 1% or less.
Furthermore, the third gist of the present invention relates to a polarizing film comprising the polyvinyl alcohol film.
And the 4th summary of this invention is related with the polarizing plate which provides a protective film in the at least single side | surface of the said polarizing film.

Since the polyvinyl alcohol film of the present invention is excellent in thickness accuracy, a polarizing film excellent in polarizing properties can be obtained, and is particularly preferably used as a raw material for a thin and long polarizing film, and has a degree of polarization. An in-plane uniform polarizing film can be obtained.

FIG. 1 is an explanatory view showing a discharge state (flight time) of a polyvinyl alcohol-based resin aqueous solution from a T-shaped slit die discharge port to the cast drum surface. FIG. 2 is an explanatory view showing a landing state of the polyvinyl alcohol resin aqueous solution on the cast drum surface. FIG. 3 is an explanatory view showing the discharge direction of the aqueous polyvinyl alcohol resin solution from the T-shaped slit die to the cast drum.

The polyvinyl alcohol film of the present invention is a polyvinyl alcohol film having a thickness of 5 to 60 μm, a width of 2 m or more, and a length of 2 km or more, wherein the variation coefficient of the thickness of the entire film surface is 1% or less. It is a polyvinyl alcohol film.

Here, the coefficient of variation is obtained by the following method.
<Measurement method>
First, the thickness of the arbitrary location of a polyvinyl alcohol-type film is measured.
・ Measurement of the flow direction (MD direction) was performed at the center and both ends in the width direction (TD direction) in increments of 0.3 mm using a Keyence Corporation “spectral interference film thickness meter SI-T80”. Measure all points.
・ Measurement in the TD direction is performed by using a continuous film thickness meter TOF-5R01 manufactured by Yamabun Denki Co., Ltd., and measuring 4000 points at each of the front, center, and end portions in the MD direction.
Next, the standard deviation is calculated from the following formula for the measured thickness value.

Finally, the coefficient of variation is calculated from the following equation.
Coefficient of variation (CV) = 100 x SD / x

In the present invention, “on the entire surface of the film” means measurement with respect to the total length and the entire width. Specifically, in the MD direction, at least three places in the center and both ends, and in the center and both ends in the TD direction. It means at least 3 places.

The polyvinyl alcohol film of the present invention is required to have a thickness variation coefficient of 1% or less by the above method, preferably 0.7% or less, particularly preferably 0.5% or less, and still more preferably 0.8%. 4% or less. When the variation coefficient of thickness is larger than 1%, the performance of the polarizing film is deteriorated and the object of the present invention cannot be achieved.
The lower limit of the coefficient of variation of the thickness is usually 0.01%, preferably 0.05%, particularly preferably 0.1%.

The polyvinyl alcohol-based film of the present invention as described above is manufactured by discharging an aqueous solution of a polyvinyl alcohol-based resin onto a cast drum rotating from a T-shaped slit die and continuously drying it. Preferably there is.

As such a polyvinyl alcohol resin, an unmodified polyvinyl alcohol 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 (for example, 10 mol% or less, preferably 5 mol% or less) of vinyl acetate and a copolymerizable component may be used. it can. Examples of components copolymerizable with vinyl acetate include unsaturated carboxylic acids (including salts, esters, amides, nitriles, etc.), and olefins having 2 to 30 carbon atoms (eg, ethylene, propylene, n-butene). , Isobutene, etc.), vinyl ethers, unsaturated sulfonates and the like. Moreover, the modified polyvinyl alcohol-type resin obtained by chemically modifying the hydroxyl group after saponification can also be used.

Also, as the polyvinyl alcohol resin, a polyvinyl alcohol resin having a 1,2-diol structure in the side chain can be used. Such a polyvinyl alcohol resin having a 1,2-diol structure in the side chain includes, for example, (i) a method of saponifying a copolymer of vinyl acetate and 3,4-diacetoxy-1-butene, and (ii) acetic acid. A method of saponifying and decarboxylating a copolymer of vinyl and vinyl ethylene carbonate; and (iii) saponifying and decarboxylating a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane. It is obtained by a method of ketalization, (iv) a method of saponifying a copolymer of vinyl acetate and glycerol monoallyl ether, or the like.

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

The average saponification degree of the polyvinyl alcohol resin used in the present invention is usually preferably 98 mol% or more, particularly preferably 99 mol% or more, further preferably 99.5 mol% or more, and particularly preferably 99.mol%. It is 8 mol% or more. When the average degree of saponification is too small, there is a tendency that sufficient optical performance cannot be obtained when a polyvinyl alcohol film is used as a polarizing film.
Here, the average saponification degree in the present invention is measured according to JIS K 6726.

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

A polyvinyl alcohol-based resin aqueous solution is prepared using the polyvinyl alcohol-based resin, the aqueous solution is cast on a rotating cast drum (drum-type roll), and film formation and drying are performed by a casting method. Alcohol-type resin can be manufactured continuously, for example, can be manufactured with the following processes.
(A) A step of forming a film by a casting method.
(B) The process of heating and drying the formed film.
(C) A step of winding the dried film on a roll after slitting.

Hereinafter, the step (A) will be described.
In the step (A), the polyvinyl alcohol resin is washed with a solvent such as water, dehydrated with a centrifuge, etc., and a polyvinyl alcohol resin wet cake having a water content of 50% by weight or less It is preferable to do. When the water content is too large, it tends to be difficult to obtain a desired aqueous solution concentration.
Such polyvinyl alcohol resin wet cake is dissolved in warm water or hot water to prepare a polyvinyl alcohol resin aqueous solution.

The method for preparing the polyvinyl alcohol-based resin aqueous solution is not particularly limited. For example, the polyvinyl alcohol-based resin aqueous solution may be prepared by using a heated multi-screw extruder. The polyvinyl alcohol-based resin wet cake thus prepared is charged and water vapor is blown into the can to prepare an aqueous solution having a desired concentration and dissolution.

In addition to polyvinyl alcohol-based resins, polyvinyl alcohol-based resin aqueous solutions include plastics commonly used such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, polyethylene glycol, and trimethylol propane as necessary. It is preferable from the viewpoint of the film-forming property of the polyvinyl alcohol film to contain an agent and a nonionic, anionic or cationic surfactant.

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

Next, the obtained polyvinyl alcohol resin aqueous solution is defoamed. Examples of the defoaming method include stationary defoaming and defoaming with a multi-screw extruder. The multi-screw extruder is not particularly limited as long as it is a multi-screw extruder having a vent. Usually, a twin-screw extruder having a vent is used.

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

The temperature of the polyvinyl alcohol resin aqueous solution at the exit of the T-type slit die is preferably 80 to 100 ° C., and particularly preferably 85 to 98 ° C.
If the temperature of the aqueous polyvinyl alcohol resin solution is too low, it tends to cause poor flow, and if it is too high, it tends to foam.

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

The discharge speed of the polyvinyl alcohol resin aqueous solution discharged from the T-type slit die onto the cast drum is preferably 0.5 to 5 m / min, particularly preferably 0.8 to 4 m / min, and more preferably 1 to 3 m / min.
If the discharge speed is too slow, the productivity tends to decrease, and if it is too fast, the fluency tends to be difficult.

In addition, it is preferable that the flight time for the polyvinyl alcohol-based resin aqueous solution to reach the cast drum surface from the T-type slit die discharge port is 0.1 to 0.7 seconds. From the point of view, it is particularly preferably 0.2 to 0.6 seconds, and more preferably 0.2 to 0.4 seconds from the viewpoint of avoiding environment-induced shape distortion described later.
If the flight time is too short or too long, the thickness variation tends to increase.

The flight time is determined by the discharge speed from the T-type slit die 1, the distance A from the discharge port 1a of the T-type slit die 1 to the surface of the cast drum 2 (grounding point P), and the lip opening of the T-type slit die 1. And the viscosity of the polyvinyl alcohol resin aqueous solution can be adjusted (see FIG. 1).

In the present invention, in order to reduce the thickness fluctuation of the obtained polyvinyl alcohol film, it is preferable to set a time for relieving the geometric distortion caused by stress release and a time for avoiding the environmental geometric distortion as much as possible.

Regarding the former, for example, when the polyvinyl alcohol-based resin aqueous solution 3 is discharged vertically from the top of the cast drum 2 rotating from above, the polyvinyl alcohol-based resin aqueous solution 3 immediately after discharge generates a tensile stress on the cast drum side R, and is free. Compressive stress is generated on the surface side S (see FIG. 2). When the aqueous solution comes in contact with the cast drum 2 in a short time, the difference in both stresses causes a variation in thickness. However, even if the polyvinyl alcohol-based resin aqueous solution 3 having a relatively high viscosity is provided with an appropriate stress relaxation time, the thickness variation occurs. The influence of can be reduced. The lower limit value of the flight time of the present invention is set in consideration of the stress relaxation time.

The diameter of the cast drum is preferably 2 to 5 m, particularly preferably 2.4 to 4.5 m, and more preferably 2.8 to 4 m.
If the diameter is too small, the drying length is insufficient and the speed tends not to be obtained, and if it is too large, the transportability tends to decrease.

The width of the cast drum is preferably 2 m or more, particularly preferably 3 m or more, more preferably 4 m or more, and particularly preferably 5 to 6 m.
If the width of the cast drum is too small, the productivity tends to decrease.

The rotational speed of such a cast drum is preferably 3 to 50 m / min, particularly preferably 4 to 40 m / min, and further preferably 5 to 35 m / min.
If the rotational speed is too slow, the productivity tends to decrease, and if it is too fast, drying tends to be insufficient.

The surface temperature of such a cast drum is preferably 40 to 99 ° C., particularly preferably 60 to 97 ° C.
If the surface temperature is too low, drying tends to be poor, and if it is too high, foaming tends to occur.

Next, the step (B) will be described. A process (B) is a process of heating and drying the formed film.

The film formed on the cast drum is dried by alternately contacting the front and back surfaces of the film with a plurality of hot rolls. The surface temperature of the hot roll is usually 40 to 150 ° C, preferably 50 to 140 ° C. If the surface temperature is too low, drying tends to be poor, and if the surface temperature is too high, drying tends to be excessive, and appearance defects such as undulation tend to be caused.
The hot roll is, for example, a roll having a diameter of 0.2 to 2 m, the surface of which is hard chrome plated or mirror-finished. Usually, 2 to 30 rolls, preferably 10 to 25 rolls are used for drying. preferable.

In the present invention, it is preferable to heat-treat the film after drying with a hot roll. The heat treatment temperature is preferably 60 to 150 ° C, particularly preferably 70 to 140 ° C. If the heat treatment temperature is too low, the water resistance of the polyvinyl alcohol-based film tends to decrease, causing retardation fluctuations. If it is too high, the stretchability during the production of the polarizing film tends to decrease. Examples of such a heat treatment method include a method of contacting with a high-temperature hot roll, a method of using a floating dryer, and the like.

The film that has been dried and heat-treated as necessary becomes a product (polyvinyl alcohol film of the present invention) through the step (C). Step (C) is a step in which both ends of the film are slit and wound on a roll.

Here, a method for preparing a polyvinyl alcohol-based film by preparing a polyvinyl alcohol-based resin aqueous solution so far, casting the aqueous solution on a rotating cast drum (drum-type roll), forming a film by a casting method, and drying the film. However, it is also possible to cast a polyvinyl alcohol-based resin aqueous solution on a resin film or a metal belt, to form a film, and to dry.

Thus, the polyvinyl alcohol film of the present invention is obtained.

The polyvinyl alcohol film of the present invention must have a thickness of 5 to 60 μm, particularly preferably 5 to 50 μm, more preferably 5 to 30 μm from the viewpoint of thinning, and particularly from the viewpoint of avoiding breakage. The thickness is preferably 10 to 30 μm.

The polyvinyl alcohol film of the present invention must have a width of 2 m or more, particularly preferably 3 m or more from the viewpoint of increasing the area, and more preferably 4 to 6 m from the viewpoint of avoiding breakage.

The polyvinyl alcohol film of the present invention is required to have a length of 2 km or more, particularly preferably 3 km or more, and more preferably 4 km or more from the viewpoint of increasing the area.
The upper limit of the length of the film is preferably 50 km or less, particularly preferably 40 km or less, and further preferably 30 km or less from the viewpoint of avoiding breakage.

As described above, the polyvinyl alcohol film of the present invention has a thickness variation coefficient of 1% or less over the entire surface of the film, but the thickness variation coefficient in the film flow direction (MD direction) is the polarization performance of the polarizing film. It is preferable that it is 0.7% or less at a point, Especially preferably, it is 0.6% or less, More preferably, it is 0.5% or less.
Further, the variation coefficient of the thickness in the width direction (TD direction) is preferably 0.7% or less, particularly preferably 0.6% or less, and more preferably 0.5 in terms of the polarizing performance of the polarizing film. % Or less.
Here, when the thickness of the polyville alcohol-based film is thin, the degree of polarization tends to be non-uniform in the flow direction in the production of the polarizing film, and the thickness accuracy in the flow direction (MD direction) of the original fabric becomes important.

The thickness variation coefficient of the present invention will be described in detail.

The thickness variation coefficient of the film is: smoothness of cast drum, uniform rotation of cast drum, uniformity of opening of T-shaped slit die slit, discharge accuracy of polyvinyl alcohol resin aqueous solution, cast drum of polyvinyl alcohol resin aqueous solution The discharge angle, the time and distance until the polyvinyl alcohol-based resin aqueous solution contacts the cast drum, the stable contact between the polyvinyl alcohol-based resin aqueous solution and the cast drum, the drying conditions in the cast drum and the subsequent heat roll, and the like.
Among these, stable contact between the polyvinyl alcohol-based resin aqueous solution and the cast drum is important, and when the contact line (touch line) moves back and forth in the MD direction, thickness variation occurs in the MD direction of the film. Further, when the contact line fluctuates in the TD direction, the thickness varies in the TD direction of the film. In order to stabilize such a touch line, the following method of adjusting [I] to [V] is effective and preferable.

[I] “Flight distance A of the polyvinyl alcohol-based resin aqueous solution from the discharge port 1a of the T-shaped slit die 1 to the surface of the cast drum 2 (contact point P)” (see FIG. 1).
The longer the flight distance A is, the more easily the polyvinyl alcohol resin aqueous solution is affected by the rotation of the cast drum 2 and the airflow of the environment. Therefore, the distance from the discharge port 1a to the cast drum 2 should be as close as possible. desirable.
In the present invention, the shortest distance B from the discharge port 1a of the T-shaped slit die 1 to the surface of the cast drum 2 (grounding point P) is preferably 2 mm or less, particularly preferably 1.5 mm or less, more preferably 1 mm. It is as follows. If the shortest distance B is too large, the thickness variation of the film tends to increase.

[II] “Discharge direction D of the polyvinyl alcohol-based resin aqueous solution from the T-shaped slit die 1 to the cast drum 2 (see FIG. 3).”
If the polyvinyl alcohol-based resin aqueous solution is dripped by gravity before being discharged onto the cast drum 2 and grounded from above, the touch line is likely to be disturbed. For example, when the tangent line of the cast drum 2 at the ground contact point P and the discharge direction D are parallel, the discharge liquid (polyvinyl alcohol-based resin aqueous solution) is difficult to come down smoothly due to gravity.
In the present invention, the discharge direction from the T-type slit die 1 to the cast drum 2 (the direction in which the lip surface inside the T-type slit die faces) D, the discharge port 1a of the T-type slit die 1 and the cast drum 2 The angle θ formed with the plane E connecting the central axes C is preferably 40 ° or less, particularly preferably 1 to 30 °, more preferably 2 to 20 °, and particularly preferably 3 to 10 °. When the angle θ is too large, the thickness variation of the film tends to increase.

[III] “Air knife and / or air chamber.”
This is a general technique for forcibly stabilizing the touch line in resin extrusion. A known method can be used as the air knife or the air chamber.

[IV] “T-type slit die horizontality.”
Since the width of the film to be formed is 2 m or more, the width of the T-type slit die is also 2 m or more. In such a case, the influence of gravity cannot be ignored, and it is effective to use a highly rigid material for the T-shaped slit die and to lift the central portion of the T-shaped slit die in the width direction.
In the present invention, the T-type slit die is preferably lifted by one or more lifting tools. Specifically, the deflection due to gravity at the center in the width direction of the T-type slit die is 0.3 mm or less. The thickness is preferably 0.2 mm or less, more preferably 0.1 mm or less. In order to reduce the amount of such deflection, there are methods such as increasing the number of suspension tools or changing the lifting position.

[V] “The spatial displacement of the cast drum.”
As a matter of course, the liquid discharged from the T-shaped slit die cannot be grounded stably if the cast drum is spatially swung. In the present invention, the swing of the rotating cast drum from top to bottom, back and forth (MD direction), and left and right (TD direction) is preferably ± 50 μm or less, particularly preferably ± 40 μm or less, more preferably ± 30 μm. It is as follows. If the shaking is too great, the thickness variation of the film tends to increase. In order to reduce the fluctuation of the cast drum, there are methods such as reducing the weight of the cast drum or increasing the rotation accuracy of the motor.

The polyvinyl alcohol film of the present invention is excellent in thickness accuracy, is suitably used as an optical polyvinyl alcohol film, and is particularly preferably used as a raw material for a polarizing film.

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

The polarizing film of the present invention is produced through steps such as swelling, dyeing, boric acid crosslinking, stretching, washing, and drying by unwinding the polyvinyl alcohol film from a roll and transferring it in the horizontal direction.

The swelling process is performed before the dyeing process. In addition to washing the surface of the polyvinyl alcohol film by the swelling step, there is also an effect of preventing uneven staining by swelling the polyvinyl alcohol film. In the swelling step, water is usually used as the treatment liquid. If the main component is water, the treatment solution may contain a small amount of an iodide compound, an additive such as a surfactant, alcohol, or 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 bringing the film into contact with a liquid containing iodine or a dichroic dye. Usually, an iodine-potassium iodide aqueous solution is used. The iodine concentration is suitably 0.1-2 g / L, and the potassium iodide concentration is 1-100 g / L. The dyeing time is practically 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 aqueous solvent.

The boric acid crosslinking 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 it is preferable that potassium iodide coexists in the solution from the viewpoint of stabilizing the polarization performance. The temperature during the treatment is preferably about 30 to 70 ° C., and the treatment time is preferably about 0.1 to 20 minutes. If necessary, the 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 a uniaxial direction. At this time, a slight stretching (stretching to prevent shrinkage in the width direction or more) may be performed in a direction perpendicular to the stretching direction. The temperature during stretching is preferably 40 to 170 ° C. Furthermore, the draw ratio may be finally set within the above range, and the drawing operation may be performed not only in one stage but also in any stage of the manufacturing process.

The washing step is performed, for example, by immersing a polyvinyl alcohol film in an aqueous solution of iodide such as water or potassium iodide, thereby removing deposits generated on the surface of the film. When using an aqueous potassium iodide solution, the concentration of potassium iodide may be 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. In addition, you may perform combining water washing | cleaning and washing | cleaning by potassium iodide aqueous solution suitably.

The drying process may be performed in the atmosphere at 40 to 80 ° C. for 1 to 10 minutes.

Further, the polarization degree 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 secured.
The degree of polarization generally refers to the light transmittance (H ₁₁ ) measured at a wavelength λ and two polarized films in a state where two polarizing films are superposed so that their orientation directions are the same. It is calculated according to the following formula 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 single transmittance of the polarizing film of the present invention is preferably 42% or more. If the single transmittance is too low, it tends to be impossible to achieve high brightness of the liquid crystal display.
The single transmittance is a value obtained by measuring the light transmittance of a polarizing film using a spectrophotometer.

Thus, although the polarizing film of the present invention is obtained, the polarizing film of the present invention is suitable for producing a polarizing plate with little color unevenness.
Hereinafter, the manufacturing method of the polarizing plate of this invention is demonstrated.

The polarizing film of the present invention becomes a polarizing plate by bonding an optically isotropic resin film as a protective film on one or both sides 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 a sheet.

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

In addition, for the purpose of thinning the polarizing film, instead of the protective film, a curable resin such as urethane resin, acrylic resin, urea resin or the like is applied on one side or both sides of the polarizing film and cured to obtain a polarizing plate. You can also

The polarizing film and polarizing plate obtained by the present invention have excellent thickness variation coefficient, no color unevenness, and excellent in-plane uniformity of polarization performance, portable information terminals, personal computers, televisions, projectors, signage, electronic desktops Computers, electronic clocks, word processors, electronic paper, game machines, videos, cameras, photo albums, thermometers, audio, liquid crystal display devices such as automobile and machinery instruments, sunglasses, anti-glare glasses, stereoscopic glasses, wearable displays, It is preferably used for a reflection reducing layer for display elements (CRT, LCD, organic EL, electronic paper, etc.), optical communication equipment, medical equipment, building materials, toys and the like.

EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not limited to a following example, unless the summary is exceeded.
In the examples, “parts” and “%” mean weight basis.
About each physical property, it carried out as follows.

(1) Thickness The thickness of the polyvinyl alcohol film in the MD direction and the TD direction was measured.
・ Measurement in the flow direction (MD direction) is 0.3mm increments using Keyence's "Spectral Interference Type Film Thickness Meter SI-T80", center and both ends (20cm inside from both ends) in the width direction (TD direction) ) Measure 60,000 points in each of the three locations.
・ Measurement in the TD direction is performed by using a continuous film thickness meter TOF-5R01 manufactured by Yamabun Denki Co., Ltd., and measuring 4000 points at each of the front, center, and end portions in the MD direction.

(2) Thickness variation coefficient It calculated from the following formula using the measured value of the thickness.

(3) Degree of polarization (%)
A strip sample having a stretching direction of 200 mm × a width direction of 40 mm is cut out from the central portion in the width direction of the obtained polarizing film, and a degree of polarization of 10 points at a pitch of 10 mm in the stretching direction using “RETS-1100A” manufactured by Otsuka Electronics Co., Ltd. It was measured.

<Example 1>
(Manufacture of polyvinyl alcohol film)
1000 kg of polyvinyl alcohol resin having a weight average molecular weight of 142,000 and a saponification degree of 99.8 mol%, water of 2000 kg, and 100 kg of glycerin as a plasticizer are added, and the temperature is raised to 150 ° C. while stirring, and the resin concentration is 25%. A polyvinyl alcohol-based resin aqueous solution that was adjusted and uniformly dissolved was obtained. Next, the polyvinyl alcohol-based resin aqueous solution is supplied to a twin screw extruder and defoamed, and then the temperature of the aqueous solution is set to 95 ° C., and a discharge speed of 2.5 m / min is applied to a cast drum rotating from a T-type slit die discharge port. To form a film. The shortest distance from the T-type slit die discharge port to the cast drum surface is 2 mm, the discharge direction from the T-type slit die to the cast drum (the direction in which the lip surface inside the T-type slit die faces), and the T-type slit The angle formed by the die discharge port and the plane connecting the center axis of the cast drum is 15 ° (see Table 1).
Next, the obtained film was peeled off from the cast drum, and dried using a hot roll and heat treated using a floating dryer. Next, both ends were cut off with a slit so as to have a width of 4 m to obtain a polyvinyl alcohol film.
Table 2 shows the characteristics of the obtained polyvinyl alcohol film.

Next, using the polyvinyl alcohol film obtained above, a polarizing film was obtained in the following manner, and the following evaluation was performed. The evaluation results are shown in Table 3.

(Manufacture of polarizing film)
The obtained polyvinyl alcohol film was stretched 1.7 times while being immersed in a water bath at a water temperature of 25 ° C. Next, the film was stretched 1.6 times while immersed in a 28 ° C. aqueous solution of iodine 0.5 g / L and potassium iodide 30 g / L, and then boric acid 40 g / L and potassium iodide 30 g / L. While being immersed in an aqueous solution (55 ° C.), boric acid treatment was performed while uniaxially stretching 2.1 times. Thereafter, the film was washed with an aqueous potassium iodide solution and dried to obtain a polarizing film having a total draw ratio of 5.8 times. Table 3 shows the polarization characteristics of the obtained polarizing film.

<Examples 2, 3, 5 to 7, Comparative Examples 1 to 4>
In Example 1, it carried out similarly except having changed the film-forming conditions as shown in Table 1, and obtained the polyvinyl alcohol-type film, and also obtained the polarizing film similarly to Example 1. FIG.
About the obtained polyvinyl alcohol-type film and polarizing film, evaluation similar to Example 1 was performed. The evaluation results are shown in Table 2 and Table 3.

<Example 4>
In Example 1, the film forming conditions were changed as shown in Table 1, and when discharging the aqueous solution, an air knife was installed above the touch line of the aqueous solution (height 5 mm, angle 90 °), and the air velocity from the air knife Except that air was discharged at 10 kPa to stabilize the touch line, the same procedure as in Example 1 was performed to obtain a polyvinyl alcohol film, and a polarizing film was obtained as in Example 1.
About the obtained polyvinyl alcohol-type film and polarizing film, evaluation similar to Example 1 was performed. The evaluation results are shown in Table 2 and Table 3.

Although the polyvinyl alcohol films of Examples 1 to 7 are thin, wide, and long, the thickness variation coefficient in both the MD direction and the TD direction is 1% or less. On the other hand, the polyvinyl alcohol films of Comparative Examples 1 to 4 have a large variation coefficient of thickness, and the polarization degree of the obtained polarizing film is non-uniform in the plane.
Also, the shortest distance (mm) from the T-type slit die discharge port to the cast drum surface, the discharge direction from the T-type slit die to the cast drum (direction in which the lip surface inside the T-type slit die faces) and the T-type slit It can be seen that a polyvinyl alcohol film having a small thickness variation coefficient can be produced by using an angle (°) between the die discharge port and a plane connecting the central axis of the cast drum and using an air knife.

In the above embodiments, specific forms in the present invention have been described. However, the above embodiments are merely examples and are not construed as limiting. Various modifications apparent to those skilled in the art are contemplated to be within the scope of this invention.

The polarizing film and polarizing plate obtained by the present invention have excellent thickness variation coefficient and excellent in-plane uniformity of polarization performance, portable information terminals, personal computers, televisions, projectors, signage, electronic desk calculators, electronic watches , Word processors, electronic paper, game machines, videos, cameras, photo albums, thermometers, audio, liquid crystal display devices such as automobile and machinery instruments, sunglasses, anti-glare glasses, stereoscopic glasses, wearable displays, display elements (CRT) , LCD, organic EL, electronic paper, etc.) for reflection reduction layers, optical communication equipment, medical equipment, building materials, toys and the like.

Claims (8)

1. A polyvinyl alcohol film having a thickness of 5 to 60 μm, a width of 2 m or more, and a length of 2 km or more, wherein the coefficient of variation in thickness over the entire film is 1% or less.
2. 2. The polyvinyl alcohol according to claim 1, wherein the polyvinyl alcohol film has a thickness of 30 μm or less and a coefficient of variation in thickness in the flow direction (MD direction) of the polyvinyl alcohol film is 0.7% or less. Alcohol film.
3. A method for producing a polyvinyl alcohol film obtained by discharging an aqueous solution of a polyvinyl alcohol resin onto a cast drum rotating from a T-shaped slit die and continuously drying the film. A method for producing a polyvinyl alcohol film, wherein the coefficient of variation is 1% or less.
4. 4. The method for producing a polyvinyl alcohol film according to claim 3, wherein the shortest distance from the T-shaped slit die to the drum surface of the cast drum is 2 mm or less.
5. The angle between the discharge direction from the T-shaped slit die to the cast drum (the direction in which the lip surface inside the T-shaped slit die faces) and the plane connecting the T-shaped slit die discharge port and the central axis of the cast drum is 40 ° The method for producing a polyvinyl alcohol film according to claim 3 or 4, wherein:
6. 6. The method for producing a polyvinyl alcohol film according to claim 3, wherein the touch line where the aqueous solution contacts the cast drum is stabilized by an air knife.
7. A polarizing film comprising the polyvinyl alcohol film according to claim 1 or 2.
8. A polarizing plate comprising a protective film provided on at least one side of the polarizing film according to claim 7.

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