JP2016203623A - Method for producing polyvinyl alcohol-based film, polyvinyl alcohol-based film, and polarizing film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a polyvinyl alcohol-based film capable of obtaining a polarizing film excellent in thickness accuracy and polarization characteristics; a polyvinyl alcohol-based film; and a polarizing film.SOLUTION: There is provided a method for producing a polyvinyl alcohol-based film which is obtained by discharging and flow-casting an aqueous solution of a polyvinyl alcohol-based resin onto a rotating cast drum from a T-type slit die to form a film, and continuously drying the film, and has a thickness of 5-40 μm, where an angle θ formed by a lip surface inside of the T-type slit die and a tangent of a landing point of the polyvinyl alcohol-based resin aqueous solution on the cast drum is 10° or less.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a polyvinyl alcohol film. More specifically, the present invention provides a polyvinyl alcohol film that is excellent in thickness accuracy, is excellent in making a polarizing film wide, long and thin, and can produce a polarizing film without uneven polarization. The present invention relates to a method for producing a base film, a polyvinyl alcohol film obtained by such a method, and a polarizing film.

  Conventionally, a polyvinyl alcohol film is prepared by dissolving a polyvinyl alcohol resin in a solvent such as water to prepare a film-forming stock solution, and then forming the film by a solution casting method (casting method) and using a metal heating roll or the like. And then dried. 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 component of a liquid crystal display, and in recent years, its use has been expanded to a device 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 much wider and longer than the conventional product and that is wide, long, and thin, and a polyvinyl alcohol film that is the raw material. . If the polyvinyl alcohol film is not flat due to uneven thickness, in-plane uniformity of polarization performance is insufficient, stress is generated in the assembled liquid crystal cell, and the liquid crystal cell itself is deformed or the durability is lowered. cause. 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 (lateral 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 4 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 4 km or more, the thickness variation in the TD direction is further increased.

  Even when only the thickness variation in the TD direction is reduced, when a polarizing film is produced by unwinding a film from a polyvinyl alcohol film roll, the dyeing, stretching, There is a problem that uniform treatment becomes difficult in each step such as acid treatment, and the degree of polarization becomes uneven in the plane of the polarizing film.

  In the example of Patent Document 2, the film thickness variation is 1.5 μm, but the film thickness is as large as 75 μm, and it is difficult to cope with the thinning of the polarizing film, and the thickness is reduced to 60 μm or less. In such a case, there is a problem that sufficient thickness accuracy cannot be secured.

  In addition, when the thickness variation of the polyvinyl alcohol film is large and uneven, it is difficult to wind up or unwind from the roll, and this phenomenon further increases when the film absorbs moisture during storage and transportation. There is also a problem that most of the film must be discarded before the polarizing film is manufactured.

  Therefore, in the present invention, it is an object of the present invention to provide a method for producing a polyvinyl alcohol film capable of obtaining a polarizing film having excellent thickness accuracy and polarization characteristics, and further provide a polyvinyl alcohol film and a polarizing film. It is what.

  However, as a result of intensive studies in view of such circumstances, the present inventors, as a result, formed an aqueous solution of a polyvinyl alcohol-based resin by discharging and flowing onto a rotating drum from a T-shaped slit die to form a T-shaped film. Paying attention to the discharge angle when discharging the polyvinyl alcohol-based aqueous solution from the slit die to the cast drum, and making the angle smaller than usual, a polyvinyl alcohol-based film that can obtain a polarizing film with excellent thickness accuracy and polarization characteristics Was found and the present invention was completed.

  That is, the gist of the present invention is that an aqueous solution of a polyvinyl alcohol-based resin is formed by discharging and flowing onto a cast drum rotating from a T-shaped slit die, and continuously drying to obtain a thickness of 5 to 40 μm. A method for producing a polyvinyl alcohol film, characterized in that an angle formed between a lip surface inside a T-shaped slit die and a tangent at a landing point of an aqueous polyvinyl alcohol resin solution on a cast drum is 10 ° or less. The present invention relates to a method for producing a polyvinyl alcohol film.

  Moreover, this invention also provides the polarizing film which consists of a polyvinyl alcohol-type film manufactured by the said manufacturing method, and also this polyvinyl alcohol-type film.

  Since the polyvinyl alcohol film obtained by the production method of the present invention is excellent in thickness accuracy, it is preferably used as a raw material for a wide, long and thin polarizing film, and a polarizing film having a uniform degree of polarization in the plane is obtained. Can do.

FIG. 1 is an explanatory view showing an angle θ formed between a lip surface inside a T-shaped slit die and a tangent line at a landing point of a polyvinyl alcohol resin aqueous solution on a cast drum. FIG. 2 is an explanatory view showing a landing state of the polyvinyl alcohol resin aqueous solution on the cast drum surface.

The present invention is described in detail below.
The present invention relates to a polyvinyl alcohol film having a thickness of 5 to 40 μm obtained by forming a film by discharging and flowing an aqueous solution of a polyvinyl alcohol resin onto a cast drum rotating from a T-shaped slit die and continuously drying the film. When producing a polyvinyl alcohol film, the angle between the lip surface inside the T-shaped slit die and the tangent at the landing point of the polyvinyl alcohol resin aqueous solution on the cast drum is 10 ° or less. is there.

  As the polyvinyl alcohol resin, an unmodified polyvinyl alcohol resin, that is, a resin produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate is used. If necessary, a resin obtained by saponifying a copolymer of vinyl acetate and a small amount (usually 10 mol% or less, preferably 5 mol% or less) of a copolymerizable component with vinyl acetate 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 (for example, 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.

  Moreover, as a polyvinyl alcohol-type resin, the polyvinyl alcohol-type resin which has a 1, 2- diol structure in a side chain can also 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 for saponifying and decarboxylating a copolymer of vinyl and vinyl ethylene carbonate, (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-based 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-based resin is used as an optical film, and if it is too large, it tends to be difficult to stretch the polyvinyl alcohol-based film when manufacturing a polarizing film. There is. In addition, the weight average molecular weight of the said polyvinyl alcohol-type 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. If 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 a polyvinyl alcohol-type resin used for this invention, you may use together 2 or more types from which modified | denatured seed | species, modified | denatured amount, weight average molecular weight, average saponification degree differ.

The polyvinyl alcohol-based film of the present invention is prepared by preparing a polyvinyl alcohol-based resin aqueous solution using the above-mentioned polyvinyl alcohol-based resin, and discharging and casting the aqueous solution on a cast drum (drum-type roll) to form a film. Is manufactured continuously. For example, it is manufactured by the following steps.
(A) Step of forming a film by a casting method (B) Step of heating and drying the formed film (C) Step of winding the wound film after slitting the dried film

  Hereinafter, the step (A) will be described.

In the step (A), the polyvinyl alcohol resin described above is first washed with a solvent such as water, dehydrated with a centrifuge, etc., and wet with a polyvinyl alcohol resin having a water content of 50% by weight or less. A cake is preferred. When the water content is too large, it tends to be difficult to obtain a desired aqueous solution concentration.
Such a polyvinyl alcohol resin wet cake is dissolved in warm water or hot water to prepare an aqueous polyvinyl alcohol resin 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 film forming property to contain an agent and a nonionic, anionic, and / or cationic surfactant.

  The resin concentration of the polyvinyl alcohol-based resin aqueous solution thus obtained is preferably 15 to 60% by weight, particularly preferably 17 to 55% by weight, and 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-based 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-based resin aqueous solution at the exit of the T-shaped slit die is preferably 80 to 100 ° C, particularly preferably 85 to 98 ° C.
If the temperature of the aqueous polyvinyl alcohol resin solution is too low, it tends to cause poor flow, and if it is too high, it tends to foam.

  Here, in the method for producing a polyvinyl alcohol film of the present invention, an angle θ formed between the lip surface inside the T-shaped slit die and the tangent at the landing point of the polyvinyl alcohol resin aqueous solution on the cast drum (see FIG. 1). Is adjusted to be within 10 °, and by adjusting the angle θ to such a range, the thickness variation of the obtained polyvinyl alcohol film can be suppressed, and the thickness accuracy of the film can be improved. Become.

  The angle θ is preferably 9 ° or less, and particularly preferably 8 ° or less. If the angle θ is too large, the thickness variation of the resulting polyvinyl alcohol film increases, and the object of the present invention cannot be achieved. The lower limit of the angle θ is usually 1 °.

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-based resin aqueous solution discharged from the T-type slit die onto the cast drum is preferably 0.2 to 5 m / min, particularly preferably 0.4 to 4 m / min, more preferably 0.8. 6-3 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.

The diameter of the cast drum is preferably 2 to 5 m, particularly preferably 2.4 to 4.5 m, and further 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 such a cast drum is preferably 4 m or more, particularly preferably 4.5 m or more, more preferably 5 m or more, and particularly preferably 5 to 6 m.
If the width of the cast drum is too small, the productivity tends to decrease.

The rotation speed of the 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, the drying tends to be insufficient.

The surface temperature of such a cast drum is preferably 40 to 99 ° C, particularly preferably 60 to 95 ° 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 a hot roll is 40-150 degreeC normally, Preferably it is 50-140 degreeC. 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 whose surface is hard chrome plated or mirror-finished, and drying is usually performed using 2 to 30, preferably 10 to 25 rolls. 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, and 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 be insufficient or the retardation may be shaken. 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 subjected to drying and heat treatment 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.

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

  The polyvinyl alcohol film obtained by the production method of the present invention needs to have a thickness of 5 to 40 μm, preferably 5 to 30 μm from the viewpoint of thinning, and particularly preferably 10 from the viewpoint of avoiding breakage. ˜30 μm.

  The polyvinyl alcohol film obtained by the production method of the present invention preferably has a width of 4 m or more, particularly preferably 4.5 m or more, more preferably 5 m or more from the viewpoint of increasing the area, particularly from the viewpoint of avoiding breakage. It is preferably 4.5 to 6 m.

  The polyvinyl alcohol film obtained by the production method of the present invention is preferably 4 km or more in length, particularly preferably 4.5 km or more, more preferably 5 km or more, from the viewpoint of transport weight, from the viewpoint of increasing the area. Particularly preferred is 5 to 50 km.

The polyvinyl alcohol film obtained by the production method of the present invention preferably has a thickness accuracy within ± 5%, particularly preferably within ± 4%, and more preferably within ± 3%. If the thickness accuracy is out of the above range, the performance of the polarizing film tends to deteriorate.
For example, when the thickness of the polyvinyl alcohol film is 30 μm, the thickness accuracy of the entire film surface is preferably within ± 1.5 μm.
Such thickness accuracy is obtained by scanning the center and both ends of the film in the width direction (positions 10 cm inside from both ends of the film to the center) over the entire length, and the average thickness, maximum thickness, and minimum thickness. It is the value calculated | required by the method of measuring.

  The thickness accuracy of the polyvinyl alcohol film is the smoothness of the cast drum, the uniform rotation of the cast drum, the uniformity of the lip opening of the T-shaped slit die, the discharge accuracy of the polyvinyl alcohol resin aqueous solution, the polyvinyl alcohol resin aqueous solution It is governed by the discharge angle to the cast drum, the time and distance until the polyvinyl alcohol resin aqueous solution reaches the cast drum, stable contact between the polyvinyl alcohol resin aqueous solution and the cast drum, and the drying conditions of the cast drum and the subsequent hot roll. It can be achieved by adjusting and managing these with high accuracy.

  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) is moved back and forth in the flow direction (MD direction), the thickness of the film varies in the MD direction. Further, when the contact line fluctuates in the width direction (TD direction), the thickness varies in the TD direction of the film.

In order to stabilize such a touch line, it is preferable that the polyvinyl alcohol resin aqueous solution is softly landed on the surface of the cast drum. Generally, the polyvinyl alcohol resin aqueous solution immediately after discharge generates tensile stress on the cast drum side, Compressive stress is generated on the free surface side, and the difference between the two stresses causes thickness variation (see FIG. 2).
In order to reduce the influence on the thickness variation, the discharge direction of the polyvinyl alcohol resin aqueous solution from the T-type slit die to the cast drum is made closer to the tangential direction of the cast drum at the landing point of the polyvinyl alcohol resin aqueous solution and the cast drum. Is effective.
As a matter of course, the T-type slit die discharge port needs to be close to the cast drum so that dripping due to gravity does not occur.

  The polyvinyl alcohol film obtained by the production method 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, the manufacturing method of the polarizing film obtained using the polyvinyl alcohol-type film of this invention is demonstrated.

  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 the 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 suitably 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, stretching in a uniaxial direction is preferably performed 3 to 10 times, preferably 3.5 to 6 times. 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 the polyvinyl alcohol film in an aqueous iodide solution such as water or potassium iodide, and can remove precipitates generated on the surface of the film. When using a potassium iodide aqueous solution, the potassium iodide concentration 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.

  What is necessary is just to perform a drying process for 1 to 10 minutes at 40-80 degreeC in air | atmosphere.

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 is generally determined by the light transmittance (H ₁₁ ) measured at the wavelength λ and the two polarized films in a state where two polarizing films are overlapped so that the orientation directions thereof are the same. 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 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 single 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 polarization degree unevenness.
Hereinafter, the manufacturing method of the polarizing plate of this invention is demonstrated.

  The polarizing film of the present invention is bonded to one surface or both surfaces of the polarizing film as a protective film by using an optically isotropic resin film 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 to one side or both sides of the protective film and cured to obtain a polarizing plate. You can also

  The polarizing film and polarizing plate obtained according to the present invention have excellent thickness accuracy, little polarization degree unevenness, and excellent in-plane uniformity of polarization performance, such as portable information terminals, personal computers, televisions, projectors, signage, and 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.
Each physical property was measured as follows.

<Measurement conditions>
(1) Thickness accuracy (%)
The thickness of the polyvinyl alcohol film was measured as follows.
Measurement in the flow direction (MD direction) is performed in 0.3 mm increments using Keyence's “Spectral Interference Type Film Thickness Meter SI-T80”, with three locations in the center and both ends (20 cm inside from both ends) in the TD direction. Each was measured for 60,000 points.
The measurement in the width direction (TD direction) was performed using a “continuous film thickness meter TOF-5R01” manufactured by Yamabun Denshi Co., Ltd., the leading part (10 m inside from the tip), the center part, and the terminal part (10 m inside from the terminal). Were measured at 5000 points over 5 m in 1 mm increments.
Next, the thickness accuracy (%) was calculated from the average value Ta of all measured values in the MD direction and the TD direction and the thickness Tf most deviated from the average value according to the following formula.
Thickness accuracy (%) = 100 × | Tf−Ta | / Ta

(2) Polarization degree unevenness (%), single transmittance (%)
From the obtained polarizing film, a strip sample having a stretching direction of 50 mm × a width direction of 1 m was cut out, and using Otsuka Electronics Co., Ltd .: RETS-1100A, the polarization degree and the single transmittance over the entire width at a pitch of 10 mm in the width direction. It was measured.
Further, a strip sample having a stretching direction of 200 mm × a width direction of 40 mm was cut out from the central portion in the width direction of the obtained polarizing film, and Otsuka Electronics Co., Ltd .: RETS-1100A was used to extend the entire length at a pitch of 10 mm in the stretching direction. The degree of polarization was measured.
Using both of the obtained data, the degree of polarization unevenness (%) was calculated from the average value Pa of the degree of polarization and the degree of polarization Pf most deviated from the average value according to the following formula. Polarization degree unevenness (%) = 100 × | Pf−Pa | / Pa

<Example 1>
(Manufacture of polyvinyl alcohol film)
Polyvinyl alcohol resin having a weight average molecular weight of 142,000, saponification degree of 99.8 mol%, 1,000 kg of water, 2,000 kg of water, and 100 kg of glycerin as a plasticizer were added, and the temperature was raised to 150 ° C. with stirring, and the resin concentration The concentration was adjusted to 25% to obtain a uniformly dissolved polyvinyl alcohol resin aqueous solution. Next, the polyvinyl alcohol-based resin aqueous solution is supplied to a twin-screw extruder and defoamed, and then the aqueous solution temperature is set to 95 ° C., and it flows from the T-type slit die discharge port to the cast drum at a discharge speed of 1.3 m / min. Then, a film was formed. The angle θ formed by the lip surface inside the T-shaped slit die and the tangent line at the landing point of the polyvinyl alcohol resin aqueous solution on the cast drum is 10 °.

  Next, the obtained film was peeled off from the cast drum, and dried with a hot roll and heat-treated with a floating dryer. Finally, both ends were cut off with a slit and wound to obtain a roll-shaped polyvinyl alcohol film (thickness 30 μm, width 5 m, length 5 km). The properties of the obtained polyvinyl alcohol film are shown in Table 1.

(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 an aqueous solution of 0.5 g / L of iodine and 30 g / L of potassium iodide, and then composed of 40 g / L of boric acid and 30 g / L of potassium iodide. Was immersed in an aqueous solution (55 ° C.) and simultaneously treated with boric acid while being uniaxially stretched 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 2 shows the polarization characteristics of the obtained polarizing film.

<Example 2>
(Manufacture of polyvinyl alcohol film)
A roll-shaped polyvinyl alcohol system is formed in the same manner as in Example 1 except that the angle θ formed between the lip surface inside the T-shaped slit die and the tangent at the landing point of the polyvinyl alcohol resin aqueous solution on the cast drum is 5 °. A film (thickness 30 μm, width 5 m, length 5 km) was obtained. The properties of the obtained polyvinyl alcohol film are shown in Table 1.

(Manufacture of polarizing film)
A polarizing film was obtained from the obtained polyvinyl alcohol film in the same manner as in Example 1. Table 2 shows the polarization characteristics of the obtained polarizing film.

<Comparative Example 1>
(Manufacture of polyvinyl alcohol film)
A roll-shaped polyvinyl alcohol system is the same as in Example 1 except that the angle θ formed by the lip surface inside the T-shaped slit die and the tangent line at the landing point of the polyvinyl alcohol resin aqueous solution on the cast drum is 30 °. A film (thickness 30 μm, width 5 m, length 5 km) was obtained. The properties of the obtained polyvinyl alcohol film are shown in Table 1.

(Manufacture of polarizing film)
A polarizing film was obtained from the obtained polyvinyl alcohol film in the same manner as in Example 1. Table 2 shows the polarization characteristics of the obtained polarizing film.

While the polyvinyl alcohol films of Examples 1 and 2 have good thickness accuracy of 5% or less, the polyvinyl alcohol film of Comparative Example 1 is inferior in thickness accuracy.
And it turns out that the polarizing property of the polarizing film obtained from each polyvinyl alcohol-type film is superior to Comparative Example 1 in Examples 1 and 2.

  The polarizing film and polarizing plate obtained by the present invention are electronic paper, game machines, videos, cameras, photo albums, thermometers, audio, polarization degree unevenness, and excellent in-plane uniformity of polarization performance. Terminals, personal computers, televisions, projectors, signage, electronic desk calculators, electronic watches, word processors, 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 (6)

  A method for producing a polyvinyl alcohol film having a thickness of 5 to 40 μm obtained by discharging and flowing 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, characterized in that an angle formed between a lip surface inside the T-shaped slit die and a tangent at a landing point of the polyvinyl alcohol resin aqueous solution on the cast drum is 10 ° or less. .   2. The method of producing a polyvinyl alcohol alcohol film according to claim 1, wherein the width of the polyvinyl alcohol film is 4 m or more and the length is 4 km or more.   The method for producing a polyvinyl alcohol film according to claim 1 or 2, wherein the rotation speed of the cast drum is 3 to 50 m / min.   The method for producing a polyvinyl alcohol film according to any one of claims 1 to 3, wherein the viscosity of the aqueous solution of the polyvinyl alcohol resin during discharge is 50 to 200 Pa · s.   A polyvinyl alcohol film obtained by the method for producing a polyvinyl alcohol film according to claim 1.   A polarizing film comprising the polyvinyl alcohol film according to claim 5.