JP5036191B2 - Polyvinyl alcohol film and method for producing the same - Google Patents

Description

  The present invention relates to a polyvinyl alcohol film and a method for producing the same. More specifically, the present invention relates to a polyvinyl alcohol film having a low birefringence and a method for producing the same.

  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 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 demand for a polarizing film that is more excellent in polarization performance, in particular in-plane uniformity of polarization performance, than conventional products. In order to achieve in-plane uniformity, the polyvinyl alcohol film, which is the raw material of the polarizing film, must not have birefringence. As a measure against this, the moisture content of the film when peeling from the casting substrate can be reduced. It has been proposed to use a polyvinyl alcohol film produced by setting it to less than 10% by weight (see, for example, Patent Document 1).

  By the way, in the molded body that is injection-molded in the mold, the molded body that is polymerized and cured in the mold, and the film that is dried and formed on the cast drum, Some sort of stress strain occurs. This stress strain is optically observed as retardation. In a polyvinyl alcohol film having a thickness of several tens of μm produced by a casting method, a retardation of several tens of nm is usually generated. It is this in-plane unevenness of the retardation that becomes a problem when a polarizing film is produced from a polyvinyl alcohol film. Usually, in-plane unevenness tends to occur in the width direction, and the retardation tends to be large at the film end where stress is concentrated and small at the center. In recent years, the polyvinyl alcohol film has been widened to a width of 3 m or more from the viewpoint of productivity, and the difference between the retardation at the end in the width direction and the retardation at the center has been a problem. A film having this difference exceeding 5 nm is insufficient as a raw material used for manufacturing a polarizing film in consideration of the recent increase in area and definition.

  In the technique disclosed in Patent Document 1, the minimum retardation value of the polyvinyl alcohol film obtained in the examples exceeds 15 nm, and the retardation unevenness exceeds 5 nm. Above all, it could not cope with a film having a width of 3 m or more, and the low birefringence of the film was still insufficient. As described above, the conventional polyvinyl alcohol film does not correspond to the increase in area and definition of the polarizing film, and a polyvinyl alcohol film having a low birefringence is required.

JP-A-6-138319

  The present invention relates to a polyvinyl alcohol film having a low birefringence, corresponding to an increase in area and definition of a polarizing film, a polyvinyl alcohol film having excellent polarization performance, and having no color unevenness, and production thereof It aims to provide a method.

  The present invention is a polyvinyl alcohol film having a width of 3 m or more produced by a casting method, the retardation is 15 nm or less, and the difference between the retardation at the end in the width direction and the retardation at the center is 5 nm or less. It relates to a certain polyvinyl alcohol film.

  Furthermore, in this invention, it is preferable to use the polyvinyl alcohol-type resin of the weight average molecular weight 120,000-300000, and it is preferable that the thickness of a film is 30-70 micrometers.

The present invention also provides:
(A) The step of cooling the heated polyvinyl alcohol film to 10 to 40 ° C. , and (B) The step of heating to 50 to 100 ° C. before winding the cooled polyvinyl alcohol film on a roll. The present invention relates to a method for producing an alcohol film.

  In the said manufacturing method, it is preferable that cooling of a process (A) is made | formed by 10-40 degreeC ventilation.

  The present invention also relates to a polyvinyl alcohol film used for obtaining an original film of a polarizing film.

  Furthermore, the present invention relates to a polarizing film comprising the polyvinyl alcohol film, and further to a polarizing plate.

  Since the polyvinyl alcohol film obtained by the production method of the present invention has a low birefringence, it is excellent in in-plane uniformity of polarization performance capable of large area and high definition, and for the production of a polarizing film without color unevenness. Can be preferably used.

  The polyvinyl alcohol film of the present invention is a polyvinyl alcohol film having a width of 3 m or more produced by a casting method, a retardation of 15 nm or less, preferably 12 nm or less, more preferably 10 nm or less, and an end in the width direction. The difference between this retardation and the retardation at the center is 5 nm or less, preferably 4 nm or less, more preferably 3 nm or less. When the retardation value exceeds 15 nm, the birefringence of the film is high, and it is insufficient as a raw material used for the production of a polarizing film in consideration of large area and high definition. When the difference in retardation at the center exceeds 5 nm, the birefringence unevenness of the film is large, and it is insufficient as a raw material used for manufacturing a polarizing film in consideration of large area and high definition.

The method for producing a polyvinyl alcohol film of the present invention comprises preparing a polyvinyl alcohol resin aqueous solution using a polyvinyl alcohol resin, and casting the aqueous solution onto an endless belt or a drum roll, preferably a drum roll. After that, it is produced by drying, heat treatment as necessary, etc., and then the following steps:
(A) It is manufactured by a step of cooling the heated polyvinyl alcohol film to 10 to 40 ° C. , and (B) a step of heating to 50 to 100 ° C. before winding the cooled polyvinyl alcohol film on a roll. .

  Hereafter, the manufacturing method which consists of said process (A) and process (B) is demonstrated.

  The polyvinyl alcohol resin used for the production of the polyvinyl alcohol film is usually produced by saponifying polyvinyl acetate obtained by polymerizing vinyl acetate. The film of the present invention is not limited to such a resin, and a resin obtained by saponifying a copolymer of vinyl acetate and a component copolymerizable with a small amount of vinyl acetate can also be used. Examples of the component copolymerizable with vinyl acetate include unsaturated carboxylic acids and salts, esters, amides or nitriles thereof; olefins having 2 to 30 carbon atoms such as ethylene, propylene, n-butene and isobutene; vinyl ether Class; unsaturated sulfonates and the like can be used.

  Further, as the polyvinyl alcohol resin, it is also preferable to use a polyvinyl alcohol resin having a 1,2-glycol bond in the side chain, and the polyvinyl alcohol resin having a 1,2-glycol bond in the side chain is, for example, ( A) a method of saponifying a copolymer of vinyl acetate and 3,4-diacetoxy-1-butene, (a) a method of saponifying and decarboxylating a copolymer of vinyl acetate and vinyl ethylene carbonate, A method of saponifying and deketalizing a copolymer of vinyl acetate and 2,2-dialkyl-4-vinyl-1,3-dioxolane, and (d) a copolymer of vinyl acetate and glycerol monoallyl ether. It is obtained by a saponification method or the like.

  The weight average molecular weight of the polyvinyl alcohol-based resin is preferably 120,000 to 300,000, more preferably 130000 to 260000, and still more preferably 135,000 to 200,000, and particularly preferably 140000 to 180,000 in terms of improving the degree of polarization of the polarizing film. If the weight average molecular weight is less than 120,000, sufficient optical performance cannot be obtained when a polyvinyl alcohol-based resin is used as an optical film, and if it exceeds 300,000, stretching becomes difficult when the film is used as a polarizing film, and industrial production. Is difficult and undesirable. In addition, the weight average molecular weight of polyvinyl alcohol-type resin is a weight average molecular weight measured by GPC-LALLS method.

  The saponification degree of the polyvinyl alcohol resin is preferably 97 to 100 mol%, more preferably 98 to 100 mol%, and still more preferably 99 to 100 mol%. When the saponification degree is less than 97 mol%, sufficient optical performance cannot be obtained when a polyvinyl alcohol-based resin is used as an optical film, which is not preferable.

  In the production method of the present invention, first, a polyvinyl alcohol resin wet cake obtained by adjusting the water content of the polyvinyl alcohol resin is dissolved in 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 resin aqueous solution may be prepared using a multi-screw extruder. In a dissolving can equipped with a vertical circulation flow generation type stirring blade, water vapor is contained in the can. Can be blown to dissolve the hydrous polyvinyl alcohol resin wet cake to prepare an aqueous solution. In addition to polyvinyl alcohol-based resins, polyvinyl alcohol-based resin aqueous solutions include generally used plasticizers such as glycerin, diglycerin, triglycerin, ethylene glycol, triethylene glycol, and polyethylene glycol, as well as nonions. From the viewpoint of mechanical properties and productivity, it is preferable to contain a hydrophilic, anionic or cationic surfactant.

  The concentration of the aqueous polyvinyl alcohol resin solution thus obtained is preferably 15 to 60% by weight, more preferably 17 to 55% by weight, and still more preferably 20 to 50% by weight. If the concentration is less than 15% by weight, the drying load increases, resulting in poor production capacity. If the concentration exceeds 60% by weight, the viscosity becomes too high and uniform dissolution cannot be achieved.

  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 the T-type slit die by a certain amount, and then cast and formed into a film by a casting method.

  The resin temperature at the exit of the T-shaped slit die is preferably 80 to 100 ° C, more preferably 85 to 98 ° C. If the resin temperature at the exit of the T-shaped slit die is less than 80 ° C., poor flow occurs, and if it exceeds 100 ° C., foaming is not preferable.

  The polyvinyl alcohol-based resin aqueous solution is cast with a drum-type roll or an endless belt, but is preferably a drum-type roll from the viewpoints of widening, lengthening, and film thickness uniformity.

  The diameter of the drum type roll is preferably 2000 to 5000 mm, more preferably 2400 to 4500 mm, and particularly preferably 2800 to 4000 mm. If the diameter of the drum-type roll is less than 2000 mm, the drying length is insufficient and the speed does not come out, and if it exceeds 5000 mm, it is difficult to manufacture the equipment. The width of the drum-type roll is preferably 1000 to 5000 mm, more preferably 2000 to 4500 mm, and particularly preferably 3000 to 4300 mm. If the width of the drum-type roll is less than 1000 mm, the productivity is inferior. The rotation speed of the drum-type roll is preferably 5 to 30 m / min, and more preferably 6 to 20 m / min. If the rotational speed is less than 5 m / min, the productivity is inferior, and if it exceeds 30 m / min, drying is insufficient, which is not preferable. Moreover, it is preferable that the surface temperature of a drum type roll is 70-99 degreeC, and it is more preferable that it is 75-97 degreeC. If the surface temperature is less than 70 ° C, drying is poor, and if it exceeds 99 ° C, foaming is not preferable.

  The film formed by the drum type roll is subsequently dried. The drying method is not particularly limited, and can be performed, for example, by passing the front and back surfaces of the film alternately through a plurality of drying rolls. The diameter of the drying roll is preferably 100 to 1000 mm, more preferably 150 to 900 mm, and particularly preferably 200 to 800 mm. If the diameter of the drying roll is less than 100 mm, an enormous number is required, and if it exceeds 1000 mm, film conveyance becomes unstable, which is not preferable. The number of drying rolls is usually 2 to 30. The surface temperature of the drying roll is not particularly limited, but is preferably 60 to 100 ° C, more preferably 65 to 90 ° C. If the surface temperature is less than 60 ° C., the drying will be poor, and if it exceeds 100 ° C., it will be too dry, resulting in poor appearance.

  After drying, the resulting film is heat treated as necessary. The heat treatment method is not particularly limited, and can be performed by, for example, a floating method or a contact heating method using a roll.

In the method for producing the polyvinyl alcohol film of the present invention, the film heated by the drying treatment or the heat treatment as described above is cooled to 10 to 40 ° C. (step (A)).

The cooling method is not particularly limited. For example, cooling can be performed by a method using air (floating method) or a method using a roll, but in order to avoid an increase in birefringence due to a rapid temperature drop, a method using air especially is used. Is preferable, and a method using air blowing at 10 to 40 ° C is more preferable. Cooling is performed at a temperature above the temperature at which condensation occurs and below the temperature at which molecular chain rearrangement occurs. That is, the cooling temperature, 10 to 40 ° C., good Mashiku is 20 to 35 ° C.. Although the cooling time is not particularly limited, it is preferably 5 seconds or longer in consideration of heat transfer to the inside of the film, and is preferably 3 minutes or shorter so as not to reduce productivity. More preferably, it is 8 seconds-2 minutes, More preferably, it is 10 seconds-1 minute.

  The polyvinyl alcohol film cooled in the step (A) is heated to 50 to 100 ° C. before being wound on a roll (step (B)).

  The heating method is not particularly limited, and for example, heating can be performed by a method using air (floating method) or a method using a roll. However, in order to avoid an increase in birefringence accompanying a rapid temperature rise, It is preferable to heat with. The heating is performed at a temperature not lower than a temperature at which the molecular chain undergoes rearrangement and not higher than a temperature at which crystallization occurs and a new stress strain is generated. That is, the heating temperature is preferably 50 to 100 ° C, more preferably 60 to 90 ° C, and further preferably 70 to 80 ° C. The heating time is not particularly limited, but is preferably 5 seconds or longer in consideration of heat transfer into the film, and preferably 3 minutes or shorter so as not to reduce productivity. More preferably, it is 8 seconds-2 minutes, More preferably, it is 10 seconds-1 minute. Further, the time between the cooling step (step (A)) and the heating step (step (B)), for example, the time from reaching the cooling temperature to the next heating temperature is 3 in consideration of productivity. Within minutes is preferred.

  The stress strain generated in the resin during the film forming process is reduced by an appropriate heat treatment, preferably by repeating the heat treatment several times. Therefore, in the method for producing a polyvinyl alcohol film of the present invention, the heat cycle comprising a cooling step (step (A)) and a heating step (step (B)) is at least once, preferably 2 to 5 times, More preferably, it is preferably carried out 3 to 4 times. This stress relaxation phenomenon due to heat cycle is because the molecular chains that make up the polymer rearrange and settle in a stable state without strain, and the free volume inside the polymer regenerates so as to relax the strain. Expected to be placed.

  Next, the manufacturing method of the polarizing film of this invention using the polyvinyl alcohol-type film of this invention is demonstrated.

  The film thickness of the polyvinyl alcohol film used for the production of the polarizing film is preferably 30 to 100 μm, more preferably 30 to 90 μm, and particularly preferably 30 to 70 μm in terms of improving the polarizing performance. If the film thickness is less than 30 μm, stretching is difficult, and if it exceeds 100 μm, the film thickness accuracy is lowered, which is not preferable.

  The polarizing film of the present invention is produced through processes such as normal dyeing, stretching, boric acid crosslinking and heat treatment. As a method for producing a polarizing film, a polyvinyl alcohol film is stretched and dyed by dipping in an iodine or dichroic dye solution, and then treated with a boron compound. After stretching and dyeing at the same time, a boron compound treatment is performed. There are a method, a method of dyeing with iodine or a dichroic dye and stretching, and then a method of treating with a boron compound, a method of dyeing and then stretching in a solution of a boron compound, etc., which can be appropriately selected and used. As described above, the polyvinyl alcohol film (unstretched film) may be stretched and dyed and further subjected to boron compound treatment separately or simultaneously, but during at least one of the dyeing step and the boron compound treatment step. It is desirable from the viewpoint of productivity to carry out uniaxial stretching.

  Stretching is preferably performed 3 to 10 times, preferably 4 to 7 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 stretching temperature is preferably selected from 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 film is dyed 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 0.1 to 2 g / L, the potassium iodide concentration is 10 to 50 g / L, and the potassium iodide / iodine weight ratio is 20 to 100. Is appropriate. 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. As the contact means, any means such as dipping, coating, spraying and the like can be applied.

  The dyed film is then treated with a boron compound. As the boron compound, boric acid and borax are practical. The boron compound is used in the form of an aqueous solution or a water-organic solvent mixture at a concentration of about 0.5 to 2 mol / L, and it is practically desirable for a small amount of potassium iodide to coexist in the solution. The treatment method is preferably an immersion method, but of course, an application method and a spray method can also be carried out. The temperature during the treatment is preferably about 50 to 70 ° C., and the treatment time is preferably about 5 to 20 minutes. If necessary, the stretching operation may be performed during the treatment.

  The polarizing film thus obtained can be used as a polarizing plate by laminating and bonding an optically isotropic polymer film or sheet as a protective film on one or both surfaces thereof. As the protective film, for example, a film such as cellulose triacetate, cellulose diacetate, polycarbonate, polymethyl methacrylate, polystyrene, polyethersulfone, polyarylene ester, poly-4-methylpentene, polyphenylene oxide, cyclo or norbornene polyolefin or the like A sheet is raised.

  In addition, for the purpose of reducing the thickness of the polarizing film, instead of the protective film, a curable resin such as a urethane resin, an acrylic resin, or a urea resin may be applied and laminated on the surface or both surfaces thereof.

  A polarizing film (including at least one surface laminated with a protective film or a curable resin) has a transparent pressure-sensitive adhesive layer formed on a surface of the polarizing film as required by a generally known method. In some cases, it may be put to practical use. Examples of the pressure-sensitive adhesive layer include acrylic acid esters such as butyl acrylate, ethyl acrylate, methyl acrylate, 2-ethylhexyl acrylate, and α-monoolefin carboxylic acids such as acrylic acid, maleic acid, itaconic acid, Copolymers with methacrylic acid, crotonic acid, etc. (including those added with vinyl monomers such as acrylonitrile, vinyl acetate, and styrene) inhibit the polarizing properties of the polarizing film. However, the present invention is not limited to this, and any pressure-sensitive adhesive having transparency can be used. For example, polyvinyl ether or rubber may be used.

  The polarizing film of the present invention is excellent in in-plane polarization performance, such as an electronic desk calculator, an electronic watch, a word processor, a personal computer, a portable information terminal, a liquid crystal display device such as an automobile or machinery instrument, sunglasses, It is preferably used for eye protection glasses, stereoscopic glasses, a reflection reducing layer for display elements (CRT, LCD, etc.), 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 “%” are based on weight unless otherwise specified.

  The measurement of a weight average molecular weight is as follows.

  Measurement is performed under the following conditions by the GPC-LALLS method.

1) GPC
Apparatus: Waters type 244 gel permeation chromatograph column: Tosoh Corporation TSK-gel-GMPW _XL (inner diameter 8 mm, length 30 cm, 2 pieces)
Solvent: 0.1 M Tris buffer (pH 7.9)
Flow rate: 0.5 ml / min Temperature: 23 ° C
Sample concentration: 0.040%
Filtration: 0.45 μm Mysori Disc W-25-5 manufactured by Tosoh Corporation
Injection volume: 0.2ml
Detection sensitivity (differential refractive index detector): 4 times

2) LALLS
Apparatus: Chromatrix KMX-6 type low angle laser light scattering photometer Temperature: 23 ° C
Wavelength: 633nm
Second virial coefficient × concentration: 0 mol / g
Refractive index density change (dn / dc): 0.159 ml / g
Filter: MILLIPORE 0.45 μm filter HAWP01300
Gain: 800mV

Example 1
200 kg of 18 ° C. water was placed in a 500 L tank, and while stirring, 40 kg of a polyvinyl alcohol resin having a weight average molecular weight of 142000 and a saponification degree of 99.8 mol% was added, and stirring was continued for 15 minutes. Then, after draining water, 200 kg of water was further added and stirred for 15 minutes. The obtained slurry was dehydrated to obtain a polyvinyl alcohol-based resin wet cake having a water content of 43% by weight.

  70 kg of the obtained polyvinyl alcohol resin wet cake was put in a dissolution can, and 4.2 kg of glycerin as a plasticizer, 42 g of polyoxyethylene laurylamine as a release agent, and 10 kg of water were added. Water vapor was blown from the bottom of the can, and the temperature was raised to 150 ° C. with stirring. After uniform dissolution, a polyvinyl alcohol resin aqueous solution having a concentration of 25% by weight was obtained by adjusting the concentration.

Next, the polyvinyl alcohol resin aqueous solution (liquid temperature 147 ° C.) was supplied from the gear pump 1 to the twin screw extruder, defoamed, and then discharged from the gear pump 2. The discharged polyvinyl alcohol-based resin aqueous solution was cast from a T-type slit die onto a drum-type roll to form a film. The conditions for such casting film formation are as follows.
Drum type roll Diameter: 3200 mm, width: 4200 mm, rotation speed: 10 m / min, surface temperature: 90 ° C., T-type slit die outlet resin temperature: 95 ° C.

The moisture content of the obtained film was 20% by weight. Drying was performed while alternately passing the front and back surfaces of this film through a drying roll under the following conditions.
Drying roll Diameter: 320 mm, width: 4200 mm, number: 10 pieces, rotation speed: 10 m / min, surface temperature: 70 ° C.

  Thereafter, further heat treatment (condition: floating dryer (120 ° C., length 6 m, transport speed 10 m / min)) was performed.

The retardation value of the film obtained by the heat treatment was 30 nm. A heat cycle was performed while the front and back surfaces of the film were alternately passed through four rolls under the following conditions.
First cooling roll Diameter: 1000 mm, width: 4200 mm, rotation speed: 10 m / min, surface temperature: 25 ° C., cooling time: 11 seconds First heating roll diameter: 1000 mm, width: 4200 mm, rotation speed: 10 m / min, surface Temperature: 70 ° C., heating time: 11 seconds Second cooling roll Diameter: 1000 mm, width: 4200 mm, rotation speed: 10 m / min, surface temperature: 25 ° C., cooling time: 11 seconds Second heating roll Diameter: 1000 mm, width: 4200 mm, rotation speed: 10 m / min, surface temperature: 70 ° C., heating time: 11 seconds

  After the obtained polyvinyl alcohol film (F-1) (width 4000 mm, thickness 50 μm) was cooled with cold air at 20 ° C. for 20 seconds, the retardation value was measured.

  For the retardation, the obtained film was cut into a width (TD) 4000 mm × flow (MD) 50 mm to prepare a strip sample, and “KOBRA-21SDH” (manufactured by Oji Scientific Instruments, measurement wavelength 590 nm) was used. Then, it was determined by measuring over the entire width at a pitch of 100 mm in the width direction (the start point and the end point are 50 mm inside the film end). The average value of retardation is an average value of 40 points measured. Further, the difference between the end portion and the center portion is a numerical value on the side having a larger retardation difference among the start point to the 20th point or the 21st point to the end point. As a result of the measurement, the average value of retardation was 12 nm, and the difference between the end portion and the central portion was 4 nm.

  Using the obtained polyvinyl alcohol-based film, a polarizing film was produced as follows, and light transmittance, polarization degree, and color unevenness were evaluated as follows.

(Production of polarizing film)
The obtained polyvinyl alcohol film was immersed in an aqueous solution consisting of 0.2 g / L of iodine and 15 g / L of potassium iodide for 240 seconds at 30 ° C., and then 60 g / L of boric acid and 30 g / L of potassium iodide. While being immersed in an aqueous solution (55 ° C.) of the composition, boric acid treatment was performed for 5 minutes while simultaneously uniaxially stretching 6 times. Then, it dried and obtained the polarizing film. Table 1 shows the optical characteristics of the obtained polarizing film.

(Light transmittance of polarizing film)
Measurement is performed using a high-speed multi-wavelength birefringence measuring apparatus (manufactured by Otsuka Electronics Co., Ltd .: RETS-2000 wavelength: 550 nm).

(Degree of polarization)
It calculated according to following formula (1).
[(H ₁₁ −H ₁ ) / (H ₁₁ + H ₁ )] ^1/2 (1)
H ₁₁ : Light transmittance at 550 nm measured in a state in which two polarizing films are overlapped so that their alignment directions are the same direction H ₁ : Two polarizing films are in a direction in which the alignment directions are orthogonal to each other The light transmittance at 550 nm measured in the superimposed state

(Color unevenness)
After sandwiching the polarizing film between two polarizing plates in a crossed Nicol state (single transmittance 43.5%, polarization degree 99.9%) at an angle of 45 °, using a light box with a surface illuminance of 14,000 lux The optical color unevenness was observed in the transmission mode and evaluated according to the following criteria.
○ ・ ・ ・ No color unevenness × ・ ・ ・ Color unevenness

Example 2
A polyvinyl alcohol film (F-2) (width 4000 mm, thickness 50 μm) was obtained in the same manner as in Example 1 except that the heat cycle was performed in the floating system under the following conditions. The average value of retardation of this film was 10 nm, and the difference between the end portion and the central portion was 3 nm.
First cooling chamber: 20 ° C, 30 seconds, length 5m
First heating chamber: 70 ° C, 30 seconds, length 5m

  A polarizing film was produced and evaluated in the same manner as in Example 1 using the obtained polyvinyl alcohol film.

Example 3
A polyvinyl alcohol film (F-3) (width 4000 mm, thickness 75 μm) was obtained in the same manner as in Example 1 except that the following 4 points were changed. The average retardation value of this film was 15 nm, and the difference between the end portion and the central portion was 5 nm.
-Concentration of polyvinyl alcohol resin aqueous solution: 30% by weight
・ Rotating speed of casting drum: 8 m / min ・ Speed of heat cycle process: 8 m / min ・ Stretching ratio for manufacturing polarizing film: 4 times

  A polarizing film was produced and evaluated in the same manner as in Example 1 using the obtained polyvinyl alcohol film.

Comparative Example 1
A polyvinyl alcohol film (F′-1) (width 4000 mm, thickness 50 μm) was obtained in the same manner as Example 1 except that the heat cycle was not performed. The average retardation value of this film was 30 nm, and the difference between the end portion and the central portion was 20 nm.

  A polarizing film was produced and evaluated in the same manner as in Example 1 using the obtained polyvinyl alcohol film.

Claims (8)

It is a polyvinyl alcohol film having a width of 3 m or more produced by a casting method, wherein the retardation is 15 nm or less, and the difference between the retardation at the end in the width direction and the retardation at the center is 5 nm or less. A polyvinyl alcohol film. The polyvinyl alcohol film according to claim 1, wherein the polyvinyl alcohol film has a weight average molecular weight of 120,000 to 300,000. The polyvinyl alcohol film according to claim 1 or 2, wherein the film has a thickness of 30 to 70 µm. (A) including a step of cooling the heated polyvinyl alcohol film to 10 to 40 ° C. , and (B) a step of heating to 50 to 100 ° C. before winding the cooled polyvinyl alcohol film on a roll. The method for producing a polyvinyl alcohol film according to claim 1, 2 or 3. The method for producing a polyvinyl alcohol film according to claim 4, wherein the cooling in the step (A) is performed by blowing air at 10 to 40 ° C. 4. The polyvinyl alcohol film according to claim 1, which is used for obtaining an original film of a polarizing film. A polarizing film comprising the polyvinyl alcohol film according to claim 1, 2, 3 or 6. A polarizing plate comprising a protective film provided on at least one surface of the polarizing film according to claim 7.