TW202210563A - Method for manufacturing polarizing film and polyvinyl alcohol-based film - Google Patents

Abstract

An objective of this invention is to provide a method for manufacturing polarizing film and a polyvinyl alcohol-based film capable of manufacturing a polarizing film in a stable process.

The method for manufacturing a polarizing film according to one embodiment includes a dipping step of immersing a long polyvinyl alcohol-based film in a treatment liquid while transporting a polyvinyl alcohol-based film, wherein, in the dipping step, the fluctuation width (%) of the width change rate (%) in the width direction of the polyvinyl alcohol-based film before and after being immersed in the treatment liquid is 0.5% or less.

Description

Manufacturing method of polarizing film and polyvinyl alcohol-based film

The present invention relates to a method for producing a polarizing film and a polyvinyl alcohol-based film.

As described in Patent Documents 1 and 2, a polarizing film is produced by subjecting a polyvinyl alcohol-based film to processes such as swelling, dyeing, crosslinking, stretching, washing, and drying, for example. In the above-mentioned treatment of the polyvinyl alcohol-based film, the polyvinyl alcohol-based film may be immersed in a treatment liquid. For example, in the swelling treatment, the polyvinyl alcohol-based film is immersed in water that is a treatment liquid.

[Prior Art Literature]

[Patent Literature]

Patent Document 1: Japanese Patent Laid-Open No. 2016-48382

Patent Document 2: Japanese Patent Laid-Open No. 2017-102438

In the production of the conventional polarizing film, the film may be cracked, etc., and the production process may be unstable.

Therefore, the objective of this invention is to provide the manufacturing method of the polarizing film which can manufacture a polarizing film by a stable process, and a polyvinyl alcohol-type film.

The inventors of the present invention have found that when a polarizing film is produced, if the fluctuation range of the width change rate before and after the immersion is too large, cracking of the film occurs. Therefore, the following manufacturing method of the polarizing film is completed.

A method for producing a polarizing film, which belongs to one aspect of the present invention, is a method for producing a polarizing film while conveying a polyvinyl alcohol-based film, and the method includes a dipping step of immersing a long polyvinyl alcohol-based film in a treatment solution, However, in the said immersion step, the fluctuation range (%) of the width change rate (%) of the width direction of the said polyvinyl alcohol type film before and after the said polyvinyl alcohol type film is immersed in the said process liquid is 0.5% or less.

In the said manufacturing method, in the said immersion process, the fluctuation range (%) of the width change rate (%) of the width direction of a polyvinyl alcohol-type film is 0.5% or less. In this case, since the above-mentioned fluctuation range is small, the film is not easily broken during the production of the polarizing film, and the polarizing film can be produced in a stable process.

In the immersion step, the polyvinyl alcohol-based film may be immersed in the treatment solution while imparting a constant tension (ie, constant tension) with a nip roll before and after dipping in the treatment solution.

In the said immersion process, the said polyvinyl alcohol-type film can be immersed in the said processing liquid, maintaining the rotation speed ratio of the nip roll before and after immersion in the said processing liquid to be constant.

The above-mentioned dipping step can be performed in the swelling treatment of the above-mentioned polyvinyl alcohol-based film. The swelling treatment is carried out in the initial stage of production of the polarizing film. Therefore, it becomes easy to change the width|variety of a polyvinyl-alcohol-type film by swelling process. Therefore, it is effective to perform swelling treatment in the above-mentioned dipping step.

The above-mentioned width change rate can be calculated by the formula (1), and the above-mentioned fluctuation range can be calculated by the formula (2).

Width change rate=|(W2-W1)/W1|×100...(1)

Variation = maximum value of width change rate - minimum value of width change rate...(2)

(In the formula (1), W1 [mm] represents the width of the polyvinyl alcohol-based film before immersion in the treatment liquid, and W2 [mm] represents the width of the polyvinyl alcohol-based film after immersion in the treatment liquid. Formula In (2), the maximum value of the width change rate and the minimum value of the width change rate are the values when the above-mentioned width change rate is continuously calculated for 60 seconds or more).

According to another aspect of the present invention, the polyvinyl alcohol-based film is immersed in water under conditions including a predetermined temperature and a predetermined immersion time while conveying the long polyvinyl alcohol-based film. The variation range (%) of the width change rate (%) in the width direction of the polyvinyl alcohol-based film is 0.5% or less, wherein the predetermined temperature is a temperature selected from the range of 15°C to 40°C, and the predetermined immersion time is The immersion time selected from the range of 20 seconds to 120 seconds, and the fluctuation range (%) is the difference between the maximum value of the width change rate and the minimum value of the width change rate when the width change rate is continuously obtained for 60 seconds or more.

The said polyvinyl alcohol-type film is used for manufacture of a polarizing film, for example. The method of manufacturing a polarizing film includes a dipping step of immersing a long polyvinyl alcohol-based film in a treatment liquid. When the above-mentioned immersion step is performed using the above-mentioned polyvinyl alcohol-based film, before and after the above-mentioned polyvinyl alcohol-based film is immersed in the above-mentioned treatment liquid, the width (%) of the width change rate (%) in the width direction of the above-mentioned polyvinyl alcohol-based film is easy to change become 0.5% or less. in this case Next, since the above-mentioned fluctuation range is small, the film is not easily broken during the production of the polarizing film, and the polarizing film can be produced in a stable process.

The fluctuation range (%) may be the following: The polyvinyl alcohol-based film is conveyed while maintaining the tension selected from the range of 5 N/m to 100 N/m at a temperature including the predetermined temperature and the predetermined dipping. The fluctuation range when immersed in the above-mentioned water under the condition of time.

The above-mentioned fluctuation range (%) may be the following: before and after immersion in the above-mentioned water, the above-mentioned polyvinyl alcohol is maintained in a state where the rotational speed ratio of the nip roll is maintained at a rotational speed ratio selected from the range of 1.1 to 4.0. The range of fluctuation when the film is immersed in the water under conditions including the predetermined temperature and the predetermined immersion time while being conveyed.

The above-mentioned fluctuation range (%) may be 0.3% or less.

The length of the longitudinal direction of the said polyvinyl alcohol-type film is, for example, 1000 m or more.

According to the present invention, a polarizing film can be produced in a stable process.

2: Film (polyvinyl alcohol-based film)

2a: end

4: polarizing film

6: Raw material roll

8: Treatment liquid

10: Manufacturing device

11: Pinch roller

12: Guide roller

13 ₁ : Swelling treatment section

13 ₂ : Dyeing Processing Section

13 ₃ : Cross-Linking Treatment Section

13 ₄ : Cleaning Treatment Section

13 ₅ : Drying section

30: Width measurer

31: End detection part

40: Computing Department

FIG. 1 is a schematic diagram for explaining the manufacturing method of the polarizing film of one embodiment.

FIG. 2 is for explaining the dipping process with which the manufacturing method of a polarizing film is equipped.

FIG. 3 is for explaining an example of the acquisition method of the width change rate and the fluctuation range.

Embodiments of the present invention will be described below with reference to the drawings. In the drawings, the same symbols are attached to the same or corresponding parts, and repeated descriptions are omitted. The dimensional ratios in the drawings do not necessarily correspond to those described.

(first embodiment)

The manufacturing method of the polarizing film of 1st Embodiment is demonstrated. FIG. 1 is a schematic diagram illustrating an example of a method for producing a polarizing film according to an embodiment of the present invention.

In the first embodiment, while conveying the long polyvinyl alcohol-based film 2 (hereinafter simply referred to as "film 2"), the film 2 being conveyed is subjected to swelling treatment, dyeing treatment, cross-linking treatment, stretching treatment and drying. The polarizing film 4 is manufactured by this process. The stretching treatment may be performed on the film 2 in any of the treatments (eg, cross-linking treatment), or may be performed on the film 2 in parallel while performing a plurality of treatments.

The material of the film 2 may be a known polyvinyl alcohol-based resin used for the production of polarizing films, and is preferably a saponified polyvinyl alcohol-based resin. The range of the degree of saponification is preferably 80.0 to 100.0 mol %, more preferably 90.0 to 99.5 mol %, and still more preferably 93.0 to 99.5 mol %. The degree of saponification refers to the value defined by the formula: degree of saponification (mol%) = (number of hydroxyl groups) / (number of hydroxyl groups + number of acetate groups) × 100, and can be determined according to the method specified in JIS K 6726 (1994). ask for. The average degree of polymerization of the polyvinyl alcohol-based resin is preferably from 100 to 10,000, more preferably from 1,000 to 10,000. The average degree of polymerization is a value obtained by the method of JIS K 6726 (1994).

The length in the longitudinal direction of the film 2 is, for example, 1000 m or more. When the length in the longitudinal direction of the film 2 is 1000 m or more, the length in the longitudinal direction of the film 2 is, for example, 30000 m or less, or preferably 20000 m or less. The length of the width direction (direction orthogonal to the longitudinal direction) of the film 2 is, for example, 1300 mm to 5000 mm. The thickness of the film 2 is, for example, 10 μm to 100 μm.

The film 2 is not particularly limited, and can be produced by a known method such as a melt extrusion method or a solvent casting method. For example, film forming can be performed by coating a polyvinyl alcohol-based resin solution on a base film composed of a thermoplastic resin (polyolefin-based resin, (meth)acrylic-based resin, etc.), and drying the resin solution. The obtained film 2 is peeled from the base film, whereby the film 2 can be obtained.

The above-mentioned base film may be provided with a primer layer as needed. The thickness of the base film can be appropriately determined, and is preferably 1 μm to 500 μm, more preferably 1 μm to 300 μm. The width and length of the base film can be appropriately determined according to the size of the intended film 2 .

A polyvinyl alcohol-based resin solution can be obtained by dissolving the above-described polyvinyl alcohol-based resin in water, alcohol, or the like. The viscosity of the solution is preferably 500cps to 10000cps, more preferably 1000cps to 7000cps, still more preferably 1000cps to 5000cps.

The application of the polyvinyl alcohol-based resin solution can be appropriately selected from known methods such as roll coating methods such as gravure coating and die coating methods. The film thickness of the applied polyvinyl alcohol-based resin solution may be, for example, 50 μm to 1000 μm.

In the above drying, the temperature of the resin is usually 30°C to 200°C, and the drying time can be carried out for 2 minutes to 24 hours. When the film thickness of the applied polyvinyl alcohol-based resin solution is thick, it can be dried with hot air above 80°C for 1 to 10 minutes to remove a certain degree of moisture, and then dried with hot air at 30°C to less than 50°C. .

The coating and drying of the polyvinyl alcohol-based resin solution, and the release of the film 2 from the base film can be carried out continuously while conveying the base film.

In the above drying, a tension of 45 N or more per 1 m of the length in the width direction of the base film may be applied to the base film in the conveyance direction of the base film (the longitudinal direction of the base film). In the above drying, It can be stretched so that the dimensional change rate in the longitudinal direction of the base film before and after drying exceeds 0%, that is, tension can be applied in the longitudinal direction so as to be plastically deformed.

The dimensional change rate may be 0.01% or more, or 0.1% or more. Moreover, the dimensional change rate in the drying step is preferably 5% or less, more preferably 1% or less.

The thickness of the resulting film 2 may be 5 μm to 100 μm. Moreover, the width of the polyvinyl alcohol-based resin layer (film 2) after drying may be 400 mm to 5000 mm.

In FIG. 1, the case where the polarizing film 4 is obtained by preparing the film 2 as the raw material roll 6 and performing each process with respect to the film 2 pulled out from the raw material roll 6 is shown. When the film 2 is produced by the above-mentioned method (melt extrusion method, solvent casting method, etc.), for example, the film 2 produced by the above-mentioned method (melt extrusion method, solvent casting method, etc.) can be continuously conveyed, and placed thereon. Each of the above-mentioned processes is performed during conveyance.

An example of the manufacturing method of the polarizing film 4 is demonstrated based on the form shown in FIG. First, the outline of the manufacturing apparatus 10 of the polarizing film 4 is demonstrated. The manufacturing apparatus 10 includes a plurality of nip rolls 11 , a plurality of guide rolls 12 , a swelling treatment section 13 ₁ , a dyeing treatment section 13 ₂ , a crosslinking treatment section 13 ₃ , a washing treatment section 13 ₄ , and a drying treatment section 13 ₅ .

The plurality of nip rolls 11 and the plurality of guide rolls 12 constitute the conveying mechanism of the film 2 . By appropriately arranging a plurality of nip rolls 11 and a plurality of guide rolls 12, the conveyance path of the film 2 is constituted.

The swelling treatment part 13 ₁ is a part that performs swelling treatment on the film 2 . The swelling treatment part 131 has _a treatment tank in which a treatment liquid for swelling treatment is stored. The membrane 2 is subjected to swelling treatment by immersing the membrane ₂ in the treatment liquid contained in the swelling treatment section 131 . In the first embodiment, the nip roll 11 and the two guide rolls 12 arranged before and after the film 2 is immersed in the treatment liquid form a film conveyance path for immersing the film 2 in the treatment liquid.

The purpose of performing the above swelling treatment is to remove foreign matter on the surface of the film 2 , remove the plasticizer in the film 2 , impart easy dyeability in subsequent steps, and plasticize the film 2 , and the like. The conditions of the swelling treatment can be determined within a range that can achieve the above-mentioned objectives and within a range where the extreme dissolution of the film 2, devitrification and other undesirable situations do not occur. In the swelling processing part 131, swelling processing is performed by immersing the film ₂ in, for example, a processing liquid having a temperature of 10°C to 50°C (preferably 15°C to 40°C). The time (dipping time) of the swelling treatment is about 5 seconds to 300 seconds, preferably about 20 seconds to 120 seconds. _An example of the treatment liquid in the swelling treatment part 131 is water. Therefore, the swelling treatment can also serve as the water washing treatment of the film 2 .

The dyeing part 13 ₂ is a part for dyeing the film 2 . The dyeing treatment section ₁₃₂ has a treatment tank in which a treatment liquid for dyeing treatment is stored. The film 2 is dyed by immersing the film ₂ in the treatment liquid contained in the dyeing treatment section 132 . In the first embodiment, the nip roll 11 and the two guide rolls 12 arranged before and after the film 2 is immersed in the treatment liquid form a film conveyance path for immersing the film 2 in the treatment liquid.

The treatment liquid of the dyeing treatment section 132 in the first embodiment is an aqueous solution of a dichroic dye, and the film ₂ is dyed with the dichroic dye in the dyeing treatment. In general, the dyeing process by the dichroic dye is aimed at making the film 2 adsorb the dichroic dye and the like. The processing conditions can be determined according to the desired optical properties within the range that can achieve the purpose and within the range where the extreme dissolution and devitrification of the film 2 do not occur. Examples of dichroic dyes used for dyeing are iodine and dichroic dyes.

When using iodine as a dichroic dye, for example, at a temperature of 10°C to 50°C (preferably 15°C to 40°C), 0.003 to 0.2 parts by weight of iodine and 0.1 part by weight of potassium iodide are contained relative to 100 parts by weight of water The film 2 is immersed in an aqueous solution of 10 to 10 parts by weight for 10 to 600 seconds (preferably 30 to 300 seconds) to perform dyeing treatment. Other iodides such as zinc iodide can also be used in place of potassium iodide. Other iodides and potassium iodide can also be used in combination. In addition, it can be combined with compounds other than iodide, boric acid, chlorine Zinc and cobalt chloride coexist. If it is a processing liquid containing 0.003 weight part or more of iodine with respect to 100 weight part of water, it can be regarded as a processing liquid for dyeing.

When using a dichroic dye as a dichroic dye, for example, at a temperature of 20°C to 80°C (preferably 30°C to 60°C), 0.001 part by weight of the dichroic dye is contained relative to 100 parts by weight of water The film 2 is immersed for 10 seconds to 600 seconds (preferably 20 seconds to 300 seconds) in an aqueous solution of 0.1 parts by weight, thereby performing dyeing treatment. The aqueous solution of the dichroic dye to be used may contain dyeing auxiliaries and the like, and may also contain inorganic salts such as sodium sulfate, surfactants, and the like. Only one type of dichroic dye may be used, or two or more types of dichroic dyes may be used in combination according to a desired hue.

The cross-linking treatment portion 13 ₃ is a portion for performing the cross-linking treatment on the film 2 . The crosslinking processing part ₁₃₃ has a processing tank in which a processing liquid for crosslinking processing is stored. The film 2 is subjected to a crosslinking treatment by immersing the film ₂ in the treatment liquid contained in the crosslinking treatment section 133 . In the first embodiment, the nip roll 11 and the two guide rolls 12 arranged before and after the film 2 is immersed in the treatment liquid form a film conveyance path for immersing the film 2 in the treatment liquid.

The purpose of performing the cross-linking treatment is to perform water resistance or hue adjustment (preventing the film 2 from being blue, etc.) by cross-linking.

The treatment liquid used in the crosslinking treatment section ₁₃₃ is, for example, an aqueous solution containing 1 to 10 parts by weight of boric acid with respect to 100 parts by weight of water. When the dichroic dye used in the dyeing treatment is iodine, the treatment liquid used in the crosslinking treatment section 13 to ₃ preferably contains iodide in addition to boric acid, and the amount thereof is, for example, 1 weight part with respect to 100 parts by weight of water parts to 30 parts by weight. Examples of the iodide include potassium iodide, zinc iodide, and the like. Compounds other than iodide, zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfate, etc. may coexist.

In the crosslinking treatment _of the crosslinking treatment section 133, the concentrations of boric acid and iodide, and the temperature of the treatment liquid can be appropriately changed according to the purpose.

For example, when the purpose of the cross-linking treatment is to achieve water resistance by cross-linking and the polyvinyl alcohol-based resin film is sequentially subjected to swelling treatment, dyeing treatment and cross-linking treatment, the solution containing the cross-linking agent in the treatment liquid is, for example, It is an aqueous solution whose concentration is boric acid/iodide/water=3 to 10/1 to 20/100 in weight ratio. If necessary, other cross-linking agents such as glyoxal and glutaraldehyde may be used instead of boric acid, or boric acid and other cross-linking agents may be used in combination. The temperature of the treatment solution when the film 2 is dipped is usually about 50°C to 70°C, preferably 53°C to 65°C, and the immersion time of the film 2 is usually about 10 seconds to 600 seconds, preferably 20 seconds to 300 seconds, More preferably, it is 20 seconds to 200 seconds. When performing dyeing treatment and cross-linking treatment in sequence on the film 2 stretched before the swelling treatment, the temperature of the treatment solution is usually about 50°C to 85°C, preferably 55°C to 80°C.

When the purpose of the cross-linking treatment is to adjust the hue and, for example, iodine is used as a dichroic dye, the concentration by weight of boric acid/iodide/water=1 to 5/3 to 30/100 containing the cross-linking agent can be used. The solution was used as a treatment liquid. The temperature of the treatment liquid when the film 2 is dipped is usually about 10 to 45° C., and the immersion time of the film 2 is usually about 1 to 300 seconds, preferably 2 to 100 seconds.

The cleaning treatment portion 134 is a portion for performing cleaning treatment on the film ₂ after the crosslinking treatment. The cleaning treatment unit ₁₃₄ has a treatment tank in which a treatment liquid for cleaning treatment is stored. The membrane 2 is subjected to a cleaning treatment by immersing the membrane ₂ in the treatment liquid contained in the cleaning treatment section 134 .

In the first embodiment, the nip roll 11 and the two guide rolls 12 arranged before and after the film 2 is immersed in the treatment liquid form a film conveyance path for immersing the film 2 in the treatment liquid. Examples of the treatment liquid in the cleaning treatment include water, an aqueous solution containing potassium iodide, and an aqueous solution containing boric acid. The temperature of the treatment liquid is usually about 2 to 40°C, and the treatment time (immersion time) is usually about 2 to 120 seconds.

The drying process part ₁₃₅ is a part which performs drying process with respect to the film 2. As shown in FIG. In the first embodiment, the drying processing unit ₁₃₅ is a drying device. In the drying processing section 135, the film ₂ cleaned by the cleaning processing section 134 is carried in, and the film ₂ is dried when the film ₂ passes through the drying processing section 135. In the first embodiment, the film conveyance path for drying the film 2 by the drying device is formed by the nip rolls ₁₁ arranged before and after the drying treatment section 135. In order to support and convey the film 2 , the guide rollers ₁₂ may be appropriately arranged in the drying process section 135 . The drying by the drying treatment section ₁₃₅ can be performed, for example, for ₃₀ seconds to 600 seconds in the drying treatment section 135 maintained at a temperature of, for example, 40°C to 100°C. FIG. 1 schematically shows the drying processing unit 13 ₅ . The drying treatment section 13 to ₅ is not particularly limited as long as it can dry the moisture adhering to the film 2 , and may be a publicly known one that is generally used when manufacturing a polarizing film.

In the manufacturing apparatus 10, the film 2 is subjected to a uniaxial stretching process by utilizing the difference in rotational speed of at least two of the plurality of nip rolls 11 (the nip roll 11 on the upstream side and the nip roll 11 on the downstream side). Extended processing. In this case, the two nip rolls 11 that contribute to the above-described uniaxial stretching process function as a stretching process section.

For example, the stretching treatment of the uniaxial stretching treatment can be performed by utilizing the difference in rotational speed between the nip roll ₁₁ arranged before the crosslinking treatment section 133 and the nip roll ₁₁ arranged after the crosslinking treatment section 133. In this case, since the stretching treatment can be performed in parallel with the crosslinking treatment, the crosslinking treatment section ₁₃₃ also functions as a stretching treatment section. The stretching treatment is also effective for suppressing the occurrence of wrinkles in the film 2 .

In addition to mainly performing the stretching treatment with two nip rolls 11 arranged before and after one treatment section (eg, the above-mentioned crosslinking treatment section 13 ₃ ), the stretching treatment may be further gradually carried out with other nip rolls 11 .

The manufacturing apparatus 10 may additionally have an extension processing part for performing extension processing. In this case, the stretching treatment part is arranged, for example, in the subsequent stage _of the crosslinking treatment part 133 (for example, between the crosslinking treatment part ₁₃₃ and the washing treatment part ₁₃₄ ).

The manufacturing apparatus 10 may have a plurality of at least one treatment section among the swelling treatment section 13 ₁ , the dyeing treatment section 13 ₂ , the crosslinking treatment section 13 ₃ , the washing treatment section 13 ₄ , and the drying treatment section 13 ₅ . For example, the manufacturing apparatus 10 may include a plurality of crosslinking processing units 13 ₃ . The manufacturing apparatus 10 may be provided with an extension processing part.

An example of the manufacturing method of the polarizing film 4 using the manufacturing apparatus 10 mentioned above is demonstrated. First, the film 2 is pulled out from the raw material roll 6 . The pulled-out film 2 is conveyed in the longitudinal direction of the film 2 along a conveyance path formed by a plurality of nip rolls 11 and a plurality of guide rolls 12 . The conveyance speed is, for example, 1 m/min to 60 m/min, or 1.5 m/min to 50 m/min. In the conveyance path of the film 2, from the raw material roll 6 side, a swelling processing part 13 ₁ , a dyeing processing part 13 ₂ , a crosslinking processing part 13 ₃ , a washing processing part 13 ₄ and a drying processing part 13 ₅ are provided. Moreover, as mentioned above, at least two nip rolls 11 also have the function as a drawing process part. Therefore, by conveying the film 2 along the conveyance path, the film 2 can be subjected to swelling treatment, dyeing treatment, cross-linking treatment, washing treatment, and drying treatment, and can also be subjected to stretching treatment. Thereby, the linear polarization characteristic is imparted to the film 2, and the polarizing film 4 is obtained.

The swelling treatment, dyeing treatment, crosslinking treatment, and cleaning treatment described above can be performed by immersing the film 2 in a treatment liquid. Therefore, the manufacturing method of the polarizing film 4 has a plurality of immersion steps (the step of immersing the film 2 in the treatment liquid). In other words, swelling treatment, dyeing treatment, crosslinking treatment, and washing treatment are performed in the above-mentioned plural dipping steps. In the manufacturing method of the polarizing film 4 of the first embodiment, at least one of the plural dipping steps corresponding to the swelling treatment, the dyeing treatment, the cross-linking treatment and the cleaning treatment is applied. One immersion step is implemented as the immersion step described with reference to FIG. 2 (hereinafter, referred to as "immersion step A" for convenience of description).

FIG. 2 is a schematic diagram for explaining the impregnation step A. FIG. In FIG. 2 , a state where the treatment liquid 8 is stored in the treatment tank and the film 2 is immersed while being transported into the treatment liquid 8 is schematically shown. The treatment liquid 8 is a treatment liquid corresponding to the treatment performed in the dipping step A in the swelling treatment, dyeing treatment, crosslinking treatment, and washing treatment. Like the respective treatment sections shown in FIG. 1 , the conveyance path of the film 2 is formed by the nip rolls 11 and two guide rolls 12 before and after the treatment liquid 8 . In the conveying path, the nip roll 11 through which the film 2 passes before being immersed in the treatment liquid 8 is called a nip roll 11 _UP , and the nip roll 11 through which the film 2 passes after the film 2 is immersed in the treatment liquid 8 is called a nip roll. Roll 11 _DOWN .

In the immersion step A, the film 2 may be immersed in the treatment liquid while maintaining the tension applied to the film 2 .

The tension in the impregnation step A is usually 3 N/m to 2000 N/m, preferably 5 N/m to 1500 N/m. When the impregnation step is a swelling step, it is more preferably 3 N/m to 200 N/m, and still more preferably 5 N/m to 100 N/m.

In the dipping step A, a constant tension can be imparted to the film 2 using, for example, the nip rolls 11 _UP and 11 _DOWN shown in FIG. 2 . For example, a constant tension can be imparted by adjusting the rotational speed of the nip roller 11 _UP and the rotational speed of the nip roller 11 _DOWN . As described above, the film 2 can be immersed in the treatment liquid 8 while maintaining the tension by applying a constant tension.

In order to adjust the tension, for example, the tension can be detected by a tension detector. As the tension detector, a general commercial item can be used. For example, the tension can be detected according to principles such as a differential transformer method, a strain gage method, and the like. The tension is continuously monitored with a tension detector. For example, the detected tension value is sent to the control system. In the control system, when the detected tension value exceeds the set value, the amount is measured, and the signal is sent to the driving device (such as servo motor, etc.) that drives the nip roller 11 _DOWN to control the drive, so that the tension value is kept constant.

In the immersion step A, the film 2 can be immersed in the processing liquid while maintaining the rotational speed ratio of the nip rolls before and after the immersion in the processing liquid 8 to be constant. The above-mentioned rotational speed ratio is the ratio of the rotational speed of the nip roll 11 _UP before immersion in the treatment liquid 8 and the rotational speed of the nip roll 11 _DOWN after the immersion in the treatment liquid 8 . In this way, by immersing the film 2 in the processing liquid while maintaining the above-mentioned rotation speed ratio constant, the film 2 can be immersed in the processing liquid while maintaining the tension.

In the immersion step A, the following condition α is satisfied.

<Condition α>

The variation width (%) of the width change rate (%) in the width direction of the film 2 before and after the film 2 is immersed in the treatment liquid 8 is 0.5% or less.

In one embodiment, the width change rate and the variation range are calculated according to equations (1) and (2).

Width change rate=|(W2-W1)/W1|×100...(1)

Variation = maximum value of width change rate - minimum value of width change rate...(2)

In the formula (1), W1 [mm] represents the width of the film 2 before immersion in the treatment liquid 8 , and W2 [mm] represents the width of the film 2 after the immersion in the treatment liquid 8 .

In the formula (2), the maximum value of the width change rate and the minimum value of the width change rate are, for example, values when the width change rate is continuously obtained for a fixed period.

The said fixed period is, for example, 60 seconds or more, preferably 1 hour or more, and more preferably 24 hours or more. The upper limit of the fixed period is the time when the immersion step A is performed with respect to the entire length of the film 2 , and can be determined based on the entire length of the film 2 and the conveyance speed. For example, the upper limit is 200 hours.

The width W1 of the film 2 before the film 2 is immersed in the treatment liquid 8 is, for example, the width of the film 2 at the position x1 as shown in FIG. 2 . In FIG. 2, the position x1 is the position after the film 2 passes through the nip roll 11 _UP . The width W2 of the film 2 after the film 2 is immersed in the treatment liquid 8 is, for example, the width of the film 2 at the position x2 shown in FIG. 2 . The position x2 represents the position after the film 2 has passed the nip roll 11 _DOWN .

In order to obtain (or calculate) the above-mentioned width change rate and fluctuation range, the manufacturing apparatus 10 may include two width measuring devices 30 and a calculating part 40 shown in FIG.

An example of a method of obtaining the width change rate and the variation width using the width measuring device 30 and the calculation unit 40 will be described. Hereinafter, a series of steps from the measurement of the width W1 and the width W2 to the acquisition of the width change rate and the width of the change will be collectively referred to as a "monitoring step".

The two width measuring devices 30 are devices for continuously measuring the width of the film 2, respectively. One of the two width measuring devices 30 is a width measuring device (hereinafter referred to as "width measuring device 30 _UP ") for measuring the width W1 of the film 2 before being immersed in the treatment liquid 8, and among the two width measuring devices 30 The other is a measuring device (hereinafter referred to as "width measuring device 30 _DOWN ") for measuring the width W2 of the film 2 after the film 2 is immersed in the treatment liquid 8.

The width measuring device 30 _UP and the width measuring device 30 _DOWN each have two end portion detection portions 31 . One of the two end portion detection portions 31 is a detection portion for detecting one end portion 2 a in the width direction of the film 2 , and the other is It is a detection part which detects the other edge part 2b (the edge part on the opposite side to the said edge part 2a) in the width direction of the film 2. Each edge part detection part 31 detects the edge part of the film 2 optically and non-contact, for example. Each end portion detection unit 31 may be configured to obtain an image of the end portion to be detected and its vicinity. For example, the end detection unit 31 may have an imaging unit such as a camera.

The calculation unit 40 calculates the width W1 based on the detection results of the two end detection parts 31 of the width measuring device 30 _UP , and calculates the width W1 based on the detection results of the two end detection parts 31 of the width measuring device 30 _DOWN . Width W2. For example, the calculation part 40 determines the positions of the end parts 2a and 2b according to the detection results (eg, image data) of the two end part detection parts 31 of the width measuring device 30 _UP , thereby calculating the width W1 and the width W2.

The calculation unit 40 calculates the width change rate from the width W1, the width W2 and the formula (1), and calculates the variation width from the formula (2).

In the first embodiment, the width W1 may be measured at the position x1 and then the width W2 may be measured at the time point when the film position where the width W1 is measured is transported to the position x2, or the width W2 may be measured at the same time point (ie, at the same time) at the position x1 The width W1 is the same as the width W2 at position x2.

The "same time point" may have a slight deviation within the scope of the gist of the present invention. The time difference between the measurement at the position x1 and the measurement at the position x2 may be within 1 minute, may be within 30 seconds, may be within 20 seconds, or may be Can be within 10 seconds.

With regard to the monitoring step, it is preferably carried out automatically. In the first embodiment, it is preferable to efficiently automate and measure the width W1 at the position x1 and the width W2 at the position x2 at the same time.

The above-mentioned condition α can be realized by, for example, adjusting the temperature of the treatment liquid 8 , the immersion time (treatment time), the tension applied to the film 2 , and the like.

In the monitoring in the monitoring step, when the fluctuation range exceeds 0.5%, each processing condition and the like can be adjusted according to the processing of the monitoring step, whereby the width change rate can be adjusted so as to satisfy the above-mentioned condition α. For example, the temperature of the treatment liquid 8, the immersion time (treatment time), the tension applied to the film 2, etc. can be adjusted so as to satisfy the above-mentioned condition α, or the film 2 used in the production of the polarizing film 4 can be replaced. More specifically, in the monitoring step at the time of swelling treatment, if the width change rate is small, it can be increased by increasing the width. The fluctuation range can be reduced by means of the solution temperature, tension reduction, and processing time. On the other hand, when the width change rate is large, the fluctuation range can be reduced by reducing the processing solution temperature, increasing the tension, and shortening the processing time. In the cross-linking step, when the film 2 is immersed in the treatment liquid and implemented as the dipping step A and the monitoring step is performed, in addition to the above-mentioned manufacturing conditions (temperature, tension, processing time), the content of the cross-linking agent or the arrangement of the guide rollers can be used. and so on to adjust the range of changes.

The manufacturing method of the polarizing film 4 has an immersion step A which satisfies the above-mentioned condition α. Therefore, in the immersion step A, even if the film 2 is immersed in the treatment liquid 8, the fluctuation range of the width change rate of the film 2 is 0.5% or less. In this case, since the fluctuation range of the membrane 2 is small, the membrane 2 is less likely to be broken, for example, in the dipping step A or in a step after the dipping step A. As a result, the polarizing film 4 can be stably produced. Since the fluctuation range of the film 2 is small, defects and poor appearance due to width changes can be reduced. As a result, a high-quality polarizing film 4 can be produced.

It is effective to carry out the impregnation step A in the swelling treatment. As shown in FIG. 1, the swelling process is a process performed on the upstream side (the initial stage of a manufacturing process) among a plurality of processes performed sequentially with respect to the film 2. As shown in FIG. Therefore, in the swelling treatment, the width of the film 2 is easily changed, so that the quality of the produced polarizing film 4 can be improved by performing the above-mentioned dipping step A in the swelling treatment. In addition, the film 2 is less likely to be broken during the swelling treatment and the subsequent treatments. When the immersion step A is performed in the swelling treatment, the temperature of the treatment liquid (eg, water) is preferably 15°C to 40°C, and the immersion time is preferably 20 seconds to 120 seconds.

As described above, the immersion step A satisfying the condition α may be performed in at least one immersion step among a plurality of immersion steps corresponding to swelling treatment, dyeing treatment, crosslinking treatment, and washing treatment.

(Second Embodiment)

Regarding the second embodiment, the long polyvinyl alcohol-based film 2 will be described. In the second embodiment, the polyvinyl alcohol-based film 2 is also simply referred to as "film 2". Examples of the material, length and thickness of the film 2 of the second embodiment are the same as those of the first embodiment.

The film 2 is a film that satisfies the following condition β.

<Condition β>

When the film 2 is immersed in water at a predetermined temperature and a predetermined immersion time while conveying the polyvinyl alcohol-based film, the variation (%) of the width change rate (%) in the width direction of the film 2 before and after the immersion is 0.5% or less film.

From the viewpoint of further suppressing the color unevenness of the polarizing film produced using the film 2, the above-mentioned fluctuation range (%) in the condition β is preferably 0.3% or less.

The above-mentioned predetermined temperature is a temperature selected from a range of 15° C. to 40° C., and the above-mentioned predetermined dipping time is a dipping time selected from a range of 20 seconds to 120 seconds.

The above-mentioned width change rate (%) can be calculated in the same manner as the above-mentioned formula (1), for example. The water in the description of the above-mentioned condition β corresponds to the treatment liquid 8 in the immersion step A of the first embodiment.

The above-mentioned fluctuation range (%) is the difference between the maximum value of the width change rate and the minimum value of the width change rate when the above-mentioned width change rate is continuously obtained for 60 seconds or more.

In the above-mentioned condition β, the polyvinyl alcohol-based film is usually conveyed in a state in which the tension is fixed, or in a state in which the rotation speed ratio of the nip rolls before and after being immersed in water is fixed. When the said polyvinyl-alcohol-type film is conveyed in the state which made the tension|tensile_strength fixed, it is set as the state which maintains the tension|tensile_strength selected from the range of 5N/m to 100N/m normally. When the said polyvinyl-alcohol-type film is conveyed in the state which fixed the rotation speed ratio before and after immersion in water, it is set as the state which maintains the rotation speed ratio selected from the range of 1.1-4.0 normally. The above-mentioned rotation speed ratio is the ratio of the rotation speed of the nip roll before immersion in water and the rotation speed of the nip roll after immersion in water.

For example, in the immersion step A described using FIG. 2 , the film 2 is immersed in the treatment liquid 8 at the above-mentioned predetermined temperature and predetermined immersion time using the treatment liquid 8 as water, and the width change rate is calculated in the same manner as in the immersion step A. (%) fluctuation range (%), by which it can be verified whether the film 2 is a film that satisfies the condition β. As described above, when the film 2 is transported in a state where the tension is fixed or the above-mentioned rotation speed ratio before and after immersion in water is fixed, the method of fixing the tension or the above-mentioned rotation before and after immersion in water The method in which the speed ratio is fixed can be the same as in the impregnation step A.

The said film 2 can be manufactured by, for example, a melt extrusion method, a solvent casting method, etc., by adjusting manufacturing conditions so that the said condition (beta) may be satisfied, for example. In addition, when the film 2 is prepared by a method including the steps of applying and drying the polyvinyl alcohol-based resin solution, from the viewpoint of easily preparing a film satisfying the above-mentioned condition β, the temperature is preferably 30° C. or higher and Dry for more than 30 minutes at a temperature below 50°C.

When the polarizing film 4 is produced using the film 2 that satisfies the condition β, for example, a high-quality polarizing film can be obtained by performing the swelling treatment by performing the dipping step (dipping step A) that satisfies the condition α, for example. In addition, in the production of the polarizing film 4, since the film 2 is not easily broken, the polarizing film 4 can be stably produced.

This point will be explained concretely with Example 1, Example 2, and Comparative Example 1. In Example 1, Example 2, and Comparative Example 1, polyvinyl alcohol-based films were produced. For convenience of description, in Example 1, Example 2, and Comparative Example 1, the produced polyvinyl alcohol-based film is referred to as "film 2".

[Example 1]

<Manufacture of Film 2>

(Fabrication of base film)

1 weight of a nucleating agent composed of high-density polyethylene was blended with homopolypropylene ("Sumitomo nobrene (registered trademark) FLX80E4" manufactured by Sumitomo Chemical Co., Ltd., melting point Tm=163°C), which is a propylene homopolymer. %, and make polypropylene with nucleating agent added. This and the propylene/ethylene random copolymer "Sumitomo nobrene (registered trademark) W151" containing about 5% by weight of ethylene units were co-extruded using a multi-layer extrusion molding machine to produce a three-layer structure of long polypropylene. It is a laminated film in which resin layers made of the above-mentioned nucleating agent-added polypropylene are arranged on both sides of a resin layer made of "Sumitomo nobrene (registered trademark) W151", and this is used as a base film. The total thickness of the base film is 100 μm, and the thickness ratio of each layer (polypropylene with nucleating agent/W151/polypropylene with nucleating agent) is 3/4/3.

(Adjustment of coating solution)

Polyvinyl alcohol powder (“Z-200” manufactured by Nippon Synthetic Chemical Industry Co., Ltd., average molecular weight 1100, average saponification degree 99.5 mol%) was dissolved in hot water at 95°C, and adjusted to a polyvinyl alcohol aqueous solution with a concentration of 3% by weight . In the obtained aqueous solution, 1 part by weight of a crosslinking agent ("Sumirez Resin (registered trademark) 650" manufactured by Sumitomo Chemical Co., Ltd.) was mixed with 2 parts by weight of polyvinyl alcohol to prepare a coating liquid for forming a primer layer.

In addition, polyvinyl alcohol powder (“PVA124” manufactured by Kuraray Co., Ltd., average degree of polymerization 2400, average saponification degree 98.0 to 99.0 mol%) was dissolved in hot water at 95°C to prepare a concentration of 8% by weight of polyvinyl alcohol A coating solution for forming a polyvinyl alcohol-based resin layer in an aqueous solution.

(coating and drying)

The corona treatment was performed on one side of the base film while continuously conveying the base film in the longitudinal direction. Next, the above-mentioned coating liquid for primer layer was continuously applied on the corona-treated surface using a gravure coater, and dried at 60° C. for 3 minutes to form a primer layer with a thickness of 0.2 μm on one side. Next, while conveying the film in the longitudinal direction, the above-mentioned polyvinyl alcohol-based coating was continuously coated on each primer layer using a Comma coater. The coating liquid for resin layer formation was dried using 80 degreeC hot air for 5 minutes. After that, it was completely dried using hot air at 30° C., whereby a polyvinyl alcohol-based resin layer with an average thickness of 30 μm was formed on the primer layer, which was wound around a shaft to obtain a base film and a film laminated on the base film. 2. Raw material roll for laminated film. The length of the laminated film wound around the raw material roll was 5000 m, and the width of the laminated film was 800 mm. The laminated film constituting the raw material roll is formed by laminating the laminated film 2 on the base film. Therefore, as shown in Table 1, a film 2 having a width of 800 mm, a thickness (average thickness) of 30 μm, and a length of 5000 m was obtained.

[Table 1]

<Manufacture of polarizing film>

The film 2 was pulled out while peeling the base film from the above-mentioned laminated film, and was immersed for 30 seconds while applying a tension of 10 N/m in a swelling treatment tank containing 30° C. pure water as a swelling treatment (swelling step). Next, as a dyeing treatment, dipping for 60 seconds in a dyeing treatment tank containing an aqueous solution of 30° C. containing iodine and potassium iodide while uniaxially extending to 2.2 times, dipping in a dyeing treatment tank containing potassium iodide/boric acid/water at a weight ratio of 12/4.4 /100/100/100 of a 55°C aqueous solution in a boric acid treatment tank to perform water resistance treatment, and uniaxially stretch until the cumulative stretching ratio from the raw material roll becomes 5.5 times. Then, it immersed in the tank which added 40 degreeC boric-acid aqueous solution, and was immersed in the washing|cleaning process tank which added 12 degreeC pure water, and it dried at 70 degreeC for 3 minutes in a drying furnace after that, and produced a polarizing film.

In Example 1, while conveying the film 2, the width of the film 2 was continuously measured over the entire length of the film 2 before and after the swelling step. The width of the film 2 refers to the film 2 (position x1 and position x2 in the example of FIG. 2 ) conveyed on the rolls of the nip rolls provided before and after the swelling treatment tank, and both ends of the film are The parts were irradiated with LED light, and the positions of both ends of the film were measured from the difference in luminance between the film 2 and the reflected light in the roll, and calculated from the measured positions. The width measurement time points before and after the swelling treatment tank were the same. 2 is a schematic diagram for explaining the dipping step, the film 2 is separated from the roller at the position x2. However, in Example 1, as described above, the dyeing treatment is performed after the swelling treatment, so The film partially touches the roll even at position x2. Moreover, according to the width|variety of the film 2 measured continuously in this way, the width change rate is sequentially calculated, and the fluctuation|variation width is calculated as the difference of the maximum value and the minimum value in this width change rate. The calculation results of the fluctuation range are shown in Table 2. Table 2 also shows the conditions of the swelling step. The width change rate and the variation width are calculated using the formulas (1) and (2).

In the production of the above-mentioned polarizing film in Example 1, as shown in Table 2, the film 2 did not break.

<Evaluation of uneven color conditions>

The manufactured polarizing film was arranged in a crossed polarizer state with respect to the linear polarizing filter in a dark room. Thereafter, the polarizing film was irradiated with a backlight of 6000 cd/m ² through the above-mentioned linear polarizing filter, and the color unevenness of the polarizing film was visually observed. Next, the level (intensity) of the color unevenness was determined in three stages of "1", "2", and "3" based on visual sensory inspection. The evaluation "1" indicates that the unevenness is the least, the evaluation "3" indicates that the unevenness is the most serious, and the evaluation "2" indicates that the unevenness is intermediate between the evaluation "1" and the evaluation "3". In the above-mentioned sensory inspection, the color unevenness was evaluated in the above-mentioned three stages by comparing with the graded sample samples according to the color unevenness grade (intensity). The polarizing film produced in Example 1 was evaluated as "1".

[Example 2]

In Example 2, in the same manner as in Example 1, a raw material roll of a laminated film having a base film and the film 2 laminated on the base film was produced. The width, thickness and length of the film 2 obtained in Example 2 are shown in Table 1 above, and are the same as the width, thickness and length of the film 2 in Example 1. In Example 2, the film 2 was pulled out while peeling the base film from the above-mentioned laminated film, and was immersed in a swelling treatment tank containing pure water at 35°C for 20 seconds while applying a tension of 15 N/m as a swelling treatment (swelling step) , except that a polarizing film was produced in the same manner as in Example 1.

In the production of the above-mentioned polarizing film in Example 2, as shown in Table 2, the film 2 did not break.

<Evaluation of uneven color conditions>

The color unevenness of the polarizing film produced was evaluated in the same manner as in Example 1. The evaluation result of the polarizing film produced in Example 2 was "2".

Also in Example 2, in the same manner as in Example 1, the width of the film 2 was measured before and after the swelling step, and the fluctuation range was calculated. The calculation results of the fluctuation range are shown in Table 2.

[Comparative Example 1]

In Comparative Example 1, the film 2 was produced in the same manner as in Example 1, except that the coating liquid for forming a polyvinyl alcohol-based resin layer was dried using hot air at 100° C. for 15 minutes. Specifically, the raw material roll of the laminated film which has a base film and the film 2 laminated|stacked on the base film is manufactured. The width, thickness and length of the film 2 obtained in Comparative Example 1 are as shown in Table 1 above, and are the same as the width, thickness and length of the film 2 in Example 1. The film 2 was pulled out while peeling the base film from the raw material roll thus produced, and a polarizing film was produced in the same manner as in Example 1.

In Comparative Example 1, in the production of the above-mentioned polarizing film, as shown in Table 2, the film 2 was broken. In Comparative Example 1, after the film 2 was broken, a polarizing film was produced while pulling out the film 2 again from the same raw material roll. In Comparative Example 2, the film 2 was broken twice until it was used to 2000 m.

<Evaluation of uneven color conditions>

In the same manner as in Example 1, the color unevenness of the produced polarizing films was evaluated. The evaluation result of the polarizing film produced in Comparative Example 1 was "3".

Also in Comparative Example 1, in the same manner as in Example 1, the width of the film 2 was measured before and after the swelling step, and the fluctuation range was calculated. The calculation results of the fluctuation range are shown in Table 2. Table 2 also shows the conditions of the swelling step. In Table 2, 0.69 (%) is the fluctuation range until the first crack occurs, and 0.75% is the fluctuation range until the second crack occurs.

[Table 2]

In Example 1 and Example 2, as described above, the width of the film 2 before and after the swelling treatment was continuously measured over the entire length of the film 2, and the fluctuation range was calculated. Therefore, the fluctuation range of Example 1 and Example 2 is the difference between the maximum value of the width change rate and the minimum value of the width change rate when the width change rate is continuously obtained for 60 seconds or more. As described above, pure water is used in the swelling treatment.

Therefore, in the swelling treatment described above, the immersion of the membrane 2 in pure water corresponds to the immersion of the membrane 2 in the condition β. As shown in Table 2, according to the swelling step conditions and width measurement results of Example 1 and Example 2 The calculated fluctuation range (%) shows that the films 2 produced in Examples 1 and 2 satisfy the condition β. In addition, the swelling process of Example 1 and Example 2 corresponds to the dipping process A which satisfies the condition α. On the other hand, according to the fluctuation range (%) calculated from the swelling step conditions and width measurement results of Comparative Example 1, it was found that the film 2 produced in Comparative Example 1 did not satisfy the condition β, and the swelling step of Comparative Example 1 also did not satisfy the condition β. Condition α is not satisfied.

In addition, in Example 1 and Example 2 in which the swelling treatment was performed by performing the immersion step A satisfying the condition α, the film 2 was not easily broken during the production of the polarizing film, so that the polarizing film could be stably produced. In Example 1 and Example 2, since the membrane 2 satisfies the condition β, the immersion step A is easily performed.

As can be seen from the results of Examples 1 and 2 shown in Table 2, by setting the variation range (%) to be 0.3% or less, the color unevenness can be further suppressed.

The embodiments of the present invention are described above. However, the present invention is not limited to the above-mentioned embodiments, and the present invention includes the scope shown in the scope of the patent application, and includes all modifications within the meaning and scope equivalent to the scope of the patent application.

When the manufacturing method of the polarizing film includes the cleaning treatment in the cleaning treatment section 134 shown in FIG. ₁ , in the cleaning treatment, a method of spraying with the treatment liquid as a shower, or a method of combining dipping and spraying can be used. and so on, the cleaning of the membrane 2 is performed.

The method of measuring the width of the film 2 is not limited to the illustrated method. For example, the width can be measured by measuring equipment such as a laser displacement meter and an LED displacement meter. The width can also be calculated from the image obtained by photographing the entire film 2 with a camera or the like. As shown in FIG. 3, in the method of obtaining the positions of the end portions 2a, 2b of the film 2, it is only necessary to arrange devices for measuring the positions of the end portions 2a, 2b at the end portions 2a, 2b of the film 2, respectively. It is preferable from the viewpoint of space or machine management (maintenance and inspection, etc.).

The stretching treatment in the stretching treatment section is not limited to the wet stretching method, and a dry stretching method can also be used. In the above-described embodiments, the order of the processes exemplified in order to manufacture the polarizing film can be appropriately changed or combined within the scope not departing from the gist of the present invention. The number of processing tanks included in each processing unit may be one or plural.

The above-described embodiments and various modifications can be appropriately combined without departing from the gist of the present invention.

2: Film (polyvinyl alcohol-based film)

4: polarizing film

6: Raw material roll

10: Manufacturing device

11: Pinch roller

12: Guide roller

13 ₁ : Swelling treatment section

13 ₂ : Dyeing Processing Section

13 ₃ : Cross-Linking Treatment Section

13 ₄ : Cleaning Treatment Section

13 ₅ : Drying section

Claims (10)

A method for producing a polarizing film, which is a method for producing a polarizing film while conveying a polyvinyl alcohol-based film, the method comprising a dipping step of immersing a long polyvinyl alcohol-based film in a treatment solution, Wherein, in the dipping step, the variation range (%) of the width change rate (%) in the width direction of the polyvinyl alcohol-based film before and after the polyvinyl alcohol-based film is immersed in the treatment solution is 0.5% or less. The method for producing a polarizing film according to claim 1, wherein in the dipping step, the polyvinyl alcohol-based film is immersed in the processing liquid while applying a constant tension with a nip roll before and after the immersion in the processing liquid middle. The method for producing a polarizing film according to claim 1, wherein in the dipping step, the polyvinyl alcohol-based film is dipped while maintaining a constant rotation speed ratio of the nip rolls before and after dipping in the treatment solution. in the aforementioned treatment solution. The method for producing a polarizing film according to any one of claims 1 to 3, wherein the dipping step is performed during the swelling treatment of the polyvinyl alcohol-based film. The method for producing a polarizing film according to any one of claims 1 to 4, wherein the width change rate is calculated by formula (1), and the variation range is calculated by formula (2), Width change rate=|(W2-W1)/W1|×100...(1) Variation = maximum value of width change rate - minimum value of width change rate...(2) In the formula (1), W1 [mm] represents the width of the polyvinyl alcohol-based film before immersion in the treatment liquid, W2 [mm] represents the width of the polyvinyl alcohol-based film after immersion in the treatment liquid, In the formula (2), the maximum value of the width change rate and the minimum value of the width change rate are the values when the aforementioned width change rate is continuously obtained for 60 seconds or more. A polyvinyl alcohol-based film in the width direction of the polyvinyl alcohol-based film before and after the immersion when a long polyvinyl alcohol-based film is immersed in water under conditions including a predetermined temperature and a predetermined immersion time while being conveyed If the variation (%) of the width change rate (%) is less than 0.5%, Wherein, the aforementioned predetermined temperature is a temperature selected from the range of 15°C to 40°C, The aforementioned predetermined immersion time is an immersion time selected from the range of 20 seconds to 120 seconds, The variation range (%) is the difference between the maximum value of the width change rate and the minimum value of the width change rate when the width change rate is continuously obtained for 60 seconds or more. The polyvinyl alcohol-based film according to claim 6, wherein the variation range (%) is the following: The range of fluctuation when the polyvinyl alcohol-based film is immersed in the water under conditions including the predetermined temperature and the predetermined immersion time while being conveyed while maintaining a tension selected from the range of 5 N/m to 100 N/m . The polyvinyl alcohol-based film according to claim 6, wherein the variation range (%) is the following: Before and after being immersed in the water, the polyvinyl alcohol-based film is conveyed at a temperature including the predetermined temperature and the The fluctuation range when immersed in the aforementioned water under the condition of a predetermined immersion time. The polyvinyl alcohol-based film according to any one of claims 6 to 8, wherein the fluctuation range (%) is 0.3% or less. The polyvinyl alcohol-based film according to any one of claims 6 to 9, wherein the length of the polyvinyl alcohol-based film in the longitudinal direction is 1000 m or more.

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