Method for producing polarized film

Abstract

[Problem] To provide a method for manufacturing a polarizing film that can produce a polarizing film with a good appearance through a stable process. [Solution] A method for manufacturing a polarizing film according to one embodiment is such that, in the crosslinking process, N stretching treatments (N is an integer greater than or equal to 1) are performed on a polyvinyl alcohol-based film, and the N stretching treatments are performed within a range satisfying Equation (1) and Equation (2). α=(ab) / a···(1) 0.28≤αmax≤0.42···(2) (In Equation (1), a and b represent the average thickness [μm] in the width direction of the polyvinyl alcohol-based film before and after the nth (n is an integer from 1 to N) stretching treatment, and the average thickness in the width direction is the average thickness of the center and both ends in the width direction of the polyvinyl alcohol-based film. In Equation (2), αmax is the maximum value of N αs obtained in N stretching treatments.)

Description

Technology Field

[0001] The present invention relates to a method for manufacturing a polarizing film. Background Technology

[0002] A polarizing film is manufactured by carrying a polyvinyl alcohol-based film and, in addition to a stretching treatment, performing, for example, a dyeing treatment, a crosslinking treatment, a drying treatment, etc. on the polyvinyl alcohol-based film (e.g., see Patent Document 1). Prior art literature

[0003] Patent Document 1: Japanese Patent Publication No. 2002-40256 The problem to be solved

[0004] When a stretching treatment is performed on a polyvinyl alcohol-based film, the thickness of the polyvinyl alcohol-based film decreases as the film is stretched. Consequently, there were cases where the polyvinyl alcohol-based film broke during the manufacture of a polarizing film, or where more color stains occurred, causing the appearance to deteriorate.

[0005] Therefore, the present invention aims to provide a method for manufacturing a polarizing film having a good appearance through a stable process. means of solving the problem

[0006] A method for manufacturing a polarizing film according to the present invention comprises a swelling process, a dyeing process, and a crosslinking process. In the crosslinking process, N stretching treatments (N is an integer greater than or equal to 1) are performed on a polyvinyl alcohol-based film, and the N stretching treatments are performed within a range satisfying Equation (1) and Equation (2).

[0007] α=(ab) / a···(1)

[0008] 0.28≤αmax≤0.42···(2)

[0009] (In Equation (1), a represents the average thickness [μm] in the width direction of the polyvinyl alcohol-based film before the n-th stretching treatment (where n is an integer from 1 to N), and b represents the average thickness [μm] in the width direction of the polyvinyl alcohol-based film after the n-th stretching treatment; the average thickness in the width direction is the average of the thickness of the central part and the thickness of both ends in the width direction of the polyvinyl alcohol-based film. In Equation (2), αmax is the maximum value of N αs obtained for the N stretching treatments.)

[0010] In this case, N stretching treatments are performed within a range satisfying the above equations (1) and (2). Therefore, a polarizing film with a good appearance can be manufactured through a stable process.

[0011] The stretching ratio for each of the above N stretching treatments may be 1.001 or more and 4.00 or less.

[0012] In each of the above N stretching processes, the polyvinyl alcohol-based film may be stretched by a nip roll positioned before and after each stretching process. In this case, for example, the polyvinyl alcohol-based film may be stretched by the difference in rotational speed of the nip roll positioned before and after each stretching process.

[0013] Each of the above N stretching treatments may be performed within a range satisfying the following equation (3).

[0014] 0.1≤Δa / Δb≤1.1···(3)

[0015] (In Equation (3), Δa represents the difference between the maximum and minimum values ​​of the thickness in the width direction of the polyvinyl alcohol-based film before the nth stretching treatment, and Δb represents the difference between the maximum and minimum values ​​of the thickness in the width direction of the polyvinyl alcohol-based film after the nth stretching treatment.)

[0016] By performing N stretching treatments to further satisfy Equation (3), the breakage of the polyvinyl alcohol-based film can be further suppressed. In addition, since defects such as color stains or wrinkles are less likely to occur, it is easier to manufacture a polarizing film with an even better appearance. Effects of the invention

[0017] According to the present invention, a method for manufacturing a polarizing film having a good appearance can be provided through a stable process. Brief explanation of the drawing

[0018] FIG. 1 is a schematic diagram illustrating a method for manufacturing a polarizing film according to one embodiment. Figure 2 is a diagram illustrating the measurement location of the thickness in the width direction of the film. Figure 3 is a diagram illustrating an example of a method for measuring the thickness of a film. FIG. 4 is a graph showing the conditions and thickness measurement results of Examples 1 to 4 and Comparative Examples 1 to 6. FIG. 5 is a graph showing the results of Examples 1 to 4 and Comparative Examples 1 to 6. Specific details for implementing the invention

[0019] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, identical or equivalent parts are denoted by the same reference numerals, and redundant descriptions are omitted. The dimensional ratios of the drawings do not necessarily correspond to those of the description.

[0020] FIG. 1 is a schematic diagram illustrating an example of a method for manufacturing a polarizing film according to an embodiment of the present invention.

[0021] In this embodiment, a polarizing film (4) is manufactured by conveying a long polyvinyl alcohol-based film (2) (hereinafter simply referred to as "film (2)") and performing swelling treatment, dyeing treatment, crosslinking treatment, stretching treatment, washing treatment, and drying treatment on the film (2) being conveyed.

[0022] When linear polarization characteristics are imparted to the film (2), the film (2) functions as a polarizing film (4). Hereinafter, for convenience of explanation, unless otherwise noted, the film (2) after all processing in the manufacture of the polarizing film is finished is referred to as the polarizing film (4), and the film before all processing is finished is referred to as the film (2).

[0023] The material of the film (2) may be a known polyvinyl alcohol-based resin used in the manufacture of polarizing films, and 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 even more preferably 93.0 to 99.5 mol%. The degree of saponification is a numerical value defined by the formula: degree of saponification (mol%) = (number of hydroxyl groups) / (number of hydroxyl groups + number of acetic acid groups) × 100, and can be obtained by the method specified in JIS K 6726 (1994). The average degree of polymerization of the polyvinyl alcohol-based resin is preferably 100 to 10000, and more preferably 1000 to 10000. The average degree of polymerization is a numerical value obtained according to the method specified in JIS K 6726 (1994).

[0024] The length of the film (2) in the longitudinal direction is, for example, 1000 m or more. When the length of the film (2) in the longitudinal direction is 1000 m or more, the length of the film (2) in the longitudinal direction is, for example, 30000 m or less, and preferably 20000 m or less. An example of the length (L) (see FIG. 2) in the width direction (direction perpendicular to the longitudinal direction) of the film (2) is 1300 mm to 5000 mm. An example of the thickness of the film (2) (film (2) constituting the fabric roll (6) described later) before the aforementioned multiple treatments are performed is 10 μm to 100 μm. The film (2) can be manufactured by melt extrusion, solvent casting, etc. The film (2) may be a purchased film or a film that has undergone prior treatments such as stretching or lamination. FIG. 1 illustrates a case in which a film (2) is prepared as a fabric roll (6), and a polarizing film (4) is obtained by performing the aforementioned plurality of treatments on the film (2) unwound from the fabric roll (6). When the film (2) is manufactured by the above method (melt extrusion method, solvent casting method, etc.), for example, the film (2) manufactured according to the above method (melt extrusion method, solvent casting method, etc.) may be continuously conveyed, and the above plurality of treatments may be performed during the conveying.

[0025] Based on the form shown in FIG. 1, an example of a method for manufacturing a polarizing film (4) is described. First, a schematic of a manufacturing apparatus (10) for a polarizing film (4) is described. The manufacturing apparatus (10) is equipped with a plurality of nip rolls (11), a plurality of guide rolls (12), a swelling treatment unit (131), a dyeing treatment unit (132), a crosslinking treatment unit (133), a washing treatment unit (134), and a drying treatment unit (135).

[0026] A plurality of nip rolls (11) and a plurality of guide rolls (12) constitute a conveying mechanism for the film (2). By appropriately arranging the plurality of nip rolls (11) and the plurality of guide rolls (12), a conveying path for the film (2) is formed.

[0027] The nip roll (11) has the function of imparting rotational force of the two rolls to the film (2) by sandwiching the film (2) between the two rolls and also pressing it. The nip roll (11) also has the function of changing the transport direction of the film (2).

[0028] The guide roll (12) supports the film (2) and has the function of changing the direction of transport of the film (2).

[0029] The swelling treatment section (131) is a section that performs swelling treatment on the film (2). The swelling treatment section (131) has a treatment tank in which a treatment solution for swelling treatment is stored. By immersing the film (2) in the treatment solution of the swelling treatment section (131), swelling treatment is performed on the film (2). In this embodiment, a transport path for the film immersed in the treatment solution is formed by a nip roll (11) and two guide rolls (12) arranged before and after the film (2) is immersed in the treatment solution.

[0030] The above swelling treatment is performed for purposes such as removing foreign substances from the surface of the film (2), removing plasticizers from the film (2), providing ease of dyeing in subsequent processes, and plasticizing the film (2). The conditions for the swelling treatment can be determined within a range that can achieve these purposes and also within a range where problems such as extreme dissolution or devitrification of the film (2) do not occur. In the swelling treatment section (131), the swelling treatment is performed by immersing the film (2) in a treatment solution, for example, at a temperature of 10°C to 50°C, preferably 15°C to 40°C. The time for the swelling treatment is approximately 5 seconds to 300 seconds, preferably 20 seconds to 120 seconds. An example of the treatment solution in the swelling treatment section (131) is water. Therefore, the swelling treatment can also serve as a water washing treatment for the film (2).

[0031] The dyeing treatment section (132) is a section that performs a dyeing treatment on the film (2). The dyeing treatment section (132) has a treatment tank in which a treatment solution for dyeing treatment is stored. By immersing the film (2) in the treatment solution of the dyeing treatment section (132), a dyeing treatment is performed on the film (2). In this embodiment, a conveying path for the film immersed in the treatment solution is formed by a nip roll (11) and two guide rolls (12) arranged before and after the film (2) is immersed in the treatment solution.

[0032] The treatment solution of the dyeing treatment unit (132) in this embodiment is an aqueous solution of a dichromatic pigment, and in the dyeing treatment, the film (2) is dyed with the dichromatic pigment. The dyeing treatment with a conventional dichromatic pigment is performed for purposes such as adsorbing the dichromatic pigment onto the film (2). The treatment conditions are determined according to the desired optical properties within a range that can achieve such purposes and also within a range where problems such as extreme dissolution or devitrification of the film (2) do not occur. Examples of dichromatic pigments used for dyeing are iodine and dichromatic dyes.

[0033] When iodine is used as a dichromatic pigment, the dyeing treatment is performed by immersing the film (2) for 10 to 600 seconds, preferably 30 to 300 seconds, in an aqueous solution containing 0.003 to 0.2 parts by weight of iodine and 0.1 to 10 parts by weight of potassium iodide per 100 parts by weight of water, at a temperature of, for example, 10°C to 50°C, preferably 15°C to 40°C. Instead of potassium iodide, other iodides, such as zinc iodide, may be used. Other iodides may be used in combination with potassium iodide. In addition, compounds other than iodides, boric acid, zinc chloride, cobalt chloride, etc., may be coexisted. A treatment solution containing 0.003 parts by weight or more of iodine per 100 parts by weight of water can be considered a treatment solution for dyeing.

[0034] When a water-soluble dichromatic dye is used as the dichromatic pigment, the dyeing treatment is performed by immersing the film (2) in an aqueous solution containing 0.001 parts by weight to 0.1 parts by weight of the dichromatic dye per 100 parts by weight of water at a temperature of, for example, 20°C to 80°C, preferably 30°C to 60°C, for 10 seconds to 600 seconds, preferably 20 seconds to 300 seconds. The aqueous solution of the dichromatic dye used may contain a dyeing agent, etc., or may contain an inorganic salt such as sodium sulfate, a surfactant, etc. Only one type of dichromatic dye may be used, or two or more types of dichromatic dyes may be used in combination depending on the desired color.

[0035] The crosslinking treatment section (133) is a section that performs crosslinking treatment on the film (2). The crosslinking treatment section (133) has a treatment tank in which a treatment solution for crosslinking treatment is stored. By immersing the film (2) in the treatment solution of the crosslinking treatment section (133), crosslinking treatment is performed on the film (2). In this embodiment, a transport path for the film immersed in the treatment solution is formed by a nip roll (11) and two guide rolls (12) arranged before and after the film (2) is immersed in the treatment solution.

[0036] Crosslinking treatment is a treatment performed for purposes such as water resistance or color adjustment (preventing the film (2) from turning bluish) by crosslinking.

[0037] The treatment solution used in the crosslinking treatment unit (133) is, for example, an aqueous solution containing 1 to 10 parts by weight of boric acid per 100 parts by weight of water. When the dichromatic pigment used in the dyeing treatment is iodine, the treatment solution used in the crosslinking treatment unit (133) preferably contains an iodide in addition to boric acid, and the amount thereof is, for example, 1 to 30 parts by weight per 100 parts by weight of water. Examples of iodides include potassium iodide and zinc iodide. Compounds other than iodides, zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, sodium sulfate, etc. may also be coexisted.

[0038] In the cross-linking treatment in the cross-linking treatment section (133), the concentration of boric acid and iodide, the temperature of the treatment solution, the treatment time, the distance between rolls, etc., can be appropriately changed according to the purpose.

[0039] For example, when the purpose of the crosslinking treatment is to make the film (2) water resistant through crosslinking, and the swelling treatment, dyeing treatment, and crosslinking treatment are performed in this order, the crosslinking agent-containing solution of the treatment solution is, for example, an aqueous solution with a weight ratio of boric acid / iodide / water = 3 to 10 / 1 to 20 / 100. If necessary, other crosslinking agents such as glyoxal or glutaraldehyde may be used instead of boric acid, or boric acid and other crosslinking agents may be used in combination. The temperature of the treatment solution when immersing the film (2) is typically about 50°C to 70°C, preferably 53°C to 65°C, and the immersion time of the film (2) is typically about 10 seconds to 600 seconds, preferably 20 seconds to 300 seconds, more preferably 20 seconds to 200 seconds. When dyeing and crosslinking treatments are performed in this order on a film (2) that has been stretched in advance before swelling treatment, the temperature of the treatment solution is typically 50°C to 85°C, preferably 55°C to 80°C.

[0040] The purpose of the crosslinking treatment is color adjustment, and for example, when iodine is used as a dichromatic pigment in the dyeing treatment section (132), a crosslinking agent-containing solution with a weight ratio of boric acid / iodide / water = 1 to 5 / 3 to 30 / 100 can be used as the treatment solution. The temperature of the treatment solution when immersing the film (2) is typically about 10 to 45°C, and the immersion time of the film (2) is typically about 1 to 300 seconds, preferably 2 to 100 seconds.

[0041] The cleaning treatment unit (134) is a part that performs cleaning treatment on the film (2) after crosslinking treatment. The cleaning treatment unit (134) has a treatment tank in which a treatment solution for cleaning treatment is stored. By immersing the film (2) in the treatment solution of the cleaning treatment unit (134), cleaning treatment is performed on the film (2). In this embodiment, a conveying path for the film immersing the film (2) in the treatment solution is formed by nip rolls (11) and two guide rolls (12) arranged before and after the film (2) is immersed in the treatment solution. Examples of the treatment solution for cleaning treatment include water, an aqueous solution containing potassium iodide, and an aqueous solution containing boric acid. The temperature of the treatment solution is typically about 2°C to 40°C, and the treatment time (immersion time) is typically about 2 seconds to 120 seconds.

[0042] The drying treatment unit (135) is a part that performs a drying treatment on the film (2). In this embodiment, the drying treatment unit (135) is a drying device. The film (2) that has been cleaned in the cleaning treatment unit (134) is introduced into the drying treatment unit (135), and the film (2) is dried while passing through the drying treatment unit (135). In this embodiment, a transport path for the film that dries the film (2) within the drying treatment unit (135) is formed by nip rolls (11) arranged before and after the drying treatment unit (135). Guide rolls (12) may be appropriately arranged within the drying treatment unit (135) to support and transport the film (2). Drying by the drying treatment unit (135) is performed for 30 seconds to 600 seconds in the drying treatment unit (135) maintained at a temperature of 40°C to 100°C. In FIG. 1, a drying treatment unit (135) is schematically shown. The drying treatment unit (135) is not particularly limited as long as it can dry moisture attached to the film (2), and it may be any known type commonly used in the manufacture of polarizing films.

[0043] When manufacturing a polarizing film using the above manufacturing device (10), first, the film (2) is unwound from the fabric roll (6). The unwound film (2) is conveyed along a conveying path formed by a plurality of nip rolls (11) and a plurality of guide rolls (12) in the long direction of the film (2). Examples of conveying speeds may be 1 m / min to 60 m / min or 1.5 m / min to 50 m / min. In the conveying path of the film (2), a swelling treatment section (131), a dyeing treatment section (132), a crosslinking treatment section (133), a cleaning treatment section (134), and a drying treatment section (135) are provided from the side of the fabric roll (6). Accordingly, by conveying the film (2) along the conveying path, swelling treatment (swelling process), dyeing treatment (dyeing process), crosslinking treatment (crosslinking process), cleaning treatment (cleaning process), and drying treatment (drying process) are performed on the film (2). In addition, in the method for manufacturing a polarizing film, N stretching treatments (where N is an integer greater than or equal to 1) are performed on the film (2) during the crosslinking process. The upper limit of N is not particularly limited, but N may be an integer less than or equal to 7. By performing the aforementioned plurality of treatments on the film (2), linear polarization characteristics are imparted to the film (2), and a polarizing film (4) is obtained. The thickness of the polarizing film (4) is, for example, 2 μm to 50 μm. Preferably, it is 5 μm to 40 μm.

[0044] The above N stretching treatments are described. The N stretching treatments are performed within the range satisfying Equation (1) and Equation (2).

[0045] α=(ab) / a···(1)

[0046] 0.28≤αmax≤0.42···(2)

[0047] In Equation (1), a represents the average thickness [μm] in the width direction of the film (2) before the nth stretching treatment (n is any integer from 1 to N), and b represents the average thickness [μm] in the width direction of the film (2) after the nth stretching treatment.

[0048] The average value of the thickness in the width direction is the average value of the thickness of the central part and the thickness of both ends in the width direction of the film (2), as shown in FIG. 2. FIG. 2 is a diagram for explaining the measurement location of the thickness in the width direction of the film (2) and schematically shows a cross-section perpendicular to the length direction of the film (2). As exemplified in FIG. 2, the thickness of the central part may be the thickness at one point within a range of 5% or less from the central position in the width direction of the film (2) (within the area (A1) indicated by hatching in FIG. 2), and the thickness of each of the two ends may also be the thickness at one point within a range of 5% or less from a pair of edges in the width direction of the film (2) (areas (A2) and (A3) indicated by hatching in FIG. 2).

[0049] In Equation (2), αmax is the maximum value of N αs obtained for N stretching treatments.

[0050] The stretching ratio for each of the N stretching processes is, for example, 1.001 or more and 4.00 or less, and may be 1.01 or more and 3.00 or less, and preferably 1.05 or more and 2.50 or less. Each of the N stretching processes can be performed using nip rolls (11) positioned before and after each stretching process. The stretching process can be performed by utilizing the difference in rotational speed between the nip rolls (11) positioned before and after the stretching process. The nip rolls (11) contributing to each stretching process function as stretching processing units.

[0051] An example of a stretching treatment is described in the case where a stretching treatment is performed together with a crosslinking treatment (crosslinking process) performed in the crosslinking treatment unit (133) shown in FIG. 1. In this case, a stretching treatment is performed using nip rolls (11) arranged before and after the crosslinking treatment unit (133). In Equation (1), a is the average value [μm] of the thickness along the width direction of the film (2) at position (x1), and b is the average value [μm] of the thickness along the width direction of the film (2) at position (x2). For convenience of explanation, as shown in FIG. 1, the upstream nip roll (11) among the two nip rolls (11) contributing to one stretching treatment is the nip roll (11 UP It is called a ) and the downstream nip roll (11) is called a nip roll (11 DOWN It is called ). The position (x1) is where the film (2) is nip roll (11 UP This is the position after passing through the nip roll (11 DOWN It is the position after passing through ).

[0052] Referring to FIG. 3, a method for calculating α represented by Equation (1) in a single stretching process is explained. To calculate α, the manufacturing device (10) may have a pair of thickness measuring parts (30) and a calculation part (40).

[0053] One of the thickness measuring parts (30) of a pair of thickness measuring parts (hereinafter, "thickness measuring part (30) UP The other thickness measuring unit (30) (hereinafter referred to as the “thickness measuring unit (30)”) measures the thickness of the film (2) before stretching treatment, and the other thickness measuring unit (30) (hereinafter referred to as the “thickness measuring unit (30) DOWN ) is called )”, and measures the thickness of the film (2) after stretching treatment.

[0054] For example, the thickness measuring unit (30 UP ) and thickness measuring unit (30 DOWNEach of the three thickness gauges (31) has three thickness gauges (31). The three thickness gauges (31) are arranged along the width direction of the film (2) so that the thickness of the central part and both ends of the film (2) can be measured. The thickness gauges (31) are not limited as long as they can measure the thickness of the film (2). The thickness gauges (31) are, for example, non-contact thickness gauges (e.g., optical thickness gauges). As the thickness gauges (31), for example, a spectroscopic interference displacement type multilayer film thickness gauge from Giens (e.g., SI-T80, etc.) can be used. The thickness measurement may be performed by a method (traverse method) in which the thickness gauges are moved in the width direction of the film (2) (a direction perpendicular to the transport direction).

[0055] The thickness may be measured at position (x1), and then the thickness may be measured at the time when the film location where the thickness was measured at position (x1) is returned to position (x2), or the thickness measurement at position (x1) and the thickness measurement at position (x2) may be measured at the same timing (i.e., simultaneously). The "same timing" may have a slight discrepancy within a range that does not deviate from the spirit of the present invention. Depending on the return speed, the time difference between the measurement at position (x1) and the measurement at position (x2) may be within 1 minute, within 30 seconds, within 20 seconds, or within 10 seconds.

[0056] The calculation unit (40) is the thickness measuring unit (30 UP ) and thickness measuring unit (30 DOWN Based on the result of ), α (=(ab) / a) in Equation (1) is calculated. The calculation unit (40) is the thickness measuring unit (30 UP ) and thickness measuring unit (30 DOWN Based on the result of ), a and b may be calculated and then α may be calculated using them, or α may be calculated directly by inserting the equation for calculating a and b into Equation (1).

[0057] Here, the stretching treatment and the method for calculating α have been explained using the example of performing the stretching treatment once in the crosslinking process, but multiple stretching treatments may be performed in the crosslinking process. Specifically, three or more nip rolls (11) may be arranged in the area where the crosslinking process is performed within the conveying path of the film (2). In this case, stretching treatment may be performed between two adjacent nip rolls (11) among the three or more nip rolls (11) arranged in the area where the crosslinking process is performed. When performing multiple stretching treatments, the thickness of the film (2) after the upstream stretching treatment among the two adjacent stretching treatments may be used as the thickness of the film (2) before the downstream stretching treatment. When performing multiple stretching treatments, the calculation unit (40) has a pair of thickness measuring units (30) corresponding to each stretching treatment UP ) and thickness measuring unit (30 DOWN A common one may be used for ). Or, a pair of thickness measuring parts (30) corresponding to each stretching treatment may be used. UP ) and thickness measuring unit (30 DOWN One output unit (40) may be placed for ).

[0058] When manufacturing a polarizing film, a stretching treatment is performed such that αmax, which is the maximum of the α obtained for each of the N stretching treatments, satisfies Equation (2). Hereinafter, the series of processes up to the calculation of α corresponding to each stretching treatment (including the measurement of thickness for the calculation of α) and the acquisition of αmax may be referred to as a “monitoring process.”

[0059] In the method for manufacturing a polarizing film according to the present embodiment, N stretching treatments are performed on the film (2) to satisfy Equation (1) and Equation (2). Therefore, even if the polarizing film (4) is continuously manufactured while unwinding the film (2) from the fabric roll (6) and conveying it, the film (2) is difficult to break, and color stains in the manufactured polarizing film (4) can be suppressed, so the manufactured polarizing film (4) has a good appearance. That is, in the method for manufacturing a polarizing film according to the present embodiment, a polarizing film (4) having a good appearance can be stably manufactured.

[0060] The color stains of the polarizing film (4) can be evaluated as follows. After placing the polarizing film (4) in a cross-Nicole state with respect to a straight polarizing filter in a dark room, the polarizing film (4) is illuminated by a backlight. The state of color stains (including a state where no color stains occur) on the polarizing film (4) illuminated in this way is evaluated. The straight polarizing filter may be placed on either the backlight side or the side opposite to the backlight (observation side) with respect to the polarizing film (4).

[0061] In a method for manufacturing a polarizing film having a monitoring process that monitors in real time whether N stretching treatments satisfy Equation (1) and Equation (2), if N stretching treatments do not satisfy Equation (1) and Equation (2), for example, the manufacturing of the polarizing film (4) may be stopped. When manufacturing is stopped, it is preferable to adjust the conditions contributing to the stretching state among the manufacturing conditions of the polarizing film (4) (e.g., stretching ratio, temperature of the treatment solution in which the film (2) is immersed, immersion time, etc.) so that αmax satisfies Equation (2). In addition, manufacturing may be continued while adjusting the manufacturing conditions so that αmax satisfies Equation (2). By doing so, it is possible to prevent the film (2) from breaking during the manufacturing of the polarizing film (4) or to suppress the manufacturing of a polarizing film (4) that becomes a defective product. Therefore, the polarizing film (4) can be manufactured in a stable process. In addition, it is easy to uniformly manufacture a polarizing film (4) with stable quality. In addition, the material cost of the polarizing film (4) can be reduced. In addition, since a good quality polarizing film (4) can be manufactured efficiently, the manufacturing yield of the polarizing film (4) is improved.

[0062] Typically, in the manufacture of a polarizing film having high optical properties, multiple stretching treatments are often performed. Therefore, when multiple stretching treatments are performed in the manufacture of the polarizing film (4), it is possible to manufacture a polarizing film (4) having a good appearance through a stable process as described above while maintaining high optical properties.

[0063] In the method for manufacturing a polarizing film, each of the N stretching treatments may be performed to further satisfy Equation (3) in order to suppress color stains in the manufactured polarizing film (4).

[0064] 0.1≤Δa / Δb≤1.1···(3)

[0065] In equation (3), Δa represents the difference between the maximum and minimum thickness values ​​in the width direction of the film (2) before the nth stretching treatment, and Δb represents the difference between the maximum and minimum thickness values ​​in the width direction of the film (2) after the nth stretching treatment.

[0066] The above Δa and Δb can be calculated by obtaining the thickness distribution in the width direction of the film (2) before the nth stretching treatment and the film (2) after the nth stretching treatment, respectively. The thickness distribution can be obtained, for example, by arranging a number of thickness gauges (31) shown in FIG. 3 along the width direction of the film (2) suitable for obtaining the thickness distribution. The thickness distribution may also be obtained by measuring the thickness in a traverse manner.

[0067] When the manufacturing method of a polarizing film has the aforementioned monitoring process, it is preferable to monitor whether Equation (1) and Equation (2) are satisfied in the monitoring process and whether Equation (3) is satisfied. If Equation (3) is not satisfied, conditions contributing to the stretching process are adjusted to satisfy Equation (3). For example, the installation state of the nip roll (11) contributing to the stretching process, the transport state of the film (2), etc. are adjusted. When monitoring whether Equation (1) and Equation (2) are satisfied in the monitoring process, it is preferable to monitor whether Equation (1) and Equation (2) are satisfied using the measurement result of the thickness in the width direction measured to obtain the thickness distribution.

[0068] When N stretching treatments are performed to further satisfy Equation (3), the influence of variations in the thickness of the film (2) in the width direction before and after each stretching treatment can be further reduced. As a result, the film (2) is more difficult to break during the manufacture of the polarizing film (4). In addition, since defects such as color stains or wrinkles are less likely to occur on the film (2), it is easier to manufacture a polarizing film (4) with a good appearance.

[0069] The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments, and is intended to include the scope indicated by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. For example, the stretching method in the stretching treatment is not limited to a method using two nip rolls (11) as long as the film (2) can be stretched. The stretching treatment is not limited to a wet stretching method, and a dry stretching method may be employed. In addition to the above N stretching treatments (stretching treatments satisfying Equation (1) and Equation (2)) in the crosslinking process, stretching treatment may also be performed in other processes (e.g., swelling process, dyeing process, etc.). The above embodiments and various modifications may be appropriately combined within a range that does not deviate from the spirit of the present invention. Furthermore, in order to manufacture a polarizing film, the film (2) may be subjected to at least a swelling treatment, a dyeing treatment, a crosslinking treatment, and a stretching treatment.

[0070] [Example]

[0071] The present invention will be further explained below using examples and comparative examples. In the following description as well, the film for manufacturing a polarizing film will be referred to as "film (2)". The present invention is not limited to the following examples.

[0072] (Example 1)

[0073] <Manufacturing of Polarizing Films>

[0074] A polarizing film was produced using a polyvinyl alcohol film with a thickness of 75 μm (Pobal film VF-PS#7500 manufactured by Kuraray Co., Ltd., degree of polymerization 2,400, degree of saponification 99.9 mol% or more) as a long film (2), by the following method.

[0075] The film (2) is unwound from the fabric roll on which the film (2) is wound, and the film (2) is immersed in pure water at 30°C while maintaining tension so as not to loosen, thereby sufficiently swelling the film (2) (swelling process). Next, uniaxial stretching is performed until the cumulative stretching ratio from the fabric reaches 2.4 times while immersed in an aqueous solution containing iodine and potassium iodide (dyeing process), and then uniaxial stretching is performed 1.75 times while immersed in an aqueous solution at 56°C with a weight ratio of potassium iodide / boric acid / water of 12 / 4.2 / 100 (first crosslinking process: cumulative stretching ratio from the fabric reaches 4.2 times). Subsequently, uniaxial stretching is performed 1.3 times while immersed in an aqueous solution of the same composition and temperature (second crosslinking process: cumulative stretching ratio from the fabric reaches 5.5 times). Next, the film was uniaxially stretched 1.05 times while immersed in an aqueous solution at 40°C with a weight ratio of potassium iodide / boric acid / water of 9 / 2.9 / 100 (third crosslinking process: the cumulative stretching ratio from the fabric was 5.7 times), then immersed in pure water at 5°C (washing process), and dried at 70°C for 3 minutes (drying process) to obtain a polarizing film. No breakage of the film (2) occurred during the manufacture of the polarizing film.

[0076] In the manufacture of the above polarizing film, the total of three stretching treatments performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were N stretching treatments in the crosslinking process described in the above embodiment. Hereinafter, the aqueous solution used in the first crosslinking process is referred to as aqueous solution A.

[0077] Thickness Measurement

[0078] During the manufacture of the polarizing film, the thickness of the film (2) being transported was measured at three locations—the center and both ends in the film width direction—before and after each process using a non-contact thickness gauge (SI-T80 manufactured by Giens Co., Ltd.). The measurement results of the thickness of the film (2) after treatment in each of the dyeing process, the first crosslinking process, the second crosslinking process, and the third crosslinking process were as shown in FIG. 4. The "initial thickness" in FIG. 4 is the thickness of the film (2) prepared for manufacturing the polarizing film (the thickness of the film (2) before swelling treatment), and the "thickness" in each process was the average value of the thicknesses at the three locations. The "thickness" of each process shown in FIG. 4 was the thickness obtained at the same timing.

[0079] Calculation of α and specification of αmax

[0080] Based on Equation (1), α corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process was calculated. α corresponding to the stretching treatment performed in the first crosslinking process (hereinafter referred to as “α1”) was calculated using the thickness of the film (2) after the dyeing process and the thickness of the film (2) after the first crosslinking process as a and b in Equation (1). Likewise, α corresponding to the stretching treatment performed in the second crosslinking process (hereinafter referred to as “α2”) was calculated using the thickness of the film (2) after the first crosslinking process and the thickness of the film (2) after the second crosslinking process as a and b in Equation (1). Likewise, α (hereinafter referred to as “α3”) corresponding to the stretching treatment performed in the third crosslinking process was calculated using the thickness of the film (2) after the second crosslinking process and the thickness of the film (2) after the third crosslinking process as a and b in Equation (1). The calculated α1, α2, and α3, and the maximum value among them, αmax, were as shown in FIG. 5.

[0081] <Calculation of Δa / Δb>

[0082] As explained in the thickness measurement above, the difference Δt1, difference Δt2, difference Δt3, and difference Δt4 between the maximum and minimum values ​​among the measurement results at three locations—the center and both ends—in the film width direction obtained after treatment in each of the dyeing process, the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The calculation results were as shown in FIG. 4. Using the difference Δt1, difference Δt2, difference Δt3, and difference Δt4 shown in FIG. 4, Δa / Δb corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process was calculated. Hereinafter, Δa / Δb is referred to as β.

[0083] Δa / Δb (hereinafter referred to as "β1") corresponding to the stretching treatment performed in the first crosslinking process was calculated using the difference Δt1 after the dyeing process and the difference Δt2 after the first crosslinking process as Δa and Δb. Similarly, Δa / Δb (hereinafter referred to as "β2") corresponding to the stretching treatment performed in the second crosslinking process was calculated using the difference Δt2 after the first crosslinking process and the difference Δt3 after the second crosslinking process as Δa and Δb. Likewise, Δa / Δb (hereinafter referred to as "β3") corresponding to the stretching treatment performed in the third crosslinking process was calculated using the difference Δt3 after the second crosslinking process and the difference Δt4 after the third crosslinking process as Δa and Δb. The calculation results were as shown in FIG. 5.

[0084] Evaluation of Color Stains

[0085] The manufactured polarizing film was placed in a cross-Nicole state with respect to a linear polarizing filter in a dark room. Subsequently, a backlight of 6000 cd / m² was irradiated onto the polarizing film through the linear polarizing filter, and the color stains on the polarizing film were observed by visual inspection. Then, the level (intensity) of the color stains was determined in three stages, “1,” “2,” and “3,” through a sensory evaluation based on visual inspection. Evaluation “1” indicates the weakest stain, evaluation “3” indicates the strongest stain, and evaluation “2” indicates an intermediate level between evaluation “1” and evaluation “3.” In the sensory evaluation, the color stains were evaluated in three stages as described above by comparing them with a sample of a level determined according to the level (intensity) of the color stains. The polarizing film manufactured in Example 1 was evaluation “1.”

[0086] (Example 2)

[0087] <Manufacturing of Polarizing Films>

[0088] A polarizing film was obtained in the same manner as in Example 1, except that a polyvinyl alcohol film with a thickness of 30 μm (Poval film VF-PE#3000 manufactured by Kuraray Co., Ltd., degree of polymerization 2,400, degree of saponification 99.9 mol% or more) with a thickness of 30 μm was used as the film (2). No breakage of the film occurred during the manufacture of the polarizing film.

[0089] Thickness Measurement

[0090] In the same manner as in Example 1, the thickness of the film (2) being transported was measured at three locations, including the center and both ends in the film width direction, before and after each process. The measurement results of the thickness of the film (2) after treatment in each of the dyeing process, the first crosslinking process, the second crosslinking process, and the third crosslinking process were as shown in FIG. 4. The thickness of each process in FIG. 4 is the average thickness, which is the same as in the case of Example 1.

[0091] Calculation of α and specification of αmax

[0092] In the same manner as in Example 1, α1, α2, and α3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The calculated α1, α2, and α3, and the maximum value among them, αmax, were as shown in FIG. 5.

[0093] Calculation of β (=Δa / Δb)

[0094] In the same manner as in Example 1, Δt1, Δt2, Δt3, and Δt4 were calculated, and β1, β2, and β3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The results of the calculation were as shown in FIGS. 4 and FIGS. 5.

[0095] Evaluation of Color Stains

[0096] The color stain of the manufactured polarizing film was evaluated in the same way as in Example 1. The evaluation result of the polarizing film manufactured in Example 2 was “1”.

[0097] (Example 3)

[0098] <Manufacturing of Polarizing Films>

[0099] A polarizing film was manufactured in the same manner as in Example 1, except that the temperature of aqueous solution A in the first and second crosslinking processes was changed to 58°C. No breakage of the film (2) occurred during the manufacture of the polarizing film.

[0100] Thickness Measurement

[0101] In the same manner as in Example 1, the thickness of the film (2) being transported was measured at three locations, including the center and both ends in the film width direction, before and after each process. The measurement results of the thickness of the film (2) after treatment in each of the dyeing process, the first crosslinking process, the second crosslinking process, and the third crosslinking process were as shown in FIG. 4. The thickness of each process in FIG. 4 is the average thickness, which is the same as in the case of Example 1.

[0102] Calculation of α and specification of αmax

[0103] In the same manner as in Example 1, α1, α2, and α3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The calculated α1, α2, and α3, and the maximum value among them, αmax, were as shown in FIG. 5.

[0104] Calculation of β (=Δa / Δb)

[0105] In the same manner as in Example 1, Δt1, Δt2, Δt3, and Δt4 were calculated, and β1, β2, and β3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The results of the calculation were as shown in FIGS. 4 and FIGS. 5.

[0106] Evaluation of Color Stains

[0107] The color stain of the manufactured polarizing film was evaluated in the same way as in Example 1. The evaluation result of the polarizing film manufactured in Example 3 was “1”.

[0108] (Example 4)

[0109] <Manufacturing of Polarizing Films>

[0110] A polarizing film was manufactured in the same manner as in Example 1, except that the temperature of aqueous solution A in the first and second crosslinking processes was changed to 62°C. No breakage of the film (2) occurred during the manufacture of the polarizing film.

[0111] Thickness Measurement

[0112] In the same manner as in Example 1, the thickness of the film (2) being transported was measured at three locations, including the center and both ends in the film width direction, before and after each process. The measurement results of the thickness of the film (2) after treatment in each of the dyeing process, the first crosslinking process, the second crosslinking process, and the third crosslinking process were as shown in FIG. 4. The thickness of each process in FIG. 4 is the average thickness, which is the same as in the case of Example 1.

[0113] Calculation of α and specification of αmax

[0114] In the same manner as in Example 1, α1, α2, and α3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The calculated α1, α2, and α3, and the maximum value among them, αmax, were as shown in FIG. 5.

[0115] Calculation of β (=Δa / Δb)

[0116] In the same manner as in Example 1, Δt1, Δt2, Δt3, and Δt4 were calculated, and β1, β2, and β3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The results of the calculation were as shown in FIGS. 4 and FIGS. 5.

[0117] Evaluation of Color Stains

[0118] The color stain of the manufactured polarizing film was evaluated in the same way as in Example 1. The evaluation result of the polarizing film manufactured in Example 4 was “2”.

[0119] (Comparative Example 1)

[0120] <Manufacturing of Polarizing Films>

[0121] A polarizing film was manufactured in the same manner as in Example 1, except that an aqueous solution with a weight ratio of potassium iodide / boric acid / water of 12 / 2 / 100 was used in the first and second crosslinking processes. No breakage of the film (2) occurred during the manufacture of the polarizing film. The aqueous solution used in the first and second crosslinking processes of Comparative Example 1 is referred to as aqueous solution B.

[0122] Thickness Measurement

[0123] In the same manner as in Example 1, the thickness of the film (2) being transported was measured at three locations, including the center and both ends in the film width direction, before and after each process. The measurement results of the thickness of the film (2) after treatment in each of the dyeing process, the first crosslinking process, the second crosslinking process, and the third crosslinking process were as shown in FIG. 4. The thickness of each process in FIG. 4 is the average thickness, which is the same as in the case of Example 1.

[0124] Calculation of α and specification of αmax

[0125] In the same manner as in Example 1, α1, α2, and α3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The calculated α1, α2, and α3, and the maximum value among them, αmax, were as shown in FIG. 5.

[0126] Calculation of β (=Δa / Δb)

[0127] In the same manner as in Example 1, Δt1, Δt2, Δt3, and Δt4 were calculated, and β1, β2, and β3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The results of the calculation were as shown in FIGS. 4 and FIGS. 5.

[0128] Evaluation of Color Stains

[0129] The color stain of the manufactured polarizing film was evaluated in the same way as in Example 1. The evaluation result of the polarizing film manufactured in Comparative Example 1 was “3”.

[0130] (Comparative Example 2)

[0131] <Manufacturing of Polarizing Films>

[0132] A polarizing film was manufactured in the same manner as in Example 1, except that an aqueous solution with a weight ratio of potassium iodide / boric acid / water of 12 / 6.5 / 100 was used in the first and second crosslinking processes. The aqueous solution used in the first and second crosslinking processes of Comparative Example 2 is referred to as aqueous solution C. During the manufacture of the polarizing film, breakage of the film (2) occurred frequently, making it impossible to obtain a stable polarizing film. In Comparative Example 2, when breakage of the film (2) occurred, the film (2) was unwound again from the raw material roll, and the manufacture of the polarizing film continued until the next breakage occurred.

[0133] Thickness Measurement

[0134] In the same manner as in Example 1, the thickness of the film (2) being transported was measured at three locations, including the center and both ends in the film width direction, before and after each process. The measurement results of the thickness of the film (2) after treatment in each of the dyeing process, the first crosslinking process, the second crosslinking process, and the third crosslinking process were as shown in FIG. 4. The thickness of each process in FIG. 4 is the average thickness, which is the same as in the case of Example 1.

[0135] Calculation of α and specification of αmax

[0136] In the same manner as in Example 1, α1, α2, and α3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The calculated α1, α2, and α3, and the maximum value among them, αmax, were as shown in FIG. 5.

[0137] Calculation of β (=Δa / Δb)

[0138] In the same manner as in Example 1, Δt1, Δt2, Δt3, and Δt4 were calculated, and β1, β2, and β3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The results of the calculation were as shown in FIGS. 4 and FIGS. 5.

[0139] Evaluation of Color Stains

[0140] The color stain of the polarizing film manufactured until the film (2) broke was evaluated in the same way as in Example 1. The evaluation result of the polarizing film manufactured in Comparative Example 2 was “1”.

[0141] (Comparative Example 3)

[0142] <Manufacturing of Polarizing Films>

[0143] A polarizing film was manufactured in the same manner as in Example 1, except that the polyvinyl alcohol film used in Example 2 was used as the film (2), and the first crosslinking process and the second crosslinking process were performed using aqueous solution B. No breakage of the film (2) occurred during the manufacture of the polarizing film.

[0144] Thickness Measurement

[0145] In the same manner as in Example 1, the thickness of the film (2) being transported was measured at three locations, including the center and both ends in the film width direction, before and after each process. The measurement results of the thickness of the film (2) after treatment in each of the dyeing process, the first crosslinking process, the second crosslinking process, and the third crosslinking process were as shown in FIG. 4. The thickness of each process in FIG. 4 is the average thickness, which is the same as in the case of Example 1.

[0146] Calculation of α and specification of αmax

[0147] In the same manner as in Example 1, α1, α2, and α3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The calculated α1, α2, and α3, and the maximum value among them, αmax, were as shown in FIG. 5.

[0148] Calculation of β (=Δa / Δb)

[0149] In the same manner as in Example 1, Δt1, Δt2, Δt3, and Δt4 were calculated, and β1, β2, and β3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The results of the calculation were as shown in FIGS. 4 and FIGS. 5.

[0150] Evaluation of Color Stains

[0151] The color stain of the manufactured polarizing film was evaluated in the same way as in Example 1. The evaluation result of the polarizing film manufactured in Comparative Example 3 was “3”.

[0152] (Comparative Example 4)

[0153] A polarizing film was manufactured in the same manner as in Example 1, except that the polyvinyl alcohol film used in Example 2 was used as the film (2), and the first crosslinking process and the second crosslinking process were performed using an aqueous solution C. During the manufacture of the polarizing film, breakage of the film (2) occurred frequently, so the polarizing film could not be obtained stably. In Comparative Example 4, when breakage of the film (2) occurred, the film (2) was unwound again from the raw material roll, and the manufacture of the polarizing film was continued until the next breakage occurred.

[0154] Thickness Measurement

[0155] In the same manner as in Example 1, the thickness of the film (2) being transported was measured at three locations, including the center and both ends in the film width direction, before and after each process. The measurement results of the thickness of the film (2) after treatment in each of the dyeing process, the first crosslinking process, the second crosslinking process, and the third crosslinking process were as shown in FIG. 4. The thickness of each process in FIG. 4 is the average thickness, which is the same as in the case of Example 1.

[0156] Calculation of α and specification of αmax

[0157] In the same manner as in Example 1, α1, α2, and α3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The calculated α1, α2, and α3, and the maximum value among them, αmax, were as shown in FIG. 5.

[0158] Calculation of β (=Δa / Δb)

[0159] In the same manner as in Example 1, Δt1, Δt2, Δt3, and Δt4 were calculated, and β1, β2, and β3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The results of the calculation were as shown in FIGS. 4 and FIGS. 5.

[0160] Evaluation of Color Stains

[0161] The color stain of the polarizing film manufactured until the film (2) broke was evaluated in the same way as in Example 1. The evaluation result of the polarizing film manufactured in Comparative Example 4 was “1”.

[0162] (Comparative Example 5)

[0163] A polarizing film was manufactured in the same manner as in Example 1, except that the temperature of aqueous solution A in the first and second crosslinking processes was changed to 50°C. During the manufacture of the polarizing film, breakage of the film (2) occurred frequently, and the polarizing film could not be obtained stably. In Comparative Example 5, when breakage of the film (2) occurred, the film (2) was unwound again from the raw material roll, and the manufacture of the polarizing film was continued until the next breakage occurred.

[0164] Thickness Measurement

[0165] In the same manner as in Example 1, the thickness of the film (2) being transported was measured at three locations, including the center and both ends in the film width direction, before and after each process. The measurement results of the thickness of the film (2) after treatment in each of the dyeing process, the first crosslinking process, the second crosslinking process, and the third crosslinking process were as shown in FIG. 4. The thickness of each process in FIG. 4 is the average thickness, which is the same as in the case of Example 1.

[0166] Calculation of α and specification of αmax

[0167] In the same manner as in Example 1, α1, α2, and α3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The calculated α1, α2, and α3, and the maximum value among them, αmax, were as shown in FIG. 5.

[0168] Calculation of β (=Δa / Δb)

[0169] In the same manner as in Example 1, Δt1, Δt2, Δt3, and Δt4 were calculated, and β1, β2, and β3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The results of the calculation were as shown in FIGS. 4 and FIGS. 5.

[0170] Evaluation of Color Stains

[0171] The color stain of the polarizing film manufactured until the film (2) broke was evaluated in the same way as in Example 1. The evaluation result of the polarizing film manufactured in Comparative Example 5 was “1”.

[0172] (Comparative Example 6)

[0173] A polarizing film was manufactured in the same manner as in Example 1, except that the temperature of aqueous solution A in the first and second crosslinking processes was changed to 45°C. During the manufacture of the polarizing film, breakage of the film (2) occurred frequently, and the polarizing film could not be obtained stably. In Comparative Example 6, when breakage of the film (2) occurred, the film (2) was unwound again from the raw material roll, and the manufacture of the polarizing film was continued until the next breakage occurred.

[0174] Thickness Measurement

[0175] In the same manner as in Example 1, the thickness of the film (2) being transported was measured at three locations, including the center and both ends in the film width direction, before and after each process. The measurement results of the thickness of the film (2) after treatment in each of the dyeing process, the first crosslinking process, the second crosslinking process, and the third crosslinking process were as shown in FIG. 4. The thickness of each process in FIG. 4 is the average thickness, which is the same as in the case of Example 1.

[0176] Calculation of α and specification of αmax

[0177] In the same manner as in Example 1, α1, α2, and α3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The calculated α1, α2, and α3, and the maximum value among them, αmax, were as shown in FIG. 5.

[0178] Calculation of β (=Δa / Δb)

[0179] In the same manner as in Example 1, Δt1, Δt2, Δt3, and Δt4 were calculated, and β1, β2, and β3 corresponding to the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process were calculated. The results of the calculation were as shown in FIGS. 4 and FIGS. 5.

[0180] Evaluation of Color Stains

[0181] The color stain of the polarizing film manufactured until the film (2) broke was evaluated in the same way as in Example 1. The evaluation result of the polarizing film manufactured in Comparative Example 6 was “2”.

[0182] [Overall Evaluation]

[0183] As shown in FIG. 5, based on the αmax of Examples 1 to 4, in Examples 1 to 4, the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process was performed while satisfying Equation (1) and Equation (2). Also, in Examples 1 to 4, no breakage of the film (2) occurred during the manufacture of the polarizing film. That is, in Examples 1 to 4, the polarizing film could be manufactured stably. In addition, in the polarizing film manufactured in Examples 1 to 4, the evaluation of color stains was evaluation “1” or evaluation “2”.

[0184] Meanwhile, based on the αmax of Comparative Examples 1 to 6, the stretching treatment performed in the first crosslinking process, the second crosslinking process, and the third crosslinking process in Comparative Examples 1 to 6 did not satisfy Equation (1) and Equation (2). In Comparative Examples 1 and 3, no breakage of the film (2) occurred during the manufacture of the polarizing film, but the evaluation of the color stain was evaluation "3". In Comparative Examples 2, 4 to 6, breakage of the film (2) occurred during the manufacture of the polarizing film, so the polarizing film could not be manufactured stably.

[0185] Accordingly, from the results of Examples 1 to 4 and Comparative Examples 1 to 6, it can be understood that by performing N stretching treatments to satisfy Equation (1) and Equation (2), a polarizing film with suppressed color stains, i.e., a polarizing film with a good appearance, can be stably manufactured.

[0186] In addition, when comparing the results of Δa / Δb in Examples 1 to 4, it can be understood that when all Δa / Δb calculated for N stretching treatments satisfy Equation (3), color staining is further suppressed. Explanation of the symbols

[0187] 2… film (polyvinyl alcohol-based film), 4… polarizing film, 11… nip roll.

Claims

Claim 1 A method for manufacturing a polarizing film comprising a swelling process, a dyeing process, and a crosslinking process, wherein in the crosslinking process, N stretching treatments (N is an integer greater than or equal to 2) are performed on a polyvinyl alcohol-based film, and the N stretching treatments are performed within a range satisfying Equation (1) and Equation (2), and the N stretching treatments include immersing the polyvinyl alcohol-based film in a treatment solution. α=(ab) / a···(1) 0.28≤αmax≤0.42···(2) (In Equation (1), a represents the average value of the thickness in the width direction [μm] of the polyvinyl alcohol-based film before the n-th stretching treatment (n is an integer from 1 to N), and b represents the average value of the thickness in the width direction [μm] of the polyvinyl alcohol-based film after the n-th stretching treatment, and the average value of the thickness in the width direction is the polyvinyl alcohol-based film It is the average value of the thickness of the central part and the thickness of both ends in the width direction. In Equation (2), αmax is the maximum value of N αs obtained for the N stretching treatments. Claim 2 A method for manufacturing a polarizing film according to claim 1, wherein the stretching ratio in each of the N stretching treatments is 1.001 or more and 4.00 or less. Claim 3 A method for manufacturing a polarizing film according to claim 1 or 2, wherein in each of the N stretching treatments, the polyvinyl alcohol-based film is stretched by a nip roll disposed before and after each stretching treatment. Claim 4 A method for manufacturing a polarizing film according to claim 1 or 2, wherein each of the N stretching treatments is performed within a range satisfying the following formula (3). 0.1 ≤ Δa / Δb ≤ 1.1 ··· (3) (wherein Δa represents the difference between the maximum and minimum values ​​of the thickness in the width direction of the polyvinyl alcohol-based film before the nth stretching treatment, and Δb represents the difference between the maximum and minimum values ​​of the thickness in the width direction of the polyvinyl alcohol-based film after the nth stretching treatment.)

Images