Method for manufacturing polarizing film

Abstract

The present invention provides a method for manufacturing a polarizing film capable of manufacturing a polarizing film with a good appearance in a stable process. In one embodiment of the method for manufacturing a polarizing film, N stretching treatments (N is an integer greater than 1) are performed on a polyvinyl alcohol film in a cross-linking process, and the N stretching treatments are performed within the range satisfying formulas (1) and (2). (In formula (1), a and b represent the average value [μm] of the thickness in the width direction of the polyvinyl alcohol film before and after the nth (n is an arbitrary integer from 1 to N) stretching treatment, and the average value of the thickness in the width direction is the average value of the thickness of the central portion and both end portions in the width direction of the polyvinyl alcohol film. In formula (2), αmax is the maximum value of N α obtained in the N stretching treatments.) α=(a‑b) / a···(1)0.28≤αmax≤0.42···(2).

Description

Technical Field

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

[0002] The polarizing film is produced by subjecting the polyvinyl alcohol-based film to stretching treatment, dyeing treatment, crosslinking treatment, drying treatment, and the like while conveying the polyvinyl alcohol-based film (see, for example, Patent Document 1).

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 2002-40256 Summary of the Invention

[0006] Problems to be solved by the invention

[0007] When a polyvinyl alcohol film is stretched, the film is stretched, thereby reducing its thickness. This may cause the film to break during the production of a polarizing film, or may cause more color unevenness, deteriorating the appearance.

[0008] Therefore, an object of the present invention is to provide a method for producing a polarizing film that can produce a polarizing film having a good appearance in a stable process.

[0009] Means used to solve problems

[0010] The manufacturing method of the polarizing film of the present invention is a manufacturing method of the polarizing film including a swelling process, a dyeing process and a cross-linking process. In the above-mentioned cross-linking process, the polyvinyl alcohol film is subjected to N stretching treatments (N is an integer greater than 1), and the above-mentioned N stretching treatments are carried out within the range of satisfying formulas (1) and (2).

[0011] α=(ab) / a···(1)

[0012] 0.28≤αmax≤0.42···(2)

[0013] (In formula (1), a represents the average value [μm] of the thickness in the width direction of the polyvinyl alcohol film before the n-th (n is an integer from 1 to N) stretching treatment, and b represents the average value [μm] of the thickness in the width direction of the polyvinyl alcohol film after the n-th stretching treatment. The average value of the thickness in the width direction is the average value of the thickness in the center portion and the thickness at both ends of the polyvinyl alcohol film in the width direction. In formula (2), αmax is the maximum value of the N α values ​​obtained for the N stretching treatments.)

[0014] In this case, N stretching processes are performed within a range satisfying the above-mentioned formulas (1) and (2).

[0015] Therefore, a polarizing film having a good appearance can be manufactured in a stable process.

[0016] The stretching ratio of each of the N stretching treatments may be 1.001 or more and 4.00 or less.

[0017] The N stretching processes may each stretch the polyvinyl alcohol film using nip rollers disposed before and after each stretching process. In this case, for example, the polyvinyl alcohol film may be stretched using a rotation speed difference between the nip rollers disposed before and after each stretching process.

[0018] The N stretching treatments described above can be performed within a range satisfying the following formula (3).

[0019] 0.1≤Δa / Δb≤1.1···(3)

[0020] (In formula (3), Δa represents the difference between the maximum and minimum values ​​of the thickness in the width direction of the polyvinyl alcohol film before the n-th stretching treatment, and Δb represents the difference between the maximum and minimum values ​​of the thickness in the width direction of the polyvinyl alcohol film after the n-th stretching treatment.)

[0021] By performing N stretching treatments so as to satisfy Formula (3), the polyvinyl alcohol film can be further prevented from breaking. Furthermore, since defects such as color unevenness and streaks are less likely to occur, a polarizing film with a better appearance can be easily produced.

[0022] Effects of the Invention

[0023] According to the present invention, a method for producing a polarizing film capable of producing a polarizing film having a good appearance in a stable process can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic diagram for explaining a method for producing a polarizing film according to one embodiment.

[0025] Figure 2 It is a figure for demonstrating the measurement position of the thickness in the width direction of a film.

[0026] Figure 3 This is a diagram for explaining an example of a method for measuring the thickness of a film.

[0027] Figure 4 It is a graph showing the conditions and thickness measurement results of Examples 1 to 4 and Comparative Examples 1 to 6.

[0028] Figure 5This is a graph showing the results of Examples 1 to 4 and Comparative Examples 1 to 6.

[0029] Description of Reference Numerals

[0030] 2 films (polyvinyl alcohol-based films), 4 polarizing films, 11 nip rollers. DETAILED DESCRIPTION

[0031] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. The same reference numerals are used for identical or corresponding parts in the drawings, and repeated descriptions are omitted. The dimensional ratios in the drawings are not necessarily the same as those in the description.

[0032] Figure 1 This is a schematic diagram illustrating an example of a method for producing a polarizing film according to one embodiment of the present invention.

[0033] In this embodiment, a long polyvinyl alcohol-based film 2 (hereinafter referred to as "film 2") is transported while being subjected to swelling treatment, dyeing treatment, crosslinking treatment, stretching treatment, washing treatment, and drying treatment to produce a polarizing film 4.

[0034] 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 specified, the film 2 after all processes in the production of the polarizing film are completed is referred to as the polarizing film 4, and the film before all processes are completed is referred to as the film 2.

[0035] The material of film 2 can be any known polyvinyl alcohol resin used in the manufacture of polarizing films, preferably a saponified polyvinyl alcohol 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 further preferably 93.0 to 99.5 mol%. The so-called degree of saponification is a value defined by the formula: degree of saponification (mol%) = (number of hydroxyl groups) / (number of hydroxyl groups + number of acetic acid groups) × 100, which can be calculated using the method specified in JIS K 6726 (1994). The average degree of polymerization of the polyvinyl alcohol resin is preferably 100 to 10,000, more preferably 1,000 to 10,000. The average degree of polymerization is a value calculated using the method specified in JIS K 6726 (1994).

[0036] The length of the longitudinal direction of the film 2 is, for example, 1000 m or more. When the length of the longitudinal direction of the film 2 is 1000 m or more, the length of the longitudinal direction of the film 2 is, for example, 30000 m or less, preferably 20000 m or less. The length L of the width direction (direction perpendicular to the longitudinal direction) of the film 2 (refer to Figure 2) is 1300 mm to 5000 mm. The thickness of the film 2 (the film 2 constituting the raw material roll 6 described later) before the above-mentioned multiple treatments is, for example, 10 μm to 100 μm. The film 2 can be produced by melt extrusion, solvent casting, or the like. The film 2 can also be a purchased film or a film that has been previously treated by stretching, lamination, or the like. Figure 1 , the figure shows a case where a film 2 is prepared as a raw material roll 6, and the film 2 pulled out from the raw material roll 6 is subjected to the above-mentioned multiple treatments to obtain a polarizing film 4. 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 transported and the above-mentioned multiple treatments can be performed during the transportation.

[0037] based on Figure 1 An example of a method for manufacturing a polarizing film 4 will be described using the method shown. First, an overview of an apparatus 10 for manufacturing the polarizing film 4 will be described. The apparatus 10 includes a plurality of nip rollers 11, a plurality of guide rollers 12, 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.

[0038] The plurality of nip rollers 11 and the plurality of guide rollers 12 constitute a conveyance mechanism for the film 2. By appropriately arranging the plurality of nip rollers 11 and the plurality of guide rollers 12, a conveyance path for the film 2 is constituted.

[0039] The nip roller 11 has a function of applying a rotational force of the two rollers to the film 2 by sandwiching the film 2 and pressing the film 2. The nip roller 11 also has a function of changing the conveyance direction of the film 2.

[0040] The guide roller 12 has a function of supporting the film 2 and changing the conveyance direction of the film 2 .

[0041] The swelling treatment section 131 performs a swelling treatment on the film 2. The swelling treatment section 131 includes a treatment tank containing a treatment liquid used for the swelling treatment. The film 2 is swelled by being immersed in the treatment liquid contained in the swelling treatment section 131. In this embodiment, a transport path for the film 2 immersed in the treatment liquid is formed by a nip roller 11 and two guide rollers 12 positioned before and after the film 2 is immersed in the treatment liquid.

[0042] The above-mentioned swelling treatment is performed for the purposes of removing foreign matter from the surface of the film 2, removing plasticizers in the film 2, imparting dyeability in subsequent steps, and plasticizing the film 2. The conditions of the swelling treatment can be determined within a range that can achieve these purposes and within a range that does not cause undesirable conditions such as extreme dissolution or devitrification of the film 2. In the swelling treatment section 131, the swelling treatment is performed by immersing the film 2 in a treatment liquid having a temperature of, for example, 10°C to 50°C, preferably 15°C to 40°C. The swelling treatment time is about 5 seconds to 300 seconds, preferably about 20 seconds to 120 seconds. An example of the treatment liquid in the swelling treatment section 131 is water. Therefore, the swelling treatment can also serve as a water washing treatment for the film 2.

[0043] The dyeing section 132 performs dyeing on the film 2. The dyeing section 132 includes a treatment tank containing a treatment liquid used for the dyeing process. The film 2 is dyed by being immersed in the treatment liquid contained in the dyeing section 132. In this embodiment, a nip roller 11 and two guide rollers 12 are positioned before and after the film 2 is immersed in the treatment liquid to form a transport path for the film 2 immersed in the treatment liquid.

[0044] The treatment liquid used in the dyeing treatment section 132 of this embodiment is an aqueous solution of a dichroic dye. During the dyeing treatment, the film 2 is dyed with the dichroic dye. Conventional dyeing treatments using dichroic dyes are performed for purposes such as adsorption of the dichroic dye onto the film 2. Treatment conditions can be determined based on desired optical properties within a range that achieves this purpose and does not cause undesirable conditions such as extreme dissolution or devitrification of the film 2. Examples of dichroic dyes used for dyeing include iodine and dichroic dyes.

[0045] When iodine is used as the dichroic pigment, the film 2 is immersed 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, for a period of 10 to 600 seconds, preferably 30 to 300 seconds, at a temperature of 10°C to 50°C, preferably 15°C to 40°C, to perform the dyeing treatment. Other iodides, such as zinc iodide, may be used in place of potassium iodide. Other iodides may also be used in combination with potassium iodide. Furthermore, compounds other than iodides, boric acid, zinc chloride, cobalt chloride, and the like may also be coexisted. Any treatment solution containing 0.003 parts by weight or more of iodine per 100 parts by weight of water may be considered a treatment solution for dyeing.

[0046] When a water-soluble dichroic dye is used as the dichroic pigment, the film 2 is dyed by immersing it in an aqueous solution containing 0.001 to 0.1 parts by weight of the dichroic dye per 100 parts by weight of water at a temperature of 20°C to 80°C, preferably 30°C to 60°C, for 10 to 600 seconds, preferably 20 to 300 seconds. The aqueous solution of the dichroic dye used may contain a dyeing auxiliary, an inorganic salt such as sodium sulfate, a surfactant, etc. A single dichroic dye may be used, or two or more dichroic dyes may be used in combination depending on the desired color tone.

[0047] The cross-linking treatment section 133 performs a cross-linking treatment on the film 2. The cross-linking treatment section 133 includes a treatment tank containing a treatment liquid used for the cross-linking treatment. The film 2 is cross-linked by being immersed in the treatment liquid in the cross-linking treatment section 133. In this embodiment, a nip roller 11 and two guide rollers 12 are positioned before and after the film 2 is immersed in the treatment liquid to form a transport path for the film 2 immersed in the treatment liquid.

[0048] The cross-linking treatment is performed for the purpose of improving water resistance by cross-linking, adjusting color tone (preventing the film 2 from turning blue, etc.), and the like.

[0049] 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 per 100 parts by weight of water. If the dichroic dye used in the dyeing process is iodine, the treatment liquid used in the crosslinking treatment section 133 preferably contains an iodide in addition to the boric acid, with the amount of iodide being, 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, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, and sodium sulfate, may also be present.

[0050] In the cross-linking treatment in the cross-linking treatment section 133 , the concentrations of boric acid and iodide, the temperature of the treatment liquid, the treatment time, the distance between rollers, and the like can be appropriately changed according to the purpose.

[0051] For example, when the purpose of cross-linking treatment is to utilize cross-linking water resistance and the film 2 is subjected to swelling treatment, dyeing treatment and cross-linking treatment in sequence, the liquid containing a cross-linking agent as the treatment liquid is, for example, an aqueous solution having a concentration of boric acid / iodide / water = 3 to 10 / 1 to 20 / 100 by weight. As needed, other cross-linking agents such as glyoxal or glutaraldehyde can be used instead of boric acid, or boric acid and other cross-linking agents can be used in combination. The temperature of the treatment liquid when immersing the film 2 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, and more preferably 20 seconds to 200 seconds. When dyeing treatment and cross-linking treatment are sequentially applied to the film 2 that has been stretched in advance before the swelling treatment, the temperature of the treatment liquid is usually about 50°C to 85°C, preferably 55°C to 80°C.

[0052] When the purpose of the crosslinking treatment is to adjust the color tone and iodine is used as a dichroic dye in the dyeing treatment section 132, a liquid containing a crosslinking agent having a concentration by weight of boric acid / iodide / water = 1 to 5 / 3 to 30 / 100 can be used as the treatment liquid. The temperature of the treatment liquid when immersing the film 2 is generally about 10 to 45°C, and the immersion time of the film 2 is generally about 1 to 300 seconds, preferably 2 to 100 seconds.

[0053] The cleaning treatment section 134 is a section that performs a cleaning treatment on the film 2 after the cross-linking treatment. The cleaning treatment section 134 has a treatment tank that stores a treatment liquid for the cleaning treatment. The film 2 is cleaned by immersing the film 2 in the treatment liquid possessed by the cleaning treatment section 134. In this embodiment, a film transport path for immersing the film 2 in the treatment liquid is formed by using a clamping roller 11 and two guide rollers 12 arranged before and after the film 2 is immersed 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 generally around 2°C to 40°C, and the treatment time (immersion time) is generally around 2 seconds to 120 seconds.

[0054] The drying process section 135 is a section that performs a drying process on the film 2. In the present embodiment, the drying process section 135 is a drying device. The film 2 that has been cleaned in the cleaning process section 134 is loaded into the drying process section 135, and the film 2 is dried while passing through the drying process section 135. In the present embodiment, the clamping rollers 11 arranged before and after the drying process section 135 are used to form a film transport path for drying the film 2 in the drying process section 135. In order to support and transport the film 2 in the drying process section 135, guide rollers 12 can be appropriately arranged. Drying in the drying process section 135 is performed for 30 seconds to 600 seconds in the drying process section 135 maintained at a temperature of 40°C to 100°C. Figure 1, a drying process section 135 is schematically shown. The drying process section 135 is not particularly limited as long as it can dry the moisture adhering to the film 2, and may be a known drying process section generally used in the production of polarizing films.

[0055] When the polarizing film is manufactured using the above-mentioned manufacturing device 10, first, the film 2 is pulled out from the raw material roll 6. The pulled-out film 2 is transported along a transport path formed by a plurality of clamping rollers 11 and a plurality of guide rollers 12 and along the length direction of the film 2. An example of the transport speed can be 1m / min to 60m / min, or 1.5m / min to 50m / min. In the transport path of the film 2, starting from the raw material roll 6 side, a swelling treatment section 131, a dyeing treatment section 132, a cross-linking treatment section 133, a cleaning treatment section 134 and a drying treatment section 135 are provided. Therefore, by transporting the film 2 along the transport path, the film 2 is subjected to swelling treatment (swelling process), dyeing treatment (dyeing process), cross-linking treatment (cross-linking process), cleaning treatment (cleaning process) and drying treatment (drying process). In addition, in the method for manufacturing the polarizing film, in the above-mentioned cross-linking process, N stretching treatments (N is an integer greater than 1) are applied to the film 2. The upper limit value of N is not particularly limited, but N can be an integer less than 7. By subjecting the film 2 to the above-mentioned multiple treatments, linear polarization characteristics are imparted to the film 2, thereby obtaining a polarizing film 4. The thickness of the polarizing film 4 is, for example, 2 μm to 50 μm, and preferably 5 μm to 40 μm.

[0056] The N stretching processes are described below. The N stretching processes are performed within a range satisfying equations (1) and (2).

[0057] α=(ab) / a···(1)

[0058] 0.28≤αmax≤0.42···(2)

[0059] In formula (1), a represents the average thickness [μm] of the film 2 in the width direction before the nth (n is an integer from 1 to N) stretching treatment, and b represents the average thickness [μm] of the film 2 in the width direction after the nth stretching treatment.

[0060] The average thickness in the width direction is Figure 2 denoted by , is an average value of the thickness at the center and the thickness at both ends of the film 2 in the width direction. Figure 2 2 is a diagram for explaining the measurement position of the thickness in the width direction of the film 2, schematically showing a cross section perpendicular to the longitudinal direction of the film 2. Figure 2 As shown in the example, the thickness of the central portion may be within a range of 5% or less relative to the central position in the width direction of the film 2 ( Figure 2The thickness of one location (in the region A1 indicated by hatching) may be the thickness of each of the two ends in the width direction of the film 2 and within a range of 5% or less relative to a pair of edges ( Figure 2 The thickness of a region A2 and a region A3 indicated by hatching is shown in FIG.

[0061] In the formula (2), αmax is the maximum value of N α obtained for N stretching processes.

[0062] The stretch ratio of each of the N stretching treatments is, for example, 1.001 or greater and 4.00 or less, or 1.01 or greater and 3.00 or less, and preferably 1.05 or greater and 2.50 or less. The N stretching treatments can be performed using the nip rollers 11 positioned before and after each stretching treatment. The stretching treatments can be performed by utilizing the difference in rotation speed of the nip rollers 11 positioned before and after the stretching treatments. The nip rollers 11 involved in each stretching treatment function as a stretching treatment unit.

[0063] In with Figure 1 An example of stretching treatment will be described in which the cross-linking treatment (cross-linking process) performed in the cross-linking treatment section 133 shown in FIG. 1 is performed together with the cross-linking treatment (cross-linking process). In this case, the stretching treatment is performed using the clamping rollers 11 arranged before and after the cross-linking treatment section 133. In formula (1), a is the average value [μm] of the thickness along the width direction of the film 2 at the position x1, and b in formula (1) is the average value [μm] of the thickness along the width direction of the film 2 at the position x2. For the convenience of explanation, as Figure 1 As shown in the figure, the upstream side nip roller 11 among the two nip rollers 11 involved in one stretching process is referred to as the nip roller 11. UP , the clamping roller 11 on the downstream side is referred to as the clamping roller 11 DOWN Position x1 is where the film 2 passes through the clamping roller 11 UP Position x2 is where the film 2 passes through the clamping roller 11. DOWN After the position.

[0064] Reference Figure 3 The method for calculating α represented by the formula (1) in one stretching process will be described. In order to calculate α, the manufacturing apparatus 10 may include a pair of thickness measuring unit 30 and calculating unit 40 .

[0065] One of the pair of thickness measuring units 30 (hereinafter referred to as "thickness measuring unit 30") UP ”) measures the thickness of the film 2 before the stretching process, and the thickness measuring unit 30 on the other side (hereinafter referred to as “the thickness measuring unit 30 DOWN ”) to measure the thickness of the film 2 after the stretching treatment.

[0066] For example, the thickness measuring unit 30UP and thickness measuring unit 30 DOWN Each has three thickness gauges 31. Three thickness gauges 31 are configured along the width direction of the film 2 in a manner that the thickness of the central portion and both end portions of the film 2 can be measured. The thickness gauge 31 is not limited as long as it can measure the thickness of the film 2. The thickness gauge 31 is, for example, a non-contact thickness gauge (such as an optical thickness gauge). As the thickness gauge 31, for example, a spectroscopic interference displacement multilayer film thickness measuring device (such as SI-T80, etc.) of KEYENCE can be used. The thickness measurement can be performed by moving the thickness gauge along the width direction (direction orthogonal to the conveying direction) of the film 2 (a method for measuring the thickness) (transverse type (Japanese: Traverse type)).

[0067] The thickness can be measured at position x1, and then measured when the portion of the film whose thickness was measured at position x1 is transported to position x2. Alternatively, the thickness measurements at position x1 and at position x2 can be performed at the same time (i.e., simultaneously). The "same time" may vary slightly without departing from the spirit of the present invention. While it depends on the transport speed and is not particularly limited, the time difference between the measurement at position x1 and the measurement at position x2 can be within approximately 1 minute, within 30 seconds, within 20 seconds, or within 10 seconds.

[0068] The calculation unit 40 calculates the thickness of the UP and thickness measuring unit 30 DOWN The calculation unit 40 can calculate the α (= (ab) / a) in the formula (1) based on the thickness measurement unit 30. UP and thickness measuring unit 30 DOWN After calculating a and b from the results, α can be calculated using a and b. Alternatively, the formula for calculating a and b can be introduced into formula (1) to directly calculate α.

[0069] Here, the stretching treatment and the calculation method of α are described by taking the case where a stretching treatment is performed once in the cross-linking process as an example. However, multiple stretching treatments may be performed in the cross-linking process. Specifically, three or more clamping rollers 11 may be arranged in the area where the cross-linking process is performed in the conveying path of the film 2. In this case, the stretching treatment may be performed between two adjacent clamping rollers 11 among the three or more clamping rollers 11 arranged in the area where the cross-linking process is performed. In the case of performing multiple stretching treatments, the thickness of the film 2 after the stretching treatment on the upstream side of the two adjacent stretching treatments may be used as the thickness of the film 2 before the stretching treatment on the downstream side. In the case of performing multiple stretching treatments, with respect to the calculation unit 40, a pair of thickness measuring units 30 corresponding to each stretching treatment may be calculated. UP and thickness measuring unit 30 DOWNAlternatively, a pair of thickness measuring units 30 corresponding to each stretching process may be used. UP and thickness measuring unit 30 DOWN One calculation unit 40 is provided.

[0070] When manufacturing a polarizing film, the stretching process is performed so that the maximum αmax among the α obtained in each of the N stretching processes satisfies formula (2). Hereinafter, the series of steps until the calculation of α corresponding to each stretching process (including the measurement of the thickness used for α calculation) and the acquisition of αmax are sometimes referred to as a "monitoring process."

[0071] In the method for producing a polarizing film of this embodiment, the film 2 is subjected to N stretching treatments in a manner that satisfies equations (1) and (2). Therefore, even when the film 2 is continuously produced while being pulled from the raw material roll 6 and transported, the film 2 is unlikely to break. Furthermore, color unevenness in the produced polarizing film 4 can be suppressed, resulting in a good appearance. In other words, the method for producing a polarizing film of this embodiment can stably produce a polarizing film 4 having a good appearance.

[0072] Color unevenness of the polarizing film 4 can be evaluated as follows. After placing the polarizing film 4 in a crossed Nicol prism configuration relative to the linear polarizing filter in a darkroom, illuminate the polarizing film 4 with backlight. The polarizing film 4 thus illuminated is evaluated for color unevenness (including a state where no color unevenness occurs). The linear polarizing filter can be placed on either the backlight side or the side opposite to the backlight (the viewing side) relative to the polarizing film 4.

[0073] In a method for manufacturing a polarizing film having a monitoring process for real-time monitoring of whether N stretching processes satisfy equations (1) and (2), when the N stretching processes do not satisfy equations (1) and (2), the manufacture of the polarizing film 4 can be interrupted, for example. In the case of interrupting the manufacture, the conditions related to the stretching state (such as the stretching ratio, the temperature of the treatment liquid for immersing the film 2, and the immersion time) in the manufacture of the polarizing film 4 can be adjusted so that αmax satisfies equation (2). In addition, for example, the manufacture can be continued while adjusting the manufacturing conditions so that αmax satisfies equation (2). In this way, the breakage of the film 2 during the manufacture of the polarizing film 4 can be prevented, and the manufacture of the polarizing film 4 that becomes a defective product can be suppressed. Therefore, the polarizing film 4 can be manufactured in a stable process. In addition, it is easy to uniformly manufacture the polarizing film 4 with stable quality. Furthermore, the material cost of the polarizing film 4 can be reduced. Moreover, since the polarizing film 4 as a qualified product can be manufactured efficiently, the manufacturing yield of the polarizing film 4 is improved.

[0074] Typically, multiple stretching processes are often performed in the manufacture of polarizing films having high optical properties. Therefore, by performing multiple stretching processes in the manufacture of the polarizing film 4, a polarizing film 4 having a good appearance can be manufactured in a stable process as described above while maintaining high optical properties.

[0075] In the method for producing a polarizing film, from the viewpoint of suppressing color unevenness of the produced polarizing film 4 , N stretching treatments may be performed so as to satisfy the formula (3).

[0076] 0.1≤Δa / Δb≤1.1···(3)

[0077] In formula (3), Δa represents the difference between the maximum and minimum thicknesses in the width direction of the film 2 before the nth stretching process, and Δb represents the difference between the maximum and minimum thicknesses in the width direction of the film 2 after the nth stretching process.

[0078] The above-mentioned Δa and Δb can be calculated by obtaining the thickness distribution of the film 2 before the n-th stretching process and the film 2 after the n-th stretching process in the width direction. The thickness distribution can be calculated by arranging a number of film 2 along the width direction of the film 2 suitable for obtaining the thickness distribution. Figure 3 The thickness distribution can be obtained by measuring the thickness using the thickness gauge 31 shown.

[0079] In the case where the method for manufacturing the polarizing film has the aforementioned monitoring step, it is sufficient to monitor whether equations (1) and (2) are satisfied and to monitor whether equation (3) is satisfied during the monitoring step. If equation (3) is not satisfied, the conditions involved in the stretching process are adjusted in a manner that satisfies equation (3). For example, the setting state of the clamping roller 11 involved in the stretching process, the conveying state of the film 2, etc. are adjusted. In the case of monitoring whether equations (1) and (2) are satisfied during the monitoring step, it is sufficient to monitor whether equations (1) and (2) are satisfied using the measurement results of the thickness in the width direction measured to obtain the thickness distribution.

[0080] When N stretching treatments are performed in a manner that also satisfies formula (3), the influence of variations in the thickness in the width direction of the film 2 before and after each stretching treatment can be further reduced. As a result, the film 2 is less likely to break during the manufacture of the polarizing film 4. Furthermore, since defects such as color unevenness and streaks are less likely to occur in the film 2, it is easier to manufacture a polarizing film 4 with a good appearance.

[0081] The embodiments of the present invention have been described above. However, the present invention is not limited to the above-mentioned embodiments, and is intended to include the scope given by the scope of the technical solution for protection, and is intended to include all changes within the meaning and scope equivalent to the scope of the technical solution for protection. For example, the stretching method in the stretching treatment is not limited to the method using two clamping rollers 11 as long as it can stretch the film 2. The stretching treatment is not limited to the wet stretching method, and a dry stretching method can also be used. In addition to the above-mentioned N stretching treatments in the cross-linking process (stretching treatments that satisfy formulas (1) and (2)), stretching treatment can also be performed in other processes (such as swelling process, dyeing process, etc.). The above-mentioned embodiments and various modifications can be appropriately combined without departing from the scope of the main purpose of the present invention. In addition, in order to manufacture a polarizing film, it is sufficient to perform at least swelling treatment, dyeing treatment, cross-linking treatment and stretching treatment on the film 2.

[0082] Example

[0083] The present invention will be further described below using examples and comparative examples. In the following description, the film used to produce the polarizing film is also referred to as "film 2." The present invention is not limited to the following examples.

[0084] (Example 1)

[0085] Polarizing Film Manufacturing

[0086] A polarizing film was produced by the following method using a 75 μm-thick polyvinyl alcohol film (Poval film VF-PS#7500 manufactured by Kuraray Co., Ltd., polymerization degree 2400, saponification degree 99.9 mol % or more) as the long film 2 .

[0087] The film 2 is pulled out from a wound-up roll and immersed in 30°C pure water while maintaining a taut, non-relaxed state to fully swell the film 2 (swelling step). It is then uniaxially stretched while immersed in an aqueous solution containing iodine and potassium iodide until the cumulative stretch ratio reaches 2.4 times (dyeing step). Afterwards, it is uniaxially stretched to 1.75 times while immersed in a 56°C aqueous solution of potassium iodide / boric acid / water at a weight ratio of 12 / 4.2 / 100 (first crosslinking step: cumulative stretch ratio of 4.2 times from the original material). Subsequently, it is uniaxially stretched to 1.3 times while immersed in an aqueous solution of the same composition and temperature (second crosslinking step: cumulative stretch ratio of 5.5 times from the original material). Next, the film was immersed in a 40°C aqueous solution of potassium iodide / boric acid / water at a weight ratio of 9 / 2.9 / 100, and then uniaxially stretched to 1.05 times (third crosslinking step: cumulative stretch ratio from the original material was 5.7 times). The film was then immersed in pure water at 5°C (washing step) and dried at 70°C for 3 minutes (drying step) to obtain a polarizing film. No breakage of the film 2 occurred during the production of the polarizing film.

[0088] In the production of the polarizing film, the three stretching processes performed in the first, second, and third crosslinking steps constitute the N stretching processes in the crosslinking step described in the above embodiment.

[0089] <Thickness measurement>

[0090] In the production of polarizing films, a non-contact thickness gauge (SI-T80 manufactured by KEYENCE) was used to measure the thickness of the film 2 being transported at three locations in the film width direction before and after each step. The thickness measurement results of the film 2 after each treatment in the dyeing step, the first cross-linking step, the second cross-linking step, and the third cross-linking step are shown in FIG. Figure 4 shown. Figure 4 The “initial thickness” in the figure is the thickness of the film 2 prepared for producing the polarizing film (the thickness of the film 2 before swelling treatment), and the “thickness” in each step is the average value of the thicknesses at the three locations. Figure 4 The "thickness" in each step shown is the thickness obtained at the same time.

[0091] <Calculation of α and Determination of αmax>

[0092] Based on formula (1), α corresponding to the stretching treatment implemented in the first cross-linking process, the second cross-linking process, and the third cross-linking process is calculated. The thickness of the film 2 after the dyeing process and the thickness of the film 2 after the first cross-linking process are used as a and b in formula (1) to calculate α corresponding to the stretching treatment implemented in the first cross-linking process (hereinafter referred to as "α1"). Similarly, the thickness of the film 2 after the first cross-linking process and the thickness of the film 2 after the second cross-linking process are used as a and b in formula (1) to calculate α corresponding to the stretching treatment implemented in the second cross-linking process (hereinafter referred to as "α2"). Similarly, the thickness of the film 2 after the second cross-linking process and the thickness of the film 2 after the third cross-linking process are used as a and b in formula (1) to calculate α corresponding to the stretching treatment implemented in the third cross-linking process (hereinafter referred to as "α3"). The calculated α1, α2, and α3, and αmax, which is the maximum value among them, are as follows. Figure 5 shown.

[0093] <Calculation of Δa / Δb>

[0094] As described above in the thickness measurement, the differences Δt1, Δt2, Δt3, and Δt4 between the maximum and minimum values ​​of the three measurement results at the center and both ends in the width direction of the film obtained after the dyeing step, the first cross-linking step, the second cross-linking step, and the third cross-linking step are calculated. Figure 4 As shown. Use Figure 4 The differences Δt1, Δt2, Δt3, and Δt4 shown are used to calculate Δa / Δb corresponding to the stretching treatments performed in the first, second, and third cross-linking steps. Δa / Δb is hereinafter referred to as β.

[0095] The difference Δt1 after the dyeing process and the difference Δt2 after the first cross-linking process are used as Δa and Δb to calculate the Δa / Δb (hereinafter referred to as "β1") corresponding to the stretching treatment performed in the first cross-linking process. Similarly, the difference Δt2 after the first cross-linking process and the difference Δt3 after the second cross-linking process are used as Δa and Δb to calculate the Δa / Δb (hereinafter referred to as "β2") corresponding to the stretching treatment performed in the second cross-linking process. Similarly, the difference Δt3 after the second cross-linking process and the difference Δt4 after the third cross-linking process are used as Δa and Δb to calculate the Δa / Δb (hereinafter referred to as "β3") corresponding to the stretching treatment performed in the third cross-linking process. The calculation results are as follows. Figure 5 shown.

[0096] <Evaluation of color unevenness>

[0097] The polarizing film thus produced was placed in a dark room in a crossed Nicol prism state relative to a linear polarizing filter. Thereafter, the polarizing film was irradiated with 6000 cd / m 2The polarizing film was visually observed for color unevenness under a backlight. The level (intensity) of color unevenness was then assessed using a visual sensory inspection using a scale of "1," "2," and "3." A rating of "1" indicated the weakest unevenness, "3" indicated the strongest unevenness, and "2" was intermediate between "1" and "3." In the sensory inspection, color unevenness was assessed on a three-level scale as described above by comparing it with a sample sample determined based on the level (intensity) of color unevenness.

[0098] The polarizing film produced in Example 1 was evaluated as "1".

[0099] (Example 2)

[0100] Polarizing Film Manufacturing

[0101] A polarizing film was obtained in the same manner as in Example 1 except that a 30 μm-thick polyvinyl alcohol film (Poval film VF-PE#3000 manufactured by Kuraray Co., Ltd., polymerization degree 2400, saponification degree 99.9 mol % or more) was used as film 2. No film breakage occurred during the production of the polarizing film.

[0102] <Thickness measurement>

[0103] The thickness of the film 2 during transport was measured at three locations in the center and at both ends in the film width direction before and after each step in the same manner as in Example 1. The thickness of the film 2 after each of the dyeing step, the first cross-linking step, the second cross-linking step, and the third cross-linking step was measured as follows: Figure 4 shown. Figure 4 The thickness in each step is the average thickness, which is the same as in Example 1.

[0104] <Calculation of α and Determination of αmax>

[0105] α1, α2, and α3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step, and the third cross-linking step were calculated in the same manner as in Example 1. The calculated α1, α2, and α3, and αmax, which is the maximum value among them, are shown in FIG. Figure 5 shown.

[0106] <Calculation of β (=Δa / Δb)>

[0107] The difference Δt1, difference Δt2, difference Δt3 and difference Δt4 were calculated in the same manner as in Example 1, and β1, β2 and β3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step and the third cross-linking step were calculated. Figure 4 and Figure 5 shown.

[0108] <Evaluation of color unevenness>

[0109] Color unevenness of the produced polarizing film was evaluated in the same manner as in Example 1. The evaluation result of the polarizing film produced in Example 2 was "1".

[0110] (Example 3)

[0111] Polarizing Film Manufacturing

[0112] A polarizing film was produced in the same manner as in Example 1, except that the temperature of the aqueous solution A in the first and second cross-linking steps was changed to 58° C. During the production of the polarizing film, the film 2 did not break.

[0113] <Thickness measurement>

[0114] The thickness of the film 2 during transport was measured at three locations in the center and at both ends in the film width direction before and after each step in the same manner as in Example 1. The thickness of the film 2 after each of the dyeing step, the first cross-linking step, the second cross-linking step, and the third cross-linking step was measured as follows: Figure 4 shown. Figure 4 The thickness in each step is the average thickness, which is the same as in Example 1.

[0115] <Calculation of α and Determination of αmax>

[0116] α1, α2, and α3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step, and the third cross-linking step were calculated in the same manner as in Example 1. The calculated α1, α2, and α3, and αmax, which is the maximum value among them, are shown in FIG. Figure 5 shown.

[0117] <Calculation of β (=Δa / Δb)>

[0118] The difference Δt1, difference Δt2, difference Δt3 and difference Δt4 were calculated in the same manner as in Example 1, and β1, β2 and β3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step and the third cross-linking step were calculated. Figure 4 and Figure 5 shown.

[0119] <Evaluation of color unevenness>

[0120] Color unevenness of the produced polarizing film was evaluated in the same manner as in Example 1. The evaluation result of the polarizing film produced in Example 3 was "1".

[0121] (Example 4)

[0122] Polarizing Film Manufacturing

[0123] A polarizing film was produced in the same manner as in Example 1, except that the temperature of the aqueous solution A in the first cross-linking step and the second cross-linking step was changed to 62° C. No breakage of the film 2 occurred during the production of the polarizing film.

[0124] <Thickness measurement>

[0125] The thickness of the film 2 during transport was measured at three locations in the center and at both ends in the film width direction before and after each step in the same manner as in Example 1. The thickness of the film 2 after each of the dyeing step, the first cross-linking step, the second cross-linking step, and the third cross-linking step was measured as follows: Figure 4 shown. Figure 4 The thickness in each step is the average thickness, which is the same as in Example 1.

[0126] <Calculation of α and Determination of αmax>

[0127] α1, α2, and α3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step, and the third cross-linking step were calculated in the same manner as in Example 1. The calculated α1, α2, and α3, and αmax, which is the maximum value among them, are shown in FIG. Figure 5 shown.

[0128] <Calculation of β (=Δa / Δb)>

[0129] The difference Δt1, difference Δt2, difference Δt3 and difference Δt4 were calculated in the same manner as in Example 1, and β1, β2 and β3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step and the third cross-linking step were calculated. Figure 4 and Figure 5 shown.

[0130] <Evaluation of color unevenness>

[0131] Color unevenness of the produced polarizing film was evaluated in the same manner as in Example 1. The evaluation result of the polarizing film produced in Example 4 was "2".

[0132] (Comparative Example 1)

[0133] Polarizing Film Manufacturing

[0134] A polarizing film was produced in the same manner as in Example 1, except that an aqueous solution having a weight ratio of potassium iodide / boric acid / water of 12 / 2 / 100 was used in the first and second crosslinking steps. No breakage of the film 2 occurred during the production of the polarizing film. The aqueous solution used in the first and second crosslinking steps of Comparative Example 1 is referred to as aqueous solution B.

[0135] <Thickness measurement>

[0136] The thickness of the film 2 during transport was measured at three locations in the center and at both ends in the film width direction before and after each step in the same manner as in Example 1. The thickness of the film 2 after each of the dyeing step, the first cross-linking step, the second cross-linking step, and the third cross-linking step was measured as follows: Figure 4 shown. Figure 4 The thickness in each step is the average thickness, which is the same as in Example 1.

[0137] <Calculation of α and Determination of αmax>

[0138] α1, α2, and α3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step, and the third cross-linking step were calculated in the same manner as in Example 1. The calculated α1, α2, and α3, and αmax, which is the maximum value among them, are shown in FIG. Figure 5 shown.

[0139] <Calculation of β (=Δa / Δb)>

[0140] The difference Δt1, difference Δt2, difference Δt3 and difference Δt4 were calculated in the same manner as in Example 1, and β1, β2 and β3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step and the third cross-linking step were calculated. Figure 4 and Figure 5 shown.

[0141] <Evaluation of color unevenness>

[0142] Color unevenness of the produced polarizing film was evaluated in the same manner as in Example 1. The evaluation result of the polarizing film produced in Comparative Example 1 was “3”.

[0143] (Comparative Example 2)

[0144] Polarizing Film Manufacturing

[0145] A polarizing film was produced in the same manner as in Example 1, except that an aqueous solution containing potassium iodide, boric acid, and water in a weight ratio of 12 / 6.5 / 100 was used in the first and second cross-linking steps. The aqueous solution used in the first and second cross-linking steps of Comparative Example 2 is referred to as aqueous solution C. During the production of the polarizing film, film 2 frequently broke, preventing stable polarizing film production. In Comparative Example 2, if film 2 broke, it was re-unrolled from the raw material roll, and polarizing film production continued until the next break occurred.

[0146] <Thickness measurement>

[0147] The thickness of the film 2 during transport was measured at three locations in the center and at both ends in the film width direction before and after each step in the same manner as in Example 1. The thickness of the film 2 after each of the dyeing step, the first cross-linking step, the second cross-linking step, and the third cross-linking step was measured as follows: Figure 4 shown. Figure 4 The thickness in each step is the average thickness, which is the same as in Example 1.

[0148] <Calculation of α and Determination of αmax>

[0149] α1, α2, and α3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step, and the third cross-linking step were calculated in the same manner as in Example 1. The calculated α1, α2, and α3, and αmax, which is the maximum value among them, are shown in FIG. Figure 5 shown.

[0150] <Calculation of β (=Δa / Δb)>

[0151] The difference Δt1, difference Δt2, difference Δt3 and difference Δt4 were calculated in the same manner as in Example 1, and β1, β2 and β3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step and the third cross-linking step were calculated. Figure 4 and Figure 5 shown.

[0152] <Evaluation of color unevenness>

[0153] Color unevenness of the polarizing film produced until the film 2 broke was evaluated in the same manner as in Example 1. The evaluation result of the polarizing film produced in Comparative Example 2 was "1".

[0154] (Comparative Example 3)

[0155] Polarizing Film Manufacturing

[0156] A polarizing film was produced 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 and second crosslinking steps were performed using aqueous solution B. The film 2 did not break during the production of the polarizing film.

[0157] <Thickness measurement>

[0158] The thickness of the film 2 during transport was measured at three locations in the center and at both ends in the film width direction before and after each step in the same manner as in Example 1. The thickness of the film 2 after each of the dyeing step, the first cross-linking step, the second cross-linking step, and the third cross-linking step was measured as follows: Figure 4 shown. Figure 4 The thickness in each step is the average thickness, which is the same as in Example 1.

[0159] <Calculation of α and Determination of αmax>

[0160] α1, α2, and α3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step, and the third cross-linking step were calculated in the same manner as in Example 1. The calculated α1, α2, and α3, and αmax, which is the maximum value among them, are shown in FIG. Figure 5 shown.

[0161] <Calculation of β (=Δa / Δb)>

[0162] The difference Δt1, difference Δt2, difference Δt3 and difference Δt4 were calculated in the same manner as in Example 1, and β1, β2 and β3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step and the third cross-linking step were calculated. Figure 4 and Figure 5 shown.

[0163] <Evaluation of color unevenness>

[0164] Color unevenness of the produced polarizing film was evaluated in the same manner as in Example 1. The evaluation result of the polarizing film produced in Comparative Example 3 was “3”.

[0165] (Comparative Example 4)

[0166] A polarizing film was produced 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 that the first and second crosslinking steps were carried out using aqueous solution C. During the production of the polarizing film, breakage of the film 2 occurred frequently, and a stable polarizing film could not be obtained. In Comparative Example 4, when a breakage of the film 2 occurred, the film 2 was re-unrolled from the raw material roll, and the production of the polarizing film continued until the next breakage occurred.

[0167] <Thickness measurement>

[0168] The thickness of the film 2 during transport was measured at three locations in the center and at both ends in the film width direction before and after each step in the same manner as in Example 1. The thickness of the film 2 after each of the dyeing step, the first cross-linking step, the second cross-linking step, and the third cross-linking step was measured as follows: Figure 4 shown. Figure 4 The thickness in each step is the average thickness, which is the same as in Example 1.

[0169] <Calculation of α and Determination of αmax>

[0170] α1, α2, and α3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step, and the third cross-linking step were calculated in the same manner as in Example 1. The calculated α1, α2, and α3, and αmax, which is the maximum value among them, are shown in FIG. Figure 5 shown.

[0171] <Calculation of β (=Δa / Δb)>

[0172] The difference Δt1, difference Δt2, difference Δt3 and difference Δt4 were calculated in the same manner as in Example 1, and β1, β2 and β3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step and the third cross-linking step were calculated. Figure 4 and Figure 5 shown.

[0173] <Evaluation of color unevenness>

[0174] Color unevenness of the polarizing film produced until the film 2 broke was evaluated in the same manner as in Example 1. The evaluation result of the polarizing film produced in Comparative Example 4 was "1".

[0175] (Comparative Example 5)

[0176] A polarizing film was produced in the same manner as in Example 1, except that the temperature of the aqueous solution A in the first and second cross-linking steps was changed to 50°C. During the production of the polarizing film, film 2 frequently broke, and a stable polarizing film could not be produced. In Comparative Example 5, when film 2 broke, the film 2 was re-unrolled from the raw material roll, and polarizing film production continued until the next break occurred.

[0177] <Thickness measurement>

[0178] The thickness of the film 2 during transport was measured at three locations in the center and at both ends in the film width direction before and after each step in the same manner as in Example 1. The thickness of the film 2 after each of the dyeing step, the first cross-linking step, the second cross-linking step, and the third cross-linking step was measured as follows: Figure 4 shown. Figure 4 The thickness in each step is the average thickness, which is the same as in Example 1.

[0179] <Calculation of α and Determination of αmax>

[0180] α1, α2, and α3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step, and the third cross-linking step were calculated in the same manner as in Example 1. The calculated α1, α2, and α3, and αmax, which is the maximum value among them, are shown in FIG. Figure 5 shown.

[0181] <Calculation of β (=Δa / Δb)>

[0182] The difference Δt1, difference Δt2, difference Δt3 and difference Δt4 were calculated in the same manner as in Example 1, and β1, β2 and β3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step and the third cross-linking step were calculated. Figure 4 and Figure 5 shown.

[0183] <Evaluation of color unevenness>

[0184] Color unevenness of the polarizing film produced until the film 2 broke was evaluated in the same manner as in Example 1. The evaluation result of the polarizing film produced in Comparative Example 5 was "1".

[0185] (Comparative Example 6)

[0186] A polarizing film was produced in the same manner as in Example 1, except that the temperature of the aqueous solution A in the first and second cross-linking steps was changed to 45°C. During the production of the polarizing film, film 2 frequently broke, and a stable polarizing film could not be produced. In Comparative Example 6, when film 2 broke, the film 2 was re-unrolled from the raw material roll, and polarizing film production continued until the next break occurred.

[0187] <Thickness measurement>

[0188] The thickness of the film 2 during transport was measured at three locations in the center and at both ends in the film width direction before and after each step in the same manner as in Example 1. The thickness of the film 2 after each of the dyeing step, the first cross-linking step, the second cross-linking step, and the third cross-linking step was measured as follows: Figure 4 shown. Figure 4 The thickness in each step is the average thickness, which is the same as in Example 1.

[0189] <Calculation of α and Determination of αmax>

[0190] α1, α2, and α3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step, and the third cross-linking step were calculated in the same manner as in Example 1. The calculated α1, α2, and α3, and αmax, which is the maximum value among them, are shown in FIG. Figure 5 shown.

[0191] <Calculation of β (=Δa / Δb)>

[0192] The difference Δt1, difference Δt2, difference Δt3 and difference Δt4 were calculated in the same manner as in Example 1, and β1, β2 and β3 corresponding to the stretching treatments performed in the first cross-linking step, the second cross-linking step and the third cross-linking step were calculated. Figure 4 and Figure 5 shown.

[0193] <Evaluation of color unevenness>

[0194] Color unevenness of the polarizing film produced until the film 2 broke was evaluated in the same manner as in Example 1. The evaluation result of the polarizing film produced in Comparative Example 6 was "2".

[0195] [Comprehensive evaluation]

[0196] like Figure 5 As shown, based on αmax of Examples 1 to 4, in Examples 1 to 4, the stretching treatments performed in the first cross-linking step, the second cross-linking step, and the third cross-linking step were performed while satisfying equations (1) and (2). In addition, in Examples 1 to 4, no breakage of the film 2 occurred during the manufacture of the polarizing film. In other words, in Examples 1 to 4, the polarizing film can be stably manufactured. In addition, in the polarizing films manufactured in Examples 1 to 4, the evaluation of color unevenness was "1" or "2".

[0197] On the other hand, based on the αmax values ​​of Comparative Examples 1 to 6, the stretching treatments performed in the first, second, and third crosslinking steps of Comparative Examples 1 to 6 did not satisfy Equations (1) and (2). In Comparative Examples 1 and 3, although no breakage of the film 2 occurred during the production of the polarizing film, the color unevenness evaluation was rated "3." In Comparative Examples 2, 4 to 6, breakage of the film 2 occurred during the production of the polarizing film, making it impossible to stably produce the polarizing film.

[0198] Therefore, based on the results of Examples 1 to 4 and Comparative Examples 1 to 6, it can be understood that by implementing N stretching treatments in a manner that satisfies Formulas (1) and (2), a polarizing film with suppressed color unevenness, that is, a polarizing film with a good appearance, can be stably manufactured.

[0199] Furthermore, when the results of Δa / Δb in Examples 1 to 4 are compared, it can be understood that when all Δa / Δb calculated for N stretching processes satisfy the formula (3), color unevenness is further suppressed.

Claims

1. A method for producing a polarizing film, comprising a swelling step, a dyeing step, and a cross-linking step. In the cross-linking step, the polyvinyl alcohol film is subjected to N stretching treatments, where N is an integer greater than 1. The N stretching processes are performed within the range satisfying equations (1) and (2): α=(ab) / a···(1) 0.28≤αmax≤0.42···(2) In formula (1), a represents the average thickness of the polyvinyl alcohol film in the width direction before the n-th stretching treatment, the unit of the average is μm, and n is an arbitrary integer from 1 to N. b represents the average value of the thickness of the polyvinyl alcohol film in the width direction after the n-th stretching treatment, and the unit of the average value is μm. The average value of the thickness in the width direction is the average value of the thickness of the central portion and the thickness of both ends of the polyvinyl alcohol film in the width direction; In formula (2), αmax is the maximum value of N α values ​​obtained for the N stretching processes.

2. The method for manufacturing a polarizing film according to claim 1, wherein: The stretching ratio of each of the N stretching treatments is 1.001 or more and 4.00 or less.

3. The method for producing a polarizing film according to claim 1 or 2, wherein: In the N stretching treatments, the polyvinyl alcohol film is stretched by nip rolls arranged before and after each stretching treatment.

4. The method for producing a polarizing film according to any one of claims 1 to 3, wherein The N stretching processes are performed within the range satisfying the following formula (3): 0.1≤Δa / Δb≤1.1···(3) In formula (3), Δa represents the difference between the maximum and minimum thicknesses in the width direction of the polyvinyl alcohol film before the n-th stretching treatment, 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 n-th stretching process.

Images