Method for manufacturing a polarizing film

Abstract

The present invention provides a method for manufacturing a polarizing film, which reduces fluctuations in optical characteristics in the width direction of the polarizing film. The method for manufacturing a polarizing film, which is manufactured from a polyvinyl alcohol-based resin film, sequentially includes a treatment step of treating the polyvinyl alcohol-based resin film by bringing a treatment liquid into contact with the polyvinyl alcohol-based resin film, and a washing step of washing the polyvinyl alcohol-based resin film by bringing a washing liquid into contact with the polyvinyl alcohol-based resin film. The washing step is a step in which the polyvinyl alcohol-based resin film is divided into a one-end portion including one end in the width direction of the polyvinyl alcohol-based resin film, a other-end portion including the other end in the width direction, and a central portion inside in the width direction with respect to the one-end portion and the other-end portion, and the polyvinyl alcohol-based resin film is washed such that the degree of washing of the one-end portion is greater than the degree of washing of the other-end portion, and the degree of washing of the other-end portion is greater than the degree of washing of the central portion.

Description

Technical Field

[0001] This invention relates to a method for manufacturing a polarizing film made of polyvinyl alcohol-based resin film. Background Technology

[0002] Polarizing films, obtained by adsorbing dichroic pigments onto a polyvinyl alcohol-based resin film and orienting it, have long been known. Polarizing films can be used, for example, as materials for polarizing plates. Polarizing plates are typically manufactured by bonding a protective film, such as triacetyl cellulose, to at least one side, preferably both sides, of the polarizing film using an adhesive formed from an aqueous solution of a polyvinyl alcohol-based resin. Polarizing plates are used, for example, as display screens in liquid crystal display devices (LCDs) such as LCD televisions, personal computer monitors, and mobile phones.

[0003] In the manufacturing process of polarizing films, efforts have been made to suppress fluctuations in the optical properties of the polarizing film in the width direction. For example, Japanese Patent Application Publication No. 2013-140345 (Patent Document 1) and Taiwan Patent Publication No. 201142375 (Patent Document 2) proposed a solution in which, after dyeing and other treatments on a polyvinyl alcohol-based resin film, the polarizing film is manufactured by washing it with water, and the fluctuations in the optical properties are suppressed by making the washing temperature at both ends of the polarizing film in the width direction different from the washing temperature at the center of the polarizing film in the width direction.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2013-140345

[0007] Patent Document 2: Taiwan Patent Publication No. 201142375 Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] The object of the present invention is to provide a method for manufacturing a polarizing film that can reduce fluctuations in the optical properties of the polarizing film in the width direction.

[0010] means for solving problems

[0011] The present invention provides a method for manufacturing the polarizing film shown below.

[0012] [1] A method for manufacturing a polarizing film, comprising the following steps:

[0013] The processing step involves contacting the processing solution with the polyvinyl alcohol-based resin film for treatment; and

[0014] The washing process involves bringing the washing solution into contact with the polyvinyl alcohol-based resin membrane for washing.

[0015] The washing process is as follows: after dividing the polyvinyl alcohol resin film into an end portion including one end in the width direction of the polyvinyl alcohol resin film, an end portion including the other end in the width direction, and a central portion that is inside the width direction relative to the one end portion and the other end portion, the polyvinyl alcohol resin film is washed to a greater degree than the washing degree of the other end portion, and the washing degree of the other end portion is greater than the washing degree of the central portion.

[0016] [2] The method for manufacturing a polarizing film according to [1], wherein the washing process is performed in a manner that satisfies i) below.

[0017] i) When the temperature of the washing liquid in contact with one end is set to T1 [°C], the temperature of the washing liquid in contact with the other end is set to T2 [°C], and the temperature of the washing liquid in contact with the central part is set to T3 [°C], T1, T2 and T3 satisfy the relationship T1>T2>T3.

[0018] [3] The method for manufacturing a polarizing film according to [2] is wherein the washing process is performed in a manner that satisfies i) and at least any of a1) and b1) below.

[0019] a1) 1 ≤ T1 - T2 ≤ 15

[0020] b1)2≤T2-T3≤30

[0021] [4] The method for manufacturing a polarizing film according to [2] or [3], wherein the washing process is performed in a manner that satisfies i) and c1) below.

[0022] c1)3≤T3≤25

[0023] [5] A method for manufacturing a polarizing film according to any one of [1] to [4], wherein the washing process is performed in a manner that satisfies ii) below.

[0024] ii) When the volume of the washing liquid per unit width and per unit time in contact with one end is set to M1 [L / m / min], the volume of the washing liquid per unit width and per unit time in contact with the other end is set to M2 [L / m / min], and the volume of the washing liquid per unit width and per unit time in contact with the central part is set to M3 [L / m / min], M1, M2, and M3 satisfy the relationship M1 > M2 > M3.

[0025] [6] A method for manufacturing a polarizing film according to any one of [1] to [5], wherein the washing process includes the operation of continuously discharging the washing liquid from the outlet toward the polyvinyl alcohol-based resin film using a washing device having a plurality of outlets for discharging the washing liquid.

[0026] [7] According to the method for manufacturing a polarizing film described in [6], in the operation of continuously discharging the washing liquid, the polyvinyl alcohol resin film is transported upward in a vertical direction, and the height position of the outlet for discharging the washing liquid toward the central portion is lower than the height position of the outlet for discharging the washing liquid toward one end and the height position of the outlet for discharging the washing liquid toward the other end.

[0027] Invention Effects

[0028] According to the present invention, a method for manufacturing a polarizing film can be provided, which can reduce fluctuations in the optical properties of the polarizing film in the width direction. Attached Figure Description

[0029] Figure 1 This is a partial cross-sectional perspective view of a washing apparatus having multiple outlets for discharging the washing liquid used in the washing process described above, illustrating an example of a two-stage washing process of the first method.

[0030] Figure 2 (a) is a schematic cross-sectional side view of the washing apparatus, illustrating an example of the two-stage washing process of the first method; (b) and (c) are schematic top views of the multiple outlets of the washing apparatus for discharging washing liquid.

[0031] Figure 3 This is a partial cross-sectional perspective view of a washing apparatus having multiple outlets for discharging the washing liquid used in the washing process described above, illustrating an example of the three-stage washing process of the second method.

[0032] Figure 4(a) is a schematic cross-sectional side view of the washing device, illustrating an example of the three-stage washing process of the second method. (b), (c) and (d) are schematic top views of the multiple outlets of the washing device for discharging washing liquid.

[0033] Explanation of reference numerals in the attached figures

[0034] 1. Polyvinyl alcohol resin film; 2. Conveying guide roller; 31. Washing bath; 32. Washing liquid; 41, 42, 43. Spraying device; 411, 411a, 411b, 411c, 421, 421a, 421b, 421c, 431, 431a, 431b, 431c. Spray nozzles; 5. Dewatering roller. Detailed Implementation

[0035] Hereinafter, with reference to the accompanying drawings, a method for manufacturing a polarizing film according to one aspect of the present invention (hereinafter also referred to as "the manufacturing method of this embodiment") will be described.

[0036] [Manufacturing method of polarizing film]

[0037] One embodiment of the present invention provides a method for manufacturing a polarizing film (the manufacturing method of this embodiment) that uses a polyvinyl alcohol (PVA) resin film to form the polarizing film. The method comprises, in sequence, a processing step in which a processing liquid is brought into contact with the PVA resin film for processing; and a washing step in which a washing liquid is brought into contact with the PVA resin film for washing. The washing step involves dividing the PVA resin film into an end portion including one end in the width direction, an end portion including the other end in the width direction, and a central portion that is inward relative to both the one end portion and the other end portion in the width direction, and then washing the PVA resin film to a greater degree than washing the other end portion, and washing the other end portion to a greater degree than washing the central portion.

[0038] <Polyvinyl alcohol-based resin film>

[0039] The aforementioned polarizing film can be obtained, for example, by adsorbing dichroic pigments (iodine, dichroic dyes, etc.) onto a uniaxially stretched polyvinyl alcohol-based resin film and then orienting it. A polyvinyl alcohol-based resin film refers to a film containing a polyvinyl alcohol-based resin, such as a saponified polyvinyl acetate-based resin. As a polyvinyl acetate-based resin, in addition to polyvinyl acetate as a homopolymer of vinyl acetate, copolymers of vinyl acetate with other monomers that can be copolymerized therewith (e.g., ethylene-vinyl acetate copolymers) can also be mentioned. Other monomers that can be copolymerized include, for example, unsaturated carboxylic acids, olefins, vinyl ethers, and unsaturated sulfonic acids. The degree of polymerization of the polyvinyl alcohol-based resin is typically about 1000 to 10000, preferably about 1500 to 5000. The degree of saponification is typically 85 mol% or more, preferably 90 mol% or more, and more preferably 99 to 100 mol%. These polyvinyl alcohol resins can be modified, for example, using aldehyde-modified polyvinyl formal, polyvinyl acetal, polyvinyl butyral, etc.

[0040] The polyvinyl alcohol (PVA) resin film used as the starting material in the manufacturing method of this embodiment can be an unstretched PVA resin film (raw material film) with a thickness of 65 μm or less (e.g., 60 μm or less), preferably 50 μm or less, more preferably 35 μm or less, and even more preferably 30 μm or less. This allows for the production of thin polarizing films, which are in increasing market demand. The width of the raw material film is not particularly limited, but is, for example, 400 mm or more and 8000 mm or less, preferably 2000 mm or more and 5500 mm or less. When the width of the raw material film is 2000 mm or more, although optical properties such as hue are prone to fluctuation in the width direction of the polarizing film, the manufacturing method of this embodiment can reduce such fluctuations. The thickness of the unstretched PVA resin film is preferably 20 μm or more. The raw material film is prepared, for example, as a roll of long, unstretched PVA resin film (raw material roll). It should be noted that the unstretched PVA resin film (raw material film) is typically supplied as a roll film.

[0041] The aforementioned polyvinyl alcohol (PVA) resin film can be laminated onto a substrate film supporting it. That is, the PVA resin film can be prepared as a laminate of a substrate film and a PVA resin film laminated thereon. In this case, the PVA resin film can be manufactured, for example, by coating at least one side of the substrate film with a coating liquid containing a PVA resin and then drying it.

[0042] As the substrate film, for example, a film containing a thermoplastic resin can be used. Specifically, the film is formed from a light-transmitting thermoplastic resin, preferably an optically transparent thermoplastic resin, such as a polyolefin resin like a chain-like polyolefin resin (polypropylene resin, etc.), a cyclic polyolefin resin like a norbornene resin, a cellulose resin like triacetyl cellulose or diacetyl cellulose, a polyester resin like polyethylene terephthalate or polybutylene terephthalate, a polycarbonate resin, a (meth)acrylic resin like methyl methacrylate, a polystyrene resin, a polyvinyl chloride resin, an acrylonitrile-butadiene-styrene resin, an acrylonitrile-styrene resin, a polyvinyl acetate resin, a polyvinylidene chloride resin, a polyamide resin, a polyacetal resin, a modified polyphenylene ether resin, a polysulfone resin, a polyethersulfone resin, a polyaryl ester resin, a polyamide-imide resin, or a polyimide resin.

[0043] <Processing Procedure>

[0044] The manufacturing method of this embodiment includes, in sequence, a processing step in which a processing solution is brought into contact with the polyvinyl alcohol-based resin film for processing; and a washing step in which a washing solution is brought into contact with the polyvinyl alcohol-based resin film for washing. The processing step is a process involving various treatments performed on the raw material film, such as swelling treatment, dyeing treatment, and crosslinking treatment (boric acid treatment).

[0045] Specifically, the above-mentioned processing steps may include: a swelling step, in which a swelling solution is brought into contact with the original film to perform a swelling treatment; a dyeing step, in which a dyeing solution is brought into contact with the swollen polyvinyl alcohol resin film (hereinafter also referred to as "swollen film") to perform a dyeing treatment; a crosslinking step, in which a crosslinking solution is brought into contact with the dyed polyvinyl alcohol resin film (hereinafter also referred to as "dyed film") to perform a crosslinking treatment; and a color correction step, in which a color correction solution is brought into contact with the crosslinked polyvinyl alcohol resin film (hereinafter also referred to as "crosslinked film") to perform a color adjustment treatment. In this specification, the polyvinyl alcohol resin film after color adjustment treatment is also referred to as a "color correction film".

[0046] In the above-described processing steps, it is preferable to include a uniaxial stretching operation on the film before or during any of the aforementioned swelling steps, etc. For example, as an example of the above-described processing steps, a swelling step, a dyeing step, a crosslinking step, and a color-correction step can be performed sequentially after uniaxially stretching the unstretched raw film in air or an inert gas (dry stretching). Alternatively, an example can be performed by sequentially performing a swelling step, a dyeing step, a crosslinking step, and a color-correction step on an unstretched raw film, and then performing a wet uniaxial stretching operation on the film before or during the crosslinking step. Furthermore, other steps may be included in the above-described processing steps as long as the effects of the present invention are achieved.

[0047] In the above-described processing steps, for example, while the raw material film is being wound from the raw material roll, it is continuously transported along the film transport path of a conventionally known polarizing film manufacturing apparatus, thereby immersing it in each processing liquid (hereinafter also referred to as a "processing bath") contained in each processing tank used for the swelling process, dyeing process, crosslinking process, and color correction process described above. After the raw material film is pulled out from the processing tank, in the above-described polarizing film manufacturing method, it can be transferred to the washing process described later. The above-described processing steps are not limited to the method of immersing the raw material film in a processing bath, as long as the processing liquid is brought into contact with the raw material film; other methods of bringing the processing liquid into contact with the surface of the raw material film are also possible. As another method of bringing the processing liquid into contact with the surface of the raw material film, a method using spraying the processing liquid to bring the processing liquid into contact with the surface of the raw material film is suitable. Here, "spraying" of the processing liquid refers to the flow of processing liquid discharged from multiple outlets, in which the processing liquid can be in particulate, mist, or liquid stream form. When the treatment liquid is brought into contact with the surface of the original membrane by the above-mentioned spraying, a spraying device is usually used. The spraying device may have multiple outlets, and the treatment liquid is discharged from the outlets to form a liquid flow. On the other hand, when the above-mentioned treatment process is carried out by immersing the original membrane in a treatment bath, the treatment bath for performing a treatment (e.g., crosslinking treatment) is not limited to one; for example, the original membrane may be immersed in two or more treatment baths in sequence.

[0048] Here, the term "processing tank" in this specification refers to the collective term including a swelling tank, a dyeing tank, a cross-linking tank, and a color-correcting tank. Furthermore, the term "processing liquid" refers to the collective term including a swelling liquid, a dyeing liquid, a cross-linking liquid, and a color-correcting liquid, and the term "processing bath" refers to the collective term including a swelling bath, a dyeing bath, a cross-linking bath, and a color-correcting bath. Hereinafter, using the conventionally known polarizing film manufacturing apparatus described above as an example of the processing steps of the manufacturing method of this embodiment, a method of continuously transporting the raw film along a film transport path and sequentially performing the above-described swelling step, dyeing step, cross-linking step, and color-correcting step will be described.

[0049] (Swelling process)

[0050] The swelling process is performed to remove foreign matter from the surface of the raw film, remove plasticizers from the raw film, impart dyeability, and plasticize the raw film. The processing conditions are determined within the range that achieves the above objectives and does not produce undesirable conditions such as extreme dissolution and devitrification of the raw film.

[0051] In the aforementioned swelling process, the raw material film is continuously wound from the raw material roll while being transported along a film transport path constructed by guide rollers and clamping rollers. After being immersed in a swelling bath (a swelling solution contained in a swelling tank) for a specified time, the raw material film is pulled out as a swollen film, thereby performing the swelling treatment. As for the swelling solution used in the swelling bath, in addition to pure water, an aqueous solution containing boric acid (Japanese Patent Application Laid-Open No. 10-153709), chloride (Japanese Patent Application Laid-Open No. 06-281816), inorganic acid, inorganic salt, water-soluble organic solvent, alcohol, etc., in the range of approximately 0.01 to 10% by mass can also be used.

[0052] The temperature of the swelling bath is, for example, about 10–50°C, preferably about 10–40°C, and more preferably about 15–30°C. The immersion time of the raw material membrane is preferably about 10–300 seconds, and more preferably about 20–200 seconds. Furthermore, when the raw material membrane is a polyvinyl alcohol-based resin membrane that has been pre-stretched in a gas, the temperature of the swelling bath is, for example, about 20–70°C, preferably about 30–60°C. The immersion time of the raw material membrane is preferably about 30–300 seconds, and more preferably about 60–240 seconds.

[0053] In the aforementioned swelling process, wrinkles are easily formed in the swollen film because the raw film swells along its width. One method for transporting the swollen film while removing these wrinkles is to use known expansion rollers such as expander rolls, spiral rolls, or crown rolls as guide rollers, or known expansion devices such as fabric guides, bending rollers, or tenter ferrules. Another method to suppress wrinkle formation is to perform a stretching process. For example, by utilizing the difference in circumferential speed of each clamping roller in the film transport path, a uniaxial stretching process can be performed on the raw film in the swelling bath.

[0054] In the aforementioned swelling process, since the membrane also swells and expands in the transport direction, in order to eliminate membrane slack in the transport direction without actively stretching the original membrane, it is preferable to control the speed of the clamping rollers positioned before and after the swelling bath, for example. Furthermore, to stabilize membrane transport in the swelling bath, it is also useful to control the water flow in the swelling bath using a spray device and to use an EPC device (Edge Position Control device: a device that detects the ends of the membrane and prevents membrane kinking).

[0055] After the swelling treatment described above, the swollen membrane pulled out of the swelling bath is introduced along the membrane transport path into the dyeing bath used in the dyeing process described below.

[0056] (Dyeing process)

[0057] The dyeing process is performed to adsorb dichroic pigments such as iodine onto the swollen polyvinyl alcohol resin film and to orient it. Processing conditions are determined within a range that achieves the above objectives without causing extreme dissolution or devitrification of the film. The dyeing process involves immersing the swollen film, transported along a film transport path constructed by guide rollers and clamping rollers, in a dyeing bath (a dyeing solution contained in a dyeing tank) for a specified time, and then pulling it out as a dyed film. To improve the dyeability of dichroic pigments, the swollen film provided to the dyeing process is preferably a film that has undergone at least some degree of uniaxial stretching treatment. Preferably, the uniaxial stretching treatment is performed during the dyeing process instead of the uniaxial stretching treatment prior to the dyeing process, or the uniaxial stretching treatment is performed during the dyeing process based on the uniaxial stretching treatment prior to the dyeing process.

[0058] Iodine is preferably used as the dichroic pigment in the staining bath. For example, an aqueous solution with a concentration of approximately 0.003–0.3 / approximately 0.1–10 / 100 by mass ratio of iodine / potassium iodide / water can be used. Other iodides, such as zinc iodide, can be used instead of potassium iodide, or potassium iodide can be used in combination with other iodides. Furthermore, compounds other than iodides, such as boric acid, zinc chloride, and cobalt chloride, can coexist. The addition of boric acid distinguishes the dichroic pigment from the crosslinking solution described later. In this specification, an aqueous solution containing approximately 0.003 or more parts by mass of dichroic pigment per 100 parts by mass of water can be considered a staining solution. The temperature of the staining bath during immersion of the swollen membrane is typically around 10–45°C, preferably 10–40°C, more preferably 20–35°C, and the immersion time of the swollen membrane is typically around 30–600 seconds, preferably 60–300 seconds.

[0059] Dichroic dyes can be used instead of iodine as dichroic pigments. For example, an aqueous solution with a concentration of approximately 0.001 to 0.1 / 100 (by mass) can be used as the staining solution in the staining bath. Staining auxiliaries, such as inorganic salts like sodium sulfate and surfactants, can coexist in this staining solution. Only one dichroic dye can be used, or two or more dichroic dyes can be used in combination.

[0060] As described above, in the dyeing process, the swollen film can be uniaxially stretched in the dyeing bath. The uniaxial stretching of the swollen film can be achieved by setting a circumferential speed difference between the clamping rollers arranged before and after the dyeing bath.

[0061] In the above dyeing process, in order to remove wrinkles and transport the polyvinyl alcohol resin film simultaneously, similar to the swelling process described above, known rollers with widening functions, such as the stretching roller, twisted roller, and medium-high roller, can be used, or known widening devices such as fabric guides, bending rollers, and tenter ferrules can be used. Another method for suppressing wrinkle formation, similar to the swelling process, is to perform a stretching process.

[0062] After the dyeing treatment described above, the dyed membrane pulled from the dyeing bath is introduced along the membrane transport path into the crosslinking bath used in the crosslinking process described below. In this case, the dyed membrane is preferably coarsely washed by a spray device before being introduced into the crosslinking bath, thereby washing away any remaining dyeing solution adhering to the dyed membrane.

[0063] (Cross-linking process)

[0064] The crosslinking process is performed to impart water resistance to the dyed film. The crosslinking process can be performed once or multiple times. The crosslinking process can be performed by immersing the dyed film, which is transported along a film transport path constructed by guide rollers and clamping rollers, in a crosslinking bath (a crosslinking solution contained in a crosslinking tank) for a specified time, and then pulling it out as a crosslinked film.

[0065] As the crosslinking liquid, a solution in which the crosslinking agent is dissolved in a solvent can be used. Examples of crosslinking agents include boric acid, boron compounds such as borax, glyoxal, and glutaraldehyde. They can be used alone or in combination of two or more. For example, water can be used as the solvent, and an organic solvent compatible with water may also be included. The concentration of the crosslinking agent in the crosslinking liquid is not limited, but is preferably in the range of 1 to 20% by mass, more preferably 4 to 15% by mass.

[0066] As a crosslinking solution, for example, it can be an aqueous solution containing about 1 to 10 parts by mass of boric acid relative to 100 parts by mass of water. When the dichroic pigment used in the dyeing process is iodine, the crosslinking solution preferably contains an iodide in addition to boric acid, and the amount of iodide can be, for example, 1 to 30 parts by mass relative to 100 parts by mass of water. Examples of iodides include potassium iodide and zinc iodide. In addition, compounds other than iodides can also coexist, such as zinc chloride, cobalt chloride, zirconium chloride, sodium thiosulfate, potassium sulfite, and sodium sulfate.

[0067] In the crosslinking process, the concentrations of boric acid and iodide, as well as the temperature of the crosslinking bath, can be appropriately varied depending on the purpose. For example, an aqueous solution with a mass ratio of boric acid / iodide / water of 3–10 / 1–20 / 100 can be used. Depending on the need, other crosslinking agents can be used instead of boric acid, or boric acid can be used in combination with other crosslinking agents. The temperature of the crosslinking bath when impregnating the dyed film is typically around 50–70°C, preferably 53–65°C, and the impregnation time of the dyed film is typically around 10–600 seconds, preferably 20–300 seconds, and more preferably 20–200 seconds. Furthermore, when dyeing and crosslinking treatments are sequentially performed on polyvinyl alcohol-based resin films that have been pre-stretched before swelling treatment, the temperature of the crosslinking bath is typically around 50–85°C, preferably 55–80°C.

[0068] Crosslinking treatment can be performed multiple times, typically 2 to 5 times. In this case, the composition and temperature of each crosslinking bath can be the same or different, as long as they are within the aforementioned range. Alternatively, uniaxial stretching treatment can be performed in the crosslinking bath by utilizing the difference in circumferential speed of each clamping roller.

[0069] In the crosslinking process, in order to remove wrinkles and transport the polyvinyl alcohol resin film simultaneously, just as in the swelling process, known rollers with widening functions, such as the aforementioned stretching rollers, twisting rollers, and medium-high rollers, can be used, or known widening devices such as fabric guides, bending rollers, and tenter ferrules can be used. Another method for suppressing wrinkle formation, similar to the swelling process, is to perform a stretching process.

[0070] After the above crosslinking treatment, the crosslinked membrane pulled out from the crosslinking bath is introduced into the color-correcting bath used in the color-correcting process described below along the membrane transport path.

[0071] (Touch-up process)

[0072] The color correction process is performed to adjust the hue of the cross-linked film. This process involves immersing the cross-linked film, transported along a film transport path constructed by guide rollers and clamping rollers, in a color correction bath (containing a color correction solution in a color correction tank) for a predetermined time, and then pulling it out as a color correction film. The color correction solution, for example, if the dichroic pigment in the dyeing solution is iodine, can be an aqueous solution with a concentration of boric acid / iodide / water in a mass ratio of 1–5 / 3–30 / 100. The temperature of the color correction bath during immersion of the cross-linked film is typically around 20–65°C, and the immersion time is typically around 1–300 seconds, preferably 2–100 seconds.

[0073] The uniaxial stretching process can also be performed in the color correction bath by utilizing the difference in circumferential speed between the clamping rollers positioned before and after the color correction bath. The color correction film pulled out from the color correction bath is introduced into the washing bath used in the washing process described below along the film transport path.

[0074] (Stretching process)

[0075] Here, as described above, the polyvinyl alcohol-based resin film is preferably subjected to uniaxial stretching treatment, either wet or dry, during a series of processing steps and the washing step described later (i.e., before and after any one or more processing and washing steps and / or during any one or more processing and washing steps). Specific methods for uniaxial stretching treatment include, for example, inter-roll stretching where a circumferential speed difference is set between two clamping rollers constituting the film transport path (e.g., two clamping rollers arranged before and after the processing bath) to perform longitudinal uniaxial stretching, hot roller stretching as described in Japanese Patent No. 2731813, stretching with a spreader, etc., with inter-roll stretching being preferred. The uniaxial stretching process can be performed multiple times from the raw polyvinyl alcohol-based resin film used as the starting material until the polarizing film is obtained. Stretching treatment is also effective in suppressing the formation of wrinkles in the film.

[0076] The final cumulative stretching ratio of the polarizing film based on the aforementioned raw material film is typically around 4.5 to 7 times, preferably 5 to 6.5 times. The stretching process can be performed in any optional processing step, and if the stretching process is performed in two or more processing steps, it can also be performed in any optional processing step.

[0077] <Washing Process>

[0078] The manufacturing method of this embodiment, as described above, sequentially includes: a treatment step in which a treatment liquid is brought into contact with the polyvinyl alcohol-based resin film (hereinafter also simply referred to as "film") for treatment; and a washing step in which a washing liquid is brought into contact with the polyvinyl alcohol-based resin film for washing. The washing step is performed after the treatment step, for example, after the color-correction step. In the washing step, residual chemicals such as boric acid and iodine adhering to the film from the crosslinking step, color-correction step, etc., are removed. The washing process can be performed, for example, by immersing the color-corrected film in a washing bath and bringing the washing liquid into contact with the film. Alternatively, the washing process can be performed by discharging the washing liquid from a spray device instead of immersing the film in a washing bath, thereby bringing the washing liquid into contact with the film, or by combining immersion in the washing bath and discharging the washing liquid from the spray device.

[0079] In the washing process, for the purpose of removing wrinkles while conveying the polyvinyl alcohol (PVA) resin film, known rollers with spreading functions such as the aforementioned stretching rollers, twisted rollers, and medium-high rollers, or known spreading devices such as fabric guides, bending rollers, and stretching clips, can be used. Additionally, a stretching process can be performed in the above washing process to suppress wrinkle formation. As the washing liquid used in the above washing process, water or any conventionally known washing liquid used in washing such PVA resin films can be used. It should be noted that after the above washing process, a liquid removal mechanism for removing washing liquid from the surface of the PVA resin film includes, for example, a clamping roller. Besides the clamping roller, a mechanism for removing liquid by blowing air onto the film, or a scraper that removes liquid in contact with the film, can also be used.

[0080] In the manufacturing method of this embodiment, specifically, the washing process is as follows: The polyvinyl alcohol (PVA) resin film is divided into three parts based on its width: one end containing one end in the width direction, another end containing the other end in the width direction, and a central portion that is inward relative to the one end and the other end in the width direction. Then, the PVA resin film is washed with a higher degree of washing at the one end than at the other end, and a higher degree of washing at the other end than at the central portion. Even when the degree of washing gradually increases from the center of the film's width direction towards one end and the other end, as long as the relationship between the degree of washing at one end, the other end, and the central portion satisfies the above-described conditions, the process remains within the scope of this embodiment.

[0081] While the details of the cause of fluctuations in optical properties along the width of the polarizing film are not clear, the following factors are presumed: In the aforementioned processing steps, the processing liquid penetrates more easily from the end face compared to the surface of the polyvinyl alcohol-based resin film, resulting in a difference in the amount of processing liquid absorbed between the ends and the center of the film. This presumably leads to a difference in the degree of processing between the ends and the center of the film, causing fluctuations in the optical properties along the width of the polarizing film. Alternatively, it is presumed that fluctuations in the thickness of the film along the width direction are caused by the aforementioned uniaxial stretching process, and these thickness fluctuations also lead to differences in the degree of processing, resulting in fluctuations in the optical properties along the width of the polarizing film. Based on the factors presumed above, the inventors devised a method to homogenize the difference in the degree of processing between the ends and the center of the film by washing the polyvinyl alcohol-based resin film, which is divided into "one end / center / other end" based on its width. This invention thus completes the present invention.

[0082] Here, in this specification, the "one end" and "the other end" of the polyvinyl alcohol (PVA) resin film in the width direction can be arbitrarily determined. For example, the "one end" and "the other end" of the long PVA resin film after the above-described treatment process can be fixed or varied during the washing process. In this case, the "one end" of the PVA resin film in the width direction is preferably the side that is treated to a greater extent by the treatment liquid, and the "other end" of the film in the width direction is preferably the side that is treated to a lesser extent by the treatment liquid compared to the "one end". The degree of treatment by the treatment liquid can be determined, for example, based on the orthogonal hue b-value.

[0083] For one end and the other end of the polyvinyl alcohol-based resin film in the width direction, based on the width of the polyvinyl alcohol-based resin film, the proportions are respectively set to 5% to 30%, preferably 5% to 25%. When the proportions of the ends with a greater degree of washing than the central portion are within the aforementioned numerical range relative to the width of the film, the effect of the present invention, which makes optical properties such as hue uniform in the width direction, can be sufficiently obtained. It is preferable that one end and the other end of the polyvinyl alcohol-based resin film in the width direction be of the same length.

[0084] A more specific washing method for the aforementioned washing process can be described by immersing a polyvinyl alcohol-based resin film in a washing bath and releasing washing liquid from a spray device toward the film, thereby bringing the washing liquid into contact with the film. Particularly in this embodiment, considering the different degrees of washing at one end, the other end, and the center of the film, it is preferable to use a method where washing liquid is released from a spray device at multiple locations along the film's transport direction, bringing the washing liquid into contact with the film. As a washing method in this case, it is preferable to first, on the upstream side of the film's transport direction, make the degree of washing at the center of the film greater than at one end and the other end, or to make the degree of washing uniform throughout the width direction of the film. Then, on the downstream side of the film's transport direction, make the degree of washing at one end of the film greater than at the other end, and the degree of washing at the other end greater than at the center. Using this washing method, fluctuations in the optical properties in the width direction of the film in contact with the washing liquid can be suppressed.

[0085] (First method)

[0086] As a first method for washing one end of the polyvinyl alcohol-based resin film to a greater degree than washing the other end of the film, and washing the other end of the film to a greater degree than washing the central portion of the film, the following method can be exemplified. That is, in the manufacturing method of this embodiment, the washing process is preferably performed in a manner that satisfies i) below.

[0087] i) When the temperature of the washing liquid contacting one end is set to T1 [°C], the temperature of the washing liquid contacting the other end is set to T2 [°C], and the temperature of the washing liquid contacting the central portion is set to T3 [°C], T1, T2, and T3 satisfy the relationship T1 > T2 > T3. That is, the relationship shown in i) above means that the higher the temperature of the washing liquid contacting the polyvinyl alcohol resin film, the greater the degree of washing of the film.

[0088] The washing process described above is preferably performed in a manner that satisfies i) above, and at least any one of a1) and b1) below. The washing process is more preferably performed in a manner that satisfies both a1) and b1) below.

[0089] a1) 1 ≤ T1 - T2 ≤ 15

[0090] b1)2≤T2-T3≤30.

[0091] The washing process described above is preferably performed in a manner that satisfies i) above and c1) below.

[0092] c1)3≤T3≤25.

[0093] It should be noted that the temperature of the washing solution in contact with the polyvinyl alcohol-based resin film is preferably set to 65°C or below. This is because if the temperature of the washing solution is higher than 65°C, the optical performance of the polarizing film obtained by the manufacturing method of this embodiment, and the polarizing plate using the polarizing film, is easily reduced. Furthermore, if the temperature of the washing solution in contact with the polyvinyl alcohol-based resin film is less than 3°C, the washing is insufficient, and the polarizing film obtained by the manufacturing method of this embodiment may not be able to adequately suppress fluctuations in the optical properties in the width direction.

[0094] When using the first method, the temperature T3 of the washing liquid contacting the central portion of the polyvinyl alcohol-based resin film is preferably a temperature that satisfies c1) above, i.e., 3 to 25°C. More preferably, the temperature T3 of the washing liquid contacting the central portion of the polyvinyl alcohol-based resin film satisfies c2) below.

[0095] c2)5≤T3≤20.

[0096] In the first approach, the temperature T2 of the washing liquid contacting the other end of the polyvinyl alcohol-based resin film can satisfy b1) in the relationship with T3, i.e., 2 ≤ T2 - T3 ≤ 30°C. Specifically, for the temperature T2, considering c1), it is preferably 5 to 55°C. More preferably, the temperature T2 of the washing liquid contacting the other end of the polyvinyl alcohol-based resin film satisfies b2) in the relationship with T3.

[0097] b2)2≤T2-T3≤20.

[0098] In the first approach, the temperature T1 of the washing liquid at one end in contact with the polyvinyl alcohol-based resin film can satisfy the condition described in a1) in relation to T2, i.e., 1 ≤ T1 - T2 ≤ 15°C. Specifically, for the temperature T1, considering the conditions described in c1) and b1), it is preferably 6 to 65°C. More preferably, the temperature T1 of the washing liquid at one end in contact with the polyvinyl alcohol-based resin film satisfies the condition described in b2) in relation to T2.

[0099] a2)3≤T1-T2≤10.

[0100] When T1, T2, and T3 satisfy the relationship T1 > T2 > T3, and also satisfy the relationships [a1), b1), and c1], the difference in the degree of processing between the ends and the center of the film during the processing step is further homogenized during the washing step. This allows the color adjustment effect of the washing step to be fully realized, thereby reducing fluctuations in the optical properties of the polarizing film in the width direction, which is therefore preferable.

[0101] In the first embodiment, the washing process preferably includes the following operation: using a washing device equipped with multiple outlets for discharging the washing liquid, the washing liquid is continuously discharged from the outlets toward the polyvinyl alcohol-based resin membrane. Specifically, in the first embodiment, a two-stage washing process is performed: upstream of the polyvinyl alcohol-based resin membrane in the transport direction, the washing intensity of the central portion of the membrane is greater than that of one end and the other end; downstream of the membrane in the transport direction, the washing intensity of one end of the membrane is greater than that of the other end, and the washing intensity of the other end is greater than that of the central portion.

[0102] The following uses Figure 1 and Figure 2 The washing process of the first method will be explained. Figure 1 This is a partial cross-sectional perspective view of a washing apparatus having multiple outlets for discharging the washing liquid used in the washing process described above, illustrating an example of a two-stage washing process of the first method. Figure 2 (a) is a schematic cross-sectional side view of the washing apparatus, illustrating an example of the two-stage washing process of the first method. Figure 2 (b) and Figure 2 (c) is a schematic top view showing the multiple outlets of the washing device for discharging washing liquid.

[0103] like Figure 1 and Figure 2As shown in (a), the polyvinyl alcohol resin film 1 (hereinafter also referred to as "film 1") is conveyed by the conveying guide roller 2 in the direction of the arrow in the figure. Thus, after being immersed in the washing liquid 32 in the washing bath 31, film 1 passes sequentially through the first spray device 41 and the second spray device 42. Multiple spray nozzles 411 and 421 are arranged along the width direction of film 1 in the spray devices 41 and 42, respectively. After passing through the spray devices 41 and 42, film 1 is sent to the dewatering roller 5 to remove moisture adhering to its surface. Here, the washing liquid 32 in the washing bath 31 and the washing liquid 32 discharged from the spray devices 41 and 42 preferably have the same or similar composition.

[0104] The spray devices 41 and 42 are configured to release the washing liquid in a manner that allows adjustment of the temperature and discharge amount according to the position of the membrane 1 in the width direction. In the above washing process, by performing washing treatment using the washing liquid 32 in the washing bath 31 and washing treatment using the spray devices 41 and 42, the difference in the degree of treatment between the ends and the center of the membrane 1 in the width direction that has come into contact with the washing liquid can be homogenized, thereby reducing fluctuations in the optical properties of the polarizing film. It should be noted that, in the first embodiment, the order of the washing treatment using the washing liquid 32 in the washing bath 31 and the washing treatment using the spray devices 41 and 42 is not particularly limited.

[0105] In the washing process using the above-mentioned spray devices 41 and 42, such as Figure 2 As shown in (c), in the first section of the spray device 41, washing liquid 32 is discharged from the spray nozzle 411b, which corresponds to the central portion of the membrane 1. Furthermore, as... Figure 2 As shown in (b), in the second section of the spray device 42, the washing liquid 32 is released from the spray nozzle 421a, which corresponds to one end of the membrane 1, and the spray nozzle 421c, which corresponds to the other end of the membrane 1, while the washing liquid 32 is not released from the spray nozzle 421b, which corresponds to the central part of the membrane 1.

[0106] In the washing process using the above-described spray devices 41 and 42, the temperature T3 of the washing liquid that contacts the center of the membrane 1 after being discharged from the spray nozzle 411b into the membrane 1 in the first spray device 41 can be selected from, for example, from 3 to 25°C, to satisfy the above-described c1). Furthermore, the temperature T2 of the washing liquid that contacts the other end of the membrane 1 after being discharged from the spray nozzle 421c into the membrane 1 in the second spray device 42 can be selected from, for example, from 5 to 55°C, satisfying the above-described b1) in relation to T3, and satisfying 2 ≤ T2 - T3 ≤ 30. Additionally, the temperature T1 of the washing liquid that contacts the other end of the membrane 1 after being discharged from the spray nozzle 421a into the membrane 1 in the second spray device 42 can be selected from, for example, from 6 to 65°C, satisfying the above-described a1) in relation to T2, and satisfying 1 ≤ T1 - T2 ≤ 15. By performing this washing process, the generation of uneven washing in the width direction of the membrane 1 that has come into contact with the washing liquid can be suppressed.

[0107] Here, the first approach is not limited to the approach described above. In the first spray device 41 or the second spray device 42, the washing liquid 32 may be released into the membrane 1 from all the spray nozzles in a manner satisfying the relationship T1 > T2 > T3. Alternatively, the washing liquid may be released only into one end and the other end of the membrane 1 from the spray nozzles 411a and 411c of the first spray device 41 and the spray nozzles 421a and 421c of the second spray device 42. In the latter case, it is preferable to wash the membrane 1 with the washing liquid 32 in the washing bath 31 before the washing treatment using the spray devices 41 and 42. This is because, in order to adjust the optical properties of the polarizing film, it is preferable to also perform some degree of washing on the central portion of the membrane 1. Furthermore, even if the temperature of the washing liquid that comes into contact with the membrane after being released from the spray nozzle gradually increases from the center of the membrane in the width direction to one end and the other end, the washing process does not deviate from the first mode as long as the temperature of the washing liquid satisfies the relationship T1>T2>T3.

[0108] (Second method)

[0109] As a second method for making the degree of washing of one end of the polyvinyl alcohol-based resin film greater than the degree of washing of the other end of the film, and making the degree of washing of the other end of the film greater than the degree of washing of the central portion of the film, the following method can be exemplified. That is, in the manufacturing method of this embodiment, the washing process is preferably performed in a manner that satisfies ii) below.

[0110] ii) When the volume of the washing liquid per unit width and per unit time in contact with one end is set to M1 [L / m / min], the volume of the washing liquid per unit width and per unit time in contact with the other end is set to M2 [L / m / min], and the volume of the washing liquid per unit width and per unit time in contact with the central portion is set to M3 [L / m / min], M1, M2, and M3 satisfy the relationship M1 > M2 > M3. That is, the relationship shown in ii) above means that the greater the volume of washing liquid per unit width and per unit time in contact with the polyvinyl alcohol resin film, the greater the degree of washing of the film.

[0111] The values ​​of M1, M2, and M3, which are the volume of washing liquid per unit width and per unit time (minute), are not particularly limited as long as the relationship M1 > M2 > M3 is satisfied. However, from the viewpoint of manufacturing efficiency of polarizing film, it is preferable to select M1 from 1 to 20 L / m / min, M2 from 1 to 17 L / m / min, and M3 from 1 to 15 L / m / min, respectively, in a manner that satisfies the relationship M1 > M2 > M3.

[0112] In the second embodiment, the washing process preferably includes the following operation: using a washing device equipped with multiple outlets for discharging the washing liquid, the washing liquid is continuously discharged from the outlets toward the polyvinyl alcohol-based resin film. Particularly in the second embodiment, during the continuous discharge of the washing liquid, the polyvinyl alcohol-based resin film is preferably transported vertically upwards, and the height of the outlet discharging the washing liquid toward the central portion is lower than the height of the outlet discharging the washing liquid toward one end and the height of the outlet discharging the washing liquid toward the other end. More specifically, the continuous discharge of the washing liquid is preferably performed in a three-stage washing process, where the height of the outlet discharging the washing liquid toward the central portion is lower than the height of each outlet discharging the washing liquid toward the other end and the first end, and the height of the outlet discharging the washing liquid toward the other end is lower than the height of the outlet discharging the washing liquid toward the first end. Therefore, in the washing process performed using the washing device described above, the relationship shown in ii) above can be easily satisfied.

[0113] The following uses Figure 3 and Figure 4 The washing process of the second method will be explained. Figure 3This is a partial cross-sectional perspective view of a washing apparatus having multiple outlets for discharging the washing liquid used in the washing process described above, illustrating an example of the three-stage washing process of the second method. Figure 4 (a) is a schematic cross-sectional side view of the washing apparatus described above, illustrating an example of the three-stage washing process of the second method. Figure 4 (b) Figure 4 (c) and Figure 4 (d) is a schematic top view showing the multiple outlets of the washing device for discharging washing liquid.

[0114] like Figure 3 and Figure 4 As shown in (a), membrane 1 (polyvinyl alcohol resin membrane 1) is conveyed by guide roller 2 in the direction of the arrow in the figure. Thus, after being immersed in the washing liquid 32 in the washing bath 31, membrane 1 passes sequentially through the first spray device 41, the second spray device 42, and the third spray device 43. In the spray devices 41, 42, and 43, multiple spray nozzles 411, 421, and 431 are arranged along the width direction of membrane 1, respectively. After passing through the spray devices 41, 42, and 43, membrane 1 is sent to the dewatering roller 5 to remove moisture adhering to its surface. Here, in the second embodiment, as in the first embodiment, the washing liquid 32 in the washing bath 31 and the washing liquid 32 discharged from the spray devices 41 and 42 preferably have the same or similar composition.

[0115] Furthermore, similar to the first method described above, the spray devices 41, 42, and 43 can be configured to release the washing liquid in a manner that adjusts the temperature and amount of the washing liquid according to the position of the membrane 1 in the width direction. The washing process described above, by performing washing treatment using the washing liquid 32 in the washing bath 31 and washing treatment using the spray devices 41, 42, and 43, uniformizes the difference in the degree of treatment between the ends and the center of the membrane 1 in the width direction that has come into contact with the washing liquid, thereby reducing fluctuations in the optical properties of the polarizing film. It should be noted that in the second method, the order of the washing treatment using the washing liquid 32 in the washing bath 31 and the washing treatment using the spray devices 41, 42, and 43 is not particularly limited.

[0116] In the washing process using the aforementioned spray devices 41, 42, and 43, such as Figure 4 As shown in (d), in the first section of the spray device 41, washing liquid 32 is released from the spray nozzle 411b, which corresponds to the central portion of the membrane 1, while washing liquid 32 is not released from the spray nozzles 411a and 411c, which correspond to one end and the other end of the membrane 1. Then, as... Figure 4As shown in (c), in the second section of the spray device 42, washing liquid 32 is discharged from the spray nozzle 421c corresponding to the other end of the membrane 1, and washing liquid 32 is not discharged from the spray nozzles 421a and 421b corresponding to one end and the central part of the membrane 1. Furthermore, as... Figure 4 As shown in (b), in the third section of the spray device 43, the washing liquid 32 is released from the spray nozzle 431a, which corresponds to one end of the membrane 1, and the washing liquid 32 is not released from the spray nozzles 431b and 431c, which do not correspond to one end and the central part of the membrane 1.

[0117] In the washing process using the above-described spray devices 41, 42, and 43, for example, the amount of washing liquid 32 discharged per unit width and per unit time from the spray nozzle 411b toward the center of the membrane 1 in the first spray device 41 can be the same as the amount of washing liquid 32 discharged per unit width and per unit time from the spray nozzle 421c toward the other end of the membrane 1 in the second spray device 42, and the amount of washing liquid 32 discharged per unit width and per unit time from the spray nozzle 431a toward one end of the membrane 1 in the third spray device 43, for example, set to 1 to 20 L / m / min. More preferably, the amount of washing liquid 32 discharged per unit width and per unit time toward one end, the other end, and the center of the membrane 1 can be set to 3 to 10 L / m / min.

[0118] By performing this washing, the volume M1 ([L / m / min]) of the washing liquid 32 per unit width and per unit time at one end of the contact membrane 1 can be greater than the volume M2 ([L / m / min]) of the washing liquid 32 per unit width and per unit time at the other end of the contact membrane 1, and the volume M2 can be greater than the volume M3 ([L / m / min]) of the washing liquid 32 per unit width and per unit time at the central portion of the contact membrane 1, thus satisfying the relationship M1 > M2 > M3. Therefore, the degree of washing at one end of the polyvinyl alcohol resin membrane 1 can be greater than the degree of washing at the other end, and the degree of washing at the other end can be greater than the degree of washing at the central portion.

[0119] This is because, in the second method, the membrane 1 is first transported vertically upwards. In this transport state, the height of the spray nozzle discharging the washing liquid 32 towards the center of the membrane 1 is lower than the height of the spray nozzle discharging the washing liquid 32 towards the other end of the membrane 1, and the height of the spray nozzle discharging the washing liquid 32 towards the other end of the membrane 1 is lower than the height of the spray nozzle discharging the washing liquid 32 towards one end of the membrane 1. In this case, the distance the washing liquid 32 falls across the membrane 1 is longest at one end, second longest at the other end, and shortest at the center. Therefore, the higher the height of the spray nozzle (outlet) discharging the washing liquid 32 towards the membrane 1, the longer the distance the washing liquid 32 falls across the membrane 1. As a result, the amount of washing liquid 32 per unit width and per unit time in contact with the membrane 1 can be increased. It should be noted that the amount of washing liquid 32 per unit width and per unit time (M1, M2 and M3) at one end, the other end and the center of the contact membrane 1 can also be controlled by adjusting the amount of washing liquid 32 released from the spray devices 41, 42 and 43 and adjusting the conveying speed of the membrane 1.

[0120] In this second approach, to further improve the uniformity of the optical properties of the polyvinyl alcohol-based resin film in the width direction, it is preferable that the temperature of the washing liquid discharged from the spray device increases in the order of the third, second, and first stages, or that the discharge amount of the washing liquid per unit width and per unit time from the spray device increases in the order of the third, second, and first stages. Wherein, if the temperature and discharge amount of the washing liquid discharged from the spray devices in the first, second, and third stages are set to be the same, it is also possible that in the second approach, the washing liquid is discharged to the center using the spray device in the first stage, to the other end using the spray device in the second stage, and to one end using the spray device in the third stage, so that the degree of washing at one end of the polyvinyl alcohol-based resin film is greater than the degree of washing at the other end, and the degree of washing at the other end is greater than the degree of washing at the center.

[0121] (Choice between Method 1 and Method 2)

[0122] In the aforementioned washing process, as a method for establishing differences in the degree of washing at one end, the other end, and the center of the polyvinyl alcohol-based resin film as described above, examples include the temperature difference of the washing liquid contacting the film and the difference in the volume of washing liquid per unit width and unit time of contact with the film. In this case, as a method for establishing differences in the degree of washing, the temperature difference is generally preferred over the volume difference. This is because when a temperature difference is established between the washing liquid contacting one end, the other end, and the center of the film, it is easy to create differences in the degree of washing, and it is easier to achieve uniformity of optical properties in the width direction in the resulting polarizing film.

[0123] Therefore, in the manufacturing method of this embodiment, the first method is set as the first option for the washing process. When the first method cannot be used for the washing process, the second method is recommended as the preferred management method for the washing process. Of course, when implementing the first method as the washing process, if the spray device of the first stage and / or the spray device of the second stage are set to differ in the amount of washing liquid per unit width and per unit time at one end, the other end, and the center of the membrane, the uniformity of the optical properties in the width direction of the polyvinyl alcohol-based resin film will be further improved, and this is therefore more preferred. Furthermore, when implementing the second method as the washing process, if the spray devices of the first, second, and third stages are set to differ in the temperature of the washing liquid at one end, the other end, and the center of the membrane, the uniformity of the optical properties in the width direction of the polyvinyl alcohol-based resin film will be further improved, and this is also more preferred.

[0124] As described above, in the manufacturing method of this embodiment, during the washing process following the processing steps (swelling step, dyeing step, crosslinking step, and color-correction step, etc.), the degree of washing at one end of the polyvinyl alcohol-based resin film is greater than that at the other end, and the degree of washing at the other end is greater than that at the central portion. Therefore, the difference in the degree of processing between the ends and the center portion of the polyvinyl alcohol-based resin film in the width direction during the processing steps is homogenized during the washing process, thereby reducing fluctuations in the optical properties of the polarizing film in the width direction.

[0125] <Other Processes>

[0126] The manufacturing method of this embodiment may include steps other than the processing and washing steps described above. Examples of additional steps and treatments that can be added include immersion treatment in an aqueous solution containing zinc chloride, which does not contain boron compounds (zinc treatment step). For the zinc treatment described above, it is preferable to perform it after the crosslinking step described above.

[0127] (Drying process)

[0128] In addition, as another process, a drying process can be included after the washing process to dry the polyvinyl alcohol-based resin film. There are no particular limitations on the drying method used in the above-mentioned drying process; for example, a drying oven can be used for hot air drying. In this case, the drying temperature is, for example, around 30–100°C, and the drying time is, for example, around 30–600 seconds. The drying process of the polyvinyl alcohol-based resin film can also be performed using a far-infrared heater. A polarizing film can be produced as described above. The thickness of the polarizing film is, for example, around 5–50 μm.

[0129] <Methods for evaluating the uniformity of optical properties in the width direction of polarization films>

[0130] Whether the polarizing film obtained using the manufacturing method of this embodiment reduces the fluctuation of its optical properties in the width direction (evaluation of the uniformity of optical properties in the width direction) can be evaluated, for example, by using the following method. First, at an arbitrary position in the transport direction of the polarizing film, the orthogonal hue b-value is calculated at five points in the width direction at that position (one end, the other end, the center, the midpoint between one end and the center, and the midpoint between the other end and the center). Then, for the orthogonal hue b-values ​​at the above five points, the difference Δ(MAX-MIN) between the maximum value (MAX) and the minimum value (MIN) is used as an index of the uniformity of optical properties. If the ratio of the difference Δ(MAX-MIN) to the maximum value (MAX) (difference Δ(MAX-MIN) / maximum value (MAX) × 100%) is within 40%, the fluctuation of optical properties is evaluated as reduced; if the ratio is greater than 40%, the reduction of the fluctuation of optical properties is evaluated as insufficient.

[0131] Here, the cross-tone b-value can be determined by measuring the UV-Vis transmission spectra of linearly polarized light incident along the transmission axis and linearly polarized light incident along the absorption axis using polarizing films mounted in a UV-Vis spectrophotometer V-7100 manufactured by Nippon Spectrophotometer Co., Ltd. The software installed in the aforementioned UV-Vis spectrophotometer V-7100 calculates the cross-tone b-value based on the values ​​of the aforementioned UV-Vis transmission spectra. The "b-value" of the cross-tone b-value refers to the b-value in Hunter's Lab color system. That is, the cross-tone b-value is equivalent to the b-value of the hue of light transmitted to the opposite side when two polarizing films are overlapped in a manner with their absorption axes orthogonal and natural light is incident from one side.

[0132] <Polarizing film>

[0133] Using the manufacturing method of this embodiment, a polarizing film that suppresses fluctuations in optical properties along the width direction can be obtained. For the visibility-correcting monomer transmittance Ty of the above-mentioned polarizing film, considering the balance with the visibility-correcting polarization degree Py, it is preferably 40-47%, more preferably 41-45%. The visibility-correcting polarization degree Py is preferably 99.9% or more at any position along the width direction, more preferably 99.95% or more. It should be noted that the difference between the maximum and minimum values ​​of the visibility-correcting polarization degree Py along the width direction is preferably 0.0015% or less, and the smaller the difference, the more preferred.

[0134] The visibility-corrected monomer transmittance (Ty) and visibility-corrected polarization degree (Py) can be determined using the following method. First, for the aforementioned polarization film, the MD transmittance and TD transmittance in the wavelength range of 380–780 nm are measured using a spectrophotometer with an integrating sphere ("V7100" manufactured by Nippon Spectrophotometer Co., Ltd.). The monomer transmittance and polarization degree at each wavelength are then calculated based on the following formula:

[0135] Monomer transmittance (%) = (MD + TD) / 2

[0136] Degree of polarization (%) = {(MD-TD) / (MD+TD)} × 100

[0137] The term "MD transmittance" here refers to the transmittance when the direction of polarized light emitted from the Glan-Thomson prism is parallel to the transmission axis of the polarizing film sample, and is expressed as "MD" in the above formula. Similarly, the term "TD transmittance" refers to the transmittance when the direction of polarized light emitted from the Glan-Thomson prism is orthogonal to the transmission axis of the polarizing film sample, and is expressed as "TD" in the above formula. Then, regarding the above-mentioned monomer transmittance and degree of polarization, based on JIS Z 8701:1999 "Methods for representing colors - XYZ color system and X..." 10 Y 10 Z 10 Visibility correction is performed on a 2-degree field of view (C light source) of the "color system", from which the visibility-corrected single-unit transmittance (Ty) and the visibility-corrected polarization degree (Py) can be calculated.

[0138] The width of the aforementioned polarizing film is, for example, 50 mm or more and 5000 mm or less, preferably 500 mm or more and 4000 mm or less. The resulting polarizing film can be wound sequentially on a winding roller to form a roll, or it can be directly supplied to the process of making a polarizing plate without being wound (the process of laminating a protective film, etc., on one or both sides of the polarizing film).

[0139] <Polarizing plate>

[0140] A polarizing plate can be obtained by attaching a protective film to at least one side of a polarizing film prepared as described above via an adhesive. Examples of protective films include films containing acetylcellulose resins such as triacetylcellulose and diacetylcellulose; films containing polyester resins such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate; polycarbonate resin films; cyclic olefin resin films; acrylic resin films; and films containing chain olefin resins such as polypropylene resins.

[0141] To improve the adhesion between the polarizing film and the protective film, surface treatments such as corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer coating, and saponification can be applied to the bonding surfaces of the polarizing film and / or the protective film. Examples of adhesives used for bonding the polarizing film and the protective film include UV-curable adhesives, aqueous solutions of polyvinyl alcohol resins or aqueous solutions containing crosslinking agents, and water-based adhesives such as urethane emulsion adhesives. UV-curable adhesives can be mixtures of acrylic compounds and photoradical polymerization initiators, or mixtures of epoxy compounds and photocationic polymerization initiators. Alternatively, cationicly polymerizable epoxy compounds and radically polymerizable acrylic compounds can be used together, with both photocationic polymerization initiators and photoradical polymerization initiators serving as initiators.

[0142] As described above, the manufacturing method of this embodiment can obtain a polarizing film that reduces fluctuations in optical properties in the width direction. Polarizing plates using this type of polarizing film are effective for various display devices, such as liquid crystal display devices.

Claims

1. A method for manufacturing a polarizing film, comprising the following steps: The processing step involves contacting the polyvinyl alcohol-based resin film with the processing liquid to perform the treatment. as well as The washing process involves bringing the washing solution into contact with the polyvinyl alcohol-based resin membrane for washing. The washing process is as follows: after dividing the polyvinyl alcohol (PVA) resin film into an end portion containing one end in the width direction, an end portion containing the other end in the width direction, and a central portion that is inward relative to the one end and the other end in the width direction, the PVA resin film is washed to a greater degree than the one end and the other end, and the degree of washing of the other end is greater than the degree of washing of the central portion. The polyvinyl alcohol-based resin film exhibits fluctuations in thickness along its width direction. The washing process includes the following operation: using a washing device equipped with multiple outlets for discharging the washing liquid, the washing liquid is continuously discharged from the outlets toward the polyvinyl alcohol-based resin membrane. During the continuous discharge of the washing liquid, the polyvinyl alcohol-based resin membrane is transported vertically upwards. The washing process involves continuously discharging the washing liquid, meaning that the height of the outlet discharging the washing liquid towards the central portion is lower than the height of each outlet discharging the washing liquid towards the other end and the first end, and the height of the outlet discharging the washing liquid towards the other end is lower than the height of the outlet discharging the washing liquid towards the first end. The washing process is performed in a manner that satisfies at least one of the following conditions i) and a1) and b1): i) When the temperature of the washing liquid in contact with one end is set to T1, the temperature of the washing liquid in contact with the other end is set to T2, and the temperature of the washing liquid in contact with the central part is set to T3, T1, T2, and T3 satisfy the relationship T1 > T2 > T3, where the units of T1, T2, and T3 are °C. a1) 1≤T1-T2≤15 b1) 2≤T2-T3≤30.

2. The method for manufacturing a polarizing film according to claim 1, wherein the washing process is performed in a manner that satisfies i) and c1) below: c1) 3≤T3≤25.

3. The method for manufacturing a polarizing film according to claim 1 or 2, wherein the washing step is performed in a manner that satisfies ii) below. ii) When the amount of washing liquid per unit width and per unit time in contact with one end is set as M1, the amount of washing liquid per unit width and per unit time in contact with the other end is set as M2, and the amount of washing liquid per unit width and per unit time in contact with the central portion is set as M3, M1, M2, and M3 satisfy the relationship M1 > M2 > M3. in, The units for M1, M2 and M3 are L / (m·min).

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