Method for manufacturing polarizing film and polarizing plate

Abstract

To provide a method of manufacturing a polarizing film which, when exposed to a high temperature environment exceeding 95°C, exhibits superior yellowing suppression effect when an interlayer-filling configuration is adopted and suppresses single transmittance deterioration when the interlayer-filling configuration is not adopted.SOLUTION: A method of manufacturing a polarizing film from a polyvinyl alcohol-based resin film is provided, the method comprising a treatment step of bringing the polyvinyl alcohol-based resin film into contact with a treatment solution containing nitrate, boric acid, and zinc iodide and having a pH of 4.3 or higher at 25°C.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a polarizing film and a polarizing plate. [Background technology]

[0002] In recent years, the applications of image display devices including polarizing plates have expanded, and they are now used not only in mobile devices such as mobile phones and tablet terminals, but also in in-vehicle image display devices such as car navigation devices and rearview monitors. Accordingly, image display devices are required to have higher durability in harsher environments (for example, high-temperature environments) than has been required in the past.

[0003] Patent Document 1 discloses that a polarizing plate having excellent optical properties and excellent durability against heat and humidity can be provided by lowering the pH of a polarizer. Patent Document 1 also describes a method for lowering the pH of a polarizer by adjusting the pH of a hardening solution to 1.5 to 3.2.

[0004] Another known method for improving the durability of polarizers is to add metal chlorides such as zinc chloride to the polarizer treatment solution, but to reduce environmental impact, there is a demand for chlorine-free solutions (e.g., chlorine content of 900 ppm or less), and achieving this while still maintaining the high durability required for polarizing plates has been a challenge. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-62458 [Patent Document 2] Japanese Patent Application Publication No. 11-174417 Summary of the Invention [Problem to be solved by the invention]

[0006] When a polarizing film included in a polarizing plate has an interlayer filling structure, it can yellow when exposed to a high-temperature environment due to the progression of polyenation of polyvinyl alcohol. This yellowing tends to progress more significantly under particularly severe temperature conditions exceeding 95°C (e.g., 105°C). The interlayer filling structure refers to a structure in which a polarizing plate disposed on the viewing-side surface of an image display panel and a transparent member such as glass disposed on the viewing side are bonded together with a pressure-sensitive adhesive or a UV-curable adhesive (see, for example, Patent Document 2).

[0007] Furthermore, when the polarizing film included in the polarizing plate is exposed to a high temperature environment exceeding 95°C, if the interlayer filling structure is not adopted, the polyenization of polyvinyl alcohol does not progress, but the absorbance on the short wavelength side of visible light increases, resulting in a problem of a decrease in the single transmittance.

[0008] In the past, when trying to suppress the polyenization of polyvinyl alcohol that occurs when an interlayer filling structure is used, there was a trade-off in that the single transmittance of the polarizing film would decrease if the interlayer filling structure was not used, and it was necessary to produce an optimal polarizing film individually for each structure to be used.

[0009] An object of the present invention is to provide a method for producing a polarizing film that, when exposed to a high-temperature environment exceeding 95°C, exhibits excellent yellowing suppression effect when an interlayer filling structure is employed, and that can suppress a decrease in single-unit transmittance when an interlayer filling structure is not employed. Another object of the present invention is to provide a method for producing a polarizing plate that includes the polarizing film. [Means for solving the problem]

[0010] The present invention provides the following methods for producing a polarizing film and a polarizing plate. [1] A method for producing a polarizing film from a polyvinyl alcohol-based resin film, comprising: a treatment step of contacting the polyvinyl alcohol-based resin film with a treatment liquid, The method for producing a polarizing film, wherein the treatment solution contains a nitrate, boric acid, and zinc iodide, and has a pH of 4.3 or higher at 25°C. [2] The method for producing a polarized film according to [1], wherein the treatment liquid has a pH of 6.5 or less at 25°C. [3] a step of producing a polarized film by the method for producing a polarized film according to any one of [1] and [2]; and a laminating step of laminating a protective film to one or both surfaces of the polarizing film via an adhesive layer. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a method for producing a polarizing film that, when exposed to a high-temperature environment exceeding 95°C, has an excellent effect of suppressing yellowing when an interlayer filling structure is used, and can suppress a decrease in single-unit transmittance when an interlayer filling structure is not used, and a method for producing a polarizing plate including the polarizing film. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a cross-sectional view schematically illustrating an example of a polarizing film manufacturing method and a polarizing film manufacturing apparatus used in the method according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the following embodiments. In the drawings, the scales of the components are appropriately adjusted to make them easier to understand, and the scales of the components shown in the drawings do not necessarily match the scales of the actual components.

[0014] <Method of manufacturing polarizing film> In this embodiment, the polarizing film is a uniaxially stretched polyvinyl alcohol resin film to which a dichroic pigment (iodine or a dichroic dye) is adsorbed and aligned. The polyvinyl alcohol resin constituting the polyvinyl alcohol resin film is typically obtained by saponifying a polyvinyl acetate resin. The degree of saponification is typically about 85 mol % or more, preferably about 90 mol % or more, and more preferably about 99 mol % or more. Examples of polyvinyl acetate resins include polyvinyl acetate, which is a homopolymer of vinyl acetate, as well as copolymers of vinyl acetate with other monomers copolymerizable therewith. Examples of other copolymerizable monomers include unsaturated carboxylic acids, olefins, vinyl ethers, and unsaturated sulfonic acids. The degree of polymerization of the polyvinyl alcohol resin is typically about 1,000 to 10,000, and preferably about 1,500 to 5,000.

[0015] These polyvinyl alcohol resins may be modified, and for example, polyvinyl formal, polyvinyl acetal, polyvinyl butyral, etc. modified with aldehydes may also be used.

[0016] In this embodiment, a polyvinyl alcohol-based resin film (raw film) having a thickness of typically 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, is used as the starting material for producing the polarizing film. This makes it possible to obtain thin polarizing films, for which market demand is increasing. The width of the raw film is not particularly limited and can be, for example, 400 to 6000 mm. The raw film is prepared, for example, as a roll (raw film roll) of long polyvinyl alcohol-based resin film.

[0017] Polarized film can be continuously produced as a long polarized film by unwinding the long raw film from the raw film roll and continuously transporting it along a film transport path in a polarized film manufacturing apparatus, carrying out a predetermined treatment step in which the film is immersed in a treatment liquid (hereinafter also referred to as a "treatment bath") contained in a treatment tank and then withdrawn, followed by a drying step. Note that the treatment step is not limited to a method of immersing the film in a treatment bath as long as it is a method of treating the film by contacting the film with the treatment liquid, and may be a method of treating the film by attaching the treatment liquid to the film surface by spraying, flowing, dropping, or the like. When the treatment step is performed by a method of immersing the film in a treatment bath, the number of treatment baths used in one treatment step is not limited to one, and one treatment step may be completed by sequentially immersing the film in two or more treatment baths.

[0018] Examples of the treatment liquid include a swelling liquid, a dyeing liquid, a crosslinking liquid, a color-complementing liquid, and a cleaning liquid. Examples of the treatment process include a swelling process in which a swelling liquid is brought into contact with the raw film to perform a swelling treatment; a dyeing process in which a dyeing liquid is brought into contact with the film after the swelling treatment to perform a dyeing treatment; a crosslinking process in which a crosslinking liquid is brought into contact with the film after the dyeing treatment to perform a crosslinking treatment; a color-complementing liquid is brought into contact with the film after the crosslinking treatment to perform a color-complementing treatment; and a cleaning process in which a cleaning liquid is brought into contact with the film after the color-complementing treatment to perform a cleaning treatment. Between these series of treatment processes (i.e., before, after, or during one or more of the treatment processes), a wet or dry uniaxial stretching treatment is performed. Other treatment processes may be added as necessary.

[0019] The manufacturing method of the present invention includes a step of treating a film with a treatment solution containing nitrate, boric acid, and zinc iodide and having a pH of 4.3 or higher at 25°C. The treatment solution is typically an aqueous solution. Because the treatment solution contains nitrate, boric acid, and zinc iodide, it can be used for crosslinking treatments to crosslink films after dyeing, or for complementary color treatments to adjust the hue of the film. Furthermore, because the pH range of the treatment solution is limited, it can also be used for pH adjustment treatments to adjust the pH of the film. By undergoing this treatment step, the yellowing suppression effect can be achieved even in high-temperature environments exceeding 95°C when an interlayer packing structure is used. Therefore, compared to methods that achieve yellowing suppression by including chlorides, the environmental impact of chlorine can be reduced. Furthermore, by undergoing this treatment step, the decrease in single-piece transmittance can be effectively suppressed even in high-temperature environments exceeding 95°C when an interlayer packing structure is not used.

[0020] The nitrate may include at least one selected from the group consisting of aluminum nitrate, copper nitrate, sodium nitrate, potassium nitrate, zinc nitrate, and magnesium nitrate. The nitrate preferably includes zinc nitrate.

[0021] The concentration of the nitrate in the treatment solution is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, per 100 parts by mass of water, and preferably 30 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less, per 100 parts by mass of water.

[0022] The concentration of chloride in the treatment liquid is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and even more preferably 1 part by mass or less, per 100 parts by mass of water. The treatment liquid does not have to contain chloride.

[0023] An example of a method for producing a polarizing film according to the present invention will be described in detail below with reference to FIG. 1. FIG. 1 is a cross-sectional view schematically illustrating an example of a method for producing a polarizing film according to the present invention and a polarizing film production apparatus used therein. The polarizing film production apparatus shown in FIG. 1 is configured such that raw film 10 made of a polyvinyl alcohol resin is continuously unwound from raw film roll 11 and transported along a film transport path, causing the film to pass sequentially through a swelling bath (swelling liquid contained in a swelling tank) 13, a dyeing bath (dyeing liquid contained in a dyeing tank) 15, a crosslinking bath (crosslinking liquid contained in a crosslinking tank) 17, a complementary color bath (complementary color liquid contained in a complementary color tank) 18, and a washing bath (washing liquid contained in a washing tank) 19, all of which are provided on the film transport path, and finally through a drying oven 21. The arrows in FIG. 1 indicate the film transport direction.

[0024] 1, the term "treatment tank" is a general term including a swelling tank, a dyeing tank, a crosslinking tank, a color-complementing tank, and a washing tank, the term "treatment liquid" is a general term including a swelling liquid, a dyeing liquid, a crosslinking liquid, a color-complementing liquid, and a washing liquid, and the term "treatment bath" is a general term including a swelling bath, a dyeing bath, a crosslinking bath, a color-complementing bath, and a washing bath. The swelling bath, dyeing bath, crosslinking bath, color-complementing bath, and washing bath respectively constitute a swelling section, a dyeing section, a crosslinking section, a color-complementing section, and a washing section in the manufacturing apparatus of this embodiment.

[0025] The film transport path of the polarizing film manufacturing equipment can be constructed by appropriately arranging, in addition to the treatment baths described above, guide rolls 30-48, 60, and 61 that support the film being transported or can even change the film transport direction, and nip rolls 50-55 that press and clamp the film being transported, imparting driving force to the film through their rotation, or even changing the film transport direction. Guide rolls and nip rolls can be placed before, after, or within each treatment bath, allowing the film to be introduced into, immersed in, and withdrawn from the treatment bath (see Figure 1). For example, one or more guide rolls can be provided in each treatment bath, and the film can be immersed in each treatment bath by transporting it along these guide rolls.

[0026] The polarized film manufacturing apparatus shown in Figure 1 has nip rolls arranged before and after each treatment bath (nip rolls 50 to 54), which makes it possible to perform inter-roll stretching in one or more treatment baths, in which longitudinal uniaxial stretching is performed by creating a difference in peripheral speed between the nip rolls arranged before and after the treatment bath. Each step will be described below.

[0027] (Swelling process) The swelling process is carried out for the purposes of removing foreign matter from the surface of the raw film 10, removing plasticizers in the raw film 10, imparting ease of dyeing, plasticizing the raw film 10, etc. The treatment conditions are determined within a range in which the above-mentioned purposes can be achieved and in which defects such as extreme dissolution or devitrification of the raw film 10 do not occur.

[0028] 1, the swelling step can be carried out by continuously unwinding raw film 10 from raw film roll 11 and transporting it along a film transport path, immersing raw film 10 in swelling bath 13 for a predetermined time, and then pulling it out. In the example of FIG. 1, from the time raw film 10 is unwound until it is immersed in swelling bath 13, raw film 10 is transported along a film transport path formed by guide rolls 60, 61 and nip roll 50. During the swelling treatment, raw film 10 is transported along a film transport path formed by guide rolls 30 to 32 and nip roll 51.

[0029] As the swelling liquid for the swelling bath 13, in addition to pure water, it is also possible to use an aqueous solution containing boric acid (Japanese Patent Application Laid-Open No. 10-153709), chlorides (Japanese Patent Application Laid-Open No. 06-281816), inorganic acids, inorganic salts, water-soluble organic solvents, alcohols, etc. in an amount of approximately 0.01 to 10 mass %.

[0030] The temperature of the swelling bath 13 is, for example, 10 to 50°C, preferably 10 to 40°C, and more preferably 15 to 30°C. The immersion time of the raw film 10 is preferably about 10 to 300 seconds, and more preferably 20 to 200 seconds. When the raw film 10 is a polyvinyl alcohol resin film that has been stretched in a gas in advance, the temperature of the swelling bath 13 is, for example, 20 to 70°C, and preferably 30 to 60°C. The immersion time of the raw film 10 is preferably about 30 to 300 seconds, and more preferably about 60 to 240 seconds.

[0031] The swelling treatment can easily cause problems such as the raw film 10 swelling in the width direction and wrinkling. One method for conveying the film while removing these wrinkles is to use rolls with a width-expanding function, such as expander rolls, spiral rolls, or crown rolls, as the guide rolls 30, 31, and / or 32, or to use other width-expanding devices, such as cross guiders, bend bars, or tenter clips. Another method for suppressing wrinkles is to perform a stretching treatment. For example, uniaxial stretching can be performed in the swelling bath 13 by utilizing the difference in peripheral speed between the nip rolls 50 and 51.

[0032] During the swelling treatment, the film also swells and expands in the film transport direction, so if the film is not actively stretched, it is preferable to take measures to eliminate slack in the film in the transport direction, such as controlling the speed of nip rolls 50, 51 placed before and after the swelling bath 13. Furthermore, for the purpose of stabilizing the film transport in the swelling bath 13, it is also useful to control the water flow in the swelling bath 13 with an underwater shower or to use an EPC device (Edge Position Control device: a device that detects the edge of the film and prevents the film from meandering) in combination.

[0033] In the example shown in FIG. 1, the film drawn out from the swelling bath 13 passes through a guide roll 32, a nip roll 51 and a guide roll 33 in this order before being introduced into the dye bath 15.

[0034] (dying process) The dyeing process is carried out for the purpose of adsorbing and orienting a dichroic dye into the polyvinyl alcohol-based resin film after swelling treatment. The treatment conditions are determined within a range that achieves the purpose and does not cause defects such as extreme dissolution or devitrification of the film. Referring to FIG. 1, the dyeing process can be carried out by conveying the film along a film conveyance path formed by nip roll 51, guide rolls 33 to 36, and nip roll 52, immersing the swelling film in dye bath 15 (a treatment solution contained in a dyeing tank) for a predetermined period of time, and then withdrawing the film. To enhance the dyeability of the dichroic dye, the film used in the dyeing process is preferably a film that has been subjected to at least some degree of uniaxial stretching treatment. Alternatively, it is preferable to perform uniaxial stretching during the dyeing process instead of or in addition to the uniaxial stretching treatment before the dyeing process.

[0035] When iodine is used as the dichroic dye, the dyeing solution of the dye bath 15 can be, for example, an aqueous solution having a mass ratio of iodine / potassium iodide / water of about 0.003-3 / about 0.1-10 / 100. Potassium iodide can be replaced with other iodides, such as zinc iodide, or potassium iodide can be used in combination with other iodides. Compounds other than iodides, such as boric acid, zinc chloride, and cobalt chloride, can also be present. When boric acid is added, the addition of boric acid is distinguished from the crosslinking treatment and complementary color treatment described below in that it contains iodine. Any aqueous solution containing at least about 0.003 parts by mass of iodine per 100 parts by mass of water can be considered a dye bath 15. The temperature of the dye bath 15 when the film is immersed is usually 10 to 45°C, preferably 10 to 40°C, and more preferably 20 to 35°C, and the immersion time of the film is usually 30 to 600 seconds, and preferably 60 to 300 seconds.

[0036] When a water-soluble dichroic dye is used as the dichroic pigment, the dyeing solution of the dye bath 15 can be, for example, an aqueous solution having a mass ratio of dichroic dye / water of about 0.001 to 0.1 / 100. This dye bath 15 may contain a dyeing assistant, for example, an inorganic salt such as sodium sulfate or a surfactant. Only one dichroic dye may be used alone, or two or more dichroic dyes may be used in combination. The temperature of the dye bath 15 when the film is immersed is, for example, 20 to 80°C, preferably 30 to 70°C, and the immersion time of the film is usually 30 to 600 seconds, preferably 60 to 300 seconds.

[0037] As described above, in the dyeing step, the film can be uniaxially stretched in the dye bath 15. The uniaxial stretching of the film can be performed by, for example, creating a difference in peripheral speed between the nip rolls 51 and 52 arranged before and after the dye bath 15.

[0038] In the dyeing treatment, as in the swelling treatment, in order to transport the polyvinyl alcohol-based resin film while removing wrinkles from the film, rolls having a width-expanding function such as expander rolls, spiral rolls, and crown rolls can be used as the guide rolls 33, 34, 35, and / or 36, or other width-expanding devices such as cross guiders, bend bars, and tenter clips can be used. Another means for suppressing the occurrence of wrinkles is to perform a stretching treatment, as in the swelling treatment.

[0039] In the example shown in FIG. 1, the film drawn out from the dye bath 15 passes through a guide roll 36, a nip roll 52, and a guide roll 37 in this order before being introduced into the crosslinking bath 17.

[0040] (Crosslinking process) The crosslinking step is a treatment for crosslinking the film to improve water resistance, etc. Referring to Fig. 1, the crosslinking step can be carried out by conveying the dyed film along a film conveyance path formed by nip roll 52, guide rolls 37 to 40, and nip roll 53a, immersing the dyed film in crosslinking bath 17 (crosslinking liquid contained in a crosslinking tank) for a predetermined time, and then pulling it out.

[0041] The crosslinking liquid can be a solution in which a crosslinking agent is dissolved in a solvent. Examples of crosslinking agents include boron compounds such as boric acid and borax, glyoxal, and glutaraldehyde. These agents can be used alone or in combination of two or more. The solvent can be, for example, water, but may also contain an organic solvent that is compatible with water. The concentration of the crosslinking agent in the crosslinking solution is not limited thereto, but is preferably in the range of 1 to 20% by mass, and more preferably 6 to 15% by mass.

[0042] The crosslinking liquid may be an aqueous solution containing, for example, about 1 to 10 parts by mass of boric acid per 100 parts by mass of water. When the dichroic dye used in the dyeing process is iodine, the crosslinking liquid preferably contains an iodide in addition to boric acid, and the amount may be, for example, 1 to 30 parts by mass per 100 parts by mass of water. Examples of iodides include potassium iodide and zinc iodide. Two or more types of iodides may be contained. Furthermore, compounds other than iodides, such as sodium thiosulfate, potassium sulfite, and sodium sulfate, may also be present. Furthermore, a nitrate may also be present. The nitrate may include at least one selected from the group consisting of aluminum nitrate, copper nitrate, sodium nitrate, potassium nitrate, zinc nitrate, and magnesium nitrate. The nitrate preferably includes zinc nitrate.

[0043] In the crosslinking treatment, the concentrations of boric acid and iodide and the temperature of the crosslinking bath 17 can be appropriately changed depending on the purpose. The crosslinking liquid can be, for example, an aqueous solution with a mass ratio of boric acid / iodide / water = 3 to 10 / 1 to 20 / 100. If necessary, another crosslinking agent may be used instead of boric acid, or boric acid may be used in combination with another crosslinking agent. The temperature of the crosslinking bath 17 when the film is immersed is usually 50 to 70°C, preferably 53 to 65°C, and the immersion time of the film is usually 10 to 600 seconds, preferably 20 to 300 seconds, and more preferably 20 to 200 seconds. Furthermore, when a polyvinyl alcohol-based resin film that has been stretched before the swelling treatment is subjected to a dyeing treatment and a crosslinking treatment in this order, the temperature of the crosslinking bath 17 is usually 50 to 85°C, preferably 55 to 80°C.

[0044] The crosslinking treatment may be carried out multiple times, for example, 2 to 5 times. In this case, the composition and temperature of each crosslinking bath used may be the same or different as long as they are within the above ranges. Uniaxial stretching treatment can also be carried out in the crosslinking bath 17 by utilizing the difference in peripheral speed between the nip roll 52 and the nip roll 53a.

[0045] In the crosslinking treatment, as in the swelling treatment, in order to transport the polyvinyl alcohol-based resin film while removing wrinkles from the film, rolls having a width-expanding function such as expander rolls, spiral rolls, and crown rolls can be used as the guide rolls 37, 38, 39, and / or 40, or other width-expanding devices such as cross guiders, bend bars, and tenter clips can be used. Another means for suppressing the occurrence of wrinkles is to perform a stretching treatment, as in the swelling treatment.

[0046] (complementary color process) The complementary color process is a process for adjusting the hue of the film. Referring to Fig. 1, the complementary color process can be carried out by conveying the film along a film conveyance path formed by nip roll 53a, guide rolls 41 to 44, and nip roll 53b, immersing the film after the crosslinking process in complementary color bath 18 (complementary color liquid contained in a complementary color tank) for a predetermined time, and then pulling it out.

[0047] The complementary color solution may be an aqueous solution containing, for example, about 1 to 10 parts by mass of boric acid per 100 parts by mass of water. When the dichroic dye used in the dyeing process is iodine, the complementary color solution preferably contains an iodide in addition to boric acid, and the amount may be, for example, 1 to 30 parts by mass per 100 parts by mass of water. Examples of iodides include potassium iodide and zinc iodide. Two or more types of iodides may be contained. Furthermore, compounds other than iodides, such as sodium thiosulfate, potassium sulfite, and sodium sulfate, may also be present. Furthermore, a nitrate may also be present. The nitrate may include at least one selected from the group consisting of aluminum nitrate, copper nitrate, sodium nitrate, potassium nitrate, zinc nitrate, and magnesium nitrate. The nitrate preferably includes zinc nitrate.

[0048] For example, when iodine is used as the dichroic dye in the complementary color solution, the concentration can be such that the mass ratio of boric acid / iodide / water is 1 to 5 / 3 to 30 / 100. The temperature of the complementary color bath 18 when the film is immersed is usually 10 to 45°C, and the immersion time of the film is usually 1 to 300 seconds, preferably 2 to 100 seconds.

[0049] The complementary color treatment may be performed multiple times, for example, 2 to 5 times. In this case, the composition and temperature of each complementary color bath used may be the same or different as long as they are within the above ranges. In addition, uniaxial stretching treatment can be performed in the complementary color bath 18 by utilizing the difference in peripheral speed between the nip rolls 53a and 53b.

[0050] In the complementary color treatment, in order to transport the polyvinyl alcohol-based resin film while removing wrinkles from the film, as in the swelling treatment, rolls having a width-expanding function such as expander rolls, spiral rolls, and crown rolls may be used as the guide rolls 41, 42, 43, and / or 44, or other width-expanding devices such as cross guiders, bend bars, and tenter clips may be used. Another means for suppressing the occurrence of wrinkles is to perform a stretching treatment, as in the swelling treatment.

[0051] In the example shown in FIG. 1, the film drawn out from the complementary color bath 18 passes through the guide roll 44 and the nip roll 53 b in this order before being introduced into the washing bath 19 .

[0052] (Cleaning process) In the example shown in FIG. 1, a cleaning step is included after the color complementing step. The cleaning treatment is performed for the purpose of removing excess chemicals such as boric acid and iodine adhering to the polyvinyl alcohol-based resin film. The cleaning step is performed, for example, by immersing the color complementing-treated polyvinyl alcohol-based resin film in a cleaning bath 19. Note that the cleaning step can also be performed by spraying a cleaning solution onto the film as a shower instead of immersing the film in the cleaning bath 19, or by combining immersion in the cleaning bath 19 with spraying the cleaning solution.

[0053] 1 shows an example of a case where a polyvinyl alcohol resin film is subjected to a cleaning treatment by immersing it in a cleaning bath 19. The temperature of the cleaning bath 19 in the cleaning treatment is usually 2 to 40°C, and the immersion time of the film is usually 2 to 120 seconds.

[0054] In addition, in the washing treatment, for the purpose of transporting the polyvinyl alcohol-based resin film while removing wrinkles, rolls having a width-expanding function such as expander rolls, spiral rolls, and crown rolls, or other width-expanding devices such as cross guiders, bend bars, and tenter clips can be used as the guide rolls 45, 46, 47, and / or 48. In addition, in the film washing treatment, a stretching treatment may be performed to suppress the occurrence of wrinkles.

[0055] (Stretching process) As described above, the raw film 10 is subjected to a wet or dry uniaxial stretching process during the series of processing steps (i.e., before, after, and / or during any one or more processing steps). Specific methods for the uniaxial stretching process include, for example, inter-roll stretching, in which longitudinal uniaxial stretching is performed by applying a peripheral speed difference between two nip rolls (e.g., two nip rolls disposed before and after a processing bath) that constitute a film transport path, hot roll stretching as described in Japanese Patent No. 2731813, tenter stretching, etc., with inter-roll stretching being preferred. The uniaxial stretching process can be performed multiple times from the raw film 10 until the polarizing film 23 is obtained. As described above, the stretching process is also advantageous in suppressing the occurrence of wrinkles in the film.

[0056] The final cumulative stretching ratio of the polarizing film 23 based on the raw film 10 is usually 4.5 to 7 times, and preferably 5 to 6.5 times. The stretching step may be carried out in any of the processing steps, and even when the stretching treatment is carried out in two or more processing steps, the stretching treatment may be carried out in any of the processing steps.

[0057] (Processing process) The processing step is a step of bringing the film into contact with a processing solution, which contains nitrate, boric acid, and zinc iodide and has a pH of 4.3 or higher at 25°C. The pH of the processing solution at 25°C is preferably 4.6 or higher, more preferably 4.7 or higher, and even more preferably 4.8 or higher. The pH of the processing solution at 25°C is preferably 6.8 or lower, more preferably 6.5 or lower, and even more preferably 6.3 or lower. The processing step is, for example, the complementary color step described above. When the processing step is a complementary color step, the description of the complementary color step described above can be applied directly to the processing step. The processing solution contains nitrate, boric acid, and zinc iodide and has a pH of 4.3 or higher. For example, an aqueous solution with a mass ratio of nitrate / boric acid / zinc iodide / water of 0.5-2.2 / 1-10 / 1-30 / 100 can be used as the processing solution. The treatment liquid may contain, for example, 1 to 20 parts by mass of potassium iodide per 100 parts by mass of water.

[0058] The treatment liquid usually further contains a pH adjuster. Examples of pH adjusters include inorganic bases such as potassium hydroxide and sodium hydroxide. The amount of pH adjuster added to the treatment liquid is adjusted so that the pH at 25°C is 4.3 or higher. By including a treatment step, this embodiment can provide a polarizing film that exhibits excellent yellowing suppression effect even in high-temperature environments exceeding 95°C when an interlayer filling structure is used, and that can effectively suppress a decrease in single-unit transmittance when an interlayer filling structure is not used.

[0059] (Second processing step) The present invention may include a second treatment step. This is a step of contacting the film with a second treatment liquid. The second treatment liquid contains nitrate, boric acid, and iodide and has a pH of less than 4.3 at 25°C. The second treatment step is, for example, the crosslinking step described above. When the second treatment step is a crosslinking step, the description of the crosslinking step described above can be applied to the second treatment step as is. The second treatment liquid contains nitrate, boric acid, and iodide and has a pH of less than 4.3 at 25°C. The second treatment liquid may be an aqueous solution having a mass ratio of nitrate / boric acid / iodide / water of 0.5 to 2.2 / 3 to 10 / 1 to 20 / 100. The pH of the second treatment liquid may be adjusted to less than 4.3 by adjusting the amounts of nitrate, boric acid, and iodide added, or by adding an acid pH adjuster. The second treatment liquid is preferably adjusted so that the pH at 25° C. is less than 4.3 by adjusting the amount of boric acid added.

[0060] When the second treatment step is included, it is preferably performed before the treatment step, and the treatments are preferably performed in the order of the second treatment step, then the treatment step, and when the cleaning step is included, it is preferably performed in the order of the second treatment step, then the treatment step, then the cleaning step. The second treatment step and the treatment step are, for example, a crosslinking step and a complementary color step, a first complementary color step and a second complementary color step, a first crosslinking step and a second crosslinking step, etc.

[0061] (drying process) Finally, it is preferable to carry out a treatment to dry the polyvinyl alcohol-based resin film. The drying of the film is not particularly limited, but can be carried out using a drying oven 21 as shown in the example of FIG. 1. The drying oven 21 can be equipped with, for example, a hot air dryer. The drying temperature is, for example, 30 to 100°C, and the drying time is, for example, 30 to 600 seconds. The treatment to dry the polyvinyl alcohol-based resin film can also be carried out using a far-infrared heater.

[0062] (Other processing steps for polyvinyl alcohol-based resin films) Treatments other than those described above may also be added, such as immersion in an aqueous iodide solution that does not contain boric acid.

[0063] The polarizing film obtained as described above may be wound up onto a winding roll to form a roll, or may be directly subjected to the method for producing a polarizing plate without being wound up. The polarizing film in the form of a roll may then be subjected to the method for producing a polarizing plate. One embodiment of the method for producing a polarizing plate includes a laminating step of laminating a protective film to one or both sides of the polarizing film via an laminating layer.

[0064] <Polarizing film> The polarizing film according to this embodiment is preferably a polyvinyl alcohol-based resin film dyed with a dichroic dye, and the thickness of the polarizing film is preferably 5 μm or more and 60 μm or less, and more preferably 7 μm or more and 30 μm or less.

[0065] The luminosity-corrected single transmittance Ty of the polarizing plate is preferably 35 to 47%, more preferably 36 to 45%, taking into consideration the balance with the luminosity-corrected polarization degree Py. The luminosity-corrected polarization degree Py is preferably 99.9% or more, more preferably 99.95% or more. The polarizing film constituting such a polarizing plate can be obtained by the above-mentioned production method.

[0066] The chlorine content of the polarizing film can be, for example, 900 ppm or less. Although there is no lower limit, the chlorine content of the polarizing film can be 0 ppm or more. The chlorine content of the polarizing film can be reduced by reducing the amount of chloride in the treatment solution used in the polarizing film manufacturing process.

[0067] <Polarizing plate> The polarizing plate according to this embodiment can be obtained by laminating a protective film to one or both sides of the polarizing film via an adhesive layer. Examples of the protective film include films made of acetylcellulose-based resins such as triacetylcellulose and diacetylcellulose; films made of polyester-based resins such as polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate; polycarbonate-based resin films, cycloolefin-based resin films; acrylic-based resin films; and films made of linear olefin-based resins such as polypropylene-based resins.

[0068] To improve the adhesion between the polarizing film and the protective film, the bonding surface of the polarizing film and / or the protective film may be subjected to a surface treatment such as corona treatment, flame treatment, plasma treatment, ultraviolet irradiation, primer coating, or saponification. The bonding layer interposed between the polarizing film and the protective film can be formed using an adhesive or pressure-sensitive adhesive. Examples of adhesives include active energy ray-curable adhesives such as ultraviolet-curable adhesives, aqueous solutions of polyvinyl alcohol resins or aqueous solutions containing crosslinkers, and water-based adhesives such as urethane emulsion adhesives. Water-based adhesives may contain zinc compounds such as zinc nitrate. The ultraviolet-curable adhesive can be a mixture of an acrylic compound and a photoradical polymerization initiator, or a mixture of an epoxy compound and a photocationic polymerization initiator. Alternatively, a cationically polymerizable epoxy compound and a radically polymerizable acrylic compound can be used in combination, and a photocationic polymerization initiator and a photoradical polymerization initiator can be used in combination as initiators.

[0069] <Image display device> The polarizing plate can be used in an image display device. Examples of image display elements used in image display devices include liquid crystal display elements and organic EL display elements. When constructing a liquid crystal display device, the polarizing plate of the present invention may be disposed on the viewing side, on the backlight side, or on both the viewing side and the backlight side. The image display device of the present invention can be used in mobile devices such as televisions, personal computers, mobile phones, and tablet terminals. Furthermore, the image display device of the present invention has a high suppression effect against yellowing or a high suppression effect against a decrease in single-unit transmittance in high-temperature environments, and can exhibit stable image display function for a long period of time. Therefore, the image display device is particularly suitable for in-vehicle applications that are likely to be exposed to harsh temperature conditions. Examples of in-vehicle applications include image display devices used in car navigation systems, speedometers, touch panels for air conditioners, back monitors, and rear monitors. [Example]

[0070] The present invention will be explained in more detail below by showing examples, but the present invention is not limited to these examples.

[0071] (1) Measurement of polarizing film thickness: Measurements were made using a Nikon digital micrometer "MH-15M."

[0072] (2) Measurement of the luminosity-corrected single transmittance of a polarizing plate: Measurement was carried out using a spectrophotometer with an integrating sphere ("V7100" manufactured by JASCO Corporation, 2-degree field of view; C light source).

[0073] (3) pH measurement: Measurement was performed using a pH meter (HORIBA "D-54").

[0074] (4) Yellow index (yellowness) measurement: We used a Konica Minolta spectrophotometer, CM-3700A. Reflected light was measured in SCI mode to determine the tristimulus values ​​(X, Y, Z), and the yellow index (YI) was calculated using the following formula based on the ASTM E 313-73 standard. YI=100×(1.000-0.847×Z / Y)

[0075] Example 1 (1) Preparation of polarizing film 1 The polarized film of Example 1 was produced from a polyvinyl alcohol-based resin film using the production apparatus shown in Figure 1. Specifically, a 45 μm-thick, long polyvinyl alcohol (PVA) raw film (trade name "VF-PE#4500" manufactured by Kuraray Co., Ltd., saponification degree 99.9 mol% or higher) was continuously transported while being unwound from a roll, immersed in a swelling bath of pure water at 23°C for 110 seconds, and uniaxially stretched to 2.1 times its original size (swelling step). The film was then pulled out of the swelling bath and immersed in a dye bath at 23°C containing an iodine-containing dye solution with a mass ratio of iodine / boric acid / water of 1.0 / 0.5 / 100 for 163 seconds, and uniaxially stretched to 1.22 times its original size (dyeing step). Next, the film was pulled out of the dye bath and immersed in a crosslinking bath containing potassium iodide / boric acid / water in a mass ratio of 2.3 / 3.7 / 100 at 59°C for 92 seconds, where it was uniaxially stretched to 2.24 times its original size (stretching and crosslinking process).

[0076] The film was then removed from the crosslinking bath and immersed in a 45°C complementary bath containing a complementary solution (a solution of zinc nitrate hexahydrate, zinc iodide, potassium iodide, boric acid, and water in a 0.55 / 1.76 / 2.7 / 5.0 / 100 (mass ratio) with potassium hydroxide added) for 14 seconds, whereupon it was uniaxially stretched to 1.02 times its original size (complementary step, treatment step). The film was then dried in a 55°C drying oven for 90 seconds (drying step). The pH of the complementary solution at 25°C was 4.30. The resulting polarized film 1 had a thickness of 19 μm.

[0077] (2) Preparation of Adhesive 1 50 parts by mass of a modified PVA resin containing acetoacetyl groups ("Gohsenex Z-410" manufactured by Mitsubishi Chemical Corporation) was dissolved in 950 parts by mass of pure water, heated at 90°C for 2 hours, and then cooled to room temperature to obtain a PVA solution for adhesive (hereinafter referred to as "PVA solution A").

[0078] The PVA solution A prepared above, pure water, a 40 mass % glyoxal solution, and methanol were mixed together so as to give the following contents per 100 mass parts of adhesive, thereby preparing adhesive 1. PVA content: 3 parts by mass Methanol content: 36 parts by mass Glyoxal content: 0.3 parts by mass Pure water: 60.7 parts by mass

[0079] (3) Preparation of Transparent Protective Film 1 A commercially available cellulose acylate film "TD40N" (manufactured by Fujifilm Corporation, thickness 40 μm) was immersed in a 1.5 mol / L NaOH aqueous solution (saponification solution) maintained at 55°C for 2 minutes, and then washed with water. The film was then immersed in a 0.05 mol / L sulfuric acid aqueous solution at 25°C for 30 seconds, and then passed through a water washing bath under running water for 30 seconds to neutralize the film. The film was then drained three times with an air knife to remove the water, and then allowed to dry in a drying zone at 70°C for 15 seconds to produce a saponified film, designated Transparent Protective Film 1.

[0080] (4) Preparation of polarizing plate 1 Transparent protective film 1 was laminated to both sides of polarizing film 1 via adhesive 1 using a roll laminator, and then heat treatment was performed at 65°C for 12 minutes to dry adhesive 1, thereby obtaining polarizing plate 1. The luminous efficiency corrected single transmittance of polarizing plate 1 was 40.8%.

[0081] (5) Adjustment of the moisture content of polarizing plate 1 (polarizing film 1) The moisture content of polarizing plate 1 was adjusted by storing it under conditions of a temperature of 20° C. and a relative humidity of 40% for 72 hours. The moisture content was measured using the Karl Fischer method after 66, 69, and 72 hours of storage, and the moisture content was the same at each stage. Therefore, the moisture content of polarizing plate 1 after moisture content adjustment can be considered to be the same as the equilibrium moisture content in the above storage environment. When the moisture content of polarizing plate 1 reaches equilibrium in a certain storage environment, the moisture content of polarizing film 1 in polarizing plate 1 can also be considered to have reached equilibrium in that storage environment. Furthermore, when the moisture content of polarizing film 1 in polarizing plate 1 reaches equilibrium in a certain storage environment, the moisture content of polarizing plate 1 can also be considered to have reached equilibrium in that storage environment.

[0082] <Comparative Example 1> (1) Preparation of polarizing film 2 The same treatments as in Example 1 were carried out up to the stretching and crosslinking steps. The film was then removed from the crosslinking bath and immersed in a complementary color bath at 45°C for 14 seconds, consisting of a solution of zinc nitrate hexahydrate / potassium iodide / boric acid / water in a 2.20 / 2.7 / 5.0 / 100 (mass ratio) to which potassium hydroxide aqueous solution was added, and uniaxially stretched to 1.02 times its original size (complementary color step, treatment step). The film was then dried by retaining it in a drying oven at 55°C for 90 seconds (drying step). The pH of the complementary color solution at 25°C was 3.40. The resulting polarized film 2 had a thickness of 19 μm.

[0083] (2) Preparation of polarizing plate 2 Transparent protective film 1 was laminated to both sides of polarizing film 2 via adhesive 1 using a roll laminator, and then heat treatment was performed at 65°C for 12 minutes to dry adhesive 1, thereby obtaining polarizing plate 2. The luminous efficiency corrected single transmittance of polarizing plate 2 was 40.8%.

[0084] (3) Adjustment of the moisture content of polarizing plate 2 (polarizing film 2) The moisture content of polarizing plate 2 was adjusted by storing it under conditions of a temperature of 20° C. and a relative humidity of 40% for 72 hours. The moisture content was measured using the Karl Fischer method after 66, 69, and 72 hours of storage, and the moisture content was the same at each stage. Therefore, the moisture content of polarizing plate 2 after moisture content adjustment can be considered to be the same as the equilibrium moisture content in the above-mentioned storage environment. When the moisture content of polarizing plate 2 reaches equilibrium in a certain storage environment, the moisture content of polarizing film 2 in polarizing plate 2 can also be considered to have reached equilibrium in that storage environment. Furthermore, when the moisture content of polarizing film 2 in polarizing plate 2 reaches equilibrium in a certain storage environment, the moisture content of polarizing plate 2 can also be considered to have reached equilibrium in that storage environment.

[0085] <Evaluation> (1) High temperature durability evaluation - Yellow Index YI - An acrylic adhesive (product number "#7" manufactured by Lintec Corporation) was applied to both sides of the moisture-adjusted polarizing plates 1 and 2. The polarizing plates were then cut to a size of 110 mm x 60 mm so that the absorption axis was parallel to the long side. An alkali-free glass ("EAGLE XG" manufactured by Corning, size 120 mm x 70 mm) was laminated to the adhesive surface of each plate to prepare evaluation samples. The evaluation samples were autoclaved for 15 minutes at 50°C and 5 atmospheres, and then subjected to a high-temperature durability test at 105°C for 168 hours. After 168 hours, Polarizing Plate 1 had a YI of 46. After 168 hours, Polarizing Plate 2 had a YI of 40. Both Polarizing Plates 1 and 2 had YIs of 50 or less, demonstrating excellent high-temperature durability in terms of YI despite being chlorine-free.

[0086] (2) High temperature durability evaluation - Single unit transmittance - An acrylic adhesive (product number "#7" manufactured by Lintec Corporation) was applied to one side of the moisture-adjusted polarizers 1 and 2. The polarizers were then cut into 40mm x 35mm pieces with the absorption axis parallel to the long side. Non-alkali glass (50mm x 40mm, manufactured by Corning Incorporated, "EAGLE XG") was then laminated to the adhesive surface to prepare evaluation samples. The evaluation samples were autoclaved for 15 minutes at 50°C and 5 atmospheres, and then spectroscopic measurements were performed using a V-7100 spectrophotometer (JASCO Corporation). The same samples were also subjected to similar spectroscopic measurements after 1000 hours at 105°C. The change in the single-unit transmittance at 480 nm after 1000 hours was 0.02% for polarizer 1 and 5.06% for polarizer 2. Polarizer 1 exhibited less change in single-unit transmittance after high-temperature durability testing. [Explanation of symbols]

[0087] 10 Raw film made of polyvinyl alcohol resin, 11 Raw film roll, 13 Swelling bath, 15 Dyeing bath, 17 Crosslinking bath, 18 Complementary color bath, 19 Cleaning bath, 21 Drying oven, 23 Polarizing film, 30 to 48, 60, 61 Guide rolls, 50 to 52, 53a, 53b, 54, 55 Nip rolls.

Claims

1. A method for producing a polarizing film from a polyvinyl alcohol-based resin film, comprising: a dyeing step of bringing the polyvinyl alcohol-based resin film into contact with a dyeing solution to perform a dyeing treatment; a crosslinking step in which a crosslinking treatment is carried out by bringing a crosslinking liquid into contact with the dyed film; a color complementing step in which a color complementing liquid is brought into contact with the film after the crosslinking treatment to perform a color complementing treatment; in this order, the complementary color solution is an aqueous solution prepared using nitrate, boric acid, zinc iodide, potassium iodide, and a pH adjuster, and has a pH of 4.3 or more and 6.5 or less at 25°C; the nitrate comprises zinc nitrate; a content ratio of the nitrate, boric acid, zinc iodide, potassium iodide, and water in the complementary color solution of nitrate / boric acid / zinc iodide / potassium iodide / water=0.5 to 2.2 / 1 to 10 / 1 to 30 / 1 to 20 / 100 by mass;

2. a step of producing a polarized film by the method for producing a polarized film according to claim 1; and a laminating step of laminating a protective film to one or both surfaces of the polarizing film via an adhesive layer.

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