Polarizer manufacturing method

Abstract

To provide a method of manufacturing a polarizer, which allows for sufficiently improving single transmittance even if a humidification time is reduced during a humidification step.SOLUTION: A polarizer manufacturing method according to an embodiment of the present invention comprises a first dying step of dying a polyvinyl alcohol resin film with a dichroic substance, a stretching step of stretching the polyvinyl alcohol resin film in a boric acid aqueous solution after the first dying step, and a second dying step of eluting boric acid from the polyvinyl alcohol resin film after the stretching step and dying the polyvinyl alcohol resin film with a dichroic substance.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a polarizer. [Background technology]

[0002] A polarizer is typically used in image display devices. Such a polarizer is produced, for example, by dyeing a polyvinyl alcohol-based resin film with a dichroic substance and then stretching the polyvinyl alcohol-based resin film in an aqueous boric acid solution. In recent years, there has been an increasing demand for improved optical properties of polarizers. To address this, a technique has been proposed in which the polarizer is humidified to improve its degree of polarization (see, for example, Patent Document 1). However, the technique described in Patent Document 1 leaves room for improvement in the single transmittance of the polarizer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-78780 Summary of the Invention [Problem to be solved by the invention]

[0004] The present invention has been made to solve the above-mentioned conventional problems, and a main object of the present invention is to provide a method for producing a polarizer that can sufficiently improve the single-piece transmittance even when subjected to a humidification step with a reduced humidification time. [Means for solving the problem]

[0005] A method for producing a polarizer according to an embodiment of the present invention includes: a first dyeing step of dyeing a polyvinyl alcohol-based resin film with a dichroic substance; a stretching step of stretching the polyvinyl alcohol-based resin film after the first dyeing step in an aqueous boric acid solution; and a second dyeing step of eluting boric acid from the polyvinyl alcohol-based resin film after the stretching step and dyeing the polyvinyl alcohol-based resin film with a dichroic substance. In one embodiment, the polyvinyl alcohol-based resin film after the second dyeing step has a boric acid content of 10% by mass or less and a single body transmittance of 35% or less. In one embodiment, the method for producing a polarizer further includes a humidifying step of humidifying the polyvinyl alcohol-based resin film after the second dyeing step in an atmosphere at a temperature of 40° C. to 100° C. and a humidity of 50% RH or more. In one embodiment, the humidification time in the humidification step is 60 minutes or less. In one embodiment, the method for producing a polarizer includes, in this order: a laminate-producing step of applying a coating liquid containing a polyvinyl alcohol-based resin and a halide onto a long thermoplastic resin substrate to produce a laminate including a polyvinyl alcohol-based resin layer as a polyvinyl alcohol-based resin film and the thermoplastic resin substrate; an auxiliary stretching step of stretching the laminate in air; the first dyeing step; the stretching step; the second dyeing step; and a drying shrinking step of shrinking the polyvinyl alcohol-based resin film after the second dyeing step in a width direction perpendicular to the long direction while transporting the polyvinyl alcohol-based resin film in the long direction. In one embodiment, the humidifying step is carried out after the heat shrinking step. In one embodiment, in the second dyeing step, the polyvinyl alcohol-based resin film after the stretching step is immersed in a second dyeing solution containing a dichroic substance, the concentration of the dichroic substance in the second dyeing solution being 0.8% by mass or less, and the immersion time in the second dyeing step is 5 minutes or less. [Effects of the Invention]

[0006] According to the embodiment of the present invention, a polarizer can be manufactured that can sufficiently improve the single-piece transmittance even when subjected to a humidification step with a reduced humidification time. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram illustrating a method for manufacturing a polarizer according to one embodiment of the present invention. [Figure 2] Fig. 2(a) is a schematic cross-sectional view of one embodiment of the polyvinyl alcohol-based resin film shown in Fig. 1. Fig. 2(b) is a schematic cross-sectional view of another embodiment of the polyvinyl alcohol-based resin film shown in Fig. 1. [Figure 3] FIG. 3 is a graph showing the relationship between the single transmittance and the polarization degree in the examples and the comparative examples. [Figure 4] FIG. 4 is a graph showing the relationship between the humidification time and the single-piece transmittance in the examples and comparative examples. DETAILED DESCRIPTION OF THE INVENTION

[0008] Representative embodiments of the present invention will be described below, but the present invention is not limited to these embodiments.

[0009] A. Overview of polarizer manufacturing methods A method for producing a polarizer according to one embodiment of the present invention includes: a first dyeing step of dyeing a polyvinyl alcohol-based resin film (hereinafter referred to as a PVA-based resin film) with a dichroic substance; a stretching step of stretching the PVA-based resin film after the first dyeing step in an aqueous boric acid solution; and a second dyeing step of eluting boric acid from the PVA-based resin film after the stretching step and dyeing the PVA-based resin film with a dichroic substance. The present inventors have found that the single-piece transmittance can be sufficiently improved by sufficiently humidifying the PVA-based resin film after the stretching process. However, a significantly long humidifying time is required to sufficiently improve the single-piece transmittance, which causes a problem of reduced efficiency in producing polarizers. In this regard, the present inventors have found that by performing a second dyeing step on a PVA-based resin film after a stretching step and sufficiently reducing the content of boric acid and the single-piece transmittance in the PVA-based resin film after the second dyeing step, the single-piece transmittance of a polarizer can be sufficiently improved in a significantly short humidification time, and have completed the present invention. More specifically, in the second dyeing step, the boric acid content and the simple substance transmittance of the PVA-based resin film after the second dyeing step (specifically, the PVA-based resin film after the second dyeing step and immediately before the humidifying step) can be adjusted to be below the following upper limits. The content of boric acid in the PVA-based resin film after the second dyeing step is, for example, 10% by mass or less, preferably 9.0% by mass or less, more preferably 7.0% by mass or less, even more preferably 4.0% by mass or less, and particularly preferably 1.0% by mass or less. The content of boric acid in the PVA-based resin film after the second dyeing step is typically 0.1% by mass or more. The PVA-based resin film after the second dyeing step has a single-piece transmittance of, for example, 35% or less, preferably 25% or less, more preferably 20% or less. The PVA-based resin film after the second dyeing step has a single-piece transmittance of, for example, 1% or more. When the PVA-based resin film after the second dyeing step has a boric acid content not more than the upper limit and a single-piece transmittance not more than the upper limit, water can smoothly penetrate into the PVA-based resin film and the dichroic material can smoothly elute from the PVA-based resin film when the PVA-based resin film after the second dyeing step is subjected to a humidifying step. Therefore, even if the humidifying time is shortened, the single-piece transmittance of the humidified PVA-based resin film (i.e., polarizer) can be efficiently improved.

[0010] In one embodiment, the method for producing a polarizer further includes a crosslinking step of contacting the PVA-based resin film after the first dyeing step with a boric acid aqueous solution before the stretching step. Contacting the PVA-based resin film after the first dyeing step with a boric acid aqueous solution can stably prevent the PVA-based resin film from dissolving in the boric acid aqueous solution during the stretching step. Specifically, boric acid can generate tetrahydroxyborate anions in the aqueous solution and form hydrogen bonds with the PVA-based resin, thereby forming crosslinks, or can dehydrate and condense with the hydroxyl groups of the PVA-based resin to form borate esters, thereby forming crosslinks. As a result, water resistance can be imparted to the PVA-based resin film.

[0011] In one embodiment, the method for producing a polarizer further includes a humidifying step of humidifying the PVA resin film after the second dyeing step in an atmosphere at a temperature of 40° C. to 100° C. and a humidity of 50% RH or higher. The humidifying temperature is, for example, 50° C. or higher, preferably 60° C. or higher, more preferably 70° C. or higher, and even more preferably 80° C. or higher, and for example, 90° C. or lower. The humidifying humidity is preferably 70% RH or higher, more preferably 80% RH or higher, and typically 100% RH or lower, and preferably 95% RH or lower. When a polarizer is produced by humidifying the PVA-based resin film after the second dyeing step in the above atmosphere, the single transmittance of the polarizer can be improved while suppressing a decrease in the degree of polarization of the polarizer.

[0012] In one embodiment, the humidification time in the humidifying step is, for example, 120 minutes or less, preferably 80 minutes or less, more preferably 60 minutes or less, even more preferably 40 minutes or less, particularly preferably 20 minutes or less, particularly preferably 10 minutes or less, and most preferably 5 minutes or less. When the humidification time is equal to or less than the upper limit, the production efficiency of the polarizer can be stably improved. The humidification time in the humidifying step is typically 30 seconds or longer, and preferably 1 minute or longer. When the humidification time is equal to or longer than the lower limit, the single transmittance and polarization degree of the polarizer can be stably improved.

[0013] In one embodiment, the method for producing a polarizer includes, in this order, a laminate-preparing step, an auxiliary stretching step, the first dyeing step described above, the stretching step described above, the second dyeing step described above, and a drying shrinking step. In the laminate-preparing step, a coating liquid containing a PVA-based resin and a halide is applied to a long thermoplastic resin substrate to produce a laminate including a PVA-based resin layer as a PVA-based resin film and a thermoplastic resin substrate. In the auxiliary stretching step, the laminate is stretched in air. In the drying shrinking step, the PVA-based resin film after the second dyeing step is shrunk in the width direction perpendicular to the long direction while being transported in the long direction. This method can provide a polarizer with reduced thickness and excellent optical properties. Specifically, by incorporating the auxiliary stretching step, the crystallinity of the PVA-based resin can be enhanced, even when the PVA-based resin is coated on a thermoplastic resin, thereby achieving high optical properties. Furthermore, by simultaneously enhancing the orientation of the PVA-based resin in advance, problems such as a decrease in orientation or dissolution of the PVA-based resin when immersed in water in the subsequent dyeing or stretching steps can be prevented, thereby achieving high optical properties. Furthermore, when the PVA-based resin layer is immersed in a liquid, the PVA-based resin molecules are less likely to be disoriented and their orientation is less likely to be impaired than when the PVA-based resin layer does not contain a halide. This can improve the optical properties of a polarizer obtained through treatment steps, such as a dyeing step and a stretching step, in which the laminate is immersed in a liquid. Furthermore, the drying shrinkage step shrinks the PVA-based resin film in the width direction, thereby improving the optical properties of the polarizer.

[0014] The above-described method for producing a polarizer may include a swelling step and / or a hue adjusting step. In the swelling step, typically, the PVA-based resin film before the first dyeing step is immersed in a swelling bath (swelling liquid). This allows foreign matter on the surface of the PVA-based resin film to be removed, and also allows the plasticizer in the PVA-based resin film to be removed. The swelling liquid is preferably a boric acid aqueous solution containing boric acid. When the swelling liquid is a boric acid aqueous solution, it can impart water resistance to the PVA-based resin film. In this case, the swelling step is called an insolubilization step. In the hue adjusting step, typically, the PVA resin film after the second dyeing step is immersed in a hue adjusting bath (hue adjusting solution) before the humidifying step, which allows the PVA resin film to be cleaned and the polarizer to be adjusted to have a desired hue.

[0015] B. Details of polarizer manufacturing method 1 is a schematic diagram illustrating a method for producing a polarizer according to one embodiment of the present invention. In the illustrated method for producing a polarizer, the swelling step (insolubilization step), first dyeing step, crosslinking step, stretching step, second dyeing step, hue adjusting step, and drying shrinkage step are carried out successively. More specifically, a long PVA-based resin film 1 is transported from a raw web roll 21 toward a take-up roll 22, and between the raw web roll 21 and the take-up roll 22, the PVA-based resin film 1 is subjected to a swelling step (insolubilization step), a first dyeing step, a crosslinking step, a stretching step, a second dyeing step, a hue adjusting step, and a drying shrinkage step in this order. In one embodiment, the PVA-based resin film 1 is immersed in a swelling bath 2A (swelling liquid), a first dyeing bath 2B' (first dyeing liquid), a crosslinking bath 2C (crosslinking liquid), a stretching bath 2D (stretching liquid), a second dyeing bath 2B'' (second dyeing liquid), and a hue adjusting bath 2E (hue adjusting liquid) in this order by a plurality of rollers 24, and then transported to pass through a heat drying section 23. As will be described in detail later, when a PVA-based resin film is included in the laminate, the laminate including the PVA-based resin film is immersed in each of the above-mentioned baths (each of the liquids) to bring the PVA-based resin film into contact with each of the liquids. Thereafter, the above-mentioned humidifying step is carried out after the drying shrinkage step.

[0016] B-1. PVA resin film In the raw film roll 21, the PVA resin film 1 (hereinafter referred to as raw film 11) before undergoing each of the above-mentioned steps is wound in a roll shape. The crystallization index of the PVA-based resin in the raw film 11 is, for example, 1.4 or more, preferably 1.6 or more, and more preferably 1.8 or more. The crystallization index of the PVA-based resin is typically 3.0 or less. The crystallization index of the PVA-based resin can be measured by the ATR method using a Fourier transform infrared spectrophotometer. The raw film 11 may be a single-layer resin film as shown in FIG. 2(a), or may be laminated to a thermoplastic resin substrate 12 (hereinafter referred to as resin substrate 12) as shown in FIG. 2(b).

[0017] Specific examples of single-layer resin films include hydrophilic polymer films such as PVA films, partially formalized PVA films, and partially saponified ethylene-vinyl acetate copolymer films, as well as polyene-based oriented films such as dehydrated PVA and dehydrochlorinated polyvinyl chloride. When the raw film 11 is a single-layer resin film, its thickness is, for example, 20 μm or more, preferably 30 μm or more, and for example, 65 μm or less, preferably 60 μm or less.

[0018] When the raw film 11 is laminated on a thermoplastic resin substrate 12, the raw film 11 may be a PVA-based resin film supported on the resin substrate 12, or may be a PVA-based resin layer 13 formed by coating on the resin substrate 12. When the raw film 11 is a PVA-based resin layer 13 formed by coating on the resin substrate 12, the PVA-based resin layer 13 is formed on the resin substrate 12 by the above-described laminate production step. More specifically, a coating liquid containing a PVA-based resin and a halide is applied to a long resin substrate 12 by any appropriate method, and if necessary, dried at a temperature of, for example, 50°C or higher to produce a laminate 10 comprising a PVA-based resin layer 13 and a resin substrate 12.

[0019] Any appropriate material may be used as the constituent material of the resin substrate 12. A typical constituent material of the resin substrate is an amorphous (non-crystallized) polyethylene terephthalate resin, and preferably an amorphous (hard to crystallize) polyethylene terephthalate resin. Specific examples of amorphous polyethylene terephthalate resins include copolymers further containing isophthalic acid as a dicarboxylic acid and copolymers further containing cyclohexanedimethanol as a glycol. The glass transition temperature (Tg) of the resin substrate is, for example, 170°C or lower, preferably 120°C or lower. When the Tg of the resin substrate is equal to or lower than the upper limit, crystallization of the PVA-based resin layer can be suppressed while the stretchability of the laminate can be sufficiently ensured. Furthermore, the glass transition temperature (Tg) of the resin substrate is typically 60°C or higher. This can suppress problems such as deformation of the resin substrate (for example, the occurrence of unevenness, sagging, and wrinkles) when a coating liquid is applied to the resin substrate and dried. The glass transition temperature (Tg) is measured in accordance with JIS K 7121. The thickness of the resin substrate before stretching is, for example, 20 μm or more, preferably 50 μm or more, and for example, 300 μm or less, preferably 200 μm or less. The surface of the resin substrate may be subjected to any appropriate surface treatment (for example, corona treatment), or an easy-adhesion layer may be formed on the surface, which can improve the adhesion between the resin substrate and the PVA-based resin layer.

[0020] The coating liquid is typically a solution in which a PVA resin and a halide are dissolved in a solvent. Any appropriate resin can be used as the PVA-based resin. Examples include polyvinyl alcohol and ethylene-vinyl alcohol copolymer. Polyvinyl alcohol can be obtained by saponifying polyvinyl acetate. Ethylene-vinyl alcohol copolymer can be obtained by saponifying ethylene-vinyl acetate copolymer. The saponification degree of the PVA-based resin is usually 85 mol% to 100 mol%, preferably 95.0 mol% to 99.95 mol%, more preferably 99.0 mol% to 99.93 mol%, and even more preferably 99.0 mol% to 99.5 mol%. The saponification degree can be measured in accordance with JIS K 6726-1994. By using a PVA-based resin with such a saponification degree, a thin polarizing film with excellent durability can be obtained. If the saponification degree is too high, gelation may occur.

[0021] The average degree of polymerization of the PVA-based resin can be appropriately selected depending on the purpose. The average degree of polymerization is, for example, 1000 or more, preferably 1500 or more, more preferably 2000 or more, and even more preferably 3000 or more, and is, for example, 10000 or less, preferably 6000 or less, and even more preferably 4300 or less. The average degree of polymerization can be measured in accordance with JIS K 6726-1994.

[0022] In one embodiment, the PVA-based resin may contain acetoacetyl-modified PVA. The content of acetoacetyl-modified PVA in the PVA-based resin is, for example, 5% by mass or more, preferably 8% by mass or more, and for example, 20% by mass or less, preferably 12% by mass or less. If the PVA-based resin contains acetoacetyl-modified PVA, the mechanical strength of the polarizer can be improved.

[0023] The content of the PVA resin in the coating liquid is, for example, 3 to 20 parts by mass relative to 100 parts by mass of the solvent. With such a resin concentration, a uniform coating film that adheres tightly to the resin substrate can be formed.

[0024] Any suitable halide may be used. Typical examples of the halide include iodide and sodium chloride. Examples of the iodide include potassium iodide, sodium iodide, and lithium iodide, and preferably potassium iodide. The content of the halide in the coating solution is, relative to 100 parts by mass of the PVA resin, for example, 5 parts by mass or more, preferably 10 parts by mass or more, and for example, 20 parts by mass or less, preferably 15 parts by mass or less. When the content of the halide is within this range, the finally obtained polarizer can be prevented from becoming cloudy.

[0025] Examples of solvents include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine. These can be used alone or in combination. Of the solvents, water is preferred. The coating liquid may contain additives. Examples of additives include plasticizers and surfactants. Examples of plasticizers include polyhydric alcohols such as ethylene glycol and glycerin. Examples of surfactants include nonionic surfactants.

[0026] The thickness of the PVA-based resin layer formed from such a coating liquid before stretching is, for example, 3 μm or more, preferably 5 μm or more, and for example, 40 μm or less, preferably 30 μm or less.

[0027] The laminate 10 including the PVA resin layer 13 and the resin substrate 12 is preferably subjected to the auxiliary stretching step described above in advance, where the laminate 10 is stretched in the longitudinal direction at the above-described stretch ratio in mid-air. That is, the auxiliary stretching step is carried out after the laminate preparation step and before the first dyeing step, and in the illustrated example, after the laminate preparation step and before the swelling step. The stretching temperature in the auxiliary stretching step is typically equal to or higher than the glass transition temperature (Tg) of the PVA resin, for example, equal to or higher than 95° C., and preferably equal to or higher than 120° C. The stretching temperature in the auxiliary stretching step is typically equal to or lower than 150° C. The stretching ratio of the laminate in the auxiliary stretching step is 2.1 times or more, preferably 2.3 times or more. If the stretching ratio of the laminate is equal to or greater than this lower limit, the orientation of the PVA-based resin layer contained in the laminate can be improved, and the PVA-based resin layer can be stably prevented from dissolving in the stretching bath in the stretching step. The upper limit of the stretching ratio of the laminate in the auxiliary stretching step is typically 4 times or less. The in-air stretching method in the auxiliary stretching step may be fixed-end stretching (for example, a method of stretching using a tenter stretching machine) or free-end stretching (for example, a method of uniaxially stretching the laminate by passing it between rolls with different peripheral speeds).

[0028] B-2. Swelling process (insolubilization process) The raw film 11 (raw film alone or raw film included in a laminate) is subjected to the swelling step (insolubilization step) before the first dyeing step, as necessary. Hereinafter, the PVA resin film that has been subjected to the swelling step (insolubilization step) is referred to as a swelling-treated film 1a. In the swelling step, the raw film 11 is typically immersed in a swelling liquid (swelling bath). The swelling liquid may be pure water or a boric acid aqueous solution. When the swelling liquid is a boric acid aqueous solution (i.e., an insolubilizing liquid), the content of boric acid in the insolubilizing liquid is, for example, 1 part by mass or more and 10 parts by mass or less per 100 parts by mass of water. The temperature of the swelling bath is, for example, 10° C. or higher, preferably 20° C. or higher, and for example, 60° C. or lower, preferably 50° C. or lower. The immersion time in the swelling step is, for example, 10 seconds or higher, preferably 20 seconds or higher, and for example, 200 seconds or lower, preferably 60 seconds or lower.

[0029] B-3. ​​First dyeing process In the first dyeing step, the PVA resin film 1 (preferably the swelling-treated film 1a) is dyed with a dichroic substance. Specifically, the PVA resin film 1 (preferably the swelling-treated film 1a) is brought into contact with a first dyeing solution to adsorb the dichroic substance. Hereinafter, the PVA resin film after the first dyeing step and before the stretching step will be referred to as a first dyed film 1b'. Examples of dichroic substances include iodine and organic dyes. The dichroic substances can be used alone or in combination. Among the dichroic substances, iodine is preferred. The first dyeing solution is typically an aqueous iodine solution. The iodine content in the first dyeing solution is, for example, 0.05 parts by mass or more, preferably 0.5 parts by mass or more, and for example, 3 parts by mass or less, per 100 parts by mass of water. The first staining liquid preferably further contains an iodine compound, which can improve the solubility of iodine in water. Examples of iodine compounds include potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, and titanium iodide. The iodine compounds can be used alone or in combination. Among the iodine compounds, potassium iodide is preferred. The mass ratio of iodine to the iodine compound (iodine:iodine compound) in the first dye solution is, for example, 1:5 to 1:20, and preferably 1:5 to 1:10, which can impart excellent optical properties to the polarizer.

[0030] In the illustrated first dyeing step, the PVA resin film 1 (swelling-treated film 1a) is immersed in the first dye bath. The temperature of the first dye bath is, for example, 10° C. or higher, preferably 20° C. or higher, and for example, 50° C. or lower, preferably 40° C. or lower. The immersion time in the first dyeing step (first dyeing time) is, for example, 5 seconds or longer, preferably 30 seconds or longer, and for example, 300 seconds or shorter, preferably 90 seconds or shorter. When the first dyeing step is performed immediately after the insolubilization step, boric acid may be mixed into the first dyeing bath from the insolubilization bath. If boric acid is mixed into the first dyeing bath, the boric acid concentration in the first dyeing bath may change over time, which may affect the boric acid content in the polarizer. Therefore, the boric acid content in the first dyeing bath is preferably adjusted to 4 parts by mass or less per 100 parts by mass of water. The method of adsorbing the dichroic material in the first dyeing step is not limited to the immersion method described above. For example, the first dyeing solution may be applied to the raw film, or the first dyeing solution may be sprayed onto the raw film.

[0031] B-4. Crosslinking process In the crosslinking step, before the stretching step, the first dyed film 1b' is brought into contact with a boric acid aqueous solution as a crosslinking liquid. Typically, the first dyed film 1b' is immersed in the boric acid aqueous solution (crosslinking bath). Hereinafter, the first dyed film 1b' that has been subjected to the crosslinking step will be referred to as a crosslinked film 1c. The content of boric acid in the crosslinking liquid is, for example, 1 part by mass or more, preferably 3 parts by mass or more, and for example, 10 parts by mass or less, preferably 8 parts by mass or less, relative to 100 parts by mass of water. The crosslinking liquid preferably further contains the above-mentioned iodine compound, which can prevent the iodine adsorbed to the first dyed film from eluting. The content of the iodine compound in the crosslinking liquid is, for example, 0.1 parts by mass or more, preferably 1 part by mass or more, and for example, 8 parts by mass or less, preferably 5 parts by mass or less, relative to 100 parts by mass of water. The mass ratio of boric acid to the iodine compound (boric acid:iodine compound) in the crosslinking liquid is, for example, 1:1 to 1:3, preferably 1:1.5 to 1:2. The temperature of the crosslinking bath is, for example, 20° C. or higher, preferably 30° C. or higher, and for example, 60° C. or lower, preferably 50° C. or lower. The immersion time in the crosslinking step is, for example, 5 seconds or higher, preferably 10 seconds or higher, and for example, 200 seconds or lower, preferably 60 seconds or lower.

[0032] B-5.Stretching process In the stretching step, the first dyed film 1b' (preferably the crosslinked film 1c) is stretched in the longitudinal direction in a boric acid aqueous solution as a stretching bath. Stretching the first dyed film in the boric acid aqueous solution can prevent the first dyed film from dissolving in the boric acid aqueous solution. Hereinafter, the PVA resin film after the stretching step and before the second dyeing step will be referred to as a stretched film 1d. The stretching ratio in the stretching step varies depending on whether or not an auxiliary stretching step is performed on the raw film. When the auxiliary stretching step is not performed on the raw film (i.e., when the raw film is a single-layer resin film or a resin film supported on a resin substrate), the stretching ratio in the stretching step is, for example, 4.5 to 7 times, and preferably 5 to 6.5 times. When the auxiliary stretching step is performed on the raw film (i.e., when the raw film is a PVA-based resin layer formed by coating on a resin substrate), the stretching ratio in the stretching step is, for example, 1.5 to 4 times, and preferably 1.5 to 3 times. The product of the stretching ratio in the auxiliary stretching step and the stretching ratio in the stretching step is, for example, 4.5 to 7 times, and preferably 5 to 6.5 times. By stretching at the above-mentioned stretching ratio, it is possible to impart extremely excellent optical properties to the polarizer.

[0033] The content of boric acid in the drawing liquid (boric acid aqueous solution) is, for example, 0.5 parts by mass or more, preferably 0.7 parts by mass or more, and for example, 5 parts by mass or less, preferably 3 parts by mass or less, relative to 100 parts by mass of water. The concentration of boric acid in the stretching solution (boric acid aqueous solution) is, for example, 5% by mass or less, preferably 3% by mass or less, and more preferably 1.5% by mass or less. When the concentration of boric acid in the stretching solution is equal to or less than the above upper limit, the content of boric acid in the polarizer before the humidifying step can be stably reduced even if the second dyeing time is reduced, as described below. Therefore, the production efficiency of the polarizer can be improved. The concentration of boric acid in the stretching solution is typically 0.1% by mass or more, and preferably 3% by mass or less. The stretching liquid preferably further contains the above-mentioned iodine compound, which can prevent iodine adsorbed to the first dyed film from eluting. The content of the iodine compound in the stretching solution is, for example, 0.1 part by mass or more, preferably 1 part by mass or more, and for example, 10 parts by mass or less, preferably 6 parts by mass or less, relative to 100 parts by mass of water. The mass ratio of boric acid to the iodine compound (boric acid:iodine compound) in the stretching solution is, for example, 1:0.5 to 1:1.2, preferably 1:0.6 to 1:1. The temperature of the stretching bath is, for example, 40°C or higher, preferably 60°C or higher, and, for example, 85°C or lower, preferably 80°C or lower, more preferably 65°C or lower. When the temperature of the stretching bath is not higher than the above upper limit, dissolution of the PVA-based resin layer in the stretching bath can be stably prevented even if the concentration of boric acid in the stretching bath is not higher than the above upper limit. The immersion time in the stretching step is, for example, 15 seconds to 300 seconds.

[0034] B-6.Second dyeing process In the second dyeing step, boric acid is eluted from the stretched film 1d and the stretched film 1d is dyed with the dichroic substance. Specifically, the stretched film 1d is brought into contact with a second dyeing solution to elute boric acid from the stretched film 1d and to adsorb the dichroic substance onto the stretched film 1d. Hereinafter, the PVA resin film after the second dyeing step will be referred to as a second dyed film 1b''. The second staining liquid is explained in the same manner as the first staining liquid described in Section B-3. The concentration of the dichroic substance (iodine) in the second staining liquid is, for example, 5% by mass or less, preferably 2% by mass or less, and more preferably 0.8% by mass or less. If the concentration of the dichroic substance in the second staining liquid is below the above-mentioned upper limit, the humidification time can be shortened even if the immersion time in the second staining step (second staining time) is below the upper limit described below. The concentration of the dichroic substance (iodine) in the second staining liquid is typically 0.1% by mass or more, and preferably 0.2% by mass or more. In the illustrated second dyeing step, the stretched film 1d is immersed in the second dye bath. The temperature of the second dye bath is, for example, 10°C or higher, preferably 20°C or higher, and, for example, 50°C or lower, preferably 40°C or lower. The immersion time in the second dyeing step (second dyeing time) is, for example, 30 seconds or more, preferably 1 minute or more, and for example, 300 minutes or less, preferably 200 minutes or less, more preferably 60 minutes or less, even more preferably 30 minutes or less, and particularly preferably 5 minutes or less. If the immersion time in the second dyeing step is at least as long as this lower limit, boric acid can be stably eluted from the stretched film, and the stretched film can be sufficiently dyed. If the immersion time in the second dyeing step is at most as long as this upper limit, the production efficiency of polarizers can be stably improved.

[0035] B-7.Hue adjustment process In the hue adjusting step, the second dyed film 1b'' is typically immersed in a hue adjusting bath (hue adjusting solution). Hereinafter, the second dyed film that has been subjected to the hue adjusting step will be referred to as a hue adjusted film 1e. The color-adjusting liquid is typically an aqueous solution of an iodine compound, which is a solution in which the above-mentioned iodine compound is dissolved in water. The content of the iodine compound in the hue-adjusting liquid is, for example, 0.5 parts by mass or more, preferably 2 parts by mass or more, and for example, 10 parts by mass or less, preferably 6 parts by mass or less, per 100 parts by mass of water. The color-adjusting solution is substantially free of boric acid. In this specification, "substantially free of boric acid" means that boric acid is not intentionally introduced into the color-adjusting bath, and more specifically, means that the concentration of boric acid in the color-adjusting solution is 0.1% by mass or less. The temperature of the hue adjusting bath is, for example, 0° C. or higher, preferably 10° C. or higher, and for example, 40° C. or lower, preferably 30° C. or lower. The immersion time in the hue adjusting step is, for example, 5 seconds or higher, preferably 10 seconds or higher, and for example, 200 seconds or lower, preferably 60 seconds or lower.

[0036] B-8. Drying shrinkage process In the drying shrinkage step, typically, the second dyed film 1b'' (preferably the hue adjusting film 1e) is heated while being transported in the longitudinal direction. Hereinafter, the second dyed film that has been subjected to the drying shrinkage step will be referred to as the dried shrinkage film 1f. In one embodiment, the ranges of the boric acid content and the ranges of the single-piece transmittance in the dried shrinkage film 1f are the same as the ranges of the boric acid content and the ranges of the single-piece transmittance in the PVA-based resin film after the second dyeing step described above. In the illustrated example, the drying shrinkage step is carried out by a heating and drying section 23. The heating and drying section may be a zone heating type in which the entire interior of the heating and drying section is heated, or a heating roll drying type in which the transport rolls are heated. Preferably, the heating and drying section uses both of these methods. The internal temperature of the heating and drying section is, for example, 70°C or higher, preferably 80°C or higher, and, for example, 120°C or lower, preferably 100°C or lower. The surface temperature of the heating roll is, for example, 60°C or higher, preferably 70°C or higher, and, for example, 100°C or lower, preferably 80°C or lower. Drying using a heated roll efficiently prevents the PVA resin film (laminate) from curling due to heat, enabling efficient production of a polarizer with excellent appearance. In addition, in the drying shrinkage step, the PVA resin film shrinks in the width direction perpendicular to the longitudinal direction. The shrinkage rate in the width direction of the PVA-based resin film in the drying shrinkage step is, for example, 2% or more, preferably 4% or more. If the shrinkage rate in the width direction is equal to or more than this lower limit, the orientation of the PVA and the PVA / dichroic substance complex (iodine complex) can be improved, and the optical properties of the polarizer can be improved. The shrinkage rate of the PVA-based resin film in the width direction is typically 10% or less, preferably 8% or less, and more preferably 6% or less. When the shrinkage rate in the width direction is equal to or less than the upper limit, the occurrence of defects in appearance such as wrinkles in the polarizer can be suppressed. Thereafter, the second dyed film 1b'' (preferably the dry shrinkable film 1f) is wound into a roll as required to form a take-up roll 22.

[0037] B-9. Humidification process In the humidifying step, typically, the second dyed film (laminate including the second dyed film) that has been wound up is left standing in the above-mentioned atmosphere for the above-mentioned humidifying time. The range of the boric acid content in the second dyed film immediately before the humidifying step is the same as the range of the boric acid content in the PVA-based resin film after the second dyeing step described above. The range of the single-piece transmittance (initial single-piece transmittance) of the stretched film immediately before the humidifying step is the same as the range of the single-piece transmittance of the PVA-based resin film after the second dyeing step described above. The polarization degree (initial polarization degree) of the stretched film immediately before the humidifying step is, for example, 98.0% or more, preferably 99.5% or more, and more preferably 99.8% or more. The second dyed film immediately before the humidifying step may be a second dyed film that has not been subjected to the hue adjusting step and the drying shrinkage step, or may be a second dyed film (hue adjusted film or dry shrinkage film) that has been subjected to the hue adjusting step and / or the drying shrinkage step. The second dyed film immediately before the humidifying step is preferably a dry shrinkage film that has been subjected to the hue adjusting step and the drying shrinkage step.

[0038] C.Polarizer In this way, a humidified polarizer is manufactured. More specifically, when the raw film is a single-layer resin film, a single-layer polarizer is manufactured, and when the raw film is laminated on a resin substrate, a polarizing plate having a polarizer / resin substrate configuration is manufactured. The thickness of the polarizer is, for example, 80 μm or less, preferably 15 μm or less, more preferably 12 μm or less, and even more preferably 8 μm or less. When the raw film is a PVA-based resin layer formed by coating on a resin substrate, the thickness of the polarizer can be 8 μm or less. The thickness of the polarizer is typically 1 μm or more, preferably 3 μm or more.

[0039] The polarizer preferably exhibits absorption dichroism at any wavelength between 380 nm and 780 nm. Such a polarizer has excellent single transmittance and degree of polarization. The single transmittance of the polarizer is, for example, 40.0% to 46.0%, preferably 41.0% to 46.0%, and more preferably 42.0% to 46.0%. The degree of polarization of the polarizer is preferably 97.0% or more, more preferably 99.0% or more, and even more preferably 99.5% or more. [Example]

[0040] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The methods for measuring each property are as follows.

[0041] (1) Measurement of single unit transmittance and polarization degree The resin substrate of the laminate (test piece) obtained in each Example and Comparative Example was peeled from the polarizer. Subsequently, the single transmittance (Ts), parallel transmittance (Tp), and crossed transmittance (Tc) of the polarizer film were measured using an ultraviolet-visible spectrophotometer (manufactured by JASCO Corporation, product name "V7100"). Furthermore, the polarization degree (P) of the polarizer was calculated using the following formula (1). Polarization degree (P)(%)={(Tp-Tc) / (Tp+Tc)}1 / 2×100...(1) The above Ts, Tp and Tc are Y values ​​measured under a 2-degree visual field (C light source) according to JIS Z8701 and corrected for visibility.

[0042] (2) Measurement of crystallization index In each example and comparative example, the crystallization index of the PVA-based resin layer was measured by the ATR method using a Fourier transform infrared spectrophotometer. Specifically, the measurement was carried out using polarized light as the measurement light, and the peak at 1141 cm -1 Intensity (IC) and 1440cm -1 The crystallization index was calculated using the intensity (IR) according to the following formula (2). Crystallization index = (IC / IR) (2)

[0043] (3) Measurement of the boric acid content in the polarizer before the humidification process In each example and comparative example, the content of boric acid in the polarizer before the humidifying step can be calculated as the amount of boric acid contained in the polarizer per unit mass using, for example, the neutralization method using the following formula. [Method for measuring the boric acid content (mass%) in polarizer] A polarizer (approximately 0.2 g) that had been dried at 120°C for 2 hours was dissolved in water, and a small amount of mannitol and BTB solution was added dropwise to the resulting solution. The solution was then neutralized with a 0.1 mol / L NaOH aqueous solution using a burette, and the boric acid content of the polarizer was calculated using the following formula. Boric acid content of polarizer (mass%) = C × V × Mw / M × 100 C: Concentration of NaOH solution (mol / L) V: Amount of NaOH solution dropped (L) Mw: Molecular weight of boric acid (g / mol) M: Polarizer mass (g) after drying at 120°C for 2 hours

[0044] <<Examples 1 to 5>> A long, amorphous isophthalic copolymerized polyethylene terephthalate film (thickness: 100 μm) having a Tg of about 75° C. was used as the thermoplastic resin substrate, and one side of the resin substrate was subjected to a corona treatment. A PVA aqueous solution (coating liquid) was prepared by dissolving 100 parts by mass of a PVA-based resin prepared by mixing polyvinyl alcohol (polymerization degree 4200, saponification degree 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFFIMER") in a 9:1 ratio, to which 13 parts by mass of potassium iodide was added, in water. The above PVA aqueous solution was applied to the corona treated surface of the resin substrate and dried at 60° C. to form a PVA resin layer with a thickness of 13 μm, thereby producing a laminate. The obtained laminate was uniaxially stretched in the longitudinal direction (machine direction) at an auxiliary stretching ratio of 2.4 times in an oven at 130°C (auxiliary stretching step). The PVA-based resin layer contained in the laminate after the auxiliary stretching and before the swelling step was subjected to the measurement of the crystallization index as described above. The crystallization index of the PVA-based resin layer was 1.82. Next, the laminate was immersed in an insolubilizing bath (a boric acid aqueous solution obtained by mixing 4 parts by mass of boric acid with 100 parts by mass of water) at a liquid temperature of 40°C for 30 seconds (swelling step, insolubilizing step). Next, the laminate was immersed in a first dye bath (an aqueous iodine solution obtained by mixing iodine and potassium iodide in a weight ratio of 1:7 with 100 parts by mass of water) at a liquid temperature of 30°C for 60 seconds (first dyeing step). Next, the laminate was immersed in a crosslinking bath (a boric acid aqueous solution obtained by mixing 3 parts by mass of potassium iodide and 5 parts by mass of boric acid with 100 parts by mass of water) at a liquid temperature of 40°C for 30 seconds (crosslinking step). Thereafter, the laminate was immersed in a stretching bath (a boric acid aqueous solution obtained by blending 5 parts by mass of potassium iodide with 100 parts by mass of water and blending boric acid to a concentration shown in Table 1) at a temperature shown in Table 1, and uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds to a total stretch ratio of 5.5 (stretching process). Next, the laminate was immersed in a second dye bath (an aqueous iodine solution obtained by mixing iodine and potassium iodide in a weight ratio of 1:7 with 100 parts by mass of water) at a liquid temperature of 30°C (second dyeing step) for the immersion time shown in Table 1. The iodine concentration in the second dye bath (second dyeing liquid) is shown in Table 1. Thereafter, the laminate was immersed in a hue adjusting bath (aqueous solution obtained by mixing 4 parts by mass of potassium iodide with 100 parts by mass of water) at a liquid temperature of 20°C (hue adjusting step). Thereafter, the laminate was brought into contact with a heated SUS roll whose surface temperature was maintained at about 75°C while being dried in an oven maintained at about 90°C (drying shrinkage step). In this way, a polarizer with a thickness of approximately 5 μm was formed on the resin substrate, and a polarizing plate having a polarizer / resin substrate configuration was obtained. The polarizer contained in the polarizing plate after the drying shrinkage process and before the humidification process was subjected to the measurement of the boric acid content described above. The results are shown in Table 1. Next, the polarizing plate was subjected to the above-described measurement of the single-piece transmittance and the degree of polarization, and the single-piece transmittance and the degree of polarization at a humidification time of 0 minutes (that is, the initial single-piece transmittance and the initial degree of polarization) were measured. Next, the polarizing plate was stretched across a transport roll and placed in a high-temperature, high-humidity oven maintained at a temperature of 85° C. and a humidity of 85% RH to obtain a humidified polarizing plate (humidification step). More specifically, the polarizing plate was subjected to the measurement of the single transmittance and polarization degree described above for each humidification time shown in Table 1. The single transmittance and polarization degree were measured for each humidification time. The results are shown in Table 1, Figures 3 and 4. The humidification time required for the single-piece transmittance to reach 43.5% was calculated by plotting the humidification time on the horizontal axis and the single-piece transmittance on the vertical axis as shown in Figure 4, connecting each point with a straight line, and calculating the intersection of this line with the single-piece transmittance of 43.5%, and evaluating it according to the following criteria. The results are shown in Table 1. 〇: Humidification time until the single unit transmittance reaches 43.5% is 20 minutes or less ×: The humidification time required for the single unit transmittance to reach 43.5% exceeds 20 minutes.

[0045] <<Comparative Example 1>> A humidified polarizing plate was obtained in the same manner as in Example 1, except that the second dyeing step was not carried out and the hue adjusting step was carried out on the laminate after the stretching step.

[0046] [Table 1]

[0047] [evaluation] As is clear from Table 1 and Figure 4, by performing the second dyeing process on the PVA-based resin film after the stretching process, the boron content in the PVA-based resin film after the second dyeing process can be reduced to 10% by mass or less, and the single-piece transmittance can be reduced to 35% or less. This allows moisture to smoothly penetrate into the polarizer in the humidifying process, and improves the deiodination properties of the polarizer. As a result, even if the humidifying time is shortened, it is possible to efficiently improve the single-piece transmittance of the polarizer and produce a polarizer that can suppress a decrease in the degree of polarization. [Industrial Applicability]

[0048] The manufacturing method according to the embodiment of the present invention can be suitably used for manufacturing a polarizer to be applied to an image display device. [Explanation of symbols]

[0049] 1 PVA resin film 10 Laminate 12 Thermoplastic resin base material

Claims

1. a first dyeing step of dyeing the polyvinyl alcohol-based resin film with a dichroic substance; a stretching step of stretching the polyvinyl alcohol-based resin film after the first dyeing step in an aqueous boric acid solution; a second dyeing step of dissolving boric acid from the polyvinyl alcohol-based resin film after the stretching step and dyeing the polyvinyl alcohol-based resin film with a dichroic substance; a humidifying step of humidifying the polyvinyl alcohol-based resin film after the second dyeing step in an atmosphere at a temperature of 40°C to 100°C and a humidity of 50% RH or more, the polyvinyl alcohol-based resin film after the second dyeing step has a boric acid content of 10% by mass or less and a single transmittance of 35% or less.

2. The method for producing a polarizer according to claim 1 , wherein the humidifying time in the humidifying step is 60 minutes or less.

3. a laminate preparation step of applying a coating liquid containing a polyvinyl alcohol-based resin and a halide onto a long thermoplastic resin substrate to prepare a laminate including a polyvinyl alcohol-based resin layer as the polyvinyl alcohol-based resin film and the thermoplastic resin substrate; an auxiliary stretching step of stretching the laminate in air; The first dyeing step; the stretching step; The second dyeing step; a heat shrinking step of shrinking the polyvinyl alcohol-based resin film dyed in the second dyeing step in a width direction perpendicular to the longitudinal direction while transporting the polyvinyl alcohol-based resin film in the longitudinal direction.

4. The method for producing a polarizer according to claim 3 , wherein the humidifying step is performed after the heat shrinking step.

5. In the second dyeing step, the polyvinyl alcohol-based resin film after the stretching step is immersed in a second dyeing solution containing a dichroic material, a concentration of the dichroic material in the second dye solution is 0.8% by mass or less; The method for producing a polarizer according to claim 1 , wherein the immersion time in the second dyeing step is 5 minutes or less.

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