Method for manufacturing polarizers

Abstract

To provide a method for manufacturing a polarizer that can sufficiently improve its individual transmittance and suppress a decrease in polarization degree, even when the humidification time is reduced during the humidification process. [Solution] The method for manufacturing a polarizer according to an embodiment of the present invention includes: a dyeing step of dyeing a polyvinyl alcohol-based resin film with a dichroic substance; and a stretching step of stretching the polyvinyl alcohol-based resin film after the dyeing step in a stretching bath that does not substantially contain boric acid.

Description

Technical Field

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

Background Art

[0002] Typically, a polarizer is used in an image display device. Such a polarizer is manufactured, 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 improving the optical properties (particularly the single transmittance and the degree of polarization) of polarizers. Therefore, a technique for improving the degree of polarization of a polarizer by humidifying the polarizer has been proposed (for example, Patent Document 1). However, in the technique described in Patent Document 1, there is still room for improvement in improving the single transmittance of the polarizer.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The present invention has been made to solve the above conventional problems, and its main object is to provide a method capable of manufacturing a polarizer that can sufficiently improve the single transmittance and suppress a decrease in the degree of polarization even when the humidification time is reduced when subjected to a humidification step.

Means for Solving the Problems

[0005] In the method for manufacturing a polarizer according to an embodiment of the present invention, a dyeing step of dyeing a polyvinyl alcohol-based resin film with a dichroic substance; and a stretching step of stretching the polyvinyl alcohol-based resin film after the dyeing step in a stretching bath substantially free of boric acid. In one embodiment, the boric acid content in the polyvinyl alcohol-based resin film after the stretching process is 7.0% by mass or less. In one embodiment, the method for manufacturing the polarizer further includes a crosslinking step in which the polyvinyl alcohol-based resin film after the dyeing step is brought into contact with an aqueous boric acid solution, prior to the stretching step. In one embodiment, the method for manufacturing the polarizer further includes a humidification step of humidifying the polyvinyl alcohol-based resin film after the stretching step in an atmosphere with a temperature of 40°C to 100°C and a humidity of 50% RH or higher. In one embodiment, the humidification time in the humidification process is 60 minutes or less. In one embodiment, the method for manufacturing the polarizer includes, in this order: a laminate manufacturing step of applying a coating solution containing a polyvinyl alcohol-based resin and a halide onto a long thermoplastic resin substrate to produce a laminate comprising a polyvinyl alcohol-based resin layer as a polyvinyl alcohol-based resin film and the thermoplastic resin substrate; an auxiliary stretching step of air-stretching the laminate; the dyeing step; the stretching step; and a drying shrinkage step of shrinking the laminate after the stretching step in a width direction perpendicular to the length direction while conveying it in the length direction. In one embodiment, the humidification step is performed after the heat shrinkage step. In one embodiment, the stretching ratio of the laminate in the auxiliary stretching step is 2.1 times or more. [Effects of the Invention]

[0006] According to embodiments of the present invention, it is possible to manufacture a polarizer that can sufficiently improve its single-unit transmittance and suppress a decrease in polarization degree, even when the humidification time is reduced during the humidification process. [Brief explanation of the drawing]

[0007] [Figure 1] Figure 1 is a schematic diagram illustrating a method for manufacturing a polarizer according to one embodiment of the present invention. [Figure 2]Figure 2(a) is a schematic cross-sectional view of one embodiment of the polyvinyl alcohol-based resin film shown in Figure 1. Figure 2(b) is a schematic cross-sectional view of another embodiment of the polyvinyl alcohol-based resin film shown in Figure 1. [Figure 3] Figure 3 is a graph showing the relationship between the transmittance and polarization degree in the examples and comparative examples. [Figure 4] Figure 4 is a graph showing the relationship between humidification time and individual transmittance in the examples and comparative examples. [Figure 5] Figure 5 is a graph showing the change in individual transmittance during the humidification process in the examples and comparative examples. [Modes for carrying out the invention]

[0008] The following describes representative embodiments of the present invention, but the present invention is not limited to these embodiments.

[0009] A. Overview of polarizer manufacturing method A method for producing a polarizer according to one embodiment of the present invention includes: a dyeing step of dyeing a polyvinyl alcohol-based resin film (hereinafter referred to as a PVA-based resin film) with a dichroic substance; and a stretching step of stretching the PVA-based resin film after the dyeing step in a stretching bath that is substantially free of boric acid. In this specification, "substantially free of boric acid" means that boric acid is not intentionally introduced into the stretching bath, and more specifically, that the concentration of boric acid in the stretching bath is less than 0.5% by mass. The concentration of boric acid in the stretching bath is preferably 0.3% by mass or less, more preferably 0.1% by mass or less. The inventors have found that by sufficiently humidifying the PVA resin film after the stretching process, the transmittance of the film itself can be significantly improved, and the decrease in polarization degree can be suppressed. However, a significantly long humidification time is required to sufficiently improve the transmittance of the film itself, which leads to the problem of reduced manufacturing efficiency of polarizers. In this regard, the present inventors have discovered that by stretching the PVA-based resin film after the dyeing process in a stretching bath that substantially does not contain boric acid, thereby reducing the boric acid content in the PVA-based resin film after the stretching process, the single-layer transmittance can be sufficiently improved with a remarkably short humidification time, and have completed the present invention. More specifically, when the PVA resin film after the dyeing process is stretched in a stretching bath that is substantially free of boric acid, the boric acid content in the PVA resin film after the stretching process (specifically, the PVA resin film after the stretching process and immediately before the humidification process) can be adjusted as follows. The boric acid content in the PVA resin film after the stretching process is, for example, 7.0% by mass or less, preferably 5.0% by mass or less, more preferably 3.0% by mass or less, and even more preferably 2.0% by mass or less. When the boric acid content is below the above upper limit, when the stretched PVA resin film is subjected to the humidification process, moisture can be smoothly absorbed into the PVA resin film, and dichroic substances can be smoothly eluted from the PVA resin film. Therefore, even if the humidification time is reduced, the transmittance of the PVA resin film (i.e., the polarizer) after humidification can be efficiently improved, and the decrease in the polarization degree of the polarizer can be suppressed. Typically, the boric acid content in the PVA resin film after the stretching process is 0.1% by mass or more.

[0010] In one embodiment, the method for manufacturing a polarizer further includes a crosslinking step in which the PVA resin film after the dyeing step is brought into contact with an aqueous boric acid solution before the stretching step. Bringing the PVA resin film after the dyeing step into an aqueous boric acid solution suppresses the dissolution of the PVA resin film in the stretching bath during the stretching step. Specifically, boric acid can form crosslinks by generating tetrahydroxyborate anions in the aqueous solution and forming hydrogen bonds with the PVA resin, or by forming boric acid esters through dehydration condensation with the hydroxyl groups of the PVA resin. As a result, water resistance can be imparted to the PVA resin film.

[0011] In one embodiment, the method for manufacturing a polarizer further includes a humidifying step of humidifying the PVA-based resin film after the stretching step in an atmosphere at a temperature of 40°C to 100°C and a humidity of 50%RH or more. The humidifying temperature is, for example, 50°C or higher, preferably 60°C or higher, more preferably 70°C or higher, still more preferably 80°C or higher, and for example, 90°C or lower. The humidifying humidity is preferably 70%RH or more, more preferably 80%RH or more, typically 100%RH or lower, preferably 95%RH or lower. When the PVA-based resin film after the stretching step is humidified in the above atmosphere to manufacture a polarizer, it is possible to improve the single transmittance of the polarizer while suppressing a decrease in the polarization degree of the polarizer.

[0012] In one embodiment, the humidifying time in the humidifying step is, for example, 120 minutes or less, preferably 80 minutes or less, more preferably 60 minutes or less, still more preferably 40 minutes or less, particularly preferably 20 minutes or less, most preferably 10 minutes or less, and most preferably 5 minutes or less. When the humidifying time is below such an upper limit, it is possible to stably improve the manufacturing efficiency of the polarizer. Also, the humidifying time in the humidifying step is typically 30 seconds or more, preferably 1 minute or more. When the humidifying time is above such a lower limit, it is possible to stably improve the single transmittance and polarization degree of the polarizer.

[0013] In one embodiment, the method for manufacturing a polarizer includes, in this order, a laminate manufacturing step; an auxiliary stretching step; the above-described dyeing step; the above-described stretching step; and a drying and shrinking step. In the laminate manufacturing step, a coating solution containing a PVA-based resin and a halide is applied onto a long thermoplastic resin substrate to manufacture a laminate including a PVA-based resin layer as a PVA-based resin film and the thermoplastic resin substrate. In the auxiliary stretching step, the laminate is stretched in the air. In the drying and shrinking step, the PVA-based resin film after the stretching step is shrunk in the width direction orthogonal to the longitudinal direction while being conveyed in the longitudinal direction. According to such a method, a polarizer with reduced thickness and excellent optical properties can be provided. That is, by introducing an auxiliary stretching step, even when a PVA-based resin is coated on a thermoplastic resin, it becomes possible to enhance the crystallinity of the PVA-based resin and achieve high optical properties. At the same time, by enhancing the orientation of the PVA-based resin in advance, problems such as a decrease in the orientation and dissolution of the PVA-based resin when immersed in water in subsequent dyeing and stretching steps can be prevented, and high optical properties can be achieved. Furthermore, when the PVA-based resin layer is immersed in a liquid, compared with the case where the PVA-based resin layer does not contain a halide, the disorder of the orientation of PVA-based resin molecules and the decrease in orientation can be suppressed. Thereby, the optical properties of the polarizer obtained through processing steps such as a dyeing step and a stretching step, which are performed by immersing the laminate in a liquid, can be improved. Additionally, by shrinking the PVA-based resin film in the width direction through a drying shrinkage step, the optical properties of the polarizer can be improved.

[0014] In one embodiment, 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 above such a lower limit, the orientation of the PVA-based resin layer contained in the laminate can be improved, and the dissolution of the PVA-based resin layer in the stretching bath in the stretching step can be stably suppressed. The upper limit of the stretching ratio of the laminate in the auxiliary stretching step is typically 4 times or less.

[0015] The above method for manufacturing a polarizer may include a swelling step and / or a hue adjustment step. In the swelling step, typically, the PVA-based resin film before the dyeing step is immersed in a swelling bath (swelling liquid). Thereby, foreign substances on the surface of the PVA-based resin film can be removed, and the plasticizer in the PVA-based resin film can be removed. The swelling liquid is preferably an aqueous boric acid solution containing boric acid. When the swelling liquid is an aqueous boric acid solution, water resistance can be imparted to the PVA-based resin film. In this case, the swelling step is referred to as an insolubilization step. In the hue adjustment process, typically, the PVA resin film after the stretching process is immersed in a hue adjustment bath before the humidification process. This allows the PVA resin film to be cleaned and the polarizer to be adjusted to have the desired hue.

[0016] B. Details of polarizer manufacturing method Figure 1 is a schematic diagram illustrating a method for manufacturing a polarizer according to one embodiment of the present invention. In the polarizer manufacturing method shown in the illustration, the swelling step (insolubilization step), dyeing step, crosslinking step, stretching step, hue adjustment step, and drying shrinkage step described above are carried out in succession. More specifically, a long PVA resin film 1 is conveyed from the raw material roll 21 towards the winding roll 22, and between the raw material roll 21 and the winding roll 22, a swelling process (insolubilization process), a dyeing process, a crosslinking process, a stretching process, a hue adjustment process, and a drying shrinkage process are sequentially performed on the PVA resin film 1. In one embodiment, the PVA resin film 1 is immersed in a swelling bath 2A (swelling solution), a dyeing bath 2B (dyeing solution), a crosslinking bath 2C (crosslinking solution), a stretching bath 2D (stretching solution), and a hue adjustment bath 2E (hue adjustment solution) in this order by a plurality of rollers 24, and then conveyed to pass through the heating and drying section 23. As will be described in more detail later, if the PVA resin film is included in a laminate, the laminate containing the PVA resin film is immersed in each of the above baths (each liquid) to bring the PVA resin film into contact with each bath (each liquid). Subsequently, the humidification process described above is carried out after the drying and shrinkage process.

[0017] B-1. PVA resin film In the raw material roll 21, the PVA resin film 1 (hereinafter referred to as the raw material film 11) is wound into a roll shape before each of the above processes is carried out. The crystallization index of the PVA resin in the raw film 11 is, for example, 1.6 or higher, preferably 1.8 or higher. If the crystallization index of the PVA resin is above this lower limit, even if the PVA resin film is stretched in a stretching bath that substantially does not contain boric acid, the dissolution of the PVA resin film in the stretching bath can be suppressed more stably. The crystallization index of the PVA resin is typically 3.0 or lower. The crystallization index of the PVA resin can be measured using the ATR method with a Fourier transform infrared spectrophotometer. The raw film 11 may be a single-layer resin film as shown in Figure 2(a), or it may be laminated on a thermoplastic resin substrate 12 (hereinafter referred to as resin substrate 12) as shown in Figure 2(b).

[0018] Specific examples of single-layer resin films include hydrophilic polymer films such as PVA-based films, partially formalized PVA-based films, and partially saponified ethylene-vinyl acetate copolymer films, as well as polyene-based oriented films such as dehydrated PVA products and dehydrochlorinated polyvinyl chloride products. If the raw material 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.

[0019] When the base film 11 is laminated onto a thermoplastic resin substrate 12, the base film 11 may be a PVA-based resin film supported by the resin substrate 12, or it may be a PVA-based resin layer 13 applied to the resin substrate 12. If the raw film 11 is a PVA-based resin layer 13 applied to a resin substrate 12, the PVA-based resin layer 13 is formed on the resin substrate 12 by the laminate manufacturing process described above. More specifically, a coating solution containing a PVA-based resin and a halogenated compound is applied to a long resin substrate 12 by any suitable method, and if necessary, it is dried at, for example, 50°C or higher to produce a laminate 10 comprising a PVA-based resin layer 13 and a resin substrate 12.

[0020] The constituent materials of the resin substrate 12 can be any suitable material. Typical constituent materials of the resin substrate include amorphous (non-crystallized) polyethylene terephthalate resins, and preferably amorphous (difficult to crystallize) polyethylene terephthalate resins. 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 below the above upper limit, crystallization of the PVA-based resin layer can be suppressed while ensuring sufficient stretchability of the laminate. Typically, the glass transition temperature (Tg) of the resin substrate is 60°C or higher. This suppresses defects such as deformation of the resin substrate (e.g., unevenness, sagging, or wrinkles) when the coating liquid is applied to and dried on the resin substrate. 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 (e.g., corona treatment), and an easy-adhesion layer may be formed thereon. This can improve the adhesion between the resin substrate and the PVA-based resin layer.

[0021] The coating solution is typically a solution of PVA-based resin and a halogenated compound dissolved in a solvent. Any suitable resin can be used for the PVA-based resin. Examples include polyvinyl alcohol and ethylene-vinyl alcohol copolymer. Polyvinyl alcohol is obtained by saponifying polyvinyl acetate. Ethylene-vinyl alcohol copolymer is obtained by saponifying ethylene-vinyl acetate copolymer. The degree of saponification 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 degree of saponification can be measured according to JIS K 6726-1994. By using a PVA-based resin with such a degree of saponification, a thin polarizing film with excellent durability can be obtained. If the degree of saponification is too high, there is a risk of gelation.

[0022] The average degree of polymerization of PVA resins can be appropriately selected depending on the purpose. For example, the average degree of polymerization is 1000 or higher, preferably 1500 or higher, more preferably 2000 or higher, and even more preferably 3000 or higher, and for example, 10000 or lower, preferably 6000 or lower, and even more preferably 4300 or lower. The average degree of polymerization can be measured in accordance with JIS K 6726-1994.

[0023] 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.

[0024] The PVA-based resin content in the coating solution is, for example, 3 to 20 parts by mass per 100 parts by mass of solvent. With such a resin concentration, a uniform coating film that adheres closely to the resin substrate can be formed.

[0025] Any suitable halide can be used as the halide. Typical examples of halides include iodide and sodium chloride. Examples of iodides include potassium iodide, sodium iodide, and lithium iodide, with potassium iodide being preferred. The halogen content in the coating solution is, 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, per 100 parts by mass of PVA resin. When the halogen content is within this range, the resulting polarizer will not become cloudy.

[0026] 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. Among the solvents, water is preferred. Additives may be added to the coating solution. 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.

[0027] The thickness of the PVA-based resin layer formed from such a coating solution 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.

[0028] Furthermore, the laminate 10 comprising the PVA-based resin layer 13 and the resin substrate 12 is preferably subjected to the auxiliary stretching step described above in advance and air-stretched in the longitudinal direction at the stretching ratio described above. That is, the auxiliary stretching step is performed after the laminate manufacturing step and before the dyeing step, and in the illustrated example, it is performed after the laminate manufacturing step and before the swelling step. The stretching temperature in the auxiliary stretching process is typically above the glass transition temperature (Tg) of the PVA resin, for example, 95°C or higher, preferably 120°C or higher. Alternatively, the stretching temperature in the auxiliary stretching process is typically 150°C or lower. The aerial stretching method in the auxiliary stretching process may be fixed-end stretching (for example, stretching using a tenter stretcher) or free-end stretching (for example, uniaxial stretching by passing the laminate between rolls with different peripheral speeds).

[0029] B-2. Swelling process (immobilization process) The raw film 11 described above (raw film alone or raw film included in a laminate) is subjected to the swelling process (immobilization process) described above before the dyeing process, if necessary. In the following, the PVA resin film that has undergone the swelling process (immobilization process) will be referred to as the swollen treated film 1a. In the swelling process, the raw film 11 is typically immersed in a swelling solution (swelling bath). The swelling solution may be pure water or an aqueous boric acid solution. When the swelling solution is an aqueous boric acid solution (i.e., an insolubilizing solution), the boric acid content in the insolubilizing solution is, for example, 1 to 10 parts by mass 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 process is, for example, 10 seconds or more, preferably 20 seconds or more, and for example, 200 seconds or less, preferably 60 seconds or less.

[0030] B-3. ​​Dyeing process In the dyeing process, the PVA resin film 1 (preferably a swollen film 1a) is dyed with a dichroic substance. Specifically, the dyeing solution is brought into contact with the PVA resin film 1 (preferably a swollen film 1a) to adsorb the dichroic substance. Hereinafter, the PVA resin film after the dyeing process and before the stretching process will be referred to as the dyed film 1b. Examples of dichroic substances include iodine and organic dyes. Dichroic substances can be used alone or in combination. Among dichroic substances, iodine is preferred. The staining solution is typically an iodine aqueous solution. The iodine content in the staining 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 staining solution preferably further contains an iodine compound. The inclusion of an iodine compound in the staining solution 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. Iodine compounds can be used individually or in combination. Among the iodine compounds, potassium iodide is preferred. The mass ratio of iodine to iodine compound (iodine:iodine compound) in the dyeing bath is, for example, 1:5 to 1:20, preferably 1:5 to 1:10. This can impart excellent optical properties to the polarizer.

[0031] In the dyeing process shown in the illustration, the PVA resin film 1 (swelled film 1a) is immersed in the dyeing bath. The temperature of the dyeing 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 dyeing process is, for example, 5 seconds or more, preferably 30 seconds or more, and for example, 300 seconds or less, preferably 90 seconds or less. Furthermore, if the dyeing process is carried out immediately after the insolubilization process described above, there is a risk that boric acid may be mixed from the insolubilization bath into the dyeing bath. If boric acid is mixed into the dyeing bath, the boric acid concentration in the dyeing solution will change over time, which may affect the boric acid content in the polarizer. For this reason, the boric acid content in the dyeing solution is preferably adjusted to be 4 parts by mass or less per 100 parts by mass of water. Furthermore, the method of adsorbing dichroic substances in the dyeing process is not limited to the immersion method described above. For example, the dyeing solution may be applied to the raw film, or the dyeing solution may be sprayed onto the raw film.

[0032] B-4.Crosslinking process In the crosslinking process, before the stretching process, the dyed film 1b is brought into contact with a boric acid aqueous solution, which serves as the crosslinking solution. Typically, the dyed film 1b is immersed in the boric acid aqueous solution (crosslinking bath). In the following, the dyed film 1b that has undergone the crosslinking process will be referred to as crosslinked film 1c. The boric acid content in the crosslinking solution is adjusted as appropriate to ensure that the boric acid content in the stretched film is within the above range. The boric acid content in the crosslinking solution 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, per 100 parts by mass of water. The crosslinking solution preferably further contains the iodine compound described above. The presence of an iodine compound in the crosslinking solution suppresses the elution of iodine adsorbed onto the dyed film. The content of the iodine compound in the crosslinking solution 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, per 100 parts by mass of water. The mass ratio of boric acid to the iodine compound (boric acid:iodine compound) in the crosslinking solution is, for example, 1:1 to 1:3, and 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 process is, for example, 5 seconds or more, preferably 10 seconds or more, and for example, 200 seconds or less, preferably 60 seconds or less.

[0033] B-5.Stretching process In the stretching process, the dyed film 1b (preferably the crosslinked film 1c) is stretched in the longitudinal direction in a stretching bath (stretching solution) that is substantially free of boric acid. In the following, the PVA resin film after the stretching process and before the humidification process will be referred to as the stretched film 1d. The stretching ratio in the stretching process varies depending on whether or not an auxiliary stretching process is performed on the raw film. If an auxiliary stretching process is not performed on the raw film (i.e., the raw film is a single layer resin film or a resin film supported on a resin substrate), the stretching ratio in the stretching process is, for example, 4.5 times or more and 7 times or less, preferably 5 times or more and 6.5 times or less. When an auxiliary stretching process is performed on the base film (i.e., when the base film is a PVA-based resin layer coated and formed on a resin substrate), the stretching ratio in the stretching process is, for example, 1.5 times or more and 4 times or less, preferably 1.5 times or more and 3 times or less. Furthermore, the product of the stretching ratio in the auxiliary stretching process and the stretching ratio in the stretching process is, for example, 4.5 times or more and 7 times or less, preferably 5 times or more and 6.5 times or less. By stretching at the stretching ratio described above, extremely excellent optical properties can be imparted to the polarizer.

[0034] The stretching solution is typically an aqueous solution of an iodine compound. The aqueous solution of an iodine compound is a solution in which the above-mentioned iodine compound is dissolved in water. As described above, the aqueous solution of an iodine compound substantially does not contain boric acid. The content of the iodine compound in the stretching solution 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 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. The immersion time in the stretching process is, for example, 15 seconds or more and 300 seconds or less.

[0035] B-6.Hue adjustment process In the hue adjustment process, typically, the stretched film 1d is immersed in a hue adjustment bath (hue adjustment solution). In the following, the stretched film that has undergone the hue adjustment process will be referred to as hue-adjusted film 1e. The hue adjusting solution is typically an aqueous solution of an iodine compound. The aqueous solution of an iodine compound is a solution in which the above-mentioned iodine compound is dissolved in water. The aqueous solution of an iodine compound substantially does not contain boric acid. The content ratio of the iodine compound in the hue adjusting solution 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 temperature of the hue adjustment 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 adjustment process is, for example, 5 seconds or more, preferably 10 seconds or more, and for example, 200 seconds or less, preferably 60 seconds or less.

[0036] B-7. Drying shrinkage process In the drying shrinkage process, typically, the stretched film 1d (preferably the hue adjustment film 1e) is heated while being transported in the longitudinal direction. Hereinafter, the stretched film that has undergone the drying shrinkage process will be referred to as the dried shrinkage film 1f. In one embodiment, the range of boric acid content in the dried shrinkage film 1f is the same as the range of boric acid content in the stretched film described above. In the illustrated example, the drying shrinkage process is carried out by the heating and drying section 23. The heating and drying section may be a zone heating system in which the entire interior of the heating and drying section is heated, or a heated roll drying system in which the conveying rolls are heated. Preferably, both types of heating and drying sections are used. 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. By drying using heated rolls, the heat curl of the stretched film (laminated material) can be efficiently suppressed, enabling the efficient production of polarizers with superior appearance. Furthermore, during the drying shrinkage process, the stretched film shrinks in the width direction perpendicular to the length direction. The shrinkage rate in the width direction of the stretched film during the drying shrinkage process is, for example, 2% or more, preferably 4% or more. If the shrinkage rate in the width direction is above this lower limit, the orientation of PVA and PVA / dichroic substance complex (iodine complex) can be improved, and the optical properties of the polarizer can be improved. The shrinkage rate in the width direction of the stretched film is typically 10% or less, preferably 8% or less, and more preferably 6% or less. If the shrinkage rate in the width direction is below this upper limit, it is possible to suppress the occurrence of appearance defects such as wrinkles in the polarizer. Subsequently, the stretched film 1d (preferably the dried shrinkage film 1f) is wound into a roll as needed to form a winding roll 22.

[0037] B-8. Humidification process In the humidification process, typically, the stretched film (a laminate containing the stretched film) that has been wound up is left to stand in the atmosphere described above for the humidification time described above. The range of boric acid content in the stretched film immediately before the humidification process is the same as the range of boric acid content in the PVA resin film after the stretching process described above. The transmittance of the stretched film immediately before the humidification process is, for example, 37.0% to 43.0%, preferably 39.0% to 41.0%. The degree of polarization of the stretched film immediately before the humidification process is, for example, 98.0% or higher, preferably 99.5% or higher, and more preferably 99.8% or higher. The stretched film immediately before the humidification process may be a stretched film in which the hue adjustment process and drying shrinkage process have not been performed, or it may be a stretched film in which the hue adjustment process and / or drying shrinkage process have been performed (hue adjustment film or drying shrinkage film). Preferably, the stretched film immediately before the humidification process is a drying shrinkage film in which the hue adjustment process and drying shrinkage process have been performed.

[0038] C.Polarizer As described above, a humidified polarizer is manufactured. More specifically, if the raw material film is a single layer resin film, a single layer polarizer is manufactured, and if the raw material film is laminated onto a resin substrate, a polarizer 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 base film is a PVA-based resin layer coated and formed on a resin substrate, the thickness of the polarizer can be 8 μm or less. Typically, the thickness of the polarizer is 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-element transmittance and polarization degree. The single-element 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 polarization degree of the polarizer is preferably 97.0% or higher, more preferably 99.0% or higher, and even more preferably 99.5% or higher. [Examples]

[0040] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The measurement methods for each characteristic are as follows.

[0041] (1) Measurement of individual transmittance and polarization The resin substrate of the laminate (test piece) obtained in each example and comparative example was peeled off from the polarizer. Subsequently, the single-layer transmittance (Ts), parallel transmittance (Tp), and orthogonal transmittance (Tc) of the polarizer single film were measured using a UV-Vis spectrophotometer (JASCO Corporation, product name "V7100"). Furthermore, the polarization degree (P) of the polarizer was calculated using the following equation (1). Polarization degree (P)(%)={(Tp-Tc) / (Tp+Tc)}1 / 2×100...(1) Note that the above Ts, Tp, and Tc are Y values ​​measured using a 2-degree field of view (C light source) according to JIS Z8701, and corrected for luminous efficiency.

[0042] (2) Measurement of crystallization index In each example and each 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 intensity (IC) at 1141 cm , -1 , -1 , , , , , of the obtained spectrum and the intensity (IR) at 1440 cm -1 were used to calculate the crystallization index according to the following formula (2). Crystallization index = (IC / IR) ··· (2)

[0043] (3) Measurement of the boric acid content ratio in the polarizer before the humidification process In each example and each comparative example, the boric acid content ratio in the polarizer before the humidification process can be calculated as the amount of boric acid contained in the polarizer per unit mass using, for example, the neutralization method according to the following formula. [Measurement method of boric acid content ratio in the polarizer] The polarizer (about 0.2 g) dried at 120°C for 2 hours was dissolved in water, and 0.1 mol / L NaOH aqueous solution was titrated by neutralization using a burette into an aqueous solution to which a small amount of mannitol and BTB solution was dropped, and the boric acid content ratio of the polarizer was calculated based on the following formula. Boric acid content ratio of the polarizer (mass%) = C × V × Mw / M × 100 C: Concentration of NaOH aqueous solution (mol / L) V: Dropwise amount of NaOH aqueous solution (L) Mw: Molecular weight of boric acid (g / mol) M: Mass of the polarizer after drying at 120°C for 2 hours (g) Also, in order to simply measure the boric acid content ratio, the boric acid amount index can be measured using FT-IR, and the boric acid content ratio in the polarizer can also be measured. [Measurement of boric acid content in the polarizer using FT-IR] Regarding the polarizers obtained in the examples and comparative examples, using a Fourier transform infrared spectrophotometer (FTIR) (manufactured by Perkin Elmer, trade name "frontire"), the intensity of the boric acid peak (665 cm -1 ) and the reference peak (2941 cm -1The intensity of the boric acid peak was measured. From the obtained boric acid peak intensity and reference peak intensity, the boric acid content index was calculated using the following formula. Furthermore, using the calculated boric acid content index, a calibration curve can be created with the boric acid content (mass%) obtained by titration, and the boric acid content can be calculated using FT-IR. (Borate content index) = (Borate peak 665 cm) -1 (Intensity) / (Reference peak 2941cm) -1 (Strength)

[0044] <<Examples 1 and 2>> As a thermoplastic resin substrate, an amorphous isophthalic copolymer polyethylene terephthalate film (thickness: 100 μm) in a long length with a Tg of approximately 75°C was used, and one side of the resin substrate was subjected to corona treatment. A PVA aqueous solution (coating solution) was prepared by dissolving 100 parts by mass of a PVA-based resin, which was prepared by mixing polyvinyl alcohol (degree of polymerization 4200, degree of saponification 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "Gosephymer") in a 9:1 ratio, with 13 parts by mass of potassium iodide. A PVA aqueous solution was applied to the corona-treated surface of a resin substrate and dried at 60°C to form a PVA-based resin layer with a thickness of 13 μm, thereby producing a laminate. The resulting laminate was uniaxially stretched in the longitudinal direction (longitudinal direction) in an oven at 130°C at the auxiliary stretching ratio shown in Table 1 (auxiliary stretching step). The PVA resin layer contained in the laminate after auxiliary stretching and before the swelling step was subjected to the crystallization index measurement described above. The results are shown in Table 1. Next, the laminate was immersed for 30 seconds in an insolubilization bath at a liquid temperature of 40°C (a boric acid aqueous solution obtained by mixing 4 parts by mass of boric acid with 100 parts by mass of water) (swelling step, insolubilization step). Next, the laminate was immersed for 60 seconds in a dyeing bath at a liquid temperature of 30°C (an iodine aqueous solution obtained by mixing iodine and potassium iodide in a weight ratio of 1:7 with 100 parts by mass of water) (dyeing process). Next, the laminate was immersed for 30 seconds in a crosslinking bath at a liquid temperature of 40°C (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) (crosslinking step). Subsequently, the laminate was immersed in a stretching bath at a liquid temperature of 70°C that was substantially free of boric acid (an aqueous solution obtained by mixing 5 parts by mass of potassium iodide with 100 parts by mass of water; boric acid concentration 0% by mass), and uniaxial stretching was performed between rolls with different peripheral speeds so that the total stretching ratio in the longitudinal direction was 5.5 times (stretching process). Subsequently, the laminate was immersed in a hue adjustment bath at a liquid temperature of 20°C (an aqueous solution obtained by mixing 4 parts by mass of potassium iodide with 100 parts by mass of water) (hue adjustment step). Subsequently, the laminate was dried in an oven maintained at approximately 90°C while being brought into contact with a SUS (stainless steel) heated roll whose surface temperature was maintained at approximately 75°C (drying shrinkage process). In this way, a polarizer with a thickness of approximately 5 μm was formed on a resin substrate, and a polarizer plate having a polarizer / resin substrate configuration was obtained. The polarizer contained in the polarizer 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 plates were subjected to the measurement of the individual transmittance and polarization degree described above, and the individual transmittance and polarization degree were measured at a humidification time of 0 minutes. Next, with the polarizing plate stretched over a conveyor roll, it was placed in a high-temperature, high-humidity oven maintained at a temperature of 60°C and a humidity of 90% RH to obtain a humidified polarizing plate (humidification step). More specifically, the polarizing plates were subjected to the above-mentioned individual transmittance and polarization degree measurements at each humidification time shown in Table 1. This allowed for the measurement of individual transmittance and polarization degree at each humidification time. The results are shown in Table 1, Figure 3, and Figure 4. Furthermore, the change in the transmittance of the individual components ΔTs during the humidification process was calculated and evaluated according to the following criteria. The results are shown in Table 1 and Figure 5. Humidification time from 0 to 20 minutes; ○: Change in single-element transmittance ΔTs is 1.2% or more. ×: Change in single-unit transmittance ΔTs is less than 1.2% Humidification time from 0 to 40 minutes; ○: Change in single-element transmittance ΔTs is 1.5% or more. ×: Change in single-unit transmittance ΔTs is less than 1.5% Humidification time ranges from 0 to 60 minutes; ○: Change in single-element transmittance ΔTs is 2.0% or more. ×: Change in single-unit transmittance ΔTs is less than 2.0%

[0045] <<Comparative Examples 1-3>> A polarizing plate having a polarizer / resin substrate configuration was obtained in the same manner as in Example 2, except that the boric acid was dissolved in the stretching bath so that the boric acid concentration was as shown in Table 1.

[0046] [Table 1]

[0047] [evaluation] As is clear from Table 1, Figures 4 and 5, by stretching a PVA-based resin film in a stretching bath that substantially does not contain boric acid, the boron content in the manufactured polarizer can be reduced to 7.0% by mass or less. This allows moisture to penetrate the polarizer smoothly during the humidification process and improves the deiodination properties of the polarizer. As a result, even with reduced humidification time, it is possible to efficiently improve the transmittance of the polarizer and manufacture a polarizer that suppresses a decrease in polarization degree. [Industrial applicability]

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

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

Claims

[Claim 1] A dyeing process in which a polyvinyl alcohol-based resin film is dyed with a dichroic substance, A method for producing a polarizer, comprising a stretching step of stretching the polyvinyl alcohol-based resin film after the dyeing step in a stretching bath that substantially does not contain boric acid.

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