Manufacturing methods for polarizing components and polarizing plates

Abstract

A method for manufacturing a polarizing element and a polarizing plate is provided, wherein the polarizing element and polarizing plate are capable of suppressing display unevenness in an image display device. The method for manufacturing a polarizing element according to an embodiment of the present invention includes: a dyeing step, wherein a polyvinyl alcohol-based resin film is dyed using a dichroic substance; a crosslinking step, wherein the polyvinyl alcohol-based resin film after the dyeing step is contacted with an aqueous boric acid solution; and a stretching step, wherein the polyvinyl alcohol-based resin film after the crosslinking step is stretched in a stretching bath. The ratio of the orthogonal absorbance A580_c of the polyvinyl alcohol-based resin film at a wavelength of 580 nm after the crosslinking step and before the stretching step to the orthogonal absorbance A580_b of the polyvinyl alcohol-based resin film at a wavelength of 580 nm after the dyeing step and before the crosslinking step is 0.91 or higher.

Description

Technical Field

[0001] This invention relates to a method for manufacturing polarizing elements and a method for manufacturing polarizing plates. Background Technology

[0002] In image display devices, polarizing elements are typically used. For example, such polarizing elements are formed by, for instance, by, forming a polyvinyl alcohol (PVA) resin layer on a resin substrate, immersing a laminate containing the resin substrate and the PVA resin layer in an aqueous boric acid solution to prevent the PVA resin layer from dissolving, dyeing the PVA resin layer with a dichroic substance, and then stretching the laminate in an aqueous boric acid solution, thereby forming it on a resin substrate (e.g., Patent Document 1).

[0003] In recent years, there has been a demand for higher resolution image display devices using polarizing elements, with a desire for further reduction in display unevenness. However, for image display devices equipped with the polarizing element described in Patent Document 1, there is still room for improvement in reducing display unevenness.

[0004] Existing technical documents

[0005] Patent Literature

[0006] Patent Document 1: International Publication No. 2010 / 100917 Summary of the Invention

[0007] The problem the invention aims to solve

[0008] The present invention was made to solve the above-mentioned problems, and its main objective is to provide a method for manufacturing a polarizer and a polarizing plate that can suppress display unevenness in an image display device.

[0009] Solution for solving the problem

[0010] The method for manufacturing a polarizing element according to an embodiment of the present invention includes: a dyeing step, in which a polyvinyl alcohol-based resin film is dyed using a dichroic substance; a crosslinking step, in which the polyvinyl alcohol-based resin film after the dyeing step is contacted with an aqueous boric acid solution; and a stretching step, in which the polyvinyl alcohol-based resin film after the crosslinking step is stretched in a stretching bath. The ratio of the orthogonal absorbance A580_c of the polyvinyl alcohol-based resin film at a wavelength of 580 nm after the crosslinking step and before the stretching step to the orthogonal absorbance A580_b of the polyvinyl alcohol-based resin film at a wavelength of 580 nm after the dyeing step and before the crosslinking step is 0.91 or higher.

[0011] In one embodiment, a polarizer with a monomer transmittance of 42% or more and 45% or less is manufactured.

[0012] In one embodiment, the method for manufacturing the polarizing element includes, in sequence: a laminate fabrication step, in which a coating liquid containing a polyvinyl alcohol resin and a halide is coated onto a strip-shaped thermoplastic resin substrate to fabricate a laminate having a polyvinyl alcohol resin layer and the aforementioned thermoplastic resin substrate as the aforementioned polyvinyl alcohol resin film; an auxiliary stretching step, in which the aforementioned laminate is stretched in the air; the aforementioned dyeing step; the aforementioned crosslinking step; the aforementioned stretching step; and a drying and shrinking step, in which the polyvinyl alcohol resin layer after the aforementioned stretching step is conveyed along the length direction while being shrunk along the width direction orthogonal to the aforementioned length direction.

[0013] In one embodiment, the thermoplastic resin substrate is a polyethylene terephthalate film.

[0014] In one embodiment, a polarizing element with a thickness of less than 12 μm is manufactured.

[0015] In one embodiment, the stretching bath is an aqueous solution of boric acid.

[0016] Another aspect of the invention involves a method for manufacturing a polarizing plate, which includes attaching a protective film to a polarizing element manufactured using the above-described method.

[0017] The effects of the invention

[0018] According to embodiments of the present invention, polarizing elements and polarizing plates capable of suppressing display unevenness in an image display device can be manufactured. Attached Figure Description

[0019] Figure 1 This is a schematic diagram illustrating a method for manufacturing a polarizing element according to one embodiment of the present invention.

[0020] Figure 2 middle, Figure 2 (a) is Figure 1 A cross-sectional schematic diagram of one embodiment of the polyvinyl alcohol-based resin film is shown. Figure 2 (b) is Figure 1 A cross-sectional schematic diagram of another embodiment of the polyvinyl alcohol-based resin film is shown.

[0021] Description of Reference Numerals

[0022] 1 PVA-based resin film

[0023] 10-layer stack

[0024] 12 Thermoplastic resin substrate Detailed Implementation

[0025] The following describes representative embodiments of the present invention, but the present invention is not limited to these embodiments.

[0026] A. Overview of the manufacturing method of polarizing elements

[0027] A method for manufacturing a polarizing element according to one embodiment of the present invention includes: a dyeing step, in which a polyvinyl alcohol-based resin film (hereinafter referred to as a PVA-based resin film) is dyed using a dichroic substance; a crosslinking step, in which the PVA-based resin film after the dyeing step is contacted with an aqueous boric acid solution; and a stretching step, in which the PVA-based resin film after the crosslinking step is stretched in a stretching bath. The ratio (A580_c / A580_b) of the orthogonal absorbance A580_c of the PVA-based resin film at a wavelength of 580 nm after the crosslinking step and before the stretching step is 0.91 or more, preferably 0.92 or more, more preferably 0.97 or more, and extremely preferably 0.99 or more.

[0028] For image display devices using polarizers dyed with dichroic substances, uneven dyeing in the polarizer can cause uneven display in the image display device. Therefore, the inventors have conducted in-depth research on improving the uniformity of dyeing in the polarizer and discovered that adjusting the ratio of orthogonal absorbance before and after the crosslinking process to a specific range can suppress potential stripe-like dyeing unevenness in the polarizer, thereby improving the uniformity of dyeing in the polarizer, and thus completing the present invention. More specifically, when A580_c / A580_b is at or above the aforementioned lower limit, dyeing unevenness in the polarizer can be significantly suppressed.

[0029] In one embodiment, the upper limit of A580_c / A580_b is typically 2.5 or less, preferably 1.50 or less, and more preferably 1.15 or less. When A580_c / A580_b is below the above-mentioned upper limit, uneven dyeing in the polarizer can be suppressed more significantly.

[0030] In addition, the orthogonal absorbance is calculated based on the orthogonal transmittance Tc described later using the following formula.

[0031] Orthogonal absorbance = log10(100 / Tc)

[0032] In one embodiment, the PVA-based resin film is not contacted with the boric acid aqueous solution before the dyeing process, but is instead contacted with the dyeing bath (dyeing solution) during the dyeing process. When the PVA-based resin film is contacted with the boric acid aqueous solution, the boric acid forms crosslinks, thus imparting water resistance to the PVA-based resin film. Therefore, in the manufacturing method of polarizing components, the PVA-based resin film is typically contacted with the boric acid aqueous solution before the dyeing process to inhibit the dissolution of the PVA-based resin film in the dyeing solution.

[0033] However, when the PVA-based resin film is pre-contaminated with a boric acid aqueous solution, boric acid may detach from the PVA-based resin film and dissolve into the dyeing solution during the dyeing process. This could potentially affect the distribution of the dichroic substance complex with the PVA-based resin formed during the dyeing process. Therefore, it is speculated that this will adversely affect the uniformity of dyeing in the polarizing element.

[0034] On the other hand, when the PVA-based resin film is not in contact with the boric acid aqueous solution before the dyeing process, but in contact with the dyeing solution, the complex of the dichroic substance and the PVA-based resin can be evenly distributed, and the uneven dyeing in the polarizer can be stably suppressed.

[0035] In one embodiment, the method for manufacturing a polarizing element sequentially includes: a laminate fabrication step; an auxiliary stretching step; the aforementioned dyeing step; the aforementioned crosslinking step; the aforementioned stretching step; and a drying and shrinking step. In the laminate fabrication step, a coating liquid containing a PVA-based resin and a halide is applied to a strip-shaped thermoplastic resin substrate to fabricate a laminate having a PVA-based resin layer and a thermoplastic resin substrate as a PVA-based resin film. In the auxiliary stretching step, the laminate is stretched in the air. In the drying and shrinking step, the PVA-based resin layer after the stretching step is conveyed along its length direction while being shrunk along its width direction, which is orthogonal to the length direction.

[0036] According to this method, polarizing elements with reduced thickness and excellent optical properties can be provided. Specifically, by incorporating an auxiliary stretching process, the crystallinity of the PVA-based resin can be improved even when coating a thermoplastic resin with PVA-based resin, resulting in high optical properties. Furthermore, by simultaneously improving the orientation of the PVA-based resin beforehand, problems such as decreased orientation and dissolution of the PVA-based resin during subsequent dyeing and immersion in liquid can be prevented, achieving high optical properties again. Additionally, in the crosslinking process, contacting the PVA-based resin layer with a boric acid aqueous solution imparts water resistance to the PVA-based resin layer, suppressing its dissolution in the stretching bath during the subsequent stretching process. Moreover, when the PVA-based resin layer is immersed in liquid, compared to when the PVA-based resin layer does not contain halides, orientation disorder and decreased orientation of PVA-based resin molecules can be suppressed. Therefore, the optical properties of polarizing elements obtained through processing steps such as dyeing and stretching in which the laminate is immersed in liquid can be improved. Furthermore, by drying and shrinking the PVA-based resin film along its width, the optical properties of the polarizer can be improved.

[0037] The manufacturing method of the aforementioned polarizing element may include a hue adjustment process.

[0038] In the hue adjustment process, typically, a PVA-based resin film after the stretching process (preferably a PVA-based resin film after the stretching process and before the drying and shrinking process) is immersed in a hue adjustment bath. This allows the PVA-based resin film to be cleaned and the hue adjusted so that the polarizer has the desired hue.

[0039] B. Details of the manufacturing method for polarizing elements

[0040] Figure 1 This is a schematic diagram illustrating a method for manufacturing a polarizing element according to one embodiment of the present invention. In the manufacturing method of the polarizing element illustrated in the figure, the above-described dyeing process, cross-linking process, stretching process, hue adjustment process, and drying and shrinkage process are performed continuously.

[0041] More specifically, a long strip of PVA-based resin film 1 is conveyed from the original roll 21 to the take-up roll 22. Between the original roll 21 and the take-up roll 22, the PVA-based resin film 1 undergoes a dyeing process, a crosslinking process, a stretching process, a hue adjustment process, and a drying and shrinking process in sequence. In one embodiment, the PVA-based resin film 1 is conveyed by a heating and drying section 23 after being sequentially immersed in 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) using multiple rollers 24. It should be noted that, as will be explained later, when the PVA-based resin film is a PVA-based resin layer contained in a laminate, the PVA-based resin layer is brought into contact with each of the aforementioned baths (liquids) by immersing the laminate containing the PVA-based resin layer in each of the aforementioned baths (liquids).

[0042] B-1. PVA-based resin film

[0043] In the original roll 21, the PVA-based resin film 1 (hereinafter referred to as the original roll film 11) before the above-mentioned processes are carried out is wound into a roll.

[0044] The crystallinity index of the PVA-based resin in the original film 11 is, for example, 1.6 or higher, preferably 1.8 or higher. When the crystallinity index of the PVA-based resin is at or above such a lower limit, even if the PVA-based resin film is not in contact with the boric acid aqueous solution but is immersed in the dyeing bath, the dissolution of the PVA-based resin film in the dyeing solution can be suppressed. The crystallinity index of the PVA-based resin is typically 3.0 or lower. The crystallinity index of the PVA-based resin can be determined using a Fourier transform infrared spectrophotometer by the ATR method.

[0045] Original film 11 can be as Figure 2 As shown in (a), it is a single-layer resin film, or it can be as shown in (a). Figure 2 As shown in (b), it is laminated onto the thermoplastic resin substrate 12 (hereinafter referred to as resin substrate 12).

[0046] 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-oriented films such as dehydrated PVA products and dehydrochlorinated polyvinyl chloride products.

[0047] When the original film 11 is a single-layer resin film, its thickness is, for example, 20 μm or more, preferably 30 μm or more, for example, 65 μm or less, preferably 60 μm or less.

[0048] It should be noted that when the original film 11 is a single-layer resin film or a resin film supported by a resin substrate, an insolubility process can be performed before the dyeing process to impart water resistance to the PVA-based resin film by contacting it with a boric acid aqueous solution. The boric acid content in the boric acid aqueous solution 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 boric acid aqueous solution is, for example, 10°C or more and 60°C or less. The contact time between the PVA-based resin film and the boric acid aqueous solution is, for example, 10 seconds or more and 200 seconds or less. Furthermore, the PVA-based resin film can also be stretched during the insolubility process.

[0049] When the original film is a single-layer resin film or a resin film supported by a resin substrate, even with insoluble treatment, A580_c / A580_b can be stably adjusted to the above range, ensuring that the uniformity of dyeing in the polarizer is at a practical level. However, as described above, avoiding contact with the boric acid aqueous solution before the dyeing process can further reduce dyeing unevenness in the polarizer, which is therefore preferable.

[0050] When the original roll film 11 is laminated on the thermoplastic resin substrate 12, the original roll film 11 can be a PVA-based resin film supported by the resin substrate 12, or it can be a PVA-based resin layer 13 coated on the resin substrate 12.

[0051] When the original film 11 is a PVA-based resin layer 13 formed by coating on a resin substrate 12, the PVA-based resin layer 13 is formed on the resin substrate 12.

[0052] More specifically, a coating liquid containing PVA-based resin and halogen is applied to a strip-shaped resin substrate 12 using any suitable method, and then dried at, for example, 50°C or above as needed, thereby producing a laminate 10 having a PVA-based resin layer 13 and a resin substrate 12.

[0053] The resin substrate 12 can be made of any suitable material. A typical example of a resin substrate is a polyethylene terephthalate film. Examples of its constituent materials include amorphous (uncrystallized) polyethylene terephthalate resins, and more preferably amorphous (non-crystallizing) polyethylene terephthalate resins. Specific examples of amorphous polyethylene terephthalate resins include copolymers further comprising isophthalic acid as a dicarboxylic acid, and copolymers further comprising cyclohexanediol as a diol.

[0054] The glass transition temperature (Tg) of the resin substrate is, for example, 170°C or less, preferably 120°C or less. When the Tg of the resin substrate is below the above-mentioned upper limit, crystallization of the PVA-based resin layer can be suppressed, and the tensile strength of the laminate can be sufficiently ensured. In addition, the glass transition temperature (Tg) of the resin substrate is typically 60°C or more. Therefore, when the coating liquid is applied to the resin substrate and dried, defects such as deformation of the resin substrate (e.g., unevenness, relaxation, wrinkling) can be suppressed. It should be noted that the glass transition temperature (Tg) is measured according to JIS K 7121.

[0055] The thickness of the resin substrate before stretching is, for example, 20 μm or more, preferably 50 μm or more, for example, 300 μm or less, preferably 200 μm or less.

[0056] The surface of the resin substrate can undergo any suitable surface treatment (e.g., corona treatment) to form an easy-to-adhere layer. This improves the adhesion between the resin substrate and the PVA-based resin layer.

[0057] The coating solution is typically a solution made by dissolving PVA-based resin and halides in a solvent.

[0058] PVA-based resins can be any suitable resin. Examples include polyvinyl alcohol and ethylene-vinyl alcohol copolymers. Polyvinyl alcohol is obtained by saponifying polyvinyl acetate. Ethylene-vinyl alcohol copolymers are obtained by saponifying ethylene-vinyl acetate copolymers. The degree of saponification of PVA-based resins is typically 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 determined according to JIS K 6726-1994. By using PVA-based resins with such a degree of saponification, thin polarizing films with excellent durability can be obtained. Excessive saponification may lead to gelation.

[0059] The average degree of polymerization of PVA-based resins can be appropriately selected according to the purpose. For example, the average degree of polymerization is 1000 or more, preferably 1500 or more, more preferably 2000 or more, and even more preferably 3000 or more; for example, it is 10000 or less, preferably 6000 or less, and even more preferably 4300 or less. It should be noted that the average degree of polymerization can be determined according to JIS K 6726-1994.

[0060] In one embodiment, the PVA-based resin may contain acetyl-modified PVA. The content of acetyl-modified PVA in the PVA-based resin is, for example, 5% by mass or more, preferably 8% by mass or more, for example, 20% by mass or less, preferably 12% by mass or less. If the PVA-based resin contains acetyl-modified PVA, the mechanical strength of the polarizing element can be improved.

[0061] The PVA-based resin content in the coating solution is, for example, 3 to 20 parts by mass relative to 100 parts by mass of the solvent. At such a resin concentration, a uniform coating film adhering closely to the resin substrate can be formed.

[0062] Any suitable halide can be used as the halide. Representative examples of halides include iodides and sodium chloride. Examples of iodides include potassium iodide, sodium iodide, and lithium iodide, with potassium iodide being a preferred example.

[0063] The content of halides 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, relative to 100 parts by mass of PVA-based resin. If the content of halides is within such a range, it is possible to suppress the occurrence of whitening in the final polarizing element.

[0064] Examples of solvents include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyols such as trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine. These can be used alone or in combination. Water is a preferred solvent.

[0065] Additives may be incorporated into the coating solution. Examples of additives include plasticizers and surfactants. Examples of plasticizers include polyols such as ethylene glycol and glycerin. Examples of surfactants include nonionic surfactants.

[0066] The thickness of the PVA-based resin layer formed by such a coating liquid before stretching is, for example, 3 μm or more, preferably 5 μm or more, for example, 40 μm or less, preferably 30 μm or less.

[0067] Furthermore, the laminate 10 comprising the PVA-based resin layer 13 and the resin substrate 12 is preferably pre-stretched in the air along its length at a predetermined stretch ratio. That is, the auxiliary stretching process is performed after the laminate fabrication process and before the dyeing process.

[0068] The stretching temperature in the auxiliary stretching process is typically above the glass transition temperature (Tg) of the PVA-based resin, for example, 95°C or higher, preferably 120°C or higher. The stretching temperature in the auxiliary stretching process is typically below 150°C.

[0069] The stretching ratio of the laminate in the auxiliary stretching process is 2.1 times or more, preferably 2.3 times or more. When the stretching ratio of the laminate is above this 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 dyeing solution can be stably suppressed. It should be noted that the upper limit of the stretching ratio of the laminate in the auxiliary stretching process is typically 4 times or less.

[0070] The aerial stretching method in the auxiliary stretching process can be fixed-end stretching (e.g., stretching using a tenter frame) or free-end stretching (e.g., unidirectional stretching by passing the laminate through rollers with different circumferential speeds).

[0071] B-2. Dyeing Process

[0072] In the dyeing process, the PVA-based resin film 1 (original roll film 11) is dyed using a dichroic substance. Specifically, the dyeing solution is brought into contact with the PVA-based resin film 1, causing the dichroic substance to be adsorbed. The dichroic substance forms a complex with the PVA-based resin. In the dyeing process illustrated in the figure, the PVA-based resin film 1 is immersed in a dyeing bath (dyeing solution). Hereinafter, the PVA-based resin film after the dyeing process and before the crosslinking process will be referred to as dyed film 1b.

[0073] Examples of dichroic substances include iodine and organic dyes. Dichroic substances can be used alone or in combination. Iodine is a preferred example among dichroic substances.

[0074] The staining solution is typically an aqueous solution of iodine. 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, relative to 100 parts by mass of water.

[0075] The staining solution preferably also contains iodine compounds. When the staining solution contains iodine compounds, the solubility of iodine in water can be increased.

[0076] 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 alone or in combination. Potassium iodide is a preferred example among iodine compounds.

[0077] The mass ratio of iodine to iodine compound in the dyeing solution (iodine:iodine compound) is, for example, 1:5 to 1:20, preferably 1:5 to 1:10. This imparts excellent optical properties to the polarizing element.

[0078] Alternatively, the dyeing solution may contain boric acid. The concentration of boric acid in the dyeing solution is, for example, 1.0% by mass or less, preferably 0.5% by mass or less, more preferably 0.45% by mass or less, extremely preferably 0.30% by mass or less, for example, 0% by mass or more, preferably 0.1% by mass or more. When the concentration of boric acid in the dyeing solution is within the above range, uneven dyeing in the polarizer can be suppressed more stably.

[0079] The temperature of the dyeing bath is, for example, 10°C or higher, preferably 20°C or higher, for example, 50°C or lower, preferably 40°C or lower. The immersion time in the dyeing process is, for example, 5 seconds or higher, preferably 30 seconds or higher, for example, 300 seconds or lower, preferably 90 seconds or lower, more preferably 60 seconds or lower.

[0080] It should be noted that the adsorption method for dichroic substances in the dyeing process is not limited to the above-mentioned impregnation. For example, the original roll of film can be coated with the above-mentioned dyeing solution, or the original roll of film can be sprayed with the above-mentioned dyeing solution.

[0081] In addition, when the original film is a single-layer resin film or a resin film supported by a resin substrate, the PVA-based resin film can also be stretched during the dyeing process.

[0082] B-3. ​​Crosslinking process

[0083] In the crosslinking process, the dyed film 1b is contacted with an aqueous boric acid solution, which serves as the crosslinking liquid. Typically, the dyed film 1b is immersed in the aqueous boric acid solution (crosslinking bath). Hereinafter, the PVA-based resin film after the crosslinking process and before the stretching process will be referred to as the crosslinked film 1c.

[0084] When the dyed film is contacted with an aqueous boric acid solution, the boric acid can crosslink by forming a tetrahydroxyborate anion in the aqueous solution and forming hydrogen bonds with the PVA-based resin, or by undergoing dehydration condensation with the hydroxyl groups of the PVA-based resin to form a borate ester. This suppresses the dissolution of the PVA-based resin and dichroic substances (preferably iodine). Therefore, the ratio (A580_c / A580_b) of the crosslinked film 1c at 580 nm to the crosslinked absorbance A580_b of the dyed film 1b at 580 nm can be adjusted to at least the aforementioned lower limit.

[0085] In one embodiment, the ratio (A480_c / A480_b) of the crosslinked film 1c at a wavelength of 480 nm to the crosslinked absorbance A480_b of the dyed film 1b at a wavelength of 480 nm is typically 1.10 or more, preferably 1.20 or more, and more preferably 1.24 or more. It should be noted that the upper limit of A480_c / A480_b is typically 2.5 or less, preferably 2.1 or less, and more preferably 1.50 or less. When A580_c / A580_b is above the aforementioned lower limit, and A480_c / A480_b is above the aforementioned lower limit and / or below the aforementioned upper limit, uneven dyeing in the polarizer can be suppressed more significantly.

[0086] The ratio of boric acid in the crosslinking solution to water is, for example, 1 part by mass or more, preferably 3 parts by mass or more, for example, 10 parts by mass or less, preferably 8 parts by mass or less.

[0087] The crosslinking solution preferably also contains the aforementioned iodine compound. If the crosslinking solution contains an iodine compound, it can inhibit the dissolution of iodine adsorbed on the dyed film.

[0088] The iodine compound content in the crosslinking solution is, for example, 0.1 parts by mass or more, preferably 1 part by mass or more, 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 iodine compound to boric acid in the crosslinking solution (iodine compound: boric acid) is, for example, 1:1 to 1:3, preferably 1:1.5 to 1:2.

[0089] The temperature of the crosslinking bath is, for example, 20°C or higher, preferably 30°C or higher, for example, 60°C or lower, preferably 50°C or lower. The immersion time in the crosslinking process is, for example, 5 seconds or higher, preferably 10 seconds or higher, for example, 200 seconds or lower, preferably 60 seconds or lower.

[0090] In addition, when the original film is a single-layer resin film or a resin film supported by a resin substrate, the PVA-based resin film can also be stretched during the crosslinking process.

[0091] B-4. Stretching process

[0092] In the stretching process, the crosslinked film 1c is stretched along its length in a stretching bath (stretching liquid). Hereinafter, the PVA-based resin film after the stretching process will be referred to as the stretched film 1d.

[0093] The stretching ratio in the stretching process varies depending on whether an auxiliary stretching process is performed on the original roll film. When no auxiliary stretching process is performed on the original roll film (i.e., when the original roll film is a single-layer resin film or a resin film supported by 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. It should be noted that when the PVA-based resin film is stretched in the dyeing process and / or crosslinking process (and consequently, the insoluble process as needed), the stretching ratio in the stretching process is adjusted such that the product of the stretching ratios in these processes is within the above-mentioned range.

[0094] When an auxiliary stretching process is performed on the original roll film (i.e., when the original roll film is a PVA-based resin layer coated 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.

[0095] By stretching the polarizer at the stretching ratio described above, extremely superior optical properties can be imparted to it.

[0096] The stretching fluid is typically an aqueous solution of boric acid. The ratio of boric acid in the stretching fluid (aqueous solution of boric acid) relative to 100 parts by mass of water is, for example, 1 part by mass or more, preferably 3 parts by mass or more, for example, 10 parts by mass or less, preferably 8 parts by mass or less.

[0097] The stretching fluid preferably also contains the aforementioned iodine compound. When the stretching fluid contains an iodine compound, it can inhibit the dissolution of iodine adsorbed on the cross-linked film.

[0098] The content of iodine compound in the stretching solution is, for example, 0.1 parts by mass or more, preferably 1 part by mass or more, 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 iodine compound in the stretching solution (boric acid: iodine compound) is, for example, 1:0.5 to 1:1.2, preferably 1:0.6 to 1:1.

[0099] 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.

[0100] B-5. Hue Adjustment Process

[0101] In the hue adjustment process, the stretched film 1d is typically immersed in a hue adjustment bath (hue adjustment solution). Hereinafter, the film in which the hue adjustment process has been performed during the stretching process will be referred to as hue adjustment film 1e.

[0102] A typical hue-adjusting solution is an aqueous solution of an iodine compound. This aqueous solution is obtained by dissolving the aforementioned iodine compound in water. The aqueous solution of the iodine compound is substantially free of boric acid. The content of the iodine compound in the hue-adjusting solution relative to 100 parts by mass of water is, for example, 0.5 parts by mass or more, preferably 2 parts by mass or more, for example, 10 parts by mass or less, preferably 6 parts by mass or less.

[0103] The temperature of the hue adjustment bath is, for example, 0°C or higher, preferably 10°C or higher, 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 higher, preferably 10 seconds or higher, for example, 200 seconds or lower, preferably 60 seconds or lower.

[0104] B-6. Drying and Shrinking Process

[0105] In the drying and shrinking process, typically, the stretched film 1d (preferably the color-adjusting film 1e) is heated while being conveyed along its length. Hereinafter, the film in which the drying and shrinking process has been performed in the stretched film will be referred to as the drying and shrinking film 1f.

[0106] In the example shown, the drying and shrinking process is performed by a heated drying section 23. The heated drying section can be a region-based heating method where the entire interior of the section is heated, or a heated roller drying method where the conveyor rollers are heated. Both methods are preferred.

[0107] The internal temperature of the heating and drying section is, for example, 70°C or higher, preferably 80°C or higher, for example, 120°C or lower, preferably 100°C or lower.

[0108] The surface temperature of the heating roller is, for example, 60°C or higher, preferably 70°C or higher, for example, 100°C or lower, preferably 80°C or lower.

[0109] By using heated rollers for drying, heat warping of the stretched film (laminated film) can be effectively suppressed, resulting in the efficient manufacture of polarizing parts with excellent appearance. Furthermore, during the drying shrinkage process, the stretched film shrinks along its width direction, orthogonal to its length direction, through contact with the heated rollers.

[0110] The shrinkage rate in the width direction of the stretched film during the drying and shrinking process is, for example, 2% or more, preferably 4% or more. When the shrinkage rate in the width direction is at or above this lower limit, the orientation of PVA and PVA / dichroic material complex (iodine complex) can be improved, and the optical properties of the polarizer can be improved.

[0111] 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. When the shrinkage rate in the width direction is below this upper limit, it is possible to suppress appearance defects such as wrinkles in the polarizing element.

[0112] Then, the stretched film 1d (preferably the dried shrink film 1f) is wound into a roll as needed to form a roll 22.

[0113] C. Polarizing components

[0114] The above methods can be used to manufacture polarizing elements. More specifically, when the original film is a single-layer resin film, a single-layer polarizing element can be manufactured; when the original film is laminated on a resin substrate, a polarizing plate with a polarizing element / resin substrate structure can be manufactured (i.e., the resin substrate is used as a protective layer for the polarizing element).

[0115] The thickness of the polarizing element 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 original film is a PVA-based resin layer formed by coating a resin substrate, the thickness of the polarizing element (i.e., the PVA-based resin layer) can be 12 μm or less, preferably 8 μm or less. It should be noted that the thickness of the polarizing element is typically 1 μm or more, preferably 3 μm or more.

[0116] The polarizer preferably exhibits absorption dichroism at any wavelength from 380 nm to 780 nm. Such a polarizer has excellent monomer transmittance and polarization. The monomer transmittance of the polarizer is, for example, 41.0% or more and 46.0% or less, preferably 42.0% or more and 45.0% or less. The polarization of the polarizer is preferably 97.0% or more, more preferably 99.0% or more, and even more preferably 99.5% or more. According to the above-described method for manufacturing the polarizer, uneven dyeing can be significantly suppressed in polarizers with monomer transmittance and polarization within the above-described ranges.

[0117] Alternatively, a polarizing element can be manufactured by laminating any suitable protective film corresponding to the purpose onto the polarizing element manufactured as described above. It should be noted that when the polarizing element is laminated onto a resin substrate, the resin substrate can also be peeled off from the polarizing element and a protective film can be laminated onto its peeled surface.

[0118] Specific examples of materials that can be the main component of a protective film include cellulose resins such as triacetyl cellulose (TAC), polyester resins, polyvinyl alcohol resins, polycarbonate resins, polyamide resins, polyimide resins, polyethersulfone resins, polysulfone resins, polystyrene resins, polynorbornene resins, polyolefin resins, (meth)acrylic resins, acetate resins, and other transparent resins. Additionally, thermosetting or UV-curing resins such as (meth)acrylic resins, urethane resins, (meth)acrylate urethane resins, epoxy resins, and silicone resins can also be used. It should be noted that "(meth)acrylic resins" refers to acrylic resins and / or methacrylic resins. Furthermore, glassy polymers such as siloxane polymers can also be used. Additionally, polymer films described in Japanese Patent Application Publication No. 2001-343529 (WO01 / 37007) can also be used. As a material for the film, resin compositions containing thermoplastic resins with substituted or unsubstituted imide groups on the side chains and thermoplastic resins with substituted or unsubstituted phenyl and nitrile groups on the side chains can be used, for example: resin compositions having alternating copolymers formed from isobutylene and N-methylmaleimide and acrylonitrile-styrene copolymers. The polymer film can be, for example, an extruded product of the above resin compositions.

[0119] Example

[0120] The present invention will be specifically described below through examples, but the present invention is not limited to these examples. The methods for measuring each characteristic are described below.

[0121] (1) Determination of orthogonal absorbance of PVA-based resin films

[0122] In each embodiment and comparative example, the orthogonal transmittance Tc of PVA-based resin films was measured using a UV-Vis spectrophotometer (manufactured by Nippon Spectrophotometer Co., Ltd., V7100) after the dyeing process and before the crosslinking process, and after the crosslinking process and before the stretching process. Tc was the Y value obtained by measuring the film using a 2-degree field of view (C light source) of JIS Z8701 and correcting for visual sensitivity.

[0123] In addition, using Tc measured at the wavelengths shown in Table 1, the orthogonal absorbance was calculated using the following formula. The results are shown in Table 1.

[0124] Orthogonal absorbance = log10(100 / Tc)

[0125] (2) Determination of monomer permeability

[0126] The monomer transmittance (Ts) of the polarizer single films obtained in each example and comparative example was determined using a UV-Vis spectrophotometer (manufactured by Nippon Spectrophotometer Co., Ltd., product name "V7100"). The results are shown in Table 1. It should be noted that when the polarizer is included in the polarizing plate, the resin substrate of the polarizing plate (test piece) is peeled off from the polarizer beforehand.

[0127] (3) Determination of crystallinity index

[0128] In each embodiment and comparative example, the crystallinity index of the PVA-based resin layer (PVA-based resin film) was determined by ATR method using a Fourier transform infrared spectrophotometer. Specifically, polarized light was used as the measurement light, and the 1141 cm⁻¹ of the obtained spectrum was used for the measurement. -1 Strength (IC) and 1440cm -1 The intensity (IR) is calculated according to the following formula (1) to obtain the crystallization index.

[0129] Crystallization index = (IC / IR)···(1)

[0130] (4) Evaluation of uneven staining

[0131] The uneven dyeing (stripe unevenness) of the polarizers obtained in each embodiment and comparative example was visually observed from a position 50 mm away in the vertical direction, and evaluated according to the following criteria. The results are shown in Table 1.

[0132] A: No unevenness can be observed.

[0133] B: Slight unevenness was observed.

[0134] C: Unevenness was observed.

[0135] <<Examples 1-6>>

[0136] As a thermoplastic resin substrate, a strip-shaped amorphous polyethylene terephthalate copolymer film (thickness: 100 μm) with a Tg of about 75°C is used to perform corona treatment on one side of the resin substrate.

[0137] A PVA aqueous solution (coating solution) was prepared by adding 13 parts by mass of potassium iodide to 100 parts by mass of a PVA-based resin made by mixing polyvinyl alcohol (degree of polymerization 4200, degree of saponification 99.2 mol%) and acetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFIMER") in a 9:1 ratio and dissolving it in water.

[0138] The above-mentioned PVA aqueous solution is coated on the corona-treated surface of the resin substrate and dried at 60°C to form a PVA-based resin layer with a thickness of 13 μm, thereby producing a laminate (laminate production process).

[0139] The resulting laminate was unidirectionally stretched in an oven at 130°C along its longitudinal direction (length direction) at an auxiliary stretching ratio of 2.4 (auxiliary stretching process). It should be noted that the PVA-based resin layer contained in the laminate after auxiliary stretching and before the dyeing process was used for the determination of the crystallinity index. The crystallinity index of the PVA-based resin layer was 1.82.

[0140] Next, the laminate was immersed in a staining bath (an iodine aqueous solution prepared by mixing iodine and potassium iodide in a 1:7 mass ratio relative to 100 parts by mass of water) at a liquid temperature of 30°C for 60 seconds (staining step). It should be noted that in Examples 2-6, the staining bath contained boric acid. Table 1 shows the boric acid concentration in the staining solution.

[0141] Next, the laminate is immersed in a crosslinking bath at a liquid temperature of 40°C (an aqueous solution of boric acid prepared by mixing 3 parts by mass of potassium iodide and 5 parts by mass of boric acid relative to 100 parts by mass of water) for 30 seconds (crosslinking process).

[0142] It should be noted that the PVA resin layer contained in the laminate was subjected to the above-mentioned orthogonal absorbance measurement after the dyeing process and before the crosslinking process, and after the crosslinking process and before the stretching process.

[0143] Then, while immersing the laminate in a stretching bath at a liquid temperature of 70°C (a boric acid aqueous solution prepared by mixing 5 parts by mass of potassium iodide and 4 parts by mass of boric acid relative to 100 parts by mass of water), it is unidirectionally stretched along the longitudinal direction (long side direction) between rollers with different circumferential speeds so that the total stretch ratio is 5.5 times (stretching process).

[0144] Then, the laminate is immersed in a color adjustment bath at a liquid temperature of 20°C (an aqueous solution of 4 parts by mass of potassium iodide mixed with 100 parts by mass of water) (color adjustment process).

[0145] Then, the laminate is dried in an oven at approximately 90°C while its contact surface temperature is maintained at approximately 75°C using SUS heated rollers (drying shrinkage process).

[0146] This process forms a polarizing element with a thickness of approximately 5.0 μm on a resin substrate, resulting in a polarizing plate having a polarizing element / resin substrate composition. The polarizing element contained in this polarizing plate is then used for the determination of the transmittance of the aforementioned monomer. Furthermore, the uneven dyeing (stripe unevenness) of this polarizing element is evaluated as described above.

[0147] <<Example 7>>

[0148] The water ratio was adjusted to achieve a monomer transmittance of 44%. Otherwise, a polarizing plate with a polarizing element / resin substrate structure was obtained in the same manner as in Example 1. The polarizing element contained in this polarizing plate was used for the above-mentioned monomer transmittance measurement. In addition, the uneven dyeing (streaking unevenness) of the polarizing element was evaluated as described above.

[0149] <<Example 8>>

[0150] As the base film, a 30 μm thick polyvinyl alcohol (PVA) based film was used. The following processes were performed on the PVA based resin film in the following order.

[0151] (Insoluble process) As the treatment liquid for the insoluble bath, a 4% by mass boric acid aqueous solution is used. The above-mentioned PVA-based resin film is conveyed into the insoluble bath (boric acid aqueous solution), immersed in the boric acid solution adjusted to 30°C for 60 seconds to swell, and then uniaxially stretched to a stretch ratio of 1.6 times.

[0152] (Dyeing Process) As the treatment solution for the dyeing bath, an iodine dyeing solution with a concentration of 0.3% by mass (iodine:potassium iodide, mass ratio = 0.5:8) is used. The PVA-based resin film that has undergone the above-mentioned insoluble treatment is transported to the dyeing bath and immersed in the iodine dyeing solution adjusted to 30°C for 60 seconds, while being unidirectionally stretched to a stretch ratio of 2.4 times relative to the original length, and dyeing is performed simultaneously.

[0153] (Cross-linking process) As the treatment solution for the cross-linking bath, an aqueous solution of boric acid containing 5% by mass of boric acid and 3% by mass of potassium iodide is used. The PVA-based resin film that has undergone the above dyeing treatment is transported to the cross-linking bath, immersed in the aqueous solution of boric acid adjusted to 40°C for 45 seconds, and cross-linked by boric acid while being unidirectionally stretched to a total stretch ratio of 3.3 times relative to the original length.

[0154] It should be noted that the PVA-based resin film was subjected to the above-mentioned orthogonal absorbance measurement after the dyeing process and before the crosslinking process, and after the crosslinking process and before the stretching process.

[0155] (Stretching Process) As the treatment solution for the stretching bath, an aqueous boric acid solution containing 4% by mass boric acid and 5% by mass potassium iodide is used. The PVA-based resin film that has undergone the above crosslinking treatment is conveyed into the stretching bath and, while being immersed in the aqueous boric acid solution adjusted to 60°C for 30 seconds, is unidirectionally stretched to a total stretch ratio of 6 times relative to the original length.

[0156] (Color Adjustment Process) As the treatment solution for the color adjustment bath, an aqueous solution containing 3% by mass of potassium iodide is used. The PVA-based resin film that has undergone the above stretching treatment is conveyed to the color adjustment bath and immersed in the aqueous solution adjusted to 27°C for 10 seconds.

[0157] (Drying and Shrinking Process) Next, the PVA resin film that has undergone the above-mentioned hue adjustment treatment is dried in an oven at 60°C for 4 minutes to obtain a polarizing part.

[0158] This process yields a polarizing element composed of a PVA-based resin film. The polarizing element is then subjected to the aforementioned monomer transmittance measurement. Furthermore, the uneven dyeing (stripe unevenness) of the polarizing element is evaluated as described above.

[0159] <<Example 9>>

[0160] The laminated body after the auxiliary stretching process was fed into the dyeing process without performing the insoluble process, and a polarizing element was obtained in the same manner as in Example 8. The polarizing element was then subjected to the above-mentioned monomer transmittance measurement. In addition, the dyeing unevenness (stripe unevenness) of the polarizing element was evaluated as described above.

[0161] <<Comparative Example 1>>

[0162] After assisted stretching and before the dyeing process, the laminate was immersed in an insoluble bath (an aqueous solution of boric acid prepared 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 (i.e., an insoluble process was performed). Otherwise, a polarizing plate having a polarizing element / resin substrate structure was obtained in the same manner as in Example 1. The polarizing element contained in this polarizing plate was used for the above-mentioned monomer transmittance measurement. In addition, the dyeing unevenness (stripe unevenness) of this polarizing element was evaluated as described above.

[0163] Comparative Example 2

[0164] After assisted stretching and before the dyeing process, the laminate was immersed in an insoluble bath (an aqueous solution of boric acid prepared 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 (i.e., an insoluble process was performed). Otherwise, a polarizing plate having a polarizing element / resin substrate structure was obtained in the same manner as in Example 7. The polarizing element contained in this polarizing plate was used for the above-mentioned monomer transmittance measurement. In addition, the dyeing unevenness (stripe unevenness) of this polarizing element was evaluated as described above.

[0165] [Table 1]

[0166]

[0167] [evaluate]

[0168] As can be clearly seen from Table 1, when the ratio (A580_c / A580_b) of the orthogonal absorbance A580_c of the PVA-based resin film after the crosslinking process and before the stretching process to the orthogonal absorbance A580_b of the PVA-based resin film after the dyeing process and before the crosslinking process is 0.91 or higher, it is possible to manufacture polarizing parts with suppressed dyeing unevenness.

[0169] Industrial availability

[0170] The manufacturing method of the embodiments of the present invention can be suitably used in the manufacture of polarizing elements and polarizing plates used in image display devices.

Claims

1. A method for manufacturing a polarizing element, comprising: The dyeing process involves dyeing the polyvinyl alcohol-based resin film using a dichroic substance. The cross-linking process involves contacting the polyvinyl alcohol resin film after the dyeing process with a boric acid aqueous solution. as well as The stretching process involves stretching the polyvinyl alcohol-based resin film after the cross-linking process in a stretching bath. The ratio of the orthogonal absorbance A580_c of the polyvinyl alcohol resin film at 580 nm after the crosslinking process and before the stretching process to the orthogonal absorbance A580_b of the polyvinyl alcohol resin film at 580 nm after the dyeing process and before the crosslinking process is greater than or equal to 0.92 and less than 1.

21. The ratio of the orthogonal absorbance A480_c of the polyvinyl alcohol resin film at 480 nm after the crosslinking process and before the stretching process to the orthogonal absorbance A480_b of the polyvinyl alcohol resin film at 480 nm after the dyeing process and before the crosslinking process is 1.10 or more and 1.50 or less.

2. The method for manufacturing a polarizing element according to claim 1, wherein the polarizing element has a single-cell transmittance of 42% or more and 45% or less.

3. The method for manufacturing a polarizing element according to claim 1 or 2, comprising the following steps: In the laminate manufacturing process, a coating liquid containing polyvinyl alcohol resin and halogens is applied to a strip of thermoplastic resin substrate to create a laminate having a polyvinyl alcohol resin layer and the thermoplastic resin substrate as the polyvinyl alcohol resin film. An auxiliary stretching process is used to stretch the laminated body in mid-air. The dyeing process; The crosslinking process; The stretching process; as well as In the drying and shrinking process, the polyvinyl alcohol resin layer after the stretching process is conveyed along the length direction while being shrunk along the width direction orthogonal to the length direction.

4. The method for manufacturing a polarizing element according to claim 3, wherein, The thermoplastic resin substrate is a polyethylene terephthalate film.

5. The method for manufacturing a polarizing element according to claim 3, wherein the polarizing element has a thickness of less than 12 μm.

6. The method for manufacturing a polarizing element according to claim 1 or 2, wherein, The stretching bath is an aqueous solution of boric acid.

7. A method for manufacturing a polarizing plate, comprising: A protective film is applied to the polarizer manufactured using the method for manufacturing polarizers according to any one of claims 1 to 6.

Images