Polarizer and polarizing plate

Abstract

The present invention provides a thin polarizing plate having excellent heat resistance. The polarizing plate of the present invention is composed of an iodine-containing polyvinyl alcohol-based resin film, wherein the free boric acid content is 0.4% by weight or less.

Description

TECHNICAL FIELD

[0001] The present application relates to a polarizer and a polarizing plate. BACKGROUND

[0002] In a liquid crystal display device which is a representative image display device, a polarizer (in essence, a polarizing plate including a polarizer) is disposed on both sides of a liquid crystal cell due to its image forming method. The polarizer is typically manufactured by dyeing a polyvinyl alcohol (PVA)-based resin film with a dichroic substance such as iodine (for example, Patent Documents 1 and 2). In recent years, the demand for thinness of image display devices has increased. Therefore, further thinning of the polarizer is also required. However, the thinner the polarizer, the lower the heat resistance, and there is a problem that the optical properties easily change in a high-temperature environment.

[0003] PRIOR ART DOCUMENTS

[0004] PATENT DOCUMENTS

[0005] Patent Document 1: Japanese Patent No. 5048120

[0006] Patent Document 2: Japanese Patent Application Laid-Open No. 2013-156391 SUMMARY

[0007] PROBLEMS TO BE SOLVED BY THE INVENTION

[0008] The present application has been achieved in order to solve the conventional problems, and its main object is to provide a polarizer which is thin and has very excellent heat resistance.

[0009] MEANS FOR SOLVING PROBLEMS

[0010] The polarizer of the present application is composed of a polyvinyl alcohol-based resin film containing iodine, in which

[0011] The free boric acid content is 0.4% by weight or less.

[0012] In one embodiment, the thickness of the polarizer described above is 7 μm or less.

[0013] In one embodiment, the iodine content of the polarizer described above is 10% by weight to 25% by weight.

[0014] According to another aspect of the present application, a polarizing plate is provided. The polarizing plate includes the polarizer described above, and a protective film laminated on one side or both sides of the polarizer.

[0015] EFFECTS OF THE INVENTION

[0016] According to the present application, the content of free boric acid contained in the polarizer is set to 0.4% by weight or less, and a polarizer which has been long awaited but has not been achieved until now, which is thin and has very excellent heat resistance, can be achieved. DETAILED DESCRIPTION

[0017] Hereinafter, embodiments of the present application will be described, but the present application is not limited to these embodiments.

[0018] A. Polarizer

[0019] A-1. Outline of Polarizer

[0020] The polarizer of the embodiments of the present application is composed of a polyvinyl alcohol (PVA)-based resin film containing iodine, in which the free boric acid content is 0.4% by weight or less.

[0021] The free boric acid content of the polarizer is preferably 0.39% by weight or less, more preferably 0.38% by weight or less. The lower limit of the free boric acid content is, for example, 0.01% by weight. As described above, one of the features of the present application is not directed to the content of all the boric acid contained in the polarizer, but is directed to the content of the free boric acid. By making the free boric acid content of the polarizer within the above range, the heat resistance thereof can be made excellent, and the change in optical properties (e.g., monochromatic transmittance) in a high-temperature environment can be suppressed. The free boric acid content of the polarizer can be typically determined using an inductively coupled plasma emission spectrometry (ICP-AES) method and according to the following method. That is, the polarizer is frozen and pulverized to obtain a measurement sample, and the measurement sample is mixed with a 2-ethyl-l,3-hexanediol / chloroform (volume ratio: 10 / 90) mixed solution, and the mixture is filtered to obtain a filtrate, and the boron content in the filtrate is quantified using an ICP emission analysis device. The obtained boron content is regarded as all from boric acid, and the boric acid in the filtrate is regarded as all from the free boric acid of the polarizer, and the boron content is converted into the free boric acid content of the polarizer. In one embodiment, the free boric acid content of the polarizer can be adjusted to be within the above range by performing a drying process at a lower temperature (preferably 50°C or lower) than the conventional heating temperature in the drying step in the manufacturing method of the polarizer as described later.

[0022] The upper limit of the thickness of the polarizer is 7 μm in one embodiment, 3 μm in another embodiment, and 2 μm in still another embodiment. The lower limit of the thickness is 0.5 μm in one embodiment, 0.6 μm in another embodiment, and 0.8 μm in still another embodiment. According to the embodiments of the present application, even if the polarizer is thin in thickness, the desired monochromatic transmittance as described later can be achieved.

[0023] The iodine content of the polarizer can be appropriately set to achieve both sufficient polarization performance and optimal single-cell transmittance. The iodine content is preferably 10% to 25% by weight, more preferably 15% to 25% by weight. According to embodiments of the present invention, exceptionally good heat resistance, previously difficult to achieve, can be realized in polarizers with the extremely high iodine content described above. More specifically, changes in optical properties in high-temperature environments can be significantly suppressed in polarizers with extremely high iodine content. In this specification, "iodine content" refers to the total amount of iodine contained in the polarizer (PVA-type resin film). More specifically, iodine in the polarizer is expressed as iodide ions (I₂O₃). - ), polyiodide ions (I3) - I5 - Iodine ions exist in various forms, including polyiodide (PVA) and triiodide (I3). The iodine content mentioned in this specification refers to the amount of iodine containing all these forms. Iodine content can be calculated using, for example, a standard curve method based on fluorescence X-ray analysis. It should be noted that polyiodide ions exist in a polarizing microscope as PVA-iodine complexes. By forming such complexes, dichroism can be observed in the visible light wavelength range. Specifically, the complex of PVA with triiodide ions (PVA·I3)... - It has an absorption peak near 470 nm; the complex of PVA and pentaiodide ions (PVA·I5) - The polyiodide ion exhibits an absorption peak near 600 nm. As a result, polyiodide ions can absorb light across a broad visible spectrum depending on their morphology. On the other hand, iodide ions (I...) - It has an absorption peak around 230 nm, which is not substantially related to the absorption of visible light. Therefore, the polyiodide ions existing in the complex state with PVA are mainly related to the absorption performance of the polarizer.

[0024] The single-unit transmittance (Ts) of the polarizer is preferably 30.0% to 43.0%, more preferably 35.0% to 41.0%. The polarization degree of the polarizer is preferably 99.9% or higher, more preferably 99.95% or higher, and even more preferably 99.98% or higher. By setting a lower single-unit transmittance and a higher polarization degree, the contrast can be improved, and blacks can be displayed more deeply, thus achieving an image display device with excellent image quality. It should be noted that the single-unit transmittance is a value measured using a spectrophotometer with an integrating sphere. The single-unit transmittance is the Y value obtained by measuring the 2-degree field of view (C light source) of JIS Z8701 and performing visibility correction. For example, it can be measured using a spectrophotometer with an integrating sphere (manufactured by Japan Spectrophotometer Co., Ltd., product name: V7100).

[0025] A-2. PVA-based resin film

[0026] Examples of PVA resins that form PVA-type resin films include polyvinyl alcohol (PVA) and ethylene-vinyl alcohol copolymers. PVA is obtained by saponifying polyvinyl acetate. Ethylene-vinyl alcohol copolymers are obtained by saponifying ethylene-vinyl acetate copolymers. The degree of saponification of PVA resins is typically 85 mol% or more and less than 100 mol%, preferably 95.0 mol% to 99.95 mol%, more preferably 99.0 mol% to 99.93 mol%. The degree of saponification can be determined according to JIS K6726-1994. By using PVA resins with such a degree of saponification, polarizers with excellent durability can be obtained. However, if the degree of saponification is too high, there is a concern about gelation.

[0027] The average degree of polymerization of PVA resins can be appropriately selected according to the purpose. The average degree of polymerization is typically 1000–10000, preferably 1200–4500, and more preferably 1500–4300. It should be noted that the average degree of polymerization can be determined according to JISK 6726-1994.

[0028] There is no particular limitation on the thickness of PVA-type resin films; it can be set according to the desired polarizer thickness. For example, the thickness of PVA-type resin films can range from 10 μm to 200 μm.

[0029] In one embodiment, the PVA-based resin film may also be a PVA-based resin layer formed on a substrate. The laminate of the substrate and the PVA-based resin layer can be obtained, for example, by coating a coating liquid containing the aforementioned PVA-based resin onto the substrate, or by laminating a PVA-based resin film onto the substrate.

[0030] B. Manufacturing method of polarizer

[0031] B-1. Overview of the manufacturing method of polarizers

[0032] The method for manufacturing a polarizer according to an embodiment of the present invention includes at least stretching and dyeing a PVA-based resin film. Typically, this manufacturing method includes a step of preparing a PVA-based resin film, a stretching step, a dyeing step, a crosslinking step, a cleaning step, and a drying step. Additionally, a swelling step may be included before the stretching step as needed. The steps using the PVA-based resin film can be performed in any suitable order and at any appropriate time. Therefore, the steps can be performed in the order described above, or in a different order. A single step can also be performed multiple times as required. Furthermore, steps other than those described above (e.g., a non-dissolving step) can be performed at any suitable time. It should be noted that if the PVA-based resin layer on which the PVA-based resin film is formed is a substrate, a laminate of the substrate and the PVA-based resin layer can be supplied to the above steps.

[0033] The following describes each process, but as mentioned above, the processes can be performed in any appropriate order and are not restricted by the order in which they are described.

[0034] B-2. Stretching process

[0035] In the stretching process, PVA-based resin films are typically stretched unidirectionally to 3 to 7 times their original strength. The stretching direction can be either the long side (MD direction) or the width (TD direction) of the film. The stretching method can be dry stretching, wet stretching, or a combination of both. Additionally, PVA-based resin films can be stretched during crosslinking, swelling, or dyeing processes. It should be noted that the stretching direction can correspond to the absorption axis direction of the resulting polarizer.

[0036] B-3. ​​Swelling Process

[0037] The swelling process is usually performed before the dyeing process. The swelling process can be performed, for example, by immersing a PVA resin film in a swelling bath. The swelling bath is typically made of water such as distilled water or pure water. The swelling bath may also contain any suitable other components besides water. Other components include solvents such as alcohols, additives such as surfactants, and iodides. Examples of iodides include potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, and titanium iodide. Potassium iodide is preferred. The temperature of the swelling bath is, for example, 20°C to 45°C. The immersion time is, for example, 10 seconds to 300 seconds.

[0038] B-4. Dyeing Process

[0039] The dyeing process involves dyeing a PVA-based resin film with a dichroic substance. Preferably, this is done by allowing the film to adsorb the dichroic substance. Examples of adsorption methods include: immersing the PVA-based resin film in a dyeing solution containing the dichroic substance, applying the dyeing solution to the PVA-based resin film, and spraying the dyeing solution onto the PVA-based resin film. Immersing the PVA-based resin film in the dyeing solution is preferred because it allows for good adsorption of the dichroic substance.

[0040] Examples of dichroic substances include iodine and dichroic dyes. Iodine is preferred. When iodine is used as the dichroic substance, an aqueous iodine solution is preferably used as the dyeing solution. The iodine content of the aqueous iodine solution is preferably 0.04 to 5.0 parts by weight relative to 100 parts by weight of water. To improve the solubility of iodine in water, it is preferable to incorporate an iodide into the aqueous iodine solution. Potassium iodide is preferably used as the iodide. The iodide content is preferably 0.3 to 15 parts by weight relative to 100 parts by weight of water.

[0041] The temperature of the dyeing solution during dyeing can be set to any appropriate value, such as 20℃ to 50℃. When immersing the PVA resin film in the dyeing solution, the immersion time is, for example, 5 seconds to 5 minutes.

[0042] B-5. Crosslinking process

[0043] In the crosslinking process, boron compounds are typically used as crosslinking agents. Examples of boron compounds include boric acid and borax. Boric acid is preferred. In the crosslinking process, boron compounds are usually used in the form of aqueous solutions.

[0044] When using an aqueous boric acid solution, the boric acid concentration of the solution is, for example, 1% to 15% by weight, preferably 1% to 10% by weight. The aqueous boric acid solution may further contain iodides such as potassium iodide, zinc compounds such as zinc sulfate and zinc chloride.

[0045] The crosslinking process can be carried out by any suitable method. Examples include: immersing a PVA-type resin film in an aqueous solution containing a boron compound, coating an aqueous solution containing a boron compound onto a PVA-type resin film, or spraying an aqueous solution containing a boron compound onto a PVA-type resin film. Immersion in an aqueous solution containing a boron compound is preferred.

[0046] The solution temperature used for crosslinking is, for example, 25°C or higher, preferably 30°C to 85°C, and more preferably 40°C to 70°C. The immersion time is, for example, 5 seconds to 800 seconds, preferably 8 seconds to 500 seconds.

[0047] B-6. Cleaning Procedure

[0048] The cleaning process is typically performed after the crosslinking process. The cleaning process typically involves immersing the PVA resin membrane in a cleaning solution. A representative example of a cleaning solution is pure water. Potassium iodide can also be added to pure water.

[0049] The temperature of the cleaning solution is, for example, 5°C to 50°C. The immersion time is, for example, 1 second to 300 seconds.

[0050] B-7. Drying Process

[0051] The drying process can be carried out by any suitable method. Examples of drying methods include natural drying, forced-air drying, reduced-pressure drying, and heat drying. Heat drying is preferred. From the viewpoint of shortening drying time, when performing heat drying, the heating temperature is preferably below 50°C, more preferably below 45°C, and particularly preferably below 40°C. The lower limit of the heating temperature is not particularly limited, but is a lower limit temperature that can be set in the heat drying apparatus. For example, it is 30°C. Furthermore, the drying time is, for example, 20 seconds to 10 minutes. In one embodiment, heat drying is performed in two or more stages. In this case, it is preferable that the heating temperature of at least one stage is within the above-mentioned range. By setting the heating temperature within the above-mentioned range during heat drying, a polarizer with excellent heat resistance can be obtained.

[0052] C. Polarizing plate

[0053] The polarizer of this invention is typically used with a protective film laminated on one or both sides (i.e., as a polarizer). In actual use, the polarizer has an adhesive layer as its outermost layer. The adhesive layer typically forms the outermost layer on the image display device side. A diaphragm is temporarily bonded to the adhesive layer in a peelable state, protecting the adhesive layer until actual use, and can be rolled up.

[0054] The protective film can be any suitable resin film. Examples of resin film forming materials include: (meth)acrylic resins, cellulose resins such as cellulose diacetate and cellulose triacetate, cyclic olefin resins such as norbornene resins, olefin resins such as polypropylene, ester resins such as polyethylene terephthalate resins, polyamide resins, polycarbonate resins, and their copolymers. It should be noted that "(meth)acrylic resins" refers to acrylic resins and / or methacrylic resins.

[0055] In one embodiment, the above-mentioned (meth)acrylic resin uses a (meth)acrylic resin having a glutarimide structure. (Meth)acrylic resins having a glutarimide structure (hereinafter also referred to as glutarimide resins) are described, for example, in the following documents: Japanese Patent Application Publication No. 2006-309033, Japanese Patent Application Publication No. 2006-317560, Japanese Patent Application Publication No. 2006-328329, Japanese Patent Application Publication No. 2006-328334, Japanese Patent Application Publication No. 2006-337491, Japanese Patent Application Publication No. 2006-337492, Japanese Patent Application Publication No. 2006-337493, Japanese Patent Application Publication No. 2006-337569, Japanese Patent Application Publication No. 2007-009182, Japanese Patent Application Publication No. 2009-161744, and Japanese Patent Application Publication No. 2010-284840. These documents are incorporated herein by reference.

[0056] When manufacturing a polarizer using a laminate of a substrate and a PVA-based resin layer, the protective film can be used directly without peeling off the substrate. Alternatively, the protective film can be applied to the polarizer after peeling off the substrate.

[0057] Example

[0058] The present invention will now be specifically described through examples, but the present invention is not limited to these examples. Furthermore, the methods for measuring each characteristic are described below.

[0059] (1) Change in monomer transmittance Ts

[0060] In the examples and comparative examples, a reflective polarizer (manufactured by 3M Corporation, trade name "DBEF") was bonded to the polarizer side of the laminate obtained in the examples and comparative examples, with a 1.0 μm thick adhesive layer in between. Next, after peeling off the thermoplastic resin substrate, a 1.3 mm thick alkali-free glass was bonded to the peeled surface with a 20 μm thick acrylic adhesive layer as a test sample. The test sample was heated at 80°C for 200 hours (heating test). The monomer transmittance of the polarizer before the test and after the heating test was measured using a spectrophotometer with an integrating sphere (manufactured by Nippon Spectrophotometer Co., Ltd., product name: V7100). Then, the change in monomer transmittance Ts was calculated using the following formula based on the monomer transmittance Ts0 before heating and the monomer transmittance Ts1 after heating.

[0061] Ts(%) = Ts1 - Ts0

[0062] (2) Iodine content

[0063] For the polarizers of the laminates obtained in the examples and comparative examples, the fluorescence X-ray intensity (kcps) was measured using a fluorescence X-ray analysis apparatus (Rigaku Corporation, trade name "ZSX-PRIMUS II", measuring diameter: ψ20 mm). On the other hand, the thickness (μm) of the polarizer was measured using a spectrophotometer (Otsuka Electronics Corporation, trade name "MCPD-3000"). Based on the obtained fluorescence X-ray intensity and thickness, the iodine content (wt%) was calculated using the following formula.

[0064] (Iodine concentration) = 20.5 × (fluorescent X-ray intensity) / (film thickness)

[0065] It should be noted that the coefficient used to calculate iodine content varies depending on the measuring device, and this coefficient can be obtained using an appropriate standard curve.

[0066] (3) Free boric acid content

[0067] The polarizers obtained in the examples and comparative examples were cut into approximately 5 mm squares using scissors, and 50 mg of each square, along with a steel ball, were filled into the sample container. The sample was then cryogenically pulverized using a JFC-300 cryogenic pulverizer (manufactured by Japan Analysis Industries Co., Ltd.) with liquid nitrogen as the refrigerant, under conditions of pre-cooling for 7 minutes and vibration for 5 minutes. The pulverized sample was placed at room temperature for approximately 30 minutes. The resulting sample was mixed with 3.5 mL of a 2-ethyl-1,3-hexanediol / chloroform (volume ratio: 10 / 90) mixture and placed at room temperature for 24 hours. The resulting mixture was filtered using a 0.45 μm filter. After removing chloroform from the filtrate on a hot plate, the residue was transferred to a Teflon (registered trademark) decomposition vessel, acid was added, and the vessel was sealed. The decomposition vessel was irradiated with microwaves for pressurized acid hydrolysis. After decomposition, ultrapure water was added to bring the volume to 25 mL, and the sample was analyzed according to the quantitative conditions for boric acid described below.

[0068] <Quantitative conditions for boric acid>

[0069] Device Name: ICP Luminescence Analyzer SPS-3520UV (Manufactured by Hitachi High-Tech Science Co.)

[0070] Measurement wavelength: B 249.848nm

[0071] The obtained boron content is assumed to be entirely derived from boric acid and converted into boric acid content. This boric acid content is then used as the free boric acid content of the polarizer.

[0072] [Example 1]

[0073] A thermoplastic resin substrate was prepared using an amorphous polyethylene terephthalate (IPA) copolymer film (100 μm thickness) with a water absorption rate of 0.75% and a Tg of 75℃. One side of the substrate was corona-treated, and an aqueous solution containing polyvinyl alcohol (degree of polymerization 4200, degree of saponification 99.2 mol%) and acetyl-modified PVA (degree of polymerization 1200, degree of acetyl-modification 4.6%, degree of saponification ≥ 99.0 mol%, manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFIMER Z200") in a 9:1 ratio was coated onto the corona-treated surface at 25℃ and then dried to form an 11 μm thick PVA resin layer, thus creating a laminate.

[0074] The resulting laminate was stretched at 140°C in a gas atmosphere of 4.5 times its normal strength in a direction orthogonal to the long side of the laminate (stretching treatment).

[0075] Next, the laminate was immersed in a staining bath (an aqueous solution of 1.4% by weight iodine and 9.8% by weight potassium iodide) at a liquid temperature of 25°C for 12 seconds to perform staining (staining treatment).

[0076] Next, the laminate is immersed in a cleaning bath (pure water) at a liquid temperature of 25°C for 6 seconds (first cleaning treatment).

[0077] Next, immerse in a crosslinking bath (an aqueous solution of 1% boron and 1% potassium iodide) at a liquid temperature of 60°C for 16 seconds (crosslinking treatment).

[0078] Next, the laminate was immersed in a cleaning bath (a 1% by weight aqueous solution of potassium iodide) at a temperature of 25°C for 3 seconds (second cleaning treatment).

[0079] Then, the laminate was dried in an oven at 25°C for 8 seconds (first drying process).

[0080] Finally, the laminate was dried in an oven at 25°C for 13 seconds (second drying process) to obtain a laminate with a PVA resin layer (polarizer) with a thickness of 1.2 μm. The obtained polarizer had an iodine content of 18.5 wt%, a free boric acid content of 0.32 wt%, and a monomer transmittance of 40.0%.

[0081] The resulting laminates were evaluated as described in (1) above. The results are shown in Table 1.

[0082] [Example 2]

[0083] The laminate was dried in an oven at 30°C in both the first and second drying processes, otherwise, a laminate with a polarizing lens was obtained in the same manner as in Example 1. The obtained polarizing lens had an iodine content of 18.8% by weight, a free boric acid content of 0.32% by weight, and a monomer transmittance of 39.9%. The obtained laminate was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[0084] [Example 3]

[0085] The laminate was dried in an oven at 40°C in both the first and second drying processes, otherwise, a laminate with a polarizing lens was obtained in the same manner as in Example 1. The obtained polarizing lens had an iodine content of 18.6 wt%, a free boric acid content of 0.39 wt%, and a monomer transmittance of 39.9%. The obtained laminate was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[0086] [Comparative Example 1]

[0087] In the second drying process, the laminate was dried in an oven at 60°C. Otherwise, a laminate with a polarizing lens was obtained in the same manner as in Example 3. The obtained polarizing lens had an iodine content of 19.1 wt%, a free boric acid content of 0.45 wt%, and a monomer transmittance of 39.9%. The obtained laminate was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[0088] [Comparative Example 2]

[0089] In the first drying process, the laminate was dried in an oven at 50°C. Otherwise, a laminate with a polarizing lens was obtained in the same manner as in Comparative Example 1. The obtained polarizing lens had an iodine content of 19.2 wt%, a free boric acid content of 0.48 wt%, and a monomer transmittance of 39.9%. The obtained laminate was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[0090] [Comparative Example 3]

[0091] In the first drying process, the laminate was dried in an oven at 60°C. Otherwise, a laminate with a polarizing lens was obtained in the same manner as in Comparative Example 1. The obtained polarizing lens had an iodine content of 19.3 wt%, a free boric acid content of 0.57 wt%, and a monomer transmittance of 40.0%. The obtained laminate was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[0092] [Table 1]

[0093]

[0094] As shown in Table 1, the polarizer of the embodiment of the present invention exhibits significantly smaller changes in monomer transmittance after heating compared to the polarizer of the comparative example, demonstrating excellent heat resistance. Specifically, the Ts of the polarizer of the embodiment is 0.75% to 0.80%, and at this level of Ts, no heat resistance issues arise in practical use. The polarizer of the embodiment is thin, and the change in monomer transmittance in a high-temperature environment is significantly suppressed. It is speculated that this excellent effect is achieved by suppressing the free boric acid content of the obtained polarizer to a low level, thereby preventing polyolefin formation in a high-temperature environment. This newly discovered problem can be solved by actually manufacturing very thin polarizers (e.g., less than 7 μm thick), which were previously very difficult to manufacture, resulting in an unexpectedly excellent effect.

[0095] Industrial applicability

[0096] The polarizer of this invention can be widely used in the liquid crystal panels of LCD TVs, LCD monitors, mobile phones, digital cameras, camcorders, mobile game consoles, car navigation systems, copiers, printers, fax machines, watches, microwave ovens, etc.

Claims

1. A polarizer comprising a polyvinyl alcohol resin film containing iodine, wherein, The free boric acid content is ≥0.01% by weight and ≤0.4% by weight, and the iodine content is 15% by weight to 25% by weight. The thickness of the polarizer is less than 7 μm.

2. A polarizer comprising: a polarizer as described in claim 1, and a protective film laminated on one or both sides of the polarizer.

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