Polarizing plate, polarizing plate roll, and method for producing polarizing film

Abstract

Provided is a polarizing plate which has excellent optical characteristics and in which variations in optical characteristics are suppressed. The polarizing plate according to the present invention has a polarizing film and a protective layer disposed on at least one side of the polarizing film, the thickness of the polarizing film being 8 [mu] m or less, the unit transmittance being 44.0% or more, the degree of polarization being 99.50% or more, and the difference between the maximum value and the minimum value of the unit transmittance of the polarizing plate in a 50 cm2 region being 0.2% or less. Another polarizing plate according to the present invention has a polarizing film and a protective layer disposed on at least one side of the polarizing film, the thickness of the polarizing film being 8 [mu] m or less, the unit transmittance being 44.0% or more, the degree of polarization being 99.50% or more, the width of the polarizing plate being 1000 mm or more, and the difference between the maximum value and the minimum value of the unit transmittance at positions along the width direction being 0.3% or less.

Description

[0001] This application is a divisional application of application No. 201880059533.3 filed on September 6, 2018, the title of which is "Polarizing plate, Polarizing plate roll, and Method for manufacturing polarizing film", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to a polarizing plate, a polarizing plate roll, and a method for manufacturing a polarizing film. BACKGROUND

[0003] In a liquid crystal display device which is a representative image display device, a polarizing film is disposed on both sides of a liquid crystal cell due to its image forming method. In addition, with the popularization of thin display devices, a display device (OLED) equipped with an organic EL panel, a display device (QLED) using a display panel using an inorganic light emitting material such as a quantum dot have been proposed. These panels have a metal layer having high reflectivity, and are prone to problems such as reflection of external light, and mirroring of a background. Therefore, it is known that these problems are prevented by providing a circular polarizing plate having a polarizing film and a λ / 4 wave plate on the visual side. As a method for manufacturing a polarizing film, for example, a method in which a laminate having a resin substrate and a polyvinyl alcohol (PVA)-based resin layer is stretched, and then dyeing treatment is performed to obtain a polarizing film on the resin substrate has been proposed (for example, Patent Document 1). According to such a method, a polarizing film having a thin thickness is obtained, and therefore, as a thin polarizing film that can contribute to the thinning of image display devices in recent years, it has attracted attention. However, as described above, the optical properties of the existing thin polarizing film are insufficient, and further improvement in the optical properties of the thin polarizing film is required.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT DOCUMENTS

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2001-343521 SUMMARY

[0007] PROBLEMS TO BE SOLVED BY THE INVENTION

[0008] The present application has been made to solve the above-described conventional problems, and a main object thereof is to provide a polarizing plate, a polarizing plate roll, and a method for manufacturing a polarizing film, which have excellent optical properties, and in which the deviation in the optical properties is suppressed.

[0009] MEANS OF SOLVING THE PROBLEM

[0010] The polarizing plate of the present application has a polarizing film, and a protective layer disposed on at least one side of the polarizing film, the thickness of the polarizing film is 8 μm or less, the monomer transmittance is 44.0% or more, and the degree of polarization is 99.50% or more, and the polarizing plate has a monomer transmittance of 44.0% or more at 50 cm 2 The difference between the maximum value and the minimum value of the monomer transmittance in the region is 0.2% or less.

[0011] The polarizing plate of the present application has a polarizing film having a thickness of 8 μm or less, a single transmittance of 44.0% or more, and a degree of polarization of 99.50% or more, and a protective layer provided on at least one side of the polarizing film, the polarizing plate having a width of 1000 mm or more, and a difference between a maximum value and a minimum value of the single transmittance at positions along the width direction being 0.3% or less.

[0012] In one embodiment, the polarizing film has a single transmittance of 44.5% or less, and a degree of polarization of 99.95% or less.

[0013] According to another aspect of the present application, there is provided a polarizing plate roll, which is obtained by winding the above-described polarizing plate in a roll shape.

[0014] According to still another aspect of the present application, there is provided a method for manufacturing a polarizing film having a thickness of 8 μm or less, a single transmittance of 44.0% or more, and a degree of polarization of 99.50% or more, the method including: forming a polyvinyl alcohol-based resin layer containing iodide or sodium chloride and a polyvinyl alcohol-based resin on one side of a long thermoplastic resin substrate, thereby manufacturing a laminate; and sequentially subjecting the laminate to a gas atmosphere-assisted stretching treatment, a dyeing treatment, a stretching treatment in an aqueous solution, and a drying shrinkage treatment, in which the laminate is transported in a length direction while being heated, thereby being shrunk by 2% or more in a width direction.

[0015] In one embodiment, the polarizing film has a single transmittance of 44.5% or less, and a degree of polarization of 99.95% or less.

[0016] In one embodiment, the content of the iodide or sodium chloride in the polyvinyl alcohol-based resin layer is 5 parts by weight to 20 parts by weight with respect to 100 parts by weight of the polyvinyl alcohol-based resin.

[0017] In one embodiment, the stretching ratio in the gas atmosphere-assisted stretching treatment is 2.0 times or more.

[0018] In one embodiment, the drying shrinkage treatment process is a process in which heating is performed using a heating roll.

[0019] In one embodiment, the temperature of the heating roll is 60°C to 120°C, and the shrinkage ratio of the laminate in the width direction due to the drying shrinkage treatment is 2% or more.

[0020] Effects of the Invention

[0021] According to the present invention, a polarizing plate having excellent optical properties with suppressed variations in optical properties can be provided, including a polarizing film having a thickness of 8 μm or less, a single-piece transmittance of 44.0% or greater, and a polarization degree of 99.50% or greater. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention.

[0023] Figure 2 This is a schematic diagram showing an example of a drying and shrinking process using a heating roller.

[0024] Figure 3 This is a graph showing the optical properties of polarizing plates obtained in Examples and Comparative Examples.

[0025] Explanation of symbols

[0026] 10 Polarizing film

[0027] 20 1st protective layer

[0028] 30 Second protective layer

[0029] 100 polarizers DETAILED DESCRIPTION

[0030] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these embodiments.

[0031] A. Polarizer

[0032] Figure 1 This is a schematic cross-sectional view of a polarizing plate according to an embodiment of the present invention. The polarizing plate 100 comprises a polarizing film 10, a first protective layer 20 disposed on one side of the polarizing film 10, and a second protective layer 30 disposed on the other side of the polarizing film 10. The thickness of the polarizing film is 8 μm or less, the single-body transmittance is 44.0% or more, and the polarization degree is 99.50% or more. One of the first protective layer 20 and the second protective layer 30 may be omitted. It should be noted that one of the first protective layer and the second protective layer may be a resin substrate (described later) used for the manufacture of the above-mentioned polarizing film.

[0033] The polarizing plate can be in a long strip shape or in a single sheet shape. In the case of a long strip shape, it is preferable to wind it into a roll shape to make a polarizing plate roll. The polarizing plate has excellent optical properties, and the deviation of the optical properties is small. In one embodiment, the width of the polarizing plate is 1000 mm or more, and the difference (D1) between the maximum value and the minimum value of the single transmittance at positions along the width direction is 0.3% or less. The upper limit of D1 is preferably 0.29%, more preferably 0.28%. The smaller D1 is, the more preferable it is, but the lower limit is, for example, 0.01%. When D1 is in the above range, a polarizing plate having excellent optical properties can be industrially produced. In another embodiment, the polarizing plate has a difference (D2) between the maximum value and the minimum value of the single transmittance in a 50 cm 2 region of the polarizing plate is 0.2% or less. The upper limit of D2 is preferably 0.15%, more preferably 0.1%. The smaller D2 is, the more preferable it is, but the lower limit is, for example, 0.01%. When D2 is in the above range, the luminance unevenness in the display screen can be suppressed when the polarizing plate is used for an image display device.

[0034] A-1. Polarizing film

[0035] As described above, the polarizing film has a thickness of 8 μm or less, a single transmittance of 44.0% or more, and a degree of polarization of 99.50% or more. Generally, there is a trade-off relationship between the single transmittance and the absorbance, and if the single transmittance is increased, the absorbance decreases, and if the absorbance is increased, the single transmittance decreases. Therefore, it has been difficult to put a thin polarizing film having optical properties satisfying a single transmittance of 44.0% or more and a degree of polarization of 99.50% or more into practical use. The polarizing film of one embodiment of the present application has excellent optical properties with a single transmittance of 44.0% or more and a degree of polarization of 99.50% or more, as described above. Furthermore, by using the polarizing film of the present embodiment, a polarizing plate in which the deviation of the optical properties is suppressed can be achieved. Achieving such a thin polarizing film (polarizing plate) is one of the results of the present application. Such a polarizing film (polarizing plate) can be used for an image display device, and can be suitably used for a circularly polarizing plate for an organic EL display device, in particular.

[0036] The thickness of the polarizing film is preferably 1 μm to 8 μm, more preferably 1 μm to 7 μm, and further preferably 2 μm to 5 μm.

[0037] The polarizing film preferably exhibits absorption dichroism at any wavelength in the wavelength range of 380 nm to 780 nm. The single transmittance of the polarizing film is preferably 44.5% or less. The degree of polarization of the polarizing film is preferably 99.70% or more, more preferably 99.80% or less. On the other hand, the upper limit of the degree of polarization is preferably 99.95%. The above-mentioned single transmittance is typically a Y value obtained by measurement using an ultraviolet-visible spectrophotometer and visibility correction. The above-mentioned degree of polarization is typically calculated based on parallel transmittance Tp and orthogonal transmittance Tc obtained by measurement using an ultraviolet-visible spectrophotometer and visibility correction, by the following formula.

[0038] Degree of polarization (%) = {(Tp - Tc) / (Tp + Tc)} 1 / 2 x 100

[0039] In one embodiment, the transmittance of a thin polarizing film of 8 μm or less is typically measured using an ultraviolet-visible spectrophotometer with a polarizing film (refractive index of surface: 1.53) and a protective film (refractive index: 1.50) as a measurement target. The reflectance at the interface of each layer varies depending on the refractive index of the surface of the polarizing film and / or the refractive index of the surface in contact with the air interface of the protective film, and as a result, the measured value of the transmittance sometimes varies. Therefore, for example, in the case where a protective film having a refractive index other than 1.50 is used, the measured value of the transmittance can be corrected depending on the refractive index of the surface in contact with the air interface of the protective film. Specifically, the correction value C of the transmittance is represented by the following formula using the reflectance Rl (transmission axis reflectance) of polarized light parallel to the transmission axis at the interface between the protective film and the air layer.

[0040] C = Rl - R0

[0041] R0 = ((1.50 - 1) 2 / (1.50 + 1) 2 ) x (Tl / 100)

[0042] Rl = ((n1 - 1) 2 / (n1 + 1) 2 ) x (Tl / 100)

[0043] wherein R0is the transmittance axis reflectance in the case where a protective film having a refractive index of 1.50 is used, nl is the refractive index of the protective film used, and Tl is the transmittance of the polarizing film. For example, in the case where a substrate having a surface refractive index of 1.53 (a cyclic olefin-based film, a film with a hard coat layer, etc.) is used as the protective film, the correction amount C is about 0.2%. In this case, 0.2% is added to the transmittance obtained by measurement, whereby the transmittance in the case where a protective film having a surface refractive index of 1.50 is used can be converted. Note that, according to the calculation based on the above formula, the change amount of the correction value C when the transmittance Tl of the polarizing film is changed by 2% is 0.03% or less, and the influence of the transmittance of the polarizing film on the value of the correction value C is limited. In addition, in the case where the protective film has an absorption other than surface reflection, appropriate correction can be made according to the amount of absorption.

[0044] As the polarizing film, any appropriate polarizing film can be used. The polarizing film is typically produced using a laminate of two or more layers.

[0045] As a specific example of the polarizing film obtained using a laminate, a polarizing film obtained using a laminate of a resin substrate and a PVA-based resin layer formed by coating on the resin substrate can be given. The polarizing film obtained using a laminate of a resin substrate and a PVA-based resin layer formed by coating on the resin substrate can be produced by, for example, coating a PVA-based resin solution on a resin substrate, drying it, and forming a PVA-based resin layer on the resin substrate to obtain a laminate of the resin substrate and the PVA-based resin layer; and stretching and dyeing the laminate to make the PVA-based resin layer into a polarizing film. In the present embodiment, the stretching typically includes stretching by immersing the laminate in an aqueous boric acid solution. In addition, the stretching can further include, as needed, stretching the laminate in a gas atmosphere at a high temperature (e.g., 95°C or higher) before the stretching in the aqueous boric acid solution. The obtained laminate of the resin substrate / polarizing film can be used as it is (i.e., the resin substrate can be used as a protective layer of the polarizing film), or the resin substrate can be peeled from the laminate of the resin substrate / polarizing film, and the peeled surface can be used by laminating any appropriate protective layer as needed. Details of such a method for producing a polarizing film are described in, for example, Japanese Patent Application Publication No. 2012-73580. The entire contents of this publication are incorporated herein by reference.

[0046] The manufacturing method of the polarizing film of the present application includes: forming a polyvinyl alcohol-based resin layer containing a halide and a polyvinyl alcohol-based resin on one side of a long thermoplastic resin substrate to produce a laminate; and sequentially applying an auxiliary stretching treatment in a gas atmosphere, a dyeing treatment, a stretching treatment in an aqueous solution, and a dry shrinkage treatment to the laminate, in which the laminate is heated while being transported in the longitudinal direction to shrink it by 2% or more in the width direction. Thus, a polarizing film having a thickness of 8 μm or less, a single transmittance of 44.0% or more, and a polarization degree of 99.50% or more, which has excellent optical properties and in which the variation in the optical properties is suppressed, can be obtained. That is, by introducing the auxiliary stretching, the crystallinity of the PVA can be improved even when the PVA is applied to the thermoplastic resin, and high optical properties can be achieved. In addition, by simultaneously improving the orientation of the PVA in advance, the orientation of the PVA can be prevented from being reduced or dissolved when immersed in water in the dyeing step and the stretching step, and high optical properties can be achieved. Furthermore, in the case where the PVA-based resin layer is immersed in a liquid, the orientation disorder of the polyvinyl alcohol molecules and the reduction in the orientation can be suppressed compared to the case where the PVA-based resin layer does not contain a halide. Thus, the optical properties of the polarizing film obtained by the treatment steps in which the laminate is immersed in a liquid in the dyeing treatment and the stretching treatment in an aqueous solution, etc. can be improved. Furthermore, the optical properties can be improved by shrinking the laminate in the width direction by the dry shrinkage treatment.

[0047] A-2. Protective layer

[0048] The first protective layer and the second protective layer can be formed of any appropriate film that can be used as a protective layer of a polarizing film. As specific examples of the material that is the main component of the film, there can be mentioned a transparent resin such as a cellulose-based resin such as cellulose triacetate (TAC), a polyester-based resin, a polyvinyl alcohol-based resin, a polycarbonate-based resin, a polyamide-based resin, a polyimide-based resin, a polyethersulfone-based resin, a polysulfone-based resin, a polystyrene-based resin, a polynorbornene-based resin, a polyolefin-based resin, a (meth)acrylic-based resin, an acetate-based resin, and the like. In addition, there can be mentioned a thermosetting resin such as a (meth)acrylic-based resin, a urethane-based resin, a (meth)acrylic urethane-based resin, an epoxy-based resin, a silicone-based resin, and the like, or an ultraviolet-curable resin, and the like. In addition thereto, there can be mentioned a glassy polymer such as a silicone-based polymer. In addition, a polymer film described in Japanese Patent Application Publication No. 2001-343529 (WO 01 / 37007) can be used. As the material of the film, a resin composition containing, for example, a thermoplastic resin having a substituted or unsubstituted imide group in a side chain and a thermoplastic resin having a substituted or unsubstituted phenyl group and a nitrile group in a side chain can be used, and there can be mentioned, for example, a resin composition having an alternating copolymer of isobutylene and N-methyl maleimide and an acrylonitrile-styrene copolymer. The polymer film can be, for example, an extrusion-molded product of the above-described resin composition.

[0049] When the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (outer protective layer) disposed on the side opposite to the display panel is typically 300 μm or less, preferably 100 μm or less, more preferably 5 μm to 80 μm, and further preferably 10 μm to 60 μm. Note that, in the case where surface treatment is performed, the thickness of the outer protective layer is the thickness including the thickness of the surface treatment layer.

[0050] When the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (inner protective layer) disposed on the side of the display panel is preferably 5 μm to 200 μm, more preferably 10 μm to 100 μm, and further preferably 10 μm to 60 μm. In one embodiment, the inner protective layer is a phase difference layer having an arbitrary appropriate phase difference value. In this case, the in-plane phase difference Re(550) of the phase difference layer is, for example, 110 nm to 150 nm. "Re(550)" is the in-plane phase difference measured at 23°C with light having a wavelength of 550 nm, and can be calculated by the formula: Re = (nx - ny) x d. Here, "nx" is the refractive index in the direction in which the in-plane refractive index is largest (i.e., the slow axis direction), "ny" is the refractive index in the direction orthogonal to the slow axis in the plane (i.e., the fast axis direction), "nz" is the refractive index in the thickness direction, and "d" is the thickness (nm) of the layer (film).

[0051] B. Method for manufacturing a polarizing film

[0052] The manufacturing method of the polarizing film of one embodiment of the present application includes: forming a polyvinyl alcohol-based resin layer (PVA-based resin layer) containing a halide and a polyvinyl alcohol-based resin (PVA-based resin) on one side of a long thermoplastic resin substrate to manufacture a laminate; and sequentially performing a gas atmosphere auxiliary stretching treatment, a dyeing treatment, a stretching treatment in an aqueous solution, and a dry shrinkage treatment on the laminate, in which the laminate is heated while being transported in the longitudinal direction to shrink by 2% or more in the width direction. The content of the halide in the PVA-based resin layer is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA-based resin. The dry shrinkage treatment is preferably performed using a heating roll, and the temperature of the heating roll is preferably 60 to 120°C. The laminate subjected to the dry shrinkage treatment preferably shrinks by 2% or more in the width direction. According to this manufacturing method, the polarizing film described in item A above can be obtained. In particular, a laminate including a PVA-based resin layer containing a halide is manufactured, a plurality of stages of stretching including a gas atmosphere auxiliary stretching and a stretching in an aqueous solution are performed on the laminate, and the stretched laminate is heated using a heating roll, whereby a polarizing film having excellent optical properties (typically, monomer transmittance and degree of polarization) and a small variation in optical properties can be obtained. Specifically, the use of a heating roll in the dry shrinkage treatment enables the laminate to be uniformly shrunk as a whole while being transported. Thus, the optical properties of the obtained polarizing film can be improved, and a polarizing film having excellent optical properties can be stably produced, and a variation in optical properties (particularly, monomer transmittance) of the polarizing film can be suppressed.

[0053] B-1. Manufacture of a laminate

[0054] As a method of manufacturing a laminate of a thermoplastic resin substrate and a PVA-based resin layer, any appropriate method can be used. It is preferable to coat a coating liquid containing a halide and a PVA-based resin on the surface of a thermoplastic resin substrate and dry it, thereby forming a PVA-based resin layer on the thermoplastic resin substrate. As described above, the content of the halide in the PVA-based resin layer is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA-based resin.

[0055] As a method of coating the coating liquid, any appropriate method can be used. Examples include roll coating, spin coating, wire bar coating, dip coating, die coating, curtain coating, spray coating, and blade coating (doctor blade coating and the like). The coating / drying temperature of the coating liquid is preferably 50°C or higher.

[0056] The thickness of the PVA-based resin layer is preferably 3 to 40 μm, and further preferably 3 to 20 μm.

[0057] Before the PVA-based resin layer is formed, the thermoplastic resin substrate can be subjected to surface treatment (e.g., corona treatment, etc.), or an easy-adhesion layer can be formed on the thermoplastic resin substrate. By performing such treatment, the adhesion of the thermoplastic resin substrate to the PVA-based resin layer can be improved.

[0058] B-1-1. Thermoplastic resin substrate

[0059] The thickness of the thermoplastic resin substrate is preferably 20 μm to 300 μm, more preferably 50 μm to 200 μm. When less than 20 μm, there is a concern that it will be difficult to form the PVA-based resin layer. When more than 300 μm, for example, in the stretching treatment in the aqueous solution described later, it takes a long time for the thermoplastic resin substrate to absorb water, and there is a concern that a large load will be required for stretching.

[0060] The water absorption of the thermoplastic resin substrate is preferably 0.2% or more, further preferably 0.3% or more. The thermoplastic resin substrate absorbs water, and the water functions as a plasticizer, and the thermoplastic resin substrate can be plasticized. As a result, the stretching stress can be greatly reduced, and stretching can be performed at a high magnification. On the other hand, the water absorption of the thermoplastic resin substrate is preferably 3.0% or less, further preferably 1.0% or less. By using such a thermoplastic resin substrate, it is possible to prevent a significant decrease in the dimensional stability of the thermoplastic resin substrate during production, and to prevent deterioration in the appearance of the obtained polarizing film and the like. In addition, it is possible to prevent the substrate from breaking during stretching in the aqueous solution, and to prevent the PVA-based resin layer from peeling from the thermoplastic resin substrate. Note that the water absorption of the thermoplastic resin substrate can be adjusted, for example, by introducing a modifying group into the constituent material. The water absorption is a value obtained in accordance with JIS K 7209.

[0061] The glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 120°C or less. By using such a thermoplastic resin substrate, it is possible to suppress crystallization of the PVA-based resin layer, and to sufficiently ensure the stretchability of the laminate. Furthermore, if the plasticization of the thermoplastic resin substrate by water and the stretching in the aqueous solution are considered, it is more preferable that the glass transition temperature be 100°C or less, further preferably 90°C or less. On the other hand, the glass transition temperature of the thermoplastic resin substrate is preferably 60°C or more. By using such a thermoplastic resin substrate, it is possible to prevent deformation (e.g., occurrence of unevenness, relaxation, wrinkles, etc.) and the like of the thermoplastic resin substrate when the coating / drying of the above-described coating liquid containing the PVA-based resin is performed, and it is possible to produce the laminate favorably. In addition, it is possible to perform stretching of the PVA-based resin layer favorably at an appropriate temperature (e.g., around 60°C). Note that the glass transition temperature of the thermoplastic resin substrate can be adjusted, for example, by using a crystalline material in which a modifying group is introduced into the constituent material, and by heating. The glass transition temperature (Tg) is a value obtained in accordance with JIS K 7121.

[0062] As the material constituting the thermoplastic resin substrate, any appropriate thermoplastic resin can be used. As the thermoplastic resin, for example, ester resins such as polyethylene terephthalate resins, cyclic olefin resins such as norbornene resins, olefin resins such as polypropylene, polyamide resins, polycarbonate resins, copolymer resins thereof, and the like can be listed. Among these, norbornene resins and amorphous polyethylene terephthalate resins are preferred.

[0063] In one embodiment, an amorphous (un-crystallized) polyethylene terephthalate resin is preferably used. Among these, an amorphous (not easily crystallized) polyethylene terephthalate resin is particularly preferred. As specific examples of the amorphous polyethylene terephthalate resin, copolymers further containing isophthalic acid and / or cyclohexane dicarboxylic acid as a dicarboxylic acid, and copolymers further containing cyclohexane dimethanol, diethylene glycol as a diol can be listed.

[0064] In a preferred embodiment, the thermoplastic resin substrate is constituted by a polyethylene terephthalate resin having isophthalic acid units. This is because such a thermoplastic resin substrate is very excellent in stretchability, and crystallization at the time of stretching can be suppressed. It is considered that this is because by introducing isophthalic acid units, a large bend can be imparted to the main chain. The polyethylene terephthalate resin has terephthalic acid units and ethylene glycol units. The proportion of isophthalic acid units is preferably 0.1 mol% or more, and further preferably 1.0 mol% or more, with respect to the total of all repeating units. This is because a thermoplastic resin substrate that is very excellent in stretchability can be obtained. On the other hand, the proportion of isophthalic acid units is preferably 20 mol% or less, and more preferably 10 mol% or less, with respect to the total of all repeating units. By setting the proportion to such a proportion, the crystallinity can be favorably increased in the drying shrinkage treatment described later.

[0065] The thermoplastic resin substrate can be stretched in advance (before the PVA-based resin layer is formed). In one embodiment, the thermoplastic resin substrate in a long strip shape can be stretched in the transverse direction. The transverse direction is preferably a direction orthogonal to the stretching direction of the laminate described later. Note that in the present specification, "orthogonal" means that it also includes the case of being substantially orthogonal. Among these, "substantially orthogonal" means that it includes the case of being 90° ± 5.0°, preferably 90° ± 3.0°, and further preferably 90° ± 1.0°.

[0066] The stretching temperature of the thermoplastic resin substrate is preferably Tg - 10°C to Tg + 50°C with respect to the glass transition temperature (Tg). The stretching ratio of the thermoplastic resin substrate is preferably 1.5 times to 3.0 times.

[0067] As the stretching method of the thermoplastic resin substrate, any appropriate method can be adopted. Specifically, it can be fixed-end stretching, or it can be free-end stretching. The stretching mode can be dry, or it can be wet. The stretching of the thermoplastic resin substrate can be performed in one stage, or it can be performed in multiple stages. In the case of being performed in multiple stages, the stretching ratio described above is the product of the stretching ratios of the respective stages.

[0068] B-1-2. Coating liquid

[0069] The coating liquid contains a halide and a PVA-based resin as described above. The coating liquid described above can be typically a solution obtained by dissolving the halide and the PVA-based resin described above in a solvent. As the solvent, for example, water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, ethylenediamine, diethylenetriamine, and the like can be exemplified. They can be used alone, or two or more of them can be used in combination. Among these, water is preferred. The PVA-based resin concentration of the solution is preferably 3 parts by weight to 20 parts by weight with respect to 100 parts by weight of the solvent. If the resin concentration is such, a uniform coating film that adheres to the thermoplastic resin substrate can be formed. The content of the halide in the coating liquid is preferably 5 parts by weight to 20 parts by weight with respect to 100 parts by weight of the PVA-based resin.

[0070] An additive can be incorporated in the coating liquid. As the additive, for example, a plasticizer, a surfactant, and the like can be exemplified. As the plasticizer, for example, polyhydric alcohols such as ethylene glycol and glycerol can be exemplified. As the surfactant, for example, a nonionic surfactant can be exemplified. These additives are used for the purpose of further improving the uniformity, dyeability, and stretchability of the obtained PVA-based resin layer.

[0071] As the PVA-based resin described above, any appropriate resin can be adopted. For example, polyvinyl alcohol and ethylene-vinyl alcohol copolymer can be exemplified. The polyvinyl alcohol can be obtained by saponifying polyvinyl acetate. The ethylene-vinyl alcohol copolymer is obtained by saponifying ethylene-vinyl acetate copolymer. The saponification degree of the PVA-based resin is generally 85 mol% to 100 mol%, preferably 95.0 mol% to 99.95 mol%, and further preferably 99.0 mol% to 99.93 mol%. The saponification degree can be found in accordance with JIS K 6726-1994. By using a PVA-based resin having such a saponification degree, a polarizing film excellent in durability can be obtained. In the case where the saponification degree is too high, there is a concern of gelation.

[0072] The average polymerization degree of the PVA-based resin can be appropriately selected according to the purpose. The average polymerization degree is usually 1000 to 10000, preferably 1200 to 4500, and further preferably 1500 to 4300. Note that the average polymerization degree can be calculated according to JIS K 6726-1994.

[0073] As the halide, any appropriate halide can be used. Examples include iodides and chlorides. As the iodide, examples include potassium iodide, sodium iodide, and lithium iodide. Of these, potassium iodide is preferred.

[0074] The amount of the halide in the coating liquid is preferably 5 parts by weight to 20 parts by weight relative to 100 parts by weight of the PVA-based resin, and more preferably 10 parts by weight to 15 parts by weight relative to 100 parts by weight of the PVA-based resin. When the amount of the halide exceeds 20 parts by weight relative to 100 parts by weight of the PVA-based resin, there is a case where the halide exudes and the resulting polarizing film becomes cloudy.

[0075] Generally, the orientation of the polyvinyl alcohol molecules in the PVA-based resin layer improves by stretching the PVA-based resin layer. However, if the stretched PVA-based resin layer is immersed in a liquid containing water, the orientation of the polyvinyl alcohol molecules becomes disordered and the orientation decreases. In particular, in the case where the laminate of the thermoplastic resin and the PVA-based resin layer is stretched in a boric acid aqueous solution, the orientation decreases significantly when the laminate is stretched at a relatively high temperature in order to stabilize the stretching of the thermoplastic resin. For example, the stretching of a PVA film monomer in a boric acid aqueous solution is usually performed at 60°C, in contrast to which the stretching of the laminate of A-PET (thermoplastic resin substrate) and the PVA-based resin layer is performed at a high temperature of around 70°C. In this case, the orientation of PVA at the initial stage of stretching decreases before it increases by stretching in the aqueous solution. In contrast, by producing a laminate of the PVA-based resin layer containing the halide and the thermoplastic resin substrate, and performing high-temperature stretching (auxiliary stretching) in air before stretching the laminate in the boric acid aqueous solution, the crystallization of the PVA-based resin in the PVA-based resin layer of the laminate after the auxiliary stretching can be promoted. As a result, in the case where the PVA-based resin layer is immersed in a liquid, the orientation disorder and the decrease in the orientation of the polyvinyl alcohol molecules can be suppressed compared to the case where the PVA-based resin layer does not contain the halide. Thus, the optical properties of the polarizing film obtained by the process of immersing the laminate in a liquid such as dyeing treatment and stretching treatment in an aqueous solution can be improved.

[0076] B-2. Auxiliary stretching treatment in a gas atmosphere

[0077] In particular, in order to obtain high optical properties, a method of two-stage stretching in which dry stretching (auxiliary stretching) is combined with stretching in a boric acid aqueous solution is selected. Like the two-stage stretching, the auxiliary stretching is introduced, whereby stretching can be performed while suppressing crystallization of the thermoplastic resin base material, the problem of a decrease in stretchability due to excessive crystallization of the thermoplastic resin base material at the time of stretching in the boric acid aqueous solution later can be solved, and the laminate can be stretched at a higher stretch ratio. Further, when the PVA-based resin is applied to the thermoplastic resin base material, in order to suppress the influence of the glass transition temperature of the thermoplastic resin base material, the application temperature needs to be lowered compared to the case where the PVA-based resin is applied to a metal drum as usual, as a result of which a problem of a decrease in crystallization of the PVA-based resin, failure to obtain sufficient optical properties, and the like can occur. In contrast to this, by introducing the auxiliary stretching, even when the PVA-based resin is applied to the thermoplastic resin, the crystallinity of the PVA-based resin can be increased, and high optical properties can be achieved. In addition, by simultaneously increasing the orientation of the PVA-based resin in advance, when immersed in water in the dyeing process and the stretching process later, a decrease in the orientation of the PVA-based resin, problems of dissolution, and the like can be prevented, and high optical properties can be achieved.

[0078] The stretching method of the auxiliary stretching in the gas atmosphere can be a fixed-end stretching (for example, a method of stretching using a tenter stretching machine), or a free-end stretching (for example, a method of unidirectionally stretching the laminate by passing it between rollers having different circumferential speeds), and in order to obtain high optical properties, the free-end stretching can be actively employed. In one embodiment, the stretching treatment in the gas atmosphere includes a heating roll stretching process of transporting the above-described laminate in the longitudinal direction thereof while stretching by the difference in the circumferential speed between heating rolls. The stretching treatment in the gas atmosphere typically includes a zone stretching process and a heating roll stretching process. Note that the order of the zone stretching process and the heating roll stretching process is not limited, and the zone stretching process can be performed first, or the heating roll stretching process can be performed first. The zone stretching process can also be omitted. In one embodiment, the zone stretching process and the heating roll stretching process are performed sequentially. In another embodiment, in a tenter stretching machine, the end portions of the film are held, the distance between the tenter stretching machines is expanded in the transport direction to stretch the laminate (the expansion of the distance between the tenter stretching machines becomes the stretch ratio). At this time, the distance between the tenter stretching machines in the width direction (perpendicular direction with respect to the transport direction) is arbitrarily close to the set distance. It is preferable that the distance between the tenter stretching machines in the width direction be set so as to be closer to the free-end stretching with respect to the stretch ratio in the transport direction. In the case of the free-end stretching, the shrinkage in the width direction = (1 / stretch ratio) 1 / 2 is calculated.

[0079] The gas atmosphere-assisted stretching can be performed in one stage or in multiple stages. In the case of multiple stages, the stretching ratio is the product of the stretching ratios of the respective stages. The stretching direction in the gas atmosphere-assisted stretching is preferably substantially the same as the stretching direction in the water solution stretching.

[0080] The stretching ratio in the gas atmosphere-assisted stretching is preferably 2.0 to 3.5. The maximum stretching ratio in the case of combining the gas atmosphere-assisted stretching and the water solution stretching is preferably 5.0 or more, more preferably 5.5 or more, and further preferably 6.0 or more, with respect to the original length of the laminate. In the present specification, the "maximum stretching ratio" refers to the stretching ratio immediately before the laminate is broken, and, in the case where the breaking of the laminate is confirmed, the "maximum stretching ratio" refers to a value 0.2 less than the value.

[0081] The stretching temperature of the gas atmosphere-assisted stretching can be set to any appropriate value depending on the forming material of the thermoplastic resin substrate, the stretching method, and the like. The stretching temperature is preferably the glass transition temperature (Tg) of the thermoplastic resin substrate or more, further preferably the glass transition temperature (Tg) of the thermoplastic resin substrate + 10°C or more, and particularly preferably Tg + 15°C or more. On the other hand, the upper limit of the stretching temperature is preferably 170°C. By performing the stretching at such a temperature, the rapid progress of the crystallization of the PVA-based resin can be suppressed, and the adverse effects caused by the crystallization (for example, the orientation of the PVA-based resin layer by the stretching is hindered) can be suppressed.

[0082] B-3. Insolubilization treatment

[0083] The insolubilization treatment is performed after the gas atmosphere-assisted stretching treatment, as necessary, before the water solution stretching treatment and the dyeing treatment. The above-described insolubilization treatment is typically performed by immersing the PVA-based resin layer in a boric acid aqueous solution. By performing the insolubilization treatment, water resistance can be imparted to the PVA-based resin layer, and the decrease in the orientation of PVA when immersed in water can be prevented. The concentration of the boric acid aqueous solution is preferably 1 part by weight to 4 parts by weight with respect to 100 parts by weight of water. The liquid temperature of the insolubilization bath (boric acid aqueous solution) is preferably 20°C to 50°C.

[0084] B-4. Dyeing treatment

[0085] The above-described dyeing treatment is typically performed by dyeing the PVA-based resin layer with iodine. Specifically, the dyeing is performed by adsorbing iodine to the PVA-based resin layer. As the adsorption method, for example, a method in which the PVA-based resin layer (laminate) is immersed in a dyeing solution containing iodine, a method in which the PVA-based resin layer is coated with the dyeing solution, a method in which the PVA-based resin layer is sprayed with the dyeing solution, and the like can be given. The method in which the laminate is immersed in the dyeing solution (dyeing bath) is preferred. This is because iodine can be well adsorbed.

[0086] The dyeing solution is preferably an aqueous iodine solution. The content of iodine is preferably 0.05 parts by weight to 0.5 parts by weight, relative to 100 parts by weight of water. In order to improve the solubility of iodine in water, it is preferable to incorporate an iodide into the aqueous iodine solution. As the iodide, for example, potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, titanium iodide, and the like can be exemplified. Among these, potassium iodide is preferable. The content of the iodide is preferably 0.1 parts by weight to 10 parts by weight, more preferably 0.3 parts by weight to 5 parts by weight, relative to 100 parts by weight of water. In order to suppress the dissolution of the PVA-based resin, the liquid temperature at the time of dyeing with the dyeing solution is preferably 20°C to 50°C. In the case where the PVA-based resin layer is immersed in the dyeing solution, in order to ensure the transmittance of the PVA-based resin layer, the immersion time is preferably 5 seconds to 5 minutes, more preferably 30 seconds to 90 seconds.

[0087] The dyeing conditions (concentration, liquid temperature, immersion time) can be set in such a manner that the monomer transmittance of the polarizing film finally obtained is 44.0% or more, and the degree of polarization is 99.50% or more. As such dyeing conditions, it is preferable to use an aqueous iodine solution as the dyeing solution, and to set the ratio of the contents of iodine and potassium iodide in the aqueous iodine solution to be 1:5 to 1:20. The ratio of the contents of iodine and potassium iodide in the aqueous iodine solution is preferably 1:5 to 1:10. Thereby, a polarizing film having the optical characteristics as described above can be obtained.

[0088] When the dyeing treatment is continuously performed after the treatment (typically, insolubilization treatment) of immersing the laminate in a treatment bath containing boric acid, the boric acid concentration of the dyeing bath changes over time by mixing the boric acid contained in the treatment bath into the dyeing bath, as a result of which the dyeability sometimes becomes unstable. In order to suppress the destabilization of the dyeability as described above, the upper limit of the boric acid concentration of the dyeing bath is adjusted to be preferably 4 parts by weight, more preferably 2 parts by weight, relative to 100 parts by weight of water. On the other hand, the lower limit of the boric acid concentration of the dyeing bath is preferably 0.1 parts by weight, more preferably 0.2 parts by weight, further preferably 0.5 parts by weight, relative to 100 parts by weight of water. In one embodiment, the dyeing treatment is performed using a dyeing bath to which boric acid is incorporated in advance. Thereby, the change ratio of the boric acid concentration in the case where the boric acid of the above-described treatment bath is mixed into the dyeing bath can be reduced. The content of the boric acid incorporated in advance into the dyeing bath (i.e., the content of the boric acid not coming from the above-described treatment bath) is preferably 0.1 parts by weight to 2 parts by weight, more preferably 0.5 parts by weight to 1.5 parts by weight, relative to 100 parts by weight of water.

[0089] B-5. Crosslinking Treatment

[0090] According to necessity, the cross-linking treatment is performed after the dyeing treatment and before the stretching treatment in an aqueous solution. The above cross-linking treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. By performing the cross-linking treatment, water resistance can be imparted to the PVA-based resin layer, and in the subsequent stretching in an aqueous solution, the decrease in the orientation of PVA when immersed in water at a high temperature can be prevented. The concentration of the aqueous boric acid solution is preferably 1 part by weight to 5 parts by weight with respect to 100 parts by weight of water. In addition, when the cross-linking treatment is performed after the above dyeing treatment, an iodide is preferably further added. By adding the iodide, the elution of iodine adsorbed to the PVA-based resin layer can be suppressed. The amount of the iodide to be added is preferably 1 part by weight to 5 parts by weight with respect to 100 parts by weight of water. The iodide is specifically exemplified above. The liquid temperature of the cross-linking bath (aqueous boric acid solution) is preferably 20°C to 50°C.

[0091] B-6. Stretching treatment in an aqueous solution

[0092] The stretching treatment in an aqueous solution is performed by immersing the laminate in a stretching bath. According to the stretching treatment in an aqueous solution, stretching can be performed at a temperature lower than the glass transition temperature of the above thermoplastic resin substrate, PVA-based resin layer (typically about 80°C), and stretching can be performed at a high magnification while suppressing the crystallization of the PVA-based resin layer. As a result, a polarizing film having excellent optical properties can be produced.

[0093] The stretching method of the laminate can employ any appropriate method. Specifically, it can be fixed-end stretching, or it can be free-end stretching (for example, a method in which the laminate is passed between rollers having different circumferential speeds to perform unidirectional stretching), and free-end stretching is preferably selected. The stretching of the laminate can be performed in one stage, or it can be performed in multiple stages. In the case of being performed in multiple stages, the stretching magnification of the laminate (maximum stretching magnification) described later is the product of the stretching magnifications of the respective stages.

[0094] The stretching in an aqueous solution is preferably performed by immersing the laminate in an aqueous boric acid solution (stretching in an aqueous boric acid solution). By using an aqueous boric acid solution as the stretching bath, rigidity against the tension applied at the time of stretching, and water resistance against dissolution in water can be imparted to the PVA-based resin layer. Specifically, boric acid can generate tetraborate anions in an aqueous solution, and cross-linking with the PVA-based resin through hydrogen bonds can be performed. As a result, rigidity and water resistance can be imparted to the PVA-based resin layer, and stretching can be performed well, and a polarizing film having excellent optical properties can be produced.

[0095] The above aqueous boric acid solution is preferably obtained by dissolving boric acid and / or a boric acid salt in water as a solvent. The boric acid concentration is preferably 1 part by weight to 10 parts by weight, more preferably 2.5 parts by weight to 6 parts by weight, and particularly preferably 3 parts by weight to 5 parts by weight, relative to 100 parts by weight of water. By setting the boric acid concentration to 1 part by weight or more, the dissolution of the PVA-based resin layer can be effectively suppressed, and a polarizing film with higher performance can be produced. Note that an aqueous solution obtained by dissolving a boron compound other than boric acid or a boric acid salt, such as borax, glyoxal, or glutaraldehyde, in a solvent can also be used.

[0096] It is preferable to incorporate an iodide in the above stretching bath (aqueous boric acid solution). By incorporating an iodide, the elution of iodine adsorbed to the PVA-based resin layer can be suppressed. Specific examples of the iodide are described above. The iodide concentration is preferably 0.05 parts by weight to 15 parts by weight, and more preferably 0.5 parts by weight to 8 parts by weight, relative to 100 parts by weight of water.

[0097] The stretching temperature (liquid temperature of the stretching bath) is preferably 40°C to 85°C, and more preferably 60°C to 75°C. If the temperature is such, the dissolution of the PVA-based resin layer can be suppressed, and stretching at a high magnification can be performed. Specifically, as described above, the glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 60°C or higher, in view of the relationship with the PVA-based resin layer. In this case, if the stretching temperature is lower than 40°C, there is a concern that the stretching cannot be performed well even if the plasticization of the thermoplastic resin substrate by water is taken into consideration. On the other hand, the higher the temperature of the stretching bath, the higher the solubility of the PVA-based resin layer, and there is a concern that an excellent optical performance cannot be obtained. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

[0098] The stretching magnification by the stretching in the aqueous solution is preferably 1.5 times or more, and more preferably 3.0 times or more. The total stretching magnification of the laminate is preferably 5.0 times or more, and further preferably 5.5 times or more, relative to the original length of the laminate. By achieving such a high stretching magnification, a polarizing film with very excellent optical performance can be produced. Such a high stretching magnification can be achieved by using the stretching in the aqueous solution (stretching in the aqueous boric acid solution).

[0099] B-7. Drying shrinkage treatment

[0100] The above-mentioned drying shrinkage treatment can be carried out by regional heating by heating the entire area, or by heating the transport roller (using a so-called heating roller) (heating roller drying method). Preferably, both are used. By using a heating roller for drying, the heating curling of the laminate can be efficiently suppressed, thereby manufacturing a polarizing film with excellent appearance. Specifically, by drying the laminate along the heating roller, the crystallization of the above-mentioned thermoplastic resin substrate can be efficiently promoted, the crystallinity can be increased, and the crystallinity of the thermoplastic resin substrate can be well increased even at a relatively low drying temperature. As a result, the rigidity of the thermoplastic resin substrate is increased, and it becomes a state that can withstand the shrinkage of the PVA-based resin layer caused by drying, which can suppress curling. In addition, by using a heating roller, the laminate can be dried while being kept in a flat state, therefore, not only can curling be suppressed, but also the generation of wrinkles can be suppressed. At this time, the laminate is shrunk in the width direction by the drying shrinkage treatment, thereby improving the optical properties. This is because the orientation of PVA and PVA / iodine complex can be effectively improved. The shrinkage rate of the laminate in the width direction by the drying shrinkage treatment is preferably 1% to 10%, more preferably 2% to 8%, and particularly preferably 4% to 6%. By using heated rollers, the laminate can be continuously shrunk in the width direction while being transported, thereby achieving high productivity.

[0101] Figure 2 This is a schematic diagram illustrating an example of a drying and shrinking process. During the drying and shrinking process, the laminate 200 is dried while being transported by transport rollers R1-R6 and guide rollers G1-G4, both heated to a predetermined temperature. In the illustrated example, the transport rollers R1-R6 are positioned to alternately and continuously heat the PVA resin layer surface and the thermoplastic resin substrate surface. However, the transport rollers R1-R6 may alternatively be positioned to continuously heat only one side of the laminate 200 (e.g., the thermoplastic resin substrate surface).

[0102] The drying conditions can be controlled by adjusting the heating temperature of the conveying roller (temperature of the heating roller), the number of heating rollers, the contact time with the heating roller, etc. The temperature of the heating roller is preferably 60°C to 120°C, more preferably 65°C to 100°C, and particularly preferably 70°C to 80°C. The crystallinity of the thermoplastic resin can be well increased, curling can be well suppressed, and an optical laminate with excellent durability can be produced. It should be noted that the temperature of the heating roller can be measured by a contact thermometer. In the illustrated example, 6 conveying rollers are provided, but there is no particular limitation as long as there are multiple conveying rollers. Conveying rollers are usually provided in 2 to 40 numbers, preferably 4 to 30 numbers. The contact time (total contact time) between the laminate and the heating roller is preferably 1 second to 300 seconds, more preferably 1 to 20 seconds, and further preferably 1 to 10 seconds.

[0103] The heating roll can be provided in a heating furnace (e.g., an oven) or in a general manufacturing line (at room temperature). It is preferable to provide it in a heating furnace equipped with a blowing mechanism. By combining the drying using the heating roll and the hot air drying, it is possible to suppress a sharp change in temperature between the heating rolls and easily control the shrinkage in the width direction. The temperature of the hot air drying is preferably 30°C to 100°C. In addition, the hot air drying time is preferably 1 second to 300 seconds. The air speed of the hot air is preferably about 10 m / s to 30 m / s. Note that the air speed is the air speed in the heating furnace and can be measured by a mini-blade type digital anemometer.

[0104] B-8. Other treatments

[0105] The cleaning treatment is preferably performed after the stretching treatment in the aqueous solution and before the drying shrinkage treatment. The above cleaning treatment is typically performed by immersing the PVA-based resin layer in an aqueous potassium iodide solution.

[0106] Examples

[0107] Hereinafter, the present application will be specifically described by way of examples, but the present application is not limited to these examples. The measurement methods of each property are described below. Note that "parts" and "%" in the examples and comparative examples are on a weight basis, unless otherwise specified.

[0108] (1) Thickness

[0109] An interference film thickness meter (Otsuka Electronics Co., Ltd., product name "MCPD-3000") was used for the measurement.

[0110] (2) Monomer transmittance and degree of polarization

[0111] For the polarizing plate (protective film / polarizing film) of the examples and comparative examples, the monomer transmittance Ts, parallel transmittance Tp, and orthogonal transmittance Tc measured using an ultraviolet-visible spectrophotometer (JASCO Corporation, V-7100) were used as Ts, Tp, and Tc of the polarizing film, respectively. These Ts, Tp, and Tc are Y values obtained by measuring a 2-degree field of view (C light source) according to JIS Z8701 and performing visibility correction. Note that the refractive index of the protective film was 1.50, and the refractive index of the surface of the polarizing film opposite to the protective film was 1.53.

[0112] From the obtained Tp and Tc, the degree of polarization P was calculated by the following equation.

[0113] Degree of polarization P (%) = {(Tp - Tc) / (Tp + Tc)} x 100 1 / 2

[0114] ​Note that the spectrophotometer can be, for example, LPF-200 manufactured by Otsuka Electronics Co., Ltd. As an example, for Sample 1 to Sample 3 of the polarizing plate each of which was produced in the same manner as in the following examples, the monomer transmittance Ts and the degree of polarization P were measured using V-7100 and LPF-200, and the measured values are shown in Table 1. As shown in Table 1, the difference between the measured value of the monomer transmittance of V-7100 and the measured value of the monomer transmittance of LPF-200 was 0.1% or less, and the same measurement results were obtained in each case where either of the spectrophotometers was used.

[0115]

[0116] Note that, for example, in a case where a polarizing plate subjected to anti-glare (AG) surface treatment and having an adhesive having diffusion performance is used as a measurement target, different measurement results can be obtained depending on the spectrophotometer, and in this case, the difference in the measurement value depending on the spectrophotometer is compensated by numerical conversion based on the measurement values of the same polarizing plate measured by each of the spectrophotometers.

[0117] (3) Variation in optical characteristics of long polarizing plate

[0118] From the long polarizing plate used in the examples and reference examples, a measurement sample was cut out at five positions at equal intervals along the width direction, and the monomer transmittance of the central portion of each of the five measurement samples was measured in the same manner as in the above (2). Next, the difference between the maximum value and the minimum value among the monomer transmittances measured at the respective measurement positions was calculated, and this value was set as the variation in the optical characteristics of the long polarizing plate (the difference between the maximum value and the minimum value of the monomer transmittance in the position along the width direction of the long polarizing plate).

[0119] (4) Variation in optical characteristics of single polarizing plate

[0120] From the long polarizing plate used in the examples and reference examples, a measurement sample of 100 mm x 100 mm was cut out, and the variation in the optical characteristics of the single polarizing plate (50 cm 2 ) was calculated. Specifically, the monomer transmittance was measured at a total of five positions, which were the position of about 1.5 cm to 2.0 cm inward from the midpoint of each of the four edges of the measurement sample and the central portion, in the same manner as in the above (2). Next, the difference between the maximum value and the minimum value among the monomer transmittances measured at the respective measurement positions was calculated, and this value was set as the variation in the optical characteristics of the single polarizing plate (the difference between the maximum value and the minimum value of the monomer transmittance in the 50 cm 2 region).

[0121] [Example 1]

[0122] 1. Production of polarizing film

[0123] As the thermoplastic resin substrate, a long, amorphous copolymerized isophthalic acid polyethylene terephthalate film (thickness: 100 μm) having a water absorption of 0.75% and a Tg of about 75°C was used. One side of the resin substrate was subjected to corona treatment.

[0124] A PVA aqueous solution (coating solution) was prepared by adding 13 parts by weight of potassium iodide to 100 parts by weight of a PVA-based resin prepared by mixing 9 parts by weight of polyvinyl alcohol (degree of polymerization: 4200, degree of saponification: 99.2 mol%) and 1 part by weight of acetyl acetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFIMER Z410").

[0125] The PVA aqueous solution described above was applied to the corona-treated side of the resin substrate, and dried at 60°C, thereby forming a PVA-based resin layer having a thickness of 13 μm, and a laminate was prepared.

[0126] The obtained laminate was uniaxially stretched in the longitudinal direction (lengthwise direction) to 2.4 times in an oven at 130°C between rollers having different circumferential speeds (stretching treatment assisted by a gas atmosphere).

[0127] Next, the laminate was immersed in a solubilization bath (boric acid aqueous solution obtained by adding 4 parts by weight of boric acid to 100 parts by weight of water) at a liquid temperature of 40°C for 30 seconds (solubilization treatment).

[0128] Next, the laminate was immersed in a dyeing bath (aqueous iodine solution obtained by adding iodine and potassium iodide at a weight ratio of 1:7 to 100 parts by weight of water) at a liquid temperature of 30°C for 60 seconds while adjusting the concentration so that the monomer transmittance (Ts) of the resulting polarizing film became 44% or more (dyeing treatment).

[0129] Next, the laminate was immersed in a crosslinking bath (aqueous boric acid solution obtained by adding 3 parts by weight of potassium iodide and 5 parts by weight of boric acid to 100 parts by weight of water) at a liquid temperature of 40°C for 30 seconds (crosslinking treatment).

[0130] Then, while the laminate was immersed in an aqueous boric acid solution (boric acid concentration: 4.0% by weight) at a liquid temperature of 70°C, it was uniaxially stretched between rollers having different circumferential speeds so that the total stretching ratio in the longitudinal direction (lengthwise direction) became 5.5 times (stretching treatment in an aqueous solution).

[0131] Then, the laminate was immersed in a washing bath (aqueous solution obtained by adding 4 parts by weight of potassium iodide to 100 parts by weight of water) at a liquid temperature of 20°C (washing treatment).

[0132] Then, while being dried in an oven maintained at 90°C, it was contacted with a SUS-made heating roller maintained at a surface temperature of 75°C for about 2 seconds (drying shrinkage treatment). The shrinkage ratio of the laminate in the width direction by the drying shrinkage treatment was 5.2%.

[0133] In this way, a polarizing film having a thickness of 5 μm was formed on the resin substrate. Further, the same procedure was repeated to produce a total of 9 polarizing films.

[0134] 2. Production of Polarizing Plates

[0135] On the surface of each of the polarizing films obtained above (the side opposite to the resin substrate), an acrylic film (surface refractive index 1.50, 40 μm) was attached as a protective film by means of an ultraviolet-curable adhesive. Specifically, the adhesive was applied in such a manner that the total thickness of the adhesive became 1.0 μm, and attachment was performed using a roll machine. Then, UV light was irradiated from the side of the protective film to cure the adhesive. Next, after cutting both end portions, the resin substrate was peeled off to obtain 9 polarizing plates having a constitution of protective film / polarizing film in a long strip shape (width: 1300 mm).

[0136] [Example 2]

[0137] In the drying shrinkage treatment, the oven temperature was set to 70°C and the heating roller temperature was set to 70°C, and otherwise, 18 polarizing films and polarizing plates were produced in the same manner as in Example 1. The shrinkage ratio of the laminate in the width direction by the drying shrinkage treatment was 2.5%.

[0138] [Comparative Example 1]

[0139] In the PVA aqueous solution (coating liquid), potassium iodide was not added, the stretching ratio in the gas atmosphere-assisted stretching treatment was set to 1.8 times, and in the drying shrinkage treatment, a heating roller was not used, and otherwise, 5 polarizing films and polarizing plates were produced in the same manner as in Example 1.

[0140] [Comparative Example 2]

[0141] The stretching ratio in the gas atmosphere-assisted stretching treatment was set to 1.8 times, and in the drying shrinkage treatment, a heating roller was not used, and otherwise, 6 polarizing films and polarizing plates were produced in the same manner as in Example 1.

[0142] [Reference Example 1]

[0143] The polarizing film obtained in the same manner as in Comparative Example 2 was left in a constant-temperature constant-humidity chamber set to a temperature of 60°C and a humidity of 90% RH for 30 minutes. Then, a polarizing plate was produced in the same manner as in Example 1.

[0144] The monomer transmittance and the degree of polarization were measured for each polarizing plate of the examples and the comparative examples. The results are shown in Table 2 and Figure 3 .

[0145]

[0146] The polarizing film obtained by the manufacturing method of the comparative example cannot satisfy both the monomer transmittance of 44.0% or more and the degree of polarization of 99.50% or more. In contrast, the polarizing film obtained by the manufacturing method of the examples has excellent optical characteristics of the monomer transmittance of 44.0% or more and the degree of polarization of 99.50% or more.

[0147] The deviation of the optical characteristics of the long strip-shaped and single sheet-shaped polarizing plates was measured for each polarizing plate of Example 1 and Reference Example 1. The results are shown in Table 3.

[0148]

[0149] The deviation of the monomer transmittance of the long strip-shaped polarizing plate obtained by the manufacturing method of the examples was 0.3% or less, the deviation of the monomer transmittance of the single sheet-shaped polarizing plate obtained by the manufacturing method of the examples was 0.2% or less, and the deviation of the optical characteristics was suppressed to a degree that was not problematic. On the other hand, in the polarizing plates of the reference examples obtained by the process of performing a moisture treatment on the polarizing film, the deviation of the optical characteristics of the long strip-shaped and single sheet-shaped polarizing plates was large in both cases.

[0150] Industrial Applicability

[0151] The polarizing plate having the polarizing film of the present application can be suitably used for a circular polarizing plate for liquid crystal display devices and organic EL display devices, inorganic EL display devices.

Claims

1. A method for manufacturing a polarizing film, wherein the polarizing film has a thickness of 8 μm or less, a single-body transmittance of 44.0% or more, and a degree of polarization of 99.50% or more, the method comprising: A polyvinyl alcohol resin layer containing iodide or sodium chloride and polyvinyl alcohol resin is formed on one side of a long thermoplastic resin substrate to prepare a laminate; as well as The laminate is sequentially subjected to auxiliary stretching treatment in a gas atmosphere, dyeing treatment, stretching treatment in an aqueous solution, and drying and shrinking treatment. In the drying and shrinking treatment, the laminate is heated while being transported in the longitudinal direction, thereby shrinking it by more than 2% in the width direction. 2 . The manufacturing method according to claim 1 , which is a method for manufacturing a polarizing film having a single-body transmittance of 44.5% or less and a polarization degree of 99.95% or less.

3. The manufacturing method according to claim 1 or 2, wherein: In the polyvinyl alcohol resin layer, the content of the iodide or sodium chloride is 5 parts by weight to 20 parts by weight based on 100 parts by weight of the polyvinyl alcohol resin.

4. The production method according to any one of claims 1 to 3, wherein The stretching ratio in the auxiliary stretching treatment in the gas atmosphere is 2.0 times or more.

5. The production method according to any one of claims 1 to 4, wherein The drying shrinkage treatment step is a step of applying heat using a heating roller.

6. The manufacturing method according to claim 5, wherein: The temperature of the heating roller is 60° C. to 120° C., and the shrinkage rate of the laminate in the width direction caused by the drying shrinkage treatment is 2% or more.

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