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 43.5% or more, the degree of polarization being 99.940% 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 0.15% 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 43.5% or more, the degree of polarization being 99.940% 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 an application with an application date of September 6, 2018, an application number of 201880059541.8, and an invention title of "Polarizing Plate, Roll of Polarizing Plate, and Method for Manufacturing Polarizing Film". Technical Field

[0002] The present invention relates to a polarizing plate, a roll of polarizing plate, and a method for manufacturing a polarizing film. Background Art

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

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001 - 343521 Summary of the Invention

[0007] Problems to be Solved by the Invention

[0008] The present invention has been made to solve the above-mentioned existing problems, and its main object is to provide a polarizing plate, a roll of polarizing plate, and a method for manufacturing a polarizing film having excellent optical characteristics and suppressed deviation in optical characteristics.

[0009] Means for Solving the Problems

[0010] The polarizing plate of 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 is 8 μm or less, the monomer transmittance is 43.5% or more, the degree of polarization is 99.940% or more, and the difference between the maximum value and the minimum value of the monomer transmittance within a 50 cm 2 region is 0.15% or less.

[0011] The polarizing plate of 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 is 8 μm or less, the monomer transmittance is 43.5% or more, and the degree of polarization is 99.940% or more. The width of the polarizing plate is 1000 mm or more, and the difference between the maximum value and the minimum value of the monomer transmittance at positions along the width direction is 0.3% or less.

[0012] In one embodiment, the monomer transmittance of the polarizing film is 44.0% or less, and the degree of polarization is 99.990% or less.

[0013] According to another aspect of the present invention, there is provided a roll of polarizing plates, which is formed by winding the above-mentioned polarizing plates into a roll shape.

[0014] According to still another aspect of the present invention, there is provided a method for manufacturing a polarizing film, which is a method for manufacturing a polarizing film having a thickness of 8 μm or less, a monomer transmittance of 43.5% or more, and a degree of polarization of 99.940% or more. The method includes: forming a polyvinyl alcohol resin layer containing an iodide or sodium chloride and a polyvinyl alcohol resin on one side of a strip-shaped thermoplastic resin substrate to form a laminate; and sequentially performing an auxiliary stretching treatment in a gas atmosphere, a dyeing treatment, a stretching treatment in an aqueous solution, and a drying shrinkage treatment on the laminate. In the drying shrinkage treatment, the laminate is heated while being transported in the length direction, so that it shrinks by 2% or more in the width direction.

[0015] In one embodiment, the monomer transmittance is 44.0% or less, and the degree of polarization is 99.990% or less.

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

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

[0018] In one embodiment, the drying shrinkage treatment step is a step of heating using a heating roll.

[0019] In one embodiment, the temperature of the heating roll 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.

[0020] Effects of the Invention

[0021] According to the present invention, a polarizer having a polarizing film with a thickness of 8 μm or less, a monomer transmittance of 43.5% or more, and a degree of polarization of 99.940% or more can be provided. The polarizer has excellent optical properties and the deviation of the optical properties is suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic cross-sectional view of a polarizer according to an embodiment of the present invention.

[0023] Figure 2 It is a schematic view showing an example of a drying shrinkage treatment using a heating roll.

[0024] Figure 3 It is a graph showing the optical properties of the polarizers obtained in the examples and comparative examples.

[0025] SYMBOL DESCRIPTION

[0026] 10 Polarizing film

[0027] 20 First protective layer

[0028] 30 Second protective layer

[0029] 100 Polarizer DETAILED DESCRIPTION OF THE EMBODIMENTS

[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 It is a schematic cross-sectional view of a polarizer according to an embodiment of the present invention. The polarizer 100 has 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 monomer transmittance is 43.5% or more, and the degree of polarization is 99.940% 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) for manufacturing the polarizing film.

[0033] The polarizer can be strip-shaped or single-sheet-shaped. When the polarizer is strip-shaped, it is preferably wound into a roll to form a polarizer roll. The polarizer has excellent optical properties and small deviation in optical properties. In one embodiment, the width of the polarizer is 1000 mm or more, and the difference (D1) between the maximum and minimum values of the monomer transmittance at positions along the width direction is 0.3% or less. The upper limit of D1 is preferably 0.25%, more preferably 0.2%. The smaller D1 is, the more preferable it is, but the lower limit is, for example, 0.01%. When D1 is within the above range, a polarizer with excellent optical properties can be industrially produced. In another embodiment, the polarizer has a difference (D2) between the maximum and minimum values of the monomer transmittance within a 50 cm 2 region of 0.15% or less. The upper limit of D2 is preferably 0.1%, more preferably 0.08%. The smaller D2 is, the more preferable it is, but the lower limit is, for example, 0.01%. When D2 is within the above range, uneven brightness in the display screen can be suppressed when the polarizer is used in an image display device.

[0034] A-1. Polarizing film

[0035] As described above, the thickness of the polarizing film is 8 μm or less, the monomer transmittance is 43.5% or more, and the degree of polarization is 99.940% or more. Generally, there is a trade-off relationship between the monomer transmittance and the absorbance. If the monomer transmittance is increased, the absorbance will decrease, and if the absorbance is increased, the monomer transmittance will decrease. Therefore, it has been difficult to practically use a thin polarizing film that satisfies the optical properties of a monomer transmittance of 43.5% or more and a degree of polarization of 99.940% or more. Achieving a thin polarizing film (polarizer) with excellent optical properties such as a monomer transmittance of 43.5% or more and a degree of polarization of 99.940% or more and with suppressed deviation in optical properties is one of the achievements of the present invention. A polarizer having such a polarizing film can be used in an image display device and is suitably used as a backside polarizer for a liquid crystal 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 range of 380 nm to 780 nm. The monomer transmittance of the polarizing film is preferably 44.0% or less. The degree of polarization of the polarizing film is preferably 99.950% or more and preferably 99.960% or less. On the other hand, the upper limit of the degree of polarization is preferably 99.990%. The above monomer transmittance is typically the Y value obtained by measuring with a UV-visible spectrophotometer and performing visibility correction. The above degree of polarization is typically calculated based on the parallel transmittance Tp and the orthogonal transmittance Tc obtained by measuring with a UV-visible spectrophotometer and performing visibility correction, and is calculated by the following formula.

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

[0039] In one embodiment, the transmittance of the thin polarizing film of 8 μm or less is typically measured using a UV-visible spectrophotometer with the laminate of the polarizing film (refractive index of the surface: 1.53) and the protective film (refractive index: 1.50) as the measurement object. The reflectance at the interface of each layer changes according to 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 changes. Therefore, for example, when using a protective film with a refractive index other than 1.50, the measured value of the transmittance can be corrected according to the refractive index of the surface in contact with the air interface of the protective film. Specifically, the correction value C of the transmittance uses the reflectance R1 (transmittance axis reflectance) of the polarized light parallel to the transmittance axis at the interface between the protective film and the air layer, and is expressed by the following formula.

[0040] C = R1 - R0

[0041] R0 = ((1.50 - 1) 2 / (1.50 + 1) 2 ) × (T1 / 100)

[0042] R1 = ((n1 - 1) 2 / (n1 + 1) 2 ) × (T1 / 100)

[0043] Here, R0 is the transmissive axis reflectance when using a protective film with a refractive index of 1.50, n1 is the refractive index of the protective film used, and T1 is the transmittance of the polarizing film. For example, when using a substrate with a surface refractive index of 1.53 (such as a cycloolefin film, a film with a hard coat, etc.) as the protective film, the correction amount C is approximately 0.2%. In this case, adding 0.2% to the transmittance obtained by measurement can convert it to the transmittance when using a protective film with a surface refractive index of 1.50. It should be noted that according to the calculation based on the above formula, when the transmittance T1 of the polarizing film changes by 2%, the change amount of the correction value C 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, when the protective film has absorption other than surface reflection, appropriate correction can be made according to the absorption amount.

[0044] As the polarizing film, any suitable polarizing film can be used. Representatively, the polarizing film can be made of 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 coated on the resin substrate can be cited. The polarizing film obtained using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate can be produced by the following method: for example, coating a PVA-based resin solution on the resin substrate and drying it to form a PVA-based resin layer on the resin substrate to obtain a laminate of the resin substrate and the PVA-based resin layer; stretching and dyeing the laminate to make the PVA-based resin layer into a polarizing film. In the present embodiment, stretching typically includes immersing the laminate in an aqueous boric acid solution for stretching. In addition, stretching may further include stretching the laminate in a gas atmosphere at a high temperature (for example, 95°C or higher) before stretching in the aqueous boric acid solution as needed. The obtained laminate of the resin substrate / polarizing film can be used directly (that is, the resin substrate can be used as a protective layer of the polarizing film), or the resin substrate can be peeled off from the laminate of the resin substrate / polarizing film, and any suitable protective layer can be laminated on the peeled surface for use. The details of such a method for manufacturing a polarizing film are described, for example, in Japanese Patent Laid-Open No. 2012-73580. The entire description of this publication can be cited as a reference in this specification.

[0046] The manufacturing method of the polarizing film of the present invention includes: forming a polyvinyl alcohol resin layer containing a halide and a polyvinyl alcohol resin on one side of a strip-shaped thermoplastic resin substrate to form a laminate; and sequentially performing an auxiliary stretching treatment, a dyeing treatment, a stretching treatment in an aqueous solution, and a drying shrinkage treatment on the above laminate. In the drying shrinkage treatment, the laminate is heated while being transported in the length direction, so that it shrinks by more than 2% in the width direction. Thus, a polarizing film with a thickness of 8 μm or less, a monomer transmittance of 43.5% or more, and a degree of polarization of 99.940% or more can be obtained. This polarizing film has excellent optical properties and the deviation of the optical properties is suppressed. That is, by introducing auxiliary stretching, even when coating PVA on a thermoplastic resin, the crystallinity of PVA can be improved, and high optical properties can be achieved. In addition, by simultaneously improving the orientation of PVA in advance, problems such as a decrease in the orientation of PVA and dissolution can be prevented when immersed in water in subsequent dyeing and stretching processes, and high optical properties can be achieved. Furthermore, when the PVA-based resin layer is immersed in a liquid, compared with the case where the PVA-based resin layer does not contain a halide, the orientation disorder and the decrease in orientation of polyvinyl alcohol molecules can be suppressed. Thus, the optical properties of the polarizing film obtained by treatment processes such as dyeing treatment and stretching treatment in an aqueous solution, where the laminate is immersed in a liquid, can be improved. In addition, by drying and shrinking the laminate in the width direction, the optical properties can be improved.

[0047] A-2. Protective layer

[0048] The first protective layer and the second protective layer can be formed of any suitable film that can be used as a protective layer of the polarizing film. Specific examples of the material that is the main component of this film include transparent resins such as cellulose resins such as cellulose triacetate (TAC), polyesters, polyvinyl alcohols, polycarbonates, polyamides, polyimides, polyethersulfones, polysulfones, polystyrenes, polynorbornenes, polyolefins, (meth)acrylic acids, and acetates. In addition, thermosetting resins or ultraviolet curable resins such as (meth)acrylic acids, urethanes, (meth)acrylic urethanes, epoxies, and silicones can also be listed. In addition to these, glassy polymers such as siloxane polymers can also be listed. In addition, the polymer film described in Japanese Patent Application Laid-Open No. 2001-343529 (WO01 / 37007) can also be used. As the material of this film, for example, a resin composition containing a thermoplastic resin having a substituted or unsubstituted imide group in the side chain and a thermoplastic resin having a substituted or unsubstituted phenyl group and a nitrile group in the side chain can be used. Specific examples include a resin composition having an alternating copolymer formed of isobutene and N-methylmaleimide and an acrylonitrile-styrene copolymer. This polymer film can be, for example, an extruded product of the above 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. It should be noted 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 display panel side 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 retardation layer having an arbitrary appropriate retardation. In this case, the in-plane retardation Re(550) of the retardation layer is, for example, 110 nm to 150 nm. "Re(550)" is the in-plane retardation measured at a wavelength of 550 nm at 23°C and can be obtained by the formula: Re = (nx - ny) × d. Here, "nx" is the refractive index in the direction in which the refractive index in the plane reaches the maximum (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] A manufacturing method of a polarizing film according to an embodiment of the present invention includes: forming a polyvinyl alcohol resin layer (PVA resin layer) containing a halide and a polyvinyl alcohol resin (PVA resin) on one side of a strip-shaped thermoplastic resin substrate to form a laminate; and sequentially performing an auxiliary stretching treatment, a dyeing treatment, a stretching treatment in an aqueous solution, and a drying shrinkage treatment on the laminate. In the drying shrinkage treatment, the laminate is heated while being transported in the length direction, so that it shrinks by more than 2% in the width direction. With respect to 100 parts by weight of the PVA resin, the content of the halide in the PVA resin layer is preferably 5 to 20 parts by weight. The drying shrinkage treatment is preferably performed using a heating roller, and the temperature of the heating roller is preferably 60°C to 120°C. According to such a manufacturing method, the above-mentioned polarizing film can be obtained. In particular, a laminate including a PVA resin layer containing a halide is produced, and the stretching of the above laminate is performed by multi-stage stretching including auxiliary stretching in a gas atmosphere and stretching in an aqueous solution, and the stretched laminate is heated using a heating roller, whereby a polarizing film having excellent optical properties (representatively, monomer transmittance and degree of polarization) and with the deviation of the optical properties suppressed can be obtained. Specifically, by using a heating roller in the drying shrinkage treatment process, the laminate can be uniformly shrunk as a whole while being transported. Thereby, not only can the optical properties of the obtained polarizing film be improved, but also the stable production of a polarizing film with excellent optical properties can be achieved, and the deviation of the optical properties (especially the monomer transmittance) of the polarizing film can be suppressed.

[0053] B-1. Fabrication of laminate

[0054] As a method for fabricating a laminate of a thermoplastic resin substrate and a PVA resin layer, any suitable method can be adopted. It is preferable to coat a coating liquid containing a halide and a PVA resin on the surface of the thermoplastic resin substrate and dry it, thereby forming a PVA resin layer on the thermoplastic resin substrate. As described above, with respect to 100 parts by weight of the PVA resin, the content of the halide in the PVA resin layer is preferably 5 to 20 parts by weight.

[0055] As a coating method of the coating liquid, any suitable method can be adopted. Examples include: roll coating method, spin coating method, wire-wound bar coating method, dip coating method, die coating method, curtain coating method, spraying method, knife coating method (such as doctor blade coating method), etc. The coating / drying temperature of the above coating liquid is preferably 50°C or higher.

[0056] The thickness of the PVA resin layer is preferably 3 μm to 40 μm, more preferably 3 μm to 20 μm.

[0057] Before forming the PVA-based resin layer, surface treatment (e.g., corona treatment, etc.) can be performed on the thermoplastic resin substrate, or an easy-bonding layer can be formed on the thermoplastic resin substrate. By performing such treatment, the adhesion between the thermoplastic resin substrate and 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 it is less than 20 μm, there is a concern that it is difficult to form the PVA-based resin layer. When it exceeds 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 an excessive load is required for stretching.

[0060] The water absorption rate of the thermoplastic resin substrate is preferably 0.2% or more, more preferably 0.3% or more. When the thermoplastic resin substrate absorbs water, the water acts like a plasticizer and can be plasticized. As a result, the tensile stress can be significantly reduced, and stretching can be performed at a high magnification. On the other hand, the water absorption rate of the thermoplastic resin substrate is preferably 3.0% or less, more preferably 1.0% or less. By using such a thermoplastic resin substrate, it is possible to prevent adverse conditions such as a significant reduction in the dimensional stability of the thermoplastic resin substrate during manufacturing, which may deteriorate the appearance of the obtained polarizing film. In addition, it is possible to prevent the substrate from breaking during stretching in the aqueous solution and the PVA-based resin layer from peeling off from the thermoplastic resin substrate. It should be noted that the water absorption rate of the thermoplastic resin substrate can be adjusted, for example, by introducing a modified group into the constituent material. The water absorption rate 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 lower. By using such a thermoplastic resin substrate, crystallization of the PVA-based resin layer can be suppressed, and the stretchability of the laminate can be sufficiently ensured. In addition, considering the plasticization of the thermoplastic resin substrate using water and good stretching in the aqueous solution, it is more preferably 100 °C or lower, further preferably 90 °C or lower. On the other hand, the glass transition temperature of the thermoplastic resin substrate is preferably 60 °C or higher. By using such a thermoplastic resin substrate, when coating / drying the above-mentioned coating liquid containing the PVA-based resin, it is possible to prevent adverse conditions such as deformation (e.g., occurrence of unevenness, slack, wrinkles, etc.) of the thermoplastic resin substrate, and a laminate can be well produced. In addition, stretching of the PVA-based resin layer can be well performed at an appropriate temperature (e.g., around 60 °C). It should be noted that the glass transition temperature (Tg) of the thermoplastic resin substrate can be adjusted, for example, by using a crystallization material into which a modified group is introduced into the constituent material and heating. The glass transition temperature (Tg) is a value obtained in accordance with JIS K 7121.

[0062] As a constituent material of the thermoplastic resin substrate, any suitable thermoplastic resin can be used. Examples of the thermoplastic resin include ester resins such as polyethylene terephthalate resins, cycloolefin resins such as norbornene resins, olefin resins such as polypropylene, polyamide resins, polycarbonate resins, copolymer resins thereof, and the like. Among these, norbornene resins and amorphous polyethylene terephthalate resins are preferred.

[0063] In one embodiment, an amorphous (uncrystallized) polyethylene terephthalate resin is preferably used. Among them, an amorphous (difficult-to-crystallize) polyethylene terephthalate resin is particularly preferably used. Specific examples of the amorphous polyethylene terephthalate resin include copolymers further containing isophthalic acid and / or cyclohexanedicarboxylic acid as dicarboxylic acids, and copolymers further containing cyclohexanedimethanol and diethylene glycol as diols.

[0064] In a preferred embodiment, the thermoplastic resin substrate is composed of a polyethylene terephthalate resin having an isophthalic acid unit. This is because such a thermoplastic resin substrate has excellent stretchability and can suppress crystallization during stretching. It is considered that by introducing the isophthalic acid unit, a large bend can be imparted to the main chain. The polyethylene terephthalate resin has a terephthalic acid unit and an ethylene glycol unit. The content ratio of the isophthalic acid unit is preferably 0.1 mol% or more, more preferably 1.0 mol% or more, based on the total of all repeating units. This is because a thermoplastic resin substrate with very excellent stretchability can be obtained. On the other hand, the content ratio of the isophthalic acid unit is preferably 20 mol% or less, more preferably 10 mol% or less, based on the total of all repeating units. By setting the content ratio in this way, the crystallinity can be improved well in the drying shrinkage treatment described later.

[0065] The thermoplastic resin substrate can be pre-stretched (before forming the PVA resin layer). In one embodiment, the long strip-shaped thermoplastic resin substrate can be stretched in the transverse direction. The transverse direction is preferably orthogonal to the stretching direction of the laminate described later. It should be noted that in this specification, "orthogonal" means including substantially orthogonal cases. Among them, "substantially orthogonal" means including cases of 90°±5.0°, preferably 90°±3.0°, and more 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 a method for stretching a thermoplastic resin substrate, any appropriate method can be adopted. Specifically, it can be fixed-end stretching or free-end stretching. The stretching method can be dry or wet. The stretching of the thermoplastic resin substrate can be carried out in one stage or in multiple stages. In the case of multiple stages, the above stretching ratio is the product of the stretching ratios of each stage.

[0068] B-1-2. Coating liquid

[0069] As described above, the coating liquid contains a halide and a PVA-based resin. Typically, the above coating liquid can be a solution obtained by dissolving the above halide and the above PVA-based resin in a solvent. Examples of the solvent include: water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, amines such as ethylenediamine and diethylenetriamine. They can be used alone or two or more of them can be used in combination. Among these, water is preferred. The concentration of the PVA-based resin in 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] Additives can be incorporated into the coating liquid. Examples of the additives include: plasticizers, surfactants, etc. Examples of the plasticizer include: polyhydric alcohols such as ethylene glycol and glycerol. Examples of the surfactant include: nonionic surfactants. 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 above PVA-based resin, any appropriate resin can be adopted. For example, polyvinyl alcohol and ethylene-vinyl alcohol copolymer can be cited. Polyvinyl alcohol can be obtained by saponifying polyvinyl acetate. 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 more preferably 99.0 mol% to 99.93 mol%. The saponification degree can be determined in accordance with JIS K 6726-1994. By using a PVA-based resin with such a saponification degree, a polarizing film with excellent durability can be obtained. In the case where the saponification degree is too high, there is a concern of gelation.

[0072] The average degree of polymerization of the PVA-based resin can be appropriately selected according to the purpose. The average degree of polymerization is generally 1000 to 10000, preferably 1200 to 4500, and more preferably 1500 to 4300. It should be noted that the average degree of polymerization can be determined according to JIS K 6726-1994.

[0073] As the above-mentioned halide, any suitable halide can be used. For example, iodides and sodium chloride can be cited. As iodides, potassium iodide, sodium iodide, and lithium iodide can be cited. Among these, potassium iodide is preferred.

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

[0075] Generally, by stretching the PVA-based resin layer, the orientation of polyvinyl alcohol molecules in the PVA-based resin is improved. However, if the stretched PVA-based resin layer is immersed in a liquid containing water, there is a case where the orientation of polyvinyl alcohol molecules is disordered and the orientation is lowered. In particular, when stretching a laminate of a thermoplastic resin and a PVA-based resin layer in an aqueous boric acid solution, when stretching the laminate in the aqueous boric acid solution at a relatively high temperature to stabilize the stretching of the thermoplastic resin, the tendency of the above-mentioned degree of orientation to decrease is significant. For example, the stretching of the PVA film monomer in the aqueous boric acid solution is usually carried out at 60 °C. In contrast, the stretching of the laminate of A-PET (thermoplastic resin substrate) and the PVA-based resin layer is carried out at a high temperature such as around 70 °C. In this case, the orientation of PVA at the initial stage of stretching will decrease before rising through stretching in the aqueous solution. In contrast, by producing a laminate of a PVA-based resin layer containing a halide and a thermoplastic resin substrate, and performing high-temperature stretching (auxiliary stretching) in the air before stretching the laminate in the aqueous boric acid solution, the crystallization of the PVA-based resin in the PVA-based resin layer of the laminate after auxiliary stretching can be promoted. As a result, when the PVA-based resin layer is immersed in a liquid, compared with the case where the PVA-based resin layer does not contain a halide, the disorder of the orientation of polyvinyl alcohol molecules and the decrease in the orientation can be suppressed. Thereby, the optical properties of the polarizing film obtained through treatment processes such as dyeing treatment and stretching treatment in an aqueous solution, and immersing the laminate in a liquid can be improved.

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

[0077] Specifically, in order to obtain high optical properties, a two-stage stretching method that combines dry stretching (auxiliary stretching) and stretching in an aqueous boric acid solution is selected. By introducing auxiliary stretching as in the two-stage stretching, stretching can be performed while suppressing crystallization of the thermoplastic resin substrate, and the problem of reduced stretchability caused by excessive crystallization of the thermoplastic resin substrate during subsequent stretching in the aqueous boric acid solution can be solved, and the laminate can be stretched at a higher magnification. Conventionally, when a PVA-based resin is coated on a thermoplastic resin substrate, in order to suppress the influence of the glass transition temperature of the thermoplastic resin substrate, the coating temperature needs to be lowered compared with the case of coating the PVA-based resin on a metal drum. As a result, there is a problem that the crystallization of the PVA-based resin is relatively reduced and sufficient optical properties cannot be obtained. In contrast, by introducing auxiliary stretching, even when a PVA-based resin is coated on a 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, problems such as a decrease in the orientation of the PVA-based resin and dissolution when immersed in water during subsequent dyeing and stretching processes can be prevented, and high optical properties can be achieved.

[0078] The stretching method of auxiliary stretching in a gas atmosphere can be fixed-end stretching (for example, a method of stretching using a tenter), or free-end stretching (for example, a method of performing unidirectional stretching by passing the laminate between rolls with different circumferential speeds). In order to obtain high optical properties, free-end stretching can be actively adopted. In one embodiment, the stretching treatment in a gas atmosphere includes a heating roll stretching step of stretching the laminate while transporting it in the length direction and by the difference in circumferential speed between heating rolls. The stretching treatment in a gas atmosphere typically includes a zone stretching step and a heating roll stretching step. It should be noted that the order of the zone stretching step and the heating roll stretching step is not limited, and the zone stretching step can be performed first, or the heating roll stretching step can be performed first. The zone stretching step can also be omitted. In one embodiment, the zone stretching step and the heating roll stretching step are performed in sequence. In addition, in another embodiment, in a tenter, the film end is held, and the distance between the tenters is expanded in the transport direction for stretching (the expansion of the distance between the tenters becomes the stretching ratio). At this time, the distance between the tenters in the width direction (perpendicular to the transport direction) is arbitrarily set to be close. Preferably, it can be set in a manner closer to free-end stretching with respect to the stretching ratio in the transport direction. In the case of free-end stretching, the shrinkage rate in the width direction = (1 / stretching ratio) 1 / 2 is calculated.

[0079] The auxiliary stretching in a gas atmosphere can be carried out in one stage or in multiple stages. In the case of multiple stages, the stretching ratio is the product of the stretching ratios of each stage. The stretching direction in the auxiliary stretching in a gas atmosphere is preferably substantially the same as the stretching direction in the stretching in an aqueous solution.

[0080] The stretching ratio in the auxiliary stretching in a gas atmosphere is preferably 2.0 to 3.5 times. In the case of combining the auxiliary stretching in a gas atmosphere and the stretching in an aqueous solution, the maximum stretching ratio with respect to the original length of the laminate is preferably 5.0 times or more, more preferably 5.5 times or more, and further preferably 6.0 times or more. In this specification, the "maximum stretching ratio" means the stretching ratio just before the laminate breaks. Additionally, when confirming the stretching ratio at which the laminate breaks, the "maximum stretching ratio" means a value 0.2 smaller than this value.

[0081] The stretching temperature in the auxiliary stretching in a gas atmosphere can be set to any appropriate value according to the forming material of the thermoplastic resin substrate, the stretching method, etc. The stretching temperature is preferably above the glass transition temperature (Tg) of the thermoplastic resin substrate, more preferably above the glass transition temperature (Tg) of the thermoplastic resin substrate + 10°C, and particularly preferably above Tg + 15°C. On the other hand, the upper limit of the stretching temperature is preferably 170°C. By stretching at such a temperature, the rapid crystallization of the PVA-based resin can be suppressed, and the adverse conditions caused by this crystallization (for example, hindering the orientation of the PVA-based resin layer due to stretching) can be suppressed.

[0082] B-3. Insolubilization treatment

[0083] If necessary, the insolubilization treatment is carried out after the auxiliary stretching treatment in a gas atmosphere and before the stretching treatment in an aqueous solution and the dyeing treatment. The above insolubilization treatment is typically carried out by immersing the PVA-based resin layer in a boric acid aqueous solution. By carrying out the insolubilization treatment, water resistance can be imparted to the PVA-based resin layer, and the orientation of PVA can be prevented from decreasing when immersed in water. The concentration of this boric acid aqueous solution is preferably 1 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 dyeing treatment is typically carried out by dyeing the PVA-based resin layer with iodine. Specifically, it is carried out by adsorbing iodine onto the PVA-based resin layer. As this adsorption method, for example, a method of immersing the PVA-based resin layer (laminate) in a dyeing solution containing iodine; a method of coating the dyeing solution on the PVA-based resin layer; a method of spraying the dyeing solution on the PVA-based resin layer, etc. are exemplified. The method of immersing the laminate in the dyeing solution (dyeing bath) is preferred. This is because iodine can be adsorbed well.

[0086] The above-mentioned staining solution is preferably an aqueous iodine solution. The compounding amount 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 increase the solubility of iodine in water, an iodide is preferably compounded in the aqueous iodine solution. Examples of the iodide include potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, titanium iodide, etc. Among these, potassium iodide is preferred. The compounding amount 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 during staining with the staining solution is preferably 20°C to 50°C. When the PVA-based resin layer is immersed in the staining 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 staining conditions (concentration, liquid temperature, immersion time) can be set such that the monomer transmittance of the finally obtained polarizing film is 43.5% or more and the degree of polarization is 99.940% or more. As such staining conditions, it is preferable to use an aqueous iodine solution as the staining solution, and set the content ratio of iodine to potassium iodide in the aqueous iodine solution to 1:5 to 1:20. The content ratio of iodine to potassium iodide in the aqueous iodine solution is preferably 1:5 to 1:10. Thus, a polarizing film having the above-described optical characteristics can be obtained.

[0088] When the staining treatment is continuously performed after the treatment (typically insolubilization treatment) of immersing the laminate in a treatment bath containing boric acid, by mixing the boric acid contained in the treatment bath into the staining bath, the boric acid concentration in the staining bath changes over time. As a result, the dyeability sometimes becomes unstable. In order to suppress the instability of the dyeability as described above, the upper limit of the boric acid concentration in the staining bath is adjusted to 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 in the staining 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 staining treatment is performed using a staining bath in which boric acid is pre-compounded. Thus, the change ratio of the boric acid concentration in the case where the boric acid in the above treatment bath is mixed into the staining bath can be reduced. The compounding amount of boric acid pre-compounded in the staining bath (that is, the content of boric acid not from the above 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] As needed, a crosslinking treatment is carried out after the dyeing treatment and before the stretching treatment in an aqueous solution. The above crosslinking treatment is typically carried out by immersing the PVA-based resin layer in an aqueous boric acid solution. By carrying out the crosslinking treatment, water resistance can be imparted to the PVA-based resin layer, and during the subsequent stretching in an aqueous solution, a decrease in the orientation of PVA when immersed in hot water can be prevented. The concentration of this aqueous boric acid solution is preferably 1 part by weight to 5 parts by weight relative to 100 parts by weight of water. In addition, when carrying out the crosslinking treatment after the above dyeing treatment, an iodide is preferably further incorporated. By incorporating an iodide, elution of iodine adsorbed on the PVA-based resin layer can be suppressed. The incorporation amount of the iodide is preferably 1 part by weight to 5 parts by weight relative to 100 parts by weight of water. Specific examples of the iodide are as described above. The liquid temperature of the crosslinking bath (aqueous boric acid solution) is preferably 20°C to 50°C.

[0091] B-6. Stretching Treatment in Aqueous Solution

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

[0093] Any appropriate method can be adopted for the stretching method of the laminate. Specifically, it can be fixed-end stretching or free-end stretching (for example, a method of unidirectionally stretching the laminate by passing it between rolls with different circumferential speeds), and free-end stretching is preferably selected. The stretching of the laminate can be carried out in one stage or in multiple stages. In the case of carrying out in multiple stages, the stretching ratio (maximum stretching ratio) of the laminate described later is the product of the stretching ratios of each stage.

[0094] The stretching in an aqueous solution is preferably carried out 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 that can withstand the tension applied during stretching and water resistance that does not dissolve in water can be imparted to the PVA-based resin layer. Specifically, boric acid can generate tetrahydroxyborate anions in an aqueous solution and crosslink with the PVA-based resin through hydrogen bonds. As a result, rigidity and water resistance can be imparted to the PVA-based resin layer, stretching can be carried out well, and a polarizing film having excellent optical properties can be manufactured.

[0095] The above-mentioned boric acid aqueous solution is preferably obtained by dissolving boric acid and / or borate in water as a solvent. The concentration of boric acid is preferably 1 to 10 parts by weight, more preferably 2.5 to 6 parts by weight, and particularly preferably 3 to 5 parts by weight with respect 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 characteristics can be manufactured. It should be noted that an aqueous solution obtained by dissolving a boron compound such as borax or glyoxal, glutaraldehyde, etc., other than boric acid or borate in a solvent can also be used.

[0096] An iodide is preferably added to the above-mentioned stretching bath (boric acid aqueous solution). By adding an iodide, the elution of iodine adsorbed on the PVA-based resin layer can be suppressed. Specific examples of the iodide are as described above. The concentration of the iodide is preferably 0.05 to 15 parts by weight, more preferably 0.5 to 8 parts by weight with respect to 100 parts by weight of water.

[0097] The stretching temperature (the liquid temperature of the stretching bath) is preferably 40°C to 85°C, more preferably 60°C to 75°C. If the temperature is within this range, the dissolution of the PVA-based resin layer can be suppressed, and stretching can be performed at a high magnification. Specifically, as described above, considering the relationship with the formation of the PVA-based resin layer, the glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 60°C or higher. In this case, when the stretching temperature is lower than 40°C, there is a concern that good stretching cannot be achieved even considering the plasticization of the thermoplastic resin substrate by water. 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 excellent optical characteristics cannot be obtained. The impregnation time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

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

[0099] B-7. Drying shrinkage treatment

[0100] The above-mentioned drying shrinkage treatment can be carried out by area heating by heating the entire area, or by heating the conveying rollers (using so-called heating rollers) (heating roller drying method). It is preferred to use both. By drying using heating rollers, the heating curl of the laminate can be efficiently suppressed, thereby manufacturing a polarizing film with excellent appearance. Specifically, by drying the laminate in a state along the heating rollers, the crystallization of the above-mentioned thermoplastic resin substrate can be efficiently promoted, and the crystallinity can be increased. Even at a relatively low drying temperature, the crystallinity of the thermoplastic resin substrate can be increased well. As a result, the rigidity of the thermoplastic resin substrate increases, and it becomes a state capable of withstanding the shrinkage of the PVA-based resin layer caused by drying, and curling can be suppressed. In addition, by using heating rollers, the laminate can be dried while being kept in a flat state. Therefore, not only curling can 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, whereby the optical characteristics can be improved. This is because the orientation of PVA and the PVA / iodine complex can be effectively improved. The shrinkage rate of the laminate in the width direction by the drying shrinkage treatment is preferably 2% to 10%, more preferably 2% to 8%, and particularly preferably 4% to 6%. By using heating rollers, the laminate can be continuously shrunk in the width direction while being conveyed, and high productivity can be achieved.

[0101] Figure 2 FIG. is a schematic view showing an example of the drying shrinkage treatment. In the drying shrinkage treatment, the laminate 200 is dried while being conveyed by the conveying rollers R1 to R6 and the guide rollers G1 to G4 heated to a given temperature. In the illustrated example, the conveying rollers R1 to R6 are arranged so as to continuously heat the surface of the PVA resin layer and the surface of the thermoplastic resin substrate alternately, but the conveying rollers R1 to R6 may be arranged, for example, so as to continuously heat only one surface of the laminate 200 (for example, the thermoplastic resin substrate surface).

[0102] By adjusting the heating temperature of the conveying rollers (the temperature of the heating rollers), the number of heating rollers, the contact time with the heating rollers, etc., the drying conditions can be controlled. The temperature of the heating rollers 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 increased well, curling can be suppressed well, and an optical laminate with very excellent durability can be manufactured. It should be noted that the temperature of the heating rollers 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 a plurality of conveying rollers. Usually, 2 to 40 conveying rollers are provided, and preferably 4 to 30 conveying rollers are provided. The contact time (total contact time) of the laminate with the heating rollers is preferably 1 second to 300 seconds, more preferably 1 to 20 seconds, and further preferably 1 to 10 seconds.

[0103] The heating roller can be disposed in a heating furnace (such as an oven), or can be disposed in a normal manufacturing line (at room temperature). It is preferably disposed in a heating furnace equipped with a blowing mechanism. By combining the drying using the heating roller and hot air drying, a sharp temperature change between the heating rollers can be suppressed, and the shrinkage in the width direction can be easily controlled. 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 wind speed of the hot air is preferably about 10 m / s to 30 m / s. It should be noted that this wind speed is the wind speed in the heating furnace and can be measured by a mini vane type digital anemometer.

[0104] B-8. Other Treatments

[0105] It is preferable to perform a cleaning treatment after the stretching treatment in an 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] C. Summary

[0107] The thickness of the polarizing film of the present invention is 8 μm or less, the monomer transmittance is 43.5% or more, and the degree of polarization is 99.940% or more.

[0108] In one embodiment, the monomer transmittance of the polarizing film is 44.0% or less, and the degree of polarization is 99.990% or less.

[0109] According to another aspect of the present invention, there is provided a polarizing plate having a polarizing film and a protective layer disposed on at least one side of the polarizing film.

[0110] According to another aspect of the present invention, there is provided a method for manufacturing a polarizing film, which is the method for manufacturing the above polarizing film, and the method includes: forming a PVA-based resin layer containing a halide and a polyvinyl alcohol-based resin on one side of a strip-shaped thermoplastic resin substrate to form a laminate; and sequentially performing an auxiliary stretching treatment, a dyeing treatment, a stretching treatment in an aqueous solution, and a drying shrinkage treatment on the laminate. In the drying shrinkage treatment, the laminate is heated while being transported in the length direction so that it shrinks by 2% or more in the width direction.

[0111] In one embodiment, in the above PVA-based resin layer, the content of the halide is 5 parts by weight to 20 parts by weight relative to 100 parts by weight of the PVA-based resin.

[0112] In one embodiment, the stretching ratio in the auxiliary stretching treatment in the gas atmosphere is 2.0 times or more.

[0113] In one embodiment, the drying shrinkage treatment step is a step of heating using a heating roller.

[0114] In one embodiment, 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.

[0115] Examples

[0116] Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited to these examples. The measurement methods for each characteristic are as described below. It should be noted that unless otherwise specified, "parts" and "%" in the examples and comparative examples are based on weight.

[0117] (1) Thickness

[0118] Measured using an interference film thickness meter (manufactured by Otsuka Electronics Co., Ltd., product name "MCPD-3000").

[0119] (2) Monomer transmittance and degree of polarization

[0120] For the polarizing plates (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 (V-7100 manufactured by JASCO Corporation) were taken as Ts, Tp, and Tc of the polarizing film. These Ts, Tp, and Tc are Y values obtained by measuring with a 2-degree field of view (C light source) of JIS Z8701 and performing visibility correction. It should be noted that the refractive index of the protective film is 1.50, and the refractive index of the surface of the polarizing film on the side opposite to the protective film is 1.53.

[0121] Based on the obtained Tp and Tc, the degree of polarization P was calculated by the following formula.

[0122] Degree of polarization P (%) = {(Tp - Tc) / (Tp + Tc)} 1 / 2 × 100

[0123] It should be noted that equivalent measurements can be performed using an LPF-200 or the like manufactured by Otsuka Electronics Co., Ltd. As an example, for samples 1 to 3 of polarizing plates having the same configuration as the following examples, the monomer transmittance Ts and the degree of polarization P were obtained by measurements using V-7100 and LPF-200, and their measured values are shown in Table 1. As shown in Table 1, it can be seen that the difference between the measured values of the monomer transmittance of V-7100 and the measured values of the monomer transmittance of LPF-200 is 0.1% or less, and equivalent measurement results are obtained regardless of which spectrophotometer is used.

[0124]

[0125] Note that, for example, when a polarizing plate with an anti-glare (AG) surface treatment and an adhesive having diffusion properties is used as a measurement object, different measurement results can be obtained depending on the spectrophotometer. In this case, numerical conversion is performed based on the measured values when the same polarizing plate is measured with each spectrophotometer, thereby compensating for the difference in measured values depending on the spectrophotometer.

[0126] (3) Deviation of the optical properties of the strip-shaped polarizing plate

[0127] From the strip-shaped polarizing plates used in the examples and reference examples, measurement samples were 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 (2) above. Then, the difference between the maximum value and the minimum value among the monomer transmittances measured at each measurement position was calculated, and this value was set as the deviation of the optical properties of the strip-shaped polarizing plate (the difference between the maximum value and the minimum value of the monomer transmittance at the positions along the width direction of the strip-shaped polarizing plate).

[0128] (4) Deviation of the optical properties of the sheet-shaped polarizing plate

[0129] From the strip-shaped polarizing plates used in the examples and reference examples, a 100 mm × 100 mm measurement sample was cut out, and the deviation of the optical properties of the sheet-shaped polarizing plate (50 cm 2 ) was determined. Specifically, the monomer transmittance at a total of five positions, namely, at positions approximately 1.5 cm to 2.0 cm inward from the midpoints of each of the four sides of the measurement sample and at the central portion, was measured in the same manner as in (2) above. Next, the difference between the maximum value and the minimum value among the monomer transmittances measured at each measurement position was calculated, and this value was set as the deviation of the optical properties of the sheet-shaped polarizing plate (the difference between the maximum value and the minimum value of the monomer transmittance within the 50 cm 2 area).

[0130] [Example 1]

[0131] 1. Production of the polarizing film

[0132] As the thermoplastic resin substrate, an amorphous isophthalic acid copolyethylene terephthalate film (thickness: 100 μm) in strip shape with a water absorption rate of 0.75% and a Tg of about 75 °C was used. Corona treatment was performed on one side of the resin substrate.

[0133] In 100 parts by weight of a PVA-based resin obtained by mixing polyvinyl alcohol (degree of polymerization 4200, saponification degree 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFIMER Z410") in a ratio of 9:1, 13 parts by weight of potassium iodide was added to prepare a PVA aqueous solution (coating solution).

[0134] The above-mentioned PVA aqueous solution was coated on the corona-treated surface of the resin substrate and dried at 60 °C, thereby forming a PVA-based resin layer with a thickness of 13 μm to produce a laminate.

[0135] In an oven at 130 °C, between rollers with different circumferential speeds, the obtained laminate was unidirectionally stretched to 2.4 times in the longitudinal direction (length direction) at the free end (auxiliary stretching treatment in a gas atmosphere).

[0136] Next, the laminate was immersed in an insolubilization bath at a liquid temperature of 40 °C (a boric acid aqueous solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) for 30 seconds (insolubilization treatment).

[0137] Next, in a dyeing bath at a liquid temperature of 30 °C (an iodine aqueous solution obtained by mixing iodine and potassium iodide in a weight ratio of 1:7 with 100 parts by weight of water), the laminate was immersed for 60 seconds while adjusting the concentration so that the monomer transmittance (Ts) of the finally obtained polarizing film became 43.5% or more (dyeing treatment).

[0138] Next, the laminate was immersed in a crosslinking bath at a liquid temperature of 40 °C (a boric acid aqueous solution obtained by mixing 3 parts by weight of potassium iodide and 5 parts by weight of boric acid with 100 parts by weight of water) for 30 seconds (crosslinking treatment).

[0139] Then, while immersing the laminate in a boric acid aqueous solution at a liquid temperature of 70 °C (boric acid concentration 4.0% by weight), between rollers with different circumferential speeds, unidirectional stretching was performed so that the total stretching ratio in the longitudinal direction (length direction) became 5.5 times (stretching treatment in an aqueous solution).

[0140] Then, the laminate was immersed in a cleaning bath at a liquid temperature of 20 °C (an aqueous solution obtained by mixing 4 parts by weight of potassium iodide with 100 parts by weight of water) (cleaning treatment).

[0141] Then, while drying in an oven maintained at 90 °C, the laminate was brought into contact with a SUS heating roller with a surface temperature maintained at 75 °C for about 2 seconds (dry shrinkage treatment). The shrinkage rate of the laminate in the width direction was 5.2% through the dry shrinkage treatment.

[0142] In this way, a polarizing film with a thickness of 5 μm was formed on the resin substrate. In addition, the same process was repeated to produce a total of 14 polarizing films.

[0143] 2. Production of Polarizer

[0144] On the surface of each of the obtained polarizing films (the side opposite to the resin substrate), an acrylic film (surface refractive index 1.50, 40 μm) as a protective film was adhered by an ultraviolet curable adhesive. Specifically, the adhesive was applied so that the total thickness of the curable adhesive became 1.0 μm, and lamination was performed using a roll machine. Then, UV light was irradiated from the protective film side to cure the adhesive. Next, after cutting both ends, the resin substrate was peeled off to obtain 14 long strip-shaped polarizers (width: 1300 mm) having a structure of protective film / polarizing film.

[0145] [Example 2]

[0146] In the dry shrinkage treatment, the oven temperature was set to 70 °C and the heating roll temperature was set to 70 °C. Otherwise, 4 polarizing films and polarizers were produced in the same manner as in Example 1. The shrinkage rate of the laminate in the width direction due to the dry shrinkage treatment was 2.5%.

[0147] [Comparative Example 1]

[0148] Potassium iodide was not added to the PVA aqueous solution (coating solution), the drawing ratio in the assisted stretching treatment in the gas atmosphere was set to 1.8 times, and a heating roll was not used in the dry shrinkage treatment. Otherwise, 8 polarizing films and polarizers were produced in the same manner as in Example 1.

[0149] [Comparative Example 2]

[0150] The drawing ratio in the assisted stretching treatment in the gas atmosphere was set to 1.8 times, and a heating roll was not used in the dry shrinkage treatment. Otherwise, 6 polarizing films and polarizers were produced in the same manner as in Example 1.

[0151] [Reference Example 1]

[0152] The polarizing film obtained in the same manner as in Comparative Example 2 was kept in a thermo-hygrostat region set at a temperature of 60 °C and a humidity of 90% RH for 30 minutes. Then, a polarizer was produced in the same manner as in Example 1.

[0153] The monomer transmittance and degree of polarization of each polarizer of the examples and comparative examples were measured. The results are shown in Table 2 and Figure 3 .

[0154]

[0155] The polarizing films obtained by the manufacturing methods of the comparative examples could not satisfy both a monomer transmittance of 43.5% or more and a degree of polarization of 99.940% or more. In contrast, the polarizing films obtained by the manufacturing methods of the examples had excellent optical properties with a monomer transmittance of 43.5% or more and a degree of polarization of 99.940% or more.

[0156] The deviations in the optical properties of the strip-shaped and single-piece polarizers were measured for the polarizers of the examples and reference examples. The results are shown in Table 3.

[0157]

[0158] The deviation in the monomer transmittance of the strip-shaped polarizer obtained by the production method of the examples was 0.3% or less, and the deviation in the monomer transmittance of the single-piece polarizer obtained by the production method of the examples was 0.15% or less, and the deviation in the optical properties was suppressed to an acceptable level. On the other hand, in the polarizers of the reference examples obtained through the step of humidifying the polarizing film, the deviations in the optical properties of both the strip-shaped and single-piece polarizers were large.

[0159] Industrial Applicability

[0160] The polarizer of the present invention can be suitably used as a circular polarizer for liquid crystal display devices, organic EL display devices, and 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 monomer transmittance of 43.5% or more, and a degree of polarization of 99.940% or more. The method includes: forming a polyvinyl alcohol resin layer containing an iodide or sodium chloride and a polyvinyl alcohol resin on one side of a strip-shaped thermoplastic resin substrate to form a laminate; and successively performing an auxiliary stretching treatment in a gas atmosphere, a dyeing treatment, a stretching treatment in an aqueous solution, and a drying and shrinking treatment on the laminate. In the drying and shrinking treatment, the laminate is heated while being transported in the length direction to cause it to shrink by 2% or more in the width direction.

2. The manufacturing method according to claim 1, which is a method for manufacturing a polarizing film having a monomer transmittance of 44.0% or less and a degree of polarization of 99.990% 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 the sodium chloride is 5 to 20 parts by weight relative to 100 parts by weight of the polyvinyl alcohol resin.

4. The manufacturing 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 manufacturing method according to any one of claims 1 to 4, wherein the drying and shrinking treatment step is a step of heating 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 and shrinking treatment is 2% or more.

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