Polyvinyl alcohol resin film and method for manufacturing polyvinyl alcohol resin film

Abstract

To provide a polyvinyl alcohol resin film capable of preventing defects of a film surface from occurring and excellent in appearance, and a method for manufacturing the same.SOLUTION: A polyvinyl alcohol resin film has: a thickness of 45 μm or less; a water absorption of 1-10%; a static friction coefficient of 2.55 or less to a stainless plate after humidification for 2 hours at 40°C and 90% RH; a long size; a width of 2000 mm or more and an average polymerization degree of 3000 or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a polyvinyl alcohol-based resin film and a method for producing the polyvinyl alcohol-based resin film. [Background technology]

[0002] Polyvinyl alcohol-based resin films are used in various applications such as packaging materials (for example, Patent Document 1). Another known application of polyvinyl alcohol-based resin films is polarizers used in image display devices and the like. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2021-143297 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, there has been a demand for optical components that meet higher standards of appearance, such as those for high-definition organic electroluminescence (EL) displays and lens-enlarged projection for virtual reality (VR) applications. This has led to a demand for polyvinyl alcohol-based resin films that are less susceptible to scratches on the film surface and have an excellent appearance. [Means for solving the problem]

[0005] 1. A polyvinyl alcohol-based resin film according to an embodiment of the present invention comprises a polyvinyl alcohol-based resin having a thickness of 45 μm or less, a water absorption rate of 1% to 10%, a static friction coefficient against a stainless steel plate after humidity conditioning at 40°C and 90% RH for 2 hours of 2.55 or less, a long shape, a width of 2000 mm or more, and an average degree of polymerization of 3000 or more. 2. The polyvinyl alcohol-based resin film described in 1 above may have an arithmetic mean surface roughness Ra of 0.05 μm or less. 3. The polyvinyl alcohol resin film according to 1 or 2 above may have a thickness variation in the width direction of 1 μm or less. 4. The polyvinyl alcohol-based resin film according to any one of the above items 1 to 3 may be a layer formed by applying a solution containing the polyvinyl alcohol-based resin to a substrate. 5. The polyvinyl alcohol-based resin film according to any one of 1 to 4 above may be used to produce a polarizer. 6. In another embodiment of the present invention, there is provided a method for producing a polyvinyl alcohol-based resin film having a static friction coefficient of 2.55 or less against a stainless steel plate after conditioning at 40°C and 90% RH for 2 hours, which method includes the steps of applying a solution containing a polyvinyl alcohol-based resin to a substrate to form a polyvinyl alcohol-based resin layer and producing an intermediate laminate, and dry-stretching the intermediate laminate while heating it at 130°C or higher, in that order. 7. In yet another aspect of the present invention, there is provided a method for producing a polarizer, which includes dyeing, with a dichroic substance, a polyvinyl alcohol-based resin film having a static friction coefficient of 2.55 or less against a stainless steel plate after humidity conditioning at 40°C and 90% RH for two hours. 8. The method for producing a polarizer described in 7 above may further include the steps of applying a solution containing a polyvinyl alcohol-based resin to a substrate to form a polyvinyl alcohol-based resin layer, thereby producing an intermediate laminate, and dry-stretching the intermediate laminate while heating it at 130°C or higher, in this order, to obtain the polyvinyl alcohol-based resin film. [Effects of the Invention]

[0006] According to an embodiment of the present invention, a polyvinyl alcohol-based resin film that is excellent in appearance and in which the occurrence of scratches on the film surface is suppressed even when subjected to various manufacturing processes, and a manufacturing method thereof are provided. [Brief explanation of the drawings]

[0007] [Figure 1]1A to 1C are schematic diagrams illustrating a method for manufacturing a polarizer according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0008] A. Polyvinyl alcohol resin film A polyvinyl alcohol (hereinafter also referred to as PVA)-based resin film according to an embodiment of the present invention comprises a polyvinyl alcohol-based resin having a thickness of 45 μm or less, a water absorption rate of 1% to 10%, a static friction coefficient against a stainless steel plate of 2.55 or less after humidity conditioning at 40°C and 90% RH for 2 hours, a long shape, a width of 2000 mm or more, and an average degree of polymerization of 3000 or more. Polyvinyl alcohol-based resin films are used in a variety of applications. Long resin films are typically transported while in contact with rolls and subjected to various processes. As a result, scratches may be formed on the surface of the PVA-based resin film, deteriorating the film's appearance. One factor contributing to the deterioration of the film's appearance is thought to be changes in the environment (e.g., humidity) in which the film is placed, which changes the resistance of the PVA-based resin film surface and causes scratches on the PVA-based resin film surface due to friction caused by contact with the rolls. These scratches are minor and do not pose a problem in conventional image display devices. Even when the polyvinyl alcohol-based resin film of the embodiment of the present invention is transported while being in contact with a roll, the influence of the production environment and the type of production process on the resistance of the film surface can be suppressed, and scratches on the PVA-based resin film surface can be suppressed.

[0009] The thickness of the PVA-based resin film is 45 μm or less, preferably 30 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less. The thickness of the PVA-based resin film is, for example, 1 μm or more. If the thickness of the PVA-based resin film is within the above range, the occurrence of scratches on the film surface is suppressed, and a polyvinyl alcohol-based resin film with excellent appearance can be obtained.

[0010] The water absorption rate of the PVA-based resin film is 1% to 10%, preferably 2% to 9%, more preferably 3% to 8%, even more preferably 3.5% to 7%, and particularly preferably 4% to 6%. If the water absorption rate of the PVA-based resin film is within the above range, the occurrence of scratches on the film surface is suppressed, and a polyvinyl alcohol-based resin film with excellent appearance can be obtained. In this specification, the water absorption rate of the PVA-based resin film refers to a value measured by the following method. <Method for measuring water absorption> A measurement sample measuring 1 cm square is cut out from the PVA-based resin film. This measurement sample is placed in a moisture adsorption / desorption measuring device (for example, Hiden's product name "IGA-SORP"). The measurement sample is then held at 23°C and 0% RH for 600 minutes, and then held at 40°C and 90% RH for 120 minutes. The weight after holding at 23°C and 0% RH is designated W1, and the weight after holding at 40°C and 90% RH for 120 minutes is designated W2, and the weight is calculated using the following formula. Water absorption rate (%)=(W2-W1) / W1×100

[0011] The static friction coefficient of the PVA-based resin film against a stainless steel plate after conditioning at 40°C and 90% RH for 2 hours is 2.55 or less, more preferably 2.5 or less, even more preferably 1.5 or less, and particularly preferably 1.5 or less. The smaller the static friction coefficient, the better, for example, 0.15 or more. In this specification, the static friction coefficient against a stainless steel plate after conditioning at 40°C and 90% RH for 2 hours refers to a value measured by the following method. <Static friction coefficient after conditioning at 40°C and 90% RH for 2 hours> A PVA resin film (15 cm × 4 cm) was conditioned at 40°C and 90% RH for 2 hours. The conditioned PVA resin film was then fixed to a sliding piece weighing 200 g and placed on a smooth stainless steel plate (SUS304, surface roughness Ra = 0.05 μm) with a contact area of 40 cm. 2 The static friction force is measured in accordance with JIS K 7125 using an autograph (for example, Shimadzu Corporation, product name "AG-IS"), and the static friction coefficient is calculated using the following formula. Coefficient of static friction = static friction force / 1.96

[0012] The PVA-based resin film is long and has a width of 2000 mm or more. The width of the PVA-based resin film is preferably 2100 mm or more. The width of the PVA-based resin film is, for example, 2800 mm or less. A wide resin film may have a variation in thickness in the width direction, which may result in more significant scratches due to contact with the roll. The PVA-based resin film of the present invention is suppressed from having scratches on the film surface even when the width is 2000 mm or more, and a polyvinyl alcohol-based resin film with excellent appearance can be obtained. In this specification, long means an elongated shape in which the length is sufficiently longer than the width, and includes, for example, an elongated shape in which the length is 10 times or more, preferably 50 times or more, the width.

[0013] The PVA-based resin film preferably has an arithmetic mean surface roughness Ra of 0.05 μm or less, more preferably 0.03 μm or less, and even more preferably 0.02 μm or less. If the arithmetic mean roughness Ra is within the above range, the occurrence of scratches on the film surface is suppressed, and a polyvinyl alcohol-based resin film with excellent appearance can be obtained. The arithmetic mean surface roughness Ra of the PVA-based resin film is, for example, 0.001 μm or more.

[0014] The PVA-based resin film preferably has a thickness variation in the width direction of 1 μm or less, more preferably 0.5 μm or less, and even more preferably 0.3 μm or less. When the thickness variation in the width direction of the PVA-based resin film is within the above range, the occurrence of scratches on the film surface is suppressed, and a polyvinyl alcohol-based resin film with excellent appearance can be obtained.

[0015] The crystallinity of the PVA-based resin film is preferably 45% to 55%, more preferably 48% to 53%. If the crystallinity is within the above range, for example, when used as a polarizer, edge discoloration can be suppressed. The crystallinity of the PVA-based resin film can be measured by the following method.

[0016] (Method for evaluating the crystallinity of PVA) Any appropriate analytical device can be used for the X-ray analysis, including, for example, the "SmartLab" X-ray diffraction device manufactured by Rigaku Corporation and the "HyPix3000" 2D detector manufactured by Rigaku Corporation. A 150 μm thick PVA-based resin layer is used as the sample. If the PVA-based resin layer is less than 150 μm thick, the PVA-based resin layers are stacked to a total thickness of approximately 150 μm. X-rays with a wavelength of 1.5406 Å are transmitted through the sample perpendicular to the thickness direction, and scattered light is detected using a two-dimensional detector placed on the opposite side of the sample from the light source, resulting in a two-dimensional scattering image. After background correction, the obtained scattering image is integrated over the entire azimuthal angle (360°) around the beam center to obtain a one-dimensional profile of the integrated X-ray intensity versus the scattering angle 2θ. Next, waveform separation of the crystalline and amorphous peaks is performed on the obtained one-dimensional profile within the scattering angle 2θ range of 14° to 28.5°, and the crystallinity (%) is calculated using the following formula: Crystallinity (%) = crystalline peak area / (crystalline peak area + amorphous peak area) × 100

[0017] The PVA-based resin film can be formed using any appropriate PVA-based resin. Any appropriate resin can be used as the PVA-based resin. Examples include polyvinyl alcohol and ethylene-vinyl alcohol copolymer. Polyvinyl alcohol can be obtained by saponifying polyvinyl acetate. Ethylene-vinyl alcohol copolymer can be obtained by saponifying ethylene-vinyl acetate copolymer.

[0018] The saponification degree of the PVA-based resin is usually 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 PVA-based resin film with excellent durability can be obtained. If the saponification degree is too high, the coating liquid is likely to gel, and when a PVA-based resin film is produced by coating, it may be difficult to form a uniform coating film.

[0019] The average degree of polymerization of the PVA-based resin is 3000 or more, preferably 3500 or more, more preferably 3800 or more, and even more preferably 4000 or more. The occurrence of scratches on the film surface is suppressed, and a polyvinyl alcohol-based resin film with excellent appearance can be obtained. The average degree of polymerization of the PVA-based resin is, for example, 4500 or less. The average degree of polymerization can be determined in accordance with JIS K 6726-1994.

[0020] The PVA-based resin film contains any other appropriate component(s) in addition to the PVA-based resin. The other component(s) preferably contains a halide. Any appropriate halide can be used as the halide. Examples of the halide include iodide and sodium chloride. Examples of the iodide include potassium iodide, sodium iodide, and lithium iodide. Among these, potassium iodide is preferred. The content of the halide can be set to any appropriate value. For example, the content of the halide in the PVA-based resin solution used to form the PVA-based resin layer (and consequently the PVA-based resin layer) is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA-based resin.

[0021] B. Method for manufacturing polyvinyl alcohol resin film A method for producing a polyvinyl alcohol-based resin film according to an embodiment of the present invention includes, in this order: applying a solution containing a polyvinyl alcohol-based resin to a substrate to form a polyvinyl alcohol-based resin layer and thereby preparing an intermediate laminate; dry-stretching the intermediate laminate while heating it at 130°C or higher; and conveying the intermediate laminate in contact with a guide roll. According to the method for producing a polyvinyl alcohol-based resin according to an embodiment of the present invention, a polyvinyl alcohol-based resin film can be obtained that has a static friction coefficient of 2.55 or less against a stainless steel plate after humidity conditioning at 40°C and 90% RH for 2 hours. A known method for adjusting the static friction coefficient of a resin film is to use a lubricant such as glycerin. According to the method for producing a polyvinyl alcohol-based resin film according to an embodiment of the present invention, the static friction coefficient of the resin film surface can be adjusted without adding a lubricant or the like.

[0022] B-1. Preparation of intermediate laminate The method for producing a polyvinyl alcohol-based resin film includes applying a solution containing a polyvinyl alcohol-based resin to a substrate to form a polyvinyl alcohol-based resin layer, and producing an intermediate laminate. If the PVA-based resin layer that will ultimately become the PVA-based resin film is a layer formed by applying a solution containing a polyvinyl alcohol-based resin to a substrate, the static friction coefficient against a stainless steel plate after humidity conditioning at 40°C and 90% RH for 2 hours can be further reduced.

[0023] The intermediate laminate is produced by forming a resin layer on a resin substrate. The resin substrate may have any suitable configuration as long as it can support the resin layer from one side. Examples of materials for forming the resin substrate include ester-based resins such as polyethylene terephthalate-based resins, cycloolefin-based resins, olefin-based resins such as polypropylene, polyamide-based resins, polycarbonate-based resins, and copolymer resins thereof. Among these, preferred are cycloolefin-based resins (e.g., norbornene-based resins) and amorphous polyethylene terephthalate-based resins. Specific examples of amorphous polyethylene terephthalate-based resins include copolymers further containing isophthalic acid as a dicarboxylic acid and copolymers further containing cyclohexanedimethanol as a glycol. The thickness of the resin substrate is preferably 20 μm to 300 μm, and more preferably 50 μm to 200 μm.

[0024] The resin substrate may be previously subjected to a surface modification treatment (e.g., corona treatment, etc.), or an easy-adhesion layer may be formed on the resin substrate. By performing such treatment, the adhesion between the resin substrate and the PVA-based resin layer can be improved. The surface modification treatment and / or the formation of the easy-adhesion layer may be performed before or after stretching the resin substrate, which is performed as needed.

[0025] Any appropriate method can be used to form the PVA-based resin layer. Preferably, the PVA-based resin layer is formed by applying a coating liquid containing a PVA-based resin to a resin substrate that has been subjected to a stretching treatment and drying the coating liquid. The thickness of the PVA-based resin layer is 45 μm or less, preferably 30 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less. The thickness of the PVA-based resin film is preferably 1 μm or more, more preferably 3 μm or more, and even more preferably 5 μm or more.

[0026] The coating liquid is typically a solution in which the PVA resin is dissolved in a solvent. Examples of the solvent include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine. These may be used alone or in combination of two or more. Of these, water is preferred. The concentration of the PVA resin in the coating liquid is preferably 3 to 20 parts by weight per 100 parts by weight of the solvent. Such a resin concentration allows the formation of a uniform coating film that adheres closely to the resin substrate.

[0027] Additives may be blended into the coating solution. Examples of additives include plasticizers and surfactants. Examples of plasticizers include polyhydric alcohols such as ethylene glycol and glycerin. Examples of surfactants include nonionic surfactants. These can be used to further improve the uniformity, dyeability, and stretchability of the resulting PVA-based resin layer.

[0028] Any appropriate method can be used to apply the coating liquid, such as roll coating, spin coating, wire bar coating, dip coating, die coating, curtain coating, spray coating, and knife coating (such as comma coating).

[0029] B-2.Dry stretching The resulting intermediate laminate is then dry-stretched while being heated at 130°C or higher. Dry stretching may be fixed-end stretching (e.g., a method of stretching using a tenter stretching machine) or free-end stretching (e.g., a method of uniaxially stretching the laminate by passing it between rolls with different peripheral speeds). Preferably, dry stretching is performed by fixed-end stretching using a tenter stretching machine. Fixed-end stretching is performed by gripping the film edges in a tenter stretching machine equipped with multiple clips as gripping means and widening the distance between the tenters in the machine direction (the widening of the distance between the tenters represents the stretch ratio).

[0030] The stretching temperature for dry stretching is 130°C or higher, preferably 140°C or higher, and more preferably 150°C or higher. The stretching temperature is, for example, 170°C or lower. The stretching temperature can be adjusted by any appropriate method. For example, dry stretching may be performed in an oven set at the above temperature, or stretching may be performed while applying hot air set at the above temperature, or stretching may be performed while in contact with a roll heated to the above temperature.

[0031] The stretching ratio of the dry stretching can be set to any appropriate value. The stretching ratio of the dry stretching is preferably 2.2 times or more, more preferably 2.3 times or more, and even more preferably 2.5 times or more. When the stretching ratio of the dry stretching is within the above range, the occurrence of scratches on the film surface is suppressed, and a polyvinyl alcohol-based resin film with excellent appearance can be obtained.

[0032] In one embodiment, the intermediate laminate may be shrunk in the width direction during the dry stretching process. Performing both longitudinal stretching and width shrinkage during the dry stretching process can prevent defects such as film breakage and enable stretching (longitudinal stretching) at a higher temperature. In this embodiment, longitudinal stretching may be followed by width shrinkage, or width shrinkage may be performed followed by longitudinal stretching, or longitudinal stretching and width shrinkage may be performed simultaneously. When longitudinal stretching and width shrinkage are performed simultaneously, for example, the rails of a tenter stretching machine may be tapered so that the rail-to-rail distance continuously decreases. That is, in the stretching device, the rail-to-rail distance (the width of the laminate) may be reduced from the width W1 before the stretching process to the predetermined film width W2 while the clip spacing in the conveying direction is increased from L1 to the predetermined width L2. Details of such longitudinal stretching and width shrinkage methods are described, for example, in Japanese Patent No. 6,563,201. These publications are incorporated herein by reference in their entireties.

[0033] In this embodiment, the tenter stretching apparatus may be, for example, a stretching apparatus equipped with a pair of rails having a straight section with a constant rail-to-rail distance and a tapered section with a continuously decreasing rail-to-rail distance, and multiple clips that can run on each rail while varying the clip spacing. With such a stretching apparatus, the intermediate laminate can be stretched and shrunk by changing the clip spacing in the conveying direction (the distance between the clips on the same rail) and the clip spacing in the width direction (the distance between the clips on different rails) while holding both side edges of the intermediate laminate with the clips. The longitudinal stretch ratio (L2 / L1) can be controlled by adjusting the clip spacing in the conveying direction (the gripping spacing in the gripping step) L1 before the stretching process and the clip spacing in the conveying direction L2 at the end of dry stretching. The shrinkage rate in the width direction can be controlled by adjusting the amount of change in the rail-to-rail distance. Specifically, by controlling the rail-to-rail distance W2 at the end of the widthwise shrinking process relative to the rail-to-rail distance W1 during the widthwise shrinking process, the shrinkage rate in the widthwise direction (%) ({1-(W2) / (W1)} x 100) can be controlled. For example, the smaller the ratio of W2 to W1, the greater the shrinkage rate that can be obtained.

[0034] When the stretching includes longitudinal stretching and widthwise shrinkage, the longitudinal stretching ratio is preferably 2.0 to 3.5 times, more preferably 2.4 to 3.0 times. When the stretching ratio is within the above range, the stretchability (total stretching ratio) of the intermediate laminate is further improved, and good stretching can be achieved even at high temperatures. Furthermore, the shrinkage ratio in the widthwise direction is preferably 25% or more, more preferably 30% or more, and even more preferably 32% or more. Furthermore, the shrinkage ratio in the widthwise direction is, for example, 50% or less. When the shrinkage ratio in the widthwise direction is within the above range, the intermediate laminate can be stretched at a higher stretching ratio.

[0035] When longitudinal stretching and widthwise shrinking are performed, the method may include a first dry stretching / shrinking step in which dry stretching is performed in the longitudinal direction while heating and shrinking in the width direction, and a second dry stretching / shrinking step in which dry stretching the intermediate laminate in the longitudinal direction while heating at a temperature lower than the maximum heating temperature in the first dry stretching / shrinking step and shrinking in the width direction. By including the first and second dry stretching / shrinking steps, the orientation of the resulting PVA-based resin film can be adjusted.

[0036] B-3.Transportation by contact with guide rolls The dry-stretched PVA-based resin film is then transported to any appropriate next step while in contact with a guide roll. The guide roll is used, for example, to move, change direction, and control the speed of the long film. The guide roll is made of any appropriate material. Typically, the guide roll is a metal roll made of stainless steel (SUS), iron, aluminum, or the like. The PVA-based resin layer formed on the intermediate laminate comes into contact with the guide roll during transport, and local scratches can occur on the resin film surface due to friction between the surface of the PVA-based resin layer and the surface of the guide roll. These scratches can cause problems in the appearance of the final product (e.g., a polarizer). According to the PVA-based resin film production method of an embodiment of the present invention, even when a step of transporting the film while in contact with a guide roll is included, the appearance of the final product (a product using a PVA-based resin film) can be improved.

[0037] C. Polarizer manufacturing method A method for producing a polarizer according to an embodiment of the present invention includes dyeing, with a dichroic substance, a polyvinyl alcohol-based resin film having a static friction coefficient of 2.55 or less against a stainless steel plate after conditioned at 40°C and 90% RH for two hours. As described above, a PVA-based resin film having a static friction coefficient of 2.55 or less against a stainless steel plate after conditioned at 40°C and 90% RH for two hours can suppress the occurrence of scratches on the film surface. Therefore, the method for producing a polarizer according to an embodiment of the present invention can produce a polarizer with excellent appearance.

[0038] 1 is a schematic diagram illustrating a method for producing a polarizer according to one embodiment of the present invention. In the illustrated method for producing a polarizer, the dyeing step, crosslinking step, stretching step, hue adjusting step, and drying shrinkage step are performed continuously. More specifically, a long PVA-based resin film 1 is transported from a raw fabric roll 21 toward a take-up roll 22. Between the raw fabric roll 21 and the take-up roll 22, the PVA-based resin film 1 is sequentially subjected to the dyeing step, crosslinking step, stretching step, hue adjusting step, and drying shrinkage step. In one embodiment, the PVA-based resin film 1 is immersed in a dye bath 2B (dyeing liquid), a crosslinking bath 2C (crosslinking liquid), a stretching bath 2D (stretching liquid), and a hue adjusting bath 2E (hue adjusting liquid) by a plurality of rollers 24, and then transported through a heat drying section 23. When the PVA-based resin film is a PVA-based resin layer contained in a laminate, the laminate including the PVA-based resin layer is immersed in each of the above-mentioned baths (each of the liquids) to bring the PVA-based resin layer into contact with each of the baths (each of the liquids).

[0039] As shown in the illustrated example, in a typical polarizer manufacturing method, a PVA-based resin film is subjected to each process while being in contact with multiple rolls. If the PVA-based resin film used as the raw material for a polarizer has a static friction coefficient of 2.55 or less against a stainless steel plate after being conditioned at 40°C and 90% RH for 2 hours, the PVA-based resin film is prevented from being scratched on its surface even when being subjected to each process while being in contact with multiple rolls, and a polarizer with excellent appearance can be obtained.

[0040] The PVA-based resin film used in the method for producing a polarizer according to an embodiment of the present invention may have a static friction coefficient of 2.55 or less against a stainless steel plate after 2 hours of humidity conditioning at 40°C and 90% RH, and any appropriate PVA-based resin film may be used. Preferably, the PVA-based resin film described in Section A above may be used. The PVA-based resin film described in Section A above may be obtained by the method described in Section B above. The method for producing a polarizer according to an embodiment of the present invention may further include the steps of preparing a PVA-based resin film, i.e., applying a solution containing a polyvinyl alcohol-based resin to a substrate to form a polyvinyl alcohol-based resin layer and producing an intermediate laminate; dry-stretching the intermediate laminate while heating at 130°C or higher; and transporting the intermediate laminate in contact with guide rolls. The PVA-based resin film may be used as a single layer, or may be used in the form of a PVA-based resin layer formed on any appropriate substrate, i.e., in the form of a laminate having a PVA-based resin layer.

[0041] C-1. Dyeing process In the dyeing step, the PVA-based resin film is dyed with a dichroic substance. Preferably, this is done by adsorbing the dichroic substance into the PVA-based resin layer. Examples of the adsorption method include immersing the PVA-based resin layer (laminate) in a dyeing solution containing the dichroic substance, applying the dyeing solution to the PVA-based resin layer, and spraying the dyeing solution onto the PVA-based resin layer. Preferably, the laminate is immersed in a dyeing solution containing the dichroic substance, because this allows for good adsorption of the dichroic substance. Both sides of the laminate may be immersed in the dyeing solution, or only one side may be immersed.

[0042] Examples of the dichroic substance include iodine and organic dyes. These may be used alone or in combination of two or more. The dichroic substance is preferably iodine. When iodine is used as the dichroic substance, the dye solution is preferably an iodine aqueous solution. The amount of iodine to be added is preferably 0.1 to 1.0 part by weight per 100 parts by weight of water. In order to increase the solubility of iodine in water, it is preferable to add an iodide salt to the iodine aqueous solution. Examples of iodide salts include potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, and titanium iodide. Among these, potassium iodide and sodium iodide are preferred. The amount of iodide salt to be added is preferably 0.3 to 15 parts by weight per 100 parts by weight of water.

[0043] The temperature of the dye solution during dyeing is preferably 20° C. to 40° C. When the PVA-based resin layer is immersed in the dye solution, the immersion time is preferably 5 to 300 seconds. Under these conditions, the dichroic substance can be sufficiently adsorbed into the PVA-based resin layer.

[0044] The process for producing a polarizer may include any appropriate steps. Examples of other steps include an insolubilization step, a dyeing step, a crosslinking step, a stretching step, a washing step, and a drying step (adjusting the moisture content, drying shrinkage). The other steps may be performed at any appropriate timing.

[0045] The insolubilizing step and the crosslinking step are typically carried out by immersing the PVA resin layer in an aqueous solution of boric acid. The washing step is typically carried out by immersing the PVA resin layer in an aqueous solution of potassium iodide. The drying temperature in the drying step is preferably 30°C to 100°C.

[0046] In the stretching step, stretching is performed by any appropriate method. The stretching step may be performed simultaneously with or separately from the dyeing step, the insolubilizing step, and / or the crosslinking step. When performed separately, the separate stretching step may be performed at any appropriate timing. Stretching is preferably performed by dry-stretching (in-air auxiliary stretching) a laminate having a PVA-based resin layer and then wet-stretching it. In one embodiment, the method for producing a polarizer may be performed consecutively with the process for producing a PVA-based resin film. In this embodiment, the process for dry-stretching an intermediate laminate having a PVA-based resin layer, for example, the above-mentioned process for dry-stretching while heating at 130°C or higher, also functions as in-air auxiliary stretching in the process for producing a polarizer, thereby improving the stretchability of the intermediate laminate (PVA-based resin film).

[0047] Wet intermediate stretching is typically performed by immersing the laminate in a stretching bath. The wet stretching method may be fixed-end stretching or free-end stretching (for example, a method in which the laminate is uniaxially stretched by passing it between rolls with different peripheral speeds). The stretching of the laminate may be performed in one stage or multiple stages. When performed in multiple stages, the stretch ratio of the laminate described below is the product of the stretch ratios in each stage.

[0048] The wet stretching is preferably performed by immersing the laminate in an aqueous boric acid solution (stretching in boric acid solution). The aqueous boric acid solution is preferably obtained by dissolving boric acid and / or a borate in water as a solvent. The boric acid concentration is preferably 1 to 10 parts by weight, more preferably 2.5 to 6 parts by weight, and even more preferably 3 to 5 parts by weight, per 100 parts by weight of water. By setting the boric acid concentration to 1 part by weight or more, dissolution of the PVA-based resin layer can be effectively suppressed, allowing the production of a polarizer with better properties. Note that, in addition to boric acid or a borate, an aqueous solution obtained by dissolving a boron compound such as borax, glyoxal, glutaraldehyde, or the like in a solvent can also be used.

[0049] Preferably, an iodide is added to the drawing bath (boric acid aqueous solution). Adding an iodide can prevent iodine adsorbed in the PVA resin layer from leaching out. Examples of iodides include potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, and titanium iodide. The concentration of the iodide is preferably 0.05 to 15 parts by weight, more preferably 0.5 to 8 parts by weight, per 100 parts by weight of water.

[0050] The stretching temperature (liquid temperature of the stretching bath) is preferably 40°C or higher, more preferably 60°C or higher. At such a temperature, the film can be stretched at a high ratio while suppressing dissolution of the PVA-based resin layer. The stretching temperature is, for example, 75°C or lower. The higher the stretching temperature, the higher the solubility of the PVA-based resin layer, which may result in failure to obtain excellent optical properties. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

[0051] The stretching ratio in wet stretching is preferably 1.5 times or more, more preferably 3.0 times or more. The total stretching ratio of the laminate (stretching ratio obtained by combining auxiliary air stretching and underwater stretching) is preferably 5.0 times or more, more preferably 5.5 times or more, relative to the original length of the laminate. By achieving such a high stretching ratio, a polarizer with extremely excellent optical properties can be produced. Such a high stretching ratio can be achieved by employing underwater stretching (stretching in boric acid water).

[0052] The drying is carried out by any suitable method. In one embodiment, it is preferable to further include heating the film while transporting it in the longitudinal direction to cause it to shrink in the width direction. The drying and shrinking treatment is preferably carried out in the order of dry stretching, dyeing, wet stretching, and dry shrinking.

[0053] The shrinkage rate of the laminate in the width direction due to drying is preferably 1% to 10%, more preferably 2% to 8%, and even more preferably 4% to 6%. By using a heated roll, the laminate can be continuously shrunk in the width direction while being transported, thereby achieving high productivity.

[0054] Drying conditions can be controlled by adjusting the heating temperature of the transport rolls (heating roll temperature), the number of heating rolls, the contact time with the heating rolls, etc. The temperature of the heating rolls is preferably 60°C to 120°C, more preferably 65°C to 100°C, and even more preferably 70°C to 90°C. Such temperatures can increase the crystallinity of the thermoplastic resin, suppress curling, and impart extremely excellent durability to the laminate. The temperature of the heating rolls can be measured with a contact thermometer. The contact time between the laminate and the heating rolls (total contact time) is preferably 1 second to 300 seconds, more preferably 1 second to 20 seconds, and even more preferably 1 second to 10 seconds.

[0055] The heating rolls may be installed in a heating furnace (e.g., an oven) or in a normal production line (under room temperature). Preferably, they are installed in a heating furnace equipped with a blower. By using both heating roll drying and hot air drying, it is possible to suppress abrupt temperature changes between the heating rolls, and to easily control shrinkage in the width direction. The hot air drying temperature is preferably 30°C to 100°C. The hot air drying time is preferably 1 second to 300 seconds. The hot air speed is preferably about 10 m / s to 30 m / s. Note that this air speed is the air speed inside the heating furnace and can be measured with a mini-vane type digital anemometer.

[0056] D. Polarizer The polarizer (PVA-based resin layer of the long laminate) produced by the manufacturing method including the above-mentioned other steps is essentially a PVA-based resin film in which a dichroic material is adsorbed and aligned. The polarizer preferably exhibits absorption dichroism at any wavelength between 380 nm and 780 nm. The polarizer's single transmittance (Ts) is preferably 39% or more, more preferably 40% or more, even more preferably 41% or more, and particularly preferably 42% or more. The theoretical upper limit of the single transmittance is 50%, and the practical upper limit is 46%. The single transmittance (Ts) is the Y value measured using a 2-degree visual field (C light source) according to JIS Z8701 and corrected for luminosity. It can be measured, for example, using a product named "V-7100" manufactured by JASCO Corporation. The degree of polarization of the polarizer is preferably 99% or more, more preferably 99.90% or more, and even more preferably 99.95% or more.

[0057] The thickness of the polarizer is, for example, 20 μm or less, preferably 12 μm or less, and may be 8 μm or less, while the thickness of the PVA resin layer is preferably 1 μm or more, more preferably 2 μm or more.

[0058] The polarizer may be used in any appropriate manner. Specifically, the polarizer may be used as a single-layer PVA-based resin film, as a laminate of a resin substrate and a PVA-based resin film, or as a laminate in which a protective film is disposed on at least one of the PVA-based resin film and the PVA-based resin film (i.e., a polarizing plate).

[0059] E. Polarizing plate The polarizing plate includes a polarizer and a protective film disposed on at least one side of the polarizer. Examples of materials for the protective film include cellulose-based resins such as diacetyl cellulose and triacetyl cellulose, (meth)acrylic resins, cycloolefin-based resins, olefin-based resins such as polypropylene, ester-based resins such as polyethylene terephthalate-based resins, polyamide-based resins, polycarbonate-based resins, and copolymer resins thereof.

[0060] The thickness of the protective film is preferably 10 μm to 80 μm. The protective film is typically laminated to the polarizer via an adhesive layer (specifically, an adhesive layer or a pressure-sensitive adhesive layer). The adhesive layer is typically formed of a PVA-based adhesive or an activation energy ray-curable adhesive. The pressure-sensitive adhesive layer is typically formed of an acrylic pressure-sensitive adhesive. When a laminate of a resin substrate / PVA-based resin film (polarizer) is used, the resin substrate can be preferably peeled off after laminating a protective film on the surface of the polarizer opposite to the resin substrate. If necessary, another protective film can be laminated on the peeled surface. By peeling off the resin substrate, curling can be more reliably suppressed.

[0061] In practice, polarizing plates have a pressure-sensitive adhesive layer as their outermost layer. This pressure-sensitive adhesive layer is typically the outermost layer on the image display device side. A release liner is temporarily and removably attached to the pressure-sensitive adhesive layer to protect the pressure-sensitive adhesive layer until actual use and to enable roll formation.

[0062] The polarizing plate may further include any appropriate optically functional layer depending on the purpose. Representative examples of optically functional layers include a retardation film (optical compensation film) and a surface treatment layer. For example, a retardation film may be disposed between the protective film and the pressure-sensitive adhesive layer (not shown). The optical properties of the retardation film (e.g., refractive index ellipsoid, in-plane retardation, thickness retardation) may be appropriately set depending on the purpose, characteristics of the image display device, etc. For example, when the image display device is an IPS-mode liquid crystal display device, a retardation film having a refractive index ellipsoid of nx>ny>nz and a retardation film having a refractive index ellipsoid of nz>nx>ny may be disposed. The retardation film may also serve as a protective film. In this case, the protective film disposed on the image display device side may be omitted. Conversely, the protective film may have an optical compensation function (i.e., it may have a refractive index ellipsoid, in-plane retardation, and thickness retardation appropriate for the purpose). Here, "nx" is the refractive index in the direction in which the refractive index within the film plane is maximum (i.e., the slow axis direction), "ny" is the refractive index in the direction perpendicular to the slow axis within the film plane, and "nz" is the refractive index in the thickness direction.

[0063] The surface treatment layer may be disposed further outside the outer protective film (not shown). Typical examples of the surface treatment layer include a hard coat layer, an anti-reflection layer, and an anti-glare layer. The surface treatment layer is preferably a layer with low moisture permeability, for example, for the purpose of improving the humidity durability of the polarizer. The hard coat layer is provided for the purpose of preventing scratches on the polarizing plate surface. The hard coat layer can be formed, for example, by adding a cured film made of an appropriate ultraviolet-curable resin such as an acrylic or silicone resin, which has excellent hardness and slip properties, to the surface. The hard coat layer preferably has a pencil hardness of 2H or more. The anti-reflection layer is a low-reflection layer provided for the purpose of preventing reflection of external light on the polarizing plate surface. Examples of anti-reflection layers include a thin-layer type, as disclosed in JP 2005-248173 A, which prevents reflection by utilizing the cancellation effect of reflected light due to light interference, and a surface structure type, as disclosed in JP 2011-2759 A, which imparts a microstructure to the surface to achieve low reflectance. Anti-glare layers are provided for purposes such as preventing external light from reflecting off the polarizing plate surface and impairing the visibility of light transmitted through the polarizing plate. Anti-glare layers are formed by imparting a micro-convex / concave structure to the surface using appropriate methods, such as surface roughening methods such as sandblasting or embossing, or incorporating transparent fine particles. The anti-glare layer may also function as a diffusion layer (e.g., a viewing angle widening function) to diffuse light transmitted through the polarizing plate and widen the viewing angle. Instead of providing a surface treatment layer, a similar surface treatment may be applied to the surface of the outer protective film. [Example]

[0064] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples.

[0065] [Example 1] A long, amorphous, isophthalic-copolymerized polyethylene terephthalate film (thickness: 100 μm) with a Tg of approximately 75°C was used as the thermoplastic resin substrate. One side of the resin substrate, measuring 2000 mm or wider, was subjected to a corona treatment. A PVA-based resin (a 9:1 mixture of polyvinyl alcohol (degree of polymerization: 4200, degree of saponification: 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFIRM", degree of polymerization: 2500) was mixed with 100 parts by weight of the PVA resin, and 13 parts by weight of potassium iodide was added. The resulting mixture was dissolved in water to prepare a PVA aqueous solution (coating solution). The above PVA aqueous solution was applied to the corona treated surface of the resin substrate and dried at 60° C. to form a PVA resin layer with a thickness of 13 μm, thereby producing a laminate. The resulting laminate was stretched 2.7 times in the longitudinal direction and shrunk 37.5% in the transverse direction at 150°C using a tenter stretching device to obtain a laminate having a PVA-based resin film (thickness 8 μm) (in-air auxiliary stretching). Stretching and shrinkage were performed simultaneously.

[0066] Next, the laminate having the PVA resin film was immersed in an insolubilizing bath (a boric acid aqueous solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40°C for 30 seconds (insolubilizing treatment). Next, the film was immersed in a dye bath (an aqueous iodine solution obtained by mixing iodine and potassium iodide in a weight ratio of 1:7 with 100 parts by weight of water) at a liquid temperature of 30°C for 60 seconds while adjusting the concentration so that the single transmittance (Ts) of the finally obtained polarizer would be a desired value (dyeing treatment). Next, the sample was immersed in a crosslinking bath (a boric acid aqueous solution obtained by blending 3 parts by weight of potassium iodide and 5 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40°C for 30 seconds (crosslinking treatment). The laminate was then immersed in a boric acid aqueous solution (boric acid concentration 4 wt %, potassium iodide concentration 5 wt %) at a liquid temperature of 64°C and uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds to a total stretch ratio of 5.5 times (underwater stretching treatment). Thereafter, the laminate was immersed in a cleaning bath (aqueous solution obtained by mixing 3 parts by weight of potassium iodide with 100 parts by weight of water) at a liquid temperature of 20° C. (cleaning treatment). Thereafter, the film was dried in an oven maintained at about 90°C, and brought into contact with a stainless steel (SUS) heated roll whose surface temperature was maintained at about 75°C (drying shrinkage treatment). In this way, a laminate having a PVA-based resin layer (polarizer) with a thickness of 5 μm on a resin substrate was obtained.

[0067] [Examples 2 to 3] A laminate having a PVA-based resin layer (polarizer) was obtained in the same manner as in Example 1, except that the stretching temperature and stretch ratio of the auxiliary in-air stretching were changed as shown in Table 1.

[0068] (Comparative Example 1) A laminate having a PVA-based resin layer (polarizer) was obtained in the same manner as in Example 1, except that the stretching temperature and stretch ratio of the auxiliary in-air stretching were changed as shown in Table 1.

[0069] (Comparative Example 2) A 30 μm-thick PVA-based resin film (manufactured by Kuraray, product name "PE3000") was stretched 1.2 times in the machine direction while immersed in a water bath at 30°C for 1 minute. It was then dyed by immersion in a 30°C aqueous solution containing 0.04 wt% iodine and 0.3 wt% potassium, and stretched twice its original length. This stretched film was then further stretched to 3 times its original length while immersed in a 30°C aqueous solution containing 3 wt% boric acid and 3 wt% potassium iodide. It was then further stretched to 6.0 times its original length while immersed in a 60°C aqueous solution containing 4 wt% boric acid and 5 wt% potassium iodide, and dried at 70°C for 2 minutes to obtain a 12 μm-thick PVA-based resin layer (polarizer).

[0070] (Comparative Example 3) A polarizer was obtained in the same manner as in Comparative Example 2, except that a PVA-based resin film having a thickness of 45 μm was used instead of the 30 μm-thick PVA-based resin film.

[0071] [evaluation] The long laminates obtained in the examples and comparative examples were evaluated as follows, and the results are shown in Table 1. 1. Thickness Thicknesses of 10 μm or less were measured using an interference film thickness meter (Otsuka Electronics Co., Ltd., product name "MCPD-3000"), and thicknesses of more than 10 μm were measured using a digital micrometer (Anritsu Corporation, product name "KC-351C").

[0072] 2.Water absorption rate For Examples 1 to 3 and Comparative Example 1, a 1 cm square measurement sample was cut from the PVA-based resin layer peeled from the laminate after in-air stretching and shrinking. For Comparative Examples 2 and 3, a 1 cm square measurement sample was cut from the PVA-based resin film used in producing the polarizer. The measurement sample was placed in a moisture absorption / desorption measuring device (manufactured by Hiden Corporation, product name "IGA-SORP") and held at 23°C and 0% RH for 600 minutes, followed by 120 minutes at 40°C and 90% RH. The weight after holding at 23°C and 0% RH was designated W1, and the weight after holding at 40°C and 90% RH for 120 minutes was designated W2. The water absorption was calculated using the following formula: Water absorption rate (%)=(W2-W1) / W1×100

[0073] 3. Coefficient of static friction In Examples 1 to 3 and Comparative Example 1, the PVA-based resin layer peeled from the laminate after air-stretching and shrinking was cut into a 15 cm × 4 cm sample. In Comparative Examples 2 and 3, the PVA-based resin film used in the manufacture of the polarizer was cut into a 15 cm × 4 cm sample. The sample was conditioned at 40°C and 90% RH for 2 hours, and the PVA-based resin film was fixed to a sliding piece weighing 200 g and placed on a smooth stainless steel plate (made of SUS304, surface roughness Ra = 0.05 μm) with a contact area of 40 cm. 2 The static friction force was measured in accordance with JIS K 7125 using an autograph (manufactured by Shimadzu Corporation, product name "AG-IS"), and the static friction coefficient was calculated using the following formula. Coefficient of static friction = static friction force / 1.96 As Reference Examples 1 to 5, the static friction coefficients of the PVA-based resin layers (Reference Examples 1 to 3) peeled from the laminates after the air-stretching and shrinking process of Examples 1 to 3 and the PVA-based resin films used in Comparative Examples 2 and 3 (Reference Examples 4 and 5) were measured in the same manner as above, except that they were not subjected to humidity conditioning.

[0074] 4. Frequency of scratches The polarizers obtained in the Examples and Comparative Examples were cut into 10 cm squares to serve as samples. A searchlight (GENTOS, product name "MG-886R") was positioned perpendicular to the MD direction of the sample, and the light was irradiated onto the surface of the sample from a position where the angle between one side of the sample and the light irradiated from the searchlight was 45° and the distance between the surface of the sample and the searchlight was 10 cm. The presence or absence of scratches on the polarizer surface was visually confirmed. Scratches were defined as those with a depth of 0.05 μm or more, a length of 0.2 mm or more, and a width of 0.2 μm or more. Zero scratches were rated as ⊚ (best), one to five scratches were rated as ◯ (good), and more than five scratches were rated as × (room for improvement). In Examples 1 to 3 and Comparative Example 1, light was irradiated from the resin substrate side.

[0075] [Table 1]

[0076] [evaluation] In Examples 1 to 3 of the present invention, the occurrence of fine scratches on the polarizer surface was suppressed. [Industrial Applicability]

[0077] The use of the polyvinyl alcohol-based resin film according to the embodiment of the present invention can prevent scratches on the film surface and provide a product with excellent appearance. The polarizer produced using the polyvinyl alcohol-based resin film according to the embodiment of the present invention can be suitably used in image display devices that require higher precision.

Claims

1. The thickness is 45 μm or less, The water absorption rate is 1% to 10%. The static friction coefficient against a stainless steel plate after 2 hours of humidity conditioning at 40°C and 90% RH is 2.55 or less, It is long and has a width of 2000 mm or more, A polyvinyl alcohol-based resin film comprising a polyvinyl alcohol-based resin having an average degree of polymerization of 3,000 or more.

2. The polyvinyl alcohol-based resin film according to claim 1, having an arithmetic mean surface roughness Ra of 0.05 μm or less.

3. The polyvinyl alcohol-based resin film according to claim 2, wherein the thickness variation in the width direction is 1 μm or less.

4. The polyvinyl alcohol-based resin film according to claim 3 , which is a layer formed by applying a solution containing the polyvinyl alcohol-based resin to a substrate.

5. The polyvinyl alcohol-based resin film according to claim 1 , which is used for producing a polarizer.

6. A method for producing a polyvinyl alcohol-based resin film, the method comprising the steps of: applying a solution containing a polyvinyl alcohol-based resin to a substrate to form a polyvinyl alcohol-based resin layer, thereby producing an intermediate laminate; and dry-stretching the intermediate laminate while heating it at 130°C or higher. The method has a static friction coefficient of 2.55 or less against a stainless steel plate after conditioning at 40°C and 90% RH for 2 hours.

7. A method for producing a polarizer, comprising dyeing, with a dichroic substance, a polyvinyl alcohol-based resin film having a static friction coefficient of 2.55 or less against a stainless steel plate after humidity conditioning at 40°C and 90% RH for 2 hours.

8. 8. The method for producing a polarizer according to claim 7, further comprising: applying a solution containing a polyvinyl alcohol-based resin to a substrate to form a polyvinyl alcohol-based resin layer, thereby producing an intermediate laminate; and dry-stretching the intermediate laminate while heating it at 130°C or higher, in this order, to obtain the polyvinyl alcohol-based resin film.

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