Method for manufacturing polarizer

Abstract

To provide a method for manufacturing a polarizer, capable of providing a polarizer in which generation of curling is suppressed even when the polarizer is thin.SOLUTION: The method for manufacturing a polarizer according to an embodiment of the present invention sequentially includes: obtaining a laminate by forming a polyvinyl alcohol-based resin layer on a substrate formed using a thermoplastic resin with a glass transition temperature Tg of 60°C to 73°C; dry-stretching the laminate; and wet-stretching the laminate. The difference between the shrinkage ratio of the thermoplastic resin substrate in the wet stretching and the shrinkage ratio of the laminate in the wet stretching is 2.0% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a polarizer. [Background technology]

[0002] Image display devices, such as liquid crystal display devices and electroluminescence (EL) display devices (e.g., organic EL display devices and inorganic EL display devices), are rapidly becoming popular. A polarizing plate including a polarizer is typically used in the image display panel mounted on the image display device. Polarizers are typically manufactured by subjecting a resin film made of a polyvinyl alcohol-based resin to swelling, dyeing, crosslinking, and stretching treatments (see, for example, Patent Document 1, etc.). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2017 / 138551 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, image display devices have been made thinner, and thus thinner polarizers are in demand. However, thin polarizers may curl, which may cause problems when they are attached to other components. Furthermore, if curling occurs during the polarizer manufacturing process, the productivity of the polarizer itself may decrease. Therefore, there is a demand for a manufacturing method that can suppress curling even when manufacturing thin polarizers. [Means for solving the problem]

[0005] 1. A method for producing a polarizer according to an embodiment of the present invention includes the steps of forming a polyvinyl alcohol-based resin layer on a substrate formed using a thermoplastic resin having a glass transition temperature Tg of 60°C to 73°C to produce a laminate; dry-stretching the laminate; and wet-stretching the laminate, in this order, wherein the difference between the shrinkage percentage of the thermoplastic resin substrate during the wet-stretching and the shrinkage percentage of the laminate during the wet-stretching is 2.0% or less. 2. In the method for producing a polarizer described in 1 above, the thermoplastic resin may be an amorphous polyethylene terephthalate resin. 3. In the method for producing a polarizer according to the above 1 or 2, the thermoplastic resin may be a polyethylene terephthalate resin containing 3.0 mol % to 10.0 mol % of diethylene glycol units relative to the total of all repeating units. 4. In the method for producing a polarizer according to any one of the above 1 to 3, the total stretching ratio of the laminate may be 5.0 times or more. 5. In the method for producing a polarizer according to any one of the above items 1 to 4, the dry stretching may be biaxial stretching. 6. In the method for producing a polarizer according to any one of the above 1 to 5, the polyvinyl alcohol-based resin layer after the dry stretching may have a crystallinity of 35% to 55%. [Effects of the Invention]

[0006] According to an embodiment of the present invention, a method for manufacturing a polarizer can be provided that can provide a polarizer in which curling is suppressed even when the polarizer is thin. DETAILED DESCRIPTION OF THE INVENTION

[0007] A. Polarizer manufacturing method A method for producing a polarizer according to an embodiment of the present invention includes the steps of forming a polyvinyl alcohol-based resin layer on a substrate (hereinafter also referred to as a thermoplastic resin substrate) formed using a thermoplastic resin having a glass transition temperature Tg of 60°C to 73°C to produce a laminate; dry-stretching the laminate; and wet-stretching the laminate, in this order. In the method for producing a polarizer according to an embodiment of the present invention, the difference between the shrinkage rate of the laminate during wet-stretching and the shrinkage rate of the thermoplastic resin substrate during wet-stretching is 2.0% or less. Thin polarizers tend to curl, which can cause problems during lamination. Furthermore, the crystallinity of polarizers is increased to suppress edge discoloration. Increasing the crystallinity makes the PVA-based resin layer more likely to curl and the edges to bend during the polarizer production process (for example, the wet-stretching process), which can reduce the productivity of polarizers. According to the method for producing a polarizer of the present invention, a substrate formed from a thermoplastic resin having a glass transition temperature Tg of 60°C to 73°C is used, and as long as the difference between the shrinkage rate of the thermoplastic resin substrate during wet stretching and the shrinkage rate of the laminate during wet stretching is 2.0% or less, the substrate and the PVA-based resin layer can be uniformly stretched in a laminated state, and curling of the resulting polarizer can be suppressed. Furthermore, even when the crystallinity of the polarizer is increased, a decrease in productivity can be suppressed.

[0008] A-1. Preparation of laminate In a method for producing a polarizer according to an embodiment of the present invention, a polyvinyl alcohol-based resin layer is first formed on a substrate made of a thermoplastic resin having a glass transition temperature Tg of 60°C to 73°C, thereby producing a laminate. The laminate can be produced, for example, by applying a coating liquid containing a PVA-based resin to the substrate to form a PVA-based resin layer. The thickness of the thermoplastic resin substrate is preferably 20 μm to 300 μm, and more preferably 50 μm to 200 μm. If the thickness is less than 20 μm, it may be difficult to form the PVA-based resin layer. If the thickness exceeds 300 μm, for example, in the wet stretching (also referred to as underwater stretching) described below, it may take a long time for the thermoplastic resin substrate to absorb water, and an excessive load may be required for stretching.

[0009] The thermoplastic resin substrate is formed using a thermoplastic resin having a glass transition temperature (Tg) of 60°C to 73°C. When the Tg is within the above range, curling of the polarizer can be suppressed, and the productivity of the polarizer can be stabilized. Furthermore, defects such as deformation of the thermoplastic resin substrate (e.g., generation of unevenness, sagging, wrinkles, etc.) can be suppressed during the application and drying of a coating liquid containing a PVA resin, and a laminate can be produced satisfactorily. Furthermore, wet stretching of the resin layer can be satisfactorily carried out at a suitable temperature (e.g., about 60°C). The glass transition temperature of the thermoplastic resin can be measured, for example, by using a differential scanning calorimeter (DSC) using a thermoplastic resin pellet as a sample. The glass transition temperature (Tg) is a value determined in accordance with JIS K 7121.

[0010] The thermoplastic resin may have a glass transition temperature Tg within the above range, and any suitable resin may be used. Examples of the thermoplastic resin include ester resins such as polyethylene terephthalate (PET) resins, cycloolefin resins such as norbornene resins, olefin resins such as polypropylene, polyamide resins, polycarbonate resins, and copolymer resins thereof. Amorphous (uncrystallized) polyethylene terephthalate resins are preferred.

[0011] Polyethylene terephthalate resins are typically obtained by dehydration condensation of a polycarboxylic acid such as a dicarboxylic acid and a polyalcohol such as a diol. Any appropriate components can be used as the dicarboxylic acid and diol. Examples of dicarboxylic acids include aromatic dicarboxylic acids such as isophthalic acid and phthalic acid, and aliphatic dicarboxylic acids such as adipic acid, suberic acid, sebacic acid, dimer acid, dodecanedioic acid, cyclohexanedicarboxylic acid, and ester derivatives thereof. Examples of diols include ethylene glycol and diethylene glycol.

[0012] The polyethylene terephthalate resin preferably contains diethylene glycol units (structural units derived from diethylene glycol) in an amount of 3.0 mol % to 10.0 mol %, more preferably 4.0 mol % to 10.0 mol %, based on the total amount of all repeating units. When the diethylene glycol unit content is within the above range, the Tg of the substrate can be adjusted to an appropriate value, and curling of the resulting polarizer can be suppressed. Furthermore, curling and edge folding of the PVA resin layer can be suppressed during the polarizer production process (e.g., a wet stretching process), thereby improving the productivity of the polarizer.

[0013] Any appropriate component may be further included to adjust the Tg of the thermoplastic resin. Examples of the optional component include neopentyl glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, triethylene glycol, polyalkylene glycol, 2,2-bis(4-hydroxyethoxyphenyl)propane, glycerin, trimethylolpropane, trimethylolethane, ethylene oxide, and propylene oxide. These components are used in any appropriate ratio so that the glass transition temperature Tg of the thermoplastic resin falls within the above-mentioned range.

[0014] The thermoplastic resin substrate may be stretched in advance (for example, before forming the PVA-based resin layer). In one embodiment, the long thermoplastic resin substrate is stretched in the transverse direction. The transverse direction is preferably a direction perpendicular to the stretching direction of the laminate described below. In this specification, "perpendicular" also includes a case where the direction is substantially perpendicular. Here, "substantially perpendicular" includes a case where the direction is 90°±5.0°, preferably 90°±3.0°, and more preferably 90°±1.0°. The stretching temperature of the thermoplastic resin substrate is preferably Tg-10°C to Tg+55°C relative to the glass transition temperature (Tg) of the thermoplastic resin substrate. The stretching ratio of the thermoplastic resin substrate is preferably 1.5 times to 3.0 times. Any appropriate method can be used to stretch the thermoplastic resin substrate. Specifically, either fixed-end stretching or free-end stretching may be used. The stretching method may be a dry method or a wet method. The stretching may be carried out in one stage or in multiple stages. When the stretching is carried out in multiple stages, the stretching ratio is the product of the stretching ratios in each stage.

[0015] As described above, the PVA-based resin layer is formed by applying a coating liquid containing a PVA-based resin to a thermoplastic resin substrate. Examples of solvents 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. Among these, water is preferred. The content of the PVA-based resin in the coating liquid is preferably 3 to 20 parts by weight per 100 parts by weight of the solvent. Within this range, a uniform coating film that adheres tightly to the thermoplastic resin substrate can be formed.

[0016] Examples of the PVA resin include polyvinyl alcohol and ethylene-vinyl alcohol copolymer. Polyvinyl alcohol is obtained by saponifying polyvinyl acetate. Ethylene-vinyl alcohol copolymer is obtained by saponifying ethylene-vinyl acetate copolymer. The saponification degree of the PVA 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 %. By using a PVA resin with such a saponification degree, a polarizer with excellent durability can be obtained. If the saponification degree is too high, gelation may occur. The saponification degree can be determined in accordance with JIS K 6726-1994.

[0017] The average degree of polymerization of the PVA resin is usually 1000 to 10000, preferably 1200 to 4500, and more preferably 1500 to 4300. The average degree of polymerization can be determined in accordance with JIS K 6726-1994.

[0018] The coating solution preferably contains a halide. Any appropriate halide can be used as the halide. Examples include iodides such as potassium iodide, sodium iodide, and lithium iodide, and chlorides such as sodium chloride. Among these, potassium iodide is preferred. The use of a halide can provide a polarizer with excellent optical properties. Specifically, crystallization of the PVA-based resin after the dry stretching (also referred to as the air-assisted stretching) described below is promoted, and the disturbance of the orientation and the decrease in the orientation of the polyvinyl alcohol molecules are suppressed in the subsequent wet treatment (for example, the dyeing and underwater stretching described below), thereby providing a polarizer with excellent optical properties.

[0019] In preparing the coating solution, the halide is preferably blended in an amount of 5 to 20 parts by weight, more preferably 10 to 15 parts by weight, per 100 parts by weight of the PVA-based resin. Specifically, the content of the halide in the resulting PVA-based resin layer is preferably 5 to 20 parts by weight, more preferably 10 to 15 parts by weight, per 100 parts by weight of the PVA-based resin. If the amount of the halide relative to the PVA-based resin is too high, for example, the halide may bleed out, causing the resulting polarizer to become cloudy.

[0020] 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 are used, for example, to improve the uniformity, dyeability, and stretchability of the resulting PVA-based resin layer.

[0021] Examples of methods for applying the coating liquid include roll coating, spin coating, wire bar coating, dip coating, die coating, curtain coating, spray coating, and knife coating (comma coating, etc.). The temperature for applying and drying the coating liquid is preferably 50°C or higher.

[0022] The thickness of the PVA resin layer is preferably 3 μm to 40 μm, and more preferably 3 μm to 20 μm.

[0023] Before forming the PVA-based resin layer, the thermoplastic resin substrate may be subjected to a surface treatment (e.g., corona treatment, etc.), or an easy-adhesion layer may 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.

[0024] A-2.Dry stretching (midair auxiliary stretching) The resulting laminate is then subjected to auxiliary stretching by dry stretching. This auxiliary stretching allows stretching while suppressing crystallization of the thermoplastic resin substrate, thereby solving the problem of reduced stretchability due to excessive crystallization of the thermoplastic resin substrate during underwater stretching, and allows the laminate to be stretched at a higher magnification. Furthermore, when a thermoplastic resin substrate is used, the coating temperature may be set low, which can lead to a problem of relatively low crystallization of the PVA-based resin, resulting in insufficient optical properties. In contrast, the introduction of auxiliary stretching can increase the crystallinity of the PVA-based resin, even when a thermoplastic resin is used. Furthermore, by increasing the orientation of the PVA-based resin in advance, problems such as reduced orientation and dissolution of the PVA-based resin during subsequent wet processing can be prevented. This allows for the production of a polarizer with excellent optical properties.

[0025] The auxiliary in-air stretching method may be fixed-end stretching (e.g., stretching using a tenter stretching machine) or free-end stretching (e.g., uniaxial stretching by passing the laminate between rolls with different peripheral speeds). The auxiliary in-air stretching may be uniaxial stretching or biaxial stretching. In one embodiment, free-end stretching is employed. For example, heated roll stretching is employed, in which the laminate is stretched by the difference in peripheral speed between heated rolls while being transported in its longitudinal direction. In one embodiment, the auxiliary in-air stretching includes a zone stretching step in a heated space (zone) and a heated roll stretching step. The order of the zone stretching step and the heated roll stretching step is not limited, but for example, the zone stretching step and the heated roll stretching step are performed in this order. In another embodiment, the film is stretched in a tenter stretching machine by gripping the film edges and widening the distance between the tenters in the machine direction (the widening of the distance between the tenters corresponds to the stretch ratio). In this case, the distance of the tenter in the width direction (perpendicular to the machine direction) is preferably set so as to be closer to the free-end stretching ratio in the machine direction. In the case of free-end stretching, the shrinkage ratio in the width direction is calculated by the formula: shrinkage ratio in the width direction = (1 / stretching ratio) 1 / 2 It is calculated as follows.

[0026] The draw ratio of the auxiliary in-air stretching is preferably 2.0 to 3.5. The auxiliary in-air stretching may be carried out in one stage or in multiple stages. When carried out in multiple stages, the draw ratio is the product of the draw ratios in each stage. The stretching direction in the auxiliary in-air stretching is preferably approximately the same as the stretching direction in underwater stretching described below.

[0027] The stretching temperature for the auxiliary in-air stretching is set to any appropriate value depending on, for example, the thermoplastic resin substrate and the stretching method used. The stretching temperature is preferably equal to or higher than the glass transition temperature (Tg) of the thermoplastic resin substrate, more preferably equal to or higher than Tg + 10°C, and even more preferably equal to or higher than Tg + 15°C. On the other hand, the upper limit of the stretching temperature is preferably 170°C. By stretching at such a temperature, rapid crystallization of the PVA-based resin can be suppressed, and problems caused by the crystallization (for example, interference with the orientation of the PVA-based resin layer due to stretching) can be suppressed.

[0028] In one embodiment, the laminate is dry-stretched while being heated at 130°C or higher. The dry stretching may be fixed-end stretching (for example, a method of stretching using a tenter stretching machine) or free-end stretching (for example, 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 increasing the distance between the tenters in the machine direction (the increase in the distance between the tenters represents the stretch ratio).

[0029] 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. A stretching temperature within the above range can provide a polarizer with a higher degree of crystallinity. The stretching temperature can be adjusted by any appropriate method. For example, dry stretching may be performed in an oven set to the above temperature, stretching may be performed while applying hot air set to the above temperature, or stretching may be performed while in contact with a roll heated to the above temperature.

[0030] The stretching ratio in the dry stretching can be set to any appropriate value. The stretching ratio in the dry stretching is preferably 2.0 to 3.5, and more preferably 2.4 to 3.0. When the stretching ratio in the dry stretching is within the above range, the total stretching ratio in the method for producing a polarizer can be made higher.

[0031] In one embodiment, the 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] The crystallinity of the polyvinyl alcohol-based resin layer is preferably 35% to 55%, more preferably 45% to 53%. When the crystallinity of the PVA-based resin layer is within the above range, discoloration from the edges is suppressed, and a polarizer can be obtained. In this specification, the crystallinity of PVA refers to a value measured by the following evaluation method.

[0036] (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

[0037] A-3.Wet stretching (underwater stretching) After dry stretching (auxiliary air stretching), the laminate is subjected to wet stretching (underwater stretching). Underwater stretching is typically performed by immersing the laminate in a stretching bath. Underwater stretching allows stretching at a temperature lower than the glass transition temperatures (typically, about 80°C) of the thermoplastic resin substrate and the PVA-based resin layer, and the PVA-based resin layer can be stretched at a high magnification while suppressing crystallization. As a result, a polarizer with excellent optical properties can be obtained. As described above, the laminate used in the method for producing a polarizer of the present invention has a PVA-based resin layer formed on a substrate formed using a thermoplastic resin having a glass transition temperature Tg of 60°C to 73°C. When such a laminate is used, the thermoplastic resin substrate and the PVA-based resin layer are uniformly stretched in the laminated state during wet stretching, thereby suppressing curling and edge folding. As a result, a decrease in the productivity of polarizers can be suppressed.

[0038] The underwater stretching method may be fixed-end stretching or free-end stretching (for example, a method in which the laminate is passed between rolls with different peripheral speeds for uniaxial stretching). Free-end stretching is preferably used. The stretching of the laminate may be carried out in one stage or in multiple stages. When stretching is carried out in multiple stages, the stretch ratio of the laminate described below is the product of the stretch ratios in each stage.

[0039] The underwater stretching is preferably performed by immersing the laminate in an aqueous boric acid solution (stretching in boric acid solution). Using an aqueous boric acid solution as a stretching bath can impart to the PVA-based resin layer rigidity sufficient to withstand the tension applied during stretching and water resistance sufficient to prevent dissolution in water. Specifically, boric acid generates tetrahydroxyborate anions in the aqueous solution, which can crosslink with the PVA-based resin through hydrogen bonding. As a result, the PVA-based resin layer is imparted with rigidity and water resistance, allowing it to be stretched well, resulting in a polarizer with excellent optical properties.

[0040] The boric acid aqueous 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 adjusting the boric acid concentration to 1 part by weight or more, dissolution of the PVA-based resin layer can be effectively suppressed, allowing for the production of a polarizer with better performance. 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.

[0041] 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.

[0042] 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. Specifically, as described above, the glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 60°C or higher in relation to the formation of the PVA-based resin layer. In this case, if the stretching temperature is lower than 40°C, good stretching may not be possible, even taking into account the plasticization of the thermoplastic resin substrate by water. On the other hand, the stretching temperature is, for example, 70°C or lower, preferably 67°C or lower, and more preferably 65°C or lower. The higher the stretching temperature, the higher the solubility of the PVA-based resin layer, which may result in poor optical properties. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

[0043] The stretching ratio in underwater stretching is preferably 1.5 times or more, more preferably 2.0 times or more. The total stretching ratio of the laminate (stretching ratio obtained by combining the auxiliary air-stretching and underwater stretching) is preferably 5.0 times or more, more preferably 5.5 times or more, and even more preferably 6.0 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).

[0044] The difference between the shrinkage rate of the laminate during wet stretching and the shrinkage rate of the thermoplastic resin substrate during wet stretching is 2.0% or less, preferably 1.0% or less, and more preferably 0.5% or less. The difference between the shrinkage rate of the laminate and the shrinkage rate of the thermoplastic resin substrate during wet stretching is, for example, -3% or more, preferably -2.5% or more. When the difference between the shrinkage rate of the thermoplastic resin substrate during wet stretching and the shrinkage rate of the laminate is within the above range, the laminate can be uniformly stretched in a laminated state, and curling and edge folding can be suppressed. In this specification, the shrinkage rate during wet stretching refers to a value calculated by {(width immediately before wet stretching - width after drying treatment) / width immediately before wet stretching} x 100.

[0045] A-4. Other processes The method for producing a polarizer according to an embodiment of the present invention may further include other appropriate steps in addition to the dry stretching and wet stretching. Examples of such other steps include an insolubilization step, a dyeing step, a crosslinking step, a stretching step other than the above stretching step, a washing step, and a drying step (adjusting the moisture content). The other steps may be performed at any appropriate timing.

[0046] Dyeing is typically performed by adsorbing iodine to the PVA-based resin layer. Examples of methods for adsorbing iodine include immersing the PVA-based resin layer (laminate) in a dyeing solution containing iodine, applying the dyeing solution to the PVA-based resin layer, and spraying the dyeing solution onto the PVA-based resin layer. The preferred method is immersing the laminate in a dyeing solution (dye bath), as this allows for good adsorption of iodine.

[0047] The dyeing solution is preferably an aqueous iodine solution. The amount of iodine blended is preferably 0.05 to 1.0 part by weight per 100 parts by weight of water. To increase the solubility of iodine in water, it is preferable to blend an iodide into the aqueous iodine solution. Specific examples of iodide are as described above. Potassium iodide is preferably used. The amount of iodide blended is preferably 0.1 to 10 parts by weight, more preferably 0.3 to 5 parts by weight, per 100 parts by weight of water. The temperature of the dyeing solution during dyeing is preferably 20°C to 50°C to suppress dissolution of the PVA-based resin. When the PVA-based resin layer is immersed in the dyeing solution, the immersion time is preferably 5 seconds to 5 minutes, more preferably 30 to 90 seconds, to ensure the transmittance of the PVA-based resin layer.

[0048] The dyeing conditions (concentration, solution temperature, immersion time) can be set, for example, so that the final polarizer has a single transmittance of 42.0% or more and a polarization degree of 99.98% or more. As such dyeing conditions, for example, in the iodine aqueous solution serving as the dyeing solution, the ratio of the contents of iodine and potassium iodide is preferably 1:5 to 1:20, and more preferably 1:5 to 1:10.

[0049] When dyeing is performed immediately after immersing a laminate in a treatment bath containing boric acid (for example, an insolubilization treatment described below), the boric acid may be mixed into the dye bath, changing the boric acid concentration in the dye bath and causing instability in the dyeability. To prevent this instability in the dyeability, the boric acid concentration in the dye bath is adjusted to preferably 4 parts by weight or less, more preferably 3 parts by weight or less, per 100 parts by weight of water. On the other hand, the boric acid concentration in the dye bath is preferably 0.1 parts by weight or more, more preferably 0.2 parts by weight or more, and even more preferably 0.5 parts by weight or more, per 100 parts by weight of water. In one embodiment, dyeing is performed using a dye bath containing boric acid in advance. This embodiment can reduce the rate of change in boric acid concentration when boric acid is mixed into the dye bath. The amount of boric acid to be added to the dye bath in advance (the content of boric acid not derived from the treatment bath) is preferably 0.1 to 2 parts by weight, and more preferably 0.5 to 1.5 parts by weight, per 100 parts by weight of water.

[0050] If necessary, after the above-mentioned auxiliary air-stretching, an insolubilization treatment is carried out before underwater stretching and dyeing. The insolubilization treatment is typically carried out by immersing the PVA-based resin layer in an aqueous boric acid solution. By carrying out the insolubilization treatment, water resistance is imparted to the PVA-based resin layer, and it is possible to prevent a decrease in the orientation of PVA when immersed in water. The concentration of the aqueous boric acid solution in the insolubilization treatment is preferably 1 to 4 parts by weight per 100 parts by weight of water. The temperature of the insolubilization treatment (liquid temperature of the aqueous boric acid solution) is preferably 20 to 50°C.

[0051] If necessary, a crosslinking treatment is carried out after dyeing and before underwater stretching. The crosslinking treatment is typically carried out by immersing the PVA-based resin layer in an aqueous boric acid solution. The crosslinking treatment imparts water resistance to the PVA-based resin layer, thereby preventing a decrease in the orientation of the PVA during the subsequent underwater stretching. The concentration of the aqueous boric acid solution in the crosslinking treatment is preferably 1 to 5 parts by weight per 100 parts by weight of water. It is preferable to add an iodide to the aqueous boric acid solution. Adding an iodide can suppress the elution of iodine adsorbed to the PVA-based resin layer. Specific examples of iodides are as described above. The amount of iodide added is preferably 1 to 5 parts by weight per 100 parts by weight of water. The temperature of the crosslinking treatment (liquid temperature of the aqueous boric acid solution) is preferably 20°C to 50°C.

[0052] The temperature of the immersion liquid is preferably 20° C. to 50° C. The immersion time of the resin film (laminate) in the immersion liquid can be, for example, 1 second to 30 seconds.

[0053] After the water washing, it is preferable to dry the resin film (laminate). Drying can be performed by any appropriate method. Specifically, it may be performed by heating the entire zone (zone heating method) or by heating the transport roll (heating roll method). A heating roll method is preferably used, and more preferably both. By using a heating roll, heat curling of the laminate can be efficiently suppressed, allowing for the production of a high-quality polarizer. Specifically, drying the laminate while it is aligned with a heating roll can efficiently promote crystallization of the thermoplastic resin substrate, thereby increasing the crystallinity. Even at a relatively low drying temperature, the crystallinity of the thermoplastic resin substrate can be effectively increased. As a result, the rigidity of the thermoplastic resin substrate increases, allowing it to withstand shrinkage of the PVA-based resin layer due to drying, thereby suppressing curling. Furthermore, by using a heating roll, the laminate can be dried while being maintained flat, thereby suppressing not only curling but also wrinkles.

[0054] Drying can shrink the laminate in the width direction, improving its optical properties. This is because it can effectively increase the orientation of the PVA and the PVA / iodine complex. 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, achieving high productivity.

[0055] 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 80°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.

[0056] 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.

[0057] B. Polarizer The polarizer obtained by the polarizer manufacturing method according to the embodiment of the present invention preferably exhibits absorption dichroism at any wavelength between 380 nm and 780 nm. The single transmittance (Ts) of the polarizer 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 a Y value measured using a 2-degree visual field (C light source) according to JIS Z8701 and corrected for luminosity. For example, it can be measured 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.

[0058] The thickness of the polarizer is, for example, 12 μm or less, preferably 8 μm or less, and may be 6 μm or less, while the thickness of the PVA resin layer is preferably 1 μm or more, more preferably 2 μm or more.

[0059] 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).

[0060] C. 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.

[0061] 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 preferably be 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.

[0062] 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.

[0063] The polarizing plate may further have any appropriate optically functional layer depending on the purpose. Typical examples of the optically functional layer include a retardation film (optical compensation film) and a surface treatment layer. 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. [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 200 μm thick film was prepared using polyethylene terephthalate (isophthalic acid unit: 2.5 mol%, terephthalic acid unit: 47.5 mol%, ethylene glycol unit: 40 mol%, diethylene glycol unit: 10.0 mol%) with a glass transition temperature Tg of 63°C, and used as the resin substrate. One side of the resin substrate was subjected to corona treatment. A PVA aqueous solution (coating solution) was prepared by dissolving 100 parts by weight of a PVA-based resin made by mixing polyvinyl alcohol (polymerization degree 4200, saponification degree 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFIRM") in a 9:1 ratio, and adding 13 parts by weight of potassium iodide in water. 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 of the laminate at 150°C using a tenter stretching device. Next, the laminate was cut into a 100 mm strip in the TD direction, and held with a jig so that the length in the MD direction (longitudinal direction of the laminate) was 120 mm. 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 final polarizer obtained would be 43.5% (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). Next, the laminate was immersed in an aqueous boric acid 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 sample was dried in an oven maintained at about 65°C (drying 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. After drying, the shrinkage of the thermoplastic resin substrate was 71%, and the shrinkage of the laminate was 68.5%.

[0066] [Example 2] A 200 μm thick film was prepared using polyethylene terephthalate (isophthalic acid unit: 2.5 mol%, terephthalic acid unit: 47.5 mol%, ethylene glycol unit: 43 mol%, diethylene glycol unit: 7.0 mol%) with a glass transition temperature Tg of 67°C, and used as the resin substrate. A laminate having a PVA-based resin layer (polarizer) was obtained in the same manner as in Example 1, except that the above resin substrate was used.

[0067] [Example 3] A 200 μm thick film was prepared using polyethylene terephthalate (isophthalic acid unit: 2.5 mol%, terephthalic acid unit: 47.5 mol%, ethylene glycol unit: 45 mol%, diethylene glycol unit: 5.0 mol%) with a glass transition temperature Tg of 70°C, and used as the resin substrate. A laminate having a PVA-based resin layer (polarizer) was obtained in the same manner as in Example 1, except that the above resin substrate was used.

[0068] [Example 4] A 200 μm thick film was prepared using polyethylene terephthalate (isophthalic acid unit: 2.5 mol%, terephthalic acid unit: 47.5 mol%, ethylene glycol unit: 46 mol%, diethylene glycol unit: 4.0 mol%) with a glass transition temperature Tg of 72°C, and used as the resin substrate. A laminate having a PVA-based resin layer (polarizer) was obtained in the same manner as in Example 1, except that the above resin substrate was used.

[0069] [Example 5] A laminate having a PVA-based resin layer (polarizer) was obtained in the same manner as in Example 3, except that the stretching conditions for the auxiliary in-air stretching were changed as shown in Table 1.

[0070] (Comparative Example 1) A 200 μm thick film was prepared using polyethylene terephthalate (isophthalic acid unit: 2.5 mol%, terephthalic acid unit: 47.5 mol%, ethylene glycol unit: 37.5 mol%, diethylene glycol unit: 12.5 mol%) with a glass transition temperature Tg of 59°C, and used as the resin substrate. A laminate having a PVA-based resin layer (polarizer) was obtained in the same manner as in Example 1, except that the above resin substrate was used.

[0071] (Comparative Example 2) A 200 μm thick film was prepared using polyethylene terephthalate (isophthalic acid unit: 2.5 mol%, terephthalic acid unit: 47.5 mol%, ethylene glycol unit: 47.2 mol%, diethylene glycol unit: 2.8 mol%) with a glass transition temperature Tg of 74°C, and used as the resin substrate. A laminate having a PVA-based resin layer (polarizer) was obtained in the same manner as in Example 1, except that the above resin substrate was used.

[0072] (Comparative Example 3) A 200 μm thick film was prepared using polyethylene terephthalate (isophthalic acid unit: 2.5 mol%, terephthalic acid unit: 47.5 mol%, ethylene glycol unit: 48 mol%, diethylene glycol unit: 2 mol%) with a glass transition temperature Tg of 75°C, and used as the resin substrate. A laminate having a PVA-based resin layer (polarizer) was obtained in the same manner as in Example 1, except that the above resin substrate was used.

[0073] Comparative Example 4 A 200 μm thick film was prepared using polyethylene terephthalate (isophthalic acid unit: 2.5 mol%, terephthalic acid unit: 47.5 mol%, ethylene glycol unit: 48 mol%, diethylene glycol unit: 2 mol%) with a glass transition temperature Tg of 75°C, and used as the resin substrate. A laminate having a PVA-based resin layer (polarizer) was obtained in the same manner as in Example 5, except that the above resin substrate was used.

[0074] [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").

[0075] 2. Glass transition temperature The glass transition temperature Tg of the thermoplastic resin was measured in accordance with JIS K 7121 using a differential scanning calorimeter (Hitachi High-Tech Science Corporation, product name "DSC7000"). Specifically, pellets (approximately 7 mg) of the polyethylene terephthalate used to prepare the thermoplastic resin substrate in the Examples and Comparative Examples were heated (first run)-cooled-heated (second run) at a rate of 10°C / min in the temperature range of 30°C or higher and 300°C or lower. The glass transition temperature of the polyethylene terephthalate was determined from the measurement results of the first run. Note that if it is difficult to measure using thermoplastic resin pellets, a sample can be taken from the film-like thermoplastic resin substrate and measured in the same manner, and the glass transition temperature can be determined from the results of the second run.

[0076] 3. Shrinkage rate Using a ruler, the widths of the substrate and the laminate after wet stretching in each Example and Comparative Example were measured, and the shrinkage ratio was calculated using the following formula: When wrinkles occurred in the laminate due to shrinkage, the wrinkles were smoothed out and the width was measured. The shrinkage rate is a value calculated by the following formula: shrinkage rate = {(width immediately before being subjected to wet stretching - width after drying) / (width immediately before being subjected to wet stretching)} x 100. Further, from the obtained shrinkage percentages, the difference between the shrinkage percentage of the laminate and the shrinkage percentage of the thermoplastic resin substrate (shrinkage percentage of the laminate - shrinkage percentage of the thermoplastic resin substrate) was calculated.

[0077] 4.Break detection The laminates obtained in the Examples and Comparative Examples were placed on a horizontal table so that the thermoplastic resin substrate side of the laminate was in contact with the table. A weight (18 cm long, 1.5 cm wide, 1.0 cm high, 250 g weight) was then placed on the end of the film in the width direction. The weight was then removed from the laminate, and the length of the folded portion of the laminate was measured using a ruler. The length of the folded portion was evaluated according to the following criteria. × (room for improvement): 4.0 mm or more △(Acceptable): 2.5mm~4.0mm Good (Good): 0mm to 2.5mm

[0078] 5. Curl determination In each example and comparative example, the laminate was placed on a horizontal table immediately after removal from the oven with the thermoplastic resin substrate facing the table and left to stand for 1 hour. The amount of curl (mm) in the width direction of the laminate was then measured with a ruler. A positive curl was defined as a curl in the inward direction (the film was U-shaped), and a negative curl was defined as a curl outward (the film was inverted U-shaped). Films that became cylindrical were deemed unmeasurable, and films that developed galvanized iron-like wrinkles and could not be measured were deemed to have galvanized iron-iron wrinkles. × (Room for improvement): ±10mm or more, or galvanized iron wrinkles △(Acceptable): ±5mm~10mm Good: Less than ±5mm

[0079] [Table 1]

[0080] [evaluation] In Examples 1 to 5 of the present invention, the occurrence of curling and folding of the polarizer was suppressed. [Industrial Applicability]

[0081] According to the method for producing a polarizer of an embodiment of the present invention, a polarizer can be obtained that is thin but suppresses curling. Furthermore, curling and folding during the production process can be suppressed, and productivity of the polarizer can be improved.

Claims

1. The method includes, in this order, forming a laminate by forming a polyvinyl alcohol-based resin layer on a substrate formed using a thermoplastic resin having a glass transition temperature Tg of 60°C to 73°C; dry-stretching the laminate; and wet-stretching the laminate, a difference between a shrinkage percentage of the laminate during the wet stretching and a shrinkage percentage of the thermoplastic resin substrate during the wet stretching being 2.0% or less.

2. The method for producing a polarizer according to claim 1 , wherein the thermoplastic resin is an amorphous polyethylene terephthalate-based resin.

3. 2. The method for producing a polarizer according to claim 1, wherein the thermoplastic resin is a polyethylene terephthalate resin containing 3.0 mol % to 10.0 mol % of diethylene glycol units relative to the total of all repeating units.

4. The method for producing a polarizer according to claim 1 , wherein the laminate has a total stretching ratio of 5.0 times or more.

5. The method for producing a polarizer according to claim 1 , wherein the dry stretching is biaxial stretching.

6. 2. The method for producing a polarizer according to claim 1, wherein the polyvinyl alcohol-based resin layer after the dry stretching has a crystallinity of 35% to 55%.

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