Polarizing film manufacturing method

Abstract

To provide a polarizing film having both high optical characteristics and excellent appearance.SOLUTION: A method for producing a polarizing film includes a step of obtaining a resin film having a first film thickness (T1) through treatment using water, an adjustment step of reducing the film thickness of the resin film to a second film thickness (T2) from the first film thickness (T1), and a drying step of drying a resin film having the second film thickness (T2), wherein a ratio (T2 / T1) of the second film thickness (T2) to the first film thickness (T1) is less than 1, the film thickness of the resin film is reduced to a third film thickness (T3) from the second film thickness (T2) by the drying step, a ratio (T3 / T2) of the third film thickness (T3) to the second film thickness (T2) is 0.90 or less, and the adjustment is performed so that the resin film is placed under an environment of humidity of 35%RH or more.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for producing a polarizing film. [Background technology]

[0002] Liquid crystal display devices, a typical example of image display devices, have polarizing films disposed on both sides of a liquid crystal cell due to their image formation method. Furthermore, with the widespread use of flat panel displays, displays incorporating organic electroluminescence (EL) panels (OLEDs) and displays using display panels incorporating inorganic light-emitting materials such as quantum dots (QLEDs) have been proposed. These panels have highly reflective metal layers, which can easily cause problems such as external light reflection and background glare. Therefore, it is known that providing a circular polarizing plate comprising a polarizing film and a λ / 4 plate on the viewing side can prevent these problems. One proposed method for producing a polarizing film is, for example, stretching a laminate comprising a resin substrate and a polyvinyl alcohol (PVA)-based resin layer, followed by dyeing the laminate to obtain a polarizing film on the resin substrate (see, for example, Patent Document 1). This method can produce a thin polarizing film, and has therefore attracted attention as a potential contribution to the recent trend toward thinner image display devices.

[0003] However, a thin polarizing film has a problem in that it is not easy to achieve both high optical properties and good appearance. Specifically, the higher the optical properties, the more likely problems with appearance tend to occur. Poor appearance of a polarizing film may affect the display properties of an image display device. For example, if streaks occur on the polarizing film, they may be visually recognized as poor appearance (texture) in the configuration of a laminated film (e.g., a circular polarizing plate). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-343521 Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention has been made to solve the above problems, and a main object of the present invention is to provide a polarizing film that combines high optical properties with an excellent appearance. [Means for solving the problem]

[0006] According to an embodiment of the present invention, there is provided a method for manufacturing a polarizing film, the method comprising the steps of obtaining a resin film having a first thickness (T1) through a treatment with water, reducing the thickness of the resin film from the first thickness (T1) to a second thickness (T2), and drying the resin film having the second thickness (T2), wherein the ratio (T2 / T1) of the second thickness (T2) to the first thickness (T1) is less than 1, the thickness of the resin film is reduced from the second thickness (T2) to a third thickness (T3) by the drying, and the ratio (T3 / T2) of the third thickness (T3) to the second thickness (T2) is 0.90 or less, and the adjusting step is performed by placing the resin film in an environment with a humidity of 35% RH or higher. In one embodiment, the resin film is placed in an environment with a temperature of less than 40°C to perform the adjustment. In one embodiment, the difference between the temperature at which the drying is carried out and the temperature at which the conditioning is carried out is 25°C or more. In one embodiment, the difference between the humidity at which the conditioning is performed and the humidity at which the drying is performed is 30% RH or more. In one embodiment, the resin film is dried in an environment at a temperature of 60° C. or higher and a humidity of 10% RH or lower. In one embodiment, the first film thickness (T1), the second film thickness (T2), and the third film thickness (T3) satisfy the relationship (T2 / T1) / (T3 / T2)≧1. In one embodiment, the first film thickness (T1) is 5 μm or more. In one embodiment, a polarizing film having a thickness of 7 μm or less is obtained by the above production method. [Effects of the Invention]

[0007] According to the embodiment of the present invention, a polarizing film having both high optical properties and excellent appearance can be obtained. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a schematic cross-sectional view showing the general configuration of a laminate according to one embodiment of the present invention. [Figure 2] 1 is a graph showing the relationship between the film thickness and moisture content of a resin film after treatment with water. [Figure 3] 1A to 1C are schematic diagrams illustrating an example of a manufacturing process for a polarizing film. [Figure 4] FIG. 2 is a schematic diagram showing an example of drying using a heating roll in a drying zone. [Figure 5] 1 is a schematic cross-sectional view showing the general configuration of a polarizing plate according to one embodiment of the present invention. [Figure 6] 1 is an observation photograph of the polarizing plate of Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these embodiments.

[0010] (Definition of terms and symbols) The definitions of terms and symbols used in this specification are as follows. (1) Refractive index (nx, ny, nz) "nx" is the refractive index in the direction in which the in-plane refractive index is greatest (i.e., the slow axis direction), "ny" is the refractive index in the direction perpendicular to the slow axis in the plane (i.e., the fast axis direction), and "nz" is the refractive index in the thickness direction. (2) In-plane phase difference (Re) "Re(λ)" is the in-plane retardation measured with light of wavelength λ nm at 23°C. For example, "Re(550)" is the in-plane retardation measured with light of wavelength 550 nm at 23°C. Re(λ) is calculated by the formula: Re(λ)=(nx-ny)×d, where d (nm) is the thickness of the layer (film). (3) Retardation in the thickness direction (Rth) "Rth(λ)" is the retardation in the thickness direction measured with light of wavelength λ nm at 23°C. For example, "Rth(550)" is the retardation in the thickness direction measured with light of wavelength 550 nm at 23°C. Rth(λ) is calculated by the formula: Rth(λ) = (nx - nz) × d, where d (nm) is the thickness of the layer (film). (4) Nz coefficient The Nz coefficient is calculated by Nz=Rth / Re.

[0011] A method for producing a polarizing film according to one embodiment of the present invention includes a step of obtaining a resin film having a first thickness (T1) through a treatment using water, a step of adjusting the thickness of the resin film from the first thickness (T1) to a second thickness (T2), and a drying step of drying the resin film having the second thickness (T2).

[0012] A. Resin film The resin film can be obtained, for example, by forming a resin layer (typically, a polyvinyl alcohol-based resin layer) on a resin substrate to produce a laminate, and then stretching the laminate and dyeing it with a dichroic substance such as iodine (for example, dyeing by adsorption of iodine).

[0013] A-1.Laminate 1 is a schematic cross-sectional view showing the general configuration of a laminate according to one embodiment of the present invention. The laminate 1 has a thermoplastic resin substrate (e.g., a long piece) 2 and a polyvinyl alcohol (PVA)-based resin layer 3. Preferably, the laminate 1 is produced by forming the PVA-based resin layer 3 containing a PVA-based resin and a halide on the thermoplastic resin substrate 2. Specifically, the PVA-based resin layer 3 is formed by applying a coating liquid containing a PVA-based resin and a halide to the thermoplastic resin substrate 2 and drying it.

[0014] The thickness of the thermoplastic resin substrate is preferably 20 μm to 300 μm, more preferably 50 μm to 200 μm. If the thickness is less than 20 μm, it may be difficult to form a PVA-based resin layer. If the thickness is more than 300 μm, for example, in 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.

[0015] The water absorption of the thermoplastic resin substrate is preferably 0.2% or more, and more preferably 0.3% or more. Such a thermoplastic resin substrate can absorb water, which acts as a plasticizer and plasticizes the substrate. As a result, the stretching stress can be significantly reduced, allowing for high stretching ratios. On the other hand, the water absorption of the thermoplastic resin substrate is preferably 3.0% or less, and more preferably 1.0% or less. This water absorption can prevent problems such as a significant decrease in the dimensional stability of the thermoplastic resin substrate during production, which can lead to poor quality of the resulting polarizing film. It can also prevent breakage of the thermoplastic resin substrate and peeling of the PVA-based resin layer during underwater stretching. The water absorption of the thermoplastic resin substrate can be adjusted, for example, by introducing a modifying group into the constituent materials. The water absorption is a value determined in accordance with JIS K 7209.

[0016] The glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 120°C or lower. By using such a thermoplastic resin substrate, the stretchability of the laminate can be sufficiently ensured while suppressing crystallization of the PVA-based resin layer. Furthermore, considering the plasticization of the thermoplastic resin substrate with water and the smooth underwater stretching, the Tg is more preferably 100°C or lower, and even more preferably 90°C or lower. On the other hand, the Tg of the thermoplastic resin substrate is preferably 60°C or higher. Such a Tg prevents problems such as deformation of the thermoplastic resin substrate (e.g., the occurrence of unevenness, sagging, wrinkles, etc.) during the application and drying of the coating liquid, allowing for the production of a satisfactory laminate. Furthermore, the resin layer can be satisfactorily stretched at a suitable temperature (e.g., about 60°C). The Tg of the thermoplastic resin substrate can be adjusted, for example, by introducing a modifying group into the constituent material or by heating using a crystallizing material. The glass transition temperature (Tg) is a value determined in accordance with JIS K 7121.

[0017] Any suitable thermoplastic resin can be used as the constituent material of the thermoplastic resin substrate. Examples of the thermoplastic resin include ester-based resins such as polyethylene terephthalate-based resins, cycloolefin-based resins such as norbornene-based resins, olefin-based resins such as polypropylene, polyamide-based resins, polycarbonate-based resins, and copolymer resins thereof. Among these, norbornene-based resins and amorphous polyethylene terephthalate-based resins are preferred.

[0018] In one embodiment, amorphous (non-crystallized) polyethylene terephthalate resins are preferably used. Among them, amorphous (hard to crystallize) polyethylene terephthalate resins are particularly preferably used. Specific examples of amorphous polyethylene terephthalate resins include copolymers further containing isophthalic acid and / or cyclohexanedicarboxylic acid as dicarboxylic acids, and copolymers further containing cyclohexanedimethanol or diethylene glycol as glycols.

[0019] In another embodiment, a polyethylene terephthalate resin having an isophthalic acid unit is preferably used. This is because it has extremely excellent stretchability and can suppress crystallization during stretching. This is thought to be because the introduction of the isophthalic acid unit imparts a large curvature to the main chain. The polyethylene terephthalate resin has a terephthalic acid unit and an ethylene glycol unit. The content of the isophthalic acid unit is preferably 0.1 mol% or more, more preferably 1.0 mol% or more, based on the total of all repeating units. This is because a thermoplastic resin substrate with extremely excellent stretchability can be obtained. On the other hand, the content of the isophthalic acid unit is preferably 20 mol% or less, more preferably 10 mol% or less, based on the total of all repeating units. This is because it can satisfactorily increase the crystallinity during drying, as described below.

[0020] 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+50°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.

[0021] The coating liquid is typically a solution in which a PVA resin and a halide are dissolved in a solvent. 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 can be used alone or in combination. Of these, water is preferred. The content of the PVA resin in the coating liquid is preferably 3 to 20 parts by weight per 100 parts by weight of the solvent. This range allows the formation of a uniform coating film that adheres closely to the thermoplastic resin substrate. The content of the halide in the coating liquid is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA resin.

[0022] 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 polarizing film 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.

[0023] 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.

[0024] 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 produce a polarizing film with excellent optical properties. Specifically, crystallization of the PVA resin after the in-air auxiliary stretching described below is promoted, and disturbance of the orientation of polyvinyl alcohol molecules and deterioration of the orientation are suppressed in subsequent wet treatments (e.g., dyeing and stretching in water described below), resulting in a polarizing film with excellent optical properties.

[0025] 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 halide relative to the PVA-based resin is too high, for example, the halide may bleed out, causing the resulting polarizing film to become cloudy.

[0026] 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.

[0027] 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.

[0028] The thickness of the PVA resin layer is preferably 3 μm to 40 μm, and more preferably 3 μm to 20 μm.

[0029] 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.

[0030] A-2. Stretching The stretching is preferably performed by dry stretching (in-air auxiliary stretching) of the laminate followed by underwater stretching. 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 stretching in boric acid water, and allows the laminate to be stretched at a higher stretching ratio. 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, 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 a polarizing film with excellent optical properties to be obtained.

[0031] The method of the auxiliary in-air stretching 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). Free-end stretching is preferably used. For example, heated roll stretching is used, 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 by gripping the edges of the film in a tenter stretching machine 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 between the tenters in the width direction (perpendicular to the machine direction) is preferably set so that it is 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.

[0032] 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.

[0033] 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.

[0034] The underwater stretching is typically performed by immersing the laminate in a stretching bath. Underwater stretching allows stretching to be performed at a temperature lower than the glass transition temperature (typically about 80°C) of the thermoplastic resin substrate or 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 polarizing film with excellent optical properties can be obtained.

[0035] 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.

[0036] 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 the 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 satisfactorily, resulting in a polarizing film with excellent optical properties.

[0037] 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 polarizing film with better properties. In addition to boric acid or a borate, aqueous solutions obtained by dissolving boron compounds such as borax, glyoxal, glutaraldehyde, etc. in a solvent can also be used.

[0038] 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.

[0039] The stretching temperature (liquid temperature of the stretching bath) is preferably 40°C or higher, more preferably 60°C or higher, and may be 65°C or higher. At such a temperature, stretching can be performed at a high magnification, resulting in a polarizing film with excellent optical properties. Specifically, as described above, the glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 60°C or higher, considering the formation of the PVA-based resin layer. In this case, if the stretching temperature is lower than 40°C, satisfactory stretching may not be possible, even considering the plasticization of the thermoplastic resin substrate by water. Even when stretching at such a temperature, a polarizing film with excellent appearance can be obtained by adjusting the film thickness as described below. Meanwhile, the stretching temperature is preferably 75°C or lower, more preferably 70°C or lower, and may be 65°C or lower. The higher the stretching temperature, the higher the solubility of the PVA-based resin layer, potentially preventing the achievement of high optical properties. At such a stretching temperature, a polarizing film with better appearance can be obtained. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

[0040] The stretching ratio in underwater stretching is preferably 1.5 times or more, and more preferably 3.0 times or more. The total stretching ratio of the laminate (stretching ratio obtained by combining the auxiliary in-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 polarizing film 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).

[0041] A-3. Dyeing The dyeing is typically carried out 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.

[0042] The dyeing solution is preferably an aqueous iodine solution. The amount of iodine blended is preferably 0.05 to 0.5 parts 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.

[0043] The dyeing conditions (concentration, solution temperature, immersion time) can be set, for example, so that the final polarizing film 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 iodine content to the potassium iodide content is preferably 1:5 to 1:20, and more preferably 1:5 to 1:10.

[0044] 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 2 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 that contains 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.

[0045] A-4. Other processing 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.

[0046] 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.

[0047] Preferably, after the underwater stretching, washing is carried out, typically by immersing the PVA-based resin layer in an aqueous potassium iodide solution.

[0048] B. Resin film thickness The resin film after the water treatment has a first thickness (T1), and the thickness of such a resin film is adjusted. As shown in Figure 2, the thickness of the resin film may be correlated with its moisture content. Specifically, the in-plane dimensional change of the resin film due to water absorption is constrained by the resin substrate with low water absorption, and the resin film is thought to expand in the thickness direction depending on the amount of water absorbed. Therefore, it is thought that there is a correlation between the thickness of the resin film after water treatment (e.g., after the above-mentioned washing) and its moisture content. The graph in Figure 2 plots data on the thickness and moisture content of the resin film after water treatment when the laminate was stretched in water under various stretching conditions (specifically, the boric acid concentration in the stretching bath) shown in Table 1 below. The approximation curve in the graph is an approximation curve obtained by the least squares method so that the plotted data becomes an exponential function. The moisture content in Figure 2 was calculated using the following formula based on the dry weight method. Moisture content of resin film = (weight of resin film after treatment with water - weight of resin film after drying) / weight of resin film after drying

[0049] [Table 1]

[0050] The first film thickness (T1) is, for example, 4.0 μm or more, preferably 4.5 μm or more, more preferably 5 μm or more, and may be 6 μm or more, or 7 μm or more. On the other hand, the first film thickness (T1) is, for example, 20 μm or less, preferably 12 μm or less.

[0051] FIG. 3 is a schematic diagram illustrating an example of a manufacturing process for a polarizing film. A laminate 1 composed of a resin substrate and a PVA-based resin layer is immersed in a bath 101 of a boric acid solution by a conveying roll (insolubilization treatment), and then immersed in a bath 102 of an aqueous solution of a dichroic substance (iodine) and potassium iodide (dyeing treatment). It is then immersed in a bath 103 of an aqueous solution of boric acid and potassium iodide (crosslinking treatment). Next, while immersed in a stretching bath 104 of a boric acid solution, the laminate 1 is stretched in the conveying direction by applying tension with rolls having different speed ratios (underwater stretching treatment). The underwater stretched laminate 1 is then immersed in a bath 105 of a potassium iodide solution and washed (washing treatment). Although not shown, the laminate 1 may be subjected to the above-mentioned auxiliary in-air stretching before the insolubilization treatment, for example.

[0052] After treatment with water (having passed through the water bath), the laminate 1 is conveyed to a conditioning zone 110 and then to a drying zone 120 .

[0053] By passing through the adjustment zone 110, the thickness of the resin film (PVA-based resin layer) of the laminate 1 can be adjusted from the first thickness (T1) to the second thickness (T2) (adjusting step). Specifically, at the entrance of the adjustment zone 110, the resin film has the first thickness (T1), and at the exit of the adjustment zone 110 (entrance to the drying zone 120), the resin film has the second thickness (T2).

[0054] The second film thickness (T2) is preferably 3.5 μm or more and 8.6 μm or less, more preferably 5.5 μm or more and 8.3 μm or less. The ratio (T2 / T1) of the second film thickness (T2) to the first film thickness (T1) is preferably 0.85 or more, more preferably 0.86 or more. Meanwhile, T2 / T1 is less than 1, preferably 0.95 or less, more preferably 0.93 or less, and even more preferably 0.90 or less.

[0055] The temperature in the adjustment zone 110 is preferably less than 40°C, more preferably 35°C or less, and may be 30°C or less. On the other hand, the temperature in the adjustment zone 110 is preferably 20°C or more, and may be 22°C or more. By placing the resin film in an environment with such a temperature, the second thickness (T2) can be satisfactorily achieved over a predetermined period of time. The humidity in the adjustment zone 110 is preferably 35% RH or more, more preferably 40% RH or more. On the other hand, the humidity in the adjustment zone 110 is, for example, 65% RH or less. By placing the resin film in an environment with such a humidity, the second thickness (T2) can be satisfactorily achieved over a predetermined period of time.

[0056] The time required to pass through the conditioning zone 110 is, for example, 5 seconds to 4 minutes. The time required to pass through the conditioning zone 110 corresponds, for example, to the time required for the resin film to be placed under a predetermined environmental condition. Note that the temperature and humidity in the conditioning zone 110 do not need to be constant, but are preferably maintained within the above-mentioned temperature and humidity ranges.

[0057] The rate of decrease in the film thickness (R1) in the adjusting step is preferably 3.6 μm / min or less, more preferably 3.2 μm / min or less, and even more preferably 3.0 μm / min or less. On the other hand, R1 is, for example, 2.0 μm / min or more.

[0058] By passing through the drying zone 120, the resin film reduces its thickness from the second thickness (T2) to a third thickness (T3) (drying step). Specifically, at the entrance of the drying zone 120, the resin film has the second thickness (T2), and at the exit of the drying zone 120, the resin film has the third thickness (T3).

[0059] The third film thickness (T3) is preferably 3.0 μm or more and 7.0 μm or less, more preferably 4.0 μm or more and 6.5 μm or less. The ratio (T3 / T2) of the third film thickness (T3) to the second film thickness (T2) is 0.90 or less, preferably 0.85 or less, more preferably 0.80 or less, and even more preferably 0.75 or less. Meanwhile, T3 / T2 is, for example, 0.70 or more, preferably 0.72 or more. The ratio (T2 / T1) / (T3 / T2) of the above (T2 / T1) to (T3 / T2) is preferably 1 or more. Furthermore, the ratio (T3 / T1) of the third film thickness (T3) to the first film thickness (T1) is preferably 0.80 or less, more preferably 0.75 or less. Meanwhile, T3 / T1 is, for example, 0.50 or more.

[0060] The film thickness reduction rate (R2) during the drying step is preferably 0.8 μm / min or more, more preferably 1.2 μm / min or more, and even more preferably 1.6 μm / min or more, while R2 is preferably 3.0 μm / min or less.

[0061] Drying can be performed by any appropriate method. For example, the drying zone 120 may be entirely heated (zone heating method), or the transport rolls in the drying zone 120 may be heated (heat roll method). The heat roll method is preferably employed, and more preferably, both methods are employed. By using a heat roll, heat curling of the laminate can be efficiently suppressed, resulting in the production of a high-quality polarizing film. Specifically, drying the laminate while it is aligned with the heat 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, enabling it to withstand shrinkage of the resin film due to drying, thereby suppressing curling. Furthermore, by using a heat roll, the laminate can be dried while being maintained flat, thereby suppressing not only curling but also wrinkling.

[0062] Drying can shrink the laminate in the width direction, improving its optical properties. For example, 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 7%. By using a heated roll, the laminate can be continuously shrunk in the width direction while being transported, achieving high productivity.

[0063] 4 is a schematic diagram showing an example of drying using heated rolls in a drying zone. In the illustrated example, the laminate 1 is dried while being transported by transport rolls R1 to R6 heated to a predetermined temperature and guide rolls G1 to G4. In the illustrated example, the transport rolls are arranged so as to alternately and continuously heat the resin film surface and the thermoplastic resin substrate surface of the laminate 1, but the transport rolls may also be arranged so as to continuously heat only one surface of the laminate (for example, the thermoplastic resin substrate surface).

[0064] In one embodiment, 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 excellent durability to the laminate. Furthermore, the thickness of the resin film can be favorably achieved. The temperature of the heating rolls can be measured with a contact thermometer. In the illustrated example, six transport rolls are provided, but the number of transport rolls is not particularly limited as long as there is more than one. The number of transport rolls is usually 2 to 40, and preferably 4 to 30. The contact time between the laminate and the heating rolls (total contact time) is preferably 1 to 300 seconds, more preferably 1 to 20 seconds, and even more preferably 1 to 10 seconds.

[0065] The drying zone 120, which may be provided with heating rolls, is preferably heated. For example, the drying zone 120 is a space inside a heating furnace (e.g., an oven). This configuration can suppress abrupt temperature changes between the heating rolls, making it easy to control shrinkage in the width direction. The temperature in the drying zone 120 is preferably 60°C or higher, more preferably 70°C or higher, even more preferably 80°C or higher, and particularly preferably 85°C or higher. On the other hand, from the viewpoint of suppressing the occurrence of wrinkles, the temperature in the drying zone 120 is preferably 105°C or lower, more preferably 95°C or lower. The humidity in the drying zone 120 is preferably 10% RH or lower, more preferably 5% RH or lower. On the other hand, the humidity in the drying zone 120 is, for example, 1% RH or higher. It is preferable that the heating furnace be in a state of blowing air. In this case, the wind speed of the hot air is, for example, about 10 m / s to 30 m / s. The wind speed in the heating furnace can be measured using a mini-vane type digital anemometer.

[0066] The temperature in the drying zone 120 is preferably higher than the temperature in the conditioning zone 110. The difference between the temperature in the drying zone 120 and the temperature in the conditioning zone 110 (the difference between the temperature at which drying is performed and the temperature at which conditioning is performed) is preferably 25°C or higher and 70°C or lower, more preferably 40°C or higher, even more preferably 50°C or higher, and particularly preferably 55°C or higher. The humidity in the conditioning zone 110 is preferably higher than the humidity in the drying zone 120. The difference between the humidity in the conditioning zone 110 and the humidity in the drying zone 120 (the difference between the humidity at which conditioning is performed and the humidity at which drying is performed) is preferably 30% RH or higher and 70% RH or lower, more preferably 35% RH or higher.

[0067] The time required to pass through the drying zone 120 is, for example, 5 seconds to 4 minutes. The time required to pass through the drying zone 120 corresponds, for example, to the time required for the resin film to be placed under a predetermined environmental condition. In the drying zone 120, the temperature and humidity do not need to be constant, but are preferably maintained within the above-mentioned temperature and humidity ranges.

[0068] By drying the resin film that has undergone the adjustment step, a polarizing film that combines high optical properties and excellent appearance can be obtained. The present inventors have found that, for example, by controlling the film thickness of the resin film before drying, it is possible to achieve both optical properties and appearance, which are thought to be in a trade-off relationship.

[0069] C. Polarizing film The polarizing film obtained according to an embodiment of the present invention is composed of a PVA-based resin film containing a dichroic material such as iodine. The thickness of the polarizing film is, for example, 10 μm or less, preferably 8 μm or less, more preferably 7 μm or less, and even more preferably 6 μm or less. According to an embodiment of the present invention, a polarizing film having such a thickness can achieve both high optical properties and excellent appearance. Meanwhile, the thickness of the polarizing film is preferably 1 μm or more, more preferably 2 μm or more.

[0070] The polarizing film preferably exhibits absorption dichroism at any wavelength between 380 nm and 780 nm. The single transmittance (Ts) of the polarizing film is preferably 41.0% or more, more preferably 42.0% or more, and even more preferably 42.5% or more. On the other hand, the single transmittance of the polarizing film is, for example, 44.2% or less. The polarization degree (P) of the polarizing film is preferably 99.95% or more, more preferably 99.98% or more, and even more preferably 99.99% or more. On the other hand, the polarization degree of the polarizing film is, for example, 99.996% or less.

[0071] The single transmittance is typically a Y value measured using an ultraviolet-visible spectrophotometer and corrected for visibility. The polarization degree is typically calculated using the following formula based on the parallel transmittance Tp and crossed transmittance Tc measured using an ultraviolet-visible spectrophotometer and corrected for visibility. Degree of polarization (%)={(Tp-Tc) / (Tp+Tc)} 1 / 2 ×100

[0072] The boric acid content of the polarizing film is preferably 25% or less, more preferably 20% or less. By having such a boric acid content, better optical properties can be achieved. Even with such a boric acid content, an excellent appearance can be achieved by adjusting the film thickness. The boric acid content of the polarizing film is preferably 10% or more, more preferably 13% or more, and even more preferably 16% or more. Such a boric acid content can result in an even more excellent appearance. The boric acid content of the polarizing film can be adjusted, for example, by adjusting the boric acid concentration during the water stretching.

[0073] D. Polarizing plate A polarizing plate according to one embodiment of the present invention has the above polarizing film and a protective layer or a retardation layer disposed on at least one side of the polarizing film.

[0074] 5 is a schematic cross-sectional view showing the general configuration of a polarizing plate according to one embodiment of the present invention. Polarizing plate 100 includes a polarizing film 10 having a first main surface 10a and a second main surface 10b facing each other, a protective layer 20 disposed on the first main surface 10a side of polarizing film 10, and a retardation layer 30 and a pressure-sensitive adhesive layer 40 disposed on the second main surface 10b side of polarizing film 10. In this embodiment, retardation layer 30 can function as a protective layer for polarizing film 10.

[0075] The protective layer 20 can be formed of any appropriate film that can be used as a protective layer for a polarizing film. Specific examples of materials that can be the main component of such films include cellulose-based resins such as triacetyl cellulose (TAC), and transparent resins such as polyesters, polyvinyl alcohols, polycarbonates, polyamides, polyimides, polyethersulfones, polysulfones, polystyrenes, and cycloolefins such as polynorbornenes, polyolefins, (meth)acrylics, and acetates. The above-mentioned resin substrates may also be used as protective layers for polarizing films.

[0076] The polarizing plate 100 is typically placed on the viewing side of an image display device. Therefore, the protective layer 20 may be subjected to surface treatments such as hard coat (HC) treatment, anti-reflection treatment, anti-sticking treatment, and anti-glare treatment, as needed. The thickness of the protective layer 20 is preferably 5 μm to 80 μm, more preferably 10 μm to 40 μm, and even more preferably 10 μm to 30 μm. When a surface treatment is applied, the thickness of the protective layer 20 includes the thickness of the surface treatment layer.

[0077] As the retardation layer 30, any appropriate configuration can be adopted. In one embodiment, an alignment and curing layer of a liquid crystal compound (liquid crystal alignment and curing layer) is used for the retardation layer 30. By using a liquid crystal compound, the difference between nx and ny of the obtained retardation layer can be made significantly larger than that of a non-liquid crystal material, so that the thickness of the retardation layer for obtaining a desired in-plane retardation can be significantly reduced. In this specification, the "alignment and curing layer" refers to a layer in which a liquid crystal compound is aligned in a predetermined direction within the layer and the alignment state is fixed. Note that the "alignment and curing layer" is a concept that includes an alignment and curing layer obtained by curing a liquid crystal monomer.

[0078] Typically, the retardation layer 30 includes a layer having a refractive index characteristic showing a relationship of nx > ny = nz. Note that "ny = nz" includes not only the case where ny and nz are exactly equal but also the case where they are substantially equal. Therefore, within a range that does not impair the effects of the present invention, ny > nz or ny < nz may occur. The Nz coefficient of the retardation layer is preferably 0.9 to 1.5, more preferably 0.9 to 1.3.

[0079] As the adhesive layer 40, any appropriate configuration can be adopted. Specific examples include acrylic adhesives, rubber adhesives, silicone adhesives, polyester adhesives, urethane adhesives, epoxy adhesives, and polyether adhesives. By adjusting the type, number, combination, and blending ratio of the monomers forming the base resin of the adhesive, as well as the blending amount of the crosslinking agent, reaction temperature, reaction time, etc., an adhesive having desired characteristics according to the purpose can be prepared. The base resin of the adhesive may be used alone or in combination of two or more. The base resin is preferably an acrylic resin (specifically, the adhesive layer is preferably composed of an acrylic adhesive). The thickness of the adhesive layer is, for example, 10 μm to 20 μm.

[0080] Each member constituting the polarizing plate may be laminated via any appropriate adhesive layer (not shown). Specific examples of the adhesive layer include an adhesive layer and a pressure-sensitive adhesive layer. Specifically, the retardation layer 30 may be attached to the polarizing film 10 via an adhesive layer (preferably using an active energy ray-curable adhesive) or a pressure-sensitive adhesive layer.

[0081] Although not shown, in practice, a release liner is attached to the surface of the pressure-sensitive adhesive layer 40. The release liner can be temporarily attached until the polarizing plate is ready for use. By using a release liner, for example, the pressure-sensitive adhesive layer is protected and the polarizing plate can be formed into a roll.

[0082] The polarizing plate may be in a long shape or in a sheet shape. In this specification, "long" refers to an elongated shape in which the length is sufficiently longer than the width, for example, an elongated shape in which the length is 10 times or more, preferably 20 times or more, the width. A long polarizing plate can be wound into a roll. [Example]

[0083] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified, the thicknesses are values ​​measured by the following measurement method. (Thickness) Thicknesses of 10 μm or less were measured using a scanning electron microscope (manufactured by JEOL Ltd., product name "JSM-7100F"), and thicknesses of more than 10 μm were measured using a digital micrometer (manufactured by Anritsu Corporation, product name "KC-351C").

[0084] [Example 1] (Resin film production) The thermoplastic resin substrate was a long, amorphous isophthalic copolymerized polyethylene terephthalate film (thickness: 100 μm) with a water absorption rate of 0.75% and a Tg of approximately 75° C. One side of the resin substrate was subjected to a corona treatment. A PVA aqueous solution (coating solution) was prepared by adding 13 parts by weight of potassium iodide to 100 parts by weight of a PVA-based resin prepared by mixing polyvinyl alcohol (polymerization degree 4200, saponification degree 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFFIMER Z410") in a weight ratio of 9:1. 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 uniaxially stretched at its free end to 3.0 times its original size in the machine direction (longitudinal direction) between rolls with different peripheral speeds in an oven at 130°C (auxiliary in-air stretching). Next, the laminate 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 (iodine aqueous 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 polarizing film would be 42.5% or higher (dyeing). 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 an aqueous boric acid solution (boric acid concentration 4 wt %, potassium iodide concentration 5 wt %) at a liquid temperature of 70°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). Thereafter, the laminate was immersed in a cleaning bath (aqueous solution obtained by mixing 4 parts by weight of potassium iodide with 100 parts by weight of water) at a liquid temperature of 20°C (cleaning). In this way, a resin film was obtained on the resin substrate.

[0085] (Adjusting film thickness) The laminate was then placed in an environment of 23.4°C and 47% RH (conditioning zone) for 20 seconds to reduce the thickness of the resin film.

[0086] (Dry) The laminate was then placed in an oven (drying zone) maintained at 90°C and 2% RH for 1 minute, during which it was brought into contact with a SUS heated roll placed in the oven and maintained at a surface temperature of 75°C for approximately 2 seconds. In this way, a polarizing film having a thickness of 5 μm was obtained on the resin substrate. The shrinkage rate of the laminate in the width direction due to drying was 6.1%.

[0087] The temperature and humidity inside the oven (drying zone) and the temperature and humidity when adjusting the film thickness before entering the oven (adjustment zone) were measured using a temperature and humidity data logger (manufactured by Testo, product name "175 H1").

[0088] (Preparation of polarizing plate) An HC-TAC film (32 μm thick) was attached to the polarizing film side of the laminate of the resin substrate and polarizing film using a UV-curable adhesive, and then the resin substrate was peeled off from the polarizing film to obtain a polarizing plate. The HC-TAC film was a TAC film (25 μm thick) with a hard coat (HC) layer (7 μm thick) formed on it, and was attached so that the TAC film faced the polarizing film side.

[0089] [Example 2] A polarizing film and a polarizing plate were obtained in the same manner as in Example 1. The temperature of the adjustment zone was 24.1°C and the humidity was 45% RH.

[0090] [Example 3] A polarizing film and a polarizing plate were obtained in the same manner as in Example 1, except that the thickness of the PVA-based resin layer contained in the laminate was 15 μm and the laminate was immersed in a boric acid aqueous solution at a liquid temperature of 64° C. and stretched in water. The temperature and humidity of the adjustment zone were 22.8° C. and 47% RH, respectively.

[0091] [Example 4] A polarizing film and a polarizing plate were obtained in the same manner as in Example 1, except that the laminate was immersed in an aqueous boric acid solution at a liquid temperature of 64°C and stretched in water. The temperature in the adjustment zone was 23.1°C and the humidity was 44% RH.

[0092] [Comparative Example 1] A polarizing film and a polarizing plate were obtained in the same manner as in Example 1, except that the laminate was placed in an environment of 37.7° C. and 23% RH for 20 seconds when adjusting the film thickness.

[0093] Comparative Example 2 A polarizing film and a polarizing plate were obtained in the same manner as in Comparative Example 1. The temperature of the adjustment zone was 37.5°C and the humidity was 25% RH.

[0094] The following evaluations were carried out for the Examples and Comparative Examples. The evaluation results are summarized in Table 2. <Evaluation> 1. Resin film thickness The thickness of the resin film was measured (in-line measurement) using a spectral interference film thickness meter (Ocean Insight, spectrometer "USB2000+", light source "HL-2000", fiber "OCF-103995"). Measurements were taken at the entrance of the conditioning zone, the exit of the conditioning zone (entrance of the drying zone), and the exit of the drying zone, and film thicknesses T1, T2, and T3 were determined. When measuring the film thickness of the resin film formed on the resin substrate, the film thickness meter was placed on the resin substrate side. 2. Boric acid content The spectrum of the polarizing film was measured using a Fourier transform infrared spectrometer (manufactured by PerkinElmer, model "Frontier FT-IR"), and the content of boric acid in the polarizing film was calculated from the spectrum obtained. Specifically, the 2940 cm -1 and 665 cm originating from boric acid esters -1 The measurement samples were collected at the exit of the drying zone. 3. Single transmittance and polarization degree The polarizing plates of the examples and comparative examples were measured using an ultraviolet-visible spectrophotometer (V-7100, manufactured by JASCO Corporation) to determine the single transmittance Ts, parallel transmittance Tp, and crossed transmittance Tc, respectively, which were designated as Ts, Tp, and Tc of the polarizing film. These Ts, Tp, and Tc are Y values ​​measured using a 2-degree visual field (C light source) according to JIS Z8701 and corrected for visibility. From the obtained Tp and Tc, the degree of polarization P was calculated using the following formula. Polarization degree P(%)={(Tp-Tc) / (Tp+Tc)} 1 / 2 ×100 4. Appearance The appearance (presence or absence of streaks) of the polarizing plates of the examples and comparative examples was visually observed. (Evaluation criteria) Good: No visible streaks Poor: Streaks are visible

[0095] [Table 2]

[0096] In the polarizing plate of Comparative Example 1, streaky marks (along the stretching direction of the polarizing film) as shown in FIG. 6 were observed. [Industrial Applicability]

[0097] The polarizing film according to the embodiment of the present invention is suitably used in image display devices such as liquid crystal display devices, organic EL display devices, and inorganic EL display devices. [Explanation of symbols]

[0098] 1. Laminate 2 Thermoplastic resin base material 3 Resin layer (resin film) 10 Polarizing film 20 protective layer 100 Polarizer

Claims

1. a step of obtaining a resin film having a first thickness (T1) through a treatment using water; an adjusting step of reducing the thickness of the resin film from the first thickness (T1) to a second thickness (T2); a drying step of drying the resin film having the second film thickness (T2), the resin film is made of a polyvinyl alcohol-based resin, a ratio (T2 / T1) of the second thickness (T2) to the first thickness (T1) is less than 1; the thickness of the resin film is reduced from the second thickness (T2) to a third thickness (T3) by the drying, and a ratio (T3 / T2) of the third thickness (T3) to the second thickness (T2) is 0.90 or less; The resin film is subjected to the conditioning in an environment where the temperature is less than 40°C and the humidity is 35% RH or more. A method for manufacturing a polarizing film.

2. A manufacturing method as described in claim 1, wherein the temperature at which the drying is performed is higher than the temperature at which the adjustment is performed, and the difference between the temperature at which the drying is performed and the temperature at which the adjustment is performed is 25°C or more.

3. A manufacturing method described in claim 1 or 2, wherein the humidity at which the adjustment is performed is higher than the humidity at which the drying is performed, and the difference between the humidity at which the adjustment is performed and the humidity at which the drying is performed is 30% RH or more.

4. The method according to claim 1 , wherein the resin film is dried in an environment at a temperature of 60° C. or higher and a humidity of 10% RH or lower.

5. 5. The manufacturing method according to claim 1, wherein the first film thickness (T1), the second film thickness (T2), and the third film thickness (T3) satisfy the relationship (T2 / T1) / (T3 / T2)≧1.

6. The method according to claim 1 , wherein the first thickness (T1) is 5 μm or more.

7. The method according to claim 1 , wherein a polarizing film having a thickness of 7 μm or less is obtained.

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