Polarizing film, polarizing plate, and method for manufacturing the polarizing film

Abstract

Provided is a polarizing film in which breakage in the direction of the absorption axis is suppressed. The polarizing film of the present invention is composed of a polyvinyl alcohol-based resin film containing a dichroic substance, and the orientation function is 0.30 or less. In one embodiment, the thickness of the polarizing film is 8 μm or less. The polarizing plate of the present invention has the above-mentioned polarizing film, and a protective layer disposed on at least one side of the polarizing film.

Description

[0001] This application is a divisional application of the application filed on February 6, 2020, with application number 202080012812.1 and entitled "Polarizing film, polarizing plate, and method for manufacturing the polarizing film". Technical Field

[0002] This invention relates to a polarizing film, a polarizing plate, and a method for manufacturing the polarizing film. Background Technology

[0003] In liquid crystal display devices, which are representative image display devices, polarizing films are disposed on both sides of the liquid crystal cells due to their image formation method. As a method for manufacturing the polarizing film, for example, the following method has been proposed: stretching a laminate having a resin substrate and a polyvinyl alcohol (PVA)-based resin layer, followed by a dyeing process, thereby obtaining a polarizing film on the resin substrate (e.g., Patent Document 1). Using this method, a thin polarizing film can be obtained, and therefore it has attracted attention as a method that contributes to the thinning of image display devices in recent years. However, as described above, the thin polarizing film is prone to cracking (fracture) along the absorption axis direction.

[0004] Existing technical documents

[0005] Patent documents

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

[0007] The problem the invention aims to solve

[0008] The present invention was made to solve the above-mentioned existing problems, and its main objective is to provide a polarizing film that suppresses breakage along the absorption axis.

[0009] Solution for solving the problem

[0010] The polarizing film of the present invention is composed of a polyvinyl alcohol-based resin film containing dichroic substances, and the orientation function is 0.30 or less.

[0011] In one embodiment, the thickness of the polarizing film is 8 μm or less.

[0012] In one embodiment, the polarizing film has a monomer transmittance of 40.0% or more and a polarization of 99.0% or more.

[0013] In one embodiment, the puncture strength of the polarizing film is 30 gf / μm or higher.

[0014] In another embodiment of the present invention, the polarizing film is composed of a polyvinyl alcohol-based resin film containing dichroic substances, and has a puncture strength of 30 gf / μm or higher.

[0015] According to another aspect of the present invention, a polarizing plate is provided. The polarizing plate has: the polarizing film described above, and a protective layer disposed on at least one side of the polarizing film.

[0016] According to another aspect of the present invention, a method for manufacturing the above-described polarizing film is provided. The method includes: forming a polyvinyl alcohol (PVA) resin layer containing iodide or sodium chloride and a PVA resin on one side of a strip-shaped thermoplastic resin substrate to form a laminate; and sequentially subjecting the laminate to an air-assisted stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment, wherein the drying shrinkage treatment involves heating the laminate while conveying it along its length direction, thereby causing a shrinkage of 2% or more in the width direction. The total stretching ratio of the air-assisted stretching treatment and the underwater stretching treatment is 3.0 to 4.5 times the original length of the laminate; the stretching ratio of the air-assisted stretching treatment is greater than the stretching ratio of the underwater stretching treatment.

[0017] The effects of the invention

[0018] According to the present invention, by setting the orientation function to 0.30 or less, or by setting the puncture intensity to 30 gf / μm or more, a polarizing film in which breakage along the absorption axis is suppressed can be achieved. Conventionally, polarizing films with such small orientation functions have struggled to achieve acceptable optical properties (typically, monomer transmittance and polarization). However, according to the present invention, both such a small orientation function and acceptable optical properties can be achieved. Furthermore, according to the present invention, both such a large puncture intensity and acceptable optical properties can be achieved. Attached Figure Description

[0019] Figure 1 This is a cross-sectional schematic diagram of a polarizing plate according to one embodiment of the present invention.

[0020] Figure 2 This is a schematic diagram illustrating an example of a drying shrinkage process using heated rollers. Detailed Implementation

[0021] The embodiments of the present invention will be described below, but the present invention is not limited to these embodiments.

[0022] A.Polarizing film

[0023] The polarizing film of the present invention is composed of a polyvinyl alcohol (PVA) resin film containing a dichroic substance (represented by iodine and dichroic dyes), and has an orientation function of 0.30 or less. With this configuration, cracking (damage) of the polarizing film along the absorption axis can be significantly suppressed. As a result, a polarizing film (resulting in a polarizing plate) with excellent flexibility can be obtained. Such a polarizing film (resulting in a polarizing plate) can be applied to preferably flexible image display devices, more preferably bendable image display devices, and even more preferably foldable image display devices. The orientation function is, for example, 0.25 or less, preferably 0.22 or less, more preferably 0.20 or less, even more preferably 0.18 or less, and particularly preferably 0.15 or less. The lower limit of the orientation function can be, for example, 0.05. If the orientation function is too small, acceptable monomer transmittance and / or polarization may sometimes not be obtained.

[0024] For the orientation function (f), for example using a Fourier transform infrared spectrometer (FT-IR), polarized light is used as the measurement light, and the value is determined by attenuated total reflection (ATR). Specifically, the measurement is performed with the stretching direction of the polarizing film parallel and perpendicular to the polarization direction of the measurement light, and the absorbance spectrum at 2941 cm⁻¹ is used. -1 The strength is calculated using the following formula. Here, strength I is defined as 3330 cm³. -1 The reference peak is 2941 cm. -1 / 3330cm -1 The value of f. It should be noted that f=1 indicates complete orientation, and f=0 indicates random orientation. Additionally, 2941cm is considered... -1 The peak is due to the absorption caused by the vibration of the PVA backbone (-CH2-) in the polarizing film.

[0025] f = (3 <cos 2 θ>-1) / 2

[0026] = (1-D) / [c(2D+1)]

[0027] = -2×(1-D) / (2D+1)

[0028] in,

[0029] c=(3cos 2 β-1) / 2, 2941cm -1 Under the condition of vibration, β=90°.

[0030] θ: Angle of the molecular chain relative to the stretching direction

[0031] β: Angle of transition dipole moment relative to the molecular chain axis

[0032] D = (I ⊥ ) / (I / / (In this case, the more oriented the PVA molecule, the larger D becomes.)

[0033] I ⊥ : Measure the absorption intensity when the polarization direction of the light is perpendicular to the stretching direction of the polarizing film.

[0034] I / / : Measure the absorption intensity when the polarization direction of the light is parallel to the stretching direction of the polarizing film.

[0035] The thickness of the polarizing film is preferably 8 μm or less, more preferably 7 μm or less, further preferably 5 μm or less, particularly preferably 3 μm or less, and especially preferably 2 μm or less. The lower limit of the polarizing film thickness can be, for example, 1 μm. In one embodiment, the thickness of the polarizing film can be 2 μm to 6 μm; in another embodiment, it can be 2 μm to 4 μm; in yet another embodiment, it can be 2 μm to 3 μm; in yet another embodiment, it can be 5.5 μm to 7.5 μm; and in yet another embodiment, it can be 6 μm to 7.2 μm. By making the polarizing film very thin, thermal shrinkage can be minimized. It is speculated that this configuration also helps to suppress breakage in the absorption axis direction.

[0036] The polarizing film preferably exhibits absorption dichroism at any wavelength from 380 nm to 780 nm. The single-cell transmittance of the polarizing film is preferably 40.0% or more, more preferably 41.0% or more. The upper limit of single-cell transmittance can be, for example, 49.0%. In one embodiment, the single-cell transmittance of the polarizing film is 40.0% to 45.0%. The polarization degree of the polarizing film is preferably 99.0% or more, more preferably 99.4% or more. The upper limit of polarization degree can be, for example, 99.999%. In one embodiment, the polarization degree of the polarizing film is 99.0% to 99.99%. According to the present invention, although the orientation function is very small as described above, such practically acceptable single-cell transmittance and polarization degree can be achieved. This is presumably due to the manufacturing method described later. It should be noted that the single-cell transmittance is representative of the Y value obtained by measuring using a UV-Vis spectrophotometer and performing visual sensitivity correction. The polarization is representative of the parallel transmittance Tp and orthogonal transmittance Tc, which are obtained by measuring with a UV-Vis spectrophotometer and correcting for visual sensitivity. They are calculated using the following formula.

[0037] Polarization (%) = {(Tp-Tc) / (Tp+Tc)} 1 / 2 ×100

[0038] The puncture strength of the polarizing film is 30 gf / μm or more, preferably 35 gf / μm or more, more preferably 40 gf / μm or more, further preferably 45 gf / μm or more, and particularly preferably 50 gf / μm or more. The upper limit of the puncture strength can be, for example, 80 gf / μm. By setting the puncture strength of the polarizing film within such a range, cracking of the polarizing film along the absorption axis can be significantly suppressed. As a result, a polarizing film with excellent flexibility (resulting in a polarizing plate) can be obtained. Puncture strength represents the resistance of the polarizing film to rupture when punctured with a specified strength. Puncture strength can be expressed, for example, as the strength (fracture strength) at which the polarizing film ruptures when a specified needle is mounted on a compression testing machine and punctured by the needle at a specified speed. It should be noted that, based on the unit, puncture strength refers to the puncture strength per unit thickness (1 μm) of the polarizing film.

[0039] The polarizing film, as described above, is composed of a PVA-based resin film containing iodine. Preferably, the PVA-based resin constituting the PVA-based resin film (essentially a polarizing film) comprises an acetyl-modified PVA-based resin. With this configuration, a polarizing film having the desired puncture strength can be obtained. When the total PVA-based resin is set to 100% by weight, the blending amount of the acetyl-modified PVA-based resin is preferably 5% to 20% by weight, more preferably 8% to 12% by weight. When the blending amount is in this range, the puncture strength can be set to a more suitable range.

[0040] A typical polarizing film can be manufactured using a laminate of two or more layers. Specific examples of polarizing films obtained using a laminate include those using a resin substrate and a PVA-based resin layer coated on the resin substrate. A polarizing film obtained using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate can be manufactured, for example, as follows: a PVA-based resin solution is coated onto a resin substrate and dried to form a PVA-based resin layer on the resin substrate, resulting in a laminate of the resin substrate and the PVA-based resin layer; the laminate is then stretched and dyed to form a polarizing film from the PVA-based resin layer. In this embodiment, it is preferable to form a polyvinyl alcohol (PVA) resin layer comprising a halide and a polyvinyl alcohol-based resin on one side of the resin substrate. Stretching typically includes immersing the laminate in an aqueous boric acid solution and then stretching it. Furthermore, stretching preferably also includes air stretching the laminate at a high temperature (e.g., 95°C or higher) before stretching in the aqueous boric acid solution. In embodiments of the present invention, the total stretching ratio is, for example, 3.0 to 4.5 times, which is significantly smaller than usual. Even with such a total stretching ratio, a polarizing film with acceptable optical properties can be obtained through the combination of halogen addition and drying shrinkage treatment. Furthermore, in embodiments of the present invention, the stretching ratio of air-assisted stretching is greater than that of stretching in boric acid water. By adopting such a configuration, a polarizing film with acceptable optical properties can be obtained even with a small total stretching ratio. Furthermore, the laminate is preferably subjected to a drying shrinkage treatment by heating while conveying the laminate along its length direction, thereby shrinking it by 2% or more in the width direction. In one embodiment, the method for manufacturing the polarizing film includes: sequentially performing air-assisted stretching treatment, dyeing treatment, water stretching treatment, and drying shrinkage treatment on the laminate. By introducing assisted stretching, even when PVA is coated on a thermoplastic resin, the crystallinity of PVA can be improved, and high optical properties can be achieved. Furthermore, by simultaneously improving the orientation of PVA beforehand, problems such as reduced orientation and dissolution of PVA can be prevented during subsequent dyeing and stretching processes when immersed in water, thus achieving high optical properties. Moreover, when the PVA-based resin layer is immersed in liquid, compared to when the PVA-based resin layer does not contain halogens, the disorder of polyvinyl alcohol molecule orientation and the reduction of orientation can be suppressed. Therefore, the optical properties of the polarizing film obtained by immersing the laminate in liquid through dyeing and underwater stretching processes can be improved. Furthermore, by using a drying shrinkage treatment to shrink the laminate in the width direction, optical properties can be improved. The resulting resin substrate / polarizing film laminate can be used directly (i.e., the resin substrate can be used as a protective layer for the polarizing film), or the resin substrate can be peeled off from the resin substrate / polarizing film laminate and any suitable protective layer for the purpose can be laminated on the peeled surface for use. Details regarding the manufacturing method of the polarizing film are described later in section C.

[0041] B.Polarizing plate

[0042] Figure 1 This is a cross-sectional schematic diagram of a polarizing plate according to one embodiment of the present invention. The polarizing plate 100 includes: a polarizing film 10, a first protective layer 20 disposed on one side of the polarizing film 10, and a second protective layer 30 disposed on the other side of the polarizing film 10. The polarizing film 10 is the polarizing film of the present invention described in paragraph A above. One of the first protective layer 20 and the second protective layer 30 may be omitted. It should be noted that, as described above, one of the first protective layer and the second protective layer can be a resin substrate used in the manufacture of the polarizing film.

[0043] The first and second protective layers are formed from any suitable thin film that can be used as a protective layer for a polarizing film. Specific examples of materials that are the main components of the thin film include cellulose resins such as cellulose triacetate (TAC), polyester resins, polyvinyl alcohol resins, polycarbonate resins, polyamide resins, polyimide resins, polyethersulfone resins, polysulfone resins, polystyrene resins, polynorbornene resins, polyolefin resins, (meth)acrylic acid resins, acetate resins, and other transparent resins. Additionally, thermosetting resins or UV-curing resins such as (meth)acrylic acid resins, urethane resins, (meth)acrylate urethane resins, epoxy resins, and silicone resins can also be used. Furthermore, glassy polymers such as siloxane polymers can also be used. Additionally, polymer films described in Japanese Patent Application Publication No. 2001-343529 (WO01 / 37007) can also be used. As a material for this film, for example, a resin composition containing a thermoplastic resin having substituted or unsubstituted imide groups on the side chains and a thermoplastic resin having substituted or unsubstituted phenyl and nitrile groups on the side chains can be used. Examples include resin compositions having alternating copolymers formed from isobutylene and N-methylmaleimide and acrylonitrile-styrene copolymers. The polymer film can be, for example, an extruded product of the above-mentioned resin composition.

[0044] When the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (outer protective layer) disposed on the side opposite to the display panel is typically 300 μm or less, preferably 100 μm or less, more preferably 5 μm to 80 μm, and even more preferably 10 μm to 60 μm. It should be noted that when surface treatment is performed, the thickness of the outer protective layer includes the thickness of the surface treatment layer.

[0045] When the polarizer 100 is applied to an image display device, the thickness of the protective layer (inner protective layer) disposed on the display panel side is preferably 5 μm to 200 μm, more preferably 10 μm to 100 μm, and even more preferably 10 μm to 60 μm. In one embodiment, the inner protective layer is a phase difference layer having any suitable phase difference value. In this case, the in-plane phase difference Re(550) of the phase difference layer is, for example, 110 nm to 150 nm. “Re(550)” is the in-plane phase difference measured at 23°C with light of wavelength 550 nm, and is obtained by the formula: Re = (nx - ny) × d. Here, “nx” is the refractive index in the direction of maximum in-plane refractive index (i.e., the slow axis direction), “ny” is the refractive index in the direction orthogonal to the slow axis in-plane (i.e., the fast axis direction), “nz” is the refractive index in the thickness direction, and “d” is the thickness (nm) of the layer (thin film).

[0046] C. Manufacturing method of polarizing film

[0047] A method for manufacturing a polarizing film according to one embodiment of the present invention includes: forming a polyvinyl alcohol (PVA) resin layer comprising a halide and a polyvinyl alcohol (PVA) resin on one side of a strip-shaped thermoplastic resin substrate to form a laminate; and sequentially subjecting the laminate to an air-assisted stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment, wherein the drying shrinkage treatment involves heating the laminate while conveying it along its length direction, thereby causing a shrinkage of 2% or more in the width direction. The halide content in the PVA resin layer is preferably 5 to 20 parts by weight relative to 100 parts by weight of the PVA resin. The drying shrinkage treatment is preferably performed using a heated roller, and the temperature of the heated roller is preferably 60°C to 120°C. The shrinkage rate in the width direction of the laminate caused by the drying shrinkage treatment is preferably 2% or more. According to this manufacturing method, the polarizing film described in item A above can be obtained. In particular, by fabricating a laminate containing a PVA-based resin layer containing halogens, stretching the laminate in a multi-stage process including air-assisted stretching and underwater stretching, and heating the stretched laminate with a heating roller, a polarizing film with excellent optical properties (represented by monomer transmittance and unit absorbance) can be obtained.

[0048] C-1. Fabrication of Layered Structures

[0049] Any suitable method can be used as a method for manufacturing a laminate of a thermoplastic resin substrate and a PVA-based resin layer. Preferably, a coating liquid containing a halide and a PVA-based resin is applied to the surface of the thermoplastic resin substrate and then dried, thereby forming a PVA-based resin layer on the thermoplastic resin substrate. As described above, the halide content in the PVA-based resin layer is preferably 5 to 20 parts by weight relative to 100 parts by weight of the PVA-based resin.

[0050] Any suitable method can be used as the coating method for the coating liquid. For example, roller coating, spin coating, wire rod coating, dip coating, mold coating, curtain coating, spray coating, knife coating (comma coating, etc.) can be listed. The coating and drying temperature of the above coating liquid is preferably 50°C or higher.

[0051] The thickness of the PVA-based resin layer is preferably 2μm to 30μm, and more preferably 2μm to 20μm. By making the thickness of the PVA-based resin layer before stretching very thin and reducing the total stretching ratio as described later, it is possible to obtain a polarizing film with acceptable monomer transmittance and polarization even with a very small orientation function.

[0052] Before forming the PVA-based resin layer, the thermoplastic resin substrate can be surface-treated (e.g., corona treatment), or an easy-to-adhere layer can be formed on the thermoplastic resin substrate. Such treatments can improve the adhesion between the thermoplastic resin substrate and the PVA-based resin layer.

[0053] C-1-1. Thermoplastic resin substrate

[0054] Any suitable thermoplastic resin film can be used as the thermoplastic resin substrate. Details regarding the thermoplastic resin substrate are described, for example, in Japanese Patent Application Publication No. 2012-73580. The entire contents of that publication are incorporated herein by reference.

[0055] C-1-2. Coating liquid

[0056] The coating solution, as described above, comprises a halide and a PVA-based resin. A representative example of the coating solution is a solution formed by dissolving the halide and the PVA-based resin in a solvent. Examples of solvents include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyols such as trimethylolpropane, ethylenediamine, and amines such as diethylenetriamine. These can be used alone or in combination of two or more. Water is preferred among these. The concentration of the PVA-based resin in the solution is preferably 3 to 20 parts by weight relative to 100 parts by weight of the solvent. At such a resin concentration, a uniform coating film that adheres closely to the thermoplastic resin substrate can be formed. The content of the halide in the coating solution is preferably 5 to 20 parts by weight relative to 100 parts by weight of the PVA-based resin.

[0057] Additives can be incorporated into the coating solution. Examples of additives include, for example, plasticizers and surfactants. Examples of plasticizers include, for example, polyols such as ethylene glycol and glycerin. Examples of surfactants include, for example, nonionic surfactants. They can be used to further improve the uniformity, dyeability, and tensile strength of the resulting PVA-based resin layer.

[0058] As the aforementioned PVA-based resin, any suitable resin can be used. Examples include polyvinyl alcohol and ethylene-vinyl alcohol copolymers. Polyvinyl alcohol is obtained by saponifying polyvinyl acetate. Ethylene-vinyl alcohol copolymers are obtained by saponifying ethylene-vinyl acetate copolymers. The degree of saponification of the PVA-based resin is typically 85 mol% to 100 mol%, preferably 95.0 mol% to 99.95 mol%, and more preferably 99.0 mol% to 99.93 mol%. The degree of saponification can be determined according to JIS K 6726-1994. By using a PVA-based resin with such a degree of saponification, a polarizing film with excellent durability can be obtained. If the saponification is too high, there is a concern about gelation. As mentioned above, the PVA-based resin preferably comprises a PVA-based resin modified with acetyl groups.

[0059] The average degree of polymerization of PVA-based resins can be appropriately selected according to the purpose. The average degree of polymerization is typically 1000–10000, preferably 1200–4500, and more preferably 1500–4300. It should be noted that the average degree of polymerization can be determined according to JIS K 6726-1994.

[0060] Any suitable halide can be used as the aforementioned halide. For example, iodides and sodium chloride can be listed. As iodides, potassium iodide, sodium iodide, and lithium iodide can be listed, for example. Among these, potassium iodide is preferred.

[0061] The amount of halide in the coating solution is preferably 5 to 20 parts by weight relative to 100 parts by weight of PVA-based resin, and more preferably 10 to 15 parts by weight relative to 100 parts by weight of PVA-based resin. If the amount of halide exceeds 20 parts by weight relative to 100 parts by weight of PVA-based resin, the halide may sometimes leach out, resulting in a cloudy polarizing film.

[0062] Typically, stretching a PVA-based resin layer increases the orientation of polyvinyl alcohol (PVA) molecules. However, immersing the stretched PVA-based resin layer in a water-containing liquid can sometimes lead to disordered PVA molecule orientation and decreased orientation. This is particularly true when stretching a laminate of thermoplastic resin and PVA-based resin layers in boric acid water. In order to stabilize the stretching of the thermoplastic resin, the laminate is stretched at a relatively high temperature in boric acid water, resulting in a significant tendency to decrease orientation. For example, the stretching of PVA film itself in boric acid water is usually performed at 60°C, while the stretching of a laminate of A-PET (thermoplastic resin substrate) and PVA-based resin layers is performed at a much higher temperature, around 70°C. In this case, the orientation of PVA decreases in the initial stage of stretching before it increases during the water stretching process. To address this, a laminate consisting of a PVA-based resin layer containing halogens and a thermoplastic resin substrate is fabricated. The laminate is then subjected to high-temperature stretching in air (assisted stretching) before being stretched in boric acid solution, thereby promoting the crystallization of the PVA-based resin in the PVA-based resin layer of the laminate after assisted stretching. As a result, when the PVA-based resin layer is immersed in liquid, compared to the case where the PVA-based resin layer does not contain halogens, the disordered orientation and reduced orientation of polyvinyl alcohol molecules can be suppressed. Therefore, the optical properties of the polarizing film obtained through processing steps such as dyeing and underwater stretching by immersing the laminate in liquid can be improved.

[0063] C-2. Aerial Assisted Stretching Treatment

[0064] In particular, to obtain high optical properties, a two-stage stretching method combining dry stretching (assisted stretching) and stretching in boric acid solution is preferred. By introducing assisted stretching as in two-stage stretching, stretching can be performed while suppressing the crystallization of the thermoplastic resin substrate. Furthermore, when coating a PVA-based resin onto a thermoplastic resin substrate, in order to suppress the influence of the glass transition temperature of the thermoplastic resin substrate, the coating temperature needs to be lowered compared to the usual case of coating PVA-based resin on a metal roller. As a result, the crystallization of the PVA-based resin is relatively lower, resulting in insufficient optical properties. To address this, by introducing assisted stretching, even when coating a PVA-based resin onto a thermoplastic resin, the crystallinity of the PVA-based resin can be improved, achieving high optical properties. In addition, by simultaneously improving the orientation of the PVA-based resin beforehand, problems such as the reduction of the orientation and dissolution of the PVA-based resin during immersion in water in subsequent dyeing and stretching processes can be prevented, achieving high optical properties.

[0065] The aerial stretching method can be fixed-end stretching (e.g., stretching using a tenter frame) or free-end stretching (e.g., unidirectional stretching by passing the laminate through rollers with different circumferential speeds). Free-end stretching is preferred for achieving high optical properties. In one embodiment, the aerial stretching process includes a heated roller stretching step, which stretches the laminate by utilizing the difference in circumferential speed between heated rollers while conveying it along its length. A typical aerial stretching process includes a zone stretching step and a heated roller stretching step. It should be noted that the order of the zone stretching step and the heated roller stretching step is not limited; the zone stretching step can be performed first, or vice versa. The zone stretching step can also be omitted. In one embodiment, the zone stretching step and the heated roller stretching step are performed sequentially. In another embodiment, in a tenter frame, stretching is performed by holding the film ends and widening the distance between the tenter frames along the flow direction (the widening of the distance between the tenter frames is the stretching ratio). At this point, the distance of the tenter frame in the width direction (perpendicular to the flow direction) is set in an arbitrarily close manner. Preferably, the stretch ratio in the flow direction can be set in a manner closer to free-end stretching. In the case of free-end stretching, the shrinkage rate in the width direction = (1 / stretch ratio). 1 / 2 To calculate.

[0066] Aerial assisted stretching can be performed in one stage or in multiple stages. In the case of multiple stages, the stretching ratio is the product of the stretching ratios of each stage. Preferably, the stretching direction in aerial assisted stretching is approximately the same as that in underwater stretching.

[0067] The stretching ratio in the air-assisted stretching is preferably 1.0 to 4.0 times, more preferably 1.5 to 3.5 times, and even more preferably 2.0 to 3.0 times. When the stretching ratio of the air-assisted stretching is in this range, when combined with the stretching in water, the total stretching ratio can be set to a desired range, and the desired orientation function can be achieved. As a result, a polarizing film in which breakage along the absorption axis is suppressed can be obtained. Furthermore, as mentioned above, the stretching ratio of the air-assisted stretching is greater than the stretching ratio of the stretching in boric acid water. By adopting such a configuration, a polarizing film with acceptable optical properties can be obtained even if the total stretching ratio is small.

[0068] The stretching temperature for air-assisted stretching can be set to any suitable value depending on the forming material of the thermoplastic resin substrate, the stretching method, etc. The stretching temperature is preferably above the glass transition temperature (Tg) of the thermoplastic resin substrate, more preferably above the glass transition temperature (Tg) of the thermoplastic resin substrate +10°C, and particularly preferably above Tg +15°C. On the other hand, the upper limit of the stretching temperature is preferably 170°C. By stretching at such a temperature, the rapid crystallization of the PVA-based resin can be suppressed, thereby suppressing undesirable conditions caused by such crystallization (e.g., hindering the orientation of the PVA-based resin layer caused by stretching).

[0069] C-3. Insoluble treatment, dyeing treatment and cross-linking treatment

[0070] As needed, an insoluble treatment is performed after the air-assisted stretching treatment and between the underwater stretching treatment and the dyeing treatment. A typical example of this insoluble treatment is immersing the PVA-based resin layer in an aqueous boric acid solution. A typical example of this dyeing treatment is dyeing the PVA-based resin layer using a dichroic substance (typically iodine). As needed, a crosslinking treatment is performed after the dyeing treatment and before the underwater stretching treatment. A typical example of this crosslinking treatment is immersing the PVA-based resin layer in an aqueous boric acid solution. Details regarding the insoluble treatment, dyeing treatment, and crosslinking treatment are, for example, described in Japanese Patent Application Publication No. 2012-73580 (as described above).

[0071] C-4. Underwater stretching treatment

[0072] The underwater stretching process involves immersing the laminate in a stretching bath. This process allows stretching to be performed at a temperature lower than the glass transition temperature (typically around 80°C) of the aforementioned thermoplastic resin substrate and PVA-based resin layer, enabling stretching while suppressing crystallization of the PVA-based resin layer. As a result, polarizing films with excellent optical properties can be manufactured.

[0073] The stretching method for the laminate can be any suitable method. Specifically, it can be fixed-end stretching or free-end stretching (e.g., a method of unidirectional stretching by passing the laminate through rollers with different circumferential speeds). Free-end stretching is preferred. The stretching of the laminate can be performed in one stage or in multiple stages. In the case of multiple stages, the total stretching ratio is the product of the stretching ratios of each stage.

[0074] Water stretching is preferably performed by immersing the laminate in an aqueous boric acid solution (boric acid water stretching). By using an aqueous boric acid solution as the stretching bath, the PVA-based resin layer can be endowed with rigidity that withstands the tension applied during stretching and water resistance that prevents it from dissolving in water. Specifically, boric acid in aqueous solution generates tetrahydroxyboronic acid anions, which crosslink with the PVA-based resin through hydrogen bonds. As a result, the PVA-based resin layer can be endowed with rigidity and water resistance, allowing for good stretching and enabling the manufacture of polarizing films with excellent optical properties.

[0075] The aforementioned aqueous boric acid solution is preferably obtained by dissolving boric acid and / or borate in water as a solvent. The boric acid concentration relative to 100 parts by weight of water is preferably 1 to 10 parts by weight, more preferably 2.5 to 6 parts by weight, and particularly preferably 3 to 5 parts by weight. By setting the boric acid concentration to 1 part by weight or more, the dissolution of the PVA-based resin layer can be effectively suppressed, resulting in a polarizing film with higher properties. It should be noted that aqueous solutions obtained by dissolving boron compounds such as borax, glyoxal, glutaraldehyde, etc., in a solvent other than boric acid or borate can also be used.

[0076] It is preferable to mix an iodide into the above-mentioned stretching bath (boric acid aqueous solution). By mixing in an iodide, the dissolution of iodine adsorbed on the PVA-based resin layer can be suppressed. Specific examples of iodides are as described above. The concentration of the iodide relative to 100 parts by weight of water is preferably 0.05 parts by weight to 15 parts by weight, more preferably 0.5 parts by weight to 8 parts by weight.

[0077] The stretching temperature (liquid temperature of the stretching bath) is preferably 40°C to 85°C, more preferably 60°C to 75°C. At such temperatures, stretching can be performed at a high ratio while suppressing the 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, considering its relationship with the formation of the PVA-based resin layer. In this case, if the stretching temperature is below 40°C, there is a concern that even considering the plasticization of the thermoplastic resin substrate by water, stretching may not be performed well. On the other hand, the higher the temperature of the stretching bath, the higher the solubility of the PVA-based resin layer, thus raising concerns about not obtaining excellent optical properties. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

[0078] The stretching ratio based on underwater stretching is preferably 1.0 to 3.0 times, more preferably 1.0 to 2.0 times, and even more preferably 1.0 to 1.5 times. When the stretching ratio in underwater stretching is within this range, the total stretching ratio can be set to the desired range, and the desired orientation function can be achieved. As a result, a polarizing film in which breakage along the absorption axis is suppressed can be obtained. The total stretching ratio (the sum of the stretching ratios when combining air-assisted stretching and underwater stretching) is, as described above, for example, 3.0 to 4.5 times the original length of the laminate, preferably 3.0 to 4.0 times, more preferably 3.0 to 3.5 times. By adding a halide to the coating liquid, adjusting the stretching ratios of air-assisted stretching and underwater stretching, and appropriately combining drying shrinkage treatment, even at such a total stretching ratio, a polarizing film with acceptable optical properties can be obtained.

[0079] C-5. Drying and shrinkage treatment

[0080] The aforementioned drying shrinkage treatment can be performed by regional heating, which involves heating the entire area, or by heating the conveyor rollers (using a so-called heated roller) (heated roller drying method). Both methods are preferred. By using a heated roller for drying, heat curling of the laminate can be effectively suppressed, resulting in a polarizing film with excellent appearance. Specifically, by drying the laminate along the heated roller, the crystallization of the thermoplastic resin substrate can be effectively promoted, thereby increasing the crystallinity. Even at lower drying temperatures, the crystallinity of the thermoplastic resin substrate can be significantly increased. As a result, the rigidity of the thermoplastic resin substrate increases, making it able to withstand the shrinkage of the PVA-based resin layer caused by drying, thus suppressing curling. Furthermore, by using a heated roller, the laminate can be dried while maintaining a flat state, thus suppressing not only curling but also wrinkle formation. At this time, the drying shrinkage treatment causes the laminate to shrink in the width direction, thereby improving optical properties. This is because the orientation of PVA and the PVA / iodine complex can be effectively improved. The shrinkage rate in the width direction of the laminate caused by the drying shrinkage treatment is preferably 1% to 10%, more preferably 2% to 8%, and particularly preferably 4% to 6%.

[0081] Figure 2 This is a schematic diagram illustrating an example of a drying shrinkage process. In the drying shrinkage process, the laminate 200 is dried while being conveyed by conveyor rollers R1-R6 heated to a predetermined temperature and guide rollers G1-G4. In the example shown, the conveyor rollers R1-R6 are configured to alternately and continuously heat the surface of the PVA resin layer and the surface of the thermoplastic resin substrate. However, for example, the conveyor rollers R1-R6 may also be configured to continuously heat only one surface of the laminate 200 (e.g., the thermoplastic resin substrate surface).

[0082] The drying conditions can be controlled by adjusting the heating temperature of the conveyor rollers (temperature of the heating rollers), the number of heating rollers, and the contact time with the heating rollers. The temperature of the heating rollers is preferably 60°C to 120°C, more preferably 65°C to 100°C, and particularly preferably 70°C to 80°C. This allows for the production of optical laminates that effectively increase the crystallinity of the thermoplastic resin, thereby effectively suppressing curling, and exhibiting excellent durability. It should be noted that the temperature of the heating rollers can be measured using a contact thermometer. The example shown has six conveyor rollers, but there are no particular limitations as long as there are multiple conveyor rollers. The number of conveyor rollers is typically set from 2 to 40, preferably 4 to 30. The contact time between the laminate and the heating rollers (total contact time) is preferably 1 second to 300 seconds, more preferably 1 to 20 seconds, and even more preferably 1 to 10 seconds.

[0083] The heating rollers can be placed inside a heating furnace (e.g., an oven) or in a typical production line (at room temperature). Preferably, they are placed inside a heating furnace equipped with air supply. By combining heating roller-based drying and hot air drying, abrupt temperature changes between the heating rollers can be suppressed, and shrinkage in the width direction can be easily controlled. The preferred temperature for hot air drying is 30°C to 100°C. Furthermore, the preferred hot air drying time is 1 second to 300 seconds. The preferred air velocity is approximately 10 m / s to 30 m / s. It should be noted that this air velocity refers to the air velocity inside the heating furnace and can be measured using a miniature blade-type digital anemometer.

[0084] C-6. Other treatments

[0085] The cleaning process is preferably performed after the stretching treatment in water and before the drying and shrinkage treatment. A typical example of this cleaning process is to immerse the PVA-based resin layer in an aqueous potassium iodide solution.

[0086] Example

[0087] The present invention will now be specifically described through examples, but the present invention is not limited to these examples. The methods for measuring each characteristic are as follows. It should be noted that, unless otherwise specified, "parts" and "%" in the examples and comparative examples are based on weight.

[0088] (1) Thickness

[0089] The thickness was measured using a gyroscope (manufactured by Otsuka Electronics Co., Ltd., product name "MCPD-3000"). The calculation wavelength range used in the thickness calculation was 400nm to 500nm, and the refractive index was 1.53.

[0090] (2) Orientation function

[0091] For the polarizing films obtained in the examples and comparative examples, Fourier transform infrared spectrometer (FT-IR) (manufactured by PerkinElmer, trade name: "Frontier") was used. Attenuated total reflection (ATR) measurements were performed on the surface of the polarizing film using polarized infrared light as the measurement light. Germanium was used as the microcrystal to bond the polarizing film, and the incident angle of the measurement light was set to 45°. The orientation function was calculated according to the following steps: The incident polarized infrared light (measurement light) was used as polarized light (s-polarized light) vibrating parallel to the surface of the sample to which the germanium crystals were bonded. The stretching direction of the polarizing film was configured perpendicular (⊥) and parallel ( / / ) to the polarization direction of the measurement light, and the absorbance spectra were measured. Based on the obtained absorbance spectra, the attenuated total reflection (ATR) function was calculated at (3330 cm⁻¹). -1 (Strength) as a reference (2941cm) -1 Intensity) I. I ⊥ The absorbance spectrum obtained is based on the absorbance spectrum obtained when the polarizing film is arranged perpendicular (⊥) to the stretching direction of the measuring light. (2941cm) -1 Strength) / (3330cm) -1 Intensity). Additionally, I / / This is the absorbance spectrum obtained when the polarizing film is arranged parallel to ( / / ) the stretching direction of the polarizing film relative to the polarization direction of the measurement light (2941cm). -1 Strength) / (3330cm) -1 Strength). Here, (2941cm) -1 The intensity is 2770 cm⁻¹, which will be the bottom of the absorbance spectrum. -1 and 2990cm -1 2941cm as the baseline -1 absorbance, (3330cm) -1 Strength) is 2990cm -1 and 3650cm -1 3330cm as the baseline -1 The absorbance. Using the obtained I... ⊥ and I / / The orientation function f is calculated according to Equation 1. It should be noted that f = 1 represents perfect orientation, and f = 0 represents random orientation. Additionally, 2941cm is considered... -1 The peak is attributed to the absorption of vibrations in the PVA backbone (-CH2-) in the polarizing film. Additionally, it is believed that the 3330 cm⁻¹ peak is due to... -1 The peak is due to the absorption caused by the vibration of the hydroxyl groups in PVA.

[0092] (Equation 1)f=(3)<cos2θ> -1) / 2

[0093] = (1-D) / [c(2D+1)]

[0094] in,

[0095] c=(3cos 2 β-1) / 2

[0096] As mentioned above, 2941cm was used -1 In this case,

[0097] θ: Angle of the molecular chain relative to the stretching direction

[0098] β: Angle of transition dipole moment relative to the molecular chain axis

[0099] D = (I ⊥ ) / (I / / )

[0100] I ⊥ : Measure the absorption intensity when the polarization direction of the light is perpendicular to the stretching direction of the polarizing film.

[0101] I / / : Measure the absorption intensity when the polarization direction of the light is parallel to the stretching direction of the polarizing film.

[0102] (3) Monomer transmittance and polarization

[0103] The polarizing film was peeled off from the laminate of the polarizing film / thermoplastic resin substrate used in the examples and comparative examples. The monomer transmittance Ts, parallel transmittance Tp, and orthogonal transmittance Tc of the polarizing film, measured using a UV-Vis spectrophotometer (V-7100 manufactured by Nippon Spectrophotometer Co., Ltd.), were taken as the Ts, Tp, and Tc of the polarizing film, respectively. These Ts, Tp, and Tc are Y values ​​obtained by measuring with a 2-degree field of view (C light source) according to JIS Z 8701 and correcting for visual perception.

[0104] Based on the obtained Tp and Tc, the polarization P is calculated using the following formula.

[0105] Polarization P (%) = {(Tp-Tc) / (Tp+Tc)} 1 / 2 ×100

[0106] It should be noted that the same measurements can also be performed using spectrophotometers such as the "LPF-200" manufactured by Otsuka Electronics Co., Ltd., confirming that the same measurement results can be obtained when using any spectrophotometer.

[0107] (4) Fracture strength

[0108] The polarizing film was peeled from the laminate of the polarizing film / thermoplastic resin substrate used in the examples and comparative examples, and placed on a compression testing machine (Kato-tech, product name "NDG5" needle penetration force tester) equipped with a needle. The film was punctured at room temperature (23℃±3℃) at a puncture speed of 0.33 cm / s, and the strength at which the polarizing film ruptured was taken as the breaking strength (puncture strength). For evaluation, the breaking strength of 10 specimens was measured and the average value was used. It should be noted that the needle used had a tip diameter of... A 0.5R needle was used. The polarizing film was clamped and fixed from both sides using a fixture with a circular opening of approximately 11 mm in diameter. The test was conducted by piercing the center of the opening with a needle. The breaking strength per unit thickness was used as an indicator of the ease of rupture, and the following criteria were used for evaluation.

[0109] ○: Fracture strength exceeds 30 gf / μm

[0110] ×: Fracture strength below 30 gf / μm

[0111] [Example 1]

[0112] As the thermoplastic resin substrate, a strip-shaped amorphous polyethylene terephthalate (PET) copolymer film (thickness: 100 μm) with a water absorption rate of 0.75% and a Tg of approximately 75 °C was used. One side of the resin substrate was subjected to corona treatment (treatment conditions: 55 W·min / m). 2 ).

[0113] 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 composed of polyvinyl alcohol (degree of polymerization 4200, degree of saponification 99.2 mol%) and acetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "GOHSEFIMER Z410") in a ratio of 9:1.

[0114] The above-mentioned PVA aqueous solution is coated on the corona-treated surface of the resin substrate and dried at 60°C, thereby forming a PVA-based resin layer with a thickness of 13 μm, and a laminate is produced.

[0115] The resulting laminate was subjected to unidirectional stretching at the free end to 2.4 times its original length between rollers with different circumferential speeds in an oven at 130°C (air-assisted stretching treatment).

[0116] Next, the laminate is immersed in an insoluble bath (an aqueous solution of boric acid prepared 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 (insoluble treatment).

[0117] Next, the concentration was adjusted in a staining bath at a liquid temperature of 30°C (an aqueous solution of iodine and potassium iodide mixed in a weight ratio of 1:7 relative to 100 parts by weight of water to obtain a monomer transmittance (Ts) of 41.6% for the final polarizing film, and the film was immersed for 60 seconds (staining treatment).

[0118] Next, immerse the sample in a crosslinking bath at 40°C (a boric acid aqueous solution prepared by mixing 3 parts by weight of potassium iodide and 5 parts by weight of boric acid relative to 100 parts by weight of water) for 30 seconds (crosslinking treatment).

[0119] Subsequently, the laminate was immersed in a boric acid aqueous solution (boric acid concentration 4.0 wt%, potassium iodide 5.0 wt%) at a liquid temperature of 62°C and unidirectionally stretched along the longitudinal direction (length direction) between rollers with different circumferential speeds, with a total stretch ratio of 3.0 times (water stretching treatment: the stretch ratio of water stretching treatment is 1.25 times).

[0120] Subsequently, the laminate was immersed in a cleaning bath at a liquid temperature of 20°C (an aqueous solution of 4 parts by weight of potassium iodide mixed with 100 parts by weight of water) (cleaning treatment).

[0121] Subsequently, while drying in an oven maintained at 90°C, it is brought into contact with SUS heated rollers with a surface temperature maintained at 75°C for approximately 2 seconds (drying shrinkage treatment). The width-direction shrinkage rate of the laminate resulting from the drying shrinkage treatment is 2%.

[0122] This process forms a polarizing film with a thickness of 7.1 μm on the resin substrate.

[0123] The orientation function, monomer transmittance, polarization degree and breaking strength of the obtained polarizing film are shown in Table 1.

[0124] [Example 2]

[0125] The stretching ratio for the underwater stretching treatment was set to 1.45 times, and the total stretching ratio was set to 3.5 times. Otherwise, the procedure was the same as in Example 1 to prepare a polarizing film with a thickness of 6.6 μm. The resulting polarizing film was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[0126] [Example 3]

[0127] The stretching ratio for the underwater stretching treatment was set to 1.67 times, and the total stretching ratio was set to 4.0 times. Otherwise, the procedure was the same as in Example 1 to prepare a polarizing film with a thickness of 6.1 μm. The resulting polarizing film was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0128] [Example 4]

[0129] The stretching ratio for the underwater stretching treatment was set to 1.87 times, and the total stretching ratio was set to 4.5 times. Otherwise, the procedure was the same as in Example 1 to prepare a polarizing film with a thickness of 5.6 μm. The resulting polarizing film was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[0130] [Comparative Example 1]

[0131] The stretching ratio in the water was set to 2.4 times, the total stretching ratio to 5.5 times, and the temperature of the stretching bath to 70°C. Otherwise, the procedure was the same as in Example 1 to prepare a polarizing film with a thickness of 5.0 μm. It should be noted that the width retention of the obtained polarizing film was 48% (width shrinkage rate was 52%). The obtained polarizing film was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[0132] [Comparative Example 2]

[0133] A polarizing film with a width retention rate of 43% (and a width shrinkage rate of 57%) was fabricated in the same manner as in Comparative Example 1. The resulting polarizing film was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[0134] [Comparative Example 3]

[0135] A long roll of a 30 μm thick PVA-based resin film (manufactured by KURARAY CO.,LTD, product name "PE3000") was unidirectionally stretched along its long side using a roller stretching machine at a total stretch ratio of 6.0. Simultaneously, swelling, dyeing, crosslinking, and cleaning processes were performed, followed by drying to produce a 12 μm thick polarizing element. The resulting polarizing element was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0136] [Table 1]

[0137]

[0138] As can be clearly seen from Table 1, the polarizing film of the embodiments of the present invention has practically acceptable monomer transmittance and polarization, and its tensile strength along the absorption axis is very high. Such tensile strength means that the polarizing film is not easily cracked along the absorption axis.

[0139] Industrial availability

[0140] The polarizing film and polarizing plate of the present invention are suitable for use in liquid crystal display devices.

[0141] Explanation of reference numerals in the attached figures

[0142] 10 polarizing film

[0143] 20 First protective layer

[0144] 30 Second protective layer

[0145] 100 polarizing plates

Claims

1. A polarizing film comprising a polyvinyl alcohol-based resin film containing a dichroic substance, The polyvinyl alcohol-based resin film contains polyvinyl alcohol resin with an average degree of polymerization of 4200 or higher. The monomer transmittance is above 40.0%, and the polarization is above 99.0%. Orientation function is above 0.05 and below 0.

25. The orientation function is determined using a Fourier transform infrared spectrometer. Polarized light is used as the measurement light, and attenuated total internal reflection spectrometry is performed with the stretching direction of the polarizing film parallel and perpendicular to the polarization direction of the measurement light. The absorbance spectrum obtained is then used at 2941 cm⁻¹. -1 The strength is calculated using the following formula (Equation 1): (Equation 1)f=(3) <cos 2 θ>-1) / 2 = (1-D) / [c(2D+1)] = -2×(1-D) / (2D+1) In Equation 1, θ represents the angle of the molecular chain relative to the stretching direction; c represents (3cosθ). 2 β-1) / 2; β represents the angle of the transition dipole moment relative to the molecular chain axis, 2941 cm. -1 In the case of vibration, β is 90°; D represents (I ⊥ ) / (I / / I is the absorbance spectrum obtained relative to 3330 cm⁻¹ -1 The absorption intensity of the reference peak is 2941 cm⁻¹. -1 Absorption intensity, expressed as (2941 cm⁻¹) -1 (strength) / (3330cm) -1 The value of (intensity); I ⊥ I, I represents the stretching direction of the polarizing film when it is arranged perpendicular to the polarization direction of the measured light. / / The term I refers to the stretching direction of the polarizing film when it is arranged parallel to the polarization direction of the measured light.

2. The polarizing film according to claim 1, wherein the thickness is 8 μm or less.

3. The polarizing film according to claim 1 or 2, wherein the puncture strength is 35 gf / μm or higher.

4. A polarizing film comprising a polyvinyl alcohol-based resin film containing a dichroic substance, The monomer transmittance is above 40.0%, and the polarization is above 99.0%. Orientation function is ≥0.05 and ≤0.25, puncture strength is ≥35gf / μm. The orientation function is determined using a Fourier transform infrared spectrometer. Polarized light is used as the measurement light, and attenuated total internal reflection spectrometry is performed with the stretching direction of the polarizing film parallel and perpendicular to the polarization direction of the measurement light. The absorbance spectrum obtained is then used at 2941 cm⁻¹. -1 The strength is calculated using the following formula (Equation 1): (Equation 1)f=(3) <cos 2 θ>-1) / 2 = (1-D) / [c(2D+1)] = -2×(1-D) / (2D+1) In Equation 1, θ represents the angle of the molecular chain relative to the stretching direction; c represents (3cosθ). 2 β-1) / 2; β represents the angle of the transition dipole moment relative to the molecular chain axis, 2941 cm. -1 In the case of vibration, β is 90°; D represents (I ⊥ ) / (I / / I is the absorbance spectrum obtained relative to 3330 cm⁻¹ -1 The absorption intensity of the reference peak is 2941 cm⁻¹. -1 Absorption intensity, expressed as (2941 cm⁻¹) -1 (strength) / (3330cm) -1 The value of (intensity); I ⊥ I, I represents the stretching direction of the polarizing film when it is arranged perpendicular to the polarization direction of the measured light. / / The term I refers to the stretching direction of the polarizing film when it is arranged parallel to the polarization direction of the measured light.

5. A polarizing plate comprising: a polarizing film according to any one of claims 1 to 4, and a protective layer disposed on at least one side of the polarizing film.

6. A method for manufacturing the polarizing film according to any one of claims 1 to 4, comprising: A laminate is formed by forming a polyvinyl alcohol resin layer containing iodide or sodium chloride and polyvinyl alcohol resin on one side of a long strip of thermoplastic resin substrate. and The laminate was subjected to a series of treatments, including aerial assisted stretching, dyeing, underwater stretching, and drying shrinkage. The drying shrinkage process involved heating the laminate along its length, causing it to shrink by more than 2% in the width direction. The total stretching ratio of the aerial assisted stretching treatment and the underwater stretching treatment is 3.0 to 4.0 times the original length of the laminate. The stretching ratio of the aerial assisted stretching treatment is greater than that of the underwater stretching treatment.

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