JP7838066B2 - Polarizing film, polarizing plate, and method for manufacturing the polarizing film - Google Patents

Description

This invention relates to a polarizing film, a polarizing plate, and a method for manufacturing the polarizing film.

Liquid crystal display devices, a typical image display device, have polarizing films positioned on both sides of the liquid crystal cell due to their image formation method. One proposed method for manufacturing polarizing films involves stretching a laminate comprising a resin substrate and a polyvinyl alcohol (PVA) resin layer, followed by a dyeing treatment to obtain a polarizing film on the resin substrate (for example, Patent Document 1). This method allows for the production of thin polarizing films, which has attracted attention as it can contribute to the miniaturization of image display devices in recent years. However, such thin polarizing films have the problem of being prone to tearing (cracking) along the absorption axis.

Japanese Patent Publication No. 2001-343521

This invention was made to solve the above-mentioned conventional problems, and its primary objective is to provide a polarizing film in which fracture along the absorption axis is suppressed.

According to one aspect of the present invention, a polarizing film is provided which is composed of a polyvinyl alcohol-based resin film containing a dichroic substance, and satisfies the following formula (1) when the transmittance of the single unit is x% and the birefringence of the polyvinyl alcohol-based resin is y.
y<-0.011x+0.525 (1)
According to another aspect of the present invention, a polarizing film is provided which is composed of a polyvinyl alcohol-based resin film containing a dichroic substance, and satisfies the following formula (2) when the transmittance of the single unit is x% and the in-plane phase difference of the polyvinyl alcohol-based resin film is z nm.
z<-60x+2875 (2)
In one embodiment, the thickness of the polarizing film is 10 μm or less.
In one embodiment, the polarizing film has a single-unit transmittance of 40.0% or more, and a polarization degree of 99.0% or more.
According to yet another aspect of the present invention, a polarizing plate is provided, having the polarizing film and a protective layer disposed on at least one side of the polarizing film.
In one embodiment, the total thickness of the polarizing plate is 30 μm or less.
A further aspect of the present invention provides a method for manufacturing the polarizing film described above. The manufacturing method includes forming a polyvinyl alcohol-based resin layer containing iodide or sodium chloride and a polyvinyl alcohol-based resin on one side of a long thermoplastic resin substrate to form a laminate, and subjecting the laminate to an air-assisted stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment in which the laminate shrinks by 2% or more in the width direction by heating while being transported in the longitudinal direction, wherein 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, and the stretching ratio of the air-assisted stretching treatment is greater than the stretching ratio of the underwater stretching treatment.

The polarizing film of the present invention is composed of a PVA-based resin film containing a dichroic substance, and the single-layer transmittance and the birefringence of the PVA-based resin or the in-plane phase difference of the PVA-based resin film satisfy a predetermined relationship. A polarizing film satisfying this relationship can achieve practically acceptable optical properties (typically single-layer transmittance and polarization degree) while suppressing fracture along the absorption axis, due to the orientation state of the PVA-based resin, etc.

This is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. This is a schematic diagram showing an example of a drying shrinkage process using heated rollers. This figure shows the relationship between the single-layer transmittance of the polarizing films prepared in the examples and comparative examples and the birefringence of the PVA-based resin. This figure shows the relationship between the individual transmittance of the polarizing films prepared in the examples and comparative examples and the in-plane phase difference of the PVA resin film.

The following describes embodiments of the present invention, but the present invention is not limited to these embodiments. Furthermore, each embodiment can be combined as appropriate.

A. Polarizing Film A polarizing film according to one embodiment of the present invention is composed of a PVA-based resin film containing a dichroic substance, and satisfies the following formula (1) when the transmittance of the PVA-based resin is x% and the birefringence of the PVA-based resin is y. A polarizing film according to another embodiment of the present invention is composed of a PVA-based resin film containing a dichroic substance, and satisfies the following formula (2) when the transmittance of the PVA-based resin film is x% and the in-plane phase difference of the PVA-based resin film is z nm.
y<-0.011x+0.525 (1)
z<-60x+2875 (2)

The birefringence of the PVA resin in the above polarizing film (hereinafter referred to as the birefringence of PVA or Δn of PVA) and the in-plane phase difference of the PVA resin film (hereinafter referred to as the "in-plane phase difference of PVA") are both values related to the degree of orientation of the molecular chains of the PVA resin constituting the polarizing film, and can become larger as the degree of orientation increases. Because the orientation of the molecular chains of the PVA resin in the above polarizing film is gentler in the direction of the absorption axis than that of conventional polarizing films, fracture along the absorption axis is suppressed. As a result, a polarizing film (and consequently a polarizing plate) with excellent flexibility can be obtained. Such a polarizing film (and consequently a polarizing plate) can preferably be applied to a curved image display device, more preferably a bendable image display device, and even more preferably a foldable image display device. Conventionally, it has been difficult to obtain acceptable optical properties (typically, single-layer transmittance and polarization degree) with polarizing films that have a low degree of orientation. However, polarizing films that satisfy the above equations (1) and/or (2) can achieve both a lower degree of orientation of the PVA-based resin and acceptable optical properties than conventional films.

The polarizing film according to the embodiment of the present invention preferably satisfies the following formula (1a) and/or formula (2a), and more preferably satisfies the following formula (1b) and/or formula (2b).
-0.004x+0.18<y<-0.011x+0.525 (1a)
-0.003x+0.145<y<-0.011x+0.520 (1b)
-40x+1800<z<-60x+2875 (2a)
-30x+1450<z<-60x+2850 (2b)

In this specification, the in-plane phase difference of the PVA is the in-plane phase difference value of the PVA-based resin film at 23°C and a wavelength of 1000 nm. By using the near-infrared region as the measurement wavelength, the influence of iodine absorption in the polarizing film can be eliminated, making it possible to measure the phase difference. Furthermore, the birefringence (in-plane birefringence) of the PVA is the value obtained by dividing the in-plane phase difference of the PVA by the thickness of the polarizing film.

The in-plane phase difference of PVA is evaluated as follows. First, the phase difference value is measured at multiple wavelengths of 850 nm or greater. The measured phase difference value: R(λ) and wavelength: λ are plotted, and this is fitted to the following Sellmeyer equation using the least squares method. Here, A and B are fitting parameters and coefficients determined by the least squares method.
R(λ)=A+B/(λ ² -600 ² )
In this case, the phase difference value R(λ) can be separated into the in-plane phase difference of PVA (Rpva), which is wavelength-independent, and the in-plane phase difference value of iodine (Ri), which is highly wavelength-dependent, as shown below.
Rpva = A
Ri = B/(λ ² -600 ² )
Based on this separation formula, the in-plane phase difference (i.e., Rpva) of PVA at a wavelength λ = 1000 nm can be calculated. The method for evaluating the in-plane phase difference of PVA is also described in Japanese Patent Publication No. 5932760, which can be referenced as needed.
Furthermore, the birefringence (Δn) of the PVA can be calculated by dividing this phase difference by the thickness.

Examples of commercially available instruments for measuring the in-plane phase difference of PVA at the above wavelength of 1000 nm include the KOBRA-WR/IR series and KOBRA-31X/IR series manufactured by Oji Instruments Co., Ltd.

The orientation function (f) of the PVA-based resin constituting the above polarizing film 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 may be, for example, 0.05. If the orientation function is too small, an acceptable single-layer transmittance and/or degree of polarization may not be obtained.

The orientation function (f) is determined, for example, by attenuated total reflection (ATR) spectroscopy using a Fourier transform infrared spectrophotometer (FT-IR) with polarized light as the measurement light. Specifically, germanium crystallites are used to adhere the polarizing film, the incident angle of the measurement light is set to 45°, and the incident polarized infrared light (measurement light) is polarized (s-polarized) that vibrates parallel to the surface to which the germanium crystal sample is adhered. Measurements are performed with the stretching direction of the polarizing film positioned parallel and perpendicular to the polarization direction of the measurement light, and the intensity at 2941 ^cm⁻¹ of the obtained absorbance spectrum is used to calculate the orientation function (f) according to the following formula. Here, the intensity I is the value of 2941 ^cm⁻¹ / 3330 ^cm⁻¹ , with 3330 ^cm⁻¹ as the reference peak. Note that when f = 1, the orientation is perfectly oriented, and when f = 0, it is random. Furthermore, the peak at 2941 ^cm⁻¹ is thought to be an absorption caused by vibrations of the PVA main chain ( _-CH₂- ) in the polarizing film.
f=(3<cos ² θ>-1)/2
=(1-D)/[c(2D+1)]
=-2×(1-D)/(2D+1)
however,
c = (3cos ^2β -1)/2, and in the case of vibration of 2941 ^cm⁻¹ , β = 90°.
θ: Angle of the molecular chain with respect to the stretching direction β: Angle of the transition dipole moment with respect to the molecular chain axis D = (I _⊥ ) / (I _// ) (In this case, D increases as the PVA molecules are oriented)
I _⊥ : Absorption intensity when the polarization direction of the measured light and the stretching direction of the polarizing film are perpendicular I _// : Absorption intensity when the polarization direction of the measured light and the stretching direction of the polarizing film are parallel

The thickness of the polarizing film is preferably 10 μm or less, and more preferably 8 μm or less. The lower limit of the polarizing film thickness may be, for example, 1 μm. In one embodiment, the thickness of the polarizing film may be 2 μm to 10 μm, and in another embodiment, 2 μm to 8 μm. By making the thickness of the polarizing film so thin, thermal shrinkage can be greatly reduced. It is presumed that this configuration may also contribute to suppressing fracture in the absorption axis direction.

The polarizing film preferably exhibits absorption dichroism at any wavelength between 380 nm and 780 nm. The single-layer transmittance of the polarizing film is preferably 40.0% or higher, more preferably 41.0% or higher. The upper limit of the single-layer transmittance may be, for example, 49.0%. In one embodiment, the single-layer transmittance of the polarizing film is 40.0% to 45.0%. The degree of polarization of the polarizing film is preferably 99.0% or higher, more preferably 99.4% or higher. The upper limit of the degree of polarization may be, for example, 99.999%. In one embodiment, the degree of polarization of the polarizing film is 99.0% to 99.9%. One feature of the polarizing film according to the embodiment of the present invention is that, despite having an in-plane phase difference, birefringence and/or orientation function as described above, it is possible to achieve such practically acceptable single-layer transmittance and degree of polarization, even though the degree of orientation of the PVA resin constituting the polarizing film is lower than conventional films. This is presumed to be due to the manufacturing method described later. The transmittance of a polarizing film is typically measured using a UV-Vis spectrophotometer and is the Y value after luminous sensitivity correction. When measuring the transmittance of a polarizing film using a polarizing plate having the configuration of [polarizing film/resin substrate (protective layer)], the transmittance of the polarizing film is the value obtained when the refractive index of one surface of the polarizing plate is converted to 1.50 and the refractive index of the other surface is converted to 1.53. The degree of polarization is typically determined by the following formula based on the parallel transmittance Tp and orthogonal transmittance Tc, which are measured using a UV-Vis spectrophotometer and luminous sensitivity correction.
Degree of polarization (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 ×100

The puncture strength of the polarizing film is, for example, 30 gf/μm or more, preferably 35 gf/μm or more, more preferably 40 gf/μm or more, even more preferably 45 gf/μm or more, and particularly preferably 50 gf/μm or more. The upper limit of the puncture strength may be, for example, 80 gf/μm. By setting the puncture strength of the polarizing film within this range, the tearing of the polarizing film along the absorption axis can be significantly suppressed. As a result, a polarizing film (and consequently a polarizer) with excellent flexibility can be obtained. Puncture strength indicates the crack resistance of the polarizing film when it is punctured with a predetermined strength. Puncture strength can be expressed, for example, as the strength at which the polarizing film breaks (breaking strength) when a predetermined needle is attached to a compression tester and punctured into the polarizing film at a predetermined speed. As is clear from the units, puncture strength refers to the puncture strength per unit thickness (1 μm) of the polarizing film.

As described above, the polarizing film is composed of a PVA-based resin film containing a dichroic substance. Preferably, the PVA-based resin constituting the PVA-based resin film (substantially the polarizing film) includes an acetoacetyl-modified PVA-based resin. With such a configuration, a polarizing film with the desired puncture strength can be obtained. The amount of acetoacetyl-modified PVA-based resin is preferably 5% to 20% by weight, and more preferably 8% to 12% by weight, when the total PVA-based resin is considered to be 100% by weight. Within this range of amounts, the puncture strength can be set to a more suitable range.

A polarizing film can typically be made using a laminate of two or more layers. A specific example of a polarizing film obtained using a laminate is a polarizing film made using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate. A polarizing film made using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate can be made, for example, by applying a PVA-based resin solution to a resin substrate, drying it to form a PVA-based resin layer on the resin substrate, and obtaining a laminate of the resin substrate and the PVA-based resin layer; and stretching and dyeing the laminate to make the PVA-based resin layer a polarizing film. In this embodiment, preferably, a polyvinyl alcohol-based resin layer containing a halide and a polyvinyl alcohol-based resin is formed on one side of the resin substrate. Stretching typically includes immersing the laminate in an aqueous boric acid solution and stretching it. Furthermore, stretching preferably 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 preferably 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 by combining the addition of halides and a drying shrinkage treatment. Furthermore, in embodiments of the present invention, the stretching ratio of the air-assisted stretching is preferably greater than the stretching ratio of the boric acid water stretching. With this configuration, a polarizing film with acceptable optical properties can be obtained even with a small total stretching ratio. In addition, the laminate is preferably subjected to a drying shrinkage treatment in which it shrinks by 2% or more in the width direction by heating while being transported in the longitudinal direction. In one embodiment, the method for manufacturing a polarizing film includes applying an air-assisted stretching treatment, a dyeing treatment, a water stretching treatment, and a drying shrinkage treatment to the laminate in this order. By introducing auxiliary stretching, even when a PVA-based resin is applied to a thermoplastic resin, it is possible to increase the crystallinity of the PVA-based resin and achieve high optical properties. Furthermore, by simultaneously enhancing the orientation of the PVA resin beforehand, problems such as a decrease in the orientation or dissolution of the PVA resin when immersed in water during subsequent dyeing and stretching processes can be prevented, enabling the achievement of high optical properties. Moreover, when the PVA resin layer is immersed in liquid, the disorder of the orientation of polyvinyl alcohol molecules and the decrease in orientation can be suppressed compared to cases where the PVA resin layer does not contain halides. This improves the optical properties of the polarizing film obtained through processing steps involving immersion in liquid, such as dyeing and underwater stretching. Furthermore, shrinking the laminate in the width direction through a drying shrinkage treatment can further improve the optical properties. The resulting resin substrate/polarizing film laminate may be used as is (i.e., the resin substrate may be used as a protective layer for the polarizing film), or the resin substrate may be peeled off the resin substrate/polarizing film laminate, and an appropriate protective layer may be laminated onto the peeled surface according to the purpose. Details of the polarizing film manufacturing method will be described later in section C.

B. Polarizing Plates Figures 1(a) and 1(b) are schematic cross-sectional views of polarizing plates according to one embodiment of the present invention. The polarizing plate 100a shown in Figure 1(a) has a polarizing film 10 and a first protective layer 20 disposed on one side of the polarizing film 10. The polarizing plate 100b shown in Figure 1(b) has a polarizing film 10, a first protective layer 20 disposed on one side of the polarizing film 10 and a second protective layer 30 disposed on the other side of the polarizing film 10. The polarizing film 10 is the polarizing film of the present invention as described in Section A above. As described above, one of the first protective layer and the second protective layer may be a resin substrate used in the manufacture of the above polarizing film.

The total thickness of the polarizing plate described above is, for example, 150 μm or less, preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less. The lower limit of the total thickness may be, for example, 10 μm. A polarizing plate having such a total thickness can possess extremely excellent flexibility and bending durability. Such a polarizing plate with a phase difference layer can be particularly suitably applied to curved image display devices and/or bendable or foldable image display devices. Note that the total thickness of the polarizing plate refers to the sum of the thicknesses of all layers constituting the polarizing plate, excluding the adhesive layer used to adhere the polarizing plate to an external substrate such as a panel or glass (i.e., the total thickness of the polarizing plate does not include the thickness of the adhesive layer used to attach the polarizing plate to an adjacent member such as an image display cell, and the release film that may be temporarily attached to its surface).

In one embodiment, the first and/or second protective layer is formed from any suitable film that can be used as a protective layer for the polarizing film. Specific examples of materials that make up the main component of the film include cellulosic resins such as triacetylcellulose (TAC), and transparent resins such as polyester, polyvinyl alcohol, polycarbonate, polyamide, polyimide, polyethersulfone, polysulfone, polystyrene, polynorbornene, polyolefin, (meth)acrylic, and acetate. Also, thermosetting resins or UV-curing resins such as (meth)acrylic, urethane, (meth)acrylic urethane, epoxy, and silicone are also acceptable. In addition, glassy polymers such as siloxane polymers can also be used. Furthermore, the polymer film described in Japanese Patent Application Publication No. 2001-343529 (WO01/37007) can also be used. As materials for this film, for example, a resin composition containing a thermoplastic resin having substituted or unsubstituted imide groups in its side chains, and a thermoplastic resin having substituted or unsubstituted phenyl groups and nitrile groups in its side chains, can be used. Examples include a resin composition having an alternating copolymer of isobutene and N-methylmaleimide, and an acrylonitrile-styrene copolymer. The polymer film may, for example, be an extruded product of the above resin composition.

In another embodiment, the first and/or second protective layer is composed of a solidified coating film of an organic solvent solution of a thermoplastic resin (hereinafter also referred to as the thermoplastic resin coating film). Any suitable resin can be used as the thermoplastic resin, depending on the purpose. Examples include acrylic resins, epoxy resins, styrene resins, polyolefin resins, and polyester resins, with acrylic resins and epoxy resins being preferred.

When the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (outer protective layer) positioned on the opposite side of the display panel is preferably 2 μm to 80 μm, more preferably 2 μm to 40 μm, and even more preferably 2 μm to 25 μm. If the outer protective layer is a thermoplastic resin coating film, its thickness may be, for example, 10 μm or less, preferably 7 μm or less, and more preferably 5 μm or less. If surface treatment is applied, the thickness of the outer protective layer includes the thickness of the surface treatment layer.

When the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (inner protective layer) positioned on the display panel side is preferably 5 μm to 80 μm, more preferably 5 μm to 40 μm, and even more preferably 5 μm to 25 μm. In one embodiment, the inner protective layer is a phase difference layer having an arbitrary appropriate phase difference value. In this case, the in-plane phase difference Re(550) of the phase difference layer, measured with light of wavelength 550 nm at 23°C, is, for example, 110 nm to 150 nm. "Re(550)" is the in-plane phase difference measured with light of wavelength 550 nm at 23°C, and is calculated by the formula: Re = (nx - ny) × d. Here, "nx" is the refractive index in the direction where the in-plane refractive index is maximum (i.e., the slow phase axis direction), "ny" is the refractive index in the direction perpendicular to the slow phase axis in the plane (i.e., the fast phase axis direction), and "d" is the thickness of the layer (film) (nm).

C. Method for Manufacturing a Polarizing Film One embodiment of the present invention provides a method for manufacturing a polarizing film, comprising forming a polyvinyl alcohol-based resin layer (PVA-based resin layer) containing a halogenated compound and a polyvinyl alcohol-based resin (PVA-based resin) on one side of a long thermoplastic resin substrate to form a laminate, and subjecting the laminate to the following in order: air-assisted stretching, dyeing, underwater stretching, and drying shrinkage by heating while conveying in the longitudinal direction to shrink by 2% or more in the width direction. The halogenated compound content in the PVA-based resin layer is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA-based resin. The drying shrinkage is preferably performed using a heated roll, and the temperature of the heated roll is preferably 60°C to 120°C. The shrinkage rate in the width direction of the laminate due to the drying shrinkage is preferably 2% or more. Furthermore, the stretching ratio of the air-assisted stretching is preferably greater than the stretching ratio of the underwater stretching. By such a manufacturing method, the polarizing film described in Section A above can be obtained. In particular, by fabricating a laminate containing a PVA-based resin layer containing a halide, performing multi-stage stretching of the laminate including air-assisted stretching and underwater stretching, and then heating the stretched laminate with a heated roll to shrink it by 2% or more in the width direction, a polarizing film with excellent optical properties (typically, single-layer transmittance and polarization degree) can be obtained.

C-1. Preparation of Laminates Any suitable method can be used to prepare a laminate of a thermoplastic resin substrate and a PVA-based resin layer. Preferably, a coating solution containing a halogenated compound and a PVA-based resin is applied to the surface of the thermoplastic resin substrate and dried to form a PVA-based resin layer on the thermoplastic resin substrate. As described above, the halogenated compound content in the PVA-based resin layer is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA-based resin.

Any suitable method can be used to apply the coating solution. Examples include roll coating, spin coating, wire bar coating, dip coating, die coating, curtain coating, spray coating, and knife coating (comma coating, etc.). The application and drying temperature of the coating solution is preferably 50°C or higher.

The thickness of the PVA resin layer is preferably 2 μm to 30 μm, and more preferably 2 μm to 20 μm. By making the thickness of the PVA resin layer before stretching very thin in this way, and by reducing the total stretching ratio as described later, it is possible to obtain a polarizing film with acceptable single-layer transmittance and polarization degree despite a lower degree of orientation of the PVA resin than conventional methods.

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

C-1-1. Thermoplastic Resin Substrate Any suitable thermoplastic resin film can be used as the thermoplastic resin substrate. Details of the thermoplastic resin substrate are described, for example, in Japanese Patent Application Publication No. 2012-73580. The entire description of that publication is incorporated herein by reference.

C-1-2. Coating Solution The coating solution contains a halogenated compound and a PVA-based resin, as described above. Typically, the coating solution is a solution obtained by dissolving the halogenated compound and the PVA-based resin 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 two or more. Among these, water is preferred. The concentration of the PVA-based resin in the solution is preferably 3 to 20 parts by weight per 100 parts by weight of the solvent. With such a resin concentration, a uniform coating film that adheres closely to the thermoplastic resin substrate can be formed. The halogenated compound content in the coating solution is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA-based resin.

Additives may be added to 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 may be used to further improve the uniformity, dyeability, and stretchability of the resulting PVA-based resin layer.

Any suitable resin can be used as the PVA-based resin. For example, polyvinyl alcohol and ethylene-vinyl alcohol copolymers can be used. 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 usually 85 mol% to 100 mol%, preferably 95.0 mol% to 99.95 mol%, and more preferably 99.0 mol% to 99.93 mol%. The 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 degree of saponification is too high, there is a risk of gelation. As described above, the PVA-based resin preferably includes an acetoacetyl-modified PVA-based resin.

The average degree of polymerization of PVA resins can be appropriately selected depending on the purpose. The average degree of polymerization is typically 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.

Any suitable halide can be used as the above-mentioned halide. Examples include iodide and sodium chloride. Examples of iodides include potassium iodide, sodium iodide, and lithium iodide. Among these, potassium iodide is preferred.

The amount of halogen in the coating solution is preferably 5 to 20 parts by weight per 100 parts by weight of PVA resin, and more preferably 10 to 15 parts by weight per 100 parts by weight of PVA resin. If the amount of halogen exceeds 20 parts by weight per 100 parts by weight of PVA resin, the halogen may bleed out, and the resulting polarizing film may become cloudy.

Generally, stretching a PVA-based resin layer increases the orientation of polyvinyl alcohol molecules within the layer. However, immersing the stretched PVA-based resin layer in a water-containing liquid can disrupt the orientation of the polyvinyl alcohol molecules, potentially reducing their degree of orientation. This tendency to decrease orientation is particularly pronounced when stretching a laminate of a thermoplastic resin substrate and a PVA-based resin layer in boric acid water. For example, while stretching a PVA film alone in boric acid water is typically performed at 60°C, stretching a laminate of A-PET (thermoplastic resin substrate) and a PVA-based resin layer is performed at a higher temperature of around 70°C. In this case, the orientation of the PVA in the initial stages of stretching may decrease before it increases due to water stretching. In contrast, by fabricating a laminate of a PVA-based resin layer containing a halide and a thermoplastic resin substrate, and then performing high-temperature stretching (auxiliary stretching) in air before stretching the laminate in boric acid water, the crystallization of the PVA-based resin in the PVA-based resin layer of the laminate after auxiliary stretching can be promoted. As a result, when the PVA-based resin layer is immersed in a liquid, the disorder of the orientation of polyvinyl alcohol molecules and the decrease in orientation can be suppressed compared to when the PVA-based resin layer does not contain a halide. This can improve the optical properties of the polarizing film obtained through processing steps that involve immersing the laminate in a liquid, such as dyeing and water stretching.

C-2. Auxiliary Stretching in Air In particular, to obtain high optical properties, a two-stage stretching method combining dry stretching (auxiliary stretching) and stretching in boric acid water is selected. By introducing auxiliary stretching, as in two-stage stretching, it is possible to stretch the thermoplastic resin substrate while suppressing crystallization. Furthermore, when coating a PVA-based resin onto a thermoplastic resin substrate, it is necessary to lower the coating temperature compared to when coating a PVA-based resin onto a normal metal drum in order to suppress the influence of the glass transition temperature of the thermoplastic resin substrate. As a result, the crystallization of the PVA-based resin becomes relatively low, which can lead to the problem of not being able to obtain sufficient optical properties. In contrast, by introducing auxiliary stretching, it is possible to increase the crystallinity of the PVA-based resin even when coating a thermoplastic resin, making it possible to achieve high optical properties. At the same time, by increasing the orientation of the PVA-based resin in advance, it is possible to prevent problems such as a decrease in the orientation of the PVA-based resin or dissolution when immersed in water in the subsequent dyeing and stretching processes, making it possible to achieve high optical properties.

The stretching method for aerial assisted stretching may be fixed-end stretching (for example, stretching using a tenter stretcher) or free-end stretching (for example, uniaxial stretching by passing the laminate between rolls with different peripheral speeds). However, free-end stretching can be actively adopted to obtain high optical properties. In one embodiment, the aerial stretching process includes a heated roll stretching step in which the laminate is stretched by the difference in peripheral speed between heated rolls while being transported in its longitudinal direction. Typically, the aerial stretching process includes a zone stretching step and a heated roll stretching step. The order of the zone stretching step and the heated roll stretching step is not limited; the zone stretching step may be performed first, or the heated roll stretching step may be performed first. The zone stretching step may be omitted. In one embodiment, the zone stretching step and the heated roll stretching step are performed in this order. In another embodiment, the film end is gripped in a tenter stretcher and stretched by widening the distance between tenters in the flow direction (the widening of the distance between tenters becomes the stretching ratio). At this time, the distance of the tenters in the width direction (perpendicular to the flow direction) is set to be arbitrarily close. Preferably, it can be set to be closer to free-end stretching with respect to the stretching ratio in the flow direction. In the case of free-end stretching, the shrinkage rate in the width direction is calculated as (1/stretching ratio) ^1/2 .

Aerial assisted stretching may be performed in one stage or in multiple stages. When performed in multiple stages, the stretching ratio is the product of the stretching ratios of each stage. The stretching direction in aerial assisted stretching is preferably approximately the same as the stretching direction in underwater stretching.

The stretching ratio in aerial 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. If the stretching ratio of aerial assisted stretching is within this range, the total stretching ratio can be set to a desired range when combined with underwater stretching, thereby achieving the desired birefringence, in-plane phase difference, and/or orientation function. As a result, a polarizing film with suppressed fracture along the absorption axis can be obtained. Furthermore, as described above, it is preferable that the stretching ratio of aerial assisted stretching is greater than that of underwater stretching. With this configuration, a polarizing film with acceptable optical properties can be obtained even with a small total stretching ratio. More specifically, the ratio of the stretching ratio of aerial assisted stretching to that of underwater stretching (underwater stretching/aerial assisted stretching) is preferably 0.4 to 0.9, and more preferably 0.5 to 0.8.

The stretching temperature during air-assisted stretching can be set to any appropriate value depending on the forming material of the thermoplastic resin substrate, the stretching method, etc. Preferably, the stretching temperature is above the glass transition temperature (Tg) of the thermoplastic resin substrate, more preferably above the glass transition temperature (Tg) + 10°C, and particularly preferably above Tg + 15°C. On the other hand, the upper limit of the stretching temperature is preferably 170°C. Stretching at such a temperature suppresses the rapid crystallization of the PVA-based resin, thereby suppressing problems caused by such crystallization (for example, hindering the orientation of the PVA-based resin layer due to stretching).

C-3. Immobilization treatment, dyeing treatment and crosslinking treatment If necessary, an immobilization treatment is performed after the air-assisted stretching treatment and before the water stretching treatment and dyeing treatment. The above immobilization treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. The above dyeing treatment is typically performed by dyeing the PVA-based resin layer with a dichroic substance (typically iodine). If necessary, a crosslinking treatment is performed after the dyeing treatment and before the water stretching treatment. The above crosslinking treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. Details of the immobilization treatment, dyeing treatment and crosslinking treatment are described, for example, in Japanese Patent Application Publication No. 2012-73580.

C-4. Underwater stretching treatment Underwater stretching treatment is performed by immersing the laminate in a stretching bath. Underwater stretching treatment allows stretching at a temperature lower than the glass transition temperature (typically around 80°C) of the thermoplastic resin substrate or PVA-based resin layer, and the PVA-based resin layer can be stretched while suppressing its crystallization. As a result, a polarizing film with excellent optical properties can be manufactured.

Any suitable method can be used to stretch the laminate. Specifically, it may be fixed-end stretching or free-end stretching (for example, uniaxial stretching by passing the laminate between rolls with different peripheral speeds). Preferably, free-end stretching is selected. The stretching of the laminate may 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.

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 resin layer can be imparted with rigidity to withstand the tension applied during stretching and water resistance that prevents dissolution in water. Specifically, boric acid can generate tetrahydroxyborate anions in the aqueous solution, which can crosslink with the PVA resin through hydrogen bonding. As a result, the PVA resin layer can be given rigidity and water resistance, allowing for good stretching and the production of a polarizing film with excellent optical properties.

The above-mentioned aqueous boric acid solution is preferably obtained by dissolving boric acid and/or a borate in water, which is the solvent. The boric acid concentration 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, per 100 parts by weight of water. By setting the boric acid concentration to 1 part by weight or more, the dissolution of the PVA-based resin layer can be effectively suppressed, and a polarizing film with higher properties can be produced. 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.

Preferably, iodide is added to the stretching bath (boric acid aqueous solution). Adding iodide suppresses the elution of iodine adsorbed onto the PVA resin layer. Specific examples of iodide are as described above. The concentration of 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.

The stretching temperature (liquid temperature of the stretching bath) is preferably 40°C to 85°C, more preferably 60°C to 75°C. At this temperature, high-magnification stretching is possible 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, in relation to the formation of the PVA-based resin layer. In this case, if the stretching temperature falls below 40°C, even considering the plasticization of the thermoplastic resin substrate by water, good stretching may not be possible. On the other hand, the higher the temperature of the stretching bath, the higher the solubility of the PVA-based resin layer, which may result in poor optical properties. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

The stretching ratio during underwater stretching is preferably 1.0 to 2.2 times, more preferably 1.1 to 2.0 times, even more preferably 1.1 to 1.8 times, and even more preferably 1.2 to 1.6 times. If the stretching ratio during underwater stretching is within this range, the total stretching ratio can be set to a desired range, and the desired birefringence, in-plane phase difference, and/or orientation function can be achieved. As a result, a polarizing film with suppressed fracture along the absorption axis 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, preferably 3.0 to 4.5 times, more preferably 3.0 to 4.3 times, and even more preferably 3.0 to 4.0 times, relative to the original length of the laminate. By appropriately combining the addition of halides to the coating solution, the adjustment of the stretching ratios for air-assisted stretching and underwater stretching, and the drying shrinkage treatment, a polarizing film with acceptable optical properties can be obtained even with such a total stretching ratio.

C-5. Drying Shrinkage Treatment The above drying shrinkage treatment may be performed by zone heating, which involves heating the entire zone, or by heating the conveying rolls (using so-called heated rolls) (heated roll drying method). Preferably, both methods are used. By drying using heated rolls, heat curling of the laminate can be efficiently suppressed, and a polarizing film with excellent appearance can be manufactured. Specifically, by drying the laminate while it is aligned with the heated rolls, the crystallization of the thermoplastic resin substrate can be efficiently promoted, increasing the degree of crystallinity, and even at relatively low drying temperatures, the degree of crystallinity of the thermoplastic resin substrate can be increased well. As a result, the rigidity of the thermoplastic resin substrate increases, making it able to withstand the shrinkage of the PVA-based resin layer due to drying, and curling is suppressed. Furthermore, by using heated rolls, the laminate can be dried while maintaining a flat state, so not only curling but also wrinkles can be suppressed. At this time, the optical properties of the laminate can be improved by shrinking it in the width direction by the drying shrinkage treatment. This is because the orientation of PVA and PVA/iodine complex can be effectively increased. The shrinkage rate in the width direction of the laminate due to the drying shrinkage treatment is preferably 1% to 10%, more preferably 2% to 8%, and particularly preferably 4% to 6%.

Figure 2 is a schematic diagram showing an example of a drying shrinkage process. In the drying shrinkage process, the laminate 200 is dried while being transported by conveyor rolls R1 to R6 and guide rolls G1 to G4, which are heated to a predetermined temperature. In the illustrated example, the conveyor rolls R1 to R6 are arranged to continuously heat the PVA resin layer surface and the thermoplastic resin substrate surface alternately. However, for example, the conveyor rolls R1 to R6 may be arranged to continuously heat only one side of the laminate 200 (for example, the thermoplastic resin substrate surface).

The drying conditions can be controlled by adjusting the heating temperature of the conveying rolls (temperature of the heating rolls), the number of heating rolls, and the contact time with the heating rolls. The heating roll temperature is preferably 60°C to 120°C, more preferably 65°C to 100°C, and particularly preferably 70°C to 80°C. This effectively increases the crystallinity of the thermoplastic resin, effectively suppresses curling, and allows for the production of optical laminates with extremely high durability. The heating roll temperature can be measured using a contact thermometer. While the illustrated example shows six conveying rolls, there are no particular limitations on the number of conveying rolls. Typically, 2 to 40 conveying rolls are provided, preferably 4 to 30. The contact time between the laminate and the heating rolls (total contact time) is preferably 1 second to 300 seconds, more preferably 1 to 20 seconds, and even more preferably 1 to 10 seconds.

The heating rolls may be installed in a heating furnace (e.g., an oven) or in a normal production line (at room temperature). Preferably, they are installed in a heating furnace equipped with a blowing mechanism. By using both heating roll drying and hot air drying, abrupt temperature changes between the heating rolls can be suppressed, and shrinkage in the width direction can be easily controlled. The temperature for hot air drying is preferably 30°C to 100°C. The hot air drying time is preferably 1 second to 300 seconds. The hot air velocity is preferably about 10 m/s to 30 m/s. This velocity is the velocity inside the heating furnace and can be measured using a mini-vane type digital anemometer.

C-6. Other treatments Preferably, a washing treatment is performed after the underwater stretching treatment and before the drying shrinkage treatment. The above washing treatment is typically performed by immersing the PVA resin layer in an aqueous potassium iodide solution.

The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The measurement methods for each characteristic are as follows. Unless otherwise specified, "parts" and "%" in the examples and comparative examples are based on weight.
(1) Thickness was measured using an interferometer (Otsuka Electronics Co., Ltd., product name "MCPD-3000"). The calculation wavelength range used for thickness calculation was 400 nm to 500 nm, and the refractive index was set to 1.53.
(2) In-plane phase difference (Re) of PVA
For the polarizing films (polarizing films alone) obtained by peeling off the resin substrate from the polarizing film/thermoplastic resin substrate laminates obtained in the examples and comparative examples, the in-plane phase difference (RPVA) of PVA at a wavelength of 1000 nm was evaluated using a phase difference measuring device (Oji Instruments Co., Ltd., product name "KOBRA-31X100/IR") (according to the principle described, this is the value obtained by subtracting the in-plane phase difference of iodine (Ri) from the total in-plane phase difference at a wavelength of 1000 nm). The absorption edge wavelength was set to 600 nm.
(3) Birefringence of PVA (Δn)
The birefringence (Δn) of the PVA was calculated by dividing the in-plane phase difference of the PVA measured in (2) above by the thickness of the polarizing film.
(4) Single-layer transmittance and polarization degree The single-layer transmittance Ts, parallel transmittance Tp, and orthogonal transmittance Tc were measured for the polarizing film (polarizing film alone) obtained by peeling off the resin substrate from the polarizing film/thermoplastic resin substrate laminate obtained in the examples and comparative examples using a UV-Vis spectrophotometer (JASCO Corporation "V-7100"). These Ts, Tp, and Tc are Y values measured using a 2-degree field of view (C light source) according to JIS Z8701 and corrected for luminous sensitivity. From the obtained Tp and Tc, the polarization degree P was determined by the following formula.
Polarization degree P (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 ×100
Furthermore, equivalent measurements can be performed using spectrophotometers such as the "LPF-200" manufactured by Otsuka Electronics Co., Ltd., and it has been confirmed that equivalent measurement results can be obtained regardless of which spectrophotometer is used.
(5) Breaking Strength The polarizing film was peeled off from the polarizing film/thermoplastic resin substrate laminate obtained in the examples and comparative examples, and placed on a compression testing machine (manufactured by Kato Tech Co., Ltd., product name "NDG5", needle penetration force measurement specification) equipped with a needle. Under room temperature conditions (23℃ ± 3℃), the film was punctured at a puncture speed of 0.33 cm/second, and the strength at which the polarizing film broke was defined as the breaking strength. The evaluation value was the average value obtained by measuring the breaking strength of 10 sample pieces. The needle used had a tip diameter of 1 mmφ and a radius of 0.5R. For the polarizing film to be measured, a jig with a circular opening of approximately 11 mm in diameter was fixed by sandwiching it from both sides of the polarizing film, and the test was performed by puncturing the center of the opening with a needle. The breaking strength per unit thickness (puncture strength) was used as an indicator of resistance to breakage and was evaluated according to the following criteria.
Excellent: Puncture strength of 45 gf/μm or higher Good: Puncture strength of 30 gf/μm or higher but less than 45 gf/μm Poor: Puncture strength less than 30 gf/μm

[Example 1]
As the thermoplastic resin substrate, an amorphous isophthalic copolymer polyethylene terephthalate film (thickness: 100 μm) in a long length, 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/ ^m² ).
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 (degree of polymerization 4200, degree of saponification 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., trade name "Gosephymer Z410") in a 9:1 ratio.
A PVA aqueous solution was applied to the corona-treated surface of a resin substrate and dried at 60°C to form a PVA-based resin layer with a thickness of 13 μm, thereby producing a laminate.
The resulting laminate was uniaxially stretched 2.4 times in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds in an oven at 130°C (air-assisted stretching).
Next, the laminate was immersed for 30 seconds in an insolubilization bath at a liquid temperature of 40°C (a boric acid aqueous solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) (insolubilization treatment).
Next, the polarizing film was immersed for 60 seconds in a staining bath at a liquid temperature of 30°C (an iodine aqueous solution obtained by mixing iodine and potassium iodide in a weight ratio of 1:7 with 100 parts by weight of water) while adjusting the concentration so that the final polarizing film's single-element transmittance (Ts) was 40.5% (staining treatment).
Next, the material was immersed for 30 seconds in a crosslinking bath at a liquid temperature of 40°C (a boric acid aqueous solution obtained by mixing 3 parts by weight of potassium iodide and 5 parts by weight of boric acid with 100 parts by weight of water) (crosslinking treatment).
Subsequently, the laminate was immersed in a boric acid aqueous solution (boric acid concentration 4.0% by weight, potassium iodide 5.0% by weight) at a liquid temperature of 62°C, and uniaxially stretched between rolls with different peripheral speeds so that the total stretching ratio in the longitudinal direction was 3.0 times (underwater stretching treatment: the stretching ratio in the underwater stretching treatment was 1.25 times).
Subsequently, the laminate was immersed in a washing bath at a liquid temperature of 20°C (an aqueous solution obtained by mixing 4 parts by weight of potassium iodide with 100 parts by weight of water) (washing treatment).
Subsequently, the laminate was dried in an oven maintained at 90°C while being brought into contact with a SUS heated roll with a surface temperature maintained at 75°C for approximately 2 seconds (drying shrinkage treatment). The shrinkage rate in the width direction of the laminate due to the drying shrinkage treatment was 2%.
In this way, a polarizing film with a thickness of 7.4 μm was formed on the resin substrate.

The obtained polarizing film was measured for its single-layer transmittance, degree of polarization, and fracture strength. The results, along with the birefringence and in-plane phase difference of the PVA constituting the polarizing film, are summarized in Table 1.

[Examples 2-4]
A polarizing film (thickness: 7.4 μm) was formed on a resin substrate in the same manner as in Example 1, except that a staining bath with a different iodine concentration (weight ratio of iodine to potassium iodide = 1:7) was used. The obtained polarizing film was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[Examples 5-8]
A polarizing film (thickness: 6.7 μm) was formed on a resin substrate in the same manner as in Example 1, except that the stretching ratio in water was set to 1.46 times (resulting in a total stretching ratio of 3.5 times) and a dyeing bath with a different iodine concentration was used (weight ratio of iodine to potassium iodide = 1:7). The obtained polarizing film was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[Examples 9-12]
A polarizing film (thickness: 6.2 μm) was formed on a resin substrate in the same manner as in Example 1, except that the stretching ratio in water was set to 1.67 times (resulting in a total stretching ratio of 4.0 times) and a dyeing bath with a different iodine concentration was used (weight ratio of iodine to potassium iodide = 1:7). The obtained polarizing film was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[Examples 13-16]
A polarizing film (thickness: 6.0 μm) was formed on a resin substrate in the same manner as in Example 1, except that the stretching ratio in water was set to 1.88 times (resulting in a total stretching ratio of 4.5 times) and a dyeing bath with a different iodine concentration was used (weight ratio of iodine to potassium iodide = 1:7). The obtained polarizing film was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[Comparative Examples 1-4]
A polarizing film (thickness: 5.5 μm) was formed on a resin substrate in the same manner as in Example 1, except that the stretching ratio in water was set to 2.29 times (resulting in a total stretching ratio of 5.5 times) and a dyeing bath with a different iodine concentration was used (weight ratio of iodine to potassium iodide = 1:7). The obtained polarizing film was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

As is clear from Table 1, the polarizing films of the examples have practically acceptable transmittance and polarization degree, and satisfy equations (1) and (2). Such polarizing films exhibit very high puncture strength and resistance to tearing along the absorption axis.

Furthermore, Figures 3 and 4 show the relationship between the single-element transmittance of the polarizing films obtained in the examples and comparative examples and the Δn of the PVA, and the relationship between the single-element transmittance of the polarizing films and the in-plane phase difference of the PVA, respectively. As shown in Figures 3 and 4, even if the birefringence or in-plane phase difference is similar (resulting in similar orientation), it can be seen that when the single-element transmittance is high, the film is prone to tearing along the absorption axis. Therefore, in order to effectively suppress fracture of the polarizing film along the absorption axis, it is important to adjust the single-element transmittance (resulting in the amount of dichroic substance adsorbed) in addition to the orientation of the PVA-based resin. Furthermore, it can be seen that polarizing films satisfying equation (1) and/or equation (2) are those in which these adjustments have been suitably performed, and that fracture along the absorption axis can be suitably suppressed.

The polarizing film and polarizing plate of the present invention are suitably used in liquid crystal display devices.

10 Polarizing film 20 First protective layer 30 Second protective layer 100 Polarizing plate

Claims (6)

It is composed of a polyvinyl alcohol-based resin film containing a dichroic substance,
When the transmittance of the single element is x%, the birefringence of the polyvinyl alcohol-based resin is y, and the in-plane phase difference of the polyvinyl alcohol-based resin film is z nm, the following equations (1) and/or (2) are satisfied,
y<-0.011x+0.525 (1)
z<-60x+2875 (2)
A method for manufacturing a polarizing film having a single-layer transmittance of 41.0% or more and a polarization degree of 99.4% or more,
The process involves forming a laminate by creating a polyvinyl alcohol-based resin layer containing iodide or sodium chloride and a polyvinyl alcohol-based resin on one side of a long thermoplastic resin substrate, and then subjecting the laminate to the following processes in this order: air-assisted stretching, dyeing, underwater stretching, and drying shrinkage by heating while conveying in the longitudinal direction, thereby shrinking by 2% or more in the width direction.
The total stretching ratio of the aerial auxiliary stretching process and the underwater stretching process is 3.0 to 3.5 times the original length of the laminate.
The stretching ratio of the aerial auxiliary stretching process is greater than the stretching ratio of the underwater stretching process.
Manufacturing method. The method for manufacturing a polarizing film according to claim 1, wherein the ratio of the stretching ratio of the aerial assisted stretching to the stretching ratio of the underwater stretching (underwater stretching/aerial assisted stretching) is 0.4 to 0.9. The method for manufacturing a polarizing film according to claim 1, wherein the stretching ratio in the aerial assisted stretching is 2.0 to 3.0 times. The method for manufacturing a polarizing film according to claim 1, wherein the stretching ratio in the underwater stretching is 1.1 to 1.6 times. The method for manufacturing a polarizing film having a thickness of 10 μm or less, as described in claim 1. The polarizing film is a polarizing film having a puncture strength of 30 gf/μm or more.
The method for manufacturing a polarizing film according to claim 1, wherein the puncture strength is the strength required to break the polarizing film when a needle with a tip diameter of 1 mmφ and a radius of 0.5R is punctured into the polarizing film at a speed of 0.33 cm/second.