JP7502001B2 - Polarizing film, polarizing plate, and method for producing the polarizing film - Google Patents

Description

The present invention relates to a polarizing film, a polarizing plate, and a method for producing the polarizing film.

In a liquid crystal display device, which is a typical image display device, a polarizing film is arranged on both sides of a liquid crystal cell due to its image formation method. As a method for manufacturing a polarizing film, for example, a method has been proposed in which a laminate having a resin substrate and a polyvinyl alcohol (PVA)-based resin layer is stretched and then dyed to obtain a polarizing film on the resin substrate (for example, Patent Document 1). This method can obtain a thin polarizing film, and has attracted attention as a possible contribution to the thinning of image display devices in recent years. However, there is a demand for further improvement in the durability of thin polarizing films in high-temperature and high-humidity environments.

JP 2001-343521 A

The present invention has been made to solve the above-mentioned problems of the conventional art, and its main objective is to provide a polarizing film and a polarizing plate that have excellent durability in high-temperature and high-humidity environments, and a method for manufacturing such a polarizing film.

The polarizing film of the present invention is composed of a polyvinyl alcohol-based resin film containing iodine, and has an absorbance Abs ₂₄₀ at a wavelength of 470 nm after a durability test for 240 hours at a temperature of 60° C. and a relative humidity of 95%, which satisfies the following relationship with the absorbance Abs ₀ before the durability test:
_Abs240 / _Abs0 >0.90
In one embodiment, the polarizing film has a single transmittance of 43.0% or more.
In one embodiment, the polarizing film has a thickness of 8 μm or less.
According to another aspect of the present invention, there is provided a polarizing plate having the above polarizing film and a protective layer disposed on at least one side of the polarizing film.
According to yet another aspect of the present invention, there is provided a method for producing the polarizing film, comprising the steps of forming a polyvinyl alcohol-based resin layer on one side of a long thermoplastic resin substrate to form a laminate, stretching and dyeing the laminate to form the polyvinyl alcohol-based resin layer into a polarizing film, and contacting the polarizing film with a treatment liquid having a pH of 3.0 or less.
In one embodiment, the method includes applying the treatment liquid to the polarizing film, hi another embodiment, the method includes immersing the polarizing film in the treatment liquid.
In one embodiment, the production method includes forming a polyvinyl alcohol-based resin layer containing iodide or sodium chloride and a polyvinyl alcohol-based resin on one side of the thermoplastic resin substrate.
In one embodiment, the manufacturing method includes 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 is heated while being transported in the longitudinal direction, thereby shrinking the laminate by 2% or more in the width direction, in that order.
In one embodiment, the drying and shrinking treatment is performed using a heating roll. In this case, the temperature of the heating roll is, for example, 60° C. to 120° C.
Another method for producing a polarizing film of the present invention includes: stretching and dyeing a polyvinyl alcohol-based resin film to form the polyvinyl alcohol-based resin film into a polarizing film; and contacting the polarizing film with a treatment liquid having a pH of 3.0 or less.

According to the present invention, a polarizing film having excellent durability in a high-temperature and high-humidity environment can be obtained by contacting the polarizing film with a treatment liquid having a pH of 3.0 or less. Specifically, the polarizing film according to the embodiment of the present invention has an absorbance Abs ₂₄₀ at a wavelength of 470 nm after a durability test for 240 hours at a temperature of 60° C. and a relative humidity of 95%, which satisfies the following relationship with the absorbance Abs ₀ before the durability test:
_Abs240 / _Abs0 >0.90
That is, the polarizing film according to the embodiment of the present invention does not show a significant decrease in absorbance at a wavelength of 470 nm even after a heat and humidity durability test. This means that the polarizing film according to the embodiment of the present invention suppresses the decrease in polarization performance in a high-temperature and high-humidity environment to a practically acceptable level. Although the polarization performance of a polarizing film (particularly a thin polarizing film) usually decreases significantly in a high-temperature and high-humidity environment, the embodiment of the present invention solves this problem and can provide a polarizing film (particularly a thin polarizing film) that has excellent durability in a high-temperature and high-humidity environment.

1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. FIG. 2 is a schematic diagram showing an example of a drying shrinkage treatment using a heating roll.

The following describes embodiments of the present invention, but the present invention is not limited to these embodiments.

A. Polarizing Film A polarizing film according to an embodiment of the present invention is made of a polyvinyl alcohol (PVA)-based resin film containing iodine, and its absorbance Abs ₂₄₀ at a wavelength of 470 nm after a durability test for 240 hours at a temperature of 60° C. and a relative humidity of 95% satisfies the following relationship with respect to its absorbance Abs ₀ before the durability test:
_Abs240 / _Abs0 >0.90
This indicates that in the polarizing film according to the embodiment of the present invention, destruction of the PVA-I ₃ ^-complex having absorption near 470 nm during a heat and humidification durability test is suppressed. Although not theoretically clear, such excellent durability can be achieved by contacting the polarizing film with a treatment liquid having a pH of 3.0 or less. Abs ₂₄₀ /Abs ₀ is preferably 0.92 or more, more preferably 0.93 or more, and even more preferably 0.95 or more. The upper limit of Abs ₂₄₀ /Abs ₀ may be, for example, 1.50. The absorbance is typically orthogonal absorbance. The orthogonal absorbance is calculated from the following formula based on the orthogonal transmittance Tc measured when calculating the degree of polarization described below.
Orthogonal absorbance = log10 (100/Tc)
The absorbance Abs ₀ before the durability test is the absorbance of the polarizing film in a normal state, and the Abs ₀ of the polarizing film at a wavelength of 470 nm is, for example, less than 5.0, preferably 3.0 or less, and more preferably 2.2 or less. The lower limit of Abs ₀ may be, for example, 1.0.

In one embodiment, the polarizing film has an absorbance Abs ₂₄₀ at a wavelength of 600 nm after a durability test for 240 hours at a temperature of 60° C. and a relative humidity of 95%, which satisfies the following relationship with the absorbance Abs ₀ before the durability test:
_Abs240 / _Abs0 >1.00
This indicates that in the polarizing film according to the embodiment of the present invention, the PVA-I ₅ ^-complex having absorption near 600 nm is not destroyed even in a heat and humidity durability test, but rather can be increased. The PVA-I ₅ ^-complex is destroyed in a high-temperature and high-humidity environment, and the polarization performance of a polarizing film is usually expected to decrease in a high-temperature and high-humidity environment, so the excellent durability of the polarizing film according to the embodiment of the present invention is unexpectedly excellent. Abs ₂₄₀ /Abs ₀ is preferably 1.05 or more, more preferably 1.10 or more, even more preferably 1.15 or more, particularly preferably 1.20 or more, and particularly preferably 1.25 or more. The upper limit of Abs ₂₄₀ /Abs ₀ can be, for example, 2.00. The Abs ₀ of the polarizing film at a wavelength of 600 nm is, for example, less than 5.0, preferably 4.3 or less, more preferably 4.0 or less. The lower limit of Abs ₀ can be, for example, 2.0.

The thickness of the polarizing film is preferably 8 μm or less, more preferably 7 μm or less, even more preferably 5 μm or less, and particularly preferably 3 μm or less. The lower limit of the thickness of the polarizing film may be 1 μm in one embodiment, and 2 μm in another embodiment. As described below, such a thickness can be achieved, for example, by producing a polarizing film using a laminate of a resin substrate and a PVA-based resin layer formed by coating on the resin substrate. When the polarizing film is produced from a single resin film, the thickness of the polarizing film can be, for example, 12 μm to 35 μm.

The polarizing film preferably exhibits absorption dichroism at any wavelength of 380 nm to 780 nm. The single transmittance of the polarizing film is preferably 42.0% or more, more preferably 42.5% or more, even more preferably 43.0% or more, particularly preferably 43.5% or more, and particularly preferably 44.0% or more. On the other hand, the single transmittance is preferably 47.0% or less, more preferably 46.0% or less. The polarization degree of the polarizing film is preferably 99.95% or more, more preferably 99.99% or more. On the other hand, the polarization degree is preferably 99.998% or less. According to the embodiment of the present invention, it is possible to achieve both a high single transmittance and a high polarization degree, and to realize excellent durability in a high temperature and high humidity environment as described above. The single transmittance is typically a Y value measured using an ultraviolet-visible spectrophotometer and corrected for luminosity. The single transmittance is a value 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 polarization degree is typically calculated by the following formula based on the parallel transmittance Tp and the crossed transmittance Tc measured using an ultraviolet-visible spectrophotometer and subjected to luminosity correction.
Degree of polarization (%)={(Tp−Tc)/(Tp+Tc)} ^1/2 ×100

In one embodiment, the transmittance (single transmittance) of a thin polarizing film of 8 μm or less is typically measured using an ultraviolet-visible spectrophotometer with a laminate of a polarizing film (refractive index of surface: 1.53) and a protective layer (protective film) (refractive index: 1.50) as the measurement target. Depending on the refractive index of the surface of the polarizing film and/or the refractive index of the surface in contact with the air interface of the protective layer, the reflectance at the interface of each layer may change, and as a result, the measured value of the transmittance may change. Therefore, for example, when a protective layer having a refractive index other than 1.50 is used, the measured value of the transmittance may be corrected depending on the refractive index of the surface in contact with the air interface of the protective layer. Specifically, the correction value C of the transmittance is expressed by the following formula using the reflectance R ₁ (transmission axis reflectance) of polarized light parallel to the transmission axis at the interface between the protective layer and the air layer.
C=R ₁ -R ₀
R ₀ = ((1.50 - 1) ² / (1.50 + 1) ² ) × (T ₁ / 100)
_R1 = (( _n1 - 1) ² / ( _n1 + 1) ² ) x ( _T1 / 100)
Here, R ₀ is the transmission axis reflectance when a protective layer with a refractive index of 1.50 is used, n ₁ is the refractive index of the protective layer used, and T ₁ is the transmittance of the polarizing film. For example, when a substrate with a surface refractive index of 1.53 (such as a cycloolefin film or a film with a hard coat layer) is used as a protective layer, the correction amount C is about 0.2%. In this case, by adding 0.2% to the transmittance obtained by measurement, it is possible to convert a polarizing film with a surface refractive index of 1.53 into the transmittance when a protective layer with a refractive index of 1.50 is used. According to the calculation based on the above formula, the change in the correction value C when the transmittance T ₁ of the polarizing film is changed by 2% is 0.03% or less, and the influence of the transmittance of the polarizing film on the value of the correction value C is limited. In addition, when the protective layer has absorption other than surface reflection, an appropriate correction can be made according to the amount of absorption.

The polarizing film may be produced using a single resin film, or may be produced using a laminate of two or more layers. A specific example of a polarizing film obtained using a laminate is a polarizing film obtained using a laminate of 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 produced, for example, by applying a PVA-based resin solution to a resin substrate, drying the substrate 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; stretching and dyeing the laminate to make the PVA-based resin layer into a polarizing film. In an embodiment of the present invention, the polarizing film is brought into contact with a treatment liquid having a pH of 3.0 or less. This makes it possible to achieve excellent durability in a high-temperature and high-humidity environment as described above. 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. The stretching typically includes immersing the laminate in an aqueous boric acid solution and stretching it. Furthermore, the stretching may further include air-stretching the laminate at a high temperature (e.g., 95°C or higher) before stretching in the boric acid aqueous solution, if necessary. In addition, in this embodiment, the laminate is preferably subjected to a drying shrinkage treatment in which the laminate is heated while being conveyed in the longitudinal direction, thereby shrinking the laminate by 2% or more in the width direction. Typically, the manufacturing method of this embodiment includes subjecting the laminate to an air-assisted stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment in this order. By introducing the auxiliary stretching, it is possible to increase the crystallinity of PVA even when PVA is applied onto a thermoplastic resin, and it is possible to achieve high optical properties. At the same time, by increasing the orientation of PVA in advance, problems such as a decrease in the orientation of PVA or dissolution can be prevented when the PVA is immersed in water in the subsequent dyeing step or stretching step, and it is possible to achieve high optical properties. Furthermore, 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 a treatment process in which the laminate is immersed in a liquid, such as a dyeing treatment and an underwater stretching treatment. Furthermore, the optical properties can be improved by shrinking the laminate in the width direction by a drying shrinkage treatment. The obtained 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 from the resin substrate/polarizing film laminate and any suitable protective layer depending on the purpose may be laminated on the peeled surface. Details of the manufacturing method of the polarizing film will be described later in Section C.

B. Polarizing Plate FIG. 1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. The polarizing plate 100 has a polarizing film 10, a first protective layer 20 arranged on one side of the polarizing film 10, and a second protective layer 30 arranged on the other side of the polarizing film 10. The polarizing film 10 is the polarizing film of the present invention described in the above section A. One of the first protective layer 20 and the second protective layer 30 may be omitted. 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 first and second protective layers are formed of any suitable film that can be used as a protective layer for a polarizing film. Specific examples of materials that are the main components of the film include cellulose-based resins such as triacetyl cellulose (TAC), and transparent resins such as polyesters, polyvinyl alcohols, polycarbonates, polyamides, polyimides, polyethersulfones, polysulfones, polystyrenes, polynorbornenes, polyolefins, (meth)acrylics, and acetates. Other examples include thermosetting resins or ultraviolet-curing resins such as (meth)acrylics, urethanes, (meth)acrylic urethanes, epoxy resins, and silicones. Other examples include glassy polymers such as siloxane polymers. Polymer films described in JP 2001-343529 A (WO01/37007) can also be used. The material for this film can be, for example, a resin composition containing a thermoplastic resin having a substituted or unsubstituted imide group in the side chain and a thermoplastic resin having a substituted or unsubstituted phenyl group and a nitrile group in the side chain, such as a resin composition having an alternating copolymer of isobutene and N-methylmaleimide, and an acrylonitrile-styrene copolymer. The polymer film can be, for example, an extrusion molded product of the above resin composition.

When the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (outer protective layer) disposed on the opposite side 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. Note that, if a surface treatment has been 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) arranged 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 retardation layer having any appropriate retardation value. In this case, the in-plane retardation Re(550) of the retardation layer is, for example, 110 nm to 150 nm. "Re(550)" is the in-plane retardation measured with light having a wavelength of 550 nm at 23°C, and is calculated by the formula: Re = (nx - ny) x d. Here, "nx" is the refractive index in the direction in which the in-plane refractive index is maximum (i.e., the slow axis direction), "ny" is the refractive index in the direction perpendicular to the slow axis in the plane (i.e., the fast axis direction), "nz" is the refractive index in the thickness direction, and "d" is the thickness (nm) of the layer (film).

C. Manufacturing method of polarizing film A manufacturing method of a polarizing film according to one embodiment of the present invention includes: applying a PVA-based resin solution to one side of a long thermoplastic resin substrate and drying the solution to form a PVA-based resin layer to form a laminate; stretching and dyeing the laminate to form the PVA-based resin layer into a polarizing film; and contacting the polarizing film with a treatment liquid having a pH of 3.0 or less. By contacting the polarizing film with a treatment liquid having a pH of 3.0 or less, a polarizing film having excellent durability in a high-temperature and high-humidity environment can be realized. Preferably, the PVA-based resin solution further contains a halide. Preferably, the manufacturing method includes subjecting the laminate to an auxiliary air-stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment in which the laminate is heated while being transported in the longitudinal direction to shrink the laminate by 2% or more in the width direction, in this order. The content of the halide in the PVA-based resin solution (as a result, 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 treatment is preferably carried out using a heating roll, and the temperature of the heating roll is preferably 60° C. to 120° C. The shrinkage rate in the width direction of the laminate due to the drying shrinkage treatment is preferably 2% or more. According to such a production method, the polarizing film described in the above item A can be obtained. In particular, a laminate including a PVA-based resin layer containing a halide is produced, the laminate is stretched in multiple stages including auxiliary air stretching and underwater stretching, and the laminate after stretching is heated with a heating roll, whereby a polarizing film having excellent optical properties (typically, single transmittance and unit absorbance) can be obtained.

C-1. Preparation of Laminate Any appropriate method may be adopted as a method for preparing 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 to form a PVA-based resin layer on the thermoplastic resin substrate. As described above, the content of the halide 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 appropriate method can be used to apply the coating liquid. 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 liquid is preferably 50°C or higher.

The thickness of the PVA-based resin layer is preferably 3 μm to 40 μm, and more preferably 3 μm to 20 μm.

Before forming the PVA-based resin layer, the thermoplastic resin substrate may be subjected to a surface treatment (e.g., corona treatment, etc.), or an easy-adhesion layer may be formed on the thermoplastic resin substrate. By carrying out such treatment, it is possible to 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 may be used as the thermoplastic resin substrate. Details of the thermoplastic resin substrate are described in, for example, JP 2012-73580 A. The entire disclosure of this publication is incorporated herein by reference.

C-1-2. Coating liquid The coating liquid contains a halide and a PVA-based resin as described above. The coating liquid is typically a solution in which the halide and the PVA-based resin are dissolved in a solvent. Examples of the solvent include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine. These can be used alone or in combination of two or more. Of these, water is preferred. 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. With such a resin concentration, a uniform coating film that is in close contact with the thermoplastic resin substrate can be formed. The content of the halide in the coating liquid is preferably 5 to 20 parts by weight relative to 100 parts by weight of the PVA-based resin.

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

Any suitable resin may be used as the PVA-based resin. Examples include polyvinyl alcohol and ethylene-vinyl alcohol copolymer. Polyvinyl alcohol is obtained by saponifying polyvinyl acetate. Ethylene-vinyl alcohol copolymer is obtained by saponifying ethylene-vinyl acetate copolymer. The saponification degree of the PVA-based resin is usually 85 mol% to 100 mol%, preferably 95.0 mol% to 99.95 mol%, and more preferably 99.0 mol% to 99.93 mol%. The saponification degree can be determined in accordance with JIS K 6726-1994. By using a PVA-based resin with such a saponification degree, a polarizing film with excellent durability can be obtained. If the saponification degree is too high, there is a risk of gelation.

The average degree of polymerization of the PVA-based resin can be appropriately selected depending on the purpose. The average degree of polymerization is usually 1,000 to 10,000, preferably 1,200 to 4,500, and more preferably 1,500 to 4,300. The average degree of polymerization can be determined in accordance with JIS K 6726-1994.

As the halide, any suitable halide may be used. Examples include iodide and sodium chloride. Examples of iodide include potassium iodide, sodium iodide, and lithium iodide. Of these, potassium iodide is preferred.

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

Generally, the orientation of the polyvinyl alcohol molecules in the PVA-based resin increases when the PVA-based resin layer is stretched, but when the stretched PVA-based resin layer is immersed in a liquid containing water, the orientation of the polyvinyl alcohol molecules may become disordered, and the orientation may decrease. In particular, when a laminate of a thermoplastic resin and a PVA-based resin layer is stretched in boric acid water, the tendency for the degree of orientation to decrease is remarkable when the laminate is stretched in boric acid water at a relatively high temperature to stabilize the stretching of the thermoplastic resin. For example, while a PVA film alone is generally stretched in boric acid water at 60°C, a laminate of A-PET (thermoplastic resin substrate) and a PVA-based resin layer is stretched at a high temperature of around 70°C, in this case, the orientation of the PVA at the beginning of the stretching may decrease before it increases due to the stretching in water. In response to this, by preparing a laminate of a PVA-based resin layer containing a halide and a thermoplastic resin substrate, and then stretching the laminate at high temperature (auxiliary stretching) in air before stretching it in boric acid water, 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 orientation of the polyvinyl alcohol molecules can be prevented from being disturbed and the orientation can be prevented from being reduced, compared to when the PVA-based resin layer does not contain a halide. This can improve the optical properties of the polarizing film obtained by immersing the laminate in a liquid through a process such as a dyeing process and an underwater stretching process.

C-2. Auxiliary Air Stretching Treatment In particular, in order to obtain high optical properties, a two-stage stretching method is selected that combines dry stretching (auxiliary stretching) and stretching in boric acid water. By introducing auxiliary stretching like two-stage stretching, it is possible to stretch while suppressing crystallization of the thermoplastic resin substrate, which solves the problem of the thermoplastic resin substrate being excessively crystallized and reduced in stretchability in the subsequent stretching in boric acid water, and the laminate can be stretched at a higher magnification. Furthermore, when applying 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, it is necessary to lower the application temperature compared to when applying a PVA-based resin onto a normal metal drum, which may result in a problem that the crystallization of the PVA-based resin is relatively low and sufficient optical properties cannot be obtained. On the other hand, by introducing auxiliary stretching, it is possible to increase the crystallinity of the PVA-based resin even when applying a PVA-based resin onto a thermoplastic resin, and it is possible to achieve high optical properties. At the same time, by increasing the orientation of the PVA-based resin in advance, problems such as a decrease in the orientation of the PVA-based resin or dissolution when the PVA-based resin is immersed in water in the subsequent dyeing and stretching processes can be prevented, and high optical properties can be achieved.

The stretching method of the auxiliary air stretching may be fixed end stretching (for example, a method of stretching using a tenter stretching machine) or free end stretching (for example, a method of uniaxially stretching a laminate between rolls with different peripheral speeds), but in order to obtain high optical properties, free end stretching may be actively adopted. In one embodiment, the air stretching process includes a heated roll stretching process in which the laminate is stretched by the difference in peripheral speed between heated rolls while being transported in its longitudinal direction. The air stretching process typically includes a zone stretching process and a heated roll stretching process. The order of the zone stretching process and the heated roll stretching process is not limited, and the zone stretching process may be performed first, or the heated roll stretching process may be performed first. The zone stretching process may be omitted. In one embodiment, the zone stretching process and the heated roll stretching process are performed in this order. In another embodiment, the film is stretched by gripping the film end in a tenter stretching machine and widening the distance between the tenters in the flow direction (the widening of the distance between the tenters is the stretching ratio). In this case, the tenter distance in the width direction (perpendicular to the machine direction) is set to be as close as possible. Preferably, it can be set to be as close as possible to the free end stretching ratio in the machine direction. In the case of free end stretching, the shrinkage ratio in the width direction is calculated as follows: (1/stretching ratio) ^1/2 .

The air-assisted stretching may be performed in one step or in multiple steps. When the air-assisted stretching is performed in multiple steps, the stretching ratio is the product of the stretching ratios in each step. The stretching direction in the air-assisted stretching is preferably approximately the same as the stretching direction in the underwater stretching.

The stretching ratio in the air-assisted stretching is preferably 2.0 to 3.5 times. The maximum stretching ratio when air-assisted stretching and underwater stretching are combined is preferably 5.0 times or more, more preferably 5.5 times or more, and even more preferably 6.0 times or more, relative to the original length of the laminate. In this specification, the "maximum stretching ratio" refers to the stretching ratio just before the laminate breaks, and refers to a value 0.2 lower than the stretching ratio at which the laminate breaks, which is determined separately.

The stretching temperature for the in-air auxiliary stretching can be set to any appropriate value depending on the material of the thermoplastic resin substrate, the stretching method, etc. The stretching temperature is preferably equal to or higher than the glass transition temperature (Tg) of the thermoplastic resin substrate, more preferably equal to or higher than the glass transition temperature (Tg) of the thermoplastic resin substrate + 10°C, and particularly preferably equal to or higher than Tg + 15°C. On the other hand, the upper limit of the stretching temperature is preferably 170°C. By stretching at such a temperature, the crystallization of the PVA-based resin can be prevented from progressing rapidly, and problems caused by the crystallization (for example, preventing the orientation of the PVA-based resin layer by stretching) can be prevented.

C-3. Insolubilization treatment, dyeing treatment, and crosslinking treatment If necessary, an insolubilization treatment is performed after the auxiliary air stretching treatment and before the underwater stretching treatment or the dyeing treatment. The insolubilization treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. The 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 underwater stretching treatment. The crosslinking treatment is typically performed by immersing the PVA-based resin layer in an aqueous boric acid solution. Details of the insolubilization treatment, dyeing treatment, and crosslinking treatment are described, for example, in JP-A-2012-73580 (above).

C-4. Underwater stretching treatment Underwater stretching treatment is performed by immersing the laminate in a stretching bath. According to the underwater stretching treatment, stretching can be performed at a temperature lower than the glass transition temperature (typically, about 80° C.) of the thermoplastic resin substrate or the PVA-based resin layer, and the PVA-based resin layer can be stretched at a high magnification while suppressing crystallization. As a result, a polarizing film having excellent optical properties can be produced.

The stretching method of the laminate can be any appropriate method. Specifically, it may be fixed-end stretching or free-end stretching (for example, a method of uniaxially stretching the laminate by passing it between rolls with different peripheral speeds). Free-end stretching is preferably selected. The stretching of the laminate may be performed in one stage or in multiple stages. When it is performed in multiple stages, the stretching ratio (maximum stretching ratio) of the laminate described below is the product of the stretching ratios in each stage.

The underwater stretching is preferably performed by immersing the laminate in an aqueous solution of boric acid (underwater stretching in boric acid). By using an aqueous solution of boric acid as the stretching bath, it is possible to impart to the PVA-based resin layer the rigidity required to withstand the tension applied during stretching, and the water resistance required to prevent dissolution in water. Specifically, boric acid can generate tetrahydroxyborate anions in the aqueous solution and crosslink with the PVA-based resin through hydrogen bonds. As a result, the PVA-based resin layer is imparted with rigidity and water resistance, allowing it to be stretched well, and a polarizing film with excellent optical properties can be produced.

The boric acid aqueous solution is preferably obtained by dissolving boric acid and/or a borate in water, which is a 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 making the boric acid concentration 1 part by weight or more, dissolution of the PVA-based resin layer can be effectively suppressed, and a polarizing film with higher characteristics can be produced. In addition to boric acid or a borate, an aqueous solution obtained by dissolving a boron compound such as borax, glyoxal, glutaraldehyde, or the like in a solvent can also be used.

Preferably, iodide is added to the stretching bath (boric acid aqueous solution). By adding iodide, it is possible to suppress the elution of iodine adsorbed in the PVA-based 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 such a temperature, the PVA-based resin layer can be stretched at a high ratio while suppressing dissolution. Specifically, as described above, the glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 60°C or higher in relation to the formation of the PVA-based resin layer. In this case, if the stretching temperature is below 40°C, even if the plasticization of the thermoplastic resin substrate by water is taken into consideration, there is a risk that the substrate cannot be stretched well. On the other hand, the higher the temperature of the stretching bath, the higher the solubility of the PVA-based resin layer, and the higher the risk that excellent optical properties cannot be obtained. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

The stretching ratio in underwater stretching is preferably 1.5 times or more, more preferably 3.0 times or more. The total stretching ratio of the laminate is preferably 5.0 times or more, more preferably 5.5 times or more, relative to the original length of the laminate. By achieving such a high stretching ratio, a polarizing film with extremely excellent optical properties can be produced. Such a high stretching ratio can be achieved by employing the underwater stretching method (stretching in boric acid water).

C-5. Drying and shrinking treatment The drying and shrinking treatment may be performed by zone heating, which heats the entire zone, or by heating the transport roll (using a so-called heating roll) (heating roll drying method). Preferably, both are used. By drying using a heating roll, it is possible to efficiently suppress the heat curl of the laminate and produce a polarizing film with excellent appearance. Specifically, by drying the laminate in a state where it is aligned with the heating roll, it is possible to efficiently promote the crystallization of the thermoplastic resin substrate and increase the crystallinity, and even at a relatively low drying temperature, it is possible to satisfactorily increase the crystallinity of the thermoplastic resin substrate. As a result, the rigidity of the thermoplastic resin substrate increases and it becomes in a state where it can withstand the shrinkage of the PVA-based resin layer due to drying, and curling is suppressed. In addition, by using a heating roll, it is possible to dry the laminate while maintaining it in a flat state, so that it is possible to suppress not only curling but also wrinkles. At this time, the laminate can be shrunk in the width direction by the drying and shrinking treatment, thereby improving the optical properties. This is because it is possible to effectively increase the orientation of the PVA and the PVA/iodine complex. 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 the drying shrinkage process. In the drying shrinkage process, the laminate 200 is dried while being transported by transport rolls R1 to R6 heated to a predetermined temperature and guide rolls G1 to G4. In the illustrated example, the transport rolls R1 to R6 are arranged so as to alternately and continuously heat the surface of the PVA resin layer and the surface of the thermoplastic resin substrate, but, for example, the transport rolls R1 to R6 may be arranged so as to continuously heat only one surface of the laminate 200 (for example, the thermoplastic resin substrate surface).

Drying conditions can be controlled by adjusting the heating temperature of the transport rolls (temperature of the heating rolls), the number of heating rolls, the contact time with the heating rolls, etc. The temperature of the heating rolls is preferably 60°C to 120°C, more preferably 65°C to 100°C, and particularly preferably 70°C to 80°C. The crystallization degree of the thermoplastic resin can be increased well, curling can be suppressed well, and an optical laminate with extremely excellent durability can be produced. The temperature of the heating rolls can be measured by a contact thermometer. In the illustrated example, six transport rolls are provided, but there is no particular restriction as long as there are multiple transport rolls. The number of transport rolls is usually 2 to 40, preferably 4 to 30. The contact time (total contact time) between the laminate and the heating rolls is preferably 1 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 means. By using both drying with the heating rolls and hot air drying, it is possible to suppress abrupt temperature changes between the heating rolls, and it is possible to easily control shrinkage in the width direction. The hot air drying temperature is preferably 30°C to 100°C. The hot air drying time is preferably 1 second to 300 seconds. The hot air speed is preferably about 10 m/s to 30 m/s. The air speed is the air speed in the heating furnace, and can be measured by a mini-vane type digital anemometer.

C-6. Contact with treatment liquid As described above, a laminate of a thermoplastic resin substrate and a polarizing film can be obtained. In an embodiment of the present invention, the polarizing film is contacted with a treatment liquid having a pH of 3.0 or less. In one embodiment, the laminate can be directly contacted with the treatment liquid to bring the polarizing film into contact with the treatment liquid. In this case, typically, the thermoplastic resin substrate can be directly used as a protective layer for the polarizing film. Alternatively, a resin film (to be a protective layer) may be attached to the polarizing film surface of the laminate that has been contacted with the treatment liquid to prepare a laminate of protective layer/polarizing film/thermoplastic resin substrate, and the thermoplastic resin substrate may be peeled off from the laminate to prepare a polarizing plate having a configuration of protective layer/polarizing film. In another embodiment, a resin film (to be a protective layer) is attached to the polarizing film surface of the laminate to prepare a laminate of protective layer/polarizing film/thermoplastic resin substrate, and the thermoplastic resin substrate is peeled off from the laminate to prepare a laminate of protective layer/polarizing film (polarizing plate). The polarizing film can be contacted with the treatment liquid by contacting the obtained polarizing plate with the treatment liquid.

The polarizing film may be brought into contact with the treatment liquid by any suitable method. Representative examples include application of the treatment liquid to the polarizing film and immersion of the polarizing film (effectively, a laminate or a polarizing plate) in the treatment liquid. Any suitable method may be adopted as the application method. Specific examples include the method described in section C-1 as the application method of the coating liquid. Immersion may also be carried out by any suitable manner. For example, the treatment liquid may be added to the cleaning bath of the cleaning treatment, a bath of the treatment liquid may be used instead of the cleaning bath, or a bath of the treatment liquid may be provided separately from the cleaning bath. Note that the cleaning treatment is typically carried out after the underwater stretching treatment and before the drying shrinkage treatment. When a separate bath of the treatment liquid is provided, the bath of the treatment liquid may be provided between the cleaning bath and the drying shrinkage treatment equipment (i.e., contact with the treatment liquid may be carried out between the cleaning treatment and the drying shrinkage treatment), or may be provided downstream of the means for peeling off the thermoplastic resin substrate (i.e., contact with the treatment liquid may be carried out after the thermoplastic resin substrate is peeled off).

As the treatment liquid, any suitable acidic liquid can be used as long as the pH is 3.0 or less. Specific examples of treatment liquids include hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and citric acid. The treatment liquid is preferably an aqueous solution of a strong acid. Specific examples of strong acids include hydrochloric acid, sulfuric acid, and nitric acid. The lower the pH of the treatment liquid (the stronger the acidity), the more preferable it is. Specifically, the pH is preferably 2.7 or less, more preferably 2.5 or less, even more preferably 2.0 or less, and particularly preferably 1.5 or less.

The acid concentration of the treatment solution is preferably 0.02% to 3.0% by weight, more preferably 0.04% to 2.0% by weight, and even more preferably 0.1% to 1.0% by weight.

The treatment liquid may contain a water-soluble resin (e.g., a PVA-based resin). The water-soluble resin may function as a binder. The concentration of the water-soluble resin in the treatment liquid is preferably 3% by weight to 5% by weight. In this case, a treatment layer can be formed by applying and drying the treatment liquid. By forming such a treatment layer, a polarizing film having the desired durability can be obtained. The thickness of the treatment layer is preferably 1.7 μm or less, and more preferably 0.2 μm to 1.4 μm.

After contact with the treatment liquid, drying may be carried out as necessary. The drying temperature is preferably 40°C to 90°C, and more preferably 50°C to 70°C.

C-7. Modifications Sections C-1 to C-6 have described a manufacturing method using a laminate of a resin substrate and a PVA-based resin layer formed by coating on the resin substrate, but the present invention can also be applied to a manufacturing method using a single PVA-based resin film. Such a manufacturing method typically involves subjecting a long PVA-based resin film to swelling, dyeing, crosslinking and washing treatments while uniaxially stretching the film in the long direction using a roll stretching machine, and finally subjecting the film to a drying treatment. The contact with the treatment liquid can typically be performed by immersion in a washing bath containing the treatment liquid, immersion in a treatment bath after washing, or application of the treatment liquid after washing.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. The methods for measuring each property are as follows. In the examples and comparative examples, "parts" and "%" are by weight unless otherwise specified.
(1) Thickness: Measurement was performed using an interference thickness meter (manufactured by Otsuka Electronics Co., Ltd., product name "MCPD-3000").
(2) Single transmittance and cross absorbance For the polarizing plates (protective layer/polarizing film) of the Examples and Comparative Examples, the single transmittance Ts, parallel transmittance Tp, and cross transmittance Tc measured using an ultraviolet-visible spectrophotometer (LPF-200 manufactured by Otsuka Electronics) were taken as Ts, Tp, and Tc of the polarizing film, respectively. These Ts, Tp, and Tc are Y values measured using a 2-degree visual field (C light source) according to JIS Z8701 and corrected for visibility. The refractive index of the protective film was 1.50, and the refractive index of the surface of the polarizing film opposite to the protective film was 1.53.
In addition, the orthogonal absorbance was calculated using the measured Tc at each wavelength according to the following formula.
Orthogonal absorbance = log10 (100/Tc)
The orthogonal absorbance Abs ₀ was determined from the orthogonal transmittance Tc at a measurement wavelength of 470 nm using an "LPF-200" manufactured by Otsuka Electronics Co., Ltd. Note that it is possible to perform equivalent measurements of Abs ₀ using an "V-7100" manufactured by JASCO Corporation.
Next, the polarizing plate was subjected to a durability test at a temperature of 60° C. and a relative humidity of 95% for 240 hours. After the durability test, the orthogonal absorbance Abs ₂₄₀ was determined in the same manner as above.

[Example 1]
The thermoplastic resin substrate used was a long amorphous isophthalic copolymerized polyethylene terephthalate film (thickness: 100 μm) having a water absorption rate of 0.75% and a Tg of about 75° C. One side of the resin substrate was subjected to a corona treatment (treatment conditions: 55 W·min/m ² ).
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 4,200, degree of saponification 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., product name "GOHSEFFIMER Z410") in a ratio of 9:1.
The above 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 having a thickness of 20 μm, thereby producing a laminate.
The obtained laminate was uniaxially stretched at its free end to 2.4 times its original size in the machine direction (longitudinal direction) between rolls having different peripheral speeds in an oven at 130° C. (auxiliary air stretching treatment).
Next, the laminate was immersed in an insolubilizing bath (a boric acid aqueous solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40° C. for 30 seconds (insolubilizing treatment).
Next, the film was immersed in a dye bath (an aqueous iodine solution obtained by mixing iodine and potassium iodide in a weight ratio of 1:7 with 100 parts by weight of water) at a liquid temperature of 30° C. for 60 seconds while adjusting the concentration so that the single transmittance (Ts) of the final polarizing plate would be 44.0% (dyeing treatment).
Next, the plate was immersed in a crosslinking bath (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) at a liquid temperature of 40° C. for 30 seconds (crosslinking treatment).
Thereafter, the laminate was immersed in an aqueous boric acid solution (boric acid concentration: 4.0 wt %, potassium iodide: 5 wt %) at a liquid temperature of 70° C., and uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls having different peripheral speeds to a total stretch ratio of 5.5 times (underwater stretching treatment).
Thereafter, the laminate was immersed in a cleaning bath (an aqueous solution obtained by mixing 4 parts by weight of potassium iodide with 100 parts by weight of water, pH=6) at a liquid temperature of 20° C. (cleaning treatment).
Thereafter, while drying in an oven maintained at 90° C., the laminate was brought into contact with a SUS heated roll having a surface temperature maintained at 75° C. for about 2 seconds (drying shrinkage treatment). The shrinkage rate of the laminate in the width direction due to the drying shrinkage treatment was 2%.
In this way, a polarizing film having a thickness of 5.0 μm was formed on the resin substrate, a cycloolefin film (manufactured by ZEON, product name "G-Film") was attached to the surface of the polarizing film as a protective layer (protective film) with a UV-curable adhesive (thickness 1.0 μm), and then the resin substrate was peeled off to obtain a laminate having a protective layer/polarizing film configuration. The single transmittance (Ts) of the obtained laminate was 44.0%, which is a value obtained by correcting the actual measured value by +0.2% and converting it to a state of 1.53/1.50 because the surface refractive index of the polarizing film/protective layer constituting the laminate is 1.53/1.53.
Next, a treatment liquid (pH = 1.3) obtained by dissolving 0.3 wt % of hydrochloric acid and 3.5 wt % of PVA (JC-25) in water was applied to the surface of the polarizing film of the laminate to a thickness of 0.6 μm, and dried at 60° C. for 4 minutes to form a treatment layer.
In this manner, the polarizing plate of this example was obtained.

The single transmittance and Abs ₂₄₀ /Abs ₀ of the obtained polarizing plate (effectively a polarizing film) are shown in Table 1.

[Examples 2 to 10]
A polarizing plate was produced by adjusting the single transmittance of the polarizing film, the method of contacting with the treatment liquid, the pH of the treatment liquid, the type of acid contained in the treatment liquid, and the thickness of the treatment layer as shown in Table 1. Table 1 shows the single transmittance and Abs ₂₄₀ /Abs ₀ of the obtained polarizing plate (effectively a polarizing film).

[Example 11]
A polarizing plate was produced in the same manner as in Example 1, except that the treatment solution did not contain a PVA-based resin (i.e., no treatment layer was formed) and that the pH of the treatment solution was 0.9. Table 1 shows the single transmittance and Abs ₂₄₀ /Abs ₀ of the obtained polarizing plate (effectively a polarizing film).

[Example 12]
The laminate of thermoplastic resin substrate/PVA-based resin layer was subjected to the auxiliary air stretching treatment, insolubilization treatment, dyeing treatment, crosslinking treatment and underwater stretching treatment in the same manner as in Example 1. The laminate that had been stretched underwater was immersed in a treatment bath (pH=1.6) at a liquid temperature of 20° C. (contact with the treatment liquid). The treatment bath was prepared by adding hydrochloric acid to a normal washing bath (aqueous solution obtained by mixing 4 parts by weight of potassium iodide with 100 parts by weight of water).
Thereafter, while drying in an oven maintained at 90° C., the laminate was brought into contact with a SUS heated roll having a surface temperature maintained at 75° C. for about 2 seconds (drying shrinkage treatment). The shrinkage rate of the laminate in the width direction due to the drying shrinkage treatment was 2%.
Next, a cycloolefin film (manufactured by ZEON, product name "G-Film") was attached to the surface of the polarizing film as a protective layer (protective film) with a UV-curable adhesive (thickness 1.0 μm), and then the resin substrate was peeled off to obtain a polarizing plate having a protective layer/polarizing film configuration. The single transmittance and _Abs240 / _Abs0 of the obtained polarizing plate (effectively a polarizing film) are shown in Table 1.

[Comparative Example 1]
Except for not contacting the polarizing plate with the treatment liquid, a polarizing plate was produced in the same manner as in Example 1. Table 1 shows the single transmittance and Abs ₂₄₀ /Abs ₀ of the obtained polarizing plate (effectively a polarizing film).

[Comparative Example 2]
A polarizing plate was produced in the same manner as in Comparative Example 1, except that the single transmittance of the polarizing film was 45.0%. Table 1 shows the single transmittance and Abs ₂₄₀ /Abs ₀ of the obtained polarizing plate (effectively a polarizing film).

[Comparative Examples 3 to 8]
Polarizing plates were produced by adjusting the single transmittance of the polarizing film, the method of contacting with the treatment liquid, the pH of the treatment liquid, the type of acid contained in the treatment liquid, and the thickness of the treatment layer (if formed) as shown in Table 1. Table 1 shows the single transmittance and Abs ₂₄₀ /Abs ₀ of the obtained polarizing plate (effectively a polarizing film).

[Example 13]
A long roll of a 55 μm-thick PVA-based resin film (manufactured by Nippon Gosei Co., Ltd., product name "PS7500") was uniaxially stretched in the long direction by a roll stretching machine to a total stretch ratio of 6.0 times, while simultaneously undergoing swelling, dyeing, crosslinking and washing treatments, and finally drying treatment to produce a polarizing film with a thickness of 23 μm. After the washing treatment and before the drying treatment, the same treatment liquid as in Example 1 was applied to one surface of the PVA-based resin film (polarizing film) in the same manner as in Example 1. The single transmittance and Abs ₂₄₀ /Abs ₀ of the obtained polarizing film are shown in Table 1.

[Example 14]
A polarizing film having a thickness of 23 μm was prepared in the same manner as in Example 13, except that the PVA-based resin film (polarizing film) was passed through a treatment bath similar to that in Example 12 instead of a cleaning bath for the cleaning treatment (thus, no treatment liquid was applied after the cleaning treatment). The single transmittance and Abs ₂₄₀ /Abs ₀ of the obtained polarizing film are shown in Table 1.

As is clear from Table 1, the polarizing films of the examples of the present invention have an Abs ₂₄₀ /Abs ₀ ratio of more than 0.90 after the durability test, and the deterioration of the polarization performance in a high-temperature, high-humidity environment is suppressed. That is, the polarizing films of the examples of the present invention have excellent durability in a high-temperature, high-humidity environment. In particular, the polarizing film of Example 10 has an Abs ₂₄₀ /Abs ₀ ratio of more than 1.0, and the polarization performance is improved in a high-temperature, high-humidity environment. This is an unexpected excellent effect that goes against common technical knowledge. The polarizing films of Comparative Examples 1 and 2, which were not contacted with a treatment liquid, and the polarizing films of Comparative Examples 3 to 8, which were contacted with a treatment liquid having a pH of more than 3.0, all had an Abs ₂₄₀ /Abs ₀ ratio of 0.88 or less. In Comparative Example 6, in which boric acid was used as the treatment liquid, the treatment liquid gelled, and contact itself was impossible.

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

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

Claims (6)

A method for producing a polarizing film, comprising the steps of:
The polarizing film is made of a polyvinyl alcohol-based resin film containing iodine, and the absorbance Abs ₂₄₀ at a wavelength of 470 nm after a durability test for 240 hours at a temperature of 60° C. and a relative humidity of 95% satisfies the following relationship with the absorbance Abs ₀ before the durability test:
The production method comprises:
forming a polyvinyl alcohol-based resin layer on one side of a long thermoplastic resin substrate to form a laminate;
the laminate is subjected to an auxiliary air-stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment in which the laminate is heated while being transported in the longitudinal direction to shrink the laminate by 2% or more in the width direction, in that order, to form the polyvinyl alcohol-based resin layer into a polarizing film; and contacting the polarizing film with a treatment liquid having a pH of 2.0 or less.
Including,
The contact with the treatment liquid is carried out after the drying shrinkage treatment.
Production method:
_Abs240 / _Abs0 >0.90 The manufacturing method according to claim 1, which includes applying the treatment liquid to the polarizing film. The manufacturing method according to claim 1, which includes immersing the polarizing film in the treatment liquid. The manufacturing method according to any one of claims 1 to 3, in which a polyvinyl alcohol-based resin layer containing iodide or sodium chloride and a polyvinyl alcohol-based resin is formed on one side of the thermoplastic resin substrate. The manufacturing method according to any one of claims 1 to 4, wherein the drying and shrinking treatment is carried out using a heated roll. The method according to claim 5, wherein the temperature of the heating roll is 60°C to 120°C.

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