JP7590878B2 - Manufacturing method of polarizing film - Google Patents

Description

The present invention relates to a method for producing a polarizing film.

In recent years, image display devices such as liquid crystal display devices and electroluminescence (EL) display devices (e.g., organic EL display devices, inorganic EL display devices) have rapidly become widespread. In organic EL display devices, it is known that problems such as external light reflection and background glare can be prevented by arranging a circular polarizer including a λ/4 plate on the viewing side of the organic EL cell (e.g., Patent Documents 1 and 2).

On the other hand, organic EL display devices consume a lot of power to emit light, so there is a demand for energy saving.

JP 2002-311239 A JP 2002-372622 A

The present invention has been made to solve the above-mentioned problems in the conventional art, and its main objective is to provide a polarizing film that can reduce the power consumption of an organic EL display device.

According to one aspect of the present invention, there is provided a method for producing a polarizing film, comprising the steps of subjecting a polyvinyl alcohol-based resin film to a dyeing treatment and a stretching treatment, and contacting a surface of the polyvinyl alcohol-based resin film with an aqueous solvent, in this order, wherein the ratio (ΔTs(λ)) of the transmittance of the polyvinyl alcohol-based resin film after contact with the aqueous solvent at a wavelength of λ nm to the transmittance of the polyvinyl alcohol-based resin film before contact satisfies the relationship ΔTs(415)>ΔTs(470)>ΔTs(550).
In one embodiment, the temperature of the aqueous solvent is from 20°C to 70°C.
In one embodiment, the moisture content of the polyvinyl alcohol-based resin film that is brought into contact with the aqueous solvent is 15% by weight or less.
In one embodiment, the polyvinyl alcohol-based resin film that is brought into contact with the aqueous solvent has a thickness of 12 μm or less.
In one embodiment, subjecting the polyvinyl alcohol-based resin film to the dyeing treatment and stretching treatment includes forming a polyvinyl alcohol-based resin film containing a halide 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 is heated while being transported in the longitudinal direction to shrink the laminate by 2% or more in the width direction, in that order.
In one embodiment, the production method is a method for producing a polarizing film having a haze of 1% or less.

According to the manufacturing method of the polarizing film of the embodiment of the present invention, a polyvinyl alcohol (PVA)-based resin film that has been subjected to a dyeing process and a stretching process is subjected to a contact process with an aqueous solvent. As a result, the transmittance of the PVA-based resin film at least in the wavelength region of 415 nm to 550 nm increases, and the ratio of the transmittance after contact to the transmittance of the PVA-based resin film before contact with an aqueous solvent at a wavelength of λ nm (ΔTs(λ) = Ts(λ) after contact / Ts(λ) before contact, hereinafter, ΔTs(λ) may be referred to as the "increase in transmittance") satisfies the relationship ΔTs(415)>ΔTs(470)>ΔTs(550). The polarizing film obtained by such a manufacturing method can transmit light on the short wavelength side more actively than light on the long wavelength side. Therefore, by using such a polarizing film, even if the amount of blue light emission, which consumes a large amount of power, is reduced, it is possible to suppress the decrease in brightness in the short wavelength region, and as a result, it is possible to achieve both energy saving and high brightness in an organic EL display device.

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. Manufacturing Method of Polarizing Film A manufacturing method of a polarizing film according to an embodiment of the present invention includes subjecting a polyvinyl alcohol (PVA)-based resin film to a dyeing treatment and a stretching treatment (step I), and contacting a surface of the PVA-based resin film with an aqueous solvent (step II), in this order, and the ratio (ΔTs(λ)) of the transmittance of the polyvinyl alcohol-based resin film after contact with the aqueous solvent at a wavelength of λ nm to the transmittance before contact satisfies the relationship ΔTs(415)>ΔTs(470)>ΔTs(550). In the dyed PVA-based resin film, iodine exists in the form of I ^- , I ₂ , I ₃ ^{- ,} PVA-I ₃ -complex, PVA-I ₅ -complex ^, etc., where I ^- , I ₂ and I ₃ ^- have absorption in the ultraviolet region (e.g., wavelengths around 290 nm to 360 nm), and PVA-I ₃ ^-complex and PVA-I ₅ ^-complex have absorption at wavelengths around 470 nm and 600 nm, respectively. Therefore, the relationship ΔTs(415)>ΔTs(470)>ΔTs(550) before and after contact with an aqueous solvent is considered to indicate that the proportions of I ^- , I ₂ , I ₃ ^- , and PVA-I ₃ ^- complexes relative to the total iodine present in the PVA-based resin film have decreased (in other words, the proportion of PVA-I ₅ ^- complexes has increased).

A-1. Step I
In step I, the PVA-based resin film is subjected to a dyeing treatment and a stretching treatment to obtain a PVA-based resin film (hereinafter, sometimes referred to as an "unbleached original film") that exhibits absorption dichroism at any wavelength of 380 nm to 780 nm. The unbleached original film is typically in a state that can function as a polarizing film.

In one embodiment, the transmittance (single transmittance: Ts) of the unbleached raw film is preferably 41.0% or more, more preferably 42.0% or more, and even more preferably 42.5% or more. On the other hand, the transmittance of the unbleached raw film is preferably 46.0% or less, more preferably 45.0% or less. The polarization degree of the unbleached raw film is preferably 98.0% or more, more preferably 99.0% or more, and even more preferably 99.9% or more. On the other hand, the polarization degree of the unbleached raw film is preferably 99.998% or less. The transmittance is typically a Y value measured using an ultraviolet-visible spectrophotometer and corrected for luminosity. The polarization degree is typically calculated by the following formula based on the parallel transmittance Tp and crossed transmittance Tc measured using an ultraviolet-visible spectrophotometer and corrected for luminosity.
Degree of polarization (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 ×100

In one embodiment, the transmittance of a thin polarizing film (unbleached original film) having a thickness of 12 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 corrected 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)
R ₁ =((n ₁ -1) ² /(n ₁ +1) ² )×(T ₁ /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 transmittance (Ts ₄₁₅ ) of the unbleached original film at a wavelength of 415 nm may be, for example, less than 40%.

The moisture content of the unbleached base film is typically 15% by weight or less, preferably 12% by weight or less, more preferably 10% by weight or less, and even more preferably 1% to 5% by weight. If the moisture content of the unbleached base film is within this range, dissolution, wrinkling, etc. can be prevented when the film comes into contact with the aqueous solvent in step II.

The thickness of the unbleached base film is typically 25 μm or less, preferably 12 μm or less, more preferably 1 μm to 12 μm, even more preferably 1 μm to 7 μm, and even more preferably 2 μm to 5 μm.

In step I, a single-layer PVA-based resin film is subjected to a dyeing process and a stretching process, thereby producing an unbleached base film. Alternatively, a laminate of two or more layers including a PVA-based resin layer (PVA-based resin film) is subjected to a dyeing process and a stretching process, thereby producing an unbleached base film. An unbleached base film produced using a laminate of two or more layers can preferably maintain excellent optical properties (typically, single-layer transmittance and polarization degree) while avoiding the occurrence of wrinkles, even after contact with an aqueous solvent.

A-1-1. Preparation of unbleached raw film using laminate of two or more layers Preparation of unbleached raw film using laminate of two or more layers can be performed, for example, by subjecting a PVA-based resin film containing a halide and a PVA-based resin to a dyeing process and a stretching process in the state of a laminate with a long thermoplastic resin substrate. Specifically, the unbleached raw film can be prepared by a method including forming a PVA-based resin layer (PVA-based resin film) containing a halide and a PVA-based resin on one side of a long thermoplastic resin substrate to form a laminate, and subjecting the laminate to an air-assisted stretching process, a dyeing process, an underwater stretching process, and a drying shrinkage process in which the laminate is heated while being conveyed 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 layer is preferably 5 to 20 parts by weight relative to 100 parts by weight of the PVA-based resin. The drying shrinkage treatment is preferably carried out 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 treatment is preferably 2% or more. According to such a production method, it is possible to obtain an unbleached base film having a high degree of orientation of the PVA-based resin and excellent optical properties.

A-1-1-1. 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, the adhesion between the thermoplastic resin substrate and the PVA-based resin layer can be improved.

The thickness of the thermoplastic resin substrate is preferably 20 μm to 300 μm, more preferably 50 μm to 200 μm. If it is less than 20 μm, it may be difficult to form a PVA-based resin layer. If it is more than 300 μm, for example, in the underwater stretching process described below, it may take a long time for the thermoplastic resin substrate to absorb water, and excessive load may be required for stretching.

The thermoplastic resin substrate preferably has a water absorption rate of 0.2% or more, more preferably 0.3% or more. The thermoplastic resin substrate can absorb water, and the water can act as a plasticizer to plasticize the substrate. As a result, the stretching stress can be significantly reduced, and the substrate can be stretched at a high ratio. On the other hand, the water absorption rate of the thermoplastic resin substrate is preferably 3.0% or less, more preferably 1.0% or less. By using such a thermoplastic resin substrate, it is possible to prevent problems such as a significant decrease in the dimensional stability of the thermoplastic resin substrate during production, which leads to a deterioration in the appearance of the resulting unbleached base film. In addition, it is possible to prevent the substrate from breaking during underwater stretching, and the PVA-based resin layer from peeling off from the thermoplastic resin substrate. The water absorption rate of the thermoplastic resin substrate can be adjusted, for example, by introducing a modifying group into the constituent material. The water absorption rate is a value determined in accordance with JIS K 7209.

The glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 120°C or less. By using such a thermoplastic resin substrate, the stretchability of the laminate can be sufficiently ensured while suppressing the crystallization of the PVA-based resin layer. Furthermore, in consideration of the plasticization of the thermoplastic resin substrate by water and the good underwater stretching, it is more preferable that the temperature is 100°C or less, and even more preferable that the temperature is 90°C or less. On the other hand, the glass transition temperature of the thermoplastic resin substrate is preferably 60°C or more. By using such a thermoplastic resin substrate, defects such as deformation of the thermoplastic resin substrate (e.g., generation of unevenness, sagging, wrinkles, etc.) when applying and drying the coating liquid containing the PVA-based resin can be prevented, and the laminate can be produced well. In addition, the stretching of the PVA-based resin layer can be performed well at a suitable temperature (e.g., about 60°C). The glass transition temperature of the thermoplastic resin substrate can be adjusted, for example, by introducing a modified group into the constituent material and heating using a crystallization material. The glass transition temperature (Tg) is a value determined in accordance with JIS K 7121.

Any suitable thermoplastic resin may be used as the material for the thermoplastic resin substrate. Examples of thermoplastic resins include ester-based resins such as polyethylene terephthalate-based resins, cycloolefin-based resins such as norbornene-based resins, olefin-based resins such as polypropylene, polyamide-based resins, polycarbonate-based resins, and copolymer resins of these. Of these, norbornene-based resins and amorphous polyethylene terephthalate-based resins are preferred.

In one embodiment, amorphous (uncrystallized) polyethylene terephthalate resins are preferably used. Among them, amorphous (hardly crystallized) polyethylene terephthalate resins are particularly preferably used. Specific examples of amorphous polyethylene terephthalate resins include copolymers further containing isophthalic acid and/or cyclohexanedicarboxylic acid as dicarboxylic acids, and copolymers further containing cyclohexanedimethanol or diethylene glycol as glycols.

In a preferred embodiment, the thermoplastic resin substrate is made of a polyethylene terephthalate resin having an isophthalic acid unit. This is because such a thermoplastic resin substrate has excellent stretchability and can suppress crystallization during stretching. This is thought to be due to the introduction of an isophthalic acid unit, which imparts a large bend to the main chain. The polyethylene terephthalate resin has a terephthalic acid unit and an ethylene glycol unit. The content of the isophthalic acid unit is preferably 0.1 mol% or more, more preferably 1.0 mol% or more, based on the total of all repeating units. This is because a thermoplastic resin substrate with extremely excellent stretchability can be obtained. On the other hand, the content of the isophthalic acid unit is preferably 20 mol% or less, more preferably 10 mol% or less, based on the total of all repeating units. By setting such a content ratio, the crystallinity can be increased satisfactorily in the drying shrinkage treatment described below.

The thermoplastic resin substrate may be stretched in advance (before the PVA-based resin layer is formed). In one embodiment, the long thermoplastic resin substrate is stretched in the lateral direction. The lateral direction is preferably a direction perpendicular to the stretching direction of the laminate described below. In this specification, "perpendicular" also includes the case where the direction is substantially perpendicular. Here, "substantially perpendicular" includes the case where the direction is 90°±5.0°, and is preferably 90°±3.0°, and more preferably 90°±1.0°.

The stretching temperature of the thermoplastic resin substrate is preferably Tg-10°C to Tg+50°C relative to the glass transition temperature (Tg). The stretching ratio of the thermoplastic resin substrate is preferably 1.5 to 3.0 times.

Any appropriate method may be adopted as the method for stretching the thermoplastic resin substrate. Specifically, it may be fixed-end stretching or free-end stretching. The stretching method may be dry or wet. The thermoplastic resin substrate may be stretched in one stage or in multiple stages. When stretched in multiple stages, the above-mentioned stretch ratio is the product of the stretch ratios in each stage.

As described above, the coating liquid contains a halide and a PVA-based resin. 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 per 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 per 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, an unbleached base film having 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 unbleached base film may become cloudy.

Generally, the orientation of the polyvinyl alcohol molecules in the PVA-based resin layer 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 substrate 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 substrate. 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, a laminate of a PVA-based resin layer containing a halide and a thermoplastic resin substrate is prepared, and the laminate is stretched at high temperature (auxiliary stretching) in air before being stretched in boric acid water, so that 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 unbleached base film obtained through a process in which the laminate is immersed in a liquid, such as a dyeing process and an underwater stretching process.

A-1-1-2. 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 to 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 on 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 to a thermoplastic resin substrate, 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 process or stretching process 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 by passing it 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 step 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 step and a heated roll stretching step. The order of the zone stretching step and the heated roll stretching step is not limited, and 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 laminate is stretched by gripping the end of the laminate 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 (1/stretching ratio) ^1/2 .

The air-assisted stretching may be performed in one stage or in multiple stages. When the air-assisted stretching is performed in multiple stages, the stretching ratio is the product of the stretching ratios in each stage. 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 of the auxiliary air 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 suppressed from progressing rapidly, and defects due to the crystallization (for example, the orientation of the PVA-based resin layer due to stretching can be suppressed). The crystallization index of the PVA-based resin after the auxiliary air stretching is preferably 1.3 to 1.8, more preferably 1.4 to 1.7. The crystallization index of the PVA-based resin can be measured by the ATR method using a Fourier transform infrared spectrophotometer. Specifically, the measurement is carried out using polarized light as the measuring light, and the crystallization index is calculated according to the following formula using the intensities at 1141 cm ⁻¹ and 1440 cm ⁻¹ of the obtained spectrum.
Crystallization index = (I _C /I _R )
however,
I _C : Intensity at 1141 cm ⁻¹ when the measurement light is incident and measured. I _R : Intensity at 1440 cm ⁻¹ when the measurement light is incident and measured.

A-1-1-3. Insolubilization 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 solution of boric acid. By performing the insolubilization treatment, water resistance is imparted to the PVA-based resin layer, and it is possible to prevent a decrease in the orientation of the PVA when immersed in water. The concentration of the aqueous solution of boric acid is preferably 1 part by weight to 4 parts by weight per 100 parts by weight of water. The liquid temperature of the insolubilization bath (aqueous solution of boric acid) is preferably 20°C to 50°C.

A-1-1-4. Dyeing The dyeing is typically performed by dyeing the PVA-based resin layer with a dichroic substance (typically iodine). Specifically, the dyeing is performed by allowing the PVA-based resin layer to adsorb iodine. Examples of the adsorption method include a method of immersing the PVA-based resin layer (laminate) in a dyeing solution containing iodine, a method of applying the dyeing solution to the PVA-based resin layer, and a method of spraying the dyeing solution onto the PVA-based resin layer. A method of immersing the laminate in a dyeing solution (dye bath) is preferred, because iodine can be well adsorbed in this method.

The dyeing solution is preferably an aqueous iodine solution. The amount of iodine is preferably 0.05 to 0.5 parts by weight per 100 parts by weight of water. In order to increase the solubility of iodine in water, it is preferable to add an iodide to the aqueous iodine solution. Examples of iodides include potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, and titanium iodide. Of these, potassium iodide is preferable. The amount of iodide is preferably 0.1 to 10 parts by weight, more preferably 0.3 to 5 parts by weight, per 100 parts by weight of water. The temperature of the dyeing solution during dyeing is preferably 20°C to 50°C in order to suppress dissolution of the PVA-based resin. When the PVA-based resin layer is immersed in the dye solution, the immersion time is preferably 5 seconds to 5 minutes, and more preferably 30 seconds to 90 seconds, in order to ensure the transmittance of the PVA-based resin layer.

The dyeing conditions (concentration, liquid temperature, immersion time) can be set so that the single transmittance of the unbleached base film finally obtained is the desired value. As such dyeing conditions, preferably, an iodine aqueous solution is used as the dyeing solution, and the content ratio of iodine and potassium iodide in the iodine aqueous solution is 1:5 to 1:20. The content ratio of iodine and potassium iodide in the iodine aqueous solution is preferably 1:5 to 1:10. This makes it possible to obtain an unbleached base film having the optical properties described below.

When a dyeing process is performed continuously after a process (typically, an insolubilization process) in which the laminate is immersed in a treatment bath containing boric acid, the boric acid concentration of the dyeing bath may change over time due to the boric acid contained in the treatment bath being mixed into the dyeing bath, resulting in unstable dyeability. In order to suppress the above-mentioned instability of dyeability, the upper limit of the boric acid concentration of the dyeing bath is adjusted to preferably 4 parts by weight, more preferably 2 parts by weight, per 100 parts by weight of water. On the other hand, the lower limit of the boric acid concentration of the dyeing bath is preferably 0.1 parts by weight, more preferably 0.2 parts by weight, and even more preferably 0.5 parts by weight, per 100 parts by weight of water. In one embodiment, the dyeing process is performed using a dyeing bath in which boric acid has been blended in advance. This can reduce the rate of change in the boric acid concentration when the boric acid of the treatment bath is mixed into the dyeing bath. The amount of boric acid preliminarily mixed into the dye bath (i.e., the content of boric acid not derived from the treatment bath) is preferably 0.1 to 2 parts by weight, and more preferably 0.5 to 1.5 parts by weight, per 100 parts by weight of water.

A-1-1-5. Crosslinking treatment 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. The crosslinking treatment imparts water resistance to the PVA-based resin layer, and prevents the orientation of the PVA from decreasing when the layer is immersed in high-temperature water in the subsequent underwater stretching. The concentration of the aqueous boric acid solution is preferably 1 to 5 parts by weight per 100 parts by weight of water. In addition, when the crosslinking treatment is performed after the dyeing treatment, it is preferable to further add an iodide. By adding an iodide, it is possible to suppress the elution of iodine adsorbed to the PVA-based resin layer. The amount of iodide added is preferably 1 to 5 parts by weight per 100 parts by weight of water. Specific examples of iodide are as described above. The liquid temperature of the crosslinking bath (aqueous boric acid solution) is preferably 20°C to 50°C.

A-1-1-6. 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, an unbleached base film having excellent optical properties can be produced.

The method of stretching 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 stretch ratio (maximum stretch ratio) of the laminate described below is the product of the stretch 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, the PVA-based resin layer can be given rigidity that can withstand the tension applied during stretching and water resistance that does not dissolve 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 can be given rigidity and water resistance, and can be stretched well, producing an unbleached base film with excellent optical properties.

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 an unbleached base film with higher properties 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, etc. 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 parts by weight to 15 parts by weight, more preferably 0.5 parts by weight 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 by 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, it is possible to produce an unbleached base film with extremely excellent optical properties. Such a high stretching ratio can be achieved by adopting the underwater stretching method (stretching in boric acid water).

A-1-1-7. Drying and shrinking treatment In the drying and shrinking treatment, for example, a laminate of a long thermoplastic resin substrate and a PVA-based resin film is heated while being conveyed in the longitudinal direction, so that the laminate shrinks by 2% or more in the width direction. In the drying and shrinking treatment, the PVA-based resin film is preferably dried until the moisture content is 15% by weight or less, and from the viewpoint of obtaining a stable appearance, it is more preferable to dry the PVA-based resin film until the moisture content is 12% by weight or less, even more preferably 10% by weight or less, and even more preferably 1 to 5% by weight.

The drying shrinkage treatment may be performed by zone heating, which heats the entire zone, or by heating the transport roll (using a so-called heating roll) (heat 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 an unbleached base 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, the laminate can be dried while being maintained in a flat state, so that not only curling but also the occurrence of wrinkles can be suppressed. At this time, the laminate can be shrunk in the width direction by the drying shrinkage treatment, thereby improving the optical properties. This is because the orientation of the PVA and the PVA/iodine complex can be effectively increased. The shrinkage rate of the laminate in the width direction due to the drying shrinkage treatment is preferably 1% to 10%, more preferably 2% to 8%, and particularly preferably 4% to 6%. By using a heated roll, the laminate can be continuously shrunk in the width direction while being transported, achieving high productivity.

Figure 1 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 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-based 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 20°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.

A-1-1-8. Other Treatments Preferably, after the underwater stretching treatment and before the drying shrinkage treatment, a washing treatment is carried out. The washing treatment is typically carried out by immersing the PVA-based resin layer in an aqueous potassium iodide solution.

In step II, the contact between the unbleached film and the aqueous solvent may be performed by contacting only one side of the unbleached film with the aqueous solvent, or both sides may be contacted with the aqueous solvent. Thus, in one embodiment, the unbleached film produced using the above laminate can be subjected to step II as a laminate of [unbleached film/thermoplastic resin substrate]. In another embodiment, a laminate of [protective layer/unbleached film/thermoplastic resin substrate] is produced by laminating a protective layer to the surface of the unbleached film of the laminate of [unbleached film/thermoplastic resin substrate], and the thermoplastic resin substrate is peeled off from the laminate to produce a laminate (polarizing plate) of [protective layer/unbleached film], and the resulting laminate can be subjected to step II. In yet another embodiment, any suitable functional layer (retardation layer, adhesive layer, etc.) can be provided on the substrate side of the laminate of [unbleached film/thermoplastic resin substrate] or on the protective layer side of the laminate of [protective layer/unbleached film], and the laminate obtained can be subjected to step II.

A-1-2. Preparation of unbleached raw film using single-layer PVA-based resin film Preparation of unbleached raw film using single-layer PVA-based resin film can be performed by dyeing and stretching (typically, uniaxial stretching using a roll stretching machine in an aqueous boric acid solution) a long PVA-based resin film having self-supporting properties (i.e., not requiring support by a substrate), and then drying until the moisture content is preferably 15% by weight or less, more preferably 12% by weight or less, even more preferably 10% by weight or less, and even more preferably 1 to 5% by weight. The dyeing is performed, for example, by immersing the PVA-based resin film in an aqueous iodine solution. The stretching ratio of the uniaxial stretching is preferably 3 to 7 times. The stretching may be performed after the dyeing treatment or while the dyeing is being performed. Alternatively, the stretching may be performed after the dyeing treatment. If necessary, the PVA-based resin film is subjected to a swelling treatment, a crosslinking treatment, a washing treatment, or the like. For example, by immersing the PVA-based resin film in water and rinsing it before dyeing, it is possible not only to clean off dirt and antiblocking agents on the surface of the PVA-based resin film but also to swell the PVA-based resin film and prevent uneven dyeing and the like.

As described above, the contact of the unbleached film with the aqueous solvent in step II may be performed by contacting only one side of the unbleached film with the aqueous solvent, or may be performed by contacting both sides with the aqueous solvent. Thus, in one embodiment, the unbleached film prepared using the above-mentioned single-layer PVA-based resin film can be directly subjected to step II. In another embodiment, a protective layer is attached to one side of the unbleached film to prepare a laminate of [protective layer/unbleached film], and the laminate can be subjected to step II. In yet another embodiment, the laminate of [protective layer/unbleached film] can be provided with any appropriate functional layer (phase difference layer, adhesive layer, etc.) on the protective layer side, and then subjected to step II.

A-2. Step II
In step II, an aqueous solvent is brought into contact with the surface of the PVA-based resin film (unbleached base film) that has been subjected to step I. Upon contact with the aqueous solvent, polyiodine ions that form I ^- , I ₂ , I ₃ ^- and PVA-I ₃ ^- complexes are preferentially eluted from the unbleached base film over polyiodine ions that form PVA-I ₅ ^- complexes, resulting in a greater increase in transmittance on the short wavelength side, and as a result, the rate of increase in transmittance at a wavelength of λ nm (ΔTs(λ)) can satisfy the relationship ΔTs(415)>ΔTs(470)>ΔTs(550).

As the aqueous solvent, any suitable solvent can be used as long as it can elute the dichroic substance (typically iodine) from the unbleached base film. The aqueous solvent can be, for example, water or a mixture of water and a water-soluble organic solvent. Preferred examples of the water-soluble organic solvent include lower monoalcohols having 1 to 4 carbon atoms, such as methanol, ethanol, n-propyl alcohol, and isopropyl alcohol, and polyhydric alcohols, such as glycerin and ethylene glycol.

The method of contacting with the aqueous solvent is not particularly limited, and any appropriate method such as immersion, spraying, or coating can be used. Immersion is preferred from the viewpoint of uniformly contacting the entire surface of the unbleached base film with the aqueous solvent.

The contact time with the aqueous solvent and the temperature of the aqueous solvent during contact can be appropriately set according to the desired Ts ₄₁₅ , Ts ₄₇₀ and Ts ₅₅₀ , etc. By increasing the contact time or the temperature of the aqueous solvent, the transmittance (particularly Ts ₄₁₅ ) tends to increase. The contact time can be, for example, 10 minutes or less, preferably 60 seconds to 9 minutes, more preferably 60 seconds to 4 minutes. The temperature of the aqueous solvent can be preferably 20°C to 70°C, more preferably 30°C to 65°C, and even more preferably 40°C to 60°C.

If necessary, a drying treatment may be carried out after contact with the aqueous solvent. The drying temperature may be, for example, 20°C to 100°C, and preferably 30°C to 80°C. The moisture content of the polarizing film after drying is typically 15% by weight or less, preferably 12% by weight or less, more preferably 10% by weight or less, and even more preferably 1% by weight to 5% by weight.

B. Polarizing Film The polarizing film obtained by the method for producing a polarizing film described in Section A is composed of a PVA-based resin film containing a dichroic substance (typically, iodine), and has a transmittance higher than that of an unbleached original film at least in the wavelength region of 415 nm to 550 nm. Specifically, the increase rate of transmittance at a wavelength of λ nm (ΔTs(λ)) satisfies the relationship ΔTs(415)>ΔTs(470)>ΔTs(550). The increase rate of transmittance at a wavelength of 415 nm (ΔTs(415)) may be, for example, more than 1.05, preferably 1.1 or more, and more preferably 1.10 to 2.2. If ΔTs(415) is within this range, the amount of blue light emission, which consumes a large amount of power, can be reduced, contributing to energy saving of an organic EL display device.

The Ts ₄₁₅ and Ts ₅₅₀ of the polarizing film may be any appropriate value depending on the purpose. Ts ₄₁₅ may be, for example, 40% or more, preferably 41% or more, more preferably 42% or more, and may be, for example, 80% or less, preferably 60% or less, more preferably 50% or less. Ts ₅₅₀ may be, for example, 40% or more, preferably 42% or more, more preferably 43% or more, and may be, for example, 70% or less, preferably 60% or less, more preferably 50% or less.

The polarizing film preferably exhibits absorption dichroism at any wavelength between 380 nm and 780 nm. The transmittance (single transmittance: Ts) of the polarizing film is preferably 41% or more, more preferably 42% or more, and even more preferably 42.5% or more. On the other hand, the transmittance of the polarizing film is, for example, 65% or less, preferably 50% or less, and more preferably 48% or less. The polarization degree of the polarizing film is, for example, 40.0% or more, preferably 90.0% or more, more preferably 94.0% or more, even more preferably 96.0% or more, even more preferably 99.0% or more, even more preferably 99.5% or more, and preferably 99.998% or less. The transmittance and polarization degree are determined in the same manner as the transmittance and polarization degree of the unbleached original film.

The haze of the polarizing film is preferably 1% or less, more preferably 0.8% or less, and even more preferably 0.6% or less. If the haze is within this range, an organic EL display device with a high contrast ratio can be obtained.

The iodine concentration in the polarizing film is preferably 3% by weight or more, more preferably 4% by weight to 10% by weight, and more preferably 4% by weight to 8% by weight. In this specification, the term "iodine concentration" means the total amount of iodine contained in the polarizing film. More specifically, iodine exists in the polarizing film in the form of I ^- , I ₂ , I ₃ ^- , PVA-I ₃ ^-complex , PVA-I ₅ -complex ^, etc., and the iodine concentration in this specification means the concentration of iodine including all of these forms. The iodine concentration can be calculated, for example, from the fluorescent X-ray intensity by fluorescent X-ray analysis and the film (polarizing film) thickness.

The thickness of the polarizing film is typically 25 μm or less, preferably 12 μm or less, more preferably 1 μm to 12 μm, even more preferably 1 μm to 7 μm, and even more preferably 2 μm to 5 μm.

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 a product called "Linear Gauge Model D-10HS" (manufactured by Ozaki Manufacturing Co., Ltd.).
(2) Single transmittance and polarization degree For the laminates of the PVA-based resin film (polarizing film or unbleached original film) and the protective layer obtained in the Examples and Comparative Examples, the single transmittance Ts, parallel transmittance Tp, and crossed transmittance Tc measured from the PVA-based resin film side using an ultraviolet-visible spectrophotometer ("LPF-200" manufactured by Otsuka Electronics Co., Ltd.) were taken as Ts, Tp, and Tc of the PVA-based resin film, respectively. These Ts, Tp, and Tc are Y values measured with a 2-degree visual field (C light source) according to JIS Z8701 and corrected for luminosity. The refractive index of the protective layer was 1.53, and the refractive index of the surface of the polarizing film opposite to the protective layer was 1.53.
From the obtained Tp and Tc, the degree of polarization P was calculated according to the following formula.
Polarization degree P (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 ×100
The Ts measured at wavelengths of 415 nm, 470 nm and 550 nm were designated as Ts ₄₁₅ , Ts ₄₇₀ and Ts ₅₅₀ , respectively.
It should be noted that equivalent measurements can be made using a spectrophotometer such as the "V-7100" manufactured by JASCO Corporation, and it has been confirmed that equivalent measurement results can be obtained regardless of which spectrophotometer is used.
(3) Moisture content The unbleached base film immediately after drying (when stretched as a laminate, the stretched substrate is peeled off) was cut into a size of 100 mm x 100 mm or more, and the weight before the treatment was measured using an electronic balance. It was then placed in a heating oven maintained at 120°C for 2 hours, and the weight after removal (weight after treatment) was measured, and the moisture content was calculated using the following formula.
Moisture content [%] = (weight before treatment - weight after treatment) / weight before treatment x 100
(4) Haze: Measured using a Haze Meter (NDH-5000) manufactured by Nippon Denshoku Industries Co., Ltd., in accordance with JIS K7136.

[Example 1-1]
1. Preparation of polarizing film and polarizing plate A long roll of a 30 μm thick PVA-based resin film (manufactured by Kuraray, product name "PE3000") was stretched 2.2 times in the conveying direction while immersed in a 30° C. water bath, and then stretched 3 times based on the original length of the film that was not stretched at all while dyeing it by immersing it in a 30° C. aqueous solution with an iodine concentration of 0.04 wt % and a potassium concentration of 0.3 wt %. Next, this stretched film was further stretched to 3.3 times based on the original length while immersing it in a 30° C. aqueous solution with a boric acid concentration of 3 wt % and a potassium iodide concentration of 3 wt %, and then further stretched to 6 times based on the original length while immersing it in a 60° C. aqueous solution with a boric acid concentration of 4 wt % and a potassium iodide concentration of 5 wt %, and finally dried for 5 minutes in an oven maintained at 60° C. to prepare a 12 μm thick polarizing film (unbleached original film a1). The moisture content of the resulting unbleached base membrane a1 was 10.0% by weight, and the single body transmittance was 42.5%.

A PVA-based resin aqueous solution (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., product name "Gosefimer (registered trademark) Z-200", resin concentration: 3 wt%) was applied to one side of the obtained unbleached base film a1, and a cycloolefin-based film (manufactured by Nippon Zeon Co., Ltd., Zeonor, thickness: 25 μm) was laminated to obtain an optical laminate having a configuration of [unbleached base film a1/protective layer]. Note that a protective layer provided with a hard coat layer may also be used as the protective layer, and examples of such protective layers include a cycloolefin-based film with a hard coat layer (manufactured by Zeon Co., Ltd., product name "G-Film", total thickness 27 μm (film thickness 25 μm + hard coat layer thickness 2 μm)).

The optical laminate was cut to a size of 45 mm x 50 mm, and attached to a glass plate via an acrylic adhesive layer (thickness 15 μm) so that the surface on the unbleached base film side was exposed. The laminate was then immersed in water at 23°C for 31 hours. It was then dried at 50°C for 5 minutes to obtain a polarizing plate having a structure of [polarizing film A1/protective layer].

[Example 1-2]
A polarizing plate having a structure of [polarizing film A2/protective layer] was obtained in the same manner as in Example 1-1, except that the plate was immersed in water at 55° C. for 9 minutes instead of in water at 23° C. for 31 hours.

[Examples 1-3]
A polarizing plate having a structure of [polarizing film A3/protective layer] was obtained in the same manner as in Example 1-1, except that the plate was immersed in water at 60° C. for 4 minutes instead of in water at 23° C. for 31 hours.

[Examples 1 to 4]
A polarizing plate having a structure of [polarizing film A4/protective layer] was obtained in the same manner as in Example 1-1, except that the plate was immersed in water at 65° C. for 3 minutes instead of in water at 23° C. for 31 hours.

[Comparative Example 1]
An optical laminate having a structure of [unbleached original film a1/protective layer] prepared in the same manner as in Example 1-1 was used as a polarizing plate.

[Example 2-1]
As a thermoplastic resin substrate, a long amorphous isophthalic copolymerized polyethylene terephthalate film (thickness: 100 μm) having a Tg of about 75° C. was used, and one side of the resin substrate was subjected to a corona treatment.
A PVA aqueous solution (coating solution) was prepared by adding 13 parts by weight of potassium iodide to 100 parts by weight of a PVA-based resin prepared by mixing polyvinyl alcohol (polymerization degree 4,200, saponification degree 99.2 mol%) and acetoacetyl-modified PVA (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., product name "GOHSEFFIMER") in a ratio of 9:1, and dissolving the resultant in water.
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 13 μm, thereby producing a laminate.
The obtained laminate was uniaxially stretched 2.4 times in the longitudinal direction (machine direction) in an oven at 130° C. (auxiliary in-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 42.3% (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 wt %, potassium iodide concentration: 5 wt %) at a liquid temperature of 70° C., and uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls with different peripheral speeds to a total stretch ratio of 5.5 times (underwater stretching 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) at a liquid temperature of 20° C. (cleaning treatment).
Thereafter, while drying in an oven maintained at about 90° C., the laminate was brought into contact with a SUS heated roll having a surface temperature maintained at about 75° C. (drying shrinkage treatment). The shrinkage rate of the laminate in the width direction due to the drying shrinkage treatment was 2%.
In this manner, an unbleached original film having a moisture content of 4.5% and a thickness of 5 μm was formed on the resin substrate, and a cycloolefin film (Zeonor, manufactured by Zeon Corporation, thickness: 25 μm) was bonded to the surface of the unbleached original film with a UV-curable adhesive (thickness: 1.0 μm). The resin substrate was then peeled off to obtain an optical laminate having a configuration of [unbleached original film b1/protective layer].

The optical laminate was cut to a size of 45 mm x 50 mm, and attached to a glass plate via an acrylic adhesive layer (thickness 15 μm) so that the surface on the unbleached base film side was exposed. The laminate was then immersed in water at 50°C for 9 minutes. It was then dried at 50°C for 5 minutes to obtain a polarizing plate having a structure of [polarizing film B1/protective layer].

[Example 2-2]
A polarizing plate having a structure of [polarizing film B2/protective layer] was obtained in the same manner as in Example 2-1, except that the film was immersed in water at 55° C. for 3 minutes instead of in water at 50° C. for 9 minutes.

[Example 2-3]
A polarizing plate having a structure of [polarizing film B3/protective layer] was obtained in the same manner as in Example 2-1, except that the film was immersed in water at 60° C. for 2 minutes instead of in water at 50° C. for 9 minutes.

[Example 2-4]
A polarizing plate having a structure of [polarizing film B4/protective layer] was obtained in the same manner as in Example 2-1, except that the plate was immersed in water at 60° C. for 3 minutes instead of in water at 50° C. for 9 minutes.

[Comparative Example 2]
An optical laminate having a structure of [unbleached original film b1/protective layer] prepared in the same manner as in Example 2-1 was used as a polarizing plate.

The unbleached original films and polarizing films obtained in the above Examples and Comparative Examples were evaluated for various properties. The results are shown in Table 1.

As can be seen from Table 1, according to the manufacturing method of the embodiment, the increase rate of the transmittance (ΔTs(λ)) of the PVA-based resin film at a wavelength of λ nm before and after contact with water satisfies the relationship ΔTs(415)>ΔTs(470)>ΔTs(550), and it is understood that the increase rate of the transmittance at a wavelength of 415 nm is large. The polarizing film obtained by such a manufacturing method has practically acceptable optical properties (typically, single transmittance and degree of polarization), and the transmittance of short wavelength light is increased.

The polarizing film of the present invention can be suitably used in image display devices such as liquid crystal displays and EL displays, in particular organic EL displays.

Claims (6)

The method includes, in this order, subjecting the polyvinyl alcohol-based resin film to a dyeing treatment and a stretching treatment, and immersing the polyvinyl alcohol-based resin film in an aqueous solvent at 20° C. to 70° C. for 60 seconds to 10 minutes,
a ratio (ΔTs(λ)) of a transmittance of the polyvinyl alcohol-based resin film after immersion in the aqueous solvent to a transmittance of the polyvinyl alcohol-based resin film before immersion in the aqueous solvent at a wavelength of λ nm satisfies the relationship ΔTs(415)>ΔTs(470)>ΔTs(550). The method according to claim 1, wherein the temperature of the aqueous solvent is from 30°C to 65°C . 3. The method according to claim 1, wherein the polyvinyl alcohol-based resin film to be immersed in the aqueous solvent has a moisture content of 15% by weight or less. The method according to claim 1 , wherein the polyvinyl alcohol-based resin film to be immersed in the aqueous solvent has a thickness of 12 μm or less. The polyvinyl alcohol-based resin film is subjected to a dyeing treatment and a stretching treatment,
A polyvinyl alcohol-based resin film containing a halide and a polyvinyl alcohol-based resin is formed on one side of a long thermoplastic resin substrate to form a laminate; and
The method according to any one of claims 1 to 4, further comprising 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 cause the laminate to shrink by 2% or more in the width direction, in that order. The method according to claim 1 , which is a method for producing a polarizing film having a haze of 1% or less.