TWI900481B - Manufacturing method of polarizing film - Google Patents

Abstract

The present invention provides a polarizing film with excellent durability in high-temperature and high-humidity environments. The polarizing film is composed of an iodine-containing polyvinyl alcohol-based resin film. Furthermore, after a 240-hour durability test at 60°C and 95% relative humidity, the absorbance at a wavelength of 470 nm (Abs ₂₄₀ ) relative to the absorbance before the test (Abs ₀₎ satisfies the following relationship: Abs ₂₄₀ /Abs ₀ > 0.90. In one embodiment, the single-unit transmittance of the polarizing film is 43.0% or greater.

Description

Manufacturing method of polarizing film

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

Liquid crystal displays, a typical image display device, have polarizing films placed on both sides of the liquid crystal cell due to their image formation method. For example, a method has been proposed for producing polarizing films by stretching a laminate comprising a resin substrate and a polyvinyl alcohol (PVA) resin layer, followed by dyeing to form a polarizing film on the resin substrate (e.g., Patent Document 1). This method enables the production of thinner polarizing films, contributing to the recent trend toward thinner image display devices and attracting considerable attention. However, thin polarizing films are required to exhibit enhanced durability in high-temperature and high-humidity environments.

Prior Art Literature Patent Literature

Patent Document 1: Japanese Patent Application Publication No. 2001-343521

This invention was developed to address the aforementioned issues. Its primary purpose is to provide a polarizing film, a polarizing plate, and a method for manufacturing the polarizing film that exhibit excellent durability in high-temperature and high-humidity environments.

The polarizing film of the present invention is composed of an iodine-containing polyvinyl alcohol resin film; and after a 240-hour durability test at a temperature of 60°C and a relative humidity of 95%, the absorbance Abs ₂₄₀ at a wavelength of 470nm relative to the absorbance Abs ₀ before the durability test satisfies the following relationship: Abs ₂₄₀ /Abs ₀ > 0.90

In one embodiment, the single-unit transmittance of the polarizing film is greater than 43.0%.

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

According to another aspect of the present invention, a polarizing plate is provided. The polarizing plate comprises the aforementioned 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, a method for manufacturing the aforementioned polarizing film is provided. The method comprises the following steps: 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 exposing the polarizing film to a treatment solution having a pH of 3.0 or less.

In one embodiment, the manufacturing method includes applying the treatment liquid to the polarizing film. In another embodiment, the manufacturing method includes immersing the polarizing film in the treatment liquid.

In one embodiment, the manufacturing method comprises 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 the following steps: sequentially subjecting the laminate to an air-assisted stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying and shrinking treatment. The drying and shrinking treatment involves heating the laminate while conveying it in the longitudinal direction, thereby shrinking it by more than 2% in the width direction.

In one embodiment, the drying and shrinking process is performed using a heating roller. The temperature of the heating roller is, for example, 60°C to 120°C.

Another method for manufacturing a polarizing film according to the present invention includes the following steps: stretching and dyeing a polyvinyl alcohol-based resin film to form a polarizing film; and contacting the polarizing film with a treatment solution having a pH of 3.0 or less.

According to the present invention, by exposing a polarizing film to a treatment solution having a pH below 3.0, a polarizing film with excellent durability in a high temperature and high humidity environment can be obtained. Specifically, after a 240-hour durability test at a temperature of 60°C and a relative humidity of 95%, the absorbance Abs ₂₄₀ at a wavelength of 470nm relative to the absorbance Abs ₀ before the durability test satisfies the following relationship: Abs ₂₄₀ /Abs ₀ > 0.90

In other words, the absorbance of the polarizing film of the embodiment of the present invention at a wavelength of 470nm remains minimal even after a heat and humidity durability test. This means that the degradation of the polarizing performance of the polarizing film of the embodiment of the present invention in a high temperature and high humidity environment has been suppressed to a level that is acceptable for practical use. The polarizing performance of polarizing films (especially thin polarizing films) typically degrades significantly in high temperature and high humidity environments. However, the embodiment of the present invention solves this problem and provides a polarizing film (especially thin polarizing film) with excellent durability in high temperature and high humidity environments.

10:Polarizing film

20: First protective layer

30: Second protective layer

100:Polarizing plate

200: Layered body

R1~R6: Conveyor Rollers

G1~G4: guide roller

Figure 1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention.

Figure 2 is a schematic diagram showing an example of a drying and shrinking process using heated rollers.

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

A.Polarizing film

The polarizing film of an embodiment of the present invention is formed of an iodine-containing polyvinyl alcohol (PVA) resin film. After a 240-hour durability test at a temperature of 60°C and a relative humidity of 95%, the absorbance at a wavelength of 470 nm (Abs ₂₄₀₎ satisfies the following relationship relative to the absorbance before the durability test (Abs ₀ ).

Abs ₂₄₀ /Abs ₀ >0.90

This indicates that in the polarizing film of an embodiment of the present invention, the PVA-I ₃ ^-complex having absorption near 470 nm is suppressed from being destroyed by the heat and humidification durability test. Although theoretically unclear, the excellent durability can be achieved by exposing the polarizing film to a treatment solution with a pH of 3.0 or less. Abs ₂₄₀ /Abs ₀ is preferably 0.92 or greater, more preferably 0.93 or greater, and even more preferably 0.95 or greater. The upper limit of Abs ₂₄₀ /Abs ₀ can be, for example, 1.50. In addition, the absorbance is typically the cross absorbance. The cross absorbance can be obtained using the following formula based on the cross transmittance Tc measured when obtaining the degree of polarization described later.

Orthogonal absorbance = log10(100/Tc)

The absorbance Abs ₀ before the durability test is the absorbance of the polarizing film under normal conditions. The Abs ₀ of the polarizing film at a wavelength of 470 nm is, for example, less than 5.0, preferably less than 3.0, and more preferably less than 2.2. The lower limit of Abs ₀ can be, for example, 1.0.

In one embodiment, after a polarizing film is subjected to a durability test at a temperature of 60° C. and a relative humidity of 95% for 240 hours, the absorbance Abs ₂₄₀ at a wavelength of 600 nm satisfies the following relationship relative to the absorbance Abs ₀ before the durability test.

Abs ₂₄₀ /Abs ₀ >1.00

This indicates that the PVA-I ₅ ^-complex , which absorbs near 600 nm, in the polarizing film of embodiments of the present invention is not destroyed even in heat and humidity durability tests, but instead increases. PVA-I ₅ ^-complexes are destroyed in high-temperature, high-humidity environments, and the polarizing performance of polarizing films is generally expected to deteriorate in such environments. However, the excellent durability of the polarizing film of embodiments of the present invention is unexpected and exceptional. Abs _{240 /} Abs ₀ is preferably 1.05 or greater, more preferably 1.10 or greater, more preferably 1.15 or greater, particularly preferably 1.20 or greater, and even more preferably 1.25 or greater. The upper limit of Abs ₂₄₀ / Abs ₀ can be, for example, 2.00. Furthermore, the Abs ₀ of the polarizing film at a wavelength of 600 nm is, for example, less than 5.0, preferably less than 4.3, and more preferably less than 4.0. The lower limit of Abs ₀ can be, for example, 2.0.

The thickness of the polarizing film is preferably 8 μm or less, preferably 7 μm or less, more preferably 5 μm or less, and even more preferably 3 μm or less. In one embodiment, the lower limit of the thickness of the polarizing film may be 1 μm, and in another embodiment, it may be 2 μm. As described below, this thickness can be achieved by using, for example, a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate to form the polarizing film. When the polarizing film is made 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 between 380 nm and 780 nm. The single-element transmittance of the polarizing film is preferably 42.0% or higher, more preferably 42.5% or higher, more preferably 43.0% or higher, particularly preferably 43.5% or higher, and particularly preferably 44.0% or higher. On the other hand, the single-element transmittance is preferably 47.0% or lower, more preferably 46.0% or lower. The polarization degree of the polarizing film is preferably 99.95% or higher, more preferably 99.99% or higher. On the other hand, the polarization degree is preferably 99.998% or lower. According to embodiments of the present invention, both high single-element transmittance and high polarization degree are achieved, and as mentioned above, excellent durability can be achieved in high temperature and high humidity environments. The above single-element transmittance is typically the Y value obtained by measuring with a UV-Vis spectrophotometer and correcting for sensitivity. Furthermore, the single-element transmittance is the value obtained by converting the refractive index of one surface of the polarizing plate to 1.50 and the refractive index of the other surface to 1.53. The above polarization degree is typically calculated using the following formula based on the parallel transmittance Tp and the cross transmittance Tc obtained by measuring with a UV-Vis spectrophotometer and correcting for sensitivity.

Polarization degree (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 × 100

In one embodiment, the transmittance (single-body transmittance) of a thin polarizing film (less than 8 μm) is typically measured using a UV-visible spectrophotometer using a laminate of a polarizing film (surface refractive index: 1.53) and a protective layer (protective film) (refractive index: 1.50). Depending on the refractive index of the polarizing film surface and/or the refractive index of the protective layer's surface in contact with air, the reflectivity at the interface between the layers will vary, resulting in variations in the measured transmittance. Therefore, for example, when using a protective layer with a refractive index other than 1.50, the measured transmittance can be corrected based on the refractive index of the protective layer's surface in contact with air. 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) R ₁ =((n ₁ -1) ² /(n ₁ +1) ² )×(T ₁ /100)

Here, _R0 is the transmission axis reflectivity when using a protective layer with a refractive index of 1.50, _n1 is the refractive index of the protective layer used, and _T1 is the transmittance of the polarizing film. For example, when using a substrate with a surface refractive index of 1.53 (such as a cycloolefin film or a film with a hard coat) as the protective layer, the correction value C is approximately 0.2%. In this case, adding 0.2% to the measured transmittance can convert the polarizing film with a surface refractive index of 1.53 to the transmittance when using a protective layer with a surface refractive index of 1.50. Furthermore, according to the above formula, the change in the correction value C after changing the transmittance _T1 of the polarizing film by 2% is less than 0.03%, indicating that the influence of the polarizing film's transmittance on the correction value C is limited. Furthermore, when the protective layer has absorption other than surface reflection, appropriate correction can be performed according to the amount of absorption.

Polarizing films can be made from a single resin film or a laminate of two or more layers. A specific example of a polarizing film made from a laminate is a laminate of a resin substrate and a PVA-based resin layer coated onto the resin substrate. A polarizing film obtained by laminating 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 the resin substrate and drying it to form the PVA-based resin layer on the resin substrate, thereby obtaining a laminate of the resin substrate and the PVA-based resin layer; and then stretching and dyeing the laminate to form the PVA-based resin layer into a polarizing film. In an embodiment of the present invention, the polarizing film is exposed to a treatment solution having a pH of 3.0 or less. This achieves the aforementioned excellent durability in high-temperature and high-humidity environments. A polyvinyl alcohol resin layer comprising a halogenated compound and a polyvinyl alcohol resin is preferably formed on one side of a resin substrate. Stretching typically comprises immersing the laminate in an aqueous boric acid solution and stretching the laminate. Optionally, stretching may further comprise stretching the laminate in the air at a high temperature (e.g., above 95°C) before stretching in the aqueous boric acid solution. Furthermore, in this embodiment, it is preferred that the laminate be subjected to a dry shrinkage treatment in which the laminate is heated while being transported in the longitudinal direction so as to shrink by more than 2% in the width direction. Typically, the manufacturing method of this embodiment comprises sequentially subjecting the laminate to an air-assisted stretching treatment, a dyeing treatment, an underwater stretching treatment, and a dry shrinkage treatment. By introducing assisted stretching, PVA crystallinity is enhanced even when coated on a thermoplastic resin, achieving high optical properties. Furthermore, by pre-enhancing the PVA's orientation, problems such as loss of orientation or dissolution during subsequent dyeing or stretching steps can be prevented, ultimately achieving high optical properties. Furthermore, immersing the PVA resin layer in a liquid further suppresses the orientational disturbance of the polyvinyl alcohol molecules and the resulting loss of orientation compared to a PVA resin layer without halides. Consequently, the optical properties of polarizing films obtained through treatments such as dyeing and underwater stretching, where the laminate is immersed in a liquid, can be enhanced. Furthermore, shrinking the laminate in the width direction through a drying and shrinking process can improve optical properties. The resulting resin substrate/polarizing film laminate can be used directly (i.e., the resin substrate can also be used as a protective layer for the polarizing film), or the resin substrate can be peeled off from the resin substrate/polarizing film laminate and any appropriate protective layer can be applied to the peeled surface for subsequent use, depending on the intended application. Details on the polarizing film manufacturing method are described in Section C.

B.Polarizing plate

Figure 1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. Polarizing plate 100 comprises a polarizing film 10, a first protective layer 20 disposed on one side of polarizing film 10, and a second protective layer 30 disposed on the other side of polarizing film 10. Polarizing film 10 is the polarizing film of the present invention described in Section A above. Either the first protective layer 20 or the second protective layer 30 may be omitted. Furthermore, as mentioned above, one of the first protective layer and the second protective layer may be a resin substrate used in the manufacture of the polarizing film.

The first and second protective films are formed of any suitable film that can be used as a protective layer for a polarizing film. Specific examples of the material constituting the main component of these films include cellulose resins such as triacetate cellulose (TAC), polyesters, polyvinyl alcohols, polycarbonates, polyamides, polyimides, polyether sulfones, polysulfones, polystyrenes, polynorbornenes, polyolefins, (meth)acrylic acid, and acetate-based transparent resins. Other examples include thermosetting resins or UV-curing resins such as (meth)acrylic acid, urethane, (meth)acrylic urethane, epoxy, and silicone. Other examples include glassy polymers such as silicone polymers. Furthermore, the polymer film described in Japanese Patent Application Publication No. 2001-343529 (WO01/37007) can also be used. For example, the material for this film can include a resin composition containing a thermoplastic resin having substituted or unsubstituted imide groups in its pendant chains and a thermoplastic resin having substituted or unsubstituted phenyl and nitrile groups in its pendant chains. Examples include a resin composition containing an alternating copolymer of isobutylene and N-methylmaleimide and an acrylonitrile-styrene copolymer. The polymer film can be, for example, an extruded product of the above resin composition.

When the polarizing plate 100 is used in an image display device, the thickness of the protective layer (outer protective layer) disposed on the side opposite 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. Furthermore, when surface treatment is applied, the thickness of the outer protective layer includes the thickness of the surface treatment layer.

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

C. Manufacturing method of polarizing film

A method for producing a polarizing film according to one embodiment of the present invention comprises the following steps: coating a PVA-based resin solution on one side of a long thermoplastic resin substrate and drying the solution to form a PVA-based resin layer, thereby producing a laminate; stretching and dyeing the laminate to produce a polarizing film from the PVA-based resin layer; and exposing the polarizing film to a treatment solution having a pH of 3.0 or less. Exposure of the polarizing film to a treatment solution having a pH of 3.0 or less enables the production of a polarizing film with excellent durability in high-temperature and high-humidity environments. Preferably, the PVA-based resin solution further contains a halogenated compound. Preferably, the above-mentioned manufacturing method includes the following steps: sequentially subjecting the laminate to an air-assisted stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying and shrinking treatment. The drying and shrinking treatment comprises heating the laminate while conveying it in the longitudinal direction, thereby shrinking it by at least 2% in the width direction. The halogenated compound content in the PVA resin solution (or, more specifically, the PVA resin layer) is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA resin. The drying and shrinking treatment is preferably performed using a heating roller, and the heating roller temperature is preferably 60°C to 120°C. The resulting widthwise shrinkage of the laminate after the drying and shrinking treatment is preferably at least 2%. According to the manufacturing method, the polarizing film described in Section A above can be obtained. In particular, a polarizing film having excellent optical properties (typically, unit transmittance and unit polarization degree) can be obtained by the following method: After preparing a laminate comprising a PVA-based resin layer containing a halogenated compound, the laminate is stretched through a multi-stage stretching process including air-assisted stretching and underwater stretching, and the stretched laminate is then heated using a heating roller.

C-1. Making a laminate

The laminate of the thermoplastic resin substrate and the PVA-based resin layer can be prepared by any appropriate method. Preferably, a coating liquid containing a halogenated compound and a PVA-based resin is applied to the surface of the thermoplastic resin substrate and dried to form the PVA-based resin layer on the thermoplastic resin substrate. As mentioned above, the halogenated compound content in the PVA-based resin layer is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA-based resin.

The coating liquid can be applied by any appropriate method. Examples include roller coating, spin coating, wire rod coating, dip coating, die coating, curtain coating, spray coating, and doctor blade coating (comma coating, etc.). The application and drying temperature of the coating liquid should preferably be above 50°C.

The thickness of the PVA resin layer should be between 3μm and 40μm, more preferably between 3μm and 20μm.

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

C-1-1. Thermoplastic resin substrate

The thermoplastic resin substrate can be any suitable thermoplastic resin film. Details of thermoplastic resin film substrates are described, for example, in Japanese Patent Application Laid-Open No. 2012-73580. This publication is incorporated herein by reference in its entirety.

C-1-2. Coating liquid

The coating liquid comprises the halogenated compound and the PVA-based resin as described above. The coating liquid is typically a solution of the halogenated compound and the PVA-based resin dissolved in a solvent. Examples of the solvent include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyols such as trihydroxymethylpropane, and amines such as ethylenediamine and diethylenetriamine. These solvents may be used alone or in combination. Water is particularly preferred. The PVA-based resin concentration in the solution is preferably 3 to 20 parts by weight per 100 parts by weight of the solvent. At this resin concentration, a uniform coating film can be formed that adheres closely to the thermoplastic resin substrate. The halogen content in the coating solution is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA resin.

Additives may also be blended into the coating liquid. Examples of additives include plasticizers and surfactants. Plasticizers include polyols such as ethylene glycol and glycerol. Surfactants include non-ionic surfactants. These additives can be used to further enhance the uniformity, dyeability, and extensibility of the resulting PVA-based resin layer.

Any suitable PVA resin can be used as the above-mentioned PVA resin. Examples include polyvinyl alcohol and ethylene-vinyl alcohol copolymer. Polyvinyl alcohol can be obtained by saponifying polyvinyl acetate. Ethylene-vinyl alcohol copolymer can be obtained by saponifying ethylene-vinyl acetate copolymer. The saponification degree of PVA resin is generally 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. Using a PVA resin with this saponification degree can produce a polarizing film with excellent durability. Excessively high saponification degrees may cause gelation.

The average degree of polymerization of PVA-based resins can be appropriately selected depending on the intended purpose. It is generally between 1,000 and 10,000, preferably between 1,200 and 4,500, and more preferably between 1,500 and 4,300. The average degree of polymerization can be determined according to JIS K 6726-1994.

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

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

Generally speaking, stretching a PVA resin layer increases the orientation of the polyvinyl alcohol molecules within the layer. However, if the stretched PVA resin layer is immersed in an aqueous solution, the orientation of the polyvinyl alcohol molecules becomes disrupted, and this orientation decreases. This tendency to decrease orientation is particularly pronounced when stretching a laminate of a thermoplastic resin and a PVA resin layer in boric acid water at relatively high temperatures to stabilize the elongation of the thermoplastic resin. For example, stretching of a PVA film monomer in boric acid water is generally performed at 60°C. In contrast, stretching of a laminate composed of an A-PET (thermoplastic resin substrate) and a PVA-based resin layer is performed at a relatively high temperature, around 70°C. At this temperature, the orientation of the PVA at the beginning of stretching decreases before it increases due to stretching in water. To address this issue, by preparing a laminate composed of a halogenated PVA-based resin layer and a thermoplastic resin substrate and stretching the laminate at a high temperature in air (assisted stretching) before stretching in boric acid water, crystallization of the PVA resin in the PVA-based resin layer of the laminate after assisted stretching can be promoted. As a result, when the PVA resin layer is immersed in a liquid, the orientation disorder of the polyvinyl alcohol molecules and the reduction in orientation are suppressed compared to the case where the PVA resin layer does not contain halides. This can improve the optical properties of polarizing films obtained through treatment steps such as dyeing and underwater stretching, where the laminate is immersed in a liquid.

C-2. Air-assisted extended processing

In particular, to achieve high optical properties, a two-stage stretching method combining dry stretching (assisted stretching) and stretching in boric acid water is chosen. In this two-stage stretching method, by introducing the assisted stretching, the crystallization of the thermoplastic resin substrate can be suppressed while stretching is performed. This solves the problem of reduced stretchability caused by excessive crystallization of the thermoplastic resin substrate during subsequent stretching in boric acid water, thereby allowing the laminate to be stretched at a higher magnification. Furthermore, when coating PVA resins on thermoplastic resin substrates, the coating temperature must be lower than when coating PVA resins on conventional metal rollers to minimize the effect of the thermoplastic substrate's glass transition temperature. This results in a relatively slow crystallization of the PVA resin, preventing the achievement of sufficient optical properties. By introducing assisted stretching, the crystallinity of the PVA resin can be enhanced even when coating the resin on a thermoplastic resin, achieving high optical properties. Furthermore, by pre-enhancing the orientation of the PVA resin, problems such as loss of orientation or dissolution of the PVA resin during subsequent dyeing or stretching steps can be avoided, thereby achieving high optical properties.

In-flight stretching can be performed using either fixed-end stretching (e.g., using a tenter stretching machine) or free-end stretching (e.g., uniaxial stretching by passing the laminate between rollers with different circumferential speeds). However, to achieve high optical properties, free-end stretching is preferred. In one embodiment, the in-flight stretching process includes a heated roller stretching step, in which the laminate is stretched while being transported along its longitudinal direction, utilizing the differential circumferential speed between the heated rollers. The in-flight stretching process typically includes a zone stretching step and a heated roller stretching step. In addition, the order of the zone stretching step and the heating roller stretching step is not limited. The zone stretching step can be performed first, or the heating roller stretching step can be performed first. The zone stretching step can also be omitted. In one embodiment, the zone stretching step and the heating roller stretching step are performed in sequence. In another embodiment, the film ends are grasped in a tenter stretching machine, and the distance between the tenters is expanded in the direction of travel to perform stretching (the increase in the distance between the tenters is the stretching ratio). At this time, the distance between the tenters in the width direction (perpendicular to the direction of travel) is set so that they can be arbitrarily approached. Preferably, the stretching ratio relative to the direction of travel can be set to achieve approach using free end stretching. When the free end is extended, the shrinkage rate in the width direction is calculated as (1/extension ratio) ^1/2 .

Air-assisted stretching can be performed in a single stage or in multiple stages. When performed in multiple stages, the stretching ratio is the product of the stretching ratios in each stage. The stretching direction in air-assisted stretching should be roughly the same as the stretching direction in underwater stretching.

The stretching ratio for air-assisted stretching is preferably 2.0x to 3.5x. The maximum stretching ratio when combining air-assisted stretching with underwater stretching is preferably at least 5.0x, preferably at least 5.5x, and even more preferably at least 6.0x, relative to the original length of the laminate. The "maximum stretching ratio" in this specification refers to the stretching ratio immediately before the laminate fractures. This is a value 0.2 lower than the stretching ratio at which the laminate fractures, after separately confirming the stretching ratio.

The stretching temperature for air-assisted stretching can be set to any appropriate value depending on the thermoplastic resin substrate material, stretching method, and other factors. The stretching temperature is preferably above the glass transition temperature (Tg) of the thermoplastic resin substrate, more preferably above the glass transition temperature (Tg) of the thermoplastic resin substrate + 10°C, and particularly preferably above Tg + 15°C. On the other hand, the upper limit of the stretching temperature is preferably 170°C. Stretching at this temperature suppresses the rapid crystallization of the PVA resin, thereby preventing adverse effects caused by this crystallization (e.g., interference with orientation of the PVA resin layer due to stretching).

C-3. Insolubilization, dyeing, and cross-linking treatments

If necessary, an insolubilization treatment is performed after the air-assisted stretching treatment and before the underwater stretching treatment or 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 can typically be 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 Japanese Patent Application Laid-Open No. 2012-73580 (cited above).

C-4. Extended treatment in water

Underwater stretching is performed by immersing the laminate in a stretching bath. This allows stretching at temperatures lower than the glass transition temperature (typically around 80°C) of the thermoplastic resin substrate or PVA resin layer, enabling high-ratio stretching while suppressing crystallization of the PVA resin layer. The result is a polarizing film with excellent optical properties.

The laminate can be stretched using any appropriate method. Specifically, it can be fixed-end stretching or free-end stretching (for example, a method in which the laminate is stretched uniaxially by passing it between rollers with different circumferential speeds). Free-end stretching is preferred. Stretching of the laminate can be performed in a single stage or in multiple stages. When stretching is performed in multiple stages, the stretch ratio (maximum stretch ratio) of the laminate, described below, is the product of the stretch ratios of each stage.

Underwater stretching is preferably performed by immersing the laminate in an aqueous boric acid solution (boric acid underwater stretching). Using an aqueous boric acid solution as the stretching bath imparts the PVA resin layer with the rigidity required to withstand the tension during stretching and the water resistance required to remain insoluble in water. Specifically, boric acid in aqueous solution forms tetrahydroxyboric acid anions, which crosslink with the PVA resin via hydrogen bonds. This imparts rigidity and water resistance to the PVA resin layer, allowing for excellent stretching and resulting in a polarizing film with superior optical properties.

The boric acid aqueous solution is preferably obtained by dissolving boric acid and/or a boric acid salt in water as a solvent. The boric acid concentration is preferably 1 to 10 parts by weight, more preferably 2.5 to 6 parts by weight, and particularly preferably 3 to 5 parts by weight per 100 parts by weight of water. Setting the boric acid concentration to 1 part by weight or higher effectively inhibits the dissolution of the PVA resin layer, resulting in a polarizing film with higher performance. In addition to boric acid or a boric acid salt, aqueous solutions obtained by dissolving boron compounds such as borax, glyoxal, glutaraldehyde, etc. in a solvent may also be used.

It is preferable to add an iodide to the stretching bath (boric acid aqueous solution). This addition can suppress the dissolution of iodine adsorbed in the PVA-based resin layer. Specific examples of iodide are 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 (stretching bath temperature) is preferably between 40°C and 85°C, more preferably between 60°C and 75°C. This temperature suppresses dissolution of the PVA resin layer while enabling high stretching ratios. Specifically, as mentioned above, the glass transition temperature (Tg) of the thermoplastic resin substrate should ideally be above 60°C in relation to the formation of the PVA resin layer. If the stretching temperature is below 40°C, even if water is used to plasticize the thermoplastic resin substrate, good stretching may not be possible. On the other hand, higher stretching bath temperatures increase the solubility of the PVA resin layer, potentially preventing the achievement of excellent optical properties. The laminate should be immersed in the stretching bath for a period of 15 seconds to 5 minutes.

The stretching ratio for underwater stretching is preferably 1.5 times or greater, more preferably 3.0 times or greater. The total stretching ratio of the laminate is preferably 5.0 times or greater, more preferably 5.5 times or greater, relative to the original length of the laminate. Achieving such a high stretching ratio enables the production of a polarizing film with excellent optical properties. This high stretching ratio can be achieved by using an underwater stretching method (boric acid underwater stretching).

C-5. Drying and shrinking treatment

The drying and shrinking process can be performed by heating the entire area (regional heating) or by heating the transport rollers (so-called heating roller drying). Using both methods is preferred. Using heating rollers for drying effectively suppresses heat warping of the laminate, resulting in a polarizing film with excellent appearance. Specifically, drying the laminate along the heating rollers effectively promotes crystallization of the thermoplastic resin substrate, increasing its degree of crystallization. Even at relatively low drying temperatures, the degree of crystallization of the thermoplastic resin substrate can be effectively increased. As a result, the rigidity of the thermoplastic resin substrate increases, allowing it to withstand the shrinkage of the PVA resin layer during drying, thereby suppressing curling. Furthermore, the use of heated rollers allows the laminate to be maintained flat during drying, thereby suppressing not only curling but also wrinkling. The drying and shrinking treatment allows the laminate to shrink in the width direction, improving optical properties. This is because it effectively enhances the orientation of the PVA and PVA/iodine complex. The widthwise shrinkage rate of the laminate after drying and shrinking is preferably 1% to 10%, more preferably 2% to 8%, and most preferably 4% to 6%.

Figure 2 schematically illustrates an example of a drying and shrinking process. During the drying and shrinking process, conveyor rollers R1-R6 and guide rollers G1-G4, heated to a predetermined temperature, are used to convey the laminate 200 while drying it. In the illustrated example, the conveyor rollers R1-R6 are positioned to alternately and continuously heat the PVA resin layer surface and the thermoplastic resin substrate surface. However, the conveyor rollers R1-R6 can also be positioned to continuously heat only one surface of the laminate 200 (e.g., the thermoplastic resin substrate surface).

Drying conditions can be controlled by adjusting the heating temperature of the conveyor rollers (heating roller temperature), the number of heating rollers, and the contact time with the heating rollers. The temperature of the heating rollers is preferably 60°C to 120°C, more preferably 65°C to 100°C, and particularly preferably 70°C to 80°C. This can effectively increase the crystallization degree of the thermoplastic resin and effectively suppress curling, while producing an optical laminate with excellent durability. The temperature of the heating rollers can also be measured with a contact thermometer. In the example shown in the figure, six conveyor rollers are provided, but there is no particular limitation as long as the number of conveyor rollers is the same. The number of conveyor rollers is generally 2 to 40, with 4 to 30 being preferred. The contact time (total contact time) between the laminate and the heating roller should ideally be 1 to 300 seconds, preferably 1 to 20 seconds, and even more preferably 1 to 10 seconds.

The heating rollers can be installed inside a furnace (such as an oven) or on a standard manufacturing line (at room temperature). They are preferably installed inside a furnace equipped with an air supply mechanism. By combining drying with the heating rollers and hot air drying, rapid temperature fluctuations between the rollers can be suppressed, making it easier to control shrinkage in the width direction. The hot air drying temperature should ideally be between 30°C and 100°C. The hot air drying time should ideally be between 1 and 300 seconds. The hot air velocity should ideally be between 10 and 30 m/s. This velocity is measured within the furnace and can be measured with a mini fan-type digital anemometer.

C-6. Contact with treatment fluid

In this manner, a laminate of a thermoplastic resin substrate and a polarizing film can be obtained. In one embodiment of the present invention, the polarizing film is exposed to a treatment solution having a pH of 3.0 or less. In one embodiment, the polarizing film and the treatment solution can be brought into contact by directly contacting the laminate. In this case, the thermoplastic resin substrate can be used directly as a protective layer for the polarizing film. Alternatively, a resin film (serving as a protective layer) may be laminated to the surface of the polarizing film of the laminate that has been in contact with the treatment liquid to produce a laminate of protective layer/polarizing film/thermoplastic resin substrate. The thermoplastic resin substrate may then be peeled off from the laminate to produce a polarizing plate having a protective layer/polarizing film structure. In another embodiment, a resin film (serving as a protective layer) may be laminated to the surface of the polarizing film of the laminate to produce a laminate of protective layer/polarizing film/thermoplastic resin substrate. The thermoplastic resin substrate may then be peeled off from the laminate to produce a laminate of protective layer/polarizing film (polarizing plate). By contacting the resulting polarizing plate with a treatment liquid, the polarizing film can be brought into contact with the treatment liquid.

The polarizing film and the treatment liquid can be brought into contact with each other using any appropriate method. Representative examples include applying the treatment liquid to the polarizing film or immersing the polarizing film (actually, a laminate or polarizing plate) in the treatment liquid. Any appropriate method can be used for the application. A specific example is the method described in Section C-1 as a method for applying the coating liquid. Immersion can also be performed using any appropriate method. For example, the treatment liquid can be added to the cleaning bath, a treatment liquid bath can be used instead of the cleaning bath, or a treatment liquid bath can be provided separately from the cleaning bath. Furthermore, the cleaning treatment is typically performed after the stretching treatment in water and before the drying and shrinking treatment. When a treatment liquid bath is provided, it can be placed between the cleaning bath and the drying and shrinking treatment equipment (i.e., contact with the treatment liquid can also be carried out between the cleaning and drying and shrinking treatments), or it can be placed downstream of the mechanism for stripping the thermoplastic resin substrate (i.e., contact with the treatment liquid can also be carried out after stripping the thermoplastic resin substrate).

Any suitable acidic liquid can be used as long as the pH of the treatment liquid is below 3.0. 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 (the stronger the acidity), the better. Specifically, the pH is preferably below 2.7, more preferably below 2.5, more preferably below 2.0, and most preferably below 1.5.

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 also contain a water-soluble resin (such as a PVA-based resin). The water-soluble resin can function as a binder. The concentration of the water-soluble resin in the treatment liquid is preferably 3% to 5% by weight. The treatment liquid is then applied and dried to form a treatment layer. Forming this treatment layer also allows for a polarizing film with the desired durability described above. The thickness of the treatment layer is preferably 1.7 μm or less, more preferably 0.2 μm to 1.4 μm.

After contact with the treatment solution, drying can be performed as needed. The drying temperature is preferably between 40°C and 90°C, more preferably between 50°C and 70°C.

C-7. Variations

Sections C-1 to C-6 describe a method for producing a laminate using a resin substrate and a PVA-based resin layer coated on the resin substrate. However, the present invention can also be applied to a method for producing a laminate using a single PVA-based resin film. The method typically comprises the following steps: uniaxially stretching a long strip of PVA-based resin film in the longitudinal direction using a roll stretching machine, simultaneously performing swelling, dyeing, crosslinking, and cleaning treatments, and finally drying. Contact with the treatment solution can typically be achieved by immersion in a cleaning bath containing the treatment solution, immersion in a treatment bath after cleaning, or application of the treatment solution after cleaning.

Implementation Examples

The present invention is described below in detail using examples, but the present invention is not limited to these examples. The measurement methods for various properties are described below. Furthermore, unless otherwise specified, "parts" and "%" in the examples and comparative examples are by weight.

(1)Thickness

Measurements were performed using an interferometer film thickness meter (manufactured by Otsuka Electronics Co., Ltd., product name: "MCPD-3000").

(2) Single body transmittance and orthogonal absorbance

The polarizing plates (protective layers/polarizing films) of the Examples and Comparative Examples were measured using a UV-visible spectrophotometer (LPF-200, manufactured by Otsuka Electronics). The measured single-element transmittance Ts, parallel transmittance Tp, and cross transmittance Tc were used as the Ts, Tp, and Tc of the polarizing film, respectively. These Ts, Tp, and Tc values were measured using a 2-degree field of view (illuminant C) in accordance with JIS Z8701 and corrected for visual sensitivity to obtain Y values. The refractive index of the protective film was 1.50, and the refractive index of the polarizing film surface opposite the protective film was 1.53.

Furthermore, using the Tc values measured at each wavelength, the orthogonal absorbance was calculated using 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 the "LPF-200" manufactured by Otsuka Electronics Co., Ltd. Similar measurements of Abs ₀ can also be performed using the "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. The orthogonal absorbance Abs ₂₄₀ after the durability test was determined in the same manner as above.

[Example 1]

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

To 100 parts by weight of a PVA-based resin, prepared by mixing polyvinyl alcohol (DP 4200, DS 99.2 mol%) and acetyl-modified PVA (trade name "GOHSEFIMER Z410" manufactured by Nippon Gosei Kagaku Kogyo Co., Ltd.) in a 9:1 ratio, 13 parts by weight of potassium iodide was added to prepare a PVA aqueous solution (coating solution).

The PVA aqueous solution was applied to the corona-treated surface of the resin substrate and dried at 60°C to form a 20 μm thick PVA resin layer to produce a laminate.

The obtained laminate was subjected to free-end uniaxial stretching by 2.4 times in the longitudinal direction (longitudinal direction) between rollers of different peripheral speeds in an oven at 130°C (air-assisted stretching treatment).

Next, the laminate was immersed in an insolubilization bath (a boric acid aqueous solution prepared by mixing 4 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40°C for 30 seconds (insolubilization treatment).

Next, the polarizing plate was immersed in a dye bath (an iodine aqueous solution obtained by mixing iodine and potassium iodide at a weight ratio of 1:7 relative to 100 parts by weight of water) at a temperature of 30°C for 60 seconds (dyeing process).

Next, it was immersed in a crosslinking bath (a boric acid aqueous solution prepared by mixing 3 parts by weight of potassium iodide and 5 parts by weight of boric acid per 100 parts by weight of water) at a liquid temperature of 40°C for 30 seconds (crosslinking treatment).

The laminate was then immersed in a boric acid aqueous solution (boric acid concentration 4.0% by weight, potassium iodide 5% by weight) at 70°C while being uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls of different circumferential speeds to a total stretching ratio of 5.5 times (underwater stretching treatment).

Afterwards, the laminate was immersed in a cleaning bath at a temperature of 20°C (an aqueous solution containing 4 parts by weight of potassium iodide per 100 parts by weight of water, pH = 6) (cleaning treatment).

Afterwards, the laminate was dried in an oven maintained at 90°C while contacting a SUS heating roller maintained at 75°C for approximately 2 seconds (drying and shrinking treatment). The resulting shrinkage rate in the width direction of the laminate during the drying and shrinking treatment was 2%.

Following the above method, a 5.0μm-thick polarizing film was formed on a resin substrate. A cycloolefin film (ZEON G-Film) was then laminated to the polarizing film surface as a protective layer (protective film) using a UV-curable adhesive (1.0μm thick). The resin substrate was then peeled off to obtain a laminate consisting of a protective layer/polarizing film. The single-unit transmittance (Ts) of the resulting laminate was 44.0%. This was calculated by correcting the actual measured value by +0.2% to convert it to a value of 1.53/1.50, based on the surface refractive indices of the polarizing film/protective layer constituting the laminate being 1.53/1.53.

Next, a treatment solution (pH = 1.3) was prepared by dissolving 0.3% by weight of hydrochloric acid and 3.5% by weight of PVA (JC-25) in water. This solution was applied to the surface of the laminated polarizing film to a thickness of 0.6 μm and dried at 60°C for 4 minutes to form a treatment layer.

The polarizing plate of this embodiment is obtained in the above manner.

Table 1 shows the single-unit transmittance and Abs ₂₄₀ /Abs ₀ of the obtained polarizing plate (actually a polarizing film).

[Examples 2-10]

Polarizing plates were produced by adjusting the monomer transmittance of the polarizing film, the method of contact with the treatment solution, the pH of the treatment solution, the type of acid contained in the treatment solution, and the thickness of the treatment layer as shown in Table 1. Table 1 shows the monomer transmittance and Abs ₂₄₀ /Abs ₀ of the resulting polarizing plates (essentially polarizing films).

[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 resin (i.e., no treatment layer was formed) and the pH of the treatment solution was set to 0.9. Table 1 shows the monomer transmittance and Abs ₂₄₀ /Abs ₀ of the resulting polarizing plate (essentially a polarizing film).

[Example 12]

Similar to Example 1, a thermoplastic resin substrate/PVA resin layer laminate was subjected to an air-assisted stretching treatment, an insolubilization treatment, a dyeing treatment, a crosslinking treatment, and an underwater stretching treatment. The laminate, after the underwater stretching treatment, was immersed in a treatment bath (pH = 1.6) at a temperature of 20°C (contacting the treatment solution). The treatment solution was prepared by adding hydrochloric acid to a conventional cleaning bath (an aqueous solution prepared by mixing 4 parts by weight of potassium iodide with 100 parts by weight of water).

Afterwards, the laminate was dried in an oven maintained at 90°C while contacting a SUS heating roller maintained at 75°C for approximately 2 seconds (drying and shrinking treatment). The resulting shrinkage rate in the width direction of the laminate during the drying and shrinking treatment was 2%.

Next, a cycloolefin film (ZEON G-Film) was bonded to the polarizing film surface as a protective layer (protective film) using a UV-curable adhesive (1.0 μm thick). The resin substrate was then peeled off to obtain a polarizing plate with a protective layer/polarizing film structure. Table 1 shows the single-unit transmittance and Abs ₂₄₀ /Abs ₀ of the resulting polarizing plate (essentially a polarizing film).

[Comparative example 1]

A polarizing plate was prepared in the same manner as in Example 1, except that the plate was not in contact with the treatment solution. Table 1 shows the single transmittance and Abs ₂₄₀ /Abs ₀ of the obtained polarizing plate (essentially 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 set to 45.0%. Table 1 shows the single transmittance and Abs ₂₄₀ /Abs ₀ of the resulting polarizing plate (essentially a polarizing film).

<Comparative Examples 3~8>

Polarizing plates were produced by adjusting the monomer transmittance of the polarizing film, the method of contact with the treatment solution, the pH of the treatment solution, the type of acid contained in the treatment solution, and the thickness of the treatment layer (if formed) as shown in Table 1. Table 1 shows the monomer transmittance and Abs ₂₄₀ /Abs ₀ of the resulting polarizing plates (essentially polarizing films).

[Example 13]

A 55μm-thick roll of PVA resin film (PS7500, manufactured by Nippon Gosei Co., Ltd.) was uniaxially stretched in the longitudinal direction to a total stretch ratio of 6.0x using a roll stretching machine. Simultaneously, the film was subjected to swelling, dyeing, crosslinking, and cleaning, and finally dried to produce a 23μm-thick polarizing film. After cleaning and before drying, the same treatment solution as in Example 1 was applied to one side of the PVA resin film (polarizing film) in the same manner as in Example 1. The resulting polarizing film has a single-unit transmittance and Abs ₂₄₀ /Abs ₀ ratio, shown in Table 1.

[Example 14]

Instead of using the cleaning bath, the PVA resin film (polarizing film) was passed through the same treatment bath as in Example 12 (therefore, no treatment solution was applied after the cleaning treatment). A 23 μm thick polarizing film was produced in the same manner as in Example 13. Table 1 shows the single-unit transmittance and Abs ₂₄₀ /Abs ₀ of the resulting polarizing film.

Table 1 clearly shows that the polarizing films of the examples of the present invention exhibited an Abs ₂₄₀ /Abs ₀ ratio greater than 0.90 after the durability test, significantly suppressing degradation of polarization performance in high-temperature, high-humidity environments. In other words, the polarizing films of the examples of the present invention exhibit excellent durability in high-temperature, high-humidity environments. In particular, the polarizing film of Example 10 exhibited an Abs ₂₄₀ /Abs ₀ ratio greater than 1.0, demonstrating enhanced polarization performance in high-temperature, high-humidity environments. This exceptional performance is unexpected and contradicts conventional wisdom. The polarizing films of Comparative Examples 1 and 2, which were not exposed to the treatment solution, and the polarizing films of Comparative Examples 3-8, which were exposed to a treatment solution with a pH greater than 3.0, all had _Abs240 / _{Abs0 values} below 0.88. Furthermore, in Comparative Example 6, which used boric acid as the treatment solution, the treatment solution gelled and could not be applied.

Industrial Availability

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

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

Claims (6)

A method for manufacturing a polarizing film, wherein the polarizing film is composed of an iodine-containing polyvinyl alcohol-based resin film. After a 240-hour durability test at a temperature of 60°C and a relative humidity of 95%, the absorbance at a wavelength of 470 nm (Abs ₂₄₀ ) relative to the absorbance before the durability test (Abs ₀ ) satisfies the following relationship: Abs ₂₄₀ /Abs ₀ > 0.90. The manufacturing method comprises the following steps: forming a polyvinyl alcohol-based resin layer on one side of a long thermoplastic resin substrate to form a laminate; The laminate is sequentially subjected to an air-assisted stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying and shrinking treatment. The drying and shrinking treatment involves heating the laminate while conveying it along its length, thereby shrinking it by more than 2% in its width direction, thereby converting the polyvinyl alcohol-based resin layer into a polarizing film. The polarizing film is then exposed to a treatment solution having a pH of less than 2.0. This contact with the treatment solution occurs after the drying and shrinking treatment. The manufacturing method of claim 1 comprises: applying the aforementioned treatment liquid on the aforementioned polarizing film. The manufacturing method of claim 1 comprises: immersing the polarizing film in the treatment solution. The manufacturing method of any one of claims 1 to 3, wherein a polyvinyl alcohol-based resin layer comprising iodide or sodium chloride and a polyvinyl alcohol-based resin is formed on one side of the thermoplastic resin substrate. The manufacturing method of any one of claims 1 to 3, wherein the drying and shrinking treatment is performed using a heated roller. The manufacturing method of claim 5, wherein the temperature of the heating roller is 60°C to 120°C.