JP7555836B2 - Manufacturing method for polarizing plate, manufacturing method for image display device, and method for adjusting transmittance of polarizing film - Google Patents

Description

The present invention relates to a method for manufacturing a polarizing plate, a method for manufacturing an image display device, and a method for adjusting the transmittance of 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. Due to the image formation method used, liquid crystal display devices have polarizing plates arranged on both sides of the liquid crystal cell. It is also known that in organic EL display devices, a circular polarizing plate including a λ/4 plate is arranged on the viewing side of the organic EL cell to prevent problems such as external light reflection and background glare (e.g., Patent Documents 1 and 2).

A polarizing plate is usually constructed by disposing a protective layer on at least one side of a polarizing film made by dyeing and stretching a polyvinyl alcohol-based resin film, and is attached to an image display cell such as a liquid crystal cell or an organic EL cell via an adhesive layer.

JP 2002-311239 A JP 2002-372622 A

Conventionally, there has been no known method for adjusting the transmittance of a polarizing film after its production, and therefore it has not been possible to adjust the transmittance of the polarizing film after the polarizing plate is attached to the image display cell. On the other hand, since the brightness of the image display device can vary due to individual differences in the quality of the image display cell, backlight unit, etc., there has been a demand for the development of a method for adjusting the transmittance of the polarizing film after the protective layer is laminated or after it is attached to the image display cell.

The present invention has been made to solve the above-mentioned problems in the conventional art, and its main objective is to provide a method for adjusting the transmittance of a polarizing film after its production.

According to one aspect of the present invention, there is provided a method for producing a polarizing plate, comprising, in this order, a step of subjecting a polyvinyl alcohol-based resin film to a dyeing treatment and a stretching treatment in an aqueous boric acid solution, followed by drying until the moisture content is 15% by weight or less to obtain a primary polarizing film, and a step of contacting an aqueous solvent with the surface of the primary polarizing film to change the transmittance to obtain a secondary polarizing film.
In one embodiment, the aqueous solvent is contacted with the exposed surface of the primary polarizing film, which has one surface exposed and the other surface protected.
In one embodiment, the step of obtaining the primary polarizing film includes subjecting a polyvinyl alcohol-based resin film containing a halide and a polyvinyl alcohol-based resin, in the form of a laminate with a long thermoplastic resin substrate, to an air-assisted stretching treatment, a dyeing treatment, a stretching treatment in an aqueous boric acid solution, and a drying and shrinking treatment in this order, and the drying and shrinking treatment includes heating the laminate while transporting it in the longitudinal direction, thereby shrinking the laminate by 2% or more in the width direction, and drying the polyvinyl alcohol-based resin film until its moisture content becomes 15% by weight or less.
In one embodiment, the halide is iodide or sodium chloride.
In one embodiment, the primary polarizing film has a thickness of 12 μm or less.
According to another aspect of the present invention, there is provided a method for manufacturing an image display device, comprising the steps of: laminating a polarizing plate, which is made of a polyvinyl alcohol-based resin film containing a dichroic substance and which includes, in this order, a polarizing film having a moisture content of 15% by weight or less, a protective layer, and a pressure-sensitive adhesive layer, on an image display cell via the pressure-sensitive adhesive, so as to expose the surface of the polarizing film opposite to the side on which the protective layer is disposed; and bringing an aqueous solvent into contact with the exposed surface of the polarizing film to change the transmittance.
In one embodiment, the image display device is a liquid crystal display device or an organic EL display device.
According to yet another aspect of the present invention, there is provided a method for adjusting the transmittance of a polarizing film, the method including a step of bringing an aqueous solvent into contact with a surface of a polarizing film which is made of a polyvinyl alcohol-based resin film containing a dichroic substance and has a moisture content of 15% by weight or less.

According to the present invention, the transmittance of a polarizing film can be changed after the fact by contacting an aqueous solvent with the surface of the polarizing film produced by dyeing and stretching a polyvinyl alcohol-based resin film. In addition, the transmittance can be changed by contacting an aqueous solvent with the surface of a polarizing film whose appearance has been stabilized by drying until the moisture content is below a predetermined value after stretching in an aqueous boric acid solution, thereby making it possible to finely adjust the transmittance. As a result, for example, the variation in brightness between image display devices can be reduced, which is useful when it is desired to align the appearance of multiple display screens, and in particular when multiple display screens are combined to display images (large public displays, digital signage, etc.).

FIG. 2 is a schematic diagram showing an example of a drying shrinkage treatment using a heating roll. FIG. 2 is a schematic cross-sectional view illustrating an example of a polarizing plate that can be produced in the process of producing a polarizing plate. FIG. 2 is a schematic cross-sectional view illustrating an example of a polarizing plate that can be produced in the process of producing a polarizing plate. FIG. 2 is a schematic cross-sectional view illustrating an example of a polarizing plate that can be produced in the process of producing a polarizing plate. 1 is a schematic cross-sectional view of an example of a polarizing plate that can be produced by a process of protecting an exposed surface of a secondary polarizing film. 1 is a schematic cross-sectional view of an example of a polarizing plate that can be produced by a process of protecting an exposed surface of a secondary polarizing film.

The following describes embodiments of the present invention, but the present invention is not limited to these embodiments. Note that the polarizing plate obtained by the manufacturing method of the polarizing plate according to the embodiment of the present invention includes at least a polarizing film, and preferably includes a polarizing film and a protective layer disposed on one or both sides of the polarizing film.

A. Manufacturing method of polarizing plate The manufacturing method of polarizing plate according to the embodiment of the present invention includes the steps of dyeing a polyvinyl alcohol (PVA) resin film and stretching it in a boric acid aqueous solution (stretching in boric acid water), and then drying it until the moisture content is 15% by weight or less to obtain a primary polarizing film, and then contacting an aqueous solvent with the surface of the primary polarizing film to change the transmittance to obtain a secondary polarizing film, in this order. According to the manufacturing method of polarizing plate, the transmittance can be changed afterwards to a desired value by contacting an aqueous solvent with the surface of a polarizing film (primary polarizing film) once produced to decolorize it. In addition, by contacting the polarizing film (primary polarizing film) in a state of being highly oriented and having a stabilized appearance through the stretching in boric acid water and drying treatment with an aqueous solvent, the transmittance can be suitably adjusted while avoiding excessive reduction in the degree of polarization, wrinkles, dissolution, etc.

In the method for manufacturing a polarizing plate according to an embodiment of the present invention, the transmittance may be changed by contacting only one side of the primary polarizing film with an aqueous solvent, or the transmittance may be changed by contacting both sides with an aqueous solvent.

In one embodiment of the method for producing a polarizing plate according to the present invention, the transmittance is changed by contacting an aqueous solvent with the exposed surface of a primary polarizing film in a state where one surface is exposed and the other surface is protected. The method for producing a polarizing plate according to this embodiment can include, for example, between the steps of obtaining a primary polarizing film and obtaining a secondary polarizing film, a step of producing a polarizing plate in which one surface of the primary polarizing film is exposed and the other surface is protected by a protective layer.

A-1. Step of Obtaining Primary Polarizing Film In the step of obtaining a primary polarizing film, a PVA-based resin film is dyed and stretched in boric acid water, and then dried until the moisture content is 15% by weight or less to obtain a primary polarizing film. The primary polarizing film may be prepared using a single layer of PVA-based resin film, or may be prepared using a laminate of two or more layers including a PVA-based resin layer (PVA-based resin film). A primary polarizing film prepared using a laminate of two or more layers can favorably maintain excellent optical properties (typically, single transmittance and polarization degree) while avoiding the occurrence of wrinkles, even after contact with an aqueous solvent.

A-1-1. Preparation of a primary polarizing film using a laminate of two or more layers Preparation of a primary polarizing film using a laminate of two or more layers can be performed, for example, by a method including subjecting a PVA-based resin film containing a halide and a PVA-based resin, in the form of a laminate with a long thermoplastic resin substrate, to an air-assisted stretching treatment, a dyeing treatment, a stretching treatment in an aqueous boric acid solution, and a drying shrinkage treatment in this order. A laminate of a thermoplastic resin substrate and a PVA-based resin film can be obtained, for example, by 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. The drying shrinkage treatment includes, for example, heating the laminate of the long thermoplastic resin substrate and the PVA-based resin film while transporting it in the longitudinal direction, thereby shrinking it by 2% or more in the width direction, and drying the PVA-based resin film until the moisture content of the PVA-based resin film becomes 15% by weight or less. The content of the halide in the PVA-based resin layer is preferably 5 to 20 parts by weight based on 100 parts by weight of the PVA-based resin. The drying shrinkage treatment is preferably performed using a heating roll, and the temperature of the heating roll is preferably 60° C. to 120° C. According to such a production method, a primary polarizing film having a high degree of orientation of the PVA-based resin and excellent optical properties can be obtained.

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, it is possible to improve the adhesion between the thermoplastic resin substrate and the PVA-based resin layer.

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 exceeds 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 polarizing film. It is also 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 transverse direction. The transverse 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 stretching of the thermoplastic resin substrate may be performed in one stage or in multiple stages. When it is performed in multiple stages, the above-mentioned stretching ratio is the product of the stretching 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, a polarizing film with excellent durability can be obtained. If the saponification degree is too high, there is a risk of gelation.

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

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

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

Generally, the orientation of the polyvinyl alcohol molecules in the PVA-based resin 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 is disturbed and the orientation is reduced, compared to when the PVA-based resin layer does not contain a halide. This can improve the optical properties of the polarizing film obtained 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, and the problem of the thermoplastic resin substrate being excessively crystallized and reduced in stretchability in the subsequent stretching in boric acid water can be solved, 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, and as a result, the crystallization of the PVA-based resin is relatively low, and a problem that sufficient optical properties cannot be obtained may occur. 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 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 step or in multiple steps. When the air-assisted stretching is performed in multiple steps, the stretching ratio is the product of the stretching ratios in each step. The stretching direction in the air-assisted stretching is preferably approximately the same as the stretching direction in the underwater stretching.

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

The stretching temperature 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 proceeding 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 with respect to 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 polarizing film finally obtained is the desired value. As for 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 a primary polarizing 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, a polarizing film having excellent optical properties can be produced.

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

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

The boric acid aqueous solution is preferably obtained by dissolving boric acid and/or a borate in water, which is a solvent. The boric acid concentration is preferably 1 to 10 parts by weight, more preferably 2.5 to 6 parts by weight, and particularly preferably 3 to 5 parts by weight, per 100 parts by weight of water. By making the boric acid concentration 1 part by weight or more, dissolution of the PVA-based resin layer can be effectively suppressed, and a polarizing film with higher characteristics can be produced. In addition to boric acid or a borate, an aqueous solution obtained by dissolving a boron compound such as borax, glyoxal, glutaraldehyde, or the like in a solvent can also be used.

Preferably, iodide is added to the stretching bath (boric acid aqueous solution). By adding iodide, it is possible to suppress the elution of iodine adsorbed in the PVA-based resin layer. Specific examples of iodide are as described above. The concentration of iodide is preferably 0.05 to 15 parts by weight, more preferably 0.5 to 8 parts by weight, per 100 parts by weight of water.

The stretching temperature (liquid temperature of the stretching bath) is preferably 40°C to 85°C, more preferably 60°C to 75°C. At such a temperature, the PVA-based resin layer can be stretched at a high ratio while suppressing dissolution. Specifically, as described above, the glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 60°C or higher in relation to the formation of the PVA-based resin layer. In this case, if the stretching temperature is below 40°C, even if the plasticization of the thermoplastic resin substrate by water is taken into consideration, there is a risk that the substrate cannot be stretched well. On the other hand, the higher the temperature of the stretching bath, the higher the solubility of the PVA-based resin layer, and the higher the risk that excellent optical properties cannot be obtained. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

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

The drying shrinkage treatment includes, for example, heating the laminate of the long thermoplastic resin substrate and the PVA-based resin film while conveying it in the longitudinal direction, thereby shrinking the laminate by 2% or more in the width direction, and drying the PVA-based resin film until the moisture content of the PVA-based resin film is 15% by weight or less. From the viewpoint of obtaining a stable appearance, it is preferable to dry the laminate until the moisture content is 12% by weight or less, 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 a primary polarizing film with excellent appearance. Specifically, by drying the laminate in a state where it is aligned with the heating roll, it is possible to efficiently promote the crystallization of the thermoplastic resin substrate and increase the crystallinity, and even at a relatively low drying temperature, it is possible to satisfactorily increase the crystallinity of the thermoplastic resin substrate. As a result, the rigidity of the thermoplastic resin substrate increases and it becomes in a state where it can withstand the shrinkage of the PVA-based resin layer due to drying, and curling is suppressed. In addition, by using a heating roll, 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 30°C to 100°C. The hot air drying time is preferably 1 second to 300 seconds. The hot air speed is preferably about 10 m/s to 30 m/s. The air speed is the air speed in the heating furnace, and can be measured by a mini-vane type digital anemometer.

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.

A-1-2. Preparation of a primary polarizing film using a single-layer PVA-based resin film Preparation of a primary polarizing film using a single-layer PVA-based resin film can be performed by dyeing and stretching (typically, uniaxial stretching using a roll stretching machine) a long PVA-based resin film having self-supporting properties (i.e., not requiring support by a substrate) in boric acid water, and then drying until the moisture content is 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. The dyeing is performed, for example, by immersing the PVA-based resin film in an iodine aqueous 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 film is being dyed. Alternatively, the film may be stretched and then dyed. 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.

A-1-3. Primary polarizing film The primary polarizing film preferably exhibits absorption dichroism at any wavelength of 380 nm to 780 nm. The transmittance (single transmittance: Ts) of the primary polarizing film is preferably 41.5% or more, more preferably 42.0% or more, and even more preferably 42.5% or more. On the other hand, the transmittance of the primary polarizing film is preferably 46.0% or less, more preferably 45.0% or less. The degree of polarization of the primary polarizing 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 degree of polarization of the primary polarizing 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 degree of polarization 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 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 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, 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 thickness of the primary polarizing film is typically 25 μm or less, preferably 12 μm or less, more preferably 1 μm to 8 μm, even more preferably 1 μm to 7 μm, and even more preferably 2 μm to 5 μm. A small thickness has the advantage that the polarizing film is less likely to wrinkle when it comes into contact with an aqueous solvent.

The moisture content of the primary polarizing 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 is within the above range, the transmittance can be changed without significant loss of appearance when contacted with an aqueous solvent.

A-2. Step of preparing a polarizing plate In the step of preparing a polarizing plate, a polarizing plate having a configuration in which a protective layer and an optional functional layer are laminated on one side of a primary polarizing film and the other side is an exposed surface is prepared. Any appropriate functional layer can be selected depending on the purpose, and specific examples thereof include a retardation layer, a pressure-sensitive adhesive layer, and the like. The step of preparing the polarizing plate is an optional step. Therefore, depending on the purpose, a laminate having a configuration of [thermoplastic resin substrate/primary polarizing film] prepared by the method described in A-1-1 or a primary polarizing film prepared using a single-layer PVA-based resin film described in A-1-2 can be directly subjected to the step of obtaining a secondary polarizing film.

2A to 2C are schematic cross-sectional views each illustrating an example of a polarizing plate that can be produced in the process of producing a polarizing plate. The polarizing plate 100A shown in FIG. 2A includes a polarizing film (primary polarizing film) 10 and a protective layer 20 disposed on one side thereof, and the side of the polarizing film (primary polarizing film) 10 opposite to the side on which the protective layer 20 is provided is an exposed surface. The polarizing plate 100A can be obtained, for example, by bonding a protective layer via an adhesive layer or pressure-sensitive adhesive layer to the primary polarizing film side surface of a laminate having a configuration of [thermoplastic resin substrate/primary polarizing film] produced by the method described in A-1-1, and then peeling off the thermoplastic resin substrate. Alternatively, the polarizing plate 100A can be obtained by bonding a protective layer via an adhesive layer or pressure-sensitive adhesive layer to one surface of a primary polarizing film produced by the method described in A-1-2.

2B includes a polarizing film (primary polarizing film) 10, a protective layer 20, a retardation layer 30, and an adhesive layer 40 in this order, and the side of the polarizing film (primary polarizing film) 10 opposite to the side on which the protective layer 20 is provided is an exposed surface. The polarizing plate 100B can be obtained, for example, by bonding the retardation layer 30 to the protective layer 20 side surface of the polarizing plate 100A via an adhesive layer or an adhesive layer, and then providing an adhesive layer 40 on the surface of the retardation layer 30. Alternatively, the polarizing plate 100B can also be obtained by bonding the retardation layer 30 to the thermoplastic resin substrate side surface of a laminate having the above-mentioned [thermoplastic resin substrate/primary polarizing film] via an adhesive layer or an adhesive layer, and then providing an adhesive layer 40 on the surface of the retardation layer 30. In this case, the thermoplastic resin substrate functions as the protective layer 20. The retardation layer 30 in the illustrated example may be a single-layer structure, or may be a laminate structure in which two or more retardation layers are laminated.

The polarizing plate 100C shown in FIG. 2C includes a polarizing film (primary polarizing film) 10, a protective layer 20, and an adhesive layer 40 in this order, with the side of the polarizing film (primary polarizing film) 10 opposite to the side on which the protective layer 20 is provided being an exposed surface. The polarizing plate 100C can be obtained, for example, by providing an adhesive layer 40 on the surface of the polarizing plate 100A on the protective layer 20 side. Alternatively, the polarizing plate 100C can also be obtained by providing an adhesive layer 40 on the thermoplastic resin substrate side surface of a laminate having the above-mentioned [thermoplastic resin substrate/primary polarizing film] configuration. In this case, the thermoplastic resin substrate functions as the protective layer 20.

Although not shown, it is preferable that a release film be temporarily attached to the surface of the adhesive layer 40 until the polarizing plate is used.

The protective layer 20 is formed of any suitable film that can be used as a protective layer for a polarizing film. The retardation layer 30 can be, for example, a thermoplastic resin film or a liquid crystal alignment solidification layer. Any suitable adhesive can be used as the adhesive for forming the adhesive layer 40, and among them, an acrylic adhesive having an acrylic polymer as the base polymer is preferably used. Such protective layers, retardation layers, and adhesive layers are well known to those skilled in the art, so detailed explanations thereof will be omitted.

A-3. Step of Obtaining a Secondary Polarizing Film In the step of obtaining a secondary polarizing film, the transmittance of the primary polarizing film is changed by contacting the surface of the primary polarizing film with an aqueous solvent. Specifically, a secondary polarizing film having a desired transmittance can be obtained by bleaching the primary polarizing film by contacting it with an aqueous solvent.

As the aqueous solvent, any suitable aqueous solvent can be used as long as it can dissolve the dichroic material from the primary polarizing 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 content of the water-soluble organic solvent in the aqueous solvent is, for example, 20% by weight or less, preferably 10% by weight or less, and more preferably 5% by weight or less.

The method of contact with the aqueous solvent is not particularly limited, and any appropriate method such as immersion, spraying, or coating can be used. Spraying or coating is preferred for partial transmittance adjustment, and immersion is preferred for full-surface transmittance adjustment.

The contact time with the aqueous solvent and the temperature of the aqueous solvent during contact can be appropriately set according to the desired change in transmittance. The change in transmittance tends to be greater by increasing the contact time or the temperature of the aqueous solvent. The contact time can be, for example, 10 minutes or less, preferably 1 second to 5 minutes, and more preferably 2 seconds to 3 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, the polarizing film may be dried after contact with the aqueous solvent. The drying temperature may be, for example, 30°C to 100°C, preferably 30°C to 80°C. The moisture content of the polarizing film (secondary 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.

The secondary polarizing film preferably exhibits absorption dichroism at any wavelength between 380 nm and 780 nm. The transmittance (single transmittance: Ts) of the secondary polarizing film can be appropriately adjusted according to the purpose. The transmittance of the secondary polarizing film is preferably 41.5% or more, more preferably 42% or more, and even more preferably 42.5% or more. On the other hand, the transmittance of the secondary polarizing film is, for example, 70% or less, preferably 50% or less, and more preferably 46% or less. In one embodiment, the secondary polarizing film may have a transmittance that is, for example, 0.1% to 1.5% higher than that of the primary polarizing film. In addition, the polarization degree of the secondary polarizing film is, for example, 90% or more, preferably 92.0% 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 above transmittance and polarization degree are values obtained in the same manner as the transmittance and polarization degree of the primary polarizing film.

The thickness of the secondary polarizing film is typically 25 μm or less, preferably 12 μm or less, more preferably 1 μm to 8 μm, even more preferably 1 μm to 7 μm, and even more preferably 2 μm to 5 μm. The thickness of the secondary polarizing film can be substantially the same as that of the primary polarizing film.

A-4. Other steps The method for producing a polarizing plate may further include any appropriate step, as necessary. For example, after the step of obtaining a secondary polarizing film, the method may further include a step of changing the transmittance by contacting an aqueous solvent with the surface of the secondary polarizing film. The step of changing the transmittance may be repeated two or more times. In addition, for example, the method may include a step of laminating any appropriate layer (protective layer, retardation layer, adhesive layer, etc.) on the exposed surface of the finally obtained polarizing film (for example, secondary polarizing film) to protect the exposed surface.

Figures 3A and 3B are schematic cross-sectional views of an example of a polarizing plate that can be obtained after the process of protecting the exposed surface of the polarizing film. Polarizing plate 100D includes a first protective layer 20a, a polarizing film (secondary polarizing film) 10, a second protective layer 20b, a retardation layer 30, and an adhesive layer 40 in this order. Polarizing plate 100D can be obtained, for example, by providing a second protective layer 20b, a retardation layer 30, and an adhesive layer 40 in this order on the exposed surface of the polarizing film 10 of polarizing plate 100A (Figure 2A) that has undergone the process of obtaining a secondary polarizing film. Alternatively, it can be obtained by laminating a first protective layer 20a to the exposed surface of the polarizing film 10 of polarizing plate 100B (Figure 2B) that has undergone the process of obtaining a secondary polarizing film.

Polarizing plate 100E includes a first protective layer 20a, a polarizing film (secondary polarizing film) 10, a second protective layer 20b, and an adhesive layer 40 in this order. In polarizing plate 100E, second protective layer 20b may have a desired phase difference and function as a phase difference layer (e.g., a λ/4 plate) as described later. Polarizing plate 100E can be obtained, for example, by providing second protective layer 20b and adhesive layer 40 in this order on the exposed surface of polarizing film 10 of polarizing plate 100A (FIG. 2A) that has undergone a process of changing the transmittance. Alternatively, polarizing plate 100E can be obtained by laminating first protective layer 20a to the exposed surface of polarizing film 10 of polarizing plate 100C (FIG. 2C) that has undergone a process of changing the transmittance.

In one embodiment, the adhesive layer 40 is used to bond the polarizing plates 100D and 100E to an image display cell (e.g., a liquid crystal cell, an organic EL cell). In this embodiment, the first protective layer 20a becomes a protective layer (outer protective layer) that is disposed on the opposite side to the image display cell when the polarizing plate is applied to an image display device, and the second protective layer 20b becomes a protective layer (inner protective layer) that is disposed on the image display cell side.

The thickness of the outer protective layer 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. If a surface treatment has been applied, the thickness of the outer protective layer includes the thickness of the surface treatment layer.

The thickness of the inner protective layer 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 optically isotropic. In this specification, "optically isotropic" means that the in-plane retardation Re(550) is 0 nm to 10 nm, and the thickness direction retardation Rth(550) is -10 nm to +10 nm. Here, "Re(550)" is the in-plane retardation measured with light having a wavelength of 550 nm at 23°C, and is calculated by the formula: Re = (nx-ny) x d. Also, "Rth(550)" is the thickness direction retardation measured with light having a wavelength of 550 nm at 23°C, and is calculated by the formula: Rth(λ) = (nx-nz) x d. Here, "nx" is the refractive index in the direction in which the in-plane refractive index is maximum (i.e., the slow axis direction), "ny" is the refractive index in the direction perpendicular to the slow axis in the plane (i.e., the fast axis direction), "nz" is the refractive index in the thickness direction, and "d" is the thickness (nm) of the layer (film).

In another embodiment, the inner protective layer is a retardation layer having any appropriate retardation value. The in-plane retardation Re(550) of the inner protective layer in this embodiment can be, for example, 110 nm to 150 nm. By arranging such an inner protective layer so that the angle between its slow axis and the absorption axis of the polarizing film is, for example, 35° to 55°, preferably 38° to 52°, more preferably 40° to 50°, even more preferably 42° to 48°, and particularly preferably 44° to 46°, clockwise or counterclockwise, the inner protective layer can function as a circular polarizing plate.

The retardation layer 30 may be a retardation layer having a desired in-plane retardation and/or thickness retardation depending on the purpose. For example, when the inner protective layer is optically isotropic, the in-plane retardation Re(550) of the retardation layer may be 110 nm to 150 nm. By arranging such a retardation layer so that the angle between its slow axis and the absorption axis of the polarizing film is, for example, 35° to 55°, preferably 38° to 52°, more preferably 40° to 50°, even more preferably 42° to 48°, and particularly preferably 44° to 46°, clockwise or counterclockwise, the retardation layer can function as a circular polarizing plate.

B. Manufacturing method of image display device The manufacturing method of the image display device according to the embodiment of the present invention includes the steps of: laminating a polarizing plate, which is made of a polyvinyl alcohol-based resin film containing a dichroic material and has a moisture content of 15% by weight or less, a protective layer, and an adhesive layer, in this order, on an image display cell via the adhesive layer, and making the surface of the polarizing film opposite to the side on which the protective layer is arranged an exposed surface; and contacting the exposed surface of the polarizing film with an aqueous solvent to change the transmittance, in this order. According to this manufacturing method of the image display device, the transmittance of the polarizing film can be adjusted after being attached to the image display cell, so that an image display device having a desired brightness can be obtained regardless of the quality individual differences of the image display cell, backlight unit, etc.

B-1. Step (I)
The polarizing plate laminated on the image display cell may have any configuration as long as it is made of a polyvinyl alcohol-based resin film containing a dichroic material, contains a polarizing film having a moisture content of 15% by weight or less, a protective layer, and a pressure-sensitive adhesive layer in this order, and the surface of the polarizing film on which the protective layer is not disposed can be exposed. For example, the polarizing plate may be attached to the image display cell in a state where the exposed surface of the polarizing film is protected with a surface protective film, and the surface protective film may be peeled off before contact with an aqueous solvent to expose the polarizing film.

In one embodiment, the polarizing plate laminated to the image display cell may further include a retardation layer. Examples of the polarizing plate laminated to the image display cell include polarizing plates having the configurations illustrated in Figures 2B and 2C. In addition, the descriptions of the primary polarizing film, protective layer, retardation layer, and adhesive layer described in Section A can be applied to the polarizing film, protective layer, retardation layer, and adhesive layer contained in the polarizing plate, respectively.

Preferable examples of image display devices manufactured by the above manufacturing method include liquid crystal display devices and organic EL display devices. Therefore, liquid crystal cells or organic EL cells can be preferably used as the image display cells. A plurality of image display devices may be combined to display a single image as a whole and used as digital signage.

B-2. Process (II)
In the step of changing the transmittance, an aqueous solvent is brought into contact with the exposed surface of the polarizing film to change the transmittance. Specifically, the dichroic material is decolorized through contact with the aqueous solvent, thereby changing the transmittance of the polarizing film. The transmittance of the image display cell can be adjusted to a desired value by changing the transmittance. The same explanation as in A-3 can be applied to the process of changing the transmittance. From the viewpoint of preventing this, the method of contacting the polarizing film with the aqueous solvent is preferably coating or spraying. may be applied.

The manufacturing method of the image display device may further include a step of protecting the exposed surface of the polarizing film, if necessary, after the step of changing the transmittance. The exposed surface of the polarizing film may be protected by laminating a protective layer, a support substrate, or the like, onto the exposed surface via an adhesive layer, such as an adhesive layer or a pressure-sensitive adhesive layer.

C. Method for Adjusting Transmittance of Polarizing Film According to another aspect of the present invention, there is provided a method for adjusting the transmittance of a polarizing film, comprising a step of contacting an aqueous solvent with a surface of a polarizing film made of a PVA-based resin film containing a dichroic material and having a moisture content of 15% by weight or less. According to the method for adjusting the transmittance of a polarizing film, the polarizing film is decolorized by contact with the aqueous solvent, and as a result, the transmittance of the polarizing film can be adjusted to a desired value (typically increased).

As the polarizing film to be brought into contact with the aqueous solvent, the primary polarizing film described in A-1 is preferably used. In addition, the same explanation as in A-3 can be applied to the contact between the polarizing film and the aqueous solvent.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. The methods for measuring each property are as follows. In the examples and comparative examples, "parts" and "%" are by weight unless otherwise specified.
(1) Thickness: Measurement was performed using an interference thickness meter (manufactured by Otsuka Electronics Co., Ltd., product name "MCPD-3000").
(2) Single transmittance and polarization degree For the laminates (polarizing plates) of the polarizing 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 polarizing film side using an ultraviolet-visible spectrophotometer ("LPF-200" manufactured by Otsuka Electronics Co., Ltd.) were taken as Ts, Tp, and Tc of the polarizing film, respectively. These Ts, Tp, and Tc are Y values measured using a 2-degree visual field (C light source) according to JIS Z8701 and corrected for 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
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 primary polarizing film immediately after drying (when the laminate is stretched, the stretching 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. The film was then placed in a heating oven maintained at 120°C for 2 hours, and the weight after removal (weight after the 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

[Example 1-1]
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 produce a polarizing film (primary polarizing film) with a thickness of 12 μm. The moisture content of the obtained primary polarizing film was 10 wt %. The single transmittance of the polarizing film was 42.5%. Next, a PVA-based resin aqueous solution (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., product name "GOHSEFIRMER (registered trademark) Z-200", resin concentration: 3% by weight) was applied to one side of the polarizing film, and a cycloolefin-based film (manufactured by Nippon Zeon Corporation, Zeonor, thickness: 25 μm) was attached thereto, thereby obtaining a polarizing plate (before treatment) having a configuration of [primary polarizing film/protective layer].

The above polarizing plate (before treatment) was cut to a size of 100 mm x 100 mm, and attached to a glass plate via an acrylic adhesive layer (thickness 15 μm) so that the surface on the primary polarizing film side was exposed. The plate was then immersed in water at 55°C for 6 minutes. It was then dried at 50°C for 5 minutes to obtain a polarizing plate (after treatment) having a configuration of [secondary polarizing film/protective layer].

[Example 1-2]
A polarizing plate (after processing) having a structure of [secondary polarizing film/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 immersing in water at 55° C. for 6 minutes.

[Examples 1-3]
A polarizing plate (after processing) having a structure of [secondary polarizing film/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 55°C for 6 minutes.

[Examples 1 to 4]
A polarizing plate (after processing) having a structure of [secondary polarizing film/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 55°C for 6 minutes.

[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. The resulting mixture was dissolved 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 machine direction (longitudinal 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, a polarizing film (primary polarizing film) having a moisture content of 4.5% by weight and a thickness of 5 μm was formed on the resin substrate, and a cycloolefin-based film (Zeonor, manufactured by Zeon Corporation, thickness: 25 μm) was bonded to the surface of the primary polarizing film with a UV-curable adhesive (thickness: 1.0 μm). The resin substrate was then peeled off to obtain a polarizing plate (before treatment) having a configuration of [primary polarizing film/protective layer].

The above polarizing plate (before treatment) was cut to a size of 100 mm x 100 mm, and attached to a glass plate via an acrylic adhesive layer (thickness 15 μm) so that the surface on the primary polarizing film side was exposed. The plate 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 (after treatment) having a configuration of [secondary polarizing film/protective layer].

[Example 2-2]
A polarizing plate (after processing) having a structure of [secondary polarizing film/protective layer] was obtained in the same manner as in Example 2-1, except that the plate 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 (after processing) having a structure of [secondary polarizing film/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 1 minute instead of in water at 50°C for 9 minutes.

[Example 2-4]
A polarizing plate (after processing) having a structure of [secondary polarizing film/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 2 minutes instead of in water at 50°C for 9 minutes.

[Example 2-5]
A polarizing plate (after processing) having a structure of [secondary polarizing film/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.

[Example 3-1]
A polarizing plate (before treatment) having a structure of [primary polarizing film/protective layer] was obtained in the same manner as in Example 2-1, except that the iodine concentration in the dye bath was changed and the transmittance of the resulting polarizing film was adjusted to 44.3%. The moisture content of the resulting primary polarizing film was 4.5% by weight.

The above polarizing plate (before treatment) was cut to a size of 100 mm x 100 mm, and attached to a glass plate via an acrylic adhesive layer (thickness 15 μm) so that the surface on the primary polarizing film side was exposed. The plate was then immersed in water at 50°C for 6 minutes. It was then dried at 50°C for 5 minutes to obtain a polarizing plate (after treatment) having a configuration of [secondary polarizing film/protective layer].

[Example 3-2]
A polarizing plate (after processing) having a structure of [secondary polarizing film/protective layer] was obtained in the same manner as in Example 3-1, except that the plate was immersed in water at 55°C for 3 minutes instead of in water at 50°C for 6 minutes.

The transmittance and the polarization degree of the polarizing plate (before treatment) and the polarizing plate (after treatment) obtained in the above examples were measured. The results are shown in Table 1.

As is clear from Table 1, according to the manufacturing method of the embodiment, the transmittance of the polarizing film can be changed after the protective layer is laminated to produce the polarizing plate.

The manufacturing method of the polarizing plate of the present invention is suitable for use in the manufacture of image display devices.

REFERENCE SIGNS LIST 10 Polarizing film 20 Protective layer 30 Retardation layer 40 Pressure-sensitive adhesive layer 100 Polarizing plate

Claims (8)

A step of dyeing the polyvinyl alcohol-based resin film and stretching the same in an aqueous boric acid solution, and then drying the film until the moisture content is 15% by weight or less to obtain a primary polarizing film; and a step of contacting an aqueous solvent with the surface of the primary polarizing film to change the transmittance to obtain a secondary polarizing film.
in that order ,
The method for producing a polarizing plate , wherein the secondary polarizing film has a single-piece transmittance that is 0.1% to 3.2% higher than that of the primary polarizing film . The method for producing a polarizing plate according to claim 1 , wherein the aqueous solvent is brought into contact with an exposed surface of the primary polarizing film in a state in which one surface is entirely exposed and the other surface is entirely protected. the step of obtaining the primary polarizing film includes subjecting a polyvinyl alcohol-based resin film containing a halide and a polyvinyl alcohol-based resin, in a state of being laminated with a long thermoplastic resin substrate, to an air-assisted stretching treatment, a dyeing treatment, a stretching treatment in an aqueous boric acid solution, and a drying shrinkage treatment in this order;
3. The method for producing a polarizing plate according to claim 1 or 2, wherein the drying and shrinking treatment comprises heating the laminate while transporting it in the longitudinal direction, thereby shrinking the laminate by 2% or more in the width direction and drying the laminate until the moisture content of the polyvinyl alcohol-based resin film becomes 15% by weight or less. The method for producing a polarizing plate according to claim 3, wherein the halide is iodide or sodium chloride. The method for producing a polarizing plate according to any one of claims 1 to 4, wherein the thickness of the primary polarizing film is 12 μm or less. a step of laminating a polarizing plate including a polarizing film, which is made of a polyvinyl alcohol-based resin film containing a dichroic material and has a moisture content of 15% by weight or less, a protective layer, and an adhesive layer, in this order, on an image display cell via the adhesive layer , so that the surface of the polarizing film opposite to the side on which the protective layer is disposed is an exposed surface; and a step of contacting an aqueous solvent with the exposed surface of the polarizing film to change the transmittance.
A method for manufacturing an image display device, comprising the steps of: The method for manufacturing an image display device according to claim 6, wherein the image display device is a liquid crystal display device or an organic EL display device. A method for adjusting the transmittance of a polarizing film, the method comprising the step of contacting an aqueous solvent with a surface of a polarizing film which is made of a polyvinyl alcohol-based resin film containing a dichroic material and has a moisture content of 15% by weight or less,
The method for adjusting the single transmittance of the polarizing film is made 0.1% to 3.2% higher than that before contact with the aqueous solvent.