JP2024114773A - Polarizing film and method for producing the same - Google Patents

Abstract

[Problem] To provide a polarizing film that has an excellent appearance and can contribute to improving the display characteristics of an image display device. [Solution] The polarizing film according to an embodiment of the present invention is made of a resin film containing iodine, has a thickness of 7 μm or less, and has a ratio (Rc400/Rc680) of the surface reflectance Rc400 of light with a wavelength of 400 nm in the absorption axis direction to the reflectance Rc680 of light with a wavelength of 680 nm in the absorption axis direction, which exceeds 1. [Selected Figure] Figure 1

Description

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

In a liquid crystal display device, which is a representative image display device, a polarizing film is arranged on both sides of a liquid crystal cell due to its image formation method. In addition, with the spread of thin displays, displays equipped with an organic electroluminescence (EL) panel (OLED) and displays using a display panel using inorganic light-emitting materials such as quantum dots (QLED) have been proposed. These panels have a highly reflective metal layer, and are prone to problems such as external light reflection and background reflection. It is known that these problems can be prevented by providing a circular polarizing plate having a polarizing film and a λ/4 plate on the viewing side. As a method for manufacturing a polarizing film, for example, a method has been proposed in which a laminate having a resin substrate and a polyvinyl alcohol (PVA)-based resin layer is stretched and then dyed to obtain a polarizing film on the resin substrate (for example, Patent Document 1). According to such a method, a thin polarizing film can be obtained, and it has attracted attention as a method that can contribute to the thinning of image display devices in recent years. However, a thin polarizing film has a poor appearance, and when used in an image display device, sufficient display characteristics may not be obtained.

JP 2001-343521 A

The present invention has been made to solve the above problems, and its main objective is to provide a polarizing film that has excellent appearance and can contribute to improving the display characteristics of image display devices.

According to an embodiment of the present invention, there is provided a polarizing film, which is made of a resin film containing iodine, has a thickness of 7 μm or less, and has a ratio (Rc 400 /Rc 680 ) of a reflectance Rc ₄₀₀ of a surface of the polarizing film at a wavelength of 400 nm in the absorption axis direction to a reflectance Rc ₆₈₀ of a surface of the polarizing film at a wavelength of ₆₈₀ nm in the absorption axis direction _, which exceeds 1.
In one embodiment, the surface has an Rc ₆₈₀ of 5% or less.
In one embodiment, the surface has an Rc ₄₀₀ of 4.8% or greater.
In one embodiment, the polarizing film has a gradient distribution region at an end portion on the front surface side, in which the amount of iodine increases from the front surface toward the back surface.
In one embodiment of the present invention, the polarizing film has a smaller amount of iodine on the front surface side than on the back surface side.
In one embodiment, the polarizing film has a single transmittance of 42.0% or more and a polarization degree of 99.98% or more.
According to another aspect of the present invention, there is provided a method for producing the above polarizing film, which comprises washing with water a surface of a resin film containing iodine and having a moisture content of 15% by weight or less.
In one embodiment, the resin film has an iodine concentration of 5% by weight or more.
In one embodiment, the resin film is a resin layer formed on a resin substrate.
In one embodiment, the production method includes underwater stretching the resin layer at 67° C. or less.
In one embodiment, the manufacturing method includes heating the resin layer using a heating roll.
According to yet another aspect of the present invention, there is provided a polarizing plate comprising the above polarizing film and a protective layer or a retardation layer disposed on at least one side of the polarizing film.

According to the present invention, a polarizing film with excellent appearance can be obtained by controlling the reflection characteristics of the surface. Furthermore, such a polarizing film can contribute to improving the display characteristics of image display devices.

1 is a schematic cross-sectional view of a polarizing film according to one embodiment of the present invention. FIG. 2 is a schematic diagram showing an example of drying using a heating roll. 1 is a schematic cross-sectional view illustrating a general configuration of a polarizing plate according to a first embodiment of the present invention. FIG. 4 is a schematic cross-sectional view showing an outline of a configuration of a polarizing plate according to a second embodiment of the present invention. 1 is a graph showing the iodine ion intensity in the thickness direction of the polarizing films of Example 1, Comparative Example 1, and Reference Example 2.

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

(Definition of terms and symbols)
The definitions of terms and symbols used in this specification are as follows.
(1) Refractive index (nx, ny, nz)
"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), and "nz" is the refractive index in the thickness direction.
(2) In-plane phase difference (Re)
"Re(λ)" is the in-plane retardation measured with light having a wavelength of λ nm at 23° C. For example, "Re(550)" is the in-plane retardation measured with light having a wavelength of 550 nm at 23° C. Re(λ) is calculated by the formula: Re(λ)=(nx−ny)×d, where d (nm) is the thickness of the layer (film).
(3) Retardation in the thickness direction (Rth)
"Rth(λ)" is the retardation in the thickness direction measured with light having a wavelength of λ nm at 23° C. For example, "Rth(550)" is the retardation in the thickness direction measured with light having a wavelength of 550 nm at 23° C. Rth(λ) is calculated by the formula: Rth(λ)=(nx-nz)×d, where d (nm) is the thickness of the layer (film).
(4) Nz Coefficient The Nz coefficient is calculated by Nz=Rth/Re.
(5) Angle When referring to an angle in this specification, the angle includes both clockwise and counterclockwise angles with respect to a reference direction. Thus, for example, "45°" means ±45°.

A. Polarizing Film Fig. 1 is a schematic cross-sectional view of a polarizing film according to one embodiment of the present invention. In Fig. 1, hatching is omitted from the cross section of the polarizing film in order to make the drawing easier to see. The polarizing film 10 has a first main surface (front surface) 10a and a second main surface (back surface) 10b. The polarizing film 10 has a gradient distribution region at the end on the front surface 10a side, where the amount of iodine increases from the front surface 10a toward the back surface 10b.

The polarizing film 10 is composed of a resin film containing iodine. Examples of the resin film that can be used include hydrophilic polymer films such as polyvinyl alcohol (PVA)-based films, partially formalized PVA-based films, and partially saponified ethylene-vinyl acetate copolymer-based films.

The thickness of the polarizing film 10 is 7 μm or less, and preferably 6 μm or less. A polarizing film of this thickness tends to have a high iodine concentration. On the other hand, the thickness of the polarizing film is preferably 1 μm or more, and more preferably 2 μm or more.

The polarizing film 10 preferably exhibits absorption dichroism at any wavelength between 380 nm and 780 nm. The single transmittance (Ts) of the polarizing film 10 is preferably 41.0% or more, more preferably 42.0% or more, and even more preferably 42.5% or more. On the other hand, the single transmittance of the polarizing film 10 is, for example, 44.2% or less. The polarization degree (P) of the polarizing film 10 is preferably 99.95% or more, more preferably 99.98% or more, and even more preferably 99.99% or more. On the other hand, the polarization degree of the polarizing film 10 is, for example, 99.996% or less.

The single transmittance is typically a Y value measured using an ultraviolet-visible spectrophotometer and subjected to luminosity correction. The polarization degree is typically calculated by the following formula based on the parallel transmittance Tp and the crossed transmittance Tc measured using an ultraviolet-visible spectrophotometer and subjected to luminosity correction.
Degree of polarization (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 ×100

The ratio (Rc ₄₀₀ /Rc ₆₈₀ ) of the reflectance Rc ₄₀₀ of light having a wavelength of 400 nm in the absorption axis direction to the reflectance Rc ₆₈₀ of light having a wavelength of 680 nm in the absorption axis direction of the surface 10a of the polarizing film 10 exceeds 1, preferably is 1.3 or more, and more preferably is 1.5 or more. By satisfying such a relationship, the reflected hue can be well controlled and the appearance can be excellent. Specifically, redness can be suppressed and the appearance can be excellent. As a result, for example, an image display device having excellent visibility can be provided. Here, the polarizing film 10 may be arranged so that its surface 10a is the viewing side of the image display device or the opposite side to the viewing side, and either arrangement can provide excellent appearance. By arranging the surface 10a of the polarizing film 10 to be the viewing side of the image display device, the appearance and display characteristics can be extremely excellent. On the other hand, the Rc ₄₀₀ /Rc ₆₈₀ of the surface 10a of the polarizing film 10 is, for example, 2 or less.

The Rc ₄₀₀ of the surface 10a of the polarizing film 10 is, for example, 4.8% or more, preferably 4.9% or more, more preferably 5% or more, and even more preferably 5.3% or more. On the other hand, the Rc ₄₀₀ of the surface 10a of the polarizing film 10 is, for example, 6% or less. The Rc ₆₈₀ of the surface 10a of the polarizing film 10 is, for example, 5% or less, preferably 4.9% or less, more preferably 4.5% or less, and even more preferably 4% or less. On the other hand, the Rc ₆₈₀ of the surface 10a of the polarizing film 10 is, for example, 3% or more.

The reflectance Rp ₄₀₀ of the surface 10a of the polarizing film 10 for light having a wavelength of 400 nm in the transmission axis direction is, for example, 4.5% to 5%. The reflectance Rp ₆₈₀ of the surface 10a of the polarizing film 10 for light having a wavelength of 680 nm in the transmission axis direction is, for example, 4.3% to 4.8%.

The above Rc and Rp are the ratios of the reflected light intensity to the incident light intensity when light is incident on the surface of the polarizing film (resin film) at a specified angle and the reflected light is detected in the absorption axis direction and the transmission axis direction, respectively.

For example, the polarizing film 10 has a smaller amount of iodine on the front surface 10a side than on the back surface 10b side. Specifically, the polarizing film 10 has, in this order from the front surface side, a first region 11 and a second region 12 in which the distribution state of iodine is different. The first region 11 is a gradient distribution region in which the amount of iodine increases from the front surface 10a toward the back surface 10b. In the second region 12, iodine is uniformly distributed. Here, uniform means, for example, that the intensity derived from iodine (e.g., iodine ions) detected by analysis is within a range of -20% to +20% from the average value. The thickness of the first region 11 is preferably 2% to 50% of the thickness of the polarizing film 10, and more preferably 10% to 40%. The thickness of the first region 11 may be 20% or more of the thickness of the polarizing film 10. Specifically, the thickness of the first region 11 is preferably 100 nm or more and 2.7 μm or less, and more preferably 500 nm or more and 2 μm or less. The thickness of the first region 11 may be 1 μm or more. According to such a range, for example, excellent optical characteristics (single transmittance and polarization degree) and excellent appearance can be achieved.

Although not shown, for example, the polarizing film may have, from the front surface side, a first region, a second region, and a third region in this order, which have different iodine distribution states. Specifically, the polarizing film may have a first region in which the amount of iodine increases from the front surface to the back surface, a second region in which iodine is uniformly distributed, and a third region in which the amount of iodine decreases from the front surface to the back surface.

B. Manufacturing method The polarizing film is obtained by washing the surface of a resin film containing iodine and having a predetermined moisture content. The moisture content of the resin film (before washing) is 15% by weight or less, preferably 12% by weight or less, more preferably 9% by weight or less, and even more preferably 6% by weight or less. On the other hand, the moisture content of the resin film is, for example, 3% by weight or more. By washing the resin film having such a moisture content, the polarizing film can be well produced. Specifically, the reflection characteristics (e.g., reflectance, hue) of the surface can be well controlled while maintaining excellent optical properties. For example, the reflectance in the long wavelength region of 550 nm or more can be reduced to suppress redness and make it possible to visually recognize blueness.

The iodine concentration of the resin film is, for example, 5% by weight or more, and may be 5.5% by weight or more, or may be 6% by weight or more. The iodine concentration of the resin film is, for example, 8% by weight or less. The thickness of the resin film is, for example, 7 μm or less, and may be 6 μm or less. The thickness of the resin film is preferably 1 μm or more, and more preferably 2 μm or more. One of the features of the present invention is that an excellent appearance can be achieved with such an iodine concentration and thickness.

B-1. Resin film The resin film having the above-mentioned predetermined moisture content can be obtained, for example, by forming a resin layer (typically, a polyvinyl alcohol-based resin layer) on a resin substrate to produce a laminate, stretching this laminate (resin layer) and dyeing it with iodine, and then drying the laminate (resin layer).

B-1-1. Laminate In one embodiment, a polyvinyl alcohol (PVA)-based resin layer containing a PVA-based resin and a halide is formed on a thermoplastic resin substrate (e.g., a long substrate) to produce the laminate. Specifically, a coating liquid containing a PVA-based resin and a halide is applied onto the thermoplastic resin substrate, followed by drying to produce the laminate.

The thickness of the thermoplastic resin substrate is preferably 20 μm to 300 μm, and 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 described below, it may take time for the thermoplastic resin substrate to absorb water, and an excessive load may be required for stretching.

The water absorption rate of the thermoplastic resin substrate is preferably 0.2% or more, more preferably 0.3% or more. Such a 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. With such a water absorption rate, it is possible to prevent problems such as a significant decrease in the dimensional stability of the thermoplastic resin substrate during production, which deteriorates the quality of the resulting polarizing film. In addition, it is possible to prevent the thermoplastic resin substrate from breaking and the PVA-based resin layer from peeling off during underwater stretching. 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, the Tg is more preferably 100°C or less, and even more preferably 90°C or less. On the other hand, the Tg of the thermoplastic resin substrate is preferably 60°C or more. With such a Tg, it is possible to prevent defects such as deformation of the thermoplastic resin substrate (e.g., generation of unevenness, sagging, wrinkles, etc.) when the coating liquid is applied and dried, and to produce a good laminate. In addition, the stretching of the resin layer can be performed well at a suitable temperature (e.g., about 60°C). The Tg of the thermoplastic resin substrate can be adjusted, for example, by introducing a modified group into the constituent material and heating it using a crystallization material. The glass transition temperature (Tg) is a value obtained 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 another embodiment, a polyethylene terephthalate resin having an isophthalic acid unit is preferably used. This is because it has excellent stretchability and crystallization during stretching can be suppressed. 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. This is because the crystallinity can be favorably increased during drying, which will be described later.

The thermoplastic resin substrate may be stretched in advance (for example, before forming the PVA-based resin layer). 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°, 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) of the thermoplastic resin substrate. The stretching ratio of the thermoplastic resin substrate is preferably 1.5 times to 3.0 times. Any appropriate method can be adopted as the stretching method of the thermoplastic resin substrate. Specifically, it may be fixed-end stretching or free-end stretching. The stretching method may be a dry method or a wet method. Stretching may be performed in one stage or multiple stages. When stretching in multiple stages, the stretch ratio is the product of the stretch ratios in each stage.

The coating liquid is typically a solution in which a PVA-based resin and a halide 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 content of the PVA-based resin in the coating liquid is preferably 3 to 20 parts by weight per 100 parts by weight of the solvent. This range allows the formation of a uniform coating film that is in close contact with the thermoplastic resin substrate. 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.

Examples of the PVA-based resin 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%. 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 saponification degree can be determined in accordance with JIS K 6726-1994.

The average degree of polymerization of the PVA-based resin 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.

Any suitable halide may be used as the halide. Examples include iodides such as potassium iodide, sodium iodide, and lithium iodide, and chlorides such as sodium chloride. Of these, potassium iodide is preferred. By using a halide, a polarizing film having excellent optical properties can be obtained. Specifically, the crystallization of the PVA-based resin after the air-assisted stretching described below is promoted, and the disturbance of the orientation of the polyvinyl alcohol molecules and the decrease in the orientation are suppressed in the subsequent wet treatment (for example, the dyeing and underwater stretching described below), and a polarizing film having excellent optical properties can be obtained.

In preparing the coating solution, it is preferable to mix 5 to 20 parts by weight of the halide with respect to 100 parts by weight of the PVA-based resin, and more preferably 10 to 15 parts by weight. Specifically, the content of the halide in the resulting PVA-based resin layer is preferably 5 to 20 parts by weight with respect to 100 parts by weight of the PVA-based resin, and more preferably 10 to 15 parts by weight. If the amount of the halide relative to the PVA-based resin is too high, for example, the halide may bleed out, causing the resulting polarizing film to become cloudy.

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 are used, for example, to improve the uniformity, dyeability, and stretchability of the resulting PVA-based resin layer.

Examples of methods for applying the coating liquid 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.

B-1-2. Stretching The stretching is preferably performed by dry stretching (air-assisted stretching) of the laminate, followed by underwater stretching. The auxiliary stretching allows stretching while suppressing crystallization of the thermoplastic resin substrate, solving the problem of reduced stretchability due to excessive crystallization of the thermoplastic resin substrate during stretching in boric acid water, and allows the laminate to be stretched at a higher magnification. In addition, when a thermoplastic resin substrate is used, the coating temperature may be set low, which may cause a problem that the crystallization of the PVA-based resin is relatively low and sufficient optical properties cannot be obtained. In response to this, the introduction of auxiliary stretching can increase the crystallinity of the PVA-based resin even when a thermoplastic resin is used. In addition, by increasing the orientation of the PVA-based resin in advance, problems such as reduced orientation and dissolution of the PVA-based resin during subsequent wet processing can be prevented. In this way, a polarizing film having excellent optical properties can be obtained.

The method of the air-assisted stretching may be fixed-end stretching (e.g., a method of stretching using a tenter stretching machine) or free-end stretching (e.g., a method of uniaxially stretching a laminate between rolls with different peripheral speeds). Preferably, free-end stretching is adopted. For example, heated roll stretching is adopted in which the laminate is stretched by the difference in peripheral speed between heated rolls while being transported in its longitudinal direction. In one embodiment, the air-assisted stretching includes a zone stretching step in a heat space (zone) and a heated roll stretching step. The order of the zone stretching step and the heated roll stretching step is not limited, but for example, the zone stretching step and the heated roll stretching step are performed in this order. In another embodiment, the tenter stretching machine stretches the film by gripping the film end and widening the distance between the tenters in the flow direction (the widening of the distance between the tenters is the stretching ratio). At this time, the distance of the tenter in the width direction (perpendicular to the flow direction) is preferably set so as to be closer to the free-end stretching than the stretching ratio in the flow direction. In the case of free end stretching, the shrinkage in the transverse direction is calculated by the formula: shrinkage in the transverse direction=(1/stretch ratio) ^1/2 .

The stretching ratio of the auxiliary air stretching is preferably 2.0 to 3.5 times. The auxiliary air stretching may be performed in one stage or in multiple stages. When performed in multiple stages, the stretching ratio is the product of the stretching ratios in each stage. The stretching direction in the auxiliary air stretching is preferably approximately the same as the stretching direction in the underwater stretching described below.

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

The underwater stretching is typically performed by immersing the laminate in a stretching bath. Underwater stretching allows stretching 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 with excellent optical properties can be obtained.

The underwater stretching method 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 used. The laminate may be stretched in one stage or in multiple stages. When stretched in multiple stages, the stretch ratio of the laminate described below is the product of the stretch ratios in each stage.

The underwater stretching is preferably performed by immersing the laminate in an aqueous solution of boric acid (underwater stretching in boric acid). By using an aqueous solution of boric acid as the stretching bath, 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, it is possible to impart rigidity and water resistance to the PVA-based resin layer, allowing it to be stretched well, and to obtain a polarizing film with excellent optical properties.

The boric acid aqueous solution is preferably obtained by dissolving boric acid and/or a borate in water, which is a solvent. The boric acid concentration is preferably 1 to 10 parts by weight, more preferably 2.5 to 6 parts by weight, and even more 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, an iodide is added to the stretching bath (boric acid aqueous solution). By adding an iodide, it is possible to suppress the elution of iodine adsorbed in the PVA-based resin layer. 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. The concentration of the iodide is preferably 0.05 to 15 parts by weight, and 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 or higher, more preferably 60°C or higher. At such a temperature, the film can be stretched at a high ratio while suppressing dissolution of the PVA-based resin layer. Specifically, as described above, the glass transition temperature (Tg) of the thermoplastic resin substrate is preferably 60°C or higher in relation to the formation of the PVA-based resin layer. In this case, if the stretching temperature is below 40°C, even if the plasticization of the thermoplastic resin substrate by water is taken into consideration, the film may not be stretched well. On the other hand, the stretching temperature is, for example, 70°C or lower, preferably 67°C or lower, more preferably 65°C or lower. The higher the stretching temperature, the higher the solubility of the PVA-based resin layer, and the less likely it is that excellent optical properties can be obtained. In addition, such a stretching temperature can suppress swelling and dissolution of the PVA-based resin layer during the water washing described below, and a polarizing film with excellent surface properties can be obtained. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

The stretching ratio by underwater stretching is preferably 1.5 times or more, and more preferably 3.0 times or more. The total stretching ratio of the laminate (stretching ratio of the combined air-assisted stretching and underwater stretching) 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. By achieving such a high stretching ratio, a polarizing film with extremely excellent optical properties can be produced. Such a high stretching ratio can be achieved by employing an underwater stretching method (stretching in boric acid water).

B-1-3. Dyeing The dyeing is typically performed by adsorbing iodine to the PVA-based resin layer. Examples of methods for adsorbing iodine 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. Specific examples of iodide are as described above. Potassium iodide is preferably used. 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 dyeing solution, the immersion time is preferably 5 seconds to 5 minutes, more preferably 30 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, for example, so that the single transmittance of the polarizing film finally obtained is 42.0% or more and the polarization degree is 99.98% or more. As for such dyeing conditions, for example, in the iodine aqueous solution serving as the dyeing solution, the ratio of the iodine and potassium iodide contents is preferably 1:5 to 1:20, and more preferably 1:5 to 1:10.

When dyeing is performed continuously after a treatment (for example, an insolubilization treatment described later) in which the laminate is immersed in a treatment bath containing boric acid, the boric acid may be mixed into the dye bath, causing a change in the boric acid concentration in the dye bath and making the dyeability unstable. In order to suppress such instability in the dyeability, the boric acid concentration in the dye bath is preferably adjusted to 4 parts by weight or less, more preferably 2 parts by weight or less, per 100 parts by weight of water. On the other hand, the boric acid concentration in the dye bath is preferably 0.1 parts by weight or more, more preferably 0.2 parts by weight or more, and even more preferably 0.5 parts by weight or more, per 100 parts by weight of water. In one embodiment, the dyeing is performed using a dye bath containing boric acid in advance. According to such an embodiment, the rate of change in the boric acid concentration when boric acid is mixed into the dye bath can be reduced. The amount of boric acid to be added to the dye bath in advance (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.

B-1-4. Other Treatments If necessary, an insolubilization treatment is performed after the above-mentioned air-assisted stretching and before underwater stretching and dyeing. 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 in the insolubilization treatment is preferably 1 part by weight to 4 parts by weight per 100 parts by weight of water. The temperature of the insolubilization treatment (liquid temperature of the aqueous solution of boric acid) is preferably 20°C to 50°C.

If necessary, a crosslinking treatment is performed after dyeing and before underwater stretching. The crosslinking treatment is typically performed by immersing the PVA-based resin layer in an aqueous solution of boric acid. By performing the crosslinking 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 during the subsequent underwater stretching. The concentration of the aqueous solution of boric acid in the crosslinking treatment is preferably 1 to 5 parts by weight per 100 parts by weight of water. It is preferable to add an iodide to the aqueous solution of boric acid. By adding an iodide, it is possible to suppress the elution of iodine adsorbed to the PVA-based resin layer. Specific examples of iodides are as described above. The amount of iodide added is preferably 1 to 5 parts by weight per 100 parts by weight of water. The temperature of the crosslinking treatment (liquid temperature of the aqueous solution of boric acid) is preferably 20°C to 50°C.

Preferably, washing is performed after underwater stretching and before drying, which will be described later. Washing is typically performed by immersing the PVA-based resin layer in an aqueous potassium iodide solution.

B-1-5. Drying The drying can be performed in any suitable manner and under any suitable conditions as long as a resin film having the predetermined moisture content can be obtained. Specifically, the drying can be performed by heating the entire zone (zone heating method) or by heating the transport roll (heating roll method). The heating roll method is preferably adopted, and more preferably both are adopted. By using the heating roll, it is possible to efficiently suppress the heat curl of the laminate and produce a polarizing film with excellent quality. 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 the heating roll, it is possible to dry the laminate while maintaining it in a flat state, so that not only curling but also wrinkles can be suppressed.

Drying can shrink the laminate in the width direction, improving the optical properties. This is because it can effectively increase the orientation of the PVA and the PVA/iodine complex. The shrinkage rate of the laminate in the width direction due to drying is preferably 1% to 10%, more preferably 2% to 8%, and even more 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 2 is a schematic diagram showing an example of drying using heated rolls. In the illustrated example, 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 even more preferably 70°C to 80°C. Such a temperature can increase the crystallinity of the thermoplastic resin to suppress curling and provide the laminate with extremely excellent durability. In addition, the moisture content of the resin film can be well achieved. The temperature of the heating rolls can be measured with 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.

B-2. Washing with water The washing with water is carried out, for example, by contacting the surface of the resin film with water. For example, the resin film is washed with water by immersing it in a water bath. When the resin film is immersed in a water bath, the back surface of the resin film is preferably protected with any appropriate protective substrate. In one embodiment, the resin substrate is used as the protective substrate. Specifically, the resin film is immersed in a water bath without peeling the resin substrate from the resin film (in the state of the laminate). In another embodiment, a protective layer described later is used as the protective substrate. For example, after laminating a protective layer on the surface of the resin film of the laminate, the resin substrate is peeled off from the resin film to prepare a laminate of the protective layer and the resin film, and this laminate is immersed in a water bath. When immersed in a water bath, the resin film may be in a long shape or a sheet shape.

The temperature of the water bath (water to be contacted) is, for example, 20°C or higher, preferably 25°C or higher, more preferably 30°C or higher, even more preferably 35°C or higher, and particularly preferably 40°C or higher. At such a temperature, for example, a polarizing film that satisfies the above reflectance can be produced in a short time. On the other hand, the temperature of the water bath (water to be contacted) is preferably 60°C or lower, more preferably 50°C or lower. At such a temperature, for example, the obtained polarizing film has excellent surface properties and can retain excellent optical properties.

The immersion time in the water bath (contact time) is set according to, for example, the above temperature, the thickness of the resin film, etc. The immersion time in the water bath is preferably 15 seconds to 5 minutes, and more preferably 30 seconds to 3 minutes.

The water bath (the water that is brought into contact) may contain additives such as boric acid.

After the above water washing, the polarizing film may be subjected to a drying process. The drying temperature is, for example, 30°C to 60°C. The drying time is, for example, 15 seconds to 3 minutes.

C. Polarizing Plate A polarizing plate according to one embodiment of the present invention has the above polarizing film, and a protective layer or a retardation layer disposed on at least one side of the polarizing film.

Figure 3 is a schematic cross-sectional view showing the outline of the polarizing plate according to the first embodiment of the present invention. The polarizing plate (polarizing plate with retardation layer) 100 has a polarizing film 10, a protective layer 20, a retardation layer 30, and an adhesive layer 40 in this order. In the polarizing plate 100, the protective layer 20 is arranged only on the back surface 10b side of the polarizing film 10, and no protective layer is arranged on the front surface 10a side (e.g., the viewing side). However, in practice, the front surface 10a of the polarizing film 10 is protected by any appropriate protective material (not shown). For example, after processing is performed on the polarizing film 10 of the polarizing plate 100, a protective material is laminated on the polarizing film 10. The retardation layer 30 may be a single layer, or may have a laminated structure in which two or more layers are laminated.

Figure 4 is a schematic cross-sectional view showing the general configuration of a polarizing plate according to a second embodiment of the present invention. The polarizing plate (polarizing plate with a retardation layer) 110 has, in this order from the viewing side, a protective layer 20 arranged on the back surface 10b side of the polarizing film 10, a polarizing film 10, a retardation layer 30 arranged on the front surface 10a side of the polarizing film 10, and an adhesive layer 40. This embodiment differs from the first embodiment in that the retardation layer 30 can function as a protective layer for the polarizing film 10 and the retardation layer 30 is arranged on the front surface 10a side of the polarizing film 10.

Although not shown, the polarizing plate may further have other functional layers. The type, characteristics, number, combination, arrangement, etc. of the functional layers that the polarizing plate may have may be appropriately set according to the purpose. For example, the polarizing plate may further have a conductive layer or an isotropic substrate with a conductive layer. A polarizing plate having a conductive layer or an isotropic substrate with a conductive layer (polarizing plate with a retardation layer) is applied to, for example, a so-called inner touch panel type input display device in which a touch sensor is incorporated inside an image display panel. As another example, the polarizing plate may further have other retardation layers. The optical characteristics (e.g., refractive index characteristics, in-plane retardation, Nz coefficient, photoelastic coefficient), thickness, arrangement, etc. of the other retardation layers may be appropriately set according to the purpose. As a specific example, the viewing side of the polarizing film 10 may be provided with other retardation layers (typically, a layer that imparts (elliptical) polarizing function, a layer that imparts ultra-high retardation) that improve visibility when viewed through polarized sunglasses. By having such a layer, excellent visibility can be achieved even when the display screen is viewed through polarized lenses such as polarized sunglasses. Therefore, the resulting polarizing plate (polarizing plate with retardation layer) can be suitably applied to image display devices that can be used outdoors.

Each member constituting the polarizing plate may be laminated via any suitable adhesive layer (not shown). Specific examples of the adhesive layer include an adhesive layer and a pressure-sensitive adhesive layer. Specifically, the retardation layer 30 may be attached to the polarizing film 10 or the protective layer 20 via an adhesive layer (preferably using an active energy ray-curable adhesive), or may be attached to the polarizing film 10 or the protective layer 20 via a pressure-sensitive adhesive layer. When the retardation layer 30 has a laminated structure of two or more layers, each retardation layer is, for example, attached via an adhesive layer (preferably using an active energy ray-curable adhesive).

Although not shown, in practice, a release film (separator) is attached to the surface of the adhesive layer 40. The release film can be temporarily attached until the polarizing plate is used. By using the release film, for example, the adhesive layer is protected and the polarizing plate can be formed into a roll.

The polarizing plate may be long or in the form of a sheet. In this specification, "long" refers to an elongated shape in which the length is sufficiently long compared to the width, for example, a long and thin shape in which the length is 10 times or more, preferably 20 times or more, the width. A long polarizing plate can be wound into a roll.

C-1. Protective layer The protective layer 20 may be formed of any suitable film that can be used as a protective layer for a polarizing film. Specific examples of materials that are the main components of the film include cellulose-based resins such as triacetyl cellulose (TAC), and transparent resins such as polyesters, polyvinyl alcohols, polycarbonates, polyamides, polyimides, polyethersulfones, polysulfones, polystyrenes, polynorbornenes, polyolefins, (meth)acrylics, and acetates.

The polarizing plate is typically disposed on the viewing side of the image display device. Therefore, the protective layer 20 may be subjected to surface treatments such as hard coat (HC) treatment, anti-reflection treatment, anti-sticking treatment, and anti-glare treatment, if necessary (for example, in the form shown in FIG. 4).

The thickness of the protective layer 20 is preferably 5 μm to 80 μm, more preferably 10 μm to 40 μm, and even more preferably 10 μm to 30 μm. If the above-mentioned surface treatment has been applied, the thickness of the protective layer 20 includes the thickness of the surface treatment layer.

In one embodiment, the protective layer disposed between the polarizing film 10 and the retardation layer 30 is preferably optically isotropic. In this specification, "optically isotropic" means that the in-plane retardation Re(550) is 0 nm to 10 nm, and the retardation in the thickness direction Rth(550) is -10 nm to +10 nm.

In one embodiment, the resin substrate can be used as a protective layer for the polarizing film. For example, in the embodiment shown in FIG. 3, the resin substrate can be used as a protective layer as is, thereby reducing the number of manufacturing steps.

C-2. Retardation layer Any suitable configuration may be adopted as the retardation layer 30. In one embodiment, an orientation solidified layer of a liquid crystal compound (liquid crystal orientation solidified layer) is used as the retardation layer 30. By using a liquid crystal compound, the difference between nx and ny of the obtained retardation layer can be significantly increased compared to non-liquid crystal materials, so the thickness of the retardation layer for obtaining a desired in-plane retardation can be significantly reduced. In this specification, the "orientation solidified layer" refers to a layer in which the liquid crystal compound is oriented in a predetermined direction within the layer and the orientation state is fixed. The "orientation solidified layer" is a concept that includes an orientation hardened layer obtained by hardening a liquid crystal monomer as described later. In the retardation layer, typically, rod-shaped liquid crystal compounds are aligned in the slow axis direction of the retardation layer (homogeneous orientation).

The thickness of the retardation layer depends on its structure (whether it is a single layer or has a laminated structure), but is preferably 8 μm or less, and more preferably 5 μm or less. On the other hand, the thickness of the retardation layer is, for example, 1 μm or more. Note that when the retardation layer has a laminated structure, the "thickness of the retardation layer" means the sum of the thicknesses of the individual retardation layers. Specifically, the "thickness of the retardation layer" does not include the thickness of the adhesive layer.

The liquid crystal alignment solidified layer can be formed by performing an alignment treatment on the surface of a predetermined substrate, applying a coating liquid containing a liquid crystal compound to the surface to align the liquid crystal compound in a direction corresponding to the alignment treatment, and fixing the alignment state. Any appropriate alignment treatment can be adopted as the alignment treatment. Specific examples include mechanical alignment treatment, physical alignment treatment, and chemical alignment treatment. Specific examples of mechanical alignment treatment include rubbing treatment and stretching treatment. Specific examples of physical alignment treatment include magnetic field alignment treatment and electric field alignment treatment. Specific examples of chemical alignment treatment include oblique deposition and photoalignment treatment. Any appropriate conditions can be adopted as the treatment conditions for various alignment treatments depending on the purpose.

The alignment of liquid crystal compounds is achieved by treating them at a temperature that exhibits a liquid crystal phase according to the type of liquid crystal compound. By carrying out such temperature treatment, the liquid crystal compounds assume a liquid crystal state, and the liquid crystal compounds are aligned according to the alignment treatment direction of the substrate surface.

In one embodiment, the alignment state is fixed by cooling the liquid crystal compound aligned as described above. When the liquid crystal compound is a polymerizable monomer or a crosslinkable monomer, the alignment state is fixed by subjecting the liquid crystal compound aligned as described above to a polymerization treatment or a crosslinking treatment.

Specific examples of liquid crystal compounds and details of the method for forming the alignment solidification layer are described in JP 2006-163343 A. The disclosure of this publication is incorporated herein by reference.

When the retardation layer 30 is a single layer, the retardation layer 30 can function as, for example, a λ/4 plate. Specifically, the Re(550) of the retardation layer is preferably 100 nm to 180 nm, more preferably 110 nm to 170 nm, and even more preferably 110 nm to 160 nm. The thickness of the retardation layer can be adjusted so as to obtain the desired in-plane retardation of the λ/4 plate. When the retardation layer is the above-mentioned liquid crystal alignment solidification layer, the thickness is, for example, 1.0 μm to 2.5 μm. In this embodiment, the angle between the slow axis of the retardation layer and the absorption axis of the polarizing film is preferably 40° to 50°, more preferably 42° to 48°, and even more preferably 44° to 46°. In this embodiment, the retardation layer preferably exhibits an inverse dispersion wavelength characteristic in which the retardation value increases according to the wavelength of the measurement light. In this embodiment, the polarizing plate may further include a layer (another retardation layer, not shown) that exhibits a refractive index characteristic of nz>nx=ny.

When the retardation layer 30 has a laminated structure, the retardation layer 30 has a two-layer laminated structure in which, for example, an H layer and a Q layer are arranged in order from the polarizing film 10 side. The H layer can typically function as a λ/2 plate, and the Q layer can typically function as a λ/4 plate. Specifically, the Re(550) of the H layer is preferably 200 nm to 300 nm, more preferably 220 nm to 290 nm, and even more preferably 230 nm to 280 nm; the Re(550) of the Q layer is preferably 100 nm to 180 nm, more preferably 110 nm to 170 nm, and even more preferably 110 nm to 150 nm. The thickness of the H layer can be adjusted so as to obtain the desired in-plane retardation of the λ/2 plate. When the H layer is the above-mentioned liquid crystal alignment solidified layer, its thickness is, for example, 2.0 μm to 4.0 μm. The thickness of the Q layer can be adjusted so as to obtain the desired in-plane retardation of the λ/4 plate. When the Q layer is the above-mentioned liquid crystal alignment solidified layer, its thickness is, for example, 1.0 μm to 2.5 μm. In this embodiment, the angle between the slow axis of the H layer and the absorption axis of the polarizing film is preferably 10° to 20°, more preferably 12° to 18°, and even more preferably 12° to 16°; the angle between the slow axis of the Q layer and the absorption axis of the polarizing film is preferably 70° to 80°, more preferably 72° to 78°, and even more preferably 72° to 76°. When the retardation layer 30 has a laminated structure, each layer (for example, the H layer and the Q layer) may exhibit a reverse dispersion wavelength characteristic in which the retardation value increases according to the wavelength of the measurement light, may exhibit a positive wavelength dispersion characteristic in which the retardation value decreases according to the wavelength of the measurement light, or may exhibit a flat wavelength dispersion characteristic in which the retardation value hardly changes depending on the wavelength of the measurement light.

The retardation layer 30 (or each layer when it has a laminated structure) typically exhibits the following refractive index characteristic: nx>ny=nz. Note that "ny=nz" does not only include the case where ny and nz are completely equal, but also includes the case where they are substantially equal. Therefore, there may be cases where ny>nz or ny<nz, as long as the effect of the present invention is not impaired. The Nz coefficient of the retardation layer is preferably 0.9 to 1.5, and more preferably 0.9 to 1.3.

As described above, the retardation layer is preferably a liquid crystal alignment solidified layer. Examples of the liquid crystal compound include liquid crystal compounds whose liquid crystal phase is a nematic phase (nematic liquid crystals). Examples of such liquid crystal compounds that can be used include liquid crystal polymers and liquid crystal monomers. The mechanism by which the liquid crystallinity of the liquid crystal compound is expressed may be either lyotropic or thermotropic. The liquid crystal polymer and liquid crystal monomer may be used alone or in combination.

When the liquid crystal compound is a liquid crystal monomer, the liquid crystal monomer is preferably a polymerizable monomer and a crosslinkable monomer. This is because the orientation state of the liquid crystal monomer can be fixed by polymerizing or crosslinking (i.e., curing) the liquid crystal monomer. After the liquid crystal monomer is aligned, for example, the liquid crystal monomers can be polymerized or crosslinked with each other, thereby fixing the above-mentioned orientation state. Here, a polymer is formed by polymerization, and a three-dimensional network structure is formed by crosslinking, but these are non-liquid crystals. Therefore, the formed retardation layer does not undergo transition to a liquid crystal phase, a glass phase, or a crystalline phase due to temperature changes, which are specific to liquid crystal compounds. As a result, the retardation layer becomes a retardation layer that is not affected by temperature changes and has excellent stability.

The temperature range in which the liquid crystal monomer exhibits liquid crystallinity varies depending on the type of the monomer. Specifically, the temperature range is preferably 40°C to 120°C, more preferably 50°C to 100°C, and most preferably 60°C to 90°C.

As the liquid crystal monomer, any suitable liquid crystal monomer can be used. For example, the polymerizable mesogen compounds described in JP-A-2002-533742 (WO00/37585), EP358208 (US5211877), EP66137 (US4388453), WO93/22397, EP0261712, DE19504224, DE4408171, GB2280445, etc. can be used. Specific examples of such polymerizable mesogen compounds include BASF's product name LC242, Merck's product name E7, and Wacker-Chem's product name LC-Sillicon-CC3767. As the liquid crystal monomer, a nematic liquid crystal monomer is preferable.

C-3. Adhesive Layer Any suitable structure may be adopted for the adhesive layer 40. Specific examples include acrylic adhesives, rubber adhesives, silicone adhesives, polyester adhesives, urethane adhesives, epoxy adhesives, and polyether adhesives. By adjusting the type, number, combination, and compounding ratio of monomers forming the base resin of the adhesive, as well as the compounding amount of the crosslinking agent, reaction temperature, reaction time, and the like, an adhesive having desired properties according to the purpose can be prepared. The base resin of the adhesive may be used alone or in combination of two or more kinds. The base resin is preferably an acrylic resin (specifically, the adhesive layer is preferably composed of an acrylic adhesive). The thickness of the adhesive layer is, for example, 10 μm to 20 μm.

C-4. Preparation of Polarizing Plate A polarizing plate can be typically obtained by laminating various layers such as a retardation layer on the polarizing film after the above-mentioned water washing.

The polarizing plate 100 shown in FIG. 3 can be obtained, for example, by using the resin substrate of the laminate of the resin substrate and the polarizing film after the water washing as it is as the protective layer 20, and sequentially laminating the retardation layer 30 and the adhesive layer 40 on the resin substrate. Also, for example, the polarizing plate 100 can be obtained by laminating the protective layer 20 on the resin film side of the laminate of the resin substrate and the resin film, peeling the resin substrate from the resin film to obtain a laminate, washing the laminate with water, and sequentially laminating the retardation layer 30 and the adhesive layer 40 on the protective layer 20 side. Also, for example, the polarizing plate 100 may be obtained by laminating the retardation layer 30 or the retardation layer 30 and the adhesive layer 40 on the laminate of the protective layer 20 and the resin film, and then washing with water.

The polarizing plate 110 shown in FIG. 4 can be obtained, for example, by laminating a protective layer 20 on the resin film side of the laminate of the resin substrate and the resin film, peeling the resin substrate from the resin film to obtain a laminate, washing the resulting laminate with water, and sequentially laminating a retardation layer 30 and a pressure-sensitive adhesive layer 40 on the polarizing film 10 side.

The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The thickness, iodine concentration and moisture content of the resin film are values measured by the following measurement methods. Furthermore, unless otherwise specified, "parts" and "%" in the examples and comparative examples are based on weight.
1. Thickness Thicknesses of 10 μm or less were measured using a scanning electron microscope (manufactured by JEOL Ltd., product name "JSM-7100F"), and thicknesses of more than 10 μm were measured using a digital micrometer (manufactured by Anritsu Corporation, product name "KC-351C").
2. Iodine concentration of resin film The fluorescent X-ray intensity (kcps) of iodine element was measured using a fluorescent X-ray analyzer (manufactured by Rigaku Corporation, product name "ZSX Primus IV", measurement diameter: ψ20 mm), and the iodine concentration (wt%) was calculated by the following formula. Here, the coefficient for calculating the iodine concentration was obtained using a calibration curve.
Iodine concentration (wt%) = 20.5 x fluorescent X-ray intensity / resin film thickness (kcps/μm)
3. Moisture content of resin film The resin film alone (resin film peeled off from the resin substrate) was dried at 120°C for 2 hours, and the amount of moisture contained in the resin film was determined by measuring the weight change before and after drying, and the moisture content was calculated.

[Example 1]
(Preparation of resin film)
The thermoplastic resin substrate used was a long amorphous isophthalic copolymerized polyethylene terephthalate film (thickness: 100 μm) having a water absorption rate of 0.75% and a Tg of about 75° C. One side of the resin substrate was subjected to a corona treatment.
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 Z410") in a ratio of 9:1.
The above PVA aqueous solution was applied to the corona-treated surface of a resin substrate and dried at 60° C. to form a PVA-based resin layer having a thickness of 13 μm, thereby producing a laminate.
The obtained laminate was uniaxially stretched at its free end 2.4 times in the machine direction (longitudinal direction) between rolls having different peripheral speeds in an oven at 130° C. (auxiliary air stretching).
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 respect to 100 parts by weight of water) having a liquid temperature of 30° C. for 60 seconds while adjusting the concentration so that the single transmittance (Ts) of the finally obtained polarizing film would be 43% or more (dyeing).
Next, the piece was immersed in a crosslinking bath (a boric acid aqueous solution obtained by mixing 3 parts by weight of potassium iodide and 5 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40° C. for 30 seconds (crosslinking treatment).
Thereafter, the laminate was immersed in an aqueous boric acid solution (boric acid concentration: 4.0% by weight) at a liquid temperature of 64° C. and uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls having different peripheral speeds to a total stretch ratio of 5.5 times (underwater stretching).
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).
Thereafter, while drying in an oven maintained at 90° C., the laminate was brought into contact with a SUS heated roll whose surface temperature was maintained at 75° C. for about 2 seconds (drying). The shrinkage rate in the width direction of the laminate due to the drying shrinkage treatment was 5.2%.
In this manner, a resin film having a thickness of 5.4 μm, an iodine concentration of 6.4%, and a moisture content of 4.4% was formed on the resin substrate.

(Water washing 1)
Next, a 20 μm-thick acrylic film having a lactone ring structure was bonded to one side of the obtained resin film (the side on which the resin substrate was not disposed) via an ultraviolet-curable adhesive, and then the resin substrate was peeled off from the resin film. The laminate of the acrylic film and the resin film was immersed in a water bath at 43° C. for 2 minutes to wash one side of the resin film (the peeled side) with water and dried at 50° C., thereby obtaining a laminate of an acrylic film and a polarizing film.

(Water wash 2)
Next, a 27 μm-thick HC-COP film was bonded to one side of the obtained resin film (the side on which the resin substrate was not disposed) via an ultraviolet-curable adhesive, and then the resin substrate was peeled off from the resin film, and the laminate of the HC-COP film and the resin film was immersed in a water bath at 43° C. for 2 minutes to wash one side (the peeled side) of the resin film with water and dried at 50° C. to obtain a laminate of the HC-COP film and the polarizing film. The HC-COP film was a film in which an HC layer (thickness 2 μm) was formed on a cycloolefin resin (COP) film (thickness 25 μm), and the COP film was bonded to the resin film side.

(Preparation of Retardation Layer)
A liquid crystal composition (coating liquid) was prepared by dissolving 10 g of a polymerizable liquid crystal exhibiting a nematic liquid crystal phase (manufactured by BASF: product name "Paliocolor LC242", represented by the following formula) and 3 g of a photopolymerization initiator for the polymerizable liquid crystal compound (manufactured by BASF: product name "Irgacure 907") in 40 g of toluene.

The surface of a polyethylene terephthalate (PET) film (thickness 38 μm) was rubbed with a rubbing cloth and subjected to an orientation treatment. The orientation treatment direction was set to be 15° from the viewing side with respect to the direction of the absorption axis of the polarizing film when it was laminated to the polarizing plate. The liquid crystal coating liquid was applied to this orientation treatment surface with a bar coater, and the liquid crystal compound was aligned by heating and drying at 90 ° C for 2 minutes. The liquid crystal layer thus formed was irradiated with light of 1 mJ / cm ² using a metal halide lamp, and the liquid crystal layer was hardened to form a liquid crystal alignment solidified layer A (H layer) on the PET film. The thickness of the liquid crystal alignment solidified layer A was 2.5 μm, and the in-plane retardation Re (550) was 270 nm. Furthermore, the liquid crystal alignment solidified layer A showed a refractive index characteristic of nx > ny = nz.

A liquid crystal alignment solidified layer B (Q layer) was formed on a PET film in the same manner as above, except that the coating thickness was changed and the orientation treatment direction was set to a 75° angle from the viewing side with respect to the absorption axis direction of the polarizing film. The thickness of the liquid crystal alignment solidified layer B was 1.5 μm, and the in-plane retardation Re (550) was 140 nm. Furthermore, the liquid crystal alignment solidified layer B exhibited refractive index characteristics of nx>ny=nz.

(Preparation of Polarizing Plate A)
The obtained liquid crystal alignment solidified layer A (H layer) and liquid crystal alignment solidified layer B (Q layer) were transferred in this order to the acrylic film side of the laminate of the acrylic film and the polarizing film. At this time, the transfer (lamination) was performed so that the angle between the absorption axis of the polarizing film and the slow axis of the alignment solidified layer A was 15°, and the angle between the absorption axis of the polarizing film and the slow axis of the alignment solidified layer B was 75°. Each transfer was performed via an ultraviolet-curing adhesive (thickness 1.0 μm). Then, a 15 μm thick adhesive layer was formed on the liquid crystal alignment solidified layer B to obtain a polarizing plate A.

(Preparation of Polarizing Plate B)
The obtained liquid crystal alignment solidified layer A (H layer) and liquid crystal alignment solidified layer B (Q layer) were transferred in this order to the polarizing film side of the laminate of the HC-COP film and the polarizing film. At this time, the transfer (lamination) was performed so that the angle between the absorption axis of the polarizing film and the slow axis of the alignment solidified layer A was 15°, and the angle between the absorption axis of the polarizing film and the slow axis of the alignment solidified layer B was 75°. Each transfer was performed via an ultraviolet-curing adhesive (thickness 1.0 μm). Then, a pressure-sensitive adhesive layer with a thickness of 15 μm was formed on the liquid crystal alignment solidified layer B to obtain a polarizing plate B.

[Example 2]
A polarizing film and a polarizing plate were obtained in the same manner as in Example 1, except that the laminate was washed by immersing it in a water bath at 40° C. for 2 minutes.

[Example 3]
A polarizing film and a polarizing plate were obtained in the same manner as in Example 1, except that the laminate was immersed in an aqueous boric acid solution at a liquid temperature of 70° C. and stretched in water to produce a resin film with a thickness of 5.4 μm, an iodine concentration of 6.6%, and a moisture content of 4.5%, and that the laminate was washed with water by immersing in a water bath at 25° C. for 5 minutes. Note that polarizing plate B was not produced due to the surface properties (swelling) of the polarizing film after washing with water.

[Example 4]
A polarizing film and a polarizing plate were obtained in the same manner as in Example 1, except that the laminate was immersed in an aqueous boric acid solution at a liquid temperature of 67° C. and stretched in water to produce a resin film with a thickness of 4.8 μm, an iodine concentration of 5.5%, and a moisture content of 4%, and that the laminate was washed by immersing it in a water bath at 43° C. for 1 minute.

[Example 5]
A polarizing film and a polarizing plate were obtained in the same manner as in Example 1, except that the laminate was immersed in an aqueous boric acid solution at a liquid temperature of 67° C. and stretched in water to produce a resin film with a thickness of 4.8 μm, an iodine concentration of 5.5%, and a moisture content of 4%, and that the laminate was washed by immersing it in a water bath at 35° C. for 2 minutes.

[Comparative Example 1]
A polarizing film and a polarizing plate were obtained in the same manner as in Example 1, except that the resin film was not washed with water.

[Comparative Example 2]
A polarizing film and a polarizing plate were obtained in the same manner as in Example 4, except that the resin film was not washed with water.

[Comparative Example 3]
A polarizing film and a polarizing plate were obtained in the same manner as in Example 3, except that the resin film was not washed with water.

[Reference Example 1]
(Preparation of resin film)
A long roll of a PVA-based resin film having a thickness of 30 μm was uniaxially stretched in the longitudinal direction at 66° C. using a roll stretching machine so that the total stretch ratio became 6.5 times, while simultaneously subjecting the film to swelling, dyeing, crosslinking and washing treatments, and finally subjecting the film to drying treatment, thereby producing a resin film having a thickness of 12 μm, an iodine concentration of 2.9% and a moisture content of 11%.

(Water washing 1)
A 20 μm-thick acrylic film having a lactone ring structure was bonded to one side of the obtained resin film via an ultraviolet-curable adhesive to produce a laminate, and the laminate of the film and the resin film was immersed in a water bath at 43° C. for 2 minutes to wash the surface of the obtained resin film (the side to which the film was not bonded) with water and dried at 50° C. to obtain a laminate of a film and a polarizing film.

(Water wash 2)
A 27 μm-thick HC-COP film was attached to one side of the obtained resin film via an ultraviolet-curable adhesive to produce a laminate, and the laminate of the film and the resin film was immersed in a water bath at 43° C. for 2 minutes to wash the surface of the obtained resin film (the side to which the film was not attached) with water and dried at 50° C. to obtain a laminate of the film and the polarizing film.

(Preparation of Polarizing Plate A)
The obtained liquid crystal alignment solidified layer A (H layer) and liquid crystal alignment solidified layer B (Q layer) were transferred in this order to the acrylic film side of the laminate of the acrylic film and the polarizing film. At this time, the transfer (lamination) was performed so that the angle between the absorption axis of the polarizing film and the slow axis of the alignment solidified layer A was 15°, and the angle between the absorption axis of the polarizing film and the slow axis of the alignment solidified layer B was 75°. Each transfer was performed via an ultraviolet-curing adhesive (thickness 1.0 μm). Then, a 15 μm thick adhesive layer was formed on the liquid crystal alignment solidified layer B to obtain a polarizing plate A.

(Preparation of Polarizing Plate B)
The obtained liquid crystal alignment solidified layer A (H layer) and liquid crystal alignment solidified layer B (Q layer) were transferred in this order to the polarizing film side of the laminate of the HC-COP film and the polarizing film. At this time, the transfer (lamination) was performed so that the angle between the absorption axis of the polarizing film and the slow axis of the alignment solidified layer A was 15°, and the angle between the absorption axis of the polarizing film and the slow axis of the alignment solidified layer B was 75°. Each transfer was performed via an ultraviolet-curing adhesive (thickness 1.0 μm). Then, a pressure-sensitive adhesive layer with a thickness of 15 μm was formed on the liquid crystal alignment solidified layer B to obtain a polarizing plate B.

[Reference Example 2]
A polarizing film and a polarizing plate were obtained in the same manner as in Reference Example 1, except that the laminate was not washed with water.

The following evaluations were carried out for the Examples and Comparative Examples. The evaluation results are summarized in Tables 1 and 2.
<Evaluation>
1. Single transmittance and polarization degree For the polarizing films (polarizing film/acrylic film or HC-COP film) of the Examples and Comparative Examples, the single transmittance Ts, parallel transmittance Tp, and crossed transmittance Tc measured using an ultraviolet-visible spectrophotometer (V-7100, manufactured by JASCO Corporation) were taken as Ts, Tp, and Tc of the polarizing film, respectively. These Ts, Tp, and Tc are Y values measured using a 2-degree visual field (C light source) according to JIS Z8701 and corrected for visibility.
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
2. Reflectance (Rc and Rp)
The reflectance of the surface (washed surface) of the polarizing film in each of the Examples and Comparative Examples was measured using a spectrophotometer (U-4100, manufactured by Hitachi High-Tech Corporation) in terms of the reflectance in the absorption axis direction (Rc) and the reflectance in the transmission axis direction (Rp). At that time, the surface opposite to the surface washed with water was attached to a blackboard so that only the surface reflection could be measured. The incident angle of the light source (polarized light) was set to 5°, and the measurement wavelength was set to 380 nm to 780 nm.
3. Surface Properties The surface properties of the polarizing films of the Examples and Comparative Examples (presence or absence of unevenness due to swelling of the resin film) were visually observed.
(Evaluation Criteria)
Good: No unevenness is observed. Bad: Unevenness is observed. 4. Color of polarizing plate (a ^* and b ^* )
The polarizing plates (circular polarizing plates) of the examples and comparative examples were attached to an aluminum sheet, and the reflected hue was measured in SCE mode using a spectrophotometer (manufactured by Konica Minolta, cm-2600d).

Both the examples and the comparative examples provide excellent optical properties (single transmittance and polarization degree). In the examples, redness is suppressed, providing an excellent appearance.

The iodine ion intensity in the thickness direction was measured for the polarizing films obtained in Example 1, Comparative Example 1, and Reference Example 2. The measurement was performed using a time-of-flight secondary ion mass spectrometer (TOF-SIMS) (manufactured by ION-TOF, product name: TOF-SIMS 5) and Bi ₃₂ ⁺ as the primary ion. The measurement results (a graph in which the horizontal axis is converted into the thickness of the polarizing film) are shown in FIG. 5. As shown in FIG. 5, it was confirmed that the polarizing film of Example 1 has a gradient distribution region at the end on the front surface side, where the amount of iodine increases from the front surface toward the back surface. The iodine ion intensity on the vertical axis corresponds to the iodine concentration.

The polarizing film according to one embodiment of the present invention is suitable for use in image display devices such as liquid crystal display devices, organic EL display devices, and inorganic EL display devices.

REFERENCE SIGNS LIST 10 Polarizing film 20 Protective layer 30 Retardation layer 40 Pressure-sensitive adhesive layer 100 Polarizing plate 110 Polarizing plate

Claims (12)

It is made of a resin film containing iodine,
The thickness is 7 μm or less,
the ratio (Rc ₄₀₀ /Rc ₆₈₀ ) of the reflectance Rc ₄₀₀ of light having a wavelength of 400 nm in the absorption axis direction to the reflectance Rc ₆₈₀ of light having a wavelength of 680 nm in the absorption axis direction of the surface exceeds 1;
Polarizing film. 2. The polarizing film according to claim 1, wherein the Rc ₆₈₀ of the surface is 5% or less. 3. The polarizing film according to claim 1, wherein the Rc ₄₀₀ of the surface is 4.8% or more. The polarizing film according to any one of claims 1 to 3, which has a gradient distribution region at the end of the front surface side, in which the amount of iodine increases from the front surface toward the back surface. The polarizing film according to any one of claims 1 to 4, wherein the amount of iodine on the front side is less than the amount of iodine on the back side. The polarizing film according to any one of claims 1 to 5, having a single transmittance of 42.0% or more and a polarization degree of 99.98% or more. washing the surface of a resin film containing iodine and having a moisture content of 15% by weight or less with water;
A method for producing the polarizing film according to claim 1 , comprising: The manufacturing method according to claim 7, wherein the iodine concentration of the resin film is 5% by weight or more. The manufacturing method according to claim 7 or 8, wherein the resin film is a resin layer formed on a resin substrate. The manufacturing method according to claim 9, which includes stretching the resin layer in water at 67°C or less. The manufacturing method according to claim 9 or 10, which includes heating the resin layer using a heating roll. The polarizing film according to any one of claims 1 to 6,
and a protective layer or a retardation layer disposed on at least one side of the polarizing film.

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