JP7488252B2 - Polarizing film, polarizing plate, and method for producing the polarizing film - Google Patents

Description

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

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

JP 2001-343521 A

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

The polarizing film of the present invention is made of a polyvinyl alcohol-based resin film containing iodine, has a thickness of 8 μm or less, and contains 5 ppm to 350 ppm of alcohol having a boiling point of less than 100° C.
In one embodiment, the alcohol having a boiling point of less than 100° C. is at least one selected from the group consisting of methanol, ethanol, n-propyl alcohol, and isopropyl alcohol.
According to another aspect of the present invention, there is provided a polarizing plate having the above polarizing film and a protective layer disposed on at least one side of the polarizing film.
According to yet another aspect of the present invention, there is provided a method for producing the polarizing film, comprising: forming a polyvinyl alcohol-based resin layer on one side of a long thermoplastic resin substrate to form a laminate; stretching and dyeing the laminate to form the polyvinyl alcohol-based resin layer into a polarizing film; and introducing an alcohol having a boiling point of less than 100° C. into the polarizing film.
In one embodiment, the production method includes immersing the polarizing film in a treatment liquid containing an alcohol having a boiling point of less than 100°C.
In one embodiment, the production method further includes heating the laminate after introducing the alcohol having a boiling point of less than 100° C. into the polarizing film.
In one embodiment, the stretching comprises underwater stretching.

According to the present invention, by introducing alcohol having a boiling point of less than 100°C into a polarizing film, a thin polarizing film having excellent durability in a high-temperature and high-humidity environment can be obtained.

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

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

A. Polarizing Film The polarizing film according to the embodiment of the present invention is composed of a polyvinyl alcohol (PVA)-based resin film containing iodine, has a thickness of 8 μm or less, and contains 5 ppm to 350 ppm of alcohol having a boiling point of less than 100° C. (hereinafter, may be referred to as low-boiling alcohol). When the polarizing film contains a predetermined amount of such a low-boiling alcohol, it is possible to obtain a polarizing film that is thin and has excellent durability in a high-temperature and high-humidity environment. As will be described later in Section C regarding the manufacturing method, such a low-boiling alcohol can be typically introduced into the polarizing film between the underwater stretching treatment and the drying shrinkage treatment. It is presumed that the introduction of such a low-boiling alcohol can improve the durability in a high-temperature and high-humidity environment by the following mechanism: (i) the drying efficiency is increased due to the low-boiling alcohol during the drying shrinkage treatment, and the orientation of the PVA is improved; and (ii) the PVA-iodine complex is stabilized due to the low-boiling alcohol in the obtained polarizing film, so that the deterioration of the optical properties during humidification can be suppressed. Furthermore, in the production of a thin polarizing film, the crystallization of PVA may be insufficient, but by introducing a low-boiling alcohol, good crystallization of PVA can be achieved. As a result, even in a thin polarizing film in which the iodine concentration is remarkably higher and the stability of iodine is insufficient than in a conventional (thick) polarizer with the same single transmittance, excellent durability can be achieved in a high-temperature and high-humidity environment. The content of the low-boiling alcohol in the polarizing film is, for example, 8 ppm to 320 ppm, preferably 20 ppm to 200 ppm, more preferably 40 ppm to 150 ppm, and even more preferably 50 ppm to 120 ppm. If the content is too small, the effect of the low-boiling alcohol may not be obtained. If the content is too large, the amount introduced during production increases, which may increase the amount of volatilization into the working environment, thereby increasing safety risks.

Typical examples of low-boiling alcohols include lower monoalcohols with 1 to 4 carbon atoms. Specific examples include methanol, ethanol, n-propyl alcohol, isopropyl alcohol, and tert-butyl alcohol. Low-boiling alcohols may be used alone or in combination of two or more. Preferred are methanol, ethanol, n-propyl alcohol, and isopropyl alcohol. These have low boiling points, which improves the drying efficiency in the drying process described below, and are advantageous in improving the properties of the resulting polarizing film.

As described above, the thickness of the polarizing film is 8 μm or less, preferably 7 μm or less, more preferably 5 μm or less, and even more preferably 3 μm or less. The lower limit of the thickness of the polarizing film may be 1 μm in one embodiment and 2 μm in another embodiment.

The polarizing film preferably exhibits absorption dichroism at any wavelength of 380 nm to 780 nm. The single transmittance of the polarizing film is preferably 42.0% or more, more preferably 42.5% or more, and even more preferably 43.0% or more. On the other hand, the single transmittance is preferably 47.0% or less, more preferably 46.0% or less. The polarization degree of the polarizing film is preferably 99.90% or more, more preferably 99.95% or more. On the other hand, the polarization degree is preferably 99.998% or less. According to the embodiment of the present invention, it is possible to achieve both a high single transmittance and a high polarization degree, and to realize excellent durability in a high temperature and high humidity environment as described above. The single transmittance is typically a Y value measured using an ultraviolet-visible spectrophotometer and corrected for luminosity. The single transmittance is a value when the refractive index of one surface of the polarizing plate is converted to 1.50 and the refractive index of the other surface is converted to 1.53. The degree of polarization is typically calculated by the following formula based on the parallel transmittance Tp and the crossed transmittance Tc which are measured using an ultraviolet-visible spectrophotometer and subjected to luminosity correction.
Degree of polarization (%)={(Tp−Tc)/(Tp+Tc)} ^1/2 ×100

The polarizing film preferably has a change in polarization degree ΔP after a durability test at a temperature of 60° C. and a relative humidity of 95% for 120 hours of -0.70% or more, more preferably -0.55% or more, and even more preferably -0.20% or more. The upper limit of ΔP may be, for example, 0.0% or more, and ΔP may be, for example, 0.10% or less. ΔP is expressed by the following formula:
ΔP = P ₁₂₀ - P ₀
In the above formula, _P120 is the polarization degree after the durability test, and _P0 is the polarization degree before the durability test (the polarization degree described above). That is, the polarizing film according to the embodiment of the present invention has a feature that the polarization degree decreases little in a high-temperature and high-humidity environment, and may even increase.

The polarizing film may be produced using a single resin film, or may be produced using a laminate of two or more layers. A specific example of a polarizing film obtained using a laminate is a polarizing film obtained using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate. A polarizing film obtained using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate can be produced, for example, by applying a PVA-based resin solution to a resin substrate, drying the substrate to form a PVA-based resin layer on the resin substrate, and obtaining a laminate of the resin substrate and the PVA-based resin layer; stretching and dyeing the laminate to make the PVA-based resin layer into a polarizing film. In an embodiment of the present invention, a low-boiling alcohol is introduced into the polarizing film. This makes it possible to achieve excellent durability in a high-temperature and high-humidity environment as described above. Preferably, a polyvinyl alcohol-based resin layer containing a halide and a polyvinyl alcohol-based resin is formed on one side of the resin substrate. The stretching typically includes immersing the laminate in an aqueous boric acid solution and stretching it. Furthermore, the stretching may further include air-stretching the laminate at a high temperature (e.g., 95°C or higher) before stretching in the boric acid aqueous solution, if necessary. In addition, in this embodiment, the laminate is preferably subjected to a drying shrinkage treatment in which the laminate is heated while being conveyed in the longitudinal direction, thereby shrinking the laminate by 2% or more in the width direction. Typically, the manufacturing method of this embodiment includes subjecting the laminate to an air-assisted stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment in this order. By introducing the auxiliary stretching, it is possible to increase the crystallinity of PVA even when PVA is applied onto a thermoplastic resin, and it is possible to achieve high optical properties. At the same time, by increasing the orientation of PVA in advance, problems such as a decrease in the orientation of PVA or dissolution can be prevented when the PVA is immersed in water in the subsequent dyeing step or stretching step, and it is possible to achieve high optical properties. Furthermore, when the PVA-based resin layer is immersed in a liquid, the disorder of the orientation of polyvinyl alcohol molecules and the decrease in orientation can be suppressed compared to when the PVA-based resin layer does not contain a halide. This can improve the optical properties of the polarizing film obtained through a treatment step in which the laminate is immersed in a liquid, such as a dyeing treatment and an underwater stretching treatment. Furthermore, the optical properties can be improved by shrinking the laminate in the width direction by a drying shrinkage treatment. The obtained resin substrate/polarizing film laminate may be used as it is (i.e., the resin substrate may be used as a protective layer for the polarizing film), or the resin substrate may be peeled off from the resin substrate/polarizing film laminate and any suitable protective layer according to the purpose may be laminated on the peeled surface. Details of the method for producing the polarizing film will be described later in Section C.

B. Polarizing Plate FIG. 1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. The polarizing plate 100 has a polarizing film 10, a first protective layer 20 arranged on one side of the polarizing film 10, and a second protective layer 30 arranged on the other side of the polarizing film 10. The polarizing film 10 is the polarizing film of the present invention described in the above section A. One of the first protective layer 20 and the second protective layer 30 may be omitted. As described above, one of the first protective layer and the second protective layer may be a resin substrate used in the manufacture of the above polarizing film.

The first and second protective layers are formed of any suitable film that can be used as a protective layer for a polarizing film. Specific examples of materials that are the main components of the film include cellulose-based resins such as triacetyl cellulose (TAC), and transparent resins such as polyesters, polyvinyl alcohols, polycarbonates, polyamides, polyimides, polyethersulfones, polysulfones, polystyrenes, polynorbornenes, polyolefins, (meth)acrylics, and acetates. Other examples include thermosetting resins or ultraviolet-curing resins such as (meth)acrylics, urethanes, (meth)acrylic urethanes, epoxy resins, and silicones. Other examples include glassy polymers such as siloxane polymers. Polymer films described in JP 2001-343529 A (WO01/37007) can also be used. As the material for this film, for example, a resin composition containing a thermoplastic resin having a substituted or unsubstituted imide group in the side chain and a thermoplastic resin having a substituted or unsubstituted phenyl group and a nitrile group in the side chain can be used, for example, a resin composition containing an alternating copolymer of isobutene and N-methylmaleimide, and an acrylonitrile-styrene copolymer. The polymer film can be, for example, an extrusion molded product of the above resin composition.

When the polarizing plate 100 is applied to an image display device, the thickness of the protective layer (outer protective layer) disposed on the opposite side to the display panel is typically 300 μm or less, preferably 100 μm or less, more preferably 5 μm to 80 μm, and even more preferably 10 μm to 60 μm. Note that, if a surface treatment has been applied, the thickness of the outer protective layer includes the thickness of the surface treatment layer.

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

C. Manufacturing method of polarizing film A manufacturing method of a polarizing film according to one embodiment of the present invention includes: applying a PVA-based resin solution to one side of a long thermoplastic resin substrate and drying the solution to form a PVA-based resin layer to form a laminate; stretching and dyeing the laminate to form the PVA-based resin layer into a polarizing film; and introducing a low-boiling alcohol into the polarizing film. By introducing a low-boiling alcohol, a polarizing film having excellent durability in a high-temperature and high-humidity environment can be realized. Preferably, the PVA-based resin solution further contains a halide. Preferably, the manufacturing method includes subjecting the laminate to an auxiliary air-stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment in which the laminate is heated while being transported in the longitudinal direction to shrink the laminate by 2% or more in the width direction, in this order. In this case, the introduction of the low-boiling alcohol can be preferably performed between the underwater stretching treatment and the drying shrinkage treatment. The content of the halide in the PVA-based resin solution (as a result, the PVA-based resin layer) is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA-based resin. The drying shrinkage treatment is preferably carried out using a heating roll, and the temperature of the heating roll is preferably 60° C. to 120° C. The shrinkage rate in the width direction of the laminate due to the drying shrinkage treatment is preferably 2% or more. According to such a production method, the polarizing film described in the above item A can be obtained. In particular, a laminate including a PVA-based resin layer containing a halide is produced, the laminate is stretched in multiple stages including auxiliary air stretching and underwater stretching, and the laminate after stretching is heated with a heating roll, whereby a polarizing film having excellent optical properties (typically, single transmittance and unit absorbance) can be obtained.

C-1. Preparation of Laminate Any appropriate method may be adopted as a method for preparing a laminate of a thermoplastic resin substrate and a PVA-based resin layer. Preferably, a coating liquid containing a halide and a PVA-based resin is applied to the surface of the thermoplastic resin substrate, and then dried to form a PVA-based resin layer on the thermoplastic resin substrate. As described above, the content of the halide in the PVA-based resin layer is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA-based resin.

Any suitable 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 coating and drying temperature of the coating liquid is preferably 50°C or higher.

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

Before forming the PVA-based resin layer, the thermoplastic resin substrate may be subjected to a surface treatment (e.g., corona treatment, etc.), or an easy-adhesion layer may be formed on the thermoplastic resin substrate. By carrying out such treatment, it is possible to improve the adhesion between the thermoplastic resin substrate and the PVA-based resin layer.

C-1-1. Thermoplastic resin substrate Any suitable thermoplastic resin film may be used as the thermoplastic resin substrate. Details of the thermoplastic resin substrate are described in, for example, JP 2012-73580 A. The entire disclosure of this publication is incorporated herein by reference.

C-1-2. Coating liquid The coating liquid contains a halide and a PVA-based resin as described above. The coating liquid is typically a solution in which the halide and the PVA-based resin are dissolved in a solvent. Examples of the solvent include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyhydric alcohols such as trimethylolpropane, and amines such as ethylenediamine and diethylenetriamine. These can be used alone or in combination of two or more. Of these, water is preferred. The concentration of the PVA-based resin in the solution is preferably 3 to 20 parts by weight relative to 100 parts by weight of the solvent. With such a resin concentration, a uniform coating film that is in close contact with the thermoplastic resin substrate can be formed. The content of the halide in the coating liquid is preferably 5 to 20 parts by weight relative to 100 parts by weight of the PVA-based resin.

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

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

The average degree of polymerization of the PVA-based resin can be appropriately selected depending on the purpose. The average degree of polymerization is usually 1000 to 10000, preferably 1200 to 4500, and more preferably 1500 to 4300. 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 iodide and sodium chloride. Examples of iodide include potassium iodide, sodium iodide, and lithium iodide. Of these, potassium iodide is preferred.

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

Generally, the orientation of the polyvinyl alcohol molecules in the PVA-based resin increases when the PVA-based resin layer is stretched, but when the stretched PVA-based resin layer is immersed in a liquid containing water, the orientation of the polyvinyl alcohol molecules may become disordered, and the orientation may decrease. In particular, when a laminate of a thermoplastic resin and a PVA-based resin layer is stretched in boric acid water, the tendency for the degree of orientation to decrease is remarkable when the laminate is stretched in boric acid water at a relatively high temperature to stabilize the stretching of the thermoplastic resin. For example, while a PVA film alone is generally stretched in boric acid water at 60°C, a laminate of A-PET (thermoplastic resin substrate) and a PVA-based resin layer is stretched at a high temperature of around 70°C, in this case, the orientation of the PVA at the beginning of the stretching may decrease before it increases due to the stretching in water. In response to this, 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 the crystallization of the PVA-based resin in the PVA-based resin layer of the laminate after the 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.

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

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

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

The stretching ratio in the auxiliary air stretching is preferably 2.0 to 3.5 times. The maximum stretching ratio when auxiliary air 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, of the original length of the laminate. In this specification, the "maximum stretching ratio" refers to the stretching ratio just before the laminate breaks, and refers to a value 0.2 lower than the stretching ratio at which the laminate breaks, which is determined separately.

The stretching temperature for the 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 prevented from progressing rapidly, and defects due to the crystallization (for example, preventing the orientation of the PVA-based resin layer by stretching) can be prevented.

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

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

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

The underwater stretching is preferably performed by immersing the laminate in an aqueous solution of boric acid (underwater stretching in boric acid). By using an aqueous solution of boric acid as the stretching bath, the PVA-based resin layer can be given rigidity that can withstand the tension applied during stretching and water resistance that does not dissolve in water. Specifically, boric acid can generate tetrahydroxyborate anions in the aqueous solution and crosslink with the PVA-based resin through hydrogen bonds. As a result, the PVA-based resin layer can be given rigidity and water resistance, and can be stretched well, resulting in the production of 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 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 there is a risk that excellent optical properties cannot be obtained. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

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

C-5. Introduction of low boiling point alcohol In an embodiment of the present invention, a low boiling point alcohol is introduced after the underwater stretching treatment (and typically before the drying shrinkage treatment described later). The introduction of the low boiling point alcohol can be carried out in any suitable manner. For example, the laminate may be immersed in a treatment liquid containing a low boiling point alcohol, or the treatment liquid containing a low boiling point alcohol may be applied to the polarizing film surface of the laminate. Typically, the introduction of the low boiling point alcohol can be carried out by immersion. The immersion can be carried out in any suitable manner. For example, a low boiling point alcohol may be added to a cleaning bath in the cleaning treatment to form a bath of the treatment liquid, a bath of the treatment liquid may be used instead of the cleaning bath, or a bath of the treatment liquid may be provided separately from the cleaning bath. Typically, a low boiling point alcohol can be added to a cleaning bath (cleaning liquid) in the cleaning treatment. The concentration of the low boiling point alcohol in the treatment liquid (cleaning liquid) is preferably 5% by weight to 35% by weight.

C-6. Drying and shrinking treatment The drying and shrinking treatment is preferably carried out after the introduction of the low boiling point alcohol. By carrying out the drying and shrinking treatment after the introduction of the low boiling point alcohol, the drying efficiency can be increased, and as a result, the orientation of the PVA can be increased.

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 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 in the width direction of the laminate due to the drying shrinkage treatment is preferably 1% to 10%, more preferably 2% to 8%, and particularly preferably 4% to 6%.

Figure 2 is a schematic diagram showing an example of a drying shrinkage process. In the drying shrinkage process, the laminate 200 is dried while being transported by transport rolls R1-R6 heated to a predetermined temperature and guide rolls G1-G4. In the illustrated example, the transport rolls R1-R6 are arranged so as to alternately and continuously heat the surface of the PVA resin layer and the surface of the thermoplastic resin substrate, but, for example, the transport rolls R1-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. At such a temperature, the appearance of the resulting polarizing film can be maintained in a good condition. Furthermore, due to the synergistic effect with the effect of the low boiling point alcohol, the orientation of the PVA can be enhanced while maintaining the appearance. In addition, 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 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 roll may be installed in a heating furnace (e.g., an oven) or in a normal production line (at room temperature). It is preferably installed in a heating furnace equipped with a blowing means. By using drying with a heating roll and hot air drying in combination, it is possible to suppress abrupt temperature changes between the heating rolls, and it is possible to easily control the shrinkage in the width direction. The temperature of the hot air drying is preferably 30°C to 100°C. At such a temperature, the appearance of the obtained polarizing film can be maintained in a good condition. Furthermore, due to the synergistic effect with the effect of the low boiling point alcohol, it is possible to increase the orientation of the PVA while maintaining the appearance. The hot air drying time is preferably 1 second to 300 seconds. The wind speed of the hot air is preferably about 10 m/s to 30 m/s. The wind speed is the wind speed in the heating furnace and can be measured by a mini-vane type digital anemometer.

C-7. Others The laminate of thermoplastic resin substrate/polarizing film obtained as described above may be used as a polarizing plate as it is (the thermoplastic resin substrate may be used as a protective layer); a protective layer may be attached to the polarizing film surface of the laminate, and then the thermoplastic resin substrate may be peeled off to use the laminate as a polarizing plate having a protective layer/polarizing film structure; or another protective layer may be attached to the peeled surface of the thermoplastic resin substrate to use the laminate as a polarizing plate having a protective layer/polarizing film/protective layer structure.

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) Alcohol concentration of polarizing film The polarizing films obtained in the examples and comparative examples were cut into ¹⁰ cm2 pieces to be used as measurement samples. These measurement samples were sealed in 20 mL headspace vials, and then heated in a headspace sampler (HSS) at 175°C for the ethanol-containing sample and at 215°C for the isopropyl alcohol-containing sample for 30 minutes. 1 mL of the gas phase after heating was injected into a gas chromatograph (manufactured by Agilent Technologies, product name "6890N"), and the amount of alcohol contained was calculated from the peak area corresponding to each alcohol type using the following calibration curve.
Ethanol calibration curve y = 4.743E ^{+ 00} x + 3.105E ^-02
Isopropyl alcohol calibration curve y = 4.565E ^{+ 00} x + 8.922E ^-03
(3) Single transmittance and polarization degree For the polarizing plates (protective film/polarizing film) of the examples and comparative examples, the single transmittance Ts, parallel transmittance Tp, and crossed transmittance Tc measured using an ultraviolet-visible spectrophotometer (LPF-200 manufactured by Otsuka Electronics) were taken as Ts, Tp, and Tc of the polarizing film, respectively. These Ts, Tp, and Tc are Y values measured using a 2-degree visual field (C light source) according to JIS Z8701 and corrected for visibility. The refractive index of the protective film was 1.50, and the refractive index of the surface of the polarizing film opposite to the protective film was 1.53. The polarization degree was calculated from the obtained Tp and Tc using the following formula.
Degree of polarization (%)={(Tp−Tc)/(Tp+Tc)} ^1/2 ×100
Next, the polarizing plate was subjected to a durability test for 120 hours at a temperature of 85° C. and a relative humidity of 85%. The crossed transmittance polarization degree P ₁₂₀ after the durability test was determined in the same manner as above. The durability test was performed by placing a test sample in a humidifying oven, the test sample having the polarizing film side of the polarizing plate attached to a glass plate via an adhesive layer.
(4) Humidity Durability ΔP was calculated from the polarization degrees P ₀ and P ₁₂₀ before and after the durability test in (3) above using the following formula.
ΔP = P ₁₂₀ - P ₀
Since ΔP is a parameter that changes depending on the single transmittance, it is necessary to compare polarizing plates with substantially the same single transmittance before the test. Therefore, the durability under a high temperature and high humidity environment was evaluated using Comparative Example 1 as a standard as follows.
◎: ΔP is significantly larger than that of Comparative Example 1 (absolute value in the negative direction is significantly smaller)
○: ΔP is larger than that of Comparative Example 1 (the absolute value in the negative direction is smaller)
△: ΔP is equivalent to Comparative Example 1 ×: ΔP is smaller than Comparative Example 1 (absolute value in the negative direction is larger)

[Example 1]
As the thermoplastic resin substrate, a long amorphous isophthalic copolymerized polyethylene terephthalate film (thickness: 100 μm) having a Tg of about 75° C. was used. 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 to 2.4 times its original size in the machine direction (longitudinal direction) between rolls having different peripheral speeds in an oven at 130° C. (auxiliary air stretching treatment).
Next, the laminate was cut into a size of 15 cm x 10 cm in the auxiliary stretching axis direction, and the short sides of the cut laminate piece were fixed with a dedicated stretching jig and immersed in an insolubilizing bath (aqueous boric acid solution obtained by mixing 3 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 30°C for 30 seconds (insolubilization 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) 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 43.0%±0.2% (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 30° C. for 30 seconds (crosslinking treatment).
Thereafter, the laminate was immersed in an aqueous boric acid solution (boric acid concentration: 4.0% by weight, potassium iodide concentration: 5.0% by weight) at a liquid temperature of 70° C. and uniaxially stretched in the longitudinal direction (longitudinal direction) to a total stretch ratio of 5.5 times (underwater stretching treatment).
Thereafter, the laminate was immersed in a treatment bath (aqueous solution of 3 wt % potassium iodide and 5 wt % ethanol) at a liquid temperature of 20° C. for 3 seconds to wash the laminate and introduce ethanol into the PVA-based resin layer (polarizing film) (washing treatment and introduction of ethanol).
Thereafter, the substrate was dried in an oven maintained at 60° C. for 4 minutes (drying treatment).
In this way, a polarizing film having a thickness of 5.0 μm was formed on the resin substrate. A cycloolefin film (manufactured by ZEON, product name "G-Film") was attached to the surface of the polarizing film as a protective layer (protective film) with a UV-curable adhesive (thickness 1.0 μm), and then the resin substrate was peeled off to obtain a polarizing plate having a protective layer/polarizing film configuration. The ethanol concentration in the polarizing film of the obtained polarizing plate was 45 ppm.

The single transmittance and ΔP of the obtained polarizing plate (effectively, the polarizing film) are shown in Table 1. Furthermore, the evaluation results of (4) above are also shown in Table 1.

[Example 2]
A polarizing plate was produced in the same manner as in Example 1, except that the ethanol concentration in the treatment bath was 20% by weight. The obtained polarizing plate (or polarizing film) was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[Example 3]
A polarizing plate was produced in the same manner as in Example 1, except that the ethanol concentration in the treatment bath was 25% by weight. The obtained polarizing plate (or polarizing film) was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[Example 4]
A polarizing plate was produced in the same manner as in Example 1, except that the ethanol concentration in the treatment bath was 2% by weight. The obtained polarizing plate (or polarizing film) was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[Example 5]
A polarizing plate was prepared in the same manner as in Example 4. The obtained polarizing plate was subjected to a durability test at a temperature of 60° C. and a relative humidity of 95% for 120 hours to obtain ΔP. The durability test procedure was as described in (3) above. The obtained ΔP was evaluated for durability based on Comparative Example 3 (described later) as follows. The results are shown in Table 1.
◎: ΔP is significantly larger than that of Comparative Example 3 (absolute value in the negative direction is significantly smaller)
○: ΔP is larger than that of Comparative Example 3 (the absolute value in the negative direction is smaller)
△: ΔP is equivalent to Comparative Example 3. ×: ΔP is smaller than Comparative Example 3 (absolute value in the negative direction is larger).

[Example 6]
A polarizing plate was produced in the same manner as in Example 1. The obtained polarizing plate was subjected to the same evaluation as in Example 5. The results are shown in Table 1.

[Example 7]
A polarizing plate was produced in the same manner as in Example 2. The obtained polarizing plate was subjected to the same evaluation as in Example 5. The results are shown in Table 1.

[Example 8]
A polarizing plate was produced in the same manner as in Example 3. The obtained polarizing plate was subjected to the same evaluation as in Example 5. The results are shown in Table 1.

[Example 9]
A polarizing plate was produced in the same manner as in Example 3, except that the immersion time in the treatment bath was 10 seconds. The obtained polarizing plate was subjected to the same evaluation as in Example 5. The results are shown in Table 1.

[Example 10]
A polarizing plate was produced in the same manner as in Example 5, except that the concentration of the dye bath was adjusted so that the single transmittance of the polarizing film was 42.0%±0.2%, and the type of alcohol in the cleaning treatment bath was isopropyl alcohol, and its concentration was 5% by weight. The obtained polarizing plate was subjected to a durability test similar to that in Example 5, and evaluated according to the following criteria. The results are shown in Table 1.
◎: ΔP is significantly larger than that of Comparative Example 4 (absolute value in the negative direction is significantly smaller)
○: ΔP is larger than that of Comparative Example 4 (the absolute value in the negative direction is smaller)
△: ΔP is equivalent to Comparative Example 4. ×: ΔP is smaller than Comparative Example 4 (absolute value in the negative direction is larger)

[Example 11]
A polarizing plate was produced in the same manner as in Example 10, except that the isopropyl alcohol concentration in the treatment bath was 20% by weight. The obtained polarizing plate was subjected to the same evaluation as in Example 10. The results are shown in Table 1.

[Example 12]
A polarizing plate was produced in the same manner as in Example 10, except that the isopropyl alcohol concentration in the treatment bath was 25% by weight. The obtained polarizing plate was subjected to the same evaluation as in Example 10. The results are shown in Table 1.

[Comparative Example 1]
A polarizing plate was produced in the same manner as in Example 1, except that no low-boiling alcohol was added to the cleaning bath (cleaning solution). The obtained polarizing plate (or polarizing film) was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[Comparative Example 2]
A polarizing plate was produced in the same manner as in Example 1, except that a low-boiling alcohol was not added to the cleaning bath (cleaning solution) and that the temperature of the drying oven after the partial cleaning bath was set to 130° C. in order to improve drying efficiency. However, due to wrinkles and/or dents in the film caused by heat shrinkage, it was not possible to obtain a measurement sample that could be evaluated.

[Comparative Example 3]
A polarizing plate was produced in the same manner as in Comparative Example 1. The obtained polarizing plate was subjected to the same evaluation as in Example 5. The results are shown in Table 1.

[Comparative Example 4]
A polarizing plate was produced in the same manner as in Comparative Example 1, except that the concentration of the dye bath was adjusted so that the single transmittance of the polarizing film was 42.0%±0.2%. The obtained polarizing plate was subjected to the same evaluation as in Example 10. The results are shown in Table 1.

[Reference Example 1]
A long roll of a PVA-based resin film (manufactured by Kuraray, product name "PS4500") having a thickness of 45 μm was uniaxially stretched in the longitudinal direction by a roll stretching machine so that the total stretch ratio was 6.0 times, while simultaneously performing swelling, dyeing, crosslinking and washing treatments, and finally performing drying treatment to produce a polarizing film having a thickness of 17 μm. That is, a thick polarizing film without low boiling point alcohol was produced. A cycloolefin-based film (manufactured by ZEON, product name "ZT12") and an acrylic resin-based film as protective layers (protective films) were bonded to both sides of the polarizing film with a UV-curable adhesive (thickness 1.0 μm), and further an adhesive layer was provided on the surface of the cycloolefin-based film to obtain a polarizing plate having a configuration of protective layer/polarizing film/protective layer/adhesive layer. Next, the same evaluation as in Example 1 was performed except that the durability test conditions were a temperature of 65° C., a relative humidity of 90%, and a test time of 500 hours, and ΔP was obtained by the following formula. The results are shown in Table 2. The durability of Reference Example 2 described below was evaluated based on the ΔP of this Reference Example.
ΔP = P ₅₀₀ - P ₀

[Reference Example 2]
The long roll of the laminate of PVA-based resin/resin substrate used in Example 1 was uniaxially stretched in the long direction by a roll stretching machine so that the total stretch ratio was 2.4 times, while simultaneously undergoing swelling, dyeing, crosslinking and washing treatments, and finally drying treatment to produce a polarizing film with a thickness of 5 μm. That is, a thin polarizing film without low boiling point alcohol was produced. On the surface of the polarizing film, an adhesive layer was provided on the protective layer in the same manner as in Reference Example 1 to obtain a polarizing plate having a configuration of protective layer/polarizing film/protective layer/adhesive layer. Except for using this polarizing plate, the evaluation was performed in the same manner as in Reference Example 1, and was evaluated according to the following criteria. The results are shown in Table 2.
◎: ΔP is significantly larger than that of Reference Example 1 (absolute value in the negative direction is significantly smaller)
○: ΔP is larger than that of Reference Example 1 (the absolute value in the negative direction is smaller)
△: ΔP is equivalent to Reference Example 1 ×: ΔP is smaller than Reference Example 1 (absolute value in the negative direction is larger)

As is clear from Table 1, the polarizing plate (polarizing film) of the embodiment of the present invention has excellent durability in a high-temperature, high-humidity environment due to the inclusion of a specified amount of low-boiling-point alcohol. Furthermore, as is clear from Table 2, durability in a high-temperature, high-humidity environment is an issue specific to thin polarizing films.

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

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

Claims (4)

The film is made of a polyvinyl alcohol-based resin film containing iodine, has a thickness of 8 μm or less, and contains 5 ppm to 350 ppm of alcohol having a boiling point of less than 100° C .;
The alcohol having a boiling point of less than 100° C. is at least one selected from the group consisting of methanol, ethanol, n-propyl alcohol, and isopropyl alcohol .
Polarizing film. A polarizing plate comprising the polarizing film according to claim 1 and a protective layer disposed on at least one side of the polarizing film. A method for producing the polarizing film according to claim 1 , comprising the steps of:
forming a polyvinyl alcohol-based resin layer on one side of a long thermoplastic resin substrate to form a laminate;
stretching and dyeing the laminate to form the polyvinyl alcohol-based resin layer into a polarizing film;
introducing an alcohol having a boiling point of less than 100° C. into the polarizing film by immersing the polarizing film in a treatment liquid containing the alcohol having a boiling point of less than 100° C.; and heating the laminate into which the alcohol having a boiling point of less than 100° C. has been introduced.
Including,
The alcohol having a boiling point of less than 100° C. is at least one selected from the group consisting of methanol, ethanol, n-propyl alcohol, and isopropyl alcohol.
Production method. The method of claim 3 , wherein the stretching comprises underwater stretching.

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