JP2024114770A - Polarizing plate and image display device including said polarizing plate - Google Patents

Abstract

To provide a polarizing plate which is extremely thin and yet is less susceptible to cracking of a deformed portion thereof.SOLUTION: A polarizing plate of the present invention comprises a polarizer and a protective layer and has a shape other than a rectangle. The protective layer consists of a resin film. The polarizer consists of a PVA-based resin film containing a dichroic substance. The polarizer satisfies the following expression (1) in one embodiment: y<-0.011x+0.52 ...(1), satisfies the following expression (2) in another embodiment: z<-60x+2850 ...(2), and satisfies the following expression (3) in another embodiment: f<-0.018x+1.1 ...(3), where x% represents a single transmittance, y represents a birefringence of the PVA-based resin, znm represents an in-plane retardation of the PVA-based resin film, and f represents an orientation function of the PVA-based resin. In yet another embodiment, the polarizer has a puncture strength of 30 gf/μm or greater.SELECTED DRAWING: Figure 3

Description

The present invention relates to a polarizing plate and an image display device including the polarizing plate.

In recent years, image display devices such as liquid crystal display devices and electroluminescence (EL) display devices (e.g., organic EL display devices, inorganic EL display devices) have rapidly become widespread. Due to the image formation method of the image display device, a polarizing plate is arranged on at least one side of the image display device. In recent years, as the demand for thinner image display devices has increased, the demand for thinner polarizing plates has also increased. However, in recent years, there are cases where it is desired to process polarizing plates into shapes other than rectangular (irregular shape processing: for example, formation of notches and/or through holes). However, there is a problem in that cracks are likely to occur in the irregularly processed parts of thin polarizing plates.

JP 2001-343521 A

The present invention was made to solve the above-mentioned problems in the past, and its main objective is to provide a polarizing plate that is extremely thin yet suppresses the occurrence of cracks in the irregularly shaped processed parts.

A polarizing plate according to one embodiment of the present invention has a polarizer and a protective layer disposed on at least one side of the polarizer, and has an irregular shape other than a rectangle. The protective layer is made of a resin film. The polarizer is made of a polyvinyl alcohol-based resin film containing a dichroic material, and satisfies the following formula (1) when the single transmittance is x% and the birefringence of the polyvinyl alcohol-based resin is y:
y<-0.011x+0.525 (1).
A polarizing plate according to another embodiment of the present invention has a polarizer and a protective layer disposed on at least one side of the polarizer, and has an irregular shape other than a rectangle. The protective layer is made of a resin film. The polarizer is made of a polyvinyl alcohol-based resin film containing a dichroic material, and satisfies the following formula (2) when the single transmittance is x% and the in-plane retardation of the polyvinyl alcohol-based resin film is z nm:
z<-60x+2875 (2).
A polarizing plate according to yet another embodiment of the present invention has a polarizer and a protective layer disposed on at least one side of the polarizer, and has an irregular shape other than a rectangle. The protective layer is made of a resin film. The polarizer is made of a polyvinyl alcohol-based resin film containing a dichroic material, and satisfies the following formula (3) when the single transmittance is x% and the orientation function of the polyvinyl alcohol-based resin is f:
f<-0.018x+1.11 (3).
A polarizing plate according to yet another embodiment of the present invention includes a polarizer and a protective layer disposed on at least one side of the polarizer, the protective layer having an irregular shape other than rectangular. The protective layer is made of a resin film. The polarizer is made of a polyvinyl alcohol-based resin film containing a dichroic material, and the piercing strength of the polarizer is 30 gf/μm or more.
In one embodiment, the polarizer has a thickness of 10 μm or less.
In one embodiment, the polarizer has a single transmittance of 40.0% or more and a polarization degree of 99.0% or more.
In one embodiment, the irregular shape is selected from the group consisting of a through hole, a V-notch, a U-notch, a recess that resembles a boat shape when viewed in a plan view, a rectangular recess when viewed in a plan view, an R-shaped recess that resembles a bathtub shape when viewed in a plan view, and combinations thereof.
In one embodiment, the radius of curvature of the U-notch is 5 mm or less.
In one embodiment, the polarizing plate further includes a reflective polarizer on the opposite side of the protective layer to the polarizer.
According to another aspect of the present invention, there is provided an image display device, the image display device including the above polarizing plate.

According to an embodiment of the present invention, in a polarizing plate having an irregular shape (irregularly processed portion), by controlling the orientation state of the polyvinyl alcohol (PVA) resin of the polarizer, it is possible to realize a polarizing plate that is extremely thin yet suppresses the occurrence of cracks in the irregularly processed portion. Furthermore, such a polarizer (and thus the polarizing plate) can exhibit optical properties that are acceptable for practical use.

1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. 3 is a schematic plan view illustrating an example of an irregular shape or irregularly processed portion in a polarizing plate according to an embodiment of the present invention. FIG. 10A to 10C are schematic plan views illustrating modified examples of irregular shapes or irregularly processed portions in a polarizing plate according to an embodiment of the present invention. 11A to 11C are schematic plan views illustrating further modified examples of the irregular shape or irregularly processed portion in the polarizing plate according to the embodiment of the present invention. 11A to 11C are schematic plan views illustrating further modified examples of the irregular shape or irregularly processed portion in the polarizing plate according to the embodiment of the present invention. FIG. 2 is a schematic diagram showing an example of a drying shrinkage treatment using a heated roll in a method for producing a polarizer that can be used in a polarizing plate according to an embodiment of the present invention. FIG. 2 is a schematic perspective view of an example of a reflective polarizer that can be used in a polarizing plate according to an embodiment of the present invention. 1 is a graph showing the relationship between the single transmittance of the polarizers produced in the examples and comparative examples and the birefringence of the PVA-based resin. 1 is a graph showing the relationship between the single transmittance of the polarizers produced in the Examples and Comparative Examples and the in-plane retardation of the PVA-based resin films. 1 is a graph showing the relationship between the single transmittance of the polarizers produced in the examples and comparative examples and the orientation function of the PVA-based resin.

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

A. Overall Configuration of 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 in the illustrated example has a polarizer 10 and a protective layer 20 arranged on one side of the polarizer 10. Depending on the purpose, another protective layer (not shown) may be provided on the polarizer 10 opposite the protective layer 20. The protective layer 20 is made of a resin film, and is typically attached to the polarizer via an adhesive layer (not shown). In one embodiment, the polarizing plate 100 may further have a reflective polarizer (not shown) on the protective layer 20 opposite the polarizer 10. The polarizing plate may be used as a viewer-side polarizing plate of an image display device, or may be used as a rear-side polarizing plate. When the polarizing plate has a reflective polarizer, the polarizing plate is typically used as a rear-side polarizing plate. In this case, the reflective polarizer may be arranged on the outside (opposite the image display cell).

The polarizing plate according to the embodiment of the present invention has an irregular shape other than a rectangle. In this specification, "having an irregular shape other than a rectangle" means that the planar shape of the polarizing plate is a shape other than a rectangle. The irregular shape is typically an irregularly processed part that has been processed into an irregular shape. Therefore, a "polarizing plate having an irregular shape other than a rectangle" (hereinafter, sometimes referred to as an "irregularly shaped polarizing plate") includes not only the case where the entire irregularly shaped polarizing plate (i.e., the outer edge that defines the planar shape of the polarizing plate) is other than rectangular, but also the case where an irregularly shaped part is formed in a part spaced inward from the outer edge of a rectangular polarizing plate. In a polarizing plate, such an irregularly shaped part is prone to cracking, but according to an embodiment of the present invention, such cracking can be significantly suppressed. More specifically, it is as follows. In a normal (i.e., non-irregular) polarizing plate (effectively a polarizer), cracks often occur along the absorption axis (stretching direction) of the polarizer. On the other hand, in the irregularly shaped part, an L-shaped crack (a crack oblique to the absorption axis) may occur. According to an embodiment of the present invention, as described below, by making the orientation of the molecular chains of the PVA-based resin in the polarizer in the absorption axis direction more gentle than in conventional polarizers, it is possible to significantly suppress not only normal cracks but also these L-shaped cracks.

As shown in Figs. 2 and 3, examples of irregular shapes (irregularly processed parts) include those with corners chamfered in an R shape, through holes, and cut parts that become recesses when viewed in a plan view. Representative examples of recesses include a shape that resembles a boat shape, a rectangle, an R shape that resembles a bathtub shape, a V-shaped notch, and a U-shaped notch. Another example of irregular shapes (irregularly processed parts) is a shape that corresponds to the meter panel of an automobile, as shown in Figs. 4 and 5. This shape includes a part whose outer edge is formed in an arc shape along the rotation direction of the meter needle and whose outer edge forms a V-shape (including an R-shape) that is convex inward in the planar direction. Needless to say, the shape of the irregular shape (irregularly processed parts) is not limited to the illustrated example. For example, the shape of the through hole may be any appropriate shape (for example, an ellipse, a triangle, a square, a pentagon, a hexagon, or an octagon) depending on the purpose other than the approximately circular shape shown in the illustrated example. In addition, the through hole may be provided in any appropriate position depending on the purpose. The through hole may be provided at the approximate center of the longitudinal end of the rectangular polarizing plate as shown in FIG. 3, at a predetermined position of the longitudinal end, or at a corner of the polarizing plate; not shown, it may be provided at the short-side end of the rectangular polarizing plate; as shown in FIG. 4 or FIG. 5, it may be provided at the center of the irregular polarizing plate. As shown in FIG. 3, a plurality of through holes may be provided. Furthermore, the shapes of the illustrated examples may be appropriately combined according to the purpose. For example, a through hole may be formed at any position of the irregular polarizing plate in FIG. 2; a V-shaped notch and/or a U-shaped notch may be formed at any appropriate position of the outer edge of the irregular polarizing plate in FIG. 4 or FIG. 5. Such an irregular polarizing plate can be suitably used in image display devices such as automobile meter panels, smartphones, tablet PCs, or smart watches. In addition, for example, when the irregular shape includes an R shape, the curvature radius is, for example, 0.2 mm or more, for example, 1 mm or more, or for example, 2 mm or more. On the other hand, the curvature radius is, for example, 10 mm or less, or for example, 5 mm or less. For example, if the irregular shape is a U-shaped notch, its radius of curvature (the radius of curvature of the U-shaped portion) is, for example, 5 mm or less, for example, 1 mm to 4 mm, or for example, 2 mm to 3 mm.

The irregular shape (irregularly shaped processed portion) can be formed by any suitable method. Specific examples of forming methods include cutting with an end mill, punching with a punching blade such as a Thomson blade, and cutting by irradiating with laser light. These methods may be combined.

The components of a polarizing plate, the polarizer, protective layer, and reflective polarizer, are explained below.

B. Polarizer The polarizer is composed of a PVA-based resin film containing a dichroic material. In one embodiment, the polarizer satisfies the following formula (1) when the single transmittance is x% and the birefringence of the polyvinyl alcohol-based resin constituting the polarizer is y. In one embodiment, the polarizer satisfies the following formula (2) when the single transmittance is x% and the in-plane retardation of the polyvinyl alcohol-based resin film constituting the polarizer is z nm. In one embodiment, the polarizer satisfies the following formula (3) when the single transmittance is x% and the orientation function of the polyvinyl alcohol-based resin constituting the polarizer is f. In one embodiment, the piercing strength of the polarizer is 30 gf/μm or more.
y<-0.011x+0.525 (1)
z<-60x+2875 (2)
f<-0.018x+1.11 (3)

The birefringence of the PVA-based resin in the polarizer (hereinafter referred to as the birefringence of PVA or Δn of PVA), the in-plane retardation of the PVA-based resin film (hereinafter referred to as the "in-plane retardation of PVA"), the orientation function of the PVA-based resin (hereinafter referred to as the "orientation function of PVA"), and the piercing strength of the polarizer are all values related to the degree of orientation of the molecular chains of the PVA-based resin constituting the polarizer. Specifically, the birefringence, in-plane retardation, and orientation function of PVA can become larger with an increase in the degree of orientation, and the piercing strength can decrease with an increase in the degree of orientation. In the polarizer according to the embodiment of the present invention (i.e., a polarizer satisfying the above formulas (1) to (3) or the piercing strength), the orientation of the molecular chains of the PVA-based resin in the absorption axis direction is gentler than in conventional polarizers, and therefore, heat shrinkage in the absorption axis direction is suppressed. As a result, such a polarizer (and thus a polarizing plate) can suppress the occurrence of cracks in the irregularly shaped processed portion while being extremely thin. In addition, such a polarizer (and thus a polarizing plate) is also excellent in flexibility and bending durability, and can therefore be preferably applied to curved image display devices, more preferably bendable image display devices, and even more preferably foldable image display devices. Conventionally, it has been difficult to obtain acceptable optical properties (typically, single transmittance and degree of polarization) with polarizers having a low degree of orientation, but the polarizer used in the embodiment of the present invention can achieve both a lower degree of orientation of the PVA-based resin than conventional ones and acceptable optical properties.

The polarizer preferably satisfies the following formula (1a) and/or formula (2a), and more preferably satisfies the following formula (1b) and/or formula (2b).
-0.004x+0.18<y<-0.011x+0.525 (1a)
-0.003x+0.145<y<-0.011x+0.520 (1b)
-40x+1800<z<-60x+2875 (2a)
-30x+1450<z<-60x+2850 (2b)

In this specification, the in-plane retardation of the PVA is the in-plane retardation value of the PVA-based resin film at 23°C and a wavelength of 1000 nm. By using the near-infrared region as the measurement wavelength, it is possible to eliminate the influence of iodine absorption in the polarizer and to measure the retardation. In addition, the birefringence (in-plane birefringence) of the PVA is the value obtained by dividing the in-plane retardation of the PVA by the thickness of the polarizer.

The in-plane retardation of PVA is evaluated as follows: First, retardation values are measured at multiple wavelengths of 850 nm or more, and the measured retardation values: R(λ) are plotted against wavelengths: λ, which are then fitted to the Sellmeier equation below by the least squares method, where A and B are fitting parameters and are coefficients determined by the least squares method.
R(λ)=A+B/(λ ² -600 ² )
In this case, the retardation value R(λ) can be separated into an in-plane retardation value (Rpva) of PVA that has no wavelength dependency and an in-plane retardation value (Ri) of iodine that has strong wavelength dependency, as shown below.
Rpva = A
Ri = B/(λ ² -600 ² )
Based on this separation formula, the in-plane retardation (i.e., Rpva) of PVA at a wavelength λ=1000 nm can be calculated. The method for evaluating the in-plane retardation of PVA is also described in Japanese Patent No. 5932760, which can be referred to as necessary.
Moreover, the birefringence (Δn) of the PVA can be calculated by dividing this retardation by the thickness.

Commercially available devices for measuring the in-plane retardation of PVA at a wavelength of 1000 nm include the KOBRA-WR/IR series and KOBRA-31X/IR series manufactured by Oji Measurement Co., Ltd.

The orientation function (f) of the polarizer preferably satisfies the following formula (3a), and more preferably satisfies the following formula (3b): If the orientation function is too small, an acceptable single-unit transmittance and/or degree of polarization may not be obtained.
-0.01x+0.50<f<-0.018x+1.11 (3a)
-0.01x+0.57<f<-0.018x+1.1 (3b)

The orientation function (f) is obtained, for example, by using a Fourier transform infrared spectrophotometer (FT-IR), measuring polarized light as the measurement light, and measuring attenuated total reflection spectroscopy (ATR). Specifically, germanium is used as the crystallite to which the polarizer is attached, the angle of incidence of the measurement light is 45° incidence, and the polarized infrared light (measurement light) to be incident is polarized light (s-polarized light) that vibrates parallel to the surface to which the germanium crystal sample is attached, and the measurement is performed in a state in which the stretching direction of the polarizer is arranged parallel and perpendicular to the polarization direction of the measurement light, and the intensity of 2941 cm ⁻¹ of the obtained absorbance spectrum is used to calculate according to the following formula. Here, the intensity I is a value of 2941 cm ⁻¹ /3330 cm ⁻¹ , with 3330 cm ⁻¹ as the reference peak. Note that when f=1, it is fully oriented, and when f=0, it is random. The peak at 2941 cm ⁻¹ is believed to be absorption caused by vibration of the main chain (—CH ₂ —) of PVA in the polarizer.
f=(3<cos ² θ>-1)/2
=(1-D)/[c(2D+1)]
=-2×(1-D)/(2D+1)
however,
c=(3 cos ² β-1)/2, and for the 2941 cm ⁻¹ vibration, β=90°.
θ: Angle of molecular chain with respect to stretching direction β: Angle of transition dipole moment with respect to molecular chain axis D = (I _⊥ ) / (I _// ) (In this case, D becomes larger as the PVA molecules are oriented)
I _⊥ : Absorption intensity when the polarization direction of the measurement light and the stretching direction of the polarizer are perpendicular I _// : Absorption intensity when the polarization direction of the measurement light and the stretching direction of the polarizer are parallel

The thickness of the polarizer is preferably 10 μm or less, and more preferably 8 μm or less. The lower limit of the polarizer thickness may be, for example, 1 μm. In one embodiment, the thickness of the polarizer may be 2 μm to 10 μm, and in another embodiment, 2 μm to 8 μm. By making the polarizer thickness so thin, it is possible to make the thermal shrinkage very small. It is presumed that such a configuration may also contribute to suppressing the occurrence of cracks in the irregularly processed portion of the polarizing plate.

The polarizer preferably exhibits absorption dichroism at any wavelength of 380 nm to 780 nm. The single transmittance of the polarizer is preferably 40.0% or more, more preferably 41.0% or more. The upper limit of the single transmittance may be, for example, 49.0%. In one embodiment, the single transmittance of the polarizer is 40.0% to 45.0%. The degree of polarization of the polarizer is preferably 99.0% or more, more preferably 99.4% or more. The upper limit of the degree of polarization may be, for example, 99.999%. In one embodiment, the degree of polarization of the polarizer is 99.0% to 99.9%. A polarizer according to an embodiment of the present invention is characterized in that the degree of orientation of the PVA-based resin constituting the polarizer is lower than that of a conventional polarizer, and the polarizer has the above-mentioned in-plane retardation, birefringence, and/or orientation function, yet can achieve such a practically acceptable single transmittance and degree of polarization. This is presumably due to the manufacturing method described below. 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 piercing strength of the polarizer is, for example, 30 gf/μm or more, preferably 35 gf/μm or more, more preferably 40 gf/μm or more, even more preferably 45 gf/μm or more, and particularly preferably 50 gf/μm or more. The upper limit of the piercing strength can be, for example, 80 gf/μm. By setting the piercing strength of the polarizer in such a range, it is possible to significantly suppress the occurrence of cracks in the irregularly processed part and the tearing of the polarizer along the absorption axis direction. As a result, a polarizer (and, as a result, a polarizing plate) with very excellent flexibility can be obtained. The piercing strength indicates the crack resistance of the polarizer when the polarizer is pierced with a predetermined strength. The piercing strength can be expressed, for example, as the strength (breaking strength) at which the polarizer breaks when a predetermined needle is attached to a compression tester and the needle is pierced into the polarizer at a predetermined speed. As is clear from the units, the piercing strength means the piercing strength per unit thickness (1 μm) of the polarizer.

As described above, the polarizer is composed of a PVA-based resin film containing a dichroic material. Preferably, the PVA-based resin constituting the PVA-based resin film (effectively the polarizer) contains an acetoacetyl-modified PVA-based resin. With this configuration, a polarizer having the desired puncture strength can be obtained. The amount of the acetoacetyl-modified PVA-based resin is preferably 5% to 20% by weight, and more preferably 8% to 12% by weight, when the entire PVA-based resin is taken as 100% by weight. If the amount is within this range, the puncture strength can be set to a more suitable range.

A polarizer can be typically produced using a laminate of two or more layers. A specific example of a polarizer obtained using a laminate is a polarizer obtained using a laminate of a resin substrate and a PVA-based resin layer coated on the resin substrate. A polarizer 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 polarizer. In this embodiment, a polyvinyl alcohol-based resin layer containing a halide and a polyvinyl alcohol-based resin is preferably formed on one side of the resin substrate. The stretching typically includes immersing the laminate in an aqueous boric acid solution to stretch the laminate. Furthermore, the stretching preferably further includes stretching the laminate in the air at a high temperature (e.g., 95°C or higher) before stretching in the aqueous boric acid solution. In an embodiment of the present invention, the total stretching ratio is preferably 3.0 to 4.5 times, which is significantly smaller than normal. Even at such a total stretching ratio, a polarizer having acceptable optical properties can be obtained by adding a halide and combining it with a drying shrinkage treatment. Furthermore, in an embodiment of the present invention, the stretching ratio of the auxiliary air stretching is preferably larger than the stretching ratio of the boric acid water stretching. With this configuration, a polarizer having acceptable optical properties can be obtained even if the total stretching ratio is small. In addition, the laminate is preferably subjected to 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 one embodiment, the method for producing a polarizer includes subjecting the laminate to an auxiliary air stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment in this order. By introducing the auxiliary stretching, even when a PVA-based resin is applied onto a thermoplastic resin, it is possible to increase the crystallinity of the PVA-based resin and 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 or dissolution of the PVA-based resin when immersed in water in the subsequent dyeing process or stretching process can be prevented, and high optical properties can be achieved. Furthermore, when the PVA-based resin layer is immersed in a liquid, the disorder of the orientation of the polyvinyl alcohol molecules and the decrease in the 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 polarizer obtained through a treatment process in which the laminate is immersed in a liquid, such as a dyeing process and an underwater stretching process. Furthermore, the optical properties can be improved by shrinking the laminate in the width direction by a drying shrinkage process. The obtained resin substrate/polarizer laminate may be used as it is (i.e., the resin substrate may be used as a protective layer for the polarizer), or the resin substrate may be peeled off from the resin substrate/polarizer laminate, and any suitable protective layer according to the purpose may be laminated on the peeled surface. Details of the method for producing a polarizer will be described in Section C.

C. Manufacturing Method of Polarizer The manufacturing method of the polarizer preferably includes forming a polyvinyl alcohol-based resin layer (PVA-based resin layer) containing a halide and a polyvinyl alcohol-based resin (PVA-based resin) on one side of a long thermoplastic resin substrate to form a laminate, and subjecting the laminate to an auxiliary air stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying shrinkage treatment in this order, 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. The content of the halide in the PVA-based resin layer is preferably 5 parts by weight to 20 parts by weight with respect to 100 parts by weight of the PVA-based resin. The drying shrinkage treatment is preferably performed using a heating roll, and the temperature of the heating roll is preferably 60° C. to 120° C. The shrinkage rate in the width direction of the laminate due to the drying shrinkage treatment is preferably 2% or more. Furthermore, the stretching ratio of the auxiliary air stretching is preferably larger than the stretching ratio of the underwater stretching. According to such a manufacturing method, the polarizer described in the above section B can be obtained. In particular, a laminate including a PVA-based resin layer containing a halide is prepared, the laminate is stretched in multiple stages including auxiliary air stretching and underwater stretching, and the stretched laminate is heated with a heating roll to shrink the laminate by 2% or more in the width direction, whereby a polarizer having excellent optical properties (typically, single transmittance and degree of polarization) 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 appropriate method can be used to apply the coating liquid. Examples include roll coating, spin coating, wire bar coating, dip coating, die coating, curtain coating, spray coating, and knife coating (comma coating, etc.). The application and drying temperature of the coating liquid is preferably 50°C or higher.

The thickness of the PVA-based resin layer is preferably 2 μm to 30 μm, and more preferably 2 μm to 20 μm. By making the thickness of the PVA-based resin layer before stretching very thin in this way, and by reducing the total stretching ratio as described below, it is possible to obtain a polarizer that has an acceptable single transmittance and polarization degree, even though the degree of orientation of the PVA-based resin is lower than in the past.

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 polarizer with excellent durability can be obtained. If the saponification degree is too high, there is a risk of gelation. As described above, the PVA-based resin preferably includes an acetoacetyl-modified PVA-based resin.

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

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

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

Generally, the orientation of the polyvinyl alcohol molecules in the PVA-based resin layer increases when the PVA-based resin layer is stretched, but when the stretched PVA-based resin layer is immersed in a liquid containing water, the orientation of the polyvinyl alcohol molecules may become disordered, and the orientation may decrease. In particular, when a laminate of a thermoplastic resin substrate and a PVA-based resin layer is stretched in boric acid water, the tendency for the degree of orientation to decrease is remarkable when the laminate is stretched in boric acid water at a relatively high temperature to stabilize the stretching of the thermoplastic resin substrate. For example, while a PVA film alone is generally stretched in boric acid water at 60°C, a laminate of A-PET (thermoplastic resin substrate) and a PVA-based resin layer is stretched at a high temperature of around 70°C, in this case, the orientation of the PVA at the beginning of the stretching may decrease before it increases due to the stretching in water. In response to this, a laminate of a PVA-based resin layer containing a halide and a thermoplastic resin substrate is prepared, and the laminate is stretched at high temperature (auxiliary stretching) in air before being stretched in boric acid water, so that crystallization of the PVA-based resin in the PVA-based resin layer of the laminate after auxiliary stretching can be promoted. As a result, when the PVA-based resin layer is immersed in a liquid, the orientation of the polyvinyl alcohol molecules can be prevented from being disturbed and the orientation can be prevented from being reduced, compared to when the PVA-based resin layer does not contain a halide. This can improve the optical properties of the polarizer obtained by immersing the laminate in a liquid through a process 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. Furthermore, when applying a PVA-based resin on a thermoplastic resin substrate, it is necessary to lower the application temperature compared to when applying a PVA-based resin on a normal metal drum in order to suppress the influence of the glass transition temperature of the thermoplastic resin substrate, and as a result, the crystallization of the PVA-based resin becomes relatively low, and a problem that sufficient optical properties cannot be obtained may occur. On the other hand, by introducing auxiliary stretching, it is possible to increase the crystallinity of the PVA-based resin even when applying a PVA-based resin on 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 can be prevented when immersed in water in the subsequent dyeing process or stretching process, and it is possible to achieve high optical properties.

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 (1/stretching ratio) ^1/2 .

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

The stretching ratio in the auxiliary air stretching is preferably 1.0 to 4.0 times, more preferably 1.5 to 3.5 times, and even more preferably 2.0 to 3.0 times. If the stretching ratio of the auxiliary air stretching is in such a range, the total stretching ratio can be set in the desired range when combined with underwater stretching, and the desired birefringence, in-plane retardation, and/or orientation function can be realized. As a result, a polarizer (and, as a result, a polarizing plate) in which crack generation in the irregularly shaped processed portion is suppressed can be obtained. Furthermore, as described above, the stretching ratio of the auxiliary air stretching is preferably larger than the stretching ratio of the underwater stretching. With this configuration, a polarizer having acceptable optical properties can be obtained even if the total stretching ratio is small. More specifically, the ratio of the stretching ratio of the auxiliary air stretching to the stretching ratio of the underwater stretching (underwater stretching/auxiliary air stretching) is preferably 0.4 to 0.9, and more preferably 0.5 to 0.8.

The stretching temperature for the in-air auxiliary 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 problems caused by 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 in, for example, JP 2012-73580 A.

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 while suppressing crystallization. As a result, a polarizer having excellent optical properties can be produced.

The laminate can be stretched by any suitable method. 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). Free-end stretching is preferably selected. The laminate may be stretched in one stage or in multiple stages. When stretched in multiple stages, the total stretch ratio 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 (stretching in boric acid water). By using an aqueous solution of boric acid as the stretching bath, it is possible to impart to the PVA-based resin layer the rigidity required to withstand the tension applied during stretching and the water resistance required to prevent dissolution in water. Specifically, boric acid can generate tetrahydroxyborate anions in the aqueous solution and crosslink with the PVA-based resin through hydrogen bonds. As a result, the PVA-based resin layer is imparted with rigidity and water resistance, allowing it to be stretched well, and a polarizer with excellent optical properties can be produced.

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

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

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

The stretching ratio in underwater stretching is preferably 1.0 to 2.2 times, more preferably 1.1 to 2.0 times, even more preferably 1.1 to 1.8 times, and even more preferably 1.2 to 1.6 times. If the stretching ratio in underwater stretching is in such a range, the total stretching ratio can be set in the desired range, and the desired birefringence, in-plane retardation, and/or orientation function can be realized. As a result, a polarizer (and, as a result, a polarizing plate) in which crack generation in the irregular processing part is suppressed can be obtained. As described above, the total stretching ratio (the sum of the stretching ratios when the auxiliary air stretching and the underwater stretching are combined) is preferably 3.0 to 4.5 times, more preferably 3.0 to 4.3 times, and even more preferably 3.0 to 4.0 times, relative to the original length of the laminate. By appropriately combining the addition of a halide to the coating solution, adjustment of the stretching ratios of the air-assisted stretching and underwater stretching, and drying and shrinking treatment, it is possible to obtain a polarizer with acceptable optical properties even at such total stretching ratios.

C-5. Drying shrinkage treatment 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 polarizer 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, it is possible to dry the laminate while maintaining it in a flat state, so that it is possible to suppress not only curling but also wrinkles. 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 2% to 6%.

Figure 6 is a schematic diagram showing an example of a drying shrinkage process. In the drying shrinkage process, the laminate 200 is dried while being transported by transport rolls R1 to R6 heated to a predetermined temperature and guide rolls G1 to G4. In the illustrated example, the transport rolls R1 to R6 are arranged so as to alternately and continuously heat the surface of the PVA resin layer and the surface of the thermoplastic resin substrate, but, for example, the transport rolls R1 to R6 may be arranged so as to continuously heat only one surface of the laminate 200 (for example, the thermoplastic resin substrate surface).

Drying conditions can be controlled by adjusting the heating temperature of the transport rolls (temperature of the heating rolls), the number of heating rolls, the contact time with the heating rolls, etc. The temperature of the heating rolls is preferably 60°C to 120°C, more preferably 65°C to 100°C, and particularly preferably 70°C to 80°C. The crystallization degree of the thermoplastic resin can be increased well, curling can be suppressed well, and an optical laminate with extremely excellent durability can be produced. The temperature of the heating rolls can be measured by a contact thermometer. In the illustrated example, six transport rolls are provided, but there is no particular restriction as long as there are multiple transport rolls. The number of transport rolls is usually 2 to 40, preferably 4 to 30. The contact time (total contact time) between the laminate and the heating rolls is preferably 1 to 300 seconds, more preferably 1 to 20 seconds, and even more preferably 1 to 10 seconds.

The heating rolls may be installed in a heating furnace (e.g., an oven) or in a normal production line (at room temperature). Preferably, they are installed in a heating furnace equipped with a blowing means. By using both drying with the heating rolls and hot air drying, it is possible to suppress abrupt temperature changes between the heating rolls, and it is possible to easily control shrinkage in the width direction. The hot air drying temperature is preferably 30°C to 100°C. The hot air drying time is preferably 1 second to 300 seconds. The hot air speed is preferably about 10 m/s to 30 m/s. The air speed is the air speed in the heating furnace, and can be measured by a mini-vane type digital anemometer.

C-6. Other Treatments Preferably, after the underwater stretching treatment and before the drying shrinkage treatment, a washing treatment is performed. The washing treatment is typically performed by immersing the PVA-based resin layer in an aqueous potassium iodide solution.

D. Protective layer The protective layer is made of a resin film. The resin film (protective layer) can be formed of any suitable material depending on the purpose. Specific examples of materials for forming the protective layer include cellulose-based resins such as triacetyl cellulose (TAC), polyester-based, polyvinyl alcohol-based, polycarbonate-based, polyamide-based, polyimide-based, polyethersulfone-based, polysulfone-based, polystyrene-based, polynorbornene-based, polyolefin-based, (meth)acrylic-based, acetate-based, and other transparent resins; thermosetting resins or ultraviolet-curing resins such as (meth)acrylic-based, urethane-based, (meth)acrylic urethane-based, epoxy-based, and silicone-based; and glassy polymers such as siloxane-based polymers. Preferably, the protective layer is made of a TAC or (meth)acrylic-based resin film.

The thickness of the protective layer is preferably 5 μm to 80 μm, more preferably 10 μm to 40 μm, and even more preferably 10 μm to 30 μm.

E. Reflective Polarizer As described above, the reflective polarizer can be provided on the opposite side of the protective layer 20 to the polarizer 10. The reflective polarizer has a function of transmitting polarized light in a specific polarization state (polarization direction) and reflecting light in other polarization states. The reflective polarizer may be of a linear polarization separation type or a circular polarization separation type. Hereinafter, a linear polarization separation type reflective polarizer will be described as an example. An example of a circular polarization separation type reflective polarizer is a laminate of a film with a cholesteric liquid crystal fixed thereon and a λ/4 plate.

Figure 7 is a schematic perspective view of an example of a reflective polarizer. The reflective polarizer is a multilayer laminate in which a layer A having birefringence and a layer B having substantially no birefringence are alternately stacked. For example, the total number of layers in such a multilayer laminate can be 50 to 1000. In the illustrated example, the refractive index nx in the x-axis direction of the A layer is larger than the refractive index ny in the y-axis direction, and the refractive index nx in the x-axis direction of the B layer and the refractive index ny in the y-axis direction are substantially the same. Therefore, the refractive index difference between the A layer and the B layer is large in the x-axis direction and substantially zero in the y-axis direction. As a result, the x-axis direction is the reflection axis, and the y-axis direction is the transmission axis. The refractive index difference between the A layer and the B layer in the x-axis direction is preferably 0.2 to 0.3. The x-axis direction corresponds to the stretching direction of the reflective polarizer in the manufacturing method of the reflective polarizer.

The A layer is preferably made of a material that exhibits birefringence when stretched. Typical examples of such materials include naphthalene dicarboxylic acid polyesters (e.g., polyethylene naphthalate), polycarbonates, and (meth)acrylic resins (e.g., polymethyl methacrylate). Polyethylene naphthalate is preferred. The B layer is preferably made of a material that does not substantially exhibit birefringence even when stretched. Typical examples of such materials include copolyesters of naphthalene dicarboxylic acid and terephthalic acid.

A reflective polarizer transmits light having a first polarization direction (e.g., p-waves) at the interface between the A layer and the B layer, and reflects light having a second polarization direction (e.g., s-waves) that is perpendicular to the first polarization direction. At the interface between the A layer and the B layer, part of the reflected light is transmitted as light having the first polarization direction, and part is reflected as light having the second polarization direction. This reflection and transmission is repeated many times inside the reflective polarizer, thereby increasing the light utilization efficiency.

In one embodiment, the reflective polarizer may include a reflective layer R as the outermost layer on the side opposite the image display cell, as shown in FIG. 7. By providing the reflective layer R, it is possible to further utilize light that is ultimately unused and returns to the outermost part of the reflective polarizer, thereby further increasing the light utilization efficiency. The reflective layer R typically exhibits a reflective function by a multilayer structure of polyester resin layers.

The total thickness of the reflective polarizer can be appropriately set depending on the purpose, the total number of layers contained in the reflective polarizer, etc. The total thickness of the reflective polarizer is preferably 10 μm to 150 μm.

As the reflective polarizer, for example, those described in JP-A-9-507308 and JP-A-2013-235259 can be used. As the reflective polarizer, a commercially available product may be used as is, or a commercially available product may be used after secondary processing (e.g., stretching). Examples of commercially available products include DBEF and APF, both manufactured by 3M.

F. Image display device The polarizing plate can be applied to an image display device. Therefore, the embodiment of the present invention includes an image display device using such a polarizing plate. Representative examples of image display devices include liquid crystal display devices and electroluminescence (EL) display devices (e.g., organic EL display devices and inorganic EL display devices). The image display device preferably has an irregular shape other than a rectangle. In such image display devices, the effects of the embodiment of the present invention are remarkable. Specific examples of image display devices having an irregular shape include an automobile meter panel, a smartphone, a tablet PC, and a smart watch.

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. Unless otherwise specified, "parts" and "%" in the examples and comparative examples are by weight.

(1) Thickness: Measured using an interference film thickness meter (manufactured by Otsuka Electronics Co., Ltd., product name "MCPD-3000") The calculation wavelength range used for thickness calculation was 400 nm to 500 nm, and the refractive index was 1.53.
(2) In-plane retardation (Re) of PVA
For the polarizer (single polarizer) obtained by peeling and removing the resin substrate from the laminate of polarizer/thermoplastic resin substrate obtained in the examples and comparative examples, the in-plane retardation (Rpva) of PVA at a wavelength of 1000 nm was evaluated using a retardation measurement device (manufactured by Oji Scientific Instruments, product name "KOBRA-31X100/IR") (this is a numerical value obtained by subtracting the in-plane retardation (Ri) of iodine from the total in-plane retardation at a wavelength of 1000 nm, according to the principle explained above). The absorption edge wavelength was set to 600 nm.
(3) Birefringence of PVA (Δn)
The birefringence (Δn) of the PVA was calculated by dividing the in-plane retardation of the PVA measured in (2) above by the thickness of the polarizer.
(4) Single transmittance and polarization degree For a polarizer (single polarizer) obtained by peeling and removing the resin substrate from the polarizer/thermoplastic resin substrate laminate obtained in the Examples and Comparative Examples, the single transmittance Ts, parallel transmittance Tp, and crossed transmittance Tc were measured using an ultraviolet-visible spectrophotometer ("V-7100" manufactured by JASCO Corporation). 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 polarization degree P was calculated according to the following formula.
Polarization degree P (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 ×100
It should be noted that equivalent measurements can also be made using a spectrophotometer such as the Otsuka Electronics LPF-200, and it has been confirmed that equivalent measurement results can be obtained regardless of which spectrophotometer is used.
(5) Puncture strength (breaking strength per unit thickness)
The polarizer was peeled off from the laminate of polarizer/thermoplastic resin substrate obtained in the examples and comparative examples, and placed on a compression tester (manufactured by Kato Tech Co., Ltd., product name "NDG5" needle penetration force measurement specifications) equipped with a needle, and pierced at a piercing speed of 0.33 cm/sec under a room temperature (23°C ± 3°C) environment. The strength at which the polarizer broke was taken as the breaking strength. The evaluation value was the breaking strength of 10 sample pieces, and the average value was used. The needle used had a tip diameter of 1 mmφ and 0.5R. The polarizer to be measured was fixed by clamping it from both sides with a jig having a circular opening with a diameter of about 11 mm, and the needle was pierced into the center of the opening to perform the test.
(6) Orientation function of PVA For the polarizer (single polarizer) obtained by peeling and removing the resin substrate from the laminate of polarizer/thermoplastic resin substrate obtained in the examples and comparative examples, a Fourier transform infrared spectrophotometer (FT-IR) (manufactured by Perkin Elmer, product name: "Frontier") was used to measure attenuated total reflection spectroscopy (ATR) of the polarizer surface on the surface opposite to the surface on which the resin substrate was peeled off, using polarized infrared light as the measurement light. Germanium was used as the crystallite to which the polarizer was in close contact, and the angle of incidence of the measurement light was set to 45°. The orientation function was calculated by the following procedure. The polarized infrared light (measurement light) to be incident was polarized light (s-polarized light) that vibrated parallel to the surface to which the germanium crystal sample was in close contact, and the absorbance spectrum of each was measured in a state in which the stretching direction of the polarizer was arranged perpendicular (⊥) and parallel (//) to the polarization direction of the measurement light. From the obtained absorbance spectrum, (2941 cm ^-1 intensity) I was calculated using (3330 cm -1 intensity) as a reference. _{I ⊥} is (2941 cm -1 intensity)/(3330 cm ^-1 intensity) obtained from the absorbance spectrum obtained when the stretch direction of the polarizer is arranged perpendicular (⊥) to the polarization direction of the measurement light. I _// is (2941 cm ^-1 intensity)/(3330 cm ^-1 intensity) obtained from the absorbance spectrum obtained when the stretch direction ^of the polarizer is arranged parallel (//) to ^the polarization direction of the measurement light. Here, (2941 cm ^-1 intensity) is the absorbance at 2941 cm ^-1 when 2770 cm ^-1 and 2990 cm ^-1 , which are the bottoms of the absorbance spectrum, are used as the baseline, and (3330 cm ^-1 intensity) is the absorbance at 3330 cm ^-1 when 2990 cm ^-1 and 3650 cm ^-1 are used as the baseline. The obtained I _⊥ and I _// were used to calculate the orientation function f according to formula 1. Note that f = 1 indicates complete orientation, and f = 0 indicates random. The peak at 2941 cm ^-1 is said to be absorption caused by vibration of the main chain (-CH ₂ -) of PVA in the polarizer. The peak at 3330 cm ^-1 is said to be absorption caused by vibration of the hydroxyl group of PVA.
(Formula 1) f=(3<cos ² θ>−1)/2
=(1-D)/[c(2D+1)]
However, c = (3 cos ² β-1)/2
So, when 2941 cm ⁻¹ is used as above, β=90°⇒y=−2×(1−D)/(2D+1).
θ: angle of molecular chain with respect to stretching direction β: angle of transition dipole moment with respect to molecular chain axis D=( _I⊥ )/(I _// )
I _⊥ : Absorption intensity when the polarization direction of the measurement light and the stretching direction of the polarizer are perpendicular I _// : Absorption intensity when the polarization direction of the measurement light and the stretching direction of the polarizer are parallel (7) Crack occurrence rate A surface protective film was temporarily attached to the reflective polarizer surface of the polarizing plate obtained in the examples and comparative examples. Then, a separator was temporarily attached to the adhesive layer. This laminate was cut into about 130 mm x about 70 mm. At this time, it was cut so that the absorption axis of the polarizer was in the short direction. A U-shaped notch with a width of 5 mm, a depth (length of the recess) of 6.85 mm, and a curvature radius of 2.5 mm was formed in the center of the short side of the cut laminate. The U-shaped notch was formed by end mill processing. The outer diameter of the end mill was 4 mm, the feed rate was 500 mm/min, the rotation speed was 35000 rpm, and the cutting amount and the number of cutting times were rough cutting 0.2 mm/time and finish cutting 0.1 mm/time, totaling two times. The separator was peeled off from the laminate with the U-shaped notch formed, and the laminate was attached to a glass plate (thickness 1.1 mm) via an acrylic adhesive layer. Finally, the surface protective film was peeled off to obtain a test sample having a structure of reflective polarizer/adhesive layer/protective layer/polarizer/adhesive layer/glass plate. This test sample was subjected to a heat shock test in which the test sample was held at -40°C for 30 minutes and then held at 85°C for 30 minutes for 300 cycles, and the presence or absence of L-shaped cracks after the test was visually confirmed. This evaluation was performed using three polarizing plates, and the number of polarizing plates in which cracks (substantially L-shaped cracks) occurred was evaluated.

[Example 1]
1. Preparation of Polarizer As a thermoplastic resin substrate, a long amorphous isophthalic copolymerized polyethylene terephthalate film (thickness: 100 μm) was used, which had 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 (treatment condition: 55 W·min/m ² ).
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 immersed in an insolubilizing bath (a boric acid aqueous solution obtained by mixing 4 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40° C. for 30 seconds (insolubilizing treatment).
Next, the film was immersed in a dye bath (an aqueous iodine solution obtained by mixing iodine and potassium iodide in a weight ratio of 1:7 with 100 parts by weight of water) having a liquid temperature of 30° C. for 60 seconds while adjusting the concentration so that the single transmittance (Ts) of the finally obtained polarizer would be 40.5% (dyeing treatment).
Next, the plate was immersed in a crosslinking bath (a boric acid aqueous solution obtained by mixing 3 parts by weight of potassium iodide and 5 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40° C. for 30 seconds (crosslinking treatment).
Thereafter, the laminate was immersed in an aqueous boric acid solution (boric acid concentration: 4.0% by weight, potassium iodide: 5.0% by weight) at a liquid temperature of 62°C and uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls having different peripheral speeds so that the total stretching ratio was 3.0 times (underwater stretching treatment: the stretching ratio in the underwater stretching treatment was 1.25 times).
Thereafter, the laminate was immersed in a cleaning bath (an aqueous solution obtained by mixing 4 parts by weight of potassium iodide with 100 parts by weight of water) at a liquid temperature of 20° C. (cleaning treatment).
Thereafter, while drying in an oven maintained at 90° C., the laminate was brought into contact with a SUS heated roll having a surface temperature maintained at 75° C. for about 2 seconds (drying shrinkage treatment). The shrinkage rate of the laminate in the width direction due to the drying shrinkage treatment was 2%.
In this manner, a polarizer having a thickness of 7.4 μm was formed on the resin substrate.

2. Preparation of polarizing plate A TAC film (thickness 20 μm) was attached to the polarizer surface of the resin substrate/polarizer laminate via an ultraviolet-curable adhesive (thickness 1.0 μm). Furthermore, a reflective polarizer was attached to the TAC film surface via an acrylic adhesive (thickness 5 μm). Next, the resin substrate was peeled off, and an acrylic adhesive layer (thickness 15 μm) was provided on the peeled surface. In this way, a polarizing plate having a configuration of reflective polarizer/adhesive layer/protective layer/polarizer/adhesive layer was obtained.

[Examples 2 to 4]
A polarizer (thickness: 7.4 μm) was formed on a resin substrate in the same manner as in Example 1, except that a dye bath having a different iodine concentration (weight ratio of iodine to potassium iodide=1:7) was used. The following procedure was the same as in Example 1, to obtain a polarizing plate having a structure of reflective polarizer/adhesive layer/protective layer/polarizer/adhesive layer.

[Examples 5 to 8]
A polarizer (thickness: 6.7 μm) was formed on a resin substrate in the same manner as in Example 1, except that the stretching ratio in the underwater stretching was 1.46 times (resulting in a total stretching ratio of 3.5 times) and a dye bath with a different iodine concentration (weight ratio of iodine to potassium iodide = 1:7) was used. The following procedure was the same as in Example 1, and a polarizing plate having a structure of reflective polarizer/adhesive layer/protective layer/polarizer/adhesive layer was obtained.

[Examples 9 to 12]
A polarizer (thickness: 6.2 μm) was formed on a resin substrate in the same manner as in Example 1, except that the stretching ratio in the underwater stretching was 1.67 times (as a result, the total stretching ratio was 4.0 times) and a dye bath with a different iodine concentration (weight ratio of iodine to potassium iodide = 1:7) was used. The following procedure was the same as in Example 1, and a polarizing plate having a structure of reflective polarizer/adhesive layer/protective layer/polarizer/adhesive layer was obtained.

[Examples 13 to 16]
A polarizer (thickness: 6.0 μm) was formed on a resin substrate in the same manner as in Example 1, except that the stretching ratio in the underwater stretching was 1.88 times (resulting in a total stretching ratio of 4.5 times) and a dye bath with a different iodine concentration (weight ratio of iodine to potassium iodide = 1:7) was used. The following procedure was the same as in Example 1, and a polarizing plate having a structure of reflective polarizer/adhesive layer/protective layer/polarizer/adhesive layer was obtained.

[Comparative Examples 1 to 4]
A polarizer (thickness: 5.5 μm) was formed on a resin substrate in the same manner as in Example 1, except that the stretching ratio in the underwater stretching was 2.29 times (resulting in a total stretching ratio of 5.5 times) and a dye bath with a different iodine concentration (weight ratio of iodine to potassium iodide = 1:7) was used. The following procedure was the same as in Example 1, and a polarizing plate having a structure of reflective polarizer/adhesive layer/protective layer/polarizer/adhesive layer was obtained.

The polarizing plates obtained in the examples and comparative examples were subjected to the above evaluations (2) to (7). The results are shown in Table 1.

As is clear from Table 1, the polarizing plate of the embodiment suppresses the occurrence of cracks in the irregularly shaped portion (U-shaped notch portion).

8 to 10 show the relationship between the single transmittance of the polarizer obtained in the examples and the comparative examples and the Δn, in-plane retardation, or orientation function of PVA, respectively. As shown in FIG. 8 to FIG. 10, even if the birefringence, in-plane retardation, or orientation function are the same (resulting in the same degree of orientation), when the single transmittance is high, cracks are likely to occur in the irregularly processed part. For example, when Δn is around 35 (×10 ⁻³ ) in FIG. 8, when the single transmittance is greater than about 44.2%, it does not satisfy formula (1), and as a result, cracks occur as in Comparative Example 4. Therefore, in order to effectively suppress the occurrence of cracks in the irregularly processed part, it is important to adjust the single transmittance (resulting in the amount of adsorption of the dichroic material) in addition to the degree of orientation of the PVA-based resin. Also, it is understood that the polarizer satisfying formula (1), formula (2), and/or formula (3) is one in which these adjustments have been suitably performed, and the occurrence of cracks in the irregularly processed part can be suitably suppressed.

The polarizing plate of the present invention is used in image display devices, and is particularly suitable for use in image display devices having irregular shapes, such as automobile meter panels, smartphones, tablet PCs, and smart watches.

Claims (4)

A polarizing plate having a polarizer and a protective layer disposed on at least one side of the polarizer,
The polarizing plate has an irregular shape other than a rectangle,
The protective layer is made of a resin film,
The polarizer is a polarizing plate that is composed of a polyvinyl alcohol-based resin film containing a dichroic material, and satisfies the following formula (1) when the single transmittance is x% and the birefringence of the polyvinyl alcohol-based resin is y:
y<-0.011x+0.525 (1). A polarizing plate having a polarizer and a protective layer disposed on at least one side of the polarizer,
The polarizing plate has an irregular shape other than a rectangle,
The protective layer is made of a resin film,
The polarizer is composed of a polyvinyl alcohol-based resin film containing a dichroic material, and satisfies the following formula (2) when the single transmittance is x% and the in-plane retardation of the polyvinyl alcohol-based resin film is z nm:
z<-60x+2875 (2). A polarizing plate having a polarizer and a protective layer disposed on at least one side of the polarizer,
The polarizing plate has an irregular shape other than a rectangle,
The protective layer is made of a resin film,
The polarizer is a polarizing plate that is composed of a polyvinyl alcohol-based resin film containing a dichroic material, and satisfies the following formula (3) when the single transmittance is x% and the orientation function of the polyvinyl alcohol-based resin is f:
f<-0.018x+1.11 (3) A polarizing plate having a polarizer and a protective layer disposed on at least one side of the polarizer,
The polarizing plate has an irregular shape other than a rectangle,
The protective layer is made of a resin film,
The polarizer is made of a polyvinyl alcohol-based resin film containing a dichroic material, and has a piercing strength of 30 gf/μm or more.

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