TWI893141B - Polarizing plate, polarizing plate with phase difference layer, and image display device - Google Patents

Abstract

The present invention provides a polarizing plate that suppresses cracking during heating despite being thin. The polarizing plate of the present invention comprises a polarizer formed of a polyvinyl alcohol-based resin film containing a dichroic substance and a protective layer disposed on one side of the polarizer; the protective layer is formed of a resin film having a thickness of 10µm or less. In one embodiment, when the monomer transmittance of the polarizer is x% and the birefringence of the polyvinyl alcohol-based resin is y, the following equation (1) is satisfied. In one embodiment, when the monomer transmittance of the polarizer is x% and the in-plane phase difference of the polyvinyl alcohol-based resin film is znm, the following equation (2) is satisfied. In one embodiment, the polarizer satisfies the following formula (3) when its monomer transmittance is x% and the orientation function of the polyvinyl alcohol-based resin film is f. In one embodiment, the puncture 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)

Description

Polarizing plate, polarizing plate with phase difference layer, and image display device

The present invention relates to a polarizing plate, a polarizing plate with a phase difference layer, and an image display device.

In recent years, image display devices, typified by liquid crystal displays and electroluminescent (EL) displays (e.g., organic and inorganic EL displays), have rapidly gained popularity. Image display devices generally utilize a polarizing plate and a retardation plate, each comprising a polarizer and a protective layer to protect the polarizer. In practical applications, a polarizing plate with a retardation layer, in which the polarizing plate and retardation plate are integrated, is widely used (e.g., Patent Document 1). Recently, with the increasing demand for thinner image display devices, the demand for thinner polarizing plates and polarizing plates with a retardation layer has also increased.

Methods for thinning polarizers have been proposed, including thinning the protective layer and laminating the protective layer only on one side of the polarizer. However, these methods fail to adequately protect the polarizer and are prone to cracking due to heat. Prior Art Patent

Patent document 1: Japanese Patent Publication No. 2015-210474

Problems to be Solved by the Invention This invention was developed to address the aforementioned conventional problems. Its primary purpose is to provide a polarizing plate that, even when very thin, suppresses cracking caused by heating.

Means for Solving the Problem According to one aspect of the present invention, a polarizing plate is provided, comprising a polarizer formed of a polyvinyl alcohol-based resin film containing a dichroic substance and a protective layer disposed on one side of the polarizer. The polarizer satisfies the following equation (1) when its single-unit transmittance is x% and the birefringence of the polyvinyl alcohol-based resin is y. The protective layer is formed of a resin film having a thickness of 10µm or less. y＜-0.011x+0.525　　　　　(1) According to one aspect of the present invention, a polarizing plate is provided, which comprises a polarizer composed of a polyvinyl alcohol resin film containing a dichroic substance and a protective layer disposed on one side of the polarizer; the polarizer satisfies the following formula (2) when its single body transmittance is x% and the in-plane phase difference of the polyvinyl alcohol resin film is znm; the protective layer is composed of a resin film having a thickness of 10µm or less. z＜-60x+2875　　　　　　　　　(2) According to one aspect of the present invention, a polarizing plate is provided, which has a polarizer composed of a polyvinyl alcohol resin film containing a dichroic substance and a protective layer arranged on one side of the polarizer; the polarizer satisfies the following formula (3) when its monomer transmittance is x% and the orientation function of the polyvinyl alcohol resin is f; the protective layer is composed of a resin film having a thickness of less than 10µm. f＜-0.018x+1.11　　　　　　(3). According to one aspect of the present invention, a polarizing plate is provided, comprising a polarizer formed of a polyvinyl alcohol-based resin film containing a dichroic substance and a protective layer disposed on one side of the polarizer. The polarizer has a puncture strength of 30 gf/µm or greater, and the protective layer is formed of a resin film having a thickness of 10 µm or less. In one embodiment, the resin film comprises at least one resin selected from epoxy resins, (meth)acrylic resins, polyester resins, and polyurethane resins. In one embodiment, the resin film is formed of a photoion-cured epoxy resin, and the softening temperature of the resin film is above 100°C. In one embodiment, the resin film is formed from a cured product obtained from an epoxy resin coating solution in an organic solvent, and the softening temperature of the resin film is 100°C or higher. In one embodiment, the resin film is formed from a cured product obtained from an organic solvent coating solution of a thermoplastic (meth)acrylic resin, and the softening temperature of the resin film is 100°C or higher. In one embodiment, the thermoplastic (meth)acrylic resin comprises at least one member selected from the group consisting of a lactone ring unit, a glutaric anhydride unit, a glutarimide unit, a maleic anhydride unit, and a maleimide unit. In one embodiment, the protective layer has an iodine adsorption amount of 25% by weight or less. In one embodiment, the polarizer has a thickness of 10 μm or less. In one embodiment, the polarizer is wound into a roll. According to another aspect of the present invention, the polarizer includes the polarizer and a retardation layer; the retardation layer is disposed on the side of the polarizer opposite to the side on which the protective layer is disposed. In one embodiment, the retardation layer is laminated on the polarizer via an adhesive layer. In one embodiment, the Re(550) of the retardation layer is 100 nm to 190 nm, the Re(450)/Re(550) ratio is greater than 0.8 and less than 1, and the angle formed between the slow axis of the retardation layer and the absorption axis of the polarizer is 40° to 50°. According to another aspect of the present invention, an image display device is provided, comprising the aforementioned polarizing plate or a polarizing plate with a retardation layer.

Effects of the Invention: The polarizing plate of the present invention utilizes a polarizer with a controlled orientation of a polyvinyl alcohol (PVA) resin. This allows for the suppression of cracking during heating, even when using an extremely thin resin film as a protective layer. Furthermore, the polarizer exhibits optical properties acceptable for practical use. Therefore, even with its extremely thinness, the polarizing plate of the present invention can achieve both acceptable optical properties and the suppression of cracking during heating.

The following describes the embodiments of the present invention, but the present invention is not limited to these embodiments. Furthermore, the various embodiments can be appropriately combined.

(Definition of terms and symbols) The definitions of terms and symbols in this specification are as follows. (1) Refractive index (nx, ny, nz) "nx" is the refractive index in the direction where the in-plane refractive index reaches the maximum (i.e., the slow axis direction), "ny" is the refractive index in the direction perpendicular to the slow axis (i.e., the fast axis direction), and "nz" is the refractive index in the thickness direction. (2) In-plane phase difference (Re) "Re(λ)" is the in-plane phase difference measured at 23°C using light of a wavelength of λnm. For example, "Re(550)" is the in-plane phase difference measured at 23°C using light of a wavelength of 550nm. Re(λ) can be obtained by the formula: Re(λ) = (nx-ny) × d when the thickness of the layer (thin film) is d(nm). (3) Retardation in the thickness direction (Rth) "Rth(λ)" is the retardation in the thickness direction measured at 23°C using light of wavelength λnm. For example, "Rth(550)" is the retardation in the thickness direction measured at 23°C using light of wavelength 550nm. Rth(λ) can be calculated by the formula: Rth(λ) = (nx - nz) × d when the thickness of the layer (film) is d (nm). (4) Nz coefficient The Nz coefficient can be calculated by Nz = Rth/Re. (5) Angle When angles are mentioned in this specification, the angles include both clockwise and counterclockwise directions relative to the reference direction. Therefore, for example, "45°" means ±45°.

A. Polarizing Plate A-1. Overview of Polarizing Plate Figure 1 is a schematic cross-sectional view of a polarizing plate according to one embodiment of the present invention. The illustrated polarizing plate 100 comprises a polarizer 10, a first protective layer 20 disposed on one side of the polarizer 10, and a second protective layer 30 disposed on the other side. The polarizer 10 is formed of a polyvinyl alcohol-based resin film containing a dichroic substance. The first protective layer 20 is formed of a resin film having a thickness of 10µm or less. The second protective layer 30 may be omitted depending on the intended purpose. Furthermore, although not shown, a hard coat layer may be provided on the side of the first protective layer 20 opposite the polarizer 10, and an adhesive layer may be provided between the first protective layer 20 and the polarizer 10, as needed.

In one embodiment, the polarizer 10 satisfies the following equation (1) when the single-body transmittance is x% and the birefringence of the polyvinyl alcohol resin constituting the polarizer is y. In one embodiment, the polarizer 10 satisfies the following equation (2) when the single-body transmittance is x% and the in-plane phase difference of the polyvinyl alcohol resin film constituting the polarizer is znm. In one embodiment, the polarizer 10 satisfies the following equation (3) when the single-body transmittance is x% and the orientation function of the polyvinyl alcohol resin film constituting the polarizer is f. In one embodiment, the puncture strength of the polarizer is greater than 30gf/µm. y＜-0.011x+0.525　　(1) z＜-60x+2875　　　(2) f＜-0.018x+1.11　　(3)

The total thickness of the polarizing plate 100 is, for example, 20 μm or less, preferably 15 μm or less, more preferably 12 μm or less, and even more preferably 10 μm or less. Furthermore, the total thickness of the polarizing plate is, for example, 5 μm or more.

The layers or optical films that comprise the polarizing plate may be bonded together via a bonding layer or may be formed without a bonding layer. The bonding layer may include a bonding agent layer or an adhesive layer. In embodiments of the present invention, a bonding agent layer may be suitably employed. This configuration allows for a thinner polarizing plate. The bonding agent that comprises the bonding agent layer may typically be an active energy ray-curable bonding agent (e.g., a UV-curable bonding agent).

In embodiments of the present invention, the thickness of the polarizing plate can be made extremely thin. Therefore, it can be suitably applied to flexible image display devices. Preferably, the image display device has a curved shape (essentially a curved display screen) and/or is bendable or foldable. Specific examples of image display devices include liquid crystal display devices and electroluminescent (EL) display devices (e.g., organic EL display devices and inorganic EL display devices). Of course, the above description does not prevent the polarizing plate of the present invention from being applied to general image display devices.

After the above-mentioned polarizing plate is placed in an environment of 60°C and 95% RH for 500 hours, the change ΔTs of the single transmittance Ts and the change ΔP of the polarization degree P should each be very small. The single transmittance Ts can be measured using, for example, an ultraviolet-visible spectrophotometer (manufactured by JASCO Corporation, product name "V7100"). The polarization degree P is calculated using the following formula from the single transmittance (Ts), parallel transmittance (Tp) and orthogonal transmittance (Tc) measured using an ultraviolet-visible spectrophotometer. Polarization degree (P) (%) = {(Tp-Tc)/(Tp+Tc)} ^{1 /2} ×100 In addition, the above-mentioned Ts, Tp and Tc are Y values obtained by measuring the 2-degree field of view (light source C) of JIS Z 8701 and performing visual sensitivity correction. Furthermore, Ts and P are essentially properties of the polarizer. ΔTs and ΔP can each be calculated using the following formulas: ΔTs(%) = _Ts500 - _Ts0 ΔP(%) = _P500 - _{P0 (} where _Ts0 is the initial transmittance before placement, _Ts500 is the initial transmittance after placement, _P0 is the polarization before placement (initial), and _P500 is the polarization after placement. ΔTs is preferably 3.0% or less, more preferably 2.5% or less, more preferably 2.0% or less, and even more preferably 1.5% or less. ΔP is preferably -5.0% to 0%, more preferably -3.0% to 0%, more preferably -1.0% to 0%, and even more preferably -0.5% to 0%.

The polarizing plate of the present invention can be in the form of a single sheet or a long strip. As used herein, "long strip" refers to a long, slender shape that is sufficiently long relative to its width. For example, this includes a long strip that is at least 10 times, and preferably at least 20 times, its width. Long strips of polarizing plates can be wound into a roll.

A-2. Polarizer The polarizer is formed of a polyvinyl alcohol resin film containing a dichroic substance. In one embodiment, the polarizer satisfies the following equation (1) when the single-element transmittance is x% and the birefringence of the polyvinyl alcohol resin constituting the polarizer is y. In one embodiment, the polarizer satisfies the following equation (2) when the single-element transmittance is x% and the in-plane retardation of the polyvinyl alcohol resin film constituting the polarizer is znm. In one embodiment, the polarizer satisfies the following equation (3) when the single-element transmittance is x% and the orientation function of the polyvinyl alcohol resin film constituting the polarizer is f. In one embodiment, the puncture strength of the polarizer is 30 gf/µm or greater. y＜-0.011x+0.525　　(1) z＜-60x+2875　　　(2) f＜-0.018x+1.11　　(3)

In polarizers made from polyvinyl alcohol (PVA) resin films containing dichroic substances, the birefringence of the PVA resin (hereinafter referred to as PVA birefringence or PVA Δn), the in-plane retardation of the PVA resin film (hereinafter referred to as "PVA in-plane retardation"), the orientation function of the PVA resin (hereinafter referred to as "PVA orientation function"), and the puncture strength of the polarizer are all values related to the degree of orientation of the molecular chains of the PVA resin constituting the polarizer. Specifically, the birefringence, in-plane retardation, and orientation function of PVA increase with increasing orientation, while the puncture strength decreases with increasing orientation. The polarizer used in the present invention (i.e., a polarizer satisfying the above formulas (1) to (3) or the puncture strength) has a PVA-based resin whose molecular chain is more gently oriented in the absorption axis direction than conventional polarizers, thereby suppressing thermal shrinkage in the absorption axis direction. As a result, a polarizing plate can be obtained that is extremely thin but suppresses cracking when heated. Furthermore, because the polarizer is also highly flexible, a polarizing plate with excellent flexibility and anti-bending durability can be obtained, and is preferably applicable to curved image display devices, more preferably to bendable image display devices, and even more preferably to foldable image display devices. In the past, polarizers with low orientation levels struggled to achieve acceptable optical properties (typically, single-element transmittance and polarization). However, the polarizer used in the present invention achieves both acceptable optical properties and a lower orientation level for PVA-based resins than previously possible.

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 PVA is the in-plane retardation value of a PVA resin film at 23°C and a wavelength of 1000 nm. By setting the measurement wavelength in the near-infrared region, the influence of iodine absorption in the polarizer can be eliminated and the retardation can be measured. Furthermore, the birefringence (in-plane birefringence) of PVA is the value obtained by dividing the in-plane retardation of PVA by the thickness (nm) of the polarizer.

The in-plane retardation of PVA is evaluated as follows. First, the retardation values are measured at multiple wavelengths above 850 nm. The measured retardation values, R(λ), are plotted against the wavelength, λ, and then fitted to the following Sellmeier equation using the least squares method. Here, A and B are fitting parameters, coefficients determined using the least squares method. R(λ) = A + B / (λ ² - 600 ² ) At this point, the retardation value R(λ) can be separated into the wavelength-independent in-plane retardation of PVA (Rpva) and the wavelength-dependent in-plane retardation of iodine (Ri) as follows: Rpva = A Ri = B / (λ ² - 600 ² ) Based on this separation formula, the in-plane retardation of PVA at a wavelength of λ = 1000 nm (i.e., Rpva) can be calculated. Furthermore, the evaluation method for the in-plane phase difference of PVA is also described in Japanese Patent No. 5932760, which can be referred to as needed. Furthermore, the birefringence (Δn) of the PVA can be calculated by dividing the phase difference by the thickness.

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

The orientation function (f) of the polarizer used in the present invention preferably satisfies the following formula (3a), and more preferably satisfies the following formula (3b). If the orientation function is too small, it may be impossible to obtain an acceptable single-unit transmittance and/or polarization degree. -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 determined, for example, by attenuated total reflection (ATR) spectroscopy using a Fourier transform infrared spectrophotometer (FT-IR) with polarized light as the measurement light. Specifically, germanium is used as the microcrystal for bonding the polarizer, the measurement light is incident at an angle of 45°, and the incident polarized infrared light (measurement light) is polarized parallel to the plane of the germanium crystal sample that is bonding the sample (s-polarization). Measurements are performed with the polarizer extending in both parallel and perpendicular directions to the polarization direction of the measurement light. The intensity at 2941 cm ^-1 in the resulting absorbance spectrum is then used to calculate the orientation function according to the following formula. Here, intensity I is referenced to the peak at 3330 cm ^-1 and is calculated as 2941 cm ^-1 /3330 cm ^-1 . Furthermore, f = 1 indicates perfect alignment, while f = 0 indicates random alignment. Furthermore, we believe the peak at 2941 cm ^-1 is due to absorption from the vibration of the PVA main chain ( _-CH2- ) in the polarizer. f = (3 < ^cos2θ > -1)/2 = (1-D)/[c(2D+1)] = -2 × (1-D)/(2D+1). However, with c = ( ^3cos2β -1)/2, β = 90° for the vibration at 2941 cm- ¹ . θ: The angle of the molecular chain relative to the extension direction β: The angle of the transition dipole moment relative to the molecular chain axis D = (I _⊥ ) / (I _// ) (At this time, the more oriented the PVA molecules are, the larger D is) I _⊥ : The absorption intensity when the polarization direction of the measured light is perpendicular to the extension direction of the polarizer I _// : The absorption intensity when the polarization direction of the measured light is parallel to the extension direction of the polarizer

The thickness of the polarizer is preferably 10µm or less, more preferably 8µm or less. The lower limit of the polarizer thickness can be, for example, 1µm. In one embodiment, the thickness of the polarizer can be 2µm to 10µm, and in another embodiment, it can be 2µm to 8µm. By making the polarizer very thin as described, thermal shrinkage can be minimized. This configuration is also believed to help suppress cracking caused by heating.

The polarizer preferably exhibits absorption dichroism at any wavelength between 380 nm and 780 nm. The single-unit transmittance of the polarizer is preferably 40.0% or greater, more preferably 41.0% or greater. The single-unit transmittance may, for example, be 49.0% or less. In one embodiment, the single-unit transmittance of the polarizer is 40.0% to 45.0%. The polarization degree of the polarizer is preferably 99.0% or greater, more preferably 99.4% or greater. The polarization degree may, for example, be 99.999% or less. In one embodiment, the polarization degree of the polarizer is 99.0% to 99.99%. One of the characteristics of the polarizer used in the present invention is that even if the orientation degree of the PVA-based resin constituting the polarizer is lower than before and has the in-plane phase difference, birefringence and/or orientation function as described above, the monomer transmittance and polarization degree that are allowable in practical use can still be achieved. We speculate that this is due to the manufacturing method described later. In addition, the monomer transmittance is represented by the Y value obtained by measuring and correcting the visual sensitivity using an ultraviolet-visible spectrophotometer. The polarization degree can be obtained by the following formula based on the parallel transmittance Tp and the orthogonal transmittance Tc obtained by measuring and correcting the visual sensitivity using an ultraviolet-visible spectrophotometer. Polarization degree (%) = {(Tp-Tc)/(Tp+Tc)} ^{1 /2} ×100

The puncture 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, more preferably 45 gf/µm or more, and particularly preferably 50 gf/µm or more. The upper limit of the puncture strength can be, for example, 80 gf/µm. By setting the puncture strength of the polarizer to the above range, cracks in the polarizer during heating and splitting of the polarizer along the absorption axis can be significantly suppressed. As a result, a polarizer (in other words, a polarizing plate) with very excellent flexibility can be obtained. The puncture strength indicates the polarizer's resistance to cracking when the polarizer is punctured with a predetermined strength. Puncture strength can be expressed, for example, as the strength (fracture strength) required to break a polarizer when a predetermined needle is installed in a compression tester and pierced at a predetermined speed. Furthermore, as is apparent from the unit, puncture strength refers to the strength per unit thickness (1µm) of the polarizer.

As described above, the polarizer is formed from a PVA-based resin film containing a dichroic substance. The PVA-based resin forming the PVA-based resin film (essentially the polarizer) preferably includes a PVA-based resin modified with an acetyl group. With this configuration, a polarizer having the desired puncture strength can be obtained. When the total weight of the PVA-based resin is 100%, the blending amount of the acetyl-based resin modified with an acetyl group is preferably 5% to 20% by weight, more preferably 8% to 12% by weight. When the blending amount is within this range, the puncture strength can be set to a more suitable range.

Polarizers are typically manufactured using a laminate of two or more layers. A specific example of a polarizer using a laminate is a laminate comprising a resin substrate and a PVA-based resin layer coated on the resin substrate. A polarizer obtained by laminating a resin substrate and a PVA-based resin layer coated on the resin substrate can be produced, for example, by applying a PVA-based resin solution to the resin substrate and drying the solution to form the PVA-based resin layer on the resin substrate, thereby obtaining a laminate of the resin substrate and the PVA-based resin layer; and then stretching and dyeing the laminate to form the polarizer. In this embodiment, a polyvinyl alcohol-based resin layer containing a halogenated compound and a polyvinyl alcohol-based resin is preferably formed on one side of the resin substrate. Stretching typically includes immersing the laminate in an aqueous boric acid solution and then stretching it. Furthermore, the stretching preferably further includes the following step: stretching the laminate in the air at a high temperature (e.g., above 95°C) before stretching in the boric acid aqueous solution. In the embodiment of the present invention, the total stretching ratio is preferably 3.0 to 4.5 times, which is significantly smaller than the general ratio. Even with the total stretching ratio, a polarizer with acceptable optical properties can be obtained by combining the addition of halides and dry shrinkage treatment. Furthermore, in the embodiment of the present invention, the stretching ratio of the air-assisted stretching is preferably greater than the stretching ratio of the stretching in boric acid water. By making the above-mentioned structure, a polarizer with acceptable optical properties can be obtained even if the total stretching ratio is small. Furthermore, the laminate is preferably subjected to a drying and shrinking treatment in which the laminate is heated while being transported along its longitudinal direction, thereby shrinking the laminate by more than 2% in its width direction. In one embodiment, the method for manufacturing a polarizer comprises sequentially subjecting the laminate to an air-assisted stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying and shrinking treatment. By introducing the auxiliary stretching, the crystallinity of the PVA resin can be improved even when the PVA resin is coated on a thermoplastic resin, thereby achieving high optical properties. Furthermore, by simultaneously improving the orientation of the PVA resin in advance, problems such as a reduction in orientation or dissolution of the PVA resin when immersed in water during the subsequent dyeing or stretching steps can be prevented, thereby achieving high optical properties. Furthermore, immersing the PVA resin layer in a liquid can further suppress the orientational disorder of the polyvinyl alcohol molecules and the reduction of their orientation compared to a PVA resin layer without halides. This can improve the optical properties of polarizers produced through treatments such as dyeing and underwater stretching, where the laminate is immersed in a liquid. Furthermore, shrinking the laminate in the width direction during drying and shrinking can further enhance optical properties. The resulting resin substrate/polarizer laminate can be used directly (i.e., the resin substrate can also be used as a protective layer for the polarizer), or the resin substrate can be peeled off from the resin substrate/polarizer laminate and then used after applying any appropriate protective layer to the peeled area. Details of the polarizer manufacturing method are described in Section A-3.

A-3. Polarizer Manufacturing Method A polarizer manufacturing method according to one embodiment of the present invention comprises the following steps: forming a polyvinyl alcohol resin layer (PVA resin layer) containing a halogenated compound and a polyvinyl alcohol resin (PVA resin) on one side of a long thermoplastic resin substrate to form a laminate; and sequentially subjecting the laminate to an air-assisted stretching treatment, a dyeing treatment, an underwater stretching treatment, and a drying and shrinking treatment. The drying and shrinking treatment involves heating the laminate while conveying it along its longitudinal direction, thereby shrinking it by 1% to 10% in its width direction. The halogenated compound content in the PVA resin layer is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA resin. The drying and shrinking treatment is preferably performed using a heated roller, and the temperature of the heated roller is preferably 60°C to 120°C. The shrinkage rate of the laminate in the width direction after the drying and shrinking treatment is preferably 1% to 10%. The polarizer described in Section A-2 above can be obtained according to the above manufacturing method. In particular, a polarizer having excellent optical properties (represented by single-body transmittance and polarization degree) can be obtained by the following method: after manufacturing a laminate including a PVA-based resin layer containing a halogenated compound, the laminate is stretched through a multi-stage stretching process including air-assisted stretching and underwater stretching, and the stretched laminate is then heated using a heating roller.

A-3-1. Laminated Product Preparation Any appropriate method can be used to prepare a laminate of a thermoplastic resin substrate and a PVA-based resin layer. Preferably, a coating liquid containing a halogenated compound and a PVA-based resin is applied to the surface of the thermoplastic resin substrate and dried to form the PVA-based resin layer on the thermoplastic resin substrate. As mentioned above, the halogenated compound content in the PVA-based resin layer is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA-based resin.

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

The thickness of the PVA resin layer is preferably 2µm to 30µm, more preferably 2µm to 20µm. By making the thickness of the PVA resin layer very thin before stretching as described above and reducing the total stretching ratio as described below, a polarizer can be obtained that has acceptable monomer transmittance and polarization degree even with a very small orientation function.

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

A-3-1-1. Thermoplastic Resin Substrate The thermoplastic resin substrate can be any suitable thermoplastic resin film. Details regarding thermoplastic resin film substrates are described, for example, in Japanese Patent Publication No. 2012-73580 or Japanese Patent No. 6470455. The entire disclosure of these publications is incorporated herein by reference.

A-3-1-2. Coating Liquid The coating liquid comprises the halogenated compound and the PVA-based resin as described above. The coating liquid is typically a solution of the halogenated compound and the PVA-based resin dissolved in a solvent. Examples of the solvent include water, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, various glycols, polyols such as trihydroxymethylpropane, and amines such as ethylenediamine and diethylenetriamine. These solvents may be used alone or in combination. Water is preferred. The PVA-based resin concentration in the solution is preferably 3 to 20 parts by weight per 100 parts by weight of the solvent. At this resin concentration, a uniform coating film can be formed that adheres closely to the thermoplastic resin substrate. The halogen content in the coating solution is preferably 5 to 20 parts by weight relative to 100 parts by weight of the PVA resin.

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

Any suitable resin can be used as the above-mentioned PVA resin. Examples include polyvinyl alcohol and ethylene-vinyl alcohol copolymer. Polyvinyl alcohol can be obtained by saponifying polyvinyl acetate. Ethylene-vinyl alcohol copolymer can be obtained by saponifying ethylene-vinyl acetate copolymer. The saponification degree of PVA resin is generally 85 mol% to 100 mol%, preferably 95.0 mol% to 99.95 mol%, and more preferably 99.0 mol% to 99.93 mol%. The saponification degree can be determined in accordance with JIS K 6726-1994. By using a PVA resin having the above-mentioned saponification degree, a polarizer with excellent durability can be obtained. When the saponification degree is too high, there is a risk of gelling. As mentioned above, the PVA-based resin preferably includes a PVA-based resin modified with an acetyl group.

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

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

The amount of halogenated compound in the coating solution should be 5 to 20 parts by weight, more preferably 10 to 15 parts by weight, relative to 100 parts by weight of the PVA resin. If the amount of halogenated compound exceeds 20 parts by weight, the halogenated compound may overflow, resulting in a cloudy, white polarizer.

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

A-3-2. Air-Assisted Stretching To achieve particularly high optical properties, a two-stage stretching method combining dry stretching (assisted stretching) and stretching in boric acid water is often used. In this two-stage stretching method, the introduction of the auxiliary stretching allows stretching to be performed while suppressing crystallization of the thermoplastic resin substrate. Furthermore, when coating PVA resin on a thermoplastic resin substrate, the coating temperature must be lower than when coating PVA resin on a conventional metal roller to minimize the effect of the thermoplastic resin substrate's glass transition temperature. This results in a relatively slow crystallization of the PVA resin, preventing the achievement of sufficient optical properties. By introducing assisted stretching, the crystallinity of the PVA resin can be enhanced even when it is coated on a thermoplastic resin, achieving high optical properties. Furthermore, by pre-enhancing the orientation of the PVA resin, problems such as loss of orientation or dissolution during subsequent dyeing or stretching steps can be avoided, thus achieving high optical properties.

The stretching method for in-flight assisted stretching can 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 the laminate by passing it between rollers with different circumferential speeds). In order to obtain high optical properties, free-end stretching can be actively adopted. In one embodiment, the in-flight stretching process includes a heating roller stretching step, which is to stretch the laminate while transporting it along its long side and utilizing the circumferential speed difference between the heating rollers. The in-flight stretching process typically includes a zone stretching step and a heating roller stretching step. In addition, the order of the zone stretching step and the heating roller stretching step is not limited, and the zone stretching step can be performed first, or the heating roller stretching step can be performed first. The regional stretching step can also be omitted. In one embodiment, the regional stretching step and the heated roller stretching step are performed sequentially. In another embodiment, the film ends are gripped in a tenter stretching machine, and the distance between the tenters is expanded in the direction of travel to stretch (the increase in the distance between the tenters is the stretching ratio). At this time, the distance between the tenters in the width direction (perpendicular to the direction of travel) is set so that they can be arbitrarily approached. It is preferable to set the stretching ratio relative to the direction of travel to use free-end stretching for approach. In the case of free-end stretching, the shrinkage ratio in the width direction is calculated as (1/stretching ratio) ^1/2 .

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

The stretching ratio of the air-assisted stretching is preferably 1.0 times to 4.0 times, more preferably 1.5 times to 3.5 times, and even more preferably 2.0 times to 3.0 times. If the stretching ratio of the air-assisted stretching is within the range, the total stretching ratio can be set to the desired range when combined with the underwater stretching, and the desired orientation function can be achieved. As a result, a polarizing plate with a phase difference layer in which cracks caused by heating are suppressed can be obtained. Moreover, as mentioned above, the stretching ratio of the air-assisted stretching is preferably greater than the stretching ratio of the stretching in boric acid water. By making the above-mentioned structure, even if the total stretching ratio is small, a polarizer with acceptable optical properties can still be obtained. More specifically, the ratio of the stretching ratio of the air-assisted stretching to the stretching ratio of the underwater stretching (underwater stretching/air-assisted stretching) is preferably 0.4-0.9, more preferably 0.5-0.8.

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

A-3-3. Insolubilization, Dyeing, and Crosslinking Treatments If necessary, an insolubilization treatment is performed after the air-assisted stretching treatment and before the underwater stretching treatment or dyeing treatment. The insolubilization treatment is typically performed by immersing the PVA resin layer in an aqueous boric acid solution. The dyeing treatment is typically performed by dyeing the PVA resin layer with a dichroic substance (typically iodine). If necessary, a crosslinking treatment is performed after the dyeing treatment and before the underwater stretching treatment. The crosslinking treatment can typically be performed by immersing the PVA resin layer in an aqueous boric acid solution. Details of the insolubilization treatment, dyeing treatment, and crosslinking treatment are described, for example, in Japanese Patent Publication No. 2012-73580 (cited above).

A-3-4. Underwater Stretching Underwater stretching is performed by immersing the laminate in a stretching bath. This allows stretching at temperatures lower than the glass transition temperature (typically around 80°C) of the thermoplastic resin substrate or PVA resin layer, suppressing crystallization of the PVA resin layer. This results in a polarizer with excellent optical properties.

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

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

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

An iodide is preferably added to the stretching bath (boric acid aqueous solution). This addition can suppress the dissolution of iodine adsorbed in the PVA-based resin layer. Specific examples of the iodide are described above. The concentration of the 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 this temperature, the dissolution of the PVA-based resin layer can be suppressed, while at the same time, stretching at a high rate can be achieved. Specifically, as mentioned above, in relation to the formation of the PVA-based resin layer, the glass transition temperature (Tg) of the thermoplastic resin substrate is preferably above 60°C. At this time, if the stretching temperature is lower than 40°C, even if the thermoplastic resin substrate is plasticized with water, it may not 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 it may not be possible to obtain excellent optical properties. The immersion time of the laminate in the stretching bath is preferably 15 seconds to 5 minutes.

The stretching ratio for underwater stretching is preferably 1.0 to 2.2 times, more preferably 1.1 to 2.0 times, more preferably 1.1 to 1.8 times, and even more preferably 1.2 to 1.6 times. If the stretching ratio for underwater stretching is within the aforementioned range, the total stretching ratio can be set to the desired range, thereby achieving the desired birefringence, in-plane phase difference, and/or orientation function. As a result, a polarizing plate with a phase difference layer in which cracking caused by heating is suppressed can be obtained. As described above, the total stretching ratio (the sum of the stretching ratios for the air-assisted stretching and the underwater stretching) 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 halides to the coating solution, adjusting the stretching ratios of the air-assisted stretching and underwater stretching, and performing a drying and shrinking treatment, a polarizer with acceptable optical properties can be obtained even at the aforementioned total stretching ratio.

A-3-5. Drying and Shrinking Treatment The drying and shrinking treatment can be performed by heating the entire area (zone heating) or by heating the transport rollers (so-called heated roller drying). Using both methods is preferred. Drying with heated rollers effectively suppresses heat-induced warping of the laminate, resulting in a polarizer with excellent appearance. Specifically, drying the laminate with the heated rollers effectively promotes crystallization of the thermoplastic resin matrix, increasing its crystallinity. Even at relatively low drying temperatures, the crystallinity of the thermoplastic resin matrix can be significantly increased. As a result, the rigidity of the thermoplastic resin substrate increases, making it capable of withstanding the shrinkage of the PVA-based resin layer due to drying, thereby suppressing curling. Furthermore, by using heated rollers, the laminate can be dried while maintaining a flat state, thereby suppressing not only curling but also wrinkling. In this case, the laminate shrinks in the width direction through the drying and shrinking treatment, thereby improving optical properties. This is because it effectively improves the orientation of the PVA and PVA/iodine complex. The shrinkage rate in the width direction of the laminate obtained by the drying and shrinking treatment is preferably 1% to 10%, more preferably 2% to 8%, and most preferably 4% to 6%.

Figure 2 schematically illustrates an example of a drying and shrinking process. During the drying and shrinking process, conveyor rollers R1-R6 and guide rollers G1-G4, heated to predetermined temperatures, are used to convey the laminate 50 while drying it. In the illustrated example, the conveyor rollers R1-R6 are positioned to alternately and continuously heat the PVA resin layer and the thermoplastic resin substrate. However, the conveyor rollers R1-R6 can also be positioned to continuously heat only one side of the laminate 50 (e.g., the thermoplastic resin substrate).

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

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

A-3-6. Other Treatments After the water stretching treatment and before the drying and shrinking treatment, a cleaning treatment is preferably performed. This cleaning treatment can typically be performed by immersing the PVA resin layer in an aqueous potassium iodide solution.

A-4. First Protective Layer The thickness of the first protective layer is 10µm or less. A first protective layer thickness of 10µm or less contributes to a thinner polarizer. Conventionally, to protect the polarizer by tracking its shrinkage during heating, a protective layer with a thickness of 20µm or greater has been used. However, the polarizer used in the present embodiment, as described above, has a lower degree of orientation in the PVA-based resin than conventional polarizers. Consequently, shrinkage due to heating is minimized, and even with a protective layer thickness of 10µm or less, cracking during heating can be suppressed.

The thickness of the first protective layer is preferably 7µm or less, more preferably 5µm or less, and even more preferably 3µm or less. For example, the thickness of the first protective layer is 1µm or more.

The first protective layer is composed of a resin film. Any appropriate resin can be used as the resin forming the resin film, depending on the intended purpose. Specific examples include thermoplastic resins such as (meth)acrylic acid resins and cellulose triacetate (TAC); polyester resins, polyurethane resins, polyvinyl alcohol resins, polycarbonate resins, polyamide resins, polyimide resins, polyether sulfone resins, polysulfone resins, polystyrene resins, polynorbornene resins, polyolefin resins, and acetate resins; thermosetting resins or active energy ray-curing resins such as (meth)acrylic acid resins, urethane resins, (meth)acrylic urethane resins, epoxy resins, and polysilicone resins; and glassy polymers such as silicone polymers. In one embodiment, the resin forming the resin film may be at least one resin selected from epoxy resins, (meth)acrylic resins, polyester resins, and urethane resins.

The resin film constituting the first protective layer may be, for example, a molded product of a molten resin, a solidified product of a coating film of a resin solution obtained by dissolving or dispersing the resin in an aqueous solvent or an organic solvent, or a cured product of a curable resin (e.g., a photoion-cured product).

In one embodiment, the first protective layer is composed of at least one selected from the group consisting of a cured product of a coating film of an organic solvent solution of a thermoplastic (meth)acrylic resin (hereinafter, (meth)acrylic resin may be simply referred to as "acrylic resin"), a photo-cured product of an epoxy resin, and a cured product of a coating film of an organic solvent solution of an epoxy resin. This is described in detail below.

A-4-1. Cured Film of Thermoplastic Acrylic Resin in an Organic Solvent Solution In one embodiment, the first protective layer is formed from a cured film of a thermoplastic acrylic resin in an organic solvent solution. From the perspective of wet durability, the softening temperature of the first protective layer in this embodiment is preferably 100°C or higher, preferably 115°C or higher, more preferably 120°C or higher, and particularly preferably 125°C or higher. Furthermore, from the perspective of formability, the softening temperature is preferably 300°C or lower, preferably 250°C or lower, more preferably 200°C or lower, and particularly preferably 160°C or lower.

A-4-1-1. Acrylic Resin The glass transition temperature (Tg) of acrylic resins is preferably above 100°C. As a result, the softening temperature of the first protective layer is also approximately 100°C or above. When the Tg of an acrylic resin is above 100°C, a polarizing plate including a protective layer obtained from such a resin exhibits not only excellent crack resistance but also excellent durability under humidity. The Tg of acrylic resins is preferably above 110°C, more preferably above 115°C, even more preferably above 120°C, and particularly preferably above 125°C. On the other hand, the Tg of acrylic resins is preferably below 300°C, more preferably below 250°C, even more preferably below 200°C, and particularly preferably below 160°C. When the Tg of the acrylic resin is within the above range, the moldability is good.

Any appropriate acrylic resin can be used as long as it has the Tg as described above. The acrylic resin typically contains (meth)acrylic acid alkyl ester as the main component as a monomer unit (repeating unit). In this specification, "(meth)acrylic acid" means acrylic acid and/or methacrylic acid. The (meth)acrylic acid alkyl ester constituting the main skeleton of the acrylic resin can be exemplified by a linear or branched alkyl group with 1 to 18 carbon atoms. These can be used alone or in combination. In addition, any appropriate copolymer monomer can be introduced into the acrylic resin by copolymerization. The repeating unit derived from the (meth)acrylic acid alkyl ester is represented by the following general formula (1):

[Chemical formula 1]

In general formula (1), ^R4 represents a hydrogen atom or a methyl group, and ^R5 represents a hydrogen atom or an optionally substituted aliphatic or alicyclic hydrocarbon group having 1 to 6 carbon atoms. Examples of the substituent include halogens and hydroxyls. Specific examples of the alkyl (meth)acrylate include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, tert-butyl (meth)acrylate, n-hexyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, benzyl (meth)acrylate, dicyclopentyloxyethyl (meth)acrylate, dicyclopentyl (meth)acrylate, chloromethyl (meth)acrylate, 2-chloroethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2,3,4,5,6-pentahydroxyhexyl (meth)acrylate, 2,3,4,5-tetrahydroxypentyl (meth)acrylate, methyl 2-(hydroxymethyl)acrylate, ethyl 2-(hydroxymethyl)acrylate, and methyl 2-(hydroxyethyl)acrylate. In the general formula (1), R ⁵ is preferably a hydrogen atom or a methyl group. Therefore, the particularly preferred alkyl (meth)acrylate is methyl acrylate or methyl methacrylate.

The acrylic resin may contain only a single alkyl (meth)acrylate unit, or may contain a plurality of alkyl (meth)acrylate units in which R ⁴ and R ⁵ in the above general formula (1) are different from each other.

The content of alkyl (meth)acrylate units in acrylic resins should preferably be 50 mol% to 98 mol%, more preferably 55 mol% to 98 mol%, even more preferably 60 mol% to 98 mol%, particularly preferably 65 mol% to 98 mol%, and most preferably 70 mol% to 97 mol%. A content below 50 mol% may not fully realize the benefits of the alkyl (meth)acrylate units (such as high heat resistance and high transparency). A content above 98 mol% may cause the resin to become brittle and easily cracked, preventing it from fully realizing its high mechanical strength, leading to reduced productivity.

The acrylic resin preferably has repeating units containing a ring structure. Examples of repeating units containing a ring structure include lactone ring units, glutaric anhydride units, glutarimide units, maleic anhydride units, and maleimide (N-substituted maleimide) units. The acrylic resin may contain only one repeating unit containing a ring structure, or two or more repeating units containing a ring structure.

The lactone ring unit is preferably represented by the following general formula (2):

[Chemical formula 2] In general formula (2), R ¹ , R ² , and R ³ each independently represent a hydrogen atom or an organic residue having 1 to 20 carbon atoms. Furthermore, the organic residue may also contain an oxygen atom. The acrylic resin may contain only a single lactone ring unit, or may contain a plurality of lactone ring units in which R ¹ , R ² , and R ³ in the general formula (2) are different from each other. Acrylic resins containing lactone ring units are described, for example, in Japanese Patent Publication No. 2008-181078, and this specification incorporates the description of that publication by reference.

The glutarimide unit is preferably represented by the following general formula (3):

[Chemical formula 3]

In the general formula (3), R ¹¹ and R ¹² each independently represent hydrogen or an alkyl group having 1 to 8 carbon atoms, and R ¹³ represents hydrogen, an alkyl group having 1 to 18 carbon atoms, a cycloalkyl group having 3 to 12 carbon atoms, or an aryl group having 6 to 10 carbon atoms. In the general formula (3), preferably, R ¹¹ and R ¹² each independently represent hydrogen or a methyl group, and R ¹³ represents hydrogen, a methyl group, a butyl group, or a cyclohexyl group. More preferably, R ¹¹ represents a methyl group, R ¹² represents hydrogen, and R ¹³ represents a methyl group. The acrylic resin may contain only a single pentylimide unit, or may contain a plurality of pentylimide units in which R ¹¹ , R ¹² , and R ¹³ in the general formula (3) are different from each other. Acrylic resins having glutarimide units are described, for example, in Japanese Patent Application Laid-Open Nos. 2006-309033, 2006-317560, 2006-328334, 2006-337491, 2006-337492, 2006-337493, and 2006-337569, the disclosures of which are incorporated herein by reference. Regarding the glutaric anhydride units, the above description regarding the glutarimide units applies, except that the nitrogen atom substituted by R ¹³ in the above general formula (3) is replaced by an oxygen atom.

The structures of the maleic anhydride unit and the maleimide (N-substituted maleimide) unit can be identified by their names, and thus a detailed description thereof will be omitted.

The content of repeating units containing a ring structure in acrylic resins is preferably 1 mol% to 50 mol%, more preferably 10 mol% to 40 mol%, and even more preferably 20 mol% to 30 mol%. If the content is too low, the Tg may be lower than 100°C, and the resulting protective layer may have insufficient heat resistance, solvent resistance, and surface hardness. If the content is too high, formability and transparency may be insufficient.

The acrylic resin may also contain repeating units other than the (meth)acrylate unit and the repeating unit containing a ring structure. Examples of the repeating units include repeating units derived from vinyl monomers copolymerizable with the monomers constituting the above-mentioned units (other vinyl monomer units). Examples of other vinyl monomers include acrylic acid, methacrylic acid, crotonic acid, 2-(hydroxymethyl)acrylic acid, 2-(hydroxyethyl)acrylic acid, acrylonitrile, methacrylonitrile, ethacrylonitrile, allyl glycidyl ether, maleic anhydride, itaconic anhydride, N-methylmaleimide, N-ethylmaleimide, N-cyclohexylmaleimide, aminoethyl acrylate, propylaminoethyl acrylate, dimethylaminoethyl methacrylate, ethyl methacrylate, Examples of the vinyl monomers include propylamine, cyclohexylaminoethyl methacrylate, N-vinyldiethylamine, N-acetylvinamine, allylamine, methylallylamine, N-methylallylamine, 2-isopropenyl-oxazoline, 2-vinyl-oxazoline, 2-acryloyl-oxazoline, N-phenylmaleimide, phenylaminoethyl methacrylate, styrene, α-methylstyrene, p-epoxypropylstyrene, p-aminostyrene, and 2-phenylvinyl-oxazoline. These monomers may be used alone or in combination. The type, amount, combination, and content ratio of other vinyl monomer units may be appropriately determined according to the intended purpose.

The weight-average molecular weight of the acrylic resin is preferably 1,000-2,000,000, more preferably 5,000-1,000,000, more preferably 10,000-500,000, particularly preferably 50,000-500,000, and most preferably 60,000-150,000. The weight-average molecular weight can be determined, for example, by gel permeation chromatography (GPC system, manufactured by Tosoh Corporation) in terms of polystyrene. Tetrahydrofuran can be used as the solvent.

Acrylic resins can be used by appropriately combining the above-mentioned monomer units and polymerizing them by any appropriate polymerization method.

In the embodiment of the present invention, acrylic resins and other resins can also be used in combination. That is, the monomer components constituting the acrylic resin can be copolymerized with the monomer components constituting the other resins, and the copolymer can be used to form the protective layer described below; or a blend of the acrylic resin and other resins can be used to form the protective layer. Examples of other resins include thermoplastic resins such as styrene resins, polyethylene, polypropylene, polyamide, polyphenylene sulfide, polyetheretherketone, polyester, polysulfone, polyphenylene oxide, polyacetal, polyimide, and polyetherimide. The type and blending amount of the resins used in combination can be appropriately set according to the purpose and the desired properties of the resulting film. For example, a styrene resin (preferably an acrylonitrile-styrene copolymer) can be used as a phase difference control agent.

The acrylic resin content in the first protective layer of this embodiment is preferably 50% to 100% by weight, more preferably 60% to 100% by weight, further preferably 70% to 100% by weight, and particularly preferably 80% to 100% by weight. If the content is less than 50% by weight, the inherent high heat resistance and high transparency of the acrylic resin may not be fully realized.

The first protective layer of this embodiment can be formed by, for example, applying an organic solvent solution of acrylic resin on the surface of the polarizer to form a coating film and curing the coating film.

Any suitable organic solvent that can dissolve or uniformly disperse the acrylic resin can be used as the organic solvent. Specific examples of the organic solvent include ethyl acetate, toluene, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), cyclopentanone, and cyclohexanone.

The acrylic resin concentration in the solution is preferably 3 to 20 parts by weight relative to 100 parts by weight of the solvent. This concentration allows for a uniform coating film that adheres closely to the polarizer.

The solution can be applied to any appropriate substrate or to a polarizer. When applied to a substrate, the cured product of the coating film formed on the substrate is transferred to the polarizer. When applied to a polarizer, the first protective layer is formed directly on the polarizer by drying (curing) the coating film. It is preferred that the solution be applied to the polarizer to form the first protective layer directly on the polarizer. With the above-mentioned structure, the adhesive layer or the pressure-sensitive adhesive layer required for transfer can be omitted, so the polarizer can be made thinner. Any appropriate method can be used for applying the solution. Specific examples include roller coating, spin coating, wire rod coating, dip coating, die coating, curtain coating, spray coating, and scraper coating (such as the notched wheel coating method).

The first protective layer is formed by drying (curing) the applied solution. The drying temperature is preferably below 100°C, more preferably between 50°C and 70°C. A drying temperature within this range prevents adverse effects on the polarizer. The drying time varies depending on the drying temperature. For example, the drying time can be between 1 and 10 minutes.

A-4-2. Photocurable Epoxy Resin In one embodiment, the first protective layer is formed from a photocurable epoxy resin. This protective layer suppresses cracking and achieves excellent moisture durability. As mentioned above, the first protective layer is a photocurable epoxy resin, so the protective layer-forming composition includes a photocatalytic polymerization initiator. A photocatalytic polymerization initiator is a photosensitive agent that functions as a photoacid generator, typically an ionic onium salt composed of a cationic portion and a cationic portion. In the onium salt, the cationic portion absorbs light, while the anionic portion becomes an acid generator. The acid generated from the photocationic polymerization initiator causes ring-opening polymerization of the epoxy groups. The resulting photocationic cured product has a high softening temperature of the first protective layer, which reduces iodine adsorption. This results in a polarizing plate with suppressed cracking and excellent durability under humidification.

From the perspective of humidification durability, the softening temperature of the first protective layer of this embodiment is preferably 100°C or higher, more preferably 110°C or higher, more preferably 120°C or higher, and particularly preferably 125°C or higher. From the perspective of formability, it is preferably 300°C or lower, more preferably 250°C or lower, more preferably 200°C or lower, and particularly preferably 160°C or lower.

A-4-2-1. Epoxy Resin Any suitable epoxy resin can be used, and epoxy resins having aromatic or aliphatic rings are particularly preferred. In this embodiment, an epoxy resin having at least one selected from the group consisting of an aromatic skeleton and a hydrogenated aromatic skeleton is preferably used. Examples of aromatic skeletons include benzene, naphthalene, and fluorene. The epoxy resin may be used singly or in combination of two or more. An epoxy resin having a biphenyl or bisphenol skeleton as the aromatic skeleton, or a hydrogenated epoxy resin thereof, is particularly preferred. The use of such epoxy resins provides a polarizing plate having superior durability and flexibility. The following is a detailed description of an epoxy resin having a biphenyl skeleton as a representative example.

In one embodiment, the epoxy resin having a biphenyl skeleton includes an epoxy resin having the following structure. The epoxy resin having a biphenyl skeleton may be used alone or in combination of two or more. [Chemical Formula 4] (wherein, R ¹⁴ to R ²¹ independently represent a hydrogen atom, a linear or branched substituted or unsubstituted alkyl group having 1 to 12 carbon atoms, or a halogen element).

R ¹⁴ to R ²¹ each independently represent a hydrogen atom, a linear or branched substituted or unsubstituted alkyl group having 1 to 12 carbon atoms, or a halogen element. Examples of the linear or branched substituted or unsubstituted alkyl group having 1 to 12 carbon atoms include methyl, ethyl, n-propyl, isopropyl, n-butyl, dibutyl, tertiary butyl, n-pentyl, isopentyl, neopentyl, tertiary pentyl, cyclopentyl, n-hexyl, isohexyl, cyclohexyl, n-heptyl, cycloheptyl, methylcyclohexyl, n-octyl, cyclooctyl, n-nonyl, 3,3,5-trimethylcyclohexyl, n-decyl, cyclodecyl, n-undecyl, n-dodecyl, cyclododecyl, phenyl, benzyl, methylbenzyl, dimethylbenzyl, trimethylbenzyl, naphthylmethyl, phenethyl, and 2-phenylisopropyl. The linear or branched substituted or unsubstituted alkyl group with 1 to 12 carbon atoms is preferably an alkyl group with 1 to 4 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, or n-butyl. Halogen elements may include fluorine and bromine.

In one embodiment, the epoxy resin having a biphenyl skeleton is an epoxy resin represented by the following formula: [Chemical Formula 5] (wherein, R ¹⁴ to R ²¹ are as described above, and n represents an integer from 0 to 6).

In one embodiment, the epoxy resin having a biphenyl skeleton is an epoxy resin having only a biphenyl skeleton. By using an epoxy resin having only a biphenyl skeleton, the durability of the resulting protective layer can be further improved.

In one embodiment, the epoxy resin having a biphenyl backbone may also contain chemical structures other than the biphenyl backbone. Examples of chemical structures other than the biphenyl backbone include a bisphenol backbone, an alicyclic structure, and an aromatic ring structure. In this embodiment, the ratio (molar ratio) of chemical structures other than the biphenyl backbone is preferably less than that of the biphenyl backbone.

Commercially available epoxy resins having a biphenyl skeleton may also be used. Examples of commercially available epoxy resins include those manufactured by Mitsubishi Chemical Co., under the trade names jER YX4000, jER YX4000H, jER YL6121, jER YL664, jER YL6677, jER YL6810, and jER YL7399.

The above-mentioned epoxy resin (epoxy resin after photoion curing) preferably has a glass transition temperature (Tg) of 100°C or higher. As a result, the softening temperature of the protective layer is also almost above 100°C. If the Tg of the epoxy resin is above 100°C, the polarizing plate including the obtained protective layer tends to have excellent durability. The Tg of the epoxy resin is preferably above 110°C, more preferably above 120°C, and particularly preferably above 125°C. On the other hand, the Tg of the epoxy resin is preferably below 300°C, more preferably below 250°C, more preferably below 200°C, and particularly preferably below 160°C. If the Tg of the epoxy resin is within the above range, the formability can be excellent.

The epoxy equivalent weight of the epoxy resin is preferably 100 g/equivalent or greater, more preferably 150 g/equivalent or greater, and even more preferably 200 g/equivalent or greater. Furthermore, the epoxy equivalent weight of the epoxy resin is preferably 3000 g/equivalent or less, more preferably 2500 g/equivalent or less, and even more preferably 2000 g/equivalent or less. By keeping the epoxy equivalent weight within this range, a more stable protective layer (a fully cured protective layer with minimal residual monomers) can be obtained. In this specification, "epoxy equivalent weight" refers to "the mass of the epoxy resin containing one equivalent of epoxy groups" and can be measured in accordance with JIS K7236.

In this embodiment, the above-mentioned epoxy resin can also be used in combination with other resins. That is, a blend of the above-mentioned epoxy resin (e.g., an epoxy resin having at least one selected from the group consisting of an aromatic skeleton and a hydrogenated aromatic skeleton) and other resins can also be used to form the protective layer. Other resins include thermoplastic resins such as styrene resins, polyethylene, polypropylene, polyamide, polyphenylene sulfide, polyetheretherketone, polyester, polysulfone, polyphenylene oxide, polyacetal, polyimide, and polyetherimide, as well as hardening resins such as acrylic resins and cyclohexane resins. Acrylic resins and cyclohexane resins are preferably used. The type and amount of the resin to be used can be appropriately determined according to the purpose and the desired properties of the resulting film. For example, a styrene-based resin can be used as a phase difference control agent.

Any suitable (meth)acrylic compound can be used as the acrylic resin. Examples of the (meth)acrylic compound include a (meth)acrylic compound having one (meth)acrylic group in the molecule (hereinafter referred to as a "monofunctional (meth)acrylic compound") and a (meth)acrylic compound having two or more (meth)acrylic groups in the molecule (hereinafter referred to as a "polyfunctional (meth)acrylic compound"). These (meth)acrylic compounds can be used alone or in combination of two or more. These acrylic resins are described, for example, in Japanese Patent Publication No. 2019-168500. The entire disclosure of this publication is incorporated herein by reference.

Any suitable compound having one or more cyclohexyl groups in the molecule can be used as the cyclohexyloxybutane resin. Examples thereof include 3-ethyl-3-hydroxymethylcyclohexyloxybutane, 3-ethyl-3-(2-ethylhexyloxymethyl)cyclohexyloxybutane, 3-ethyl-3-(phenoxymethyl)cyclohexyloxybutane, 3-ethyl-3-(cyclohexyloxymethyl)cyclohexyloxybutane, 3-ethyl-3-(oxiranylmethoxy)cyclohexyloxybutane, and (meth)acrylate (3-ethylcyclohexyl-3-yl)methyl ester. Oxycyclobutane compounds containing one oxocyclobutane group; oxocyclobutane compounds containing two or more oxocyclobutane groups in the molecule, such as 3-ethyl-3{[(3-ethyloxocyclobutane-3-yl)methoxy]methyl}oxocyclobutane, 1,4-bis[(3-ethyl-3-oxocyclobutane)methoxymethyl]benzene, and 4,4'-bis[(3-ethyl-3-oxocyclobutane)methoxymethyl]biphenyl. These oxocyclobutane resins may be used alone or in combination of two or more.

Preferred cyclohexane resins include 3-ethyl-3-hydroxymethylcyclohexane, 1,4-bis[(3-ethyl-3-cyclohexane)methoxymethyl]benzene, 3-ethyl-3-(2-ethylhexyloxymethyl)cyclohexane, 3-ethyl-3-(oxiranylmethoxy)cyclohexane, (3-ethylcyclohexane-3-yl)methyl (meth)acrylate, and 3-ethyl-3{[(3-ethylcyclohexane-3-yl)methoxy]methyl}cyclohexane. These cyclohexane resins are readily available and have excellent diluting properties (low viscosity) and compatibility.

In one embodiment, from the perspective of compatibility and adhesion, an oxycyclobutane resin having a molecular weight of 500 or less and being liquid at room temperature (25°C) is preferably used. In one embodiment, an oxycyclobutane compound containing two or more oxycyclobutane groups in the molecule, or an oxycyclobutane compound containing one oxycyclobutane group and one (meth)acryloyl group or one epoxy group in the molecule is preferably used. 3-Ethyl-3-{[(3-ethyloxycyclobutane-3-yl)methoxy]methyl}oxycyclobutane, 3-Ethyl-3-(oxiranylmethoxy)oxycyclobutane, and (3-ethyloxycyclobutane-3-yl)methyl (meth)acrylate are more preferably used. By using these cyclohexane oxychloride resins, the hardening and durability of the protective layer can be improved.

Commercially available oxycyclobutane resins can also be used. Specifically, ARON OXETANE OXT-101, ARON OXETANE OXT-121, ARON OXETANE OXT-212, and ARON OXETANE OXT-221 (all manufactured by Toagosei Co., Ltd.) can be used. Preferably, ARON OXETANE OXT-101 and ARON OXETANE OXT-221 can be used.

In the first protective layer of this embodiment, the content of the epoxy resin (e.g., an epoxy resin having at least one selected from the group consisting of an aromatic skeleton and a hydrogenated aromatic skeleton) is preferably 50% to 100% by weight, more preferably 60% to 100% by weight, more preferably 70% to 100% by weight, and particularly preferably 80% to 100% by weight. If the content is less than 50% by weight, the protective layer may not exhibit sufficient heat resistance and adhesion to the polarizer.

When using an epoxy resin with a biphenyl or bisphenol backbone and a cyclohexane oxybutane resin together, the cyclohexane oxybutane resin content is preferably 1 to 50 parts by weight, more preferably 5 to 45 parts by weight, and even more preferably 10 to 40 parts by weight, per 100 parts by weight of the epoxy resin and cyclohexane oxybutane resin combined. This range improves curability and enhances adhesion between the protective layer and the polarizer.

A-4-2-2. Photocationic Polymerization Initiators Photocationic polymerization initiators are photosensitive agents that function as photoacid generators. Representative examples include ionic onium salts composed of a cationic portion and an anionic portion. In these onium salts, the cationic portion absorbs light, while the anionic portion serves as a source of acid. The acid generated by the photocationic polymerization initiator causes ring-opening polymerization of epoxy groups. Any suitable compound that can cure epoxy resins having at least one selected from the group consisting of aromatic skeletons and hydrogenated aromatic skeletons upon irradiation with light, such as ultraviolet light, can be used as the photocationic polymerization initiator. The photocatalytic polymerization initiator may be used alone or in combination of two or more.

Examples of the photocatalytic polymerization initiator include triphenylcorbium hexafluoroantimonylate, triphenylcorbium hexafluorophosphate, p-(phenylthio)phenyldiphenylcorbium hexafluoroantimonylate, p-(phenylthio)phenyldiphenylcorbium hexafluoroantimonylate, 4-chlorophenyldiphenylcorbium hexafluorophosphate, 4-chlorophenyldiphenylcorbium hexafluoroantimonylate, bis[4-(diphenylcorbyl)phenyl]sulfide bishexafluorophosphate, bis[4-(diphenylcorbyl)phenyl]sulfide bishexafluoroantimonylate, (2,4-cyclopentadien-1-yl)[(1-methylethyl)benzene]-Fe-hexafluorophosphate, and diphenyliodonium hexafluoroantimonylate. Preferred photocatalytic polymerization initiators are triphenylsulfonium hexafluoroantimonate salts and diphenyliodonium hexafluoroantimonate salts.

Commercially available photocatalytic polymerization initiators may also be used. Examples include triphenylstemium salt-based hexafluoroantimony type SP-170 (manufactured by ADEKA), CPI-101A (manufactured by SAN-APRO), WPAG-1056 (manufactured by Wako Junyaku Industries), and diphenyliodonium salt-based hexafluoroantimony type WPI-116 (manufactured by Wako Junyaku Industries).

The amount of the photocatalytic polymerization initiator is preferably 0.1 to 3 parts by weight, more preferably 0.25 to 2 parts by weight, per 100 parts by weight of the epoxy resin (e.g., an epoxy resin having at least one selected from the group consisting of an aromatic skeleton and a hydrogenated aromatic skeleton). If the amount of the photocatalytic polymerization initiator is less than 0.1 parts by weight, the resin may not be sufficiently cured even after exposure to light (ultraviolet light).

The first protective layer of this embodiment can be formed, for example, by applying a composition comprising the above-mentioned epoxy resin and a photocatalytic polymerization initiator to form a coating film, and then irradiating the coating film with light (e.g., ultraviolet rays).

The solvent contained in the above composition can be any suitable solvent that can dissolve or uniformly disperse the epoxy resin and the hardener. Specific examples of the solvent include ethyl acetate, toluene, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), cyclopentanone, and cyclohexanone.

The epoxy resin concentration in the above composition is preferably 10 to 30 parts by weight relative to 100 parts by weight of the solvent. This resin concentration allows for a uniform coating film that adheres closely to the polarizer.

The above-mentioned composition can be applied to any appropriate substrate or to a polarizer. When applied to a substrate, the cured coating formed on the substrate is transferred to the polarizer. When applied to a polarizer, the coating is cured, for example, by irradiation with light, thereby forming a first protective layer directly on the polarizer. It is preferable to apply the above-mentioned composition to the polarizer and form the first protective layer directly on the polarizer. If the above-mentioned structure is adopted, the adhesive layer or the adhesive layer required for transfer can be omitted, so the polarizer can be made thinner. The method of applying the composition is as described above.

When curing the coating by light irradiation, the coating can be irradiated with light (typically, ultraviolet light) using any appropriate light source at any appropriate irradiation dose. Examples of ultraviolet light sources include low-pressure mercury lamps, high-pressure mercury lamps, ultra-high-pressure mercury lamps, electrodeless lamps, halogen lamps, carbon arc lamps, xenon lamps, metal halogen lamps, chemical lamps, black light lamps, and LED lamps. The ultraviolet irradiation dose is, for example, 2 mJ/ ^cm² to 3000 mJ/ ^cm² , preferably 10 mJ/ ^cm² to 2000 mJ/ ^cm² . Specifically, when using a high-pressure mercury lamp as the light source, the irradiation dose is generally between 5mJ/ ^cm2 and 3000mJ/ ^cm2 , preferably between 50mJ/ ^cm2 and 2000mJ/ ^cm2 . When using an electrodeless lamp as the light source, the irradiation dose is generally between 2mJ/ ^cm2 and 2000mJ/ ^cm2 , preferably between 10mJ/ ^cm2 and 1000mJ/ ^cm2 .

The irradiation time can be set to any appropriate value depending on the type of light source, the distance between the light source and the coated surface, the coating thickness, and other conditions. Irradiation times typically range from several seconds to several tens of seconds, but can also be as short as a fraction of a second. Light can be applied from any appropriate direction. To prevent uneven curing, irradiation is preferably applied from the side of the coated surface of the protective layer-forming composition.

After exposure to light such as ultraviolet radiation, a heat treatment may be further performed to complete the curing process of the photoreaction. The heat treatment may be performed at any appropriate temperature and for any appropriate time. The heating temperature is, for example, 80°C to 250°C, preferably 100°C to 150°C. The heating time is, for example, 10 seconds to 2 hours, preferably 5 minutes to 1 hour.

A-4-3. Cured Film of an Epoxy Resin Organic Solvent Solution In one embodiment, the first protective layer is formed from a cured film of an epoxy resin organic solvent solution. From the perspective of wet durability, the softening temperature of the first protective layer in this embodiment is preferably 100°C or higher, preferably 110°C or higher, more preferably 120°C or higher, and particularly preferably 125°C or higher. Furthermore, from the perspective of formability, the softening temperature is preferably 300°C or lower, preferably 250°C or lower, more preferably 200°C or lower, and particularly preferably 160°C or lower.

A-4-3-1. Epoxy Resin In this embodiment, the epoxy resin preferably has a glass transition temperature (Tg) of 100°C or higher. As a result, the softening temperature of the protective layer is also approximately 100°C or higher. When the Tg of the epoxy resin is above 100°C, polarizing plates containing a protective layer made of such a resin tend to have excellent durability. The Tg of the epoxy resin is preferably 110°C or higher, more preferably 120°C or higher, and particularly preferably 125°C or higher. On the other hand, the Tg of the epoxy resin is preferably 300°C or lower, more preferably 250°C or lower, more preferably 200°C or lower, and particularly preferably 160°C or lower. If the Tg of epoxy resin is within the above range, the moldability can be excellent.

Any appropriate epoxy resin may be used as long as it has the Tg described above. Epoxy resins typically refer to resins having an epoxy group in their molecular structure. Preferred epoxy resins include those having an aromatic ring in their molecular structure. By using epoxy resins having an aromatic ring, an epoxy resin having a higher Tg can be obtained. Examples of aromatic rings in epoxy resins having an aromatic ring in their molecular structure include a benzene ring, a naphthalene ring, and a fluorene ring. Epoxy resins may be used alone or in combination of two or more. When using two or more epoxy resins, you can also use a combination of epoxy resins containing aromatic epoxy and epoxy resins that do not contain aromatic epoxy.

Epoxy resins with aromatic rings in their molecular structure can be specifically listed as: bisphenol A diglycidyl ether type epoxy resin, bisphenol F diglycidyl ether type epoxy resin, bisphenol S diglycidyl ether type epoxy resin, resorcinol diglycidyl ether type epoxy resin, Hydroquinone diglycidyl ether type epoxy resin, terephthalate diglycidyl ester type epoxy resin, diphenoxyethanol fluorene diglycidyl ether type epoxy resin, bisphenol fluorene diglycidyl ether type epoxy resin, biscresol fluorene diglycidyl ether type epoxy resin, etc. have two Epoxy resins containing epoxy groups; epoxy resins having three epoxy groups, such as phenolic epoxy resins, N,N,O-triglycyrrhizic-p- or -m-aminophenol epoxy resins, N,N,O-triglycyrrhizic-4-amino-m- or -5-amino-o-cresol epoxy resins, and 1,1,1-(triglycyrrhizic-propyloxyphenyl)methane epoxy resins; epoxy resins having four epoxy groups, such as glycyrrhizic-amine epoxy resins (e.g., diaminodiphenylmethane type, diaminodiphenylsulfonium type, and interrupted diamine type). Furthermore, hexahydrophthalic anhydride type epoxy resin, tetrahydrophthalic anhydride type epoxy resin, dialcic acid type epoxy resin, p-oxybenzoic acid type epoxy resin, or other glycidyl ester type epoxy resin can also be used.

The weight average molecular weight of the epoxy resin is preferably 1,000 to 2,000,000, more preferably 5,000 to 1,000,000, more preferably 10,000 to 500,000, particularly preferably 50,000 to 500,000, and most preferably 60,000 to 150,000. The weight average molecular weight can be determined, for example, by gel permeation chromatography (GPC system, manufactured by Tosoh Corporation) in terms of polystyrene. Tetrahydrofuran can be used as the solvent.

The epoxy equivalent weight of the epoxy resin is preferably 1000 g/equivalent or greater, more preferably 3000 g/equivalent or greater, and even more preferably 5000 g/equivalent or greater. Furthermore, the epoxy equivalent weight of the epoxy resin is preferably 30,000 g/equivalent or less, more preferably 25,000 g/equivalent or less, and even more preferably 20,000 g/equivalent or less. By keeping the epoxy equivalent weight within this range, a more stable protective layer can be obtained. In this specification, "epoxy equivalent weight" means "the mass of the epoxy resin containing one equivalent of epoxy groups" and can be measured in accordance with JIS K7236.

In this embodiment, epoxy resins can also be used in combination with other resins. That is, a blend of epoxy resins and other resins can also be used to form the protective layer. Other resins include thermoplastic resins such as styrene-based resins, polyethylene, polypropylene, polyamide, polyphenylene sulfide, polyetheretherketone, polyester, polysulfone, polyphenylene oxide, polyacetal, polyimide, and polyetherimide. The type and blending amount of the resin used in combination can be appropriately set according to the purpose and the desired properties of the resulting film. For example, a styrene-based resin can be used in combination as a phase difference control agent.

The epoxy resin content in the first protective layer of this embodiment is preferably 50% to 100% by weight, more preferably 60% to 100% by weight, more preferably 70% to 100% by weight, and even more preferably 80% to 100% by weight. If the content is less than 50% by weight, the protective layer may not provide adequate heat resistance and adhesion to the polarizer.

The first protective layer of this embodiment can be formed, for example, by applying an organic solvent solution containing the aforementioned epoxy resin to form a coating film, and then curing the coating film. The epoxy resin concentration in the organic solvent solution is preferably 3 to 20 parts by weight per 100 parts by weight of the solvent. This resin concentration allows for a uniform coating film that adheres closely to the polarizer.

The organic solvent may be any suitable solvent that can dissolve or uniformly disperse the epoxy resin. Specific examples of the solvent include ethyl acetate, toluene, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), cyclopentanone, and cyclohexanone.

The solution can be applied to any appropriate substrate or to a polarizer. When applied to a substrate, the cured product of the coating film formed on the substrate is transferred to the polarizer. When applied to a polarizer, the first protective layer is formed directly on the polarizer by drying (curing) the coating film. It is preferable to apply the solution to the polarizer and form the first protective layer directly on the polarizer. If the structure is as described above, the adhesive layer or the adhesive layer required for transfer can be omitted, so the polarizer can be made thinner. The method of applying the solution is as described above.

By drying (curing) the solution coating, a protective layer is formed as a cured product of the coating. The drying temperature is preferably below 100°C, more preferably between 50°C and 70°C. A drying temperature within this range prevents adverse effects on the polarizer. The drying time varies depending on the drying temperature. For example, the drying time can be between 1 and 10 minutes.

A-4-4. Composition and Properties of the Protective Layer In one embodiment, the first protective layer, as described above, is composed of at least one selected from the group consisting of a cured product obtained from a coating film of a thermoplastic acrylic resin in an organic solvent solution, a photocured epoxy resin, and a cured product obtained from a coating film of an organic solvent solution of an epoxy resin. This protective layer can be made significantly thinner than an extruded film. As described above, the thickness of the first protective layer is 10µm or less. Furthermore, although the theoretical reasons are still unclear, this protective layer shrinks less during film formation than cured products of other thermosetting resins or active energy ray-curing resins (such as UV-curing resins), and contains no residual monomers. This has the advantage of suppressing degradation of the film itself and preventing residual monomers from adversely affecting the polarizing plate (polarizer). Furthermore, its hygroscopicity and permeability are lower than those of cured products of aqueous coatings such as aqueous solutions or dispersions, resulting in excellent humidification resistance. As a result, a highly durable polarizing plate can be achieved, maintaining its optical properties even in heated and humidified environments.

The first protective layer is preferably substantially optically isotropic. In this specification, "substantially optically isotropic" means that the in-plane phase difference Re(550) is 0nm~10nm and the phase difference Rth(550) in the thickness direction is -20nm~+10nm. The in-plane phase difference Re(550) is preferably 0nm~5nm, more preferably 0nm~3nm, and particularly preferably 0nm~2nm. The phase difference Rth(550) in the thickness direction is preferably -5nm~5nm, more preferably -3nm~3nm, and particularly preferably -2nm~2nm. If the Re(550) and Rth(550) of the first protective layer are within the above ranges, when the polarizing plate including the protective layer is applied to an image display device, it is possible to prevent adverse effects on the display characteristics.

The first protective layer preferably has a light transmittance of 3µm at 380nm. Specifically, the light transmittance is preferably 85% or higher, more preferably 88% or higher, and even more preferably 90% or higher. Light transmittance within this range ensures the desired transparency. Light transmittance can be measured, for example, according to ASTM D-1003.

The lower the haze of the first protective layer, the better. Specifically, the haze is preferably below 5%, more preferably below 3%, even more preferably below 1.5%, and most preferably below 1%. A haze below 5% imparts excellent transparency to the film. Furthermore, even when used as a side-viewing polarizer in an image display device, the displayed content remains clearly visible.

The YI of the first protective layer at a thickness of 3µm is preferably 1.27 or less, preferably 1.25 or less, more preferably 1.23 or less, and even more preferably 1.20 or less. A YI greater than 1.3 may result in insufficient optical transparency. Alternatively, the YI can be calculated using the following formula from the tristimulus values (X, Y, Z) measured using a high-speed integrating sphere spectroscopic transmittance meter (trade name DOT-3C, manufactured by Murakami Color Technology Laboratory): YI = [(1.28X - 1.06Z) / Y] × 100

The b-value (a hue scale based on the Hunter colorimetric system) of the first protective layer at a thickness of 3 μm is preferably less than 1.5, and preferably less than 1.0. A b-value exceeding 1.5 may result in an undesirable color tone. The b-value can be obtained, for example, by cutting a sample of the film constituting the protective layer into a 3 cm square and measuring the hue using a high-speed integrating sphere spectroscopic transmittance meter (trade name DOT-3C, manufactured by Murakami Color Technology Laboratory) and evaluating the hue according to the Hunter colorimetric system.

The iodine adsorption amount of the first protective layer is preferably 25 wt% or less, more preferably 10 wt% or less, more preferably 6.0 wt% or less, and particularly preferably 3.0 wt% or less. The smaller the iodine adsorption amount, the better. If the iodine adsorption amount is within the above range, a polarizing plate with better durability can be obtained. The iodine adsorption amount can be measured by the following method. A protective layer (thickness 3µm) is formed on a substrate (PET film) to obtain a PET film with a protective layer. The obtained PET film with a protective layer is cut into 1 cm×1 cm (1 ^cm2 ) to make a sample, which is taken into a topside vial (20 mL capacity) and weighed. Then, a screw-capped bottle (1.5 mL capacity) containing 1 mL of iodine solution is also placed in the topside vial and tightly capped. Afterwards, the empty vial was placed in a desiccator at 65°C and heated for 6 hours, so that the gaseous _I2 would be adsorbed on the sample. Afterwards, the sample was collected into a ceramic boat and burned using an automatic sample combustion device, and the generated gas was collected into 10 mL of absorption liquid. After collection, the absorption liquid was adjusted to 15 mL with pure water, and IC quantitative analysis was performed on the original solution or the appropriately diluted liquid. In addition, the iodine adsorption amount when the same measurement was performed only on PET film was almost 0. Based on the iodine weight obtained by IC quantitative analysis and the weight of the protective layer monomer ("weight of PET film with protective layer" - "weight of PET film"), the iodine adsorption amount (weight %) was calculated using the following formula. Iodine adsorption amount (weight %) = iodine weight obtained by IC quantitative analysis / weight of protective layer monomer × 100. The following measuring devices can be used for analysis, for example. [Measuring device] Automatic sample combustion device: Mitsubishi Chemical Analytech, "AQF-2100H" IC (anion): Thermo Fisher Scientific, "ICS-3000"

The first protective layer (e.g., a cured product of a coating film or a photo-cured product) may contain any appropriate additives according to the purpose. Specific examples of additives include: UV absorbers; levelers; antioxidants such as hindered phenols, phosphorus, and sulfur; stabilizers such as light stabilizers, weather stabilizers, and heat stabilizers; reinforcing materials such as glass fiber and carbon fiber; near-infrared absorbers; tris(dibromopropyl) phosphate, triallyl phosphate, and the like. Flame retardants such as esters and antimony oxide; antistatic agents such as anionic, cationic, and nonionic surfactants; colorants such as inorganic pigments, organic pigments, and dyes; organic or inorganic fillers; resin modifiers; organic or inorganic fillers; plasticizers; lubricants; antistatic agents; and flame retardants. Additives can be added during resin polymerization or during the formation of the protective layer. The type, amount, combination, and dosage of additives can be appropriately determined based on the intended purpose.

A-5. Second Protective Layer In one embodiment, the second protective layer is formed from any suitable film that can be used as a protective layer for a polarizer. Specific examples of the material constituting the main component of this film include cellulose resins such as triacetate cellulose (TAC), or transparent resins such as polyesters, polyvinyl alcohols, polycarbonates, polyamides, polyimides, polyether sulfones, polysulfones, polystyrenes, polynorbornenes, polyolefins, (meth)acrylics, and acetates. Other examples include thermosetting or UV-curing resins such as (meth)acrylics, urethanes, (meth)acrylic urethanes, epoxies, and silicones. Other examples include glassy polymers such as silicone polymers. The thickness of the second protective layer in this embodiment is preferably 5µm to 80µm, more preferably 5µm to 40µm, and even more preferably 5µm to 25µm.

In another embodiment, the second protective layer can be a cured product of a coating of a resin solution, or a cured product of a curable resin (e.g., a photo-curable product). The same description as for the first protective layer applies to the second protective layer of this embodiment.

A-6. Other Layers A highly adhesive layer may be formed on the polarizer side of the protective layer (typically, the first protective layer). The adhesive layer may, for example, comprise a water-based polyurethane and an oxazoline-based crosslinker. Forming this adhesive layer improves the adhesion between the protective layer and the polarizer. Furthermore, a hard coat layer may be formed on the side of the protective layer (typically, the first protective layer) opposite the polarizer. The hard coat layer may be formed such that the combined thickness of the protective layer (e.g., a cured coating) and the hard coat layer is no greater than 10µm, preferably no greater than 7µm, and more preferably no greater than 5µm. The hard coat layer can be formed on the visible side of the protective layer. When both the easy-adhesion layer and the hard coat layer are formed, they can be formed on different sides of the protective layer.

B. Polarizing Plate with Retardation Layer B-1. Overall Structure of Polarizing Plate with Retardation Layer Figure 3 is a schematic cross-sectional view of a polarizing plate with retardation layer according to one embodiment of the present invention. The polarizing plate with retardation layer 200a of this embodiment comprises a polarizing plate 100 and a retardation layer 110. The polarizing plate 100 comprises a polarizer 10 and a first protective layer 20 disposed on one side thereof. The retardation layer 110 is disposed on the side of the polarizer 10 opposite to the side on which the first protective layer 20 is disposed. In the polarizing plate with retardation layer 200a, the retardation layer 110 also functions as a protective layer for the polarizer 10. The phase shift layer 110 is typically laminated onto the polarizing plate 100 via a bonding layer (not shown). The bonding layer is a bonding agent layer or adhesive layer, and an adhesive layer (e.g., an acrylic adhesive layer) is preferred for workability. Although not shown, the polarizing plate 100 may also include a second protective layer on the phase shift layer 110 side of the polarizer 10 as needed.

As shown in Figure 4 , another embodiment of a polarizing plate with a phase shift layer 200b may also include another phase shift layer 120 and/or a conductive layer or an isotropic substrate with a conductive layer 130. The phase shift layer 120 and the conductive layer or the isotropic substrate with a conductive layer 130 may be positioned outside the phase shift layer 110 (on the side opposite the polarizing plate 100). The phase shift layer 120 exhibits a refractive index characteristic that satisfies the relationship nz>nx=ny. The phase shift layer 120 and the conductive layer or the isotropic substrate with a conductive layer 130 are positioned sequentially, starting from the phase shift layer 110 side. The other phase difference layer 120 and the conductive layer or the isotropic substrate 130 with a conductive layer represent arbitrary layers that can be provided as needed, and either or both can be omitted. For convenience, the phase difference layer 110 is sometimes referred to as the first phase difference layer, and the other phase difference layer 120 is sometimes referred to as the second phase difference layer. Furthermore, when a conductive layer or an isotropic substrate with a conductive layer is provided, the polarizer with a phase difference layer can be used in a so-called touch-sensitive panel input display device, in which a touch sensor is integrated between an image display unit (e.g., an organic EL unit) and the polarizer.

In the embodiment of the present invention, the Re(550) of the first retardation layer 110 is preferably 100 nm to 190 nm, and the Re(450)/Re(550) is preferably greater than 0.8 and less than 1. Furthermore, the angle formed between the slow axis of the first retardation layer 110 and the absorption axis of the polarizer 10 is typically 40° to 50°.

The above embodiments may be combined as appropriate, and modifications apparent in the industry may be incorporated into the components of the above embodiments. For example, the configuration of the isotropic substrate 130 with a conductive layer disposed outside the second retardation layer 120 may be replaced with an optically equivalent configuration (e.g., a laminate of the second retardation layer and the conductive layer).

The polarizing plate with a retardation layer of the present invention may further include other retardation layers. The optical properties (e.g., refractive index, in-plane retardation, Nz coefficient, photoelastic coefficient), thickness, and placement of the other retardation layers can be appropriately configured according to the intended purpose.

B-2. Polarizing Plate Polarizing plate 100 is the polarizing plate described in section A.

B-3. First Retardation Layer The first retardation layer 110 can have any appropriate optical and/or mechanical properties depending on the intended purpose. The first retardation layer typically has a slow axis. In one embodiment, the angle θ formed between the slow axis of the first retardation layer and the absorption axis of the polarizer 10 is, as described above, 40° to 50°, preferably 42° to 48°, and more preferably approximately 45°. If the angle θ is within this range, by forming the first retardation layer into a λ/4 plate, as described later, a polarizing plate with a retardation layer having excellent circular polarization properties (resulting in excellent anti-reflection properties) can be obtained.

The first phase difference layer preferably exhibits a refractive index characteristic that satisfies the relationship nx>ny≧nz. The first phase difference layer is typically provided to impart antireflection properties to the polarizing plate, and in one embodiment, it can function as a λ/4 plate. In this case, the in-plane phase difference Re(550) of the first phase difference layer is preferably 100nm to 190nm, more preferably 110nm to 170nm, and even more preferably 130nm to 160nm. Furthermore, "ny=nz" here encompasses not only the case where ny and nz are completely identical, but also the case where they are substantially identical. Therefore, a situation where ny<nz is possible without compromising the effects of the present invention.

The Nz coefficient of the first retardation layer is preferably 0.9-3, more preferably 0.9-2.5, further preferably 0.9-1.5, and particularly preferably 0.9-1.3. By satisfying these conditions, the resulting polarizing plate with a retardation layer can achieve excellent reflected hue when used in an image display device.

The first phase difference layer can exhibit an inverse dispersion wavelength characteristic in which the phase difference value increases with the wavelength of the measurement light, a positive wavelength dispersion characteristic in which the phase difference value decreases with the wavelength of the measurement light, or a flat wavelength dispersion characteristic in which the phase difference value hardly changes with the wavelength of the measurement light. In one embodiment, the first phase difference layer exhibits an inverse dispersion wavelength characteristic. In this case, the Re(450)/Re(550) ratio of the phase difference layer is preferably 0.8 or more and less than 1, more preferably 0.8 or more and 0.95 or less. With such a configuration, very excellent anti-reflection properties can be achieved.

The first retardation layer preferably comprises a resin having an absolute photoelastic coefficient of 2× ^{10-11 m2} /N or less, more preferably between 2.0× ^{10-13 m2} /N and 1.5 ^{×10-11 m2} /N, and even more preferably between 1.0× ^{10-12 m2} /N and 1.2 ^{×10-11 m2} /N. When the absolute photoelastic coefficient falls within this range, retardation changes due to shrinkage stress during heating are less likely to occur. As a result, thermal nonuniformity in the resulting image display device can be effectively prevented.

The first phase difference layer is typically formed of a stretched film of a resin film. In one embodiment, the thickness of the first phase difference layer is preferably 70µm or less, more preferably 45µm to 60µm. If the thickness of the first phase difference layer is within the above range, curling during heating can be effectively suppressed, and curling during bonding can be effectively adjusted. Furthermore, as described later, in an embodiment in which the first phase difference layer is formed of a polycarbonate resin film, the thickness of the first phase difference layer is preferably 40μm or less, more preferably 10μm to 40μm, and more preferably 20μm to 30μm. By being formed of a polycarbonate resin film having the above thickness, the first phase difference layer can suppress curling and help improve anti-bending durability and reflective hue.

The first retardation layer can be formed from any suitable resin film that satisfies the aforementioned properties. Representative examples of such resins include polycarbonate resins, polyester carbonate resins, polyester resins, polyvinyl acetal resins, polyarylate resins, cycloolefin resins, cellulose resins, polyvinyl alcohol resins, polyamide resins, polyimide resins, polyether resins, polystyrene resins, and acrylic resins. These resins can be used alone or in combination (e.g., by blending or copolymerization). When the first retardation layer is formed of a resin film exhibiting reverse dispersion wavelength characteristics, a polycarbonate resin or a polyester carbonate resin (hereinafter sometimes simply referred to as a polycarbonate resin) can be suitably used.

Any appropriate polycarbonate resin can be used as the polycarbonate resin as long as the effects of the present invention are achieved. For example, the polycarbonate resin comprises structural units derived from a fluorene-based dihydroxy compound, structural units derived from an isosorbide-based dihydroxy compound, and structural units derived from at least one dihydroxy compound selected from the group consisting of alicyclic diols, alicyclic dimethanols, di-, tri- or polyethylene glycols, and alkylene glycols or spiroglycerol. The polycarbonate resin preferably includes structural units derived from fluorene-based dihydroxy compounds, structural units derived from isosorbide-based dihydroxy compounds, structural units derived from alicyclic dimethanols, and/or structural units derived from di-, tri-, or polyethylene glycol. More preferably, the polycarbonate resin includes structural units derived from fluorene-based dihydroxy compounds, structural units derived from isosorbide-based dihydroxy compounds, and structural units derived from di-, tri-, or polyethylene glycol. The polycarbonate resin may also include structural units derived from other dihydroxy compounds as needed. In addition, the details of the polycarbonate resin that can be suitably used in the present invention are described in, for example, Japanese Patent Publication No. 2014-10291, Japanese Patent Publication No. 2014-26266, Japanese Patent Publication No. 2015-212816, Japanese Patent Publication No. 2015-212817, and Japanese Patent Publication No. 2015-212818, and these descriptions are incorporated herein by reference.

The glass transition temperature of the polycarbonate resin is preferably 110°C to 150°C, more preferably 120°C to 140°C. If the glass transition temperature is too low, heat resistance tends to deteriorate, potentially causing dimensional changes after film formation, or reducing the image quality of the resulting organic EL panel. If the glass transition temperature is too high, film forming stability may deteriorate or the transparency of the film may be compromised. The glass transition temperature can be determined in accordance with JIS K 7121 (1987).

The molecular weight of the aforementioned polycarbonate resin can be expressed as its concentrated viscosity. This viscosity is measured using an Ooalloy viscometer at 20.0°C ± 0.1°C using methylene chloride as a solvent, after precisely adjusting the polycarbonate concentration to 0.6 g/dL. The lower limit of the concentrated viscosity is generally preferably 0.30 dL/g, and more preferably 0.35 dL/g or higher. The upper limit of the concentrated viscosity is generally preferably 1.20 dL/g, more preferably 1.00 dL/g, and even more preferably 0.80 dL/g. If the concentrated viscosity is below the aforementioned lower limit, the mechanical strength of the molded product may be reduced. On the other hand, if the specific viscosity exceeds the upper limit, the fluidity during molding may decrease, which may cause problems in productivity or moldability.

Commercially available polycarbonate resin films may also be used. Specific examples of commercially available products include "PURE-ACE WR-S," "PURE-ACE WR-W," and "PURE-ACE WR-M" manufactured by Teijin Co., Ltd., and "NRF" manufactured by Nitto Denko Co., Ltd.

The first phase difference layer can be obtained, for example, by stretching a film formed from the above-mentioned polycarbonate resin. The method of forming a film from a polycarbonate resin can adopt any appropriate molding method. Specific examples include: compression molding, transfer molding, injection molding, extrusion molding, blow molding, powder molding, FRP molding, casting and coating (such as casting), calendering molding, hot pressing, etc. Extrusion molding or casting and coating is preferred. This is because the smoothness of the obtained film can be improved, thereby obtaining good optical uniformity. The molding conditions can be appropriately set according to the composition or type of the resin used, the desired properties of the phase difference layer, etc. Furthermore, as mentioned above, polycarbonate resins are widely available in the market as film products, and thus these commercially available films can be directly subjected to the stretching process.

The thickness of the resin film (unstretched film) can be set to any appropriate value depending on the desired thickness of the first retardation layer, the desired optical properties, the stretching conditions described below, etc. It is preferably 50µm to 300µm.

The above-mentioned stretching can adopt any appropriate stretching method and stretching conditions (such as stretching temperature, stretching ratio, stretching direction). Specifically, various stretching methods such as free end stretching, fixed end stretching, free end shrinking, fixed end shrinking, etc. can be used separately, or they can be used simultaneously or successively. Regarding the stretching direction, it can also be carried out along various directions or dimensions such as the length direction, width direction, thickness direction, and oblique direction. The stretching temperature is preferably Tg-30℃~Tg+60℃ relative to the glass transition temperature (Tg) of the resin film, and more preferably Tg-10℃~Tg+50℃.

By appropriately selecting the stretching method and stretching conditions, a retardation film having the desired optical properties (such as refractive index properties, in-plane retardation, and Nz coefficient) can be obtained.

In one embodiment, the retardation film can be produced by uniaxially stretching a resin film or by fixed-end uniaxial stretching. A specific example of fixed-end uniaxial stretching is stretching the resin film in its widthwise (lateral) direction while moving it in its longitudinal direction. The stretching ratio is preferably 1.1x to 3.5x.

In another embodiment, the phase difference film can be produced by continuously stretching a long strip of resin film obliquely in the direction of the aforementioned angle θ relative to the long side direction. By adopting the oblique stretching, a long strip of stretched film having an orientation angle of angle θ relative to the long side direction of the film (having a slow axis in the direction of angle θ) can be obtained. For example, when laminating with a polarizer, a roll-to-roll process can be performed, thereby simplifying the manufacturing steps. In addition, the angle θ can be the angle formed by the absorption axis of the polarizer and the slow axis of the phase difference layer in the polarizing plate with the phase difference layer. As mentioned above, the angle θ is preferably 40°~50°, more preferably 42°~48°, and more preferably about 45°.

The stretching machine used for oblique stretching can be a tenter-type stretching machine, which applies conveying forces, stretching forces, or pulling forces at different speeds in the transverse and/or longitudinal directions. Tenter-type stretching machines include transverse uniaxial stretching machines and simultaneous biaxial stretching machines. Any suitable stretching machine can be used as long as it can continuously and obliquely stretch a long strip of resin film.

By appropriately controlling the left and right speeds in the stretching machine, a phase difference layer (essentially a long strip of phase difference film) having the desired in-plane phase difference and a slow axis in the desired direction can be obtained.

The stretching temperature of the film can vary depending on the desired in-plane retardation value and thickness of the retardation layer, the type of resin used, the film thickness, the stretching ratio, and other factors. Specifically, the stretching temperature is preferably between Tg-30°C and Tg+30°C, more preferably between Tg-15°C and Tg+15°C, and most preferably between Tg-10°C and Tg+10°C. By stretching at these temperatures, a first retardation layer having properties suitable for the present invention can be obtained. Tg is the glass transition temperature of the film's constituent material.

B-4. Second Retardation Layer As described above, the second retardation layer can be a so-called positive C-plate, whose refractive index characteristics exhibit the relationship nz>nx=ny. Using a positive C-plate as the second retardation layer effectively prevents oblique reflections, thereby enhancing the anti-reflection function over a wide viewing angle. In this case, the thickness-direction retardation Rth(550) of the second retardation layer is preferably -50nm to -300nm, more preferably -70nm to -250nm, more preferably -90nm to -200nm, and most preferably -100nm to -180nm. Here, "nx=ny" includes not only the case where nx and ny are strictly equal, but also the case where nx and ny are substantially equal. That is, the in-plane retardation Re(550) of the second retardation layer can be smaller than 10 nm.

The second phase difference layer having the refractive index characteristic of nz＞nx=ny can be formed of any appropriate material. The second phase difference layer is preferably composed of a thin film containing a liquid crystal material fixed in a homeotropic alignment. The liquid crystal material (liquid crystal compound) that can achieve homeotropic alignment can be a liquid crystal monomer or a liquid crystal polymer. Specific examples of the liquid crystal compound and the method for forming the phase difference layer include the liquid crystal compound and the method for forming the phase difference layer described in paragraphs [0020] to [0028] of Japanese Patent Gazette No. 2002-333642. At this time, the thickness of the second phase difference layer is preferably 0.5μm~10µm, more preferably 0.5μm~8µm, and even more preferably 0.5μm~5µm.

B-5. Conductive Layer or Isotropic Substrate with Conductive Layer The conductive layer can be formed by depositing a metal oxide film on any suitable substrate using any suitable film-forming method (e.g., vacuum evaporation, sputtering, CVD, ion plating, spraying, etc.). Examples of the metal oxide include indium oxide, tin oxide, zinc oxide, indium-tin composite oxide, tin-antimony composite oxide, zinc-aluminum composite oxide, and indium-zinc composite oxide. Indium-tin composite oxide (ITO) is particularly preferred.

When the conductive layer comprises a metal oxide, the thickness of the conductive layer is preferably 50 nm or less, more preferably 35 nm or less. The lower limit of the thickness of the conductive layer is preferably 10 nm.

The conductive layer can be transferred from the substrate to the first phase difference layer (or the second phase difference layer if a second phase difference layer is present), and the conductive layer can serve alone as a constituent layer of the polarizing plate with phase difference layer. Alternatively, the conductive layer and the substrate can be laminated onto the first phase difference layer (or the second phase difference layer if a second phase difference layer is present) in the form of a laminate (substrate with conductive layer). Preferably, the substrate is optically isotropic, so the conductive layer can serve as an isotropic substrate with a conductive layer for the polarizing plate with phase difference layer.

Any suitable isotropic substrate can be used as the optically isotropic substrate. Examples of materials constituting the isotropic substrate include materials having a main skeleton composed of resins without a conjugated system, such as norbornene-based resins or olefin-based resins, and materials having a cyclic structure, such as a lactone ring or a glutarimido ring, in the main chain of an acrylic resin. Using such materials can minimize the phase difference exhibited by molecular chain orientation when forming the isotropic substrate. The thickness of the isotropic substrate is preferably 50µm or less, more preferably 35µm or less. The lower limit of the thickness of the isotropic substrate is, for example, 20µm.

The conductive layer and/or the conductive layer of the isotropic substrate with the conductive layer can be patterned as needed. Patterning can form conductive and insulating portions. This results in electrodes, which can function as touch sensing electrodes for detecting contact with the touch panel. Any appropriate patterning method can be used. Specific examples include wet etching and screen printing.

C. Image Display Device The aforementioned polarizing plate or polarizing plate with a phase difference layer can be applied to an image display device. Therefore, the present invention includes an image display device having the aforementioned polarizing plate or polarizing plate with a phase difference layer. Representative examples of image display devices include liquid crystal display devices and electroluminescent (EL) display devices (e.g., organic EL display devices, inorganic EL display devices). The image display device of an embodiment of the present invention has the polarizing plate described in item A or the polarizing plate with a phase difference layer described in item B on its viewing side. The polarizing plate with a phase difference layer is laminated such that the phase difference layer is on the side of an image display unit (e.g., a liquid crystal unit, an organic EL unit, an inorganic EL unit) (such that the polarizer is on the viewing side). In one embodiment, the image display device has a curved shape (essentially a curved display screen) and/or is bendable or foldable. In such an image display device, the effect of the polarizing plate with a phase difference layer of the present invention is more significant.

EXAMPLES The present invention is described below using examples, but the present invention is not limited to these examples. The measurement methods for various properties are described below. In addition, unless otherwise noted, "parts" and "%" in the examples are by weight.

(1) Thickness The thickness of the polarizer was measured using an interference film thickness meter (manufactured by Otsuka Electronics Co., Ltd., product name "MCPD-3000"). The wavelength range used for thickness calculation is 400nm~500nm, and the refractive index is set to 1.53. In addition, the thickness of the protective layer was measured using an interference film thickness meter (manufactured by Otsuka Electronics Co., Ltd., product name "MCPD-3000"), with the calculation wavelength range and refractive index appropriately selected. The thickness of the easy-to-bond layer was obtained by observation using a scanning electron microscope (SEM). Thicknesses greater than 10μm were measured using a digital micrometer (manufactured by Anritsu Co., Ltd., product name "KC-351C"). (2) Orientation Function of PVA: For the polarizer (polarizer unit) obtained by stripping the resin substrate from the polarizer/thermoplastic resin substrate laminate used in the Examples and Comparative Examples, attenuated total reflection (ATR) spectroscopy of the polarizer surface was performed using a Fourier transform infrared spectrophotometer (FT-IR) (manufactured by Perkin Elmer, trade name: "Frontier"), using polarized infrared light as the measurement light. Germanium was used as the microcrystal for adhering the polarizer, and the incident angle of the measurement light was set to 45°. The orientation function was calculated according to the following procedure. The incident polarized infrared light (measurement light) was set to oscillate parallel to the surface of the germanium crystal sample in close contact (s-polarization). Absorbance spectra were measured with the polarizer positioned perpendicular (⊥) and parallel (//) to the polarization direction of the measurement light. From the resulting absorbance spectra, the intensity at (2941 cm ^-1 ) was calculated, using the intensity at (3330 cm-1) as a reference. _I⊥ is the ratio (2941 cm ^- 1)/(3330 cm ^{- 1} ) obtained from the absorbance spectrum obtained when the polarizer was positioned perpendicular (⊥) to the polarization direction of the measurement light. Furthermore, I _// is the (2941 cm ^-1 intensity) / (3330 cm ^-1 intensity) obtained from the absorbance spectrum obtained when the polarizer's extension direction is parallel (//) to the polarization direction of the measurement light. Here, (2941 cm ^-1 intensity) refers to the absorbance at 2941 cm ^-1 at the bottom of the absorbance spectrum, using 2770 cm ^-1 and 2990 cm ^-1 as baselines, and (3330 cm ^-1 intensity) refers to the absorbance at 3330 cm ^-1 using 2990 cm ^{-1 and 3650 cm -1 as} baselines. Using the obtained I _⊥ and I _// , the orientation function f is calculated according to Equation 1. f = 1 indicates perfect orientation, and f = 0 indicates random orientation. The peak at 2941 cm ^-1 can be attributed to the absorption of the PVA main chain ( _-CH2- ) vibration in the polarizer. Furthermore, the peak at 3330 cm ^-1 can be attributed to the absorption of the hydroxyl group vibration in PVA. (Equation 1) f = (3 < ^cos2θ > -1)/2 = (1-D)/[c(2D+1)] However, using c = ( ^3cos2β -1)/2, as in the case of 2941 cm ^-1 , β = 90° ⇒ f = -2 × (1-D)/(2D+1). θ: Angle of the molecular chain relative to the extension direction β: Angle of the transition dipole moment relative to the molecular chain axis D = (I _⊥ ) / (I _// ) I _⊥ : Absorption intensity when the polarization direction of the measured light is perpendicular to the extension direction of the polarizer I _// : Absorption intensity when the polarization direction of the measured light is parallel to the extension direction of the polarizer (3) In-plane phase difference (Re) of PVA The polarizer (polarizer unit) obtained by removing the resin substrate from the polarizer/thermoplastic resin laminate obtained in the embodiment and the comparative example was evaluated for the in-plane phase difference (Rpva) of PVA at a wavelength of 1000 nm using a phase difference measuring device (manufactured by Oji Scientific Instruments, Ltd., under the name "KOBRA-31X100/IR"). (According to the principle described, the value obtained by subtracting the in-plane phase difference (Ri) of iodine from the total in-plane phase difference at a wavelength of 1000 nm) was obtained. The absorption end wavelength was set to 600 nm. (4) Birefringence (Δn) of PVA The birefringence (Δn) of PVA was calculated by dividing the in-plane phase difference of PVA measured in (3) above by the thickness of the polarizer. (5) Puncture Strength The polarizer was peeled off from the polarizer/resin substrate laminate used in the Examples and Comparative Examples and placed on a compression tester equipped with a needle (manufactured by KATO TECH CO., LTD., product name "NDG5" needle penetration test specification). At room temperature (23°C ± 3°C), the polarizer was punctured at a puncture speed of 0.33 cm/s. The strength at which the polarizer broke was used as the fracture strength (puncture strength). The evaluation value was the fracture strength of 10 sample pieces and the average value was used. In addition, the needle used had a tip diameter of 1 mmφ and a 0.5R. For the polarizer to be measured, a clamp with a circular opening of 11 mm in diameter is clamped and fixed from both sides of the polarizer, and then a needle is inserted into the center of the opening for testing. (6) Cracks The polarizer or polarizer with a phase difference layer obtained in the embodiment and the comparative example is cut into a size of 100 mm × 100 mm. The cut sample is attached to a glass plate (thickness 1.1 mm) through an acrylic adhesive layer of 15 μm in thickness with the protective layer on the outside. The sample attached to the glass plate is placed in an oven at 85°C for 120 hours, and then visually checked to see if there are any cracks in the absorption axis direction (MD direction) of the polarizer. The evaluation was performed using three polarizing plates with phase difference layers. If even one of the plates had cracks, it was evaluated as "cracked", and if all three plates had no cracks, it was evaluated as "none". (7) Single body transmittance and polarization degree The single body transmittance Ts, parallel transmittance Tp, and orthogonal transmittance Tc of the polarizer (polarizer single body) obtained from the polarizer/thermoplastic resin substrate laminate obtained in the Examples and Comparative Examples after the resin substrate was removed were measured using an ultraviolet-visible spectrophotometer ("V-7100" manufactured by JASCO Corporation). These Ts, Tp, and Tc were measured using a 2-degree field of view (light source C) in accordance with JIS Z8701 and the Y values obtained by performing sensitivity correction. The polarization degree P was calculated from the obtained Tp and Tc using the following formula. Polarization degree P (%) = {(Tp-Tc)/(Tp+Tc)} ^{1 /2} × 100. A spectrophotometer such as "LPF-200" manufactured by Otsuka Electronics Co., Ltd. can also be used for equivalent measurements. Regardless of the spectrophotometer used, equivalent measurement results can be confirmed. (8) Wet durability: A test piece (50 mm × 50 mm) was cut out from the polarizing plate or the polarizing plate with a phase difference layer obtained in the Examples and Comparative Examples. The test piece was formed with two sides facing the direction perpendicular to the absorption axis of the polarizer and the absorption axis. The test piece was bonded to an alkali-free glass plate with the protective layer facing outward using an adhesive to prepare a test sample. The single transmittance (Ts), parallel transmittance (Tp) and orthogonal transmittance (Tc) of the test sample were measured in the same manner as in (7) using an ultraviolet-visible spectrophotometer (manufactured by JASCO Corporation, product name "V7100") to obtain the polarization degree (P). At this time, the measurement light was incident from the protective layer side. Next, the test sample was placed in an oven at 60°C and 95% RH for 500 hours for heating and humidification (wet heat test). The change in polarization degree ΔP was obtained from the polarization degree P ₀ before the wet heat test and the polarization degree P ₅₀₀ after the wet heat test using the following formula. ΔP (%) = P ₅₀₀ -P ₀ The obtained ΔP results were evaluated according to the following criteria. Good: ΔP is -5.0% to 0% Poor: ΔP is less than -5.0% or discoloration occurs (9) Softening temperature of protective layer A protective layer (thickness: 3µm) was formed on one side of the polarizer (total elongation ratio: 5.5 times) produced in Comparative Example 1 in the same manner as the formation of the protective layer in each embodiment and comparative example. Local thermal analysis (nano-TA measurement) was performed on the surface of the protective layer of the obtained polarizing plate [protective layer/polarizer] to calculate the softening temperature of the protective layer. The measuring apparatus and measuring conditions are as follows. Measuring device: Made by Hitachi High-Tech Science Co., product name "AFM5300E//Nano-TA2" Measuring mode: Contact mode Probe: AN2-200 Measuring area: 8µm□ Scanning measurement Gas environment: Atmospheric pressure (10) Iodine adsorption amount A protective layer (thickness: 3µm) was formed on one side of a PET film in the same manner as the protective layer in each embodiment and comparative example. The obtained PET film with a protective layer was cut into 1cm×1cm ( ^1cm2 ) to prepare a sample, which was collected into a headspace vial (20mL capacity) and weighed. Next, place a screw-capped bottle (1.5 mL capacity) containing 1 mL of iodine solution (1% iodine concentration by weight, 7% potassium iodide concentration by weight) into the headspace vial and securely cap it. The headspace vial is then placed in a desiccator at 65°C for 6 hours (this allows the gaseous _I₂ to adsorb onto the sample). The sample is then transferred to a ceramic boat and burned using an automatic sample combustion device. The resulting gas is then collected in 10 mL of absorption liquid. After collection, the absorption liquid is diluted to 15 mL with pure water, and IC quantitative analysis is performed on the original solution or a suitably diluted solution. In addition, the iodine adsorption amount after the same measurement was performed on the PET film alone was almost 0, so the iodine adsorption amount (weight %) was calculated from the following formula based on the iodine weight obtained by IC quantitative analysis and the weight of the protective layer monomer ("weight of the PET film with the protective layer" - "weight of the PET film"). Iodine adsorption amount (weight %) = iodine weight obtained by IC quantitative analysis / weight of the protective layer monomer × 100. In addition, the measuring device is as follows. [Measuring device] Automatic sample combustion device: "AQF-2100H" manufactured by Mitsubishi Chemical Analytech Co., Ltd. IC (anion): "ICS-3000" manufactured by Thermo Fisher Scientific Co., Ltd.

[Example 1-1] 1. Preparation of a Polarizer/Resin Substrate Laminate: The resin substrate was a long, amorphous polyethylene terephthalate (PET) film (thickness: 100µm) with a water absorption of 0.75% and a Tg of approximately 75°C. One side of the resin substrate was subjected to a corona treatment. 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 composed of a 9:1 mixture of polyvinyl alcohol (DP4200, saponification degree 99.2 mol%) and acetyl-modified PVA (manufactured by Mitsubishi Chemical Co., trade name "GOHSEFIMER Z410") . The PVA aqueous solution was applied to the corona-treated surface of the resin substrate and dried at 60°C to form a 13μm-thick PVA resin layer, producing a laminate. The resulting laminate was then subjected to free-end uniaxial stretching to 2.4 times its original length in the longitudinal direction (longitudinal direction) between rollers of varying circumferential speeds in an oven at 130°C (in-air assisted stretching). The laminate was then immersed in an insolubilization bath (a boric acid aqueous solution containing 4 parts by weight of boric acid per 100 parts by weight of water) at 40°C for 30 seconds (insolubilization treatment). Next, the polarizer was immersed in a dye bath (an aqueous iodine solution prepared by mixing iodine and potassium iodide at a weight ratio of 1:7 per 100 parts by weight of water) at a temperature of 30°C for 60 seconds, with the concentration adjusted to achieve a single-unit transmittance (Ts) of 41.5% (dyeing treatment). The polarizer was then immersed in a crosslinking bath (an aqueous boric acid solution prepared by mixing 3 parts by weight of potassium iodide and 5 parts by weight of boric acid per 100 parts by weight of water) at a temperature of 40°C for 30 seconds (crosslinking treatment). The laminate was then immersed in a boric acid aqueous solution (boric acid concentration 4.0% by weight) at a temperature of 70°C and uniaxially stretched in the longitudinal direction (longitudinal direction) between rollers of varying circumferential speeds (underwater stretching treatment). The total stretch ratio was 2.4 x 1.46 = 3.5. The laminate was then immersed in a cleaning bath (an aqueous solution of 4 parts by weight of potassium iodide per 100 parts by weight of water) at a temperature of 20°C (cleaning treatment). The laminate was then dried in an oven maintained at 90°C while contacting a SUS heating roller maintained at a surface temperature of 75°C for approximately 2 seconds (drying and shrinking treatment). The laminate was dried and shrunk to a widthwise shrinkage rate of 2%. Following the above method, a 6.7μm-thick polarizer was formed on a resin substrate, producing a polarizer/resin substrate laminate. The polarizer's single-unit transmittance (initial single-unit transmittance) _Ts0 was 41.5%, and its polarization (initial polarization) _P0 was 99.99%.

2. Forming the Adhesive Layer On the polarizer side of the resulting laminate, a polyurethane aqueous dispersion resin (SUPERFLEX SF210, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) was applied to a thickness of 0.1µm to form an adhesive layer.

3. Preparation of the Protective Layer 20 parts by weight of a 100% polymethyl methacrylate acrylic resin (B-728, manufactured by Kusumoto Chemicals Co., Ltd.) was dissolved in 80 parts by weight of methyl ethyl ketone to obtain a 20% acrylic resin solution. This acrylic resin solution was applied to the adhesive layer surface of the laminate using a wire bar. The coated film was dried at 60°C for 5 minutes to form a protective layer composed of a cured product of the coated film. The protective layer had a thickness of 3µm. The resin substrate was then peeled from the laminate to obtain a polarizing plate consisting of a protective layer (cured product of the coated film), an adhesive layer, and a polarizer.

[Example 1-2] A polarizing plate having a structure of hard coating layer/protective layer (cured product of coating film)/easy bonding layer/polarizer was obtained in the same manner as in Example 1-1, except that the protective layer had a thickness of 2 µm and a hard coating layer (thickness 3 µm) was further formed on the surface of the protective layer opposite to the easy bonding layer. The hard coating layer was formed by mixing 70 parts by weight of dihydroxymethyl-tricyclodecane diacrylate (produced by Kyoei Chemicals, trade name: LIGHT ACRYLATE DCP-A), 20 parts by weight of isoborneol acrylate (produced by Kyoei Chemicals, trade name: LIGHT ACRYLATE IB-XA), 10 parts by weight of 1,9-nonanediol diacrylate (produced by Kyoei Chemicals, trade name: LIGHT ACRYLATE 1.9NA-A), and a photopolymerization initiator (produced by BASF, trade name: IRGACURE 907) by weight, mixed with an appropriate solvent, and the resulting coating liquid was applied to the protective layer in such a way that it would become 3µm after curing. The solvent was then dried and irradiated with ultraviolet light using a high-pressure mercury lamp in a nitrogen environment at a cumulative light intensity of 300mJ/ ^cm2 .

[Example 1-3] A polarizing plate having a hard coat/protective layer (cured product of the coating film)/easy-adhesive layer/polarizer structure was obtained in the same manner as in Example 1-1, except that a lactone ring unit-containing polymethyl methacrylate acrylic resin (lactone ring unit content: 30 mol%) was used instead of the 100% polymethyl methacrylate acrylic resin. The protective layer had a thickness of 2µm, and a hard coat layer (3µm thick) was further formed on the side of the protective layer opposite the adhesive layer. The hard coat layer was formed in the same manner as in Example 1-2.

[Example 1-4] A polarizing plate having a hard coat/protective layer (cured product of the coating film)/easy-adhesive layer/polarizer structure was obtained in the same manner as in Example 1-1, except that a polymethyl methacrylate acrylic resin containing glutarimido ring units (4 mol% of glutarimido ring units) was used instead of the 100% polymethyl methacrylate acrylic resin. The protective layer had a thickness of 2µm, and a hard coat (3µm thick) was further formed on the side of the protective layer opposite the adhesive layer. The hard coat was formed in the same manner as in Example 1-2.

[Example 1-5] A polarizing plate having a protective layer (cured product of the coating film)/easy-adhesion layer/polarizer structure was obtained in the same manner as in Example 1-1, except that an acrylic resin comprising a copolymer of methyl methacrylate and butyl methacrylate (molar ratio 80/20) was used instead of the 100% polymethyl methacrylate acrylic resin.

[Example 1-6] A polarizing plate having a hard coat/protective layer (cured coating film)/easy-adhesive layer/polarizer structure was obtained in the same manner as in Example 1-1, except that an acrylic resin comprising a copolymer of methyl methacrylate and butyl methacrylate (molar ratio 80/20) was used instead of the 100% polymethyl methacrylate acrylic resin, the protective layer having a thickness of 2µm, and a hard coat (3µm thick) formed on the side of the protective layer opposite the adhesive layer. The hard coat was formed in the same manner as in Example 1-2.

[Example 1-7] A polarizing plate having a protective layer (cured product of the coating film)/easy-adhesion layer/polarizer structure was obtained in the same manner as in Example 1-1, except that an acrylic resin comprising a copolymer of methyl methacrylate and ethyl acrylate (molar ratio 55/45) (manufactured by Kusumoto Chemicals, product name "B-722") was used instead of the 100% polymethyl methacrylate acrylic resin.

[Example 1-8] A polarizing plate having a protective layer (cured product of the coating film)/easy-adhesion layer/polarizer structure was obtained in the same manner as in Example 1-1, except that an acrylic resin comprising a copolymer of methyl methacrylate and butyl methacrylate (molar ratio 35/65) (manufactured by Kusumoto Chemicals, product name "B-734") was used instead of the 100% polymethyl methacrylate acrylic resin.

[Example 2-1] 1. Preparation of a Polarizer/Resin Substrate Laminate: A polarizer having a thickness of 6.7 μm was formed on a resin substrate in the same manner as in Example 1-1 to produce a polarizer/resin substrate laminate. 2. Preparation of a Protective Layer: 15 parts of an epoxy resin having a biphenyl skeleton (manufactured by Mitsubishi Chemical Co., trade name: jER (registered trademark) YX4000) were dissolved in 83.8 parts of methyl ethyl ketone to obtain an epoxy resin solution. 1.2 parts of a photocatalytic polymerization initiator (San-Apro Ltd., trade name: CPI (registered trademark)-100P) was added to the resulting epoxy resin solution to obtain a protective layer forming composition. The resulting protective layer-forming composition was applied directly (i.e., without forming an easy-adhesion layer) to the surface of the polarizer using a wire rod and dried at 60°C for 3 minutes. Next, the film was irradiated with UV light using a high-pressure mercury lamp at a cumulative dose of 600 mJ/ ^cm² to form a protective layer. The protective layer had a thickness of 3 µm. The resulting laminate was then peeled from the resin substrate to obtain a polarizing plate consisting of a protective layer (photoion-cured epoxy resin layer) and a polarizer.

[Example 2-2] 15 parts of an epoxy resin having a biphenyl skeleton (manufactured by Mitsubishi Chemical Co., trade name: jER (registered trademark) YX4000) and 10 parts by weight of an oxycyclobutane resin (manufactured by Toagosei Co., Ltd., trade name: ARON OXETANE (registered trademark) OXT-221) were dissolved in 73 parts of methyl ethyl ketone to obtain an epoxy resin solution. To this epoxy resin solution, 2 parts of a photocatalytic polymerization initiator (manufactured by San-Apro Ltd., trade name: CPI (registered trademark)-100P) were added to obtain a protective layer-forming composition. Polarizing plates having a protective layer (photoion-cured epoxy resin layer)/polarizer configuration were obtained in the same manner as in Example 2-1, except that the epoxy resin solution was used to form a protective layer composition, dyeing baths of various iodine concentrations (weight ratio of iodine to potassium iodide = 1:7), and the stretching ratio during underwater stretching during polarizer production was set to 1.25 times (total stretching ratio: 3.0 times).

[Example 2-3] Polarizing plates with a protective layer (photoion-cured epoxy resin layer)/polarizer structure were obtained in the same manner as in Example 2-2, except that dye baths with varying iodine concentrations (weight ratio of iodine to potassium iodide = 1:7) were used and the underwater stretching magnification during polarizer production was set to 1.46x (total stretching magnification: 3.5x).

[Example 2-4] Polarizing plates with a protective layer (photoion-cured epoxy resin layer)/polarizer structure were obtained in the same manner as in Example 2-2, except that dye baths with varying iodine concentrations (weight ratio of iodine to potassium iodide = 1:7) were used and the underwater stretching magnification during polarizer production was set to 1.67x (total stretching magnification: 4.0x).

[Example 2-5] Polarizing plates with a protective layer (photoion-cured epoxy resin layer)/polarizer structure were obtained in the same manner as in Example 2-2, except that dye baths with varying iodine concentrations (weight ratio of iodine to potassium iodide = 1:7) were used and the underwater stretching magnification during polarizer production was set to 1.88x (total stretching magnification: 4.5x).

[Example 2-6] A polarizing plate having a protective layer (photoion-curable epoxy resin layer)/polarizer structure was obtained in the same manner as in Example 2-1, except that a bisphenol-type epoxy resin (manufactured by Mitsubishi Chemical Co., trade name: jER (registered trademark) 828) was used instead of the epoxy resin having a biphenyl backbone.

[Example 2-7] A polarizing plate having a protective layer (photoion-curable epoxy resin layer)/polarizer structure was obtained in the same manner as in Example 2-3, except that a bisphenol-type epoxy resin (manufactured by Mitsubishi Chemical Co., trade name: jER (registered trademark) 828) was used instead of the epoxy resin having a biphenyl backbone.

[Example 2-8] A polarizing plate having a protective layer (photoion-cured epoxy layer of epoxy resin)/polarizer structure was obtained in the same manner as in Example 2-1, except that a hydrogenated bisphenol-based epoxy resin (manufactured by Mitsubishi Chemical Co., trade name: jER (registered trademark) YX8000) was used instead of the epoxy resin having a biphenyl backbone.

[Example 2-9] A polarizing plate having a protective layer (photoion-cured epoxy resin layer)/polarizer structure was obtained in the same manner as in Example 2-3, except that a hydrogenated bisphenol-based epoxy resin (manufactured by Mitsubishi Chemical Co., trade name: jER (registered trademark) YX8000) was used instead of the epoxy resin having a biphenyl backbone.

[Example 2-10] 1. Preparation of Polarizing Plate A polarizing plate consisting of a protective layer (photoion-cured epoxy resin layer) and a polarizer was obtained in the same manner as in Example 2-3.

2. Preparation of the first phase difference layer: Polymerization was carried out using a batch polymerization apparatus consisting of two vertical reactors equipped with stirring blades and a reflux cooler controlled at 100°C. The reaction mixture consisted of 29.60 parts by mass (0.046 mol) of bis[9-(2-phenoxycarbonylethyl)fluoren-9-yl]methane, 29.21 parts by mass (0.200 mol) of isosorbide (ISB), 42.28 parts by mass (0.139 mol) of spiroglycerol (SPG), 63.77 parts by mass (0.298 mol) of diphenyl carbonate (DPC), and 1.19× ^10⁻² parts by mass (6.78× ^10⁻⁵ mol) of calcium acetate monohydrate as a catalyst. After the reactor was depressurized and purged with nitrogen, it was heated with a heating medium and stirred when the internal temperature reached 100°C. 40 minutes after the start of the temperature increase, the internal temperature reached 220°C. While maintaining this temperature, the pressure was reduced to 13.3 kPa over 90 minutes. Phenol vapor, a by-product of the polymerization reaction, was introduced into a 100°C reflux cooler to return a small amount of monomeric components contained in the phenol vapor to the reactor. Uncondensed phenol vapor was then recovered in a 45°C condenser. Nitrogen was introduced into the first reactor to temporarily return the pressure to atmospheric pressure. The oligomerized reaction solution in the first reactor was then transferred to the second reactor. Next, the temperature and pressure in the second reactor were raised and reduced, reaching an internal temperature of 240°C and a pressure of 0.2 kPa over 50 minutes. Polymerization was then continued until the predetermined stirring power was reached. At this point, nitrogen was introduced into the reactor to restore the pressure, and the resulting polyester carbonate resin was extruded into water. The resulting strands were then cut into pellets.

The resulting polyester carbonate resin pellets were vacuum-dried at 80°C for 5 hours. Then, a 130μm-thick long resin film was produced using a film-forming apparatus equipped with a uniaxial extruder (Toshiba Machine Co., Ltd., cylinder set temperature: 250°C), a T-die (200mm width, set temperature: 250°C), a cooling roll (set temperature: 120-130°C), and a winder. The resulting long resin film was then stretched while adjusting the film to achieve a predetermined retardation, resulting in a 48μm-thick retardation film. Stretching conditions were a widthwise stretching temperature of 143°C and a stretching ratio of 2.8x. The obtained retardation film had a Re(550) of 141 nm, a Re(450)/Re(550) of 0.86, and an Nz coefficient of 1.12.

3. Second phase difference layer A phase difference film ("MCP-N" manufactured by Dai Nippon Printing Co., Ltd.) having a refractive index characteristic satisfying the relationship nz>nx=ny and a thickness direction phase difference Rth(550) of -135 nm was used as the second phase difference layer.

4. Preparation of a Polarizing Plate with a Retardation Layer First, a second retardation layer is bonded to the first retardation layer using a UV-curable adhesive. Next, the first retardation layer of the first/second retardation layer laminate is bonded to the polarizer surface of the polarizing plate using a 12µm-thick acrylic adhesive. During bonding, the slow axis of the first retardation layer forms a 45° angle with the absorption axis of the polarizer. This completes a polarizing plate with a retardation layer consisting of a protective layer (photoion-curable epoxy resin layer), polarizer, adhesive layer, first retardation layer, and second retardation layer.

[Example 3-1] 1. Preparation of a Polarizer/Resin Substrate Laminate A polarizer with a thickness of 6.7 μm was formed on a resin substrate in the same manner as in Example 1-1 to produce a polarizer/resin substrate laminate. 2. Preparation of a Protective Layer 20 parts of epoxy resin 1 (manufactured by Mitsubishi Chemical Co., trade name: jER (registered trademark) 1256B40, weight-average molecular weight: 40,000, epoxy equivalent weight: 7,350) was dissolved in 80 parts of methyl ethyl ketone to obtain an epoxy resin solution (20%). The epoxy resin solution was applied to the polarizer surface of the laminate using a wire rod. The coated film was dried at 60°C for 3 minutes to form a protective layer consisting of a cured epoxy resin coating. The protective layer had a thickness of 3µm. The resin substrate was then peeled from the laminate to obtain a polarizing plate consisting of a protective layer (cured epoxy resin coating) and a polarizer.

[Example 3-2] A polarizing plate having a protective layer (cured layer of epoxy resin coating)/polarizer configuration was obtained in the same manner as in Example 3-1, except that epoxy resin 2 (manufactured by Mitsubishi Chemical Co., trade name: jER (registered trademark) YX6954BH30, weight-average molecular weight: 36,000, epoxy equivalent weight: 13,000) was used instead of epoxy resin 1.

[Example 4] A polarizing plate having a protective layer (cured coating film)/polarizer configuration was obtained in the same manner as in Example 1-1, except that no adhesive layer was formed (i.e., the protective layer was formed directly on the polarizer) and a water-based polyester resin (manufactured by Nippon Gosei Kagaku Co., Ltd., product name "POLYESTER WR905") was used.

[Example 5] A polarizing plate having a protective layer (cured coating)/polarizer configuration was obtained in the same manner as in Example 1-1, except that no easy-adhesion layer was formed (i.e., the protective layer was formed directly on the polarizer) and a water-based polyurethane resin (manufactured by Daiichi Kogyo Seiyaku Co., Ltd., product name "SUPERFLEX SF210") was used.

[Comparative Example 1] A polarizing plate having a protective layer (cured product of the coating film)/easy-adhesion layer/polarizer structure was obtained in the same manner as in Example 1-1, except that the underwater stretching ratio during polarizer production was set to 2.29 times (total stretching ratio: 5.5 times).

[Comparative Example 2] A polarizing plate having a hard coat layer/protective layer (cured coating film)/easy-adhesion layer/polarizer structure was obtained in the same manner as in Examples 1-2, except that the underwater stretching ratio during polarizer production was set to 2.29 times (total stretching ratio: 5.5 times).

[Comparative Example 3] Dyeing baths with varying iodine concentrations (weight ratio of iodine to potassium iodide = 1:7) and a polarizer stretching ratio of 2.29x (total stretching ratio: 5.5x) were used. A polarizing plate having a protective layer (photoion-curable epoxy resin layer), an easy-to-adhesive layer, and a polarizer structure was obtained in the same manner as in Examples 2-3.

[Comparative Example 4] A polarizing plate with a retardation layer was obtained in the same manner as in Example 2-10, except that the underwater stretching ratio during polarizer production was set to 2.29 times (total stretching ratio: 5.5 times). The structure consisted of a protective layer (photoion-curable epoxy resin layer), a polarizer, an adhesive layer, a first retardation layer, and a second retardation layer.

[Comparative Example 5] A polarizing plate having a protective layer (cured product of the coating film)/easy-adhesion layer/polarizer structure was obtained in the same manner as in Examples 1-7, except that the underwater stretching ratio during polarizer production was set to 2.29 times (total stretching ratio: 5.5 times).

[Comparative Example 6] A polarizing plate having a protective layer (acrylic film)/polarizer configuration was obtained in the same manner as in Example 1-1, except that the underwater stretching magnification during polarizer production was set to 2.29x (total stretch magnification: 5.5x) and an acrylic film (refractive index: 1.50, thickness: 20µm) with a single-side adhesive treatment was used as the protective layer. The acrylic film was directly bonded to the polarizer surface using a UV-curable adhesive. Specifically, a curable adhesive was applied to a total thickness of 1.0µm and bonded using a roller. UV light was then applied from the side of the acrylic film to cure the adhesive.

The evaluation results are shown in Tables 1 to 3. [Table 1]

[Table 2]

[Table 3]

As shown in Tables 1-3, polarizing plates using a PVA-based resin polarizer with a predetermined orientation (thus, a polarizer with a predetermined puncture strength) exhibit minimal cracking during heating, even with a minimal protective layer thickness. Furthermore, polarizing plates with this specific protective layer exhibit excellent durability even when exposed to humidity.

Furthermore, Figures 5 to 7 show the relationship between the single transmittance of the polarizers used in the Examples and Comparative Examples (polarizers with a total stretch ratio of 3.0, 3.5, 4.0, 4.5, or 5.5 times) and the Δn of the PVA, the in-plane retardation, or the orientation function. As shown in Figures 5 to 7 and Tables 1 to 3, it can be seen that polarizers that satisfy equations (1), (2), and/or (3) exhibit a puncture strength below a predetermined value, and polarizing plates made using such polarizers can suppress cracking during heating even when the protective layer thickness is extremely small.

Industrial Applicability The polarizing plate of the present invention is suitable for use in image display devices. Examples of such devices include: portable devices such as PDAs, smartphones, mobile phones, clocks, digital cameras, and portable game consoles; office automation equipment such as computer monitors, laptops, and copiers; home appliances such as video cameras, televisions, and microwaves; in-vehicle devices such as rearview cameras, car navigation systems, and car stereos; display devices such as digital signage and store information guide screens; alarm devices such as surveillance screens; and nursing and medical devices such as monitors and medical devices.

10: Polarizer 20: First protective layer 30: Second protective layer 50: Laminated body 100: Polarizer 110: Retardation layer, first retardation layer 120: Second retardation layer, second retardation layer 130: Conductive layer, isotropic substrate with conductive layer 200a, 200b: Polarizer with retardation layer G1-G4: Guide rollers R1-R6: Transport rollers

Figure 1 is a schematic cross-sectional view of a polarizing plate according to an embodiment of the present invention. Figure 2 is a schematic view showing an example of a drying and shrinking process using heated rollers in the production of polarizers. Figure 3 is a schematic cross-sectional view of a polarizing plate with a retardation layer according to an embodiment of the present invention. Figure 4 is a schematic cross-sectional view of a polarizing plate with a retardation layer according to an embodiment of the present invention. Figure 5 is a graph showing the relationship between the single-unit transmittance of polarizers used in Examples and Comparative Examples and the birefringence of PVA-based resins. Figure 6 is a graph showing the relationship between the single-unit transmittance of polarizers used in Examples and Comparative Examples and the in-plane retardation of PVA-based resin films. FIG7 is a graph showing the relationship between the monomer transmittance of the polarizer used in the embodiment and the comparative example and the orientation function of the PVA-based resin.

10:Polarizer

20: First protective layer

30: Second protective layer

100:Polarizing plate

Claims (11)

A polarizing plate comprises a polarizer formed of a polyvinyl alcohol resin film containing a dichroic substance and a protective layer disposed on one side of the polarizer; The polarizer satisfies the following formula (1) when its single body transmittance is x% and the birefringence of the polyvinyl alcohol resin is y; y＜-0.011x+0.525　　　　　(1) The birefringence of the polyvinyl alcohol resin is a value obtained by dividing the in-plane phase difference of the polyvinyl alcohol resin at a wavelength of 1000 nm by the thickness of the polarizer; The single body transmittance of the polarizer is 40.0% or more and the polarization degree is 99.0% or more; The thickness of the polarizer is 10µm or less; The protective layer is formed of a resin film having a thickness of 1µm or more and 3µm or less; The resin film is composed of a photo-cured epoxy resin, a cured epoxy resin coating film obtained from an organic solvent solution, or a cured thermoplastic (meth)acrylic resin coating film obtained from an organic solvent solution. The resin film has a softening temperature of 100°C or higher. A polarizing plate comprises a polarizer formed of a polyvinyl alcohol resin film containing a dichroic substance and a protective layer disposed on one side of the polarizer; When the polarizer has a single-body transmittance of x% and an in-plane phase difference of the polyvinyl alcohol resin film at a wavelength of 1000 nm of znm, the polarizer satisfies the following formula (2); z＜-60x+2875　　　　　　　　　(2) The single-body transmittance of the polarizer is 40.0% or more, and the polarization degree is 99.0% or more; The thickness of the polarizer is 10µm or less; The protective layer is formed of a resin film having a thickness of 1µm or more and 3µm or less; The resin film is composed of a photo-cured epoxy resin, a cured epoxy resin coating film obtained from an organic solvent solution, or a cured thermoplastic (meth)acrylic resin coating film obtained from an organic solvent solution. The resin film has a softening temperature of 100°C or higher. A polarizing plate comprises a polarizer formed of a polyvinyl alcohol-based resin film containing a dichroic substance and a protective layer disposed on one side of the polarizer; The polarizer satisfies the following formula (3) when its single body transmittance is x% and the orientation function of the polyvinyl alcohol-based resin is f; f＜-0.018x+1.11　　　　　(3) The single body transmittance of the polarizer is 40.0% or more and the polarization degree is 99.0% or more; The thickness of the polarizer is 10µm or less; The protective layer is formed of a resin film having a thickness of 1µm or more and 3µm or less; The resin film is composed of a photo-cured epoxy resin, a cured epoxy resin coating film obtained from an organic solvent solution, or a cured thermoplastic (meth)acrylic resin coating film obtained from an organic solvent solution. The resin film has a softening temperature of 100°C or higher. A polarizing plate comprising a polarizer formed of a polyvinyl alcohol-based resin film containing a dichroic substance and a protective layer disposed on one side of the polarizer; The polarizer has a puncture strength of 30 gf/µm or greater; The puncture strength of the polarizer is defined as the strength at which the polarizer breaks when a needle with a tip diameter of 1 mmφ and 0.5 R is pierced through the polarizer at a speed of 0.33 cm/sec; The polarizer has a single-element transmittance of 40.0% or greater and a degree of polarization of 99.0% or greater; The polarizer has a thickness of 10 µm or less; The protective layer is formed of a resin film having a thickness of 1 µm or greater and 3 µm or less; The resin film is composed of a photo-cured epoxy resin, a cured epoxy resin coating film obtained from an organic solvent solution, or a cured thermoplastic (meth)acrylic resin coating film obtained from an organic solvent solution. The resin film has a softening temperature of 100°C or higher. The polarizing plate of any one of claims 1 to 4, wherein the thermoplastic (meth)acrylic resin comprises at least one selected from the group consisting of a lactone ring unit, a glutaric anhydride unit, a glutarimide unit, a maleic anhydride unit, and a maleimide unit. A polarizing plate according to any one of claims 1 to 4, wherein the iodine adsorption amount of the protective layer is less than 25 wt %, and the iodine adsorption amount is measured using the protective layer formed on a PET film with a thickness of 3 μm. The polarizing plate of any one of claims 1 to 4, wherein the polarizing plate is wound into a roll. A polarizing plate with a phase difference layer, comprising the polarizing plate according to any one of claims 1 to 7 and a phase difference layer; wherein the phase difference layer is disposed on the side of the polarizer opposite to the side on which the protective layer is disposed. The polarizing plate with a phase difference layer as claimed in claim 8, wherein the phase difference layer is laminated on the polarizing plate via an adhesive layer. The polarizing plate with a phase difference layer as claimed in claim 8 or 9, wherein the Re(550) of the phase difference layer is 100 nm to 190 nm, and the Re(450)/Re(550) is greater than 0.8 and less than 1; and the angle formed by the slow axis of the phase difference layer and the absorption axis of the polarizer is 40° to 50°. An image display device comprises the polarizing plate of any one of claims 1 to 7 or the polarizing plate with a phase difference layer of any one of claims 8 to 10.