TWI895249B - Polarizing plate with phase difference layer and image display device using the same - Google Patents

Abstract

A polarizing plate with a phase difference layer is provided that is thin, has excellent processability, and has excellent optical properties. The polarizing plate with a phase difference layer of the present invention comprises a polarizing plate and a phase difference layer, wherein the polarizing plate comprises a polarizing film and a protective layer located on at least one side of the polarizing film. The polarizing film is composed of a polyvinyl alcohol-based resin film containing a dichroic substance, has a thickness of less than 8 μm, a single body transmittance of greater than 44.5%, and a polarization degree of greater than 99.0%. 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. The angle formed by the slow axis of the phase difference layer and the absorption axis of the polarizing film is 40° to 50°.

Description

Polarizing plate with phase difference layer and image display device using the same

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

Invention Background

In recent years, image display devices, typically liquid crystal display devices and electroluminescent (EL) display devices (e.g., organic EL display devices, inorganic EL display devices), have rapidly become popular. Image display devices typically use polarizing plates and phase difference plates. In practical applications, polarizing plates with phase difference layers, which are integrated polarizing plates and phase difference plates, are widely used (e.g., Patent Document 1). Recently, as the demand for thinner image display devices has increased, the demand for thinner polarizing plates with phase difference layers has also increased. Furthermore, in recent years, the demand for curved image display devices and/or flexible or bendable image display devices has increased, and there is a demand for further thinning and greater flexibility of polarizing plates and polarizing plates with phase difference layers. To reduce the thickness of polarizing plates with retardation layers, efforts are underway to reduce the thickness of the protective layer and retardation film, which significantly impact the thickness of the polarizing film. However, as the protective layer and retardation film are thinned, the shrinkage of the polarizing film becomes significantly greater, leading to warping of the image display device and reduced operability of the polarizing plate with retardation layer.

To address the aforementioned issues, the polarizing film must also be thinned. However, simply reducing the thickness of the polarizing film degrades its optical properties. More specifically, one or both of the polarization degree and the single-element transmittance, which are in a mutually exclusive relationship, are reduced to levels that are unacceptable for practical applications. Consequently, the optical properties of the polarizing plate with a retardation layer also become insufficient.

Prior Art Literature

Patent Literature

Patent Document 1: Japanese Patent No. 3325560

Summary of the Invention

This invention was developed to address the aforementioned problems. Its main purpose is to provide a thin polarizing plate with a phase difference layer that is easy to process and exhibits excellent optical properties.

The polarizing plate with a phase difference layer of the present invention comprises a polarizing plate and a phase difference layer. The polarizing plate includes a polarizing film and a protective layer located on at least one side of the polarizing film. The polarizing film is composed of a polyvinyl alcohol-based resin film containing a dichroic substance, has a thickness of less than 8 μm, a single body transmittance of greater than 44.5%, and a polarization degree of greater than 99.0%. The Re(550) of the phase difference layer is 100 nm to 190 nm, 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 polarizing film is 40° to 50°.

In one embodiment, the protective layer is formed of a substrate having an elastic modulus of 3000 MPa or greater.

In one embodiment, the total thickness of the polarizing plate with a retardation layer is 90 μm or less, the front reflection hue is 3.5 or less, and the protective layer is made of a resin film with an elastic modulus of 3000 MPa or more.

In one embodiment, the protective layer is composed of a cellulose triacetate resin film.

In one embodiment, the polarizing plate includes the polarizing film and the protective layer disposed only on one side of the polarizing film, and the phase difference layer is bonded to the polarizing film via an adhesive layer.

In one embodiment, the phase difference layer is composed of a polycarbonate resin film.

In one embodiment, the phase difference layer is composed of a polycarbonate resin film having a thickness of 40 μm or less.

In one embodiment, the difference between the maximum and minimum values of the single transmittance of the polarizing film in an area of 50 cm ² is less than 0.2%.

In one embodiment, the width of the polarizing plate with a retardation layer is greater than 1000 mm, and the difference between the maximum and minimum values of the single transmittance along the width direction of the polarizing film is less than 0.3%.

In one embodiment, the polarizing film has a single-unit transmittance of 45.0% or less and a degree of polarization of 99.9% or less.

In one embodiment, the polarizing plate with a phase difference layer further comprises another phase difference layer outside the phase difference layer, and the refractive index characteristics of the other phase difference layer exhibit the relationship nz>nx=ny.

In one embodiment, the polarizing plate with a retardation layer further comprises a conductive layer or an isotropic substrate with a conductive layer outside the retardation layer.

In one embodiment, the polarizing plate with a retardation layer is in the form of an elongated strip. The polarizing film has an absorption axis in the longitudinal direction, and the retardation layer is an obliquely stretched film having a slow axis at an angle of 40° to 50° relative to the longitudinal direction. In another embodiment, the polarizing plate with a retardation layer is wound into a roll.

According to another aspect of the present invention, an image display device is provided. The image display device comprises the aforementioned polarizing plate with a phase difference layer.

In one embodiment, the image display device is an organic electroluminescent display device or an inorganic electroluminescent display device.

According to the present invention, a polarizing film with excellent optical properties, despite being thin, can be obtained by combining the following methods: adding a halogen (typically potassium iodide) to a polyvinyl alcohol (PVA) resin, performing a two-stage stretching process consisting of air-assisted stretching and underwater stretching, and drying and shrinking using heated rollers. Using this polarizing film, a thin polarizing plate with a retardation layer can be achieved that is easy to handle and exhibits excellent optical properties.

10:Polarizing plate

11:Polarizing film

12: First protective layer

13: Second protective layer

20: Phase difference layer (first phase difference layer)

50: Another phase difference layer (second phase difference layer)

60: Conductive layer or isotropic substrate with conductive layer attached

100, 101: Polarizing plate with phase difference layer

200: Layered body

G1~G4: guide roller

R1~R6: Conveyor Rollers

Figure 1 is a schematic cross-sectional view of a polarizing plate with a phase difference layer according to one embodiment of the present invention.

Figure 2 is a schematic cross-sectional view of a polarizing plate with a phase difference layer according to another embodiment of the present invention.

Figure 3 is a schematic diagram showing an example of a drying and shrinking process using heated rollers in the manufacturing method of the polarizing film used in the polarizing plate with a phase difference layer of the present invention.

Form used to implement the invention

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

(Definition of terms and symbols)

The definitions of terms and symbols in this manual are as follows.

(1) Refractive index (nx, ny, nz)

"nx" is the refractive index in the direction of maximum in-plane refractive index (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 retardation measured at 23°C using light of wavelength λ nm. For example, "Re(550)" is the in-plane retardation measured at 23°C using light of wavelength 550 nm. Re(λ) can be calculated using 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 phase difference in the thickness direction measured at 23°C using light of wavelength λ nm. For example, "Rth(550)" is the phase difference in the thickness direction measured at 23°C using light of wavelength 550 nm. Rth(λ) can be calculated using the formula: Rth(λ) = (nx - nz) × d, when the layer (film) thickness is d (nm).

(4) Nz coefficient

The Nz coefficient can be calculated as Nz=Rth/Re.

(5) Angle

When angles are mentioned in this manual, they include both clockwise and counterclockwise angles relative to a reference direction. Therefore, for example, "45°" means ±45°.

A. Overall structure of the polarizing plate with phase difference layer

FIG1 is a schematic cross-sectional view of a polarizing plate with a phase difference layer according to an embodiment of the present invention. The polarizing plate with a phase difference layer 100 according to this embodiment comprises a polarizing plate 10 and a phase difference layer 20. The polarizing plate 10 comprises a polarizing film 11, a first protective layer 12 disposed on one side of the polarizing film 11, and a second protective layer 13 disposed on the other side of the polarizing film 11. Depending on the purpose, one of the first protective layer 12 and the second protective layer 13 may be omitted. For example, when the phase difference layer 20 can function as a protective layer for the polarizing film 11, the second protective layer 13 may also be omitted. In the embodiment of the present invention, the polarizing film is typically formed of a polyvinyl alcohol-based resin film containing a dichroic substance. The polarizing film has a thickness of 8 μm or less, a single-element transmittance of 44.5% or more, and a polarization degree of 99.0% or more.

As shown in Figure 2, another embodiment of a polarizing plate with a phase shift layer 101 may also include another phase shift layer 50 and/or a conductive layer or an isotropic substrate with a conductive layer 60. The phase shift layer 50 and the conductive layer or the isotropic substrate with a conductive layer 60 are typically disposed outside the phase shift layer 20 (on the side opposite the polarizing plate 10). The refractive index characteristics of the phase shift layer typically exhibit the relationship nz > nx = ny. The phase shift layer 50 and the conductive layer or the isotropic substrate with a conductive layer 60 are typically disposed sequentially, starting from the phase shift layer 20 side. The other phase difference layer 50 and the conductive layer or the isotropic substrate 60 with a conductive layer represent arbitrary layers that can be provided as needed; either or both can be omitted. For convenience, the phase difference layer 20 is sometimes referred to as the first phase difference layer, and the other phase difference layer 50 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 polarizing plate with a phase difference layer can be used in a so-called internal touch panel input display device, in which a touch sensor is integrated between an image display unit (e.g., an organic EL unit) and a polarizing plate.

In the embodiment of the present invention, the Re(550) of the first phase difference layer 20 is 100nm~190nm, and the Re(450)/Re(550) is greater than 0.8 and less than 1. In addition, the angle formed by the slow axis of the first phase difference layer 20 and the absorption axis of the polarizing film 11 is 40°~50°.

The above embodiments may be combined as appropriate, and modifications apparent in the industry may be made to the components of the above embodiments. For example, the configuration of the isotropic substrate 60 with a conductive layer disposed outside the second retardation layer 50 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 also include other retardation layers. The optical properties (e.g., refractive index, in-plane retardation, Nz coefficient, photoelastic coefficient), thickness, and placement of these other retardation layers can be appropriately configured according to the intended purpose.

The polarizing plate with a phase difference layer of the present invention can be in the form of a thin sheet or a long strip. The so-called "long strip" in this specification means a long and thin shape that is long enough relative to its width. For example, it includes a long and thin shape that is more than 10 times the length of the width, and preferably more than 20 times the length. The long strip-shaped polarizing plate with a phase difference layer can be rolled into a roll. When the polarizing plate with a phase difference layer is in the form of a long strip, both the polarizing plate and the phase difference layer are in the form of a long strip. In this case, the polarizing film preferably has an absorption axis in the direction of the long strip. The first phase difference layer is preferably an obliquely stretched film having a slow axis in a direction at an angle of 40° to 50° relative to the direction of the long strip. As long as the polarizing film and the first retardation layer have the above-described configuration, a polarizing plate with a retardation layer can be produced by roll-to-roll production.

In practical use, an adhesive layer (not shown) can be placed on the side of the phase difference layer opposite the polarizing plate, and the polarizing plate with phase difference layer can be attached to the image display unit. Furthermore, a release film is preferably temporarily attached to the surface of the adhesive layer before the polarizing plate with phase difference layer is used. This temporary attachment of the release film protects the adhesive layer while forming a roll.

The front-reflected hue (√(a ^*2 + b ^*2 )) of a polarizing plate with a retardation layer is preferably 3.5 or less, and preferably 3.0 or less. When the front-reflected hue is within this range, undesirable coloration can be suppressed, resulting in a polarizing plate with a retardation layer having excellent reflective properties.

The total thickness of the polarizing plate with a phase difference layer is preferably 140 μm or less, preferably 120 μm or less, more preferably 100 μm or less, more preferably 90 μm or less, and even more preferably 85 μm or less. The lower limit of the total thickness can be, for example, 30 μm. According to embodiments of the present invention, an extremely thin polarizing plate with a phase difference layer as described above can be achieved. The polarizing plate with a phase difference layer can have excellent flexibility and bending durability. The polarizing plate with a phase difference layer is particularly suitable for use in curved image display devices and/or image display devices that can be bent or folded. In addition, the total thickness of the polarizing plate with a retardation layer refers to the total thickness of all layers that constitute the polarizing plate with a retardation layer, excluding the adhesive layer used to adhere the polarizing plate with a retardation layer to an external adherend such as a panel or glass. (That is, the total thickness of the polarizing plate with a retardation layer does not include the thickness of the adhesive layer used to attach the polarizing plate with a retardation layer to an adjacent component such as an image display unit, and the thickness of the release film that may be temporarily adhered to the surface of the polarizing plate with a retardation layer.)

The following is a more detailed explanation of the components of a polarizing plate with a retardation layer.

B.Polarizing plate

B-1.Polarizing film

As described above, the polarizing film 11 has a thickness of 8 μm or less, a single element transmittance of 44.5% or greater, and a degree of polarization of 99.0% or greater. Generally speaking, single element transmittance and degree of polarization are in a trade-off relationship. Therefore, increasing single element transmittance decreases polarization, and increasing polarization decreases single element transmittance. Therefore, conventional thin polarizing films that meet the optical properties of a single element transmittance of 44.5% or greater and a degree of polarization of 99.0% or greater have been difficult to implement in practical applications. One of the features of the present invention is the use of a thin polarizing film that exhibits excellent optical properties of a single element transmittance of 44.5% or greater and a degree of polarization of 99.0% or greater, while suppressing variations in the optical properties.

The thickness of the polarizing film should be between 1μm and 8μm, preferably between 1μm and 7μm, and even more preferably between 2μm and 5μm.

The polarizing film preferably exhibits absorption dichroism at any wavelength between 380 nm and 780 nm. The single-element transmittance of the polarizing film is preferably 45.0% or less. The polarization degree of the polarizing film is preferably 99.2% or greater, more preferably 99.4% or greater. On the other hand, the polarization degree is preferably 99.9% or less. The single-element transmittance is typically the Y value measured using a UV-visible spectrophotometer and calibrated for optical efficiency. The polarization degree is typically calculated using the following formula based on the parallel transmittance Tp and the cross transmittance Tc measured using a UV-visible spectrophotometer and calibrated for optical efficiency.

Polarization degree (%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 × 100

In one embodiment, the transmittance of thin polarizing films (less than 8 μm) is measured using a UV-visible spectrophotometer, typically using a laminate of the polarizing film (surface refractive index: 1.53) and a protective film (refractive index: 1.50). Due to variations in the refractive index of the polarizing film surface and/or the refractive index of the protective film's surface in contact with air, the reflectivity at the interfaces between the layers can vary, resulting in variations in the measured transmittance. Therefore, when using a protective film with a refractive index other than 1.50, for example, the transmittance can be corrected based on the refractive index of the protective film's surface in contact with air. Specifically, the correction value C of the transmittance is expressed by the following formula using the reflectance R ₁ (transmission axis reflectance) of polarized light parallel to the transmission axis at the interface between the protective film and the air layer.

C=R ₁ -R ₀

R ₀ =((1.50-1) ² /(1.50+1) ² )×(T ₁ /100)

R ₁ =((n ₁ -1) ² /(n ₁ +1) ² )×(T ₁ /100)

Here, _R0 is the transmission axis reflectivity when using a protective film with a refractive index of 1.50, _n1 is the refractive index of the protective film, and _T1 is the transmittance of the polarizing film. For example, when using a substrate with a surface refractive index of 1.53 (such as a cycloolefin film or a film with a hard coat) as the protective film, the correction value C is approximately 0.2%. In this case, adding 0.2% to the measured transmittance can be converted to the transmittance when using a protective film with a surface refractive index of 1.50. Furthermore, according to the above formula, the change in the correction value C after changing the transmittance _T1 of the polarizing film by 2% is less than 0.03%, indicating that the influence of the polarizing film's transmittance on the correction value C is limited. Furthermore, if the protective film has absorption other than surface reflection, appropriate correction can be made based on the amount of absorption.

In one embodiment, the width of the polarizing plate with a phase difference layer is greater than 1000 mm, so the width of the polarizing film is also greater than 1000 mm. At this time, the difference (D1) between the maximum and minimum values of the single transmittance at a position along the width direction of the polarizing film is preferably less than 0.3%, preferably less than 0.25%, and more preferably less than 0.2%. The smaller D1 is, the better, and the lower limit of D1 can be, for example, 0.01%. As long as D1 is within the above range, polarizing plates with phase difference layers having excellent optical properties can be industrially produced. In another embodiment, the difference (D2) between the maximum and minimum values of the single transmittance of the polarizing film in an area of 50 ^cm2 is preferably less than 0.2%, preferably less than 0.1%, and more preferably less than 0.05%. The smaller D2 is, the better, and the lower limit of D2 can be, for example, 0.01%. As long as D2 is within the above range, the brightness variation of the display screen can be suppressed when the polarizing plate with a phase difference layer is used in an image display device.

Any suitable polarizing film can be used as the polarizing film. Typically, the polarizing film can be produced using a laminate of two or more layers.

A specific example of a polarizing film obtained using a laminate is a laminate comprising a resin substrate and a PVA-based resin layer coated on the resin substrate. A polarizing film obtained using a laminate comprising 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, drying the solution to form the PVA-based resin layer on the resin substrate, and then stretching and dyeing the laminate to produce a polarizing film. In this embodiment, stretching typically includes immersing the laminate in a boric acid aqueous solution and stretching it. Optionally, stretching may further include stretching the laminate in-air at a high temperature (e.g., above 95°C) before stretching in the boric acid aqueous solution. The resulting resin substrate/polarizing film laminate can be used directly (i.e., the resin substrate can also be used as a protective layer for the polarizing film), or the resin substrate can be peeled off from the resin substrate/polarizing film laminate and any appropriate protective layer can be laminated on the peeled surface, depending on the intended purpose, before use. Details of the polarizing film manufacturing method are described, for example, in Japanese Patent Publication No. 2012-73580. The entire disclosure of this publication is incorporated herein by reference.

The method for manufacturing a polarizing film typically includes the following steps: forming a polyvinyl alcohol-based resin layer containing a halogenated compound and a polyvinyl alcohol-based 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 at least 2% in its widthwise direction. This method provides a polarizing film with excellent optical properties and suppressed optical property variation. The polarizing film has a thickness of 8 μm or less, a single-element transmittance of 44.5% or greater, and a degree of polarization of 99.0% or greater. In other words, by introducing assisted stretching, the crystallinity of the PVA can be enhanced even when it is coated on a thermoplastic resin, achieving high optical properties. Furthermore, by pre-enhancing the PVA's orientation, problems such as loss of orientation or dissolution during immersion in water during 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 resulting loss of orientation compared to a PVA resin layer without halides. Consequently, the optical properties of polarizing films obtained through treatments such as dyeing and underwater stretching, which involve immersing the laminate in a liquid, can be improved. Furthermore, by shrinking the laminate in width through a drying and shrinking process, optical properties can be improved.

B-2. Protective layer

The first protective layer 12 and the second protective layer 13 are each formed of any suitable film that can be used as a protective layer for a polarizing film. Specific examples of the material constituting the main component of these films include cellulose resins such as triacetate cellulose (TAC), polyesters, polyvinyl alcohols, polycarbonates, polyamides, polyimides, polyether sulfones, polysulfones, polystyrenes, polynorbornenes, polyolefins, (meth)acrylic acid, and acetate-based transparent resins. Other examples include thermosetting resins such as (meth)acrylic acid, urethane, (meth)acrylic urethane, epoxy, and silicone resins, as well as UV-curing resins. Other examples include glassy polymers such as silicone polymers. In addition, the polymer film described in Japanese Patent Application Publication No. 2001-343529 (WO01/37007) can also be used. As the material of the film, for example, a resin composition containing a thermoplastic resin having a substituted or unsubstituted imide group in the side chain and a thermoplastic resin having a substituted or unsubstituted phenyl group and a nitrile group in the side chain can be used. For example, a resin composition containing an alternating copolymer composed of isobutylene and N-methylmaleimide and an acrylonitrile-styrene copolymer can be cited. The polymer film can be, for example, an extruded product of the above-mentioned resin composition. In one embodiment, the protective layer (especially the protective layer on the visual side) contains a TAC resin. By using a TAC-based resin film as a protective layer, bending durability can be improved.

The polarizing plate with a retardation layer of the present invention is typically positioned on the viewing side of an image display device, as described below, and the first protective layer 12 is typically positioned on that viewing side. Therefore, the first protective layer 12 may also be subjected to surface treatments such as hard coating, anti-reflection treatment, anti-sticking treatment, and anti-glare treatment, as needed. Furthermore,/or alternatively, the first protective layer 12 may also be treated to improve visibility when viewed through polarized sunglasses (typically, imparting (elliptical) circular polarization or imparting ultra-high retardation). By applying these treatments, excellent visibility can be achieved even when viewing a displayed image through polarized lenses such as polarized sunglasses. Therefore, polarizing plates with phase difference layers can also be suitably used in outdoor image display devices.

The thickness of the first protective layer is preferably 5μm to 80μm, preferably 10μm to 40μm, and more preferably 10μm to 35μm. When surface treatment is applied, the thickness of the outer protective layer includes the thickness of the surface treatment layer.

In one embodiment, the second protective layer 13 is preferably optically isotropic. In this specification, "optically isotropic" means that the in-plane retardation Re(550) is 0nm to 10nm, and the thickness direction retardation Rth(550) is -10nm to +10nm. In one embodiment, the second protective layer 13 is a retardation layer having any appropriate retardation value. In this case, the in-plane retardation Re(550) of the retardation layer is, for example, 110nm to 150nm. The thickness of the second protective layer is preferably 5μm to 80μm, preferably 10μm to 40μm, and more preferably 10μm to 30μm. From the perspective of thinness and weight reduction, it is ideal to omit the second protective layer.

B-3. Polarizing Film Manufacturing Method

Polarizing films can be produced, for example, by a manufacturing method comprising the following steps: forming a polyvinyl alcohol-based resin layer (PVA-based resin layer) on one side of a long thermoplastic resin substrate to form a laminate. The PVA-based resin layer contains a halogenated compound and a PVA-based resin; 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 longitudinally, thereby shrinking it by at least 2% in the width direction. 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 drying and shrinking treatment is preferably carried out using a heating roller, and the temperature of the heating roller is preferably 60°C to 120°C. The shrinkage rate in the width direction obtained by the drying and shrinking treatment of the laminate is preferably not less than 2%. According to the manufacturing method, the polarizing film described in the above-mentioned item B-1 can be obtained. In particular, a polarizing film having excellent optical properties (represented by single-unit transmittance and polarization degree) and suppressed optical property variations can be obtained by the following method: After preparing a laminate including a PVA-based resin layer containing a halogenated substance, the laminate is stretched by a multi-stage stretching process including air-assisted stretching and underwater stretching, and the stretched laminate is then heated by a heating roller. Specifically, by using heated rollers during the drying and shrinking process, the laminate can be uniformly shrunk while being transported. This not only improves the optical properties of the resulting polarizing film, but also enables the stable production of polarizing films with excellent optical properties and suppresses variations in the film's optical properties (particularly the transmittance of individual cells).

B-3-1. Making a laminate

The laminate of the thermoplastic resin substrate and the PVA-based resin layer can be prepared by any appropriate method. 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 roller coating, spin coating, wire rod coating, dip coating, die coating, curtain coating, spray coating, and doctor blade coating (comma coating, etc.). The application and drying temperature of the coating liquid should preferably be above 50°C.

The thickness of the PVA resin layer should be between 3μm and 40μm, more preferably between 3μm and 20μm.

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

B-3-1-1. Thermoplastic resin substrate

The thickness of the thermoplastic resin substrate should preferably be between 20μm and 300μm, more preferably between 50μm and 200μm. If it is less than 20μm, it may be difficult to form the PVA resin layer. If it is greater than 300μm, for example, the thermoplastic resin substrate may take a long time to absorb water during the underwater stretching treatment described later, and there is a risk of excessive load on the stretching process.

The water absorption rate of the thermoplastic resin substrate is preferably 0.2% or higher, more preferably 0.3% or higher. When the thermoplastic resin substrate absorbs water, the water activates the plasticizer, causing plasticization. This significantly reduces the elongation stress, enabling high elongation rates. On the other hand, the water absorption rate of the thermoplastic resin substrate is preferably 3.0% or lower, more preferably 1.0% or lower. Using this type of thermoplastic resin substrate prevents problems such as a significant decrease in the dimensional stability of the thermoplastic resin substrate during manufacturing, which could deteriorate the appearance of the resulting polarizing film. It also prevents the substrate from cracking during stretching in water, or the PVA resin layer from peeling off from the thermoplastic resin substrate. The water absorption rate of thermoplastic resin substrates can be adjusted, for example, by introducing a modifying group into the constituent material. The water absorption rate is determined according to JIS K 7209.

The glass transition temperature (Tg) of the thermoplastic resin substrate is preferably below 120°C. By using such a thermoplastic resin substrate, the crystallization of the PVA-based resin layer can be suppressed while the elongation of the laminate is fully ensured. In addition, considering that the thermoplastic resin substrate is plasticized by water and can be well elongated in water, it is preferably below 100°C, and more preferably below 90°C. On the other hand, the glass transition temperature of the thermoplastic resin substrate is preferably above 60°C. By using such a thermoplastic resin substrate, it is possible to prevent the thermoplastic resin substrate from deforming (such as unevenness, sagging or wrinkling) when applying and drying the coating liquid containing the above-mentioned PVA-based resin, thereby preventing the laminate from being well produced. Furthermore, the PVA resin layer can be stretched well at an appropriate temperature (e.g., around 60°C). Furthermore, the glass transition temperature of a thermoplastic resin substrate can be adjusted by, for example, heating it with a crystallizing material that can introduce modified groups into the constituent material. The glass transition temperature (Tg) is determined in accordance with JIS K 7121.

The thermoplastic resin substrate can be made of any suitable thermoplastic resin. Examples of thermoplastic resins include ester resins such as polyethylene terephthalate resins, cycloolefin resins such as norbornene resins, olefin resins such as polypropylene, polyamide resins, polycarbonate resins, and copolymers thereof. Of these, norbornene resins and amorphous polyethylene terephthalate resins are particularly preferred.

In one embodiment, an amorphous (uncrystallized) polyethylene terephthalate resin is preferably used. Among them, an amorphous (difficult to crystallize) polyethylene terephthalate resin is particularly preferred. Specific examples of amorphous polyethylene terephthalate resins include copolymers containing isophthalic acid and/or cyclohexanedicarboxylic acid as dicarboxylic acids, or copolymers containing cyclohexanedimethanol or diethylene glycol as glycols.

In a preferred embodiment, the thermoplastic resin substrate is composed of a polyethylene terephthalate resin containing isophthalic acid units. This is because such a thermoplastic resin substrate has excellent extensibility and can suppress crystallization during extension. We speculate that this is due to the introduction of isophthalic acid units to impart a large degree of curvature to the main chain. Polyethylene terephthalate resins contain terephthalic acid units and ethylene glycol units. The content ratio of isophthalic acid units relative to the total of all repeating units is preferably 0.1 mol% or more, and more preferably 1.0 mol% or more. This is because a thermoplastic resin substrate with excellent extensibility can be obtained. On the other hand, the content of isophthalic acid units relative to the total of all repeating units is preferably 20 mol% or less, and more preferably 10 mol% or less. This content ratio effectively increases the degree of crystallization during the drying and shrinking treatment described below.

The thermoplastic resin substrate may also be pre-stretched (before forming the PVA-based resin layer). In one embodiment, stretching is performed in the transverse direction of the long thermoplastic resin substrate. The transverse direction is preferably perpendicular to the direction of stretching of the laminate described below. Furthermore, the term "perpendicular" as used herein includes substantially perpendicular. Here, "substantially perpendicular" includes 90° ± 5.0°, preferably 90° ± 3.0°, and more preferably 90° ± 1.0°.

The elongation temperature of thermoplastic resin substrates should preferably be between Tg-10°C and Tg+50°C relative to the glass transition temperature (Tg). The elongation ratio of thermoplastic resin substrates should preferably be between 1.5x and 3.0x.

The thermoplastic resin substrate can be stretched using any appropriate method. Specifically, it can be fixed-end stretching or free-end stretching. The stretching method can be dry or wet. The thermoplastic resin substrate can be stretched in a single stage or in multiple stages. When stretched in multiple stages, the stretching ratio described above is the product of the stretching ratios of each stage.

B-3-1-2. Coating liquid

The coating liquid comprises a halogenated compound and a PVA-based resin as described above. The coating liquid is typically a solution formed by dissolving the halogenated compound and the PVA-based resin 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 can be used alone or in combination. Water is particularly preferred. The PVA-based resin concentration in the solution is preferably 3 to 20 parts by weight relative to 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 liquid is preferably 5 to 20 parts by weight relative to 100 parts by weight of the PVA resin.

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

Any suitable PVA 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 is determined in accordance with JIS K 6726-1994. Using a PVA resin with this saponification degree can produce a polarizing film with excellent durability. Excessively high saponification degrees may cause gelation.

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 according to JIS K 6726-1994.

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

The amount of halogen in the coating solution is preferably 5 to 20 parts by weight per 100 parts by weight of the PVA resin, more preferably 10 to 15 parts by weight. If the amount of halogen exceeds 20 parts by weight per 100 parts by weight of the PVA resin, the halogen may overflow, resulting in a cloudy white appearance in the resulting polarizing film.

Generally speaking, stretching a PVA resin layer increases the orientation of the polyvinyl alcohol molecules within the layer. However, immersing the stretched PVA resin layer in an aqueous solution can disrupt the orientation of the polyvinyl alcohol molecules, reducing this orientation. This tendency to reduce 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 generally performed at 60°C. In contrast, stretching of a laminate of an A-PET (thermoplastic resin substrate) and a PVA-based resin layer is performed at a relatively high temperature, around 70°C. At this temperature, the orientation of the PVA at the beginning of stretching decreases before it increases due to stretching in water. To address this issue, after preparing a laminate of a halogenated PVA-based resin layer and a thermoplastic resin substrate, the laminate is stretched in air at a high temperature (assisted stretching) before stretching in boric acid water. This promotes crystallization of the PVA-based resin in the PVA-based resin layer of the laminate after assisted stretching. As a result, when the PVA resin layer is immersed in a liquid, the orientation disorder of the polyvinyl alcohol molecules and the reduction in orientation are suppressed compared to the case where the PVA resin layer does not contain halides. This can improve the optical properties of polarizing films obtained through treatment steps such as dyeing and underwater stretching, where the laminate is immersed in a liquid.

B-3-2. Air-assisted extended processing

In particular, to achieve high optical properties, a two-stage stretching method combining dry stretching (assisted stretching) and stretching in boric acid water is chosen. In this two-stage stretching method, by introducing the assisted stretching, stretching is performed while suppressing crystallization of the thermoplastic resin substrate. This solves the problem of reduced stretchability caused by excessive crystallization of the thermoplastic resin substrate during subsequent stretching in boric acid water, thereby enabling higher stretching rates for the laminate. Furthermore, when coating PVA-based resins on thermoplastic resin substrates, the coating temperature must be lower than when coating PVA-based resins on conventional metal rollers to mitigate the effects of the thermoplastic substrate's glass transition temperature. This results in a relatively slow crystallization of the PVA-based resin, preventing the achievement of sufficient optical properties. By introducing assisted stretching, the crystallinity of the PVA-based resin can be enhanced even when coating the resin 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 of the PVA resin during subsequent dyeing or stretching steps can be avoided, thereby achieving high optical properties.

In-flight stretching can be performed using either fixed-end stretching (e.g., using a tenter stretching machine) or free-end stretching (e.g., uniaxial stretching by passing the laminate between rollers with different circumferential speeds). However, to achieve high optical properties, free-end stretching is preferred. In one embodiment, the in-flight stretching process includes a heated roller stretching step, in which the laminate is conveyed along its longitudinal direction while simultaneously stretching the laminate using the differential circumferential speeds between the heated rollers. The in-flight stretching process typically includes a zone stretching step and a heated roller stretching step. In addition, the order of the zone stretching step and the heating roller stretching step is not limited. The zone stretching step can be performed first, or the heating roller stretching step can be performed first. The zone stretching step can also be omitted. In one embodiment, the zone stretching step and the heating roller stretching step are performed in sequence. In another embodiment, the film ends are grasped in a tenter stretching machine, and the distance between the tenters is expanded in the direction of travel to perform stretching (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. Preferably, the stretching ratio relative to the direction of travel can be set to achieve approach using free end stretching. When the free end is extended, the shrinkage rate in the width direction is calculated as (1/extension 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 roughly the same as the stretching direction in underwater stretching.

The stretching ratio for air-assisted stretching is preferably 2.0x to 3.5x. The maximum stretching ratio when combining air-assisted stretching with underwater stretching is preferably at least 5.0x, preferably at least 5.5x, and even more preferably at least 6.0x, relative to the original length of the laminate. The "maximum stretching ratio" in this specification refers to the stretching ratio immediately before the laminate fractures. This is a value 0.2 lower than the stretching ratio at which the laminate fractures, after separately confirming the stretching ratio.

The stretching temperature of the air-assisted stretching can be set to any appropriate value according to the forming material of the thermoplastic resin substrate, the stretching method, etc. The stretching temperature is preferably above the glass transition temperature (Tg) of the thermoplastic resin substrate, more preferably above the glass transition temperature (Tg) of the thermoplastic resin substrate + 10°C, and particularly preferably above Tg + 15°C. On the other hand, the upper limit of the stretching temperature is preferably 170°C. By stretching at the above temperature, the rapid progress of crystallization of the PVA-based resin can be suppressed, thereby suppressing the adverse conditions caused by the crystallization (for example, the orientation of the PVA-based resin layer is hindered by stretching). The crystallization index of the PVA-based resin after air-assisted stretching is preferably 1.3~1.8, more preferably 1.4~1.7. The crystallization index of PVA resins can be measured using the ATR method with a Fourier transform infrared spectrophotometer. Specifically, polarized light is used as the measuring light, and the crystallization index is calculated using the intensities at 1141 ^cm⁻¹ and 1440 ^cm⁻¹ of the resulting spectrum according to the following formula.

Crystallization index = ( _IC / _IR )

However, _IC : the intensity at 1141 cm ^-1 when the measurement light is incident and measured, and _IR : the intensity at 1440 cm ^-1 when the measurement light is incident and measured.

B-3-3. Insolubility Treatment

If necessary, an insolubilization treatment may be performed after the air-assisted stretching treatment and before the underwater stretching or dyeing treatment. This insolubilization treatment typically involves immersing the PVA resin layer in an aqueous boric acid solution. This insolubilization treatment imparts water resistance to the PVA resin layer and prevents loss of PVA orientation during immersion in water. The concentration of the aqueous boric acid solution is preferably 1 to 4 parts by weight per 100 parts by weight of water. The temperature of the insolubilization bath (aqueous boric acid solution) is preferably 20°C to 50°C.

B-3-4. Dyeing Process

The dyeing treatment typically involves dyeing the PVA resin layer with a dichroic substance (typically iodine). Specifically, this is accomplished by allowing the iodine to adsorb into the PVA resin layer. Examples of such adsorption methods include immersing the PVA resin layer (laminated body) in an iodine-containing dye solution, applying the dye solution to the PVA resin layer, and spraying the dye solution onto the PVA resin layer. Immersing the laminate in the dye solution (dye bath) is preferred because it effectively adsorbs iodine.

The dyeing solution is preferably an aqueous iodine solution. The iodine content is preferably 0.05 to 0.5 parts by weight per 100 parts by weight of water. To increase the solubility of iodine in water, an iodide is preferably added to the aqueous iodine solution. Examples of iodides include potassium iodide, lithium iodide, sodium iodide, zinc iodide, aluminum iodide, lead iodide, copper iodide, barium iodide, calcium iodide, tin iodide, and titanium iodide. Potassium iodide is particularly preferred. The iodide content is preferably 0.1 to 10 parts by weight, more preferably 0.3 to 5 parts by weight, per 100 parts by weight of water. To inhibit the dissolution of the PVA-based resin, the dyeing solution should be maintained at a temperature of 20°C to 50°C during dyeing. To ensure the transmittance of the PVA resin layer when immersing it in the dye solution, the immersion time should ideally be 5 seconds to 5 minutes, and 30 seconds to 90 seconds is optimal.

The dyeing conditions (concentration, liquid temperature, and immersion time) are set so that the resulting polarizing film has a single-element transmittance of 44.5% or greater and a degree of polarization of 99.0% or greater. The dyeing conditions preferably use an iodine aqueous solution as the dyeing solution, with the iodine-to-potassium iodide content ratio in the iodine aqueous solution set to 1:5 to 1:20. The iodine-to-potassium iodide content ratio in the iodine aqueous solution is preferably 1:5 to 1:10. This produces a polarizing film with the aforementioned optical properties.

When the laminate is immersed in a treatment bath containing boric acid (typically, an insolubilization treatment) and then subsequently dyed, the boric acid contained in the treatment bath mixes into the dye bath, causing the boric acid concentration in the dye bath to fluctuate over time, resulting in unstable dyeability. To prevent this destabilization, the upper limit of the boric acid concentration in the dye bath is preferably 4 parts by weight, more preferably 2 parts by weight, per 100 parts by weight of water. On the other hand, the lower limit of the boric acid concentration in the dye bath is preferably 0.1 parts by weight, more preferably 0.2 parts by weight, and even more preferably 0.5 parts by weight, per 100 parts by weight of water. In one embodiment, the dyeing process is performed using a dye bath pre-mixed with boric acid. This can reduce the rate of change in boric acid concentration when the boric acid in the treatment bath is added to the dye bath. The amount of boric acid pre-mixed into the dye bath (i.e., the amount of boric acid not from the treatment bath) is preferably 0.1 to 2 parts by weight, more preferably 0.5 to 1.5 parts by weight, per 100 parts by weight of water.

B-3-5. Cross-linking treatment

If necessary, a crosslinking treatment may be performed after the dyeing treatment and before the underwater stretching treatment. The crosslinking treatment may be performed by immersing the PVA resin layer in an aqueous boric acid solution. By performing the crosslinking treatment, the PVA resin layer is rendered water-resistant, thereby preventing the PVA from losing its orientation when immersed in high-temperature water during the subsequent underwater stretching. The concentration of the aqueous boric acid solution is preferably 1 to 5 parts by weight relative to 100 parts by weight of water. Furthermore, when performing the crosslinking treatment after the dyeing treatment, an iodide may be further added. By adding an iodide, the dissolution of iodine adsorbed on the PVA resin layer may be suppressed. The amount of iodide added is preferably 1 to 5 parts by weight relative to 100 parts by weight of water. Specific examples of iodide are as described above. The temperature of the crosslinking bath (boric acid aqueous solution) should be between 20°C and 50°C.

B-3-6. Extended treatment in water

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, enabling high-ratio stretching while suppressing crystallization of the PVA resin layer. The result is a polarizing film 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 is stretched uniaxially by passing it between rollers with different circumferential speeds). Free-end stretching is preferred. Stretching of the laminate can be performed in a single stage or in multiple stages. When stretching is performed in multiple stages, the stretch ratio (maximum stretch ratio) of the laminate, described below, is the product of the stretch ratios of 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 during stretching and water-resistance, making it insoluble in water. 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 a polarizing film 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 a polarizing film 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 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 (stretching bath temperature) is preferably between 40°C and 85°C, more preferably between 60°C and 75°C. This temperature suppresses dissolution of the PVA resin layer while enabling high stretching ratios. Specifically, as mentioned above, considering the formation of the PVA resin layer, the glass transition temperature (Tg) of the thermoplastic resin substrate should ideally be above 60°C. If the stretching temperature is below 40°C, even if water is used to plasticize the thermoplastic resin substrate, good stretching may not be possible. On the other hand, higher stretching bath temperatures increase the solubility of the PVA resin layer, potentially preventing the achievement of excellent optical properties. The laminate should be immersed in the stretching bath for a period of 15 seconds to 5 minutes.

The stretching ratio during underwater stretching is preferably 1.5 times or greater, more preferably 3.0 times or greater. The total stretching ratio of the laminate is preferably 5.0 times or greater, more preferably 5.5 times or greater, relative to the original length of the laminate. Achieving such a high stretching ratio enables the production of a polarizing film with excellent optical properties. This high stretching ratio can be achieved by using an underwater stretching method (boric acid underwater stretching).

B-3-7. Drying and shrinkage treatment

The drying and shrinking process can be performed by heating the entire area (zone heating) or by heating the transport rollers (so-called heating roller drying). Using both methods is preferred. Using heating rollers for drying effectively suppresses heat-induced warping of the laminate, resulting in a polarizing film with excellent appearance. Specifically, drying the laminate while it travels along the heating rollers effectively promotes crystallization of the thermoplastic resin substrate, increasing its degree of crystallization. This can significantly increase the degree of crystallization of the thermoplastic resin substrate even at relatively low drying temperatures. As a result, the rigidity of the thermoplastic resin substrate increases, allowing it to withstand 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%. By using heated rollers, the laminate can be conveyed while continuously shrinking in its width, achieving high productivity.

Figure 3 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 200 while drying it. In the illustrated example, the conveyor rollers R1-R6 are positioned to alternately and continuously heat the PVA resin layer surface and the thermoplastic resin substrate surface. However, the conveyor rollers R1-R6 can also be positioned to continuously heat only one surface of the laminate 200 (e.g., the thermoplastic resin substrate surface).

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 rollers. The heating roller temperature is preferably between 60°C and 120°C, more preferably between 65°C and 100°C, and even more preferably between 70°C and 80°C. This effectively increases the crystallization degree of the thermoplastic resin and effectively suppresses warping, while producing optical laminates with excellent durability. The heating roller temperature can be measured using a contact thermometer. The example shown in the figure shows six conveyor rollers, but any number is acceptable. The number of conveyor rollers is typically between two and 40, with four to 30 being preferred. The contact time between the laminate and the heating roller (total contact time) is preferably 1 to 300 seconds, preferably 1 to 20 seconds, and even more preferably 1 to 10 seconds.

The heating rollers can be installed inside a furnace (such as an oven) or on a standard manufacturing line (at room temperature). They are preferably installed inside a 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 ideally be between 30°C and 100°C. The hot air drying time should ideally be between 1 and 300 seconds. The hot air velocity should ideally be between 10 and 30 m/s. This velocity is measured within the furnace and can be measured with a mini fan-type digital anemometer.

B-3-8. Other Processing

It is recommended to perform a cleaning treatment after the water stretching treatment and before the drying and shrinking treatment. This cleaning treatment can typically be performed by immersing the PVA resin layer in an aqueous potassium iodide solution.

C. 1st phase difference layer

The first retardation layer 20 can have any appropriate optical and/or mechanical properties depending on the intended purpose. The first retardation layer 20 typically has a slow axis. In one embodiment, the angle θ formed between the slow axis of the first retardation layer 20 and the absorption axis of the polarizing film 11 is, as described above, 40° to 50°, preferably 42° to 48°, and more preferably approximately 45°. As long as 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 (and consequently, excellent anti-reflection properties) can be obtained.

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

The Nz coefficient of the first retardation layer is preferably 0.9-3, preferably 0.9-2.5, more preferably 0.9-1.5, and most 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 measured light, a normal wavelength dispersion characteristic in which the phase difference value decreases with the wavelength of the measured light, or a flat wavelength dispersion characteristic in which the phase difference value hardly changes with the wavelength of the measured light. In one embodiment, the first phase difference layer exhibits an inverse dispersion wavelength characteristic. In this case, the Re(450)/Re(550) of the phase difference layer is preferably greater than 0.8 and less than 1, and more preferably greater than 0.8 and less than 0.95. With the above 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, preferably between 2.0× ^{10-13 m²} /N and 1.5 ^{×10-11 m²} /N, and 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 are less likely to occur when heat-induced compression stress is applied. As a result, thermal nonuniformity in the resulting image display device can be effectively prevented.

The first retardation layer is typically formed of a stretched resin film. In one embodiment, the thickness of the first retardation layer is preferably 70 μm or less, and preferably 45 μm to 60 μm. When the thickness of the first retardation layer is within this range, curling during heating can be effectively suppressed and curling during lamination can be effectively adjusted. Furthermore, as described below, in embodiments in which the first retardation layer is formed of a polycarbonate resin film, the thickness of the first retardation layer is preferably 40 μm or less, and preferably 10 μm to 40 μm, and more preferably 20 μm to 30 μm. Forming the first retardation layer of a polycarbonate resin film having this thickness suppresses curling and contributes to improved flexural durability and reflected hue.

The first retardation layer 20 can be formed of 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 phase difference 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.

As long as the effects of the present invention are achieved, any appropriate polycarbonate resin may be used as the polycarbonate resin. For example, the polycarbonate resin may comprise 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. Preferably, the polycarbonate resin comprises 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 comprises 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 comprise structural units derived from other dihydroxy compounds as needed. In addition, details of polycarbonate resins that can be suitably used in the present invention are described, for example, in Japanese Patent Application Laid-Open No. 2014-10291, Japanese Patent Application Laid-Open No. 2014-26266, Japanese Patent Application Laid-Open No. 2015-212816, Japanese National Publication No. 2015-212817, and Japanese National 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, and 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 resins can be expressed as a thick 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 thick viscosity is generally 0.30 dL/g, preferably 0.35 dL/g or higher. The upper limit of the thick viscosity is generally 1.20 dL/g, preferably 1.00 dL/g, and preferably 0.80 dL/g. A thick viscosity below this lower limit may result in reduced mechanical strength of the resulting molded product. On the other hand, if the specific viscosity exceeds the upper limit, the fluidity during molding will decrease, which may lead to problems with productivity or moldability.

Commercially available polycarbonate resin films can 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 20 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 cast film), 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 film, etc. Furthermore, as mentioned above, polycarbonate resins are widely available in the market as film products, so 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 phase difference layer, the desired optical properties, the stretching conditions described below, and other factors. A suitable value is 50 μm to 300 μm.

The stretching described above may be performed using any appropriate stretching method and conditions (e.g., stretching temperature, stretching ratio, and stretching direction). Specifically, various stretching methods, such as free-end stretching, fixed-end stretching, free-end shrinking, and fixed-end shrinking, may be used individually, simultaneously, or sequentially. Stretching may be performed along various directions or dimensions, such as the longitudinal direction, width direction, thickness direction, or diagonal direction. The stretching temperature is preferably between Tg-30°C and Tg+60°C relative to the glass transition temperature (Tg) of the resin film, and preferably between Tg-10°C and Tg+50°C.

By appropriately selecting the stretching method and stretching conditions, a retardation film having the desired optical properties (e.g., 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 a method in which the resin film is stretched in the widthwise (lateral) direction while being moved along its longitudinal direction. The stretching ratio is preferably 1.1x to 3.5x.

In another embodiment, the retardation film can be produced by continuously stretching a long strip of resin film obliquely at the aforementioned angle θ relative to its longitudinal direction. This oblique stretching produces a long, stretched film having an orientation angle θ relative to its longitudinal direction (with a slow axis in the direction of angle θ). This allows for roll-to-roll lamination with a polarizing film, simplifying the manufacturing process. Furthermore, angle θ can be the angle formed between the absorption axis of the polarizing film and the slow axis of the retardation layer in a polarizing plate with a retardation layer. As mentioned above, angle θ is preferably 40° to 50°, preferably 42° to 48°, and more preferably approximately 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 will 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.

D. Second phase difference layer

As described above, the second phase difference layer can be a so-called positive C-plate whose refractive index characteristics exhibit the relationship nz>nx=ny. By using a positive C-plate as the second phase difference layer, oblique reflections can be effectively prevented, and the anti-reflection function can be extended to a wider viewing angle. In this case, the phase difference Rth(550) in the thickness direction of the second phase difference layer is preferably -50nm to -300nm, 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 exactly equal, but also the case where nx and ny are substantially equal. In other words, the in-plane phase difference Re(550) of the second phase difference layer can be less than 10nm.

The second retardation layer, which has a refractive index characteristic of nz > nx = ny, can be formed of any appropriate material. The second retardation layer is preferably formed of a thin film containing a liquid crystal material fixed in homeotropic alignment. The liquid crystal material (liquid crystal compound) capable of 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 retardation layer include the liquid crystal compound and the method for forming the retardation layer described in paragraphs [0020] to [0028] of Japanese Patent Application Laid-Open No. 2002-333642. In this case, the thickness of the second retardation layer is preferably 0.5 μm to 10 μm, more preferably 0.5 μm to 8 μm, and more preferably 0.5 μm to 5 μm.

E. Conductive layer or isotropic substrate with conductive layer attached

The conductive layer can be formed by forming 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 retardation layer (or the second retardation layer if a second retardation layer is present), and the conductive layer can serve alone as a constituent layer of the polarizing plate with a retardation layer. Alternatively, the conductive layer and substrate can be laminated onto the first retardation layer (or the second retardation layer if a second retardation layer is present) in the form of a laminate (substrate with conductive layer). Ideally, the substrate is optically isotropic, so the conductive layer can serve as an isotropic substrate with a conductive layer for the polarizing plate with a retardation layer.

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

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 portions and insulating portions. Consequently, electrodes can be formed. These electrodes can function as touch sensing electrodes for sensing contact with the touch panel. Any appropriate patterning method can be used. Specific examples include wet etching and screen printing.

F. Image display device

The polarizing plates with a phase difference layer described in Items A through E above can be applied to image display devices. Therefore, the present invention includes an image display device using the polarizing plates 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 and inorganic EL display devices). The image display device according to an embodiment of the present invention includes the polarizing plates with a phase difference layer described in Items A through E above on its viewing side. The polarizing plates with a phase difference layer are laminated such that the phase difference layer is on the side of the image display unit (e.g., liquid crystal unit, organic EL unit, inorganic EL unit) (e.g., the polarizing film 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 polarizing plate with a phase difference layer of the present invention exhibits a more pronounced effect.

Implementation Examples

The present invention is described below in detail 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 specified, "parts" and "%" in the examples and comparative examples are by weight.

(1)Thickness

Thicknesses below 10 μm were measured using an interferometer thickness meter (MCPD-3000, manufactured by Otsuka Electronics Co., Ltd.). Thicknesses greater than 10 μm were measured using a digital micrometer (KC-351C, manufactured by Anritsu Corporation).

(2) Single body transmittance and polarization degree

The polarizing film/protective layer laminates (polarizing plates) used in the Examples and Comparative Examples were measured using a UV-visible spectrophotometer (V-7100 manufactured by JASCO Corporation). The measured single-element transmittance Ts, parallel transmittance Tp, and cross transmittance Tc were used as the polarizing film's Ts, Tp, and Tc, respectively. These Ts, Tp, and Tc values were measured using a 2-degree field (illuminant C) in accordance with JIS Z8701 and calibrated for luminous efficacy. The refractive index of the protective layer was 1.50, and the refractive index of the polarizing film's surface opposite the protective layer was 1.53.

The polarization degree P is calculated from the obtained Tp and Tc using the following formula.

Polarization degree P(%) = {(Tp-Tc)/(Tp+Tc)} ^1/2 × 100

Alternatively, equivalent measurements can be performed using a spectrophotometer such as the LPF-200 manufactured by Otsuka Electronics Co., Ltd. As an example, samples 1 through 3, each having the same configuration as the polarizing plates described in the following examples, were measured using the V-7100 and LPF-200. The measured values for the single-element transmittance Ts and polarization degree P are listed in Table 1. As shown in Table 1, the difference between the single-element transmittance measured using the V-7100 and the LPF-200 is less than 0.1%, demonstrating that equivalent measurement results can be obtained using either spectrophotometer.

For example, when measuring polarizing plates with an anti-glare (AG) surface treatment or an adhesive with diffusing properties, different spectrophotometers will produce different measurement results. However, by using the values obtained when measuring the same polarizing plate with different spectrophotometers as a benchmark for numerical conversion, the difference in the values obtained by the spectrophotometers can be compensated.

(3) The optical properties of long strip polarizing films vary.

From the polarizing plate used in the Examples and Comparative Examples, five test samples were cut out at equal intervals along the width direction. The single transmittance of the central portion of each of the five test samples was measured in the same manner as in (2) above. The difference between the maximum and minimum single transmittance values measured at each measurement position was then calculated, and this value was used as the optical characteristic variation of the long polarizing film.

(4) Optical properties of thin polarizing films vary

A 100 mm x 100 mm sample was cut from the polarizing plate used in the Examples and Comparative Examples, and the optical characteristic variation of the sheet-like polarizing film (50 cm ² ) was determined. Specifically, the single transmittance was measured at five locations, approximately 1.5 cm to 2.0 cm inward from the midpoint of each of the four sides of the sample, and at the center, using the same method as described in (2) above. The difference between the maximum and minimum single transmittance values measured at each measurement location was then calculated, and this value was used as the optical characteristic variation of the sheet-like polarizing film.

(5) Curve

The polarizing plates with retardation layers obtained in the Examples and Comparative Examples were cut into 110 mm x 60 mm dimensions. Cutting was performed with the absorption axis of the polarizing film as the longitudinal direction. The cut polarizing plates with retardation layers were bonded to 120 mm x 70 mm, 0.2 mm thick glass plates with adhesive to create test specimens. The test specimens were placed in an oven maintained at 85°C for 24 hours and then removed to measure the warp. The test specimens were placed on a flat surface with the glass plate facing downward. The height of the highest point above the surface was measured as the warp.

(6) Bending durability

The polarizing plates with retardation layers obtained in the Examples and Comparative Examples were cut into 50 mm x 100 mm pieces. Cutting was performed with the absorption axis of the polarizing film oriented along its shorter side. The cut polarizing plates with retardation layers were subjected to a folding test using a folding tester (CL09 type-D01, manufactured by Yusa) equipped with a constant temperature and humidity chamber at 20°C and 50% RH. Specifically, the polarizing plates with retardation layers were repeatedly bent parallel to the absorption axis, with the retardation layer side facing outward. The number of bends required to produce cracks, peeling, or film breaks that would cause image defects was measured, and evaluation was performed according to the following criteria (bending diameter: 2 mm).

<Evaluation Criteria>

Less than 10,000 times: defective

More than 10,000 times and less than 30,000 times: Good

More than 30,000 times: Excellent

(7) Elastic coefficient

The film to be measured is formed into a tensile test dumbbell-shaped sheet with a parallel width of 10 mm and a length of 40 mm according to JIS K6734:2000. The tensile modulus is then determined by performing a tensile test according to JIS K7161:1994. The longitudinal direction generally corresponds to the stretching direction of the polarizing film.

[Example 1]

1. Make polarizing film

The thermoplastic resin substrate is a long, amorphous polyethylene terephthalate (PET) film (thickness: 100 μm) with a water absorption of 0.75% and a Tg of approximately 75°C. One side of the resin substrate is treated with a corona treatment.

To 100 parts by weight of a PVA-based resin prepared by mixing polyvinyl alcohol (DP 4200, DS 99.2 mol%) and acetyl-modified PVA (manufactured by Nippon Gosei Kagaku Kogyo Co., Ltd., trade name "GOHSEFIMER Z410") in a ratio of 9:1, 13 parts by weight of potassium iodide was added and dissolved in water to prepare a PVA aqueous solution (coating solution).

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 to produce a laminate.

The resulting laminate was then subjected to free-end uniaxial stretching by 2.4 times in the longitudinal direction (longitudinal direction) between rollers of different circumferential speeds in an oven at 130°C (in-flight assisted stretching).

Next, the laminate was immersed in an insolubilization bath (a boric acid aqueous solution prepared by mixing 4 parts by weight of boric acid with 100 parts by weight of water) at a liquid temperature of 40°C for 30 seconds (insolubilization treatment).

Next, the film was immersed in a dye bath (an iodine aqueous solution obtained 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 (dyeing process).

Next, it was immersed in a crosslinking bath (a boric acid aqueous 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 liquid 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 wt%) at a temperature of 70°C and uniaxially stretched in the longitudinal direction (longitudinal direction) between rolls of different circumferential speeds to a total stretching ratio of 5.5 times (underwater stretching treatment).

Afterwards, the laminate was immersed in a cleaning bath (an aqueous solution containing 4 parts by weight of potassium iodide per 100 parts by weight of water) at a liquid temperature of 20°C (cleaning treatment).

Afterwards, the laminate was dried in an oven maintained at 90°C while contacting a SUS heating roller maintained at 75°C for approximately 2 seconds (drying and shrinking treatment). The shrinkage rate in the width direction of the laminate after the drying and shrinking treatment was 5.2%.

Through the above process, a polarizing film with a thickness of 5.0μm was formed on the resin substrate.

2. Make polarizing plates

A cycloolefin film (28 μm thick, 2100 MPa elastic modulus) with a hard coating (refractive index 1.53) is applied to the surface of the polarizing film (opposite to the resin substrate) using a UV-curable adhesive as a protective layer. Specifically, the curable adhesive is applied to a total thickness of 1.0 μm and bonded using a roller. UV light is then applied from the protective layer side to cure the adhesive. The ends are then cut and the resin substrate is peeled off, resulting in a long polarizing plate (1300 mm wide) consisting of a protective layer/adhesive layer/polarizing film. The single-unit transmittance of a polarizing plate (essentially a polarizing film) is 44.54%, and the degree of polarization is 99.703%. Furthermore, the optical property variation of a long polarizing film is 0.18%, while that of a thin polarizing film is 0.05%.

3. Fabrication of the phase difference film that constitutes the phase difference layer

3-1. Polymerization of Polyester Carbonate Resins

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. This temperature was maintained while the pressure was reduced to 13.3 kPa 90 minutes after reaching 220°C. Phenol vapor, a by-product of the polymerization reaction, was introduced into a 100°C reflux cooler to return a small amount of monomer contained in the phenol vapor to the reactor. Uncondensed phenol vapor was then introduced into a 45°C condenser for recovery. 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 increased and reduced, reaching 240°C and 0.2 kPa after 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.

3-2. Preparation of Phase Difference Film

The resulting polyester carbonate resin pellets were vacuum-dried at 80°C for 5 hours. Then, a 130μm thick long resin film strip was formed using a film-forming apparatus equipped with a uniaxial extruder (manufactured by 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 strip was stretched while adjusting to achieve the desired retardation, resulting in a 48μm thick retardation film. Stretching conditions were applied in the width direction at a temperature of 143°C and a stretch 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.

4. Making a polarizing plate with a phase difference layer

The phase difference film obtained in step 3 was bonded to the surface of the polarizing film of the polarizing plate obtained in step 2. above via an acrylic adhesive (thickness 5 μm). At this time, the polarizing film was bonded so that the absorption axis of the polarizing film and the slow axis of the phase difference film formed an angle of 45°. In the above manner, a polarizing plate with a phase difference layer having a structure of protective layer/bonding layer/polarizing film/adhesive layer/phase difference layer was obtained. The total thickness of the polarizing plate with a phase difference layer obtained was 87 μm. The polarizing plate with a phase difference layer obtained was subjected to the evaluations of steps (5) and (6) above. The warp was 3.4 mm. The results are shown in Table 2.

[Example 2-1]

1. Make polarizing film

A polarizing film with a thickness of 5.0 μm was formed on a resin substrate in the same manner as in Example 1.

2. Make polarizing plates

A polarizing plate with a protective layer/bonding layer/polarizing film structure was produced in the same manner as in Example 1, except that a triacetate cellulose (TAC) film with a hard coat (hard coat thickness 7 μm, TAC thickness 25 μm, modulus of elasticity: 3600 MPa) was used as the protective layer.

3. Fabrication of the phase difference film that constitutes the phase difference layer

A polyester carbonate resin pellet was obtained in the same manner as in Example 1, except that 0.7 parts by mass of PMMA was melt-kneaded. After vacuum drying at 80°C for 5 hours, a 105 μm thick long resin film was produced using a film forming apparatus equipped with a uniaxial extruder (manufactured by Toshiba Machine Co., Ltd., cylinder temperature setting: 250°C), a T-die (200 mm width, temperature setting: 250°C), a cooling roll (temperature setting: 120-130°C), and a winder. The resulting long resin film was then stretched 2.8 times in the width direction at 138°C while adjusting the film to achieve a predetermined retardation, resulting in a 38 μm thick retardation film. The Re(550) of the obtained retardation film is 144nm, and the Re(450)/Re(550) is 0.86.

4. Making a polarizing plate with a phase difference layer

The retardation film obtained in step 3 was bonded to the surface of the polarizing film of the polarizing plate obtained in step 2 above, using an acrylic adhesive (5 μm thick). The polarizing film's absorption axis and the retardation film's slow axis were bonded so that they formed a 45° angle. This yielded a polarizing plate with a retardation layer, comprising a protective layer/bonding layer/polarizing film/adhesive layer/retardation layer. The total thickness of the resulting polarizing plate with a retardation layer was 81 μm. The resulting polarizing plate with a retardation layer was subjected to the same evaluations as in Example 1. The results are listed in Table 2.

[Example 2-2]

A 105 μm thick strip of polyester carbonate resin film obtained in the same manner as in Example 2-1 was adjusted to obtain a predetermined retardation while being stretched in the width direction to obtain a retardation film having a thickness of 38 μm. The Re(550) of the obtained retardation film was 140 nm.

A polarizing plate with a retardation layer was obtained in the same manner as in Example 2-1, except that the aforementioned retardation film was used as the retardation layer. The structure of the polarizing plate with a retardation layer was: protective layer/bonding layer/polarizing film/adhesive layer/retardation layer. The total thickness of the resulting polarizing plate with a retardation layer was 81 μm. The resulting polarizing plate with a retardation layer was evaluated in the same manner as in Example 1. The results are listed in Table 2.

[Example 2-3]

A 105 μm thick strip of polyester carbonate resin film obtained in the same manner as in Example 2-1 was adjusted to obtain a predetermined phase difference and then stretched in the width direction to obtain a 38 μm thick phase difference film. The Re(550) of the obtained phase difference film was 149 nm.

A polarizing plate with a retardation layer was obtained in the same manner as in Example 2-1, except that the aforementioned retardation film was used as the retardation layer. The structure of the polarizing plate with a retardation layer was: protective layer/bonding layer/polarizing film/adhesive layer/retardation layer. The total thickness of the resulting polarizing plate with a retardation layer was 81 μm. The resulting polarizing plate with a retardation layer was evaluated in the same manner as in Example 1. The results are listed in Table 2.

[Example 3]

A polycarbonate resin film obtained in the same manner as in Example 1 was obliquely stretched according to the method of Example 2 in Japanese Patent Application Publication No. 2014-194483 to obtain a retardation film having a thickness of 58 μm. The resulting retardation film had an Re(550) of 144 nm, a Re(450)/Re(550) of 0.86, an Nz coefficient of 1.21, and an orientation angle (in the direction of the slow axis) relative to the longitudinal direction of 45°. This retardation film was laminated with the polarizing plate of Example 1 in a roll-to-roll manner via an acrylic adhesive (5 μm thick), thereby obtaining a polarizing plate with a retardation layer having a structure of protective layer/adhesive layer/polarizing film/adhesive layer/retardation layer. The total thickness of the resulting polarizing plate with a retardation layer was 97 μm. The resulting polarizing plate with a retardation layer was subjected to the same evaluation as in Example 1. The warp was 4.1 mm. The results are listed in Table 2.

[Comparative example 1]

1. Making polarizers

A 30μm-thick polyvinyl alcohol (PVA) resin film with an average degree of polymerization of 2,400 and a saponification degree of 99.9 mol% was prepared. The film was immersed in a 20°C swelling bath (water bath) between rollers with different peripheral speed ratios for 30 seconds to swell and stretch 2.4 times in the transport direction (swelling step). The film was then immersed in a 30°C dye bath (an aqueous solution containing 0.03% iodine and 0.3% potassium iodide by weight) to achieve the desired monomer transmittance after stretching. The film was then stretched 3.7 times in the transport direction relative to the original PVA film (a PVA film that had not been stretched in the transport direction) (dyeing step). The immersion time was approximately 60 seconds. Next, the dyed PVA film was immersed in a 40°C crosslinking bath (a 3.0 wt% boric acid and 3.0 wt% potassium iodide aqueous solution) while being stretched to 4.2 times the original PVA film's length in the transport direction (crosslinking step). The resulting PVA film was then immersed in a 64°C stretching bath (a 4.0 wt% boric acid and 5.0 wt% potassium iodide aqueous solution) for 50 seconds, stretched to 6.0 times the original PVA film's length in the transport direction (stretching step), and then immersed in a 20°C cleaning bath (a 3.0 wt% potassium iodide aqueous solution) for 5 seconds (cleaning step). The washed polyvinyl alcohol film was dried at 30°C for 2 minutes to produce a polarizer (thickness 12 μm).

2. Make polarizing plates

The following aqueous solution was used as the adhesive: an acetylacetyl-containing polyvinyl alcohol resin (average degree of polymerization 1,200, saponification degree 98.5 mol%, acetylacetylation degree 5 mol%) and hydroxymethyl melamine. Using this adhesive to a thickness of 0.1 μm, a triacetate cellulose (TAC) film with a hard coat (7 μm hard coat, 25 μm TAC, modulus of elasticity: 3600 MPa) was laminated to one side of the polarizer using a roll laminator. A 25 μm thick TAC film was then laminated to the other side of the polarizer. The layers were then heat-dried in an oven (60°C for 5 minutes) to produce a polarizing plate with a structure consisting of protective layer 1 (32 μm thick)/adhesive layer/polarizer/adhesive layer/protective layer 2.

3. Making a polarizing plate with a phase difference layer

A retardation film was laminated to the surface of the protective layer 2 of the polarizing plate obtained in step 2. above in the same manner as in Example 1, producing a polarizing plate with a retardation layer having a structure consisting of protective layer 1/bonding layer/polarizer/bonding layer/protective layer 2/adhesive layer/retardation layer. The resulting polarizing plate with a retardation layer had a total thickness of 122 μm. The resulting polarizing plate with a retardation layer was evaluated in the same manner as in Example 1. The warp was 5.3 mm.

[Comparative example 2]

1. Making polarizers

A polarizer (thickness 12 μm) was produced in the same manner as in Comparative Example 1.

2. Make polarizing plates

A polarizing plate with the following structure was prepared in the same manner as in Comparative Example 1: protective layer 1 (32 μm thick)/bonding layer/polarizer/bonding layer/protective layer 2 (25 μm thick).

3. Fabrication of the first and second directional solidified layers constituting the phase difference layer

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

The surface of the polyethylene terephthalate (PET) film (thickness 38 μm) was wiped with a wiping cloth and an orientation treatment was performed. The orientation treatment direction was set to be 15° relative to the absorption axis direction of the polarizer when it was bonded to the polarizing plate and viewed from the viewing side. The above-mentioned liquid crystal coating liquid was applied to the orientation treatment surface using a rod coater and heated and dried at 90°C for 2 minutes to orient the liquid crystal compound. A metal halogen lamp was used to irradiate the liquid crystal layer formed in the above manner with 1 mJ/ ^cm2 of light to harden the liquid crystal layer, thereby forming a liquid crystal orientation solidification layer A on the PET film. The thickness of the liquid crystal orientation solidification layer A is 2.5 μm, and the in-plane phase difference Re(550) is 270 nm. In addition, the liquid crystal orientation solidification layer A has a refractive index distribution of nx>ny=nz.

A liquid crystal orientation cured layer B was formed on the PET film in the same manner as above, except that the coating thickness was changed and the orientation treatment direction was set to be 75° relative to the absorption axis of the polarizer when viewed from the viewing side. The thickness of the liquid crystal orientation cured layer B was 1.5 μm, and the in-plane retardation Re(550) was 140 nm. Furthermore, the liquid crystal orientation cured layer B had a refractive index distribution of nx>ny=nz. Furthermore, the Re(450)/Re(550) ratio of the liquid crystal orientation cured layers A and B was 1.11.

4. Making a polarizing plate with a phase difference layer

The liquid crystal oriented solidified layer A and the liquid crystal oriented solidified layer B obtained in the above 3. are sequentially transferred to the surface of the protective layer 2 of the polarizing plate obtained in the above 2. At this time, the transfer (lamination) is performed in such a way that the angle formed by the absorption axis of the polarizer and the slow axis of the oriented solidified layer A is 15 degrees, and the angle formed by the absorption axis of the polarizer and the slow axis of the oriented solidified layer B is 75 degrees. In addition, each transfer (lamination) is performed through an ultraviolet curing adhesive (thickness 1μm). According to the above method, a polarizing plate with a phase difference layer having a structure of protective layer 1/bonding layer/polarizer/bonding layer/protective layer 2/bonding layer/phase difference layer (first oriented solidified layer/bonding layer/second oriented solidified layer) is obtained. The total thickness of the resulting polarizing plate with a retardation layer was 75 μm. The resulting polarizing plate with a retardation layer was subjected to the same evaluation as in Example 1. The results are listed in Table 2.

[Comparative example 3]

A polarizing film was produced in the same manner as in Example 1, except that potassium iodide was not added to the PVA aqueous solution (coating solution), the stretching ratio in the air-assisted stretching treatment was set to 1.8 times, and no heated rollers were used in the drying and shrinking treatment. However, the PVA resin layer dissolved during the dyeing and underwater stretching treatments, making it impossible to produce a polarizing film. Consequently, a polarizing plate with a retardation layer could not be produced.

[evaluate]

Comparison of the Examples with the Comparative Examples clearly demonstrates that the polarizing films of the Examples of the present invention exhibit superior optical properties and significantly suppress warping after a heat test. Furthermore, thinning the polycarbonate resin to less than 40 μm, setting the polarizing plate thickness to less than 85 μm, and using a substrate with an elastic modulus of 3000 MPa or higher, preferably a TAC film as a protective layer, further enhances the bending properties.

Industrial Availability

The polarizing plate with a phase difference layer of the present invention can be suitably used as a circular polarizing plate for liquid crystal display devices, organic EL display devices, and inorganic EL display devices.

10:Polarizing plate

11:Polarizing film

12: First protective layer

13: Second protective layer

20: Phase difference layer (first phase difference layer)

100: Polarizing plate with phase difference layer

Claims (10)

A method for manufacturing a polarizing plate with a phase difference layer, wherein the polarizing plate with a phase difference layer comprises a polarizing plate and a phase difference layer, wherein the polarizing plate comprises a polarizing film and a protective layer located on at least one side of the polarizing film; the polarizing film is composed of a polyvinyl alcohol-based resin film containing a dichroic substance, and has a thickness of less than 8 μm, a single body transmittance of greater than 44.5%, and a polarization degree of greater than 99.0%; 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; the angle formed by the slow axis of the phase difference layer and the absorption axis of the polarizing film is 40° to 50°; The manufacturing method includes the following steps: forming a polyvinyl alcohol-based resin layer containing iodide or sodium chloride and a polyvinyl alcohol-based 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 to produce the polarizing film. The drying and shrinking treatment involves heating the laminate in a heating furnace using a heating roller while conveying the laminate along its longitudinal direction. The heating roller is kept in contact with the laminate for a total time of 1 to 20 seconds, thereby shrinking the laminate by 2% to 10% in the width direction. The temperature of the heating roller is 65°C to 100°C. The method for manufacturing a polarizing plate with a phase difference layer as claimed in claim 1, wherein the protective layer is composed of a cellulose triacetate resin film. The method for manufacturing a polarizing plate with a phase difference layer as claimed in claim 1 or 2, wherein the phase difference layer is composed of a polycarbonate resin film. The method for manufacturing a polarizing plate with a phase difference layer as claimed in claim 1 or 2, wherein the difference between the maximum and minimum values of the single transmittance of the polarizing film in an area of 50 ^cm2 is less than 0.2%. The method for manufacturing a polarizing plate with a retardation layer as claimed in claim 1 or 2, wherein the width of the polarizing film is greater than 1000 mm, and the difference between the maximum and minimum values of the single-element transmittance along the width direction is less than 0.3%. The method for manufacturing a polarizing plate with a retardation layer as claimed in claim 1 or 2, wherein the single-unit transmittance of the polarizing film is 45.0% or less, and the degree of polarization is 99.9% or less. A method for manufacturing a polarizing plate with a retardation layer as claimed in claim 1 or 2, wherein the polarizing plate with a retardation layer further comprises another retardation layer outside the retardation layer, wherein the refractive index characteristics of the other retardation layer exhibit the relationship nz>nx=ny; the manufacturing method comprising: disposing the other retardation layer outside the retardation layer. The method for manufacturing a polarizing plate with a retardation layer as claimed in claim 1 or 2, wherein the polarizing plate with a retardation layer further comprises a conductive layer or an isotropic substrate with a conductive layer outside the retardation layer; the manufacturing method comprises: disposing the conductive layer or the isotropic substrate with a conductive layer outside the retardation layer. A method for manufacturing a polarizing plate with a retardation layer as claimed in claim 1 or 2, wherein the polarizing plate with a retardation layer is in the form of an elongated strip; The method comprises laminating an elongated polarizing film and an elongated retardation layer by roll-to-roll method; the elongated polarizing film has an absorption axis in the longitudinal direction, and the elongated retardation layer is an obliquely stretched film having a slow axis at an angle of 40° to 50° relative to the longitudinal direction. The method for manufacturing a polarizing plate with a phase difference layer as claimed in claim 9 comprises: winding the aforementioned long strip of polarizing plate with a phase difference layer into a roll.