TWI899049B - Circular polarizer, strip-shaped wide-band λ/4 plate, organic electroluminescent display device, and liquid crystal display device - Google Patents

Abstract

A circularly polarizing plate comprises, in order: a polarizing film, a λ/2 plate having a slow axis at an angle Θh relative to the transmission axis of the polarizing film, and a λ/4 plate having a slow axis at an angle Θq relative to the transmission axis of the polarizing film, wherein the angle Θh of the λ/2 plate and the angle Θq of the λ/4 plate satisfy the following equations (A1), (A2), and (A3): Θq±10°＝2Θh＋45°　　　(A1), 27°＜Θh＜43°　　　(A2), 97°＜Θq＜133°　　　(A3), The degree of wavelength dispersion of the λ/2 plate is different from the degree of wavelength dispersion of the λ/4 plate, and the NZ coefficient NZq of the λ/4 plate satisfies NZq≤0.0.

Description

Circular polarizer, strip-shaped wide-band λ/4 plate, organic electroluminescent display device, and liquid crystal display device

The present invention relates to a circular polarizer, a strip-shaped wide-bandwidth λ/4 plate, an organic electroluminescent display device, and a liquid crystal display device.

Conventionally, organic electroluminescent displays (hereinafter referred to as "organic EL displays") and liquid crystal displays have sometimes been equipped with circularly polarizing plates to reduce external light reflection on the display surface. These circularly polarizing plates typically use a combination of a polarizing film and a λ/4 plate. However, conventional λ/4 plates typically function as λ/4 wavelength plates only for light within a specific, narrow wavelength range. Therefore, while circularly polarizing plates can reduce external light reflection within this specific, narrow wavelength range, they are difficult to reduce external light reflection beyond this range.

In contrast, in recent years, wideband λ/4 plates, which combine a λ/4 plate with a λ/2 plate, have been proposed (Patents 1-3). This wideband λ/4 plate can function as a λ/4 plate over a wide wavelength range, thus achieving a circularly polarizing plate that reduces external light reflection over a wide wavelength range.

Patent Literature

Patent Document 1: Japanese Patent Publication No. H05-100114

Patent Document 2: Japanese Patent Publication No. 2007-004120

Patent Document 3: Japanese Patent Publication No. 2013-235272

In a circularly polarizing plate that combines a polarizing film with a wideband λ/4 plate, the directions of the optical axes—the transmission axis of the polarizing film, the slow axis of the λ/2 plate, and the slow axis of the λ/4 plate—are required to be adjusted so that these optical axes form a specified angle.

However, when a circularly polarizing plate is viewed from an oblique direction other than the straight-on direction, the viewing angle of the optical axis may deviate from the specified angle. Therefore, while conventional circularly polarizing plates can reduce external light reflection in the straight-on direction, they are sometimes ineffective in reducing external light reflection in oblique directions other than the straight-on direction. In particular, circularly polarizing plates equipped with a wideband λ/4 plate have a greater number of optical axes than conventional circularly polarizing plates because they include both a λ/4 plate and a λ/2 plate. Consequently, the viewing angle of the optical axis of circularly polarizing plates equipped with a wideband λ/4 plate deviates further than that of conventional circularly polarizing plates without a λ/2 plate, and their ability to reduce external light reflection in oblique directions tends to be inferior.

The present invention was developed in response to the aforementioned problems. Its purpose is to provide: a circularly polarizing plate that effectively reduces external light reflection in both the frontal and tilted directions; a wide-band λ/4 plate that effectively reduces external light reflection in both the frontal and tilted directions; and an organic EL display device and a liquid crystal display device using the circularly polarizing plate or wide-band λ/4 plate.

The inventors of this invention conducted intensive research to address the aforementioned issues. They discovered that by combining polarizing film, a λ/2 plate, and a λ/4 plate, and by appropriately adjusting the optical axis, wavelength dispersion, and NZ coefficient, they could produce a circularly polarizing plate with excellent reflection suppression in both the front and tilted directions. This led to the completion of the present invention.

That is, the present invention includes the following.

[1] A circularly polarizing plate comprising, in order: a polarizing film, a λ/2 plate having a slow axis at an angle Θh relative to the transmission axis of the polarizing film, and a λ/4 plate having a slow axis at an angle Θq relative to the transmission axis of the polarizing film, wherein the aforementioned angle Θh of the λ/2 plate and the aforementioned angle Θq of the λ/4 plate satisfy the following equations (A1), (A2), and (A3): Θq±10°=2Θh+45° (A1)

27°<Θh<43° (A2)

97°<Θq<133° (A3)

The degree of wavelength dispersion of the aforementioned λ/2 plate is different from that of the aforementioned λ/4 plate. The NZ coefficient NZq of the aforementioned λ/4 plate satisfies NZq ≤ 0.0.

[2] The circularly polarizing plate described in [1], wherein the in-plane phase difference Reh(400) of the λ/2 plate before a wavelength of 400 nm, the in-plane phase difference Reh(550) of the λ/2 plate before a wavelength of 550 nm, the in-plane phase difference Req(400) of the λ/4 plate before a wavelength of 400 nm, and the in-plane phase difference Req(550) of the λ/4 plate before a wavelength of 550 nm satisfy the following formula (B): Reh(400)/Reh(550)<Req(400)/Req(550) (B).

[3] The circularly polarizing plate as described in [1] or [2], wherein the in-plane phase difference Reh(400) of the λ/2 plate before a wavelength of 400 nm, the in-plane phase difference Reh(550) of the λ/2 plate before a wavelength of 550 nm, the in-plane phase difference Req(400) of the λ/4 plate before a wavelength of 400 nm, and the in-plane phase difference Req(550) of the λ/4 plate before a wavelength of 550 nm satisfy the following formula (C): 0.04<Req(400)/Req(550)-Reh(400)/Reh(550)<1.0 (C).

[4] The circularly polarizing plate according to any one of [1] to [3], wherein the NZ coefficient NZh of the aforementioned λ/2 plate satisfies 1.0≦NZh≦1.3, and the NZ coefficient NZq of the aforementioned λ/4 plate satisfies -1.5≦NZq≦0.0.

[5] A circularly polarizing plate as described in any one of [1] to [4], wherein the λ/4 plate comprises a layer made of a material having a negative intrinsic birefringence value.

[6] A circularly polarizing plate as described in any one of [1] to [5], wherein the λ/2 plate comprises a layer made of a material having a positive intrinsic birefringence value.

[7] A circularly polarizing plate as described in any one of [1] to [6], wherein the circularly polarizing plate is in the form of a strip, and the transmission axis of the polarizing film is in the width direction of the circularly polarizing plate.

[8] A long strip of wide-band λ/4 plate, comprising: a λ/2 plate having a slow axis at an angle Θh relative to the width direction of the wide-band λ/4 plate, and a λ/4 plate having a slow axis at an angle Θq relative to the width direction of the wide-band λ/4 plate, wherein the angle Θh of the λ/2 plate and the angle Θq of the λ/4 plate satisfy the following equations (A1), (A2), and (A3): Θq ± 10° = 2Θh + 45° (A1)

27°<Θh<43° (A2)

97°<Θq<133° (A3)

The degree of wavelength dispersion of the aforementioned λ/2 plate is different from that of the aforementioned λ/4 plate. The NZ coefficient NZq of the aforementioned λ/4 plate satisfies NZq ≤ 0.0.

[9] The long strip wide-band λ/4 plate as described in [8], wherein the aforementioned λ/2 plate is an obliquely stretched film.

[10] A long strip-shaped wide-band λ/4 plate as described in [8] or [9], wherein the λ/4 plate is an obliquely stretched film.

[11] An organic electroluminescent display device comprising: a circularly polarizing plate as described in any one of [1] to [7], or a wide-bandwidth λ/4 thin film obtained by cutting out a long wide-bandwidth λ/4 plate as described in any one of [8] to [10].

[12] A liquid crystal display device comprising: a circularly polarizing plate as described in any one of [1] to [7], or a wide-band λ/4 thin film obtained by cutting out a long wide-band λ/4 plate as described in any one of [8] to [10].

The present invention provides: a circularly polarizing plate that effectively reduces external light reflection in both the front and tilted directions; a wide-band λ/4 plate that effectively reduces external light reflection in both the front and tilted directions; and organic EL display devices and liquid crystal display devices using the circularly polarizing plate or wide-band λ/4 plate.

10: Reflective surface

11:Normal direction

12: Baseline Direction

20: Observation direction

100: Circular polarizing plate

110:Polarizing film

111: Penetrating Axis

112, 113: axis

120:λ/2 plate

121: Slow Axis

130:λ/4 plate

131: Slow Axis

140: Wideband λ/4 plate

Θh, Θq: Angles

ρ: polar angle

Φ: azimuth

Figure 1 is a perspective exploded view of a circularly polarizing plate according to one embodiment of the present invention.

FIG2 is a three-dimensional schematic diagram illustrating the evaluation model used when calculating color space coordinates in the simulations of the embodiment and the comparative example.

The following disclosures describe the present invention in detail using embodiments and examples. However, the present invention is not limited to the embodiments and examples shown below and may be modified and implemented without departing from the scope of the patent application and its equivalents.

In the following description, "long strip" film refers to a film having a length of at least five times its width, preferably 10 times or more. Specifically, it refers to a film long enough to be rolled up for storage or transportation. There is no particular upper limit on the length of a long strip film; for example, it can be set at no more than 100,000 times its width.

In the following description, the in-plane retardation Re of a film, unless otherwise noted, is expressed as Re = (nx - ny) × d. Furthermore, the thickness-direction retardation Rth of a film, unless otherwise noted, is expressed as Rth = [(nx + ny) / 2 - nz] × d. Furthermore, the NZ coefficient of a film, unless otherwise noted, is expressed as (nx - nz) / (nx - ny). Here, nx represents the refractive index in the direction perpendicular to the thickness direction of the film (in-plane direction) and having the maximum refractive index. ny represents the refractive index in the in-plane direction perpendicular to the nx direction. nz represents the refractive index in the thickness direction. d represents the film thickness. The measurement wavelength is 590 nm unless otherwise noted.

In the following description, the term "intrinsic birefringence" is positive, unless otherwise noted, meaning that the refractive index in the direction of extension is greater than the refractive index in the direction perpendicular to it. Furthermore, the term "intrinsic birefringence" is negative, unless otherwise noted, meaning that the refractive index in the direction of extension is less than the refractive index in the direction perpendicular to it. The value of intrinsic birefringence can be calculated from the dielectric constant distribution.

In the following description, "(meth)acrylic acid" includes "acrylic acid," "methacrylic acid," and combinations thereof.

In the following description, the oblique direction of the long film, unless otherwise noted, refers to the in-plane direction of the film and is neither parallel nor perpendicular to the width direction of the film.

In the following description, the front direction of a film, unless otherwise noted, refers to the direction normal to the principal plane of the film. Specifically, it refers to the direction with a polar angle of 0° and an azimuth angle of 0° of the principal plane.

In the following description, the tilt direction of a film, unless otherwise noted, refers to a direction that is neither parallel nor perpendicular to the principal plane of the film. Specifically, it refers to a direction within the range where the polar angle of the principal plane is greater than 0° and less than 90°.

In the following description, the orientations of components are referred to as "parallel," "perpendicular," and "orthogonal." Unless otherwise noted, these orientations may include errors within a range of, for example, ±5°, without impairing the effectiveness of the present invention.

In the following description, "polarizing plate," "λ/2 plate," and "λ/4 plate," unless otherwise noted, refer not only to rigid components but also to flexible components such as resin films.

In the following description, the angles of the optical axes (transmission axis, slow axis, etc.) of each thin film in a component comprising multiple thin films are angles when the film is viewed from the thickness direction, unless otherwise noted.

In the following description, the slow axis of a film refers to the slow axis within the plane of the film, unless otherwise specified.

[1. Layer structure of circularly polarizing plate]

Figure 1 is a perspective exploded view of a circularly polarizing plate 100 according to one embodiment of the present invention. In Figure 1 , an axis 112 extending in the same direction as the transmission axis 111 of the polarizing film 110 is depicted as a dotted chain on the surface of the λ/2 plate 120. Furthermore, in Figure 1 , an axis 113 extending in the same direction as the transmission axis 111 of the polarizing film 110 is depicted as a dotted chain on the surface of the λ/4 plate 130.

As shown in FIG1 , a circularly polarizing plate 100 according to one embodiment of the present invention includes a polarizing film 110 , a λ/2 plate 120 , and a λ/4 plate 130 in order along the thickness direction of the circularly polarizing plate 100 .

Polarizing film 110 is a polarizing plate having a transmission axis 111. It transmits linearly polarized light with a polarization direction parallel to transmission axis 111, while absorbing other polarized light. The polarization direction of linearly polarized light refers to the polarization direction of the electric field of the linearly polarized light. This polarization direction of linearly polarized light is sometimes referred to as the "polarization axis."

The λ/2 plate 120 is an optical component with a specified phase difference. This λ/2 plate 120 has a slow axis 121 at a specified angle θh relative to the transmission axis 111 of the polarizing film 110.

The λ/4 plate 130 is an optical component that has a specific phase difference different from that of the λ/2 plate 120. This λ/4 plate 130 has a slow axis 131 that forms a specific angle θq with respect to the transmission axis 111 of the polarizing film 110.

The direction at which the slow axis 131 of the λ/4 plate 130 forms an angle Θq with respect to the transmission axis 111 of the polarizing film 110 is generally the same as the direction at which the slow axis 121 of the λ/2 plate 120 forms an angle Θh with respect to the transmission axis 111 of the polarizing film 110. Therefore, for example, if the slow axis 121 of the λ/2 plate 120 forms an angle Θh clockwise with respect to the transmission axis 111 of the polarizing film 110 when viewed in the thickness direction, the slow axis 131 of the λ/4 plate 130 will generally form an angle Θq clockwise with respect to the transmission axis 111 of the polarizing film 110. Furthermore, for example, when viewed in the thickness direction, if the slow axis 121 of the λ/2 plate 120 forms a counterclockwise angle Θh with respect to the transmission axis 111 of the polarizing film 110, the slow axis 131 of the λ/4 plate 130 generally forms a counterclockwise angle Θq with respect to the transmission axis 111 of the polarizing film 110.

In the circularly polarizing plate 100 having this structure, the layer comprising the λ/2 plate 120 and the λ/4 plate 130 functions as a wideband λ/4 plate 140. This wideband λ/4 plate 140 imparts an in-plane phase difference of approximately one-quarter of the wavelength of light passing through it over a wide wavelength range. Therefore, the circularly polarizing plate 100 functions as a circularly polarizing plate capable of absorbing either right-handed or left-handed circularly polarized light over a wide wavelength range while transmitting the remaining light.

The circularly polarizing plate 100 can be a cut sheet of film or a long strip of film. In the case of a long strip of film, the transmission axis 111 of the polarizing film 110 is generally aligned with the width of the circularly polarizing plate 100.

〔2.Polarizing film〕

Polarizing films typically have a polarizing layer and, if necessary, a protective film layer to protect the polarizing layer.

As the polarizer layer, for example, a film of a suitable vinyl alcohol polymer that has been subjected to a suitable treatment in a suitable order and manner can be used. Examples of such vinyl alcohol polymers include polyvinyl alcohol and partially formalized polyvinyl alcohol. Examples of film treatments include dyeing treatments using dichroic substances such as iodine and dichroic dyes, stretching treatments, and crosslinking treatments. Usually, in the stretching treatment used to manufacture the polarizer layer, the long strip of film before stretching is stretched along the long side direction, so that the polarizer layer obtained has a transmission axis parallel to the width direction of the polarizer layer. This polarizer layer is one that allows linear polarized light with a polarization direction parallel to the transmission axis to pass through, and preferably one with excellent polarization degree. The thickness of the polarizer layer is generally 5μm~80μm, but is not limited to this.

Any transparent film can be used as the protective film layer for protecting the polarizer layer. Films made of resins with excellent transparency, mechanical strength, thermal stability, and moisture shielding properties are preferred. Examples of such resins include acetate resins such as cellulose triacetate, polyester resins, polyether resins, polycarbonate resins, polyamide resins, polyimide resins, polyolefin resins, cycloolefin resins, and (meth)acrylic resins. Among these, acetate resins, cycloolefin resins, and (meth)acrylic resins are preferred in terms of low birefringence. Cycloolefin resins are particularly preferred in terms of transparency, low moisture absorption, dimensional stability, and lightness.

Polarizing film can be cut into sheets or in strips to suit the shape of the circular polarizer.

For example, the polarizing film can be manufactured by laminating a polarizer layer and a protective film layer. Adhesives may be used during lamination, if desired. Furthermore, when manufacturing the polarizing film as a long strip, the long strip of polarizer layer and the long strip of protective film layer can be aligned parallel to each other along their longitudinal directions and then laminating them using roll-to-roll lamination, thereby improving manufacturing efficiency. Furthermore, when manufacturing cut sheets of polarizing film, the long strip of polarizing film can be cut into a desired shape to produce the cut sheets.

[3. λ/2 plate]

A λ/2 plate is an optical component with an in-plane retardation of typically 200 nm or greater and typically 300 nm or less at a measurement wavelength of 590 nm. This in-plane retardation allows the combination of a λ/2 plate and a λ/4 plate to create a broadband λ/4 plate. Consequently, the circularly polarizing plate of this embodiment exhibits the ability to absorb either right-handed or left-handed circularly polarized light across a wide wavelength range while transmitting the remaining light. Consequently, this circularly polarizing plate can reduce reflection of light across a broad wavelength range in both the straight-on and oblique directions. In order to effectively reduce external light reflection in the oblique direction, the in-plane retardation of the λ/2 plate at a measured wavelength of 590nm is preferably 210nm or greater, preferably 220nm or greater, and preferably 280nm or less, preferably 270nm or less.

The angle Θh of the slow axis of the λ/2 plate and the angle Θq of the slow axis of the λ/4 plate satisfy the following equation (A1).

Θq±10°=2Θh+45° (A1)

The angles Θh and Θq of the two slow axes can be fine-tuned within this range, particularly to optimize front-side characteristics. More specifically, "2Θh + 45°" is typically at least "Θq - 10°," preferably at least "Θq - 7°," and particularly preferably at least "Θq - 5°." It is also typically at most "Θq + 10°," preferably at most "Θq + 7°," and particularly preferably at most "Θq + 5°."

Generally speaking, when a multilayer film is combined with a λ/4 plate having a slow axis at an angle Θ(λ/4) relative to a certain reference direction and a λ/2 plate having a slow axis at an angle Θ(λ/2) relative to the aforementioned reference direction, and the formula (D) is satisfied: "Θ(λ/4)=2Θ(λ/2)+45°", the multilayer film becomes a broadband λ/4 plate. The broadband λ/4 plate can impart an in-plane phase difference of approximately 1/4 wavelength of the wavelength of light passing through the multilayer film over a wide wavelength range (see Japanese Patent Publication No. 2007-004120). In the circularly polarizing plate according to this embodiment, the λ/2 plate and the λ/4 plate satisfy a relationship approximately as expressed by equation (D) above. This allows the portion comprising the λ/2 and λ/4 plates to function as a broadband λ/4 plate. Consequently, the circularly polarizing plate absorbs circularly polarized light across a wide wavelength range, thereby reducing external light reflection.

The angle Θh of the slow axis of the λ/2 plate satisfies the following equation (A2).

27°<Θh<43° (A2)

More specifically, the angle θh of the slow axis of the λ/2 plate is typically greater than 25°, preferably greater than 26°, and particularly greater than 27°, and is typically less than 45°, preferably less than 44°, and particularly preferably less than 43°. By keeping the angle θh within this range, the circularly polarizing plate can reduce external light reflection in both the front and oblique directions. In particular, the reflection suppression effect in the oblique direction is significantly enhanced.

The NZ coefficient (NZh) of the λ/2 plate preferably satisfies 1.0 ≤ NZh ≤ 1.3. More specifically, the NZ coefficient (NZh) of the λ/2 plate is preferably 1.0 or greater, preferably 1.3 or less, more preferably 1.25 or less, and particularly preferably 1.2 or less. A λ/2 plate having an NZh within the aforementioned range effectively reduces external light reflection in both the front and oblique directions. In particular, the reflection suppression effect in the oblique direction is significantly enhanced.

A resin film is typically used to create a λ/2 plate with the aforementioned optical properties. A thermoplastic resin is preferred as this resin. Furthermore, the λ/2 plate can be a single-layer resin film or a multilayer resin film with two or more layers.

Among them, in terms of ease of manufacturing, it is preferable that the λ/2 plate has a layer made of a material with a positive intrinsic birefringence value. As a material with a positive intrinsic birefringence value, a resin with a positive intrinsic birefringence value is usually used. Such a resin with a positive intrinsic birefringence value includes a polymer with a positive intrinsic birefringence value. If we want to give examples of such polymers, we can cite: polyolefins such as polyethylene and polypropylene; polyesters such as polyethylene terephthalate and polybutylene terephthalate; polyaryl sulfides such as polyphenylene sulfide; polyvinyl alcohol; polycarbonate; polyarylate; cellulose ester polymer; polyether sulfide; polysulfide; polyarylate; polyvinyl chloride ... Cycloolefin polymers such as olefin polymers; rod-shaped liquid crystal polymers, etc. These polymers may be used alone or in combination of two or more in any ratio. Furthermore, the polymers may be homopolymers or copolymers. Among these, polycarbonate polymers are preferred for their excellent retardation and low-temperature elongation, while cycloolefin polymers are preferred for their excellent mechanical properties, heat resistance, transparency, low moisture absorption, dimensional stability, and lightness.

As the polycarbonate polymer, any polymer having a structural unit containing a carbonate bond (-O-C(=O)-O-) can be used. Examples of polycarbonate polymers include bisphenol A polycarbonate, branched bisphenol A polycarbonate, and o-o-o-o-tetramethylbisphenol A polycarbonate.

Cycloolefin polymers are polymers whose structural units have an alicyclic structure. Examples of cycloolefin polymers include: (1) Olefin polymers, (2) monocyclic cycloolefin polymers, (3) cyclic covalent diene polymers, (4) vinyl aliphatic cycloolefin polymers, etc. Olefin polymers are particularly suitable due to their good formability. Olefin polymers, for example: Ring-opening polymers of monomers of olefin structure, containing Ring-opening copolymers of monomers with olefin structures and other monomers capable of ring-opening copolymerization, and hydrogenated products thereof; containing Addition polymers of monomers with olefin structures, containing Addition copolymers of monomers with olefin structures and other copolymerizable monomers. Among these, those containing Preferably, the ring-opening polymer hydrogenates of olefin-structured monomers are used.

The cycloolefin polymer can be selected from, for example, the polymers disclosed in Japanese Patent Publication No. 2002-321302.

Cycloolefin resins containing cycloolefin polymers are available in a variety of commercially available products, so those with desired properties can be appropriately selected for use. Examples of commercially available products include "ZEONOR" (manufactured by Zeon Co., Ltd.), "ARTON" (manufactured by JSR Corporation), "APEL" (manufactured by Mitsui Chemicals), and "TOPAS" (manufactured by Polyplastics).

The weight-average molecular weight (Mw) of the polymer contained in the resin with a positive intrinsic birefringence value is preferably 10,000 or greater, more preferably 15,000 or greater, and particularly preferably 20,000 or greater, and preferably 100,000 or less, more preferably 80,000 or less, and particularly preferably 50,000 or less. When the weight-average molecular weight is within this range, the mechanical strength and moldability of the λ/2 plate are well balanced, resulting in a suitable result. The weight-average molecular weight mentioned above is the weight-average molecular weight in terms of polyisoprene or polystyrene, measured by gel permeation chromatography using cyclohexane as the solvent (toluene may also be used if the sample is insoluble in cyclohexane).

The molecular weight distribution (weight average molecular weight (Mw) / number average molecular weight (Mn)) of the polymer contained in the resin having a positive intrinsic birefringence value is preferably 1.2 or greater, more preferably 1.5 or greater, and particularly preferably 1.8 or greater, and preferably 3.5 or less, more preferably 3.0 or less, and particularly preferably 2.7 or less. By setting the molecular weight distribution above the lower limit of the aforementioned range, polymer productivity can be improved, reducing manufacturing costs. Furthermore, by setting it below the upper limit, the amount of low-molecular components can be reduced, thereby suppressing relaxation during high-temperature exposure and improving the stability of the λ/2 plate.

The proportion of the polymer in the resin with a positive intrinsic birefringence value is preferably 50% to 100% by weight, more preferably 70% to 100% by weight, and even more preferably 90% to 100% by weight. By setting the polymer proportion within this range, the λ/2 plate can achieve sufficient heat resistance and transparency.

Resins with positive intrinsic birefringence must contain, in addition to the aforementioned polymers, a blending agent. Examples of blending agents include: pigments, dyes, and other colorants; plasticizers; fluorescent whitening agents; dispersants; thermal stabilizers; light stabilizers; UV absorbers; antistatic agents; antioxidants; microparticles; and surfactants. These ingredients can be used alone or in combination of two or more in any ratio.

The glass transition temperature (TgP ₎ of a resin with a positive intrinsic birefringence value is preferably 100°C or higher, more preferably 110°C or higher, and particularly preferably 120°C or higher, and preferably 190°C or lower, more preferably 180°C or lower, and particularly preferably 170°C or lower. By setting the glass transition temperature (TgP ₎ of a resin with a positive intrinsic birefringence value above the lower limit of the aforementioned range, the durability of the λ/2 sheet in high-temperature environments can be improved. Furthermore, by setting it below the upper limit, stretching processing can be facilitated.

The absolute value of the photoelastic coefficient of a resin with a positive intrinsic birefringence is preferably 10× ^10-12 Pa ^-1 or less, more preferably 7× ^10-12 Pa ^-1 or less, and even more preferably 4× ^10-12 Pa ^-1 or less. This minimizes the variation in the in-plane retardation of the λ/2 plate. Here, the photoelastic coefficient C is expressed as C = Δn/σ, where birefringence is defined as Δn and stress is defined as σ.

The total light transmittance of a λ/2 plate is preferably 80% or higher. Light transmittance should be measured in accordance with JIS K0115 using a spectrophotometer (V-570 Ultraviolet, Visible, and Near-Infrared Spectrophotometer, manufactured by JASCO Corporation).

The haze of a λ/2 plate is preferably 5% or less, more preferably 3% or less, and even more preferably 1% or less. Ideally, it should be 0%. The haze value should be the average value of five measurements taken using a Nippon Denshoku Industries "NDH-300A Turbidity Meter" in accordance with JIS K7361-1997.

The amount of volatile components contained in the λ/2 plate is preferably 0.1% by weight or less, more preferably 0.05% by weight or less, and even more preferably 0.02% by weight or less, and ideally zero. Reducing the amount of volatile components improves the dimensional stability of the λ/2 plate and minimizes the temporal variation of optical properties such as retardation.

Here, volatile components are trace amounts of substances with a molecular weight of 200 or less present in the film, such as residual monomers and solvents. The amount of volatile components can be quantified by dissolving the film in chloroform and analyzing it by gas chromatography, as the total value of substances with a molecular weight of 200 or less present in the film.

The saturated water absorption of the λ/2 plate is preferably 0.03% by weight or less, more preferably 0.02% by weight or less, and even more preferably 0.01% by weight or less. Ideally, it is zero. If the saturated water absorption of the λ/2 plate is within the above range, the temporal variation of optical properties such as in-plane retardation can be minimized.

Here, saturated water absorption refers to the percentage of the increase in mass of a film specimen after immersion in 23°C water for 24 hours relative to the mass of the film specimen before immersion.

The thickness of the λ/2 plate is preferably 10 μm or greater, more preferably 15 μm or greater, and even more preferably 30 μm or greater, and preferably 100 μm or less, more preferably 80 μm or less, and even more preferably 60 μm or less. This improves the mechanical strength of the λ/2 plate.

As described above, the λ/2 plate can be manufactured, for example, by preparing a first pre-stretching film made of a thermoplastic resin and stretching the first pre-stretching film to exhibit a desired phase difference. Specifically, if the λ/2 plate comprises a layer made of a resin with positive intrinsic birefringence, the λ/2 plate can be manufactured using a manufacturing method comprising (a) a first step of preparing the first pre-stretching film comprising a layer made of a resin with positive intrinsic birefringence, and (b) a second step of stretching the prepared first pre-stretching film to obtain the λ/2 plate.

In the first step (a), a first pre-stretched film having a layer made of a resin having a positive intrinsic birefringence value is prepared. The first pre-stretched film can be produced by melt forming or solution casting, with melt forming being preferred. Among melt forming methods, extrusion, inflation, or compression molding are preferred, with extrusion being particularly preferred.

Typically, the first pre-stretched film is prepared as a long resin film. By preparing the first pre-stretched film as a long resin film, some or all of the various steps in the λ/2 plate manufacturing process can be performed on the production line, enabling simple and efficient manufacturing.

After preparing a first pre-stretched film in step (a), the second step (b) of stretching this pre-stretched film is performed. Typically, since stretching in step (b) produces the desired retardation in a layer made of a resin with positive intrinsic birefringence, a λ/2 plate can be obtained by forming a stretched film.

The stretching method used in the second step (b) can be arbitrarily selected depending on the optical properties desired to be achieved through stretching. Accordingly, the second step (b) can be performed as either uniaxial stretching (stretching in a single direction) or biaxial stretching (stretching in two directions). Generally, uniaxial stretching in the second step (b) improves the uniaxiality of the layer formed from a resin with a positive intrinsic birefringence value, thereby allowing the NZ coefficient (NZh) of the λ/2 plate to approach 1.0. On the other hand, biaxial stretching in the second step (b) reduces the uniaxiality of the layer formed from a resin with a positive intrinsic birefringence value, thereby allowing the NZ coefficient (NZh) of the λ/2 plate to move away from 1.0.

(b) The stretching in the second step preferably includes stretching in an oblique direction. A manufacturing method that includes stretching in an oblique direction can produce a λ/2 plate as an obliquely stretched film. An obliquely stretched film is a film produced by a manufacturing method that includes stretching in an oblique direction. Typically, an obliquely stretched film exhibits a slow axis that is neither parallel nor perpendicular to its widthwise direction. Therefore, the λ/2 plate as an obliquely stretched film can facilitate the production of a circularly polarizing plate by laminating the obliquely stretched film with a polarizing film having a transmission axis in the widthwise direction and a λ/4 plate via roll-to-roll bonding.

The stretching ratio in the second step (b) is preferably 1.1 times or greater, more preferably 1.3 times or greater, and particularly preferably 1.5 times or greater, and preferably 4 times or less, more preferably 3 times or less, and particularly preferably 2.5 times or less. When stretching in two or more directions, it is desirable that the sum of the stretching ratios in each direction falls within the aforementioned range. By limiting the stretching ratio in the second step (b) to the aforementioned range, a λ/2 plate with desired optical properties can be easily obtained.

The stretching temperature in the second step (b) is preferably Tg _P °C or higher, more preferably Tg _P + 2°C or higher, and particularly preferably Tg _P + 5°C or higher. It is preferably Tg _P + 40°C or lower, more preferably Tg _P + 35°C or lower, and particularly preferably Tg _P + 30°C or lower. Here, Tg _P represents the glass transition temperature of the resin, which indicates a positive intrinsic birefringence value. By setting the stretching temperature in the second step (b) within the aforementioned range, the molecules contained in the film before the first stretching are reliably oriented, making it easier to obtain a λ/2 plate with desired optical properties.

Furthermore, in the aforementioned method for manufacturing a λ/2 plate, any additional steps other than the aforementioned steps may be performed.

For example, when using a long strip of pre-stretched film to produce a long λ/2 sheet, a trimming process can be performed to cut the λ/2 sheet into the desired shape. This trimming process yields a cut sheet of the desired shape.

Furthermore, a process such as placing a protective layer on a λ/2 plate can also be performed.

[4. λ/4 plate]

A λ/4 plate is an optical component with an in-plane retardation of typically 75 nm or greater and typically 154 nm or less at a measurement wavelength of 590 nm. This in-plane retardation allows a wideband λ/4 plate to be combined with a λ/2 plate. Consequently, the circularly polarizing plate of this embodiment exhibits the ability to absorb either right-handed or left-handed circularly polarized light across a wide wavelength range while transmitting the remaining light. Consequently, this circularly polarizing plate can reduce reflection of light across a broad wavelength range in both the front and oblique directions. In order to effectively reduce external light reflection in the oblique direction, the in-plane retardation of the λ/4 plate at a measured wavelength of 590nm is preferably 80nm or greater, preferably 90nm or greater, and preferably 138nm or less, preferably 128nm or less.

The angle Θq of the slow axis of the λ/4 plate satisfies the following equation (A3).

97°<Θq<133° (A3)

More specifically, the angle θq of the slow axis of the λ/4 plate is typically greater than 95°, preferably greater than 96°, and particularly preferably greater than 97°, and is typically less than 135°, preferably less than 134°, and particularly preferably less than 133°. By keeping the angle θq within this range, the circularly polarizing plate can reduce external light reflection in both the front and oblique directions. In particular, the reflection suppression effect in the oblique direction is significantly enhanced.

The NZ coefficient (NZq) of a λ/4 plate typically satisfies NZq ≤ 0.0, preferably -1.5 ≤ NZq ≤ 0.0. More specifically, the NZ coefficient (NZq) of a λ/4 plate is preferably -1.5 or higher, more preferably -1.2 or higher, and particularly preferably -1.0 or higher. It is typically 0.0 or lower, preferably -0.1 or lower, and particularly preferably -0.15 or lower. A λ/4 plate having an NZq within the aforementioned range can reduce external light reflection in both the front and oblique directions. In particular, the reflection suppression effect in the oblique direction is significantly enhanced.

A λ/4 plate has a wavelength dispersion that differs significantly from that of a λ/2 plate. Here, the degree of wavelength dispersion of a particular film is expressed as the in-plane retardation at a wavelength of 400 nm divided by the in-plane retardation at a wavelength of 550 nm. According to this, when the in-plane phase difference of the λ/2 plate at a wavelength of 400 nm is set to Reh(400), the in-plane phase difference of the λ/2 plate at a wavelength of 550 nm is set to Reh(550), the in-plane phase difference of the λ/4 plate at a wavelength of 400 nm is set to Req(400), and the in-plane phase difference of the λ/4 plate at a wavelength of 550 nm is set to Req(550), the degree of wavelength dispersion of the λ/2 plate is expressed as "Reh(400)/Reh(550)", and the degree of wavelength dispersion of the λ/4 plate is expressed as "Req(400)/Req(550)". By combining the λ/2 plate and the λ/4 plate having different degrees of wavelength dispersion so that the slow axes thereof have specified angles Θh and Θq, the reflection of external light in the front direction of the circularly polarizing plate can be reduced.

Furthermore, the in-plane phase differences Reh(400) and Reh(550) of the λ/2 plate at wavelengths of 400nm and 550nm, and the in-plane phase differences Req(400) and Req(550) of the λ/4 plate at wavelengths of 400nm and 550nm, preferably satisfy the following formula (B). This effectively reduces external light reflection in the front direction of the circularly polarizing plate.

Reh(400)/Reh(550)<Req(400)/Req(550) (B)

Furthermore, the in-plane phase differences Reh(400) and Reh(550) of the λ/2 plate at wavelengths of 400nm and 550nm, and the in-plane phase differences Req(400) and Req(550) of the λ/4 plate at wavelengths of 400nm and 550nm preferably satisfy the following formula (C).

0.04<Req(400)/Req(550)-Reh(400)/Reh(550)<1.0 (C)

More specifically, the difference between the wavelength dispersion of the λ/2 plate and the wavelength dispersion of the λ/4 plate, "Req(400)/Req(550)-Reh(400)/Reh(550)", is preferably greater than 0.04, more preferably greater than 0.1, and particularly preferably greater than 0.15. It is also preferably less than 1.0, more preferably less than 0.95, and even more preferably less than 0.9. This effectively reduces external light reflection in the front direction of the circularly polarizing plate.

A resin film is typically used as a λ/4 plate having the aforementioned optical properties. A thermoplastic resin is preferred as this resin. Furthermore, the λ/4 plate can be a single-layer resin film or a multilayer resin film having two or more layers.

Among them, in terms of ease of manufacturing, it is preferred that the λ/4 plate has a layer made of a material with a negative intrinsic birefringence value. As a material with a negative intrinsic birefringence value, a resin with a negative intrinsic birefringence value is generally used. Such a resin with a negative intrinsic birefringence value includes a polymer with a negative intrinsic birefringence value. If we want to give examples of such polymers, we can cite: a polystyrene polymer including a homopolymer of styrene or a styrene derivative, and a copolymer of styrene or a styrene derivative and any monomer; a polyacrylonitrile polymer; a polymethyl methacrylate polymer; or a multi-component copolymer of these; etc. Moreover, as the aforementioned arbitrary monomers that can be copolymerized with styrene or a styrene derivative, for example: acrylonitrile, maleic anhydride, methyl methacrylate and butadiene can be cited as the preferred ones. Moreover, these polymers can be used alone or in combination of two or more in any ratio.

Among these, polystyrene polymers are preferred due to their high retardation visibility. Furthermore, copolymers of styrene or a styrene derivative with maleic anhydride are particularly preferred due to their high heat resistance. In this case, the amount of maleic anhydride units per 100 parts by weight of the polystyrene polymer is preferably 5 parts by weight or greater, more preferably 10 parts by weight or greater, and particularly preferably 15 parts by weight or greater, and preferably 30 parts by weight or less, more preferably 28 parts by weight or less, and particularly preferably 26 parts by weight or less. The maleic anhydride units referred to above refer to structural units having a structure formed by polymerizing maleic anhydride.

The proportion of the polymer in the resin having a negative intrinsic birefringence value is preferably 50% to 100% by weight, more preferably 70% to 100% by weight, and even more preferably 90% to 100% by weight. By setting the polymer proportion within the aforementioned range, the λ/4 plate exhibits suitable optical properties.

Resins with negative intrinsic birefringence must contain a blending agent in addition to the aforementioned polymer. Examples of blending agents are the same as those required for resins with positive intrinsic birefringence. A single blending agent may be used, or two or more may be combined in any ratio.

The glass transition temperature (Tg _{N )} of a resin with negative intrinsic birefringence is preferably 80°C or higher, more preferably 90°C or higher, even more preferably 100°C or higher, particularly preferably 110°C or higher, and particularly preferably 120°C or higher. Having such a high Tg _N of a resin with negative intrinsic birefringence reduces directional relaxation of the resin. While the upper limit of the Tg _N of a resin with negative intrinsic birefringence is not particularly limited, it is typically 200°C or lower.

Some resins with negative intrinsic birefringence have low mechanical strength. For example, resins containing polystyrene polymers tend to have low mechanical strength. Therefore, a λ/4 plate comprising a layer made of a resin with negative intrinsic birefringence preferably includes a protective layer that protects the layer made of the resin and is combined with the layer made of the resin with negative intrinsic birefringence.

Any layer may be used as long as the effects of the present invention are not significantly impaired. For example, a layer made of a resin with a positive intrinsic birefringence value may be used as the protective layer. In this case, from the perspective of facilitating adjustment of the retardation in the λ/4 plate, it is preferable that the in-plane retardation and thickness-direction retardation of the protective layer be close to zero. For example, one method of achieving close to zero in the in-plane retardation and thickness-direction retardation of the protective layer is to make the glass transition temperature of the resin included in the protective layer lower than the glass transition temperature Tg _N of a resin with a negative intrinsic birefringence value.

Furthermore, the protective layer may be provided on only one side of the layer made of the resin having an inherently negative birefringence value, or on both sides.

The optimal total light transmittance of a λ/4 plate is 80% or higher.

The haze of a λ/4 plate is preferably below 5%, more preferably below 3%, and even more preferably below 1%. Ideally, it should be 0%.

The amount of volatile components contained in the λ/4 plate is preferably 0.1% by weight or less, more preferably 0.05% by weight or less, and even more preferably 0.02% by weight or less, and ideally zero. Reducing the amount of volatile components improves the dimensional stability of the λ/4 plate and minimizes the temporal variation of optical properties such as retardation.

The saturated water absorption of the λ/4 plate is preferably 0.03% by weight or less, more preferably 0.02% by weight or less, and even more preferably 0.01% by weight or less, and ideally zero. If the saturated water absorption of the λ/4 plate is within the above range, the temporal variation of optical properties such as in-plane retardation can be minimized.

The thickness of the λ/4 plate is preferably 3 μm or greater, more preferably 5 μm or greater, and preferably 80 μm or less, more preferably 75 μm or less, and particularly preferably 70 μm or less. By setting the thickness of the λ/4 plate at or above the lower limit of the aforementioned range, the desired retardation can be easily achieved. Furthermore, by setting it below the upper limit, the thickness of the circularly polarizing plate can be reduced.

As described above, the λ/4 plate can be manufactured, for example, by preparing a second pre-stretching film made of a thermoplastic resin and stretching the second pre-stretching film to exhibit the desired phase difference. Specifically, if the λ/4 plate includes a layer made of a resin with a negative intrinsic birefringence value, the λ/4 plate can be manufactured using a manufacturing method comprising a third step of (c) preparing a second pre-stretching film including a layer made of a resin with a negative intrinsic birefringence value, and a fourth step of (d) stretching the prepared second pre-stretching film to obtain the λ/4 plate.

In the third step (c), a second pre-stretched film having a layer made of a resin having a negative intrinsic birefringence value is prepared. The second pre-stretched film can be produced by melt forming or solution casting, with melt forming being preferred. Among melt forming methods, extrusion, inflation, or compression molding are preferred, with extrusion being particularly preferred.

Furthermore, when manufacturing the second pre-stretched film as a multilayer film, such as a multilayer film comprising a layer made of a resin with negative intrinsic birefringence and a protective layer, coextrusion molding methods such as the coextrusion T-die method, the coextrusion inflation method, and the coextrusion layer pressing method can be used; film layer pressing methods such as the dry layer pressing method; and coating molding methods such as coating a resin solution constituting the next layer onto a certain layer. Among these methods, coextrusion molding is preferred from the perspectives of excellent manufacturing efficiency and preventing volatile components such as solvents from remaining in the λ/4 plate. Among these coextrusion molding methods, the coextrusion T-die method is particularly preferred. Furthermore, the T-die co-extrusion method can be implemented using either a feed block or a multi-manifold method. However, the multi-manifold method is preferred in terms of reducing variations in layer thickness.

Typically, the second pre-stretched film is prepared as a long resin film. By preparing the second pre-stretched film as a long resin film, some or all of the various steps in the λ/4 plate manufacturing process can be performed on the production line, enabling simple and efficient manufacturing.

After preparing a second pre-stretched film in step (c), the fourth step (d) of stretching this second pre-stretched film is performed. Typically, since stretching in step (d) produces the desired retardation in a layer made of a resin with negative intrinsic birefringence, a λ/4 plate can be obtained by forming a stretched film.

The stretching method in step (d) (4) can be arbitrarily selected depending on the desired optical properties to be achieved. Accordingly, step (d) (4) can be performed either as uniaxial stretching (stretching in a single direction) or as biaxial stretching (stretching in two directions). Typically, uniaxial stretching in step (d) (4) increases the uniaxiality of the layer formed from a resin with a negative intrinsic birefringence value, thereby bringing the NZ coefficient (NZq) of the λ/4 plate closer to 0.0. On the other hand, biaxial stretching in step (d) (4) decreases the uniaxiality of the layer formed from a resin with a negative intrinsic birefringence value, thereby reducing the NZ coefficient (NZq) of the λ/4 plate from 0.0.

(d) The stretching in the fourth step preferably includes stretching in an oblique direction. A manufacturing method that includes stretching in an oblique direction can produce a λ/4 plate as an obliquely stretched film. Typically, an obliquely stretched film exhibits a slow axis that is neither parallel nor perpendicular to its widthwise direction. Therefore, the λ/4 plate as an obliquely stretched film can facilitate the production of a circularly polarizing plate by laminating the obliquely stretched film with a polarizing film having a transmission axis in the widthwise direction and a λ/2 plate via roll-to-roll bonding.

The stretching ratio in the fourth step (d) is preferably 1.1 times or greater, more preferably 1.15 times or greater, and particularly preferably 1.2 times or greater, and preferably 4 times or less, more preferably 3 times or less, and particularly preferably 2 times or less. When stretching in two or more directions, it is desirable that the sum of the stretching ratios in each direction falls within the aforementioned range. By limiting the stretching ratio in the fourth step (d) to the aforementioned range, a λ/4 plate with desired optical properties can be easily obtained.

The stretching temperature in the fourth step (d) is preferably Tg _N °C or higher, more preferably Tg _N + 2°C or higher, and particularly preferably Tg _N + 5°C or higher. It is preferably Tg _N + 40°C or lower, more preferably Tg _N + 35°C or lower, and particularly preferably Tg _N + 30°C or lower. Here, Tg _N represents the glass transition temperature of the resin, which has a negative intrinsic birefringence value. By setting the stretching temperature in the fourth step (d) within this range, the molecules contained in the film before the second stretching are reliably oriented, making it easier to obtain a λ/4 plate with desired optical properties.

Furthermore, in the aforementioned method for manufacturing a λ/4 plate, any additional steps other than those described above may be performed.

For example, any of the steps described in the λ/2 plate manufacturing method can be performed.

[5. Any layer]

The circularly polarizing plate according to this embodiment may include any layers other than the polarizing film, λ/2 plate, and λ/4 plate, as long as the effects of the present invention are not significantly impaired.

For example, circularly polarizing plates must have a protective film layer to prevent scratches. Furthermore, for example, circularly polarizing plates must have a bonding layer or adhesive layer to bond the polarizing film to the λ/2 plate, and to bond the λ/2 plate to the λ/4 plate.

[6. Physical Properties of Circularly Polarizing Plates]

The circularly polarizing plate of this embodiment, when placed on a surface that reflects light, can reduce external light reflection in both the front and oblique directions. In particular, the circularly polarizing plate of this embodiment effectively reduces external light reflection in the oblique direction.

Because external light reflection can be suppressed as described above, the use of the circularly polarizing plate can suppress unintended coloring of the display surface of an image display device. If the display surface experiences significant external light reflection, there is a possibility that the display surface will be colored by the color of the reflected light. In contrast, suppressing external light reflection reduces the amount of reflected light, thereby suppressing this coloring.

Furthermore, the circularly polarizing plate of this embodiment, thanks to its combination of a λ/2 plate and a λ/4 plate, functions as a wideband λ/4 plate. This suppresses external light reflection across a wide wavelength range in the visible light region. Therefore, by suppressing reflection across such a wide wavelength range, coloration of the display surface can also be effectively suppressed.

The degree of coloration is expressed in the L*a*b* color space and can be evaluated by measuring the color difference (ΔE*ab) between the coloration measured when viewing a display surface with a circularly polarizing plate from an oblique angle and the coloration of a non-reflective black display surface. This coloration can be determined by measuring the spectrum of light reflected from the display surface and multiplying it by the spectral sensitivity corresponding to the human eye (color matching function) to determine the tristimulus values X, Y, and Z. The coordinates a*, b*, and L* in the L*a*b* color space are then calculated. Furthermore, the aforementioned color difference ΔE*ab can be calculated using the following formula (X) from the color space coordinates (a0*, b0*, L0*) when the display surface is not illuminated by external light and the color space coordinates (a1*, b1*, L1*) when the display surface is illuminated by external light.

Generally speaking, the coloration of a display surface caused by reflected light may vary depending on the viewing angle. Therefore, when viewing the display from an oblique angle, the measured color space coordinates may vary depending on the viewing angle, and the color difference ΔE*ab may also vary. Therefore, when evaluating the degree of coloration when viewing a display from an oblique angle, it is best to use the average of the color differences ΔE*ab obtained from multiple azimuth angles to evaluate the coloration. Specifically, the color difference ΔE*ab is measured at 5° intervals in the azimuth direction, within the range of the azimuth angle Φ (see Figure 2) from 0° to 360°. The degree of coloration is evaluated based on the average of the measured ΔE*ab values. The smaller the average value, the less coloring the display surface will have when viewed from an oblique direction.

[7. Manufacturing Method of Circularly Polarizing Plate]

A circularly polarizing plate according to one embodiment of the present invention can be manufactured by laminating the aforementioned polarizing film, a λ/2 plate, and a λ/4 plate. The polarizing film, the λ/2 plate, and the λ/4 plate are laminated together after adjusting the directions of their optical axes so that the slow axes of the λ/2 plate and the slow axes of the λ/4 plate form a desired angle relative to the transmission axis of the polarizing film. Specifically, the circularly polarizing plate according to this embodiment can be manufactured by a manufacturing method comprising: laminating the polarizing film and the λ/2 plate so that the slow axis of the λ/2 plate forms a predetermined angle θh relative to the transmission axis of the polarizing film; and laminating the λ/2 plate and the λ/4 plate so that the slow axis of the λ/4 plate forms a predetermined angle θq relative to the transmission axis of the polarizing film.

In the aforementioned manufacturing method, either the step of laminating the multi-layer film and the λ/2 plate or the step of laminating the λ/2 plate and the λ/4 plate can be performed first, or both steps can be performed simultaneously.

Furthermore, adhesives or bonding agents may be used during bonding as needed.

To achieve efficient production through roll-to-roll lamination, the polarizing film, λ/2 plate, and λ/4 plate can also be laminated while still in the form of long films. Furthermore, to facilitate adjustment of the optical axis, circularly polarizing plates can be manufactured by cutting sheets of polarizing film, λ/2 plate, and λ/4 plate from long strips of polarizing film, λ/2 plate, and λ/4 plate and laminating the cut sheets.

[8. Wideband λ/4 Plate]

One embodiment of the present invention relates to a long, wideband λ/4 plate. In a long, circularly polarizing plate having a polarizing film with a transmission axis in its widthwise direction, the structure is identical to that of the polarizing film. Therefore, this wideband λ/4 plate comprises the aforementioned λ/2 plate and a λ/4 plate. Furthermore, the λ/2 plate has a slow axis at a predetermined angle θh relative to the widthwise direction of the wideband λ/4 plate. Furthermore, the λ/4 plate has a slow axis at a predetermined angle θq relative to the widthwise direction of the wideband λ/4 plate.

This wideband λ/4 plate provides at least the following advantages.

‧The broadband λ/4 plate can impart an in-plane phase difference of approximately 1/4 wavelength to light that passes through the plate in the front direction over a wide wavelength range.

‧The broadband λ/4 plate can impart an in-plane phase difference of approximately 1/4 wavelength to light that passes through the plate obliquely over a wide wavelength range.

Therefore, by combining a wide-band λ/4 plate with a polarizing film, the aforementioned circularly polarizing plate can be realized, which can reduce the reflection of light in a wide wavelength range in both the front and oblique directions.

[9. Organic electroluminescent display device]

An organic EL display device according to one embodiment of the present invention comprises the aforementioned circularly polarizing plate or a wide-bandwidth λ/4 film sheet cut from a long wide-bandwidth λ/4 plate.

When an organic EL display device is equipped with a circular polarizer, it is typically provided on the display surface. This allows the circular polarizer to function as an anti-reflection film for the organic EL display device. Specifically, by placing the circular polarizer on the display surface of the organic EL display device with the polarizing film facing the viewing side, light incident from outside the device can be prevented from being reflected within the device and emitted outside. Consequently, glare on the display surface of the display device can be suppressed. Specifically, of the light incident from outside the device, only a portion of the linearly polarized light passes through the polarizing film, where it then becomes circularly polarized by passing through the λ/2 plate and the λ/4 plate. Circularly polarized light is reflected by reflective components within the display device (such as the reflective electrodes in the organic EL element) and passes through the λ/4 plate and λ/2 plate again, becoming linearly polarized light with a polarization direction perpendicular to that of the incident linearly polarized light. This prevents the light from passing through the polarizing film. This achieves an anti-reflection function.

Furthermore, when the organic EL display device includes a wide-bandwidth λ/4 film sheet, the organic EL display device must include the wide-bandwidth λ/4 film sheet at any position.

[10. Liquid crystal display device]

A liquid crystal display device according to one embodiment of the present invention comprises the aforementioned circularly polarizing plate or a wide-bandwidth λ/4 film obtained by cutting out a long wide-bandwidth λ/4 plate.

When a liquid crystal display device is equipped with a circular polarizer, it is typically installed on the display surface. This allows the circular polarizer to function as an anti-reflection film for the liquid crystal display device. Specifically, by placing the circular polarizer on the display surface of the liquid crystal display device with the polarizing film facing the viewing side, light incident from outside the device is prevented from being reflected within the device and emitted outside. Consequently, glare on the display surface of the display device can be reduced.

When a liquid crystal display device is equipped with a wideband λ/4 film, it is typically installed on the viewing side of the liquid crystal panel. This allows the wideband λ/4 film to function as a film that improves the display's visibility for viewers wearing polarized sunglasses. Specifically, a circularly polarizing plate is positioned closer to the display surface than the polarizer on the viewing side of the liquid crystal panel. The slow axis of the λ/2 plate of the wideband λ/4 film is set at an angle θh relative to the transmission axis of the polarizer on the viewing side. This allows linearly polarized light that passes through the viewing-side polarizer to be converted into circularly polarized light via the broadband λ/4 film, enabling stable viewing of light emitted from the display through polarized sunglasses.

《Implementation Examples》

The following examples are provided to illustrate the present invention in detail. However, the present invention is not limited to the following examples and may be implemented with modifications as long as they do not depart from the scope of the patent application and its equivalents.

In the following descriptions, "%" and "parts" are by weight unless otherwise noted. Furthermore, the operations described below were performed at room temperature and pressure unless otherwise noted.

[Evaluation Method]

(Measurement Method of Phase Difference and NZ Coefficient)

Using a retardation meter (AxoScan, manufactured by Axometrics), the in-plane retardation and thickness-direction retardation of the film were measured at multiple points spaced 50 mm apart across the width of the film. The average of the measured values at these points was calculated and used as the in-plane retardation and thickness-direction retardation of the film. Measurements were performed at a wavelength of 590 nm. The NZ coefficient was calculated from the ratio of the obtained in-plane retardation to the thickness-direction retardation.

(Method for measuring the degree of wavelength dispersion)

Using the aforementioned retardation measurement method, the film's in-plane retardation at wavelengths of 400nm and 550nm was measured. The film's wavelength dispersion was then calculated by dividing the in-plane retardation at 400nm by the in-plane retardation at 550nm.

(Calculation method of color difference through simulation)

Using "LCD Master" manufactured by Shintech as simulation software, the circularly polarizing plates produced in each of the Examples and Comparative Examples were modeled and the following calculations were performed.

The simulation model was set up with a structure that included a circularly polarizing plate consisting of a λ/4 plate, a λ/2 plate, and a polarizing film, arranged in this order from the reflective surface side of a mirror having a planar reflective surface. The λ/4 plate and λ/2 plate were those used in the Examples and Comparative Examples. Furthermore, a commonly used polarizing film with a polarization degree of 99.99% was used as the polarizing film. Furthermore, an aluminum mirror was used as the mirror.

FIG2 is a three-dimensional schematic diagram illustrating the evaluation model used when calculating color space coordinates in the simulations of the embodiment and the comparative example.

As shown in Figure 2, the color space coordinates observed on the reflective surface 10 of the mirror equipped with a circular polarizer when illuminated by a D65 light source (not shown) are calculated. Furthermore, the color space coordinates when not illuminated are set to a0*=0, b0*=0, and L0*=0. The color difference ΔE*ab is then calculated using the aforementioned equation (X) from the color space coordinates when (i) illuminated and (ii) not illuminated.

The aforementioned chromatic aberration ΔE*ab is calculated in the observation direction 20 at a polar angle ρ of 0° relative to the reflective surface 10 to obtain the chromatic aberration ΔE*ab in the frontal direction. The polar angle ρ represents the angle relative to the normal direction 11 of the reflective surface 10.

Furthermore, the aforementioned chromatic aberration ΔE*ab is calculated in the observation direction 20 at a polar angle ρ of 60° relative to the reflecting surface 10. This calculation at a polar angle ρ = 60° is performed multiple times by moving the observation direction 20 in 5° increments along the azimuth direction within the range of an azimuth angle Φ of 0° or greater but less than 360°. The azimuth angle Φ represents the angle between a direction parallel to the reflecting surface 10 and a reference direction 12 parallel to the reflecting surface 10. The chromatic aberration ΔE*ab calculated in these multiple observation directions 20 is then averaged to obtain the chromatic aberration ΔE*ab in the oblique direction at a polar angle ρ = 60°.

(Evaluation method of circularly polarizing plates by visual inspection from the front)

Prepare a mirror with a flat reflective surface. Attach a λ/4 plate from a circularly polarizing plate to the reflective surface of this mirror.

Under clear daylight, the circularly polarizing plate was visually inspected on a mirror. Observation was performed from the front, with the polar angle of the plate at 0° and the azimuth angle at 0°. If color was observed, the result was judged as "poor," and if no color was observed, it was judged as "good."

(Evaluation method of circularly polarizing plates by visual observation in an oblique direction)

Prepare a mirror with a flat reflective surface. Attach a λ/4 plate from a circularly polarizing plate to the reflective surface of this mirror.

Under clear sunlight, the circularly polarizing plate on the mirror was visually observed. Observation was performed at a 60° polar angle and at an azimuth angle of 0° to 360°. The results of the observations were comprehensively evaluated based on the quality of the reflected brightness and coloration, and the Examples and Comparative Examples were ranked. The ranked Examples and Comparative Examples were then assigned a score corresponding to their ranking (12 points for first place, 11 points for second place, and finally 1 point for last place).

Multiple people conducted the aforementioned observations, and a total score was calculated for each embodiment and comparative example. The embodiments and comparative examples were ranked in order of the aforementioned total scores, and within this total score range, the leading group was evaluated in the order of A, B, C, D, and E.

[Examples 1, 3, and 6]

(Manufacturing of polarizing film)

Prepare a long strip of iodine-dyed, pre-stretched polyvinyl alcohol resin film. Stretching the pre-stretched film along its longitudinal direction at a 90° angle relative to its widthwise direction yields a long strip of polarizing film. This polarizing film has an absorption axis along its longitudinal direction and a transmission axis along its widthwise direction.

(Manufacturing of λ/2 Plates)

A long strip of cycloolefin resin film (ZEONOR Film, manufactured by Zeon Co., Ltd., Japan, glass transition temperature 126°C; thickness 45 μm) obtained by melt extruding a cycloolefin polymer into a film was prepared as the pre-stretching film.

This cycloolefin resin film was stretched in its widthwise direction to produce a long λ/2 sheet. The stretching conditions for the widthwise stretching treatment were set at a temperature of 120°C to 150°C and a stretching ratio of 2.0x to 5.0x to produce a λ/2 sheet with the properties shown in Table 1 below.

The λ/2 plate obtained was evaluated using the aforementioned method.

(Manufacturing of λ/4 Plates)

As a material with a negative intrinsic birefringence value, a styrene-maleic acid copolymer resin ("Daylark D332" manufactured by NOVA CHEMICAL, glass transition temperature 130°C, oligomer content 3 wt%) was prepared.

As the acrylic resin for the protective layer, prepare "SUMIPEX HT-55X" (glass transition temperature 105°C) manufactured by Sumitomo Chemical Co., Ltd.

As a bonding agent, a modified ethylene-vinyl acetate copolymer ("MODIC AP A543" manufactured by Mitsubishi Chemical Corporation, with a Ficat softening point of 80°C) was prepared.

The prepared styrene-maleic acid copolymer resin, acrylic resin, and adhesive were co-extruded to obtain a long, pre-stretched film having, in order, an acrylic resin layer, an adhesive layer, a styrene-maleic acid copolymer resin layer, an adhesive layer, and an acrylic resin layer. The thickness of the styrene-maleic acid copolymer resin layer in this pre-stretched film was 40 μm.

The pre-stretched film was then stretched in its widthwise direction to produce a long λ/4 sheet. The stretching conditions for the widthwise stretching treatment were set within a stretching temperature range of 110°C to 140°C and a stretching ratio of 1.5x to 4.0x to produce a λ/4 sheet with the properties shown in Table 1. The resulting λ/4 sheet exhibited no retardation between the acrylic resin layer and the adhesive layer.

The λ/4 plate obtained was evaluated using the aforementioned method.

(Manufacturing of circularly polarizing plates)

Long strips of polarizing film, long strips of λ/2 plate, and long strips of λ/4 plate are cut to obtain cut sheets of polarizing film, cut sheets of λ/2 plate, and cut sheets of λ/4 plate. These cut sheets of polarizing film, cut sheets of λ/2 plate, and cut sheets of λ/4 plate are bonded together using an adhesive (Nitto Denko Corporation's "CS9621") to obtain a circularly polarizing plate comprising, in this order, a polarizing film, an adhesive layer, a λ/2 plate, an adhesive layer, and a λ/4 plate. The lamination is performed so that, as viewed from the polarizing film side, the "clockwise angle Θh" of the slow axis of the λ/2 plate relative to the transmission axis of the polarizing film and the "clockwise angle Θq" of the slow axis of the λ/4 plate relative to the transmission axis of the polarizing film are as shown in Table 1.

The circularly polarizing plate obtained was evaluated using the aforementioned method.

[Examples 2, 4-5, 7-8, and Comparative Examples 1-2]

Circularly polarizing plates were manufactured and evaluated by following the same procedures as in Example 1, except for the following changes:

First, the stretching operation of the pre-stretched film during the λ/4 sheet manufacturing process was modified as follows. Specifically, the pre-stretched film was stretched in both the longitudinal and widthwise directions. The stretching conditions for the longitudinal direction were set within a stretching temperature range of 110°C to 140°C and a stretching ratio of 1.1x to 2.0x to produce a λ/4 sheet with the properties shown in Table 1. Furthermore, the stretching conditions for the widthwise direction were set within a stretching temperature range of 110°C to 140°C and a stretching ratio of 1.5x to 4.0x to produce a λ/4 sheet with the properties shown in Table 1.

Second: Change the lamination angles of the cut polarizing film, cut λ/2 plate, and cut λ/4 plate to produce a circularly polarizing plate with the structure shown in Table 1.

[Comparative Examples 3 and 4]

(Manufacturing of λ/4 Plates)

Prepare a long strip of cycloolefin resin film (ZEONOR film manufactured by Zeon Co., Ltd., Japan, glass transition temperature 126°C; thickness 45 μm) similar to that used in the λ/2 plate in Example 1 as the pre-stretching film.

This cycloolefin resin film was stretched in its widthwise direction to produce a long λ/4 sheet. The stretching conditions for the widthwise stretching treatment were set at a temperature of 120°C to 150°C and a stretching ratio of 1.5 to 2.5 times to produce a λ/4 sheet with the properties shown in Table 1 below.

The λ/4 plate obtained was evaluated using the aforementioned method.

(Manufacturing of circularly polarizing plates)

The aforementioned λ/4 plate made of cycloolefin resin was used in place of the λ/4 plate produced in Example 1. Furthermore, the lamination angles of the cut polarizing film, cut λ/2 plate, and cut λ/4 plate were varied as shown in Table 1. Aside from these changes, circularly polarizing plates were produced and evaluated using the same procedures as in Example 1.

〔result〕

The results of the Examples and Comparative Examples are shown in Table 1 below. In Table 1, the abbreviations have the following meanings.

Reh: In-plane phase difference of a λ/2 plate at a measured wavelength of 590nm.

Rthh: Phase difference in the thickness direction of a λ/2 plate at a measurement wavelength of 590nm.

Θh: The clockwise angle of the slow axis of the λ/2 plate relative to the transmission axis of the polarizing film, as viewed from the polarizing film side.

NZh: NZ coefficient of the λ/2 plate.

Req: Measuring the in-plane phase difference of a λ/4 plate at a wavelength of 590nm.

Rthq: Phase difference along the thickness of a λ/4 plate at a measured wavelength of 590nm.

Θq: The clockwise angle of the slow axis of the λ/4 plate relative to the transmission axis of the polarizing film, as viewed from the polarizing film side.

NZq: NZ coefficient of the λ/4 plate.

Difference in wavelength dispersion: The difference in wavelength dispersion between the λ/2 plate and the λ/4 plate.

100: Circular polarizer 110: Polarizing film 111: Transmission axis 112, 113: Axis 120: λ/2 plate 121: Slow axis 130: λ/4 plate 131: Slow axis 140: Wideband λ/4 plate Θh, Θq: Angles

Claims (10)

A circular polarizing plate, which comprises, in order: a polarizing film; a λ/2 plate having a slow axis in a direction at an angle Θh relative to the transmission axis of the polarizing film; and a λ/4 plate having a slow axis in a direction at an angle Θq relative to the transmission axis of the polarizing film; wherein the angle Θh of the λ/2 plate and the angle Θq of the λ/4 plate satisfy the following equations (A1), (A2) and (A3): Θq±10°＝2Θh＋45°　　　(A1)27°＜Θh＜43°　　　(A2)97°＜Θq＜133°　　　(A3)The in-plane phase difference R of the λ/2 plate at a wavelength of 400nm eh(400), the in-plane phase difference Reh(550) of the λ/2 plate at a wavelength of 550nm, the in-plane phase difference Req(400) of the λ/4 plate at a wavelength of 400nm, and the in-plane phase difference Req(550) of the λ/4 plate at a wavelength of 550nm satisfy the following formula (C): 0.04＜Req(400)／Req(550)－Reh(400)／Reh(550)＜1.0　　　(C); the NZ coefficient NZh of the λ/2 plate satisfies 1.0≦NZh≦1.3, and the NZ coefficient NZq of the λ/4 plate satisfies NZq≦－0.1. The circularly polarizing plate as described in claim 1, wherein the NZ coefficient NZq of the λ/4 plate satisfies -1.5≦NZq≦-0.1. The circularly polarizing plate as claimed in claim 1, wherein the λ/4 plate has a layer made of a material having a negative intrinsic birefringence value. The circularly polarizing plate as claimed in claim 1, wherein the λ/2 plate comprises a layer made of a material having a positive intrinsic birefringence value. The circularly polarizing plate as described in claim 1, wherein the circularly polarizing plate is in the shape of an elongated strip, and the transmission axis of the polarizing film is located in the width direction of the circularly polarizing plate. A long strip wide-band λ/4 plate, which is a long strip wide-band λ/4 plate and comprises: a λ/2 plate having a slow axis in a direction at an angle Θh relative to the width direction of the wide-band λ/4 plate; and a λ/4 plate having a slow axis in a direction at an angle Θq relative to the width direction of the wide-band λ/4 plate; wherein the angle Θh of the λ/2 plate and the angle Θq of the λ/4 plate satisfy the following equations (A1), (A2) and (A3): Θq±10°＝2Θh＋45°　　　(A1)27°＜Θh＜43°　　　(A2)97°＜Θq＜133°　　　(A3)At the wavelength of 400nm The in-plane phase difference Reh(400) of the λ/2 plate, the in-plane phase difference Reh(550) of the λ/2 plate at a wavelength of 550nm, the in-plane phase difference Req(400) of the λ/4 plate at a wavelength of 400nm, and the in-plane phase difference Req(550) of the λ/4 plate at a wavelength of 550nm satisfy the following formula (C): 0.04＜Req(400)／Req(550)－Reh(400)／Reh(550)＜1.0　　　(C); the NZ coefficient NZh of the λ/2 plate satisfies 1.0≦NZh≦1.3, and the NZ coefficient NZq of the λ/4 plate satisfies NZq≦－0.1. The long strip-shaped wide-band λ/4 plate as described in claim 6, wherein the λ/2 plate is an obliquely stretched film. The long strip-shaped wide-band λ/4 plate as described in claim 6 or 7, wherein the λ/4 plate is an obliquely stretched film. An organic electroluminescent display device comprises: a circularly polarizing plate as described in any one of claims 1 to 5, or a wide-band λ/4 thin film obtained by cutting out a long wide-band λ/4 plate as described in any one of claims 6 to 8. A liquid crystal display device comprising: a circularly polarizing plate as described in any one of claims 1 to 5, or a wide-band λ/4 film sheet obtained by cutting out a long wide-band λ/4 plate as described in any one of claims 6 to 8.