Optical laminate, elliptically polarizing plate, and organic el display device

Abstract

Provided is an optical laminate including a first liquid crystal layer and a second liquid crystal layer. The first liquid crystal layer and the second liquid crystal layer contain a liquid crystal composition that is in a twist alignment in which the thickness direction of the optical laminate serves as the helical axis. The chiral pitch of the liquid crystal composition in the first liquid crystal layer and the second liquid crystal layer is 2-40 μm. The alignment direction of the liquid crystal composition at the surface of the first liquid crystal layer facing the second liquid crystal layer is different from the alignment direction of the liquid crystal composition at the surface of the second liquid crystal layer facing the first liquid crystal layer.

Description

Optical laminate, elliptically polarizing plate, and organic EL display device

[0001] The present disclosure relates to an optical laminate, an elliptically polarizing plate, and an organic EL display device.

[0002] Conventionally, optical laminates that change the polarization state of incident light are known. One example is an optical laminate in which multiple layers, including a liquid crystal layer containing a twist-oriented liquid crystal compound, are stacked. Such optical laminates are applied to display devices that display images, etc. For example, Patent Document 1 discloses that a "wideband λ / 4 retarder" is realized by using a multilayer retarder that includes an optically anisotropic layer containing a twist-oriented liquid crystal compound. Furthermore, Patent Document 1 discloses that a circular polarizer comprising a retarder and a polarizing film is used for anti-reflection purposes in image display devices.

[0003] In particular, in Patent Document 1, the in-plane slow axis of the exposed surface of the first optically anisotropic layer (first liquid crystal layer) is rotated by a predetermined angle relative to the in-plane slow axis of the surface on the alignment film side. By further coating a liquid crystal compound on the surface of the first optically anisotropic layer, the liquid crystal compound can be aligned along the in-plane slow axis of the exposed surface, which is at a predetermined angle relative to the in-plane slow axis of the surface on the alignment film side of the first optically anisotropic layer. In the retardation plate thus formed, the in-plane slow axis of the first optically anisotropic layer on the surface facing the second optically anisotropic layer (second liquid crystal layer) is parallel to the in-plane slow axis of the second optically anisotropic layer.

[0004] Japanese Patent Application Laid-Open No. 2014-209219

[0005] The optical laminate is required to have a variety of performances for changing the polarization state of incident light. For example, the optical laminate may be required to accurately change linearly polarized light having a wavelength within a specific numerical range into circularly polarized light. Therefore, there is a demand for an optical laminate with improved design freedom that can achieve a variety of performances for changing the polarization state of incident light.

[0006] The present disclosure has been made in consideration of the above points, and aims to provide an optical laminate with improved design freedom.

[0007] Embodiments of the present disclosure relate to the following [1] to

[10] .

[0008] [1] An optical laminate comprising a first liquid crystal layer and a second liquid crystal layer, wherein the first liquid crystal layer and the second liquid crystal layer contain a liquid crystal composition that is twist-oriented with a thickness direction of the optical laminate as a helical axis, the chiral pitch of the liquid crystal composition in the first liquid crystal layer and the second liquid crystal layer is 2 μm or more and 40 μm or less, and the alignment direction of the liquid crystal composition on the surface of the first liquid crystal layer facing the second liquid crystal layer is different from the alignment direction of the liquid crystal composition on the surface of the second liquid crystal layer facing the first liquid crystal layer.

[0009] [2] The optical laminate according to [1], wherein the alignment direction of the liquid crystal composition in the surface of the first liquid crystal layer facing the second liquid crystal layer is non-perpendicular to the alignment direction of the liquid crystal composition in the surface of the second liquid crystal layer facing the first liquid crystal layer.

[0010] [3] The optical laminate according to [1] or [2], comprising: a first laminate including the first liquid crystal layer; a second laminate including the second liquid crystal layer; and a bonding layer that bonds the first laminate and the second laminate.

[0011] [4] The optical laminate according to [3], wherein the first laminate further includes a first alignment film, and the first liquid crystal layer is formed on the first alignment film; and the second laminate further includes a second alignment film, and the second liquid crystal layer is formed on the second alignment film.

[0012] [5] The optical laminate according to any one of [1] to [4], wherein the direction of twist alignment of the liquid crystal composition in the first liquid crystal layer is opposite to the direction of twist alignment of the liquid crystal composition in the second liquid crystal layer.

[0013] [6] The optical laminate according to [5], wherein the ratio of the larger absolute value of the twist amount of the twist-oriented liquid crystal composition in the first liquid crystal layer to the smaller absolute value of the twist amount of the twist-oriented liquid crystal composition in the second liquid crystal layer is 1.21 or more and 4.74 or less.

[0014] [7] The optical laminate according to any one of [1] to [6], wherein the ratio of the larger absolute value of the twist amount of the twist-aligned liquid crystal composition in the first liquid crystal layer to the smaller absolute value of the twist amount of the twist-aligned liquid crystal composition in the second liquid crystal layer is 1 or more and 11 or less.

[0015] [8] A Mueller matrix M describing a change in polarization state brought about by the optical stack for polarized light that is transmitted in the thickness direction through the second liquid crystal layer and the first liquid crystal layer in this order in a first direction perpendicular to the thickness direction. ｓ and a Stokes vector S describing the polarization state of polarized light incident on the optical laminate, which is linearly polarized light that vibrates in the first direction and has an arbitrary wavelength of 450 nm or more and 650 nm or less and passes through the second liquid crystal layer and the first liquid crystal layer in this order in the thickness direction. ｉｎ and the Stokes vector S of the polarized light emitted from the optical laminate obtained by the product of ｏｕｔ The optical laminate according to any one of [1] to [7], wherein the first direction exists in which the absolute value of the right-handed circularly polarized light intensity S3' is 0.95 or more.

[0016] [9] An elliptical polarizing plate comprising: the optical laminate according to [8]; and a polarizing plate superimposed on the optical laminate so that the direction in which the absorption axis extends is perpendicular to the first direction when observed from the thickness direction.

[0017]

[10] An organic EL display device comprising the elliptically polarizing plate according to [9].

[0018] According to the embodiments of the present disclosure, an optical laminate with improved design freedom can be provided.

[0019] FIG. 1 is a cross-sectional view of an elliptically polarizing plate according to an embodiment. FIG. 2 is a cross-sectional view of an elliptically polarizing plate according to an embodiment. FIG. 3 is a diagram showing an observed state of a portion of an optical laminate according to an embodiment. FIG. 4 is a diagram showing an observed state of a portion of a first liquid crystal layer according to an embodiment. FIG. 5 is a diagram showing an observed state of a portion of a second liquid crystal layer according to an embodiment. FIG. 6 is a diagram showing an observed state of a liquid crystal composition according to an embodiment. FIG. 7 is a diagram showing an observed state of a portion of a virtual liquid crystal layer. FIG. 8 is a cross-sectional view of a display device according to an embodiment. FIG. 9A is a diagram showing a method for manufacturing an optical laminate according to an embodiment. FIG. 9B is a diagram showing a method for manufacturing an optical laminate according to an embodiment. FIG. 10 is a cross-sectional view of an elliptically polarizing plate according to Modification 1. FIG. 11 is a cross-sectional view of a display device according to Modification 2.

[0020] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. In the drawings accompanying this specification, the scale and aspect ratios of the actual objects have been appropriately changed and exaggerated for the sake of ease of illustration and understanding.

[0021] In this disclosure, the term "alignment force" refers to the action of aligning the liquid crystal composition in a liquid crystal layer in a specific direction. In this disclosure, "horizontal alignment" refers to alignment in a direction perpendicular to the thickness direction d1 of the optical laminate 1. In this disclosure, "(meth)acrylic" refers to either acrylic or methacrylic. Terms used in this disclosure that specify shape and geometric conditions, as well as their degree, such as "parallel," "vertical," and "same," as well as values ​​of length and angle, are interpreted without being bound by strict meaning, but rather as including a range within which similar functions can be expected. In this specification, terms such as "film" and "plate" are not distinguished from each other solely based on differences in name. Furthermore, in this disclosure, the term "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits.

[0022] In the present disclosure, the in-plane retardation (Re) can be calculated from Nx, Ny, and the thickness d of the liquid crystal layer by the following formula: In-plane retardation (Re)=(Nx-Ny)×d

[0023] In this disclosure, the in-plane retardation (Re) of the liquid crystal layer is a value measured using a retardation measuring device (KOBRA-WR, manufactured by Oji Scientific Instruments Co., Ltd.). In this disclosure, the refractive indices Nx and Ny are measured using an Abbe refractometer (NAR-4T, manufactured by Atago Co., Ltd.) with a sodium lamp (λ = 589 nm) as a light source. When measuring wavelength dependency, measurements can be made using a multi-wavelength Abbe refractometer DR-M2 (manufactured by Atago Co., Ltd.) in combination with an interference filter. Values ​​from the Polymer Handbook (John Wiley & Sons, Inc.) and catalogs for various optical films can also be used. Examples of average refractive index values ​​for major optical films are listed below: cellulose acylate (1.48), cycloolefin polymer (1.52), polycarbonate (1.59), polymethyl methacrylate (1.49), and polystyrene (1.59).

[0024] In the present disclosure, the refractive index of the retardation layer in the direction in which the in-plane slow axis extends is defined as Nx, and the refractive index of the retardation layer in the direction perpendicular to the extension direction of the in-plane slow axis and the thickness direction of the retardation layer is defined as Ny. The in-plane slow axis is an axis line that faces the axial direction of the highest refractive index along the plane of the retardation layer.

[0025] I. Optical Laminate FIGS. 1 and 2 are cross-sectional views showing an elliptical polarizer 70 including an optical laminate 1 according to the present embodiment. As shown in FIGS. 1 and 2, the optical laminate 1 includes a first liquid crystal layer 11 and a second liquid crystal layer 21. In the example shown in FIGS. 1 and 2, the optical laminate 1 includes a first laminate 10 including the first liquid crystal layer 11 and a second laminate 20 including the second liquid crystal layer 21. In the example shown in FIGS. 1 and 2, the first laminate 10 further includes a first alignment film 12. The first liquid crystal layer 11 is formed on the first alignment film 12. The second laminate 20 further includes a second alignment film 22. The second liquid crystal layer 21 is formed on the second alignment film 22. In the example shown in FIGS. 1 and 2, the first alignment film 12, the first liquid crystal layer 11, the second alignment film 22, and the second liquid crystal layer 21 are laminated in this order.

[0026] In the examples shown in FIGS. 1 and 2 , the optical stack 1 further includes a bonding layer 30 that bonds the first stack 10 and the second stack 20. The optical stack 1 may further include a substrate 60, as shown in FIG. 2 . In the example shown in FIG. 1 , a first alignment film 12, a first liquid crystal layer 11, a bonding layer 30, a second alignment film 22, and a second liquid crystal layer 21 are laminated in this order. In the example shown in FIG. 2 , a substrate 60, a first alignment film 12, a first liquid crystal layer 11, a bonding layer 30, a second alignment film 22, and a second liquid crystal layer 21 are laminated in this order. The optical stack 1 has a first surface 1a and a second surface 1b located on the opposite side to the first surface 1a. In the optical stack 1 including the substrate 60 shown in FIG. 2 , the second surface 1b is a surface formed by the substrate 60. The first surface 1a is a surface opposite to the surface formed by the substrate 60. 1 , in which the optical laminate 1 does not include the substrate 60, the second surface 1b is a surface formed by peeling the substrate 60 from other portions of the optical laminate 1, as will be described later. The first surface 1a is a surface opposite to the surface formed by peeling the substrate 60 from other portions of the optical laminate 1. In the optical laminate 1 of this embodiment, when a polarizing plate 71 is superimposed on the optical laminate 1 to produce an optical member such as an elliptically polarizing plate 70, as will be described later, the first surface 1a faces the polarizing plate 71.

[0027] 1. First Alignment Film The first alignment film 12 is a layer that regulates the alignment of the liquid crystal composition. The first alignment film 12 has an alignment regulating force that aligns the liquid crystal composition contained in the first liquid crystal layer 11. In the present embodiment, the first alignment film 12 horizontally aligns the liquid crystal composition contained in the first liquid crystal layer 11. Various configurations that can exert an alignment regulating force on the liquid crystal composition can be applied as the configuration of the first alignment film 12. As an example, the first alignment film 12 is produced by subjecting a polyimide resin layer to a rubbing treatment. The first alignment film 12 may also be produced by subjecting a so-called photo-alignment film material layer to a photo-alignment treatment.

[0028] 2. First Liquid Crystal Layer The first liquid crystal layer 11 is a layer containing a liquid crystal composition. The direction in which the liquid crystal composition is aligned in a liquid crystal layer such as the first liquid crystal layer 11 is also referred to as the alignment direction. Unless otherwise specified, the alignment direction of the liquid crystal composition is the alignment direction of the liquid crystal composition perpendicular to the thickness direction d1 when the optical laminate 1 is observed from the thickness direction d1. In this embodiment, the liquid crystal compositions contained in the first liquid crystal layer 11 and the second liquid crystal layer 21 are aligned in a direction perpendicular to the thickness direction d1. In the first liquid crystal layer 11, the liquid crystal composition is twist-aligned with the thickness direction d1 of the optical laminate 1 as the helical axis. That is, the liquid crystal composition is aligned such that the alignment direction gradually changes from one side of the surface of the first liquid crystal layer 11 to the other side in the thickness direction d1. Therefore, the alignment direction of the liquid crystal composition differs depending on the position in the thickness direction d1 of the first liquid crystal layer 11.

[0029] The first liquid crystal layer 11 may exhibit a chiral nematic liquid crystal phase or a cholesteric liquid crystal phase having a so-called helical structure. For example, the liquid crystal composition used in the first liquid crystal layer 11 is a liquid crystal composition exhibiting a nematic liquid crystal phase. To form the phase, the composition used in the first liquid crystal layer 11 may be a mixture of a liquid crystal composition exhibiting a nematic liquid crystal phase and a chiral agent, as described below. The liquid crystal composition used in the first liquid crystal layer 11 may contain a liquid crystal compound exhibiting a cholesteric liquid crystal phase. In this case, the first liquid crystal layer 11 may be a cholesteric liquid crystal layer. A cholesteric liquid crystal layer is a layer composed of liquid crystal molecules exhibiting cholesteric regularity. In this specification, a cholesteric liquid crystal layer may be referred to as a "twisted liquid crystal layer." The molecular weight of the liquid crystal composition contained in the first liquid crystal layer 11 is, for example, 250 to 2000.

[0030] The liquid crystal composition contained in the first liquid crystal layer 11 may be a polymerizable liquid crystal compound having a polymerizable group. That is, the first liquid crystal layer 11 may contain a polymerizable liquid crystal compound. A liquid crystal composition containing a polymerizable liquid crystal compound having a polymerizable group can be used. The first liquid crystal layer 11 may contain a polymerizable liquid crystal composition. In this case, the polymerizable liquid crystal composition contained in the first liquid crystal layer 11 preferably exhibits liquid crystallinity and contains a polymerizable liquid crystal compound having a polymerizable group in the molecule. The polymerizable liquid crystal compound contained in the liquid crystal composition can be appropriately selected from conventionally known polymerizable liquid crystal compounds that can be twist-aligned as described below. The polymerizable liquid crystal composition may consist of a single liquid crystal compound or a mixture of two or more liquid crystal compounds.

[0031] When the liquid crystal composition contained in the first liquid crystal layer 11 contains a polymerizable liquid crystal compound, the type of polymerizable group of the polymerizable liquid crystal compound is not particularly limited. The polymerizable group of the polymerizable liquid crystal compound is preferably a functional group capable of an addition polymerization reaction. The polymerizable group of the polymerizable liquid crystal compound is preferably a polymerizable ethylenically unsaturated group or a ring-polymerizable group. More specifically, the polymerizable group of the polymerizable liquid crystal compound is preferably a (meth)acryloyl group, a vinyl group, a styryl group, an allyl group, or the like, and particularly preferably a (meth)acryloyl group.

[0032] The liquid crystal composition contained in the first liquid crystal layer 11 contains a polymerizable liquid crystal compound, and the polymerizable liquid crystal compound having a polymerizable group can be fixed by polymerization or the like to form the first liquid crystal layer 11. This makes it possible to reduce changes in the first liquid crystal layer 11 that occur due to temperature, humidity, and the like.

[0033] As an example, the first liquid crystal layer 11 of the present embodiment exhibits positive dispersion. The in-plane retardation of the first liquid crystal layer 11 at a wavelength of 550 nm is defined as Re Ａ１（５５０） The in-plane retardation of the first liquid crystal layer 11 at a wavelength of 450 nm is defined as Re Ａ１（４５０） When the first liquid crystal layer 11 exhibits normal dispersion, the following formula (i) is established. From formula (i), Re Ａ１（５５０） Re Ａ１（４５０）It can be seen that the Re when the first liquid crystal layer 11 exhibits normal dispersion is smaller than 1 / 2. Ａ１（４５０） / Re Ａ１（５５０） The value of Re may be greater than 1.05. Ａ１（６５０） When the first liquid crystal layer 11 exhibits normal dispersion, Re Ａ１（６５０） Re Ａ１（５５０） It may be smaller.

[0034] The first liquid crystal layer 11 of the present embodiment may exhibit reverse dispersion. When the first liquid crystal layer 11 exhibits reverse dispersion, the following formula (ix) holds. From formula (ix), Re Ａ１（５５０） Re Ａ１（４５０） It can be seen that the first liquid crystal layer 11 exhibits reverse dispersion. Ａ１（４５０） / Re Ａ１（５５０） When the first liquid crystal layer 11 exhibits reverse dispersion, the value of Re Ａ１（６５０） Re Ａ１（５５０） It may be larger.

[0035] The first liquid crystal layer 11 of the present embodiment may exhibit flat dispersion. When the first liquid crystal layer 11 exhibits flat dispersion, the following formula (x) holds.

[0036] The material and formation method of the first liquid crystal layer 11 are not particularly limited as long as they are materials and formation methods that can form the first liquid crystal layer 11 on the above-described first alignment film 12. For example, the first liquid crystal layer 11 can be a liquid crystal layer obtained by forming a low-molecular-weight liquid crystal compound into a nematic alignment in the liquid crystal state and then fixing the alignment by photocrosslinking or thermal crosslinking. The first liquid crystal layer 11 can also be a liquid crystal layer obtained by forming a high-molecular-weight liquid crystal compound into a nematic alignment in the liquid crystal state and then fixing the alignment by cooling. As the material and formation method of the first liquid crystal layer 11, conventionally known materials and formation methods for liquid crystal layers exhibiting positive dispersion can be used.

[0037] The first liquid crystal layer 11 may be made of a normal dispersion liquid crystal, particularly a polymerizable liquid crystal compound exhibiting normal dispersion. For example, a polymerizable liquid crystal compound exhibiting normal dispersion is used to form the first liquid crystal layer 11 exhibiting normal dispersion. The normal dispersion liquid crystal is a liquid crystal having an Re450 / Re550 ratio greater than 1.00. In the normal dispersion liquid crystal, the Re450 / Re550 may be greater than 1.05. The first liquid crystal layer 11 may be made of a reverse dispersion liquid crystal, particularly a polymerizable liquid crystal compound exhibiting reverse dispersion. For example, a polymerizable liquid crystal compound exhibiting reverse dispersion is used to form the first liquid crystal layer 11 exhibiting reverse dispersion. The reverse dispersion liquid crystal is a liquid crystal having an Re450 / Re550 ratio less than 1.00. In the reverse dispersion liquid crystal, the Re450 / Re550 may be less than 0.95. The first liquid crystal layer 11 may be made of a flat dispersion liquid crystal, particularly a polymerizable liquid crystal compound exhibiting flat dispersion. As an example, a polymerizable liquid crystal compound exhibiting flat dispersion is used to form the first liquid crystal layer 11 exhibiting flat dispersion. The flat dispersion liquid crystal is a liquid crystal having an Re450 / Re550 ratio of 0.95 or more and 1.05 or less.

[0038] Flat-dispersion liquid crystals tend to be easier to manufacture at lower cost than reverse-dispersion liquid crystals with an Re450 / Re550 ratio of less than 0.95. Furthermore, flat-dispersion liquid crystals tend to have better durability than reverse-dispersion liquid crystals with an Re450 / Re550 ratio of less than 0.95. The present inventors have discovered that an elliptical polarizer including an optical laminate 1 in which flat-dispersion liquid crystals or reverse-dispersion liquid crystals are used as the material for the first liquid crystal layer 11 and the second liquid crystal layer 21 described later is particularly excellent in terms of the effect of making the color less observable, as described below. In particular, they have discovered that even when flat-dispersion liquid crystals are used as the material for the first liquid crystal layer 11 and the second liquid crystal layer 21 described later, an effect similar to that obtained when reverse-dispersion liquid crystals with an Re450 / Re550 ratio of less than 0.95 are used can be obtained. Therefore, from the viewpoint of obtaining the effect of making the color less observable, it is preferable to use flat-dispersion liquid crystals or reverse-dispersion liquid crystals as the material for the first liquid crystal layer 11 and the second liquid crystal layer 21. From the viewpoint of achieving both the effect of making color less observable and the reduction in cost and improvement in durability, it is preferable to use flat dispersion liquid crystal as the material for the first liquid crystal layer 11 and the second liquid crystal layer 21 .

[0039] The liquid crystal composition may contain at least one of rod-shaped liquid crystals and discotic liquid crystals. At least one of rod-shaped liquid crystals and discotic liquid crystals can be used as the polymerizable liquid crystal compound contained in the liquid crystal composition. The liquid crystal composition may contain two or more rod-shaped liquid crystals, two or more discotic liquid crystals, or a mixture of rod-shaped liquid crystals and discotic liquid crystals. The types of rod-shaped liquid crystals and discotic liquid crystals used are not particularly limited.

[0040] When the first liquid crystal layer 11 contains a liquid crystal compound exhibiting a cholesteric liquid crystal phase and is a twisted liquid crystal layer, examples of the twisted liquid crystal layer include a cured product of a curable composition containing a liquid crystalline monomer or oligomer having a polymerizable group, and a liquid crystalline polymer in a glassy state. Among the above, the twisted liquid crystal layer is preferably a cured product of a curable composition containing a liquid crystalline monomer or oligomer having a polymerizable group. When the twisted liquid crystal layer is a cured product of the curable composition, the liquid crystal molecules can be optically fixed while remaining in the cholesteric liquid crystal state, improving handleability. The curable composition may be either ionizing radiation-curable or heat-curable, but from the viewpoint of the fixation described above, an ionizing radiation-curable composition is preferred. In this specification, "ionizing radiation" refers to electromagnetic waves or charged particle beams that have an energy quantum capable of polymerizing or crosslinking molecules. Typically, ultraviolet (UV) or electron beams (EB) are used. However, other types of electromagnetic waves, such as X-rays and gamma rays, as well as charged particle beams, such as alpha rays and ion beams, can also be used.

[0041] Examples of the polymerizable group include ethylenically unsaturated bond groups such as (meth)acryloyl, vinyl, and allyl groups, as well as epoxy and oxetanyl groups. From the viewpoint of polymerizability, a (meth)acryloyl or vinyl group is preferred, and a (meth)acryloyl group is more preferred. The liquid crystalline monomer or oligomer having a polymerizable group may have at least one of the above-mentioned polymerizable groups, but from the viewpoint of obtaining a twisted liquid crystal layer in which liquid crystalline molecules are optically fixed by three-dimensional crosslinking, it is preferred that the liquid crystalline monomer or oligomer have two or more polymerizable groups, and more preferred is a bifunctional liquid crystalline monomer or oligomer having polymerizable groups at both ends.

[0042] Examples of liquid crystalline monomers having a polymerizable group include those disclosed in JP-A-7-258638 and JP-A-10-508882. Examples of liquid crystalline oligomers having a polymerizable group include cyclic organopolysiloxane compounds having a cholesteric phase, such as those disclosed in JP-A-57-165480.

[0043] Specific examples of the liquid crystal monomer having a polymerizable group include a liquid crystal monomer having acryloyl groups at both ends, which is represented by the following structural formula (I).

[0044]

[0045] The twisted liquid crystal layer is preferably a cured product of a curable composition containing a liquid crystalline monomer or oligomer having a polymerizable group and a chiral agent. When the liquid crystalline monomer or oligomer is converted into a liquid crystal layer at a predetermined temperature, it becomes a nematic state. Adding a chiral agent to the liquid crystal layer results in a chiral nematic liquid crystal (i.e., cholesteric liquid crystal). The helical pitch of the helical structure of the liquid crystal molecules contained in the twisted liquid crystal layer can be adjusted by changing the chiral power by changing the type of chiral agent used or by changing the amount of chiral agent added.

[0046] The twisted liquid crystal layer may be made of a discotic liquid crystal. The twisted liquid crystal layer may use a chiral discotic compound as described in, for example, JP-A No. 2000-086591, or a copolymer of a non-chiral discotic liquid crystal compound and a chiral discotic compound having a polymerizable group as described in, for example, JP-A Nos. 2000-111734, 2000-171637, and 2000-347039.

[0047] From the viewpoint of obtaining a twisted liquid crystal layer in which the liquid crystal molecules are optically fixed by three-dimensional crosslinking, the twisted liquid crystal layer is more preferably a cured product of a curable composition containing a liquid crystalline monomer or oligomer having a polymerizable group and a chiral agent having a polymerizable group. From the viewpoint of obtaining a twisted liquid crystal layer in which the liquid crystalline molecules are optically fixed by three-dimensional crosslinking, the chiral agent having a polymerizable group is preferably a chiral agent having two or more polymerizable groups, more preferably a bifunctional chiral agent having polymerizable groups at both ends. Examples of polymerizable groups include ethylenically unsaturated bond groups such as (meth)acryloyl groups, vinyl groups, and allyl groups, as well as epoxy groups and oxetanyl groups. From the viewpoint of polymerizability, (meth)acryloyl groups or vinyl groups are preferred, and (meth)acryloyl groups are more preferred.

[0048] Examples of chiral agents include chiral compounds disclosed in JP-A-7-258638 and JP-A-10-508882. Commercially available chiral agents include "Paliocolor (registered trademark) LC756" (manufactured by BASF), a chiral agent having acryloyl groups as polymerizable groups at both ends.

[0049] The amount of chiral agent in the twisted liquid crystal layer is not particularly limited as long as it is an amount that can obtain the desired optical properties for the twisted liquid crystal layer. However, when the total amount of the liquid crystalline monomer, liquid crystalline oligomer, and chiral agent in the curable composition used to form the twisted liquid crystal layer is taken as 100% by mass, the amount of chiral agent blended is preferably 1% by mass or less, and more preferably 0.1% by mass or more and 0.5% by mass or less.

[0050] The curable composition used to form the twisted liquid crystal layer is preferably one that is cured by irradiation with the above-mentioned ionizing radiation. When an electron beam is used as the ionizing radiation, the acceleration voltage can be appropriately selected depending on the material used and the layer thickness, but curing is usually preferably carried out at an acceleration voltage of about 70 to 300 kV. When ultraviolet light is used as the ionizing radiation, ultraviolet light having a wavelength of 190 to 380 nm is usually emitted. There are no particular limitations on the ultraviolet light source, and examples that can be used include high-pressure mercury lamps, low-pressure mercury lamps, metal halide lamps, and carbon arc lamps.

[0051] When the curable composition used to form the twisted liquid crystal layer is an ultraviolet-curable composition, it preferably further contains a photopolymerization initiator. This is because it enables the liquid crystalline monomer or oligomer having a polymerizable group and the chiral agent having a polymerizable group in the curable composition to be cured by ultraviolet irradiation. Examples of the photopolymerization initiator include one or more selected from acetophenone, benzophenone, α-hydroxyalkylphenone, Michler's ketone, benzoin, benzyl dimethyl ketal, benzoyl benzoate, α-acyloxime ester, thioxanthones, etc. The above photopolymerization initiators can be used alone or in combination of two or more. The amount of photopolymerization initiator in the curable composition is preferably 1 to 10 parts by mass, more preferably 2 to 8 parts by mass, based on 100 parts by mass of the total amount of the liquid crystalline monomer, liquid crystalline oligomer, and chiral agent.

[0052] The curable composition used to form the twisted liquid crystal layer may further contain other components such as a photopolymerization accelerator, a lubricant, a plasticizer, a filler, an antistatic agent, an antiblocking agent, a crosslinking agent, a light stabilizer, an ultraviolet absorber, an antioxidant, a conductive agent, a refractive index adjuster, a solvent, and a leveling agent, as long as the effects of the present invention are not impaired.

[0053] When the material constituting the twisted liquid crystal layer is a liquid crystalline polymer, specific examples thereof include polymers in which mesogen groups exhibiting liquid crystallinity have been introduced into the main chain, side chain, or both the main chain and the side chain, and polymeric cholesteric liquid crystals in which cholesteryl groups have been introduced into the side chain, such as the liquid crystalline polymers disclosed in JP-A-9-133810 and JP-A-11-293252. As the liquid crystalline polymer, a cholesteric liquid crystalline polymer having chiral properties may be used, or a mixture of a nematic liquid crystalline polymer and a cholesteric liquid crystalline polymer may be used. Such liquid crystalline polymers change state depending on the temperature. For example, if the polymer has a glass transition temperature of 90°C and an isotropic transition temperature of 200°C, the polymer will exhibit a cholesteric liquid crystalline state between 90°C and 200°C, and when cooled to room temperature, the polymer can be solidified into a glassy state while maintaining the cholesteric structure.

[0054] The thickness of the twisted liquid crystal layer is not particularly limited as long as the twisted liquid crystal layer has the desired optical properties. The thickness of the twisted liquid crystal layer is, for example, 0.5 μm or more. The thickness of the twisted liquid crystal layer is, for example, 3.5 μm or less.

[0055] In this embodiment, the first alignment film 12 is located closer to the second surface 1 b side of the optical laminate 1 in the thickness direction d1 than the first liquid crystal layer 11 .

[0056] 3. Second Alignment Film The second alignment film 22 is a layer that regulates the alignment of the liquid crystal composition. The second alignment film 22 has an alignment regulation force that aligns the liquid crystal composition contained in the second liquid crystal layer 21. In the present embodiment, the second alignment film 22 horizontally aligns the liquid crystal composition contained in the second liquid crystal layer 21. Various configurations that can exert an alignment regulation force on the liquid crystal composition can be applied as the configuration of the second alignment film 22. The method for producing the second alignment film 22 is, for example, the same as the method for producing the first alignment film 12.

[0057] 4. Second Liquid Crystal Layer The second liquid crystal layer 21 is a layer containing a liquid crystal composition. In the second liquid crystal layer 21, the liquid crystal composition is twist-oriented with the thickness direction d1 of the optical laminate 1 as the helical axis. That is, the liquid crystal composition is oriented such that the orientation direction gradually changes from one side of the surface of the second liquid crystal layer 21 to the other side in the thickness direction d1. Therefore, the orientation direction of the liquid crystal composition differs depending on the position in the thickness direction d1 of the second liquid crystal layer 21.

[0058] Except as otherwise specified, the material and forming method of the second liquid crystal layer 21 can be the same as the material and forming method of the first liquid crystal layer 11. Except as otherwise specified, the structure of the second liquid crystal layer 21 can be the same as the structure of the first liquid crystal layer 11.

[0059] As an example, the second liquid crystal layer 21 of the present embodiment exhibits positive dispersion. The in-plane retardation of the second liquid crystal layer 21 at a wavelength of 550 nm is defined as Re Ａ２（５５０） The in-plane retardation of the second liquid crystal layer 21 at a wavelength of 450 nm is defined as Re Ａ２（４５０） When the second liquid crystal layer 21 exhibits normal dispersion, the following formula (ii) holds. From formula (ii), Re Ａ２（５５０） Re Ａ２（４５０） It can be seen that the Re when the second liquid crystal layer 21 exhibits normal dispersion is smaller than 1 / 2. Ａ２（４５０） / Re Ａ２（５５０） The value of Re may be greater than 1.05. Ａ２（６５０） When the second liquid crystal layer 21 exhibits normal dispersion, Re Ａ２（６５０） Re Ａ２（５５０） It may be smaller.

[0060] The second liquid crystal layer 21 of the present embodiment may exhibit reverse dispersion. When the second liquid crystal layer 21 exhibits reverse dispersion, the following formula (xi) holds. From formula (xi), Re Ａ２（５５０） Re Ａ２（４５０） In other words, it can be seen that the second liquid crystal layer 21 exhibits reverse dispersion. Ａ２（４５０） / Re Ａ２（５５０）When the second liquid crystal layer 21 exhibits reverse dispersion, the value of Re Ａ２（６５０） Re Ａ２（５５０） It may be larger.

[0061] The second liquid crystal layer 21 of the present embodiment may exhibit flat dispersion. When the second liquid crystal layer 21 exhibits flat dispersion, the following formula (xii) holds.

[0062] In this embodiment, the second alignment film 22 is located closer to the second surface 1b of the optical laminate 1 in the thickness direction d1 than the second liquid crystal layer 21.

[0063] In this embodiment, the first liquid crystal layer 11 is located closer to the second surface 1b in the thickness direction d1 of the optical stack 1 than the second liquid crystal layer 21. In this embodiment, the first stack 10 is located closer to the second surface 1b in the thickness direction d1 of the optical stack 1 than the second stack 20.

[0064] 1 and 2, the surface of the first liquid crystal layer 11 facing the second liquid crystal layer 21 is referred to as the first opposing surface 11a. The surface of the second liquid crystal layer 21 facing the first liquid crystal layer 11 is referred to as the second opposing surface 21a. In the present embodiment, the first opposing surface 11a and the second opposing surface 21a face each other with the second alignment film 22 sandwiched therebetween. In the example shown in FIGS. 1 and 2, the first opposing surface 11a and the second opposing surface 21a face each other with the second alignment film 22 and the bonding layer 30 sandwiched therebetween.

[0065] 5. Bonding Layer The optical laminate 1 of this embodiment further includes a bonding layer 30 that bonds the first laminate 10 and the second laminate 20 together. In the example shown in FIGS. 1 and 2 , the bonding layer 30 bonds the first liquid crystal layer 11 and the second alignment film 22 together, thereby bonding the first laminate 10 and the second laminate 20 together. As an example, the bonding layer 30 is an adhesive layer (adhesive layer) located between the first laminate 10 and the second laminate 20. In this case, the adhesive or adhesive for the adhesive layer (adhesive layer) can be appropriately selected from conventionally known adhesives. As the adhesive or adhesive for the adhesive layer (adhesive layer), any adhesive type can be suitably used, such as a pressure-sensitive adhesive (adhesive), a two-component curing adhesive, an ultraviolet-curing adhesive, a heat-curing adhesive, or a hot-melt adhesive. The adhesive for the adhesive layer may be an adhesive composition having a (meth)acrylic resin as a base polymer, preferably from the viewpoints of transparency, weather resistance, heat resistance, etc. The thickness of the adhesive layer (bonding layer) is determined depending on its adhesive strength, etc., but may be, for example, 1 μm to 50 μm, preferably 2 μm to 45 μm, more preferably 3 μm to 40 μm, more preferably 4 μm to 35 μm, more preferably 5 μm to 35 μm, and even more preferably 5 μm to 25 μm. The thickness of the adhesive layer (bonding layer) is, for example, 5 μm.

[0066] 6. Substrate The optical laminate 1 may further include a substrate 60, as shown in FIG. 2, for example. The substrate 60 of this embodiment is directly adjacent to the first alignment film 12 as shown in FIG. 2. The substrate 60 is preferably transparent. As an example, the substrate 60 is a transparent polymer substrate, i.e., a transparent resin substrate made of a polymer. In this case, the substrate 60 can be made of any of polyester-based resins, cellulose-based resins, acrylic resins, and olefin-based resins. Polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) can be used as polyester-based resins. Polyethylene terephthalate can be used in particular as polyester-based resins. Triacetyl cellulose (TAC) can be used in particular as cellulose-based resins. Cycloolefin polymer (COP) can be used in particular as olefin-based resins. That is, the substrate 60 can be made of any of polyethylene terephthalate, triacetyl cellulose, acrylic resins, and cycloolefin polymers. The substrate 60 may be a glass plate.

[0067] The substrate 60 preferably has a transmittance of 80% or more, and more preferably 90% or more, in the visible light region. The transmittance of the substrate 60 can be measured according to JIS K7361-1 (Test method for total light transmittance of plastic transparent materials).

[0068] The thickness of the substrate 60 is not particularly limited as long as it can support the optical laminate 1 depending on the application of the optical laminate 1, but is usually about 10 μm to 200 μm. The thickness of the substrate 60 is preferably 25 μm to 125 μm, more preferably 30 μm to 110 μm, more preferably 30 μm to 100 μm, and even more preferably 40 μm to 100 μm. The thickness of the substrate 60 is, for example, 100 μm.

[0069] The substrate 60 may be a release substrate that can be peeled off from the first alignment film 12. Thereby, when attaching the optical laminate 1 to another member, after attaching the surface of the optical laminate 1 that is not constituted by the substrate 60 to the other member, peeling can be caused at the interface between the substrate 60 and the first alignment film 12, and the substrate 60 can be removed from the portion of the optical laminate 1 other than the substrate 60. For example, when producing an elliptical polarizing plate 70 using the optical laminate 1 as described below, after bonding the optical laminate 1 to a polarizing plate 71 described below, the substrate 60 can be removed from the portion of the optical laminate 1 other than the substrate 60.

[0070] 7. Alignment Direction of Liquid Crystal Composition As described above, in the first liquid crystal layer 11 and the second liquid crystal layer 21, the liquid crystal composition is twist-aligned with the thickness direction d1 of the optical stack 1 as the helical axis. Details of the alignment direction of the liquid crystal composition in the first liquid crystal layer 11 and the second liquid crystal layer 21 will be described with reference to FIGS. 3, 4, and 5. FIG. 3 is a diagram showing the first liquid crystal layer 11, the bonding layer 30, the second alignment film 22, and a portion of the second liquid crystal layer 21 of the optical stack 1 shown in FIGS. 1 and 2, observed from a direction perpendicular to the thickness direction d1 of the optical stack 1. FIG. 4 is a diagram showing the first liquid crystal layer 11 shown in FIG. 3, observed from the first surface 1a side of the optical stack 1 along the thickness direction d1. That is, FIG. 4 is a diagram showing the first liquid crystal layer 11 shown in FIG. 3, observed from the direction of the arrow indicated by the symbol A1 in FIG. 3. Fig. 5 is a diagram showing a part of the second liquid crystal layer 21 shown in Fig. 3 observed from the first surface 1a side of the optical laminate 1 along the thickness direction d1. That is, Fig. 5 is a diagram showing a part of the second liquid crystal layer 21 shown in Fig. 3 observed from the direction of the arrow labeled A1 in Fig. 3. In Figs. 3, 4, and 5, as well as Fig. 6 described below, the liquid crystal composition contained in the first liquid crystal layer 11 is labeled with the symbol 13, and the liquid crystal composition contained in the second liquid crystal layer 21 is labeled with the symbol 23.

[0071] 3, 4, and 5, the liquid crystal composition 13 is twist-aligned with a helical axis L1 extending in the thickness direction d1 of the optical laminate 1. The liquid crystal composition 23 is twist-aligned with a helical axis L2 extending in the thickness direction d1 of the optical laminate 1.

[0072] The chiral pitch P1 of the liquid crystal composition 13 in the first liquid crystal layer 11 and the chiral pitch P2 of the liquid crystal composition 23 in the second liquid crystal layer 21 are 2 μm or more and 40 μm or less. The chiral pitch is the thickness of the portion where the alignment direction of the twist-aligned liquid crystal composition changes by 360° when the change in the alignment direction is observed along the thickness direction d1. In the example shown in FIG. 3 , the change in the alignment direction of the liquid crystal composition 13 is less than 360° throughout the first liquid crystal layer 11. Furthermore, the change in the alignment direction of the liquid crystal composition 23 is less than 360° throughout the second liquid crystal layer 21. In the example shown in FIG. 3 , the chiral pitch P1 of the liquid crystal composition 13 is calculated from the twist amount θ1 (°) of the liquid crystal composition 13 and the film thickness d (film thickness d11) of the first liquid crystal layer 11, which will be described later, using the following formula (iii): Furthermore, the chiral pitch P2 of the liquid crystal composition 23 is calculated from the twist amount θ2 (°) of the liquid crystal composition 23 and the film thickness d (film thickness d21) of the second liquid crystal layer 21, which will be described later, by the following formula (iv).

[0073] The alignment direction of the liquid crystal composition 13 located on the first opposing surface 11a of the first liquid crystal layer 11, designated by reference numeral 131 in Figures 3 and 4, is designated as alignment direction da. The alignment direction of the liquid crystal composition 23 located on the second opposing surface 21a of the second liquid crystal layer 21, designated by reference numeral 231 in Figures 3 and 5, is designated as alignment direction db. In this case, the alignment direction da and the alignment direction db are different. That is, the alignment direction da of the liquid crystal composition 13 on the first opposing surface 11a of the first liquid crystal layer 11 is different from the alignment direction db of the liquid crystal composition 23 on the second opposing surface 21a of the second liquid crystal layer 21. In this embodiment, the optical laminate 1 includes a first alignment film 12 and a second alignment film 22, thereby making it possible to make the alignment direction da and the alignment direction db different.

[0074] The alignment direction da of the liquid crystal composition 13 on the first opposing surface 11a of the first liquid crystal layer 11 and the alignment direction db of the liquid crystal composition 23 on the second opposing surface 21a of the second liquid crystal layer 21 may be non-perpendicular. "The alignment directions da and db are non-perpendicular," regardless of the above-mentioned statement that "terms such as 'parallel,' 'vertical,' and 'same,' and values ​​of length and angle, etc., are not limited to their strict meanings but are interpreted to include a range within which similar functions can be expected," is interpreted as follows. "The alignment directions da and db are non-perpendicular" means that the angle θ3, which is an angle of 90° or less between the alignment directions da and db, is 89.9° or less. When the alignment directions da and db are non-perpendicular, the angle θ3 may be 89.8° or less, or may be 89.7° or less. In particular, from the viewpoint of improving production efficiency during mass production of the optical laminate 1, it is preferable that the alignment directions da and db are non-perpendicular.

[0075] The alignment direction da of the liquid crystal composition 13 on the first opposing surface 11a and the alignment direction db of the liquid crystal composition 23 on the second opposing surface 21a can be determined by the following method. ｓ Furthermore, an optical stack 1 including a first liquid crystal layer 11 and a second liquid crystal layer 21 is modeled using a twisted liquid crystal model. The optical stack 1 is modeled so that the Δn, film thickness d, and twisted alignment state of the first liquid crystal layer 11, which will be described later, and the Δn, film thickness d, and twisted alignment state of the second liquid crystal layer 21, which will be described later, can be adjusted. The Mueller matrix M of the modeled optical stack 1 is ｓ can be calculated from the settings of Δn, film thickness d, and twist alignment state of the first liquid crystal layer 11, and the settings of Δn, film thickness d, and twist alignment state of the second liquid crystal layer 21. Using the modeled optical stack 1, the measured Mueller matrix M ｓ By performing an inverse analysis of the Mueller matrix M, the orientation direction da and the orientation direction db can be determined. ｓWhen the values ​​of Δn in the first liquid crystal layer 11 and Δn in the second liquid crystal layer 21 of the optical laminate 1 to be measured are known, the inverse analysis can be performed by the following method. The known values ​​are set as Δn in the first liquid crystal layer 11 and Δn in the second liquid crystal layer 21 of the modeled optical laminate 1. Next, the Mueller matrix M ｓ is the measured Mueller matrix M of the optical laminate 1 ｓ The settings of the film thickness d and the twist orientation mode of the first liquid crystal layer 11, and the film thickness d and the twist orientation mode of the second liquid crystal layer 21 are specified so as to match. When the values ​​of Δn in the first liquid crystal layer 11 and Δn in the second liquid crystal layer 21 of the optical stack 1 to be measured are known, this corresponds to the case where the optical stack 1 to be measured is produced while controlling Δn by itself. The Mueller matrix M ｓ When the values ​​of Δn in the first liquid crystal layer 11 and Δn in the second liquid crystal layer 21 of the optical laminate 1 to be measured are not known, the inverse analysis can be performed by the following method. ｓ is the measured Mueller matrix M of the optical laminate 1 ｓ The settings of Δn, film thickness d, and twist orientation of the first liquid crystal layer 11 and the settings of Δn, film thickness d, and twist orientation of the second liquid crystal layer 21 are specified so that the specified settings are consistent with the Mueller matrix M ｓ is realized in the optical laminate 1 to be measured, it is possible to identify the state of twist alignment of the first liquid crystal layer 11 and the second liquid crystal layer 21 in the optical laminate 1 to be measured. From the identified state of twist alignment of the first liquid crystal layer 11 and the second liquid crystal layer 21, it is possible to identify the alignment direction da and the alignment direction db.

[0076] Considering common technical knowledge, it is believed that the optical stack 1 is endowed with optical properties such that the directions corresponding to the alignment direction da and the alignment direction db are identified by the above-mentioned identification method using the Mueller matrix polarimeter AxoScan (registered trademark) only when the optical stack 1 includes the first liquid crystal layer 11 and the second liquid crystal layer 21. For this reason, it is clear that the optical stack 1 having optical properties such that the directions corresponding to the alignment direction da and the alignment direction db are identified by the above-mentioned identification method includes the first liquid crystal layer 11 and the second liquid crystal layer 21. In particular, even without examining the components of each layer of the optical stack 1, it is clear that the optical stack 1 includes the first liquid crystal layer 11 containing a liquid crystal composition and the second liquid crystal layer 21 containing a liquid crystal composition.

[0077] 6 is a diagram showing the liquid crystal composition 13 denoted by reference numeral 131 in FIGS. 3 and 4 and the liquid crystal composition 23 denoted by reference numeral 231 in FIGS. 3 and 5, observed from the first surface 1a side of the optical laminate 1 along the thickness direction d1. The angle θ3 formed between the alignment direction da and the alignment direction db shown in FIG. 6 is, for example, 5° or more. The angle θ3 may be 10° or more. The angle θ3 is an angle formed between the alignment direction da and the alignment direction db of 90° or less. The angle θ3 is always expressed as 0 or a positive value.

[0078] The alignment direction of the liquid crystal composition 13 located on the surface 11b opposite to the first opposing surface 11a of the first liquid crystal layer 11, designated by reference numeral 132 in Fig. 3 and Fig. 4, is designated as alignment direction dc. The alignment direction of the liquid crystal composition 23 located on the surface 21b opposite to the second opposing surface 21a of the second liquid crystal layer 21, designated by reference numeral 232 in Fig. 3 and Fig. 5, is designated as alignment direction dd. The alignment direction dc of the liquid crystal composition 13 on the surface 11b and the alignment direction dd of the liquid crystal composition 23 on the surface 21b can be determined by the same method as the method for determining the alignment direction da and the alignment direction db described above. That is, the alignment direction dc and the alignment direction dd can be determined by calculating the Mueller matrix M of the optical laminate 1 using a Mueller matrix polarimeter AxoScan (registered trademark) manufactured by Axometrics. ｓ and the Mueller matrix M ｓThe amount of twist θ1 (°) of the liquid crystal composition 13 can be determined by analyzing the above. Furthermore, when the change in the alignment direction of the liquid crystal composition 13 is observed along the thickness direction d1 from the surface 11b to the first opposing surface 11a, the amount of change in the alignment direction of the liquid crystal composition 13 is defined as the twist amount θ1 (°) of the liquid crystal composition 13. In the example shown in FIG. 4, the twist amount θ1 (°) corresponds to the angle between the alignment direction dc and the alignment direction da. When the change in the alignment direction of the liquid crystal composition 23 is observed along the thickness direction d1 from the second opposing surface 21a to the surface 21b, the amount of change in the alignment direction of the liquid crystal composition 23 is defined as the twist amount θ2 (°) of the liquid crystal composition 23. In the example shown in FIG. 5, the twist amount θ2 (°) corresponds to the angle between the alignment direction db and the alignment direction dd.

[0079] The positive and negative values ​​of the twist amounts θ1 (°) and θ2 (°) are defined as follows. Consider the case where the first liquid crystal layer 11 and the second liquid crystal layer 21 are observed from the first surface 1a of the optical laminate 1 along the thickness direction d1, as shown in FIGS. 4 and 5 . In this case, when the alignment directions of the liquid crystal compositions 13 and 23 change counterclockwise from the second surface 1b toward the first surface 1a, the twist amounts θ1 (°) and θ2 (°) are defined as positive values. When the alignment directions of the liquid crystal compositions 13 and 23 change clockwise from the second surface 1b toward the first surface 1a, the twist amounts θ1 (°) and θ2 (°) are defined as negative values. In the example shown in FIG. 4 , the alignment direction of the liquid crystal composition 13 changes counterclockwise from the second surface 1b toward the first surface 1a. Therefore, the twist amount θ1 (°) shown in Fig. 4 is a positive value. In the example shown in Fig. 5, the alignment direction of the liquid crystal composition 23 changes clockwise from the second surface 1b side toward the first surface 1a side. Therefore, the twist amount θ2 (°) shown in Fig. 5 is a negative value.

[0080] In the examples shown in Figures 4 and 5, the twist amount θ1 (°) is a positive value and the twist amount θ2 (°) is a negative value. As a result, the direction of twist alignment of the liquid crystal composition 13 in the first liquid crystal layer 11 is opposite to the direction of twist alignment of the liquid crystal composition 23 in the second liquid crystal layer 21. The twist alignment direction is the direction in which the alignment directions of the liquid crystal composition 13 and the liquid crystal composition 23 change from the second surface 1b side toward the first surface 1a side when the first liquid crystal layer 11 and the second liquid crystal layer 21 are observed from the first surface 1a side of the optical laminate 1 along the thickness direction d1. In the example shown in Figure 4, the twist alignment direction of the liquid crystal composition 13 in the first liquid crystal layer 11 is the direction of the arrow labeled d2. In the example shown in Figure 5, the twist alignment direction of the liquid crystal composition 23 in the second liquid crystal layer 21 is the direction of the arrow labeled d3. Although not shown, the twist amount θ1 (°) may be negative and the twist amount θ2 (°) may be positive, so that the direction of twist alignment of liquid crystal composition 13 is opposite to the direction of twist alignment of liquid crystal composition 23. Furthermore, although not shown, the twist direction of liquid crystal composition 13 may be the same as the direction of twist alignment of liquid crystal composition 23. In this case, the twist amounts θ1 (°) and θ2 (°) may both be positive or negative.

[0081] The following relationship may be established between the twist amount θ1 (°) of the twist-aligned liquid crystal composition 13 in the first liquid crystal layer 11 and the twist amount θ2 (°) of the twist-aligned liquid crystal composition 23 in the second liquid crystal layer 21. The ratio of the larger absolute value of the twist amount θ1 (°) or the smaller absolute value of the twist amount θ2 (°) to the smaller absolute value may be 1.21 or more and 4.74 or less. In the examples shown in Figures 4 and 5, the absolute value of the twist amount θ1 (°) is larger than the absolute value of the twist amount θ2 (°). In this case, the ratio of the absolute value of the twist amount θ1 (°) to the absolute value of the twist amount θ2 (°) may be 1.21 or more and 4.74 or less. Although not shown, when the absolute value of the twist amount θ2 (°) is greater than the absolute value of the twist amount θ1 (°), the ratio of the absolute value of the twist amount θ2 (°) to the absolute value of the twist amount θ1 (°) may be 1.21 or more and 4.74 or less. The ratio of the larger absolute value of the twist amount θ1 (°) or the smaller absolute value of the twist amount θ2 (°) may be 1.83 or more and 4.74 or less, or 1.83 or more and 2.57 or less.

[0082] The ratio of the larger absolute value of the twist amount θ1 (°) and the smaller absolute value of the twist amount θ2 (°) to the larger absolute value of the smaller absolute value may be 1 or greater and 11 or less. More preferably, this ratio is 1 or greater and 5 or less. In an optical laminate 1 in which the direction of twist alignment of the liquid crystal composition 13 in the first liquid crystal layer 11 is opposite to the direction of twist alignment of the liquid crystal composition 23 in the second liquid crystal layer 21, this ratio may be within the above-mentioned numerical range. In an optical laminate 1 in which the direction of twist alignment of the liquid crystal composition 13 in the first liquid crystal layer 11 is the same as the direction of twist alignment of the liquid crystal composition 23 in the second liquid crystal layer 21, this ratio may be within the above-mentioned numerical range.

[0083] The alignment direction of the liquid crystal composition 13 on the surface (surface 11b) of the first liquid crystal layer 11 that contacts the first alignment film 12 is determined according to the alignment regulating force of the first alignment film 12. The alignment direction of the liquid crystal composition 23 on the surface (second opposing surface 21a) of the second liquid crystal layer 21 that contacts the second alignment film 22 is determined according to the alignment regulating force of the second alignment film 22. The alignment directions of the liquid crystal compositions 13 and 23 in other parts of the first liquid crystal layer 11 and the second liquid crystal layer 21 can be adjusted by adding a chiral agent and a component that adjusts the alignment direction of the liquid crystal composition 13 and the liquid crystal composition 23.

[0084] 8. Effects of the Optical Laminate In the optical laminate 1 of the present embodiment, the alignment direction da of the liquid crystal composition 13 on the first opposing surface 11a of the first liquid crystal layer 11 is different from the alignment direction db of the liquid crystal composition 23 on the second opposing surface 21a of the second liquid crystal layer 21. This improves the degree of freedom in designing the optical laminate 1 compared to, for example, a case where the alignment directions da and db are constrained to be parallel. This makes it possible to provide an optical laminate 1 with improved design freedom that can achieve a variety of performance capabilities in terms of the ability to change the polarization state of incident light.

[0085] The effects of the optical laminate 1 of this embodiment will be further explained by comparison with the descriptions in prior art documents. Japanese Patent Application Laid-Open No. 2023-27201 describes an optical element comprising a plurality of optically anisotropic layers in the thickness direction having an in-plane orientation pattern in which the orientation of the optical axis derived from the liquid crystal compound changes while continuously rotating along at least one direction in the plane (see claim 1, etc.). The in-plane orientation pattern in Japanese Patent Application Laid-Open No. 2023-27201 is, in particular, a pattern oriented and fixed along axis A so that the angle between the optical axis 30A and axis A gradually changes from φ to φ+180° or φ-180° (see paragraph 0015). As a result, Japanese Patent Application Laid-Open No. 2023-27201 realizes a diffraction element that diffracts incident light (see paragraphs 0031, 0047, 0048). In JP 2023-27201 A, in order to form a corresponding in-plane alignment pattern in each of a plurality of optically anisotropic layers stacked in the thickness direction, it is considered to be essentially essential to align the alignment directions of the liquid crystal composition on opposing surfaces of the optically anisotropic layers. When the alignment directions of the liquid crystal composition on opposing surfaces of liquid crystal layers (optically anisotropic layers) are made different, as in the optical laminate 1 of the present embodiment, it becomes necessary to bond the liquid crystal layers together so that the in-plane alignment patterns of the opposing liquid crystal layers completely correspond. Because such bonding is practically difficult, it is virtually impossible to form an in-plane alignment pattern corresponding to each liquid crystal layer. Furthermore, JP 2014-209219 A, mentioned above as Patent Document 1, describes a retardation plate for a circular polarizer comprising a first optically anisotropic layer and a second optically anisotropic layer, wherein the first optically anisotropic layer and the second optically anisotropic layer contain a liquid crystal compound that is twisted and aligned with the helical axis in the thickness direction. JP 2014-209219 A particularly describes that the in-plane slow axis of the surface of the first optically anisotropic layer facing the second optically anisotropic layer is parallel to the in-plane slow axis of the surface of the second optically anisotropic layer facing the first optically anisotropic layer (see claim 1, paragraph

[0025] of the Claims). In contrast, in the optical laminate 1 of the present embodiment, the alignment direction da of the liquid crystal composition 13 at the first opposing surface 11a of the first liquid crystal layer 11 is made different from the alignment direction db of the liquid crystal composition 23 at the second opposing surface 21a of the second liquid crystal layer 21.In particular, in the optical laminate 1 of this embodiment, the alignment direction da and the alignment direction db are made different and non-perpendicular to each other, thereby making it possible to provide an optical laminate 1 with improved design freedom that can achieve a variety of performances in terms of the ability to change the polarization state of incident light.

[0086] In particular, the optical laminate 1 may be required to accurately convert linearly polarized light having a wavelength within a specific numerical range into circularly polarized light. For example, as described below, the optical laminate 1 may be combined with a polarizing plate, particularly a linear polarizing plate, to produce an elliptical polarizing plate comprising the optical laminate 1 and a linear polarizing plate. By using the elliptical polarizing plate in a display device, particularly an organic EL display device (organic electroluminescence display device), it is possible to reduce the reflection of external light on the display surface of the display device. In other words, the elliptical polarizing plate can be used as an external light anti-reflection film. In this case, the optical laminate 1 can accurately convert linearly polarized light having a wavelength within a specific numerical range into circularly polarized light, thereby more effectively reducing the reflection of external light on the display surface of the display device.

[0087] As a comparative example, consider a case where the optical laminate 1 has low accuracy in converting linearly polarized light having a certain wavelength into circularly polarized light, and the optical laminate 1 converts linearly polarized light having that wavelength into elliptically polarized light that is different from circularly polarized light. In this case, the effect of the elliptically polarizing plate in reducing the reflection of external light having that wavelength is weakened. Therefore, when the elliptically polarizing plate is observed from the thickness direction d1 of the optical laminate 1 and from the side that will be viewed when the optical laminate 1 is used in a display device, etc., a color corresponding to that wavelength can be observed. In particular, when the elliptically polarizing plate is used in a display device, even when the display device is not displaying an image on the display surface and the display surface is black, a color can be observed when the display surface is observed through the elliptically polarizing plate. In contrast, because the optical laminate 1 can accurately convert linearly polarized light having a wavelength within a specific numerical range into circularly polarized light, a color is less likely to be observed on the elliptically polarizing plate.

[0088] By differentiating the alignment direction da from the alignment direction db as described above, it is possible to provide the optical laminate 1 that can accurately convert linearly polarized light into circularly polarized light. In particular, it is possible to provide the optical laminate 1 that can accurately convert linearly polarized light having a wavelength of 450 nm or more and 650 nm or less into circularly polarized light.

[0089] The optical laminate 1, which can accurately convert linearly polarized light having a wavelength of 450 nm or more and 650 nm or less into circularly polarized light, will be described in detail. The change in polarization state brought about by the optical laminate 1 for polarized light passing through the optical laminate 1, particularly polarized light passing through the second liquid crystal layer 21 and the first liquid crystal layer 11 in this order in the thickness direction d1, is expressed by the matrix shown in the following formula (v), i.e., the Mueller matrix M ｓ It can be described by

[0090] The polarization state of polarized light incident on the optical laminate 1, which passes through the second liquid crystal layer 21 and the first liquid crystal layer 11 in this order in the thickness direction d1, can be described by the vector shown in the following formula (vi), i.e., a four-dimensional vector. The vector shown in formula (vi) can be expressed as the Stokes vector S ｉｎ The Stokes vector S ｉｎ In the equation, S0 is the average light intensity of the incident polarized light. S1 is the horizontal linear polarization intensity of the incident polarized light. S2 is the 45-degree linear polarization intensity of the incident polarized light. S3 is the right-handed circular polarization intensity of the incident polarized light.

[0091] The polarization state of the polarized light emitted from the optical laminate 1 can be described by the vector shown in the following formula (vii), i.e., a four-dimensional vector. The polarized light emitted from the optical laminate 1 is the polarized light emitted from the optical laminate 1 when the above-mentioned incident polarized light passes through the optical laminate 1 in the thickness direction d1. The vector shown in formula (vii) can be expressed as the Stokes vector S ｏｕｔ The Stokes vector S ｏｕｔ In the equation, S0' is the average light intensity of the output polarized light. S1' is the horizontal linear polarization intensity of the output polarized light. S2' is the 45-degree linear polarization intensity of the output polarized light. S3' is the right-handed circular polarization intensity of the output polarized light.

[0092] The Mueller matrix M shown in equation (v) ｓ and the Stokes vector S shown in equation (vi) ｉｎ From the product of these, the Stokes vector S ｏｕｔ In other words, the following equation (viii) holds: In further terms, the Mueller matrix M ｓ is a matrix determined so that equation (viii) holds when any incident polarized light that passes through the second liquid crystal layer 21 and the first liquid crystal layer 11 in this order in the thickness direction d1 passes through the optical stack 1 in the thickness direction d1.

[0093] Mueller matrix M of optical laminate 1 ｓ can be measured by a Mueller matrix polarimeter AxoScan® manufactured by Axometrics.

[0094] Based on the above, let us consider the conditions for the optical laminate 1 that can accurately convert linearly polarized light having a wavelength of 450 nm or more and 650 nm or less into circularly polarized light. First, let us consider the case where the incident polarized light is linearly polarized light that oscillates in a first direction d4 perpendicular to the thickness direction d1 of the optical laminate 1. In this case, the Stokes vector S of the outgoing polarized light is ｏｕｔ is the Mueller matrix M based on equation (viii). ｓ and the Stokes vector S of the incident polarization ｉｎ The Stokes vector S of the outgoing polarized light can be calculated from the optical laminate 1 obtained by multiplying ｏｕｔIn this case, if the absolute value of the right-handed circularly polarized light intensity S3' is large, it can be said that the optical laminate 1 can accurately convert linearly polarized light vibrating in the first direction d4 into circularly polarized light. In particular, if the absolute value of the right-handed circularly polarized light intensity S3' is 0.95 or more, it can be said that the optical laminate 1 can convert linearly polarized light vibrating in the first direction d4 into circularly polarized light with sufficient accuracy. In particular, if the absolute value of the right-handed circularly polarized light intensity S3' is 0.95 or more, it can be said that the optical laminate 1 can convert linearly polarized light vibrating in the first direction d4 into circularly polarized light with high accuracy to the extent that the following effects can be obtained. If the absolute value of the right-handed circularly polarized light intensity S3' is 0.95 or more, it can be said that a color is unlikely to be observed when an elliptical polarizer including the optical laminate 1 is observed from the thickness direction d1 of the optical laminate 1 and from the side that will be viewed when the optical laminate 1 is used in a display device or the like. In particular, if the absolute value of the right-handed circularly polarized light intensity S3' is 0.95 or more, when the elliptically polarizing plate is used in a display device, even when the display device is not displaying an image on the display surface and the display surface is black, color tinge is unlikely to be observed.

[0095] When the value of the right-handed circular polarization intensity S3' is 1, the optical stack 1 converts linearly polarized light vibrating in the first direction d4 into right-handed circularly polarized light. When the value of the right-handed circular polarization intensity S3' is -1, the optical stack 1 converts linearly polarized light vibrating in the first direction d4 into left-handed circularly polarized light. The optical stack 1, which can accurately convert linearly polarized light vibrating in the first direction d4 into circularly polarized light, may convert the linearly polarized light into right-handed circularly polarized light, or may convert the linearly polarized light into left-handed circularly polarized light. If the right-handed circular polarization intensity S3' is 0.95 or more, it can be said that the optical stack 1 can convert linearly polarized light vibrating in the first direction d4 into right-handed circularly polarized light with sufficient accuracy. If the right-handed circular polarization intensity S3' is -0.95 or less, it can be said that the optical stack 1 can convert linearly polarized light vibrating in the first direction d4 into left-handed circularly polarized light with sufficient accuracy. When the absolute value of the right-handed circularly polarized light intensity S3' is 0.95 or more, this includes cases where the optical laminate 1 can convert the linearly polarized light into right-handed circularly polarized light with sufficient precision, and cases where the optical laminate 1 can convert the linearly polarized light into left-handed circularly polarized light with sufficient precision.

[0096] Furthermore, consider the case where the incident polarized light is linearly polarized light that vibrates in the first direction d4 and has an arbitrary wavelength of 450 nm to 650 nm. In this case, if the absolute value of the right-handed circularly polarized light intensity S3′ is 0.95 or more, it can be said that the optical laminate 1 can convert linearly polarized light that vibrates in the first direction d4 and has an arbitrary wavelength of 450 nm to 650 nm into circularly polarized light with sufficient accuracy.

[0097] For this reason, if there exists a first direction d4 such that it can be said that "when the incident polarized light is linearly polarized light that oscillates in the first direction d4 and has an arbitrary wavelength of 450 nm or more and 650 nm or less, the absolute value of the right-handed circularly polarized light intensity S3' is 0.95 or more," it can be said that the optical laminate 1 can convert linearly polarized light that oscillates in the first direction d4 and has an arbitrary wavelength of 450 nm or more and 650 nm or less into circularly polarized light with sufficient accuracy. From the above, the existence of the first direction d4 is a condition for the optical laminate 1 to be able to convert linearly polarized light that has a wavelength of 450 nm or more and 650 nm or less into circularly polarized light with sufficient accuracy.

[0098] The present inventors have conducted extensive research and found that the optical laminate 1, which has improved design freedom due to the alignment direction da and the alignment direction db being different as described above, can be designed so that the first direction d4 exists. That is, the inventors have found that the optical laminate 1, which has improved design freedom due to the alignment direction da and the alignment direction db being different, can accurately convert linearly polarized light having a wavelength of 450 nm or more and 650 nm or less into circularly polarized light.

[0099] When the optical laminate 1 is required to change linearly polarized light into circularly polarized light with high precision, it is preferable that the direction of twist alignment of the liquid crystal composition 13 in the first liquid crystal layer 11 is opposite to the direction of twist alignment of the liquid crystal composition 23 in the second liquid crystal layer 21. This enables the optical laminate 1 to change linearly polarized light having a wavelength of 450 nm or more and 650 nm or less into circularly polarized light with particularly high precision.

[0100] The present inventors have found that, with respect to an elliptically polarizing plate including the optical laminate 1, from the viewpoint of obtaining the effect of making it difficult to observe a color tint, which will be described later, it is preferable that the direction of twist alignment of liquid crystal composition 13 be opposite to the direction of twist alignment of liquid crystal composition 23. The present inventors have found that, particularly when angle θ3, which is an angle of 90° or less between alignment directions da and db, is close to 90°, it is preferable that the direction of twist alignment of liquid crystal composition 13 be opposite to the direction of twist alignment of liquid crystal composition 23. In particular, the present inventors have found that, particularly when angle θ3 is 85° or greater, it is preferable that the direction of twist alignment of liquid crystal composition 13 be opposite to the direction of twist alignment of liquid crystal composition 23. In other words, it was found that an optical laminate 1 in which the angle θ3 is 85° or more and the direction of the twist orientation of liquid crystal composition 13 is opposite to the direction of the twist orientation of liquid crystal composition 23 is preferable from the standpoint of color hue to an optical laminate 1 in which the angle θ3 is 85° or more and the direction of the twist orientation of liquid crystal composition 13 is the same direction as the direction of the twist orientation of liquid crystal composition 23.

[0101] When the optical laminate 1 is required to convert linearly polarized light into circularly polarized light with high precision, and when the direction of twist alignment of liquid crystal composition 13 is opposite to the direction of twist alignment of liquid crystal composition 23, it is preferable that the following condition be further satisfied: The ratio of the larger absolute value of the twist amount θ1 (°) and the larger absolute value of the twist amount θ2 (°) to the smaller absolute value is preferably 1.21 or more and 4.74 or less. The ratio of the larger absolute value of the twist amount θ1 (°) and the larger absolute value of the twist amount θ2 (°) to the smaller absolute value is preferably 1.83 or more and 3.46 or less, and more preferably 1.83 or more and 2.57 or less. This enables the optical laminate 1 to convert linearly polarized light having a wavelength of 450 nm or more and 650 nm or less into circularly polarized light with particularly high precision.

[0102] According to the above-described optical laminate 1, linearly polarized light having a wavelength of 450 nm or more and 650 nm or less can be converted into circularly polarized light with sufficient precision, particularly without providing any liquid crystal layers other than the first liquid crystal layer 11 and the second liquid crystal layer 21. This allows the thickness of the optical laminate 1 to be reduced, and further allows the amounts of materials and the number of steps required for manufacturing the optical laminate 1 to be reduced.

[0103] When manufacturing an elliptical polarizing plate, it is also possible to use a retardation plate including a half-wave plate and a quarter-wave plate as described in JP-A-10-68816 instead of the optical laminate 1. That is, it is also possible to manufacture an elliptical polarizing plate including the retardation plate and a linear polarizing plate. In this case, the linear polarizing plate, the half-wave plate, and the quarter-wave plate are stacked in this order in the elliptical polarizing plate. An elliptical polarizing plate using the retardation plate can achieve the following effects. When an elliptical polarizing plate including the retardation plate is used in a display device or the like, external light is converted into linearly polarized light by passing through the linear polarizing plate, and then passes through the half-wave plate and the quarter-wave plate in this order. The wavelength of light to which a half-wave retardation is imparted by the half-wave plate and a half-wave retardation is imparted by the quarter-wave plate is referred to as the reference wavelength. When linearly polarized light having a reference wavelength passes through, the polarization direction of the light is rotated when the light passes through the half-wave plate, and becomes circularly polarized when the light passes through the quarter-wave plate. As a result, linearly polarized light having a reference wavelength is converted into circularly polarized light by passing through the half-wave plate and the quarter-wave plate. Next, consider the case where linearly polarized light having a wavelength shorter than the reference wavelength passes through. In this case, when the light passes through the half-wave plate, the retardation of the half-wave plate is excessive to convert the linearly polarized light into linearly polarized light with a rotated polarization direction. Next, when light having a shorter wavelength passes through the quarter-wave plate, the retardation of the quarter-wave plate is excessive to convert the linearly polarized light into circularly polarized light. Since the excess retardation of the half-wave plate and the excess retardation of the quarter-wave plate cancel each other out, as a result, the light having a short wavelength is converted into circularly polarized light or elliptically polarized light that is relatively close to circularly polarized light. Next, consider the case where linearly polarized light having a wavelength longer than the reference wavelength passes through. In this case, when light passes through the half-wave plate, the retardation of the half-wave plate is insufficient to convert the linearly polarized light into linearly polarized light with a rotated polarization direction. When light having a longer wavelength passes through the quarter-wave plate, the retardation of the quarter-wave plate is insufficient to convert the linearly polarized light into circularly polarized light.The retardation deficiency of the half-wave plate and the retardation deficiency of the quarter-wave plate cancel each other out, resulting in light with a long wavelength being converted into circularly polarized light or elliptically polarized light that is relatively close to circularly polarized light. Thus, in an elliptical polarizer having a retardation plate with a half-wave plate and a quarter-wave plate, light within a certain range of wavelengths centered on a reference wavelength can be converted into circularly polarized light or elliptically polarized light that is relatively close to circularly polarized light by passing light through the half-wave plate and the quarter-wave plate. It is also conceivable that such an elliptical polarizer can be used in a display device or the like to prevent reflection of external light. In the above-mentioned elliptical polarizer retardation plate, the accuracy of converting linearly polarized light into circularly polarized light can be improved by adjusting the retardation of the half-wave plate and the quarter-wave plate.

[0104] However, research by the present inventors has revealed that a color tint is observed in an elliptical polarizer equipped with a retarder having such a half-wave plate and a quarter-wave plate. The reason for this is thought to be that the retarder having a half-wave plate and a quarter-wave plate does not have sufficient precision in converting linearly polarized light into circularly polarized light.

[0105] In contrast, with an elliptical polarizing plate including the optical laminate 1 of the present embodiment, the color can be made less observable than with an elliptical polarizing plate including a retardation plate having a half-wave plate and a quarter-wave plate. In particular, by increasing the precision with which the optical laminate 1 changes linearly polarized light into circularly polarized light compared to the precision of a retardation plate having a half-wave plate and a quarter-wave plate, the color can be made less observable.

[0106] The reason why the accuracy of converting linearly polarized light into circularly polarized light in the optical laminate 1 of this embodiment can be higher than that of a retardation plate having a half-wave plate and a quarter-wave plate will be explained with reference to Figure 7. Figure 7 is a diagram showing a part of a virtual liquid crystal layer 40 containing a liquid crystal composition 41, observed from a direction perpendicular to the thickness direction of the liquid crystal layer 40. The film thickness of the liquid crystal layer 40 is taken as film thickness d. The refractive index difference between the long axis direction d5 and the short axis direction d6 of the liquid crystal molecules constituting the liquid crystal composition 41 of the liquid crystal layer 40 is taken as Δn. In this specification, the reference wavelength of Δn is 550 nm.

[0107] If the liquid crystal composition 41 in the liquid crystal layer 40 is not twist-oriented but homogeneously oriented, the retardation of the liquid crystal layer 40 can be expressed as Δn×d. On the other hand, if the liquid crystal composition 41 in the liquid crystal layer 40 is twist-oriented, the retardation of the liquid crystal layer 40 cannot be expressed as Δn×d. The present inventors have found that even when the liquid crystal composition 41 is twist-oriented, the optical properties of the liquid crystal layer 40 can be adjusted by adjusting the value of Δn×d. In particular, they have found that a liquid crystal layer 40 having desired optical properties can be obtained by adjusting the value of Δn×d and the twist alignment mode of the liquid crystal composition 41. In this case, adjusting the twist alignment mode includes adjusting the direction of the twist alignment described above. Furthermore, adjusting the twist alignment mode includes adjusting the twist amount described above. Furthermore, adjusting the twist alignment mode includes adjusting the alignment direction of the liquid crystal composition, such as the alignment direction da or alignment direction db described above.

[0108] In the optical laminate 1 of the present embodiment, it is believed that the accuracy of converting linearly polarized light into circularly polarized light can be increased by adjusting the value of Δn×d in each of the first liquid crystal layer 11 and the second liquid crystal layer 21 and the mode of twist alignment of the liquid crystal compositions (liquid crystal compositions 13 and 23) as follows: First, the value of Δn×d in each of the first liquid crystal layer 11 and the second liquid crystal layer 21 is adjusted so that the optical laminate 1 can convert linearly polarized light having a specific wavelength into circularly polarized light with high accuracy. Then, the mode of twist alignment of the liquid crystal compositions 13 and 23 is adjusted so that linearly polarized light having not only the specific wavelength but any wavelength within a specific numerical range can be converted into circularly polarized light with high accuracy.

[0109] In the optical laminate 1 of the present embodiment, the alignment direction da of the liquid crystal composition 13 at the first opposing surface 11a of the first liquid crystal layer 11 is different from the alignment direction db of the liquid crystal composition 23 at the second opposing surface 21a of the second liquid crystal layer 21. This provides a high degree of freedom in adjusting the twist alignment aspects of the liquid crystal compositions 13 and 23. This allows the twist alignment aspects of the liquid crystal compositions 13 and 23 to be adjusted so as to more accurately change linearly polarized light having an arbitrary wavelength within a specific numerical range into circularly polarized light.

[0110] In the optical laminate 1 of the present embodiment, the first liquid crystal layer 11 and the second liquid crystal layer 21 contain twist-aligned liquid crystal compositions (liquid crystal compositions 13 and 23). Therefore, as described above, by adjusting the values ​​of Δn×d of the first liquid crystal layer 11 and the second liquid crystal layer 21 and the state of twist alignment of the liquid crystal compositions 13 and 23, the accuracy of converting linearly polarized light into circularly polarized light can be increased. Furthermore, in the optical laminate 1 of the present embodiment, the alignment direction da and the alignment direction db are different. Therefore, there is a high degree of freedom in adjusting the state of twist alignment of the liquid crystal compositions 13 and 23. As described above, in the optical laminate 1 of the present embodiment, the values ​​of Δn×d of the first liquid crystal layer 11 and the second liquid crystal layer 21 and the state of twist alignment of the liquid crystal compositions 13 and 23 can be adjusted, and there is a high degree of freedom in adjusting the state of twist alignment of the liquid crystal compositions 13 and 23. As described above, it is considered that the optical laminate 1 of the present embodiment can achieve higher accuracy in converting linearly polarized light into circularly polarized light than the above-mentioned retardation plate, which increases the accuracy in converting linearly polarized light into circularly polarized light by adjusting the retardation of the half-wave plate and the quarter-wave plate.

[0111] In the optical laminate 1 of the present embodiment, the value of Δn×d of the first liquid crystal layer 11 and the second liquid crystal layer 21 and the mode of twist alignment of the liquid crystal compositions 13 and 23 can be adjusted, and there is a high degree of freedom in adjusting the mode of twist alignment of the liquid crystal compositions 13 and 23. With such optical laminate 1, even when a performance of changing the polarization state of incident light other than the performance of changing linearly polarized light to circularly polarized light is to be exhibited, it is thought that the performance can be exhibited with high precision by adjusting the value of Δn×d and the mode of twist alignment.

[0112] II. Elliptical Polarizing Plate The present disclosure provides an elliptical polarizing plate 70 including the optical laminate 1 of the present embodiment described above and a polarizing plate 71 superimposed on the optical laminate 1. The concept of an elliptical polarizing plate encompasses the concept of a circular polarizing plate.

[0113] 1 and 2 includes the optical laminate 1 of the present embodiment and a polarizing plate 71. In the example shown in Fig. 1 and 2, the polarizing plate 71 is located adjacent to the optical laminate 1. The elliptical polarizing plate 70 may include an adhesive layer (not shown) located between the optical laminate 1 and the polarizing plate 71, if necessary.

[0114] In the elliptical polarizer 70, the second liquid crystal layer 21 is disposed closer to the polarizer 71 than the first liquid crystal layer 11. In the elliptical polarizer 70 shown in Figures 1 and 2, the first alignment film 12, the first liquid crystal layer 11, the bonding layer 30, the second alignment film 22, the second liquid crystal layer 21, and the polarizer 71 are laminated in this order.

[0115] In the present embodiment, the polarizing plate 71 is a plate-like plate that transmits only light vibrating in a specific direction. The polarizing plate 71 may be any polarizing plate appropriately selected from conventionally known polarizing plates. In the present embodiment, the polarizing plate 71 is a linear polarizing plate. As an example, the linear polarizing plate that is the polarizing plate 71 includes a polarizer and a polarizer protective layer provided on at least one side of the polarizer. The polarizer is, for example, a stretched film or stretched layer to which a dye having absorption anisotropy is adsorbed. The polarizer may also be a film formed by applying and curing a dye having absorption anisotropy. The dye having absorption anisotropy is, for example, a dichroic dye. Specific examples of the dichroic dye include iodine and dichroic organic dyes. Examples of stretched films to which a dye having absorption anisotropy is adsorbed include polyvinyl alcohol films, polyvinyl formal films, polyvinyl acetal films, and saponified ethylene-vinyl acetate copolymer films that are dyed with iodine or a dye and stretched. For details of the linear polarizer used, see, for example, paragraphs 0025 to 0059 of JP 2021-51287 A. The thickness of the polarizer is, for example, 2 μm to 100 μm, and preferably 10 μm to 60 μm.

[0116] In this embodiment, the adhesive layer (bonding layer) located between the optical laminate 1 and the polarizing plate 71 can be the adhesive layer (bonding layer) described above as used in the bonding layer 30.

[0117] The elliptically polarizing plate 70 of the present embodiment may further include other layers included in known optical members, such as known circularly polarizing plates and known elliptically polarizing plates, in addition to the optical laminate 1, the polarizing plate, and the pressure-sensitive adhesive layer (adhesive layer). The other layers are not particularly limited as long as they are layers included in known optical members. Examples of the other layers include a retardation layer, an antireflection layer, a diffusion layer, an antiglare layer, an antistatic layer, and a protective film.

[0118] In the elliptical polarizing plate 70, the polarizing plate 71 is superimposed on the optical laminate 1 so that the direction in which the absorption axis extends is perpendicular to the first direction d4 when observed from the thickness direction d1 of the optical laminate 1. This allows the polarizing plate 71 to pass light vibrating in the first direction d4. This allows linearly polarized light vibrating in the first direction d4 to be incident on the optical laminate 1, which the optical laminate 1 can convert to circularly polarized light with high precision. In particular, when the elliptical polarizing plate 70 is used as an external light anti-reflection film, external light can be converted into linearly polarized light vibrating in the first direction d4 and then incident on the optical laminate 1. This allows the elliptical polarizing plate 70 to effectively reduce reflection of external light.

[0119] The elliptically polarizing plate 70 of this embodiment can be suitably used as an optical member for suppressing reflection of external light in a display device.

[0120] III. Display Device The display device 100 of the present embodiment includes the optical laminate 1 of the present embodiment or an optical member including the optical laminate 1. FIG. 8 is a cross-sectional view showing an example of a display device 100 including an elliptically polarizing plate 70 of the present embodiment. The display device 100 shown in FIG. 8 includes the optical laminate 1 and a display device main body 102 that displays images, etc. The second surface 1b of the optical laminate 1 faces the display device main body 102. The optical member included in the display device 100 of the present embodiment is, for example, an elliptically polarizing plate 70 including the optical laminate 1 of the present embodiment and a polarizing plate 71. The display device 100 of the present embodiment can be manufactured using the elliptically polarizing plate 70 of the present embodiment. The display device 100 shown in FIG. 8 is an organic EL display device (organic electroluminescence display device) 101. That is, in the example shown in FIG. 8, the organic EL display device 101 includes the elliptically polarizing plate 70 of the present embodiment. The display device 100 may be a touch panel including a touch sensor. According to the organic EL display device 101 including the elliptically polarizing plate 70 of this embodiment, it is possible to reduce reflection of external light on the display surface.

[0121] The display device 100 of the present embodiment may be one that is expected to be rotated by a user. For example, the display device 100 may be one that can be rotated around an axis perpendicular to the display surface. Examples of display devices that are expected to be rotated by a user include smartphones and tablet terminals.

[0122] IV. Methods for Manufacturing Optical Laminate and Elliptical Polarizer A method for manufacturing the optical laminate 1 of this embodiment will be described. As an example, a method for manufacturing the optical laminate 1 in which the optical laminate 1 is superimposed on a polarizer 71 to form an elliptical polarizer 70 as shown in FIGS. 1 and 2 will be described. A method for manufacturing the elliptical polarizer 70 will also be described. As an example, a method for manufacturing the optical laminate 1 in which twisted liquid crystal layers are formed as the first liquid crystal layer 11 and the second liquid crystal layer 21 will be described. In this case, there are no particular limitations on the method for forming the twisted liquid crystal layer, and known methods can be used. In particular, a case in which the twisted liquid crystal layer to be formed is a cured product of an ionizing radiation-curable composition containing the above-mentioned liquid crystalline monomer or oligomer will be described below as an example.

[0123] First, a substrate 60 such as a polymer film or a glass plate is prepared. Next, a first alignment film 12 is formed on the substrate 60. The first alignment film 12 can be produced by a conventionally known method. For example, the first alignment film 12 can be formed by forming a polyimide film on the substrate 60 and rubbing it. The first alignment film 12 may also be formed by forming a film of a polymer compound that will become a photo-alignment film on the substrate 60 and irradiating it with polarized UV (ultraviolet light). The first alignment film 12 may also be formed by using a stretched PET (polyethylene terephthalate) film. The first alignment film 12 may also be formed by patterning using a mask.

[0124] Next, a twisted liquid crystal layer is formed as the first liquid crystal layer 11 on the first alignment film 12. In this process, an ionizing radiation-curable composition for forming a twisted liquid crystal layer, which contains a liquid crystalline monomer or oligomer, a chiral agent, and other components such as a photopolymerization initiator and a solvent, is applied to the first alignment film 12, and the liquid crystal molecules (liquid crystalline monomer and oligomer) are aligned by the alignment-regulating force of the first alignment film 12. Next, the liquid crystalline monomer or oligomer is three-dimensionally crosslinked by irradiating the aligned film with ionizing radiation, thereby obtaining a twisted liquid crystal layer, which is a cured product of the curable composition. Examples of methods for applying the curable composition include various known methods such as spin coating, dipping, spraying, die coating, bar coating, roll coating, meniscus coating, flexographic printing, screen printing, and bead coating. When the curable composition contains a solvent, it is preferable to dry the applied curable composition at, for example, 30 to 120° C. for 10 to 120 seconds.

[0125] When the twisted liquid crystal layer is made of the aforementioned liquid crystal polymer, a composition containing the liquid crystal polymer is applied to the first alignment film 12 by the aforementioned method in the same manner as above, and the polymer is aligned by the alignment restricting force of the first alignment film 12. After drying as necessary, the liquid crystal polymer can be fixed in a glassy state by cooling, thereby obtaining a twisted liquid crystal layer.

[0126] In this manner, the first liquid crystal layer 11 can be formed on the first alignment film 12. In this manner, the laminate 4 having the first alignment film 12 and the first liquid crystal layer 11 is fabricated, as shown in Fig. 9A.

[0127] Next, a release substrate 61 is prepared. The material of the release substrate 61 is, for example, the same as the material of the substrate 60 described above. Next, a second alignment film 22 is formed on the release substrate 61. Next, a second liquid crystal layer 21 is formed on the second alignment film 22. The method of forming the second alignment film 22 and the second liquid crystal layer 21 can be the same as the method of forming the first alignment film 12 and the first liquid crystal layer 11 described above, except that the liquid crystal composition 23 is adjusted so as to have a desired twist alignment. In this way, a laminate 5 having the second alignment film 22 and the second liquid crystal layer 21, as shown in FIG. 9B, is produced.

[0128] Next, a step of preparing a polarizing plate 71 is performed. As an example of the step of preparing a polarizing plate 71, a case where a stretched film having an absorption anisotropy dye adsorbed thereon is used as a polarizer will be described. A stretched film having an absorption anisotropy dye adsorbed thereon can typically be manufactured through the steps of uniaxially stretching a polyvinyl alcohol-based resin film, dyeing the polyvinyl alcohol-based resin film with a dichroic dye to adsorb the dichroic dye, treating the polyvinyl alcohol-based resin film having the adsorbed dichroic dye with a boric acid aqueous solution, and washing the film with water after the boric acid aqueous solution treatment. The polarizing plate 71 can be manufactured by laminating a polarizer protective layer to one or both sides of the obtained polarizer. The polarizing plate 71 can be prepared, for example, by referring to paragraphs 0025 to 0059 of JP 2021-51287 A.

[0129] Next, the laminate 5 shown in FIG. 9B is laminated on a polarizing plate 71. When laminating the laminate 5 and the polarizing plate 71, the laminate 5 and the polarizing plate 71 are laminated so that the surface of the laminate 5 opposite the surface formed by the release substrate 61 faces the polarizing plate 71. As an example, the laminate 5 and the polarizing plate 71 can be laminated by bonding the surface of the laminate 5 and the surface of the polarizing plate 71 with an adhesive layer (adhesive layer). Next, the release substrate 61 is peeled off from the second alignment film 22 of the laminate 5. Next, the second alignment film 22 and the laminate 4 shown in FIG. 9A are bonded via the bonding layer 30. At this time, the second alignment film 22 and the laminate 4 are bonded so that the surface of the laminate 4 opposite the surface formed by the substrate 60 faces the second alignment film 22. When the second alignment film 22 and the laminate 4 are bonded by the bonding layer 30, the orientation of the second liquid crystal layer 21 relative to the first liquid crystal layer 11 is adjusted so that the alignment directions of the liquid crystal compositions 13 and 23 are as described above. This makes it possible to manufacture the optical laminate 1 in a state where it is superimposed on the polarizing plate 71 as shown in FIG. 2 . In other words, it is possible to manufacture the elliptical polarizing plate 70 shown in FIG. 2 . Next, the substrate 60 is peeled off from the first alignment film 12. This makes it possible to manufacture the optical laminate 1 in a state where it is superimposed on the polarizing plate 71 as shown in FIG. 1 . In other words, it is possible to manufacture the elliptical polarizing plate 70 shown in FIG. 1 .

[0130] <Modifications> Next, various modifications of the present embodiment will be described with reference to Figures 10 and 11. In Figures 10 and 11, the same parts as those in the embodiment shown in Figures 1 to 9B are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0131] <Modification 1> In the above embodiment, the optical laminate 1 including the bonding layer 30 that bonds the first laminate 10 and the second laminate 20 has been described. However, the layer configuration of the optical laminate 1 is not limited to this. Fig. 10 is a cross-sectional view showing an elliptically polarizing plate 70 including the optical laminate 1 of Modification 1.

[0132] As shown in FIG. 10 , the optical stack 1 of Modification 1 does not include a bonding layer 30 that bonds the first stack 10 and the second stack 20. In the optical stack 1 of Modification 1, the first stack 10 is directly adjacent to the second stack 20. In the example shown in FIG. 10 , the first liquid crystal layer 11 is directly adjacent to the second alignment film 22. As a result, the first stack 10 is directly adjacent to the second stack 20. Like the optical stack 1 of the above-described embodiment, the optical stack 1 of Modification 1 also includes a first stack 10 including a first alignment film 12 and the first liquid crystal layer 11, and a second stack 20 including a second alignment film 22 and the second liquid crystal layer 21. In the optical stack 1 of Modification 1, the first alignment film 12, the first liquid crystal layer 11, the second alignment film 22, and the second liquid crystal layer 21 are also stacked in this order.

[0133] The degree of freedom in design can also be improved by the optical laminate 1 of Modification 1. The optical laminate 1 of Modification 1 can also convert linearly polarized light having a wavelength within a specific numerical range into circularly polarized light with high precision.

[0134] Next, a method for manufacturing the optical laminate 1 shown in Fig. 10 will be described. Additionally, a method for manufacturing the elliptical polarizer 70 shown in Fig. 10 will be described. First, the laminate 4 shown in Fig. 9A is prepared. Next, the second alignment film 22 is formed on the first liquid crystal layer 11 of the laminate 4 shown in Fig. 9A by a method similar to the method for forming the second alignment film 22 on the release substrate 61 described above. Next, the second liquid crystal layer 21 is formed on the second alignment film 22 by a method similar to the method for forming the second liquid crystal layer 21 on the second alignment film 22 described above.

[0135] An elliptical polarizing plate 70 shown in Fig. 10 can be manufactured by overlaying a polarizing plate 71 on the optical laminate 1 of Modification 1 so that the polarizing plate 71 faces the first surface 1a, and peeling off the substrate 60 from the first alignment film 12. The elliptical polarizing plate 70 manufactured in this manner also provides the same effect as the elliptical polarizing plate 70 of the above-described embodiment. Furthermore, the elliptical polarizing plate 70 of Modification 1 can be used to manufacture a display device 100 as shown in Fig. 10.

[0136] <Modification 2> In the above-described embodiment, the case where the optical laminate 1 is used in the elliptically polarizing plate 70 for suppressing external light reflection in the display device 100 has been described. Furthermore, the case where the display device 100 is an organic EL display device 101 has been described. However, the use of the optical laminate 1 and the form of the display device 100 are not limited to this. Figure 11 is a cross-sectional view showing a display device 100 including the optical laminate 1 of modification 2.

[0137] In the example shown in Fig. 11 , the display device 100 is a liquid crystal display device 103. In this case, the display device main body 102 emits linearly polarized light. In this way, when a portion of the display device 100 is configured to emit linearly polarized light, the optical laminate 1 may be used as an optical element for converting the linearly polarized light emitted from the portion of the display device 100 into circularly polarized light. In the example shown in Fig. 11 , the first surface 1a of the optical laminate 1 faces the portion of the display device 100 that emits linearly polarized light (the display device main body 102). In this case, too, the optical laminate 1 can convert linearly polarized light into circularly polarized light with high precision.

[0138] 11 , the optical laminate 1 converts linearly polarized light emitted from a part of the display device 100 that emits linearly polarized light (the display device main body 102) into circularly polarized light. This prevents the user from being unable to see the light from the display device 100 depending on the angle of rotation, even when the user of the display device 100 is wearing polarized sunglasses and the display device 100 is rotated by the user.

[0139] Next, specific examples for confirming the effects of the optical laminate 1 of the present disclosure will be described.

[0140] In Examples 1-21 and Comparative Examples 1-3, an elliptically polarizing plate 70 having the optical laminate 1 shown in Fig. 1 was produced by the same method as in Examples 1-21 and Comparative Examples 1-3, except that the conditions were set as shown in Table 1 below. Furthermore, in Examples 22-23, an elliptically polarizing plate 70 having the optical laminate 1 shown in Fig. 1 was produced by the same method as in Examples 1-21 and Comparative Examples 1-3, except that the conditions were set as shown in Table 2 below.

[0141] Specifically, first, a resin substrate 60 was prepared. Next, a first alignment film 12 was formed on the substrate 60. The first alignment film 12 was produced by rubbing a polyimide resin layer. Next, a composition containing liquid crystal composition 13, which was used for the first liquid crystal layer 11, was applied to the surface of the first alignment film 12 to form a coating film. In Examples 1-21 and Comparative Examples 1-3, the composition used for the first liquid crystal layer 11 included a coating solution prepared by diluting liquid crystal composition 13, a liquid crystal material with Re450 / Re550=1.09, with a solvent (MEK / MIBK=5 / 5). In Example 22, the composition used for the first liquid crystal layer 11 included a coating solution prepared by diluting liquid crystal composition 13, a liquid crystal material with Re450 / Re550=1.00, with a solvent (MEK / cyclohexanone=5 / 5). In Example 23, the composition used for the first liquid crystal layer 11 included a coating solution prepared by diluting liquid crystal composition 13 (Re450 / Re550 = 0.86) with a solvent (THF / cyclohexanone = 5 / 5). "Re450 / Re550" is the value obtained by dividing the in-plane retardation at a wavelength of 450 nm of a homogeneously aligned liquid crystal layer using the liquid crystal material by the in-plane retardation at a wavelength of 550 nm of the liquid crystal layer. In Examples 1-21 and Comparative Examples 1-3, the Re450 / Re550 ratio of the liquid crystal materials used was greater than 1, indicating that the liquid crystal materials used were normal dispersion liquid crystals. In Examples 1-21 and Comparative Examples 1-3, the Re450 / Re550 ratio of the liquid crystal materials used was particularly greater than 1.05. In Example 22, the Re450 / Re550 of the liquid crystal material used was 0.95 or more and 1.05 or less, which indicates that the liquid crystal material used was a flat dispersion liquid crystal. In Example 23, the Re450 / Re550 of the liquid crystal material used was less than 1, which indicates that the liquid crystal material used was a reverse dispersion liquid crystal. In Example 23, the Re450 / Re550 of the liquid crystal material used was particularly less than 0.95. The composition used for the first liquid crystal layer 11 further contained a polymerization initiator, a chiral agent, a crosslinking agent, and a leveling agent as other components.The amount of chiral agent added to the composition containing liquid crystal composition 13 used in the first liquid crystal layer 11 was adjusted so that the liquid crystal composition 13 would have the twist alignment shown in Tables 1 and 2 below. The adjustment of the twist alignment of liquid crystal composition 13 by adjusting the amount of chiral agent added was performed as follows. Multiple compositions containing the same liquid crystal composition were prepared, except for the amount of chiral agent added. Next, multiple liquid crystal layers were prepared from each of the multiple compositions. The state of twist alignment of the liquid crystal in each of the multiple liquid crystal layers was then detected, and a calibration curve was created showing the relationship between the detected state of twist alignment of the liquid crystal and the amount of chiral agent added. The desired twist alignment was obtained by adjusting the amount of chiral agent added based on this calibration curve. The formed coating was then heated and dried. The coating was then cooled to a temperature at which it could be solidified, and polymerization was allowed to proceed by a curing process. The curing process was performed by irradiating the coating with ultraviolet light (light irradiation process) to cure the polymerizable liquid crystal compound contained in liquid crystal composition 13 through a photopolymerization reaction. The curing treatment fixed the alignment direction of the liquid crystal composition 13, thereby obtaining a first liquid crystal layer 11 containing the liquid crystal composition 13 in a twisted alignment as shown in Tables 1 and 2 below. In this manner, the first liquid crystal layer 11 was formed on the first alignment film 12. In this manner, a laminate 4 having the first alignment film 12 and the first liquid crystal layer 11, as shown in FIG. 9A , was produced.

[0142] Next, a release substrate 61 was prepared. The release substrate 61 was made of the same resin as the substrate 60 described above. Next, a second alignment film 22 was formed on the release substrate 61. Next, a second liquid crystal layer 21 was formed on the second alignment film 22. The method for forming the second alignment film 22 and the second liquid crystal layer 21 was the same as the method for forming the second alignment film 22 and the second liquid crystal layer 21 described above, except that the liquid crystal composition 23 was adjusted so as to be twistedly aligned as shown in Tables 1 and 2 described below. In this way, a laminate 5 having the second alignment film 22 and the second liquid crystal layer 21 shown in FIG. 9B was produced.

[0143] Furthermore, a polarizing plate 71 was prepared. The polarizing plate 71 was prepared using a stretched film adsorbed with a dye having absorption anisotropy as a polarizer by the method described in the above embodiment.

[0144] Next, the laminate 5 and the polarizing plate 71 were laminated. When laminating the laminate 5 and the polarizing plate 71, the laminate 5 and the polarizing plate 71 were laminated so that the surface of the laminate 5 opposite the surface formed by the release substrate 61 faced the polarizing plate 71. In this process, the laminate 5 and the polarizing plate 71 were laminated by bonding the surface of the laminate 5 and the surface of the polarizing plate 71 with an adhesive layer. Next, the release substrate 61 was peeled off from the second alignment film 22 of the laminate 5. Next, the second alignment film 22 and the laminate 4 were bonded via an adhesive layer 30. At this time, the second alignment film 22 and the laminate 4 were bonded so that the surface of the laminate 4 opposite the surface formed by the substrate 60 faced the second alignment film 22. As the adhesive layer 30, an adhesive layer similar to the adhesive layer bonding the surface of the laminate 5 and the surface of the polarizing plate 71 was used. When the second alignment film 22 and the laminate 4 were bonded by the bonding layer 30, the orientation of the second liquid crystal layer 21 relative to the first liquid crystal layer 11 was adjusted so that the alignment directions of the liquid crystal compositions 13 and 23 were as shown in Tables 1 and 2 described below. In this manner, the elliptical polarizer 70 shown in FIG. 2 was produced. Next, the substrate 60 was peeled off from the first alignment film 12, and the first alignment film 12 was bonded to a display surface of a display device having a black display surface. The display surface and the first alignment film 12 were bonded using an adhesive layer similar to the adhesive layer bonding the surface of the laminate 5 to the surface of the polarizer 71. In this manner, the elliptical polarizer 70 shown in FIG. 1 was produced, bonded to the black display surface of the display device.

[0145] The "Refractive index difference Δn" column in Tables 1 and 2 shows the refractive index difference between the major axis direction and the minor axis direction of the liquid crystal molecules constituting the liquid crystal composition 13 in the first liquid crystal layer 11 and the liquid crystal composition 23 in the second liquid crystal layer 21 when the reference wavelength is 550 nm. In Examples 1-21 and Comparative Examples 1-3, the same liquid crystal molecules are used for the liquid crystal compositions 13 and 23. Therefore, in both Examples 1-21 and Comparative Examples 1-3, the refractive index difference Δn in the first liquid crystal layer 11 and the refractive index difference Δn in the second liquid crystal layer 21 are the same. In Example 22, the same liquid crystal molecules are used for the liquid crystal compositions 13 and 23. Therefore, in Example 22, the refractive index difference Δn in the first liquid crystal layer 11 and the refractive index difference Δn in the second liquid crystal layer 21 are the same. In Example 23, the same liquid crystal molecules are used for the liquid crystal compositions 13 and 23. Therefore, in Example 23, the refractive index difference Δn in the first liquid crystal layer 11 and the refractive index difference Δn in the second liquid crystal layer 21 are the same. The "First Liquid Crystal Layer Thickness d" column in Tables 1 and 2 shows the thickness d of the first liquid crystal layer 11. The "Second Liquid Crystal Layer Thickness d" column in Tables 1 and 2 shows the thickness d of the second liquid crystal layer 21. The "Alignment Direction da" column in Tables 1 and 2 shows the angle that the alignment direction da makes with respect to the extension direction of the absorption axis of the polarizer 71. The "Alignment Direction db" column in Tables 1 and 2 shows the angle that the alignment direction db makes with respect to the extension direction of the absorption axis of the polarizer 71. The "Alignment Direction dc" column in Tables 1 and 2 shows the angle that the alignment direction dc makes with respect to the extension direction of the absorption axis of the polarizer 71. The "Alignment direction dd" column in Tables 1 and 2 shows the angle that the alignment direction dd makes with the extension direction of the absorption axis of the polarizing plate 71. The "Twist amount θ1" column in Tables 1 and 2 shows the value of the twist amount θ1 described above. The "Twist amount θ2" column in Tables 1 and 2 shows the value of the twist amount θ2 described above. The "Ratio of absolute twist amount values" column in Tables 1 and 2 shows the ratio of the larger absolute value of the twist amount θ1 and the smaller absolute value of the twist amount θ2. The "Relationship between alignment direction da and alignment direction db" column in Tables 1 and 2 shows whether the alignment direction da and the alignment direction db are different or the same.The "Twist Orientation Direction" column in Tables 1 and 2 indicates whether the twist orientation direction of the liquid crystal composition 13 in the first liquid crystal layer 11 is opposite to or the same as the twist orientation direction of the liquid crystal composition 23 in the second liquid crystal layer 21.

[0146]

[0147]

[0148] The positive and negative meanings of the angles formed by the alignment directions da, db, dc, and dd with respect to the direction in which the absorption axis of the polarizing plate 71 extends will now be explained. Consider the case in which the first liquid crystal layer 11 and the second liquid crystal layer 21 are observed from the first surface 1a side of the optical laminate 1 along the thickness direction d1. In this case, when the angle formed by the alignment direction with respect to the direction in which the absorption axis of the polarizing plate 71 extends is α° (α is a positive value), the rotation angle of a virtual line segment extending in the direction in which the absorption axis extends, rotated counterclockwise until it becomes parallel to the alignment direction, is α°. When the angle formed by the alignment direction with respect to the direction in which the absorption axis of the polarizing plate 71 extends is -α°, the rotation angle of a virtual line segment extending in the direction in which the absorption axis extends, rotated clockwise until it becomes parallel to the alignment direction, is α°.

[0149] In Tables 1 and 2, the angles formed by the alignment direction da, the alignment direction db, the alignment direction dc, and the alignment direction dd with respect to the direction in which the absorption axis of the polarizing plate 71 extends are rounded to the nearest integer. However, in reality, the angles formed by the alignment direction da, the alignment direction db, the alignment direction dc, and the alignment direction dd have values ​​in the first decimal place and below. Considering the values ​​in the first decimal place, the alignment direction da and the alignment direction db are non-perpendicular in Example 1-23.

[0150] In the "Alignment direction da" column of Table 3, the angle of the alignment direction da with respect to the extension direction of the absorption axis of the polarizing plate 71 is rounded off to one decimal place. In the "Alignment direction db" column of Table 3, the angle of the alignment direction db with respect to the extension direction of the absorption axis of the polarizing plate 71 is rounded off to one decimal place. In the "Alignment direction dc" column of Table 3, the angle of the alignment direction dc with respect to the extension direction of the absorption axis of the polarizing plate 71 is rounded off to one decimal place. In the "Alignment direction dd" column of Table 3, the angle of the alignment direction dd with respect to the extension direction of the absorption axis of the polarizing plate 71 is rounded off to one decimal place. The "da-db" column in Table 3 shows the value obtained by subtracting the angle that the alignment direction db makes with the extension direction of the absorption axis of the polarizing plate 71 from the angle that the alignment direction da makes with the extension direction of the absorption axis of the polarizing plate 71. The "θ3" column in Table 3 shows the value of the angle θ3. As described above, the angle θ3 is an angle of 90° or less between the alignment directions da and db, and is always expressed as 0 or a positive value. It can be seen from Table 3 that the alignment directions da and db are non-perpendicular in Example 1-23.

[0151]

[0152] A front color evaluation test was conducted on the optical laminates 1 of Examples 1-23 and Comparative Examples 1-3. In the front color evaluation test, subjects observed the elliptically polarizing plate 70 bonded to the black display surface of a display device from the thickness direction d1 of the optical laminate 1 and from the side that would be viewed when the optical laminate 1 is used in a display device, etc. Then, they were asked to evaluate whether a color was observed or whether only the black color of the display surface was observed without any color observed. The evaluation was performed on a five-point scale, A to E, as follows. The results of the front color evaluation test are shown in the "Front Color" column of Table 1. A: No color was observed at all, and only the black color of the display surface was observed. B: Although a color was not completely absent, it was barely observed, and it was impossible to distinguish whether the color was reddish or blueish. C: A color that could be distinguished as reddish or blueish was observed, but the reddish or blueish tint was slight and not noticeable unless carefully examined. D: A reddish or blueish tint was observed that could be recognized without paying special attention, but it was not so reddish or blueish that it prevented the display surface from being recognized as black overall. E: A strong reddish or blueish tint was observed that prevented the display surface from being recognized as black overall.

[0153] In particular, for the front color evaluation test, a reference sample serving as a reference for black was prepared by the following method. Black vinyl tape (product name "Black Vinyl Tape NO200-38-21," 38 mm wide x 10 m long, manufactured by Yamato Co., Ltd.) and a 2.5 mm thick, 50 mm square glass plate were prepared. Next, a 50 mm length of the vinyl tape was cut out. The vinyl tape was then attached to the center of the glass plate, spanning the opposing edges of the glass plate, without creating wrinkles. In this manner, a reference sample serving as a reference for black was prepared. For the front color evaluation test, subjects were asked to compare the black color observed when observing the surface of the reference sample to which the vinyl tape was attached with the elliptically polarizing plate 70. They were then asked to evaluate whether a color was observed in comparison with the black of the reference sample, or whether no color was observed and only a black color similar to that of the reference sample was observed as the black of the display surface. The five-level evaluation scale (A-E) described above means the following in more detail. A: Compared to the reference sample, no color was observed at all, and only the same black color as the reference sample was observed as the black color of the display surface. B: Compared to the reference sample, some color was not observed at all, but almost no color was observed, and it was impossible to distinguish whether the color was reddish or blueish. C: Compared to the reference sample, a color was observed that could be distinguished as reddish or blueish, but the reddish or blueish color was slight and not noticeable unless careful. D: Compared to the reference sample, a reddish or blueish color was observed that was noticeable even without careful attention, but it was not so strong that it prevented the display surface from being perceived as black overall. E: Compared to the reference sample, a strong reddish or blueish color was observed that prevented the display surface from being perceived as black overall.

[0154] The front color evaluation test was conducted under the following conditions. A workbench was placed with the work surface 70 cm above the floor in a bright room illuminated by an LED bar fixture (product name "TENQOO recessed 40-type W300 (LEER-43002-LS)" manufactured by Toshiba Lighting & Technology Corporation). The illuminance on the work surface of the workbench was maintained between 800 lx and 900 lx. An elliptically polarizing plate 70 bonded to the black display surface of the display device and a reference sample were placed on the work surface of the workbench and the subject was asked to observe them. The positional relationship between the observation objects, such as the elliptically polarizing plate 70 and the reference sample, the room lighting, and the subject was such that the room lighting would not be reflected on the observation objects when the subject observed them from the front.

[0155] In the front color evaluation test, the elliptically polarizing plate 70 is directly viewed by the subject. On the other hand, in a product that actually may be distributed and that incorporates the elliptically polarizing plate 70, a panel such as glass may be placed between the elliptically polarizing plate 70 and the subject. However, even if a panel is placed between the elliptically polarizing plate 70 and the subject, it is difficult to imagine that the superiority or inferiority in the front color evaluation test would be reversed. For this reason, it is thought that an elliptically polarizing plate 70 that is evaluated as excellent in the front color evaluation test would have a difficult-to-observe color even when incorporated into a product that actually may be distributed.

[0156] The results of the evaluation test for front color tint for the optical laminates 1 of Example 1-23 and Comparative Example 1-3 revealed the following: In Example 1-23, in which the alignment direction da and the alignment direction db were different, the evaluation result was any of A to D, whereas in Comparative Example 1-3, in which the alignment direction da and the alignment direction db were the same, the evaluation result was E. This shows that in Example 1-23, in which the alignment direction da and the alignment direction db were different, color tint was less observable than in Comparative Example 1-3, in which the alignment direction da and the alignment direction db were the same.

[0157] When comparing Examples 1-21 and Comparative Examples 1-3 in which the same liquid crystal molecules are used in liquid crystal compositions 13 and 23, it was found that in Example 1-21 in which the alignment direction da and the alignment direction db are different, when the twist alignment directions are opposite, color tends to be less observable than when the twist alignment directions are the same. In particular, Examples 2-12 and 18-21 were evaluated as either A or C, whereas Examples 13-17 were evaluated as D.

[0158] When comparing Examples 1-21 and Comparative Examples 1-3 in which the same liquid crystal molecules are used for liquid crystal composition 13 and liquid crystal composition 23, Examples 1-12 and 18-21 in which the alignment direction da and the alignment direction db are different and the twist alignment directions are opposite, Examples 2-12 and 18-21 in which the ratio of the absolute value of the twist amount is 1.21 or more and 4.74 or less gave evaluation results A to C, whereas Example 1 in which the ratio of the absolute value of the twist amount is more than 4.74 gave evaluation result D. Furthermore, Examples 3-11 and 18-21 in which the ratio of the absolute value of the twist amount is 1.83 or more and 3.46 or less gave evaluation results A or B, whereas Examples 2 and 12 in which the ratio of the absolute value of the twist amount is more than 3.46 or less than 1.83 gave evaluation result C. Furthermore, Examples 5-11, in which the ratio of the absolute value of the twist amount was 1.83 or more and 2.57 or less, received an evaluation result of A, whereas Examples 3 and 4, in which the ratio of the absolute value of the twist amount was greater than 2.57, received an evaluation result of B.

[0159] In Example 1, in which normal dispersion liquid crystal was used for liquid crystal compositions 13 and 23, the evaluation result was D, whereas in Example 22, in which flat dispersion liquid crystal was used for liquid crystal compositions 13 and 23, and in Example 23, in which reverse dispersion liquid crystal was used for liquid crystal compositions 13 and 23, the evaluation result was A.

[0160] The front color of Example 5-11, which received an evaluation result of A, was evaluated in more detail. Specifically, with regard to the front color of Example 5-11, Examples 5-11 in which the black color of the display surface was particularly clear were evaluated as "A1," Examples 5-11 in which the black color of the display surface was intermediate in clarity were evaluated as "A2," and Examples 5-11 in which the black color of the display surface was poor in clarity were evaluated as "A3." This evaluation was performed using the same method as the front color evaluation test described above. As a result, Examples 5, 7, and 9 were evaluated as "A1," Examples 8 and 10 as "A2," and Examples 6 and 11 as "A3." This indicates that no tendency can be observed, such as the black color of the display surface becoming clearer as the alignment direction da and the alignment direction db become closer to perpendicular. Furthermore, it was found that the black color of the display surface can be more clearly perceived by changing the value obtained by subtracting the angle formed by the alignment direction db with respect to the extension direction of the absorption axis of the polarizing plate 71 from the angle formed by the alignment direction da with respect to the extension direction of the absorption axis of the polarizing plate 71. Because the optical laminate 1 of the present disclosure has a high degree of design freedom, it is possible to change the value obtained by subtracting the angle formed by the alignment direction db from the angle formed by the alignment direction da. Therefore, it can be understood that the black color of the display surface can be more clearly perceived by changing the value obtained by subtracting the angle formed by the alignment direction db from the angle formed by the alignment direction da.

[0161] The components disclosed in the above-described embodiment and each modification may be combined as needed, or some components may be omitted from all the components shown in the above-described embodiment and each modification.

[0162] REFERENCE SIGNS LIST 1 optical laminate 10 first laminate 11 first liquid crystal layer 12 first alignment film 13 liquid crystal composition 20 second laminate 21 second liquid crystal layer 22 second alignment film 23 liquid crystal composition 30 bonding layer 70 elliptically polarizing plate 71 polarizing plate 100 display device 101 organic EL display device

Claims

1. An optical laminate comprising a first liquid crystal layer and a second liquid crystal layer, wherein the first liquid crystal layer and the second liquid crystal layer contain a liquid crystal composition that is twist-oriented with the thickness direction of the optical laminate as its helical axis, the chiral pitch of the liquid crystal composition in the first liquid crystal layer and the second liquid crystal layer is 2 μm or more and 40 μm or less, and the alignment direction of the liquid crystal composition on the surface of the first liquid crystal layer facing the second liquid crystal layer is different from the alignment direction of the liquid crystal composition on the surface of the second liquid crystal layer facing the first liquid crystal layer.

2. The optical laminate according to claim 1, wherein the alignment direction of the liquid crystal composition on the surface of the first liquid crystal layer facing the second liquid crystal layer is non-perpendicular to the alignment direction of the liquid crystal composition on the surface of the second liquid crystal layer facing the first liquid crystal layer.

3. The optical stack described in claim 1, comprising: a first stack including the first liquid crystal layer; a second stack including the second liquid crystal layer; and a bonding layer bonding the first stack and the second stack.

4. The optical stack according to claim 3, wherein the first stack further includes a first alignment film, and the first liquid crystal layer is formed on the first alignment film; and the second stack further includes a second alignment film, and the second liquid crystal layer is formed on the second alignment film.

5. The optical laminate according to claim 1, wherein the direction of twist alignment of the liquid crystal composition in the first liquid crystal layer is opposite to the direction of twist alignment of the liquid crystal composition in the second liquid crystal layer.

6. The optical laminate described in claim 5, wherein the ratio of the larger absolute value of the twist amount of the twist-oriented liquid crystal composition in the first liquid crystal layer to the smaller absolute value of the twist amount of the twist-oriented liquid crystal composition in the second liquid crystal layer is 1.21 or more and 4.74 or less.

7. The optical laminate described in claim 1, wherein the ratio of the larger absolute value of the twist amount of the twist-oriented liquid crystal composition in the first liquid crystal layer to the smaller absolute value of the twist amount of the twist-oriented liquid crystal composition in the second liquid crystal layer is 1 or more and 11 or less.

8. A Mueller matrix M that describes a change in the polarization state that the optical stack brings about for polarized light that is transmitted in the thickness direction through the second liquid crystal layer and the first liquid crystal layer in this order in a first direction perpendicular to the thickness direction. ｓ and a Stokes vector S describing the polarization state of polarized light incident on the optical laminate, which is linearly polarized light that vibrates in the first direction and has an arbitrary wavelength of 450 nm or more and 650 nm or less and passes through the second liquid crystal layer and the first liquid crystal layer in this order in the thickness direction. ｉｎ and the Stokes vector S of the polarized light emitted from the optical laminate obtained by the product of ｏｕｔ 8. The optical laminate according to claim 1, wherein there is the first direction in which the absolute value of the right-handed circularly polarized light intensity S3' is 0.95 or more.

9. An elliptical polarizing plate comprising: the optical laminate according to claim 8; and a polarizing plate superimposed on the optical laminate so that the direction in which the absorption axis extends is perpendicular to the first direction when observed from the thickness direction.

10. An organic EL display device comprising the elliptically polarizing plate according to claim 9.

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