Multilayer retardation film and method for producing same

Abstract

The multilayer retardation film comprises a first retardation layer and a second retardation layer. The first retardation layer has an in-plane retardation of more than 0 nm and not more than 60 nm and a thickness-direction retardation of at least 50 nm and not more than 150 nm. The second retardation layer has an in-plane retardation of at least 100 nm and not more than 150 nm, a thickness-direction retardation of equal to or greater than -150 nm and equal to or less than -50 nm, and an NZ coefficient NZ of more than -0.5 and not more than 0.0. The angle formed by the slow axis direction of the second retardation layer with the slow axis direction of the first retardation layer is not greater than 10°.

Description

Multilayer retardation film and method for producing the same

[0001] The present invention relates to a multilayer retardation film and a method for producing the same.

[0002] In a liquid crystal display device including a first polarizer, a liquid crystal cell, a second polarizer, and a light source in this order, a retardation film is provided between the liquid crystal cell and the first polarizer or between the liquid crystal cell and the second polarizer, thereby improving the display contrast of the liquid crystal display device (see Patent Documents 1 to 6, etc.). Also, a specific polymer that can be used as a material for an optical compensation film of a liquid crystal display device is known (see Patent Document 7, etc.). It is also known to produce a broadband film constituting a circular polarizer by a method including a step of stretching a predetermined multilayer film (see Patent Document 8, etc.).

[0003] Japanese Patent No. 5170093, Japanese Patent No. 5282821 (Corresponding Publication: U.S. Patent Application Publication No. 2012 / 0140154), Japanese Patent No. 4855081, Japanese Patent Application Laid-Open No. 2023-054647, Japanese Patent Application Laid-Open No. 2009-139747 (Corresponding Publication: U.S. Patent Application Publication No. 2010 / 0309414), Japanese Patent Application Laid-Open No. 2009-192612, Japanese Patent Publication No. 2010-522900 (Corresponding Publication: International Publication No. 2008 / 121580), Japanese Patent No. 7059936 (U.S. Patent Application Publication No. 2019 / 0293852)

[0004] A retardation film having a multilayer structure (hereinafter also referred to as a multilayer retardation film) can be used to improve the contrast of display in a liquid crystal display device. However, the manufacturing of a multilayer retardation film may be complicated. For example, in a manufacturing method in which each layer included in a multilayer retardation film is separately prepared and then laminated, it is complicated to adjust the slow axes of each layer to have a predetermined relationship.

[0005] On the other hand, the multilayer retardation film can be manufactured by co-stretching. Specifically, the multilayer retardation film can be manufactured by a method including preparing a multilayer film in a stage preceding the multilayer retardation film and co-stretching each layer constituting the multilayer film. According to the manufacturing method using co-stretching, the number of steps for manufacturing the multilayer retardation film can be reduced, so that improvement in manufacturing efficiency can be expected. However, conventionally, it has not been possible to manufacture a multilayer retardation film with excellent contrast by using co-stretching.

[0006] Therefore, there is a need for a novel multilayer retardation film that can be incorporated into a liquid crystal display device to realize a display with excellent contrast and can be produced by co-stretching; and a method for producing such a multilayer retardation film.

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that the above-mentioned problems can be solved by a multilayer retardation film including a first retardation layer having specific optical properties and a second retardation layer having specific optical properties in a predetermined arrangement, and have completed the present invention. The present invention provides the following.

[0008] <1> A multilayer retardation film comprising a first retardation layer and a second retardation layer, wherein the first retardation layer has an in-plane retardation of more than 0 nm and not more than 60 nm and a thickness direction retardation of 50 nm to 150 nm, the second retardation layer has an in-plane retardation of 100 nm to 150 nm and a thickness direction retardation of -150 nm to -50 nm and an NZ coefficient NZ of more than -0.5 to 0.0, and the angle formed by the slow axis direction of the second retardation layer with respect to the slow axis direction of the first retardation layer is 10° or less. <2> The multilayer retardation film according to <1>, wherein the first retardation layer contains a resin having positive intrinsic birefringence, and the second retardation layer contains a resin having negative intrinsic birefringence. <3> The method for producing the multilayer retardation film according to <1> or <2>, comprising the steps of: a first step of preparing a long resin layer (A) containing a resin having a positive intrinsic birefringence and having an orientation angle with respect to the longitudinal direction in the range of 90°±10°; a second step of forming a resin layer (B) containing a resin having a negative intrinsic birefringence on the resin layer (A) to obtain a multilayer film; and a third step of stretching the multilayer film in a stretching direction forming an angle of 0° or more and 5° or less with respect to the longitudinal direction to obtain a long multilayer retardation film containing the first retardation layer and the second retardation layer. <4> The method for producing the multilayer retardation film according to <3>, wherein the second step comprises applying a resin liquid containing the resin having a negative intrinsic birefringence and an organic solvent onto the resin layer (A) to form a resin liquid layer, and drying the resin liquid layer.

[0009] The present disclosure also provides the following: <5> A polarizing plate comprising the multilayer retardation film according to <1> or <2> and a polarizer. <6> A liquid crystal display device comprising the multilayer retardation film according to <1> or <2>, a polarizer, and a liquid crystal cell.

[0010] According to the present invention, a novel multilayer retardation film that can be incorporated into a liquid crystal display device to realize a display with excellent contrast and can be produced by co-stretching; and a method for producing such a multilayer retardation film; can be provided.

[0011] FIG. 1 is a perspective view schematically showing a multilayer retardation film according to one embodiment of the present invention.

[0012] The present invention will be described in detail below with reference to embodiments and examples. However, the present invention is not limited to the embodiments and examples shown below, and can be implemented with any modifications within the scope of the claims of the present invention and their equivalents. The components of the embodiments shown below can be combined as appropriate. For example, any numerical value selected from the group of numerical values ​​listed as the lower limit and any numerical value selected from the group of numerical values ​​listed as the upper limit can be combined as appropriate.

[0013] In addition, in the drawings, the same components are denoted by the same reference numerals, and their description may be omitted.

[0014] In the following description, the term "long" for a certain member (e.g., a film or layer) means that the length is 5 times or more, preferably 10 times or more, the width, specifically a length that can be wound into a roll for storage or transportation. There is no particular upper limit to the length, and it can be, for example, 100,000 times or less the width.

[0015] In the following description, unless otherwise specified, the slow axis of a film or layer refers to the slow axis in the plane of the film or layer.

[0016] In the following description, unless otherwise specified, the orientation angle of a film or layer refers to the angle that the slow axis of the film or layer makes with a reference direction perpendicular to the thickness direction. For long films and layers, the longitudinal direction is used as the reference direction unless otherwise specified.

[0017] In the following description, the angle formed by the optical axis (slow axis, transmission axis, absorption axis, etc.) of each layer in a member having multiple layers represents the angle when the layer is viewed from the thickness direction, unless otherwise specified.

[0018] In the following description, unless otherwise specified, the front direction of a film means the normal direction of the main surface of the film, and specifically refers to the direction of the polar angle of 0° and the azimuthal angle of 0° of the main surface.

[0019] In the following description, unless otherwise specified, the tilt direction of a film means a direction that is neither parallel nor perpendicular to the main surface of the film, and specifically refers to a direction in which the polar angle of the main surface is in the range of greater than 0° and less than 90°.

[0020] In the following description, unless otherwise specified, a material with positive intrinsic birefringence means a material whose refractive index in the stretching direction is greater than that in the direction perpendicular to the stretching direction. Furthermore, a material with negative intrinsic birefringence means a material whose refractive index in the stretching direction is smaller than that in the direction perpendicular to the stretching direction. The value of intrinsic birefringence can be calculated from the dielectric constant distribution.

[0021] In the following description, the term "(meth)acrylate" encompasses "acrylate," "methacrylate," and combinations thereof.

[0022] In the following description, the in-plane retardation Re of a layer is a value expressed by Re = (nx - ny) x d unless otherwise specified. Furthermore, the birefringence Δn of a layer is a value expressed by Δn = nx - ny, and therefore expressed as Δn = Re / d, unless otherwise specified. Furthermore, the retardation Rth in the thickness direction of a layer is a value expressed by Rth = [{(nx + ny) / 2} - nz] x d unless otherwise specified. Furthermore, the NZ coefficient NZ of a layer is a value expressed by NZ = (nx - nz) / (nx - ny), and can be calculated by NZ = Rth / Re + 0.5, unless otherwise specified. Here, nx represents the refractive index in the in-plane direction of the layer that gives the maximum refractive index. Unless otherwise specified, "in-plane direction" represents the direction perpendicular to the thickness direction. ny represents the refractive index in the in-plane direction of the layer that is perpendicular to the nx direction. nz represents the refractive index in the thickness direction of the layer. d represents the layer thickness. The measurement wavelength is 590 nm unless otherwise specified.

[0023] In the following description, unless otherwise specified, the directions of elements as "parallel," "vertical," and "orthogonal" may include an error within a range that does not impair the effects of the present invention, for example, within a range of ±5°, for example, ±3°, ±2°, or ±1°.

[0024] In the following description, unless otherwise specified, the term "polarizing plate" includes not only rigid members but also flexible members such as resin films.

[0025] In the following description, unless otherwise specified, the term "adhesive" includes not only adhesives in the narrow sense (adhesives having a shear storage modulus of 1 MPa to 500 MPa at 23° C. after energy ray irradiation or heat treatment) but also pressure-sensitive adhesives having a shear storage modulus of less than 1 MPa at 23° C. Therefore, the term "adhesive layer" includes not only adhesive layers in the narrow sense but also pressure-sensitive adhesive layers.

[0026] <1. Multilayer Retardation Film> <1.1. Overview> A multilayer retardation film according to one embodiment of the present invention comprises a first retardation layer and a second retardation layer, wherein the first retardation layer has an in-plane retardation of more than 0 nm and not more than 60 nm and a thickness direction retardation of 50 nm to 150 nm, the second retardation layer has an in-plane retardation of 100 nm to 150 nm and a thickness direction retardation of -150 nm to -50 nm and an NZ coefficient NZ of more than -0.5 to 0.0, and the angle formed by the slow axis direction of the second retardation layer with respect to the slow axis direction of the first retardation layer is 10° or less. The multilayer retardation film according to this embodiment can be incorporated into a liquid crystal display device to realize a display with excellent contrast and reduced color unevenness, and can be easily manufactured.

[0027] 1 is a perspective view schematically illustrating a multilayer retardation film according to one embodiment of the present invention. As shown in FIG. 1, the multilayer retardation film 100 has a slow axis A １１０ a first retardation layer 110 having a slow axis A １２０ The first retardation layer 110 includes a second retardation layer 120 having a slow axis A １１０ The slow axis A of the second retardation layer 120 with respect to the direction １２０ The direction of is angle θ １－２ This is what is happening.

[0028] the slow axis A of the first retardation layer １１０ The slow axis A of the second retardation layer １２０ The angle θ between the directions of１－２ is usually 10° or less, preferably 8° or less, more preferably 5° or less, and even more preferably 3° or less, and is ideally 0°, but may be greater than 0°. １－２ When the thickness is within the above range, a liquid crystal display device including the multilayer retardation film can realize a display with excellent contrast.

[0029] The multilayer retardation film 100 of the present embodiment does not include any layer between the first retardation layer 110 and the second retardation layer 120, and the first retardation layer 110 and the second retardation layer 120 are directly connected. Here, "directly" means that no layer is present between the first retardation layer and the second retardation layer.

[0030] When the multilayer structure having two retardation layers and the angle between the slow axes of the two retardation layers is obtained by forming the two retardation layers as separate layers and laminating the formed two retardation layers so that the slow axes are approximately parallel, the manufacturing process of the multilayer structure may be complicated.On the other hand, the multilayer retardation film of the present embodiment has the first retardation layer and the second retardation layer contained therein having the specific optical properties described above, and can be easily manufactured by the manufacturing method including the first step to the third step described later.

[0031] The in-plane retardation and thickness direction retardation of the first retardation layer can be adjusted, for example, in a manufacturing method including the first to third steps described below, by adjusting the in-plane retardation and thickness direction retardation of the resin layer (A) prepared in the first step, or by adjusting the stretching ratio in the third step. The in-plane retardation and thickness direction retardation of the second retardation layer can be adjusted, for example, in a manufacturing method including the first to third steps described below, by adjusting the thickness of the resin layer (B) formed in the second step, or by adjusting the stretching ratio in the third step. Angle θ １－２ can be adjusted, for example, by adjusting the stretching ratio in the third step in a production method including the first to third steps described below.

[0032] The multilayer retardation film 100 of this embodiment is long. A long multilayer retardation film can be efficiently laminated to a long optical element (e.g., a polarizer) using a roll-to-roll method. However, in another embodiment, the multilayer retardation film may be a sheet.

[0033] <1.2. First Retardation Layer> (Optical Properties of First Retardation Layer) The first retardation layer usually has an in-plane retardation of more than 0 nm and not more than 60 nm. The in-plane retardation of the first retardation layer is preferably not more than 50 nm, more preferably not more than 40 nm, and even more preferably not more than 30 nm, and the lower limit may be not less than 0.1 nm, not less than 1.0 nm, or not less than 2.0 nm.

[0034] The first retardation layer usually has a thickness direction retardation of 50 nm or more and 150 nm or less. The thickness direction retardation of the first retardation layer is preferably 55 nm or more, more preferably 60 nm or more, even more preferably 65 nm or more, and is preferably 140 nm or less, more preferably 130 nm or less, even more preferably 120 nm or less.

[0035] When the in-plane retardation and thickness direction retardation of the first retardation layer are within the above ranges, the display of a liquid crystal display device including the multilayer retardation film can have excellent contrast.

[0036] (Orientation angle of first retardation layer) When the multilayer retardation film is long, the orientation angle of the first retardation layer relative to the longitudinal direction of the multilayer retardation film is preferably within the range of 90 ° ± 10 °, more preferably within the range of 90 ° ± 8 °, and even more preferably within the range of 90 ° ± 5 °. When the orientation angle of the long first retardation layer is within the above range, a long multilayer retardation film and a long linear polarizing film having an absorption axis in the longitudinal direction are laminated so that their longitudinal directions coincide with each other, thereby making it possible to efficiently produce a long polarizing plate.

[0037] (Thickness of First Retardation Layer) The thickness T1 of the first retardation layer can be set to a thickness that provides a desired retardation. The thickness T1 of the first retardation layer can be set to, for example, 5 μm or more, for example, 10 μm or more, and can be set to, for example, 200 μm or less, for example, 100 μm or less.

[0038] (Material of the First Retardation Layer) The first retardation layer is usually formed of a thermoplastic resin and contains a thermoplastic resin. Hereinafter, the thermoplastic resin that forms the first retardation layer will also be referred to as resin (1). The first retardation layer may contain only resin (1). The thermoplastic resin usually contains a thermoplastic polymer. From the viewpoint of easily manufacturing a multilayer retardation film using a manufacturing method including the first to third steps described below, the thermoplastic resin that can be contained in the first retardation layer is preferably a resin with a positive intrinsic birefringence. Examples of thermoplastic polymers that can be contained in the first retardation layer include cyclic olefin polymers; polyolefins such as polyethylene and polypropylene; polyesters such as polyethylene terephthalate and polybutylene terephthalate; polyarylene sulfides such as polyphenylene sulfide; polyvinyl alcohol; polycarbonate; polyarylate; cellulose ester; polyethersulfone; polysulfone; polyarylsulfone; polyvinyl chloride; rod-shaped liquid crystal polymers; and the like. Resins containing these polymers usually have a positive intrinsic birefringence.

[0039] These polymers may be used singly or in combination of two or more kinds in any ratio.

[0040] Among these polymers, cyclic olefin polymers are preferred. Here, the cyclic olefin polymer refers to a polymer having structural units obtained by polymerizing a cyclic olefin, or a hydrogenated product thereof. The cyclic olefin may or may not have a substituent.

[0041] A cyclic olefin polymer contains a cyclic structure in its molecule. Typically, a cyclic olefin polymer has an alicyclic structure in the repeating unit of the polymer. The cyclic olefin polymer may be a polymer having an alicyclic structure in the main chain, a polymer having an alicyclic structure in the side chain, a polymer having alicyclic structures in the main chain and the side chain, or a mixture of two or more of these in any ratio. From the viewpoint of mechanical strength and heat resistance, the cyclic olefin polymer is preferably a polymer having an alicyclic structure in the main chain.

[0042] Examples of the alicyclic structure include saturated alicyclic hydrocarbon (cycloalkane) structures, unsaturated alicyclic hydrocarbon (cycloalkene, cycloalkyne) structures, etc. Among these, from the viewpoints of mechanical strength and heat resistance, cycloalkane structures and cycloalkene structures are preferred, and cycloalkane structures are particularly preferred.

[0043] The number of carbon atoms constituting the alicyclic structure is preferably 4 or more, more preferably 5 or more, and preferably 30 or less, more preferably 20 or less, particularly preferably 15 or less, per alicyclic structure. When the number of carbon atoms constituting the alicyclic structure is within the above range, mechanical strength, heat resistance, and moldability are well balanced.

[0044] In the cyclic olefin polymer, the proportion of repeating units having an alicyclic structure to all repeating units is preferably 55% by weight or more, more preferably 70% by weight or more, even more preferably 90% by weight or more, and usually 100% by weight or less. When the proportion of repeating units having an alicyclic structure to all repeating units is within this range, the transparency and heat resistance are good.

[0045] Among cyclic olefin polymers, norbornene-based polymers are preferred. Examples of norbornene-based polymers include ring-opening polymers of monomers having a norbornene structure and their hydrogenated products; and addition polymers of monomers having a norbornene structure and their hydrogenated products. Examples of ring-opening polymers of monomers having a norbornene structure include ring-opening homopolymers of one type of monomer having a norbornene structure, ring-opening copolymers of two or more types of monomers having a norbornene structure, and ring-opening copolymers of a monomer having a norbornene structure and any monomer copolymerizable therewith. Examples of addition polymers of monomers having a norbornene structure include addition homopolymers of one type of monomer having a norbornene structure, addition copolymers of two or more types of monomers having a norbornene structure, and addition copolymers of a monomer having a norbornene structure and any monomer copolymerizable therewith. Among these, hydrogenated ring-opening polymers of monomers having a norbornene structure, addition copolymers of monomers having a norbornene structure and α-olefins, and hydrogenated addition copolymers of monomers having a norbornene structure and α-olefins are preferred.

[0046] Examples of the monomer having a norbornene structure include bicyclo[2.2.1]hept-2-ene (common name: norbornene), tricyclo[4.3.0.1]hept-2-ene, and the like. ２，５ ]deca-3,7-diene (common name: dicyclopentadiene), 7,8-benzotricyclo[4.3.0.1 ２，５ ]dec-3-ene (common name: methanotetrahydrofluorene), tetracyclo[4.4.0.1 ２，５ .1 ７，１０ ]dodec-3-ene (common name: tetracyclododecene), and derivatives of these compounds (for example, those having a substituent on the ring). Examples of the substituent include an alkyl group, an alkylene group, and a polar group. These substituents may be the same or different, and a plurality of them may be bonded to the ring. The monomer having a norbornene structure may be used alone or in combination of two or more.

[0047] Examples of the polar group include a heteroatom or an atomic group having a heteroatom. Examples of the heteroatom include an oxygen atom, a nitrogen atom, a sulfur atom, a silicon atom, and a halogen atom. Specific examples of the polar group include a carboxyl group, a carbonyloxycarbonyl group, an epoxy group, a hydroxyl group, an oxy group, an ester group, a silanol group, a silyl group, an amino group, a nitrile group, and a sulfonic acid group.

[0048] Examples of the monomer capable of ring-opening copolymerization with the monomer having a norbornene structure include monocyclic olefins such as cyclohexene, cycloheptene, and cyclooctene, and derivatives thereof; cyclic conjugated dienes such as cyclohexadiene and cycloheptadiene, and derivatives thereof; etc. The monomer capable of ring-opening copolymerization with the monomer having a norbornene structure may be used alone or in combination of two or more.

[0049] A ring-opening polymer of a monomer having a norbornene structure can be produced, for example, by polymerizing or copolymerizing the monomer in the presence of a ring-opening polymerization catalyst.

[0050] In the addition copolymer of a monomer having a norbornene structure and an α-olefin, examples of the α-olefin include α-olefins having 2 to 20 carbon atoms, such as ethylene, propylene, and 1-butene, and derivatives thereof. Among these, ethylene is preferred. One type of α-olefin may be used alone, or two or more types may be used in combination.

[0051] An addition polymer of a monomer having a norbornene structure can be produced, for example, by polymerizing or copolymerizing the monomer in the presence of an addition polymerization catalyst.

[0052] The hydrogenated products of the ring-opening polymer and the addition polymer described above can be produced, for example, by hydrogenating, preferably to 90% or more, the carbon-carbon unsaturated bonds in a solution of the ring-opening polymer and the addition polymer in the presence of a hydrogenation catalyst containing a transition metal such as nickel or palladium.

[0053] Examples of trade names of norbornene-based polymers include "ZEONOR" and "ZEONEX" manufactured by Zeon Corporation; "ARTON" manufactured by JSR Corporation; and "APEL" manufactured by Mitsui Chemicals, Inc.

[0054] The norbornene polymers may be used alone or in combination of two or more.

[0055] The weight average molecular weight Mw of the cyclic olefin polymer is preferably 10,000 or more, more preferably 15,000 or more, particularly preferably 20,000 or more, and is preferably 100,000 or less, more preferably 80,000 or less, particularly preferably 50,000 or less. When the weight average molecular weight is within such a range, the mechanical strength and moldability of the resin containing the cyclic olefin polymer are well balanced.

[0056] In this specification, the weight average molecular weight (Mw) can be measured using gel permeation chromatography (GPC). Examples of solvents used in GPC include cyclohexane, toluene, and tetrahydrofuran. When GPC is used, the weight average molecular weight is measured as a relative molecular weight, for example, in terms of polyisoprene or polystyrene.

[0057] The amount of the cyclic olefin polymer is preferably 50% by weight to 100% by weight, more preferably 70% by weight to 100% by weight, and even more preferably 90% by weight to 100% by weight, based on 100% by weight of the resin (1). When the amount of the cyclic olefin polymer is within the above range, the resin (1) can have high heat resistance and transparency.

[0058] Resin (1) may contain any component other than the polymer. Examples of the optional component that may be contained in resin (1) include colorants such as pigments and dyes; plasticizers; fluorescent brighteners; dispersants; heat stabilizers; light stabilizers; ultraviolet absorbers; antistatic agents; antioxidants; fine particles; surfactants; etc. One type of optional component may be used alone, or two or more types may be used in combination.

[0059] The glass transition temperature Tg of the resin (1) is preferably 100°C or higher, more preferably 110°C or higher, even more preferably 120°C or higher, and preferably 190°C or lower, more preferably 180°C or lower, and even more preferably 170°C or lower. When the glass transition temperature Tg of the resin (1) is equal to or higher than the lower limit of the above range, the durability of the first retardation layer in a high-temperature environment can be improved. When the glass transition temperature Tg is equal to or lower than the upper limit, the stretching process for obtaining the first retardation layer can be carried out smoothly.

[0060] The glass transition temperatures of resin (1) and resin (2) described later can be measured using a differential scanning calorimeter (for example, "DSC6220" manufactured by SII Nanotechnology Inc.) at a temperature rise rate of 10°C / min in accordance with JIS K6911.

[0061] <1.3. Second Retardation Layer> (Optical Properties of Second Retardation Layer) The second retardation layer has an in-plane retardation of usually 100 nm or more, preferably 105 nm or more, and usually 150 nm or less, preferably 140 nm or less, more preferably 130 nm or less.

[0062] The second retardation layer has a thickness direction retardation of usually −150 nm or more, preferably −140 nm or more, more preferably −130 nm or more, and usually −50 nm or less, preferably −60 nm or less, more preferably −70 nm or less.

[0063] The second retardation layer has an NZ coefficient NZ that is usually greater than −0.5, preferably −0.4 or more, more preferably −0.3 or more, and is usually 0.0 or less, preferably −0.1 or less.

[0064] When the in-plane retardation, thickness direction retardation, and NZ coefficient of the second retardation layer are within the above ranges, the display contrast of a liquid crystal display device including the multilayer retardation film is excellent.

[0065] (Orientation angle of second retardation layer) When the multilayer retardation film is long, the orientation angle of the second retardation layer relative to the longitudinal direction of the multilayer retardation film is preferably within the range of 90 ° ± 10 °, more preferably within the range of 90 ° ± 8 °, and even more preferably within the range of 90 ° ± 5 °. When the orientation angle of the long second retardation layer is within the above range, a long multilayer retardation film and a long linear polarizing film having an absorption axis in the longitudinal direction are laminated so that their longitudinal directions coincide with each other, thereby making it possible to efficiently produce a long polarizing plate.

[0066] (Thickness of second retardation layer) The thickness T2 of the second retardation layer is preferably 10 μm or less, more preferably 8 μm or less, and even more preferably 7 μm or less. Also, it is preferably 1 μm or more, more preferably 2 μm or more, even more preferably 3 μm or more, and even more preferably 4 μm or more. When the thickness T2 of the second retardation layer is in this range, the second retardation layer can be manufactured more smoothly, its thickness precision is improved, and color unevenness can be effectively reduced.

[0067] (Material of the second retardation layer) The second retardation layer is usually formed of a thermoplastic resin and contains a thermoplastic resin. Hereinafter, the thermoplastic resin forming the second retardation layer is also referred to as resin (2). The thermoplastic resin that can be contained in the second retardation layer is preferably a resin with negative intrinsic birefringence. Examples of thermoplastic polymers that can be contained in the second retardation layer include acrylonitrile polymers; (meth)acrylate polymers; aromatic vinyl polymers; and multicomponent copolymers thereof. Resins containing these polymers usually have negative intrinsic birefringence.

[0068] These polymers may be used singly or in combination of two or more kinds in any ratio.

[0069] Among these polymers, aromatic vinyl polymers are preferred. Here, the term "aromatic vinyl polymer" refers to a polymer having a structural unit obtained by polymerizing aromatic vinyl. The structural unit obtained by polymerizing aromatic vinyl is also called an aromatic vinyl unit. The aromatic vinyl may or may not have a substituent.

[0070] Examples of aromatic vinyls include aromatic vinyls having a single ring, such as styrene; aromatic vinyls having fused rings; aromatic vinyls having ring assemblies; and compounds in which the hydrogen atoms of these compounds are substituted with substituents. An example of an aromatic vinyl having a single ring is styrene. Examples of aromatic vinyls having fused rings include vinyl naphthalenes, such as 1-vinyl naphthalene and 2-vinyl naphthalene; and vinyl anthracenes, such as 1-vinylanthracene, 2-vinylanthracene, and 9-vinylanthracene. Examples of aromatic vinyls having ring assemblies include vinyl biphenyls, such as 4-vinyl biphenyl; and vinyl terphenyls. Examples of substituents include halogen atoms, such as chlorine atoms; and alkyl groups, such as methyl and ethyl groups. Among these, aromatic vinyls having ring assemblies are preferred, vinyl biphenyls are more preferred, and 4-vinyl biphenyl is even more preferred.

[0071] The proportion of aromatic vinyl units contained in 100% by weight of the aromatic vinyl polymer is preferably 70% by weight or more, more preferably 80% by weight or more, and even more preferably 90% by weight or more. The upper limit is usually 100% by weight or less, and may be 99% by weight or less.

[0072] The amount of the aromatic vinyl polymer is preferably 50% by weight to 100% by weight, more preferably 70% by weight to 100% by weight, and even more preferably 90% by weight to 100% by weight, based on 100% by weight of the resin (2). When the amount of the aromatic vinyl polymer is within the above range, the layer of the resin (2) can exhibit a high birefringence Δn and a parameter Rth / d, which will be described later.

[0073] The weight-average molecular weight Mw of the polymer contained in resin (2) is preferably 10,000 or more, more preferably 15,000 or more, even more preferably 20,000 or more, even more preferably 50,000 or more, even more preferably 60,000 or more, even more preferably 100,000 or more, and is preferably 500,000 or less, more preferably 300,000 or less, even more preferably 200,000 or less. When the weight-average molecular weight is within this range, the mechanical strength and solubility in solvents of resin (2) containing the polymer are well balanced.

[0074] Resin (2) may contain any component other than the polymer. Examples of the optional component that may be contained in resin (2) include the examples listed as the optional components that may be contained in resin (1). One type of optional component may be used alone, or two or more types may be used in combination.

[0075] The glass transition temperature Tg of resin (2) is preferably 100°C or higher, more preferably 110°C or higher, even more preferably 120°C or higher, and preferably 200°C or lower, more preferably 180°C or lower, and even more preferably 160°C or lower. When the glass transition temperature Tg of resin (2) is equal to or higher than the lower limit of the above range, the durability of the second retardation layer in a high-temperature environment can be improved. When it is equal to or lower than the upper limit, the stretching process for obtaining the second retardation layer can be carried out smoothly. The glass transition temperature Tg of resin (2) may be higher than the glass transition temperature of resin (1). When the glass transition temperature Tg of resin (2) is within the above range, the orientation relaxation of the second retardation layer is usually reduced, and the display contrast of a liquid crystal display device can be particularly excellent.

[0076] When the multilayer retardation film is manufactured by a manufacturing method including the first step, the second step, and the third step described later, from the viewpoint of smoothly adjusting the optical properties of the first retardation layer and the second retardation layer by co-stretching, it is preferable that the glass transition temperature Tg of the resin (1) contained in the first retardation layer and the glass transition temperature Tg of the resin (2) contained in the second retardation layer are not too far apart. Specifically, the absolute value |ΔTg| of the difference between the glass transition temperature Tg of the resin (1) and the glass transition temperature Tg of the resin (2) is preferably 50 ° C. or less, more preferably 40 ° C. or less, particularly preferably 30 ° C. or less.

[0077] The birefringence Δn of the second retardation layer is preferably 0.010 or more, more preferably 0.013 or more, and even more preferably 0.016 or more at a measurement wavelength of 590 nm.The upper limit is preferably 0.025 or less, more preferably 0.020 or less.When the birefringence Δn is within the above range, it becomes easier to adjust the thickness of the second retardation layer appropriately when manufacturing the second retardation layer, and the surface condition of the second retardation layer can be effectively improved, and color unevenness can be effectively reduced.The birefringence Δn usually represents the magnitude of the refractive index anisotropy in the in-plane direction.

[0078] The parameter Rth / d of the second retardation layer is preferably -0.07 or more, more preferably -0.05 or more, even more preferably -0.04 or more, and preferably 0.00 or less, at a measurement wavelength of 590 nm. The parameter Rth / d is a value obtained by dividing the thickness direction retardation (Rth) by the thickness d, and usually represents the magnitude of the refractive index anisotropy in the thickness direction. As represented by the parameter Rth / d, the second retardation layer preferably has a negative refractive index anisotropy with a large absolute value in the thickness direction. In this case, even if the second retardation layer has a small thickness, it can have a negative thickness direction retardation Rth with a sufficiently large absolute value.

[0079] In one embodiment, the first retardation layer preferably contains a resin having a positive intrinsic birefringence, and the second retardation layer preferably contains a resin having a negative intrinsic birefringence. This allows the multilayer retardation film to be easily obtained by a manufacturing method including the first step, the second step, and the third step described below.

[0080] <1.4. Optional Layer> The multilayer retardation film may contain an optional layer in addition to the first retardation layer and the second retardation layer. Examples of the optional layer include an adhesive layer for bonding the multilayer retardation film to an optional optical element, and a thin film provided between the first retardation layer and the second retardation layer. Specific examples of the thin film include an anchor layer for improving the peel strength between the first retardation layer and the second retardation layer. The thin film preferably has optical isotropy. Specifically, the in-plane retardation of the optional thin film is preferably 5 nm or less, more preferably 4 nm or less, even more preferably 3 nm or less, particularly preferably 2 nm or less, at a measurement wavelength of 590 nm, and is usually 0 nm or more, and may be 0 nm.

[0081] When a thin film is provided between the first retardation layer and the second retardation layer, the thickness of any thin film is preferably less than 2.0 μm, more preferably less than 1.8 μm, and even more preferably less than 1.5 μm, from the viewpoint of making the multilayer retardation film thin. The lower limit of the thickness of the thin film is preferably as thin as possible, and can be, for example, 0.1 μm.

[0082] An optional layer may be provided between the first retardation layer and the second retardation layer, or an optional layer may not be provided. It is preferable that no optional layer is provided and the second retardation layer is directly provided on the first retardation layer.

[0083] <1.5. Physical Properties of Multilayer Retardation Film> The in-plane retardation of the multilayer retardation film is preferably 80 nm or more, more preferably 100 nm or more, even more preferably 120 nm or more, and is preferably 200 nm or less, more preferably 180 nm or less, even more preferably 160 nm or less. When the in-plane retardation of the multilayer retardation film is in the above range, the display contrast in a liquid crystal display device can be particularly excellent.

[0084] When the multilayer retardation film is long, the orientation angle of the multilayer retardation film with respect to the longitudinal direction of the multilayer retardation film is preferably in the range of 90 ° ± 10 °, more preferably in the range of 90 ° ± 8 °, even more preferably in the range of 90 ° ± 5 °, and even more preferably in the range of 90 ° ± 3 °. When the orientation angle of the multilayer retardation film is within the above range, by laminating a long multilayer retardation film and a long linear polarizer having a transmission axis in the width direction so that their longitudinal directions coincide with each other, the angle between the slow axis direction of the multilayer retardation film and the transmission axis of the linear polarizer can be easily set to 0 ° or approximately 0 °.

[0085] The total light transmittance of the multilayer retardation film is preferably 80% or more, more preferably 85% or more, particularly preferably 90% or more, and is usually 100% or less. The total light transmittance can be measured using an ultraviolet-visible spectrometer in the wavelength range of 400 nm to 700 nm.

[0086] <2. Manufacturing method of multilayer retardation film> The multilayer retardation film can be simply manufactured by a manufacturing method including a step (later described third step) of co-stretching a multilayer film including a layer (later described resin layer (A)) for forming a first retardation layer and a layer (later described resin layer (B)) for forming a second retardation layer.Specifically, the multilayer retardation film can be manufactured by a method including the following first step, second step and third step in this order.

[0087] The first step is to prepare a long resin layer (A) containing a resin having a positive intrinsic birefringence and having an orientation angle relative to the longitudinal direction in the range of 90 ° ± 10 °. The second step is to form a resin layer (B) containing a resin having a negative intrinsic birefringence on the resin layer (A) to obtain a multilayer film. The third step is to stretch the multilayer film in a stretching direction forming an angle of 0 ° or more and 5 ° or less with respect to the longitudinal direction to obtain a long multilayer retardation film containing the first retardation layer and the second retardation layer.

[0088] According to the production method including the first step to the third step in this order, it is easy to set the angle between the slow axis of the first retardation layer and the slow axis of the second retardation layer within a predetermined range, and it is possible to simply produce a multilayer retardation film.

[0089] That is, according to the preferred method for producing a multilayer retardation film, the resin layer (A) and the resin layer (B) are stretched together in the third step. Therefore, the number of stretching processes can be reduced. Since the number of steps required for producing a multilayer retardation film can be reduced, efficient production can be achieved. Furthermore, in a method in which the first retardation layer and the second retardation layer are laminated together after their respective production, a deviation in the direction of the slow axis due to lamination may occur. On the other hand, in a production method in which the resin layer (A) and the resin layer (B) are co-stretched by stretching a multilayer film to obtain a multilayer retardation film, a deviation in the direction of the slow axis due to lamination does not occur. Therefore, it is easy to precisely control the direction of the slow axis of each of the first retardation layer and the second retardation layer. As a result, a multilayer retardation film that can provide particularly excellent display contrast in a liquid crystal display device can be obtained. Each step will be described below.

[0090] <2.1. First step> The long resin layer (A) prepared in the first step contains a resin having a positive intrinsic birefringence. The resin layer (A) is stretched in the third step to become the first retardation layer. Therefore, when producing a multilayer retardation film by the production method of this embodiment, the resin (1) contained in the first retardation layer can be the same resin as the resin contained in the resin layer (A).

[0091] Examples of resins having positive intrinsic birefringence contained in the resin layer (A) include resins containing the polymers listed as examples of thermoplastic polymers that can be contained in the first retardation layer.As resins having positive intrinsic birefringence contained in the resin layer (A), resins containing cyclic olefin polymers are preferred.Examples and preferred examples of cyclic olefin polymers that can be contained in the resin layer (A) include the same examples as the examples and preferred examples of cyclic olefin polymers that can be contained in the resin (1).

[0092] The orientation angle of the long resin layer (A) with respect to the longitudinal direction is in the range of 90°±10°. The orientation angle of the resin layer (A) with respect to the longitudinal direction is preferably in the range of 90°±8°, more preferably in the range of 90°±5°, and even more preferably in the range of 90°±3°.

[0093] The in-plane retardation of the resin layer (A) may be appropriately set within a range in which the desired in-plane retardation of the first retardation layer is obtained, depending on the stretching conditions such as the stretching ratio in the third step. In one embodiment, the in-plane retardation of the resin layer (A) is preferably 150 nm or less, more preferably 130 nm or less, and even more preferably 100 nm or less, and is preferably 50 nm or more, more preferably 60 nm or more, and even more preferably 70 nm or more.

[0094] The thickness direction retardation of the resin layer (A) may be appropriately set within a range in which a desired thickness direction retardation of the first retardation layer is obtained, depending on the stretching conditions such as the stretching ratio in the third step. In one embodiment, the thickness direction retardation of the resin layer (A) is preferably 40 nm or more, more preferably 45 nm or more, and even more preferably 50 nm or more, and is preferably 150 nm or less, more preferably 140 nm or less, and even more preferably 130 nm or less.

[0095] The resin layer (A) can be produced by any method. For example, the resin layer (A) can be produced by stretching a pre-stretched film as a long resin film containing a resin having a positive intrinsic birefringence in a direction approximately perpendicular to the longitudinal direction of the pre-stretched film (i.e., approximately the width direction of the pre-stretched film). More specifically, the resin layer (A) can be produced by stretching the pre-stretched film preferably in the range of 90°±10°, more preferably in the range of 90°±8°, even more preferably in the range of 90°±5°, and even more preferably in the range of 90°±3°, relative to the longitudinal direction of the pre-stretched film.

[0096] When a pre-stretched film containing a resin with positive intrinsic birefringence is stretched, a slow axis usually appears in the stretching direction, so it is preferable to set the stretching direction of the pre-stretched film in a direction parallel to the orientation angle of the resin layer (A).

[0097] The pre-stretched film can be produced by a melt molding method or a solution casting method. More specific examples of the melt molding method include extrusion molding, press molding, inflation molding, injection molding, blow molding, and stretch molding. Among these methods, in order to obtain a resin layer (A) excellent in mechanical strength and surface precision, extrusion molding, inflation molding, and press molding are preferred, and among these, extrusion molding is particularly preferred from the viewpoint of being able to produce the resin layer (A) efficiently and simply.

[0098] The stretching ratio in the stretching to obtain the resin layer (A) is preferably 1.1 times or more, more preferably 1.2 times or more, and preferably 5.5 times or less, more preferably 5.0 times or less. The stretching temperature in the stretching to obtain the resin layer (A) is preferably TgA°C or higher, more preferably "TgA + 2°C" or higher, particularly preferably "TgA + 5°C" or higher, and preferably "TgA + 40°C" or lower, more preferably "TgA + 35°C" or lower, particularly preferably "TgA + 30°C" or lower. Here, TgA represents the glass transition temperature of the resin contained in the resin layer (A) that has positive intrinsic birefringence. Stretching can usually be performed using a tenter stretching machine while continuously transporting the pre-stretched film in the longitudinal direction.

[0099] <2.2. Second step> The resin layer (B) formed in the second step is formed on the long resin layer (A), so it is usually long. The resin layer (B) is stretched together with the resin layer (A) in the third step, and can become the second retardation layer. Therefore, when producing a multilayer retardation film by the production method of this embodiment, the resin (2) contained in the second retardation layer can be the same resin as the resin contained in the resin layer (B).

[0100] Examples of the resin having negative intrinsic birefringence contained in the resin layer (B) include resins containing the polymers listed as examples of the thermoplastic polymer that can be contained in the second retardation layer. As the resin having negative intrinsic birefringence contained in the resin layer (B), a resin containing an aromatic vinyl polymer is preferred.

[0101] When a resin containing an aromatic vinyl polymer is used to form a resin layer (B) by a layer formation method such as a coating method, the resin layer (B) can have a large refractive index nz in the thickness direction. Therefore, the resin layer (B) can have a parameter Rth / d that has a large absolute negative value, and therefore the retardation in the thickness direction can also have a large absolute negative value.

[0102] According to the stretching in the third step, the thickness direction retardation Rth of the resin layer (B) is maintained at a negative value, and usually, the absolute value of the negative value can be increased.Therefore, the thickness direction retardation Rth of the second retardation layer obtained by stretching the resin layer (B) can be a negative value with a sufficiently large absolute value.In this way, one of the advantages of the manufacturing method according to this embodiment is that a second retardation layer having a negative thickness direction retardation Rth can be obtained by the simple process of forming and stretching the resin layer (B).

[0103] Since the orientation angle of the second retardation layer can be easily controlled within a desired range by stretching the resin layer (B) in the third step, it is preferable that the in-plane retardation is 0 nm or close to 0 nm. Specifically, the in-plane retardation of the resin layer (B) is preferably 10 nm or less, more preferably 5 nm or less, and even more preferably 3 nm or less, and is usually 0 nm or more, and may be 0 nm.

[0104] The thickness direction retardation of the resin layer (B) may be appropriately determined depending on the type of resin contained in the resin layer (B). In one embodiment, the thickness direction retardation of the resin layer (B) is preferably −40 nm or less, more preferably −50 nm or less, and even more preferably −60 nm or less. The lower limit of the thickness direction retardation of the resin layer (B) is preferably as small as possible, and may be, for example, −150 nm or more, such as −140 nm or more, or −130 nm or more. In another embodiment, the thickness direction retardation of the resin layer (B) is preferably 40 nm or more, more preferably 50 nm or more, and even more preferably 60 nm or more, and is preferably 150 nm or less, more preferably 140 nm or less, and even more preferably 130 nm or less.

[0105] The thickness of the resin layer (B) may be appropriately set so that the second retardation layer obtained by stretching the resin layer (B) can exhibit the desired retardation. In one embodiment, the thickness of the resin layer (B) is preferably 20 μm or less, more preferably 15 μm or less, and even more preferably 10 μm or less, and is preferably 3 μm or more, more preferably 5 μm or more.

[0106] In the second step, the resin layer (B) is formed on the resin layer (A). Here, the resin layer (B) is formed directly on the resin layer (A) or indirectly via an optional layer such as a thin film. Here, "directly" means that there is no optional layer between the resin layer (A) and the resin layer (B).

[0107] The formation of the resin layer (B) in the second step preferably includes: (2-1) applying a resin liquid containing the resin having negative intrinsic birefringence and an organic solvent onto the resin layer (A) to form a resin liquid layer; and (2-2) drying the resin liquid layer. Steps (2-1) and (2-2) are usually performed in this order.

[0108] By including the step (2-1) and the step (2-2) in the second step, it is possible to form a resin layer (B) that is thin and has a small in-plane retardation.

[0109] Examples of organic solvents used in the resin liquid include cyclopentanone, methyl ethyl ketone, toluene, etc. The organic solvents may be used alone or in combination of two or more.

[0110] Examples of methods for applying the resin liquid include curtain coating, extrusion coating, roll coating, spin coating, dip coating, bar coating, spray coating, slide coating, print coating, gravure coating, die coating, and gap coating.

[0111] After applying the resin liquid onto the resin layer (A), the resin liquid is dried to remove the organic solvent, thereby forming the resin layer (B) on the resin layer (A). Drying can be performed by a drying method such as natural drying, heat drying, reduced-pressure drying, or reduced-pressure heat drying.

[0112] <2.3. Third step> In the third step, the multilayer film including the resin layer (A) and the resin layer (B) obtained in the second step is stretched. By stretching the multilayer film in the third step, the first retardation layer is obtained from the resin layer (A), and the second retardation layer is obtained from the resin layer (B).

[0113] The stretching of the multilayer film in the third step is usually performed in only one direction forming an angle of 0° to 5° with respect to the longitudinal direction of the multilayer film. The stretching direction may be an angle of 0° with respect to the longitudinal direction of the multilayer film (i.e., a direction that coincides with the longitudinal direction of the multilayer film).

[0114] The orientation angle of the long resin layer (A) prepared in the first step relative to the longitudinal direction of the resin layer (A) usually does not change in the second step, and even in the resin layer (A) after the second step, it is usually in the range of 90 ° ± 10 ° relative to the longitudinal direction.Therefore, the stretching direction of the multilayer film in the third step is perpendicular or almost perpendicular to the slow axis of the resin layer (A) provided in the multilayer film.Therefore, the in-plane retardation of the first retardation layer obtained from the resin layer (A) by stretching in the third step is usually smaller than the in-plane retardation of the resin layer (A) prepared in the first step, but the direction of the slow axis of the first retardation layer is usually the same or almost the same direction as the slow axis of the resin layer (A), and is perpendicular or almost perpendicular to the stretching direction.On the other hand, the second retardation layer obtained from the resin layer (B) by stretching in the third step has a slow axis that appears in a direction perpendicular or almost perpendicular to the stretching direction. Therefore, by stretching the multilayer film including the resin layer (A) and the resin layer (B) in the third step, the angle θ formed by the slow axis direction of the second retardation layer with respect to the slow axis direction of the first retardation layer １－２ may be typically 10° or less, typically 0° or greater than 0°.

[0115] The stretching ratio in the third step may be appropriately set depending on the optical properties such as the in-plane retardation of the prepared resin layer (A), and is preferably 1.10 times or more, more preferably 1.15 times or more, particularly preferably 1.20 times or more, and preferably 2.00 times or less, more preferably 1.80 times or less, particularly preferably 1.60 times or less. When the stretching ratio in the third step is equal to or greater than the lower limit of the above range, the occurrence of wrinkles can be suppressed. When the stretching ratio is equal to or less than the upper limit, the direction of the slow axis can be easily controlled.

[0116] The stretching temperature in the third step is preferably TgA - 20°C or higher, more preferably TgA - 10°C or higher, even more preferably TgA - 5°C or higher, and is preferably TgA + 30°C or lower, more preferably TgA + 25°C or lower, even more preferably TgA + 20°C or lower. Here, TgA represents the glass transition temperature of a resin having positive intrinsic birefringence contained in the resin layer (A).

[0117] The stretching temperature in the third step is preferably TgB-50°C or higher, more preferably TgB-40°C or higher, particularly preferably TgB-30°C or higher, and is preferably TgB+30°C or lower, more preferably TgB+25°C or lower, particularly preferably TgB+20°C or lower. Here, TgB represents the glass transition temperature of a resin having negative intrinsic birefringence contained in the resin layer (B).

[0118] The stretching in the third step is preferably carried out by free uniaxial stretching. Here, free uniaxial stretching refers to stretching in a certain direction, and stretching without applying restraining force in any direction other than the stretching direction. Therefore, for example, free uniaxial stretching in the longitudinal direction of a multilayer film refers to stretching in the longitudinal direction without restraining the end of the width direction of the multilayer film. By carrying out free uniaxial stretching in the third step, the slow axis direction of each of the first retardation layer and the second retardation layer can be easily controlled.

[0119] The stretching in the third step described above can be carried out using, for example, a tenter stretching machine or a roll stretching machine, and it is preferable to use a roll stretching machine. Free uniaxial stretching can be easily carried out using a roll stretching machine. Free uniaxial stretching using a roll stretching machine is usually carried out while continuously transporting a long multilayer film in the longitudinal direction. As the roll stretching machine, for example, one described in International Publication No. 2016 / 047465 can be used.

[0120] <2.4. Optional Step> The manufacturing method of the multilayer retardation film may further include an optional step in combination with the first step, the second step, and the third step. For example, the manufacturing method of the multilayer retardation film may include a step of providing a protective layer on the surface of the multilayer retardation film. Furthermore, for example, the manufacturing method of the multilayer retardation film may include a step of performing a surface treatment such as corona treatment or plasma treatment on one or more surfaces of any layer such as the resin layer (A), the resin layer (B), and the thin film at any time. Also, for example, after the first step, it may include a step of forming a thin film on the resin layer (A). The thin film can be formed, for example, by a method including applying a coating liquid containing a resin as a material for the thin film and a solvent onto the resin layer (A).

[0121] 3. Uses of the Multilayer Retardation Film The multilayer retardation film can be suitably used as an optical element constituting a liquid crystal display device. A liquid crystal display device including the multilayer retardation film has excellent contrast even when the display surface is observed from an oblique direction.

[0122] The multilayer retardation film can be combined with a polarizer to form a polarizing plate and incorporated into a liquid crystal display device. The polarizer to be combined with the multilayer retardation film can be a film that can transmit one of two linearly polarized light beams whose vibration directions intersect at right angles and absorb or reflect the other. Here, the vibration direction of linearly polarized light refers to the vibration direction of the electric field of the linearly polarized light. Such a film usually has a transmission axis of polarized light, and can transmit linearly polarized light beams whose vibration direction is parallel to the transmission axis, and can absorb or reflect linearly polarized light beams whose vibration direction is perpendicular to the transmission axis.

[0123] Any linear polarizer can be used as the polarizer. An example of a linear polarizer is a linear polarizing film. Specific examples of linear polarizing films include a film obtained by adsorbing iodine or a dichroic dye onto a polyvinyl alcohol film and then uniaxially stretching the film in a boric acid bath; and a film obtained by adsorbing iodine or a dichroic dye onto a polyvinyl alcohol film, stretching the film, and further modifying a portion of the polyvinyl alcohol units in the molecular chain to polyvinylene units. Among these, a polarizer containing polyvinyl alcohol is preferred as the linear polarizer.

[0124] Typically, in the stretching process for producing a polarizer, a film before stretching is stretched in the longitudinal direction. Therefore, the resulting polarizer may have an absorption axis parallel to the longitudinal direction of the polarizer. In addition, the absorption axis and transmission axis of the polarizer are generally perpendicular when viewed from the thickness direction. The polarizer is preferably one that can absorb linearly polarized light having a vibration direction parallel to the absorption axis, and is particularly preferably one that has an excellent degree of polarization. The thickness of the polarizer is generally 5 μm to 80 μm, but is not limited thereto.

[0125] Since the polarizer is usually a flexible film, it may be a laminate provided with a protective film from the viewpoint of improving handleability and durability. The multilayer retardation film may be attached to a polarizer to form a polarizing plate including the multilayer retardation film and the polarizer, and the multilayer retardation film may exhibit a polarizer protection function.

[0126] In a polarizing plate including a multilayer retardation film and a polarizer, the angle between the transmission axis of the polarizer and the slow axis of the first retardation layer included in the multilayer retardation film is preferably in the range of 0°±20°, more preferably in the range of 0°±10°, and even more preferably in the range of 0°±5°. When the angle between the slow axis of the first retardation layer and the transmission axis of the polarizer is within the above range, when a polarizing plate including the multilayer retardation film and a polarizer is incorporated into a liquid crystal display device, the display contrast of the liquid crystal display device can be particularly excellent.

[0127] In addition, in a polarizing plate including a multilayer retardation film and a polarizer, the angle between the transmission axis of the polarizer and the slow axis of the second retardation layer included in the multilayer retardation film is preferably in the range of 0°±20°, more preferably in the range of 0°±10°, and even more preferably in the range of 0°±5°. When the angle between the slow axis of the second retardation layer and the transmission axis of the polarizer is within the above range, when a polarizing plate including a multilayer retardation film and a polarizer is incorporated into a liquid crystal display device, the display contrast of the liquid crystal display device can be particularly excellent.

[0128] Here, the angular relationship between the slow axes of the first retardation layer and the second retardation layer in the polarizing plate and the transmission axis of the polarizer is defined such that a shift in one direction is positive and a shift in the other direction is negative, and the positive and negative directions are defined in common for the first retardation layer, the second retardation layer, and the polarizer, which are components of the polarizing plate.

[0129] A liquid crystal display device usually includes a first polarizer disposed on the viewing side, a liquid crystal cell, a second polarizer, and a light source, in this order. The multilayer retardation film can be disposed between the first polarizer and the liquid crystal cell, or between the second polarizer and the liquid crystal cell, or both.

[0130] The transmission axis of the first polarizer and the transmission axis of the second polarizer can be generally arranged to be perpendicular to each other.

[0131] Examples of display modes of the liquid crystal cell include in-plane switching (IPS) mode, vertical alignment (VA) mode, multi-domain vertical alignment (MVA) mode, continuous spin wheel alignment (CPA) mode, hybrid alignment nematic (HAN) mode, twisted nematic (TN) mode, super twisted nematic (STN) mode, and optically compensated bend (OCB) mode, with the IPS mode being preferred.

[0132] In the IPS mode, liquid crystal molecules are aligned parallel to the surfaces of the substrates that make up the liquid crystal cell. By applying an electric field to the substrates, the orientation direction of the liquid crystal molecules is rotated in a plane parallel to the substrates, thereby adjusting the light passing through the liquid crystal cell.

[0133] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to the examples shown below, and can be practiced with any modifications within the scope of the claims of the present invention and their equivalents.

[0134] In the following description, the "%" and "parts" that represent amounts are by weight unless otherwise specified. Furthermore, the operations described below were carried out at room temperature (20°C ± 15°C) and atmospheric pressure (1 atm) unless otherwise specified.

[0135] <Evaluation Method> (Thickness) The thickness of the laminate and each layer was measured using a film thickness measurement system ("F20" manufactured by Filmetrics).

[0136] (Retardation, NZ Coefficient) Retardation was measured using a phase difference meter (Axometrics' "AxoScan") at a temperature of 23° C. When determining the physical properties of each layer of an inseparable multilayer structure, the sample was measured from multiple directions and calculated by fitting analysis using the attached multilayer analysis software.

[0137] (Method for measuring the direction of the slow axis) The direction of the slow axis of each layer constituting the multilayer film or multilayer retardation film was measured using a retardation meter ("AxoScan" manufactured by Axometrics). The direction of the slow axis relative to the longitudinal direction of the long multilayer film or multilayer retardation film was determined as an orientation angle.

[0138] (Glass Transition Temperature) Measurement was carried out using a differential scanning calorimeter (for example, "DSC6220" manufactured by SII Nanotechnology Inc.) at a temperature rise rate of 10°C / min in accordance with JIS K6911.

[0139] (Contrast) The contrast of the liquid crystal display devices obtained in Examples and Comparative Examples was measured using a display goniophotometer ("DMS803" manufactured by Instrument Systems). Specifically, the luminance (unit: nit) in the black display state and the luminance (unit: nit) in the white display state were measured from a direction at a polar angle of 60° and an azimuthal angle (here, the angle with the absorption axis of the first polarizer as 0°) of 45° relative to the display surface, and the contrast was calculated from the ratio (white display state luminance / black display state luminance).

[0140] (Color Unevenness) The liquid crystal display devices obtained in the examples and comparative examples were visually inspected in a dark room. The liquid crystal display devices were set to a black display state, and while the azimuth angle (here, the angle with the absorption axis of the first polarizer at 0°) was changed in the range of 0° to 360°, it was confirmed whether color unevenness was visible from directions at polar angles of 0° and 60° with respect to the display surface. Cases where color unevenness was not visible were evaluated as "absent," and cases where color unevenness was visible were evaluated as "present."

[0141] Example 1 (1-1. First step: preparation of resin layer (A)) (1-1-1. production of pre-stretched film) Pelletized norbornene resin (manufactured by Zeon Corporation; glass transition temperature 126°C) as a resin with positive intrinsic birefringence was dried at 100°C for 5 hours. The dried resin was supplied to an extruder, passed through a polymer pipe and a polymer filter, and extruded into a sheet form from a T-die onto a casting drum. The extruded resin was cooled to obtain a long pre-stretched film with a thickness of 80 µm. The obtained pre-stretched film was wound onto a roll and recovered.

[0142] (1-1-2. Stretching of Pre-Stretched Film) The pre-stretched film was pulled out from the roll and continuously supplied to a tenter stretching machine. Then, using this tenter stretching machine, the pre-stretched film was stretched in the width direction of the pre-stretched film at a stretching temperature of 135°C and a stretching ratio of 3.5 times to obtain a long stretched film as the resin layer (A). The orientation angle of the obtained stretched film was 90° (width direction) with the longitudinal direction as the reference 0°, the in-plane retardation Re was 80 nm, and the thickness direction retardation Rth was 60 nm. The obtained stretched film was wound up on a roll and recovered.

[0143] (1-2. Second Step: Formation of Resin Layer (B)) A liquid composition was prepared as a resin liquid containing poly(4-vinylbiphenyl) (MW = 115,000, glass transition point 138 ° C.) as a resin having negative intrinsic birefringence. This liquid composition contained cyclopentanone as an organic solvent, and the concentration of poly(4-vinylbiphenyl) in the liquid composition was 15 wt %. A stretched film serving as the resin layer (A) was pulled out from the roll, and the resin liquid was applied onto this stretched film to form a layer of the resin liquid. Thereafter, the formed layer of the resin liquid was dried, and a layer of poly(4-vinylbiphenyl) (thickness 7.6 μm) serving as the resin layer (B) was formed on the resin layer (A). This resulted in a multilayer film comprising a stretched film serving as the resin layer (A) and a layer of poly(4-vinylbiphenyl) serving as the resin layer (B). The resin layer (B) thus obtained had an in-plane retardation Re of 2 nm and a thickness direction retardation Rth of −60 nm. The multilayer film thus obtained was wound around a roll and collected.

[0144] (1-3. Third Step: Stretching of Multilayer Film) The multilayer film was pulled out from the roll and continuously fed to a longitudinal stretching machine. Then, using this longitudinal stretching machine, the multilayer film was subjected to free uniaxial stretching at a stretching temperature of 135°C and a stretching ratio of 1.3 times in the longitudinal direction. This resulted in a multilayer retardation film comprising a first retardation layer (thickness: 20 μm) and a second retardation layer (thickness: 6.7 μm). The first retardation layer was obtained by stretching a stretched film as the resin layer (A) in the longitudinal direction. The second retardation layer was obtained by stretching a layer of poly(4-vinylbiphenyl) as the resin layer (B) in the longitudinal direction. The multilayer retardation film was a co-stretched film of the resin layer (A) and the resin layer (B). The second retardation layer had a birefringence Δn of 0.018. Furthermore, the Rth / d of the second retardation layer was -0.0122.

[0145] (1-4. Production of Polarizing Film) A long linear polarizing film having an absorption axis in the longitudinal direction was prepared. This linear polarizing film and the surface of the multilayer retardation film on the first retardation layer side were bonded together with their longitudinal directions parallel to each other. This bonding was performed using a pressure-sensitive adhesive ("CS-9621" manufactured by Nitto Denko Corporation). This resulted in a polarizing film comprising a linear polarizing film, a first retardation layer, and a second retardation layer in this order. A rectangular polarizing plate was cut out from the polarizing film.

[0146] (1-5. Preparation of Liquid Crystal Display Device for Evaluation) A liquid crystal display device (Apple iPad (10th Generation) (registered trademark)) equipped with an IPS mode liquid crystal cell was prepared. This liquid crystal display device included a viewer-side polarizer (corresponding to the first polarizer), a liquid crystal cell, and a rear-side polarizer (corresponding to the second polarizer). The transmission axis of the viewer-side polarizer and the transmission axis of the rear-side polarizer were orthogonal (forming an angle of 90°), the liquid crystal cell was in a black display state when no voltage was applied, and the transmission axis of the viewer-side polarizer was parallel to the alignment direction of the liquid crystal molecules of the liquid crystal cell in the black display state (forming an angle of 0°). Each of the viewer-side polarizer and the rear-side polarizer was combined with a protective film to form a polarizing plate. The protective film on the liquid crystal cell side of the rear-side polarizer was a film made of an isotropic material having no retardation.

[0147] This liquid crystal display device was disassembled, and the polarizing plate containing the viewer-side polarizer was peeled off to expose the surface of the liquid crystal cell. The surface of this liquid crystal cell and the rectangular polarizing plate obtained in (1-4) were bonded together via an adhesive ("CS9621" manufactured by Nitto Denko Corporation). During bonding, the polarizing plate was oriented so that the linear polarizing film was on the viewer side. Furthermore, during bonding, the relationship between these was adjusted so that the transmission axis of the linear polarizing film was at an angle perpendicular to the transmission axis of the rear-side polarizer (an angle of 90°). As a result, a liquid crystal display device was obtained that included, in this order, a linear polarizing film as a first polarizer, a first retardation layer, a second retardation layer, a liquid crystal cell, a protective film, and a second polarizer.

[0148] The optical in-plane axis directions of the layers, with the transmission axis direction of the first polarizer being taken as the reference 0°, are as follows: First polarizer transmission axis: 0° First retardation layer slow axis: 0° (width direction in a long polarizing film) Second retardation layer slow axis: 0° (width direction in a long polarizing film) Liquid crystal molecule alignment direction of the liquid crystal cell in the black display state: 0° Protective film slow axis: none (isotropic) Second polarizer transmission axis: 90°

[0149] The contrast of the obtained liquid crystal display device was measured by the above-mentioned method, and the presence or absence of color unevenness was confirmed.

[0150] Comparative Example 1 A long pre-stretched film was obtained in the same manner as in (1-1-1) above. In (1-1-2) above, the pre-stretched film was pulled out from the roll and continuously fed to a simultaneous biaxial stretching machine. The pre-stretched film was then stretched by this simultaneous biaxial stretching machine at a stretching temperature of 138°C to a stretch ratio of 2.2 times in the width direction and 1.5 times in the longitudinal direction to obtain a long stretched film as the resin layer (A). The orientation angle of the obtained stretched film was 90°, with the longitudinal direction being taken as the reference 0°, and the in-plane retardation Re was 28 nm and the thickness direction retardation Rth was 57 nm. The obtained stretched film was wound up on a roll and recovered.

[0151] The obtained stretched film was treated in the same manner as in (1-2) to (1-3) above, to obtain a multilayer retardation film as a co-stretched film having, in this order, a first retardation layer obtained by stretching the resin layer (A) and a second retardation layer obtained by stretching the resin layer (B).

[0152] A rectangular polarizing plate was obtained in the same manner as in (1-4) above, except for the following changes: A linear polarizing film and the surface of the multilayer retardation film on the first retardation layer side were attached to each other with their longitudinal directions perpendicular to each other.

[0153] A liquid crystal display device was obtained in the same manner as in (1-5) above.

[0154] The optical in-plane axis directions of the layers, with the transmission axis direction of the first polarizer being taken as the reference 0°, are as follows: First polarizer transmission axis: 0° First retardation layer slow axis: 0° (width direction in a long polarizing film) Second retardation layer slow axis: 90° (longitudinal direction in a long polarizing film) Liquid crystal molecule alignment direction of the liquid crystal cell in the black display state: 0° Protective film slow axis: none (isotropic) Second polarizer transmission axis: 90°

[0155] The contrast of the obtained liquid crystal display device was measured by the above-mentioned method, and the presence or absence of color unevenness was confirmed.

[0156] <Results> The results are shown in the table below. The orientation angle in the table below is the angle formed by the in-plane slow axis direction of each layer with respect to the longitudinal direction of the long multilayer retardation film. In the table below, the abbreviations have the following meanings: Re: in-plane retardation Rth: thickness direction retardation θ １－２ NZ: NZ coefficient: angle formed by the slow axis direction of the second retardation layer with respect to the slow axis direction of the first retardation layer.

[0157]

[0158] From the above results, it is understood that the multilayer retardation film according to Example 1 is excellent in display contrast in a liquid crystal display device and can reduce color unevenness.

[0159] 100 Multilayer retardation film 110 First retardation layer 120 Second retardation layer A １１０ , A １２０ Slow axis

Claims

1. A multilayer retardation film comprising a first retardation layer and a second retardation layer, wherein the first retardation layer has an in-plane retardation of more than 0 nm and not more than 60 nm and a thickness direction retardation of 50 nm to 150 nm, the second retardation layer has an in-plane retardation of 100 nm to 150 nm and a thickness direction retardation of -150 nm to -50 nm and an NZ coefficient NZ of more than -0.5 to 0.0, and the angle formed by the slow axis direction of the second retardation layer with respect to the slow axis direction of the first retardation layer is 10° or less.

2. The multilayer retardation film according to claim 1, wherein the first retardation layer contains a resin having a positive intrinsic birefringence, and the second retardation layer contains a resin having a negative intrinsic birefringence.

3. A method for producing a multilayer retardation film according to claim 1 or 2, comprising, in this order: a first step of preparing a long resin layer (A) containing a resin having a positive intrinsic birefringence and having an orientation angle with respect to the longitudinal direction in the range of 90°±10°; a second step of forming, on said resin layer (A), a resin layer (B) containing a resin having a negative intrinsic birefringence to obtain a multilayer film; and a third step of stretching said multilayer film in a stretching direction forming an angle of 0° or more and 5° or less with respect to the longitudinal direction to obtain a long multilayer retardation film containing said first retardation layer and said second retardation layer.

4. The method for producing a multilayer retardation film according to claim 3, wherein the second step comprises: applying a resin liquid containing the resin having negative intrinsic birefringence and an organic solvent onto the resin layer (A) to form a layer of the resin liquid; and drying the layer of the resin liquid.

Images