WO2025249080A1 - Multilayer retardation film and method for manufacturing same - Google Patents

Abstract

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 60 nm or less and a retardation in the thickness direction of 50-150 nm, the second retardation layer has an in-plane retardation of 100-150 nm and a retardation in the thickness direction of -150 nm to -50 nm, the angle formed by the slow axis direction of the second retardation layer and the slow axis direction of the first retardation layer is 10° or less, and the second retardation layer contains a resin containing a polymer containing a 4-vinylbiphenyl monomer unit.

Description

Multilayer retardation film and method for producing the same

The present invention relates to a multilayer retardation film and a method for manufacturing the same.

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 known that a broadband wavelength film constituting a circularly polarizing plate can be produced by a method including a step of stretching a predetermined multilayer film (see, for example, Patent Document 8).

Patent No. 5170093 Japanese Patent No. 5282821 (corresponding publication: U.S. Patent Application Publication No. 2012/0140154) Patent No. 4855081 Japanese Patent Application Laid-Open No. 2023-054647 Japanese Patent Application Publication No. 2009-139747 (corresponding publication: U.S. Patent Application Publication No. 2010/0309414) JP 2009-192612 A JP 2010-522900 A (corresponding publication: WO 2008/121580 A) Japanese Patent No. 7059936 (U.S. Patent Application Publication No. 2019/0293852)

Retardation films with a multilayer structure (hereinafter also referred to as multilayer retardation films) can be used to improve the display contrast in liquid crystal display devices. Multilayer retardation films can be manufactured using co-stretching. Specifically, multilayer retardation films can be manufactured by a method that involves preparing a multilayer film, which is a precursor to the multilayer retardation film, and co-stretching each layer that makes up the multilayer film. Manufacturing methods that use co-stretching can reduce the number of steps required to manufacture a multilayer retardation film, which is expected to improve manufacturing efficiency.

However, multilayer retardation films manufactured using methods that include co-stretching can sometimes exhibit poor display contrast in liquid crystal display devices. Furthermore, because it is difficult to prepare the multilayer film that precedes the multilayer retardation film, it can sometimes be difficult to obtain a multilayer retardation film by co-stretching.

Therefore, there is a need for a new multilayer retardation film that can be incorporated into a liquid crystal display device to achieve a display with excellent contrast; and a method for manufacturing such a multilayer retardation film.

As a result of intensive research to solve the above-mentioned problems, the present inventors 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 containing a specific resin and having specific optical properties, in a predetermined arrangement, and have completed the present invention.
The present invention provides the following:

<1> A first retardation layer and a second retardation layer are included,
the first retardation layer has an in-plane retardation of more than 0 nm and 60 nm or less and a thickness direction retardation of 50 nm or more and 150 nm or less,
the second retardation layer has an in-plane retardation of 100 nm or more and 150 nm or less and a thickness direction retardation of −150 nm or more and −50 nm or less,
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;
A multilayer retardation film, wherein the second retardation layer contains a resin containing a polymer containing a 4-vinylbiphenyl monomer unit.
<2> The multilayer retardation film according to <1>, wherein the first retardation layer contains a resin containing a cyclic olefin polymer.
<3> The multilayer retardation film according to <1> or <2>, wherein the proportion of 4-vinylbiphenyl monomer units contained in 100% by weight of the polymer containing the 4-vinylbiphenyl monomer units is 70% by weight or more.
<4> A method for producing the multilayer retardation film according to any one of <1> to <3>,
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 polymer containing a 4-vinylbiphenyl monomer unit on the resin layer (A) to obtain a multilayer film;
A third step of stretching the multilayer film in a stretching direction forming an angle of 0° to 5° with respect to the longitudinal direction to obtain a long multilayer retardation film including the first retardation layer and the second retardation layer;
A method for producing a multilayer retardation film, comprising the steps of:
<5> The second step
The method for producing the multilayer retardation film according to <4>, comprising: applying a resin liquid containing a polymer containing the 4-vinylbiphenyl monomer unit and an organic solvent onto the resin layer (A) to form a resin liquid layer; and drying the resin liquid layer.
<6> The method for producing a multilayer retardation film according to <4> or <5>, wherein the resin having a positive intrinsic birefringence is a resin containing a cyclic olefin polymer.

The present disclosure also provides the following:
<7> A polarizing plate comprising the multilayer retardation film according to any one of <1> to <3> and a polarizer.
<8> A liquid crystal display device comprising the multilayer retardation film according to any one of <1> to <3>, a polarizer, and a liquid crystal cell.

The present invention provides a novel multilayer retardation film that can be incorporated into a liquid crystal display device to achieve a display with excellent contrast; and a method for manufacturing such a multilayer retardation film.

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

The present invention will be described in detail below, showing embodiments and examples. However, the present invention is not limited to the embodiments and examples shown below, and can be modified and implemented as desired without departing from 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 lower limit values can be combined as appropriate with any numerical value selected from the group of numerical values listed as upper limit values.

Furthermore, in the figures, identical components are given the same reference numerals and their explanations may be omitted.

In the following description, a "long" component (e.g., a film or layer) means that its length is at least five times its width, preferably 10 times or more, and specifically that it is long enough to 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 its width.

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.

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.

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

In the following description, unless otherwise specified, the front direction of a film refers to the normal direction of the principal surface of the film, and more specifically, the direction of the principal surface at a polar angle of 0° and an azimuthal angle of 0°.

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

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

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

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. "In-plane direction" represents the direction perpendicular to the thickness direction unless otherwise specified. 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 thickness of the layer. The measurement wavelength is 590 nm unless otherwise specified.

In the following description, unless otherwise specified, the orientation of elements as "parallel," "perpendicular," 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°.

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

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

<1. Multilayer Retardation Film>
<1.1. Overview>
The multilayer retardation film according to one embodiment of the present invention is
a first retardation layer and a second retardation layer;
the first retardation layer has an in-plane retardation of more than 0 nm and 60 nm or less and a thickness direction retardation of 50 nm or more and 150 nm or less,
the second retardation layer has an in-plane retardation of 100 nm or more and 150 nm or less and a thickness direction retardation of −150 nm or more and −50 nm or less,
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 second retardation layer contains a resin containing a polymer containing a 4-vinylbiphenyl monomer unit.
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.

Fig. 1 is a perspective view schematically showing a multilayer retardation film according to one embodiment of the present invention. As shown in Fig. 1, the multilayer retardation film 100 includes a first retardation layer 110 having a slow axis _A110 and a second retardation layer ₁₂₀ having a slow axis A120. The direction of the slow axis A120 of the second retardation layer ₁₂₀ forms an angle _θ1-2 with the direction of the slow axis A110 of the first retardation layer ₁₁₀ .

The angle θ _1-2 formed by the direction of the slow axis A ₁₂₀ of the second retardation layer with respect to the direction of the slow axis A ₁₁₀ of the first retardation layer 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 exceed 0°. When the angle θ _1-2 is within the above range, a liquid crystal display device including the multilayer retardation film can achieve display with excellent contrast.

The multilayer retardation film 100 of this 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 there is no layer between the first retardation layer and the second retardation layer.

The in-plane retardation and thickness direction retardation of the first retardation layer can be adjusted, for example, by adjusting the in-plane retardation and thickness direction retardation of the resin layer (A) prepared in the first step in a production method including the first step to the third step described later, 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, by adjusting the thickness of the resin layer (B) formed in the second step in a production method including the first step to the third step described later, or by adjusting the stretching ratio in the third step.
The 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.

The multilayer retardation film 100 of this embodiment is long. The long multilayer retardation film can be efficiently bonded to a long optical element (for example, a polarizer) by a roll-to-roll method.
However, in another embodiment, the multilayer retardation film may be in the form of a sheet.

<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.

The first retardation layer typically 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.

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 equipped with a multilayer retardation film can have excellent contrast.

(Orientation angle of first retardation layer)
When the multilayer retardation film is long, the orientation angle of the first retardation layer with respect 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 polarizing plate can be efficiently produced by laminating a long multilayer retardation film and a long linear polarizing film having an absorption axis in the longitudinal direction so that their longitudinal directions are aligned.

(Thickness of First Retardation Layer)
The thickness T1 of the first retardation layer can be set to a value 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.

(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 forming the first retardation layer is also 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 producing the multilayer retardation film by a production 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 having 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; etc. Resins containing these polymers usually have positive intrinsic birefringence.

These polymers may be used alone or in combination of two or more in any ratio.

Among these polymers, cyclic olefin polymers are preferred. Here, 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.

Cyclic olefin polymers contain a cyclic structure within their molecules. Typically, cyclic olefin polymers have an alicyclic structure within the repeating units of the polymer. Cyclic olefin polymers can be polymers having an alicyclic structure in the main chain, polymers having an alicyclic structure in the side chain, polymers having alicyclic structures in the main chain and side chain, or mixtures of two or more of these in any ratio. From the standpoint of mechanical strength and heat resistance, cyclic olefin polymers containing an alicyclic structure in the main chain are preferred.

Examples of alicyclic structures include saturated alicyclic hydrocarbon (cycloalkane) structures and unsaturated alicyclic hydrocarbon (cycloalkene, cycloalkyne) structures. Among these, from the standpoint of mechanical strength and heat resistance, cycloalkane structures and cycloalkene structures are preferred, with cycloalkane structures being particularly preferred.

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, and particularly preferably 15 or less per alicyclic structure. When the number of carbon atoms constituting the alicyclic structure is within this range, a high level of balance between mechanical strength, heat resistance, and moldability is achieved.

In cyclic olefin polymers, the ratio 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 is usually 100% by weight or less. When the ratio of repeating units having an alicyclic structure to all repeating units is within this range, the polymer has good transparency and heat resistance.

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. Of 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.

Examples of monomers having a norbornene structure include bicyclo[2.2.1]hept-2-ene (common name: norbornene), tricyclo[4.3.0.1 ^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 (e.g., those having a substituent on the ring). Examples of the substituent include alkyl groups, alkylene groups, and polar groups. These substituents may be the same or different, and multiple substituents may be bonded to the ring. One type of monomer having a norbornene structure may be used alone, or two or more types may be used in combination.

Types of polar groups include, for example, heteroatoms and atomic groups containing heteroatoms. Examples of heteroatoms include oxygen atoms, nitrogen atoms, sulfur atoms, silicon atoms, and halogen atoms. Specific examples of polar groups include carboxyl groups, carbonyloxycarbonyl groups, epoxy groups, hydroxyl groups, oxy groups, ester groups, silanol groups, silyl groups, amino groups, nitrile groups, and sulfonic acid groups.

Examples of monomers capable of ring-opening copolymerization with monomers 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; and the like. One type of monomer capable of ring-opening copolymerization with monomers having a norbornene structure may be used alone, or two or more types may be used in combination.

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.

In addition copolymers 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. Of these, ethylene is preferred. One type of α-olefin may be used alone, or two or more types may be used in combination.

Addition polymers of monomers having a norbornene structure can be produced, for example, by polymerizing or copolymerizing the monomers in the presence of an addition polymerization catalyst.

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

Trade names for norbornene-based polymers include, for example, "ZEONOR" and "ZEONEX" manufactured by Zeon Corporation; "ARTON" manufactured by JSR Corporation; and "APEL" manufactured by Mitsui Chemicals, Inc.

A single norbornene polymer may be used alone, or two or more types may be used in combination.

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

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 using GPC, the weight average molecular weight is measured as a relative molecular weight, for example, in terms of polyisoprene or polystyrene.

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

Resin (1) may contain optional components other than the polymer. Examples of optional components that may be contained in resin (1) include colorants such as pigments and dyes; plasticizers; fluorescent brighteners; dispersants; heat stabilizers; light stabilizers; UV 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.

The glass transition temperature Tg of 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. If the glass transition temperature Tg of resin (1) is at or above the lower limit of the above range, the durability of the first retardation layer in high-temperature environments can be improved. Furthermore, if it is at or below the upper limit, the stretching process to obtain the first retardation layer can be carried out smoothly.

The glass transition temperatures of resin (1) and the vinyl biphenyl resins described below can be measured using a differential scanning calorimeter (e.g., the DSC6220 manufactured by SII Nanotechnology Inc.) in accordance with JIS K6911 at a heating rate of 10°C/min.

<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.

The second retardation layer has a thickness direction retardation of typically -150 nm or more, preferably -140 nm or more, more preferably -130 nm or more, and typically -50 nm or less, preferably -60 nm or less, more preferably -70 nm or less.

When the in-plane retardation and thickness direction retardation of the second retardation layer are within the above ranges, the display contrast of a liquid crystal display device equipped with a multilayer retardation film can be made particularly excellent.

The second retardation layer has an NZ coefficient NZ of preferably −0.5 or more, more preferably more than −0.5, even more preferably −0.4 or more, still more preferably −0.3 or more, and preferably 0.0 or less, more preferably −0.1 or less.
When the NZ coefficient NZ of the second retardation layer is within the above range, the display contrast of a liquid crystal display device including the multilayer retardation film can be made particularly excellent.

(Orientation angle of second retardation layer)
When the multilayer retardation film is long, the orientation angle of the second retardation layer with respect 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 polarizing plate can be efficiently produced by laminating a long multilayer retardation film and a long linear polarizing film having an absorption axis in the longitudinal direction so that their longitudinal directions are aligned.

(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.

(Material of the second retardation layer)
The second retardation layer contains a resin including a polymer containing a 4-vinylbiphenyl monomer unit. Hereinafter, a polymer containing a 4-vinylbiphenyl monomer unit will also be referred to as a 4-vinylbiphenyl-based polymer. Furthermore, a resin including a polymer containing a 4-vinylbiphenyl monomer unit will also be referred to as a vinylbiphenyl-based resin. The second retardation layer contains a vinylbiphenyl-based resin, and may contain only a vinylbiphenyl-based resin. A vinylbiphenyl-based resin is usually a thermoplastic resin. Furthermore, a vinylbiphenyl-based resin usually has negative intrinsic birefringence.

Vinyl biphenyl resins can be used to form resin layers with a large absolute negative value for thickness direction retardation Rth using common resin layer formation methods, such as coating methods. Typically, when a resin liquid containing a vinyl biphenyl resin is prepared and then coated and dried to form a vinyl biphenyl resin layer, the resulting resin layer can have a large absolute negative thickness direction retardation Rth. Furthermore, the thickness direction retardation Rth thus obtained is not lost when the resulting resin layer is stretched in one direction. Therefore, after stretching, the Rth value can be maintained or even increased to a larger absolute negative value. This property is unique to vinyl biphenyl resins. By utilizing this property, the multilayer retardation film of this embodiment can be manufactured with a reduced number of steps.

The 4-vinylbiphenyl monomer unit represents a repeating unit having a structure formed by polymerizing 4-vinylbiphenyl represented by formula (1), and specifically represents a repeating unit represented by formula (2). Typically, 4-vinylbiphenyl monomer units are formed by polymerization of 4-vinylbiphenyl, but the 4-vinylbiphenyl monomer unit also includes repeating units formed by other formation methods.

The proportion of 4-vinylbiphenyl monomer units contained in 100% by weight of the 4-vinylbiphenyl 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. When the amount of 4-vinylbiphenyl monomer units is within this range, the birefringence expression of the vinylbiphenyl resin can be effectively enhanced, making it possible to thin the thickness of the second retardation layer to a level that satisfies processability, and achieving particularly excellent display contrast in liquid crystal display devices. Typically, the proportion of 4-vinylbiphenyl monomer units contained in 100% by weight of the 4-vinylbiphenyl polymer corresponds to the ratio (feed ratio) of the 4-vinylbiphenyl to 100% by weight of all monomers used in the polymerization of the 4-vinylbiphenyl polymer.

The 4-vinylbiphenyl polymer may contain any monomer unit other than the 4-vinylbiphenyl monomer unit. The arbitrary monomer unit refers to a monomer unit having a structure formed by polymerizing any monomer other than 4-vinylbiphenyl. The optional monomer may be a monomer compound polymerizable with 4-vinylbiphenyl (e.g., radically polymerizable), such as (meth)acrylic acid ester monomers such as methyl acrylate and methyl methacrylate; diene compound monomers such as butadiene and isoprene; and maleimide monomers such as N-phenylmaleimide. One type of optional monomer may be used alone, or two or more types may be used in combination.

The 4-vinylbiphenyl polymer may be used alone or in combination of two or more types.

The weight-average molecular weight Mw of the 4-vinylbiphenyl polymer 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 preferably 500,000 or less, more preferably 300,000 or less, and particularly preferably 200,000 or less. When the weight-average molecular weight is within this range, the mechanical strength and solubility in solvents of the 4-vinylbiphenyl polymer are highly balanced.

The amount of 4-vinylbiphenyl polymer is preferably 50% by weight or more, more preferably 70% by weight or more, and even more preferably 90% by weight or more, relative to 100% by weight of the vinylbiphenyl resin, and is usually 100% by weight or less. When the amount of 4-vinylbiphenyl polymer is within this range, the birefringence of the vinylbiphenyl resin is effectively enhanced, resulting in particularly excellent display contrast in liquid crystal display devices.

The vinylbiphenyl resin may contain optional components other than the 4-vinylbiphenyl polymer in combination with the 4-vinylbiphenyl polymer. Examples of optional components include polymers other than the 4-vinylbiphenyl polymer and the same examples 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.

The glass transition temperature Tg of the vinyl biphenyl resin 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. The glass transition temperature Tg of the vinyl biphenyl resin may be higher than the glass transition temperature of resin (1). When the glass transition temperature Tg of the vinyl biphenyl resin is within the above range, the birefringence expression of the vinyl biphenyl resin can be effectively enhanced, resulting in particularly excellent display contrast in liquid crystal display devices. In addition, alignment relaxation of the second retardation layer can usually be reduced.

When a multilayer retardation film is manufactured by a manufacturing method including the first, second, and third steps described below, from the viewpoint of smoothly adjusting the optical properties of the first and second retardation layers 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 vinyl biphenyl resin 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 vinyl biphenyl resin is preferably 50°C or less, more preferably 40°C or less, and particularly preferably 30°C or less.

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 optimize the thickness of the second retardation layer during manufacturing, effectively improving the surface condition of the second retardation layer and effectively reducing color unevenness. The birefringence Δn usually represents the magnitude of the refractive index anisotropy in the in-plane direction.

The parameter Rth/d of the second retardation layer is preferably in the range of -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 retardation in the thickness direction (Rth) by the thickness d, and typically 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.

1.4. Optional Layer
The multilayer retardation film may contain any 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 any 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.

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 perspective of making the multilayer retardation film thin. The thinner the lower limit of the thin film thickness, the better, and it can be, for example, 0.1 μm.

An optional layer may or may not be provided between the first retardation layer and the second retardation layer. It is preferable that the second retardation layer is directly attached to the first retardation layer.

<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 of the liquid crystal display device can be particularly excellent.

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 respective longitudinal directions are aligned, the angle between the slow axis of the multilayer retardation film and the transmission axis of the linear polarizer can easily be set to 0° or nearly 0°.

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

<2. Method for producing multilayer retardation film>
The multilayer retardation film can be produced by any method. The multilayer retardation film may be produced by producing the first retardation layer and the second retardation layer as separate films and laminating these films, but the multilayer retardation film can be produced by a method that preferably includes the following first step, second step, and third step in this order.

The first step is to prepare a long resin layer (A) containing a resin having a positive intrinsic birefringence and having an orientation angle in the range of 90°±10° with respect to the longitudinal direction.
The second step is a step of forming a resin layer (B) containing a polymer containing a 4-vinylbiphenyl monomer unit on the resin layer (A) to obtain a multilayer film.
The third step is a step of stretching the multilayer film in a stretching direction forming an angle of 0° to 5° with respect to the longitudinal direction to obtain a long multilayer retardation film including the first retardation layer and the second retardation layer.

A manufacturing method that includes the first to third steps in this order makes it 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 specified range, allowing for the simple manufacture of a multilayer retardation film.

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. In addition, 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.

<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).

Examples of resins with 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. Resins with positive intrinsic birefringence contained in the resin layer (A) are preferably resins containing cyclic olefin polymers. 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) above.

The orientation angle of the long resin layer (A) relative to the longitudinal direction is in the range of 90°±10°. The orientation angle of the resin layer (A) relative 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°.

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

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

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, which is a long resin film containing a resin with 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.

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 parallel to the orientation angle of the resin layer (A).

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

The stretching ratio in the stretching to obtain 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 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 resin layer (A) that has positive intrinsic birefringence. Stretching can usually be performed using a tenter stretching machine while continuously transporting the unstretched film in the longitudinal direction.

<2.2. Second process>
The resin layer (B) formed in the second step is usually long, since it is formed on the long resin layer (A).
The resin layer (B) contains a resin containing a polymer containing a 4-vinylbiphenyl monomer unit, and is usually formed from a resin containing a polymer containing a 4-vinylbiphenyl monomer unit. Therefore, the resin layer (B) that is usually formed may contain only a resin containing a polymer containing a 4-vinylbiphenyl monomer unit.

The resin layer (B) can be stretched together with the resin layer (A) in the third step to form the second retardation layer. Therefore, when a multilayer retardation film is produced using the production method of this embodiment, the resin containing a polymer containing a 4-vinylbiphenyl monomer unit contained in the second retardation layer can be the same resin as the resin contained in the resin layer (B). Examples and preferred examples of the resin contained in the resin layer (B) include the same examples and preferred examples as those listed as the 4-vinylbiphenyl-based resin contained in the second retardation layer.

When a resin layer (B) is formed using a layer formation method such as a coating method, vinylbiphenyl resins containing 4-vinylbiphenyl polymers can increase the refractive index nz in the thickness direction of the resin layer (B). Therefore, the parameter Rth/d of the resin layer (B) can have a large absolute negative value, and the retardation in the thickness direction can also have a large absolute negative value.

By stretching in the third step, the thickness direction retardation Rth of the resin layer (B) is maintained at a negative value, and typically the absolute value of this 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 of this embodiment is that a second retardation layer with a negative thickness direction retardation Rth can be obtained by the simple process of forming and stretching the resin layer (B).

Since the orientation angle of the second retardation layer can be easily controlled within the desired range by stretching in the third step, it is preferable that the resin layer (B) have an in-plane retardation of 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.

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). When the resin layer (B) contains a resin containing a polymer containing a 4-vinylbiphenyl monomer unit, and is formed from a resin containing a polymer containing a 4-vinylbiphenyl monomer unit, 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, but may be, for example, −150 nm or more, for example, −140 nm or more, or for example, −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, even more preferably 60 nm or more, and preferably 150 nm or less, more preferably 140 nm or less, even more preferably 130 nm or less.

The thickness of the resin layer (B) may be appropriately set so that the second phase difference layer obtained by stretching the resin layer (B) exhibits 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, even more preferably 10 μm or less, and is preferably 3 μm or more, more preferably 5 μm or more.

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

The formation of the resin layer (B) in the second step is preferably carried out by
(2-1) applying a resin liquid containing a polymer containing 4-vinylbiphenyl monomer units 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.

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

Examples of organic solvents used in the resin liquid include cyclopentanone, methyl ethyl ketone, and toluene. Furthermore, one type of organic solvent may be used alone, or two or more types may be used in combination.

Examples of methods for applying 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.

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, vacuum drying, or vacuum heat drying.

<2.3. Third step>
In the third step, the multilayer film having 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).

The stretching of the multilayer film in the third step is usually carried out in only one direction that forms an angle of 0° or more and 5° or less with respect to the longitudinal direction of the multilayer film. The stretching direction may also 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).

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 the resin layer (A) after the second step 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 appearing in a direction perpendicular or nearly 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 _1-2 formed by the slow axis direction of the second retardation layer with respect to the slow axis direction of the first retardation layer can be usually 10° or less, usually 0° or more.

The stretching ratio in the third step may be set appropriately 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, and particularly preferably 1.20 times or more, and is preferably 2.00 times or less, more preferably 1.80 times or less, and particularly preferably 1.60 times or less. When the stretching ratio in the third step is at least the lower limit of the above range, the occurrence of wrinkles can be suppressed. Furthermore, when it is at most the upper limit, the direction of the slow axis can be easily controlled.

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 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 the resin contained in resin layer (A) that has positive intrinsic birefringence.

Furthermore, the stretching temperature in the third step is preferably TgB - 50°C or higher, more preferably TgB - 40°C or higher, and particularly preferably TgB - 30°C or higher, and is preferably TgB + 30°C or lower, more preferably TgB + 25°C or lower, and particularly preferably TgB + 20°C or lower. Here, TgB represents the glass transition temperature of the polymer containing 4-vinylbiphenyl monomer units contained in resin layer (B).

The stretching in the third step is preferably performed by free uniaxial stretching. Here, free uniaxial stretching refers to stretching in a certain direction without applying a 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 edges of the multilayer film in the width direction. By performing free uniaxial stretching in the third step, the slow axis directions of the first retardation layer and the second retardation layer can be easily controlled.

The stretching in the third step described above can be carried out using, for example, a tenter stretching machine or a roll stretching machine, with a roll stretching machine being preferred. 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 the long multilayer film in the longitudinal direction. As a roll stretching machine, for example, one described in WO 2016/047465 can be used.

2.4. Optional steps
The manufacturing method of the multilayer retardation film may further include any 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).

<3. Uses of 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.

The multilayer retardation film can be combined with a polarizer to form a polarizing plate, which can be incorporated into a liquid crystal display device.
As a polarizer to be combined with a multilayer retardation film, 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 can be used. 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.

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 it in a boric acid bath; and a film obtained by adsorbing iodine or a dichroic dye onto a polyvinyl alcohol film, stretching it, and further modifying some of the polyvinyl alcohol units in the molecular chain to polyvinylene units. Of these, a polarizer containing polyvinyl alcohol is preferred as the linear polarizer.

In the stretching process for producing a polarizer, the unstretched film is typically stretched in the longitudinal direction. Therefore, the resulting polarizer may exhibit an absorption axis parallel to the longitudinal direction of the polarizer. Furthermore, the absorption axis and transmission axis of the polarizer are generally perpendicular when viewed in the thickness direction. A polarizer that can absorb linearly polarized light with a vibration direction parallel to the absorption axis is preferred, and one with a particularly high degree of polarization is particularly preferred. The thickness of the polarizer is typically 5 μm to 80 μm, but is not limited to this.

Since a polarizer is usually a flexible film, it may be a laminate provided with a protective film from the viewpoint of improving handling properties 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.

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 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°. If the angle between the slow axis of the first retardation layer and the transmission axis of the polarizer is within this 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 made particularly excellent.

Furthermore, 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 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 this range, when a polarizing plate including a multilayer retardation film and a polarizer is incorporated into a liquid crystal display device, the display contrast in the liquid crystal display device can be made particularly excellent.

Here, the angular relationship between the slow axes of the first and second retardation layers in the polarizing plate and the transmission axis of the polarizer is defined as a shift in one direction being positive and a shift in the other direction being negative, and these positive and negative directions are defined commonly for the first and second retardation layers and polarizer, which are components of the polarizing plate.

A liquid crystal display device typically comprises a first polarizer arranged on the viewing side, a liquid crystal cell, a second polarizer, and a light source, in that order. The multilayer retardation film can be arranged between the first polarizer and the liquid crystal cell, or between the second polarizer and the liquid crystal cell, or both.

The transmission axis of the first polarizer and the transmission axis of the second polarizer can usually be arranged to be perpendicular to each other.

Examples of display modes for liquid crystal cells 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 IPS mode being preferred.

In IPS mode, liquid crystal compounds are arranged parallel to the surfaces of the substrates that make up the liquid crystal cell. By applying an electric field to the substrates and rotating the orientation of the liquid crystal compounds in a plane parallel to the substrates, the light passing through the liquid crystal cell is adjusted.

The present invention will now be described in more detail with reference to the following examples. However, the present invention is not limited to the examples shown below, and can be modified and implemented as desired without departing from the scope of the claims of the present invention and their equivalents.

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

(Retardation, NZ coefficient)
The retardation was measured at a temperature of 23° C. using a phase difference meter (Axometrics "AxoScan").
When determining the physical properties of each layer of an inseparable composite, the sample was measured from multiple directions and calculated by fitting analysis using the included multi-layer analysis software.

(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 (Axometrics'"AxoScan"). The direction of the slow axis relative to the longitudinal direction of the long multilayer film or multilayer retardation film was determined as the orientation angle.

(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.

(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 at 0°) of 45° relative to the display surface, and the contrast was calculated from the ratio (white display state luminance/black display state luminance).

(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."

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

(1-1-2. Stretching of pre-stretched film)
The pre-stretched film was pulled out from the roll and continuously fed to a tenter stretching machine. The pre-stretched film was then 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 using this tenter stretching machine, 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 around a roll and recovered.

(1-2. Second step: Formation of resin layer (B))
A liquid composition was prepared as a resin liquid containing poly(4-vinylbiphenyl) (Sigma-Aldrich, MW = 115,000, glass transition temperature 138°C) as a resin with 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%.
The stretched film serving as the resin layer (A) was pulled out from the roll, and the resin liquid was applied onto the stretched film to form a layer of the resin liquid. The formed layer of the resin liquid was then 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 in-plane retardation Re of the resulting resin layer (B) was 2 nm, and the thickness direction retardation Rth was −60 nm. The resulting multilayer film was wound onto a roll and collected.

(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 is a co-stretched film of the resin layer (A) and the resin layer (B).

(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 first retardation layer side surface of the multilayer retardation film 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 having 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.

(1-5. Fabrication 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 with no retardation.

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 rectangular polarizing plate obtained in (1-4) was then bonded to the surface of this liquid crystal cell 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 elements was adjusted so that the transmission axis of the linear polarizing film was perpendicular to the transmission axis of the rear-side polarizer (forming an angle of 90°). In this way, 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.

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

The contrast of the resulting LCD device was measured using the method described above, and the presence or absence of color unevenness was confirmed.

<Comparative Example 1>
As a resin with negative intrinsic birefringence, polystyrene (Sigma-Aldrich, weight average molecular weight Mw = 280,000, glass transition point 100 ° C.) was prepared. An evaluation test was performed in which this polystyrene film was stretched 1.3 times at 135 ° C. to confirm the birefringence Δn exhibited by the stretching, which was 0.00031. Therefore, in order to obtain a second retardation layer having an in-plane retardation Re of about 120 nm similar to that of Example 1, it was confirmed that it is required to form a second retardation layer with a thickness of 387.1 μm.

Therefore, we attempted to manufacture a multilayer film and a multilayer retardation film using the same method as in Example 1, except that a resin liquid containing polystyrene was used instead of poly(4-vinylbiphenyl) as the resin liquid, and the coating thickness of the resin liquid was changed so that a second retardation layer with a thickness of 387.1 μm was obtained after stretching.

However, because the required coating thickness was too thick, it was not possible to form a polystyrene layer of the desired thickness, making it impossible to produce a multi-layer film.

<Comparative Example 2>
As a resin with negative intrinsic birefringence, poly(4-methylstyrene) (Sigma-Aldrich, weight average molecular weight Mw = 72,000, glass transition point 106 ° C.) was prepared. An evaluation test was conducted in which a film of this poly(4-methylstyrene) was stretched 1.3 times at 135 ° C. to confirm the birefringence Δn exhibited by the stretching, which was 0.00037. Therefore, in order to obtain a second retardation layer having an in-plane retardation Re of about 120 nm similar to that of Example 1, it was confirmed that a second retardation layer with a thickness of 324.3 μm was required.

Therefore, we attempted to manufacture a multilayer film and a multilayer retardation film using the same method as in Example 1, except that a resin liquid containing poly(4-methylstyrene) was used instead of poly(4-vinylbiphenyl) and the coating thickness of the resin liquid was changed so that a second retardation layer with a thickness of 324.3 μm was obtained after stretching.

However, because the required coating thickness was too thick, it was not possible to form a poly(4-methylstyrene) layer with the desired thickness, making it impossible to produce a multilayer film.

<Comparative Example 3>
As a resin with negative intrinsic birefringence, poly(4-chlorostyrene) (Sigma-Aldrich, weight average molecular weight Mw = 75,000, glass transition point 106 ° C.) was prepared. An evaluation test was conducted in which this poly(4-chlorostyrene) film was stretched 1.3 times at 135 ° C. to confirm the birefringence Δn exhibited by the stretching, which was 0.00071. Therefore, in order to obtain a second retardation layer having an in-plane retardation Re of about 120 nm similar to that in Example 1, it was confirmed that it is required to form a second retardation layer with a thickness of 169.0 μm.

Therefore, we attempted to manufacture a multilayer film and a multilayer retardation film using the same method as in Example 1, except that a resin liquid containing poly(4-chlorostyrene) was used instead of poly(4-vinylbiphenyl) and the coating thickness of the resin liquid was changed so that a second retardation layer with a thickness of 169.0 μm was obtained after stretching.

However, because the required coating thickness was too thick, it was not possible to form a poly(4-chlorostyrene) layer with the desired thickness, making it impossible to produce a multilayer film.

<Comparative Example 4>
As a resin with negative intrinsic birefringence, a styrene-maleic anhydride copolymer (Polyscope "XIRAN3500", styrene: maleic anhydride = 3:1, weight average molecular weight Mw = 80,000, glass transition point 130 ° C.) was prepared. An evaluation test was conducted in which a film of this copolymer was stretched 1.3 times at 135 ° C. to confirm the birefringence Δn developed by the stretching, which was 0.00016. Therefore, in order to obtain a second retardation layer having an in-plane retardation Re of about 120 nm similar to that of Example 1, it was confirmed that it is required to form a second retardation layer with a thickness of 750.0 μm.

Therefore, we attempted to manufacture a multilayer film and a multilayer retardation film using the same method as in Example 1, except that a resin liquid containing a styrene-maleic anhydride copolymer was used instead of poly(4-vinylbiphenyl) as the resin liquid, and the coating thickness of the resin liquid was changed so that a second retardation layer with a thickness of 750.0 μm was obtained after stretching.

However, because the required coating thickness was too thick, it was not possible to form a layer of styrene-maleic anhydride copolymer with the desired thickness, making it impossible to produce a multilayer film.

Comparative Example 5
As a resin with negative intrinsic birefringence, poly(2-vinylnaphthalene) (Sigma-Aldrich, weight average molecular weight Mw = 175,000, glass transition point 135 ° C.) was prepared. A resin solution containing poly(2-vinylnaphthalene) was used instead of poly(4-vinylbiphenyl) as the resin solution, and the thickness of the resin layer (B) in the second step was changed to 24.1 μm by changing the coating conditions. A multilayer film and a multilayer retardation film were obtained and evaluated in the same manner as in Example 1.
The obtained multilayer film had a stretched film as the resin layer (A) and a layer of poly(2-vinylnaphthalene) as the resin layer (B). The obtained resin layer (B) had an in-plane retardation Re of 1 nm and a thickness direction retardation Rth of 0 nm.
In the obtained multilayer retardation film, the first retardation layer had a thickness of 20.0 μm, and the second retardation layer had a thickness of 21.1 μm and a birefringence Δn of 0.0055.

<Results>
The results of the above-mentioned Examples and Comparative Examples are shown in the following table. In the table, the meanings of the abbreviations are as follows:
Orientation angle: the angle formed by the in-plane slow axis direction of each layer with respect to the longitudinal direction of a long multilayer retardation film.
Re: in-plane retardation Rth: retardation in the thickness direction 120 nm×10 ⁻³ /Δn: thickness (μm) of the second retardation layer required to achieve a retardation Re of 120 nm
d: thickness of the second retardation layer Δn: birefringence θ _1-2 : angle formed by the slow axis direction of the second retardation layer with respect to the slow axis direction of the first retardation layer.
*1: Measurement was not possible because a multilayer film could not be produced.
Tg: glass transition temperature Mw: weight average molecular weight

These results demonstrate that the multilayer retardation film of Example 1, which comprises a first retardation layer having a specified in-plane retardation and thickness retardation, and a second retardation layer containing a vinyl biphenyl resin and having a specified in-plane retardation and thickness retardation, provides excellent display contrast in liquid crystal display devices and reduces color unevenness.

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

Claims (6)

a first retardation layer and a second retardation layer;
the first retardation layer has an in-plane retardation of more than 0 nm and 60 nm or less and a thickness direction retardation of 50 nm or more and 150 nm or less,
the second retardation layer has an in-plane retardation of 100 nm or more and 150 nm or less and a thickness direction retardation of −150 nm or more and −50 nm or less,
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, wherein the second retardation layer contains a resin containing a polymer containing a 4-vinylbiphenyl monomer unit. The multilayer retardation film according to claim 1, wherein the first retardation layer contains a resin containing a cyclic olefin polymer. The multilayer retardation film of claim 1, wherein the proportion of 4-vinylbiphenyl monomer units contained in 100% by weight of the polymer containing the 4-vinylbiphenyl monomer units is 70% by weight or more. A method for producing the multilayer retardation film according to any one of claims 1 to 3,
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 polymer containing a 4-vinylbiphenyl monomer unit on the resin layer (A) to obtain a multilayer film;
A third step of stretching the multilayer film in a stretching direction forming an angle of 0° to 5° with respect to the longitudinal direction to obtain a long multilayer retardation film including the first retardation layer and the second retardation layer;
A method for producing a multilayer retardation film, comprising the steps of: The second step
A method for producing a multilayer retardation film according to claim 4, comprising: applying a resin liquid containing a polymer containing the 4-vinylbiphenyl monomer unit and an organic solvent onto the resin layer (A) to form a resin liquid layer; and drying the resin liquid layer. The method for producing a multilayer retardation film according to claim 4, wherein the resin having a positive intrinsic birefringence is a resin containing a cyclic olefin polymer.