JP2024177584A - Method for manufacturing retardation film - Google Patents

Abstract

To provide a manufacturing method capable of easily manufacturing a retardation film capable of obtaining a circular polarization plate capable of suppressing a coloring due to a reflection of external light in both of a front direction and an inclination direction of a display surface.SOLUTION: A manufacturing method of a retardation film having prescribed optical characteristics includes: a first step of preparing a multilayer film including a resin layer (A) formed with a thermoplastic resin A having a positive intrinsic birefringence value and a resin layer (B) formed with a thermoplastic resin B having a negative intrinsic birefringence value; and a second step of drawing the multilayer film by two or more times at different drawing temperatures to obtain a retardation film. A sum of an NZ coefficient NZA of the resin layer (A) of the retardation film and an NZ coefficient NZB of the resin layer (B) is within a prescribed range. An absolute value |TgA-TgB| of a difference between a glass transition temperature TgA of the thermoplastic resin A and a glass transition temperature TgB of the thermoplastic resin B is 5°C or more.SELECTED DRAWING: None

Description

The present invention relates to a method for manufacturing a retardation film, a retardation film manufactured by the manufacturing method, and a circular polarizing plate including the retardation film.

Retardation films may be provided in image display devices such as organic electroluminescence image display devices (hereinafter, occasionally referred to as "organic EL image display devices") and liquid crystal image display devices. Some such retardation films have a multi-layer structure comprising two or more layers. As a method for manufacturing such retardation films having a multi-layer structure, a method using a co-stretching method may be adopted (see Patent Documents 1 to 5).

JP 2012-73646 A JP 2009-192844 A JP 2009-192845 A JP 2009-223163 A JP 2002-40258 A

In some image display devices, a circular polarizing plate is provided to reduce the reflection of external light on the display surface. Such a circular polarizing plate is generally a film that combines a linear polarizer and a λ/4 plate. However, most conventional λ/4 plates can actually only function as a λ/4 plate in a specific narrow wavelength range. Therefore, while most conventional circular polarizing plates can reduce the reflection of external light in a specific narrow wavelength range, it is difficult to reduce the reflection of external light outside of that range. When external light is reflected on the display surface, the display surface may be colored by the color of the reflected light.

In order to suppress the coloring of the display surface as described above, a circular polarizing plate that can reduce the reflection of external light over a wide wavelength range is required. Such a circular polarizing plate can be manufactured, for example, by using a broadband λ/4 plate that can function as a λ/4 plate over a wide wavelength range. As such a broadband λ/4 plate, a retardation film that combines multiple layers is known, for example, a retardation film that combines a λ/2 plate and a λ/4 plate. A circular polarizing plate equipped with a retardation film that can function as a broadband λ/4 plate can reduce the reflection of external light over a wide wavelength range in the front direction perpendicular to the display surface, and therefore can suppress the coloring of the display surface.

However, in tilt directions that are neither parallel nor perpendicular to the display surface, the phase difference value may deviate from the ideal value, or the optical axes of each layer may deviate, making it impossible to reduce the reflection of external light over a wide wavelength range. Therefore, in order to realize a circular polarizing plate that can suppress coloring due to reflection of external light in the tilt direction, the phase difference film is required to have an NZ coefficient in a specific range greater than 0.0 and less than 1.0.

The layers contained in a retardation film that satisfies the above-mentioned requirements usually differ in some or all of their optical properties, such as the direction of the slow axis, the in-plane retardation, and the NZ coefficient. For this reason, the above-mentioned retardation film has generally been manufactured by manufacturing each layer separately and then laminating the layers together. However, such a conventional manufacturing method tends to increase the number of steps, and to increase the effort and cost, since each layer is manufactured separately. In addition, it is required to accurately match the lamination angle when laminating each layer, which requires the effort of adjusting the angle, and this also tends to increase the effort.

Under these circumstances, there is a demand for the development of a method for easily producing a retardation film that can provide a circular polarizing plate that can suppress coloring due to reflection of external light in both the front direction and the inclined direction of the display surface.
The present invention has been devised in view of the above-mentioned problems, and aims to provide a manufacturing method for easily manufacturing a retardation film that can provide a circular polarizing plate that can suppress coloring due to reflection of external light in both the front direction and the inclined direction of the display surface; a retardation film manufactured by the manufacturing method; and a circular polarizing plate including this retardation film.

The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that a retardation film capable of solving the above-mentioned problems can be obtained by a production method including stretching a multilayer film having a resin layer (A) and a resin layer (B) containing a thermoplastic resin A and a thermoplastic resin B having glass transition temperatures TgA and TgB that differ by 5° C. or more at different stretching temperatures two or more times, thereby completing the present invention.
That is, the present invention includes the following.

[1] A method for producing a retardation film that satisfies the following formula (1), the following formula (2), and the following formula (3),
The manufacturing method comprises:
A first step of preparing a multilayer film having a resin layer (A) formed of a thermoplastic resin A having a positive intrinsic birefringence value and a resin layer (B) formed of a thermoplastic resin B having a negative intrinsic birefringence value;
A second step of stretching the multilayer film two or more times to obtain the retardation film including the resin layer (A) having a slow axis and the resin layer (B) having a slow axis substantially perpendicular to the slow axis of the resin layer (A),
The second step comprises:
A first stretching step of stretching the multilayer film at a stretching temperature Ts1;
A second stretching step of stretching the multilayer film at a stretching temperature Ts2 different from the stretching temperature Ts1,
The method for producing a retardation film, wherein the absolute value of the difference between the glass transition temperature TgA of the thermoplastic resin A and the glass transition temperature TgB of the thermoplastic resin B, |TgA-TgB|, is 5°C or more.
100nm≦Re _T (550)≦180nm (1)
Re _T (450)<Re _T (550)<Re _T (650) (2)
0.0< _NZT <1.0 (3)
(however,
Re _T (450) represents the in-plane retardation of the retardation film at a wavelength of 450 nm;
Re _T (550) represents the in-plane retardation of the retardation film at a wavelength of 550 nm;
Re _T (650) represents the in-plane retardation of the retardation film at a wavelength of 650 nm;
_NZT represents the NZ coefficient of the retardation film.
[2] The method for producing a retardation film according to [1], wherein an absolute value of a difference between the stretching temperature Ts1 and the stretching temperature Ts2, |Ts1-Ts2|, is 5°C or more.
[3] The method for producing a retardation film according to [1] or [2], wherein the stretching direction in the first stretching step and the stretching direction in the second stretching step are approximately perpendicular to each other.
[4] The method for producing a retardation film according to any one of [1] to [3], wherein the retardation film satisfies the following formula (4):
{Re _Q (450)/Re _Q (550)}-{Re _H (450)/Re _H (550)}>0.08 (4)
(however,
Re _H (450) represents the in-plane retardation of the high retardation layer at a wavelength of 450 nm;
Re _H (550) represents the in-plane retardation of the high retardation layer at a wavelength of 550 nm;
Re _Q (450) represents the in-plane retardation of the low retardation layer at a wavelength of 450 nm;
Re _Q (550) represents the in-plane retardation of the low retardation layer at a wavelength of 550 nm;
The high retardation layer represents one of the resin layers (A) and (B) included in the retardation film, which has a larger in-plane retardation at a wavelength of 550 nm,
The low retardation layer represents the layer having a smaller in-plane retardation at a wavelength of 550 nm, out of the resin layer (A) and the resin layer (B) included in the retardation film.
[5] At a wavelength of 550 nm, the resin layer (A) of the retardation film has an in-plane retardation larger than that of the resin layer (B) of the retardation film;
The method for producing a retardation film according to any one of [1] to [4], wherein the glass transition temperature TgA of the thermoplastic resin A is lower than the glass transition temperature TgB of the thermoplastic resin B.
[6] At a wavelength of 550 nm, the resin layer (B) of the retardation film has an in-plane retardation larger than that of the resin layer (A) of the retardation film;
The method for producing a retardation film according to any one of [1] to [4], wherein the glass transition temperature TgB of the thermoplastic resin B is lower than the glass transition temperature TgA of the thermoplastic resin A.
[7] A retardation film produced by the method according to any one of [1] to [6].
[8] A circular polarizing plate comprising a linear polarizer and the retardation film according to [7].

The present invention provides a method for easily producing a retardation film that can provide a circular polarizing plate that can suppress coloring due to reflection of external light in both the front direction and the tilted direction of the display surface; a retardation film produced by the method; and a circular polarizing plate that includes this retardation film.

FIG. 1 is a perspective view showing a schematic view of an evaluation model set when calculating color space coordinates in simulations of examples and comparative examples.

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 may be modified as desired without departing from the scope of the claims of the present invention and their equivalents.

In the following description, the in-plane retardation Re indicates a value expressed by Re = (nx - ny) x d, unless otherwise specified. The thickness direction retardation Rth is a value expressed by Rth = {(nx + ny) / 2 - nz} x d, unless otherwise specified. Furthermore, the NZ coefficient NZ represents a value expressed by NZ = Rth / Re + 0.5, unless otherwise specified, and can therefore be expressed as NZ = (nx - nz) / (nx - ny). nx represents the refractive index in the direction perpendicular to the thickness direction (in-plane direction) and giving the maximum refractive index (slow axis direction), ny represents the refractive index in the in-plane direction and perpendicular to the direction of nx, nz represents the refractive index in the thickness direction, and d represents the thickness. The measurement wavelength is 550 nm, unless otherwise specified. The in-plane retardation, thickness direction retardation, and NZ coefficient can be measured using a retardation meter (Axometrics' "AxoScan").

In the following description, the slow axis of a layer refers to the slow axis in the in-plane direction of that layer, unless otherwise specified.

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

In the following description, unless otherwise specified, the front direction of a surface means the normal direction of the surface, specifically the direction of the surface with a polar angle of 0° and an azimuth angle of 0°.

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

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 effect of the present invention, for example, within a range of ±5°.

In the following description, a "long" film refers to a film that is 5 times or more longer than its width, preferably 10 times or more longer, and specifically refers to a film that is long enough to be wound into a roll for storage or transportation. There is no particular upper limit to the length of a long film, and it can be, for example, 100,000 times or less than its width.

In the following description, the longitudinal direction of a long film is usually parallel to the flow direction of the film on the production line. Also, the width direction of a long film is usually perpendicular to the thickness direction and perpendicular to the longitudinal direction.

In the following description, unless otherwise specified, the terms "polarizing plate," "circular polarizing plate," "plate," "lambda/2 plate," and "lambda/4 plate" include not only rigid members, but also flexible members such as resin films.

In the following description, "polymer having positive inherent birefringence" and "resin having positive inherent birefringence" mean "polymer whose refractive index in the stretching direction is greater than that in the direction perpendicular to the stretching direction" and "resin whose refractive index in the stretching direction is greater than that in the direction perpendicular to the stretching direction", respectively. Also, "polymer having negative inherent birefringence" and "resin having negative inherent birefringence" mean "polymer whose refractive index in the stretching direction is smaller than that in the direction perpendicular to the stretching direction" and "resin whose refractive index in the stretching direction is smaller than that in the direction perpendicular to the stretching direction", respectively. The inherent birefringence can be calculated from the dielectric constant distribution.

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

[1. Manufactured Retardation Film]
The retardation film produced by the production method according to one embodiment of the present invention satisfies the following formulas (1), (2), and (3). By combining this retardation film with a linear polarizer, a circular polarizing plate capable of suppressing coloring due to reflection of external light in both the front direction and the tilt direction of the display surface can be obtained.

100nm≦Re _T (550)≦180nm (1)
Re _T (450)<Re _T (550)<Re _T (650) (2)
0.0< _NZT <1.0 (3)
(however,
Re _T (450) represents the in-plane retardation of the retardation film at a wavelength of 450 nm;
Re _T (550) represents the in-plane retardation of the retardation film at a wavelength of 550 nm;
Re _T (650) represents the in-plane retardation of the retardation film at a wavelength of 650 nm;
_NZT represents the NZ coefficient of the retardation film.

The formula (1) will be described in detail. The in-plane retardation Re _T (550) of the retardation film at a wavelength of 550 nm is usually 100 nm or more, preferably 115 nm or more, particularly preferably 125 nm or more, and usually 180 nm or less, preferably 160 nm or less, particularly preferably 150 nm or less. When the in-plane retardation Re _T (550) is in such a range, the retardation film can function as a λ/4 plate. Therefore, by combining the retardation film with a linear polarizer, a circular polarizing plate capable of suppressing reflection of external light can be obtained.

The in-plane retardation Re _T (550) satisfying the formula (1) can be obtained, for example, by appropriately adjusting the in-plane retardation of each layer, such as the resin layer (A) and the resin layer (B), contained in the retardation film, and the direction of the slow axis of each layer.

The formula (2) will be described in detail. The in-plane retardation Re _T (450), Re _{T (550) and Re T (} 650) of the retardation film at wavelengths of 450 nm, 550 nm and 650 nm satisfy Re _T (450) < Re _T (550) < Re _T (650). The in-plane retardation of the retardation film satisfying this formula (2) usually exhibits reverse wavelength dispersion. Specifically, the retardation film usually has a larger in-plane retardation as the measured wavelength is longer. Therefore, this retardation film can function as a broadband λ / 4 plate that can uniformly convert the polarization state of light passing through the retardation film in a wide wavelength range. Therefore, by combining the retardation film with a linear polarizer, a circular polarizing plate that can suppress coloring due to reflection of external light can be obtained.

The in-plane retardation values Re _T (450), Re _T (550) and Re _T (650) satisfying the formula (2) can be obtained, for example, by appropriately adjusting the in-plane retardation values of each layer, such as the resin layer (A) and the resin layer (B), contained in the retardation film, and the directions of the slow axes of each layer.

The formula (3) will be described in detail. The NZ coefficient NZ _T of the retardation film is usually larger than 0.0, preferably larger than 0.2, particularly preferably larger than 0.3, and usually less than 1.0, preferably less than 0.8, particularly preferably less than 0.7. When the retardation film has an NZ coefficient NZ _T in the above range, the retardation film has appropriately adjusted birefringence in both the in-plane direction and the thickness direction. Therefore, the circular polarizing plate obtained by combining the retardation film with a linear polarizer can suppress coloring due to reflection of external light in both the front direction and the tilt direction of the display surface.

Unless otherwise specified, the NZ coefficient _NZT of a retardation film is expressed by " _NZT = { _RthT (550) / ReT (550)} + 0.5" using the in-plane retardation _ReT (550) and thickness direction retardation _RthT (550) of the retardation film at a wavelength of 550 nm. The NZ coefficient _NZT satisfying the formula ( ₃ ) can be obtained, for example, by appropriately adjusting the NZ coefficient of each layer, such as the resin layer (A) and the resin layer (B), contained in the retardation film.

In the manufacturing method according to this embodiment, a retardation film satisfying the above-mentioned formulas (1) to (3) is manufactured as a retardation film having a combination of a resin layer (A) containing a thermoplastic resin A and a resin layer (B) containing a thermoplastic resin B.

The combination of thermoplastic resin A and thermoplastic resin B is selected so that the sign of the intrinsic birefringence value of thermoplastic resin A is different from the sign of the intrinsic birefringence value of thermoplastic resin B. Specifically, a combination of thermoplastic resin A having a positive intrinsic birefringence value and thermoplastic resin B having a negative intrinsic birefringence value is used. When the combination of thermoplastic resin A and thermoplastic resin B is used to carry out the manufacturing method described below, a retardation film satisfying the above-mentioned formulas (1) to (3) can be easily manufactured.

The thermoplastic resin A having a positive intrinsic birefringence value usually contains a polymer having a positive intrinsic birefringence value. Examples of this polymer include 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; polyether sulfone; polysulfone; polyarylsulfone; polyvinyl chloride; alicyclic structure-containing polymers; rod-shaped liquid crystal polymers; and the like. These polymers may be used alone or in combination of two or more types in any ratio. Among them, alicyclic structure-containing polymers, cellulose esters, and polycarbonates are preferred, and alicyclic structure-containing polymers are particularly preferred.

The alicyclic structure-containing polymer is a polymer containing an alicyclic structure in the repeating unit, and is usually an amorphous polymer. As the alicyclic structure-containing polymer, either a polymer containing an alicyclic structure in the main chain or a polymer containing an alicyclic structure in the side chain can be used. Examples of the alicyclic structure include a cycloalkane structure and a cycloalkene structure, and from the viewpoint of thermal stability, a cycloalkane structure is preferred. The number of carbon atoms contained in one alicyclic structure is preferably 4 or more, more preferably 5 or more, and particularly preferably 6 or more, and is preferably 30 or less, more preferably 20 or less, and particularly preferably 15 or less.

In the alicyclic structure-containing polymer, the proportion of repeating units containing an alicyclic structure is preferably 50% by weight or more, more preferably 70% by weight or more, and particularly preferably 90% by weight or more. When the proportion of repeating units containing an alicyclic structure is within the above range, a retardation film with excellent heat resistance can be obtained.

Examples of the polymer containing an alicyclic structure include (1) norbornene-based polymers, (2) monocyclic cyclic olefin polymers, (3) cyclic conjugated diene polymers, (4) vinyl alicyclic hydrocarbon polymers, and hydrogenated products thereof. Among these, cyclic olefin polymers and norbornene-based polymers are preferred, and norbornene-based polymers are particularly preferred. Examples of the norbornene-based polymers include ring-opening polymers of monomers containing a norbornene structure, ring-opening copolymers of monomers containing a norbornene structure and other monomers capable of ring-opening copolymerization, and hydrogenated products thereof; addition polymers of monomers containing a norbornene structure, and addition copolymers of monomers containing a norbornene structure and other monomers capable of copolymerization. Among these, from the viewpoint of transparency, ring-opening polymer hydrogenated products of monomers containing a norbornene structure are particularly preferred. The alicyclic structure-containing polymer may be selected from the polymers disclosed in, for example, JP-A-2002-321302.

Examples of cellulose esters include lower fatty acid esters of cellulose (e.g., cellulose acetate, cellulose acetate butyrate, and cellulose acetate propionate). Lower fatty acids refer to fatty acids having 6 or fewer carbon atoms per molecule. Cellulose acetates can include triacetyl cellulose (TAC) and cellulose diacetate (DAC).

The total acyl group substitution degree of the cellulose ester is preferably 2.20 or more and 2.70 or less, more preferably 2.40 or more and 2.60 or less. Here, the total acyl group can be measured in accordance with ASTM D817-91. The weight average polymerization degree of the cellulose ester is preferably 350 or more and 800 or less, more preferably 370 or more and 600 or less.

Polycarbonates usually have repeating units containing carbonate bonds (-O-C(=O)-O-). Examples of polycarbonates include polymers having structural units derived from dihydroxy compounds and carbonate structures (structures represented by -O-(C=O)-O-). Examples of dihydroxy compounds include bisphenol A. The structural units derived from dihydroxy compounds contained in polycarbonates may be of one type or two or more types.

The weight average molecular weight (Mw) of the polymer contained in the thermoplastic resin A 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 in such a range, the mechanical strength and moldability of the resin layer (A) are highly balanced. The weight average molecular weight is the weight average molecular weight in terms of polyisoprene or polystyrene measured by gel permeation chromatography (GPC) using cyclohexane as a solvent. However, if the sample does not dissolve in cyclohexane, toluene may be used as the GPC solvent.

The molecular weight distribution (weight average molecular weight (Mw)/number average molecular weight (Mn)) of the polymer contained in thermoplastic resin A is preferably 1.2 or more, more preferably 1.5 or more, particularly preferably 1.8 or more, and is preferably 3.5 or less, more preferably 3.0 or less, particularly preferably 2.7 or less. When the molecular weight distribution is equal to or greater than the lower limit of the above range, the productivity of the polymer can be increased and the manufacturing cost can be reduced. When the molecular weight distribution is equal to or less than the upper limit, the amount of low molecular weight components is reduced, thereby suppressing relaxation during exposure to high temperatures and improving the stability of resin layer (A).

The proportion of the polymer in the thermoplastic resin A is preferably 50% by weight to 100% by weight, more preferably 70% by weight to 100% by weight, and particularly preferably 90% by weight to 100% by weight. When the proportion of the polymer is within the above range, the resin layer (A) can have sufficient heat resistance and transparency.

The thermoplastic resin A may further contain optional components in combination with the above-mentioned polymer. Examples of optional components include stabilizers such as antioxidants, heat stabilizers, light stabilizers, weather stabilizers, ultraviolet absorbers, and near-infrared absorbers; plasticizers; and the like. These components may be used alone or in combination of two or more types in any ratio.

The thermoplastic resin B having a negative intrinsic birefringence value usually contains a polymer having a negative intrinsic birefringence value. Examples of this polymer include homopolymers of styrene or styrene derivatives, and polystyrene-based polymers including copolymers of styrene or styrene derivatives with any monomer; polyacrylonitrile polymers; polymethyl methacrylate polymers; poly(2-vinylnaphthalene); or multicomponent copolymers thereof. Examples of optional monomers that can be copolymerized with styrene or styrene derivatives include acrylonitrile, maleic anhydride, methyl methacrylate, and butadiene. These polymers may be used alone or in combination of two or more in any ratio.

Among the polymers having a negative intrinsic birefringence value, polystyrene-based polymers are preferred from the viewpoint of high retardation expression. Among them, copolymers of styrene or styrene derivatives and maleic anhydride are particularly preferred from the viewpoint of high heat resistance. In this copolymer, the amount of maleic anhydride units is preferably 5 parts by weight or more, more preferably 10 parts by weight or more, particularly preferably 15 parts by weight or more, and preferably 30 parts by weight or less, more preferably 28 parts by weight or less, particularly preferably 26 parts by weight or less, per 100 parts by weight of polystyrene-based polymer. Maleic anhydride units refer to structural units having a structure formed by polymerizing maleic anhydride.

The proportion of the polymer in the thermoplastic resin B is preferably 50% by weight to 100% by weight, more preferably 70% by weight to 100% by weight, and particularly preferably 90% by weight to 100% by weight. When the proportion of the polymer is within the above range, the resin layer (B) can exhibit appropriate optical properties.

Thermoplastic resin B may further contain optional components in combination with the above-mentioned polymer. Examples of optional components include the same examples as the optional components that may be contained in thermoplastic resin A. One type of optional component may be used alone, or two or more types may be used in combination in any ratio.

The combination of the thermoplastic resin A contained in the resin layer (A) and the thermoplastic resin B contained in the resin layer (B) is selected so that the absolute value of the difference between the glass transition temperature TgA of the thermoplastic resin A and the glass transition temperature TgB of the thermoplastic resin B, |TgA-TgB|, is within a predetermined range. Specifically, the absolute value of the difference, |TgA-TgB|, is usually 5°C or higher, more preferably 10°C or higher, and particularly preferably 13°C or higher. When a combination of thermoplastic resins having glass transition temperatures different from each other to the above degree is used as the thermoplastic resin A and the thermoplastic resin B, and the manufacturing method described below is performed, a retardation film satisfying the above-mentioned formulas (1) to (3) can be easily manufactured. There is no particular limit to the upper limit of the absolute value of the difference, |TgA-TgB|, and from the viewpoint of smoothly stretching the multilayer film, it is preferably 40°C or lower, more preferably 30°C or lower, and particularly preferably 25°C or lower.

The specific values of the glass transition temperature TgA of thermoplastic resin A and the glass transition temperature TgB of thermoplastic resin B are arbitrary within the range in which the absolute value of the difference between them, |TgA-TgB|, satisfies the above-mentioned requirements. For example, the lower temperature Tg(low) of the glass transition temperatures TgA and TgB is preferably 100°C or higher, more preferably 105°C or higher, particularly preferably 110°C or higher, and preferably 140°C or lower, more preferably 135°C or lower, and particularly preferably 130°C or lower. In addition, the higher temperature Tg(high) of the glass transition temperatures TgA and TgB is preferably 120°C or higher, more preferably 125°C or higher, particularly preferably 130°C or higher, and preferably 160°C or lower, more preferably 155°C or lower, and particularly preferably 150°C or lower. When using thermoplastic resin A and thermoplastic resin B having such glass transition temperatures TgA and TgB, the production of a retardation film can be particularly easily performed.

In particular, when the resin layer (A) of the retardation film has a larger in-plane retardation than the resin layer (B) of the retardation film at a wavelength of 550 nm, the glass transition temperature TgA of the thermoplastic resin A is preferably lower than the glass transition temperature TgB of the thermoplastic resin B. With such a combination of thermoplastic resin A and thermoplastic resin B, it is particularly easy to manufacture a retardation film having a resin layer (A) with a relatively large in-plane retardation and a resin layer (B) with a relatively small in-plane retardation.

In addition, when the resin layer (B) of the retardation film has a larger in-plane retardation at a wavelength of 550 nm than the resin layer (A) of the retardation film, it is preferable that the glass transition temperature TgB of the thermoplastic resin B is lower than the glass transition temperature TgA of the thermoplastic resin A. With such a combination of thermoplastic resin A and thermoplastic resin B, it is particularly easy to manufacture a retardation film having a resin layer (A) with a relatively small in-plane retardation and a resin layer (B) with a relatively large in-plane retardation.

The glass transition temperature Tg can be measured using a differential scanning calorimeter (D.S.C. DSC6220SII manufactured by Nano Technology Co., Ltd.) at a heating rate of 10°C/min according to JIS K 6911.

The resin layer (A) of the retardation film has a slow axis. The resin layer (B) of the retardation film has a slow axis that is approximately perpendicular to the slow axis of the resin layer (A). The slow axis of the resin layer (B) is "approximately perpendicular" to the slow axis of the resin layer (A) means that the angle between the slow axis of the resin layer (A) and the slow axis of the resin layer (B) is in a specific range close to 90°. Specifically, the angle between the slow axis of the resin layer (A) and the slow axis of the resin layer (B) is usually 85° or more, preferably 87° or more, more preferably 88° or more, and particularly preferably 89° or more, and is usually 95° or less, preferably 93° or less, more preferably 92° or less, and particularly preferably 91° or less. In the manufacturing method described below, a retardation film that satisfies the above-mentioned formulas (1) to (3) can be obtained as a multilayer film having a combination of resin layer (A) and resin layer (B) that have the slow axes in the above-mentioned relationship.

In particular, in a long retardation film, it is preferable that one of the slow axis of the resin layer (A) and the slow axis of the resin layer (B) forms an angle of a specific range close to 45° with respect to the width direction of the retardation film. Specifically, the angle is preferably 40° or more, more preferably 42° or more, even more preferably 43° or more, particularly preferably 44° or more, and preferably 50° or less, more preferably 48° or less, even more preferably 47° or less, and particularly preferably 46° or less. Furthermore, in this case, it is preferable that the other of the slow axis of the resin layer (A) and the slow axis of the resin layer (B) forms an angle of a specific range close to 135° with respect to the width direction of the retardation film. Specifically, the angle is preferably 130° or more, more preferably 132° or more, even more preferably 133° or more, particularly preferably 134° or more, and preferably 140° or less, more preferably 138° or less, even more preferably 137° or less, and particularly preferably 136° or less. A typical long linear polarizer has an absorption axis parallel or perpendicular to the width direction of the linear polarizer. A long retardation film having a resin layer (A) and a resin layer (B) having a slow axis in a direction that forms an angle within the above range with respect to the width direction can be simply laminated to the typical linear polarizer with the width direction of the retardation film parallel to the width direction of the linear polarizer to obtain a circular polarizing plate. Therefore, the retardation film and the linear polarizer can be laminated by roll-to-roll, so that the circular polarizing plate can be manufactured particularly easily.

The NZ coefficient NZ _A of the resin layer (A) and the NZ coefficient NZ _B of the resin layer (B) are preferably set appropriately so that the NZ coefficient NZ _T of the retardation film falls within the formula (3). The NZ coefficient NZ _A of the resin layer (A) is a value represented by "NZ _A = (nx _A - _{nz A} ) / (nx _A - ny _A )", and is therefore represented by "NZ _A = (Rth _A / Re _A ) + 0.5". nx _A represents the refractive index in the in-plane direction of the resin layer (A) that gives the maximum refractive index. ny _A represents the refractive index in the in-plane direction of the resin layer (A) that is perpendicular to the direction that gives nx _A. nz _A represents the refractive index in the thickness direction of the resin layer (A). Re _A represents the in-plane retardation of the resin layer (A), and is therefore represented by "Re _A = (nx _A - ny _A ) x d _A ". Rth _A represents the thickness direction retardation of the resin layer (A), and is therefore represented by "Rth _A = {(nx _A + ny _A ) / 2 - nz _A } x d _A ". d _A represents the thickness of the resin layer (A). In addition, the NZ coefficient NZ _B of the resin layer (B) is a value represented by "NZ _B = (nx _B - nz _B ) / (nx _B - ny _B )", and is therefore represented by "NZ _B = (Rth _B / Re _B ) + 0.5". nx _B represents the refractive index in the in-plane direction of the resin layer (B) that gives the maximum refractive index. ny _B represents the refractive index in the in-plane direction of the resin layer (B) and perpendicular to the direction giving nx _B. nz _B represents the refractive index in the thickness direction of the resin layer (B). Re _B represents the in-plane retardation of the resin layer (B), and is therefore represented by "Re _B = (nx _B - ny _B ) x d _B ". Rth _B represents the retardation in the thickness direction of the resin layer (B), and is therefore represented by "Rth _B = {(nx _B + ny _B ) / 2 - nz _B } x d _B ". d _B represents the thickness of the resin layer (B).

The NZ coefficient _{NZ_A} of the resin layer (A) is preferably 1.00 or more. Therefore, the refractive indexes _{ny_A} and _{nz_A} of the resin layer (A) preferably satisfy the relationship _{ny_A} ≧ _{nz_A} . The NZ coefficient _{NZ_B} of the resin layer (B) is preferably less than 0.0. Therefore, the refractive indexes _{nx_B} and _{nz_B} of the resin layer (B) preferably satisfy the relationship _{nz_B} > _{nx_B} . According to such a combination of the resin layer (A) and the resin layer (B), the NZ coefficient _{NZ_T} of the entire retardation film can be easily adjusted within the range of formula (3).

More specifically, the NZ coefficient NZ _A of the resin layer (A) is preferably 1.00 or more, more preferably 1.05 or more, and preferably 1.30 or less, more preferably 1.20 or less. The refractive indices nx _A , ny _A , and nz _A of the resin layer (A) having an NZ coefficient NZ _A of 1.00 or more can have a relationship of nx _A > ny _A ≧ nz _A. Thus, the resin layer (A) can function as a positive A plate or a negative B plate.

The NZ coefficient NZ _B of the resin layer (B) is preferably -2.0 or more, more preferably -1.5 or more, and preferably less than 0.0, more preferably -0.2 or less, particularly preferably -0.4 or less. The refractive indices nx _B , ny _B and nz _B of the resin layer (B) having an NZ coefficient of less than 0.0 in this way can have a relationship of nz _B > nx _B > ny _B. Thus, the resin layer (B) can be a layer in which the refractive indices nx _B , ny _B and nz _B in the three directions are different (i.e., a layer having biaxiality). The resin layer (B) can also function as a positive B plate.

The sum NZ A +NZ _B of the NZ _coefficient NZ _A of the resin layer (A) and the NZ coefficient NZ _B of the resin layer (B) is preferably within a specific range. Specifically, the sum "NZ _A +NZ _B " is preferably -0.3 or more, more preferably 0.0 or more, particularly preferably 0.15 or more, and preferably 0.8 or less, more preferably 0.75 or less, particularly preferably 0.65 or less. When the sum "NZ _A +NZ _B " is within the above range, a retardation film can be realized that can obtain a circular polarizing plate that can effectively suppress coloring due to reflection of external light in both the front direction and the tilt direction of the display surface.

It is preferable that the retardation film satisfies the following formula (4).
{Re _Q (450)/Re _Q (550)}-{Re _H (450)/Re _H (550)}>0.08 (4)
(however,
Re _H (450) represents the in-plane retardation of the high retardation layer at a wavelength of 450 nm;
Re _H (550) represents the in-plane retardation of the high retardation layer at a wavelength of 550 nm;
Re _Q (450) represents the in-plane retardation of the low retardation layer at a wavelength of 450 nm;
Re _Q (550) represents the in-plane retardation of the low retardation layer at a wavelength of 550 nm;
The high retardation layer refers to the layer having a larger in-plane retardation at a wavelength of 550 nm out of the resin layer (A) and the resin layer (B) included in the retardation film,
The low retardation layer refers to the layer having a smaller in-plane retardation at a wavelength of 550 nm out of the resin layer (A) and the resin layer (B) included in the retardation film.

In formula (4), "Re _Q (450)/Re _Q (550)" represents the wavelength dispersion of the low retardation layer. In formula (4), "Re _H (450)/Re _H (550)" represents the wavelength dispersion of the high retardation layer. Therefore, formula (4) represents that there is a difference in the wavelength dispersion of the in-plane retardation between the low retardation layer and the high retardation layer of the retardation film. More specifically, it represents that the in-plane retardation of the low retardation layer has a larger wavelength dispersion than the in-plane retardation of the high retardation layer. The parameter "Re _Q (450)/Re _Q (550)-Re _H (450)/Re _H (550)" representing the difference in wavelength dispersion is preferably greater than 0.08, more preferably greater than 0.09, particularly preferably greater than 0.10, and the upper limit is not particularly limited, and is preferably less than 2.0, more preferably less than 1.5, particularly preferably less than 1.2. In this way, a retardation film having a resin layer (A) and a resin layer (B) having different wavelength dispersion of in-plane retardation can easily obtain reverse dispersion characteristics in a laminated state.

"Re _Q (450)/Re _Q (550)" is preferably 1.05 or more, more preferably 1.08 or more, particularly preferably 1.1 or more, and there is no particular upper limit, but it is preferably 2.0 or less, more preferably 1.7 or less, particularly preferably 1.5 or less. When "Re _Q (450)/Re _Q (550)" is in the above range, a circularly polarizing plate that can effectively suppress coloring due to reflection of external light can be obtained.

" _ReH (450)/ _ReH (550)" is preferably 0.92 or more, more preferably 0.95 or more, particularly preferably 0.98 or more, and is preferably 1.2 or less, more preferably 1.1 or less, particularly preferably 1.05 or less. When " _ReH (450)/ _ReH (550)" is in the above range, a circularly polarizing plate that can effectively suppress coloring due to reflection of external light can be obtained.

The in-plane retardation Re _Q (550) of the low retardation layer at a wavelength of 550 nm is preferably 80 nm or more, more preferably 100 nm, particularly preferably 110 nm or more, and preferably 170 nm or less, more preferably 150 nm or less, particularly preferably 140 nm or less. When the in-plane retardation Re _Q (550) is within the above range, the low retardation layer can function as a λ/4 plate. And, according to the retardation film having the low retardation layer, a circular polarizing plate that can effectively suppress coloring due to reflection of external light can be obtained.

The in-plane retardation Re _H (550) of the high retardation layer at a wavelength of 550 nm is preferably 220 nm or more, more preferably 240 nm, particularly preferably 250 nm or more, and preferably 310 nm or less, more preferably 290 nm or less, particularly preferably 280 nm or less. When the in-plane retardation Re _H (550) is within the above range, the high retardation layer can function as a λ/2 plate. And, according to the retardation film having the high retardation layer, a circular polarizing plate that can effectively suppress coloring due to reflection of external light can be obtained.

The difference between the in-plane retardation Re _H (550) and the in-plane retardation Re _Q (550), "Re _H (550) - Re _Q (550)", is preferably 100 nm or more, more preferably 110 nm or more, and preferably 180 nm or less, more preferably 160 nm or less. When "Re _H (550) - Re _Q (550)" is within the above range, a circularly polarizing plate that can effectively suppress coloring due to reflection of external light can be obtained.

The retardation film may have any layer other than the resin layer (A) and the resin layer (B) as necessary. An example of the optional layer is an optional layer having optical isotropy. The optional layer having optical isotropy usually has an in-plane retardation of 10 nm or less at a wavelength of 550 nm, and examples of the optional layer include a protective film layer for protecting the resin layer (A) and the resin layer (B); an adhesive layer for bonding each layer such as the resin layer (A) and the resin layer (B); and the like. Another example of the optional layer is an optional layer having optical anisotropy. There is no restriction on the optical properties of the optional layer having optical anisotropy as long as the entire retardation film satisfies the formulas (1) to (3). As a specific example, the optical properties of the optional layer may be set so that the combination of the optional layer having optical anisotropy and one or both of the resin layer (A) and the resin layer (B) can function as a λ/4 plate or a λ/2 plate.

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

The haze of the retardation film is preferably 5% or less, more preferably 3% or less, particularly preferably 1% or less, and ideally 0%. Haze can be measured using a haze meter in accordance with JIS K7361-1997.

The retardation film may be a sheet of film or a long film.

There are no particular limitations on the thickness of the retardation film. From the viewpoint of thinning, the specific thickness of the retardation film is preferably 5 μm or more, more preferably 10 μm or more, and particularly preferably 15 μm or more, and is preferably 200 μm or less, more preferably 150 μm or less, and particularly preferably 100 μm or less.

There are no particular limitations on the thickness of each layer, such as the resin layer (A) and the resin layer (B), that the retardation film has. The thickness of the resin layer (A) and the resin layer (B) is preferably 0.5 μm or more, more preferably 1 μm or more, and is preferably 150 μm or less, more preferably 100 μm or less, independently of each other.

[2. Overview of the method for producing a retardation film]
In a manufacturing method according to one embodiment of the present invention, the above-mentioned retardation film is
A first step of preparing a multilayer film having a resin layer (A) formed from a thermoplastic resin A and a resin layer (B) formed from a thermoplastic resin B;
A second step of stretching the multilayer film two or more times;
The composition is produced by a production method including the steps of:

The resin layer (A) and the resin layer (B) of the multilayer film prepared in the first step have optical properties different from the optical properties of the resin layer (A) and the resin layer (B) of the retardation film. Specifically, the resin layer (A) and the resin layer (B) of the multilayer film usually do not have a large optical anisotropy. Therefore, the resin layer (A) and the resin layer (B) of the multilayer film usually do not have a retardation, or if they do, the value is small. In addition, the resin layer (A) and the resin layer (B) of the multilayer film usually do not have a slow axis, or if they do, the slow axis is not approximately vertical.

Therefore, in the manufacturing method according to this embodiment, the multilayer film is subjected to a second step including two or more stretchings. The stretching in the second step adjusts the optical properties of the resin layer (A) and the resin layer (B), and a retardation film having the desired optical properties described above is obtained. For example, the stretching in the second step causes the resin layer (A) and the resin layer (B) to have large optical anisotropy, and a retardation film satisfying formulas (1), (2), and (3) is obtained. In addition, the stretching in the second step causes the resin layer (A) and the resin layer (B) to have slow axes perpendicular to each other. If the resin layer (A) and the resin layer (B) of the multilayer film have slow axes, the directions of the slow axes are adjusted by the stretching in the second step, and the slow axes become approximately perpendicular.

[3. First step: Preparation of multi-layer film]
In the first step, a multilayer film is prepared, which includes a resin layer (A) formed from a thermoplastic resin A and a resin layer (B) formed from a thermoplastic resin B. The thermoplastic resin A and the thermoplastic resin B are as described in the section on the retardation film.

Usually, the resin layer (A) and the resin layer (B) of the multilayer film prepared in the first step do not have a large optical anisotropy. Therefore, the phase difference of the resin layer (A) and the resin layer (B) is usually small. To give a specific range, the in-plane phase difference of the resin layer (A) and the resin layer (B) at 550 nm is preferably 0 nm to 20 nm, more preferably 0 nm to 10 nm, and particularly preferably 0 nm to 5 nm, respectively and independently.

The multilayer film may be a sheet film, but is preferably a long film. By preparing the multilayer film as a long film, some or all of the steps in the manufacture of the retardation film can be carried out in-line, making the manufacture simple and efficient.

There are no limitations to the method for producing a multilayer film. For example, a multilayer film can be produced by a co-extrusion method, a co-casting method, a coating method in which a liquid composition containing the material of one layer is applied onto a certain layer, and the like. Among these, a method in which a liquid composition containing a thermoplastic resin B is applied onto a resin layer (A) and the applied liquid composition is dried as necessary is preferred. The method for producing this multilayer film is described below.

There is no limitation on the manufacturing method of the resin layer (A). The resin layer (A) can be manufactured by, for example, a melt molding method or a solution casting method. Among these, the melt molding method is preferred. Among the melt molding methods, the extrusion molding method, the inflation molding method, or the press molding method is preferred, and the extrusion molding method is particularly preferred. According to these methods, the resin layer (A) can be manufactured as a long film.

After preparing the resin layer (A), a liquid composition containing thermoplastic resin B is applied onto the resin layer (A). The liquid composition may contain thermoplastic resin B and a solvent. The solvent is preferably one that can dissolve or disperse thermoplastic resin B, and is particularly preferably one that can dissolve thermoplastic resin B. The solvent may be one type alone or two or more types may be combined in any ratio. The concentration of thermoplastic resin B in the liquid composition is preferably adjusted so that the viscosity of the liquid composition falls within a range suitable for application, and may be, for example, 1% by weight to 50% by weight.

There are no limitations on the method for applying the liquid composition. Examples of application methods include curtain coating, extrusion coating, roll coating, spin coating, dip coating, bar coating, spray coating, slide coating, print coating, gravure coating, die coating, gap coating, and dipping.

By applying a liquid composition containing thermoplastic resin B, a layer of the liquid composition is formed on resin layer (A). Therefore, if necessary, the layer of the liquid composition can be dried to remove the solvent, forming resin layer (B) on resin layer (A), thereby obtaining a multilayer film. There are no limitations on the drying method, and drying methods such as heat drying and reduced pressure drying can be used.

[4. Second step: Stretching of multilayer film]
After preparing the multilayer film in the first step, a second step is carried out, which includes stretching the multilayer film two or more times.
A first stretching step of stretching the multilayer film at a stretching temperature Ts1;
and a second stretching step of stretching the multilayer film at a stretching temperature Ts2 different from the stretching temperature Ts1, in this order. By performing such stretching two or more times at different temperatures, the above-mentioned retardation film can be obtained.

It is preferable that the absolute value of the difference between the stretching temperature Ts1 in the first stretching step and the stretching temperature Ts2 in the second stretching step, |Ts1-Ts2|, is within a specific range. Specifically, the absolute value of the difference, |Ts1-Ts2|, is preferably 5°C or more, more preferably 8°C or more, particularly preferably 10°C or more, and is preferably 25°C or less, more preferably 22°C or less, particularly preferably 20°C or less. When the absolute value of the difference between the stretching temperature Ts1 and the stretching temperature Ts2, |Ts1-Ts2|, is within the above range, the retardation film can be produced particularly easily.

Of the stretching temperatures Ts1 and Ts2, the lower temperature is preferably in a specific temperature range close to Tg(low). Specifically, this specific temperature range is preferably Tg(low)-5°C or higher, more preferably Tg(low)-3°C or higher, particularly preferably Tg(low)-1°C or higher, and preferably Tg(low)+20°C or lower, more preferably Tg(low)+15°C or lower, particularly preferably Tg(low)+12°C or lower. Tg(low) represents the lower of the glass transition temperatures TgA and TgB. When the lower of the stretching temperatures Ts1 and Ts2 is within the above range, the orientation of the polymer molecules contained in both the resin layer (A) and the resin layer (B) can be effectively promoted to greatly change their refractive indices, and therefore the retardation film can be manufactured particularly easily.

Of the stretching temperatures Ts1 and Ts2, the higher temperature is preferably in a specific temperature range close to Tg(high). Specifically, this specific temperature range is preferably Tg(high)-5°C or higher, more preferably Tg(high)-3°C or higher, particularly preferably Tg(high)-1°C or higher, and preferably Tg(high)+20°C or lower, more preferably Tg(high)+15°C or lower, particularly preferably Tg(high)+12°C or lower. Tg(high) represents the higher of the glass transition temperatures TgA and TgB. When the higher of the stretching temperatures Ts1 and Ts2 is within the above range, the orientation of the polymer molecules progresses significantly in the resin layer formed of the thermoplastic resin with the higher glass transition temperature of the resin layer (A) and the resin layer (B). Therefore, the refractive index can be changed significantly. On the other hand, in the resin layer formed from the thermoplastic resin with a lower glass transition temperature, the stretching temperature is sufficiently higher than the glass transition temperature, so the orientation of the polymer molecules proceeds relatively little or does not proceed at all. Therefore, the refractive index does not change, or if it does change, the amount of change is small. This simplifies the adjustment of the optical properties of the resin layer (A) and the resin layer (B), making it particularly easy to manufacture the retardation film.

In the second step, the stretching may be performed at a high temperature first, or at a low temperature first. Thus, the stretching temperature Ts1 in the first stretching step may be lower than the stretching temperature Ts2 in the second stretching step. Also, the stretching temperature Ts1 in the first stretching step may be higher than the stretching temperature Ts2 in the second stretching step. In particular, from the viewpoint of easily manufacturing the retardation film, it is preferable that the stretching temperature Ts1 in the first stretching step is higher than the stretching temperature Ts2 in the second stretching step.

In the second step, the stretching in the first stretching step and the stretching in the second stretching step are usually performed in different stretching directions. In this case, the stretching in the first stretching step and the stretching in the second stretching step can both be performed as unidirectional stretching. In particular, the stretching in the second step is preferably performed so that the stretching direction in the first stretching step and the stretching direction in the second stretching step are approximately perpendicular. The fact that the two stretching directions are "approximately perpendicular" means that the angle between the stretching directions is in a specific range close to 90°. Specifically, the angle between the stretching directions is preferably 85° or more, more preferably 87° or more, even more preferably 88° or more, particularly preferably 89° or more, and preferably 95° or less, more preferably 93° or less, even more preferably 92° or less, and particularly preferably 91° or less. When the stretching direction in the first stretching step and the stretching direction in the second stretching step are approximately perpendicular, the production of the retardation film can be particularly easily performed.

In particular, when a long multilayer film is stretched in the second step, it is preferable that one of the stretching directions in the first stretching step and the second stretching step forms an angle of a specific range close to 45° with respect to the width direction of the multilayer film. Specifically, the angle is preferably 40° or more, more preferably 42° or more, even more preferably 43° or more, particularly preferably 44° or more, and preferably 50° or less, more preferably 48° or less, even more preferably 47° or less, and particularly preferably 46° or less. Furthermore, in this case, it is preferable that the other of the stretching direction in the first stretching step and the stretching direction in the second stretching step forms an angle of a specific range close to 135° with respect to the width direction of the retardation film. Specifically, the angle is preferably 130° or more, more preferably 132° or more, even more preferably 133° or more, particularly preferably 134° or more, and preferably 140° or less, more preferably 138° or less, even more preferably 137° or less, and particularly preferably 136° or less. When stretching at the above stretching angle, the resulting retardation film can usually be laminated to a long linear polarizer by roll-to-roll to produce a circular polarizing plate, making it particularly easy to produce a circular polarizing plate.

It is preferable that the stretching ratio in the second stretching step is appropriately set so that the desired retardation film is obtained. The stretching ratios in each stretching step may be the same or different. Thus, the stretching ratio in the first stretching step and the stretching ratio in the second stretching step may be the same or different.

In the first stretching step and the second stretching step, the stretching ratio in the stretching step in which stretching is performed at a relatively low stretching temperature is preferably 1.2 times or more, more preferably 1.3 times or more, particularly preferably 1.35 times or more, and is preferably 3.0 times or less, more preferably 2.5 times or less, particularly preferably 2.0 times or less. When the stretching ratio in the stretching step in which stretching is performed at a relatively low stretching temperature is within the above range, the retardation film can be produced particularly easily.

In the first stretching step and the second stretching step, the stretching ratio in the stretching step in which stretching is performed at a relatively high stretching temperature is preferably 1.05 times or more, more preferably 1.1 times or more, particularly preferably 1.12 times or more, and is preferably 2.0 times or less, more preferably 1.5 times or less, particularly preferably 1.3 times or less. When the stretching ratio in the stretching step in which stretching is performed at a relatively high stretching temperature is within the above range, the retardation film can be produced particularly easily.

The second step may include an optional stretching step in which the multilayer film is further stretched in combination with the first stretching step and the second stretching step. The stretching conditions in the optional stretching step may be appropriately set within a range in which the desired retardation film can be obtained.

According to the second step described above, the resin layer (A) and the resin layer (B) of the multilayer film can exhibit the optical properties required for the resin layer (A) and the resin layer (B) of the retardation film. Therefore, the retardation film can be obtained as a stretched multilayer film by the stretching in the second step. The inventors speculate that the mechanism by which the retardation film is obtained by the stretching in the second step is as follows. However, the technical scope of the present invention is not limited by the mechanism described below.

The second step includes drawing at a drawing temperature Ts1 and drawing at a drawing temperature Ts2 different from the drawing temperature Ts1.
By stretching at the lower temperature of the stretching temperatures Ts1 and Ts2, the polymer molecules contained in the resin layer (A) and the resin layer (B) of the multilayer film are oriented in the stretching direction. Due to this orientation, in the resin layer (A) containing the thermoplastic resin A having a positive intrinsic birefringence value, the refractive index in the direction parallel to the stretching direction becomes large, and the refractive index in the direction perpendicular to the stretching direction becomes small. In addition, in the resin layer (B) containing the thermoplastic resin B having a negative intrinsic birefringence value, the refractive index in the direction perpendicular to the stretching direction becomes large, and the refractive index in the direction parallel to the stretching direction becomes small, due to the orientation.

On the other hand, the polymer molecules contained in the resin layer (A) and the resin layer (B) may be oriented in the stretching direction by stretching at the higher of the stretching temperatures Ts1 and Ts2. This orientation may increase the refractive index in the direction parallel to the stretching direction in the resin layer (A) and decrease the refractive index in the direction perpendicular to the stretching direction. Also, the refractive index in the direction perpendicular to the stretching direction in the resin layer (B) may increase and decrease the refractive index in the direction parallel to the stretching direction. However, the degree of orientation may differ from that of stretching at a lower temperature. Specifically, in the resin layer formed of the thermoplastic resin with the higher glass transition temperature of the resin layer (A) and the resin layer (B), the orientation of the polymer molecules proceeds relatively more, but in the resin layer formed of the thermoplastic resin with the lower glass transition temperature, the orientation of the polymer molecules proceeds relatively less or does not proceed at all.

Therefore, by combining stretching at stretching temperature Ts1 and stretching at stretching temperature Ts2, the degree of change in the refractive index and the direction in which the change in the refractive index occurs can be adjusted for each of the resin layers (A) and (B). Usually, adjustments are made so that one of the resin layers (A) and (B) has high uniaxiality and the other of the resin layers (A) and (B) has high biaxiality. As a result of such adjustments, resin layers (A) and (B) are obtained in which the optical properties such as the in-plane retardation, thickness direction retardation, and slow axis direction are within appropriate ranges, and the above-mentioned retardation film is obtained.

5. Optional steps
The method for producing a retardation film may further include any step in combination with the first step and the second step described above. For example, when a long retardation film is obtained using a long multilayer film, the method for producing a retardation film may include a trimming step of cutting the obtained retardation film into a desired shape. According to the trimming step, a sheet of retardation film having a desired shape is obtained. In addition, the method for producing a retardation film may include, for example, a step of providing a protective layer on the retardation film.

[6. Preferred First Example]
Hereinafter, a first preferred example of the manufacturing method according to the above-mentioned embodiment will be described. In this first example, a retardation film is manufactured using a thermoplastic resin A and a thermoplastic resin B having glass transition temperatures TgA and TgB that satisfy TgA<TgB.

In the manufacturing method according to the first example, a first step is performed to prepare a multilayer film having a resin layer (A) formed from the thermoplastic resin A and a resin layer (B) formed from the thermoplastic resin B. Then, a second step is performed, which includes a first stretching step in which the multilayer film is stretched in one direction at a stretching temperature Ts1, and a second stretching step in which the multilayer film is stretched in another direction at a stretching temperature Ts2. In the second step of the manufacturing method according to the first example, the stretching direction in the first stretching step and the stretching direction in the second stretching step are approximately perpendicular.

In this first example, stretching at the lower of the stretching temperatures Ts1 and Ts2 results in a large degree of orientation of the polymer molecules in the stretching direction in both resin layer (A) and resin layer (B) of the multilayer film. As a result, the refractive index in the stretching direction of resin layer (A) becomes large, and the refractive index in the direction perpendicular to the stretching direction becomes small. On the other hand, the refractive index in the stretching direction of resin layer (B) becomes small, and the refractive index in the direction perpendicular to the stretching direction becomes large.

Furthermore, when stretching is performed at the higher of the stretching temperatures Ts1 and Ts2, the orientation of the polymer molecules in the stretching direction progresses significantly in the resin layer (B), but the orientation of the polymer molecules in the stretching direction progresses to a small extent or does not progress at all in the resin layer (A). Therefore, in the resin layer (A), the refractive index does not change, or if it does change, the change is small. On the other hand, in the resin layer (B), the refractive index in the stretching direction becomes small and the refractive index in the direction perpendicular to the stretching direction becomes large.

In the first example, the stretching direction in the first stretching step and the stretching direction in the second stretching step are approximately perpendicular. Therefore, by combining the first stretching step and the second stretching step, the uniaxiality is increased in the resin layer (A) and the biaxiality is increased in the resin layer (B). In this way, a desired retardation film can be obtained as a film having a resin layer (A) with a large uniaxiality and a resin layer (B) with a large biaxiality. The manufacturing method according to this first example can be suitably used for manufacturing a retardation film having a resin layer (A) as a high retardation layer and a resin layer (B) as a low retardation layer.

[7. Preferred Second Example]
Hereinafter, a second preferred example of the manufacturing method according to the above-mentioned embodiment will be described. In this second example, a retardation film is manufactured using a thermoplastic resin A and a thermoplastic resin B having glass transition temperatures TgA and TgB that satisfy TgA>TgB.

In the manufacturing method according to the second example, the first and second steps are performed in the same manner as in the manufacturing method according to the first example. Therefore, in the second step of the manufacturing method according to the second example, the stretching direction in the first stretching step and the stretching direction in the second stretching step are approximately perpendicular.

In this second example, as in the first example, stretching at the lower of the stretching temperatures Ts1 and Ts2 results in a large degree of orientation of the polymer molecules in the stretching direction in both resin layer (A) and resin layer (B) of the multilayer film. As a result, in resin layer (A), the refractive index in the stretching direction increases, and the refractive index in the direction perpendicular to the stretching direction decreases. On the other hand, in resin layer (B), the refractive index in the stretching direction decreases, and the refractive index in the direction perpendicular to the stretching direction increases.

In contrast, when stretching is performed at the higher of the stretching temperatures Ts1 and Ts2, the orientation of the polymer molecules in the stretching direction progresses significantly in the resin layer (A), but the orientation of the polymer molecules in the stretching direction progresses to a small extent or does not progress at all in the resin layer (B). Therefore, in the resin layer (A), the refractive index in the stretching direction increases and the refractive index in the direction perpendicular to the stretching direction decreases. On the other hand, in the resin layer (B), the refractive index does not change, or if it does change, the change is small.

In the second example, the stretching direction in the first stretching step and the stretching direction in the second stretching step are approximately perpendicular. Therefore, by combining the first stretching step and the second stretching step, the biaxiality is increased in the resin layer (A) and the uniaxiality is increased in the resin layer (B). In this way, a desired retardation film can be obtained as a film having a resin layer (A) with a large biaxiality and a resin layer (B) with a large uniaxiality. The manufacturing method according to the second example can be suitably used for manufacturing a retardation film having a resin layer (A) as a low retardation layer and a resin layer (B) as a high retardation layer.

[8. Main advantages of the method for manufacturing retardation film]
According to the above-mentioned manufacturing method, the resin layer (A) and the resin layer (B) of the multilayer film are stretched simultaneously to produce a desired retardation film having the stretched resin layer (A) and the resin layer (B). Therefore, since it is not necessary to prepare the resin layer (A) and the resin layer (B) separately, the number of steps can be reduced, and the labor and cost can be suppressed. In addition, since it is not necessary to bond the resin layer (A) and the resin layer (B), which requires precise adjustment of the bonding angle, the labor can also be suppressed.
By using the retardation film thus obtained, it is possible to manufacture a circularly polarizing plate capable of suppressing coloring due to reflection of external light in both the front direction and the inclined direction of the display surface.

[9. Circularly polarizing plate]
A circular polarizing plate according to one embodiment of the present invention includes a linear polarizer and a retardation film. By providing this circular polarizing plate on the display surface of an image display device, reflection of external light can be suppressed. The circular polarizing plate including the retardation film described above can suppress reflection of external light and effectively suppress coloring in both cases where the display surface is viewed from the front direction and the display surface is viewed from an inclined direction.

The circular polarizing plate may include a linear polarizer, a resin layer (A), and a resin layer (B) in this order. The circular polarizing plate may include a linear polarizer, a resin layer (B), and a resin layer (A) in this order. The specific order may be set according to the in-plane retardation of the resin layer (A) and the resin layer (B).

In the circular polarizing plate, the angle between the width direction of the linear polarizer and one of the slow axes of the resin layer (A) and the slow axis of the resin layer (B) is preferably in a specific range close to 45°. Specifically, the angle is preferably 40° or more, more preferably 42° or more, even more preferably 43° or more, particularly preferably 44° or more, and preferably 50° or less, more preferably 48° or less, even more preferably 47° or less, and particularly preferably 46° or less. Furthermore, in this case, the angle between the width direction of the linear polarizer and the other of the slow axes of the resin layer (A) and the slow axis of the resin layer (B) is preferably in a specific range close to 135°. Specifically, the angle is preferably 130° or more, more preferably 132° or more, even more preferably 133° or more, particularly preferably 134° or more, and preferably 140° or less, more preferably 138° or less, even more preferably 137° or less, and particularly preferably 136° or less.

Any linear polarizer can be used as the linear polarizer. Examples of linear polarizers include a film obtained by adsorbing iodine or a dichroic dye onto a polyvinyl alcohol film, followed by uniaxial stretching 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 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.

When natural light is incident on the linear polarizer, only one polarized light is transmitted through the linear polarizer 130. The degree of polarization of the linear polarizer 130 is not particularly limited, but is preferably 98% or more, and more preferably 99% or more.
The thickness of the linear polarizer is preferably 5 μm to 80 μm.

The circular polarizing plate described above may further include an optional layer. Examples of the optional layer include a polarizer protective film layer; an adhesive layer for bonding the linear polarizer and the retardation film; a hard coat layer such as an impact-resistant polymethacrylate resin layer; a matte layer for improving the slipperiness of the film; a reflection suppressing layer; an antifouling layer; an antistatic layer; and the like. Only one of these optional layers may be provided, or two or more layers may be provided.

[10. Image display device]
The above-mentioned circular polarizing plate can be provided in an image display device. In particular, it is preferable that the circular polarizing plate is provided in an organic EL image display device. This organic EL image display device includes a circular polarizing plate and an organic electroluminescence element (hereinafter, sometimes referred to as "organic EL element" as appropriate). This organic EL image display device usually includes a linear polarizer, a retardation film, and an organic EL element in this order.

The organic EL element includes a transparent electrode layer, a light-emitting layer, and an electrode layer in this order, and the light-emitting layer can emit light when a voltage is applied from the transparent electrode layer and the electrode layer. Examples of materials constituting the organic light-emitting layer include polyparaphenylenevinylene-based, polyfluorene-based, and polyvinylcarbazole-based materials. The light-emitting layer may also have a laminate of layers with different light-emitting colors, or a mixed layer in which a dye layer is doped with a different dye. Furthermore, the organic EL element may have functional layers such as a hole injection layer, a hole transport layer, an electron injection layer, an electron transport layer, an equipotential surface forming layer, and a charge generation layer.

The image display device can suppress reflection of external light on the display surface. Specifically, only a portion of the linearly polarized light incident from outside the device passes through the linear polarizer, and then passes through the retardation film to become circularly polarized light. The circularly polarized light is reflected by a component that reflects light in the image display device (such as a reflective electrode in an organic EL element) and passes through the retardation film again to become linearly polarized light with a vibration direction perpendicular to the vibration direction of the incident linearly polarized light, and does not pass through the linear polarizer. Here, the vibration direction of linearly polarized light means the vibration direction of the electric field of the linearly polarized light. This achieves the function of suppressing reflection.

Because the retardation film has the optical properties described above, the organic EL image display device can exert its anti-reflection function not only in the front direction of the display surface, but also in the inclined direction. This makes it possible to effectively suppress the reflection of external light and suppress coloring in both the front direction and the inclined direction of the display surface.

The degree of coloring can be evaluated by the color difference ΔE ^* ab between the chromaticity measured by observing a reflective display surface and the chromaticity of a non-reflective black display surface. The chromaticity can be obtained by measuring the spectrum of light reflected by the display surface, multiplying the spectrum by the spectral sensitivity (color matching function) corresponding to the human eye to obtain tristimulus values X, Y, and Z, and calculating the chromaticity (a ^* , b ^* , L ^* ). The color difference ΔE ^* ab can be obtained from the chromaticity (a0 ^* , b0 ^* , L0 ^* ) when the display surface is not illuminated by external light and the chromaticity (a1 ^* , b1 ^* , L1 ^* ) when it is illuminated by external light, according to the following formula (X).

In addition, generally, the coloring of the display surface due to reflected light may vary depending on the azimuth angle of the observation direction. Therefore, when observed from the inclined direction of the display surface, the measured chromaticity may vary depending on the azimuth angle of the observation direction, and the color difference ΔE ^* ab may also vary. Therefore, in order to evaluate the degree of coloring when observed from the inclined direction of the display surface as described above, it is preferable to evaluate the coloring by the average value of the color differences ΔE ^* ab obtained by observing from multiple azimuth angles. Specifically, the color difference ΔE ^* ab is measured at 5° intervals in the azimuth angle direction in the range of azimuth angles φ (see FIG. 1) of 0° or more and less than 360°, and the degree of coloring is evaluated by the average value (average color difference) of the measured color differences ΔE ^* ab. The smaller the average color difference, the smaller the coloring of the display surface when observed from the inclined direction of the display surface.

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 modified and carried out as desired without departing from the scope of the claims of the present invention and the scope of equivalents thereto.
In the following description, the units "%" and "parts" are by weight unless otherwise specified. Furthermore, the operations described below were carried out at room temperature and pressure unless otherwise specified.

[Evaluation method]
(Method of measuring phase difference and NZ coefficient)
Using a retardation meter (Axometrics'"AxoScan"), the in-plane retardation Re at wavelengths of 450 nm, 550 nm, and 650 nm of the evaluation objects (retardation film, resin layer (A) and resin layer (B)); and the thickness direction retardation Rth at a wavelength of 550 nm were measured. When determining the physical properties of each layer of the resin layer (A) and the resin layer (B), the sample was measured from multiple directions and calculated by fitting analysis using the attached multi-layer analysis software. In addition, the NZ coefficient was calculated using the in-plane retardation Re and thickness direction retardation Rth at a wavelength of 550 nm.

(Method of Measuring Orientation Angle)
The directions of the slow axes of the resin layers (A) and (B) included in the retardation film were measured using a retardation meter (Axometrics'"AxoScan"). The angle between the slow axis and the width direction of the retardation film was calculated as the orientation angle θ (0°≦θ<180°).

(Calculation method of color difference by simulation)
The circularly polarizing plates manufactured in each of the Examples and Comparative Examples were modeled using "LCD Master" manufactured by Shintech Co., Ltd. as simulation software, and the following calculations were performed.

In the simulation model, a circular polarizing plate was provided on the reflection surface of a mirror having a flat reflection surface. In Examples 1 to 4 and Comparative Example 1, the circular polarizing plate was set to have a low retardation layer, a high retardation layer, and a linear polarizer in this order from the reflection surface side. In Comparative Example 2, the circular polarizing plate was set to have a norbornene resin layer, a styrene-maleic anhydride copolymer resin layer, a norbornene resin layer, and a linear polarizer in this order from the reflection surface side. The low retardation layer, high retardation layer, norbornene resin layer, and styrene-maleic anhydride copolymer resin layer obtained in each Example and Comparative Example were set. In addition, a commonly used polarizing plate with a polarization degree of 99.99% was set as the linear polarizer. In addition, an ideal mirror that can mirror-reflect incident light with a reflectance of 100% was set as the mirror.

FIG. 1 is a perspective view that illustrates an evaluation model that is set when calculating color space coordinates in simulations of the examples and comparative examples.
As shown in Fig. 1, the color space coordinates observed on the reflective surface 10 of a mirror provided with a circular polarizer were calculated when illuminated by a D65 light source (not shown). The color space coordinates when not illuminated by the light source were set as a0 ^* = 0, b0 ^* = 0, and L0 ^* = 0. Then, the color difference ΔE ^* ab was calculated from (i) the color space coordinates when illuminated by the light source and (ii) the color space coordinates when not illuminated by the light source.

The color difference ΔE ^* ab was calculated in an observation direction 20 where the polar angle ρ with respect to the reflecting surface 10 is 0°, to obtain the color difference ΔE ^* ab in the front direction. The polar angle ρ represents the angle with respect to the normal direction 11 of the reflecting surface 10.

The color difference ΔE ^* ab was calculated in an observation direction 20 with a polar angle ρ of 60° relative to the reflecting surface 10. The calculation at the polar angle ρ=60° was performed multiple times by moving the observation direction 20 in the azimuth direction and the azimuth angle φ in increments of 5° within a range of 0° or more and less than 360°. The azimuth angle φ represents the angle that a direction parallel to the reflecting surface 10 makes with a certain reference direction 12 parallel to the reflecting surface 10. The color difference ΔE ^* ab in the calculated multiple observation directions 20 was then averaged to obtain the color difference ΔE ^* ab in the tilt direction with a polar angle ρ=60°.

(Method of visually evaluating circularly polarizing plates in the front direction)
An image display device (Apple Watch (registered trademark) manufactured by Apple Inc.) equipped with an organic EL image display device was prepared. This image display device was disassembled, and the polarizing plate attached to the surface of the organic EL image display device was peeled off to expose the reflective electrode. The surface of this reflective electrode was attached to the surface of the circular polarizing plate obtained in each of the examples and comparative examples opposite to the linear polarizer via an adhesive (CS9621 manufactured by Nitto Denko Corporation). This resulted in a sample equipped with the reflective electrode, adhesive, and circular polarizing plate in this order.

The circular polarizer sample was exposed to sunlight on a sunny day, and the circular polarizer on the reflective electrode was visually observed. The observation was performed in the front direction of the circular polarizer with a polar angle of 0° and an azimuth angle of 0°. If a chromatic color was visible as a result of the observation, it was judged as "poor", and if a chromatic color was not visible, it was judged as "good".

(Method of Visually Evaluating Circularly Polarizing Plates in an Inclined Direction)
The circular polarizer sample prepared in the above (method of visually evaluating a circular polarizer in the front direction) was illuminated with sunlight on a sunny day, and the circular polarizer on the reflective electrode was visually observed. The observation was performed at a polar angle of 60° and an azimuth angle of 0° to 360°. As a result of the observation, the reflection luminance and coloring were comprehensively judged to be superior or inferior, and the Examples and Comparative Examples were ranked. The ranked Examples and Comparative Examples were then given points corresponding to their ranks (6 points for 1st place, 5 points for 2nd place, ... 1 point for lowest).

The above observations were performed by a large number of people, and the total score of the given points was calculated for each Example and Comparative Example. The Examples and Comparative Examples were ranked in order of the above total score, and within the range of the total score, the top groups were rated in the order of A, B, C, D, and E.
[Example 1]
(1-1. Formation of resin layer (A))
Pellet-shaped norbornene-based resin (manufactured by Zeon Corporation; glass transition temperature 126°C) was dried at 100°C for 5 hours to obtain thermoplastic resin A. This thermoplastic resin A was fed to an extruder, passed through a polymer pipe and a polymer filter, and extruded from a T-die into a sheet on a casting drum. The extruded thermoplastic resin A was cooled to obtain a long resin film having a thickness of 100 μm as resin layer (A). The obtained resin layer (A) was wound into a roll and collected.

(1-2. Formation of Resin Layer (B))
In a dried, nitrogen-substituted pressure reactor, 500 ml of toluene as a solvent and 0.29 mmol of n-butyl lithium as a polymerization catalyst were added, and then 35 g of 2-vinylnaphthalene was added and reacted at 25°C for 1 hour. As a result, a reaction product containing poly(2-vinylnaphthalene) as a homopolymer of 2-vinylnaphthalene was obtained. This reaction product was poured into a large amount of 2-propanol to precipitate poly(2-vinylnaphthalene), which was then separated. The obtained poly(2-vinylnaphthalene) was dried at 200°C for 24 hours using a vacuum dryer to obtain a thermoplastic resin B. The weight average molecular weight of poly(2-vinylnaphthalene) measured by GPC was 250,000. The glass transition temperature of poly(2-vinylnaphthalene) measured by a differential scanning calorimeter was 142°C.

Poly(2-vinylnaphthalene) and 1,3-dioxolane were mixed to obtain a liquid composition containing thermoplastic resin B. The concentration of poly(2-vinylnaphthalene) in this liquid composition was 15% by weight.

The resin layer (A) was pulled out from the roll, and the liquid composition was applied onto the pulled out resin layer (A). The applied liquid composition was then dried to form a layer (thickness 12 μm) of poly(2-vinylnaphthalene) as resin layer (B) on the resin layer (A). This resulted in a multilayer film having a resin layer (A) formed of a norbornene-based resin and a resin layer (B) formed of poly(2-vinylnaphthalene). The obtained multilayer film was wound into a roll and collected.

[1-3. First stretching process]
The multilayer film was pulled out from the roll and fed to a longitudinal tenter stretching machine. The tenter stretching machine was used to stretch the multilayer film at an angle of 135° with respect to the width direction of the multilayer film. The multilayer film was stretched in the stretching direction at a stretching temperature of 140° C. and a stretching ratio of 1.10. The stretched multilayer film was cooled to room temperature, and then wound up in a roll and collected.

[1-4. Second stretching process]
The multilayer film stretched in the first stretching step was pulled out from the roll, and the pulled out multilayer film was supplied to a tenter stretching machine. The film was stretched at a stretching temperature of 128° C. and a stretching ratio of 1.50 in a stretching direction forming an angle of 45° with respect to the film, to obtain a long retardation film. The retardation film thus obtained had a resin layer (A) and a resin layer (B) as a low retardation layer. The optical properties (in-plane retardation, thickness direction retardation, etc.) of each layer contained in the retardation film were The retardation, the direction of the slow axis, and the NZ coefficient were measured by the method described above.

[1-5. Production of circularly polarizing plate]
A long polyvinyl alcohol resin film dyed with iodine was prepared. This film was stretched in a longitudinal direction at an angle of 90° to the width direction of the film to obtain a linear polarizer as a long polarizing film. This linear polarizer had an absorption axis in the longitudinal direction of the linear polarizer and a transmission axis in the width direction of the linear polarizer.

The linear polarizer and the retardation film were bonded together via an optically isotropic adhesive ("CS9621" manufactured by Nitto Denko Corporation) to obtain a long circularly polarizing plate. The bonding was performed with the width direction of the linear polarizer and the width direction of the retardation film parallel to each other. The obtained circularly polarizing plate had a linear polarizer, a resin layer (A) as a high retardation layer, and a resin layer (B) as a low retardation layer in this order.
The obtained circularly polarizing plate was evaluated by the method described above.

[Examples 2 to 4]
In the step (1-1), the extrusion conditions of the extruder were adjusted to change the thickness of the long resin layer (A) as shown in Table 1.
In the step (1-2), the thickness of the resin layer (B) was changed as shown in Table 1 by adjusting the coating amount of the liquid composition.
Furthermore, in the step (1-3), the stretching ratio was changed as shown in Table 1.
Except for the above, the retardation film and the circularly polarizing plate were produced and evaluated in the same manner as in Example 1.

[Comparative Example 1]
In the step (1-1), the extrusion conditions of the extruder were adjusted to change the thickness of the long resin layer (A) as shown in Table 1.
In the step (1-2), the thickness of the resin layer (B) was changed as shown in Table 1 by adjusting the coating amount of the liquid composition.
Furthermore, in the step (1-3), the stretching temperature and the stretching ratio were changed as shown in Table 1.
Except for the above, the retardation film and the circularly polarizing plate were produced and evaluated in the same manner as in Example 1.

[Comparative Example 2]
A retardation film was produced by the same method as in Example 5 of JP-A No. 2002-40258.

Specifically, a norbornene-based resin (manufactured by Zeon Corporation, glass transition temperature 136°C) was prepared as thermoplastic resin A having a positive intrinsic birefringence value. A styrene-based maleic anhydride copolymer resin (Dylark D332, manufactured by Nova Chemical, glass transition temperature 130°C) was prepared as thermoplastic resin B having a negative intrinsic birefringence value. These resins were previously dried under nitrogen purging to reduce the moisture content.

A film forming device ("LABO PLASTOMILI" manufactured by Toyo Seiki) capable of co-extruding thermoplastic resin A and thermoplastic resin B to produce a multilayer film having a layer structure of "layer of thermoplastic resin A / layer of thermoplastic resin B / layer of thermoplastic resin A" was prepared. The norbornene-based resin and styrene-maleic anhydride copolymer resin were supplied to this film forming device, and co-extrusion was performed to obtain a multilayer film (thickness 186 μm) having a three-layer structure of "layer of norbornene-based resin / layer of styrene-maleic anhydride copolymer resin / layer of norbornene-based resin".

This multilayer film was stretched at a stretching temperature of 120°C and a stretching ratio of 1.65 times in a stretching direction at an angle of 45° to the width direction of the multilayer film using a tenter stretching machine to obtain a long retardation film. The obtained retardation film had a three-layer structure of "norbornene-based resin layer (thickness 36 μm) / styrene-maleic anhydride copolymer resin layer (thickness 39 μm) / norbornene-based resin layer (thickness 38 μm)". The optical properties (in-plane retardation, thickness direction retardation, direction of slow axis, NZ coefficient) of the obtained retardation film and each layer contained in the retardation film were measured by the above-mentioned method. Since the retardation film obtained in Comparative Example 2 had two layers of norbornene-based resin as thermoplastic resin A, the total values of the two layers are listed in the columns of in-plane retardation, thickness direction retardation, and NZ coefficient of the resin layer (A) in Table 2 described later.

This retardation film and the same long linear polarizer as used in Example 1 were bonded together via an optically isotropic adhesive ("CS9621" manufactured by Nitto Denko Corporation) to obtain a long circular polarizing plate. The bonding was performed with the width direction of the linear polarizer and the width direction of the retardation film parallel to each other. The obtained circular polarizing plate had a linear polarizer, a layer of a norbornene-based resin, a layer of a styrene-maleic anhydride copolymer resin, and a layer of a norbornene-based resin in this order.
The obtained circularly polarizing plate was evaluated by the method described above.

[result]
The manufacturing conditions of the retardation films in the above-mentioned Examples and Comparative Examples are shown in Table 1, and the evaluation results are shown in Table 2. In the following Tables 1 and 2, the meanings of the abbreviations are as follows.
"COP": Norbornene-based resin.
"VN": poly(2-vinylnaphthalene).
"PSt": styrene-based maleic anhydride copolymer resin.
"Positive" in the column for intrinsic birefringence value: the intrinsic birefringence value is positive.
"Negative" in the intrinsic birefringence value column: the intrinsic birefringence value is negative.
"Stretching angle": the angle that the stretching direction makes with the width direction of the multilayer film.
"Orientation angle θ": the angle that the slow axis makes with respect to the width direction of the retardation film.

In the above-mentioned examples and comparative examples, the visual evaluation in the front direction is judged in two stages, "good" and "poor," whereas the visual evaluation in the inclined direction is judged in five stages, from "A" to "E." The reason for the different judgment criteria is as follows.
In other words, the brightness of the reflected light is sufficiently lower in the front direction than in the tilt direction. Therefore, if there is no strong coloring, the observer cannot recognize the coloring. Therefore, a two-stage judgment is adopted based on whether the coloring can be seen or not.
On the other hand, the brightness of the reflected light is high in the tilted direction. Therefore, even if the color is weak, the observer can easily recognize the color. Therefore, a detailed evaluation of superiority and inferiority on a five-point scale was adopted.

[Consider]
In Examples 1 to 4, the first stretching step is performed at a temperature sufficiently higher than the glass transition temperature TgA of the thermoplastic resin A. Therefore, the orientation of the polymer molecules contained in the resin layer (A) progresses little in the first stretching step and progresses much in the second stretching step. Therefore, the resin layer (A) of the retardation film has a high uniaxiality and a slow axis parallel to the stretching direction in the second stretching step.

On the other hand, the orientation of the polymer molecules contained in the resin layer (B) containing the thermoplastic resin B having a glass transition temperature TgB higher than the glass transition temperature TgA of the thermoplastic resin A progresses significantly in both the first and second stretching steps. Therefore, the resin layer (B) of the retardation film has high biaxiality. In addition, in Examples 1 to 4, the second stretching step is stretched at a higher stretch ratio than the first stretching step, so the resin layer (B) of the retardation film has a slow axis perpendicular to the stretching direction in the second stretching step.

The retardation film including these resin layers (A) and (B) satisfies formulas (1) to (3) as a whole, as shown in Table 2. Therefore, a circular polarizing plate including this retardation film can suppress coloring due to reflection of external light in both the front direction and the tilted direction of the display surface of an image display device including the circular polarizing plate.

10 Reflecting surface 11 Normal direction of reflecting surface 12 Reference direction 20 Observation direction φ Azimuth angle ρ Polar angle

Claims (9)

A method for producing a retardation film that satisfies the following formula (1), the following formula (2), and the following formula (3),
The manufacturing method comprises:
A first step of preparing a multilayer film having a resin layer (A) formed of a thermoplastic resin A having a positive intrinsic birefringence value and a resin layer (B) formed of a thermoplastic resin B having a negative intrinsic birefringence value;
A second step of stretching the multilayer film two or more times to obtain the retardation film including the resin layer (A) having a slow axis and the resin layer (B) having a slow axis substantially perpendicular to the slow axis of the resin layer (A),
When the NZ coefficient of the resin layer (A) having the slow axis is NZ _A and the NZ coefficient of the resin layer (B) having the slow axis is NZ _B , the sum of the NZ _A and the NZ _B , "NZ _A + NZ _B ", is 0.15 or more and 0.8 or less,
The second step comprises:
A first stretching step of stretching the multilayer film at a stretching temperature Ts1;
A second stretching step of stretching the multilayer film at a stretching temperature Ts2 different from the stretching temperature Ts1,
The method for producing a retardation film, wherein the absolute value of the difference between the glass transition temperature TgA of the thermoplastic resin A and the glass transition temperature TgB of the thermoplastic resin B, |TgA-TgB|, is 5°C or more.
100nm≦Re _T (550)≦180nm (1)
Re _T (450)<Re _T (550)<Re _T (650) (2)
0.0< _NZT <1.0 (3)
(however,
Re _T (450) represents the in-plane retardation of the retardation film at a wavelength of 450 nm;
Re _T (550) represents the in-plane retardation of the retardation film at a wavelength of 550 nm;
Re _T (650) represents the in-plane retardation of the retardation film at a wavelength of 650 nm;
_NZT represents the NZ coefficient of the retardation film. The method for producing a retardation film according to claim 1, wherein the absolute value of the difference between the stretching temperature Ts1 and the stretching temperature Ts2, |Ts1-Ts2|, is 5°C or more. The method for producing a retardation film according to claim 1 or 2, wherein the stretching direction in the first stretching step and the stretching direction in the second stretching step are approximately perpendicular. The method for producing a retardation film according to any one of claims 1 to 3, wherein the retardation film satisfies the following formula (4):
{Re _Q (450)/Re _Q (550)}-{Re _H (450)/Re _H (550)}>0.08 (4)
(however,
Re _H (450) represents the in-plane retardation of the high retardation layer at a wavelength of 450 nm;
Re _H (550) represents the in-plane retardation of the high retardation layer at a wavelength of 550 nm;
Re _Q (450) represents the in-plane retardation of the low retardation layer at a wavelength of 450 nm;
Re _Q (550) represents the in-plane retardation of the low retardation layer at a wavelength of 550 nm;
The high retardation layer represents one of the resin layers (A) and (B) included in the retardation film, which has a larger in-plane retardation at a wavelength of 550 nm,
The low retardation layer represents the layer having a smaller in-plane retardation at a wavelength of 550 nm, out of the resin layer (A) and the resin layer (B) included in the retardation film. At a wavelength of 550 nm, the resin layer (A) of the retardation film has a larger in-plane retardation than the resin layer (B) of the retardation film,
The method for producing a retardation film according to any one of claims 1 to 4, wherein the glass transition temperature TgA of the thermoplastic resin A is lower than the glass transition temperature TgB of the thermoplastic resin B. At a wavelength of 550 nm, the resin layer (B) of the retardation film has a larger in-plane retardation than the resin layer (A) of the retardation film,
The method for producing a retardation film according to any one of claims 1 to 4, wherein the glass transition temperature TgB of the thermoplastic resin B is lower than the glass transition temperature TgA of the thermoplastic resin A. The method for producing a retardation film according to any one of claims 1 to 6, wherein the NZ _A is 1.00 or more and 1.30 or less. The method for producing a retardation film according to any one of claims 1 to 6, wherein the _NZB is -2.0 or more and less than 0.0. The method for producing a retardation film according to any one of claims 1 to 6, wherein the NZ _A is 1.00 or more and 1.30 or less, and the NZ _B is -2.0 or more and less than 0.0.

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