Method for manufacturing retardation film

Abstract

To provide a retardation film which can suppress the occurrence of display unevenness when being applied to an image display device.SOLUTION: A retardation film is a positive B plate in which the refractive index nx in the slow axis direction in the plane, the refractive index ny in the advance phase axis direction in the plane and the refractive index nz in the thickness direction satisfy nz>nx>ny. The thickness of the retardation film is preferably 3-9 μm. The front retardation Re of the retardation film is preferably 12-30 nm. In the retardation film, the ratio X(%) of the range of the thickness to the average thickness in the range of 10 cm×10 cm and the front retardation Re (nm) preferably satisfy 0.5≤X≤4.5-0.11×Re.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention is a retardation filter. Mu's Manufacturing method By law Regarding. [Background technology]

[0002] Retardation films are used in displays such as liquid crystal display devices for the purpose of optical compensation such as improving contrast and widening the viewing angle. For example, Patent Document 1 describes that by combining a retardation film (negative B plate) having a refractive index anisotropy of nx>ny>nz with a retardation film (positive B plate) having a refractive index anisotropy of nz>nx>ny, it is possible to reduce light leakage when an in-plane switching (IPS) liquid crystal display device is viewed from an oblique direction.

[0003] Retardation films used for optical compensation require uniformity in film thickness and optical properties. Therefore, solution casting methods are widely used to produce retardation films, and are particularly suitable for producing thin films. In solution casting methods, a resin solution (dope) in which a polymer is dissolved in a solvent is applied to a support, and the solvent is then removed by heating and drying, etc., to form a laminate in which the coating film is tightly laminated on the support. The coating film is then peeled off from the support and stretched in at least one direction to impart optical anisotropy, thereby obtaining a retardation film. When the coating film (film) is thin, the support may be peeled off after stretching a laminate in which the coating film is tightly laminated on the support, as described in Patent Document 2. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-139747 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-109924 Summary of the Invention [Problem to be solved by the invention]

[0005] As described in Patent Document 1, a polarizing plate formed by laminating a polarizer, a negative B plate, and a positive B plate is suitable for use in optical compensation of IPS-mode liquid crystal display devices. However, in liquid crystal display devices with this configuration, display unevenness may be observed when the screen is viewed from an oblique direction. In recent years, screens have become increasingly high-definition and bright, and even slight unevenness can become a noticeable quality issue.

[0006] The present inventors have investigated the causes of the display unevenness and found that the in-plane variation in the front retardation of the positive B plate (phase difference unevenness) is the cause of the display unevenness.

[0007] In view of the above, an object of the present invention is to provide a retardation film that has small in-plane retardation variations and can suppress the occurrence of display unevenness when applied to an image display device. [Means for solving the problem]

[0008] The retardation film of the present invention is a positive B plate in which the refractive index nx in the in-plane slow axis direction, the refractive index ny in the in-plane fast axis direction, and the refractive index nz in the thickness direction satisfy nz>nx>ny. The thickness of the retardation film is preferably 3 to 9 μm. The in-plane retardation Re of the retardation film is preferably 12 to 30 nm. The thickness direction retardation Rth of the retardation film is preferably -60 to -135 nm.

[0009] The in-plane variation X (%) of the retardation film, defined as the ratio of the thickness range (the difference between the maximum and minimum thickness values) to the average thickness in a 10 cm × 10 cm area, preferably satisfies 0.5≦X≦4.5−0.11×Re, where Re is the in-plane retardation (unit: nm) of the retardation film.

[0010] The amount of the remaining solvent in the retardation film may be 0.5 to 2.0% by weight.

[0011] The retardation film can be produced, for example, by applying a solution containing a polymer having negative intrinsic birefringence and an organic solvent onto a support to form a coating film (coating film forming step), drying and removing the organic solvent by heating to form a laminate in which the coating film is tightly laminated on the support (drying step), and stretching the coating film in at least one direction (stretching step).The support to which the solution is applied may be a resin film.

[0012] The organic solvent in the solution may have a boiling point of 50 to 120° C. The solid concentration of the solution may be 10% by weight or more.

[0013] During drying, it is preferable to carry out first heating at a temperature equal to or lower than the boiling point of the organic solvent in the solution, and then carry out second heating at a temperature 40° C. or higher than the boiling point of the organic solvent. The heating time in the first heating is, for example, 20 to 40 seconds, and the heating time in the second heating is, for example, 10 to 120 seconds.

[0014] When stretching the coating film, the support may be left unpeeled and the coating film may be tightly laminated on the support and stretched. Stretching may be free-end uniaxial stretching.

[0015] A polarizing plate is obtained by laminating a retardation film and a polarizer. The polarizing plate may include, in addition to the polarizer and the retardation film, an optically anisotropic element (negative B plate) having a refractive index anisotropy of nx>ny>nz.

[0016] The liquid crystal display device includes the above polarizing plate on one side of a liquid crystal cell, which may be an IPS liquid crystal cell. [Effects of the Invention]

[0017] When the retardation film of the present invention is applied to an image display device, it can suppress the occurrence of display unevenness. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 2 is a cross-sectional view of a polarizing plate according to an embodiment. [Figure 2] FIG. 1 is a conceptual diagram illustrating the configuration of a liquid crystal panel according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] [Retardation film (positive B plate)] <Characteristics of retardation film> A retardation film according to one embodiment of the present invention is a positive B plate having a refractive index anisotropy of nz>nx>ny, where nx is the in-plane refractive index in the slow axis direction, ny is the in-plane refractive index in the fast axis direction, and nz is the refractive index in the thickness direction.

[0020] To produce a retardation film having refractive index anisotropy in which the refractive index nz in the thickness direction is larger than the refractive index nx in the slow axis direction, a polymer material having negative intrinsic birefringence is preferably used. A polymer having negative intrinsic birefringence refers to a polymer whose refractive index in the orientation direction becomes relatively small when the polymer is oriented by stretching or the like.

[0021] Examples of polymers having negative intrinsic birefringence include those in which chemical bonds or functional groups with large polarization anisotropy, such as aromatic or carbonyl groups, are introduced into the side chains of the polymer, and specific examples include acrylic resins, styrene resins, maleimide resins, and fumarate ester resins.

[0022] The in-plane retardation of the retardation film: Re=(nx-ny)×d is preferably 12 to 30 nm, and may be 15 to 28 nm. The thickness direction retardation of the retardation film: Rth=(nx-nz)×d is preferably −65 to −135 nm, more preferably −70 to −130 nm, and may be −75 to −125 nm. d is the thickness of the retardation film, and nx, ny, and nz are as described above. In this specification, the in-plane retardation and thickness direction retardation are values ​​at a wavelength of 590 nm, unless otherwise specified.

[0023] When Re and Rth are within the above ranges, light leakage in black display tends to be reduced when the liquid crystal display device is viewed obliquely, particularly at an angle of 45 degrees (azimuth angles of 45 degrees, 135 degrees, 225 degrees, and 315 degrees) relative to the absorption axis of the polarizer. In addition, when Re is small, display unevenness tends to be less visible even when the thickness of the retardation film varies greatly.

[0024] The thickness d of the retardation film is preferably 3 to 9 μm. When the thickness of the retardation film is 3 μm or more, a sufficient thickness direction retardation Rth can be realized by the polymer material. When the thickness of the retardation film is 9 μm or less, it is advantageous for thinning.

[0025] To suppress display unevenness, it is preferable that the retardation film has small in-plane thickness variation. The in-plane thickness variation is evaluated based on the ratio of the thickness range (difference between the maximum and minimum values) to the average thickness d of a 10 cm × 10 cm sample. The in-plane variation X is defined as X (%) = 100 × {(maximum thickness - minimum thickness) / average thickness}.

[0026] The in-plane thickness variation X is preferably 3.0% or less, more preferably 2.5% or less, and may be 2.0% or less or 1.7% or less. The smaller X, the more likely it is that display unevenness is suppressed. On the other hand, in order to bring the in-plane thickness variation X closer to 0, it is necessary to reduce the film formation speed (line speed of roll-to-roll transport) and lengthen the drying time, which results in poor productivity. Furthermore, if the line speed is reduced to reduce the thickness variation X, horizontal stripes (stripes extending in a direction perpendicular to the transport direction) are more likely to occur. From the viewpoint of suppressing the occurrence of horizontal stripes, the in-plane thickness variation X of the retardation film is preferably 0.5% or more. X may be 0.55% or more.

[0027] As described above, the smaller the in-plane thickness variation X of the retardation film, the more the display unevenness tends to be suppressed, but the allowable range depends on the front retardation Re of the retardation film. When the front retardation Re is small, the allowable range of the in-plane thickness variation X is large. In other words, the smaller Re is, the less the unevenness is visually recognized even when X is large.

[0028] The in-plane thickness variation X (%) of the retardation film and the front retardation Re (nm) preferably satisfy the relationship X≦4.5−0.11×Re. From the viewpoint of suppressing the occurrence of horizontal stripes and suppressing display unevenness, the in-plane thickness variation X of the retardation film preferably satisfies 0.5≦X≦4.5−0.11×Re. X is more preferably (4.3−0.11×Re) or less, and even more preferably (4.1−0.11×Re) or less.

[0029] <Method of manufacturing retardation film> The method for producing the retardation film of the present invention is not particularly limited, but from the viewpoint of reducing thickness variations to a thickness of 9 μm or less, a method in which a film produced by a solution casting method is stretched is preferred.

[0030] In solution casting, a solution (dope) of the polymer that constitutes the retardation film is applied onto a support to form a coating film, and the organic solvent is dried and removed by heating to form a laminate in which the coating film is tightly laminated on the support. The resulting coating film is peeled off from the support, or, in the state of a laminate in which the coating film and the support are integrated without peeling off the support, the coating film is stretched in at least one direction to impart optical anisotropy to the coating film.

[0031] Since the retardation film of the present invention has a small thickness, the film (coating film) before stretching may be difficult to handle in the state of a single layer peeled from the support. Therefore, a method of stretching a laminate in which the dried coating film is tightly laminated on the support is preferred.

[0032] The dope is a solution of a resin material for forming a retardation film, and contains a resin material (polymer) and an organic solvent. As described above, in the present invention, a polymer having negative intrinsic birefringence is used as the resin material. The dope may contain additives such as a leveling agent, a plasticizer, an ultraviolet absorber, and an anti-degradant, as needed.

[0033] The organic solvent is not particularly limited as long as it dissolves the resin material but not the support, and various solvents commonly used in solution casting can be used. From the viewpoint of appropriately controlling the amount of remaining solvent, the boiling point of the organic solvent is preferably 50 to 120°C, and may be 60 to 100°C. Specific examples of organic solvents include ketones such as acetone (boiling point: 56°C), methyl ethyl ketone (boiling point: 80°C), methyl isopropyl ketone (boiling point: 94°C), diethyl ketone (boiling point: 102°C), methyl isobutyl ketone (boiling point: 116°C), and methyl propyl ketone (boiling point: 102°C). Two or more organic solvents may be mixed. When a mixed solvent is used, the boiling point of the mixed solvent is preferably within the above range.

[0034] From the viewpoint of solvent removal efficiency, the solid content of the dope is preferably 10% by weight or more, and may be 13% by weight or more or 15% by weight or more. The solid content of the dope is generally 50% by weight or less, and may be 40% by weight or less, 35% by weight or less, or 30% by weight or less.

[0035] The support to which the dope is applied includes a glass substrate, a metal substrate such as SUS, a metal drum, a metal belt, a resin film, etc. When the coating film after drying the solvent is stretched together with the support, a resin film is used as the support.

[0036] The resin film used as the support preferably has excellent thermal stability and mechanical strength. Examples of resin materials include polyester, polyolefin, polycycloolefin, polyamide, polycarbonate, vinyl chloride, vinylidene chloride, imide polymers, and sulfone polymers. Among these, polyester resins are preferred because of their high solvent resistance.

[0037] Examples of polyester resins include polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), and polyesters in which the glycol components and / or dicarboxylic acids of the monomer units constituting these polyesters are partially or entirely substituted with other monomer components. Crystalline polyesters are preferred from the viewpoint of increasing the mechanical strength of the film.

[0038] The support preferably has excellent stretchability at the heating temperature (e.g., about 140°C) during the stretching step after coating formation. The tensile modulus at 140°C is preferably 100 to 1,000 MPa, more preferably 300 to 800 MPa. Examples of crystalline polyester films with such a tensile modulus include biaxially stretched films of crystalline polyesters in which the glycol components and / or dicarboxylic acids of the monomer units constituting the polyester are partially or completely substituted with other monomer components. Examples of polyesters with substituted glycol components include glycol-modified polyesters in which a portion of the linear glycol, such as ethylene glycol in PET or 1,4-butanediol in PBT, is substituted with 1,2-cyclohexanedimethanol or 1,4-cyclohexanedimethanol. Examples of polyesters with substituted dicarboxylic acid components include dicarboxylic acid-modified polyesters in which terephthalic acid in PET or 2,6-naphthalenedicarboxylic acid in PEN is substituted with isophthalic acid, orthophthalic acid, 2,5-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, or the like. Among these, polyethylene terephthalate / isophthalate copolymers in which part of the terephthalic acid in PET is substituted with isophthalic acid are preferred.

[0039] The mechanical properties, such as modulus of elasticity, and thermal properties of polyethylene-terephthalate / isophthalate copolymers can be adjusted by changing the ratio of the terephthalic acid component to the isophthalic acid component, and increasing the ratio of the isophthalic acid component tends to decrease the modulus of elasticity at 140°C and improve stretchability. Furthermore, like PET, polyethylene-terephthalate / isophthalate copolymers can be crystallized by stretching, and therefore have excellent mechanical strength and high solvent resistance, making them suitable as supports for solution casting.

[0040] The thickness of the support is not particularly limited as long as it has both self-supporting properties and flexibility. The thickness of the support is generally about 20 μm to 200 μm, preferably 30 μm to 150 μm, and more preferably 35 μm to 100 μm.

[0041] The method for applying the dope onto the support is not particularly limited, and various coating methods can be applied, such as knife roll coating, kiss roll coating, gravure coating, reverse coating, spray coating, Mayer bar coating, air knife coating, curtain coating, lip coating, die coating, etc. The thickness of the film to be formed may be set depending on the optical properties (retardation value) required for the retardation film, and is set, for example, so that the film thickness after drying is about 3 μm to 15 μm.

[0042] After applying the dope onto the support, the organic solvent is removed by heating to obtain a laminate in which a coating film of a polymer having negative intrinsic birefringence is tightly adhered to the support. The heating temperature and heating time during drying are not particularly limited. The heating temperature does not need to be constant, and a temperature profile in which the temperature is increased or decreased stepwise may be used.

[0043] From the perspective of reducing thickness unevenness and appropriately decreasing the residual solvent amount, it is preferable to perform heating at a temperature below the boiling point of the organic solvent and then at a temperature 40°C or more higher than the boiling point. The heating time at a temperature below the boiling point (primary heating) is preferably 20 to 40 seconds. Subsequently, the heating time at a temperature 40°C or more higher than the boiling point (secondary heating) is preferably 10 to 120 seconds. Between the primary heating and the secondary heating, heating may be performed at a temperature higher than the boiling point and less than the boiling point + 40°C. The temperature of the primary heating is preferably 40°C or more, and may be 50°C or more, 60°C or more, or 65°C or more. The temperature of the secondary heating is preferably 230°C or less, more preferably 200°C or less, and even more preferably 180°C or less.

[0044] When the coating film on the support is dried, the molecular chains of the polymer tend to orient in the in-plane direction. When a polymer having negative intrinsic birefringence is oriented in the plane, the refractive index nz in the thickness direction of the coating film becomes relatively large with respect to the refractive index in the plane, and a positive C-plate characteristic having a refractive index anisotropy of nx ≒ ny < nz (Rth is a negative value) is exhibited.

[0045] By stretching the above coating film, a positive B-plate having a refractive index anisotropy of nz > nx > ny is obtained, in which the refractive index in the stretching direction becomes smaller. The coating film may be stretched after peeling from the support, but as described above, from the perspective of handling properties and the like, a method of stretching a laminate in which the coating film is closely laminated on the support without peeling the support is preferable.

[0046] The stretching method is not particularly limited, and examples include longitudinal uniaxial stretching (free-end uniaxial stretching) in which both ends in the width direction of the film are not fixed and stretching is performed in the longitudinal direction, transverse stretching in which both ends in the width direction of the film are gripped with a tenter clip or the like and stretched in the width direction, and simultaneous biaxial stretching in which both ends in the width direction of the film are gripped and stretched in the width direction while changing the moving speed of the gripping tool such as a tenter clip in the longitudinal direction to also stretch in the longitudinal direction. Sequential biaxial stretching in which longitudinal stretching and transverse stretching are performed in sequence may also be performed.

[0047] In the case of uniaxial stretching at the free end, as the film is stretched in the longitudinal direction, a shrinking effect occurs in the width and thickness directions. Therefore, when the polymer constituting the coating film has negative intrinsic birefringence, the refractive index (ny) in the longitudinal direction decreases, while the refractive indices (nx) in the width direction and (nz) in the thickness direction increase. In uniaxial stretching at the free end, generally, the shrinkage rate in the width direction is equivalent to that in the thickness direction, and the decrease rate (or increase rate) of the refractive index in the width direction is equivalent to that in the thickness direction. Due to the orientation during drying on the support, when the coating film has refractive index anisotropy of nx = ny < nz, the refractive index anisotropy of nz > nx is retained before and after stretching. Therefore, when the absolute value of Rth of the coating film is large or the stretching ratio is small, a positive B plate having refractive index anisotropy of nz > nx > ny can be obtained by uniaxial stretching at the free end.

[0048] The stretching temperature is not particularly limited, but it is preferably a temperature at which both the support and the coating film formed thereon can be stretched, and it is set according to the type of polymer constituting the coating film (retardation film) and the thermal properties of the support, etc. The stretching temperature is generally about 100°C to 200°C, preferably about 120°C to 180°C.

[0049] The stretching ratio is preferably 1.01 times or more, more preferably 1.03 times or more. In uniaxial stretching at the free end, the larger the stretching ratio, the greater the tendency for the front retardation Re to increase. The stretching ratio is generally 3 times or less, and may be 2.5 times or less or 2 times or less. As described above, from the viewpoint of suppressing display unevenness, it is preferable that the Re of the retardation film is small, and the stretching ratio may be 1.5 times or less, 1.3 times or less or 1.2 times or less.

[0050] The residual solvent amount of the stretched coating film (retardation film) is preferably 0.5 to 2.0% by weight. If the residual solvent amount is 2.0% or less, the changes in Re and Rth of the retardation film due to heating are small, and the reliability is excellent. Also, when the residual solvent amount is 0.5% by weight or more, the thickness unevenness becomes small and the occurrence of horizontal streaks tends to be suppressed. By adjusting the drying conditions of the coating film, the residual solvent amount of the stretched retardation film can be controlled within an appropriate range.

[0051] [Polarizing plate and liquid crystal display device] The retardation film of the present invention may be laminated integrally with a polarizer to form a polarizing plate. A polarizing plate can be obtained by laminating the retardation film to one main surface of the polarizer via an appropriate adhesive layer or pressure-sensitive adhesive layer. Another film may be laminated between the polarizer and the retardation film.

[0052] Examples of polarizers include hydrophilic polymer films such as polyvinyl alcohol films, partially formalized polyvinyl alcohol films, and partially saponified ethylene-vinyl acetate copolymer films, which are uniaxially stretched after adsorbing dichroic substances such as iodine or dichroic dyes, and polyene-based oriented films such as dehydrated polyvinyl alcohol and dehydrochlorinated polyvinyl chloride.

[0053] Among these, polyvinyl alcohol (PVA) polarizers are preferred because of their high polarization degree. For example, a PVA polarizer can be obtained by dyeing a polyvinyl alcohol film with iodine and stretching it, and then aligning the film in a predetermined direction.

[0054] The PVA-based polarizer may be a thin polarizer having a thickness of 10 μm or less. Examples of thin polarizers include thin polarizing films described in JP-A-51-069644, JP-A-2000-338329, WO2010 / 100917, Japanese Patent No. 4691205, and Japanese Patent No. 4751481. Such thin polarizers can be obtained, for example, by stretching a PVA-based resin layer and a resin substrate for stretching in a laminate state, and then dyeing the laminate with iodine.

[0055] The arrangement angle between the polarizer and the retardation film is not particularly limited. For example, when the retardation film is used for the purpose of optical compensation to suppress light leakage when the liquid crystal display device is viewed from an oblique direction, it is preferable to arrange the polarizer and the retardation film so that the absorption axis direction of the polarizer and the slow axis direction of the retardation film are parallel to or perpendicular to each other.

[0056] FIG. 1 is a cross-sectional view of a polarizing plate according to one embodiment of the present invention, and FIG. 2 is a conceptual diagram of the configuration of a liquid crystal panel in which the polarizing plate 100 of FIG. 1 is disposed on one surface of a liquid crystal cell 10. As shown in FIG.

[0057] 1 includes a first optically anisotropic element 60 and a second optically anisotropic element 70, arranged in this order, on one side of a polarizer 30. A transparent film serving as a polarizer protective film may be attached to the other side of the polarizer 30 via an appropriate adhesive or pressure-sensitive adhesive layer. The polarizing plate may be laminated with an adhesive or pressure-sensitive adhesive layer for attachment to a liquid crystal cell or the like.

[0058] In the polarizing plate 100, the first optically anisotropic element 60 arranged closer to the polarizer 30 is a negative B plate having a refractive index anisotropy of nx>ny>nz. The second optically anisotropic element 70 arranged further from the polarizer is a positive B plate having a refractive index anisotropy of nz>nx>ny, and the retardation film of the present invention is used.

[0059] A material having positive intrinsic birefringence is preferably used as the material for the negative B plate. A polymer having positive intrinsic birefringence refers to a polymer whose refractive index in the direction of orientation becomes relatively large when the polymer is oriented by stretching or the like. Examples of polymers having positive intrinsic birefringence include polycarbonate resins, polyester resins such as polyethylene terephthalate and polyethylene naphthalate, polyarylate resins, sulfone resins such as polysulfone and polyethersulfone, sulfide resins such as polyphenylene sulfide, polyimide resins, cyclic polyolefin (polynorbornene) resins, polyamide resins, polyolefin resins such as polyethylene and polypropylene, and cellulose esters.

[0060] 2, a polarizing plate 100 is disposed on one surface of a liquid crystal cell 10, and a polarizing plate including a polarizer 40 is disposed on the other surface, thereby forming a liquid crystal panel 200. A liquid crystal display device is formed by combining this liquid crystal panel 200 with a light source (not shown). The light source may be disposed on the polarizing plate 100 side of the present invention, or on the polarizing plate including the polarizer 40 side.

[0061] 2, arrows 35 and 45 indicate the absorption axis directions of polarizers 30 and 40, and arrows 63 and 73 indicate the slow axis directions of optically anisotropic elements 60 and 70. As shown in Fig. 2, in polarizing plate 100, the slow axis direction 63 of first optically anisotropic element 60 (negative B plate) and the slow axis direction 73 of second optically anisotropic element 70 (positive B plate) are parallel to each other, and it is preferable that these slow axis directions 63 and 73 are perpendicular to the absorption axis direction of polarizer 30.

[0062] The liquid crystal cell 10 has a liquid crystal layer between a pair of substrates. In a typical configuration, a color filter and a black matrix are provided on one substrate, and a switching element for controlling the electro-optical properties of the liquid crystal is provided on the other substrate. The liquid crystal cell 10 is preferably one in which the liquid crystal is homogeneously aligned in the absence of an electric field. An example of a liquid crystal cell in which the liquid crystal is homogeneously aligned in the absence of an electric field is an in-plane switching (IPS) mode liquid crystal cell. In IPS mode liquid crystal cells, nematic liquid crystal is generally used as the liquid crystal material.

[0063] The alignment direction (initial alignment direction) 11 of the liquid crystal molecules in the liquid crystal cell in the absence of an electric field is preferably parallel to the slow axis direction 63 of the first optically anisotropic element and the slow axis direction 73 of the second optically anisotropic element, and perpendicular to the absorption axis direction 35 of the polarizer 30. The absorption axis direction of the polarizer 40 disposed on the surface of the liquid crystal cell 10 opposite the polarizing plate 100 is perpendicular to the absorption axis direction 35 of the polarizer 30 of the polarizing plate 100.

[0064] The in-plane retardation Re and thickness direction retardation Rth of the first optically anisotropic element 60, which is a negative B plate, are not particularly limited and may be adjusted appropriately according to the optical design. In one embodiment, the sum of the in-plane retardation of the first optically anisotropic element 60 and the in-plane retardation of the second optically anisotropic element 70 is preferably 90 to 180 nm, more preferably 100 to 170 nm, and the sum of the thickness direction retardation of the first optically anisotropic element 60 and the thickness direction retardation of the second optically anisotropic element 70 is preferably 30 to 100 nm, more preferably 40 to 80 nm. Depending on the values ​​of Re and Rth of the second optically anisotropic element 70 (the retardation film of the present invention, which is a positive B plate), the Re and Rth of the first optically anisotropic element may be adjusted so that the sum of Re and the sum of Rth are within the above ranges.

[0065] By setting the optical anisotropy of the first optically anisotropic element 60 and the second optically anisotropic element 70, which are disposed between the liquid crystal cell 10 and the polarizer 30, within the above range, light leakage is reduced when the liquid crystal display device is viewed from an oblique direction, particularly at an angle of 45 degrees (azimuth angles of 45 degrees, 135 degrees, 225 degrees, and 315 degrees) relative to the absorption axis of the polarizer. Furthermore, by using the retardation film of the present invention as the second optically anisotropic element 70, display unevenness is suppressed, and the visibility of the liquid crystal display device can be improved. [Example]

[0066] EXAMPLES The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0067] [Synthesis of polymer and preparation of dope] An autoclave equipped with a stirrer, condenser, nitrogen inlet, and thermometer was charged with 48 parts by weight of hydroxypropyl methylcellulose (Shin-Etsu Chemical Co., Ltd., trade name: Metrose 60SH-50), 15,601 parts by weight of distilled water, 8,161 parts by weight of diisopropyl fumarate, 240 parts by weight of 3-ethyl-3-oxetanylmethyl acrylate, and 45 parts by weight of the polymerization initiator t-butyl peroxypivalate. Nitrogen bubbling was performed for 1 hour, followed by stirring at 49°C for 24 hours to carry out radical suspension polymerization. The mixture was then cooled to room temperature, and the resulting suspension containing polymer particles was centrifuged. The resulting polymer was washed twice with distilled water and twice with methanol, then dried under reduced pressure to obtain a white fumarate ester-based resin.

[0068] The obtained fumaric acid ester-based resin was dissolved in methyl ethyl ketone to prepare a solution with a solid content of 20% by weight. Further, 5 parts by weight of tributyl trimellitate as a plasticizer was added to 100 parts by weight of the fumaric acid ester-based resin to prepare a dope.

[0069] [Preparation Examples 1 to 14: Preparation of Retardation Films] A 75 μm thick polyester film (biaxially stretched film of polyethylene terephthalate / isophthalate copolymer) was used as the support film. While conveying the support at the line speed shown in Table 1, the dope was coated onto the support so that the film thickness after drying would be about 8 μm, and the support was heated under the first heating conditions shown in Table 1, and then further heated under the second heating conditions to obtain a laminate in which a coating film of a fumaric acid ester resin was tightly laminated on the support.

[0070] The above laminate was free-end uniaxially stretched at the temperature and stretch ratio shown in Table 1 to obtain a laminate in which a retardation film (positive B plate) having a refractive index anisotropy of nz>nx>ny was tightly laminated on the support.

[0071] [evaluation] <Lettering> A laminate of the support and retardation film was cut into a 5cm square. The retardation film was attached to a glass plate with an acrylic adhesive, and then the support was peeled off to prepare a measurement sample. Using this sample, the in-plane retardation was measured at a wavelength of 590nm using a polarization / retardation measurement system (Axometrics' "AxoScan"), as well as the retardation measured with the sample tilted 40° around the slow axis direction as the center of rotation. From these measurements, the in-plane retardation (Re = (nx - ny) × d) and the thickness retardation (Rth = (nx - nz) × d) were calculated.

[0072] <Residual solvent amount> After peeling the retardation film from the support, it was cut into a 10 cm square, its weight W0 was measured, and then it was heated in an oven at 150°C for 30 minutes. After heating, its weight W1 was measured, and the amount of solvent remaining in the retardation film was calculated using the following formula. Residual solvent amount (wt%) = 100 × (W0 - W1) / W0

[0073] <Film thickness> The laminate of the support and the retardation film was cut into a 10 cm square, and the support was peeled off. The thickness of the retardation film was measured at lattice points (81 points in total) spaced 1 cm apart using a digital microgauge, and the average thickness d and the maximum thickness d max , and the minimum thickness d min The thickness variation was calculated based on the following formula. Thickness variation (%) = 100 × (d max -d min ) / d

[0074] <Appearance> A linear polarizer was roll-to-roll laminated to the retardation film side of a laminate consisting of a support and a retardation film, with an adhesive layer interposed between them, and then the support was peeled off. This sample was cut into a 20 cm x 30 cm rectangle with the long side aligned with the retardation film's stretch direction (the polarizer's absorption axis direction). The rectangular sample was placed on a light box with the retardation film side facing up. Another linear polarizer was placed on top of the rectangular sample in a crossed Nicol configuration. Visual inspection was performed at an azimuth angle of 45° relative to the polarizer's absorption axis direction and at polar angles (angles between the polarizer and the film normal) ranging from 0° to 40° to check for unevenness and horizontal streaks (stripes extending perpendicular to the stretch direction). Samples without unevenness or horizontal streaks were rated as "OK," while samples with unevenness or horizontal streaks were rated as "NG."

[0075] Table 1 shows the manufacturing conditions and evaluation results of the retardation film in each manufacturing example.

[0076] [Table 1]

[0077] Among Production Examples 1 to 3, unevenness was observed in Production Example 3, which had a large in-plane variation in thickness, whereas no unevenness was observed in Production Examples 1 and 2, which had smaller in-plane variation in thickness than Production Example 3. The same was true in comparisons with Production Examples 4 to 7, Production Examples 8 to 11, and Production Examples 12 to 14, where the in-plane variation in thickness was small, no unevenness was observed and good visibility was demonstrated. On the other hand, in Production Examples 4 and 7, which had a long heating time and an in-plane variation in thickness of less than 0.5%, no unevenness was observed, but poor appearance due to horizontal streaks occurred.

[0078] Comparing these results, it can be seen that, from the viewpoint of suppressing the visibility of unevenness, it is preferable that the in-plane variation in thickness of the retardation film is small, but if the heating time is extended in order to reduce the variation in thickness, horizontal streaks tend to occur.

[0079] In Production Example 11, unevenness was observed even though the in-plane thickness variation was equivalent to that of Production Example 2. Furthermore, in Production Example 14, unevenness was observed even though the in-plane thickness variation was smaller than that of Production Example 2. These results show that when the front retardation Re is small, the tolerance for thickness variation is wide and unevenness tends to be less visible.

Claims

1. A method for producing a retardation film, wherein a refractive index nx in an in-plane slow axis direction, a refractive index ny in an in-plane fast axis direction, and a refractive index nz in a thickness direction satisfy nz>nx>ny, a coating film forming step of applying a solution containing a polymer having negative intrinsic birefringence and an organic solvent onto a support to form a coating film; a drying step of drying and removing the organic solvent by heating to obtain a laminate in which the coating film is tightly laminated on the support; and a stretching step of stretching the coating film in at least one direction; in order, In the drying step, a first heating is performed at a temperature equal to or lower than the boiling point of the organic solvent, and then a second heating is performed at a temperature 40° C. or higher than the boiling point of the organic solvent; a heating time in the first heating step is 20 to 40 seconds, and a heating time in the second heating step is 10 to 120 seconds; The retardation film after the stretching step is The thickness is 3 to 9 μm, The front retardation Re is 12 to 30 nm, The ratio X (%) of the thickness range to the average thickness in a 10 cm × 10 cm area and the front retardation Re (nm) are 0.5≦X≦4.5−0.11×Re A method for manufacturing a retardation film that satisfies the above relationship.

2. A method for manufacturing a retardation film according to claim 1, wherein the retardation film has a thickness direction retardation Rth of -60 to -135 nm.

3. A method for producing a retardation film described in claim 1 or 2, wherein the amount of residual solvent in the retardation film is 0.5 to 2.0% by weight.

4. The method for producing a retardation film according to any one of claims 1 to 3, wherein the support is a resin film.

5. The method for producing a retardation film according to claim 4 , wherein the stretching step stretches a laminate in which the coating film is tightly laminated on the support.

6. The method for producing a retardation film according to any one of claims 1 to 5, wherein free-end uniaxial stretching is performed in the stretching step.

7. The method for producing a retardation film according to any one of claims 1 to 6, wherein the boiling point of the organic solvent is 50 to 120°C.

8. The method for producing a retardation film according to any one of claims 1 to 7, wherein the solution has a solids concentration of 10% by weight or more.

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