Method for manufacturing retardation film

Abstract

To provide a retardation film with a small NZ coefficient.SOLUTION: A retardation film provided herein has an in-plane refractive index nx in a slow axis direction, an in-plane refractive index ny in a fast axis direction, and a refractive index nz in a thickness direction that satisfy nx>nz>ny. A method of manufacturing the retardation film involves stretching a laminate comprising heat-shrinkable film laminated on at least one surface of a polymer film in one direction while heating. Preferably, the amount of residual solvent in the polymer film before stretching is 0.1-3.5 wt.%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a retardation film. [Background technology]

[0002] Retardation films are used in displays such as liquid crystal display devices for optical compensation such as improving contrast and widening the viewing angle, and for blocking external light reflected by metal electrodes (antireflection). Retardation films using non-liquid crystal polymers are given optical anisotropy by stretching the polymer film in at least one direction.

[0003] When a polymer film is stretched longitudinally (free-end uniaxial stretching), the polymer molecular chains are oriented in the stretching direction, and shrinkage occurs in directions perpendicular to the stretching direction, i.e., the width and thickness directions. When a polymer film with a positive intrinsic refractive index is stretched longitudinally, the refractive index in the longitudinal direction (nx) increases, while the refractive index in the width direction (ny) and the refractive index in the thickness direction (nz) decrease. In free-end uniaxial stretching, the shrinkage rate in the width direction (width reduction rate) and the thickness reduction rate are approximately the same, so a retardation film (positive A plate) with refractive index anisotropy of nx > ny ≒ nz is obtained.

[0004] When a polymer film is laminated with a heat-shrinkable film on at least one side thereof and the laminate is stretched while being heated, the shrinkage force of the heat-shrinkable film causes a larger amount of shrinkage in the width direction (the direction perpendicular to the stretching direction) than in conventional free-end uniaxial stretching. The refractive index ny in the width direction (the direction of the fast axis) becomes smaller, and the refractive index nz in the thickness direction becomes relatively larger, resulting in a retardation film with a refractive index anisotropy of nx>nz>ny (see, for example, Patent Document 1). Retardation films with a refractive index anisotropy of nx>nz>ny exhibit small changes in retardation with respect to the viewing angle, and are used as optical compensation films for liquid crystal display devices and λ / 4 plates for circular polarizers. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-72309 Summary of the Invention [Problem to be solved by the invention]

[0006] In a stretching method that uses the shrinkage force of a heat-shrinkable film to increase the amount of shrinkage in the width direction, a retardation film with a smaller NZ coefficient, defined as NZ = (nx - nz) / (nx - ny), can be obtained by using a heat-shrinkable film with a large thermal shrinkage rate. However, increasing the amount of thermal shrinkage of the heat-shrinkable film can cause problems such as wrinkles during stretching and increased variation in the in-plane optical properties of the retardation film after stretching, so there is a limit to how much the NZ coefficient can be reduced while maintaining the quality of the retardation film by simply controlling the physical properties of the heat-shrinkable film. [Means for solving the problem]

[0007] One aspect of the present invention is a method for producing a retardation film, in which the in-plane refractive index nx in the slow axis direction, the in-plane refractive index ny in the fast axis direction, and the refractive index nz in the thickness direction satisfy nx>nz>ny. A laminate in which a heat-shrinkable film is laminated on at least one surface of a polymer film is stretched in one direction while heating, and the heat-shrinkable film is peeled off and removed, thereby obtaining a retardation film that satisfies nx>nz>ny.

[0008] The amount of the solvent remaining in the polymer film before stretching is preferably 0.1 to 3.5% by weight. The difference Δn between the refractive index nx in the in-plane slow axis direction of the polymer film before stretching and the refractive index nz in the thickness direction is xz is preferably 0.0010 or less. The thickness of the polymer film before stretching is preferably 15 μm or more.

[0009] The retardation film may have an NZ coefficient defined as NZ=(nx-nz) / (nx-ny) of 0.7 or less. [Effects of the Invention]

[0010] According to the production method of the present invention, a retardation film having a small NZ coefficient can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0011] A retardation film according to one embodiment of the present invention has a refractive index anisotropy of nx>nz>ny, where nx is the refractive index in the in-plane slow axis direction, ny is the refractive index in the in-plane fast axis direction, and nz is the refractive index in the thickness direction. A retardation film having a refractive index anisotropy of nx>nz>ny can be obtained by stretching a laminate obtained by laminating a heat-shrinkable film on a polymer film (unstretched film) in one direction.

[0012] As the material for the polymer film, a non-liquid crystal polymer material having positive intrinsic birefringence is preferably used. When a polymer having positive intrinsic birefringence is oriented by stretching or the like, the refractive index in the orientation direction becomes relatively large. Examples of non-liquid crystal polymers having positive intrinsic birefringence include polycarbonate-based resins, polyester-based resins such as polyethylene terephthalate and polyethylene naphthalate, polyarylate-based resins, sulfone-based resins such as polysulfone and polyethersulfone, sulfide-based resins such as polyphenylene sulfide, polyimide-based resins, cyclic polyolefin-based (polynorbornene-based) resins, polyamide resins, polyolefin-based resins such as polyethylene and polypropylene, and cellulose esters.

[0013] In one embodiment, a cyclic polyolefin resin is used as the material for the polymer film. Cyclic polyolefins have excellent transparency and heat resistance, as well as excellent chemical resistance, making them suitable as optical film materials for displays.

[0014] Examples of cyclic polyolefin resins include those described in JP-A-1-240517, JP-A-3-14882, and JP-A-3-122137. Specific examples include ring-opening (co)polymers of cyclic olefins, addition polymers of cyclic olefins, copolymers (typically random copolymers) of cyclic olefins with α-olefins such as ethylene and propylene, and graft polymers and hydrogenated products of these modified with unsaturated carboxylic acids or their derivatives. Commercially available cyclic polyolefin resins include "ZEONOR" and "ZEONEX" manufactured by Zeon Corporation, "ARTON" manufactured by JSR Corporation, "APEL" manufactured by Mitsui Chemicals, and "TOPAS" manufactured by TOPAS ADVANCED POLYMERS.

[0015] The cyclic polyolefin film preferably contains 50% by weight or more of a cyclic polyolefin resin, more preferably 70% by weight or more, even more preferably 80% by weight or more, and may even be 90% by weight or more or 95% by weight or more.

[0016] A preferred method for producing a polymer film is a solution casting method. In the solution casting method, 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 or the like to form a laminate in which the coating film is tightly laminated on the support. Thereafter, the coating film is peeled off from the support, and if necessary, further heated to remove the solvent, thereby obtaining a polymer film.

[0017] The dope may contain additives such as a leveling agent, a plasticizer, an ultraviolet absorber, and an anti-degradation agent, as necessary. The type of solvent is not particularly limited as long as it dissolves the resin material but does not dissolve the support. The solid content and viscosity of the dope may be appropriately set depending on the type and molecular weight of the resin, the thickness of the retardation film, etc.

[0018] The support may be a metal drum roll, a metal belt (endless belt), a plastic film, or the like. A heat-shrinkable film may also be used as the support. The method for applying the dope onto the support is not particularly limited, and may be knife roll coating, kiss roll coating, gravure coating, reverse coating, spray coating, Mayer bar coating, air knife coating, curtain coating, lip coating, die coating, or the like.

[0019] The drying conditions for the solvent on the support and after peeling from the support are not particularly limited, and drying is generally performed at a temperature near the boiling point of the solvent constituting the dope or higher than the boiling point of the solvent. The amount of solvent remaining in the film after drying is preferably 0.1 to 3.5 wt %, and may be 0.1 to 3.0 wt %. As described below, adjusting the amount of solvent remaining in the film improves the stretchability when stretching is performed using the shrinkage effect of the heat-shrinkable film, and tends to reduce the NZ coefficient of the retardation film.

[0020] From the viewpoint of heat resistance, the glass transition temperature of the polymer film is preferably 100°C or higher, more preferably 120°C or higher, and may be 130°C or higher or 135°C or higher. From the viewpoint of stretching processability, the glass transition temperature of the polymer film is preferably 200°C or lower, more preferably 180°C or lower, and may be 170°C or lower, 160°C or lower, or 155°C or lower. The glass transition temperature of the polymer film is the inflection point of a DSC curve obtained by differential scanning calorimetry (DSC).

[0021] The thickness of the polymer film before stretching may be set depending on the optical properties (retardation value) required for the retardation film, and is generally about 5 μm to 300 μm. From the viewpoint of obtaining a retardation film with a larger retardation, the thickness of the polymer film is preferably 15 μm or more, more preferably 20 μm or more. When the polymer film formed by solution casting has a small thickness, the polymer molecules tend to be oriented in-plane during film formation, and as a result, the birefringence Δn in the thickness direction increases. xzTherefore, from the viewpoint of obtaining a retardation film with a small NZ coefficient, the thickness of the polymer film before stretching is preferably 15 μm or more.

[0022] The in-plane retardation of the polymer film before stretching (Re=(nx-ny)×d) is preferably 10 nm or less. The thickness direction retardation of the polymer film before stretching (Rth=(nx-nz)×d) is preferably 150 nm or less, more preferably 120 nm or less. nx, ny, and nz are as described above, and d is the thickness.

[0023] Birefringence in the thickness direction of the polymer film before stretching: Δn xz The value of nx-nz is preferably 0.0010 or less. x may be 0.0002 or more. When a film is produced on a support by solution casting, the thickness of the coating film decreases as the solvent evaporates (volume reduction of the coating film), but the in-plane dimensions remain almost unchanged. Therefore, stress is generated at the interface between the coating film (polymer film) and the support due to the force that causes the coating film to shrink as the solvent evaporates. This stress acts to orient the polymer molecular chains in-plane, so films formed by solution casting often have a refractive index anisotropy of nx ≒ ny > nz.

[0024] When using a polymer material with a large intrinsic birefringence or when the film thickness is small, the polymer molecules tend to be oriented in-plane when the solvent evaporates, resulting in a large Δn xz Therefore, polymer materials with small intrinsic birefringence, such as cyclic polyolefins, are used, and the Δn of the polymer film before stretching is reduced. xz is preferably within the above range.

[0025] A laminate is formed by laminating a heat-shrinkable film on a polymer film. The heat-shrinkable film may be laminated on one side of the polymer film or on both sides of the polymer film. For example, a laminate of a polymer film and a heat-shrinkable film is formed by attaching a heat-shrinkable film to the surface of a polymer film via an appropriate adhesive layer. As described above, a laminate may be formed in which a polymer film (coating film) to be stretched is tightly laminated on the heat-shrinkable film by using the heat-shrinkable film as a supporting substrate and applying a polymer solution (dope) thereon and drying the solvent.

[0026] The heat-shrinkable film is not particularly limited as long as it is heat-shrinkable in a direction perpendicular to the stretching direction when laminated and integrated with the polymer film and stretched. The material constituting the heat-shrinkable film is not particularly limited, but a material that heat-shrinks near the glass transition temperature of the polymer film is preferred. Polyolefins such as polyethylene and polypropylene, and polyesters are preferably used as materials for the heat-shrinkable film because they are highly versatile and inexpensive.

[0027] The heat-shrinkable film preferably has a shrinkage stress of 0.5 N / 4 mm or more at the glass transition temperature of the polymer film. The heat-shrinkable film may have an anisotropic shrinkage rate.

[0028] A retardation film having a refractive index anisotropy of nx>nz>ny is obtained by stretching a laminate of a polymer film (unstretched film) and a heat-shrinkable film in one direction in a heating furnace while conveying the laminate. The stretching ratio is set, for example, in the range of about 1.01 to 2 times depending on the desired optical properties. The heating temperature during stretching is generally in the range of about ±10°C of the glass transition temperature of the polymer film.

[0029] When a single film without a laminated heat-shrinkable film is stretched at a stretching ratio of p times along the conveying direction, the width and thickness decrease at the same ratio, so the width and thickness become (1 / √p) times those before stretching, respectively. When stretching along the conveying direction with a heat-shrinkable film laminated, since the heat-shrinkable film shrinks in the width direction, the shrinkage amount in the width direction becomes larger than that in the case of a single polymer film. Therefore, when the stretching ratio is p times, the width of the film after stretching becomes smaller than (1 / √p) times that before stretching, and the thickness of the film after stretching becomes larger than (1 / √p) times that before stretching. Along with this, the refractive index ny in the width direction (fast axis direction) becomes smaller, and the refractive index nz in the thickness direction becomes relatively larger, so a retardation film having a refractive index anisotropy of nx>nz>ny can be obtained.

[0030] For a retardation film having a refractive index anisotropy of nx>nz>ny, the NZ coefficient defined by NZ = (nx - nz) / (nx - ny) satisfies 0 < NZ < 1. The NZ coefficient of the retardation film is preferably 0.7 or less, more preferably 0.6 or less, and may be 0.2 or more or 0.3 or more. A retardation film with NZ = 0.5 has no change in retardation depending on the viewing angle and is used as an optical compensation film for liquid crystal display devices and a retardation film for circular polarizing plates.

[0031] The greater the contraction action in the width direction due to the contraction force of the heat-shrinkable film during stretching, the more likely the NZ of the retardation film is to decrease. Furthermore, in the present invention, by using a polymer film with the residual solvent amount within the above range as the stretching target, compared with the case of using a film without solvent (for example, a film formed by a melting method) or a film with a large amount of residual solvent as the stretching target, the stretching processability is excellent and the NZ coefficient tends to be small.

[0032] In order to produce a retardation film with a large front retardation Re using a polymer film with a small thickness, the stretching ratio is increased to increase the birefringence Δn xyHowever, when the stretching ratio is increased, the refractive index nx in the stretching direction (slow axis direction) increases significantly, and the difference Δn between nx and nz xz =nx-nz tends to become large, making it difficult to reduce the NZ coefficient defined as NZ=(nx-nz) / (nx-ny).

[0033] One possible reason why the NZ coefficient can be reduced even at high stretch ratios (high birefringence) when polymer films contain residual solvent is that the solvent softens the film, improving its stretchability. When the film becomes softer, it is more likely to follow the shrinkage of the heat-shrinkable film, reducing the refractive index ny in the width direction (fast axis direction), and thus increasing the refractive index nz in the thickness direction. Therefore, (nx - ny) tends to be small and (nx - nz) tends to be large, which is thought to result in a smaller NZ coefficient. On the other hand, when the amount of residual solvent is excessively high, the NZ coefficient does not decrease sufficiently. This is thought to be due in part to the fact that the orientation of the polymer molecular chains is weak (easily relaxed) even when the film undergoes dimensional changes, making it difficult for refractive index anisotropy to develop.

[0034] The in-plane retardation Re of the stretched retardation film is, for example, about 15 nm to 400 nm, and may be 100 nm or more, 150 nm or more, or 200 nm or more. When the retardation film is used as a half-wave plate (λ / 2 plate), the in-plane retardation at a wavelength of 550 nm is preferably 240 to 300 nm, more preferably 250 to 290 nm. As described above, the NZ coefficient of the retardation film is preferably 0.7 or less, and a retardation film with a smaller NZ coefficient can be produced by stretching a polymer film containing a predetermined amount of solvent.

[0035] The front retardation and NZ coefficient of a retardation film are appropriately set depending on the application of the retardation film, the optical design of the image display device, etc. Applications of a retardation film with an NZ coefficient of less than 1 include optical compensation in liquid crystal display devices and circular polarizing plates for blocking reflected light in organic EL display devices.

[0036] For example, when an IPS-mode liquid crystal display device is viewed obliquely at an angle of 45 degrees to the absorption axis of the polarizer (azimuth angles of 45 degrees, 135 degrees, 225 degrees, and 315 degrees), light leakage in the black display is large, and contrast reduction and color shift are likely to occur. By placing a retardation film with a front retardation of 1 / 2 the wavelength λ and an NZ coefficient of 0.5 between the liquid crystal cell and the polarizer, black brightness in the oblique direction can be reduced and contrast can be improved.

[0037] In organic EL display devices, a circular polarizer is placed on the viewing side of the organic EL cell to prevent light from being reflected by the metal electrodes and perceived as a mirror from the outside. A circular polarizer has a configuration in which a λ / 4 plate with a front retardation of ¼ the wavelength λ is placed on one side of a polarizer (the side facing the organic EL cell). A retardation film with a refractive index anisotropy of nx>nz>ny exhibits little change in retardation with respect to the viewing angle. Therefore, using a retardation film with a refractive index anisotropy of nx>nz>ny as the λ / 4 plate of a circular polarizer can improve the blocking of light not only from the front (normal direction) of the display device but also from oblique directions.

[0038] In-plane birefringence Δn of retardation film xy is 1.0 x 10 -3 The in-plane birefringence Δn xy =nx-ny is the difference between the refractive index nx in the in-plane slow axis direction and the refractive index ny in the in-plane fast axis direction, and is the value obtained by dividing the in-plane retardation Re by the thickness. As mentioned above, in a stretched retardation film, the larger the stretching ratio, the larger Δn tends to be, and the larger Δn is, the larger the in-plane retardation can be achieved with a smaller thickness. The Δn of a retardation film is 1.3×10 -3 or more, or 1.5 x 10-3 It may be more than that.

[0039] As described above, the retardation film can be used as an optical film for image display devices for the purpose of optical compensation of liquid crystal display devices, etc. The retardation film may be integrally laminated 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] The arrangement angle between the polarizer and the retardation film is not particularly limited. For example, when a retardation film is used for the purpose of optical compensation to suppress light leakage when a liquid crystal display device is viewed from an oblique direction, it is preferable to arrange both of them so that the absorption axis direction of the polarizer and the slow axis direction of the retardation film are parallel or perpendicular to each other. When a polarizer and a retardation film are laminated to form a circular polarizing plate, it is preferable to arrange both of them so that the angle between the absorption axis direction of the polarizer and the slow axis direction of the retardation film is 45°. Note that the arrangement angle does not need to be strictly within the above range and may include an error of about ±2°.

[0044] A transparent film as a polarizer protective film may be attached to the other surface of the polarizer via an appropriate adhesive layer or pressure-sensitive adhesive layer. An optical film other than the above-mentioned retardation film and polarizer protective film may be laminated on the polarizing plate. An adhesive layer or pressure-sensitive adhesive layer for laminating the polarizing plate to an image display cell or the like may be laminated on the polarizing plate.

[0045] The retardation film and the polarizing plate can be used as an optical film for an image display device. For example, an image display panel can be obtained by bonding a polarizing plate having a retardation film to the surface of an image display cell via an appropriate adhesive. When the image display cell is a liquid crystal cell, a liquid crystal display device can be formed by further combining it with a backlight as a light source. [Example]

[0046] 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.

[0047] [Synthesis Example 1] Into a reaction vessel purged with nitrogen, 21 parts by weight of dicyclopentadiene and 8-methyl-8-carboxymethyltetracyclo[4.4.0.1 2,5 .1 7,1078 parts by weight of ]-3-dodecene, 1 part by weight of 2-norbornene, 14.7 parts by weight of 1-hexene as a molecular weight modifier, and 150 parts by weight of toluene as a solvent were added and heated to 107 °C. 0.4 parts by weight of a toluene solution of ethylaluminum (0.6 mol / L) and 1.8 parts by weight of a toluene solution of methanol-modified tungsten hexachloride (0.025 mol / L) were added to this solution, and the reaction was carried out for 1 hour at 107 °C to obtain a ring-opened polymer. 0.04 parts by weight of Ru[4-CH3(CH2)4C6H4CO2]H(CO)[P(C6H5)3] was added as a hydrogenation catalyst to 360 parts by weight of the resulting ring-opened polymer solution, and the reaction was carried out for 3 hours at a hydrogen gas pressure of 9 to 10 MPa and a temperature of 160 to 165 °C. After the reaction was completed, the resulting product (hydrogenated product) was precipitated in methanol and vacuum dried to obtain a cyclic polyolefin resin (weight average molecular weight: 46,000, glass transition temperature: 148°C). The resulting resin was melt-kneaded using a twin-screw extruder, extruded into strands, cooled with water, and passed through a feeder rudder to obtain pellets.

[0048] [Comparative Example 1] A 20 μm thick unstretched film was prepared by melt extrusion using pellets of cyclic polyolefin resin ("ARTON R5000" manufactured by JSR). The unstretched film had a front retardation Re of 0 nm and a thickness retardation Rth of 1 nm. A heat-shrinkable biaxially stretched propylene film ("TORAYFAN" manufactured by Toray) was attached to both sides of the unstretched film via an adhesive to obtain a laminate. This laminate was then subjected to free-end uniaxial stretching (longitudinal stretching) at a temperature of 150°C and a magnification of 1.3 times, after which the heat-shrinkable films attached to both sides were peeled off to obtain a retardation film.

[0049] [Comparative Examples 2 and 3] A retardation film was obtained in the same manner as in Comparative Example 1, except that the thickness of the unstretched film was changed as shown in Table 1.

[0050] [Comparative Examples 4 and 5] In Comparative Example 4, "ARTON RH4900" was used instead of JSR's "ARTON R5000," and in Comparative Example 5, pellets of the cyclic polyolefin resin obtained in Synthesis Example 1 were used, and the film thickness and stretching conditions were changed as shown in Table 1. Otherwise, a retardation film was obtained in the same manner as in Comparative Example 1.

[0051] Comparative Example 6 Cyclic olefin polymer (COP) resin pellets ("ARTON R5000" manufactured by JSR) were dissolved in methylene chloride, and an unstretched film with a thickness of 65 μm was produced by a solution film-forming method. The amount of residual solvent in this unstretched film was 4% by weight. Heat-shrinkable biaxially stretched propylene films ("TORAYFAN" manufactured by Toray) were bonded to both sides of this unstretched film via an adhesive, and stretched in the same manner as in Comparative Example 1 to obtain a retardation film.

[0052] [Example 1] Cyclic olefin polymer (COP) resin pellets ("ARTON R5000" manufactured by JSR) were dissolved in methylene chloride, and an unstretched film with a thickness of 20 μm was produced by a solution film-forming method. The amount of residual solvent in this unstretched film was 0.2 wt %. Heat-shrinkable biaxially stretched propylene films ("TORAYFAN" manufactured by Toray) were bonded to both sides of this unstretched film via an adhesive, and stretched in the same manner as in Comparative Example 1 to obtain a retardation film.

[0053] [Examples 2 and 3] A retardation film was obtained in the same manner as in Example 1, except that the thickness of the unstretched film was changed as shown in Table 1.

[0054] [Examples 4 and 5] In Example 4, "ARTON RH4900" was used instead of JSR's "ARTON R5000," and in Example 5, pellets of the cyclic polyolefin resin obtained in Synthesis Example 1 were used, and the film thickness and stretching conditions were changed as shown in Table 1. Otherwise, a retardation film was obtained in the same manner as in Example 1.

[0055] [Example 6] Cyclic olefin polymer (COP) resin pellets ("ARTON R5000" manufactured by JSR) were dissolved in methylene chloride, and the solution was applied to a heat-shrinkable biaxially stretched propylene film ("TORAYFAN" manufactured by Toray) and dried to obtain a laminate in which an unstretched film was tightly laminated onto the heat-shrinkable film. This laminate was then stretched longitudinally at a temperature of 155°C and a stretching ratio of 1.3, after which the heat-shrinkable film was peeled off and removed to obtain a retardation film.

[0056] [evaluation] <Residual solvent amount> The unstretched film was cut into a 10 cm square, and its weight W0 was measured. After that, it was heated in an oven at 150°C for 30 minutes, and its weight W1 after heating was measured. The amount of remaining solvent in the retardation film was calculated by the following formula. In Example 6, the measurement was performed using an unstretched film from which the heat-shrinkable film had been peeled off as a sample. Residual solvent amount (wt%) = 100 × (W0 - W1) / W0

[0057] <Phase difference characteristics> The unstretched and stretched retardation films were cut into 50mm x 50mm pieces and measured using a polarization / retardation measurement system (Axometrics "AxoScan") at a wavelength of 550nm. The in-plane retardation and the retardation were measured with the sample tilted 40° around the slow axis. From these measurements, the in-plane retardation at 550nm was calculated as Re = (nx - ny) x d, the thickness retardation as Rth = (nx - nz) x d, the thickness birefringence as Δnxz = nx - nz, and the NZ coefficient as NZ = (nx - nz) / (nx - ny). nx is the in-plane refractive index along the slow axis, ny is the in-plane refractive index along the fast axis, nz is the refractive index along the thickness, and d is the thickness.

[0058] Table 1 shows the production conditions and evaluation results of the unstretched films and retardation films of the examples and comparative examples.

[0059] [Table 1]

[0060] In Comparative Example 1, which used a polymer film produced by melt casting, the NZ coefficient of the retardation film after stretching was 0.53, whereas in Example 1, which used a polymer film produced by solution casting with a residual solvent amount of 0.2 wt %, the NZ coefficient of the retardation film after stretching was 0.43, and it is clear that Example 1 has a larger in-plane retardation and a smaller NZ coefficient than Comparative Example 1. Similar trends were also observed in the comparisons between Comparative Example 2 and Example 2, between Comparative Example 3 and Example 3, between Comparative Example 4 and Example 4, and between Comparative Example 5 and Example 5.

[0061] In Example 6, in which a laminate in which a heat-shrinkable film was tightly laminated on one side of a polymer film by applying a polymer solution onto the heat-shrinkable film was stretched, the NZ coefficient was also smaller than when a polymer film produced by melt-casting was used (Comparative Example 3), as in Example 3.

[0062] On the other hand, in Comparative Example 6, which used a polymer film with a residual solvent amount of 4 wt %, the NZ coefficient of the retardation film after stretching was equivalent to that of Comparative Example 2, which used a polymer film produced by melt casting.

[0063] These results show that a retardation film having a smaller NZ coefficient can be produced by stretching a laminate of a polymer film having a residual solvent amount within a predetermined range and a heat-shrinkable film.

Claims

1. A method for producing a retardation film in which 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 nx>nz>ny, and a front retardation of 250 to 290 nm, a stretching step of stretching a laminate in which heat-shrinkable films are laminated on both sides of a cyclic polyolefin film at a stretching ratio p in a conveyance direction while heating the laminate, The cyclic polyolefin film before stretching has a difference Δn between the refractive index nx in the in-plane slow axis direction and the refractive index nz in the thickness direction. ｘｚ is 0.0002 to 0.0010, and the amount of residual solvent is 0.1 to 3.5% by weight, The shrinkage stress of the heat-shrinkable film at the glass transition temperature of the cyclic polyolefin film is 0.5 N / 4 mm or more, The thickness d1 of the cyclic polyolefin film before stretching and the thickness d2 of the cyclic polyolefin film after stretching satisfy d2>d1 / (√p). A method for manufacturing a retardation film.

2. The method for producing a retardation film according to claim 1 , wherein the cyclic polyolefin film has a thickness of 15 μm or more before stretching.

3. 3. The method for producing a retardation film according to claim 1, wherein the retardation film has an NZ coefficient defined as NZ=(nx-nz) / (nx-ny) of 0.7 or less.

Images