JP7777931B2 - Method for manufacturing retardation film - Google Patents

Description

The present invention relates to a method for manufacturing a retardation film.

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

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. With free-end uniaxial stretching, the shrinkage rate in the width direction (width reduction rate) and the thickness reduction rate are approximately the same, resulting in a retardation film (positive A plate) with a refractive index anisotropy of nx > ny ≒ nz.

When a heat-shrinkable film is laminated on at least one side of a polymer film and the laminate is stretched while being heated, the shrinkage force of the heat-shrinkable film causes a greater 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 (fast axis direction) 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 have small changes in phase difference with viewing angle, and are used as optical compensation films in liquid crystal display devices and λ/4 plates for circular polarizers.

Japanese Patent Application Laid-Open No. 2006-72309

In stretching methods that utilize the shrinkage force of a heat-shrinkable film to increase the amount of shrinkage in the width direction, using a heat-shrinkable film with a large thermal shrinkage rate results in a retardation film with a smaller NZ coefficient, defined as NZ = (nx - nz) / (nx - ny). However, increasing the amount of thermal shrinkage of the heat-shrinkable film can cause problems such as wrinkles during stretching and greater variation in the in-plane optical properties of the retardation film after stretching. Therefore, there are limits 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.

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.

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

The NZ coefficient of the retardation film, defined as NZ = (nx - nz) / (nx - ny), may be 0.7 or less.

The manufacturing method of the present invention produces a retardation film with a small NZ coefficient.

A retardation film according to one embodiment of the present invention has a refractive index anisotropy of nx > nz > ny. 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 with a refractive index anisotropy of nx > nz > ny can be obtained by stretching a laminate in one direction, in which a heat-shrinkable film is laminated on a polymer film (unstretched film).

A non-liquid crystal polymer material with positive intrinsic birefringence is preferably used as the material for the polymer film. When a polymer with 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 with 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.

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

Examples of cyclic polyolefin resins include those described in Japanese Patent Application Laid-Open Nos. 1-240517, 3-14882, and 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.

The cyclic polyolefin film preferably contains 50% or more by weight of cyclic polyolefin resin. The content of cyclic polyolefin resin in the cyclic polyolefin film is more preferably 70% or more by weight, even more preferably 80% or more by weight, and may be 90% or more by weight, or even 95% or more by weight.

The solution casting method is preferred as a method for producing polymer films. In this 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, 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 if necessary, further heated to remove the solvent, yielding a polymer film.

The dope may contain additives such as leveling agents, plasticizers, UV absorbers, and anti-degradation agents, as needed. The type of solvent is not particularly limited, as long as it dissolves the resin material but not the support. The solid content and viscosity of the dope can be set appropriately depending on the type and molecular weight of the resin, the thickness of the retardation film, etc.

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 to the support is not particularly limited, and methods that can be used include knife roll coating, kiss roll coating, gravure coating, reverse coating, spray coating, Mayer bar coating, air knife coating, curtain coating, lip coating, and die coating.

The conditions for drying the solvent on the support and after peeling from the support are not particularly limited, and drying is generally carried out at a temperature near the boiling point of the solvent that constitutes 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% by weight, and may be 0.1 to 3.0% by weight. 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.

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 stretchability, 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 the DSC curve obtained by differential scanning calorimetry (DSC).

The thickness of the polymer film before stretching may be set depending on the optical properties (retardation value) required of 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 thickness of a polymer film formed by solution casting is small, the polymer molecules tend to be oriented in-plane during film formation, which increases the birefringence Δn _xz in the thickness direction, and the NZ coefficient may not be sufficiently reduced by stretching. Therefore, 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.

The front retardation of the polymer film before stretching: Re = (nx - ny) x d is preferably 10 nm or less. The thickness direction retardation of the polymer film before stretching: Rth = (nx - nz) x 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.

The thickness-direction birefringence of the polymer film before stretching, Δn _xz = nx - nz, is preferably 0.0010 or less. Δn _x of the polymer film may be 0.0002 or more. When a film is produced on a support by solution casting, the thickness of the coating film decreases with the evaporation of the solvent (volume reduction of the coating film), but the in-plane dimensions change very little. 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, and therefore films formed by solution casting often have a refractive index anisotropy of nx ≒ ny > nz.

When a polymer material with a large intrinsic birefringence is used or when the film thickness is small, the polymer molecules tend to be oriented in-plane when the solvent evaporates, which tends to increase Δn _xz , and the NZ coefficient may not be sufficiently reduced by stretching. Therefore, it is preferable to use a polymer material with a small intrinsic birefringence, such as a cyclic polyolefin, and to set the Δn _xz of the polymer film before stretching within the above range.

A laminate is formed by laminating a heat-shrinkable film onto a polymer film. The heat-shrinkable film may be laminated onto one side of the polymer film, or may be laminated onto 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) to be stretched is tightly laminated onto the heat-shrinkable film, using the heat-shrinkable film as a supporting substrate, and a polymer solution (dope) is applied thereon, followed by drying the solvent.

There are no particular restrictions on the heat-shrinkable film, so long as it heat-shrinks in a direction perpendicular to the stretching direction when laminated and stretched with the above-mentioned polymer film. There are no particular restrictions on the material that makes up the heat-shrinkable film, but it is preferable that it be made of a material that heat-shrinks near the glass transition temperature of the polymer film. Due to their versatility and low cost, polyolefins such as polyethylene and polypropylene, and polyesters are preferably used as materials for the heat-shrinkable film.

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.

A phase difference film with 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 approximately 1.01 to 2 times depending on the desired optical properties. The heating temperature during stretching is generally within the range of approximately ±10°C of the glass transition temperature of the polymer film.

When a film without a heat-shrinkable film laminated thereto is stretched in the conveying direction at a stretching ratio of p, the width and thickness decrease at the same rate, resulting in a width and thickness that are each (1/√p) times their pre-stretching ratios. When a film laminated with a heat-shrinkable film is stretched in the conveying direction, the heat-shrinkable film shrinks in the width direction, resulting in a greater shrinkage in the width direction than in the case of a polymer film laminated thereto. Therefore, when the stretching ratio is p, the width of the stretched film is smaller than (1/√p) times its pre-stretching ratio, and the thickness of the stretched film is larger than (1/√p) times its pre-stretching ratio. Consequently, 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, resulting in a retardation film with a refractive index anisotropy of nx > nz > ny.

A retardation film with a refractive index anisotropy of nx > nz > ny has an NZ coefficient defined as NZ = (nx - nz) / (nx - ny) that 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 exhibits no change in retardation with viewing angle and is used as an optical compensation film for liquid crystal displays and as a retardation film for circular polarizing plates.

The greater the shrinkage effect in the width direction due to the shrinkage force of the heat-shrinkable film during stretching, the smaller the NZ coefficient of the retardation film tends to be. Furthermore, in the present invention, by stretching a polymer film with a residual solvent amount within the above-mentioned range, the stretching processability tends to be superior and the NZ coefficient tends to be smaller than when a film that does not contain solvent (for example, a film formed by a melting method) or a film with a large amount of residual solvent is stretched.

In order to produce a retardation film with a large in-plane retardation Re using a thin polymer film, it is necessary to increase the stretching ratio to increase the birefringence Δn _xy . However, when the stretching ratio is increased, the refractive index nx in the stretching direction (slow axis direction) increases significantly, and accordingly, the difference between nx and nz, Δn _xz = nx - nz, tends to increase, making it difficult to reduce the NZ coefficient defined as NZ = (nx - nz) / (nx - ny).

One possible reason why the NZ coefficient can be reduced even at high stretch ratios (high birefringence) when a polymer film contains residual solvent is that the inclusion of 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 become sufficiently small. This is thought to be due in part to the fact that refractive index anisotropy is less likely to be expressed due to the low orientation of the polymer molecular chains (easily relaxed) even when the film undergoes dimensional changes.

The front retardation Re of the stretched retardation film is, for example, approximately 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 front retardation at a wavelength of 550 nm is preferably 240 to 300 nm, and more preferably 250 to 290 nm. As mentioned above, the NZ coefficient of the retardation film is preferably 0.7 or less, and by stretching a polymer film containing a specified amount of solvent, a retardation film with an even smaller NZ coefficient can be produced.

The front retardation and NZ coefficient of a retardation film are set appropriately depending on the application of the retardation film and the optical design of the image display device. Applications of retardation films 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.

For example, when an IPS-type 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 significant, and contrast reduction and color shifts are likely to occur. By placing a phase difference 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 oblique directions can be reduced and contrast can be improved.

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 λ/4 plate with a front retardation of ¼ the wavelength λ placed on one side of the polarizer (the side facing the organic EL cell). A retardation film with a refractive index anisotropy of nx>nz>ny shows little change in retardation with viewing angle. Therefore, using a retardation film with a refractive index anisotropy of nx>nz>ny as the λ/4 plate of the circular polarizer improves the blocking of light not only from the front (normal direction) of the display device, but also from oblique directions.

The in-plane birefringence Δn _xy of the retardation film may be 1.0×10 ⁻³ or more. 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 described above, in a stretched retardation film, the greater the stretching ratio, the greater the Δn tends to be, and the greater the Δn, the greater the in-plane retardation can be achieved with a smaller thickness. The Δn of the retardation film may be 1.3×10 ⁻³ or more, or 1.5×10 ⁻³ or more.

As described above, the retardation film can be used as an optical film for image display devices, such as for optical compensation of liquid crystal display devices. A polarizing plate can be formed by laminating a retardation film and a polarizer together. A polarizing plate can be obtained by attaching a retardation film to one main surface of a polarizer via an appropriate adhesive or pressure-sensitive adhesive layer. Another film may be laminated between the polarizer and the retardation film.

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 have been 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.

Among these, polyvinyl alcohol (PVA) polarizers are preferred because of their high polarization degree. These polarizers are made by adsorbing a dichroic substance such as iodine or a dichroic dye onto a polyvinyl alcohol film, such as polyvinyl alcohol or partially formalized polyvinyl alcohol, and then aligning the film in a predetermined direction. For example, a PVA polarizer can be obtained by dyeing a polyvinyl alcohol film with iodine and stretching it.

A thin polarizer having a thickness of 10 μm or less can also be used as the PVA-based polarizer. Examples of thin polarizers include the thin polarizing films described in JP-A Nos. 51-069644, 2000-338329, WO 2010/100917, Japanese Patent Nos. 4691205 and 4751481. Such thin polarizers can be obtained, for example, by stretching a laminate of a PVA-based resin layer and a resin substrate for stretching, followed by iodine dyeing.

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 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 or perpendicular. When a polarizer and a retardation film are laminated to form a circular polarizing plate, it is preferable to arrange the polarizer and the retardation film 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°.

A transparent film serving as a polarizer protective film may be attached to the other surface of the polarizer via an appropriate adhesive 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 or pressure-sensitive adhesive layer may be laminated on the polarizing plate for bonding to an image display cell, etc.

Retardation films and polarizing plates can be used as optical films for image display devices. For example, an image display panel can be obtained by attaching a polarizing plate with a retardation film to the surface of an image display cell via an appropriate adhesive. If 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.

The present invention will be explained in more detail below using examples and comparative examples, but the present invention is not limited to these examples.

[Synthesis Example 1]
A nitrogen-purged reaction vessel was charged with 21 parts by weight of dicyclopentadiene, 78 parts by weight of 8-methyl-8-carboxymethyltetracyclo[ ^{4.4.0.12,5.17,10} ]-3- ^dodecene , and 1 part by weight of 2-norbornene as monomers, 14.7 parts by weight of 1-hexene as a molecular weight modifier, and 150 parts by weight of toluene as a solvent, and heated to 107° C. To this solution, 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, and the mixture was allowed to react at 107° C. for 1 hour to obtain a ring-opened polymer. To 360 parts by weight of the obtained ring-opening polymer solution, 0.04 parts by weight of Ru[4-CH ₃ (CH ₂ ) ₄ C ₆ H ₄ CO ₂ ]H(CO)[P(C ₆ H ₅ ) ₃ ] was added as a hydrogenation reaction catalyst, 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 completion of the reaction, the obtained 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 obtained resin was melt-kneaded using a twin-screw extruder, extruded into a strand, and after water cooling, passed through a feeder rudder to obtain pellets.

[Comparative Example 1]
A 20 μm thick unstretched film was produced 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 bonded 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, and the heat-shrinkable films bonded to both sides were peeled off and removed to obtain a retardation film.

[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.

[Comparative Examples 4 and 5]
In Comparative Example 4, "ARTON RH4900" was used instead of "ARTON R5000" manufactured by JSR, 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.

[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.

[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.

[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.

[Examples 4 and 5]
In Example 4, "ARTON RH4900" was used instead of "ARTON R5000" manufactured by JSR, 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. Except for these, a retardation film was obtained in the same manner as in Example 1.

[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 on the heat-shrinkable film. This laminate was then stretched longitudinally at a temperature of 155°C and a stretching ratio of 1.3, and the heat-shrinkable film was then peeled off to obtain a retardation film.

[evaluation]
<Residual Solvent Amount>
The unstretched film was cut into a 10 cm square, and its weight _W0 was measured. After that, the unstretched film 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×(W ₀ −W ₁ )/W ₀

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

The production conditions and evaluation results for the unstretched films and retardation films of the examples and comparative examples are shown in Table 1.

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 content of 0.2 wt %, the NZ coefficient of the retardation film after stretching was 0.43. This shows that Example 1 has a larger in-plane retardation and a smaller NZ coefficient than Comparative Example 1. Similar trends were observed in the comparisons between Comparative Example 2 and Example 2, Comparative Example 3 and Example 3, Comparative Example 4 and Example 4, and Comparative Example 5 and Example 5.

In Example 6, in which a heat-shrinkable film was applied to one side of a polymer film by coating the film with a polymer solution and then stretching the resulting laminate, the NZ coefficient was smaller than in the case of Example 3 (Comparative Example 3), in which a polymer film produced by melt-casting was used.

On the other hand, in Comparative Example 6, which used a polymer film with a residual solvent content of 4% by weight, 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.

These results show that by stretching a laminate of a polymer film with a residual solvent content within a specified range and a heat-shrinkable film, it is possible to produce a retardation film with a smaller NZ coefficient.

Claims (3)

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 _xz between the refractive index nx in the in-plane slow axis direction and the refractive index nz in the thickness direction of the film of 0.0002 to 0.0010, and a residual solvent amount of 0.1 to 3.5 wt %;
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. 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. 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.