Method for manufacturing phase difference films

Abstract

To provide a retardation film small in variations in an orientation angle of an optical axis.SOLUTION: A retardation film satisfies an expression of nx>nz>ny, where a refractive index in an in-plane slow axis direction is denoted by nx, a refractive index in an in-plane fast axis direction is denoted by ny, and a refractive index in a thickness direction is denoted by nz. In manufacturing the retardation film, a laminate (10) being formed such that a heat-shrinkable film is stacked on at least one surface of a polymer film having a glass transition temperature Tg1 is stretched in one direction while being heated. A value (T-Tg1) / d1 obtained by dividing a difference (T-Tg1) between the glass transition temperature Tg1 of the polymer film and a stretching temperature T by a thickness d1 of the polymer film before being stretched is 0.03°C / μm or more. A stretching time (heating time at a temperature of Tg1 or more in a stretching process) is 40 seconds or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a retardation film.

Background Art

[0002] In displays such as liquid crystal display devices, a retardation film is used for optical compensation such as improving contrast and expanding the viewing angle, and for shielding external light reflected by metal electrodes (anti-reflection). A retardation film using a non-liquid crystal polymer is imparted with optical anisotropy by stretching the polymer film in at least one direction.

[0003] [[ID=1,6]]When a polymer film is longitudinally stretched (uniaxially stretched with a free end), the molecular chains of the polymer are oriented in the stretching direction, and a contraction effect occurs in the direction perpendicular to the stretching direction, that is, the width direction and the thickness direction. When a film of a polymer having a positive refractive index is longitudinally stretched, the refractive index (nx) in the longitudinal direction increases, and the refractive indices (ny) in the width direction and (nz) in the thickness direction decrease. In uniaxial stretching with a free end, since the shrinkage rate in the width direction (the rate of decrease in width) and the rate of decrease in thickness are substantially the same, a retardation film (positive A plate) having refractive index anisotropy of nx>ny≒nz can be obtained.

[0004] When the laminate is stretched while heating in a state where a heat-shrinkable film is laminated on at least one surface of the polymer film, due to the influence of the shrinkage force of the heat-shrinkable film, the amount of shrinkage in the width direction (the direction perpendicular to the stretching direction) becomes larger than that in normal uniaxial stretching with a free end. Since the refractive index ny in the width direction (the fast axis direction) becomes smaller and the refractive index nz in the thickness direction becomes relatively larger, a retardation film having refractive index anisotropy of nx>nz>ny can be obtained (see, for example, Patent Document 1 and Patent Document 2). A retardation film having refractive index anisotropy of nx>nz>ny has a small change in retardation depending on the viewing angle, and is used as an optical compensation film for a liquid crystal display device and a λ / 4 plate for a circular polarizing plate.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 2006-72309 [Patent Document 2] Japanese Patent Publication No. 2006-91836 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Phase difference films used in image display devices are required to exhibit minimal changes in optical properties even when exposed to high-temperature environments for extended periods during use. In particular, phase difference films used in automotive image display devices require even greater durability at high temperatures, and in recent years, it has been required that no changes in optical properties occur even after high-temperature durability tests exceeding 100°C.

[0007] Phase difference films that utilize the shrinkage force of heat-shrinkable films to achieve refractive index anisotropy of nx>nz>ny are more susceptible to changes in retardation due to heating compared to phase difference films that achieve refractive index anisotropy of nx>ny≈nz by uniaxial stretching at the free end, and therefore require improved heat durability.

[0008] In view of the above, the present invention aims to provide a phase difference film having refractive index anisotropy nx>nz>ny and exhibiting small retardation changes due to heating. [Means for solving the problem]

[0009] One aspect of the present invention is a method for manufacturing a phase difference film in which the refractive index nx in the slow phase axis direction, the refractive index ny in the fast phase 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 having a glass transition temperature Tg1 is stretched in one direction while being heated, and the heat-shrinkable film is peeled off to obtain a phase difference film that satisfies nx>nz>ny.

[0010] During stretching, the polymer film is stretched at a stretching temperature T that is higher than the glass transition temperature Tg1 of the polymer film. The value obtained by dividing the difference between the glass transition temperature Tg1 and the stretching temperature T (T-Tg1) by the thickness d1 of the polymer film before stretching (T-Tg1) / d1 is 0.03℃ / μm or greater. In the stretching process, the heating time at a temperature above Tg1 is 40 seconds or more.

[0011] The frontal retardation of the phase difference film may be 230-320 nm. The NZ coefficient of the phase difference film, defined as NZ = (nx-nz) / (nx-ny), may be 0.4-0.8. [Effects of the Invention]

[0012] The phase difference film of the present invention exhibits minimal change in optical properties even when exposed to high-temperature environments for extended periods, and has excellent heat resistance. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic cross-sectional view showing the process of uniaxial stretching of a laminate of a polymer film and a heat-shrinkable film at its free end. [Figure 2] This graph shows the rate of change in retardation after heating tests of the phase difference films of the examples and comparative examples. [Modes for carrying out the invention]

[0014] A phase difference film according to one embodiment of the present invention has refractive index anisotropy of nx>nz>ny, where nx is the refractive index in the in-plane slow phase axis direction, ny is the refractive index in the in-plane fast phase axis direction, and nz is the refractive index in the thickness direction. A phase difference film having refractive index anisotropy of nx>nz>ny is obtained by stretching a laminate obtained by laminating a heat-shrinkable film onto a polymer film (unstretched film) in one direction.

[0015] 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 other means, the refractive index in the orientation direction becomes relatively larger. Examples of non-liquid crystal 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.

[0016] 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.

[0017] Examples of cyclic polyolefin resins include those described in Japanese Patent Publication No. 1-240517, Japanese Patent Publication No. 3-14882, and Japanese Patent Publication No. 3-122137. Specific examples include ring-opening (co)polymers of cyclic olefins, addition polymers of cyclic olefins, copolymers of cyclic olefins with α-olefins such as ethylene and propylene (typically random copolymers), and graft polymers and hydrides obtained by modifying these with unsaturated carboxylic acids or their derivatives. Commercially available cyclic polyolefin resins include "Zeonor" and "Zeonex" from Nippon Zeon, "Arton" from JSR, "Appel" from Mitsui Chemicals, and "Topas" from TOPAS ADVANCEDPOLYMERS.

[0018] The cyclic polyolefin-based film preferably contains 50% by weight or more of a cyclic polyolefin-based resin. The content of the cyclic polyolefin-based resin in the cyclic polyolefin-based film is more preferably 70% by weight or more, still more preferably 80% by weight or more, and may be 90% by weight or more or 95% by weight or more.

[0019] As a method for producing the polymer film, known methods such as a solution casting method and a melt extrusion method can be adopted. The film may contain additives such as an ultraviolet absorber, a stabilizer, a lubricant, and a plasticizer.

[0020] The thickness d1 of the film is not particularly limited, but generally it is about 5 μm to 300 μm. The thickness d1 of the film may be 200 μm or less, 150 μm or less, 120 μm or less, or 100 μm or less. As described later, the smaller the thickness d1 of the polymer film and the higher the stretching temperature, the more excellent the heat resistance of the retardation film tends to be. From the viewpoint of retardation performance, the thickness d1 of the film may be 50 μm or more, 70 μm or more, or  80 μm or more.

[0021] 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 or 160°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).

[0022] The heat-shrinkable film is not particularly limited as long as it shrinks in the direction perpendicular to the stretching direction when laminated and stretched on the above polymer film. The material constituting the heat-shrinkable film is not particularly limited, but a material that shrinks near the glass transition temperature of the polymer film is preferred. From the viewpoint of excellent versatility and low cost, polyolefins such as polyethylene and polypropylene, and polyesters are preferably used as the material of the heat-shrinkable film.

[0023] The heat-shrinkable film preferably has a shrinkage stress of 0.5 N / 4 mm or more at the glass transition temperature Tg1 of the polymer film. The heat-shrinkable film may have anisotropic shrinkage rates. When the heat-shrinkable film has anisotropic shrinkage rates, it is preferable that the shrinkage stress in the direction perpendicular to the stretching direction of the polymer film is 0.5 N / 4 mm or more at the glass transition temperature Tg1 of the polymer film.

[0024] The heat-shrinkable film may be laminated on one surface of the polymer film or on both surfaces of the polymer film. A laminate of the polymer film and the heat-shrinkable film is formed by bonding the heat-shrinkable film to the surface of the polymer film through an appropriate adhesive layer. A laminate in which a polymer film (coating film) to be stretched is closely laminated on the heat-shrinkable film may be formed by applying a polymer solution on the heat-shrinkable film as a support substrate and drying the solvent.

[0025] FIG. 1 is a cross-sectional view schematically showing a state in which a laminate 10 of a polymer film and a heat-shrinkable film is uniaxially stretched at a free end. A pair of nip rolls 51 and 52 are provided on the inlet side (upstream) of the heating furnace 30, and a pair of nip rolls 61 and 62 are provided on the outlet side (downstream) of the heating furnace 30. By making the peripheral speed of the outlet-side nip rolls 61 and 62 larger than the peripheral speed of the inlet-side nip rolls 51 and 52, the laminate 10 is stretched in the conveying direction in the heating furnace 30. The stretching ratio (the peripheral speed ratio of the outlet-side nip roll to the inlet-side nip roll) is set in a range of, for example, about 1.01 times to 2 times according to the target optical characteristics.

[0026] The heating furnace 30 may have multiple heating zones whose temperatures can be individually adjusted along the transport direction (stretching direction) of the laminate 10. Figure 1 shows a configuration in which the heating furnace 30 has an upstream first zone 31 and a downstream second zone 32. The heating furnace may have three or more heating zones. Each heating zone may be separated by a partition wall. When the heating furnace is divided into multiple heating zones, the heating temperatures in each heating zone may be the same or different.

[0027] When a single film without a heat-shrinkable film laminated on it is stretched along the transport direction at a stretching ratio of p, the width and thickness decrease at the same ratio, so the width and thickness become (1 / √p) times the original width and thickness, respectively. When a film with a heat-shrinkable film laminated on it is stretched along the transport direction, the heat-shrinkable film shrinks in the width direction, so the amount of shrinkage in the width direction is greater than in the case of a single polymer film. Therefore, when the stretching ratio is p, the width of the stretched film becomes smaller than (1 / √p) times the original width, and the thickness of the stretched film becomes larger than (1 / √p) times the original thickness. Consequently, the refractive index ny in the width direction (phase-advancing axis direction) becomes smaller, and the refractive index nz in the thickness direction becomes relatively larger, so a phase difference film with refractive index anisotropy nx>nz>ny is obtained.

[0028] The heating temperature (stretching temperature) T in the heating furnace 30 is higher than the glass transition temperature Tg1 of the polymer film. The stretching temperature is the highest temperature within the laminate 10 to 10 mm range in the heating furnace 30. If a temperature distribution exists along the transport direction in the heating furnace 30, the film stretching proceeds most easily at the highest temperature location. If the heating temperatures of multiple heating zones are different, the temperature of the highest heating zone is taken as the stretching temperature T.

[0029] The stretching temperature T is preferably (Tg1+3)°C or higher, more preferably (Tg1+5)°C or higher, even more preferably (Tg1+7)°C or higher, and may also be (Tg1+9)°C or higher, (Tg1+10)°C or higher, or (Tg1+11)°C or higher. The higher the stretching temperature, the more the phase difference film tends to have superior heat resistance. On the other hand, if the stretching temperature is excessively high, the phase difference formation due to stretching may be low, and the desired frontal retardation may not be obtained. Therefore, the stretching temperature T is preferably (Tg1+20)°C or lower, and more preferably (Tg1+15)°C or lower.

[0030] The value obtained by dividing the difference between the glass transition temperature Tg1 and the stretching temperature T of the polymer film (T-Tg1) by the thickness d1 of the polymer film before stretching, (T-Tg1) / d1, is 0.03°C / μm or greater. Preferably, (T-Tg1) / d1 is 0.05°C / μm or greater, more preferably 0.07°C / μm or greater, and even more preferably 0.10°C / μm or greater. The larger (T-Tg1) / d1, the better the heat resistance of the phase difference film, and the smaller the change in frontal retardation when heated at high temperatures tends to be.

[0031] The heating time (stretching time) of the laminate 10 in the heating furnace 30 is preferably 40 seconds or more, and may be 50 seconds or more. If the heating time is excessively short, the heat durability of the phase difference film may be insufficient, even if the (T-Tg1) / d1 value is large. When the heating time (stretching) is short, the polymer molecular chains are rapidly stretched, making it easy for strain to remain in the phase difference film. When the phase difference film is heated, the orientation of the polymer molecular chains is relaxed when the strain is released. Therefore, it is thought that phase difference films with large strains are more susceptible to relaxation of molecular chain orientation by heating, and consequently, retardation tends to decrease.

[0032] The longer the stretching time, the smaller the change in retardation due to heating of the phase difference film tends to be. On the other hand, from the viewpoint of productivity and phase difference formation, the stretching time is preferably 360 seconds or less, and may be 300 seconds or less.

[0033] When the temperature is uniform along the stretching direction (transport direction) in the heating furnace 30, the temperature of the heating furnace is the stretching temperature, and the time during which the laminate 10 stays in the heating furnace is the stretching time. When there is a temperature distribution along the stretching direction in the heating furnace 30, for example, when the heating furnace 30 has a plurality of heating zones 31, 32 and the temperatures of each heating zone are different, the highest temperature in the heating furnace is the stretching temperature T, and the time during which the temperature in the heating furnace is Tg1 or higher is the stretching time. For example, when the temperature T1 of the first zone 31 is lower than Tg1 and the temperature T2 of the second zone 32 is higher than Tg1, the residence time of the laminate 10 in the second zone 32 is the stretching time.

[0034] After stretching the laminate in the heating furnace, by peeling and removing the heat-shrinkable film from the polymer film, a retardation film can be obtained. The front retardation Re of the 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 front retardation at a wavelength of 550 nm is preferably 230 to 320 nm, more preferably 240 to 310 nm, and may be 250 to 300 nm.

[0035] 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 0.2 to 0.8, more preferably 0.3 to 0.7, and even more preferably 0.4 to 0.6. A retardation film with an NZ coefficient close to 0.5 has no change in retardation depending on the viewing angle and is used as an optical compensation film for a liquid crystal display device or a retardation film for a circular polarizing plate.

[0036] The greater the shrinkage in the width direction due to the shrinkage force of the heat-shrinkable film during stretching, the smaller the NZ coefficient of the phase difference film tends to be. The higher the stretching temperature T, the greater the shrinkage of the heat-shrinkable film, and therefore the smaller the NZ coefficient tends to be. The lower the stretching temperature, the larger the NZ coefficient (closer to 1) tends to be, and the greater the frontal retardation tends to be. Also, at the same stretching temperature, the larger the stretching ratio, the greater the frontal retardation and the larger the NZ coefficient tends to be.

[0037] The frontal retardation Re of a phase difference film is expressed as the product of the in-plane birefringence Δn = nx - ny and the thickness d, Δn × d. Since the thickness d1 of the film before stretching tends to increase as the thickness d of the film after stretching increases, the frontal retardation Re tends to increase as d1 increases.

[0038] As described above, in the present invention, a high stretching temperature T and a small film thickness d1 result in a large (T-Tg1) / d1, and a phase difference film with small changes in frontal retardation due to heating is obtained. On the other hand, when the stretching temperature T is high and the thickness d1 is small, the frontal retardation Re tends to be small. Therefore, in order to obtain a phase difference film with high retardation that can be used as a λ / 2 plate, it is necessary to increase the stretching ratio.

[0039] From the viewpoint of obtaining a phase difference film that can be applied as a λ / 2 plate while (T-Tg1) / d1 satisfies the above range, the stretching ratio is preferably 1.18 times or more, more preferably 1.22 times or more, even more preferably 1.24 times or more, and may be 1.26 times or more, 1.28 times or more, or 1.30 times or more. As mentioned above, the stretching ratio is generally 2 times or less, and may be 1.8 times or less, 1.6 times or less, 1.5 times or less, or 1.4 times or less.

[0040] Phase difference films can be applied as optical films for image display devices, such as for optical compensation in liquid crystal displays. Applications of phase difference films with an NZ coefficient less than 1 include optical compensation in liquid crystal displays and circular polarizers for shielding reflected light in organic EL displays. The frontal retardation and NZ coefficient of the phase difference film are set appropriately according to the application of the phase difference film and the optical design of the image display device.

[0041] For example, in IPS type liquid crystal display devices, when viewed from an oblique direction at an angle of 45 degrees with respect to the absorption axis of the polarizer (azimuth angles of 45, 135, 225, and 315 degrees), significant light leakage in black displays occurs, easily leading to a decrease in contrast and color shift. By placing a phase difference film between the liquid crystal cell and the polarizer, where the frontal retardation is 1 / 2 of the wavelength λ and the NZ coefficient is 0.5, the black brightness in oblique directions can be reduced and contrast can be improved.

[0042] A polarizing plate may be formed by laminating a phase difference film with a polarizer. A polarizing plate can be obtained by bonding a phase difference film to one main surface of a polarizer via an appropriate adhesive layer or tack layer. Another film may be laminated between the polarizer and the phase difference film.

[0043] Examples of polarizers include hydrophilic polymer films such as polyvinyl alcohol-based films, partially formalized polyvinyl alcohol-based 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 or dehydrochlorinated polyvinyl chloride.

[0044] Among these, polyvinyl alcohol (PVA) polarizers are preferred because they have a high degree of polarization. These polarizers are made by adsorbing dichroic substances such as iodine or dichroic dyes onto a polyvinyl alcohol-based film, such as polyvinyl alcohol or partially formalized polyvinyl alcohol, and oriented in a predetermined direction. For example, a PVA polarizer can be obtained by iodine dyeing and stretching a polyvinyl alcohol-based film.

[0045] As a PVA-based polarizer, a thin polarizer with a thickness of 10 μm or less can also be used. Examples of thin polarizers include the thin polarizing films described in Japanese Patent Publication No. 51-069644, Japanese Patent Publication No. 2000-338329, WO2010 / 100917, Japanese Patent Specification No. 4691205, and Japanese Patent Specification No. 4751481. Such thin polarizers can be obtained, for example, by stretching a laminate of a PVA-based resin layer and a stretchable resin substrate and then staining it with iodine.

[0046] The arrangement angle of the polarizer and the phase difference film is not particularly limited. For example, when using a phase difference film for optical compensation to suppress light loss when viewing a liquid crystal display device from an oblique direction, it is preferable to arrange them so that the absorption axis direction of the polarizer and the slow phase axis direction of the phase difference film are parallel or perpendicular. When forming a circular polarizer by laminating the polarizer and the phase difference film, it is preferable to arrange them so that the angle between the absorption axis direction of the polarizer and the slow phase axis direction of the phase difference 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°.

[0047] A transparent film serving as a polarizer protective film may be laminated to the other side of the polarizer via an appropriate adhesive or tack layer. Optical films other than the phase difference film and polarizer protective film may be laminated onto the polarizing plate. An adhesive or tack layer may be laminated onto the polarizing plate for bonding with an image display cell or the like.

[0048] Phase difference films and polarizing plates can be used as optical films for image display devices. For example, an image display panel can be obtained by bonding a polarizing plate equipped with a phase difference film to the surface of an image display cell using 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. [Examples]

[0049] 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.

[0050] [Examples of film manufacturing] <Manufacturing Example 1> Unstretched films with thicknesses of 133 μm, 153 μm, 174 μm, or 194 μm were prepared by melt extrusion using pellets of cyclic polyolefin resin (JSR's "ARTON R5000," glass transition temperature: 136°C).

[0051] <Manufacturing Example 2> Unstretched films with thicknesses of 92 μm, 133 μm, 153 μm, 174 μm, or 194 μm were prepared by melt extrusion using pellets of cyclic polyolefin resin (JSR "ARTON RH4900", glass transition temperature: 141°C).

[0052] [Film stretching] A laminate was obtained by bonding a heat-shrinkable biaxially oriented propylene film (Toray's "Trefan") to both sides of the unstretched film obtained in Production Examples 1 and 2 via an adhesive. After this laminate was uniaxially stretched (longitudinal stretching) at the free end using a roll stretcher under the conditions shown in Table 1, the heat-shrinkable films bonded to both sides were peeled off to obtain a phase difference film.

[0053] [evaluation] <Phase difference characteristics> Ten 50mm x 50mm samples were cut from the center of the widthwise direction of the phase difference film, and each sample was attached to an adhesive glass plate. Using a polarization and phase difference measurement system (Axometrics "AxoScan"), frontal retardation and retardation with the sample tilted 40° around the slow phase axis were measured at a measurement wavelength of 550nm. From these measurements, the frontal retardation at 550nm: Re = (nx - ny) × d and the NZ coefficient: NZ = (nx - nz) / (nx - ny) were calculated. nx is the refractive index in the slow phase axis direction in the plane, ny is the refractive index in the fast phase axis direction in the plane, nz is the refractive index in the thickness direction, and d is the thickness. The average values ​​of the frontal retardation Re and NZ coefficient of the 10 samples were used as the Re and NZ for the phase difference films of the examples and comparative examples.

[0054] <Heating Test> Of the 10 samples whose phase difference characteristics were measured, 5 were placed in an 85°C oven and 5 in a 105°C oven and heated for 500 hours. After that, the frontal retardation Re1 was measured again, and for each sample, the percentage change (%) from the frontal retardation Re0 before heating was calculated as 100 × (Re1 - Re0) / Re0. The average percentage change of the 5 samples heated at 85°C and the average percentage change of the frontal retardation Re of the 5 samples heated at 105°C were calculated.

[0055] Table 1 shows the glass transition temperature Tg1 and thickness d1 of the unstretched films in the examples and comparative examples, the stretching conditions (stretching temperature T, stretching ratio, and stretching time), and the evaluation results of the phase difference films. Figure 2 shows the plot of (T-Tg1) / d1 on the x-axis and the rate of change of Re after the heating test on the y-axis for Examples 1-8, 10 and Comparative Examples 1, 3-6, where the stretching time was 60 seconds.

[0056] [Table 1]

[0057] In Examples 1-8, 10 and Comparative Examples 1, 3-6, where the stretching time was 60 seconds, a larger value of (T-Tg1) / d1 tended to correlate with a smaller change in Re after the heating test, indicating superior heat resistance. Figure 2 shows a high correlation between (T-Tg1) / d1 and the change in Re after the heating test, and the same trend is observed in Examples 1-8 and Comparative Example 1, which used the film of Production Example 1 with a glass transition temperature of 136°C, and in Examples 10 and Comparative Examples 3-5, which used the film of Production Example 2 with a glass transition temperature of 141°C.

[0058] In Comparative Example 2, where the stretching time was changed to 30 seconds while maintaining the same stretching temperature and stretching ratio as Example 5, the rate of change in Re after the heating test was larger compared to Example 5. On the other hand, in Example 9, where the stretching time was changed to 300 seconds while maintaining the same stretching temperature and stretching ratio as Example 5, it exhibited excellent heating durability equivalent to that of Example 5.

[0059] These results indicate that by having a large (T-Tg1) / d1 ratio and ensuring sufficient stretching time, a phase difference film with minimal change in frontal retardation due to heating and excellent heat resistance can be obtained. [Explanation of Symbols]

[0060] 10 Laminate 30 Furnace 31, 32 Heating Zones 51, 52 Inlet side nip roll 61, 62 Exit side nip roll

Claims

1. A method for manufacturing a phase difference film in which the refractive index nx in the slow phase axis direction, the refractive index ny in the fast phase axis direction, and the refractive index nz in the thickness direction satisfy nx > nz > ny, The process involves a stretching step in which a laminate, in which a heat-shrinkable film is laminated on at least one surface of a polymer film, is conveyed from upstream to downstream in a heating furnace while being heated, between an inlet nip roll provided on the upstream side of the heating furnace and an outlet nip roll provided on the downstream side of the heating furnace with a higher peripheral speed than the inlet nip roll, and stretched in the conveying direction. The polymer film has a thickness of d １ , glass transition temperature Tg １ And, In the stretching process, the Tg between the inlet nip roll and the outlet nip roll １ The heating time at a higher stretching temperature T is 50 to 360 seconds. (T-Tg 1) / d 1 is 0.03°C / μm or higher. A method for manufacturing a phase difference film.

2. A method for manufacturing a phase difference film according to claim 1, wherein the frontal retardation of the phase difference film is 230 to 320 nm.

3. A method for manufacturing a phase difference film according to claim 1 or 2, wherein the NZ coefficient of the phase difference film, defined as NZ = (nx - nz) / (nx - ny), is 0.4 to 0.

6.

4. A method for producing a phase difference film according to any one of claims 1 to 3, wherein the polymer film is a cyclic polyolefin film.

5. A method for manufacturing a phase difference film according to any one of claims 1 to 4, wherein the stretching ratio in the stretching step is 1.18 to 2 times.

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