Method for manufacturing a retardation film

Abstract

To provide a retardation film capable of suppressing display unevenness when used for image display devices.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 (10) comprising heat-shrinkable film laminated on at least one surface of a polymer film having a glass transition temperature Tg1 in one direction while heating. The stretching involves heating the laminate at a first temperature in a range of (Tg1-4) to (Tg1+5)°C for 20 seconds or more, and then at a second temperature in a range of Tg1 to (Tg1+5)°C for 20 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, retardation films are used for optical compensation such as improving contrast and expanding the viewing angle, and for shielding external light reflected by metal electrodes (antireflection). A retardation film using a non-liquid crystalline polymer is imparted with optical anisotropy by stretching the polymer film in at least one direction.

[0003] When a polymer film is longitudinally stretched (uniaxially stretched at a free end), the molecular chains of the polymer are oriented in the stretching direction, and a shrinking action 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 at a free end, since the shrinkage rate in the width direction (reduction rate of the width) and the reduction rate of the 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 shrinkage amount in the width direction (direction perpendicular to the stretching direction) becomes larger than that in normal uniaxial stretching at a free end. Since 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, 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 or a λ / 4 plate for a circular polarizing plate.

Prior Art Documents

Patent Documents

[0005] Patent Document 1 Japanese Patent Application Laid-Open No. 2006-72309 Patent Document 2 Japanese Patent Application Laid-Open No. 2006-91836 SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0006] A retardation film having refractive index anisotropy of nx > nz > ny by utilizing the shrinkage force of a heat-shrinkable film is likely to have variations in in-plane retardation characteristics, and as a result, display unevenness may occur in an image display device. In recent years, the brightness and dynamic range of image display devices have been increasing, and even slight unevenness has become a quality issue. Therefore, there is a demand for a retardation film having uniform retardation characteristics and less unevenness. MEANS FOR SOLVING THE PROBLEMS

[0007] One aspect of the present invention is a method for manufacturing a retardation film in which the refractive index nx in the in-plane slow axis direction, the refractive index ny in the in-plane fast axis direction, and the refractive index nz in the thickness direction satisfy 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 of Tg 1 is stretched in one direction while being heated, and the heat-shrinkable film is peeled off and removed, whereby a retardation film satisfying nx > nz > ny can be obtained.

[0008] At the time of stretching, it is preferably heated at a first temperature of (Tg 1 - 4) to (Tg 1 + 5)°C for 20 seconds or more, and then heated at a second temperature of Tg 1 to (Tg 1 + 5)°C for 20 seconds or more. The second temperature is preferably 1°C or higher than the first temperature.

[0009] The front retardation of the retardation film may be 240 to 300 nm. The NZ coefficient defined by NZ = (nx - nz) / (nx - ny) of the retardation film may be 0.3 to 0.7.

Advantages of the Invention

[0010] When the retardation film of the present invention is applied to an image display device, the occurrence of display unevenness can be suppressed.

Brief Description of the Drawings

[0011]

Figure 1

Embodiments for Carrying Out the Invention

[0012] The retardation film according to an 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. By stretching a laminate obtained by laminating a heat-shrinkable film on a polymer film (unstretched film) in one direction, a retardation film having a refractive index anisotropy of nx > nz > ny can be obtained.

[0013] As the material of the polymer film, a non-liquid crystalline polymer material having a positive intrinsic birefringence is preferably used. A polymer having a positive intrinsic birefringence has a relatively large refractive index in the orientation direction when the polymer is oriented by stretching or the like. Examples of non-liquid crystalline polymers having a 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.

[0014] In one embodiment, a cyclic polyolefin resin is used as the material of the polymer film. Cyclic polyolefin is excellent in transparency, heat resistance, and chemical resistance, and is suitable as an optical film material for displays.

[0015] Examples of the cyclic polyolefin resin include resins described in JP-A-1-240517, JP-A-3-14882, JP-A-3-122137, etc. Specific examples include ring-opening (co)polymers of cyclic olefins, addition polymers of cyclic olefins, copolymers (typically random copolymers) of cyclic olefins and α-olefins such as ethylene and propylene, and graft polymers and hydrides obtained by modifying these with unsaturated carboxylic acids or their derivatives. Commercially available products of cyclic polyolefin resins include "Zeonor" and "Zeonex" manufactured by Nippon Zeon, "Arton" manufactured by JSR, "Apel" manufactured by Mitsui Chemicals, "Topas" manufactured by TOPAS ADVANCED POLYMERS, etc.

[0016] The cyclic polyolefin-based film preferably contains 50% by weight or more of the cyclic polyolefin resin. The content of the cyclic polyolefin 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.

[0017] 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 thickness of the film is not particularly limited, but is generally about 5 μm to 300 μm. The film may contain additives such as an ultraviolet absorber, a stabilizer, a lubricant, and a plasticizer.

[0018] From the perspective 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 perspective of drawability, 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).

[0019] The heat-shrinkable film is not particularly limited as long as it shrinks in the direction perpendicular to the drawing direction when laminated and drawn on the above polymer film. The material constituting the heat-shrinkable film is not particularly limited, but those that shrink near the glass transition temperature of the polymer film are 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.

[0020] The heat-shrinkable film has a shrinkage stress at the glass transition temperature Tg of the polymer film 1 of preferably 0.5 N / 4 mm or more. The heat-shrinkable film may have anisotropy in the shrinkage rate. When the heat-shrinkable film has anisotropy in the shrinkage rate, at the glass transition temperature Tg of the polymer film 1 the shrinkage stress in the direction perpendicular to the drawing direction of the polymer film is preferably 0.5 N / 4 mm or more.

[0021] The heat-shrinkable film may be laminated on one surface of the polymer film or on both surfaces of the polymer film. By laminating the heat-shrinkable film on the surface of the polymer film through an appropriate adhesive layer, a laminate of the polymer film and the heat-shrinkable film is formed. Using the heat-shrinkable film as a support substrate, a polymer solution is applied thereon and the solvent is dried to form a laminate in which the polymer film (coating film) to be drawn is closely laminated on the heat-shrinkable film.

[0022] 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.

[0023] When a single film without a heat-shrinkable film laminated thereon 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 amount of shrinkage 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 refractive index anisotropy of nx>nz>ny can be obtained.

[0024] The retardation film having refractive index anisotropy of nx>nz>ny satisfies 0<NZ<1, where the NZ coefficient is defined as NZ=(nx - nz) / (nx - ny). The NZ coefficient of the retardation film is preferably 0.3 to 0.7, and more preferably 0.4 to 0.6. The 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 a liquid crystal display device or a retardation film for a circular polarizing plate. The larger 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 become smaller.

[0025] The heating temperature (drawing temperature) in the heating furnace 30 is the glass transition temperature Tg of the polymer film 1 or higher, and preferably (Tg 1 + 5)°C or lower. The heating temperature in the drawing process is the maximum temperature within a range of 10 mm from the laminate 10 in the heating furnace 30. When the drawing temperature is within the range of Tg 1 to (Tg 1 + 5)°C, the NZ coefficient of the retardation film is controlled within an appropriate range, and unevenness tends to be suppressed.

[0026] The heating time of the laminate 10 in the heating furnace 30, that is, the residence time of the laminate 10 in the heating furnace 30, is preferably 40 seconds or more. When the heating time is excessively short, unevenness is likely to occur due to the rapid drawing and contraction of the polymer film.

[0027] The heating furnace 30 may have a plurality of individually temperature-adjustable heating zones along the conveyance direction (drawing direction) of the laminate 10. In FIG. 1, a form in which the heating furnace 30 has a first zone 31 on the upstream side and a second zone 32 on the downstream side is shown. The heating furnace may be provided with three or more heating zones. Each heating zone may be partitioned by a partition wall.

[0028] In one embodiment, the heating temperature in the first zone 31 is in the range of (Tg 1 - 4) to (Tg 1 + 5)°C, and the heating temperature in the second zone 32 is Tg 1 to (Tg 1 + 5)°C. The heating time in the first zone 31 and the second zone 32 is preferably 20 seconds or more, and may be 25 seconds or more or 30 seconds or more.

[0029] The heating temperature in the first zone 31 may be the same as or different from the heating temperature in the second zone 32. Preferably, the heating temperature of the second zone 32 is higher than the heating temperature of the first zone 31. It is preferable that the heating temperature of the second zone 32 is 1°C or more higher than the heating temperature of the first zone 31. The heating temperature of the second zone 32 may be 2°C or more or 3°C or more higher than the heating temperature of the first zone 31. The temperature difference between the first zone 31 and the second zone 32 is preferably 10°C or less, more preferably 8°C or less, and may be 6°C or less or 5°C or less.

[0030] When there is a temperature distribution along the conveying direction in the heating furnace 30, stretching of the film tends to proceed at the location where the temperature is the highest. When the heating furnace 30 is divided into two heating zones, the first zone 31 and the second zone 32, and the temperature of the second zone 32 is higher than the temperature of the first zone 31, stretching mainly proceeds in the second zone 32, and the first zone 31 serves as a preheating zone. When stretching in the heating furnace, after heating at a relatively low first temperature for 20 seconds or more and then heating at a relatively high second temperature for 20 seconds or more, the unevenness of the retardation film tends to be reduced.

[0031] As described above, the first temperature is (Tg 1 -4) to (Tg 1 +5)°C is preferable, and the second temperature is Tg 1 to (Tg 1 +5)°C is preferable. The first temperature may be the same as the second temperature, but preferably the second temperature is higher than the first temperature, and preferably the second temperature is 1°C or more higher than the first temperature.

[0032] The heating time at the first temperature and the heating time at the second temperature may be the same or different. In FIG. 1, since the lengths of the first zone 31 and the second zone 32 are the same, the heating time in the first zone 31 (heating time at the first temperature) and the heating time in the second zone 32 (heating time at the second temperature) are substantially the same. A difference may be provided in the lengths of the first zone and the second zone to provide a difference in the heating time at the first temperature in the first zone and the heating time at the second temperature in the second zone.

[0033] In FIG. 1, an example in which the heating furnace 30 has two heating zones 31 and 32 is shown, but the heating furnace 30 may not be divided into a plurality of zones. For example, if the heating temperature of the heating furnace is Tg 1 ~(Tg 1 +5)° C., and the heating time in the heating furnace is 40 seconds or more, then "(Tg 1 -4)~(Tg 1 +5)° C. for 20 seconds or more, and then Tg 1 ~(Tg 1 +5)° C. for 20 seconds or more" satisfies the condition.

[0034] The heating furnace may be divided into three or more heating zones. The temperatures of all the heating zones may be the same or different among the three or more heating zones. For example, the temperatures of the three or more heating zones may be set such that the temperature gradually increases from the upstream side to the downstream side. Among the three or more heating zones, the temperatures of two or more heating zones may be the same. Even when the heating furnace has three or more heating zones, the temperatures of the respective heating zones may be adjusted so as to satisfy the condition of "heating at a first temperature of (Tg 1 -4)~(Tg 1 +5)° C. for 20 seconds or more, and then heating at a second temperature of Tg 1 ~(Tg 1 +5)° C. for 20 seconds or more".

[0035] As described above, in a preferred embodiment, the first heating temperature is relatively low and the second heating temperature is relatively high. The first heating temperature is preferably (Tg 1 -3)~(Tg 1 +3)° C., and the second heating temperature is preferably (Tg 1 +3)~(Tg 1 +5)° C.

[0036] The higher the second heating temperature, the more tendency there is to reduce the unevenness of the retardation film. On the other hand, if the second heating temperature is excessively high, molecular orientation due to stretching is difficult to occur, and the front retardation may not be sufficiently exhibited. As described above, (Tg 1By ensuring that the heating time at the second heating temperature of +5°C or lower is 20 seconds or more and performing heating (preheating) at the first temperature for 20 seconds or more before the second heating, it is possible to achieve both suppression of unevenness and phase difference expressibility.

[0037] If it is possible to ensure that the heating time at the first heating temperature and the heating time at the second heating temperature are each 20 seconds or more, heating may be performed within a temperature range other than the above in the heating furnace before heating at the first heating temperature, between heating at the first heating temperature and heating at the second heating temperature, and after heating at the second temperature. For example, preheating may be performed at a temperature lower than the first temperature before heating at the first heating temperature. Also, heating may be performed at a temperature lower than the second temperature after heating at the second temperature.

[0038] After stretching the laminate in the heating furnace, the heat-shrinkable film is peeled off from the polymer film to obtain a retardation film. 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 240 to 300 nm, and more preferably 250 to 290 nm. As described above, the NZ coefficient of the retardation film is preferably 0.3 to 0.7, and more preferably 0.4 to 0.6.

[0039] The front retardation and NZ coefficient of the retardation film are appropriately set according to the use of the retardation film, the optical design of the image display device, etc. For example, a λ / 2 plate with an NZ coefficient of 0.3 to 0.7 is suitably used for optical compensation of an IPS-mode liquid crystal display device.

[0040] In the IPS mode liquid crystal display device, when viewed obliquely at an angle of 45 degrees (azimuth angles of 45 degrees, 135 degrees, 225 degrees, and 315 degrees) with respect to the absorption axis of the polarizer, there is a large amount of light leakage in the black display, and it is easy to cause a decrease in contrast and color shift. By disposing a retardation film having a front retardation of 1 / 2 of the wavelength λ and an NZ coefficient of 0.5 between the liquid crystal cell and the polarizer, the black luminance in the oblique direction can be reduced and the contrast can be improved.

[0041] The thickness of the retardation film is not particularly limited, but from the viewpoint of workability such as strength and handleability, 5 to 300 μm is preferable. In order to increase the front retardation, the thickness of the retardation film is preferably 10 μm or more, more preferably 20 μm or more, and may be 30 μm or more, 40 μm or more, or 50 μm or more. The thickness of the retardation film may be 250 μm or less or 200 μm or less.

[0042] The in-plane birefringence Δn of the retardation film is 1.0×10 -3 or more. The in-plane birefringence Δn = nx - ny is the difference between the refractive index nx in the slow axis direction in the plane and the refractive index ny in the fast axis direction in the plane, and is the value obtained by dividing the front retardation Re by the thickness. In the stretched retardation film, the larger the stretching ratio, the larger the Δn tends to be, and the larger the Δn, the larger the front retardation can be realized with a small thickness. The Δn of the retardation film may be 1.3×10 -3 or more, or 1.5×10 -3 or more.

[0043] As described above, the retardation film can be applied as an optical film for an image display device for the purpose of optical compensation of a liquid crystal display device or the like. The retardation film may be laminated and integrated with the polarizer to form a polarizing plate. By bonding the retardation film to one main surface of the polarizer via an appropriate adhesive layer or pressure-sensitive adhesive layer, a polarizing plate can be obtained. Another film may be laminated between the polarizer and the retardation film.

[0044] Examples of the polarizer include a hydrophilic polymer film such as a polyvinyl alcohol-based film, a partially formalized polyvinyl alcohol-based film, and an ethylene-vinyl acetate copolymer-based partially saponified film, to which a dichroic substance such as iodine or a dichroic dye is adsorbed and uniaxially stretched, and a polyene-based oriented film such as a dehydrated product of polyvinyl alcohol or a dehydrochlorinated product of polyvinyl chloride.

[0045] Among them, a polyvinyl alcohol (PVA)-based polarizer obtained by adsorbing a dichroic substance such as iodine or a dichroic dye to a polyvinyl alcohol-based film such as polyvinyl alcohol or a partially formalized polyvinyl alcohol and orienting it in a predetermined direction is preferred because it has a high degree of polarization. For example, a PVA-based polarizer can be obtained by subjecting a polyvinyl alcohol-based film to iodine staining and stretching.

[0046] As the PVA-based polarizer, a thin polarizer with a thickness of 10 μm or less can also be used. Examples of the thin polarizer include thin polarizing films described in, for example, JP-A-51-069644, JP-A-2000-338329, WO2010 / 100917 pamphlet, Japanese Patent No. 4691205, Japanese Patent No. 4751481, and the like. Such a thin polarizer can be obtained, for example, by stretching a laminate of a PVA-based resin layer and a stretching resin substrate and subjecting it to iodine staining.

[0047] 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 obliquely, it is preferable to arrange the two so that the absorption axis direction of the polarizer and the slow axis direction of the retardation film are parallel or perpendicular.

[0048] As described above, in the retardation film extending along the conveyance direction, the conveyance direction is the slow axis direction. Since the stretching direction of the PVA-based polarizer is the absorption axis direction, by laminating the retardation film and the polarizer roll-to-roll, a polarizing plate can be obtained in which the absorption axis direction of the polarizer and the slow axis direction of the retardation film are parallel. Further, by laminating the polarizer oriented by transverse stretching and the above-described retardation film roll-to-roll, a polarizing plate can be obtained in which the absorption axis direction of the polarizer and the slow axis direction of the retardation film are orthogonal.

[0049] On the other surface of the polarizer, a transparent film as a polarizer protection film may be bonded via an appropriate adhesive layer or pressure-sensitive adhesive layer. An optical film other than the above-described retardation film and polarizer protection film may be laminated on the polarizing plate. An adhesive layer or pressure-sensitive adhesive layer for bonding to an image display cell or the like may be laminated on the polarizing plate.

[0050] 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 including 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 is formed by further combining a backlight as a light source.

[0051] As described above, in the retardation film of the present invention, the occurrence of unevenness is suppressed by adjusting the stretching conditions. Therefore, by using the retardation film of the present invention, it is possible to provide an image display device in which display unevenness is suppressed and visibility is excellent.

Example

[0052] Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these examples.

[0053] [Production Example of Retardation Film] Using pellets of cyclic polyolefin resin (JSR's "ARTON RH4900", glass transition temperature: 141°C), an unstretched film with a thickness of 80 μm was produced by the melt extrusion method. A biaxially stretched propylene film having heat shrinkability ("Trefan" manufactured by Toray Industries, Inc.) was laminated on both sides of this film via an adhesive to obtain a laminate.

[0054] This laminate was longitudinally stretched at a draw ratio of 1.25 times using a roll stretcher equipped with a heating furnace having eight temperature zones along the stretching direction. After that, the heat-shrinkable films laminated on both sides were peeled off and removed to obtain a stretched retardation film.

[0055] As shown in Table 1, in Production Examples 1 to 5, 7, and 9 to 11, the first to fourth zones were set at the same temperature (first temperature), the fifth to sixth zones were set at the same temperature (second temperature), and the conveyance speed (delivery speed) of the film was adjusted so that the time for the film to pass through the heating furnace (total stretching time) was 60 seconds. In Production Examples 6 and 8, the temperatures of the first to eighth zones were the same. In Production Examples 12 to 15, the temperatures of each zone were set as shown in Table 1, and the conveyance speed of the film was changed to adjust the total stretching time. In Table 1, the stretching time at that temperature is shown in parentheses.

[0056] [Evaluation] [Retardation characteristics] The retardation film was cut out into a size of 50 mm × 50 mm, and the front retardation at a measurement wavelength of 550 nm and the retardation in a state where the sample was inclined by 40° with the slow axis direction in the plane as the rotation center were measured using a polarization / retardation measurement system ("AxoScan" manufactured by Axometrics). From these measured values, the front retardation at a wavelength of 550 nm: Re = (nx - ny) × d and the NZ coefficient: NZ = (nx - nz) / (nx - ny) were calculated. nx is the refractive index in the slow axis direction in the plane, ny is the refractive index in the fast axis direction in the plane, nz is the refractive index in the thickness direction, and d is the thickness.

[0057] [Non-uniformity] The retardation film of the production example and the linear polarizer were bonded together via an adhesive layer such that the slow axis direction of the retardation film was parallel to the absorption axis direction of the polarizer. This sample was cut into a size of 20 cm × 20 cm and placed on top of a light box with the surface on the retardation film side facing upward. Another linear polarizer was overlaid thereon so as to form crossed Nicols, and visually observed within the range of an azimuth angle of 45° and a polar angle (the angle formed with the normal of the film) of 0 to 40° with respect to the absorption axis direction of the polarizer, and the presence or absence of unevenness was evaluated according to the following criteria. A: No unevenness was visually observed B: Slight unevenness was observed C: Unevenness was clearly visually observed

[0058] Table 1 shows the stretching conditions (heating temperature and heating time) of Production Examples 1 to 15 and the evaluation results of the retardation films.

[0059]

Table 1

[0060] 136 °C: (Tg 1 -5 °C) in the first to fourth zones set at 30 seconds, 146 °C: (Tg 1 +5 °C) In Production Example 1 where stretching was performed while heating for 30 seconds in the fifth to eighth zones set, significant unevenness was observed in the retardation film. In Production Examples 2 to 5 where the temperature of the first to fourth zones (first temperature) was increased, a tendency for the unevenness to decrease was observed as the temperature increased, but in Production Examples 6 and 7 where the first temperature was further increased, a tendency for the unevenness to become prominent was observed as the temperature increased.

[0061] In Production Example 8 where the temperature of all the first to eighth zones was set to 140 °C, the unevenness of the retardation film was prominent. Also, the NZ coefficient of the retardation film of Production Example 8 was 0.9, which was larger than that of other examples. In Production Example 8, it is considered that due to the low stretching temperature, the contraction action in the width direction by the heat-shrinkable film was not sufficiently exhibited and the NZ coefficient did not sufficiently decrease.

[0062] In Production Examples 9 to 11 where the temperature (second temperature) in the 5th to 8th zones was increased, unevenness tended to decrease as the temperature increased. However, in Production Example 11 where the second temperature was 148 °C: (Tg 1 + 7 °C), due to the high stretching temperature, the retardation was small and the phase difference developability was insufficient.

[0063] In Production Example 12 where the heating time at 140 °C (first temperature) was shortened to 10 seconds, unevenness was more prominent than in Production Example 3. On the other hand, in Production Example 13 where the heating time at the first temperature was 50 seconds, unevenness was reduced compared to Production Example 3. From these results, it can be seen that unevenness tends to decrease by increasing the heating time at the relatively low first temperature.

[0064] In Production Example 14 where the heating time at 146 °C (second temperature) was shortened to 10 seconds, the NZ coefficient of the retardation film was larger than in Production Example 3. On the other hand, in Production Example 15 where the heating time at the second temperature was 50 seconds, it showed an NZ coefficient equivalent to that of Production Example 3. From these results, it can be seen that within the range where phase difference developability can be ensured, by increasing the second temperature and lengthening its heating time, a retardation film with less unevenness and a smaller NZ coefficient can be obtained.

Explanation of Signs

[0065] 10 Laminated body 30 Heating furnace 31, 32 Heating zones 51, 52 Inlet side nip rolls 61, 62 Outlet side nip rolls

Claims

Claim 1 A method for manufacturing a retardation film in which the refractive index nx in the slow axis direction in the plane, the refractive index ny in the fast axis direction in the plane, 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 conveyed in a heating furnace while being stretched along the conveyance direction by an inlet-side nip roll located upstream of the inlet of the heating furnace and an outlet-side nip roll located downstream of the outlet of the heating furnace and rotating at a peripheral speed greater than that of the inlet-side nip roll, having a stretching step, The glass transition temperature of the polymer film is Tg １ and The heating furnace has a plurality of heating zones whose temperatures can be adjusted individually. In the stretching step, after heating at a first temperature of (Tg １ −4) to (Tg １ +4)°C for 20 seconds or more, it is heated at a second temperature that is 1°C or more higher than the first temperature and within the range of Tg １ to (Tg １ +5)°C for 20 seconds or more. A method for manufacturing a retardation film. Claim 2 The method for manufacturing a retardation film according to claim 1, wherein the polymer film is a cyclic polyolefin-based film. Claim 3 The method for manufacturing a retardation film according to claim 1 or 2, wherein the front retardation of the retardation film is 240 to 300 nm. Claim 4 The method for manufacturing a retardation film according to any one of claims 1 to 3, wherein the NZ coefficient defined by NZ = (nx - nz) / (nx - ny) of the retardation film is 0.3 to 0.7.

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