JP7534904B2 - Manufacturing method of stretched film - Google Patents

Description

The present invention relates to a method for producing a stretched film.

Conventionally, in image display devices such as liquid crystal display devices (LCDs) and organic electroluminescence display devices (OLEDs), retardation films such as λ/4 plates have been used to improve display characteristics and prevent reflection. Retardation films are generally produced by stretching a resin film to control the in-plane retardation (for example, Patent Documents 1 and 2).

The retardation film may exhibit different wavelength dispersion characteristics (specifically, inverse wavelength dispersion characteristics in which the retardation value increases with an increase in the wavelength of the measurement light, positive wavelength dispersion characteristics in which the retardation value decreases with an increase in the wavelength of the measurement light, or flat wavelength dispersion characteristics in which the retardation value hardly changes with the wavelength of the measurement light) depending on the constituent material of the film. Among these, retardation films that exhibit inverse wavelength dispersion characteristics generally have brittle resins as constituent materials. Therefore, in the manufacture of such films, sufficient preheating is required before stretching in order to impart the desired retardation, and residual stress due to shrinkage is relaxed after stretching in order to ensure heating reliability. However, such a manufacturing method is prone to unevenness in the orientation angle, and the obtained retardation film has room for further improvement in terms of orientation.

Patent No. 5975776 Patent No. 5594125

The present invention has been made to solve the above-mentioned problems in the past, and its purpose is to provide a method for manufacturing a retardation film with improved alignment axis precision and uniaxiality.

According to one aspect of the present invention, there is provided a method for producing a stretched film, comprising: a first step including, while running a long thermoplastic resin film in the longitudinal direction, preheating, stretching in the width direction, shrinking in the width direction, and winding up into a roll; and a second step including, while unwinding the rolled thermoplastic resin film and running it in the longitudinal direction, preheating, stretching in the width direction, and shrinking in the width direction, in that order, wherein when a stretch ratio in the width direction in the first step is E1 and a total stretch ratio in the width direction is E, the following formula (1) is satisfied:
E×0.55<E1<E×0.75 (1)
In one embodiment, the production method is a method for producing a stretched film that satisfies the following formulas (A) and (B).
0.8<R(450)/R(550)<1 (A)
1<R(650)/R(550)<1.2 (B)
(In formulas (A) and (B), R(450), R(550) and R(650) are the front retardations of the film measured at 23° C. using light having wavelengths of 450 nm, 550 nm and 650 nm, respectively.)
In one embodiment, the total stretching ratio in the width direction is 2.4 to 3.2 times.
In one embodiment, the stretching ratio in the width direction in the first step is 1.4 to 2.2 times.
In one embodiment, the thermoplastic resin film contains at least one selected from a polycarbonate-based resin, a polyvinyl acetal resin, a cellulose ester-based resin, a polyester-based resin, and a polyester carbonate-based resin.

According to the manufacturing method of the present invention, a long thermoplastic resin film is stretched and shrunk in the width direction while traveling in the opposite directions in the first and second steps, and further, by setting the ratio of the stretching ratio in the first step to the total stretching ratio within a predetermined range, a retardation film with improved both orientation axis precision and uniaxiality can be obtained.

FIG. 2 is a schematic plan view illustrating a first step in one embodiment of the present invention. FIG. 4 is a schematic plan view illustrating a second step in one embodiment of the present invention. FIG. 4 is a diagram showing the relationship between the stretching ratio and the birefringence in the first step. FIG. 4 is a diagram showing the relationship between the stretching ratio and the orientation angle unevenness in the first step.

The following describes preferred embodiments of the present invention, but the present invention is not limited to these embodiments.

A. Manufacturing Method of Stretched Film The manufacturing method of a stretched film according to an embodiment of the present invention includes a first step including, in this order, preheating, stretching in the width direction, shrinking in the width direction, and winding up a roll while running a long thermoplastic resin film in the longitudinal direction, and a second step including, in this order, preheating, stretching in the width direction, and shrinking in the width direction while unwinding and running the rolled thermoplastic resin film in the longitudinal direction, and the second step satisfies the following formula (1), where E1 is the stretch ratio in the width direction in the first step, and E is the total stretch ratio in the width direction.
E×0.55<E1<E×0.75 (1)

According to the above-mentioned method for producing a stretched film, the starting end of the long thermoplastic resin film in the first step becomes the end end in the second step, and the running direction of the thermoplastic resin film in the first step and the running direction in the second step are opposite to each other. In this way, the long thermoplastic resin film is stretched and shrunk in the width direction while running in the opposite directions in the first step and the second step, and the ratio of the stretching ratio in the first step to the total stretching ratio in the width direction is set within a predetermined range, thereby obtaining a retardation film with improved alignment axis precision and uniaxiality. The reason for obtaining such an effect is considered to be as follows, although it does not limit the present invention in any way. That is, by dividing the stretching and shrinking into multiple steps and setting the stretching ratio at the first stretching to a high value, the in-plane alignment of the resin molecular chains can be improved, and as a result, the uniaxiality is thought to be improved. In addition, by reversing the running direction during each stretching and shrinking, it is thought that the alignment axis precision is improved by offsetting the unevenness in the alignment angle caused by the bowing phenomenon, etc.

The total stretching ratio E in the width direction in the manufacturing method of the stretched film can be appropriately set according to the purpose. In an embodiment in which the target stretched film is a retardation film exhibiting reverse wavelength dispersion characteristics, the total stretching ratio E in the width direction can be, for example, 2.4 to 3.2 times, preferably 2.6 to 3.0 times, and more preferably 2.7 to 2.9 times. The total stretching ratio is the product of the stretching ratios in all the width direction stretching performed in the manufacturing method of the stretched film. Therefore, for example, when the manufacturing method of the stretched film includes only the stretching in the first step and the stretching in the second step as the stretching in the width direction, the total stretching ratio E is obtained as the product of the stretching ratio E1 in the width direction in the first step and the stretching ratio E2 in the width direction in the second step.

A-1. Thermoplastic resin film As the material for forming the thermoplastic resin film, any suitable thermoplastic resin can be used depending on the purpose. For example, polycarbonate-based resin, polyvinyl acetal resin, cycloolefin-based resin, acrylic resin, cellulose ester-based resin, cellulose-based resin, polyester-based resin, polyester carbonate-based resin, olefin-based resin, polyurethane-based resin, etc. are mentioned. Preferably, it is polycarbonate-based resin, polyvinyl acetal resin, cellulose ester-based resin, polyester-based resin, polyester carbonate-based resin. This is because if it is these resins, a retardation film showing the wavelength dependency of so-called reverse dispersion can be obtained. These resins may be used alone or in combination depending on the desired characteristics.

As the polycarbonate-based resin, any suitable polycarbonate-based resin can be used. For example, a polycarbonate-based resin containing a structural unit derived from a dihydroxy compound is preferred. Specific examples of the dihydroxy compound include 9,9-bis(4-hydroxyphenyl)fluorene, 9,9-bis(4-hydroxy-3-methylphenyl)fluorene, 9,9-bis(4-hydroxy-3-ethylphenyl)fluorene, 9,9-bis(4-hydroxy-3-n-propylphenyl)fluorene, 9,9-bis(4-hydroxy-3-isopropylphenyl)fluorene, 9,9-bis(4-hydroxy-3-n-butylphenyl)fluorene, 9,9-bis(4-hydroxy-3-sec-butylphenyl)fluorene, 9,9-bis(4-hydroxy-3-tert-butylphenyl)fluorene, 9,9-bis(4-hydroxy-3-cyclohexylphenyl)fluorene, 9,9-bis(4-hydroxy-3-phenylphenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)phenyl)fluorene, and 9,9-bis( 4-(2-hydroxyethoxy)-3-methylphenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)-3-isopropylphenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)-3-isobutylphenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)-3-tert-butylphenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)-3-cyclohexylphenyl)fluorene phenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)-3-phenylphenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)-3,5-dimethylphenyl)fluorene, 9,9-bis(4-(2-hydroxyethoxy)-3-tert-butyl-6-methylphenyl)fluorene, 9,9-bis(4-(3-hydroxy-2,2-dimethylpropoxy)phenyl)fluorene, etc. In addition to the structural units derived from the dihydroxy compounds described above, the polycarbonate resin may contain structural units derived from dihydroxy compounds such as isosorbide, isomannide, isoidet, spiroglycol, dioxane glycol, diethylene glycol (DEG), triethylene glycol (TEG), polyethylene glycol (PEG), and bisphenols.

Details of the polycarbonate-based resins described above are described, for example, in JP 2012-67300 A and Japanese Patent No. 3325560. The descriptions in these patent documents are incorporated herein by reference.

The glass transition temperature of polycarbonate resins is preferably 110°C or higher and 250°C or lower, and more preferably 120°C or higher and 230°C or lower. If the glass transition temperature is too low, the heat resistance tends to be poor, and dimensional changes may occur after film formation. If the glass transition temperature is too high, the molding stability during film formation may be poor, and the transparency of the film may be impaired. The glass transition temperature is determined in accordance with JIS K 7121 (1987).

Any suitable polyvinyl acetal resin can be used as the polyvinyl acetal resin. Typically, the polyvinyl acetal resin can be obtained by a condensation reaction of at least two types of aldehyde compounds and/or ketone compounds with a polyvinyl alcohol resin. Specific examples of polyvinyl acetal resins and detailed manufacturing methods are described in, for example, JP 2007-161994 A. The description is incorporated herein by reference.

The thickness of the thermoplastic resin film (before stretching) is, for example, 10 μm to 300 μm, preferably 20 μm to 200 μm, and more preferably 30 μm to 150 μm.

A-2. First Step The first step includes preheating a long thermoplastic resin film (hereinafter sometimes simply referred to as a "resin film") while running it in the longitudinal direction, stretching it in the width direction, shrinking it in the width direction, and winding it up into a roll, in that order. If necessary, the first step may further include stabilizing the resin film after shrinking in the width direction. In addition, the end portion may be slit before winding to adjust the film width to a desired value.

In one embodiment, a thermoplastic resin film is stretched in the width direction using a tenter-type stretching device while traveling in the longitudinal direction, and then shrunk in the width direction. FIG. 1 is a schematic plan view illustrating the first step in this embodiment. While being transported in the longitudinal direction at a predetermined speed, the thermoplastic resin film 1 is held at its widthwise ends by left and right clips 20L, 20R at the entrance of the stretching device 100a, and travels through the preheating zone A1, stretching zone B1, shrinkage zone C1, and stabilization zone D1 in that order. Thereafter, the thermoplastic resin film 1 is released from the grip by the left and right clips 20L, 20R, and wound into a roll by the winding device 200.

A-2-1. Stretching device The stretching device 100a shown in FIG. 1 includes a pair of left and right rails 10L, 10R, and multiple pairs of left and right clips 20L, 20R that run along the left and right rails 10L, 10R and grip the widthwise ends of the resin film 1. Although not shown, the stretching device 100a typically further includes a heating means (such as an oven) capable of adjusting the resin film to a predetermined temperature in each zone described below. In this specification, the left rail as viewed from the inlet side of the film is referred to as the left rail 10L, and the right rail as viewed from the right rail 10R. In the stretching device 100a, a preheating zone A1, a stretching zone B1, a shrinking zone C1, and a stabilization zone D1 are provided in this order from the inlet side of the film to the outlet side. Note that these zones respectively mean zones in which the resin film to be stretched is substantially preheated, stretched, shrunk, and stabilized, and do not necessarily mean mechanically or structurally independent compartments. Also, it should be noted that the length ratios of the various zones in the drawing apparatus of FIG. 1 may differ from the actual length ratios.

In the stretching device 100a, the left and right rails 10L, 10R are configured to be symmetrical in a plan view, and in the preheating zone A, the left and right rails 10L, 10R are configured to extend approximately parallel to the longitudinal direction of the resin film 1 at a distance corresponding to the initial width of the resin film 1 to be stretched. In the stretching zone B1, the left and right rails 10L, 10R are configured to gradually increase in distance from the preheating zone A1 toward the shrinking zone C1 until the distance corresponds to the width of the resin film 1 after stretching. In the shrinking zone C1, the left and right rails 10L, 10R are configured to gradually decrease in distance from the stretching zone B1 toward the stabilization zone D1. In the stabilization zone D1, the left and right rails 10L, 10R are configured to be approximately parallel to each other at a distance corresponding to the width of the resin film 1 after shrinkage. Typically, the pair of left and right clips 20L, 20R run at approximately the same speed so that the line connecting the pair of left and right clips is approximately perpendicular to the running direction of the resin film.

The stretching device used in the manufacturing method according to the embodiment of the present invention is not limited to the stretching device shown in the above-mentioned example. For example, the stretching device may be one in which a preheating zone A1, a stretching zone B1, and a shrinkage zone C1 are provided in this order from the inlet side to the outlet side of the film, and which does not have a stabilization zone D1.

A-2-2. Gripping A long resin film is gripped at both ends by left and right clips at the film intake of the stretching device. Typically, the resin film is gripped at both ends by the left and right clips at a constant clip pitch that is equal to each other. Preferably, the resin film is gripped so that the line connecting the pair of left and right clips when gripping both ends is approximately perpendicular to the running direction of the resin film. The resin film whose both ends are gripped by the left and right clips is sent to the preheating zone A1 by the movement of the clips.

A-2-3. Preheating In the preheating zone A1, the left and right rails 10L, 10R are configured to be approximately parallel to each other (extending in a direction approximately parallel to the running direction of the resin film) with a separation distance corresponding to the initial width of the resin film 1 to be stretched as described above, so that the resin film is basically heated without being stretched transversely or longitudinally. However, the separation distance between the left and right clips may be slightly increased to avoid problems such as the resin film being deflected by preheating and coming into contact with the nozzle in the oven.

In preheating, the resin film is heated to a temperature T1 (°C). The temperature T1 is preferably equal to or higher than the glass transition temperature (Tg) of the resin film, more preferably equal to or higher than Tg + 2°C, and even more preferably equal to or higher than Tg + 5°C. On the other hand, the heating temperature T1 is preferably equal to or lower than Tg + 40°C, and more preferably equal to or lower than Tg + 30°C. Although it varies depending on the resin film used, the temperature T1 is, for example, 70°C to 190°C, and preferably 80°C to 180°C.

The heating time to the above temperature T1 and the holding time at temperature T1 can be appropriately set according to the constituent material of the resin film and the manufacturing conditions (e.g., the conveying speed of the resin film). The resin film may also be heated in stages. These heating times and holding times can be controlled by adjusting the moving speed of the clip, the length of the preheating zone, the temperature of the preheating zone, etc. The preheating time can be, for example, 8 to 180 seconds.

In the stretching zone B1, the resin film is stretched in the width direction. Specifically, the resin film is stretched in the width direction by traveling through the stretching zone B1, which is configured so that the distance between the left and right rails gradually increases downstream in the conveying direction.

The stretching ratio E1 in the width direction (E1=film width after stretching (W1)/initial film width (W0)) can be appropriately selected depending on the final stretching ratio E. Specifically, the stretching ratio E1 in the width direction is selected so as to satisfy the following formula (1), and is preferably set so as to satisfy formula (2), and more preferably formula (3). The above film width means the film width including the portions gripped by the left and right clips.
E×0.55<E1<E×0.75 (1)
E×0.6<E1<E×0.7 (2)
E×0.6<E1<E×0.65 (3)

In one embodiment, the stretching ratio E1 in the width direction can be, for example, 1.4 to 2.2 times, preferably 1.5 to 2.1 times, and more preferably 1.6 to 2.0 times.

The stretching speed [(film width after stretching (W1) - initial film width (W0)) / time required for stretching process] is preferably 30 mm/sec or less, more preferably 10 mm/sec to 25 mm/sec, and even more preferably 12 mm/sec to 20 mm/sec. By setting such a stretching speed, it is possible to achieve both heating reliability and orientation axis precision.

The above stretching is typically performed at a temperature T2. The temperature T2 is preferably Tg-20°C to Tg+30°C relative to the glass transition temperature (Tg) of the resin film, more preferably Tg-10°C to Tg+20°C, and particularly preferably about Tg. Although it varies depending on the resin film used, the temperature T2 is, for example, 70°C to 180°C, preferably 80°C to 170°C. The difference (T1-T2) between the above temperatures T1 and T2 is preferably ±2°C or more, more preferably ±5°C or more. In one embodiment, T1>T2, and therefore the resin film heated to temperature T1 in preheating can be cooled to temperature T2.

A-2-5. Shrinkage In the shrinkage zone C1, the resin film is shrunk in the width direction. Specifically, the resin film is shrunk in the width direction by traveling through the shrinkage zone C1, which is configured so that the distance between the left and right rails gradually decreases toward the downstream of the conveying direction. This shrinkage process relieves residual stress in the resin film, thereby improving heating reliability.

The shrinkage ratio in the width direction [film width after shrinkage (W2)/film width after stretching (W1)] is preferably 0.92 to 1.0, more preferably 0.95 to 1.0, and even more preferably 0.98 to 1.0. With such a shrinkage ratio, high heating reliability can be obtained.

The shrinkage is typically performed at a temperature T3. The temperature T3 is typically equal to or lower than the glass transition temperature (Tg) of the resin film, preferably Tg-20°C to Tg°C, more preferably Tg-15°C to Tg°C, and even more preferably Tg-10°C to Tg°C.

A-2-6. Stabilization In the stabilization zone D1, typically, the resin film is maintained at a temperature T4 without being stretched or shrunk. This stabilizes the orientation state of the resin film. Typically, the temperature T4 is equal to or lower than the glass transition temperature (Tg) °C of the resin film, preferably equal to or lower than Tg-1 °C, and more preferably room temperature to Tg-3 °C. In one embodiment, the temperature T4 is equal to the temperature T3 °C. The stabilization treatment time can be appropriately set depending on the constituent material of the resin film and the manufacturing conditions (for example, the conveying speed of the resin film). The stabilization time can be, for example, 5 seconds to 60 seconds.

A-2-7. Release The resin film that has been subjected to the above shrinkage and further optional stabilization is released from the left and right clips near the exit of the stretching device. Release from the clips can be performed, for example, in a room temperature (about 25° C.) environment.

A-2-8. Winding The resin film released from the clips is wound into a roll by a winding device in the usual manner.

A-3. Second step The second step includes preheating, stretching in the width direction, and shrinking in the width direction, in this order, while unwinding the rolled thermoplastic resin film wound in the first step and running it in the longitudinal direction. If necessary, the second step may further include stabilizing the resin film after shrinking in the width direction.

In one embodiment, the roll-shaped thermoplastic resin film wound in the first step is unwound and run in the longitudinal direction, while being stretched in the width direction using a tenter-type stretching device, and then shrunk in the width direction. FIG. 2 is a schematic plan view illustrating the second step in this embodiment. The thermoplastic resin film 1 wound into a roll in the first step is unwound from the unwound device 300 and conveyed in the longitudinal direction at a predetermined speed, while the ends in the width direction are held by the left and right clips 20L, 20R at the entrance of the stretching device 100b, and run through the preheating zone A2, the stretching zone B2, the shrinkage zone C2, and the stabilization zone D2 in that order. The thermoplastic resin film 1 is then released from the grip by the left and right clips 20L, 20R.

A-3-1. Stretching device The stretching device 100b shown in FIG. 2 has a basically similar configuration to the stretching device 100a shown in FIG. 1. Specifically, the stretching device 100b includes a pair of left and right rails 10L, 10R, and a plurality of pairs of left and right clips 20L, 20R that run along the left and right rails 10L, 10R and grip the ends of the resin film 1 in the width direction. Typically, the stretching device 100b further includes a heating means (such as an oven) capable of adjusting the resin film to a predetermined temperature in each zone described later (not shown). In the stretching device 100b, a preheating zone A2, a stretching zone B2, a shrinking zone C2, and a stabilization zone D2 are provided in this order from the inlet side to the outlet side of the film. Note that these zones respectively mean zones where the resin film to be stretched is substantially preheated, stretched, shrunk, and stabilized, and do not necessarily mean mechanically or structurally independent sections. It should be noted that the ratio of the lengths of the respective zones in the stretching device in FIG. 2 may differ from the actual ratio of the lengths.

In the stretching device 100b, the left and right rails 10L, 10R are configured to be symmetrical in a plan view, and in the preheating zone A2, the left and right rails 10L, 10R are configured to extend approximately parallel to the longitudinal direction of the resin film 1 at a distance corresponding to the initial width of the resin film 1 to be stretched (the width of the resin film unwound in step 2). In the stretching zone B2, the left and right rails 10L, 10R are configured to gradually expand from the preheating zone A2 side toward the shrinking zone C2 until the distance corresponds to the width of the resin film 1 after stretching. In the shrinking zone C2, the left and right rails 10L, 10R are configured to gradually reduce the distance from the stretching zone B2 side toward the stabilization zone D2 side. In the stabilization zone D2, the left and right rails 10L, 10R are configured to be approximately parallel to each other at a distance corresponding to the width of the resin film 1 after shrinkage. Typically, the pair of left and right clips 20L, 20R run at approximately the same speed so that the line connecting the pair of left and right clips is approximately perpendicular to the running direction of the resin film.

The stretching device used in the manufacturing method according to the embodiment of the present invention is not limited to the stretching device shown in the above-mentioned example. For example, the stretching device may be one in which a preheating zone A2, a stretching zone B2, and a shrinkage zone C2 are provided in this order from the inlet side to the outlet side of the film, and which does not have a stabilization zone D2.

A-3-2. Gripping The long resin film is gripped at both ends by left and right clips at the film intake of the stretching device. The gripping of the long resin film by the left and right clips can be performed in the same manner as in the first process.

A-3-3. Preheating In the preheating zone A2, the resin film is preheated. The same explanation as for the preheating in the first step can be applied to the preheating of the resin film.

In the stretching zone B2, the resin film is stretched in the width direction. Specifically, the resin film is stretched in the width direction by traveling through the stretching zone B2, which is configured so that the distance between the left and right rails gradually increases downstream in the conveying direction.

The stretching ratio E2 in the width direction (E1 = film width after stretching (W4) / initial film width (W3)) can be appropriately set according to the final stretching ratio E. In one embodiment, the stretching ratio E2 in the width direction can be, for example, 1.3 to 1.9 times, preferably 1.4 to 1.8 times, and more preferably 1.5 to 1.7 times. The initial film width (W3) can correspond to the film width (W2) after shrinkage in the first step, but since slight shrinkage may occur during or after stabilization, or the ends may be slit, the relationship W3 ≦ W2 can be satisfied.

Regarding the stretching speed and stretching temperature, the same explanation as for the stretching in the first step applies.

A-3-5. Shrinkage In the shrinkage zone C2, the resin film is shrunk in the width direction. Specifically, the resin film is shrunk in the width direction by traveling through the shrinkage zone C2, which is configured so that the distance between the left and right rails gradually decreases toward the downstream of the conveying direction. This shrinkage process relieves residual stress in the resin film, thereby improving heating reliability.

The shrinkage ratio in the width direction [film width after shrinkage (W5)/film width after stretching (W4)] is preferably 0.92 to 1.0, more preferably 0.95 to 1.0, and even more preferably 0.98 to 1.0. Such a shrinkage ratio can provide high heating reliability.

Regarding the shrinkage temperature, the same explanation as for shrinkage in the first step applies.

In the stabilization zone D2, the resin film is typically maintained at a predetermined stabilization temperature without being stretched or shrunk. The same explanation as for the stabilization in the first step can be applied to the stabilization temperature and stabilization time.

The resin film that has been subjected to the above shrinkage and, optionally, stabilization is released from the left and right clips near the exit of the stretching device. The release from the clips can be performed, for example, in a room temperature (about 25° C.) environment.

The resin film (stretched film) released from the clips can be wound into a roll by a winding device. Alternatively, it can be used for the next process (e.g., punching to a specified size, lamination with another long optical film, etc.) without being wound.

B. Stretched Film The stretched film obtained by the manufacturing method described in A is typically a retardation film having optical anisotropy. The thickness of the stretched film is preferably 30 μm to 70 μm, more preferably 35 μm to 55 μm, and even more preferably 35 μm to 45 μm.

The retardation film preferably has a refractive index characteristic that satisfies the relationship nx>ny. In addition, the retardation film preferably has high in-plane orientation, and for example, the birefringence Δn (Δn=nx-ny) measured at a wavelength of 550 nm is preferably 0.0020 to 0.0050, more preferably 0.0025 to 0.0050, even more preferably 0.0030 to 0.0045, and even more preferably 0.0035 to 0.0045.

The retardation film can preferably function as a λ/4 plate. The front retardation Re(550) of the retardation film is preferably 100 nm to 160 nm, more preferably 135 nm to 155 nm. In this specification, nx is the refractive index in the direction in which the in-plane refractive index is maximum (i.e., the slow axis direction), ny is the refractive index in the direction perpendicular to the slow axis in the plane (i.e., the fast axis direction), and nz is the refractive index in the thickness direction. Re(λ) is the front retardation of the film measured with light having a wavelength of λ nm at 23° C. Therefore, Re(550) is the front retardation of the film measured with light having a wavelength of 550 nm at 23° C. Re(λ) is calculated by the formula: Re(λ)=(nx-ny)×d, where d(nm) is the thickness of the film.

The retardation film may exhibit any suitable index ellipsoid as long as it has the relationship nx>ny. Preferably, the index ellipsoid of the retardation film exhibits the relationship nx>ny≧nz. The Nz coefficient of the retardation film is preferably 1 to 1.20, more preferably 1 to 1.19, even more preferably 1 to 1.18, and even more preferably 1 to 1.17. The Nz coefficient is calculated by Nz = Rth(λ)/Re(λ). Here, Rth(λ) is the retardation in the thickness direction of the film measured with light of wavelength λ nm at 23°C, and is calculated by the formula: Rth(λ) = (nx-nz) x d.

The retardation film typically exhibits the wavelength dependency of so-called reverse dispersion. Specifically, the front retardation satisfies the relationship of Re(450)<Re(550)<Re(650). Preferably, the retardation film satisfies the following formulas (A) and (B), and more preferably further satisfies (C).
0.8<R(450)/R(550)<1 (A)
1<R(650)/R(550)<1.2 (B)
0.8<R(450)/R(550)≦0.95 (C)

The absolute value of the photoelastic coefficient of the retardation film is preferably 2×10 ⁻¹² (m ² /N) to 100×10 ⁻¹² (m ² /N), and more preferably 2×10 ⁻¹² (m ² /N) to 50×10 ⁻¹² (m ² /N).

The retardation film has excellent alignment axis precision. The difference between the alignment axis (slow axis) angle at the widthwise end and the alignment axis (slow axis) angle at the widthwise center can be, for example, within ±3°, preferably within ±2°, and more preferably within ±1.8°.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. The measurement and evaluation methods used in the examples are as follows.

(1) Orientation angle (direction in which the slow axis appears)
The width direction center and both ends of the retardation film obtained in the examples and comparative examples were cut into a square shape of 50 mm wide and 50 mm long so that one side was parallel to the width direction of the film to prepare a sample. This sample was measured using a Mueller matrix polarimeter (manufactured by Axometrics, product name "Axoscan") to measure the orientation angle at a wavelength of 550 nm and 23 ° C. The orientation angle was measured with the sample placed parallel to the measurement table. The difference between the orientation angle of the width direction center and the orientation angle of both ends was obtained and evaluated as the orientation angle unevenness (orientation angle Δ).
(2) Front retardation Re, thickness direction retardation Rth, and Δn
In the same manner as in (1) above, the measurement was performed at 23° C. using an “Axoscan” manufactured by Axometrics Inc. Note that Δn is a value at a wavelength of 550 nm.
(3) Nz coefficient Calculated based on the formula: Nz = Rth(550)/Re(550).
(4) Thickness: Measurement was performed using a microgauge type thickness meter (manufactured by Mitutoyo Corporation).

[Example 1]
(Preparation of polycarbonate resin film)
Polymerization was carried out using a batch polymerization apparatus consisting of two vertical reactors equipped with stirring blades and a reflux condenser controlled at 100°C. 9,9-[4-(2-hydroxyethoxy)phenyl]fluorene (BHEPF), isosorbide (ISB), diethylene glycol (DEG), diphenyl carbonate (DPC), and magnesium acetate tetrahydrate were charged in a molar ratio of BHEPF/ISB/DEG/DPC/magnesium acetate = 0.348/0.490/0.162/1.005/1.00 x ^10-5 . After the inside of the reactor was sufficiently replaced with nitrogen (oxygen concentration 0.0005 to 0.001 vol%), heating was carried out with a heat medium, and stirring was started when the inside temperature reached 100°C. The internal temperature was allowed to reach 220° C. 40 minutes after the start of the temperature rise, and while controlling to maintain this temperature, pressure reduction was started, and the pressure was reduced to 13.3 kPa 90 minutes after reaching 220° C. Phenol vapor by-produced during the polymerization reaction was guided to a reflux condenser at 100° C., and a small amount of monomer components contained in the phenol vapor was returned to the reactor, and uncondensed phenol vapor was guided to a condenser at 45° C. and recovered.

Nitrogen was introduced into the first reactor to restore the pressure to atmospheric pressure, and the oligomerized reaction liquid in the first reactor was then transferred to the second reactor. Next, heating and decompression in the second reactor were started, and the internal temperature was 240°C and the pressure was 0.2 kPa in 50 minutes. Polymerization was then allowed to proceed until a predetermined stirring power was reached. When the predetermined power was reached, nitrogen was introduced into the reactor to restore the pressure, and the reaction liquid was extracted in the form of strands and pelletized with a rotary cutter to obtain polycarbonate resin A with a copolymer composition of BHEPF/ISB/DEG = 34.8/49.0/16.2 [mol %]. The reduced viscosity of this polycarbonate resin was 0.430 dL/g and the glass transition temperature was 140°C.

The resulting polycarbonate resin was vacuum dried at 80°C for 5 hours, and then a polycarbonate resin film with a thickness of 130 μm was produced using a film-making device equipped with a single-screw extruder (Isuzu Chemical Engineering Co., Ltd., screw diameter 25 mm, cylinder set temperature: 220°C), a T-die (width 900 mm, set temperature: 220°C), a chill roll (set temperature: 120-130°C) and a winder.

(Production of Retardation Film)
The polycarbonate resin film obtained as described above was subjected to a first step using a stretching apparatus as shown in Fig. 1, and then to a second step using a stretching apparatus as shown in Fig. 2 to obtain a retardation film. The details of each step are as follows.

(First step)
A long polycarbonate resin film (thickness 130 μm, width (W0) 610 mm) was conveyed in the longitudinal direction at a speed of 1500 mm/sec while the left and right ends were held with left and right clips.

Then, while traveling through the preheating zone, the film temperature was raised in three stages (144°C, 149°C, and 154°C), and each temperature was maintained for approximately 1.5 seconds, thereby preheating the resin film.

Next, the resin film was stretched in the width direction by running it through a stretching zone where the distance between the left and right clips was increased. The stretching temperature (film temperature) was 143°C, and the stretch ratio (W1/W0) was 1.55 times.

Then, the resin film was shrunk in the width direction by running it through a shrinkage zone where the distance between the left and right clips was reduced. The shrinkage temperature (film temperature) was 138°C, and the shrinkage ratio (W2/W1) was 0.98 times.

Next, at the exit of the stretching device (ambient temperature 25°C), the resin film was released from the left and right clips, and both ends including the clipped parts were slit by about 100 mm, and then wound into a roll by a winding device.

(Second step)
The roll-shaped polycarbonate resin film (thickness 84 μm, width (W3) 767 mm) obtained in the first step was unwound and run in the opposite direction to the running direction in the first step at a speed of 1500 mm/sec while the left and right ends were held with left and right clips.

Then, while traveling through the preheating zone, the film temperature was raised in three stages (144°C, 149°C, and 154°C), and each temperature was maintained for approximately 1.5 seconds, thereby preheating the resin film.

Next, the resin film was stretched in the width direction by running it through a stretching zone where the distance between the left and right clips was increased. The stretching temperature (film temperature) was 143°C, and the stretch ratio (W4/W3) was 1.81 times (total stretch ratio: 2.8 times).

Then, the resin film was shrunk in the width direction by running it through a shrinkage zone where the distance between the left and right clips was reduced. The shrinkage temperature (film temperature) was 138°C, and the shrinkage ratio (W5/W4) was 0.98 times.

Then, at the exit of the stretching device (ambient temperature 25°C), the resin film was released from the left and right clips.

In this way, a phase difference film (thickness 47 μm) was obtained.

[Example 2]
A retardation film (thickness 47 μm) was obtained in the same manner as in Example 1, except that the stretching ratio (W1/W0) in the first step was 1.75 times and the stretching ratio (W4/W3) in the second step was 1.6 times.

[Example 3]
A retardation film (thickness 47 μm) was obtained in the same manner as in Example 1, except that the stretching ratio (W1/W0) in the first step was 1.95 times and the stretching ratio (W4/W3) in the second step was 1.44 times.

[Example 4]
A retardation film (thickness 47 μm) was obtained in the same manner as in Example 1, except that the stretching ratio (W1/W0) in the first step was 2.1 times and the stretching ratio (W4/W3) in the second step was 1.34 times.

[Example 5]
A retardation film (thickness: 40 μm) was obtained in the same manner as in Example 1, except that the thickness of the film before stretching was 110 μm.

[Example 6]
A retardation film (thickness: 36 μm) was obtained in the same manner as in Example 2, except that the thickness of the film before stretching was 100 μm.

[Example 7]
A retardation film (thickness: 38 μm) was obtained in the same manner as in Example 3, except that the thickness of the film before stretching was 105 μm.

[Example 8]
A retardation film (thickness: 42 μm) was obtained in the same manner as in Example 4, except that the thickness of the film before stretching was 120 μm.

[Comparative Example 1]
A retardation film (thickness 47 μm) was obtained in the same manner as in Example 1, except that the stretching ratio (W1/W0) in the first step was 2.8 times and the second step was not performed.

[Comparative Example 2]
A retardation film (thickness 47 μm) was obtained in the same manner as in Example 1, except that the stretching ratio (W1/W0) in the first step was 1.4 times and the stretching ratio (W4/W3) in the second step was 2.0 times.

[Comparative Example 3]
A retardation film (thickness 47 μm) was obtained in the same manner as in Example 1, except that the stretching ratio (W1/W0) in the first step was 2.45 times and the stretching ratio (W4/W3) in the second step was 1.14 times.

[Reference Example 1]
A retardation film (thickness 30 μm) was obtained in the same manner as in Example 1 except that an isosorbide polycarbonate resin film (manufactured by Mitsubishi Chemical Corporation, product name “Durabio”, thickness 85 μm, Tg = 124 ° C.) was used, the preheating temperature in the first step was 140 ° C., the stretching temperature was 137 ° C., the stretching ratio (W1/W0) was 2.8 times, and the second step was not performed.

[Reference Example 2]
A retardation film (thickness 18 μm) was obtained in the same manner as in Example 1, except that a cyclic olefin resin film (manufactured by JSR Corporation, product name “Arton”, thickness 50 μm, Tg = 137°C) was used, the preheating temperature in the first step was 147°C, the stretching temperature was 142°C, the stretching ratio (W1/W0) was 2.8 times, and the second step was not performed.

The retardation films obtained in the above Examples, Comparative Examples, and Reference Examples were subjected to the above measurements (1) to (3). The results are shown in Table 1. In addition, for the retardation films obtained in the Examples and Comparative Examples, the relationship between the stretching ratio in the first step and the birefringence and the relationship between the stretching ratio in the first step and the orientation angle unevenness are shown in Figures 3 and 4, respectively.

<Evaluation>
As shown in Table 1 and Figures 3 and 4, the retardation films obtained in the examples are superior in both alignment axis precision and alignment (in-plane alignment and uniaxiality) compared with the retardation films obtained in the comparative examples. Also, as shown in Reference Examples 1 and 2, it is found that the resin film exhibiting positive wavelength dispersion characteristics and the resin film exhibiting flat wavelength dispersion characteristics are unlikely to cause orientation angle unevenness even in a manufacturing method that includes only one stretching and one shrinking.

The retardation film obtained by the manufacturing method of the present invention is suitable for use in image display devices such as liquid crystal displays (LCDs) and organic electroluminescence displays (OLEDs).

1 Thermoplastic resin film 10 Rail 20 Clip 100 Stretching device

Claims (3)

A method for producing a stretched film that satisfies the following formulas (A) and (B),
0.8<R(450)/R(550)<1 (A)
1<R(650)/R(550)<1.2 (B)
(In formulas (A) and (B), R(450), R(550) and R(650) are the front retardations of the film measured at 23° C. using light having wavelengths of 450 nm, 550 nm and 650 nm, respectively.)
A first step includes, in this order, preheating a long thermoplastic resin film while running it in a longitudinal direction, stretching it in a width direction, shrinking it in the width direction, and winding it up into a roll;
a second step including, in this order, preheating, stretching in the width direction, and shrinking in the width direction while unwinding and running the roll-shaped thermoplastic resin film in the longitudinal direction;
When the stretching ratio in the width direction in the first step is E1 and the total stretching ratio in the width direction is E, the following formula (1) is satisfied:
E×0.55<E1<E×0.75 (1)
The method for producing a stretched film, wherein the stretching ratio in the width direction in the first step is 1.4 to 2.2 times. The method according to claim 1 , wherein the total stretching ratio in the width direction is 2.4 to 3.2 times. The method according to claim 1 or 2 , wherein the thermoplastic resin film contains at least one resin selected from the group consisting of polycarbonate-based resins, polyvinyl acetal resins, cellulose ester-based resins, polyester-based resins, and polyester carbonate-based resins.