JP7513410B2 - Method for manufacturing retardation film - Google Patents

Description

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

Circular polarizing plates are used in image display devices such as liquid crystal displays (LCDs) and organic electroluminescence displays (OLEDs) to improve display characteristics and prevent reflection. A circular polarizing plate is typically formed by laminating a polarizer and a retardation film (typically a λ/4 plate) so that the absorption axis of the polarizer and the slow axis of the retardation film form an angle of 45°. Conventionally, retardation films have typically been produced by uniaxial or biaxial stretching in the vertical and/or horizontal directions, so that the slow axis is often expressed in the horizontal (width direction) or vertical (length direction) direction of a long-length original film. As a result, in order to produce a circular polarizing plate, it was necessary to cut the retardation film so that it forms an angle of 45° with the width or length direction and bond them together one by one. In addition, in order to ensure the broadband properties of the circular polarizing plate, two retardation films, a λ/4 plate and a λ/2 plate, may be laminated. In this case, the λ/2 plate must be laminated at an angle of 75° to the absorption axis of the polarizer, and the λ/4 plate must be laminated at an angle of 15° to the absorption axis of the polarizer. Even in this case, when producing a circular polarizing plate, it was necessary to cut the retardation film at 15° and 75° and laminate them one by one. In order to solve this problem (specifically, to enable so-called roll-to-roll lamination), a technology has been proposed in which the left and right ends of a long film in the width direction are held by left and right clips of a variable pitch type whose vertical clip pitch changes, and the clip pitch of at least one of the left and right clips is changed to stretch the film in an oblique direction (hereinafter also referred to as "oblique stretching"), thereby expressing the slow axis of the retardation film in an oblique direction (for example, Patent Document 1).

On the other hand, in recent years, there has been a strong demand for thinner image display devices, and therefore a strong demand for thinner retardation films. For this reason, attention has been focused on resin films with large birefringence Δn (hence, the thickness required to obtain the desired in-plane retardation value is small). However, there is a problem in that obliquely stretched films of resin films with large birefringence Δn have large variations in the in-plane retardation in the width direction.

Patent No. 4845619

The present invention has been made to solve the above problems, and its main objective is to provide a method for producing an obliquely stretched retardation film with small variation in in-plane retardation in the width direction without any problems, using a resin film with a large birefringence Δn.

A method for producing a retardation film according to an embodiment of the present invention includes: holding the left and right ends in the width direction of a long film having a birefringence Δn of 0.025 or less in an unstretched state with left and right clips of a variable pitch type whose longitudinal clip pitch changes; moving the left and right clips while changing at least one of the clip pitches to obliquely stretch the film; heating the film to a predetermined temperature to fix the stretched state; and releasing the film from the left and right clips. In this production method, the temperature difference between the left and right sides when fixing the stretched state is 7° C. or more, and includes correcting slack in the film during the oblique stretching.
In one embodiment, the lower of the left side temperature and the right side temperature when fixing the stretched state is Tg-20° C. or higher, where Tg is the glass transition temperature of the long film.
In one embodiment, the birefringence Δn of the film in an unstretched state is 0.008 or more.
In one embodiment, the long film contains a polycarbonate-based resin, and the polycarbonate-based resin contains a structural unit represented by the following formula (V):
In one embodiment, the manufacturing method includes correcting slack in the long film during oblique stretching, and the correction of the slack includes fixing the stretched state of the film, transporting the film with a roll, and detecting the amount of slack in the film between the transport rolls and the areas where slack has occurred; and making a correction to change the clip pitch of at least one of the left and right clips upstream of the transport line based on the detection results.
In one embodiment, the thickness of the retardation film obtained by the above-mentioned production method is 15 μm to 45 μm, the in-plane retardation Re(550) is 100 nm to 200 nm, the angle between the slow axis direction and the longitudinal direction is 40° to 50° or 130° to 140°, and the variation of the in-plane retardation Re(550) in the width direction is within 6% of the average value of the in-plane retardation Re(550) in the width direction.

According to an embodiment of the present invention, in a method for manufacturing an obliquely stretched retardation film using a resin film with a large birefringence Δn, by setting the temperature difference between the left and right sides when fixing the stretched state (during so-called heat fixing) to 7°C or more, and by performing slack correction during oblique stretching, it is possible to manufacture an obliquely stretched retardation film with small variation in the in-plane retardation in the width direction without problems such as breakage.

1A to 1C are schematic diagrams illustrating a method for producing a retardation film according to one embodiment of the present invention. FIG. 1 is a schematic plan view illustrating an overall configuration of an example of a stretching device that can be used in a method for producing a retardation film according to an embodiment of the present invention. 3 is a schematic plan view of a main portion for explaining a link mechanism for changing a clip pitch in the stretching device of FIG. 2. FIG. 3 is a schematic plan view of a main portion for explaining a link mechanism for changing a clip pitch in the stretching device of FIG. 2. FIG. FIG. 4 is a schematic diagram illustrating a method for measuring the amount of slack. FIG. 2 is a schematic diagram showing a clip pitch profile in a method for producing a retardation film according to one embodiment of the present invention. FIG. 11 is a schematic diagram showing a clip pitch profile in a method for producing a retardation film according to another embodiment of the present invention.

Representative embodiments of the present invention are described below, but the present invention is not limited to these embodiments. In this specification, "vertical clip pitch" means the center-to-center distance in the running direction of vertically adjacent clips. In addition, the left-right relationship in the width direction of a long film means the left-right relationship in the transport direction of the film, unless otherwise specified.

A. Method for manufacturing a retardation film The method for manufacturing a retardation film according to an embodiment of the present invention includes holding the left and right ends of a long film in the width direction with left and right clips of a variable pitch type in which the clip pitch in the vertical direction changes, respectively; moving the left and right clips while changing the clip pitch of at least one of them to obliquely stretch the film; heating the film to a predetermined temperature to fix (thermally fix) the stretched state; and releasing the film from the left and right clips. Typically, the film held by the clips is preheated and then subjected to oblique stretching. The manufacturing method typically includes correcting the slack of the film during oblique stretching. The correction of the slack includes, for example, fixing the stretched state of the film, transporting the film with a roll, detecting the amount of slack of the film between the transport rolls and the part where the slack occurs; and performing a correction to change the clip pitch of at least one of the left and right clips upstream of the transport line based on the detection result. In an embodiment of the present invention, a resin film having a birefringence Δn of 0.025 or less in an unstretched state is used as a long film (film to be stretched). Furthermore, in an embodiment of the present invention, the temperature difference between the left and right sides during heat setting (specifically, the temperature difference between the left heat setting zone and the right heat setting zone) is 7° C. or more. The birefringence Δn is calculated from the formula: nx-ny. Here, nx is the refractive index in the direction in which the in-plane refractive index is maximum (i.e., the slow axis direction), and ny is the refractive index in the in-plane direction perpendicular to the slow axis (i.e., the fast axis direction).

Figure 1 is a schematic diagram illustrating a method for producing a retardation film according to one embodiment of the present invention. The obliquely stretched film 1, which is obliquely stretched in the stretching device 100 and then released from the clip, is sent out from the outlet of the stretching device 100, transported in a roll using transport rolls 200a, 200b, 200c, and 200d, and wound up in the winding section 300. When the film 1 is transported in a roll, the amount of slack, etc. is detected between the transport rolls, and based on the detection result, a correction is made to change the clip pitch of at least one of the left and right clips upstream of the transport line. As a result, the difference in length between the left and right ends of the stretched film obtained after the correction is reduced, and a long obliquely stretched film with reduced slack can be obtained.

The above-mentioned gripping of the film with the clips, preheating, diagonal stretching, heat setting, and release from the clips can be carried out, for example, using a tenter-type simultaneous biaxial stretching device equipped with left and right clips that can move at different speeds while gripping the left and right ends of the long film in the width direction.

2 is a schematic plan view illustrating the overall configuration of an example of a stretching device that can be used in the manufacturing method of a retardation film according to an embodiment of the present invention. The stretching device 100 has an endless loop 10L and an endless loop 10R, which have a number of clips 20 for holding the film, symmetrically arranged on the left and right sides in a plan view. In this specification, the endless loop on the left side as viewed from the inlet side of the film is referred to as the left endless loop 10L, and the endless loop on the right side is referred to as the right endless loop 10R. The clips 20 of the left and right endless loops 10L and 10R move in a loop shape guided by the reference rail 70. The clip 20 of the left endless loop 10L moves in a counterclockwise direction, and the clip 20 of the right endless loop 10R moves in a clockwise direction. In the stretching device, a gripping zone A, a preheating zone B, a stretching zone C, and a heat fixing and release zone D are provided in this order from the inlet side of the sheet to the outlet side. Each of these zones refers to a zone where the film to be stretched is substantially gripped, preheated, diagonally stretched, heat-set, and released, and does not refer to a mechanically or structurally independent section. Also, please note that the length ratio of each zone in the stretching device in FIG. 2 is different from the actual length ratio.

Although not shown in FIG. 2, a zone for performing any appropriate processing may be provided between the stretching zone C and the heat setting and release zone D, as necessary. Such processing may include a lateral shrinkage process. Also, although not shown, the stretching device typically includes a heating device (e.g., various ovens such as hot air, near infrared, and far infrared) for creating a heated environment from the preheating zone B to the heat setting and release zone D.

In the gripping zone A and preheating zone B of the stretching device 100, the left and right endless loops 10L, 10R are configured to be approximately parallel to each other at a distance corresponding to the initial width of the film to be stretched. In the stretching zone C, the distance between the left and right endless loops 10L, 10R gradually increases from the preheating zone B side toward the heat setting and opening zone D until it corresponds to the width of the film after stretching. In the heat setting and opening zone D, the left and right endless loops 10L, 10R are configured to be approximately parallel to each other at a distance corresponding to the width of the film after stretching. However, the configuration of the left and right endless loops 10L, 10R is not limited to the illustrated example. For example, the left and right endless loops 10L, 10R may be configured to be approximately parallel to each other at a distance corresponding to the initial width of the film to be stretched from the gripping zone A to the heat setting and opening zone D.

The clip (left clip) 20 of the left endless loop 10L and the clip (right clip) 20 of the right endless loop 10R can move cyclically independently. For example, the drive sprockets 11, 12 of the left endless loop 10L are driven to rotate in the counterclockwise direction by the electric motors 13, 14, and the drive sprockets 11, 12 of the right endless loop 10R are driven to rotate in the clockwise direction by the electric motors 13, 14. As a result, a running force is applied to the clip carrying member of the drive roller (not shown) engaged with these drive sprockets 11, 12. As a result, the left endless loop 10L moves cyclically in the counterclockwise direction, and the right endless loop 10R moves cyclically in the clockwise direction. By independently driving the left electric motor and the right electric motor, the left endless loop 10L and the right endless loop 10R can be moved cyclically independently.

Furthermore, the clip (left clip) 20 of the left endless loop 10L and the clip (right clip) 20 of the right endless loop 10R are each a variable pitch type. That is, the vertical clip pitch of the left and right clips 20, 20 can change independently as they move. A variable pitch type configuration can be achieved by employing a drive system such as a pantograph system, a linear motor system, or a motor chain system. Below, a link mechanism (pantograph mechanism) is described as an example.

Figures 3 and 4 are schematic plan views of the main parts to explain the link mechanism that changes the clip pitch in the stretching device of Figure 2, with Figure 3 showing the state where the clip pitch is at its minimum and Figure 4 showing the state where the clip pitch is at its maximum.

As shown in Figs. 3 and 4, a clip carrying member 30 is provided that is elongated and rectangular in a plan view and that carries each clip 20 individually. Although not shown, the clip carrying member 30 is formed into a strong frame structure with a closed cross section by an upper beam, a lower beam, a front wall (wall on the clip side), and a rear wall (wall opposite the clip). The clip carrying member 30 is provided so that it can roll on the running surfaces 81 and 82 by running wheels 38 at both ends. In Figs. 3 and 4, the running wheels on the front wall side (running wheels that roll on the running surface 81) are not shown. The running surfaces 81 and 82 are parallel to the reference rail 70 over the entire area. A long hole 31 is formed along the longitudinal direction of the clip carrying member on the rear side (opposite the clip side (hereinafter, the anti-clip side)) of the upper beam and the lower beam of the clip carrying member 30, and a slider 32 is engaged so as to be slidable in the longitudinal direction of the long hole 31. A first shaft member 33 is provided vertically penetrating the upper and lower beams near the end of the clip carrying member 30 on the clip 20 side. Meanwhile, a second shaft member 34 is provided vertically penetrating the slider 32 of the clip carrying member 30. One end of a main link member 35 is pivotally connected to the first shaft member 33 of each clip carrying member 30. The other end of the main link member 35 is pivotally connected to the second shaft member 34 of the adjacent clip carrying member 30. In addition to the main link member 35, one end of a sub-link member 36 is pivotally connected to the first shaft member 33 of each clip carrying member 30. The other end of the sub-link member 36 is pivotally connected to the middle part of the main link member 35 by a pivot 37. Due to the link mechanism of the main link member 35 and the sub-link member 36, as shown in FIG. 3, the vertical pitch between the clip carrying members 30 (resulting in the clip pitch) becomes smaller as the slider 32 moves toward the rear side (anti-clip side) of the clip carrying member 30, and as shown in FIG. 4, the vertical pitch between the clip carrying members 30 (resulting in the clip pitch) becomes larger as the slider 32 moves toward the front side (clip side) of the clip carrying member 30. The positioning of the slider 32 is performed by the pitch setting rail 90. As shown in FIG. 3 and FIG. 4, the clip pitch becomes larger as the distance between the reference rail 70 and the pitch setting rail 90 becomes smaller.

By obliquely stretching a film using the above-mentioned stretching device, an obliquely stretched retardation film (typically a retardation film having a slow axis in a direction oblique to the longitudinal direction) can be produced. Specific embodiments of the above-mentioned stretching device are described, for example, in JP-A-2008-44339, the entirety of which is incorporated herein by reference. Each step will be described in detail below.

A-1. Gripping of film by clips In the gripping zone A (the entrance for film intake into the stretching device 100), both side edges of the film to be stretched are gripped by the clips 20 of the left and right endless loops 10L, 10R at equal and constant clip pitches or at different clip pitches. The film is sent to the preheating zone B by the movement of the clips 20 of the left and right endless loops 10L, 10R (effectively, the movement of each clip support member guided by the reference rail 30).

A-2. Preheating In the preheating zone B, the left and right endless loops 10L, 10R are configured to be approximately parallel to each other with a distance corresponding to the initial width of the film to be stretched as described above, so that the film is basically heated without being stretched transversely or longitudinally. However, the distance between the left and right clips (distance in the width direction) may be slightly increased to avoid problems such as the film bending due to preheating and contacting the nozzle in the oven.

In preheating, the 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 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 film used, the temperature T1 is, for example, 70°C to 190°C, and preferably 80°C to 180°C.

The time required to heat up to the temperature T1 and the time required to hold the temperature T1 can be appropriately set depending on the material of the film and the manufacturing conditions (e.g., the film transport speed). These heat-up times and holding times can be controlled by adjusting the moving speed of the clip 20, the length of the preheating zone, the temperature of the preheating zone, etc.

In the stretching zone C, the left and right clips 20 are moved while changing the clip pitch in the longitudinal direction of at least one of them, to stretch the film obliquely. More specifically, the clip pitch of the left and right clips is increased or decreased at different positions, or the clip pitch of the left and right clips is changed (increased and/or decreased) at different change speeds, to thereby stretch the film obliquely.

The diagonal stretching may include lateral stretching. In this case, the diagonal stretching may be performed while increasing the distance (distance in the width direction) between the left and right clips, for example, as in the illustrated example. Alternatively, unlike the illustrated example, the diagonal stretching may be performed while maintaining the distance between the left and right clips.

When the oblique stretching includes transverse stretching, the stretching ratio in the transverse direction (TD) (the ratio of the width W _final of the film after oblique stretching to the initial width W _initial of the film (W _final /W _initial )) is preferably 1.05 to 6.00, more preferably 1.10 to 5.00.

In one embodiment, the diagonal stretching can be performed by increasing or decreasing the clip pitch of each clip to a predetermined pitch, with the position where the clip pitch of one of the left and right clips begins to increase or decrease being different from the position where the clip pitch of the other clip begins to increase or decrease in a different vertical position. For the diagonal stretching of this embodiment, reference can be made to, for example, the descriptions in Patent Document 1 and JP 2014-238524 A.

In another embodiment, the diagonal stretching can be performed by fixing the clip pitch of one of the left and right clips, increasing or decreasing the clip pitch of the other clip to a predetermined pitch, and then returning it to the initial clip pitch. For the diagonal stretching of this embodiment, see, for example, JP 2013-54338 A and JP 2014-194482 A.

In yet another embodiment, the diagonal stretching may be performed by (i) increasing the clip pitch of one of the left and right clips while decreasing the clip pitch of the other clip, and (ii) changing the clip pitch of each clip so that the decreased clip pitch and the increased clip pitch are equal to a predetermined pitch. For the diagonal stretching of this embodiment, for example, reference may be made to the description in JP 2014-194484 A. The diagonal stretching of this embodiment may include: increasing the clip pitch of one clip while increasing the distance between the left and right clips, while decreasing the clip pitch of the other clip, to diagonally stretch the film (first diagonal stretching step); and maintaining or decreasing the clip pitch of one clip while increasing the distance between the left and right clips so that the clip pitches of the left and right clips are equal, and increasing the clip pitch of the other clip, to diagonally stretch the film (second diagonal stretching step).

Oblique stretching is typically performed at a temperature T2. The temperature T2 is preferably Tg-20°C to Tg+30°C, more preferably Tg-10°C to Tg+20°C, and particularly preferably about Tg, relative to the glass transition temperature (Tg) of the film to be stretched. Although it varies depending on the 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 film heated to temperature T1 in the preheating zone can be cooled to temperature T2.

The above-mentioned lateral shrinkage process is carried out after the diagonal stretching. For the process after the diagonal stretching, refer to paragraphs 0029 to 0032 of JP2014-194483A.

A-4. Heat setting and clip release In the heat setting and release zone D, the film is heat treated to fix the stretched state (heat setting). When heat setting, the clip pitch in the longitudinal direction may be reduced to relieve stress. Heat setting may typically be performed at a temperature T3. The temperature T3 varies depending on the film to be stretched, and may be T2≧T3 or T2<T3. In general, when the film is an amorphous material, T2≧T3, and when the film is a crystalline material, crystallization may be performed by setting T2<T3. When T2≧T3, the difference between the temperatures T2 and T3 (T2-T3) is preferably 0°C to 50°C. The heat treatment (heat setting) time is typically 10 seconds to 10 minutes.

In an embodiment of the present invention, the temperature difference between the left and right sides in the heat fixing (specifically, the temperature difference between the left heat fixing zone and the right heat fixing zone) is 7°C or more, preferably 10°C or more, as described above. On the other hand, the temperature difference is preferably 25°C or less, more preferably 22°C or less. By providing such a temperature difference in the heat fixing, the variation in the in-plane phase difference in the width direction of the obtained retardation film can be set within a desired range. If the temperature difference is too small, the variation in the in-plane phase difference in the width direction of the obtained retardation film may not be sufficiently suppressed. If the temperature difference is too large, the film on the low temperature side may slacken. Note that, by performing the slack correction described later, slack can be suppressed even if the temperature difference is increased. The high temperature side may be the left side or the right side. Here, the temperature of the left heat fixing zone and the temperature of the right heat fixing zone refer to the set temperatures of the heating device for making the corresponding zones into a heating environment, respectively.

The lower of the left and right temperatures during heat setting is preferably Tg-20°C or higher, more preferably Tg-20°C to Tg+30°C, even more preferably Tg-10°C to Tg+20°C, and particularly preferably about Tg. If the temperature is within this range, there is an advantage that the film does not sag significantly.

After heat setting, the film may be cooled below Tg, if necessary.

After heat setting and optional cooling, the film is released from the clips at any position in the heat setting and release zone D. In the heat setting and release zone D, neither transverse nor longitudinal stretching is typically performed.

The stretched film is released from the clips and sent out from the exit of the stretching device for roll transport, as described below.

In one embodiment, air may be blown onto the film in the latter half of the heat fixing and release zone D and before the clips are released. By blowing air, slack in the film can be further suppressed. The air speed when blowing air is preferably 15 m/s to 35 m/s, more preferably 20 m/s to 30 m/s. The air is preferably hot air. The temperature of the air may correspond to, for example, the heat fixing temperature. More specifically, the temperature of the air on the left side may be the same as the heat fixing temperature on the left side, and the temperature of the air on the right side may be the same as the heat fixing temperature on the right side. The air may be blown, for example, by blowing air at a set temperature and a set air speed from upper and/or lower nozzles.

A-5. Roller Transport In roll transport, the amount of slack in the stretched film between the transport rolls and the location where slack occurs are detected.

In one embodiment, the left and right ends in the width direction of the stretched film released from the clips are cut and removed, and then the amount of slack and the area where slack has occurred are detected. By detecting the amount of slack and the area where slack has occurred with both ends removed, more accurate detection results can be obtained.

The width of each end to be cut and removed can be, independently, for example, 20 mm to 600 mm, preferably 100 mm to 500 mm. The ends can be cut and removed by conventional slitting.

In one embodiment, the amount of slack and the location where slack has occurred can be detected by detecting the difference between the original running position of the film during roll transport and the actual running position of the film. For example, the detection can be performed by detecting the difference in the position (transport height) in the width direction of the film at the midpoint between the transport rolls.

FIG. 5 is a schematic diagram for explaining an example of a method for detecting the amount of slack and the portion where slack occurs. As shown in FIG. 5, an ultrasonic displacement sensor 400 is disposed below the center and left and right ends in the width direction of the stretched film 1 at the midpoint between two adjacent transport rolls 200b, 200c, and the distance from the ultrasonic displacement sensor 400 to the stretched film 1 is measured, and the difference (L _MAX -L _MIN ) between the maximum distance (L _MAX ) and the minimum distance (L _MIN ) can be regarded as the amount of slack. The portion where the minimum distance is given is detected as the portion where slack occurs. Note that the cause of slack in the obliquely stretched film is that the stretching process (timing, number of times, order, thermal history, etc. of stretching or contraction) at the left and right ends of the film during oblique stretching is different from each other, resulting in uneven deformation amounts at both ends after clip release. Therefore, the portion where slack occurs is usually one of the ends. Therefore, the detection points of slack can be limited to the left and right ends in the width direction of the stretched film 1. In this case, a film without slack is transported in advance, the distance (L ₀ ) from the ultrasonic displacement sensor to the film is measured, and the amount of slack can be determined as the difference between the distance between the left and right ends and the ultrasonic displacement sensor and L _0. Note that although an ultrasonic displacement sensor has been described as an example of a slack detection means, slack can be detected using any appropriate detection means (for example, using a laser Doppler velocimeter to determine the passing speed of the film at normal portions and slack portions, and calculating the difference in length from that, etc.).

The distance (D) between the transport rolls during the above detection is not particularly limited, but can be, for example, 500 mm to 2000 mm, and preferably 700 mm to 1500 mm.

The film tension during the above detection is not particularly limited, but can be, for example, 50 N/m to 400 N/m, and preferably 100 N/m to 200 N/m. If the transport tension is too high, the film may elastically deform during transport, making it difficult to detect slack. On the other hand, if the transport tension is too low, the tension itself may not be stable, and the measured slack value may not be stable.

The roll transport may be performed in a non-heated environment. The ambient temperature during roll transport may be, for example, about 15°C to 40°C, or about 20°C to 30°C.

A-6. Correction by changing clip pitch The correction by changing the clip pitch is a so-called feedback correction, and is performed by changing the clip pitch of at least one of the left and right clips upstream of the conveying line so as to reduce the amount of slack based on the detection results of the amount of slack and the portion where the slack occurs. For example, when the detected amount of slack is equal to or greater than a predetermined value, a correction is performed to change the clip pitch, and when it is less than the predetermined value, the oblique stretching can be continued without correction. Specifically, the above correction can be performed when the amount of slack detected at a roll distance of 1000 mm is, for example, 3 mm or more, 5 mm or more, 10 mm or more, or 15 mm or more. By performing feedback correction (slack correction), it is possible to manufacture an obliquely stretched retardation film with small variation in in-plane retardation in the width direction using a resin film with large birefringence Δn without defects such as breakage.

The above-mentioned correction for changing the clip pitch (hereinafter, also simply referred to as "feedback correction") can be performed by any appropriate method as long as the effect of the present invention can be obtained. Feedback correction can be performed, for example, by increasing the clip pitch of the clip that holds the end far from the part where slack occurs, by decreasing the clip pitch of the clip that holds the end near the part where slack occurs, or by combining these. However, since decreasing the clip pitch may not shrink the film but may only cause it to slacken, it is preferable to perform feedback correction by increasing the clip pitch of the clip that holds the end far from the part where slack occurs. More specifically, if the part where slack occurs is either the left or right end of the stretched film, feedback correction can be suitably performed by increasing the clip pitch of the clip that holds the other end.

In the above feedback correction, the timing of changing the clip pitch is not particularly limited as long as the effect of the present invention can be obtained. In one embodiment, the clip pitch can be changed to the corrected clip pitch at any timing after the film upstream of the conveying line moves to the oblique stretching zone and before it is released from the clip. The corrected clip pitch is applied from any time point after the preceding clip on the upstream of the conveying line passes the midpoint of the running section of the oblique stretching zone until the film is released from the clip, more preferably from the time point when the preceding clip passes 1/2 to 9/10 of the running section of the oblique stretching zone until the film is released from the clip. More specifically, the application of the feedback correction is started from any time point after the preceding clip on the upstream of the conveying line passes the midpoint of the running section of the oblique stretching zone, preferably from the time point when the preceding clip passes 1/2 to 9/10 of the running section of the oblique stretching zone, and the clip pitch is changed so that the desired correction amount is obtained at the end point of the oblique stretching zone. It is also preferable to maintain the correction amount until the film is released from the clips even after moving from the oblique stretching zone to the release zone. In the second half of the oblique stretching, particularly at the end, at least one clip pitch is maintained constant or changes at a small rate, so the effects of the present invention can be preferably obtained by correcting the clip pitch at that timing.

When applying the feedback correction in the oblique stretching zone, the target film is preferably heated to Tg°C to Tg+20°C, more preferably Tg+3°C to Tg+10°C, and even more preferably Tg+4°C to Tg+8°C. The effect of the present invention can be suitably obtained by applying feedback correction at a temperature equal to or slightly higher than Tg. In one embodiment, the film that passes through the oblique stretching zone while undergoing feedback correction at the above temperature and transitions to the release zone is heat-set while maintaining the amount of correction applied in the oblique stretching zone, and then cooled as necessary, before being released from the clips. Heat-setting and cooling are as described in Section A-4.

6A is a schematic diagram showing a clip pitch profile in a method for producing a retardation film according to one embodiment of the present invention. In the illustrated example, the clip pitches of the left and right clips X and Y in the preheating zone B are both set to _P1 , and in the initial oblique stretching before feedback correction, as soon as the film enters the oblique stretching zone C, the clip pitch of one clip X starts to increase and the clip pitch of the other clip Y starts to decrease, and the clip pitch of clip X is increased to _P2 , and after the clip pitch of clip Y is decreased to _P3 , the clip pitch of clip X is maintained at _P2 while the clip pitch of clip Y is increased to _P2 . The left and right clips X and Y move to the heat fixing and release zone D with the clip pitch _P2 to release the film. After that, as a result of feedback correction based on the amount of slack during roll transport of the film, the clip pitch of clip X is gradually increased from _P2 to _P2 ' in the oblique stretching zone C. As described later, in the heat setting and release zones, the clip cuts of clips X and Y are maintained at P ₂ ' and P ₂ , respectively, and the correction amount (P ₂ '-P ₂ ) at the end point of the oblique stretching zone is maintained.

6B is a schematic diagram showing a profile of a clip pitch in a method for producing a retardation film according to another embodiment of the present invention. In the illustrated embodiment, oblique stretching is performed in a pattern similar to that of the embodiment shown in FIG. 6A, and the clip pitches of the left and right clips X and Y are both reduced from P ₂ to P ₃ during heat setting in the heat setting and release zone D, and then the film is released. Thereafter, as a result of feedback correction based on the amount of slackness during roll transport of the film, the clip pitch of clip X is gradually increased from P ₂ to P ₂ ' in the oblique stretching zone C, and in the heat setting and release zone, the clip pitch of clip X is reduced from P ₂ ' to P ₃ ', and the clip pitch of clip Y is reduced from P ₂ to P ₃ . Note that, as described later, in the heat setting and release zone, the clip pitches of clips X and Y are reduced so that the correction amount (P ₂ '-P ₂ ) at the end point of the oblique stretching zone is maintained, and the relationship of P ₃ '-P ₃ =P ₂ '-P ₂ is satisfied.

In the oblique stretching zone, it is preferable that the change to the corrected clip pitch (change to _P2 ') proceeds gradually from the point where the application of feedback correction starts to the end point (in Figs. 6A and 6B, from the point where 2/3 of the way through the oblique stretching zone to the end point). It is also preferable that the correction amount at the end point of the oblique stretching (|clip pitch before correction at the end point of the oblique stretching--clip pitch after correction at the end point of the oblique stretching|) is maintained from the end point of the oblique stretching zone to the release of the clip. For example, in the profile shown in Figs. 6A and 6B, the difference between the clip pitch of clip X and the clip pitch of clip Y is maintained constant (i.e., _P2' - _P2 ) from the end point of the oblique stretching zone to the release of the clip. By changing the clip pitch in this manner, the effect of the present invention can be suitably obtained.

The clip pitch can be changed by adjusting the distance between the reference rail and the pitch setting rail, as described above. These adjustments can be made with or without stopping the conveyor line.

The amount of clip pitch correction at the end of the oblique stretching in the feedback correction (|clip pitch before correction at the end of the oblique stretching--clip pitch after correction at the end of the oblique stretching|) can be appropriately set according to the amount of slack, etc. The amount of clip pitch correction is preferably an amount that exceeds the difference in length between the left and right ends of the stretched film between the transport rolls, and can be more preferably 1.4 to 5.0 times the difference in length, even more preferably 1.6 to 4.0 times, and even more preferably 1.8 to 3.0 times. If the amount of clip pitch correction is less than or equal to the difference in length between the left and right ends, the amount of slack reduction may be insufficient.

The difference in length L' (unit: mm) between the left and right ends of the stretched film between the transport rolls can be calculated by substituting the length L (unit: mm) of the stretched film between the transport rolls, calculated based on the following formulas (1) and (2), into the following formula (3).
(In the above formula, d represents the detected amount of slack (unit: mm), w represents the mass per meter of the film (unit: g), g represents the gravitational acceleration, S represents the distance between the transport rolls (unit: mm), and H represents the tension (unit: N/m) applied to the end side where slack is occurring, calculated from formula (1).)

In one embodiment, the amount of slackness reduced by the feedback correction (the amount of slackness of the stretched film obtained before feedback correction - the amount of slackness of the stretched film obtained after feedback correction, measured at a distance of 1000 mm between the transport rolls) may be, for example, 3 mm or more, preferably 5 mm or more, more preferably 8 mm or more, and even more preferably 10 mm or more. The amount of slackness of the stretched film obtained after feedback correction may be, for example, less than 15 mm, preferably 10 mm or less, more preferably 8 mm or less, even more preferably 5 mm or less, and even more preferably less than 3 mm.

B. Film to be stretched In an embodiment of the present invention, a resin film having a birefringence Δn of 0.025 or less in an unstretched state is used as the film to be stretched. The birefringence Δn of the resin film is preferably 0.022 or less, more preferably 0.020 or less, and even more preferably 0.018 or less. On the other hand, the birefringence Δn of the resin film is preferably 0.004 or more, more preferably 0.007 or more, even more preferably 0.010 or more, and particularly preferably 0.012 or more. According to an embodiment of the present invention, a resin film having such a large birefringence Δn can be used to obtain an obliquely stretched retardation film having a small variation in the in-plane retardation in the width direction without any problems such as breakage. If the birefringence Δn is small, the problem of variation in the in-plane retardation in the width direction does not occur, but the film thickness for obtaining the desired in-plane retardation becomes large. If the birefringence Δn is too large, the variation in the in-plane retardation in the width direction may not be controlled.

As the material constituting the resin film, any suitable resin may be adopted as long as it has the birefringence Δn as described above. Specific examples include polycarbonate-based resins, polyester carbonate-based resins, polyester-based resins, polyvinyl acetal-based resins, polyarylate-based resins, cyclic olefin-based resins, cellulose-based resins, polyvinyl alcohol-based resins, polyamide-based resins, polyimide-based resins, polyether-based resins, polystyrene-based resins, and acrylic-based resins. These resins may be used alone or in combination (e.g., blended or copolymerized). Polycarbonate-based resins or polyester carbonate-based resins (hereinafter sometimes simply referred to as polycarbonate-based resins) are preferred. This is because the effects of the embodiments of the present invention are remarkable.

The polycarbonate resin preferably contains a structural unit derived from a dihydroxy compound having a bond structure represented by the following formula (I).

An example of a dihydroxy compound is a compound represented by the following formula (II). Such dihydroxy compounds include isosorbide, isomannide, and isoidite, which are stereoisomeric. These may be used alone or in combination of two or more.

The above dihydroxy compound may be used in combination with another dihydroxy compound, for example, an alicyclic dihydroxy compound represented by the following formula (III).
HOCH ₂ -R ¹ -CH ₂ OH ... (III)
In formula (III), R ¹ represents a cycloalkylene group having 4 to 20 carbon atoms. The alicyclic dihydroxy compound may be, for example, tricyclodecane dimethanol or pentacyclopentadecanedimethanol. These include various isomers in which R ¹ in formula (III) is represented by the following formula (IV) (wherein n represents 0 or 1).

In one embodiment, the polycarbonate-based resin includes a structural unit represented by the following formula (V): That is, the polycarbonate-based resin may be a copolymer of diphenyl carbonate, isosorbide, and tricyclodecane dimethanol.

Details of polycarbonate-based resins are described, for example, in JP 2012-031370 A, the disclosure of which is 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).

C. Obtained retardation film The retardation film obtained by stretching the film to be stretched preferably has a refractive index characteristic of nx>ny. In one embodiment, the retardation film can preferably function as a λ/4 plate. In this case, the in-plane retardation Re (550) of the retardation film (λ/4 plate) is preferably 100 nm to 200 nm, more preferably 120 nm to 160 nm, and even more preferably 130 nm to 150 nm. In another embodiment, the retardation film can preferably function as a λ/2 plate. In this case, the in-plane retardation Re (550) of the retardation film (λ/2 plate) is preferably 230 nm to 310 nm, more preferably 240 nm to 300 nm, and even more preferably 260 nm to 290 nm. By appropriately setting the conditions of the oblique stretching, a retardation film having a desired in-plane retardation can be obtained. Needless to say, according to the embodiment of the present invention, a retardation film (a retardation film other than a λ/4 plate and a λ/2 plate) having any suitable in-plane retardation according to the purpose can be obtained. In this specification, Re(λ) is the in-plane retardation of the film measured with light having a wavelength of λ nm at 23° C. Therefore, Re(550) is the in-plane 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 has a variation in the in-plane retardation Re (550) in the width direction of preferably 6% or less, more preferably 5% or less, even more preferably 4% or less, and particularly preferably 2.2% or less, relative to the average value of the in-plane retardation Re (550) in the width direction. According to an embodiment of the present invention, a diagonally stretched retardation film having a small variation in the in-plane retardation in the width direction can be obtained without defects such as breakage, using a resin film having a large birefringence Δn. The variation can be obtained, for example, as follows. At any position in the longitudinal direction of the obtained long-sized retardation film, the in-plane retardation Re (550) at positions selected at 10 mm intervals along the entire width direction is measured. The average value is calculated from the measured values, and the maximum and minimum values are picked up, and the variation is calculated by the following formula.
Variation (%) = (maximum value - minimum value) / average value x 100

The retardation film has a slow axis because the refractive index characteristics show the relationship of nx>ny as described above. The direction of the slow axis (the angle between the slow axis direction and the longitudinal direction of the obtained retardation film) can be controlled to any appropriate direction (typically, a direction oblique to the longitudinal direction) according to the purpose by appropriately setting the conditions of the oblique stretching. For example, when the retardation film can function as a λ/4 plate, the angle between the slow axis direction and the longitudinal direction is preferably 40° to 50° or 130° to 140°, more preferably 42° to 48° or 132° to 138°, and even more preferably 44° to 46° or 134° to 136°. For example, when the retardation film can function as a λ/2 plate, the angle between the slow axis direction and the longitudinal direction is preferably 10° to 20° or 100° to 110°, more preferably 12° to 18° or 102° to 108°, even more preferably 14° to 16° or 104° to 106°; or preferably 70° to 80° or 160° to 170°, more preferably 72° to 78° or 162° to 168°, even more preferably 74° to 76° or 164° to 166°.

The thickness of the retardation film can vary depending on the desired in-plane retardation. For example, when the retardation film can function as a λ/4 plate, the thickness of the retardation film is preferably 15 μm to 45 μm, more preferably 25 μm to 40 μm, and even more preferably 30 μm to 40 μm. For example, when the retardation film can function as a λ/4 plate, the thickness of the retardation film is preferably 15 μm to 40 μm, and more preferably 15 μm to 30 μm. In an embodiment of the present invention, by using a resin film having a predetermined birefringence Δn in an unstretched state as the film to be stretched, the desired in-plane retardation can be achieved with a thickness significantly thinner than that of a normal retardation film (stretched film of a resin film).

The retardation film preferably exhibits a so-called flat wavelength dependency. Specifically, the in-plane retardation satisfies the relationship Re(450) ≒ Re(550) ≒ Re(650). Re(450)/Re(550) is preferably 0.97 to 1.03, more preferably 0.98 to 1.02. Re(550)/Re(650) is preferably 0.97 to 1.03, more preferably 0.98 to 1.02.

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 5×10 ⁻¹² (m ² /N) to 50×10 ⁻¹² (m ² /N).

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) Thickness: The thickness was measured using a dial gauge (manufactured by PEACOCK, product name "DG-205 type pds-2").
(2) In-plane retardation and birefringence The in-plane retardation Re(550) was measured using an Axoscan manufactured by Axometrics Inc. The birefringence Δn was calculated by dividing the in-plane retardation Re(550) by the film thickness.
(3) Orientation angle (slow axis direction)
A sample was prepared by cutting out a square of 50 mm width and 50 mm length from the center of the film to be measured, with one side parallel to the width direction of the film. This sample was measured using an Axoscan manufactured by Axometrics, and the orientation angle at a wavelength of 590 nm was measured.
(4) Glass transition temperature (Tg)
Measurement was performed in accordance with JIS K 7121.
(5) Variation In the long-length retardation films obtained in the examples and comparative examples, the in-plane retardation Re(550) was measured at positions selected at 10 mm intervals along the entire width direction at any position in the long-length direction. The average value was calculated from the measured values, and the maximum and minimum values were picked up, and the variation was calculated by the following formula.
Variation (%) = (maximum value - minimum value) / average value x 100
(6) Breakage The state of the film when it came out of the tenter stretching machine was visually observed and evaluated according to the following criteria.
○: No break or crack ×: Break and/or crack (7) Color Unevenness The retardation films obtained in the Examples and Comparative Examples were laminated to a commercially available polarizing plate to prepare a circular polarizing plate, and the circular polarizing plate was laminated to a commercially available organic EL panel. The hue was visually confirmed and evaluated according to the following criteria.
○: No color unevenness was observed. ×: Color unevenness was observed.

<Production Example 1>
81.98 parts by mass of isosorbide (hereinafter sometimes abbreviated as "ISB"), 47.19 parts by mass of tricyclodecane dimethanol (hereinafter sometimes abbreviated as "TCDDM"), 175.1 parts by mass of diphenyl carbonate (hereinafter sometimes abbreviated as "DPC"), and 0.979 parts by mass of a 0.2% by mass aqueous solution of cesium carbonate as a catalyst were charged into a reaction vessel, and in a nitrogen atmosphere, as the first step of the reaction, the temperature of the heating vessel was heated to 150°C, and the raw materials were dissolved while stirring as necessary (about 15 minutes). Next, the pressure was changed from normal pressure to 13.3 kPa, and the temperature of the heating vessel was increased to 190°C over one hour, while the generated phenol was extracted from the reaction vessel. After the entire reaction vessel was kept at 190°C for 15 minutes, in the second step, the pressure in the reaction vessel was set to 6.67 kPa, the heating tank temperature was raised to 230°C in 15 minutes, and the generated phenol was extracted outside the reaction vessel. As the stirring torque of the stirrer increased, the temperature was raised to 250°C in 8 minutes, and the pressure in the reaction vessel was allowed to reach 0.200 kPa or less in order to remove the further generated phenol. After the predetermined stirring torque was reached, the reaction was terminated, and the reaction product was extruded into water to obtain polycarbonate copolymer pellets (PC resin 1).

After PC resin 1 was vacuum dried at 80°C for 5 hours, resin film 1 was produced using a film-making device equipped with a single-screw extruder (manufactured by Toshiba Machine Co., Ltd., cylinder set temperature: 250°C), a T-die (width 200 mm, set temperature: 250°C), a chill roll (set temperature: 120-130°C) and a winder. The birefringence Δn of the obtained resin film 1 was 0.015.

<Production Example 2>
A polycarbonate copolymer pellet (PC resin 2) was obtained in the same manner as in Production Example 1, except that the amounts of ISB were changed to 39.6 parts by mass, spiroglycol (hereinafter sometimes abbreviated as "SPG") to 22.7 parts by mass, bis[9-(2-phenoxycarbonylethyl)fluoren-9-yl]methane (hereinafter sometimes abbreviated as "BRFM") to 21.1 parts by mass, DPC to 15.9 parts by mass, and cesium carbonate 0.2 mass% aqueous solution to 0.5 parts by mass as a catalyst. A resin film 2 was produced from PC resin 2 in the same manner as in Production Example 1. The birefringence Δn of the obtained resin film 2 was 0.004.

<Production Example 3>
A commercially available polycarbonate-based resin film (manufactured by Mitsubishi Chemical Corporation, product name "DURABIO D5380R") was used as the resin film 3. The birefringence Δn of the resin film 3 was 0.031.

Example 1
(Diagonal stretching before feedback correction)
The resin film 1 obtained in Production Example 1 was obliquely stretched using a stretching device as shown in Figures 2 to 4 to obtain a retardation film. Specifically, the polyester carbonate resin film was preheated to 145 ° C. in the preheating zone of the stretching device. In the preheating zone, the clip pitch (P ₁ ) of the left and right clips was 125 mm. Next, as the film entered the oblique stretching zone C, the clip pitch of the right clip was increased and the clip pitch of the left clip was decreased, so that the clip pitch of the right clip was increased to P ₂ and the clip pitch of the left clip was decreased to P _3. At this time, the clip pitch change rate (P ₂ /P ₁ ) of the right clip was 1.42, the clip pitch change rate (P ₃ /P ₁ ) of the left clip was 0.78, and the transverse stretching ratio with respect to the original width of the film was 1.45 times. Next, while maintaining the clip pitch of the right clip at P ₂ , the clip pitch of the left clip was increased and increased from P ₃ to P ₂ . During this time, the clip pitch change rate ( _P2 / _P3 ) of the left clip was 1.82, and the transverse stretching ratio with respect to the original width of the film was 1.9 times. The oblique stretching zone C was set at Tg+3.2°C (143.2°C).

Then, in the heat setting and release zone D, heat setting was performed for 60 seconds. During heat setting, the temperature of the left heat setting zone was set to 132°C, and the temperature of the right heat setting zone was set to 115°C. That is, the temperature difference between the left and right sides during heat setting was 17°C. During the latter half of heat setting, air was blown onto the film from both above and below at a left temperature of 132°C and a right temperature of 115°C (i.e., the same temperature as the heat setting temperature) and at a wind speed of 30 m/sec. The heat-set film was then cooled to 100°C, after which the left and right clips were released.

(Roll transport)
After being released from the clips and fed out of the stretching device, 250 mm of each of the both side edges of the stretched film were cut off. The film with both ends cut off was transported by rolls, and the amount of slack between the transport rolls and the location where slack occurred were detected. As a result, slack occurred at the left end, and the amount of slack was 18.0 mm. In addition, the difference L' in length between both ends of the stretched film before correction, calculated based on the above formulas (1) to (3), was 0.95 mm.

(Feedback compensation)
Between the time when the film had passed 3/4 of the running section of the oblique stretching zone C and the time when the film reached the end point, the clip pitch of the right clip was gradually increased to _P2 ' (clip pitch correction amount ( _P2' - _P2 ): 0.3 mm), and oblique stretching was continued by changing the clip pitch profile so that the clip was maintained while heat-fixing and cooling (100°C) were performed as described above and the clip was then released. In other words, the clip pitch when the obliquely stretched film was released from the clip after feedback correction was _P2 ' on the right side and _P2 on the left side.

The in-plane retardation Re(550) of the obtained retardation film was 147 nm, the thickness was 30 μm, and the angle between the slow axis direction and the longitudinal direction was 45°. The obtained retardation film was subjected to the above evaluations (5) to (7). The results are shown in Table 1.

Example 2
A retardation film was obtained in the same manner as in Example 1, except that the temperature of the left heat setting zone in the heat setting was set to 132°C and the temperature of the right heat setting zone was set to 122°C (the temperature difference between the left and right sides in the heat setting was 10°C), and the wind speed was set to 25 m/sec. The in-plane retardation Re(550) of the obtained retardation film was 147 nm, the thickness was 30 μm, and the angle between the slow axis direction and the longitudinal direction was 45°. The obtained retardation film was subjected to the above evaluations (5) to (7). The results are shown in Table 1.

Example 3
A retardation film was obtained in the same manner as in Example 1, except that the temperature of the left heat setting zone in the heat setting was set to 131°C, the temperature of the right heat setting zone was set to 124°C (the temperature difference between the left and right sides in the heat setting was 7°C), and the wind speed was set to 25 m/sec. The in-plane retardation Re(550) of the obtained retardation film was 140 nm, the thickness was 40 μm, and the angle between the slow axis direction and the longitudinal direction was 45°. The obtained retardation film was subjected to the above evaluations (5) to (7). The results are shown in Table 1.

Example 4
A retardation film was obtained in the same manner as in Example 1, except that the resin film 2 obtained in Production Example 2 was used, and the temperature of the left heat fixing zone in the heat fixing was set to 137°C, and the temperature of the right heat fixing zone was set to 130°C (the temperature difference between the left and right sides in the heat fixing was 7°C). The in-plane retardation Re(550) of the obtained retardation film was 140 nm, the thickness was 57 μm, and the angle between the slow axis direction and the longitudinal direction was 45°. The obtained retardation film was subjected to the above evaluations (5) to (7). The results are shown in Table 1.

Example 5
A retardation film was obtained in the same manner as in Example 4, except that the wind speed was set to 25 m/sec. The in-plane retardation Re(550) of the obtained retardation film was 137 nm, the thickness was 40 μm, and the angle between the slow axis direction and the longitudinal direction was 45°. The obtained retardation film was subjected to the above evaluations (5) to (7). The results are shown in Table 1.

<Comparative Example 1>
A retardation film was obtained in the same manner as in Example 1, except that the temperature of the left heat fixing zone in the heat setting was set to 132°C, and the temperature of the right heat fixing zone was set to 132°C (the temperature difference between the left and right sides in the heat setting was 0°C). The in-plane retardation Re(550) of the obtained retardation film was 135 nm, the thickness was 30 μm, and the angle between the slow axis direction and the longitudinal direction was 45°. The obtained retardation film was subjected to the above evaluations (5) to (7). The results are shown in Table 1.

<Comparative Example 2>
A retardation film was obtained in the same manner as in Example 1, except that no slack correction (feedback correction) was performed and the wind speed was set to 10 m/sec. The in-plane retardation Re(550) of the obtained retardation film was 137 nm, the thickness was 30 μm, and the angle between the slow axis direction and the longitudinal direction was 45°. The obtained retardation film was subjected to the above evaluations (5) to (7). The results are shown in Table 1.

<Comparative Example 3>
A retardation film was obtained in the same manner as in Example 1, except that the resin film 3 obtained in Production Example 3 was used, and the temperature of the left heat fixing zone in the heat fixing was set to 132°C, and the temperature of the right heat fixing zone was set to 112°C (the temperature difference between the left and right sides in the heat fixing was 20°C). The in-plane retardation Re(550) of the obtained retardation film was 144 nm, the thickness was 30 μm, and the angle between the slow axis direction and the longitudinal direction was 45°. The obtained retardation film was subjected to the above evaluations (5) to (7). The results are shown in Table 1.

<Evaluation>
As is clear from Table 1, in the manufacturing method of an obliquely stretched retardation film using a resin film having a predetermined birefringence Δn, by setting the temperature difference between the left and right sides during heat setting to a predetermined value or more and performing slack correction during oblique stretching, it is possible to manufacture an obliquely stretched retardation film having small variation in in-plane retardation in the width direction without any defects such as breakage.

The manufacturing method according to the embodiment of the present invention is suitable for use in the manufacture of retardation films, and as a result, can contribute to the manufacture of image display devices such as liquid crystal displays (LCDs) and organic electroluminescence displays (OLEDs).

REFERENCE SIGNS LIST 1 Stretched film 10L Endless loop 10R Endless loop 20 Clip 100 Stretching device 200 Transport roll 300 Winding section 400 Ultrasonic displacement sensor

Claims (6)

The left and right ends in the width direction of a long film having a birefringence Δn of 0.004 or more and 0.025 or less in an unstretched state are gripped by left and right clips of a variable pitch type whose clip pitch in the longitudinal direction is variable;
moving the left and right clips while changing the clip pitch of at least one of them to obliquely stretch the film;
heating the film to a predetermined temperature to fix the stretched state; and
releasing the film from the left and right clips;
the temperature difference between the left and right sides when fixing the stretched state is 7° C. or more;
and correcting slack in the film during the oblique stretching.
A method for manufacturing a retardation film. The method for producing a retardation film according to claim 1, wherein the lower of the left side temperature and the right side temperature when fixing the stretched state is Tg-20°C or higher, where Tg is the glass transition temperature of the long film. The method for producing a retardation film according to claim 1 or 2, wherein the birefringence Δn of the film in an unstretched state is 0.008 or more. The method for producing a retardation film according to any one of claims 1 to 3, wherein the long film contains a polycarbonate-based resin, and the polycarbonate-based resin contains a structural unit represented by the following formula (V):
The correction of the slack includes: fixing the stretched state of the film, transporting the film with rolls, and detecting the amount of slack in the film between the transport rolls and the portion where the slack occurs; and
performing a correction to change the clip pitch of at least one of the left and right clips upstream of the conveying line based on the detection result;
The method for producing the retardation film according to claim 1 , comprising: The thickness of the obtained retardation film is 15 μm to 45 μm, the in-plane retardation Re (550) is 100 nm to 200 nm, the angle between the slow axis direction and the longitudinal direction is 40 ° to 50 ° or 130 ° to 140 °, and the variation of the in-plane retardation Re (550) in the width direction is within 6% of the average value of the in-plane retardation Re (550) in the width direction. The method for producing a retardation film according to any one of claims 1 to 5.

Images