Biaxially oriented polyester film roll and method for producing biaxially oriented polyester film roll

Abstract

To make it possible to obtain a biaxially oriented polyester film roll that can be stably processed when the film is processed by heating during the production process of the film, and that can be stably processed when processing the film master roll.SOLUTION: A method for producing a biaxially oriented polyester film includes: a biaxial stretching step; and a relaxation step in which the film is subjected to a relaxation treatment in a film longitudinal direction (MD) after the biaxial stretching step. The relaxation treatment is carried out by gripping width direction edges of the film with clips and controlling intervals of the clips, and a relaxation rate per one meter of film running distance in the relaxation step is more than 0% and 1.5% or less, and a relaxation speed is 0.02% / sec or more and 0.20% / sec or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented polyester film roll and a method for producing a biaxially oriented polyester film roll. [Background technology]

[0002] Polyester films stretched biaxially, in the machine direction (MD) and the transverse direction (TD), perpendicular to the machine direction within the film plane, have excellent mechanical properties and are used in a variety of fields. Among the polyesters that make up polyester films, polyethylene terephthalate (PET) and polyethylene 2,6-naphthalate (PEN) are particularly well-suited for their excellent mechanical and thermal properties, and PET in particular is inexpensive, making them widely used.

[0003] In particular, in recent years, it has increasingly been used instead of glass as the substrate of transparent conductive laminates used in touch panels, electronic paper, etc. Some transparent conductive laminates use a polyester film as the substrate, on which an ITO (indium tin oxide) film is formed by sputtering, either directly or via an anchor layer. The polyester film used here is generally heat-processed. Furthermore, in the manufacturing process of transparent electrodes for touch panels, transparent conductive films on which a transparent conductive film made of ITO is formed undergo many heating steps, such as annealing, an ITO crystallization step, a resist printing step, and an etching step, so the substrate of the transparent conductive laminate is required to be resistant to heat deformation.

[0004] In polyester film, which is biaxially stretched polyester, the stretching process results in molecular orientation, improving mechanical properties such as strength. However, biaxially stretched films have the property that residual strain remains in the molecular chains, and when heated, the strain is released, causing the film to shrink. This shrinkage characteristic is generally a hindrance. In particular, when post-processing, such as for ITO applications, involves a heating process, it is necessary to minimize the amount of heat shrinkage of polyester film.

[0005] Therefore, after biaxial stretching, the molecular chain strain is relieved by heat treatment (also called heat setting) in a tenter. Generally, the thermal shrinkage rate decreases depending on the heat treatment temperature, but this heat treatment alone cannot completely remove the molecular chain strain. One method for removing this residual strain is to perform offline heat treatment after polyester film formation. However, this method increases equipment costs, incurs post-processing costs, and reduces the flatness of the film. Therefore, a method for removing residual strain in-line has been adopted in which the tenter rail width is tapered to cause slight shrinkage in the TD. However, this method does not remove residual strain in the MD, and various methods for removing residual strain in the MD have been investigated. For example, Patent Document 1 mentions a longitudinal relaxation treatment by narrowing the clip spacing as a means of reducing the thermal shrinkage rate of polyester film itself.

[0006] Furthermore, polyester films are required to suppress not only the absolute value of the heat shrinkage rate but also the variation (R). If the heat shrinkage rate is constant, it is possible to process the film while taking the rate of change into account. However, if variations (R) occur in the heat shrinkage rates in the MD and TD, stable processing becomes difficult. Therefore, suppression of the variation (R) in the MD and TD of the film is required. Patent Document 2 describes that, with the aim of suppressing variations in physical properties, by adjusting the stretching ratio and the temperature during stretching within specific ranges, a film can be obtained in which the difference between the maximum and minimum values ​​of the elongation stress, the modulus of elasticity, and the average and standard deviation of the thickness are within specific ranges in each of four directions: 45°, 90°, and 135° clockwise from any direction on the film surface, with the direction set to 0°. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-276190 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-226841 Summary of the Invention [Problem to be solved by the invention]

[0008] In Patent Document 1, the method of reducing the absolute value of the heat shrinkage rate by relaxing in the longitudinal direction is effective, but it was found that if the relaxation section in the relaxation treatment is short or the film-forming speed is too fast, there is a problem of variation (R) in the heat shrinkage rate of the final product. Furthermore, in Patent Document 2, since the film is for cold forming purposes and is intended to improve the ductility and heat resistance of the laminated metal foil, the focus is mainly on the stress during elongation, and the range of the heat shrinkage rate due to heating is set broadly. For films for optical applications, the heat shrinkage rate during heating must be controlled more strictly.

[0009] In view of the above, an object of the present invention is to provide a biaxially oriented polyester film roll that suppresses curling during heat processing in the production and processing of the film and enables stable processing during processing of the film roll. [Means for solving the problem]

[0010] In order to solve the above problems, the present invention has the following configuration.

[0011] (1) A method for producing a biaxially oriented polyester film, comprising a biaxial stretching step and a relaxation step in which a relaxation treatment is performed in the longitudinal direction (MD) of the film after the biaxial stretching step, wherein the relaxation treatment is performed by gripping the width direction ends of the film with clips and controlling the distance between the clips, and wherein the relaxation rate per meter of film running distance in the relaxation step is more than 0% and not more than 1.5%, and the relaxation speed is 0.02% / sec or more and 0.20% / sec or less.

[0012] (2) The method for producing a biaxially oriented polyester film according to (1), wherein the total relaxation rate in the relaxation step is 0.5% or more and 3.0% or less.

[0013] (3) The method for producing a biaxially oriented polyester film according to (1) or (2), wherein the relaxation temperature in the relaxation step is 130°C or higher and 220°C or lower.

[0014] (4) A biaxially oriented polyester film roll in which the difference (R) between the maximum and minimum values ​​of the thermal shrinkage rate L (%) measured at 45° to the longitudinal direction (MD) of the film at three points: the center of the width direction (TD) of the biaxially oriented polyester film and 250 mm inward from both ends of the width direction, is 0.40% or less.

[0015] (5) A biaxially oriented polyester film roll according to (4), in which the difference (R) between the maximum and minimum values ​​when the thermal shrinkage rate L (%) in the longitudinal direction (MD) and transverse direction (TD) of the biaxially oriented polyester film is measured at three points, namely, the center in the transverse direction (TD) and 250 mm inward from both ends in the transverse direction, is 0.2% or less.

[0016] (6) The biaxially oriented polyester film roll according to (4) or (5), having a thickness of 38 μm or more and 250 μm or less. [Effects of the Invention]

[0017] According to the present invention, it is possible to obtain a biaxially oriented polyester film roll that can be stably processed when the film is processed by heating during the production process of the film, and that can be stably processed when the film is processed into a raw roll. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be described in detail below.

[0019] The polyester constituting the film used in the present invention is a polymer obtained by condensation polymerization of a diol and a dicarboxylic acid, the dicarboxylic acid being typified by terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, adipic acid, sebacic acid, etc. The diol being typified by ethylene glycol, trimethylene glycol, tetramethylene glycol, cyclohexanedimethanol, etc.

[0020] Specific examples of such polyesters include polymethylene terephthalate, polyethylene terephthalate, polytetramethylene terephthalate, polyethylene-p-oxybenzoate, poly-1,4-cyclohexanedimethylene terephthalate, and polyethylene-2,6-naphthalate. These polyesters may be homopolymers or copolymers, and the copolymerization components may include diol components such as diethylene glycol, neopentyl glycol, and polyalkylene glycol, and dicarboxylic acid components such as adipic acid, sebacic acid, phthalic acid, isophthalic acid, and 2,6-naphthanedicarboxylic acid. In the present invention, polyethylene terephthalate and polyethylene-2,6-naphthalate are particularly preferred from the standpoints of mechanical strength, heat resistance, chemical resistance, and durability, and of these, polyethylene terephthalate is most preferred due to its low cost.

[0021] Furthermore, various additives such as antioxidants, antistatic agents, crystal nucleating agents, inorganic particles, and organic particles may be added to the polyester as required.

[0022] In addition, in the present invention, it is also a preferred embodiment that the biaxially stretched polyester film has a laminated structure. Examples of the laminated structure include lamination by coextrusion of polymers and lamination structures obtained by a method of applying a coating liquid onto a polyester film substrate. The latter process of applying a coating liquid can be selected as needed, such as before stretching the polyester film substrate, during the stretching process, or after stretching and heat treatment. However, by performing the process within the film production process, such as before stretching or during the stretching process, the process can be simplified.

[0023] As a method for applying the coating liquid onto the polyester film substrate, various coating methods can be used, such as reverse coating, gravure coating, rod coating, bar coating, Mayer bar coating, die coating, spray coating, etc. Although not particularly limited, in consideration of the uniformity of application and adhesiveness of the coating film (resin layer) formed by application, the surface of the polyester film substrate may be subjected to corona discharge in advance.

[0024] These laminate structures are primarily used to impart surface properties appropriate for the intended application. For example, they can impart adhesion to inks and toners, or antistatic properties to suppress static electricity. Furthermore, when the biaxially oriented polyester film obtained by the present invention is used as an optical film substrate, excellent adhesion to materials that undergo post-processing, such as prism lens processing, hard coating, and anti-reflection processing, is required. To improve adhesion between the biaxially oriented polyester film obtained by the present invention and the materials to be subjected to such post-processing, it is preferable to provide a polymeric adhesive layer (resin layer) on at least one side of the biaxially stretched polyester film, the polymeric adhesive layer being composed primarily of one or more resins selected from the group consisting of polyester resins, acrylic resins, urethane resins, and polyamide resins. Various additives, such as antioxidants, crystal nucleating agents, inorganic particles, and organic particles, may also be added to this polymeric adhesive layer. In particular, porous silica, which has a refractive index similar to that of the resin in the adhesive layer, imparts lubricity to the film surface while maintaining transparency, making it effective for improving the handleability of the polyester film.

[0025] In the present invention, when the heat shrinkage percentage L (%) in the direction forming an angle of 45° to the longitudinal direction (MD) of the biaxially oriented polyester film is measured at three points, namely, the center in the width direction (TD) and 250 mm inward from both widthwise ends, the difference (R) between the maximum and minimum values ​​is 0.40% or less. The difference (R) between the maximum and minimum values ​​when the heat shrinkage percentage L (%) in the direction forming an angle of 45° to the longitudinal direction (MD) of the film is measured is more preferably 0.3% or less, and even more preferably 0.28% or less. If the difference (R) between the maximum and minimum values ​​when the heat shrinkage percentage L (%) in the direction forming an angle of 45° to the longitudinal direction (MD) of the film exceeds 0.40%, curl variation (R) during film processing becomes large, resulting in a reduced yield of the final product. When the thermal shrinkage percentage L (%) of the above-mentioned film is measured in a direction at an angle of 45° to the MD, the difference (R) between the maximum and minimum values ​​at three points in the width direction of the film can be achieved by appropriately controlling the relaxation rate per meter of film running distance, relaxation speed, total relaxation rate, and relaxation process temperature in the longitudinal direction (MD) relaxation treatment, and by appropriately controlling the stretching and relaxation treatment in the transverse direction (TD).

[0026] The MD and TD heat shrinkage rates of the biaxially oriented polyester film obtained by the present invention are preferably 1.0% or less, and more preferably 0.7% or less, when left standing at 150°C for 30 minutes. Setting the heat shrinkage rate within the above range is advantageous in that it suppresses an increase in the amount of film curl and a decrease in yield due to heat treatment during film processing. The MD and TD heat shrinkage rates of the biaxially oriented polyester film can be achieved by appropriately controlling the relaxation rate per meter of film running distance, relaxation speed, total relaxation rate, and relaxation process temperature in the longitudinal direction (MD) relaxation treatment, and by appropriately controlling the stretching and relaxation treatment in the transverse direction (TD).

[0027] In the biaxially oriented polyester film of the present invention, the difference (R) between the maximum and minimum heat shrinkage values ​​L (%) in the MD and TD of the film at three points in the width direction of the film, as measured by the method described below, is preferably 0.2% or less, and more preferably 0.15% or less. The difference (R) between the maximum and minimum heat shrinkage values ​​L (%) in the MD and TD of the film at three points in the width direction of the film can be achieved by appropriately controlling the relaxation rate per meter of film running distance, relaxation speed, total relaxation rate, and relaxation process temperature in the longitudinal direction (MD) relaxation treatment, and by appropriately controlling the stretching and relaxation treatment in the width direction (TD).

[0028] The thickness of the biaxially oriented polyester film of the present invention is preferably 38 μm or more and 250 μm or less, more preferably 50 μm or more and 220 μm or less. A film having the above thickness range exhibits excellent handleability while suppressing curling during a heating step, and is therefore suitable for use as a substrate for a transparent conductive laminate.

[0029] The length in the transverse direction (TD) of the biaxially oriented polyester film of the present invention is preferably 1.0 m or more and 3.0 m or less, more preferably 1.0 m or more and 2.0 m or less.

[0030] The length in the machine direction (MD) of the biaxially oriented polyester film roll of the present invention is not particularly limited, but is preferably 100 m or more and 10,000 m or less.

[0031] Here, the biaxially oriented polyester film refers to a film that is oriented in both the machine direction (MD) and the transverse direction (TD) of the film. The biaxially oriented polyester film can be obtained, for example, by stretching the film in the machine direction (MD) and the transverse direction (TD) of the film and, if necessary, subjecting it to heat treatment.

[0032] Specific examples of stretching include a method in which a substantially unoriented film obtained by melt-extruding polyester into a sheet is stretched in MD and then stretched in TD, a method in which it is stretched in TD and then stretched in MD, or a method in which it is stretched in MD and TD simultaneously. MD stretching and TD stretching may also be combined multiple times.

[0033] Here, we will explain the case of sequential stretching, in which the film is stretched in MD and then stretched in TD. MD stretching is often performed by using a difference in roll peripheral speed. The stretching ratio varies depending on the type of resin, but is usually preferably 2 to 15 times. When polyethylene terephthalate is used as the resin constituting the film, a stretching ratio of 2 to 7 times is preferred, more preferably 3 to 5 times, and even more preferably 3 to 4 times. The stretching temperature is preferably higher than the glass transition temperature of the resin constituting the film but lower than the glass transition temperature + 100°C; specifically, a temperature of 70°C to 120°C is preferred, and a temperature of 80°C to 110°C is more preferred.

[0034] The uniaxially stretched film thus obtained may be subjected to a surface treatment such as corona treatment, flame treatment, or plasma treatment as needed, and then may be imparted with properties such as easy slippage, easy adhesion, and antistatic properties by in-line coating.

[0035] In the subsequent TD stretching, a tenter method is usually used, and the film is stretched in the width direction while being conveyed while being held at both ends with clips. The stretching ratio varies depending on the type of resin, but is usually preferably 2 to 15 times. When polyethylene terephthalate is used as the resin constituting the film, a ratio of 2 to 5 times is preferably used, and more preferably 3 to 4.5 times. The stretching temperature is preferably from the glass transition temperature of the resin constituting the film to the glass transition temperature + 120°C, specifically preferably from 70°C to 140°C, and more preferably from 80°C to 130°C.

[0036] Heat treatment is preferably performed to impart low heat shrinkage and flatness to polyester film stretched in both MD and TD. However, heat treatment alone does not provide sufficient low heat shrinkage and flatness. That is, cooling from such high temperatures reversibly shrinks the thermal expansion at high temperatures as the film cools, accumulating strain and resulting in additional heat shrinkage in a range from the glass transition temperature to 150°C. Therefore, to suppress this heat shrinkage, a relaxation treatment is preferably performed during the cooling process from the tenter heat treatment to absorb the reversible shrinkage associated with this cooling. After heat treatment, a TD relaxation treatment is preferably performed to reduce the tenter rail width, and an MD relaxation treatment is preferably performed to reduce the clip spacing.

[0037] The MD relaxation rate per meter of film running distance in the relaxation step of the present invention is greater than 0% and not more than 1.5%. The lower limit of the MD relaxation rate per meter of film running distance is preferably 0.1% or more, more preferably 0.15% or more. By setting the MD relaxation rate per meter of film running distance within the above range, the MD relaxation zone length is not unnecessarily long, which is preferable.

[0038] Next, by optimizing the MD relaxation distance and film-forming speed, it is possible to suppress an increase in the variation (R) in the thermal shrinkage rate within and between products, and in turn, an increase in the TD variation (R) in the amount of curl when heat-processing is performed during film manufacturing and processing, resulting in a preferred embodiment.

[0039] Therefore, the longitudinal direction (MD) relaxation rate in the manufacturing method of the biaxially oriented polyester film of the present invention is 0.02% / sec or more and 0.20% / sec or less. The MD relaxation rate is preferably 0.03 sec or more and 0.19% / sec or less. When the MD relaxation rate is in the above range, the TD variation (R) of the heat shrinkage of the biaxially oriented polyester film can be suppressed, and the TD variation (R) of the curl amount during heat processing in the film manufacturing process can be suppressed. The reason for the suppression of the TD variation (R) of the heat shrinkage of the biaxially oriented polyester film is not clear, but it is presumed to be because the MD relaxation rate in the tenter is sufficiently small compared to the orientation relaxation rate of the molecular chains. Furthermore, if the MD relaxation rate exceeds 0.20% / sec, the TD variation (R) of the curl amount during heat processing increases, so the relaxation rate is preferably in the above range. On the other hand, if the MD relaxation rate is less than 0.02% / sec, the MD relaxation zone length becomes unnecessarily long, increasing equipment costs, so the above range is preferred.

[0040] The total relaxation rate in the MD relaxation treatment is preferably 0.5% or more and 3.0% or less, more preferably 0.8% or more and 1.5% or less. By setting the total relaxation rate in the MD relaxation treatment within this range, a biaxially oriented polyester film that combines low heat shrinkage and flatness can be obtained. In addition, the MD relaxation zone length is not unnecessarily long, which is preferable.

[0041] The relaxation step temperature in the MD relaxation treatment is preferably 130°C or higher and 220°C or lower, more preferably 130°C or higher and 180°C or lower. The relaxation step temperature may be multi-staged. Setting the relaxation step temperature to 130°C or higher results in favorable film orientation relaxation and heat shrinkage during the relaxation step. Setting the relaxation step temperature to 220°C or lower provides a sufficient temperature difference from the film melting point, preventing slack during film transport, which is favorable. Setting the relaxation step temperature to 180°C or lower reduces residual distortion in the low-temperature range and further suppresses the variation (R) in heat shrinkage at 150°C. The tenter outlet temperature is preferably between 80°C and 200°C, more preferably between 120°C and 160°C. [Evaluation method] (1) Heat shrinkage rate at 150°C for 30 minutes Three 250mm square samples were taken from polyester film, 250mm inward from both ends in the width direction and with the width center at the center of the sample width. Cross marks were made on each sample at approximately 200mm intervals, and the spacing between the marks was measured using a length measuring device combining a Nippon Kogaku Co., Ltd. universal projector and a Mitoyo Shoji Co., Ltd. linear scale (accuracy 0.001mm) to determine L0 (mm). The samples were then heated in an oven heated to 150°C for 30 minutes, allowed to cool to room temperature (23°C, 65%), and the spacing between the marks was measured again using the length measuring device to determine L (mm). The thermal shrinkage was calculated as (L0 - L) × 100 / L0 (%), and the average value (Ave) of the three widths and the difference (R) between the maximum and minimum values ​​of the three samples were calculated. The longitudinal direction of the film was set as 0° (MD), and the heat shrinkage was also measured at angles of 45° and 90° (TD) counterclockwise. (2) Curl resistance of transparent conductive film Hard coat layers were formed on both sides of the film in the following manner, and a transparent conductive layer was further formed on the surface of one of the hard coat layers to obtain a transparent conductive film.

[0042] (Arrangement of hard coat layer) The following raw materials were diluted with propylene glycol monomethyl ether (PGMA) and mixed, and each raw material was dispersed in a solvent to prepare a coating material with a non-volatile content of 25.5% by mass. The coating material obtained here was applied to one side of a film so that the film thickness after drying would be 1.0 μm. After removing the solvent from the coating film in a drying oven set at 80°C, the coating material was dried using a UV treatment device with an integrated light dose of 400 mJ / cm. 2 The coating film was cured by irradiating it with ultraviolet light of 1000 kJ / cm.sup.2 at ...

[0043] Reactive group-decorated colloidal silica (dispersion medium: propylene glycol monomethyl ether acetate, non-volatile content: 40% by weight): 100 parts by weight Dipentaerythritol hexaacrylate: 48 parts by mass 1,6-Hexanediol diacrylate: 12 parts by mass Photopolymerization initiator (1-hydroxycyclohexyl phenyl ketone): 2.5 parts by mass (Arrangement of transparent conductive layer) Transparent conductive films were fabricated by sputtering a transparent conductive layer onto one side of a hard-coated film with hard-coat layers on both sides. Specifically, a target containing indium oxide and 5% tin oxide was used to deposit a transparent conductive layer composed of a composite oxide of indium oxide and tin oxide on one side of the hard-coat layer in a mixed atmosphere of 98% argon gas by volume and 2% oxygen gas by volume. A 200mm square sample was then fabricated with its four sides aligned along the length and width of the film. The transparent conductive layer was placed on top in an oven set to 140°C under untension for 90 minutes, and then cooled at room temperature for 10 minutes to produce a transparent conductive film. The fabricated transparent conductive film was placed on a flat surface, and the distance between the flat surface and the top of the transparent conductive film was measured. The average of the four measurements was calculated, and the average of the three measurements across the width was used to calculate the curl amount. Curl resistance was evaluated as follows: A: curl amount less than 10 mm; B: curl amount 10 mm or more but less than 15 mm; C: curl amount 15 mm or more. (3) Difference between maximum and minimum curl amount in the film width direction (R) Three 200mm square samples were taken across the width of the polyester film, 250mm inward from both ends in the width direction (TD) and at the center of the width direction. The four edges were taken along the longitudinal and width directions of the film. Using these cut samples, transparent conductive films were fabricated using the same method as described in (2) Curl Resistance of Transparent Conductive Films. The transparent conductive film was placed on a flat surface, and the distance between the flat surface and the top of the transparent conductive film was measured. The average of the four measurements was calculated as the curl amount. The curl amount of each sample was then measured at three points across the width, and the difference between the maximum and minimum curl amounts in the width direction (R) was evaluated. A rating was given for a film with a difference (R) of less than 5mm between the maximum and minimum curl amounts in the width direction, a rating of B for a difference (R) of 5mm to 10mm, and a rating of C for a difference (R) of 10mm or more. (4) Thickness The film was cut to A4 size, and measurements were taken at any 20 points using a dial gauge (Mitutoyo Corporation "No2110S-10"), and the average value was taken as the thickness (μm). [Example]

[0044] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples. [Example 1] Pellets of polyethylene terephthalate with an intrinsic viscosity of 0.65 were vacuum-dried at 180°C for 5 hours, then fed into an extruder heated to 270-300°C and formed into a sheet from a T-die. This sheet was then electrostatically solidified onto a cooling drum with a surface temperature of 25°C to obtain an unstretched polyester film. The unstretched polyester film obtained was heated with a group of heating rolls at 70-100°C and stretched 3.1 times in the longitudinal direction in one step, and then cooled with a group of rolls at 20-50°C to obtain a uniaxially stretched polyester film. Both sides of this uniaxially stretched polyester film were coated with a 6 μm thick coating of a 4.5 wt % water-dispersible polyester resin containing a lubricant (0.5 wt % solids of 0.1 μm particle size colloidal silica) using a bar coater. The coated polyester film was then introduced into a tenter. While holding both ends of the coated polyester film with clips, it was preheated in a hot air atmosphere heated to 90°C and transversely stretched 3.6 times in the width direction in a hot air atmosphere heated to 120°C. The film thus obtained was then heat-treated in the tenter at 230°C. The resulting biaxially oriented polyester film was then slowly cooled to 195°C and relaxed at a 6% relaxation rate in the TD direction by reducing the tenter rail width. The film was then relaxed at 195°C in the MD relaxation zone, where the tenter clip spacing was reduced, at a 1% relaxation rate and a relaxation rate of 0.07% / sec. The polyester film was then removed from the tenter, and both edge portions of the film were trimmed and wound up to obtain a 2000m long roll of biaxially oriented polyester film with a thickness of 200µm and a film width and length of 1.6m. The heat shrinkage of the resulting polyester film is shown in Table 1. A biaxially oriented polyester film with small variation in heat shrinkage (R), excellent curl resistance, and small variation in curl amount (R) across the film width was obtained. [Examples 2 to 14, Comparative Examples 1 to 6] A film was obtained in the same manner as in Example 1, except for the changes shown in Table 1. The obtained evaluation results and the like are as shown in Table 1.

[0045] [Table 1] The films of the Examples were excellent in 45° heat shrinkage variation (R) and in TD curl resistance variation (R), whereas the films of the Comparative Examples were poor in at least one of the required properties (45° heat shrinkage variation (R), curl resistance, and cross-machine (TD) variation (R)). [Industrial Applicability]

[0046] According to the present invention, a biaxially oriented polyester film roll having excellent processing stability can be provided, which is suitable as a base film for optical films that require a heating step in post-processing.

Claims

1. A method for producing a biaxially oriented polyester film, comprising a biaxial stretching step and a relaxation step in which a relaxation treatment is performed in the longitudinal direction (MD) of the film after the biaxial stretching step, wherein the relaxation treatment is performed by gripping the width direction ends of the film with clips and controlling the distance between the clips, and wherein the relaxation rate per meter of film running distance in the relaxation step is more than 0% and not more than 1.5%, and the relaxation speed is 0.02% / sec or more and 0.20% / sec or less.

2. The method for producing a biaxially oriented polyester film according to claim 1, wherein the total relaxation rate in the relaxation step is 0.5% or more and 3.0% or less.

3. The method for producing a biaxially oriented polyester film according to claim 1 or 2, wherein the relaxation temperature in the relaxation step is 130°C or higher and 220°C or lower.

4. A biaxially oriented polyester film roll, in which the difference (R) between the maximum and minimum values ​​when the heat shrinkage rate L (%) is measured in a direction at an angle of 45° counterclockwise to the longitudinal direction (MD) of the film at a total of three points, one at the center in the width direction (TD) of the biaxially oriented polyester film and one 250 mm inward from both ends in the width direction, is 0.40% or less.

5. 5. The biaxially oriented polyester film roll according to claim 4, wherein the difference (R) between the maximum and minimum heat shrinkage rates L (%) in the longitudinal direction (MD) and the transverse direction (TD) of the biaxially oriented polyester film is 0.2% or less when the heat shrinkage rates L (%) in the longitudinal direction (MD) and the transverse direction (TD) of the film are measured at three points, one at the center in the transverse direction (TD) and one at a position 250 mm inward from each end of the transverse direction.

6. The biaxially oriented polyester film roll according to claim 4 or 5, having a thickness of 38 μm or more and 250 μm or less.

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