Biaxially oriented polyester film and method for producing the same

Abstract

To provide a polyester film that offers both consistent light blocking and resistance to delamination, with enhanced recyclability.SOLUTION: A biaxially oriented polyester film comprises carbon black of 0.01 mass% or more and 5.50 mass% or less. The carbon black has a planar orientation index, Mc, of 1.10 or more and 2.00 or less. The crystalline orientation index, χi, of film is 5.0 or more and 13.0 or less as determined by wide-angle X-ray diffraction.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a biaxially oriented polyester film that achieves both light shielding uniformity, resistance to splitting, and recyclability, and a method for producing the same.

Background Art

[0002] Polyester (especially polyethylene terephthalate, polyethylene 2,6-naphthalenedicarboxylate, etc.) resins are excellent in mechanical properties, thermal properties, chemical resistance, electrical properties, and moldability, and are used in various applications. Among polyester films formed from such polyester, biaxially oriented polyester films are used as materials for solar cell backsheets, electrical insulation materials for water heater motors, electrical insulation materials for motors such as those for car air conditioners and drive motors used in hybrid vehicles, tape materials, capacitor materials, packaging materials, building materials, photographic applications, graphic applications, thermal transfer applications, and various other applications due to their mechanical and electrical properties.

[0003] Among these applications, in tape materials used as light shielding members inside electronic devices such as smartphones and smartwatches, as the functions of cameras and optical sensors are improved, the requirements for light shielding properties are becoming more stringent. In particular, since even slight non-uniformity of a light shielding member, such as light shielding spots caused by obliquely incident light and fine spots of reflected light due to non-uniformity of light shielding components, directly affects the performance as an optical module, a member having more uniform light shielding characteristics is required. In addition, with the miniaturization and thinning of electronic devices, efficient utilization of the internal volume of the device is necessary, and as the usage environments in complex shapes adapted to the forms of each module are increasing, the durability when the member is deformed and used has become important.

[0004] Conventionally, as means for improving light shielding properties in tape materials, methods such as providing a printing layer containing a large amount of black pigment on a polyester film (Patent Documents 1 and 2) and studies on light shielding films containing inorganic pigments inside a polyester film (Patent Document 3) have been conducted.

Prior Art Documents

[0005] [Patent Document 1] International Publication No. 2010 / 106999 [Patent Document 2] Japanese Patent Publication No. 2016-196527 [Patent Document 3] Japanese Patent Publication No. 2017-210557 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, while the films described in Patent Documents 1 and 2 achieve high light-shielding properties in thin films, the repeated processing leads to a high frequency of defects and other issues, resulting in problems with uniform light-shielding. Furthermore, there are environmental pollution problems caused by the printing ink layer, and the presence of the printing layer makes film recycling difficult during production, resulting in a significant environmental burden. In addition, the film described in Patent Document 3 contains a large amount of inorganic pigment added to enhance light-shielding properties, which leads to the formation of fine cracks and cleavage in the film when used in complex shapes, causing contamination. In particular, when recycled polyester raw materials are used at high concentrations, the film strength decreases, making it difficult to achieve both cleavage resistance and light-shielding properties.

[0007] In view of the prior art, the object of the present invention is to provide a polyester film that achieves both uniform light shielding and cleavage resistance, and is also highly recyclable. [Means for solving the problem]

[0008] A preferred embodiment of the present invention for solving the above problems is as follows. [1] A biaxially oriented polyester film containing 0.01% by mass or more and 5.50% by mass or less of carbon black, wherein the plane orientation index Mc of the carbon black is 1.10 or more and 2.00 or less, and the crystal orientation index χi of the film determined by wide-angle X-ray diffraction is 5.0 or more and 13.0 or less. [2] The biaxially oriented polyester film according to [1], wherein the average primary particle diameter a (nm) of the carbon black is 10 nm or more and 150 nm or less. [3] The biaxially oriented polyester film according to [1] or [2], wherein the aggregate size b (nm) and average primary particle diameter a (nm) of the carbon black satisfy formula (2). b / a ≥ 10 ···(2) [4] A biaxially oriented polyester film according to any one of [1] to [3], wherein the absolute value of the dielectric constant difference at a frequency of 15 GHz in two orthogonal directions within the film plane is 0.00 or more and 0.20 or less. [5] A method for producing a biaxially oriented polyester film according to any one of [1] to [4], wherein a polyester film containing carbon black and having a surface orientation index Mc of carbon black of 1.03 or higher is used as a raw material in a mass ratio of 20% to 40%. [Effects of the Invention]

[0009] According to the present invention, a polyester film can be provided that exhibits excellent light-shielding uniformity and also excellent cleavage resistance. [Brief explanation of the drawing]

[0010] [Figure 1] A schematic diagram showing a biaxially oriented polyester film with a small plane orientation index Mc, viewed from an oblique angle. [Figure 2] A schematic diagram showing a biaxially oriented polyester film with a large plane orientation index Mc, viewed from an oblique angle. [Figure 3] Cross-sectional image of a biaxially oriented polyester film with a plane orientation index Mc of 1.25. [Figure 4]Binary image in an example of a biaxially oriented polyester film with a surface orientation index Mc of 1.25. [Figure 5] Power spectrum in an example of a biaxially oriented polyester film with a surface orientation index Mc of 1.25. [Figure 6] Orientation intensity distribution in an example of a biaxially oriented polyester film with a surface orientation index Mc of 1.25. [Figure 7] Film cross-sectional image in an example of a biaxially oriented polyester film with a surface orientation index Mc of 1.06. [Figure 8] Binary image in an example of a biaxially oriented polyester film with a surface orientation index Mc of 1.06. [Figure 9] Power spectrum in an example of a biaxially oriented polyester film with a surface orientation index Mc of 1.06. [Figure 10] Orientation intensity distribution in an example of a biaxially oriented polyester film with a surface orientation index Mc of 1.06.

Modes for Carrying Out the Invention

[0011] The present invention will be described in detail below.

[0012] <Biaxially Oriented Polyester Film> Preferred embodiments of the polyester resin constituting the biaxially oriented polyester film of the present invention (hereinafter, may also be simply referred to as "the film of the present invention") are described below.

[0013] Polyester resin refers to a polymer having ester bonds as its main chain, but the polyester resin used in the present invention is preferably a polyester resin having a structure in which dicarboxylic acid and diol are condensed polymerized. Examples of dicarboxylic acid components include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, diphenyldicarboxylic acid, diphenylsulfondicarboxylic acid, diphenoxyethanedicarboxylic acid, and 5-sodiumsulfondicarboxylic acid; aliphatic dicarboxylic acids such as oxalic acid, succinic acid, adipic acid, sebacic acid, dimer acid, maleic acid, and fumaric acid; alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid; and oxycarboxylic acids such as parahydroxybenzoic acid. Furthermore, examples of dicarboxylic acid ester derivative components include esterified products of the above-mentioned dicarboxylic acid compounds, such as dimethyl terephthalate, diethyl terephthalate, 2-hydroxyethyl methyl terephthalate, dimethyl 2,6-naphthalenedicarboxylate, dimethyl isophthalate, dimethyl adipate, diethyl maleate, and dimethyl dimerate. Examples of diol components include aliphatic dihydroxy compounds such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and 2,2-dimethyl-1,3-propanediol (neopentyl glycol); polyoxyalkylene glycols such as diethylene glycol, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol; alicyclic dihydroxy compounds such as 1,4-cyclohexanedimethanol and spiroglycol; and aromatic dihydroxy compounds such as bisphenol A and bisphenol S. Each of these may be used individually or in combination of two or more. Furthermore, if it does not affect the film-forming properties, it may also be a copolymer of one or more trimellitic acid, pyromellitic acid, and their ester derivatives in small amounts.

[0014] Specific examples of polyester resins include polyethylene terephthalate (hereinafter abbreviated as PET), polyethylene-2,6-naphthalenedicarboxylate, polypropylene terephthalate, polybutylene terephthalate, and poly-1,4-cyclohexylenedimethylene terephthalate, which are readily available at low cost and have good film-forming properties, making them particularly suitable for use.

[0015] Furthermore, the polyester resin may be a homopolymer or a copolymer. Examples of copolymer components in the copolymer include aromatic dicarboxylic acids, aliphatic dicarboxylic acids, alicyclic dicarboxylic acids, and diol components having 2 to 15 carbon atoms. Examples of these include isophthalic acid, adipic acid, sebacic acid, phthalic acid, sulfonic acid base-containing isophthalic acid, and their ester-forming compounds, ethylene glycol, 1,4-butanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, spiroglycol, and polyalkylene glycols with a number-average molecular weight of 400 to 20,000.

[0016] The biaxially oriented polyester film of the present invention preferably contains 0.01% by mass or more and 5.50% by mass or less of carbon black from the viewpoint of controlling light shielding properties. A carbon black content of 0.01% by mass or more provides sufficient light shielding properties, while a carbon black content of 5.50% by mass or less improves cleavage resistance when a complex shape is formed. A carbon black concentration of 1.00% by mass or more and 4.50% by mass or less is more preferable, and a concentration of 2.00% by mass or more and 4.00% by mass or less is particularly preferable. From the viewpoint of light shielding uniformity, 3.10 parts by mass or more is more preferable.

[0017] As for the type of carbon black, furnace black, channel black, acetylene black, Ketjen black, etc., can be used. While known types of carbon black can be used, it is preferable that its average primary particle diameter a (nm) is 10 nm or more and 150 nm or less. More preferably, it is 20 nm or more and 130 nm or less. An average primary particle diameter a of 10 nm or more can suppress deterioration of dispersibility in the film due to an increase in surface area, and an average primary particle diameter a of 150 nm or less can suppress the generation of voids near the carbon black, thereby increasing the strength of the film. Here, "average primary particle diameter" as used in this invention refers to the number-average particle diameter.

[0018] In the biaxially oriented polyester film of the present invention, it is preferable that the average aggregation size b (nm) and average primary particle diameter a (nm) of carbon black satisfy the following formula.

[0019] b / a ≥ 10 ···(2) Equation (2) corresponds to the number of primary particles that form carbon black aggregates. Generally, from the viewpoint of filtration stability during extrusion and uniformity of film appearance, it is preferable for carbon black to be uniformly and finely dispersed in the three-dimensional space of the film, and it is common to control equation (2) to a smaller value. However, the larger the number of primary particles that form carbon black aggregates represented by equation (2), the easier it is to form a continuous orientation of carbon black in the planar direction during the film stretching process, which is preferable as it makes it easier to control the planar orientation index Mc of carbon black, as described later. Furthermore, it is even more preferable if the range of equation (3) below is satisfied.

[0020] b / a ≥ 15 ···(3) The average aggregation size b (nm) and average primary particle diameter a (nm) of the carbon black will be determined by the method described in the examples.

[0021] The biaxially oriented polyester film of the present invention preferably has a carbon black plane orientation index Mc of 1.10 or more and 2.00 or less. The plane orientation index Mc is higher when the carbon black is oriented in the plane direction of the film, and is close to 1.00 when it is unoriented. By having a plane orientation index Mc of 1.10 or more, light shielding properties can be obtained even when the carbon black concentration is low. By having a plane orientation index Mc of 2.00 or less, cleavage in the orientation direction of the carbon black can be suppressed.

[0022] By controlling the plane orientation index Mc within the aforementioned range, the carbon black can be distributed more densely on the film plane, thereby efficiently suppressing the transmission of light, especially from oblique angles, and achieving high light-shielding uniformity even at low concentrations of carbon black. As shown in the figure, when the plane orientation index Mc is small (Figure 1), there may be areas where light is not shielded by the carbon black when viewed from an oblique angle. However, when the plane orientation index Mc is relatively large and the carbon black is oriented to some extent (Figure 2), the carbon black shields the light when viewed from an oblique angle, thus achieving high light-shielding uniformity.

[0023] While the plane orientation index Mc can be increased in general biaxial orientation film manufacturing methods by using exceptionally high stretch ratio conditions, this also increases the orientation of the polyester resin matrix itself, leading to the problem of increased cleavage. As a result of diligent research by the inventors, they discovered that by using recycled raw materials obtained by crushing films in which carbon black is oriented and the plane orientation index Mc of carbon black is 1.03 or higher into flakes, the carbon black is oriented at the stage of the unstretched sheet extruded from the die, and by performing stretching afterward, it is possible to achieve both suppression of matrix resin orientation and promotion of carbon black orientation.

[0024] From a similar viewpoint, the carbon black plane orientation index Mc is more preferably 1.15 or more and 1.80 or less, and most preferably 1.20 or more and 1.60 or less. The method for setting the carbon black plane orientation index Mc to a specific range is not particularly limited, but examples include using a biaxially oriented polyester film having a carbon black plane orientation index Mc of 1.03 or more as a raw material for the film after crushing it, at a mass ratio of 20% to 40%. When a film containing carbon black is used as a recycled raw material by melting or chemical processing to produce recycled chips, the degree of orientation of the carbon black may decrease, resulting in insufficient properties. However, by using a film with oriented carbon black as a raw material, the polyester matrix becomes unoriented during melt extrusion, while the carbon black maintains its oriented state to some extent. This allows the degree of carbon black plane orientation to further increase in the subsequent biaxial stretching process, making it possible to achieve the specific preferred range. The method for measuring the carbon black plane orientation index Mc is as described in the examples.

[0025] The biaxially oriented polyester film of the present invention preferably has a crystal orientation index χi of 5.0 to 13.0. χi is a parameter indicating the orientation state in the crystals of the matrix resin component, and by setting it within the range of 5.0 to 13.0, a light-shielding film with sufficient strength and less susceptibility to cleavage can be obtained. A χi of 6.5 to 12.0 is more preferable, and a χi of 7.0 to 11.0 is most preferable. The method for setting χi within the above range is not particularly limited, but one example is the manufacturing method using the stretch ratio conditions described later.

[0026] As a method for setting Mc and χi within the aforementioned ranges, the following method can be preferably exemplified. First, recycled raw material obtained by crushing a film in which carbon black is oriented and the carbon black plane orientation index Mc is 1.03 or higher into flakes is used in an amount of 20% to 40% by mass in 100% by mass of the film raw material. Then, by applying the stretching ratio conditions described later, the carbon black plane orientation index Mc and the orientation of the film's matrix resin component can be controlled to a preferred range according to the principle described above. When high stretching conditions are used to increase the plane orientation index Mc, the orientation of the matrix resin becomes too high, making it difficult to set χi within a preferred range, and there is a problem that film cleavage is likely to occur. In addition, although it is sometimes possible to reduce the amount of light transmitted even within the general range of stretching ratios by adding carbon black at a high concentration, the degree of plane orientation is low, so light tends to escape in oblique directions, and the high concentration of carbon black may cause problems such as cleavage.

[0027] The biaxially oriented polyester film of the present invention preferably has a crystallinity of 38.0% or less. This embodiment improves the brittleness of the film and enhances its cleavage resistance. The crystallinity of the biaxially oriented polyester film can be determined by the method described in the examples. As a method to achieve a crystallinity of 38.0% or less, a recycled material obtained by crushing a film in which the plane orientation index Mc of carbon black is 1.03 or higher into flakes is used in an amount of 20% to 40% by mass of the film material, while applying the stretching ratio conditions described later. Note that the orientation of the polyester resin can be relaxed by heat fixing (thermal relaxation) after biaxial orientation, but this tends to increase the crystallinity.

[0028] In the biaxially oriented polyester film of the present invention, it is preferable that the absolute value of the dielectric constant difference in two orthogonal directions within the film plane is 0.00 or more and 0.20 or less.

[0029] The dielectric constant is an index that correlates with the orientation of carbon black and matrix resin. When carbon black is highly oriented in the plane, but with little bias in its in-plane distribution, cleavage caused by stress concentration in the orientation direction when complex shapes are formed can be reduced. It is more preferable for the absolute value of the dielectric constant difference to be between 0.00 and 0.10. There are no particular limitations on how to set the absolute value of the dielectric constant difference to a specific range, but for example, it can be achieved by setting the draft ratio when extruding the polymer to a specific range to suppress the orientation of carbon black in the flow direction in the unstretched film, and then balancing the biaxial stretch ratio under appropriate conditions.

[0030] The biaxially oriented polyester film of the present invention preferably has a break elongation of 30% or more when subjected to the cleavage resistance evaluation described in the examples.

[0031] <Method for manufacturing biaxially oriented polyester film> Next, preferred embodiments of the method for producing the biaxially oriented polyester film of the present invention will be described with specific examples. However, the film of the present invention is not limited to those obtained by the following manufacturing method.

[0032] The raw materials to be used are vacuum-dried to a moisture content of 50 ppm or less, and then supplied to a single-screw extruder for melt extrusion. At this time, it is preferable to control the resin temperature to 265°C to 295°C. After that, it is cooled and solidified to produce an unstretched (unoriented) PET film. The extruded unstretched sheet is cooled and solidified in close contact on a cooled drum to obtain an unstretched laminated film. At this time, it is preferable to apply static electricity to make the film adhere to the drum in order to obtain a uniform film, and from the viewpoint of keeping the dielectric constant within a specific range, it is preferable that the draft ratio (= die lip gap KL (mm) / unstretched film thickness MT (mm)) when extruding the molten polymer from the die is between 2.0 and 19.0. If the draft ratio is less than 2.0, the polymer is in contact with the die for a long time, making it easy for defects such as streaks to occur, and if the draft ratio is greater than 19.0, it may be difficult to make the dielectric constant uniform in each direction in the plane.

[0033] In the raw materials used for the biaxially oriented polyester film of the present invention, it is preferable to use polyester film having a carbon black plane orientation index Mc of 1.03 or higher in a mass ratio of 20% to 40%. By using a film in which carbon black is oriented as a raw material by crushing or other means, the polyester matrix becomes unoriented during melt extrusion, while the carbon black maintains an oriented state to some extent. As a result, the degree of plane orientation of the carbon black is further increased in the subsequent biaxial stretching process, making it possible to achieve the aforementioned preferred range. It is more preferable that the plane orientation index Mc of the carbon black-containing raw material be 1.07 or higher, and most preferable that be 1.15 or higher. Furthermore, by using polyester film with a carbon black plane orientation index Mc of 1.03 or higher in a mass ratio of 20% to 40%, it is possible to stably obtain a film with the aforementioned preferred carbon black plane orientation index Mc without process defects such as bridging of the film raw material due to it not being in chip form, resulting in supply failures or screw engagement failures. Furthermore, from the viewpoint of raw material supply stability and screw engagement stability, it is preferable to use a chip-shaped raw material obtained by melt-kneading a polyester film having a carbon black surface orientation index Mc of 1.03 or higher in combination with the pulverized or otherwise processed raw material. In addition, by adopting this embodiment, the distribution of the surface orientation index Mc in the resulting film is broadened, which can improve light shielding uniformity and cleavage resistance.

[0034] The unstretched film is heated to above the glass transition temperature (Tg) of the polymer by roll heating, or infrared heating if necessary, and stretched in the longitudinal direction (hereinafter referred to as MD) (MD stretching). In the case of sequential biaxial stretching, MD stretching is performed using the difference in peripheral speed of two or more rolls. The MD stretching ratio is preferably 1.5 to 3.8 times. By setting it to 1.5 times or more, more preferably 2.0 times or more, and even more preferably 2.8 times or more, a film with excellent mechanical properties can be obtained. Furthermore, by setting it to 3.8 times or less, more preferably 3.5 times or less, the occurrence of breakage during film formation can be prevented. In addition, the MD stretching temperature is preferably between the film's Tg and Tg + 20°C from the viewpoint of uniform stretching and suppressing roll adhesion. Furthermore, the stretching speed is preferably between 1,000% / min and 200,000% / min.

[0035] After MD stretching, the film can be subsequently stretched in a direction perpendicular to MD (hereinafter referred to as TD) (TD stretching), and then subjected to sequential heat setting and heat relaxation treatments to obtain a biaxially oriented film. These treatments are performed while the film is running. At this time, the preheating and stretching temperature for TD stretching is preferably above the glass transition temperature Tg of the polymer and below Tg+50°C. The TD stretching ratio is preferably 2.5 to 3.9 times. By setting it to 2.5 times or more, more preferably 3.0 times or more, a film with excellent mechanical properties can be obtained with high production efficiency. By setting it to 3.9 times or less, more preferably 3.5 times or less, the occurrence of breakage during film formation can be suppressed. Furthermore, it is desirable that the stretching speed in the TD direction be 1,000% / min or more and 200,000% / min or less.

[0036] Furthermore, the film is heat-treated after biaxial stretching. The heat treatment can be carried out by any conventionally known method, such as in an oven or on a heated roll. A heat treatment temperature of 150°C to 240°C can impart high dimensional stability. The heat treatment time can be any range that does not degrade the properties, preferably 5 seconds to 60 seconds, more preferably 10 seconds to 40 seconds, and most preferably 15 seconds to 30 seconds. After the heat treatment, it is preferable to perform a 3-15% relaxation treatment in the width direction or the longitudinal direction. After that, it is cooled uniformly and slowly, cooled to room temperature, and wound onto a roll.

[0037] Although this explanation uses the sequential biaxial stretching method as an example, either the sequential biaxial stretching method or the simultaneous biaxial stretching method may be used. [Examples]

[0038] The present invention will be described in detail below with reference to examples. Note that each characteristic value was measured by the following method.

[0039] (1) Film thickness, film layer composition Five cross-sections were cut from the central part of the film in the width direction. These were observed using a scanning electron microscope (Hitachi Field Emission Scanning Electron Microscope (FE-SEM) S-4000) at magnification of 500 to 5,000 times, ensuring that the film was contained within the field of view in the thickness direction. From the resulting cross-sectional images, the film thickness and, in the case of a laminated structure, the film layer configuration (thickness of each layer) were determined.

[0040] (2) Resin composition Dissolve the film in hexafluoroisopropanol (HFIP), 1 H-NMR and 13 The content of each monomer residue and by-product diethylene glycol can be quantified using 1C-NMR. In the case of laminated films, the components constituting each layer can be collected and evaluated by scraping off each layer of the film according to the laminate thickness. For the film of the present invention, the resin composition was calculated from the mixing ratio during film manufacturing.

[0041] (3) Carbon black content Based on JIS K6813 (2002), the following procedure is followed: Weigh the sample and determine its mass as m1. Place the weighed sample on a sample boat and heat it in a cylindrical electric furnace preheated to 550°C under a nitrogen atmosphere for 45 minutes. Allow the sample boat to cool under a nitrogen atmosphere for 10 minutes, then transfer it to a desiccator and allow it to cool to room temperature. Weigh the sample boat and determine its mass as m2. Place the sample boat in a muffle furnace and ash it at 900°C until no trace of carbon black remains. Allow the sample boat to cool to room temperature in the desiccator and weigh the sample boat to determine its mass as m3. The carbon black content (mass %) is calculated using the following formula (a). This trial is repeated five times, and the arithmetic mean is used to determine the carbon black content (mass %). (a)(m2-m3) / m1×100 If the film had a laminated structure with different compositions, it was selectively sampled based on the thickness of each layer obtained in (1), and each layer was analyzed to identify the specific layer.

[0042] (4) Planar orientation index of carbon black Mc The film of the present invention was frozen and cut so that the cross-section was in any direction of the film × the film thickness direction, obtaining ultrathin section samples for observation of the resin layer cross-section. Sampling was performed in five directions, changing the direction by 30° from the above-mentioned arbitrary direction. Images of the sample cross-sections were taken using a TEM (transmission electron microscope: Hitachi, Ltd. H7100FA model) at a magnification of 20,000x and a field of view of 1000nm × 1000nm and used for analysis.

[0043] Images obtained by TEM were binarized (using Scion Image software) to extract only carbon black. The binarized images were then Fourier transformed using analysis software (FiberOri8single03), and the angular distribution of orientation intensity was calculated from the resulting power spectrum, with the thickness direction as the reference point (0°). Furthermore, the angular distribution of orientation intensity was approximated by an ellipse using the same software, and the major / minor axis ratio of the resulting ellipse was taken as the surface orientation index Mc of the carbon black. These operations were performed similarly for each sample with a different orientation, and the arithmetic mean of the data obtained from a total of 6 points was calculated. In addition, 5 points were sampled from different locations on the film, and the arithmetic mean of the surface orientation index Mc at each point was adopted as the value for the sample.

[0044] (5) Crystal orientation index χi The sample was placed in the sample holder of an X-ray diffractometer manufactured by Rigaku Denki Co., Ltd. When diffraction peaks were observed by reflection while varying the width direction of the film and the incident angle of the X-rays, the diffraction intensities at 26° and 22.5°, corresponding to diffraction angles 2Θ of the (100) and (11(-)0) planes of the polyethylene terephthalate crystal, were defined as I1 and II2, respectively, and the crystal orientation index χi was calculated from their ratio (II2 / I1).

[0045] (6) Average primary particle size and aggregation size of carbon black The equivalent circular particle size was determined for particles in images observed at a magnification of 100,000x using a scanning electron microscope (SEM). 100 particles were randomly selected from those with an equivalent circular particle size of 0.0002 to 1 μm, and the average of these 100 particles' equivalent circular particle sizes was calculated to determine the average primary particle size of the carbon black. Furthermore, aggregates of 10 or more carbon black particles in contact with each other were extracted from the SEM images observed at 10,000x magnification. The length in both the thickness and surface directions of each aggregate was measured, and the average value was calculated. The same calculation was performed for 10 different fields of view, and the average value across all observed fields (10 fields of view observed at 10,000x magnification) was defined as the carbon black aggregate size.

[0046] (7) Dielectric constant Dielectric constant measurements were performed at 15 GHz using a microwave transmission molecular orientation analyzer (MOA series) manufactured by Oji Instruments Co., Ltd. Measurements were taken at points 5 cm away from an arbitrary location on a straight line in the longitudinal and width directions of the film, and the arithmetic mean of a total of nine measurements was taken as the dielectric constant. In cases where the direction was unclear or not defined, measurements were taken on an arbitrary straight line and on a straight line perpendicular to it.

[0047] (8) Light shielding uniformity Polyester film was cut to a size of 500mm x 500mm, and transmitted light tests were performed in a darkroom under fluorescent lighting. The unevenness of the light-blocking state when the film was viewed from the front and at 45° and 85° angles was visually judged according to the following criteria. No light leakage or shading is observed from either the front or oblique viewpoint: A Slight light leakage and shading are visible from an oblique viewpoint, but no light leakage or shading is visible from a frontal viewpoint:B Light leakage and shading are clearly visible from an oblique viewpoint, but not from a frontal viewpoint:C Both frontal and oblique views clearly show light leakage and shading:D The uniformity of light blocking is good for grades A through C, with A being the best among them.

[0048] (9) Cleavage resistance A polyester film was cut to a size of 10 mm (width) x 150 mm (length), and the cut sample was folded perpendicular to the cutting direction at the center point (75 mm position) of the 150 mm end. The folded sample was pressed with a press machine at a pressure of 0.3 MPa for 5 seconds, then removed, and the fold was straightened before use as a sample. Using a "TENSILON" (registered trademark) universal testing machine RTG-1210 (A&D Co., Ltd.), the sample was set with a chuck distance of 50 mm and a tensile speed of 300 mm / min, so that the aforementioned fold point was included in the tensile length, and the elongation at break was evaluated according to the following criteria to determine the cleavage resistance. Measurements were taken in the length and width directions with n=5, and the average value of the total 10 points was adopted. When measuring in the width direction, the polyester film was cut to a size of 10 mm (length) x 150 mm (width), and measured in the same manner as above. In this evaluation, cleavage is assessed by inducing fine cracks (cleavage) in the film by bending and measuring the elongation at break. The tendency for the elongation at break to decrease in proportion to the ease with which cleavage occurs is observed. Elongation at break of 60% or more: A Elongation at break of 50% or more, but less than 60%: B Elongation at break of 40% or more, but less than 50%: C Elongation at break of 30% or more, but less than 40%: D Elongation at break is less than 30%: E Cleavage resistance is good for grades A through D, with A being the best among them.

[0049] (10) Recyclability The content of carbon black-containing recovered polyester in the polyester film of the present invention was evaluated according to the following criteria. A: Content of 30% by mass or more B: Content is 20% by mass or more but less than 30% by mass C: Content is 12% by mass or more and less than 20% by mass D: Content is less than 12% by mass.

[0050] (11) Degree of crystallinity In accordance with JIS K7122 (1987), a differential scanning calorimetry robot DSC-RDC220 manufactured by Seiko Electronics Industries, Ltd. was used, and a "disk session" SSC / 5200 was used for data analysis. A 5 mg film sample was heated from room temperature to 300°C at a heating rate of 20°C / min on an aluminum tray and held at 300°C for 5 minutes. At that time, the endothermic peak heat ΔHm, cold crystallization heat ΔHc, and fusion heat ΔHm of perfect crystal PET were obtained by measurement. 0 The following formula was used to calculate the value from (140.1 J / g). Crystallinity (%)=(ΔHm-ΔHc) / ΔHm 0 ×100.

[0051] (raw materials) The polyester resin used had the following properties:

[0052] (Raw material A: Polyester 1) 100 parts by mass of dimethyl terephthalate, 57.5 parts by mass of ethylene glycol, 0.03 parts by mass of magnesium acetate dihydrate, and 0.03 parts by mass of antimony trioxide were melted at 150°C under a nitrogen atmosphere. The molten mixture was heated to 230°C over 3 hours while stirring, and methanol was distilled off to complete the transesterification reaction. After the transesterification reaction, an ethylene glycol solution (pH 5.0) prepared by dissolving 0.005 parts by mass of phosphoric acid in 0.5 parts by mass of ethylene glycol was added. The intrinsic viscosity of the polyester composition at this point was less than 0.2. Subsequently, the polymerization reaction was carried out at a final temperature of 285°C and a vacuum of 0.1 Torr to obtain polyethylene terephthalate with an intrinsic viscosity of 0.65 and a terminal carboxyl group content of 34 equivalents / ton.

[0053] (Ingredient B: Carbon Black Master 1) Polyester A was mixed with 75% by mass of Mitsubishi Chemical's Furnace Black (#3050B) as carbon black in a co-rotating twin-screw kneading extruder (L / D=40) equipped with a kneading paddle section and a vacuum vent. The mixture was melt-extruded at 290°C with a residence time of 90 seconds, a screw rotation speed of 300 rpm, a screw rotation speed fluctuation rate of 4%, and the material was extruded in strand form. After cooling with water at 25°C, the material was immediately cut into chips to obtain carbon black master 1 with an intrinsic viscosity of 0.58 dl / g.

[0054] (Material C: Recycled Polyester 1) A biaxially oriented polyester film with a carbon black concentration of 4.5% by mass and a plane orientation index Mc of 1.04 was pulverized, resulting in a bulk density of 0.38 g / cm³. 3 A flaked recycled polyester raw material 1.

[0055] (Material D: Recycled Polyester 2) A biaxially oriented polyester film with a carbon black concentration of 4.5 mass% and a plane orientation index Mc of 1.04 was cut and supplied to a co-rotating type vented twin-screw kneading extruder (manufactured by Japan Steel Works, screw diameter 30 mm, screw length / screw diameter = 45.5) equipped with three kneading paddles heated to 280°C, where it was melt-kneaded to obtain chip-shaped recycled polyester B. The obtained recycled chips had a glass transition temperature of 78°C, a melting point of 255°C, an intrinsic viscosity of 0.61 dl / g, and a carboxyl end group content of 70 eq. / t. The plane orientation index Mc of the carbon black in raw material D was determined by the same method as in (4) Plane orientation index Mc of carbon black and was found to be less than 1.03.

[0056] (Material E: Recycled polyester 3) A biaxially oriented polyester film with a carbon black concentration of 4.5% by mass and a plane orientation index Mc of 1.18 was pulverized, resulting in a bulk density of 0.38 g / cm³. 3 Flake-like recycled polyester raw material 3.

[0057] (Example 1) Polyester 1, carbon black master 1, and recycled polyesters 1 and 2 were blended in the proportions shown in Table 1 and then supplied to a vented extruder (L / D=28) under a nitrogen atmosphere. The mixture was melt-kneaded at 280°C while degassing the vacuum vent section, filtered through a filter set to 290°C, and then melt-extruded through the die nozzle of a T-die set to 280°C to obtain an unstretched film with a die-lip gap of 4.5 mm and an unstretched film thickness of 0.3 mm (draft ratio 15). The film was then cooled and solidified while applying an electrostatic charge to a cast drum with a surface temperature of 25°C to obtain an unstretched film.

[0058] This unstretched film was stretched in the longitudinal direction at a rate of 3.0 times at a temperature of 90°C using a longitudinal stretcher consisting of multiple heated roll groups, utilizing the difference in peripheral speed of the rolls. Then, both ends of the film were gripped with clips and guided into a stentor, where it was stretched in the width direction at a stretching temperature of 95°C and a stretching ratio of 3.6 times. A heat treatment was then performed at 225°C for 6 seconds to obtain a polyester film with a thickness of 50 μm. The evaluation results of the obtained polyester film are shown in Table 2.

[0059] (Examples 2-7, 13, Comparative Examples 1-5, 7) A polyester film with a thickness of 50 μm was obtained in the same manner as in Example 1, using the raw material formulations and conditions described in Tables 1 and 2. The evaluation results of the obtained polyester film are shown in Table 2. (Examples 8 and 10) A polyester film with a thickness of 50 μm was obtained in the same manner as in Example 1, except that the polyester film from Example 1 was crushed and used as a raw material, and the raw material composition and conditions were as described in Tables 1 and 2. The evaluation results of the obtained polyester film are shown in Table 2. (Example 9) A polyester film with a thickness of 50 μm was obtained in the same manner as in Example 1, except that the polyester film of Comparative Example 4 was crushed and used as a raw material, and the raw material composition and conditions were as described in Tables 1 and 2. The evaluation results of the obtained polyester film are shown in Table 2. (Example 12) A polyester film with a thickness of 50 μm was obtained in the same manner as in Example 1, except that the gap between the nozzle lip was 1.2 mm and the unstretched film thickness was 0.3 mm (draft ratio 4). The evaluation results of the obtained polyester film are shown in Table 2. (Comparative Example 6) In the manufacturing process of Example 1, the clip gripping portion after transverse stretching was slit, and the resulting edge was crushed and used as a raw material. A polyester film with a thickness of 50 μm was obtained in the same manner as in Example 1, except that the raw material composition and conditions were as described in Tables 1 and 2. The evaluation results of the obtained polyester film are shown in Table 2.

[0060] [Table 1]

[0061] [Table 2] [Industrial applicability]

[0062] According to the present invention, a polyester film suitable as a light-shielding material can be obtained that achieves both uniform light-shielding properties and cleavage resistance, and is also highly recyclable.

Claims

1. A biaxially oriented polyester film containing 0.01% by mass or more and 5.50% by mass or less of carbon black, wherein the plane orientation index Mc of the carbon black is 1.10 or more and 2.00 or less, and the crystal orientation index χi of the film, determined by wide-angle X-ray diffraction, is 5.0 or more and 13.0 or less.

2. The biaxially oriented polyester film according to claim 1, wherein the average primary particle diameter a (nm) of the carbon black is 10 nm or more and 150 nm or less.

3. The biaxially oriented polyester film according to claim 1 or 2, wherein the aggregation size b (nm) and average primary particle diameter a (nm) of the carbon black satisfy formula (2). b / a≧10...(2)

4. The biaxially oriented polyester film according to claim 1 or 2, wherein the absolute value of the dielectric constant difference at a frequency of 15 GHz in two orthogonal directions within the film surface is 0.00 or more and 0.20 or less.

5. A method for producing a biaxially oriented polyester film according to claim 1 or 2, wherein a polyester film containing carbon black and having a carbon black surface orientation index Mc of 1.03 or higher is used as a raw material in a mass ratio of 20% to 40%.

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