polyester film

Abstract

Provided is a polyester film that uses recycled resin, has fewer foreign substances on the film surface, and is excellent in color tone. [Solution] When the film is structured, in order from one surface to the other surface in the thickness direction, as a surface layer portion A, an intermediate layer portion B, and a surface layer portion C, the polyester film satisfies the following (I) to (III). (I) The polyester resin of the surface layer portion A and / or the surface layer portion C contains a copolymerization component. (II) The amount of the copolymerization component contained in the polyester resin of the surface layer portion A and / or the surface layer portion C is larger than that of the intermediate layer portion B. (III) When the amounts of the compound represented by the following chemical formula contained in the surface layer portion A, the intermediate layer portion B, and the surface layer portion C are respectively ST-A, ST-B, and ST-C, ST-A < ST-B and / or ST-C < ST-B, and ST-B is 0.1 ppm or more and less than 20 ppm. TIFF2026142904000006.tif29144 (In the formula, R1 and R2 are each independently a substituent containing hydrogen, an alkyl group, an alcohol, an ester, or an ether. n is an integer of 1 or more.)

Description

Technical Field

[0001] The present invention relates to a polyester film. Background Art

[0002] Polyester is excellent in mechanical properties, thermal properties, chemical resistance, electrical properties and moldability, and is used in various applications. Among polyesters, particularly polyethylene terephthalate (hereinafter referred to as PET) is excellent in transparency and processability, so it is widely used in applications requiring high quality such as optical films and release films. However, process films such as release films are discarded after use, so reduction of environmental burden has been demanded in recent years.

[0003] As a measure for reducing environmental burden, there is thermal recycling in which discarded polyester resin is burned to obtain thermal energy. However, when thermal recycling is performed, carbon dioxide is generated and polyester raw materials are lost, so new petroleum raw materials need to be used to reproduce polyester.

[0004] In response to these problems, Patent Document 1 discloses a technique related to films recovered from PET bottles. Prior Art Documents Patent Documents

[0005] Patent Document 1 Japanese Patent Laid-Open No. 2017-7175 Summary of the Invention Problems to be Solved by the Invention

[0006] Patent Document 1 discloses a laminated film using a polyester resin recycled from PET bottles. However, repeated material recycling that involves remelting the polyester resin promotes thermal decomposition, hydrolysis, and oxidative decomposition of the polyester resin, which causes problems of quality degradation such as coloration, generation of foreign substances, and decrease in mechanical strength due to molecular weight reduction. [Means for Solving the Problems]

[0007] As a result of intensive studies to solve the above problems, the present inventors have arrived at a polyester film that uses a recycled resin, has few foreign substances on the film surface, and is excellent in color tone.

[0008] The object of the present invention is achieved by the following means. (1) A polyester film containing a polyester resin as a main component, which is formed using a recycled resin, wherein when the film is divided in the thickness direction from one surface toward the other surface into a surface layer portion A, an intermediate layer portion B, and a surface layer portion C in this order, the polyester film satisfies the following (I) to (III). (I) The polyester resin of the surface layer portion A and / or the surface layer portion C contains a copolymerization component. (II) The amount of the copolymerization component contained in the polyester resin of the surface layer portion A and / or the surface layer portion C is larger than that in the intermediate layer portion B. (III) When the amounts of Chemical Formula 1 (Chem 1) contained in the surface layer portion A, the intermediate layer portion B, and the surface layer portion C are respectively defined as ST-A, ST-B, and ST-C, ST-A<ST-B and / or ST-C<ST-B, and ST-B is 0.1 ppm or more and less than 20 ppm.

[0009] [Chemical Formula]

[0010] (Chem 1) R1 and R2 are each independently a substituent containing hydrogen, an alkyl group, an alcohol, an ester, or an ether. n is an integer of 1 or more. The polyester film according to (1), wherein when the amounts of cyclic trimer contained in surface layer portion A, intermediate layer portion B and surface layer portion C are C3-A, C3-B and C3-C respectively, C3-A<C3-B and / or C3-C<C3-B is satisfied. The polyester film according to (1), wherein when the total amounts of terephthalic acid (TPA), monohydroxyethyl terephthalate (MHET) and bis-β-hydroxyethyl terephthalate (BHET) contained in surface layer portion A, intermediate layer portion B and surface layer portion C are OLG-A, OLG-B and OLG-C respectively, OLG-A<OLG-B and / or OLG-C<OLG-B is satisfied. The polyester film according to (1), wherein the content of the copolymerization component contained in surface layer portion A and / or surface layer portion C is 0.01 mol% or more and 10 mol% or less. The polyester film according to (1), wherein the copolymerization component comprises any one of neopentyl glycol, 1,4-cyclohexanedimethanol, butanediol, propylene glycol, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, and dimethyl sodium 5-sulfoisophthalate. The polyester film according to (1), comprising a polyester resin composition derived from chemical recycling of depolymerized polyester and / or a polyester resin composition derived from material recycling of remelted polyester. The polyester film according to (1), wherein surface layer portion A and / or surface layer portion C comprises a polyester resin composition derived from chemical recycling. The polyester film according to (1), wherein the raw materials for the chemical recycling and the material recycling are any of used films, bottles, containers and fibers. The polyester film according to (1), which is a biaxially stretched polyester film for process release. [Effects of the Invention]

[0011] The present invention provides a polyester film having less foreign matter on the film surface and excellent color tone by applying a recycled resin containing a copolymerization component to a surface layer. [Modes for carrying out the invention]

[0012] The present invention will be described in detail below.

[0013] In polyester films, the surface layer refers to the portion with a thickness ratio of 5% or less from the surface of the polyester film, and the intermediate layer refers to the portion with a thickness ratio of ±45% from the center in the thickness direction of the polyester.

[0014] The polyester film of the present invention is a film made using a polyester resin composition obtained by polycondensation of a dicarboxylic acid component and a diol component. Various dicarboxylic acid components can be used as the dicarboxylic acid component, such as aromatic dicarboxylic acids, linear aliphatic dicarboxylic acids, and alicyclic dicarboxylic acids. Among these, aromatic dicarboxylic acids and their ester-forming derivative components are preferred from the viewpoint of mechanical properties, heat resistance, and hydrolysis resistance of the polyester resin composition. In particular, terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, and their ester-forming derivative components are preferred from the viewpoint of polymerizability and mechanical properties, with terephthalic acid being the most preferred.

[0015] Various diols can be used as the diol component. For example, aliphatic diols such as ethylene glycol, 1,2-propanediol, 1,3-propanediol, butanediol, and neopentyl glycol; alicyclic diols such as cyclohexanedimethanol and cyclohexanediethanol; and aromatic cyclic diols such as bisphenol A, bisphenol S, styrene glycol, 9,9-bis(4-(2-hydroxyethoxy)phenyl)fluorene and 9,9-bis(4-hydroxyphenyl)fluorene. Among these, ethylene glycol is particularly preferred from the viewpoint of mechanical properties.

[0016] The polyester film of the present invention is required to contain a copolymerization component in the polyester resin of the surface layer portion A and / or the surface layer portion C. By containing the copolymerization component in the surface layer, it is possible to suppress the generation of cyclic trimers that precipitate on the surface when the molded article is heated and cause foreign matter problems. Furthermore, it is required that the amount of the copolymerization component contained in the polyester resin of the surface layer portion A and / or the surface layer portion C is larger than that in the intermediate layer portion B. When the amount of the copolymerization component contained in the surface layer portion A and the surface layer portion C is larger, foreign matters can be reduced without impairing mechanical properties resulting from the crystallinity, melting point and the like of the film. Here, the amount of the copolymerization component means the total content of components other than the main component, and the "main component" refers to the component present in the largest amount. In the case of a polyester copolymer formed using a dicarboxylic acid component and a diol component, it means the sum of the proportion of components other than the main component in the dicarboxylic acid component and the proportion of components other than the main component in the diol component. For example, in the case of a polyester copolymer composed of 90 mol% of dicarboxylic acid component I, 10 mol% of dicarboxylic acid component II, 90 mol% of diol component I, and 10 mol% of diol component II, the amount of the copolymerization component is 20 mol%.

[0017] The amount of the copolymerization component contained in the surface layer portion A and / or the surface layer portion C is preferably 0.01 mol% or more and 10 mol% or less. When it exceeds 10 mol%, the crystallinity decreases, and when it is less than 0.01 mol%, the effect of suppressing the generation of cyclic trimers becomes small.

[0018] As the copolymerization component, one or more types selected from isophthalic acid, 2,6-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, dimethyl sodium 5-sulfoisophthalate, etc. can be used as the dicarboxylic acid component, and one or more types selected from neopentyl glycol, 1,4-cyclohexanedimethanol, butanediol, propylene glycol, etc. can be used as the glycol component. In particular, isophthalic acid is preferable as the copolymerization component.

[0019] The recycled resin in the present invention refers to a polyester resin obtained through chemical recycling or material recycling. The polyester film of the present invention is required to contain the recycled resin, but may also contain a virgin polyester resin.

[0020] The polyester resin composition derived from chemical recycling refers to a polyester resin composition obtained by depolymerizing used polyesters such as PET bottles, polyester films, clothes and containers, and scraps generated in molding processing steps, subjecting the depolymerized product to purification treatment and other processes, and then repolymerizing the product. When a polyester film is used as the raw material for recycling, it is preferable to use a polyester film for process release.

[0021] Methods for depolymerizing a polyester resin composition include, but are not limited to: a method in which depolymerization is performed with a glycol compound such as ethylene glycol to obtain polyester monomers such as bishydroxyethyl terephthalate and oligomers thereof, followed by repolymerization; a method in which depolymerization is performed with a glycol compound such as ethylene glycol, followed by methanolysis to obtain dimethyl terephthalate and the like, followed by repolymerization; and a method in which dicarboxylic acid monomers such as terephthalic acid are obtained by hydrolysis, followed by repolymerization. In chemical recycling, polyester is depolymerized into monomers and oligomers such as polyester monomers like bishydroxyethyl terephthalate and oligomers thereof, dimethyl terephthalate, and dicarboxylic acid monomers like terephthalic acid, followed by purification and repolymerization using these depolymerized products as raw materials, so that physical properties equivalent to those of a virgin polyester resin composition that has not been recycled can be obtained. A virgin polyester resin composition is an unused polyester resin composition produced using a petroleum-derived raw material or a bio-derived raw material. If necessary, a new dicarboxylic acid component or glycol component may be mixed into the low polymer depolymerized through chemical recycling.

[0022] The polyester resin composition derived from material recycling refers to the polyester resin obtained when used polyesters such as PET bottles, polyester films, clothes, and containers, as well as scraps generated in molding processes are collected, crushed, washed, and subjected to foreign matter removal as necessary, and then the polyester resin composition formed into flakes or pelletized through melt molding is melted to form a film or the like. When a recycled polyester film is used as the raw material, it is preferable to use a biaxially stretched polyester film for process release. Polyester resin compositions derived from material recycling have a history of multiple thermoforming processes and use as molded products, so the polyester resin itself has progressed in degradation, and has lower quality than virgin polyester resin compositions.

[0023] In the polyester film of the present invention, when the contents of chemical formula 1 (Chem 1) contained in the surface layer portion A, the intermediate layer portion B, and the surface layer portion C are defined as ST-A, ST-B, and ST-C respectively, ST-A < ST-B and / or ST-C < ST-B, and ST-B is required to be 0.1 ppm or more and less than 20 ppm.

[0024]

Chem.

[0025] (Chemical Formula 1) is generated when polyester undergoes thermal degradation. In material recycling, repeated thermal history is applied through processes such as melt molding and pelletizing, making it easy for (Chemical Formula 1) to be generated. On the other hand, in chemical recycling, purification is performed after depolymerization to remove the impurity (Chemical Formula 1), resulting in a lower amount of (Chemical Formula 1) compared to polyester resin derived from material recycling. (Chemical Formula 1) causes a decrease in the color tone of the film. Also, because it is a low molecular weight substance, it bleeds out on the film surface, causing defects. In this invention, by using polyester resin derived from chemical recycling in the surface layer, the bleed-out of (Chemical Formula 1) on the film surface is reduced, and a film with fewer foreign matter defects can be obtained. If the amount of ST-B exceeds the above upper limit, it may lead to a decrease in the color tone and physical properties such as mechanical properties of the polyester film.

[0026] When molded products such as films and fibers are heated or exposed to solvents, cyclic trimers precipitate on the surface of the molded product, causing surface defects and stains, which can be problematic. A higher amount of copolymer reduces the probability of three ethylene terephthalate units linking together consecutively, thus reducing the amount of cyclic trimers generated. Therefore, increasing the amount of copolymer in the polyester resin of the surface layer results in a film that exhibits less surface precipitation even when used under heating conditions.

[0027] In material recycling, polyester resins are repeatedly subjected to thermal processes such as melt molding and pelletizing. This thermal history leads to thermal decomposition, hydrolysis, and oxidative decomposition, causing the molecular chains of the polyester to break and generating polyester monomers and oligomers. When the polyester is polyethylene terephthalate, terephthalic acid (TPA), monohydroxyethyl terephthalate (MHET), and bis-β-hydroxyethyl terephthalate (BHET) are generated. Since polyester oligomers and monomers are sublimable, if their content is high, they sublimate during polyester film molding, causing process contamination and surface defects in the film. In this invention, by using a polyester resin with a low content of linear oligomers in the surface layer, i.e., a polyester resin derived from chemical recycling, it is possible to obtain a film with fewer foreign matter defects on the film surface.

[0028] In the polyester film of the present invention, the polyester resin composition derived from chemical recycling is preferably 7% by weight or more, and more preferably 20% by weight or more. Furthermore, from the viewpoint of environmental impact and cost, the upper limit is preferably 80% by weight or less, and more preferably 60% by weight or less. Similarly, the polyester resin composition derived from material recycling is preferably 20% by weight or more, and more preferably 40% by weight or more. From the viewpoint of film quality, the upper limit is preferably 90% by weight or less, and more preferably 80% by weight or less. In the present invention, by using both a polyester resin composition derived from chemical recycling and a polyester resin composition derived from material recycling, it is possible to achieve high film quality while reducing the environmental impact as a recycled film.

[0029] Furthermore, in order to reduce the environmental impact, the total amount of the polyester resin composition derived from chemical recycling and the polyester resin composition derived from material recycling is preferably 80% by weight or more of the total polyester film, more preferably 90% by weight or more, and even more preferably 100% by weight.

[0030] The polyester film of the present invention preferably contains a polyester resin composition derived from chemical recycling in one of its surface layers. In chemical recycling, even if the polyester resin composition has been subjected to heat history during molding and material recycling, the purification process makes it possible to restore its quality to the same level as that of a virgin polyester resin composition. Furthermore, particles and other additives can be added as needed during repolymerization. Therefore, by using a polyester resin composition derived from chemical recycling in the surface layer in particular, a high-quality polyester film can be obtained.

[0031] The polyester film of the present invention may be in any stretched form, such as an unstretched film, a uniaxially oriented film, or a biaxially oriented film, but from the viewpoint of mechanical strength, a biaxially oriented film is preferred. Furthermore, the polyester film of the present invention can be suitably used as a biaxially oriented polyester film for process release. This is because the film for this application becomes unnecessary after release and can be suitably utilized as a raw material for recycling, and further recycling it for this application is preferable from the viewpoint of a circular economy. Specifically, such process release films can be suitably used as release films for the manufacture of multilayer ceramic capacitors (MLCCs), films for dry film resists, films for polarizing plate release, and films for optical release.

[0032] The following describes an example of a method for producing a polyester film according to the present invention: a method for obtaining a biaxially oriented polyester film.

[0033] Polyester resin derived from chemical recycling can be obtained by the following preparation method, but is not limited thereto: Biaxially oriented polyester film and / or PET bottles used for process mold release are recovered and depolymerized with ethylene glycol in any mixing ratio. Solid impurities are removed by filtering the depolymerized solution, and decolorization is performed using an adsorbent such as activated carbon to obtain polyester monomers and oligomers. These can then be repolymerized to obtain polyester resin derived from chemical recycling. The amount of copolymer is adjusted based on the known copolymer content of PET bottles; if a higher copolymer ratio is desired, copolymer is added during repolymerization.

[0034] Furthermore, polyester resin derived from material recycling can be obtained by the following preparation methods, but is not limited to them. It can be obtained by recovering biaxially oriented polyester film for process release and / or PET bottles, cutting and washing them, remelting and pelletizing them. For material recycling, the amount of copolymer component is adjusted from the known copolymer component amounts of PET bottles.

[0035] Next, polyester films can be obtained by the following methods, but are not limited to them. Other methods that can be used include a method in which polyester resin is heated and melted in an extruder and extruded from a die onto a cooled cast drum to form a sheet (melt casting method), and a method in which polyester resin is dissolved in a solvent, the solution is extruded from a die onto a support such as a cast drum or endless belt to form a film, and then the solvent is dried off from the film layer to form a sheet (solution casting method).

[0036] Furthermore, in the case of laminated films, a method is preferably used in which the polyester resin of each layer to be laminated is fed into a separate extruder, melted, and then combined, and co-extruded from a die onto a cooled cast drum to process it into a sheet (a method of melt film formation by co-extrusion). This method will be described in detail below.

[0037] First, polyester resin is fed into the extruder corresponding to each layer and heated, melted, and extruded. The layers are laminated using a confluence block and co-extruded from the die onto a cast drum cooled to a surface temperature of 10-60°C. The film is then cooled and solidified using electrostatic force to create an unstretched film. At this time, it is preferable to filter the polyester resin melted in the extruder. Since even very small foreign matter can become large protrusions and defects in the film, it is effective to use a high-precision filter that captures 95% or more of foreign matter larger than 5 μm.

[0038] Next, the unstretched film is guided to a group of rolls heated to a temperature of 70-140°C and stretched 3-4 times in the longitudinal direction (vertical direction, i.e., the direction in which the sheet moves), and then cooled in a group of rolls heated to a temperature of 20-50°C. Subsequently, the sheet is guided to a tenter while both ends are held with clips and stretched 3-4 times in the direction perpendicular to the longitudinal direction (width direction) in an atmosphere heated to a temperature of 80-240°C. After stretching, a relaxation treatment of 0.1-5% in the longitudinal and width directions may also be applied. As for the biaxial stretching method, in addition to the sequential biaxial stretching method in which the longitudinal and width directions are stretched separately as described above, either the simultaneous biaxial stretching method in which the longitudinal and width directions are stretched at the same time is acceptable. [Examples]

[0039] The present invention will be described in more detail below with reference to the following examples. The physical properties in the examples were measured by the following methods. When measuring the physical properties of the surface layer and the intermediate layer, the corresponding layers were cut and analyzed.

[0040] (1) Copolymerization amount (unit: mole%) 30 mg of polyester resin was dissolved in 1 mL of deuterated chloroform and measured using nuclear magnetic resonance (NMR) spectroscopy. The following apparatus and measurement conditions were used. Equipment: JEOL Ltd. AL-400 Measured nuclide: 1H Measurement temperature: 25℃ Total number of times: 128.

[0041] (2) Content of (1) ST amount (unit: ppm) Bis-(2-hydroxyethyl) terephthalate, obtained by depolymerizing polyester resin, was quantified by high-performance liquid chromatography (HPLC). Depolymerization was carried out by the following method. 100 parts by mass of polyester resin, 200 parts by mass of ethylene glycol, and 0.5 parts by mass of sodium hydroxide were placed in a reaction vessel, and depolymerization was carried out while gradually increasing the temperature. When the internal temperature reached 203°C, the depolymerization was stopped and allowed to cool to 25°C. Solid-liquid separation was performed to obtain a depolymer containing impurities as solid matter. To this depolymer, 1000 parts by mass of water was added for every 100 parts by mass of the polyester resin used, dissolved at 100°C, and then filtered using a 1.0 μm filter. The obtained filtrate was cooled to crystallize BHET, and wet BHET crystals were obtained as solid matter by filtration using 5B filter paper. The obtained wet BHET crystals were dried to obtain BHET. HPLC was performed using the following method. Equipment: Ultimate 3000 (manufactured by Thermo Fisher Scientific) Column: Reverse-phase column Mobile phase: A: 10 mmol / L ammonium acetate aqueous solution, B: Acetonitrile gradient conditions The content of compound (1) was calculated using a calibration curve created by HPLC.

[0042] (3) Cyclic trimer content C3 amount (unit: ppm) The measurements were performed using liquid chromatography with orthochlorophenol as the solvent. Equipment: Shimadzu LC-10ADvp Column: YMC-Pack ODS-A 150×4.6mm S-5μm Column temperature: 40℃ Flow rate: 1.3ml / min Detector: 240nm Eluent: Solution A: 35% pure water, Solution B: 75% methanol.

[0043] (4) Linear oligomer content (OLG amount) (unit: ppm) The total amount of terephthalic acid, monohydroxyethyl terephthalate, and bishydroxyethyl terephthalate was used as the linear oligomer content. 0.1 g of polyester resin was weighed and dissolved in 2 mL of a 1 / 1 volume mixture of HFIP (hexafluoro-2-propanol) / chloroform. The solution was then transferred to a beaker, 3 mL of chloroform was added, and then 40 mL of methanol was gradually added. The solution was then filtered through a paper filter (ADVANTEC No. 2) and concentrated to dryness. 0.5 mL of DMF (N,N-dimethylformamide) was added to the residue to dissolve and decompose it, and ethanol was added to bring the volume to 5 mL. The solution filtered through a 0.45 μm pore size PTFE membrane filter was used as the sample solution. The obtained sample solution was analyzed by LC / UV to determine the content of terephthalic acid, monohydroxyethyl terephthalate, and bishydroxyethyl terephthalate.

[0044] (5) Evaluation of foreign body defects Polyester film 1m 2 The samples were cut to size, a spotlight was used as the light source, and bright spots based on light scattering using reflected and transmitted light were observed. Foreign object defects were marked with a pen and counted. The number of defects was evaluated according to the following criteria, with ◎ and ○ being considered passing grades. ◎: Less than 5 items ○: 5 or more but less than 15 △: 15 or more.

[0045] (6) Color tone of polyester film A sample measuring 100mm x 100mm was cut from the polyester film to be measured at an arbitrary point. Using a Konica Minolta CM-3600d spectrophotometer, the sample was set so that the angle between the normal to the film plane and the incident light was 0°. The chromaticity b* at an arbitrary point on the sample was measured using transmitted light under the conditions of a target mask with a measurement diameter of φ25.4mm. The same measurement was then repeated four times, with the sample moved at least 30mm away from the center of the measurement point (measurements were taken at five arbitrary points). The color tone b* was evaluated according to the following criteria, with ◎ and ○ being considered passing grades. ◎: Less than 1.5 ○: 1.5 or higher and less than 3.0 △: 3.0 or higher.

[0046] (7) Precipitation of cyclic trimers Polyester film was cut into 5cm x 5cm pieces and heated in a hot air dryer at 150°C for 60 minutes. The film surface was then observed with a scanning electron microscope at 1000x magnification, and the number of cyclic trimer precipitates of 1 μm or larger on randomly selected film surfaces was counted. The precipitation properties of cyclic trimers were evaluated according to the following criteria, with ◎ and ○ being considered passing grades. In addition, cyclic trimers were identified using a Fourier transform infrared spectrometer (Thermo Fisher Scientific Nicolet 6700). ◎: Less than 5 items ○: 5 or more but less than 10 △: 10 or more.

[0047] (Reference example) Resins (i) to (xi) having the physical properties shown in Table 1 were used.

[0048] [Table 1]

[0049] The polyester resin derived from chemical recycling was obtained by recovering biaxially oriented polyester film and / or PET bottles used for process release as raw materials, mixing the raw materials in an arbitrary ratio, and depolymerizing them with ethylene glycol. Solid impurities were removed by filtering the depolymerized solution, decolorization was performed using activated carbon, and the resulting polyester resin was repolymerized. The amount of copolymerized components was adjusted based on the known copolymerized component amounts of PET bottles, and isophthalic acid, 1,4-cyclohexanedimethanol, and 2,6-naphthalenedicarboxylic acid were added as appropriate during repolymerization.

[0050] Furthermore, the polyester resin derived from material recycling is a polyester resin produced by recovering biaxially oriented polyester film and / or PET bottles used for process release, cutting and washing them, remelting them, and pelletizing them. For material recycling, the amount of copolymer components was adjusted based on the known copolymer component amounts of PET bottles.

[0051] (Example 1) As resins constituting the surface layers A and C of the polyester film, resin (iii) was blended in a ratio of 50 parts by weight and resin (viii) in a ratio of 50 parts by weight. After vacuum drying at 160°C for 2 hours, the mixture was fed into the extruder for surface layer A and the extruder for surface layer C. Similarly, as resin constituting the intermediate layer B of the polyester film, resin (iX) was vacuum dried at 160°C for 2 hours and then fed into the extruder for intermediate layer B. The respective raw materials were melted at 280°C in the extruder and then combined and laminated in a lamination confluence block to form a three-layer laminate with surface layer A, intermediate layer B, and surface layer C in the thickness direction of the film. Subsequently, the laminate was extruded onto a casting drum with a surface temperature of 25°C to create a laminated sheet with a three-layer structure. Next, the sheet was preheated with a group of heated rolls, then stretched 3.3 times in the longitudinal direction (vertical direction, i.e., the direction of sheet movement) at a temperature of 90°C, and then cooled with a group of temperature rolls at 25°C to obtain a uniaxially oriented film. The obtained uniaxially oriented film was held at both ends with clips and stretched 3.5 times in the direction perpendicular to the longitudinal direction (width direction) in the 110°C heating zone of a tenter. Subsequently, it was fixed at a temperature of 230°C for 10 seconds in the heat treatment zone of the tenter. Then, after uniform slow cooling in the cooling zone, it was wound up to obtain a polyester film with a thickness of 30 μm. The physical properties of the obtained polyester film are shown in Table 2.

[0052] The polyester film obtained in Example 1 was of a quality suitable for use as a release film for processes, etc.

[0053] (Examples 2-4) A polyester film was obtained in the same manner as in Example 1, except that the type of resin used for surface layers A and C was changed and the copolymerization amount was changed, as shown in Table 2. The physical properties of the obtained polyester film are shown in Table 2.

[0054] The film obtained in Example 2 had a lower copolymer content in surface layers A and C, resulting in a greater tendency for cyclic trimers to precipitate during heat treatment compared to Example 1. However, it was of a quality suitable for use as a release film in processes.

[0055] The film obtained in Example 3 was of a quality suitable for use as a release film in the manufacturing process.

[0056] The film obtained in Example 4 had a high copolymer content in surface layers A and C. A high copolymer content lowers the melting point, so when the film is formed at the same temperature, thermal decomposition occurs, increasing the amount of OLG and ST. Therefore, although the evaluation of foreign matter defects was inferior to that of Example 1, the quality was suitable for use as a release film in processes, etc.

[0057] (Examples 5 and 6) A polyester film was obtained in the same manner as in Example 1, except that the types of resin used for surface layers A and C were changed as shown in Table 2, and the types of copolymer components were also changed. The physical properties of the obtained polyester film are shown in Table 2.

[0058] The polyester films obtained in Examples 5 and 6 were of a quality suitable for use as release films in processes, etc.

[0059] (Comparative Example 1) A polyester film was obtained in the same manner as in Example 1, except that the type of resin used for surface layers A and C was changed as shown in Table 2.

[0060] The polyester film obtained in Comparative Example 1 did not contain copolymer components in surface layers A and C, resulting in a large amount of cyclic trimer precipitation during heat treatment.

[0061] (Comparative Example 2) A polyester film was obtained in the same manner as in Example 1, except that the type of resin used in the intermediate layer B was changed as shown in Table 2.

[0062] The polyester film obtained in Comparative Example 2 had a higher copolymer content in the intermediate layer B compared to the surface layers A and C. As mentioned above, a higher copolymer content lowers the melting point, and when the film is formed at the same temperature, thermal decomposition occurs, increasing the OLG and ST content. Because the ST content in the intermediate layer B increased, the overall ST content of the film also increased, resulting in a high b value in the color evaluation of the polyester film obtained in Comparative Example 2.

[0063] (Comparative Example 3) A polyester film was obtained in the same manner as in Example 1, except that the types of resin used for surface layer A, surface layer C, and intermediate layer B were changed as shown in Table 2.

[0064] The polyester film obtained in Comparative Example 3 had a high amount of OLG in the surface layer and many foreign matter defects because the surface layer A and surface layer C used resin derived from material recycling.

[0065] (Comparative Example 4) A polyester film was obtained in the same manner as in Example 1, except that the type of resin used in the intermediate layer B was changed as shown in Table 2.

[0066] The polyester film obtained in Comparative Example 4 had a high amount of ST in the intermediate layer B and a high b value in the color evaluation.

[0067] [Table 2] [Industrial applicability]

[0068] The polyester film obtained in this manner is useful for optical applications, agricultural materials, horticultural materials, fishing materials, civil engineering and construction materials, stationery, medical supplies, automotive parts, electrical and electronic components, and other applications, and is particularly suitable as a release film for processes where high quality is required.

Claims

1. A polyester film having polyester resin as its main component, made using recycled resin, and when the film is divided in the thickness direction from one surface to the other into a surface layer A, an intermediate layer B, and a surface layer C, the polyester film satisfies the following conditions (I) to (III). (I) The polyester resin of surface layer A and / or surface layer C contains copolymer components. (II) The amount of copolymer components contained in the polyester resin of surface layer A and / or surface layer C is greater than that of intermediate layer B. (III) When the amounts of chemical formula 1 (Chemical Formula 1) contained in surface layer A, intermediate layer B, and surface layer C are ST-A, ST-B, and ST-C respectively, ST-A < ST-B and / or ST-C < ST-B, and ST-B is 0.1 ppm or more and less than 20 ppm. 【Chemistry 1】 (Chemical Formula 1) R1 and R2 are each independent substituents including hydrogen, alkyl groups, alcohols, esters, and ethers. n is an integer of 1 or more.

2. The polyester film according to claim 1, wherein when the amounts of cyclic trimers contained in the surface layer A, the intermediate layer B, and the surface layer C are C3-A, C3-B, and C3-C, respectively, C3-A < C3-B and / or C3-C < C3-B.

3. The polyester film according to claim 1, wherein when the amounts of terephthalic acid (TPA), monohydroxyethyl terephthalate (MHET), and bis-β-hydroxyethyl terephthalate (BHET) contained in surface layer A, intermediate layer B, and surface layer C are denoted as OLG-A, OLG-B, and OLG-C, respectively, OLG-A < OLG-B and / or OLG-C < OLG-B.

4. The polyester film according to claim 1, wherein the amount of copolymer component contained in surface layer A and / or surface layer C is 0.01 mol% or more and 10 mol% or less.

5. The polyester film according to claim 1, wherein the copolymer component comprises any of neopentyl glycol, 1,4-cyclohexanedimethanol, butanediol, propylene glycol, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, or dimethyl sodium 5-sulfisoisophthalate.

6. The polyester film according to claim 1, comprising a polyester resin composition derived from chemical recycling that depolymerizes polyester and / or a polyester resin composition derived from material recycling that remelts polyester.

7. The polyester film according to claim 1, wherein the surface layer A and / or surface layer C contains a polyester resin composition derived from chemical recycling.

8. The polyester film according to claim 1, wherein the source of the chemical recycling and material recycling is any of used film, bottles, containers, or fibers.

9. The polyester film according to claim 1, which is a biaxially oriented polyester film for process release.

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