Biaxially oriented polyester film and its manufacturing method

Abstract

To provide a biaxially oriented polyester film which has little variation or distortion in molecular orientation and thermal shrinkage rate in a film width direction and has uniform physical properties, making it suitable for various processing applications and which is also environmentally friendly, and a method for producing the same.SOLUTION: A biaxially oriented polyester film made of a polyester resin composition containing polyester resin and particles made from recycled PET bottles satisfies all of the following requirements (1) to (4). (1) A crystallization temperature upon heating is 120°C or higher and 139°C or lower. (2) An amount of change in orientation angle in a width direction of the film is between 0° and 10° per 0.5 m. (3) When heat-treated at 150°C for 30 minutes, a thermal shrinkage rate across an entire width of the film is 0.5% to 2.0% in a longitudinal direction and -1.0% to 1.0% in a transverse direction. (4) A content of an isophthalic acid component relative to 100 mol % of all dicarboxylic acid components in the polyester resin composition is 0.01 mol % or more and 3.0 mol % or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented polyester film and a method for producing the same. More specifically, the present invention relates to a biaxially oriented polyester film that is suitable for a variety of processing applications due to its small variation in molecular orientation and thermal shrinkage rate and small distortion in the film width direction, and uniform physical properties in the film width direction, and that is environmentally friendly because it uses polyester resin recycled from the market and society, including PET bottles (hereinafter sometimes referred to as "polyester resin recycled from PET bottles"), and a method for producing the same. [Background technology]

[0002] Polyester resins such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), which are thermoplastic resins with excellent heat resistance and mechanical properties, are used in a wide variety of fields, including plastic films, electronics, energy, packaging materials, and automobiles. Among plastic films, biaxially oriented polyester film is widely used in industrial and packaging fields due to its excellent balance of mechanical strength, heat resistance, dimensional stability, chemical resistance, optical properties, and cost.

[0003] In recent years, with the growing demand for a recycling-oriented society, the use of recycled raw materials has been promoted in the field of materials. In the case of polyester resin, as mentioned above, recycled PET beverage bottles are being used, and their utilization methods are attracting attention. It is said that using recycled PET bottle materials leads to a reduction in CO2 emissions, and there is a desire to increase the proportion of recycled PET bottle materials used, even if only slightly, from the perspective of the global environment.

[0004] In recent years, secondary processing steps for polyester films, such as coating, printing, and vapor deposition, have been increasing in speed and the width and length of the substrate film to improve productivity. In particular, when the film width is increased, the effects of heat and tension during processing become greater, making it difficult to achieve uniform physical properties across the width, and it is difficult to obtain a biaxially oriented polyester film suitable for various processing applications. In particular, there is a demand for reductions in the variation in molecular orientation and heat shrinkage and distortion across the film width.

[0005] For example, Patent Document 1 discloses a biaxially oriented polyethylene terephthalate film made from recycled PET bottle materials. This technology makes it possible to obtain a biaxially oriented polyester film that has excellent thermal stability, little residual components of the cleaning solution used in producing recycled PET bottle raw materials, little foreign matter, and stable resistivity when melted, without compromising the productivity or quality of the film.

[0006] However, while the publication mentions biaxially oriented polyethylene terephthalate film made from recycled PET bottles and an alkaline washing method for obtaining a film with excellent thermal stability and little foreign matter, it does not mention the molecular orientation in the film's width direction or variations in heat shrinkage or distortion. Furthermore, a method has been proposed for obtaining biaxially oriented polyethylene terephthalate film made from recycled PET bottles, in which recycled PET bottles are mixed with a polyester resin derived from fossil fuels, the melt-extruded unstretched sheet is heated to 100°C using a group of heated rolls and an infrared heater, stretched 4.0 times in the longitudinal direction using a group of rolls, and then stretched 4.2 times in the width direction at 130°C, followed by heat setting and further heat relaxation. However, the inventors found that polyester resins made from recycled PET bottles are generally recycled through processes such as crushing collected used PET bottles, removing foreign matter by washing, and decontaminating them at high temperatures, and that because their temperature-induced crystallization temperature is typically around 130°C, which is lower than the temperature-induced crystallization temperature of polyester resins derived from fossil fuels, they are prone to breakage during stretching at stretching temperatures higher than 130°C. On the other hand, if the stretching temperature is lowered to prevent breakage during stretching, differences in the longitudinal stretching stress caused by widthwise stretching are likely to occur between the edges held by the clips and the central portion where the restraining force is relatively weak, resulting in a bowing phenomenon in which physical properties are non-uniform across the width of the film, and the problem of variations in molecular orientation and thermal shrinkage and distortion in the width direction of the film is easily generated is not fully considered.

[0007] The bowing phenomenon referred to here is, in geometric terms, a straight line drawn across the width of the film at the entrance of the tenter, which changes to a bow-like curve with the center of the film lagging behind at the exit of the tenter. The resulting biaxially oriented polyester film has a molecular chain orientation whose main axis is more inclined from the center to the edges, which causes variations and distortion in the width direction in thermal shrinkage characteristics, etc., which are thought to be caused by differences in the effects of the stretching stress in the longitudinal direction of the film, the shrinkage stress in the longitudinal direction generated in the heat setting process, and the tension generated in the tenter.

[0008] Various measures to reduce the bowing phenomenon have been studied. For example, Patent Document 2 proposes a method of appropriately combining film-forming conditions such as longitudinal and widthwise stretching conditions, heat setting conditions, and heat relaxation conditions, in which an unstretched sheet is heated with an infrared heater so that both ends of the sheet are 3°C warmer than the center, and the sheet is stretched in two stages, with a first stage stretching ratio of 1.4 and a second stage stretching ratio of 2.86 at a stretching temperature of 115°C (total stretch ratio: 4.0), followed by stretching in the width direction at a stretching temperature of 115°C and a stretch ratio of 4.3, followed by heat setting at 235°C and heat relaxation treatment by 5% in the width direction, thereby reducing the difference in physical properties in the width direction of the film. However, in order to reduce the difference in physical properties in the width direction of the film, a method of stretching in the longitudinal direction in multiple stages between multiple rolls is used, but this method does not fully consider the problems that the large-scale production equipment is disadvantageous in terms of production costs, and that scratches with the rolls are likely to occur, which can damage the quality of the biaxially oriented polyester film.In addition, a polyester film that has little variation in molecular orientation and heat shrinkage rate and distortion in the width direction of the film, and has uniform physical properties in the width direction of the film, is suitable for various processing applications, and is environmentally friendly by using polyester resin recycled from PET bottles has not yet been realized. The inventors have conducted studies and found that simply applying the production conditions described in Patent Document 2 to produce the film described in Patent Document 1 does not result in a sufficient reduction in the bowing phenomenon. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 6036099 [Patent Document 2] Patent No. 6070842 Summary of the Invention [Problem to be solved by the invention]

[0010] The object of the present invention is to overcome the problems of the conventional technology and to provide an environmentally friendly biaxially oriented polyester film and a method for producing the same, which uses a polyester resin made from recycled PET bottles, has little variation in molecular orientation and heat shrinkage rate or distortion in the film width direction, and has uniform physical properties in the film width direction, making it suitable for a variety of processing applications. [Means for solving the problem]

[0011] As a result of extensive research, the present inventors have found that in a biaxially oriented polyester film obtained by biaxially stretching a polyester resin composition in which a polyester resin made from recycled PET bottles is blended with a polyester resin containing particles, a melt-extruded unstretched sheet is heated and biaxially stretched. In the process of stretching, the sheet is stretched in the longitudinal direction and then stretched in the width direction in a tenter, and a cooling region is provided before the heat-setting region, where heat-setting and heat-relaxation treatments are performed. This makes it possible to obtain a biaxially oriented polyester film that uses polyester resin made from recycled PET bottles, has little variation in molecular orientation and heat shrinkage rate, and little distortion in the width direction of the film, and has uniform physical properties in the width direction of the film, making it suitable for a variety of processing applications and environmentally friendly.

[0012] That is, the present invention comprises the following configurations. 1. A biaxially oriented polyester film made of a polyester resin composition containing polyester resin and particles made from recycled PET bottles, which satisfies all of the following requirements (1) to (4): (1) The temperature-rising crystallization temperature measured by a differential scanning calorimeter (DSC) is 120°C or higher and 139°C or lower. (2) The change in orientation angle in the width direction of the film is 0° or more and 10° or less per 0.5 m. (3) When heat-treated at 150°C for 30 minutes across the entire width of the film, the heat shrinkage is in the range of 0.5% to 2.0% in the longitudinal direction and -1.0% to 1.0% in the transverse direction. (4) The content of the isophthalic acid component relative to 100 mol % of all dicarboxylic acid components in the polyester resin composition is 0.01 mol % or more and 3.0 mol % or less. Here, the sample collection locations for requirements (2) and (3) shall be the center position across the entire width and positions at intervals of 500 mm from the center position toward both ends. If 500 mm intervals cannot be secured near both ends, samples shall be collected at the end positions where collection is possible. Furthermore, requirement (2) requires that the maximum and minimum values ​​of each measurement position data be within a range, and requirement (3) requires that the maximum value of each change amount between two adjacent sample collection positions be within a range.

[0013] 2. The biaxially oriented polyester film described in 1., characterized in that the content of polyester resin recycled from PET bottles in the polyester resin composition constituting the biaxially oriented polyester film is 50% by mass or more and 100% by mass or less.

[0014] 3. The biaxially oriented polyester film according to 1. or 2., wherein the polyester resin recycled from PET bottles is a mechanically recycled polyester resin and / or a chemically recycled polyester resin.

[0015] 4. A method for producing a biaxially oriented polyester film, comprising a polyester raw material resin melt extrusion step, a biaxial stretching step, a heat setting step, a heat relaxation step, and a step of winding up the biaxially oriented polyester film into a roll, characterized in that in the biaxial stretching step, after the film is stretched in the longitudinal direction, it is stretched in the width direction in a tenter, and a cooling zone is provided before the heat setting zone. [Effects of the Invention]

[0016] The present invention can provide an environmentally friendly polyester film and a method for producing the same, which uses a polyester resin made from recycled PET bottles, has little variation in molecular orientation and heat shrinkage rate in the width direction of the film, and has uniform physical properties in the width direction of the film, making it suitable for a variety of processing applications. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a plan view illustrating an example of a biaxial stretching step (tenter apparatus) and a heat setting treatment step for producing a biaxially oriented polyester film of the present invention. [Figure 2] FIG. 10 is a plan view illustrating the biaxial stretching step (tenter device) and the heat setting treatment step of Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be described in detail below. [Polyester resin made from recycled PET bottles] The biaxially oriented polyester film of the present invention preferably uses a polyester resin made from recycled PET bottles, as described below. By using a polyester resin made from recycled PET bottles, the proportion of recycled materials in the film can be increased, making it possible to obtain an environmentally friendly film. As the polyester resin made from recycled PET bottles in the present invention, either a polyester resin obtained by a physical recycling method (hereinafter sometimes referred to as mechanically recycled polyester resin) in which used PET bottles collected from the market or society are sorted, crushed, and washed to thoroughly remove surface dirt and foreign matter, and then exposed to high temperatures to thoroughly wash out contaminants remaining inside the resin, and then the resin is re-pelletized (hereinafter sometimes referred to as chemically recycled polyester resin) or a polyester resin obtained by decomposing polyester resin contained in used packaging containers to the monomer level, removing contaminants, and then polymerizing it again can be suitably used.

[0019] The polyester resin used in the biaxially oriented polyester film of the present invention, which is made from recycled PET bottles, is mainly made from recycled containers mainly made of polyethylene terephthalate. For example, recycled beverage containers for tea drinks, soft drinks, etc. are preferably used. The polyester resin may be appropriately oriented, and colorless bottles are preferred, but it may also contain a small amount of coloring components.

[0020] Recycled raw materials from the market or society, including PET bottles, are preferably polyesters manufactured and molded by conventional polymerization and solid-state polymerization methods, preferably composed primarily of polyethylene terephthalate, and may contain other polyester components or copolymer components. They may contain metal compounds such as antimony, germanium, and titanium as catalysts, and phosphorus compounds as stabilizers. Polyesters for PET bottles typically contain germanium as a catalyst, and when recycled PET bottle raw materials are used to make films, the film will contain 1 ppm or more of germanium. However, since this is merely the catalyst content, it is usually no more than 100 ppm, and usually no more than 50 ppm.

[0021] Mechanically recycled polyester resin and chemically recycled polyester resin will be described below.

[0022] [Mechanically recycled polyester resin] Collected used recycled PET bottles are sorted to prevent contamination with other materials and waste, and after removing labels and other debris, they are crushed into flakes. These flakes often contain foreign matter. Consumers may also fill used PET bottles with chemicals, such as pharmaceuticals and solvents. Examples include dishwashing detergents, insecticides, herbicides, pesticides, and various oils. Because conventional cleaning cannot adequately remove chemicals adsorbed to the surface of PET bottles, alkaline cleaning is preferred. The alkali metal hydroxide solution used in this cleaning process is either sodium hydroxide or potassium hydroxide. In such cleaning processes, a preliminary cleaning may be performed before alkaline cleaning. If alkaline washing is not performed, foreign matter will remain in the raw resin material, which can contaminate the film and cause breakage during film production, reducing productivity. Furthermore, the foreign matter will remain in the film, affecting its appearance and causing printing defects in the subsequent printing process.

[0023] The concentration of the aqueous solution of alkali metal hydroxide used in the washing step is usually in the range of 1 to 10% by weight, although this depends on the temperature, time, and stirring conditions. The washing time is in the range of 10 to 100 minutes, and it is preferable to carry out the washing with stirring to enhance the effect.

[0024] It is preferable to perform rinsing and drying after the alkali washing. The alkali washing and rinsing may be repeated several times. If the components of the aqueous solution of alkali metal hydroxide used for washing in the alkali washing step remain in the flakes, the physical properties of the final film may be affected by the subsequent melt extrusion step in the pelletizing step or the melt extrusion step during film formation.

[0025] The sodium and potassium concentrations in the film finally obtained using polyester resins recycled from these PET bottles are preferably greater than 0 ppm and less than 150 ppm, more preferably 3 to 120 ppm, and even more preferably 5 to 80 ppm. A sodium or potassium concentration in the film greater than 150 ppm is undesirable because it reduces the film's heat resistance and thermal stability and causes coloration. Furthermore, a complete absence of sodium or potassium is undesirable because it reduces the effect of suppressing the production of diethylene glycol. Furthermore, polyester resins recycled from PET bottles may contain small amounts of these components, making it difficult to completely eliminate them.

[0026] During this washing process, the PET bottle flakes are partially hydrolyzed by the aqueous solution of alkali metal hydroxide. Furthermore, the degree of polymerization of the resin decreases due to the heat generated during the molding of the PET bottles. Furthermore, after the collected PET bottles are crushed for reuse, they are melted again and pelletized, which reduces the degree of polymerization due to the effects of heat and moisture. While they can be reused as is, depending on their intended use, a decrease in the degree of polymerization can lead to poor moldability, strength, transparency, heat resistance, and other properties, making them unusable as is.

[0027] In such cases, in order to restore the reduced degree of polymerization, it is preferable to crush and wash the PET bottles into flakes or melt the flakes and pelletize them, and then subject them to solid-state polymerization.

[0028] In the solid-state polymerization step, washed flakes or flakes melt-extruded and pelletized can be subjected to continuous solid-state polymerization in an inert gas such as nitrogen gas or a rare gas at 180 to 245°C, preferably 200 to 240°C.

[0029] It is desirable to adjust the flake or pellet conditions so that the final polyester resin recycled from PET bottles has an intrinsic viscosity of 0.55 to 0.90 dl / g, preferably 0.60 to 0.85 dl / g.

[0030] The process for pelletizing the flakes will be described below: The flakes are melted, extruded, cooled, and pelletized using an extruder equipped with a degassing means and a filtering means.

[0031] The melting step in the extruder is usually carried out by melt-kneading at 260 to 300°C, preferably 265 to 295°C. The flakes obtained by crushing PET bottles before being added must be sufficiently dried, and drying is preferably carried out under conditions where the moisture content is 5 to 200 ppm, preferably 10 to 100 ppm, and even more preferably 15 to 50 ppm. If the flakes contain a large amount of moisture, a hydrolysis reaction will proceed during the melting step, reducing the intrinsic viscosity of the resulting polyester resin. As a degassing means, it is preferable to have at least one vacuum vent in the resin melting zone.

[0032] The extruder preferably has a filter as a filtering means capable of filtering out solid foreign matter having a particle size of 25 μm or more, preferably 15 μm or more, more preferably 10 μm or more, from the molten resin.

[0033] The molten resin that has passed through the filter passes through a die and is cooled in water, after which it is cut into pellets of the desired shape and granulated.

[0034] [Chemically recycled polyester resin] The method for producing the chemically recycled polyester resin used in the present invention is not particularly limited, but specific examples include, as described in JP 2000-169623 A, in which collected used PET bottles are sorted, crushed, and washed to remove foreign matter, and then depolymerized to break them down into raw materials or intermediate materials for PET resin, which are then purified and polymerized to produce new PET resin. Examples of depolymerization include adding ethylene glycol (EG) in the presence of a catalyst to return the material to bis-2-hydroxyethyl terephthalate (BHET), an intermediate material used in resin production, which is then purified and repolymerized to produce PET; and, as described in JP 2000-302707 A, heat-treating polyethylene terephthalate in a nonaqueous organic solvent in the presence of a catalyst containing oxidized iron to produce terephthalic acid and ethylene glycol, which are then repolymerized. A feature of chemically recycled polyester resin is that foreign matter and other materials are removed during the depolymerization / repolymerization process, allowing it to be recycled into polyester resin of the same high quality as virgin resin.This makes it more hygienic than the mechanically recycled polyester resin mentioned above, making it particularly suitable for use in food packaging.

[0035] The chemically recycled polyester resin used in this invention is made from bales of compressed used PET bottles. These PET bottle bales are produced by a known method currently adopted by municipalities. Other polyethylene terephthalate waste or PET bottle flakes can also be used as starting materials instead of PET bottle bales.

[0036] A PET bottle bale made by compressing PET bottle waste to reduce its volume is placed in a crusher, and hot water, room temperature water, or hot water or room temperature water containing detergent is poured in to crush the PET bottles underwater.

[0037] Next, the mixture of PET bottle flakes and washing water discharged from the crusher is immediately subjected to gravity separation to separate the metal, stone, glass, sand and flakes.The flakes are then separated from the washing water, rinsed with ion-exchanged water and centrifuged for dehydration.

[0038] The crude polyethylene terephthalate flakes obtained in the above pretreatment step are depolymerized, melted, and simultaneously hydrolyzed to produce a polyethylene terephthalate melt with a low degree of polymerization, which is then depolymerized with excess ethylene glycol to produce a mixed solution of crude BHET and crude ethylene glycol.

[0039] After the depolymerization reaction is complete, the mixture of crude BHET and crude ethylene glycol is cooled and filtered to remove unreacted linear and cyclic oligomers as high-melting precipitates, coagulation of remaining plastics other than polyethylene terephthalate, and solid foreign matter such as metals. The mixture is then subjected to adsorption and ion exchange treatment to remove colored substances and dissolved ions, thereby removing the foreign matter contained in the crude BHET.

[0040] The mixed solution of crude BHET and crude ethylene glycol obtained through the pre-purification step is subjected to distillation and evaporation to separate and distill off ethylene glycol, thereby obtaining concentrated BHET; alternatively, the mixed solution is cooled to 10°C or less to crystallize BHET, and then the ethylene glycol and BHET are subjected to solid-liquid separation to obtain concentrated BHET. This concentrated BHET is then evaporated in vacuum at a temperature of more than 190°C and 250°C or less so that the residence time of the concentrated BHET in the evaporator is 10 minutes or less, thereby obtaining purified bis-β-hydroxyethyl terephthalate.

[0041] After high-purity purified BHET is obtained as described above, this purified BHET is charged into a melt polycondensation reactor to obtain high-purity polyethylene terephthalate polymer.

[0042] [Polyester resin composition] The biaxially oriented polyester film of the present invention is made of a polyester resin composition containing the following polyester resin as a main component. The polyester resin constituting the biaxially oriented polyester film of the present invention is a polymer synthesized from a dicarboxylic acid or its ester-forming derivative and a diol or its ester-forming derivative. Examples include polyethylene terephthalate, polybutylene terephthalate, and polyethylene-2,6-naphthalate, with polyethylene terephthalate being preferred from the viewpoints of mechanical properties, heat resistance, cost, etc. The term "main component" as used herein means that the content in the polyester resin composition is 80% by weight or more, preferably 90% by weight or more, more preferably 95% by weight or more, and most preferably 98% by weight or more.

[0043] These polyester resins may also be copolymerized with other components as long as the objectives of the present invention are not impaired. Specifically, copolymerization components include dicarboxylic acid components such as isophthalic acid, naphthalenedicarboxylic acid, 4,4-diphenyldicarboxylic acid, adipic acid, sebacic acid, and their ester-forming derivatives. Diol components include diethylene glycol, hexamethylene glycol, neopentyl glycol, and cyclohexanedimethanol. Polyoxyalkylene glycols such as polyethylene glycol and polypropylene glycol are also included. The copolymerization amount is preferably 10 mol % or less, more preferably 5 mol % or less, and most preferably 3 mol % or less, per constituent repeating unit.

[0044] Among the polyester resins constituting the biaxially oriented polyester film of the present invention, methods for producing fossil fuel-derived polyester resins other than mechanically recycled polyester resins and chemically recycled polyester resins include a method in which the above-mentioned dicarboxylic acid or its ester-forming derivative and diol or its ester-forming derivative are first used as main starting materials, and then the resulting polyester resin is subjected to an esterification or transesterification reaction according to a conventional method, followed by a polycondensation reaction at high temperature and reduced pressure.

[0045] The intrinsic viscosity of the polyester resin constituting the biaxially oriented polyester film of the present invention is preferably in the range of 0.50 to 0.90 dl / g, more preferably 0.55 to 0.80 dl / g, from the viewpoints of film formability and recollection.

[0046] The polyester resin composition of the present invention preferably contains at least one type of particle selected from the group consisting of inorganic particles, organic particles, and mixtures thereof.

[0047] Examples of inorganic particles used include particles made of silica (silicon oxide), alumina (aluminum oxide), titanium dioxide, calcium carbonate, kaolin, crystalline glass filler, kaolin, talc, alumina, silica-alumina composite oxide particles, and barium sulfate. Examples of organic particles include acrylic resin particles, melamine resin particles, silicone resin particles, and particles made of cross-linked polystyrene. Among these, particles made of silica (silicon oxide), calcium carbonate, or alumina (aluminum oxide), or particles made of polymethacrylate, polymethyl acrylate, or derivatives thereof are preferred, and particles made of silica (silicon oxide) or calcium carbonate are more preferred, with silica (silicon oxide) being particularly preferred in terms of reducing haze.

[0048] The weight average particle size of the particles in the present invention is a value measured by a cole counter, and is preferably 0.5 to 4.0 μm, more preferably 0.8 to 3.8 μm, and even more preferably 1.5 to 3.0 μm. If the weight average particle size of the particles is less than 0.5 μm, the formation of surface irregularities will be insufficient, resulting in a decrease in the slipperiness of the film and in the inability to uniformly remove air trapped in the film when it is wound onto a roll, which will likely result in poor appearance such as wrinkles and pimples on the surface of air bubbles, and will likely result in poor winding properties. If the weight average particle size of the particles exceeds 4.0 μm, the formation of coarse protrusions is likely to impair the quality of the film, such as causing printing defects.

[0049] Furthermore, the polyester resin composition of the present invention may contain small amounts of other polymers, antioxidants, heat stabilizers, antistatic agents, ultraviolet absorbers, plasticizers, pigments, or other additives, as long as the object of the present invention is not impaired.

[0050] The biaxially oriented polyester film obtained as described above preferably contains an isophthalic acid component in an amount of 0.01 mol % or more and 3.0 mol % or less relative to 100 mol % of all dicarboxylic acid components in the film. The crystallinity of polyesters generally used in PET bottles is controlled to improve the appearance of the bottles, and as a means for this, polyesters containing 10 mol % or less of isophthalic acid components are sometimes used. Therefore, the biaxially oriented polyester film of the present invention contains a certain amount of material containing an isophthalic acid component.

[0051] The lower limit of the amount of isophthalic acid relative to the total dicarboxylic acid components constituting the polyester resin contained in the film is preferably 0.01 mol%, more preferably 0.05 mol%, even more preferably 0.1 mol%, and particularly preferably 0.15 mol%. As mentioned above, polyester resins recycled from the market or society, including PET bottles, contain a large amount of isophthalic acid. Therefore, if the isophthalic acid component constituting the polyester resin in the film is less than 0.01 mol%, it becomes difficult to produce a polyester film with a high proportion of recycled resin, which is not desirable. The upper limit of the amount of isophthalic acid relative to the total dicarboxylic acid components constituting the polyester resin contained in the film is preferably 3.0 mol%, more preferably 2.5 mol%, and even more preferably 2.0 mol%. If the amount exceeds 3.0 mol%, the crystallinity decreases, which may reduce the mechanical strength of the film, which is not desirable.

[0052] The upper limit of the intrinsic viscosity of the polyester resin recycled from PET bottles is preferably 0.90 dL / g, more preferably 0.80 dL / g, even more preferably 0.75 dL / g, and particularly preferably 0.69 dL / g. If the intrinsic viscosity exceeds 0.90 dL / g, the resin becomes difficult to extrude from the extruder, which may reduce productivity, and is therefore not preferred.

[0053] In the biaxially oriented polyester film of the present invention, the lower limit of the content of polyester resin recycled from PET bottles relative to the total amount of polyester resin constituting the film is preferably 50% by weight, more preferably 70% by weight, even more preferably 90% by weight, and particularly preferably 100% by weight. If it is less than 50% by weight, the content is poor for utilizing recycled resin, and is not very desirable in terms of contributing to environmental protection. Furthermore, polyester resin recycled from PET bottles can also be used as a masterbatch (high-concentration resin) when adding lubricants or additives such as inorganic particles to improve the film's functionality.

[0054] [Method of manufacturing biaxially oriented polyester film and film roll] The biaxially oriented polyester film of the present invention can be obtained, for example, by supplying and mixing polyester resin chips made from recycled PET bottles and chips of a polyester resin composition mainly composed of polyester resin into an extruder equipped with a hopper, melt-extruding the mixture through the extruder to form an unstretched sheet, and then stretching the unstretched PET sheet. Suitable examples are given below, but the present invention is not limited to these.

[0055] The biaxially oriented polyester film of the present invention may have a single layer, two layers, three layers, or a laminate structure of four or more layers. In the case of a two or more layer structure, each layer contains the polyester resin, inorganic particles, and a resin other than the polyester resin as described above as constituent components, but the type or content of any of the constituent components of adjacent layers is different. In the case of a single-layer structure, the surface layer (A) in the present invention is the entire biaxially oriented polyester film. In the case of a two-layer structure, the surface layer (A) in the present invention is one or both layers, and in the case of a three-layer structure, the surface layer (A) in the present invention is one or both layers.

[0056] In the case of a three-layer structure, if the compositions that make up the film are represented as A, A', B, and C, then the film can have a structure of, for example, A / B / C, A / B / A, or A / B / A'. However, if there is no need to change the surface properties of both sides, it is preferable to have an A / B / A structure, in which both layers are designed to have the same composition, as this is easier to manufacture. Here, A and A' do not have the same composition.

[0057] In particular, in the case of a three-layer structure, even if there are no inorganic particles in the base layer (B), the surface roughness of the film can be controlled by controlling the amount of particles added only to the surface layer (A), and the content of inorganic particles in the film can be reduced, which is preferable because it also improves the problem of odor components escaping through voids (air gaps) that form at the boundary between the inorganic particles and the polyester resin, resulting in a decrease in aroma retention. Furthermore, it is easy to mix and use recycled raw materials from the edge portions generated during the film production process or recycled raw materials from other film production processes into the base layer (B) as appropriate, as long as it does not adversely affect the properties of the film surface, which is also advantageous in terms of cost.

[0058] As mentioned above, when a film has a two-layer or more structure, chemically recycled polyester resin and mechanically recycled polyester resin can be laminated as separate layers. For example, in the case of a three-layer structure (A / B / A), by disposing a chemically recycled polyester resin in the outermost layer (A) and a mechanically recycled polyester resin in the central layer (B), it is possible to reduce defects in the film by having the chemically recycled polyester resin with fewer impurities in the surface layer. On the other hand, by disposing a mechanically recycled polyester resin in the outermost layer (A) and a chemically recycled polyester resin in the central layer (B), the mechanically recycled polyester resin with a higher isophthalic acid content in the surface layer can improve adhesion with sealants, etc.

[0059] The lower limit of the particle content in the surface layer (A) of the biaxially oriented polyester film of the present invention is 500 ppm by weight, more preferably 600 ppm by weight, and particularly preferably 700 ppm by weight. If the particle content is less than 500 ppm by weight, the slipperiness and winding properties of the film surface may decrease, which is not preferable.

[0060] The content of inorganic particles in all layers of the biaxially oriented polyester film of the present invention is preferably 100 ppm or more and 1000 ppm or less, more preferably 800 ppm or less. If the content of inorganic particles is less than 100 ppm, the slipperiness is reduced, which can cause problems during roll running, winding, rewinding, slitting, etc. during the film production process, leading to scratches on the film surface, the occurrence of winding wrinkles, and static electricity. If the content of inorganic particles exceeds 1000 ppm, the arithmetic mean height Sa and maximum protrusion height Sp of the film surface tend to increase, so caution is required. Furthermore, the number of voids in the film increases, which can easily cause a decrease in transparency.

[0061] When melt-extruding the polyester resin and polyester resin composition made from recycled PET bottles, it is preferable to dry them using a dryer such as a hopper dryer or paddle dryer, or a vacuum dryer. After drying the polyester resin and polyester resin composition made from recycled PET bottles in this way, they are melted and extruded into a film using an extruder at a temperature of 200 to 300°C, which is equal to or higher than the melting point of the polyester resin. Alternatively, the polyester resin, particles, and optional additives may be fed using separate extruders, merged, melted, and extruded into a sheet. When extruding the molten resin composition, any existing method such as a T-die method or a tubular method can be used.

[0062] The extruded molten polyester resin can then be rapidly cooled to obtain an unstretched sheet. A suitable method for rapidly cooling the molten polyester resin is to cast the molten polyester resin onto a rotating drum from a die and rapidly cool and solidify it to obtain a substantially unoriented resin sheet. The temperature of the rotating drum is preferably set to 40°C or lower.

[0063] Furthermore, the biaxially oriented polyester film of the present invention can be obtained by combining the obtained unstretched sheet with the following processes, such as a biaxial stretching process in the longitudinal direction and the width direction, a heat setting process, and a heat relaxation process. The details are explained below: The longitudinal direction means the direction in which the unstretched sheet travels, and the width direction means the direction perpendicular to the longitudinal direction.

[0064] The stretching method can be simultaneous biaxial stretching, in which stretching in the longitudinal direction and the width direction is performed simultaneously, or sequential biaxial stretching, in which stretching in either the longitudinal direction or the width direction is performed first. However, sequential biaxial stretching is most preferred from the viewpoints of high productivity due to a fast film-forming speed and excellent thickness uniformity of the final biaxially oriented polyester film. The film-forming speed referred to here means the running speed (m / min) of the biaxially oriented polyester film when it is taken up around a master roll after the stretching step.

[0065] The temperature during stretching of the unstretched sheet in the longitudinal direction is preferably in the range of 80 to 130° C., and the stretching ratio is preferably in the range of 3.3 to 4.7 times. When the stretching temperature is 80°C or higher and the stretching ratio is 4.7 times or lower, the shrinkage stress in the longitudinal direction is reduced, the bowing phenomenon is reduced, and the molecular orientation and thermal shrinkage rate variation and distortion in the width direction of the obtained biaxially oriented polyester film are reduced, which is preferable.

[0066] Furthermore, when stretching an unstretched sheet in the longitudinal direction, a method of stretching by heating using an infrared heater or the like is preferred over a method of stretching in multiple stages between multiple rolls, in view of the scale of the production equipment, manufacturing costs, and damage defects such as adhesive damage and transfer damage caused by the rolls, as this method makes it easier to obtain high temperatures, facilitates localized heating, and reduces damage defects caused by the rolls.

[0067] The film obtained by stretching the unstretched sheet in the longitudinal direction may be subjected to a surface treatment such as corona treatment or plasma treatment as necessary, and then a resin dispersion or resin solution may be applied to at least one surface of the film to impart functions such as easy slippage, easy adhesion, and antistatic properties.

[0068] When the film obtained by stretching an unstretched sheet in the longitudinal direction is stretched in the width direction, the film is introduced into a tenter device, both ends of the film obtained by stretching the unstretched sheet in the longitudinal direction are held with clips, the film is heated to a predetermined temperature with hot air, and the distance between the clips is increased while the film is transported in the longitudinal direction, thereby stretching the film in the width direction.

[0069] In the biaxially oriented polyester film of the present invention, the temperature-raised crystallization temperature of the polyester resin recycled from PET bottles relative to the total amount of polyester resin constituting the film is usually around 130° C., and therefore the film is prone to breakage during stretching at a stretching temperature higher than 130° C. Furthermore, if the stretching temperature is lowered to prevent breakage during stretching, the longitudinal stretching stress caused by stretching in the width direction is likely to differ between the edges held by the clips and the central part where the restraining force is relatively weak, resulting in a bowing phenomenon in which physical properties become non-uniform across the width of the film, and thus variations in molecular orientation and heat shrinkage and distortion in the width direction of the film are likely to occur.

[0070] In order to improve the nonuniformity of physical properties in the width direction of the film caused by such breakage and bowing during stretching, it is preferable in the present invention to stretch the film in the longitudinal direction in the biaxial stretching step, then stretch it in the width direction in a tenter, and to provide a cooling zone before the heat setting zone. The cooling zone before the heat setting zone refers to a cooling step in which the temperature of the film stretched in the width direction is lowered after stretching in the width direction in the tenter, for example, by blocking the hot air from the heating zone with a shielding plate such as an aluminum plate inside the tenter before the heat setting zone, or by blocking the hot air and then actively feeding compressed air. The temperature of the film in the cooling region is preferably equal to or lower than the glass transition temperature (Tg) of the film. When stretching in the width direction in the tenter, the central portion has a relatively smaller restraining force than the end portions held by the clips, and therefore deformation due to longitudinal stretching stress caused by width direction stretching and longitudinal shrinkage stress caused by heat setting treatment becomes large, and the bowing phenomenon is likely to increase. However, by cooling the film to a temperature equal to or lower than the glass transition temperature, the rigidity of the film is increased, film deformation is suppressed, and variations in molecular orientation and heat shrinkage rate and distortion in the width direction of the film caused by the bowing phenomenon are reduced, which is preferable. The temperature in the cooling region is preferably 60° C. or higher. By cooling the film to a temperature of 55° C. or higher, the thermal shrinkage of the film in the longitudinal and transverse directions can be suppressed, which is preferable.

[0071] The preheating temperature during stretching in the width direction is preferably in the range of 100° C. to 130° C. When the preheating temperature during stretching is 100° C. or higher, the shrinkage stress generated during stretching in the longitudinal direction is reduced, the bowing phenomenon is reduced, and the molecular orientation and thermal shrinkage variation and distortion in the width direction of the obtained biaxially oriented polyester film are reduced, which is preferable. The temperature during widthwise stretching is preferably in the range of 105°C to 135°C. When the stretching temperature is 105°C or higher, the longitudinal stretching stress caused by widthwise stretching is reduced, and the bowing phenomenon is less likely to increase, which is preferable. Furthermore, when the stretching temperature is 135°C or lower, even when a polyester resin recycled from PET bottles, which has a temperature-rise crystallization temperature of about 130°C, is used, breakage during stretching is less likely to occur, which is preferable. The stretching ratio in the width direction is preferably in the range of 3.5 to 5.0 times. When the stretching ratio in the width direction is 3.5 times or more, a high yield is likely to be obtained in terms of material balance, the mechanical strength is not reduced, and thickness unevenness in the width direction is less likely to increase, which is preferable. Furthermore, when the stretching ratio in the width direction is 5.0 times or less, breakage during film formation by stretching is less likely to occur, which is preferable.

[0072] The biaxial stretching process in the longitudinal and width directions is followed by a heat setting process. The heat setting temperature of the film obtained by stretching the unstretched sheet in the longitudinal direction and then stretching it in the width direction is preferably 220°C or higher and 250°C or lower. When the heat setting temperature is 220°C or higher, the heat shrinkage rate in both the longitudinal and transverse directions is not too high, and the thermal dimensional stability during secondary processing is improved, which is preferable. On the other hand, when the heat setting temperature is 250° C. or less, bowing is unlikely to increase, and variations in molecular orientation and heat shrinkage rate and distortion in the film width direction are reduced, which is preferable.

[0073] A heat relaxation treatment step is then carried out, which may be carried out separately from the heat setting step after the heat setting step, or may be carried out simultaneously with the heat setting step. The relaxation rate in the film width direction in the heat relaxation treatment step is preferably 4% or more and 8% or less. When the relaxation rate is 4% or more, the heat shrinkage rate in the width direction of the resulting biaxially oriented polyester film does not become too high, and the dimensional stability during secondary processing is improved, which is preferable. On the other hand, if the relaxation rate is 8% or less, the longitudinal stretching stress caused by the widthwise stretching of the central part of the film is not too large, and the bowing phenomenon is less likely to increase, which is preferable.

[0074] In the heat relaxation treatment step, the film obtained by stretching the unstretched sheet in the longitudinal direction is stretched in the width direction, and during the time until the film is stretched in the width direction by heat relaxation, the restraining force in the width direction is reduced, causing the film to slacken under its own weight, and the film may also swell due to the accompanying air currents of the hot air blown out from the nozzles installed above and below the film, so the film is in a state where it is very susceptible to vertical fluctuations, and the change in the orientation angle of the resulting biaxially oriented polyester film is likely to fluctuate greatly. One way to alleviate these problems is to keep the film parallel by adjusting the wind speed of the hot air blown out from the upper and lower nozzles, for example.

[0075] The biaxially oriented polyester film of the present invention may be subjected to corona discharge treatment, glow discharge treatment, flame treatment, or surface roughening treatment, and may also be subjected to known anchor coating treatment, printing, decoration, etc., as long as the object of the present invention is not impaired.

[0076] The wide biaxially oriented polyester film stretched by the above method is wound on a winder to produce a master roll. The width of the master roll is preferably 5,000 mm or more and 10,000 mm or less. A roll width of 5,000 mm or more is preferred because it reduces the cost per film area in subsequent secondary processing such as slitting, vapor deposition, and printing. The master roll preferably has a winding length of 10,000 m or more and 100,000 m or less. A winding length of 5,000 m or more is preferred because it reduces the cost per film area in subsequent secondary processing such as slitting, vapor deposition, and printing. The winding width of the film roll slit from the master roll is preferably 400 mm or more and 3000 mm or less. A winding width of 400 mm or more reduces the need for frequent film roll replacement during the printing process, which is preferable from the standpoint of cost. A longer winding width is preferable, but a width of 3000 mm or less is preferable because the roll width does not become too large and the roll weight does not become too heavy, preventing deterioration in handleability. The film roll length is preferably 2000 m or more and 65000 m or less. A film roll length of 2000 m or more is preferable from the standpoint of cost, as it reduces the need to frequently replace the film roll during the printing process. While a longer film roll length is preferable, a film roll length of 65000 m or less is preferable because it prevents the roll diameter from becoming too large and the roll weight from becoming too heavy, preventing deterioration in handleability. The core used for the film roll is not particularly limited, and typically, a cylindrical core made of plastic, metal, or cardboard with a diameter of 3 inches (37.6 mm), 6 inches (152.2 mm), 8 inches (203.2 mm), etc. can be used.

[0077] [Characteristics of biaxially oriented polyester film and film roll] The biaxially oriented polyester film of the present invention, which is made of a polyester resin composition containing particles and a polyester resin recycled from PET bottles, preferably satisfies all of the following requirements (1) to (4). Each of these will be described in detail. (1) The temperature-rising crystallization temperature measured by a differential scanning calorimeter (DSC) is 120°C or higher and 139°C or lower. (2) The change in orientation angle in the width direction of the film is 0° or more and 10° or less per 0.5 m. (3) When heat-treated at 150°C for 30 minutes across the entire width of the film, the heat shrinkage is in the range of 0.5% to 2.0% in the longitudinal direction and -1.0% to 1.0% in the transverse direction. (4) The content of the isophthalic acid component relative to 100 mol % of all dicarboxylic acid components in the polyester resin composition is 0.01 mol % or more and 3.0 mol % or less.

[0078] [Temperature-raised crystallization temperature (Tc1)] The temperature-rise crystallization temperature of the biaxially oriented polyester film of the present invention is preferably 139° C. or lower, more preferably 135° C. or lower. The temperature-rise crystallization temperature of polyester resins recycled from PET bottles is usually about 130° C., which is lower than the temperature-rise crystallization temperature of polyester resins derived from fossil fuels. Therefore, if the temperature-rise crystallization temperature in the film is higher than 139° C., it becomes difficult to produce a polyester film with a high proportion of recycled resin, which is not desirable.

[0079] [Change in orientation angle in the film width direction] The biaxially oriented polyester film of the present invention preferably exhibits a change in orientation angle per 0.5 m relative to the film width direction of 0° to 10°, more preferably 0° to 8°, and even more preferably 0° to 6°. The orientation angle here refers to the tilt of the main axis of molecular chain orientation relative to the width direction. If the change in orientation angle is greater than 10° per 0.5 m, the predominant molecular orientation direction within the film width will vary significantly. Therefore, when a film roll made by winding a wide film is used, runnability will be impaired and diagonal wrinkles will likely occur during secondary processing such as vapor deposition or printing, resulting in localized processing irregularities and processing defects, potentially compromising film quality. Here, samples are taken at the center of the width direction and at 500 mm intervals from the center toward both ends. If 500 mm intervals cannot be secured near both ends, samples are taken at the available edge positions. Furthermore, the above range is considered to be satisfied when the maximum change in each amount calculated between two adjacent sample taking positions is within the range.

[0080] [Heat shrinkage rate in the longitudinal and transverse directions] The biaxially oriented polyester film of the present invention preferably exhibits a heat shrinkage of 0.5% to 2.0% in the longitudinal direction and -1.0% to 1.0% in the transverse direction when heat-treated at 150°C for 30 minutes across its entire width, more preferably 0.8% to 1.8% in the longitudinal direction and -0.8% to 0.8% in the transverse direction. Maintaining the heat shrinkage within the above ranges in the longitudinal and transverse directions ensures dimensional stability against heat during secondary processing such as vapor deposition and printing, preventing thermal deformation and wrinkling and reducing film quality degradation. Here, samples are taken across the entire width at the center position in the transverse direction and at 500 mm intervals from the center toward both ends. If 500 mm intervals cannot be ensured near both ends, samples are taken at the edge positions where possible. Furthermore, the maximum and minimum values ​​of data at each measurement position are considered to be within the range if both values ​​satisfy the above ranges.

[0081] [Film thickness] The biaxially oriented polyester film of the present invention preferably has a film thickness of 5 to 40 μm. A thickness of 5 μm or more is preferable because the strength and stiffness of the film are not reduced and the film roll is less likely to wrinkle when wound up by a winder. On the other hand, a film thickness of 40 μm or less provides sufficient strength and stiffness, and from a cost perspective, it is preferable to make the film thinner. The film thickness is more preferably 8 to 30 μm, and particularly preferably 9 to 20 μm. [Example]

[0082] A. The polyester resin was evaluated as follows: [Glass transition temperature (Tg)] Using a differential scanning calorimeter (DSC6220 model, manufactured by SII NanoTechnology Inc.), 5 mg of the resin sample was melted to 280°C under a nitrogen atmosphere, held at that temperature for 5 minutes, then rapidly cooled with liquid nitrogen. Measurements were then performed at a heating rate of 20°C / min from room temperature.

[0083] [Intrinsic viscosity (IV)] 0.2 g of polyester resin was dissolved in 50 ml of a mixed solvent of phenol / 1,1,2,2-tetrachloroethane (60 / 40 (weight ratio)), and the viscosity was measured using an Ostwald viscometer at 30°C. The unit is dl / g.

[0084] [Content of terephthalic acid and isophthalic acid components in raw polyester and polyester that constitutes the film] A sample solution was prepared by dissolving chloroform D (manufactured by Eurisop) and trifluoroacetic acid D1 (manufactured by Eurisop) in a 10:1 (volume ratio) mixture, and proton NMR of the sample solution was measured using an NMR (GEMINI-200; manufactured by Varian) at a temperature of 23°C and an accumulation count of 64. In the NMR measurement, the peak intensity of a predetermined proton was calculated, and the content (mol %) of terephthalic acid components and isophthalic acid components in 100 mol % of acid components was calculated.

[0085] B. The polyester film was evaluated as follows. Sampling of films for evaluation of properties other than the processability evaluation of the film roll shown below was carried out from the surface layer of the biaxially oriented polyester film roll of the present invention.

[0086] [Film thickness] Measurement was carried out using a dial gauge in accordance with JIS K7130-1999 Method A.

[0087] [Heated crystallization temperature] An 8 mg sample cut out from the evaluation film was measured using a differential scanning calorimeter (DSC60) manufactured by Shimadzu Corporation. The sample temperature is (1) Heat from 25°C to 320°C at 10°C / min (2) Rapid cooling to 25°C at 300°C / min (3) Keep at 25°C for 3 minutes (4) Heat from 25°C to 320°C at 10°C / min The peak top temperature of the exothermic curve for temperature-promoted crystallization in (4) was determined as the temperature-promoted crystallization temperature (Tc1).

[0088] [Heat shrinkage rate in the longitudinal and transverse directions] A sample of 10 mm wide and 250 mm long was cut from the evaluation film in the longitudinal and transverse directions, and marks were made at 200 mm intervals. The spacing (A) between the marks was measured under a constant tension of 5 gf. The film was then heat-treated at 150°C for 30 minutes without load, after which the spacing (B) between the marks was measured under a constant tension of 5 gf, and the thermal shrinkage was calculated using equation (1). Measurements were taken across the entire width of the resulting biaxially oriented polyester film, at the center and at 500 mm intervals from the center toward both ends. When 500 mm intervals could not be ensured near both ends, measurements were taken at measurable edge positions. The maximum and minimum thermal shrinkage values ​​in the longitudinal and transverse directions across the entire width of the film were calculated based on the thermal shrinkage values ​​thus determined. Heat shrinkage rate (%) = {(AB) / A} × 100 Formula (1)

[0089] [Change in orientation angle in the film width direction] The evaluation film was cut into 100mm x 100mm pieces, and the orientation angle of the molecular chain main axis was determined using an MOA-6004 molecular orientation analyzer manufactured by Oji Instruments Co., Ltd., with the axis in the width direction of the film as the reference. A counterclockwise tilt relative to the width direction of the film was defined as +, and a clockwise tilt as -. Measurements were taken across the entire width of the obtained biaxially oriented polyester film, at the center and at 500mm intervals from the center toward both ends. When 500mm intervals could not be ensured near both ends, measurements were taken at measurable edge positions, and the measurement was divided by the distance between the measured samples to convert to per 0.5m using formula (2). Change in orientation angle per 0.5m (° / 0.5m) = |Difference in orientation angle between two adjacent points (°)| ÷ Measured sample interval distance (mm) × 500 (mm / m) Equation (2) The maximum value of the calculated changes between two adjacent sample collection positions was taken as the evaluation result of the change in the film.

[0090] [Evaluation of film roll processability and appearance] Gravure printing was performed on the obtained film roll (winding length 60,000 m, width 800 mm) using a gravure printing machine (manufactured by Higashitani Iron Works Co., Ltd.), and the processability of the film during printing processing and the appearance after processing were evaluated with a rating of ◯ or ×. If there was no sagging or wrinkles and the running property was good, it was rated ◯. If there was sagging or wrinkles or the running property could not be said to be good, it was rated ×.

[0091] The raw resin chips used in the present examples and comparative examples are described in detail below. (Polyester resin A): Mechanically recycled polyester resin The mechanically recycled polyester resin made from recycled PET bottles and used in the production of the biaxially oriented polyester film described below was synthesized using the following method. After washing the remaining beverage and other foreign matter from the PET beverage bottles, they were crushed to obtain flakes. The resulting flakes were washed with a 3.5 wt% sodium hydroxide solution at a flake concentration of 10 wt% at 85°C for 30 minutes under stirring. After the alkaline wash, the flakes were removed and washed with distilled water at a flake concentration of 10 wt% at 25°C for 20 minutes under stirring. This water wash was repeated two more times, each time replacing the distilled water. After washing, the flakes were dried and melted in an extruder. The finer foreign matter was further filtered out twice using filters with successively smaller mesh sizes, and finally filtered out a third time using a filter with the smallest mesh size (50 μm), to obtain a polyester resin A with an intrinsic viscosity of 0.69 dl / g and an isophthalic acid content of 3.0 mol%.

[0092] (Polyester resin B) The fossil fuel-derived PET resin used in the production of the biaxially oriented polyester film described below was terephthalic acid / ethylene glycol = 100 / / 100 (mol %) (manufactured by Toyobo Co., Ltd., intrinsic viscosity 0.62 dl / g).

[0093] (Polyester Resin C): Chemically recycled polyester resin The chemically recycled polyester resin recycled from PET bottles used in the production of the biaxially oriented polyester film described below was synthesized using the following method. The sorted and collected PET bottle bales were placed in a wet crusher, and 1,000 liters of water to which 500g of liquid dish detergent was added was circulated within the wet crusher while being crushed. Materials with high specific gravity, such as metal, sand, and glass, were allowed to settle in a gravity separator connected to the crusher, and flakes were extracted from the top layer. These flakes were rinsed with pure water and centrifuged to produce recovered flakes.

[0094] The recovered flakes were melted in a wet state, and 30 kg of the melted flakes was placed in a stirred autoclave and charged into a preheated mixture of 150 kg of ethylene glycol and 150 g of zinc acetate dihydrate. After removing fractions with boiling points lower than those of ethylene glycol, such as water and acetic acid, the mixture was reacted for 4 hours at a temperature of 195-200°C using a reflux condenser.

[0095] After the reaction was completed, the temperature of the contents of the reactor was lowered to 97 to 98°C, and the contents were filtered while hot to remove suspended matter and precipitates.

[0096] The filtrate after hot filtration was further cooled, and after confirming that the crude BHET was completely dissolved, it was passed through an activated carbon bed at 50 to 51°C and then through an anion / cation exchange mixed bed over 30 minutes for pre-purification treatment.

[0097] The above pre-purified solution was again charged into the stirring autoclave and heated to distill off excess ethylene glycol under atmospheric pressure, thereby obtaining a molten solution of concentrated BHET.

[0098] The resulting molten liquid of concentrated BHET was allowed to cool naturally while stirring under a nitrogen gas atmosphere, and then removed from the autoclave to obtain a block of concentrated BHET flakes.

[0099] The block of fine particles was again heated to 130°C and melted, then fed to a thin-film vacuum evaporator using a metering pump, evaporated, cooled, and condensed to obtain purified BHET.

[0100] This purified BHET was used as a raw material to carry out melt polymerization, and a chemically recycled polyester resin C having an intrinsic viscosity of 0.696 dl / g was obtained.

[0101] (Polyester Resin D) The esterification reactor was heated to 200°C, and a slurry of terephthalic acid (86.4 parts by mass) and ethylene glycol (64.4 parts by mass) was charged, followed by addition of antimony trioxide (0.017 parts by mass) and triethylamine (0.16 parts by mass) as catalysts with stirring. The reactor was then heated to a temperature of 0.34 MPa, and a pressurized esterification reaction was carried out at 240°C. The pressure in the esterification reactor was then returned to normal, and magnesium acetate tetrahydrate (0.071 parts by mass) was added, followed by trimethyl phosphate (0.014 parts by mass). The temperature was then raised to 260°C over 15 minutes, followed by trimethyl phosphate (0.012 parts by mass) and then sodium acetate (0.0036 parts by mass). After 15 minutes, the mixture was dispersed using a high-pressure disperser, and 0.72 parts by weight of an ethylene glycol slurry of irregular silica particles with an average particle size of 2.7 μm was added, based on the particle content. These silica particles were obtained by preparing an ethylene glycol slurry in advance, centrifuging it to remove 35% of the coarse particles, and then filtering it through a metal filter with a 5 μm mesh size. After 15 minutes, the resulting esterification reaction product was transferred to a polycondensation reactor and subjected to a polycondensation reaction under reduced pressure at 280°C, yielding a polyester resin D with an intrinsic viscosity of 0.62 dl / g. (Polyester Resin E) The esterification reactor was heated to 200°C, and a slurry of terephthalic acid (86.4 parts by mass) and ethylene glycol (64.4 parts by mass) was charged, followed by addition of antimony trioxide (0.017 parts by mass) and triethylamine (0.16 parts by mass) as catalysts with stirring. The reactor was then heated to a temperature of 0.34 MPa, and a pressurized esterification reaction was carried out at 240°C. The pressure in the esterification reactor was then returned to normal, and magnesium acetate tetrahydrate (0.071 parts by mass) was added, followed by trimethyl phosphate (0.014 parts by mass). The temperature was then raised to 260°C over 15 minutes, followed by trimethyl phosphate (0.012 parts by mass) and then sodium acetate (0.0036 parts by mass). After 15 minutes, the mixture was dispersed using a high-pressure disperser, and 30.0 parts by weight of an ethylene glycol slurry of irregular silica particles with an average particle size of 2.7 μm was added, based on the particle content. These silica particles were obtained by preparing an ethylene glycol slurry in advance, centrifuging it to remove 35% of the coarse particles, and then filtering it through a metal filter with a 5 μm mesh size. After 15 minutes, the resulting esterification reaction product was transferred to a polycondensation reactor and subjected to a polycondensation reaction under reduced pressure at 280°C, yielding a polyester resin E with an intrinsic viscosity of 0.61 dl / g.

[0102] The raw resin chips are as shown in Table 1. The abbreviations in the table are as follows: TPA: Terephthalic acid EG: Ethylene glycol

[0103] [Table 1]

[0104] [Example 1] A three-layer film was produced using three extruders. The base layer (B) was composed of 95.0% by mass of polyester resin A and 5.0% by mass of polyester resin D, and the surface layer (A) was composed of 87.5% by mass of polyester resin A and 12.5% ​​by mass of polyester resin D. After drying the respective raw resins, the mixed resin forming the surface layer (A) was melt-extruded from the first and third extruders at a resin temperature of 285°C. The mixed resin forming the base layer (B) was melted from the second extruder at a resin temperature of 285°C. The layers were then laminated in a T-die in the order of surface layer (A) / base layer (B) / surface layer (A) from the side contacting the casting drum to a thickness ratio of 1 / 10 / 1 (μm). The layers were then extruded from a T-shaped die and cooled and solidified on a casting drum with a surface temperature of 25°C to obtain an unstretched polyethylene terephthalate sheet. At this time, static electricity was applied using a wire electrode with a diameter of 0.15 mm, and the film was brought into close contact with a cooling drum to obtain a three-layer unstretched film. The resulting unstretched film was heated to 120° C. with an infrared heater and stretched in one stage in the longitudinal direction at a stretching ratio of 4.0 times. The film was then stretched in the width direction in a tenter-type transverse stretching machine at a preheating temperature of 120°C, a stretching temperature of 130°C, and a stretch ratio of 4.2. In the cooling zone provided before the heat-setting zone following the tenter, aluminum plates attached to the top, bottom, left, and right sides were used to block hot air, and compressed air was sent in to cool the film down to a temperature of 70°C. The film was then heat-set at 245°C, subjected to a 5% thermal relaxation treatment in the width direction, and the surface layer (A) on the side that came into contact with the chill roll was stretched at 40 W·min / m 2 The film was then wound up into a roll using a winder to produce a master roll (60,000 m long, 8,000 mm wide) of biaxially oriented polyester film with a thickness of 12 μm. The biaxially oriented polyester film was unwound from the obtained master roll and slit to a width of 800 mm onto a core with a diameter of 6 inches (152.2 mm). The film roll was wound up while applying surface pressure to the film roll with a contact roll and tension to the film with a two-axis turret winder. The raw material composition and film-forming conditions of the obtained film, as well as the physical properties and evaluation results of the obtained film are shown in Table 2. The film was evaluated on the surface layer (A) on the side that came into contact with the chill roll.

[0105] [Example 2] A biaxially stretched film was produced in the same manner as in Example 1, except that the width direction preheating temperature was changed to 105°C, to obtain a biaxially oriented polyester film having a thickness of 12 µm. The raw material composition and film production conditions of the obtained film, as well as the physical properties and evaluation results of the obtained film, are shown in Table 2. The film was evaluated on the surface layer (A) on the side that came into contact with the chill roll.

[0106] [Example 3] A biaxially stretched film was produced in the same manner as in Example 1, except that the width direction thermal relaxation rate was changed to 8%, to obtain a biaxially oriented polyester film having a thickness of 12 μm. The raw material composition and film production conditions of the obtained film, as well as the physical properties and evaluation results of the obtained film, are shown in Table 2. The film was evaluated on the surface layer (A) on the side that came into contact with the chill roll.

[0107] [Example 4] A biaxially stretched film was produced in the same manner as in Example 1, except that the raw materials were changed so that the polyester resin A in the surface layer (A) was 99.7% by mass and the polyester resin E was 0.3% by mass, and the polyester resin A in the base layer (B) was 99.88% by mass and the polyester resin E was 0.12% by mass, to obtain a biaxially oriented polyester film with a thickness of 12 μm. The raw material composition and film production conditions of the obtained film, as well as the physical properties and evaluation results of the obtained film, are shown in Table 2. The film was evaluated on the surface of Layer A on the side that came into contact with the chill roll.

[0108] [Example 5] A biaxially stretched film was produced in the same manner as in Example 1, except that the raw materials were changed so that the polyester resin A in the surface layer (A) was 72.2% by mass and the polyester resin D was 27.8% by mass, and the polyester resin A in the base layer (B) was 72.2% by mass and the polyester resin D was 27.8% by mass, to obtain a biaxially oriented polyester film with a thickness of 12 μm. The raw material composition and film production conditions of the obtained film, as well as the physical properties and evaluation results of the obtained film, are shown in Table 2. The film was evaluated on the surface of Layer A that came into contact with the chill roll.

[0109] [Example 6] A biaxially stretched film was produced in the same manner as in Example 1, except that the polyester resin A used in the surface layer (A) and base layer (B) in Example 1 was changed to a polyester resin A / polyester resin C ratio of 50 / 50, to obtain a biaxially oriented polyester film with a thickness of 12 μm. The raw material composition and film production conditions of the obtained film, as well as the physical properties and evaluation results of the obtained film, are shown in Table 2. The film was evaluated on the surface layer (A) on the side that came into contact with the chill roll.

[0110] [Example 7] A biaxially stretched film was produced in the same manner as in Example 1, except that the polyester resin A used in the surface layer (A) and base layer (B) in Example 1 was changed to a ratio of polyester resin A / polyester resin C = 10 / 90, to obtain a biaxially oriented polyester film with a thickness of 12 μm. The raw material composition and film production conditions of the obtained film, as well as the physical properties and evaluation results of the obtained film, are shown in Table 2. The film was evaluated on the surface layer (A) on the side that came into contact with the chill roll.

[0111] [Example 8] A biaxially stretched film was produced in the same manner as in Example 1, except that polyester resin A was used for the surface layer (A) and polyester resin C was used for the base layer (B), to obtain a biaxially oriented polyester film having a thickness of 12 μm. The raw material composition and film production conditions of the obtained film, as well as the physical properties and evaluation results of the obtained film, are shown in Table 2. The film was evaluated on the surface layer (A) on the side that came into contact with the chill roll.

[0112] [Example 9] A biaxially stretched film was produced in the same manner as in Example 1, except that polyester resin C was used for the surface layer (A) and polyester resin A was used for the base layer (B), to obtain a biaxially oriented polyester film having a thickness of 12 μm. The raw material composition and film production conditions of the obtained film, as well as the physical properties and evaluation results of the obtained film, are shown in Table 2. The film was evaluated on the surface layer (A) on the side that came into contact with the chill roll.

[0113] [Comparative Example 1] A biaxially stretched film was produced in the same manner as in Example 1, except that the polyester resin for the surface layer (A) was changed to polyester resin B, the longitudinal stretching temperature was 130°C, the width direction preheating temperature was 130°C, and the width direction stretching temperature was 140°C, resulting in a biaxially oriented polyester film with a thickness of 12 μm. The raw material composition and film production conditions of the obtained film, as well as the physical properties and evaluation results of the obtained film, are shown in Table 2. The film was evaluated on the surface layer (A) on the side that came into contact with the chill roll.

[0114] Comparative Example 2 A biaxially stretched film was produced in the same manner as in Example 1, except that a cooling zone was not provided before the heat setting zone in the biaxial stretching step, to obtain a biaxially oriented polyester film having a thickness of 12 μm. The raw material composition and film production conditions of the obtained film, as well as the physical properties and evaluation results of the obtained film, are shown in Table 2. The film was evaluated on the surface layer (A) on the side that came into contact with the chill roll.

[0115] Comparative Example 3 A biaxially stretched film was produced in the same manner as in Example 1, except that the cooling zone temperature was changed to 50°C before the heat setting zone, to obtain a biaxially oriented polyester film having a thickness of 12 μm. The raw material composition and film production conditions of the obtained film, as well as the physical properties and evaluation results of the obtained film, are shown in Table 2. The film was evaluated on the surface layer (A) on the side that came into contact with the chill roll.

[0116] Comparative Example 4 A biaxially stretched film was produced in the same manner as in Example 1, except that the transverse stretching temperature was changed to 145°C, to obtain a biaxially oriented polyester film having a thickness of 12 µm. The raw material composition and film production conditions of the obtained film, as well as the physical properties and evaluation results of the obtained film, are shown in Table 2. The film was evaluated on the surface layer (A) on the side that came into contact with the chill roll.

[0117] Comparative Example 5 A biaxially stretched film was produced in the same manner as in Example 1, except that the raw materials used were: 91.7% by mass of polyester resin A and 8.3% by mass of polyester resin C for the surface layer (A); 91.7% by mass of polyester resin A and 8.3% by mass of polyester resin C for the base layer (B); the longitudinal stretching temperature was 115°C; the width direction stretching temperature was 115°C; the heat setting temperature was 235°C; preheating in the width direction was not performed; and a cooling zone was not provided before the heat setting zone. A biaxially oriented polyester film with a thickness of 12 μm was obtained. The raw material composition and film production conditions of the obtained film, as well as the physical properties and evaluation results of the obtained film, are shown in Table 2. The film was evaluated on the surface of Layer A that contacted the chill roll.

[0118] [Table 2A]

[0119] [Table 2B]

[0120] [Table 2C]

[0121] As shown in Table 2, the films of Examples 1 to 5 had a temperature-rising crystallization temperature, a change in orientation angle in the width direction, a heat shrinkage rate in the longitudinal and width directions across the entire width, and an isophthalic acid component content that were within the specified ranges. Therefore, the films had little variation or distortion in the molecular orientation and heat shrinkage rate in the width direction of the film, and the physical properties in the width direction of the film were uniform, making them suitable for various processing applications. Furthermore, the use of polyester resin made from recycled PET bottles made them environmentally friendly biaxially oriented polyester films.

[0122] In Comparative Example 1, the change in orientation angle in the width direction of the obtained film and the thermal shrinkage rates in the longitudinal and width directions across the entire width were within the specified ranges, making it suitable for various processing applications. However, since the film was made of a conventional polyester resin derived from fossil fuels and had a low content of isophthalic acid components, it was inferior as an environmentally friendly polyester film.

[0123] In Comparative Example 2, no cooling zone was provided before the heat setting zone, and although the heating crystallization temperature, the heat shrinkage rate in the longitudinal and width directions across the entire width, and the content of isophthalic acid component were within the specified ranges, the change in orientation angle in the width direction was large, and the film processability evaluation was poor.

[0124] In Comparative Example 3, the temperature-rising crystallization temperature, the change in orientation angle in the width direction, and the content of isophthalic acid component of the obtained film were within the specified ranges, but the heat shrinkage rates in the longitudinal and width directions across the entire width were outside the specified ranges, and the dimensional stability against heat was poor, so the film processability evaluation was poor.

[0125] In Comparative Example 4, although the temperature-raised crystallization temperature, the change in orientation angle in the width direction, the thermal shrinkage in the longitudinal and width directions across the entire width, and the content of isophthalic acid component were all within the specified ranges, the temperature during width direction stretching was high, making the film prone to breakage during stretching film formation, and it was not possible to stably obtain a master roll with a winding length of 60,000 m, making the film inferior as a film roll for industrial continuous production.

[0126] In Comparative Example 5, although the heating crystallization temperature, the heat shrinkage rate in the longitudinal and width directions across the entire width, and the content of isophthalic acid component were within the specified ranges, the change in orientation angle in the width direction was large, and therefore the film processability evaluation was poor. [Industrial Applicability]

[0127] The biaxially oriented polyester film of the present invention has little variation in molecular orientation and thermal shrinkage rate or distortion in the film width direction, and has uniform physical properties in the film width direction, making it suitable for a variety of processing applications.In addition, by using polyester resin made from recycled PET bottles, it is possible to provide an environmentally friendly biaxially oriented polyester film and a method for producing the same. This technology can be widely applied in the field of packaging films, such as food packaging and gas barrier films, and is expected to make a significant contribution to the industrial sector, given the strong demand for reducing environmental impact these days.

Claims

1. A biaxially oriented polyester film made of a polyester resin composition containing a polyester resin recycled from polyethylene terephthalate containers and particles, the biaxially oriented polyester film having an outermost layer A and a central layer B, at least Layer A containing a polyester resin recycled from polyethylene terephthalate containers and particles, and satisfying all of the following requirements (1) to (4): (1) a temperature-raised crystallization temperature measured by a differential scanning calorimeter (DSC) of 120°C or higher and 139°C or lower; and (2) a change in orientation angle in the width direction of the film of 0° or higher and 10° or lower per 0.5 m. (3) When the film is heat-treated at 150°C for 30 minutes, the thermal shrinkage across the entire width is between 0.5% and 2.0% in the longitudinal direction and between -1.0% and 1.0% in the transverse direction. (4) The content of isophthalic acid components relative to 100 mol% of all dicarboxylic acid components in the polyester resin composition is between 0.01 mol% and 3.0 mol%. In requirements (2) and (3), samples are taken at the center of the entire width and at intervals of 500 mm from the center toward both ends. If 500 mm intervals cannot be ensured near both ends, samples are taken at available edge positions. Requirement (2) requires that the maximum and minimum values ​​of data at each measurement position be within a range, and requirement (3) requires that the maximum change in each amount of change between two adjacent sample collection positions be within a range.

2. 2. The biaxially oriented polyester film according to claim 1, characterized in that the content of polyester resin recycled from polyethylene terephthalate containers in the polyester resin composition constituting the biaxially oriented polyester film is 50% by mass or more and 100% by mass or less.

3. 3. The biaxially oriented polyester film according to claim 1, wherein the polyester resin recycled from the polyethylene terephthalate container is a mechanically recycled polyester resin and / or a chemically recycled polyester resin.

4. 4. A method for producing a biaxially oriented polyester film according to any one of claims 1 to 3, comprising the steps of melt-extruding a polyester raw material resin, biaxially stretching, heat-setting, heat-relaxation, and winding the biaxially oriented polyester film into a roll, wherein in the biaxial stretching step, after the film is stretched in the longitudinal direction, it is stretched in the width direction in a tenter, and a cooling zone is provided before the heat-setting zone.

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