Regenerated biaxially oriented polyester film

Abstract

To provide a regenerated biaxially oriented polyester film which causes little deterioration in quality by coloration and contaminated foreign substances.SOLUTION: A regenerated biaxially oriented polyester film contains 50 mass% or more and 100 mass% or less of a regenerated polyester resin A, wherein the regenerated polyester resin A is a polyester resin obtained by removing a functional layer X from a laminated polyester film having the functional layer X on at least one surface thereof, and satisfies requirements 1 and 2. Requirement 1: A b value of the regenerated polyester resin A is 14.3 or less. Requirement 2: A surface area parameter S of the regenerated polyester A calculated by Expression 1 is 3.0 m2 / kg or more and 15.0 m2 / kg or less. Expression 1: S=Σ((P / 100) / (T×10-6×1400)). S: surface area parameter (m2 / kg) of laminated polyester film as raw material. P: percentage content (mass%) each thickness of laminated polyester film as raw material. T: thickness (μm) of laminated polyester film as raw material.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a recycled polyester film, more specifically to a recycled biaxially oriented polyester film that contains a recycled polyester resin, has excellent color tone, and has little internal foreign matter. [Background technology]

[0002] Conventionally, waste plastics have been disposed of by landfilling, dumping in the ocean, incineration, etc., but it is becoming increasingly difficult to secure landfill sites, and ocean dumping is an environmental problem because waste plastics do not decompose. In addition, while waste plastics can be used as heat by incineration, there is the problem that the carbon dioxide emissions lead to global warming.

[0003] In light of the recent heightening of environmental concerns, there is a need for recycling, such as reusing and regenerating waste plastics, and research and development into this field is being actively carried out. In addition, since most plastics are produced using fossil fuels, there is a need to develop recycling methods in order to make effective use of resources.

[0004] Meanwhile, polyester film, which is a kind of plastic film, is useful as a base film, and is often used as a laminate film having various functional layers laminated on one or both sides. The functional layers include an easy-adhesion layer, a release layer, a hard coat layer, a tacky adhesive layer, a decorative layer, a light-shielding layer, a polarizing layer, an ultraviolet-shielding layer, and the like, and a laminate film in which a material corresponding to the functional layer is laminated on a polyester film is used.

[0005] However, such laminated films have high hurdles to recycling as described below, and are rarely reused after use, but are discarded, incinerated, etc. For example, if a laminated film with a functional layer laminated thereon is remelted as it is to be recycled, the material constituting the functional layer will be mixed into the molten polymer, causing an unpleasant odor during extrusion or a decrease in the melt viscosity of the polymer, which will cause breakage during film formation. Even if a film can be formed, deterioration in quality due to coloring of the resulting film or contamination with foreign matter is inevitable. Even if the functional layer is physically scraped off and removed, and then melt extruded, problems such as clogging of the filter by the remaining functional layer during the filtration process during extrusion and failure to form a normal film will occur.

[0006] As a method for recycling laminated films, there are techniques disclosed in, for example, Patent Documents 1 and 2. The technique of Patent Document 1 is to separate and recover only the base film by dissolving the soluble resin layer and washing with a solvent that does not dissolve the base film after use of a laminated film formed by laminating an easily soluble resin layer and a surface functional layer in this order on at least one side of a base film. The base film separated and recovered by this technique is remelted and recycled as the resin composition that constituted the base film. The technique of Patent Document 2 is a recycled film made from recycled resin generated from a biaxially oriented polyester film in which a polymeric easy-adhesion layer is formed as a functional layer. In the technique of Patent Document 2, a recycled film with little coloring can be obtained by a method such as reducing the content of recycled resin to 40% by weight or less, drying the film raw material under reduced pressure in the absence of oxygen, or melting the recycled resin in the absence of oxygen. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2004-169005 A [Patent Document 2] JP 2001-302994 A Summary of the Invention [Problem to be solved by the invention]

[0008] The method disclosed in Patent Document 1 is based on the premise of a laminated film in which a soluble resin layer and a surface functional layer are laminated in this order on the surface of a base film, as described above, and aims to remove the functional layer by dissolving the soluble resin layer. In other words, it cannot be used for most laminated polyester films that do not have a soluble resin layer, and has a problem in terms of versatility. The method disclosed in Patent Document 2 obtains recycled resin without removing the functional layer, so there is a significant possibility of foreign matter being mixed into the recycled film, and when the content of the recycled resin exceeds 40% by mass, especially when it is 50% by mass or more, there is a problem that the recycled film becomes more discolored.

[0009] In view of the above circumstances, the present invention aims to provide a recycled biaxially oriented polyester film which is less susceptible to deterioration in quality due to discoloration, inclusion of foreign matter, etc. [Means for solving the problem]

[0010] The present invention is intended to solve the above problems and comprises the following configuration. (1) A recycled biaxially oriented polyester film containing 50% by mass or more and 100% by mass or less of recycled polyester resin A, wherein the recycled polyester resin A is a polyester resin obtained by removing a functional layer X from a laminated polyester film having a functional layer X on at least one side thereof, and the recycled biaxially oriented polyester film satisfies requirements 1 and 2. Requirement 1: The b value of recycled polyester resin A is 14.3 or less. Requirement 2: The surface area parameter S of the recycled polyester A calculated by formula 1 is 3.0 m 2 / kg or more 15.0m 2 / kg or less. Equation 1: S=Σ((P / 100) / (T×10 -6 ×1400)) however, S: Surface area parameter of the raw material laminated polyester film (m 2 / kg) P: Content (mass%) of each thickness of the raw material laminated polyester film T: Thickness of the laminated polyester film (μm) (2) The recycled biaxially oriented polyester film according to (1), wherein the functional layer X is a polymeric adhesive layer. (3) The recycled biaxially oriented polyester film according to (1) or (2), wherein the functional layer X contains at least one resin selected from the group consisting of polyester-based resins, acrylic-based resins, and urethane-based resins. (4) The recycled biaxially oriented polyester film according to any one of (1) to (3), wherein the functional layer X contains at least one resin selected from the group consisting of polyester-based resins, acrylic-based resins, and urethane-based resins, and at least one crosslinking agent selected from the group consisting of melamine-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, and carbodiimide-based crosslinking agents. (5) The recycled biaxially oriented polyester film according to any one of (1) to (4), characterized in that it has a functional layer Y on at least one surface thereof. Effect of the Invention

[0011] According to the present invention, it is possible to provide a recycled biaxially oriented polyester film which is less susceptible to deterioration in quality due to discoloration, inclusion of foreign matter, etc. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The recycled biaxially oriented polyester film of the present invention is a recycled biaxially oriented polyester film containing 50% by mass or more and 100% by mass or less of recycled polyester resin A, characterized in that the recycled polyester resin A is a polyester resin obtained by removing a functional layer X from a laminated polyester film having a functional layer X on at least one side thereof, and satisfies requirements 1 and 2. The recycled biaxially oriented polyester film of the present invention will be described in detail below. Requirement 1: The b value of recycled polyester resin A is 14.3 or less. Requirement 2: The surface area parameter S of recycled polyester resin A calculated by formula 1 is 3.0 m 2 / kg or more 15.0m 2 / kg or less. Equation 1: S=Σ((P / 100) / (T×10 -6 ×1400)) however, S: Surface area parameter of the raw material laminated polyester film (m 2 / kg) P: Content (mass%) of each thickness of the raw material laminated polyester film T: Thickness (μm) of the raw laminated polyester film.

[0013] The recycled biaxially oriented polyester film of the present invention is characterized by containing 50% by mass or more and 100% by mass or less of recycled polyester resin A. Here, containing 50% by mass or more and 100% by mass or less of recycled polyester resin A means that the recycled polyester resin A is contained in an amount of 50% by mass or more and 100% by mass or less when the total components of the recycled biaxially oriented polyester film are taken as 100% by mass. By making the content of recycled polyester resin A 50% by mass or more, the environmental load of the recycled biaxially oriented polyester film can be reduced and it is possible to satisfy the Ecomark certification standard. From the viewpoint of recyclability, the content of recycled polyester resin A in the recycled biaxially oriented polyester film is more preferably 80% by mass or more and 100% by mass or less. In addition, the term "film" in the present invention is used to mean a two-dimensional structure mainly composed of a thermoplastic resin, such as a sheet, a plate, and a membrane, and the main component refers to a component contained in an amount of more than 50% by mass when the entire structure is taken as 100% by mass. In the present invention, when a plurality of components corresponding to thermoplastic resins are contained, even if each component is less than 50% by mass, as long as the total of these components exceeds 50% by mass, the thermoplastic resin is treated as the main component.

[0014] <Recycled polyester resin A> The recycled polyester resin A used in the recycled biaxially oriented polyester film of the present invention is a polyester resin obtained by removing a functional layer X from a laminated polyester film having a functional layer X on at least one side. By obtaining the recycled polyester resin A by removing the functional layer X, it is possible to reduce deterioration in quality due to coloration, foreign matter contamination, etc. of the recycled biaxially oriented polyester film of the present invention containing 50% by mass or more of the recycled resin.

[0015] (Functional layer X) The functional layer X is not particularly limited, and examples thereof include an easy-adhesion layer, a release layer, a hard coat layer, a tacky adhesive layer, a decorative layer, a light-shielding layer, a polarizing layer, and an ultraviolet-shielding layer, but is preferably an easy-adhesion layer or a release layer from the viewpoint of easily securing the raw material of the recycled polyester resin A. The easy-adhesion layer or the release layer is often provided by an in-line coating method, and by using scraps and rejected products, which are generated in large quantities in the film manufacturing process, as the raw material of the recycled polyester resin A, it becomes easier to secure the raw material.

[0016] Moreover, it is more preferable that the functional layer X is an easy-adhesion layer. The easy-adhesion layer is often provided on a relatively thick film, and by using this as a raw material, it becomes easier to adjust the surface area parameter S of the recycled polyester resin A described later to a suitable range.

[0017] The easy-adhesion layer that can be used as the functional layer X is not particularly limited, and any known easy-adhesion layer can be used, but it is particularly preferable that the easy-adhesion layer is a polymer easy-adhesion layer from the viewpoint of easy removal of the easy-adhesion layer when obtaining the recycled polyester resin A. Here, the polymer easy-adhesion layer refers to an easy-adhesion layer containing 50% by mass or more and 100% by mass or less of a polymer compound, and the polymer compound refers to a synthetic resin.

[0018] The functional layer X preferably contains at least one resin selected from the group consisting of polyester resins, acrylic resins, and urethane resins. Among them, from the viewpoint of easy removal of the functional layer X when obtaining the recycled polyester resin A, it is preferable to contain a polyester resin or an acrylic resin among the above-mentioned resin group. When the functional layer X contains a polyester resin or an acrylic resin, the functional layer X and the polyester base layer are adhered to each other by hydrogen bonds at the interface, and the functional layer X can be removed relatively easily when obtaining the recycled polyester resin A. On the other hand, when the functional layer X contains a urethane resin, the functional layer X and the polyester base layer are adhered to each other by chemical bonds at the interface, and the functional layer X is difficult to remove when obtaining the recycled polyester resin A, which may cause quality deterioration due to coloring of the recycled film or contamination with foreign matter.

[0019] The functional layer X can contain at least one crosslinking agent selected from the group consisting of melamine-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, and carbodiimide-based crosslinking agents. From the viewpoint of easy removal of the functional layer X when obtaining the recycled polyester resin A, it is preferable to contain a melamine-based crosslinking agent, an epoxy-based crosslinking agent, or an oxazoline-based crosslinking agent among the above-mentioned crosslinking agent groups. When the functional layer X contains a melamine-based crosslinking agent, an epoxy-based crosslinking agent, or an oxazoline-based crosslinking agent, the functional layer X and the polyester base layer are adhered to each other by hydrogen bonds at the interface between the functional layer X and the polyester base layer, and the functional layer X can be removed relatively easily when obtaining the recycled polyester resin A. On the other hand, when the functional layer X contains a carbodiimide-based crosslinking agent, the functional layer X and the polyester base layer are adhered to each other by chemical bonds at the interface between the functional layer X and the polyester base layer, and the functional layer X is difficult to remove when obtaining the recycled polyester resin A, which may cause quality deterioration due to coloring of the recycled film or contamination with foreign matter.

[0020] (Removal of functional layer X) The method for removing the functional layer X is not particularly limited, and known methods such as a method for physically scraping off the functional layer using an abrasive or a method for peeling and removing the functional layer using a cleaning agent can be adopted, but a method for peeling and removing the functional layer X using a cleaning agent is preferred. By removing the functional layer X using a cleaning agent, the recovery rate of the recycled polyester resin A is improved and the remaining functional layer X is reduced. Therefore, when it is made into a recycled biaxially oriented polyester film, it is easy to obtain a high-quality film with little internal foreign matter.

[0021] The cleaner is preferably an aqueous cleaner containing an alkalizing agent (a) and a compound (b) having at least one hydroxyl group. By using an aqueous cleaner containing an alkalizing agent (a) and a compound (b) having at least one hydroxyl group, the functional layer X can be more effectively removed, less functional layer X remains, and a high-quality film with less foreign matter can be obtained when the polyester film is formed.

[0022] (Alkalizing agent (a)) The alkalizing agent (a) is a substance that makes the cleaning agent alkaline and is also called an alkali agent. The alkalizing agent may be an inorganic alkalizing agent or an organic alkalizing agent.

[0023] Examples of inorganic alkalizing agents include hydroxides of alkali metals such as sodium hydroxide, potassium hydroxide, rubidium hydroxide, and cesium hydroxide; hydroxides of alkaline earth metals such as calcium hydroxide and barium hydroxide; carbonates of alkali metals such as sodium carbonate and potassium carbonate; phosphates of alkali metals such as trisodium phosphate, sodium pyrophosphate, sodium tripolyphosphate, sodium tetrapolyphosphate, tripotassium phosphate, potassium pyrophosphate, and potassium tripolyphosphate; silicates of alkali metals such as sodium orthosilicate, sodium metasilicate, and potassium silicate; and ammonia. It is preferable to use hydroxides of alkali metals, and sodium hydroxide and potassium hydroxide are more preferable from the viewpoint of availability, and potassium hydroxide is particularly preferable from the viewpoint of cleaning properties. In addition, the inorganic alkalizing agent can be used alone or in combination of two or more. In particular, it is preferable to use a combination of potassium hydroxide and sodium hydroxide from the viewpoint of effect and handling.

[0024] Examples of organic alkalizing agents include organic amine compounds such as N,N-bis(2-hydroxyethyl)-N-cyclohexylamine, diazabicycloundecene, diazabicyclononene, monomethylamine, dimethylamine, trimethylamine, monoethanolamine, diethanolamine, triethanolamine, morpholine, 2-(dimethylamino)ethanol, 2-(diethylamino)ethanol, 1-amino-2-propanol, and triisopropanolamine, among which monoethanolamine, diethanolamine, and triethanolamine are preferred from the viewpoint of versatility, monoethanolamine and diethanolamine are more preferred from the viewpoint of availability, and monoethanolamine is particularly preferred from the viewpoint of cleaning properties. Note that organic alkalizing agents may include compounds having at least one hydroxyl group, and as long as the acidity constant (pKa) of the compound is 30 or more, it can be treated as an alkalizing agent.

[0025] Furthermore, from the viewpoint of cleaning properties, it is also preferable to use an inorganic alkalizing agent in combination with an organic alkalizing agent. Specifically, a combination of at least one inorganic alkalizing agent selected from sodium hydroxide and potassium hydroxide and at least one organic alkalizing agent selected from monoethanolamine, diethanolamine, triethanolamine, morpholine, 2-(dimethylamino)ethanol, 2-(diethylamino)ethanol, 1-amino-2-propanol, and triisopropanolamine is more preferable, and a combination of at least one inorganic alkalizing agent selected from sodium hydroxide and potassium hydroxide and at least one organic alkalizing agent selected from monoethanolamine and diethanolamine is particularly preferable.

[0026] (Compound (b) having at least one hydroxyl group) Examples of the compound (b) having at least one hydroxyl group include alcohols and phenols.

[0027] Examples of alcohols include monohydric alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol, benzyl alcohol, ethylene glycol monomethyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, and propylene glycol monoethyl ether; dihydric alcohols such as ethylene glycol, diethylene glycol, and propylene glycol; and polyhydric alcohols such as glycerin.

[0028] Examples of phenols include phenol, xylenol, salicylic acid, picric acid, naphthol, catechol, resorcinol, hydroquinone, pyrogallol, phloroglucinol, dibutylhydroxytoluene, bisphenol A, cresol, estradiol, eugenol, gallic acid, guaiacol, phenolphthalein, serotonin, dopamine, adrenaline, noradrenaline, thymol, tyrosine, and hexahydroxybenzene.

[0029] These may be used alone or in combination of two or more kinds. In particular, from the viewpoint of enhancing the peeling and cleaning effect, it is preferable to use two or more kinds in combination, and among these, it is more preferable to use two or more kinds of alcohols in combination.

[0030] Among these, monohydric alcohols are preferred from the viewpoint of maintaining the cleaning ability without impairing the alkalinity of the cleaning agent. In particular, from the viewpoint of cleaning ability, methyl alcohol, ethyl alcohol, propyl alcohol and benzyl alcohol are more preferred. These alcohols have high cleaning ability because protons are easily ionized to generate alkoxides. From the viewpoint of low volatility and usable temperature range, benzyl alcohol is particularly preferred.

[0031] The cleaning agent according to the present invention is preferably an aqueous cleaning agent. The aqueous cleaning agent can increase the flash point, so it is relatively safe, and since the equipment for producing the recycled polyester resin A does not need to be explosion-proof, it is possible to produce the recycled polyester resin A using relatively inexpensive equipment. In addition, since no organic solvent is required, or even if an organic solvent is used, a relatively small amount is sufficient, it is easy to reduce the environmental load.

[0032] In addition to the above-mentioned alkalizing agent (a) and the compound (b) having at least one hydroxyl group, various additives can be blended into the cleaning agent of the present invention. For example, a surfactant, an antioxidant, a rust inhibitor, a pH adjuster, a preservative, a viscosity adjuster, an antifoaming agent, etc. can be added.

[0033] The method for peeling and removing the functional layer X by using a cleaning agent is not particularly limited, but examples thereof include an immersion method in which the functional layer X is immersed in a cleaning tank containing a cleaning agent, a coating method in which a cleaning agent in a solution state is applied, a spraying method in which a cleaning agent in a solution state or a vaporized cleaning agent is sprayed, etc. Among these, it is preferable to adopt the immersion method from the viewpoint of the penetration of the cleaning agent into the functional layer.

[0034] The temperature of the cleaning agent in the immersion method is preferably 20°C or higher. When the temperature of the cleaning agent is 20°C or higher, the viscosity of the cleaning solution is low and it is easy to penetrate into the functional layer, so that good cleaning properties are easily obtained. The temperature of the cleaning agent in the immersion method is more preferably 40°C or higher, even more preferably 50°C or higher, and particularly preferably 60°C or higher. When the cleaning agent is used in a solution state, the upper limit of the temperature of the cleaning agent is a temperature below the boiling point. That is, when a water-based cleaning agent is used as the cleaning agent, the upper limit is 100°C. In addition, microwave irradiation may be performed in the peeling cleaning in the immersion method. The immersion time can be appropriately adjusted depending on the configuration of the functional layer X and the cleaning agent, the temperature of the cleaning agent, etc., and usually, an immersion time of 10 minutes to 60 minutes can sufficiently remove the functional layer X.

[0035] In the immersion method, examples of a method for immersing a laminated polyester film having a functional layer X in a cleaning tank containing a cleaning agent include a method in which the laminated polyester film having a functional layer X is cut into flakes and introduced into the cleaning tank, and a method in which a film roll made of a laminated polyester film having a functional layer X is directly introduced into the cleaning tank while being continuously unwound.

[0036] When a method of cutting a laminated polyester film having a functional layer X into flakes and introducing them into a cleaning tank is adopted, a method of continuously introducing the pre-cut flakes into the cleaning tank using a belt conveyor or the like may be adopted, or a method of continuously unwinding a film roll made of a laminated polyester film having a functional layer X, cutting the film into flakes, and continuously introducing the film directly into the cleaning tank may be adopted. In either case, there are advantages such as making it easier to perform the step of peeling and removing the functional layer X in a batch manner and allowing the immersion time to be set relatively long.

[0037] Furthermore, in the case of adopting a method in which a film roll made of a laminated polyester film having a functional layer X is continuously unwound, cut into flakes, and continuously introduced directly into a cleaning tank, the laminated polyester film is less likely to be contaminated, which has the advantage of facilitating management of raw materials and making it easier to control the surface area parameter S of the recycled polyester resin A described below within a suitable range. In order to efficiently remove the functional layer X from the laminated polyester film, equipment equipped with physical means such as ultrasonic waves, microbubbles, nanobubbles, water flow, and compressed cold air may be provided in the cleaning tank.

[0038] When a method is adopted in which a film roll made of a laminated polyester film having a functional layer X is continuously unwound and directly introduced into a cleaning tank, continuous cleaning processing is easily performed, and high production efficiency is easily obtained. In addition, in order to efficiently remove the functional layer X from the laminated polyester film, as in the method of introducing the cut pieces into a flake shape into a cleaning tank, equipment equipped with physical means such as ultrasonic waves, microbubbles, nanobubbles, water flow, and compressed cold air may be provided in the cleaning tank, but this method has the advantage that it is easy to provide a roll brush, which is difficult to employ in the method of introducing the cut pieces into a flake shape into a cleaning tank, and thus the functional layer X can be more effectively removed.

[0039] (Production of Recycled Polyester Resin A) The recycled polyester resin A used in the recycled biaxially oriented polyester film of the present invention is obtained by removing the functional layer X as described above, but it is preferable to provide a rinsing step, a drying step, and a recycling pelletizing step in this order after removing the functional layer X. Each step will be described below.

[0040] The rinsing step according to the present invention refers to a step of rinsing away the cleaning agent and free components derived from the functional layer X attached to the film from which the functional layer X has been removed, using a rinsing liquid. The rinsing liquid is not particularly limited as long as it can wash away the cleaning agent and free components derived from the functional layer X, but when a water-based cleaning agent is used as the cleaning agent, water can be used as the rinsing liquid. The temperature of the rinsing step is preferably around room temperature from the viewpoint of efficient rinsing away, specifically, preferably 5 to 50°C, more preferably 5 to 30°C. Examples of methods for rinsing away the cleaning agent and free components derived from the functional layer X include a spraying method in which a rinsing liquid is sprayed onto the film from which the functional layer X has been removed, and an immersion method in which the film is immersed in a rinsing tank containing a rinsing liquid.

[0041] The rinsing step according to the present invention may be performed multiple times. As described above, it is a preferred embodiment that the cleaning agent contains an alkalizing agent (a). However, if the alkalizing agent (a) remains after the rinsing step, it may be easy to promote decomposition of the polyester resin during the production of recycled chips or recycled film, and the stability of the production process may deteriorate when producing a recycled biaxially oriented polyester film, and gel foreign matter may increase. Therefore, it is preferable to thoroughly wash away the alkalizing agent (a) by performing the rinsing step multiple times. From the same viewpoint, it is also a preferred embodiment to add an acidifying agent to the rinsing solution for the purpose of neutralizing the alkalizing agent (a). More specifically, a more preferred embodiment can be exemplified in which the rinsing step is performed twice, and the alkalizing agent (a) is neutralized using a rinsing solution to which an acidifying agent has been added in the first rinsing step, and water is used as the rinsing solution in the second rinsing step.

[0042] It is preferable to provide a drying step after the rinsing step. The drying step can remove the cleaning agent and rinsing liquid remaining on the film, and furthermore, it is easier to suppress hydrolysis in the regenerative pelletization step described below. The conditions of the drying step are not particularly limited, and drying at 140 to 160°C for about 4 to 8 hours is usually sufficient. As a drying method, a general method such as heating drying using an infrared heater or oven, hot air drying using a hot air dryer, or microwave heating drying can be used, but from the viewpoint of more efficient drying, it is preferable to dry under reduced pressure using a vacuum dryer.

[0043] It is preferable to provide a regenerated pelletizing process after the drying process. The regenerated pellets can be obtained by the regenerated pelletizing process, and by using these as the regenerated polyester resin A, a regenerated polyester resin A having excellent storage properties and feedability to an extruder during the production of a regenerated film can be obtained. A known method can be adopted for the regenerated pelletizing process. For example, the dried flakes are fed to an extruder set at 290°C, melt-extruded and discharged in the form of a strand, and then cooled with water at a temperature of 25°C and immediately cut to obtain the regenerated pelletized regenerated polyester resin A.

[0044] (b value of recycled polyester resin A) It is important that the recycled polyester resin A used in the recycled biaxially oriented polyester film of the present invention has a b value of 14.3 or less. If the b value of the recycled polyester resin A exceeds 14.3, the coloring of the recycled biaxially oriented polyester film becomes significant and it is not suitable for practical use. In order to make the b value of the recycled polyester resin A 14.3 or less, the surface area parameter S of the recycled polyester resin A described below must be 3.0 m 2 / kg or more 15.0m 2 / kg or less, and appropriately selecting the components constituting the functional layer X as described above is effective (these methods can be used in combination as appropriate). From the above viewpoints, the b value of the recycled polyester resin A is preferably 13.2 or less, more preferably 12.7 or less, and particularly preferably 12.1 or less. In addition, taking into consideration the thermal history that the recycled polyester resin A is subjected to, 9.5 is the substantial lower limit of the b value. The b value of the recycled polyester resin A is measured by the method described in the examples.

[0045] (Surface area parameter S of recycled polyester resin A) The recycled polyester resin A used in the recycled biaxially oriented polyester film of the present invention has a surface area parameter S of 3.0 m 2 / kg or more 15.0m 2 It is important that the surface area parameter S of the recycled polyester resin A is the total value of the surface area of ​​the laminated polyester film that is the raw material when producing the recycled polyester resin A, and is calculated by the following formula 1. Equation 1: S=Σ((P / 100) / (T×10 -6 ×1400)) however, S: Surface area parameter of the raw material laminated polyester film (m 2 / kg) P: Content (mass%) of each thickness of the raw material laminated polyester film T: Thickness (μm) of the raw laminated polyester film.

[0046] For example, when recycled polyester resin A is produced using 50% by mass of a 50 μm-thick laminated polyester film and 50% by mass of a 100 μm-thick laminated polyester film as raw materials, the surface area parameter S of recycled polyester resin A is 10.7 m 2 / kg.

[0047] The surface area parameter S of recycled polyester resin A is 15.0 m 2If the surface area parameter S of the recycled polyester resin A exceeds 15.0 m / kg, the area exposed to oxygen in the air during production of the recycled polyester resin A increases, accelerating oxidation, increasing the b value of the recycled polyester resin A, and causing significant coloring of the recycled biaxially oriented polyester film, making it unsuitable for practical use. 2 In order to achieve a surface area of ​​11.9 m2 / kg or less, it is effective to increase the thickness of the laminated polyester film used as the raw material in producing the recycled polyester resin A, to reduce the content of relatively thin laminated polyester film, or to increase the content of relatively thick laminated polyester film. From the above viewpoint, the upper limit of the surface area parameter S of the recycled polyester resin A is set to 11.9 m2. 2 / kg, and 10.7m 2 / kg is more preferable, and 9.0m 2 The lower limit of the surface area parameter S of the recycled polyester resin A is 3.0 m / kg from the viewpoint of the supplyability of the laminated polyester film used as a raw material in producing the recycled polyester resin A. 2 / kg is the practical lower limit.

[0048] The laminated polyester film used as a raw material for producing the recycled polyester resin A preferably contains 90% by mass or more of laminated polyester film having a thickness of 50 μm or more and 1000 μm or less. By containing 90% by mass or more of laminated polyester film having a thickness of 50 μm or more and 1000 μm or less, the b value of the recycled polyester resin A and coloration of the recycled biaxially oriented polyester film are easily suppressed, and the production efficiency of the recycled polyester resin A is excellent. With regard to the thickness and content of the laminated polyester film, it is more preferable that the content of the laminated polyester film having a thickness of 50 μm or more and 1000 μm or less is 95% by mass or more, further preferably that the content of the laminated polyester film having a thickness of 50 μm or more and 1000 μm or less is 99% by mass or more, and particularly preferably that the content of the laminated polyester film having a thickness of 50 μm or more and 250 μm or less is 99% by mass or more.

[0049] Furthermore, it is also a particularly preferred embodiment that the laminated polyester film used as a raw material in producing the recycled polyester resin A contains 60 mass% or less of films having a thickness of 50 μm or more and less than 75 μm, or does not contain any films having a thickness of 50 μm or more and less than 75 μm.

[0050] <Recycled biaxially oriented polyester film> It is important that the recycled biaxially oriented polyester film of the present invention is biaxially oriented. The term "biaxial orientation" as used herein refers to a film that shows a biaxially oriented pattern in wide-angle X-ray diffraction. A biaxially oriented film can generally be obtained by biaxial stretching, that is, stretching an unstretched sheet by about 2.5 to 5.0 times in each of the longitudinal and transverse directions, and then subjecting it to heat treatment to complete the crystal orientation. In addition, as the biaxial stretching method, a sequential biaxial stretching method or a simultaneous biaxial stretching method may be used. Furthermore, after the biaxial stretching, the film may be stretched again in the longitudinal or transverse direction. By making the film into a biaxially oriented film, it is easy to obtain high crystallinity, and a polyester film with excellent dimensional stability can be obtained, and it can be used in a wide variety of applications such as optical films and films for industrial materials.

[0051] A preferred embodiment of the recycled biaxially oriented polyester film of the present invention has a functional layer Y on at least one side. The functional layer Y is not particularly limited, but at least a part of the components of the functional layer X are common, and the greater the degree of commonality, the more preferable. In other words, the most preferred embodiment is that the functional layer Y and the functional layer X are similar functional layers. By making the functional layer Y and the functional layer X have a high degree of commonality in their components, it becomes easier to reuse scraps and rejected products that are generated in large quantities in the manufacturing process of the recycled biaxially oriented polyester film of the present invention as raw materials for the recycled polyester resin A, which has the advantage of making it easier to establish a circular recycling scheme.

[0052] The recycled biaxially oriented polyester film of the present invention preferably has a film b value of less than 14.0. A low film b value means less coloring, and if the film b value is less than 14.0, it is easy to adopt it to applications where the quality requirements for the film color tone are high, such as optical film applications and decorative film applications. The film b value can be controlled by lowering the b value of the recycled polyester resin A or reducing the content of the recycled polyester resin A, and these methods may be used in combination as appropriate. From the above viewpoint, the film b value is more preferably less than 13.0, and particularly preferably less than 11.0. The lower limit of the film b value is 8.0, which is a substantial lower limit, from the viewpoint of the content of the recycled polyester resin A. The film b value is measured by the method described in the examples.

[0053] The recycled biaxially oriented polyester film of the present invention has an internal foreign matter content of 1,000,000 pieces / m 2 It is preferable that the amount of internal foreign matter is less than 1,000,000 pieces / m 2 If it is less than this, it will be easy to use in applications where there are high quality requirements for the amount of internal foreign matter, such as optical film applications and decorative film applications. The amount of internal foreign matter can be controlled by appropriately adjusting the method for peeling and removing the functional layer X and various conditions when producing recycled polyester resin A, or by reducing the amount of recycled polyester resin A contained. The amount of internal foreign matter is 500,000 pieces / m 2 More preferably, it is less than 200,000 particles / m 2 It is particularly preferable that the amount of internal foreign matter is less than 4,000 pieces / m from the viewpoint of the content of recycled polyester resin A. 2 The internal foreign matter according to the present invention refers to flat or corrugated foreign matter observed inside the film, which is caused by the remaining components of the functional layer X during the production of the recycled polyester resin A, and the amount of the internal foreign matter is measured by the method described in the Examples.

[0054] Internal foreign matter amount: 1,000,000 pieces / m 2Examples of methods for making the recycled polyester resin A less than or within the above preferred range include reducing the content of recycled polyester resin A, lowering the surface area parameter S of recycled polyester resin A, making functional layer X easier to remove by washing, appropriately optimizing the method and conditions for removing functional layer X as described above, and using a laminated polyester film having functional layer X on only one side as the raw material for recycled polyester resin A, and these can be used in combination as appropriate.

[0055] Next, the method for producing the recycled biaxially oriented polyester film of the present invention will be described using an example in which homopolyethylene terephthalate (hereinafter sometimes abbreviated as PET) resin is used as the polyester resin, although the recycled biaxially oriented polyester film of the present invention is not limited thereto.

[0056] First, 50-100% recycled PET pellets and 0-50% PET pellets are dry-blended in a ratio of 0-50% by mass, vacuum-dried, fed into an extruder, melted at 260-300°C, extruded into a sheet from a T-shaped die, wrapped around a mirror-finished casting drum with a surface temperature of 10-60°C using an electrostatic casting method, and cooled and solidified to produce an unstretched PET film. This unstretched film is stretched 2.5-5.0 times in the machine direction (the direction in which the film travels, also called the "longitudinal direction") between rolls heated to 70-100°C. Next, this uniaxially oriented film is held by a clip and introduced into a preheating zone, where it is heated to a temperature of 75 to 95 ° C., and then continuously stretched in the transverse direction (the direction perpendicular to the film's traveling direction in the film plane, also called the "width direction") by 2.5 to 5.0 times, preferably 3.0 to 5.0 times, in a heating zone at 90 to 115 ° C., followed by heat treatment for 5 to 60 seconds in a heating zone at 200 to 240 ° C., and passing through a cooling zone at 100 to 200 ° C. to obtain a biaxially oriented PET film (corresponding to the recycled biaxially oriented polyester film of the present invention) in which crystal orientation has been completed. During the above heat treatment, a 3 to 12% relaxation treatment may be performed as necessary. Biaxial stretching may be either sequential stretching or simultaneous biaxial stretching, and after longitudinal and transverse stretching, re-stretching may be performed in either the longitudinal or transverse direction. The biaxially oriented PET film thus obtained is trimmed and removed at both ends in the width direction, and then wound into an intermediate product, which is then cut into a desired width using a slitter and wound around a cylindrical core to obtain a PET film roll of the desired length. Note that the both ends of the film may be embossed during winding to improve the roll appearance.

[0057] The recycled biaxially oriented polyester film of the present invention may have a functional layer Y on one or both sides. The functional layer Y may be a multi-layer of two or more layers on one side, or when the functional layer Y is provided on both sides, it may be different on one side and the other side. For example, it is possible to complement multiple functions such as easy slip and easy adhesion, or easy slip and oligomer block properties with one functional layer Y, and it is also possible to adopt a configuration in which a functional layer Y that complements easy slip and easy adhesion is provided on one side, a hard coat layer is provided thereon, and a functional layer Y that complements both easy slip and oligomer block properties is provided on the other side. There are no limitations on the coating surface, function, number and type of functions complemented by one functional layer Y, combination of the number of layers, etc.

[0058] When providing a functional layer Y on the surface of the recycled biaxially oriented polyester film of the present invention, the method of providing the functional layer Y can be either a method of providing the functional layer Y by coating in a separate process from the polyester film production process, that is, a so-called offline coating method, or a method of obtaining a polyester film provided with the functional layer Y in one go by coating during the polyester film production process, that is, a so-called in-line coating method. In the recycled biaxially oriented polyester film of the present invention, it is preferable to provide the functional layer Y by the in-line coating method from the viewpoints of cost and uniformity of the coating thickness, and the solvent of the coating liquid used in this case is preferably aqueous from the viewpoints of environmental pollution and explosion resistance.

[0059] When providing a functional layer Y on the surface of the recycled biaxially oriented polyester film of the present invention, a method for producing a polyester film using the inline coating method, in which the following steps 1) to 4) are carried out in this order, is particularly preferred from the viewpoint of expressing the properties of the functional layer Y and increasing productivity. 1) A coating process in which a coating liquid for forming a functional layer Y is applied to at least one side of a base polyester film. 2) A heat drying process in which the coating liquid applied to the base polyester film is heated and dried to form the functional layer Y. 3) A stretching process for stretching the process film on which the functional layer Y has been formed. 4) A heat treatment process in which the stretched process film is heated and heat-treated.

[0060] The coating method in the coating step described in the above item 1) is not particularly limited, and for example, a reverse coating method, a spray coating method, a bar coating method, a gravure coating method, a rod coating method, a die coating method, etc. can be used. From the viewpoint of reducing the thickness unevenness of the functional layer Y, it is preferable to use a gravure coating method and a bar coating method, and it is particularly preferable to use a bar coating method using a measuring bar. Regarding the measuring bar used when using the bar coating method using a measuring bar, its diameter is not particularly limited, but it is usually in the range of 10 to 30 mm. In addition, the method of making a groove for measurement may adopt a wire bar method in which a wire is wound around a cylindrical member, or a method in which a spiral groove is dug on the surface of the member. In addition, from the viewpoint of coating uniformity, it is preferable to use a measuring bar with a deflection of 150 μm or less, and it is particularly preferable to use a measuring bar with a deflection of 100 μm or less.

[0061] In the heat drying step described in item 2), it is preferable to adjust the temperature of the film surface of the functional layer Y side after heat drying to a range of 75 to 95°C. If the temperature of the film surface of the functional layer Y side after heat drying is less than 75°C, the film will not be preheated sufficiently, and the film will tend to be easily torn in the subsequent stretching step. In addition, if the temperature of the film surface of the functional layer Y side after heat drying exceeds 95°C, the uniformity of the functional layer Y during stretching may be impaired, and the thickness of the functional layer Y will tend to vary greatly.

[0062] The heat drying method in the heat drying step is not particularly limited, and examples thereof include a method of blowing hot air and a method of heating with a non-contact heater, but from the viewpoint of uniformity of heat drying, it is preferable to adopt a method of blowing hot air. Furthermore, it is preferable to use a method in which the film surface of the functional layer Y surface after the heat drying step is measured with a non-contact thermometer, and the wind speed and temperature of the hot air in the heat drying step are controlled to a target value determined so that the measurement result falls within the above-mentioned temperature range (if the temperature is higher than the target temperature, the wind speed is reduced or the temperature is reduced; if the temperature is lower than the target temperature, the wind speed is increased or the temperature is increased) in terms of suppressing fluctuations in the thickness of the functional layer Y during long-term continuous production.

[0063] In the in-line coating method, the film is usually stretched transversely after drying by heating in an oven, but since both ends in the width direction are held by clips, the coating is often performed only on the center in the width direction. Therefore, the film temperature during stretching may differ between the center in the width direction of the film where coating is performed and both ends in the width direction of the film where coating is not performed. In this case, both ends in the width direction of the film, which have a relatively high temperature, are more likely to be stretched. In addition, when the functional layer Y contains various crosslinking agents, the coating thickness tends to decrease over time due to clogging of the measuring bar or gravure roll due to long-term continuous production. In this case, the film temperature at the center of the film during transverse stretching increases, so that the center of the film in the width direction tends to be relatively easily stretched, and the effective ratio of the transverse stretching at the center of the width direction of the film increases, so that the thickness of the functional layer Y after transverse stretching may become thin. In view of these actual circumstances, it is a preferred embodiment to control the film temperature after the heat drying process to a constant level over time in the width direction or during continuous production.

[0064] In the stretching step of the above item 3), it is preferable to stretch the process film on which the coating layer has been formed 2.5 to 5.0 times, preferably 3.0 to 5.0 times, in the film width direction while blowing hot air set at 90 to 115°C onto the film. When the temperature of the hot air is 90°C or higher, film breakage during stretching is reduced, while a hot air temperature of 115°C or lower reduces deterioration of the thickness uniformity of the functional layer Y due to heating. In addition, a film transverse stretching ratio of 2.5 times or higher reduces deterioration of strength in the width direction and thickness unevenness in the width direction, while a film transverse stretching ratio of 5.0 times or lower reduces film breakage during stretching.

[0065] In the heat treatment step of the above item 4), a method of blowing hot air set at 200°C to 240°C for 5 to 60 seconds is preferable. In this step, the crystallization of the processed film is promoted, thereby fixing the secondary and tertiary structures of the polymers constituting the film, and the heat deformation resistance, thermal dimensional stability, chemical resistance, etc. of the film can be improved. The temperature and time of the heat treatment step can be adjusted within the above ranges according to the target film properties. EXAMPLES

[0066] The configuration and effects of the present invention will be described in more detail below with reference to examples. Note that the present invention is not limited to the following examples. Before describing each example, the evaluation method for each item will be described.

[0067] (1) Resin b value (-) Test pieces measuring 100 mm wide x 100 mm long x 1 mm thick were prepared using an injection molding machine, and the b value was measured using transmitted light with a colorimeter ("ZE 6000" manufactured by Nippon Denshoku Industries Co., Ltd.) The preparation and measurement of the test pieces were repeated three times, and the average of the obtained values ​​was taken as the b value of the resin.

[0068] (2) Surface area parameter S (m 2 / kg) It was calculated using the following formula 1. Equation 1: S=Σ((P / 100) / (T×10 -6 ×1400)) however, S: Surface area parameter of the raw material laminated polyester film (m 2 / kg) P: Content (mass%) of each thickness of the raw material laminated polyester film T: Thickness of the laminated polyester film (μm) Incidentally, T: the thickness (μm) of the laminated polyester film used as the raw material was measured in the same manner as in (3) below.

[0069] (3) Film thickness (μm) Using a dial gauge ("No2110S-10" manufactured by Mitutoyo Corporation), measurements were taken at any 20 points, and the average of the obtained values ​​was taken as the film thickness (μm).

[0070] (4) Film b value (-) The films were stacked to a total thickness of 1 mm, and this was used as a sample, and the b value was measured using transmitted light with a colorimeter ("SE 2000" manufactured by Nippon Denshoku Industries Co., Ltd.) based on JIS Z 8722 (2000). The preparation and measurement of the sample were repeated three times, and the average of the obtained values ​​was used as the film b value.

[0071] (5) Amount of internal foreign matter (pieces / m 2 ) A film sample was cut into a square of 10 cm square, a rectangular marking of 5 mm x 50 mm was put on one surface, and the surface opposite to the surface with the rectangular marking was filled with an oil-based marker ("Magic Ink" (registered trademark) extra thick black, manufactured by Teranishi Chemical Industry Co., Ltd.) so that the entire marking area was filled. Next, the marking area was observed from the surface with the marking under conditions of 50 times magnification and incident light using a polarizing microscope ("ECLIPSE" (registered trademark) LV100, manufactured by Nikon Corporation). At this time, the focus was adjusted to the film surface and the observation was performed, and if a brightly shining foreign object was observed, the focus was shifted in the depth direction of the film, and if the foreign object was focused inside the film rather than on the film surface, it was judged to be an internal foreign object. The entire marking area was observed, the number of internal foreign objects was counted, and the same evaluation was repeated using another sample, and the average value of three times was converted into an area to determine the amount of internal foreign objects. In each example and each comparative example, the following resins, coating liquids, laminated polyester films, etc. were used.

[0072] (6) Recyclability The recycled biaxially oriented polyester film was evaluated based on the content of recycled polyester resin A according to the following criteria. A: The content of recycled polyester resin A was 80% by mass or more. B: The content of recycled polyester resin A was 50% by mass or more and less than 80% by mass. C: The content of recycled polyester resin A was less than 50% by mass.

[0073] (7) Lack of coloring Based on the evaluation results of the film b value in (4) above, the film was evaluated according to the following criteria. A: The film b value was less than 11.0. B: The film b value was 10.0 or more and less than 13.0. C: The film b value was 13.0 or more and less than 14.0. ·D: The film b value was 14.0 or more.

[0074] (8) Fewer foreign bodies Based on the evaluation results of (5) Amount of internal foreign matter, the evaluation was made according to the following criteria. A: The amount of internal foreign matter is 200,000 pieces / m 2 It was less than. B: The amount of internal foreign matter is 200,000 pieces / m 2 More than 500,000 pieces / m 2 It was less than. C: Internal foreign matter amount is 500,000 pieces / m 2 More than 1 million pieces / m 2 It was less than. D: The amount of internal foreign matter is 1,000,000 pieces / m 2 That was all.

[0075] <Virgin polyester resin B1> PET pellets (intrinsic viscosity 0.63 dl / g) containing no externally added particles were used.

[0076] <Virgin polyester resin B2> PET pellets (intrinsic viscosity 0.63 dl / g) containing 0.03 mass % of silica particles having a particle size of 1.4 μm and 0.01 mass % of silica particles having a particle size of 2.6 μm as externally added particles were used.

[0077] <Coating Liquid A> In a nitrogen gas atmosphere, 40 mol parts of 2,6-naphthalenedicarboxylic acid, 50 mol parts of terephthalic acid, 5 mol parts of sodium 5-sulfoisophthalate as dicarboxylic acid components, 95 mol parts of ethylene glycol and 5 mol parts of diethylene glycol as glycol components were charged into an ester exchange reactor, 100 mol parts of tetrabutyl titanate (catalyst) was added to the total dicarboxylic acid components per 1 million parts by mass, and an esterification reaction was carried out for 5 hours at 160 to 240 ° C., after which the distillate was removed. Thereafter, 5 mol parts of trimellitic acid and 100 mol parts of tetrabutyl titanate were further added to the total dicarboxylic acids per 1 million parts by mass, and the distillate was removed at 240 ° C. until the reaction product became transparent, and then a polycondensation reaction was carried out under reduced pressure at 220 to 280 ° C. to obtain polyester (A). Thereafter, 100.0 parts by mass of polyester (A), 50.0 parts by mass of a melamine-based crosslinking agent (Nicalac (registered trademark) MW12LF manufactured by Nippon Carbide Industries Co., Ltd.: containing 50% by mass of active ingredient and 17% by mass of isopropyl alcohol) calculated as an active ingredient, 2.2 parts by mass of an aqueous dispersion obtained by mixing 40 parts by mass of colloidal silica (particle size 140 nm), 0.6 parts by mass of an aqueous dispersion obtained by mixing 20 parts by mass of silica particles (particle size 300 nm), 3.0 parts by mass of ethylene glycol mono-n-butyl ether, and 92.0 parts by mass of water were mixed to obtain coating liquid A.

[0078] <Coating liquid B> Coating liquid B was obtained in the same manner as coating liquid A, except that acrylic (A) was an acrylic resin copolymer obtained by copolymerizing 64.0 parts by mass of methyl methacrylate, 34 parts by mass of ethyl acrylate, 1 part by mass of acrylic acid, and 1 part by mass of acrylonitrile, and polyester (A) was changed to acrylic (A).

[0079] <Coating liquid C> Coating liquid C was obtained in the same manner as for coating liquid A, except that 100.0 parts by mass of polyester (A) was changed to 70.0 parts by mass of acrylic (A) and 30.0 parts by mass of polyester (A).

[0080] <Coating liquid D> Under a nitrogen gas atmosphere, 70 parts by mass of 1,6-hexane diisocyanate as an aliphatic polyisocyanate compound, 30 parts by mass of polyisobutylene glycol as a polyol compound, 60 parts by mass of acetonitrile and 30 parts by mass of N-methylpyrrolidone as a solvent were charged into a reactor, 0.06 parts by mass of stannous octylate was added as a catalyst, and the reaction liquid temperature was adjusted to 75 to 78 ° C. and reacted for 7 hours to obtain urethane (A). Then, 2.2 parts by mass of an aqueous dispersion obtained by mixing 40 parts by mass of colloidal silica (particle size 140 nm), 0.6 parts by mass of an aqueous dispersion obtained by mixing 20 parts by mass of silica particles (particle size 300 nm), 3.0 parts by mass of ethylene glycol mono-n-butyl ether, and 92.0 parts by mass of water were mixed to obtain coating liquid D.

[0081] <Coating Liquid E> Coating liquid E was obtained in the same manner as for coating liquid A, except that the crosslinking agent was changed from a melamine-based crosslinking agent (Nippon Carbide Industries Co., Ltd.'s "Nicalac" (registered trademark) 50.0 parts by mass (effective ingredient equivalent)) to a carbodiimide compound (Nisshinbo Chemical Inc.'s "Carbodilite" (registered trademark) V02-L2 (solid content concentration 40 mass%, solvent: water)) (effective ingredient equivalent: 50.0 parts by mass).

[0082] <Laminated polyester film> The virgin polyester resin B1 was dried in a vacuum at 160°C for 4 hours, and then fed to an extruder and melt-extruded at 285°C. The molten resin was filtered through a filter with an average opening of 5μm made of sintered and compressed stainless steel fibers, and then filtered through a filter made of sintered stainless steel powder with an average opening of 14μm. The molten resin composition was then extruded into a sheet from a T-shaped die, and wrapped around a mirror-finished casting drum with a surface temperature of 20°C using an electrostatic casting method, and cooled and solidified. The unstretched film thus obtained was preheated to 70°C with a preheating roll, and then heated to 90°C from above and below using a radiation heater, while stretching 3.1 times in the longitudinal direction using the difference in peripheral speed between the rolls, and then cooled to 25°C with a cooling roll to obtain a uniaxially oriented film. Both sides of this uniaxially oriented film were subjected to a corona discharge treatment in air, and the surface tension of the film was set to 55mN / m. Next, at least one coating liquid selected from the group consisting of the coating liquid A, coating liquid B, coating liquid C, coating liquid D, and coating liquid E was appropriately selected (those used in each Example and Comparative Example are shown in Table 1), and was applied to one or both sides of the uniaxially oriented film using a bar coater. The metering wire bar used had a diameter of 13 mm and a wire diameter of 0.1 mm (#4). The uniaxially oriented film coated with the coating liquid was held with a clip and introduced into an oven, where it was heated and dried with hot air at a temperature of 120°C and a wind speed of 20 m / min. It was then continuously introduced into a stretching process, and stretched 3.7 times in the width direction while being heated with hot air at a temperature of 100°C and a wind speed of 15 m / min. The obtained biaxially oriented film was then continuously heat-treated with hot air at a temperature of 230°C and a wind speed of 20 m / min for 15 seconds, and then subjected to a 5% relaxation treatment while being cooled from 230°C to 120°C, and then cooled to 50°C. Then, both ends in the width direction were removed and the film was wound up to obtain a polyester film intermediate product having a film thickness of 38 μm to 250 μm. The polyester film intermediate product was then slit into a width of 1500 mm, and the film was wound up to 2000 m while being neutralized with an AC voltage application type neutralizer at the portion where the film was wound around the core, to obtain a film roll made of a laminated polyester film.

[0083] <Recycled polyester resin A1-18> The laminated polyester film was continuously unwound from a film roll, and cut into flakes of 0.5 to 3 cm square, with an average of about 1.5 cm square, into flakes with an internal volume of about 7 m. 3 1600 kg was added to the cleaning tank, and 4500 L of warm water was added. Next, 480 L of sodium hydroxide aqueous solution (active ingredient 50 mass%) as an alkalizing agent (a), 20 L of ethyl alcohol as a compound (b) having at least one hydroxyl group, and 1 kg of a polyoxyethylene derivative ("Emulgen" (registered trademark) A-500 manufactured by Kao Corporation) as a surfactant were added, and the temperature in the cleaning tank was raised to 90 ° C. while rotating the stirring blade installed in the cleaning tank. While rotating the stirring blade, the cleaning process was performed for 30 minutes after the temperature in the cleaning tank reached 90 ° C., and then, while discharging the cleaning liquid from the lower part of the cleaning tank, pure water was supplied into the cleaning tank at a water supply rate of 1000 L / min from the shower device installed in the upper part of the cleaning tank, and shower washing was performed for 12 minutes. Next, the washed flakes were dried in a vacuum at 160°C for 4 hours, and then fed to an extruder set at 290°C, and the molten resin was filtered through a filter with an average opening of 10 μm made of sintered and compressed stainless steel fibers. The molten resin was then discharged in a strand shape, cooled with water at a temperature of 25°C, and immediately cut to obtain recycled polyester resins A1 to 17. Table 1 shows the contents, film thickness, and blending amount of the functional layer X of the laminated polyester film used in producing recycled polyester resins A1 to 17. Here, the A side in Table 1 means the surface that comes into contact with the casting drum in the film production process, and the B side means the surface that does not come into contact with the casting drum in the film production process. The surface area parameter S and the b value of the resin of the recycled polyester resin A of the recycled polyester resins A1 to 17 were the values ​​shown in Table 1. It should be noted that an attempt was made to produce recycled polyester resin A with the contents shown in Table 1 as recycled polyester resin A18, but the cutting of the film did not go well, and production was not possible.

[0084] [Table 1]

[0085] In the table, laminated polyester films 1 to 3 represent the laminated polyester films that are the raw material for recycled polyester resin A. In addition, since functional layer X is formed by coating and has an extremely thin thickness, the content of laminated polyester films 1 to 3 is substantially maintained in recycled polyester resin A. In addition, sides A and B are optional when both sides are coated, and when only one side is coated, the side to be coated is designated as side A.

[0086] Example 1 As a raw material, a dry blend of recycled polyester resin A1 and virgin polyester resin B1 in a mass ratio of 1:1 was used, dried in a vacuum at 160 ° C for 4 hours, and then fed to an extruder and melt-extruded at 285 ° C. The molten resin was filtered through a filter with an average opening of 5 μm made by sintering and compressing stainless steel fibers, and then through a filter made of sintered stainless steel powder with an average opening of 14 μm, and then extruded into a sheet from a T-shaped die, and wrapped around a mirror-finished casting drum with a surface temperature of 20 ° C. using an electrostatic casting method, and cooled and solidified. The unstretched film thus obtained was preheated to 70 ° C. with a preheating roll, and then heated to 90 ° C. from above and below using a radiation heater, while stretching 3.1 times in the longitudinal direction using the difference in peripheral speed between the rolls, and then cooled to 25 ° C. with a cooling roll to obtain a uniaxially oriented film. Corona discharge treatment was performed on both sides of this uniaxially oriented film in air, and the surface tension of the film was set to 55 mN / m. Next, the coating liquid A was applied to both sides of the uniaxially oriented film as the functional layer Y using a bar coater. The metering wire bar used had a diameter of 13 mm and a wire diameter of 0.1 mm (#4). The uniaxially oriented film coated with the coating liquid was held with a clip and introduced into an oven, where it was heated and dried with hot air at a temperature of 120°C and a wind speed of 20 m / min. It was then continuously introduced into a stretching process, and stretched 3.7 times in the width direction while being heated with hot air at a temperature of 100°C and a wind speed of 15 m / min. The obtained biaxially oriented film was then continuously heat-treated with hot air at a temperature of 230°C and a wind speed of 20 m / min for 15 seconds, and then subjected to a 5% relaxation treatment while cooling from 230°C to 120°C, and then cooled to 50°C. The two ends in the width direction were then trimmed and removed, and the film was wound up to obtain a polyester film intermediate product with a film thickness of 50 μm. The polyester film intermediate product was then slit to a width of 1500 mm, and the film was wound up to 2000 m while being neutralized with an AC voltage application type neutralizer at the portion where the film was wound around the core, to obtain a film roll made of the recycled biaxially oriented polyester film of the present invention. The evaluation results of the recycled biaxially oriented polyester film of the present invention obtained here are shown in Table 2.

[0087] (Examples 2 to 29) A film roll made of the recycled biaxially oriented polyester film of the present invention was obtained in the same manner as in Example 1, except that the raw materials, functional layer Y, and film thickness were changed as shown in Table 2. In addition, side A in Table 2 means the surface that comes into contact with the casting drum in the film manufacturing process, and side B means the surface that does not come into contact with the casting drum in the film manufacturing process. The evaluation results of the recycled biaxially oriented polyester film of the present invention obtained here are shown in Table 2.

[0088] (Comparative Examples 1 to 9) A film roll made of a biaxially oriented polyester film was obtained in the same manner as in Example 1, except that the raw materials, functional layer Y, and film thickness were changed to those shown in Table 2. The evaluation results of the biaxially oriented polyester film obtained here are shown in Table 2.

[0089] [Table 2] [Industrial Applicability]

[0090] According to the present invention, it is possible to provide a recycled biaxially oriented polyester film that is less susceptible to deterioration in quality due to coloration, contamination with foreign matter, etc. Because of its excellent properties, the recycled biaxially oriented polyester film of the present invention can be suitably used for optical film applications, decorative film applications, industrial material film applications, etc.

Claims

1. A recycled biaxially oriented polyester film containing 50% by mass or more and 100% by mass or less of recycled polyester resin A, A recycled biaxially oriented polyester film, characterized in that the recycled polyester resin A is a polyester resin obtained by removing a functional layer X from a laminated polyester film having a functional layer X on at least one side thereof, and satisfies requirements 1 and 2. Requirement 1: The b value of the recycled polyester resin A is 14.3 or less. Requirement 2: The surface area parameter S calculated by Equation 1 is 3.0 m ２ / kg or more 15.0m ２ / kg or less. Formula 1: S=Σ((P / 100) / (T×10-6×1400)) however, S: Surface area parameter (m ２ / kg) P: Content (mass%) per thickness of the raw material laminated polyester film T: Thickness (μm) of the raw material laminated polyester film

2. The recycled biaxially oriented polyester film according to claim 1, wherein the functional layer X is a polymeric easy-adhesion layer.

3. 3. The recycled biaxially oriented polyester film according to claim 1, wherein the functional layer X contains at least one resin selected from the group consisting of polyester resins, acrylic resins, and urethane resins.

4. 3. The recycled biaxially oriented polyester film according to claim 1, wherein the functional layer X contains at least one resin selected from the group consisting of polyester-based resins, acrylic-based resins, and urethane-based resins, and at least one crosslinking agent selected from the group consisting of melamine-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, and carbodiimide-based crosslinking agents.

5. The recycled biaxially oriented polyester film according to claim 1 or 2, characterized in that it has a functional layer Y on at least one side.