Polyester film and method for recycling polyester film

Abstract

The present invention addresses the problem of providing a polyester film which has excellent material recyclability. A polyester film according to the present invention comprises: a base material film that contains a polyester resin; and a functional layer that is provided on at least one surface of the base material film. The self-hydrolysis resistance parameter SHS represented by formula 1 satisfies 1.50 or less.　Formula 1: SHS = 0.245([IV]f -1.47 - [IV]i -1.47) (In the formula, [IV]i represents the limiting viscosity IV of the polyester film. [IV]f represents the limiting viscosity IV after heat-treating the polyester film at 300°C for 2 hours in a nitrogen atmosphere.)

Description

Polyester film and method for recycling polyester film

[0001] The present invention relates to a polyester film having excellent recyclability in material recycling and a recycling method thereof.

[0002] Films including functional layers having various functions on the surface of a substrate film such as a synthetic resin are used in fields such as electronic components, optical components, labels, and release agents. Among these films, used films, films that do not meet specifications, and films that have been damaged during distribution are usually discarded (hereinafter, such films may be referred to as films to be discarded). However, in order to make effective use of resources, it is desirable to recycle films to be discarded as raw materials for substrate films.

[0003] An example of such a technology is disclosed in Patent Document 1. Patent Document 1 discloses that a water-soluble resin layer is provided as an undercoat layer for the functional layer, and the water-soluble layer is dissolved during a process of washing the film in a water bath after use, thereby removing the functional layer from the base film. This process allows only the base film to be recovered, and a highly pure recycled raw material can be obtained.

[0004] Japanese Patent Application Laid-Open No. 2021-115862

[0005] However, the method of Patent Document 1 requires a large amount of cleaning solution to remove the functional layer, and the waste liquid after the cleaning process must be purified to a level that allows it to be discharged, which results in a problem of a large environmental impact. The inventors have considered solving the problem of the large environmental impact caused by the waste liquid treatment of Patent Document 1 by material recycling a film to be discarded with the functional layer remaining without performing a process of removing the functional layer. As a result, they have found that, depending on the type and amount of the functional layer remaining in the film to be discarded, thermal degradation of the base resin may occur when the base film is material recycled, and that resin degradation may reduce material recyclability, such as by reducing the melt viscosity due to a decrease in molecular weight, resulting in deterioration of film formability, or by causing a decrease in quality due to coloration.

[0006] An object of the present invention is to provide a polyester film having excellent material recyclability. More specifically, an object of the present invention is to provide a polyester film that suppresses thermal degradation during material recycling and is excellent as a recycled raw material. A preferred object of the present invention is to provide a polyester film having excellent material recyclability for used polyester films that still have functional layers, and a method for recycling the polyester film.

[0007] The gist of the present invention is as follows: [1] A polyester film comprising a base film containing a polyester resin and a functional layer provided on at least one surface of the base film, wherein the autohydrolysis resistance parameter SHS expressed by the following formula 1 satisfies 1.50 or less: SHS = 0.245 {[IV] ｆ －１．４７ -[IV] ｉ －１．４７}...Formula 1 (wherein, [IV] ｉ indicates the intrinsic viscosity IV of the polyester film. [IV] ｆindicates the intrinsic viscosity IV after the polyester film is heat-treated at 300°C for 2 hours under a nitrogen atmosphere.) [2] The polyester film according to [1], which has a functional layer containing a melamine resin, and the content of the functional layer is 1 ppm or more and 20,000 ppm or less relative to the total mass of the polyester film. [3] The polyester film according to [2], wherein the functional layer containing a melamine resin is a release layer. [4] The polyester film according to [2] or [3], which has a functional layer containing a melamine resin on one surface of the base film, and wherein a layer forming the other surface of the base film (e.g., surface layer B described below) contains inorganic particles. [5] The polyester film according to [1], which has a functional layer containing an acrylic resin, and the content of the functional layer is 1 ppm or more and 11,000 ppm or less relative to the total mass of the polyester film. [6] The polyester film according to [5], wherein the functional layer containing an acrylic resin is a lubrication layer. [7] The polyester film according to [5] or [6], wherein the functional layer containing an acrylic resin further contains a crosslinking agent. [8] The polyester film according to [1], wherein one surface of the base film has a functional layer containing a melamine resin as the functional layer, and the other surface of the base film has a functional layer containing an acrylic resin as the functional layer, the total content of the functional layers being 1 ppm to 11,000 ppm relative to the total mass of the polyester film. [9] The polyester film according to [8], wherein the functional layer containing a melamine resin is a release layer, and the functional layer containing an acrylic resin is a lubrication layer.

[10] The polyester film according to [1], wherein one surface of the base film has a functional layer containing a silicone resin as the functional layer, and the other surface of the base film has a functional layer containing an acrylic resin as the functional layer, the content of the functional layer containing an acrylic resin being 1 ppm to 11,000 ppm relative to the total mass of the polyester film.

[11] The polyester film according to

[10] , wherein the functional layer containing a silicone resin is a release layer, and the functional layer containing an acrylic resin is a lubrication layer.

[12] The polyester film according to any one of [1] to

[11] , which is used for release film applications.

[13] A method for using the polyester film according to any one of [1] to

[11] as a release film.

[14] A method for recycling a polyester film, in which a used polyester film is converted back into a polyester film through a melting step and an extrusion step, comprising a determining step of determining the autohydrolysis resistance parameter SHS represented by Equation 1 prior to melting the used polyester film, and not subjecting a polyester film having an autohydrolysis resistance parameter SHS of more than 1.50 to the melting step. SHS = 0.245 {[IV]. ｆ －１．４７ -[IV] ｉ －１．４７}...Formula 1 (wherein, [IV] ｉ indicates the intrinsic viscosity IV of the polyester film. [IV] ｆ indicates the intrinsic viscosity IV after the polyester film is heat-treated at 300°C for 2 hours under a nitrogen atmosphere.

[0008] According to the present invention, a polyester film having excellent material recyclability and a method for recycling the polyester film can be provided. According to a preferred embodiment of the present invention, a polyester film having excellent material recyclability and a method for recycling the polyester film can be provided for a used polyester film having a functional layer still attached thereto.

[0009] 1. Polyester Film The polyester film of the present invention comprises a substrate film containing a polyester resin and a functional layer provided on at least one surface of the substrate film. The polyester film of the present invention is characterized in that the autohydrolysis resistance parameter SHS, expressed by Equation 1, satisfies 1.50 or less: SHS = 0.245 {[IV] ｆ －１．４７ -[IV] ｉ －１．４７}...Formula 1 (wherein, [IV] ｉ indicates the intrinsic viscosity IV of the polyester film. [IV] ｆindicates the intrinsic viscosity IV after the polyester film is heat-treated at 300°C for 2 hours under a nitrogen atmosphere. To measure the intrinsic viscosity IV of a polyester film, a solvent capable of dissolving the polyester film may be used. A preferred solvent is a mixed solvent of phenol / tetrachloroethane (mass ratio 60:40). The intrinsic viscosity can be measured using a capillary viscometer such as an Ubbelohde viscometer.

[0010] The autohydrolysis resistance parameter SHS of the present invention is useful as an index for indicating whether a polyester film can be regenerated as a polyester film by melting and extruding it in the material recycling of the polyester film. That is, a polyester film having an autohydrolysis resistance parameter SHS expressed by Equation 1 of more than 1.50 is considered to be a material with poor material recyclability and cannot be recycled, whereas a polyester film having an autohydrolysis resistance parameter SHS expressed by Equation 1 of 1.50 or less is considered to be a material with excellent material recyclability and can be recycled.

[0011] The autohydrolysis resistance parameter SHS is 1.50 or less, preferably 1.45 or less, more preferably 1.40 or less, even more preferably 1.35 or less, and even more preferably 1.30 or less, and is, for example, 0.25 or more, preferably 0.30 or more, and more preferably 0.35 or more. That is, the autohydrolysis resistance parameter SHS is preferably 0.25 to 1.50, more preferably 0.30 to 1.45, even more preferably 0.35 to 1.40, even more preferably 0.35 to 1.35, and particularly preferably 0.35 to 1.30. When the autohydrolysis resistance parameter SHS satisfies the above range, the polyester film can be recycled as a recyclable material.

[0012] Intrinsic viscosity of polyester film [IV] ｉis preferably 0.45 dl / g or more, more preferably 0.50 dl / g or more, even more preferably 0.55 dl / g or more, still more preferably 0.57 dl / g or more, and is preferably 0.85 dl / g or less, more preferably 0.80 dl / g or less, even more preferably 0.75 dl / g or less, still more preferably 0.70 dl / g or less, and particularly preferably 0.65 dl / g or less. That is, [IV] ｉ is preferably 0.45 to 0.85 dl / g, more preferably 0.50 to 0.80 dl / g, even more preferably 0.55 to 0.75 dl / g, still more preferably 0.57 to 0.70 dl / g, and particularly preferably 0.57 to 0.65 dl / g. [IV] ｉ When the above range is satisfied, the polyester film is more useful in terms of recycling. ｉ When the viscosity is 0.45 dl / g or more, breakage is less likely to occur in the stretching step of recycling. ｉ When the viscosity is 0.85 dl / g or less, the cutting property when cutting into a predetermined width of a recycled product is good, and dimensional defects are unlikely to occur.

[0013] Intrinsic viscosity of polyester film [IV] ｆ is preferably more than 0.23 dl / g, more preferably 0.24 dl / g or more, even more preferably 0.25 dl / g or more, still more preferably 0.26 dl / g or more, and is preferably 0.70 dl / g or less, more preferably 0.65 dl / g or less, even more preferably 0.60 dl / g or less, still more preferably 0.55 dl / g or less, particularly preferably 0.50 dl / g or less, and most preferably 0.43 dl / g or less. That is, [IV] ｆ is preferably more than 0.23 dl / g and not more than 0.70 dl / g, more preferably 0.24 to 0.65 dl / g, even more preferably 0.25 to 0.60 dl / g, still more preferably 0.26 to 0.55 dl / g, particularly preferably 0.26 to 0.50 dl / g, and most preferably 0.26 to 0.43 dl / g. [IV] ｆ When the above range is satisfied, the polyester film is more useful in terms of recycling.

[0014] The polyester film of the present invention is preferably one that can be used for recycling, and more preferably one that can be used for material recycling.

[0015] The substrate film (also referred to as substrate polyester film) included in the polyester film of the present invention is preferably a film made of a polyester resin such as polyethylene terephthalate or polyethylene naphthalate, and more preferably a film made of polyethylene terephthalate.

[0016] Although polyethylene terephthalate may be copolymerized with a small amount of other dicarboxylic acid components or diol components than ethylene terephthalate, it is preferable that the polyethylene terephthalate is composed only of terephthalic acid and ethylene glycol.

[0017] The substrate film and the polyester film may be either a uniaxially oriented polyester film or a biaxially oriented polyester film.

[0018] The base film of the polyester film preferably has a thickness of 12 to 80 μm, more preferably 12 to 50 μm, and even more preferably 15 to 31 μm.

[0019] The substrate film of the polyester film may be a single layer or a multilayer structure of two or more layers. In the case of a polyester film used in applications requiring smoothness, such as a release film for the production of electronic components, the substrate film of the polyester film is preferably a laminated polyester film having a surface layer A substantially free of inorganic particles on at least one side. In the case of a laminated polyester film having a multilayer structure of two or more layers, it is preferable to have a surface layer B, which may contain particles, on the side opposite the surface layer A, which substantially does not contain inorganic particles. The layer structure is such that the layer on the side where the functional layer (preferably a release layer) is provided is the surface layer A, the layer on the opposite side is the surface layer B, and the other core layer is the core layer C. The layer structure in the thickness direction is a laminate structure such as surface layer A / surface layer B, or surface layer A / core layer C / surface layer B. In the case of a polyester film having a functional layer, the layer structure in the thickness direction may be a laminate structure such as functional layer (release layer) / surface layer A / surface layer B, or functional layer (release layer) / surface layer A / core layer C / surface layer B. The core layer C may be a single layer or a multilayer structure consisting of multiple layers. The surface layer B may also be a layer that does not contain particles. In this case, in order to impart slipperiness for winding the film into a roll, it is preferable to provide a functional lubrication layer on the surface layer B. The lubrication layer may be a coating layer D that contains particles and a binder.

[0020] In the present invention, "substantially free of inorganic particles" is defined as the amount of inorganic particles being 50 ppm or less relative to the total mass of the polyester film having a functional layer when inorganic elements are quantified by fluorescent X-ray analysis. The inorganic particle content of the layer that is substantially free of inorganic particles is preferably 10 ppm or less, and most preferably below the detection limit, relative to the total mass of the polyester film having a functional layer.

[0021] When the substrate film of the polyester film is a laminate film, the surface layer B, which forms the opposite side of the surface layer A on which a functional layer (particularly a release layer) is provided, preferably contains inorganic particles, more preferably silica particles and / or calcium carbonate particles, from the viewpoints of the film's slipperiness and ease of air escape, when no lubrication layer is provided as a functional layer. When the surface layer B contains particles (preferably inorganic particles), the particle content is preferably 500 to 20,000 ppm, more preferably 500 to 15,000 ppm, and may even be 3,000 to 15,000 ppm, in total, based on the total mass of the surface layer B. In one embodiment, the surface layer B contains 500 to 10,000 ppm of silica particles and / or calcium carbonate particles, based on the total mass of the surface layer B. When surface layer B contains silica particles and / or calcium carbonate particles within the above range, the entire layer of the base film (preferably a polyethylene terephthalate film) contains 500 to 10,000 ppm of silica particles and / or calcium carbonate particles relative to the total mass of surface layer B, which is mainly composed of polyester (preferably polyethylene terephthalate). Herein, when referring to the content of particles contained in all layers or when referring to the same meaning, if the base film is a laminate film, it means the total amount of particles contained in surface layer A, surface layer B, and the core layer C, which is provided as needed.

[0022] When the surface layer B does not contain particles, it is also preferable to provide the surface layer B with a functional lubrication layer (for example, a coating layer D containing particles) to provide lubricity.

[0023] <Functional Layer> The polyester film of the present invention preferably comprises a functional layer on at least one surface of the substrate film. The polyester film of the present invention more preferably comprises a functional layer on the surface of the surface layer of a laminate film or on the surface of a monolayer film (substrate layer). Here, the laminate film or monolayer film corresponds to the substrate film. The functional layer is preferably one or more layers selected from a lubrication layer, a release layer, an adhesion layer, and an antistatic layer, and the polyester film more preferably comprises at least a release layer as the functional layer. The total content of the functional layers (total functional layer concentration) is preferably 24,000 ppm or less, more preferably 20,000 ppm or less, even more preferably 19,500 ppm or less, and even more preferably 19,000 ppm or less, based on the total mass of the polyester film. When a functional layer containing a melamine resin is included as a functional layer, the content of the functional layer containing the melamine resin is preferably 1 ppm to 20,000 ppm, more preferably 1 ppm to 19,500 ppm, and even more preferably 1 ppm to 19,000 ppm, relative to the total mass of the polyester film. When a functional layer containing a melamine resin is included as a functional layer, the total content of the functional layer (including the content of the functional layer, if any, other than the melamine resin-containing functional layer) is preferably 1 ppm to 20,000 ppm, more preferably 1 ppm to 19,500 ppm, and even more preferably 1 ppm to 19,000 ppm, relative to the total mass of the polyester film. When a functional layer containing an acrylic resin is included as a functional layer, the content of the functional layer containing the acrylic resin is preferably 1 ppm to 11,000 ppm, more preferably 3,000 ppm to 11,000 ppm, relative to the total mass of the polyester film. When the functional layer contains an acrylic resin, the total content of the functional layer is preferably 1 ppm or more and 11,000 ppm or less relative to the total mass of the polyester film. The lower limit is preferably 3,000 ppm or more, and when the functional layer contains an acrylic resin, the total content of the functional layer is more preferably 3,000 ppm or more and 11,000 ppm or less relative to the total mass of the polyester film.When the functional layer includes a functional layer containing a melamine resin and a functional layer containing an acrylic resin (preferably, a functional layer containing a melamine resin on one surface of the base film and a functional layer containing an acrylic resin on the other surface of the base film), the total content of the functional layer containing the melamine resin and the functional layer containing the acrylic resin is preferably 1 ppm to 11,000 ppm, more preferably 2,000 ppm to 10,000 ppm, even more preferably 3,000 ppm to 9,000 ppm, and still more preferably 4,000 ppm to 9,000 ppm. When the functional layer includes a functional layer containing a melamine resin and a functional layer containing an acrylic resin, the total content of the functional layers is preferably 11,000 ppm or less, more preferably 10,000 ppm or less, and even more preferably 9,000 ppm or less, relative to the total mass of the polyester film. The lower limit may be 1 ppm or more, preferably 2000 ppm or more, more preferably 3000 ppm or more, and even more preferably 4000 ppm or more. These upper and lower limits can be appropriately combined, and when the functional layer includes a functional layer containing a melamine resin and a functional layer containing an acrylic resin, the total content of the functional layers is preferably 1 ppm or more and 11000 ppm or less, more preferably 2000 ppm or more and 11000 ppm or less, even more preferably 3000 ppm or more and 10000 ppm or less, and even more preferably 4000 ppm or more and 9000 ppm or less, based on the total mass of the polyester film. When the functional layer comprises a functional layer containing a silicone resin and a functional layer containing an acrylic resin (preferably, a functional layer containing a silicone resin is provided on one surface of the base film and a functional layer containing an acrylic resin is provided on the other surface of the base film), the content of the functional layer containing an acrylic resin is preferably 1 ppm or more and 11,000 ppm or less relative to the total mass of the polyester film.The lower limit is preferably 3000 ppm or more, and when the functional layer includes a functional layer containing a silicone resin and a functional layer containing an acrylic resin, the content of the functional layer containing the acrylic resin is more preferably 3000 ppm or more and 11000 ppm or less relative to the total mass of the polyester film. When the functional layer includes a melamine resin, if the functional layer concentration is 20000 ppm or less, the autohydrolysis resistance parameter SHS is unlikely to exceed 1.50, resin deterioration during material recycling is suppressed, and when a substrate film is re-formed using recycled raw materials obtained by material recycling, film formability is improved. In other words, if the functional layer concentration is 20000 ppm or less, the SHS is unlikely to exceed 1.50, and the polyester film can be recycled with good material recyclability. On the other hand, when the functional layer includes an acrylic resin, if the content of the acrylic resin-containing functional layer is 11,000 ppm or less, the autohydrolysis resistance parameter SHS is unlikely to exceed 1.50, resin deterioration during material recycling is suppressed, and when a substrate film is again formed using recycled raw materials obtained by material recycling, film formability is improved. In other words, if the content of the acrylic resin-containing functional layer is 11,000 ppm or less, the SHS is unlikely to exceed 1.50, and the polyester film can be recycled with good material recyclability.

[0024] Furthermore, when the functional layer has a functional layer containing an acrylic resin and a functional layer containing a melamine resin, if the total content of the functional layer containing an acrylic resin and the functional layer containing a melamine resin is 11,000 ppm or less, the autohydrolysis resistance parameter SHS is unlikely to exceed 1.50, resin deterioration during material recycling is suppressed, and when a substrate film is again formed using recycled raw materials obtained by material recycling, film formability is improved. In other words, if the total content of the functional layer containing an acrylic resin and the functional layer containing a melamine resin is 11,000 ppm or less, the SHS is unlikely to exceed 1.50, and the polyester film can be recycled with good material recyclability.

[0025] When a polyester film has a release layer as a functional layer, the surface (front surface) of the base film of the polyester film on which the release layer is provided may be classified as the front surface (front face) or first surface of the base film, and the surface opposite to the surface on which the release layer is provided (preferably the surface on which the lubrication layer is provided) may be classified as the back surface or second surface of the base film.

[0026] The thickness of each functional layer is preferably 1 nm or more, more preferably 2 nm or more, even more preferably 3 nm or more, and even more preferably 4 nm or more, relative to a 100 μm thickness of the polyester film. It is preferably less than 1930 nm, more preferably 1900 nm or less, even more preferably 1800 nm or less, even more preferably 1700 nm or less, and particularly preferably 1600 nm or less. That is, the thickness of each functional layer is preferably 1 nm or more but less than 1930 nm, more preferably 2 to 1900 nm, even more preferably 3 to 1800 nm, even more preferably 4 to 1700 nm, and particularly preferably 4 to 1600 nm, relative to a 100 μm thickness of the polyester film. When the polyester film of the present invention has a functional layer having the above-described thickness range, it is likely to have an autohydrolysis resistance parameter (SHS) of 1.50 or less.

[0027] <Slippery Layer> The polyester film of the present invention may have a slippery layer on at least one surface (preferably one surface) of the base film containing the above-mentioned polyester resin. The slippery layer preferably contains at least a binder resin. The slippery layer may further contain particles. The slippery layer is a layer provided to improve the slip properties of the front and back surfaces of the polyester film.

[0028] (Binder Resin in Slippery Layer) The binder resin constituting the slippery layer in the present invention is not particularly limited, but preferably contains an acrylic resin.

[0029] The content of the acrylic resin in the lubrication layer is preferably 20% by mass or more and 95% by mass or less of the total solid content. More preferably, it is 30% by mass or more and 90% by mass or less. If it is 20% by mass or more, the crosslinking component (e.g., carboxyl group) is not too small, and the crosslinking density is not low, which is preferable. If it is 95% by mass or less, when the lubrication layer contains a crosslinking agent, the amount of the crosslinking agent to be crosslinked is not too small, and the crosslinking density is not low, which is preferable.

[0030] (Particles in the Slippery Layer) The slippery layer preferably contains particles (also called lubricant particles) to impart slipperiness to the surface. The particles may be inorganic or organic. Examples of particles include (1) inorganic particles such as silica and calcium carbonate, and (2) organic particles such as acrylic particles. Silica is particularly preferably used to impart appropriate slipperiness to the slippery layer.

[0031] (Crosslinking Agent in the Slippery Layer) To form a crosslinked structure in the slippery layer, the slippery layer preferably contains a crosslinking agent, more preferably at least one crosslinking agent selected from an oxazoline-based crosslinking agent or a carbodiimide-based crosslinking agent. The inclusion of a crosslinking agent (particularly an oxazoline-based crosslinking agent and / or a carbodiimide-based crosslinking agent) improves adhesion to a substrate film containing a polyester resin and promotes crosslinking with the carboxyl groups of the acrylic resin in the slippery layer, thereby improving the coating strength of the slippery layer and, as a result, suppressing unwinding electrification when the polyester film roll is unwound. The content of the crosslinking agent in the slippery layer is preferably 5% by mass or more and 80% by mass or less of the total solids content. More preferably, it is 10% by mass or more and 70% by mass or less. A content of 5% by mass or more is preferred because it prevents a decrease in the crosslink density of the resin in the slippery layer. A content of 80% by mass or less is preferred because it prevents a decrease in the amount of carboxyl groups in the acrylic resin, which is the target of crosslinking by the crosslinking agent, resulting in a low crosslink density.

[0032] <Release Layer> The release layer is preferably formed from a resin or a mixture of a resin and a release agent. Materials used as the resin of the release layer may include resins such as silicone resin, cyclic olefin resin, acyclic olefin resin, fluororesin, and melamine resin, and preferably includes silicone resin and / or melamine resin.

[0033] The silicone resin is a resin having a silicone structure in the molecule, and examples of the resin to be used include UV-curable silicones manufactured by Shin-Etsu Chemical Co., Ltd. (X62-7028A / B, X62-7052, X62-7205, X62-7622, ​​X62-7629, X62-7660, etc.), UV-curable silicones manufactured by Momentive Performance Materials, Inc. (TPR6502, TPR6501, TPR6500, UV9300, UV9315, XS56-A2982, UV9430, etc.), and UV-curable silicones manufactured by Arakawa Chemical Industries, Ltd. (Silicolyse UV POLY200, POLY215, POLY201, KF-UV265AM, etc.).

[0034] Examples of the melamine resin include alkylated melamine formaldehyde resins such as melamine formaldehyde resin, methylated melamine formaldehyde resin, and butylated melamine formaldehyde resin, etherified melamine formaldehyde resin, epoxy-modified melamine formaldehyde resin, urea melamine resin, and acrylic melamine resin.

[0035] When a melamine resin is used in the release layer, it may be combined with a release agent. The release agent is preferably a silicone-based release agent. As the silicone-based release agent, polyorganosiloxane or the like can be suitably used.

[0036] Whether the melamine resin is used alone or in combination with a release agent, the proportion of the melamine resin in the total mass of the polyester film is preferably 20,000 ppm (particularly 19,000 ppm) or less, which makes it easier to satisfy the autohydrolysis resistance parameter SHS of 1.50 or less.

[0037] <Easy-Adhesion Layer> The polymer such as a resin constituting the easy-adhesion layer is not particularly limited as long as it is a resin or the like that provides easy adhesion, and specific examples of the polymer that can be used include polyester resins, acrylic resins, urethane resins, polyvinyl resins (such as polyvinyl alcohol), polyalkylene glycols, polyalkyleneimines, methyl cellulose, hydroxy cellulose, and starches.

[0038] <Antistatic Layer> The antistatic layer can be formed by directly applying an antistatic agent or by dispersing an antistatic agent in a binder component.

[0039] The polyester film of the present invention is preferably a film used in release film applications, and the polyester film of the present invention is preferably used as a used film to regenerate substrate films for release films, and more preferably used to regenerate substrate films for release films for multilayer ceramic capacitors.

[0040] 2. Polyester Film Recycling Method One aspect of the present invention is a polyester film recycling method for converting used polyester film into polyester film again through a melting step and an extrusion step, which includes a step of determining the autohydrolysis resistance parameter SHS, expressed by Equation 1, prior to melting the used polyester film, and determining whether or not a polyester film having an autohydrolysis resistance parameter SHS of more than 1.50 is subjected to the melting step. SHS = 0.245 {[IV] ｆ －１．４７ -[IV] ｉ －１．４７}...Formula 1 (wherein, [IV] ｉ indicates the intrinsic viscosity IV of the polyester film. [IV] ｆindicates the intrinsic viscosity IV after the polyester film is heat-treated at 300°C for 2 hours under a nitrogen atmosphere.) According to the present invention, it is possible to determine whether a used polyester film has poor recyclability at any stage before melting the used polyester film, for example, at the stage of collecting the used polyester film or at the stage of preparing the polyester film. Furthermore, according to a preferred embodiment of the present invention, it is possible to determine whether a used polyester film still having a functional layer (preferably a release layer and / or an easy-slip layer) has poor recyclability.

[0041] Another aspect of the present invention is a method for recycling polyester films, which comprises blending a plurality of used polyester films and remaking the resulting polyester film through a melting step and an extrusion step, wherein the used polyester films satisfying the autohydrolysis resistance parameter SHS of 1.50 or less, expressed by Equation 1, account for 50 mass % or more of the total used polyester films. SHS = 0.245 {[IV] ｆ －１．４７ -[IV] ｉ －１．４７}...Formula 1 (wherein, [IV] ｉ indicates the intrinsic viscosity IV of the polyester film. [IV] ｆ indicates the intrinsic viscosity IV after the polyester film is heat-treated at 300°C for 2 hours under a nitrogen atmosphere.) According to the present invention, it is possible to determine whether a mixture containing a used polyester film has excellent recyclability. Furthermore, according to a preferred embodiment of the present invention, it is possible to determine whether a mixture containing a used polyester film that still has a functional layer (preferably a release layer and / or a lubricity layer) has excellent recyclability.

[0042] The used polyester film may be a used polyester film that has been distributed in the market, or may be a waste polyester film generated during the manufacture of products.

[0043] The autohydrolysis resistance parameter SHS, [IV], is expressed by Equation 1. ｉ and [IV] ｆ The preferred range may be the same as above.

[0044] Used polyester films having an autohydrolysis resistance parameter SHS of 1.50 or less, as expressed by Formula 1, account for 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and still more preferably 90% by mass or more of all used polyester films. Used polyester films having an autohydrolysis resistance parameter SHS of 1.50 or less, as expressed by Formula 1, may account for 100% by mass of all used polyester films.

[0045] The recycling method of the present invention preferably includes a step of pulverizing a used polyester film. The pulverization step is preferably performed without removing the functional layer of the polyester film. The surface of the functional layer may contain adhesives, ceramic green sheets, impurities, etc. This step can reduce the amount of waste compared to conventional recycling techniques, and shorten the time required to produce resin pellets and films.

[0046] The recycling method of the present invention preferably includes a step of chipping or pelletizing the pulverized polyester film to form recycled chips or recycled pellets.

[0047] Chips or pellets obtained by melting (e.g., melting at 280°C) ground polyester film preferably have a predetermined intrinsic viscosity IV. The intrinsic viscosity IV of the chips or pellets is preferably 0.51 dl / g or more, more preferably 0.52 dl / g or more, even more preferably 0.53 dl / g or more, even more preferably 0.54 dl / g or more, and even more preferably 0.55 dl / g or more, and is preferably 0.70 dl / g or less, more preferably 0.65 dl / g or less. That is, the intrinsic viscosity IV of the chips or pellets is preferably 0.51 to 0.70 dl / g, more preferably 0.52 to 0.65 dl / g, even more preferably 0.53 to 0.65 dl / g, even more preferably 0.54 to 0.65 dl / g, and even more preferably 0.55 to 0.65 dl / g.

[0048] As for the chips or pellets, a plurality of chips or pellets may be used, or chips or pellets derived from virgin polyester may be mixed. The recycling method of the present invention preferably includes a step of forming a mixture of the recycled polyester (A) and the virgin polyester (B) into a film. The film formation can be performed by melting the polyester mixture in an extruder, extruding it into a film, and cooling it on a rotating cooling drum to obtain an unstretched film, and then uniaxially or biaxially stretching the unstretched film.

[0049] The mixture of recycled polyester (A) and virgin polyester (B) is preferably used in both the surface layer and the core layer constituting the laminated polyester film, which is the base film, and is also preferably used in the single-layer polyester film (base layer), which is the base film.

[0050] This application claims the benefit of priority based on Japanese Patent Application No. 2024-054648 filed on March 28, 2024, Japanese Patent Application No. 2024-153400 filed on September 5, 2024, Japanese Patent Application No. 2024-188601 filed on October 25, 2024, Japanese Patent Application No. 2024-227838 filed on December 24, 2024, and Japanese Patent Application No. 2025-003753 filed on January 9, 2025. Japanese Patent Application No. 2024-054648 filed on March 28, 2024, Japanese Patent Application No. 2024-153400 filed on September 5, 2024, Japanese Patent Application No. 2024-188601 filed on October 25, 2024, Japanese Patent Application No. 2024-227838 filed on December 24, 2024, and Japanese Patent Application No. 2025-003753 filed on January 9, 2025 The entire contents of the specification are incorporated by reference into this application.

[0051] The present invention will be described in more detail below with reference to examples. In the following, unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass".

[0052] <Measurement of intrinsic viscosity IV and autohydrolysis resistance parameter SHS of polyester film> <<Intrinsic viscosity [IV] of untreated polyester film ｉ The polyester film was pulverized in a handy mill (pulverizer) to a powder of 20 mesh or less, and dried under reduced pressure at 130°C overnight. The IV of the polyester film was measured at 30°C using the sample after drying under reduced pressure ([IV] ｉ <<Intrinsic viscosity [IV] of polyester film after specific heat treatment ｆ >> The polyester film was placed in an ampoule and dried under reduced pressure at 130°C overnight. After replacing the atmosphere with nitrogen, the opening of the ampoule was sealed. The ampoule containing the film pieces was heated in a salt bath (sodium nitrate / potassium nitrate = 1 / 3) at 300°C for 2 hours. After cooling, the heated sample was allowed to cool, and the resin (polyester resin derived from the polyester film) removed from the ampoule was freeze-pulverized. The polyester resin was filtered through a glass filter and dried under reduced pressure at 130°C overnight. The IV was then measured at 30°C ([IV] ｆ <<Autohydrolysis Resistance Parameter (SHS)>> The autohydrolysis resistance parameter (SHS) was calculated by the following formula: SHS = 0.245 {[IV] ｆ －１．４７ -[IV] ｉ －１．４７ In addition, phenol / tetrachloroethane (mass ratio 60:40) was used as a solvent for dissolving the polyester when measuring the IV.

[0053] <Evaluation of Winding Properties> A polyester film having a width of 1000 mm and a length of 4500 m was wound into a roll using a winding device, and the appearance of the roll was evaluated visually. Specifically, the number of convex wrinkles formed by air infiltration visible from the roll surface was observed per product (one roll) and evaluated using the following criteria: ◯ (Good): 0 to 5 wrinkles or less △ (Acceptable): 6 to 20 wrinkles or less × (Unacceptable): 21 wrinkles or more

[0054] <Method for producing recycled pellets> A biaxially stretched film (polyester film) was put into a single-screw crusher and crushed using a 4 mm aperture screen at a rate of 100 kg / hour to obtain a crushed film product. The crushed product was then fed into a twin-screw extruder, melted at 280°C, and extruded through a die to obtain recycled pellets (hereinafter sometimes referred to as "re-pellets").

[0055] <Evaluation of film-forming properties> Recycled pellets and particle-free polyethylene terephthalate pellets (b) described below were mixed in a 50:50 ratio, dried under reduced pressure (3 Torr) at 180 ° C for 8 hours, and then fed to an extruder and melted at 285 ° C. The polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy 10 μm particle 95% cut), extruded into a sheet form from a nozzle, and then wrapped around a casting drum with a surface temperature of 30 ° C. using an electrostatic casting method, and cooled and solidified to form an unstretched film. The film-forming properties of the unstretched film at this time were evaluated according to the following criteria: ○ (Good): An unstretched film could be obtained. × (Unsatisfactory): The viscosity was low and a film could not be formed, and an unstretched film could not be obtained.

[0056] <Evaluation of Color Value of Recycled Pellets (Re-Pellets)> The color b value of the recycled pellets was measured using a desktop color difference meter ZE-5000 manufactured by Nippon Denshoku Co., Ltd. The color b value of the recycled pellets was measured five times, and the average value of the three central points excluding the maximum and minimum values ​​of the measurement results was taken as the color b value of the recycled pellets.

[0057] <Measurement of intrinsic viscosity IV of recycled pellets (re-pellets)> The recycled pellets were pulverized in a handy mill (pulverizer) to obtain powder of 20 mesh or less, and dried under reduced pressure at 130°C overnight, and the intrinsic viscosity IV was measured at 30°C ([IV]).

[0058] Reference Example 1 Preparation of Calcium Carbonate Particle-Containing Polyethylene Terephthalate Pellets (a) Calcium carbonate particles (manufactured by Maruo Calcium Co., Ltd.) having an average particle size of 0.6 μm as measured with a light transmission particle size distribution analyzer (SA-CP3, manufactured by Shimadzu Corporation) were charged into ethylene glycol, and the resulting mixture was filtered through a viscose rayon filter having a 95% cutoff diameter of 30 μm to obtain an ethylene glycol slurry of calcium carbonate particles.

[0059] Polyethylene terephthalate pellets (a) containing calcium carbonate particles were obtained by the following method. An esterification reactor was heated, and when the temperature reached 200°C, a slurry consisting of 86.4 parts by mass of terephthalic acid and 64.4 parts by mass of ethylene glycol was charged, and 0.03 part by mass of antimony trioxide as a catalyst, 0.088 part by mass of magnesium acetate tetrahydrate, and 0.16 part by mass of triethylamine were added with stirring. Next, the temperature was increased under pressure, and the gauge pressure was increased to 3.5 kgf / cm. ２ A pressurized esterification reaction was carried out under conditions of 240°C and 240°C. Thereafter, the pressure inside the esterification reactor was returned to normal pressure, and 0.040 parts by mass of trimethyl phosphate was added. The temperature was then raised to 260°C, and 15 minutes after the addition of trimethyl phosphate, the ethylene glycol slurry of calcium carbonate particles was added so that the concentration was 20,000 ppm relative to the produced polyester. After 15 minutes, the obtained esterification reaction product was transferred to a polycondensation reactor, and a polycondensation reaction was carried out under reduced pressure at 280°C. After completion of the polycondensation reaction, the product was filtered using a Naslon filter (manufactured by Nippon Seisen Co., Ltd.) with a 95% cutoff diameter of 28 μm, yielding polyethylene terephthalate pellets (a) containing calcium carbonate particles with an intrinsic viscosity of 0.62 dl / g.

[0060] (Preparation of Polyethylene Terephthalate Pellets (b)) A continuous esterification reactor comprising a three-stage complete mixing vessel having a stirrer, a partial condenser, a raw material inlet, and a product outlet was used as the esterification reactor. Terephthalic acid (TPA) was supplied at a rate of 2 ton / hr, ethylene glycol (EG) was supplied at 2 moles per mole of TPA, and antimony trioxide was supplied in an amount such that the Sb atom concentration in the produced PET was 160 ppm. A slurry of these was continuously supplied to a first esterification reactor of the esterification reactor, and the reaction was carried out at normal pressure for an average residence time of 4 hours at 255°C. Next, the reaction product in the first esterification reactor was continuously removed from the system and fed to a second esterification reactor, and EG distilled off from the first esterification reactor was fed to the second esterification reactor in an amount of 8 mass% based on the produced polymer (produced PET). Further, an EG solution containing magnesium acetate in an amount such that the Mg atoms would be 65 ppm based on the produced PET, and an EG solution containing trimethyl phosphate (TMPA) in an amount such that the P atoms would be 20 ppm based on the produced PET, were added, and the reaction was carried out at atmospheric pressure for an average residence time of 1.5 hours at 260° C. Next, the reaction product in the second esterification reactor was continuously removed from the system and fed to a third esterification reactor, and further, an EG solution containing TMPA in an amount such that the P atoms would be 20 ppm based on the produced PET, was added, and the reaction was carried out at atmospheric pressure for an average residence time of 0.5 hours at 260° C. The esterification reaction product produced in the third esterification reactor was continuously supplied to a three-stage continuous polycondensation reactor to carry out polycondensation, and further filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 90% cutoff of 5 μm particles) to obtain polyethylene terephthalate pellets (b) having an intrinsic viscosity of 0.620 dl / g.

[0061] (Production of laminated polyester film X1) The above polyethylene terephthalate pellets (a) were mixed with particle-free polyethylene terephthalate pellets (b) in a predetermined ratio, and dried under reduced pressure (3 Torr) at 180 ° C. for 8 hours. Then, the particle-free polyethylene terephthalate pellets (b) were fed to extruder 1, extruder 2, and the core layer (intermediate layer: polyethylene terephthalate pellets) was fed to extruder 3, and melted at 285 ° C. These three polymers were each filtered through a stainless steel sintered filter material (nominal filtration accuracy: 10 μm particles, 95% cut) and stacked in a three-layer confluence block equipped with a rectangular lamination section. Then, the extruded film was formed into a sheet by a die, and the sheet was wound around a casting drum with a surface temperature of 30 ° C. using an electrostatic casting method, and cooled and solidified to form an unstretched film.

[0062] This unstretched film was stretched 3.3 times in the longitudinal direction at 85°C, and then this uniaxially stretched film was stretched 4.0 times in the width direction using a tenter and heat-treated at 230°C for 5 seconds to obtain a laminated polyester film X1 having a total thickness of 31 µm, with a surface layer B containing calcium carbonate particles accounting for 20% of the film thickness, a surface layer A containing substantially no particles accounting for 40% of the film thickness, and a core layer accounting for 40% of the film thickness. The particle content of the surface layer B containing calcium carbonate particles was 5000 ppm relative to the total mass of the surface layer B.

[0063] (Example 1) A functional layer was formed on a laminated polyester film X1 (substrate film) using a coating liquid 1 having the following composition: (Coating Liquid 1) Normal hexane 97.98 parts by mass UV cation-curable silicone resin (UV9315, manufactured by Toshiba Silicon Co., Ltd.) 2.00 parts by mass Bis(alkylphenyl)iodonium hexafluoroantimonate 0.02 parts by mass (Formation of Functional Layer) The coating liquid 1 was applied onto the surface layer A of the laminated polyester film X1 using a wire bar, dried at 100°C for 30 seconds, and then irradiated with ultraviolet light (300 mJ / cm) using an ultraviolet irradiation device. ２ ) to obtain a polyester film having a release layer with a thickness of 17 nm.

[0064] Example 2 Coating solution 2 having the following composition was applied to the surface layer A of laminated polyester film X1 using reverse gravure so that the thickness of the release layer after drying would be 104 nm, and the coating was dried at 140°C for 15 seconds to obtain a polyester film having a release layer. Because methyl ethyl ketone and toluene are volatile solvents, they all become gases upon drying and evaporate into the atmosphere. The ratio of the mass of the release layer to the total mass of the polyester film (release film) was 0.36%. (Coating liquid 2) Methyl ethyl ketone 49.45 parts by mass Toluene 49.45 parts by mass Full-ether type methylated melamine (manufactured by Sanwa Chemical Co., Ltd., product name MW-30M, non-volatile content 100%) 0.95 parts by mass Release agent (single-terminal carboxyl-modified polydimethylsiloxane, X22-3710, solid content 100%, manufactured by Shin-Etsu Chemical Co., Ltd., alkyl group interposed between dimethylsiloxane and carboxyl group) 0.05 parts by mass p-toluenesulfonic acid (manufactured by Hitachi Chemical Co., Ltd., product name Dryer 900) 0.10 parts by mass

[0065] Example 3 The coating solution 2 was applied to the surface layer A of the laminated polyester film X1 using reverse gravure so that the thickness of the release layer after drying would be 503 nm, and the coating was dried at 140°C for 15 seconds to obtain a polyester film having a release layer. As in Example 2, the methyl ethyl ketone and toluene were all converted to gas by this drying and evaporated into the atmosphere. The ratio of the mass of the release layer to the total mass of the polyester film (release film) was 1.74%.

[0066] Example 4 The coating solution 2 was applied to the surface layer A of the laminated polyester film X1 using reverse gravure so that the thickness of the release layer after drying would be 549 nm, and the coating was dried at 140°C for 15 seconds to obtain a polyester film having a release layer. As in Example 2, the methyl ethyl ketone and toluene were all converted to gas by this drying and evaporated into the atmosphere. The ratio of the mass of the release layer to the total mass of the polyester film (release film) was 1.91%.

[0067] (Example 5) (Production of Acrylic Polyol Resin) A four-necked flask equipped with a stirrer, reflux condenser, thermometer, and nitrogen inlet tube was charged with 231 parts by mass of methyl methacrylate (MMA), 100 parts by mass of hydroxyethyl methacrylate (HEMA), 33 parts by mass of methacrylic acid (MAA), and 1153 parts by mass of isopropyl alcohol (IPA), and the temperature in the flask was raised to 80 ° C. while stirring. Stirring was carried out for 3 hours while maintaining the temperature in the flask at 80 ° C., and then 0.5 parts by mass of 2,2-azobis-2-methyl-N-2-hydroxyethylpropionamide was added to the flask. After nitrogen substitution was carried out while heating the flask to 120 ° C., the mixture was stirred at 120 ° C. for 2 hours. Next, a reduced pressure operation of 1.5 kPa was carried out at 120 ° C., and unreacted raw materials and solvent were removed to obtain an acrylic polyol resin. The pressure inside the flask was returned to atmospheric pressure, and the flask was cooled to room temperature, and 1,976 parts by mass of an aqueous IPA solution (water content: 50% by mass) was added and mixed. Thereafter, triethylamine was added using a dropping funnel while stirring, and the acrylic polyol resin was neutralized until the pH of the solution reached a range of 5.5 to 7.5, thereby obtaining an acrylic polyol resin solution with a solids concentration of 20%.

[0068] (Production of Oxazoline-Based Crosslinking Agent) A flask equipped with a stirrer, reflux condenser, nitrogen inlet tube, and thermometer was charged with 460.6 parts by mass of isopropyl alcohol and heated to 80°C while gently flowing nitrogen gas. A previously prepared monomer mixture consisting of 126 parts by mass of methyl methacrylate, 210 parts by mass of 2-isopropenyl-2-oxazoline, and 84 parts by mass of methoxypolyethylene glycol acrylate, and an initiator solution consisting of 21 parts by mass of the polymerization initiator 2,2'-azobis(2-methylbutyronitrile) ("ABN-E" manufactured by Nippon Hydrazine Kogyo Co., Ltd.) and 189 parts by mass of isopropyl alcohol were each added dropwise from the dropping funnel over a period of 2 hours to react, and the reaction continued for 5 hours after the completion of the dropwise addition. Nitrogen gas was continuously flowed during the reaction, and the temperature inside the flask was maintained at 80±1°C. The reaction mixture was then cooled to obtain a solution of an oxazoline-based crosslinking agent (a resin having oxazoline groups) with a solids concentration of 25%. The amount of oxazoline groups in the resulting resin having oxazoline groups was 4.3 mmol / g, and the number average molecular weight measured by GPC (gel permeation chromatography) was 20,000.

[0069] (Preparation of Coating Liquid 3) Coating liquid 3 having the following composition was prepared. (Coating Liquid 3) Water 41.86 parts by mass Isopropyl alcohol 35.00 parts by mass Acrylic polyol resin (solid content concentration 20%) 16.57 parts by mass Oxazoline-based crosslinking agent (solid content concentration 25%) 5.68 parts by mass Colloidal silica (manufactured by Nissan Chemical Industries, trade name MP2040) 0.59 parts by mass Surfactant (fluorine-based, solid content concentration 10%) 0.30 parts by mass

[0070] (Production of Polyester Film) Particle-free polyethylene terephthalate pellets (b) were dried under reduced pressure (3 Torr) at 180 ° C for 8 hours, then fed to an extruder and melted at 285 ° C. The polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy 10 μm particle 95% cut), extruded into a sheet from a die, and then wrapped around a casting drum at a surface temperature of 30 ° C. using an electrostatic casting method, and cooled and solidified to form an unstretched film. This unstretched film was stretched 3.3 times in the longitudinal direction at 85 ° C. to obtain a uniaxially stretched film. Coating liquid 3 was applied to one side of this uniaxially stretched film (hereinafter also referred to as the back side) using a bar coater, and then dried at 80 ° C. for 15 seconds. Here, the coating amount was adjusted so that the thickness of the coating layer (functional layer) after final stretching and drying was 0.1 μm. Next, this uniaxially stretched film was stretched 4.0 times in the width direction using a tenter and heat-treated at 230° C. for 5 seconds to obtain a polyester film having a thickness of 31 μm.

[0071] (Reference Example 2) A biaxially stretched polyester film having no coating layer (functional layer) was obtained in the same manner as in the production of the polyester film of Example 5, except that after uniaxial stretching, the film was stretched in the width direction without applying Coating Solution 3.

[0072] Example 6 A biaxially stretched polyester film was obtained in the same manner as in Example 5, except that the coating amount of Coating Solution 3 was adjusted so that the thickness of the coating layer (functional layer) after final stretching and drying would be 0.38 μm.

[0073] (Example 7) Coating liquid 2 was applied to the surface (hereinafter also referred to as the surface) of the base film of the polyester film of Example 5 that was not coated with coating liquid 3, in the same manner as in Example 2, and dried so that the film thickness of the release layer after drying was 0.1 μm (104 nm), to obtain a polyester film.

[0074] Example 8 A polyester film was obtained in the same manner as in Example 7, except that Coating Solution 2 was applied so that the thickness of the release layer after drying would be 0.14 μm.

[0075] (Example 9) The coating liquid 1 was applied to the surface (front side) of the substrate film of the polyester film of Example 5, on which the coating liquid 3 was not applied, using a wire bar. The coated film was dried at 100°C for 30 seconds, and then irradiated with ultraviolet light (300 mJ / cm) using an ultraviolet irradiation device. ２ ) to obtain a polyester film having a release layer of 17 nm (0.02 μm).

[0076] (Example 10) In the production of the polyester film of Example 9, a polyester film was obtained in the same manner as in Example 9, except that the coating solution 1 to be applied to the surface of the base film of the polyester film of Example 5 was applied so that the film thickness after drying would be 0.3 μm.

[0077] (Example 11) In the production of the polyester film of Example 9, a polyester film was obtained in the same manner as in Example 9, except that the coating solution 1 to be applied to the surface of the base film of the polyester film of Example 5 was applied so that the film thickness after drying would be 0.6 μm.

[0078] (Example 12) The coating liquid 1 was applied to the surface (hereinafter also referred to as the front surface) of the base film of the polyester film of Example 6, on which the coating liquid 3 was not applied, using a wire bar. The coated film was dried at 100°C for 30 seconds, and then irradiated with ultraviolet light (300 mJ / cm) using an ultraviolet irradiation device. ２ ) to obtain a polyester film having a release layer of 0.6 μm.

[0079] Comparative Example 1 A polyester film having a release layer was obtained in the same manner as in Example 2, except that Coating Solution 2 was applied so that the film thickness of the release layer was 628 nm. As in Example 2, methyl ethyl ketone and toluene all turned into gas upon drying and evaporated into the atmosphere. The ratio of the mass of the release layer to the total mass of the polyester film (release film) was 2.17%.

[0080] Comparative Example 2 A biaxially stretched polyester film was obtained in the same manner as in Example 5, except that the coating amount of Coating Solution 3 was adjusted so that the thickness of the coating layer (functional layer) after final stretching and drying would be 0.42 μm.

[0081] (Comparative Example 3) Coating liquid 2 was applied to the surface (surface) of the base film of the polyester film of Example 6 that was not coated with coating liquid 3, in the same manner as in Example 2, so that the thickness of the release layer after drying was 0.1 μm, and then dried to obtain a polyester film.

[0082] (Comparative Example 4) A polyester film was obtained by coating and drying in the same manner as in Example 2, except that coating liquid 2 was applied to the surface (surface) of the base film of the polyester film of Example 5 on the side not coated with coating liquid 3 so that the thickness of the release layer after drying would be 0.5 μm.

[0083] (Comparative Example 5) The coating liquid 1 was applied to the surface of the base film of the polyester film of Comparative Example 2, on the side not coated with the coating liquid 3, using a wire bar, and the coated film was dried at 100°C for 30 seconds, and then irradiated with ultraviolet light (300 mJ / cm) using an ultraviolet irradiation device. ２ ) to obtain a polyester film having a release layer of 0.6 μm.

[0084] The film configurations, functional layers, functional layer particles, functional layer ratios, film winding properties, intrinsic viscosities before and after heating, SHS, re-pellet intrinsic viscosity, re-pellet b value, and film formability used in Reference Examples 1 and 2, Examples 1 to 12, and Comparative Examples 1 to 5 are shown in Tables 1 and 2. In Table 1, "(surface layer A / core layer / surface layer B)" refers to the laminated polyester film X1.

[0085]

[0086]

Claims

1. A polyester film comprising a substrate film containing a polyester resin and a functional layer provided on at least one surface of the substrate film, wherein the autohydrolysis resistance parameter SHS expressed by the formula 1 satisfies 1.50 or less. SHS = 0.245 {[IV] ｆ －１．４７ -[IV] ｉ －１．４７ }...Formula 1 (wherein, [IV] ｉ indicates the intrinsic viscosity IV of the polyester film. [IV] ｆ indicates the intrinsic viscosity IV after the polyester film is heat-treated at 300°C for 2 hours under a nitrogen atmosphere.

2. The polyester film according to claim 1, wherein the functional layer comprises a melamine resin, and the content of the functional layer is 1 ppm or more and 20,000 ppm or less based on the total mass of the polyester film.

3. The polyester film according to claim 2, wherein the functional layer containing the melamine resin is a release layer.

4. The polyester film according to claim 2, wherein one surface of the base film has a functional layer containing the melamine resin, and a layer forming the other surface of the base film contains inorganic particles.

5. The polyester film according to claim 1, wherein the functional layer comprises an acrylic resin, and the content of the functional layer is 1 ppm or more and 11,000 ppm or less relative to the total mass of the polyester film.

6. The polyester film according to claim 5, wherein the functional layer containing an acrylic resin is a slip layer.

7. The polyester film according to claim 5, wherein the functional layer containing the acrylic resin further contains a crosslinking agent.

8. The polyester film according to claim 1, wherein one surface of the base film has a functional layer containing a melamine resin as the functional layer, and the other surface of the base film has a functional layer containing an acrylic resin as the functional layer, and the total content of the functional layers is 1 ppm or more and 11,000 ppm or less relative to the total mass of the polyester film.

9. The polyester film according to claim 8, wherein the functional layer containing a melamine resin is a release layer, and the functional layer containing an acrylic resin is a slip layer.

10. The polyester film according to claim 1, wherein one surface of the base film has a functional layer containing a silicone resin as the functional layer, and the other surface of the base film has a functional layer containing an acrylic resin as the functional layer, and the content of the functional layer containing the acrylic resin is 1 ppm or more and 11,000 ppm or less relative to the total mass of the polyester film.

11. The polyester film according to claim 10, wherein the functional layer containing a silicone resin is a release layer, and the functional layer containing an acrylic resin is a slip layer.

12. The polyester film according to any one of claims 1 to 11, which is used as a release film.

13. A polyester film recycling method for converting used polyester film into polyester film again through a melting step and an extrusion step, comprising a step of determining the autohydrolysis resistance parameter SHS expressed by Equation 1 prior to melting the used polyester film, and determining whether or not polyester film having an autohydrolysis resistance parameter SHS of more than 1.50 is subjected to the melting step. SHS = 0.245 {[IV] ｆ －１．４７ -[IV] ｉ －１．４７ }...Formula 1 (wherein, [IV] ｉ indicates the intrinsic viscosity IV of the polyester film. [IV] ｆ indicates the intrinsic viscosity IV after the polyester film is heat-treated at 300°C for 2 hours under a nitrogen atmosphere.

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